Nuclear magnetic resonance studies of zeolites

0079-6565/~$0130+ .50 Copyright@ 1984. Pergamon Press Ltd.

Progress in NMR Spectroscopy, Vol.16,pp.237-309, 1984 PrintedinGreatBritain. All rights reserved.

NUCLEAR MAGNETIC RESONANCE STUDIES OF ZEOLITES J. KLINOWSKI Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEP, England (Received 12 March 1984)

CONTENTS 1. Introduction 2. Outline of the Structure and Properties of Zeolites 3. Spectroscopic Considerations 3.1 General 3.2. Dipolar interactions 3.3. Chemical shift anisotropy 3.4. Quadrupolar interactions 4. *%i NMR Studies 4.1. Silicate solutions 4.2. Solid silicates and aluminosilicates 4.3. Determination of the composition of the aluminosilicate framework using *%i MAS NMR 4.4. Silicon-aluminium ordering in zeolites X and Y 4.5. The structure of zeolite A 4.6. *%i MAS NMR of gallosilicate zeolites 4.7. Highly siliceous zeolites 4.8. Resolving crystallographically non-equivalent tetrahedral sites 4.9. Factors determining resolution and lineshape of NMR signals 4.10. 29Si relaxation times in the solid state 5. “Al NMR Studies 6. Monitoring of Chemical Modification of Zeolites 6.1. Decationation and ultrastabilization 6.2. ‘H MAS NMR studies of zeolitic acidity 6.3. The mechanism of dehydroxylation of zeolites 6.4. Isomorphous substitution using silicon tetrachloride vapour 7. Precursors in Zeolite Synthesis 8. NMR Studies of Exchangeable Cations 9. Guest Molecules in Zeolites 9.1. General considerations 9.2. Water sorption and mobility 9.3. Multinuclear studies of sorbed species 10. Future Prospects Acknowledgements References

237 238 242 242 242 243 243 245 245 246 250 255 257 260 263 265 270 271 271 278 278 284 285 286 290 293 296 296 296 299 302 304 304

1. INTRODUCTION Both from the practical and from the academic point of view, zeolites are a remarkable class of materials. These hydrated porous aluminosilicates, composed of the three most abundant elements in the lithosphere, have found widespread industrial application as highly active catalysts, sorbents and ion exchangers-hence their appeal to the engineer. They fascinate the scientist because their well-defined geometry involving regular systems of intracrystalline channels and cavities with high total surface area and considerable void volume when dehydrated makes them ideal model systems in several areas of chemistry and surface physics. 237

238

J.KLINOWSKI

Modern zeolite science is roughly contemporary with NMR; the first synthetic zeolite without a natural counterpart was prepared by R. M. Barrer, a pioneer in the field, in 1946-7. The 50’s and 60’s saw intensive efforts directed at synthesising new zeolites which culminated in the preparation of Linde A in 1956 and zeolites X and Y in 1964-just before the advent of Fourier transform NMR and the inception af multinuclear applications of the technique. Further milestones were the discovery of the catalytic activity of zeolites in 1960 and the preparation of the first high-silica zeolites in the early 70’s. It was begun to be felt at that time that new techniques were required to study the structure and properties of synthetic zeolites which are usually obtained as polycrystalline powders. While single crystals of natural minerals readily lend themselves to conventional methods of structural investigation, the new synthetic zeolites were of necessity studied by the less powerful powder X-ray diffractometry, which required the support of indirect methods, such as sorption studies. Fortunately, the development of high-resolution solid-state NMR techniques, such as magic-angle spinning and cross-polarization soon gave zeolite chemistry a new and powerful structural tool. Zeolites are attractive objects to an NMR spectroscopist. The early work involved studying water of hydration and organics sorbed on zeolites using ‘H resonance and wide-line spectroscopy of cations such as ‘05T1+ and 23Na+. Later it became possible to observe 29Si, 27A1 (and recently also r’0) in the zeolitic framework. “Si studies in particular, pursued vigorously by several groups around the world, have provided many impressive new insights into the structure and chemistry of zeolites. It is fortunate that the relatively short spin-lattice relaxation times of the nucleus in crystalline aluminosilicates made such results possible. Magic-angle-spinning studies of protons in zeolites made progress towards elucidating the nature of catalytic acidity. The most recent advances are the use of probe molecules, such as various organics and sorbed xenon and observing ‘H, 2H, 13C 15N and iz9Xe resonances, in order to characterize intracrystalline environments; and the resolution of 29Si signals originating from crystallographically non-equivalent silicon atoms and relating them to structural parameters. I have no doubt that many further problems in zeolite science will be answered with the help of NMR. Some of the most pressing of these are: (i) Si, Al ordering in the zeolitic framework (known to be intimately related to catalytic activity); (ii) The magnetic and crystallographic equivalence, or otherwise, of the various Si and Al sites; (iii) Factors determining the acidity of zeolites; (iv) The mechanism of the removal of aluminium from the zeolitic framework and the nature of extra-framework Al; (v) The position and mobility of exchangeable cations and their interaction with guest molecules; (vi) The mobility, diffusivity, configuration and reactivity of guest molecules in zeolites; (vii) The relationship between NMR spectra and structure. Can structural details, such as T-O-T angles (T = Si or Al), be gleaned from NMR? This is the first attempt at a comprehensive overview of all aspects of NMR in zeolites.

2. OUTLINE

OF THE STRUCTURE

AND PROPERTIES

OF ZEOLITES

The name “zeolite” (from the Greek [EW = to boil and azeoa = stone) was coined by Cronstedt(‘J in 1756 to describe the behaviour of the newly discovered mineral stilbite. When heated, stilbite loses water rapidly and thus seems to boil. Zeolites are framework aluminosilicates built from cornersharing SiOq- and AlO:- tetrahedra and containing regular systems of intracrystalline cavities and channels of molecular dimensions. The net negative change of the framework, equal to the number of the constituent aluminium atoms, is balanced by exchangeable cations, M”‘, typically sodium, located in the channels which normally also contain water. The general oxide formula of a zeolite is M,,,(A102),(SiOz)y

. m&O.

It is invariably found that y 2 x. The simplest interpretation

of this, given that each silicate and

239

Nuclear magnetic resonance studies of zeolites TABLE 1. NMR

Isotope ‘H 2H ‘Li “B 1% ‘SN I’0 ‘9F 23Na =A1 *%i lz9Xe Z05T1

Spin Natural abundance 112 3;2 312 112 112 512 112 312 512 l/2 112 l/2

99.98 1.5x lo-* 92.58 80.42 1.108 0.365 3.7 x lo-* 100.00 100.00 100.00 4.70 26.44 70.50

properties of the nuclei discussed in this review. Relative sensitivity 1.00 9.65 x lo- 3 0.29 0.17 1.59 x lo-* 1.04 x 10-s 2.91 x lo-* 0.83 9.25 x lo- * 0.21 7.84 x 1O-3 2.12 x 1o-2 0.19

NMR frequency in a field of 2.3488T 100.00

Electric quadrupole moment Q (in multiples of e x 1O-24 cm’) -

15.351 38.863 32.084 25.144 10.133

2.73 x lo-’ -3.0x 1o-2 3.55 x 1o-2 -

13.557 94.077

-2.6 x lo-’ -

26.451 26.057 19.865 27.660 57.708

0.14-O. 15 0.149 -

aluminate tetrahedron is linked, via oxygen bridges, to four other tetrahedra, is that aluminate tetrahedra cannot be neighbours in a zeolitic framework, i.e. that Al-O-Al linkages are forbidden. This requirement, known as the Loewenstein rule,@) will be discussed in detail later in this review. There are at present around 40 identified species of zeolite minerals (with the %/Al ratio y/x = 1 to 5 depending on the structure) and at least 120 synthetic species with a very wide range of aluminium contents. Zeolites are prepared under mild (60400°C) hydrothermal conditions in strongly basic media. The type and concentration of the base are important structure-directing factors and a variety of organic bases are now being used in zeolite synthesis.‘3) The ZSM series (for Zeolite Socony Mobil) of highly siliceous zeolites is prepared from solutions containing alkylammonium bases. Other elements, such as Ga and Ge, can substitute for Si and Al in the zeolitic framework, and there are claims that many other elements can also do so. Recently, a whole new class of non-silicate zeolite-type crystalline aluminophosphates has been reported (4) but comparatively little is known about their properties. Zeolites display a number of interesting physical and chemical properties. The three classes of phenomena which are of greatest practical importance are the ability to sorb organic and inorganic substances, to act as cation exchangers and to catalyse a wide variety of reactions. The most important aspects of these are described below. The zeolitic channel systems, which may be one-, two- or three-dimensional (Figs 1,19 and 38) and may occupy more than SOo/0of crystal volume, are normally filled with water. When water is removed (which can usually be done reversibly) other species such as gaseous elements, CO,, CS,, ammonia, alkali metal vapours, hydrocarbons, alkanols and many other organic and inorganic species may be accommodated in the intracrystalline space. Depending on pore diameter and on molecular dimensions, this process is often highly selective, and gives rise to the alternative name for zeolites: molecular sieves. Thus dehydrated chabazite, with pore openings less than 5A wide, can sorb water, methanol, ethanol and formic acid, but not acetone, ether or benzene. By contrast, synthetic zeolite omega, with channels more than 7.4A wide, can sorb molecules smaller than, and including (n-C,F,),N. Thus zeolitic sorption is a powerful method for the resolution of mixtures. For example, Ca-exchanged zeolite Linde A can separate n-paraffins from other hydrocarbons. In addition to the effect of pore size alone, polar molecules are in general selectively sorbed in the presence of non-polar molecules. The reason for this is that zeolitic crystals are usually themselves highly polar. Commercial applications of molecular sieving are wide and include thorough drying of organics, separation of hydrocarbons and of N, and 0, in air and the removal of NH, and CS, from industrial gases. Cations neutralising the electrical charge of the aluminosilicate framework can be exchanged for other cations from solutions. Zeolites often possess high ion-exchange selectivities for certain cations,

240

J. KLINOWSKI (a)

Truncated

octahedron

(b)

Zeolite A

Fauiasite

Sodalite

(zeolites

X and Y)

FIG. 1. (a) The cubooctahedral building block (also known as “sodalite cage” or “b-cage”). Tetrahedral atoms (denoted by open circles) are located at the corners of polygons with oxygen atoms (not shown) approximately half-way between them. (b) The structure of zeolite A formed by linking the sodalite cages through double four-membered rings (compare Fig. 2). (c) The structure of sodalite formed by direct face-sharing of fourmembered rings in the neighbouring sodalite cages. (d) The faujasite structure formed by linking the sodalite cages through double six-membered rings. Exchangeable non-framework cations are not shown for clarity.

and this is used for their isolation and concentration. For example, NHf is efficiently removed from solution by clinoptilolite, a zeolite found in large sedimentary deposits. Other applications are the collection of harmful products of nuclear fission (such as 13’Cs and “Sr), water softening, the treatment of brackish water and the recovery of precious elements. Zeolites can act as sieves for ions just as they do for molecules in the course of sorption. For example, in zeolite Na-A sodium can be exchanged for n-alkylammonium cations, but not for branched alkylammonium or tetramethylammonium ions. Finally, molecule sieving properties of zeolites can be modified by ion exchange. Thus Na-A sorbs both N, and 0,, while Ca-A sorbs nitrogen preferentially to oxygen. But it is the ability to catalyse a wide range of reactions, such as cracking, hydrocracking, oxidation and isomerisation of hydrocarbons which by far overshadows all other applications of zeolites. Rare-earth exchanged and hydrogen forms (prepared indirectly by thermal decomposition of the

Nuclear magnetic resonance studies of zeolites

S4R

D4R

S8R

S6R

(b)

241

D6R

i: 633 P

a

D8R

D6R

FIG. 2. (a) The secondary building units in zeolite structures according to Meier. (io4) The positions of tetrahedral (T) silicon and aluminium atoms are shown in open circles; oxygen atoms (not shown) lie approximately hall-way between them. (b) Some polyhedra found in zeolite frameworks: (io5) the “a-cage” (26hedron of type I) or truncated cubooctahedron; the sodalite cage; double &membered ring; double 6-membered ring (hexagonal prism); the 18-hedron (“y-cage”); the 11-hedron (“s-cage” or “cancrinite cage”). ammonium form) of some zeolites, such as synthetic faujasite, mordenite, gmelinite and chabazite, have a cracking activity which is orders of magnitude greater than that of conventional silica/alumina catalysts. Zeolite-based catalysis was first discoven&‘) in 1960 and two years later cracking catalysts based on zeolite Y were introduced. They have now almost completely displaced conventional catalysts. The synthetic zeolite ZSM-5, introduced in 1972,@)is an even more powerful catalyst. Its high silica content @i/AI is typically 30) gives it high thermal stability, while the channel diameter, ca TABLE2. Classification of zeolites according to secondary building units. The number of tetrahedral atoms forming the main channel system in each zeolite is given together with free aperture of the channels in A. S4R phillipsite gismondine

8 8

3.9 X4.4 2.8 x 4.9

8 12 8 12 12 6

3.6 X 5.2 6.9 3.2x.5.1 7.4 7.4 2.2

D4R type A (Syn) type ZK-4 (Syn)

8 8

4.2 4.2

4-1

8 8 8

2.6 x 3.9 2.6 x 3.9 2.6 x 3.9

S6R erionite offretite levyne mazzite omega (Syn) Losod

natrolite scolecite thomsonite

(Syn): Synthetic. JPNHRS lh:3,4-D

D6R chabazite gmelinite faujasite type X (Syn) type Y (Syn) type ZK-5 (Syn) type L (Syn)

8 12 12 12 12 8 12

3.1 x 4.2 7.0 7.4 7.4 7.4 3.9 7.1

51

mordenite dachiardite ferrierite ZSM-5 (Syn) silicalite (Syn)

12 10 10 10 10

6.7 x 7.0 3.7 x 6.7 4.3 x 5.5 5.4 x 5.6 5.2 x 5.8

441

heulandite clinoptilolite stilbite barrerite

10 10 10 10

4.4 x 7.2 4.4 x 7.2 4.1 x 6.2 4.1 x 6.2

242

J.KLINOWSKI

is very convenient for many applications, particularly in the petroleum industry. The lo-membered channels of ZSM-5 are responsible for the quite striking shape selectivity: only certain reactants may penetrate the channel system and only certain products may diffuse out of it. This shape selectivity can be “tuned” even more finely by ion exchange. Catalytic properties of ZSM-5 include the ability to synthesise gasoline from methanol in a single step. Silicalite, a material which is isostructural with ZSM-5, but contains only small amounts of aluminium is, by contrast to most other zeolites, non-polar (i.e. hydrophobic) but organophilic. It is successfully used in the removal of dissolved organics from water. Zeolites containing transition metal ions (such as Cr 3+, Ag+ and Cu’+) are active as oxidation catalysts. Comprehensive reviews dealing with various aspects of the structure,“-lO) sorption,‘*) catalysis”“5) and other chemical properties of zeolites are available in the literature. 5.5&

3. SPECTROSCOPIC

CONSIDERATIONS

3.1. General In general, the nuclear spin Hamiltonian H=

is represented by the sum:06*r7)

HZ+HRF+HD+H’+HCS+HSR+HQ

(1)

where HZ accounts for the Zeeman interaction of the nuclear magnetic moment with the applied field B,; HRF results from the interaction between nuclear spin and the time-dependent radio frequency field B,; HD-from the direct dipole-dipole internuclear interactions; H’-the electron-mediated (indirect) interactions; Hcs -the chemical shift; HSR-the coupling of spins with the magnetic moment produced by the angular momentum of the molecule; and @-the quadrupolar interactions. HZ and HRF are under the experimentalist’s control, while the remaining terms depend on the nucleus in question and its environment. For the solids under consideration H” and HSR are unimportant and will henceforth be disregarded. 3.2. Dipolar Interactions

The dipole Hamiltonian

for the coupling between two nuclei i and j is:(“*‘s) HD = $yiyjh2r; 3 (Ii . Ij - 31,,1,,) (3 cos’ Oil- 1)

where yi, yj are nuclear gyromagnetic ratios, rij the internuclear vector, and eij is the angle between rij and B, which is by convention directed along the z-axis. It is seen from eqn. (2) that: (i) As the isotropic average co? Bij = l/3, thermal motion of molecules in fluids averages the dipolar interaction to zero; (ii) Dipolar interaction is strongly dependent on the internuclear distance and is therefore important only for nuclei in close proximity. The average distance between magnetically dilute nuclei such as 2gSi and 13C is large and so homonuclear dipolar interaction is unimportant. In zeolites the same applies to the 27Al-27Al interaction in view of the fact that the aluminate tetrahedra are never neighbours in the framework; (iii) If i and j are diferent nuclei (for instance 13C and ‘H) the heteronuclear dipolar interaction, which is often strong, can be removed by “dipolar decoupling” which consists of irradiating nucleus j (say ‘H) at its resonance frequency while observing nucleus i (say 13C). The timeaveraged value of the Hamiltonian is then zero. (iv) If the sample is rapidly spun at an angle A to the magnetic field, the time-averaged value of the angle between all internuclear vectors and the magnetic field Bij = A. When 8, = 54’44 (so that 3 cos2 8, - 1 = 0) the time-averaged value of HD is zero. This technique is known as magic-angle spinning (MAS).07~1Y~20~ In certain cases, notably those involving strong homonuclear proton-proton

interactions, the size

Nuclear magnetic resonance studies of zeolites

243

of the Hamiltonian is such that it cannot be reduced by MAS at rotation speeds achievable at present. Such strong interactions are reduced by multiple-pulse methods.“” 3.3. Chemical Shft Anisotropy The electrons modify the magnetic field experienced by the nucleus. The chemical shift interaction is described by the Hamiltonian ps

= hyI.a.B,

(3)

where 0 is a second rank tensor known as the chemical shielding tensor. In large magnetic fields u is symmetric and can be described by three principal values, ell, uzz, es3 and the three angles which define the orientation of the principal axes. It has been shown u’J’) that when the sample is spun at the angle j? to the magnetic field B, the time-averaged value of the tensor component along B, is (a,,)

=~0sin2j9+~(3c0s2j?--1)

i

ajj~os2~j

(4)

j=l

where 0 =

36711+~22+~,,)

and xj is the angle between the spinning axis and each of the three principal axes. We see that at the magic angle, when cos’ /? = l/3, sin2 /I = 2/3 and the time-averaged value of u,, is reduced to the scalar isotropic value u. Chemical shift anisotropy is thus removed. 3.4. Quadrupolar Interactions Nuclei with spin I 2 1 possess a quadrupole moment eQ and may interact with electric field gradients (EFG) present in the solid. The EFG is described by the traceless symmetrical tensor vij =

aZvfax,ax,

(5)

where V is the electrical potential and xi and xj are Cartesian coordinates. Taking V’, f V,, < V,. the tensor is described by five quantities three of which specify the orientation and two of which describe the magnitude and shape of the EFG: eq = V,, ? = (v,,-- ~J/vzZ’ In the principal axis system of Vij in which the tensor is diagonal the quadrupolar single spin is

(6) Hamiltonian

for a

(7) Three cases are now possible (i) HZ w HQ (ii) HZ < HQ (iii) HZ > HQ. No general analytical method is possible for obtaining the eigenstates and eigenvalues of Ho in case (i), while (ii) belongs to pure quadrupole resonance. We shall consider the usual “high field” case in which HZ >-zHQ and also take t) = 0 for simplicity (which assumes a field gradient of cylindrical symmetry) and we denote:

3e2qQ VQ= 2h1(21- 1) VL= $I,( 1- U)/2H a = I(I+

1).

(8)

244

J.KLINOWSKI

The various energy levels E,(‘), EC” m and E!$ (superscripts denote order) of eqn. (7) are obtained using perturbation theory. Instead of a single resonance frequency vL = (EC! 1 - ,!?:))//I as in the case of spin $ nuclei, there are now several resonance frequencies: v, = VL+ VP)+ v$

(9)

In the case of non-integer spin nuclei, such as “Al or 23Na, we observe only the central -*-i transition as the other transitions are spread over too wide a frequency range. The first- and second-order frequency shifts are: 1 v?’ = -VQ m-; (3cosZ82 ( > vi;,, = -___ l:v

c-i L (

1)

(1-cosze)(9cos~0-1).

(10)

>

Equations (10) lead to the following conclusions: (i) The first-order frequency shift vanishes for m = 3, which means that the central transition for non-integer spin nuclei is not affected to first order by quadrupolar interactions. It is clearly advantageous to work with these, as the - 1~0 and 0~ 1 transitions for integer spin nuclei are always shifted. (ii) The first-order shift is scaled by f(3 cos’ 0 - 1) and is therefore always reduced by MA.5 (iii) The second-order shift is not scaled by (3~0s~ 8-- 1). It increases with vi and is inversely proportional to the magnetic field. Bearing in mind that the dispersion of the chemical shift which is normally the parameter we endeavour to measure, is directly proportional to Bo, it follows that it is to our advantage to work at high fields, where the chemical shifts are larger and quadrupolar effects smaller. As the second-order frequency shift is always present, the feasibility of obtaining useful spectra depends on the magnitude of VQ.

The following strategy therefore emerges for the study of quadrupolar nuclei : use MAS (to remove dipolar coupling, chemical shift anisotropy and first-order quadrupolar effects), work at high fields (to minimise second-order effects), observe the central transition of nuclei with non-integer spin. It is often found that spin 8 nuclei have very long Tl relaxation times (up to several hours in some cases) particularly in amorphous solids. Quadrupolar nuclei however generally relax quite fast, which makes them of special interest in the study of the solid state. Expressions for the frequency of the central transition with fast MAS have been derived by Kundla et ~1.‘~~)and Samoson et ~1.‘~~)As re-written by Kentgens et CI~.‘~~) they are: v1/2

=

--v,+$g

a-; L (

[A(a)cos4~+B(a)cos2~+C(cr)], >

where A(u) = ~-5~COS2a+~92cos~2n B(cr) = -3+~~z+~~cos2n-;~2cos22~ 7 5 1 C(a) = 6-j~cos2~+18~2cos~2~, and ~1and B are the polar angles of the spinning axis with respect to the principal axes of the quadrupole tensor. As is shown in Fig. 3 there are singularities in the calculated powder pattern where av,,,/aa = 0 and av,,2/&!I = 0. Recently Samoson and LippmaaC2@ used a two-dimensional NMR technique for the study of half-integer quadrupole nuclei in powders without resorting to MAS. They found that central transition excitation spectra provide a useful means of selective determination of quadrupole interaction parameters in correlation with those describing other NMR interactions.

Nuclear magnetic resonance studies of zeolites

245

0

-v--y

---\I-$

FIG. 3. Theoretical line shape of the ) ++ -f transition of a quadrupolar magic-angle spinning for different values of the asymmetry parameter q.@”

nuclear spin in a powder with fast

A = -(+J(v&)[I(I+l)-$1.

It has been shown(2732*) that, for systems in which the second-order quadrupole interaction dominates the breadth of the central transition, optimum line narrowing is achieved by sample rotation at angles other than the magic angle. The theory of VASS (for Variable Angle Sample Spinning) has been given@‘) and the technique may in the future prove to be useful in the study of nuclei with high quadrupolar moments in zeolites. It has been successfully applied(29) to the 27A1 NMR study of the crystallization from a vitreous precursor of synthetic cordierite, Mg,Al,Si,O,,; and in work with various materials where the quadrupolar effects are much larger than in zeolites.(28) The influence of the various types of interactions on the NMR spectra of rotating solids has been fully discussed in the literature.(17*19~20*30)

4. 29SiNMR STUDIES 4.1. Silicate Solutions 29Si NMR studies of solutions are not easy because of the long spin-lattice relaxation times of the nucleus and a negative nuclear Overhauser enhancement. The 29Si-‘H dipole-dipole relaxation is inefficient because in most compounds the internuclear distance is large. Fortunately, since 1972 the problem of relaxation can often be overcome by resorting to cross-polarization (CP) methods.‘3’) The full range of 29Si chemical shifts is over SOOppm, but most shifts are to be found in a narrower range of ca 120 ppm.(32) Tetramethylsilane (TMS) is the accepted reference compound. The range of chemical shift anisotropies for 29Si is relatively small as compared with that for lsC, and in silicates the shielding tensor is nearly symmetric. High-resolution 29Si NMR has been extensively used in solution(32-44) enabling the structure of the various silicate anions present in alkali metal silicates, tetraalkylammonium silicates and solutions of silicic acids to be established. The total range of chemical shifts found is from -60 to - 120ppm from TMS; this is split up into well-separated intervals corresponding to silicon atoms in monosilicates, i.e. in isolated SiOi- groups (denoted by Q’), disilicates and chain end groups (Q’); middle groups in chains (Q’), chain branching sites (Q3) and fully cross-linked framework sites (Q4). All silicate anions can be described using combinations of Q” units, where the superscript refers to the number of silicon atoms linked, via oxygen bridges, to the central silicon.

J. KLINOWSKI

246

4.2. Solid Silicates and Aluminosilicates Conventional NMR studies of solid silicates give spectra with very broad lines. For example, the “Si signal from solid Na,Si,O, is 208ppm wide.‘45) Still, Holzman et a1.(46)were able to measure chemical shifts for silica (- 113 ppm from TMS), cristobalite (- 113 ppm), quartz (- 109 ppm) and several multicomponent glasses (-93 ppm). In 1972 Gibby et al.(47)determined principal elements of the “Si chemical shielding tensor for a number of organosilicon compounds at - 186% using cross-polarization. More recently, Grimmer et al.‘48) employed the technique to measure shielding anisotropies in two polycrystalline compounds containing (Si,07)6- and (Si,O,,)*- anions. Pioneering studies in high-resolution “Si NMR spectroscopy of solid silicon compounds are due to two research groups working in close collaboration-in Tallinn (led by Endel Lippmaa) and Berlin (Gunther Engelhardt). First, they used MAS combined with cross-polarization to obtain spectra of organosilicon compounds. (4g) Later they carried out a systematic study(50) of numerous silicates composed of different types of Q units. In solids of known composition containing more than one type of Q unit separate lines were observed in the requisite intensity ratio. For instance, the spectrum of xonotlite, Ca,(OH),(Si,O,,), a double-chain silicate branched at every third silicon

1

_Q3-QZ-Q2_&Q2_Q2_

n

[ contains two 2gSi signals, at - 86.8 and -97.8 ppm, in the 2 : 1 intensity ratio (Fig. 4). Lippmaa et al.“‘) concluded originally that the ranges of chemical shift corresponding to each kind of Q” unit are narrower and non-overlapping. Subsequent work involving a large number of silicates of various types(51) revealed that chemical shift ranges in silicates do in fact overlap significantly, with the exception of those for Q3 and Q4 units. Figure 5 summarizes the information available at present. The “Q notation” used above is not sufficient to describe the basic building units in aluminosilicates such as zeolites. While in framework silicates the environment of each silicon atom is always Q” (4 Si), in framework aluminosilicates there are five possibilities described by the formula Q4[nAl, (4 - n)Si] where n = 0, 1,2,3,4. We shall for simplicity denote these five basic units as Si(nA1) or Si[(4 - n)Si], which expresses the fact that each silicon atom is linked, via oxygens, to n aluminium and (4 - n) silicon neighbours. a*

i Xonotlite

Ca6(OH12[Si647] I

I a3

-60

ppm FIG.

4. High-resolution

-$_

(-J*_a*_

I _ Q3-

a*_QL

) n

-80 -100 -120

from

TMS

‘?3i MAS NMR spectrum

of xonotlite

at 39.74 MHzJsO)

241

Nuclear magnetic resonance studies of zeolites

I

I

Q’(SiOSi=180°) I&&ID QZ

I

-60

I

I

I

-70

I

1

-60

I

-90

I

I

I

I

-100

-110

ppm from

TMS

I

I

-120

FIG. 5. %i chemical shift ranges for silicates with different degree of condensation of SiO:- units, described using Q” notation (see text). The diagram is based on data from the study of 60 silicate minerals; the number of materials examined in each Q” range is given in brackets below. Q”-nesosilicates (22); Q’--sorosilicates (5); Q’(SiOSi = 180”)-sorosilicates linkages (4); Q2-inosilicates (17); Q3-phyllosilicates (7); Q4-silica polymorphs.

with nearly linear Si-0-Si

I am grateful to Dr. G. Engelhardt for supplying this figure in advance of publication of Ref. 51. Lippmaa et al. GO) found that when one or more Si atoms in a Q4 unit are replaced by Al atoms, a significant paramagnetic shift results, i.e. the “Si chemical shift becomes less negative. In general, the substitution Si[(n - l)Al] - Si(nA1) brings about a low-field shift of ca 5 ppm. The spectra of aluminosilicates were again in good agreement with known crystal structures. (50,53)For instance, the spectrum of the natural zeolite thomsonite, Na,Ca,(A1,,Si,,0,,). 24H,O, in which all Si atoms are crystallographically equivalent and alternate with Al atoms in the framework, contains a single signal at - 83.5 ppm, assigned to Si(4Al); while the spectrum of natural natrolite, Na,Al,Si,O,, .2H,O,

-60

ppm

-80

-100

from

-120

TM8

FIG. 6. High-resolution “Si MAS NMR spectrum of natrolite at 39.74MHz. (‘OrOpen circles denote Al atoms, closed circles Si atoms.

J. KLINOWSKI

248

which is known to contain two kinds of Si atoms, Si(3Al) and Si(2Al) in the 2 : 1 population ratio, shows two signals in the same ratio of intensity (Fig. 6). The spectra of zeolites X and Y (synthetic faujasites) show, depending on composition, all five Si(nA1) signals (Fig. 9), while the spectrum of zeolite A (with Si/Al = 1.00) has just one signal (Fig. 14) which indicates that the environment of each Si atom is identical, i.e. that there is ordering in the zeolitic framework. The most surprising result of the early 29Si MAS NMR experiments on zeolites is the magnitude of the shift in zeolite A: the signal is found at -89.0* 1 ppm, and coincides with the Si(3Al) signal in synthetic faujasites, the spectra of which could be unambiguously interpreted in terms of Si(nA1) units. This signal was expected at ca -84ppm, which would correspond to Si (4Al) units, i.e. to strict alternation of Si and Al atoms in the zeolitic framework. Lippmaa et u/.(53’concluded therefore that zeolite A consists of Si(3AI) units, which means that each Si atom in the structure is linked to three Al atoms and one Si atom and vice versa: each Al is linked to three Si and one other Al, thus breaking the Loewenstein rule which forbids Al-O-Al linkages. The structure of zeolite A is considered fully in Section 4.5. It seemed at first, on the basis of the spectra of fourteen different zeolites, that the Si(4Al) and Si (3Al) chemical shift ranges are non-overlapping, so that even a single-peak spectrum could be assigned 4

(a)

3

analcite

2

f%A_

h

zeolite

2

mordenit e

ppm FIG. 7.

from

TMS

High-resolution %i MAS NMR spectra of three synthetic zeolites at 79.80MH~.“‘~’ Si(nA1) signals are identified by the n above the peaks. (a) analcite; (b) zeolite Z; (c) mordenite.

Nuclear magnetic resonance studies of zeolites

249

TABLE3(a). Parameters of “Si MAS NMR spectra of zeolites for which the individual spectral signals can be directly assigned to Si(nA1)structural units (see text). “Si chemical shifts (ppm from TMS) Zeohte

Idealised unit cell composition

Si/Al

Si(4Al)

Na-A

CNa12AllZSilZ048 ~27W%

1.0

- 88.9

Li-A(BW)

Li,A1,SiLO,, .4H,O

1.0

-80.1

analcime

Nai,Al,$i,,O,,

.16H,O

cancrinite

Na,Al,Si,O,,

.CaCO, .2H,O

chabazite

Ca5Al10Si26072 .40H,O

gismondine

Ca,Al,SisO,,

gmelinite

2.0 1.0

- 85.4

2.6 1.0

- 89.9

Na,Al,Si,~O,s

.24HsO

2.0

- 86.8

laumontite

Ca,Al,Sii,O,,

.16H,O

2.0

leucite

KAlSi,O,

.18H,O

Losod

Na,,Al,sSi,,O.,,

mordenite

NasAl,Si400,6 . 24H20

natrolite

Na,.sAl,$i,,Os,~

RHO

(Na,Cs),,Al,,Si,,O,,

scolecite

Ca,Al,,Si,,O,,

sodalite

Na,AJ&O,,

thomsonite

Na,Ca,Al,,Si,,O,,

ZK-5

Na,cAl,,Si,,O,,,

ZSM-5

Na,Al,Si,,O,,,

16H,O .44H,O

.24H,O

.2NaCl 24HZ0 .98H,O .xH,O

1.0 5.0

Si(2Al)

Si(lAl)

-92.0 -

-96.3:

-108.0

99.48 - 104.8 -

-110.0 -

- 88.9

1.5

-97.4

-101.0

52

- 100.0

- 105.5

-111.6*

170

- 102.7;

- 108.0

276

-

52.57

-95.4

-92.5

- 97.2

1.5

-

- 86.0 - 88.6

-95.3

- 83.5

2.2

- 87.5

31.0

52

53

-

53 -

- 92.0

276

77

- 87.7*

1.0

52

-91.6

-

- 84.8

53

- 108.0 -

3.0

1.0

53

- 102.7

-

-

77

52

-97.1* - 85.2

Ref.

77

- 101.3

-92.4 -81.0

Si(OA1)

-

-94.0

.16HrO

2.0

Si(3Al)

- 97.6; -

-

-

53

- 103.5

- 108.6

276

- 101.8

- 111.8*

276

*Denotes the largest peak.

TABLE3(b). Parameters of ?Si MAS NMR spectra of zeolites for which the individual signals are composites and cannot be assigned to Si(nAl) structural units (see text). All the zeolites listed contain at least two non-equivalent kinds of tetrahedral site for silicon. All resonances from silicalite correspond to non-equivalent Si(4Si) units, but cannot at present be individually assigned to site groups. Idealised unit cell composition

Zeolite clinoptilolite

Na,K3Al,Si,,0,Z

erionite

NagAl$i,,O,,

ferrierite

‘24HzO

Si/Al

*%i chemical shifts (ppm from TMS)

Ref.

5.0

- 100.6; - 106.9:; - 112.8

.27H,O

3.0

-92.4; -97.9; - 102.4; - 107.0; - 112.3

Na,Al&,O,,

.8H,O

5.0

- 100.0; - 105.9;; - 110.5

heulandite

Ca,AlsSi,,O,,

.24H,O

3.5

-95.0; -99.0*; - 105.3; - 108.0

53

(K,Na)-L

KsNa~Al,Si2,0,,

3.0

-92.6; -96.6; - 101.2*; - 106.5

276

offretite

Na,Al 4Si 140 36 .14H,O (TMA),Na,Al,Si,,O,, .28H,O

3.5

-93.5; -97.5; - 102.3; - 107.2; - 112.5

omega

5.0

-93.0; -98.3*; - 105.7; - 113.1

silicalite

(SiO&

-

-109.2; -111.3; -112.0; -112.6; -113.1*; -113.9; -114.5; -115.3; -116.3

*Denotes the largest peak.

.21H,O

53 52 276

52 276 79

J. KLINOWSKI

250

Al 0 AIO~OSI

Al 0 AbosiO*l

:I

Al

SI

SiMAI)

SXIAI)

SKPAI)

4:o

32

22

si

Al 0 SiOSiOSi 0 SI

Al 0 *IO;OSu

0 SiOzOSi SI

SI(IAI)

SNOAI)

1.3

04

I

I

Si(OAI) Si(lAI) I

I

Si(2AI)

I

Si(3AI)

I

I

I

Si(4AI) -80

I

I

1 -90

I

I

I -100

I -110

ppm

from

I

TMS

FIG. 8. Ranges of “Si chemical shift for Si(nA1)building blocks in framework aluminosilicates. from the magnitude of the chemical shift alone. This has since been shown not to be the case. Lippmaa et a2.(53) correctly assigned signals in the spectra of most zeolites, even though they concluded, in the case of gmelinite and chabazite, that these materials also contain Al-O-Al bridges. We shall discuss the Loewenstein rule (also known as “aluminium avoidance principle”) in Sections 4.4. and 4.5, and show that these early results were incorrect. Many more 29Si spectra of natural and synthetic zeolites have been measured by various workers since the pioneering papers by Lippmaa et al. and Engelhardt et al. appeared. The results are listed in Table 3 in two categories. The first contains spectra which can be satisfactorily interpreted (see Section 4.3) the second the few known cases in which such a simple interpretation is not possible. Briefly, the reason for this is that some zeolites contain two or more kinds of crystallographically non-equivalent tetrahedral site, each corresponding to a distinct value of 6, the 29Si chemical shift. When the chemical shift difference, A& between two such sites with silicons surrounded by the same number of Al atoms is comparable with the chemical shift difference between Si(nA1) and Si[(n+ l)Al] units, spectral lines overlap and cannot be assigned simply. This effect, which has recently been turned into advantage in structural investigations of zeolites, is discussed fully in Section 4.8. The currently accepted ranges of chemical shift for various Si(nA1) environments in Q’ aluminosilicates are given in Fig. 8. 4.3. Determination of the Composition of the Aluminosilicate

Framework

using 29Si MAS NMR

As we have seen in the preceding section and in Figs 6-9, the 29Si MAS NMR spectra of zeolites consist of one to five signals corresponding quantitatively to different Si(nA1) building blocks. It follows that when a spectrum (i) contains more than one signal; (ii) is correctly interpreted in terms of Si(nA1) units, and (iii) no Al-O-Al linkages are present, it must be possible to calculate the Si/Al ratio in the sample from the 29Si spectrum alone. The justification for this conclusion is as follows. In the absence of A-O-Al linkages the environment of every Al atom is Al(4Si). Each Si-O-Al linkage in a Si(nA1) unit therefore incorporates iA atoms, and the whole unit n/4 Al atoms. The Si/Al ratio in the aluminosilicate framework is thus:‘s5-s8) 4 (Si/Al)NMs = 1 ISi(nAl) “to : ISi (13) n=cl

where Isi(,At) is the intensity

I

of the NMR signal attributable

to Si(nA1) units. Equation

(13) is

Nuclear magnetic resonance studies of zeolites

251

structure-independent and applies to all zeolites provided the assumptions made in its derivation are justified. It can, by implication, serve as a test for the correctness of spectral assignments and the absence of Al-O-Al linkages.

The validity of eqn. (13) has first been tested in the case of zeolites X and Y (synthetic faujasites) which can be obtained in a range of compositions (Si/Al from 1.0 to ca 2.75). Figure 9 gives “Si MAS NMR spectra of a series of zeolites X and Y obtained at 79.80MHz. The spectra were computersimulated using Gaussian peak shapes, and the areas of the individual deconvoluted signals were measured. Recognizing the fact that both the shape and the position of signals are affected by the neighbouring signals, this procedure allows “corrected” halfwidths and chemical shifts to be obtained. The results are given in Tables 4 and 5. As is evident from a comparison of the second and the last columns in Table 5, there is very good agreement between the Si/Al ratios obtained by X-ray

Si /Al

ppm

from TMS

FIG. 9. High-resolution ‘%i MAS NMR spectra of synthetic zeolites Na-X and Na-Y at 79.80MH~‘~~’ Experimental spectra are given in the left-hand columns; Si(nA1) signals are identified by the n above the peaks. Computer-simulated spectra based on Gaussian peak shapes and corresponding with each experimental spectrum are given in the right-hand columns. Individual deconvoluted peaks are drawn in dotted lines.

252

J. KLINOWSKI

Si /Al F

G

A A 3

2

2.0 2 1

2.35

:(’

L

H

A 3

\ : ,’ ::’ ,’ *

1 0 ‘>,’

2.56 JY 2 1 3

I

_fk

2.61

0

J

2.75 A

-8-10 p p m from TMS FIG. 9

(continued)

fluorescence and those calculated from the spectra. The individual spectral signals have therefore been interpreted correctly, and the Loewenstein rule is obeyed in the structure. Melchior et ~1.‘~~)gave an elegant graphical demonstration of this. They pointed out that if the distribution of Si and Al in the framework were purely random, the average number of Al neighbours per Si atom would be equal to the average overall population: A = 4/( 1 + R) where R = Si/Al. If however, Al-O-Al linkages were forbidden, the average number of Al neighbours would be A = 4/R. Figure 10 clearly shows that the latter is the case. Figure 11 demonstrates that high-field spectra of synthetic chabazite and gmelinite,@‘) which on the basis of lower resolution spectra were thought to contain Al-O-Al IinkageP) do in fact satisfy eqn. (13). It now seems that the Loewenstein rule is obeyed by all zeolites (compare Section 4.5 dealing with the structure of zeolite A)-at least on a spatially averaged basis. It is of course possible that occasional Al-O-Al linkages are present as structural defects.

-

- 83.7 84.4 0.7

1.35

1.59

1.67

1.87

2.00

2.35

2.56

2.61t

2.75

2:: ASI (ppm)

2

3

4

5

6

7

8

9

10

--88.1 89.6 1.5

-88.7 - 88.6 88.5 - 89.0 88.7 - 88.6 88.6 - 88.6 - 89.6 - 89.7

-88.3 - 88.2

-88.1 -88.3

-88.1: -88.1

-88.1 - 88.5 88.6 - 88.4

Si(3Al)

-93.0 -95.0 2.0

-93.7 -93.8 - 94.9 -95.0

-93.6 -93.6

-93.7; -93.7 93.9 -94.1

-93.3* -93.3

-93.0 -93.1 - 93.2* 93.2

-94.0 -93.3

-93.1 -93.4

Si(2Al)

and corrected shift/ppm

faujasites

- - loo.4 97.7 2.7

- 100.4* - 100.4

-99.1* -99.1 99.3; - 99.3

-98.7 -98.6 99.6* -99.6

-98.1 -98.1

-98.6 -98.5 97.6 -97.7 97.9 -98.0

-91.9 -98.1

Si(lA1)

102.3 102.6

102.4 102.4

105.8 106.0

103:o) 102.8 103.4 103.6 104.5 105.3 104.1 104.6 104.9 105.1

- 101.9 106.0 4.1

(-

(I ;;$?

(- 101.0) - 101.9

-

Si(OA1)

2.85

3.15

3.0

3.15

2.85

3.1

3.5

3.0

2.95

2.4

Si(3Al)

3.4

3.0

3.2

3.0

2.9

3.25

3.7

3.1

3.0

2.5

4.5

4.8

5.4

5.2

4.2

3.65

3.3

3.0

3.0

1.0

Si(lA1)

3.05

2.5

2.3

2.7

3.3

3.5

3.4

3.4

2.2

1.0

Si(OA1)

peaks/ppm

for each Si(nA1) signal, and the span of these values

-

-

-

2.4

2.05

2.2

2.15

2.15

1.1

Si(4Al)

of simulated

Si(nA1) signals.‘5s)

Si(2Al)

of simulated

Widths at half-height

with widths at half-height

chemical

together

(lower numbers)

shifts in synthetic

* Peaks of highest intensity (c$ Table 5) the positions of which are fixed during simulation. t Denotes composition determined by analytical electron microscopy (energy dispersive X-ray analysis). and AS denote, respectively, the minimum and maximum values of corrected chemical shifts found $ &i”, L (A6 = 6,, -S,r”).

-

-

83.8 -83.7 - 84.0 - 84.0

-84.1* -84.1 83.9 - 83.8 83.9 -83.9

1.19

- 83.9* - 83.9

Si(4Al)

(upper numbers)

values of 29Si chemical

Experimental

and corrected

1

Sample no.

Si/AI (by XRF)

TABLE 4. Experimental

ib $ & J 8 g a R’ ;; 2 “0 =: 8

x F rt: 3 c B g

J. KLINOWSKI

254

TABLE 5. High-resolution *%i MAS NMR peak intensities (corrected for spinning sidebands 100) in synthetic faujasites determined by computer simulation, together with Si/Al determined from the spectra.@*) Normalized

peak intensities

and normalized to ratios, (Si/Alh,,,,

(ZIs,(nAI, = lOO)*

Sample no.

Si/Al (by XRF)

Si(4AI)

Si(3Al)

Si(2Al)

Si(lA1)

Si(OA1)

(Si/Al)nMa

1 2 3 4 5 6 7 8 9 10

1.19 1.35 1.59 1.67 1.87 2.00 2.35 2.56 2.61t 2.75

64.0 33.9 19.9 12.5 8.7 7.5 0 0 0 0

26.5 33.5 36.1 33.5 29.5 24.5 14.7 10.0 12.3 9.0

6.2 21.5 27.5 33.8 36.4 36.5 38.3 34.7 37.7 39.5

1.4 8.9 11.7 15.3 19.9 25.8 41.8 49.2 44.2 43.0

1.9 2.2 4.8 4.9 5.4 5.7 5.2 6.1 5.8 8.5

1.14 1.39 1.57 1.71 1.85 1.98 2.46 2.69 2.56 2.69

*For convenience,

when handling

the experimental j0

t

Composition

determined

by energy-dispersive

results, peak intensities lSi(nAl) =

were normalized

to 100, i.e.

loo.

X-ray analysis

using electron

microscopy.

Equation (13) provides the zeolite chemist with a powerful quantitative method for the determination offramework composition of zeolites. By comparing (Si/A1)NMa values with the results of chemical analysis, which gives bulk composition, the amount of non-framework (6-coordinated) aluminium can be calculated. This is of particular value in the study of chemically modified zeolites (see Section 6). Equation (13) works well for materials with framework Si/Al ratios below ca 10. For more sil-

I

I

2.0

2.5

I

1.5

Si/Al

3.0

(R)

FIG. 10. Plots of the average number, A, of aluminium neighbours for a silicon atom calculated for the “truly random” and the “Loewensteinian” distribution of Si, Al in a range of compositions of synthetic zeolites X and Y!s9) Experimental points were calculated from the first moment of the spectra assuming constant half-width and regular spacing of Si(nA1) signals.

255

Nuclear magnetic resonance studies of zeolites (Si/Al$,,

=

2.37

(.%/AI),,,

=

2.48

(Si/AI&,

=

2.30

(SilAl),,,

=

2.43

, -80

I

-90

(a) gmelinite

(b) 2

)

chabazite

I

-100

I

-110

I

-120

ppm from TMS FIG. 11. 29Si MAS NMR spectra at 79.80 MHz of two synthetic zeolites each containing only one crystallographic

kind of tetrahedral site.“‘) (a) gmelinite; (b) chabazite.

iceous zeolites, the 29Si MAS NMR spectrum is dominated by the Si(4Al) signal and the estimation of composition becomes inaccurate. In these cases, framework and non-framework aluminium can be determined from “Al NMR spectra (Section 5). 4.4. Silicon-Aluminium Ordering in Zeolites X and Y The considerations of the preceding section prompt the question whether 29Si NMR can be of assistance in determining the ordering (if any) of Si and Al atoms in zeolites, beyond the restrictions of the Loewenstein rule. The answer is a qualified yes. First, it should be noted that MAS NMR yields spatially and temporally averaged information, and the spectrum, apart from the cases where there is only one signal, does not by itself imply long-range order. It does, however, provide valuable subsidiary information. The obvious case to be considered first is that of synthetic faujasites, which come in a range of compositions, and for which a considerable amount of spectral information is available. Evidence of Si,Al ordering in zeolites X and Y is provided by the presence of discontinuities in the plot of the (cubic) lattice parameter versus the Si/Al ratio,@” which indicates stepwise rather than gradual change in Si,Al distribution. This effect is even more pronounced in synthetic faujasitic gallosilicates.(62) Once the existence of Si,Al ordering is accepted, the possible Si,Al ordering schemes may be

256

J. KLINOWSKI

x,.,,i3bi: bl:/bl: $1

X,.,,(Zt# M:/M: M: /kl: I$)*

E=39&

E=390

FIG. 12. Two of the possible Si,AI ordering schemes for zeolite X with Si/Al = 1.18. The ratio of intensities Si(4AI):Si(3AI): Si(2Al):Si(lAl):Si(OAI) corresponding to each scheme is given in the upper right-hand corner. E is the calculated electrostatic energy for the double-cage in units of (qe)‘/a. The asterisk denotes the scheme preferred by the authors of Ref. 58 from which the figure is taken. p # 0 denotes net dipole moment in double sodalite cage.

constructed. The areas under the peaks in the NMR spectrum are directly proportional to the populations of the respective structural units in the sample; it is therefore possible to estimate these from the experimental data (see Fig. 9) and to compare with the relative numbers of such units contained in models involving different, Si,Al ordering schemes. Klinowski et al.@*)examined a great number of such models, and found that for most Si/Al ratios more than one ordering scheme is compatible with the Si(nAl) intensities determined by “Si MAS NMR. They chose between the various ordering schemes on the basis of three criteria: (i) degree of agreement between actual spectral intensities and those required by the given model; (ii) compliance with cubic symmetry and the correct unit cell repeat (a0 = 24.7 A); (iii) minimum electrostatic repulsion within the aluminosilicate framework. Figures 12 and 13 show the preferred ordering schemes for Si/Al = 1.18 and 1.67, respectively. The approach described above does offer support for the view that a discontinuity in the unit cell parameter should occur at a well-defined Si/Al ratio: it was found that the electrostatic repulsion energy per number of Al atoms in the unit cell changes abruptly at Si/Al = 2.0. The question of Si,AI ordering in zeolites X and Y was considered fully by Melchior et a1.(5g)and by Engelhardt et al. (55)While the details of their preferred models are sometimes different, the broad conclusions are similar to those reached by Klinowski et a1.(58) As was stated above, “Si MAS NMR does not by itself imply Si,Al ordering in zeolites. It is therefore of interest to calculate relative intensities of the spectra if the distribution of tetrahedral atoms were random, but subject to the restrictions of the Loewenstein rule. Calculating the average relative populations of the five Si(nA1) building blocks is equivalent to calculating the expected

Nuclear magnetic resonance studies of zeolites

251

(a)

FIG. 13. Two of the possible Si,Al ordering schemes CM)for zeolite Y with %/AI =

1.67.

intensities in the 29Si spectrum. The probability of the occurrence of an Si-O-Al linkage is p = l/R, and that of an Si-0-Si linkage 1 -p. The probabilities of the five possible configurations are therefore : p4 4(1 -p)p3 6(1 -p)2p2 4(1 -~)~p (1 -P)~

for for for for for

Si(4Al) Si(3Al) Si(2Al) Si(lA1) Si(OA1).

Expected relative intensities can thus be calculated for the whole range of Si/Al ratios. There are then two possibilities: (i) calculated and measured intensities are significantly different; (ii) calculated and measured intensities are similar. When (i) applies, the Si,Al distribution clearly cannot be random. In case (ii) however, both ordering and disorder are possible. This is because “ordered” and “disordered” distributions do not require intrinsically different numbers of Si(nA1) units. In other words, it is possible to construct, for a given Si/Al ratio, a perfectly ordered model which requires the same spectral intensities as a random (but Loewensteinian) model. Thus NMR can disprove randomness but cannot prove it. A comparison of the observed Zsi(nAi)intensities with those calculated from the above formulae reveals rather poor agreement for 1 < R < 2, but a better agreement as R increases. Detailed analysis of 29Si MAS NMR intensities in the faujasite spectra in the light of the “random” model have been given by Peters,‘i3@ Mikovskyo3’) and Vega.038) 4.5. The Structure of Zeolite A Single-crystal X-ray diffraction measurements(63p64) on zeolite Na-A (with Si/AI = 1.00) led to the conclusion that the (cubic) space group of this zeolite is Fmk, and that the Si and Al atoms alternate

J. KLINOWSKI

258

-89.2ppm

Zeolite Si/AI-

1

-80

A 1 .OO

I

I

I

-80

-100

-120

ppm from TMS FIG. 14. 29Si MAS NMR spectrum of zeolite Na-A at 79.80 MHz.

the framework. The presence of the single 29Si resonance in the spectrum of zeolite A (Fig. 14) proves the existence of Si,Al ordering, but the chemical shift of the signal (- 89.2 + 1 ppm from TMS) is unexpected: the peak coincides with the Si(3Al), rather than Si(4Al), signal in zeolite X which is also built of sodalite units. Accordingly, the signal in the spectrum of zeolite A was assigned(50~52) to Si(3Al) building blocks. This means that, contrary to the results of X-ray studies, each Si atom in zeolite A would be linked to three Al atoms and one Si atom and vice versa: each Al atom would be linked to three Si atoms and one other Al atom. The suggested non-Loewensteinian structure of zeolite A(“’ involved alternating Si,Al distribution within the cubooctahedral sodalite cages, but Al-O-Al bridges in the double four-membered rings joining the cages. Since each tetrahedral atom is part of one such ring, each Al atom would be involved in an Al-O-Al linkage. However, this structure must be rejected on the grounds of incorrect unit cell repeat. Having confirmed the NMR results of Lippmaa et u~.,‘~O’ Bursill et 01.‘~~)re-examined the structure of zeolite A using high-resolution electron microscopy and neutron diffraction. They discovered a small rhombohedral distortiorP) m ’ dehydrated Na-A, and also found that many electron diffraction patterns seemed at variance with the Si(4Al) ordering scheme. They finally proposed another alternative structure for zeolite A, which is shown in Fig. 15(b). However, the problem had to be examined still further, when it was shown@‘) that hydrated Na-A is perfectly cubic, and that rhombohedral distortion in dehydrated Tl-A and Ag-A is extremely sma11.‘68*69) Distortion observed in Na-A is thus caused by factors other than Si,Al ordering. The controversy was finally resolved by the results of independent 29Si MAS NMR experiments by Thomas et al.(56) and Melchior et a/.“‘) who examined the spectra of zeolite ZK-4, which is isostructural with zeolite A, but has the Si/Al ratio greater than unity. The reasoning behind these experiments was as follows. The 29Si NMR spectrum of ZK-4 must contain more than one resonance, because Si/Al > 1. It had been established earlier (55*58)that for a given aluminosilicate structure the 2ySi chemical shifts for each kind of Si(nA1) unit are only marginally affected by variation in Si/Al ratio. Signals corresponding to the same type of Si(nA1) unit in zeolite A and zeolite ZK-4 should therefore coincide. Thus, assignment of the signal in the spectrum of zeolite A should be possible by matching it with one of the peaks in the spectrum of ZK-4. In turn, the correctness of the assignment of peaks in the latter zeolite can be checked by comparing the Si/Al ratio as obtained from the spectrum with the results of chemical analysis (see Section 4.3). The 29Si MAS NMR spectrum of ZK-4 does indeed contain several peaks (Fig. 16) and good agreement between (Si/Al)NMR and throughout

Nuclear magnetic resonance studies of zeolites

259

(b)

FIG. 15. (a) The original model for the structure of zeohte A: two sodalite cages (cubooctahedra) are linked through double four-membered rings. At the vertices of the cubooctahedra Si( 0) and Al( 0) atoms alternate, so that each Si is surrounded tetrahedrally by four Al atoms, oia oxygen bridges which are not shown for simplicity. This double unit repeats itself three-dimensionally resulting in the cubic Fm3c space group. Exchangeable cations are not shown. (b) An alternative model in which cubooctahedra are slightly distorted (not shown) yielding a rhombohedral space group R3. In this ordering scheme each Si atom is surrounded by three Al atoms and one Si atom, and each Al atom by three Si atoms and one AI atom. Thus AI-O-AI bridges are an integral feature of the structure. Notwithstanding earlier interpretations, ‘?Si MAS NMR supports the ordering scheme (a) (see text).

-70 I 1

-90 I ppm from

-110 II TMS

FIG. 16. ‘%i MAS NMR spectra of two uncalcined samples of zeohte ZK-4 at 79.80MH~.‘~~) Numbers above NMR signals are n in Si(nA1)for the most likely assignment. Signals marked with “m” are caused by small amounts of SiO:- (metasilicate) impurity. (a) (Si/A&ar = 1.77; (b) (Si/AQxa, = 1.56.

260

J. KLINOWSKI TABLE 6. “Si MAS NMR chemical shifts (in ppm from TMS) for the single signal observed in various cationic forms of zeolite A.@@ All values +0.5 ppm.

Cationic form Na-A Ba-A Ag-A Ag-A with enclathrated AgNO, TI-A Li-A

*%i chemical shift (ppm from TMS) - 88.9 -90.5 - 87.5 - 88.2 -88.8 -85.1

(Si/Al)Xaa ratios is obtained when the resonance at - 89.0 + 1 ppm is assigned to Si (4Al) rather than Si(3Al). The original structure of zeolite A is therefore vindicated, and it is appropriate that a controversy caused by an NMR result was finally resolved by NMR. Freude et u1.(71*153) argue that “Al MAS NMR of zeolite A also supports the Si(4Al) assignment. They measured “Al spectra of zeolites X, Y and A in various cationic forms. Chemical shifts for the sodium forms are 62.6,63.7 and 60.2 ppm from Al (H,O)z+ respectively. As no Al-O-Al linkages are present in zeolites X and Y, the environment of each Al atom is Al(4Si). Since 29Si and 27A1nuclei in the zeolitic framework have the same electronic environment, and a decrease of ca Sppm in 29Si chemical shift is observed on going from Si(3Si) to Si(4Si) in X and Y, the “Al chemical shift when going from Al(3Si) to Al(4Si) should also decrease. This conclusion is borne out by 27A1 chemical shifts in aluminosilicate glasses. In other words, if the environment of aluminium in zeolite A were Al(3Si), its “Al chemical shift should be higher than that of zeolite Y. In fact, the opposite is observed: 27A1chemical shifts of all cationic forms of zeolite A are lower than those for the corresponding forms of zeolites X and Y. When presenting the same argument earlier Freude and Behrens(72) arrived at the opposite conclusion by confusing the directions of scales of nuclear screening. To summarise, the “Si resonance of the Si(4AI) unit in faujasite is at ca - 83.9 ppm, but in zeolite A, which is structurally related (see Fig. l), at ca - 89.0 ppm. Thomas et a1.(56’were the first to suggest that the unusually low chemical shift in zeolite A is due to the presence of a unique structural unit: strained double four-membered rings (compare Fig. 1 and Table 2) with Si-O-Al angles of 129, 152, 152 and 177”. The presence of the nearly linear linkages modifies the bonding, which in turn affects the value of the chemical shift. This problem will be considered fully in Section 4.8. Early MAS NMR work indicated”‘) that apart from zeolite A, zeolite Losod also displays Si(3Al) ordering, and that sodalite and cancrinite may occur in either Si(4Al) or Si(3Al) ordering schemes. This is now thought to be incorrect. In Losod, cancrinite and sodalite Si/Al = 1.00, so that the assignment of the single-peak spectra was based on the value of the chemical shift alone. It has since become apparent (see Fig. 8) that the chemical shift ranges corresponding to Si(nA1) and Si[(n f l)Al] building units overlap considerably; furthermore Ref. 77 compared chemical shifts of different cationic forms of cancrinite. It is now clear (see Table 6) that 29Si chemical shifts are not insensitive to the type of cation. The origin of this effect is as yet uncertain, but it may be caused by the distortion of Si-O-T bonds (T = Si or Al) in the framework by highly polarizable cations, such as Li +. 4.6. 29Si MAS NMR of Gallosilicate Zeolites It has been known for many years that Al and Si in the zeolitic framework can be substituted by Ga and Ge respectively in the course of direct synthesis. Several (Si,Ga) and (Ge,Ga) zeolites (often containing Al as the third tetrahedral component) as well as (Ge,Al) materials have been reported, and their overall structures were shown to be closely similar to their aluminosilicate counterparts. In particular, gallium analogues of sodalite and faujasite have been prepared by Selbin and Mason(73) and Ktih1,‘62)gallium thomsonite by Barrer et ~l.,“~) and gallium analcime by Ponomareva et aL(75)

Nuclear magnetic resonance studies of zeolites

261

n

a

b

-90

-70 ppm FIG.

from

-110

TMS

17. 29Si MAS NMR spectra at 39.5MHz.“@ (a) zeolite ZK-4; (b) zeolite Na-X; (c) (Si,Ga)-sodalite.

Vaughan et a1.(76)and Thomas et aLcz3’) measured “Si MAS NMR spectra of the sodium forms of (Si,Ga)-sodalites and (Si,Ga)-faujasites. All preparations in Ref. 76 contained some aluminium (attempts at preparing completely Al-free compounds were unsuccessful), but the amounts involved were so small (less than 5 %) that the influence of Al on “Si spectra was negligible. (Si/Ga)NMs ratios calculated from a formula similar to eqn. (13) agreed closely with chemical analysis, which shows that in every case the gallosilicate equivalent of the Loewenstein rule applies (i.e. that no Ga-0-Ga linkages are present), and that the assignment of NMR resonances is correct. The composition of (Si,Ga)-sodalite was such that Si/Ga > 1, while (SiAl)-sodalite crystallizes with SijAl = 1.00. More than one “Si signal must therefore be observed (Fig. 17). It was found that the TABLE 7. A

comparison of relative intensities in the ‘%i spectra of ZK-4, Na-X and

(Si,Ga)-sodalite. Sample ZK-4 (Si,AI)-X (Si,Ga)-sodalite

(“) T denotes Al or Ga, as appropriate for each sample. Composition (Si/AI) = 1.32 (Si/AI) = 1.27 (Si/Ga) = 1.28

1Si(4T)/1Si(3T)

1.23 1.74 1.25

1Si(3T)IISi(2T)

3.18 2.16 1.98

262

J. KLINOWSKI

TABLE8. Comparison of the chemical shifts, 6 (in ppm from TMS) for the sodium forms of aluminofaujasite (Si,Al)-X and gallofaujasite (Si,Ga)-X of different compositions. W) T = Al or Ga. A denotes the difference (&o, - (6&, for signals with the same value of T. Chemical shift (6) Sample

(SP/T)NMR

(Si,AI)-X (Si,Al)-X (Si,Ga)-X

1.14 1.17 1.11

A

Si(4T)

Si(3T)

Si(2T)

Si(lT)

Si(OT)

- 83.9 - 84.6 - 17.7

-88.1 - 89.0 - 84.2

-93.1 - 94.2 - 90.3

-97.9 -98.8 - 96.4

- 102.4 - 103.1 - 102.8

6.6

4.4

3.4

2.0

distribution of signal intensities corresponding different from that measured in aluminosilicate

0

to the various Si(nGa) units in gallo-sodalite was zeolites ZK-4 and X of similar Si/T ratio (T = Al or

Ga). Results given in Table 7 indicate that the distribution of Si and Ga in gallosodalites, while always Loewensteinian, is different from the distribution of Si and Al in aluminosilicate zeolites. The 2gSi chemical shifts in gallium zeolites show interesting trends. Firstly, as seen from Table 8, they span a wider range (25.1 ppm in (Si,Ga)-X) than in the corresponding aluminosilicates (18.5 ppm in (Si,Al)-X). Secondly, the Si(nGa) atoms are deshielded (i.e. their chemical shift is less negative) in comparison with Si(nA1) silicons with the same n in aluminium zeolites (Fig. 17 and 18). The difference is proportional to n : Si(OGa) and Si(OA1)shifts are very similar, while the difference between Si(4Ga) and Si(4Al) is 6.6 ppm. This indicates that it is Ga in the first tetrahedral coordination sphere which is the main factor here. The origin of this effect is unknown, but Vaughan et ~1.~~~) suggest that, as in aluminosilicate zeolites, it may be related to the magnitude of the T-O-T angle. The “non-bonded radius” approach by O’Keefe and Hyde (‘s) indicates that Si-0-Ga angles would generally be smaller than %-O-Al angles, and consequently gallium substitution would tend to deshield the silicon atom.

J

-70

*

1

-90

.

I_

-110

, -70

ppm

*

1

-90

from

1

1

-110

, -70

.

1

-90

.

1

-110

TMS

containing small amounts of aluminium FIG. 18. *%i MAS NMR spectra of (Si,Ga)-faujasiteP6) Si/(Ga +Al) ratios of (a) 1.11; (b) 1.26; and (c) 2.00 compared with the spectra of their (Si,Al) analogues ratios of(d) 1.24; (e) 1.44; and (f) 1.95.

and with with Si/Al

Nuclear magnetic resonance studies of zeolites

263

The “Ga MAS NMR spectra of gallosilicate zeolites are rather too broad (FWHM ca 60 ppm) for ready interpretation.(23s) 4.1. Highly Siliceous Zeolites In naturally occurring zeolites the Si/Al ratio is always less than ca 5, but materials with much lower Al contents can be prepared in the laboratory. (9) Some of them are potent catalysts, and have the added bonus of high thermal stability due to high silicon content. In 1972 the Mobil Oil Corporation synthesised and patented a range of highly siliceous zeolites. ZSM-5 with the unit cell formula Na,Al,Si,, _XO192.16H,O with x < 27 and typically about 3 (corresponding to Si/Al = 31) is the best known member of this range;(6,‘08) ZSM-11 is another. The structure of ZSM-5 is based on two interlinked channel systems formed by lo-membered rings ca 5.5A in diameter (see Fig. 19). Zig-zag channels are running in the [lOO] direction; straight channels in the [OlO] direction. ZSM-5 possesses remarkable sorptive properties, stemming from the desirable aperture size and large internal volume. It is a powerful acid catalyst, capable of converting methanol into gasoline and the benzene/ethylene mixture into ethylbenzene. A crystalline microporous material called silicalite, isostructural with ZSM-5 but containing only traces of aluminium, was synthesized by the Union Carbide Corporation and described by Grose and Flanigen@“) and Flanigen et al.(“‘) Unlike ordinary zeolites, silicalite is organophilic and hydrophobic and can remove from water a

(b)

FIG. 19. The structure of ZSM-S/silicalite.“06r (a) Secondary building units (indicated by bold lines) each composed of 12 tetrahedral atoms are linked into chains, one of which is shown in the c direction. (b) The chains are interlinked to form a three-dimensional framework in which there are lO-membered ring openings (5.5 A in diameter) running in the [OlO] direction. In this portion of the ac structural projection, 0 denotes a tetrahedral site.

J. KLINOWSKI

264

I

I

I

I

-80

-100

-120

-140

ppm from TMS FIG. 20. ‘?Si MAS NMR spectrum

of zeolite ZSM-5 (Si/Al = 33.3) at 79.80 MHz.@~)

variety of dissolved organic compounds. Both ZSM-5 and silicalite display remarkable “shape selectivity”: because of the geometry of the channels only certain reactants may enter and diffuse through the crystals, and only certain products may diffuse out of the intracrystalline space. In the light of what has been said in Sections 4.2 to 4.6 about the appearance and interpretation of

-105

-110

-112

-100

-lC

-110 ppm

from

FIG. 21. (a) High-resolution

-114

-116

-118

-115

-120

-115

TMS

-105

-105

-110

-110

-114

-112

-114

-112

ppm

from

-116

-115

-118

-118

TMS

?Si MAS NMR spectrum of silicalite at 79.80 MHz. “‘) 6550 free induction decays were accumulated; repetition time Ssec. (b) The spectrum given in (a) can be computer-simulated using the minimum number of nine Gaussian-shaped peaks, shown individually below the simulated spectrum. The areas of the peaks are from left to right, in the ratio 0.98:2.70:2.19:2.63: 10.35: 1.30: 1.61:1.87:0.82 (see text).

Nuclear magnetic resonance studies of zeolites

265

29Si MAS NMR spectra of zeolites, one might expect the spectrum of a highly siliceous zeolite to be uncomplicated displaying a single Si(4Si) signal, sometimes with a smaller Si(3Si) resonance, depending on the Si/Al ratio. The spectrum of a typical sample of ZSM-5 is indeed featureless (Fig. 20). The discovery by Fyfe et al. (79)that the spectrum of a sample of silicalite with a particularly low aluminium content shows considerable fine structure (Fig. 21) was therefore something of a surprise. The resolution of the spectrum is exceptional, superior to that observed in any other zeolite. The complete spread of all the components is ca 6ppm, and the chemical shifts of all the peaks are characteristic of Si(4Si) groupings in highly siliceous materials. The observed multiplicity must arise from the crystallographically non-equivalent tetrahedral environments of the Si(4Si) sites. The spectrum may be simulated by a minimum of nine Gaussian signals, the intensities of which are approximately in the ratio 1: 3 : 2 : 3 : 10 : 1: 1: 2 : 1 (Fig. 21(b)). The relative intensities did not show perceptible change over a lO-fold increase in the delay time of the experiments, which indicates that they are quantitatively reliable. Using the intensities of the well-resolved lowest- and highest-field signals as base units of one, the total intensity of the peaks in the Si(4Si) multiplet is found to be approximately 24. This suggests that the space group of silicalite is one which contains 24 non-equivalent sites in the structural repeat unit, rather than the Pmma space group, which had been favoured earlier, but which has only 12 distinct sites. It is premature, at this stage, to attempt to assign individual 29Si signals to specific crystallographic sites, mainly because the details of the structure of silicalite are not sufficiently well established. The 27A1 MAS NMR spectrum of silicalite, indicating that, contrary to earlier claims, aluminium is present in the tetrahedral environment, will be discussed in Section 5.

4.8. Resolving Crystallographically Non-Equivalent Tetrahedral Sites A few cases have come to light where crystallographic non-equivalence of tetrahedral sites for silicon has been reflected in the 29Si MAS NMR spectra of zeolites. The most complex and the best resolved example is the spectrum of silicalite which was discussed in the preceding Section, anotherthe spectrum of the well-ordered natural zeolite scolecite, t5’) which contains two signals corresponding to non-equivalent Si(3Al) units (see Fig. 22). The correctness of the assignment of the latter spectrum is supported by the agreement between (Si/Al)N ~a and (Si/Al)XRF and also by the reference to the structure of scolecite as determined by F5lth and Hansen. (*l) Using the numbering of Si and Al sites as given in that reference, Klinowski et ~1.‘~‘) assigned the two Si(3Al) signals to the nonequivalent Si,(Al,Al,Al,Si,) and Si,(Al,Al,Al,Si,) silicon atoms. However, apart from silicalite, the spectra of synthetic zeolites do not reveal signals which may be assigned to non-equivalent silicon sites. In particular, no fine detail is observed in the spectra of zeolite ZSM-5 (Si/Al ratio typically ca 50) isostructural with silicalite, and of similar degree of crystallinity (see Fig. 20). However, studies of highly siliceous zeolites have revealed that when Al is isomorphously replaced by Si in the course of treatment with silicon tetrachloride vapour or ultra2

scolecite CaA11Si,010.3H,0

-70

-00

-110

ppm from TMS FIG. 22. *%i MAS NMR spectrum of natural scolecite from Poona, India, at 79.80 MHz.(~‘)

266

J. KLINOWSKI

h-7

Zeolite Omega

Zeolite Y n

c--

,,Jk_,-A,.,d\;_

-00 -5-l

400

-no -110

-90

-100

-110

-120

-100

-105

-110

3,s

-120

-115

ppm from TMS FIG. 23. %i

stabilization

MAS NMR

spectra at 79.80

MHz

of parent zeolites and their dealuminated forms (seetext).‘85)

(see Section 6) the resolution of the spectrum improves, and both the width and the of the Si(4Si) signal are affected. Since the overall zeolitic structure is unchanged, these observations strongly suggest that the resolution of the 29Si MAS NMR spectra of zeolites is ultimately governed by the amount of Al present and/or its distribution in the framework. The spectrum sharpens up when the Al content decreases (silicalite vs ZSM-5; dealuminated Y vs Na-Y) and the Si(4Al) signal in the perfectly alternating Si,Al framework of zeolite A is exceptionally narrow. Since, however, zeolites Y and A each contain only one type of crystallographic tetrahedral site, this effect should be studied using materials which do show such non-equivalence. Fyfe et .1.“**s3) and Thomas et ul.(84,85)tested this hypothesis by gradually decreasing the Al content of several carefully chosen zeolites, while monitoring the 29Si spectrum. Aluminium can be removed from zeolites by acid washing and other chemical means, but this creates framework imperfections. Isomorphous replacement of Al using SiCl, treatment (see Section 6.4) is completely successful only with zeolite Y. 29Si MAS NMR reveals that crystalline high-silica zeolites which preserve the structure of the parent material are best prepared by hydrothermal treatment of the ammonium-exchanged form (for details see Section 6.1). It is obvious that such isomorphous replacement of Al by Si without loss of crystallinity must simplify the 29Si spectra; if the degree of dealumination is high enough, only Si(4Si) resonances will remain. These arguments have been borne out by experiments. Figure 23 gives the spectra of the zeolites omega, mordenite, offretite and ZSM-5 before and after hydrothermal treatment.“‘) The framework Si/Al ratios in the products were of the order of several hundred. The intensities of the two signals in the spectrum of the siliceous zeolite omega are in the 2 : 1 ratio. While omega is more appropriately called synthetic mazzite, (86) both the previously suggested structure@‘) and the structure of mazzite@s) call for two non-equivalent tetrahedral sites in that chemical

shift

Nuclear magnetic resonance studies of zeolites TABLE 9.

Zeohte omega

Structural parameters of some zeohtes and the chemical shift for the Si(4Si) resonances.(85) T-O-T angle*

*?ji chemical shift (ppm from TMS)

Signal assignment

140.8 151.9

- 106.0 - 114.4

T, TI

142.5 151.3

- 109.7 - 115.2

T, T*

6.2

150.4 152.3 156.01

-

Si/Al ratio (parent material) 4.24

offretite mordenite

267

Y

2.61

144.8

TMA-sodalite

4.82

158.0

ZK-4

1.66

148.0

112.2 113.1 115.0 107.1

-116.2 -111.0

TI T, T,+T, -

* This value represents the mean of the four T-O-T angles which define a central tetrahedral site. TAverage value for 158.1°(Si,) and 153.9’(Si,). population ratio, and accordingly Thomas et ul.(s5) were able to assign the signals to these sites (see Table 9). A similar situation obtains in dealuminated offretite. The structure of mordenite, on the other hand, is known to contain four non-equivalent types of site in the 2 : 1: 1: 2 population ratio, and it is clear that the largest signal at - 115 ppm in Fig. 23 must be a composite. The spectrum of dealuminated ZSM-5 is essentially identical to that of silicalite, but as was already said in Section 4.7, the assignment of the individual signals to specific types of site is not yet possible. As the data bank of “Si chemical shift values has grown, Thomas et ,1.@@ suggested that a correlation exists between the chemical shift values and the T-O-T angles in zeohtic frameworks. As we have seen in Section 4.5, the Si(3AI) signal in zeolites X and Y is coincidental with the Si(4AI) signal

in zeolites A and ZK-4, and they suggested that this related to the larger value of the Si-O-Al angle in the latter structures. Soon after, Grimmer et a1.@” measured the unusually negative (- 128.5 ppm from TMS) chemical shift in the (non-zeolitic) mineral zunyite and commented on the apparent relationship between 6 and the Si-0-Si angle in zunyite which is almost 180”. Spectral information on several dealuminated zeolites(85) as well as on ZK-4 and TMA-sodalite studied by Jarman@‘) who also considered the quantitative relationship between 6 and 8, is given in Table 9. The average values of 13for hydrothermally dealuminated zeolite Y and dealuminated acid-washed mordenite, as determined by X-ray diffraction, are very close to those in non-

0

Zeolite

0 A

Offretnte Mordmte

R

‘r -120

1

110

14s

0

Zeollte

@

TMA

+

ZK-4

I 150

Y

Sodahte

I 155

\ 160

8 I deg FIG.

24. Relationships between the mean T-O-T angle and isotropic %i chemical shift for the Si(4Si) signals in zeolites.‘gl’

268

J. KLINOWSKI

dealuminated materials, and Thomas et a1.‘*‘)assumed that this is also true for other dealuminated zeolites. They proposed the following linear correlation between 6 in degrees and 6 the shift in ppm from TMS (see Fig. 24): 6 = - 25.44 - 0.57938. This line also fits the exceptional 0 for zunyite as well as several naturally occurring polymorphs of silica.‘gO) Equation 14 correlates the chemical shift of the Si(4Al) signal with the magnitude of the T-O-T angle. There is evidence, however, that similar relationships exist for Si(nA1) signals with n # 0. Indeed, this line of inquiry was triggered off by the large difference in the Si(4Al) chemical shifts between zeolites A and X. In an attempt to extend these arguments to alljue Si(nA1) signals, Ramdas and Klinowski(gl) used the accumulated NMR data on many zeolites to derive a semi-empirical relationship between the isotropic chemical shift of a Si(nA1) signal and Cdrr, the total (non-bonded) Si . . T distance (T = Si or Al) in Angstroms calculated from the T-O-T angle, assuming constant Si-0 and Al-O bond lengths of 1.62 and 1.75A respectively: 6 = 143.03 -20.34Xdn,

(15)

where Zdrr = [3.37n + 3.24(4 - n)] sin i.

The procedure was as follows. First, 6 was plotted as a function of Zdn for Si(OA1) units. An approximately linear relationship was found (see the lowest plot in Fig. 25). Next, assuming the same slope and including an additional term, 7.95 n, to account for the paramagnetic contribution of n tetrahedral aluminium atoms to the chemical shift of the central silicon, the remaining four lines in Fig. 25 were drawn for Si(nA1) units with n = 1,2,3 and 4 (these lines are therefore predictions rather than fits to experimental points). It is evident that the experimental data are close to the theoretical lines, despite the empirical character of the expressions. Ramdas and Klinowski suggest that their work may serve as a basis for the reliable estimation of the average value of the T-O-T angle for each kind of silicon in an aluminosilicate of unknown structure (including amorphous materials) from “Si MAS NMR alone.

-SO-

-llO-

11.75

12.25

12.75

13.25

Zd FIG. 25. Plots of the isotropic “Si.NMR chemical shift (in ppm from TMS), versus Ed,, (in A), the calculated . T non-bonded distance, for Si atoms in the five kinds of Si(nA1) tetrahedral environments. Aluminosilicates corresponding to the various points can be identified by consulting Ref. 91 from which the figure is taken.

total Si

269

Nuclear magnetic resonance studies of zeolites

It is now easy to see why 29Si MAS NMR signals in the spectra of certain zeolites, such as zeolite omega, lead to incorrect (Si/Al)NMa values when assigned to individual Si(nA1) units in the simple fashion described in Section 4.3. When A6, the chemical shift difference between signals from non-equivalent Si(nA1) units with the same value of n (8.4ppm in the case of Si,(OAl) and Si,(OAl) in zeolite omega), is similar to the shift difference between Si(nA1) and Si[(n+ l)Al] units, the effects of Al substitution and crystallographic non-equivalence overlap. The signals are composites, and the cannot be directly read off the spectrum. In the case of zeolite omega, the actual intensities Isi spectrum is the sum of two mutually overlapping families of signals, which we denoteoo3) Si,(OAl), Si,(lAl), Sir(2Al) and Si,(OAl), Si,(lAl) and Si,(2Al). When A6 is observable, but much less than the difference between Si(nA1) and Si[(nk l)Al] chemical shifts, a broadening of the spectral linewidths is observed. For example, the “Si spectrum of chemically untreated mordenite is broad because it is a superimposition offour sets of signals. Simplification of NMR information brought about by dealumination will in the future enable spectroscopists to identify such overlaps and reach a correct interpretation of the spectra.

17.96 (a)

Si (2AL) 79.80 MHz

(b) Si(3AI)

J I

-80

FIG. 26. %i

I

I

-100 ppm from

-120 TMS

MAS NMR spectra of zeolite Na-Y @i/Al = 2.61) at two magnetic fields.“35) (a) 17.96MHz spectrum; (b) 79.80MHz spectrum.

270

J. KLINOWSKI

4.9. Factors Determining Resolution and Lineshape of NMR Signals

The earliest “Si MAS NMR spectra of zeolites, measured in a magnetic field of 2.35T or lower, showed modest resolution. The considerable improvement in resolution on increasing the field (compare Fig. 26(a) and (b)) must therefore be due to the removal of some field-dependent effect. Resolution increases until fields of ca 4.70 T are reached, and no further improvement is observed (as opposed to S/N) above this value. Dipolar 2gSi-2gSi coupling is unlikely to be responsible, in view of the magnetic dilution of the 2gSi nucleus; the weakness of the interaction, which is in any case removable by MAS; and the fact that dipolar interactions are field-independent. Melchiore’2) was the interaction. Such interfirst to suggest that the likely cause is the 2gSi-27A1 dipole-quadrupole actions give rise to splitting and broadening of 13C MAS NMR spectra of cl-carbon atoms in aminoacids,(g3~y7) which disappears when i4N (spin 1) is substituted by 15N (spin 3). In zeolites an analogous effect is caused by the deviation of the quantization axis for 27A1from the direction of the applied magnetic field caused by the quadrupolar interaction; the dipolar Hamiltonian then contains angle-dependent terms other than (3 cos’ 0 - 1) and is not averaged to zero by MAS. It has been shown(g8~100)that these interactions, which are inversely proportional to the cube of the internuclear distance, are also strongly dependent on the magnetic field. It seems likely that the 2gSi-27Al coupling is significant mainly in Si(nA1) units with n # 0 (note that the Si(4Si) signal is narrow even at low fields) and that it is eliminated in the fields of 4.70T or larger. It has been shown before that the removal of aluminium from the framework leads to very marked narrowing of NMR signals. When Al is absent, the Si(4Si) lines are very narrow indeed (0.7 ppm or less). Fyfe et al.cs3) studied the effect of dealumination on linewidths of Si(4Si) signals in several zeolites. They found that substantial line narrowing occurs at Si/Al > 100, which indicates that the effect must be long-range in nature. They suggest that it is caused by a chemical shift interaction which is due to distribution of Al in the second-nearest and further coordination shells of silicon. Following Meier and Moeck,““) Klinowski(“‘) considered this effect in terms of “coordination sequences” Ni, giving the number of T-atoms in the i-th coordination sphere of the atom under observation. Thus Ni = 4 (by definition); while for the faujasite structure N, = 9, N, = 16, N, = 25 and N5 = 37. Consider the average number, Nf’, of aluminium atoms in each coordination sphere (see Table 10). The Nf’ aluminium atoms in the i-th coordination sphere may be distributed among the Ni T-sites in many different magnetically non-equivalent ways, each giving rise to a different chemical shift for the central Si atom: each Si(nA1) peak is in fact a sum of many narrower signals. Intrinsic crystallographic non-equivalence of Si atoms in certain zeolites is superimposed on this effect. Klinowski and Andersonuo3) considered the range of the chemical shift effect necessary to account for the experimental results. They noted that “n-th coordination sphere” rarely implies distances of the order of n&r, where drr is the average distance between neighbouring T-atoms. For example, Si atoms facing one another across the lo-membered channels in ZSM-5, i.e. in one another’s jfth coordination sphere, are ca 5.5 A apart, while dm = 3.OA. Th ey then calculated the average distance, (Tsi_AI in various zeolites for different Al contents. For example, in ZSM-5 with Si/Al = 33, asi-*, = 5.7 A. This quantity is inversely proportional to the cube of the Si/Al ratio and thus increases slowly with dealumination, which is why very high Si/Al ratios must be reached before the effect becomes negligible.

TABLE10. Average number, Nf’. of Al atoms in the i-th coordination sphere of a Si(4Si) atom in faujasites of different Si/AI ratio.““)

Na-Y Dealuminated zeolite Y

2.61 55.0

0 0

2.61 0.17

4.64 0.30

7.24 0.47

10.72 0.69

25.21 1.63

Nuclear magnetic resonance studies of zeolites

271

Hays et &u65) were the first to observe that thermal treatment of ammonium-exchanged ZSM-5 improves the resolution of the “Si MAS NMR spectrum to the point when it becomes indistinguishable from that of silicalite; treating Na-exchanged ZSM-5 has no such effect. In order to explain this unexpected result, some of the same authorso66) monitored heat treatment of numerous samples of H+, NHf and TPA+-exchanged ZSMJ, as well as Na+-exchanged samples. For the first category they observed a marked improvement in resolution, and argue that it is due to the removal, at high temperatures, of adjacent silanol groups present in the structure as crystallographic defects, resulting in ring closure in the silicate framework. 4.10. 2gSi Relaxation Times in the Solid State The chemical shift anisotropy of the 2gSi nucleus is generally small, and thus unlike in 13C solid-state spectroscopy at high fields, no sideband problems are encountered in MAS studies of silicates. 2gSi spin-lattice relaxation times of crystalline aluminosilicates are remarkably short when compared with those for i3C in solids so that relatively brief pulse delay times, typically less than 20 set, are adequate. Most spectra of synthetic zeolites were acquired with recycle times of 5 sec. Although there has been no systematic study of TI in aluminosilicates it is generally assumed that it is controlled by spin-diffusion from paramagnetic centres. The two recent studies of relaxation times concern layer silicates. Barron et al.(log) found that T, varies very considerably between different minerals, with an upper limit of ca 1.3 hr in the case of nacrite. There is little correlation between the total Fe3+ content of four samples and their respective TI values for 2gSi or ‘H. Watanabe et al.(“‘) investigated the influence of paramagnetic impurities on 2gSi MAS NMR spectra of twelve clay minerals. They found TI to be always less than 1 set, i.e. orders of magnitude less than reported by Barron et al.(log) There is a marked tendency for the linewidth to increase with Fe3+ concentration, demonstrating that dipolar interactions between 2gSi and the electron spin of the Fe3+ ion is largely responsible for line broadening. T, was found to be inversely proportional to the concentration of paramagnetic species. More work is clearly needed to resolve this controversy. It seems doubtful whether the short TI values in zeolites are in fact caused by paramagnetic impurities. Degree of crystallinity appears to be very important: when a sample of a synthetic zeolite is made amorphous by thermal treatment, TI increases by orders of magnitude, while the concentration of paramagnetics remains unchanged. Sometimes MAS spectra of minerals show particularly broad lines and spinning sidebands. Oldfield et al.,““) who measured such spectra for 2gSi, 27A1and 23Na in the feldspar sanidine, (K,Na)AISi,O,, argue that these effects are due not to chemical shift anisotropy, but rather to the presence of large magnetic susceptibility broadening. 5. 27A1 NMR STUDIES

27AI is a very favourable nucleus for NMR investigations: it has a 100 % natural abundance, with I = 5/2 and a chemical shift range of ca 450ppm. Cl’ 2, The linewidth of the 27A1resonance signal is a sensitive function of the symmetry of the nuclear environment. Useful chemical information can be obtained from the spectra provided quadrupole coupling and chemical shift effects can be separated. There are three situations in which this is possible: in solution, in single crystals and under conditions of fast specimen rotation. With quadrupolar nuclei of non-integer spin, the central (i- -)) transition, the only one which is normally observed, is independent of the quadrupolar interaction to first order as illustrated in Fig. 27, but is affected by second order quadrupolar effects which are inversely proportional to the magnetic field (see Section 3.4). The best spectra will therefore be obtained at very high magnetic field strengths. The 27A1resonance is useful in the study of ionic solutions, since the rates of exchange around the trivalent cation are so slow that separate species can often be observed.” 12)In particular, the spectra of alkaline aluminate compounds have been extensively studied.013-‘21’ It was found that in aluminate anions four-coordinated aluminium (with respect to oxygen) resonates at 60-80 ppm from

J. KLINOWSKI

272

s/2

_--5,2

__-------

T 3/*

-----___

%

----___

-----+

-‘A

3/2

---_ -_

1 I

x?

---_ --._---_

-B-y2

-3

/2

$

------_______r~~~

__--

-5

_--

2

__--

/2

FIG. 27. Energy level diagram for a spin S/2 nucleus showing the effect of the first-order quadrupolar interaction on the Zeeman energy levels. Frequency of the central transition (shown in bold lines) is independent of the

quadrupolar interaction to first order, but is subject to second-order quadrupolar effects(seetext).

the six-coordinated Al in Al(H,O)z+. Thus 27A1 NMR is a sensitive tool for determining the coordination of aluminium. Conventional single-crystal studies of aluminosilicates date back to the early 50’s, and principal components of the quadrupolar coupling tensors together with asymmetry parameters have been determined for a number of compounds (see Table 11). Wide-line NMR measurements of the dependence of the 27A1 and 23Na quadrupolar coupling constants in analcime, zeolites Na-X and Na-A on the water content were carried out by Gabuda et ~1.‘~~‘) Alkali and alkaline-earth aluminosilicates are insoluble, and Miiller et a1.‘131) resorted to tetramethylammonium (TMA) aluminosilicates to measure 27A1 chemical shifts in aluminosilicate solutions. Solutions with different Si/Al ratios and the pure TMA aluminate solution were studied. The molar ratio TMAOH : Si : Al varied from 3 : 0 : 2 to 9 : 6 : 2. Theoretically there are 15 distinct Q”(mSi) units with Q = Al (n from 0 to 4 and m from 0 to n). However, dimeric aluminate anions are

TABLE11. Nuclear quadrupole coupling constants eQVzz/h and asymmetry parameter q in some aluminosilicates. Coordination number of aluminium

Mineral

eQVJh

MHz

‘I

Ref. 122

6

2.950

0.94

euclase HBeAlSiO,

6

5.173

0.698

123

beryl Be,Al,Si,O,,

6

3.093

0

124

4

spodumene LiAISizOs

.2H,O

1.663

0.5029

125

albite NaAlSi,Os

4

3.29

0.26

126

microcline KAlSi,O,

4

3.21

0.21

126

kyanite A12Si0,

6 6 6 6

10.04 9.37 6.53 3.70

0.27 0.38

127 127 127 127

sillimanite Al,SiOS

6 4

8.93 6.17

not determined

128 128

andalusite Al,SiO,

6 5

0.08 0.69

129 129

natrolite Na,Al,Si,O,,

15.7 5.9

0.59 0.89

Nuclear magnetic resonance studies of zeohtes

273

found only in very concentrated solutions and even then in very small quantities, which led the authors to suggest that the Loewenstein rule is obeyed in aluminate and aluminosilicate anions. The exclusion of Al-O-Al linkages limits the number of possibilities to five Q”(nSi) structural units with n = 0,1,2,3 and 4. Several resonances were identified in the spectra, and the proposed interpretation of the various lines is that in the series where Q denotes the central Ccoordinated Al atom the chemical shift decreases as follows:

Q0

Q’(lSi)

Q’(2Si)

Q3(3Si)

79.5

74.3

69.5

64.2

ppm from Al(H,O)z+

where numbers below the Q symbols denote chemical shifts of the proposed groupings. No Q4(4Si) signal, which according to the above series would be expected at ca 55 ppm, was observed. Miiller et a1.032) were the first to carry out a systematic investigation of Z7Al MAS NMR spectra of polycrystalline aluminates (at 70.4 MHz). They found that the isotropic “Al chemical shifts depend primarily on the coordination of aluminium with respect to oxygen. For the tetrahedral coordination, chemical shifts of 55 to 80ppm from AI(H,O)z+ are observed, while octahedral Al resonates at O-22 ppm. These results are in full agreement with the studies of Akitt et al.@13) in aqueous solutions of different aluminium species. As the structure of a number of aluminates has not been fully established by X-rays because of the difficulty of obtaining single crystals of the necessary size, this result is of considerable value for structural elucidation. Thus only 6-coordinated Al was found(‘33) in 2CaO. A&O,. 8H,O and CaO Al,O, lOH,O, both of unknown structure. The presence of a range of 27A1 chemical shifts both for 4- and 6-coordinated aluminium indicates that the shift is influenced not only by the coordination number, but by other effects such as the composition of the second coordination sphere and the nature of the cation. The first 27A1 MAS NMR study of zeolites was carried out by Freude and Behrens.t72i They measured, first, chemical shifts and half-widths of signals from stationary samples of zeolites Na-A, Tl-A, Na-Y and TI-Y at 16MHz. For MAS frequencies vn such that vR > v$/v, the central line of the 27Al resonance is reduced to about l/3 of its original value. Freude and Behrens next calculated the quadrupole frequencies and the shifts of the centre of gravity of each line due to the quadrupolar interaction, at 70 MHz. Then, apparent line positions and line-widths, Sz$ and Gv:~~*~ were measured experimentally using MAS at 70 MHz. The corrected chemical shift value at 70 MHz was then calculated from the relationships 6,, = 6,, They were several ppm different from the apparent values (see Table 12). Samoson et a!.(’ 34) consider the line narrowmg of 27A1MAS spectra in detail. In terms of quantities defined in eqn. (8) the characteristic linewidth, s, for a quadrupolar nucleus is vQ

vQ

=

(v!

-

vL)/vL

6,.

s = v~[z(z+1)-~]/12vL.

(16)

For 27Al I = 512 and we have s = 3v$/4vr..

(17)

In order for a line to be narrowed by MAS the spinning frequency VRmust be greater than s. It follows that the largest interaction which can be narrowed in an 11.7T magnetic field (in which vL = TABLE12. Calculation of the corrected value of “Al chemical shift in various zeolites from the spectra of stationary samples at 16 MHz and of rotating samples at 70 MHz. t”) For explanation of symbols see text. P Gv:yy*s 81° w2 6, Sample kHz kaz Hz pim p;m ppm Na-A (hydrated) Tl-A (hydrated) Tl-A (dehydrated) Na-Y (hydrated) Tl-Y (hydrated)

4.2 2.9 8.4 5.1 4.1

220 183 310 256 217

-2.7 -1.8 -5.3 -3.5 -2.6

414

56.0

58.7

282

55.5

51.3

730 564 577

51.1 57.9 59.1

56.4 61.4 61.7

214

J. KLINOWSKI

130.32 MHz for 27Al) must obey the relationship: v&r

< 173.76MHz.

(18)

Samoson et al. illustrate their argument with the 27Al MAS NMR spectrum of sillimanite. For the signal from 6-coordinated Al in sillimanite s = 9.1 kHz, (12*)which means that the signal cannot be narrowed by MAS; however, for Ccoordinated Al s = 5.3 kHz ( < vR), and a clear-cut lineshape which compares well with theoretical calculations, is indeed visible in the spectrum. Fyfe et a1.“35) measured high-resolution solid-state 27Al MAS NMR spectra of a number of zeolites at 104.22 MHz. All the spectra contained one narrow peak with a chemical shift ranging, in different materials, from 51.5 to 65.0ppm from Al(H,O)z+ (see Table 13). In dealuminated zeolites an additional signal is observed corresponding to 6-coordinated Al in the zeolitic channels. Figure 28 shows the very substantial improvement in the quality of the spectrum on increasing the resonance frequency from 23.45 MHz for 27Al (proton frequency 90MHz) to 104.22MHz. The improvement, which is due to the reduction of the second-order quadrupolar interaction, involves the increased intensity and symmetry as well as the reduction of the width of the peak. Fyfe et uL(‘~~’give the maximum error in their observed chemical shifts as less than 1.5 ppm, calculated from the widths of peaks in stationary spectra and assuming that low-field broadening is entirely quadrupolar in origin. At 104.22 MHz the line widths in all zeolite samples are substantially the same, and all chemical shifts fall within the 50-70ppm range reported for 4-coordinated Al by Milller et a1.(‘32) and by Freude and Behrens.‘72) Chemical shift in zeolites X and Y remains virtually constant while the Al content is halved; on the other hand, shifts in different zeolites with the same Al content are sometimes quite different, although highly siliceous materials correspond in general to low values of chemical shift.

TABLE 13. Parameters

of 27A1 MAS NMR spectra of zeolites at 104.22 MHz.“~” Chemical shifts are given in ppm from AI(H,O):+.

Zeolite Na-A Na-X Na-Y

NH;-Y dealuminated Na-Y Na-cancrinite hydrate Na,Li-cancrinite hydrate mordenite (large port) mordenite (small port) sodalite hydrate natural sodalite Losod ZSM-5 gmetinite ferrierite chabazite

Si/Al

6

1.00 1.19 1.35 1.59 1.67 1.87 1.95 2.00 2.35 2.45 2.56 2.61 2.15 2.61 55 1.00 1.08 6.62 5.59 1.00 1.00 1.00 20-40 2.40 5.00 2.22

58.6 61.3 61.5 61.3 61.0 61.2 61.3 61.2 61.4 61.4 61.4 61.2 61.4 61.6 54.8 58.9 62.7 55.1 55.1 61.0 65.0 55.8 51.5-54.8 59.2 54.3 58.4

Avw Hz

390 565 671 612 596 641 501 586 550 414 538 560 531 549 1428 583 595 610 562 471 306 831 700 549 879 598

Nuclear magnetic resonance studies of zeolites

215

6 104.22 MHz

(a 1

L

I

I

I

100

0

-100

I

200

ppm

from

Al(H,O)Q

FIG. 28.

High-resolution “AI MAS NMR spectra ofzeolite Na-Y @i/Al = 2.61) at two magnetic fields.‘135’(a) At 104.22 MHz. The chemical shift is 61.3ppm from Al(H,O)z’. The two signals of low intensity, equidistant from the main peak, are spinning sidebands. (b) At 23.45MHz.

Fyfe et CX~.(‘~~) also recorded spectra of samples of zeolite Y dealuminated using silicon tetrachloride vapour. This method is discussed in detail in Section 6.4;here we shall only say that (i) an additional “Al signal appears at O.O+2ppm, which is due to residual 6-coordinated Al and (ii) there is a large change in the chemical shift of tetrahedrally coordinated Al-from 61.3 ppm in the parent material to 54.8 ppm in the dealuminated product. The latter effect indicates that the chemical shift is related to the neighbourhood of the AlO:- tetrahedron. The first-order tetrahedral neighbourhood of an Al atom is Al(4Si) (the Loewenstein rule), so that the actual magnitude of the shift must be governed by geometric factors such as T-O-T angles and by the composition of further tetrahedral coordination shells. In particular, the direction of the change in chemical shift on dealumination is in agreement with the arguments of Freude et CZZ.(‘~) (see Section 4.5) that “Al shift should decrease as the number of Al atoms in the neighbourhood of a central Al atom decreases. “Al MAS NMR spectra of chemically untreated zeolites are thus much simpler than their *‘Si counterparts. This is a direct consequence of the fact that while five types of Si(nA1) environments are possible for the silicon atom, only one possibility exists for the aluminium. However, while Si in zeolites is always present in Ccoordination, Al can be 4- or 6-coordinated, and *‘Al MAS NMR is a very sensitive quantitative probe for this. In other words, *‘Al NMR is most valuable in probing the coordination, quantity and location of Al atoms in chemically treated zeolites, but less useful than 2gSi NMR for direct structural determination. *‘Al MAS NMR is sometimes able to distinguish crystallographically non-equivalent tetrahedral Al atoms. One such example is zeolite omega, where two separate types of site are observed at 130.32MHz, in agreement with the structure of this zeolite (see Section 6.4). However, these sites cannot be separated at 52.11 MHz, which indicates that second-order quadrupolar effects are large (see Fig. 39).

216

J. KLINOWSKI

Important structural information is provided by the “Al MAS NMR spectrum of silicalite, a porous solid isostructural with zeolite ZSM-5 (see Section 4.7). It has been argued oo7) that silicalite has no aluminium and no cations in this structure and therefore no ion-exchange properties. In other words, it was claimed not to be a zeolite and the original patent @‘) describes it as a “porous form of silica”, and asserts that such aluminium as may be present is in the form of Al,O, impurity. Because it is not possible to monitor the coordination of very small amounts of Al (of the order of hundreds of ppm) by X-ray crystallographic methods, these claims could not be tested directly. Fyfe et LL(‘~)have demonstrated that : (i) (ii) (iii) (iv)

the aluminium in silicalite is readily detectable by *‘Al MAS NMR; all the aluminium is tetrahedrully coordinated to oxygen; there are at least two distinct types of tetrahedral framework sites occupied by the aluminium; the Si/Al ratio in the sample can be estimated from the spectra.

Figure 29 gives the *‘Al MAS NMR spectrum of the same sample of silicalite which produced the 29Si spectrum in Fig. 21. The fairly broad signal centered at ca 55.6 ppm from external Al(H,O)z+ is clearly characteristic of tetrahedral coordination. The signal shows Rne structure due to at least two components, at 54.5 and 56.7 ppm, indicating the presence of crystallographically non-equivalent sites for tetrahedral Al. It is interesting to note that when zeolite ZSM-5 is thoroughly dealuminated by a hydrothermal treatment (s2) to reach Si/Al = 800, the same splitting of the residual tetrahedral 27A1 signal is seen in the spectrum. Because of the 100 % isotopic abundance of 27A1and its very short spin-lattice relaxation time, even

54.5

I

(a)

56.7

2’5

100 ppm FIG. 29. High-resolution

50 from

0 Al(H20,:+

“AI MAS NMR spectrum of silicalite at 104.22 MHz. were accumulated; repetition time 0.1 sec.

v9) 176 ,214 free induction

decays

Nuclear magnetic resonance studies of zeolites

211

traces of aluminium are detectable by MAS NMR. A quantitative determination of Al concentration in the sample can be carried out(79) by adding a known amount of aluminous reference compound and comparing absolute intensities of the spectra before and after addition. Alternative i y, if the sample and the reference contain Al in different coordination, the intensities of the two peaks can be directly compared in a single spectrum. on the electric Kentgens et ~1.‘~~)used “Al MAS NMR to study the effect of hydration/dehydration field gradient at the 27A1 nucleus in zeolites ZSM-5 and H-Y at three different magnetic fields corresponding to proton frequencies of 300, 180 and 60MHz. Figure 30 shows their spectra of H-ZSM-5. The initial decrease of the linewidth of the signal in the hydrated samples on lowering the magnetic field suggests that at the highest field the linewidth is not controlled by quadrupolar effects; the authors suggested that a distribution of “Al chemical shifts is responsible, to be overtaken, at lower fields, by quadrupolar effects. It is also seen in Fig. 30 that for dehydrated samples of H-ZSM-5 quadrupolar effects are always dominant. The same applies to both the hydrated and the dehydrated samples of zeolite H-Y (not shown). Figure 31 shows the spectra of Na-ZSM-5 as a function of time of hydration. In dehydrated samples quadrupole interactions are so strong that the tetrahedral Al line disappears, to reappear on rehydration. Dehydration has no effect on octahedrally bound Al. These findings are in broad agreement with the wide-line measurements by Genser(149) who from HYDRATED

DEHYDRATED

100 Hz

FIG. 30. *‘Al MAS NMR spectra of zeolite H-ZSM-5 at three magnetic field strengths.‘25) The spectra of hydrated materials, represent the following number of FID’s (from top to bottom): 10,000; 130,000 and 350,000 while the spectra of the dehydrated materials needed 250,000 and 180,000 FID’s. At 15.6 MHz no signal was obtained of the dehydrated material after 750,000 FID’s.

J. KLINOWSKI

278

0

1OPW

FIG. 31. “Al MAS NMR spectra of zeolite Na-ZSM-5 as a function of rehydration spectra in the series represent 25,000 FID’s each.

time at 78.2 MHz.“‘)

All

the second moment of the 27A1line in hydrated zeolite Y calculated VQ= 390 kHz, but was not able to observe a signal in dehydrated zeolite. Gabuda et nl. (‘30) observed increased values of vQ after dehydration of zeolites. For the hydrated and dehydrated analcime the values are 270 and 390 kHz, respectively; for Na-X 165 and 285 kHz; and for Na-A 75 and 165 kHz. The authors of Ref. 25 explain the observed strong quadrupolar effects by a distortion of AlO:tetrahedra on dehydration, possibly caused by the closeness of the “bare” cation. They justify the postulate of a distribution of 27A1chemical shifts with an observation of several aluminous species with distinct T1 values and chemical shifts. More work is needed to elucidate these important effects fully. 6. MONITORING

OF CHEMICAL

MODIFICATION

OF ZEOLITES

It is often of interest to modify the composition of a zeolite while retaining the topology and crystallinity of the parent structure. Studies of such processes have been concerned particularly with synthetic faujasites, and involve acid-leaching and treatment with complexing agents such as EDTA or acetylacetone. Results of particular interest are obtained using the two methods described in the following sections. The first involves thermal treatment of ammonium-exchanged zeolite Y to produce extremely effective cracking catalyst with high thermal stability. The second depends on the isomorphous replacement of framework aluminium with silicon using SiCl, vapour at elevated temperatures. 29Si 27A1and ‘H MAS NMR have been used to monitor the course of these important processes in the solid state. 6.1. Decationation and Ultrastabilization Dehydration and thermal treatment of ammonium-exchanged zeolite Y under high vacuum at temperatures up to 600°C are known as “decationation”. During this process water is removed, while

Nuclear magnetic resonance studies of zeolites

279

the cation decomposes to give off ammonia; crystalline zeolite, nominally in the “hydrogen” form, is left behind. At higher temperatures, hydroxyl groups are also eliminated from the crystal as water; this latter process is known as “dehydroxylation”. McDaniel and Maher(139*140*152)were the first to report that upon thermal treatment of NH,-Y, under a particular set of conditions, thermal stability of the zeolite is considerably increased. The product retains crystallinity at temperatures in excess of lOOWC, while the decomposition of the sodium form of the zeolite takes place at CQ 800°C. This process is known as “ultrastabilization”. Ultrastable zeolite Y is very well suited as a catalyst for hydrocracking reactions-much more so than the “as-prepared” zeolite, which is too acidic and has insufficient thermal stability. Ultrastable faujasitic catalysts are a cornerstone of the petroleum industry, and it is not surprising that much effort has been devoted to the study of their properties and methods of preparation. The following summarises the most important observations: (i) Thermally treated NH,-Y has a greatly increased thermal stability; (ii) The process is facilitated by low sodium contents of the zeolite; (iii) The atmosphere above the sample during treatment is very important, proceeds more easily when carried out in the presence of water vapour;

6

\

NH:

-_Si-O--kf

/

/

\

_-OS_Si-

B

\

/Si-0-HH

H-0-d \

/

I \ + NH3 + AI(

Second *t.ge WO”,, >

\ii/

\>Si-0-!i-0-S:0

+4H,O

9

FIG. 32. The proposed(‘41)course of ultrastabilization of zeolite NH,-Na-Y.

and the process

J. KLINOWSKI

286

(iv) The properties of the product depend on the bed geometry during treatment, and three distinct types of treatment are usually distinguished: Shallow bed (SB)-the zeolite layer is less than 3mm thick and is slowly heated to the activation temperature under vacuum. Normal bed (NB)-the zeolite layer is thicker, but is also heated under vacuum. Deep bed (DB)-thick layer of zeolite is gradually heated under atmospheric pressure. The DB process gives the most stable product. (v) The unit cell dimension of the ultrastable zeolite is smaller than in the parent material; (vi) The DB zeolite has a greatly reduced ion-exchange capacity, which indicates that framework Al has been removed. It can be subsequently leached out of the zeolite with acids and other reagents; (vii) Framework vacancies created by the removal of Al are subsequently re-occupied by Si.

(a)

I

I

I

1

0

-100

Tetrahedral

b)

h

Si(OAll

u

Octahedral

-L

i"

x

1 -00

I -90

-100

(d)

1

ppm from TtlS

I

-110

I

-120

I

200

I

100

pprn from [AlUiIO\13'

Fro. 33. High-resolution “Si (at 79.80MHz) and 27A1 (at 104.22 MHz) MAS NMR studies of the ultrastabilization of zeolite Y.(14’)(a ) p arent zeolite NH,-Na-Y; (b) after calcining in air for 1 hr at 4OOT; (c) after heating to 7WC for 1 hr in the presence of steam; (d) after repeated ion exchange, heating and prolonged leaching with nitric acid.

Nuclear magnetic resonance studies of zeolites

281

The above observations are consistent with the reaction given in Fig. 32.041) However, despite extensive studies employing a range of techniques, many questions remained unanswered. The two most important are the mechanism of Al removal in stage I and the origin of the Si required in stage II. It can be argued, for instance, that silicon which re-occupies vacancies left by aluminium must create vacancies of its own elsewhere in the framework. Ultrastabilization has been studied using solid state NMR by several research groups. 042-145* 147-148.153-155) Klinow&i et .1>‘42) used 2gSi and 27Al MAS NMR in tandem to examine a series of four samples subjected to different types of treatment, while determining the Si/Al ratio from spectral intensities using eqn. (13). 2gSi NMR clearly shows (Fig. 33) how Al is removed from the framework, and how the resulting vacancies are subsequently reoccupied. The starting material (sample 1) had Si/Al = 2.61. Sample 2 was prepared by calcining sample 1 in air at 400°C for one hour. Its 2gSi spectrum is significantly different with (Si/A1)NMR= 3.37. However, chemical analysis shows no change in composition-the “missing” Al is now in 6-coordination (Fig. 33(b)), and there is a consequent loss of ion-exchange capacity. Sample 3 was prepared at 700°C in the presence of steam, and its 2gSi spectrum is completely transformed, giving (Si/Al)NMR = 6.89. Sample 4 was made by repeated application of the procedure, followed by prolonged leaching with acid. The product has Si/Al > 50 (by chemical analysis) and the unit cell parameter reduced by 1.58 % in comparison with sample 1. Its 2gSi spectrum shows one sharp Si(OA1)peak at - 106.9 ppm (compared with - 107.4 ppm in quartz) and a very small broad signal at ca - 101.3 ppm, attributable to the residual Si(lA1) units. Sample 4 is very crystalline to X-rays, and its 2gSi spectrum clearly shows that framework vacancies have been re-occupied. If this were not the case, it would be more complex, reflecting a range of possible environments for Si atoms including one, two or three neighbouring hydroxyl groups. The 27Al MAS NMR spectrao42) show directly how the occluded 6-coordinated Al builds up at the expense of the 4-coordinated Al in the framework. The spectrum of sample 4 contains a broad residual tetrahedral peak and an extremely sharp octahedral signal due to motionally free Al(H,O)z+ in the cationic positions, not removed by leaching with acid. In an independent 2gSi MAS NMR study, Maxwell et a1.o43) reached very similar conclusions. As we have seen in Section 4.8 other zeolites also undergo ultrastabilization during which Al is isomorphously replaced by Si. While Refs. 142 and 143 establish the re-occupation of the framework vacancies, the questions of the precise mechanism of Al removal, the nature of the intermediate defect structure and of the origin of the Si for lattice reconstitution remain. Gas sorption studies 046) indicate that materials prepared in a manner similar to that for sample 4 contain a secondary mesopore system with pore radii in the range V-19& suggesting that tetrahedral sites are reconstituted with silicon which, contrary to earlier speculations, does not come only from the surface or from amorphous parts of the sample, but also from its bulk, which may involve the elimination of the entire sodalite cages. Engelhardt et al.(‘44) used 2gSi MAS NMR with cross-polarization (CP) in order to detect “surface” Si atoms attached to one or two hydroxyl groups. From spectra without CP they determined, using eqn. (13),framework Si/Al ratios, and by difference with the results of chemical analysis, the amount of non-framework Al. Si(3Si)(OH) groups were found at - lOOppm, and Si(2Si)(OH,) at - 90.5 ppm, although the former signal coincides with that of Si(lA1,3Si) groupings. Their spectra are given in Fig. 34. Zeolite Y treated hydrothermally at 540°C for three hours shows the presence of Si(3Si)(OH) groups due to defect sites in the CP spectrum (Fig. 34(f)), although their absolute amounts could not be determined because the enhancement factor (CP efficiency) is not known. When the so treated sample is extracted with 0.1~ HCl at lOO”C,not only interstitial but also framework Al is removed and many “hydroxyl nests” are formed (Fig. 34(h)). They cannot be healed, as at such a low temperature migration of silicon-bearing species required by Fig. 32 must be insignificant. For repeatedly DB-treated and acid-extracted samples, 2gSi spectra with and without CP are similar, which indicates that almost all vacancies are healed (not shown). The extent of dealumination was found 045) to be limited by the degree of ammonium exchange of the starting material, and also to depend on the temperature and water vapour pressure during treatment. Depending on the conditions, any desired composition of the product up to Si/Al m 8 can be obtained and no preferred Si/Al ratios were found. For a given temperature the degree of

282

J. KLINOWSKI Stb?All

h

k

m

I 1 1.A i

-80 --z---

I

-120 -80

-100

-1

I

-120 -80

I

-100 -120 -80

ppm

from

,

-100

I

LJ

I

-120 -en

-100

-120 -80

-100 -120

TYS

FIG. 34. A high-resolution 29Si MAS NMR study of progressive ultrastabilization of zeolite Y (Si/Al = 2.37).044) Upper spectra without, lower spectra with cross-polarization. (a) and (b) zeolite Na-Y (sample 1); (c) and (d) sample 1 after 50 % NH,-exchange (sample 2); (e) and (f) sample 2 after DB treatment at 54O’C for 3 hr (sample 3); (g) and (h) sample 3 after extraction with 0.1~ HCI for 3.5 hr at 100°C (sample 4); (i) and (lt) sample 3 after two-fold ammonium exchange and DB treatment at 815°C for 3 hr (sample 5); (1)and (m) sample 5 after extraction with 0.1~ HCl for 3.5 hr at 1ooOC(sample 6).

dealumination increases approximately linearly with the degree of NH,-exchange, but with a different slope for each temperature. The degree of dealumination is always far below the degree of exchange which indicates that a considerable number of acidic protons (“structural OH groups” produced by the decomposition of NH:) must be retained. At low water vapour pressures, the extent of dealumination is limited by the availability of water. In another “Si MAS NMR study, Engelhardt et a1.(147)considered the problem of Si,Al ordering in dealuminated zeolites Y in the range of compositions (2.5 ,( Si/Al < 5.8). They found that the relative spectral intensities are independent of the method of dealumination (SB, DB, acid extraction) or conditions of thermal treatment, but depend only on the final value of B/Al ratio. As was pointed out in Section 4.4, this does not necessarily mean that the ordering is the same, although this is a strong possibility. This is because “random” distribution and various different ordering schemes may in principle give rise to the same relative spectral intensities. Bosacek et al.(14@ used wide-line 27A1 NMR measurements of stationary samples to measure the EFG at the nuclear site in decationated zeolites. In zeolite Na-Y they measured a line half-width of to vQ = 840 kHz; 6V1,2 = 61 kHz (for vL = 16MHz) which leads, via the theoretical considerationso”) the calculated field gradient was 2.9 V/A’. In hydrated samples this gradient is partially averaged by random reorientation of water molecules, giving BvilZ = 5.7 kHz and vQ = 256 kHz. When zeolite NH,-Na-Y was treated at 4OO’C under DB conditions a decrease in the number of

Nuclear magnetic resonance studies of zeolites

283

TABLE 14. Number of 27A1atoms observedby NMR in samples of NH,-Na-Y decationated at 400°C under deep bed conditions.“48) The total Al content is 6.7 atoms per cavity.

Degree of ammonium exchange

Number of Al atoms per cavity*

Number of Na+ ions per cavity7

,i$ 502 70% 90%

6.9 + 6.1 f 0.5 1.0 5.8 k 0.5 2.8 f 1.0 1.8 * 1.0

6.7 4.4 3.4 2.0 0.7

*Determined from “Al NMR signal intensity. t Determined by chemical analysis. observable Al atoms was found as the degree of ammonium exchange increased from 0 to 90%. In the latter case, only ca l/3 of Al present in the zeolite is observed by 27A1 NMR (see Table 14). The

authors estimate 1.2 MHz for the unobservable Al. However, extra lattice Al can be detected by contacting the zeolite with 38 y0 solution of acetylacetone in ethanol, whereupon mobile Al(acac), complexes are formed, and a very narrow 27A1 NMR line results; the solution does not al&t framework aluminium. It was found that the amount of 6-coordinated (i.e. extra-framework) Al increases from 5 y0 in 84 De Na-Y 300 SB zeolite to 50 y0 in 84 De Na-Y 500 DB zeolite (in this notation the first number refers to the degree of ammonium exchange, the second to temperature of treatment; “De” means “decationated” and the final symbol distinguishes between the deep bed and the shallow bed process). Freude et al.(‘53) carried out a systematic study of the relative amounts of 4- and 6-coordinated Al in thermally treated zeolite Y, using wide-line and MAS 27Al NMR at 16 and 70.34 MHz, respectively. Table 15 gives the results calculated per one faujasitic supercage (l/8 of the unit cell). It is evident that loss of 27A1 line intensity takes place in treated zeolites in comparison with the parent material, evidently due to extra-framework Al being in an environment of low symmetry. It is of interest to consider the possible status of this “invisible” aluminium. It could be present vQ

>

TABLE 15. Numbers of OH groups per cavity (l/8 of unit cell), of framework aluminium atoms (Al&&), of extra-framework aluminium (AIA-Awm) and of aluminium in the form of mobile hydrated complexes (A#$$). Al(acac), is the number of extra-framework aluminium atoms per cavity which can be extracted using an acetylacetone/ethanol solution. AlA = 6.7 is the number of Al atoms per cavity in the parent material (determined by chemical analysis).“53)

Dehydrated zeolite OH

AINMR mppm AlA-AIER,

Rehydrated zeolite, 0 = 1 APR aoppm AV-Alg$,

Al&$

acac + eth.

CAUacacM

Na-Y 85 DeNa-Y 300 DB 85 DeNa-Y 300 SB

6.7 + 0.0 5.8 f 0.3 4.7kO.4 7.0_+0.3 5.5 _+0.4

0.0 * 0.0

6.7 + 0.0 4.9 + 0.2 4.9 f 0.2

0.0 + 0.0

0.0 + 0.0

2.0 f 0.4 1.1 kO.4

1.8 +0.2 1.8 _+0.2

1.4kO.2 1.3kO.2

0.0 * 0.05 1.5 f 0.2 1.3kO.2

85 DeNa-Y 400 DB 85 DeNa-Y 400 SB

4.4 f 0.3 3.7 + 0.4 6.7 + 0.3 5.8 * 0.4

2.9 + 0.4 0.9 * 0.4

3.6kO.2 4.9 * 0.2

3.lkO.2 1.8kO.2

1.1 f 0.2 1.4 + 0.2

3.4 & 0.3 0.4kO.l

85 DeNa-Y 500 DB 85 DeNa-Y 450 SB*

2.8 f 0.2 2.7 _+0.4 6.5 + 0.3 5.9 + 0.4

3.9 _+0.4 0.8 + 0.4

1.7kO.2 4.5 + 0.2

5.OkO.2 2.2 & 0.2

0.4+0.1 1.1 kO.2

3.3 + 0.3 l.OkO.2

85 DeNa-Y 600 DB 85 DeNa-Y 600 SB

1.8 +0.3 0.7+0.3

1.1 kO.4 1.2+0.4

5.6 f 0.4 5.5 f 0.4

1.1 kO.2 2.6 f 0.2

5.6 f 0.2 4.1 kO.2

0.1+0.1 0.1+0.1

5.120.4 5.3 + 0.4

85 DeNa-Y 700 DB 85 DeNa-Y 700 SB

1.4 f 0.3 0.8 f 0.4 0.7kO.4

5.9 + 0.4 6.0 + 0.4

1.4kO.2 1.5 kO.2

5.3 + 0.2 5.2 f 0.2

0.0*0.1 0.1+0.1

-

* As SB treatment at temperatures above 450°C damages the zeolitic framework, results for the 450 SB sample are given.

284

J. KLINOWSKI

as AI(O Al(OH)2+, AI(O Al,Os or some poiymeric aluminous species. Resing and Rubinstein”“) observed a loss of intensity of the 27Al signal on hydrolysis of zeolite Na-X and interpreted this as due to the formation of Al(OH), complexes. However, Freude et c~l.(‘~~) think that the hydroxide is unlikely to be present in stabilized zeolites, which are strong solid acids, as it is not favoured in acidic aqueous solutions. The “low symmetry environment” may be the surface of the crystallites or of the secondary pore system. Indeed, Lohse and Mildebrath(15’) found Alz03 clusters inside the mesopore system formed as a result of the proposed “condensation of lattice defects” during thermal treatment,(14@ while Dwyer et al.(‘58~‘5g)and Ward and Lunsford(160) reported an enrichment in Al at the external surface of the particles of ultrastable zeolite Y using techniques other than NMR. 6.2. ‘H MAS NMR Studies of Zeolitic Acidity

High-resolution ‘H MAS NMR is the most advanced tool for the measurement of zeolitic acidity, which is essential for the understanding of the mechanisms underlying many zeolite-mediated catalytic reactions. The difficulties involved in high-resolution proton work in the solid state are: (i) the strong dipolar interactions which necessitate the use of multiple-pulse line narrowing techniques and (ii) the narrow range of the chemical shifts for the proton. As a result, only a limited number of lines can be resolved. Despite these problems, the Leipzig group in particular was able to use the technique to obtain very important information on the chemical status of hydroxyls in zeolites and gels. Freude et ~1.“~~) obtained ‘H MAS NMR spectra of DB-treated zeolite Y at 60MHz and considered the magnitude of the proton chemical shift as a measure of Bransted acidity. They point out that hydroxyl groups in the non-acidic silica gel resonate at ca 1.6ppm from TMS, while the chemical shift of the bare proton is 30.94 ppm. (15@By comparing the spectra of their zeolite samples with those of various inorganic materials, they concluded that terminal Si-0 hydroxyls and hydroxyls attached to extra-framework aluminium resonate at ca 2 ppm, and the acidic hydroxyls at 6-10 ppm. They consider their spectra as distribution functions of acidity. Table 16 gives the details of the spectra of the various zeolites with and without sorbed deuterated pyridine. The shift of spectral lines to lower field in pyridine-treated samples is attributed to the formation of hydrogen bonds between OH groups and the organic base. In another study, Freude et CI~.(‘~~)used ‘H MAS NMR at 270MHz with the WHH-4 pulse

TABLE 16. ‘H MAS NMR chemical shifts (in ppm from TMS) and line widths of hydroxyl groups in various

dehydrated sorbents.(‘55’ Adsorbent

Loading

Relative intensity (%) Chemical shift (ppm)

Line width (Hz)

silica gel

-

100

1.6

170

~-A1203

-

100

2.0

290

1.8

210

-

22 78

6.0

510

15 85 68 32

1.7 9.9 1.6 6.0

220 800 260 550

60 40 100

1.5 9.1 1.6

230 440 220

50

1.8 8.6 1.7 8.4

280 830 290 850

amorphous alumino silicate DeNaY

pyridine CaNaY pyridine H-mordenite pyridine -

. 50 50 50

Nuclear magnetic resonance studies of zeolites

285

4.1

500 03

450DB 4A

fl.8 ?.l

1.8

:__

-A6 (ppm

1

FIG. 35. High-resolution ‘H NMR spectra with MAS and WHH-4 pulse sequence of 88 % NH,-exchanged zeolite NH,-Na-Y at various activation conditions quoted as temperature (400, 450 and SOOT) and deep bed (DB) or shallow bed (SB).“s4) sequence to obtain high-resolution spectra of thermally treated zeolites (see Fig. 35). Three distinct lines are present, and their chemical shifts with assignments suggested by the authors are as follows:

(i) 2.0ppm-due to terminal OH groups and hydroxyl groups attached to extra-framework Al. This line was significant only in sample 500 SB, where it amounts to 40% of total spectral intensity. (ii) 4.2-5.0 ppm due to structural hydroxyl groups. (iii) 6.8-8.0 ppm 1 In addition, some samples also gave a signal at 7.1 ppm from the residual NH: cations; the amount of the latter was determined by thermodesorption and subtracted from the intensity of line (iii). Thus the sum of intensities of (ii) and (iii) gave the true total content of acidic hydroxyl groups. They have T2 of 60-75 psec, while sample 500 SB contains an additional FID component due to extra-framework hydroxyls. Scholle et al.“63,‘64) used ‘H MAS NMR to study the acidity of the hydroxyl groups in zeolite H-ZSM-5 and its borosilicate “equivalent”, known as H-boralite, at various water contents. They were able to distinguish terminal and water hydroxyls from acidic hydroxyl groups in the framework, although the resolution of their spectra was lower than that achieved by the Leipzig/Jena workers. H-ZSM-5 was found to be more acidic than boralite. 6.3. The Mechanism of Dehydroxylation of Zeolites When zeolite H-Y obtained by decationation of NH4-Y is heated further, water is irreversibly lost from the framework. The dehydroxylated zeolite Y displays Bransted and Lewis acid properties. The

286

J. KLINOWSKI

following hypothetical mechanism of this process, suggested by Uytterhoeven well established in the literature: H

H

I

I

et ~1.“~‘) has become

/“\si/o~~.‘A~/o\si/o~~..*~/o\si/o\ /\ /\ /\ /\ /\

I /“\,/o\~/o\g I\ /\

/\

*L/o\,i/o\ /\ /\

+H,O

The scheme requires that : (i) A defect structure involving 3-coordinated Si and Al is formed, with Lewis acidity being due to the latter; (ii) The amount of Ccoordinated Al decreases with increasing degree of dehydroxylation. After complete dehydroxylation halfA atoms remain Ccoordinated; (iii) The number of 4-coordinated Al atoms is always greater than or equal to the number of structural hydroxyl groups (because two OH groups disappear per each framework Al atom). The scheme has been repeatedly questioned in the light of X-ray and IR spectroscopic results which do not support it. Based on their IR measurements, Jacobs and Beyero62) proposed an extraframework Al,0 species acting as a Lewis acid, in place of the hypothetical 3-coordinated framework Al. But the strongest arguments against the scheme of Uytterhoeven et al. come from NMR work.(‘48*1s4)The amounts of “terminal” and “structural” hydroxyls were separately measured using ‘H NMR, while the amount of 4-coordinated Al was readily obtained from “Al MAS NMR. No 3-coordinated Si or Al were observed; the amounts of structural hydroxyls and 4-coordinated Al are always equal; and much less than half of Ccoordinated Al was found after complete dehydroxylation. It is therefore clear that dehydroxylation is always accompanied by the release, of Al from the framework. It seems that when the Al atom in the vicinity of a structural OH group is lost from the framework, the group is simultaneously dehydroxylated. DB treatment produces four times as much extra-framework Al as SB treatment at the same temperature. 6.4. Isomorphous Substitution Using Silicon Tetrachloride Vapour As was mentioned earlier, silicon-rich zeolitic materials are attractive as catalysts and catalytic supports in a variety of commercial processes involving hydrocarbon cracking, synthesis and isomerisation. Aluminium can be removed from the zeolitic framework using various methods, one of which (ultrastabilization) has been described in the preceding Sections. A new method of dealuminating synthetic faujasites by reaction with BCI, vapour at elevated temperatures has been reported by Beyer and Belenykaja.(16’) Klinowski et ~l.(‘~~-“~) applied “Si and 27A1MAS NMR to the study of this remarkable reaction. Dehydrated zeolite Na-Y (Si/Al ratio 2.61) was treated (16’) at 56O’C with dry argon saturated (at room temperature) with SiCl, for three hours. Aluminium was successively substituted in the zeolite framework by silicon and removed in part from the crystals in the form of volatile AICI, observed as white vapour. The zeolite was then flushed, also at 560°C with dry argon, and the temperature was gradually reduced. The product was then repeatedly washed with water.

287

Nuclear magnetic resonance studies of zeolites

SilAl = 2.61

(a)

Si(2AI)

I

I -80

-90

I

I

-100

-110

1 -120

Si(OAI1

(b)

I

-80

Si /Al = 55

I

-90

I

I

-100 ppm

-110

I

-120

from TMS

FIG. 36. Deahnnination of zeolite Na-Y using SKI, vapour studied by ?ii MAS NMR spectroscopy at 79.80MHz. (a) Parent material; (b) After “complete” dealumination (corresponding to the 27A1 MAS NMR spectrum in Fig. 37(d)).

Each step in the substitution

reaction, taking Na-Y with an Si/AI ratio of y/x as starting material,

can be written Nax(A10~)x(SiO~),+SiCl~--*Na,_l(A10~),_1(SiO~),+1

+AlCl:, +NaCl

As indicated by NMR (see below) part of the aluminium remains in the solid as NaAlCl, formed from the high-temperature reaction of NaCl with AlCl,, which gives Al(H,O)z+ on contact with water. This, together with the sodium chloride, can be successively removed by washing the product repeatedly with aqueous acid. The aluminium content of the highly crystalline product is similar to that of the ZSM zeolites. X-ray powder diffraction, IR spectroscopy and high-resolution electron microscopy all show that the crystal structure of the product is the same as that of the parent material, although the unit cell parameter decreases by 1.5 % as a consequence of the difference in size between the AlO:- and the SiOt- tetrahedra. The 29Si MAS NMR spectra given in Fig. 36 undergo a dramatic change upon dealumination. The single peak in the spectrum of the dealuminated material arises from Si(OAI), i.e. from Si(4Si)

288

J. KLINOWSKI NaCAICl.3 non framework (tetrahedral)

framework (tetrahedral)

k

framework (tetrahedral)

nonframework (octahedral)

ppm from AI(H,O)t+ FIG. 37. Dealumination of zeolite Na-Y using SiCl, vapour studied by 27A1 MAS NMR spectroscopy at 104.22 MHz.“~~) (a) parent zeolite Na-Y; (b) dealuminated material before washing; (c) after washing with dilute acid; (d) after repeated washing. Note that the aluminium jettisoned from the zeolitic framework is first of all bound tetrahedrally (as NaAlCl,, see text), but after washing adopts octahedral coordination. groupings: essentially all other groupings having been eliminated. The hump on the base line comes from amorphous material (probably silica) in this particular sample.

The progress of the same reaction can also be monitored using 27A1MAS NMR. The spectrum of the parent zeolite Na-Y (Fig. 37(a)) shows a single relatively narrow signal with a chemical shift of 61.3ppm from Al(H,O);+ corresponding to Ccoordinated Al (see Section 5). After treatment with SiCl, but before washing with water, apart from the signal from the residual framework Al and the peak from the emergent 6-coordinated Al occluded in the intracrystalline space, there is an additional signal due to NaAlCl, also occluded in the zeolite (Fig. 37(b)). The chemical shift of the latter peak, at 100.8 ppm is close to the value of 95.9 measured o69) for crystalline LiAlCl,. Upon washing the sample with water, chloroaluminate is largely removed or converted to the hydrated cation, Al(H,O)z+, which may be considered as the “exchangeable” cation. It is significant that the amount of 6coordinated Al (at 54.8 + 0.2 ppm) is about a third of the residual framework Al as required by charge balance (Fig. 37(c)). The FWHM of the tetrahedral signal is much greater than in the parent material (1428 Hz as compared with 560 Hz), which indicates a distribution of immediate environments for the aluminium atoms remaining in the framework. A comparison of Fig. 37(c) and (d) shows that most, but not all, of the 6-coordinated Al can be removed by washing. This is probably due to ion-exchange equilibrium between Na+ (and H30+ if the sample is acid-washed) and Al(H,O)z+ competing for the cationic sites. The poorer signal-to-noise in Fig. 37(b)-(d) in comparison with that in Fig. 37(a) is due to the very much lower concentration of aluminium in these samples. Silicon tetrachloride treatment can produce faujasites with very high Si/Al ratios in a single step, but works less well with other zeolites. The reasons for this are not clear. However, 27A1 MAS NMR

289

Nuclear magnetic resonance studies of zeolites

FIG. 38. Projection drawing, viewed along [OOl] of the structure of zeolite omega (synthetic mazzite). There are two distinct tetrahedral sites, one more (A) and one less (B) accessible via large channels. Unit cell indicated.

Zeolite with

R dealuminated

SiCI,.

spectrum

27AI MASNMR at 130.32

MHz

I 2

-40

-20

80

100

Lprn fr:rn

AI~H,CI~’

FIG 39. z’A1 MAS NMR spectrum at 130.32 MHz of zeolite omega dealuminated with silicon tetrachloride vapour.“‘r)

octahedral *‘AI

MASNMR

spectrum

ZSM-5 I

100

50

treated

with

silicon tetrachloride

0

-50

ppm from AI(H,O)~’ FIG. 40. 27AI MAS NMR spectrum at 52.11 MHz of zeolite ZSM-5 dealuminated with silicon tetrachloride vapour.“‘)

290

J. KLINOWSKI

shows unambiguously that other zeolites, notably mordenite,““) zeolite omega(“i) and ZSM-5”‘) are also dealuminated in this way. This is often not detectable by other methods since, after being removed from the framework, the aluminium may remain in 6-coordination in the zeolite channels and cavities, so that chemical analysis detects no change in the %/Al ratio. This is the case in SiCl,-treated mordenite; other examples include zeolite omega (Fig. 39) and ZSM-5 (Fig. 40). In zeolite omega the 6-coordinated Al is evidently extremely mobile (FWHM of the peak is only 0.2ppm) which is understandable given that this material possesses the widest channel system (channels more than 7.4A in free diameter) of any known zeolite. In the case of ZSM-5 the motion of octahedral aluminous species is restricted by the narrowness of the channels (cc 5.5A in diameter), accordingly the FWHM of the octahedral peak, measured at the same magnetic field strength as for zeolite omega, is 1.1 ppm. Figure 40 shows a lower field spectrum. It is interesting to note that the high-field spectrum in Fig. 39 contains two signals coming from crystallographically non-equivalent Al atoms-in agreement with the crystal structure of zeolite omega, providing for two distinct tetrahedral sites (Fig. 38). When the spectrum is measured at the lower magnetic field of 4.7OT (“Al frequency of 52.11 MHz) only one tetrahedral signal is observed. It is evident that the line-broadening influence of the second-order quadrupolar interaction, inversely proportional to the magnetic field strength, is reduced at 11.74T (Fig. 39). This demonstrates the advantages of high-magnetic-field solid-state NMR spectroscopy of quadrupolar nuclei. The 2%i MAS NMR spectrum of ZSM-5 extensively dealuminated with SiCl, approaches that of silicalite.‘s2**4)It has been claimed that elimination of framework Al may be effected by other volatile compounds including Ccl,, PCl,, TiCl,, CrO,Cl, and COCl, (phosgene). 7. PRECURSORS

IN ZEOLITE

SYNTHESIS

Zeolitic aluminosilicates are prepared in the laboratory by hydrothermal synthesis at moderate temperatures. The reaction mixture must contain silicon (as soluble silicate or colloidal silica), aluminium (in the form of aluminate, aluminium hydroxide or alkoxide) and must be strongly basic. The nature of the cation (i.e. of the base) has strong structure-directing influence. For example, zeolite A is formed in NaOH solutions; if however KOH is used instead, zeolite L, with a completely different structure, crystallizes. Some highly siliceous zeolites are synthesised in the presence of quarternary amines, and a large number of organic bases have been tried with a view to preparing novel zeolitic structures. The role of the base is thought to be two-fold: (i) it alters the gel chemistry; (ii) it serves as a “template” controlling the geometry of the tetrahedral units thus providing the initial building block for a particular type of structure. (3)Crystallization of a zeolite is preceded by the formation of an aluminosilicate gel and involves an “induction period”. The mechanism by which zeolites form from such gels is among the least well understood aspects of zeolite chemistry. The main reason is the complexity and heterogeneity of the synthesis mixture composed of the amorphous gel, the supernatant solution and the emergent zeolite crystals. The three phases have in the past been studied separately using diverse techniques such as the molybdate method, chromatography, trimethyl silylation, NMR, hydrogen electrode measurements and equilibrium centrifugation. (9*172)The most important questions to be answered are as follows: (i) The nature of the Al- and Si-bearing species in the mixture. Are aluminosilicate ions present? Are the secondary building units found in zeolitic frameworks already present in solution? (ii) The structure-directing role of the base and the status of the “template theory”. How can one base give rise to so many different structures? Why is the same structure sometimes obtained under completely different conditions? The chemical state of dissolved silica (if SiO, is supplied as silica sol) must influence the nucleation and growth of crystalline silicates. Cary et al. 073) dissolved isotopically enriched silica in H20/D20 and using 29Si NMR, concluded that tetrahedral dimers corresponding to pyrosilicic acid, H$i,O,, buil; of two Q’ units were present in addition to monomers (QO). The former species resonated at -9.26 ppm from TMS and accounted for up to 6 % of total spectral intensity. While silicate and aluminate solutions have been extensively studied using 29Si and 27A1NMR (see

291

Nuclear magnetic resonance studies of zeolites

4.1 and 5, mixed S/Al-bearing solutions by NMR with a view to elucidating the mechanism of zeolite synthesis. There is thus a considerable scope for further work in this important area. The work of Miiller et .1.03r) who measured “Al chemical shifts in TMA-aluminosilicate solutions has been discussed in Section 5. Briefly, they identified four kinds of Al-centred units: Q”, Q’(lSi), Q2(2Si) and Q3(3Si) and suggested that no Al-O-Al linkages are present in aluminosilicate anions. De Jong and Dibbleo74) and Dibble et a1.075) carried out a series of experiments with mixtures containing Na+ and aluminate and silicate anionic species. The solutions, 3~ in Si, 0-0.4~ in Al and with Na/Si = 3 were clear and no gels formed until the onset of zeolite nucleation. While no incontrovertible proof of the actual existence of aluminosilicate anions was given, the “Si spectra are strongly affected by the addition of Al. When the solution is 0.4~ in Al the FWHM of the Q” signal doubles in comparison with pure silicate, FWHM of the Q’ signal increased by 10 % when aluminate was added. 29Si and 27A1spectra did not change during the induction period, but when crystallization came to completion no Al was detected in the supernatant solution, which gave a typical silicate spectrum. Derouane et ~1.“‘~) studied the influence of pH and the addition of Na’, Cs+ and TPAf on “Al NMR spectra of sodium silicate/sodium aluminate solutions. At pH < 4 only octahedral Al(H,O)z’ species are formed and the spectra are not affected by the addition of either cation. At pH 7 a very broad resonance is observed at lo-25 ppm, presumably attributable to precipitated (&coordinated polymeric aluminous species. At pH 10.5 a resonance at ca 80ppm appears in the absence of silicate, corresponding to Al(OH); ; it broadens on adding TPA+, but narrows again when Na+ is also added, suggesting that Na+ interacts with aluminate species in preference to TPA+. When sodium Sections

-I

I

-74

I

,

-82

I

I

,

-90 ppm from

,

-98 TM8

,

,

-106

,

,

,

-II4

FIG. 41. 29Si MAS NMR spectra at 39.74MHz of solid intermediates in the synthesis ofzeolite Na-A.“78) (a) solid separated from the reaction mixture kept at room temperature after Smins (see text); (b) after 4hr at W’C; (c) after 6 hr at WC; (d) after 17hr at 8oOC.Chemical shifts are in ppm from TMS.

292

J. KLINOWSKI

silicate is added to the solution, a broad line appears at ca 55 ppm corresponding to Ccoordinated Al. In another experiment, aluminosilicate gels were first prepared at pH 4 whereupon pH was increased to 10.5. The resulting 27Al lines are twice as broad as when the reagents were mixed at pH 10.5. Derouane et al. propose molecular schemes to describe the formation of aluminosilicate gels under various conditions, and conclude that the role played by TPA+ as counterion depends on the availability of the alkali cations. Boxhoorn et a1.(‘77)studied silicate species present in the ZSM-5 synthesis mixture of composition 288Si0, : Al,O, : 9Na,O :42(TPA),O : 54OOH,O. 29Si NMR revealed the presence of many different species in the mixture, from monomers to branched silicate anions. When however DMSO, MeOH or EtOH were added, the spectrum simplified to a single sharp signal at -98.0 ppm suggesting the presence of a double 4-, 5- or 6-membered ring species. NMR alone cannot distinguish which of the three is present, as their chemical shifts differ only by ca 1 ppm. The guthors claim that attenuated total reflection FT IR spectroscopy and mass spectroscopy point to the assignment of the NMR signal to the double 5-membered ring, which would then serve as a precursor species for ZSM-5. Engelhardt et aL(“*) used 29Si and 27Al MAS NMR to study solid aluminosilicate gel precursors in the synthesis of zeolite A. Sodium silicate and sodium aluminate solutions were mixed to give reactant mixture of composition 2Si0, : Al,O, : 3.3Na,O : 170H,O. In one case the precipitate formed was separated after 5 min; other solid samples were prepared by heating the reaction mixture to 8oOC for 4, 6 or 17 hr prior to filtration. As shown in Fig. 41(a) the initial gel gives a broad (FWMH = 420Hz) 29Si signal at -85ppm from TMS, which is typical of Si(4Al) units in an amorphous or highly disordered environment. The spectrum does not change when CP is applied, which means that Si(3AI)(OH) units are not present in significant amounts. Partial transformation of the spectrum is observed when the gel is separated after 4hr (Fig. 41(b)), but in the sample from the mixture treated for 17 hr a narrow peak at - 89.4 ppm is observed, which is characteristic of zeolite A (see Section 4.5). 27AI MAS NMR spectra always feature a single peak at 59 ppm from Al(H,O)~’ corresponding to Ccoordinated Al. With progressing crystallization the 27Al signal narrows considerably, probably due to the removal of residual quadrupolar interactions caused by the asymmetric environment of the Al nucleus in the incompletely formed crystals.

N (CH,-

FIG. 42. “C

MAS NMR

CH,-

CH, &

spectrum with cross-polarization of tetrapropylammonium (TPA+) cation in zeolite ZSM-5 at SO.29MHz.“~*)

Nuclear magnetic resonance studies of zeolites

293

Nagy et u/.(192) used thermogravimetry in combination with “C MAS NMR to study tetrapropylammonium (TPA), tetrabutylammonium (TBA) and tetrabutylphosphonium (TBP) species enclathrated in the framework of zeolites ZSM-5 and ZSM-11 in the course of their synthesis. They observed interesting chemical shift and line width/multiplicity effects. Thermal analysis showed that in its as-prepared state ZSM-5 contains 3.3-3.8 TPA+ cations per unit cell, which are known(“‘) to be chemically intact. On the other hand, the unit cell of ZSM-11 contains only 2.6-3.0TBA+ or 2.5-2.6TBP+ cations. The spectrum given in Fig. 42 shows that the C-3 carbon resonance from TPA/ZSM-5 is split into two components of approximately equal intensity. The authors conclude that the cation is located at the cross-section of the two non-equivalent channels with two propyl groups located in each channel. In ZSM-11 the situation is more complicated, because there are two types of channel intersections of unequal size. Nagy et al. believe that for steric reasons TBA+ and TBP’ preferentially occupy the larger intesections, which explains their lower numbers in comparison with TPA+ in ZSM-5. Also using 13C MAS NMR, Jarman and Melchior(‘g6) could distinguish between TMA+ cations trapped in the a and /l (sodalite) cages in zeolite TMA-A. The observed intensities indicate near-complete occupancy of the /?-cage over the complete composition range, i.e. their preferential occupation. 13C chemical shifts (58.8 ppm from TMS for the /?-cage and 56.9 ppm for the a-cage) are insensitive to Si/Al ratio. References 192 and 196 show that 13C MAS NMR can be a sensitive probe for the nature of the intracrystalline environment. 8. NMR STUDIES OF EXCHANGEABLE

CATIONS

The sorptive, catalytic and ion-exchange properties of zeolites depend strongly on the kind, position and mobility of the charge-balancing cations. Since chemical shifts and multiplicities of lines are related to site occupancy and their widths to cationic mobility, NMR can in principle provide important information on the nature of intracrystalline environment. Unfortunately however, the cations most often found in natural and synthetic zeolites are not ideal for NMR studies. The 23Na nucleus has a large quadrupolar moment, while cations of most spin 3 nuclei (such as Y, Rh, Ag, Cd, Pb, Hg, OS and Pt) are of relatively little interest to the zeolite chemist. For these reasons much attention has been paid to 7Li, which has a small quadrupolar moment, and to zo5Tl, an attractive spin 3 nucleus. On the other hand, the 23Na resonance can be used as a sensitive probe for the electric field gradient in zeolites and the effects brought about by sorbed molecules. The early wide-line NMR experiments on quadrupolar nuclei in zeolites have been performed by Lechert and co-workers(17g-1*s) and are discussed fully in a review by Lcchert.(1s7) Briefly, the “Na spectra of zeolite X with Si/Al close to unity consist of a single featureless line ca 12 kHz wide. For synthetic faujasites with Si/Al > 1.29 the line becomes asymmetric and its intensity decreases rapidly. Such spectra are very difficult to interpret, but certain conclusions on the occupancy of the cationic sites at low Si/Al ratios can be drawn. A point charge-multipole model led Lechert(ls3) to the conclusion that not all 23Na nuclei in zeolite X are observable by NMR. Calculated linewidths for S; and S, sites are too large for these cations to be observable, and according to the model the entire signal intensity comes from 23Na in the S, cationic sites in the double 6-membered rings joining the sodalite cages. Lechert believes that the main contribution to EFG, the electric field gradient, comes from induced dipoles at the sites of oxygen atoms. Lineshapes depend on the amount of intracrystalline water, and sharp narrow lines are observed at high water contents, which might be due to the averaging of the EFG by the motion of the sorbate. The effect of sorbed polar molecules on the spectra can be explained in terms of their influence on the induced dipoles. 23Na spectra of zeolites A and Y cannot be satisfactorily interpreted by such model calculations. Until comparatively recently only broad-line NMR had been used for observing 23Na resonance. The technique is not well suited to obtaining absolute intensities, as this requires double integration of the measured derivative spectrum. Basler’lMg’ used the pulsed resonance technique at 16 MHz in which the initial intensity of the FID is proportional to the number of 23Na nuclei and can be

294

J. KLINOWSKI

calibrated against the ‘H signal in the same sample. Zeolite Na-Y with Si/Al = 2.36 contains seven Na+ per supercage, of which three are located in S, and S; sites and four in S, sites, i.e. in the middle of 6-membered rings lining the cage. The total measured 23Na intensity corresponds to four Na+ per cage, and Basler concludes therefore that the NMR signal comes from S, sites. When these sites are selectively exchanged by Ca’+, no 23Na signal is observed which tends to confirm this conclusion, in contrast to earlier work.(‘83,187) West(190) observed no 23Na resonance in dehydrated synthetic faujasites, suggesting that the EFG at the cationic site is larger than in hydrated samples because of the displacement of the cations away from their high-symmetry positions. The signal appeared when seven H,O molecules per cage are present. Fully hydrated Na-X and Na-Y have Tz of 100 and 14Opsec, respectively, while in dehydrated samples much faster transverse relaxation is observed. The ‘Li resonance in zeolites is also difficult to interpret, even though the quadrupole moment is much smaller. Lechert et ~l.(‘~~) believe that the ‘Li linewidth is controlled by the dipole-dipole interaction with *‘Al nuclei in the aluminosilicate framework. According to Herden et ~1.~‘~~)the increase of ‘Li frequency from 9 to 21 MHz does not affect the second moment of the spectra in zeolites Li-X and Li-Y, which means that the quadrupolar interaction is small. The second moment is also independent of the Si/Al ratio. The mean Li-Al distance calculated from the van Vleck formula is 2.351(. Small amounts of divalent cations reduce the movement of Li+ considerably with the activation energy for this process increasing from 30 to 60 kJ/mol. ‘05Tl is a very favourable nucleus for soiid-state NMR studies: it has I = 3, high natural abundance and high sensitivity. Its large chemical shift range makes it possible to observe individual environments of the nucleus. Thallium can be easily introduced into zeolites by cation exchange. Freude ef .1.‘193*194)measured *05Tl NMR spectra of Tl-exchanged zeolites X, Y and A as a function of water content and temperature. They interpreted the spectra of zeolite A as superimpositions of three lines, and ascribed the observed changes in lineshape to thermal motion of Tl, cations located near the centre of the 8-membered rings. With increasing water content and/or temperature the frequency of jumps between the four cationic sites in the plane of the ring increases; it is lo5 set-’ at 100% in dehydrated crystals or at room temperature with four Hz0 molecules per large cage. Jumps out of the plane (lo3 set- ’ at 200°C in dehydrated crystals or at room temperature with ten H,O molecules per cage) lead to translational diffusion of Tl+ cations through the crystal. This motion is dominant at higher water contents. 2ooO fwm

(a)

(cl

(b)

(e)

(dl

(f 1

(fJ)

i: R

R

R

R

R

(h)

LR

Ho FIG. 43. 205T1NMR spectra’190’ at 51.92MHz of dehydrated zeolites (TI,Na)-A. The reference R marks the position of the signal in hydrated (12T1)-A. (a) (12Tl)-A; (b) (lOT1,2Na)-A; (c) (8T1,4NatA; (d) (6T1,6Na)-A; (e) (STl,7Na)-A; (f) (4T1,8Na)-A; (g) (3T1,9Na)-A; (h) (2TI,lONa)-A.

Nuclear magnetic resonance studies of zeolites

/,\

295

120)

I

a

I241

b

FIG. 44. *05Tl NMR power spectra”“’ at 10.3 MHz as a function of water content for zeolites (a) (3T1,9Na)-A; (b) (12Tl)-A. The water content is given in brackets as number of H,O molecules per a-cage.

WestoQo) carried out a series of “‘Tl measurements in zeolites (Na,Tl)-A. The fine structure of the resonance line is associated with at least two specific types of cation site. Figure 43 shows that for low Tl contents one site group is preferentially exchanged by Tl+. The lines from hydrated samples are quite narrow with FWHM varying linearly from 180Hz in (2Tl.lONa)-A to SOOHz in (12Tl)-A. Figure 44 shows the spectra of samples (12Tl)-A and (3T1,9Na)-A as a function of water content. West concludes that there is rapid self diffusion of Tl+ cations in the latter sample when dehydrated, and that this motion is completely quenched with partial hydration. In general, the effect of sorbed water is to increase the mobility of the Tl+ cations, but not that of Na+ cations which remain stationary in all samples.

J. KLINOWSKI

236

9. GUEST MOLECULES

IN ZEOLITES

9.1. General Considerations Relaxation times T, and T, depend on the motion of molecules which contain the nucleiog7) and their measurement often leads to the various kinetic parameters for the adsorbed molecules, the knowledge of which is essential for the understanding of the mechanism of many zeolite-mediated processes. The diffusion coefficient of the reactants and products in a catalytic reaction, which can be determined from NMR, is often rate-limiting. Relaxation studies can also determine surface coverage by the sorbed species and provide information about the distribution of adsorption energy between the different sites on the surface of a catalyst. For these reasons a great deal of NMR work has been done with adsorbed species in zeolites in the course of the last twenty years. From the applied viewpoint the emphasis was on water and hydrocarbons as guest molecules; from the fundamental viewpoint species such as Xe, SF,, H,, CH, and NH, are of special interest. It is outside the scope of this review to survey this extensive field fully, especially since several good Instead, a summary of the most important results reviews are already in existence. ~187~1g8~201~214~237~ will be given together with a discussion of recent work, particularly that involving high-resolution NMR. 9.2. Water Sorption and Mobility

Much attention has been given to the NMR behaviour of water adsorbed on Zeolites.“g8-22g~233-236) Tl and T2 have been determined as a function of temperature and surface coverage in various zeolites,

particularly of the faujasite type. The early experiments were troubled by the very strong dependence of relaxation rates on the concentration of paramagnetic impurities. In order for the relaxation values to be meaningful, such impurities expressed as Fe content must be below ca 6 ppm. Figure 45 shows the variation of Tl and T, of water adsorbed in a particularly pure sample of zeolite Na-X.(200J The T

500 400

300

ZEOLITE

to2.0

(OK)

250

200

4.0 (“K-9

5.0

13-X

3.0 IO?‘T

FIG. 45. Proton relaxation times T, and 7’, for water adsorbed in zeolite Na-X at ~~MHz.‘~~“’ 0 denotes the

results of Kvlividze et u/.‘~*~)

291

Nuclear magnetic resonance studies of zeolites

authors of Ref. 200 account for the experimental results using a model of the intracrystalline fluid which is about 30 times as viscous as bulk water at room temperature, shows a broad distribution of molecular mobilities (the ratio T,/T, at the minimum in Tr is much larger than expected for a single correlation time) and is about as dense as liquid water. The median correlation time is t* = 2.8 . lo-r2 exp [417/(T-

189)] sec.

By observing the free induction decay following an rf pulse, two distinct exponential components of Tz could be distinguished in faujasitic zeolites. The fast decay immediately following the pulse has been attributed(2’9-221) to water inside the sodalite cages (T, = SOpsee at room temperature) and the slower decay (T, = 40msec) to water in faujasitic supercages. Signal intensity measurements indicate that over a wide range of coverages there are four H,O molecules per sodalite cage. The application of the theory of Zimmermann and B&tin (231)to those relaxation times leads to the conclusion that no exchange of water between the two reservoirs occurs on a time scale of several seconds. For the fast-decaying component of the FID Tl and T, are unaffected by the concentration of paramagnetic impurities, confirming that the motion of the molecules to these relaxation centres is restricted. On the other hand, the relaxation of water in supercages is very strongly al&ted by the paramagnetics. (200-204.214,218) Pfeifer et a1.‘*“) conclude from their measurements of Tl and T, vs temperature in samples with controlled water contents that the lifetime of sorption complexes of water is 3.5 x 10m9aec at 5O“C with non-localized cations and at - 10°C with localized ones. Water was found to be bound more strongly in faujasites with higher Si/AI ratios, which agrees with model calculations by Dempse.y(232) of the electrostatic fields around cations. At higher coverages the mobility of H,O is independent of the Si/Al ratio and is two orders of magnitude lower than in bulk water. The T2 relaxation times of 5Ousee and 40msec given above correspond to line half-widths of 6.4 kHz and 8 Hz, respectively. Whipple et ~1.‘~~‘)concluded that the linewidths of several hundred hertz which are obtained in practice must be due to bulk magnetic susceptibility effects. This type of

7 7

I

6

I

5

I

4

I

3 ppm

I

2 from

I 1

I

0

dimethylriloxane

FIG. 46. ‘H MAS NMR spectra at 1OOMHzfrom fully hydrated zeolites Na-Y, Na-mordenite (Na-M) and Na-ZSM-5.‘*2s)Chemical shifts are 5iven in ppm from dimethylsiloxane.

J. KLINOWSKI

298

line broadening is removable by MAS(228) and they were the first to obtain high-resolution spectra with linewidths similar to those expected from the T2 values. Kasai and Jones(225) applied MAS to the study of water in zeolites A, X, Y mordenite, ZSM-5 and silicalite. They found that although the signals are sometimes quite broad, their chemical shifts are characteristic of the zeolite (Fig. 46). They interpret this as the effect of the disruption of hydrogen bonding of bulk water by the zeolitic framework and of the interaction of water molecules with framework oxygens. An inverse relationship was found between the chemical shift and the Si/Al ratio. The chemical shift of water in silicalite is quite different from that in ZSM-5 and does not fit this relationship. It is, on the other hand similar to that in silica gel, which indicates that water molecules in silicalite are located only at the external surface of the crystallites, presumably hydrogen-bonded to the hydroxyl groups. The same authors measured the chemical shift of water in Ba-, Ca- and Na-mordenites. Considering the effect of the cation on the proton spectra of aqueous solutions of electrolytes(22g) one would expect larger dependence of 6 on the type of cation than is actually found. Kasai and Jones suggest therefore that in zeolites water molecules interact primarily with the framework oxygen. Chemical shifts in zeolites exchanged with divalent cations are larger than for monovalent cations because more oxygens are “exposed” in the former case. Support for this explanation comes from the relative magnitudes of heats of immersion of various zeolites measured by Barrer and Cram. (230)Relative linewidths indicate that the mobility of water is inversely related to the size and number of cations. Investigations of self-diffusion coefficients have been carried out by Parravano et CZ~.(~~~) and Karger et a1.(213~233-6)using pulsed NMR techniques. The residence time 7 of a molecule in a particular state and the self-diffusion coefficient, D, define the mean square jump length for isotropic diffusion in three dimensions : (d’)

Klrger(2’3) measured D = 2.5 x lo-‘cm’

I

7

I

6

I 5

set-’

I

4

= 607.

at - 35OC for zeolite X with 12 Hz0 molecules per

I 3

I 2

ppm

from

I 1

I

0

dimethyleiloxane

FIG. 47. ‘H MAS NMR spectra of the 50 : 50 mixture (by weight) of zeolites Na-X and Na-Y compared spectra of individual zeolites. (‘w Chemical shifts are given in ppm from dimethylsiloxane.

with the

Nuclear magnetic resonance studies of zeolites

299

supercage, and 1.4 x 10-7cmZsec-1 for 30 molecules. This leads to jump lengths of 8 and 6w respectively, compared with ca 2 A in bulk water. Residence times and jump distances in zeolites are greater than in the liquid. Riedel et a1.‘2’5) concluded from pulse-gradient experiments that the mean diffusion distance of water in Na-Y is larger than the average size of the particle. This was confirmed by the measurements of temperature dependence of linewidth conducted by Whipple et a1.‘220)Kasai and Jones(225) show the coalescence of the proton signal from the intimate mixture of zeolites with very different chemical shifts for water, which confirms this finding further. Figure 47 shows that a single line at an average value of chemical shift is obtained when zeolites Na-X and Na-Y are mixed. It is well known that high-silica zeolites such as silicalite are hydrophobic. Addition of hexane to ZSM-5 does not affect the NMR signal from water, but addition of butanol has a very marked influence. This indicates that butanol displaces water from the intracrystalline space to the outer surface of the zeolite particles. 9.3. Multinuclear Studies of Sorbed Species The early NMR studies of molecules sorbed on zeolites used ‘H and “F resonances. Truly multinuclear work involving 13C, “N and 12’Xe in particular began in the early 70’s with the advent of modern Fourier transform spectrometers. The low natural abundance of 13C (1.1%) can be compensated for by working at high fields, using cross-polarization and by resorting to isotopically enriched compounds. On the other hand, 13C has the advantage of a large range of chemical shifts (ca 250ppm as opposed to ca 1Oppm for ‘H) and gives much narrower resonance lines as a result of the absence of homonuclear dipolar interactions. 13C-lH interactions are removed by high-power decoupling and the chemical shift anisotropy (which can be considerable, particularly for carbonyl and aromatic carbons) by magic-angle spinning. The resulting 13C spectra often contain much valuable information. The same applies to ’ 5N, although the use of isotopically enriched species is normally unavoidable when working with this nucleus which has the natural abundance of only 0.365 %. Diffusional behaviour of sorbed species is studied by NMR using one of three approaches: the van Vleck method of moments; relaxation measurements and the pulsed field gradient method. An example of the use of the method of moments is the study by Lechert and Wittern(24g) of C,H, and C,H,D, adsorbed on zeolite Na-X. Analysis of second moments of ‘H resonances allowed the intraand intermolecular contributions to the spectra to be extracted. Similarly, second moments of ‘H and lgF spectra of cyclohexane, benzene, fluorobenzene and dioxane on Na-X provided information about orientation of molecules within zeolitic cavities.(246-g) The measurement of relaxation times T1 and T2 and the subsequent application of the theory formulated by Bloembergen et al.““) and extended by Kubo and Tomita(227) and Torrey(241) leads to the determination of motional and thermodynamic parameters such as mean times between molecular jumps, diffusion coefficients and activation enthalpies for translation. For example, Resing and Thompson(277-8) used this approach in their study of diffusional behaviour of SF, in zeolite Na-X by monitoring the “F resonance as a function of temperature. Pulsed field gradient NMR, (242-3) in which spin-echoes are measured in the presence of a time-dependent magnetic field gradient, has been used to determine effective diffusion coefficients, &, in beds of zeolite powder. Barrerc8) quotes the expression for the spin-echo amplitude given by Kiirgerf2 ’ 3, in the form : In t,G(ag)= In $(O) - y2a2g2At D* +

PD: y’o*g*zpD; + 1

1

where Q and g are respectively the width and amplitude of the gradient pulses at intervals At, p is the fraction of the sorbate molecules in the intercrystalline space and D*, 0: are respectively the intraand intercrystalline self-diffusion coefficients. Des, the quantity in the square bracket, is approximately equal to (d2)/6At where (d2) is the mean square displacement of the molecule over the interval At.

300

J. KLINOWSKI

There are two limiting situations:@) (i) z >->At, i.e. (d2)1/2 < crystal radius. In this case Des = D*; (ii) r -=KAt , i.e . (d2)“’ >> crystal radius. In this case D,s = pD:. Pulsed field gradient studies of methane sorbed on zeolite (Ca,Na)-A and n-butane and n-heptane on zeolite Na-X(2’3,24”5) under the conditions of case (i) above showed that D,s decreases with increasing hydrocarbon chain length and with the fractional saturation of crystals, 8. At 2oOC and 0 = 0.8, &( =D*) is 1.4 x lo-’ and 6.3 x 10-6cm2 set- ’ for n-butane and n-heptane, respectively, which is similar to the values measured in bulk liquids. An intriguing aspect of these measurements is that the values of D* determined from NMR and from sorption kinetics differ by several orders of magnitude. For example, for methane on (Ca,Na)-A the value of the diffusion coefficient determined by NMR is 2 x 10-5cm2 set-‘, and the value determined from sorption rates only 5 x lo- lo cm2 set- ‘. The values from NMR are always larger and are similar to those measured in bulk liquids. The discrepancy, which is, of course, far greater than the uncertainty of either method, remained unexplained for several years, until careful studies(222.250*264)have shown that the actual sorption rates are not determined by intracrystalline diffusion, but by diffusion outside the zeolite particles, surface barriers and/or by the rate of dissipation of the heat of sorption. NMR-derived results are therefore vindicated. Large diffusion coefficients (of the order of 10m6cm* see- ‘) can be reliably measured by sorption kinetics only in large crystals of natural zeolites. A recent NMR study (252)has indicated that surface barriers are indeed present in zeolite powders. The usefulness of 13C NMR to the study of sorption of various hydrocarbons adsorbed in zeolites has been demonstrated by Deininger and Michel(253) and Michel.(254-5) Changes of ’ 3C chemical shift with respect to bulk liquid were observed in (Na,Mg)- and (Na,Ag)-faujasites and depended on the degree of pore filling, the nature of cations and the structure of hydrocarbons. For (Na,Ag)-zeolites with high silver content, where each adsorbed molecule comes into contact with an Ag+ cation, the shifts with respect to those measured in Na-zeolites are nearly the same as in solutions of silver salts, indicating that chemical shift is determined by bonding to silver. For lower Ag contents line shifts are smaller, because of the rapid exchange of molecules between Na+ and Ag+ sites. The strength of the Ag+-olefin bond does not change significantly as the temperature is increased. The shift of carbons forming double bonds is most affected by the type of cation. (257) Large low-field shifts have been observed in CO sorbed on decationated zeolites. (261)This is believed to be caused by interactions of the hydrocarbon with the Lewis acid sites. The motionally narrowed spectrum of p-xylene adsorbed on zeolite ZSM-5(268) is consistent with translational diffusion and rotation of the molecule, while the spectrum of o-xylene shows essentially a “rigid lattice” anisotropy pattern, indicating that translational diffusion is very slow. Recent work involving 1% NMR studies of sorbed species is summarized in Table 17. There is at present only a handful of publications involving “N NMR of molecules sorbed on zeolites, but they establish beyond doubt the power of this technique for the study of zeolitic acidity and other surface phenomena. The nitrogen atom in molecules such as ammonia or pyridine has a lone pair of electrons and binds directly to the surface site. One is therefore observing large effects on a nucleus with a wide (ca 900 ppm) range of chemical shifts, rather than more indirect influence as in the case of i3C. Michel et ~1.‘~~~~‘~~’and Jtinger et ~1.‘~~~)measured the spectra of ammonia, trimethylamine, pyridine and acetonitrile on various zeolites at 9.12 MHz and found that resonance shifts indeed depended strongly on the interactions of sorbate molecules with cations and Br#nsted and Lewis acid sites. Quadrupolar interactions can offer direct information on the dynamics of organics within zeolite crystals. Eckman and Vega (270)studied the ‘H quadrupolar echo decay in perdeuterated p-xylene adsorbed on zeolite. ZSM-5. The deuterium quadrupolar interaction usually dominates the spin Hamiltonian, so that the powder pattern can be used as a test for models of molecular motion. At - 75°C and 25°C typical rigid-lattice spectra were obtained. At 100°C however, the resonance arising from the aromatic deuterons was motionally narrowed, while the methyl resonance was not. The

Nuclear

magnetic

resonance

studies of zeolites

301

TABLE 17. Recent 13C studies of sorbed species in zeolites. Zeolite

Sorbate

Study

Ref.

butene isomers

Na-X and Na-Y

T,, T2, chemical

shifts

255

propene

alkali-metal exchanged zeolites X and Y

T,, chemical shifts, NOE enhancements, effects of paramagnetics

257

but-1-ene, trans-but-Zene

(Ge,Al)-X

T,, chemical cations

258

CO, enriched

various

MAS spectra, T,, physi- and chemisorption

259

Na-A, Na-X, Na-Y, H-Y, Na-mordenite

chemical shifts vs coverage and temperature (180 to 400 K). Interaction with Lewis sites

261

Na-Y

T, and T, vs temperature NOE effects.

262

but-1-ene

Na-X, (Ag,Na)-Y

Chemical shifts. Calculations of NMR parameters from electron densities

263

ethane

(Ca,Na)-A

Tracer desorption

264

H-ZSM-5 H-mordenite

CP/MAS.

ethene, propene, isobutene, 2-methyl-but-I-ene

H-ZSM-5

Oligomerization 373K

o-xylene, pxylene

ZSM-5

Molecular

methanol

H-Y

Reaction intermediates of alkanes

pyridine

H-Y

formic acid

NH,-Y,

in “C

CO and CO,

butanol

enriched

carbonaceous catalysis

in 13C

residues from

Br#nsted H-Y

Unidentate

shifts, interactions

with

(220 to 420 K);

kinetics

Identification

of species

265

studied at 300 and

mobilities

267

at 310 K

268

in the production

acidity and bidentate

275 279

formate

species

280

conclude that p-xylene molecules re-orient about an axis which passes through the C, axes of the methyl groups. Xenon is a very useful molecular probe for adsorption studies. ‘29Xe is a spin 4 nucleus of 26.44 % natural abundance and a very wide range of chemical shifts. The shielding of the xenon atom with respect to the bare nucleus has been estimated to be 5642ppm,‘281) and the 129Xe chemical shift is extremely sensitive to physical environment as shown by its strong dependence on density in the pure phases: the liquid at 224K resonates 161 ppm downfield from the gas at zero density, whereas the solid at 161 K has its resonance at - 274 ppm. The atomic diameter of xenon is 4.6 A, i.e. comparable to the size of zeolitic channels. Ito and Fraissard(272) were the first to take advantage of these properties of lz9Xe for the study of xenon on zeolites Ca-A, Na-X, Na-Y, H-Y and Ca-Y. Because of the aperture size, xenon is not sorbed on zeolite Na-A. They interpreted the observed chemical shift/density gradients in terms of collisions between Xe atoms and between Xe and the cavity walls. De Menorval et .Z.(273) studied xenon adsorbed on zeolite Y containing particulate platinum with or without pre-chemisorbed hydrogen. They conclude that chemisorption of hydrogen occurs homogeneously on all particles of similar size and claim to be able to determine the mean size of platinum particles from their results. Ripmeester(239’ used MAS to study xenon adsorbed on zeolites Na-X and H-mordenite. In the case of faujasite containing excess sorbate lines from liquid, solid, gaseous and sorbed xenon could be distinguished (see Fig. 48). The presence of a line from adsorbed xenon at 160K shows that sorbed xenon does not freeze at the bulk xenon melting point. The line from liquid xenon measured at 170 K shifts to high field (Fig. 48(b)), suggesting that sorbed xenon is more dense than bulk liquid. authors

J. KLINOWSKI

302

a Xe on Zeohte 13x

C Xe on H Mordenite

160

K

240

-_

K

1

b

Xe on Zeollte

d Xe on

13X

_I\;z:__

H Mordenite

I

302

K

d

200ppm

,

FIG. 48. (a) and (b) rz9Xe MAS NMR spectra of excess xenon on zeolite Na-X. (*w Lines due to solid, liquid, gaseous and adsorbed xenon are marked s, I, g and a respectively. 40FID’s with 4Osee repetition rate were obtained. (c) and (d) rzsXe MAS NMR of xenon in hydrogen mordenite. 400 FID’s at 4 set repetition rate.

For H-mordenite at 240 K two broad “‘Xe resonances are observed ca 62 ppm apart. The low field line is attributed to xenon in side pockets in the structure and the high field line to xenon in the main channels. It seems that there is no exchange of xenon between the two at this temperature, while large linewidths suggest a further distribution of molecular environments. At 302K only a single line is observed at an intermediate frequency, showing that xenon now undergoes rapid exchange. In a further paper (240)Ripmeester determined the distribution of xenon between main channels and side pockets in Na+, K+, NH: and Cs+-exchanged mordenites. For steric reasons, the side pockets in Cs’-mordenite are not available for xenon sorption. Finally, it appears that i*‘Xe NMR can determine directly the surface area of zeolite samples, which may be of considerable practical importance.

10. FUTURE

PROSPECTS

The main strength of multinuclear high-resolution NMR spectroscopy of solids is that it makes all atomic components of a zeolitic crystal liable to direct investigation. While this review understandably pays much attention to *‘Si and *‘Al NMR, many other nuclei can be readily observed in the solid state (for examples see Fig. 49 and 50). Oxygen, the remaining major constituent of zeolites, which are essentially mixed oxides, can also be monitored. Klinowski et a/.“” measured l’0 MAS NMR spectra of zeolite A enriched in “0 (see Fig. SO). The spectrum of a rapidly spun sample contains a single signal (with a small quadrupolar splitting) which in the light of the recent observation(271) of three non-equivalent oxygen sites in the chain silicate diopside, CaMgSi,O,, by MAS NMR, signifies that there is only one kind of.oxygen site in zeolite A, confirming the absence of Al-O-Al linkages. These measurements revealed furthermore that the oxygen atoms in the linked SiOi- and AlO:- structural units are much more labile than was previously thought. The synthesis of molecular sieves with non-aluminosilicate frameworks (such as the recently

Nuclear magnetic resonance studies of zeolites

A

303

‘%I

zeolitmY

n

128.4 MHz

23.5 MHZ

104.2 MHz

200

100 ppm

C

from

0

-100

AlfH20)~+

from

PPm

D

Pb(NO&

IIFx0Et2

*NH4'sN03

‘54

83.4 MHz

40.5 YUZ

N

03

.& ,d,, x32

\ : . -Is0

.

NHI+

L

A

,J"r-

-600 PPm

from

-810

PbfClO4)2

200

0

-200

PPm from

-400

NO5

FIG. 49. High-resolution MAS NMR spectra of various solids.(lo6)(A) *‘Al spectra of zeolite Na-Y at 23.5MHz (7771 scans, 25 Hz line broadening, 0.1 set relaxation delay) and at 104.22 MHz (2656 scans, no line broadening, 0.1 set relaxation delay). (B) B spectrum of Corning 7070 glass at 128.4 MHz (13775 scans, 25 Hz line broadening, no relaxation delay). Note the characteristic pattern of the residual quadrupolar interaction. (C) *“‘Pb spectra of Pb(NO,), at 83.4 MHz. The powder pattern (420 scans, 2OHz line broadening, 2 set relaxation delay) is shown above the spectrum with MAS (230 scans, 2OHz line broadening, 2sec relaxation delay). (D) “N spectrum at 40.5 MHz of NH,NO, isotropically enriched in 15N (2978 scans, 40 Hz line broadening, 1 set relaxation delay). The NO; portion of the spectrum, magnified 32 times, is also shown. The small peaks in the spectra, equidistant form the central signal, are spinning sidebands.

discovered ahnninophosphate (ALPO) molecular sieves, and also gallosilicates, aluminogermanates, ferrosilicates, borosilicates and chromosilicates) clearly opens new vistas before the technique. The majority of the exchangeable cations in zeolites have an NMR-active nucleus, but very little high-resolution work has so far been done in this area. In particular, ‘H, MAS NMR will no doubt be extensively used in the future for the study of zeolitic acidity. As we have seen, guest species and reactants accommodated within the cavities and tunnels of a zeolite, or bound to its active centres, can be profitably studied by NMR. It is likely that advanced techniques such as 2D NMR and multiple-quantum NMR will be employed to resolve many hitherto inaccessible problems of zeolite chemistry. Nuclear magnetic resonance studies of zeolites are likely to flourish in the years to come.

304

J. KLINOWSKI stationary

sample

zeolite II

“0

A

MASNMR

d spinning

at 5 kHz

(b)

sb

I

400

I

200

sb

I

I

0

-200

r -400

ppm from HZ170 FIG. 50. “0 MAS NMR spectra at 54.22MHz of zeolite Na-A isotopically enriched in 170.‘57)Chemical shifts are given in ppm from Hi’0 (at natural abundance). (a) stationary sample; (b) sample spinning at ca 5 kHz. The two signals, marked sb and equidistant from the central peak are spinning sidebands. The small splitting of the central peak is due to residual quadrupolar interactions. Acknowledgements--I am grateful to Professor J. M. Thomas, FRS, Cambridge, for suggesting (in November 1980) that I take up NMR studies of zeolites, and for his subsequent help and encouragement. I wish to thank Professor C. A. Fyfe, Guelph, for instructing me in NMR, and Professor R. M. Barrer, FRS, London, for comments on the manuscript. Finally, I acknowledge support from BP Research Centre, Sunbury, without which this work would not be possible. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

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