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Chapter 7 Techniques of Zeolite Characterization
241
Chapter 7
TECHNIQUES OF ZEOLITE CHARACTERIZATION
J.H.C.
van Booff')
and
J.W. Roelofsen')
Laboratory for Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands. AKZO Chemicals B.V., Research Centre Amsterdam, P.O.Box 15, 1000 AA Amsterdam, The Netherlands.
corns 1.
Introduction
2.
Zeolite Structure and Structural Defects
3.
Pore Structure of Zeolites
4.
Chemical Composition of Zeolites
5.
Determination of the Framework Si/Al Ratio
6.
Determination of Zeolite Acidity
7.
Zeolite Stability
8.
Zeolite Morphology and Particle Size
242
1.
INTRODUCTION Before a zeolite can be used for a certain application it is necessary
to characterize this zeolite, to see if it has the desired properties for that application. If not, another synthesis method should be used or the zeolite must be modified, to meet the specifications. Zeolite synthesis, -modification, -characterization and -application thus are strongly related as schematically is indicated in Fig. 1.
SYNTHESIS
1
MODIFICATION
t
CHARACT
t
1
t
APPLICATION Fig. 1.
Position of Zeolite Characterization.
Of course not all zeolite properties are of the same importance for every application. This aspect is illustrated in Fig. 2 in which is indicated, which characteristics are of special importance for the main applications of three different zeolites. For example for the application of zeolite A in detergents particle size and morphology are extremely important while acidity and stability play a minor role. These characteristics, however, are just of crucial importance for the application of zeolite Y in cracking catalysts. For this reason we will discuss the different characterization methods in relation to the application for which the specific property is important.
243
IMPORTANT CHARACTERISTIC
T Y P E ZEOLITE A
3 F
Z E O L I T E
ZEOLITE Y
CHEMICAL COMPOSITION
0
0
FRAMEWORK
SI/AL
RATIO
- 5
0
STRUCTURE STRUCTURAL D E F E C T S PORE-S IZ E
ZSM
0 0
0
____
0
0
ACIDITY S T A B IL I T Y MORPHOLOGY PARTICLE S I Z E
I MPORTAIiT
APPLICATION
Fig. 2
0
0 ADSORBENT I O N EXCHANGER
I N DETERGENTS
CRACKING CATALYST
Most important c/mrflcteristics for thrcc i(d/-knozvn zeo/ites a'ppl icat io 11s.
SHAPE SELECTIVE CATALYST
flrld t/lclr lfmfljor
244 2.
ZEOLITE STRUCTURE AND STRUCTURAL DEFECTS For the application of zeolite ZSM-5 as a shape-selective catalyst it is
very important that the zeolite has the correct structure and has no structural defects. The best way to check zeolite structure is by X-Ray Diffraction. This method is based on the fact that every crystalline material has its own characteristic X-ray diffraction pattern.
For most of the known zeolite structures the ideal diffraction patterns have been simulated and together with other crystal data these diffraction patterns are collected in the following publication of the Structure Commission of the International Zeolite Association. Collection of Simulated XRD Powder Patterns for Zeolites by Roland van Ballmoos Butterworths 1984 To illustrate the kind of information that can be found in this book, we here give the diffraction pattern (Fig. 3 ) together with the other crystal data that are given for zeolite ZSM-5 officially named MFI (Mobil-Five).
I
0
10
20
50
2 THETA
Fig. 3
Simrlated X-ray Diffraction Pattern of MFI.
40
50
60
245
Crystal data: Space group Orthorhombic Pnma a = 20.096 A b = 19.949 = 13.428
a a
a = 90.00"
13 = 90.00' Y
=
90.00"
Additional structural information can be found in a second publication of the Structure Commission of the International Zeolite Association. Atlas of Zeolite Structure Types by W.M. Meier and D . H . Olsen Butterworths 1987 The book gives stereographic views for most of the known zeolite structures together with drawings of the cross-sections of the main channels.
framework viewed along (0101
In-ring viewed along (OlO] (straight channel)
10-ring viewed along (loo] (sinusoidal channel)
Fig. 4
Stereographic view and channel cross-sections of zeolite MFI
246
Furthermore several structural data are presented; for example for ZSM-5 or MFI is given: Framework density : 17.9 T atoms/1000 A 3 Channels
direction number of T-units
free aperture A
[OlOI
10 MR
5.3 x 5.6 (b)
[loo1
10 MR
5.1 x 5.5 (a)
Fault planes: (100) Normally the first check is the comparison of the observed X-ray diffraction pattern with the simulated pattern. If all observed peaks can be found back in the simu ated pattern an, have similar intensities the crystal structure of the zeolite under investigation is the same as the structure that is used for the pattern simulation. If extra peaks are observed or peaks are missing this is an indication that other crystalline phases are present, or that the structure is different
.
The intensity of the diffraction peaks can be used to determine the crystallinity of the sample. For that purpose the intensity of one particular peak (or a number of peaks) is compared with the intensity of the same peak (or peaks) of a standard sample. The X-ray crystallinity then can be calculated from: ( 3 4 ) X-ray crystallinity =
Intensity of peak hkl of sample -~ Intensity of peak hkl of standard x 100%
~-
Furthermore the width of the diffraction peaks gives information about the average crystallite size of the investigated zeolite sample. According to Scherrer ( 3 5 ) the relation between linewidth ( B ) and crystallite size (t) is given by the following formula.
t(in nm) =
0 , 9 X (in nm)
cos
X-ray diffraction thus can give information about crystal structure, degree of crystallinity and crystallite size. It is, however, impossible to obtain information about the presence of structural defects by this method.
247
The presence of fault planes and/or stacking faults i n the structure can be observed by high resolution electron microscopy (HREM) as shown i n Fig. 5.
J
Fig. 5
r/
Electron niicrograph of zeolite Y showing the presence of structiiral defects, indicated by arrows.
Another type of structural defects that can be present are hydroxyl groups.
-
Firstly, there are the terminal s i l a n o l groups that are always present at the outer surface of the zeolite crystals to terminate the structure. For this reason these groups are called: terminal silanol groups.
248
-
The second type of silanol groups is formed when a Si-0-Si bond is broken by a reaction with water.
I -Si-
I
As
-
I I
H2O
S i-
0-
.c--
1
-Si-OH
I broken
HO-Si-
bond
I
I
indicated in the reaction equation this type of silanol groups is
always present as pairs.
-
Finally, silanol groups can be formed if a T-atom is missing in the structure. This type of defect leads to a cluster of four silanol groups or a so-called hydroxyl nest.
I
I
-Si-
-Si-
I
I I
0
-Si-O-~i-O-Si-
I
I
I
I
?
-
I
I
0
I
H HO -Si-
-Si-OH
I
0
I I
hydroxyl
-Si-
I
All these silanol groups can be observed by I R and H NMR spectroscopy. Moreover, it is possible to distinguish between the different types upon reacting these silanol groups with trimethyl-chlorosilane and subjecting the reaction products to 29Si CP-MAS NMR (31).
As indicated in Fig. 6 trimethylchlorosilane will first react via the C1 ligand t o form the primary product. I f , however, neighbouring OH groups are present also the secondary, tertiary and quaternary products can be formed.
249
By
29
Si NMR it is possible to distinguish between these different
reaction products and the presence of the signals attributed to the secondary and tertiary products then indicates the presence of pairs of silanolgroups and hydroxyl nests.
a
-50
-100
6
Fig. 6
PPm
9Si CP-MAS-NMR spectruni of MFI after reaction with trinictliylchlorosilane at 673K showing the presence of primary, secondary and tertiary products.
3.
PORE STRUCTURE OF ZEOLITES For the use as molecular sieve or as shape selective catalyst it is
essential to characterize the zeolite's system. Information about pore size can be obtained from: 3.1
-
the crystal structure
3 . 2 - adsorption measurements
3.3
3.4
-
129xe NMR spectroscopy model compound reactions
3.1 Crvstal structure From a single crystal structure X-ray analysis of the zeolite it is possible to derive the positions of the component atomstogether with the apertures of the channels and cavities that are present in the zeolite structure, and whether the pore system is one-, two- or three-dimensional. The pore size determined for a given zeolite form the X-ray analysis pertains to perfect crystals. Often zeolite samples do not fulfil this condition due to the presence of imperfections or minor amounts of other phases inside the pore system. Therefore it is recommendable t o characterize the pore volume and pore size in a more direct way by one of the other methods.
3 . 2 Adsorotion measurement6
Adsorption measurements with molecules of different size give direct information about the dimensions of the pore system. Molecules with kinetic diameters smaller than the pore openings can be adsorbed and larger molecules can not. In boundary cases a minimum temperature of adsorption may be required.
D.W. Breck (1) presents the dimensions of a number of probe molecules, from which the following examples are compiled in Table 1.
251
TABLE I. Moleciilar diniensions of a series of probe niolecitles. Pauling
Lennard-Jones
length
width
Pm
pm
Pm
300
260
310
kinetic diameter
240
28 9
320
275
384
340
390
280
346
410
300
364
396
360
436
396
420
3 70
376
5 10
3 70
330
390
315
265
410
380
260
5 28
400
360
420
380
500
440
3 90
650
490
430
490
430
560
500
700
6 20
660
585
6 70
600 780 810 1020
The Lennard-Jones kinetir diameters are perhaps most appropriate to predict accessibility of a zeolite for a given adsorbate. For comparison; the apertures of 6, 8, 10 and 12 membered oxygen rings are about 270 pm, 430 pm, 60 pm and 770 pm, respectively.
Generally, the adsorption isotherm (the amount of gas adsorbed as function of the partial pressure of the gas at constant temperature) gives information about the pore structure. This is a method often applied for characterizing materials having meso (2-50
m ) and macropores ( > 50 nm).
252
Problems arise, however, in the characterization of micropores ( < 2 m) the dimensions of which are close to those of the adsorbing molecules. In this case the adsorption process is a filling of the micropores instead of multilayer adsorption on which the interpretation of adsorption isotherms is generally based. This, together with the requirement to measure the adsorption isotherms at very low pressures, makes it very difficult to obtain detailed information on the pore structure of zeolites by this method. At the moment two comparable methods are reported in literature by which it is possible to determine the micropore volume of zeolites from the N2 adsorption isotherm. These are:
-
t-plot method ( 2 ) in which the volume of the adsorbed N, is plotted against the statistical thickness(t) of the adsorbed layer. This results in a straight line and extrapolation to t=O gives the micropore volume.
-
as method ( 3 ) in which the volume of the adsorbed N, is plotted against
the reduced standard adsorption (as). Extrapolation to as = 0 gives again the micropore volume.
='"Xe
NMR
suet-
Fraissard and co-workers (4) recently developed "'Xe
NMR spectroscopy
as a method to obtain information on the pore system of zeolites. Due to the relatively large diameter of the xenon atom ( 4 3 6 pm) information is only obtained on the larger pores. For example in the faujasite type zeolites the supercages are accessible for xenon whereas the sodalite cages are not. The method is based on the measurement of the "'Xe
chemical shift of
the in the NMR spectrum. According to Fraissard the chemical shift of xenon adsorbed in a porous material can be expressed as:
This relation shows that the observed chemical shift is the chemical shifts originating from: -
the reference (6,)
-
the electric field (6,) the magnetic field (6,)
sum
of the
253
-
the interaction of the xenon atoms with the zeolite (6(xe-z).pz)
(pz is
related to the zeolite structure) -
the mutual interactions between the xenon atoms (6(Xe-Xe).he)
(heis
the concentration of the xenon atom, in the pores) (see Fig. 7 ) .
As the exchange of Xe between different positions is fast on the NMR timescale, the NMR signal reflects the average of the different Xe positions and interactions.
t
Fig. 7
Schematic representation of the different contributions to the shift of xenon atonzs adsorbed in the pore systeni of a zeolite.
9Xe cheniical
Fig. 8 gives for three zeolites the lZ9Xe chemical shift as a function of the xenon concentration in the zeolite.
254
/
/
0
120
6 PPm 100
extrapolated values
/
80
Fig. 8
The
Nay :
60 ppm
KL :
9 0 ppm
ZSM-5: 113 ppm
9Xe clteniical shift as function of pxefor tlzree types of zeolite.
The following information can be obtained:
-
extrapolation of the chemical shift to zero concentration gives
-
from the slope of the curves information can be obtained about the void
-
The intensity of the signal gives information about the number of
information about pore size and pore restrictions while space available to the absorbed Xe atoms. well-formed cages. lZ9Xe NMR has been applied
-
so
far in various investigations such as:
characterization of zeolite pore systems; also f o r zeolites with at that time unknown crystal structure (zeolite P ( 5 ) )
-
study of coke formation during catalytic cracking ( 6 )
-
study of the formation of Ni particles upon reduction of NiZf-exchanged Y-zeolites ( 7 ) .
255 When, however, the electric and/or the magnetic field also contribute to the chemical shift the conclusions are less accurate.
3.4 Model mmpound reactions The most direct method to obtain information about the dimensions of the internal space in zeolites available for catalytic reactions is of course the performance of model compound reactions. The advantage of this method is that information is obtained under conditions that are representative for the catalytic application. Generally, three reaction selectivity possibilities can be distinguished:
-
a reactant molecule is too large to enter the pores, causing reactant
-
a product molecule is too large to leave the pores, causing product
selectivity selectivity
-
the transition state for a certain reaction is too large to be accommodated in the pores causing transition state selectivity. In literature several reactions have been discussed by which information
on the pore system of zeolites can be obtained. Three of these reactions
will be discussed here; the first is related to reactant selectivity, while the two other examples are based on transiton state selectivity.
3.4.1 The Constraint Index (CI) Mobil Oil workers (8) introduced a test reaction in which a 1:1 mixture of n-hexane and 3-methylpentane (see Fig. 9 ) is cracked in a gas-flow setup.
n- hexane
Fig. 9
Mokciiles
6
3-methylpentane
irsed for the determination of the constraint index (CI)
256
Depending on the pore size, the relative rate of cracking of the linear versus the branched hydrocarbon molecule will change and they define: rate constant of n-hexane cracking Constraint Index (CI) = rate constant of 3methylpentane cracking ~
and supposing first order kinetics for the cracking reactions this relation is equivalent with CI =
log (fraction n-hexane remaining) log (fraction 3-methylpentane remaining)
The Constraint Index provides the following classification of the pore-system CI < 1
: large pores
1 < CI < 12 : intermediate pores
CI > 12 : small pores The determation of the CI is suitable for the characterization of medium pore zeolites. The method is, however, not sensitive for large pore zeolites. Moreover, the method proved to be temperature dependent.
-
3.4.2 The Refined Constraint Index (RCI) Jacobs et al. ( 9 ) introduced the bifunctional conversion (1% Pt on the zeolite in the proton form) n-decane as a test reaction. Under the conditions that 5% of the n-decane is isomerized the ratio of 2-methylnonane and 5methylnonane which are formed in this reaction (see Fig. 10) is determined.
n-decane
2
1
4
3
2-methylnonane
Fig. 10
8
6
5
7
10
9
li,
5-methylnonane
Reactions used for the deterniination of the refined constraint index.
257
Refined Constraint Index (RCI) =
[Z-methylnoflane] [ 5-methylnonane1
It is pointed out that more space is needed for the formation of 5-methylnonane than for the formation of Z-methylnonane. So the RCI will increase with decreasing pore width. The method proved to be especially suited for the characterization of zeolites with 10 membered ring pore systems, it is, however, not sensitive for zeolites having larger pores.
3 . 4 . 3 The Spaciousness Index (SI)
To characterize wide pore zeolites Weitkamp et al. (10) have introduced the hydrocracking of n-butylcylcohexane as a test reaction. Two of the reaction products are n-butane and isobutane and according to the authors the transition state for the formation of isobutane is larger than that for the formation of n-butane (see Fig. 11).
butylcyclohexane
n-butane
Fig. 11
i-butane
Reaction used for the determination of the spaciousness index.
The ratio of the amounts of formed n-butane and i-butane thus can be used as an indication for the available space in the zeolite pore system leading to the following definition: Spaciousness Index (SI) =
[formed i-butane] [formed n-butane] ~~
It will be clear that SI decreases with decreasing pore width. The method proved to be especially suited for the characterization of zeolites with 12 membered ring pore systems and so the SI is complementary to the RCI and CI discussed above.
258
4. CHEMICAL COMPOSITION OF ZEOLITES There are several possibilities to express the chemical composition of zeolites. For example for the sodium form of zeolite Y the following formulae can be used
2
Na,O,
2
A1,0,;
(192-x)Si02, zHzO
in which the number of A1 per unit cell can vary between 48 < x < 64. Thus for dried pure zeolites the chemical composition can be characterized by a single parameter x, the number of A1 atoms per unit cell. However, instead of the number of A1 per unit cell (NAl) often the Si/AI (silicon/aluminum) atom ratio or the SiO,/Al,O,
(silica/alumina) molecular
ratio is used to characterize the chemical composition of zeolites. Table 2 shows the interrelation of these parameters.
TABLE 2
Interrelation of the different parameters used to characterize the chemical composition of N a y zeolites.
X
192-x X
48
3
2
6
4
192-x
x
64
11.8
15.4
19.5
25.3
68.7
59.4
259
The methods for the determination of the chemical composition can be divided into two groups: 4.1
Methods based on complete dissolution of the zeolite and subsequent analysis of the solution obtained (destructive wet analysis).
4.2
Analysis
of
the
solid material
by
physical
methods
(non-destructive dry analysis)
4.1 Complete Dissolution Method An example of a frequently used method is given below: Weigh accurately a sample (1 g) of dried zeolite
(g gram).
Add sulphuric acid ( 4 0 ml) and heat until SO, fumes. Dilute with water (about 200 ml) and filter
(filtrate A).
Dry and ignite the residue at 1000°C Add hydrofluoric acid (10 ml) evaporate carefully
(a gram)
and finally ignite at 1000°C
(b gram). X Sio, =
a - b ~
g
x 100%
Add potassium peroxodisulphate (2g) to the residue and heat until a clear melt is obtained. Dissolve the melt in water (100 ml) and combine the solution with filtrate A . Determine Na and A1 i n this solution by AAS. As can be seen this method is rather laborious and therefore several more rapid physical analysis method have been developed. For the accurate analysis of exchange of cations or substitution of T-atoms a proper chemical analysis remains a prerequisite.
4.2 Physical Analvsis Methods for Zeolites
Several methods are available, all with their advantages and disadvantages ( 3 2 ) .
260
4.2.1 X-ray Fluorescence Spectroscopy In this method a suitable sample is radiated by high energy X-rays by which electrons can be expelled from the different atoms, leaving holes in low-lying orbitals. The main mechanism for relaxation then is, that an upper electron falls into this hole. The energy released may result either in the generation of radiation, which is called X-ray fluorescence, or in the ejection of another electron, the secondary electron of the so-called Auger effect (see Fig. 12).
primary e l e c t r o n (XPS)
e
primary electron
-
a
e
-
secondary e l e c t r o n (AES)
4
hVout 0
-
I I
I
I
I
I
I
I . I
I
I hvin
T
I
/
I
/
/
/
13 X-ray F l u o r e s c e n c e
Fig. 12
Auger E f f e c t
Schematic representation of the tzoo possible relaxation processes after expellation of an electron from a low lying orbital.
Both X-ray fluorescence radiation and the Auger electrons are characteristic for the emitting atoms and can be used for quantitative elemental analysis. Appropriate sample preparation and calibration is, however, essential when using this method. A suitable method for sample preparation consists of making a solid solution of the zeolite in a LiBO, melt followed by rapid cooling to a glassy material. For calibration standard zeolite samples with known composition should be used.
261 4.2.2
Proton/Particle Induced X-ray hission (PIXE) A modification of the former method can be used if high energy particles
are at one's disposal. Holes in the low-lying orbitals then can be created by a bombardment of the sample with these particles followed by a registration of the emitted X-rays. Again adequate sample preparation and calibration is essential for an accurate analysis. 4.2.3
X-ray Photoelectron Spectroscopy (XPS) Instead of measuring the emitted X-rays or secondary electrons it is
also possible to measure the energy spectrum of the primary electrons (see Fig. 1 2 ) . This is done in X-ray Photoelectron Spectroscopy and the resulting spectrum can be used for qualitative analysis and after calibration a160 for quantitative analysis of the samples. Because the escape depth of
the electrons is limited to a few
atom-layers only the surface composition of the samples is obtained. In combination with sputtering of the sample by bombarding the sample with noble gas ions it is possible the obtain a depth profile of the chemical composition of the sample. 4.2.4
Electron Probe Micro Analysis ( E m ) Modern scanning electron microscopes can have the facility to focus6 the
electron beam on a small spot on the surface of the sample, which can result in the expellation of electrons from the atoms, followed by X-ray fluorescence or the emission of Auger electrons. In this case the emitted X-rays are originating only from the small spot radiated by the electron beam and thus give information on the chemical composition of that spot (diameter about 8 nm).
By scanning the electron
beam over the sample it is possible to obtain information about the distribution of the elements in this way (11). All methods discussed above can be used to determine the chemical composition. For example when the aluminum content is determined the result is the tptal amount of aluminum present in the sample (or part of the sample). It is, however, impossible to distinguish between framework aluminum and non-framework aluminum in this way. As only the framework aluminum can give rise to the formation of Br$nnsted acidic sites whereas non-framework aluminum can provide Lewis acidity it is important to have methods available for the determination of both types and amounts of the aluminum. Some methods developed for this purpose will be discussed in the next paragraph.
262
5. DETERMINATION OF THE FRAMEWORK Si/Al RATIO. As mentioned in the introduction the main application of zeolite Y is in catalytic cracking, and for this application the framework Si/Al ratio proved to be extremely important. So most information on the determination of the framework Si/Al ratio is based on this type of zeolite, and in this
paragraph the different methods will be discussed on the basis of the results with zeolite Y.
5.1 XRD-Lattice constant As zeolite Y possesses cubic symmetry (a=b=c) the unit cell dimensions
can be characterized by the single lattice constant a. Because the A1-0 bond-length is larger than the Si-0 bond-length this lattice constant a will increase with increasing (framework) A1 content of the zeolite. Based on the results of chemical and X-ray diffraction analysis of a set of hydrated NaY zeolites made by direct synthesis, having a number of framework A1 atoms per unit cell (NA1) in the range of 48 to 77 Breck and Flanigen (12)
reported the following relation between N A ~and a (see Fig. 13).
i2 .50.-
= 1152.(
NAl
a
-
2.4191
-_-_-__ -_ _ __ _ - - - - - - - . T
a m
2 .45
-
' 0
0
0
0
0
0
0
0
/
0
0
A'
-----_-
I
I
1
I I
I I
I
I I
I
I I
I
I
I
I
I I
I
)
10
20
30
40
I
50
60 NAl
Fig. 13
I .
.
70
80
Breck-Flanigen relation betztwn the niiniber of franzcu~orkA1 atonis ycr tinit ccll NAI arid the lattice constant a for hydrated N a y zeolites.
Once this relation is known it is possible to derive NA1 of an unknown hydrated NaY zeolite from the experimentally determined lattice constant a.
263 A potential problem of this method is that the Breck-Flanigen relation
is only valid for hydrated samples and that the lattice constant is also influenced by cation exchange and substances formed during thermal treatment of the zeolite. The zeolite framework is not a rigid construction but can accomodate itself to the requirements of cations and other phases present in the framework. This is for example illustrated by Fritz et al. (13) who studied the influence of dehydration and NH,'
exchange of NaY zeolites. Upon
dehydration they observed an increase of the lattice constant from 2.466 run to 2.4765 nm while NH,'
exchange to very low sodium content lead to an
increase to 2.478 nm. Roelofsen et al. ( 1 4 ) found evidence that also the presence of rare earth ions causes deviations from the Breck-Flanigen relation. Another drawback of this method is that it is restricted to zeolites with cubic structure, for other zeolites no such relations have been reported.
5 . 2 IR spectroscopy
In the infrared spectrum of zeolites in the range of 300 - 1300 cm-'
the
lattice vibrations can be observed. These vibrations can be divided in structure sensitive and structure insensitive bands, Flanigen (15) reports the following positions of these bands: Structure insensitive vibrations
- asymmetric stretch
-
synnnetric stretch
- T-0 bending
950 - 1250 cm-' 650
-
720
cm-'
420 - 500
cm-'
Structure sensitive vibrations
- asymmetric stretch - symmetric stretch -
double ring vibrations
- pore opening vibration
1050 - 1150 cm-' 750
-
820
cm-l
500 - 650
cm-'
- 420
cm-'
300
For some of the structure sensitive bands a linear relation between the wave number and the number of lattice aluminum atoms is reported ( 1 6 ) . After calibration it is possible to use this relation to derive the number of lattice aluminum atoms from the band positions. Fig. 14 shows the results obtained by Flanigen for Y-zeolites with different degree of dealumination.
fry
264
A.O% N,,
T a.u.
= 56
II I
I
III I
I11
\ \
I
I
1
J
I
\
906
1200
600
wavenumber
Fig. 24
cm
-1
IR spectra of Y-zeolites uiith dif erent degree of dcalumination (% dealiiminatiiini denoted in Figure{
From t h i s f i g u r e i t i s p o s s i b l e t o d e r i v e t h e f o l l o w i n g r e l a t i o n between t h e p o s i t i o n of t h e s t r u c t u r e s e n s i t i v e asynnnetric s t r e t c h band ( u l ) and t h e number of l a t t i c e aluminum atoms (NA1). N A l = 0.960 (1068 - u ~ )
t h e symmetric s t r e t c h band (u2) c a n b e used.
Also t h e p o s i t i o n of Lunsford e t a l .
( 1 7 ) g i v e t h e f o l l o w i n g r e l a t i o n s t o d e r i v e N A ~from t h e
p o s i t i o n s of t h e two bands. NAl
= 0.766
(1086
-
crl) and
N A l = 1.007
(838
-
u*)
A s n o such r e l a t i o n s are r e p o r t e d f o r o t h e r z e o l i t e t y p e s t h i s method i s r e s t r i c t e d t o Y-zeolites.
265
5.3 *'Si
and "A1
MAS-NMR
Since the introduction of the Magic Angle Spinning technique to NMR spectroscopy it is possible to obtain high resolution spectra from solid samples. Lippmaa et al. (18) were the first who applied in 1981 this method to record the *'Si-spectra
of zeolites. Since then the technique has been
developed rapidly and at the moment is widely applied in zeolite research. The state of the art is very well described in the book of Engelhardt ( 1 9 ) who is also the author of one of the chapters in this book. Here only a short description of the use of this method to determine the number of framework A1 atoms will be given. Application of the "Si
MAS NMR technique to zeolites results in spectra
in which a number of separate Si peaks can be observed. For the faujasite structure in which all T-atoms (Si and Al) are crystallographically equivalent it is possible to distinguish up to five different peaks that can
be attributed to Si connected-through oxygen- to 0,1,2,3 and 4 A1 atoms, respectively (see Fig. 15).
A1 0
AlOSiOAl 0
A1
-30
A1 0
SiOSiOAl 0
A1
-90
Si
0
SiOSiOAl 0
A1
-100
Si 0
SiOSiOAl
Si n SiOSiOSi
0
0
Si
S i
-110
3 (29~i) ppm Fig. 75
9Si MAS-NMR spectrum of N a y zeolite with Si/Al= 2.5
266
As the intensity of a peak is proportional to the number of the Si atoms concerned, it is possible to calculate the total number of Si atoms by sommation of the peak intensities: 4
N~~
=
c.E I(Si-nAl) n=o
If the Lijwenstein rule is obeyed, every framework A1 atom is surrounded by 4 Si atoms. So the total number of framework A1 atoms can be calculated from:
N~~ =
c.1
n=o
-
I(Si-nAl)
The framework Si/Al ratio can then simply be calculated from:
Table 3 shows the results of this method when applied to determine the framework Si/A1 ratio of a NH,NaY zeolite before and after steaming, clearly showing the dealumination of the framework upon steaming.
r
Peak Intensities Sample
4A1 3A1
dried NH4NaY
1
11.5
steamed NH,NaY
0
4.1
2A1
1Al OAl
48.1 35.6
Si/A1 ratio
4.9
4.8
5.7 3 0 . 2 60.0
14.5
Problems can be the presence of paramagnetic cations (such as some rare earth ions), that can cause appreciable broadening of the NMR peaks, as has been reported by Roelofsen (20) and Schemer (21).
267 In principle it is possible to determine A 1 directly by 27Al MAS
NMR. In
practice, however, several problems can arise that hamper the interpretation of the results. The main reason for these problems is the fact that the A1 nucleus possesses a quadrupole moment that may give rise to strong line broadening especially in the case that the A1 is present i n a low symmetry environment. Because of the latter reason several authors report no problems with the determination of the tetrahedrally coordinated framework A 1 but the non-framework A1 is not completely "visible". Grobet et al. (22) report the addition of the Al-coordinating compound acetylacetone to the sample as a method to make visible also this part of the Al.
268 6.
DETERMINATION OF ZEOLITE ACIDITY The acidity of zeolites is mainly caused by the presence of BrOnsted
acid sites but especially after high temperature treatments also Lewis acid sites may be present (see Fig. 16).
H S i/O\
-
A1
/O\
Si
A1
4.
H20
/"\A1
Si
Br$nsted-acid sites
Fig. 16
r-7
Si
Lewis-acid site
BrOnsted and Lewis acid sites in zeolites.
For a complete characterization of zeolite acidity it is, therefore, necessary to determine number and strength of both types of acid sites. Several methods have been developed for this purpose from which the most important are:
-
titration methods
-
adsorption and desorption of bases
-
IR spectroscopy
- of OH groups
-
NMR spectroscopy
- of adsorbed bases - of OH groups -
of adsorbed bases
This paragraph will give a brief description of these methods
6.1 Titration methods The acid-strength of a BrOnsted acid BH is derived from its proton donating ability and can be expressed by the equilibrium constant of the reaction
269
Ka
BH aH+
Ka =
B-
. aB-
+
H+
fB-'cB=
aH+
~
BH *CBH
a~~
In which fi is the activity coefficient of species i and Ci its concentration
.
Then bv definition: pKa = -1ogKa = -log [aH+
-]
. fB-' cg~BH-CBH
Because for solid acids, as zeolites are, the activity coefficients are very difficult to quantify, Hammett introduced the acidity function Ho. pKa = -log [aH+
.
"3
~ B H
- log CB- = Ho CBH
-
log CBCBH
It is evident from this definition that as the limit of a dilute aqueous solution is approached the values of fg- and fgH approach 1 and Ho becomes equal to the pH. The Hammett acidity function thus behaves similar to the pH in that it becomes more negative the greater the acidity of the solution. The value reaches -10 in the most concentrated solutions of H,SO, To determine H,
or HC10,.
for solid acids Hammett used a series of aniline bases
as indicators which required acids of different strength to give color change (see Table 4).
TABLE 4
Series of Indicators to determine the Haniniett acidity function.
Hammett Indicator Neutral red Phenylazonapthylamine Butter yellow Benzeneazophenylamine Dicinnamal acetone
HO
+ 6.8 + 4.0 + 3.3
+ -
1.5
3.0
Benzal acetophenone
- 5.6
Anthraquinone
-
8.2
270
To determine the acid strength (distribution) of a zeolite the solid is
dispersed in a nonpolar solvent (e.g. n-hexane) and a series of indicators requiring successively stronger acids to become protonated is added until no color change is observed. When a color change is observed for a given indicator it is concluded that some of the sites on the zeolite correspond to the Ho characterized by that indicator. Then the procedure is repeated after addition of a known amount of a base (e.g. n-butylamine) that neutralizes the strongest acid sites. In this way an acid-strength distribution curve as illustrated in Fig. 17 can be obtained ( 2 3 ) .
I
RENaY
1.0.1
after calcination
0.8..
0.6"
$trong 0.4
..
0.2..
6
Fig. 17
4
2
0
-2
-4
-6
-8
Acid-strength distribution curves of RENaY after various pretreatnlents.
Problems with this method are: the impossibility to distinguish between BrQnsted and Lewis acidity and the fact that the indicator molecules cannot enter the zeolite pores, and thus can react only with the acid sites at the outer surface of the zeolite.
6.2 Adsorption of bases : Cal.Qrimetric measurements When an acid is neutralized by a reaction with a base the heat of neutralization is evolved. This heat of neutralization is larger the stronger the acid and so it can be used to characterize acid-strength. Aurow et al. (24) have demonstrated this method for several zeolites. Fig. 18 shows the results for H-ZSM-5 and Na-ZSM-5.
271
I
Qads
140
120
100
kJ/mol 30 60
40 2
4
Fig. 18
8
6
ads ads or bed)
cm 3 / g
10
Heat evolutiori during the adsorption of N H , on H-ZSM-5 and Na-ZSM-5 at 150°C.
A s can be seen in this figure more heat is evolved when NH, is adsorbed on H-ZSM-5
than on Na-ZSM-5 demonstrating the presence of stronger
acid-sites. A complication of this method is the accessibility of the acid sites and the long times that are needed to reach equilibrium. The fact that the heat evolved after the addition of the first dose of NH, to H-ZSM-5
is
smaller that the heat evolved after the second and third dose in the example mentioned above is explained by the authors on that basis. The first dose of NH, is supposed not to react with the strongest acid sites but mainly with the more easily accessible weaker sites, and the more difficult accessible stronger acid sites are only neutralized after the addition of the second and third dose. Furthermore, the method is rather time-consuming and
so
most
investigators prefer the more rapid method in which the opposite reaction is used.
272 6.3 Desorption o f bases. Temp As
DesorDtion
( TPD
1
discussed in the preceeding paragraph bases adsorb stronger on
stronger than on weaker acidic sites. Thus it will cost more energy or higher temperatures to desorb bases from stronger acidic than from weaker acidic sites. This principle can be used to characterize the acid sites present on a zeolite sample. First the zeolite is contacted with a base (NH, or pyridine) to neutralize the acidic sites present. Then the temperature is raised at a constant rate and the amount of desorbed base is recorded (either by mass spectrometry or by a heat-conductivity cell).
In this way a desorption
spectrum is obtained as presented in Fig. 19, for zeolite H-ZSM-5.
\
4
t irne
H-ZSM- 5 S i / A l = 30 = 10 K/min
Silicalite
LT
400
600
800
1000 &
T (10
Fig. 19
,
Temperature prograninied desorption spectrum of NH from H-ZSM-5 (Si/Al= 30)
In this spectrum two peaks can be observed. A low temperature peak (LT)
corresponding with NA, desorbing from the weaker acidic sites. This peak can also be observed when desorbing NH, from non-acidic silicalite. And a high temperature peak (HT) corresponding with the stronger acidic sites. The area of
this peak gives information about the amount of these strong acidic
sites, while the peak-maximum-temperature (T,)
gives information about its
acid-strength. Cvetanovic and Amenomiya (25) have developed a theory enabling the derivation of the heat of desorption from the peak-maximumtemperature (T,)
at a certain heating rate (B).
213 Although temperature programmed desorption of ammonia (NH,-TPD)
is a
simple and rapid method to characterize zeolite acidity, the method has its limitations. Firstly is it not possible to distinguish between ammonia desorbing from BrQnsted or Lewis acidic sites, and even desorption from non-acidic sites (like in silicalite) is recorded. For Y-zeolites a second more serious problem is, however, the possibility of diffusion limitation during the desorption of ammonia from adsorption sites in the pores and cavities of the zeolite, causing desorption at higher temperatures than corresponding with the acid-strength of the desorption-sites.
6 . 4 IR-spectroscouv of
OH g r o w
A more direct study of the Br4nsted-acidic
OH groups in zeolites is
possible by IR-spectroscopy. This method, that is extensively reviewed by Ward (261, is based on the dependence of the 0-H stretching frequency on the acidity. The weaker the 0-H bond, the lower the stretching frequency and the higher the acid-strength. Fig. 20 shows a typical IR-adsorption spectrum of a thin wafer of HNaY after dehydration.
3800
3700
3600
3640
Fig. 20
IR-Absorption spectrum of HNaY
3500
'
cm
-1
214
The spectrum shows three peaks in the 0-H stretching region. The peak at 3740 cm-' is assigned to the non-acidic silanol groups that are present at the outer surface of the zeolite crystals and at structural defects. The peaks at 3640 cm-l and 3540 cm-l correspond with OH groups having lower 0-H bond strength and
so
more acidic character. From these bands the
one at 3640 cm-I dissappears after adsorption of pyridine and so this band
is assigned to the accessible acidic OH groups. Other zeolites show similar spectra with small shifts in the positions of the peaks. Barthomeuf (27) has measured the exact position of the IR peaks corresponding with the acidic OH-groups for a series of zeolites with varying Si/Al ratio (see Fig. 21).
ZSM-5 T
Mordeni te
h 3620
3640
/
A
1
2
3
4
5
6
7
' Si/Al
Fig. 21
30
Position of the IR-band corresponding zciith the acidic OH-groups for a series of zeolites uuth varying Si/AI ratio.
From this figure it can be seen that the peak position shifts from about 3660 cm-l for zeolite X with a Si/Al ratio of 1.25 to about 3610 cm-l for Mordenite with a Si/Al ratio of 4.5. Further increase of the Si/Al ratio has only a minor effect. For ZSM-5 with a Si/A1 ratio of 30 the peak is observed at 3605 cm-'.
It must be concluded that the acid-strength of zeolites
increases with increasing Si/Al ratio till a maximum value is reached at a Si/Al ratio of about 6 to 7. To explain this result it is accepted that the
275 acid-strength of a bridging OH group in a zeolite depends on the number of neighbouring A1 atoms (see Fig. 2 2 ) .
Fig. 22
Surrounding of a bridging OH group in a zeolite.
According to the Lowenstein rule A 1 is surrounded by 4 Si atoms, but the Si atom can be surrounded by 0 to 4 A 1 atom. The bridging OH thus can have 0 to 3 A1 atoms as next-nearest neighbours. The probability of this will depend on the Si/A1 ratio of the zeolite. Maximum acidity is obtained with no A 1 atoms in the second coordination shell. This will be reached when the Si/Al ratio of the zeolite exceeds 9 or 10 ( 3 3 ) . Further increase of
Si/Al then has no more effect on the
acid-strength. Thus determination of the position of the IR absorption band belonging to the acidic OH groups can give information on the acid-strength of these groups. Because it is difficult to determine the mass of the sample from which the I R adsorption spectrum is recorded the method only gives qualitative information on the number of acid sites.
6.5 IR suectroscouv o f adsorbed bases Instead of looking to the acidic OH groups it is also possible to study the IR absorption of bases that have reacted on the acid sites of the zeolite. Frequently the reaction with pyridine is used for this purpose. As indicated in Fig. 23 pyridine can react with the BrQnsted-acid sites as well as with the Lewis acid sites of the zeolites. The reaction on the BrQnsted acid site results in the formation of a pyridinium ion while the pyridine molecule is coordinatively bound on the Lewis acid sites. Both the pyridinium ion and the coordinatively bound pyridine have characteristic IR absorption bands.
276
I _
L
B
L-acid
B-acid
L
L
1700
Fig. 23
1600
1500
1400
-1
u crn
IR spectrurri of pyridine adsorbed on partly dehydroxylated HY
In this way it is possible to indicate the presence of both Br+nsted and Lewis acid sites in zeolite samples whilst the intensity of the bands gives information on the number of these sites. It is, however, impossible to obtain information on the acid-strength. The presence of the absorption bands only shows that acid-sites are present with an acid-strength large enough to react with the basic probe molecule pyridine. Jacobs et al. (2 8) have, therefore, modified the method by using the less basic probe molecule benzene. Reaction with Br@nsted acid site then does not result in complete proton transfer to form the benzenium ion but in an interaction of the benzene with the acid proton at which the proton remains bonded to the bridging oxygen atom. As a consequence, the 0-H bond strength is weakened, which can be observed by a shift of the corresponding
IR band to lower wavenumbers. Table 5 shows the observed shifts when adsorbing benzene on different zeolites in the proton form.
277
TABLE 5
Observed shift of the position of the OH band in the ZR spectrum when interacting with benzene for these types of zeolites aOH
'OOH
re la t ive
cm-'
cm-'
i nt e n s i t y
300
50
348
50
H-ZSM-5 S i / A l = 30
3600
H-ZSM-11 S i / A l = 30
3603
220 300 340
50 15
3610
326
100
Dealuminated H-Y Si/A1 = 30
Two t y p e s of a c i d i c groups a r e observed i n H-ZSM-5.
35
One type showing a
s h i f t of 300 cm-'
when i n t e r a c t i n g w i t h benzene and a second t y p e showing a
s h i f t of 348 cm-'.
The l a r g e r s h i f t observed f o r t h e l a t t e r t y p e is caused
by a s t r o n g e r i n t e r a c t i o n w i t h t h e benzene molecule. The i n t e n s i t y r a t i o
(50:50) i n d i c a t e s t h a t e q u a l amounts of b o t h groups are p r e s e n t .
6.6 '€IMAS-NMR of OH-arom S i n c e t h e i n v e n t i o n of t h e magic a n g l e s p i n n i n g t e c h n i q u e i t i s a l s o p o s s i b l e t o o b t a i n h i g h l y r e s o l v e d NMR s p e c t r a from s o l i d samples. P f e i f e r e t a l . ( 2 9 ) have a p p l i e d t h i s t e c h n i q u e t o s t u d y t h e d i f f e r e n t t y p e s of p r o t o n s p r e s e n t i n z e o l i t e s . Fig. 24 shows a n example of a 'H MAS-NMR spectrum as o b t a i n e d by them. In t h e upper spectrum 4 t y p e s of p r o t o n s ( a , b , c , d )
can b e observed from
which 3 t y p e s d i s a p p e a r a f t e r a d d i t i o n of d e u t e r a t e d p y r i d i n e .
Peaks b , c
and d t h u s must belong t o a c i d i c p r o t o n s . From t h e s e , peak d can b e a s s i g n e d t o NH,' NH,-Y.
t h a t remained i n t h e z e o l i t e d u r i n g t h e c a l c i n a t i o n of t h e p a r e n t P e a k s b and c , however, r e s u l t form a c i d i c OH groups and c o r r e s p o n d
w i t h t h e 3640 cm-'
and 3540 cm-'
bands, r e s p e c t i v e l y ,
i n t h e I R spectrum
( s e e F i g . 20). Also i n t h i s c a s e t h e p o s i t i o n of t h e peak i n r e l a t e d t o t h e
0-H bond-strength
(The chemical s h i f t
bH i n c r e a s e s
with
decreasing
b o n d - s t r e n g t h ) and t h u s g i v e s i n f o r m a t i o n on t h e a c i d - s t r e n g t h , peak area can be used t o q u a n t i f y t h e number of a c i d groups.
while t h e
278
~~
~
15
10
5
0
-5
6, PPm
Fig.24
' H - M A S - N M R spectriini of H-Y zeolite before (---I orid after (4 the addition of deirterated pyridine.
Experimentally, the technique is rather demanding especially because the method is extremely sensitive for the presence of H,O. Summarizing, it can be said that several techniques can be used to characterize the acidity of zeolites all with their pro's and contra's (see Table 6).
TABLE 6
Available techniques for the cliaracterization of zeolite acidity. Potentials and Problenzs.
PlETliOD
T I TRAT ION
ADSORPI I OF TI'D
IR
OH
1R BFISES
NRR
lH
AMOUNT
STRENGTH
PROBLEflS ACCESSIBILITY DIFFUSION DI F FUSION NON-ACIDIC A D S O R P T I O N SAMPLE P R E P A R A T I O N SAMPLE P R E P A R A T I O N WATER A D S O R P T I O N
I
279
7.
ZEOLITE STABILITY An important requirement posed on zeolites is their stability under
operation conditions. This means amongst others thermal stability towards amorphization and dealumination. Various methods are available to check this stability, the most versatile of which is X-ray diffraction. By analysing the zeolite before and after a certain thermal treatment or certain reaction conditions with X-ray diffraction it can be determined whether or not changes in degree of crystallintiy, crystal structure or number of framework aluminum atoms have occurred. Of special interest is the High Temperature X-ray diffraction technqiue in which the diffraction pattern is continuously recorded while heating up the sample. An example of a pattern obtained in this way is presented in Fig. 25. This figure clearly shows that upon heating at about 200°C the diffraction lines shift to the left indication an increase of the lattice constant from 2.477 to 2.484 run, which is caused by dehydration of the zeolite. At about 600°C intensity changes of lines take place and new lines appear indicating a change in crystal structure. Finally collapse of the crystal structure is observed at about llOO°C when the diffraction lines disappear.
Fig. 25
Variable temperatiire X-ray diffraction pattern of rare-earth exchanged zeolite Y (14% RE,O,).
281
A second method to determine thermal stability is by differential thermal analysis (DTA) (see Fig. 2 6 ) .
1
RENaY
-.
B = 40 OC/min 1 0 2 2 OC
I
AT
\
I
I
500
1000
T
Fig. 26
OC
-
DTA pattern of RENaY (14% RE,O,)
The DTA pattern shows a exothermic reaction at about 1020°C caused by the collapse of the crystal structure.
282
8.
ZEOLITE MORPHOLOGY AND PARTICLE SIZE The size and morphology of the zeolite particles will influence the
properties. A typical example is the size and shape that are required for the application of NaA zeolite as a builder in detergent formulations. As reported (30) the particle size should be in the range of 0.1 to 10 p n and the edges of the cubic zeolite crystals should be rounded off. Also for other applications where the kinetics of adsorption or ion exchange play a role size and shape of the zeolite particles will be important. Scanning electron microscopy (SEM) is the most versatile technique to study the morphology and particle size distribution of zeolites (see Fig. 2 7 ) .
Fig. 27
Scanning Electron Micrograph of a titanium silicalite-1 sample
Other methods to determine particle size are based on sieving and sedimentation. By the dry sieving method particles larger than 5 p can b e determined, while the coulter counter sedimentation method is suitable for particles down to 0.5 p n .
283
REFERENCES 1. D.W. Breck; "Zeolite Molecular Sieves", John Wiley & Sons, 1974, p.636. 2. B.C. Lippens and J.H. de Boer; J. Catal. 4 (19651, 319. 3. S.J. Gregg and K.S.W. Sing; in "Adsorption, Surface Area and Porosity", 2nd Ed, Academic Press, 1982. 4. J. Fraissard and T. Ito; Zeolites 8, (1988). 351. 5. R. Benslama, J. Fraissard, A. Albizane, F. Fajula and F. Figueras; Zeolites 8, (19881, 196. 6. T. Ito, J.L. Bonardet, J. Fraissard, J.B. Nagy, C. Andre, Z. Gabelica and E.G. Derouane; Appl. Catal. 43, (1988), L5. 7. E.W. Scharpf, R.W: Crecely, B.C. Gates and C. Dybowski; J. Phys. Chem. 90, (19861, 9. 8. V.J. Frilette, W.O. Haag and R.M. Lago; J. Catal. 67, (1981), 218. 9. J.A. Martens and P.A. Jacobs; Zeolites 6, (1986), 334. 10. S. Ernst, R. Kumar, M. Neuber and J. Weitkamp; in "Characterization of Porous Solids". Ed. K.K. Unger et al, Elsevier, 1988. 11 C.E. Lyman, P.W. Betteridge and E.F. Moran; in "Intrazeolite Chemistry", Ed. G.D. Stucky and F.G. Dwyer, ACS Symp. Ser. 218, 1983. 12. D.W. Breck and E.M. Flanigen; in "Molecular Sieves", SOC. Chem. Ind. 1968, p.47. 13. P.O. Fritz, J.H. Lunsford and C.M. Fu; Zeolites 8, (19881, 205. 14. J.W. Roelofsen, H. Mathies and R.L. de Groot; in "Zeolites: Fact, Figures, Future", Elsevier 49, 1989, p.643. 15. E.M. Flanigen, H. Khatami and H.A. Szymanski; Adv. Chem. Ser., 10, (1971), 201. 16. E.M. Flanigen; in "ACS Monograph 171" Ed. J.A. Rabo, 1976. 17. J.R. Sohn, S.J. DeCanio, J.H. Lunsford and D.J.O. Donnell; Zeolites, 6, (19861, 225. 18. E. Lippmaa, M. Magi, A. Samoson, M. Tarmak and G. Engelhardt; J.Am. Chem.Soc. 103, (1981) 4992. 19. G. Engelhardt and D. Michel; in "High Resolution Solid State NMR of Silicates and Zeolites", John Wiley & Sons, 1987. 20. J.W. Roelofsen, H. Mathies, R.L. de Groot, H. Angad Gaur and P.C.W. van Woerkom; in "New developments in Zeolite Science and Technology" Ed. Y Murakami, A. Iijima and J.W. Ward; Kondansha Elsevier 1986, p.337. 21. P.S. Iyer, J. Scherzer and Z.C. Mester; in "Perspectives in Molecular Sieve Science", Ed. W.H. Flank and T.E. White jr., ACS Symp. Ser., 368 (19881, 48. 22. P.J. Grobet, H. Geerts, J.A. Martens and P.A. Jacobs; J. Chem. SOC. Chem. Comm. 22, (1987), 1688. 23. L. Moscou and R. Mone; J. Catal. 30, (1973), 417. 24. A . Auroux and J.C. Vedrine; in "Catalysis by acids and bases" Elsevier, 20, 1985, p. 311. 25. R.J. Cvetanovic and Y. Amenomiya; Adv. Catal. 17, (1967), 103. 26. J.W. Ward; in "ACS Monograph 171" Ed. J.A. Rabo, 1976. 27. D. Barthomeuf; in "Catalysis by Zeolites" Elsevier, 5, 1980, p. 55. 28. P.A. Jacobs, J.A. Martens, J . Weitkamp and H.K. Beyer; i n "Selectivity in Heterogeneous Catalysis", Far. Disc. Chem. SOC., 72, (1981), 351. 29. H. Pfeifer, D. Freude and M. Hunger: Zeolites, 5, (1985), 274. 30. R.A. Llenado; in "Proceedings Sixth International Zeolite Conference", Ed. D. Olson and A. Bisio, Butterworths, 1983, p.940. 31. B. Kraushaar, L.J.M. van de Ven, J.W. de Haan and J.H.C. van Hooff: in "Innovation in Zeolite Materials Science" Elsevier, 37, 1988, p. 167. 32. D.R. Corbin, B.D. Burgess Ir. , A.J. Vega and R.D. Farlee; Anal. Chem., 59, (19871, 2722. 33. W.A. Wachter; in "Proceedings Sixth International zeolite Conference", Ed. D. Olson and A. Bisio, Butterworth, 1983, p. 141. 34. AS" Standards on Catalysts, (1988), D3906-85a. 35. P. Scherrer; Gottinger Nachrichten 2, (1918), 98. I
Chapter 7
TECHNIQUES OF ZEOLITE CHARACTERIZATION
J.H.C.
van Booff')
and
J.W. Roelofsen')
Laboratory for Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands. AKZO Chemicals B.V., Research Centre Amsterdam, P.O.Box 15, 1000 AA Amsterdam, The Netherlands.
corns 1.
Introduction
2.
Zeolite Structure and Structural Defects
3.
Pore Structure of Zeolites
4.
Chemical Composition of Zeolites
5.
Determination of the Framework Si/Al Ratio
6.
Determination of Zeolite Acidity
7.
Zeolite Stability
8.
Zeolite Morphology and Particle Size
242
1.
INTRODUCTION Before a zeolite can be used for a certain application it is necessary
to characterize this zeolite, to see if it has the desired properties for that application. If not, another synthesis method should be used or the zeolite must be modified, to meet the specifications. Zeolite synthesis, -modification, -characterization and -application thus are strongly related as schematically is indicated in Fig. 1.
SYNTHESIS
1
MODIFICATION
t
CHARACT
t
1
t
APPLICATION Fig. 1.
Position of Zeolite Characterization.
Of course not all zeolite properties are of the same importance for every application. This aspect is illustrated in Fig. 2 in which is indicated, which characteristics are of special importance for the main applications of three different zeolites. For example for the application of zeolite A in detergents particle size and morphology are extremely important while acidity and stability play a minor role. These characteristics, however, are just of crucial importance for the application of zeolite Y in cracking catalysts. For this reason we will discuss the different characterization methods in relation to the application for which the specific property is important.
243
IMPORTANT CHARACTERISTIC
T Y P E ZEOLITE A
3 F
Z E O L I T E
ZEOLITE Y
CHEMICAL COMPOSITION
0
0
FRAMEWORK
SI/AL
RATIO
- 5
0
STRUCTURE STRUCTURAL D E F E C T S PORE-S IZ E
ZSM
0 0
0
____
0
0
ACIDITY S T A B IL I T Y MORPHOLOGY PARTICLE S I Z E
I MPORTAIiT
APPLICATION
Fig. 2
0
0 ADSORBENT I O N EXCHANGER
I N DETERGENTS
CRACKING CATALYST
Most important c/mrflcteristics for thrcc i(d/-knozvn zeo/ites a'ppl icat io 11s.
SHAPE SELECTIVE CATALYST
flrld t/lclr lfmfljor
244 2.
ZEOLITE STRUCTURE AND STRUCTURAL DEFECTS For the application of zeolite ZSM-5 as a shape-selective catalyst it is
very important that the zeolite has the correct structure and has no structural defects. The best way to check zeolite structure is by X-Ray Diffraction. This method is based on the fact that every crystalline material has its own characteristic X-ray diffraction pattern.
For most of the known zeolite structures the ideal diffraction patterns have been simulated and together with other crystal data these diffraction patterns are collected in the following publication of the Structure Commission of the International Zeolite Association. Collection of Simulated XRD Powder Patterns for Zeolites by Roland van Ballmoos Butterworths 1984 To illustrate the kind of information that can be found in this book, we here give the diffraction pattern (Fig. 3 ) together with the other crystal data that are given for zeolite ZSM-5 officially named MFI (Mobil-Five).
I
0
10
20
50
2 THETA
Fig. 3
Simrlated X-ray Diffraction Pattern of MFI.
40
50
60
245
Crystal data: Space group Orthorhombic Pnma a = 20.096 A b = 19.949 = 13.428
a a
a = 90.00"
13 = 90.00' Y
=
90.00"
Additional structural information can be found in a second publication of the Structure Commission of the International Zeolite Association. Atlas of Zeolite Structure Types by W.M. Meier and D . H . Olsen Butterworths 1987 The book gives stereographic views for most of the known zeolite structures together with drawings of the cross-sections of the main channels.
framework viewed along (0101
In-ring viewed along (OlO] (straight channel)
10-ring viewed along (loo] (sinusoidal channel)
Fig. 4
Stereographic view and channel cross-sections of zeolite MFI
246
Furthermore several structural data are presented; for example for ZSM-5 or MFI is given: Framework density : 17.9 T atoms/1000 A 3 Channels
direction number of T-units
free aperture A
[OlOI
10 MR
5.3 x 5.6 (b)
[loo1
10 MR
5.1 x 5.5 (a)
Fault planes: (100) Normally the first check is the comparison of the observed X-ray diffraction pattern with the simulated pattern. If all observed peaks can be found back in the simu ated pattern an, have similar intensities the crystal structure of the zeolite under investigation is the same as the structure that is used for the pattern simulation. If extra peaks are observed or peaks are missing this is an indication that other crystalline phases are present, or that the structure is different
.
The intensity of the diffraction peaks can be used to determine the crystallinity of the sample. For that purpose the intensity of one particular peak (or a number of peaks) is compared with the intensity of the same peak (or peaks) of a standard sample. The X-ray crystallinity then can be calculated from: ( 3 4 ) X-ray crystallinity =
Intensity of peak hkl of sample -~ Intensity of peak hkl of standard x 100%
~-
Furthermore the width of the diffraction peaks gives information about the average crystallite size of the investigated zeolite sample. According to Scherrer ( 3 5 ) the relation between linewidth ( B ) and crystallite size (t) is given by the following formula.
t(in nm) =
0 , 9 X (in nm)
cos
X-ray diffraction thus can give information about crystal structure, degree of crystallinity and crystallite size. It is, however, impossible to obtain information about the presence of structural defects by this method.
247
The presence of fault planes and/or stacking faults i n the structure can be observed by high resolution electron microscopy (HREM) as shown i n Fig. 5.
J
Fig. 5
r/
Electron niicrograph of zeolite Y showing the presence of structiiral defects, indicated by arrows.
Another type of structural defects that can be present are hydroxyl groups.
-
Firstly, there are the terminal s i l a n o l groups that are always present at the outer surface of the zeolite crystals to terminate the structure. For this reason these groups are called: terminal silanol groups.
248
-
The second type of silanol groups is formed when a Si-0-Si bond is broken by a reaction with water.
I -Si-
I
As
-
I I
H2O
S i-
0-
.c--
1
-Si-OH
I broken
HO-Si-
bond
I
I
indicated in the reaction equation this type of silanol groups is
always present as pairs.
-
Finally, silanol groups can be formed if a T-atom is missing in the structure. This type of defect leads to a cluster of four silanol groups or a so-called hydroxyl nest.
I
I
-Si-
-Si-
I
I I
0
-Si-O-~i-O-Si-
I
I
I
I
?
-
I
I
0
I
H HO -Si-
-Si-OH
I
0
I I
hydroxyl
-Si-
I
All these silanol groups can be observed by I R and H NMR spectroscopy. Moreover, it is possible to distinguish between the different types upon reacting these silanol groups with trimethyl-chlorosilane and subjecting the reaction products to 29Si CP-MAS NMR (31).
As indicated in Fig. 6 trimethylchlorosilane will first react via the C1 ligand t o form the primary product. I f , however, neighbouring OH groups are present also the secondary, tertiary and quaternary products can be formed.
249
By
29
Si NMR it is possible to distinguish between these different
reaction products and the presence of the signals attributed to the secondary and tertiary products then indicates the presence of pairs of silanolgroups and hydroxyl nests.
a
-50
-100
6
Fig. 6
PPm
9Si CP-MAS-NMR spectruni of MFI after reaction with trinictliylchlorosilane at 673K showing the presence of primary, secondary and tertiary products.
3.
PORE STRUCTURE OF ZEOLITES For the use as molecular sieve or as shape selective catalyst it is
essential to characterize the zeolite's system. Information about pore size can be obtained from: 3.1
-
the crystal structure
3 . 2 - adsorption measurements
3.3
3.4
-
129xe NMR spectroscopy model compound reactions
3.1 Crvstal structure From a single crystal structure X-ray analysis of the zeolite it is possible to derive the positions of the component atomstogether with the apertures of the channels and cavities that are present in the zeolite structure, and whether the pore system is one-, two- or three-dimensional. The pore size determined for a given zeolite form the X-ray analysis pertains to perfect crystals. Often zeolite samples do not fulfil this condition due to the presence of imperfections or minor amounts of other phases inside the pore system. Therefore it is recommendable t o characterize the pore volume and pore size in a more direct way by one of the other methods.
3 . 2 Adsorotion measurement6
Adsorption measurements with molecules of different size give direct information about the dimensions of the pore system. Molecules with kinetic diameters smaller than the pore openings can be adsorbed and larger molecules can not. In boundary cases a minimum temperature of adsorption may be required.
D.W. Breck (1) presents the dimensions of a number of probe molecules, from which the following examples are compiled in Table 1.
251
TABLE I. Moleciilar diniensions of a series of probe niolecitles. Pauling
Lennard-Jones
length
width
Pm
pm
Pm
300
260
310
kinetic diameter
240
28 9
320
275
384
340
390
280
346
410
300
364
396
360
436
396
420
3 70
376
5 10
3 70
330
390
315
265
410
380
260
5 28
400
360
420
380
500
440
3 90
650
490
430
490
430
560
500
700
6 20
660
585
6 70
600 780 810 1020
The Lennard-Jones kinetir diameters are perhaps most appropriate to predict accessibility of a zeolite for a given adsorbate. For comparison; the apertures of 6, 8, 10 and 12 membered oxygen rings are about 270 pm, 430 pm, 60 pm and 770 pm, respectively.
Generally, the adsorption isotherm (the amount of gas adsorbed as function of the partial pressure of the gas at constant temperature) gives information about the pore structure. This is a method often applied for characterizing materials having meso (2-50
m ) and macropores ( > 50 nm).
252
Problems arise, however, in the characterization of micropores ( < 2 m) the dimensions of which are close to those of the adsorbing molecules. In this case the adsorption process is a filling of the micropores instead of multilayer adsorption on which the interpretation of adsorption isotherms is generally based. This, together with the requirement to measure the adsorption isotherms at very low pressures, makes it very difficult to obtain detailed information on the pore structure of zeolites by this method. At the moment two comparable methods are reported in literature by which it is possible to determine the micropore volume of zeolites from the N2 adsorption isotherm. These are:
-
t-plot method ( 2 ) in which the volume of the adsorbed N, is plotted against the statistical thickness(t) of the adsorbed layer. This results in a straight line and extrapolation to t=O gives the micropore volume.
-
as method ( 3 ) in which the volume of the adsorbed N, is plotted against
the reduced standard adsorption (as). Extrapolation to as = 0 gives again the micropore volume.
='"Xe
NMR
suet-
Fraissard and co-workers (4) recently developed "'Xe
NMR spectroscopy
as a method to obtain information on the pore system of zeolites. Due to the relatively large diameter of the xenon atom ( 4 3 6 pm) information is only obtained on the larger pores. For example in the faujasite type zeolites the supercages are accessible for xenon whereas the sodalite cages are not. The method is based on the measurement of the "'Xe
chemical shift of
the in the NMR spectrum. According to Fraissard the chemical shift of xenon adsorbed in a porous material can be expressed as:
This relation shows that the observed chemical shift is the chemical shifts originating from: -
the reference (6,)
-
the electric field (6,) the magnetic field (6,)
sum
of the
253
-
the interaction of the xenon atoms with the zeolite (6(xe-z).pz)
(pz is
related to the zeolite structure) -
the mutual interactions between the xenon atoms (6(Xe-Xe).he)
(heis
the concentration of the xenon atom, in the pores) (see Fig. 7 ) .
As the exchange of Xe between different positions is fast on the NMR timescale, the NMR signal reflects the average of the different Xe positions and interactions.
t
Fig. 7
Schematic representation of the different contributions to the shift of xenon atonzs adsorbed in the pore systeni of a zeolite.
9Xe cheniical
Fig. 8 gives for three zeolites the lZ9Xe chemical shift as a function of the xenon concentration in the zeolite.
254
/
/
0
120
6 PPm 100
extrapolated values
/
80
Fig. 8
The
Nay :
60 ppm
KL :
9 0 ppm
ZSM-5: 113 ppm
9Xe clteniical shift as function of pxefor tlzree types of zeolite.
The following information can be obtained:
-
extrapolation of the chemical shift to zero concentration gives
-
from the slope of the curves information can be obtained about the void
-
The intensity of the signal gives information about the number of
information about pore size and pore restrictions while space available to the absorbed Xe atoms. well-formed cages. lZ9Xe NMR has been applied
-
so
far in various investigations such as:
characterization of zeolite pore systems; also f o r zeolites with at that time unknown crystal structure (zeolite P ( 5 ) )
-
study of coke formation during catalytic cracking ( 6 )
-
study of the formation of Ni particles upon reduction of NiZf-exchanged Y-zeolites ( 7 ) .
255 When, however, the electric and/or the magnetic field also contribute to the chemical shift the conclusions are less accurate.
3.4 Model mmpound reactions The most direct method to obtain information about the dimensions of the internal space in zeolites available for catalytic reactions is of course the performance of model compound reactions. The advantage of this method is that information is obtained under conditions that are representative for the catalytic application. Generally, three reaction selectivity possibilities can be distinguished:
-
a reactant molecule is too large to enter the pores, causing reactant
-
a product molecule is too large to leave the pores, causing product
selectivity selectivity
-
the transition state for a certain reaction is too large to be accommodated in the pores causing transition state selectivity. In literature several reactions have been discussed by which information
on the pore system of zeolites can be obtained. Three of these reactions
will be discussed here; the first is related to reactant selectivity, while the two other examples are based on transiton state selectivity.
3.4.1 The Constraint Index (CI) Mobil Oil workers (8) introduced a test reaction in which a 1:1 mixture of n-hexane and 3-methylpentane (see Fig. 9 ) is cracked in a gas-flow setup.
n- hexane
Fig. 9
Mokciiles
6
3-methylpentane
irsed for the determination of the constraint index (CI)
256
Depending on the pore size, the relative rate of cracking of the linear versus the branched hydrocarbon molecule will change and they define: rate constant of n-hexane cracking Constraint Index (CI) = rate constant of 3methylpentane cracking ~
and supposing first order kinetics for the cracking reactions this relation is equivalent with CI =
log (fraction n-hexane remaining) log (fraction 3-methylpentane remaining)
The Constraint Index provides the following classification of the pore-system CI < 1
: large pores
1 < CI < 12 : intermediate pores
CI > 12 : small pores The determation of the CI is suitable for the characterization of medium pore zeolites. The method is, however, not sensitive for large pore zeolites. Moreover, the method proved to be temperature dependent.
-
3.4.2 The Refined Constraint Index (RCI) Jacobs et al. ( 9 ) introduced the bifunctional conversion (1% Pt on the zeolite in the proton form) n-decane as a test reaction. Under the conditions that 5% of the n-decane is isomerized the ratio of 2-methylnonane and 5methylnonane which are formed in this reaction (see Fig. 10) is determined.
n-decane
2
1
4
3
2-methylnonane
Fig. 10
8
6
5
7
10
9
li,
5-methylnonane
Reactions used for the deterniination of the refined constraint index.
257
Refined Constraint Index (RCI) =
[Z-methylnoflane] [ 5-methylnonane1
It is pointed out that more space is needed for the formation of 5-methylnonane than for the formation of Z-methylnonane. So the RCI will increase with decreasing pore width. The method proved to be especially suited for the characterization of zeolites with 10 membered ring pore systems, it is, however, not sensitive for zeolites having larger pores.
3 . 4 . 3 The Spaciousness Index (SI)
To characterize wide pore zeolites Weitkamp et al. (10) have introduced the hydrocracking of n-butylcylcohexane as a test reaction. Two of the reaction products are n-butane and isobutane and according to the authors the transition state for the formation of isobutane is larger than that for the formation of n-butane (see Fig. 11).
butylcyclohexane
n-butane
Fig. 11
i-butane
Reaction used for the determination of the spaciousness index.
The ratio of the amounts of formed n-butane and i-butane thus can be used as an indication for the available space in the zeolite pore system leading to the following definition: Spaciousness Index (SI) =
[formed i-butane] [formed n-butane] ~~
It will be clear that SI decreases with decreasing pore width. The method proved to be especially suited for the characterization of zeolites with 12 membered ring pore systems and so the SI is complementary to the RCI and CI discussed above.
258
4. CHEMICAL COMPOSITION OF ZEOLITES There are several possibilities to express the chemical composition of zeolites. For example for the sodium form of zeolite Y the following formulae can be used
2
Na,O,
2
A1,0,;
(192-x)Si02, zHzO
in which the number of A1 per unit cell can vary between 48 < x < 64. Thus for dried pure zeolites the chemical composition can be characterized by a single parameter x, the number of A1 atoms per unit cell. However, instead of the number of A1 per unit cell (NAl) often the Si/AI (silicon/aluminum) atom ratio or the SiO,/Al,O,
(silica/alumina) molecular
ratio is used to characterize the chemical composition of zeolites. Table 2 shows the interrelation of these parameters.
TABLE 2
Interrelation of the different parameters used to characterize the chemical composition of N a y zeolites.
X
192-x X
48
3
2
6
4
192-x
x
64
11.8
15.4
19.5
25.3
68.7
59.4
259
The methods for the determination of the chemical composition can be divided into two groups: 4.1
Methods based on complete dissolution of the zeolite and subsequent analysis of the solution obtained (destructive wet analysis).
4.2
Analysis
of
the
solid material
by
physical
methods
(non-destructive dry analysis)
4.1 Complete Dissolution Method An example of a frequently used method is given below: Weigh accurately a sample (1 g) of dried zeolite
(g gram).
Add sulphuric acid ( 4 0 ml) and heat until SO, fumes. Dilute with water (about 200 ml) and filter
(filtrate A).
Dry and ignite the residue at 1000°C Add hydrofluoric acid (10 ml) evaporate carefully
(a gram)
and finally ignite at 1000°C
(b gram). X Sio, =
a - b ~
g
x 100%
Add potassium peroxodisulphate (2g) to the residue and heat until a clear melt is obtained. Dissolve the melt in water (100 ml) and combine the solution with filtrate A . Determine Na and A1 i n this solution by AAS. As can be seen this method is rather laborious and therefore several more rapid physical analysis method have been developed. For the accurate analysis of exchange of cations or substitution of T-atoms a proper chemical analysis remains a prerequisite.
4.2 Physical Analvsis Methods for Zeolites
Several methods are available, all with their advantages and disadvantages ( 3 2 ) .
260
4.2.1 X-ray Fluorescence Spectroscopy In this method a suitable sample is radiated by high energy X-rays by which electrons can be expelled from the different atoms, leaving holes in low-lying orbitals. The main mechanism for relaxation then is, that an upper electron falls into this hole. The energy released may result either in the generation of radiation, which is called X-ray fluorescence, or in the ejection of another electron, the secondary electron of the so-called Auger effect (see Fig. 12).
primary e l e c t r o n (XPS)
e
primary electron
-
a
e
-
secondary e l e c t r o n (AES)
4
hVout 0
-
I I
I
I
I
I
I
I . I
I
I hvin
T
I
/
I
/
/
/
13 X-ray F l u o r e s c e n c e
Fig. 12
Auger E f f e c t
Schematic representation of the tzoo possible relaxation processes after expellation of an electron from a low lying orbital.
Both X-ray fluorescence radiation and the Auger electrons are characteristic for the emitting atoms and can be used for quantitative elemental analysis. Appropriate sample preparation and calibration is, however, essential when using this method. A suitable method for sample preparation consists of making a solid solution of the zeolite in a LiBO, melt followed by rapid cooling to a glassy material. For calibration standard zeolite samples with known composition should be used.
261 4.2.2
Proton/Particle Induced X-ray hission (PIXE) A modification of the former method can be used if high energy particles
are at one's disposal. Holes in the low-lying orbitals then can be created by a bombardment of the sample with these particles followed by a registration of the emitted X-rays. Again adequate sample preparation and calibration is essential for an accurate analysis. 4.2.3
X-ray Photoelectron Spectroscopy (XPS) Instead of measuring the emitted X-rays or secondary electrons it is
also possible to measure the energy spectrum of the primary electrons (see Fig. 1 2 ) . This is done in X-ray Photoelectron Spectroscopy and the resulting spectrum can be used for qualitative analysis and after calibration a160 for quantitative analysis of the samples. Because the escape depth of
the electrons is limited to a few
atom-layers only the surface composition of the samples is obtained. In combination with sputtering of the sample by bombarding the sample with noble gas ions it is possible the obtain a depth profile of the chemical composition of the sample. 4.2.4
Electron Probe Micro Analysis ( E m ) Modern scanning electron microscopes can have the facility to focus6 the
electron beam on a small spot on the surface of the sample, which can result in the expellation of electrons from the atoms, followed by X-ray fluorescence or the emission of Auger electrons. In this case the emitted X-rays are originating only from the small spot radiated by the electron beam and thus give information on the chemical composition of that spot (diameter about 8 nm).
By scanning the electron
beam over the sample it is possible to obtain information about the distribution of the elements in this way (11). All methods discussed above can be used to determine the chemical composition. For example when the aluminum content is determined the result is the tptal amount of aluminum present in the sample (or part of the sample). It is, however, impossible to distinguish between framework aluminum and non-framework aluminum in this way. As only the framework aluminum can give rise to the formation of Br$nnsted acidic sites whereas non-framework aluminum can provide Lewis acidity it is important to have methods available for the determination of both types and amounts of the aluminum. Some methods developed for this purpose will be discussed in the next paragraph.
262
5. DETERMINATION OF THE FRAMEWORK Si/Al RATIO. As mentioned in the introduction the main application of zeolite Y is in catalytic cracking, and for this application the framework Si/Al ratio proved to be extremely important. So most information on the determination of the framework Si/Al ratio is based on this type of zeolite, and in this
paragraph the different methods will be discussed on the basis of the results with zeolite Y.
5.1 XRD-Lattice constant As zeolite Y possesses cubic symmetry (a=b=c) the unit cell dimensions
can be characterized by the single lattice constant a. Because the A1-0 bond-length is larger than the Si-0 bond-length this lattice constant a will increase with increasing (framework) A1 content of the zeolite. Based on the results of chemical and X-ray diffraction analysis of a set of hydrated NaY zeolites made by direct synthesis, having a number of framework A1 atoms per unit cell (NA1) in the range of 48 to 77 Breck and Flanigen (12)
reported the following relation between N A ~and a (see Fig. 13).
i2 .50.-
= 1152.(
NAl
a
-
2.4191
-_-_-__ -_ _ __ _ - - - - - - - . T
a m
2 .45
-
' 0
0
0
0
0
0
0
0
/
0
0
A'
-----_-
I
I
1
I I
I I
I
I I
I
I I
I
I
I
I
I I
I
)
10
20
30
40
I
50
60 NAl
Fig. 13
I .
.
70
80
Breck-Flanigen relation betztwn the niiniber of franzcu~orkA1 atonis ycr tinit ccll NAI arid the lattice constant a for hydrated N a y zeolites.
Once this relation is known it is possible to derive NA1 of an unknown hydrated NaY zeolite from the experimentally determined lattice constant a.
263 A potential problem of this method is that the Breck-Flanigen relation
is only valid for hydrated samples and that the lattice constant is also influenced by cation exchange and substances formed during thermal treatment of the zeolite. The zeolite framework is not a rigid construction but can accomodate itself to the requirements of cations and other phases present in the framework. This is for example illustrated by Fritz et al. (13) who studied the influence of dehydration and NH,'
exchange of NaY zeolites. Upon
dehydration they observed an increase of the lattice constant from 2.466 run to 2.4765 nm while NH,'
exchange to very low sodium content lead to an
increase to 2.478 nm. Roelofsen et al. ( 1 4 ) found evidence that also the presence of rare earth ions causes deviations from the Breck-Flanigen relation. Another drawback of this method is that it is restricted to zeolites with cubic structure, for other zeolites no such relations have been reported.
5 . 2 IR spectroscopy
In the infrared spectrum of zeolites in the range of 300 - 1300 cm-'
the
lattice vibrations can be observed. These vibrations can be divided in structure sensitive and structure insensitive bands, Flanigen (15) reports the following positions of these bands: Structure insensitive vibrations
- asymmetric stretch
-
synnnetric stretch
- T-0 bending
950 - 1250 cm-' 650
-
720
cm-'
420 - 500
cm-'
Structure sensitive vibrations
- asymmetric stretch - symmetric stretch -
double ring vibrations
- pore opening vibration
1050 - 1150 cm-' 750
-
820
cm-l
500 - 650
cm-'
- 420
cm-'
300
For some of the structure sensitive bands a linear relation between the wave number and the number of lattice aluminum atoms is reported ( 1 6 ) . After calibration it is possible to use this relation to derive the number of lattice aluminum atoms from the band positions. Fig. 14 shows the results obtained by Flanigen for Y-zeolites with different degree of dealumination.
fry
264
A.O% N,,
T a.u.
= 56
II I
I
III I
I11
\ \
I
I
1
J
I
\
906
1200
600
wavenumber
Fig. 24
cm
-1
IR spectra of Y-zeolites uiith dif erent degree of dcalumination (% dealiiminatiiini denoted in Figure{
From t h i s f i g u r e i t i s p o s s i b l e t o d e r i v e t h e f o l l o w i n g r e l a t i o n between t h e p o s i t i o n of t h e s t r u c t u r e s e n s i t i v e asynnnetric s t r e t c h band ( u l ) and t h e number of l a t t i c e aluminum atoms (NA1). N A l = 0.960 (1068 - u ~ )
t h e symmetric s t r e t c h band (u2) c a n b e used.
Also t h e p o s i t i o n of Lunsford e t a l .
( 1 7 ) g i v e t h e f o l l o w i n g r e l a t i o n s t o d e r i v e N A ~from t h e
p o s i t i o n s of t h e two bands. NAl
= 0.766
(1086
-
crl) and
N A l = 1.007
(838
-
u*)
A s n o such r e l a t i o n s are r e p o r t e d f o r o t h e r z e o l i t e t y p e s t h i s method i s r e s t r i c t e d t o Y-zeolites.
265
5.3 *'Si
and "A1
MAS-NMR
Since the introduction of the Magic Angle Spinning technique to NMR spectroscopy it is possible to obtain high resolution spectra from solid samples. Lippmaa et al. (18) were the first who applied in 1981 this method to record the *'Si-spectra
of zeolites. Since then the technique has been
developed rapidly and at the moment is widely applied in zeolite research. The state of the art is very well described in the book of Engelhardt ( 1 9 ) who is also the author of one of the chapters in this book. Here only a short description of the use of this method to determine the number of framework A1 atoms will be given. Application of the "Si
MAS NMR technique to zeolites results in spectra
in which a number of separate Si peaks can be observed. For the faujasite structure in which all T-atoms (Si and Al) are crystallographically equivalent it is possible to distinguish up to five different peaks that can
be attributed to Si connected-through oxygen- to 0,1,2,3 and 4 A1 atoms, respectively (see Fig. 15).
A1 0
AlOSiOAl 0
A1
-30
A1 0
SiOSiOAl 0
A1
-90
Si
0
SiOSiOAl 0
A1
-100
Si 0
SiOSiOAl
Si n SiOSiOSi
0
0
Si
S i
-110
3 (29~i) ppm Fig. 75
9Si MAS-NMR spectrum of N a y zeolite with Si/Al= 2.5
266
As the intensity of a peak is proportional to the number of the Si atoms concerned, it is possible to calculate the total number of Si atoms by sommation of the peak intensities: 4
N~~
=
c.E I(Si-nAl) n=o
If the Lijwenstein rule is obeyed, every framework A1 atom is surrounded by 4 Si atoms. So the total number of framework A1 atoms can be calculated from:
N~~ =
c.1
n=o
-
I(Si-nAl)
The framework Si/Al ratio can then simply be calculated from:
Table 3 shows the results of this method when applied to determine the framework Si/A1 ratio of a NH,NaY zeolite before and after steaming, clearly showing the dealumination of the framework upon steaming.
r
Peak Intensities Sample
4A1 3A1
dried NH4NaY
1
11.5
steamed NH,NaY
0
4.1
2A1
1Al OAl
48.1 35.6
Si/A1 ratio
4.9
4.8
5.7 3 0 . 2 60.0
14.5
Problems can be the presence of paramagnetic cations (such as some rare earth ions), that can cause appreciable broadening of the NMR peaks, as has been reported by Roelofsen (20) and Schemer (21).
267 In principle it is possible to determine A 1 directly by 27Al MAS
NMR. In
practice, however, several problems can arise that hamper the interpretation of the results. The main reason for these problems is the fact that the A1 nucleus possesses a quadrupole moment that may give rise to strong line broadening especially in the case that the A1 is present i n a low symmetry environment. Because of the latter reason several authors report no problems with the determination of the tetrahedrally coordinated framework A 1 but the non-framework A1 is not completely "visible". Grobet et al. (22) report the addition of the Al-coordinating compound acetylacetone to the sample as a method to make visible also this part of the Al.
268 6.
DETERMINATION OF ZEOLITE ACIDITY The acidity of zeolites is mainly caused by the presence of BrOnsted
acid sites but especially after high temperature treatments also Lewis acid sites may be present (see Fig. 16).
H S i/O\
-
A1
/O\
Si
A1
4.
H20
/"\A1
Si
Br$nsted-acid sites
Fig. 16
r-7
Si
Lewis-acid site
BrOnsted and Lewis acid sites in zeolites.
For a complete characterization of zeolite acidity it is, therefore, necessary to determine number and strength of both types of acid sites. Several methods have been developed for this purpose from which the most important are:
-
titration methods
-
adsorption and desorption of bases
-
IR spectroscopy
- of OH groups
-
NMR spectroscopy
- of adsorbed bases - of OH groups -
of adsorbed bases
This paragraph will give a brief description of these methods
6.1 Titration methods The acid-strength of a BrOnsted acid BH is derived from its proton donating ability and can be expressed by the equilibrium constant of the reaction
269
Ka
BH aH+
Ka =
B-
. aB-
+
H+
fB-'cB=
aH+
~
BH *CBH
a~~
In which fi is the activity coefficient of species i and Ci its concentration
.
Then bv definition: pKa = -1ogKa = -log [aH+
-]
. fB-' cg~BH-CBH
Because for solid acids, as zeolites are, the activity coefficients are very difficult to quantify, Hammett introduced the acidity function Ho. pKa = -log [aH+
.
"3
~ B H
- log CB- = Ho CBH
-
log CBCBH
It is evident from this definition that as the limit of a dilute aqueous solution is approached the values of fg- and fgH approach 1 and Ho becomes equal to the pH. The Hammett acidity function thus behaves similar to the pH in that it becomes more negative the greater the acidity of the solution. The value reaches -10 in the most concentrated solutions of H,SO, To determine H,
or HC10,.
for solid acids Hammett used a series of aniline bases
as indicators which required acids of different strength to give color change (see Table 4).
TABLE 4
Series of Indicators to determine the Haniniett acidity function.
Hammett Indicator Neutral red Phenylazonapthylamine Butter yellow Benzeneazophenylamine Dicinnamal acetone
HO
+ 6.8 + 4.0 + 3.3
+ -
1.5
3.0
Benzal acetophenone
- 5.6
Anthraquinone
-
8.2
270
To determine the acid strength (distribution) of a zeolite the solid is
dispersed in a nonpolar solvent (e.g. n-hexane) and a series of indicators requiring successively stronger acids to become protonated is added until no color change is observed. When a color change is observed for a given indicator it is concluded that some of the sites on the zeolite correspond to the Ho characterized by that indicator. Then the procedure is repeated after addition of a known amount of a base (e.g. n-butylamine) that neutralizes the strongest acid sites. In this way an acid-strength distribution curve as illustrated in Fig. 17 can be obtained ( 2 3 ) .
I
RENaY
1.0.1
after calcination
0.8..
0.6"
$trong 0.4
..
0.2..
6
Fig. 17
4
2
0
-2
-4
-6
-8
Acid-strength distribution curves of RENaY after various pretreatnlents.
Problems with this method are: the impossibility to distinguish between BrQnsted and Lewis acidity and the fact that the indicator molecules cannot enter the zeolite pores, and thus can react only with the acid sites at the outer surface of the zeolite.
6.2 Adsorption of bases : Cal.Qrimetric measurements When an acid is neutralized by a reaction with a base the heat of neutralization is evolved. This heat of neutralization is larger the stronger the acid and so it can be used to characterize acid-strength. Aurow et al. (24) have demonstrated this method for several zeolites. Fig. 18 shows the results for H-ZSM-5 and Na-ZSM-5.
271
I
Qads
140
120
100
kJ/mol 30 60
40 2
4
Fig. 18
8
6
ads ads or bed)
cm 3 / g
10
Heat evolutiori during the adsorption of N H , on H-ZSM-5 and Na-ZSM-5 at 150°C.
A s can be seen in this figure more heat is evolved when NH, is adsorbed on H-ZSM-5
than on Na-ZSM-5 demonstrating the presence of stronger
acid-sites. A complication of this method is the accessibility of the acid sites and the long times that are needed to reach equilibrium. The fact that the heat evolved after the addition of the first dose of NH, to H-ZSM-5
is
smaller that the heat evolved after the second and third dose in the example mentioned above is explained by the authors on that basis. The first dose of NH, is supposed not to react with the strongest acid sites but mainly with the more easily accessible weaker sites, and the more difficult accessible stronger acid sites are only neutralized after the addition of the second and third dose. Furthermore, the method is rather time-consuming and
so
most
investigators prefer the more rapid method in which the opposite reaction is used.
272 6.3 Desorption o f bases. Temp As
DesorDtion
( TPD
1
discussed in the preceeding paragraph bases adsorb stronger on
stronger than on weaker acidic sites. Thus it will cost more energy or higher temperatures to desorb bases from stronger acidic than from weaker acidic sites. This principle can be used to characterize the acid sites present on a zeolite sample. First the zeolite is contacted with a base (NH, or pyridine) to neutralize the acidic sites present. Then the temperature is raised at a constant rate and the amount of desorbed base is recorded (either by mass spectrometry or by a heat-conductivity cell).
In this way a desorption
spectrum is obtained as presented in Fig. 19, for zeolite H-ZSM-5.
\
4
t irne
H-ZSM- 5 S i / A l = 30 = 10 K/min
Silicalite
LT
400
600
800
1000 &
T (10
Fig. 19
,
Temperature prograninied desorption spectrum of NH from H-ZSM-5 (Si/Al= 30)
In this spectrum two peaks can be observed. A low temperature peak (LT)
corresponding with NA, desorbing from the weaker acidic sites. This peak can also be observed when desorbing NH, from non-acidic silicalite. And a high temperature peak (HT) corresponding with the stronger acidic sites. The area of
this peak gives information about the amount of these strong acidic
sites, while the peak-maximum-temperature (T,)
gives information about its
acid-strength. Cvetanovic and Amenomiya (25) have developed a theory enabling the derivation of the heat of desorption from the peak-maximumtemperature (T,)
at a certain heating rate (B).
213 Although temperature programmed desorption of ammonia (NH,-TPD)
is a
simple and rapid method to characterize zeolite acidity, the method has its limitations. Firstly is it not possible to distinguish between ammonia desorbing from BrQnsted or Lewis acidic sites, and even desorption from non-acidic sites (like in silicalite) is recorded. For Y-zeolites a second more serious problem is, however, the possibility of diffusion limitation during the desorption of ammonia from adsorption sites in the pores and cavities of the zeolite, causing desorption at higher temperatures than corresponding with the acid-strength of the desorption-sites.
6 . 4 IR-spectroscouv of
OH g r o w
A more direct study of the Br4nsted-acidic
OH groups in zeolites is
possible by IR-spectroscopy. This method, that is extensively reviewed by Ward (261, is based on the dependence of the 0-H stretching frequency on the acidity. The weaker the 0-H bond, the lower the stretching frequency and the higher the acid-strength. Fig. 20 shows a typical IR-adsorption spectrum of a thin wafer of HNaY after dehydration.
3800
3700
3600
3640
Fig. 20
IR-Absorption spectrum of HNaY
3500
'
cm
-1
214
The spectrum shows three peaks in the 0-H stretching region. The peak at 3740 cm-' is assigned to the non-acidic silanol groups that are present at the outer surface of the zeolite crystals and at structural defects. The peaks at 3640 cm-l and 3540 cm-l correspond with OH groups having lower 0-H bond strength and
so
more acidic character. From these bands the
one at 3640 cm-I dissappears after adsorption of pyridine and so this band
is assigned to the accessible acidic OH groups. Other zeolites show similar spectra with small shifts in the positions of the peaks. Barthomeuf (27) has measured the exact position of the IR peaks corresponding with the acidic OH-groups for a series of zeolites with varying Si/Al ratio (see Fig. 21).
ZSM-5 T
Mordeni te
h 3620
3640
/
A
1
2
3
4
5
6
7
' Si/Al
Fig. 21
30
Position of the IR-band corresponding zciith the acidic OH-groups for a series of zeolites uuth varying Si/AI ratio.
From this figure it can be seen that the peak position shifts from about 3660 cm-l for zeolite X with a Si/Al ratio of 1.25 to about 3610 cm-l for Mordenite with a Si/Al ratio of 4.5. Further increase of the Si/Al ratio has only a minor effect. For ZSM-5 with a Si/A1 ratio of 30 the peak is observed at 3605 cm-'.
It must be concluded that the acid-strength of zeolites
increases with increasing Si/Al ratio till a maximum value is reached at a Si/Al ratio of about 6 to 7. To explain this result it is accepted that the
275 acid-strength of a bridging OH group in a zeolite depends on the number of neighbouring A1 atoms (see Fig. 2 2 ) .
Fig. 22
Surrounding of a bridging OH group in a zeolite.
According to the Lowenstein rule A 1 is surrounded by 4 Si atoms, but the Si atom can be surrounded by 0 to 4 A 1 atom. The bridging OH thus can have 0 to 3 A1 atoms as next-nearest neighbours. The probability of this will depend on the Si/A1 ratio of the zeolite. Maximum acidity is obtained with no A 1 atoms in the second coordination shell. This will be reached when the Si/Al ratio of the zeolite exceeds 9 or 10 ( 3 3 ) . Further increase of
Si/Al then has no more effect on the
acid-strength. Thus determination of the position of the IR absorption band belonging to the acidic OH groups can give information on the acid-strength of these groups. Because it is difficult to determine the mass of the sample from which the I R adsorption spectrum is recorded the method only gives qualitative information on the number of acid sites.
6.5 IR suectroscouv o f adsorbed bases Instead of looking to the acidic OH groups it is also possible to study the IR absorption of bases that have reacted on the acid sites of the zeolite. Frequently the reaction with pyridine is used for this purpose. As indicated in Fig. 23 pyridine can react with the BrQnsted-acid sites as well as with the Lewis acid sites of the zeolites. The reaction on the BrQnsted acid site results in the formation of a pyridinium ion while the pyridine molecule is coordinatively bound on the Lewis acid sites. Both the pyridinium ion and the coordinatively bound pyridine have characteristic IR absorption bands.
276
I _
L
B
L-acid
B-acid
L
L
1700
Fig. 23
1600
1500
1400
-1
u crn
IR spectrurri of pyridine adsorbed on partly dehydroxylated HY
In this way it is possible to indicate the presence of both Br+nsted and Lewis acid sites in zeolite samples whilst the intensity of the bands gives information on the number of these sites. It is, however, impossible to obtain information on the acid-strength. The presence of the absorption bands only shows that acid-sites are present with an acid-strength large enough to react with the basic probe molecule pyridine. Jacobs et al. (2 8) have, therefore, modified the method by using the less basic probe molecule benzene. Reaction with Br@nsted acid site then does not result in complete proton transfer to form the benzenium ion but in an interaction of the benzene with the acid proton at which the proton remains bonded to the bridging oxygen atom. As a consequence, the 0-H bond strength is weakened, which can be observed by a shift of the corresponding
IR band to lower wavenumbers. Table 5 shows the observed shifts when adsorbing benzene on different zeolites in the proton form.
277
TABLE 5
Observed shift of the position of the OH band in the ZR spectrum when interacting with benzene for these types of zeolites aOH
'OOH
re la t ive
cm-'
cm-'
i nt e n s i t y
300
50
348
50
H-ZSM-5 S i / A l = 30
3600
H-ZSM-11 S i / A l = 30
3603
220 300 340
50 15
3610
326
100
Dealuminated H-Y Si/A1 = 30
Two t y p e s of a c i d i c groups a r e observed i n H-ZSM-5.
35
One type showing a
s h i f t of 300 cm-'
when i n t e r a c t i n g w i t h benzene and a second t y p e showing a
s h i f t of 348 cm-'.
The l a r g e r s h i f t observed f o r t h e l a t t e r t y p e is caused
by a s t r o n g e r i n t e r a c t i o n w i t h t h e benzene molecule. The i n t e n s i t y r a t i o
(50:50) i n d i c a t e s t h a t e q u a l amounts of b o t h groups are p r e s e n t .
6.6 '€IMAS-NMR of OH-arom S i n c e t h e i n v e n t i o n of t h e magic a n g l e s p i n n i n g t e c h n i q u e i t i s a l s o p o s s i b l e t o o b t a i n h i g h l y r e s o l v e d NMR s p e c t r a from s o l i d samples. P f e i f e r e t a l . ( 2 9 ) have a p p l i e d t h i s t e c h n i q u e t o s t u d y t h e d i f f e r e n t t y p e s of p r o t o n s p r e s e n t i n z e o l i t e s . Fig. 24 shows a n example of a 'H MAS-NMR spectrum as o b t a i n e d by them. In t h e upper spectrum 4 t y p e s of p r o t o n s ( a , b , c , d )
can b e observed from
which 3 t y p e s d i s a p p e a r a f t e r a d d i t i o n of d e u t e r a t e d p y r i d i n e .
Peaks b , c
and d t h u s must belong t o a c i d i c p r o t o n s . From t h e s e , peak d can b e a s s i g n e d t o NH,' NH,-Y.
t h a t remained i n t h e z e o l i t e d u r i n g t h e c a l c i n a t i o n of t h e p a r e n t P e a k s b and c , however, r e s u l t form a c i d i c OH groups and c o r r e s p o n d
w i t h t h e 3640 cm-'
and 3540 cm-'
bands, r e s p e c t i v e l y ,
i n t h e I R spectrum
( s e e F i g . 20). Also i n t h i s c a s e t h e p o s i t i o n of t h e peak i n r e l a t e d t o t h e
0-H bond-strength
(The chemical s h i f t
bH i n c r e a s e s
with
decreasing
b o n d - s t r e n g t h ) and t h u s g i v e s i n f o r m a t i o n on t h e a c i d - s t r e n g t h , peak area can be used t o q u a n t i f y t h e number of a c i d groups.
while t h e
278
~~
~
15
10
5
0
-5
6, PPm
Fig.24
' H - M A S - N M R spectriini of H-Y zeolite before (---I orid after (4 the addition of deirterated pyridine.
Experimentally, the technique is rather demanding especially because the method is extremely sensitive for the presence of H,O. Summarizing, it can be said that several techniques can be used to characterize the acidity of zeolites all with their pro's and contra's (see Table 6).
TABLE 6
Available techniques for the cliaracterization of zeolite acidity. Potentials and Problenzs.
PlETliOD
T I TRAT ION
ADSORPI I OF TI'D
IR
OH
1R BFISES
NRR
lH
AMOUNT
STRENGTH
PROBLEflS ACCESSIBILITY DIFFUSION DI F FUSION NON-ACIDIC A D S O R P T I O N SAMPLE P R E P A R A T I O N SAMPLE P R E P A R A T I O N WATER A D S O R P T I O N
I
279
7.
ZEOLITE STABILITY An important requirement posed on zeolites is their stability under
operation conditions. This means amongst others thermal stability towards amorphization and dealumination. Various methods are available to check this stability, the most versatile of which is X-ray diffraction. By analysing the zeolite before and after a certain thermal treatment or certain reaction conditions with X-ray diffraction it can be determined whether or not changes in degree of crystallintiy, crystal structure or number of framework aluminum atoms have occurred. Of special interest is the High Temperature X-ray diffraction technqiue in which the diffraction pattern is continuously recorded while heating up the sample. An example of a pattern obtained in this way is presented in Fig. 25. This figure clearly shows that upon heating at about 200°C the diffraction lines shift to the left indication an increase of the lattice constant from 2.477 to 2.484 run, which is caused by dehydration of the zeolite. At about 600°C intensity changes of lines take place and new lines appear indicating a change in crystal structure. Finally collapse of the crystal structure is observed at about llOO°C when the diffraction lines disappear.
Fig. 25
Variable temperatiire X-ray diffraction pattern of rare-earth exchanged zeolite Y (14% RE,O,).
281
A second method to determine thermal stability is by differential thermal analysis (DTA) (see Fig. 2 6 ) .
1
RENaY
-.
B = 40 OC/min 1 0 2 2 OC
I
AT
\
I
I
500
1000
T
Fig. 26
OC
-
DTA pattern of RENaY (14% RE,O,)
The DTA pattern shows a exothermic reaction at about 1020°C caused by the collapse of the crystal structure.
282
8.
ZEOLITE MORPHOLOGY AND PARTICLE SIZE The size and morphology of the zeolite particles will influence the
properties. A typical example is the size and shape that are required for the application of NaA zeolite as a builder in detergent formulations. As reported (30) the particle size should be in the range of 0.1 to 10 p n and the edges of the cubic zeolite crystals should be rounded off. Also for other applications where the kinetics of adsorption or ion exchange play a role size and shape of the zeolite particles will be important. Scanning electron microscopy (SEM) is the most versatile technique to study the morphology and particle size distribution of zeolites (see Fig. 2 7 ) .
Fig. 27
Scanning Electron Micrograph of a titanium silicalite-1 sample
Other methods to determine particle size are based on sieving and sedimentation. By the dry sieving method particles larger than 5 p can b e determined, while the coulter counter sedimentation method is suitable for particles down to 0.5 p n .
283
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