Characterization of aluminium siting in MOR and BEA zeolites by 27Al, 29Si NMR and FTIR spectroscopy

Studies in Surface Science and Catalysis, volume 158 J. (~ejka,N. Zilkovfiand P. Nachtigall (Editors) 9 2005 Elsevier B.V. All rights reserved.

765

Characterization of aluminium siting in MOR and BEA zeolites by 27A1, 29Si N M R and FTIR spectroscopy T.I. Korfinyi j, K. F6ttinger 2, H. Vinek 2 and J. B.Nagy 3 ~Department of Molecular Spectroscopy, Institute of Structural Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, P.O. Box 17, H- 1525 Budapest, Hungary. 2Institute of Material Chemistry, Vienna University of Technology, Veterin~irplatz 1, A- 1210 Wien, Austria. 3Laboratoire de R.M.N, Facult6s Universitaires Notre Dame de la Paix, Rue de Bruxelles 6 l, B-5000 Namur, Belgium. Commercial and dealuminated mordenites (MOR) as well as home made and cobalt modified beta (BEA) zeolites have been characterized by 29Si and 27A1 solid state Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy. The quantitative contributions of Si(nA1) and Si(OH)x sites to the NMR signal intensities were calculated from the various Si/A1 ratios and relative 29Si and 27A1 NMR signal intensities assuming twin, lone and no A1 containing periodical building units of the zeolite framework. More lone compared to twin and silicalite units were suggested for BEA than for MOR zeolites. Some octahedral A1 transformed back to tetrahedral coordination due to ion-exchange of HBEA to Co/BEA zeolites. We conclude that we are able to distinguish the Si(OH)• groups which are original defect sites or produced in a dealumination or calcination process. 1. INTRODUCTION The silicon-aluminium ordering in zeolites and the state of aluminium incorporated either in the framework or out-of-lattice (extraframework) positions can be obtained by 29Si and 27A1 NMR spectroscopy. The Si/AI ratio of zeolites and the number of crystallographically distinct sites for the five different Si(nA1) (n=0-4) configurations can be determined by 29Si NMR. The ratio of tetrahedral silicon and aluminium in the zeolite framework can be directly calculated from the line intensities in a 29Si MAS NMR spectrum by the following equation assuming that the A1-A1 avoidance rule of Loewenstein is obeyed and Si(OH)• signals are not included in the bands [1 ]:

(Si/A1)NMR = ~ Isi(nAl)/~ n/4 lsi(nAl) summation is from n : 0 to n = 4

(1)

The resulting Si/A1 ratio may strongly underestimate the actual Si/A1 ratio as defect sites (Si(OH)x groups) are generally present in the zeolite framework. IH-29Si cross-polarization (CP) makes possible the detection of silicon atoms to which one or more hydroxyl groups are attached. In the CP spectra the line intensities of silicon

766

atoms bearing OH groups are selectively and strongly enhanced. 27A1 NMR characterizes aluminium species of different (tetra- or octahedral) coordination. The line (at ca 55 ppm) of tetrahedral aluminium in the zeolitic lattice is well separated from the line (at 0 ppm) of outof-lattice octahedral aluminium, following appropriate calibration their quantities can be determined quantitatively [ 1]. The silicon-aluminium ordering in mordenites and in their dealuminated forms has been determined by Debras et al. [2]. Jia et al. [3] claimed that the appearance of octahedral A1 in the NMR spectra of beta zeolites does not necessarily involve framework aluminium extraction. Kiricsi et al. [4] assigned octahedral A1 NMR resonance to transient-state aluminium species which is leaving the framework. Bokhoven et al. [5] identified threecoordinate aluminium in MOR and BEA zeolites with in situ X-ray absorption near-edge spectroscopy (XANES). Bodart el al. [6] computed the number of SiOH groups following dealumination of mordenites by combination of their bulk Si/AI ratios with 29Si and 27A1 NMR results. They demonstrated that aluminium atoms preferentially occupy tetrahedral positions in the fourmembered rings of the mordenite structure. They suggested that during the dealumination process AI atoms are removing two by two from the four-membered rings, parallely four silanol groups per extracted A1 atom are generated and this number gradually decreases to two through a structural reorganization. The aim of this work is to develop the method of Bodart et al. [6] further in order to understand the dealumination process of MOR and BEA zeolites in a deeper level. We try to calculate the number of Si(OH)2 species assuming different dealumination mechanisms in order to reveal the origin of Si(OH)• groups. 2. EXPERIMENTAL Various commercial mordenites (Norton MORNor and Degussa MORDeg) and home-made beta zeolites with different global or bulk Si/AI ratios were studied by 29$1- and 27A1 MAS-NMR spectroscopy. Part of the MORDeg sample was dealuminated by steaming for 20 min at 873 K (MORDea0. The home-made HBEA was modified by cobalt chloride (2.6 wt% metal loading) either by solid-state ion exchange (Co/BEAssm) or by incipient wetness impregnation (Co/BEAIMp) [7, 8]. The amount of acid sites was determined by thermogravimetric (TG) analysis of NH3 desorption (NETZSCH STA 409 Luxx). The bulk Si/A1 ratios were calculated from the results of TG and PIGE [9] analysis. The NMR spectra were recorded either on a Bruker MSL 400, or Avarice 500 spectrometer. For 29Si (79.4 MHz), a 6 ~ts (| = rt/6) pulse was used with a repetition time of 6.0 s. For 27A1 (130.3 MHz), a 1 gs ( | rt/12) pulse was used with a repetition time of 0.1 s. The infrared spectra were recorded on a Bruker IFS 28 FT-IR spectrometer equipped with an MCT detector. 9

3. RESULTS The >Si NMR spectrum of mordenites with a typical Si/A1 ratio of 5 displays three broad signals for Si(2AI), Si(1A1) and Si(0A1) at about-100, -106 and-113 ppm, respectively, the relative intensities of which are 1:2:2. Highly siliceous zeolites exhibit narrower lines with distinctly different intensity distributions of the three Si(nAl) lines. Splitting of the Si(0A1) signal reflects Si atoms in crystallographycally non-equivalent T-sites [1]. The 298i NMR spectrum of MORNor sample (Fig. l a) shows broad signals for Si(2Al), Si(1A1) and Si(0A1) at about the above mentioned chemical shifts, but at different relative

767 intensities (Table 1). The lines assigned to Si(2A1) and Si(1AI) configurations (at-99 and -105 ppm in Fig. l a) involve relatively few defect sites (Si(OH)• groups) in the spectrum, because the shape of MORNor CP spectrum (not shown) is similar to the shape of its normal spectrum (Fig. l a). However the lower Si/A1NMRratio of MORNor (7.0, Table 1) than its global or bulk Si/AI ratio (8.4) indicates the presence of some defect silanol groups in this mordenite. The 29Si NMR spectrum of HBEA zeolite (Fig. 1d) shows three resonances at -104,- 111 and -115 ppm, which can be ascribed to Si(1AI), Si(0AI)A and Si(0A1)B sites, respectively (Table 1). Splitting of the signal assigned to Si(0A1) configuration is due to two groups of different crystallographic sites [10]. The CP spectrum of HBEA (not shown) confirms the presence of SiOH groups in the line at-104 ppm assigned to Si(1A1) configuration, as its intensity compared to the Si(0A1) line is higher in the CP than in the normal spectrum (Fig l d). The difference in the framework (24.7) and NMR (16.5) Si/A1 ratio of HBEA shows also the presence of defect sites (Table 1). The effect of dealumination is clearly seen in the 29Si NMR spectra of Degussa mordenite samples (Fig. 1). The broad lines of MORDeg (Fig. l b) are resolved to Si(2A1), Si(1A1), and Si(0A1) configurations (Table 1), but following dealumination (MORDeal) the relative intensity

a

d

,7

,

;

HBE

-90

-100

-110

b

-120

(ppm)

-100

-110

e

'

o

-90

-120

(ppm)

-120

(ppm)

-120

(ppm)

'

C

,

|

-90

-100

-110

-12o

-90

(ppm)

-100

-110 r !

f

MOR~D~~ ~ -90

-100

-110

Co

-120

(ppm)

-90

-100

-110

Fig. 1. 298i NMR spectra ofMORNor (a), MORI)eg (b), MORDeal (c), HBEA (d), Co/BEAssm (e) and Co/BE&Me (f) zeolites

768 Table l Bulk (mean of TG and PIGE), lattice or framework (Si/Alfram.=Si/Albuik / A1T(fraction)) and NMR (Si/AINMR calculated by Eq. (1)) Si/AI ratios, relative tetrahedral A1T and Si(nAl) (n -2, 1, 0) coordinations (%) of MOR and BEA zeolites calculated from 27A1 and 29Si NMR spectra. Sum of 2 Si(OH)2 + SiOH concentrations were calculated by Eq. (3). Zeolite

MORNor MORDeg MORD~al HBEA Co/BEAs Co/BEAI

Si/A1 27A1 Si/A1 29SiNMR 2Si(OH)2 bulk A1T fram. Si/A1NMR Si(2A1) Si(1A1) Si(0A1)A Si(0A1)B + SiOH 8.4 100 8.4 7.0 8.0 41.3 16.3 34.4 9.4 15.9 100 15.9 8.5 7.6 31.9 27.7 32.8 21.9 18.0 82.4 21.8 20.6 2.8 13.8 60.3 23.1 1.1 18.2 73.8 24.7 16.5 0 24.2 65.2 10.6 8.0 19.5 8 7 . 1 22.4 13.8 1.1 26.9 62.0 10.0 11.1 1 9 . 5 98.7 19.8 18.9 0.6 20.0 65.7 13.7 0.9

of signals assigned to the Si(2A1) and Si(1Al) configurations at-104 and-106 ppm decreases compared to the Si(0A1) line (Fig. lc). The signals assigned to Si(2A1) and Si(1A1) configurations also contain defect hydroxyl groups at a high extent in the MORDeg sample, as their relative intensities are much higher in the CP (not shown) than in the normal (Figs lb) spectra (Table 1). The big difference in the NMR (8.5 and 20.6) and framework (15.9 and 21.8) Si/A1 ratios (Table 1) clearly indicates that Equation (1) is not applicable for these Degussa mordenites due to the presence of defect silanol sites. The presence of silanol (SiOH) and strong Broensted acidic (SiOHA1) sites are clearly seen in the infrared spectra of mordenites (Fig. 2). The bands around 3740 and 3600 cm -I wavenumbers can be ascribed to the former and latter sites, respectively. The SiOHA1 band areas of MORNor and MORDeg samples are 3-4 times higher than that of MORDea~ zeolite indicating loss of acidity due to dealumination in accordance with the 29Si NMR spectra.

0,1 0

;

3727

113 ,r,,-

0

~"

,~

~a

,

615-35

,-. . . . 9 i i i .i

t

t

I

#

!

~. ~.

ORDeg

"-

"'", "",

t

"'-..

"', ",...

\%

]

'..

i

'

l

:"'-, 3600

""-..

MORNor "" ...................

i

"~. . . . . '7-", ..... "'" ....

.,-'"3,743,-" 36 O0 ,...,~

..-""

j

i -"r

"

MORDeai

0

4000

3800

3600

3400

Wavenumber

3200

(cm "l)

Fig. 2. OH region (4000-3000 cm-~) in the FTIR spectra of mordenites

3000

769 The 298i NMR spectra of Co/BEA zeolites (Fig. l) shows four resonances, which can be ascribed to Si(2A1), Si(1A1), Si(0A1)A and Si(0A1)B sites, respectively (Table 1). The Si(2AI) and Si(1A1) lines of Co/BEA zeolites include some Si(OH)x defect sites confirmed by the enhancement of the corresponding lines in the CP spectra (not shown). 27A1 NMR spectra give information on the A1 distribution in structurally distinct sites of the lattice. The 27A1NMR spectra of the mordenites and beta zeolites are shown in Fig. 3. The sharp signals at 55 ppm are attributed to aluminium in the zeolitic framework at tetrahedral coordination. The NMR spectra - except MORNor and MORDeg samples - exhibit an additional line at 0 ppm, which is assigned to extraframework aluminium in octahedral out-of-lattice positions. The relative concentration of A1 in tetrahedral positions (Table 1) were calculated from the relative integrated line areas of the 27A1NMR spectra (Fig. 3). Due to dealumination a rather high amount of octahedral aluminium was produced in MORDeal and the beta zeolite samples also contain extraframework aluminium.

HBEA

MORNor J

MORDeg _j

Co/BEAssIE

Co/BEAIMP 100

50

0

(ppm)

100

50

0 (ppm)

Fig. 3.27A1NMR spectra of mordenites (left) and beta zeolites (right).

4. DISCUSSION The periodic building unit (PBU) of MOR is a 12-membered unit composed of two finite zigzag chains and a 4-membered ring, the one of BEA is a tetragonal beta layer composed of 16-membered units connected through 4-membered rings [ 11 ]. Bodart et al. [6] suggested that two aluminium atoms are preferentially sitting in the 4-membered rings of the mordenite structure in diagonal positions, the configurations with only one A1 atom are excluded. The two silicon atoms in the 4-membered rings have Si(2AI) configurations, the four Si atoms connected to the two A1 atoms from outside the 4-membered ring have Si(1A1) configurations. Due to energetics reasons A1 should occupy the same positions in the 4membered rings of the beta zeolites. The relative abundance of these sites is 1/8 in the BEAand 1/3 in the MOR structure, consequently all aluminium atoms should sit in the 4membered rings of the MOR and BEA zeolites in two by two "twin" positions. We develop the method of Bodart et al. [6] further taking some new assumptions. As the intensity ratios of Si(1AI) to Si(2A1) lines is much higher than 2 in the 298i NMR spectra (Table 1), beside the above mentioned "twin" positions PBUs with one "lone" AI atom sitting in the 4-membered ring surrounded with 4 Si(1A1) positions are also allowed. Two kinds of

770 "twin" PBUs are distinguished in the BEA zeolites: 2 ("twin2") or 1 ("twinl") A1 atoms may sit in the 16-membered PBUs. In addition to "twin" and "lone" at highly siliceous zeolites "silicalite" PBUs are also assumed with Si(OH)2, SiOH and Si(0A1) positions only. The "silicalite" PBUs may decompose to "original" Si(OH)x groups during preparation (e.g. calcinations) of the zeolites, but irrespectively of the dealumination process. We take into account the possible production of Si(OH)• groups from Si(XA1) configurations during dealumination and preparation (silicalite): "twin":A1T + Si(2A1) + 2Si(1A1) --~ AIo + Si(OH)2 + 2SiOH "lone"" A1T + 4Si(1A1) ~ Alo + 4SiOH "silicalite" (Si(0Al) only): SiOSiOSi--~ Si(OH)2 + 2SiOH and/or SiOSi ~ 2SiOH

(2)

Assuming that "Line(2)" around -100 ppm includes Si(2A1) and Si(OH)2 lines, "Line(l)" at -105 ppm involves Si(1A1) and SiOH lines, and i f ~ Isi(nA0 is normalized to 1, then Eq. (1) is modified to the following Eq. (3): Line(2) = Si(2A1) + Si(OH)2 Line(1 ) = Si(1A1) + SiOH (Si/A1)f~amework= 4 / [2(Line(2)-Si(OH)2) +Line(1)-SiOH] 2Si(OH)2 + SiOH = 2Line(2) + L i n e ( l ) - 4 / (Si/A1)framework

(3)

The 2Si(OH)2 + SiOH concentrations calculated by Eq. (3) are shown in Table 1. The sum of silanol concentrations are all around 10%, except MORDeg, MORDeal and Co/BEAIMp samples. An assumption should be taken to the ratios of SiOH to Si(OH)2 concentrations. It is well seen from Eq. (2), that this ratio should be higher than 2 if not only "twin" units are present. We assume that during dealumination the defect Si(OH)x groups are originated with equal probability from the Si(2AI) and Si(1AI) configurations, which means that their ratios should not change remarkably during the process. The following ratios are supposed: Si(OH)2 / Si(2A1)

=

SiOH / Si(1A1)

=

Alo/AIT

(4)

A similar assumption is not necessary for the "original" defect silanol groups, because the concentration of "silicalite" PBUs and "original" Si(OH)• groups are calculated as the difference of all Si(0A1) and all Si(OH)• groups and those which are included in the "twin" and "lone" PBUs or produced during their dealumination. The computed results with the above assumptions are shown in Table 2. As there are 2-2 A1T and Si(2Al) atoms as well as 4-4 Si(1A1) and Si(0A1) species in one "twin" PBU of mordenites, the following concentration relations are valid: A1T = Si(2A1) - Si(1A1) / 2 and Alo = Si(OH)2 = SiOH / 2. The "lone" PBUs (1 A1T, 4 Si(1AI) and 7 Si(0A1)) include the excess Si(1AI) with the appropriate other atoms. The "silicalite" units are formed from the remaining Si(0AI) and "original" silanol groups. The ratio of "lone" PBUs always exceeds the concentration of "twin" units, therefore the "twin only" assumption of Bodart et al. [6] must be modified. We are able to distinguish the "original" and "dealuminated" silanol groups and the ratio of SiOH to Si(OH)2 is always much higher than two in accordance with the assumption taken in Eq. (2). "Silicalite" units do not exist in the MORNor sample, because the concentration of Si(0A1) is practically zero in this PBU. Therefore presumably the "original" silanol groups are sitting preferentially in the "lone" units, far from the A1 atoms in the MORNor zeolite.

771 Table 2 MOR and BEA zeolite compositions (%) assuming perfect, theoretical twin (BEA: twin2 or twin 1), lone and silicalite Periodical Building Units (PBU) Zeolite

PBU

twin lone silicalit twin MORDeg lone silicalit twin MORDeal lone silicalit twin2/1 HBEA lone silicalit twin2/1 Co/BEAss~E lone silicalit twin2/1 Co/BEAIMv lone silicalit MORNor

PBU A1 deal 35.6 0 64.0 0 0.3 22.8 0 25.3 0 51.9 15.0 0.9 22.0 1.3 60.1 0 0 61.4 20.7 17.8 4.9/9.8 0.7/1.4 58.1 8.2 28.0/22.4 4.2/8.3 0.1/0.1 68.8 0.8 26.2/22.0

s i site contribution Alv Alo Si(2A1) Si(OH)2 Si(1A1) SiOH Si(0A1) 5.9 0 5.9 0 11.9 0 11.9 4.7 0 18.9 0 33.2 1.2 1' 6.0 T 0.3 3.8 0 3.8 0 7.6 0 7.6 2.1 0 8.4 0 14.8 3.3 14.0 34.6 2.5 0.1 2.5 0.1 5.0 0.3 5.3 1.8 0.1 7.3 0.4 13.6 0 60.1 0 0 0 0 0 0 0 3.8 1.3 15.4 5.2 56.5 0 2.4 15.4 0.6 0.1 0.6 0.1 1.2 0.2 2.8/8.4 3.6 0.5 14.5 2.1 45.6 0.3 7.6 20.1/14.5 0.5 0 0.5 0 1.0 0 2.1/6.3 4.3 0.1 17.2 0.2 47.9 0.6 25.6/21.4

The ratio of "twin" to "lone" units is similar in the MORDeg (0.9) and MORDeal (0.7) zeolites. During dealumination of the Degussa mordenite samples both A1 containing PBUs decompose in a similar rate without preference. All (3.3 + 14.0 %) "original" silanol decomposes and parallely new "dealuminated" silanol groups form (0.1 + 0.7 %). As was already discussed in the Introduction, framework-related threefold-coordinated A1-OH intermediate species may exist in dealuminated zeolites [5], which means a one-to-one SiOH : AI(OSi)3 ratio. Indeed, the shoulder at ca. 30-40 ppm in the 27A1NMR spectrum of beta zeolites (Fig. 4) suggests the presence of deformed tetrahedral atoms, i.e; (SiO)3A1OH, in the structure. In the hydrated state water molecules are coordinatively bound to these AI(OSi)3 species leading to an equilibrium between tetrahedral and octahedral configuration, which is reflected in the 27A1NMR spectra. The extraordinary sharpness of the octahedral line in the spectrum of HBEA (Fig. 4) indicates a very high symmetry of this configuration. Exchange of the acidic protons of HBEA by cobalt ions results in an ionized tetrahedral configuration, which explains the conversion of octahedrally into tetrahedrally coordinated aluminium observed in the 27A1 NMR spectra (Fig. 4 and Table 2) in accordance with the literature [12]. Based on the data shown in Table 2 it is nut possible to suggest if the "twin2" or "twinl" units are more preferable in the structure of beta zeolites. Nevertheless a definite difference exists between MOR and BEA zeolites that only slight excess of "lone" over "twin" units is observed in the former, while a marked excess of "lone" over "twin" PBUs is present in the latter structures (Table 2).

772 5. C O N C L U S I O N S A detailed theoretical structure analysis of MOR and BEA lattices enabled us to a deeper understanding of the origin of defect sites, the mechanism of dealumination and ion-exchange through the 27A1 and 29Si NMR analysis of A1 and Si site contributions. We conclude that we are able to distinguish the Si(OH)x groups which are original defect sites or produced in a dealumination or calcination process. The dealumination of mordenite resulted in the decomposition of original defect sites and the production of new silanol groups. The ionexchange of protons to cobalt ions effected the conversion of octahedrally into tetrahedrally coordinated aluminium in BEA zeolites. All of the studied zeolites contain lone A1 sites in remarkable amount contrary to the exclusive two by two A1 sitting in 4-membered rings theoretical structure. The main difference between mordenites and beta zeolites is the presence of the highly symmetric hydrated and hydroxyl group containing octahedral aluminium in the lattice of HBEA which transforms to A1T due to ion-exchange or impregnation and the relatively high amount of defect sites in mordenites compared to beta zeolites.

ACKNOWLEDGEMENT T.I.K. is indebted to F.N.R.S. (Belgium) for the financial support.

REFERENCES [1]

[2] [3] [4] [5] [6] [7]

[s] [9] [lO] [ll] [12]

G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, New York, 1987. G. Debras, J. B.Nagy, Z. Gabelica, P. Bodart and P.A. Jacobs, Chem. Lett., (1983) 199. C. Jia, P. Massiani and D. Barthomeuf, J. Chem. Soc. Faraday Trans., 89 (1993) 3659. I. Kiricsi, C. Flego, G. Pazzuconi, W.O. Parker, R. Millini, C. Perego and G. Bellussi, J. Phys. Chem., 98 (1994) 4627. J.A. van Bokhoven, A.M.J. van der Eerden and D.C. Koningsberger, J. Am. Chem. Soc., 125 (2003) 7435. P. Bodart, J. B.Nagy, G. Debras, Z. Gabelica and P.A. Jacobs, J. Phys. Chem., 90 (1986) 5183. J. Wittayakun, B. Nuntaitawegon, N. Grisdanurak, G. Kinger, H. Vinek, in Proc. Regional Symp. on Chem. Eng. 2003, Metro Manila, Philippines, Dec 1-3, 2003. J. Wittayakun, N. Grisdanurak, G. Kinger and H. Vinek, submitted for publication. G. Debras, E.G. Derouane, J.-P. Gilson, Z. Gabelica and G. Demortier, Zeolites, 3 (1983) 37. C.B. Dam and M.E. Davis, Catal. Today, 19 (1994) 151. W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types, Elsevier, London, 1996. Y. Neinska, V. Mavrodinova, Ch. Minchev and R.M. Mihfilyi, Stud. Surf. Sci. Catal., 125 (1999) 37.