Heterogeneity of hydroxyl groups in zeolites studied by IR spectroscopy

COLLOIDS AND Colloids and Surfaces A: Physicochemicaland Engineering Aspects 105 (1995) 1-18

ELSEVIER

A

SURFACES

Heterogeneity of hydroxyl groups in zeolites studied by IR spectroscopy J. Datka *, M. Boczar, B. Gil Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland Received 19 January 1995; accepted 7 May 1995

Abstract A hypothesis has been proposed that homogeneous hydroxyl groups would be present in zeolites in which all the hydroxyls have the same number of A1 atoms in the close vicinity and also have the same bridge geometry (the same Si-O and A1-O bond distances and the same Si-OH-A1 bridge angle). However, heterogeneous hydroxyl groups would appear in zeolites in which there are hydroxyls with various A1 numbers or with various geometries. Heterogeneity of hydroxyl groups was expected in NaHA and NaHX zeolites (the same bridge geometry and the same number of A1 atoms). However, heterogeneity was expected in NaHY zeolite (the same geometry but various number of A1 atoms), in NaHZSM-5 and NaH-ferrisilicate (the same AI or Fe number but various geometries) and also in NaH-mordenites (various A1 numbers and various geometries). The heterogeneity of hydroxyl groups was studied by analysis of the IR band of hydroxyls hydrogen-bonded to aromatic hydrocarbons, their derivatives and to ethene. Principal component analysis evidenced that the splitting of the IR band of hydrogen-bonded hydroxyl groups is due to the heterogeneity of the hydroxyls and not to Fermi resonance. The results of our IR studies concerning the properties of hydroxyl groups were compared with the results of quantum-chemical calculations (MNDO method) and also the results of 298i MAS NMR studies of properties of framework Si atoms.

Keywords: Hydroxyl groups; IR spectroscopy; Zeolites

1. Introduction Hydroxyl groups in zeolites (Si-OH-A1) are active sites in many carbenium ion reactions. Their acidic strength is an important parameter characterizing the catalytic activity. This paper concerns the problem of heterogeneity of hydroxyl groups in zeolites, i.e. the presence of hydroxyls with various acidic strengths in a given zeolite structure. The acidic strength of bridging S i - O H A1 groups depends mainly on two factors: a 'chemical factor' and a 'geometrical factor'. The chemical

* Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03338-6

factor can be understood as the number of A1 atoms in the close vicinity of the Si-OH-A1 groups i.e. the number n in the formula (A10),(SiO)3_,Si-OH-AI(SiO)3. According to the results of quantum-chemical calculations of Kazanski and co-worker [ 1,2], and also to Datka et al. [ 3 ] the O - H dissociation energy depends strongly on n. The geometrical factor can be understood as the Si-O and A1-O bond distances, and the Si-OH-A1 bridge angle. The quantum-chemical calculations of Beran [4,5] and Sauer and co-workers [6,7] have evidenced a dependence of the O - H dissociation energy on both the Si-O and A1-O distances, and bridge angle. A hypothesis was therefore proposed, that

2

J. Datka et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1-18

homogeneous Si-OH-A1 groups (all with equal acidic strength) would exist in zeolites in which all the hydroxyl groups have the same number of A1 atoms in the close vicinity (the same n number) and also the same Si-O and A 1 0 bond distances and the same Si-OH-A1 bridge angle. However, heterogeneous hydroxyl groups are expected in zeolites in which there are Si-OH-A1 groups having various numbers of A1 atoms in the close vicinity (various n number for various hydroxyl groups) or of various Si-O and A1-O bond distances or various bridge angles. In order to verify this hypothesis and to obtain more data on the heterogeneity of hydroxyl groups in zeolites (NaHA), faujasites (NaHX and NaHY), NaHZSM-5, NaH-ferrisilicate and NaH-mordenites have been studied. In the case of NaHA (Si/A1 ratio =1) and NaHX (Si/A1 ratio ~1) bridging hydroxyl groups were expected to be homogeneous. Hydroxyl groups in NaHY (Si/A1 ratio ~ 2.5) were expected to be heterogeneous for chemical reasons, hydroxyl groups in NaHZSM-5, and in NaH-ferrisilicate for geometric reasons heterogeneous and hydroxyl groups in NaH-mordenites were expected to be heterogeneous for both chemical and geometric reasons. The heterogeneity of hydroxyl groups in zeolites was studied by IR spectroscopy by the analysis of bands of hydroxyl groups which are hydrogenbonded with g-electron-containing molecules and by "permanent dipole-induced dipole interaction" with n-hexane. This method was applied to the studies of acidity of surface hydroxyls by Hair and Hertl [8], Tretjakov and Filimonow [9] and by Rauxhet and Semples [10]. The increase of OH acid strength resulted in an increase of the frequency shift accompanying both kinds of interactions [11-14]. In the case of homogeneous hydroxyl groups, the hydroxyl band interacting with sorbed molecules was relatively narrow, symmetric and not split. However, broad, asymmetric, and split bands were observed in the case of heterogeneous hydroxyl groups. IR results concerning Si-OH-A1 groups were compared with the results of 29Si MAS NMR experiments which gave information on silicon and aluminium ordering in zeolite frameworks and with the results of MNDO quantum-chemical calculations of O-H dissoci-

ation energies in clusters stimulating the surrounding of Si-OH-A1. The principal component analysis (PCA) of IR spectra was performed to exclude the possibility that the splitting of the IR band of OH groups interacting with sorbed molecules was due to Fermi resonance.

2. Experimental The following parent zeolites were used: NaA (Linde), NaX (Si/A1 ratio = 1.06) obtained from the Fritz-Haber-Institute, Berlin; NaHY (Si/A1 ratio =2.56) and NaZSM-5 (Si/A1 ratio =47) synthesized in Instytut Chemii Przemyslowej, Warsaw); NaNH4-ferrisilicate synthesized by Dr. A. Cichocki from the Faculty of Chemistry of Jagiellonian University; a series of Na-faujasites with various Si/A1 ratios (1.19-7.02) obtained from the Department of Chemistry, Cambridge University, UK; Na-mordenite (Si/A1 ratios = 5.6) supplied by Chemie AG Bitterfeld-Wolfen. Parent Na-forms of zeolites were transformed into ammonium forms by treatment with NH4NO 3 and CH3COONH 4 solutions. The zeolite composition is presented in Table 1 and further details on the preparation of zeolites and their treatment is given in previous papers [ 15-19]. For IR studies the NH4-zeolites were pressed into thin wafers (3-10 mg cm -2) and activated in situ in an IR cell under vacuum at 570-770K (depending on zeolite composition) for 1 h. Most of the IR spectra were recorded by using a BRUKER IFS48 PC Fourier transform spectrometer equipped with a MCT detector. Some spectra were recorded by using a dispersion SPECORD spectrometer (Carl Zeiss Jena) working on-line with a KSR 4100 minicomputer. In many cases the second derivative of the IR spectra were calculated. Before second derivative calculation, the spectra were smoothed by the 'spline function' method (which was found to be more effective than smoothing by the SavitzkyGolay polynominals method). The presence of several submaxima in the second derivative diagram, suggested the presence of several submaxima in the spectra. In order to obtain more reliable information, the spectra were recorded at various

J. Datka et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1-18 Table 1 The composition of zeolites studied Zeolite

Na/NH4 exchange degree (%)

Si/A1 ratio

Composition

NaNH4A NaNH4X NaNH4Y NaNH4X NaNH4X NaNH4X NaNH4X NaNH4Y NaNH4Y NaNH4Y NaNH4Y NaNHaY NaNH4Y NaNH4Y NaNH4Y NaNH4ZSM-5 NaNH4-ferrisilicate NaNH4-mordenite NaNH 4 mordenite NaNH 4 mordenite NaNHa-mordenite NaNH4-mordenite

25 28 77 42 43 42 46 100 81 81 68 85 87 57 44 98 80 15 36 47 72 100

1 1.06 2.56 1.19 1.35 1.67 1.87 2.00 2.39 2.56 2.75 4.15 5.03 5.89 7.02 47 Si/Fe = 43 5.66 5.66 5.66 5.66 5.66

Nag(NH4)3 [-(A102)12(SIO2)12] Na69(NH4)24[(AI02)93(Si02)99] Na44(NH4)12[(A102)54(SiO2)laS] Na51(NH4)av[(A102)ss(SiO2h04] Na47(NH4)35[(A102)s2(SiO2h 10] Na42(NH4)30[(AIO2)72(SIO2h20] Na36(NH4)31[(A102)67(SIO2)125] (NH4)64[(A102)64(SiOzh2s] Na11(NH4)46[(A102)sv(SiO:h35] Na10(NH4)44[(A102)54(SiO2h3s] Na16(NH4)3s[(A102)51(SIO2)141] Na6(NH4hl [(AlO2)aT(SiO2h55] Na4(NH4)2s [(A102)32(SIO2)160] Naa2(NH4)16[(A102)2s(SiO2)a64] Na13(NH4)u [(A102)24(SiO2h6s] (NH4)2[(AIO2)2(SIO2)94] Nao.s(NH4h.v [(AlO2)E.2(SiO2)93.s] Na6.1(NH4)1.1[(AlO2)~.2(SiO2)40.s] Naa.5(NH4)2.7 [(AlOz)7.2(SiO2)40.s] Yaa.s(NH4)3.4[(A102)7.2(SiO2)40.s] Na2.0(NH4)5.2[(AlO2)7.2(SiO2)40.s] (NH4)7.2[(A102)7.2(SiO2)40.8]

adsorbate loadings, and only the minima present in all the second derivative diagrams were taken into consideration.

3. Results and discussion 3.1. N a H A zeolite

In NaHA zeolite with a relatively low Na/H exchange degree (below 30%) bridging hydroxyl groups are expected to be homogenous. As the Si/A1 ratio = 1, all the Si atoms are surrounded by four A1 atoms, 29Si MAS NMR spectra show [20,21] only one narrow signal of Si(4AI). All the hydroxyl groups can be represented by the formula (A10)3 Si-OH-A1 (SiO)3 (i.e. n=3). The chemical condition for the homogeneity is therefore fulfilled. Now the geometric condition will be discussed. All T atoms are in crystallographically equivalent positions, but there are three oxygen atom positions (O1-O3) (see Ref. [22]). O1 positions are in 8-rings and 4-rings, O2 positions in 8-rings and

6-rings, and 03 positions are in 6-rings and 4-rings. Recent 1H MAS NMR results of Hunger et al [23] evidenced that in NaHA zeolite of Na/H exchange degree 25% protons were located in 8-rings. Bridging hydroxyl groups in NaHA may be therefore Si-OtH-AI or Si-O2H-A1. Both of them protrude into the large cage and may be accessible to sorbed molecules. According to XRD data [22], eight Na + ions are located in 6-rings, three Na + ions in 8-rings and one Na + ion in 4-ring. The results of sorption studies of Breck et al. [24] evidenced that the Na + from 8-rings were removed first and at relatively low exchange degrees (below 30%) each of 6-rings still contained a Na + ion in the center. It might be therefore supposed that owing to electrostatic repulsion, protons formed by the decomposition of ammonium ions would not react with the oxygens in 6-rings, and the only hydroxyls formed at low exchange degrees (below 3 0 % ) would be Si-OIH-A1. As all T atoms are equivalent, all Si-OIH-A1 groups have the same geometry (T-O distance 0.1659nm and T-O-T bridge angle

4

J. Datka et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1 18

142.1°) so the 'geometric condition' for the heterogeneity is also fulfilled. In order to test the hypothesis that hydroxyl groups in NaHA zeolite are homogeneous, ethene was sorbed [ 18] in NaHA zeolite (Na/H exchange degree 25%). The hydrogen form was obtained by activation of the ammonium form in situ in an IR cell at 650 K. The spectrum of activated zeolite, zeolite with sorbed ethene and the difference spectrum are presented in Fig. 1A. The spectrum of activated NaHA shows the Si O,H-A1 band at 3620cm -1. The hydrogen-bonding with ethene resulted in a shift of the hydroxyl band to 3380cm -~. The band of Si O 1 H ~ I groups

1.0 A

,~ 0.6

0.2

b-o

-0.2 -,, 3700

,

,

,

I

. . . .

I

h

=

~

L

I

i

i

i

3500

i

3300

0.20

O. 15 0.10

,~ 0.05

0.00 ii

, l l

,

,

3500 0.2 "

,

,I

, ,,

,I

. . . .

Ii~al

3400

3300

C

0.0 -0.2 -0.,¢ , , , I , , , , I , , , l l t , B l [ , i , ,

3500

3400

3300

v tcm-I]

Fig. 1. IR spectra of hydroxyl groups hydrogen-bonded to ethene molecules in NaHA zeolite. (A) Spectrum of activated zeolite (curve a), spectrum recorded after ethene sorption (curve b) and the difference spectrum (curve b - c u r v e a). (B) Spectrum of hydoxyl groups hydrogen-bonded to ethene. (C) Second derivative diagram of spectrum presented in (B) (before second the derivative calculation the spectra were smoothed by the 'spline functions' method).

hydrogen-bonded with ethene is shown in Fig. 1B and the second derivative diagram in Fig. 1C (before the second derivative calculation, the spectrum was smoothed by the spline function method). There is only one minimum in the second derivative diagram indicating that the IR band of hydrogen-bonded hydroxyls is not split and that Si OIHA1 groups are homogenous. These results agree well with our expectation based on the composition and structure of A zeolite and on the NMR results.

3.2. NaHFaujasites Faujasite-type zeolites (NaHX and NaHY) are interesting objects for the studies of the problem of heterogeneity of OH groups. All T atoms are crystallographically equivalent, but there are four oxygen positions O1-O4. It is well known that in faujasites only O1H groups projecting into supercages are accessible to sorbed molecules and may be active sites in reactions catalyzed by zeolites. All the O1H groups, which are interesting from a catalytical point of view, therefore have the same geometry. The number of A1 atoms close to the bridge Si-O1H-A1 depends, however, on the type of zeolite. In zeolite X, in which Si/A1 ratio 1, the situation is similar as in A zeolite. All Si atoms are surrounded by four A1 atoms (the NMR spectrum shows only one signal of Si(4A1) (see Ref. [25]) so only one kind of hydroxyl group is expected [(A10)3 Si-OH-A1 (SiO)3]. However, in Y zeolite (Si/A1 ratio ~2.5) the MAS NMR spectrum shows four signals: Si(0A1), Si(1A1), Si(2A1) and Si(3A1) [25]. As Si(0al) cannot create bridging hydroxyl groups three kinds of Si O1H-A1 groups are expected in HY zeolites (n=0, 1, 2 in (A10), (SiO)3_,Si-OH-A1 (SiO)3). According to the XRD data [26] T-O distances are 0.1643 nm and the bridge angle is 138.6°. In order to verify the hypothesis that Si-O~H-AI groups in NaHX zeolite are homogeneous and that in NaHY zeolite they are heterogeneous, the IR spectra of hydroxyl groups hydrogen-bonded to benzene, fluorobenzene, chlorobenzene, toluene, and p-xylene were studied [3,15,16,27].

J. Datka et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1-18

3.3. N a H X zeolite

NaNH4 X zeolite with a Si/A1 ratio = 1.06 and a Na/NH4 exchange degree of 28% was activated at 570K. The Si-O~H-A1 group band at 3660 cm-a is present in the spectrum (Fig. 2). The hydrogen-bonding of these hydroxyl groups with aromatic hydrocarbons and their derivatives results in a shift of the hydroxyl band to lower freqeuncies, the shifted band is symmetric, narrow and not split. The results obtained with chlorobenzene are shown (as an example) in Fig. 2. The spectrum was smoothed by the spline method and the second derivative was calculated. The second

2.0

0.0 . ~ , . . . .

t,,

10

''~ .... 3600

1.50

3200

~',, 2800

B

1.00 0.50 0.00 3500

0.3

3400

3300

c

-0.3

-0.8

L l l i , = , , I

3500

. . . .

I l l l l [

3400

. . . .

3300

Fig. 2. |R spectra of hydroxyl groups in NaHX zeolite hydrogen-bonded to chlorobenzene molecules. (A) Spectrum of activated zeolite (curve a), spectrum recorded after chlorobenzene sorption (curve b) and difference the spectrum (curve b - c u r v e a). (B) Spectrum of hydroxyl groups hydrogenbonded to the chlorobenzene. (C) Second derivative diagram of spectrum presented in (B) (before second derivative calculation the spectra were smoothed by the 'spline functions' method).

5

derivative diagram shows only one minimum thus indicating that Si-OxH-A1 groups in our NaHX zeolite are homogeneous. As in the case of NaHA, this result agrees with out expectations based on the composition and structure of NaHX zeolite and also on the N M R results. All the hydroxyl groups in NaHX zeolite can be represented by the formula (A10)3Si-OH-AI(SiO)3. 3.4. NaHYzeolite

NaNH4Y zeolite with a Si/A1 ratio =2.56 and a N a / N H 4 exchange degree of 77% was activated at 730K. Two bands of Si-O1H-A1 and S~O3H-A1 groups at 3642 and 3545 cm-1 respectively are present in the spectrum (Fig. 3). As mentioned, only Si-O~H-A1 projecting into supercages can interact with sorbed molecules. The spectra of these hydroxyl groups hydrogen-bonded to chlorobenzene are presented in Fig. 3. The IR band of interacting hydroxyl groups is broader than in the case of NaHX zeolite (Fig. 2). The second derivative diagram shows three minima suggesting that the IR band of hydroxyl groups hydrogen-bonded to chlorobenzene comprises three submaxima. The same results were obtained with other adsorbates: benzene, fluorobenzene, toluene and p-xylene. In all cases the IR band of hydrogen-bonded Si-O~H-A1 comprised three submaxima. The band fit was done (according to the procedure of Pitha and Jones [28,29], the results obtained with chlorobenzene are presented in Fig. 3. The presence of three submaxima in the IR band of Si-O~H-A1 groups hydrogen-bonded to aromatic hydrocarbons and their derivatives suggests the heterogeneity and the presence of three kinds of hydroxyl groups in N a H Y zeolite (as expected). It should however, be, mentioned that in some cases the splitting of the IR band of hydrogenbonded hydroxyl groups may be due not only to heterogeneity of O H groups, but also to overlap of combination and overtone bands of lower frequency fundamentals enhanced by Fermi resonance - - this is observed, for example, in dimers of carboxylic acids in which a strong hydrogenbonding occurs. It seems that in our case of relatively week hydrogen bonds, the Fermi reso-

6

J. Datka et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1 18

!

A

•~ 1.5 i

0.5 -0.5

3600

3200

2800

the spectrum of hydroxyl bands interacting with sorbed molecules are due to one species (the case of homogeneous hydroxyl groups and band splitting due to the Fermi resonance) or to a few independent species, each representing one kind of hydroxyl groups of given acid strength. PCA has been used to solve marly problems in chemometrics [31-33] but has not yet been applied to the problem of zeolites.

B

O.80

3.5. Principal component analysis 0.40 O.OC 3450 0.5

3350

3250

C

-0.3

-1.0

, I , , , , I , , , , I , 1 , , I

3450

....

Ill

3350

3250

3350 v [cm'11

3250

D

O.8O ~ 0.40

0.00

3450

Fig. 3. IR spectra of hydroxyl groups in NaHY zeolite hydrogen-bonded to the chlorobenzene molecules. (A) Spectrum of activated zeolite (curve a), spectrum recorded after chlorobenzene sorption (curve b) and difference spectrum (curve b - c u r v e a). (B) Spectrum of hydroxyl groups hydrogenbonded to the chlorobenzene. (C) Second derivative diagram of spectrum presented in (B) (before second derivative calculation the spectra were smoothed by the 'spline functions' method). (D) The result of band fit of the spectrum presented in (B).

nance is less probable, but principal component analysis (PCA) was applied [30] in order to completely exclude the Fermi resonance interpretation. PCA can answer the question if the submaxima in

Three sets of spectra of Si-O1H-A1 groups hydrogen-bonded to the chlorobenzene (Fig. 4) were taken as input data for the PCA: a series of spectra recorded at various chlorobenzene loadings in NaHY (Si/A1 ratio =2.56), a series of spectra recorded after chlorobenzene sorption in NaHY (Si/A1 ratio = 2.56) with preadsorbed pyridine and a spectrum of hydroxyl groups in N a H Y (Si/A1 ratio = 5.89) hydrogen-bonded to chlorobenzene. The output data from the PCA were the series of eigenvalues and eigenspectra (being a linear combination of pure component spectra). They were used to determine the number of independent components in the studied system. If spectra did not contain noise (the ideal case) the component number would be the number of eigenvalues higher than zero as well as the number of eigenspectra which still showed the spectral structure. As our spectra contained some noise, the eigenvalues would never reach zero, they decreased along the series until they reached a certain threshold level corresponding to the noise. Below this level the decrease of the eigenvalues was much slower. The number of components in the system was the number of eigenvalues higher than this 'noise level' (for which the decrease was still fast). In our case the following eigenvalues have been obtained: 10.8657, 0.12756, 0.003443, 0.00041, 0.00021 ... Only three of them are higher than the 'noise level' which can be supposed to be at about 0.001. It suggests the existence of three components in our system. The same conclusion can be drawn from the inspection of the eigenspectra set (Fig. 4). Only three of eigenspectra show a typical spectral structure, the others represent the noise accumulation. This result (the presence of three independent

Z Datka et aL/Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 1-18

7

D

Si/A1=2.56 increasingIoadin!

0.15 0.00"

. . . . . . .

£ o

I

. . . .

3300 E

O.lO

:~~=~0

0.05

3300

3400

0

4

SI/AI=2.56 pyridine desorption

. . . .

~j

~

-0.04 L'-'-t , , , ~ 3300 F 0.01

B

~"

.

I

3400

, , , J-, , ~1 3400

0.1~ >.

eo <__80.1C

k5 - 0 . 0 1 ~ L

,

, ~

t

,

, ~ I

3300 001,-- G

0.0~ 3300 0 . 0 8 i - - CSi/Al=5.89

,

3400

3400 ~5

-0.01

c

0.04

3300 H

3400

,<

0.00 33OO

340O v [cm -1]

-0.01

* *• . . . .

J. . . .

3300

~x~. ~j

3400 v [cm-1]

Fig. 4. PCA of IR spectra. (A) N a H Y zeolite (Si/AI ratio=2.56) spectra recorded after 27, 35, 49, 59 and 74% loadings with chlorobenzene. (B) NaHY zeolite (Si/A1 ratio=2.56) spectra recorded upon pyridine sorption and subsequent desorption at 743, 578, 788 and 798 K, excepting the upper curve - - chlorobenzene sorbed without pyridine preadsorption. (C) NaHY zeolite (Si/AI ratio=5.89), hydroxyl groups hydrogen-bonded to chlorobenzeue. (D-H) The eigenspectra resulting from PCA. Some spectra are

multiplied by the factors given in the figures.

components in the spectra of Si-O,H-A1 groups interacting with chlorobenzene) agrees well with our earlier conclusion obtained from the analysis of the second derivativees of IR spectra (three minima in the second derivative. We assign, therefore, three kinds of hydroxyls in NaHY zeolites to the species of various number of A1 atoms close to the bridge: (SiO)3Si-OIH-AI(SiO)3,

(A10)(SiO)2Si-O1H-Al(SiO)3, and (A10)2(SiO)SiOIH-Al(SiO)3. This assignment agrees with the earlier 29Si MAS NMR results: the presence of Si(3A1), Si(2AI), Si(1A1) signals in the spectrum

[25,34,35]. Our conclusion on the heterogeneity of Si-OH-A1 groups in NaHY zeolites agrees with the results of other authors. D2wigaj et al. 1-36] as

8

J. Datka et al./Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 1-18

well as Dombrowski et al. [37] reported the splitting of IR band of free hydroxyls, Kubelkowa et al. [38] observed two submaxima in the band of hydroxyl groups interacting with CO (at low coverages CO reacted preferentially with more acidic hydroxyls). The results of T P D and microcalorimetric studies also imply the heterogeneity of acidic hydroxyls in NaHY zeolite. Mishin et al. [39] noted the stepwise dependence of NH3 differential heat of adsorption on the coverage and also the presence of four T P D peaks of N H 3 desorption. Chen et al. [40] reported the decrease of differential heat of adsorption of pyridine with coverage. Catalytic results can also be explained by assuming the heterogeneity of acidic sites in NaHY. The elimination of 10% of all Si-O~H-A1 hydroxyl groups by pyridine decreased the rate of but-l-ene isomerization by 70% and elimination of 46% of Si O~H-A1 reduced the rate by 95% [27]. Lombardo et al. [-41] observed also that small doses of ammonia poisoned the catalytic activity of NaHY in neopentane cracking.

3.6. Faujasites of various Si/Al ratios

29Si MAS N M R studies 1-25,34,35] have evidenced that five kinds of Si species exist in the faujasite framework: Si(OA1)-Si(4A1). As Si(0A1) cannot create bridging hydroxyls, four kinds of bridging hydroxyls with a various number of A1 atoms close to the bridge may exist. Our IR studies [3,15,16,27] have shown that it is in fact the case: (A10)3Si-O1H-AI(SiO)3, groups have been found as the only hydroxyl groups in NaHX zeolite (Fig. 2) and (A10)z(SiO)Si-OIH-AI(SiO)3, (A10(SiO)zSi-OIH-AI(SiO)3, and (SiO)3Si-O1HAI(SiO)3 were found in NaHY (Fig. 3). We have undertaken a study of a series of NaHFaujasites with various aluminium contents (Si/A1 ratio = 1.06-7.02). Earlier N M R studies of Klinowski [-25] and Engelhardt [35] evidenced that proportions between Si(nA1) species depended distinctly on the A1 content in zeolite. As there is the only one kind of bridging hydroxyl group corresponding to each of Si(1A1)-Si(4A1), it was interesting to check if the proportion between the amounts of our four kinds of bridging hydroxyls

would depend on the Si/A1 ratio in the same way as the proportion between the amounts of Si(nA1) species. The results of IR studies concerning hydroxyl groups were therefore compared with the results of MAS NMR studies concerning Si atoms. NaHFaujasites with Si/A1 ratio of 1.06, 1.19, 1.35, 1.67, 1.87, 2.00, 2.39, 2.56, 2.75, 4.15, 5.03, 5.89 and 7.02 were studied. Benzene, (or in another series of experiments chlorobenzene) was sorbed in zeolite and the spectra of hydrogen-bonded Si O1HA1 groups were recorded. The second derivative was calculated and the band fit was performed. The results obtained for zeolites with Si/A1 ratios = 1.06, 1.35, 1.87, 2.56, 4.15 and 7.02 are presented in Fig. 5. The 29Si MAS N M R spectra are presented also. Generally, the IR results agree with the N M R results. Both numbers and relative intensities of IR subaxima are comparable as in the case of N M R signals. Only in the case of NaHX zeolites of Si/A1 ratio = 1.35 and 1.87 are the intensities of low frequency submaxima lowered, because of spectra deformation owing to baseline corection. It was assumed that the extinction coefficients of all submaxima were the same and the relative intensities of each of submaxima in the IR spectra were calculated. The average values obtained with benzene and chlorobenzene as a function of Si/A1 ratio are presented in Fig. 6. The relative intensities of 29Si signals in MAS N M R spectra are presented in the same figure. Contributions of either Si(nA1) species or their corresponding hydroxyl groups show the same dependence on the Si/A1 ratio. Si(4A1) is the only signal and corresponding to the less acidic (A10)3Si-OIH-AI(SiO)3 are the only hydroxyls in X zeolite of Si/A1 ratio ~ 1. The contribution of both species decreases monotonically with Si/A1 ratio. The contribution of Si(3AI) as well as that of the corresponding (A10)z(SiO)Si-OIH-AI(SiO)3, increases up to a Si/AI,~ 1.8 and then decreases. The contribution of Si(2A1) and of the corresponding (A10)(SiO)zSi-O1H AI(SiO)3, increases up to a Si/A1 ratio ,~2.5 and then decreases. The contribution of Si(1A1) and from the most acidic (SiO)3Si-O1H-AI(SiO)a increases monotonically with Si/A1 ratio. The results presented in Fig. 6 indicate an agreement of 29Si MAS N M R results concerning

J. Datka et aL/Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 1-18

29Si MAS NMR

9

IR S P E C T R A (THE B A N D FIT)

BENZENE SilAI

~

CHLOROBENZENE

~

1.06

f/~

3350

3550

3300

3500

3350

3550

3300

3500

3350

3550

3300

1.87

2.56 ~-

3500

~

~

~

~

~

~ o

~ o

~o

~ o

~ o

3400

3200

3450

3250

3400

3200

3450

3250

3200

3450

3400

v [cm -1]

3250 "v [cm -11

Fig. 5. 29Si MAS NMR spectra of zeolites NaHX and NaHY with different Si/A1 ratios and the band fit results for the IR spectra of hydroxyl groups hydrogen-bonded to benzene and chlorobenzene molecules. The NMR signals Si(0A1) not forming hydroxyl groups are marked in black. OH1,Si(4A1) hydroxyls correspond to Si(4A1); OH2,Si(3A1) correspond to Si (3AI); OH3,Si(2AI) correspond to Si(2A1); OH4,Si(1A1) correspond to Si(1AI).

the environment of Si atoms and IR results concerning the acid strength of bridging hydroxyls. The contributions of Si(nA1) signals and of their corresponding Si-O1H-A1 groups shows the same dependence on Si/A1. Not only are the trends the same, but also the absolute values of the contribution of Si(nA1) and OH species are close (only in the case of N a H X with Si/A1 ratio = 1.87, some

discrepancies are seen). A generally good agreement indicates that the assignment concerning the nature of bridging hydroxyl groups was correct. It also indicates that some predictions concerning the existence of various kinds of acidic hydroxyls can be done by the analysis of 29Si MAS N M R spectra of zeolites. It may have a great importance for catalysis, because the catalytic

10

J. Datka et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1-18 sl(4at) (,4/013 S/-OH-AI (S/O)a

so

st(3AO (ato)#stol St-OH-At(StO)3

so

Ill ~

4O

0

~.B

2

0

,

,

[_

J

i

4

r

I

I

I

I

I

0

6

0

2

St(2AI) (Ato)(S/O)~ St-OH-A/(S/O) 3

80

6

Si/AI

Si/AI

O

4

stoaO ($1o)3 $1-OH-AI(StO)a

~0 80 Ill

~

4O

8 N I

0

,~[,

]

2

,

4

6

Si/AI

0

0

, ,N, IIJ 2

4

$

Si/AI

Fig. 6. The contributions of Si(nAl) signals in z9si MAS NMR spectra (black bars) and correspondingthem relative populations of different hydroxylgroups in IR spectra (dotted bars) in zeolitesNaHX and NaHY of various Si/AI ratios.

properties of zeolites can be predicted from the N M R spectra.

3.7. I R studies and quantum-chemical calculations o f O - H deprotonization energies

Our IR studies have shown that four kinds of bridging hydroxyls with various numbers of A1 atoms close to the bridge can exist in N a H faujasites: (A10)3Si-O1H-AI(SiO)3, (A10)2(SiO) Si-O1H-AI(SiO)3, (A10)(SiO)zSi-O~H-AI(SiO)3, and (SiO)3Si-O1H-AI(SiO)3. It was interesting [3] to calculate O - H deprotonization energies by a quantum-chemistry method by using four clusters stimulating the environment of zeolitic hydroxyl groups and to compare the theoretical values with experimental data derived from IR data using the Bellamy-Hallam-Williams (BHW) relation.

3.7.1. I R studies o f O - H deprotonization energies

The values of O - H deprotonization energies in NaHX and N a H Y zeolites were calculated by using the BHW relation. This is an empirical equation [42] relating 3 v values of two acids (one of them can be considered as a standard) engaged in hydrogen-bonding with the same electron donors: (3V/Vo)x~H... B = a + b(Av/Vo)x,~n... B

where X ' - O H represents the standard acid. The proton affinity (PA) values were calculated from BHW slopes (b) and PAst values by using following equation [43,44]: PA = PAst-448 log b In order to obtain Av values for O H groups in NaHX and three kinds of hydroxyl groups in NaHY benzene, chlorobenzene, fluorobenzene, tol-

J. Datka et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1-18

uene and p-xylene were sorbed. The band fit of IR bands of hydroxyl groups interacting with sorbed molecules was performed (after smoothing the IR curves and calculating the second derivatives). The zlv values were used to calculate the PA for all four kinds of bridging hydroxyl groups. The following acids were used as the standards: acetic; chloroacetic; trifluoroacetic; benzoic and phenol. In all the cases, linear BHW plots were obtained (correlation coefficients were higher than 0.9). The average values of PA obtained with all the standard acids are presented in Table 2. They will be compared with the values obtained from quantumchemical calculations.

3.7.2. Quantum-chemical MN DO calculations Quantum-chemical calculation were carried out [3] by M N D O method using an AMPAC package adapted for IBM PC/486 computers for semiempirical calculations with the M N D O Hamiltonian in standard parametrization [45]. First, the clusters for modeling the four types of the bridging hydroxyl environments were constructed [46] in the following way: [H20(OH)2 A10],(H3SiO)3_nSi-OH-AI(OSiH3)3, where n = 0 3. The geometry of the cluster and a sketch of 3-dimensional supercavity model are presented in Fig. 7. Bridging A1 and Si atoms are tetrahedrally coordinated via oxygen to other Si or AI atoms in the second coordination sphere (according to the Loevenstein rule) and terminal bonds in the third coordination sphere are saturated using the - S i l l 3 units for the terminal silicon and the -AI(OH)2(H20) units for the terminal aluminum. Bond distances and cluster geometry were optimized, keeping tetrahedral coordination of silicon

11

and aluminum atoms and the bridge angle constant, equal to its experimental value [26]. Theoretical deprotonization energies of these four clusters were calculated as the total SCF energy difference between the appropriate cluster and its deprotonated anionic form (with the same geometry as the parent cluster). As automatic geometry optimization was too crude to discriminate the O - H bond distances (Ron) in various clusters, an additional calculation for a few points on the one-dimensional O H potential energy curve near the minimum was made. The curve was then fitted to a Lennard-Jones-type potential from which the optimum Roll and deprotonization energies were calculated. They are presented in Table 2 and compared with experimental data derived from IR spectra. The experimental and theoretical PA values are practically the same. This agreement is much better in the case of our M N D O calculations than in the case of C N D O calculations [1]. According to C N D O results [ 1 ], the deprotonization energy values (1533-1444 kJ mol -~) were considerably higher than experimental ones and our M N D O results (Table 2).

3.8. NaHZSM-5 zeolites In NaHZSM-5 zeolite (Si/A1 ratio ~50) Si-OH-A1 groups were expected to be heterogeneous due to geometric reasons. All the hydroxyl groups have the same number of A1 atoms close to the bridge (all the hydroxyls can be represented as: (SiO)3Si-OH-AI(SiO)3), however, there may be hydroxyls with various bridge geometries. According to XRD data [47] T - O distances can vary from 0.152 to 0.167 nm and the bridge angle

Table 2 Experimental (IR) and theoretical (MNDO) values of deprotonization energies and optimum hydroxyl bond distances for the clusters studied Cluster

E DEPR(IR) (kJ mol- 1)

E DEPR(MNDO) (kJ mol - 1)

ROOToH (nm)

(A10)3 SiOHAI (SiO)3 (SiO)(AIO)2 SiOHA1 (SiO)3 (SiO)2(A10) SiOHA1 (SiO)3 (SiO)3 SiOHA1 (SiO)3

1419 1386 1315 1300

1393 1363 1282 1243

0.09607 0.09624 0.09626 0.09629

12

J. Datka et al./Colloids SurJacesA: Physicochem. Eng. Aspects 105 (1995) 1-18

s,,,,erco ,i

• ,%.

~Si

'\\

~

'

\

" "

jH AI /

o/\o Si-______Q O ~ .

,

AI "

o~ '

+0.21~ +0.23

Q ®

~

AI / ~ .~

(~ / + 1 . 1 2

+ +1.11

® - OSiH 3

136.8o

Si /

Q~

+1.85 + + 1 . 9 6 ~ " ~ 0

O - OSiH 3 or OAI(OH)2H20

Fig. 7. Supercavity model and the optimal parameters for the bridge of clusters used in the MNDO calculations.

from 143 to 175 ° Quantum-chemical calculations of Beran [-4,5] and of Saner and co-workers [6,7] evidenced that variation of bridge geometry implies a distinct variation of O-H deprotonization energy. We studied [44,48] the heterogeneity of Si-OH-A1 groups in NaHZSM-5 zeolite by the sorption of benzene, chlorobenzene, fluorobenzene, toluene, p-xylene, ethene (forming hydrogen bonds)

and n-hexane (interacting by 'permanent dipoleinduced dipole' interaction). The results obtained with chlorobenzene are presented in Fig. 8. The second derivative diagrams showed five minima suggesting that five kinds of hydroxyls of various acidic strengths exist in NaHZSM-5. The same conclusions were obtained using other adsorbates. The Av values obeyed the (BHW) relation independently of the size of sonde molecule and of the kind

J. Datka et al./Colloids Surfaces A. Physicochem. Eng, Aspects 105 (1995) 1 18

A

0.08 d [2 0

0.04

2~ 0.00

3200

3300 v [cm -11

3400

B

1.0

% ~L~-0.5

-2.0

i

i

r

i

3200

i

i

i

i

3300

i

~

i

L

3400

v [cm-11

C

0.08 {:c

o 0.04

0.00

3200

3300

3400

v [cmql Fig. 8. lR spectra of hydroxyl groups in zeolite NaHZSM-5 hydrogen-bonded to chlorobenzene molecules. (A) Spectra recorded after 37, 62, 77 and 87% loadings with chlorobenzene (curves a--d respectively) (B) Second derivative diagrams of spectra presented in (A) (before second derivative calculation the spectra were smoothed by the 'spline functions' method). (C) The band fit result of the spectrum of the highest loading (87%) with clorobenzene.

of interaction, PA values were calculated from BHW slopes, and following values were obtained: 1333, 1260, 1226, 1210, and l179kJ/mol 1. The presence of five kinds of hydroxyl groups of various acidic strength in NaHZSM-5 suggests that adsorption heat of the bases should decrease with the coverage (as observed in NaHY, see Ref. [39,40]. Indeed, it was observed by Auroux [49,50], but other authors noted a practically constant value of the differential heat of adsorption of NH3 [51] and

13

pyridine [40]. It is not excluded that in the case of narrow pore zeolites (such as ZSM-5), in which diffusion is much slower than in large pore ones (such as faujasites) a practically constant value of heat of adsorption is a consequence of the fact that basic molecules were sorbed 'zone by zone' neutralizing all acid sites in each zone without differentiating between strong and weak sites. Such differentiation is possible only in desorption experiments in which basic molecules are removed from the weak sites first. Recent results of Zholobenko et al. [52] obtained with H-Mordenite support this hypothesis. In adsorption experiments, ammonia molecules did not show any preference towards stronger acid sites, but in desorption they were removed from the less acidic hydroxyl groups first. The same conclusion can be also drawn from our results [53]: the neutralization of all acid sites and subsequent step-by-step desorption restored the less acidic hydroxyl groups first. The results of some authors [41] agree with our hypothesis of the heterogeneity of hydroxyl groups in NaHZSM-5, but the results of Haag [54] suggested homogeneity of acidic hydroxyls. Haag [54] observed a linear increase in catalytic activity with the amount of A1 and a decrease in activity with the amount of H + ions replaced by Cs + ions. These results can be explained well by assuming that all acidic sites are homogeneous (though it cannot be excluded that the variation of A1 or Cs contents does not influence the distribution of acid sites). However, the results of Lombardo et al. [41] suppo'~ the hypothesis of heterogeneity of hydroxyl groups in NaHZSM-5. These authors neutralized all acid sites with ammonia, desorbed ammonia step-by-step at increasing temperatures, and then studied catalytic activity of such treated zeolites. Under these experimental conditions even small amounts of ammonia present in zeolite poisoned the activity, the lethal dose was only about 15% of A1 content.

3.9. Heterogeneity of hydroxyl groups in NaHferrisilicates The heterogeneity of Si-OH-A1 groups in NaHZSM-5 zeolites was explained by the existence of crys~allographically non-equivalent T atoms in

14

Z Datka et aL/Colloids SurJaces A." Physicochem. Eng. Aspects 105 (1995) 1-18

the MFI framework and the presence of hydroxyl groups with various bridge geometries. It was interesting to study structural analogues of zeolites in which A1 atoms are substituted by another trivalent atoms, X. It was expected that S i - O H - X would be heterogenous. The heterogeneity of S i - O H - F e groups in NaH-ferrisilicates was studied by benzene sorption [55]. The results obtained with N a i l ferrisilicate activated at 670, 770 and 870 K and also with NaHZSM-5 zeolite (used as a reference) are presented in Fig. 9. In both NaH-ferrisilicate and NaHZSM-5 zeolite four submaxima are present in the spectra. It should be noted that in the case of NaHZSM-5 zeolite, five submaxima were present if chlorobenzene was used as adsorbate (Fig. 8). Only four of them can be seen if benzene was sorbed because the lowest frequency (the highest Av) submaximum, corresponding to the most acidic hydroxyl, overlaps with the bands of benzene (3000 3100 cm-t). However chlorobenzene could not be used in our study because the submaximum of the highest frequency (of the lowest (Av) would overlap the band of free O H at 3625 cm-~. The frequency shifts (Av) of all four submaxima observed in the case of N a i l ferrisilicate and NaHZSM-5 zeolite are presented in Table 3. The Av values for NaH-ferrisilicate are lower (of 20-40 cm-1) then for corresponding them hydroxyls in NaHZSM-5 indicating that the acid strength of S i - O H - F e is lower than of Si-OH-A1. This agrees with the observation of other authors (for a review see Ref. [56]) that the average acidic strength of hydroxyls in ferrisilicates was lower than in zeolites. This is due to the lower electronegativity of Fe compared with AI. Comparing the spectra in Figs. 9D and 9E, shows that the distribution of S i - O H Fe in N a H ferrisilicate is practically the same as of Si-OH-A1 in NaHZSM-5.

3.10. Dehydroxylation of NaHZSM-5 and of NaHferrisilicate Dehydroxylation is a condensation of two hydroxyl groups formating a water molecule and Lewis acid site. Our IR studies evidenced that in both NaHZSM-5 and in NaH-ferrisilicate,

A 10

c 0.8 o

0.6

i

014

i , i , I,,

3800

, ,LI

B

0.081

i , I~l,,

3400

3000 6ZOK

O~

770K

0°4 0.O0

'~

'

C

,

,

D 0.08

c~

'

. . . .

3400

,

. . . .

3400

I

. . . .

3300

3200

V/i i; 1"41

,

3300

L ,

,

,I

3200 ]

~" H-ferrisilicate a c t i v a t e d 6 7 0 K

0.04 0.00. . . . . . . . 3400 3300 3200 E 0'08 1H Z S M - 5 a c t i v a t e d 5 ~

.~ 0.04 < 0.00

. . . . . . . . .

3350

3250 v [cm-1]

3150

Fig. 9. IR spectra of hydroxyl groups in NaH-ferrisilicate and NaHZSM-5 hydrogen-bonded to benzene molecules. (A) Spectrum of activated ferrisilicalite (curve a), spectrum recorded after benzene sorption (curve b) and difference spectrum (curve b - c u r v e a). (B) Spectra of hydoxyl groups interacting with benzene in ferrisilicalite activated at 670, 770 and 870 K. (C) second derivative diagrams of spectra presented in (B) (before second derivative calculation the spectra were smoothed by the 'spline functions' method). (D) The band fit result of the spectrum of hydroxyl groups in ferrisilicate interacting with benzene. (E) The band fit results for the spectrum of hydroxyl groups in NaHZSM-5 zeolite interacting with benzene.

J. Datka et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1 18 Table 3 Frequency shifts (Av) of submaxima in the IR spectra of the Si OH-Fe groups of H-ferrisilicate and Si-OH-A1 groups in HZSM-5, interacting with benzene Group

H-ferrisilicate HZSM-5

dv (cm- ') OH(l)

OH(2)

OH(3)

OH(4)

219 245

270 309

336 370

405 435

hydroxyl groups were heterogeneous. It was interesting to study which kind of hydroxyls (the most acidic or the less acidic) would be the most prone to dehydroxylation. Benzene was sorbed in zeolite [53] and in ferrisilicate [ 55 ] activated at temperatures at which dehydroxylation had not yet occured (770 K and 670 K respectively) and also at temperatures in which about half of the hydroxyls were lost ( 1270 K and 870 K). Comparing the spectra of hydroxyl groups interacting with benzene (normalized to the same band area) indicated that in the case of NaHZSM-5, the most acidic hydroxyls were the most prone to dehydroxylation, the contribution of these hydroxyls decreased in partially dehydroxylated zeolites. Contrary to NaHZSM-5, in the case of NaH-ferrisilicate there was no order of priority in which these hydroxyls were removed. Generally, the Si-OH-Fe (in ferrisilicate) were more prone to dehydroxylation than Si-OH-A1. 3.11. NaH-mordenites Bridging hydroxyls in NaH-mordenites are expected to be heterogeneous owing both to chemical and geometrical reasons. In mordenites of Si/A1 ratio ~5, 298i MAS NMR spectra showed [35] three signals of Si(OA1), Si(1A1) and a weak Si(2A1). Two kinds of bridging hydroxyls of various number of A1 may therefore exist: (SiO)3Si-OH-AI(SiO)3 and a small amount of (A10)(SiO)2Si-OH-AI(SiO)3. According to XRD results [57] there are four crystallographically non-equivalent T atoms and ten oxygen positions. The bridge angle may vary from 143 to 180 ° . The results of some authors support the hypothesis

15

that bridging hydroxyls in NaH-mordenites are heterogeneous and the results of others do not. The results of the TPD studies of Karge and Dondur [58] suggested the presence of two kinds of Bronsted acid sites in NaH-mordenite. The same conclusion can be also drawn from the IR data of Zholobenko et al. [52] who observed two IR bands of hydroxyls in Nail mordenite and assigned them to hydroxyls in broad and in narrow channels. The microcalorimetric studies of Klyachko et al. [59,60] and also of Stach [61] revealed four kinds of Bronsted sites. It was concluded from the stepwise dependence of adsorption heat of ammonia on coverage. On the other hand, the results of microcalorimetric studies of Chen et al. [40] and Sharma et al. [51] suggest the homogeneity of acid sites in NaH-mordenites. IR studies of heterogeneity of OH groups in NaH-mordenites with various exchange degrees were undertaken recently in our laboratory and the preliminary results will be now presented. The information on the heterogeneity was obtained by a comparison of the studies of the sorption of ammonia (which reaches all the hydroxyls) and pyridine (which reacts only with the hydroxyls in broad channels) and also by considering the dependence of the acidic strength of the hydroxyls on the Na/H exchange degree. Fig. 10 presents the spectra of hydroxyl groups in activated NaH-mordenites with various exchange degrees and in zeolites in which an excess of pyridine was sorbed, and physically adsorbed pyridine was then removed. In NaH-mordenites with exchange degrees lower than about 50% all hydroxyls are situated in large channels and can react with pyridine. In NaH-mordenites of higher exchange degrees, there are also hydroxyls in narrow channels inaccessible to pyridine. Ammonia reacts with all the hydroxyls. The results obtained with pyridine sorption indicate that there are acidic hydroxyls in broad and in narrow channels that agrees with the earlier results of Zholobenko et al. [52]. In the case of H-mordenite with an exchange degree of 100%, the stretching frequencies of hydroxyls in broad and in narrow channels were 3605 and 3590 cm 1 respectively. Further information on the heterogeneity of acidic hydroxyls was obtained by following the

16

.L Datka et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1-18 A

4

B

15%

0.4

38%

36~o

0.4

°

"~

3615

*¢

o 3650

3600

3550

Z~,,, ~,

i~.~ . - , . ~ ,. , . ~ ,,

3650

3600

cm-I

C

3609

3550

cm-I D

47%

3604

72%

0.4

0

3650

3600

3550

cm-1

E

I~,,I

3650

....

Iz,,,I,,,,lll

3600

355~

c rn "1

3603

0.4

i

3650

3600

355~

c m -1

Fig. 10. IR spectra of NaH-mordenites with different exchange degrees. The spectra of activated zeolite (upper curves) and spectra recorded after pyridine sorption and subsequentive desorption at 323 K (bottom curves).

dependence of parameters characterizing acid strength on the Na/H exchange degree (Table 4). Two such parameters were considered: hydroxyl stretching frequency (which decreases with the acid strength), and hydroxyl integrated extinction Table 4 The dependence of parameters characterizing the acidic strength in NaH-mordenites, OH stretching frequency (v), and hydroxyl integrated extinction coefficient (e) as function of Na/H exchange degree Exchange degree (%1

v (cm ~)

e (cm gmol- ~)

15 36 47 72 100

3622 3616 3611 3608 3607

1.77 2.96 3,46 3,67 3,81

coefficients (which increases with the acid strength). The values of the hydroxyl extinction coefficient were determined in experiments of the sorption of measured portions of ammonia in mordenites. The data presented in Table 3 indicate that the average acidic strength of hydroxyl groups (e.g. hydroxyl groups situated both in broad and in narrow channels) increases with the Na/H exchange degree. The increase in acidic strength at lower exchange degrees concerns only the hydroxyls in broad channels. It seems probable that even in broad channels there are more and less acidic hydroxyls and that the less acidic hydroxyls are created at the lowest exchange degrees. The problem of the heterogeneity of Si-OH-A1 groups in NaH-mordenites will be examined in our laboratory by studying the spectra of hydroxyls which are hydrogenbonded to ethene and to CO.

J. Datka et al. /Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 1-18

4. Conclusions (1) In NaHA (Si/A1 ratio =1) and in NaHX zeolite (Si/A1 ratio = 1.06) all the Si atoms are surrounded by four A1 atoms, 29Si MAS NMR showed one Si(4A1) signal. All hydroxyls have four A1 atoms close to the bridge: (A10)3Si-O~H-AI(SiO)3. As all T atoms are crystallographically equivalent, all Si-OH-A1 groups have the same geometry. Our IR studies evidenced that all Si-OxH-A1 groups were homogeneous. (2) In NaHY (Si/A1 ratio =2.56) there are four 298i MAS NMR signals of Si(3A1), Si(2A1), Si(1A1), Si(0A1). As Si(0A1) cannot create bridging hydroxyls, three kinds of acidic hydroxyl group were expected: (A10)z(SiO)Si-O1H-AI(SiO)3, (A10)(SiO)zSi-O1H-AI(SiO)3 and (SiO)3Si-O1H AI(SiO)3. They were actually found in our IR studies. The band of hydroxyl groups hydrogenbonded to g-electron donors was split and PCA evidenced that this splitting was due not to Fermi resonance (like in dimmers of carboxylic acids) but to the heterogeneity of hydroxyl groups. (3) The comparison of 298i MAS NMR results concerning framework Si atoms and IR results concerning the properties of hydroxyl groups in the series of NaH-faujasites of various Si/A1 ratios (1.06-7.02) showed that contributions of Si(nA1) and of corresponding them Si-O1H-A1 depended on the same way on Si/A1 ratio. It means that the information on the acidic hydroxyls, being acidic sites in many important reactions, can be obtained from NMR data concerning framework Si atoms. (4) The deprotonization energies of hydroxyl groups with various numbers of A1 atoms close to the bridge were derived from IR data by using the BHW relation and were also calculated by a quantum-chemical method (MNDO). The experimental (IR) and the theoretical (MNDO) values were very close for all four kinds of hydroxyls present in faujasites. (5) In NaHZSM-5 zeolite (Si/A1 ratio ~50) all the hydroxyls have only one A1 atom close to the bridge: (SiO)3Si-OH-AI(SiO)3. As T atoms are crystallographically non-equivalent there may be hydroxyls with various bridge geometries. Our IR studies evidenced that hydroxyl groups were heterogeneous, five kinds of acidic hydroxyls were

17

found. Their deprotonization energies calculated from the BHW relation were 1333, 1260, 1226, 1210 and 1179 kJ mo1-1. (6) In NaH-ferrisilicate of the same structure as ZSM-5, Si-OH-Fe groups were also found to be heterogeneous. The acidic strength of Si-OH-Fe (in ferrisilicate) was lower than that of corresponding them Si-OH-A1 (in NaHZSM-5 zeolite). The distribution of Si-OH-Fe was the same as the corresponding Si-OH-A1. (7) In NaHZSM-5 dehydroxylation removed the most acidic hydroxyls first. In NaH-ferrisilicate there was no such order of preference. (8) In NaH-mordenites (Si/A1 ratio ~5) 298i MAS NMR spectra show three signals, Si(0A1) and Si(1A1) and a weak Si(2A1) signal. This suggests the presence of two kinds of hydroxyls with various numbers of A1 atoms close to the bridge: (SiO)3Si-OH-AI(SiO)3, and small amount of (A10)(SiO)zSi-OH-AI(SiO)3. As T atoms are crystallographically non-equivalent, there may be hydroxyls with various bridge geometries. Our IR studies evidenced the presence of hydroxyls at various locations (in broad and in narrow channels), the preliminary IR data suggest that the hydroxyls in broad channels are also heterogeneous.

Acknowledgment This study was partially sponsored by the grant PB 0634/P3/94/07 of Komitet Badafi Naukowych.

References [1] V.B. Kazanski, Structure and Reactivity of Modified Zeolites, Elsevier, Amsterdam, 1984, p. 61. [2] G.M. Zhidomirov and V.B. Kazanski, Adv. Catal., 34 (1985) 131. [3] J. Datka, E. Broctawik and B. Gil, J. Phys. Chem,, 98 (1994) 5622. [4] S. Beran, J. Mol. Catal. 23 (1984) 31. [5] S. Beran, Z. Phys. Chem. Neue Folge., 137 (1983) 89. [6] K.P. Schr6der, J. Sauer, M. Leslie and C.R. Catlow, Zeolites, 12 (1992) 20. [7] J. Sauer, C. K61mel, F. Hasse and R. Ahlrichs, in R. von Ballmoos, J.B. Higgins and M.M.J. Treacy (Eds.), Proc.

18

[8] [9] [10] [11] [12] [13] [14] [ 15] [16] [17] [18] [19] [20] [21]

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