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1H Broad-Line NMR at 4 K For Studying the Acidity of Solids: Application to Zeolites
Guczi, L. et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1W3 Elsevier Science Publishers B.V.All rights reserved
l H BROAD-LINE NMR AT 4 K FOR STUDYING THE ACIDITY OF SOLIDS: APPLICATION TO ZEOLITES P. Batamack, C. Doremieux-Morinand J. Fraksard
Laboratoire de Chimie des Surfaces, associe au CNRS - URA 1428, Universite P. et M. Curie; Tour 55, 3e etage, 4, Place Jussieu, 75252 Paris Cedex 05, France
Abstract Simulation of 4K broad-line 1H NMR spectra of solids with Bronsted acid properties and loaded with known amounts of water allows the determination of the concentrations of the species: H3O+, H20-HO and of the remaining initial species: OH and H 2 0 . This original method, conjugated with 1H HR-NMR (MAS) at ambient temperature, is applied to the comparison of Y and MFI zeolites without and with framework defects.
1. INTRODUCTION The 1 H HR-NMR (MAS) of solid samples in sealed tubes has been developed particularly in the last 8 years (1-6). It was hoped that the acidity of solids would be reflected by the ionicity of the Bronsted sites of the "anhydrous" samples (without any adsorbed gas but still hydroxylated) and could then be measured directly by the chemical shift. On the contrary, the low value of the isotropic chemical shift proved the low ionicity of acid OH bonds (1,2). Thus, the presence of a base is necessary to reveal the acidity. Several methods are based on the adsorption-desorption properties of the acidic solids. Among them multinuclear NMR has been extensively used, but here we shall confine ourselves to 1 H NMR (1,2,7). Later, we wished to compare the results with those obtained in the homogeneous phase and especially in aqueous solutions. For this reason we chose to adsorb controlled amounts of water on "anhydrous" H-zeolites in order to study the equilibrium: OH + H 2 0 = H3O+ + 0- (Equ. 1). Another advantage of
244
this choice is that water molecules are small enough to gain access to all acid sites. In the presence of adsorbed molecules (water, ammonia), at ambient temperature, hydrogen atoms chemically exchange between the adsorbed molecules and the Bronsted acid SiO(H)T (T for A1 or Ga), denoted ZOH in what follows, leading to signal coalescence and, consequently to average values of the chemical shifts of the various species present. For this reason, the Bronsted acidity of zeolites, defined by Equ. 1, cannot be measured by 1H MAS NMR unless it is performed at sufficiently low temperature. We obtain the concentrations of these oxygen-protonated species from their weighted contributions to the broad-line 1 H NMR spectra by studying the direct dipolar interaction between the nuclear spins of the protons, in the absence of diffusion or rotation of the atom groups, at 4 K. Previous results on HZSM-5 (8) obtained by this method and by high resolution 1 H MAS NMR spectroscopy show, using our experimental conditions, that the measured concentrations correspond to the equilibrium at ambient temperature. We assume that it is the same for all the samples studied. As some water molecules become hydrogen-bonded to ZOH, the concentration of the ( H 2 0 ...HOZ) species as a whole must also be considered and determined (9,lO). 2. EXPERIMENTAL SECTION AND DESCRIPTION OF THE METHOD
The samples belong to two main families: Y and MFI zeolites. Their characteristics have been already described except for the Ga-MFI sample, which was home-prepared with n-butylamine as template and then calcined in air to 815 K to eliminate the template; the framework Si/Al ratio, determinated by 29Si MAS NMR is 18?3 before calcination and 3223 after. Sample characteristics are collected in Table 1. A HY zeolite is denoted ND (for non-dealuminated) to remind us that it contains no extraframework aluminium. There are 47.7 ZOH groups per unit cell and no silanol groups (11,12). The HY zeolite denoted D has been dealuminated by means of an aqueous solution of (NHq)2SiFfj. It contains 35.8 ZOH and 1.5 silanol groups per unit cell (13). The DED HY sample has been doubly-steamed with exchange of the remaining sodium by ammonium between the steamings. It contains no framework A1 but amorphous silico-aluminate debris in the cavities (14). Three HZSM-5 samples were studied (8). De was synthetised without a template and mildly steamed. TPA and Bu were synthetised with TPA+ and n-butylamine as templates, respectively. They contain 4 framework aluminium and about 4 bridging hydroxyl groups per unit cell. In order to prevent dealumination, the samples of the initial hydrated
245
zeolites either in the ammonium (Y zeolite) or in the hydrogen form (ZSM-5 and Ga-MFI) were first kept under vacuum at room temperature for 24 hours, then heated at 12 Kh-l to 675 K at less than 10-2 Pa, kept under these conditions for 16 hours and finally cooled to 300 K. Water vapour is adsorbed step by step, in situ, at 300 K at a pressure below 40% of the saturation pressure; the amount of water in each sample is known by volumetry and gravimetry. After treatment at 375 K for 3 hours to ensure that the water molecules are homogeneously distributed, the samples are sealed. Table 1 Sample characteristics The symbols are the following: n B A for n-butylamine, TPA for tetrapropylammonium, HFS for ammonium hexafluorosilicate, St for steaming and Ex for sodium-ammonium exchange. HZSM-5
GaMFI
23
23
23
32
St
no
no
no
HY
Si/Al
2.4
4.4
very high
Na20% (w/w)
1.6
c1
c0.2
Dealumination treatment
no
HFS St-Ex-St
The spectra are recorded as the derivatives of the absorption signal. They are theoretically symmetrical with respect to the centre and, in practice, the two parts of the experimental spectrum are averaged; for this reason we give only one half of each spectrum. Although it is small, the probe signal is subtracted from each spectrum, both for the MAS and the broad-line experiments. The broad-line spectra were recorded and simulated as described (8,12,13). The simulated spectra correspond to the weighted sum of the contributions of the various species (H20, H3O+, H20-HOZ and OH) which
246
were calculated from the corresponding magnetic configurations (1 5-18, 9,10,19). For H 2 0 , H 3 O + and OH only one internal length parameter corresponding to the H H distance is necessary; for H 2 0 . . . H O Z or deformed H 3 O + , a magnetic configuration with three spins at the vertices of an isosceles triangle needs two distance parameters. Each of the corresponding functions is then convoluted by a Gaussian, which makes it possible to take into account interactions between the protons of the configuration considered and those belonging to neighbouring configurations, as well as to other nuclei with non-zero spin in the environment (mainly 27Al or 69Ga and 71Ga in the present case). When the effects of these nuclei are small the parameter of each Gaussian is related to a distance X close to the shortest distance between a proton of the configuration considered and a proton outside it. The calculations are normalized to a crystallographic unit cell by means of a numerical base which allows for the total number of protons i n the sample, equal to the OH groups plus twice the number of water molecules number of introduced. Moreover, because there is a large number of independent parameters for each simulation, we impose that the concentrations obtained are in agreement with the number of initial ZOH groups and the number of water molecules adsorbed. Finally, the intra-proton distances found must be in the known range corresponding to the various protonated-oxygen groups identified. The MAS NMR experiments were performed on a Bruker MSL-400 spectrometer at ambient temperature, the rotation frequency being 3 kHz. The pulse delay for recording the spectra of "anhydrous" samples is 1 mn; the other conditions have already been described (1.2.8). Chemical shifts are expressed with the usual conventions without bulk magnetic susceptibility correction. For broad-line NMR we employed a 60 MHz continuous w a v e spectrometer with phase detection and signal accumulation, using a laboratory-made probe (20).
3. RESULTS 3.1. Broad-line NMR The broad-line rigid lattice spectra always show three maxima o r shoulders (Fig.]): the first, at 1 gauss, is due to ZOH either free, hydrogen-bonded to water or having formed H3O+ ions. The "free" water molecules or those hydrogen-bonded to the ZOH contribute greatly to a second maximum at about 6 gauss. Finally, the third at about 1 0 gauss results mainly from hydroxonium ions. In samples containing very small amounts of water the first maximum is the most important, the two others being weak (Fig. 1A). Increasing the water content significantly modifies the appearance of the spectra (Fig.] B): t h e first maximum
247
106.5 H20/uc
19.0 HzO/uc A
n
2.5
5.a
7.5
1u.n
12.5
15.8
i ' ! '. I
.,__,..I
6
4
a
lk
11
,
,
I4
#
16
I
*
Ih
Figure 1. *. half-derivative of the experimental absorption spectrum of sample HY ND after adsorption of water; - : simulated spectrum; weighted contributions of: ---- : hydroxonium ions; -.-.: H20-.HOZ; - - - :A: OH; B: H20. decreases in favour of the other two; the second maximum becomes the most important for high hydration levels. The number of hydroxonium ions formed per initial Bronsted acid site, denoted N(H3O+), versus the number of adsorbed water molecules per initial Bronsted acid site, N(H20), is shown in Fig. 2, A and B for zeolites of the Y and MFI families, respectively . For the ND HY zeolite, N(H3O+) increases to about 0.2 for N(H20) = 1; it then remains about constant, i n agreement with the previously proposed equations (12) (Fig. 2A):
x ZOH + y H 2 0 ->
inf(x,y) H20-*.HOZ + / x - y / ZOH or H 2 0 (Equ. 2)
H20.**HOZ = H3O++ 20-.
(Equ. 3)
For the D HY zeolite, N(H3O+) is the same as for ND when N(H20) = 1; when the water content is increased, it remains constant to N(H20) = 2 and then increases to 0.4 for 4.4 H 2 0 per site (Fig. 2A). For the DED HY sample, containing A1 atoms only as extra-framework amorphous silica-aluminas, we showed that there is no ZOH bridge in the "anhydrous" zeolite but that adsorption of water can (re)create such bridges (14). There is no detectable hydroxonium up to N(H20) = 1.6, but for 2.2 H 2 0 per site, which is the maximum value able to enter this hydrophillic compound, N(H3O+) is about 0.2, the number of Bronsted acid sites being known from the total balance of the hydrogen atoms (21).
248
O
A
0
0
1
2..3
4
5
6
7
1s'
-.- 0
0.0
MFI; H Olas
1
. ,
2
. , 3
. , . , . , . 4
5
6
7
Figure 2. Number of hydroxonium ions formed per initial Bronsted acidic site (N(H3O+)), versus the number of adsorbed water molecules per initial Bronsted acidic site (N(H20)) for samples: A: HY family: 0 :
ND; A: D;
:DED. B: HMFI family: 0 :De; A:TPA; 0: Bu;
.
: Ga-MFI.
For the HZSM-5 samples (8,21) the situation is as follows (Fig. 2B): when N(H20) increases from 0 to 1, N(H30C) reaches 0.2 only for the De sample. It remains constant up to N(H20) equals 1.5 and 3 for the De and TPA samples, respectively; then it increases strongly to about 0.6 for N(H20) = 6. The plateau about N(H20) = 1 is replaced by a small increase for the Bu sample, followed by a stronger one from N(H2O) = 1.5; N(H3O+) reaches 0.6 for N(H2O) = 3.7. For the Ga-MFI sample, there is no hydroxonium ion for N(H20) less than 0.7. Then N(H3O+) increases continuously, reaching 0.7 for N ( H 2 0 ) equal to 5.5.
3.2. MAS NMR Some of the MAS spectra of the "anhydrous" species have been already published (8,12,14). Those of the D HY, Bu HZSM-5 and Ga-MFI samples are shown in Fig. 3 with the simulated spectra and the signals which make them up, The spectra of "anhydrous" ND and D HY samples (12) (Fig. 3A) contain the Gaussian signals at 4.3 and 5.4 ppm, characteristic of the protons of
249
ZOH bridges in HY zeolite (1,2). There are also small numbers of silanol groups (very weak for ND). After adsorption of a small amount of water on D, (N(H20) = 0.1 to 0.25) its spectrum contains a signal at 7 ppm c o r r e s p o n d i n g t o w a t e r bonded to Lewis acid sites which are not extraframework A1 atoms ( 7 ) . T h e s p e c t r u m of the "anhydrous" DED (14) contains a signal due to AlOH groups (1,2). This signal is wide, probably because of a lowering of the real symmetry of the A1 atom sites (22,23). The spectra of the "anhydrous" samples of HZSM-5 all show silanol and ZOH groups (2.3 and 4.3 ppm, respectively) (8) (Fig.3 B and C). However, in some spectra, especially that of Bu (Fig 3B), the ZOH signal is partly broadened, leading us to assume that part of the A1 atom sites h a v e a l o w symmetry (22,23). The De sample spectrum (8) contains a normal width (1 ppm) signal of AlOH at 3 ppm. For "anhydrous" Ga-MFI, in addition to normal width expected signals Figure 3. 1 H M A S - N M R for silanol and ZOH, the spectrum spectra (experimental, (Fig. 3C) includes a wide Gaussian simulated and components) signal with strong side-bands, obtained on "anhydrous" located at the same position (4.5 samples ( *: spinning sideppm) as ZOH. This signal may also bands): A: D HY; B: Bu be attributed to GaOH groups as HzSlbf-5; C: Ga-HMFI. the chemical shift is not known for these species. 4. DISCUSSION
For abscissa values less than unity, N(H3 O + ) increases continuously with N(H20) for all samples (Fig. 2A and 2B) with the exception of DED
250 HY and Ga-MFI (Fig. 2). In these cases, hydroxonium ions are only detectable for large values of N(H20) (>1.6 for DED and > 0.7 for Ga-MFI) (Fig. 2 A and B). As already mentionned, there are no initial ZOH in the "anhydrous" DED sample (14), but AlOH groups of the extraframework amorphous debris. The Gaussian 1H MAS signal of these AlOH groups,, with strong spinning side bands, is then wide (4 ppm) as is one of the 4.5 ppm signal of "anhydrous" Ga-MFI. Because of the analogies between DED and Ga-MFI, we assume that the widest l H MAS signal at 4.5 pprn in the spectrum of Ga-MFI is attributable to GaOH species rather than to SiO(H)Ga groups N(H3O+) is approximately constant for N(H20) greater than 1 for the ND HY (Fig. 2A). This is in agreement with Equ. 2 and 3 (12), which describe the results when the framework has no defect and especially no dealumination whatsoever. For all the other samples, N(H3 0 + ) increases markedly when N(H2O) is larger than 1 (Fig. 2). We have already shown in the case of D HY that this increase is to relate to defects of the framework (Lewis acid sites detected through water molecules interacting with them) (13). We assume that the Lewis centres are then aluminium atoms still bonded to the framework by less than the initial four A1-0- bonds (25). This is one type of framework defects ; many other types may happen giving rise to an increase i n the hydroxonium concentration in presence of water. For example, as already mentionned, we showed that, in amorphous silica-alumina, Bronsted sites can be formed by the interaction between AlOH groups and water molecules (14); of course the presence of an unsaturated neighbour Si atom is then needed. Our present results suggest the possibility of a lowering of the symmetry of some ZOH groups for the Bu HZSM-5 sample (wide 4.3 ppm signal); this has to be confirmed. Moreover, Staudte et al. proved that extraframework aluminium atoms are able to dissociate water molecules and create Bronsted acid sites (26). 5. CONCLUSION
Comparison of some acid zeolites using broad-line NMR at 4 K shows that the number of hydroxonium ions per initial SiO(H)T bridge of the samples, i.e. the dissociation coefficient, increases with the number of adsorbed water molecules. When the number of water molecules is greater than that of initial bridges, the hydroxonium concentration remains constant for the only sample which has no initial defect, ND HY. It increases again strongly for the other samples, whose defects have been characterized by MAS NMR. The defects that we assume to be responsible for the increase in the number of Bronsted acid sites in the
251
presence of water are the following: extraframework silico-aluminate debris, AlOH (and perhaps GaOH) species still bonded to the framework and Lewis acid sites which can be A1 atoms still bonded to the framework. We cannot exclude the presence of some other active centres, but silanol groups have never shown any activity.
To summarize, though l H MAS NMR does n o t allow to determine the strength of acid sites i n zeolites directly, i t is very helpful through the analysis of the O H groups in "anhydrous" samples and the characterization of the oxygen-protonated species bonded to the T atoms in partly hydrated samples. Broad-line 1H NMR at 4 K can be used to measure the intrinsic Bronsted acidity of the initial SiO(H)T bridges. It shows also that an extra Bronsted acidity is developed depending on zeolite characteristics, especially on the dealurnination (its degree and the nature of the species formed during the dealumination -some of them still bonded to the framework-). This development is related to the hydration degree. Clearly, both 1 H NMR methods will be useful for the future understanding of acidity and dealumination processes.
6. ACKNOWLEDGMENT We thank Dr Marie GRUIA for preparing the Ga-MFI zeolite and Prof. Dieter FREUDE for his contribution to HZSM-5 study (8).
7. REFERENCES 1 2 3
4 5 6
7
H. Pfeifer, J. Chem. SOC.,Faraday Trans. I , 84 (1988) 3777. D. Freude, Stud. Surf. Sci. Catal., 52 (1989) 169. H. Pfeifer, D. Freude and M. Hunger, Zeolites, 5 (1985) 274. G. Engelhardt, H.-G. Jerschkewitz, U . Lohse, P. Sarv, A . Samoson and E. Lippmaa, Zeolites, 7 (1987) 289. D. Freude, J. Klinowski and H. Hamdan, Chem. Phys. Letters, 149 (1988) 355. E. Brunner, H. Ernst, D. Freude, M. Hunger, C.B. Krause, D. Prager, W. Reschetilowski, W. Schwieger and K.-H. Bergk, Zeolites, 9 (1989) 282. M. Hunger, D. Freude and H. Pfeifer, J. Chem. SOC., Faraday Trans., 87 (1991) 657.
252 8 9 10 11 12 13
P. Batamack, C. DorCmieux-Morin, J. Fraissard and D. Freude, J. Phys. Chem., 95 (1991) 3790. C. DorCmieux-Morin, J. Magn. Res., 21 (1976) 419. C. DorCmieux-Morin, J. Magn. Res., 33 (1979) 505. P. Batamack, C. DorCmieux-Morin, R. Vincent and J. Fraissard, Chem. Phys. Letters, 180 (1991) 545. P. Batamack, C. DorCmieux-Morin and J. Fraissard, J. Chim. Phys. 89 (1992) 423. P. Batamack, C. Dortmieux-Morin and J. Fraissard, Cat. Letters, 11 (1991) 119.
14
P. Batamack, C. DorCmieux-Morin and J. Fraissard, Cat. Letters, 9 (1991) 403.
15 16 17 18 19
20 21 22 23 24
25
26
G.E. Pake, J. Chem. Phys., 16 (1948) 327. E.R. Andrew and R.J. Bersohn, J. Chem. Phys., 18 (1950) 159. R.E. Richards and J.A.S. Smith, Trans. Faraday SOC., 48 (1952) 675. E.R. Andrew and N.D. Finch, Proc. Phys. SOC.,70B (1957) 980. A.L. Porte, H.S. Gutowsky and J.E. Boggs, J. Chem. Phys., 36 (1962) 1695.
R. Vincent, unpublished results. P. Batamack, Doctorat Thesis, P. and M. Curie University, Paris
(1991).
J. Bohm, D. Fenzke and H. Pfeifer, J. Magn. Res., 55 (1983) 197. E. Brunner, J. Chem. SOC., Faraday Trans., 86 (1990) 3957. W. Schwieger, K.-H. Bergk, D. Freude, M. Hunger and H. Pfeifer, ACS Symposium Series, 398 (1989) 274. P. Batamack, C. DorCmieux-Morin and J. Fraissard, to be published. B. Staudte, M. Hunger and M. Nimz, Zeolites, 11 (1991) 837. Symposium Series, 398 (1989) 274.
253 DISCUSSIONS
Q: H. G. Kar e (Germany)
Y
Y
1) The lfI broad-line N M R spectra exhibit three stron 1 overla ing sections with three maxima to which the various protonated species (ZOH, ZO *--OH O--H 0+,"free" H20) contribute to a varyin extent. How accurate is the determination o? the num&r of H@+ ions [N(H O+)]derived rom these spectra by simulation taking into account the "weighted contrhtions of these oxygen-protonated species", i.e. what are the limits of error ? 2) In this study single water molecules hydrogen-bonded to bridgin Si*-OH-AI, i.e. ZOH*-OH2, were observed whereas these species were not detected by fil [l]. Could you please comment on this ? 3) It seems to be desirable to re are one series of homologous acidic zeolite samples, e.g. H-ZSMJ with decreasing A l / l l + k and minimum defects, in order to check whether or not the novel 1H NMR technique provides the expected systematic change in the strength of the acidic sites. It would then be of great interest to compare the results with those of other experimental methods which are believed to be suitable for measuring and ranking the acidity strength of solid acids such as zeolites. Could you, please, comment ?' A. Jentys, G. Warecka, M. Derewinski and J. A. Lercher, J. Phys. Chem., 93,4837 [l] (1989)
f
A: J. Fraissard 1) From the weighted contribution of the characteristic protonated s ecies we estimate the limits of error on the determination of the number of H O+ ions to tl %. The number of adsorbed water molecules on the "anhydrous" zeolite samdes is determined with an error of 25% at present. These limits are given for HY zeolite in the Figures 2A and 2B [2]. The accuracy on the number of adsorbed water molecules will be improved if we can use the same thin lass ampoule for the pretreatment and the various experiments. It is not ossible for us to comment on the results of J. Lercher et al. Concerning our study, (i) the di ferences between the theoretical spectra of the various protonated species are characteristic as long as the distances between hydrogen atoms belonging to distinct species are sufficiently greater than internal H-H distances; that this criterion is satisfied can be checked by examining the distance parameters displayed for the simulation (the displayed values for the inter-species H-H distances are lower than the true ones because of spectral enlargement due to other nuclei with non zero spin); (ii) the assumption that there is no hydrogen-bond between SiO(H)AI and H20, i.e. no ZOH--OH2, leads to no acceptable quantitative simulation of the experimental spectra. 3) We agree that new experiments on series of homolo ous acid zeolite samples are needed. We are looking forward to doing some on a series of - Z S M J zeolites with various Si/AI ratios; one very important point is that the samples must contain few defects. It would be very interesting to study other sample series: some with only one type of defect. As pointed out in the remark, we must compare the results obtained on the acid strength with those from other methods. P. Batamack, C. Doremieux-Morin, J. Fraissard, Catal. Lett., 11, 119 (1991) [2]
s
I)
P
a
Q: J. J. Fripiat (USA) Are you not afraid that adding water to a zeolite prepared at elevated temperature will hydrolyze some M-O-M bridges (M = Al, Si,etc,) and reconstruct a surface that will be different from that on which acid catalysis will create ? A: J. Fraissard We know that h drolysis of the zeolitic framework of acidic zeolites may happen after have to find a compromise to ensure that the samples have been water adsorption. homogenized before the NMR experiment and to minimize hydrolysis. Usually, we maintain the samples at 375 K for 2 or 3 hours. Under such conditions, for the ND HY zeolite which
d
254 has no initial defect, we see no sign of hydrolysis except when the number of adsorbed water molecules is larger than the number of acid hydroxyl groups. Then the 'H MAS NMR spectra of the sam les show a visible but very small signal corresponding to water molecules of the species.
m2~)6~R
Q: J. Lercher (Austria) Our IR spectra of water adsorbed on HZSM-5 (1:l stoichiometry between water and SiOHAl groups) suggest that the hydroxonium ion is formed and that it is hydrogen bonded to the surface. The difference between these results measured at 300 K and your results might be raised by the shift in the equilibrium caused by a slightly positive enthalpy of the reaction (AH 0.5 kJ/mol)
The explanation agrees also very well with recent calculations of Sauer et al. A: J. Fraissard The aim of our study is primarily to measure the hydroxonium concentration in the samples. We did not plane to determine the actual symmetry of these ions. nevertheless, we d o not claim that the hydroxonium ions d o not form hydrogen-bonds to the framework. The optimum geometry proposed by Sauer for these groups (with two hydrogen-bonds to framework oxygens, as you noted) corresponds to a theoretical model with the three H atoms at the apices of any triangle. We have not such a theoretical model programmed now. However, after simulations using an equilateral triangle, w e were able to enhance the quality of the simulations for most of the samples by using a theoretical configuration where the H30+ H atoms are assumed to be at the apices of an isosceles triangle. The two equal sides of this triangle are in agreement with the length corresponding to an average dipolar interaction for the two longest sides of Suer's model. The base of the configuration is also in agreement with Sauer's model.
Q: R. Kumar (India) 1) Does your Ga-MFI contain AI ? Table 1 of your paper shows that Si/AI ratio in GaMFI is 32. Is it Si/Ga; otherwise what is Si/Ga ? 2) Fig. 2 of your paper shows no difference between MFI and Ga-MFI while in Fig. 3 Ga-MFI exhibits two peaks or splitting in 'H-NMR at 2.3 and 4.3 ppm. However pure MFI exhibits comparatively a much less intense shoulder at 2.3 ppm apart from the peak at 4.3 ppm. Please comment.
A: J. Fraissard
1) We apologize for the error in Table 1 and in the text: in the case of Ga-MFI the given value (32) is the Si/Ga ratio and not Si/AI. Nevertheless, there is a small amount of A1 in the sample due to seeding with h4FI crystals during the synthesis. 2) There is indeed a difference between the curves obtained for Ga-MFI and AI-MFI in Figure 2B, especially for values of H20/as <1. The MAS NMR signal at 2.3 ppm given by
255 these samples is attributable to silanol groups, which, in our studies, show no interaction with water, i.e. no appearance of acidity.
Q: J. B. Nagy (Belgium) Are you going 10 extend your nice method by using partially deuterated bases, like CD,OH, NDpH, etc. ’? This could help you to measure the acidity with respect to bases of different strengths.
A: J. Fraissard We are currently trying some experiments which d o not go so far as those you propose but which are moving in the same direction.
l H BROAD-LINE NMR AT 4 K FOR STUDYING THE ACIDITY OF SOLIDS: APPLICATION TO ZEOLITES P. Batamack, C. Doremieux-Morinand J. Fraksard
Laboratoire de Chimie des Surfaces, associe au CNRS - URA 1428, Universite P. et M. Curie; Tour 55, 3e etage, 4, Place Jussieu, 75252 Paris Cedex 05, France
Abstract Simulation of 4K broad-line 1H NMR spectra of solids with Bronsted acid properties and loaded with known amounts of water allows the determination of the concentrations of the species: H3O+, H20-HO and of the remaining initial species: OH and H 2 0 . This original method, conjugated with 1H HR-NMR (MAS) at ambient temperature, is applied to the comparison of Y and MFI zeolites without and with framework defects.
1. INTRODUCTION The 1 H HR-NMR (MAS) of solid samples in sealed tubes has been developed particularly in the last 8 years (1-6). It was hoped that the acidity of solids would be reflected by the ionicity of the Bronsted sites of the "anhydrous" samples (without any adsorbed gas but still hydroxylated) and could then be measured directly by the chemical shift. On the contrary, the low value of the isotropic chemical shift proved the low ionicity of acid OH bonds (1,2). Thus, the presence of a base is necessary to reveal the acidity. Several methods are based on the adsorption-desorption properties of the acidic solids. Among them multinuclear NMR has been extensively used, but here we shall confine ourselves to 1 H NMR (1,2,7). Later, we wished to compare the results with those obtained in the homogeneous phase and especially in aqueous solutions. For this reason we chose to adsorb controlled amounts of water on "anhydrous" H-zeolites in order to study the equilibrium: OH + H 2 0 = H3O+ + 0- (Equ. 1). Another advantage of
244
this choice is that water molecules are small enough to gain access to all acid sites. In the presence of adsorbed molecules (water, ammonia), at ambient temperature, hydrogen atoms chemically exchange between the adsorbed molecules and the Bronsted acid SiO(H)T (T for A1 or Ga), denoted ZOH in what follows, leading to signal coalescence and, consequently to average values of the chemical shifts of the various species present. For this reason, the Bronsted acidity of zeolites, defined by Equ. 1, cannot be measured by 1H MAS NMR unless it is performed at sufficiently low temperature. We obtain the concentrations of these oxygen-protonated species from their weighted contributions to the broad-line 1 H NMR spectra by studying the direct dipolar interaction between the nuclear spins of the protons, in the absence of diffusion or rotation of the atom groups, at 4 K. Previous results on HZSM-5 (8) obtained by this method and by high resolution 1 H MAS NMR spectroscopy show, using our experimental conditions, that the measured concentrations correspond to the equilibrium at ambient temperature. We assume that it is the same for all the samples studied. As some water molecules become hydrogen-bonded to ZOH, the concentration of the ( H 2 0 ...HOZ) species as a whole must also be considered and determined (9,lO). 2. EXPERIMENTAL SECTION AND DESCRIPTION OF THE METHOD
The samples belong to two main families: Y and MFI zeolites. Their characteristics have been already described except for the Ga-MFI sample, which was home-prepared with n-butylamine as template and then calcined in air to 815 K to eliminate the template; the framework Si/Al ratio, determinated by 29Si MAS NMR is 18?3 before calcination and 3223 after. Sample characteristics are collected in Table 1. A HY zeolite is denoted ND (for non-dealuminated) to remind us that it contains no extraframework aluminium. There are 47.7 ZOH groups per unit cell and no silanol groups (11,12). The HY zeolite denoted D has been dealuminated by means of an aqueous solution of (NHq)2SiFfj. It contains 35.8 ZOH and 1.5 silanol groups per unit cell (13). The DED HY sample has been doubly-steamed with exchange of the remaining sodium by ammonium between the steamings. It contains no framework A1 but amorphous silico-aluminate debris in the cavities (14). Three HZSM-5 samples were studied (8). De was synthetised without a template and mildly steamed. TPA and Bu were synthetised with TPA+ and n-butylamine as templates, respectively. They contain 4 framework aluminium and about 4 bridging hydroxyl groups per unit cell. In order to prevent dealumination, the samples of the initial hydrated
245
zeolites either in the ammonium (Y zeolite) or in the hydrogen form (ZSM-5 and Ga-MFI) were first kept under vacuum at room temperature for 24 hours, then heated at 12 Kh-l to 675 K at less than 10-2 Pa, kept under these conditions for 16 hours and finally cooled to 300 K. Water vapour is adsorbed step by step, in situ, at 300 K at a pressure below 40% of the saturation pressure; the amount of water in each sample is known by volumetry and gravimetry. After treatment at 375 K for 3 hours to ensure that the water molecules are homogeneously distributed, the samples are sealed. Table 1 Sample characteristics The symbols are the following: n B A for n-butylamine, TPA for tetrapropylammonium, HFS for ammonium hexafluorosilicate, St for steaming and Ex for sodium-ammonium exchange. HZSM-5
GaMFI
23
23
23
32
St
no
no
no
HY
Si/Al
2.4
4.4
very high
Na20% (w/w)
1.6
c1
c0.2
Dealumination treatment
no
HFS St-Ex-St
The spectra are recorded as the derivatives of the absorption signal. They are theoretically symmetrical with respect to the centre and, in practice, the two parts of the experimental spectrum are averaged; for this reason we give only one half of each spectrum. Although it is small, the probe signal is subtracted from each spectrum, both for the MAS and the broad-line experiments. The broad-line spectra were recorded and simulated as described (8,12,13). The simulated spectra correspond to the weighted sum of the contributions of the various species (H20, H3O+, H20-HOZ and OH) which
246
were calculated from the corresponding magnetic configurations (1 5-18, 9,10,19). For H 2 0 , H 3 O + and OH only one internal length parameter corresponding to the H H distance is necessary; for H 2 0 . . . H O Z or deformed H 3 O + , a magnetic configuration with three spins at the vertices of an isosceles triangle needs two distance parameters. Each of the corresponding functions is then convoluted by a Gaussian, which makes it possible to take into account interactions between the protons of the configuration considered and those belonging to neighbouring configurations, as well as to other nuclei with non-zero spin in the environment (mainly 27Al or 69Ga and 71Ga in the present case). When the effects of these nuclei are small the parameter of each Gaussian is related to a distance X close to the shortest distance between a proton of the configuration considered and a proton outside it. The calculations are normalized to a crystallographic unit cell by means of a numerical base which allows for the total number of protons i n the sample, equal to the OH groups plus twice the number of water molecules number of introduced. Moreover, because there is a large number of independent parameters for each simulation, we impose that the concentrations obtained are in agreement with the number of initial ZOH groups and the number of water molecules adsorbed. Finally, the intra-proton distances found must be in the known range corresponding to the various protonated-oxygen groups identified. The MAS NMR experiments were performed on a Bruker MSL-400 spectrometer at ambient temperature, the rotation frequency being 3 kHz. The pulse delay for recording the spectra of "anhydrous" samples is 1 mn; the other conditions have already been described (1.2.8). Chemical shifts are expressed with the usual conventions without bulk magnetic susceptibility correction. For broad-line NMR we employed a 60 MHz continuous w a v e spectrometer with phase detection and signal accumulation, using a laboratory-made probe (20).
3. RESULTS 3.1. Broad-line NMR The broad-line rigid lattice spectra always show three maxima o r shoulders (Fig.]): the first, at 1 gauss, is due to ZOH either free, hydrogen-bonded to water or having formed H3O+ ions. The "free" water molecules or those hydrogen-bonded to the ZOH contribute greatly to a second maximum at about 6 gauss. Finally, the third at about 1 0 gauss results mainly from hydroxonium ions. In samples containing very small amounts of water the first maximum is the most important, the two others being weak (Fig. 1A). Increasing the water content significantly modifies the appearance of the spectra (Fig.] B): t h e first maximum
247
106.5 H20/uc
19.0 HzO/uc A
n
2.5
5.a
7.5
1u.n
12.5
15.8
i ' ! '. I
.,__,..I
6
4
a
lk
11
,
,
I4
#
16
I
*
Ih
Figure 1. *. half-derivative of the experimental absorption spectrum of sample HY ND after adsorption of water; - : simulated spectrum; weighted contributions of: ---- : hydroxonium ions; -.-.: H20-.HOZ; - - - :A: OH; B: H20. decreases in favour of the other two; the second maximum becomes the most important for high hydration levels. The number of hydroxonium ions formed per initial Bronsted acid site, denoted N(H3O+), versus the number of adsorbed water molecules per initial Bronsted acid site, N(H20), is shown in Fig. 2, A and B for zeolites of the Y and MFI families, respectively . For the ND HY zeolite, N(H3O+) increases to about 0.2 for N(H20) = 1; it then remains about constant, i n agreement with the previously proposed equations (12) (Fig. 2A):
x ZOH + y H 2 0 ->
inf(x,y) H20-*.HOZ + / x - y / ZOH or H 2 0 (Equ. 2)
H20.**HOZ = H3O++ 20-.
(Equ. 3)
For the D HY zeolite, N(H3O+) is the same as for ND when N(H20) = 1; when the water content is increased, it remains constant to N(H20) = 2 and then increases to 0.4 for 4.4 H 2 0 per site (Fig. 2A). For the DED HY sample, containing A1 atoms only as extra-framework amorphous silica-aluminas, we showed that there is no ZOH bridge in the "anhydrous" zeolite but that adsorption of water can (re)create such bridges (14). There is no detectable hydroxonium up to N(H20) = 1.6, but for 2.2 H 2 0 per site, which is the maximum value able to enter this hydrophillic compound, N(H3O+) is about 0.2, the number of Bronsted acid sites being known from the total balance of the hydrogen atoms (21).
248
O
A
0
0
1
2..3
4
5
6
7
1s'
-.- 0
0.0
MFI; H Olas
1
. ,
2
. , 3
. , . , . , . 4
5
6
7
Figure 2. Number of hydroxonium ions formed per initial Bronsted acidic site (N(H3O+)), versus the number of adsorbed water molecules per initial Bronsted acidic site (N(H20)) for samples: A: HY family: 0 :
ND; A: D;
:DED. B: HMFI family: 0 :De; A:TPA; 0: Bu;
.
: Ga-MFI.
For the HZSM-5 samples (8,21) the situation is as follows (Fig. 2B): when N(H20) increases from 0 to 1, N(H30C) reaches 0.2 only for the De sample. It remains constant up to N(H20) equals 1.5 and 3 for the De and TPA samples, respectively; then it increases strongly to about 0.6 for N(H20) = 6. The plateau about N(H20) = 1 is replaced by a small increase for the Bu sample, followed by a stronger one from N(H2O) = 1.5; N(H3O+) reaches 0.6 for N(H2O) = 3.7. For the Ga-MFI sample, there is no hydroxonium ion for N(H20) less than 0.7. Then N(H3O+) increases continuously, reaching 0.7 for N ( H 2 0 ) equal to 5.5.
3.2. MAS NMR Some of the MAS spectra of the "anhydrous" species have been already published (8,12,14). Those of the D HY, Bu HZSM-5 and Ga-MFI samples are shown in Fig. 3 with the simulated spectra and the signals which make them up, The spectra of "anhydrous" ND and D HY samples (12) (Fig. 3A) contain the Gaussian signals at 4.3 and 5.4 ppm, characteristic of the protons of
249
ZOH bridges in HY zeolite (1,2). There are also small numbers of silanol groups (very weak for ND). After adsorption of a small amount of water on D, (N(H20) = 0.1 to 0.25) its spectrum contains a signal at 7 ppm c o r r e s p o n d i n g t o w a t e r bonded to Lewis acid sites which are not extraframework A1 atoms ( 7 ) . T h e s p e c t r u m of the "anhydrous" DED (14) contains a signal due to AlOH groups (1,2). This signal is wide, probably because of a lowering of the real symmetry of the A1 atom sites (22,23). The spectra of the "anhydrous" samples of HZSM-5 all show silanol and ZOH groups (2.3 and 4.3 ppm, respectively) (8) (Fig.3 B and C). However, in some spectra, especially that of Bu (Fig 3B), the ZOH signal is partly broadened, leading us to assume that part of the A1 atom sites h a v e a l o w symmetry (22,23). The De sample spectrum (8) contains a normal width (1 ppm) signal of AlOH at 3 ppm. For "anhydrous" Ga-MFI, in addition to normal width expected signals Figure 3. 1 H M A S - N M R for silanol and ZOH, the spectrum spectra (experimental, (Fig. 3C) includes a wide Gaussian simulated and components) signal with strong side-bands, obtained on "anhydrous" located at the same position (4.5 samples ( *: spinning sideppm) as ZOH. This signal may also bands): A: D HY; B: Bu be attributed to GaOH groups as HzSlbf-5; C: Ga-HMFI. the chemical shift is not known for these species. 4. DISCUSSION
For abscissa values less than unity, N(H3 O + ) increases continuously with N(H20) for all samples (Fig. 2A and 2B) with the exception of DED
250 HY and Ga-MFI (Fig. 2). In these cases, hydroxonium ions are only detectable for large values of N(H20) (>1.6 for DED and > 0.7 for Ga-MFI) (Fig. 2 A and B). As already mentionned, there are no initial ZOH in the "anhydrous" DED sample (14), but AlOH groups of the extraframework amorphous debris. The Gaussian 1H MAS signal of these AlOH groups,, with strong spinning side bands, is then wide (4 ppm) as is one of the 4.5 ppm signal of "anhydrous" Ga-MFI. Because of the analogies between DED and Ga-MFI, we assume that the widest l H MAS signal at 4.5 pprn in the spectrum of Ga-MFI is attributable to GaOH species rather than to SiO(H)Ga groups N(H3O+) is approximately constant for N(H20) greater than 1 for the ND HY (Fig. 2A). This is in agreement with Equ. 2 and 3 (12), which describe the results when the framework has no defect and especially no dealumination whatsoever. For all the other samples, N(H3 0 + ) increases markedly when N(H2O) is larger than 1 (Fig. 2). We have already shown in the case of D HY that this increase is to relate to defects of the framework (Lewis acid sites detected through water molecules interacting with them) (13). We assume that the Lewis centres are then aluminium atoms still bonded to the framework by less than the initial four A1-0- bonds (25). This is one type of framework defects ; many other types may happen giving rise to an increase i n the hydroxonium concentration in presence of water. For example, as already mentionned, we showed that, in amorphous silica-alumina, Bronsted sites can be formed by the interaction between AlOH groups and water molecules (14); of course the presence of an unsaturated neighbour Si atom is then needed. Our present results suggest the possibility of a lowering of the symmetry of some ZOH groups for the Bu HZSM-5 sample (wide 4.3 ppm signal); this has to be confirmed. Moreover, Staudte et al. proved that extraframework aluminium atoms are able to dissociate water molecules and create Bronsted acid sites (26). 5. CONCLUSION
Comparison of some acid zeolites using broad-line NMR at 4 K shows that the number of hydroxonium ions per initial SiO(H)T bridge of the samples, i.e. the dissociation coefficient, increases with the number of adsorbed water molecules. When the number of water molecules is greater than that of initial bridges, the hydroxonium concentration remains constant for the only sample which has no initial defect, ND HY. It increases again strongly for the other samples, whose defects have been characterized by MAS NMR. The defects that we assume to be responsible for the increase in the number of Bronsted acid sites in the
251
presence of water are the following: extraframework silico-aluminate debris, AlOH (and perhaps GaOH) species still bonded to the framework and Lewis acid sites which can be A1 atoms still bonded to the framework. We cannot exclude the presence of some other active centres, but silanol groups have never shown any activity.
To summarize, though l H MAS NMR does n o t allow to determine the strength of acid sites i n zeolites directly, i t is very helpful through the analysis of the O H groups in "anhydrous" samples and the characterization of the oxygen-protonated species bonded to the T atoms in partly hydrated samples. Broad-line 1H NMR at 4 K can be used to measure the intrinsic Bronsted acidity of the initial SiO(H)T bridges. It shows also that an extra Bronsted acidity is developed depending on zeolite characteristics, especially on the dealurnination (its degree and the nature of the species formed during the dealumination -some of them still bonded to the framework-). This development is related to the hydration degree. Clearly, both 1 H NMR methods will be useful for the future understanding of acidity and dealumination processes.
6. ACKNOWLEDGMENT We thank Dr Marie GRUIA for preparing the Ga-MFI zeolite and Prof. Dieter FREUDE for his contribution to HZSM-5 study (8).
7. REFERENCES 1 2 3
4 5 6
7
H. Pfeifer, J. Chem. SOC.,Faraday Trans. I , 84 (1988) 3777. D. Freude, Stud. Surf. Sci. Catal., 52 (1989) 169. H. Pfeifer, D. Freude and M. Hunger, Zeolites, 5 (1985) 274. G. Engelhardt, H.-G. Jerschkewitz, U . Lohse, P. Sarv, A . Samoson and E. Lippmaa, Zeolites, 7 (1987) 289. D. Freude, J. Klinowski and H. Hamdan, Chem. Phys. Letters, 149 (1988) 355. E. Brunner, H. Ernst, D. Freude, M. Hunger, C.B. Krause, D. Prager, W. Reschetilowski, W. Schwieger and K.-H. Bergk, Zeolites, 9 (1989) 282. M. Hunger, D. Freude and H. Pfeifer, J. Chem. SOC., Faraday Trans., 87 (1991) 657.
252 8 9 10 11 12 13
P. Batamack, C. DorCmieux-Morin, J. Fraissard and D. Freude, J. Phys. Chem., 95 (1991) 3790. C. DorCmieux-Morin, J. Magn. Res., 21 (1976) 419. C. DorCmieux-Morin, J. Magn. Res., 33 (1979) 505. P. Batamack, C. DorCmieux-Morin, R. Vincent and J. Fraissard, Chem. Phys. Letters, 180 (1991) 545. P. Batamack, C. DorCmieux-Morin and J. Fraissard, J. Chim. Phys. 89 (1992) 423. P. Batamack, C. Dortmieux-Morin and J. Fraissard, Cat. Letters, 11 (1991) 119.
14
P. Batamack, C. DorCmieux-Morin and J. Fraissard, Cat. Letters, 9 (1991) 403.
15 16 17 18 19
20 21 22 23 24
25
26
G.E. Pake, J. Chem. Phys., 16 (1948) 327. E.R. Andrew and R.J. Bersohn, J. Chem. Phys., 18 (1950) 159. R.E. Richards and J.A.S. Smith, Trans. Faraday SOC., 48 (1952) 675. E.R. Andrew and N.D. Finch, Proc. Phys. SOC.,70B (1957) 980. A.L. Porte, H.S. Gutowsky and J.E. Boggs, J. Chem. Phys., 36 (1962) 1695.
R. Vincent, unpublished results. P. Batamack, Doctorat Thesis, P. and M. Curie University, Paris
(1991).
J. Bohm, D. Fenzke and H. Pfeifer, J. Magn. Res., 55 (1983) 197. E. Brunner, J. Chem. SOC., Faraday Trans., 86 (1990) 3957. W. Schwieger, K.-H. Bergk, D. Freude, M. Hunger and H. Pfeifer, ACS Symposium Series, 398 (1989) 274. P. Batamack, C. DorCmieux-Morin and J. Fraissard, to be published. B. Staudte, M. Hunger and M. Nimz, Zeolites, 11 (1991) 837. Symposium Series, 398 (1989) 274.
253 DISCUSSIONS
Q: H. G. Kar e (Germany)
Y
Y
1) The lfI broad-line N M R spectra exhibit three stron 1 overla ing sections with three maxima to which the various protonated species (ZOH, ZO *--OH O--H 0+,"free" H20) contribute to a varyin extent. How accurate is the determination o? the num&r of H@+ ions [N(H O+)]derived rom these spectra by simulation taking into account the "weighted contrhtions of these oxygen-protonated species", i.e. what are the limits of error ? 2) In this study single water molecules hydrogen-bonded to bridgin Si*-OH-AI, i.e. ZOH*-OH2, were observed whereas these species were not detected by fil [l]. Could you please comment on this ? 3) It seems to be desirable to re are one series of homologous acidic zeolite samples, e.g. H-ZSMJ with decreasing A l / l l + k and minimum defects, in order to check whether or not the novel 1H NMR technique provides the expected systematic change in the strength of the acidic sites. It would then be of great interest to compare the results with those of other experimental methods which are believed to be suitable for measuring and ranking the acidity strength of solid acids such as zeolites. Could you, please, comment ?' A. Jentys, G. Warecka, M. Derewinski and J. A. Lercher, J. Phys. Chem., 93,4837 [l] (1989)
f
A: J. Fraissard 1) From the weighted contribution of the characteristic protonated s ecies we estimate the limits of error on the determination of the number of H O+ ions to tl %. The number of adsorbed water molecules on the "anhydrous" zeolite samdes is determined with an error of 25% at present. These limits are given for HY zeolite in the Figures 2A and 2B [2]. The accuracy on the number of adsorbed water molecules will be improved if we can use the same thin lass ampoule for the pretreatment and the various experiments. It is not ossible for us to comment on the results of J. Lercher et al. Concerning our study, (i) the di ferences between the theoretical spectra of the various protonated species are characteristic as long as the distances between hydrogen atoms belonging to distinct species are sufficiently greater than internal H-H distances; that this criterion is satisfied can be checked by examining the distance parameters displayed for the simulation (the displayed values for the inter-species H-H distances are lower than the true ones because of spectral enlargement due to other nuclei with non zero spin); (ii) the assumption that there is no hydrogen-bond between SiO(H)AI and H20, i.e. no ZOH--OH2, leads to no acceptable quantitative simulation of the experimental spectra. 3) We agree that new experiments on series of homolo ous acid zeolite samples are needed. We are looking forward to doing some on a series of - Z S M J zeolites with various Si/AI ratios; one very important point is that the samples must contain few defects. It would be very interesting to study other sample series: some with only one type of defect. As pointed out in the remark, we must compare the results obtained on the acid strength with those from other methods. P. Batamack, C. Doremieux-Morin, J. Fraissard, Catal. Lett., 11, 119 (1991) [2]
s
I)
P
a
Q: J. J. Fripiat (USA) Are you not afraid that adding water to a zeolite prepared at elevated temperature will hydrolyze some M-O-M bridges (M = Al, Si,etc,) and reconstruct a surface that will be different from that on which acid catalysis will create ? A: J. Fraissard We know that h drolysis of the zeolitic framework of acidic zeolites may happen after have to find a compromise to ensure that the samples have been water adsorption. homogenized before the NMR experiment and to minimize hydrolysis. Usually, we maintain the samples at 375 K for 2 or 3 hours. Under such conditions, for the ND HY zeolite which
d
254 has no initial defect, we see no sign of hydrolysis except when the number of adsorbed water molecules is larger than the number of acid hydroxyl groups. Then the 'H MAS NMR spectra of the sam les show a visible but very small signal corresponding to water molecules of the species.
m2~)6~R
Q: J. Lercher (Austria) Our IR spectra of water adsorbed on HZSM-5 (1:l stoichiometry between water and SiOHAl groups) suggest that the hydroxonium ion is formed and that it is hydrogen bonded to the surface. The difference between these results measured at 300 K and your results might be raised by the shift in the equilibrium caused by a slightly positive enthalpy of the reaction (AH 0.5 kJ/mol)
The explanation agrees also very well with recent calculations of Sauer et al. A: J. Fraissard The aim of our study is primarily to measure the hydroxonium concentration in the samples. We did not plane to determine the actual symmetry of these ions. nevertheless, we d o not claim that the hydroxonium ions d o not form hydrogen-bonds to the framework. The optimum geometry proposed by Sauer for these groups (with two hydrogen-bonds to framework oxygens, as you noted) corresponds to a theoretical model with the three H atoms at the apices of any triangle. We have not such a theoretical model programmed now. However, after simulations using an equilateral triangle, w e were able to enhance the quality of the simulations for most of the samples by using a theoretical configuration where the H30+ H atoms are assumed to be at the apices of an isosceles triangle. The two equal sides of this triangle are in agreement with the length corresponding to an average dipolar interaction for the two longest sides of Suer's model. The base of the configuration is also in agreement with Sauer's model.
Q: R. Kumar (India) 1) Does your Ga-MFI contain AI ? Table 1 of your paper shows that Si/AI ratio in GaMFI is 32. Is it Si/Ga; otherwise what is Si/Ga ? 2) Fig. 2 of your paper shows no difference between MFI and Ga-MFI while in Fig. 3 Ga-MFI exhibits two peaks or splitting in 'H-NMR at 2.3 and 4.3 ppm. However pure MFI exhibits comparatively a much less intense shoulder at 2.3 ppm apart from the peak at 4.3 ppm. Please comment.
A: J. Fraissard
1) We apologize for the error in Table 1 and in the text: in the case of Ga-MFI the given value (32) is the Si/Ga ratio and not Si/AI. Nevertheless, there is a small amount of A1 in the sample due to seeding with h4FI crystals during the synthesis. 2) There is indeed a difference between the curves obtained for Ga-MFI and AI-MFI in Figure 2B, especially for values of H20/as <1. The MAS NMR signal at 2.3 ppm given by
255 these samples is attributable to silanol groups, which, in our studies, show no interaction with water, i.e. no appearance of acidity.
Q: J. B. Nagy (Belgium) Are you going 10 extend your nice method by using partially deuterated bases, like CD,OH, NDpH, etc. ’? This could help you to measure the acidity with respect to bases of different strengths.
A: J. Fraissard We are currently trying some experiments which d o not go so far as those you propose but which are moving in the same direction.