NH3 as a Probe Molecule for NMR and IR Study of Zeolite Catalyst

A Frequency-Response Study of the Diffusion and Sorption Dynamics of Ammonia in Zeolites. J. Valyon, Gy. Onyestyák, and L. V. C. Rees. Langmuir 2000 ...
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J. Phys. Chem. B 1997, 101, 1824-1830

NH3 as a Probe Molecule for NMR and IR Study of Zeolite Catalyst Acidity F. Yin, A. L. Blumenfeld, V. Gruver, and J. J. Fripiat* Department of Chemistry and Laboratory for Surface Studies, P.O. Box 413, UniVersity of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201 ReceiVed: July 22, 1996; In Final Form: NoVember 21, 1996X

The measurement of the numbers of Bro¨nsted and Lewis acid sites has been performed quantitatively using NH3 as an infrared molecular probe. The assignment of the deformation NH4+ and NH3 bands has been carried out by comparing the proton MAS NMR spectra with the corresponding infrared spectra in dealuminated acid ZSM-5 (DHZ). It has been shown that the total number of Bro¨nsted sites is equal to the number of OH bridging a silicon to an aluminum in a 4Q(1Al) cluster. This represents a variable fraction of the content in framework aluminum (FAl), depending on the zeolite composition. The bridging OH in 4Q(nAl), n > 1, clusters are not Bro¨nsted sites, in agreement with the results of a recent REDOR study. The number of Lewis sites (L:NH3) is a fraction of the nonframework aluminum content (NFAl). The Lewis sites dispersion ratio L:NH3/NFAl is between 75 and 40%, depending upon the temperature (115 or 175 °C) used for outgassing the sample after NH3 adsorption. The quantitative results obtained with NH3 IR are compared to the qualitative results obtained earlier by low-temperature CO IR. This comparison shows that, in DHZ as well as in USY, two types of Bro¨nsted sites exist. The strongest Bro¨nsted sites are related to FAl1, a computable number representing a bridging OH in a 4Q(1Al) environment with no next-nearest-neighbor aluminum.

Introduction The knowledge of the nature of the Bro¨nsted and Lewis acid sites on acid catalysts and the estimate of their concentrations are a prerequisite to any attempt to rank the catalysts by the turnover number of a model reaction, by the selectivity, etc. Among the techniques that have been successfully used in the past half of this century, infrared (IR) and NMR spectroscopies have been the most popular. Both rely on a judicious choice of a chemisorbed molecular probe. The modification of the molecular probe and the extent of its interaction with the acid sites contains the desired information. When the same molecular probe is usable by both NMR and IR, the complementary data are an even richer source of knowledge. NH3 and P(CH3)3 are excellent examples of such a combined use. By infrared spectroscopy, the distinction between ammonium on a Bro¨nsted site and of the ammonia adduct on a Lewis site1-4 and the quantization of both kinds of sites are relatively easy. The use of a base weaker than ammonia presents advantages when some scaling of the acidity is desired. Pyridine5 has been extensively studied, and CO6 meets many criteria that have promoted its extensive use as a surface probe for the study of Lewis as well as Bro¨nsted sites by infrared spectroscopy. One of the most important characteristics of any probe is the heat of chemisorption, since it rules the amount remaining on the surface after the excess physisorbed material has been removed. The chemisorption energy (or enthalpy) integrates the effects of the residual pressure, of the duration of the outgassing, and of the outgassing temperature. In view of its combined use in NMR, IR, and microcalorimetry,7 NH3 is rather unique. One of the objections often encountered toward its use is the inherent complexity of the vibrational spectra of NH4+ and coordinated NH3 when compared, for instance, to CO. Also, the base strength of NH3 is believed to level out the subtle distinctions between the acidity * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, February 15, 1997.

S1089-5647(96)01854-8 CCC: $14.00

of various sites. Finally, NH3 may dissociate on an oxide surface such as M-O-M:NH3 f MOH + MNH2.8 Dissociation of NH3 on zeolites and alumina has not been observed in this work where the desorption temperature has never exceeded 400 °C, and it will be shown that NH3 in spite of its basic strength still differentiates between Bro¨nsted sites and even Lewis sites. The results of the study of surface Bro¨nsted acidity in some dealuminated zeolites by means of the rotational echo doubleresonance (REDOR) technique using the 1H (NH4+) f 29Si interaction have been reported elsewhere.9 Two types of bridging OH have been observed in ultrastable Y (USY) and dealuminated acid mordenites (DHM). These sites differ in their acid strength; this difference results in the different way the NH3 probe molecule is absorbed, namely, either as NH4+-OZ(complete proton transfer) or as a NH3‚‚‚HOZ (hydrogen-bonded ammonia). The sites were assigned to the OH groups bridging either aluminum to silicon in a 4Q(1Al) or in a 4Q(nAl), n > 1, that is, to silicon with one or more than one aluminum neighbor. Q1 is obtained from the contributions 4Q(1Al) to the 29Si MAS spectra calculated by the binomial probability distribution 4

Q1/Si ) 1/4 4Q(1Al)/

4 Q(nAl) ) R-1(1 - R-1)3 ∑ n)0

(1)

with R ) Si/FAl, where FAl stands for framework aluminum. Q1 is calculated either per gram or per unit cell, assuming no silicon is removed from the lattice. In order to assign the physical meaning to Q1, the following assumptions were made: (i) The Lowenstein rule is obeyed. (ii) The bridging OH groups that accompany every Al atom in the framework have equal probability to appear at any one of the four bridging oxygens. (iii) The Bro¨nsted sites are those that bridge Al with four Si or a silicon with only one Al neighbor, that is, of a 4Q(1Al) type. The latter assumption is clear in the case of isolated Al, when all four Si neighbors are the 4Q(1Al) type, but not as easy to © 1997 American Chemical Society

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understand in the cluster containing two aluminums.

This cluster is contributed by six Q1 silicons of 4Q(1Al) type and one silicon of 4Q(2Al) type. Thus, the Q1 value is 6/4 ) 1.5. On the other hand, there are two bridging OH groups. The probability (i) that both of them are Bro¨nsted sites (see assumption iii) is 9/16, (ii) that both of them are nonacidic is 1/16, and (iii) that one of them is nonacidic while the other one is acidic is 6/16. Hence, the expected number of Bro¨nsted sites in this cluster is 2 × 9/16 + 1 × 6/16 ) 1.5, that is, the same as the Q1 value. The same holds true for sets of identical clusters with higher Al contents such as 4Q(3Al). The existence of nonequivalent bridging OH groups in various zeolites was reported previously in numerous publications10-14 in which the Bro¨nsted sites were distinguished by either 1H NMR or IR spectroscopy. However, in this respect, several important questions remain open. For example, there is no direct evidence that different OH groups distinguished by 1H MAS NMR necessarily exhibit different acidic properties. The other important question concerns the relationship between the acidity of the Bro¨nsted sites and the distribution of framework Al atoms. It is common knowledge15,16 that the strongest Bro¨nsted sites are associated with isolated framework Al atoms (FAl1, that is, framework Al without other Al atoms in the second sphere). It has been shown that FAl1 can be easily calculated from Si/FAl and the connectivity16 using the following equation

FAl1/Si ) R-1(1 + R-1)-β

(2)

with β equal to 9 and 12 for the Y and ZSM-5 zeolites, respectively. It would be, of course, of high practical interest to relate Q1 and FAl1 to experimental spectroscopic data obtained from probe molecules at a quantitative level. By 27Al CP MAS from the protons of chemisorbed NH3 it had been shown elsewhere17 that two types of Lewis sites exist on transition aluminas or on the nonframework aluminas in calcined and/or steamed zeolites. Both are located on coordinately unsaturated aluminum, either tetrahedrally coordinated or in pentahedral coordination. The extent to which the Lewis sites contribute to the total zeolite acidity, independently from the Bro¨nsted sites or through a synergistic action with the Bro¨nsted sites, is also an open question. In this contribution the normal modes of vibration of chemisorbed NH3 (NH4+) and the measurement of the absolute number of the different chemisorbed species will be compared to the information obtained from the NMR studies. Experimental Section Materials. The preparation and characterization of the dealuminated acid zeolite Y (DHY), mordenite (DHM), and ZSM-5 (DHZ) have been published.6,18,19 Dealumination was

carried out by calcination in air at the temperature indicated after the symbol. For instance, DHY500 was an acid Y obtained from thoroughly ammonium-exchanged Y after calcination at 450 °C. Before being used for spectroscopic characterization, all samples were pretreated in situ at 475 °C for 2 h. They were kept in vacuum afterward. The alumina used in this work is the so-called superfive aluminas described in ref 17. The preparation of the DHZ samples for quantitative 1H MAS NMR is described hereafter. The ZSM-5 zeolite used for the REDOR and 1H MAS experiments reported here was synthesized without an organic template (US Patent 4,175,114). It was ion-exchanged by NH4Ac and then calcined under vacuum (10-4 Torr). The temperature was increased at the rate of 100 °C/h toward the target temperature (400, 600, 700, and 800 °C). The samples were held at the target temperature overnight. About 30-50 Torr of pure oxygen was introduced into the system at 500 °C for 30 min (except for the sample calcined at 400 °C, for which O2 was introduced at 400 °C for a longer time) to burn out the remaining organic impurities. Ammonia (98% enriched 15NH3 from Aldrich) adsorption was performed at 115 °C, and the sample was outgassed at 115 °C for 30 min. After treatment the sample was sealed in a 5 mm glass vial. The amount of the sample in the vial was carefully weighed with an accuracy of (1 mg. FTIR. For IR study autocoherent wafers weighing 16 mg/ cm2 were prepared by pressing the powder between stainless steel surfaces. They were introduced into a cell,6b allowing the recording of transmission IR spectra between -170 and 630 °C. The low temperature was used for CO adsorption FTIR. For the present FTIR study of chemisorbed ammonia the cell was used between room temperature and 600 °C. After chemisorption at 115 °C the wafer was outgassed for 30 min at 115 °C and then at increasing temperature for the same duration. A total of 400 scans were accumulated at room temperature at each step. The spectrum recorded for the sample treated in vacuum at 475 °C was subtracted, and the difference spectrum was studied between 1270 and 1700 cm-1 (Vide infra). NMR Technique. All 1H NMR experiments were performed under MAS conditions in a 11.7 T static field with a GN500 spectrometer. The acquisition parameters were the following: the pulse width was 4 µs, the relaxation delay was 10 s, the spinning rate was 10 kHz, and the number of acquisitions was 8 or 16. There was a considerable background signal coming from the spinner and the stator, from the Teflon film that held the vial inside the spinner, and from the paint that was used to detect the spinning rate. This background signal was accumulated separately under the same conditions and then subtracted from the experimental spectra. The spectra were then rescaled with respect to the weight of the samples. The rotational echo double-resonance (REDOR) experiments allowed the reintroduction of the 29Si-H heteronuclear dipolar interactions into MAS experiments in a controlled way. For a full description of a REDOR experiment and the adjustment and processing procedure, see ref 9. Vibrational Spectra of NH3 and NH4+ NH3 Modes of Vibration. Gaseous NH3 has four fundamental modes of vibration20 as predicted for a symmetrical pyramidal structure (point group C3V) (Table 1). Among the fundamentals, the totally symmetric ν2 is split into two bands at about 950 cm-1 (inversion doubling) and is very sensitive to the environment. Wilmshurst21 found an empirical correlation between the symmetrical deformation ν2 and the electronegativity of the metal to which ammonia is coordinated. Thus, it

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TABLE 1: Fundamentals of Gaseous NH3 According to G. Herzberg (Vacuum)a mode ν1 (a) ν2 (a1) ν3 (e) ν4 (e) ν1 (a1) ν2 (e) ν3 (f2) ν4 (f2)

infrared

} }

Raman

3334.2 (vs) 3335.9 vs, | 3337.5 931.58 s, | 934 (m) 968.08 964.3 3414 ⊥ 1627.5 vs, ⊥ Fundamentals of “Free” NH4+ (Td Symmetry) 3033 1685 1685 3134 3134 1397

}

a Notation: vs ) very strong; s ) strong; m ) medium. |, parallel, and ⊥, pependicular, to the C3 symmetry axis of NH3.

Figure 1. Bending modes spectral region of NH3 chemisorbed at 115 °C and remaining after 30 min outgassing at 175 °C: (a) 1, DHY500; 2, DHZ400; 3, DHZ800. (b) 1, DHM700; 2, DHM600; 3, DHM500.

might be expected that NH3 chemisorbed on Lewis sites should be sensitive to the electron-acceptor character of the aprotic center, the frequency ν2 increasing with the electron-acceptor power. Indeed, ν2 increases by ∼40 cm-1 as the desorption temperature increases from 50 to 350 °C on Al2O3.22 Tret’yakov and Filimonov23 systematically studied the variation of ν2 with the nature of the metal oxide and then compared it to the shift of the band near 1580 cm-1 observed for pyridine adsorbed on Lewis sites on halides. Both frequencies show the same trend. However, the frequency shift is much larger for NH3 than for pyridine. By contrast, the asymmetric bending (ν4) band of ammonia is almost insensitive to the environment. Coluccia et al.24 have observed it at 1620 cm-1 in NH3 coordinated to lattice O2- and at 1630 cm-1 in ammonia coordinated to Mg2+. The corresponding values of ν2 are at 1024, 1060, and 1080 cm-1, respectively. Being so insensitive to the environment, ν4 of physisorbed NH3 might overlap with the corresponding frequency of chemisorbed NH3.25 In order to avoid this difficulty, it is necessary to record spectra at temperatures where physisorption can be neglected. Figure 1 compares the NH4+ (and NH3) bending modes on different zeolites, while Figure 2 shows the spectral evolution with respect to temperature on USY. The ν2 mode is generally observable between 1300 and 1340 cm-1 when enough NFAl is present. An interesting comparison between the information obtained from CO and NH3 FTIR has been given by Sobalik et al.26 On a vanadia-alumina catalyst reduced at increasing temperature, a CO band between 2180 and 2190 cm-1 increases in intensity and is assigned to Lewis sites. The absorption of NH3 on the same sample shows three lines in the 1200-1500 cm-1 range: one at 1425 cm-1 characteristic of NH4+ groups and two others at 1240 and 1285 cm-1 due to NH3 held coordinately by two

Figure 2. Bending modes spectral region of NH3 remaining on USY after 30 min outgassing at 115, 175, 230, and 400 °C, from top to bottom.

Figure 3. (a) Variation of the integrated absorbance of 1680 cm-1 with respect to the integrated absorbance of the NH4+ (lf) deformation band at 1400 cm-1 for DHY and DHZ. (b) Variation of the intensity of the NH3 ν2 vs the intensity of the ν4 normal mode after outgassing between 50 and 420 °C. The outgassing time at each temperature is 30 min. Higher intensities correspond to lower outgassing temperatures.

kinds of Lewis acid sites. On DHM, CO chemisorption gives rise to two bands that were assigned to two types of Lewis sites.6b On DHY700, a CO band at 2225 cm-1 assigned to Lewis sites has also been observed.25 NH4+ Modes of Vibration. For the generally accepted Td symmetry, two vibrational modes, ν4 and ν3, are easily observed (Table 1). Both are triply degenerated. Figures 1 and 2 show the bending region where the most intense ν4 is near 1450 cm-1. ν2 is near 1685 cm-1.28 ν4 and ν3 are observed easily, for instance, in a clay without Si by Al substitution in the tetrahedral layer.29 A doubling of ν4 has been observed in clays with isomorphic (Si by Al) substitution in the tetrahedral layer29 or in NH4+16C6 or NH4+15C5 crown ether complexes absorbed in clays.30 Here the splitting of ν4 in two bands has been observed in USY and DHZ zeolites, but not in DHM. Similarly, two CO stretching bands attributable to CO interacting with acidic OH are observed27 when ν4(NH4)+ is split into two bands at ∼1450 and ∼1400 cm-1. The band at 1400 cm-1 being a shoulder of the band at 1450 cm-1 is not well-defined, and thus, the error on the integrated absorbance of the low-frequency (lf) band at 1400 cm-1 is higher than for the high-frequency (hf) bending. In addition, it has been observed that the intensity of NH4+ ν2 is larger in sample when the lf ν4 band is well-defined (Figure 3a). Thus, everything happens as if two NH4+ species with different symmetries were present in zeolites. One would be characterized by ν4 at ∼1450 cm-1 and would be the most abundant or

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the only one observed (DHM). The other one would be characterized by ν4 at ∼1400 cm-1 and ν2 at 1680 cm-1. The presence of two different species in DHZ will be supported later on by 1H MAS NMR. When only one band is present in the 1450 cm-1 region as in DHM, the extinction coefficient can be unambiguously obtained by measuring the increase of the peak amplitude for known added doses of NH3 to the acid zeolites as done by Datka et al.31 However, the overlap of the two ν4 components requires the deconvolution of the band, and then, of course, it is impossible to obtain the relevant specific absorbances. The stretching region where the ν3 mode(s) (split or not) are observable is crowded by the OH modes of the dealuminated zeolites. Hence, the use of another technique, such as 1H MAS NMR, is useful to confirm the assignment of the IR bands. Estimate of the Absolute Number of Sites NH3 at Lewis acid centers are revealed by the ν4 bending mode, but since this mode is common with that of the physisorbed NH3, it was important to observe how it decreases in intensity as the outgassing temperature increases. Since ν2 between ∼1250 cm-1 in alumina and ∼1330 cm-1 in DHY, for instance, is attributable only to NH3:L and not to physisorbed NH3, at the outgassing temperature where all physisorbed species are gone, singularity should be observed in the plot I(ν2) vs I(ν4). As shown in selected cases (Figure 3b), the ν2 intensity varies almost linearly with ν4, but when the outgassing temperature is lower than 115 °C, the plot of I(ν2) vs I(ν4) levels out, suggesting some contribution of physisorbed NH3. Unfortunately, because ν2 overlaps with lattice vibration Si-O mode below ∼1280 cm-1, its intensity cannot be measured consistently. The determination of the extinction coefficient in an adsorbed phase is difficult, especially when different adsorption sites exist. As far as NH4+ is concerned, thanks to a very careful work published by Datka et al.,31 the extinction coefficient for NH4+ ν4 at 1460 cm-1 in mordenite is known and equal to 0.147 ( 0.001 cm2/µmol, while it is 0.022 ( 0.0001 cm2/µmol-1 for chemisorbed NH3 ν2 at 1622 cm-1. In our study all of the spectra have been fitted with Gaussian (Peakfit program): one for ν4 (NH3) and two for ν4 (NH4+), in the cases where the shoulder between 1400 and ∼1430 cm-1 was observed. Plots of the integrated absorbance vs intensity (or peak amplitude) have allowed us to calculate the specific integrated absorbances for these two bands. In first approximation, the lf band of NH4+ at ∼1400 cm-1 has been assumed to have the same extinction coefficient as that measured by Datka et al.31 for the hf 1450 cm-1 band. In order to obtain the Bro¨nsted and Lewis sites’ populations, the thermal desorption function giving the integrated absorbance, as such or already converted in the number of chemisorbed species, with respect to the absolute temperature, is calculated; Vide infra. From these raw data, the coverage of each category of sites has to be computed. As far as the Bro¨nsted sites are concerned, the calculation is easy, for the experimental data can be easily fitted to a Langmuir desorption isobar,

n ) nsat

a exp(bT-1) 1 + a exp(bT-1)

(3)

nsat being the desired value. Figure 4a shows by a few examples that the fitting is excellent, especially for the hf component of NH4+ ν4. The lf component generally having less than onefifth the intensity of the hf component, the calculation is less

Figure 4. (a) Application of eq 3 to the variation of the integrated absorbance of NH4 ν4 bending at 1450 cm-1 (hf) and low frequency (lf) (1400 cm-1) with respect to 1/T. Solid line: best fitting the experimental integrated absorbance, transformed in number NH4+ per gram. DHZ400: hf, ]; lf, [; DHM500: steamed, f; USY: hf, 0; lf, 9. (b) Variation of the logarithm of the integrated absorbance of the NH3 ν4 at 1620 cm-1 vs 1/T. Application of eq 4. DHM500: steamed, f; DHM600, 9; DHZ800, ×; and DHY600, ].

accurate. It can also be seen that ammonium adducts are stable up to ∼180 °C. It has been shown elsewhere32 that the variation of the heat of chemisorption of ammonia with coverage on Bro¨nsted sites belongs to the Temkin type, while the variation of the heat of chemisorption of ammonia with coverage on aluminas or nonframework aluminas in zeolites obeys a Freundlich isotherm, characteristic of a heterogeneous surface. Hence, the measurement of the number of Lewis sites at saturation with NH3 is meaningless, for the logarithmic variation typical of a Freundlich isotherm is not compatible with the concept of saturation.

ln nads ) a + b/T

(4)

Hence, a temperature limit beyond which it is reasonable to assume that chemisorbed ammonia only subsists must be arbitrarily defined: 115 °C has been proposed27 because NH3 remaining after outgassing at this temperature has a differential heat of chemisorption larger than 100 kJ/mol-1, exceeding the physical adsorption energy and the energy of most hydrogen bonds. (In all cases, the outgassing time has been 30 min.) Thus, the linear regression between the logarithm of the number of ammonia molecules contributing to the band at 1625 cm-1 and 1/T has been calculated for each sample, and from the regression (Figure 4b) the amounts chemisorbed at 115 and 175 °C have been computed. The R2 of a four data point regression was generally larger than 0.95, and obtaining the so-called “number of Lewis sites” in this way averages out most of the experimental error. Two new 29Si-1H REDOR experiments were carried out with two of the ZSM-5 samples, especially prepared for the present study and precalcined at 400 and 600 °C, respectively, prior to the NH3 saturation. The precalcination yielded samples with no significant amount of nonframework aluminum (NFAl) (see Table 2). The evolutions of the REDOR fractions with time for both samples were very close to each other and did not differ much from the one reported in our previous publication.9 The experimental points can be easily fitted with one significant component (M2Si-H ≈ 0.5 kHz2) with weight of 40-60%. The rest of the REDOR response is due to a very weak dipolar coupling between silicons and remote proton-containing species; it is not possible to assign this “background” response to any particular site. On the basis of the second moment analysis,9 it is reasonable to assign the main REDOR component to the silicons coupled to the ammonia chemisorbed as NH4+. The

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TABLE 2: Total Bro1 nsted Sites (Brt), Strong Bro1 nsted Sites (Brst), Lewis Sites (L), Theoretical Total Bro1 nsted Sites (Q1), and Strong Bro1 nsted Sites (FAl1)a Si/FAl Si/Al 29Si (ca.) NMR NFA USY550 USY500 DHY500 DHY600 DHM500 DHM500 (steamed) DHM600 DHM700 DHZ400 DHZ400 DHZ600 DHZ700 DHZ800

2.6 6.9 2.6 5.9 2.55 3.3 2.55 4.56 5.2 9.8 5.2 11 5.2 5.2 14.9 15.1 14.9 15.1 15.1

10 15 18 17.5 19 37.1 52.7

L Q1 FAl1 Brt Brst

175 °C

115 °C

3.5 3.3 4.6 4.0 2.0 3.7

5.9 8.6 6.4 7.9 2.3 4.7

13.0 11 3.8 8.4 6.2 7.1

7.9 8.3 7.9 8.6 6.7 6.3

3.7 3.5 2.1 3.0 3.2 3.2

4.8 8.5 5.9 2.7 7.5 8.5

1 2.4 2.1 nm nm nm

6.4 9.3 ∼0 0.76 ∼0 3.5 4.4

6.6 5.1 4.5 4.5 4.3 2.4 1.8

3.2 3.1 2.8 2.8 2.7 1.9 1.5

5.4 5.0 4.8 5.0 5.3 2.3 1.7

nm 3.4 5.0 0.25 3.4 5.1 1.5 ∼0 ∼0 2.0 ∼0 ∼0 1.5 ∼0 ∼0 0.44 0.7 0.9 0.25 1.9 2.3

a

Si/Al ratio from chemical analysis and from 29Si NMR. All results in 1020 sites/g.

Figure 5. (a) Total number of Bro¨nsted sites obtained from the sum of the (hf and lf) ν4(NH4+) integrated absorbance Vs the calculated Q1. The slope of the dotted line is one. (b) Variation of the integrated proton lines (see text) and the number of Bro¨nsted sites in DHZ zeolites assuming the following correspondence: (0 and 9) line at 5 and 6.5 ppm; (] and 9) line at 3 and 6 ppm; (f) line at 2.6 ppm; (×) total Bro¨nsted.

relative weight of this component gradually decreases along the 29Si CP MAS NMR spectrum when going high field from -105 to -120 ppm, as was discussed earlier. The results of the measurement of the Lewis acid sites are in Table 2. The ratios of the number of Lewis to the number of NFAl could be considered as a measurement of the degree of dispersion of the Lewis sites; it fluctuates considerably. In Figure 5a it is shown that the number of sites corresponding to the sum of the hf and lf NH4+ bending band integrated intensities vs Q1 gives a direct proportionality relationship with a slope equal to one. The experimental absolute number of Bro¨nsted sites and the predicted theoretical number are in good agreement. Another set of experiments supporting the calculation of the absolute number of Bro¨nsted and Lewis sites from the IR data has been to record the proton NMR spectra in DHZ400, 600, 700, and 800 before and after NH3 chemisorption. The purpose of these experiments was to assign the deformation bands observed in the IR spectra after NH3 chemisorption to the corresponding acid centers. For this exercise, the dealuminated acid ZSM-5 was preferred to other zeolites because no Lewis sites were present in samples calcined below 700 °C. In addition, the amount of ammonia hydrogen-bonded to OH bridging a silicon having more than one Al neighbor is negligible

Figure 6. 1H NMR MAS spectra of ZSM-5 samples calcined at (a) 400, (b) 600, (c) 700, and (d) 800 °C. The bottom traces correspond to the spectra obtained for the samples immediately after calcination; the top traces are the spectra obtained for the same samples after ammonia chemisorption. The symbols correspond to the experimental spectra, and the lines correspond to the simulation. Separate lines obtained with the deconvolution procedure are also shown. For the line assignment see text.

according to the REDOR study. All of these characteristics stem from a relatively large (Si/Al)CA ratio in the starting material. In Figure 5b the integrated intensity of the proton lines before or after loading with chemisorbed ammonia is plotted against the number of sites corresponding to either the hf of lf NH4+ bending, to the NH3 bending (Lewis sites), and to the sum of hf and lf NH4+ bending. Note that the intensity of the proton line from NH4+ is divided by four and that of NH3 by three in order to be compared to the number of sites. In Figure 5b it is clear that the linear regression (R2 ) 0.87, standard error ) (1) fits all the results whatever the origin of the line. In Figure 6 the proton MAS NMR spectra of DHZ400, 600, 700, and 800 before and after loading with NH3 are compared in order to make apparent the relationship between the proton lines and the bending vibrations shown in Figures 3 and 4. Before chemisorbing NH3 five proton lines are observed at 1.5, 3.8, 5, 2.5, and 6.4 ppm, lines a, b, c, e, and f, respectively, in Figure 6c. In Figure 6d, the lines subsisting are at 1.4 ppm and two lines with very weak intensity at 5 and 6.4 ppm. After loading with ammonia, new lines are observed at 2.6, 6, and 6.5 ppm, lines D, B, and C in Figure 6c, respectively. The following assignment is suggested: line a at ∼1.5 ppm, H “impurity” in the NMR probe and terminal SiOH; line D at 2.6 ppm, NH3 on Lewis sites corresponding to the 1625 cm-1 ν4(NH3) band; line b at 3.8 ppm and line B at 6 ppm, OH or NH4+ corresponding to the ν4(NH4+) at 1450 cm-1 (hf bending band); line c at 5 ppm and line C at 6.5 ppm, OH or NH4+ corresponding to the ν4(NH4+) at 1400 cm-1 (lf bending band). A linear correlation must exist between the number of sites obtained from NH4+ and NH3 infrared spectra and the corresponding integrated proton lines, if this assignment is correct. The agreement shown in Figure 5b has several consequences. The first is that the ∼1400 cm-1 ν4 bending corresponds to species physically distinguishable from that with the 1450 cm-1 bending. The former band may be assigned to NH4+ on the strong Bro¨nsted sites corresponding to FAl1. Previously, it had been shown that the high-frequency integrated absorbance of CO stretch was proportional to the number of the so-called

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J. Phys. Chem. B, Vol. 101, No. 10, 1997 1829

Figure 7. Variation of Q1 and FAl1 (left) and of FAl and the nonacidic bridging OH (FAl1-Q1) (right) Vs Si/FAl. It is assumed that there is no reaction between bridging acidic or nonacidic OH and NFAl. The Si/Al ratio from chemical analysis are supposed to be 2.55, 5.2, and 15 for DHY, DHM, and DHZ, taken as examples.

TABLE 3: Comparison between the Number of Bro1 nsted Sites (Total) Lewis Sites Obtained from Ammonia FTIR at 115 and 175 °C and the Results of the ASTM Tests Carried out at 175 °C; Calculated Q1 Are Also Shown (All in 1020 sites/g) L DHZ400 DHZ700 DHZ800 USY DHY500 DHY600

ASTM

Brt

Q1

175 °C

115 °C

175 °C

115 °C

5.0 2.3 1.7 8.5 5.9 2.7

4.54 2.4 1.8 8.3 7.9 8.6

∼0 0.7 1.9 3.3 4.6 4.0

∼0 0.9 2.3 8.3 6.4 7.9

5.51 2.09 1.69 9.0 10.6 6.9

9.99 3.16 2.99 16.0 18.0 10.4

FAl1 or OH bridging silicon to an aluminum which does not have next-nearest-neighbor Al.25 The second consequence is that the sum of the integrated intensities of the 6 and 6.5 ppm band must be proportional to the number of sites corresponding to the overall (lf and hf) NH4+ band (total NH4+ in Figure 5b). The sum of the sites corresponding to the lf and hf bands must be Q1 as supported by Figure 5a. The assignment of the sites corresponding to lf ν4(NH4+) to FAl1 is reasonable but not supported quantitatively. Another appreciation of the results in Table 2 is possible when comparing them to those from the ASTM test,33 as done in Table 3. This test measures the total Bro¨nsted and Lewis acidity from NH3 remaining adsorbed at 175 °C. We have also measured the amount remaining at 115 °C. As indicated previously, the saturation of the Bro¨nsted sites is easily defined by eq 3 and adequately represented by the amount retained at ∼175 °C. The Lewis sites measured by the ammonia retained at 115 °C (after 30 min outgassing) comprise a collection of sites with a distribution of heats of chemisorption larger than 100 kJ mol-1. Table 3 compares these ASTM data with Q1 and L:NH3 for a selected number of samples, at the regrettable exception of the DHM samples which were no longer available. When Q1 is approximately equal to the ASTM (175 °C) test results, both values are close to the measurable amount of Bro¨nsted sites (Brt). In DHY600, Brt is noticeably smaller than Q1 and also smaller than the ASTM (175 °C) data. Discussion Figure 7 is a good starting point for the discussion. Assuming that the poisoning of the Bro¨nsted sites provoked by the reaction between the acidic bridging OH and an OH belonging to nonframework aluminum is negligible, the total amount of Bro¨nsted sites is Q1. Q1 goes to a maximum near Si/FAl = 4 and decreases monotonously afterward.

The number of Bro¨nsted sites is significantly lower than FAl for Si/FAl 1. These molecules should have a ν4 bending frequency at ∼1620 cm-1, e.g., the same frequency as the ammonia on the Lewis sites, or L:NH3. Thus, the L:NH3 contents shown in Table 2 must also contain some NH3 hydrogen-bonded to nonacidic (or weakly acidic) bridging OH. As said previously, if the low-frequency NH3 intensity were measurable, the concentration in L:NH3 would be measured easily, and the distinction between ammonia on nonacidic bridging OH and L:NH3 would be possible. The remaining possibility is to compare L:NH3 (125 °C) and L:NH3 (175 °C), as in Table 2. NH3 chemisorbed on Lewis sites is more energetically retained than hydrogen-bonded ammonia.32 Accordingly, in the low-temperature CO FTIR, the CO stretching band near 2220 cm-1, that is the most energetically retained species, was assigned to L:CO.6b Since the REDOR experiments were performed on samples outgassed at 115 °C after NH3 chemisorption, it could be suggested that the difference between L (175 °C) and L (125 °C) contains some hydrogenbonded NH3. It is a fact that this difference is the smallest for the DHZ samples and higher for HY, as expected from compositions and the number of clusters 4Q(nAl) with n > 1. In the absence of information about the distribution of differential heats of chemisorption, one cannot go further in the interpretation of the results. The ranking of the strength suggested from the ν2(NH3) frequency as well as the comparison between L:NH3 (115 °C) and L:NH3 (175 °C) in Table 3 seems to suggest stronger Lewis sites on DHM and DHZ than on USY. The comparison of the behavior of CO and NH3 toward Lewis sites leads to a curious observation. NH3 detects more Lewis sites on USY than on DHM, but these Lewis sites are probably weaker (lower ν2 frequency). The reason why CO fails to detect Lewis acids on the Y sieves should be explored. It might be that the polar and strongly adsorbed NH3 has access to sites that are not available to the weakly polar CO, for instance, those in small cavities. It has also been shown that, in dealuminated Y and ZSM-5 zeolites, NFAl is located in the pores available to CO, clogging a part of them. On the contrary, in DHM, NFAl is spread over the entire porous volume, leaving a larger fraction of the porous space available to reagents.18 As said previously, a CO stretch has been observed between 2173 and 2176 cm-1 and in DHZ and USY, but not in DHM. Accordingly, for the latter the line at 1400 cm-1 is not observable. In DHM the NH4+ ν4 deformation at 1450 cm-1 is perfectly symmetrical, and ν2 at 1680 cm-1 is not observable in spite of the fact that the FAl1 population is not very different from that observed in other zeolites with similar Si/FAl ratios, as shown in Figure 7. As shown by the 1H NMR study, it is safe to assign the sum of the (lf and hf) NH4+ bending bands to Q1, but lf NH4+ ν4 which is, in fact, a shoulder on the hf band is not accurately defined, and its equivalence to FAl1 could be questioned. By

1830 J. Phys. Chem. B, Vol. 101, No. 10, 1997 contrast, the proton line belonging to NH3 is not ambiguous at all. It is even the strongest line in Figure 6d. It corresponds to the 1626 cm-1 NH3 ν4. Jacobs et al.34 have studied the interaction of ammonia with Bro¨nsted sites by 1H MAS NMR in HY. Before NH3 chemisorption, the acidic proton lines are at 4.5 and 3.45 ppm, in addition to a line at about 2 ppm. When some dealumination occurs, an additional line is observed at 2.9 ppm which can be assigned to hydroxyls on NFAl. The lines at 4.7 and 3.95 ppm have been assigned to the OH located in the sodalite cage or pointing toward the supercage, respectively. After loading with ammonia, a line at 6.5 ppm is assigned to NH4+ in the sodalite cage whereas a line at 6.9 ppm would be due to NH4+ in the supercage. As expected, these assignments are different from those suggested here for DHZ, considering the structural and chemical differences. The line assignment suggested for DHZ cannot be extended blindly to other zeolites. Sample no. 3 of Jacobs et al.34 has a Si/FAl ratio of 2.8. In DHZ this ratio is 17.5. Amazingly, in HY with Si/FAl ratios of 5 and 12, Jacobs et al. did not observe the 1H (NH3) line, in spite of the fact that the amount of NFAl should be comparable with that in our DHZ800. As already mentioned, one uncertainty that affects the comparison of the number of Bro¨nsted sites calculated (Q1 or FAl1) and measured from the NH4+ infrared spectra is the extent of the reaction between acidic OH and basic OH on NFAl. This question obviously deserves further study. Conclusions The simultaneous use of IR and 1H MAS NMR of NH3 chemisorption on zeolites with Bro¨nsted and Lewis sites has shown that the absolute total Bro¨nsted acidity can be computed from the NH4+ bending modes. The result is in good agreement with the number of OH bridging Si to Al in a cluster with a single Al atom (Q1). Q1 is between 60 and 36%, the number of framework aluminum (FAl) in USY and DHY. It is between about 75% (FAl) in DHM and larger than 85% in DHZ. The population on the Lewis sites on the nonframework alumina is in a ratio L:NH3/NFAl between ∼75 and 40% for NH3 retained with an energy larger than 100 kJ/mol. NH3 as well as CO IR distinguishes two types of Bro¨nsted sites in DHZ and USY. CO distinguishes two types of Lewis sites on alumina and DHM. NH3 detects Lewis sites on all zeolites containing NFAl. Acknowledgment. This work has been made possible by DOE Grant DOE-FG02-90 ER1430.

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