8786
J. Phys. Chem. 1993,97, 8786-8792
Characterization of Acidity in H-ZSM-5, H-ZSM-12, H-Mordenite, and H-Y Using Microcalorimetry D. J. Parrillo and R. J. Gorte' Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received: March 19, 1993; In Final Form: May 27, 1993
Adsorption enthalpies have been measured for ammonia, pyridine, and isopropylamine, as a function of coverage a t 480 K, on several high silica zeolites, including H-ZSM-5, H-ZSM-12, H-mordenite (H-M), and H-Y. Temperature-programmed desorption was used to determine the number of strong Br~rnstedacid sites in each material and estimate the mobilities. Except for isopropylamine in H-ZSM-5, each adsorbate exhibited a constant heat of adsorption up to the coverage of one molecule per Brmsted acid site, followed by a sharp drop in the measured heats. For isopropylamine in H-ZSM-5, the results were found to depend on Si/Al ratio and synthesis procedures, and it appears that hydrogen bonding between the adsorbate molecules affects the measured heats. For ammonia, the adsorption enthalpies were 150 kJ/mol for H-ZSM-5, H-ZSM-12, and H-Y and 160 kJ/mol for H-M. Adsorption enthalpies for pyridine were between -200 and 210 kJ/mol for H-ZSM-5, H-ZSM-12, and H-M but only 180 kJ/mol on H-Y. Heats of adsorption for isopropylamine tended to be slightly higher than that for pyridine on each sample but appeared to depend on site concentrations. The implications of these results to acid catalysis are discussed.
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Introduction It is well-known that the structure of a zeolite can strongly affect its catalytic activity.' These differences in activity are usually viewed as an indicationof the relative strength of the acid sites; however, it is also possible that variations in the catalytic activity with zeolite structure are due to factors other than the strength of the acid sites themselves. For example, if paraffin cracking rates are dependent on the concentration of species present in the pore? rates should depend on the physical adsorption isotherm, which in turn is known to depend on zeolite structure. The shape of the site could also affect rates through the configuration of the adsorption complex or transition state.4 One of the most straightforward methods for characterizing the strength of acid sites is microcal~rimetry.~-~~ The enthalpy of adsorption for simple bases is a direct measure of the strength of the acid sites and, in principle, can provide information on the distribution of strengths if the sites are heterogeneous. However, there can be difficulties with the measurements and their interpretation. First, the mobility of strong bases can be limited at lower temperatures so that equilibrium adsorption is not achieved upon exposure to a small dose of the base." Indeed, the equations describing adsorption are identical to those describing desorption in a porous material except for the boundary conditions,14J5 so that adsorption systems which show complex desorption curves will experience similar complexities in adsorption. Therefore,the temperature and samplecharacteristics used in calorimetry measurements must be carefully chosen to ensure sample equilibration. Second, factors in addition to the acidbase interactions may affect the measured adsorption enthalpies. For example, with alkylamine adsorption, van der Waals interactions with the alkyl group may add to the interaction of the amine functionalitywith the acid site.16J7 Differences in the heats observed on two different materials could be the result of differences in the van der Waals interactions. In order to better understand the effect of structure on the strength of an acid site, we investigated the adsorption of three simple bases on four zeolite structures using a combination of temperature-programmeddesorption-thermogravimetricanalysis (TPBTGA) and microcalorimetry. In order to avoid complexities associated with high alumina concentration, each of the materials we investigated had a Si/A12 ratio greater than 30, so that the A1 sites may be considered to be dilute.'* The zeolites 0022-3654/93/2091-8786%04.00/0
examined were H-ZSM-5, H-ZSM-I 2, H-M (mordenite), and H-Y, materials which have considerably different pore sizes and channel connections. TPD-TGA measurements were used to determine the number of adsorption sites and establish the experimentalconditionsnecessary for rapid mobility of the bases. Three bases, ammonia, pyridine, and isopropylamine, were used in the comparison in order to gain information on secondary interactions. The measurements show that the adsorption energies for each of the three bases on all of the zeolitm were approximately constant up to a coverage corresponding to one molecule per framework Al. Differences in the adsorption enthalpies were observed for the different structures; however, this observation depended on the probe used in making the measurements. For ammonia, the heat of adsorption was 150 kJ/mol for H-ZSM-5, H-ZSM12, and H-Y and was only slightly higher, -160 kJ/mol, on H-M. Larger differences between the zeolite samples were observed for pyridine and isopropylamine, but these could be due to secondary interactions rather than acid-base interactions. On the basis of the results, we suggest that the structure of the zeolite does not substantially affect the ability of the zeolite to donate a proton but that other factors are more responsible for the differences in catalytic activity which are found for zeolites.
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Experimental Section Data for each of the catalysts used in this study are summarized in Table I. The H-ZSM-S(A), H-ZSM-S(B) and H-ZSM-12 samples were prepared in our laboratory according to previously reported procedures. The synthesis was performed under hydrothermal conditions using NaOH (Fisher) as a base, Ludox (DuPont) as the silica source, NaAlOz (MCB), and either TPABr (Johnson Matthey) or MTEA-Cl (American Tokyo Kasei) as the templating agent. To place them in the hydrogen form, the samples were calcined in dry, flowing air for 2 h at 873 K, converted to the ammonium form by ion exchange in 1 M (NH4)2SO4(Sigma) at 363 K for 1 h, and activated in dry flowing air at 873 K for 1 h. A third ZSM-5 sample, H-ZSM-S(C), was provided by Alma and had been prepared in the Na form without a template. This sample was also placed into the hydrogen form by repeated ammonium ion exchange, followed by calcination. All of the samples appeared to be highly crystalline in X-ray diffraction and showed no anomalous peaks or amorphous halos. 19320
0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8787
Acidity in H-ZSM-5, H-ZSM-12, H-M, and H-Y TABLE I: Physical Properties of Samples Used in This Studv
[All (Irmollg) sample
bulk
[H+]"
H-ZSM-S(A) H-ZSM-5(B) H-ZSM-S(C) H-ZSM-12 H-Y H-M
470
360 180 600 120 160 800
620
555 1100
pore volume (cm)/g) 0 2
0.125 0.315 0.183
n-hexane 0.180 0.184 0.124 0.095
0.090
The Brcansted acid site concentration was determined from TPD-
TGA measurements with isopropylamine.
The pore volumes, measured by either n-hexane adsorption at room temperature and 1.5 Torr or 0 2 adsorption at 78 K and 64 Torr, were also in reasonable agreement with ideal pore volumes, except for ZSM-5(C). The H-Y sample was supplied by PQ corporation, and NH4-M was obtained from Conteka. Both samples had been steamed and leached in mineral acid to obtain high Si/Al ratios. Bulk analysis of the steamed materials was performed by atomic absorption spectroscopy, and the Si/Al ratios for the H-Y and H-M zeolites were found to be 30 (555 pmol/g of AI) and 15 (1100 pmol/g), respectively. Since it is difficult to remove extraframework A1 from a faujasite structure, the framework A1 content was estimated from XRD. The lattice parameter of the H-Y sample was determined to be 2.4269 nm using the region between 50" and 60' 28 and the (222) line of NaCl as an internal reference. From the correlation of Sohn et al., this corresponds to a framework A1 content of 290 pmollg.2' The H-Y and NH4-M samples were loaded directly into either the calorimeter or the TPD-TGA system without further pretreatment. The microcalorimeter used in these measurementswas a Calvettype instrument which was designed and constructed in our laboratory. Specific details about the instrument and the experimental procedures have been described earlier.16 The sample and reference cells of the calorimeter are Pyrex cubes, 2.5 cm in diameter. When placed in the calorimeter, these cubes are surrounded by square thermopiles at the bottom and four sides for measuring heat flow. Because of the large size of the cubes, a typical, 1-g sample results in a thin bed, 1 mm in height, so that adsorption is rapid when the sample is exposed to a gas dose at -480 K. To perform an experiment, the zeolites were first pressed into wafers, loaded into the Pyrex sample cell, and heated to 750 K in Torr for 6 h. The cells were then placed in the calorimeter, and the entire instrument was equilibrated at 480 K overnight. Gas doses of 10-20 pmol were exposed to the sample, and the signal from the thermophiles was monitored using a microcomputer until the signal returned to its original base line. This signal was then integrated with respect to time to determine the heat that was evolved. The fraction of each dose actually adsorbing in the zeolite could be calculated from the pressure above the sample after each adsorption run. Calibration of the thermopiles was performed by passing a known current through a platinum resistor placed under the sample and monitoring the resulting base line shift. The performance of the calorimeter is shown in Figure 1,which gives the raw data obtained for pulses of pyridine on H-ZSM5(A) at low and high initial coverages. Signals such as these were integrated in order toobtain the heats reported in this paper. Several features of this data are notable. First, the time required for making the measurements is relatively short. Required integration times were typically 10-20 min in order for the signal to return to base line, even for the slowest processes. Second, there was no base line shift in the data following an adsorption experiment. The base lines always returned to their originalvalue, within our ability to measure them. Third, the pulse width was dependent on the mobility of the molecules. In Figure la, which was obtained after the sample had a coverage greater than one N
0
300
600
900
1200
Time (s) Figure 1. Signal from the thermal flux meters, at (a) high and (b) low initial adsorbate coverages, following a pulse of pyridine onto H-ZSM5(A) at 480 K. The pulse sizes were 12.3 and 14.6 pmol, respectively, for the two signals.
molecule per Bronsted site, the signal is slightly narrower than the peak in Figure 1b, which was obtained on a sample having a low coverage of pyridine. The peak width at half-maximum is 106 s in (a) compared to 138 s in (b). This points out that molecular mobility in the sample is a limitation to rapid data collection. Indeed, if experiments had been performed at lower temperatures, integration would have been difficult due to the slow return of the signal to its base line. Rapid collection of evolved heat was an important design criterion in our instrument and is the major reason for choosing very thin sample beds and high adsorption temperatures. The TPD/TGA apparatus and procedures are also described in detail e1~ewhere.l~In this set of experiments, a 10-20-mg sample was placed in a Cahn microbalance, which could be evacuated to a base pressure of lo-' Torr. The samples were first heated under vacuum at 20 K/min to 750 K, exposed to adsorbates using their vapor pressure at room temperature, and then evacuated for -1 h. The TPD-TGA experiments were performed using a heating rate of 20 K/min, using a quadrupole mass spectrometer to monitor pressures above the sample.
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Results TPD-TGA. As discussed in the Introduction, it is very important to carry out calorimetrymeasurementsunder conditions which allow sufficient mobility for the molecules to migrate to the sites of interest. It is alsoimportant to know theconcentration of strong acid sites in each material so that the calorimetry results can be compared to this value. Because desorption of simple amines has been shown to be very useful for determining Brernsted acid site ~oncentrations,2*-~~ TPD-TGA curves for isopropylamine were obtained on each zeolite. The concentration of Brernsted acid sites on each sample was taken to be equal to the amount of isopropylamine which decomposed to propene and ammonia between 575 and 650 K, and these values are listed in Table I. Representative isopropylamine TPD-TGA curves are shown in Figure 2, a and b, for H-ZSM-12 and H-M samples,respectively. For high silica materials like H-ZSM-12, there was a clear demarcation between the weakly adsorbed isopropylamine ( m / e = 17,41, and 44) leaving the sample below 450 K and the propene ( m / e = 41) and ammonia ( m / e = 17) above 575 K. For materials like the H-M zeolite used in this study, which almost certainly had significant amounts of nonframeworkA1, a nontrivial amount of unreacted amine desorbed at higher temperatures, perhaps due to interactions with nonframework Al, and the concentration
Parrillo and Gorte
8788 The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 a)
P 0
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Figure 3. TPD-TGA of ammonia from H-M. b)
1000 800
so
E
600
400 200 0
300
400
500 600 T(K)
700
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Figure 2. TPD-TGA results for isopropylaminein (a) H-ZSM-12 and
(b) H-M. The mass spectrometersignals correspond to isopropylamine (m/e = 44,17, and 41), propene (m/e = 41), and ammonia ( m / e = 17).
of sites was less than the framework A1 content. Except for H-Y, however, theconcentrationofsiteswhichcausedthedecomposition of the amine approached the bulk A1 concentration. Possible reasons for the relatively large discrepancywith H-Y, 160pmol/g of acid sites versus 290 pmol/g of framework Al, have been discussed elsewhere.23.25 TPD-TGA measurements were also performed for ammonia and pyridineon selectedsamples in order to establish the conditions at which there was sufficient mobility for the samples to migrate to the strong Brernsted acid sites. Results for ammonia26 and pyridine19 on H-ZSM-5 have been published elsewhere. While TPD-TGA curyes for ammonia on the three H-ZSM-5 samples studied here differed in detail from those published earlier, the results were similar. For room-temperature adsorption, the coverage after 1-h evacuation was still significantly greater than
one molecule per framework Al. However,during the temperature ramp, ammonia desorbed in two separate features, with the second state corresponding to a coverage of one molecule per Brernsted acid site. All of the ammonia which was not associated with the A1 sites desorbed below -475 K. The peak temperature for the ammonia which was associated with the A1 ranged from 500 to >SO0 K, depending on the sample. This variation in the peak temperature can be easily explained by noting that the effective desorption rate from a porous sample is a function of particle size and site density, even when desorption is carried out in vacuum under otherwiseidentical conditions.14J5 The results do not imply that the sites in the three H-ZSM-5 samples have different strengths. The results for pyridine on H-ZSM-5 are similar to those for ammonia in that there is a well-defined desorption feature corresponding to a coverage of one per Brernsted acid site. Those molecules in excess of the the 1:l coverage desorbed below 450 K, while the 1:l state desorbed above 600 K. Due to its higher A1 content, TPD-TGA curves were also measured for ammonia and pyridine on H-M, with the results for ammonia given in Figure 3. Again, the feature at lower temperatures for ammonia can probably be disregarded, and the feature above 500 K is likely due to the same Brernsted acid sites observed with isopropylamine desorption. The coverage correspondingto the second ammonia features is -900 pmollg, which appears to beslightly higher than the Brernsted acidconcentration from isopropylamine; however, while it is possible that some sites which are accessible to ammonia are not accessible to isopropylamine, the calorimetry results to be discussed later do not show evidence for this. Also, site concentrations obtained from TGA measurements with ammonia are less reliable due to the lower molecular weight. We suggest that the adsorption sites probed by both molecules are the same. While the peak temperature for ammonia on H-M, -610 K, is significantly higher than that obtained on any of the H-ZSM-5 samples, this cannot be used to infer information on the strength of the sites. TPD-TGA results for pyridine on H-M showed only a gradual decline in mass, from -900 pmol/g after evacuation at room temperature, to730pmol/gat 500K. toacoverageof 150pmol/g at 800 K, the highest temperature we used. Again, it appears that the same sites are probed by pyridine as were probed by the other bases, and it is probably necessary to heat H-M to -500 Kin vacuum in order to achieve significantmobilities for pyridine to sample stronger sites.
Acidity in H-ZSM-5, H-ZSM-12, H-M, and H-Y
250
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8789
r-----l
250
s loo
I
i
A
c A
1
.
I
200 PO0 400 500 1" 9 Figure 4. Heats of adsorption as a function of coverage for ammonia (unfilled points) and pyridine (filled points) on H-ZSM-5(A) (squares) and H-ZSM-5(B) (circles).
400 600 800 pmol/g Figure 5. Heats of adsorption for isopropylamineon the three H-ZSM-5 samples: H-ZSM-5(A) (A),H-ZSM-5(B) (O),andH-ZSM-S(C) (U).
TABLE II: Summary of the Differential Adsorption Energies for Ammonia, Pyridine, and Isopropylamine on Each of the Zeolites Examined for Coverages below One/AP sample ammonia pvridine isopropylamine H-ZSM-S(A) 150 195 230-245 H-ZSM-S(B) 150 205 205-2 15 200-2 10 H-ZSM-5 (C) H-ZSM- 12 150 200 210
For H-ZSM-S(B), which has an acid site density of 180 pmol/g, the differential heats for ammonia and pyridine are 150 and 200 kJ/mol for coveragesbelow one per acid site. The slight difference in the heats of adsorption of pyridineon the two H-ZSM-5 samples is greater than the uncertainty of our measurements and may be caused by molecular interactions, which will be discussed in more detail later. However,theclose agreement between the differential adsorption enthalpies in the low-coverageregion strongly suggests that the sitesareequivalent instrength. Since thecatalyticactivity of H-ZSM-5 has been shown to increase linearly with A1 content for a number of reactions,Z7 the equivalence of the acid sites in H-ZSM-5 is not surprising. The difference between the heat of adsorption for ammonia and for pyridine, between 50 and 60 kJ/mol, is also a substantial fraction of the difference between the gas-phase, proton affinities of these two molecules, -62 kJ/ mo1,28 which implies that proton transfer is important in the adsorption of these two bases at the Br~nstedacid sites in H-ZSM5. Differentialheats of adsorption for isopropylamineon H-ZSM5(A), H-ZSM-S(B), and H-ZSM-S(C) are shown in Figure 5. The results for H-ZSM-S(A) exhibit an increasing heat with coverage, starting at 230 kJ/mol at zero coverage and extending to -245 kJ/mol. This data was reported in a previous paper, although no satisfactory reason could be given.16 On H-ZSMS(B), the heat of adsorption was again observed to increase with coverage, although the heats were all approximately 30 kJ/mol lower than that on H-ZSM-S(A). The adsorption enthalpies on H-ZSM-S(C) are much more constant with coverage and, again, noticeably lower than those on H-ZSM-S(A). We suggest that the main reason for the differences between the three samples is due to the proximity of the A1 sites in the samples. It has been shown that ZSM-5 samples prepared using a template crystallize with a high silica core, leaving a region at the boundary which may have Si/Al ratios on the order of Samples prepared without a template, such as H-ZSM-S(C), would presumably have a much more even A1 distribution. If we now assume that hydrogen bonding between isopropylamine molecules adsorbed at nearby sites could increase the adsorption energy, it is logical that the differential heats of adsorption should be highest on H-ZSM-S(A) and that the heats should increase with coverage. The adsorption energies should be lowest on the sample which was synthesizedwithout using a template, H-ZSM5(C). This sample should also show the lowest variation in the heat of adsorption with coverage. Similar interactions between adsorbed molecules may explain the 10 kJ/mol difference in the adsorptionenthalpies for pyridineon H-ZSM-S(A) and H-ZSM5tB). Finally, it should be noted that the proton affinity of isopropylamine is -4 kJ/mol less than that of pyridine.Z* If the heats were to scale linearly with proton affinity,16.17 the adsorption enthalpy for isopropylamine should be between 200 and 210 kJ/
100
0
~~~
~~
H-Y
150-135
185-175
195-1 85
H-M 160 200-180 220-2 15 a The differential enthalpies are reported in kJ/mol and are taken as average values from the plots which are shown as a function of coverage in Figures 4-8. Wherea range ofvalues are given, the heats wereobserved to vary from the initial value to the final value in going from a low coverageto a coverage approaching one/Al. The uncertainty in each of the measurements is approximately 5 kJ/mol.
Calorimetry. The adsorption enthalpiesfor ammonia, pyridine, and isopropylamine, measured at 480 K on each of the zeolites, are shown as a function of coveragein Figures &8 and summarized in Table 11. Before discussing the results in detail, it is important to notice that, with the exception of isopropylamineon H-ZSM5, the heats of adsorption for each of the molecules on each of the zeolites are approximately constant up to a coverage equal to the Bransted acid, site density determined from TPD-TGA and reported in Table I. Above this coverage the heats dropped sharply, and it became impossible to adsorb additional base without a substantial equilibrium pressure above the sample. It is important to comment on the apparent observation that the strength of the Br~nstedacid sites associated with framework A1 does not vary in a given zeolite. Since TPD-TGA results, carried out in high vacuum, demonstrate that the bases associated with the strong sites cannot desorb until significantly higher temperatures, at least for pyridine and isopropylamine,it may be that molecules sample the acid sites statistically and are unable to migrate to the strongest sites at 480 K. For example, if a pyridine pulse saturates the zeolite crystallites at the top of the sample bed before proceeding to the crystallites in the next layer, the observation of a constant heat of adsorption with coverage is simply proof that the calorimeter is giving consistent data. However, given that the heats of adsorption for ammonia are also constant with coverage,even though the measurementswere made at temperatures at which ammonia can desorb intovacuum, does suggest that the sites have identical adsorption energies in these high silica materials. H-ZSM-5. The heats of adsorption for ammonia and pyridine on H-ZSM-S(A) and H-ZSM-S(B) are shown in Figure 4 as a function of coverage. For H-ZSM-S(A), which has an acid site density of 360 pmol/g, the differential heats of adsorption for ammonia and pyridine were approximately constant at 150 and 210kJ/mol, respectively,forcoveragesbelowthe 1:l limit. Above this coverage, the differential heats drop sharply, as stated earlier.
0
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8790 The Journal of Physical Chemistry, Vo1. 97, No. 34, 1993
Parrillo and Gorte
200
E 150 \
Y 7
100 50
0
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pmol/g Heats of adsorotion for ammonia (0). pyridine . . . ._ isopropylamine (D) on H-ZSM-12.
( 0 ) .and
mol, close to the values observed on H-ZSM-S(C). However, it is also possible that interactions between the silica walls and the isopropyl group could contribute additional energy since the heat of adsorption for propane in silicalite has been measured to be 30 kJ/mol.go The relative importance of interactions of this type is yet to be determined. H-ZSM-12. The differential adsorption enthalpies for each of the three probe molecules in H-ZSM-12 are shown in Figure 6. The heats of adsorption are again almost constant up to a coverage of 120 bmol/g, which corresponds to one molecule per Bransted acid site, and then fall sharply at higher converages. The adsorption energies at low coverages are also very close to that observed on H-ZSM-S(B): 150 kJ/mol for ammonia, 200 kJ/mol for pyridine, and 215 kJ/mol for isopropylamine. These results suggest that the Bransted acid sites in H-ZSM-12 are virtually identical in strength to those in H-ZSM-5 in the dilute A1 regime. This agrees with the conclusions of Kofke et al., who measured TPD-TGA curves for 2-propanol in H-ZSM-5 and H-ZSM- 12.20 They reported that 2-propanol decomposed to propene and water at -405 K on both of these zeolites, while the decomposition occurred at higher temperatures on the weaker acid sites formed when Fe or Ga was incorporated into framework positions in the ZSM-5 structure.26.31 The similarity between the sites in H-ZSM-5 and H-ZSM-12 has important consequences given that the two materials have very different crystal structures. ZSM-12 consists of onedimensional, 12-membered rings, while ZSM-5 is made up of interconnecting, 10-membered rings. The fact that the sites in these two materials are so similar suggests that the bond angles and geometry do not play a major role in affecting the ability of the structure to donate a proton. H-Y. The enthalpies of adsorption as a function of coverage for all three bases in H-Y are given in Figure 7. Again, there is a drop in the heats near 160 pmol/g, the Bransted acid site density. For ammonia in the low-coverageregime, the enthalpies started at 150 kJ/mol and decreased to 135 kJ/mol at the 1:1 coverage. This slight decrease may be indicative of some heterogeneity in the sites, although the similarity in the heats with those measured on H-ZSM-5 and H-ZSM-12 suggests that the sites are very similar. The heats for pyridine and isopropylamine, -180 and 190 kJ/mol, are also slightly lower than that observed for the other zeolites. The reasons for the slightly lower heats observed with H-Y, particularly with pyridine and isopropylamine, areuncertain. One possibility is obviously that the sites in H-Y are weaker and less able to donate protons. The slight heterogeneity observed in the heats could be caused by the siting of nonframework AP2 or by segregation of the Al, if next-nearest-neighbor effects are imp~rtant.~S Alternatively, the ability of the sites to donate a proton may be identical, with other interactions which add to the adsorption energy being weaker. This would explain why the
-
-
0
200
100
50
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pmol/g Figure 7. Heats of adsorption for ammonia isopropylamine (D) on H-Y.
200
( O ) , pyridine
(a), and
I
250 1
- 150
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(&9”ooo
0 053,. 0
5 Y
.
0
“‘5
100
Qoo 0
200
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8 0 0 10001200
vmol/g Figure 8. Heats of adsorption for ammonia (0),pyridine (n), and isopropylamine (m) on H-mordenite. results for ammonia are so similar on all of the materials studied, while the heats for pyridine and isopropylamine are not. For example, it has already been shown that alkanes adsorb more strongly in ZSM-5 than in Y,3 probably because alkanes can interact more effectively with the cylindrical walls of ZSM-5. If dispersion forces between the siliceous walls of zeolite and the nonacidic component of pyridine and isopropylamine are important in setting the heats of adsorption, the heats in the more open cavities of H-Y should be lower than that found for H-ZSM-5 and H-ZSM-12. At the present time, it is not possible to resolve which of these two mechanisms explain the difference. H-M. Mordenite is normally considered to have substantially stronger acid sites than H-ZSM-5 or H-Y and is therefore of considerable interest for a comparison study.’ Like ZSM-12, the mordenite structure has one-dimensional, 12-membered-ring channels, but these are connected by eight-membered ring channels which can allow small molecules to pass from channel to channel. The calorimetry results for H-M are shown in Figure 8. Once again, the adsorption enthalpies for each of the probe moecules are approximately constant up to a coverage of one molecule per Bransted acid site, with differential heats of 160 kJ/mol for ammonia, 205-180 kJ/mol for pyridine, and -215 kJ/mol for isopropylamine. When these numbers are compared to the results for H-ZSM-5 and H-ZSM-12, one can see that the heats are very similar, with only the heat of adsorption for ammonia being substantiallyhigher. Furthermore, the agreement between our results and those of Chen et al. for ammonia and pyridine is quite good, giving support for the reliability of the data? The data obviously raises several questions. First, the heat of adsorption for ammonia is slightly higher on H-M than on H-ZSM-5, and the heat of adsorption for pyridine appears to be slightly lower. Since the coverageat which the heats drop sharply is almost the same for both bases, adsorption must occur at the
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Acidity in H-ZSM-5, H-ZSM-12, H-M, and H-Y
The Journal of Physical Chemistry, Vol. 97, NO.34, 1993 8791
same sites. Since simple proton-transfer bonding would predict that heats of adsorption for both bases would be higher on H-M, other factors are again influencing the heat of adsorption. One possibility for explainingthe apparently contradictory results for ammonia and pyridine is that most of the remaining sites on the dealuminatedH-M are in the eight-memberedrings. This might allow more effective coordination of the ammonia, particularly if hydrogen bonding of the ammonia with lattice oxygen is important.34 Pyridine, being too large to completely fit into the eight-membered rings, may exist with its end sticking into the larger pores. While this explanation is clearly speculative, the calorimetry data clearly does not justify the idea that the acid sites in H-M are stronger than that in H-ZSM-5. Second,the higher intrinsic catalytic activity usually observed in H-M can probably be explained in terms of effects other than intrinsic a ~ i d i t y . ~One possibility is that reactants can be accommodated by the eight-membered ring channels, leading to a “confinement” e f f e ~ t .This ~ confinement might maintain a high concentration of reactant near the Bransted acid site or stabilize the transition state. Much needs to be learned about the precise nature of reactions in zeolites before a firm conclusion can be reached. Finally, it should benoted that, since thedifferential adsorption enthalpies for ammonia on H-M and H-ZSM-5 are within 10% of each other, differences in desorption peak temperatures in TPD cannot be due to stronger adsorption in H-M. As pointed out earlier, desorption temperatures in porous materials are a strong function of diffusion and site density, so that desorption temperatures need to be interpreted with great care.
There are many reasons that various zeolite structures could exhibit such different activities, even if the proton affinities are similar. For some reactions, proton transfer may not be the ratelimitingstep. For example, it has beenshownthat H-D exchange for alkanes in H-Y occurs at significantly lower temperatures than that necessary for alkane cracking, suggesting that protonation of alkanes occurs more easily than hydrocarbon~racking.~ As pointed out earlier, changes in concentration of the reactants due to differences in the adsorption isotherms2 and confinement effects4could also affect the observed reaction rates. Which of these effects is most important has yet to be determined. Obviously, there is still much to be learned about zeolite acidity, and microcalorimetry is only one tool which can contributetoward our understanding. Besides those discussed in this manuscript, questionsconcerning the effect of A1 concentration,nonframework Al, hydroxyl defects, etc., on the strength of acid sites are still very much unanswered. Microcalorimetry should play an increasingly important role in addressing these issues.
Discussion It was recently reported that adsorption energies of bases like ammonia on acid sites in zeolites are not known with certainty due to the fact that a wide range of values have been reported in theliterature.35 A major reason for thisis that microcalorimetry measurements are very difficult to perform. There are always questions about whether one is actually probing the strong acid sites and whether factors other than simple acid-base interactions must be taken into considerationin interpreting the results. The uncertainty in obtaining heats of adsorptionhas been a significant problem for developing quantitative models of zeolite acidity and unifying themes for understanding acidity. We believe that the results in this paper provide reliable numbers for three reasons. First, the choice of materials with high silicaalumina ratios implies that the sites should be relatively isolated and noninteracting. Even with the materialswe examine, evidence for molecular interactions between adsorbed molecules was observed for isopropylamine in H-ZSM-5. Second, the combination of TPD-TGA measurements, in tandem with the calorimetry measurements,ensuresthat the strong Bransted acid sites are indeed the sites being probed. Third, the use of three separate probe molecules on each sample demonstrated that each molecule probed the same sites and showed that the heats of adsorption varied in a manner expected if proton transfer were important in the interaction between the bases and the acid sites. The observation that the sites in all of the materials investigated were very similar is in agreement with previous adsorptionstudies by Kofke et a1.m They had examined the adsorptionof 2-propanol on H-ZSM-5, H-ZSM-12, and H-M and found that the chemistryat the strong Bransted acid sites was virtually identical in each of the samples. 2-Propanol was found to decompose to propene and water at 405 K in TPD-TGA experiments. Furthermore,oligomerizationof propene and ethene in H-ZSM-5 was found to oligomerize under very similar conditions on H-ZSM-536 and H-Y.3’ Propene was found to oligomerize at the Bransted acid sites in both materials at room temperature, while it was necessary to heat both samples to -400 K for reaction of ethene. These experiments suggest that each of the zeolites has similar abilities to transfer a proton to the reactants.
Acknowledgement is made to the donors of the Petroleum Research Fund, administeredby the American ChemicalSociety, for the partial support of this research. This work was also supported by the NSF, Grant CBT-8720266. We thank the PQ Corporation and Alcoa for some of the samplesused in this study.
Conclusions The heats of adsorption for ammonia, pyridine, and isopropylamine in H-ZSM-5, H-ZSM-12, H-Y, and H-mordenite scale approximately with the gas-phase, proton affinities of the bases, although other factors such as van der Waals interactions appear to add to the measured heats. Surprisingly, the various structures exhibit almost the same adsorption energies, suggesting that the isolated, Bransted acid sites in high silica materials are almost identical.
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