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Heats of Adsorption of Ammonia on a Zeolite Catalyst and an Acid-Activated Clay Catalyst Determined by Flow Adsorption Microcalorimetry D. R. Brown† and A. J. Groszek*,‡ Department of Chemical and Biological Sciences, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K., and Microscal Limited, 79 Southern Row, London W10 5AL, U.K. Received July 9, 1999. In Final Form: February 1, 2000 Measurements of heats of adsorption of ammonia from a nitrogen carrier have been carried out on zeolite Y and K10 clay catalysts in the acid and Na+ forms. The equipment used was a new model of the Microscal flow calorimeter (FMC) linked to a thermal conductivity detector in which the rates of adsorption and desorption and the associated rates of heat evolution or absorption were measured simultaneously at atmospheric pressure and temperatures ranging from 150 to 207 °C. Ammonia was used as a probe interacting with the acid sites on the catalysts. The work revealed new information in surface heterogeneity of the catalysts existing under flow conditions at atmospheric pressure of a nitrogen carrier. It was found that the irreversibly adsorbed ammonia is mobile on both catalysts. Molar heats of adsorption recorded as surface coverage is increased show that the sites covered first are not necessarily those with the highest heats of adsorption, illustrating that this flow technique gives important information on the relative accessibilities of acid sites as well as their strengths.
Introduction Most workers with solid acid catalysts would agree that, currently, there is no single method for measuring the surface acidities of these materials that can be used to predict catalytic properties accurately. For this reason, several different methods are routinely used, both spectroscopic and thermochemical, each of which has particular advantages and disadvantages. Temperature-programmed desorption methods are widely used,1 but they only give comparisons between catalysts tested under identical conditions. More quantitative data can be obtained from base adsorption calorimetry, and this technique is probably accepted as the most reproducible laboratory-to-laboratory thermochemical method.2-5 In practice, however, most of the instrumental systems used for these adsorption experiments operate in static mode, so that base adsorption occurs over an extended period, and catalysts are held under vacuum prior to adsorption. There is a question over how acidities measured under these conditions relate to surface acidities under operational conditions. In the work reported here, a flow calorimetric adsorption technique, using a Microscal flow microcalorimeter (FMC), has been used to measure catalyst surface acidities. In this technique, the base is added to a flowing carrier gas stream and the catalyst adsorbs the base from the flowing carrier during a relatively short contact time. The advantage of this technique is that the composition of the carrier gas can, in principle, be chosen to mimic operational conditions for the catalyst. The humidity of † ‡
University of Huddersfield. Microscal Limited.
(1) Parillo, D. J.; Kokotailo, G. T.; Gorte, R. Appl. Catal. 1990, 67, 107. (2) Gorte, R. J.; White, D. Top. Catal. 1997, 3, 57. (3) Parillo, D. J.; Gorte, R. J. J. Phys. Chem. 1993, 97, 8786-8792. (4) Gervasini, A.; Auroux, A. J. Phys. Chem. 1993, 97, 2628-2639. (5) Auroux, A.; Muscas, M.; Coster, D. J.; Fripiat, J. J. Catal. Lett. 1994, 28, 179-186.
the gas stream could easily be controlled for example. Also, the effect of a typical reaction product, which might poison the catalyst, could be assessed. The other main difference from the static technique is that base is adsorbed under flow conditions. By controlling flow rates, it is possible to adjust the extent to which base adsorption is under kinetic rather than thermodynamic control, a factor which may be significant in operation where the accessibility as well as the strength of acid sites are both important. Parallel experiments in which the base is added to the carrier in separate pulses can also be performed, in which time is allowed between pulses for adsorption equilibrium to be reached, ensuring thermodynamic control similar to that seen in static experiments. The objective of this work is to compare the acidities of solid catalysts measured by the flow technique with reported acidities measured by static adsorption microcalorimetry and, in this way, establish the potential value of the flow technique in characterizing solid acids. Two solid acid catalysts have been used. Zeolite Y in the H+ and the Na+ forms has been chosen as an example of a well-defined solid catalyst. Calorimetric studies (static) of pyridine adsorption on H-Y and Na/H-Y6 have shown a well-defined distribution of acid site strengths in this material. Similar static experiments on adsorption of ammonia, pyridine, and isopropylamine on partially dealuminated H-Y7 showed acid strength distribution profiles which are significantly different but verify that pyridine tends to probe the same acid sites as does ammonia. The second solid acid has been the commercial acidactivated montmorillonite clay K10. This material is widely used as an acid catalyst and exhibits relatively strong acid sites.8 It differs from the zeolite in that it is highly inhomogeneous. It exhibits a broad pore size (6) Chen, D. T.; Sharma, S. B.; Filmonov, I.; Dumesic. J. A. Catal. Lett. 1992, 12, 201. (7) Parillo, D. J.; Gorte, R. J. J. Phys. Chem. 1993, 97, 8736. (8) Rhodes, C. N.; Brown, D. R. Thermochim. Acta 1997, 294, 33.
10.1021/la990897h CCC: $19.00 © 2000 American Chemical Society Published on Web 03/28/2000
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distribution9 and is expected to show a much wider distribution of acid strengths than the zeolite. An important new feature of the work in the FMC is the simultaneous determination of the rates of heat evolution and the rates of adsorption (transfer of NH3 from the N2/ NH3 mixture to the adsorbent surface). Comparison of these rates reveals that the heat evolution accompanying the adsorption process may proceed at very different rates from the rate at which NH3 is adsorbed on the catalyst surface. Comparison of the heat and adsorption profiles clearly indicates how accessible some of the strong acid sites are to NH3 and how the acidity varies with surface coverage in the presence of N2. This important kinetic information is facilitated by the relatively low time constant of the Microscal calorimeter. In the continuous adsorption/desorption method used by the authors, two adsorption/desorption cycles are carried out. Saturation is achieved in the first cycle, so the adsorption seen in the second cycle corresponds entirely to reversibly adsorbed ammonia. The irreversible adsorption is defined as adsorption which cannot be reversed when the gas flow reverts to a pure carrier gas. This is analogous to the application of vacuum to a sample in a static calorimeter. In this case, the definition of irreversibly adsorbed base is that which remains on the surface under these vacuum conditions. Subtraction of the heats and amounts of adsorption on the second cycle from the corresponding quantities determined in the first cycle gives the amounts and heats of irreversible adsorption. The irreversible effects can then be sliced into time segments corresponding to increasing degrees of surface saturation, and in this way, the differential heats of adsorption can be evaluated for a wide range of surface coverage by a base. Work of this type was recently described by one of the authors.10 Experimental Section Na+
Adsorbents. Zeolite Y was supplied in the form by Aldrich. The Si and Al contents were confirmed by elemental analysis (Butterworth Laboratories), giving a unit cell formula of Na56(Al56Si136O384)‚235H2O and an ion-exchange capacity of 3.52 mequiv g-1. The zeolite Y was used as supplied in the Na+ form. Samples were also prepared in the H+ form by repeated ion exchange with NH4+ followed by calcination at 450 °C. The procedure used was a standard method.6,11 These two catalysts are referred to as Na-Y and H-Y. K10 acid-activated clay was supplied by Sud-Chemie. It has been characterized as having a surface area (by nitrogen adsorption) of 279 m2g-1, a pore volume of 0.44 cm3g-1, and an average pore-diameter (by the BJH method) of 5.2 nm.12 This material is based on an aluminosilicate montmorillonite clay which has been treated with mineral acid. A significant proportion of the structural aluminum has been leached from the lattice, and the clay structure has been largely delaminated, exposing a much larger surface area than in the parent clay. The cation exchange capacity of dry K10 was determined (by Co2+ adsorption) to be 0.55 mequiv g-1 compared to 1.0 mequiv g-1 for the parent clay.13 The acid sites on K10 are generally accepted as being mainly associated with the exchangeable cations.14-16 It is possible that some additional defect sites, possibly coordinatively (9) Brown, D. R. Geol. Carpathia - Ser. Clays 1994, 45, 45-56. (10) Groszek, A. J.; Aharoni, C. Study of the Active Carbon-Water Interaction by Flow Adsorption Microcalorimetry. Langmuir 1999, 15, 5956-5960. (11) Corma, A.; Martinez, A.; Martinez, C. J. Catal. 1994, 146, 185. (12) Rhodes, C. N.; Brown, D. R. J. Chem. Soc., Faraday Trans. 1993, 89, 1387. (13) Rhodes, C. N.; Brown, D. R. Clay Miner. 1994, 29, 799. (14) Brown, D. R.; Rhodes, C. N. Catal. Lett. 1997, 45, 35. (15) Purnell, J. H.; Thomas, J. M.; Diddams, P.; Ballantine, J. A.; Jones, W. Catal. Lett. 1989, 2, 125. (16) Purnell, J. M.; Yun, L. Catal. Lett. 1993, 18, 235.
Figure 1. (a) Purging with nitrogen. (b) FMC cell schematic. unsaturated sites on new edges formed on de-alumination, may also exhibit some acidic properties, and these would not, of course, be lost on ion exchange with nonacidic cations. The H+ form of K10 is unstable and auto-transforms to the Al3+ form.17 It is generally accepted that the strongest Brønsted acid sites on K10 are associated with Al3+ cations, on thermal activation at 150 °C.17 In this work, the clay has been used in the as-received form (Al3+ in exchange sites) and following ion exchange with NaCl solution to convert to the Na+ form. Ion exchange by NaCl was performed twice using 1.5 M solution and stirring a 1% suspension of clay overnight. The clay was then washed free of Cl- (tested with AgNO3 solution) and dried. Double exchange using this concentration of exchange ions is known to exchange virtually all resident cations.18 These two catalysts are referred to as AlK10 and Na-K10. General Operational Procedures. All of the adsorbents had their surfaces purged with N2 at the temperatures at which the adsorption experiments were carried out: 150 °C for the K10 clay and 207 ( 2 °C for the zeolite Y. The purging was conducted for 20 h with the N2 flow fixed at 60 mL/h by flow-mass controllers. The same flow-rate was used for all of the adsorption experiments. The purging removed adsorbed water and probably contaminants such as CO2 and O2 as was clearly shown by negative heat effects due to the displacement of these gases and vapors by the carrier gas. The purging of the catalyst is accompanied by an endotherm which acts as useful indicator of the progress in the removal of the adsorbed impurities from the adsorbent. Figure 1a shows how such a purge progresses for zeolite H-Y at 208 °C. The adsorbent is initially placed in the adsorption cell, preheated to 208 °C, and left in the adsorption cell exposed to the atmosphere for about 30 min until thermal equilibrium is established. Nitrogen is then passed through the cell at the rate (17) Rhodes, C. N.; Brown, D. R. J. Chem. Soc., Faraday Trans. 1995, 91, 1031-1035. (18) Laudelout, H. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; Mineralogical Society: London, 1987.
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Figure 2. Rates of adsorption of NH3 on 23 mg of zeolite H-Y at 207 °C. of 1 mL/min at atmospheric pressure, and the heat absorption occurring during the process is recorded. As can be seen for zeolite H-Y, the heat absorption stops after about 2500 s, signaling the end of the purging process. The adsorbent is then left in the cell for 20 h under flowing nitrogen before the adsorption experiments are carried out. A Microscal Mark 4Vms flow adsorption microcalorimeter (model FMC-4110) was used to carry out the simultaneous determination of the rates of ammonia adsorption and the accompanying heats of adsorption, and was recently described in detail.19 A thermal conductivity detector was used to monitor NH3 concentration changes accompanying heat evolution and adsorption during adsorption/desorption processes. An important feature of the calorimeter is the small volume of the adsorption cell (170 µL) which, when completely filled, contains about 0.1 g of the adsorbent. For experiments with NH3, which produced very large amounts of heat during the displacement of N2, the adsorbents were “diluted” with quartz sand, the volume of the sand/catalyst mixture being adjusted so as to fill the adsorption cell completely. In the present work, about 20 mg of catalyst (weighed to 0.1 mg) was mixed with 220 mg of the quartz sand, and the mixture occupying the volume of the cell was sealed in the adsorption cell. A sketch of the cell filled with the adsorbent with a centrally placed calibration stem is shown in Figure 1b. Gases can be percolated through the cell with a very small pressure drop (less than 50 mbar), and the heat effects can be electrically calibrated in situ. Blank experiments were conducted with the cell filled with pure quartz sand. This material did produce very small heats of N2 displacement and NH3 adsorption, which were subtracted from the much larger heat effects produced by the preferential adsorption of NH3 by the catalysts. The blank experiments were also used to fix accurately the time at which the adsorption process commenced in the cell containing the adsorbent. This permitted an accurate determination of any retention times produced during adsorption and the total amount of adsorption that takes place during extended equilibration of the adsorbent with NH3. Furthermore, the blank experiments permitted matching of the heat evolution curves and the NH3 adsorption curves, resulting in accurate determinations of the integral and differential molar heats of adsorption as described previously.19 Continuous Adsorption and Desorption Experiments. A thermal conductivity detector (Valco Instruments Co. Inc.) was used to monitor continuously the concentration of NH3 in the carrier gas. The concentration profiles during adsorption and desorption were related to those obtained for the cell without adsorbent or filled with sand to evaluate the total amount of (19) Groszek, A. J. Advances in characterization of adsorbents by flow adsorption microcalorimetry. Stud. Surf. Sci. Catal. 1999, 120a, 143-175 (Adsorption and its applications in Industry and Environmental Protection - 1 Applications in industry; Da¸ browski, A., Ed.; Elsevier Science, Amsterdam).
adsorption. These procedures were recently described by one of the authors.19 Pulse Adsorption Experiments. Small amounts of NH3 were introduced into the calorimetric cell by switching off the flow of the N2 carrier and switching on the premixed 5% v/v NH3 in N2 for a fixed period of time. The switching valve was then returned to the original position and pure carrier gas passed again at a constant flow rate of 1 mL/min. The effluent from the cell was continuously monitored by the thermal conductivity detector to obtain an indication of the presence of any NH3 either unadsorbed during the brief contact between the mixture and the adsorbent in the cell, or that rapidly desorbed after the initial adsorption. Any desorption in this period was invariably associated with a negative heat effect. A lack of detector response indicated that the NH3 was irreversibly adsorbed. Reproducibility. Duplicate results were obtained for adsorptions on all of the catalysts examined in this work. The heat evolutions and the amounts of the adsorptions in all of the cases were reproducible to within (5%. This was for experiments with freshly loaded samples of catalyst mixed with quartz sand and purged as described above. The pulse experiments gave results which were reproducible to within (2%. In the saturation experiments, the profiles of heat evolution and uptake of NH3 were exactly reproduced, the heat being evolved at a relatively low rate initially compared with the adsorption rates as shown in Figures 2 and 3 below. The good reproducibility of the results suggests that for the adsorptions carried out on the catalyst-sand mixtures the channelling and dispersion effects did not play a significant role. In the second and subsequent adsorption cycles, there was, however, a noticeable difference between the amounts of adsorption and desorption, the latter being greater than the adsorptions. This could be due to displacement by N2 of small amounts of water released from the sites on which the adsorption of NH3 takes place as shown in eq 2.
Results and Discussion Typical results obtained in the flow microcalorimeter are shown in Figures 2 and 3 showing two adsorption/ desorption cycles of 5% v/v NH3 on zeolite H-Y catalyst at 207 °C and Al-K10 clay at 150 °C. In both Figures 2 and 3, the peaks record the rates of heat evolution and the corresponding rates of adsorption. The difference in the total heat evolution between the first and second cycles is taken as the heat of irreversible adsorption. The corresponding amount of irreversible NH3 adsorption is shown by the data in the second channel, the lower set of peaks, recording the total amount of NH3 uptake occurring during the percolation of the NH3/N2 mixture.
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Figure 3. Adsorption of 5% NH3 on Al-K10 clay at 150 °C for two adsorption/desorption cycles. Note both the difference in the rate of adsorption profiles between the first and second cycles and the prominent desorption peaks. Table 1. Integral Heats of Adsorption of NH3 - Two Adsorption/Desorption Cycles heat of adsorp.a
c
amt of adsorpb
sample
1st cycle
2nd cycle
1st cycle
2nd cycle
heat of irrev. adsorp.c
zeolite H-Y zeolite Na-Y Al-K10 clay Na-K10 clay
184.3 45.2 46.2 29.7
72.3 34.6 16.7 9.4
2.35 1.09 0.53 0.38
1.08 0.83 0.21 0.20
-88.20 -41.00 -92.00 -93.00
NH3 + H3O f NH4 + H2O
(2)
However, in the presence of very little water (and this is all there would be), the lattice acid site would only be partially dissociated, i.e.,
a In joules per gram of catalyst. b In mmole per gram of catalyst. In kilojoules per mole.
Table 1 gives the integral heats of adsorption of NH3 on catalysts zeolite Y and acid-activated K10 clay (H+/Al3+ and Na+ forms) at 207 °C and 150 °C, respectively. These integral heats combine reversible and irreversible adsorption. To obtain an estimate of the irreversible heats of adsorption, two adsorption/desorption cycles have been carried out, as shown in Figures 2 and 3. The adsorption recorded in the second cycle, which is reversible, is then subtracted from the adsorption amount recorded in the first cycle. This gives an estimate of irreversible adsorption (per gram of catalyst). The integral irreversible molar heats, obtained by dividing the irreversible heats and amounts of adsorption (in mmoles of NH3), are listed in the last column of Table 1. As can be seen, both the amounts and the heats of adsorption are lower for the zeolite Y catalyst in its Na+ form than those in its H+ form, but the molar heat for the K10 clay in the Na+ form is relatively high. This probably confirms the view that a significant proportion of the acid sites on K10 clay are due to lattice defects, which are not removed when the clay is ion-exchanged with Na+ ions. It is also evident that the differences in the adsorption between the first and second cycles are greater for the catalysts in their H+/Al3+ form than those in their Na+ form, confirming that there are more sites able to irreversibly adsorb NH3 on the H+/Al3+ forms of the catalysts than on the Na+ forms. The irreversibly adsorbed NH3 is assumed to occur on the Brønsted sites which are present in all of the catalysts studied in this work. Therefore, NH3 will be adsorbed as the ammonium ion. For zeolite Y-H, this would be
zeo-H + NH3 f zeo- .... NH4+
This goes without saying for a completely dry catalyst. If there is a small amount of strongly adsorbed water in the vicinity of the acid site then the process may be
(1)
zeo-H + H2O f zeo- + H3O
(3)
Therefore, even though ammonia would all react with H3O (by eq 2) the equilibrium in eq 3 would shift to the righthand side as the H3O is used up, and the enthalpy measured would reflect the strength of the zeo-H bond in just the same way as it would if the solid acid were completely dehydrated. From Figures 2 and 3, it can be seen that for all of the catalysts there is a marked difference in the rates and heats of adsorption between the first and second cycles, the latter being predominantly reversible. In the first cycle, the rates of heat evolution remain relatively low with a maximum value after 750 s of NH3 flow (flow of 5% v/v NH3 in N2), whereas the maximum rate for the second cycle is reached after about 250 s. The ascending heat evolution rates in the first cycle are relatively high initially then they slow before increasing again prior to reaching a peak value. After saturation of the most active sites, the rates decrease rapidly, reaching zero value after about 3000 s of NH3 flow. A similar situation is observed for the rates of adsorption which, however, after reaching a maximum value, remain steady for about 600 s before decreasing rapidly after saturation of the active sites. The clear differences between the rates of heat evolution and NH3 adsorption indicate that initially the displacements of N2 by NH3 produce relatively low molar heats of adsorption, but the molar heats evidently increase as the NH3 begins to access stronger or less accessible sites, possibly those in smaller micropores. No such differences exist for the desorption peaks and the second adsorption/ desorption peaks for which the interaction between NH3 and the surface is predominantly reversible. Differential molar heats of irreversible NH3 adsorption for H-Y and Al-K10 plotted against surface coverage are given in Figures 4 and 5. The plots are derived from
Heats of Adsorption of Ammonia on Catalysts
Figure 4. Differential heat of irreversible adsorption of NH3 on zeolite H-Y from 5% v/v NH3 in N2 at 207 °C obtained from two adsorption/desorption cycles.
Figure 5. Differential heats of irreversible adsorption of NH3 on Al-K10 clay from 5% v/v NH3 in N2 carrier at 150 °C obtained from two adsorption/desorption cycles.
the heat output and adsorption data shown in Figures 2 and 3 as follows. The coverage in each case was calculated by dividing the increasing amounts of irreversible adsorption, obtained by “slicing” the adsorption peaks into 1 min segments, by the total irreversible adsorption calculated from the values recorded on saturation with NH3 given in Table 1. The molar heat of adsorption for NH3 on each catalyst was similarly taken at 1 min intervals, for irreversibly adsorbed NH3. The differential molar heats of NH3 adsorption on H-Y shown in Figure 4 increase at first, reaching a maximum of 210 kJ mol-1 for surface coverages between 30 and 40% (350-450 µmol g-1), and then decrease to 50 kJ mol-1 for surface coverages exceeding 70%. Similar reductions were demonstrated by previous workers in static NH3 adsorption on an evacuated zeolite Y sample.6,7 This reduction in molar heat of adsorption at high surface coverage may be associated with relatively weak acid sites, as has been suggested for static experiments where NH3 is adsorbed essentially from a vacuum. However, in the case of flow techniques, it has to be realized that for both zeolite Y and K10 clay catalysts the last stages of adsorption may involve displacement of strongly adsorbed N2 from a proportion of micropores for which it has a relatively high affinity. The reduced heats of adsorption may be due to the difficulty NH3 has in the displacement of the adsorbed N2 from micropores having diameters less than 0.7 nm. Figure 5 shows the variation of the molar heats with coverage for a K10 clay sample. In this case, the heats
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follow a path different from that observed for zeolite Y, with the highest molar heats of adsorption obtained for coverages of about 60%. The heats then decrease and reach values lower than 50 kJ mol-1 at coverage values above 65%. The question of how mobile molecules are on the surface of their catalyst following adsorption has been addressed with experiments in which short pulses of NH3 were applied to the catalyst. Time was allowed after each pulse for heat evolution to die away completely before applying the following pulse. Pulse adsorption experiments on a K10 catalyst are illustrated in Figure 6. The first two pulses (of 7.3 and 5 µmol) produce positive heat peaks and no measurable desorption when the gas flow switches back to pure N2. A closer inspection of these adsorption peaks shows that after injection of NH3 is stopped, the heat evolution does not stop immediately, as it does during in situ electrical calibration in which the heat evolution stops when the calibrator is switched off. In contrast, the heat evolution caused by the adsorption of NH3 continues for some 400 s after the injection is stopped, indicating that the initially adsorbed probe, which does not leave the adsorbent bed (i.e., is irreversibly adsorbed) migrates with the generation of additional heat of N2 displacement. The third and fourth peaks in Figure 6 show distinct signs of desorption of the weakly adsorbed portion of NH3, the desorption-generating heat absorption. The molar heats of adsorption for the four pulses fall from 124 to 69 kJ mol-1, but after the fourth pulse, the adsorptions become increasingly reversible. The results indicate that the ability of the flow microcalorimeter used in this work to detect the small changes in the rates of heat evolution during irreversible adsorption and the subsequent NH3 migration is due to its relatively very low time constant compared with that of the microcalorimeter used previously under static conditions.6,7 Evidence for migration of adsorbed NH3 can also be seen for K10 clays in the continuous adsorption experiment in Figure 3. The first adsorption peak in Figure 3 produces rates of heat evolution reaching a maximum value in two stages followed by a rapid decrease after the adsorption is completed. The rate of NH3 adsorption (channel 2) gives a relatively steady value during the period of increasing rates of heat evolution and then falls rapidly together with the rate of heat evolution. The second adsorption peak is much smaller than the first peak, indicating the degree to which the adsorption is irreversible, which is much greater for K10 clay at 150 °C than that found for zeolite Y at 207 °C. It seems that, for both the K10 and zeolite Y catalysts, the changing values of the differential heats of adsorption as coverage increases, shown in Figures 4 and 5, are due to mobility of the initially adsorbed NH3 in the presence of N2, which leads to gradual occupation of the strongest acid sites by NH3 molecules as the adsorption process progresses. There is a marked difference in this respect between the results of static experiments and the present work carried out by continuous adsorption which indicates significant differences in the differential heats of irreversible NH3 adsorption with coverage in the range in which the statically obtained results remain constant around 150 kJ mol-1. It is possible, of course, that the difference is due to the different nature of the zeolite Y samples used in the present work and the work reported by Parillo and Gorte (who used a steamed form of zeolite H-Y with Si/Al ) 30). Further experimentation with the use of static and flow-through methods using identical catalyst samples would clarify this point. It is very
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Figure 6. Pulse adsorption of a series of injections of 5% v/v NH3 in N2 into an N2 carrier flowing through 70 mg of catalyst Al-K10 clay at 150 °C. Desorption commences after the third injection.
interesting, however, that in the continuous adsorption experiment for the adsorption of NH3, the heats of adsorption reached values which were significantly in excess of those reported by the above-mentioned authors, although their zeolite Y samples were heated to 500 °C before the static adsorption experiments were carried out. The samples used in the present work were merely purged with N2 at 207 °C before the preferential adsorption of NH3 was determined under flow conditions. Clearly further work in this area would be desirable. The flow-calorimetric results can be compared with those reported in the literature using static adsorption calorimetry on zeolite H-Y.6,7 The results of our experiment in which a series of NH3 pulses were added to the catalyst show a trend which broadly follows the results obtained in continuous adsorption as shown in Figure 5. The acid strength profiles in pulse experiments and those in which NH3 is added continuously (Figures 2 and 4) are sensitive to the accessibilities of acid sites, and the order in which sites are occupied appears to depend on site accessibility as well as strength. In the cases of both H-Y and Al-K10, it is evident that the first occupied, and therefore presumably the most accessible, sites are relatively weak compared to the bulk of acid sites (which, for H-Y, typically give a heat of neutralization with NH3 of about 150 kJ mol-1). Conclusions Simultaneous determination of the rates of heat evolution and adsorption of NH3 during the flow microcalorimetric experiments reveals new information on surface
heterogeneity of catalysts which exists under atmospheric pressures of nitrogen. An important advantage of the flow methods is that they determine not only acid strength but are sensitive to accessibility of the sites. The adsorption appears to occur initially on the most accessible sites which are not necessarily the most acidic sites in a catalyst. However, bearing in mind that the heats were those of displacement of N2 from the catalysts, it may be that the relatively low heats of adsorption at coverages below 20% may be due to the resistance to the displacement by the strongly adsorbed N2. The rates of heat evolution and irreversible adsorption of NH3 on both zeolite and acid-activated clay catalysts show that the process is kinetically controlled. The irreversibly adsorbed ammonia is mobile and in the presence of N2 continues to migrate to less accessible, but stronger surface sites, long after the supply of NH3 is cut off. The strength of acid sites obtained from the measurements of the heats of displacement of nitrogen by NH3 at atmospheric pressures are believed to be related more closely to the practical performance of catalyst used in industry than the acidities derived from the heats obtained on evacuated catalysts under static conditions. An additional advantage of flow-through calorimetry is that it can operate with carrier gases containing small amounts of water vapor and other compounds which might be present as products or poisons. LA990897H
