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Langmuir 2006, 22, 7664-7671
Microcalorimetric Study of Ammonia Chemisorption on H3PW12O40 Supported onto Mesoporous Synthetic Carbons and SBA-15 Alexei Lapkin,*,† Claire Savill-Jowitt,‡ Karen Edler,§ and Robert Brown‡ Catalysis and Reaction Engineering Group, Department of Chemical Engineering, UniVersity of Bath, Bath BA2 7AY, U.K., Department of Chemical and Biological Sciences, UniVersity of Huddersfield, Huddersfield HD1 3DH, U.K., and Department of Chemistry, UniVersity of Bath, Bath BA2 7AY, U.K. ReceiVed April 29, 2006. In Final Form: July 5, 2006 Keggin tungstophosphoric acid supported onto mesoporous and meso-microporous materials was studied by ammonia microcalorimetry. Experiments were performed at high and low temperatures to separate the contributions of chemisorption and physisorption, low-temperature data being potentially important for the ambient temperature purification processes. Analysis of thermokinetic data obtained from heat output curves at an elevated temperature allows one to distinguish the transition between reaction of ammonia with the acid sites and the onset of physical sorption. Thermokinetic data are also sensitive to the pore structure of the support materials, with faster sorption registered in the case of materials with the open mesoporous structure. The SBA-15 supported acid shows the maximum ammonia uptake and the strongest acid sites among the three materials studied. The dynamics of sorption, characterized by the thermokinetic parameter, appears to be better in the two materials possessing open pore structure, mesoporous carbon Novacarb and especially the mesoporous silica SBA-15.
Introduction Heteropoly acids, and in particular Keggin-type compounds, have been a subject of a large number of studies and have found several commercial applications as acidic and redox catalysts.1 The acids by themselves are effectively nonporous and should, therefore, be either supported on porous solids, or transformed into microporous insoluble salts to obtain reasonably active heterogeneous catalysts.2 Among many catalyst support materials, the well-structured mesoporous solids, sometimes referred to as “mesoporous zeolites”, are particularly attractive for applications in catalysis due to their high surface areas, up to 1600 m2 g-1, and pore diameters, enabling high catalyst efficiency in gas- and liquid-phase reactions, that is, avoiding the internal diffusional mass transfer resistance characteristic of catalysts supported on microporous solids.3 The use of heteropoly acids supported on structured mesoporous solids has been reported in a variety of catalytic reactions, for example, alkylation, Diels-Alder, esterification, hydration, etc.2,4,5 However, reports of potential applications of solid acid materials in separation processes are scarce,6 and, to the best of our knowledge, the concept of developing accessible active sites in the ordered mesoporous support structures has not been studied directly. There are a number of potential applications for materials with high capacity and good accessibility of active sites, such as adsorption, reactive separation, and catalysis. To develop new chemisorption materials, it is necessary to fully characterize the strength and uniformity of active sites, which can be achieved by microcalorimetric measurements. * Corresponding author. Tel.: +44 1225 383369. Fax: +44 1225 385713. E-mail:
[email protected]. † Department of Chemical Engineering, University of Bath. ‡ University of Huddersfield. § Department of Chemistry, University of Bath. (1) Kozhevnikov, I. V. Catalysis by Polyoxometalates; Wiley: Chichester, 2002; Vol. 2. (2) Okuhara, T.; Mizuno, N.; Misono, M. Appl. Catal., A 2001, 222, 63-77. (3) On, D. T.; Desplantier-Giscard, D.; Danumah, C.; Kaliaguine, S. Appl. Catal., A 2001, 222, 299-357. (4) Kozhevnikov, I. V. Catal. ReV.-Sci. Eng. 1995, 37, 311-352. (5) Liu, Q.-Y.; Wang, W.-L.; Ren, X.-Q.; Wang, Y.-R. Microporous Mesoporous Mater. 2004, 76, 51-60. (6) Katsoulis, D. E. Chem. ReV. 1998, 98, 359-387.
Microcalorimetric studies of bulk heteropoly acids7,8 supported on Aerosil and sol-gel silica8,9 using ammonia and pyridine as probe molecules have been reported in the literature. These studies were performed at high temperature (423 K), because (i) only acidity at elevated temperatures is of any relevance in catalytic applications and (ii) physical adsorption, which complicates interpretation of the data, is largely avoided. However, the potential applications of the solid acid materials in separation processes are likely to operate at low temperatures, and the data on acidity and separation performance are not available. In this work, microcalorimetric studies of tungstophosphoric acid (H3PW12O40) supported on three mesoporous supports (synthetic mesoporous carbons Sibunit and Novacarb, and silica SBA-15), performed at 303 and 423 K, are reported. In our earlier work, tungstophosphoric acid was successfully supported onto SBA15 and Novacarb, and these materials were postulated to be the most promising candidates for potential chemisorption processes, for example, in removing molecular impurities in ultrapure manufacturing environments, due to their high sorption capacity and open pore structure.10 In this paper, the concept of achieving high utilization of active sites due to uniform deposition on the surface of structured mesoporous supports is studied in more detail using the static microcalorimetry method and the analysis of thermokinetic data. Microcalorimetry enables not only the characterization of the number and strength of acid/basic sites, but also can provide an insight into the morphology of porous supports, based on the measurements of probe molecules of different sizes and affinities.11-14 Detailed analysis of the adsorption and calorimetry data also enables us to obtain insights into the location of sites of different strength, even when the differences in the strengths are not very large.15 However, it should be stressed that (7) Lefebvre, F.; Liu-Cai, F. X.; Auroux, A. J. Mater. Chem. 1994, 4, 125131. (8) Kozhevnikova, E. F.; Kozhevnikov, I. V. J. Catal. 2004, 224, 164-169. (9) Lefebvre, F.; Jouguet, B.; Auroux, A. React. Kinet. Catal. Lett. 1994, 53, 297-302. (10) Lapkin, A.; Bozkaya, B.; Mays, T.; Borello, L.; Edler, K.; Crittenden, B. Catal. Today 2003, 81, 611-622. (11) Auroux, A. Top. Catal. 2002, 19, 205-213. (12) Guil, J. M.; Guil-Lopez, R.; Perdigon-Melon, J. A.; Corma, A. Microporous Mesoporous Mater. 1998, 22, 269-279.
10.1021/la061187w CCC: $33.50 © 2006 American Chemical Society Published on Web 08/03/2006
Study of Ammonia Chemisorption on H3PW12O40
interpretation of microcalorimetry data, and especially conclusions on the distribution of the strength of active sites, can be quite ambiguous due to the integral nature of the experimental method: the heat output data are the sum of several simultaneously occurring processes with significantly different rates. Thus, in the earlier literature on microcalorimetry, it was stressed that the decreasing differential heats of sorption with loading of a probe molecule are most often associated with nonequilibrium processes, rather than true inhomogeneity of the site binding energies.16 In more recent literature, there are several reports stressing the importance of the contribution of nonequilibrium processes and the presence of weaker sites to the maximum heat of sorption, and its dependence on the probe molecule loading.15,17,18 At the same time, the treatment of thermokinetic data, which can provide an insight into the rate processes occurring during the sorption/ reaction of a probe molecule and, thus, provide additional evidence for interpretation of the heat data, is very rare in the literature.7,17 The treatment of kinetic data during microcalorimetry experiments is considerably more developed in the area of reaction calorimetry, when deconvolution of the heat output curves as a function of reactants addition and reaction time using appropriate models allows one to obtain insights into the reaction mechanisms.19 The main difference between reaction calorimetry and sorption experiments is the absence of mass transfer, or at least a close approximation to the condition of pure kinetic control, in the former, which enables a relatively simple mathematical treatment of the deconvolution. Quantitative treatment of the heat output curves in the case of sorption, especially for complex materials, would be significantly more challenging. In this study, we report that the kinetic heat output data show quite distinctive qualitative differences between sorption of ammonia onto mesoporous materials containing and free of micropores, and also provide an insight into the accessibility of the acid sites and the onset of physical sorption from the shape of the characteristic heat output time versus probe molecule loading. Experimental Section Samples of mesoporous synthetic carbon Novacarb were kindly provided by MAST Carbon Ltd. (U.K.) in the form of microspheres with diameters in the ranges 10-45 µm and 50-500 µm. Detailed information on the preparation and properties of Novacarb has been published elsewhere.20 Samples of Novacarb with the lower particle sizes were used as received. Particles with larger diameters were sieved out to obtain particle fraction 150-212 µm. The sieved fraction contains a small amount of agglomerated finer particles. To break up these agglomerates, the carbon sample was placed in water and treated ultrasonically for 1 h. Following ultrasonic treatment, the water contained some fine, slow settling particles. Fines were removed by decanting and repeated washing with demineralized water. Cleaned samples were dried in a vacuum oven at 373 K and sieved again. A sample of synthetic mesoporous carbon Sibunit was kindly provided by Boreskov Institute of Catalysis (Novosibirsk, Russia). Sibunit particles were ca. 2 mm in diameter. Detailed information on the synthesis and properties of Sibunit is available elsewhere.21 Both (13) Corma, A.; Chica, A.; Guil, J. M.; Llopis, F. J.; Mabilon, G.; PerdigonMelon, J. A.; Valencia, S. J. Catal. 2000, 189, 382-394. (14) Gervasini, A.; Auroux, A. J. Phys. Chem. 1993, 97, 2628-2639. (15) Drago, R. S.; Dias, S. C.; Torrealba, M.; Lima, L. D. J. Am. Chem. Soc. 1997, 119, 4444-4452. (16) Rideal, E. K.; Trapnell, B. M. W. Proc. R. Soc. London, Ser. A 1950, 205, 409-421. (17) O’Neil, M.; Phillips, J. J. Phys. Chem. 1987, 91, 2867-2874. (18) Parrillo, D. J.; Gorte, R. J. Thermochim. Acta 1998, 312, 125-132. (19) LeBlond, C.; Wang, J.; Larsen, R. D.; Orella, C. J.; Forman, A. L.; Landau, R. N.; Laquidara, J.; Sowa, J. R.; Blackmond, D. G.; Sun, Y.-K. Thermochim. Acta 1996, 289, 189 207. (20) Tennison, S. R. Appl. Catal., A 1998, 173, 289-311. (21) Fenelonov, V. B.; Likholobov, V. A.; Derevyankin, A. Y.; Mel’gunov, M. S. Catal. Today 1998, 42, 341-345.
Langmuir, Vol. 22, No. 18, 2006 7665 carbon samples were washed in water and ethanol and dried under vacuum at 473 K for 2-3 h prior to impregnation. Pure SBA-15 was prepared as described earlier.22 In a typical synthesis, 2 g of Pluronic (P123) copolymer (BASF) was dissolved in a mixture of 52.5 g of H2O and 12 g of HCl (36.5 wt %) at 313 K. Tetraethoxy silane (TEOS) was added dropwise, and the resultant mixture was stirred for 24 h. After being stirred, the mixture was poured into a polypropylene bottle and aged in an oven for 24 h at 363 K. Following aging, the mixture was filtered, washed with demineralized water, and dried under atmospheric conditions for 48 h. Finally, plain SBA-15 samples were calcined in air. The calcination temperature was increased from ambient to 773 K at 1 K/min and kept at that temperature for 6 h. Tungstophosphoric heteropoly acid (W-HPA) H3PW12O40 was obtained from Fisher and purified by recrystallization from demineralized water prior to impregnation. Supported acid was prepared by adsorption from ethanol-water solutions (1:1 volumetric ratio of demineralized water and 96 wt % ethanol) as described elsewhere.23 Gently stirred samples were equilibrated with solution for 72 h at ambient temperature. Sample were then filtered off and dried in air at 473 K for 2 h. The amount of acid adsorbed was evaluated using a gravimetric technique and was validated by the results of volumetric chemisorption of ammonia, assuming a stoichiometric ratio of ammonia to protons in the acid. In the standard gravimetric technique, the residual amount of acid was evaluated by weighing on a microbalance (with (0.01 mg accuracy) of dry residue from an aliquot brought to constant weight, and compared to blanks of solvent and the initial acid solution to yield the value of the sorbed acid. The atomic absorption method of analysis did not yield good results in the case of carbon supported materials due to the presence of a small amount of carbon dust in solutions, affecting the measurements, and, therefore, it was not used. A similar technique of characterizing the amount of sorbed heteropoly acid had been used in our previous work and was checked against an independent measurement by ammonia titration.10 The validation of the acid loadings measured by the gravimetric technique against the acid content calculated from the ammonia volumetric chemisorption data is warranted, because it has been shown earlier24 and in the recent microcalorimetric study25 that ammonia is sorbed stoichiometrically onto supported W-HPA when loading is above 3.6 wt % on a variety of supports. Because we used these results to validate the amount of supported acid, it was then unreasonable to back-calculate the amount of sorbed ammonia per Keggin unit. A list of prepared samples and characterization results is shown in Table 1. Adsorption microcalorimetry measurements were performed using a Tian-Calvet type Setaram C80 microcalorimeter linked to a volumetric gas handling system.26 In the experiments, ammonia pulse size was determined from a pressure transducer (MKS Baraton) connected to the manifold, whereas temporal adsorption data were recorded using a pressure transducer connected to the calorimeter cells. Samples were dried for 2 h under vacuum at 423 K prior to experiments at 423 K, and at 473 K prior to experiments at 303 K. The amount of chemisorbed ammonia was determined via readsorption experiments. The first adsorption cycle consisted of a number of pressure steps for which both heat and pressure changes were recorded. Samples were then evacuated for 2 h at the experimental temperature. This procedure was intended to remove all physically adsorbed ammonia. The readsorption in the second adsorption cycle, therefore, corresponds to physical adsorption only. The accuracy of the differential heats of sorption is estimated to be (5 kJ mol-1 for all samples apart from pure Novacarb measured at 423 K, for which the accuracy is (10 kJ mol-1 due to a very low (22) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. (23) Pizzio, L. R.; Caceres, C. V.; Blanco, M. N. Appl. Catal., A 1998, 167, 283-294. (24) Essayem, N.; Frety, R.; Goudurier, G.; Vedrine, J. C. J. Chem. Soc., Faraday Trans. 1997, 93, 3243-3248. (25) Newman, A. D.; Brown, D. R.; Siril, P.; Lee, A. F.; Wilson, K. Phys. Chem. Chem. Phys. 2006, 8, 2893-2902. (26) Hart, M.; Fuller, G.; Brown, D. R.; Park, C.; Keane, M. Catal. Lett. 2001, 72, 135.
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Table 1. Summary of Samples and Characterization Data sample H3PW12O40
HPA loading /wt % 100
SBET /m2 g-1
Vmesopore /cm3 g-1
Vmicropore /cm3 g-1
2.3
Novacarb
0
926
0.53
0.33
HPA-7/NC
6.7
369
0.34
0.12
Sibunit HPA-6/Sib
0 6.4
291 133
0.18 0.16
0.02 0.005
HPA-20/Sib
20
125
0.15
0.008
SBA HPA-38/SBA
0 38
729 143
0.63 0.11
0.09 0.004
uptake of ammonia. Both pressure pulse data and the constant baseline were used as indicators of reaching equilibrium after each pulse. Low-temperature nitrogen adsorption experiments were performed using a Coulter SA3100 volumetric system. All samples were outgassed for 2 h at 423 K. BET specific surface areas were calculated on the basis of the adsorption branch between the relative pressures 0.05-0.182. The C-values for BET isotherms varied between 80 and 232 for different samples. Mesopore volumes were calculated using the desorption branch according to the Barrett-JoynerHalenda (BJH) method. Micropore volume was calculated using the t-plot method. Scanning electron microscopy (SEM) images were obtained using a JEOL 6310 system. XRD spectra were recorded using a Bruker AXS-D8 Advance powder diffractometer.
Results and Discussion Chemisorption of Ammonia onto Bulk H3PW12O40 at 423 K. To obtain the baseline data, ammonia uptake onto the bulk unsupported tungstophosphoric acid was measured at both
Figure 1. Sorption of ammonia onto bulk H3PW12O40 at 423 and 303 K: (a) sorption isotherm; (b) differential heat of sorption as a function of ammonia uptake.
T/K 303 423 303 423 303 423 303 423 303 423 303 303 423
chemisorbed NH3 /mmol g-1 >0.8 0.8 0.013 0.235 0.067 0.114 0.064 0.26 0.20 0.45 0.38
-∆Hads0 at NH3 sorption of 0.05 mmol g-1/kJ mol-1 154 218 78 79 106 138 84 102 100 124 75 149 160
ambient and elevated temperatures; see Figure 1. The initial differential heat of ammonia sorption at 423 K (200-220 kJ mol-1) is very close to that measured earlier, following a similar sample pretreatment ensuring retention of protons in the Keggin anion.7,9 The readsorption experiment at 423 K shows that the uptake of ammonia is nearly all due to the reaction of ammonia with the acid, with only a small amount of physically sorbed ammonia registered. The maximum chemisorbed amount at 423 K corresponds to about 2.4 molecules of ammonia per Keggin unit, which is somewhat lower than the values 2.8-3.2 measured in the earlier microcalorimetry studies.8,9 The amount of physically sorbed ammonia at 303 K is only slightly higher than that at the elevated temperature; see readsorption data in Figure 1a. More significantly, the slope of the isotherm in the linear part is lower than that at 423 K. The shape of this part of the isotherm is likely to be affected by slow diffusion of ammonia into bulk acid crystallites. This is discussed in detail below, when thermokinetic data are analyzed. However, the lower heat of reaction observed at 303 K (about 150 kJ mol-1) can be attributed to (i) an appreciable contribution of physisorption and (ii) slow diffusion. A similar situation, when a slow diffusion coupled with the presence of weak sites produce lower differential heat of sorption, has been discussed in the literature.18 Adsorption of Ammonia onto Support Materials. Isotherms of ammonia adsorption onto the pure supports measured at 303 K are shown in Figure 2a. Uptake of ammonia on pure supports at high temperature is very small and is therefore not shown as isotherms. The differential heats of ammonia adsorption as a function of loading for SBA-15 and Novacarb are shown in Figure 2b (measurement of heat of adsorption in the case of Sibunit was not feasible due to large experimental errors in determining the heat flux). The sorption capacity of Sibunit appears to be considerably lower than that of Novacarb. This is consistent with the previous measurements, in which equilibrium amount of ammonia adsorption measured at 2.13 kPa ammonia and ambient temperature was 0.22 mmol g-1 for Novacarb and 0.05 mmol g-1 for Sibunit.10 This is likely to be due to the different surface chemistries of the two carbon materials: the Novacarb sample was activated in CO2 during preparation, which produces weak surface acid groups and increases the micropore volume of the carbon.20,27 According to the detailed study of acidic groups on the surface of different Novacarb materials, the CO2 activated material contains some lactonic or phenolic oxygen groups, which may be responsible for very weak acidity, but (27) Pigamo, A.; Besson, M.; Blanc, B.; Gallezot, P.; Blackburn, A.; Kozynchenko, O.; Tennison, S.; Crezee, E.; Kapteijn, F. Carbon 2002, 40, 12671278.
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Figure 3. XRD pattern of SBA-15.
Figure 2. Adsorption of ammonia onto SBA-15, Novacarb, and Sibunit: (a) sorption isotherms at 303 K; (b) differential heat of sorption at 303 and 423 K onto Novacarb and SBA-15 at 303 K as functions of ammonia uptake.
does not contain stronger carboxylic groups.27 The presence of weak surface acid groups would be expected to result in a moderate increase in chemisorption of ammonia, whereas an increase in the volume of micropores could only result in an increase in the physical sorption capacity, which would reduce the overall heat of sorption. The observed differential enthalpy of sorption by Novacarb at 303 K in the first sorption steps is relatively high (ca. 76 kJ mol-1) in comparison with the latent heat of ammonia liquefaction (ca. 23.3 kJ mol-1), a rough value for physisorbed ammonia of ca. 25 kJ‚mol-1 used in the literature,28 or even an accepted maximum value of the enthalpy of ammonia physical adsorption of ca. 37.7 kJ mol-1.29 A similar value (ca. 79 kJ mol-1) for the differential enthalpy of sorption onto Novacarb at low surface coverage (first two pulses) was measured at the elevated temperature (423 K, see Figure 2b), when physical sorption is negligible. This suggests that the Novacarb support possesses a small number of weak acid sites, equivalent to ca. 0.013 mmol g-1 ammonia uptake or ca. 0.014 µmol m-2 surface coverage (see Table 1 for the surface area data). The temperature independence of the initial heat of sorption suggests that the sites with this energy are very easily accessible. The accessibility of (28) Vedrine, J. C.; Auroux, A.; Bolis, V.; Dejaifve, P.; Naccache, C.; Wierzchowski, P.; Derouane, E. G.; Nagy, J. B.; Gilson, J.-P.; Hooff, J. H. C. v.; Berg, J. P. v. d.; Wolthuizen, J. J. Catal. 1979, 59, 248-262. (29) Hayward, D. O.; Trapnell, B. M. W. Chemisorption, 2nd ed.; Butterworth: London, 1964; p 323.
sites in different types of materials is discussed in more detail in the thermokinetic data analysis section. SBA-15 contains a small amount of micropores, which are affected by the deposition of the tungstophosphoric acid (see Table 1). However, this is a much smaller effect than in the case of Novacarb due to the small micropore volume of the silica: the most significant decrease in the pore volume upon supporting the acid is registered in the mesopores. The sorption capacity of SBA-15 at ambient temperature (Figure 2a) is considerably higher than that reported in the earlier literature (ca. 0.89 mmol g-1 at 2.13 kPa ammonia pressure)10 and considerably higher than the capacity of both studied carbon materials. The discrepancy with the earlier data is most likely due to variability between samples: good structure of the material synthesized in this study is exhibited by the high surface area ca. 729 m2 g-1 (see Table 1) and good hexagonal ordering, shown in the XRD pattern in Figure 3. The initial differential heat of sorption on SBA-15 at low loading is higher than that in the case of the Novacarb material (see Figure 2b), indicating stronger sorption interaction, but drops off quickly. There is no literature data on the microcalorimetry on SBA-15 either at high or at low temperatures. However, some comparison can be made with the studies of MCM and other ordered templated siliceous mesoporous materials. The differential heat of sorption in the first pulse is ca. 85 kJ mol-1 and decreases rapidly with further pulses. This value of sorption heat is associated with very weak acid sites, such as silanol hydrogen-bonding sites.15 However, in the case of purely siliceous MCM-41 material, the observed differential heat of sorption of ammonia at low surface coverages (up to 0.3 µmol m-2, 353 K) was only 40-50 kJ mol-1.30 These values of the apparent strength of native acid sites of pure SBA-15 support are of little interest for any practical applications, but, similar to Novacarb, the presence of weak acid sites native to the support influences the overall heat output upon sorption of a probe molecule on the acid impregnated materials. Chemisorption on Acid Impregnated Materials. Pore structure characterization data of the pure Novacarb support and the acid impregnated Novacarb are shown in Table 1. The parent material has very high surface area and contains both meso- and micropores. Both pore volumes are decreased upon impregnation with the tungstophosphoric acid, which indicates that at least part of the acid is contained within the micropores. The ammonia sorption behavior of this material, therefore, combines physical sorption in the micropores, sorption on weak acid sites of the (30) Meziani, M. J.; Zajac, J.; Jones, D. J.; Partyka, S.; Roziere, J.; Auroux, A. Langmuir 2000, 16, 2262-2268.
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Figure 4. Sorption of ammonia onto HPA impregnated Novacarb (HPA-7/Novacarb) at 303 and 423 K: (a) sorption isotherms; (b) differential heat of sorption as a function of ammonia uptake.
parent material, and sorption on strong acid sites, possibly located in micro- and mesopores. Figure 4a shows sorption and readsorption isotherms of ammonia onto tungstophosphoric acid impregnated Novacarb at low and elevated temperatures; Figure 4b shows the differential heats of sorption at both temperatures. The equilibrium ammonia uptake onto HPA impregnated Novacarb at ambient temperature is more than an order of magnitude higher than that on the pure support material. Based on the readsorption data, the amount of chemisorbed ammonia at 303 K is 0.235 mmol g-1, whereas at 423 K it is 0.067 mmol g-1. The significantly larger chemisorbed amount of ammonia at ambient temperature cannot be attributed to strong sorption in micropores or mass transfer effects (nonequilibrium effects), because the data obtained on pure Novacarb support (see Figure 2b) clearly indicate the absence of either. Therefore, acid sites of intermediate strength should exist in the sample. The initial heat of sorption is also considerably higher on the acid impregnated Novacarb as compared to the pure support, and there is a considerable difference between sorption behavior at the two temperatures. The initial differential heat of sorption at low temperature is about 120 kJ mol-1, whereas at high temperature it is about 160 kJ mol-1. Both values are lower than the enthalpies of chemical reaction of ammonia with the neat tungstophosphoric acid at the corresponding temperatures (200225 kJ mol-1 at 423 K, and 150 kJ mol-1 at 303 K; see Figure
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Figure 5. Sorption of ammonia onto HPA impregnated Sibunit at 303 and 423 K: (a) sorption isotherms; (b) differential heat of sorption as a function of ammonia uptake.
1b). It has been reported earlier that Keggin tungstophosphoric acid may interact strongly with the surface of carbon supports, resulting in lowering the acidity of the Keggin anion.31 At the same time, the infrared data show that Keggin anion retains its structure when supported onto Novacarb and Sibunit.10 Hence, both at elevated and at ambient temperatures the cumulative energy of sorption is combined from the interaction with the dispersed strong acid and the sorption onto weak acid sites of the carbon surface, identified by ammonia sorption on the pure support material. At ambient temperature, there is also an additional contribution of the physical sorption in the micropores, evidenced by the lower heat of sorption at higher ammonia coverages, approaching typical values of physisorption of ammonia. Figure 5a shows the uptake of ammonia on the acid impregnated Sibunit with two acid loadings at ambient and elevated temperatures, and Figure 5b shows the corresponding differential heats as a function of ammonia coverage. The low acid loading sample shows a behavior similar to the Novacarb supported acid sample with close value of the acid loading: a rapid decrease in the differential heat of sorption following the initial high value. However, the initial heat of sorption in the case of the 6 wt % loading Sibunit sample is significantly lower (ca. 113 kJ mol-1 (31) Timofeeva, M. N. Russ. Chem. Bull. Int. Ed. 2002, 51, 243-248.
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Figure 7. Definition of thermokinetic parameters: cooling half width at peak maximum (CHWM) and cooling time constant (τ).
Figure 6. Sorption of ammonia onto HPA impregnated SBA-15 at 303 and 423 K: (a) sorption isotherms; (b) differential heat of sorption as a function of ammonia uptake.
at 423 K) than the corresponding value in the case of the 7 wt % loading Novacarb sample (160 kJ mol-1). Similar to Novacarb, the chemisorbed amount on the HPA-6/Sibunit sample at ambient temperature is considerably higher than that at the elevated temperature (see Table 1). These data support the earlier conclusion that tungstophosphoric acid interacts strongly with the Sibunit type carbon, significantly reducing its acidity.31 Tungstophosphoric acid supported on Novacarb appears to retain more of its acidity and should therefore exhibit higher activity in Bro¨nsted-catalyzed reactions. The differential sorption heat curve measured on the higher loading sample of Sibunit at 423 K, when the contribution from physisorption is relatively small, exhibits the behavior expected from a sample containing a large number of accessible strong acid sites of a similar energy, with a characteristic plateau at low ammonia uptakes (see Figure 5b). Figure 6a shows the uptake of ammonia onto SBA-15 supported acid with a relatively high acid loading at ambient and elevated temperatures, and Figure 6b shows the corresponding differential heats as a function of ammonia uptake. The difference between the low- and the high-temperature sorption processes is most evident from the differential heats of sorption data (see Figure 6b): at 423 K, the differential heat remains constant until a certain uptake, and then drops off rapidly. A similar behavior is observed in the case of reaction of ammonia with pure tungstophosphoric acid, shown in Figure 1b. Also, the shape of the isotherm of ammonia uptake on HPA-38/SBA sample at
Figure 8. Comparison of thermokinetic parameters CWHM and τ for ammonia sorption onto HPA-20/Sibunit sample at 423 K.
423 K shows a very sharp change in the slope (see Figure 6a), coinciding with completion of reaction. No other material studied in this work had shown such well-defined behavior. The maximum observed differential heat of sorption at low ammonia coverages in the case of SBA-15 supported heteropoly acid measured at 423 K is about 165 kJ mol-1. This is lower than the value reported earlier (200 kJ mol-1) for this acid supported onto Aerosil silica material and measured at the same temperature,9 and lower than the value recorded for pure heteropoly acid (225-200 kJ mol-1); see Figure 1b. The decrease in the maximum differential heat of sorption is likely to be due to the strength of interaction between the support material and the heteropoly anion, because the plateau on the differential heat versus surface coverage plot at high temperature (see Figure 6b) suggests that the acid sites are distributed uniformly and have a similar strength. The observed behavior, that is, sharp change in the slope of the isotherm and in the slope of differential heat of sorption versus loading, excludes the possibility of site energy inhomogeneity or nonequilibrium effects. Analysis of Thermokinetic Data. Analysis of heat output curves (ballistic curves) recorded during the microcalorimetry experiments allows one to make at least qualitative judgments on the rates of processes occurring during the sorption of a test molecule. Thus, in the earlier studies the cooling time constant7 (τ) was used in the discussion of the diffusion of ammonia as a function of crystal structure of polyoxometalates, and normalized cooling width at half peak maximum17 (CWHM) was used to
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Figure 9. Comparison of differential heat of sorption and thermokinetic parameter (CWHM) at ambient (a,b,c) and elevated (d,e,f) temperatures.
identify the transition between physical and chemical sorption mechanisms of oxygen on acid activated carbons. The difference between the two parameters is shown in Figure 7. Calculation of the cooling time constant requires an assumption that the experimental cooling curve satisfies the first-order exponential decay model, given by ∆W ) W0 e-(t/τ), where τ is the thermokinetic parameter (≡time constant), W0 is the maximum power output, and ∆W is the power output at a given time t. This assumption is unlikely to be satisfied in the case of complex materials, in which several phenomena, such as diffusion into solid acid crystals, diffusion in micropores, and physical and chemical sorption, are occurring simultaneously. Therefore, parameter τ obtained from experimental data under the assumption of the first order behavior is, strictly speaking, incorrect. The CWHM parameter is not linked to a specific mathematical model. This parameter is most often used to characterize the width of
chromatographic peaks. However, as it is shown in Figure 8, the functional dependences of both parameters on ammonia uptake are practically identical. To obtain dimensionless values, both parameters can be normalized to the corresponding values relating to the cooling curves of a calorimeter obtained during calibration of the instrument (the characteristic time constant of the specific instrument used in this study is 30 s). In this work, we will use the not-normalized parameter CWHM due to its less ambiguous meaning. In the earlier microcalorimetric study, it was observed that the thermokinetic parameter (time constant, τ) is effectively independent of the increase in ammonia uptake in the case of pure tungstophosphoric acid.7 This was attributed to the formation of a microporous ammonia salt, which promotes access to the unreacted acid inside the crystalline grains. Therefore, the process of ammonia reaction with Keggin heteropoly acid is self-
Study of Ammonia Chemisorption on H3PW12O40
promoting and is accompanied by the gradual change in the pore structure. Figure 9d shows the differential heat of sorption and CWHM parameter as functions of ammonia uptake by pure tungstophosphoric acid at 423 K. The rate of sorption is nearly constant until 0.67 mmol g-1 and then increases (a decrease in the CWHM parameter or the time constant means an increase in the rate). The latter increase can be attributed to the slower diffusion into the bulk acid crystallites and thus can explain the lower than stoichiometric ratio of ammonia molecules per Keggin unit (2.4) obtained in this study. This hypothesis can be checked by measuring the rate of sorption on several acid samples with defined particle sizes. The corresponding data for pure tungstophosphoric acid at ambient temperature are shown in Figure 9a. The rate of sorption at ambient temperature is very slow until ammonia loading of 0.15 mmol g-1 and then increases gradually. The slow rate of sorption is consistent with the interpretation of the shape of the isotherm (Figure 1a) offered above. However, the increase in the rate of sorption also suggests that the contribution of physical sorption is significant even at very low ammonia loadings: an increase in the rate of sorption coinciding with the decrease in the differential heat of sorption has been attributed in the literature to the transition from chemical (a relatively slow process) to physical (a relatively fast process) sorption.17 In the case of lowtemperature sorption on tungstophosphoric acid, the slow initial rate of sorption is likely to be due to the slow diffusion of ammonia into the bulk crystalline acid at this temperature, because the observed rate of sorption at elevated temperature is considerably higher. On the basis of the rates of sorption data obtained with the unsupported acid, it should be expected that the rates of sorption onto supported materials at two different temperatures should exhibit marked differences. However, in the case of supported acid, the mass transfer limitation clearly observed at ambient temperature can be significantly reduced or even eliminated due to dispersion of the Keggin anions onto the high surface area supports, producing accessible sites. Figure 9 shows a comparison of differential heats of sorption and the thermokinetic parameter (CWHM) as functions of uptake of ammonia on the three types of chemisorbents according to the support material and temperature. Among the three supports, Novacarb is the only material possessing a significant volume of micropores. It is expected that in the case of the other two, preferentially mesoporous, materials, the dynamics of sorption should show a clearer transition from chemical to physisorption, especially at elevated temperature. The shape of the CWHM versus ammonia loading curves for all three supported materials obtained at the elevated temperature (Figure 9d-f) follows the same trend: an initial fast rate of sorption is followed by a gradual decrease in the rate, a minimum, and a gradual increase in the rate of sorption. A similar behavior was observed on a different acidic material, activated carbon, in the case of chemisorption of oxygen.17 The initial rate of sorption in all materials is only slightly lower than that in the case of bulk acid, suggesting that there is a small contribution of mass transfer. The minimum in the rate of sorption is most evident in the case of W-HPA supported onto Sibunit, Figure 9e. The relative position of the minimum is determined by the acid loading onto the support, with the higher acid loading shifting the minimum in the rate of sorption to higher ammonia surface coverages. The position of the minimum also correlates with the change in the slope of the differential heat curve, corresponding to completion of chemisorption and the onset of physical sorption. It is not entirely clear why this transition is manifested by a significant
Langmuir, Vol. 22, No. 18, 2006 7671
decrease and a minimum in the rate of sorption. It could be tentatively attributed to the increasing contribution of mass transfer: acid sites located closer to the surface of the sorbent particles react first. Contribution of physisorption becomes more significant as less accessible strong acid sites are being occupied. The slowest rate, and the largest difference between max and min rates, was observed in the case of the HPA-6/Sibunit sample. This is consistent with the offered interpretation, because Sibunit particles are considerably larger than these of both Novacarb and SBA-15. Furthermore, according to the earlier data, Sibunit also possesses a less open pore network,10 which could contribute to the slower rate of sorption. The shape of the CWHM versus ammonia loading curves at the ambient temperature (Figure 9a-c) strongly depends on the nature of the support material. In the case of HPA supported onto Novacarb, the rate of sorption and its trend are very similar to that of the bulk acid, whereas the rate of sorption on pure Novacarb is considerably faster (Figure 9a). This suggests that the acid is not well dispersed, and the rate of sorption is limited by the same mass transfer process as in the bulk acid. In the case of Sibunit and SBA-15, there are clear minima in the rates of sorption as functions of ammonia loading. The initial sorption rates are significantly faster than that on bulk acid and close to the rates observed at elevated temperature on the same materials. This suggests that Keggin anions are dispersed much better on SBA15 and Sibunit. Between the three support materials, the uptake on the two carbons can be compared for the materials with low W-HPA loadings (Figure 9d,e): in the case of Novacarb the ammonia uptake is higher, the initial differential heat of sorption is higher, and the difference in the rates of sorption over the range of ammonia coverages is small. This leads to the conclusion that accessibility of acid sites in the HPA-Novacarb material is better in comparison to Sibunit. However, the low-temperature data also show that the dispersion of the acid on Novacarb is poor. The comparison between the two materials with a higher W-HPA loading (Sibunit and SBA-15; see Figure 9e,f) clearly shows the advantages of an open pore structure of the ordered silica material: the difference between max and min rate of sorption is considerably smaller in comparison with Sibunit, suggesting that most active sites are easily accessible.
Conclusions Bulk and supported onto the three different mesoporous materials Keggin tungstophosphoric acid were studied by static ammonia microcalorimetry. Among the three support materials, SBA-15 shows the most promising performance in both potential high- and low-temperature applications, due to (i) better retention of acid strength, (ii) good dispersion of acid, and (iii) good accessibility of the acid sites. The combination of analysis of the static microcalorimetry data and the thermokinetic data allows one to differentiate between chemical and physical sorption and the effect of mass transfer on the sorption, thus providing a significant insight into the mechanisms of sorption on complex materials, and should allow one to develop better models of distribution of active sites in complex functional porous materials. Acknowledgment. A.L. is grateful to Steve Tennison and MAST Carbon Ltd. for kindly providing samples of Novacarb, and to Boreskov Institute of Catalysis for kindly providing a sample of Sibunit. Assistance with preparation of several samples by Dr. B. Bozkaya and Mr. E. Duthie is appreciated. LA061187W
