Number and Strength of Surface Acidic Sites on Porous

Jan 11, 2000 - It is also possible to detect the contributions from Si(3Si,1Al) and Si(2Si,2Al) sites. Generally, the width of NMR spectra is characte...
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Langmuir 2000, 16, 2262-2268

Number and Strength of Surface Acidic Sites on Porous Aluminosilicates of the MCM-41 Type Inferred from a Combined Microcalorimetric and Adsorption Study M. J. Meziani, J. Zajac,* D. J. Jones, S. Partyka, and J. Rozie`re Laboratoire des Agre´ gats Mole´ culaires et Mate´ riaux Inorganiques, UPRESA 5072, Universite´ Montpellier 2, Place E. Bataillon, 34095 Montpellier Cedex 5, France

A. Auroux UPR 5401, 2 Av Albert Einstein, 69626 Villeurbanne Cedex, France Received June 29, 1999. In Final Form: October 29, 1999 A combined microcalorimetry and adsorption study has been used to characterize the surface acidity of two series of MCM-41 aluminosilicates (referred to as SiAlxCn, where x is the mole Si:Al ratio and n the chain length of the surfactant template). 29Si magic angle spinning NMR spectra of a selected sample (SiAl32C16) indicates the presence of siloxane groups, Si(OSi)4, and three types of silanol groups, that is, single (SiO)3-Si-OH, hydrogen-bonded (SiO)3-SiOH‚‚‚HO-Si-(SiO)3, and geminal (SiO)2-Si(OH)2. It is also possible to detect the contributions from Si(3Si,1Al) and Si(2Si,2Al) sites. Generally, the width of NMR spectra is characteristic of amorphous materials and suggests a large variety of Si-O-Si and SiO-Al bond angles and lengths. The 27Al MAS NMR study made on the hydrogen-exchanged SiAl32C14 sample (H+-SiAl32C14) and the one rich in aluminum (SiAl8C14) clearly points to the presence of extraframework six-coordinate Al in the calcined materials. The volumetric and calorimetric measurements of gas ammonia adsorption at 353 K were used to determine the number and strength of surface acidic sites. With the exception of H+-SiAl32C14 and SiAl8C14, all samples have low surface acidity. The density of acidic sites ranges between 0.04 and 0.09 µmol m-2, increasing a little with decreasing pore size. These sites are relatively homogeneous and weak. For H+-SiAl32C14, surface acidity is enhanced as a result of the formation of strongly acidic Bro¨nsted-type sites and extraframework aluminum. The density of acidic sites increases to 0.11 µmol m-2 and the fraction of strongly acidic sites is about 60%. In the case of the SiAl8C14 sample, the density of acidic sites reaches a value of 0.3 µmol m-2. These sites are very heterogeneous: the corresponding differential enthalpy of ammonia adsorption decreases from -160 to -80 kJ mol-1. Following the pyridine-TPD study on this sample, Lewis acid sites producing surface pyridine complexes constitute the strongest acidic sites.

Introduction The conversion efficiency and selectivity of catalysts for many chemical reactions depend on the nature of the active sites on a solid surface, on their number, and on their strength. In this view, the evaluation of surface acidity or basicity is of great importance if the catalytic action of many solid materials is to be understood. The number and strength of acidic surface sites may be evaluated by a variety of physicochemical methods usually based on amine, pyridine, and ammonia titrations or chemisorptions in the liquid or vapor phase with the use of potentiometric or enthalpimetric titration, spectroscopic investigations, temperature-programmed desorption, and gravimetric adsorption measurements.1-7 Each of these methods suffers from a number of drawbacks and the range of applicability is limited to a narrow class of solids. Comparing the surface acidic characteristics derived from different methods is no easy task because different physicochemical principles underlie the experimental procedures applied. * To whom correspondence should be addressed. E-mail: [email protected]. (1) Labus, S.; Podgorecka, A. Mater. Chem. 1978, 3, 113. (2) Boehm, H. P. Discuss. Faraday Soc. 1971, 52, 264. (3) Berteau, P.; Belmon B. Catal. Today 1989, 5, 121. (4) Kiviat, F. E.; Petrakis, L. J. Phys. Chem. 1973, 77, 1232. (5) Valyon, J.; Henker, M.; Wendlandt, K. P. React. Kinet. Catal. Lett. 1989, 38, 2. (6) Brown, D. R.; Rhodes, C. N. Thermochim. Acta 1997, 294, 33. (7) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371.

The surface acidic properties of mesoporous silicates and aluminosilicates with a hexagonal porous structure have received considerable attention in recent years. In an effort to increase Bro¨nsted/Lewis surface acidity of such solids, numerous attempts have been made at replacing some silicon atoms by trivalent elements (e.g., Al, Ga, Tl, etc.) in the solid matrix.8-15 The experimental methods used for surface acidity testing include thermogravimetry of cyclohexylamine,16 IR spectroscopy of acetonitrile and pyridine preadsorbed on the solid surface,16-21 temperature-programmed desorption (TPD) (8) Yang, R. T.; Pinnavaia, T. J.; Li, W.; Zhang, W. J. Catal. 1997, 172, 488. (9) Tuel, A.; Gontier, S. Chem. Mater. 1996, 8, 114. (10) Mokaya, R.; Jones, W. Chem. Commun. 1996, 981. (11) Mokaya, R.; Jones, W. Chem. Commun. 1996, 983. (12) Mokaya, R.; Jones, W. J. Catal. 1997, 172, 211. (13) Kloetstra, K. R.; Van Bekkum, H.; Jansen, J. C. J. Chem. Soc., Chem. Commun. 1997, 2281. (14) Pinnavaia, T. J.; Zhang, W. In Mesoporous Molecular Sieves; Bonneviot, L., Be´land, F., Danumah, C., Giasson, S., Kaliaguine, K., Eds.; Elsevier: Baltimore, MD, 1998; p. 23. (15) Meziani, M. J.; Zajac, J.; Jones, D. J.; Rozie`re, J.; Partyka, S. Langmuir 1997, 13, 5409. (16) Mokaya, R.; Jones, W.; Zhaohua, L.; Alba, M. D.; Klinowski, J. Catal. Lett. 1996, 37, 113. (17) Busio, M.; Ja¨nchen, J.; Van Hooff, J. C. H. Microporous Mater. 1995, 5, 211. (18) Corma, A.; Fornes, V.; Navarro, M. T.; Perez-Pariente, J. J. Catal. 1994, 148, 569. (19) Reddy, K. R.; Araki, N.; Niwa, M. Chem. Lett. 1997, 7, 637. (20) Liepold, A.; Roos, K.; Reschetilowski, W. Chem. Eng. Sci. 1996, 51, 3007.

10.1021/la990840v CCC: $19.00 © 2000 American Chemical Society Published on Web 01/11/2000

Porous Aluminosilicates of the MCM-41 Type

of ammonia,22 FTIR-TPD of ammonia, and microcalorimetric measurements of the enthalpy of ammonia adsorption.23 Various spectroscopic studies made on MCM-41 materials allowed Lewis and Bro¨nsted acid sites to be better distinguished from each other with respect to their strength and hydrothermal stability.24-28 For example, it was shown29-31 that the vibration band of ammonium ions localized on Bro¨nsted sites appeared at 1450 cm-1, while the absorbance bands at 1620 and 1300 cm-1 were assigned to bending modes of ammonia coordinated with Lewis sites. The adsorption of pyridine on the hydrogenexchanged sample at 423 K resulted in the formation of pyridinium ion species monitored at a frequency of 1545 cm-1 and in the bonding of a pyridine complex to Lewis acid sites vibrated at 1450 cm-1.32 Moreover, the initial enthalpy value for ammonia chemisorption was used as a test for distinguishing strong acid sites from weak ones: values less negative than -80 kJ mol-1 were taken to characterize weak acid sites.33-35 In most cases, new surface sites were generated by the incorporation of aluminum into an electrically neutral silica framework. The Bro¨nsted and Lewis acid sites in these materials were assigned to “framework” and “extraframework” aluminum, respectively.32 Among the various factors which could influence the surface acidity, the cation electronegativity determining the percentage of cation-oxygen bond ionicity in oxide-walled materials, the charge and ionic radius of the cation to be incorporated, and the solid crystallinity were commonly considered to be the most important.36,37 In a previous work,15 specific surface area, porous structure parameters, and surface polarity were determined as functions of pore size and aluminum content for two series of MCM-41-type aluminosilicates by measuring adsorption from gaseous and liquid phases. Then, the effect of these factors on the mechanism of cationic surfactant adsorption from aqueous solution was established. Fundamental adsorption studies on the same samples have been continued so as to obtain more detailed (21) Liepold, A.; Roos, K.; Reschetilowski, W.; Esculcas, A. P.; Rocha, J.; Philippou, A.; Anderson, M. W. J. Chem. Soc., Faraday Trans. 1996, 92, 4623. (22) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Microporous Mater. 1993, 2, 17. (23) Kosslick, H.; Landmesser, H.; Fricke, R. J. Chem. Soc., Faraday Trans. 1997, 93, 1849. (24) Viale, S.; Garrone, E.; Di Renzo, F.; Chiche, B.; Fajula, F. Stud. Surf. Sci. Catal. 1997, 105, 533. (25) Di Renzo, F.; Chiche, B.; Fajula, F.; Viale, S.; Garrone, E. Stud. Surf. Sci. Catal. 1997, 101, 851. (26) Landmesser, H.; Kosslick, H.; Storek, W.; Fricke, R. Solid State Ionics 1997, 101, 271. (27) Feng, X. B.; Lee, J. S.; Lee, J. W.; Lee, J. Y.; Wei, D.; Haller, G. L. Chem. Eng. J. 1996, 64, 255. (28) Reddy, K. M.; Song, C. In Mesoporous Molecular Sieves; Bonneviot, L., Be´land, F., Danumah, C., Giasson, S., Kaliaguine, S., Eds.; Elsevier: Baltimore, MD, 1998; p 291. (29) Uytterhoeven, J. B.; Christner, L. G.; Hall, W. K. J. Phys. Chem. 1965, 69, 2117. (30) Kiselev, A. V.; Lygin, V. Infrared Spectra of Surface Compounds; Wiley: New York, 1975. (31) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds; Wiley-Interscience: New York, 1986. (32) Weglarski, J.; Datka, J.; He, H.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1996, 92, 5161. (33) Vedrine, J. C.; Auroux, A.; Bolis, V.; Dejaifve, P.; Naccache, C.; Wierzchowski, P.; Derouane, E. G.; Nagy, J. B.; Gilson, J.-P.; Van Hooff, J. H. C.; Van den Berg, J. P.; Wolthuizen, J. J. Catal. 1979, 59, 248. (34) Klyachko, A. L.; Kapustin, G. I.; Brueva, T. R.; Rubinstein, A. M. Zeolites 1987, 7, 119. (35) Kapustin, G. J.; Brueva, T. R.; Klyachko, A. L.; Beran, S.; Wichterlova, B. Appl. Catal. 1988, 42, 239. (36) Gervasini, A.; Bellussi, G.; Fenyvesi, J.; Auroux, A. J. Phys. Chem. 1995, 99, 5117. (37) Huang, M.; Kaliaguine, S.; Auroux, A. J. Phys. Chem. 1995, 99, 9952.

Langmuir, Vol. 16, No. 5, 2000 2263 Table 1. Specific Surface Area and Pore Structure Parameters of the MCM-41 Materials Studied solid

SBET, m2 g-1

Vpore, cm3 g-1

dpore, nm

SiAl32C8 SiAl32C12 SiAl32C14 SiAl32C16 SiAl32C18 SiC14 SiAl8C14

600 772 881 690 591 946 900

0.22 0.50 0.54 0.56 0.57 0.74 0.60

1.8 2.2 2.7 3.2 3.7 2.7 2.7

information about the surface acidity characteristics. In the present paper, the results of the amount and enthalpy measurements of the NH3 gas adsorption at 353 K are reported. The surface density and strength of acidic sites on the MCM-41 materials studied are presented as functions of porous structure and Si:Al ratio in the solid matrix. The IR spectra of the pyridine adsorbed on the sample rich in aluminum are analyzed in addition to the 27 Al MAS NMR spectra to identify the nature of strongly acidic sites. The results obtained on a hydrogen-exchanged sample (free of surface sodium) are also included. Experimental Section Materials. Aluminosilicates mesophases were prepared following the method described by Chmelka.38 The synthesis route and successive treatments were detailed in the previous article.15 Two series of MCM-41 aluminosilicates were prepared with the use of alkyltrimethylammonium bromides with different chain lengths as surfactant templates and aluminum isopropoxide as a source of aluminum with different framework Si:Al ratios. These materials are referred to as SiAlxCn, where x is the mole Si:Al ratio and n the surfactant chain length. The SiC14 represents the purely siliceous material. To recall the most important surface characteristics of the calcined samples, specific surface area and pore structure parameters have been collected in Table 1. All these parameters were determined previously.15 An additional sample, designated H+-SiAl32C14, was obtained by repeated ion exchange of the calcined SiAl32C14 sample with a 1 M aqueous solution of NH4Cl and followed by the calcination at 450 °C for 12 h. Specific surface area and porous structure parameters have not been modified during this treatment. Ammonia purchased from Air Liquid (France; purity > 99.9%) was purified by successive freeze-thaw cycles after drying on sodium wires. NMR. The coordination of aluminum and silicon in calcined phases was examined by 27Al and 29Si MAS NMR spectroscopy with the use of a Bruker spectrometer. NH3 Gas Adsorption. The differential enthalpies of NH3 gas adsorption were determined using a heat flow microcalorimeter of the Tian-Calvet type (C80 from Setaram), with an on-line injection system permitting the introduction of successive small pulses of the reactive gas. After every adsorption step, the equilibrium pressure was measured by means of a Datametrics differential pressure gauge. Successive gas doses were sent onto the sample until a final equilibrium pressure of 0.6 Torr was attained. Prior to adsorption measurements, the samples were degassed overnight at 433 K. To diminish physisorption of ammonia on the solid surface, the adsorption temperature was maintained at 353 K. At the end of the first adsorption cycle, the sample was pumped at 353 K and the secondary adsorption cycle was then performed at the same temperature. Pyridine-TPD. FTIR spectra of the pyridine chemisorbed on the solid surface were measured using a Vector 22 FTIR spectrometer. The same amount (19 mg) of each sample was pressed into a self-supporting wafer with an 18-mm diameter. The wafers were calcined under O2 vacuum (10-5 Torr) at 673 (38) Janicke, M.; Kumar, D.; Stucky, G. D.; Chmelka, B. F. In Zeolites and Related Microporous Materials: State of the Art 1994; Studies in Surface Science and Catalysis 84; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Elsevier: Amsterdam, 1994; p 243.

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Figure 2. 29Si NMR spectra of SiAl32C16: (a) as-synthesized sample; (b) calcined sample. Figure 1. 27Al MAS NMR spectra of calcined SiAl8C14 (a) and H+-SiAl32C14 (b) samples. K for 2 h and subsequently exposed to pyridine vapor at ambient temperature. Finally, the pyridine-loaded wafers were pumped for 2 h at different temperatures ranging between 298 and 673 K.

Results and Discussion NMR Studies. The 27Al MAS NMR spectra of the assynthesized and calcined SiAl32C16 sample were described in detail in the previous article.15 The main observation was that only tetrahedral aluminum appeared in the MCM-41 framework after calcination. For comparative purposes, the NMR spectra obtained on the sample rich in aluminum, that is, SiAl8C14, and the hydrogen-exchanged one, that is, H+-SiAl32C14, are shown in Figure 1. The appearance of the resonance signal at 0 ppm originating from six-coordinate Al indicates that the proportion of framework aluminum removed during various treatment procedures becomes important. To gain an idea about the nature of silanol groups present in the mesostructure, MAS 29Si NMR study has been made. The NMR spectra for as-synthesized and calcined SiAl32C16 samples are shown in Figure 2, as an example characteristic of the materials studied. For an as-synthesized sample, 29Si NMR spectra have three resonance peaks at -109, -99, and -88 ppm, attributed to Si(4Si), Si(OSi)3(OH), and Si(OSi)2(OH)2 environments, respectively. Between the last two peaks, the former may contain contributions from Si(3Si, 1Al) sites and the latter from Si(2Si, 2Al) sites. For the calcined sample, only two resonance peaks at -98 and -91 ppm can be clearly distinguished. These resonances may be assigned to the contributions from [Si(OSi)3(OH) + Si(3Si, 1Al)] and [Si(OSi)2(OH)2 + Si(2Si, 2Al) + Si(2Si, 1Al)(OH)] sites, respectively. A significant increase in the intensity of the resonance that appeared at -98 ppm reveals a high proportion of the first type of sites.

The intensity of the signal at -109 ppm decreases very strongly after calcination. Nevertheless, the absence of Si(4Si) units in the material seems little probable. The sensitivity of the 29Si chemical shift to any modification of the environment of the silicon atom is a plausible explanation for this effect. It may be that the appearance of some resonances related to Al-containing groups causes line broadening of MAS NMR spectra and diminishes the intensity of the resonance at -109 ppm. Another factor is the amount of disorder in the amorphous pore walls of MCM-41 aluminosilicates, which may result in peak broadening. For example, a change in the Si-O bond length of 0.0001 nm gives rise to a change in the chemical shift of about 1 ppm.39 It can reasonably be argued that the excess silanol groups are forced to condense during calcination and this produces a high distortion of the SiO-Si and Si-O-Al bond angles and lengths. Such distorted structures seem to be the main cause for the less symmetric environment of the silicon atom manifested by the decreased intensity of peaks in 29Si MAS NMR spectra.22,39,40 NH3 Gas Adsorption. The combined equilibrium adsorption and calorimetric measurements of the gaseous ammonia adsorption on calcined MCM-41 samples at 353 K have been used to characterize the number and strength of surface acidic sites. Ammonia is a reliable basic probe molecule for surface acidity testing on account of its small cross-sectional area of 0.141 nm2 and suitable pKa 9.24.7 The effects of the pore size, Si:Al ratio, and nature of the exchangeable cation (Na+ or H+) on the surface acidity were quantified. Figure 3a,b shows the examples of NH3 adsorption isotherms obtained in two successive adsorption cycles for two arbitrary selected samples, SiAl32C18 and SiAl8C14. The difference between both adsorption values has also been plotted versus the equilibrium bulk pressure. (39) Hudson, M. J.; Workman, A. D. In Multifunctional Mesoporous Inorganic Solids; Sequeira, C. A. C., Hudson, M. J., Eds.; Kluwer: Amsterdam, 1993; p 347. (40) Luan, Z.; Cheng, C.-F.; Zhou, W.; Klinowski, J. J. Phys. Chem. 1995, 99, 1018.

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Figure 4. Volumetric isotherms of NH3 adsorption onto MCM41 materials prepared with different pore sizes: (9) SiAl32C8; ([) SiAl32C12; (b) SiAl32C18; (2) SiAl32C14. Table 2. Total and Irreversible Adsorption Contributions Obtained from Ammonia Adsorption Measurements on the MCM-41 Materials Studied at Bulk Pressure of 0.2 Torr and Temperature of 353 K

Figure 3. NH3 adsorption isotherms obtained in two successive adsorption cycles: (a) SiAl32C18; (b) SiAl8C14.

The first cycle isotherms of NH3 adsorption reveal an important difference between the two samples. For SiAl8C14, the total quantity of adsorption increases very quickly with increasing bulk pressure in the region of very low-pressure values and, consequently, the initial isotherm part is almost vertical. This indicates the presence of numerous acidic sites, which give strong acidbase interactions with ammonia molecules. The corresponding initial increase in the NH3 adsorption on SiAl32C18 is much more gradual; this sample should be characterized by a lower surface acidity. For higher pressure values, the rate of increasing adsorption on both samples diminishes and the isotherms become quasilinear. This linear tendency may be explained by the physical adsorption on some surface sites and even the formation of a multilayer, following or paralleling the NH3 chemisorption on acidic sites. The adsorption difference between two adsorption cycles may be ascribed to irreversible adsorption of NH3 on the MCM-41 surface. At first, this quantity changes as the adsorption progresses, but then it levels off. The same conclusions can be drawn from the analysis of adsorption isotherms of both types obtained with all the samples studied. In all cases, the irreversible adsorption contribution is practically constant above 0.2 Torr. It is thus reasonable to accept the difference amount at a pressure value of 0.2 Torr as the maximum amount of NH3 irreversibly adsorbed on the surface, that is, Γirr. Because irreversible adsorption of NH3 means the localized chemi-

solid

Γtot, µmol m-2

Γirr, µmol m-2

SiAl32C8 SiAl32C12 SiAl32C14 SiAl32C18 SiC14 SiAl8C14 H+-SiAl32C14

0.35 0.30 0.24 0.30 0.16 0.79 0.29

0.085 0 0.06 0.05 0.04 0.29 0.11

sorption of single ammonia molecules on acidic sites, Γirr provides the total number of such sites on a solid surface. The total amount of NH3 adsorbed at 0.2 Torr and the amount of the adsorbate irreversibly retained on the surface are given in Table 2. It may be concluded that the surface density of acidic sites on SiAl32Cn samples is lower than 0.1 µmol m-2 and represents, at most, 18-25% of the total surface sites occupied by the adsorbing NH3 molecules up to 0.2 Torr. The number of acidic sites per unit surface area increases a little with decreasing pore size. For SiAl32C12, the value of Γirr is practically zero. The interpretation of this result is not straightforward. Above 0.1 Torr, the corresponding adsorption isotherms obtained in the two adsorption cycles superimpose, whereas the difference amount adsorbed has been found to be greater than zero at lower pressures. This means that there are some acidic sites in this sample but the adsorption behavior of NH3 is more complicated compared to other samples. The density of acidic sites on the H+-exchanged SiAl32C14 is doubled in comparison with the original sample. For the same pore size, the number of surface acidic sites is markedly increased when the solid matrix is enriched with aluminum: Γirr rises from 0.04 µmol m-2 (purely siliceous SiC14) to 0.29 µmol m-2 (SiAl8C14). The strength of surface acidic sites may be determined on the basis of the results of calorimetric measurements correlated with the adsorption isotherms. The appropriate experimental curves obtained for all the samples studied are shown in Figures 4-9. The total adsorption isotherms on four MCM aluminosilicate samples prepared with the same Si:Al ratio of 32 and varying chain lengths of the surfactant template are presented in Figure 4. In the region of low-pressure values, up to about 0.025 Torr, the evolution of the amount

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Figure 5. Differential molar enthalpy of NH3 adsorption as a function of the amount adsorbed onto MCM-41 materials prepared with different pore sizes: (9) SiAl32C8; ([) SiAl32C12; (b) SiAl32C18; (2) SiAl32C14.

Figure 6. Volumetric isotherms of NH3 adsorption onto hydrogen-exchanged (b) and original (2) SiAl32C14 sample.

adsorbed is quite similar for the four samples. The maximum adsorption quantity does not exceed 0.1 µmol m-2. The quantitative differences become more pronounced for higher pressure values. At first sight, the changes in Γ for different samples are not parallel to the related variation of the solid porosity. They would probably be explained by different surface densities of all active sites on the MCM-41 materials considered, if such sites were identified. Figure 5 shows four curves of the differential enthalpy of adsorption plotted versus the total amount of NH3 adsorbed on the surface. As a general observation, the thermal effects accompanying the adsorption of NH3 in the adsorption range studied are quite small and do not go beyond the critical value of -80 kJ mol-1, characteristic of ammonia chemisorption on strongly acidic sites. The “chemisorbed” and “physisorbed” components are even hard to distinguish. Obviously, the first point in each enthalpy curve represents the integral enthalpy value rather than the differential enthalpy one and it should not be taken into consideration. Furthermore, the first two gas doses sent onto each of the solid samples lead to an equilibrium amount adsorbed of at least 0.03 µmol m-2. As can be seen in Figure 3a for this Γ value, the

Meziani et al.

Figure 7. Differential molar enthalpy of NH3 adsorption as a function of the amount adsorbed onto hydrogen-exchanged (b) and original (2) SiAl32C14 sample.

Figure 8. Volumetric isotherms of NH3 adsorption onto MCM41 materials prepared with different Si:Al ratios: (9) SiC14; (2) SiAl32C14; (b) SiAl8C14.

chemisorption of NH3 on acidic sites is already accompanied by the reversible adsorption of the adsorbate on some other surface sites. The most likely explanation is that the four MCM-41 samples possess relatively weak and homogeneous acidic sites. The weaker chemisorption of ammonia on these sites takes place only when the equilibrium bulk pressure approaches values at which van der Waals or hydrogen-bonding adsorption might be commencing. Nevertheless, the presence of strongly acidic sites in the four aluminosilicate samples cannot be completely excluded. For example, a slightly increasing tendency in ∆adsh, observed at small Γ values for SiAl32C12 and SiAl32C14, may be attributed to some residual energetic heterogeneity of acidic sites. The difficulty in monitoring such sites, when their surface density is very low, is an inherent weakness of the experimental calorimetric procedure applied. Much smaller amounts adsorbed are needed for the strong chemisorption to be extracted from the total adsorption process. Unfortunately, each attempt to decrease the first doses of ammonia injected to the calorimetric cell has been accompanied by a significant diminution in the measurement precision.

Porous Aluminosilicates of the MCM-41 Type

Figure 9. Differential molar enthalpy of NH3 adsorption as a function of the amount adsorbed onto MCM-41 materials prepared with different Si:Al ratios: (9) SiC14; (2) SiAl32C14; (b) SiAl8C14.

For SiAl32C8 and SiAl32C18, the negative differential enthalpy increases with increasing Γ values at the beginning of the adsorption region studied. Such tendency is usually explained by mutual interactions between the molecules adsorbed on the neighboring sites. If this interpretation were accepted in the case of NH3 chemisorption, this would mean that the acidic sites are distributed unequally on the surface (e.g., regrouped on some surface patches). For example, the formation of some intermediate adsorbed species such as NH2, NH, OH, and NH4+ may lead to mutual hydrogen bonding of the type N-H‚‚‚O, N-H‚‚‚N, or O-H‚‚‚N. The magnitude of the differential enthalpy of adsorption corresponding to the reversible adsorption of NH3 is quite unexpected. In the boundary region, where the reversible component begins to dominate the total amount adsorbed, changes in ∆adsh are very small and this quantity remains practically constant, ranging between -50 and -70 kJ mol-1 for different samples. In the region of completely reversible adsorption, the degree of exothermicity slowly decreases with an increasing amount adsorbed. With the exception of SiAl32C8, the last experimental values of ∆adsh are about -45 kJ mol-1. For classical flat surfaces, enthalpy values more negative than about -40 kJ mol-1 are usually taken to signify chemisorption. Adsorption of the adsorbate in pores of comparable diameters results in the enhancement of the interaction energy.41 In the case of aluminosilicate materials possessing fine capillaries (e.g., zeolites), the negative differential enthalpy value of ≈50-60 kJ mol-1 has been ascribed to the physisorption of ammonia molecules on the pore walls, Si-O-Si and Si-O-Al bridging oxygen atoms, and the terminal surface silanol groups.42,43 Such surface sites are obviously present in the MCM-41 samples studied. Nevertheless, this possibility should be ruled out because the cylindrical pores considered are much larger than the adsorbate size. Instead, strong mutual interactions via hydrogen bonding between the ammonia molecules physisorbed within the small pores can be postulated as an explanation for the increased enthalpy of reversible adsorption. (41) Everett, D. H.; Powl, J. C. J. Chem. Soc., Faraday Trans. 1 1976, 72, 619. (42) Lohse, U.; Parlitz, B.; Patzelova, V. J. Phys. Chem. 1989, 93, 3677. (43) Teunissen, E. H.; Duijneveldt, F. B.; Van Santen, R. A. J. Phys. Chem. 1992, 96, 366.

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When the calcined SiAl32C14 sample is subsequently exchanged with NH4+ ions and recalcined, the total adsorption isotherm becomes more vertical at the beginning and the amount adsorbed increases, as shown in Figure 6. The irreversible adsorption undergoes many pronounced changes because Γirr is about 2 times greater than that for the original sample. The corresponding differential enthalpy curves are compared in Figure 7. For the H+-SiAl32C14 sample, a great fraction of the surface acidic sites are strong and heterogeneous. It was thus possible to distinguish chemisorption from physisorption on the basis of the single criterion of the adsorption enthalpy. The initial enthalpy value is about -145 kJ mol-1. With increasing ammonia loading, the value of ∆adsh decreases strongly and reaches a plateau between -50 and -60 kJ mol-1. In this adsorption region, the two enthalpy curves practically superimpose. The maximum number of strong acidic sites, yielding NH3 chemisorption enthalpies higher than -80 kJ mol-1, is assessed at 0.07 µmol m-2 for the H+exchanged sample. This value is greater than the total number of all acidic sites in SiAl32C14. The strongly exothermic adsorption on H+-SiAl32C14 may be interpreted as a result of the ammonia bonding to Bro¨nsted sites, provided that alkali-metal ions (Na+) are completely absent.44,45 The generation of surface H+ ions, which are retained as charge-balancing cations, may markedly increase the number of Bro¨nsted acid sites in the form of “bridging” Si(OH)Al hydroxy groups. Moreover, it is also possible to enhance the Lewis-type acidity by creating extraframework Al during second calcination. The influence of the Si:Al ratio on the adsorption capacity and the differential enthalpies of ammonia adsorption is illustrated in Figures 8 and 9. For SiC14 and SiAl32C14, the resulting adsorption isotherms have the same shape. In the case of the SiAl8C14 sample, the adsorption isotherm is almost vertical for low equilibrium pressure values, indicating the very strong adsorption of ammonia on numerous acidic sites. The differential enthalpy of ammonia adsorption for SiAl8C14 reaches a relatively great initial value of about -160 kJ mol-1. With an increasing quantity of adsorption, the value of ∆adsh decreases monotonically. This means that the distribution of adsorption energy values is quite broad and the sample surface is heterogeneous. Most of the acidic sites are strong because the differential molar enthalpy of adsorption remains more negative than -80 kJ mol-1 in a wide adsorption range. The energetic heterogeneity of acidic sites may be ascribed to an important flexibility of the amorphous framework and to the different contributions from sites containing Al atoms, as evidenced clearly by the results of NMR study. The unsymmetrical environments of Al and Si after calcination of the sample may result in acidic sites having different adsorption strengths. Additional Bro¨nsted sites may be created by the dissociative adsorption of water molecules during the dealumination process, yielding two Al-linked hydroxyl species Si(OH)Al and SiAl(OH)-Si, as discussed previously.24 On the purely siliceous SiC14, a quasi-constant enthalpy value of about -40 kJ mol-1 has been observed in the whole adsorption range. This means that acidic sites on this sample are very weak and homogeneous, weaker than those existing on aluminosilicate SiAl32C14. The main (44) Mucas, M.; Dutel, J. F.; Solinas, V.; Auroux, A.; Ben Taarit, Y. J. Mol. Catal. A 1996, 106, 169. (45) Mishin, L. V.; Klyachko, A. L.; Brueva, T. R.; Nissenbaum, V. D.; Karge, H. G. Kinet. Katal. 1993, 34, 835.

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sites.46 According to the literature study,16 the purely siliceous materials contain only hydrogen-bonded pyridine. It is thus clear that incorporating aluminum into the MCM-41 structure leads to the creation of acid sites of both Bro¨nsted and Lewis type. Moreover, the relative intensity of the hydrogen-bonded pyridine band decreases.16 After desorption of pyridine from the surface at 473 K, the band corresponding to the hydrogen-bonded pyridine disappears. The band attributed to the formation of pyridinium ions (Hpy+) disappears only after evacuation at 573 K. The only band which is present, even on the sample heated at 673 K, is the one corresponding to the pyridine complex bonded to Lewis-acid sites (Lpy). Therefore, these are the strongest acidic sites on the surface of SiAl8C14.

Figure 10. IR spectra of pyridine adsorbed on the SiAl8C14 sample followed by thermal treatment at (1) 373 K, (2) 473 K, (3) 573 K, and (4) 673 K. H denotes the hydrogen-bonded, B the Bronsted-site-bound, and L the Lewis-site-bound pyridine.

active sites correspond to terminal silanol groups and SiO-Si bridging oxygen atoms in the pore walls, on which the ammonia molecules are physically adsorbed by van der Waals forces or hydrogen bonding, possibly enhanced by strong lateral interactions between the adsorbed molecules. In this case, the magnitude of the differential enthalpy of adsorption cannot be used as experimental criterion for deciding the adsorption type. Pyridine-TPD Study. On the basis of the results of ammonia adsorption measurements, SiAl8C14 material rich in aluminum appears to have the highest surface acidity. Most of the acidic sites are strong. However, adsorption calorimetry is not capable of directly suggesting any details as to the site structure. In an effort to fill this gap, IR study of the weakly basic (pKb ) 8.8) pyridine adsorbed on the sample has been made. The results of IR study have to be processed with caution. The experimental procedure requires the powdered MCM41 samples to be pressed into thin wafers. Experience has taught that such a treatment causes, at least, a significant decrease in the BET specific surface area. It is worthwhile establishing whether this effect is accompanied by any other changes in the surface properties. However, the problem will not be considered here. Figure 10 shows the infrared spectra, in the frequency range of 1650-1400 cm-1, of the surface-bound pyridine activated at different temperatures (from 298 to 673 K). As can be seen, the SiAl8C14 sample exhibits the expected bands due to hydrogen-bonded pyridine (1447 and 1599 cm-1), Lewis-acid-bound pyridine (1450, 1575, and 1623 cm-1), pyridine bound on Bro¨nsted-acid sites (1545 and 1650 cm-1), and a band at 1490 cm-1 attributed to the pyridine associated with both Lewis and Bro¨nsted acid

Conclusion Adsorption microcalorimetry of basic probe molecules, combined with the measurements of the extent of adsorption, may be useful for the determination of the strength and number of acidic sites on porous MCM-41 aluminosilicates. The possibility of testing powdered samples, that is, as obtained from synthesis and calcination, gives it important advantages over other methods. Purely siliceous sample and aluminosilicate materials with a Si:Al ratio of 32 have low surface acidity. The surface density of weakly acidic sites ranges between 0.04 and 0.09 µmol m-2, the aluminosilicate MCM-41 characterized by the smallest pore diameter being at the upper part of this range. The enhanced differential enthalpy of physical adsorption on these materials makes the principal test for distinguishing chemisorption from physisorption fallible. The nature of this enhancement is not clear at present and requires further study. The surface acidity is higher on the H+-exchanged SiAl32C14 sample. The density of acidic sites is approximately doubled in comparison with the original sample. The replacement of the charge compensating sodium cations by the hydronium ions on a solid surface increases the number of Bro¨nsted acid sites in the form of “bridging” Si(OH)Al hydroxy groups. Extraframework Al, created during second calcination, may give rise to the Lewis-type acidity. There are about 0.07 µmol of strongly acidic sites per unit specific surface area. These sites are quite heterogeneous, the strongest sites producing the differential enthalpy of ammonia adsorption of -145 kJ mol-1. For SiAl8C14, the density of acidic sites is about 0.3 µmol m-2. They are very heterogeneous. The corresponding value of ∆adsh decreases from -160 to -80 kJ mol-1. The heterogeneity of acidic sites is due mainly to the flexibility of the amorphous aluminosilicate framework in the pore walls. A pyridine-TPD study has shown that Lewis acid sites forming surface complexes with the adsorbing pyridine molecules are the strongest sites in the sample. LA990840V (46) Parry, E. R. J. Catal. 1963, 2, 371.