2614
J . Phys. Chem. 1992, 96, 2614-2617
Investigation of Dealuminated Mordenites by X-ray Photoelectron Spectroscopy M. J. Remy,* M. J. Genet, G. Poncelet, Groupe de Physico-Chimie Mindrale et de Catalyse, Place Croix du Sud 2, Bte 17, 1348 Louvain-la-Neuve, Belgium
P. F. Lardinois, and P. P. Nott6 Monsanto Europe S.A. Technical Center Europe, rue Laid Burniat, 1348 Louvain-la-Neuve, Belgium (Received: April 15, 1991)
Mg Ka X-ray photoelectron spectroscopy (XPS) was used to establish the relative concentration and the chemical state of aluminum at the surface of dealuminated mordenites. A method for the binding energy calibration is proposed. Modified Auger parameters of AI have been determined for different zeolites and clay minerals as well as for dealuminated mordenites. The data are compared with those available in the literature. Ranges of values for modified Auger parameter of octahedral and tetrahedral aluminum are reported. The results obtained on highly dealuminated mordenites are consistent with the presence of surface tricoordinated AI.
,
Introduction Mordenite is used in several industrial catalytic processes.'v2 Dealumination of mordenite is nowadays a common way to improve the catalytic performances as well as the catalyst life.3-4 Actually, little is known on the chemical state of A1 near the external surface of dealuminated mordenites. Indeed, few studies have applied the XPS technique to achieve a quantitative analysis of A1 at the surface of dealuminated zeolite^.^-^ From these studies, the relation between surface A1 concentration and bulk concentration depends on the type of zeolite and the dealumination procedure. Besides, qualitative investigations using the modified Auger parameters of A1 in zeolites and other crystalline aluminosilicates have shown that tetrahedral A1 can be distinguished from octahedral A1.6-8 The great dispersion of the elemental binding energy (BE) values obtained on the same zeolite by 12 laboratories demonstrates the influence of the calibration method on the value of the binding energy.9 Indeed, several XPS studies on zeolites make use of the C 1s peak as reference for the binding energy scale,lSf2 while others refer to the Si 2p and/or Si 2s peaks.13ni4 In the present study, two methods for the calibration of the binding energy scale are compared and discussed. Data on the relative quantity and the chemical state of aluminum at the surface of dealuminated mordenites are reported. (1) Chen, N. Y . ;Degnan, T. F. Chem. Eng. Prog. 1988, Feb, 32. (2) Spencer, M. S.Selected Deuelopments in Catalysis; Jennings, J. R., Ed.; Blackwell Scientific Publications: London, 1985; Vol. 12, p 64. (3) Meyers, B. L.; Fleisch, T. H.; Ray, G. J.; Miller, J. T.; Hall, J. B. J . Catal. 1988, 110, 82. (4) Sawa, M.; Kato, K.; Hirota, K.; Niwa, M.; Murakami, Y. Appl. Cotal. 1990, 64, 297. (5) Fleisch, T. H.; Meyers, B. L.; Ray, G. J.; Hall, J. B.; Marshall, C. L. J . Catal. 1986, 99, 117. (6) Winiecki, A. M.; Suib, L. S.;Occelli, M. Langmuir 1988, 4, 512. (7) Wagner, C. D.; Passoja, D. E.; Hilery, H. F.; Kinisky, T. G.; Six, H. A.; Jansen, W. T.; Taylor, J. A. J . Vac. Sci. Technol. 1982, 21(4), 933. (8) Wagner, C. D.; Joshi, A. J . Electron Spectrosc. Relat. Phenom. 1988, 47, 283. (9) Madey, T. E.; Wagner, C. D.; Joshi, A. J . Electron Spectrosc. Relot. Phenom. 1977, IO, 359. (10) Okamoto, Y.;Ogawa, M.; Maezawa, A,; Toshinobu, 1. J . Cotal. 1988, 112, 421. ( 1 1) Merlen, E.; Lynch, J.; Risiaux, M.; Raatz, F. SurJ Interface Anal. 1990, 16, 364. (12) Suib, S.L.; Winiecki, A. M.; Kostapapas, A. Langmuir 1987, 3,483. (13) Vedrine, J. C.; Auroux, A.; Dejaifve, P.; Ducarme, V . ; Hoser, H.; Zhou, S. J . Carol. 1982, 73, 147. (14) Shyu, J. Z . ; Skopinski, E. T.; Goodwin, J . G.; Sayari, A. Appl. SurJ Sci. 1985, 21, 291.
0022-365419212096-2614$03.00/0
Experimental Section Materials. Neosyl, a precipitated silica provided by Crossfield Chemicals, was used to assess the values of Si 2p and 0 1s binding energy in silica. Pure kaolinite (K) and pyrophyllite (P) were taken as standards for measuring the Auger parameter of octahedral aluminum. Several protonated zeolites, on the other hand, were selected for determining the Auger parameter of tetrahedral aluminum: two Y zeolites (ZF110 and ZF520) and a small-port mordenite (ZM101) from Zeocat, large-port mordenites (HSZ600 and HSZ640)from Tosho, a largeport mordenite (2solon 1OOH) from P. Q. Corp., and a largeport mordenite (CBV20A) from Conteka. Dealuminated mordenites (ZM5 10, ZM760, ZM4770, ZM4951, ZM980, and ZM4838) were provided by Zeocat. Dealumination was carried out on ZM101, through a procedure combining steaming (between 520 and 700 "C) and subsequent acid treatment. I s X-ray Photoelectron Spectroscopy. The XPS analyses were performed with a Vacuum Generators ESCA 3 Mk I1 spectrometer equipped with a Tracor Northern T N 1710 signal averager. The residual pressure in the spectrometer was in the range 1 X to 5 X Pa. A Mg anode (energy 1253.6 eV) with an A1 window was used as an X-ray source; it was powered at 14 kV and the beam current was 20 mA. The constant pass energy in the hemispherical analyzer of 50 eV and the slit width of 4 mm gave an energy resolution, expressed by the fwhm of Au 4f7,2, of 2.0 eV. The powder specimen was pressed in a stainless steel sample holder of 8 mm diameter. The samples were introduced into the spectrometer without previous thermal treatment. They were outgassed overnight and analyzed at room temperature. The angle between the normal to the sample surface and the electron detection was 45O. The energy scale of the spectrometer was calibrated with Au 4f,,2 binding energy fixed at 84.0 eV. For nonzeolitic samples, the charge correction was made by f i i n g the binding energy of the C 1s peak at 284.8 eV. In the case of zeolites, the calibration method of the binding energy scale will be discussed below. The intensities were calculated by measuring the integral of each peak with the internal routine of the signal averager, after smoothing and linear background subtraction. The atomic concentration ratios were calculated by correcting the intensity ratios with the experimental sensitivity factors given by Wagner et a1.I6 This can be done because the analyzer trans(15) Hamon, C.; Bandiera, J.; Senes, M. US. Patent 4, 447, 669 (May 8. 1984).
0 1992 American Chemical Society
Investigation of Dealuminated Mordenites
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2615 TABLE I: Si 2p Values Obtained before and after AI 2p Recording of ZM980 irradiation time, Si 2p absolute kinetic energy, Si 2p area,‘ min eV au 20 1140.9 106.4 260 1141.5 99.5
VI
‘au = arbitrary units.
0 531
20
0
LO
60 Si/Al XPS
Figure 1. Evolution of the 0 Is (H) and Si 2p (A)binding energy as a function of the surface Si/AI atomic ratio using C Is peak at 284.8 eV as a reference for the binding energy calibration. The arrows repregent the range of Si 2p and 0 Is BE measured on silica.
B
bF
532
4
1
than the C 1s peak for the binding energy calibration in the case of highly dealuminated zeolites. The Si 2p, C 1s and 0 1s peaks, as discussed above, were recorded within the first 20 min of irradiation. Now, for the A1 2p and Al KLL peaks, the recording required longer accumulation times. In order to control the eventual evolution of the position of the reference peak during the recording of A1 2p and A1 KLL, the reference was scanned before and after each one of these peaks. Table I gives the absolute kinetic energy and the area of the Si 2p peak before and after recording A1 2p of sample ZM980. A difference in the absolute energies of Si 2p is observed. It is believed that the Si 2p shift originates from variations in the charge stabilization during long irradiation times. Sample damage caused by the X-ray beam would most probably modify the shape of the Si 2p signal, which in no case was observed. The mean shift of the Si 2p peak observed during the recording of the A1 2p or A1 KLL on nondealuminated samples was 0.1 eV whereas for dealuminated mordenites, which required longer acquisition times, the mean shift value was 0.45 f 0.25 eV. In order to minimize the imprecision due to this phenomenon, it is proposed to determine the BE of each one of these peaks using the mean absolute energy of two surrounding scannings of the reference. As the Si 2p peak area decreases as a function of time (Table I), the mean area of the Si 2p peaks measured before and after recording the A1 2p is used for the calculation of the Si/A1 atomic ratios. The limit of this approach lies in the assumption that the evolution of the reference peak is linear with time, which might not be correct. The validity of this assumption would require statistical studies of several experiments in which the Si 2p peak is recorded after increasing irradiation times. Nevertheless, we may consider that in most cases this approach is more accurate than using a reference peak taken at one time. Chemical State of AI by XPS. The knowledge of the chemical state of A1 at the surface of dealuminated mordenites is of great interest to understand the improvement of the catalytic behavior brought about by dealumination. 27AlMAS-NMR studies on zeolites have shown the presence of at least two types of Al: octahedral and tetrahedral A1.I8 The A1 2p binding energies measured on well-known ahminosilicate structures do not allow to distinguish octahedral from tetrahedral AL7 But the distinction can be done by using the modified Auger parameter (a’) obtained by the following equation7
3L 531
0
20
LO
60 Si/A/ XPS
Figure 2. Evolution of the 0 Is binding energy as a function of the surface Si/AI atomic ratio using Si 2p peak at 103.4 eV as a reference for the binding energy calibration. The arrow represents the range of 0 1s BE measured on silica.
mission function varies with the inverse of the photoelectron kinstic energy (as required by Wagner’s data).
Results and Discussion Calibration Method. The C Is, 0 Is, Si 2p, A1 2p XPS peaks and AI KLL Auger peak were measured on protonated mordenites and dealuminated mordenites. Figure 1, where the C 1s peak at 284.8 eV is taken as a reference for the binding energy calibration, compares the evolution of the binding energy of 0 1s and Si 2p as a function of the surface Si/A1 atomic ratio of dealuminated mordenites. It shows that the 0 1s binding energy varies from 532.2 to 533.5 eV while, for the Si 2p binding energy, the values lie between 102.9 and 104.2 eV. These results are somewhat unexpected. Indeed, with the progressive impoverishment in Al, 0 Is, and Si 2p, BE values approaching those of pure silica would be expected, according to the data reported, e.g., by Wagner et al.,’ Okamoto et al.,1° and Barr1’ for Na A, X,Y and mordenites, namely a series of zeolites with increasing Si/Al ratios (from 1 to 11). As shown in Figure 1, the 0 1s and Si 2p binding energies at high Si/Al ratios (-60) are significantly higher than in pure silica. They may hardly be interpreted in terms of changes in the covalency/ionicity character accompanying the withdrawal of framework Al, as was proposed by Barr” to account for the increasing BE values and the parallel evolution of the 0 1s and Si 2p BE vs the Si/Al ratio of the zeolites. The abnormally high values mentioned above may be due to a wrong energy scale calibration. Differential charging between adventitious carbon and zeolite matrix (wherein strong internal electric fields are developed) has often been incriminated. As a result, a BE scale referred to the C 1s line does not appear to be most adequate. As mentioned above, this is the reason why several authors instead adopt the Si 2p or the Si 2s peak as an internal r e f e r e n ~ e . ’ ~ J ~ When, alternatively, the Si 2p peak at 103.4 eV is taken as a reference for the binding energy calibration, an 0 1s value near 532.7 f 0.2 eV is obtained for all the samples, as shown in Figure 2, which is in good agreement with the 0 1s value measured on silica. Therefore the Si 2p peak appears to be a better reference
where KE(AL(KLL)) is the kinetic energy of the A1(KL23L23) Auger peak and BE(A1 2p) is the binding energy of the A1 2p XPS peak. The Bremsstrahlung radiation of an unmonochromated magnesium or aluminum X-ray source induces the A1 KLL Auger peak.I9 It is thus possible, with such X-ray sources, to measure the modified Auger parameter (a’) of Al. Besides, the a’ value is independent of the sample charging and of the binding energy attributed to the reference peak.’ It has also been shown theoretically as well a s experimentally that the a’ value of an element is a measure of its electronic interaction with the near neighbouring atoms.8 In aluminosilicates, the A1 a’ values depend on the polarizability of the near neighboring oxygen atoms.*O
(16) Wagner, C. D.; Davis, L. E.; Zeller, M. V.;Taylor, J. A.; Raymond, R. H.; Gale, L. H. Surf. Interface A n d . 1981, 3 ( 5 ) , 21 1. (17) Barr, T. L. Zeolites 1990, 10, 760.
(18) Scherzer, J. ACS Symp. Ser. 1984, 248, 157. (1 9) Castle, J. E.; West, R. H. J. Electron Spectrosc. Relat. Phenom. 1980, 18, 355.
a’ = KE(AL(KLL))
+ BE(A1 2p)
2616 The Journal of Physical Chemistry, Vol. 96, No. 6,1992 TABLE 11: XPS Results on Zeolites and Clays, Comparison with Literature Data’ 0 1s AI 2p AI(KLL) Si/A1 Si/A1 BE, BE, KE, AI a’, sample bulk” surfaceb eV eV eV eV AI IV ZMIOIC 5.8 6.8 532.6 74.8 1385.7 1460.5 6.1 5.7 532.6 74.8 1385.7 1460.5 CBV 10AC HSZ600C 5.4 6.4 532.8 75.0 1385.6 1460.6 HSZ640‘ 9.7 11.0 532.9 74.6 1385.8 1460.4 1385.1 1460.3 ZF1 loc 2.7 2.8 532.6 75.2 ZF520‘ 20.0 19.0 532.7 74.5 1385.9 1460.4 H zeolon‘ 7.0 18.0 532.7 74.6 1385.7 1460.3 H zeolonc*d 12.0 532.7 74.9 1385.4 1460.3 AI VI 1.2 532.0 74.5 1386.8 1461.3 kaolinited 1.0 1.0 532.2 74.7 kaolinitedVc 1.0 1386.6 1461.4 1.8 532.1 74.7 1386.7 1461.4 pyrophyllited 2.0 pyrophyllited,‘ 2.0 532.2 74.7 1386.8 1461.5 OValues given by the companies for the zeolites, and calculated from the theoretical formulas for kaolinite and pyrophyllite. Determined by XPS. cThe reference for the BE calibration is Si 2p at 103.4 eV. dThe reference for the BE calibration is C 1s at 284.8 eV. (Data taken from Wagner et ai.’
Remy et al.
S i / A / / n bulk
Figure 3. Dealuminated small port mordenites: surface Si/AI vs bulk Si/Al.
138LO
76
TABLE 111: XPS Results of Dealuminated Mordenites 0 1s AI 2p AI (KLL) KE, Si/AI Si/AI BE, BE, eV eV sample bulka surfaceb eV 1385.9 ZM510 11 33 532.7 74.6 1385.3 532.7 74.3 ZM760 39 56 1384.5 70 ZM4770 54 532.6 74.8 1384.9 ZM4951 75 59 532.8 74.6 1385.1 60 532.8 74.7 ZM980 89 1385.0 117 ZM4838 54 532.8 74.8
u 7L
A l 2 p binding energy lev)
AI a’, eV 1460.5 1459.6 1459.3 1459.5 1459.8 1459.8
“ Determined by X-ray fluorescence. Determined by XPS; reference for the BE calibration is Si 2p at 103.4 eV. Clays and Zeolites: Comparison with Literature Data. The A1 2p binding energies, the A1 (KLL) kinetic energies, and the modified Auger parameters (a’)were established on mordenites containing only tetrahedral aluminum (A1 IV), and on two clay minerals wherein only octahedral aluminum (A1 VI) is present. The XPS data are given in Table 11. It is clear that the A1 2p binding energies alone do not allow, as expected, to make a distinction between octahedral and tetrahedral aluminum. For the A1 (KLL) kinetic energies, there is a difference of about 1 eV between the two types of aluminum. The use of the aluminummodified Auger parameters (a’), as defined above, allows a more accurate distinction between octahedral and tetrahedral aluminum. For sake of comparison, the corresponding values reported by Wagner et al.’ on similar solids are also included in Table 11. The agreement between the two series of data is fairly good and confirms our results. Thus,an a’value between 1461.3 and 1461.5 eV is characteristic of octahedral Al, whereas tetrahedral A1 exhibits an a’ value between 1460.3 and 1460.6 eV. Dealuminated Mordenites. The detailed XPS data obtained on the series of dealuminated mordenites are given in Table 111. The surface Si/A1 ratios plotted vs bulk Si/AI ratios in Figure 3 show that there is a relative A1 enrichment at the surface of mordenites with bulk Si/Al ratios higher than 50. On the contrary, the mordenite samples with bulk %/A1 ratios lower than about 50 are relatively depleted in surface Al. Noteworthy is that dealuminated mordenites with bulk Si/A1 ratios between 39 and 117 are characterized by an almost identical Si/Al ratio at the surface. (20) West, R. H.; Castle, J. E. Surf. Interface Anal. 1982, 4 , 68. (21) Defosse, C.; Scokart, P. 0.: Rouxhet, P. G.Verres Refract. 1981, 3.5, 1, 50.
75
(22) Kasansky, V. B. Structure and Reactivity of Modified Zeolites; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1984; pp 18, 61.
Figure 4. AI chemical state plot: (0)clays; (0) zeolites; (A) dealuminated small-port mordenites with a bulk Si/AI ratio higher than 1 1 .
Plotting the Al KLL values vs the A1 2p binding energies (given in Table 11) yields the A1 chemical state plot (Figure 4). The diagonal lines in this plot correspond to the modified Auger parameter (d). Along the diagonals, which refer to the right-hand ordinate, the a’ value is constant. The experimental data are aligned around three diagonals corresponding to three different a’ values. The first one, a t an d value of 1461.5 eV, and the second one, at an d value of 1460.5 eV, are attributed to octahedral and tetrahedral Al, respectively. The third zone centered at an a’value of 1459.5 eV corresponds to dealuminated mordenites with bulk %/A1 ratios of about 40 and higher. Dealumination of this series of mordenites was performed on the protonic form using a two step process, namely a steaming followed by an acid leaching. Several studies based on different techniques have clearly established that the AI atoms extracted from the framework during the steaming step may form different A1 species within the channels of the zeolites as well as on the outer s ~ r f a c e . ~These ~ - ~ ~species are dissolved during the subsequent acid treatment, if not totally, at least to a major extent.26 The d value obtained for this series of dealuminated mordenites, namely 1459.5 eV, is lower than those observed for octahedral and tetrahedral Al. According to West and CastleZoa lower a’ value corresponds to a less polarizable oxygen neighboring. This would be the case for a tricoordinated Al, associated with Lewis acidity,21as compared with tetrahedral Al. Tricoordinated A1 was evidenced in high-silica zeolites by IR diffuse reflectance spectroscopy after low-temperature H2 adsorption,22and was proposed to explain quantitative XPS observation^.'^ The presence of tricoordinated Al, as inferred from the A1 chemical state plot, is, however, in partial contradiction with KasanskyZ7who reported that dehydroxylation of dealuminated mordenites with Si/Al ratio between 12 and 70 resulted in the formation of trimrdinated Si atoms, whereas for lower ratios both tricoordinated A1 and Si atoms were formed upon dehydroxylation. Nevertheless, the (23) Bodart, P.; Nagy, J. B.; Debras, G.;Gabelica, Z.: Jacobs, P. A. J . Phys. Chem. 1986, 90, 5183. (24) Seddon, D. Appl. Catal. 1983, 7, 327. (25) Baik Hyon Ha; Barthomeuf, D. J . Chem. SOC.Faraday Trans. 1 1979, 75, 2367. (26) Chen, N . Y . ; Smith, F. A. Inorg. Chem. 1976, 15, 2, 295. (27) Kasansky, V . B. Coral. Today 1988, 3, 367.
J . Phys. Chem. 1992, 96. 2617-2629
results reported here are a clear evidence of the presence of tricoordinated A1 in surface of dealuminated mordenites. The XPS data do not allow to ascribe these tricoordinated A1 to framework or extraframework species. Since acid leaching removes most of the extra-framework A1 resulting from the steaming, the tricoordinated A1 species may reasonably be considered as belonging to the zeolite lattice, although persisting islands of more stable A1 species in the external part of the channels or on the outer surface of the crystals cannot be ruled out. The nearly constant surface Si/Al atomic ratio (Table 111), unexpected if the steaming/acid leaching removes uniformly A1 throughout the crystals,28 implies either that A1 is extracted to a greater extent in the bulk than at the surface, or that A1 atoms reintegrate lattice vacancies in the outer zone of the crystals. A similar suggestion was already made by Chen and Smithz6but with no distinction between bulk and surface. This point needs further investigation. Whatever the explanation is, the presence of surface tricoordinated A1 is, from the catalytic viewpoint, of importance since the Lewis acid sites associated with those tricoordinated A1 may be responsible for wanted or unwanted reactions. (28) Dwyer, J.; Fitch, F. R.; Qin, G.; Vickerman, J. C. J . Phys. Chem. 1982,86,4574.
2617
Conclusions
For the XPS characterization of high silica containing zeolites, it is proposed to use Si 2p peak instead of C 1s as reference for the BE calibration. Furthermore, it is suggested to scan the reference before and after each peak requiring long accumulation times, and to calibrate, for each of these peaks, the binding energy scale using the mean absolute energy of the two surrounding references. The use of the modified Auger parameter (a') allows a clear distinction between tetrahedral and octahedral Al. For well-known aluminosilicates, the a' values obtained fall within 1461.3 and 1461.5 eV for octahedral aluminum, and between 1460.3 and 1460.6 eV for tetrahedral aluminum. In dealuminated mordenites with bulk Si/Al atomic ratios higher than 40, A1 a' values between 1459.3 and 1459.8 eV are obtained. These values, 1 eV lower than a' value of tetrahedral Al, support the presence of surface tricoordinated A1 in dealuminated mordenites.
Acknowledgment. We acknowledge Momanto Europe for supporting this research. We also thank the different companies and especially Zeocat for having supplied mordenite samples. The Service de Programmation de la Politique Scientifique Belgium is also gratefully acknowledged.
Physicochemical Characterization of V-Silicaiite G. Centi,* S. Perathoner, F. Trifirb, Department of Industrial Chemistry and Materials, V.le Risorgimento 4, 401 36 Bologna, Italy
A. Aboukais, C . F. Aissi, and M. Guelton Laboratoire de Catalyse Heterogene et Homogene URA CNRS 402, Universite des Sciences et Technologie de Lille, Flandres-Artois, Villeneuve d'Ascq Cedex. France (Received: May 6, 1991)
The coordination and nature of V sites in V-silicalitesamples prepared by hydrothermal synthesis are characterized by wide-line solid-state V - N M R , electron spin resonance (ESR), UV-visible diffuse reflectance (DR) and infrared (FT-IR) spectroscopies, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ammonia temperature-programmed desorption (NH3-TPD) and hydrogen temperature-programmedreduction (H,-TPR). Four vanadium species were detected: (i) a polynuclear vanadium oxide species containing reduced vanadium species, (ii) a nearly octahedral vanadyl (V02+)species in the zeolitic channels which interacts with Brsnsted sites, (iii) a Vs+ species in sites characterized by a nearly symmetric tetrahedral environment, and (iv) after reduction a V4+species in a nearly tetrahedral environment. The polynuclear vanadium oxide species can be removed from the zeolite by an ammonium acetate extraction and is derived from excess vanadium present in the preparation of V-silicalite. The excess vanadium remains as an amorphous oxide in pores or in external positions, as indicated by XPS. The tetrahedral V5+species can be attributed to atomically dispersed V species anchored to the silicalite framework, probably as a framework satellite, and localized inside the pore structure, as indicated by FT-IR and XPS. The UV-visible DR spectrum of this species differs from that of other tetrahedral compounds, suggesting the presence of a short V-0 bond. The FT-IR spectroscopy data obtained in the characterization of surface acidity show thc presence of strong Lewis acid sites associated with V only, inside the zeolitic channels, indicating the internal localizaticrl of these V5+ sites. Only weak silanol groups are present on the external surface of the zeolite, and these groups generate very weak Lewis sites upon evacuation. Silanol group are also formed inside the zeolite channel in the V-silicalite, as indicated by FT-IR and NH3-TPD. The nearly octahedral V02+species interacts with the OH groups associated with tetrahedral V5+ species. The model of the possible localization of V5+ sites in the zeolitic structure is also given.
Introduction
In recent years increasing attention has been directed toward the study of new zeolitic materials, especially crystalline microporous metallosilicalites, due to their interesting catalytic properties. From a general point of view, silicalite can be considered as a regular structured lattice in which lattice "defects" can be introduced up to a certain concentration. The catalytic reactivity of the zeolite depends on the type of this substitution. As is
well-known, trivalent cations, such as A13+create bridging hydroxyl groups due to a charge-compensation effect, but the nature of this hydroxyl group can be suitably tuned by substitution of the A13+ with other trivalent cations that fit well into a tetrahedral oxygen environment, such as Fe3+,Ga3+,and B3+. In this way it is possible to control the Br~nstedacidity of the zeolite.'.2 In like manner, (1) Post, M. F.; Huizinga, T.; Eneis, C. A.; Nanne, J. M.; Stork, W. H. J. In Zeolites as Catalysts, Sorbents and Detergents Builders; Karge, H.G., Weitkamp, J., Eds.; Elsevier Science Publishers: Amsterdam, 1989; p 365.
0022-365419212096-26 17%03.00/0 0 1992 American Chemical Society
