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
2618 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
the catalytic activity also can be tuned for applications such as hydrocarbon conversion or synthesis of organic compound^.^ Tetravalent ions, especially Ti4+but also Ge4+ and other elem e n t ~ ,can ~ , ~be inserted in the zeolite framework, which, in this case, does not possess acidic properties (apart from some silanol group due to the presence of defects), and the catalytic reactivity is specifically related to the reactivity properties of the heteroatoms. These properties have recently been fruitfully applied in the development of new industrial proces~es.~ Some recent data6 also suggest the possibility of inserting some Ms+ transition metals in the zeolite framework. In fact, treatment of a highly siliceous zeolite with SbCIS vapor resulted in the insertion of SbS+cations in the defect sites of the zeolite; direct hydrothermal synthesis of Sb-silicalite is not possible due to the large ionic radius of the Sb3+cation (0.076 nm). The charge effect due to the introduction of pentavalent atoms is compensated by the formation of weak Bransted sites.6 The smaller ionic radius of V3+ (0.063 nm) or of V4+ (0.059 nm), in theory, also allows V-silicalite to be obtained by direct hydrothermal synthesis of the silicalite,' however, with some difficulty because the ionic radius is at the limit of the range for isomorphic substitution of an element on the basis of the simplified version of Pauling's minimum radius ratio.* However, due to its tendency to be oxidized, vanadium can be easily transformed to Vs+, and thus in the final zeolite, different valence states of vanadium may be found which justify its redox reactivity. The synthesis of a thermally stable vanadium-containing silicalite with the MFI structure and a low aluminum content has been reported? but the authors concluded that the vanadium species (present both as VIv and Vv) are probably connected to the framework at a defect site. Furthermore, V-silicalite and V-ZSM5 samples have been prepared recently by treatment of silicalite or HZSM5 with VOCl, vapor, which results in the removal of silanol groups and formation of species resistant to hydrolytic regeneration of silanols.'O V-silicalite has been shown to have some interesting catalytic properties in the conversion of methanol to hydrocarbons,",12 NO, reduction with ammonia and substituted aromatics amm~ x i d a t i o n , l ~ butadiene -'~ oxidation to and propane (2) (a) Coudurier, G.; Vedrine, J. C. In New Developments in Zeolite Science and Technology-Proceedings of 7th International Zeolite Conference; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Elsevier Science Publishers: Amsterdam, 1986; p 643. (b) Newsam, J. M.; Vaughan, D. E. W. In New Developments in Zeolite Science and Technology-Proceedings of 7th International Zeolite Conference; Murakami, Y., Iijima, A,, Ward, J. W., Eds.; Elsevier Science Publishers: Amsterdam, 1986; p 457. (3) HBlderich, W . F. In Zeolites: Facts, Figures, Future; Jacobs, P. A., van Santen, R. A,, Eds.; Elsevier Science Publishers: Amsterdam, 1989; p 69. (4) (a) Perego, G.; Bellussi, G.; Corno, C.; Taramasso, M.; Buonomo, F.; Esposito, A. In New Developments in Zeolite Science and TechnologvProceedings of 7th International Zeolite Conference; Murakami, Y., lijima, A,, Ward, J. W., Eds.; Elsevier Science Publishers: Amsterdam, 1986; p 129. (b) Taramasso, M.; Perego, G. U S . Patent, 4,441,501 1983. (5) Gabelica, Z.; Guth, J. L. In Zeolites: Facts, Figures, Future; Jacobs, P. A,, van Santen, R. A,, Eds.; Elsevier Science Publishers: Amsterdam, 1989, p 421. (6) Yamagishi, K.; Namba, S.; Yashima, T. In Zeolites: Facts, Figures, Future; Jacobs, P. A., van Santen, R. A,, Eds.; Elsevier Science Publishers: Amsterdam, 1989; p 459. (7) Morosi, L.; Stabenow, L.; Schwarzmann, J. FRG Appl. 2131631, 1981. (8) Tielen, N.; Geelen, M.; Jacobs, P. A. Acta Phys. Chem. 1985, 31, I . (9) Rigutto, M. S.;van Bekkum, H. Appl. Catal. 1991, 68, L1. (IO) Whittington, B. I.; Anderson, J. R. J. Phys. Chem. 1991, 95, 3306. (1 I ) Inui, T.; Medhanavyn, D.; Praserthdam, P.; Fukuda, K.; Ukawa, T.; Sakamoto, A,; Miyamoto, A. Appl. Catal. 1985, 18, 311. (12) Miyamoto, A.; Medhanavyn, D.; lnui, T. Appl. Catal. 1986, 28, 89. (13) Miyamoto, A.; Medhanavyn, D.; Inui, T. Chem. Express 1986, I, 559. (14) Miyamoto, A.; Medhanavyn, D.; h i , T. In Proceedings, 9th International Congress on Catalysis; Phillips, M. J., Ternan, M., Eds.; The Chemical Institute of Canada: Ottawa, Canada, 1988; Vol. I , p 437. (1 S) Cavani, F.; Trifiro', F.; Jiru, P.; Habersberger, K.; Tvaruzkova, Z. Zeolites 1988, 8, 12. (1 6) Centi, G.; Jiru, P.; Trifiro', F. In Successful Design of Catalysts; Inui, T., Ed.; Elsevier Science Publishers: Amsterdam, 1988; p 247. (17) Tvaruzkova, Z.; Centi, G.; Jiru, P.; Trifiro', F. Appl. Catal. 1985, 19, 307. (18) Habersberger, K.; Jiru, P.; Tvaruzkova, Z.; Centi, G.; Trifiro', F. React. Kinet. Catal. Lett. 1989, 39, 95.
Centi et al. ammoxidation to acrylonitrile.2o Recently interesting results also have been obtained in propane oxidative dehydrogenation to propylenea21 The use of vanadium-containing silicalite as a catalyst in liquid-phase oxidations with H202as the reactant has also been suggested.1° In conclusion, vanadosilicalite seems to have different and/or in some cases improved catalytic performances compared to supported vanadium oxide catalysts. However, there are relatively little data9J0J2J4J8on the characterization of these samples, and the reasons for these reactivity differences are not yet understood and clear identification of the nature and localization of active vanadium species in the zeolitic structure has not been achieved. The aim of this paper is to clarify these aspects with a detailed physicochemical characterization of a V-silicalite sample with a pentasyl structure, prepared by hydrothermal template synthesis.
Experimental Section Catalyst Preparation. V-silicalite samples were prepared hydrothermally, according to a procedure which allows highly pure Al-free samples to be obtained. Tetraethylorthosilicate (TES), tetrapropylammonium hydroxide (TPA), VC13, and double-distilled water were used as starting compounds. The Si/V ratio in the starting solution was as follows: Si02/V203= 30. After recovery and drying, the solid was calcined in air by slowly increasing the temperature to 823 K. Two samples with different compositions were prepared, namely: Si02/V203= 1 17 (VSilll7) and Si02/V203= 237 (VSi1237). Elemental analysis shows that the Na+ and A13+concentrations were below the detection limit ( 1 ppm). The absence of A1 was also checked by X-ray photoelectron spectroscopy in the region of the Al, peak. The samples were then treated with an ammonium acetate solution at room temperature in order to remove extralattice vanadium. About 80% of the vanadium was removed from the VSilll7 sample after this treatment. Chemical analyses after drying and calcination indicate the following composition: Si02/V203= 545 (VSi1545). A similar composition was also found for the VSi1237 sample after similar treatment. The preparation method forms microcrystals with a mean diameter of less than 0.3 pm, as shown by scanning electron microscopy. In the same conditions, of synthesis a reference sample of pure silicalite (Sil), which also is microcrystalline, was prepared. Infrared spectra of framework vibrations of pure and V-modified silicalite are very similar to those of other pentasyl zeolites such as AI-ZSM. The presence of characteristic bands at 1220, 550, and 450 cm-I and their relative intensities (in particular, the ratio of absorbance at 550 and 450 cm-I) confirm the presence of a five-membered ring and the crystallinity of the samples22 before and after extraction of vanadium. The characteristic IR band of the V = O stretching frequency cannot be analyzed because it is masked by the stronger absorption band due to lattice vibrations of silicalite, but no new band or shoulder is observed in the IR spectrum of V-silicalite compared to that of pure silicalite. Reference samples for UV-visible diffuse reflectance spectra were prepared by hydrothermal synthesis analogous to that used for the preparation of the VSil samples, but using tetrabutylammonium hydroxide (TBA) instead of TPA as the structuredirecting agent, followed by a rapid oxidative activation procedure of the precursor obtained (calcination at 773 K in air with rapid heating), and ion exchange in 0.5 M HNO3.I8 The zeolite structure collapsed and the major crystalline phase detected was a-cristobalite (VSilCR). This sample was taken as a reference for dispersed/included Vs+ in a nonzeolitic silica structure. The N
(19) Centi, G.; Habersberger, K.; Jiru, P.; Trifiro', F.; Tvaruzkova, 2. Chem. Express 1986, I , 717. (20) Miyamoto, A,; lwamoto, Y.; Matsuda, H.; Inui, T. In Zeolites: Facts, Figures, Future; Jacobs, P. A,, van Santen, R. A,, Eds.; Elsevier Science Publishers: Amsterdam, 1989; p 1233. (21) Zatorki, L. W.; Centi, G.; Lopez Nieto, J.; Trifiro', F.; Bellussi, G.; Fattore, V. In Zeolites: Facts, Figures, Future; Jacobs, P. A., van Santen, R. A., Eds.; Elsevier Science Publishers: Amsterdam, 1989; p 1243. (22) Coudurier, G.: Naccache, C.; Vedrine, J. C. J . Chem. Soc., Chem. Commun. 1982, 1414.
Physicochemical Characterization of V-Silicalite chemical analysis of this sample indicates a Si02/Vz03ratio of 60.Two further reference samples of V supported on pure silicalite or Si02(VSil-IM and VSiO2-IM, respectively) were prepared by incipient wet impregnation of the Sil sample or of Si02 (AKZO 20 F-7), using an aqueous solution containing ammonium vanadate in such an amount as to have 0.17 wt % (as VzOs) of vanadium in the final sample. This amount roughly corresponds to that present in the VSil samples. After impregnation, the samples were dried overnight, calcined at 623 K for 12 h, and then heated further at 773 K for 3 h; the calcination temperature was increased at a rate of -323 K/h. V20s, NH4V03, and Na3V03were pure commercial products (RPE Merck). V/MgO reference samples [ortho and meta, M&(VO4)2 and M B ( V O ~ )respectively] ~, were prepared and characterized according to the procedure reported by Kung et Characterization. Wide-line solid-state vanadium nuclear magnetic resonance (51V-NMR) spectra were recorded using a CXP 100 Bruker spectrometer at 26.289 MHz.24925The spectral width was 125 kHz. A 22.5' pulse angle [14-261 and a 0.7-s repetition time were used. The spectra were obtained using quadrature detection from the accumulation of at least 40 000 transients. Differences between sample and probe free induction decays were calculated before exponential multiplication with 100-Hz line-broadening. A special insert was used to allow measurements on a vertical 1 0 " 0.d. Liquid V0Cl3 was chosen as a reference for chemical shifts (6 = 0). X-ray diffraction patterns (XRD) (powder technique) were obtained using Cu K a radiation and a Siemens D5000 computer-controlled instrument. The XRD patterns were corrected for Ka2 contributions. Unit cell parameters were obtained by a least-squares fit to the interplanar spacings of the eight strong reflections, accurately measured in the 10-35' 28 region, using a-A1203as an internal standard. The electron spin resonance (ESR) spectra were recorded on a Varian E 109 spectrometer at 9.3 GHz (X band) with a rectangular dual cavity (TEIw). Modulation at 100 and 10 W z was used in both channels with modulation amplitudes of 4 G for the catalysts and 1 G for the standard sample (strong pitch Varian, g = 2.0028). The hf power was chosen small enough to prevent any signal saturation. The spectra were systematically recorded at room temperature and 77 K. The evacuation temperature and reduction at 623 or 773 K (atmosphere of 20% H2 in helium) of the samples were obtained in a conventional gas manipulation flow apparatus to which the ESR tubes could be attached via grease-free stopcocks. Diffuse reflectance spectra (UV-visible DRS) were recorded in the UV-visible region on a Kontron Uvicon 860 instrument using barium sulfate as the reference sample. The spectra were recorded in air at room temperature and when necessary diluted with MgO in order to normalize the spectra to the same amount of vanadium. Reflectance spectra were digitalized and converted in the Kubelka-Munk functionZ6M R , ) ] , which is proportional to the absorption coefficient. Fourier-transform infrared (FT-IR) spectra (resolution 2 cm-') were recorded with a Perkin-Elmer 1750 instrument in a cell connected to evacuation and gas manipulation grease-free lines. The self-supporting disk technique was used. The usual pretreatment of the samples was evacuation a t 800 K. Adsorbate compounds were hyperpure products of Carlo Erba and S I 0 (Milano, Italy). Self-supporting disks (about 10-20 mg/cm2) for infrared examination were prepared by pressing the samples at 20 000 psi.
-
(23) Patel, D.; Kung, M. C.; Kung, H. H . In Proceedings, 9th International Congress on Catalysis; Phillips, M. J., Ternan, M., Eds.; The Chemical Institute of Canada: Ottawa, Canada, 1988; p 1554. (24) (a) Costoumer, L. R.; Taouk, B.; Le Meur, M.; Payen, E.; Guelton, M.; Grimblot, J. J . Phys. Chem. 1988, 92, 1230. (b) Taouk, B.; Guelton, M.; Grimblot, J.; Bonnelle, J. P. J . Phys. Chem. 1988, 92, 6700. (25) Centi, G.; Lena, V.; Trifir6, F.; Ghoussoub, D.; A ' h , C. F.; Guelton, M.; Bonnelle, J. P. J . Chem. Soc., Faraday Trans I 1990,86. 2775. (26) Stone, F. S. In Surface Properties and Catalysis by non-Metals; Bonnelle, J. P., Delmon, B., Derouane, E., Eds.; NATO ASI Ser. 1983, No. 105. 237.
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2619
I
5
"
.
'
"
'
,
I
10
)
'
,
,
15
I
,
2o
,
'
,
20
'
,
25
,
'
,
1
,
30
Figure 1. XRD pattern of VSi1545.
X-ray photoelectron spectroscopy (XPS) data were obtained using a LHSlO spectrometer equipped with an A1 anode which operated at 13 kV and 20 FA current emission (hv = 148.6 eV). Binding energies were corrected for charge effects by reference to the carbon peak at 284.6 eV. The Si/V intensity ratio was determined from the integral areas of the Si (2p) and V (2p1,2+3,z) peaks after subtraction of the linear background and correction for the X-ray 01,satellite. The atomic Si/V ratio was calculated from the intensity ratio after correction for the Scofield cross section, the parameter of asymmetry, and the experimental kinetic energy.24b Ammonia temperature-programmed desorption (NH3-TPD) and hydrogen temperature-programmed reduction (H2-TPR) tests were camed out in a quartz microreactor with continuous analysis of the outlet composition via a mass quadrupole detector; the heating rate was 10 K/min. Before the tests, the sample was pretreated in situ with a flow of helium at 473 K to constant weight in order to remove adsorbed species. For NH3-TPD tests, the sample was then cooled at 373 K for adsorption of NH3 (flow of 3.6 L/h containing 1% NH3 in helium) to a constant ammonia outlet concentration. After this initial ammonia sorption, the sample was equilibrated in a flow of helium (6 L/h) at 373 K until no further ammonia desorption occurred. Then the linear increase of temperature was started and the change in the intensity of the peak at m / e 17 was monitored. Corrections for contributions from water fragmentation (23% of the intensity of the mass peak a t m / e 18) were made. H2-TPR tests were carried out starting from 473 K, after equilibration with the helium flow; a flow of 2% Hz in helium (6 L/h) was used. The change in peak intensity at m / e 2 was monitored.
-
ReSultS X-ray Diffraction (XRD) Data. The XRD powder patterns of V-silicalite samples indicate in all cases the presence of only a pentasyl-type framework structure with monoclinic lattice symmetry, characteristic of silicalite- 1;&v2' no evidence was found for the presence of vanadium oxide crystallites. Reported in Figure 1 is the XRD pattern of VSi1545; similar spectra were obtained for VSi1237 and VSil117. All diffraction lines correspond to those found for pure silicalite-1 or for Tisilicalite.4a However, in these samples the most intense reflections were found in the 5" < 28 < 10" region, whereas in V-silicalite these reflections are not the most intense (Figure l), probably due to the perturbation induced by vanadium during synthesis. The values found for the cell parameters of VSi1545 were as follows: a = 20.118 A, b = 19.891 A, and c = 13.387 A. These cell parameters are similar to or not significantly higher than those found for pure silicalite-l,4a~z7and this is in agreement with that expected on the basis of the small amount of V atoms present in V-silicalite. In fact, similar values are also found for Ti-silicalite samples having siniilir compositions in terms of Ti even though the ionic radius of Ti4+is higher than that of Vs+. The analysis of the V-silicalite cell parameters, therefore, does not (27) (a) Anderson, J . R.; Foger, K.; Mole, T.; Rajadhyaksha, R. A.; Sanders, J. V . J . Catal. 1979, 58, 114. (b) Flanigen, E. M.; Bennet, J. M.; Grose, R. W.; Cohen, J . P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature (London) 1978, 271, 512.
2620 The Journal of Physical Chemistry, Vol. 96, No. 6,1992
Centi et al.
indicate whether or not V sites are actually present within the framework of the silicalite structure of VSi1545. On the other hand, in the samples with higher vanadium contents, such as VSill 17, the additional vanadium is present mainly as an external amorphous vanadium oxide phase. In fact, cell parameters similar to those of VSi1545 were found for VSill17. Wide-Line Solid-state V-NMR Spectra. The solid-state NMR spectrum of the slV isotope (spin 7/2), which possesses a nuclear electric quadrupole moment, is affected by the interaction of this moment with electrostatic field gradients created by asymmetric electronic environments. In a less asymmetric surrounding, the static NMR line shape is dominated by chemical shift anisotropy related to the three principal components of the nuclei magnetic shielding tensor. Therefore, in all cases, the NMR line shape gives useful information to differentiate between the various local coordination environments of vanadium sites, better than that obtained from analysis of the isotropic chemical shift However, comparison of the wide-line solid-state S'V-NMR line shape with those of selected chemical reference compounds is required. The 51V-NMRspectra and parameters of various model compounds with well-defined coordination environments have been previously reported,24-2s,28-30 but it is useful to summarize briefly the relationships observed between the local structure of V sites and SIV-NMRline shape. V205, which presents a nearly square-pyramidal geometry around vanadium [one short bond ( V 4 ) (0.158 nm), four axial V-0 bonds (mean value 0.180 nm), and one very long V-0 bond (0.278 nm)], shows a near-axial symmetric chemical shift tensor with maximized shielding along the axial direction. The wide-line 51V-NMRspectrum is dominated by a central line at about -300 ppm with a smaller line at -1000 ppm surrounded by very small satellites at about 800 and -2100 ppm corresponding to the first-order quadrupolar ~ p l i t t i n g . ~ ~ Metavanadates ,~~,~' Figure 2. Wide-line solid-state 5 1 V - N M Rspectra of VSil1 17, VSi1237, and VSi1545 samples. (MVZO6,where M = Mg, Zn, Pb) have a structure based on strongly distorted octahedral pairs sharing corner oxygen atoms; significantly shorter (about 0.161-0.162 nm) than that of the columns of double chains extend infinitely along the b axis.32 In corresponding K+ or (NH4)+compounds (about 0.166-0.168 nm). these compounds there are one very long (-0.26 nm), two shorter A decrease in the V-0 bond length implies a decrease in the (0.16-0.17 nm), and three moderately long (0.19-0.21 nm) V-O vectors representing the positions of the various electrons.3s In bonds. The SIV-NMRspectrum (for example, that of ZnV206) a homologous series of vanadate samples, a low-field chemical is characterized by a line shape quite different from that of V205 shift can be thus coherently interpreted as due to the different and is dominated by a main line centered at about -400 ppm effect induced on the local field sensed by the V nucleus from the surrounded by two components at about -300 and -950 ppm.28-30 decrease in the V-O bond length. The decrease in the chemical When a more symmetrical environment around V sites is present, shift from about -550 ppm for K3V04 to about -480 ppm for such as for MgV206,the spectrum is dominated by a broad nearly Tl,VO, can be thus correlated to the decrease in the V-0 bond symmetrical band centered at about -560 ppm which derives from lengths. the partial overlapping of the components due to the three anIn pyrovanadate (such as MeV207,where M = Pb, Cd, Zn, isotropic components with relatively small anisotropy values (A6 Mg) the tetrahedral V 0 4 species are linked at the tops of the = 63 - 6,) for the fully asymmetric 6-tensor.)O Decavanadate, in tetrahedra, forming [V20,l2-fragments with nonequivalent vawhich a 6-fold coordination environment around V also is present, has a spectrum close to that observed for MV206c o m p o ~ n d s . ~ ~nadium ~ ~ ~ ~atoms. ~ ~ This results in a distorted coordination environment around V and thus an asymmetric shielding tensor, which, Vanadates with 4-fold coordination can be distinguished from in turn, causes an increase in the anisotropy value of the &tensor the above V sites with 5- and 6-fold coordination, and in this case, and a change from a symmetrical line (orthovanadate) to a broader the line shapes in the SIV-NMRspectra depend considerably on main line centered at about -650 ppm (Zn2V207)with pronounced the local distortion around V sites. Monomeric orthovanadates shoulders on both the low- and high-field sides. In metavanadates such as Na3V04 and Mg3(V04)2have a structure formed by (such as MV03, where M = NH,, Na, K), the vanadium atom isolated tetrahedrally coordinated V ions in a nearly symmetrical has a strongly distorted environment and shares two bridging environment [three medium-short V-0 bonds (-0.17 nm) and oxygen with other V polyhedra. In pyrovanadate, each V site ~ ~ S'V-NMR one slightly longer V-0 bond (-0.18 r ~ m ) ] .The shares one bridging oxygen with other V sites, whereas no V-0-V s p e ~ t r are a characterized ~ ~ ~ ~ ~by~a narrow ~ ~ ~symmetrical ~ ~ line bridging oxygens are present in orthovanadate. The more distorted centered at - 4 5 0 ppm. In the case of Tl3VO,, a symmetrical environment around V in meta- compared to pyro- and ortholine is found at lower fields (-480 ppm). It should be observed vanadate gives rise to an increase in the anisotropy value of the that, in the thallium(1) ana ad ate,^^,^^ the V-0 bond length is &tensor, with a consequent further broadening, shifting, and strengthening of the lower and higher field components of the (28) Echert, H.; Wachs, I. E. J . Phys. Chem. 1989, 93, 6796. shielding tensor. (29) Chary, K. V. R.; Venkat Rao, V.; Mastikhin. V. M. J . Chem. Soc., Reported in Figure 2 are the wide-line SIV-NMRspectra of Chem. Commun. 1989, 202. (30) Lapina, 0. B.; Simakov, A. V.; Mastikhin, V. M.; Veniaminov. S.A,; VSi1117, VSi1237, and VSi1545 samples. In all samples only a Shibin, A. A. J . Mol. Card. 1989, 50, 55. symmetrical sharp line centered at -480 ppm is found. The (31) Gornostansky, S.D.; Stager, C. V. J . Chem. Phys. 1967, 46. 4959. intensity of the spectra, due to the very small amount of V present (32) Jordan, B. D.; Calvo, C. Can. J . Chem. 1974, 52, 2701 in the sample, is very low as shown by the low signal-to-noise ratio. (33) Krishnamachari, N.; Calvo, C. Can. J . Chem. 1971, 49, 1629. (34) (a) Touboul. M.; Ganne, M.; Cuche, C.; Tournoux, M. Z . Anorg. Allg. Chem. 1974, 410, 1. (b) Ganne, M.; Piffard, Y.; Tornoux, M. Can.J .
Chem. 1974, 52, 3539.
(35) Golovkin, B . G.; Fotiev, A . A . Zh. Neorg. Khim. 1973, 18, 2574.
Physicochemical Characterization of V-Silicalite
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2621
TABLE I: XPS Characterization of V-Silicalite Samples binding energy
VSi1237
V(2P,/ZtI/2), eV 516.7 516.7
VSi1545
516.8
sample VSilll7
molar ratio (xlo-)) (V~05/SiOAps (V205/Si02)chem.anal. 12.82 4.72 1.76
8.55 4.22 1.83
Apparent peaks at about -2000, -1200, and 1500 ppm are due to subtraction of probe free induction decay. No significant differences are found between the various samples, except for the decrease in the amount of V in the samples. However, in VSilll7 and possibly also in VSi1237, the narrow symmetrical sharp line overlaps a broad signal in the 0 to -1000 ppm range, which may suggest the presence in these samples of an additional Vv species in an undefined environment. On the basis of the above discussion of the relationship between W - N M R line shape and local structure of V sites, the line shape observed for VSil samples can be interpreted as being due to the presence of V5+ sites in a nearly symmetrical tetrahedral environment, with relatively short (about 0.1604165 nm) V 4 bonds. The shifting in the line position compared with that of an orthovanadate such as Na3V03can be attributed to the presence of a slightly shorter bond, according to the discussion on thallium vanadate. No clear evidence for the presence of crystalline V205 is found, at least as the predominant V5+species, but in VSilll7, the presence of another V5+species with no defined environment may be envisaged. These results are in good agreement with those found by Rigutto and van B e k k ~ m . ~ Another interesting aspect emerges from the analysis of the 5'V-NMR spectra of V-silicalite samples. Distorted tetrahedral vanadium(5+) oxide has been found, at low surface coverage, on y-A1,03, T i O , SiO,, and Mg0,24,2s330 in agreement with EXAFS and quantomechanical s t ~ d i e s . ~ However, ~,~* this species is very sensible to the presence of water vapor. After exposure of the samples to ambient conditions, the octahedral coordination is generally re-formed. The line shape change upon dehydration is not observed in V-silicalite, where stable nearly tetrahedral V5+ is evidenced by solid-state 51VN M R under ambient conditions, without line shape change depending on dry or wet air treatment. This indicates that vanadium is stabilized in nearly tetrahedral coordination by a direct specific interaction with the silicalite framework. X-ray Photoelectron (XPS) Spectroscopy. The XPS data on the characterization of V-silicalite samples are summarized in Table I. The binding energy of vanadium indicates the presence of V5+ as the main species present in all samples. The presence of a small contribution from a reduced vanadium species, however, cannot be excluded, due to the very small shift in the peak maximum (-0.5 eV) in going from V5+to the reduced vanadium species and to the low intensity of the V,, peak, which makes the analysis of peak shape difficult. The value of the binding energy for vanadium is in agreement with the value found for vanadium on The V/Si ratio for VSi1545 obtained from the XPS data corresponds well with that obtained by chemical analysis. A similar value is found after grinding of the samples in order to fracture particles. This procedure, which changes the internal versus external surface ratio, was found by Whittington and AndersonIO to increase the V/Si ratio. However, in our cases, the procedure was found not to be necessary due to the 10-15 times smaller dimensions of the zeolite crystals. The correspondence between the V/Si ratios determined by XPS and chemical analysis thus indicates a homogeneous dispersion of V in VSi1545. In contrast, in the sample with a higher V content (VSilll7), the V/Si ratio (36) Yoshida, S.; Tanaka, T.; Nishimura, Y.; Mizutani, H.; Funabiki, T. In Proceedings, 9th International Congress on Catalysis; Phillips, M. J., Ternan, M., Eds.;The Chemical Institute of Canada: Ottawa, Canada, 1988; Vol. 3, p 1473. (37) Tanaka, T.; Yamasjita, H.; Tsuchitani, R.; Funabiki, T.;Yoshida, S.
C
1
Figure 3. ESR spectra in vacuum of VSilll7 (a, 293 K; b, 77 K), VSi1237 (c, 77 K), and VSi1545 (d, 77 K).
found by XPS is higher than that found by chemical analysis. This suggests that, in VSilll7, part of the vanadium is segregated on the external surface of the silicalite crystals. Electron Spin Resonance (ESR) Studies. The ESR spectrum of VSilll7 a t 293 K is shown in Figure 3a. Apparently, the spectrum consists of a large signal L with peak-to-peak width of 110 G surrounded on the low- and high-field sides by a poorly resolved group of lines. At 77 K, the resolution of the spectrum improves considerably (Figure 3b). Superimposed on the large unstructured signal L a second signal S , with hyperfine structure, deriving from the interaction of free electrons of V4+ (3d') with the magnetic nuclear moment of 51V( I = '/*) that gives rise to an 8-fold hyperfine splitting of all anisotropic components, becomes evident. The ESR parameters are reported in Table 11. When the vanadium content decreases (VSi1237, VSi1545), the SIsignal becomes well-resolved and the intensity of the signal increases, whereas the L signal totally disappears (Figure 3c,d). Probably, as shown especially from the analysis of the shapes of low-field parallel components (mr = -7/2 and m,= - 5 / 2 ) , a second signal analogous to S,,but with slightly different values of the parallel and perpendicular components, is also present. The ESR parameters determined for the SI signal are typical for a polycrystalline sample containing V4+ ion in sites of axial symmetry. The g and A anisotropic tensor values deduced from the signal can be correlated with V02+ ions in octahedral sites with a tetragonal d i s t o r ~ i o n . ~ ~ - ~ ~
-
J . Chem. Soc., Faraday Trans. I 1988, 84, 2987.
(38) Kobayashi, H.; Yamaguchi, M.; Tanaka, T.; Nishimura, Y.; Kowakami, H.; Yoshida, S. J . Phys. Chem. 1988, 92, 2516.
(39) Tougne, P.; Legrand, A. P.; Sanchez, C.; Livage, J. J . Phys. Chem. Solids 1981, 42, 101.
2622 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
Centi et al.
TABLE 11: ESR Parameters and Type of Species Detected by ESR in VSilll7, VSi1237, and VSi1545 after Evacuation and after Reduction with 20% H2in Helium at Different Temperatures‘ type of species sample T, K gll g, 4,G A , , G W i l l 17 V3+ dimers and V 0 2 + dimers L 293 1.9686 113 77 1.9686 91 s1 77 203 80 (V 02+) octa VSi1237 or VSi1545 (V O2 s1 77 1.9355 1.9827 203 80 VSi1237 or VSi1545 after reductionb s2 77 1.9117 1.9625 154 68 (V4+),etra +
‘Spectra recorded at 298 K or 77 K as indicated. bReduction with a flow of 20% H, in helium
The broad L signal may be attributed to V4+ in dipolar interaction with other V4+ ions. However, the hyperfine splitting constant of the parallel components of the L signal (1 13 G) is almost half that of those corresponding to the SIsignal (203 G), and a slight narrowing (a decrease of -20 G of the peak-to-peak width) of the signal as the temperature decreases is observed (compare parts a and b of Figure 3). Similar spectra have been obtained in vanadyl chelates of 4,4’,4”,4”’-tetrasulfophthalocyanine4’ and in vanadyl porphyrins.48 It has been shown49~50 that, if the two ions of a dimer are equivalent and the pair axis is the same as one of the principal axes of the single ion, the spectra corresponding to those species should have the same g tensor and the A tensor of the dimeric spectrum should be half that of the single ion. The narrowing of line width, however, may be interpreted as being due to a superexchange effect between two paramagnetic sites as seen in hydrogen bronzes of vanadium pentaoxide (HzxV205,).51 For these compounds, V3+ions form by reduction and two V3+ions (configuration 3d,4s) form a pair with the configuration 3d,4s2,3d. This pairing of the 4s orbitals cancels their magnetic moment in a such a way that these two V3+ions have the same magnetic moment as two 3dI ions, namely, two V4+ ions. The overlapping of the 4s orbitals is at the origin of an exchange between two vanadium cations and the narrow ESR signal. When the temperature decreases from 298 to 77 K, the intensity ratio of the L signal is in the 4-6 range, which does not agree with the Curie law dependence. This phenomenon can be interpreted by the fact that for a given population of a paramagnetic triplet state of vanadium ion pairs, the L signal intensity will also be governed by the Bolzmann distribution following the equation P = [ 3 / q exp[-J/kU where J is the splitting between the singlet and the triplet states by coupling between the two unpaired electrons of the ion pair. This suggests that the large unstructured signal L can be attributed to the presence of isolated dimers of V3+ ions, and possibly also of VOz+ dimers. The ESR analysis of VSilll7 thus suggests the presence of an isolated nearly octahedral vanadyl species and of a vanadium oxide polynuclear species containing pairs of V3+ions and of V02+ ions. Compared to VSilll7 (Figure 3a,b), VSi1237 (Figure 3c) and VSi1545 (Figure 3d) are less heterogeneous and no polynuclear
(40) Bardoux, P.; Gourier, D.; Livage, J. Colloids Surf. 1984, 11, 119. (41) Busca, G.; Centi, G.; Marchetti, L.; TrifirB, F. Langmuir, 1986, 2, 5568. (42) Cavani, F.; Centi, G.; Foresti, E.; Trifir6, F.; Busca, G. J. Chem. SOC., Faraday Trans. I 1988, 84, 237. (43) Che, H.; Canosa, B.; Gonzalez-Elipe, A. R. J. Phys. Chem. 1986, 90, 618. (44) Meriaudeau, P.; Vedrine, J. C. Nouu. J , Chim. 1978, 2, 133. (45) Gray, H. B. Inorg. Chem. 1962, I , 1 1 1 . (46) Siegel, I. Phys. Reu. 1964, 134, 193. (47) Boyd, P. 0. W.; Smith, T. D. J. Chem. SOC.,Dalton Trans. A 1972, 839. (48) Boyd, P. D. W.; Smith, T. D.; Price, J. H.; Pilbrow, J. R. J . Chem. Phys. 1972, 56, 1253. (49) Dupegre, R. M.; Lemeuri, H.; Rassat, A. J. Am. Chem. SOC.1965, 87, 3771. (50) Meriaudeau, P.; Cleyaud, B.; Che, M . J. Phys. Chem. 1987,87, 3872. (51) Tinet, D.; Legay, M. H.; Gatineau, L.; Fripiat, J. J. J . Phys. Chem. 1986, 90, 948.
at
778 K.
vanadium oxide is present, but rather only isolated nearly octahedral V02+. It should be noted that a signal with ESR parameters very close to that of the SI signal is found in samples obtained by solid-state reaction of V205with H-ZSM5.52 The signal is attributed to isolated VIv ions in the zeolite interior near to a charge-compensating (OH) site, as shown by electron spin echo modulation experiment^.^^ The migration of V inside the zeolite and the spontaneous reduction of V5+to V4+occurring during calcination derive from the interaction of V with the strong Bransted groups associated with Al sites in the zeolite framework. Similar evidence of V5+ to V4+ reduction and analogous ESR spectra of vanadyl species was found by Whittington and Anderson’O by treatment of silicalite with VOC13vapor. Upon dehydration in vacuum, the ESR signal of the V205-ZSM5 sample changes slightly and the ESR parameters reported by the authors52correspond to those tentatively suggested to interpret the possible presence of a second signal with hyperfine structure, as seen especially in VSi1545 sample. The presence of this second signal with hyperfine structure is suggested by the broadening of the low-field parallel components, but cannot be resolved unambiguously, and for this reason the corresponding parameters are not reported. However, on the basis of the similarity with the V-ZSM5 results, the SI signal can be interpreted as due to the presence of a saturated vanadyl species and the possible second signal with hyperfine structure to the same species, but unsaturated due to the removal of water of coordination. The correspondence of ESR parameters of isolated V@+ species in V-silicalite samples with those observed in V-ZSM5 samples suggests that the vanadyl groups are probably inside the zeolitic channels of the silicalite and near OH groups. In pure silicalite, only some silanol groups on the external surface are present, due to structural defects or crystal faults created during the preparation of the zeolite. In fact, as a consequence of the process of template growing, defects and thus silanol groups must be localized mainly at the external surface of the crystals of pure silicalite. The evidence from the ESR spectrum for VSil samples (signal SI)as found for V-ZSMS samples51thus suggests that relatively strong O H groups are present inside the V-silicalite pore structure, as a consequence of specific defects created by the presence of vanadium during hydrothermal preparation or associated to the interaction of vanadium with the silicalite structure. These Bransted groups are salified by reaction with vanadium to give the SI signal of the saturated vanadyl species (and possibly a second unsaturated vanadyl species), analogous to those found in V-ZSMS and due to the reaction of V with the strong Bransted groups present in H-ZSM5.52 Effect of Reduction. In order to characterize better the nature of the vanadium species, V-silicalite samples were reduced in a H2 (20% in helium) flow at 623 and 773 K. VSilll7 after reduction at 623 K gives rise to a broad intense unstructured signal centered at g = 1.9686, with the peak-to-peak width larger than before reduction. The broadening of the signal indicates an increase in the integral of exchange and the dipolar interaction between paramagnetic centerss3 and thus an increase in near-lying V4+or V3+ sites, with an apparent disappearance of isolated vanadyl groups (S, signal). The same effect is observed (52) Sass, C.E.: Chen, X.; Kevan, L. J. Chem. Soc., Faraday Tram. 1990, 86. 189.
(53) O’Reilley, D. E.; MacIver, D. S. J. Phys. Chem. 1962, 66, 276.
Physicochemical Characterization of V-Silicalite (OOG
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2623
I
SI1545 SI1117
VSII-CR VS3-IM
VSOZ.IM
10000
30000
20000
40000
50000
wavenumber, cm-1
ESR spectra of VSi1545 after reduction at 773 K with 20% H2in helium (a, 77 K) and after subsequent admission of 10 Torr O2at Figure 4.
room temperature and subsequent evacuation) (b, 77 K).
-U
lo
.
i
c
t g,,=2.0905
OF
&
_Id--.. I
10000
I
.
20000
1
c ,
30000
40000
50000
wavenumber, cm-1
Figure 6. UV-visible diffuse reflectance spectra of V-silicalite samples and of some reference compounds. In (a) the spectra were normalized to nearly the same amount of vanadium as in the VSi1545 sample. Figure 5. ESR spectrum at 77 K of (V4+),,,, sites on VSi1545.
OF anion radicals adsorbed on
upon similar treatment, but in a flow of helium, suggesting that dehydration of vanadium oxide domains also induces an increase in near-lying paramagnetic centers. VSi1237 and VSi1545 after reduction or H e treatment at 623 K also show the presence of a very broad signal lacking hyperfine indicates a structure. The very large peak-to-peak width (W,,) very strong magnetic interaction. After reduction a t higher temperature (773 K), the unstructured signal further broadens ( W,,> 2000 G) and practically disappears in the spectrum recorded at 77 K, but a new signal with hyperfine structure appears (S2,Table 11) (Figure 4a). Compared to the SIsignal, attributed to distorted octahedral V02+,a strong decrease in the A,,and slight decrease in the A , parameters are observed (Table 11). This indicates that the axial component becomes more covalent. The g factors also change, indicating a modification in the V4+ environment. The signal S2 can thus be attributed to V4+ ions in a distorted tetrahedral e n v i r ~ n m e n t . ~ The ~ ~ ~shape ~ . ~ ~of the low-field parallel components of the S2signal in this case suggests the possible presence of a second similar signal that, however, cannot be resolved unambiguously. A new signal with superhyperfine structure appears in the spectrum recorded at 77 K after admission at 293 K of small amounts of gaseous oxygen (10 Torr) and subsequent evacuation (Figure 4b; reported in Figure 5 is the same signal at a higher scan range resolution). The new signal overlaps the S2 signal, whose intensity slightly decreases. The ESR parameters of the new signal (gl,= 2.0905, g22= 2.0295, g,, = 2.0034) clearly indicate the formation of an [02]It should be noted (54) Narayama, M.; Nasami, C. S.; Evans, L.J . Carol. 1983, 79, 237. (55) Shvets, V. A.; Kazansky, V. B. J . Card. 1972, 25, 123. (56) Che, M.; Tench, A. J. Ada Carol. 1982, 31, 77. (57) Lunsford, J. H. In Coralysis, Science ond Technology;Anderson, J.
R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1987; Vol. 8, Chapter 5, p 227.
that according to Shvets and KazanskP this adsorbed oxygen species indicates the presence of V4+ ions in a tetrahedral environment, according to the following scheme:
-
(V4+)telra+ O2
V5+.02-
This further confirms the assignment of the S2 signal to tetrahedrally coordinated V4+. UV-Visible Diffuse Reflectance Spectra. Diffuse reflectance (DR) spectra in the UV-visible region of V-silicalite samples and of selected reference compounds are reported in panels a and b of Figure 6, respectively. For this characterization technique less data are available from the literature in regard to correlations between the local structure of V sites in solid samples and their DR spectra. It is thus useful to report the spectra of selected compounds of V5+that help in the interpretation. In particular, spectra of magnesium vanadate are shown in Figure 6b to evidence the modifications in the spectra related to the change in the structural features from isolated monomeric tetrahedral VO, [magnesium orthovanadate, Mg3(V04),] to a strongly distorted octahedral environment with columns of octahedral pairs [magnesium metavanadate, Mg(V0J2]. Spectra of isolated monomeric VO, such as in Na3V04,of polymeric tetrahedral VO, sharing two bridging oxygen with other V polyhedra [NH4V03]and of V205 characterized by distorted trigonal bypyramids of V 0 5 sharing edges to form zigzag double chains, are also shown (Figure 6b). Also reported in Figure 6a are the DR spectra of other reference samples in order to indicate more clearly the spectral features characteristic of V-silicalite samples prepared by hydrothermal synthesis (VSil). In particular, DR spectra of samples prepared by impregnation with NH,VO, (and subsequent calcination) of pure sikalite and of SiO; (VSil-IM and VSi02-IM, respectively) are reported together with the DR spectrum of a sample whose zeolite structure collapsed during calcination to form the a-cristoba1ite structure (VSil-CR-see Section). All spectra of Figure 6a were normalized to nearly the same vanadium content in order to facilitate the comparison.
2624 The Journal of Physical Chemistry, Vol. 96,No. 6,1992
In general, the electronic spectra of vanadium ionss8 in the region examined are characterized for the vanadium(V) ions by the lower-energy charge-transfer (LCT) band associated with 0 to V electron transfer5*- falling in the 20000-30000-cm-' region for octahedral coordination. For lower coordination, such as tetrahedral vanadium(V), the LCT transition is expected at higher frequencies (30000-35 000 cm-1).61 The LCT transition for vanadium(1V) falls at higher frequencies (35 000-40 000-cm-' region), whereas the d-d transitions of V02+ions fall, in the region examined, at near 13 000 cm-l [b2(d,) e(d,,dy,)] and at near b,(d,~,~)].The d-d transition at higher 16000 cm-l [b2(d,) frequencies [b2(d,) al(d,z)] is generally masked by more intense CT (charge-transfer) transitions. The intensities of these d-d transitions, in fact, are generally 10-30 times lower than those of CT transitions. The spectrum of the reference compound VzOs (Figure 6b) shows three main absorption bands at -22000 cm-I, at -31 000 cm-I, and at 42000 cm-l, with a fourth partially masked absorption band at 26 000 cm-I. Following J ~ r g e n s e n , the ~ , energy of the LCT transition can be calculated by the electronegativity of ligand or metal, the number of d electrons, the difference in electronegativities between do and d' states of the metal, and the spinpairing energy parameter. On the basis of this correlation, which gives a good description of the charge-transfer spectra for V, Mo, W, and N b oxy compounds,59 the LCT band can be calculated to fall at -26 000 cm-l. The 26 000 and 3 1 OOO-cm-' bands in V,Os thus can be assigned to e(.) b,(xy) (LCT) and a2(r),bl(r) b2(xy) transitions, due to terminal or bridging oxygen, respectively. The lowest energy band at 21 500 cm-I, on the contrary, cannot be reasonably assigned to d-d transitions as given by Hanuza and Jezowska-Tr~ebiatowska,~~ but rather has been tentatively assigned to a charge transfer to a delocalized acceptor site such as conjugated sites like those in octahedral V06 chains. An analogous interpretation can be made for the spectrum of magnesium metavanadate, which shows three main absorption bands at about 26 000, 30 000, and 43 000 cm-I and a shoulder at -21 000 cm-l. In tetrahedral Vv compounds [NH4V03,Na3V04,Mg3(V04),; Figure 6b], in contrast, the LCT band is found at -30000 cm-' and the second CT absorption band at -36000 cm-I. They can be attributed to ( r ) t l (d)e and (r)t, (d)e oxygen to vanadium transitions. Relatively small changes in the positions of these bands are observed between the various reference compounds examined for Vv tetrahedral symmetry, notwithstanding the considerable changes in the coordination distortion (see also discussion of structural features of these compounds in the 51VNMR part). It should be observed, however, that the position of the (vo4)3-lowest charge-transfer transition correlates well with the V-0 bond distance.61 The following points emerge from this comparison: (i) the CT band due to the V=O double bond falls near 26 000 cm-I, (ii) bands at lower energies may be tentatively attributed to the presence of V-0-V chains, (iii) in a distorted octahedral (magnesium metavanadate) or nearly square-pyramidal (V205) environment the second CT band is expected near 3 1 000 cm-I, (iv) the LCT band due to V - 0 bonds is found at higher energies (- 30 000 cm-I) for tetrahedral Vv compounds and does not depend greatly on the distortion of environment, and (v) LCT bands in the 25 000-35 000-cm-' region are related to Vv species (for VIv species these bands are expected at higher energies, 40 000-45 OOO-cm-l region). The spectra of V-silicalite samples (Figure 6a) can be analyzed
--
-
-
-+
-
-
(58) (a) Hush,N. S.; Hobba, R. J. Prog. Inorg. Chem. 1968,10,259. (b) Kung, F.; Good, M. L. Spectrochim. Acto 1973, 29A, 707. (c) Selbin, J. Chem. Rev. 1965, 65, 153. (59) So, H.;Pope, M. T. Inorg. Chem. 1972, 11, 1441. (60) Iwamoto, M.; Furukawa, H.; Matsukami, K.; Takenaka, T.; Kagawa, S . J . Am. Chem. Soc. 1983, 105, 3719. (61) Ronde, H.; Snijders, J. G. Chem. Phys. Lett. 1977, 50. 282. (62) (a) Jerrgensen, C. K. Mol. Phys. 1959, 2, 309. (b) Jerrgensen, C. K. Struct. Bonding (Berlin) 1966, 1 , 3 . (63) Hanuza, J.; Jezowska-Trzebiatowska, B.; Oganowski, W . J . Mol. Cofol.1985, 29, 109.
Centi et al. on the basis of such considerations. VSi1545 shows a well-defined band centered at 26 000 err-' and two further broad bands at about 38 000 and 43 000 cm-I. VSil1 17 shows these bands, as well as additional bands at about 30 000 and 34 000 cm-I. It should be noted that impregnating pure silicalite or Si02 with ammonium vanadate and subsequent calcination (Figure 6a, VSil-IM and VSiO,-IM, respectively), led to quite different spectra. Main bands fall at about 38 000 and 43 OOO cm-I and a not well resolved shoulder is observed in the 26000-32000-~m-~region. The presence of an intense absorption band at 26000 cm-l is thus characteristic of V-silicalite prepared hydrothermally and, tentatively, of a V species anchored to the zeolite framework. In support of this interpretation, only a weak shoulder is found at -26000 cm-' in a sample in which the zeolite structure has collapsed (VSilCR), forming an a-cristobalite structure. According to that discussed above, the energy of the 26000-cm-' C T transitions suggest the presence of a V=O double bond. However, in VSi1545 the difference in energies between the LCT and the second CT transition (falling at -38 000 cm-I) does not agree with that expected for a distorted octahedral or squarepyramidal coordination environment. In VSilll7, bands are found in the 30 000-36 000-cm-' region, but it should be noted that in this sample a large part of V can be removed by extraction with an ammoniacal solution (see Experimental Section). In the DR spectrum after extraction (sample VSil545) these bands disappear while the band centered at 26 000 cm-I remains clearly evident. This suggests that the absorption bands in the 30 000-35 000-cm-I region and possibly a shoulder near 22 000 cm-I are associated with a species removed during the extraction procedure, which can be considered as being an amorphous polynuclear vanadium(V) oxide species, not detected by X-ray diffraction analysis. The diffuse absorption band in the 10000-20000-cm-' region also decreases after the extraction procedure (compare VSi1545 and VSill 17 DR spectra), further indication for the removal of reduced V species (VIv and VI") during this treatment. More difficult is the attribution of the Vs+ species characterized by the 26000-cm-I band. The position of this band, as stated before, suggests the presence of a short (double) vanadium-xygen bond, but the second CT band does not agree with that expected for a distorted octahedral or square-pyramidal environment, but rather is indicative of a nearly tetrahedral field. The interpretation of the UV-visible DR spectra of VSi1545 thus suggests the presence of a Vv species characterized by a short vanadium-oxygen bond (-0.16 nm) and three slightly longer (about 0.1654).170 nm) V 4 bonds, according to the correlation observed between (V04)3-charge-transfer transitions and V-O distance.6I However, this interpretation cannot be considered unequivocal, due to the lack of suitable reference compounds to confirm the analysis. Fourier-Transform Infrared (FT-IR) Spectroscopy. The presence of vanadium interacting with the silicalite framework may induce modifications in the surface acidity properties that can be characterized by IR spectroscopy using suitable probe molecules. VSi1545 was used for the tests, because an extralattice polynuclear vanadium oxide species was detected in the other VSil samples, as discussed above, that can alter the interpretation of the results. In addition, all the tests were done in comparison with a pure silicalite prepared using the same hydrothermal methodology, but without vanadium, in order to clarify better the specific modifications induced by the presence of vanadium. Reported in Figure 7 are the IR spectra in the 1500-4000-cm-' range of the VSi1545 outgassed in vacuo at increasing temperatures. In the inset the analogous results for the Si1 sample are also shown. The series of bands in the 1500-2000-~m-~region are due to overtone modes of fundamental lattice vibrations of the zeolite structure. The absence of modifications shows that the V-silicalite structure does not collapse during the thermal treatments up to temperatures of the order of 850 K. The outgassing at progressively higher temperatures in the 293-823 K range causes a progressive disappearance of the strong and broad, structureless, absorption band in the 3700-2500-cm-' range, due to progressive dehydroxylation of the solid. At the higher tem-
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2625
Physicochemical Characterization of V-Silicalite
L
2000
l5bO'
'
1OoO cm.' 1d00 '
I
1600
'
'
'1300
Figure 8. FT-IRspectra of VSi1545 (a, solid line) and of pure silicalite (b, dashed line) after adsorption of 50 Torr NH, a t room temperature and subsequent evacuation a t 323 K for 1 h (A) or after adsorption of pyridine a t room temperature and evacuation a t 423 K for 3 h (B).
i I
I
4000
I
I
I
3000
I
I
I
I
I
2000
I
I
I
-,I
L
,500
cm
Figure 7. M-IR spectra in vacuum of the vOH region of VSi1545 after different evacuation temperatures: (a) 373 K, (b) 723 K, (c) 783 K, (d) 823 K. In the inset are analogous spectra obtained after evacuation of a pure silicalite sample at different temperatures: (a) 623 K, (b) 723 K, (c) 783 K.
peratures, a sharp peak at 3726 cm-' is present due to isolated silanols.@ The same band is present in pure silicalite and is related to defects or crystal faults in the zeolite structure which form during preparation. The intensity of the bands, in fact, is higher in microcrystalline silicalite as compared to analogous samples with larger crystal dimensions.*' According to this interpretation, these silanol groups are probably situated on the external surface of the zeolite crystals. A second distinct peak is observed at 3682 cm-' which can be attributed to free, less thermally stable, OH groups probably also connected to Si atoms, deduced from the fact that they also are present in the pure silicalite (inset of Figure 7) as well as in Ti-~ilicalite.6~Analysis of the IR hydroxyl stretching region does not provide evidence of specific differences between the V-silicalite and pure silicalite. In order to characterize further differences in the Bransted acidity, the adsorption of ammonia and pyridine on VSi1545 and Sil was carried out. The spectra after evacuation at 323 and 423 K, respectively, are reported in Figure 8A and B. The adsorption of ammonia on VSi1545 (Figure 8A,a) gives rise to the formation of two distinct but weak bands at 1460 and 1702 cm-' due to ammonium ions (6,,, 6, NH4+, respectively), indicating the presence of Bransted acid sites.65d7 The position of both bands agrees well with that observed by absorption of ammonia on hydroxylated alumina.65d7 The 6, NH4+ is absent in pure silicalite (Figure 8A,b), suggesting that Bransted sites able to protonate ammonia are specifically related to the presence of vanadium in the silicalite. This 6, NH4+is thermally unstable and disappears upon evacuation at 373 K, indicating that the (64) Boccuti, M. R.; Rao, K. M.; Zecchina, A.; Leofanti, G. Petrini, G. In Structure and Reactivity of Surfaces; Morterra, L., Zecchina, A., Costa, G., Eds.; Elsevier Science Publishers: Amsterdam, 1989;p 133. ( 6 5 ) Kung, M. C.; Kung, H.H. Catal. Reu.-Sci. Eng. 1985, 27, 425. (66)Tsyganenko, A. A.; Pozdnyakov, D. V.; Filimonov, V. N. J. Mol. Sfruct. 1975, 29, 299. (67) Jobson, E.;Baiker, A.; Wokaun, A. J . Chem. SOC.,Faraday Trans. 1990,86, 1131.
Bransted acidity is relatively weak. It also can be noted that the silanol groups shown to be present in pure silicalite (see Figure 7) apparently do not give rise to the protonation of ammonia, probably because their acid strength is too weak. The NH4+bands in V-silicalite thus indicate the presence of additional stronger Bransted sites with respect to pure silicalite. These sites can protonate ammonia, but are still relatively weak so as to be thermally unstable upon evacuation at temperatures higher than 373 K. No evidence is found of the presence of chemisorbed ammonia. The adsorption of pyridine and evacuation at 423 K (Figure 8B) gives rise to different results. The bands at 1597 and 1446 cm-l are typical of pyridine chemisorbed on Lewis acid sites [@a) and ~(19b)].6~,~*-~' As is well-known, these ring vibration modes are most affected by the nature of intermolecular interactions via the nitrogen lone pair electrons and thus the extent of the shift of the v(8a) vibration (1578 cm-' in the liquid) may be taken as a measure of the strength of the Lewis ~ites.6*-~'In this case, the position of the bands indicates the presence of very weak Lewis sites on both V-silicalite and pure silicalite, reasonably formed upon dehydroxylation of silanol groups. This is confirmed by the disappearance of the bands upon evacuation at higher temperatures. A further weak band at 1581 cm-' is observed (Figure 8B) in the spectra which suggests the presence of H-bonded pyridine,65q70'71 taking into account the evacuation temperature. This suggests the presence of weak silanol groups that, however, are not able to form a pyridinium ion stable upon evacuation at 423 K, in agreement with previous results. It should furthermore be considered that the relatively stronger Bransted sites in VSi1545 as indicated by ammonia adsorption can still be too weak to protonate pyridine (from this point of view, ammonia is a stronger base than pyridine for adsorption onto a Bransted acid site") and/or are less accessible to pyridine since they are located inside the zeolite channels. In fact, the mean steric hindrance for pyridine is 0.59 nm and for ammonia 0.26 nm, compared to -0.55 nm of mean diameter of access to pore cavities for pentasyl ~eolites.'~ Probably, only in the presence of very strong Bransted sites such (68) Centi, G . ; Golinelli, G.;Busca, G.J. Phys. Chem. 1990, 94, 6813. Centi, G.;Trifir6, F.; Lorenzelli, V. J. Phys. Chem. 1986, (69) Busca, G.; 90, 1337. (70) Knozinger, H. Adu. Catal. 1976, 25, 184. (71) Boehm, H.-P.; Knozinger, H. In Cafalysis,Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1983;Chapter 2, p 39. (72) Bein, T.;Brown, K.; Brinker, C. J. In Zeolites: Facts, Figure, Future; Jacobs, P. A,, van Santen, R. A,, Eds.; Elsevier Science Publishers: Amsterdam, 1989;Vol. B, p 887.
2626 The Journal of Physical Chemistry, Vol. 96, No. 6. 1992
Centi et al.
TABLE III: Results Obtained by NHJ-TPD and H,-TPR Experiments on VSi1117, VSi1545, and Si1 Samples NH,-TPD expt
H2-TPR expt mol of NH3/mol of V 2"
mol of N H 3 / g of catalyst samvle
1'
2"
3"
1"
VSi1545 VSil117 Si1
9.77 x 10" 1.01 X 2.26 x 10-5
3.44 x 10" 3.47 X lo4
2.83 X 10"
1.66 X lo-' 7.21 X
5.84 X 2.48 X
3"
2.02 X
T.6 K
mol of H J m o l of V
910 918
0.48 0.44
K.
"First NH3-TPD desorption peak centered a t 500 K; second desorption peak centered at 625 K; third desorption peak centered a t 670 Maximum temperature of rate of reduction.
as in A1-ZSM5 is the driving force of the reaction sufficient to allow diffusion of pyridine at room temperature. On the contrary, it is known that pyridine is adsorbed more strongly (is desorbed more slowly) than NH, on Lewis acid which may explain why chemisorbed ammonia is not detected (Figure 8A). Taking into account the steric hindrance of pyridine, weak Lewis sites in the zeolite channels could not be detected using this molecule. Additional tests were thus carried out using CD3CN and tert-butyl cyanide as probe molecules to differentiate between the presence of Lewis sites in internal or external positions of the zeolite crystals and surface heterogeneities. In fact, the extent of the vCN vibration as compared to that of liquid (2259 cm-l for CD3CN and 2238 cm-l for tert-butyl cyanide)73is related to the strength of the Lewis acidity. However, due to Fermi resonance between the vCN and the (6CH, + vcX) combination band in CH3CN, a series of bands are observed in the vCN region using CH3CN.69i73*74 Due to the absence of this Fermi resonance in deuterated acetonitrile or in tert-butyl in contrast, a single band is observed after adsorption of these probe molecules on Lewis acid sites, allowing a better analysis of surface heterogeneity when these molecules are used instead of acetonitrile. In addition, the steric hindrance of acetonitrile is -0.3 nm while that of tert-butyl cyanide7*is 0.81 nm, and thus they can discriminate between Lewis acid sites internal or external to the zeolite crystals. Reported in Figure 9A and B are the spectra obtained upon adsorption of CD3CN or tert-butyl cyanide, respectively, and evacuation at room temperature. The adsorption of CD3CN on the V-silicalite and subsequent evacuation at room temperature (Figure 9A, a) gives rise to the formation of two bands at 2275 (stronger) and at 2313 cm-'. Only the former band is observed on pure silicalite (Figure 9A, b). The extent of the shift of these vCN modes as compared to liquid indicates the presence of very weak Lewis acid sites (2275 cm-l) and of medium-strong Lewis acid sites (23 13 cm-I). The frequency of the latter is very similar to that observed on vanadyl p y r o p h o ~ p h a t e ~and ~ 9 ~assigned ~ to surface-exposed unsaturated vanadyl species. The former fall at frequencies very similar to those observed in decationized Y zeolites.74 Upon adsorption of tert-butyl cyanide (Figure 9B), in contrast, a single band at 2250 cm-' is observed both on VSi1545 (a) and pure silicalite (b). The extent of the shift as compared to the vCN mode in liquid confirms the presence of very weak Lewis acid sites. By consideration of (i) the higher steric hindrance of tert-butyl cyanide, which prevents its reaction with Lewis acid sites inside the zeolite crystals, and (ii) the presence of stronger additional sites in the V-silicalite as compared to pure silicalite (as shown by deuterated acetonitrile adsorption), it can be concluded that very weak Lewis acid sites are present on the external surface of both VSi1545 and Sil, but additional stronger Lewis acid sites are present inside the zeolite channels in V-silicalite and are reasonably related to vanadium sites. Temperaturehogrammed Methods. In order to characterize further the surface acidity of V-silicalite samples, ammonia temperature-programmed desorption (NH,-TPD) tests were carried out on VSil117, VSi1545, and Sil. The results obtained are shown in Figure 10. (73) Kniizinger, H.; Krietenbrink, H.J. Chem. SOC.,Faraday Trans. 1
1975, 71, 242 1. (74) Angell, C. L.; Howell,
M.V. J. Phys. G e m . 1969, 73, 2551.
ppm NH3
I
I
0
400
IIh,
I
I
II II
si1 -__ I I
C
500
'
1
600 700 Temperature, K
I
800
._J 900
Figure 10. NH3-TPD profiles for ammonia desorption from VSil117 (a), VSi1545 (b), and Si1 (c) samples. Ammonia was adsorbed at 373 K; heating rate 10 K/min.
Two desorption peaks can be observed, centered at about 500 and 625 K,after activated NH, adsorption (373 K) in order to avoid physically adsorbed species. Present in all three samples is the lower temperature desorption peak, reasonably related to the presence of very weak silanol Bransted sites due to silicalite crystal defects. The higher temperature ammonia desorption peak is absent in pure silicalite and increases in intensity in going from VSil545 to VSil117. Quantitative data for the amount of ammonia desorbed for the various peaks and samples are reported in Table 111. The higher temperature desorption peak in VSi1545 can be attributed to V-OH or V-*(OH)-Si sites with higher Bransted acidity as compared to the former silanol sites. This interpretation is in agreement with infrared data on NH, adsorption on this sample (Figure 8A), which indicate the presence of additional
Physicochemical Characterization of V-Silicalite stronger Bronsted sites in VSi1545 as compared to Sil. However, it is possible to note that an additional peak centered at -670 K is probably present in VSilll7 that can be reasonably associated with V 4 H of the extraframework vanadium oxide present in this sample. This suggests that the peak centered a t 625 K may be attributed to silanol group with enhanced acidity due to near-lying vanadium sites [V-.(OH)-Si]. It should also be noted that, in all samples, the stronger Bronsted sites characteristic of H-ZSMS which give rise to an ammonia desorption peak centered at -800 K are not present. As shown in Table 111, the integrated area of NH,-TPD peaks indicates the presence of a limited number of sites able to protonate ammonia; this agrees with the presence of a limited number of free silanol groups due to crystal defects induced during synthesis of the zeolite. In pure silicalite, the intensity of the NH,-TPD peak centered at 500 K is higher than in V-silicalite samples, suggesting that the tetrahedral vanadium species characteristic of the V-silicalite may form at defect sites of the zeolite framework, probably hydroxyl nest^.^^'^ The integrated areas of the NH,-TPD peaks associated with vanadium (peaks centered at 625 and 670 K) indicate a low ratio of moles of NH3 desorbed per mole of vanadium present, whereas a value near to 1 can be expected, especially for VSi1545. However, it must be taken into account that for various reasons a value below 1 can be found, even if one Bronsted site is associated with each tetrahedral vanadium site characteristic of this sample. In fact, due to the low acid strength of these Bronsted sites, it is reasonable to expect that only a fraction of the sites is protonated; this agrees with infrared observations. Furthermore, part of the Bronsted sites are saturated by near-lying V02+ groups (according to ESR results) and an equilibrium between two species, depending on the partial pressure of water, may reasonably exist, even if the nearly tetrahedral coordination is maintained. It should be noted, in fact, that, before NH,-TPD tests, the samples are equilibrated in a water-free helium flow. Also reported in Table I11 are the results of the characterization of V-silicalite samples by temperature-programmed reduction with hydrogen (H2-TPR). The temperature of the maximum rate of H2consumption agrees with that found for vanadium supported on Si02,75which is generally higher than that observed for vanadium supported on other oxides such as T i 0 2 or A1203. This indicates a close relationship between the vanadium species found on the silica surface or in V-silicalite, even though it must be noted that the temperature of the maximum rate of reduction is related to the strength of the V-O-M bond (M = Si, Ti, Al) more than to the coordination of vanadium, and thus for well-dispersed species, similar results can be expected for vanadium on silica or V-silicalite. In VSill 17, a slightly higher temperature for the H2-TPR peak is found, in agreement with the possible presence of an amorphous vanadium oxide species. In fact, the maximum temperature of reduction of not-supported V205is observed at -950 K.75 Also reported in Table 111 are the estimated values of the ratio between moles of H2consumed and moles of V present. It has been shown75that V5+ on silica is reduced to V4+ under these conditions. The results reported in Table I11 thus indicate that vanadium is mainly present as a V5+ species, in agreement with the UV-visible DR and XPS results.
-
Discussion Type of Vanadium Species. The characterization of V-silicalite indicates the presence of at least three types of vanadium species: (i) a polynuclear vanadium oxide containing V in various valence states (VI1', VW,and Vv, but mainly Vv as indicated by UV-visible DR, XPS, and H2-TPR data); (ii) an octahedral V02+, preferentially interacting with O H groups localized inside the pore structure of the zeolite crystals, as indicated by ESR, FT-IR, and NH3-TPD data; and (iii) a nearly symmetrical tetrahedral V5+ species, as indicated by 51V-NMR,UV-visible DR, XPS, H,-TPR data, and attributed to a V species anchored to the zeolite (75) Erdohelyi, A,; Solymosi,
F. J . Coral. 1990, 123, 31.
The Journal of Physical Chemistry, Vol. 96, No. 6,1992 2627 framework at defect (silanol) sites. The comparison of ESR and UV-visible DR spectra of VSilll7 with those of VSi1545, in fact, clearly indicates the presence of a vanadium oxide in the former sample that can be removed by extraction a t room temperature with an ammonium acetate solution. The easy solubilization indicates that this vanadium species is probably localized, in pores or in external positions, as suggested by XPS results. This species is amorphous, since it is not detected by X-ray diffraction analysis. In the hydrothermal preparation method, V1"C13 was used as the source of vanadium. It is reasonable that part of the vanadium remains on the zeolite crystals as amorphous hydroxide, which partially oxidizes during calcination. ESR, in fact, reveals the presence of pairs of V3+and possibly also of near-lying V4+sites, whereas UV-visible DR spectra and H2-TPR results show the presence of V5+sites as well in the species removed by extraction. We can thus conclude that the species removed is a polynuclear vanadium oxide containing vanadium ions in different valence states. The amount of this amorphous vanadium oxide which remains on the zeolite after preparation depends on several not-quantified factors during the hydrothermal synthesis of Vsilicalite. However, after the extraction procedure, nearly similar amounts of V remain in the zeolite, independent of the starting amount of V. Similar results were, in fact, obtained by extraction of VSilll7 and VSi1237 samples. This suggests that vanadium species remaining on the silicalite after extraction have very different characteristics as compared to the extractable polynuclear vanadium oxide. After extraction, evidence is found for at least two distinct V species, (i) octahedral V02+ shown by ESR and (ii) tetrahedral V5+ shown by 51V-NMRand UV-visible DR spectra. Possibly two distinct ESR signals of octahedral vanadyl species can be evidenced, but they can be reasonably assigned to the same species, saturated or unsaturated by coordination water. As discussed under the ESR studies, these signals are the same as those found for V-ZSM5 samples prepared by solid-state interaction of V205 with ZSM552and assigned to octahedral vanadyl group interacting with the strong (Al)-OH groups in the zeolite pores. According to these authors,52 the ESR parameters are affected by the electrostatic field created by the charge-compensating groups in the zeolite channels, and thus their environment is different from the situation on the surface. By analogy, since the same ESR parameters were found in the V-silicalite sample, we can also assign the S I ESR signals to an octahedral vanadyl species interacting with relatively strong OH group inside the zeolite channels. It should be observed, however, that the ESR parameters4, for surface vanadyl ions on SiOl are not drastically different from those observed for the V-ZSM5 sample.6I Vanadyl species do not form directly by solid-state interaction of V2O5 with pure silicalite, but rather form only in the presence of stronger Bronsted sites such as in ZSM5. In fact, Whittington and Anderson,I0 in the comparison of results obtained by reaction of VOC13 vapor with silicalite and HZSM5, concluded that V02+ may form in the latter by reaction of (V02)+ with the strong Bronsted groups of HZSM5. In addition, they foundi0 that the V02+ in silicalite, formed by reduction of (V02)+with the HCl generated during the anchoring process, is characterized by a slow tumbling motion of the hydrated vanadyl species, due to weak anchorage with the silicalite framework. In V-ZSM5, by contrast, the V02+ is characterized by a relatively strong interaction with the framework. In our silicalite, the little effect induced by the extraction with ammonium acetate as well as ESR data indicates a relatively strong anchoring of V02+ to the silicalite framework. In V-silicalite, relatively stronger Bransted groups as compared to free silanol groups are evidenced by IR and NH,-TPD characterization to be present inside the zeolite pore structure. Tentatively, the enhanced acidity of OH groups may derive from the effects induced by V5+ sites on near-lying zeolite framework defects (silanol groups), according to the model discussed below. Their interaction with vanadium to form the vanadyl species responsible for the SI ESR signal is thus reasonable, in agreement with the suggested preferential localization of this vanadyl species
2628
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
inside the zeolite pore structure near relatively strong OH groups. The third type of vanadium species which is present in Vsilicalite is the tetrahedral Vs+ species shown by 5'V-NMR and UV-visible diffuse reflectance spectra. As indicated by UV-visible DR spectra and XPS and H2-TPR data, this is the dominating species in VSi1545 (sample after extraction), whereas octahedral V02+ is probably present only in smaller amounts. XPS results indicate that the tetrahedral V5+ species is homogeneously distributed in silicalite and is not localized at the external surface of the zeolite microcrystals. Both species are apparently not removed by the extraction procedure, suggesting their stabilization by anchorage to the zeolite. Furthermore, for the tetrahedral V5+ species, the coordination does not change upon dehydration, contrary to that observed for vanadium species on a silica ~urface.'~ In fact, EXAFS/XANES spectroscopy of vanadium oxide supported on s i l i ~ a ~shows ~ , ~ ' that vanadium is present on silica in distorted tetrahedral sites in a dehydrated state and as a polymeric array of octahedral vanadium in the hydrated state. Similar conclusions can be drawn from the results of spin-echo modulation experiment^'^ and from ESR spectra with labeled adsorption species.43 SIV-NMRresults24band diffuse reflectance ~ p e c t r a ~ * * ' ~ suggest a different picture of the surface situation, but again a drastic effect of coordination water is observed. The situation of the tetrahedral V5+species in V-silicalite is thus different as compared to that found for vanadium on S O 2 . This is also very clear from the results obtained by UV-visible diffuse reflectance (Figure 6) where the spectra of V-silicalite are compared with those of samples prepared by impregnation of S i 0 2 or pure silicalite. This suggests, furthermore, that a specific V5+species forms during the hydrothermal preparation of V-silicalite and probably this species is anchored to the silicalite framework. Possible Model of V5+ Sites Anchored to the Silicalite Framework. The information deduced from the characterization of the possible V5+ sites in the silicalite structure can be summarized as follows: (i) A nearly symmetrical tetrahedral environment is present, but is characterized by relatively a shorter vanadium-oxygen bond (about 0.1604.170 nm), as suggested by 51VN M R (Figure 2). UV-visible DRS (Figure 6) results agree with this indication, but also suggest the presence of a V=O double bond. This coordination model agrees with that indicated by EXAFS/XANES36s37 and quantum mechanical studies3*for the structure of vanadium oxide on silica in a dehydrated state. (ii) The V5+species can be reduced by treatment with H2 (ESR and H,-TPR data), and the formation of a tetrahedral V4+species can be evidenced by ESR spectroscopy (Table 11). The reduction seems to require slightly stronger conditions than those required for the reduction of the octahedral vanadyl species. (iii) Relatively weak Bronsted sites are present inside the zeolite pore structure (FT-IR and NH3-TPD data), which, however, are stronger with respect to the free silanol groups also present in pure silicalite and related to defects or crystal faults. This enhancement of the Br~nstedacidity is reasonably associated with the effect induced on the silanol groups from the presence of V5+. (iv) Strong Lewis acid sites are present inside the V-silicalite pore structure in addition to weak Lewis acid sites present on the external surface (FT-IR data). The latter sites also are present in the pure silicalite and can be thus attributed to vacancies created by dehydroxylation of the zeolite, whereas the former stronger sites are related to unsaturated V sites, tentatively to the tetrahedral V5+,since these sites represent the main species in VSi1545. A tentative model for the local coordination environment of Vs+ sites in the silicalite structure that takes into account these indications is reported in Figure 11. The model is consistent with (a) the presence of an enhanced Br~nstedacidity (the acidity of (76) Schrami-Marth, M.; Wokaum, A.; Pohl, M.; Krauss, H.-L. J . Chem. SOC.,Faraday Trans. 1991, 87, 2635.
(77) Narayana, M.; Narasimhan, C. S.; Kevan, L. J. Coral. 1983, 79, 237. (78) Hanke, W.; Bienert, R.; Jerschkewitz. H.-G. Z Anorg. Allg. Chem. 1975, 414, 109.
(79) Hanke, W.; Heise, K.; Jerschkewitz, H.-G.; Lischke, G.; Ohlmann, G . ; Parlitz, B. 2. Anorg. Allg. Chem. 1978, 438, 176.
Centi et al.
Figure 11. Tentative model of the local coordination environment of V5+ sites in the silicalite structure.
the silanol group interacting with vanadium is stronger; in agreement, Rigutto and van Bekkum9 have observed a cationexchange capacity in V-containing silicalite), (b) the previously listed observations, (c) the localization of this V5+species in the zeolite pore structure, (d) the relative stability of this species against both extraction and changes in coordination by adsorption of water molecules, (e) the monodispersion and immobility of the V5+ species (no sintering is observed even after extended calcination), and (f) the possibility of reversible redox behavior with formation of a V4+tetrahedral species. In fact, during propane oxidative dehydrogenation2' at temperatures around 780 K the V-silicalite reduces and the predominant species becomes V4+; subsequent calcination re-form the starting V5+-silicalite,as clearly shown by UV-visible diffuse reflectance spectra and 5'V-NMR solid-state spectra. An apparent contradiction is that 51V-NMRdata suggest a nearly-symmetrical tetrahedral environment, whereas UV-visible DR data suggest the presence of a slightly shorter V-0 double bond. However, it must be considered that in Na3V04,for example, not all the V-0 bonds are equivalent, but together with three V - 0 bonds of -0.170 nm, a slightly longer bond (-0.175 nm) is also present. Nevertheless, the shielding tensor is nearly symmetrical and the 51V-NMR solid-state spectrum is characterized by only a narrow symmetrical line at about -550 ppm. It is thus reasonable that the suggested V5+species in V-silicalite (three V-0 bonds of -0.165 nm and a shorter V=O bond of -0.160 nm) gives rise to a symmetrical shielding tensor around vanadium and that the slV-NMR spectrum of this V5+ species is characterized by a single symmetrical sharp line at about -480 PPm. Due to the limited number of tetrahedral V5+ sites, it is not possible to distinguish whether they are (i) real substitutional sites in the zeolite framework or (ii) sites anchored to the zeolite surface at defect sites. For these small amount of tetrahedral Vs+, the two possibilities are equivalent, but the presence of this V5+ in real substitutional sites in the zeolite framework implies the possibility of introducing larger amounts of tetrahedral Vs+ into the silicalite composition. Since this was not observed, these V sites probably form at defect sites, possibly hydroxyl nests, the formation of which can be enhanced by the presence of V during the hydrothermal synthesis. This agrees with the suggested possible structure of vanadium species as a framework satellite in V-containing ~ilicalite.~ It is also interesting to note that the formation of an 02-species is detected by ESR (Figure 5) upon adsorption of oxygen at low temperature on the tetrahedral V(1V) species formed by reduction of tetrahedral V5+. It is well-known56that these activated oxygen species are very reactive toward hydrocarbon oxidation and, in particular, in the oxidative dehydrogenation of alkanes. We have
J . Phys. Chem. 1992, 96, 2629-2632 previously reported2' an interesting selective behavior of V-silicalite in propane oxidative dehydrogenation to propylene. The formation of this activated oxygen species and, more generally, the presence of stabilized tetrahedral Vs+ sites may thus be the key factor in explaining the peculiar catalytic characteristics of V-silicalite, especially in alkane oxidative dehydrogenation. Further studies are in progress to clarify this aspect. However, it should be noted that the number of V5+sites which appear to be stabilized in defect site position is rather low, which certainly affects the activity of the V-silicalite and its possible applications. Alternative methods
2629
may enhance their amount and the catalytic performance of this system, which appears to be rather interesting, especially for the possibility of having isolated stabilized tetrahedral Vs+ sites with a redox character. Acknowledgment. We thank Dr. D. Ghoussoub, Dr. G. Wrobel, and Dr. L. Gengembre (University of Lille, France) for the NMR, XRD, and XPS measurements, and Dr. G. Bellussi (Eniricerche, Italy) for helpful and stimulating discussion as well as providing us with some samples.
*H NMR Investigations of Ion-Molecule Interactions of Aromatics Included in Zeolites'' M. A. Hepp,+**V. Ramamurthy,*qs David R. Corbin,s and Cecil Dybowski*,t Department of Chemistry and Biochemistry and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 1971 6 , and Central Research and Deuelopment, The Du Pont Company, Wilmington, Delaware 19880-0328 (Received: June 6 , 1991; In Final Form: October 4, 1991)
The sensitivity of the solid-state deuterium NMR line shape to the motion of the C-D bond is exploited to investigate the state of phenanthrene-d,, in zeolites with various counterions. NMR line-shape analysis and measurement of the relaxation time of the Zeeman order (TI) support the existence of two independent spin reservoirs. Spectral line shapes are decomposed into contributions from each spin reservoir to determine equilibrium constants for ion-phenanthrene association as a function of temperature. AH for this process decreases with increasing cation radius.
Introduction Zeolites are crystalline aluminosilicate materials with open framework structures' that find applications in catalysis, separations, detergent formulations, and, more recently, as a medium for studying photochemistry.2 The two synthetic forms of faujasite, X and Y zeolites, have the following unit cell compositions: X type: M86(A102)s6(Si02)lo6.264H20
Y type: M56(A102)56(Si02) 136-1 36H20 where M is a monovalent cation. The framework structure of faujasite is shown in Figure 1. The vertices represent silicon or aluminum atoms and the straight lines between them represent oxygen bridges. The arrows indicate three sites where chargebalancing cations may reside. Only ions at sites I1 and I11 are accessible to adsorbed organic molecules. To understand and predict the behavior of guest molecules like benzene in zeolites, the exact location and the dynamic state of the guest have been probed by many techniques: infrared spect r o ~ c o p y Raman ,~ ~pectroscopy,~ UV diffuse reflectance spectro~copy,~ NMR spectroscopy,6 neutron diffraction,' small-angle neutron scattering? adsorption techniques? and quantum chemical calculations.I0 These studies have shown that, at high loadings, there are three distinct types of benzene molecules in the s u p e r c a g r n e at the cation site (site I1 or HI),one at the 12-ring window site, and one in a cluster of benzene molecules in the supercage. At low loading levels, clustering is avoided and the distribution between the window and the cation sites is controlled by the loading level and the nature of the cation. At the cation site, a benzene molecule is stabilized through interaction of the cation with the benzene T cloud. The binding strength depends on the charge density (acidity) or the electrostatic potential of the cation. At the 12-ring window site, interaction occurs through van der Waals forces and through acid-base interactions between University of Delaware. *Present address: Labratoire de Chimie des Surfaces, Universite Pierre et Marie Curie, 4, Place Jussieu, Paris, France. $The Du Pont Co. ''Contribution Number 5719.
the C-H bonds of the benzene and the oxygens of the 12-ring window. The nature of the cation site has been shown to be (1)(a) Breck, D. W. Zeolite Molecular Sieves: Strucrure, Chemistry and Use; John Wiley and Sons: New York; 1974. (b) Dyer, A. A n Introduction to Zeolite Molecular Sieues; John Wiley and Sons: Bath, England, 1988. (c) Introduction to Zeolite Science and Practice; van Bekkum, H., Flanigan, E. M., Jansen, J. C., Eds.; Elsevier: Amsterdam, 1991. (2)(a) Ramamurthy, V. In Inclusion Phenomena and Molecular Recognition; Atwood, J., Ed.; Plenum Press: New York, 1990;p 351. (b) Turro, N.J. In Molecular Dynamics in Restricted Geometries; Klafter, J., Drake, J. M., Eds.; John Wiley: New York, 1989;p 387. (3)(a) Abramov, V. N.; Kiselev, A. V.; Lygin, V. I. Russ. J . Phys. Chem. (Eng. Transl.) 1963,37,613. (b) Geodakyan, K. T.; Kiselev, A. V.; Lygin, V. I. Russ. J . Phys. Chem. (Eng. Transl.) 1967,41, 227,476. (c) Angell, C.L.; Howell, M. V. J . Colloid Surf. Sci. 1968,28,279. (d) Coughlan, B.; Carroll, W. M.; O'Malley, P.; Nunan, J. J . Chem. SOC.,Faraday Trans. I 1981,77,3037. (e) de Mallmann, A.; Barthomeuf, D. Proc. 7th International Zeolite Conference; Murakami, Y., Iijima, A., Ward, J. W.,Eds.; Tokyo, 1986;p 609. (0 de Mallmann, A.; Barthomeuf, D. Zeolites 1986,8,292.(g) de Mallmann, A.; Barthomeuf, D. J . Chem. SOC.,Chem. Commun. 1989,129. (h) de Mallmann, A,; Dzwigaj, S.; Barthomeuf, D. In Zeolites, Facts, Figures and Future; Jacobs, P. A., van Santan, R. A,, Eds.; Elsevier: Amsterdam, 1989;p 935. (i) O'Malley, P. J. Chem. Phys. Left. 1990, 166, 340. (4)Freeman, J. J.; Unland, M. L. J . Catal. 1978,54, 183. ( 5 ) (a) Unland, M. L.; Freeman, J. J. J . Phys. Chem. 1978,82,1036. (b) Primet, M.; Garbowski, E.; Mathieu, M. V.; Imelik, B. J . Chem. Soc., Faraday Trans. I 1980, 76, 1942. (6)(a) Lechert, H.; Wittern, K. P. Ber. Bunsenges. Phys. Chem. 1978,82, 1054. (b) Borovkov, V. Yu.; Hall, W. K.; Kazanski, V. B. J . Caral. 1978,51, 437. (c) Ryoo, R.; Liu, S . B.; de Menorval, L. C.; Takegoshi, K.; Chmelka, B.; Trecoske, M.; Pines, A. J . P ~ J ' sChem. . 1987,91,6575.(d) de Menorval, L. C.; Raferty, D.; Liu, S . B.; Takegoshi, K.; Ryoo, R.; Pines, A. J . Phys. Chem. 1990, 94, 27. (7)(a) Fitch, A. N.; Jobic, H.; Renouprez, A. J . Chem. SOC.,Chem. Commun. 1985, 284. (b) Fitch, A. N.; Jobic, H.; Renouprez, A. J . Phys. Chem. 1986,90, 131 1. (c) Czjek, M.; Vogt, T.; Feuss, H. Angew. Chem., In?. Ed. Engl. 1989, 28, 770. (8)(a) Renouprez, A.; Jobic, H.; Oberthur, R. C. Zeolites 1985,5 , 222. (b) Jobic, H.;Renouprez, A,; Fitch, A. N.; Lauter, H. J . J . Chem. SOC., Faraday Trans. 1 1987,83, 3199. (9) (a) Tsitsishvili, G. V.; Andronikashvili, T. G. In Molecular Sieue Zeolites; Flanigan, E. M., Sand, L. B., Eds.; Advances in Chemistry 102; American Chemical Society: Washington, 1971;Vol. 11, p 216. (b) Basler, W.; Lechert, H . Ber. Bunsenges. Phys. Chem. 1974,78,667. (c) de Mallmann, A.; Barthomeuf, D. J . Phys. Chem. 1989,93,5636. (10) (a) Sauer, J.; Deininger, D. Zeolites 1982,2, 114. (b) Demontis, P.; Yashonath, S . ; Klein, M. L. J . Phys. Chem. 1989,93,5016.
0022-3654/92/2096-2629%03.00/0 , 0 1992 American Chemical Society I
,
~
