Structure and Adsorbate Interactions of Vanadium in a Vanadium

6860

J. Phys. Chem. B 2000, 104, 6860-6868

Structure and Adsorbate Interactions of Vanadium in a Vanadium Silicate (VS-1) Molecular Sieve A. M. Prakash and Larry Kevan* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204 ReceiVed: March 15, 2000; In Final Form: May 9, 2000

A medium pore vanadium silicate molecular sieve VS-1 with the ZSM-5 structure was studied by electron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectroscopies to determine the vanadium structure and interaction with various adsorbate molecules. The VS-1 material is compared with ion-exchanged VO2+-ZSM-5 and impregnated V/silicalite-1 materials. As-synthesized VS-1 contains two vanadyl species with distorted octahedral coordination. Deviating from earlier assignments, we assign one of these species to VO(H2O)2+ based on ESEM results. The other species is suggested to be a similar complex in which the water molecule is replaced by a hydroxyl ion interacting with a tetrapropylammonium ion. The VO2+ ions oxidize to V5+ on calcination and then form a V5+‚‚‚H2O complex after room-temperature hydration. Calcined, hydrated VS-1, after dehydration at elevated temperature, shows vanadyl species with square pyramidal geometry involving four framework oxygens. High-temperature H2 or CO treatment of a previously oxidized sample shows vanadyl species that are suggested to be VO2+ ions with weak interaction with H2 or CO molecules. When dehydrated VS-1 is contacted with D2O at room temperature, a vanadyl species identified as VO(D2O)12+ from 2D ESEM data is observed. Adsorption of C2D4 on VS-1 generates a new vanadium species, VO(C2D4)12+. When deuterated ammonia is adsorbed on dehydrated VS-1, another new vanadium species VO(ND3)12+, is observed. Adsorption of deuterated methanol on dehydrated VAPO-5 resulted in another new vanadium species.

Introduction Incorporation of transition metal ions into zeolite frameworks and other molecular sieves has been an area of intensive research in the last two decades. These materials have been subjected to various spectroscopic investigations to understand the nature of the incorporated metal ion.1 Metallosilicate molecular sieves exhibit interesting catalytic properties in various hydrocarbon conversion reactions. In particular, the titanium silicate molecular sieve TS-1 has been found to be an excellent catalyst for the selective oxidation of small molecules, such as propylene and phenol, with hydrogen peroxide.2 This discovery has prompted the scientific community to explore the catalytic behavior of other metallosilicate molecular sieves. Vanadium silicate molecular sieves have been found to be active in ammoxidation of alkanes, epoxidation of alkenes and allylic alcohols, and oxidation of aromatic compounds.3-6 The activity and selectivity of these materials are often different from those of supported vanadium oxide catalysts. It is well known that the activity and selectivity of transition-metal-modified molecular sieves are strongly dependent on the structure and location of metal ions and on their accessibility and coordination with adsorbate molecules. The VS-1 molecular sieve has same structure as that of silicate-1, ZSM-5, or TS-1. The structure code assigned for these materials by the Structure Commission of the International Zeolite Association is MFI. The MFI structure consists of two 10-ring channels intersecting with each other to form a three-dimensional porous network. One channel is straight, with a nearly circular pore opening, and the other one is sinusoidal, with an elliptical pore opening. The synthetic incorporation of vanadium into the framework of materials with an MFI structure has been studied by several research groups.7-11

The resultant material has been given various names such as VS-1, KVS-5, and V-silicalite by individual research groups. We prefer the name VS-1 in accordance with the naming of other metallosilicate materials of the MFI structure, such as TS1. The structure and location of vanadium in VS-1 has been studied using various spectroscopic methods7-11 and by cyclic voltametric measurements.12 Several factors, such as the vanadium source and the concentration of vanadium in the synthesis gel, influence the structure of vanadium in the product. In studies on V-silicalite samples prepared by hydrothermal synthesis using VCl3 as a vanadium source, Centi et al.9 identified several vanadium species. In samples synthesized with high vanadium content, a polynuclear vanadium species containing pairs of V3+ ions as well as an isolated VO2+ species were assigned by ESR measurements. In addition to these species, 51V nuclear magnetic resonance (NMR) spectra suggest a V5+ species in a tetrahedral environment. However, in samples with low vanadium content, the polynuclear vanadium species was absent. In another study, by Kornatowski et al.,10 two isolated VO2+ species with square pyramidal coordination were suggested in vanadium silicate KVS-5. This material is prepared using NaVO3 and VOSO4 as vanadium sources. It was concluded that in as-synthesized KVS5, the two VO2+ species are located at two distinct framework positions, each having coordination with four framework oxygens. On calcination in O2, the vanadyl ions are partially or completely oxidized to V5+ and can be reduced back to VO2+ under the action of a suitable reducing agent. These conclusions are mainly based on the variation in the intensity of ESR signals after various treatments. Studies on adsorbate interactions of vanadium in vanadiumincorporated molecular sieves are limited. No systematic effort

10.1021/jp000987r CCC: $19.00 © 2000 American Chemical Society Published on Web 07/01/2000

Vanadium Molecular Sieve Structure and Adsorbate Interactions has been made to identify the various complexes of vanadium with adsorbate molecules, notwithstanding the fact that this is of considerable significance for catalysis. Previously, electron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectroscopy methods have been effectively used to probe the structure and adsorbate interactions of various transition metal ions, such as Ti, Cr, Mn, Ni, and Cu incorporated into the framework of aluminophosphate-based molecular sieves.13-17 Although ESR can be used to deduce the local symmetry of the transition metal ions, analysis of the ESEM signals yields the number and coordination distances of associated ligands. Recently, we have used ESEM spectroscopy to characterize vanadium incorporated into VAPO-5 molecular sieves.18 In the present study we describe ESR and ESEM studies on a VS-1 molecular sieve to obtain more information on the structure and adsorbate interactions of vanadium in this material. Experimental Section Preparation. The VS-1 molecular sieve was prepared hydrothermally using tetrapropylammonium hydroxide as the organic template. The following chemicals were used without further purification: tetraethyl orthosilicate (98%, Aldrich), tetrapropylammonium hydroxide (20 wt % in H2O, TCI America), and vanadyl sulfate hydrate (Aldrich). Deionized water was used throughout the synthesis. A 100 cm3 stainless steel reactor lined with poly(tetrafluoroethylene) was used for the crystallization. Based on preliminary experiments, the following molar composition was optimized for the crystallization of VS-1: 1.00 SiO2/0.001 V2O5/0.36 TPA OH/35 H2O, where TPA is tetrapropylammonium. The concentration of vanadium in the synthesis gel was kept very low to avoid formation of clustered vanadium species or oxidic vanadium. It has been reported that at high vanadium concentration, the formation of extraframework polynuclear vanadium oxide species is common.9 Moreover, a low concentration of vanadium in the final product is preferred for ESR to reduce spin-spin interactions. In a typical preparation, 12 mL of TPAOH and 10 g of water were added slowly to a beaker containing 14.17 g of tetraethyl orthosilicate. The mixture was stirred for 30 min before adding 0.029 g of vanadyl sulfate trihydrate dissolved in 5 g of water in a dropwise manner. The remaining 10.1 mL of TPAOH was then added, followed by the addition of 9 g of water. After thoroughly stirring the gel for 10 min, it was heated to 343 K and stirred for 3 h. During the heating process, water was added to compensate for the loss due to evaporation. The final gel (∼60 mL) was transferred to the reactor and heated to 448 K for 96 h. After crystallization, the product was separated from the mother liquor by vacuum filtration, washed with water, and dried at 373 K overnight. For comparison, samples of both silicalite-1 and ZSM-5 were also prepared. The silicalite-1 molecular sieve was synthesized by a procedure similar to that for VS-1, but the addition of a vanadium source is omitted. The ZSM-5 material was prepared based on a literature recipe19 using tetrapropylammonim bromide as the organic template. Fumed silica (surface area 380 m2/g, Aldrich), aluminum nitrate hydrate (99.1%, Fisher), and sodium hydroxide (98.4%, Mallinckrodt) were used as respective sources for silicon, aluminum, and sodium. The following gel composition was used: 60 SiO2/ Al2O3/12 Na2O/9 TPA-Br/2500 H2O. Crystallization was achieved by heating the gel at 433 K for 36 h. To remove the organic matter, the as-synthesized materials were slowly heated to 823 K in O2 and maintaining at this temperature for 16 h. A reference sample of vanadium supported on silicalite-1 was prepared by incipient wet impregnation of calcined silicalite-1

J. Phys. Chem. B, Vol. 104, No. 29, 2000 6861 and 1 × 10-3 M aqueous solution of vanadyl sulfate hydrate. After impregnation the sample was dried overnight, calcined at 673 K in O2 for 16 h, and then heated further at 723 K for 3 h. The sample is designated as V/silicalite-1. Another reference sample of ion-exchanged VO2+-ZSM-5 was prepared by exchange of calcined ZSM-5 in 1 × 10-3 M aqueous vanadyl sulfate hydrate solution at 323 K for 16 h. After exchange, the material was filtered and washed several times with deionized water and dried at 343 K in air. This sample is designated as VO2+-ZSM-5. Sample Treatment and Measurements. Powder X-ray diffraction (XRD) patterns were recorded on a Philips PW 1840 X-ray diffractometer using CuKR radiation. Chemical analyses of the samples were carried out by electron microprobe analysis on a JEOL JXA - 8600 spectrometer operated at a beam votage of 15 kV and a current of 30 to 50 nA. Si and O were calibrated with diopsite, Ca MgSiO6; Al with anorthite, CaAl2Si2O8; and V with metallic vanadium. Prior to measurement, the samples were prepared as pressed pellets to make a dense material with a reasonably smooth surface. The electron beam was defocused to 10 µm diameter to minimize the damage caused to the specimen by heating. Data were collected from four to five randomly chosen regions and averaged to represent the bulk composition. No significant differences were seen between the regions. The precision was 573 K, the resultant spectrum shows a vanadium species, denoted as species G, with parameters g|G ) 1.931 and A|G ) 179 × 10-4 cm-1, and g⊥G ) 1.987 and A⊥G ) 64 × 10-4 cm-1 (Figure 3a). Similarly, when calcined VS-1, after high-temperature dehydration, O2 treatment and evacuation, is treated with 1 atm of CO at 673 K, the resultant spectrum shows a vanadyl species, denoted as species H, with parameters g|H ) 1.934 and A|H ) 178 × 10-4 cm-1, and g⊥H ) 1.985 and A⊥H ) 64 × 10-4 cm-1 (Figure 3b). The ESR parameters of species G and H correspond to those of VO2+ with distorted octahedral site symmetry. Figure 4 shows the ESR spectra after D2O and C2D4 are adsorbed on dehydrated VS-1 at room temperature. A single vanadium species, whose parameters are same as those of species A, is observed on adsorption of D2O on VS-1. This result suggests that species A is an aquo-vanadyl complex. The ESR spectrum recorded on VS-1 after adsorbing C2D4 shows a new vanadium species I characterized by g|I ) 1.932 and A|I ) 180 × 10-4 cm-1, and g⊥I ) 1.985 and A⊥I ) 65 × 10-4 cm-1. When deuterated ammonia is adsorbed on dehydrated VS-1 at room temperature, a new vanadium species J, with parameters g|J ) 1.935 and A|J ) 180 × 10-4 cm-1, and g⊥J ) 1.980 and A⊥J ) 64 × 10-4 cm-1 is observed (Figure 5a). Also when deuterated methanol is adsorbed on dehydrated VS-1, a new species K, with ESR parameters g|K ) 1.934 and A|K )

6864 J. Phys. Chem. B, Vol. 104, No. 29, 2000

Prakash and Kevan

Figure 7. Experimental (s) and simulated (‚‚‚) three-pulse 2D ESEM spectra of VS-1 after C2D4 adsorption on a dehydrated sample. The estimated uncertainty in the distance is (0.1 Å and that in the isotropic hyperfine coupling is (10%.

Figure 5. ESR spectra at 77 K of VS-1 (a) after ND3 adsorption on a dehydrated sample at 295 K for 24 h and (b) after CD3OH adsorption on a dehydrated sample at 295 K for 24 h.

Figure 6. Experimental (s) and simulated (‚‚‚) three-pulse 2D ESEM spectra of VS-1 after D2O adsorption on a dehydrated sample. The estimated uncertainty in the distance is (0.1 Å and that in the isotropic hyperfine coupling is (10%.

182 × 10-4 cm-1, and g⊥K ) 1.981 and A⊥K ) 64 × 10-4 cm-1 is observed. Three-pulse 2D ESEM spectra were recorded at a magnetic field of 3630 G and at a microwave frequency of 9.808 GHz for various VO2+ species. The echo signal was maximum around this field. The delay between the first and second pulses (τ) was selected so as to minimize modulation from other magnetic nuclei present in the system. In Figure 6 is given the experi-

Figure 8. Experimental (s) and simulated (‚‚‚) three-pulse 2D ESEM spectra of VS-1 after ND3 adsorption on a dehydrated sample. The estimated uncertainty in the distance is (0.1 Å and that in the isotropic hyperfine coupling is (10%.

mental and simulated 2D ESEM spectrum of VS-1 after adsorbing D2O. The interpulse time τ was selected as 0.25 µs. Simulation of the spectrum gives one deuterium at 2.8 Å and another deuterium at 3.3 Å. These values are consistent with one D2O molecule directly coordinating with VO2+ ions in species D. Figure 7 shows the experimental and simulated 2D ESEM spectrum of VS-1 after C2D4 adsorption. The magnetic field and τ values are the same as for adsorbed D2O. Simulation of the spectrum gives four deuteriums at 4.5 Å. These parameters are consistent with one ethylene molecule with weak π-bond coordination with VO2+ in species E. Figure 8 shows the experimental and simulated 2D ESEM spectrum observed for VS-1 after adsorbing ND3. Simulation of the spectrum gives three deuteriums at 3.4 Å. These parameters indicate that one molecule of ammonia coordinates with VO2+ in species F. The ESR parameters and possible assignments of various vanadyl species observed in VS-1 are given in Table 1. Discussion Tetravalent vanadium normally enters into compounds and various host lattices as V4+ and more often as the stable oxovanadium molecular ion VO2+ exhibiting paramagnetic resonance absorption due to a single unpaired electron. The electronic state of VO2+ is mainly dependent on the 3d1 electron of vanadium and therefore the energy levels of VO2+ are similar

Vanadium Molecular Sieve Structure and Adsorbate Interactions TABLE 1: ESR Parameters and Possible Assignments of Various Vanadium Species Observed in VS-1 Molecular Sieve treatment

species

assignment

g|

A| a

g⊥

A⊥a

as-synthesized

A B D G H A I J K

VO(H2O)12+ VO(OH‚‚‚TPA)2+ VO2+ VO2+‚‚‚H2 VO2+‚‚‚CO VO(D2O)12+ VO(C2D4)12+ VO(ND3)12+ VO(CD3OH)12+

1.937 1.944 1.930 1.931 1.934 1.937 1.932 1.935 1.934

173 174 179 179 178 173 180 180 182

1.986 1.995 1.982 1.987 1.985 1.986 1.985 1.980 1.981

64 65 65 64 64 64 65 64 64

dehydrated H2 CO D2O C2D4 ND3 CD3OH a

× 10-4 cm-1.

to those of V4+ ion. However, there are differences in the paramagnetic behavior of these two ions. The ionic V4+ usually has tetrahedral coordination and, because of low lying excited states, has a very short spin-lattice relaxation time. Thus, ESR spectra can normally only be observed at or below 77 K. On the other hand, the vanadyl species VO2+ normally has square pyramidal or distorted octahedral coordination and has a longer spin-lattice relaxation time. As a consequence, its ESR spectra can be observed at room temperature. The ESR spectra of both V4+ and VO2+ ions are complex, consisting of hyperfine lines generated due to magnetic interaction with the vanadium nucleus (51V: I ) 7/2 and natural abundance 99.8%) and by secondorder effects that tend to produce an asymmetric hyperfine pattern with unequal separations of the lines. The spin Hamiltonian parameters of VO2+ change from lattice to lattice. The g and A values depend on several molecular parameters and the crystal field splitting. The changes in the g and A values depend on the coordination of the VO2+ ion. The properties of a ligand, such as its electronegativity, π-bonding ability, and ligand field strength, will also influence the g and A values.25 In general, the isotropic parameters giso {)(1/3) (g| + 2g⊥)} and Aiso {)(1/3)(A| + 2A⊥)} vary with ligand bonding strength. For a ligand with strong covalent bonding, the delocalization of the unpaired electron of vanadium onto the ligand orbital is large, which reduces the spin-orbit coupling and thereby increases giso. This situation results in smaller hyperfine coupling because the interaction of the electron with the vanadium nucleus decreases. Also, the anisotropy in g (∆g ) g⊥ - g|) depends on the ligand field strength. In general, ∆g is smaller for stronger field ligands. ESR studies have been reported for several systems in which vanadium enters as VO2+ ion. For example, this ion exhibits an ESR signal with parameters g| ) 1.938, g⊥ ) 1.981 and A| 174 × 10-4 cm-1, and A⊥ ) 64 × 10-4 cm-1 in VOPO4‚ 2H2O.23 Similarly, the parameters reported23 for polycrystalline samples of VO2+ doped into ZnSO4‚7H2O are g| ) 1.940, g⊥ ) 1.977, and A| ) 187 × 10-4 cm-1, A⊥ ) 74 × 10-4 cm-1. Also, an ESR signal with parameters g| ) 1.941, g⊥ ) 1.996, and A| ) 179 × 10-4 cm-1 and A⊥ ) 70 × 10-4 cm-1 is reported for VO2+ in V2O5 supported on SiO2.24 The VO2+ ions in these systems have octahedral site symmetry with the groundstate being dxy, and ESR signals can be observed at 300 K. Similar values have been reported for VO2+ in several other crystal lattices.22 Tetravalent V4+ ion, on the other hand, has different values for the corresponding ESR parameters. For example, V4+ in ThSiO4 exhibits an ESR signal with g| ) 1.831, g⊥ ) 1.980, and A| ) 164 × 10-4 cm-1, and A⊥ ) 33 × 10-4 cm-1 at 77 K.28 Similarly, for V4+ in a SiO2 matrix, an ESR signal with g| ) 1.942, g ⊥) 1.987, and A| ) 98 × 10-4 cm-1, and A⊥ ) 42

J. Phys. Chem. B, Vol. 104, No. 29, 2000 6865 × 10-4 cm-1 is observed at 77 K.29 Vanadium has tetrahedral site symmetry with a ground state of dxy in these systems. Even in systems where V4+ has octahedral site symmetry, the ESR parameters are markedly different from those of VO2+. For example, ESR parameters of gx ) 1.915, gy ) 1.913, gz ) 1.956, and Ax ) 31 × 10-44 cm-1, Ay ) 43 × 10-4 cm-1, and Az ) 142 × 10-4 cm-1 have been reported for V4+ ion located at titania sites in TiO2 rutile single crystal.30 This ion has a short relaxation time because the spectrum is observed only at 78 K but not at 300 K. Similarly, ESR values of g| ) 1.921, g⊥ ) 1.963, and A| ) 37 × 10-4 cm-1, and A⊥ ) 135 × 10-4 cm-1 have been reported for V4+ ion in polycrystalline GeO2.31 As-synthesized VS-1 shows two species, A and B, whose ESR parameters are very similar to the values reported for VO2+ with octahedral site symmetry in various other systems. Thus, species A and B can be assigned to VO2+ with distorted octahedral coordination. Although different from species A and B, the parameters for species C observed in VO2+-ZSM-5 are most consistent with those of a vanadyl ion in an octahedral environment. The observed difference in ESR parameters between species C and species A or B suggests different locations for VO2+ in these two materials. In VO2+-ZSM-5, it is obvious that VO2+ is located at an ion-exchange site inside the channels of ZSM where it coordinates with five water molecules. Because the ESR parameters of species A and B are clearly different from those of species C, the possibility of vanadyl ion located at an ion-exchange site in VS-1 seems unlikely. The other possibility is that this ion is located at a framework site. The same conclusion has been proposed from other studies.10 In some earlier studies on VS-1, the ESR data were interpreted in terms of only one type of VO2+ ion existing.5,7 However, later studies have clearly shown that there are two vanadyl species in VS-1. Fejis et al. have assigned these two signals to both framework and ion-exchanged vanadyl species.32 The significant difference in the ESR parameters of species A and B compared with species C clearly suggests that VO2+ ions in VS-1 are not located at ion-exchange sites. In KVS-5 material, Kornatowski et al.10 have assigned these two signals to VO2+ located at two distinct framework sites with coordination with four framework oxygens. No additional coordination with water or hydroxyl groups is assumed. This latter assignment should be reevaluated in light of the present results. Even though two vanadyl species are observed in our as-synthesized VS-1, only a single species is observed in a dehydrated sample and after adsorption of various adsorbates on dehydrated VS-1. If species A and B are indeed the same vanadium species located at different framework sites, one would expect two signals after adsorption of various adsorbates. Moreover, species A in an as-synthesized sample is the same species observed after adsorption of D2O on dehydrated VS-1. This observation clearly supports the idea that species A is an aquavanadyl complex with additional coordination from framework oxygens. The VS-1 molecular sieve is synthesized by a hydrothermal method. Crystallization is achieved under basic conditions from an aqueous medium containing tetrapropylammonium (TPA) ions. Thus, formation of complexes of vanadyl ions with interacting water or hydroxyl ion is likely under such basic conditions. So we assign vanadyl species A to VO(H2O)2+ and vanadyl species B to VO(OH...TPA)2+. The number of water molecules interacting with VO2+ ion in species A is obtained from an ESEM analysis of VS-1 after adsorption of D2O on a dehydrated sample. This analogy is justified by the fact that the ESR parameters observed for species A in as-synthesized VS-1 are

6866 J. Phys. Chem. B, Vol. 104, No. 29, 2000 the same as those observed after adsorption of D2O on dehydrated VS-1. The presence of nonparamagnetic V5+ in as-synthesized VS-1 is also another possibility. In KVS-5 the presence of tetrahedrally coordinated V4+ and V5+ has been suggested in as-synthesized samples.10 Becausee the concentration of vanadium as measured by ESR is about the same as obtained by chemical analysis, the presence of any significant amount of V5+ in our sample seems excluded. It should also be mentioned that the vanadium content of VS-1 (0.023 V/unit cell) in this study is much less than the vanadium content of similarly prepared KVS-5 (1.14 and 1.08 V/unit cell).10 Moreover, unlike KVS-5, the color of VS-1 is white in both as-synthesized and calcined, hydrated forms. KVS-5 samples synthesized using VOSO4 have been reported to be dark green-gray in their as-synthesized form and yellowish/orange in their calcined, hydrated form.10 It would be interesting to compare the structure of vanadium in VS-1 with that in vanadium aluminophosphate material such as VAPO-5. In our earlier study on a VAPO-5 molecular sieve, we observed two paramagnetic vanadyl species with distorted octahedral coordination in an as-synthesized sample. Although one of these species is identified as VO(H2O)32+ based on ESEM results, the other species is suggested to be a similar complex in which one of the water ligands is replaced by hydroxyl interacting with a protonated amine. A similar assumption seems valid for species B in VS-1, where the water molecule in species A is replaced by a hydroxyl group interacting with a tetrapropylammonium ion. One major difference observed between species A in VS-1 and the VO(H2O)32+ species in VAPO-5 is the number of water ligands coordinating with the vanadyl ion. Although three water molecules are found to be coordinated with vanadyl ion in VAPO-5, only one molecule coordinates with this ion in VS-1. In both systems, octahedral coordination for vanadyl ion is suggested by ESR. The difference in the number of water molecules is most likely due to a difference in geometrical constraints within the two framework structures. The large-pore VAPO-5 structure offers less spatial constraints for coordinating with more than one water molecule with the vanadyl ion. As will be discussed later, this observed difference in coordination behavior of vanadium in these two materials is found with other adsorbate molecules also. The absence of any vanadyl species in calcined VS-1 is indicative of complete oxidation of VO2+ ion to V5+ ion during calcination in O2. On evacuation and dehydration at 673 K, a new species D, characteristic of a vanadyl ion, is observed. Reduction of metal ions such as Cu2+ and Ni2+ during dehydration at elevated temperature has been reported earlier in several systems. For example in Cu2+-exchanged SAPO-17, we have reported33 that dehydration at temperatures >573 K causes a reduction of the Cu2+ intensity by half. This loss in intensity was attributed to reduction of Cu2+ to Cu+ by residual water with water decomposition.32 A similar mechanism is possible here in VS-1 where V5+ is reduced to VO2+ by residual water with water decomposition. A possible reaction mechanism is shown in eq 1, where M represents the molecular sieve VS1:

[V5+ ) O, H2O] M f [V4+ ) O, H+, OH-] M f [V4+ ) O(OH)‚‚‚H+] M (1) The disappearance of species D after O2 treatment at 723 K is due to the oxidation of the VO2+ ions to V5+. Dehydration of hydrated VO2+-ZSM-5 produces species E, whose g and A parameters are slightly different from those of species C. These

Prakash and Kevan changes can be related to a change in the ligand nature between species C and species E. In VO2+ exchanged X and Y zeolites these changes have been described as VO2+ ions localized within a supercage having coordination with hydroxyls or framework oxygens.27 Because the 5-ring and 6-ring channels of ZSM-5 are too small for the large VO2+ ion to enter, the localization of these ions in the main 10-ring channels of ZSM-5 seems likely. The smaller hyperfine coupling of species D in comparison with that of species E could be due to greater covalent bonding between vanadium and framework oxygens in VS-1. Dehydration of V/silicalite-1 at 673 K produces species F whose ESR parameters are very close to those of species D. Formation of vanadyl ion in vanadium oxide supported on silica or alumina or zeolites has been reported by a number of authors.34-36 It has been proposed that at sufficiently low loading, the initial calcined V2O5/SiO2 system contains VO4 units that change to VO2+ with square pyramidal coordination after interaction with molecules such as water and ammonia.37 Because as-prepared V/silicalite-1 does not show any ESR signal at 77 K, we believe that all of vanadium is in the nonparamagnetic V5+ state. However, upon dehydration, V5+ reduces to VO2+ by residual water by a mechanism similar to that suggested for VS-1. The oxidation-reduction behavior of vanadium in VS-1 has been studied by several authors. An ESR signal with parameters g| ) 1.932, g⊥ ) 1.982, and A| ) 171 × 10-4 cm-1, and A⊥ ) 67 × 10-4 cm-4 was reported for a vanadium silicate of MFI structure in its as-synthesized form.11 After calcination, this signal disappeared completely due to complete conversion of VO2+ to V5+. On reduction in H2 at 573 K for 6 h, an ESR signal with parameters g| ) 1.931, g⊥ ) 1.991, and A| ) 167 × 10-4 cm-1, and A⊥ ) 64 × 10-4 cm-4 is observed showing the reversible redox behavior of vanadium in this material. Similar results have been reported for KVS-5 with the MFI structure.10 The isolated vanadyl species observed in an assynthesized sample disappears completely on oxidation in O2 at 670 K and then reappears after reduction in H2 at 570 K. However, the ESR parameters for the reduced sample were not given. In another study9 of vanadium silicalite material (named V-silicalite there), reduction at 773 K with 20% H2 in helium generates an ESR signal with parameters g| ) 1.9117, g⊥ ) 1.9625, and A| ) 142 × 10-4 cm-1, and A⊥ ) 63 × 10-4 cm-4 in samples with low vanadium content. These parameters are significantly different from those observed for an as-synthesized sample (g| ) 1.9355, g⊥ ) 1.9827, and A| ) 187 × 10-4 cm-1, and A⊥ ) 74 × 10-4 cm-4) and were assigned to V4+ ions with a distorted tetrahedral environment.9 The current result for VS-1 differs from these other results. The ESR parameters of species G and H observed in VS-1 after reduction by H2 and CO are somewhat different from those of species A. This result suggests that the coordination environments of VO2+ in species G and H are somewhat different from those for species A or D. Studies on the interaction of vanadyl ion in VS-1 with adsorbates are limited. To study the acidity of the vanadium silicate molecular sieve, these materials were studied by infrared (IR) spectroscopy after adsorption of bases such as ammonia and pyridine.9 Bands due to ammonium ions suggesting the presence of Bronsted acid sites are observed in vanadium silicate materials but not in pure silicalite. Also bands due to pyridine chemisorbed on weak Lewis acid sites are observed in both vanadium silicate and silicalite materials. The IR studies on the interaction of vanadium silicate with molecules such as CD3CN and tert-butyl cyanide suggest additional stronger Lewis acid sites due to the presence of vanadium in the framework of this material. In the present study, although the ESR parameters

Vanadium Molecular Sieve Structure and Adsorbate Interactions

J. Phys. Chem. B, Vol. 104, No. 29, 2000 6867

Figure 9. Possible structures of vanadium species in VS-1 and their reaction with adsorbates. The formal valences noted for V indicate paramagnetic (V4+) and nonparamagnetic (V5+) species.

of VO2+ change only slightly after adsorption of various molecules, direct insertion of these molecules into the first coordination sphere of vanadium ion is evident from the 2D ESEM spectra observed for these complexes. Because there has been no previous report to identify such vanadium-adsorbate complexes in terms of the adsorbate number and distance, a direct comparison of the present results with previous studies is not possible. Moreover, as pointed out earlier, several factors, including the specific metal ion and its location, the structure type, the framework charge, and the pretreatment conditions determine the coordination behavior of a metal ion with adsorbate molecules. This result is evident from the different vanadium-adsorbate complexes observed in VS-1 in comparison to VAPO-5 molecular sieve. With adsorbed C2D4, VS-1 shows a new vanadyl species I. The 2D ESEM parameters of four deuteriums at 4.5 Å are consistent with one ethylene molecule coordinating with VO2+. A possible geometry for this complex is one in which the vanadyl ion coordinates with framework oxygens and has weak π-bond coordination with ethylene to form a distorted octahedral complex. With C2D4, a similar coordination geometry is observed in VAPO-5 molecular sieve.18 In an earlier study on TS-1, only one ethlylene molecule was found to coordinate with titanium.13 Both VS-1 and TS-1 have the same MFI structure. Adsorption of ND3 on dehydrated VS-1 generates a new species J, with ESR parameters slightly different from those of species A. The 2D ESEM results show three deuteriums interacting at 3.4 Å, consistent with the coordination of one ammonia molecule to VO2+ ion. This result suggests that the immediate coordination sphere of V involves both N and O ligands. The relatively large distance between VO2+ and deuterium suggests a weak interaction for ammonia. Few compounds have been reported in which the coordination of

vanadium involves both N and O ligands. Vanadium in the single-crystal bis(N-methylsalicylaldiminato)nickel(II) and in a frozen solution of bis (2-methyl-8-quinolinolato)oxovanadium(IV) have been reported to have similar coordination environments involving both O and N ligands.38,39 The ESR parameters of these compounds are comparable to the values observed for species J. The observation that only one ND3 molecule coordinates with vanadium is consistent with similar observations for TS-1,13 whereas in VAPO-5, two ammonia molecules coordinate with a vanadyl ion.18 As for D2O adsorbate, the large pore structure of VAPO-5 offers less spatial constraints for coordination of more than one molecule of ammonia. A possible model for the various vanadium species in VS-1 based on our ESR and ESEM results is shown schematically in Figure 9. This model can explain the observed ESR behavior of VS-1. In as-synthesized VS-1, the vanadyl species is paramagnetic, with one weakly coordinated water molecule. Species B (not shown in Figure 9) is assumed to be a similar species in which the weakly coordinated water molecule is replaced by a hydoxyl group interacting with a tetrapropylammonium (TPA) ion. On calcination in O2, all the VO2+ species are oxidized to nonparamagnetic V5+ because there is no ESR signal. In the literature, a distorted tetrahedral symmetry is suggested for this species based on 51V NMR results.7,10 Because no ESR signal is observed in calcined, hydrated VS-1 either, the vanadium is still present as V5+ with probably weak coordination with one water molecule. After dehydration at elevated temperature, paramagnetic VO2+ species D is observed. Species D is assumed to be generated by thermal reduction of V5+ to V4+ by residual water accompanied by water decomposition. This species most likely has distorted square pyramidal symmetry with four oxygens of the framework. After adsorption of D2O, ND3, and C2D4 on dehydrated VS-1, complexes of

6868 J. Phys. Chem. B, Vol. 104, No. 29, 2000 VO2+ weakly coordinating with one molecule of each of these adsorbates are formed. Conclusions ESEM spectroscopy, when coupled with ESR spectroscopy, has been shown to be very effective in obtaining information about the structure and location of vanadium in VS-1 and also its coordination with various adsorbates. VS-1 and ionexchanged VO2+-ZSM-5 shows significant difference in their ESR parameters for the VO2+ species. As-synthesized VS-1 contains two VO2+ species with distorted octahedral coordination. Although one species is found to be a VO(H2O)2+ complex based on ESEM results, the other species is suggested to be VO(OH‚‚‚‚TPA)2+. Ion-exchanged VO2+-ZSM-5 shows only a single vanadyl species VO(H2O)52+ located at an ion-exchange site. During calcination of VS-1 most of the VO2+ ions are oxidized to V5+. Calcined, hydrated VS-1, after dehydration at elevated temperature, shows a vanadyl species suggested to be a VO2+ ion with no water ligands. Adsorption of D2O on dehydrated VS-1 generates a VO2+ species with the same ESR parameters as one of the two species observed in an assynthesized sample. This species is identified from 2D ESEM as VO(D2O)12+. When deuterated ethylene is adsorbed on dehydrated VS-1, another new vanadyl species is observed, which is identified as VO(C2D4)12+. Adsorption of ND3 on VS-1 generates a new vanadyl species identified as VO(ND3)12+. Acknowledgment. This research was supported by the National Science Foundation, the Robert A Welch Foundation, and the Environmental Institute of Houston. References and Notes (1) Vedrine J. C. In Zeolite Chemistry and Catalysis, Jacobs, P. A., Jaeger, N. I., Kubelkova, L., Wichterlova, B., Eds., Studies in Surface Science and Catalysis, Vol. 69; Elsevier: Amsterdam, 1991; p 25. (2) Notari, B. AdV. Catal. 1996, 41, 253. (3) Miyamoto, A.; Medhanavyn, D.; Inui, T Appl. Catal. 1986, 28, 89. (4) Habersberger, K.; Jiru, P.; Tvaruzkova, Z.; Centi, G.; Trifiro, F. React. Kinet. Catal. Lett. 1989, 39, 95. (5) Miyamoto, A.; Iwamoto, Y.; Matsuda, H.; Inui, T. In Zeolites: Facts, Figures, Future; Jacobs, P. A., van Santen R. A., Eds.; Elsevier: Amsterdam, 1989; p 1233. (6) 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: Amsterdam, 1989; p 1243. (7) Rigutto, M. S.; Van Bekkum, H. Appl. Catal. 1991, 68, L1

Prakash and Kevan (8) Bellussi, G.; Maddinelli, G.; Carati, A.; Gervasini, A.; Millini, R. In Proceedings from the Ninth International Zeolite Conference; von Ballmoos, R., Higgins, J. B., Treacy, M. M. J., Eds.; ButterworthHeinemann: Boston, 1992; p 207. (9) Centi, G.; Perathoner, S.; Trifiro, F.; Aboukais, A.; Aissi, C. F.; Guelton, M. J. Phys. Chem. 1992, 96, 2617. (10) Kornatowski J.; Wichterlova B.; Jirkovsky J.; Loffler E.; Pilz W. J. Chem. Soc., Faraday Trans. 1996, 92, 1067. (11) Sen, T.; Rajamohanan, P. R.; Ganapathy, S.; Sivasanker, S. J. Catal. 1996, 163, 354. (12) Venkatathri, N.; Vinod, M. P.; Vijayamohan, K.; Sivasanker, S. J. Chem. Soc., Faraday Trans. 1996, 92, 473. (13) Prakash, A. M.; Kevan, L. J. Catal. 1998, 178, 586. (14) Zhu, Z.; Kevan, L. Phys. Chem. Chem. Phys. 1999, 1, 199. (15) Brouet, G.; Chen, X.; Kevan, L. J. Phys. Chem. 1991, 95, 4928. (16) Hartmann, M.; Azuma, N.; Kevan, L. J. Phys. Chem. 1995, 99, 10988. (17) Munoz, T.; Prakash, M.; Kevan, L.; Balkus, K. J., Jr. J. Phys. Chem. B 1998, 102, 1379. (18) Prakash, A. M.; Kevan, L. J. Phys. Chem. B 1999, 103, 2214. (19) Argauer, R. J.; Landolt, G. R. U. S. Patent 3 702 886, 1972. (20) Anderson, M. W.; Kevan, L. J. Chem. Phys. 1987, 87, 1. (21) Van Koningsveld, H.; Van Bekkum, H.; Jansen, J. C. Acta Crystallogr. 1987, B34, 127. (22) Misra, B. N.; Kripal, R. Chem. Phys. Lett. 1977, 46, 536. (23) Ballutaud, D.; Bordes, E; Courtine, P. Mater. Res. Bull. 1982, 17, 519. (24) Narayana, M.; Kevan, L. J. Phys. C: Solid State Phys. 1983, 16, L863. (25) Viswanath, K. A. J. Chem. Phys. 1977, 67, 3744. (26) Komorek, J.; Romotowski, T.; Serwicka, E. M.; Mastikhin, V. M. Zeolites 1994, 14, 629. (27) Martini, G.; Ottaviani, M. F.; Seravalli, G. L. J. Phys. Chem. 1975, 79, 1716. (28) Di Gregorio, S.; Green Blatt, M., Pifer, J. H.; Sturge, M. D. J. Chem. Phys. 1982, 76, 2931. (29) Grunin, V. S. SoV. Phys. - Solid State 1971, 12, 1785. (30) Gerritsen, H. J.; Lewis, H. R. Phys. ReV. 1960, 119, 1010. (31) Madacsi, D. P.; Gilliam, O. R. Phys. Lett. 1972, 41A, 63. (32) Fejis, P.; Marsi, I.; Kiricsi, I.; Halasz, J.; Hannus, I.; Rockenbauer, A. Tasi, G. Korecz, L.; Schobel, G. In Zeolite Chemistry and Catalysis; Jacobs, P. A., Jaeger, N. I., Kubelkova, L., Wichterlova, B., Eds.; Studies in Surface Science and Catalysis, Vol. 69; Elsevier: Amsterdam, 1991; p 173. (33) Prakash, A. M.; Kevan, L. Langmuir 1997, 13, 5341. (34) Yoshida S.; Iguchi T.; Ishida S.; Tarama, K. Bull. Chem. Soc. Jpn. 1972, 45, 376. (35) Gao X. T.; Bare S. R.; Weckhuysen B. M.; Wachs I. E. J. Phys. Chem. B 1998, 102, 10842. (36) Catana G.; Rao R. R.; Weckhuysen, B. M.; Van Der Voort P.; Vansant E.; Schoonheydt R. A. J. Phys. Chem. B 1998, 102, 8005. (37) Van Reijen L. L.; Cossee P. Discuss. Faraday Soc. 1966, 41, 277. (38) Stoklosa, H. J.; Wasson, J. R.; McCormick, B. J. Inorg. Chem. 1974, 13, 592. (39) Hitchman, M. A.; Moores, B. W.; Belford, R. L. Inorg. Chem. 1969, 8, 1817.