Langmuir 1997, 13, 1545-1551
1545
Characterization of Silica-Supported Vanadium(V) Complexes Derived from Molecular Precursors and Their Ligand Exchange Reactions Gordon L. Rice and Susannah L. Scott* Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5 Received July 10, 1996. In Final Form: November 13, 1996X The reaction of OdVX3, where X is Cl or OiPr, with the hydroxyl-terminated silica surface gives the well-defined surface complexes tSiOVOX2. These complexes have been characterized by 51V magic angle spinning and 13C cross polarization magic angle spinning NMR spectroscopy and infrared spectroscopy. The surface complexes undergo clean ligand replacement reactions with alcohols, similar to the reactions of analogous molecular vanadium complexes and relevant to the understanding of mechanisms in catalysis.
Introduction Vanadium-containing solid catalysts are widely used for the partial oxidation of hydrocarbons,1-3 oxidation of SO2, and selective catalytic reduction (SCR) of NOx.4 Mixed oxides generally show higher catalytic activity and more selectivity than bulk, crystalline V2O5.5 The mixed oxide catalysts are believed to consist of a two-dimensional vanadium oxide overlayer stabilized by interaction with a high surface area oxide such as silica or titania. The mechanisms of the reactions catalyzed by these materials continue to be targets of active research. On silica, oxidation of organic substrates has been proposed to occur at sites containing one or more terminal vanadium-oxygen bonds.6 There is also evidence that the nature of the interaction between vanadium and the oxide support exerts a strong influence on reactivity. The atomic level structure of these materials has been studied by vibrational spectroscopy,5,7 51V solid-state NMR,8 extended X-ray absorption fine structure/X-ray absorption near edge structure (EXAFS/XANES),9 and by comparison to molecular model compounds.10,11 A variety of surface structures on silica have been proposed, with various degrees of condensation ranging from isolated mononuclear vanadium pseudotetrahedra to two-dimensional rafts of V2O5.6,12 A significant advance in controlling the molecular-level architecture of these materials is their preparation from volatile molecular precursors. Using this strategy, it should be possible to synthesize surface species whose composition is very uniform within a given sample. The homogeneous nature of the vanadium environment makes it amenable to molecular spectroscopies, as well as to kinetic studies of reactions at the gas-solid interface. In * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Bond, G. C.; Tahir, S. F. Appl. Catal. 1991, 71, 1. (2) Cullis, C. F.; Hucknall, D. J. In Specialist Periodical ReportsCatalysis; Royal Society of Chemistry: London, 1982; Vol. 5; p 273. (3) Wainwright, M. S.; Foster, N. R. Catal. Rev. 1979, 19, 211. (4) Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369. (5) Busca, G.; Centi, G.; Marchetti, L.; Trifiro, F. Langmuir 1986, 2, 568-577. (6) Oyama, S. T. Res. Chem. Intermed. 1991, 15, 165-182. (7) Wachs, I. E. J. Catal. 1990, 124, 570-573. (8) Eckert, H.; Wachs, I. E. J. Phys. Chem. 1989, 93, 6796-6805. (9) Tanaka, T.; Yamashita, H.; Tsuchitani, R.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2987. (10) Das, N.; Eckert, H.; Hu, H.; Wachs, I.; Walzer, J.; Feher, F. J. Phys. Chem. 1993, 97, 8240-8243. (11) Feher, F.; Walzer, J. Inorg. Chem. 1991, 30, 1689-1694. (12) Deo, G.; Wachs, I. E.; Haber, J. Crit. Rev. Surf. Chem. 1994, 4, 141-188.
S0743-7463(96)00679-8 CCC: $14.00
order to prepare such a material, the reaction conditions must be reproducible and carefully controlled, especially for the presence of water vapor. In particular, the reaction temperature must not exceed the thermal stability of the molecular precursors or the surface complexes. In this paper, we report the preparation and characterization of well-defined silica-supported vanadium(V) coordination complexes which are analogous to known molecular complexes. We also demonstrate the reactivity of these surface complexes in ligand exchange reactions with alcohols and tert-butyl hydroperoxide. Experimental Section Preparation and Characterization of Silica-Supported Vanadium Complexes. Surface complexes were prepared from two different vanadium precursors. OdV(OiPr)3 (Alfa-Aesar) was separated from its 2-propanol impurity by a trap-to-trap distillation using liquid N2 and CO2/acetone cold baths. A similar procedure was used to remove HCl from OdVCl3 (Aldrich). Both vanadium reagents were stored in grease-free glass bulbs equipped with high-vacuum stopcocks and were transferred into reaction vessels using standard breakseal and high vacuum techniques. Pyrogenic silica (Degussa Aerosil-200, 200 m2/g) was used in all experiments. A standard pretreatment procedure was followed in order to ensure reproducibility. For transmission infrared experiments, silica was either pressed at 125 kg/cm2 into a self-supporting disk of diameter 1.6 cm (2-4 mg of silica/ cm2) or spread in a thin film onto a 25 mm diameter ZnSe disk (0.1-0.5 mg of silica/cm2). In all other experiments, silica was compacted by pressing into pellets (20-30 mg/cm2) which were then finely ground in a mortar. In all reactions, silica-500 was prepared by calcination in 200 Torr O2 (Air Products, ultrapure carrier grade) at 500 °C for 2 h, followed by dehydroxylation for 2 h at 500 °C in dynamic vacuum (10-4 Torr). To prepare silica-200 and silica-25, the calcination step was omitted. The silica was simply heated to the appropriate temperature under dynamic vacuum for 2 h. Infrared experiments were performed in high-vacuum in situ IR cells (volume ca. 200 mL) equipped with KCl windows. Transmission spectra were recorded on a dry-air purged Mattson Research Series FTIR equipped with a DTGS detector. For both background and sample spectra, 32 scans were recorded at a resolution of 2 cm-1. The HCl and 2-propanol liberated by reactions of OdVCl3 and OdV(OiPr)3, respectively, with silica were analyzed by quantitative IR spectroscopy in the in situ cells. At the end of each experiment, chemisorbed vanadium was extracted from the silica with 1 M H2SO4 to give solutions containing ca. 0.1 mg of V/mL. These solutions were treated with 30% aqueous H2O2 (0.03 mL/mL sample solution) to form the strongly absorbing red-brown peroxovanadium complex.13
© 1997 American Chemical Society
1546 Langmuir, Vol. 13, No. 6, 1997 Its absorbance at 450 nm was converted to vanadium concentration by comparison to a calibration curve prepared under the same conditions using ammonium vanadate. Supported vanadium complexes for NMR experiments were prepared in a Schlenk tube equipped with a high vacuum stopcock and a 5 mm Pyrex NMR tube welded onto the tube at right angles. Samples were transferred in vacuo into the NMR tube and sealed off at 30 mm lengths with a torch, to give tubes containing approximately 15 mg of sample. 51V MAS (magic angle spinning) NMR, frequency 52.6 MHz, and 13C CP-MAS (cross polarization magic angle spinning) NMR, frequency 50.32 MHz, were recorded on a Bruker ASX-200 spectrometer. The 51V NMR spectra were collected using a 4.8 µs 90° pulse. The relaxation delay was 0.2 s. The samples were spun at 3000-4000 Hz, and the spin rate was varied in order to identify spinning side bands. Line shape analysis to determine true chemical shifts was not attempted; the values reported are peak maxima. The 13C CP-MAS NMR were collected using a 4 µs 90° proton pulse with a contact time of 1000 µs. The relaxation delay was 2 s. The 51V spectra were baseline corrected with a spline fit. Reactions of Silica-Supported Vanadium Complexes. All reagents were stored in glass bulbs under vacuum and degassed by three freeze-pump-thaw cycles before use. They were introduced into the reactors via vapor phase transfer through a high vacuum line (10-4 Torr) equipped with a Hg diffusion pump. tert-Butanol, allyl alcohol, and 2-propanol (Aldrich, 99%) were vacuum-distilled and stored over activated molecular sieves. tert-Butyl hydroperoxide was dried over magnesium sulfate.
Results and Discussion The temperature at which the silica is heated determines the density of surface hydroxyl groups. Fully hydroxylated silica contains approximately 4.9 OH/nm2,14 while silicas dehydroxylated at 200 and 500 °C have 2.6 and 1.2 OH/nm2, respectively.15 These silicas will be referred to as silica-25, silica-200, and silica-500, respectively, where the appended number indicates the temperature at which the silica was treated before reaction with the vanadium complexes. Stoichiometry of the Reaction of OdVX3 with Silica. When gaseous OdVX3 (X is OiPr, 1 or Cl, 2) interacts at room temperature with the surface hydroxyl groups of silica, a chemical reaction takes place in which adsorbed vanadium species are formed with concurrent liberation of protonated ligands as HX, eq. 1.
n tSiOH + OdVX3 f (tSiO)nV(O)X3-n + nHX (1) After desorption of unreacted OdVX3, the amount of chemisorbed vanadium was determined by extraction. The value of n was evaluated by determining the amount of 2-propanol or HCl liberated in eq 1 by quantitative gas phase IR spectroscopy.16 The value of n was then confirmed by quantifying the 2-propanol or HCl liberated upon hydrolysis of the chemisorbed vanadium complex.17 In all cases, we found n ) 1. The results are summarized in Table 1 for silicas activated at various temperatures. Our data indicate that the maximum amount of chemisorbed vanadium is nearly equal to the initial amount of surface hydroxyl groups, which, obviously, decreases as the dehydroxylation temperature of the silica increases. Also, the stoichiometry of eq 1 is independent of the dehydroxylation temperature of silica in the range (13) Vogel, A. I. A Textbook of Quantitative Inorganic Analysis; Longman: London, 1961; pp 790-1. (14) Zhuravlev, L. T. Langmuir 1987, 3, 316. (15) Morrow, B. A. Stud. Surf. Sci. Catal. 1990, 57A, A161-A224. (16) Neither 2-propanol nor HCl react significantly with silica at room temperature. (17) The surface complexes are extremely sensitive to water vapor. In the presence of traces of water, dramatic color changes from virtually colorless to red and eventually to yellow-green are observed.
Rice and Scott
25-500 °C. The value n ) 1 is an average; however, since physisorbed OdVX3 was desorbed from the silica surface before analysis, we can exclude significant contributions from unreacted OdVX3 and, consequently, the disubstituted species (tSiO)2VOX. Thus the molecular complexes 1 and 2 react at room temperature with only one tSiOH regardless of the density of surface hydroxyl groups. The surface reactions are formulated as
tSiOH + OdV(OiPr)3 f tSiOVO(OiPr)2 + iPrOH 1 3 (2) tSiOH + VOCl3 f tSiOVOCl2 + HCl 2 4
(3)
The absence of (tSiO)2VOX among the products can be understood from the kinetics of the molecular substitution reaction of OdVCl3 with a large excess of alcohol.18,19 The second substitution is much slower than the first. The grafting reaction on silica, which takes place in the presence of a large excess of OdVX3, favors the reaction of vicinal hydroxyl groups with two OdVX3. Equations 2 and 3 confirm the stoichiometry previously inferred for the reaction of OdVCl320 and OdV(OR)321,22 with oxide surfaces, based on a comparison of the amount of grafted vanadium to the hydroxyl content. In contrast, the reaction of OdV(OEt)3 on silica-250 and -500 was recently reported to yield a mixture of mono- and disubstituted surface vanadium complexes at 150 °C.23 In all of these studies, the surface complexes were subsequently hydrolyzed20 and/or annealed20,23 to remove the ligands. In our experience, subjecting the surface complexes 3 and 4 to temperatures above 70 °C results in a dramatic color change from colorless to purple, possibly due to reduction of vanadium.5,22 Solid-State NMR Characterization. Whereas our measurement of the stoichiometry of reactions 2 and 3, Table 1, gives only an average value for the number of ligand substitutions on the surface complexes, it is possible by NMR spectroscopy to examine the distribution of surface vanadium species. As discussed below, this approach also proved to be useful in the study of their reactivity. The 51V MAS NMR spectrum of the product of the reaction of OdV(OiPr)3 with silica-500 is shown in Figure 1a. A single broad resonance at -650 ppm was identified as the true peak maximum; all other bands are spinning side bands whose position shifted when the spin rate was varied. The resonance at -650 ppm is assigned to the monosubstituted vanadium complex, 3. However, the difference in chemical shift between 3 and the starting material OdV(OiPr)3, 1, at δ -630 ppm, is only 20 ppm. Given the peak width in Figure 1a of ca. 80 ppm, and the likelihood that other species such as (tSiO)2VO(OiPr) will have similar chemical shifts, we cannot confidently distinguish between different possible surface complexes in this spectrum. On silica-200 and silica-25, the products of the reaction of 1 with tSiOH gave spectra identical to the one shown in Figure 1a. The 13C CP-MAS NMR (18) Funk, H.; Weiss, W.; Zeising, M. Z. Anorg. Allg. Chem. 1958, 296, 36-45. (19) Mittal, R. K.; Mehrotra, R. C. Z. Anorg. Allg. Chem. 1964, 332, 189-196. (20) Hanke, W.; Bienert, R.; Jerschkewitz, H.-G. Z. Anorg. Allg. Chem. 1975, 414, 109-129. (21) Kijenski, J.; Baiker, A.; Glinski, M.; Dollenmeier, P.; Wokaun, A. J. Catal. 1986, 101, 1-11. (22) Schraml-Marth, M.; Wokaun, A. J. Chem. Soc., Faraday Trans. 1991, 87, 2635-2446. (23) Inumaru, K.; Okuhara, T.; Misono, M. J. Phys. Chem. 1991, 95, 4826-4832.
Characterization of Silica-Supported Vanadium(V) Complexes
Langmuir, Vol. 13, No. 6, 1997 1547
Table 1. Stoichiometry of the Room Temperature Reaction of OdVX3 with SiO2 dehydroxylation temperature of silica
X
500
OiPr
200 25
Cl OiPr Cl OiPr Cl
HX/V
weight % chemisorbed vanadiuma
V/tSiOHb
during graftingc
after graftingd
2.2 2.2 4.2 4.3 7.6 7.4
0.99 0.92 0.98 0.96 0.96 0.94
0.96 ( 0.05 1.1 ( 0.1 0.97 ( 0.05 0.97 ( 0.05 0.94 ( 0.05 0.95 ( 0.05
1.8 2.0 ( 0.1 1.9 1.9 ( 0.1 1.8 1.9 ( 0.1
a (0.1, based on the average of three to eight independent experiments. b Ratio of the amount of chemisorbed V to the hydroxyl content of unreacted silica at each dehydroxylation temperature (4.9, 2.6 and 1.2 OH/nm2 for silica-25, -200, and -500, respectively).14,15 c Ratio of the amount of HX liberated during grafting of OdVX3 on silica to the amount of chemisorbed vanadium. d Ratio of the amount of HX liberated by hydrolysis of surface complexes to the amount of chemisorbed vanadium.
Table 2.
spectrum of 3 on silica-500 contains a band at 84.3 ppm, assigned to the methyne carbon, and two resonances at 25.6 and 24.5 ppm, assigned to the methyl carbons of the isopropoxo ligands, for which there are apparently two slightly different environments. The 51V MAS NMR spectrum of the product of the reaction of VOCl3 with silica-500 is shown in Figure 1b. Only one resonance was observed at -295 ppm, assigned to 4. (In samples where OdVCl3 was not desorbed from the silica, physisorbed vanadium was observed at 0 ppm.) In molecular chemistry, each substitution of one chloro ligand of OdVCl3 by an alkoxo ligand results in a large upfield shift of the 51V resonance, consistent with the substitution of a purely σ-donating ligand by a σ,πdonating ligand.11,24 By comparison to the chemical shifts (24) Devore, D.; Lichtenhan, J.; Takusagawa, F.; Maatta, E. J. Am. Chem. Soc. 1987, 109, 7408-7416.
NMR Data for Molecular and Analogous Surface Complexes
molecular complex
chemical shift, ppm
analogous surface complex
peak maximum, ppma
VOCl3, 2 VO(OtBu)Cl2 VO(OtBu)2Cl VO(OiPr)3, 1 VO(OtBu)3
0 -32524 -53724 -630 -67024
tSiOVOCl2, 4 tSiOVOCl(OtBu), 5 tSiOV(OiPr)2, 3
-295 (55) -548 (75) -650 (70)
a
Figure 1. 51V MAS NMR spectra of tSiOVO(OiPr)2, 3, on silica500 (a) and tSiOVOCl2, 4, on silica-500 (b).
51V
fwhm peak widths are given in parentheses in ppm.
of mixed alkoxochlorovanadium(V) complexes, Table 2, it appears that a similar effect results from the substitution of chloro ligands by “siloxo” ligands derived from the silica surface. The ca. 300 ppm upfield shift of 4 relative to 1 is consistent with the substitution of a single chloro ligand by an oxygen donor ligand. We predict that disubstituted (tSiO)2VOCl will have a chemical shift of ca. -500 ppm. Since no peaks near -500 ppm were observed, we conclude that species other than 4 are not present in significant amounts on the silica surface.25 However, the presence of intense spinning side bands makes it difficult to exclude the possibility of a small amount (
