Vanadium alkoxides preparation

Niobium Products Co., 50 m /g). Many different synthesis methods have been used to prepare supported metal oxide catalysts. In the case of supported vanadium oxide catalysts, the catalysts were prepared by vapor phase grafting with VOCI3, nonaqueous impregnation (vanadium alkoxides), aqueous impregnation (vanadium oxalate), as well as spontaneous dispersion with crystalline V2O5 [4]. No drastic reduction of surface area of the catalysts was observed. 

This reaction has been in contrast successfully applied for the synthesis of alkoxide derivatives of acidic oxides, as the whole homologous series of vanadium alkoxides (Orlov, 1959 Prandtl, 1913), and for the preparation of a number of hydrocarbon soluble complexes with diols ofmolybdenum(Vl) (Bishop, 1979), rhenium(VI) (Edwards, 1998) and... 

Titanium—Vanadium Mixed Metal Alkoxides. Titanium—vanadium mixed metal alkoxides, VO(OTi(OR)2)2, are prepared by reaction of titanates, eg, TYZOR TBT, with vanadium acetate ia a high boiling hydrocarbon solvent. The by-product butyl acetate is distilled off to yield a product useful as a catalyst for polymeri2iag olefins, dienes, styrenics, vinyl chloride, acrylate esters, and epoxides (159,160). 

VO(OR)3 — the alkoxides of vanadyl (V) or otherwise the esters of vanad-ic acid — belong to the most easily available derivatives and can be easily purified by distillation under low vacuum. The synthesis of these derivatives — from ethoxide to t-amyloxide—via alcoholysis of V2Os was first described at the beginning ofthe twentieth century by Prandtl and Hess [1313,743], The same work contained the first description ofthe hydrolysis of VO(OR)3, which laid basis for the numerous studies of the processes of formation of sols and gels of vanadic acid. The studies of the low-valent derivatives of vanadium that are very sensitive to oxidation were started in the 1960s by the works of Bradley on V(OR)4 and V(OR)3 [226]. The preparation, structure, and magnetic properties of vanadium(II) alkoxides were investigated only quite recently. 

It is worth noting here that in contradiction to the data published by Bradley [227] on the formation of VC12(0R)2 alkoxide chlorides on alcoholysis of VC14 his reaction was proved to yield VOCl2 nROH as the major products [607, 1634]. To avoid this reaction, the vanadium chloride should be introduced as an ether solution into the reaction with NaOR for the preparation of vanadium (IV) alkoxides. 

In reporting a Ziegler-Natta catalyst, the kind of transition metal compound should not be omitted. Group 4-8 transition metal compounds, such as halides, oxyhalides, alkoxides, acetylacetonates, etc., have been used as catalyst precursors with activators such as alkyl derivatives or hydrides of group 1-4 metals. Titanium chlorides and triethylaluminium are most commonly applied for the preparation of heterogeneous catalysts in an aliphatic hydrocarbon medium. Also, vanadium oxychloride or acetylacetonate and dialkyaluminium chloride are often used for the preparation of homogeneous catalysts in an aliphatic hydrocarbon or an aromatic hydrocarbon medium. 

Comparison of the XANES spectra of the vanadium layer on Ti02 prepared by two different routes (by alkoxide precipitation and via VOCl3) shows close analogy. This indicates that the simple precipitation technique without chemical reaction between the vanadium reagent and the surface leads to a disordered monolayer on Ti02. Similar data have also been reported for the V205/y-Al203 system (147). 

Compound 445 (R = w-Bu) demonstrates the ability of azastannatranes to participate in transmetaUation reactions with metal alkoxides N=Mo(OBu-f)3, OV(OPr-i)3 or N-substituted tris(siloxy)vanadium(V) imides to prepare N=Mo(NMeCH2CH2)3N, 0=V(NMeCH2CH2)3N and RN=V(NMeCH2CH2)3N (R = SiMe3, CMe3) i, respectively, in high yields. 

Fig. 10.2. Surface reactions taking place in the preparation of layered supported vanadium oxide catalysts by flte successive reaction of vanadyl alkoxides with surface hydroxy groups.

Vanadium(V) alkoxides have been known since 1913,69 and in Table 2 we show some of the known alkoxides, the parent alcohol, and the nature of the complex in organic solvents and in aqueous solution. Alkoxides have been prepared from methanol, ethanol, isopropanol, t-butanol, silsequioxanes, cyclopentanol, cyclohexanol, norborneol, adamantanol, phenol, and other alcohols (see Table 2).70- 0 The simple complexes associate in organic solvents and dimerization in the presence of alkoxide is observed with the isopropanol complex.70 The more sterically hindred alcohols are less reactive and do not hydrolyze completely in the presence of small amounts of water.73 The first simple alkoxide to be structurally characterized was the methoxide complex (14) and the vanadium was found to be a six-coordinate dinuclear species however, the structure of this compound did not refine very well.91 This was the first report of the diamond core V—O—V—0 unit, albeit associated with two six-coordinate vanadium atoms. The diamond core V—O—V—O (11), which has since then been found to be a typical structural unit for these complexes, was distinctly asymmetric revealing a difference in the interactions between the two mononuclear parts of the molecule. The first monodentate alkoxide found to contain... 

Vanadium(IV) forms complexes with charged carboxylate, aryloxide and alkoxide functionalities as well as the neutral carbonyl group. The steric crowding of the /-butoxides allowed structural characterization of the tetraalkoxide (96) by gas-phase electron diffraction.475 Vw phenoxide complexes derived from ligands such as 2-(dimethylaminomethyl)phenol and 4-chloro-2-[(dimethylamino)methyl]phenol are active as ethylene and propylene copolymerization agents 476 Complexes with silanoxides have been prepared by the oxidation of Vnl complexes.477... 

Hydrolytic and non-hydrolytic sol-gel routes are implemented to prepare various pure and silica-dispersed vanadium- or niobium-based oxide catalysts corresponding to the compositions Nb-V, Sb-V and Nb-V-M (M = Sb, Mo, Si). Starting reagents in the hydrolytic procedure are isopropanol solutions of the metal alkoxides. The non-hydrolytic route is based on reactions between metal and Si alkoxides and hexane suspensions of niobium(V) chloride. The catalysts are tested in propane oxidative dehydrogenation. NbVOs, SbV04 and Nb2Mo30n are the major crystalline phases detected in the fresh catalysts, but structural modifications are in some cases observed after the use in the catalytic tests. At 500 C, propane conversions of 30 % and selectivities to propene between 20 and 40 % are attained. When the space velocity is decreased, acrolein is in some cases found as by-product. 

Inclusion of these cations does impart new catalytic activities, but in many cases the active site results from a metal ion that has left the framework and entered the pore space upon heating, especially in the presence of water vapour. This is thought to be the case for zinc- and gallium-containing solids used in the dehydrocyclisation of butane and propane to aromatics in the Cyclar process (Chapter 9). Boron, iron, chromium and vanadium all appear to leave the framework under harsh conditions. The incorporation of titanium and more recently tin into framework sites within silicates have become very important substitutions, because both titanosilicates and stannosilicates have been shown to contain stable Lewis acid sites of importance in selective oxidation catalysis. The metal atom can coordinate additional water molecules in the as-prepared material, but these can be removed by heating. In the synthesis of titanosilicates, titanium is usually added to the gel as the alkoxide, and synthesis performed in the absence of sodium hydroxide to avoid precipitation of sodium titanate or nanoparticulate titanium oxides. 

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