Vanadium oxide catalysts, preparation

Figure 4. X-Ray diffraction patterns for phosphorous-vanadium oxide catalysts prepared by aerosol technique at 600 C, 8 seconds residence time, and 0.8 M V in feed a. Catalyst analyzed by XRD immediately after synthesis, b. catalyst calcined at 450°C in nitrogen for 3 hrs immediately after synthesis, c. Catalyst allowed to stand for 14 days in an air tight container at ambient temperature without calcining.

Figure 19 shows the typical photoluminescencc spectrum of the anchored vanadium oxide catalyst prepared by photo-CVD methods (a), its corresponding excitation spectrum (b), and the UV absorption spectrum of the catalyst (c) (56,115,116). These absorption and photoluminescence spectra (phosphorescence) are attributed to the following charge-transfer processes on the surface vanadyl group (V=0) of the tetrahedrally coordinated VO4 species involving an electron transfer from to V and a reverse radia-... 

The results obtained with vanadium oxide catalysts prepared by the impregnation method show a remarkable contrast with those obtained with anchored vanadium oxide catalysts (63,116). As shown in Fig. 64. the yields of the photocatalytic isomerization as well as the yields of the phosphorescence of the oxide increase with the content of the vanadium ions and then decrease, even when the catalyst contains 0.1 wt% V. When the vanadium content is high, an increase in the efficiency of the radiationless deactivation due to the aggregation of the vanadium oxide species is observed. 

Maleic anhydride and the two diacid isomers were first prepared in the 1830s (1) but commercial manufacture did not begin until a century later. In 1933 the National Aniline and Chemical Co., Inc., installed a process for maleic anhydride based on benzene oxidation using a vanadium oxide catalyst (2). Maleic acid was available commercially ia 1928 and fumaric acid production began in 1932 by acid-catalyzed isomerization of maleic acid. 

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. 

The vanadium oxide species is formed on the surface of the oxide support during the preparation of supported vanadium oxide catalysts. This is evident by the consumption of surface hydroxyls (OH) [5] and the structural transformation of the supported metal oxide phase that takes place during hydration-dehydration studies and chemisorption of reactant gas molecules [6]. Recently, a number of studies have shown that the structure of the surface vanadium oxide species depends on the specific conditions that they are observed under. For example, under ambient conditions the surface of the oxide supports possesses a thin layer of moisture which provides an aqueous environment of a certain pH at point of zero charge (pH at pzc) for the surface vanadium oxide species and controls the structure of the vanadium oxide phase [7]. Under reaction conditions (300-500 C), moisture desorbs from the surface of the oxide support and the vanadium oxide species is forced to directly interact with the oxide support which results in a different structure [8]. These structural... 

The surface structure and reactivity of vanadium oxide monolayer catalysts supported on tin oxide were investigated by various physico-chemical characterization techniques. In this study a series of tin oxide supported vanadium oxide catalysts with various vanadia loadings ranging from 0.5 to 6. wt.% have been prepared and were characterized by means of X-ray diffraction, oxygen chemisorption at -78°C, solid state and nuclear magnetic resonance... 

A series of vanadium phosphate catalysts prepared by different routes and containing different phases were examined by Guliants and coworkers [23]. From this study it was concluded that the catalytically active phase is an active surface layer on VPP. Their experimental results showed VOPO4 phases to be detrimental to the performance of the catalyst. This was confirmed by Cavani and Trifiro, who suggested that V sites are responsible for the over-oxidation of maleic anhydride to carbon oxides [24]. 

The selective oxidation of n-butane and 1-butene on vanadium phosphate catalysts prepared via different routes was investigated by Cavani and coworkers [77] Precursors prepared in aqueous medium were found to have greater crystallinity than those prepared in organic solvents (the activity and selectivity of which was the same for 1-butene oxidation). However, for butane activation, the crystalline catalyst was considerably less active than the organically prepared catalyst, which had an XRD pattern showing some disorder in the (100) plane. 

Fig. 41. Sterii-Volmer plots of o/4> values for the quenching by various molecules of the phosphorescence of the vanadium oxide catalyst supported on SiO (vanadium oxide/ SiO 0.03 V wt%) prepared by an impregnation method data taken at 298 K [reproduced with permission from Anpo et at (//6)].

Thus, the results obtained with vanadium oxides not only provide useful information about the advantages of the photo-CVD Method for preparing highly dispersed vanadium oxide catalysts and far achieving a high photocatalytic activity but also directly show the significant role that the charge-transfer excited triplet state of the tetrahedrally coordinated vanadium oxide species plays in photocatalytic reactions on supported vanadium oxides (115, 116). 

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.

In addition to the bulk industrial applications of vanadium oxides in preparation of maleic anhydride from n-butate, vanadium compounds have for some time been useful catalysts in organic synthesis. The number of contributions in this area has increased tremendously aided by key contributions from several synthetic groups (Hirao, Modena, Furia, Conte, Pedersen,... 

Methane and ethane have been photooxidised to the corresponding aldehydes using a solid-supported vanadium oxide catalyst, V205/S102-IW (incipient wetness) at elevated temperatures.Both processes are highly sensitive to reaction temperature and to the method by which the catalyst is prepared. 

A. N. Volkova, A.A. Malygin, S.I. Koltsov, V.B.Aleskovskii, Method of Preparation of Vanadium Oxide Catalyst for Organic Compounds Oxidation, USSR Patent No. 42447 (1976). 

The nature of supported oxides and of the support plays a critical role in the partial oxidation of hydrocarbons since the support is not only providing a high surface area, but also dispersing the oxide. The interaction between the metal oxide overlayer and the imderlying support similarly determines the performance of the catalyst, which may also be affected by the exposed sites of the support. To fully understand these effects, a series of supported vanadium oxide catalysts at monolayer and submonolayer coverage have been prepared. The monolayer coverage was determined hy Raman spectroscopy and X -ray photoelectron spectroscopy. The activity of the supported vanadium oxide catalysts is determined by the specific support and surface vanadia coverage. 

Vanadium oxide catalysts were prepared by impregnation of the MgO-support with aqueous solutions of ammonium vanadate. Samples were dehydroxilated at 120 C, then calcinated in air stream by gradual temperature elevation [8]. 

Supported vanadium oxide catalysts may be prepared by either mixing powdered pumice with a solution of ammonium vanadate and subsequently drying, by soaking pumice in a gum solution and then shaking with ammonium vanadate powder, by impregnating porous material with a colloidal solution of vanadic acid stabilized with starch, blood, etc., or by precipitating the oxide on the porous support from solutions of the salt.20... 

The physico-chemical properties of the supports and vanadium oxide catalysts are listed in Table 2. The catalysts are labeled XVj/M, where X corresponds to the % by weight of vanadium, V to vanadium, p to the preparation method (imp=impregnation, graf=grafting) and M to the support. No marked effect of the deposition of titanium oxide on the specific surface area of the silica was detected in the case of the support TSm, while, in the case of the vanadium-based catalysts, a decrease in Sbet was observed. The vanadium surface densities are calculated as number of vanadium atoms per square nanometer of catalyst (V/nm oat) to facilitate a comparison of the samples prepared on different surface area supports [6]. The vanadium contents of the samples are quite smaller than the theoretical monovanadate monolayer coverage of 2.3 VO,/nm" [6]. 

For the preparation of aliphatic nitriles, mainly acetonitrile, 2-4 carbon olefins have been reacted over a supported vanadium oxide catalyst in a... 

Centi, G., Giamello, E., Pinelli, D., et al. (1991). Surface Structure and Reactivity of Vanadium-titanium Oxide Catalysts Prepared by Solid-state Reaction. 1. Formation of a Vanadium(IV) Interacting Layer, J. Catal, 130, pp. 220-237. 

Hutchings G.J., OUer R., Sanan s M.T. and Volta J.-C. (1994). Vanadium Phosphate Catalysts Prepared by the Reduction of VOPO4, 2H2O , in Cortes Corberan V. and Vic BeUon S. (eds). New Developments in Selective Oxidation II, Stud. Surf. Sci. Catal., 82, 213-220. 

Cavalli P, Cavani F, Manenti I, Trifiro F (1987) Ammoxidation of toluene to benzonitrile on vanadium-titanium oxides catalysts prepared by precipitation. The role of catalyst composition. Ind Eng Chem Res 26(4) 639-647... 

Fumaric acid is conveniently prepared by the oxidation of the inexpensive furfural with sodium chlorate in the presence of a vanadium pentoxide catalyst ... 

Patents claiming specific catalysts and processes for thek use in each of the two reactions have been assigned to Japan Catalytic (45,47—49), Sohio (50), Toyo Soda (51), Rohm and Haas (52), Sumitomo (53), BASF (54), Mitsubishi Petrochemical (56,57), Celanese (55), and others. The catalysts used for these reactions remain based on bismuth molybdate for the first stage and molybdenum vanadium oxides for the second stage, but improvements in minor component composition and catalyst preparation have resulted in yields that can reach the 85—90% range and lifetimes of several years under optimum conditions. Since plants operate under more productive conditions than those optimum for yield and life, the economically most attractive yields and productive lifetimes maybe somewhat lower. 

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