Vanadium, isobutane oxidation

A. Briieckner, G. Scholz, D. Heidemaim, M. Schneider, D. Herein, U. Bentrup and M. Kant, Structural evolution of H4PVM011O40.XH2O during calcination and isobutane oxidation new insights into vanadium sites by a comprehensive in situ approach, J. Catal., 245, 369-380 (2007). 

Briickner, A., Scholz, G., Heidemann, D., et al. (2(X)7). Structural Evolution of H4PVMoiiO )XH20 during Calcination and Isobutane Oxidation New Insights into Vanadium Sites by a Comprehensive In Situ Approach, J. Catal., 245, pp. 369-380. 

Heteropoly catalysts have significant activities for the oxidation of isobutane into methacrolein and methacrylic acid. The yield increased up to 6% by vanadium substitution or salt formation, as follows. With Cs2.5Ni0.08H0.34+JrPV,Mo12 - O40, the highest conversion and selectivity were observed at x 1 (355). Increases in the reaction temperature to 613 K led to increased yields, up to 9.0%. A similar increase in the yield resulted from the substitution of As for P as a heteroatom or from the addition of various transition metals (106, 356). 

This review analyzes the properties which are necessary for heterogeneous catalysts to promote the oxyfunctionalization of light paraffins to valuable chemicals. Three catalytic systems are discussed i) vanadium/phosphorus mixed oxide, the industrial catalyst for the oxidation of n-butane to maleic anhydride, which is here also examined for reactions aimed at the transformation of other hydrocarbons ii) Keggin-type heteropolycompounds, which are claimed for the oxidation of propane and isobutane, whose composition can be tuned in order to direct the reaction either to the formation of olefins or to the formation of oxygenated compounds iii) rutile-based mixed oxides, where rutile can act as the matrix for hosting transition metal ions or favour the dispersion of other metal oxides, thus promoting the different role of the various elements in the formation of acrylonitrile from propane. 

In recent years heteropolycompounds have been studied for the oxidation of propane to acrylic acid and of isobutane to methacrylie aeid. Rohm Haas Company was the first in 1981 to claim the one-step oxidation of isobutane to methacrolein and methacrylie acid (55). Even though no reference is made to heteropolycompounds, the claimed catalyst compositions correspond to Keggin-type structures. In the patents later issued by Sumitomo (56,57) an important role was claimed to be played by vanadium (in an anionic position), by cesium (in a cationic position), as well as by an excess of phosphorus with respect to the stoichiometric composition. These catalysts gave selectivities to methacrylie acid plus methacrolein close to 70 %, with isobutane conversions in the 10 to 13 % range. Besides carbon oxides, acetic acid was the main by-product. 

Characteristic features of vanadium containing heteropoly catalysts for the selective oxidation of hydrocarbons have been described. MAA yield ftom isobutyric acid was successfully enhanced by the stabilization of the vanadium-substituted heteropolyanions by forming cesium salts. As for lower alkane oxidation by using vanadium containing heteropoly catalysts, it was found that the surface of (V0)2P207 was reversibly oxidized to the Xi (8) phase under the reaction conditions of n-butane oxidation. The catalytic properties of cesium salts of 12-heteropolyacids were controlled by the substitution with vanadium, the Cs salt formation, and the addition of transition metal ions. By this way, the yield of MAA from isobutane reached 9.0%. Furthermore, vanadium-substituted 12-molybdates in solution showed 93% conversion on H2O2 basis in hydroxylation of benzene to phenol with 100% selectivity on benzene basis. 

Methacryhc acid is a major chemical conunodity produced annually via a multistep route that does, unfortunately, produce many environmentally undesirable by-products.Methacrylic acid, however, can also be direcily formed by oxidation of isobutane over polyoxometalate Keggin catalysts (e.g. H3PM012O40 and its vanadium-substituted analogs H3+xPMoi2-xVx04o) and such a green process has been investigated by several researchers. This possibility has resulted in many oxidation studies with polyoxometalate catalysts over the past two decades. 

Systems based on Keggin-type polyoxometalates (POMs) (phosphomolybdic acid derivatives) have been the most effective and widely used catalysts in gas-phase isobutane selective oxidation research. This Keggin POM catalyst application was hrst claimed in 1981 by Rohm and Haas, and after several catalyst mod-ihcations has been patented by other companies such as Asahi Kasei, Sumitomo, Mitshubishi, etc. However, in addition to a Keggin phosphomolybdate, vanadium must also be present in the anion structure in order to obtain a catalyst effective in the selective oxidation of isobutane. Moreover, the addition of - -3 metal cations to the phosphomolybdate anion increases the thermal stability of the catalyst, significantly minimizing its deactivation. " ... 

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