Synthesis of methacrylic acid by selective oxidation of isobutane, catalysed by Keggin-type polyoxometalates:: relationship between catalytic performance, reaction conditions and chemical-physical features of the catalyst

The ammonium salt of 12-molybdophosphoric acid, (NH(4))(3)PMo(12)O(40) (polyoxometalate, POM), having the Keggin-type structure was prepared and tested as a. heterogeneous catalyst for the selective oxidation of isobutane to methacrylic acid. This reaction might represent an alternative to the current industrial process for methyl methacrylate production, the acetone-cyanohydrin process, which is characterized by a no longer acceptable environmental impact. The POM catalyst was found to operate with higher selectivity at isobutane-rich conditions (26 % isobutane, 13 % oxygen), than at isobutane-lean conditions (1 % isobutane, 13 % oxygen). This was attributed to the development of a more favourable distribution of reduced Mo(5+) and oxidized Mo(6+) sites in the POM in the former case. Modifications occurring in the catalyst during time-on-stream at different reaction conditions are discussed. (C) 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.