Catalytic performance of Cs_x(NH_4)_yH_zPMo_(12)O_(40) and reland heteropolyacids in the methacrolein to methacrylic acid conversion: in situ structural study of the formation and stability of the catalytically active species

摘要

The chemical and structural changes undergone by the Keggin-type compound Cs(NH_4)PMo_(12)O_(40) with increasing time on stream under the conditions for methacrolein to methacrylic acid conversion have been studied by thermal analysis and in situ X-ray powder diffraction, which allowed the structure of the working catalysts and the changes involved in the catalyst ageing processes to be determined. The results show that, under the operating conditions of the catalytic reaction, restructuring of the (PMo_(12)O_(40))~(3-) Keggin anion takes place, leading to formation of the reduced heteropolyanions (PMo_(13)O_(41))~- and (PMo_(14)O_(42))~-. The six-electron reduced polyanion (PMo_(14)O_(42))~- represents and ene member of a series of catalytically active and thermally stable species which, by further reduction, decompose into catalytically less active MoO_3. Performance of catalysts derived from the above cesium-containing heteropolyacids is also compared to that of the anhydrous acid H_3PMo_(12)O_(40) and of the anhydride (MoO_x)_(0.5)(PMo_(14)O_(42)) (x approx= 2), for which a detailed structural analysis was carried out.