Thermally and chemically induced structural transformations of Keggin-typeheteropoly acid catalysts

Raman characterization revealed that the Keggin anion structure of H4PVMo11 O40 is inherently unstable upon heat treatment and loss of water. Vanadyl a nd molybdenyl species are expelled from the Keggin cage and defective Keggi n structures are formed. These defective structures further disintegrate to presumably Mo3O13 triads of the former Keggin. These Keggin fragments olig omerize at later stages to molybdenum oxygen clusters comparable to hepta- or octamolybdates. The final disintegration and structural reorganization p roduct is MoO3. This disintegration and recondensation process seems to be strongly affected by the heating rate and hence the presence of water in th e sample. Only partial expulsion of V occurred under moderate dehydration c onditions. The absence of water during heat treatments stabilizes the inter mediate defective structures. Raman spectroscopy proved that free polyacids are unstable under catalytic partial oxidation conditions. Therefore, it c an be suggested that intact Keggin anions are not the active species within an operating partial oxidation catalyst. From this Raman spectroscopy stud y it may be inferred that the structurally reorganized intermediates are re levant for the catalytic action. The Raman investigations of the HPA decomp osition additionally revealed a dependency of the decomposition process on the reactive atmosphere and the presence of Cs. The presence of Ca led to a partial stabilization of the structural disintegration products of PVMo11 and to the formation of the thermodynamically stable, but catalytically ina ctive Cs-3-salt. Cs also inhibited the condensation of MoO3-type oxides. O- 2 present in the gas phase also led to stabilization of the structural reor ganization intermediates. Importantly, the presence of water did not lead t o a stabilization of the intact Keggin structure. In contrast, hydrolysis o f the Keggin anions seemed to be enhanced compared to the water-free situat ion. This observation is of high importance because water is added to the F eed in industrial partial oxidation reactions. Hence, under industrial cond itions, HPA-derived catalysts are inherently unstable and cannot contain in tact Keggin anions at their active surface. Catalytic partial oxidation con ditions even led to a more pronounced structural reorganization and amorpho us suboxides of the MoO3-x type seemed to be formed. Hence, heteropolyacids have to be understood only as defined molecular precursor compound. (C) 20 01 Elsevier Science B.V. All rights reserved.