In Situ XRD Investigations of Heteropolyacid Catalysts in the Methacrolein to Methacrylic Acid Oxidation Reaction: Structural Changes during the Activation/Deactivation Process

The structural changes of CsH2PMo12O40 Keggin-type heteropolyacid under the conditions of the methacrolein to methacrylic acid oxidation have been studied by in situ X-ray powder diffraction with simultaneous measurement of the activity and selectivity of the catalysts. The results of the present studies show that under the operating conditions migration of Mo atoms from the Keggin unit into the intracrystalline pore system takes place. The working catalyst is characterized by the presence of Mo in the intracrystalline pore system of the crystal lattice. However, the migration of Mo atoms has been found to be a progressive process which, with increasing time on stream, finally results in the destruction of the Keggin units. The observed activity decline is correlated with the decomposition of the heteropolyacid under formation of catalytically less active MoO3.