Methacrolein Formation over Cesium Containing Oxidation Catalysts
Methyl methacrylate is produced on a large scale for use in the polymer industry. Synthesis of this acrylic monomer occurs almost exclusively via the acetone cyanohydrin process, which has many environmentally hazardous aspects. In particular the reactant HCN and the intermediate acetone cyanohydrin are very toxic and large amounts of ammonium sulfate, contaminated with organic compounds, are produced. Several companies now utilize alternative processes for methyl methacrylate production via methacrolein using molybdenum based heteropoly acids and isobutene oxidation. Improvements in catalytic selectivity and activity for these “green” processes can be achieved by a careful investigation of the kinetics of targeted catalysts. Furthermore, economic improvements can be made using isobutane as the gas-phase reactant. In this paper Keggin-type heteropoly compounds, which possess unique catalytic properties due to their favourable redox and acid properties, are examined.
Four cesium containing phosphomolybdate catalysts were synthesized (CsH2[PMo12O40], Cs2H[PMo12O40], Cs2.5H0.5[PMo12O40] and fully substituted Cs3[PMo12O40]) and analysed using a low-pressure steady-state technique. Gas-phase products (methacrolein, isobutene, carbon dioxide and water) from the selective oxidation of isobutane were monitored under anaerobic temperature-programmed conditions (100 to 500oC). Accurate kinetic analyses were performed and yielded activation energies for methacrolein and isobutene formation. These energy barriers are compared with parameters obtained for copper-containing phosphomolybdate oxidation and zeolite acid catalysis. The addition of cesium positively affects methacrolein production with the most active catalyst being Cs2H[PMo12O40] (69.7 ± 1.9 kJ mol-1), however further substitution negatively impacts on methacrolein formation activation. This may be due to a structural phase change associated with further substitution.