In-situ spectroscopic studies of the mechanism of solid super-acid catalysed methanol carbonylation

Acetic acid is an important industrial chemical, used in paints, plastics and as a solvent for many important reactions including the formation of many household polymers. As a result of this heavy demand, 8 million tonnes of acetic acid is produced annually, 80 % of which is produced by methanol carbonylation. Current industrial processes involve rhodium and iridium catalysed homogeneous reactions. The use of heterogeneous catalysts in the synthesis of acetic acid would offer several process advantages. This thesis explores Ru/Rh doped heteropolyacid systems for catalytic application especially in heterogeneous methanol carbonylation.
Novel compounds comprising Ru-tethered polyoxometallate Keggin anions of general formula [NEt3H]+[(Ru{η5-C5H5}{L}2)2(PW12O40)]- have been synthesised where L = PPh3 or 1/2 dppe showing high activity and selectivity in Phenyl acetylene dimerisation under a heterogeneous catalytic regime. Selectivity towards the E-enyne, not found in the homogeneous RuCl(η5-C5H5)(PPh3)2 analogue was achieved while retaining high catalytic activity (TOF 225 h-1). Although methanol carbonylation was not achieved, proof of principle was shown with catalytic activity while retaining structural integrity.
Silica supported Rh(CO)(Xantphos) exchanged phosphotungstic acids of general formulae [Rh(CO)(Xantphos)]+n[H3-nPW12O40]- have been synthesised exhibiting tuneable activity and selectivity towards vapour phase methanol carbonylation. The optimal Rh:H+(Keggin) ratio = 0.5. On-stream deactivation above 200 °C reflects Rh decomplexation and reduction to Rh metal, in conjunction with catalyst dehydration and loss of solid acidity due to undesired methyl acetate hydrolysis, alleviated by water addition and lower temperature operation. A mechanistic insight is provided using XPS, EXAFS and in-situ IR indicating the role of the organoRh and solid acid functionalities of activating CO and methanol respectively. The work examines alternative ligand systems, substituting Xantphos for Anthraphos and Terpy respectively. These systems are compared showing no improvement in stability with substitution of the Rh ligand backbone and that the Xantphos system achieved superior catalytic performance at 200 ºC.