Catalyst deactivation in the synthesis of methyl acetate from methyl ether using group VIII metal salts of phosphotungstic acid

Heteropoly acids have received much attention in the literature. Of particular importance is phosphotungstic acid which has undergone ion-exchange with several group VIII metals identified as active carbonylation catalysts in the past, namely, rhodium, iridium, palladium, and ruthenium. The resulting metal substituted phosphotungstic acid salt was then found to be active as a methanol carbonylation catalyst producing methyl acetate. Dimethyl ether can be produced much more efficiently than methanol in a liquid phase process. The potential, therefore, for production of methyl acetate from a dimethyl ether based process is of industrial importance. Specifically, conversion of dimethyl ether to methyl acetate was investigated over a variety of group VIII metal substituted phosphotungstic acid salts. The effect of active metal, support type, multiple metal loading and feed conditions were also examined. Finally, the differences in the reaction pathway for methyl acetate production from dimethyl ether versus methanol were compared, especially as it relates to catalyst deactivation.; The investigation of dimethyl ether conversion to methyl acetate is important from several aspects. First, methyl acetate has importance as a specialty chemical. It is used in the production of ethylidene diacetate, a precursor to vinyl acetate which is a monomer for polyvinyl acetate used in paints. Methyl acetate may also be converted to acetic acid via hydrocarbonylation (reductive carbonylation) or hydrolysis(49) -acetic acid is of wide-scale industrial importance as a keystone to other industrially important petrochemicals. The work presented here is unique in its examination of the causes of catalyst deactivation involved in the conversion of methyl ether to methyl acetate. In addition, very little work has been found in the literature which utilizes trivalent salts of heteropoly acids. The use of this relatively new catalyst should add to the growing knowledge base of this class of oxoaxids as well as the field of catalysis in general.; The experimental investigation was carried out in a fixed-bed mode of operation utilizing a mini-pilot plant designed and constructed for general research on gas phase reactions. Approximately 8 grams of supported catalyst was placed at the center of a tubular reactor. Catalyst batches consisted of different group VIII metals supported on a variety of silicon based supports. All reactions were carried out between 200{dollar}spcirc{dollar}C and 300{dollar}spcirc{dollar}C utilizing a methyl ether and CO gas mixture at atmospheric pressure with a dimethyl ether weight hourly space velocitie (WHSV) of 0.15 hr{dollar}sp{lcub}-1{rcub}.{dollar}; The results have shown that of the group VIII metals investigated, iridium shows the highest carbonylation activity of all the metals. No significant synergistic effects could be found among multiple metal loadings. In addition, the 150 A average pore size SiO{dollar}sb2{dollar} showed the best compatibility with the iridium-based catalyst in terms of activity with time on stream. In comparison to methanol carbonylation using the same catalyst, a dimethyl ether feedstock was found to have higher initial CO conversion levels, but rapid deactivation. Subsequent catalyst characterization showed that this deactivation is due to coke formation in conjunction with the loss of acidity of the catalyst over time. Using dimethyl ether as a feedstock will always eventually deplete the proton concentration of the catalyst. Using methanol does not result in rapid deactivation and has a great advantage over methyl ether in this case, despite the lower methyl acetate selectivity. (Abstract shortened by UMI.)...