Saucy–Marbet ketonization in a continuous fixed-bed catalytic reactor

Abstract

Saucy–Marbet ketonizations of two unsaturated alcohols (3,7-dimethyl-oct-6-en-1-yn-3-ol (dehydrolinalool (DLL)) and 2-methyl-3-butyn-2-ol (MB)) with 2-methoxypropene (MP), generally performed in a batch reactor using homogeneous acid catalysts, have been studied in a continuous fixed-bed reactor containing a commercial alkyl sulfonic acid polysiloxane catalyst. The solid acid catalyst showed an initial transient period of several hours till stable operation was reached. Similar products were observed as in corresponding homogeneous catalytic reactions indicating similar reaction pathways. However, the heterogeneous catalytic reactions showed lower selectivity to the desired products than the corresponding homogeneous reactions reported in the literature. This behaviour is traced to the complex transformation the unsaturated ether (2-methoxypropene) undergoes upon interaction with the surface of the solid acid catalyst. Furthermore, both Brønsted and Lewis-acid sites were found to be present on the catalyst surface. These factors are proposed to be the main reasons for the less efficient use of reactants observed in the heterogeneous catalytic reactions. A maximum yield of 54% to the desired product could be achieved in the heterogeneous catalytic ketonization of 3,7-dimethyl-oct-6-en-1-yn-3-ol and 45% for the 2-methyl-3-butyn-2-ol ketonization, respectively. The studies indicate that the continuous ketonization in a fixed-bed reactor is feasible, but for technical application more selective acid catalysts are necessary to compete with the generally used homogeneous catalytic routes.