Hydrotalcite-catalyzed methylation of isosorbide via dimethyl carbonate: Influence of the catalyst on the reaction mechanism

This work presents the results of the methylation of isosorbide to produce dimethyl isosorbide using dimethyl carbonate as methylating agent and reaction medium and a MgAl mixed oxide derived from the calcination of a commercial hydrotalcite as basic catalyst. This reaction was carried out using three types of reactors: a continuous liquid flow reactor and two batch reactors, the first one working at autogenous pressure and the other one at atmospheric pressure. The best results were achieved for the atmospheric pressure reactor, with the dimethyl isosorbide yield being 100 % at 110 ºC, after 8 h. Under these experimental conditions, the catalyst showed a loss of selectivity towards dimethyl isosorbide with consecutive catalytic cycles, reaching approximately 50 % dimethyl isosorbide yield after 5 catalytic cycles, but maintaining the isosorbide conversion at 100 %. The reaction mechanism most probably relies on the cooperation between the catalyst and the dimethyl carbonate. With the help of computational studies, it has been demonstrated that the adsorption of dimethyl carbonate on the catalyst takes place through the carbonyl group and that it adsorbs on either Mg^2 + or Al³⁺ ions. Finally, the assignation of the FTIR bands corresponding to the adsorbed dimethyl carbonate species revealed that it undergoes a surface reaction, leading to the formation of both methoxide and monomethyl carbonate adsorbed species. The presence of such species was identified as responsible for the changing pattern selectivity of the reaction during the reutilization of the catalyst.