La-modified mesoporous Mg–Al mixed oxides: effective and stable base catalysts for the synthesis of dimethyl carbonate from methyl carbamate and methanol

Abstract

A series of La-containing Mg–Al hydrotalcite-like (HTl) precursors with different La contents (Mg²⁺ : Al³⁺ : La³⁺ = 3 : 1 : x, where x varies from 0 to 1.0) were synthesized using a co-precipitation method followed by hydrothermal treatment. X-ray diffraction and thermogravimetric measurements demonstrated that the yield of the HTl phase decreased with increasing La content. The La-modified Mg–Al mixed oxides (HTC-La) were then obtained by thermal decomposition of the corresponding HTl precursors, and the mesoporous structure was formed during calcination. It was demonstrated that the structure and surface basic properties of the HTC-La samples strongly depended on the amount of La additive. Simultaneously, the resulting HTC-La materials were used as solid base catalysts for the synthesis of dimethyl carbonate (DMC) from methyl carbamate (MC) and methanol. Then, the correlation between their basic properties and catalytic performance was studied in detail. The incorporation of a suitable amount of La into HTC-La catalysts was beneficial for the production of DMC, and a DMC yield of 54.3% with a high DMC selectivity of 80.9% could be achieved when x was tuned to 0.5 under the optimized reaction conditions. In addition, the HTC-La catalyst could be readily recycled while maintaining high catalytic activity and selectivity for DMC. Furthermore, in situ FTIR experiments were carried out to elucidate the adsorption behaviours of the reactants. On the basis of the experimental results, a plausible basic catalytic mechanism wherein MC and methanol were activated simultaneously on the basic sites of the catalyst was proposed for this catalytic reaction.