Xuelan Zhang

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

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.

Fluorinated Mg–Al Hydrotalcites Derived Basic Catalysts for Transesterification of Glycerol with Dimethyl Carbonate

A series of fluorinated Mg–Al hydrotalcite-like (HTl) compounds were prepared by introduction of different amount of (AlF6)3− into HTl sheets via co-precipitation method. The fluorine-modified Mg–Al mixed oxides (CHT-F) with mesoporous structure were then acquired by thermal decomposition of as-prepared HTl precursors. The influence of fluorine modifier on the physicochemical properties of CHT-F samples was studied in detail and the results demonstrated that their structure and basicity were strongly related to the fluorine content. Simultaneously, the CHT-F samples were test for the glycerol carbonate (GC) production via transesterification between glycerol and dimethyl carbonate without organic solvent. For these catalysts, the role of basic sites was examined based on reactant conversion and product selectivities in overall reaction as well as in side reaction identified in this work. The catalytic results showed that the activity of CHT-F catalysts depended on the total number of surface basic sites. However, further study demonstrated that the undesired product was catalyzed by strong basic sites, resulting in decrease of GC selectivity. Thus, the introduction of appropriate amount of (AlF6)3− into the HTl structure can promote the production of GC, and the best catalytic performance was obtained over the catalyst with F:Al = 1.0. Besides, various parameters including reaction time, reaction temperature and catalyst amount were investigated to optimize the reaction conditions. Furthermore, these CHT-F catalysts possessed high stability on the basis of reusability test and catalyst characterization.Graphical Abstract