Transesterification Of Glycerol To Glycerol Carbonate Over Zr Based Oxides Prepared By Hydrothermal Process

The rapid growth of the biodiesel industry has saturated the glycerol（GL）market,and it is of great significance to transfer by-product crude glycerol into a high value-added product through chemical or biological methods.Among many glycerol derivatives,glycerol carbonate（GC）is widely used in cosmetics and coatings industries because of its low flammability,low toxicity and biodegradability.At present,the transesterification method is widely used in the synthesis of GC because of its mild reaction conditions and simple operation.It is considered to be one of the most direct and feasible ways in industry.In this paper,Mg-Zr composite metal oxides and ZrO₂catalysts with different crystalline forms were synthesized by hydrothermal process,and their catalytic performance for the transesterification of GL and DMC to GC was investigated.The Mg-Zr composite oxide catalysts were prepared by hydrothermal process to address the problem of poor stability of alkali catalysts prepared by the conventional co-precipitation process.Compared with the co-precipitation process,the catalyst prepared by hydrothermal process has larger specific surface area,smaller grain size and higher dispersion.The effects of Mg-Zr molar ratio,calcination temperature,and calcination atmosphere on the performance of the Mg-Zr composite oxide catalyst were investigated,and the results showed that Mg1Zr2-HT catalyst calcined at 600°C in nitrogen atmosphere exhibited the best catalytic performance.Under the conditions of reaction time of 90 min,reaction temperature of 90°C,catalyst dosage of 3 wt%of GL and GL/DMC molar ratio of1/5,the conversion of GL was 99%and the selectivity of GC was 96.1%,and the yield of GC was 74.5%when it was reused for the fourth time.Under the same conditions,Mg1Zr2-CP was used in this reaction,the conversion of GL was 95.2%and the selectivity of GC was 94.7%,when it was reused for the fourth time,the yield of GC was 55.2%.The reason for the difference may be that in the cyclic reaction process,Mg1Zr2-HT had good grain stability and small growth amplitude,but the grain growth range of active species in Mg1Zr2-CP is large,which will greatly reduce the effective active surface area of the catalyst,resulting in a significant decrease of catalytic performance.ZrO₂ catalysts were prepared by three different processes including sol-gel process,hydrothermal process and co-precipitation process.The results showed that the ZrO₂-HT catalyst prepared by hydrothermal process had the advantages of moderate pore size,good dispersibility and alkalinity,and the best catalytic activity.ZrO₂catalysts with different crystal forms and morphologies were synthesized by hydrothermal process.The precise regulation of ZrO₂crystal structure was realized by adjusting the parameters such as material p H value,hydrothermal time and hydrothermal temperature.The effect of ZrO₂crystal structures on GL transesterification to GC reaction was investigated.The activity evaluation results showed that the activity of tetragonal ZrO₂for GL transesterification to GC was higher than that of monoclinic ZrO₂.Under the optimal conditions,the conversion of GL was 67.1%and the selectivity of GC was 96.2%.And after four cycles,the GL conversion did not decrease significantly.