Ovine placenta, fetuses and lambs: pathological or physiological finding?

A series of Ca-Zr catalysts modified by different transition metals were prepared by the sol-gel method and their catalytic performance in the synthesis of dimethyl carbonate (DMC) from methanol and propylene carbonate (PC) by transesterification at low temperature was investigated. The results indicate that the selectivity to DMC of various transition metal-modified Ca-Zr catalysts follows the order of Co-Ca-Zr > Cu-Ca-Zr > Ca-Zr > Fe-Ca-Zr > Ni-Ca-Zr > Zn-Ca-Zr. For the transesterification over the Co-Ca-Zr catalyst, in particular, the conversion of PC reaches 84.3% with a selectivity of 94.5% to DMC after reaction for 2 h under 35 °C, a methanol/PC molar ratio of 15, and catalyst amount of 4%. Combining with the XRD, FT-IR, XPS and CO2-TPD results, it is revealed that increasing the strength of basic sites can raise the conversion of PC, whereas increasing the density of basic sites leads to a decrease of the selectivity to DMC. As a result, the Co-modified Ca-Zr (Co-Ca-Zr) catalyst, with the lowest density of surface basic sites but the highest fraction of strong basic sites, exhibits a high conversion of PC and a high selectivity to DMC for the transesterification of PC with methanol at a low temperature.