Development of a novel catalyst for direct conversion of low valued natural gas containing carbon dioxide to petrochemical feedstock

本工作的重点是开发一种新的催化体系，将含有大量CO_2的低价值天然气直接转化为乙烷和乙烯等石化原料，即CH_4与CO_2偶联生成C_2烃。在由锰氧化物和碱土金属氧化物组成的非负载型二元氧化物催化剂中，C_2选择性的增大顺序为Ca-Mn < Ba-Mn < Sr-Mn，其中Sr-Mn催化剂的C_2选择性达到80%，高于Ca-Ce和Ca-Cr催化剂。Ba-Mn和Sr-Mn的C_2选择性对原料气中CO_2分压几乎没有依赖性，而Ca基复合氧化物的C_2选择性对原料气中CO_2的分压有显著依赖性。这种差异可能源于催化活性物质的化学结构和C_2的形成机理。用高比表面积（880m^2/g）和大孔体积（1.9cm^3/g）的介孔分子筛二氧化硅（SBA-15）浸染这些氧化物组分后，平均孔径为7.4nm的介孔内几乎可以容纳所有的氧化物，且锰基复合材料的C_2选择性更大，而钙基复合材料的C_2烃时空产率更高。用传统的无定形SiO_2载体代替SBA-15载体，结果表明，SBA-15载体对C_2的选择性较大，但在最小表面积为300m^2/g的SiO_2载体上负载Ca-Ce催化剂，C_2烃的产率最高。在负载型Ca-Ce催化剂上提高选择性可能是未来研究的主题。

The present work focuses on developing a novel catalyst system for the purpose of directly converting low valued natural gas, which contains a large amount of CO_2, into petrochemical feedstock such as ethane and ethylene, in other words, for CH_4 coupling with CO_2 to produce C_2 hydrocarbons. In an unsupported binary oxide catalyst, which is composed of Mn oxide and alkaline earth metal oxide, C_2 selectivity increases in the order of Ca-Mn < Ba-Mn < Sr-Mn, and the value over the Sr-Mn catalyst reaches 80%, which is higher than those over Ca-Ce and Ca-Cr catalysts. The Ba-Mn and Sr-Mn provide almost no dependency of C_2 selectivity on partial pressure of CO_2 in feed gas, in contrast with the remarkable dependency observed with Ca based composite oxides. It is likely that this difference originates from chemical structures of catalytically active species and the mechanism of C_2 formation. When these oxide components are impregnated with a mesoporous molecular sieve silica (SBA-15) with high surface area (880m^2/g) and large pore volume (1.9cm^3/g), almost all of the oxide species can be held inside the mesopores with average pore diameter of 7.4nm, and C_2 selectivity is larger with Mn-based composites, whereas space time yield of C_2 hydrocarbons is higher with Ca-based composites. The use of conventional amorphous SiO_2 supports in place of the SBA-15 support shows that, although C_2 selectivity is larger with the latter, the yield of C_2 hydrocarbons is the highest with the Ca-Ce catalyst supported on the SiO_2 with the lowest surface area of 300m^2/g. To improve the selectivity over the supported Ca-Ce catalyst maybe the subject of future study.