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型催化剂体系，用于将低价值的含CO_2天然气直接转化为乙烷和乙烯等石油化工原料，即CH_4与CO_2偶联制C_2烃。在由Mn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，C2选择性按Ca-Mn < Ba-Mn < Sr-Mn的顺序增加，Sr-Mn催化剂的C2选择性可达80%，高于Ca-Ce和Ca-Cr催化剂。Ba-Mn和Sr-Mn的C_2选择性几乎不受原料气中CO_2分压的影响，而Ca基复合氧化物的C_2选择性则明显受原料气中CO_2分压的影响。这种差异可能源于催化活性组分的化学结构和C_2生成机理。当这些氧化物组分用具有高表面积的介孔分子筛二氧化硅（SBA-15）浸渍时，（880m^2/g）和大孔容（1.9cm^3/g）时，几乎所有的氧化物物种都能保留在平均孔径为7.4nm的介孔中，Mn基复合材料的C_2选择性较高，而钙基复合材料的C_2烃时空产率较高。用传统的无定形SiO_2载体代替SBA-15载体，结果表明，SBA-15载体的C_2选择性虽然较大，但以比表面积最小（300 m^2/g）的SiO_2为载体的Ca-Ce催化剂的C_2烃收率最高。提高负载型Ca-Ce催化剂的选择性是今后研究的方向。

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.