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

本工作致力于开发一种新型的催化剂体系，用于将含有大量二氧化碳的低值天然气直接转化为乙烷、乙烯等石油化工原料，即CH4与CO2偶联生成C2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选择性按Ca-Mn&lt；Ba-Mn&lt；sr-mN顺序增加，在锶-锰催化剂上达到80%，高于钙-Ce和钙-铬催化剂。与钙基复合氧化物的显著相关性相比，BaMn型和SrMn型催化剂对原料气中CO2分压的选择性几乎没有影响。这种差异可能源于催化活性物种的化学结构和C_2的生成机理。用具有高比表面积(880m~2/g)和大孔容(1.9 cm~(3/g))的介孔分子筛SiO_2(SBA-15)浸渍这些氧化物组分时，几乎所有的氧化物物种都可以固定在平均孔径为7.4 nm的介孔内，锰基复合材料具有较高的C2选择性，而钙基复合材料C2烃的时空产率较高。用传统的无定形SiO_2载体代替SBA-15载体的结果表明，虽然SBA-15载体具有较高的C_2选择性，但负载在SiO_2上的Ca-Ce催化剂的C_2烃的产率最高，其比表面积最低，为300m~2/g，进一步提高C_2选择性可能是今后研究的方向。

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.