甲烷无氧脱氢芳构化双促进W/MCM-22基催化剂研究

Study on Doubly Promoted W/MCM-22-based Catalysts for Dehydro-aromatization of CH_4 in the Absence of O_2

以MCM 2 2分子筛为载体 ,H2 SO4酸化的 (NH4) 2 WO4为W组分前驱物 ,以Zn和 /或Ga ,Co ,Mo为促进剂 ,研制两系列促进型W /MCM 2 2基催化剂 .在常压固定床连续流动反应器 GC测试系统评价其对CH4无氧脱氢芳构化的催化性能 ,结果显示 ,在双促进的W Co Mo(或W Ga Zn) /MCM 2 2催化剂上 ,常压、10 73K、原料气空速GHSV =15 0 0mL(STP)·h-1·(g cat.) -1的反应条件下 ,苯选择性最高达到 70 %～ 72 % ,相应甲烷转化率为～ 15 % ;反应 45 0min后 ,甲烷转化率降低到～ 5 %水平 ;7 5h平均积炭选择性～ 2 0 % .H2 TPR和NH3 TPD表征研究显示 ,在W /MCM 2 2基质催化剂中适当添加少量Co3 + / 2 + /Mon+ 或Ga3 + /Zn2 + ,一方面导致Wn+ 物种还原活化温度下降及可还原W6+ 物种数量增加 ,另一方面消去最强的一些B酸位同时诱生相当数量的中强酸位 ,这两方面的促进效应都对催化剂活性、选择性提高及结炭减缓作出贡献 .

With incorporation of Ga3+/Zn2+ (or Co3+/2+/Mon+) into a W/MCM-22 host catalyst, highly active and heat-resistant doubly promoted W/MCM-22-based catalysts for dehydro-aromatization of methane ( abbreviated as DHAM) were prepared. Over an 8% W-0.1% Co-0.4% Mo/MCM-22 or 8% W-0.1% Ga-0.6% Zn/MCM-22 catalyst, 70% similar to 72% selectivity to benzene formation at similar to 15% conversion of CH4 could be achieved in the initial similar to 3 h of reaction under reaction condition of 0.1 MPa, 1073 K, GHSV = 1500 mL(STP) (.) h(-1) (.) (g-cat.)(-1). Addition of a proper amount of CO2 (less than or equal to 1.2%) or H-2 (similar to 7.0%) to the feed-gas was found to significantly enhance conversion of CH4 and selectivity to benzene, and to improve the performance of coke-resistance of the catalysts. By correlating the results of the NH3-TPD measurements and the catalyst tests, it was confirmed that intensity and concentration of the surface B-acid sites have pronounced effects on the performance of the catalyst for DHAM reaction. It was found experimentally that addition of a proper amount of Ga3+/Zn2+( or Co3+/2+/Mon+) into W/MCM-22 host catalyst resulted not only in eliminating parts of the strong surface B-acid sites but also in generating a kind of new medium-strung acid sites, mostly B-acid sites, simultaneously. The latter could serve as the catalytically active sites for DRAM reaction, and on such medium-strong surface B-acid sites, the formation of coke would be also alleviated to a greater extent. On the other hand, the doping of the Ga3+/Zn2+ (or Co3+/2+/Mon+)-components to tungsten oxide matrix would facilitate inhibiting aggregation of the W-containing active species and improving dispersion of the W-component at the surface of catalyst, thus leading to a pronounced decrease in the reduction-temperature for the hard-to-be-reduced W6+-species and an increase in quantity of the reducible W6+-species, as evidenced by the results of the H-2-TPR study. The above two roles that Ga3+/Zn2+ (or Co3+/2+/Mon+) as promoters played were contributed to the high CH4 conversion and benzene selectivity, the alleviation of coke deposition and the improvement of the durability of the catalyst.