甲烷脱氢芳构化W/HZSM5基催化剂的制备
STUDY OF PREPARATION OF W/HZSM 5 BASED CATALYSTS FOR NON OXIDATIVE DEHYDRO AROMATIZATION OF METHANE
Abstract
[中文摘要]由H2SO4酸化的(NH4)2WO4溶液(pH=2~3)制备的WH2SO4/HZSM5催化剂对甲烷脱氢芳构化(DHAM)反应的催化活性比不经酸化的(NH4)2WO4溶液(pH=8~9)制备的W/HZSM5催化剂高得多.氧化前驱态催化剂的H2TPR研究表明,WH2SO4/HZSM5试样的还原所需温度比W/HZSM5低得多;前者可还原至较低价态的W物种在负载总W量中所占比例也比后者高得多.H2TPR及相关体系的EPR和Raman表征结果相互佐证表明,用H2SO4酸化导致负载钨酸铵前驱溶液中一些由共边正八面体配位结构单元(WO6)n-构成的聚钨酸根物种的生成;WH2SO4/HZSM5催化剂对DHAM反应催化活性之所以高,主要由于所负载的W前驱物种较大部分是(WO6)n-基的W物种,而非(WO4)2-基W物种;在较低温度下就可被还原,且在DHAM反应条件下有一部分可还原至较低价态W4+的(WO6)n-物种很可能是催化活性位的前驱组成部分.[英文摘要]It has been found experimentally that the acidification of the solution of (NH 4) 2WO 4 used for impregnation by H 2SO 4 enhances markedly the activity of the W/HZSM 5 based catalysts for non oxidative dehydro aromatization of methane (DHAM). The H 2 TPR results indicated that the temperature required for the H 2 reduction of the catalyst precursor of the W H 2SO 4/HZSM 5 derived from acidified solution of (NH 4) 2WO 4 (with pH=2~3) was much lower than that of the W/HZSM 5 derived from nonacidified solution of(NH4)2WO4(with pH =8~9).It was estimated that, in the temperature region of 298~1200 K,the propor- tions of the W-species reducible to W5+ and W4+ in the total amount of loaded W for the former were~3.2 and ~ 10 times as high as that for the latter, respectively. This along with the results of the EPR and Raman investigat ions of the related systems would be in favor of the following viewpoint, i e ,that the acidification by H2SO4 resulted in the formation of polytungstates species being built of octahedral group (WO6)n-sharing edges, via the reaction such as 6( WO4)2- + 7H+ = ( HW6O21)5- +3H2O, in the precursor solution, and that (WO6)n- derived W-species were more
easily reduced by H2 than those derived from (WO4)2- , and that the high DHAM activity was closely correlated with the polytungstate group (WO6)n- as precursor of catalytically active species.