甲烷脱氢芳构化W/HZSM5基催化剂的制备

Abstract

[中文摘要]由Ｈ２ＳＯ４酸化的（ＮＨ４）２ＷＯ４溶液（ｐＨ＝２～３）制备的ＷＨ２ＳＯ４／ＨＺＳＭ５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／ＨＺＳＭ５催化剂高得多．氧化前驱态催化剂的Ｈ２ＴＰＲ研究表明，ＷＨ２ＳＯ４／ＨＺＳＭ５试样的还原所需温度比Ｗ／ＨＺＳＭ５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．Ｈ２ＴＰＲ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；ＷＨ２ＳＯ４／ＨＺＳＭ５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 non����acidified 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.