甲烷无氧脱氢芳构化催化剂Mo/HZSM-5的研究

DEHYDROAROMATIZATION OF METHANE IN THE ABSENCE OF O 2 ON Mo/HZSM 5 CATALYSTS BEFORE AND AFTER NH 4OH EXTRACTION

催化剂Ｍｏ／ＨＺＳＭ－５在甲烷无氧脱氢芳构化反应中表现出很高的活性．用ＸＲＤ，ＢＥＴ比表面，ＮＨ３－ＴＰＤ及ＴＰＲ等手段，对氨水抽提前后催化剂上的活性Ｍｏ物种进行了研究．ＸＲＤ结果表明，Ｍｏ物种高度分散于分子筛表面．随着Ｍｏ担载量的提高，ＢＥＴ比表面积有所下降；但氨水抽提后，比表面积有很大程度的恢复．ＮＨ３－ＴＰＤ结果表明，Ｍｏ物种优先占据分子筛中的强酸位．ＴＰＲ结果显示出有ＭｏＯ３晶相存在的催化剂较易被还原；但氨水抽提后，基本上无还原峰存在．据此可以认为，分子筛上担载的Ｍｏ物种随担载量的不同，形成了多种不同的结构类型，如单层分散的四配位和六配位结构，ＭｏＯ３微晶以及Ａｌ２（ＭｏＯ４）３晶体等．其中ＭｏＯ３微晶与分子筛的结合力较弱，可被氨水抽提，它可能是甲烷无氧脱氢芳构化反应的主要活性物种．

Mo/HZSM 5 catalysts show high catalytic activity in the absence of O 2 for dehydroaromatization of CH 4. The active Mo species on Mo/HZSM 5 catalysts before and after NH 4OH extraction were investigated by XRD, BET, NH 3 TPD and TPR analyses. The XRD patterns show that the Mo species are well dispersed on the zeolite surface. The specific surface area decreases with increasing Mo loading but they can be restored by NH 4OH extraction. NH 3 TPD results suggest that the Mo species prefer to deposit on the strong acid sites of HZSM 5 zeolite. TPR profiles show that there is a kind of Mo species which is easily reduced. No TPR peak can be obviously observed after NH 4OH extraction. There are several kinds of Mo species deposited on the surface. By referring to the Mo species of MoO 3 supported on Al 2O 3, it is proposed that the dissoluble Mo species in NH 4OH solution are MoO 3 crystallites and their aggregation in octahedral coordination, while the insoluble Mo species mainly are Al 2(MoO 4) 3 and MoO 2- 4 in tetrahedral coordination. The catalytic performance of Mo/HZSM 5 catalysts before and after NH 4OH extraction illustrates that the Mo species in MoO 3 micro crystallites in octahedral coordination are active for CH 4 activation in the absence of O 2, while the Mo species in tetrahedral coordination is less active for the reaction.