Mo/HZSM-5上甲烷芳构化的诱导期钼物种的部分还原和碳物种的沉积

在无氧条件下,Mo/HZSM-5催化剂表现出良好的催化甲烷芳构化性能.采用脉冲反应,TPSR,TPR,UV Raman,XRD和BET等手段研究了反应的诱导期.BET和XRD结果表明,钼物种较好地分散于分子筛的外表面和孔道内.TPSR和TPR结果表明,在苯产生之前催化剂上至少有两种不同形式的高价钼物种被部分还原.参照MoO3/Al2O3的TPR结果,认为这两种高价钼物种可能是以多聚酸盐和微晶形式存在的MoO3.UV Raman光谱表明,催化剂表面确实存在八面体配位的多聚酸盐铝物种;而XRD没有检测到微晶铝物种的存在.脉冲反应结果表明,预还原可以缩短反应诱导期,而某种类型积碳的存在对缩短反应诱导期更为有利.根据以上结果,认为诱导期是反应活性相逐渐形成的过程,包括高价钼物种的部分还原和碳物种的沉积,而后者可能对反应更为重要.

INDUCTION PERIOD OF METHANE AROMATIZATION OVER Mo/HZSM-5 Partial Reduction of Mo Species and Deposition of Carbonaceous Species

MoO 3/HZSM 5 catalyst shows good ability in dehydrogenation and aromatization of CH 4 in the absence of O 2. The induction period of the reaction was investigated by pulse reaction method combining with TPSR, TPR, UV Raman, XRD and BET analyses. BET and XRD results showed that the Mo species were well dispersed on the surface and in the channels of the zeolite. TPSR and TPR results revealed that there were at least two kinds of Mo species that could be and needed to be reduced before the C 6H 6 formation on the catalyst. It was suggested that the two kinds of Mo species might be polymolybdate MoO 3 and crystalline MoO 3. UV Raman spectra showed the existence of octahedrally coordinated surface polymolybdate species. XRD, however, did not detect any crystalline MoO 3. In addition, pulse reaction results demonstrated that the prereduction of the catalyst could shorten the induction period and the deposition of some kind of carbonaceous species was more beneficial. It was proposed that the induction period was the process of active phase formation which included partial reduction of Mo species and deposition of carbonaceous species and the latter was more important to the title reaction.