Structure and function of metal cations in light alkane reactions catalyzed by modified H-ZSM5
Structure and function of metal cations in light alkane reactions catalyzed by modified H-ZSM5
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DOI：
10.1016/s0920-5861(96)00028-4
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发表时间：
1996-12-05
期刊：
影响因子：
5.3
通讯作者：
Iglesia, E
中科院分区：
文献类型：
--
作者：
Biscardi, JA;Iglesia, E
丙烷脱氢环二聚形成C-6+芳族化合物的速率受到一系列不可逆脱氢反应的限制，这些反应导致丙烯、高级烯烃、二烯、三烯和芳族化合物。烯烃中间体的准平衡酸催化裂化、低聚和环化反应与这些脱氢反应顺序发生。每个脱氢反应又受到处理C-H键活化步骤中形成的H原子的基本步骤的速率的限制。C-H键活化速率、重组氢脱附速率和丙烷化学转化速率由C_3H_8/C_3D_8和D-2/C_3H_8混合物在H-ZSM_5、Ga/H-ZSM_5和Zn/H-ZSM_5上反应期间的同位素再分配速率和化学转化速率测量。同位素研究表明，在H-ZSM 5、Ga/H-ZSM 5和Zn/H-ZSM 5上，丙烷脱氢环二聚反应的稳态过程中，C-H活化步骤较快，Ga和Zn物种增加了丙烷的化学转化率、重组氢脱附率和氘从D-2掺入反应产物的速率。在C-H键活化步骤中形成的氢的处置通过将H原子转移到不饱和物质以形成烷烃或转移到Ga和Zn物质而发生，Ga和Zn物质催化H原子的重组解吸以形成二氢（H-2）。在丙烷脱氢环二聚转换过程中，几个氢原子的顺序释放限制了该反应在H-ZSM 5上的速率和选择性。原位X射线吸收研究表明，Ga和Zn物种作为单体阳离子驻留在阳离子交换位点，并且在每个脱氢环二聚转换过程中，重组解吸涉及这些阳离子的还原-氧化循环。这些单体物质在Zn离子从溶液交换到H-ZSM 5上的过程中直接形成。然而，Ga 3+物种不直接从溶液交换到H-ZSM 5上，而是形成沸石外的Ga 2 O 3晶体。离子交换发生在随后的接触丙烷或氢在700-800 K通过挥发性Ga 1+物种的气相交换。
The rate of propane dehydrocyclodimerization to form C-6+ aromatics is limited by a sequence of irreversible dehydrogenation reactions leading to propene, higher alkenes, dienes, trienes, and aromatics. Quasi-equilibrated acid-catalyzed cracking, oligomerization, and cyclization reactions of alkene intermediates occur in sequence with these dehydrogenation reactions. Each dehydrogenation reaction is in turn limited by the rate of elementary steps that dispose of H-atoms formed in C-H bond activation steps. The rate of C-H bond activation, recombinative hydrogen desorption, and propane chemical conversion have been measured from the rates of isotopic redistribution and chemical conversion during reactions of C3H8/C3D8 and D-2/C3H8 mixtures on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5. Isotopic studies show that C-H activation steps are fast during steady-state propane dehydrocyclodimerization on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5.Ga and Zn species increase the rates of propane chemical conversion, recombinative hydrogen desorption, and deuterium incorporation from D-2 into reaction products. Disposal of hydrogen formed in C-H bond activation steps occurs by transfer of H-atoms to unsaturated species to form alkanes or to Ga and Zn species, which catalyze the recombinative desorption of H-atoms to form dihydrogen (H-2). The sequential release of several H-atoms during a propane dehydrocyclodimerization turnover limits the rate and selectivity of this reaction on H-ZSM5.In-situ X-ray absorption studies suggest that Ga and Zn species reside at cation exchange sites as monomeric cations and that recombinative desorption involve reduction-oxidation cycles of such cations during each dehydrocyclodimerization turnover. These monomeric species form directly during exchange of Zn ions from solution onto H-ZSM5. Ga3+ species, however, do not exchange directly from solution onto H-ZSM5, but instead form extrazeolitic Ga2O3 crystals. Ion exchange occurs during subsequent contact with propane or hydrogen at 700-800 K via vapor phase exchange of volatile Ga1+ species.