Size, shape, and site selectivities in the photochemical reactions of molecules adsorbed on pentasil zeolites. Effects of coadsorbed water
Size, shape, and site selectivities in the photochemical reactions of molecules adsorbed on pentasil zeolites. Effects of coadsorbed water
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吸附在五元硅沸石上的分子的光化学反应中的尺寸、形状和位点选择性。
DOI：
10.1021/ja00242a033
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发表时间：
1987
影响因子：
15
通讯作者：
D. Corbin
中科院分区：
文献类型：
--
作者：
N. Turro;C. Cheng;L. Abrams;D. Corbin
甲基苄基苄基酮（ACOB）在五元沸石存在下的光化学由于酮的吸附位置的不同而遵循着明显不同的途径。的产品分布，在笼效应（成对自由基组合的效率）方面，证明了吸附和扩散对光解产生的自由基物种的影响。p-ACOB容易吸附在pentasil骨架内，并产生p-AB作为主要产物。相反，o-ACOB的光解产物分布可以根据其吸附到框架中的程度而显著变化。通过在酮吸附后加入非反应性滴定剂如水，可根据骨架的铝含量系统地改变光解产物分布。通过考虑（a）五元沸石的尺寸和形状吸附，（B）五元沸石的亲水位点对水的吸附（其取决于骨架铝含量），和（c）不含骨架铝的五元沸石通道的疏水特性，完全描述了观察到的结果。在过去的二十年中，高度含硅的合成沸石分子筛的催化性能的结构的研究以及由这种结构施加的反应机理已经引起了相当大的注意。特别地，pentasil沸石家族，其中ZSM-5是杰出的成员，已经表现出非常有价值的催化性能，包括高化学选择性，低焦化（老化）倾向，2引起相当大关注的一个反应是甲醇相当显著地转化为形成汽油的异烷烃和芳族化合物3a，甲苯与甲醇的选择性烷基化反应生成p-~ylene。pentasil系列沸石具有令人印象深刻的催化选择性和使用中的稳定性，这归因于其独特的性质组合，如尺寸、形状和位置吸附选择性、化学性质以及内表面上的基质扩散。尺寸和形状吸附选择性与基质扩散相结合的一个实例是二甲苯异构体在ZSM-5中的扩散率的比较。对二甲苯的扩散率比间二甲苯或邻二甲苯的扩散率快约1000至10000倍，并且这种巨大的差异归因于相对于骨架通道的ZSM-5孔径的尺寸和形状，邻位和Meta异构体的尺寸和形状更大。骨架四面体铝原子及其相关的阳离子以相对稀的和孤立的状态存在于pentasil结构中，并且被认为是ZSM催化的位点特异性的决定因素。沸石分子筛中孔径的尺寸取决于构成孔的环中Si或Al四面体的数目。X-射线衍射分析表明，ZSM-5具有两种类型的孔，均由10个四面体元环组成，一种孔系为正弦形，横截面约为5.5埃，另一种孔系为直孔，垂直于正弦形孔系，横截面约为5.5埃。5.2 X 5.8 A.图1示意性地显示了ZSM-5内部骨架通道（空隙空间）的内表面和该空隙空间的简化拓扑结构。共吸附添加剂对反应机理的影响是催化和界面科学基础理论中具有相当实际重要性和内在兴趣的问题。特别是，水对吸附和扩散的影响
The photochemistry of methylbenzyl benzyl ketones (ACOB) in the presence of pentad zeolites follows strikingly different pathways due to the location of the adsorbed ketone. The product distribution, in terms of the cage effect (efficiency of geminate radical combination), demonstrates the effects of sorption and diffusion on the radical species produced by photolysis. p-ACOB is readily adsorbed within the pentasil framework and produces p-AB as the primary product. In contrast, the photolysis product distributions of o-ACOB can be dramatically varied depending upon the extent of its adsorption into the framework. By addition of a nonreactive titrant, such as water, after the ketone adsorption, the photolysis product distributions can be systematically varied depending upon the aluminum content of the framework. The observed results are completely described by considerations of (a) the size and shape sorption of the pentad zeolites, (b) the sorption of water by the hydrophilic sites of the pentasil zeolites (which depend upon the framework aluminum content), and (c) the hydrophobic characteristics of the pentasil channels which do not contain framework aluminum. During the last two decades considerable attention has been given to investigations of the structure of the catalytic properties of highly siliceous synthetic zeolite molecular sieves and to the reaction mechanisms imposed by such structures.' In particular, the pentasil family of zeolites, of which ZSM-5 is an outstanding member, has exhibited very valuable catalytic properties including high chemical selectivity, low coking (aging) tendency, and high turnover activity for a variety of chemical reactions.2 One reaction that has attracted considerable attention is the rather remarkable conversion of methanol into isoalkanes and aromatics that form gasoline3a as well as the selective alkylation of toluene with methanol to form p-~ylene.~~ The impressive catalytic selectivity and in-use stability of the pentasil family of zeolites are attributed to a unique combination of properties, such as size, shape, and site sorption selectivities, chemistry, and substrate diffusion on the internal ~urface.~ An example of the size and shape sorption selectivities combined with substrate diffusion is the comparison of the diffusivities of xylene isomers in ZSM-5. The diffusivity ofp-xylene is about 1000 to 10000 times faster than that of either m-xylene or of o-xylene, and this enormous difference is ascribed to the larger size and shape of the ortho and meta isomers relative to the size and shape of the ZSM-5 pore diameter of the framework channel^.^ Framework tetrahedral aluminum atoms and their associated cations occur in relatively dilute and isolated states in the pentasil structures and are believed to be the de- termining factor in the site specificity of catalysis by the ZSM The dimensions of the pore diameters in zeolite molecular sieves depend on the number of Si or AI tetrahedra in the rings that make up the pore. X-ray diffraction analyses show that ZSM-5 possesses two types of pores, both of which are composed of 10 tetrahe- dral-membered ringd one pore system is sinusoidal with a nearly circular cross section of about 5.5 A, and the other pore system is straight and perpendicular to the sinusoidal system with elliptical pores of ca. 5.2 X 5.8 A. A representation of the internal surface of the channels (void space) of the ZSM-5 internal framework and a simplified topological structure of this void space are shown schematically in Figure 1. Of considerable practical importance and also of inherent in- terest to the fundamental theories of catalytic and interface science is the effect of coadsorbed additives on reaction mechanisms.'c In particular, the influence of water on the sorption and diffusional