Study on Shape Selectivity in Hydrocarbon Oxidation catalyzed by Zedites

我们发现在烯烃的环氧化反应和烷烃的氧合反应中有显著的形状选择性。本文研究了不同位阻和电子性质基质在钛硅石TS-1和TS-2上氧化速率的影响因素。催化反应体系比较复杂。液相被分成有机相和水相。由于固体催化剂几乎完全存在于水相中，因此底物必须转移到存在催化剂和H_2O_2的水相中。这说明了有机共溶剂对疏水烃氧化的促进作用。下一步是扩散到沸石孔中。我们发现环烷烃和烯烃比线性烷烃更难氧化。因此，与沸石具有相似尺寸的分子的扩散受到包括水在内的共存分子的影响。底物的极性和沸石的亲疏/疏水性也可以考虑在内。即使底物在沸石孔隙中，待氧化基团也必须接近活性位点。在不饱和醇的氧化反应中，端双键和内双键对化学选择性的影响是完全不同的，这可以用它们对Ti活性位点的配位能力来解释。末端CH_2=CH-双键的空间要求较低，似乎能够与Ti活性位点强烈相互作用，阻止醇基接近活性位点。结果表明，羟基化活性与IR Ti=O拉伸强度成正比。因此，钛基可能由活性位点组成。另一方面，环氧化活性与Ti=O拉伸带强度之间没有关系。然而，进入活性部位并不是反应的充分条件。对于不饱和醇的氧化反应，环氧化反应活性与配位能力相反；由于活性氧对C=C双键的攻击具有亲电性，因此内双键的环氧化率高于末端双键的环氧化率。最后一个阶段是产物向孔外扩散。我们发现这个过程有时限制了氧化的速度；因此，在硅钛石上完成有机底物的H_2O_2氧化有几个先决条件。少

We have found remarkable shape selectivity in the epoxidation of alkenes and oxygenation of alkanes. Here we have investigated the factors determining the rates of oxidation of various substrates which have different steric and electronic characters over titanium silicalites TS-1 and TS-2.The catalytic reaction system is rather complicated. The liquid phase is separated into an organic phase and an aqucous phase. Since the solid catalyst exists almost exclusively in the aqueous phase, the substrates must transfer to the aqueous phase, where the catalyst and H_2O_2 are present. This accounts for the promoting effect of organic cosolvents in the oxidation of hydrophobic hydrocarbons.The next step is diffusion into zeolite pores. We have found that cyclic alkanes and alkenes are much more difficult to oxidize than linear ones. Thus the diffusion of molecules which have similar dimensions to the zeolites is affected by the coexisting molecules including water. The polarity of substrates an … More d hydrophobicity/- philicity of zeolites might also be taken into account.Even if the substrate is in the zeolite pores, the group to be oxidized must approach the active site. In the oxidation of unsaturated alcohols the influence on the chemoselectivity between terminal and internal double bonds was found quite different and interpreted in terms of their coordination ability to the Ti active site. The terminal CH_2=CH- double bonds, being less sterically demanding, appear to be able to interact strongly with the Ti active site, preventing the alcoholic group from approaching the active site.It has been revealed that the hydroxylation activity is proportional to the intensity of IR Ti=O stretching intensity. Therefore titanyl species might consist of active site. On the other hand, there is no relationship between the epoxidation activity and the intensity of the Ti=O stretching band.However, access to the active site is not the sufficient condition for the reaction. For the oxidation of unsaturated alcohols reactivity toward epoxidation was opposite to the coordination ability; higher turnover of epoxidation of the internal double bond than that of the terminal double bond would result from an electrophilic nature of the attack of active oxygen species against C=C double bonds.The final stage is the diffusion of products into the outside of the pores. We have found this process some- times limits the rate of the oxidation; the diffusion of reactants into the pore filled with products must be retard- ed. Thus it has been revealed that there are several prerequisite conditions to the completion of the H_2O_2 oxidation of organic substrates on titanium silicalites. Less