Study on Shape Selectivity in Hydrocarbon Oxidation catalyzed by Zedites

在烯烃的环氧化和烷烃的氧化反应中，我们发现了显著的择形作用。本文考察了在钛硅分子筛TS-1和TS-2上不同空间和电子性质的底物的氧化速率的影响因素。液相分离为有机相和水相。由于固体催化剂几乎只存在于水相中，底物必须转移到存在催化剂和H_2O_2的水相中。这解释了有机助溶剂对疏水性碳氢化合物氧化的促进作用。下一步是扩散到沸石孔中。我们发现环烷烃和烯烃比直链烷烃更难氧化。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。衬底的极性和…还可以考虑沸石分子筛的疏水性/亲水性，即使底物在沸石孔内，被氧化的基团也必须接近活性中心。在不饱和醇氧化反应中，末端双键和内部双键对化学选择性的影响有很大不同，其原因是它们对钛活性中心的配位能力。末端的CH_2=CH-双键对空间要求较低，能够与钛活性中心发生强烈的相互作用，阻止醇基靠近活性中心，羟化活性与IR钛=O的伸缩强度成正比。因此，钛基物种可能由活性中心组成。另一方面，环氧化活性与Ti=O伸缩带的强度无关，但进入活性中心并不是反应的充分条件。对于不饱和醇的氧化，环氧化反应的活性与配位能力相反，内部双键的环氧化转化率高于末端双键的转化率，这是由于活性氧物种对C=C双键的攻击具有亲电性，最终产物扩散到孔外。我们发现，这一过程有时会限制氧化速度；必须阻止反应物扩散到充满产物的孔中。从而揭示了在钛硅沸石上完成有机底物H_2O_2氧化的几个前提条件。较少

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