Research On MOFs Catalyst Based On Olefin Catalytic Oxidation

Olefin catalytic oxidation has been a hot research topic in recent years,because epoxidized products are mostly used in industrial production or pharmaceutical industry.For example,styrene oxide is an important intermediate in organic synthesis.Therefore,this paper mainly studied the catalytic oxidation of styrene.The catalysts commonly used for this reaction include metal catalysts,polyoxometalates catalysts,schiff base metal complex catalysts and molecular sieve catalysts.Among them,schiff base metal complex catalysts have the best performance in olefin catalytic oxidation.However homogeneous catalysts are usually difficult to separate from the reaction mixture.Due to its advantages of large specific surface area,adjustable pore structure and exposed active sites,MOFs can be used as a carrier to introduce transition metal salen complexes into the surface or pore channels of MOFs,so as to overcome the shortcomings of homogeneous catalysts.In this study,Cu salen@NH₂-MIL-101（Cr）catalyst with compact and interconnected pore structure,Ni salen@NH₂-MIL-101（Cr）catalyst with partial curly sheet and non-porous structure and Cu@Al-MIL-53-P21 catalyst with rod-like morphology were successfully prepared by hydrothermal method and one-step method.All the three catalysts obtained better catalytic effects.The conversion rate of styrene was 99%,86.7%,97%,and the selectivity of styrene oxide was 89.6%,84.8%,and 23.3%,respectively.Among them,the catalyst Cu@Al-MIL-53-P21 could achieve 75.7%benzaldehyde selectivity.In addition,a series of characterization for all the prepared samples were carried out,and the correlation between the morphology,reaction conditions and catalytic activity was discussed in detail.The following conclusions are drawn:（1）The smooth and tightly connected sheet-like morphology of the transition metal Cu salen@NH₂-MIL-101（Cr）catalyst was conducive to the uniform dispersion of the reaction substrate.At the same time,the contact area between the reaction substrate,the reactive center and the oxidant could be increased to further improve the styrene conversion and products selectivity.However,the catalytic activity of Ni salen@NH₂-MIL-101（Cr）catayst in the styrene oxidation was relatively weak due to its partially curly and non-porous sheet morphology.（2）Phenylacetaldehyde increased with the introduction of Cu salen.Due to the oxidation capacity of Cu²⁺is greater than that of Ni²⁺,and the electron density around the reacted Cu salen increased,it would force part of styrene oxide ring to form phenylacetaldehyde.（3）Different transition metals,reaction temperature,type of oxidants,mole ratio of reaction substrate to oxidants and reaction time all affected the activity of the reaction,and the optimal reaction conditions can be determined according to the needs of different products.（4）The Cu@Al-MIL-53-P21 catalyst with rod-shaped morphology had a good crystallinity and the structure was still mainly on microporous.Compared with the pure carrier NH₂-MIL-53（Al）,the functionalized catalyst has a partially cross-linked nanorod morphology,resulting in partial mesoporous structure of the catalyst.Therefore,the excessive oxidation of benzaldehyde can be effectively reduced with Cu@Al-MIL-53-P21 catalyst.