Preparation Of Schiff Base Molybdenum Complex Grafted On MCF And Its Catalytic Performance For Epoxidation Of Propylene

Propylene oxide is an important basic chemical raw material used in the production of polyether polyols,propylene glycol and various non-ionic surfactants.The co-oxidation method is the propylene oxide production technology encouraged by our country because of its technical and economic advantages.Among them,the isobutane co-oxidation method?PO/TBA process?is the epoxidation process used propylene and tert-butyl hydroperoxide?TBHP?from the oxidation product of isobutane as raw material to produce propylene oxide.The homogenous molybdenum complex catalyst used in this process has higher epoxidation efficiency,but its separation and recovery are difficult,which leads to a series of problems such as equipment fouling and decreasing in device economics.Therefore,constructing of a stable and efficient heterogeneous molybdenum catalyst applied to the propylene epoxidation reaction has become a concern of many researchers.In the preliminary work of this research group,an in-situ one-step method was used to prepare immobilized Schiff base molybdenum complexes supported on mesoporous cellular silica foams?MCF?.The supported molybdenum complex has better catalytic performance in the propylene epoxidation reaction,but has the problems of loss of active components and poor stability.In order to further improve the stability of the catalyst,in this work,a step-by-step grafting method is used to prepare the MCF-supported Schiff base molybdenum complex,and the influence of the preparation method and ligand form on the catalytic activity and stability is studied.The main work is as follows:Firstly,the Schiff base ligand formed by the condensation of the aminated modified MCF with pyridine-2-carbaldehyde was bonded in situ to the molybdenum acetylacetonate to form a Schiff base molybdenum coordination structure.It was confirmed by various characterization methods that the Mo O₂-N=CPy@MCF heterogeneous catalyst was successfully prepared.Its active center is Mo?VI?,but there is a Mo?V?complex structure from the directly coordination with the amino group.Using 1,2-dichloroethane as solvent and TBHP?dehydration?as oxygen source,the performance of propylene epoxidation catalyzed by Mo O₂-N=CPy@MCF was investigated.It was found that at the ratio of propylene and TBHP is 10:1,catalyst dosage is 1.69wt.%and the reaction is carried out at 120°C for 1 h,the catalytic performance of Mo O₂-N=CPy@MCF was better.The conversion of TBHP reached97.2%,and the selectivity of PO reached 82.4%.When investigating its stability,it is found that there is still a phenomenon of component loss.Among them,a large amount of Mo?V?is mainly lost,and only a small amount of Mo?VI?is lost.Secondly,in order to reduce the influence of Mo?V?on the structure and performance of the catalyst,the synthesis sequence of the supported catalyst was changed,pyridine-2-carbaldehyde and 3-aminopropyl-triethoxysilane?APTES?are condensed firstly,and next it is coordinated with molybdenyl acetylacetonate and then supported on the MCF carrier.Mo O₂-N=CPy@MCF was obtained by changing the molar ratio of APTES and pyridine-2-carbaldehyde,and the catalyst structure was characterized by various means.Mo?VI?is still an active molybdenum species.Using TBHP as the oxygen source with 1,2-dichloroethane as the solvent,under the condition of the ratio of propylene and TBHP is 10:1,m?catalyst?=1.69wt.%,and the reaction is carried out at 120°C for 1 h,Mo O₂-N=CPy@MCF-2 obtained with a molar ratio of amine to aldehyde of 1:2 has better catalytic performance for propylene epoxidation,the conversion rate of TBHP is 81.1%,and the selectivity of PO reaches86.1%.The lower the Mo?V?ratio,the better the catalytic performance.When examining the stability of Mo O₂-N=CPy@MCF-2,it was found that after repeated use twice,its catalytic performance did not change,nor did it show structural changes.Thirdly,in order to change the coordination structure,the catalysts MCF-N=CSa-Mo O₂and Mo O₂-salen H₂@MCF were prepared by using salicylaldehyde and 5-chloromethylsalicylaldehyde with ethylenediamine as ligand sources,separately,which were characterized by various means.The active centers of both structures are Mo?VI?,and there is a very small proportion of Mo?V?.Because of the difference in coordination elements and the influence of steric hindrance,the molybdenum content of the two samples is relatively low,but the modulation of coordination elements and symmetrical ligand structure are conducive to the enhancement of the L acidity of the molybdenum center,thereby improving the catalytic performance.Under the same reaction conditions,the catalytic efficiency of MCF-N=CSa-Mo O₂and Mo O₂-salen H₂@MCF is far higher than that of MCF-N=CPy-Mo O₂.