Influences Of Mesopores Structures Of SiO₂ On Bipyridine-proline Grafting And Asymmetric Aldol Catalytic Performances

Homogeneous catalyst suffers many problems,including difficulty in separation from the reaction mixture and recycling,and moreover the organocatalyst are usually used in a substantial quantity.From a practical and environmental point of view,it would be desirable to transfer the catalysts into heterogeneous media,and immobilization is proved to be the simplest method to achieve this heterogeneity.After the immobilization the product purification and the catalyst recycling can be facilitated,which is meaningful for the environmental protection and energy conservation,especially when the catalyst is obtained after several synthetic steps.Mesoporous silica materials have received much attention because of their large surface areas,uniform ordered mesoporous channels,high hydrothermal stability,easy functionalized surfaces property and tunable pore dimension and channel structure.More importantly,the nanopores of these mesoporous supports can influence the catalytic reaction process,resulting in the different catalytic performances.Although,some heterogeneous catalysts have achieved comparable activity to the homogeneous ones,in most cases two conflicting problems,activity and stability,perplex the synthesis of immobilized catalysts.In the catalysis process the immobilized organic catalyst must coordinate with the substrates and change configuration,which means that the mobility of the confined catalysts are of great importance to the activity.But on the other hand,to maintain the high stability,the protection of immobilized catalyst by carrier confinement against leaching is necessary.Many works focus on the interaction method and linking group between the organic catalyst and the supports.However,to the best of our knowledge,the intensive research about the synthetic strategy including the effect of different structures and morphology of mesoporous material have seldom been systemically investigated.The results obtained are as follows:1.Accordance to the BMMs ratio of our laboratory,bimodal mesoporous silicas(BMMs)were prepared under room temperature via hydrothermal route with different addition of mesitylene(TMB)using cetyltrimethylammonium bromide(CTAB)as template,tetraethoxysilane(TEOS)as silicon source,and TMB as expanding pore agent.A series of T-BMMs were designed and synthesized by controlling the addition amount of TMB and crystallization time during the sol-gel process to study the effect of addition volume and crystallization time on the structure and morphology of BMMs.The addition of TMB and different crystallization time have a significant effect on the mesoporous structure of the BMMs,especially with the increase of the amount of TMB,the primary pores gradually increase from 3.03 nm to 4.21 nm;prolonging the crystallization time is conducive to the regulation of secondary pores increase in the direction of the big pore.2.BMMs with different pore size,supported organocatalysis were prepared using metal Zn acetate as the anchor to graft the bipyridine-proline derivative.In order to further compare the effects of mesoporous structure on catalyst activity and stability,SBA-15 and SBA-16 were prepared simultaneously,and the same applies to the above heterogeneous catalyst preparation method.As catalysts for the asymmetric aldol reaction of cyclohexanone with 4-nitrobenzaldehyde,the catalytic performances were investigated in detail to demonstrate the influences of pore structure and pore size of different supports on the catalytic activities and stability.The results showed that the increase in pore size favors the loading of more Z.3.The effects of pore structure and pore size on the activity and stability of the catalysts were investigated in detail.A new series of hybrid heterogeneous catalyst based on bipyridine-proline as active sites and mesoporous silica with different pore structure,including BMMs,LBMMs,SBA-15 and SBA-16,as solid support were successfully prepared through grafting,as well as applied to the asymmetric aldol reaction.All the heterogeneous catalysts showed the comparable or even better catalytic performance to the homogeneous one.Particularly,the recycle of solid heterogeneous catalyst could be simply achieved by centrifugation and washing.However,catalyst with different structure showed different catalytic performance.Z@ZnSBA-15 was observed the best catalytic activity and stability in the recycle process compared to the other three catalysts,because of its long and large mesoporous channel which provide enough space for the much larger molecules catalysis reaction and protect the Z against leaching.These studies offered us an innovative strategy that to immobilizing the organic catalyst through weak interaction with the carriers,high performance and good stability could be expected by encapsulating the homogeneous catalysts in the solid nano-materials with long channel and large pore size.