Synthesis Of Amino Functionalized Mesoporous Silicas And The Catalytic Research In Organic Reactions

Organic modification of ordered mesoporous silica is a common method to supply as a heterogeneous catalytic material or as a support for uniformly loading small-sized metal nanoparticles.In general,mesoporous silica functionalization is accomplished by loading organic functional groups into the pores by post-grafting or co-condensation.It has been proved that the organic functional groups introduced by the two methods will cause partial blockage of mesopores and affect the mass transfer process during the reaction,resulting in decrease in the accessibility of the active sites in the organic functional groups.In this paper,in order to effectively improve the accessibility of organic functional groups,organosilica precursors with different compositions were employed as copolycondensation precursors to synthesize periodic mesoporous organosilicas(PMOs)with different structures and morphologies by sol-gel method.The construction of molecular size pores in the framework,precise regulation of catalytic active sites or in situ reduction of metal nanoparticles by the introduction of organic functional groups were in-depth studied and the effect of structures and compositions of PMOs on organic catalysis was further investigated.With hydroquione,bisphenol A and binaphthol as organic bridging groups,three different structures and morphologies of carbamate functionalized ordered PMOs materials were synthesized by sol-gel method.The effects of organic bridging groups and synthesis conditions on structures and morphologies of the PMOs materials was analyzed.The PMOs materials were applied as alkali catalysts to study the tandem reaction performance of the three-membered cyclization.The α,β-unsaturated compound is replaced by a cheaper aromatic aldehyde compound as the reaction substrate.After 5 h reaction,the conversion of aromatic aldehyde can reach to 93.7%,and the highest yield and selectivity of cyclopropanic compound can be up to 82.4% and 99.9%,respectively.The results show that the orderation of mesoporous structures,specific surface area,the size of organic bridging groups and the reaction substrate have a significant effect on the catalytic performance.Compared with homogeneous organic catalysts,the carbamate fuctionalized PMOs materials exhibit better catalytic performance and good recycling properties as heterogeneous catalysts.With hydroquinone and bisphenol A as organic bridging groups,different ratios and compositions of carbamate functionalized PMOs materials were synthesized by adjusting the molar ratio between the organosilica precursors and tetraethyl orthosilicate.By thermal treatment in aqueous dimethyl sulfoxide,the carbamate group cleaves to form a more basic amino group,and a molecular(hydroquinone or bisphenol A)size cavity is produced next to the amino group,and amino functionalized PMOs materials with cavities were obtained.Using ethyl cyanoacetate as the reaction substrate,the cavity-type PMOs materials can efficiently catalyze the Knoevenagel condensation reaction of benzaldehyde based reactants.The conversion of aldehyde molecules is close to 100% after 3 hours,indicating that the enhhanced alkalinity and the increased accessibility of the active species is more conducive to the progress of the reaction.At the same time,the size of the pore can affect the catalytic conversion efficiency of the pore-type amino functionalized PMOs.With hydroquinone as organic bridging group,four kinds of PMOs with different silicone contents were synthesized by adjusting the molar ratio between hydroquinone organosilica precursor and tetraethyl orthosilicate.Amino groups and cavities were formed in the skeleton of the mesoporous material by thermal treatment in aqueous solution of dimethyl sulfoxide.An aldehyde group was introduced into the cavity by condensation between glyoxal and amino group,and ultra-small size silver nanoparticles about 2 nm were synthesized in the cavities by silver mirror reaction,and the content of silver nanoparticles can be accurately and quantitatively controlled by optimizing the content of hydroquinone organosilica precursor in the skeleton.The catalytic results show that the silver nanoparticles in the cavities can be used as active centers to realize the catalytic process of synthesizing p-aminophenol with p-nitrophenol,and the higher the silver content,the higher the reactivity.The skeleton cavities in the PMOs material limit the free movement of the silver nanoparticles,weakening the growth process of the ripening,and keeping the size of the silver nanoparticles in a small range.Since one end of the silver nanoparticles is anchored in the skeleton of the PMOs material,neither excessive blockage of the pores nor loss of silver nanoparticles can be caused.