Synthesis And Catalytic Properties Of Titanosilicate Catalysts For Unsaturated Fatty Acid Methyl Esters Epoxidation

Titanosilicate/H2O2 catalytic system for oxidation reactions possesses a series of merits,such as high atomic efficiency,favorable recyclability,and mild reaction conditions,etc.,exhibiting a wide application prospect in biomass catalytic oxidation.However,the catalytic system is currently suffering from low catalytic efficiency,high dependence on solvent,and low selectivity,which impedes its scalable application.In view of these issues,the research of this dissertation is focused on titanosilicate catalyzing solvent-free epoxidation of typical vegetable oil-derived unsaturated fatty acid methyl esters(UFAMEs)-methyl oleate(MO).The research aims at developing high-efficiency titanosilicate for MO epoxidation mainly by the modification of surface property and the modulation of pore structure.The main research contents are as follows:Firstly,different Ti-containing silicates including small-grained hierarchical TS-1(HTS-1),large-grained HTS-1,Ti-MCM-41,and Ti-HMS,etc.were prepared by hydrothermal synthesis.Their catalytic performances for MO epoxidation oxidized by H2O2 under solvent or solvent-free conditions were evaluated.Under acetonitrile solvent conditions,small-grained HTS-1 exhibited the best catalytic performance,over which MO conversion of 100%and product selectivity of 94.9%were achieved under the optimal conditions.However,mesoporous titanosilicates showed higher catalytic activity than that of small-grained HTS-1 for MO epoxidation under solvent-free conditions,in which excellent channels diffusivity and hydrophobicity play a key role.Thus,methanol and ethanol were utilized to improve the surface hydrophobicity of HTS-1.The catalytic performances of methanol/ethanol-modified HTS-1 for solvent-free MO epoxidation were promoted.Among all the catalysts,ethanol-modified HTS-1 at ambient temperature exhibited remarkably enhanced catalytic property.The MO conversion and product selectivity for modified HTS-1 were 35 percentage points and 18 percentage points higher than those for pristine HTS-1.The improvement of catalytic performance of HTS-1 after alcohol modification can be ascribed to the remedied defects and enhanced hydrophobicity/lipophilicity realized by covalently bonding alkoxy groups to Si-OH and Ti-OH defect sites,which strengthens the interaction of substrate with active sites and then accelerates epoxidation reaction.Furthermore,reduced amorphous impurities occluded in the pore and the unchoked channels by alcohol treating are beneficial for the internal diffusion of reactants.On the other hand,in order to obtain enhanced catalytic activity and product selectivity for solvent-free MO epoxidation,organic-functionalized Ti-HMS(Cn-HMS)catalysts with different organic groups(methyl,propyl,cetyl,or phenyl)were prepared via a one-pot co-condensation route.Among all the catalysts,Cl-HMS-50 synthesized with methylsilane displayed the optimal catalytic performance.MO conversion of 95.9%and epoxide selectivity of 65.2%were achieved over Cl-HMS-50,which are higher than those of its inorganic counterpart Ti-HMS(80.5%,35.3%).The merits of methyl-functionalization are as follows:?)the enhanced hydrophobicity/lipophilicity of catalyst realized by methyl-incorporation is conducive to the accelerated adsorption of reactants and interphase mass transfer;?)the specific area and pore volume are improved by the introduction of methyl triethoxysilane precursor,facilitating the internal diffusion of reactants;?)the reduced framework defects by methyl-functionalization suppresses the side-reactions.On this basis,amphiphilic core-shell structured composite catalyst TS@PMO was fabricated by an interfacial deposition method for solvent-free MO epoxidation.The TS@PMO contains a mesoporous Ti-surface-enriched TiO2-SiO2(TS)core and a mesoporous organosilica(PMO)shell.MO conversion of 90.2%and epoxide selectivity of 70.2%were obtained over TS@PMO,which are superior to those of mesoporous TS core(63.7%,49.2%).The advantage of TS@PMO is that the PMO shell is favorable for the enrichment and transfer of substrate and H2O2 to TS core.Meanwhile,the Ti sites are enriched at the surface of TS core,which is caused by organosilane directed growth-induced etching effect,greatly enhancing the accessibility of active Ti sites.