Synthesis, Modification And Catalytic Performance Of Ti-MCM-41 For Transesterification

MCM-41 mesoporous molecular sieve has many advantages such as high surface area, periodic hexagonal pore channels, easy adjustability of pore dimension, high chemical stability, and facile modification of surface properties. It has wide applications in catalytic organic chemistry. Synthesis and functionalization of transition-metal-substituted mesoporous molecular sieves have attract much attention in materials field. This dissertation focuses on the effect of synthesis conditions of Ti-MCM-41 mesoporous sieve on catalytic performance for transesterification of dimethyl oxalate （DMO） and phenol. Investigations have been carried out on the prosity regulation and protonic acid modification of Ti-MCM-41 and analysis of its structure-activity relationship.Ti-MCM-41 materials were successfully synthesized through both hydrothermal and microwave irradiation methods, using cetyltrimethylammonium bromide as a template agent, tetraethyoxysilane and tetrabutyl titanate as Si and Ti sources respectively in an alkaline solution. Various characterization techniques, including XRD, N2 adsorption-desorption, TEM, FTIR, UV-vis, Py-FTIR, NH₃-TPD, XPS and ICP-OES were carried out to analyze the textural and physicochemical properties of these Ti-MCM-41 samples, with particular emphasis on the influences of microwave conditions on molecular sieves properties. Ti-MCM-41 molecular sieve possesses a long-range ordered hexagonal structure and titanium has incorporated into the silica framework with Ti species mostly in tetrahedral coordination. Experimental results suggested that the microwave irradiation method could provide a short crystallization period, small particle size of products, high structural order, narrow pore size distribution, large surface area, and little loss of active component. The optimum microwave-irradiation condition was keeping a continuous heating at 120℃for 40 min.The desirable catalytic activities of Ti-MCM-41 molecular sieves could be ascribed to its favorable structure and surface properties. First, the rich inner-framework Ti （Ⅳ） centers could provide plenty of weak L acid sites for the transesterification of DMO and phenol. Secondly, the mesoporous structure with high surface area and large pore volume is beneficial for the molecular diffusion in reaction system, which improves the efficiency of transesterification reaction.According the mechanism of solubilization, the porosity regulation of Ti-MCM-41 by utilizing organic swelling agents was examined. Swelling agents played a key role for pore expansion because its hydrophobic and molecular properties could affect the stability and the swelling volume of surfactant micelles. Addition of swelling agents has been proved to be an effective way for pore expansion of Ti-MCM-41 while the characteristic mesoporous structure and its chemical properties were maintained simultaneously. Thus, the catalytic activity of Ti-MCM-41 molecular sieve was accelerated greatly as its pore size expanded. N-heptane was an optimal swelling agent with which the average pore diameter was expanded form 28.6（A|°） to 48.3 （A|°） .The structure and chemical properties of S/Ti-MCM-41 were also studied intensively. Due to the strong electron inductive effect caused by the S=O bond of SO₄^2-, Lewis acid sites were intensified significantly. Moreover, the surface hydroxyl groups were further activated and formed H⁺, generating Br（o|¨）nsted acid sites. The results of catalytic test of S/Ti-MCM-41 catalysts showed that Br（o|¨）nsted acid sites could promote the transesterification reaction between DMO and phenol, although it would also lead to the formation of by-product anisole.