Study On Synthesis, Characterization And Catalytic Performance Of New Acid-base Bifunctionalized Mesoporous Materials

Ascribing to their excellent textural properties, including highly ordered pore channels, controllable narrow pore size distribution, large surface area, as well as a large amounts of Si-OH, which can facilitate the modification of their surface with metals and organic group, the functionalization of mesoporous silicas have attracted more and more attention. And in many cases, the synergism of the acid-base active centres usually contributes to promote the progress of the reaction, and results in an enhancement of the reaction velocity, in an improvement of the selectivity and in a prolongation of the catalyst life. Therefore, acid-basic bifunctional mesoporous materials, which could combine the advantages of mesoporous structure and acid-basic properties, that is, combining the specific chemical reactivity of the acid-basic groups with attractive structure properties, have generated considerable interest in their application such as shape-selective catalysis, adsorption-separation processes, and other fields. In this work, we attempt to develop new facile and efficient strategies for the acidic-basic bifunctionalization of mesoporous silicas.A new acidic-basic bifunctional approach with high efficiency and facility, named solid state ionics migration, for mesoporous silica was developed. The generation of acid-base active sites and removal of host template were achieved in a single step by this method. A series of acid-base bifunctional mesoporous nanocomposites MgO-Al2O3–SBA-15 (MA–SBA-15) have been successfully synthesized by means of this approach. The characterization results indicated that the resultant bifunctional mesoporous nanocomposites can keep mesoporous framework well, and the guest species can be efficiently introduced into the channel and fully dispersed on the surface of the host. The electric positive Mg2+ and Al3+ can form covalent bonds between the metal ions and the oxygen atoms of the Si-OH groups on the surface of the host during calcination. Hence, the Si–O–Mg and/or Si–O–Al interlinkages can conveniently establish between the mesoporous host and the introduced guests to achieve the bifuctionalization. The results of NH3, CO2-TPD and pyridine adsorption unambiguously indicated the acidic sites, including Lewis and Br?nsted acid sites, and basic sites co-exist on the surface of the mesoporous nanocomposites synthesized by the titled method. And the mechanism and inherent rules of bifuctionalization was also investigated. In addition, the resultant nanocomposite exhibits high activity for the synthesis of ethyl methyl carbonate.By the in-situ grafting method, a series of surface-modified mesoporous silicas, MgO-ZrO2–SBA-15(MZ–SBA-15), endowed with acid-base properties have been firstly successfully synthesized in one pot by the introduction of zirconium and magnesium salts into the initial mixture of synthesizing mesoporous silica (SBA-15). The characterization results indicated the resultant nanocomposites exhibit excellent acid-basic properties with well periodically ordered mesoporous backbone.Two novel mesoporous silica materials, GML-MGE and GMS-MGE, have been firstly synthesized by self-assembly route, and the influence of the crystallization temperature, crystallization time, aging temperature, removal method of template and surfactants with different chain length on mesoporous framework have been investigated. XRD, N2 adsorption-desorption isotherms and FTIR employed to characterize the structure properties of the new mesoporous silica materials, and the results showed that GML-MGE and GMS-MGE possess the typical mesoporous structure properties. Subsequently, the modification of GMS-MGE mesoporous material with metal ions (M-MGE, M=Al,Mg) was performed and the effect of different metal salt precursors on the mesoporous structure was investigated. The characterization results indicated the resultant mesoporous nanocomposites can keep mesoporous structure properties well and the active species can fully disperse on the channel surface of the host. The esterification reation of stearic acid and glycerol was employed as a probe reaction to evaluate the catalytic performance of M-MGE mesoporous material. The results suggested the M-MGE mesoporous material exhibits good performance on this reaction. The conversion of stearic acid reach 52 % and the selectivity of monoglyceric stearate can reach 96 % under the condition: reaction time 10 h, catalyst loading 5 wt %, reaction temperature 110℃.