Synthesis And Applications Of Supported Chromium-based Mesoporous Catalysts

Compared to microporous zeolites, traditional silica-based mesoporous materials have limited to use in the catalytic reactions due to lack of acid centers and catalytic active sites. At present, researchers solve the above problems mainly through introducing heteroatoms into the frameworks of mesoporous materials or synthesizing other nonsiliceous mesoporous materials with active sites themselves. However, the incorporation and fixation of heteroatoms, and the catalytic performance of final catalysts will be influenced by a lot of factors, such as the introducing methods and supports, and so on. Herein, mesoporous materials (alumina and SBA-15) and chromic nitrate nonahydrate are used as supports and source of heteroatoms respectively to synthesize supported chromium-based mesoporous materials. And they have been tested in the liquid phase oxidation of cyclohexane with hydrogen peroxide(H2O2). The influence of Cr-introducing methods on structures of framework, existing forms of chromium, and thus the catalytic performance and stability of final catalysts have been systematically investigated by several measuring methods. Moreover, in order to lower the synthesis cost of mesoporous alumina materials effectively, they are also prepared using cheaper inorganic aluminium nitrate as the precursor. We have carried out comprehensive studies and achieved the following results:(1) Aluminum isopropoxide is used as precursor, F-127as template, concentrated hydrochloric acid as pH adjustor and citric acid as additive. And ordered mesoporous alumina is synthesized by evaporation-induced self-assembly (EISA). Supported Cr-based mesoporous alumina with a variety of nAl/ncr ratios is prepared using the direct synthesis method and incipient wetness impregnation (IWI) method. Small-angle XRD, nitrogen adsorption and desorption, and TEM results show that catalysts synthesized with direct synthesis method exhibit better order degree, higher surface area and larger pore size, and they can reach264m2/g and7.7nm respectively. Moreover, from the WXRD patterns and UV-vis spectrum, it can be known that in the samples prepared using direct synthesis method chrome active components are highly dispersed and many of them are catalytically active Cr(VI). In contrast, in the catalysts synthesized with incipient wetness impregnation method chromium species will gradually form crystalline Cr2O3without catalytic activity with decreasing nAi/nCe ratios. The catalytic activity of catalysts has been studied at353K for4h, and the results show that catalysts prepared with direct synthesis method are more active and more stable.(2) Cheaper aluminum nitrate or mixture of aluminum nitrate and aluminum isopropoxide are used as precursors, F-127as template, concentrated hydrochloric acid as pH adjustor, and citric acid as additive. Another kind of ordered mesoporous alumina with high thermal stability is synthesized by modified evaporation-induced self-assembly (EISA). Small-angle XRD and nitrogen adsorption desorption results show that the mesoporous alumina prepared with pure nitrate aluminum possesses a highly ordered2D hexagonal mesostructure but its surface area is relatively low, while introducing moderate aluminum isopropoxide and using P-123as template can increase the surface area and pore volume dramatically. Further more, modified evaporation-induced self-assembly (EISA) can promote the thermal stability of mesoporous alumina materials evidently. After calcination at1000℃for1h, the materials can still maintain their high order degree, relatively high surface area and large pore volume.(3) Based on the understanding on the pH-adjusting high temperature hydrothermal (pH-aDH) grafting mechanism, both the Cr-OH and Al-OH concentration can be controlled by adjusting pH and temperture during high temperature hydrothermal treatment (HTHT) processes, and then a large amount of chromium species can be grafted into the SBA-15framework homogenously. Compared to Cr/SBA-15synthesized through traditional incipient wetness impregnation (IWI) method, samples prepared by pH-adjusting high temperature hydrothermal (pH-aDH) grafting method not only have highyl ordered mesoporous structures but also exhibit higher activity and stability in the oxidation of cyclohexane.