Synthesis Of Mesoporous Molecular Sieves And Application As Supports Of Cobalt Catalysts For Fischer-Tropsch Synthesis

From a rough comparative estimate of the sources of carbon, the reserves of coal exceed, by far, the sum of all alternative carbon sources. And the explored reserves of natural gas are increasing. Therefore the effective and clean use of coal/natural gas becomes hotspot of research. One route is to make heavy hydrocarbon via syngas. Cobalt-based catalyst is one of catalyst systems used in heavy hydrocarbon manufacture via syngas from coal/natural gas.The common supports of cobalt-based catalyst are SiO2, Al2O3, TiO2. The recent appearance of mesoporous molecular sieves with features such as high surface area, relatively large pore and narrow pore distribution gives an alternative for catalyst support. When served as support of cobalt-based catalyst, high surface area of mesoporous silica might bring high dispersion of cobalt species. Compared with microporous and amorphous support, mesoporous support with middle pore size will have special effect on Fischer-Tropsch synthesis.In this paper, over a novel mesoporous silica supported cobalt catalysts, support effect on FTS was investigated in a fixed-bed reactor(I. D. 12mm) under the conditions of P=2.0MPa, T=473-513K, H2/CO=2.0, and GHSV=500 h-1, and the support and catalysts were characterized in detail.Four novel mesoporous silica supported cobalt catalysts (Co/HMS, Co/SBA-12, Co/MSU-1 (another name was Co/MSU0195), Co/MCM-41) containing 15wt% Co were prepared. The results indicated that HMS present double pore distribution (2.9, 40nm) and SBA-12 had relatively large pore (4.10nm). Pore sizes of MSU0195 and MCM-41 were smaller (2.65, 2.85nm). Hi-TPR results suggested that the order of interaction between cobalt species and support was Co/MSU0195 ~ Co/MCM-41 > Co/SBA-12 > Co/HMS, which accorded with that of the catalysts activity. High C19+ hydrocarbon selectivity over Co/HMS and high CH4 selectivity over Co/MSU0195 and Co/MCM-41were observed. The activity and selectivity of catalysts is related to the diffusion effect of support and the interaction between cobalt species and support.Mesoporous silicas were synthesized at different synthesis temperature, which were used as supports of cobalt catalysts and compared with MSU0195. XRD results suggested that high temperature destroyed the order of mesoporous structure. The order was MSU930>MSU945>MSU960, but all not as good as MSU0195. N2 adsorption-desorption results suggested that MSU0195 had single pore size distribution basically and MSU930> MSU945, MSU960 all had double pore size distribution at range of 2-10nm. Moreover MSU930, MSU945, MSU960 had wide pore size distribution at range of 10-100nm and their contribution amount to the overall pore volume did not present monotone trend with the increasing synthesis temperature of mesoporous silica. H2-TPR present double consumption peaks below 400℃ in H2-TPR of all the catalysts. The peak height decreased following the order: Co/MSU960 > Co/MSU945> Co/MSU930 > Co/MSU0195, the trend according with that of catalyst activity. Compared with MSU0195^ MSU930, Co/MSU945, Co/MSU960 had lower CH4, C2-4 , C5-11 and higher C19+, C26+ hydrocarbon selectivity. The activity and selectivity of catalysts was related to diffusion effect of support.Based on the important effect of support on FTS of cobalt-based catalyst, synthesis of mesoporous silica MSU-1 was studied in order to better control structure properties of support. Two kinds of silica source (TEOS, sodium silicate solution) and several surfactants were compared and the system of sodium silicate solution-A(EO)9 was established. In the system, pH effect was studied in detail. Effects of synthesis conditions (such as surfactant concentration, silica concentration in reactant mixture, reaction temperature, time and calcination temperature) were firstly studied at choosen pH (≈0.8).