Study On Preparation Of Co-loaded On Hierarchical Porous Zeolite And Its Application To Fischer-tropsch Synthesis

Conversion of coals or natural gas to liquid hydrocarbon via Fischer-Tropsch (F-T)synthesis is an important way to obtain liquid fuel. The key factor which affects theproduct distribution for F-T synthesis is to prepare an active, selective and stablecatalyst. Some drawbacks of common porous supports, such as low hydrothermalstability of mesoporous molecular sieve and narrow pore structure of microporouszeolite, limit their application in F-T synthesis. Meso-and microporous molecularsieves have some advantages, sush as regular structure and high hydrothermal stability.Therefor, it is an important means to adjust the product distribution for F-T synthesisvia preparation of suitable catalyst using such molecular sieves as a support.Hierarchical porous molecular sieves (Meso-ZSM-5) were synthesized using twotemplating agents under hydrothermal conditions, and then Co/Meso-ZSM-5catalystswere prepared with incipient-wetness impregnation method and characterized byX-ray diffraction (XRD), scanning electron microscopy (SEM), temperatureprogrammed reduction (TPR), X-ray fluorescence (XRF) and N2adsorption-desorption. Activity and selectivity of Co/Meso-ZSM-5catalyst for F-Tsynthesis were investigated. Specific work as follows:Fistly, three supports (ZSM-5, Al-MCM-41and Meso-ZSM-5) with different porestructure were prepared. The difference in catalytic performance of Co/ZSM-5,Co/Al-MCM-41and Co/Meso-ZSM-5for F-T synthesis was determined. Theexperimental results indicated that the pore structure had an important influence onthe the selectivity of the catalysts.Co/Meso-ZSM-5catalyst had a high C5-18selectivity and low gaseous hydrocarbon selectivity due to the confinement effect ofmicropore and good diffusibility for long-chain hydrocarbon of mesopore inCo/Meso-ZSM-5.Secondly, effects of second crystallization temperature, molar ratio of mesoporoustemplate hexadecyl trimethyl ammonium bromide (CTAB) and tetraethoxysilane(TEOS), molar ratio of Si/Al and pH value on pore structure of Meso-ZSM-5werestudied. With the increase of second crystallization temperature, the crystallinity ofthe catalyst micropore was improved, and CO conversion and C5-18selectivityincreased. As the molar ratio of CTAB/TEOS increased, the crystallinity of ZSM-5decreases, and mesoporous pore size increased. When the molar ratio of CTAB/TEOSis equal to0.25, CO conversion and selectivity of C5-18reached the maximum. Withthe molar ratio of Si/Al increases, the crystallization of ZSM-5was becoming perfect, and the conversion of CO increases, and the selectively of gasoline components (C5-11)increased and the selectivity of diesel components (C12-18) was reduced.To further increase the mesopore size of Meso-ZSM-5, the effect of the triblockcopolymer (P123) on the pore size of zeolite was studied. The result indicated thatmesopore size of Meso-ZSM-5increased along with the increase of the amount ofP123.