Studies On SBA-16Molecular Sieves Supported Cobalt Catalysts For Fischer-tropsch Synthesis

The Fischer–Tropsch synthesis (FTS) is an important technology for the production ofclean transportation fuels and chemicals from syngas (CO+H2). Although it was firstdeveloped90years ago, some challenges still remain. The development of FTS catalystswith high activity, high stability, and, in particular, high selectivity, remains one of the keygoals of current research in this area.In this paper, ordered mesoporous molecular sieves SBA-16have been synthesizedusing P123and F127as template agent. Aluminum-substituted mesoporous SBA-16(Al-SBA-16) materials were also synthesized using “pH-adjusting” method. The catalystswere prepared by using incipient wetness impregnation method and characterized by usingX-ray diffraction(XRD), nitrogen adsorption-desorption, transmission electronmicroscopy(TEM), hydrogen temperature programmed reduction(H2-TPR), hydrogentemperature programmed desorption(H2-TPD), oxygen titration, ammonia temperatureprogrammed desorption(NH3-TPD) and solid-state27Al NMR(ssNMR). The catalyticproperties scuh as activity, stability and selectivity for FTS were investigated in afixed-bed reactor and a continuously stirred tank reactor (CSTR). The main conclusionsare as follows:(1) For SBA-16supported cobalt catalysts, most of the Co3O4nanoparticles areuniformly distributed inside the interconnected cages of SBA-16. The average diameter ofthe nanoparticles is12nm, which does not increase largely with cobalt loading. Thecatalytic activity of the Co/SBA-16and Co/SiO2catalysts was studied in a fixed-bedreactor. The activity increases with increasing cobalt loading from10to20wt%. A furtherincrease in cobalt loading from30to40wt%increased the CO conversion only slightly.The CO conversion of20%Co/SBA-16is about three times higher than that of20%Co/SiO2catalyst. This higher activity is related to the higher dispersion of cobalt onthe SBA-16surface because cobalt nanoparticles on SBA-16have a smaller particle sizeand a higher surface area. (2) A comparison of the stability of20%Co/SBA-16catalyst with that of20%Co/SiO2catalysts at a high initial CO conversion was studied in a fixed-bed reactor. The20%Co/SBA-16catalyst is more active and stable than the20%Co/SiO2catalyst, which isattributed to the high dispersion of cobalt species and low mobility of cobalt particles inthe SBA-16cages, respectively. Owing to the spatial restriction of the isolated nanocagesand smaller pore entrances of SBA-16, the aggregation and the sintering of cobaltnanoparticles were efficiently prevented. However, for20%Co/SiO2catalyst, the spatialrestriction was unable to prevent the growth of cobalt nanoparticles.The stability of the20%Co/SBA-16,20%Co/SiO2and20%Co/SBA-15catalysts withthe addition of water was investigated in a CSTR. For the20%Co/SBA-16and20%Co/SBA-15catalyst, the CO conversion was found to be higher with the water thanthat without water. For the20%Co/SiO2catalyst, the water addition did not affect the COconversion. The enhanced CO conversion by water addition was due to a diffusion effect inthe pores. After switching back to the standard operating conditions, the CO conversion ofthe20%Co/SBA-16catalyst is still higher and stability than the other two’s. The highstability of the Co/SBA-16catalyst is attributed to the effective stabilization of cobaltnanoparticles in the three-dimensional mesoporous silica cages of SBA-16—known as thepore confinement effect.(3) Al-SBA-16supports with the unique pore size, different aluminum content wereprepared using “pH-adjusting” method. The acidity of the support increases withincreasing aluminum content. The majority of the aluminum is tetrahedrally coordinatedAlO4groups and only a small amount of aluminum is present outside the framework. TheCo3O4nanoparticles have similar particle size (12nm) and are highly dispersed within theAl-SBA-16cages. The reducibility of the catalyst decreased from65.4to59.6%withdecreasing Si/Al ratios from30to10, which is attributed to the increased interactionbetween cobalt and the support. The acidity of the Al-SBA-16supports played a criticalrole in controlling the selectivity of the FTS. The selectivity shifted towards lighterproducts with lower Si/Al ratios, which was mainly attributed to the increased acidity ofthe Al-SBA-16supports, thereby hindering the olefins from undergoing furthertransformations at lower temperatures on the cobalt catalysts.