Performance Of Mesostructured Cellular Silica Foams (MCF) Supported Cobalt Catalysts For Fischer-tropsch Synthesis

Fossil energy are being depleted and natural environment suffers a greatdestruction gradually with social economic development. Particularly, global warminghas become increasingly serious. In order to sustain economic and societaldevelopment, people are paying more attention to clean energy development and greenenergy promotion. Fischer-Tropsch synthesis (FTS) is a technology for the productionof liquid fuels from natural gas, coal, and biomass to alleviate the depletion of oilresources in the near future. Therefore, the FTS has caused widespread concern inmany countries and by lots of researchers. Especially the study and development ofFTS catalysts are more active. To develop an active and stable catalyst with highselectivity has been one of the major challenge in FTS.Since the3-D mesostructured cellular silica foams (MCF) material discovered in1999, it has been found to be an efficient support in some catalytic reaction such as thesynthesis of hydrogen peroxide, alcohol esterification and so on. It is considered to bean ideal support attributing to the fact that mesostructured cellular silica foams haslarge cell pores (10-50nm) with smaller window pores (9-22nm) and large surfaceareas (1000m2g-1). The large surface area can enhance the dispersion enablingprovision of more active sites; large pore size results in production of heavyhydrocarbon, while3-D mesostructured pore is beneficial to the transportation ofreactants and products. Thus, MCF may have great potential values in the field of FTS.A series of mesostructured cellular silica foams with different pore sizes weresynthesized with a sol-gel hydrothermal synthesis method using benzene asmicroemulsion, P123as template and tetraethoxysilane (TEOS) as silicon source. Thecatalysts were prepared by incipient wetness impregnation and characterized by X-raydiffraction (XRD), nitrogen adsorption-desorption, transmission electron microscopy(TEM), scanning electron microscopy (SEM), temperature programmed reduction(TPR), and hydrogen temperature programmed desorption (H2-TPD) followed by pulseoxygen titration. The catalytic properties for FTS were tested in a fixed-bed reactor.The effects of benzene on the structure of MCF and the pore size, the cobalt loading onthe catalytic performance of cobalt based catalyst were investigated. The mainconclusions are as follows:1. The different pore sizes of MCF can be obtained by changing the content ofbenzene. The structure of SBA-15changed from2-D to3-D with increasing benzene content. The mesostructured celluar silica foams with double pores wasobtained when a certain amount of benzene was added. At higher benzene ratio, thestructure would be destroyed.2. Owing to the difference of structure, the catalytic performance was influenced bydifferent pore structure of cobalt-base catalysts. Compared to SBA-15, the catalystssupported on MCF have higher activity and produced heavy hydrocarbon. Thehigher activity and heavy hydrocarbon products were attributed to high surfacearea, large pore and volume as well as to the interconected MCF mesostructure.The cobalt metal particles were trapped in the pores and hence, exhibited gooddispersion. SBA-15has smaller pores and2-D structure, so that mass transport washindered. Reactive gas is not easy to reach the active sites of cobalt, and thereactive products are not easy to get out of the channels. As a result, MCFsupported cobalt catalysts have higher activity and produced more higherhydrocarbons as well as lower methane selectivity.3. Owing to high surface area, large pore size and connective channels, the catalystswith different cobalt loading were prepared using MCF as supports, since MCF canbe impregnated with a high content(up to30%) of cobalt by only one-step.Whereas, ordinary mesoporous materials including mesoporous molecular sieveusually impregnated multiple times to achieve high cobalt loading. This wasmainly attributed to MCF had not only high surface area, but also large pore sizeand volume. With the increase of cobalt loading, the activity was firstly increasedand then remained. This may be due to the following reasons: a. with the increaseof cobalt loading, the active sites also increased, resulting in increasing activity. b.Large particle of cobalt clusters were easy to form when the cobalt loading washigh. As a result the dispersion and activity decreased.4. Cobalt catalysts with different pore sizes were prepared by incipient wetnessimpregnation. The suface areas of MCF were hardly affected by the pore size ofcell, the catalyst retained high suface area. The cobalt particle size increased whenthe pore size was increased. This enhanced the reduction of catalysts and decreasedthe cobalt dispersion. As a result, the activity was decreased.