MIXED METAL XEROGELS AND AEROGELS FOR FISCHER-TROPSCH SYNTHESIS OF ALTERNATIVE FUELS

摘要

Fischer-Tropsch synthesis has been recognized as an important alternative technology to petroleum refining in the production of liquid fuels and chemicals from syngas derived from biomass/coal as it offers a way to convert synthesis gas (a mixture of CO and H2 obtained from coal, natural gas or biomass) to a multicomponent mixture of gasoline, diesel fuel, waxes and other specialty chemicals. F-T catalysts are derived from metals such as iron , cobalt and ruthenium . The use of iron catalysts is attractive because apart from being less expensive, the Fe based catalyst exhibits high water gas shift (WGS) activity. This enables iron F-T catalysts to work in a wide range of H2/CO feed ratios which is advantageous when using hydrogen-lean synthesis gas obtained from coal gasification. Syngas obtained from coal gasification typically has a H2:CO ratio close to 1:1 whereas a higher proportion of hydrogen in syngas of 2:1 is required by the F-T reaction stoichiometry.Applications of aerogels and xerogels comprise a growth industry . Both aerogels and xerogels are initially isolated as wet gels using traditional low temperature sol-gel techniques. In the final step, the wet gels can be dried by evaporation of the solvent from the wet gel matrix resulting in a xerogel. However, if the solvent in the wet gel is removed slowly by supercritical drying, aerogels are obtained (Figure 1). Fe/Cu/Si as well as Fe/Cu/Al, mixed element wet gels were synthesized using nitrates of iron, copper and tetraethyl orthosilicate (TEOS)/aluminum nitrate as the Fe, Cu and Si/Al precursors respectively and propylene oxide as the polycondenstaion agent. The wet gels were then dried in air (under ambient conditions) to get the xerogels and under supercritical-CO2 conditions to get the monolithic aerogels.