(539c) Preparation and Characterisation of Cobalt Catalysts for Conversion of Synthesis Gas to Diesel Fuels

In the twenty-first century, fluctuating crude oil price, depleting oil resources and increasing demand for fuels, increasing local climate change and environmental concern have made an immediate need for newer, cleaner alternative fuels. Fischer Tropsch (FT) process has received increasing attention worldwide due to technical advances and growing reserves of coal and natural gas. In this paper an effort has been made to convert syngas to mainly diesel range of hydrocarbons. Cobalt based catalysts were chosen for conversion of synthesis gas to liquid fuels as this metal has been found effective for producing long- chain normal paraffin. Five different promoted cobalt catalysts supported on silica were prepared by a combination of sol-gel and impregnation method. (10%Co/Silica (A), 10% Co/Silica using acid-base condition (B), 10%Co/Silica using chelating agent(C), 1%K/10%Co/Silica (D) and 1%CeO2/10%Co/Silica (E). The promotional effect of potassium and ceria was also studied. All these catalysts were characterised by surface area, pore volume, XRD pattern and scanning electron microscopy (SEM) and transmission electron miscroscopy (TEM) . The thermal behaviour of the catalyst is investigated using TGA analyser. It is found that most of the decomposable matter from the catalyst is removed in temperature range of 200-350 deg C. The surface area measured by using BET surface area analyser is found to be in range of 125 m2/g to 474 m2/g. The non-promoted catalyst have higher surface area as they were made solely by sol-gel process, whereas promoted catalyst has relatively lower surface area as the promoters have been impregnated on the catalysts prepared by sol-gel method. The reduction temperature of the prepared catalysts has been investigated using TPR technique. It is observed that the cobalt catalysts have two reduction peaks thus suggesting two step reductions (Co3O4 to CoO to Co). The surface morphology as studied by SEM technique suggests that the particles are spherical in shape and are well dispersed. The particles formed are in the range of 10-20 nm as analysed by TEM. The phase analysis of the catalysts is also done by XRD technique which suggests that the catalysts are amorphous in nature. The H2 chemisorptions were performed to determine the metal dispersion and active metal size.

The performance of the catalysts was evaluated in the fixed bed Bench Top Reactor System(BTRS) under the FT condition of 250 deg C, 20 atm pressure, H2:CO =2:1, W/FAo= 1000 kg. s/Nm3 for 15 hr. The investigation revealed that per pass CO conversion is in the range of 60-80%. The yield of C5+ was found to be in the range 30-40%. The highest %CO conversion and % C5+ selectivity is observed with the 10% Co/Silica catalyst (C), which was made by using chelating agent. Probably the chelating agent acts as the bidentate and bridging ligands which results high cobalt dispersion. In order to elucidate the effect of alkali the adsorption of carbon monoxide and hydrogen on promoted and unpromoted cobalt catalysts was studied. Addition of alkali causes a considerably increased adsorption of carbon monoxide and a slightly decreased one of hydrogen. The alkali promoter takes effect by an increased strength of carbon monoxide adsorption which causes an enhanced 1-alkene desorption and consequently a decreased rate of consecutive hydrogenation and of isomerisation of 1-alkenes towards 2-alkenes. The maximum coke formation was observed for the potassium promoted catalyst. It has been reported that K metal leads to the dissociative adsorption of carbon monoxide into carbon and oxygen. The coke deposition causes the early deactivation of the catalyst thus the overall % CO conversion and C5+ selectivity was lower for this catalyst. The liquid products obtained from catalysts were analysed by GCMS and Gas Chromatograph. The liquid product contained mainly C10 to C18 range of hydrocarbon along with carbon number alcohol derivatives. The kinetics of the FT synthesis over these catalysts has also been discussed.