CATALYTIC CONVERSION OF SYNGAS INTO ETHYLENE AND HIGHER HYDROCARBONS

Abstract

A rapid growth in population and industrialization has resulted in a shortage of natural resources with increasing human demands. With the rapidly depleting petroleum resources, other venues such as the utilization of coal and biomass for energy production are under intense investigation. Fischer Tropsch (FT) technology is extensively used for the conversion of coal, natural gas and biomass derived syngas (CO+H2) to fuel by utilizing transition metals as catalysts. One of the main challenges in FT synthesis is the production of higher molecular weight waxes which blocks the active sites of the catalysts, resulting in decreased catalytic activity. The catalyst supports in FT synthesis is also very important as it not only enhances the dispersion of active metal catalyst but also provide active sites for hydrogenation and cracking of higher hydrocarbons. The present study was intended to explore Montmorillonite (MMT) as a novel support material for Co-based FT synthesis to increases the surface acidity, hydrogenation and cracking of higher molecular weight hydrocarbons. The lower thermal stability and lack of porosity in MMT was overcome by replacing the sodium ion present in the interlayer of MMT clay with different metal oxides (MOs) (M=Al and Zr) to achieve high surface area and pore volume. Along with the modification of catalyst support, the effect of Mn and Ce promoters have also been investigated in this study. A series of Al and Zr-pillared montmorillonite (Al-PILC and Zr-PILC) supported Co catalysts were fabricated by impregnation and hydrothermal methods. FT reaction was carried out in fixed bed micro reactor at temperature 225 oC, 260 oC Page ix and 275 oC and pressure of 1, 5, and 10 bar. It was found that Co supported Na montmorillonite (NaMMT) had lower CO-conversion and higher CH4-selectivity while the Al-PILC and Zr-PILC supported Co catalysts gives higher CO-conversion and lower CH4-selectivity. Moreover, increase in reaction temperature from 225 oC to 275 oC resulted in higher CH4-selectivity, higher CO-conversion and decreased in selectivity towards C5+ hydrocarbons. Increase in pressure from 5 to 10 bar resulted in decreased CH4-selectivity of the catalyst but increase in C5+ hydrocarbons and CO-conversion efficiency. The Addition of Mn as promoter to the Al-PILC and Zr-PILC supported Co nanoparticles significantly increased the selectivity of catalyst toward C2-C12 hydrocarbons as a result of the cracking of long chain C21+ hydrocarbons. The addition of Mn also resulted in a decreased selectivity toward CH4. On the other hand when Ce is used as a promoter, the selectivity toward C5-C12 hydrocarbons and CH4- increased and that of C21+ selectivity decreased. Significant enhancement in CO-conversion and CH4-selectivity was observed at higher reaction temperatures (>220 oC). The increase in pressure from 1 to 10 bars eventually resulted in enhancement in C5+ hydrocarbons and decrease in CH4 and C2-C5 hydrocarbons selectivity. All of these could be attributed to the synergistic effect of electronically and geometrically modified sites on the catalyst surface, their orientations and resultant intermediates concentration.