Study On Oxygenates Formation In Fischer-Tropsch Synthesis

Fischer-Tropsch technology can be briefly defined as the means used to convert synthesis gas containing hydrogen and carbon monoxide to a mixture of hydrocarbon products. Interest in Fischer-Tropsch technology as a source of alternate fuels has been in spurts, driven by peaks in the price and availability of crude oil. The qualities of Fischer-Tropsch products are excellent and their environmental properties are being recognized as very valuable in the ongoing drive towards cleaner fuels and engines. The emphasis in this process is rather on the development of catalysts and industrial reactor. Iron-based catalysts are widely used because of their low cost, excellent flexibility and high activity for water-gas shift reaction in the production of liquid fuel from coal-based syngas with low H2/CO ratio. It is well known that the product spectrum of Fischer-Tropsch synthesis is determined by Anderson-Schulz-Flory (ASF) rule and it is difficult to produce a single product with high selectivity. Moreover, the products of Fischer-Tropsch synthesis with an iron catalyst consist of a complex multicomponent mixture of linear and branched hydrocarbons and oxygenates. A large number of oxygenates are produced with high selectivity from syngas. Many factors including additives and reaction conditions have effects on selectivity of the catalysts. However, the more recent studies on the formation of oxygenates did not focus on iron-based catalysts since the performances of iron-based catalysts were by far less interesting.This thesis conductes in situ diffuse reflectance infrared spectrometer (DRIFTS), temperature-programmed-desorption supplemented with GC-MSD and chemical trapping to investigate the effect of additives and reaction conditions on the nature of adsorbed species on the catalytic surface and the individual steps of oxygenates formation from syngas on iron-based catalysts during Fischer-Tropsch synthesis. The relationship between intermediates and reactivity of the catalyst is also discussed.(1) Effect of copper on the surface species and catalytic properties in the formation of oxygenatesLow-temperature iron-based catalysts were prepared by a combination of coprecipitaion and impregnation techniques and evaluated in a tubular fixed bed reactor. The temperature-programmed desorption (TPD) and DRIFTS were used to study influence of copper loading on the surface species and catalytic properties of Fe/Cu/K/La/SiO2catalyst in the formation of oxygenates during Fischer-Tropsch synthesis. The results showed that addition of copper improved the reducibility of catalysts, however, the adsorption features of catalysts changed significantly. New CO adsorption sites were formed when copper was added by impregnation. Typically, bridged-form CO which was thought to be more active was enhanced with the composition of100Fe/28Cu/5K/5La/17SiO2(molar ratio, relative to iron). Extra copper would suppress CO adsorption as well as oxidation of adsorbed molecules. The stability of intermediates weakened simultaneously. Intemediates such as dioxymethylene, formyl, formaldehyde and methoxy species were found on catalyst surface during Fischer-Tropsch synthesis. They were thought to be formed by CO insertion into hydroxy-metal bond and continuous hydrogenation. Such intermediates were not found on unmodified iron-based catalysts. Addition of copper provided more active sites on surface. The selectivity towards oxygenates first decreased with copper loading, then increased rapidly after reached a minimum,The trend for hydrocarbons was opposite. Moreover, the quantitative estimates of the concentration of the surface species by TPD-GC-MSD allowed the illustration of copper effect on the catalytic properties.(2) Effect of reaction conditions on the surface species and catalytic properties in the formation of oxygenatesEffect of reaction conditions including temperature and pressure on the surface species and catalytic properties on oxygenates formation during Fischer-Tropsch synthesis over a low-temperature precipitated iron-based catalyst was investigated by tubular fixed bed reactor and DRIFTS. It showed that the amount of CO2and CH4on surface increased with temperature and the selectivity of oxygenates decreased with temperature. The effect of pressure was opposite. A competition between the formation of hydrocarbons and alcohols was observed. It was found that carbene could reach a saturated extent of adsorption even at low pressure. This indicated that carbene was not the rate-controlling step in oxygenates synthesis which was confirmed by the results of reactivity measurements. It also showed that alkyl as well as methoxy species, which were thought to be C1product precursor, were greatly favored at high temperature.(3) Studies on oxygenates formation over low-temperature iron-based catalyst in Fischer-Tropsch synthesisIn-situ DRIFTS technique and chemical trapping were employed to investigate the adsorbed species over the surface of low-temperature iron-based catalysts and mechanism of oxygenates formation in Fischer-Tropsch synthesis. The similarities of surface properties were observed:(i) reactivity of alcohols with free surface hydroxyls to give alkoxy species;(ii) reactivity of oxidizing on adsorbed molecules;(iii) reactivity of basic sites such as lattice oxygen with e.g., CH3OH or CH3CHO molecules. The interaction between iron and copper made the process of the hydrogenation of carbonaceous species different on the surface. Active sites and the stability of surface species were also enhanced. Acetyl as a key intermediate for oxygenates was observed by investigation of CH3OH+CO and CH3I+CO+H2. CO insertion into CH3-metal bond or CH2-metal bond to form an acetyl was the key step. The mechanism of oxygenates formation on Fe/Cu/K/La/SiO2catalyst was discussed based on the structure and properties of surface species.(4) Studies on oxygenates formation over high-temperature iron-based catalyst in Fischer-Tropsch synthesisThe adsorbed species over the surface of high-temperature iron-based catalysts and mechanism of oxygenates formation in Fischer-Tropsch synthesis were also investigated by DRIFTS and chemical trapping. The results showed that both linear and bridged CO were observed on the surface. Numerous oxygenated precursors were also found by CO adsorption. In-situ DRIFTS observed the presence of some crucial intermediates, such as surface acetate, acetyl and methoxide. The similarities of surface properties were also observed with low-temperature iron-based catalyst. The mechanism of oxygenates formation was discussed based on the results.