Formation of higher alcohols from synthesis gas

This study investigates the selective formation of higher oxygenates from a synthesis gas mixture using a variety of copper-based heterogeneous catalysts. The catalysts are prepared by co-precipitation. Full characterisation of the catalysts, both before and after use, is achieved using primarily by spectroscopy, thermogravimetric and X-ray analysis and BET and copper surface area measurements. The catalytic activities and selectivities of the products are determined from the reaction of synthesis gas mixtures (H2/CO/CO2) in a self-built continuous flow microreactor, equipped with on-line gas chromatographic analysis, operating at moderate temperature and pressure. The activity is measured in terms of conversion from reactants and the conversion and product distribution of all products formed determine the selectivity of higher alcohols. These factors can be determined by variation in the reactor gasfeed composition, the pressure, the temperature and the gas hourly space velocity. The effect on selectivity of production by the addition of lower alcohol (methanol or ethanol) to the reactant mixture has also been examined. The variation in microreactor parameters can be related to possible correlations between catalyst structure and catalyst activity and selectivity. Preparation of the initial catalyst, Cu/Cr2O3, formed preferentially methanol; hydrocarbons and dimethyl ether were also produced. Substitution of chromium for aluminium and magnesium caused changes in product distribution; the aluminium for increasing activity and the magnesium to decrease the acidity. However, the initial preparation conditions for the Cu/MgO/Al2O3 (50/10/40) catalyst resulted in incomplete co-precipitation and a consequential decrease in activity such that no alcohols were produced. Methane was the primary product for this catalyst. Binary catalysts of Cu/MgO and Cu/Al2O3 were prepared to optimise the preparation condition to give complete co-precipitation. A main emphasis has been to establish preparation conditions, which have been shown to lead to reproducibility in catalyst structure and catalytic activity. The magnesium concentration was initially fixed at 2 % of the total metal content; however, this still resulted in the production of hydrocarbons and dimethyl ether. The magnesium concentration was gradually increased through 5 % to 7 % with respect to the copper and aluminium components. This increase in magnesium concentration resulted in a precursor with increased crystallinity and a catalyst, which gave a higher yield of higher alcohols. However, methanol was still the predominant alcohol in the total alcohol yield. At this increased magnesium concentration, hydrocarbons and dimethyl ether were still being produced. On the basis that the inhibiting step is the C1-C2 chain growth step, it has been found that this can be overcome by K-promotion of the Cu/Mg/Al203 (60/7/35) catalysts with the K+ ion at 0.005 wt %. This was especially noticeable with the catalyst prepared at pH = 8 and aged in the mother liquor for 30 mins, as a large production of ethanal was observed. Under the conditions used unlike unpromoted catalysts, no induction period in the formation of oxygenates was observed with these catalysts. However, the overall activity as measured by the consumption of the reactants was lower than that observed with unpromoted catalysts. No DME was formed with the K-promoted catalysts