Heterogeneous Catalytic Conversion of Dry Syngas to Ethanol and Higher Alcohols on Cu-Based Catalysts

Abstract

Ethanol and higher alcohols have been identified as potential fuel additives or hydrogen carriers for use in fuel cells. One method of ethanol production is catalytic conversion of syngas (a mixture of CO, H2, CO2, and H2O), derived from biomass, coal, or natural gas. Thermodynamics of CO hydrogenation shows that ethanol is favored as the sole product at conditions of practical interest, but if methane is allowed as product in this analysis, essentially no ethanol is formed at equilibrium. The kinetics of ethanol formation must therefore be maximized. Although rhodium-based catalysts give C2 + oxygenates with high selectivity, their prohibitive cost has spurred research on less expensive copper-based alternatives. Copper-based catalysts require an optimum amount of promoter to suppress undesired reactions and maximize the yields of ethanol and higher alcohols. Common promoters include alkali, transition metals and their oxides, and rare earth oxides. Careful selection of operating variables is also necessary to achieve the desired activity and selectivity. This review describes the effects of promoters, supports, and operating conditions on the performance of copper-based catalysts for conversion of dry syngas to ethanol and higher alcohols. Proposed mechanisms from the literature for ethanol and higher-alcohol synthesis are outlined.

Characterization of Modified Fischer–Tropsch Catalysts Promoted with Alkaline Metals for Higher Alcohol Synthesis

“…Since hydrogenation of CO to hydrocarbon is thermodynamically more favorable than hydrogenation to alcohols, it is understandable that alcohol formation requires selective catalysts [11][12][13][14]. Therefore, a combination of the methanol synthesis catalysts (containing Cu) and the usual F-T catalysts containing transition metal oxides such as Co, Cr can theoretically work to produce high-molecularweight alcohols by CO hydrogenation [15].…”

Revisiting Catalyst Structure and Mechanism in Methanol Synthesis

“…Higher alcohols are synthesized due to the alkali residues such as K + and Na + remaining in the catalysts from their manufacturing [31], which affects production of formyl species as the key precursor. These alkali cations and their OH -anions serve as the active centers to adsorb and activate CO and methanol and produce formate [32]. Hydrogenation of the formate produces surface formyl species, and formaldehyde can be produced either from hydrogenation of the formyl species or dehydrogenation of methanol via surface OH -.…”

“…The higher alcohol amounts can be lowered by decreasing the alkali amount on the catalyst [38,39], increasing the space velocity [34], increasing H 2 /CO ratio [36], and decreasing methanol partial pressure [4]. It has been reported that synthesis of higher alcohols are favored within 553-583 K over Cu-based catalysts with promotors [32]. The amounts of esters and ketones are most probably in equilibrium with the corresponding alcohols by hydrogenation, which is strongly affected by the hydrogen partial pressure [4].…”

Recent Situation and Strategic Development of Advanced Biofuels in Thailand

“…Alternatively, syngas can be fed to a catalytic reactor to synthesize ethanol 15) . An ideal catalyst is rhodium-based which shows both high activity and selectivity.…”