A review of recent literature to search for an efficient catalytic process for the conversion of syngas to ethanol
Alternatives to petroleum-derived fuels and chemicals are being sought in an effort to improve air quality and increase energy security through development of novel technologies for the production of synthetic fuels and chemicals using renewable energy sources such as biomass. In this context, ethanol is being considered as a potential alternative synthetic fuel to be used in automobiles or as a potential source of hydrogen for fuel cells as it can be produced from biomass. Renewable ethanol can also serve as a feedstock for the synthesis of a variety of industrial chemicals and polymers. Currently, ethanol is produced primarily by fermentation of biomass-derived sugars, especially those containing six carbons, whereas 5-carbon sugars and lignin, which are also present in the biomass, remain unusable. Gasification of biomass to syngas (CO + H2), followed by catalytic conversion of syngas, could produce ethanol in large quantities. However, the catalytic conversion of syngas to ethanol remains challenging, and no commercial process exists as of today although the research on this topic has been ongoing for the past 90 years. Both homogeneous and heterogeneous catalytic processes have been reported. The homogeneous catalytic processes are relatively more selective for ethanol. However, the need for expensive catalyst, high operating pressure, and the tedious workup procedures involved for catalyst separation and recycling make these processes unattractive for commercial applications. The heterogeneous catalytic processes for converting syngas to ethanol suffer from low yield and poor selectivity due to slow kinetics of the initial C–C bond formation and fast chain growth of the C2 intermediate. Recently, there is a growing worldwide interest in the conversion of syngas to ethanol. Significant improvements in catalyst design and process development need to be achieved to make this conversion commercially attractive. This paper reviews and critically assesses various catalytic routes reported in the recent past for the conversion of syngas to higher alcohols, with an emphasis on ethanol. The chemistry and thermodynamics of the processes, the type of catalysts developed, reactors used, and the current status of the technology are reviewed and discussed.