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 + H 2 ), 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 C 2 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.

Adsorbents based on transition metal ion-exchanged Y zeolites (with Cu, Ni, Zn, Pd, and Ce ions) were synthesized and evaluated for the adsorptive desulfurization of a model jet fuel (MJF) and a real jet fuel (JP-8). Among the adsorbents tested, Ce-exchanged Y zeolites exhibited better adsorption capacity of about 10 mg of sulfur/g of adsorbent at 80 °C with a MJF containing 510 ppmw sulfur. The same adsorbent exhibited a sulfur adsorption capacity of about 4.5 mg/g for the real JP-8 jet fuel containing about 750 ppmw sulfur. Desulfurization of MJF under flow conditions at 80 °C showed a breakthrough capacity of about 2.3 mg/g of adsorbent. Ce-exchanged zeolites exhibited higher selectivity for sulfur compounds as compared to the selectivity of aromatics, for which a comparative study indicated that the sulfur compounds are adsorbed over Ce-exchanged Y zeolites via direct sulfur−adsorbent (S−M) interaction rather than via π-complexation. While the selectivity for 2-methyl benzothiophene (2-MBT) was higher in the static adsorption studies, the adsorption selectivity decreased in the order 5-methyl benzothiophene (5-MBT) > benzothiophene (BT) > 2-MBT under dynamic conditions. This trend was correlated to the electron density on sulfur atoms derived from computer-aided molecular orbital calculations.

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