Hydrogen from coal-derived syngas: Catalytic synthesis of ethanol as a hydrogen carrier

PETR 81

James J. Spivey, jjspivey@lsu.edu, Dept. of Chemical Engineering, Louisiana State University, S. Stadium Drive, Baton Rouge, LA 70803 and Adefemi Egbebi, aegbeb1@lsu.edu, Chemical Engineering, Louisiana State University, South Stadium Dr, Baton Rouge, LA 70803.
Syngas from coal, natural gas, or biomass can be catalytically converted into hydrogen-rich liquids that can be transported to the point of use. These liquids can then be reformed into a hydrogen-rich gas for use in an energy conversion device such as a fuel cell. Although ethanol (and higher alcohols) have the advantage of containing significant stoichiometric quantities of hydrogen, the literature shows that the conversion of syngas into ethanol is plagued by poor selectivity. Methane, methanol, C2+ oxygenates, and higher hydrocarbons are the primary undesirable byproducts. Despite the fact that the hydrogenation of CO to ethanol is thermodynamically favorable at conditions that can be integrated into a gasification or reforming process (e.g., 30 atm pressure, T< ~300 deg C0): 2 CO + 4 H2→ C2H5OH + H2O

ΔGo = -29 kcal/mol

Thermodynamics also shows that if methane is allowed as a product, free energy minimization starting with a typical syngas mixture results in a product mixture containing virrtually no ethanol.

The problem is therefore one of selectivity.Among the catalysts that have been investigated for this reaction, promoted Rh seems to have the highest yield of ethanol, but the reaction products typically include significant amounts of methane and other undesired products, and ethanol selectivity is only high at low CO conversions.

Recent results on a series of Rh-promoted catalysts shows significant effects of the promoters Mn, Li, and Fe on the selectivity of ethanol, as well as differences in TiO2 and Al2O3 as supports. For example, nominally identical Rh-Mn-Li-Fe catalysts showed essentially the same methane selectivity, but there is much lower methanol yield on the TiO2-supported catalyst than on alumina (9% of total oxygenates versus 72%), but also much more acetaldehyde (43% versus 8%). The problem appears to be striking a balance between hydrogenation activity and associative/dissociative adsorption of CO.

 

2nd Symposium on Hydrogen from Renewable Sources and Refinery Applications
8:00 AM-12:20 PM, Tuesday, April 8, 2008 Morial Convention Center -- Rm. 209, Oral

Division of Petroleum Chemistry

The 235th ACS National Meeting, New Orleans, LA, April 6-10, 2008