Introduction to Heterogeneous Catalysts
In contrast to homogeneous catalysis, heterogeneous catalysis is a widely studied subject in syngas to ethanol conversion. There are several excellent review articles on the subject of synthesis of ethanol from biomass-derived syngas using heterogeneous catalysts [9,1,10,11]. The most noted article is the critical review by Spivey and Egbebi, which summarized the progress till 2007 [9], and later a second review in 2011 from the same group titled, "Heterogeneous Catalytic Conversion of Dry Syngas to Ethanol and Higher Alcohols on Cu-based catalysts," captured the more recent developments in Cu-Based catalysts [11]. Another review by Jegannathan et al. brings out the advantages and disadvantages of using different types of catalyst in biofuel production and in particular the syngas to ethanol process [10]. Also, patent literature in the field is discussed in a 2012 review article titled, "Recent Patents on the Conversion of Biomass to Fuels via Synthesis Gas." [12]. The vast majority of earlier studies on thermochemical heterogeneous catalysis route are based on the hydrogenation of CO. There are limited studies based on hydrogenation of CO2, and even fewer on hydrogenation of mixtures of CO and CO2. There are also more recent experimental studies on using CO, CO2, H2 mixtures mimicking syngas or using actual syngas itself. Rhodium and copper are the most widely studied catalysts; the other systems of interest are based on metals such as Fe, Mo, Ru, Re, and Co.