Homogeneous Catalysts
Homogeneously catalyzed reactions of syngas in the production of C2 alcohols is a relatively less explored area, and is discussed in three review articles [1-3]. Metal complexes of Co, Fe, Ru, or Rh are the most widely studied systems to produce ethanol and C2 oxygenates from syngas [2]. The solution reactions can be carried out in oxy-solvents such as glymes, N-methlpyrrolidone (NMP), sulfolane, and acetic acid; in addition to this it is common to use promoters such as acids, iodied ions and salts such as Bu4PBr in the reaction concoction. The reaction product is usually a mixture of ethanol, methanol, ethylene glycol and its derivatives like ethylene glycol ethers. These solution reactions are quite different in all respects to the heterogeneously catalyzed reactions, where the major products are usually linear hydrocarbons, and the solution reactions also require higher temperatures (>230°C) and higher syngas pressures than the heterogeneous ones. A concise list of earlier attempts to develop Ru-based homogeneous catalysts for the syngas to ethanol process is shown in Table 13.1.
This catalytic route is basically a reduction of CO by hydrogen in the syngas; soluble Ru [5] and Ru-Re [6] bimetallic complexes have also been used in several homogeneous catalysis studies. Phosphoric acid is known to promote ethanol formation with ruthe — nium-PPNCl-FIX (X= Cl, Br or I) and ruthenium-PPNCl-PPNI catalyst, and high activity for ethanol synthesis could be attained by optimization of the catalyst systems. Tanaka et al. have reported the effects of solvents and other acids added, and it was proposed that
protons accelerate the hydrogenation of carbon monoxide together with the conversion of methanol, the reaction intermediate to ethanol in Ru-PPNCl system [6].
Researchers from the Texaco Company have patented [7] a process for converting syngas to alcohol-ester fuels using a RuO catalyst mixed with Bu4PI quaternary salt, and they have obtained 60% selectivity to ethanol at 220°C and 6320 psig. In another example, a mixture of triruthenium dodecacarbonyl and tripropylphosphine oxide has been used as the catalyst with iodine as the promoter [2,7], where a mixture of methanol, ethanol, and methane was produced, and the ethanol yield was only 46 g/(L cat h) at 240°C and 4000 psig. Iron-based homogeneous catalyst has been used in N-methlpyrrolidone (NMP) in the production of ethanol from biomass-derived syngas. This process developed by the Argonne National Laboratory involved a novel selective catalytic ethanol synthesis route [8]. The first step of this process incorporates steam reforming of biomass such as switchgrass to produce syngas. Secondly, the syngas is converted to methanol using the commercial heterogeneous Cu/ZnO catalyst, and in the third step, the homogeneous catalyst system containing HFe(CO)4 is used in the carbonylation followed by hydrogenation to produce ethanol. The process operates in the temperature range between 180 and 220°C and at pressures up to 300 atm (over 4400 psig). The rate-determining step in the catalytic reaction has been reported to be the nucleophilic attack of the iron carbonyl complex on 1-methyl-2-pyr — rolidinone solvent used for the CO insertion reaction. Furthermore, they reported the production of relatively pure ethanol using this method without coproducing either water or other alcohols [8].
The overall process can be summarized in the following equation:
CH3OH(g) + 2CO(g) + H2(g) ^ C2H5OH(g) + CO2(g) AH°29g = -206.2 kJ/mol of ethanol AG°298 = -125.6 kJ/mol of ethanol (13.1)
According to this equation, the reaction produces a mixture of ethanol and carbon dioxide rather than a mixture of ethanol and water produced in the conventional methanol homologation reaction. If successfully developed, this process could become economical because it avoids the tedious separation step employed for recovering ethanol from an ethanol-water azeotropic mixture. Also, the process uses a non-noble, metal-based catalyst, which could be cost effective. However, the handling of the toxic HFe(CO)4 complex and the use of high pressure (over 4000 psig) are some of the major concerns for the practical application and commercial viability of this process.