Conversion of Syngas to Ethanol Using Chemical Catalysts
As presented in Chapter 12, there are two methods to convert biomass-derived synthesis gas to fuel grade ethanol. The first approach using microorganisms was presented in Chapter 12 and the second approach presented in this chapter utilizes chemical catalysis for the synthesis of alcohol from syngas. A general outline of this path from biomass to ethanol is shown in Figure 13.1. In this process syngas is exposed to a homogeneous or heterogeneous catalyst, which is typically a metal, metal oxide or metal-chelate. Unlike biochemical processes using enzymes, these catalytic methods are not very selective and usually give mixtures of oxygenated compounds as well as hydrocarbons. However, the reaction rates of chemical catalytic processes are orders of magnitude higher than enzyme reactions and can be used to process syngas produced from a wide range of feed stocks. This can be a significant advantage in making these processes economically competitive with biochemical methods. The types of catalysts studied can be simply divided
Homogeneous or heterogeneous chemical catalyst
Figure 13.1 Flow chart of the gasification-chemical catalysis path to cellulosic ethanol.
into homogeneous and heterogeneous categories. The most widely studied systems are heterogeneous metal catalysts at high temperatures; however, there are some reports on the use of homogeneous catalysts in solvent systems.