3.1 Thermodynamic consideration

The two major components of synthesis gas, hydrogen and carbon monoxide are the building blocks of what is often known as C1 chemistry. Conversion of syngas to liquid fuels as well as conversion rates is directly related to the composition of the catalyst. Syngas can be efficiently converted to different products as alcohols and aldehyde (Figure 2).

Although many routes are available, the most promising route at industrial level is the
production of methanol since the synthesis yields are the highest. The conversion to
synthetic liquids as methanol is strongly influenced by thermodynamic factors (Rostrup-

Nielsen, 2000).

For methanol synthesis, a stoichiometric ratio defined as (H2-CO2)/(CO+CO2), of about 2 is preferred, which implies that there should be just the stoichiometric amount of hydrogen needed for methanol synthesis. For kinetic reasons and in order to control by-products formation, a value slightly above 2 is normally preferred (Dybkjr and Christensen, 2001).

Moreover, methanol synthesis is subjected to a thermodynamic equilibrium that limits the process to low conversion per pass and therefore implies a large recycle of unconverted gas. The reaction is strongly exothermic and consequently requires significant cooling duty. Different phenomena exist at high and low pressure conditions. As example, when the pressure is relatively low, increasing temperature, CO conversion is not monotonic, and the trend is that of an increase followed by a decrease with the maximum conversion appearing near 250°C. This phenomenon is in agreement with many works in the literature (Li and Inui, 1996, Liaw and Chen, 2001, Wang et al., 2002). As the reaction temperature increases, the reaction rate gets higher and leads to the increase of CO conversion. However, methanol synthesis is an exothermic reaction and low temperature is more beneficial considering equilibrium. The conversion does not continue to increase due to the thermodynamic limitation and a decrease trend will even appear. When the system pressure is relative high, because of the relatively low CO conversion, the system is far from the thermodynamic equilibrium and under the control of reaction kinetics. In this case, CO conversion increases monotonically with an increase in temperature. From a mechanistic point of view for the methanol synthesis, two main reactions need to be in line with real world situation: CO hydrogenation and CO2 hydrogenation.