In order to reduce the pressure and temperature needed for the current methanol production process, and also to improve its thermodynamic efficiency, alternative routes to convert CO/H2 mixtures to methanol under milder conditions have been developed. Among these, the most notable is the synthesis of methanol via methyl formate, first proposed in 1919 by Christiansen [184-186].
This methanol synthesis route consists of two steps. Methanol is first carbony-lated to methyl formate, which is subsequently reacted with hydrogen to produce twice the amount of methanol. The carbonylation reaction is carried out in the liquid phase using sodium or potassium methoxide (NaOCH3 or KOCH3) as a homogeneous catalyst. It is a proven and commercially available technology used in the production of formic acid. High activities for methanol carbonylation have also been shown recently with Amberlyst and Amberlite resins used as heterogeneous catalysts. The subsequent reaction of methyl formate with hydrogen (hydrogenolysis) to synthesize methanol can be conducted either in the liquid or gas phase using generally a copper-based catalyst (copper chromite, copper supported on silica, alumina, magnesium oxide, etc.). Carbonylation and hydroge-nolysis can be carried out in two separate reactors, but are preferably combined in a single reactor. In order to run the carbonylation and hydrogenation simultaneously in a single reactor, different combinations of catalysts have been investigated; in particular CH3ONa/Cu and CH3ONa/Ni. Nickel-based systems are very active and selective, but due to the volatility and high toxicity of the Ni(CO)4 that may be formed during the reaction this process is considered difficult and hazardous to use in industrial settings. Copper systems are thus preferred because they offer similar activities and selectivities, without the toxicity problems associated with the nickel systems.
Overall, by the methyl formate route, methanol is produced from syn-gas, but at lower temperature and pressure than that employed in conventional methanol production processes. Patents from Mitsui Petrochemicals, Brookhaven National Laboratories and Shell using the methyl formate route claim to produce methanol at 80-120 °C and pressures of 10 to 50 atm. Another report states that continuous operation in a bubble reactor at 110 °C and a pressure of only 5 atm is also possible.
The problem with this process is the presence of CO2 and water in the syn-gas, which will react with sodium methoxide, deactivating the catalyst and forming undesirable byproducts. In order to minimize this deactivation, CO2 and water therefore need to be removed from the gas feed. Catalysts more tolerant to these poisons (such as the recently reported KOCH3/copper chromite systems) are also being developed . Although further improvements are still needed, the synthesis of methanol via methyl formate at low temperature and pressure, could lead to an attractive alternative to current processes operating at 200300 °C and 50-100 atm. At the same time as methyl formate is also made by the dimerization of formaldehyde, the methyl formate to methanol route could also play a role in the secondary treatment of the oxidative conversion of methane to methanol, without going through syn-gas.
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