Methanol is produced by the hydrogenation of carbon oxides over a suitable (copper oxide, zinc oxide, or chromium oxide-based) catalyst:

CO + 2H2 о CH3OH (2.5)

CO2 + 3H2 о CH3OH + H2O (2.6)

The first reaction is the primary methanol synthesis reaction, a small amount of CO2 in the feed (2-10%) acts as a promoter of this primary reaction and helps maintain catalyst activity. The stoichiometry of both reactions is satisfied when R in the following relation is 2.03 minimally (Katofsky 1993). H2 builds up in the recycle loop; this leads to an actual R value of the combined synthesis feed (makeup plus recycle feed) of 3 to 4 typically.

H2 — CO2 CO + CO2

The reactions are exothermic and give a net decrease in molar volume. Therefore, the equilibrium is favored by high pressure and low temperature. During production, heat is released and has to be removed to keep optimum catalyst life and reaction rate. 0.3% of the produced methanol reacts further to form side products such as dimethyl ether, formaldehyde, or higher alcohols (van Dijk et al. 1995).

The catalyst deactivates primarily because of loss of active copper due to physical blockage of the active sites by large by-product molecules; poisoning by halogens or sulfur in the synthesis gas, which irreversibly form inactive copper salts; and sintering of the copper crystallites into larger crystals, which then have a lower surface area-to-volume ratio.

Conventionally, methanol is produced in two-phase systems, the reactants and products forming the gas phase and the catalyst forming the solid phase. The
production of methanol from synthesis gas was first developed at BASF in Germany in 1922. This process used a zinc oxide/chromium oxide catalyst with poor selectivity, and required extremely vigorous conditions—pressures ranging from 300-1000 bar and temperatures of about 400°C. In the 1960s and 1970s the more active Cu/Zn/Al catalyst was developed allowing more energy-efficient and cost-effective plants, and larger scales. Processes under development at present focus on shifting the equilibrium to the product side to achieve higher conversion per pass. Examples are the gas/solid/solid trickle flow reactor, with a fine adsorbent powder flowing down a catalyst bed and picking up the produced methanol, and liquid phase methanol processes where reactants, product, and catalyst are suspended in a liquid. Fundamentally different could be the direct conversion of methane to methanol, but despite a century of research this method has not yet proved advantageous.