Methanol Synthesis from CO2 Hydrogenation

Abstract

In the future we will be phasing out the use of fossil fuels in favour of more sustainable forms of energy, especially solar derived forms such as hydroelectric, wind and photovoltaic. However, due to the variable nature of the latter sources which depend on time of day, and season of the year, we also need to have a way of storing such energy at peak production times for use in times of low production. One way to do this is to convert such energy into chemical energy, and the principal way considered at present is the production of hydrogen. Although this may be achieved directly in the future via photocatalytic water splitting, at present it is electrolytic production which dominates thinking. In turn, it may well be important to store this hydrogen in an energy dense liquid form such as methanol or ammonia. In this brief review it is emphasised that CO₂ is the microscopic carbon source for current industrial methanol synthesis, operating through the surface formate intermediate, although when using CO in the feed, it is CO which is hydrogenated at the global scale. However, methanol can be produced from pure CO₂ and hydrogen using conventional and novel types of catalysts. Examples of such processes, and of a demonstrator plant in construction, are given, which utilize CO₂ (which would otherwise enter the atmosphere directly) and hydrogen which can be produced in a sustainable manner. This is a fast-evolving area of science and new ideas and processes will be developed in the near future.

Keywords: Carbon dioxide hydrogenation; ZnO catalysts; copper catalysts; methanol plant; sustainable methanol.

Figure 1

Temperature programmed desorption (TPD) experiments after dosing gases at various temperatures on ZnO powder, and cooling in the gas to ambient. (Left panel) CO₂ TPD showing the activated nature of adsorption into the most stable state desorbing at 550 K; (middle panel) after adsorption of a mixture of CO₂ and H₂ at 500 K and cooling, showing mainly CO and H₂ production, but also with CO₂ and H₂O; (right panel) products after the adsorption of H₂CO at 500 K, with cooling. The product distribution is very similar to that seen after methanol dosing, and shows a variety of products evolving near‐coincidently with the formate decomposition, including formaldehyde itself, dimethyl ether, and methanol.11, 12. Adapted from ref 11,with thanks to the Royal Society of Chemistry.

Figure 2

The methanol synthesis mechanism and kinetics for the reaction on ZnO.12

Figure 3

Desorption products after exposure of a Cu/ZnO/Al₂O₃ catalyst to methanol at ambient temperature. a) dimethyl ether (45 amu x33), b) hydrogen (2 amu x1), c) CO₂(44 amu x3.3), d) methanol (29 amu), e) formaldehyde (31 amu x10), f) CO (28 amu x1).

Figure 4

Ball models of the surface methoxy (left) and formate species (right) on a metal surface; red‐oxygen, blue‐hydrogen, black‐carbon and gold, the metal.

Figure 5

Schematic of the 500 t/y mefCO2 plant built at Niederaussem to convert CO₂ from the power plant to methanol using hydrogen from solar‐generated electricity via electrolysis. Courtesy of Angel Sanchez Diaz at iDeals, Madrid.

Figure 6

CZA catalyst powder after synthesis and after drying at 110 °C (left sample) and (right) after calcination from 200 to 500 °C, every 50 °C. Catalyst preparation and photo courtesy of Dr James Hayward, Cardiff Catalysis Institute, Cardiff University.

Figure 7

Equilibrium methanol conversion and selectivity for a CO₂/H₂ mix with a ratio 1/3, and its dependence on pressure and temperature, the dashed lines represent gas phase equilibrium.40 Reprinted with permission from the American Chemical Society.

Figure 8

A simplified diagram of the main components of a ‘green’ methanol synthesis plant.