Methanol is a very versatile alcohol and used to make a great variety of products, such as paints and plastics or fuel. It is small wonder, therefore, that around the globe many research projects are devoted to the development of methods of sustainable methanol production. If, for example, a method could be found to economically convert methane into methanol, people would get a liquid fuel which can be much more easily stored and transported than natural gas or pure hydrogen. This, in turn, would contribute to reducing emissions of methane from natural gas processing plants and pipelines.
Now (2021), scientists at Brookhaven National Laboratory have developed a new method of producing methanol using a common industrial catalyst that can perform the conversion effectively with and, more importantly, also without water. While adding water keeps the reaction from getting out of control and prevents the methanol from transforming further into carbon monoxide (CO) and carbon dioxide (CO2), it also makes the process more complex and more expensive because at the temperatures needed for this reaction, the water forms large quantities of steam, which have to be controlled in an industrial setting. Therefore, the Brookhaven team wanted to find a setting where the reaction could take place without water by using a common copper-zinc oxide catalyst. Copper-zinc oxide had proved to possess the best selectivity of any catalyst tested for this reaction without the addition of water — about 30%. To find out exactly how the selectivity of 30 % was achieved, the scientists conducted an ambient-pressure x-ray photoemission spectroscopy (XPS), which uses bright beams of x-rays to make out the carbon, hydrogen, oxygen, and the metal-oxygen combinations at the active sites of the catalyst in the course of the reaction. They also examined the samples under different reaction conditions. The amount of methane, oxygen as well as the pressure and temperature was varied, and the amount of chemical species present at different stages of the reaction was monitored. The team also turned to theoretical modelling to find out which sites on the catalyst were involved in the reaction. For this reason, a scanning tunnelling microscope was used to study the atomic-level structure of the catalyst, and the structural details were then put together using computational models of the atomic arrangements. They ran density functional theory (DFT) calculations, which showed how the reactants developed when interacting with one another as well as the catalyst, and kinetic modelling on computing clusters, which analysed all the possible pathways the transformations might undergo under reaction conditions.
For many years, scientists have tried to harness the potential of methane by turning it into useful products, first and foremost fuels. In 2017, scientists showed that a catalyst consisting of small nickel oxide clusters supported on ceria–zirconia (NiO/CZ) was able to convert methane to methanol and ethanol in a steady-state process using O2 as an oxidant. Steam was needed to obtain alcohols rather than CO2 as the product of catalytic combustion. The high activity of this catalyst was because of the synergy between the small acidic nickel oxide clusters and the redox-active ceria–zirconia support, which also stabilised the small nickel oxide clusters.
In 2018, scientists developed a reaction mechanism for the methane-to-methanol reaction over a copper-zeolite. The reaction was responsible for the formation of the reaction intermediate consisting of copper, a methyl group and water. The scientists found that increasing the pre-oxidation temperature from 450°C to 550°C was responsible for a 15% increase in methanol production. Furthermore, copper-exchanged SSZ-13 zeolites, which performed well in the ammonia selective catalytic reduction reaction at 200°C, also showed a high activity in the methane-to-methanol reaction.
There are several benefits to performing the methane-to-methanol conversion reaction without water: first and foremost, excluding water from the reaction process less makes it less complex and expensive, as the steam would not have to be controlled in an industrial setting. Also, in a waterless reaction the binding between the methanol and the catalyst is strong enough to allow the methanol to form from methane, but weak enough to enable the methanol to be released from the surface as a gas before it is further oxidized to CO or CO2. This quick desorption of methanol from the surface of the catalyst also eliminates a potentially explosive step.
The team is already employing their newly-gained knowledge of the reaction mechanisms to look for ways to further improve the catalyst. Their goal is to achieve a selectivity of at least 60-70% without water. In the next step, DFT calculations and kinetic modelling will be used to test out other compositions, aiming to further improve the methane conversion and methanol selectivity.