Industrial synthesis of methanol could be on the cusp of a revolution following research carried out at the Cardiff Catalysis Institute in Wales. Researchers there have found a way to create methanol from methane using oxygen harnessed from the air — opening up a vista of greener and cheaper fuel, plastics and household chemicals.
Traditionally, methane is first converted to methanol via the production of carbon monoxide and hydrogen synthesis gas at high temperatures and pressures. This is both expensive and energy intensive.
Finding a way directly to oxidize methane to methanol has been an ongoing challenge for a century or more, and a variety of different strategies are still being used.
Some studies have focused on the cyclic gas-phase oxidation of methane with metal exchanged zeolite catalysts using nitrogen, dinitrogen oxide or steam. These, however, rely on high temperatures (200–500°C) to activate the oxidant and then to desorb produced methanol.
Others studies have looked at liquid-phase reactions that typically use milder conditions. Mercury and platinum-based enzyme complexes have both been shown to be active for methane oxidation. While the temperature is slightly lower, usually around 180°C, strongly acidic media are essential for the reaction to proceed — and a further hydrolysis step is still needed to release the methanol.
Researchers also have looked at more benign oxidants such as hydrogen peroxide, but have faced problems with agglomeration and precipitation of enzymes from the reaction solution.
Graham Hutchings, director of the Institute, explains: “The quest to find a more efficient way of producing methanol is 100 years old. Our process uses oxygen — effectively a ‘free’ product in the air around us — and combines it with hydrogen peroxide at mild temperatures, which require less energy.”
The Cardiff team previously reported on methane oxidation using gold-palladium (AuPd) nanoparticles (NPs) supported by titanium oxide under mild aqueous conditions using hydrogen peroxide as the oxidant at 50°C.
“We have already shown that gold nanoparticles supported by titanium oxide could convert methane to methanol, but we simplified the chemistry further and took away the titanium oxide powder. The results have been outstanding,” he says.
The research team’s report in Science shows that removing the support material from the catalyst leads to a huge improvement in activity— with the selectivity to primary products topping 90% and minimal carbon dioxide production.
Stuart Taylor of the Institute believes the discovery promises a process that will not only be cheaper but also much more environmentally friendly, as it both reduces energy consumption and conserves dwindling stocks of natural gas. It also opens up the prospect for the first time of easily making methanol from natural gas at the site where it’s extracted, so that it can be piped as a liquid in normal atmospheric conditions. In contrast, today, methane has to be condensed into liquid natural gas and shipped in pressurized containers, he notes.
According to Taylor, it also will simplify processes for manufacturing plastics and chemicals.
“Methanol is vital in manufacturing as it’s much more reactive than methane, and so can be more easily transformed into a wide range of fuels and chemicals. So finding an easier way to produce methanol will have a knock-on effect for all sorts of industries,” he says.
Hutchings adds: “Commercialization will take time, but our science has major implications for the preservation of natural gas reserves as fossil fuel stocks dwindle across the world. At present, global natural gas production is ca. 2.4 billion t/y and 4% of this is flared into the atmosphere — roughly 100 million tons. Cardiff Catalysis Institute’s approach to using natural gas could salvage this ‘waste’ gas, saving carbon dioxide emissions. In the U.S. there is now a switch to shale gas and our approach is well suited to using this gas as it can enable it to be liquefied so it can be readily transported.” This, in the long term, also could provide a new route of converting shale resources into higher value chemical intermediates, he concludes.
In their paper, the scientists also proposed that, in principle, other routes for generating the reactive chemical species needed could be used to facilitate this chemistry. For example, rather than using hydrogen peroxide, it would be highly desirable to devise a method of coupling the AuPd colloidal catalyst with a photochemical or electrochemical fuel cell to generate the hydroxyl radicals needed to drive methanol production.