In the quest to transition to sustainable energy through hydrogen, methanol has emerged as a promising solution to the issue of hydrogen storage and transportation.
The versatility of methanol as a hydrogen-carrier and precursor chemical for a variety of industries will make methanol a key component in reducing greenhouse gas emissions and achieving a circular chemical economy, if methanol is derived from carbon dioxide (CO₂) and renewable hydrogen.
This is accomplished by the CO₂ hydrogenation process, which involves captured CO₂ being directly reacted with renewable hydrogen. This is a recognised carbon capture and utilisation (CCU) strategy, and a method to mitigate carbon emissions.
The methanol economy
The potential for methanol is significant, especially outside of the energy sector. Methanol will play a crucial role in the global chemical industry as it can serve as a precursor to a variety of chemicals across numerous sectors, including adhesives and resins, paints and coatings, plastics, pharmaceuticals, and in solvents and cleaning products.
This represents the concept of the ‘methanol economy’, whereby methanol is the basis of the global chemical industry rather than petroleum hydrocarbons. This will enable carbon intensive industries that are vital to the modern economy to become carbon neutral. The importance of methanol is recognised by these sectors, and significantly, renewable methanol production is set to evolve over the next decade, driven largely by technology innovation.
Using CO2 hydrogenation to produce methanol presents several challenges when compared to the conventional method through natural gas.
Firstly, CO2 hydrogenation requires an additional molecule of hydrogen than the conventional route, leading to a comparatively higher hydrogen consumption. Secondly, the thermodynamics of CO2 hydrogenation are less favourable than those of the conventional process, which results in lower methanol yields. Thirdly, competing reactions during CO2 hydrogenation can generate carbon monoxide (CO), which further reduces methanol production.
Currently, the methanol yield from CO2 hydrogenation is less than 40 per cent due to these limitations, and the process struggles to compete against the conventional natural gas route as a result.
To improve yields, CO2 hydrogenation processes typically employ multi-stage reactors with methanol separators and selective absorption to recycle unreacted reagents. This increases both the capital and operating costs of renewable methanol, making the economics more unfavourable.
Membrane reactors
Technology innovation in CO2 hydrogenation is focused on an alternative reaction approach to produce methanol through a membrane reactor, which integrates the reaction and separation processes into a single process unit.
Image: University of Melbourne.
A membrane reactor utilises a selective material that actively removes the products (methanol) of the reaction directly from the reaction zone as they are formed. The reagents – in this case, CO2 and hydrogen – experience the material as a barrier and subsequently remain in the reaction zone until they react to produce methanol, which freely passes through the membrane material.
This enables the membrane reactor to overcome the thermodynamic and equilibrium limitation through in-situ reaction product removal and is based on extending the Le Chatelier’s principle.
Membrane reactors are also an example of process intensification, whereas the chemical reaction and products/reactants separation are united into one compact unit, which typically lower capital cost and increase energy efficiency. Hence, developing viable membrane reactor technology for CO2 hydrogenation to methanol has become the focus for the sector.
An innovative new technology
The University of Melbourne has developed catalyst membrane reactor technology that can effectively produce purified methanol at a rate that is over 300 per cent higher than conventional approaches. This technology is based on two core achievements, development of a structural catalyst morphology ideal for CO2 hydrogenation, and utilising high temperature performance polymeric membranes.
The catalyst is one innovation in the University of Melbourne technology and is vital to the overall process as the catalyst is used to speed up the CO2 hydrogenation reaction, without which CO2 and hydrogen would not covert to methanol in a meaningful timescale.
Copper-based catalysts are among the most widely studied for CO2 hydrogenation because copper is effective at facilitating the reaction at relatively low temperatures and pressures. For the developed membrane reactor, the catalyst is based on a copper-zinc oxide (Cu/ZnO) combination, where the zinc oxide enhances the catalytic performance.
The novelty in the University of Melbourne approach is the deposition of the catalyst as nanoparticles in an open morphology upon the membrane surface. This is achieved through a process known as flame spray pyrolysis, that vaporises and atomises the catalyst material at high temperature, which is then sprayed onto the membrane surface.
The hot catalyst material upon experiencing the membrane surface strongly adheres as nanoparticles that are melded into the open morphology are vital for allowing CO2 and hydrogen to mix with the catalyst. The positioning of the catalyst directly upon the membrane surface ensures that the produced methanol is removed directly from the catalyst reaction zone upon formation, meaning high conversion efficiencies are achieved.
The membrane is the other innovation in the technology, as the membrane needs to be selective for methanol, while CO2 and hydrogen must experience the membrane as a barrier. This is a challenge because membranes generally allow smaller molecules to pass through compared to larger molecules (known as size sieving), which is not the situation for the CO2 hydrogenation.
Methanol is a much larger molecule than both CO2 and hydrogen, hence, the design behind the material must be different to other membrane processes. This is further complicated by the high temperature of the reaction, and the fact that few membrane materials can withstand these temperatures while still having selectivity.
The innovation that the University of Melbourne has developed has enabled the utilisation of high temperature resilient polyimide and polybenzimidazole materials as the membrane. These polymers will only degrade at extreme temperatures (above 400°C) and can easily be fabricated as ultra-thin membranes.
Methanol selectivity
The methanol selectivity challenge is overcome by fabricating the membranes as non-porous layers where separation is achieved through solubility mechanisms, rather than size separation.
Methanol has a high sorption affinity for these polymers, which allows the methanol concentration to build up within these polymers and results in a high methanol flux through the membrane. Conversely, CO2 and hydrogen have very low sorption affinity for these polymers and therefore cannot accumulate within the membrane, resulting in a low flux.
This was the key to providing methanol selectivity for the membrane while ensuring operability at high temperature. An additional advantage of these polymers is that water also actively permeates through, and as water is a by-product of the CO2 hydrogenation reaction, removal of water further enhances conversion.
This combination of catalyst and high temperature membranes enables the University of Melbourne’s membrane reactor technology to successfully produce methanol between 100°C to 200°C, significantly lower than the 300°C required through a conventional reactor.
This is possible because the active removal of methanol from the reaction enables lower temperature thermodynamics, while sustaining a reasonable reaction rate. In addition, the membrane’s high selectivity ensures the methanol is separated under pressurised conditions. As such, the membrane reactor produces a methanol product that is of high purity, with almost no CO2 or hydrogen present.
Looking to the future
The next challenge for the technology is large-scale demonstration, transitioning the bench-scale system to a plant that can produce methanol on a sizeable scale. Currently, there are no large-scale CO2 hydrogenation to methanol plants globally, so there is potential for the University of Melbourne’s technology to become the global leader.
The demonstration plant will be focused on producing methanol as an intermediate in the chemical production chain to dimethyl ether and liquid hydrocarbons, as green substitutes for gasoline. This is to fully demonstrate the methanol economy, while we wait for large-scale uptake of hydrogen as an energy vector.
CO2 hydrogenation to methanol is a promising approach that offers a way to recycle carbon dioxide and produce a valuable chemical. By utilising advanced catalysts and integrating membranes, optimal membrane reactor technology has been developed, which will contribute to a more sustainable future.
Whilst there are challenges remaining, the key research and development has been achieved, and viable technology is ready for demonstration that will fully unlock the full benefits of CO2 hydrogenation to methanol.
