New plasma synthesis process for one-step conversion of CO2 and methane into higher value fuel and chemicals
Researchers from the University of Liverpool (UK), with colleagues from Dalian University of Technology (China) and the University of Hull (UK), have developed a new process for the direct, one-step activation of carbon dioxide and methane (dry reforming of methane) into higher value liquid fuels and chemicals (e.g., acetic acid, methanol, ethanol, and formaldehyde) with high selectivity at ambient conditions.
In a study published in chemistry journal Angewandte Chemie, they reported the total selectivity to oxygenates was approximately 50–60%, with acetic acid being the major component at 40.2% selectivity—the highest value reported for acetic acid thus far. The direct plasma synthesis of acetic acid from CH4 and CO2 is an ideal reaction with 100% atom economy, but is almost impossible by thermal catalysis owing to the significant thermodynamic barrier. The combination of plasma and catalyst in this process shows great potential for manipulating the distribution of liquid chemical products in a given process, they concluded.
A particularly significant route currently being developed for CO2 utilization is catalytic CO2 hydrogenation. This can produce a range of fuels and chemicals including CO, formic acid, methanol, hydrocarbons and alcohols; however, high H2 consumption (CO2 + 3H2 → CH3OH + H2O) and high operating pressure (~30-300 bar) are major challenges facing this process.
Instead of using H2, direct conversion of CO2 with CH4 (dry reforming of methane, DRM) to liquid fuels and chemicals (e.g. acetic acid) represents another promising route for both CO2 valorisation and CH4 activation. CH4 is an ideal H-supplier to replace H2 in CO2 hydrogenation as CH4 has a high H density and is available from a range of sources (e.g. natural gas, shale gas, biogas and flared gas). Moreover, it is a cheap carbon source which can increase the atom utilization of CO2 hydrogenation due to the stoichiometric ratio of C and O atoms, as well as reducing the formation of water.
… It is almost impossible to directly convert two stable and inert molecules (CO2 and CH4) into liquid fuels or chemicals in a one-step catalytic process bypassing the production of syngas.
… Non-thermal plasma (NTP) offers a unique way to enable thermodynamically unfavorable chemical reactions to occur at low temperatures due to its non-equilibrium character: the overall gas temperature in a NTP remains low, while the generated electrons are highly energetic with a typical electron temperature of 1-10 e; sufficient to activate inert molecules (e.g. CO2 and CH4) into reactive species, including radicals, excited atoms, molecules and ions.—Wang et al.
The one-step room-temperature synthesis of liquid fuels and chemicals from the direct reforming of CO2 with CH4 was achieved by using a novel atmospheric-pressure non-thermal plasma reactor with a water electrode and a low energy input.
The experiment was carried out in a coaxial dielectric barrier discharge (DBD) reactor with a novel water electrode at atmospheric pressure and room temperature. The DBD reactor consisted of a pair of coaxial glass cylinders (inner and outer glass tubes) and two coaxial electrodes. Compared to conventional cylindrical DBD reactor design, circulating water filled the space between the inner and outer glass cylinders and acted as a ground water electrode.
This novel reactor design using the water electrode could effectively remove heat generated by the discharge and maintain the reaction at around room temperature (~30 oC) for the effective synthesis of liquid oxygenates at atmospheric pressure.
The DBD reactor was connected to an AC high voltage power supply with a maximum peak voltage of 30 kV and a variable frequency of 7-12 kHz. In this work, the frequency was fixed at 9 kHz.
|Schematic diagram of experimental setup. Wang et al. Click to enlarge.|
The dry reforming of methane (DRM) reaction was carried out at the same temperature (~30 ˚C) under three different operating modes: plasma-alone, catalysis-alone and plasma-catalysis. In the catalysis-alone mode, no reaction occurred at a temperature of around 30 ˚C.
These results clearly show that non-thermal plasmas offer a promising solution to overcome the thermodynamic barrier for the direct transformation of CH4 and CO2 into a range of strategically important platform chemicals and synthetic fuels at ambient conditions. Introducing a catalyst into the plasma chemical process, known as plasma-catalysis, could tune the selectivity of target chemicals.
This is a major breakthrough technology that has great potential to deliver a step-change in future methane activation, CO2 conversion and utilization and chemical energy storage, which is also of huge relevance to the energy & chemical industry and could help to tackle the challenges of global warming and greenhouse gas effect.—Dr. Xin Tu, from the University’s Department of Electrical Engineering and Electronics
Plasma, the fourth state of matter, an electrically charged gas mixture, offers a promising and attractive alternative for the synthesis of fuels and chemicals, providing a unique way to enable thermodynamically unfavorable reactions to take place at ambient conditions.
In non-thermal plasmas, the gas temperature remains low (as low as room temperature), while the electrons are highly energetic with a typical electron temperature of 1-10 eV, which is sufficient to activate inert molecules (e.g. CO2 and CH4) present and produce a variety of chemically reactive species including radicals, excited atoms, molecules and ions. These energetic species, which are produced at a relatively low temperature, are capable of initiating a variety of different reactions.
Plasma systems have the flexibility to be scaled up and down. In addition, high reaction rate and fast attainment of steady state in a plasma process allows rapid start-up and shutdown of the plasma process compared to other thermal processes, which significantly reduces the overall energy cost and offers a promising route for the plasma process powered by renewable energy (e.g. wind and solar power) to act as an efficient chemical energy storage localized or distributed system.
The highly attractive process could also provide a promising solution to end gas flaring from oil and gas wells through the conversion of flared methane into valuable liquid fuels and chemicals which can be easily stored and transported.
- Wang, L., Yi, Y., Wu, C., Guo, H. and Tu, X. (2017) “One-Step Reforming of CO2 and CH4 into High-Value Liquid Chemicals and Fuels at Room Temperature by Plasma-Driven Catalysis,” Angew. Chem. doi: 10.1002/ange.201707131