Carbon Dioxide Utilization in the Natural Gas Industry Via Direct Conversion to Organic Carbonates

Natural gas is considered by many to be a transitional fuel with significantly lower environmental impact compared to other energy sources such as coal and oil. However, the production and utilization of natural gas still come with significant greenhouse gas emissions, including CO_2, which has made the decarbonization of the natural gas industry through CO₂ capture, sequestration, and utilization (CCSU) the highest priority for the industry. While the idea of chemical utilization of CO₂ to produce other molecules is appealing, CO₂ is a stable molecule that is challenging to convert into added-value chemicals efficiently. One means of facilitating CO₂ reactions is via catalysis, wherein the CO₂ molecules are adsorbed on the catalyst’s surface and interact with other precursors to form products. A challenging aspect of CO₂ reactions is low conversion due to thermodynamic constraints even in the presence of catalysts – as is observed in the conversion of CO­₂ to Organic Carbonates.

This paper will highlight our efforts in developing a sustainable process for the direct synthesis of dimethyl carbonate (DMC), which is a flexible molecule used in a variety of applications such as a sustainable and green solvent, an electrolyte in the battery industry, a fuel additive, and in the manufacturing of engineering plastics. The direct synthesis of DMC involves reacting methanol and CO₂ to produce DMC and water as a means of decarbonizing the natural gas industry. We utilize reaction engineering concepts and techniques to synthesize, characterize, and develop catalysts and then test the efficacy of those catalysts for the direct synthesis of DMC. We synthesized catalysts using xero-gel method and co-precipitation techniques, and the performance of these catalysts was evaluated in a slurry reactor under the batch condition of 5-30 bar pressure and 110-130 °C temperature. The catalysts exhibited significantly different conversions towards DMC, but all below 1% conversion – in line with our thermodynamic assessment (Elbashir, Choudhury and Challiwala, 2021). Furthermore, a metric we developed to study the potential for CO₂ fixation through chemical reactions indicated the process is CO₂ neutral and negative only when the reaction has a conversion above 30% when the reaction is carried out in these relatively low-pressure conditions. Thus, to approach this target, we attempt to shift the equilibrium conversion by utilizing methods for in-situ removal of water, which shifts the equilibrium in favor of the forward reaction. Our test with physical adsorbents such as activated carbon, Dowex, and 5A molecular sieves showed no improvement in the methanol conversion toward DMC. However, the addition of 2-CP as a water scavenger, even at a low-pressure reaction condition of 5 bar, showed significant improvement in methanol conversion towards DMC (Tomishige, Tamura and Nakagawa, 2018). An optimization of 2-CP system for low-pressure conditions of 5 bar is underway to approach the conversion needed to make the direct synthesis of DMC viable reaction pathways for CO₂ sequestration. Currently, we are conducting modeling, simulations, and kinetic analyses in preparation for testing the reaction in a relatively large scale (10 l/hr) Reactive Water Distillation unit.

This work is funded by a research grant from Qatar National Research Fund under its flagship NPRP program (Project No. NPRP13S-0122-200141) and co-funded by Shell Global Solutions (Qatar Shell Research Technology Center).

References

Elbashir, N. O., Choudhury, H. A. and Challiwala, M. (2021) ‘Methods of Producing Dimethyl Carbonate’. World Intellectual Property Organization International Bureau.

Tomishige, K., Tamura, M. and Nakagawa, Y. (2018) ‘CO ₂ Conversion with Alcohols and Amines into Carbonates, Ureas, and Carbamates over CeO ₂ Catalyst in the Presence and Absence of 2-Cyanopyridine’, The Chemical Record. John Wiley & Sons, Ltd. doi: 10.1002/tcr.201800117.