(77b) Investigating the Effect of Dehydrating Agents for the Direct Synthesis of Dimethyl Carbonate and Study Their Potential for Carbon Dioxide Fixation

Dimethyl carbonate (DMC) is known to be a safe and non-corrosive material that has wide range of applications. For example, it can be used as a fuel additive for gasoline and diesel to improve their combustion efficiency and reduce the pollutants emission. DMC has been manufactured through different methods such as the phosgenation of methanol and methanol oxidative carbonylation. However, these routes involve the use of toxic and hazardous materials such as carbon monoxide (CO) and phosgene. Therefore, there is growing interest in alternative routes, such as direct synthesis from carbon dioxide (CO2) and methanol (MeOH). This method is considered to be an attractive alternative, due to its low toxicity compared to the other routes. Moreover, it offers environmental benefits by utilizing CO2, thereby contributing to the reduction of atmospheric CO2 levels. According to different studies reported in the literature, DMC is considered one of the most promising products derived from CO2 utilization and it has the potential to serve as a model reaction for Carbon Capture and Utilization (CCU). Furthermore, this method does not include the formation of any toxic or corrosive materials, as water is the only by-product. However, the direct synthesis method faces several drawbacks that limit the methanol conversion towards DMC to around 1% even under favoured conditions of production. Some of these drawbacks are the high stability and low reactivity of CO2, in addition to the in-situ formed water, as a by-product, that shifts the reaction equilibrium toward the hydrolysis of DMC [1].

Therefore, to overcome the energy barrier of the reaction, an effective catalyst is required. Various catalysts have been investigated and reported in the literature, such as cerium oxide (CeO2), silicon dioxide (SiO2), and zinc oxide (ZnO), among which CeO2 exhibits the most effective performance. Simultaneously, a method for in-situ water removal is required to shift the equilibrium towards DMC production. Different methods have been suggested for in-situ water removal such as the use of dehydrating agents such as 2- cyanopyridine (2-CP), acetonitrile, and benzonitrile. These dehydrating agents reacts with water helping in shifting the reaction towards the DMC formation based on Le Châtelier's principle. 2-CP is the dehydrating agent with the highest performance as it helped in increasing the conversion above 95% with a DMC selectivity greater than to 99% [2]. However, the use of the other nitriles did not increase the conversion as 2-CP and their selectivities were lower probably due to the side reactions such as reaction of the nitriles with methanol, decomposition of the nitriles, and catalyst deactivation. For example, in the case of using benzonitrile as a dehydrating agent, it helped to overcome the equilibrium limitation and to increase the DMC conversion to 47% by reacting with water resulting in the formation of benzamide. However, the DMC formation reaction stopped at this percentage and did not go over it due to benzamide as a catalyst poison [3]. 2,2-dimethoxypropane (DMP) is another dehydrating agent that was reported in literature due to its rapid and effective action by chemical hydrolysis. However, the use of DMP reduced the DMC selectivity due to the formation of acetone as a by-product. Despite the outstanding performance of 2-CP, it still did not help in scaling up the process and making it economically feasible due to difficulties in separation and regeneration. Therefore, the search of other dehydrating agents continuous. In this work, dehydrating agents molecular such as zeolite 3A, 4A, 5A were tested due to their large surface area, thermal stability, and excellent adsorption. However, the addition of large amount of molecular sieve may lead to reduction in the catalyst activity as it can prevent the interaction of reactants with the catalyst active sites. On the other hand, the use of small amounts of molecular sieve will not be to remove the formed water and achieve optimal conversion [4]. Moreover, ethylene glycol was also tested as it showed high potential as a candidate due to its utilization in the natural gas industry to dehydrate the natural gas.

This work also aims to investigate the ability of the DMC process in the presence of different dehydrating as a CCU reaction pathway to fix CO2. This goal is achieved by using the CO2Fix parameter, which is a matric developed by Ibrahim et. al. to estimate the CO2 fixation potential for different CCU pathways. If the CO2Fix calculations resulted in a value equal to or above 1, then the process does utilize more CO2 than it produces, however, a CO2Fix value less than 1 indicate that the process produces more CO2 than it utilizes [5]. For the direct synthesis of DMC from CO2 in the presence of dehydrating agents such as 2-CP, acetonitrile, and benzonitrile helped in achieving a conversion equal to 94%, 9%, and 47%, respectively. The corresponding CO2Fix calculations were 3.68 for 2-CP, 2.92 for benzonitrile, and 0.80 for acetonitrile. Meaning that the direct synthesis method has the potential to fix CO2 in the presence of 2-CP and benzonitrile as dehydrating agents, but not in the presence acetonitrile. Furthermore, within this work, the overall direct DMC synthesis process shall be simulated to study the economic aspects.

References

1. Pyo, S.-H., Park, J. H., Chang, T.-S., & Hatti-Kaul, R. (2017a). Dimethyl carbonate as a green chemical. Current Opinion in Green and Sustainable Chemistry, 5, 61–66. https://doi.org/10.1016/j.cogsc.2017.03.012

2. Honda, M., Tamura, M., Nakagawa, Y., Nakao, K., Suzuki, K., & Tomishige, K. (2014). Organic carbonate synthesis from CO2 and alcohol over CEO2 with 2-cyanopyridine: Scope and mechanistic studies. Journal of Catalysis, 318, 95–107. https://doi.org/10.1016/j.jcat.2014.07.022

3. Tomishige, K., Tamura, M., & Nakagawa, Y. (2018). Co2 conversion with alcohols and amines into carbonates, ureas, and carbamates over CEO2 catalyst in the presence and absence of 2‐cyanopyridine. The Chemical Record, 19(7), 1354–1379. https://doi.org/10.1002/tcr.201800117

4. Faria, D. J., Moreira dos Santos, L., Bernard, F. L., Selbacch Pinto, I., Carmona da Motta Resende, M. A., & Einloft, S. (2020). Dehydrating agent effect on the synthesis of dimethyl carbonate (DMC) directly from methanol and carbon dioxide. RSC Advances, 10(57), 34895–34902. https://doi.org/10.1039/d0ra06034h

5. Ibrahim, G., Challiwala, M. S., Choudhury, H. A., Zang, G., El-Halwagi, M. M., & Elbashir, N. O. (2023). CO2Fix: An approach to assess CO2 fixation potential of CCU reaction pathways. Computers & Chemical Engineering, 178, 108398. https://doi.org/10.1016/j.compchemeng.2023.108398