Reaction of CO2 with Alcohols to Linear-, Cyclic-, and Poly-Carbonates Using Ceo2-Based Catalysts

REVIEW published: 23 June 2020 doi: 10.3389/fenrg.2020.00117 Reaction of CO2 With Alcohols to Linear-, Cyclic-, and Poly-Carbonates Using CeO2-Based Catalysts Keiichi Tomishige*, Yu Gu, Yoshinao Nakagawa and Masazumi Tamura School of Engineering, Tohoku University, Sendai, Japan Reaction of CO2 with alcohols to organic carbonates is one of non-reductive CO2 conversion methods. The catalysts are needed for this reaction, at the same time, effective H2O removal methods are also needed because the yield of organic carbonates is strongly limited by the equilibrium. The development of heterogeneous catalysts for the synthesis of dimethyl carbonate from CO2 and methanol, which is a model and typical reaction, is described. This is because heterogeneous catalysts are more suitable to the practical process than homogeneous catalysts from the viewpoint of the separation of catalysts from the products and the reusability of the catalysts. One of the reported heterogeneous catalysts is CeO2, and it has been also reported that the combination of dimethyl carbonate synthesis from CO2 and methanol with the hydration of nitriles Edited by: such as 2-cyanopyridine, where both reactions are catalyzed by CeO2, enabled high Michele Aresta, + IC2R srl, Italy yield of the carbonate. In addition, the combination of CeO2 catalyst nitriles can be Reviewed by: applied to the synthesis of a variety of linear-, cyclic (five- and six-membered ring)-, and Kenichi Shimizu, poly-carbonates from CO2 and corresponding alcohols. Hokkaido University, Japan Jordi Llorca, Keywords: carbon dioxide, ceria, alcohol, carbonate, equilibrium shift, nitrile hydration Universitat Politecnica de Catalunya, Spain *Correspondence: INTRODUCTION Keiichi Tomishige [email protected] Much attention has been recently paid to chemical utilization of CO2, although the chemicals produced from CO2 are so limited at present. Generally speaking, it is not easy to decrease the Specialty section: net CO2 emission in the production of a chemical from CO2 because of the CO2 emission due This article was submitted to to the separation and purification of CO2. On the other hand, it is possible that the CO2 emission Carbon Capture, Storage, and from the CO2-based chemical production becomes smaller than that from conventional production Utilization, methods. This can be connected to the replacement of the conventional method by CO2-based one. a section of the journal Chemical utilization of CO can be divided into two categories: one is reductive conversion Frontiers in Energy Research 2 of CO2 and the other is non-reductive conversion of CO2. One of typical methods for reductive Received: 20 March 2020 conversion of CO2 is the hydrogenation of CO2. In most cases of the CO2 hydrogenation, the Accepted: 18 May 2020 production of renewable hydrogen is required for the decrease in the CO emission and it is Published: 23 June 2020 2 thought that the development of the feasible production method for hydrogen from renewable Citation: resources needs more time. In contrast, non-reductive conversion of CO2 does not need the Tomishige K, Gu Y, Nakagawa Y and renewable hydrogen. Tamura M (2020) Reaction of CO2 With Alcohols to Linear-, Cyclic-, and One of the non-reductive conversions of CO2 includes the synthesis of organic carbonates, Poly-Carbonates Using CeO2-Based carbamates, and ureas from CO2, and alcohols, and/or amines. The synthesis of organic carbonates, Catalysts. Front. Energy Res. 8:117. carbamates, and ureas is possible from phosgene, and alcohols, and/or amines. Much higher doi: 10.3389/fenrg.2020.00117 reactivity of phosgene than CO2 enables no equilibrium limitation of these reactions and Frontiers in Energy Research | www.frontiersin.org 1 June 2020 | Volume 8 | Article 117 Tomishige et al. Reaction of CO2 With Alcohols also connected to the non-catalytic reactions. In contrast, the 2019). We adopted the DFT calculations because it is generally synthesis of organic carbonates, carbamates, and ureas from CO2, difficult to collect all the thermodynamic data such as formation and alcohols, and/or amines is commonly reversible reactions enthalpy and so on regarding various substrates and products. and can be limited by the equilibrium. At the same time, much The experimental data of the heat of reaction are also shown lower reactivity of CO2 than phosgene requires effective catalysts. in Table 1 for representative entries. Although the obtained It has been reported that CeO2-based catalysts are effective to the values can be different from the experimentally determined synthesis of organic carbonates, carbamates, and ureas from CO2, ones, the comparison of the obtained values can be suggestive. and alcohols, and/or amines (Tamura et al., 2014; Tomishige In particular, we have to be careful that all the substrate and et al., 2019, 2020). The product yield at the equilibrium is strongly product molecules are present in the gas phase. The negative influenced by the substrates such as alcohols and amines. As is and large energy change means that the reaction is highly known, the equilibrium limitation of the synthesis of organic exothermic and the positive and large one means that the reaction carbonates from CO2 and alcohols is usually much more serious is highly endothermic. Since these values are almost the same than that of the synthesis of carbamates and ureas. Therefore, the to enthalpy change, the equilibrium is also affected by entropy shift of the equilibrium is very important for high product yield change, especially when the number of molecules is changed in the synthesis of organic carbonates from CO2 and alcohols. in the reaction. Comparison between reactions with different Recently, it is found that the combination of the synthesis of changes in the number of molecules should be careful because organic carbonates from CO2 and alcohols with the hydration of the entropy term is much different between such reactions. Since nitriles to amides is effective to the shift of the equilibrium to the the standard entropy (S0) of liquid alcohols is about 150 J mol−1 product side (Honda et al., 2014a). The present review mainly K−1, the difference in the number of reactant alcohol molecules mentions the synthesis of the organic carbonates such as linear- by one corresponds to about 60 kJ mol−1 difference in free , cyclic, and poly-carbonates from CO2, and corresponding energy at 400K. alcohols combined with the hydration of the nitriles over CeO2 as a heterogeneous catalyst comparing to other related catalysts, Non-CO2-Based Synthesis Route of Linear organic dehydrants, and so on. First, various synthesis routes Organic Carbonates of organic carbonates are introduced, and challenging subjects Phosgene method is the most typical one for the synthesis of the such as the low equilibrium level, and the catalyst development linear organic carbonates from alcohols (Equation 1). are mentioned. Next, we mention the method for the removal of H2O in the synthesis of organic carbonates from CO2 and alcohols to shift the equilibrium since the production of target COCl2 + 2ROH + 2NaOH → (RO)2CO + 2NaCl + 2H2O (1) organic carbonates in higher yield is important and can enhance the impact. Then, various examples of catalyst development for The problem of this method is the toxicity of phosgene and the reaction are shown and compared, indicating that CeO2 can large amount of byproduct NaCl. The formation of DMC from be regarded as one of promising heterogeneous catalysts in terms phosgene and methanol is highly exothermic even when the of the catalytic activity such as the formation rate of organic product HCl is not neutralized (Table 1, Entry 1). This behavior is carbonates. Another superiority of CeO2 is that the combination explained by the high energy state of phosgene itself, which is also of the synthesis of DMC synthesis from CO2 and methanol with connected to high reactivity of phosgene. It should be noted that the hydration of nitriles over CeO2 enhanced the DMC yield the exothermicity becomes much larger considering the reaction remarkably, and the applicability of this system is also mentioned of HCl with NaOH for the neutralization (Table 1, Entry 2). in this review. Totally, the high energy state and high reactivity of phosgene enables the practical production of organic carbonates by the reaction with alcohols, and this reaction can proceed without SYNTHESIS ROUTES OF ORGANIC any catalysts. CARBONATES Transesterification of linear organic carbonates with alcohols is applied to the synthesis of other organic carbonates, for A variety of synthesis routes of organic carbonates have been example, DMC is the simplest linear organic carbonate and considered, and we would like to compare the routes from the transesterification of DMC with other alcohols gives various thermodynamic viewpoint. This can clarify the thermodynamic organic carbonates (Equation 2) (Ono, 1997). High yield of the difficulty level of the routes, and this is a fundamental aspect target carbonates will be obtained by large excess of DMC. This before the development of catalysts for each reaction. In this reaction has been also applied to the production of diphenyl chapter, synthesis routes of organic carbonates are divided carbonate, which can be rather endothermic due to the high into three categories: non-CO2-based route, indirect CO2-based energy of diphenyl carbonate (Table 1, Entry 7) (Ono, 1997). route, and direct CO2-based route. Table 1 lists the energy change in carbonate formation ′ ′ reactions and their related reactions calculated by DFT for the (CH3O)2CO + 2R OH → (R O)2CO + 2CH3OH (2) evaluation of the severity of the equilibrium limitation and part of calculation was carried out for this article and some calculation If DMC can be produced by CO2-based routes, this route can be has been described in our previous review (Tomishige et al., also categorized to indirect CO2-based ones. Frontiers in Energy Research | www.frontiersin.org 2 June 2020 | Volume 8 | Article 117 Tomishige et al.

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