Co-catalyst and Metal-free CO2 Fixation into Cyclic Carbonates: COPs to the Rescue

Abstract

The non-redox fixation of CO2 in cyclic and polymeric carbonates is regarded as one of the most promising technologies for small-scale CO2 utilization. Yet, most heterogeneous catalysts reported to date require the use of co-catalysts, harsh reaction conditions, and non-base metals. In this issue of Chem, Yavuz and coworkers report the one-step synthesis of a new covalent organic polymer that circumvents these issues. The non-redox fixation of CO2 in cyclic and polymeric carbonates is regarded as one of the most promising technologies for small-scale CO2 utilization. Yet, most heterogeneous catalysts reported to date require the use of co-catalysts, harsh reaction conditions, and non-base metals. In this issue of Chem, Yavuz and coworkers report the one-step synthesis of a new covalent organic polymer that circumvents these issues. Over the next decades, we, human beings, will face one of the most important challenges we have ever faced: to find a way to maintain our standards of living with an ever-increasing population without making our planet inhabitable. The rising atmospheric concentration of CO2, a direct consequence of industrialization and rising living standards, is reaching levels that will soon have extraordinary consequences for our planet. Indeed, despite the implementation of non-emitting energy-generation technologies, 35 Gtons of CO2 are emitted into the atmosphere every year,1Mac Dowell N. Fennell P.S. Shah N. Maitland G.C. The role of CO2 capture and utilization in mitigating climate change.Nat. Clim. Chang. 2017; 7: 243-249Crossref Scopus (539) Google Scholar,2McDonough W. Carbon is not the enemy.Nature. 2016; 539: 349-351Crossref PubMed Scopus (18) Google Scholar which means that, unless radical changes in policy and breakthroughs in technology occur over the next few decades, we will most likely enter the feared 2°C scenario that the Paris Agreement tried to avoid.1Mac Dowell N. Fennell P.S. Shah N. Maitland G.C. The role of CO2 capture and utilization in mitigating climate change.Nat. Clim. Chang. 2017; 7: 243-249Crossref Scopus (539) Google Scholar As William McDonough wisely pointed out 3 years ago, there is still a great misconception about carbon and its unique association with atmospheric CO2 concentrations: “carbon—the element—is not the enemy. Climate change is the result of breakdowns in the carbon cycle caused by us: it is a design failure. Anthropogenic greenhouse gases in the atmosphere make airborne carbon a material in the wrong place, at the wrong dose and for the wrong duration. It is we who have made carbon toxic …. In the right place, carbon is a resource and a tool.”2McDonough W. Carbon is not the enemy.Nature. 2016; 539: 349-351Crossref PubMed Scopus (18) Google Scholar In this line, understanding CO2 emissions as a design failure of our current energy system is the first step toward fixing this issue. More importantly, understanding and accepting that carbon, the element, is essential to our lives (we are made of carbon) and to our society (the 20th century is known as the century of polymers because of the extreme impact that these carbon-made materials had on our society) could help us realize the great opportunity offered by CO2 utilization.3Hepburn C. Adlen E. Beddington J. Carter E.A. Fuss S. Mac Dowell N. Minx J.C. Smith P. Williams C.K. The technological and economic prospects for CO2 utilization and removal.Nature. 2019; 575: 87-97Crossref PubMed Scopus (670) Google Scholar Once the challenge and opportunities are well understood, we will realize that we should talk about not a single design failure but about many. We emit anthropogenic CO2 in diverse ways, from huge energy-generation stations based on coal, oil derivates, or natural gas to smaller emissions (per emitting unit) in transportation or flaring. Obviously, because of the diverse nature and geographic location of each one of these emissions, thinking of a single solution to mitigate them would be overly simplistic. For instance, although capture and storage or utilization can be realized at large emitting points with an available network for sequestration or utilization, sequestering other important emissions (such as those derived from remote flaring) would imply a great energy input in transportation of the captured CO2.4Bui M. Adjiman C.S. Bardow A. Anthony E.J. Boston A. Brown S. Fennell P.S. Fuss S. Galindo A. Hackett L.A. et al.Carbon capture and storage (CCS): the way forward.Energy Environ. Sci. 2018; 11: 1062-1176Crossref Google Scholar It is especially for these remote emissions where technological breakthroughs such as the one reported by the Yavuz team in this issue of Chem5Subramanian S. Oppenheim J. Kim D. Nguyen T.S. Silo W.M.H. Kim B. Goddard III, W.A. Yavuz C.T. Catalytic non-redox carbon dioxide fixation in cyclic carbonates.Chem. 2019; 5: 3232-3242Abstract Full Text Full Text PDF Scopus (56) Google Scholar could find an important application niche. When remote emissions are considered, the direct capture of CO2 in high-density form (be it a liquid or a solid) is preferred. Moreover, the use of additional reductants such as hydrogen and/or harsh reaction conditions, in terms of either temperature or pressure, should be avoided. Ideally, the potential product(s), in addition to containing a high density of CO2, should have a considerable added value. The catalyst should be solid, reusable, and preferably made of abundant elements. The use of co-catalysts or other stoichiometric reactants should be prevented. Last but not least, CO2 separation and purification steps would add to the overall CO2 footprint of the capture process and are therefore not desired. Given these constrains, the non-redox fixation of CO2 in cyclic and polymeric carbonates through reaction with epoxides is one of the most promising technologies for small-scale CO2 valorization and capture. This traditionally homogenously catalyzed reaction was first reported in 19696Inoue S. Koinuma H. Tsuruta T. Copolymerization of carbon dioxide and epoxide.J. Polym. Sci. B. 1969; 7: 287Crossref Google Scholar and offers an attractive route, with a 100% atom efficiency of CO2, for the production of fine chemicals and polymers. Since then, the scientific community has explored a large number of catalysts and different epoxide substrates. Despite notable achievements, most heterogeneous catalysts reported to date still require the use of co-catalysts, are only active at high temperatures and pressures, and in many cases, use non-base metals.7Kamphuis A.J. Picchioni F. Pescarmona P.P. CO2-fixation into cyclic and polymeric carbonates: principles and applications.Green Chem. 2019; 21: 406-448Crossref Google Scholar Only recently did the same team report on a pyridyl salicylimine polymeric catalyst that showed quantitative conversion without the need for additives or co-catalysts, even for the conversion of hard substrates such as styryl epoxide.8Zhang W. Liu T. Wu H. Wu P. He M. Direct synthesis of ordered imidazolyl-functionalized mesoporous polymers for efficient chemical fixation of CO2.Chem. Commun. (Camb.). 2015; 51: 682-684Crossref PubMed Google Scholar In search of better-performing solids to catalyze this reaction, the Yavuz team discovered serendipitously the one-step synthesis of a covalent organic polymer (COP) with unprecedented activity in the non-redox fixation of CO2. COP-222 (see Figure 1), an imidazolinium polymer produced through the one-pot reaction of terephthalaldehyde and ammonium chloride, does not require the use of any co-catalyst, solvent, or harsh reaction conditions to transform mixtures of CO2 and a wide range of epoxides into their corresponding cyclic carbonates, including the highly sterically and electronically challenging cyclohexene oxide and resorcinol diglycidyl ether. The presence of both a slightly acidic quaternary amine and a basic amine seems to be key to the unique reactivity of COP-222. According to density functional theory calculations, the reaction proceeds through a nucleophilic-attack-driven epoxide-ring-opening (ND-ERO) mechanism that starts with the coordination of the epoxide to an ammonium proton and follows with the activation of CO2 by the resulting chloride-stabilized oxyanion; this is similar to the process reported by Endo et al. for different halide salts.9Kihara N. Hara N. Endo T. Catalytic activity of various salts in the reaction of 2,3-epoxypropyl phenyl ether and carbon-dioxide under atmospheric pressure.J. Org. Chem. 1993; 58: 6198-6202Crossref Scopus (441) Google Scholar Although it is still too early to assess the actual industrial impact that COP-222 might have, its outstanding catalytic performance, the fact that neither solvents nor co-catalysts are required, the use of commercially available polymer precursors, and the one-pot synthesis of the solid all point in the right direction. When it comes to limiting anthropogenic CO2 emissions, such technology could be applicable, as discussed above, in small-scale, remote locations. However, full life-cycle analyses akin to those being done for other CO2 valorization technologies,10González-Garay A. Frei M.S. Al-Qahtani A. Mondelli C. Guillén-Gosálbez G. Pérez-Ramírez J. Plant-to-planet analysis of CO2-based methanol processes.Energy Environ. Sci. 2019; https://doi.org/10.1039/C9EE01673BCrossref Google Scholar including CO2 emissions derived from the production of the corresponding epoxides, should be considered in the assessment of the actual impact of such a technology on CO2 mitigation. Moreover, other important points can be learned from this work: (1) in spite of the very low porosity of COP-222 (21 m2/g), its catalytic activity is outstanding; (2) it is not a crystalline solid, which is certainly not needed for a reaction that does not rely on shape selectivity, yet given the repetitive nature of polymer units, it should be considered a single-site catalyst; and (3) the role of serendipity in science cannot be ignored—the authors originally speculated that the one-pot reaction of terephthalaldehyde and ammonium chloride would form a porous material containing cyclic amines. The actual product turned out to be something else yet something very promising. Catalytic Non-redox Carbon Dioxide Fixation in Cyclic CarbonatesSubramanian et al.ChemNovember 14, 2019In BriefYavuz and colleagues introduced a highly active catalyst for non-redox fixation of CO2 into cyclic carbonates, a versatile product family with potential use in green polymers and solvents. The metal-free, heterogeneous imidazolinium network structure is easily made, scaled up, recycled, and inexpensive and provides quantitative selectivity and conversion yields over a wide substrate scope of epoxides. Full-Text PDF Open Archive