Capturing carbon dioxide and turning it into commercial products, such as fuels or construction materials, could become a new global industry, according to a study by researchers from UCLA, the University of Oxford and five other institutions.
Should that happen, the phenomenon would help the environment by reducing greenhouse gas emissions.
The research, published in Nature, is the most comprehensive study to date investigating the potential future scale and cost of 10 different ways to use carbon dioxide, including in fuels and chemicals, plastics, building materials, soil management and forestry.
The study considered processes using carbon dioxide captured from waste gases that are produced by burning fossil fuels or from the atmosphere by an industrial process.
And in a step beyond most previous research on the subject, the authors also considered processes that use carbon dioxide captured biologically by photosynthesis.
The research found that on average each utilization pathway could use around 0.5 gigatonnes of carbon dioxide per year that would otherwise escape into the atmosphere.
(A tonne, or metric ton, is equivalent to 1,000 kilograms, and a gigatonne is 1 billion tonnes, or about 1.1 billion U.S. tons.)
The researchers noted, however, that the potential scales and costs of using carbon dioxide varied substantially across sectors.
“The analysis we presented makes clear that carbon dioxide utilization can be part of the solution to combat climate change, but only if those with the power to make decisions at every level of government and finance commit to changing policies and providing market incentives across multiple sectors,” said Emily Carter, a distinguished professor of chemical and biomolecular engineering at the UCLA Samueli School of Engineering and a co-author of the paper.
“The urgency is huge and we have little time left to effect change.”
According to the Intergovernmental Panel on Climate Change, keeping global warming to 1.5 degrees Celsius over the rest of the 21st century will require the removal of carbon dioxide from the atmosphere on the order of 100 to 1,000 gigatonnes of carbon dioxide.
Currently, fossil carbon dioxide emissions are increasing by over 1% annually, reaching a record high of 37 gigatonnes of carbon dioxide in 2018.
“Greenhouse gas removal is essential to achieve net zero carbon emissions and stabilise the climate,” said Cameron Hepburn, one of the study’s lead authors, director of Oxford’s Smith School of Enterprise and Environment.
“We haven’t reduced our emissions fast enough, so now we also need to start pulling carbon dioxide out of the atmosphere. Governments and corporations are moving on this, but not quickly enough.
“The promise of carbon dioxide utilization is that it could act as an incentive for carbon dioxide removal and could reduce emissions by displacing fossil fuels.”
Critical to the success of these new technologies as mitigation strategies will be a careful analysis of their overall impact on the climate.
Some are likely to be adopted quickly simply because of their attractive business models. For example, in certain kinds of plastic production, using carbon dioxide as a feedstock is a more profitable and environmentally cleaner production process than using conventional hydrocarbons, and it can displace up to three times as much carbon dioxide as it uses.
Biological uses might also present opportunities to reap co-benefits
. In other areas, utilization could provide a “better choice” alternative during the global decarbonization process. One example might be the use of fuels derived from carbon dioxide, which could find a role in sectors that are harder to decarbonize, such as aviation.
The authors stressed that there is no “magic bullet” approach.
“I would start by incentivizing the most obvious solutions — most of which already exist — that can act at the gigatonne scale in agriculture, forestry and construction,” said Carter, who also is UCLA’s executive vice chancellor and provost, and the Gerhard R. Andlinger Professor in Energy and Environment Emeritus at Princeton University.
“At the same time, I would aggressively invest in R&D across academia, industry and government labs — much more so than is being done in the U.S., especially compared to China — in higher-tech solutions to capture and convert carbon dioxide to useful products that can be developed alongside solutions that already exist in agriculture, forestry and construction.”
The demand for energy of the rapid industrialization results in large-scale combustion of fossil fuel, which causes excessive emissions of carbon dioxide. As the detrimental environmental impacts of CO2 have drawn considerable attention, various strategies have been developed to mitigate CO2 accumulation in the atmosphere, among which carbon capture and storage/sequestration (CCS) is considered as a promising CO2 reducing option (Alexander et al., 2015).
Nowadays, a plethora of CO2 absorbents have been developed to facilitate CO2 capture and desorption. Nevertheless, the extensive energy needed in the absorbent regeneration and CO2 separation is not conducive to the implementation of CCS strategy.
In contrast to carbon sequestration, converting CO2 into valuable chemicals could be a sustainable option, which has been proposed by Ciamician as early as 1912 (Ciamician, 1912). In recent decades, CO2 conversion has attracted considerable concern and been intensively investigated (Rahman et al., 2017). However, in most processes for CO2 conversion, pure or high pressure CO2 is needed, implying that the CO2 from the atmosphere or industrial exhaust cannot be used as C1 source directly and thus the energy issue in CO2 capture and separation still remains.
To address the energy penalties associated with CCS strategy and realize the direct fixation of CO2 from the atmosphere or industrial exhaust, the CO2 capture and utilization (CCU) strategy, whereby the captured CO2 is used as a non-toxic, abundant, and sustainable feedstock to produce valuable organic compounds via chemical, electrochemical or photochemical reactions, was proposed and now is flourishing (Scheme 1).
By now, both organic compounds and functional materials containing the bridging-carbonato metal complexes can be obtained from atmospheric CO2 using the CCU strategy (Yang et al., 2011; Liu et al., 2012; Massoud et al., 2015). Although realizing the attractive prospect of the CO2 capture and in situ conversion in the industry scale remains a challenge (Zhang and Lim, 2015), the emergence of efficient absorbents and the development of CO2 transformation will cast light on it. In continuation of our work on the conversion of the captured CO2 into value-added organic chemicals, this review summarized the recent progress on CO2 capture and in situ conversion into organic products.
To realize the carbon capture and in situ conversion strategy, effective absorbents are always necessary. Ideally, the absorbents for CCU strategy should not only capture CO2, but also activate CO2 and even the substrate. Thus, the chemical transformation can proceed under mild conditions. Up to now, organic and inorganic bases, N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs), ionic liquids (ILs) and frustrated Lewis pairs (FLPs) have already been applied to CO2 capture and in situ conversion. A plethora of valuable organic chemicals have been obtained through the CCU strategy as shown in Scheme 2.
Due to the electrophilicity of carbon atom in CO2, the organic and inorganic bases containing strong nucleophilic atom have been widely used in CO2 trapping, where the base can interact with CO2 directly or function as a proton acceptor. The resulting CO2 capture products i.e., CO2 adducts have been employed for subsequent synthesis of various valuable chemicals.
Considering the transformations of the captured CO2 derived from primary and secondary amines and amino alcohols to isocyanates, carbamates, ureas, and oxazolidinones have been concerned by several excellent review papers (Hampe and Rudkevich, 2003; Chaturvedi and Ray, 2006; Yang et al., 2012; Tamura et al., 2014; Wang et al., 2017a,b), here we focus on the transcarboxylation effect and other transformations of the captured CO2, namely CO2 derivatives.
Synthesis of Carbamates and Ureas
In the synthesis of carbamates, the aprotic organic bases can function as CO2 absorbents and transcarboxylation agents. The initial attempt was made by Rossi group, in which CO2 is trapped by a methanol solution of commercially available tetraethylammonium hydroxide. The resulting tetraethylammonium hydrogen carbonate can be used as a surrogate of CO2 in the synthesis of carbamate. Meanwhile, the presence of tetraethylammonium ion as counterion increases the nucleophilicity of carbamate anion (Inesi et al., 1998).
Soon after, Franco group has successfully identified the DBU-CO2 complex via reacting CO2 with DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene) in anhydrous acetonitrile, implying that DBU can be used as CO2 trap reagent (Pérez et al., 2002). Moreover, the resulting reactive DBU-CO2 adduct can be utilized as transcarboxylating reagent for synthesis of N-alkyl carbamates.
Later, the same group revealed the activation capacity of CO2 by other bicyclic amidines and observed the inverse relation between the thermal stability and the transcarboxylating activity for the amidine-CO2 adducts (Scheme 3) (Pérez et al., 2004), which is the first time to investigate the activation ability of organic bases to CO2.
The combination of organic base and alcohol is an efficient CO2 capture system and the absorbed CO2 can be in situ transformed. The prototypical example is the polyethylene glycol (PEG)/superbase system developed by our group in 2011 (Yang et al., 2011). In the capture step, the superbase is used as a proton acceptor and almost equimolar CO2 per mole superbase can be absorbed (Scheme 4). The resulting liquid amidinium carbonate can directly react with n-butylamine at 110°C to afford dibutyl urea in almost quantitative yield (96%) without any other additives. This protocol can be used in the synthesis of other symmetrical urea derivatives.
In the above examples, the captured CO2 in the transcarboxylating agents can be regarded as the activated CO2 because the linear structure of CO2 is converted to bent structure, which is more liable to nucleophilic attack.
Synthesis of Oxazolidinones
The “CO2 absorption and subsequent transcarboxylation” triggers the research on CO2 capture and in situ transformation. Several years later, M. Yoshida and coworkers use DBU to enrich and activate CO2 in air and perform the first example of directly transforming atmospheric CO2 into the substituted 5-vinylideneoxazolidin-2-ones using propargylic substrate 4-(benzylamino)-2-butynyl carbonates or benzoates as a substrate (Scheme 4) (Yoshida et al., 2008). In their follow-up work, they further improve the reaction efficiency by utilizing AgNO3 as catalyst and propargylic amines as substrates (Scheme 4) (Yoshida et al., 2012).
Inspired by these works, our group designs a series of novel CO2 capture and activation systems. For example, by employing ammonium iodide as catalyst, the cycloaddition reaction of various aziridines with the captured CO2 by NH2PEG150NH2 gives rise to oxazolidinones at 40°C in >94% yield and selectivity (Scheme 4) (Yang et al., 2011).
Soon after, we report the first example of steric-hindrance-controlled CO2 absorption, where the sodium N-alkylglycinates and N-alkylalaninates dissolved in PEG150 are used to capture CO2, generating the carbamic acid rather than the ammonium carbamate (Liu et al., 2012). N-isopropylglycinate is found to be the best absorbent for the rapid and reversible capture of almost equimolar CO2. Crucially, the captured CO2 can be activated simultaneously and the resulting carbamic acid can react with either aziridine or propargyl amine to afford oxazolidinones in the presence of NH4I and AgOAc as a catalyst, respectively (Scheme 4).
Motivated by these results, we further develop potassium phthalimide as absorbent to realize equimolar CO2 capture in PEG150. Moreover, the obtained product can be used as in situ transcarboxylating reagent to synthesize oxazolidinone derivatives (Scheme 4) (Zhang et al., 2014).
Recently, Hu group subtly designs a CCU example (Yu et al., 2016), in which carbamate salts generated from CO2 and primary amines are used as substrates. The captured CO2 not only acts as a reactant but also acts as a protecting reagent for the amine to avoid poisoning of the copper catalyst. By using 5 mol% of CuI as catalyst, carbamate salts can react with aromatic aldehydes and aromatic terminal alkynes, affording the important oxazolidin-2-ones (Scheme 4).
- Cameron Hepburn, Ella Adlen, John Beddington, Emily A. Carter, Sabine Fuss, Niall Mac Dowell, Jan C. Minx, Pete Smith, Charlotte K. Williams. The technological and economic prospects for CO2 utilization and removal. Nature, 2019; 575 (7781): 87 DOI: 10.1038/s41586-019-1681-6