Integration of CO2 Capture and Utilization: Comparison
Please note this is a comparison between Version 1 by Cuijuan Zhang and Version 2 by Camila Xu.

Integrated CO2 capture and utilization aims to capture CO2 from gas streams and other emission sources and convert it into valuable chemicals or energy sources. The key is to find the match between the CO2 separation process and the CO2 utilization process, including the temperature and pressure, etc. CO2 can be captured via physical or chemical absorption depending on the interaction between CO2 and the absorbent. 

  • carbon neutrality
  • CO2 capture
  • CO2 conversion
  • integration

1. Introduction

Carbon dioxide capture, utilization, and storage (CCUS) are increasingly gaining global attention. The challenge is to meet the energy demand while balancing CO2 emissions. Several solutions have been proposed to reduce the CO2 emission, namely, increasing the utilization of eco-friendly energy sources, such as wind and solar energy, to improve the energy efficiency. However, the advancement of such technologies is currently still limited and can be an optimal option in the long-term. At present times, the CCUS technology is more effective and can be a short-term alternative.
CO2 can be captured using various technologies, such as amine absorption, porous materials adsorption and membrane separation. Since CO2 itself is a carbon source, it can be converted into valuable chemicals via dry methane reforming (DMR), CO2 hydrogenation, reverse water–gas shift reaction, etc. There are excellent reviews on both the technologies [1][2][3][1,2,3]. However, the traditional CO2 capture and utilization processes are separated, which inevitably increases the transportation cost. To reduce or even eliminate such cost, the integration of CO2 capture and utilization is greatly desirable.

2. Integration of CO2 Capture and Utilization

Integrated CO2 capture and utilization aims to capture CO2 from gas streams and other emission sources and convert it into valuable chemicals or energy sources. The key is to find the match between the CO2 separation process and the CO2 utilization process, including the temperature and pressure, etc.

2.1. Integration of CO

2

Absorption and Conversion

CO2 can be captured via physical or chemical absorption depending on the interaction between CO2 and the absorbent. The former mainly utilizes the solubility of each gas component in the solvent whereas the latter involves chemical reactions between CO2 and the absorbent. Chemical absorption is mostly employed with organic amine, hot potash, and liquid ammonia solvents considering its easy operation and mild working conditions [4]. Accordingly, it can be integrated with CO2 hydrogenation for the production of formic acid or methanol. Amine-based CO2 capture and conversion integrated reactors consist of a series of amine sorbents and metal ions that form a pincer complex, such as pincerpentaethylenehexamine (PEHA), pyrrolizidine, N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDTA) and polyethyleneimine (PEI). These are coupled with metal pincer-based homogeneous catalysts [5][6][7][8][5,6,7,8]. Due to the presence of multiple amine sites [9], PEI can be used as a superior CO2 absorber [10], which can absorb both high and low concentrations of CO2 [11]. Li et al. [12] used PEI/RhCl3·3H2O/CyPPh2 to capture CO2 and convert it in situ (Figure 1a). PEI absorbs CO2 from air in an ethylene glycol solution containing PEI and converts it into amino formate esters and alkyl carbonate esters. CO2 is hydrogenated in situ to form formate salts (TON = 260) in the presence of RhCl3-3H2O/CyPPh2, demonstrating the first in situ CO2 capture and conversion to formate. Multifunctional materials can be synthesized by modifying the PEI backbone with iminophosphine ligand functionality and subsequently metallizing it with Ir precursors. About 65% of the available primary amines on PEI can be modified to form iminophosphine/Ir (PN/Ir) for balanced CO2 capture and conversion, resulting in higher formic acid yields. PEI with relatively lower molecular weight has better CO2 capture ability and catalytic activity (Figure 1b) [13]. Kothandaraman et al. [14] reported, for the first time, a green and simplified method for in situ conversion of captured CO2 to formate in aqueous media in the presence of Ru- and Fe-based pincer complexes without excess alkali, with yields of up to 95% of the formate.
Figure 1. (a) Proposed pathways of carbon capture and subsequent hydrogenation of the captured CO2 [12]; (b) Formic acid yields in the hydrogenation of CO2 catalyzed by the PEI–PN/Ir materials as a function of temperature and MW of PEI for PEI600–PN/Ir (▪), PEI1800–PN/Ir (•), and PEI25 000–PN/Ir (▴) [13]. (c) Multiple recycling of the catalyst in biphasic reaction mixture. Yield (%) of formate is relative to the amount of CO2 captured. Ru–PNP 1: Cat 1 = 10 μmol, T = 55 °C, H2 = 50 bar, 17.2 mmol diazabicyclo[2.2.2]octane (DABCO) + 3 mL H2O (CO2 captured each cycle = 15 mmol), 3 mL additional H2O–4 mL 2-Methyltetrahydrofuran (2–MeTHF) added for hydrogenation study. Fe–PNP 4: Cat 4 = 20 μmol, T = 55 °C, H2 = 50 bar, 17.2 mmol DABCO + 3 mL H2O (CO2 captured each cycle = 15 mmol), 3 mL additional H2O–4 mL 2–MeTHF added for hydrogenation study [14]. (d) Proposed reaction sequence for CO2 capture and in situ hydrogenation to CH3OH using a polyamine [5].
In addition to formate, the captured CO2 can be converted into methanol. In 2015, Rezayee et al. [15] prepared methanol by tandem CO2 capture and in situ conversion of dimethylamine with homogeneous ruthenium complexes under basic conditions. Dimethylamine can react with CO2 and inhibit the formation of formic acid. The conversion of CO2 is >95% (Figure 1c). Moreover, Kothandaraman et al. [5] first proposed and demonstrated a system for capturing CO2 from air (~400 ppm) and converting it in situ (Figure 1d), which consisted of PEHA and Ru–PNP complex, with a methanol yield of 79%. Integrated systems for CO2 capture and conversion into methanol are still uncommon. The major obstacle is the harsh reaction conditions to produce methanol, which involves a high-pressure gas-phase catalyst reactor at relatively low temperatures of 200–300 °C and high pressures of 50–100 atm. The amine-based materials offer the potential to capture and convert CO2 under mild conditions. Due to the relatively high CO2 capture capacity of PEI, it can directly capture CO2 from the air, which also removes the limitation of constructing CO2 capture equipment. However, the high selectivity of some materials to carbon dioxide poses a challenge for the regeneration of amines. Furthermore, the toxicity and corrosiveness of amine solvents limit their industrial application. To drive future research, it is essential to explore systems that are highly efficient and recyclable, enabling CCU to establish a reliable foundation for industrial applications.

2.2. CO

2

Adsorption and Conversion Integration

Similar to absorption, adsorption separation can also be divided into chemisorption and physisorption according to the interaction between CO2 and the adsorbate, with the former forming covalent bonds whilst the latter forms bond with electrostatic attraction and van der Waals forces.  Porous organic polymers (POPs) are a series of new two- or three-dimensional networked polymeric materials formed by covalent bonding of organic small molecule substrates through specific chemical reactions, usually with microporous, mesoporous or multistage pore structures. They have promising applications in separation, sensor and catalysis [16]. The ionic porous organic polymers (IPOPs) are generally classified as porous organic materials, whose backbone typically includes anions or cations. They can be divided into IPOPs with cationic moieties, IPOPs with anionic moieties, and IPOPs with zwitterionic moieties. Common cationic moieties include imidazolium, pyridinium, viologen, and quaternary phosphonium, whereas more common anionic moieties include tetrakis(phenyl)borate and tris(catecholate) phosphate. The inclusion of these ionic moieties in porous materials can enhance their CO2 capture capacity and catalyze the in situ CO2 conversion. In 2011, tetrakis(4-ethynylphenyl)methane and diiodoimidazolium salts were used to prepare tubular microporous organic via Sonogashira coupling reaction networks bearing imidazolium salts (T-IM) (Figure 2a). The material, with a microporous structure of specific surface area (620 m2 g−1), shows good catalytic activity towards the conversion of CO2 to cyclic carbonates [17]. Wang et al. [18] utilized the Friedel–Crafts reaction to synthesize imidazole-based IPOPs. The specific surface area can reach up to 926 m2 g−1; however, its CO2 capture capacity is only 14.2 wt%. Nevertheless, the polymer exhibits good stability and repeatability. Sun et al. [19] demonstrated, for the first time, the capture and in situ conversion of CO2 into cyclic carbonates under relatively mild room temperature conditions using a metal-free solvent followed by a heterogeneous catalytic system. The effect of halogen anions (Cl, Br, and I) and quaternary phosphonium cations on the catalytic activity was investigated. The catalytic activity follows the order Cl > Br > I (Figure 2b), because the rate-controlling step of the reaction is ring opening by anion attack on the epoxide [20].
Figure 2. (a) Preparation of porous organic networks bearing imidazolium salts [17]; (b) Yields of chloropropene carbonate from the cycloaddition of epichlorohydrin and CO2 catalyzed by PIPs with corresponding QPs and PIP-Me-X, PIP-Et-X, and PIP-Bn-X (X = Cl, Br, and I). Reaction conditions: epichlorohydrin (1.0 g, 10.9 mmol), catalyst (0.05 mmol, based upon the quaternary phosphonium salt), 323 K, CO2 (ambient pressure), and 24 h [19].
In addition to IPOP, the porphyrin-based organic polymer (POP-TPP) can also be used. Xiao et al. [21] synthesized a hierarchically porous organic polymer (POP-TPP) by polymerizing vinyl-functionalized tetraphenylporphyrin monomer, and then metalated it with different metals (Co3+, Zn2+, and Mg2+). The resulting heterogeneous catalysts have rich active sites and exhibit higher activity than the homogenous Co/TPP catalyst at relatively low CO2 concentrations, primarily due to the favorable enrichment of CO2 in the porous structure (micropores and nanochannels) of Co/POP-TPP. The TOF of the catalysts decreases in the following order: Co/POP-TPP (436 h−1) > Zn/POP-TPP (326 h−1) > Mg/POP-TPP (171 h−1) (Table 1). Later, a series of high surface area hollow tubular metal (Al, Co, Fe, and Mn) porphyrin-based hypercrosslinked polymers (HCP) were synthesized via Friedel–Crafts alkylation reactions. Al-HCP can catalyze the formation of propylene carbonate with a selectivity of approximately 99% after only 1 h with 2.0 mol% TBAB catalyst [22].
Table 1.
Cycloaddition of epichlorohydrin with CO
2
to form cyclic carbonate over various catalysts at 29 °C [21].