Amine Chemistry of Porous CO2 Adsorbents

ConspectusAs renewable energy and CO2 utilization technologies progress to make a more significant contribution to global emissions reduction, carbon capture remains a critical component of the mission. Current CO2 capture technologies involve operations at point sources such as fossil fuel-based power plants or source-agnostic like in direct air capture. Each strategy has its own advantages and limitations, but in common, they all employ sorption-based methods with the use of sorbents strongly adhering to CO2. Amine solutions are the most widely used absorbents for industrial operations due to the robust chemical bonds formed between amines and CO2 under both dry and humid conditions, rendering excellent selectivity. Such strong binding, however, causes problematic regeneration. In contrast, purely physisorptive porous materials with high surface areas allow for the confinement of CO2 inside narrow pores/channels and have a lower regeneration energy demand but with decreased selectivity and capacity. The most promising solution would then be the unification of both types of sorbents in one system, which could bring about a practical adsorption-desorption process. In other words, the development of porous solid materials with tunable amine content is necessary to leverage the high contact surface of porous sorbents with the added ability to manipulate amine incorporation toward lower CO2 binding strength.To answer the call to uncover the most feasible amine chemistry in carbon capture, our group has devoted intense effort to the study of amine-based CO2 adsorbents for the past decade. Oriented along practicality, we put forth a principle for the design of our materials to be produced in no more than three synthetic steps with economically viable starting materials. Porous organic polymers with amine functionalities of various substitutions, meaning primary, secondary, and tertiary amines, were synthesized and studied for CO2 adsorption. Direct synthesis proved to be feasibly applicable for secondary and tertiary amine-incorporated porous polymers whereas primary-amine-based sorbents would be conveniently obtained via postsynthetic modifications. Sorbents based on tertiary amines exhibit purely physical adsorption behavior if the nitrogen atoms are placed adjacent to aromatic cores due to the conjugation effect that reduces the electron density of the amine. However, when such conjugation is inhibited, chemisorptive activity is observed. Secondary amine adsorbents, in turn, express a higher binding strength than tertiary amine counterparts, but both types can merit a strengthened binding by the physical impregnation of small-molecule amines. Sorbents with primary-amine tethers can be obtained via postsynthetic transformation of precursor functionalities, and for them, chemical adsorption is mainly at work. We conclude that mixed-amine systems could exhibit unprecedented binding mechanisms, resulting in exceptionally specific interactions that would be useful for the development of highly selective sorbents for CO2.

Theoretical Assessment of Carbon Dioxide Reactivity in Methylpiperidines: A Conformational Investigation

In this work, the possible mechanisms for the reactions of CO₂ with various positional isomers of methylpiperidines (MPs) (N-MP, 2-MP, 3-MP, and 4-MP) including the effect of aqueous solvation have been explored using quantum chemical methods. The major pathways investigated for CO₂ capture in aqueous amines are carbamate formation, its hydrolysis, and the bicarbonate formation (CO₂ + H₂O + MP) reaction. The calculations indicate that an axial orientation for the methyl group and an equatorial for the COO^- group could be energetically ideal in the carbamate product of MPs. The proton abstraction step in the carbamate pathway is almost barrierless for the zwitterion-amine route, while a much higher energy barrier is observed for the zwitterion-H₂O route. During carbamate hydrolysis, the addition of even two explicit water molecules does not exhibit any notable effect on the already high energy barrier associated with this reaction. This indicates that bicarbonate formation is less likely to occur via carbamate hydrolysis. The calculations suggest that, although the carbamate pathway is kinetically favored, the MP carbamate could still be a minor product, especially for sterically hindered conformations, and the bicarbonate pathway should be predominant in aqueous MPs.

Prospects for Simultaneously Capturing Carbon Dioxide and Harvesting Water from Air

Mitigation of anthropogenic climate change is expected to require large-scale deployment of carbon dioxide removal strategies. Prominent among these strategies is direct air capture with sequestration (DACS), which encompasses the removal and long-term storage of atmospheric CO₂  by purely engineered means. Because it does not require arable land or copious amounts of freshwater, DACS is already attractive in the context of sustainable development, but opportunities to improve its sustainability still exist. Leveraging differences in the chemistry of CO₂  and water adsorption within porous solids, here, the prospect of simultaneously removing water alongside CO₂  in direct air capture operations is investigated. In many cases, the co-adsorbed water can be desorbed separately from chemisorbed CO₂  molecules, enabling efficient harvesting of water from air. Depending upon the material employed and process conditions, the desorbed water can be of sufficiently high purity for industrial, agricultural, or potable use and can thus improve regional water security. Additionally, the recovered water can offset a portion of the costs associated with DACS. In this Perspective, molecular- and process-level insights are combined to identify routes toward realizing this nascent yet enticing concept.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

A Unified Approach to CO2-Amine Reaction Mechanisms

A unified CO₂-amine reaction mechanism applicable to absorption in aqueous or nonaqueous solutions and to adsorption on immobilized amines in the presence of both dry and humid conditions is proposed. Key findings supported by theoretical calculations and experimental evidence are as follows: (1) The formation of the 1,3-zwitterion, RH₂N⁺-COO^-, is highly unlikely because not only the associated four-membered mechanism has a high energy barrier, but also it is not consistent with the orbital symmetry requirements for chemical reactions. (2) The nucleophilic attack of CO₂ by amines requires the catalytic assistance of a Bro̷nsted base through a six-membered mechanism to achieve proton transfer/exchange. An important consequence of this concerted mechanism is that the N and H atoms added to the C=O double bond do not originate from a single amine group. Using ethylenediamine for illustration, detailed description of the reaction pathway is reported using the reactive internal reaction coordinate as a new tool to visualize the reaction path. (3) In the presence of protic amines, the formation of ammonium bicarbonate/carbonate does not take place through the widely accepted hydration of carbamate/carbamic acid. Instead, water behaves as a nucleophile that attacks CO₂ with catalytic assistance by amine groups, and carbamate/carbamic acid decomposes back to amine and CO₂. (4) Generalization of the catalytic assistance concept to any Bro̷nsted base established through theoretical calculations was supported by infrared measurements. A unified six-membered mechanism was proposed to describe all possible interactions of CO₂ with amines and water, each playing the role of a nucleophile and/or Bro̷nsted base, depending on the actual conditions.