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.

CO2 Capture by Diamines in Dry and Humid Conditions: A Theoretical Approach

This work is a mechanistic study of the CO2 reaction with diamines under both dry and wet conditions. All protic α,ω-diamines R1H1N1-(CH2)n-N2H2R2, with n = 1-5 and R1 and R2 = H and/or CH3, were investigated. Depending on the nature of the diamine, the reaction was found to follow one of two concerted asynchronous reaction mechanisms with a zwitterion hidden intermediate. Both mechanisms involved two processes. The first process consisted of a nucleophilic attack of the nitrogen N1 of the first amine group on the carbon of CO2, accompanied by the transfer of a hydrogen atom H1 from N1 to the nitrogen N2 of the second amine group, leading to the formation of a carbamate zwitterion. The subsequent process corresponds to the transfer of a hydrogen atom H2 from the second amine group N2 to an oxygen atom of CO2, thus ending the reaction by the formation of carbamic acid. The structure of the zwitterion hidden intermediate was determined using the reactive internal reaction coordinates (RIRC), a reaction pathway visualization tool, consisting of a 3D representation of the potential energy versus the internuclear distances N2-H1 and N2-H2, which correspond to the bond being formed and the bond being broken, respectively. The life span of the transitory species, i.e., the zwitterion, was found to depend on the nature of the second amine group. For primary amines, the life span of the zwitterion was "short", whereas for secondary amines, it was "long". The corresponding mechanisms were termed the "early" and "late" asynchronous mechanism, respectively. Regardless of the mechanism, the activation barriers were found to decrease with the length of the carbon chain linking the two amine groups, with an asymptotic behavior from n = 4. Involvement of a water molecule generates a significant catalytic effect for diamines with short carbon chains (n < 4), whereas for longer chain diamines, water has a slightly adverse effect.

Non-solvent post-modifications with volatile reagents for remarkably porous ketone functionalized polymers of intrinsic microporosity

Chemical modifications of porous materials almost always result in loss of structural integrity, porosity, solubility, or stability. Previous attempts, so far, have not allowed any promising trend to unravel, perhaps because of the complexity of porous network frameworks. But the soluble porous polymers, the polymers of intrinsic microporosity, provide an excellent platform to develop a universal strategy for effective modification of functional groups for current demands in advanced applications. Here, we report complete transformation of PIM-1 nitriles into four previously inaccessible functional groups - ketones, alcohols, imines, and hydrazones - in a single step using volatile reagents and through a counter-intuitive non-solvent approach that enables surface area preservation. The modifications are simple, scalable, reproducible, and give record surface areas for modified PIM-1s despite at times having to pass up to two consecutive post-synthetic transformations. This unconventional dual-mode strategy offers valuable directions for chemical modification of porous materials.

Systematic Modulation of Thiol Functionalities in Inexpensive Porous Polymers for Effective Mercury Removal

Through accumulation, mercury contamination in aquatic systems still poses serious health risks despite the strict regulations on drinking water and industrial discharge. One effective strategy against this is adsorptive removal, in which a suitably functionalized porous material is added to water treatment protocols. Thiol (SH) group-grafted structures perform commendably; however, insufficient attention is paid to the cost, scalability, and reusability or how the arrangement of sulfur atoms could affect the Hg^II binding strength. We used an inexpensive and scalable porous covalent organic polymer (COP-130) to systematically introduce thiol functional groups with precise chain lengths and sulfur content. Thiol-functionalized COP-130 demonstrates enhanced wettability and excellent Hg^II uptake of up to 936 mg g^-1 , with fast kinetics and exceptionally high selectivity. These Hg adsorbents are easily regenerated with HCl and can be used at least six times without loss of capacity even after treatment with strong acid, a rare performance in the domain of Hg-removal research.

A Two Step Postsynthetic Modification Strategy: Appending Short Chain Polyamines to Zn-NH2-BDC MOF for Enhanced CO2 Adsorption

Functionalizing metal-organic frameworks (MOFs) with amines is a commonly used strategy to enhance their performance in CO₂ capture applications. As such, in this work, a two-step strategy to covalently functionalize NH₂-containing MOFs with short chain polyamines was developed. In the first step, the parent MOF, Zn₄O(NH₂-BDC)₃, was exposed to bromoacetyl bromide (BrAcBr), which readily reacts with pendant -NH₂ groups on the 2-amino-1,4-benzenedicarboxylate (NH₂-BDC^2-) ligand. ¹H NMR of the digested MOF sample revealed that as much as 90% of the MOF ligands could be functionalized in the first step. Next, the MOF samples 60% of the ligands functionalized with acetyl bromide, Zn₄O(NH₂-BDC)_1.2(BrAcNH-BDC)_1.8, was exposed to several short chain amines including ethylenediamine (ED), diethylenetriamine (DETA), and tris(2-aminoethyl)amine (TAEA). Subsequent digested ¹H NMR analysis indicated that a total of 30%, 28%, and 19% of the MOF ligands were successfully grafted to ED, DETA, and TAEA, respectively. Next, the CO₂ adsorption properties of the amine grafted MOFs were studied. The best performing material, TAEA-appended-Zn₄O(NH₂-BDC)_1.2(BrAcNH-BDC)_1.8, exhibits a zero-coverage isosteric heat of CO₂ adsorption of -62.5 kJ/mol, a value that is considerably higher than the one observed for the parent framework, -21 kJ/mol. Although the boosted CO₂ affinity only leads to a slight increase in the CO₂ adsorption capacity in the low-pressure regime (0.15 bar), which is of interest in postcombustion carbon dioxide capture, the CO₂/N₂ (15/85) selectivity at 313 K is 143, a value that is ∼35 times higher than the one observed for Zn₄O(NH₂-BDC)₃, 4.1. Such enhancements are attributed to accessible primary amines, which were grafted to the MOF ligand. This hypothesis was further supported via in situ DRIFTS measurements of TAEA-Ac-Zn₄O(NH₂-BDC)_1.2(BrAcNH-BDC)_1.8 after exposure to CO₂, which revealed the chemisorption of CO₂ via the formation of hydrogen bonded carbamates/carbamic acid and CO₂^δ- species; the latter are adducts formed between CO₂ and [amineH]⁺Br^- salts that are produced during the amine grafting step.

Covalent Amine Tethering on Ketone Modified Porous Organic Polymers for Enhanced CO2 Capture

Effective removal of excess greenhouse gas CO₂ necessitates new adsorbents that can overcome the shortcomings of the current capture methods. To achieve that, porous materials are often modified post-synthetically with reactive amine functionalities but suffer from significant surface area losses. Herein, we report a successful amine post-functionalization of a highly porous covalent organic polymer, COP-130, without losing much porosity. By varying the amine substituents, we recorded a remarkable increase in CO₂ uptake and selectivity. Ketone functionality, a rarely accessible functional group for porous polymers, was inserted prior to amination and led to covalent tethering of amines. Interestingly, aminated polymers demonstrated relatively low heats of adsorption, which is useful for the rapid recyclability of materials, due to the formation of suspected intramolecular hydrogen bonding.

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.

Sequential Ammonia and Carbon Dioxide Adsorption on Pyrolyzed Biomass to Recover Waste Stream Nutrients

The amine-rich surfaces of pyrolyzed human solid waste (py-HSW) can be "primed" or "regenerated" with carbon dioxide (CO₂) to enhance their adsorption of ammonia (NH₃) for use as a soil amendment. To better understand the mechanism by which CO₂ exposure facilitates NH₃ adsorption to py-HSW, we artificially enriched a model sorbent, pyrolyzed, oxidized wood (py-ox wood) with amine functional groups through exposure to NH₃. We then exposed these N-enriched materials to CO₂ and then resorbed NH₃. The high heat of CO₂ adsorption (Q _st) on py-HSW, 49 kJ mol^-1, at low surface coverage, 0.4 mmol CO₂ g^-1, showed that the naturally occurring N compounds in py-HSW have a high affinity for CO₂. The Q _st of CO₂ on py-ox wood also increased after exposure to NH₃, reaching 50 kJ mol^-1 at 0.7 mmol CO₂ g^-1, demonstrating that the incorporation of N-rich functional groups by NH₃ adsorption is favorable for CO₂ uptake. Adsorption kinetics of py-ox wood revealed continued, albeit diminishing NH₃ uptake after each CO₂ treatment, averaging 5.9 mmol NH₃ g^-1 for the first NH₃ exposure event and 3.5 and 2.9 mmol NH₃ g^-1 for the second and third; the electrophilic character of CO₂ serves as a Lewis acid, enhancing surface affinity for NH₃ uptake. Furthermore, penetration of ¹⁵NH₃ and ¹³CO₂ measured by NanoSIMS reached over 7 μm deep into both materials, explaining the large NH₃ capture. We expected similar NH₃ uptake in py-HSW sorbed with CO₂ and py-ox wood because both materials, py-HSW and py-ox wood sorbed with NH₃, had similar N contents and similarly high CO₂ uptake. Yet NH₃ sorption in py-HSW was unexpectedly low, apparently from potassium (K) bicarbonate precipitation, reducing interactions between NH₃ and sorbed CO₂; 2-fold greater surface K in py-HSW was detected after exposure to CO₂ and NH₃ than before gas exposure. We show that amine-rich pyrolyzed waste materials have high CO₂ affinity, which facilitates NH₃ uptake. However, high ash contents as found in py-HSW hinder this mechanism.

Microporous Polymer Networks for Carbon Capture Applications

A new generation of porous polymer networks has been obtained in quantitative yield by reacting two rigid trifunctional aromatic monomers (1,3,5-triphenylbenzene and triptycene) with two ketones having electron-withdrawing groups (trifluoroacetophenone and isatin) in superacidic media. The resulting amorphous networks are microporous materials, with moderate Brunauer-Emmett-Teller surface areas (from 580 to 790 m² g^-1), and have high thermal stability. In particular, isatin yields networks with a very high narrow microporosity contribution, 82% for triptycene and 64% for 1,3,5-triphenylbenzene. The existence of favorable interactions between lactams and CO₂ molecules has been stated. The materials show excellent CO₂ uptakes (up to 207 mg g^-1 at 0 °C/1 bar) and can be regenerated by vacuum, without heating. Under postcombustion conditions, their CO₂/N₂ selectivities are comparable to those of other organic porous networks. Because of the easily scalable synthetic method and their favorable characteristics, these materials are very promising as industrial adsorbents.

Highly Efficient Catalytic Cyclic Carbonate Formation by Pyridyl Salicylimines

Cyclic carbonates as industrial commodities offer a viable nonredox carbon dioxide fixation, and suitable heterogeneous catalysts are vital for their widespread implementation. Here, we report a highly efficient heterogeneous catalyst for CO₂ addition to epoxides based on a newly identified active catalytic pocket consisting of pyridine, imine, and phenol moieties. The polymeric, metal-free catalyst derived from this active site converts less-reactive styrene oxide under atmospheric pressure in quantitative yield and selectivity to the corresponding carbonate. The catalyst does not need additives, solvents, metals, or co-catalysts, can be reused at least 10 cycles without the loss of activity, and scaled up easily to a kilogram scale. Density functional theory calculations reveal that the nucleophilicity of pyridine base gets stronger due to the conjugated imines and H-bonding from phenol accelerates the reaction forward by stabilizing the intermediate.

A catalytic role of surface silanol groups in CO2 capture on the amine-anchored silica support

A new mechanism of CO₂ capture on the amine-functionalized silica support is demonstrated using density functional theory calculations, in which the silica surface not only acts as a support to anchor amines, but also can actively participate in the CO₂ capture process through a facile proton transfer reaction with the amine groups.

Insights into the Hydrothermal Stability of Triamine-Functionalized SBA-15 Silica for CO2 Adsorption

The hydrothermal stability of triamine-grafted, large-pore SBA-15 CO₂ adsorbents was studied by using steam stripping. Following two 3 h cycles of steam regeneration, lower CO₂ uptakes, lower CO₂ /N ratios, and slower adsorption kinetics were observed relative to fresh samples, particularly at the lowest adsorption temperature (25 °C). CO₂ adsorption measurements for a selected sample exposed to 48 h of steam stripping depicted that after the initial loss during the first exposure to steam (3-6 h), the adsorptive properties stabilized. For higher adsorption temperatures (i.e., 50 and 75 °C), however, all adsorptive properties remained almost unchanged after steaming, indicating the significance of diffusional limitations. Thermogravimetric analysis and FTIR spectroscopy on grafted samples before and after steam stripping showed no amine leaching and no change in the chemical nature of the amine groups, respectively. Also, a six-cycle CO₂ adsorption/desorption experiment under dry conditions showed no thermal degradation. However, N₂ adsorption measurement at 77 K showed significant reductions in the BET surface area of the grafted samples following steaming. Based on the pore size distribution of calcined, grafted samples before and after steaming, it is proposed that exposure to steam restructured the grafted materials, causing mass transfer resistance. It is inferred that triamine-grafted, large-pore SBA-15 adsorbents are potential candidates for CO₂ capture at relatively high temperatures (50-75 °C; for example, flue gas) combined with steam regeneration.

Direct Access to Primary Amines and Particle Morphology Control in Nanoporous CO2 Sorbents

Chemical tuning of nanoporous, solid sorbents for ideal CO₂ binding requires unhindered amine functional groups on the pore walls. Although common for soluble organics, post-synthetic reduction of nitriles in porous networks often fails due to insufficient and irreversible metal hydride penetration. In this study, a nanoporous network with pendant nitrile groups, microsphere morphology was synthesized in large scale. The hollow microspheres were easily decorated with primary amines through in situ reduction by widely available boranes. The CO₂ capture capacity of the modified sorbent was increased to up to four times that of the starting nanoporous network with a high heat of adsorption (98 kJ mol^-1 ). The surface area can be easily tuned between 1 and 354 m²  g^-1 . The average particle size (ca. 50 μm) is also quite suitable for CO₂ capture applications, such as those with fluidized beds requiring spheres of micron sizes.

Robust C-C bonded porous networks with chemically designed functionalities for improved CO2 capture from flue gas

Effective carbon dioxide (CO₂) capture requires solid, porous sorbents with chemically and thermally stable frameworks. Herein, we report two new carbon-carbon bonded porous networks that were synthesized through metal-free Knoevenagel nitrile-aldol condensation, namely the covalent organic polymer, COP-156 and 157. COP-156, due to high specific surface area (650 m²/g) and easily interchangeable nitrile groups, was modified post-synthetically into free amine- or amidoxime-containing networks. The modified COP-156-amine showed fast and increased CO₂ uptake under simulated moist flue gas conditions compared to the starting network and usual industrial CO₂ solvents, reaching up to 7.8 wt % uptake at 40 °C.

Incorporation of Alkylamine into Metal-Organic Frameworks through a Brønsted Acid-Base Reaction for CO2 Capture

The escalating atmospheric CO₂ concentration is one of the most urgent environmental concerns of our age. To effectively capture CO₂ , various materials have been studied. Among them, alkylamine-modified metal-organic frameworks (MOFs) are considered to be promising candidates. In most cases, alkylamine molecules are integrated into MOFs through the coordination bonds formed between open metal sites (OMSs) and amine groups. Thus, the alkylamine density, as well as the corresponding CO₂ uptake in MOFs, are severely restricted by the density of OMSs. To overcome this limit, other approaches to incorporating alkylamine into MOFs are highly desired. We have developed a new method based on Brønsted acid-base reaction to tether alkylamines into Cr-MIL-101-SO₃ H for CO₂ capture. A systematic optimization of the amine tethering process was also conducted to maximize the CO₂ uptake of the modified MOF. Under the optimal amine tethering condition, the obtained tris(2-aminoethyl)amine-functionalized Cr-MIL-101-SO₃ H (Cr-MIL-101-SO₃ H-TAEA) has a cyclic CO₂ uptake of 2.28 mmol g^-1 at 150 mbar and 40 °C, and 1.12 mmol g^-1 at 0.4 mbar and 20 °C. The low-cost starting materials and simple synthetic procedure for the preparation of Cr-MIL-101-SO₃ H-TAEA suggest that it has the potential for large-scale production and practical applications.

Enhanced Carbon Dioxide Capture from Landfill Gas Using Bifunctionalized Benzimidazole-Linked Polymers

Tuning the binding affinity of small gases and their selective uptake by porous adsorbents are vital for effective CO2 removal from gas mixtures for environmental protection and fuel upgrading. In this study, an amine-functionalized benzimidazole-linked polymer (BILP-6-NH2) was synthesized by a combination of pre- and postsynthetic modification techniques in two steps. Presynthetic incorporation of nitro groups resulted in stoichiometric functionalization (1 nitro/phenyl) in addition to noninvasive functionalization, where more than 80% of the surface area maintained compared to BILP-6. Experimental studies presented enhanced CO2 uptake and CO2/CH4 selectivity in BILP-6-NH2 compared to BILP-6, which are governed by the synergetic effect of benzimidazole and amine moieties. DFT calculations were used to understand the interaction modes of CO2 with BILP-6-NH2 and confirmed the efficacy of amine groups. Encouraged by the enhanced uptake and selectivity in BILP-6-NH2, we have evaluated its performance in landfill gas separation under vacuum swing adsorption (VSA) settings, which resulted in very promising working capacity and sorbent selection parameters outperforming most of the best solid adsorbent in the literature.