Effect of Amine Surface Coverage on the Co-Adsorption of CO2 and Water: Spectral Deconvolution of Adsorbed Species

Water Enables Efficient CO2 Capture from Natural Gas Flue Emissions in an Oxidation-Resistant Diamine-Appended Metal-Organic Framework

Supported by increasingly available reserves, natural gas is achieving greater adoption as a cleaner-burning alternative to coal in the power sector. As a result, carbon capture and sequestration from natural gas-fired power plants is an attractive strategy to mitigate global anthropogenic CO₂ emissions. However, the separation of CO₂ from other components in the flue streams of gas-fired power plants is particularly challenging due to the low CO₂ partial pressure (∼40 mbar), which necessitates that candidate separation materials bind CO₂ strongly at low partial pressures (≤4 mbar) to capture ≥90% of the emitted CO₂. High partial pressures of O₂ (120 mbar) and water (80 mbar) in these flue streams have also presented significant barriers to the deployment of new technologies for CO₂ capture from gas-fired power plants. Here, we demonstrate that functionalization of the metal-organic framework Mg₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) with the cyclic diamine 2-(aminomethyl)piperidine (2-ampd) produces an adsorbent that is capable of ≥90% CO₂ capture from a humid natural gas flue emission stream, as confirmed by breakthrough measurements. This material captures CO₂ by a cooperative mechanism that enables access to a large CO₂ cycling capacity with a small temperature swing (2.4 mmol CO₂/g with ΔT = 100 °C). Significantly, multicomponent adsorption experiments, infrared spectroscopy, magic angle spinning solid-state NMR spectroscopy, and van der Waals-corrected density functional theory studies suggest that water enhances CO₂ capture in 2-ampd-Mg₂(dobpdc) through hydrogen-bonding interactions with the carbamate groups of the ammonium carbamate chains formed upon CO₂ adsorption, thereby increasing the thermodynamic driving force for CO₂ binding. In light of the exceptional thermal and oxidative stability of 2-ampd-Mg₂(dobpdc), its high CO₂ adsorption capacity, and its high CO₂ capture rate from a simulated natural gas flue emission stream, this material is one of the most promising adsorbents to date for this important separation.

The Role of the OH Group in Citric Acid in the Coordination with Fe3O4 Nanoparticles

The role of the C?OH group in citric acid (CA) in the molecular coordination with Fe₃O₄ nanoparticles (NPs) has been elusive for a long time. In this study, attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectral deconvolution and thermogravimetric analysis (TGA) have been used to quantitatively clarify its significance in CA adsorption and its corresponding conformation. The experimental results show that the coordination and the corresponding conformation are exclusively determined by COOH not C?OH at pH 3, where its adsorption behavior conforms to the Brunauer?Emmett?Teller (BET) multilayer model with a maximal monolayer coordination number of 2.1/nm². However, C?OH is involved in the coordination at pH 10, and CA conforms to the Langmuir monolayer model with 1.4/nm² as its maximal monolayer coordination number, which is more stable than the COOH-only coordination. Especially, the conformational transformation is observed for the first time at pH 3, where the CA molecules adjust their conformation upon elution to maximize the utilization of the available binding sites on Fe₃O₄ NPs. This finding deepens the understanding on the fundamental mechanism for the interaction between the C?OH and COOH groups containing the organic ligand and metal oxide.

Non-Porous versus Mesoporous Siliceous Materials for CO2 Capture

In this study, the adsorption properties of a Stöber silica‐based material towards CO₂ were evaluated for the first time. The use of Stöber silica as support is interesting for real technological applications mainly due to economic factors. Furthermore, a direct comparison between the non porous Stöber sample with an ordered porous material (based on MCM‐41 silica) allowed to evaluate the effect of the porosity on the CO₂ adsorption properties. In order to make silica materials reactive towards CO₂, they were functionalized by introducing amino groups via grafting of 3‐[2‐(2‐aminoethyl)aminoethyl]aminopropyltrimethoxysilane. After a qualitative study of the CO₂ adsorption, the quantitative determination of CO₂ adsorption capacity at 35 °C revealed that the mesoporous material is more efficient compared to the Stöber‐based one (adsorption capacity values of 0.49 and 0.58 mol/kg for Stöber‐based and mesoporous samples). However, since the difference in the adsorption capacity is only about 15 % and the Stöber‐based sample is considerably cheaper, the non‐porous sample should be considered as a favourable adsorbent material for CO₂ capture applications.

Conformation-Dependent Coordination of Carboxylic Acids with Fe3O4 Nanoparticles Studied by ATR-FTIR Spectral Deconvolution

The coordination of valeric acid (VA), glutaric acid (GA), and tricarballylic acid (TA) with Fe-OH on the Fe₃O₄ nanoparticle surface has been systematically studied to elucidate the effects of COOH, molecular configuration, and ligand concentration on the coordination by the combined use of attenuated total reflectance Fourier transform infrared (ATR-FTIR) and thermogravimetric analysis (TGA). The results show that the binding ability of the acids increases with the increase in the COOH number. Multiple conformations coexist for the dicarboxylic and tricarboxylic acid coordinated on the iron oxide NPs. Saturated coordination formed with only a one-, two-, or three-COOH conformation for VA, GA, and TA, respectively, occurs under ligand-scarce conditions, while unsaturated coordination formed with the mixture of uncoordinated, one-, and/or two-COOH conformations for VA, GA, and TA, respectively, exists under ligand-abundant conditions. The maximum coordination numbers for monolayer adsorption for VA, GA, and TA on Fe₃O₄ NPs are 9, 2.4, and 2.7 nm^-2, respectively. This study helps us to understand the fine coordination mechanism caused by the acid molecules with different configurations and elucidates, for the first time, the fine conformational variance incurred by the surrounding ligand with different concentrations and the way in which the ligand is added.

Oxidation-Resistant, Cost-Effective Epoxide-Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air

CO₂ adsorbents based on the reaction of pentaethylenehexamine (PEHA) or tetraethylenepentamine (TEPA) with propylene oxide (PO) were easily prepared in "one pot" by impregnation on a silica support in water. The starting materials were readily available and inexpensive, facilitating the production of the adsorbents on a large scale. The prepared polyamine/epoxide adsorbents were efficient in capturing CO₂ and could be regenerated under mild conditions (50-85 °C). They displayed a much-improved stability compared with their unmodified amine counterparts, especially under oxidative conditions. Leaching of the active organic amine became minimal or nonexistent after treatment with the epoxide. The adsorption as well as desorption kinetics were also greatly improved. The polyamine/epoxide adsorbents were able to capture CO₂ from various sources including ambient air and indoor air with CO₂ concentrations of only 400-1000 ppm. The presence of water, far from being detrimental, increased the adsorption capacity. Their use for indoor air quality purposes was explored.

Unravelling the Structure of Chemisorbed CO2 Species in Mesoporous Aminosilicas: A Critical Survey

Chemisorbent materials, based on porous aminosilicas, are among the most promising adsorbents for direct air capture applications, one of the key technologies to mitigate carbon emissions. Herein, a critical survey of all reported chemisorbed CO₂ species, which may form in aminosilica surfaces, is performed by revisiting and providing new experimental proofs of assignment of the distinct CO₂ species reported thus far in the literature, highlighting controversial assignments regarding the existence of chemisorbed CO₂ species still under debate. Models of carbamic acid, alkylammonium carbamate with different conformations and hydrogen bonding arrangements were ascertained using density functional theory (DFT) methods, mainly through the comparison of the experimental ¹³C and ¹⁵N NMR chemical shifts with those obtained computationally. CO₂ models with variable number of amines and silanol groups were also evaluated to explain the effect of amine aggregation in CO₂ speciation under confinement. In addition, other less commonly studied chemisorbed CO₂ species (e.g., alkylammonium bicarbonate, ditethered carbamic acid and silylpropylcarbamate), largely due to the difficulty in obtaining spectroscopic identification for those, have also been investigated in great detail. The existence of either neutral or charged (alkylammonium siloxides) amine groups, prior to CO₂ adsorption, is also addressed. This work extends the molecular-level understanding of chemisorbed CO₂ species in amine-oxide hybrid surfaces showing the benefit of integrating spectroscopy and theoretical approaches.

Stability of amine-functionalized CO2 adsorbents: a multifaceted puzzle

This review focuses on important stability issues facing amine-functionalized CO2 adsorbents, including amine-grafted and amine-impregnated silicas, zeolites, metal-organic frameworks and carbons. During the past couple of decades, major advances were achieved in understanding and improving the performance of such materials, particularly in terms of CO2 adsorptive properties such as adsorption capacity, selectivity and kinetics. Nonetheless, to pave the way toward commercialization of adsorption-based CO2 capture technologies, in addition to other attributes, adsorbent materials should be stable over many thousands of adsorption-desorption cycles. Adsorbent stability, which is of utmost importance as it determines adsorbent lifetime and operational costs of CO2 capture, is a multifaceted issue involving thermal, hydrothermal, and chemical stability. Here we discuss the impact of the adsorbent physical and chemical properties, the feed gas composition and characteristics, and the adsorption-desorption operational parameters on the long-term stability of amine-functionalized CO2 adsorbents. We also review important insights associated with the underlying deactivation pathways of the adsorbents upon exposure to high temperature, oxygen, dry CO2, sulfur-containing compounds, nitrogen oxides, oxygen and steam. Finally, specific recommendations are provided to address outstanding stability issues.

Silica-Supported Sterically Hindered Amines for CO2 Capture

Most studies exploring the capture of CO₂ on solid-supported amines have focused on unhindered amines or alkylimine polymers. It has been observed in extensive solution studies that another class of amines, namely sterically hindered amines, can exhibit enhanced CO₂ capacity when compared to their unhindered counterparts. In contrast to solution studies, there has been limited research conducted on sterically hindered amines on solid supports. In this work, one hindered primary amine and two hindered secondary amines are grafted onto mesoporous silica at similar amine coverages, and their adsorption performances are investigated through fixed bed breakthrough experiments and thermogravimetric analysis. Furthermore, chemisorbed CO₂ species formed on the sorbents under dry and humid conditions are elucidated using in situ Fourier-transform infrared spectroscopy. Ammonium bicarbonate formation and enhancement of CO₂ adsorption capacity is observed for all supported hindered amines under humid conditions. Our experiments in this study also suggest that chemisorbed CO₂ species formed on supported hindered amines are weakly bound, which may lead to reduced energy costs associated with regeneration if such materials were deployed in a practical separation process. However, overall CO₂ uptake capacities of the solid supported hindered amines are modest compared to their solution counterparts. The oxidative and thermal stabilities of the supported hindered amine sorbents are also assessed to give insight into their operational lifetimes.

Metallic Cobalt-Carbon Composite as Recyclable and Robust Magnetic Photocatalyst for Efficient CO2 Reduction

CO2 conversion into value-added chemical fuels driven by solar energy is an intriguing approach to address the current and future demand of energy supply. Currently, most reported surface-sensitized heterogeneous photocatalysts present poor activity and selectivity under visible light irradiation. Here, photosensitized porous metallic and magnetic 1200 CoC composites (PMMCoCC-1200) are coupled with a [Ru(bpy)3 ]Cl2 photosensitizer to efficiently reduce CO2 under visible-light irradiation in a selective and sustainable way. As a result, the CO production reaches a high yield of 1258.30 µL with selectivity of 64.21% in 6 h, superior to most reported heterogeneous photocatalysts. Systematic investigation demonstrates that the central metal cobalt is the active site for activating the adsorbed CO2 molecules and the surficial graphite carbon coating on cobalt metal is crucial for transferring the electrons from the triplet metal-to-ligand charge transfer of the photosensitizer Ru(bpy)32+ , which gives rise to significant enhancement for CO2 reduction efficiency. The fast electron injection from the excited Ru(bpy)32+ to PMMCoCC-1200 and the slow backward charge recombination result in a long-lived, charge-separated state for CO2 reduction. More impressively, the long-time stability and easy magnetic recycling ability of this metallic photocatalyst offer more benefits to the photocatalytic field.

The "Missing" Bicarbonate in CO2 Chemisorption Reactions on Solid Amine Sorbents

We have identified a hydrated bicarbonate formed by chemisorption of 13CO2 on both dimethylaminopropylsilane (DMAPS) and aminopropylsilane (APS) pendant molecules grafted on SBA-15 mesoporous silica. The most commonly used sequence in solid-state NMR, 13C CPMAS, failed to detect bicarbonate in these solid amine sorbent samples; here, we have employed a Bloch decay ("pulse-acquire") sequence (with 1H decoupling) to detect such species. The water that is present contributes to the dynamic motion of the bicarbonate product, thwarting CPMAS but enabling direct 13C detection by shortening the spin-lattice relaxation time. Since solid-state NMR plays a major role in characterizing chemisorption reactions, these new insights that allow for the routine detection of previously elusive bicarbonate species (which are also challenging to observe in IR spectroscopy) represent an important advance. We note that employing this straightforward NMR technique can reveal the presence of bicarbonate that has often otherwise been overlooked, as demonstrated in APS, that has been thought to only contain adsorbed CO2 as carbamate and carbamic acid species. As in other systems (e.g., proteins), dynamic species that sample multiple environments tend to broaden as their motion is frozen out. Here, we show two distinct bicarbonate species upon freezing, and coupling to different protons is shown through preliminary 13C-1H HETCOR measurements. This work demonstrates that bicarbonates have likely been formed in the presence of water but have gone unobserved by NMR due to the nature of the experiments most routinely employed, a perspective that will transform the way the sorption community will view CO2 capture by amines.

Detecting Proton Transfer in CO2 Species Chemisorbed on Amine-Modified Mesoporous Silicas by Using 13 C NMR Chemical Shift Anisotropy and Smart Control of Amine Surface Density

The wealth of site-selective structural information on CO₂ speciation, obtained by spectroscopic techniques, is often hampered by the lack of easy-to-control synthetic routes. Herein, an alternative experimental protocol that relies on the high sensitivity of ¹³ C chemical shift anisotropy (CSA) tensors to proton transfer, is presented to unambiguously distinguish between ionic/charged and neutral CO₂ species, formed upon adsorption of ¹³ CO₂ in amine-modified porous materials. Control of the surface amine spacing was achieved through the use of amine protecting groups during functionalisation prior to CO₂ adsorption. This approach enabled the formation of either "isolated" or "paired" carbamate/carbamic acid species, providing a first experimental NMR proof towards the identification of both aggregation states. Computer modelling of surface CO₂ -amine adducts assisted the solid-state NMR assignments and validated various hydrogen-bond arrangements occurring upon formation of isolated/aggregated carbamic acid and alkylammonium carbamate ion species. This work extends the understanding of chemisorbed CO₂ structures formed at pore surfaces and reveals structural insight about the protonation source responsible for the proton-transfer mechanism in such aggregates.

Double-Layer Structured CO2 Adsorbent Functionalized with Modified Polyethyleneimine for High Physical and Chemical Stability

CO₂ capture using polyethyleneimine (PEI)-impregnated silica adsorbents has been receiving a lot of attention. However, the absence of physical stability (evaporation and leaching of amine) and chemical stability (urea formation) of the PEI-impregnated silica adsorbent has been generally established. Therefore, in this study, a double-layer impregnated structure, developed using modified PEI, is newly proposed to enhance the physical and chemical stabilities of the adsorbent. Epoxy-modified PEI and diepoxide-cross-linked PEI were impregnated via a dry impregnation method in the first and second layers, respectively. The physical stability of the double-layer structured adsorbent was noticeably enhanced when compared to the conventional adsorbents with a single layer. In addition to the enhanced physical stability, the result of simulated temperature swing adsorption cycles revealed that the double-layer structured adsorbent presented a high potential working capacity (3.5 mmol/g) and less urea formation under CO₂-rich regeneration conditions. The enhanced physical and chemical stabilities as well as the high CO₂ working capacity of the double-layer structured adsorbent were mainly attributed to the second layer consisting of diepoxide-cross-linked PEI.

Enhancement of CO2 binding and mechanical properties upon diamine functionalization of M2(dobpdc) metal-organic frameworks

The family of diamine-appended metal-organic frameworks exemplified by compounds of the type mmen-M2(dobpdc) (mmen = N,N'-dimethylethylenediamine; M = Mg, Mn, Fe, Co, Zn; dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are adsorbents with significant potential for carbon capture, due to their high working capacities and strong selectivity for CO2 that stem from a cooperative adsorption mechanism. Herein, we use first-principles density functional theory (DFT) calculations to quantitatively investigate the role of mmen ligands in dictating the framework properties. Our van der Waals-corrected DFT calculations indicate that electrostatic interactions between ammonium carbamate units significantly enhance the CO2 binding strength relative to the unfunctionalized frameworks. Additionally, our computed energetics show that mmen-M2(dobpdc) materials can selectively adsorb CO2 under humid conditions, in agreement with experimental observations. The calculations further predict an increase of 112% and 124% in the orientationally-averaged Young's modulus E and shear modulus G, respectively, for mmen-Zn2(dobpdc) compared to Zn2(dobpdc), revealing a dramatic enhancement of mechanical properties associated with diamine functionalization. Taken together, our calculations demonstrate how functionalization with mmen ligands can enhance framework gas adsorption and mechanical properties.

CO₂ Capture with Mesoporous Silicas Modified with Amines by Double Functionalization: Assessment of Adsorption/Desorption Cycles

CO₂ adsorption on mesoporous silica modified with amine by double functionalization was studied. Adsorption microcalorimetry was used in order to investigate the influence of increasing the nitrogen surface density on double functionalized materials with respect to the only grafted materials. The distribution of sites and the rate-controlling mechanism of adsorption were evaluated. A Tian Calvet microcalorimeter coupled to a manometric setup was used to evaluate the energy distribution of adsorption sites and to calculate the thermokinetic parameters from the differential enthalpy curves. CO₂ and N₂ adsorption equilibrium isotherms at 50 and 75 °C were measured with a magnetic suspension balance, allowing for the computation of working capacity and selectivity at two temperatures. With these data, an Adsorbent Performance Indicator (API) was calculated and contrasted with other studied materials under the same conditions. The high values of API and selectivity confirmed that double functionalized mesoporous silica is a promising adsorbent for the post combustion process. The adsorption microcalorimetric study suggests a change in active sites distribution as the amine density increases. Maximum thermokinetic parameter suggests that physisorption on pores is the rate-controlling binding mechanism for the double-functionalized material.

15N Solid State NMR Spectroscopic Study of Surface Amine Groups for Carbon Capture: 3-Aminopropylsilyl Grafted to SBA-15 Mesoporous Silica

Materials composed of high-porosity solid supports, such as SBA-15, containing amine-bearing moieties inside the pores, such as 3-aminopropylsilane (APS), are envisioned for carbon dioxide capture; solid-state ¹⁵N NMR can be highly informative for studying chemisorption reactions. Two ¹⁵N-enriched samples with different APS loadings were studied to probe the identity of the pendant molecules and structure of the chemisorbed CO₂ species. ¹⁵N cross-polarization magic-angle spinning NMR provides unique information about the amines, whether they are rigid or dynamic, by measuring contact time curves and rotating frame, T_1ρ(¹⁵N), relaxation. Both carbamate and carbamic acid are formed; carbamic acid is shown to be less stable than carbamate. After desorption, a steady state for the chemisorbed reaction product is reached, leaving behind carbamate. ¹⁵N NMR monitors the evolution of the species over time. During desorption, APS is regenerated, but the ammonium propylsilane intensity does not change, leading us to conclude that carbamic acid desorbs, while carbamate (to which ammonium propylsilane is ion paired) persists. A secondary ditehtered amine present does not react with CO₂, and we posit this may be due to its rigidity. These findings demonstrate the versatility of solid-state NMR to provide information about these complex CO₂ reactions with solid amine sorbents.

Overcoming double-step CO2 adsorption and minimizing water co-adsorption in bulky diamine-appended variants of Mg2(dobpdc)

Alkyldiamine-functionalized variants of the metal-organic framework Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are promising for CO2 capture applications owing to their unique step-shaped CO2 adsorption profiles resulting from the cooperative formation of ammonium carbamate chains. Primary,secondary (1°,2°) alkylethylenediamine-appended variants are of particular interest because of their low CO2 step pressures (≤1 mbar at 40 °C), minimal adsorption/desorption hysteresis, and high thermal stability. Herein, we demonstrate that further increasing the size of the alkyl group on the secondary amine affords enhanced stability against diamine volatilization, but also leads to surprising two-step CO2 adsorption/desorption profiles. This two-step behavior likely results from steric interactions between ammonium carbamate chains induced by the asymmetrical hexagonal pores of Mg2(dobpdc) and leads to decreased CO2 working capacities and increased water co-adsorption under humid conditions. To minimize these unfavorable steric interactions, we targeted diamine-appended variants of the isoreticularly expanded framework Mg2(dotpdc) (dotpdc4- = 4,4''-dioxido-[1,1':4',1''-terphenyl]-3,3''-dicarboxylate), reported here for the first time, and the previously reported isomeric framework Mg-IRMOF-74-II or Mg2(pc-dobpdc) (pc-dobpdc4- = 3,3'-dioxidobiphenyl-4,4'-dicarboxylate, pc = para-carboxylate), which, in contrast to Mg2(dobpdc), possesses uniformally hexagonal pores. By minimizing the steric interactions between ammonium carbamate chains, these frameworks enable a single CO2 adsorption/desorption step in all cases, as well as decreased water co-adsorption and increased stability to diamine loss. Functionalization of Mg2(pc-dobpdc) with large diamines such as N-(n-heptyl)ethylenediamine results in optimal adsorption behavior, highlighting the advantage of tuning both the pore shape and the diamine size for the development of new adsorbents for carbon capture applications.

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.

A Diaminopropane-Appended Metal-Organic Framework Enabling Efficient CO2 Capture from Coal Flue Gas via a Mixed Adsorption Mechanism

A new diamine-functionalized metal-organic framework comprised of 2,2-dimethyl-1,3-diaminopropane (dmpn) appended to the Mg2+ sites lining the channels of Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) is characterized for the removal of CO2 from the flue gas emissions of coal-fired power plants. Unique to members of this promising class of adsorbents, dmpn-Mg2(dobpdc) displays facile step-shaped adsorption of CO2 from coal flue gas at 40 °C and near complete CO2 desorption upon heating to 100 °C, enabling a high CO2 working capacity (2.42 mmol/g, 9.1 wt %) with a modest 60 °C temperature swing. Evaluation of the thermodynamic parameters of adsorption for dmpn-Mg2(dobpdc) suggests that the narrow temperature swing of its CO2 adsorption steps is due to the high magnitude of its differential enthalpy of adsorption (Δhads = -73 ± 1 kJ/mol), with a larger than expected entropic penalty for CO2 adsorption (Δsads = -204 ± 4 J/mol·K) positioning the step in the optimal range for carbon capture from coal flue gas. In addition, thermogravimetric analysis and breakthrough experiments indicate that, in contrast to many adsorbents, dmpn-Mg2(dobpdc) captures CO2 effectively in the presence of water and can be subjected to 1000 humid adsorption/desorption cycles with minimal degradation. Solid-state 13C NMR spectra and single-crystal X-ray diffraction structures of the Zn analogue reveal that this material adsorbs CO2 via formation of both ammonium carbamates and carbamic acid pairs, the latter of which are crystallographically verified for the first time in a porous material. Taken together, these properties render dmpn-Mg2(dobpdc) one of the most promising adsorbents for carbon capture applications.

Superior Selective CO2 Adsorption of C3N Pores: GCMC and DFT Simulations

Development of high-performance sorbents is extremely significant for CO₂ capture due to its increasing atmospheric concentration and impact on environmental degradation. In this work, we develop a new model of C₃N pores based on GCMC calculations to describe its CO₂ adsorption capacity and selectivity. Remarkably, it exhibits an outstanding CO₂ adsorption capacity and selectivity. For example, at 0.15 bar it shows exceptionally high CO₂ uptakes of 3.99 and 2.07 mmol/g with good CO₂/CO, CO₂/H₂, and CO₂/CH₄ selectivity at 300 and 350 K, separately. More importantly, this adsorbent also shows better water stability. Specifically, its CO₂ uptakes are 3.80 and 5.91 mmol/g for and 0.15 and 1 bar at 300 K with a higher water content. Furthermore, DFT calculations demonstrate that the strong interactions between C₃N pores and CO₂ molecules contribute to its impressive CO₂ uptake and selectivity, indicating that C₃N pores can be an extremely promising candidate for CO₂ capture.

Spectroscopic Characterization of Adsorbed 13CO2 on 3-Aminopropylsilyl-Modified SBA15 Mesoporous Silica

Multiple chemisorption products are found from the interaction of CO₂ with the solid-amine sorbent, 3-aminopropyl silane (APS), bound to mesoporous silica (SBA15) using solid-state NMR and FTIR spectroscopy. We employed a combination of both ¹⁵N{¹³C} rotational-echo double-resonance (REDOR) NMR and ¹³C{¹⁵N} REDOR to determine the chemical identity of these products. ¹⁵N{¹³C} REDOR measurements are consistent with a single ¹³C-¹⁵N pair and distance of 1.45 Å. In contrast, both ¹³C{¹⁵N} REDOR and ¹³C CPMAS are consistent with multiple ¹³C products. ¹³C CPMAS shows two neighboring resonances, whose chemical shifts are consistent with carbamate (at 165 ppm) and carbamic acid. The ¹³C{¹⁵N} REDOR experiments resonant at 165 ppm show an incomplete buildup of the REDOR data to ∼90% of the expected maximum. We conclude this 10% missing intensity corresponds to a ¹³C NMR species that resonates at the identical chemical shift but that is not in dipolar contact with ¹⁵N. These data are consistent with the presence of bicarbonate, HCO₃^-, since it is commonly observed at ∼165 ppm and lacks ¹⁵N for dipolar coupling.

Role of Alumina Basicity in CO2 Uptake in 3-Aminopropylsilyl-Grafted Alumina Adsorbents

Oxide-supported amine materials are widely known to be effective CO₂ sorbents under simulated flue-gas and direct-air-capture conditions. Most work has focused on amine species loaded onto porous silica supports, though potential stability advantages may be offered through the use of porous alumina supports. Unlike silica materials, which are comparably inert, porous alumina materials can be tuned to have substantial acidity and/or basicity. Owing to their amphoteric nature, alumina supports play a more active role in CO₂ sorption than silica supports, potentially directly participating in the adsorption process. In this work, primary amines associated with 3-aminopropyltriethoxysilane are grafted onto two different mesoporous alumina materials having different levels of basicity. Adsorbent materials with different amine loadings are prepared, and the CO₂ -adsorption behavior of similar amines on the two alumina supports is demonstrated to be different. At low amine loadings, the inherent properties of the support surface play a significant role, whereas at high amine loadings, when the alumina surface is effectively blocked, the sorbents prepared on the two supports behave similarly. At high amine loadings, amine-CO₂ -amine interactions are shown to dominate, leading to adsorbed species that appear similar to the species formed over silica-supported amine materials. The sorbent properties are comprehensively characterized using N₂ physisorption analysis, in situ FTIR spectroscopy, and adsorption microcalorimetry.

Elucidation of Surface Species through in Situ FTIR Spectroscopy of Carbon Dioxide Adsorption on Amine-Grafted SBA-15

The nature of the surface species formed through the adsorption of CO₂ on amine-grafted mesoporous silica is investigated through in situ FTIR spectroscopy with the aid of ¹⁵ N dynamic nuclear polarization (DNP) and ¹³ C NMR spectroscopy. Primary, secondary, and tertiary amines are functionalized onto a mesoporous SBA-15 silica. Both isotopically labeled ¹³ CO₂ and natural-abundance CO₂ are used for accurate FTIR peak assignments, which are compared with assignments reported previously. The results support the formation of monomeric and dimeric carbamic acid species on secondary amines that are stabilized differently to the monocarbamic acid species on primary amines. Furthermore, the results from isotopically labelled ¹³ CO₂ experiments suggest the existence of two carbamate species on primary amines, whereas only one species is observed predominantly on secondary amines. The analysis of the IR peak intensities and frequencies indicate that the second carbamate species on primary amines is probably more asymmetric in nature and forms in a relatively smaller amount. Only the formation of bicarbonate ions at a low concentration is observed on tertiary amines; therefore, physisorbed water on the surface plays a role in the hydrolysis of CO₂ even if water is not added intentionally and dry gases are used. This suggests that a small amount of bicarbonate ions could be expected to form on primary and secondary amines, which are more hydrophilic than tertiary amines, and these low concentration species are difficult to observe on such samples.

Adsorption Microcalorimetry of CO2 in Confined Aminopolymers

Aminopolymers confined within mesoporous supports have shown promise as materials for direct capture of CO₂ from ambient air. In spite of this, relatively little is known about the energetics of CO₂ binding in these materials, and the limited calorimetric studies published to date have focused on materials made using molecular aminosilanes rather than amine polymers. In this work, poly(ethylenimine) (PEI) is impregnated within mesoporous SBA-15, and the heats of CO₂ adsorption at 30 °C are investigated using a Tian-Calvet calorimeter with emphasis on the role of PEI loading and CO₂ pressure in the compositional region relevant to direct capture of CO₂ from ambient air. In parallel, CO₂ uptakes of these materials are measured using multiple complementary approaches, including both volumetric and gravimetric methods, and distinct changes in uptake as a function of CO₂ pressure and amine loading are observed. The CO₂ sorption behavior is directly linked to textural data describing the porosity and PEI distribution in the materials.

The influence of particle size of amino-functionalized MCM-41 silicas on CO2 adsorption

The effect of the particle size of hybrid organic–inorganic MCM-41 silicas on the CO₂ adsorption properties has been investigated.

Enhanced Adsorption Efficiency through Materials Design for Direct Air Capture over Supported Polyethylenimine

Until recently, carbon capture and sequestration (CCS) was regarded as the most promising technology to address the alarming increase in the concentration of anthropogenic CO₂ in the atmosphere. There is now an increasing interest in carbon capture and utilization (CCU). In this context, the capture of CO₂ from air is an ideal solution to supply pure CO₂ wherever it is needed. Here, we describe innovative materials for direct air capture (DAC) with unprecedented efficiency. Polyethylenimine (PEI) was supported on PME, which is an extra-large-pore silica (pore-expanded MCM-41) with its internal surfaces fully covered by a uniform layer of readily accessible C₁₆ chains from cetyltrimethylammonium (CTMA⁺ ) cations. The CTMA⁺ layer plays a key role in enhancing the amine efficiency toward dry or humid ultradilute CO₂ (400 ppm CO₂ /N₂ ) to unprecedented levels. At the same PEI content, the amine efficiency of PEI/PME was two to four times higher than that of the corresponding calcined mesoporous silica loaded with PEI or with different combinations of C₁₆ chains and PEI. Under humid conditions, the amine efficiency of 40 wt % PEI/PME reached 7.31 mmolCO2 /g_PEI , the highest ever reported for any supported PEI in the presence of 400 ppm CO₂ . Thus, amine accessibility, which reflects both the state of PEI dispersion and the adsorption efficiency, is intimately associated with the molecular design of the adsorbent.

Poly(ethylenimine)-Functionalized Monolithic Alumina Honeycomb Adsorbents for CO2 Capture from Air

The development of practical and effective gas-solid contactors is an important area in the development of CO2 capture technologies. Target CO2 capture applications, such as postcombustion carbon capture and sequestration (CCS) from power plant flue gases or CO2 extraction directly from ambient air (DAC), require high flow rates of gas to be processed at low cost. Extruded monolithic honeycomb structures, such as those employed in the catalytic converters of automobiles, have excellent potential as structured contactors for CO2 adsorption applications because of the low pressure drop imposed on fluid moving through the straight channels of such structures. Here, we report the impregnation of poly(ethylenimine) (PEI), an effective aminopolymer reported commonly for CO2 separation, into extruded monolithic alumina to form structured CO2 sorbents. These structured sorbents are first prepared on a small scale, characterized thoroughly, and compared with powder sorbents with a similar composition. Despite consistent differences observed in the filling of mesopores with PEI between the monolithic and powder sorbents, their performance in CO2 adsorption is similar across a range of PEI contents. A larger monolithic cylinder (1 inch diameter, 4 inch length) is evaluated under conditions closer to those that might be used in large-scale applications and shows a similar performance to the smaller monoliths and powders tested initially. This larger structure is evaluated over five cycles of CO2 adsorption and steam desorption and demonstrates a volumetric capacity of 350 molCO2  m-3monolith and an equilibration time of 350 min under a 0.4 m s(-1) linear flow velocity through the monolith channels using 400 ppm CO2 in N2 as the adsorption gas at 30 °C. This volumetric capacity surpasses that of a similar technology considered previously, which suggested that CO2 could be removed from air at an operating cost as low as $100 per ton.

Mesoporous Nano-Silica Serves as the Degradation Inhibitor in Polymer Dielectrics

A new generation of nano-additives for robust high performance nanodielectrics is proposed. It is demonstrated for the first time that mesoporous material could act as “degradation inhibitor” for polymer dielectrics to sequestrate the electrical degradation products then restrain the electrical aging process especially under high temperature conditions, which is superior to the existing additives of nanodielectrics except further increasing the dielectric strength. Polyethylenimine (PEI) loaded nano-scaled mesoporous silica MCM-41 (nano-MS) is doped into the dielectric matrix to prepare the PP/MCM-41-PEI nanocomposites. PEI provides the amines to capture the electrical degradation products while the MCM-41 brackets afford large adsorption surface, bring down the activating temperature of the absorbent then enhance the absorptive capacity. The electrical aging tests confirm the contribution of the mesoporous structure to electrical aging resistance and FT-IR analysis of the electrical degraded regions demonstrates the chemical absorption especially under high temperature conditions. Take the experimental data as examples, extending the aging durability and dielectric strength of polymer dielectrics by 5 times and 16%, respectively, can have substantial commercial significance in energy storage, power electronics and power transmission areas.

Probing Intramolecular versus Intermolecular CO2 Adsorption on Amine-Grafted SBA-15

A mesoporous silica SBA-15 is modified with an array of amine-containing organosilanes including (i) propylamine, SiCH2CH2CH2NH2 (MONO), (ii) propylethylenediamine, SiCH2CH2CH2NHCH2CH2NH2 (DI), (iii) propyldiethylenetriamine, SiCH2CH2CH2NHCH2CH2NHCH2CH2NH2 (TRI), and (iv) propyltriethylenetetramine, SiCH2CH2CH2NHCH2CH2N(CH2CH2NH2)2 (TREN) and the low loading silane adsorbents (∼0.45 mmol silane/g) are evaluated for their CO2 adsorption properties, with a focus on gaining insight into the propensity for intramolecular vs intermolecular CO2 adsorption. Adsorption isotherms at low CO2 coverages are measured while simultaneously recording the heat evolved via a Tian-Calvet calorimeter. The results are compared on a silane molecule efficiency basis (mol CO2 adsorbed/mol silane) to assess the potential for intramolecular CO2 adsorption, employing two amine groups in a single silane molecule. As the number of amines in the silane molecule increases (MONO < DI < TREN ∼ TRI), the silane molecule efficiency is enhanced owing to the ability to intramolecularly capture CO2. Analysis of the CO2 uptake for samples with the surface silanols removed by capping demonstrates that cooperative uptake due to amine-CO2-silanol interactions is also possible over these adsorbents and is the primary mode of sorption for the MONO material at the studied low silane loading. As the propensity for intramolecular CO2 capture increases due to the presence of multiple amines in a single silane molecule (MONO < DI < TREN ∼ TRI), the measured heat of adsorption also increases. This study of various amine-containing silanes at low coverage is the first to provide significant, direct evidence for intramolecular CO2 capture in a single silane molecule. Furthermore, it provides evidence for the relative heats of adsorption for physisorption on a silanol laden surface (ca. 37 kJ/mol), a silanol-capped surface (ca. 25 kJ/mol), via amine-CO2-silanol interactions (ca. 46 kJ/mol), and via amine-CO2-amine interactions at low surface coverages (ca. 65 kJ/mol).

Characterization of a Mixture of CO2 Adsorption Products in Hyperbranched Aminosilica Adsorbents by (13)C Solid-State NMR

Hyperbranched amine polymers (HAS) grown from the mesoporous silica SBA-15 (hereafter "SBA-15-HAS") exhibit large capacities for CO2 adsorption. We have used static in situ and magic-angle spinning (MAS) ex situ (13)C nuclear magnetic resonance (NMR) to examine the adsorption of CO2 by SBA-15-HAS. (13)C NMR distinguishes the signal of gas-phase (13)CO2 from that of the chemisorbed species. HAS polymers possess primary, secondary, and tertiary amines, leading to multiple chemisorption reaction outcomes, including carbamate (RnNCOO(-)), carbamic acid (RnNCOOH), and bicarbonate (HCO3(-)) moieties. Carbamates and bicarbonate fall within a small (13)C chemical shift range (162-166 ppm), and a mixture was observed including carbamic acid and carbamate, the former disappearing upon evacuation of the sample. By examining the (13)C-(14)N dipolar coupling through low-field (B0 = 3 T) (13)C{(1)H} cross-polarization MAS NMR, carbamate is confirmed through splitting of the (13)C resonance. A third species that is either bicarbonate or a second carbamate is evident from bimodal T2 decay times of the ∼163 ppm peak, indicating the presence of two species comprising that single resonance. The mixture of products suggests that (1) the presence of amines and water leads to bicarbonate being present and/or (2) the multiple types of amine sites in HAS permit formation of chemically distinct carbamates.

Probing the Role of Zr Addition versus Textural Properties in Enhancement of CO₂ Adsorption Performance in Silica/PEI Composite Sorbents

Polymeric amines such as poly(ethylenimine) (PEI) supported on mesoporous oxides are promising candidate adsorbents for CO2 capture processes. An important aspect to the design and optimization of these materials is a fundamental understanding of how the properties of the oxide support such as pore structure, particle morphology, and surface properties affect the efficiency of the guest polymer in its interactions with CO2. Previously, the efficiency of impregnated PEI to adsorb CO2 was shown to increase upon the addition of Zr as a surface modifier in SBA-15. However, the efficacy of this method to tune the adsorption performance has not been explored in materials of differing textural and morphological nature. Here, these issues are directly addressed via the preparation of an array of SBA-15 support materials with varying textural and morphological properties, as well as varying content of zirconium doped into the material. Zirconium is incorporated into the SBA-15 either during the synthesis of the SBA-15, or postsynthetically via deposition of Zr species onto pure-silica SBA-15. The method of Zr incorporation alters the textural and morphological properties of the parent SBA-15 in different ways. Importantly, the CO2 capacity of SBA-15 impregnated with PEI increases by a maximum of ∼60% with the quantity of doped Zr for a "standard" SBA-15 containing significant microporosity, while no increase in the CO2 capacity is observed upon Zr incorporation for an SBA-15 with reduced microporosity and a larger pore size, pore volume, and particle size. Finally, adsorbents supported on SBA-15 with controlled particle morphology show only modest increases in CO2 capacity upon inclusion of Zr to the silica framework. The data demonstrate that the textural and morphological properties of the support have a more significant impact on the ability of PEI to capture CO2 than the support surface composition.