Understanding the Impacts of Support-Polymer Interactions on the Dynamics of Poly(ethyleneimine) Confined in Mesoporous SBA-15

Atom-level interaction design between amines and support for achieving efficient and stable CO2 capture

Amine-functionalized adsorbents offer substantial potential for CO2 capture owing to their selectivity and diverse application scenarios. However, their effectiveness is hindered by low efficiency and unstable cyclic performance. Here we introduce an amine-support system designed to achieve efficient and stable CO2 capture. Through atom-level design, each polyethyleneimine (PEI) molecule is precisely impregnated into the cage-like pore of MIL-101(Cr), forming stable composites via strong coordination with unsaturated Cr acid sites within the crystal lattice. The resulting adsorbent demonstrates a low regeneration energy (39.6 kJ/molCO2), excellent cyclic stability (0.18% decay per cycle under dry CO2 regeneration), high CO2 adsorption capacity (4.0 mmol/g), and rapid adsorption kinetics (15 min for saturation at 30 °C). These properties stem from the unique electron-level interaction between the amine and the support, effectively preventing carbamate products' dehydration. This work presents a feasible and promising cost-effective and sustainable CO2 capture strategy.

Unravelling the structure of CO2 in silica adsorbents: an NMR and computational perspective

This comprehensive review describes recent advancements in the use of solid-state NMR-assisted methods and computational modeling strategies to unravel gas adsorption mechanisms and CO2 speciation in porous CO2-adsorbent silica materials at the atomic scale. This work provides new perspectives for the innovative modifications of these materials rendering them more amenable to the use of advanced NMR methods.

Minimal Kinetic Model of Direct Air Capture of CO2 by Supported Amine Sorbents in Dry and Humid Conditions

Dilute concentration (∼400 ppm) and humidity are two important factors in the direct air capture (DAC) of CO2 by supported sorbents. In this work, a minimal DAC CO2 adsorption-kinetics model was formulated for supported amine sorbents under dry and humid conditions. Our model fits well with a recent DAC experiment with supported amine sorbent in both dry and humid conditions. Temperature and flow rate effects on breakthrough curves were quantitatively captured, and increasing temperature led to faster CO2 adsorption kinetics. Moisture was shown to broaden the breakthrough curve with slower CO2 adsorption kinetics but significantly improve the uptake capacity. The present minimal model provides a versatile platform for kinetic modeling of the DAC of CO2 on supported amine and other chemisorption systems.

Alumina Incorporation in Self-Supported Poly(ethylenimine) Sorbents for Direct Air Capture

Self-supported branched poly(ethylenimine) scaffolds with ordered macropores are synthesized with and without Al2O3 powder additive by cross-linking poly(ethylenimine) (PEI) with poly(ethylene glycol) diglycidyl ether (PEGDGE) at -196 °C. The scaffolds' CO2 uptake performance is compared with a conventional sorbent, i.e., PEI impregnated on an Al2O3 support. PEI scaffolds with Al2O3 additive show narrow pore size distribution and thinner pore walls than alumina-free materials, facilitating higher CO2 uptake at conditions relevant to direct air capture. The PEI scaffold containing 6.5 wt % Al2O3 had the highest CO2 uptake of 1.23 mmol/g of sorbent under 50% RH 400 ppm of CO2 conditions. In situ DRIFT spectroscopy and temperature-programmed desorption experiments show a significant CO2 uptake contribution via physisorption as well as carbamic acid formation, with lower CO2 binding energies in PEI scaffolds relative to conventional PEI sorbents, likely a result of a lower population of primary amines due to the amine cross-linking reactions during scaffold synthesis. The PEI scaffold containing 6.5 wt % Al2O3 is estimated to have the lowest desorption energy penalty under humid conditions, 4.6 GJ/tCO2, among the sorbents studied.

Hollow carbon-based materials for electrocatalytic and thermocatalytic CO2 conversion

Electrocatalytic and thermocatalytic CO2 conversions provide promising routes to realize global carbon neutrality, and the development of corresponding advanced catalysts is important but challenging. Hollow-structured carbon (HSC) materials with striking features, including unique cavity structure, good permeability, large surface area, and readily functionalizable surface, are flexible platforms for designing high-performance catalysts. In this review, the topics range from the accurate design of HSC materials to specific electrocatalytic and thermocatalytic CO2 conversion applications, aiming to address the drawbacks of conventional catalysts, such as sluggish reaction kinetics, inadequate selectivity, and poor stability. Firstly, the synthetic methods of HSC, including the hard template route, soft template approach, and self-template strategy are summarized, with an evaluation of their characteristics and applicability. Subsequently, the functionalization strategies (nonmetal doping, metal single-atom anchoring, and metal nanoparticle modification) for HSC are comprehensively discussed. Lastly, the recent achievements of intriguing HSC-based materials in electrocatalytic and thermocatalytic CO2 conversion applications are presented, with a particular focus on revealing the relationship between catalyst structure and activity. We anticipate that the review can provide some ideas for designing highly active and durable catalytic systems for CO2 valorization and beyond.

The Structure-Function Relationship of Branched Polyethylenimine Impregnated over Mesoporous Carbon Aerogels: An In-Depth Thermogravimetric Insight

We present a comprehensive thermogravimetric analysis (TGA) of polyethylenimine (PEI)-impregnated resorcinol-formaldehyde (RF) aerogels. While numerous studies focus on PEI-impregnated SBA, RF materials have been less examined, despite their interest and specificities. As most articles on PEI-impregnated porous materials follow typical experimental methods defined for SBA, particularities of RF-PEI materials could remain unheeded. The design of nonisothermal TGA protocols, completed with nitrogen isotherms, based on the systematic filling of the matrix delivers a fundamental understanding of the relationship between the structure and function. This study demonstrates (i) the competition between the matrix and PEI for CO2-physisorption (φ) and CO2-chemisorption (χ), (ii) the hysteresis ([Formula: see text]) of CO2 capture at low temperature attributed to the kinetic (K) hindrance of CO2 diffusion (D) through PEI film/plugs limiting the chemisorption, and (iii) the thermodynamic (θ) equilibrium limiting the capture at high temperature. At variance with SBA-PEI materials, the first layers of PEI in RF are readily available for CO2 capture given that this matrix does not covalently bind PEI as SBA. A facile method allows the discrimination between physi- and chemisorption, exhibiting how the former decreases with PEI coverage. The CO2 capture hysteresis, while seldom introduced or discussed, underlines that the commonly accepted operating temperature of the "maximum capture" is based on an incomplete experiment. Through isotherm adsorption analysis, we correlate the evolution of this maximum to the morphological distribution of PEI. This contribution highlights the specificities of RF-PEI and the advantages of our TGA protocol in understanding the structure/function relationship of this kind of material by avoiding the typical direct applications of SBA-specific protocols. The method is straightforward, does not need large-scale facilities, and is applicable to other materials. Its easiness and rapidness are suited to high-volume studies, befitting for the comprehensive evaluation of interacting factors such as the matrix's nature, pore size, and PEI weight.

Distribution and Mobility of Amines Confined in Porous Silica Supports Assessed via Neutron Scattering, NMR, and MD Simulations: Impacts on CO2 Sorption Kinetics and Capacities

ConspectusSolid-supported amines are a promising class of CO2 sorbents capable of selectively capturing CO2 from diverse sources. The chemical interactions between the amine groups and CO2 give rise to the formation of strong CO2 adducts, such as alkylammonium carbamates, carbamic acids, and bicarbonates, which enable CO2 capture even at low driving force, such as with ultradilute CO2 streams. Among various solid-supported amine sorbents, oligomeric amines infused into oxide solid supports (noncovalently supported) are widely studied due to their ease of synthesis and low cost. This method allows for the construction of amine-rich sorbents while minimizing problems, such as leaching or evaporation, that occur with supported molecular amines.Researchers have pursued improved sorbents by tuning the physical and chemical properties of solid supports and amine phases. In terms of CO2 uptake, the amine efficiency, or the moles of sorbed CO2 per mole of amine sites, and uptake rate (CO2 capture per unit time) are the most critical factors determining the effectiveness of the material. While structure-property relationships have been developed for different porous oxide supports, the interaction(s) of the amine phase with the solid support, the structure and distribution of the organic phase within the pores, and the mobility of the amine phase within the pores are not well understood. These factors are important, because the kinetics of CO2 sorption, particularly when using the prototypical amine oligomer branched poly(ethylenimine) (PEI), follow an unconventional trend, with rapid initial uptake followed by a very slow, asymptotic approach to equilibrium. This suggests that the uptake of CO2 within such solid-supported amines is mass transfer-limited. Therefore, improving sorption performance can be facilitated by better understanding the amine structure and distribution within the pores.In this context, model solid-supported amine sorbents were constructed from a highly ordered, mesoporous silica SBA-15 support, and an array of techniques was used to probe the soft matter domains within these hybrid materials. The choice of SBA-15 as the model support was based on its ordered arrangement of mesopores with tunable physical and chemical properties, including pore size, particle lengths, and surface chemistries. Branched PEI─the most common amine phase used in solid CO2 sorbents─and its linear, low molecular weight analogue, tetraethylenepentamine (TEPA), were deployed as the amine phases. Neutron scattering (NS), including small angle neutron scattering (SANS) and quasielastic neutron scattering (QENS), alongside solid-state NMR (ssNMR) and molecular dynamics (MD) simulations, was used to elucidate the structure and mobility of the amine phases within the pores of the support. Together, these tools, which have previously not been applied to such materials, provided new information regarding how the amine phases filled the support pores as the loading increased and the mobility of those amine phases. Varying pore surface-amine interactions led to unique trends for amine distributions and mobility; for instance, hydrophilic walls (i.e., attractive to amines) resulted in hampered motions with more intimate coordination to the walls, while amines around hydrophobic walls or walls with grafted chains that interrupt amine-wall coordination showed recovered mobility, with amines being more liberated from the walls. By correlating the structural and dynamic properties with CO2 sorption properties, novel relationships were identified, shedding light on the performance of the amine sorbents, and providing valuable guidance for the design of more effective supported amine sorbents.

Direct Air Capture of CO2 Using Amine/Alumina Sorbents at Cold Temperature

Rising CO2 emissions are responsible for increasing global temperatures causing climate change. Significant efforts are underway to develop amine-based sorbents to directly capture CO2 from air (called direct air capture (DAC)) to combat the effects of climate change. However, the sorbents' performances have usually been evaluated at ambient temperatures (25 °C) or higher, most often under dry conditions. A significant portion of the natural environment where DAC plants can be deployed experiences temperatures below 25 °C, and ambient air always contains some humidity. In this study, we assess the CO2 adsorption behavior of amine (poly(ethyleneimine) (PEI) and tetraethylenepentamine (TEPA)) impregnated into porous alumina at ambient (25 °C) and cold temperatures (-20 °C) under dry and humid conditions. CO2 adsorption capacities at 25 °C and 400 ppm CO2 are highest for 40 wt% TEPA-incorporated γ-Al2O3 samples (1.8 mmol CO2/g sorbent), while 40 wt % PEI-impregnated γ-Al2O3 samples exhibit moderate uptakes (0.9 mmol g-1). CO2 capacities for both PEI- and TEPA-incorporated γ-Al2O3 samples decrease with decreasing amine content and temperatures. The 40 and 20 wt % TEPA sorbents show the best performance at -20 °C under dry conditions (1.6 and 1.1 mmol g-1, respectively). Both the TEPA samples also exhibit stable and high working capacities (0.9 and 1.2 mmol g-1) across 10 cycles of adsorption-desorption (adsorption at -20 °C and desorption conducted at 60 °C). Introducing moisture (70% RH at -20 and 25 °C) improves the CO2 capacity of the amine-impregnated sorbents at both temperatures. The 40 wt% PEI, 40 wt % TEPA, and 20 wt% TEPA samples show good CO2 uptakes at both temperatures. The results presented here indicate that γ-Al2O3 impregnated with PEI and TEPA are potential materials for DAC at ambient and cold conditions, with further opportunities to optimize these materials for the scalable deployment of DAC plants at different environmental conditions.

Support Pore Structure and Composition Strongly Influence the Direct Air Capture of CO2 on Supported Amines

A variety of amine-impregnated porous solid sorbents for direct air capture (DAC) of CO₂ have been developed, yet the effect of amine-solid support interactions on the CO₂ adsorption behavior is still poorly understood. When tetraethylenepentamine (TEPA) is impregnated on two different supports, commercial γ-Al₂O₃ and MIL-101(Cr), they show different trends in CO₂ sorption when the temperature (-20 to 25 °C) and humidity (0-70% RH) of the simulated air stream are varied. In situ IR spectroscopy is used to probe the mechanism of CO₂ sorption on the two supported amine materials, with weak chemisorption (formation of carbamic acid) being the dominant pathway over MIL-101(Cr)-supported TEPA and strong chemisorption (formation of carbamate) occurring over γ-Al₂O₃-supported TEPA. Formation of both carbamic acid and carbamate species is enhanced over the supported TEPA materials under humid conditions, with the most significant enhancement observed at -20 °C. However, while equilibrium H₂O sorption is high at cold temperatures (e.g., -20 °C), the effect of humidity on a practical cyclic DAC process is expected to be minimal due to slow H₂O uptake kinetics. This work suggests that the CO₂ capture mechanisms of impregnated amines can be controlled by adjusting the degree of amine-solid support interaction and that H₂O adsorption behavior is strongly affected by the properties of the support materials. Thus, proper selection of solid support materials for amine impregnation will be important for achieving optimized DAC performance under varied deployment conditions, such as cold (e.g., -20 °C) or ambient temperature (e.g., 25 °C) operations.