Discovering Inherent Characteristics of Polyethylenimine-Functionalized Porous Materials for CO2 Capture

Developing High-Capacity Solid "Molecular Basket" Sorbents for Selective CO2 Capture and Separation

ConspectusSince carbon-based energy continues to dominate (over 80%) the global primary energy supply, carbon dioxide capture, utilization, and sequestration (CCUS) is deemed to be a promising and viable option to mitigate greenhouse gas emissions and climate change, for which CO2 capture is critical to the overall success of CCUS. Although liquid amine scrubbing is a mature technology for carbon capture, it is energy-intensive and costly due to energy consumption in solvent heating and water evaporation apart from the energy needed to break amine-CO2 bonding. To address this challenge, Song's group developed a new design approach for adsorptive CO2 capture and separation, namely, "molecular basket" sorbents (MBS), without the need for dealing with solvent heating and water evaporation. The solid MBS consisting of polymeric amines (such as PEI) immobilized into nanoporous materials (such as SBA-15) possesses a high capacity for CO2 capture with high selectivity, fast kinetics, and good regenerability. Consequently, MBS can greatly reduce energy consumption and carbon capture cost. Conventional adsorbents such as zeolites, activated carbon, alumina, and silica have low adsorption capacities, and their use of CO2 adsorption requires prior removal of moisture and cooling of flue gas (∼35 °C). On the contrast, the CO2 sorption capacity of MBS can even be promoted by the presence of moisture/steam and reaches the best performance closer to flue gas temperature (∼75 °C). This Account presents an overview of our research progress in the material development and fundamental understanding of MBS for CO2 capture and the separation of CO2 from various gas streams. It begins with an illustration of the MBS concept, followed by efforts to improve the performance and pilot-scale demonstration of MBS for CO2 capture. With the systematic characterization of MBS by various ex situ and in situ techniques, a better understanding is developed for the CO2 sorption process mechanistically. Furthermore, this Account demonstrates how the fundamental understanding of the CO2 sorption mechanism promotes the further development of more robust and advanced sorbent materials with improved CO2 sorption capacity, kinetics of sorption and desorption, and cyclic stability. Finally, an outlook is provided for the future design and development of novel sorbent materials and the CO2 sorption process for various gas streams including flue gas, biogas, air, and hydrogen streams.

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.

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.

Polyethylenimine assisted non-monotonic jamming of colloids during evaporation induced assembly and its implication on CO2 sorption characteristics

We report a detailed study of hierarchically organized silica-polyethylenimine (PEI) microspheres achieved through evaporation-induced assembly. Due to complex interactions between oppositely-charged silica nanoparticles and PEI, non-monotonic jamming of the colloidal particles is manifested. With an increase in the polymer concentration, the local volume fraction of the silica particles decreases from 0.68 to 0.43 and then increases to 0.55 with further increase. The unusual jamming behaviour of the silica colloids in the presence of PEI provides an avenue for immobilizing PEI without reducing the porosity and specific area in contrast to the conventional impregnation approach. The resultant composite microspheres show good thermal stability and CO₂ sorption characteristics. For a 33 wt% PEI loading, the microspheres exhibit a significant CO₂ capture capacity of 65 mg g^-1 even at room temperature and it is increased to 90 mg g^-1 at 75 °C. The variation in the CO₂ capture capacity at 0 °C as a function of PEI loading also demonstrated the signature of non-monotonicity owing to the structural modification in the silica-PEI microspheres. The composite microspheres demonstrated fast adsorption kinetics reaching 70% of the total capture capacity in one minute during the CO₂ capture. The CO₂ cycling adsorption-desorption studies showed good regeneration capability up to 20 cycles.

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

Supported amines are a promising class of CO₂ sorbents offering large uptake capacities and fast uptake rates. Among supported amines, poly(ethyleneimine) (PEI) physically impregnated in the mesopores of SBA-15 silica is widely used. Within these composite materials, the chain dynamics and morphologies of PEI strongly influence the CO₂ capture performance, yet little is known about chain and macromolecule mobility in confined pores. Here, we probe the impact of the support-PEI interactions on the dynamics and structures of PEI at the support interface and the corresponding impact on CO₂ uptake performance, which yields critical structure-property relationships. The pore walls of the support are grafted with organosilanes with different chemical end groups to differentiate interaction modes (spanning from strong attraction to repulsion) between the pore surface and PEI. Combinations of techniques, such as quasi-elastic neutron scattering (QENS), ¹H T₁-T₂ relaxation correlation solid-state NMR, and molecular dynamics (MD) simulations, are used to comprehensively assess the physical properties of confined PEI. We hypothesized that PEI would have faster dynamics when subjected to less attractive or repulsive interactions. However, we discover that complex interfacial interactions resulted in complex structure-property relationships. Indeed, both the chain conformation of the surface-grafted chains and of the PEI around the surface influenced the chain mobility and CO₂ uptake performance. By coupling knowledge of the dynamics and distributions of PEI with CO₂ sorption performance and other characteristics, we determine that the macroscopic structures of the hybrid materials dictate the first rapid CO₂ uptake, and the rate of CO₂ sorption during the subsequent gradual uptake stage is determined by PEI chain motions that promote diffusive jumps of CO₂ through PEI-packed domains.

Effective Dual-Functional Metal-Organic Framework (DF-MOF) as a Catalyst for the Solvent-Free Cycloaddition Reaction

A new porous metal-organic framework, [Co (oba) (bpdh)]·(DMF) (TMU-63), containing accessible nitrogen-rich diazahexadiene groups was successfully prepared with the solvothermal assembly of 5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene (4-bpdh), 4,4'-oxybis(benzoic) acid (oba), and Co(II) ions. The combination of Lewis basic functional groups and porosity leads to high performance in CO₂ adsorption and conversion in the cycloaddition reaction of epoxides under solvent-free conditions. To further enhance the catalytic efficiency of TMU-63, we introduced a highly acidic malonamide ligand into the structure via solvent-assisted ligand exchange (SALE) as a postsynthesis method. Incorporating different percentages of N¹,N³-di(pyridine-4-yl) malonamide linker (4-dpm) into TMU-63 created a new porous structure. Powder X-ray diffraction (PXRD) and NMR spectroscopy confirmed that 4-bpdh was successfully replaced with 4-dpm in the daughter MOF, TMU-63S. The catalytic activity of both MOFs was confirmed by significant amounts of CO₂ cycloaddition of epoxides under solvent-free conditions. The catalytic cycloaddition activities were found to be well-correlated with the Lewis base/Brønsted acid distributions of the materials examined in the TMU-63S series, showing that the concurrent presence of both acid and base sites was desirable for high catalytic activity. Furthermore, the heterogeneous catalysts could easily be separated out from the reaction mixtures and reused four times without loss of catalytic activity and with no structural deterioration.

Straightforward synthesis of multifunctional porous polymer nanomaterials for CO2 capture and removal of contaminants

A straightforward synthesis of multifunctional mesoporous polymer nanomaterials suitable for the removal of contaminants and CO2 capture is reported.

Universality of mesoporous coal gasification slag for reinforcement and deodorization in four common polymers

Mesoporous adsorbents and polymer deodorants are difficult to implement on a large scale because of their complicated preparation methods. Herein, a mesoporous adsorbent (CGSA) with a specific surface area of 564 m²g^-1and a pore volume of 0.807 cm³g^-1was prepared from solid waste coal gasification slag using a simple acid leaching process. The adsorption thermodynamics and adsorption kinetics results verified that the adsorption mechanism of propane on CGSA was mainly physisorption. Then the universality of CGSA in different polymers was investigated by introducing CGSA and its commercialized counterparts (CaCO₃, and zeolite) into four common polymers. When the filler content was 30 wt%, the average reinforcement effect of CGSA on the tensile, flexural, and impact strengths of the four polymers was 46.68%, 83.62%, and 211.90% higher than that of CaCO₃, respectively. Gas chromatography results also showed that CGSA significantly decreased total volatile organic compound emissions from the composites, and its optimal deodorization performance reached 69.58%, 81.33%, and 91.09% for different polymers, respectively, far exceeding that of zeolite. Therefore, this study showed that low-cost, high-performance, and multifunctional mesoporous polymer fillers with excellent universality can be manufactured from solid contaminants.

The volume expansion effect of amine during CO2 adsorption process: An experimental study combined with theoretical calculations

The volume expansion effect of amine supported on mesoporous silica during CO₂ adsorption process was found for the first time through well-designed experiments and was further confirmed by theoretical calculations. It was found that the residual pore volume of mesocellular silica foam (MCF) based solid amine sorbent (tetraethylenepentamine (TEPA) supported on MCF) gradually decreased with the increase of CO₂ uptake. Moreover, the surface area, the average diameters of window and cell of MCF show a similar changing trend. This is due to the volume expansion effect of TEPA during CO₂ adsorption process, i.e., the total volume of reaction products of TEPA and CO₂ is larger than that of pure TEPA. The products are very sticky and almost lose the liquidity totally even at 80 °C. The sticky products and the volume expansion effect may increase the mass transfer resistance and are not beneficial to higher CO₂ uptakes especially for solid amine sorbent with higher amine loading due to the decrease of pore size and the residual pore volume. DFT calculations based on simple models also indicate that the total volume of the generated products is much larger than that of unreacted amine, further confirming the volume expansion effect of amine during CO₂ adsorption process. DFT calculations also indicate that the volume is even doubled in the presence of moisture. The volume expansion effect of solid amine sorbent found in this study may help to design the sorbent with high CO₂ capture performance and less the mass transfer resistance.

Acoustic-Controlled Bubble Generation and Fabrication of 3D Polymer Porous Materials

Porous materials have a variety of applications such as catalysis, gas separation, sensing, tissue engineering, sewage treatment, and so on. However, there are still challenges in the synthesis of porous materials with light weight, high porosity, and superhydrophobicity. Herein, we demonstrate one acoustic-controlled microbubble generation method, which is used to synthesize 3D polymer porous materials. The acoustic-controlled microbubble generation based on focused surface acoustic wave (FSAW) is suitable for not only the generation of gas-in-oil microbubbles but also the gas-in-water microbubbles. The size of microbubbles can be real-time controlled by adjusting the frequency or the driving voltage of the FSAW. The as-prepared poly(vinyl alcohol) (PVA) foams composed of microbubbles can be used as a template to fabricate the PVA-based porous gel materials through freezing-thawing cyclic processing, and the various sized bubbles result in different porosity of the PVA-based porous gel materials. Moreover, excellent properties like oleophilicity and superhydrophobicity of the PVA-based porous gel materials can be obtained through a further hydrophobic modification treatment. The oil/water separation experiments have been done to demonstrate the good absorption and reliability of the modified porous gel materials, which are capable of multiple uses.