PIM-1 as a Solution-Processable "Molecular Basket" for CO2 Capture from Dilute Sources

Mind the Gap: The Role of Mass Transfer in Shaped Nanoporous Adsorbents for Carbon Dioxide Capture

Adsorptive separations by nanoporous materials are major industrial processes. The industrial importance of solid adsorbents is only expected to grow due to the increased focus on carbon dioxide capture technology and energy-efficient separations. To evaluate the performance of an adsorbent and design a separation process, the adsorption thermodynamics and kinetics must be known. However, although diffusion kinetics determine the maximum production rate in any adsorption-based separation, this aspect has received less attention due to the challenges associated with conducting diffusion measurements. These challenges are exacerbated in the study of shaped adsorbents due to the presence of porosity at different length scales. As a result, adsorbent selection typically relies mainly on adsorption properties at equilibrium, i.e., uptake capacity, selectivity and adsorption enthalpy. In this Perspective, based on an extensive literature review on mass transfer of CO2 in nanoporous adsorbents, we discuss the importance and limitations of measuring diffusion in nanoporous materials, from the powder form to the adsorption bed, considering the nature of the process, i.e., equilibrium-based or kinetic-based separations. By highlighting the lack of and discrepancies between published diffusivity data in the context of CO2 capture, we discuss future challenges and opportunities in studying mass transfer across scales in adsorption-based separations.

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.

Removal of dyes using polymers of intrinsic microporosity (PIMs): a recent approach

Removal of dyes from various industrial effluents is a great challenge, and cost-effective methods and materials with high dye removal efficacy are in high demand. Adsorption, nanofiltration and photocatalytic degradation are three major techniques that have been investigated for dye removal. PIMs are promising materials for use in these three methods based on their attributes, such as microporosity, solution processibility, high chemical stability and tunability through facile synthesis and easy postmodification. Although the number of reports on dye removal employing PIMs are limited, some of the materials have been shown to exhibit good dye separation properties, which are comparable to those of the state-of-the-art material activated carbon. In this highlight, we make an account of progress in PIMs and PIM-based composite materials in different dye removal processes over the last decade. Furthermore, we discuss the existing challenges of PIM-based materials and aim to analyze the key parameters for improving their dye removal properties.

Polymers of Intrinsic Microporosity Based on Dibenzodioxin Linkage: Design, Synthesis, Properties, and Applications

The development of polymers of intrinsic microporosity (PIMs) over the last two decades has established them as a distinct class of microporous materials, which combine the attributes of microporous solid materials and the soluble nature of glassy polymers. Due to their solubility in common organic solvents, PIMs are easily processable materials that potentially find application in membrane-based separation, catalysis, ion separation in electrochemical energy storage devices, sensing, etc. Dibenzodioxin linkage, Tröger's base, and imide bond-forming reactions have widely been utilized for synthesis of a large number of PIMs. Among these linkages, however, most of the studies have been based on dibenzodioxin-based PIMs. Therefore, this review focuses precisely on dibenzodioxin linkage chemistry. Herein, the design principles of different rigid and contorted monomer scaffolds are discussed, as well as synthetic strategies of the polymers through dibenzodioxin-forming reactions including copolymerization and postsynthetic modifications, their characteristic properties and potential applications studied so far. Towards the end, the prospects of these materials are examined with respect to their utility in industrial purposes. Further, the structure-property correlation of dibenzodioxin PIMs is analyzed, which is essential for tailored synthesis and tunable properties of these PIMs and their molecular level engineering for enhanced performances making these materials suitable for commercial usage.

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.

A Comparative Study of Different Sorbents in the Context of Direct Air Capture (DAC): Evaluation of Key Performance Indicators and Comparisons

Direct air capture can be based on an adsorption system, and the used sorbent (chemisorbents or physisorbents) influences process. In this work, two amine-functionalized sorbents, as chemisorbents, and three different metal organic frameworks, as physisorbents, are considered and compared in terms of some key performance indicators. This was carried out by developing a mathematical model describing the adsorption and desorption stages. An independent analysis was carried out in order to verify data reported in the literature. Results show that the equilibrium loading is a critical parameter for adsorption capacity, energy consumption, and cost. The considered metal organic frameworks are characterized by a lower equilibrium loading (10−4 mol/kg) compared to chemisorbents (10−1 mol/kg). For this reason, physisorbents have higher overall energy consumptions and costs, while capturing a lower amount of carbon dioxide. A reasonable agreement is found on the basis of the operating conditions of the Climeworks company, modelling the use of the same amine cellulose-based sorbent. The same order of magnitude is found for total costs (751 USD/tonneCO2 for our analysis, compared to the value of 600 USD/tonneCO2 proposed by this company).

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.

High molecular weight polyethylenimine encapsulated into a porous polymer monolithic by one-step polymerization for CO2 capture

A low-cost porous polymer monolithic with a well-interconnected 3D structure and high amino efficiency for CO₂ capture.

Boosting gas separation performance and suppressing the physical aging of polymers of intrinsic microporosity (PIM-1) by nanomaterial blending

In recent decades, polymers of intrinsic microporosity (PIMs), especially the firstly introduced PIM-1, have been actively explored for various membrane-based separation purposes and widely recognized as the next generation membrane materials of choice for gas separation due to their ultra-permeable characteristics. Unfortunately, the polymers suffer substantially the negative impacts of physical aging, a phenomenon that is primarily noticeable in high free volume polymers. The phenomenon occurs at the molecular level, which leads to changes in the physical properties, and consequently the separation performance and membrane durability. This review discusses the strategies that have been employed to manage the physical aging issue, with a focus on the approach of blending with nanomaterials to give mixed matrix membranes. A detailed discussion is provided on the types of materials used, their inherent properties, the effects on gas separation performance, and their benefits in the suppression of the aging problem.

Polymers of Intrinsic Microporosity Chemical Sorbents Utilizing Primary Amine Appendance Through Acid-Base and Hydrogen-Bonding Interactions

Here, we present novel chemical sorbents based on polymers with intrinsic microporosity (PIMs). For the first time, alkylamines were incorporated in PIMs through an acid-base interaction to create a chemisorbent. The amine-appended PIMs not only showed a nearly four-fold enhancement in CO₂ loading capacity (36.4 cc/g at 0.15 bar and 298 K) and very high CO₂/N₂ selectivity compared to neat PIM-1 but also proved to have stable performance when cycled between adsorption and desorption isotherms under both dry and humid conditions that are typical for postcombustion CO₂ capture.

Open and Hierarchical Carbon Framework with Ultralarge Pore Volume for Efficient Capture of Carbon Dioxide

Amine-impregnated adsorbents are promising candidates for the selective capture of CO₂ from flue gas. The key is to develop suitable supports possessing large pore sizes and very large pore volumes, and the material has to be facilely synthesized from readily available reagents. In this work, hierarchical carbon nanosheet (CNS) featuring large pore width (30-100 nm) and extraordinarily huge pore volume (8.41 cm³/g) was prepared through controlled carbonization of glucose and dicyandiamide. The CNS was physically impregnated with pentaethylenehexamine (PEHA) to act as adsorbents for selective capture of CO₂. Owing to the unique porosity of CNS, the amount of amine loading in CNS can be ultrahigh (6 g PEHA/g CNS) in comparison with those of known amine-impregnated adsorbents, and the CO₂ capacity in a flow of 15 v/v % of CO₂ balanced in N₂ was up to 5.0 mmol/g at 75 °C. The synthesized PEHA-CNS composite materials perform well in capturing CO₂ under humid condition and display good stability in a test of 10 adsorption-desorption cycles. It is believed that the CNS synthesized in this work has great potential to act as a support material for CO₂ adsorption.

Oxidatively-Stable Linear Poly(propylenimine)-Containing Adsorbents for CO2 Capture from Ultradilute Streams

Aminopolymer-based solid sorbents have been widely investigated for the capture of CO₂ from dilute streams such as flue gas or ambient air. However, the oxidative stability of the widely studied aminopolymer, poly(ethylenimine) (PEI), is limited, causing it to lose its CO₂ capture capacity after exposure to oxygen at elevated temperatures. Here, we demonstrate the use of linear poly(propylenimine) (PPI), synthesized through a simple cationic ring-opening polymerization, as a more oxidatively stable alternative to PEI with high CO₂ capacity and amine efficiency. The performance of linear PPI/SBA-15 composites was investigated over a range of CO₂ capture conditions (CO₂ partial pressure, adsorption temperature) to examine the tradeoff between adsorption capacity and sorption-site accessibility, which was expected to be more limited in linear polymers relative to the prototypical hyperbranched PEI. Linear PPI/SBA-15 composites were more efficient at CO₂ capture and retained 65-83 % of their CO₂ capacity after exposure to a harsh oxidative treatment, compared to 20-40 % retention for linear PEI. Additionally, we demonstrated long-term stability of linear PPI sorbents over 50 adsorption/desorption cycles with no loss in performance. Combined with other strategies for improving the oxidative stability and adsorption kinetics, linear PPI may play a role as a component of stable solid adsorbents in commercial applications for CO₂ capture.

Critical Comparison of Structured Contactors for Adsorption-Based Gas Separations

Recent advances in adsorptive gas separations have focused on the development of porous materials with high operating capacity and selectivity, useful parameters that provide early guidance during the development of new materials. Although this material-focused work is necessary to advance the state of the art in adsorption science and engineering, a substantial problem remains: how to integrate these materials into a fixed bed to efficiently utilize the separation. Structured sorbent contactors can help manage kinetic and engineering factors associated with the separation, including pressure drop, sorption enthalpy effects, and external heat integration (for temperature swing adsorption, or TSA). In this review, we discuss monoliths and fiber sorbents as the two main classes of structured sorbent contactors; recent developments in their manufacture; advantages and disadvantages of each structure relative to each other and to pellet packed beds; recent developments in system modeling; and finally, critical needs in this area of research.

Molecular Layer Deposition-Modified 5A Zeolite for Highly Efficient CO2 Capture

Effective pore mouth size of 5A zeolite was engineered by depositing an ultrathin layer of microporous TiO₂ on its external surface and appropriate pore misalignment at the interface. As a result, a slightly bigger N₂ molecule (kinetic diameter: 0.364 nm) was effectively excluded, whereas CO₂ (kinetic diameter: 0.33 nm) adsorption was only influenced slightly. The prepared composite zeolite sorbents showed an ideal CO₂/N₂ adsorption selectivity as high as ∼70, a 4-fold increase over uncoated zeolite sorbents, while maintaining a high CO₂ adsorption capacity (1.62 mmol/g at 0.5 bar and 25 °C) and a fast CO₂ adsorption rate.

Soluble Polymers with Intrinsic Porosity for Flue Gas Purification and Natural Gas Upgrading

A soluble polymer with intrinsic microporosity, 2,4-diamino-1,3,5-triazine-functionalized organic polymer, is used for the first time as a solid adsorbent, providing an easy solution to overcome the fouling issue. Promising adsorption performances including good CO₂ adsorption capacity, excellent CO₂ /N₂ and CO₂ /CH₄ selectivities, high chemical and thermal stabilities, and easiness of preparation and regeneration are shown.

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.

A nitrogen-rich, azaindole-based microporous organic network: synergistic effect of local dipole–π and dipole–quadrupole interactions on carbon dioxide uptake

A new type of microporous organic polymer with azaindole units (N-PEINK) has been designed.