Developing Versatile Contactors for Direct Air Capture of CO2 through Amine Grafting onto Alumina Pellets and Alumina Wash-Coated Monoliths

Measuring and Modeling Water and Carbon Dioxide Adsorption on Amine Functionalized Alumina under Direct Air Capture Conditions

Water vapor is an unavoidable component of ambient air that sorbents designed for atmospheric CO2 capture must contend with. Amine functionalized sorbents often exhibit an enhancement of CO2 uptake in the presence of moisture through a variety of mechanisms, and in this work, we investigate the coadsorption of water and CO2 on amine functionalized alumina. Sorbent performance is examined under varying levels of humidity and temperature using three common measurement techniques: gravimetric, volumetric, and breakthrough methods. Our findings show that water increasingly enhances CO2 adsorption up to the monolayer saturation point of water, above which no further enhancement is observed. Competitive adsorption is observed primarily at low relative humidities, and a novel dual-site isotherm model is developed that successfully describes these behaviors. Additionally, this study highlights the unique advantages of each measurement technique for accurately characterizing sorbent performance under direct air capture (DAC) conditions. These insights contribute to the understanding and optimization of amine-based sorbents in DAC applications.

Behavior, mechanisms, and applications of low-concentration CO2 in energy media

This review explores the behavior of low-concentration CO2 (LCC) in various energy media, such as solid adsorbents, liquid absorbents, and catalytic surfaces. It delves into the mechanisms of diffusion, adsorption, and catalytic reactions, while analyzing the potential applications and challenges of these properties in technologies like air separation, compressed gas energy storage, and CO2 catalytic conversion. Given the current lack of comprehensive analyses, especially those encompassing multiscale studies of LCC behavior, this review aims to provide a theoretical foundation and data support for optimizing CO2 capture, storage, and conversion technologies, as well as guidance for the development and application of new materials. By summarizing recent advancements in LCC separation techniques (e.g., cryogenic air separation and direct air carbon capture) and catalytic conversion technologies (including thermal catalysis, electrochemical catalysis, photocatalysis, plasma catalysis, and biocatalysis), this review highlights their importance in achieving carbon neutrality. It also discusses the challenges and future directions of these technologies. The findings emphasize that advancing the efficient utilization of LCC not only enhances CO2 reduction and resource utilization efficiency, promoting the development of clean energy technologies, but also provides an economically and environmentally viable solution for addressing global climate change.

Self-Supported Branched Poly(ethylenimine) Monoliths from Inverse Template 3D Printing for Direct Air Capture

3D-printed inverse templates are combined with ice templating to develop self-supported branched poly(ethylenimine) monoliths with regular channels of varying channel density and ordered macropores. A maximum uptake of 0.96 mmol of CO2/g of monolith from ambient air containing 45.5% RH is achieved from dynamic breakthrough experiments, which is a 31% increase compared to the CO2 uptake from adsorption under dry conditions for the same duration. The breakthrough experiments show characteristics of internal mass-transfer limitations. The cyclic dynamic breakthrough experiments indicate stable operation without significant loss in CO2 uptake across eight cycles. Moreover, the self-supported monolith shows minimal loss in adsorption capacity (7.7%) upon exposure to air containing 21% oxygen at 110 °C, in comparison to a conventional sorbent consisting of poly(ethylenimine) impregnated on Al2O3 (18.9%). The monoliths exhibit good mechanical stability, contributed by elastic deformation, corresponding to up to 74% strain and lower pressure drop compared to many existing monoliths in the literature.

Mass Transfer of CO2 in Amine-Functionalized Structured Contactors in Ultra-Dilute Conditions

Extracting CO2 from the atmosphere via direct air capture (DAC) provides a pathway to counteract the rising CO2 concentration in the atmosphere. Processes using amine functionalized solid sorbents have attracted considerable attention, as they exhibit high affinity toward CO2 at atmospheric concentrations. The process is significantly influenced by the mass transfer kinetics of adsorption, and accurate quantification is crucial for improving process models and DAC systems. In this study, we addressed this critical issue by quantifying the mass transfer kinetics of three amine functionalized structured sorbents: two alumina pellets with unimodal (TRI@unimodal) and bimodal (TRI@bimodal) pore size distributions, and a honeycomb mullite/alumina monolith (TRI@monolith). A modeling framework was developed to enable the use of a commercial volumetric sorption device to measure sorbent mass transfer kinetics, and to distinguish them from resistances within the device. The measurements revealed distinct mass transfer regimes, with pore diffusion playing a significant role in the bimodal pellets, whereas a surface resistance introduced by the functionalization procedure dominated in the unimodal pellets. The device was unable to capture the pore diffusion in the monolith due to instrument resistances limiting this regime. A self-limiting diffusion behavior previously reported in literature was identified in the amine layer, which decreased diffusion with increasing CO2 uptake. We estimate kinetic parameters for all three sorbent materials for use in a widely used linear driving force (LDF) model adapted for amine functionalized sorbents. The parameter describing the mass transfer in the gas phase is nearly five times larger for TRI@bimodal than for TRI@unimodal. For the mass transfer in the amine layer, the parameter increases progressively from TRI@monolith to TRI@unimodal to TRI@bimodal. The results highlight the importance of pore structure and functionalization procedure to improve DAC sorbents.

Feasibility and Effectivity of an Amine-Grafted Alumina Adsorbent for Direct Air Capture

Direct air capture (DAC) has been identified as a necessary negative emission technology (NET) to solve global warming. DAC methods have been divided into two major types: solvent absorption and sorbent adsorption. Aqueous amine absorption is the major method in postcombustion carbon capture, not in DAC because contactors blow large volumes of air over the solvent, which results in high evaporation of solvents. Solid amine adsorption has been one of the primary methods of DAC due to its low volatility of amine. Therefore, the development of DAC adsorbents is the key to improve CO2 capture efficiency. Nowadays, the research on adsorbents mainly focuses on amine impregnation and grafting. The grafting adsorbents generally have better stability than impregnation adsorbents. Silica is the most common support material for amine-grafted adsorbents. Nonetheless, silica has some defects, such as poor hydrothermal stability, which limits its employment. Alumina is a promising support material with excellent hydrothermal stability, but studies on amine-grafted alumina are still scarce. Herein, a method of 3-[2-(2-aminoethylamino)ethylamino] propyltrimethoxysilane (TRI) grafting onto γ-alumina is presented. The results of this paper suggest that alumina is a potential support material for amine grafting. The CO2 adsorption capacity of the adsorbent is 0.39 mmol g-1 at 400 ppm and 25 °C. The amine-grafted alumina has excellent thermal stability than the amine-impregnation silica adsorbent. Besides, the adsorbent exhibits stable performance during cycles, with the working capacity maintained at 83% of that of the first cycle after 60 cycles. Water adsorption capacity and selectivity indicate that TRI-Al2O3 has good selectivity at high relative humidity. These findings make amine-grafted alumina a promising DAC adsorbent.

Exploring Geometric Properties and Cycle Design in Packed Bed and Monolith Contactors Using Temperature-Vacuum Swing Adsorption Modeling for Direct Air Capture

This study presents a comprehensive comparison between the packed bed and monolith contactor configurations for direct air capture (DAC) via process modeling of a temperature-vacuum swing adsorption (TVSA) process. We investigate various design parameters to optimize performance across different contactor geometries, including pellet size, monolith wall thickness, active sorbent content in monoliths, and packed bed structure configurations, considering both a traditional long column (PB40) and multiple shorter columns configured in parallel (PB5). Our parametric analysis assesses specific exergy consumption, sorbent, and volume requirements across different operating conditions of a five-step TVSA cycle. For minimizing sorbent requirements, PB5 and monoliths with over 80% sorbent loading were the best-performing contactor designs with overlapping performance in the low-exergy region. Beyond this region, PB5 faced limitations in reducing sorbent requirements further and was constrained by a maximum velocity at which it is sensible to operate without substantially increasing the exergy demand. In contrast, monoliths decreased sorbent requirements with minimal exergy increase due to reduced mass transfer resistances and lower pressure drop associated with their thin walls. The analysis of volume requirement-specific exergy Pareto fronts revealed that PB5 was less competitive with this metric due to the requirements for additional void space in the contactor configuration. The study also revealed that optimal sorbent loading for reducing volume requirements in monoliths differed from those minimizing sorbent usage, with the most effective loading being below 100%. Thus, the optimal contactor design varies depending on the goals of minimizing sorbent and volume requirements, and the choice and design of the contactor will depend on the relative costs of these factors. Lastly, our findings challenge the assumption that higher velocities are always preferable for direct air capture, suggesting instead that the operating velocity depends on the contactor configuration.

On Comparing Packed Beds and Monoliths for CO2 Capture from Air Through Experiments, Theory, and Modeling

This study compares the performance of amine-functionalized γ-alumina sorbents in the form of 3 mm γ-alumina pellets and of a γ-alumina wash-coated monolith for CO2 capture for direct air capture (DAC). Breakthrough experiments were conducted on the two contactors to analyze the adsorption kinetics and performance for different gas feeds. A constant pattern analysis revealed dominant mass transfer resistances in the gas film and in the pores, with axial dispersion also observed, particularly at higher concentrations. A 1D, physical model was used to fit the experiments and thus to estimate mass transfer and axial dispersion coefficients, which appear to be consistent with the hypotheses derived from constant pattern analysis. A dual kinetic model to describe mass transfer was found to better describe the tail behavior in the monolith, whereas a pseudo-first-order model was sufficient to describe breakthroughs on packed beds. A substantial two-order magnitude decrease in mass transfer coefficients was noted when reducing the feed concentration from 5.6% to 400 ppm CO2, thus underscoring the significant mass transfer limitations observed in DAC. Comparison between the contactors revealed notably higher mass transfer coefficients in the monolith compared to the packed beds, which are attributed to shorter diffusion lengths and lower equilibrium capacity. While the faster mass transfer coefficients observed in the monolith experiments led to reduced specific energy consumption and increased adsorption productivity compared to the packed bed at 400 ppm, no significant improvement was observed for the same process at the higher concentration of 5.6% CO2 in the feed. This finding highlights the need to tailor the contactor design to the specific gas separation requirements. This research contributes to the understanding and quantification of mass transfer kinetics at DAC concentrations in both packed bed and monolith contactors. It demonstrates the crucial role of the contactor in DAC systems and the importance of optimizing the adsorption step to enhance productivity and DAC performance.

Synergistic Assembly of Charged Oligomers and Amino Acids at the Air-Water Interface: An Avenue toward Surface-Directed CO2 Capture

Interfaces are considered a major bottleneck in the capture of CO2 from air. Efforts to design surfaces to enhance CO2 capture probabilities are challenging due to the remarkably poor understanding of chemistry and self-assembly taking place at these interfaces. Here, we leverage surface-specific vibrational spectroscopy, Langmuir trough techniques, and simulations to mechanistically elucidate how cationic oligomers can drive surface localization of amino acids (AAs) that serve as CO2 capture agents speeding up the apparent rate of absorption. We demonstrate how tuning these interfaces provides a means to facilitate CO2 capture chemistry to occur at the interface, while lowering surface tension and improving transport/reaction probabilities. We show that in the presence of interfacial AA-rich aggregates, one can improve capture probabilities vs that of a bare interface, which holds promise in addressing climate change through the removal of CO2 via tailored interfaces and associated chemistries.

Contributions of CO2, O2, and H2O to the Oxidative Stability of Solid Amine Direct Air Capture Sorbents at Intermediate Temperature

Aminopolymer-based sorbents are preferred materials for extraction of CO2 from ambient air [direct air capture (DAC) of CO2] owing to their high CO2 adsorption capacity and selectivity at ultra-dilute conditions. While those adsorptive properties are important, the stability of a sorbent is a key element in developing high-performing, cost-effective, and long-lasting sorbents that can be deployed at scale. Along with process upsets, environmental components such as CO2, O2, and H2O may contribute to long-term sorbent instability. As such, unraveling the complex effects of such atmospheric components on the sorbent lifetime as they appear in the environment is a critical step to understanding sorbent deactivation mechanisms and designing more effective sorbents and processes. Here, a poly(ethylenimine) (PEI)/Al2O3 sorbent is assessed over continuous and cyclic dry and humid conditions to determine the effect of the copresence of CO2 and O2 on stability at an intermediate temperature of 70 °C. Thermogravimetric and elemental analyses in combination with in situ horizontal attenuated total reflection infrared (HATR-IR) spectroscopy are performed to measure the extent of deactivation, elemental content, and molecular level changes in the sorbent due to deactivation. The thermal/thermogravimetric analysis results reveal that incorporating CO2 with O2 accelerates sorbent deactivation using these sorbents in dry and humid conditions compared to that using CO2-free air in similar conditions. The in situ HATR-IR spectroscopy results of PEI/Al2O3 sorbent deactivation under a CO2-air environment show the formation of primary amine species in higher quantity (compared to that in conditions without O2 or CO2), which arises due to the C-N bond cleavage at secondary amines due to oxidative degradation. We hypothesize that the formation of bound CO2 species such as carbamic acids catalyzes C-N cleavage reactions in the oxidative degradation pathway by shuttling protons, resulting in a low activation energy barrier for degradation, as probed by metadynamics simulations. In the cyclic experiment after 30 cycles, results show a gradual loss in stability (dry: 29%, humid: 52%) under CO2-containing air (0.04% CO2/21% O2 balance N2). However, the loss in capacity during cyclic studies is significantly less than that during continuous deactivation, as expected.