CO2 Sorption Performance of Composite Polymer/Aminosilica Hollow Fiber Sorbents: An Experimental and Modeling Study

Assessing Impacts of Atmospheric Conditions on Efficiency and Siting of Large-Scale Direct Air Capture Facilities

The cost and efficiency of direct air capture (DAC) of carbon dioxide (CO2) will be decisive in determining whether this technology can play a large role in decarbonization. To probe the role of meteorological conditions on DAC we examine, at 1 × 1° resolution for the continental United States (U.S.), the impacts of temperature, humidity, atmospheric pressure, and CO2 concentration for a representative amine-based adsorption process. Spatial and temporal variations in atmospheric pressure and CO2 concentration lead to strong variations in the CO2 available in ambient air across the U.S. The specific DAC process that we examine is described by a process model that accounts for both temperature and humidity. A process that assumes the same operating choices at all locations in the continental U.S. shows strong variations in performance, with the most influential variables being the H2O gas phase volume fraction and temperature, both of which are negatively correlated with DAC productivity for the specific process that we consider. The process also shows a moderate positive correlation of ambient CO2 with productivity and recovery. We show that optimizing the DAC process at seven representative locations to reflect temporal and spatial variations in ambient conditions significantly improves the process performance and, more importantly, would lead to different choices in the sites for the best performance than models based on a single set of process conditions. Our work provides a framework for assessing spatial variations in DAC performance that could be applied to any DAC process and indicates that these variations will have important implications in optimizing and siting DAC facilities.

Recent advances in direct air capture by adsorption

Significant progress has been made in direct air capture (DAC) in recent years. Evidence suggests that the large-scale deployment of DAC by adsorption would be technically feasible for gigatons of CO₂ capture annually. However, great efforts in adsorption-based DAC technologies are still required. This review provides an exhaustive description of materials development, adsorbent shaping, in situ characterization, adsorption mechanism simulation, process design, system integration, and techno-economic analysis of adsorption-based DAC over the past five years; and in terms of adsorbent development, affordable DAC adsorbents such as amine-containing porous materials with large CO₂ adsorption capacities, fast kinetics, high selectivity, and long-term stability under ultra-low CO₂ concentration and humid conditions. It is also critically important to develop efficient DAC adsorptive processes. Research and development in structured adsorbents that operate at low-temperature with excellent CO₂ adsorption capacities and kinetics, novel gas-solid contactors with low heat and mass transfer resistances, and energy-efficient regeneration methods using heat, vacuum, and steam purge is needed to commercialize adsorption-based DAC. The synergy between DAC and carbon capture technologies for point sources can help in mitigating climate change effects in the long-term. Further investigations into DAC applications in the aviation, agriculture, energy, and chemical industries are required as well. This work benefits researchers concerned about global energy and environmental issues, and delivers perspective views for further deployment of negative-emission technologies.

Scalable Formation of Diamine-Appended Metal-Organic Framework Hollow Fiber Sorbents for Postcombustion CO2 Capture

We describe a straightforward and scalable fabrication of diamine-appended metal-organic framework (MOF)/polymer composite hollow fiber sorbent modules for CO₂ capture from dilute streams, such as flue gas from natural gas combined cycle (NGCC) power plants. A specific Mg-MOF, Mg₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), incorporated into poly(ether sulfone) (PES) is directly spun through a conventional "dry-jet, wet-quench" method. After phase separation, a cyclic diamine 2-(aminomethyl)piperidine (2-ampd) is infused into the MOF within the polymer matrix during postspinning solvent exchange. The MOF hollow fibers from direct spinning contain as high as 70% MOF in the total fibers with 98% of the pure MOF uptake. The resulting fibers exhibit a step isotherm and a "shock-wave-shock" breakthrough profile consistent with pure 2-ampd-Mg₂(dobpdc). This work demonstrates a practical method for fabricating 2-ampd-Mg₂(dobpdc) fiber sorbents that display the MOF's high CO₂ adsorption capacity while lowering the pressure drop during operation.

Propylene Enrichment via Kinetic Vacuum Pressure Swing Adsorption Using ZIF-8 Fiber Sorbents

This work presents the synthesis, characterization, and implementation of ZIF-8/cellulose acetate fiber sorbents for the cyclic kinetic adsorption separation of an equimolar propane/propylene feed. These fiber sorbents are packed as structured mass-transfer contactors and employed in a two-bed vacuum pressure swing adsorption cycle without propylene product purge. The unoptimized adsorption cycle process produces a high-pressure product of up to 81% propane purity at 31% recovery at 0 °C. The effects of adsorption step time, temperature, and feed rate are investigated and presented as a purity/recovery trade-off. This work represents the first successful demonstration of a metal-organic framework fiber sorbent in a pressure swing adsorption cycle and the first use of metal-organic frameworks in a kinetic separation cycle.

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.

Dual-Channel, Molecular-Sieving Core/Shell ZIF@MOF Architectures as Engineered Fillers in Hybrid Membranes for Highly Selective CO2 Separation

A novel core/shell porous crystalline structure was prepared using a large pore metal organic framework (MOF, UiO-66-NH₂, pore size, ∼ 0.6 nm) as core surrounded by a small pore zeolitic imidazolate framework (ZIF, ZIF-8, pore size, ∼ 0.4 nm) through a layer-by-layer deposition method and subsequently used as an engineered filler to construct hybrid polysulfone (PSF) membranes for CO₂ capture. Compared to traditional fillers utilizing only one type of porous material with rigid channels (either large or small), our custom designed core/shell fillers possess clear advantages via pore engineering: the large internal channels of the UiO-66-NH₂ MOFs create molecular highways to accelerate molecular transport through the membrane, while the thin shells with small pores (ZIF-8) or even smaller pores generated at the interface by the imperfect registry between the overlapping pores of ZIF and MOF enhance molecular sieving thus serving to distinguish slightly larger N₂ molecules (kinetic diameter, 0.364 nm) from smaller CO₂ molecules (kinetic diameter, 0.33 nm). The resultant core/shell ZIF@MOF and as-prepared hybrid PSF membranes were characterized by transmission electron microscopy, X-ray diffraction, wide-angle X-ray scattering, scanning electron microscopy, Fourier transform infrared, thermogravimetric analysis, differential scanning calorimetry, and contact angle tests. The dependence of the separation performance of the membranes on the MOF/ZIF ratio was also studied by varying the number of layers of ZIF coatings. The integrated PSF-ZIF@MOF hybrid membrane (40 wt % loading) with optimized ZIF coating cycles showed improved hydrophobicity and excellent CO₂ separation performance by simultaneously increasing CO₂ permeability (CO₂ permeability of 45.2 barrer, 710% higher than PSF membrane) and CO₂/N₂ selectivity (CO₂/N₂ selectivity of 39, 50% higher than PSF membrane), which is superior to most reported hybrid PSF membranes. The strategy of using dual-channel molecular sieving core/shell porous crystals in hybrid membranes thus provides a promising means for CO₂ capture from flue gas.

Linking Silica Support Morphology to the Dynamics of Aminopolymers in Composites

A combined computational and experimental approach is used to elucidate the effect of silica support morphology on polymer dynamics and CO₂ adsorption capacities in aminopolymer/silica composites. Simulations are based on coarse-grained molecular dynamics simulations of aminopolymer composites where a branched aminopolymer, representing poly(ethylenimine) (PEI), is impregnated into different silica mesoporous supports. The morphology of the mesoporous supports varies from hexagonally packed cylindrical pores representing SBA-15, double gyroids representing KIT-6 and MCM-48, and cagelike structures representing SBA-16. In parallel, composites of PEI and the silica supports SBA-15, KIT-6, MCM-48, and SBA-16 are synthesized and characterized, including measuring their CO₂ uptake. Simulations predict that a 3D pore morphology, such as those of KIT-6, MCM-48, and SBA-16, will have faster segmental mobility and have lower probability of primary amine and surface silanol associations, which should translate to higher CO₂ uptake in comparison to a 2D pore morphology such as that of SBA-15. Indeed, it is found that KIT-6 has higher CO₂ uptake than SBA-15 at equivalent PEI loading, even though both supports have similar surface area and pore volume. However, this is not the case for the MCM-48 support, which has smaller pores, and SBA-16, whose pore structure rapidly degrades after PEI impregnation.

Unraveling the Dynamics of Aminopolymer/Silica Composites

The structure and dynamics of a model branched polymer was investigated through molecular dynamics simulations and neutron scattering experiments. The polymer confinement, monomer concentration, and solvent quality were varied in the simulations and detailed comparisons between the calculated structural and dynamical properties of the unconfined polymer and those confined within an adsorbing and nonadsorbing cylindrical pore, representing the silica based structural support of the composite, were made. The simulations show a direct relationship in the structure of the polymer and the nonmonotonic dynamics as a function of monomer concentration within an adsorbing cylindrical pore. However, the nonmonotonic behavior disappears for the case of the branched polymer within a nonadsorbing cylindrical pore. Overall, the simulation results are in good agreement with quasi-elastic neutron scattering (QENS) studies of branched poly(ethylenimine) in mesoporous silica (SBA-15) of comparable size, suggesting an approach that can be a useful guide for understanding how to tune porous polymer composites for enhancing desired dynamical and structural behavior targeting carbon dioxide adsorption.