Carbon dioxide capture enhanced by pre-adsorption of water and methanol in UiO-66

The rapidly rising level of carbon dioxide in the atmosphere resulting from human activity is one of the greatest environmental problems facing our civilization today. Most technologies are not yet sufficiently developed to move existing infrastructure to cleaner alternatives. Therefore, techniques for capturing carbon dioxide from emission sources may play a key role at the moment. The structure of the UiO‐66 material not only meets the requirement of high stability in contact with water vapor but through the water pre‐adsorbed in the pores, the selectivity of carbon dioxide adsorption is increased. We successfully applied the recently developed methodology for water adsorption modelling. It allowed to elucidate the influence of water on CO₂ adsorption and study the mechanism of this effect. We showed that water is adsorbed in octahedral cage and stands for promotor for CO₂ adsorption in less favorable space than tetrahedral cages. Water plays a role of a mediator of adsorption, what is a general idea of improving affinity of adsorbate. On the basis of pre‐adsorption of methanol as another polar solvent, we have shown that the adsorption sites play a key role here, and not, as previously thought, only the interaction between the solvent and quadrupole carbon dioxide. Overall, we explained the mechanism of increased CO₂ adsorption in the presence of water and methanol, as polar solvents, in the UiO‐66 pores for a potential post‐combustion carbon dioxide capture application.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

CO2 capture enhancement in MOFs via the confinement of molecules

This review focuses on exploring a new approach to improve the CO₂ adsorption properties of MOFs by confining small amounts of molecules with different nature, such as: water, alcohols, amines, and even aromatic molecules.

CO2 capture enhancement for InOF-1: confinement of 2-propanol

The confinement of small amounts of i-PrOH demonstrated and enhanced CO₂ capture for InOF-1 as a result of the bottleneck effect and the formation of essential hydrogen bonds.

Metal–Organic Framework (MOF)-based CO2 Adsorbents

Rising CO2 levels in the atmosphere resulting from fossil fuel combustion is one of the most significant global environmental concerns. Carbon capture and sequestration (CCS), primarily post-combustion CO2 capture, is an essential research area to reduce CO2 levels and avoid environmental destabilization. Recently, metal–organic frameworks (MOFs) have been attracting attention in the scientific community for potential applications in gas storage and separation, including CCS, owing to their novel properties, such as a large surface area, tunable pore shape and size, and tailored chemical functionality. This chapter starts with a brief introduction about the significance of CO2 adsorption and separation, followed by how MOF-based research endeavors were initiated and explored, and why MOFs are unique for gas adsorption. Secondly, we reviewed the relationship between CO2 adsorption and MOF properties including surface area, pore size and volume, amine functionality, nature of linkers, and structural flexibility, and analyzed the reported data based on the possible adsorption mechanism. The humidity effects on CO2 capture over MOFs and implementation of MOF composites were considered as well. Finally, some conclusions on the status of the developed MOFs and perspectives for future research on MOFs for the practical application of CO2 adsorption and separation were mentioned.

Structure stability of HKUST-1 towards water and ethanol and their effect on its CO2 capture properties

Water and ethanol stabilities of the crystal structure of the Cu-based metal-organic framework (MOF) HKUST-1 have been investigated. Vapour (water and ethanol) sorption isotherms and cyclability were measured by a dynamic strategy. The ethanol sorption capacity of HKUST-1 at 303 K remained unchanged contrasting water sorption (which decreased along with the sorption experiment time). Considering the binding energy of each sorbate with the open Cu(ii) sites, obtained by the use of diffusion coefficients, we showed the superior crystal stability of the HKUST-1 framework towards ethanol. Finally, a small quantity of ethanol (pre-adsorbed) slightly enhanced CO₂ capture without crystal structure degradation.

Confinement of H2O and EtOH to enhance CO2 capture in MIL-53(Al)-TDC

EtOH adsorption–desorption properties of MIL-53(Al)-TDC along with the confinement of small amounts of water to enhance its CO₂ capture, and the reversible capture of iodine are presented in this article.

Humidity-induced CO2 capture enhancement in Mg-CUK-1

Mg-CUK-1 showed a 1.8-fold increase in CO₂ capture (from 4.6 wt% to 8.5 wt%) in the presence of 18% RH.

Confined methanol within InOF-1: CO2 capture enhancement

The CO₂ capture in InOF-1 was enhanced by confining small amounts of MeOH. DFT calculations coupled with forcefield based-MC simulations revealed that such an enhancement is due to an increase of the degree of confinement.

Confinement of alcohols to enhance CO2 capture in MIL-53(Al)

CO₂ capture of MIL-53(Al) was enhanced by confining small amounts of MeOH and i-PrOH within its micropores.

Water Adsorption Properties of NOTT-401 and CO2 Capture under Humid Conditions

The water-stable material NOTT-401 was investigated for CO₂ capture under humid conditions. Water adsorption properties of NOTT-401 were studied, and their correlation with CO₂ sequestration at different relative humidities (RHs) showed that the CO₂ capture increased from 1.2 wt % (anhydrous conditions) to 3.9 wt % under 5% RH at 30 °C, representing a 3.2-fold improvement.

CO2 capture under humid conditions in NH2-MIL-53(Al): the influence of the amine functional group

NH₂-MIL-53(Al) exhibited a considerable stronger affinity to water than MIL-53(Al). Thus, the hydrophobicity (shown by in situ FTIR) of the pores within MIL-53(Al) enhanced the CO₂ adsorption.