Different MOFs for CO2 capture

https://doi.org/10.3390/en15093473

“The ability of three different MOFs and their composites to adsorb carbon dioxide was evaluated using the approach of cyclic CO₂ adsorption analysis on a thermogravimeter under thermal adsorption conditions, considering the actuality of this process for carbon capture from hot flue gases. The amount of CO₂ adsorbed was calculated for each cycle and then the average value was used to compare different materials. The CO₂ adsorption capacities of MOF-composites, as well as pristine MOFs, biochar, montmorillonite, and aerosil, are shown in Figure 11. Porous structure, abundant functional groups, high carbon stability, low toxicity, and low cost are properties of biochar that could be a perspective to test its application as a C-containing MOF-composite for CO₂ capture and even for other purposes in the future. High stability is a characteristic of aerosils and clays, which can play an important role in the practical application of MOF-composites. As these materials can be obtained relatively easily, and the number of additives studied allows for the maintenance of the characteristic properties of MOF such as a crystalline structure, thermal stability, as well as a relatively high surface area, so it is important to study the CO₂ adsorption capacity of these materials. It should be noted that the CO₂ adsorption capacities of biochar and montmorillonite reach 0.8 mmol/g and 0.3 mmol/g, respectively, while the CO₂ adsorption capacities of MOF-composites containing them are significantly higher. Almost all CuBTC composites show a higher adsorption capacity than CuBTC (2.4 mmol/g), while the CO₂ adsorption capacity of UTSA-16 and UiO-66-BTEC composites was lower compared to UTSA-16 (3.9 mmol/g) and UiO-66-BTEC (1.5 mmol/g). CuBTC-composites show an enhanced adsorption ability of CO₂ in comparison to CuBTC, except for CuBTC-BC-30. Adsorption capacity increases in the direction CuBTC-Mt-15 < CuBTC-BC-15 < CuBTC-BC-5 < CuBTC-A-15, reaching, accordingly, 2.6 mmol/g, 3.0 mmol/g, 3.7 mmol/g, and 3.7 mmol/g. CuBTC-BC-5 and CuBTC-A-15 composites show the highest adsorption capacity in comparison to all MOF-composites obtained in this study. This is because the biochar and aerosil that were added to CuBTC did not block the pores of CuBTC itself, thus increasing the amount of porosity in the CuBTC-BC-5 and CuBTC-A-15 omposites. Microporosity is a characteristic of CuBTC, but CuBTC-A-15 and CuBTC-BC-5 also show small amounts of mesopores, which may indicate successful composite formation. In addition, micro-mesoporous materials are thought to promote CO₂ adsorption. Moreover, the obtained results show that both the used filler additive and its added amount play an important role in the MOF-composite. For example, CuBTC-BC-5 has a relatively high adsorption capacity, while as the BC content increases to 15 wt% and further to 30 wt% (based on parent MOF precursors’ total weight), the adsorption capacity of the MOF-composite decreases significantly. This trend, in turn, can be explained by the excessive number of additives, which reduces the porosity of the material by blocking part of the pores of the MOF-composite. This reduces the number of available adsorption sites and consequently the CO₂ adsorption capacity. In this study, the obtained trend is comparable with findings made by other authors; for example, too much graphene oxide in MOF-505 resulted in lower CO₂ adsorption capacity [59]. According to the reported literature [21,22,53,91], CO₂ adsorption possibly at first occurs on both open Cu cations and cage windows of the MOF, followed by further adsorption governed by the van der Waals forces. However, the electrostatic interaction between Cu cations and CO₂ quadrupole is stronger than the van der Waals interaction. Combining MOF, such as CuBTC, and clay or aerosil that contain oxygen-functional groups with the new formed composite material potentially has been complemented with additional mesopores that are favorable for diffusion and mass transfer, possibly due to the attachment of oxygen functional groups of used material to the copper in MOF structure by co-ordination bonds. Biochar can participate in bonding interactions, enhancing the co-ordination bonding with Cu ions, leading to the growth of CuBTC composite, and thus providing improved structures due to the presence of heteroatom-containing functional groups and sp² aromatic domains in biochar. The adsorption of CO₂ on the MOF-composite can be affected not only by the specific surface area but also by the increase in the dispersive forces of the surface due to the presence of the filler material with a dense atomic structure. Moreover, functional groups of filler additives may co-ordinate with Cu²⁺, resulting in imperfection and defects of crystal surface which could cause more unsaturated Cu²⁺ metal sites and thus form strong adsorptive sites that may promote CO₂ adsorption [59]. In addition, it is possible to improve the thermal and hydrolytic stability of the materials compared to the original MOF; for example, carbon rings from graphene sheets surround the metal center forming a hydrophobic barrier, thus protecting the co-ordination bonds in the MOF [92]. The addition of biochar or clay can have a similar effect, but it should be noted that the amount of the additive is important. The higher its content, the higher the thermal stability, but this may lead to a decrease in porosity and lower CO₂ adsorption capacity.”

“Figure 11. CO₂ adsorption using (a) CuBTC and CuBTC-composites containing aerosil (CuBTC-A-15), montmorillonite (CuBTC-Mt-15), and CuBTC-composites containing different amounts of biochar (CuBTC-BC-5, CuBTC-BC-15, and CuBTC-BC-30, respectively), (b) UTSA-16 and UTSA-16-composites containing aerosil (UTSA-16-A-15), biochar (UTSA-16-BC-15), and UTSA-16-composites containing different amounts of montmorillonite (UTSA-16-Mt-5, UTSA-16-Mt-15, and UTSA-16-Mt-40, respectively), (c) UiO-66-BTEC and UiO-66-BTEC-composites containing aerosil (UiO-66-BTEC-A-15), montmorillonite (UiO-66-BTEC-Mt-15), and UiO-66-BTEC-composites containing different amounts of biochar (UiO-66-BTEC-BC-5, UiO-66-BTEC-BC-15, and UiO-66-BTEC-BC-30, respectively).”