Marhaina Ismail

Carbon dioxide (CO ₂ ) capture is a significant chemical process that has attracted considerable interest in both academic and industrial sectors. It is essential for mitigating climate change and its related impacts on the environment and human health. Various technologies are implemented for CO ₂ capture, with physical adsorption using porous material standing out as one of the most widely employed methods. Gallate-based metal-organic frameworks (MOFs) are reported to offer remarkable CO ₂ adsorption capacity values, with Mg-gallate exhibiting the highest capacity, followed by Co-gallate and Ni-gallate. The mechanism of CO ₂ adsorption on gallate-based MOFs, however, lacks extensive discussion. A thorough understanding of the adsorption mechanism helps in designing and synthesizing MOFs with enhanced CO ₂ capture performance. Therefore, this work aims to discuss the mechanism of CO ₂ adsorption on gallate-based MOFs based on the experimental pure isotherms. The experimental isotherms exhibited S-shaped curves that are related to the occurrence of gate-opening effect. These S-shaped isotherms corresponded to multistep adsorption, classifying gallate-based MOFs as flexible MOFs. The flexibility of these frameworks can be controlled by the pressure and temperature, which is important for designing specific gas storage and separation systems. In addition, the intra-particle diffusion model supported that the CO ₂ adsorption occurred at the surface and mesopore of gallate-based MOFs. Given these characteristics, gallate-based MOFs can be considered as the promising physisorbent for CO ₂ capture.

In the area of adsorption, experimental multicomponent data possess challenges in comparison to pure components. By solely relying on the pure adsorption experiment, a model known as Ideal Adsorbed Solution Theory (IAST) can predict binary adsorption isotherms without requiring any experimental multicomponent data. According to our previous work, Co-gallate exhibited higher CO2 uptakes than CH4 based on pure adsorption isotherms. However, there is a lack of study on the equilibrium isotherms of binary adsorption calculated by IAST for multi CO2/CH4 compositions at different temperature conditions. Therefore, the experimental CO2 and CH4 pure adsorption isotherms were fitted with six analytical models, in which Langmuir model yielded the highest goodness-of-fit. IAST calculations via the Python package were employed to predict the equilibrium isotherms of binary CO2/CH4 adsorption at different compositions and temperatures. The predicted binary adsorption isotherms revealed that CO2 adsorption was more favourable on the surface of Co-gallate than CH4, even though those uptakes were lower than pure adsorption data due to competitive co-adsorption behaviours. Moreover, IAST selectivity suggested that CO2/CH4 separation performed better as the CO2/CH4 compositions approached unity and at lower temperature condition. IAST approach evaluated that Co-gallate can be a promising solid material for CO2/CH4 mixed gas separations.

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Biogas is mainly consisted of methane and carbon dioxide in the presence of other contaminants. The biogas purification by adsorption using metal-organic frameworks is getting attention due to the low-cost operation and high-efficiency process. Co-gallate was predicted to give a promising performance in CO2 and CH4 adsorption. However, the behaviours of CO2 and CH4 adsorption on Co-gallate are not well-explained. Therefore, this work is to synthesize Co-gallate and its performance was discussed in terms of adsorbed amount of CO2 and CH4. The experimental CO2 and CH4 pure adsorption isotherms were then fitted with equilibrium isotherm and kinetic models to describe the adsorption behaviours. Co-gallate offered a greater CO2 adsorption capacity than CH4 due to a stronger adsorbent-adsorbate interaction. The experimental pure adsorption isotherms were best fitted with Toth model compared to Langmuir, Freundlich and Sips models according to the values. Toth model described the CO2 adsorption was multilayer and heterogeneous. Thermodynamic property suggested the CO2 and CH4 adsorption were classified as exothermic process and physisorption. For kinetic models, pseudo-first order model brought the highest goodness-of-fit in terms of rate of adsorption compared to pseudo-second order and Elovich models. Pseudo-first order model reflects the adsorption rate is proportional to the number of vacant sites. It confirmed CO2 adsorption was more favourable than CH4, at lower temperature condition. In this work, the equilibrium isotherm and kinetic models were employed to select the best-fitted model in explaining the adsorption behaviours. Therefore, these behaviours of CO2 and CH4 adsorption on Co-gallate are useful in designing the future practical operation of CO2/CH4 gas adsorption.

Ideal Adsorbed Solution Theory (IAST) is a predictive model that does not require any mixture data. In gas purification and separation processes, IAST is used to predict multicomponent adsorption equilibrium and selectivity based solely on experimental single-component adsorption isotherms. In this work, the mixed gas adsorption isotherms were predicted using IAST calculations with the Python package (pyIAST). The experimental CO₂ and CH₄ single-component adsorption isotherms of Mg-gallate were first fitted to isotherm models in which the experimental data best fit the Langmuir model. The presence of CH₄ in the gas mixture contributed to a lower predicted amount of adsorbed CO₂ due to the competitive adsorption among the different components. Nevertheless, CO₂ adsorption was more favorable and resulted in a higher predicted adsorbed amount than CH₄. Mg-gallate showed a stronger affinity for CO₂ molecules and hence contributed to a higher CO₂ adsorption capacity even with the coexistence of a CO₂/CH₄ mixture. Very high IAST selectivity values for CO₂/CH₄ were obtained which increased as the gas phase mole fraction of CO₂ approached unity. Therefore, IAST calculations suggest that Mg-gallate can act as a potential adsorbent for the separation of CO₂/CH₄ mixed gas.

The separation of aromatic compound from aliphatic mixture is one of the essential industrial processes for an economically green process. In order to determine the separation efficiency of ionic liquid (IL) as a solvent in the separation, the ternary diagram of liquid-liquid extraction (LLE) 1-butyl-3-methylimidazolium dicyanamide [BMIM][N(CN)₂] with benzene and cyclohexane was studied at T=298.15 K and atmospheric pressure. The solute distribution coefficient and solvent selectivity derived from the equilibrium data were used to evaluate if the selected ionic liquid can be considered as potential solvent for the separation of benzene from cyclohexane. The experimental tie line data was correlated using non-random two liquid model (NRTL) and Margules model. It was found that the solute distribution coefficient is (0.4430-0.0776) and selectivity of [BMIM][N(CN)₂] for benzene is (53.6-13.9). The ternary diagram showed that the selected IL can perform the separation of benzene and cyclohexane as it has extractive capacity and selectivity. Therefore, [BMIM][N(CN)₂] can be considered as a potential extracting solvent for the LLE of benzene and cyclohexane.