Investigação in silico de redes metalorgânicas para separação de gases leves

Abstract

The drastic climate changes and the increasing global energy demand are driving the search for a sustainable energy matrix. In this context, natural gas and biogas are identified as fuels for the energy transition. Natural gas is the most environmentally friendly fossil fuel and widely available in reserves worldwide. It is mainly composed of CH4, CO2, light hydrocarbon fractions (C2s and C3s), and N2, with CH4 being relevant for energy conversion. On the other hand, biogas is a combustible gas generated through anaerobic fermentation of organic matter, primarily consisting of methane. To prevent corrosion of equipment and enhance the energy efficiency of these fuels, it is necessary to separate methane from impurities. Currently, cryogenic distillation is employed, involving high pressures and temperatures below the boiling point of molecules, resulting in a high-energy-cost process. A promising alternative is selective adsorption processes using microporous solids, particularly metal-organic frameworks (MOFs). These materials are comprised of metal cations and organic ligands, forming a porous network with large surface areas and high selectivity for gas separation. Anion-pillared metal-organic frameworks (APMOFs) are a class of MOFs that contain anions in their structure. SIFSIX APMOFs are composed of the SiF²−6 anion, aromatic organic ligands, and divalent cations such as Cu²+, demonstrating great potential for capturing CO2 from mixtures containing CH4. The combination of these components results in functionalized cavities with high selectivity for gas separation. In this study, the computational investigation of SIFSIX-3-Cu, SIFSIX-2-Cu-i, and SIFSIX-2-Cu MOFs for the separation of methane from ethane, ethene, propane, propene, and N2 was performed. Monte Carlo, molecular dynamics, and density functional theory methodologies were employed. The investigations indicated that the evaluated MOFs are promising, especially for the separation of methane from the C3 fraction. Adsorption studies revealed that the adsorption of propene and propane in SIFSIX-3-Cu and SIFSIX-2-Cu-i MOFs is challenging due to the small pore size compared to the kinetic diameter of these molecules. Additionally, it was observed that SIFSIX-2-Cu MOF exhibits higher adsorption capacity for larger molecules. Evaluation of molecule diffusion in MOF channels demonstrated that diffusion coefficients can be ordered as SIFSIX-2-Cu > SIFSIX-2-Cu-i > SIFSIX-3-Cu. Structural, electronic, and thermodynamic investigations revealed that molecule adsorption occurs preferentially near fluorine sites, where these atoms establish electrostatic interactions with hydrogen atoms of hydrocarbons, and molecule confinement plays an important role in the process. The combination of employed methodologies showed that SIFSIX-2-Cu-i network, due to its interpenetration, may be promising for the separation of CH4/N2 mixtures.