Neutrons for the energy transition

MMMs and MOFs

One of the approaches for gas separation is based on membranes with selective permeability – acting like sieves or filters. The goal is to achieve exceptional selectivity, minimising the methane losses, while maintaining a high permeability. The concept of MMM was developed by combining to types of membranes with different base structures (or matrices) and trying to keep the best of the two worlds: to overcome the limitations of each type while combining their desirable properties of both.

Currently, most commercial membranes are made from polymers, both amorphous and semi-crystalline polymers – rubbery or glassy materials made of large, repetitive molecular chains. Polymeric membranes are mechanically resistant, easy to process and low cost.

Conversely, certain microporous materials—such as MOFs and zeolites—exhibit remarkable selectivity in gas separation processes. These materials can be thought of as sponges, capable of retaining large amounts of gas or liquid within their cavities; however, their pores are usually on the subnanometer scale, roughly six orders of magnitude smaller. Microporous solids play a pivotal role in energy storage and conversion technologies through adsorption, a surface phenomenon in which guest molecules preferentially attach to the internal surfaces of the porous framework. The extensive internal surface area provided by their pore networks, together with the chemical tunability of their pore spaces, greatly enhances the material’s capacity for molecular storage and interaction.

In MMMs, nanoparticles of such materials are dispersed in polymeric films. This has been shown to enhance the performance porous polymer membranes for gas separation applications. Relatively unexplored remains the possibility of integration porous fillers that actively contribute to gas separation, particularly by enhancing selectivity. This is precisely where MOFs offer significant advantages over other conventional porous materials.

Metal-organic frameworks (MOFs) are porous materials that are made up of metal atoms linked by organic (carbon-based) molecules called ligands. Together, the metal ions and the ligands form crystals with large cavities, in which molecules can flow in and out. By varying the building blocks used in the MOFs, chemists can design them to chemically function in specific ways, for example capturing specific substances. It is possible to precisely modulate pore size, aperture and, more importantly, the selective interactions and/or diffusivity of gas molecules such as CO₂ and CH₄.

Neutrons: the key to the molecular-level insight

The study now published demonstrates that incorporating a specifically tuned MOF in a polymer membrane significantly enhances CO₂/CH₄ separation performance, improving both permeability and selectivity.

“The structural and physicochemical properties of the polymer membrane (PIM-I), the MOF (MOF-808 functionalised with aminoacids) and the resulting MMM were characterised using a host of different techniques, including nuclear magnetic resonance, infrared spectroscopy or small-angle X-ray scattering, just to name a few,” explains Dalia Refaat, first author of the study, “Gas separation measurements revealed a notable increase in both CO₂ permeability and CO₂/CH₄ selectivity.”

At this stage, a fundamental question remained to be answered: why? Insight into how is really happening, at molecular level, was provided by neutron scattering (INS, inelastic neutron scattering), combined with theoretical modelling (DFT, density functional theory). Neutron scattering measurements were performed at the IN1-Lagrange neutron spectrometer installed at the hot source at the ILL. INS spectra were measured in the range of energy transfers from 20 to 250 meV, with an energy resolution of Δ E/ΔE ~ 2 %

“Inelastic neutron scattering (INS) is ideal to study such systems. Indeed, Neutrons are particularly good at ‘seeing’ individual hydrogen atoms and can detect all the molecular vibrational modes where the H atoms are involved.

The analysis of inelastic neutron scattering data allows extract precise information of the molecular vibrations – especially the ones associated to hydrogen atoms – and opens the room to understand the dynamics of specific atoms or molecules and of the surroundings chemical environment disturbing them,” says Monica Jimenez-Ruiz, ILL scientist and IN1-Lagrange instrument responsible.

In this study, INS was employed to elucidate the binding modes of the amino acid to MOF-808, and to unveil the adsorption sites and their energy of interaction with CO₂ and CH₄ molecules. INS and DFT calculations confirmed that CO₂ molecules preferentially interact with the amino groups. DFT simulations further revealed that while CO₂ and CH₄ access similar adsorption sites, the interaction energy of CO₂ with amino groups is approximately three times higher than that of CH₄.

These findings demonstrate that MOF-808@AA fillers can significantly enhance the CO₂-philicity of PIM-1-based MMMs when compared to pristine PIM-1 or unmodified PIM-1/MOF-808 composites. “Overall, the combination of INS and DFT in this work provides a clear relation between structure and properties, or performance,” conclude the main authors of the study, Joaquín Coronas (INMA-CSIC, Universidad de Zaragoza, Spain) and Roberto Fernández de Luis (BCM, Basque Center for Materials, Applications and nanostructures), “It demonstrates that amino acid functionalization of MOF-808 provides a versatile and efficient approach to develop high performance, stable MMMs for advanced CO₂/CH₄ separation applications”