CO₂ Adsorption in Azobenzene Functionalized Stimuli Responsive Metal–Organic Frameworks

Recent reports of externally triggered, controlled adsorption of carbon dioxide (CO₂) have raised the prospects of using stimuli responsive metal–organic frameworks (MOFs) for energy efficient gas storage and release. Motivated by these reports, here we investigate CO₂ adsorption mechanisms in photoresponsive PCN-123 and azo-IRMOF-10 frameworks. Using a combination of grand canonical Monte Carlo and first-principles quantum mechanical simulations, we find that the CO₂ adsorption in both frameworks is substantially reduced upon light-induced isomerization of azobenzene, which is in agreement with the experimental measurements. We show that the observed behavior originates from inherently weaker interactions of CO₂ molecules with the frameworks when azobenzene groups are in cis state rather than due to any steric effects that dramatically alter the adsorption configurations. Our studies suggest that even small changes in local environment triggered by external stimuli can provide a control over the stimuli responsive gas adsorption and release in MOFs

Understanding Methanol Coupling on SrTiO₃ from First Principles

Perovskites are interesting materials for catalysis due to their great tunability. However, the correlation of many reaction processes to the termination of a perovskite surface is still unclear. In this study, we use the methanol coupling reaction on the SrTiO₃(100) surface as a probe reaction to investigate direct C–C coupling from a computational perspective. We use density functional theory to assess methanol adsorption, C–H activation, and direct C–C coupling reactions on the SrTiO₃(100) surface of different terminations. We find that, although methanol molecules dissociatively adsorb on both A and B terminations with similar strength, the dehydrogenation and C–C coupling reactions have significantly lower activation energies on the B termination than on the A termination. The predicted formation of methoxy and acetate on the SrTiO₃(100) B termination can well explain the ambient-pressure XPS data of methanol on the single-crystal SrTiO₃(100) surface at 250 °C. This work suggests that a choice of B termination of perovskites would be beneficial for the C–C coupling reaction of methanol

Porous Aromatic Frameworks Impregnated with Lithiated Fullerenes for Natural Gas Purification

Natural gas, a lower emission alternative than its fossil fuel counterparts, requires the removal of carbon dioxide, known as “sweetening”, prior to its use. In this study we computationally explore the separation of methane and carbon dioxide using a new adsorbent consisting of lithium-decorated fullerenes (Li₆C₆₀) impregnated within a series of porous aromatic frameworks (PAFs) of various pore sizes. The strong affinity of CO₂ with the impregnated frameworks, confirmed by density functional theory, leads to selective adsorption over CH₄. The impregnation can also double the CO₂ adsorption capacity compared to the bare PAF and increase selectivity of CO₂/CH₄ up to 48 for an optimum amount of Li₆C₆₀, which is above the current industry benchmark. Overall, the study reveals physical insights and proposes impregnated PAFs to be promising candidates for CO₂/CH₄ separations for natural gas purification