Contrasting-functionality-decked robust MOF for moisture-tolerant and variable-temperature CO2 adsorption with in-built urea group mediated mild condition cycloaddition

Abstract

The dire need to reduce the atmospheric carbon dioxide (CO₂) concentration has attracted worldwide attention to the capture of this greenhouse gas and its conversion into useful chemicals. Nevertheless, it is still difficult to achieve variable-temperature and humid-condition adsorption with mild condition fixation of CO₂ in metal–organic frameworks (MOFs) due to difficulties in positioning assorted task-specific sites. We introduced open metal site (OMS), hydrogen-bond operative functionality, and free amine moiety inside the pore wall of a mixed-ligand robust Cd(II) framework. Two-fold interpenetration generated high-density acid–base functionalization promotes appreciable CO₂ adsorption in the guest-free structure at elevated temperature with considerable MOF–CO₂ interaction. The aqua-robust MOF exhibits minimum loss in CO₂ uptake during multiple capture–release cycles under variable temperature and retains the adsorption capacities even upon exposure to 75% RH. The atomistic-level snapshots of temperature-induced inclusion of gas molecules inside this microporous vessel are rationalized from simulation studies, and portray diverse CO₂-philic sites. Particularly, the four-fold increased CO₂ adsorption compared to that of an un-functionalized MOF validates the prime role of pore surface engineering. Moreover, the CO₂ selectivity shows a drastic improvement upon gradually increasing the temperature, attaining a CO₂/N₂ value of 380 at 313 K. The framework further demonstrates solvent-free CO₂ conversion to cyclic carbonates in high yield with broad substrate scope and satisfactory reusability under less harsh conditions and in a rather short time. In addition to typical OMS/co-catalyst synergism, the mutual participation of antagonistic active sites in substrate interaction and activation is validated by juxtaposing the performance of a urea-free isoskeletal framework and by the relative fluorescence modification in the presence of epoxide. The results corroborate the unique organic-functionality-mediated cycloaddition mechanism, which provides important structure–function synergy in this unconventional route to non-redox CO₂ fixation.