Highly Active Ultrasmall Ni Nanoparticle Embedded Inside a Robust Metal-Organic Framework: Remarkably Improved Adsorption, Selectivity, and Solvent-Free Efficient Fixation of CO2.

Abstract

We report integrating additional functionality in an amine decorated, robust metal-organic framework (MOF) by encapsulating Ni nanoparticles (NPs). In-depth characterization of the postmodified structure confirms well-dispersed and ultrasmall NPs inside the framework pores. Although, the surface area is more reduced than pristine MOF, the CO2 uptake capacity is remarkably increased by 35% with a large 10 kJ/mol rise in adsorption enthalpy that validates favorable interactions between CO2 and NPs. In particular, CO2 adsorption selectivity over N2 and CH4 displays significant improvement (CO2/N2 = 145.7, CO2/CH4 = 12.65), while multicycle CO2 uptake demonstrates outstanding sorption recurrence. Impressively, the embedded NPs act as highly active functional sites toward solvent-free CO2 cycloaddition with epoxides in 98% yield and 99% selectivity under relatively mild conditions. The catalyst shows high recyclability without leaching of any metal-ion/NPs and greater pre-eminent activity than the unmodified analogue or contemporary reports. Of note is that outstanding conversion and selectivity are maintained for a wide range of aliphatic and aromatic epoxides, while larger substrates exhibit insignificant conversion, demonstrating admirable size selectivity. Based on the literature reports and experimental outcome, a rationalized mechanism is proposed for the reaction. This study exclusively demonstrates how strategic encapsulation of Ni NPs influences the inherent electronic properties in a MOF for highly selective CO2 adsorption and represents a step forward to sustainable CO2 valorization in terms of abundant active sites, sufficient stability, and consistent usability.