Efficient rare earth elements (REEs) separation and recovery are crucial to meet the ever-increasing demand for REEs extensively used in various high technology devices. Herein, we synthesized a highly stable chromium-based metal-organic framework (MOF) structure, Cr-MIL-101, and its derivatives with different organic functional groups (MIL-101-NH, MIL-101-ED (ED: ethylenediamine), MIL-101-DETA (DETA: diethylenetriamine), and MIL-101-PMIDA (PMIDA: N-(phosphonomethyl)iminodiacetic acid)) and explored their effectiveness in the separation and recovery of La, Ce, Nd, Sm, and Gd in aqueous solutions. The prepared materials were characterized using various analytical instrumentation. These MOFs showed increasing REE adsorption capacities in the sequence MIL-101 < MIL-101-NH < MIL-101-ED < MIL-101-DETA < MIL-101-PMIDA. MIL-101-PMIDA showed superior REE adsorption capacities compared to other MOFs, with Gd being the element most efficiently adsorbed by the material. The adsorption of Gd onto MIL-101-PMIDA was examined in detail as a function of the solution pH, initial REE concentration, and contact time. The obtained adsorption equilibrium data were well represented by the Langmuir model, and the kinetics were treated with a pseudo-second-order model. A plausible mechanism for the adsorption of Gd on MIL-101-PMIDA was proposed by considering the surface complexation and electrostatic interaction between the functional groups and Gd ions under different pH conditions. Finally, recycling tests were carried out and demonstrated the higher structural stability of MIL-101-PMIDA during the five adsorption-regeneration runs.

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Hydroxylamine-anchored covalent aromatic polymer (CAP-DAP) was synthesized from p-terphenyl and 1,3,5benzene tricarbonyl chloride, followed by subsequent functionalization with 1,3-diamino-2-propanol for CO 2 capture and metalfree catalysis in CO 2 −epoxide cycloaddition reactions. The novel CAP-DAP material was characterized using various analytical techniques. It showed very good CO 2 adsorption capacity of 153 mg/g along with a high (CO 2 /N 2 ) selectivity of 86 at 273 K/1 bar, in contrast to bare CAP, which exhibited moderate CO 2 adsorption of 136 mg/g with a CO 2 /N 2 selectivity of 47. CAP-DAP also displayed high catalytic activity for CO 2 −epoxide cycloaddition reactions under mild and solvent-free conditions. The synergistic effect between metal-free CAP-DAP and tetrabutylammonium bromide (n-Bu 4 NBr) enabled a high epoxide conversion of 98% coupled with an excellent product selectivity of 99% at 60 °C, 1 bar CO 2 , and a reaction time of 12 h. Faster reaction kinetics with reaction times <6 h was possible at 80 °C. The catalyst also showed excellent reusability and no leaching of active species was observed from the spent catalyst. Based on experimental results, a plausible reaction mechanism for CO 2 − epoxide cycloaddition reaction over CAP-DAP catalyst has been proposed.

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