Ultrastable Carboxyl-Functionalized Pore-Space-Partitioned Metal-Organic Frameworks for Gas Separation.

Abstract

Isoreticular chemistry, which enables property optimization by changing compositions without changing topology, is a powerful synthetic strategy. One of the biggest challenges facing isoreticular chemistry is to extend it to ligands with strongly coordinating substituent groups such as unbound -COOH, because competitive interactions between such groups and metal ions can derail isoreticular chemistry. It is even more challenging to have an isoreticular series of carboxyl-functionalized MOFs capable of encompassing chemically disparate metal ions. Here, with the simultaneous introduction of carboxyl functionalization and pore space partition, a family of carboxyl-functionalized materials is developed in diverse compositions from homometallic Cr³⁺ and Ni²⁺ to heterometallic Co²⁺/V³⁺, Ni²⁺/V³⁺, Co²⁺/In³⁺, Co²⁺/Ni²⁺. Cr-MOFs remain highly crystalline in boiling water. Unprecedentedly, one Cr-MOF can withstand the treatment cycle with 10m NaOH and 12m HCl, allowing reversible inter-conversion between unbound -COOH acid form and -COO^- base form. These materials exhibit excellent sorption properties such as high uptake capacity for CO₂ (100.2 cm³ g^-1) and hydrocarbon gases (e.g., 142.1 cm³ g^-1 for C₂H₂, 110.5 cm³ g^-1 for C₂H₄) at 1 bar and 298K, high benzene/cyclohexane selectivity (up to ≈40), and promising separation performance for gas mixtures such as C₂H₂/CO₂ and C₂H₂/C₂H₄.