Polyamine-Appended, Intentionally Defective Uio-66-Nh2 (Zr) Based Metal Organic Frameworks for Enhanced Low-Pressure Co­2 Capture and Humid Temperature-Swing Cycling

Abstract

Rising concentrations of CO₂ from anthropogenic activities drive the development of novel materials for CO₂ capture, with the structural tunability of solid sorbents like metal-organic frameworks (MOFs) offering unprecedented opportunities for achieving high capture capacities. This study explores the feasibility of harnessing intentionally defective UiO-66-NH₂ (Zr) MOFs post synthetically modified with polyamines to boost CO₂ uptake, along with humid temperature swing adsorption (TSA) cycling which simulates flue gas conditions with water vapor. Polyamine compounds Tris(2-aminoethyl)amine (TREN) and Polyethyleneimine (PEI) were successfully grafted onto the MOF framework, after introducing defects into the UiO-66-NH2 structure to compensate for the associated loss in porosity and surface area, with conversion efficiencies of ~40%. Adsorption tests of CO₂ at a low partial pressure of 15 kPa, and a temperature of 298 K showed that UiO-66-NH-Ac-TREN and UiO-66-NH-Ac-PEI had capture capacities of 1.55 and 1.85 mmol/g, respectively, exhibiting substantial enhancement compared to the unmodified MOFs. Furthermore, the selectivity of CO₂ over N₂ was significantly improved for a CO2/N2 (15/85) composition at 298K, with a selectivity of 183 and 377 for the TREN and PEI appended MOFs respectively. Additionally, the MOFs with polyamine modifications demonstrated humid cycling stability, boosting the uptake as well as retaining ~74% (UiO-66-NH-Ac-TREN) and ~81% (UiO-66-NH-Ac-PEI) of their initial capture capacity after 20 humid TSA cycles. The heat of adsorption for CO₂ was observed to increase nearly twofold compared to the unmodified MOF, yet it remained lower than that of conventional liquid amines, underscoring the potential of these materials for energy-efficient TSA processes.

Keywords: Metal organic Frameworks, Defect engineering, Carbon dioxide capture, Post-Synthetic Modification, Temperature Swing Adsorption