A robust metal-organic framework for gas separation

Abstract

The development of efficient sorbent materials and processes to drive chemical separations at (near) ambient conditions remains an important and challenging pipeline of research today. The separation of various gas mixtures is currently being carried out at industrial scales using energy-intensive techniques, such as cryogenic distillation. This thesis reports the application of a robust porous metal-organic framework (MOF), MFM-520 (MFM = Manchester Framework Material) in the separation of various gas mixtures of high industrial and environmental importance. The thesis also reports the study of the impact of host-guest interaction on the separation performance in MFM-520. Chapter 1 presents a literature review on gas separations, including the purification of olefins (C2H4 and C3H6) and the capture of toxic air pollutants (NO2 and SO2). The recent progress on studies of impacts of active sites, crystal engineering and host-guest chemistry on gas separation in MOFs are reviewed. Chapter 2 describes the purification of C2H4 and C3H6 in MFM-520. MFM-520 exhibits strong temperature dependence for the adsorption of alkenes and alkanes, promoting the efficient separation of C3H6/C3H8 and C2H4/C2H6 mixtures at 318 K--a temperature that is relevant with industrial processes. The study of supramolecular binding confirmed the stronger interactions and successful breakthrough experiments confirmed the selective adsorption of C2H4 and C3H6 compared with their alkane analogues Additionally, mixed matrix membranes (MMMs) have been fabricated based upon MFM-520 and these MMM realized C3H6/C3H8 purification with a separation factor of ~ 7.9 and propylene permeance of ~ 1984 Barrer, demonstrating the potential of MFM-520 in industrial applications. Chapter 3 describes the capture and conversion of NO2 in MFM-520. At 298 K and 0.01 bar, MFM-520 shows a fully reversible uptake of 4.2 mmol/g. In situ synchrotron X-ray diffraction, inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR) studies revealed that all adsorbed NO2 dimerized to N2O4 in the pores and were stablised by 24-fold weak host-guest interactions, thus facilitating the high adsorption uptake and selectivity. In addition, the treatment of captured NO2 with water in air leads to quantitative conversion into nitric acid in MFM-520. Chapter 4 presents a ‘periodic to aperiodic’ structural transition controlled by guest molecules in MFM-520. The as-synthesised MFM-520·H2O exhibits a 3D periodic structure due to the presence of guest water molecules in the pores confining the structural relaxation. Upon the removal of water molecules, the [ZnO4N] polyhedra have undergone notable distortion, leading to a subtle (3+2) D incommensurate structural modulation with achiral symmetry. Filling the pore with CO2 has minimal effect to the framework. The strong interactions between SO2 and the framework further incommensurately modulate the structure with dynamically varying modulation vectors. The structural difference reconciles with all other experimental observations on the selective adsorptions of SO2 under practical conditions. The study has uncovered a class of meta-rigid MOF materials showing a new level of structural complexity controlled by guest molecules. Chapter 5 gives an overall perspective on the results obtained in this thesis and an outlook for further investigations.

Date of Award	14 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Martin Schröder (Supervisor) & Sihai Yang (Supervisor)