Morphology control of metal-organic frameworks and their application on adsorption and catalysis

Abstract

Metal-organic frameworks (MOFs), consisting of inorganic metal centres coordinated with organic linkers, are a group of hybrid materials emerging recently and have attracted immense attention because of their exceptionally high microporosity, tuneable pore size and uniformly-structured cavities. The controlled synthesis of crystals with defined morphology is critical for their performance in terms of adsorption, gas separation, and catalysis etc. Some instructive attempts have been conducted successfully to manipulate the morphology of MOFs by adjusting reaction parameters, changing solvents and using templates or additives. Among them, the organic additives with their high efficiency and repeatability have been frequently introduced. However, organic additives tend to be difficult to be removed from MOFs micropore structures and have adverse impacts on the crystallinity due to introducing defects. Therefore, there is an increasing interest in fashioning the morphology of MOFs by inorganic coordination strategy.This project focuses on controlled synthesis of MOFs by using inorganic additives or precursors and investigates their morphology-dependent applications in terms of adsorption and catalysis. The studies include the morphology control of copper-based MOFs and the study of morphology transformation mechanism, as well as exploring the morphology dependent performance on gas adsorption and catalysis. It aims to establish guidelines for the controlled synthesis of MOFs via inorganic strategies.In the first part of the experimental chapter, we used Mg/Al layered double hydroxide (LDH) nanosheets as modulators to tune the growth orientation of HKUST-1 without either pore blockage or crystallinity degradation. Through the introduction of LDH during the hydrothermal process, HKUST-1 crystal shape transfers from octahedron with fully exposed {111} facets to tetrakaidecahedron with both {100} and {111} facets. The exposure of {100} facet with large pore size facilitates the activation of MOF, thereby providing more open metal sites. The tetrakaidecahedral HKUST-1 exhibits high acetylene uptake of 275 cm3 (STP)/g at 298 K and 1 atm, which is the highest value ever reported to the best of our knowledge. Time-dependent experiment revealed that the morphology control of LDH was due to: a) competitive coordination between aluminium ions released from LDH and copper ions with organic ligands; b) fast deprotonation of ligands induced by the hydroxyl function group on the LDH. In order to further reveal the mechanism of aluminium ion on the morphology control of copper-based MOFs, we used aluminium nitrate as an inorganic competitive coordinator to control the growth orientation of copper-based MOFs, such as HKUST-1, MOF-14 and Cu-MOF-74. Through monitoring the reactant composition, we find that the competitive coordination induced by the added aluminium nitrate mainly affects the crystal growth stage rather than the nucleation stage. The kinetic study further reveals that Al3+ competes with Cu2+ to coordinate with ligands, restraining the growth rate of certain facet and resulting in the orientated growth of copper-based MOFs. Hydroxylation of toluene was utilized as a model reaction to investigate the facet-catalytic activity for as-synthesized HKUST-1. The selectivity of the cresol increases with the morphology transformation of HKUST-1 from octahedron to cube.In the third part of the experimental chapter, we used metal oxide to accelerate the deprotonation of ligands. A universal method for controlled-synthesis of urchin-like superstructure MOFs by matching the dissolution of metal source and the crystallization of MOFs was developed through using nanosized metal oxide as precursors. Cu-MOF74, Co-BTC and Mn-BTC with urchin-like superstructures (US) were synthesized successfully by using proposed strategy. Cu-MOF-74 was selected as an example to investigate the mechanism of the formation of superstructure MOFs. Taking advantage of their unique structure, the US-MOFs are used as useful catalysts for selective catalytic reduction of NO.