Aqueous Flow Reactor and Vapour-Assisted Synthesis of Aluminium Dicarboxylate Metal-Organic Frameworks for Use as Water Adsorbents

Energy-efficient indoors temperature control can be realised through adsorption chillers or adsorption heat pumps based on the reversible adsorption and desorption of water in porous materials. Stable microporous aluminium-based metal-organic frameworks (MOFs) present promising water sorption properties for this goal. The development of synthesis routes that make use of available and affordable building blocks and avoid the use of organic solvents is crucial to advance this field. In this work, two scalable synthesis routes under mild reaction conditions were developed for aluminium-based MOFs: (1) in aqueous solutions using a continuous flow reactor and (2) through the vapour-assisted conversion of solid precursors. Fumaric acid, its methylated analogue mesaconic acid, as well as mixtures of the two were used as linkers. The synthesis conditions determine the crystal structure (symmetry and topology), either the MIL-53 or MIL-68 type with square-grid or kagome-grid topology, respectively. Fine-tuning resulted in new MOF materials thus far inaccessible through conventional synthesis routes. Furthermore, by varying the linker ratio, the water sorption properties can be continuously adjusted while retaining the sigmoidal isotherm shape advantageous for heat transformation applications.