Gel-Based Morphological Design of Zirconium Metal-organic Frameworks

TL;DR

This paper demonstrates the gel-based synthesis of zirconium MOF monoliths with hierarchical porosity, enabling their use in catalytic and adsorptive applications with improved mass transfer.

Contribution

It introduces a novel gel-based method to produce pure Zr-MOF monoliths with hierarchical pores, including shaped monolithic spheres for practical applications.

Findings

Hierarchical pore systems facilitate mass transfer in MOF monoliths.

High reactant concentrations induce gel formation of Zr-MOFs.

Monolithic spheres of UiO-66 were successfully shaped for catalytic uses.

Abstract

The ability of metal-organic frameworks (MOFs) to gelate under specific synthetic conditions opens up new opportunities in the preparation and shaping of hierarchically porous MOF monoliths, which could be directly implemented for catalytic and adsorptive applications. In this work, we present the first examples of xero- or aerogel monoliths consisting solely of nanoparticles of several prototypical Zr4+-based MOFs: UiO-66-X (X = H, NH2, NO2, (OH)2), UiO-67, MOF-801, MOF-808 and NU-1000. High reactant and water concentrations during synthesis were observed to induce the formation of gels, which were converted to monolithic materials by drying in air or supercritical CO2. Electron microscopy, combined with N2 physisorption experiments, was used to show that an irregular nanoparticle packing leads to pure MOF monoliths with hierarchical pore systems, featuring both intraparticle micropores and interparticle mesopores. Finally, UiO-66 gels were shaped into monolithic spheres of 600 micrometer diameter using an oil-drop method, creating promising candidates for packed-bed catalytic or adsorptive applications, where hierarchical pore systems can greatly mitigate mass transfer limitations.