Defective hierarchical porous copper-based MOFs synthesised via facile acid etching

Introducing hierarchical pore structure to microporous materials such as metal-organic frameworks (MOFs) can be beneficial for reactions where the rate of reaction is limited by low rates of diffusion or high pressure drop. This advantageous pore structure can be obtained by defect formation, mostly via post-synthetic acid etching, which has been studied extensively on water-stable MOFs. Here we show that a water-unstable HKUST-1 MOF can also be modified in a corresponding manner by using phosphoric acid as a size-selective etching agent and a mixture of dimethyl sulfoxide and methanol as a dilute solvent. Interestingly, we demonstrate that the etching process which is time- and acidity- dependent, can result in formation of defective HKUST-1 with extra interconnected hexagonal macropores without compromising on the bulk crystallinity. These findings suggest an intelligent scalable synthetic method for formation of hierarchical porosity in MOFs that are prone to hydrolysis, for improved molecular accessibility and diffusion for catalysis.

Figure. Nitrogen isotherms of HKUST-1 etching in phosphoric acid using DMSO and MeOH as dilute solvents at different concentrations (a) and times (b). The full isotherms in logarithmic scale in the blown-up sections show 2-stepped adsorption in the samples, which correspond to two different micropores preserved after etching.

This work was published in Scientific Reports 9, 10887 (2019)

Hierarchical metal-organic frameworks with macroporosity: current achievements and challenges

Introduction of multiple pore size regimes into metal-organic frameworks (MOFs) to form hierarchical porous structures can lead to improved performance of the material in various applications. In many cases, where interactions with bulky molecules are involved, enlarging the pore sizes of typically microporous MOF adsorbents or MOF catalysts is crucial for enhancing both mass transfer and molecular accessibility. In this review, we examine the range of synthetic strategies which have been reported thus far to prepare hierarchical MOFs or MOF composites with added macroporosity. These fabrication techniques can be either pre- or post-synthetic and include use of structural templating agents, gelation, defect formation, routes involving supercritical CO2 and 3D printing. We also discuss some challenges involved in the current techniques, which must be addressed if any of these approaches are to be taken forward for large-scale application.

This review was published in Nano-Micro Letters 11, 54 (2019)

Controlled Formation of Hierarchical Metal-Organic Frameworks using CO2 Expanded Solvent Systems

It is shown that a crystalline metal-organic framework (HKUST-1) can be rapidly synthesized from DMSO/MeOH solution with greatly reduced amounts of organic solvents using a supercritical CO2 (scCO2) solvent expansion technique. The precursor solution is stable for months under ambient conditions, and CO2-driven MOF crystallization is achieved under mild conditions (40˚C, 40-100 bar) with excellent reproducibility. As the degree of liquid phase expansion drives MOF nucleation and growth, the crystallite size and overall yield can be tuned by adjusting the CO2 pressure. Furthermore, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and gas sorption analyses showed that in the presence of scCO2, HKUST-1 crystallites with a hierarchical pore structure are generated through a post-crystallization etching process. These findings demonstrate that scCO2 is a time and material efficient route to MOF synthesis with a high level of control over the crystallization process for accessing tailored material properties.

This work was published in ACS Sustainable Chemistry & Engineering 2017, 5, 9, 7887-7893