Updated: Aug 21, 2020
Introducing additional meso- or macroporosity into traditionally microporous metal-organic frameworks (MOFs) is a very promising way to improve the catalytic performance of these materials, mostly due to the resultant reductions of diffusional barriers during reactions. Here we show that HKUST-1 can be successfully synthesised either via post-synthetic treatment (by acid-etching prepared HKUST-1 samples in phosphoric acid, referred to here as “HKUST AE”) or via in situ crystallisation (by exposing the MOF precursor solution to supercritical CO2, referred to here as “HKUST CO2”) to produce hierarchically porous structures that are highly beneficial for catalysis. These hierarchical MOFs were characterised by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and gas sorption to confirm the preservation of the microporous structure and the appearance of macropores in the crystallites. More importantly, the benefits of introducing a hierarchical porous structure into this MOF for improving the diffusion accessibility of reagents to the sample in catalysed liquid- and gas-phase reactions were quantified for the first time. It was found that the hierarchical pore structure helped to improve the catalytic performance in CO oxidation, which is evidenced by the greater extent of the reaction over HKUST CO2 compared to the commercial HKUST-1 sample over the same time period, at temperatures between 220 and 260 oC. The hierarchical porous structure proved even more beneficial in liquid phase reactions where more bulky molecules were involved; here the conversion of styrene oxide in methanolysis was used as an example. These findings serve to demonstrate the advantages of using such hierarchical porous MOFs in catalysis.
Figure. Results of CO oxidative reactions (a) and styrene oxide methanolysis reactions (b), showing an improvement in activity for the hierarchical porous MOF (red squares), compared to the normal microporous MOF (yellow triangles)