Enhanced Catalytic Activity through the Variation of Hierarchical Porosity in a Zr-Based Metal–Organic Framework

Herein, we present a study that demonstrates variation of the hierarchical porosity (HP) of metal–organic frameworks (MOFs) obtained by changes in the synthetic conditions, has significant effects on the accessibility of bulkier molecules toward the active sites in the HP MOFs and helps in overcoming the mass-transfer limitation. Two materials of UiO-66-NH₂, HP-MOF-1 and HP-MOF-3, with narrow size distribution of mesopores around 5 nm and with higher relative meso- to microporous volumes have been obtained by surfactant-assisted synthesis using the gemini surfactant, G_14–6–14, in water–ethanol mixed solvent compositions. A different pattern of HP has been observed for HP-MOF-2 and HP-MOF-4, with much broader mesoporous size distributions in the 10 to 25 nm range and lower relative meso- to microporous volumes that are obtained from similar but in the surfactant-/template-free synthetic conditions. Involvement of both the lamellar and vesicular mixed assembly of G_14–6–14 as a mesoporous template is demonstrated in the formation of HP-MOF-1 and HP-MOF-3. The broader pore-size distributions and higher external surface areas found in our HP-MOF-2 and HP-MOF-4, are attributed to the intercrystallite voids, which occur due to the aggregation of MOF nanoparticles in the template-free conditions. Dye-uptake studies in our MOF samples, involving bulky molecules of different shapes, sizes and charges, provided useful insights into the active-site accessibility and also about the mode of uptake. The HP-MOF-1 and HP-MOF-3 achieve faster kinetics and higher catalytic conversions of bulky fatty acids into corresponding methyl esters (biodiesels) compared to HP-MOF-2 and HP-MOF-4 and their microporous counterpart on account of the increased accessibility to the active sites and better mass transfer kinetics in the hierarchically connected micropores and mesopores.