Abstract:
Covalent Organic Frameworks are crystalline polymers with intrinsic porosity and
covalent linkages connecting their building blocks. They have been utilised extensively in gas storage, sensing, charge storage, catalysis, and other applications due to their great chemical stability, surface area, and tunability. Encapsulating COFs with small-sized metal, metal oxide, or oxyhydroxide nanoparticles enables them to serve as
heterogeneous catalysts with recyclability, high selectivity, yield, and ease of handling.
Lewis acidic zirconium, surface hydroxyl groups, and the equilibrium between oxide and
hydroxide ions make zirconium oxyhydroxide a potential multifunctional catalyst, and
loading of zirconium oxyhydroxide nanoparticle inside COF makes it a potential
multifunctional heterogeneous catalyst. Herein, we report the successful loading of
zirconium oxyhydroxide inside COF with precise control on the amount of loading. We
achieved a zirconium loading of nearly 7.8 wt% and found that the loaded zirconium
oxyhydroxide is converted to zirconium oxide at higher temperatures by burning the
loaded COF under air at 600°C. The saturation N2 uptake of the pristine COF was 10
mmol/g, while the saturation N2 uptake of the zirconium oxyhydroxide-loaded COF was 6
mmol/g. Zirconium oxyhydroxide loaded COF was employed as a catalyst for the
Knoevenagel condensation reaction. Our catalyst exhibits significant activity at room
temperature with a catalyst loading of 0.8 mol% and a turnover number of 103-120, and a turnover frequency of 2.78 - 6 /min using water as a solvent. The catalyst has been
proven to be recyclable for up to ten cycles. After catalysis, the crystallinity of COF has
been slightly compromised, but the chemical linkages have remained intact. Additionally,
2.2 mol% of zirconium oxyhydroxide loaded COF can efficiently catalyze the hydrolysis
of Dimethyl nitrophenyl phosphate (DMNP), a mimetic of nerve agent Soman, with a conversion rate of 37% in just over 180 minutes while the precursors COF and zirconium oxyhydroxides showed minimal-low activity.
