The effect of modulator in the synthesis of UiO-66(Zr) and UiO-67(Zr) and their performances in catalytic transfer hydrogenation reaction of α-angelica lactone to γ-valerolactone

Abstract

The synthesis of UiO-66 and UiO-67 metal–organic frameworks (MOFs) with high crystallinity, large surface area, and enhanced porosity presents a significant challenge. This study aims to investigate the effect of modulator variation on the structural and textural properties of UiO-66(Zr) and UiO-67(Zr), and to assess its impact on their catalytic performance in the catalytic transfer hydrogenation (CTH) of α-angelica lactone (AnL) to γ-valerolactone (GVL). UiO-66(Zr) and UiO-67(Zr) were synthesized via solvothermal methods, with varying amounts of modulator introduced during the process. The synthesized materials were characterized using spectroscopy and microscopy techniques. X-ray diffraction (XRD) analysis confirmed the presence of characteristic peaks for both MOFs, while FTIR spectroscopy identified the formation of Zr-O bonds, as indicated by peak broadening at wavenumbers 743 and 591 cm⁻¹ for UiO-66, and 743 and 578 cm⁻¹ for UiO-67. Field emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (FESEM-EDX) revealed that higher modulator concentrations enhanced the clarity of the octahedral shape and increased particle size. Nitrogen physisorption analysis demonstrated improvements in surface area, pore volume, and pore diameter with modulator addition. Thermogravimetric analysis (TGA) indicated that higher modulator content resulted in reduced ZrO₂ residue. The CTH reaction was conducted using 10 wt% catalyst at 90 °C for 6 h under reflux condition. All synthesized materials exhibited catalytic activity, producing GVL from AnL. Notably, UiO-66(Zr) synthesized without a modulator showed the highest activity, achieving 67% AnL conversion and 60% selectivity toward GVL. Interestingly, higher crystallinity, surface area, and pore volume were found to decrease catalytic activity and selectivity, likely due to size selectivity in GVL formation and a reduction in active sites or defects following modulator addition. The reusability tests confirmed the formation of carbon coke, likely due to polymerization reactions initiated by Brönsted acid sites, which subsequently lowered both the yield and selectivity.