Promotion of phosphoester hydrolysis by the ZrIV-based metal-organic framework UiO-67

The ZrIV-based metal-organic framework (MOF) UiO-67 has been examined as a promoter of the hydrolysis of phosphoester bonds by using sodium para-nitrophenylphosphate (pNPP) as a model substrate. The reactions were followed by 1H NMR spectroscopy and performed under mild conditions using 2–100 mol% of the MOF relative to pNPP. All of the systems studied promoted the hydrolysis of pNPP to give para-nitrophenol (pNPh) and inorganic phosphate. A reaction half-life of ca. 30 min was achieved using 17 mol% of UiO-67 at 55 °C and 30 mol% at ambient temperature; ≥98% removal of pNPP was reached within 1 h for both reaction temperatures. 1H NMR spectra of the reaction solutions, together with powder X-ray diffraction, FT-IR spectroscopy and solid-state NMR data for the recovered MOF, revealed that (i) pNPP was encapsulated and then converted to pNPh within the cavities of UiO-67, (ii) both pNPh and inorganic phosphate were retained in the solid promoter, and (iii) the MOF suffered partial structural breakdown into the components Zr6O4(OH)4 and 4,4′-biphenyldicarboxylate, with partial release of the latter into solution. These structural changes eventually compromized the recyclability of the promoter, although the material could be recovered and reused in a second cycle without loss of activity.