A hydrophobic titanium doped zirconium-based metal organic framework for photocatalytic hydrogen peroxide production in a two-phase system

Abstract

Photocatalytic synthesis of hydrogen peroxide (H₂O₂) from O₂ coupled with the synthesis of valuable products from benzyl alcohol (BA) by a hydrophobic metal organic framework (MOF) is attractive. Herein, a hydrophobic titanium doped zirconium-based MOF (OPA/Zr_100−xTi_x-MOF), whose metal Zr clusters are alkylated by octadecylphosphonic acid (OPA), is synthesized and used for photocatalytic H₂O₂ production in a two-phase system (water/BA). This allows the formation of H₂O₂ in the water phase and benzaldehyde, the oxidation product of BA, in the organic phase, respectively. The hydrophobic OPA/Zr_92.5Ti_7.5-MOF exhibited a remarkable H₂O₂ production rate of 9.7 mmol L⁻¹ h⁻¹ under the irradiation of visible light (λ > 420 nm), which is about 4.5 times higher than that of the parent zirconium-based MOF (Zr₁₀₀-MOF). The enhanced activity is attributed to the effective Ti-doping and the unique hydrophobic nature of the catalyst. The Ti species played a role in effectively promoting electron transfer from photoexcited linkers of the MOF to Ti, inhibiting the recombination of photogenerated electron–hole pairs in the hydrophobic MOF matrix. The unique hydrophobic nature, exemplified by a 139° water contact angle, prevented overreduction of H₂O₂ by spatial separation of the MOF in BA and H₂O₂ in water. The recycling test of H₂O₂ production over the catalysts demonstrated that OPA/Zr_92.5Ti_7.5-MOF displayed good stability. This study has greatly enriched the applications of hydrophobic MOFs in the field of photocatalytic energy production.