Insight into the hydrogen mobility upon Pt/ZSM-5 and its catalytic function during liquid-phase hydrogen isotopes exchange

Liquid-phase hydrogen isotopes exchange between hydrogen and water is a promising route for the hydrogen isotopes separation from water, catalyzed by the metal Pt with hydrophobic coating to surmount the flooding of reactive sites. However, poor water activation ability on Pt has limited the catalytic efficiency of Pt-based catalysts and hindered their application. Herein, we report a bifunctional Pt/ZSM-5 catalyst with hydrophobic modification by surface silylation for H2-HDO isotope catalytic exchange. The integration of metallic Pt and Brønsted acid (BA) sites in the microporous ZSM-5 boosts the catalytic efficiency of deuterium extraction by two orders of magnitude compared to commercial Pt-C at the optimum condition. In-situ spectroscopic and theoretical studies demonstrated that HDO-H2O preferentially adsorbs on BA sites (-Si-OH-Al) to form protonated water clusters stabilized by hydrogen-bonded network or to undergo the proton exchange to leave deuterium on zeolite framework as a new acidic site ([AlO4D]). Under the attractive interaction, the dissociated hydrogen on Pt clusters releases partial electrons to form positively-charged hydrogen, which further transfers to the [AlO4D] sites or protonated water clusters by interfacial spillover to produce the overcharging intermediates. These intermediates cleave their O-D bonds to form positively charged D, which ultimately spills back to Pt cluster and combines with the remaining electron to desorb in the form of HD. The synergistic effect between Pt and BA sites creates a new pathway of hydrogen transfer, which enables the more efficient hydrogen-water isotope exchange on the bifunctional Pt/ZSM-5 catalyst.