Ligand-protected and lowered-temperature hydrothermal synthesis of platinum encapsulated in TON zeolite for shape-selective hydrogenation of furfural to furfuryl alcohol

Abstract

Encapsulating metal in zeolite is an effective tactic to up-regulate the catalytic selectivity of metal/zeolite catalysts in hydrogenation reaction by virtue of the spatial confinement of the zeolite microchannels. Herein, we present the synthesis of Pt encapsulated in zeolite with TON topology (Pt@ZSM-22) by adopting the ligand-protected and lowered-temperature hydrothermal crystallization. The XRD, SEM, TEM, TG-DSC-MS, and ¹³C CP/MAS NMR are used to track the hydrothermal process. The experimental results indicate the decomposition and reduction of metal precursor ([Pt(en)₂]²⁺), which usually occurs at the harsh hydrothermal conditions, is effectively restrained at the lowered hydrothermal temperature (140 °C) and with the protection of the ligand (ethylenediamine). The intact [Pt(en)₂]²⁺ is electrostatically adsorbed on the amorphous silicate nanoparticles (the zeolite precursor) and is encapsulated inside the ZSM-22 crystals as these nanoparticles are crystallized. The highly-dispersed and uniform Pt particles embedded inside the ZSM-22 zeolite are successfully obtained by adopting the direct H₂ reduction to remove the template and reduce the [Pt(en)₂]²⁺. The hydrogenation of furfural to furfuryl alcohol was conducted to evaluate the selective hydrogenation performance of the encapsulated Pt@ZSM-22. The reaction results reveal the furfuryl alcohol selectivity reaches as high as 97.6% at a conversion of 99.5% over the encapsulated Pt@ZSM-22, which is superior to the supported Pt/ZSM-22. The excellent furfuryl alcohol selectivity reflects the shape selectivity conferred by the spatial confinement of the one-dimensional microchannels of ZSM-22. The CO-FT-IR, XPS, XAFS and FT-IR of adsorbed furfural are used to disclose the structure-activity relationship of Pt@ZSM-22. Our work not only successfully realizes the direct hydrothermal synthesis of metal encapsulation in zeolite with 1D straight channels but also demonstrates the great application potentials of such catalysts in selective catalysis.