Preparation of PdAg/CDs-ZSM-5 catalyst and performance study on furfural aqueous phase hydrogenation-rearrangement to cyclopentanone

With the huge demand for fossil resources and increasing energy consumption, the utilization of biomass as a renewable alternative to the production of chemicals and fuels has attracted much attention. Furfural (FFA), as an important biomass-based derived carbonyl compound, can be industrially produced on a large scale from lignocellulosic biomass feedstocks and converted into various high-value chemicals, liquid fuels, and functional materials through a variety of pathways, which is crucial for alleviating the global fossil resource crisis and achieving carbon peaking and carbon neutrality goals. Carbon dots (CDs) are a new type of zero-dimensional carbon-based nanomaterials with particle size usually less than 10 nm, whose core is usually composed of sp² hybridized carbon, and whose surface contains abundant functional groups such as hydroxyl, amino, and carboxyl groups, which have excellent UV-visible absorption, strong proton adsorption, and good stability and hydrophilicity. The high electron-transferring property of the surface functional groups of CDs makes them excellent electron carriers and donors, especially under UV light irradiation. Especially under the irradiation of ultraviolet light, CDs can be used as a reducing agent and stabilizer to reduce metal ions to metal monomers. Zeolite molecular sieves can effectively promote the diffusion of reactants and products in the pores and improve the catalytic activity due to their highly ordered pores, large specific surface area, suitable pore size and good hydrothermal stability. Therefore, molecular sieves can be a good choice of carrier in multiphase catalysis, and their unique domain-limited environment can provide spatial confinement for metal particles to improve the resistance of metals to sintering and prevent the leaching of active metal substances during the catalytic process. Based on this, in this paper, bimetallic PdAg/CDs-ZSM-5 catalysts were prepared by reduction with zeolite molecular sieve ZSM-5 as the carrier and CDs as the reducing and stabilizing agents via UV irradiation and applied to the aqueous-phase hydrogenation-rearrangement of FFA for the preparation of cyclopentanone (CPO) reaction. The CDs and composite catalysts were characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and pyridine infrared (Py-FTIR). The results showed that the CDs had good reducibility and abundant Lewis acidic sites, and were able to reduce Pd²⁺ and Ag⁺ to metal monomers and form PdAg nano-alloy structures. The effects of reaction temperature, reaction time and hydrogen pressure on the reaction performance of the aqueous-phase selective hydrogenation-rearrangement of FFA to produce CPO were investigated using PdAg/CDs-ZSM-5 as catalyst. It was shown that the synergistic effect between the suitable acidic sites on the composite catalyst and the PdAg alloy greatly promoted the rearrangement of the reaction intermediate FAL, thus selectively controlling the hydrogenation of FFA to produce FAL first and then further rearrangement to obtain CPO. 100% conversion of FFA was achieved at the reaction temperature of 160 ℃, 2 MPa H₂, and the target product CPO under the reaction conditions of 4 h. The selectivity of the target product CPO was 92.6%. After reusing the catalyst for 5 times, the conversion of FFA was basically unchanged, and the selectivity of CPO only decreased by 3.6%.