Palladium Nanoparticles Encaged in a Nitrogen-Rich Porous Organic Polymer: Constructing a Promising Robust Nanoarchitecture for Catalytic Biofuel Upgrading

摘要

Robust nanoarchitectures based on surfactant-free ultrafine Pd nanoparticles (NPs) (2.7-8.2 +/- 0.5 nm) have been developed by using the incipient wetness impregnation method with subsequent reduction of PdII species encaged in the 1,3,5-triazinefunctionalized nitrogen-rich porous organic polymer (POP) by employing NaBH4, HCHO, and H-2 reduction routes. The Pd-POP materials prepared by the three different synthetic methods consist of virtually identical chemical compositions but have different physical and texture properties. Strong metal-support interactions, the nanoconfinement effect of POP, and the homogeneous distribution of Pd NPs have been investigated by performing C-13 cross-polarization (CP) solid-state magic angle spinning (MAS) NMR, FTIR, and X-ray photoelectron spectroscopy (XPS), along with wide-angle powder XRD, N-2 physisorption, high-resolution (HR)-TEM, high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive X-ray (EDX) mapping spectroscopic studies. The resulting Pd-POP based materials exhibit highly efficient catalytic performance with superior stability in promoting biomass refining (hydrodeoxygenation of vanillin, a typical compound of lignin-derived bio-oil). Outstanding catalytic performance (& 98% conversion of vanillin with exclusive selectivity for hydrogenolysis product 2-methoxy-4-methylphenol) has been achieved over the newly designed Pd-POP catalyst under the optimized reaction conditions (140 degrees C, 10 bar H-2 pressure), affording a turnover frequency (TOF) value of 8.51 h(-1) and no significant drop in catalytic activity with desired product selectivity has been noticed for ten successive catalytic cycles, demonstrating the excellent stability and reproducibility of this catalyst system. A size-and location-dependent catalytic performance for the Pd NPs with small size (1.31 +/- 0.36 and 2.71 +/- 0.25 nm) has been investigated in vanillin hydrodeoxygenation reaction with our newly designed Pd-POP catalysts. The presence of well-dispersed electron-rich metallic Pd sites and highly rigid cross-linked amine-functionalized POP framework with high surface area is thought to be responsible for the high catalytic activity and improvement in catalyst stability.