Abstract
Three series of catalysts consisting of gold (Au), platinum (Pt), or gold-platinum bimetallic nanoparticles (NPs) with controlled sizes (Au NPs 10 ± 2 nm, Pt NPs 6 ± 2 nm) anchored on hierarchical micro-/meso-/macroporous silica were successfully developed and systematically evaluated for the selective oxidation of aromatic alcohols to their corresponding aldehydes. The catalysts were prepared by the sol-immobilization method using as-made Au NPs and/or Pt NPs colloids; the silica supports were prepared with controlled pore structures and the hierarchical porous structures of catalysts were created by controllable desilication via the alkaline solution of the metal colloids. The catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), and these results showed no synergistic effect between Au and Pt on boosting the catalytic performance, whereas they demonstrated a clear dependence of catalytic conversions and reaction rates on the structural porosity of Au-Pt bimetallic catalysts. Our findings could potentially inspire peer researchers and scientists to develop designer porous catalysts and processes in the selective organic conversions.
Highlights
Selective oxidation catalysis plays a significant role in the current chemical society for the production of key intermediates and valorization of biomass or bio-derived platform chemicals [1,2], contributing to green and sustainable chemistry [3]
In the aforementioned studies, including bimetallic or alloy-based catalysts, researchers primarily concentrate on electronic effects of catalysts or synergistic effects between multi-metal active sites on boosting the catalytic conversions; less attention has been paid to the steric effect of bimetallic NPs and the change in the pore structures resulted from loading of various metallic NPs
None of the aforementioned four diffraction peaks were observed on pure silica supports (Figure S1a), and the obvious diffraction peaks of both monometallic and bimetallic catalysts revealed that monometallic Au, Pt and bimetallic Au-Pt metal particles were well formed and dispersed on the silica supports
Summary
Selective oxidation catalysis plays a significant role in the current chemical society for the production of key intermediates (e.g., alcohols, aldehydes, organic acids, esters, epoxides, and ketones) and valorization of biomass or bio-derived platform chemicals [1,2], contributing to green and sustainable chemistry [3]. Selective oxidation of aromatic alcohols to aldehydes is a significant process for both laboratory and commercial fine chemical production, because the aldehydes are important organic intermediates or valuable components in the industry of perfumery, dyes, pharmaceuticals, and agrochemicals [4,5] Among these aldehydes, benzaldehyde is the most widely investigated, which is conventionally produced by hydrolysis of benzyl chloride or by vapor-phase partial oxidation of toluene, whereas these processes yield traces of chlorine impurities and a substantial quantity of waste or result in sintering of catalysts, owing to high reaction temperatures (350 ◦ C–450 ◦ C) [6,7,8]. The experimental results demonstrated that silica with larger pores showed higher catalytic performance in terms of catalytic conversion and reaction rates; the steric effect of co-existence of Au NPs and Pt NPs within silica was confirmed All these catalysts showed excellent selectivity toward benzaldehyde. These findings are helpful for catalyst design as the silica supports, with ordered porous structures (e.g., zeolites, MOFs, MCM-41 and SBA-15) are popular for enhancing stability of metal NPs within their channel structures and simultaneously offer high surface areas and inhibit agglomeration or sintering [26,27]
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