Abstract
Hierarchical zeolites have the potential to provide a breakthrough in transport limitation, which hinders pristine microporous zeolites and thus may broaden their range of applications. We have explored the use of Pd-doped hierarchical ZSM-5 zeolites for aerobic selective oxidation (selox) of cinnamyl alcohol and benzyl alcohol to their corresponding aldehydes. Hierarchical ZSM-5 with differing acidity (H-form and Na-form) were employed and compared with two microporous ZSM-5 equivalents. Characterization of the four catalysts by X-ray diffraction, nitrogen porosimetry, NH3 temperature-programmed desorption, CO chemisorption, high-resolution scanning transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy allowed investigation of their porosity, acidity, as well as Pd active sites. The incorporation of complementary mesoporosity, within the hierarchical zeolites, enhances both active site dispersion and PdO active site generation. Likewise, alcohol conversion was also improved with the presence of secondary mesoporosity, while strong Brønsted acidity, present solely within the H-form systems, negatively impacted overall selectivity through undesirable self-etherification. Therefore, tuning support porosity and acidity alongside active site dispersion is paramount for optimal aldehyde production.
Highlights
The selective oxidation of alcohols plays a key role in synthesizing carbonyl species and finds widespread applications in chemical industries, including fragrancies, flavourings, pharmaceutical and agrochemical sectors [1]
After impregnation with Pd, the Na-form hierarchical ZSM-5 presented a high Pd dispersion, while the H-form enabled high dispersion; they resulted in a small degree of Pd agglomeration
Extensive characterization confirmed that mesopores in hierarchical ZSM-5 generally benefited Pd dispersion, which elevated PdO formation at the surface of small Pd nanoparticles
Summary
The selective oxidation (selox) of alcohols plays a key role in synthesizing carbonyl species and finds widespread applications in chemical industries, including fragrancies, flavourings, pharmaceutical and agrochemical sectors [1]. Alcohol selox has entailed the use of stoichiometric oxidizing agents, including permanganates [3], chromates [4] or H2O2 [5] Such heavy metal oxidants generate high volumes of aqueous metal salt waste, increasing economic and environmental impact. Supported noble metals (such as Pd and Pt) have been employed as heterogeneous catalysts, with the ability to activate oxygen, either pure or from air, as oxidizing agent under mild operating conditions [6] Such catalytic aerobic selox reactions represent a significant reduction in environmental impact through increased atom-economy and reduced e-factor while reducing energy consumption relative to traditional stoichiometric oxidants [7]
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