Palladium-doped hierarchical ZSM-5 for catalytic selective oxidation of allylic and benzylic alcohols

Hierarchical zeolites have the potential to provide a breakthrough in transport limitation, which hinder 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 corresponded aldehyde. Hierarchical ZSM-5 with differing acidity (H-form and Na-form) were employed and compared with two microporous ZSM-5 equivalents. Characterisation of the four catalysts by X-ray diffraction, nitrogen porosimetry, NH3-TPD, CO chemisorption, high-resolution (S)TEM, X-ray photoelectron spectroscopy, 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 from the presence of secondary mesoporosity, while strong Brønsted acidity, present solely within the H-form systems, negatively impacted on overall selectivity through undesirable self-etherification. Therefore, for optimal aldehyde production, tuning support porosity and acidity, alongside active site dispersion, are paramount.