pH-dependent catalytic activity of Au and Pd-based hybrid cryogels by investigating the acid/base nature of the polymeric phase

Over the last decade, cryogels have proven to be effective catalytic supports in various hybrid systems for the conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-NP). A critical determinant influencing the conversion of nitroaromatic compounds in the presence of sodium borohydride is the solution pH as suggested from Grzeschik's model. This study aims to investigate the catalytic mechanism and the role of pH in reactions mediated by hybrid catalysts. In particular, polymeric cryogels with different acid/base properties were prepared and used as supports for the in-situ preparation of Au and Pd nanocatalysts. Notably, catalytic tests showed the significant influence of the polymeric support's acidity on 4-NP reduction, with poly(acrylic acid)-based catalysts emerging as the most effective systems. To further probe the reaction mechanism, a series of catalytic tests were carried out, and the results demonstrated the effect of pH on the reaction process, allowing to propose a novel mechanism based on an extension of Grzeschik's model. Moreover, data emphasized the pivotal role of the polymer in the catalytic mechanism, showcasing its capacity to tune catalytic activity by altering the acid/base properties of the matrix substrates.