(266g) Gold Supported On Nanocrystalline Metal Oxides Are Highly Effective Catalysts for the Oxidation of Glucose Under Base-Free Conditions

Selective oxidation of glucose to produce mono- and di-carboxylic acids is an attractive catalytic technology for the production of valuable chemical intermediates from biomass. Currently, most heterogeneously-catalyzed processes for the transformation of glucose to gluconic acid require the use of corrosive basic promoters (e.g. NaOH) to regulate the pH of the reaction, given that severe catalyst deactivation occurs once the solution enters the acidic regime. The development of a catalyst that maintains high catalytic activity and product selectivity in the absence of basic promoters is of considerable importance.  Here, we demonstrate that gold nanoparticles supported on nano-crystalline metal oxide supports are active and stable catalysts for the oxidation of glucose to gluconic acid under base-free conditions. We show that the size of the metal oxide is a critical factor to achieve catalyst stability. Thus, although Au NPs supported on micro-sized metal oxides are initially as active as Au NPs supported on nano-sized supports, only the latter remain active over several cycles without severe metal aggregation and leaching.  Similarly, only Au NPs on nano-crystalline supports are capable of performing the oxidation reaction under acidic conditions (pH = 1.6, H₂SO₄) with rates similar to those obtained under uncontrolled pH conditions, while also maintaining activity over multiple cycles. This unique feature opens an attractive avenue to pursue cascade reactions involving acid/redox sequences (e.g., hydrolysis followed by oxidation).