Gold-containing Beta zeolite in base-free glucose oxidation – The role of Au deposition procedure and zeolite dopants

Au-Beta zeolites prepared by different methods and on supports of different compositions were studied in base-free glucose oxidation to gluconic acid. The catalysts were characterized by XRD, N2 adsorption/desorption, TEM, UV–vis and FTIR spectroscopy, ICP-OES and pyridine adsorption combined with FTIR measurements in order to estimate the structural/textural and surface properties of prepared materials and their impact on the activity and selectivity in glucose oxidation. A relationship between the gold particle size, depending on the method of gold deposition, and the effectiveness of glucose oxidation to gluconic acid, expressed by glucose conversion and the selectivity to gluconic acid, was observed. The optimal average gold nanoparticles (NPs) size corresponding to the maximum glucose conversion was ∼6 nm, whereas the normalization of structure/size sensitivity to exposed surface gold atoms (TOF) moved this optimum to Au NPs of ca. 24 nm. The presence of niobium in the support reduced the gold particle size, decreased the activity in glucose oxidation and affected the selectivity of the reaction, enhancing glucuronic acid formation. Addition of cerium dopant to the zeolite support for gold resulted in an increase in glucose conversion and stabilization of the catalyst activity for six investigated reaction cycles. Deactivation of the catalysts in successive reaction runs depended on the method of gold deposition and the type of zeolite support.