Theoretical and experimental studies of gold catalysts for glucose oxidation

The glucaric acid (GLA) has been identified as a “top value-added chemical from biomass” that can be employed for many uses; for instance, it could be a precursor of adipic acid, a monomer of Nylon-6,6. GLA can be synthetized by the oxidation of glucose (GLU), passing through the intermediate gluconic acid (GLO). In recent years, a new process has been sought to obtain GLA in an economic and environmental sustainable way, in order to replace the current use of HNO3 as a stoichiometric oxidant, or electrocatalysis and biochemical synthesis, which show several disadvantages. Thereby, this work is focused on the study of catalysts based on gold nanoparticles supported on activated carbon for the oxidation reaction of GLU to GLA using O2 as an oxidant agent and NaOH as base. The sol-immobilization method leads us to obtain small and well dispersed nanoparticles, characterized by UV-Vis, XRD and TEM techniques. Repeating the reaction on different batches of catalyst, both the synthesis and the reaction were confirmed to be reproducible. The effect of the reaction time feeding GLO as reagent was studied: the results show that the conversion of GLO increases as the reaction time increases; however, the yields of GLA and others increase up to 1 hour, and then they remain constant. In order to obtain information on the catalytic mechanism at the atomistic level, a computational study based on density functional theory and atomistic modeling of the gold nano-catalyst were performed. Highly symmetric (icosahedral and cubo-octahedral) and distorted Au55 nanoparticles have been optimized along with Au(111) and Au(100) surfaces. Distorted structures were found to be more stable than symmetrical ones due to relativistic effects. On these various models the adsorptions of various species involved in the catalysis have been studied, including OH- species, GLU and GLO. The study carried out aims to provide a method for approaching to the study of nanoparticellary catalytic systems.

Ossidazione elettrocatalitica di glucosio su schiume di nickel per la produzione selettiva di acido gluconico e acido glucarico

The selective electro-oxidation of D-glucose represents a promising way for the synthesis of many platform molecules, currently produced from non-renewable fossil fuels. Electrocatalysis is gaining considerable interest as an alternative to the thermochemical process, since it allows the process to be sustainable by operating at room temperature and pressure as well as using green solvents and electricity produced from renewable sources. In this work, the activity of three electrocatalysts, based on open cell Ni foams, towards the electrochemical oxidation of glucose to gluconic acid and glucaric acid was initially compared: a Ni bare foam, and Ni foams calcined at 500 °C for one hour and electrodeposited with Ni NP nanoparticles. The calcined foam turned out to be the most efficient. Then, the performance of this Ni calcined electrocatalyst was studied varying the reaction conditions, such as the potential applied (from 0,5V to 1,0V vs SCE), D-glucose concentration (0,01M, 0,05M and 0,10M) and NaOH concentration (0,10M and 1,0M). The activity of the electrocatalysts was evaluated in terms of glucose conversion, gluconic and gluconic acid selectivity and faradic efficiency. The best results were obtained over a Ni calcined foam with a 0,05M solution of D-glucose and 0,10M of NaOH at a 0,80V vs SCE potential.