Influence of gold particle size in Au/C catalysts for base-free oxidation of glucose

Abstract

A series of gold colloids were prepared and immobilized on commercial activated carbon. The influence of the colloid preparation and stability were studied and related to the gold particle size in the final catalyst. The catalysts show an important activity in the glucose to gluconic acid oxidation reaction, leading to gluconic acid yield close to 90% in base free mild conditions (0.1 MPa O2 and 40 ◦C). The size-activity correlation and probable mechanism were also discussed. Finally, the viability of the catalyst was tested by recycling it up to four times. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Biorefinery, defined as the efficient transformation of renew- able materials to fuels and intermediate chemicals, and associated to environmental and economic benefits, has driven the research in this area to notable increase in the last decades [1–4]. Within the renewable materials the vegetal biomass, mostly constituted by carbohydrates, represents around 75% of the total renewable biomass [5]. Among the carbohydrates represented in this biomass the cellulose remains the most attractive fuel precursor, mainly due to its low price, chemical purity and because it is formed only by one monomer – glucose [6]. After cellulose depolimerazion the subse- quent transformation of glucose to valuable compounds involves a variety of processes such as hydrogenation [7], isomerization [8], dehydration [9] and oxidation [10]. Every single mentioned pro- cess or a combination of them lead to the formation of different ‘platform chemicals’. As an example, the D-Gluconic acid, derived from the oxidation of glucose at anomeric position, results to be an useful food additive and raw material for drugs and biodegradable polymers manufacturing [11,12]. Industrially D-Gluconic acid is ∗ Corresponding author. E-mail address: cristina.megias@icmse.csic.es (C. Megías-Sayago). produced by enzymatic fermentation process [13,14] for which the principal inconvenient for sustainable large-scale production is the necessity of a neutralization step in order to avoid enzymes deac- tivation by the produced acid [15]. This problem could be solved either by using a base or by the substitution of the enzymes with a heterogeneous catalyst able to oxidize glucose under mild base-free conditions by using either O2 or H2O2 as oxidants [16–19]. Although the use of base (NaOH and a relatively high pH of around 9-9.5) results in increase of heterogeneous catalyst’s activ- ity due to particle size stabilization and metal leaching suppression [20–22], a decrease in the selectivity to gluconic acid is often observed caused by the glucose to fructose isomerization process [23]. In addition, the formation of gluconate salt instead of pure glu- conic acid occurs and entails the need of cost effective post-reaction treatment to obtain the target acid. Therefore, a simple base-free heterogeneously catalyzed process able to produce selectively glu- conic acid and avoiding the problems of particle size sintering and metal leaching is highly desirable. Within the catalyst’s candidates for such a process, the most promising alternative is nanometric gold. Glucose oxidation has been carried out over both unsupported [24,25] and supported [20,26] gold catalysts with good results in activity and selectiv- ity; however, some issues must be addressed in order to improve the catalytic system. Various studies reported the base-free aero- https://doi.org/10.1016/j.cattod.2017.01.007 0920-5861/© 2017 Elsevier B.V. All rights reserved.