Gold-based catalysts have shown better performance than palladium or platinum-based ones for the oxidation of carbohydrates (
Figure 1, [
23]). One of the most interesting properties of Au is its ability to convert almost all types of aldoses (e.g., glucose, maltose, xylose) to the corresponding aldonic acids (e.g., gluconic acid). Comotti et al. [
24,
34] reported high activity and selectivity towards gluconic acid using unsupported Au nanoparticles (NPs). However, these systems showed relatively low stability [
35], which was improved by immobilization on carbon [
23,
26].
Taking into account this observation, the gold–support interaction was declared to play a crucial role in the formation of a stable catalytic system [
37,
38,
39]. Various supports were studied using Au NPs of the same size. Different catalytic activities were observed indicating that a specific metal–support interaction between Au and the support was governing NPs’ activity [
36]. On the contrary, Ishida et al. observed that the size of the Au NPs plays a more essential role than the nature of the support [
39,
40]. Excellent performance in terms of activity and recycling was observed for the Au/TiO
2 system. Cao et al. [
41] prepared 1 wt % Au/TiO
2 catalysts using different methods. Once prepared, the catalysts were tested in base-free oxidation of glucose, giving good yields of gluconic acid (64%). The catalyst prepared by the sol immobilization method, using polyvinylalcohol (PVA) as a stabilizing ligand and NaBH
4 as a reductant, showed the highest catalytic activity. The glucose reaction conditions were 0.3 MPa O
2, 160 °C for 1 h. The effects of post-synthesis treatments, i.e., heating in air (at 250–550 °C for 3 h) or treatment with water, were studied. The effect of the quantity of the stabilizing ligand PVA added during the preparation was also studied. Post-synthesis treatments were also applied to remove the residual PVA. Lower activity of Au/TiO
2 catalysts in the presence of PVA resulting from the formation of core–shell structures was reported by Villa et al. [
42]. Indeed, as described in their work, the maximum catalytic activity was reached for the catalysts calcined at 250–350 °C. A higher temperature permits to remove more PVA from the catalyst surface. Moreover, the best PVA to Au ratio was 0.1, as observed by the authors. The group also reported results obtained with magnesium oxide modified with Au using the sol immobilization method. It showed an excellent selectivity towards gluconic acid [
6]. Puyu Qi et al. [
43] used an ordered mesoporous carbon (CMK-3)-supported Au catalyst in the aerobic oxidation of glucose with O
2 under base-free conditions. Catalytic tests showed that the conversion remarkably increased, but the selectivity decreased when oxygen pressure and reaction temperature were increased. Glucose conversion to gluconic acid reached over 92% with 85% selectivity in 2 h, at 110 °C and 0.3 MPa oxygen pressure. Hydrogen peroxide was generated during the reaction, and the relationship between the hydrogen peroxide produced in situ and the formation of fructose as a byproduct was discussed. A low glucose/Au molar ratio minimized fructose formation. A 92% gluconic acid yield was obtained after 15 min of reaction when the molar ratio of glucose/Au was set to 100. The spent catalyst treated with an aqueous solution of NaOH at 90 °C could convert glucose up to 87%, which was close to the result obtained with the as-prepared catalyst, and excluded the effect of any alkaline residue. Wang et al. [
44] studied Au catalysts supported on nano- or micro-sized metal oxides in the base-free oxidation of glucose to gluconic acid. The pH of the reaction solution was kept uncontrolled or, in certain cases, lowered by the addition of a mineral acid. The authors also studied the stability of these catalysts. They observed that the irreversible deactivation of the Au catalysts was due to the leaching and sintering of the gold nanoparticles. To improve the stability and counteract the hydrothermal sintering, they studied lower Au loadings. They observed that, during the oxidation reaction, the Au surface density significantly affected their agglomeration tendency. The best stability was observed for the sample composed of 0.02 wt % Au/μCeO
2. The adsorption of different reactive species on the catalyst surface could also be responsible for the reversible deactivation of the catalyst. However, this adsorption could be removed by the calcination of the used catalyst (preferentially at 325 °C under static air) [
44]. The base-free oxidation of glucose using microwave heating was studied by Rautiainen et al. [
45]. High conversion and selectivity towards gluconic acid were obtained on Au-supported carbon catalysts. Moreover, the use of microwave heating permitted a remarkably short reaction time. Indeed, after only 10 min of reaction, a high yield of gluconic acid was obtained (76%) using H
2O
2 as an oxidant and Au/Al
2O
3 (0.09 mol % Au) as a catalyst. In addition, high turnover frequencies (TOFs = 10,000 h
−1) due to the microwave-assisted oxidation were observed. In terms of stability, the catalyst activity remained constant after four runs, even though some increase in Au particle size was observed. Miedziak et al. [
6] reported the selective synthesis of gluconic acid from glucose under mild conditions. They showed that this reaction could give good results even without any sacrificial base addition or pH control if a basic support was used. To conclude, the oxidation in a base-free medium is quite challenging, and there is still a need to enhance the efficiency of the catalysts used at low pH. Gold NPs-based catalysts permit to obtain high efficiencies in neutral or low pH conditions [
44,
45].
Table 1 summarizes the previous reports on base-free glucose oxidation on supported Au NPs catalysts.