Sol‐immobilized vs deposited‐precipitated Au nanoparticles supported on CeO₂ for furfural oxidative esterification

Abstract

BACKGROUND: The non-edible nature of lignocellulose makes this an important raw material for future biorefineries. This work concerns the oxidative esterification of furfural (a C5 compound) to alkyl furoates, which find applications in the fine chemical industry.RESULTS: Very efficient Au/CeO 2 catalysts for furfural oxidative esterification were prepared by deposition of gold colloids using polyvinyl alcohol as protective agent. The catalysts provided complete conversion and selectivity without the presence of a base. Sol-immobilized catalysts displayed better catalytic performances than Au/CeO 2 prepared by deposition-precipitation (74% conversion). It was proposed that polyvinyl alcohol molecules act not only as gold stabilizer, but they leave ceria support uncovered and available for catalysis. The reaction is extremely sensitive to temperature, but can be carried out at very low pressures. Sol-immobilized catalysts do not require any preliminary calcination to be activated and can be recovered by simple filtration: no oxidation of the exhausted catalyst is required for at least six catalytic runs. CONCLUSION:The new catalyst is active, selective, recyclable and suitable for an industrial chemistry application based on renewable resources. Furoate ester can be obtained with optimal yields by a greener process than is presently used.

New Insights on the Dynamic Role of the Protecting Agent on the Reactivity of Supported Gold Nanoparticles

“…The mass balance ranged between 68.8 % and 74.6 %, due to the substrate degradation to unidentified compounds (possibly volatile compounds or to humins). It is worth noting that, differently from what reported in the literature, the formation of the acetal was never observed under our experimental conditions employing conventional heating, neither in the case of other gold catalysts supported on different oxides The acetal comes from the addition of methanol to the carbonyl group of the aldehyde to form the −(OCH 3 ) 2 acetal group. This reaction takes place if the C=O group of 2‐FA is activated by Lewis acid sites …”

“…The former mechanism is thermodynamically more favoured owing to the low stability of the hemiacetal . This reaction path was already observed for both AuCeDP and AuCePVA catalysts under conventional heating . Conversely, the latter one is slower, due to the stability of the acetal, and was proposed previously by Corma et al …”

“…Interestingly, despite the protecting agent may compete with reactant molecules by interacting with the nanoparticle surface and blocking the active sites, the PVA‐containing catalyst displayed superior catalytic performance in time, as shown in Figure SI‐3. Such catalytic performances were maintained for 6 catalytic runs without requiring any activation procedure, as in the case of AuCeDP (oxidation at 450 °C) . Therefore, the presence of PVA plays a role in stabilizing the gold nanoparticles without preventing the reaction.…”

“…More recently, very efficient gold catalysts were prepared by deposition of gold colloids using polyvinyl alcohol as protective agent . Unexpectedly, the catalytic performance was superior than that of Au/ZrO 2 and above all of Au/CeO 2 catalysts prepared by conventional deposition‐precipitation method .…”

“…Moreover, detailed in situ Fourier transform Infrared spectroscopy measurements of adsorbed molecules at controlled temperature and pressure were performed to have precise information on the exposed sites and on the effect of the presence of polyvinyl alcohol on these sites. It was previously demonstrated that sol‐immobilized catalysts do not require any preliminary activation for at least 6 catalytic runs and can be recovered by simple filtration . In order to further optimise the reaction conditions, the 2‐FA oxidative esterification was carried out under microwave (MW) irradiation in the presence of AuCePVA, AuCeDP and bare ceria catalysts.…”