Elsevier

Catalysis Today

Volume 203, 30 March 2013, Pages 196-201
Catalysis Today

The effects of gold nanosize for the exploitation of furfural by selective oxidation

https://doi.org/10.1016/j.cattod.2012.01.033Get rights and content

Abstract

An Au/ZrO2 catalyst was prepared and characterized in order to study the role of gold nanosize in the conversion of furfural into methyl furoate by oxidative esterification with O2 and methanol, without NaCH3O. The comparison with the Au/TiO2 reference catalyst provided by the World Gold Council and with an Au/ZrO2 catalyst treated at 600 °C in oxygen emphasized very good catalytic performances. Highly dispersed Au was evidenced on Au/ZrO2 by quantitative CO chemisorption, HRTEM analysis and FTIR spectroscopy of adsorbed CO. On the contrary, the thermal treatment at 600 °C results in the presence of nanoparticles larger than 4 nm that make the catalyst poorly active and selective. The Au clusters and their ability in the activation of the O2 molecule play a key role in the reaction.

Highlights

► The oxidative esterification of furfural was investigated without the use of NaCH3O. ► Very good catalytic performances were obtained on highly dispersed Au/ZrO2, even if compared to the Au/TiO2 (WGC). ► The Au clusters play a key role in the reaction.

Introduction

The challenge for scientists today is to provide the chemical industry with new tools to convert biomass into useful chemicals in an economically viable fashion. For the C5 fraction (xylose) for example there is no well developed process yet. Furfural can be obtained from xyloses by dehydration in acidic media and it can be used in soil chemistry and as a building block in the production of Lycra®, etc. However, additional transformations of furfural are highly desired: among these, the synthesis of alkyl furoates can open very interesting perspectives for the use of xyloses, because they can be used either as solvent or extracting agents in many different industrial plants if produced in larger amounts and at low price.

Suitable catalysts for the selective oxidation are mainly supported metal samples. In particular, nanodispersed Au has been recognized as a very good catalyst for selective oxidations with molecular O2 [1]. It has been shown [2] that furfural can be converted to methyl furoate by an oxidative esterification with NaCH3O and CH3OH under mild conditions on an Au/TiO2 reference catalyst purchased by the World Gold Council (WGC). However, in order to be applied in a large scale production, the composition and the microstructure of the catalyst have to be optimized. Recently Corma and coworkers [3] have reported a base-free synthesis of methyl furoate on an Au/CeO2 catalyst, but using temperature and pressure higher than those employed in the activity test of Christensen et al. [2].

Catalysis by gold nanoparticles is a topic of current interest, as proved by the exponential growth of the papers on this subject [4]. In fact, gold supported on oxides or carbon, once considered catalytically inert, is now firmly established as an effective catalyst and the catalogue of reactions that it can catalyze is really wide [5]. However, the relationship between activity, microstructure and nature of the catalytically active gold sites is, up to now, not fully understood.

We have recently focused our attention on Au/ZrO2 catalysts [6], [7]. The choice of zirconia as support is due to its intrinsic chemical and physical characteristics that can be adjusted by choosing different precursors and synthesis conditions. Moreover, the addition of dopants, in particular sulfates, increases surface acidity, retards crystallization and enhances the surface area [8]. We have recently demonstrated [9] that sulfate addition to zirconia means a twofold advantage: (i) higher gold dispersion due to higher surface area; (ii) higher gold dispersion due to the positive role of SO42− groups that address the deposition of Au in the form of highly dispersed non metallic gold clusters in close contact with the support. However, no sulfates are present in the final catalysts anymore, due to the detachment of sulfate groups during the deposition–precipitation. So, as already discussed in depth [6], [7], sulfates do not behave as promoters of the gold active phase in the final samples, but they act as structural promoters of the support and as nucleation centers for small gold clusters. For this reason we have named the catalyst object of the paper only as Au/ZrO2 samples.

The goal of the present work is to verify the role of gold nanosize for Au/ZrO2 and Au/TiO2 samples in the oxidative esterification of furfural, without the addition of NaCH3O, that would make the process less green and more expensive [3].

Section snippets

Catalyst preparation

Au/ZrO2 was prepared by a two-step synthesis technique. Zr(OH)4 was prepared by precipitation from ZrOCl2·8H2O at constant pH (pH 8.6) and then aged for 20 h at 90 °C, washed free from chloride (AgNO3 test) and dried at 110 °C for 15 h. The hydroxide was then sulfated with (NH4)2SO4 (Merck) by incipient wetness impregnation in order to obtain a 2 wt% amount of sulfates.

Sulfated zirconium hydroxide was calcined in air (30 mL/min) at 650 °C for 3 h. Gold was added by deposition–precipitation (DP) at pH

Catalytic activity results

The oxidative esterification of furfural has been carried out in methanol without the addition of NaCH3O, that would make the process less green and more expensive [3]. Moreover, mild reaction conditions were employed (120 °C and 6 bar), comparable to those reported in the literature [3].

Apart from the methyl furoate, the only by-product found was the acetal derivate, as detected by mass spectroscopy.

In Fig. 1, section a, the conversion and selectivity to methyl furoate for AuZ are shown. The

Conclusions

Very good catalytic performances were obtained on the AuZ sample, even if compared to the Au/TiO2 catalyst provided by the World Gold Council. AuZ is very active and selective in the oxidative esterification of furfural with CH3OH: no acetal formation is observed, differently from AuWGC.

On AuZ, the presence of very small gold particles and clusters was evidenced by HRTEM analysis, quantitative CO chemisorption measurements and FTIR spectroscopy of adsorbed CO. On the contrary, a thermal

Acknowledgment

The authors gratefully acknowledge the MIUR (PRIN 2008) for financial support.

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