Elsevier

Applied Catalysis B: Environmental

Volume 129, 17 January 2013, Pages 287-293
Applied Catalysis B: Environmental

Au/ZrO2: an efficient and reusable catalyst for the oxidative esterification of renewable furfural

https://doi.org/10.1016/j.apcatb.2012.09.035Get rights and content

Abstract

Highly dispersed gold based catalysts supported on zirconia were employed in the oxidative esterification of furfural by an efficient and sustainable process. Au/ZrO2 catalysts were calcined at different temperatures in order to modulate gold nanosize. A detailed characterization was carried out for the sake of ascertaining if micro structural changes occurred, and the size issue was discussed. Catalysts stability and recycling were investigated too, and the opportunity of reusability by thermal oxidation at a proper temperature was successfully proved.

Highlights

► The oxidative esterification of furfural was investigated on Au/ZrO2 catalyst. ► Au/ZrO2 is active and selective. ► Au/ZrO2 is recyclable by thermal oxidation at a proper temperature. ► The calcination at a proper temperature allows to obtain the ideal catalyst.

Introduction

The world needs to find new ways for generating energy and platform chemicals, doing so in an economically viable fashion while limiting environmental damage. Biomass can be considered [1] a renewable resource because it can be replenished over a relatively short timescale and it is essentially limitless in supply. In particular, ligno-cellulosic materials (e.g. wood, straw, energy crops) present a good opportunity since they avoid competition with the food sector and often do not requires as much as land and fertilisers to grow. The supply of agricultural wastes, wood industry and forest wastes looks especially promising. So, the development of ligno-cellulosic bio refineries is a strategic target, but it requires the development of new methodologies and synthesis, since they are a complex and highly integrated system. As a matter of fact, the sustainability of bio refineries derives from their ability of exploiting every product, as actually occurs in the oil refineries. In the general framework, the upgrading and valorization of C5 fraction represents a specific relevant issue. Indeed, for this fraction (xylose) there are no well developed process yet. Furfural (2-FA) 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. Nevertheless, additional transformations of furfural are highly desired, that is to find new pathways for converting 2-FA to items which could be integrated into the bio refinery product chain. Among these, the synthesis of alkyl furoates (in particular methyl- or ethyl-derivatives) can open very interesting perspectives for the use of xyloses. Alkyl furoates find applications as flavor and fragrance component in the fine chemical industry.

Up to now, the upgrade of 2-FA has been little investigated in the literature [2], [3]. Traditionally, the furoate ester is prepared by oxidizing furfural with potassium permanganate, preferably using acetone as solvent, and reacting the furoic acid so formed with methyl or ethyl alcohol, in the presence of sulfuric acid. The use of this substance affects a considerable environmental impact. Recently, it has been shown [2] that furfural can be converted into methyl furoate by an oxidative esterification in the presence of a base (NaCH3O) in CH3OH under mild conditions on a Au/TiO2 reference catalyst purchased by the World Gold Council (WGC). However, in order to be applied in a large scale production, this process must be optimized, starting from the composition and the microstructure of the catalyst. Corma and co-workers have recently reported [3] a base-free synthesis of methyl furoate on a Au/CeO2 catalyst, but increasing both temperature and pressure conditions with respect the activity test of Christensen and co-workers [2]. As a matter of fact, catalysis by gold nanoparticles is a topic of current interest, as proved by the exponential growth of the papers on this subject [4], [5].

ZrO2 has been found to be a very suitable support for gold [6], [7], [8], and we have lately focused our attention on Au/ZrO2 samples [9], [10] for the LT-WGSR. Very recently, we have applied such highly dispersed catalysts to the esterification of furfural. We have found [11] excellent catalytic performances of a Au/ZrO2 sample in comparison to the Au/TiO2 catalyst provided by the World Gold Council. The noticeable differences observed between the two samples can be ascribed [11] to the presence on the Au/ZrO2 catalyst of highly dispersed Au clusters able to produce atomic oxygen by reaction with the oxygen molecule. 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 [12]. We have recently demonstrated [13] that sulfates addition to zirconia means a twofold advantage: (i) 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 [9], [10], sulfates do not behave as promoters of the gold active phase in the final samples, but they only act as structural promoters of the support. For this reason we have named the catalysts object of the present paper only as Au/ZrO2 samples.

In the present work we show a detailed and comprehensive investigation on the role of the calcination temperature of Au/ZrO2 samples on gold dispersion and consequently on the catalytic properties in the oxidative esterification of furfural. In particular, we have studied the reaction without the addition of NaCH3O, that would make the process less green and more expensive [3]. The main part of the research highlights in depth all the sample's deactivation reasons in order to assess the possibility of catalyst's recycling. The final goal of the present paper is therefore to obtain an active, selective and recyclable catalytic system for the exploitation of renewable furfural.

Section snippets

Catalyst preparation

Zr(OH)4 was prepared by precipitation from ZrOCl2·8H2O at constant pH 8.6, aged for 20 h at 90 °C [13], washed with warm water free of chloride (AgNO3 test), dried at 110 °C overnight. The hydroxide was sulfated with (NH4)2SO4 (Merck) by incipient wetness impregnation in order to obtain a 2 wt% amount of sulfates on the final support. Sulfated zirconium hydroxide was then calcined in air (30 mL/min STP) at 650 °C for 3 h. 1.5 wt% of gold was added by deposition-precipitation (dp) at pH 8.6: the oxide

The role of gold nanosize on the oxidative esterification of furfural

Catalysts have been calcined at different temperatures in order to modulate gold nanosize and investigate its role in the oxidative esterification of furfural to methyl furoate. Such reaction has been carried out in methanol without the addition of NaCH3O, that would make the process less green and more expensive [3].

A part from the methyl furoate, the only by-product that was found is the acetal derivate, as revealed by mass spectroscopy. Under the experimental conditions, the moles of

Conclusions

Au/ZrO2 calcined at a proper temperature is an efficient catalyst for the oxidative esterification of furfural. In particular, the calcination at 500 °C allows to modulate gold nanosize (below 3 nm) and above all to stabilize gold clusters, that are able to dissociate oxygen.

In addition, beside the choice of the best preliminary calcination temperature, a very important point is the stability and reusability of the catalyst. In particular, an oxidation treatment at the proper temperature of the

Acknowledgments

We thank Mrs.Tania Fantinel for technical assistance. Financial support to this work by MIUR (Cofin 2008) is gratefully acknowledged.

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