Elsevier

Journal of Catalysis

Volume 309, January 2014, Pages 241-247
Journal of Catalysis

Oxidative esterification of renewable furfural on gold-based catalysts: Which is the best support?

https://doi.org/10.1016/j.jcat.2013.10.005Get rights and content

Highlights

  • Gold-based catalysts were investigated in the oxidative esterification of furfural.

  • Catalytic performances follow the trend: Au/ZrO2 > Au/CeO2  Au/TiO2.

  • High Au dispersion and specific surface area are essential for good conversion.

  • Suitable acid–base properties of the support are important for good selectivity.

  • Au/ZrO2 fulfills the compromise and proved to be active, selective, recyclable.

Abstract

Gold-based catalysts over different supports were investigated in the oxidative esterification of furfural by employing an efficient and sustainable process. The catalytic performances follow the trend: Zirconia–Au > Ceria–Au  Titania–Au. Zirconia came out to be the best support option to promote activity, selectivity and also stability. The chemical and morphological properties observed for zirconia-supported sample seem to fulfill a good compromise between high gold dispersion and the presence of suitable acid–base properties, for good selectivity. Moreover, stability and recycling of the catalysts were also investigated.

Introduction

With diminishing fossil resources, developing new technologies to utilize versatile and renewable biomass as the alternative feedstock of energy and chemical sources has been attracting more attention than ever. In particular, the upgrading of lignocellulosic biomass wastes into fuels and higher added-value chemicals is one the most researched topics in the forthcoming concept of biorefinery [1].

The sustainability of bio-refineries derives from their ability of exploiting every product. Furfural (2-FA), a C5 compound, is industrially manufactured for a long time through hydrolysis of pentose which comes from agricultural raw materials including corncobs, oat, wheat bran, sawdust, etc. These materials are yearly renewable and not competitive with the food sector. Thus, exploring the products derived from furfural as the replacements of fossil resources is greatly attractive. In fact, utilization of furfural as the starting material could synthesize a variety of chemicals including more than 1600 commercial products [2].

Actually furfural has many different uses: it is considered an excellent solvent for many organic materials, and it can also be used as a feedstock to make gasoline, diesel, or jet fuel [3]. It is also a precursor to other desired compounds such as furfuryl alcohol (via hydrogenation), furan (via decarbonylation) and tetrahydrofuran (via hydrogenation of furan). However, additional transformations of furfural are highly desired [4]. There already have been efforts to modify furfural-based derivatives by gold catalysis [5], [6], [7], [8]. The synthesis of alkyl furoates can open very interesting perspectives for the use of xyloses because they find applications as flavor and fragrance component in the fine chemical industry.

Up to now, only a few studies have investigated the heterogeneous catalysis in oxidation of 2-FA [9], [10]. Traditionally, the 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 these substances has a substantial negative impact on the environment. It has been shown [9] that furfural can be converted to methyl furoate under mild conditions by an oxidative esterification with NaCH3O and CH3OH on the Au/TiO2 reference catalyst purchased from the World Gold Council (WGC) [11]. In fact, gold supported on oxides or carbon, once considered catalytically inert, is now firmly established as an effective catalyst [12]. Very recently, we have observed [13] good catalytic performances during esterification of furfural over a gold-supported sulfated zirconia support, especially when compared with the Au/TiO2 reference catalyst. Homogeneous oxidative esterifications by gold have also been reported, but most unfortunately, these depend on peroxides as oxidants [14]. Afterward, we have investigated a series of Au/ZrO2 catalysts calcined at different temperatures (from 150 °C up to 650 °C) in order to modulate the size of the gold nanoparticles, demonstrating that in this reaction, the catalytic activity is strictly connected to the metal dispersion. In particular, the presence of highly dispersed gold clusters ability to activate atomic oxygen is required for good catalytic performances. Moreover, the stability of these new catalysts has been also studied [15]. We demonstrated that the catalytic activity can be completely recovered when the organic residue of the exhausted sample is removed from both gold and zirconia sites [15]. Such results suggested that the support also plays a role in the furfural esterification reaction.

Therefore, we decided to investigate different oxidic supports that are commonly used in catalysis. In particular, we examined plain titania (TiO2), ceria (CeO2), and zirconia (ZrO2). TiO2 is widely used for a variety of applications because of its high photocatalytic activity, non-toxicity, good availability, low cost, and stability. Its main characteristics strongly depend on its physicochemical properties, such as surface area, crystal structure (anatase, rutile, brookite), crystallite size, and surface hydroxyl groups [16]. Ceria is characterized by a high oxygen storage capacity and reducibility [17]: we can take advantage of these properties in furfural esterification reaction, in which atomic oxygen produced on gold species play a fundamental role. Moreover, gold on cerium oxide support has been shown by IR studies to stabilize gold(III) for heterogeneous catalytic applications [18]. Finally, the choice of zirconia as a support is due to its intrinsic chemical and physical characteristics that can be adjusted by choosing different precursors and synthesis conditions [19].

The aim of the present work is to verify the role of the nature of the support in the base free oxidative esterification of furfural catalyzed by gold-based systems. In particular, the goal is to investigate the above tested catalysts by employing the main characterization techniques typically used in surface science approach.

Section snippets

Catalyst preparation

Zr(OH)4 was prepared by precipitation from ZrOCl2·8H2O at constant pH = 8.6 and then aged for 20 h at 90 °C [20]. Then, zirconium hydroxide was calcined in air (30 mL/min STP) at 650 °C for 3 h.

Ceria support was synthesized by precipitation from (NH4)2Ce(NO3)6 by urea at 100 °C in aqueous solution [21], [22]. The solution was mixed and boiled for 6 h at 100 °C, and the precipitate was washed twice in boiling deionized water and dried at 110 °C overnight. The material was then calcined in flowing air (50 

Texture, morphology, and structure of the catalysts

N2 physisorption analyses were carried out in order to determine surface areas and pore size distributions of both supports and catalysts. No significant differences between the N2 physisorption analyses of the bare supports and those of the corresponding gold-based samples have been evidenced. The isotherms for the catalysts are shown in Fig. 1, while the corresponding data are reported in Table 1.

The C–Au and T–Au samples exhibited isotherms with hysteresis loops typical of mesoporous

Conclusion

Gold-based catalysts were investigated in the oxidative esterification of furfural by an efficient and sustainable process. The furoate ester can be obtained with optimal yields by a process more environmentally friendly than the actual one.

A comparison among metals, such as Zirconium, Cerium, and Titanium, as a support for Au-based catalysis was performed. Catalytic performances followed the trend of Z–Au > C–Au  T–Au. Both chemical and morphological properties, such as (i) high dispersion of Au,

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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