Elsevier

Journal of Catalysis

Volume 326, June 2015, Pages 1-8
Journal of Catalysis

Structure–activity relationships of Au/ZrO2 catalysts for 5-hydroxymethylfurfural oxidative esterification: Effects of zirconia sulphation on gold dispersion, position and shape

https://doi.org/10.1016/j.jcat.2015.03.006Get rights and content

Highlights

  • The process of oxidative esterification of HMF to FDMC was investigated on Au/ZrO2.

  • The effects of different sulphates amount on zirconia support were studied.

  • Sulphate groups regulate the gold amount, dispersion, shape and position.

  • The reactivity is closely tied to gold shape and distribution on the support.

  • Irregular plate-like Au nanoparticles were observed and they affect selectivity.

Abstract

The oxidative esterification of 5-hydroxymethylfurfural (HMF) to furan-2,5-dimethylcarboxylate (FDMC) has been investigated on Au/ZrO2 catalysts. We have examined bare zirconia and sulphated zirconia with different amounts of sulphates in order to identify the effects of the sulphation pretreatment. The amount of sulphate groups present on zirconia regulate the gold amount and dispersion, as well as its shape and position. It has been found that the reactivity is associated not only to the Au active phase, in terms of loading and size, but it is also closely tied to its shape and distribution on the support. Irregular plate-like gold nanoparticles have been observed on the sulphated samples and they affect the selectivity to FDMC. Structure–properties relationships have been determined and a model has been proposed.

Introduction

The catalytic conversion of biomasses provides a sustainable alternative to produce non-fossil-based chemicals and biofuels. Such opportunity can reduce the dependence from petroleum-based resources [1]. Actually, carbohydrates coming from biomass can be considered the most abundant renewable resource [2]. The nonedible nature of lignocellulose, which implies no competition with food, makes this an important raw material for biorefineries in the future. According to Scheme 1, lignocellulosic biomass is a complex matrix of cellulose, hemicellulose and lignin. It contains two types of sugars: hexoses and pentoses, the ‘C6’ and ‘C5’ fractions, respectively.

The transformation of selected platform molecules is the most appropriate approach for identifying molecules that can replace those coming from petrolchemistry or for the generation of new molecules that display improved or different properties. The transformation of 5-hydroxymethylfurfural (HMF) in valuable furan derivates for biofuel and fine chemical industry is an example of this approach [3], [4]. HMF can be produced by acid-catalysed dehydration of lignocellulosic building blocks such as glucose and fructose, as well as directly from cellulose [5].

HMF is a versatile intermediate that can be further transformed into a wide variety of high performance and high value-added chemicals. Among the various conversion routes, the HMF molecule can be catalytically oxidized to 2,5 furandicarboxylic acid (FDCA). The catalytic oxidation to FDCA is an essential process, because FDCA has been recently proposed as a possible alternative for terephthalic acid [6] which is the monomer used for the production of polyethylenterephthalate (PET) plastic.

Actually, PET is mainly manufactured through the purified terephthalic acid, whose process is based on the liquid-phase oxidation of p-xylene [7], [8]. Bioaccumulation in living organisms displayed by phthalates and derived polymers has driven towards new alternative routes. The polymer that can be produced starting from FDCA (PEF) presents a lot of advantages: (i) it is obtained from a bioderived, renewable raw material, (ii) it is more easily degradable, and above all, (iii) it does not present bioaccumulation problems. However, FDCA is practically insoluble in most of the solvents industrially used. A promising alternative is the HMF oxidative esterification into the corresponding furan-2,5-dimethylcarboxylate (FDMC), according to Scheme 2.

The FDMC molecule can be easily purified by low-temperature sublimation to give high-purity FDMC, which is readily soluble in the most common solvents, and therefore, it could be even more suitable than FDCA as monomer for the replacement of terephthalic acid in plastics [9], [10].

Some reviews on the selective oxidation of furan derivates [11], [12], [13] have been published in the last years, meaning that a great attention has been paid to these reactions. The necessity of the clean production of value-added chemicals, such as FDMC from HMF, increased the demand for aerobic catalysts that use molecular oxygen as oxidant and produce only water as by-product. The use of heterogeneous catalysts would be of particular interest because HMF can be easily oxidized, being an alcohol and an aldehyde at the same time. Recently, gold catalysts have received attention as promising oxidation candidates [9], [10], [14], [15], [16], indicating that the choice of gold nanoparticles is advantageous for oxidative esterification reactions.

Good yields in the HMF oxidative esterification were obtained by using the Au/TiO2 reference catalyst provided by the World Gold Council in the presence of a base (8% CH3ONa) [9]. However, the use of the base makes the process less environmentally and economically advantageous. To overcome such problem, Corma et al. [10] tested this reaction over gold based catalysts on different supports, without using the base. Satisfactory performances were observed only in the case of one particular catalyst dispersed on ceria nanoparticles, while the other samples require reaction times ranging from 24 to 72 h to get still very low yields.

Very recently, we investigated gold catalysts supported on zirconia [17], [18], ceria and titania [19] for a base-free esterification of furfural. In particular, the comparison among Au samples with similar metal content over different supports revealed that the catalytic performances follow the trend: Au/zirconia > Au/ceria >> Au/titania. The furoate ester can be obtained with optimal yields by taking into account both Au nanosize (for good conversion) and acidity/basicity properties of the support (for good selectivity). The results for HMF oxidative esterification have shown that in reaction conditions, oxygen is fundamental to keep the active surface sites free, in particular the highly dispersed gold sites, where oxygen can be dissociated [20]. As a consequence, the dispersion of gold is a crucial point, due to the role played by basic oxygen atoms in the activation of both methanol and HMF reactants.

In the past years, we have demonstrated that sulphates act as structural promoters of the zirconia support. In particular, the addition of sulphates plays a positive role, resulting in an enhancement of the surface area of the oxide and in a higher gold dispersion [21]. However, no sulphates are present in the final catalysts anymore, due to the detachment of sulphate groups during the deposition–precipitation at pH = 8.6.

In the present paper, we investigated the role of the sulphation degree of zirconia on the nature and dispersion of supported gold species and consequently on the catalytic properties. In particular, such a screening on sulphation has been carried out in order to further improve catalytic performances of the Au/ZrO2 system for the HMF oxidative esterification reaction in the absence of NaCH3O. Au/zirconia catalysts with the same gold loading and in which with different amount of sulphates have been added before the metal introduction will be examined. The results will be compared with those obtained from a gold catalyst supported on plain zirconia and one reference catalyst provided by the World Gold Council.

Section snippets

Catalyst preparation

Zr(OH)4 was prepared by precipitation with ammonia (5 M) from ZrOCl2⋅8H2O at constant pH = 8.6, aged for 20 h at 90 °C and then dried at 110 °C for 20 h. Part of the hydroxide was sulphated with (NH4)2SO4 (Merck) by incipient wetness impregnation in order to obtain a 2 or 8 wt% amount of sulphates on the final support. Then zirconium hydroxide (Z) and sulphated zirconium hydroxides (SZ2 and SZ8) were calcined in air (30 ml/min STP) at 650 °C for 3 h.

Gold was added by deposition–precipitation (dp) at pH = 

Preliminary characterization of the supports

Very recently, we compared Au samples with similar metal content over different supports for the oxidative esterification of furfural [19]. The Au/zirconia system displayed the best catalytic performances indicating that the chemical and morphological properties observed for the zirconia-based catalyst seem to fulfil a compromise between high gold dispersion and the need of nonacid or basic sites exposed by the support. It is well known that the sulphation procedure induces an increase of the

Conclusions

Zirconia-supported catalysts are active in the HMF oxidative esterification even without the use of a base such as sodium methoxide which makes the process less sustainable from an environmental and economic point of view. The best catalytic activity was observed with gold supported on a sulphated zirconia with a low percentage of sulphate groups (2 wt% nominal sulphates content).

It has been observed that the amount of sulphates not only increases the surface area of zirconia and the gold amount

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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