Effects of synthetic parameters on the catalytic performance of Au/CeO2 for furfural oxidative esterification
Graphical abstract
Introduction
Oxidation is a key reaction in organic synthesis and will likely play a significant role in the development of value-added chemicals from biomass. The application of heterogeneous catalysis and molecular oxygen to oxidation reactions offers a green alternative to the use of traditional, toxic chemical oxidants. The potential for biomass valorization in the framework of biorefineries is enormous. However, successfully replacing petroleum-based fuels and chemicals with biomass-based products of lignocellulose will require high-yield, low-cost and energetically efficient targeted upgrading processes.
Dehydration of C6 and C5 biomass sugars under mineral acid solutions leads to the formation of furan compounds including furfural (2-FA) and hydroxymethylfurfural. The two compounds have a very high platform potential for chemical production in biorefineries. For example, oxidation of furfural allows production of relevant carboxylic acids, such as furoic and maleic acids [1]. By oxidative furfural esterification it is possible to obtain methyl-2-furoate, which is used as a flavor and fragrance component and is a higher-added-value product. Christensen et al. have investigated furfural oxidative esterification using a commercial Au/TiO2 catalyst from the World Gold Council in the presence of a base (8% CH3ONa) [2]. Subsequently Corma et al. studied the same reaction using gold-based catalysts, but avoiding the use of the base, which would make the process less green and less advantageous from an economic point of view [3]. We have extensively investigated gold-based catalysts on different supports for base-free esterification of furfural [4], [5], [6]. Very recently, we verified the real mechanism of oxidative furfural esterification with a gold-on-zirconia catalyst under our reaction conditions and identified the best process conditions in terms of pressure, temperature, nature of the oxidizing agent, and reaction time to make the process greener, safer, economic, and sustainable [7]. From literature studies, it is possible to say that the choice of gold nanoparticles is advantageous for oxidative esterification reactions, but it is necessary to improve the performance of the catalyst in order to make the process industrially feasible.
Cerium oxide is a rare earth oxide that has received a great deal of interest from researchers because of its unusual properties, which include high chemical stability, high charge transfer capability, nontoxicity, oxygen ion conductivity, and biocompatibility [8]. Cerium has two stable oxidation states, +4 and +3, and the relatively ease of switching between these two states is the essential factor in its catalytic activity. This rapid change of oxidation state is related to its ability to store and release oxygen, a property measured by the oxygen storage capacity (OSC) [9]. These characteristics make CeO2 a very interesting support for oxidation reactions [10 and references therein].
The study of an efficient Au/CeO2 catalytic system able to operate without the presence of a base, which negatively affects the sustainability of the 2-FA oxidative esterification reaction, is reported here. The goal of the present work is to investigate the effect of the different calcination temperatures to which the support and the final catalysts were submitted on both Au size and ceria support and to check the effect of such features on conversion and selectivity in the esterification reaction.
Section snippets
Synthesis of the supports
Ceria supports were synthesized by precipitation from (NH4)2Ce(NO3)6 by urea in aqueous solution [11], [12]. The solution was stirred and boiled at 100 °C for 6 h; the precipitate was washed twice in boiling deionized water and dried at 110 °C for 15 h (Ce110). Part of this material was calcined at 300 °C (Ce300) and part at 500 °C (Ce500) in flowing air (50 mL/min) for 3 h.
For the Ce90 support, the solution was mixed and boiled at 90 °C, immediately filtered, washed twice in boiling deionized water,
Preliminary characterization of the supports
First, one ceria support was synthesized according to the procedure reported in the literature [11] and simply dried at 110 °C (Ce110). A thermal TG/DTA analysis (Fig. 1) was carried out in order to have information on the behavior of the material during calcination from room temperature up to 900 °C under O2.
The TG curve indicates that the sample underwent weight loss between 50 and 200 °C, corresponding to 4 wt.%. Corresponding to this weight loss there is a large endothermic peak in the DTA
Conclusions
The results put in evidence the important role played by two different steps in the preparation of the samples:
- (i)
the temperature to which the support is calcined;
- (ii)
the final calcination temperature to which the catalyst is subjected after gold dp.
The calcination steps strongly influence the final size of the gold nanoparticles, while the ceria support is not apparently modified in size and structure (with the exception of the Ce90 sample). The calcination at 500 °C made it possible to obtain gold
Acknowledgments
We thank Mrs. Tania Fantinel for technical assistance. Financial support for this work by MIUR (Cofin 2008) is gratefully acknowledged.
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