Use of renewables for the production of chemicals: Glycerol oxidation over carbon supported gold catalysts

Abstract

As a renewable feedstock and due to its high functionality glycerol is an attractive reactant for the production of a large number of valuable compounds. We report on an environmentally friendly alternative to produce chemicals from the glycerol oxidation, which are currently produced either by stoichiometric oxidation processes or by enzymatic routes. We investigate the heterogeneously catalyzed liquid-phase oxidation of glycerol with carbon supported gold catalysts. The prepared nanosized gold catalysts are highly active, so that the reaction could be performed under atmospheric pressure. The influence of the preparation method of the catalysts has been investigated. Moreover, the support effect on the catalytic process has been studied and discussed in terms of pore structure of the investigated carbon materials. The promotor effect of platinum on Au/C catalysts was examined and it could be shown that the presence of Pt increases not only the catalyst activity but also the selectivity. By promoting the gold catalysts with platinum the selectivity to dihydroxyacetone could be increased from 26% (Au/C) to 36% (Au–Pt/C).

Introduction

Oxidation reactions are of industrial importance for the synthesis of fine chemicals, even though stoichiometric oxidizing agents (e.g. permanganate) or biotechnological processes are used and a large number of by-products are often formed which decrease the selectivity to the desired oxidation product. An environmentally friendly alternative is the oxidation in the presence of a heterogeneous catalyst and oxygen. Because of the intrinsic reactivity of nanoscaled gold particles (d_Au = 1–10 nm), supported gold catalysts are used for liquid-phase oxidations, e.g. in the glucose oxidation to gluconic acid [1]. In the present work, we report on the heterogeneously catalyzed liquid-phase oxidation of glycerol performed under atmospheric pressure and at constant pH. Because of the high functionality of this molecule a complex reaction pathway exists (Fig. 1), whose single steps can be controlled via the catalyst properties and the reaction conditions. This reaction has been already investigated with classical oxidation catalysts like palladium or platinum supported on carbon [2], [3], [4], [5], [6], [7] and since the last few years also with gold catalysts [6], [7], [8], [9], [10], [11], [12]. Hutchings and co-workers [6], [7] performed the glycerol oxidation in an autoclave reactor under oxygen pressures up to 6 bar without pH control and obtained 100% selectivity to glyceric acid (Fig. 1) at 56% conversion with a 1 wt.% Au/C catalyst. The highest yield of 84% is obtained at 6 bar oxygen pressure and with a glycerol/metal molar ratio of 214. The group of Prati and co-workers [12] could improve this result to 92% at full conversion by optimizing both the catalyst preparation method (1 wt.% Au/C prepared by citrate-protected gold-sol method) and the reaction conditions ($p_{{\text{O}}_2}=3\text{bar}$, glycerol/Au molar ratio = 500) but also without pH control (glycerol/NaOH molar ratio = 4). However, the oxidation reaction of glycerol undergoes an oxidative dehydrogenation mechanism, in which the first step consists on the dehydrogenation of the substrate molecule [5], [15]. We could confirm in a previous study of the Au/C catalyzed glycerol oxidation performed in a batch reactor under pressures up to 10 bar that (i) this first step is only occurring in presence of a base and (ii) the base concentration plays a significant role on the reaction course [13]. Consequently, we focused our following studies on the aerobic glycerol oxidation, so that the pH can be controlled during the reaction. In order to improve the catalyst properties we investigated, amongst others, the promotor effect of platinum on Au/C catalysts. It has been reported in the literature that bimetallic gold catalysts show an improved activity in the glycerol oxidation compared to the corresponding monometallic gold catalysts [14]. But these reactions have been carried out without pH control, so that the pH decreases during the reaction as a function of the product formation rate, which is proportional to the pH depending reaction rate. In this work, we report on the improvement of the Au/C catalyst properties by platinum promoting, obtained at constant pH. To the best of our knowledge, this is the first study of the influence of Pt on the catalyst performance of Au/C under atmospheric pressure and at constant pH.