Support acid-base properties as a tool for directing selectivity in the Au-Pt catalyzed base-free glycerol oxidation

Concept Glycerol is a promising chemical building block for the synthesis of fine chemicals from renewable sources [1]. In particular glycerol oxidation shows a complex reaction pathway that can lead to a large number of valuable organic compounds (glyceric acid, tartronic acid, dihydroxyacetone, etc.). Supported noble metal nanoparticles have been used as catalysts for this reaction when molecular oxygen is employed as the oxidant [2]. Depending on the reaction conditions and the noble metal employed, the reaction pathway can be directed either to the oxidation of the primary or the secondary hydroxyl group. In particular Au NPs reveal peculiar properties in terms of both activity and selectivity. However only recently it was found that the catalytic oxidation of glycerol can be carried out without the use of a basic environment, being it a great advantage from an industrial point of view. In this view it has been shown that basic supports greatly enhanced the activity of AuPt alloyed catalysts. In this work AuPt NPs based catalysts were tested in the base-free glycerol aqueous phase oxidation, using supports with different acid/base properties. Experimental results showed that the support surface properties not only influenced the catalyst activity but also greatly affected the selectivity to C3 compounds. Motivations and Objectives Recent studies [3] [4] showed that a careful choice of the support of AuPt alloyed nanoparticle lead to effective catalytic systems in terms of both activity and selectivity, even in the absence of a basic environment. In particular these works opened the way to a new-generation of bifunctional gold-based catalysts where the acid/base properties of the support play a crucial role, affecting activity and selectivity of the reaction. Herein we supported AuPtNPs on a series of supports with different acid-base character in order to clarify the role of the surface properties of the support on catalyst activity/selectivity in the base-free glycerol oxidation. Results and Discussion AuPt nanoparticles were immobilized on, MCM-41, SiO2, NiO and MgO and tested in the base-free glycerol oxidation in a batch reactor (0.3 mol L-1 aqueous glycerol solution, glycerol/metal molar ratio 500, pO2 3 atm, 16 h). Using acidic support (MCM-41, SiO2) a high selectivity to glyceraldehyde/glyceric acid (88-90%) has been obtained. Conversely, using basic supports (NiO, MgO), a low selectivity to glyceric acid was observed (35%) as a consequence of the successive oxidation to tartronic acid/ mesoxalic acids but also to the high occurrence of C-C bond oxidative cleavage. Moreover glyceraldehyde was not detected. References [1] A. Corma, S. Iborra, and A. Velty, Chem. Rev., 107 (2007) 2411-2502 [2] T. Mallat and A. Baiker, Chem. Rev., 104 (2004) 3037-3058. [3] A. Villa, G.M. Veith and L. Prati, Angew. Chem. Int. Ed., 49 (2010) 4499-4502 [4] G. L. Brett, Q. He, C. Hammond, P. J. Miedziak, N. Dimitratos, M. Sankar, A. A. Herzing, M. Conte, J. A. Lopez-Sanchez, C. J. Kiely, D. W. Knight, S. H. Taylor, and G. J. Hutchings et al., Angew. Chem. Int. Ed., 50 (2011) 10136-10139