Glycerol oxidation over gold supported catalysts – “Two faces” of sulphur based anchoring agent

• Three methods for fine control of the deposition of gold nanoparticles on oxide supports are compared.
• Thiol moieties inhibit the oxidation reaction more strongly than conventional antioxidants do.
• Glycerol oxidation is blocked by the presence of SH groups on the catalysts surface.

Three different alumina-supported catalysts were prepared according to 3 different methods, aiming at achieving fine deposition of gold particles. In methods 1 and 2, (3-aminopropyl)triethoxysilane (APTES) and (3-mercaptopropyl)triethoxysilane (MPTES) were respectively used as anchoring agents. In method 3, the gold particles were directly deposited on the support without using any anchoring agent. The performances of the so-obtained catalysts were evaluated in the reaction of glycerol partial oxidation in the liquid phase, and compared to those of a 1 wt.% Au/Al2O3 catalyst provided by Mintek. The best catalyst A3 (method 3) exhibited a 99% conversion at 60 ̊C after 2.5 h, with glyceric and glycolic acids selectivities of 60 and 20%, respectively. These performances were slightly higher than those of the reference commercial catalyst (95% conversion with glyceric and glycolic acids selectivities of 62 and 20%, respectively). In the case of A2 catalyst (method 2), in spite of very good dispersion of small nanoparticles on the surface, which is normally supposed to yield very active materials, a very strong inhibition of glycerol conversion was observed. After 2.5 h of process at 60 ̊C, the glycerol conversion did not exceed 4%. A similar behavior was also observed for TiO2- and ZnO-supported catalysts, prepared in the same manner (using MPTES). In order to get a better understanding of the parameters ruling the reactivity of the various catalytic systems, a series of experiments over the commercial catalyst was performed, in which small quantities of various sulphur derivatives and antioxidants were used in the reaction mixture to simulate the response of the various species that can be present in the lab-prepared samples (method 2). The results, together with XPS analysis, suggests that there are three possible interpretations of the observed inhibition effect: (i) a strong interaction between gold and sulphur leading to the formation of covalent AuS bonds on nanoparticle's surface; (ii) the formation of so-called SAM's (self-assembled monolayers) blocking access to surface active centers; or (iii) an inhibition of the reactivity due to antioxidation properties of S-derivatives.

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