Hydroxyapatites as tunable supports of gold-based catalysts for liquid phase selective alcohol oxidation in base-free conditions

Supported noble metal nanoparticles (NPs) have been widely investigated as catalytic systems in liquid-phase selective oxidation reactions. In particular, supported gold nanoparticles revealed peculiar catalytic performances despite an alkaline medium is usually required in order to accomplish high activity and selectivity. As avoiding basic conditions represents a great advantage from a practical point of view, lot of efforts have been recently devoted to gold based catalysts able to maintain good activity (and selectivity) even under neutral (or acidic) conditions. In this respect, it has been observed that alloying platinum or palladium to gold provides effective catalytic systems in terms of activity and selectivity [1, 2]. Moreover, it has been also shown that the acid-base properties of the support can play a crucial role [3-10]. Herein, we investigated in detail the influence of weakly basic supports as hydroxyapatites on the catalytic performances of AuPt catalysts in two selected reactions, namely glycerol and hydroxymethyl furfural (HMF) oxidations. The choice of these two substrates was connected not only to the industrial importance of the derived products but also to the facile catalyst deactivation usually observed when glycerol or HMF are attempted to be oxidized under neutral conditions. Hydroxyapatite samples (HAPs) were synthesized by co-precipitation method, adding NH4H2PO4 to Ca(NO3)2 at 80°C for different pHs adjusted during the precipitation and/or during the maturation time. Despite similar apparent basic properties (similar auto-generated solution pH around 7.1-7.3), XPS analyses revealed different surface composition from the bulk, especially in terms of Ca/P atomic ratio (XPS versus ICP analyses). Au and Pt nanoparticles were deposited on HAPs using the sol immobilisation technique whereas bimetallic Au-Pt/HAP were prepared following a two steps procedure which ensures the formation of AuPt alloy avoiding any metal segregation [11]. From a catalytic point of view HAPs have been revealed peculiar supports which promote the base-free oxidation of glycerol and HMF but limiting the C-C bond cleavage on the contrary of other basic supports (i.e.MgO). We also observed a strong synergistic effect between Au and Pt. In particular the surface enrichment of P observed by increasing the pH during precipitation of HAP, correspond to an increase of activity in both catalytic tests.