Abstract
This paper studies the cooperative effect of Au nanoparticles deposited on vanadyl pyrophosphate oxide (VPO) in the liquid phase oxidation of benzyl alcohol. VPO was prepared using the classical method by thermally treating VOHPO4·0.5H2O precursor in reacting atmosphere at 420 °C for a period of 72 h. Au nanoparticles were deposited by incipient wetness method. The catalysts were characterized by means of XRD, TEM, XPS and Raman. The bulk VPO catalyst contains vanadyl pyrophosphate phase ((VO)2P2O7), and a small amount of VOPO4. The catalytic system exhibits a high activity in the base-free liquid phase oxidation of alcohols compared to Au on activated carbon, classic catalyst used for this type of reaction. Au/VPO showed a high peculiar selectivity to benzyl benzoate (76%), an important product used in the pharmaceutical and perfume industries. This behavior might be ascribed to the presence of strong acid sites of VPO, as determined by liquid phase titration. Stability tests performed on Au/VPO showed a deactivation of 10% after the first run, but a constant conversion along the following five cycles. This phenomenon can be attributed to the increase of mean Au particle size (from 19.1 to 23.4 nm) after recycling tests as well as the partial leaching of Au and V in the reaction media. Moreover, XRD evidenced a modification in the VPO structure with the partial formation of VOHPO4·0.5H2O phase.
Highlights
The liquid phase oxidation of benzyl alcohol, using heterogeneous catalysts and oxygen as oxidant, has been extensively studied in the last years [1,2,3]
This paper studies the cooperative effect of Au nanoparticles deposited on vanadyl pyrophosphate oxide (VPO) in the liquid phase oxidation of benzyl alcohol
VPO was prepared according to the procedure reported by Luciani et al [31]. 1% Au was added by incipient wetness (IW) to VPO and for comparison to activated carbon, typical support used for liquid phase reactions
Summary
The liquid phase oxidation of benzyl alcohol, using heterogeneous catalysts and oxygen as oxidant, has been extensively studied in the last years [1,2,3]. Depending on the catalytic materials and the reaction conditions (temperature (60–160◦C), solvent, oxygen pressure), many products of industrial interest, such as benzaldehyde, benzyl benzoate, benzoic acid, and benzyl ether can be obtained [2]. The drawback related to the use Au catalyst is the need to operate under alkaline conditions, which facilitate the first step of the oxidative dehydrogenation (H abstraction), making purification a challenge and limiting the industrial application [18,19]. The supports can be active in the specific reaction increasing the synergistic effect with the active sites of the metal nanoparticles [25]
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