Pd on carbon nanotubes for liquid phase alcohol oxidation
Introduction
Metal catalyzed alcohol oxidation in the liquid phase using oxygen as the oxidant represents a well known interesting process, but its industrial exploitation was limited by the strong deactivation [1]. Several metals (ruthenium, platinum, palladium and gold) have been used as monometallic catalysts, in some cases with modifier (like Bi, Pb) [2], [3], [4], [5]. The main requirement for a high selectivity and long durability has not been yet achieved. Moreover, a variety of experimental conditions have been tested: from solventless to organic solvents or aqueous conditions. It has been observed that basic conditions improved the activity and the durability, probably due to a prolonged catalyst life. However, under basic conditions the selectivity to aldehyde in the alcohol oxidation decreased in favour of the corresponding carboxylic acid (carboxylate).
From an industrial point of view carbonaceous supports are preferable because of the facile recovery of precious metals by burning off the support. Recently, there is a growing interest in the use of CNTs as catalyst supports for metal nanoparticles in many catalytic applications; for example, CNTs supported platinum nanoparticles are used as electrocatalyst in proton-exchange membrane fuel cells [6], [7]. Moreover, Ni [8], [9] and Pd [10] nanoparticles decorated CNTs were used for hydrogen storage and for hydrogen sensing. In addition, Pd [11], Pt and Ru [12] metal nanoparticles supported on CNTs show a high catalytic activity in the liquid phase hydrogenation of cinnamaldehyde. However, only few reports on the use of CNTs instead of the widely used AC can be found in the literature. For example, Corma et al. [13] showed that Pd nanoparticles immobilised on CNTs are more active than the one supported on AC in the Heck reaction of styrene and iodobenzene and for the Suzuki coupling of phenylboronic and iodobenzene. They assigned the high activity to the different particle size obtained for the different supports. They showed that the Pd/AC is more active than Pd/CNTs for the aerobic oxidation of cinnamyl alcohol in ethanol. Moreover Karaousis et al. [14] reported higher activity of CNTs supported metal nanoparticles with respect to AC supported ones in the hydrogenation of methyl-9-octadecenoate and 2-methyl-2-pentenal. They concluded that the high catalytic performance of CNTs supported Pd is due to the large active surface of metallic palladium.
In the present work we report on the catalytic activity of CNTs and AC supported Pd nanoparticles prepared by colloidal method using PVA as protective agent [15] in the liquid phase oxidation of benzyl alcohol with the aim of preserving a high selectivity to aldehyde but enhancing the catalyst activity and reusability. Different groups showed that adding Au to Pd catalysts has a beneficial effect in the alcohol oxidation, improving not only the catalytic activity and selectivity to the desired product [16], but also the resistance to the deactivation [17], [18], [19]. We recently set up a two step methodology allowing the preparation of uniformly alloyed bimetallic Au60–Pd40 catalyst supported on activated carbon [15]. With the same procedure we prepared a bimetallic Au–Pd catalyst supported on CNTs.
Section snippets
Materials
Na2PdCl4 and NaAuCl4·2H2O were from Aldrich (99.99% purity), activated carbon from Camel (X40S; SA = 900–1100 m2/g; PV = 1.5 ml/g) and carbon nanotubes from Bayern Materials Science AG (Baytubes C150P; SA = 30 m2/g; internal diameter 13–16 nm, length 1–10 μm). NaBH4 of purity >96% from Fluka, polyvinylalcohol (PVA) (Mw = 13,000–23,000; 87–89% hydrolysed) from Aldrich were used. Gaseous oxygen from SIAD was 99.99% pure.
Monometallic catalysts
Pd sol: Na2PdCl4·2H2O (0.043 mmol) and PVA solution 2 wt% (880 μl) were added to 130 ml of H2
Catalytic activity of Pd supported on AC and CNTs
AC is a widely used catalyst support for liquid phase reaction. However, recent literature disregards AC as the support for alcohol oxidation; the most used supporting materials are oxides despite their disadvantage in precious metal recovery. We concentrated on benzylic alcohol to benzaldehyde selective oxidation which is an important reaction from an industrial point of view and has been studied under a variety of reaction conditions using different catalytic systems.
We synthesized 1 wt% Pd/AC
Conclusions
We compared the activity of Pd nanoparticles supported on AC and CNTs in the liquid phase oxidation of benzylic alcohol to benzaldehyde. Although we used a procedure (sol immobilisation) that in principle assures a similar distribution on different supports, we obtained a slight different metal distribution in the two cases. Pd nanoparticles on CNTs are smaller in size but less dispersed than on AC. Catalytic test revealed that Pd supported on CNTs behaves differently from Pd on AC. In fact,
Acknowledgements
Authors thank DAAD, Programma Vigoni and Fondazione Cariplo for financial support.
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Current address: School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK.