Pt nanoparticles deposited over carbon nanotubes for selective hydrogenation of cinnamaldehyde

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Abstract

Two kinds of carbon nanotubes (CNTs) with different inner diameter (less than 10 nm: CNTs-1 and between 60 and 100 nm: CNTs-2) were used as catalyst supports. The platinum particles were simply deposited on the outside surface (CNTs-1) and inside (CNTs-2) and were easily reduced to Pt0 by refluxing. The catalysts exhibit high activities in the selective hydrogenation of cinnamaldehyde which contains both Cdouble bondC and Cdouble bondO bonds. But the selectivity of these two catalysts was quite different under same reaction conditions. The high selective hydrogenation of Cdouble bondO bond was observed over catalyst 3%Pt/CNTs-2, while the completely hydrogenation of both Cdouble bondC and Cdouble bondO bonds was found over catalyst 3%Pt/CNTs-1.

Introduction

Selective hydrogenation of α,β-unsaturated aldehydes yields unsaturated alcohols. They are industrially valuable products and intermediates for the synthesis of fine chemicals [1], [2], [3]. The desired product in hydrogenation of cinnamaldehyde is cinnamyl alcohol, an important additive in food industry, perfumery and an intermediate in the production of certain pharmaceutics [4]. Despite economic drawbacks the conventional method used for many years for the reduction of cinnamaldehyde is based on the Meerwein–Ponndorf–Verley reaction giving the yield of cinnamyl alcohol 85–90% after purification [5]. It is apparent that the application of heterogeneous catalysis might be advantageous.

In addition, it is a suitable model reaction to investigate the effect of catalyst structure on selectivity. Many important factors can influence the activity and selectivity, such as the active metal, the catalyst support properties, the metal particle size, the solution properties and the presence of a second metal [6]. A comprehensive review of the state of the art has been published [7]. Since the reduction of the Cdouble bondC bond is easier, the selectivity towards the unsaturated alcohol is generally poor. Thus, the preparation of a selective catalyst remains a challenging task. Platinum and palladium catalysts can be used. Carbon materials are among the most widely used supports for this reaction, including graphite, carbon blacks, activated carbons, fullerenes, carbon nanotubes and carbon nanofibers [8], [9], [10], [11], [12], [13], [14]. Recent results obtained in the laboratory have shown that active phase located inside the carbon nanotube cavity exhibit an extremely high activity when compared to that observed on traditional grain size catalysts [15], [16], [17]. Tessonnier et al. reported that palladium metal nanoparticles were introduced in the carbon nanotubes, and it exhibits a high selectivity towards the Cdouble bondC bond hydrogenation. This result could be attributed to the peculiar morphology of the support, which in turn, significantly modified the adsorption properties of the metal surface itself [18].

It is expected that specific properties will be observed with the catalyst, which metal particles deposited on the different surface of the supports, i.e., inside or outside. The present article is to report the catalysts of cinnamaldehyde hydrogenation, which Pt nanoparticles supported on carbon nanotubes with different inner diameter. The X-ray photoelectron spectroscope (XPS) and transmission electron microscopy (TEM) showed that, for the CNTs with inner diameter less than 10 nm, almost all of the Pt particles were supported on the outside surface, and for the CNTs with inner diameter between 60 and 100 nm, mainly of the Pt particles were inside. These two catalysts exhibit different catalytic properties in the selective hydrogenation of cinnamaldehyde.

Section snippets

Materials

The carbon nanotubes (CNTs) were supplied by Shenzhen Nanotech Port Co. Ltd., China, in a purified form, and were previously treated using HNO3 solution before used. Cinnamaldehyde, H2PtCl6 · 6H2O and other solvents are the reagents of analytical grade. The purity of hydrogen is 99.99%.

CNTs characteristics

High-resolution TEM observations of the catalysts were conducted using the JEOL-2010 microscope, working at 200-kV accelerating voltage. The images of the starting carbon nanotubes supports showed that, the inner

Catalyst characterization

The binding energies of the Pt 4f7/2 levels are collected in Table 1. The results showed that Pt4+ was reduced to Pt0 in both catalysts, by using the simple refluxing method.

The relative amounts of oxygen-containing surface groups were inferred from the oxygen surface concentration determined by XPS, which was carried out in a VG Scientific ESCALAB MK II spectrometer using non-monochromatized Mg Kα radiation. The atomic ratio of C/O and Pt/C was listed in Table 1. The oxygenated groups of CNTs-1

Conclusion

In summary, the catalysts of platinum particles supported on carbon nanotubes with different inner diameter have different catalytic properties in the selective hydrogenation of cinnamaldehyde. The platinum particles can be easily supported on the surface and/or introduced inside the CNTs by a simple impregnation followed by refluxing in order to reduce Pt4+ to Pt0. The platinum particles located inside the CNTs, the catalyst exhibits a high selectivity towards the Cdouble bondO bond hydrogenation. Such

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