Elsevier

Applied Catalysis A: General

Volume 506, 5 October 2015, Pages 134-142
Applied Catalysis A: General

Nitrogen- and oxygen-functionalized carbon nanotubes supported Pt-based catalyst for the selective hydrogenation of cinnamaldehyde

https://doi.org/10.1016/j.apcata.2015.08.023Get rights and content

Highlights

  • Nitrogen- and oxygen-functionalized CNTs were obtained with the hydrothermal method.

  • The influence of nitrogen and oxygen species on Pt-based catalysts was investigated.

  • The catalyst was used in cinnamaldehyde hydrogenation reaction.

  • The functionalization could greatly improve the catalytic performance.

  • PtCo/N-CNTs was highly active and selective to cinnamyl alcohol at low temperature.

Abstract

Carbon nanotubes (CNTs) supported Pt catalysts were employed to study the influence of nitrogen- and oxygen-functionalized surface on catalytic performance in the liquid-phase hydrogenation of cinnamaldehyde. Nitrogen- and oxygen-functionalized CNTs were simply treated with the hydrothermal method using ammonia and hydrogen peroxide, respectively, and the metal catalyst was prepared using an impregnation-reduction-deposition method. Here, we studied and gave some insights into the influence of the functionalization on CNTs surface chemistry environment and Pt nanoparticles using XRD, Raman spectroscopy, TEM, XPS, and nitrogen sorption techniques. It was shown that the functionalization lead to the small size distribution of Pt particles and promoted the conversion from ca. 75% to ca. 95%. To obtain the high selectivity, transition metals, cobalt and nickel, were doped into the Pt catalyst and the 99.7% conversion with the 87.9% selectivity was achieved over the nitrogen-functionalized CNTs supported PtCo catalysts. The PtCo/N-CNTs remained high activity even at 40 °C and achieved 94.2% conversion and 90.3% selectivity.

Introduction

Cinnamaldehyde (CAL) is a particularly important representative of α, β-unsaturated aldehyde that contains two unsaturated bonds the carbonyl (Cdouble bondO) and olefinic (Cdouble bondC) group in molecule. The hydrogenation of CAL can generate different products, the partial one of which, cinnamyl alcohol (COL), is important intermediates in pharmaceutical and fragrance industries [1], [2]. Also, it is considered as a good model for investing the catalytic behaviors of the microstructures of heterogeneous catalysts. The selective hydrogenation is a rather complex reaction network involving several important intermediates and a number of series-parallel reactions. The main competitive products of selective hydrogenation can be produced through the hydrogenation of the conjugated Cdouble bondC or Cdouble bondO bond over the supported metal catalysts. However, obtaining the unsaturated alcohols is difficult, owing to the fact that C&9552;O bond presents a higher binding energy than C&9552;C bond (715 kJ/mol and 615 kJ/mol, respectively) [3], the reduction of the C&9552;C bond is thermodynamically more favorable than that of Cdouble bondO bond. The selectivity to the unsaturated alcohol can be correlated with the metal D-banD width. Generally, the larger the d-band, the stronger the repulsive four-electron interaction of the metal with the Cdouble bondC double bond and the stronger the attractive interaction of the metal surface with the Cdouble bondO π-system, which results in good selectivity to the unsaturated alcohol (Ir, Os > Pt > Ru > Rh > Pd) [4].

Although various kinds of metal (Pd, Ru and Au)-based catalysts were employed in the hydrogenation reaction [5], [6], [7], Pt was most commonly used for the purpose of Cdouble bondO hydrogenations in fact. Maintaining high selectivity of Pt-based catalysts while increasing their activity remains an urgent task for further development in this field. Many studies have been devoted to enhance the selectivity towards the unsaturated alcohol either by fine control of the sizes [8] and/or the exposed facets of the active metal nanoparticles [9], or by exploiting the steric constraints imposed by the environment of the active site [10], or by decorating the primary metal with a second metal component [11], or by adding promoters like an alkali element [1]. However, most methods usually involved ingenious design and complex process. Therefore, searching for appropriate materials used as supports is a simple and feasible way to prepare highly active and selective catalysts for the hydrogenation reaction.

The conventional carbon nanostructure materials, especially carbon nanotubes (CNTs), have been widely employed as support in heterogeneous catalysis, owing to the high electrical conductivity and corrosion resistance [12]. Their high surface area could lead to a high dispersion of metal nanoparticles and the perfect smooth graphitic surface could make all metal nanoparticles on CNTs accessible to the reactants. Additionally, the transfer of π-electrons from the graphitic planes of CNTs to the metal particles leads to the increase in the charge density on the metal, which decreases the probability of Cdouble bondC bond activation and favors the formation of unsaturated alcohols [13], [14]. Recently, functionalized or doped CNTs are considered as one of the promising supports due to the merits performing in the catalytic process. The functionalization mainly focuses on two aspects, nitrogen and oxygen. The functionalization treatments have excited chemists to renovate traditional catalysis materials and, actually, it has been proved that they could significantly improve CNTs interaction with solvents and dispersion, allow the grafting of nanoparticles [15], [16] and promote the adsorption of energy molecules and modulate molecular reactivity and selectivity [17], [18]. Chizari et al. prepared N-doped CNTs (synthesized using a Chemical Vapor Deposition process) supported Pd catalyst and investigated the influence of different types of incorporated nitrogen species on the liquid-phase hydrogenation of CAL [19]. Solhy et al. reported three activation procedures of Multi-walled CNTs: nitric acid oxidation, ball-milling and air oxidation and its influence on the catalytic performance of Pt/MWCNT catalysts for selective hydrogenation [20]. However, the different influence of nitrogen and oxygen species on the catalytic performance has been not researched.

Here, nitrogen- and oxygen-functionalized CNTs were simply obtained with the hydrothermal method using ammonia and hydrogen peroxide, respectively. Pt-based catalyst supported on those nanotubes was prepared with the traditional impregnation-reduction-deposition method. The main aim of the present work was to study and compare the effects of oxygen and nitrogen species on the CNTs surface chemistry environment and Pt-based nanoparticles, and to evaluate the influence of the functionalization on the catalytic performances of Pt-based catalysts for selective hydrogenation. The functionalized CNTs and prepared Pt-based catalysts were fully characterized using X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), nitrogen sorption isotherms and X-ray photoelectron spectroscopy (XPS) techniques. The performances of different Pt-based catalysts were investigated and compared under mild reaction conditions. We found that the high conversion and selectivity could be achieved over the nitrogen-functionalized CNTs supported PtCo catalysts even at the room temperature.

Section snippets

Materials and chemicals

Multi-walled CNTs prepared by the chemical vapor deposition method with Fe-Co/Al2O3 as catalyst were obtained from Boyu Gaoke New Material Technology Co., Ltd. (Beijing, China). All reagents used in the experiment were analytical reagent grade and used without further purification. H2PtCl6·6H2O, CoCl2·H2O, CAL and NaBH4 were purchased from Aladdin. Ammonia, hydrogen peroxide (30%) and concentrated HCl (12 mol/L) were purchased from Kangde Chemical Reagent (Yantai, China). Ethanol was purchased

Catalysts characterization

Fig. 1a shows the powder XRD patterns of as-prepared catalysts. The peaks at 26.1°, 43.5°, 54.1°, 77.5° were attributed to the (0 0 2), (1 0 1), (0 0 4) and (1 1 0) diffractions of hexagonal graphitic carbon (JCPDS 41-1487), respectively. The wide (0 0 2) peak showed that the CNTs possess amorphous structure with small regions of crystallinity and the regions of crystallinity are highly graphitic [21]. The position and sharpness of the C (0 0 2) peak in the Pt/N-CNTs and Pt/O-CNTs indicates that the

Conclusions

The graphite structure of the CNTs was functionalized by ammonia and hydrogen peroxide without significant damage. The functionalization not merely increased the density of the N and O element but also dramatically modified its surface functional groups, which significantly promoted the content of metal nanoparticles and lead to the small size distribution of Pt particles on the CNT surface. The N and O species increased the distribution of positively charged Pt(II) and Pt(IV) species, which

Acknowledgement

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21203113).

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      Similarly, PtCo bimetallic catalyst supported on ammonia functionalized CNTs (PtCo/N-CNT) investigated for CAL hydrogenation resulted in high selectivity for COL (Table 2, Entry 24) [69]. Presence of highly dispersed Pt on the N-functionalized CNTs with high ratio of Pt0 served as a reason for commendable activity of PtCo/N-CNTs catalyst for COL formation [69]. Su et al. supported Pt-Co bimetallic catalyst on oxidized CNTs (oCNTs) and encountered that the PtCo3-oCNTs catalysts exhibited better catalytic performance towards CAL hydrogenation to COL (Table 2, Entry 25) [70].

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