Elsevier

Journal of Catalysis

Volume 291, July 2012, Pages 149-154
Journal of Catalysis

Deactivation of Ni/TiO2 catalyst in the hydrogenation of nitrobenzene in water and improvement in its stability by coating a layer of hydrophobic carbon

https://doi.org/10.1016/j.jcat.2012.04.020Get rights and content

Abstract

Water as a green solvent to replace the conventional organic solvent presents many advantages in the organic synthesis. The hydrogenation of nitrobenzene in water has been investigated by using Ni/TiO2 catalyst in this work, and our main purpose was focused on the Ni/TiO2 catalyst activity and its stability improvement. The experimental results and analysis from the data of XRD, XPS, ICP revealed that the formation of nickel hydroxide from metallic nickel reacting with water caused a rapid deactivation of Ni/TiO2 catalyst. Based on these, we designed a catalyst with hydrophobic property to prevent the nickel active species to contact with water; thus, a hydrophobic carbon layer was coated on the surface of Ni/TiO2. As expected, the hydrophobic carbon was successfully coated on Ni/TiO2 catalysts by a hydrothermal method and they presented higher reactivity and improved stability in the present aqueous reaction system; nickel hydroxide was not detected on the used and water treated carbon-coated Ni/TiO2 samples. The improved abilities were attributed to the increased hydrophobicity of catalysts modified by carbon, which not only prevents water to contact with nickel catalytic species, but also protects the metallic nickel to be oxidized as it exposed to air.

Graphical abstract

The activity and stability of Ni/TiO2 catalyst were significantly improved after the catalyst was modified by a carbon coating. The surface hydrophilic Ni/TiO2 catalyst changed to hydrophobic (Ni/TiO2)@C after coated with carbon, which inhibited the transformation of active nickel metallic species to Ni(OH)2 in H2O.

  1. Download : Download high-res image (109KB)
  2. Download : Download full-size image

Highlights

► Formation of nickel hydroxide caused Ni/TiO2 deactivation in water. ► A hydrophobic carbon layer was successfully coated on the surface of Ni/TiO2. ► The activity and stability of the catalyst were improved significantly after carbon coating. ► The surface hydrophobicity prevented water to contact with nickel catalytic species forming nickel hydroxide.

Introduction

As a solvent, water bears a number of attractive physicochemical properties over the traditional organic solvents. It is non-flammable, non-toxic, and non-carcinogenic, and in addition, water is probably the least expensive and most easily accessible solvent, and if a biphasic reaction system is used, organic substrates could be isolated by a simple phase separation [1]. Water also has a high specific heat capacity, enabling the more facile control of an exothermic reaction, and a network of hydrogen bonds which can influence the reactivity of substrates [2]. Therefore, the use of water as a reaction medium instead of organic solvents has been an important theme of current research in response to calls for sustainable chemistry in these years [1], [3], [4]. It was reported that the organic reactions exhibited improved reaction rates and product selectivity in water with comparing to the reactions in organic solvents. For example, the reaction rate was enhanced largely when the hydrogenation of t-butylbenzene with rhodium nanoparticles encapsulated in a porous carbon shell was performed in water compared with the results in octane, acetone, and ethanol [5]. Moreover, the addition of appropriate amounts of water could increase the reaction rate, like the transfer hydrogenations of styrene and nitrobenzene over Pd-based catalyst in methanol was increased from 26.3% and 7.1% to 100% and 31.9%, respectively, when the molar ratio of water to methanol increased from 0 to 1 [6], and the reaction rates for the selective hydrogenation of p-chloronitrobenzene in ethanol over the supported catalysts such as Ru/SiO2, Fe/SiO2, Co/SiO2, Ni/SiO2, Cu/SiO2, and Ag/SiO2 were also enhanced dramatically with the addition of water [7]. Furthermore, it was also reported that the reaction rate increased with water content and became the highest in pure water for the hydrogenation of p-nitrophenol in water–ethanol and water–dioxane [8]. In our previous work, the conversion of o-chloronitrobenzene in water was about three times larger than that in ethanol over Au/TiO2 catalysts [9]. The promoting effect of water might be partially attributed to the H-bond interaction of interfacial water with reactant or transition state intermediates [10]. Most recently, we found that the hydrogenations of water-insoluble nitrobenzene and chloronitrobenzene could be performed selectively and efficiently in H2O–CO2 system over the supported Ni catalysts at 35–50 °C without using any harmful organic solvents [11]. It was found that the N–O bond of N-phenyl-hydroxylamine (PHA) was activated through interactions with water, possibly via OH⋯O and OH⋯N bonding. Water could promote the reaction step of hydrogenation of PHA to aniline, the rate-determining step in organic solvents [11]. Hydrogenation of nitro-compounds in H2O–CO2 is an environmental benign process for the production of anilines, the important chemicals in producing the polyurethanes, dyes, herbicides, pesticides, pharmaceuticals, etc., due to the high reaction rate, high product selectivity, lower temperature, easy product separation, and cheap and clean solvents of H2O–CO2.

Based on the requirements of industrial application, water is a promising reaction medium. Herein, the hydrogenation of nitrobenzene in neat water over the Ni/TiO2 catalyst was investigated. We focused our attention on the mechanism of the Ni/TiO2 catalyst deactivation and development of a new strategy to improve the catalyst stability. Based on the cause of the deactivation, we designed a catalyst with hydrophobic property by coating a hydrophobic carbon layer on the surface of Ni/TiO2 catalyst via a hydrothermal method, and the catalytic performance of the carbon modified catalyst has been discussed.

Section snippets

Catalyst preparation and characterization

Ni/TiO2 was prepared by incipient wetness impregnation using anatase TiO2 (specific surface area 120 m2 g−1, Nanjing High Technology Nano Material Co. Ltd., China) and Ni(NO3)2⋅6H2O. After dried at 120 °C, the samples were calcined at 450 °C for 4 h in air. Before the hydrogenation run, the calcined samples were reduced under H2 flow at 450 °C for 2 h.

Carbon-coated Ni/TiO2 catalysts were prepared via hydrothermal approach. Glucose and sodium chloride were dissolved in 10 mL of distilled water in the

Catalytic performances of Ni/TiO2 catalyst

When the hydrogenation of nitrobenzene was carried out in the organic solvents, several intermediates such as nitrosobenzene (NSB), N-phenylhydroxylamine (PHA), azoxybenzene (AOB), azobenzene (AB), and hydrazobenzene (HAB) are frequently produced and accumulated during the reaction [12], [13], [14], [15], [16]. Interestingly, these intermediates were less produced when nitrobenzene hydrogenation was catalyzed by Ni/TiO2 catalyst in water, as the results shown in Fig. 1. The selectivity to

Conclusions

In conclusion, the catalytic performance of Ni/TiO2 catalyst and its stability improvement have been investigated for the hydrogenation of nitrobenzene in water. The results demonstrated that the Ni/TiO2 catalyst deactivated during the reaction, and the formation of nickel hydroxide from metallic nickel reacting with H2O was the main reason, which was confirmed with the experimental results combined with the XRD, XPS, and ICP analysis. The further study showed that the activity and stability of

Acknowledgments

The authors gratefully acknowledge the financial support from NSFC 20873139 and 20086036, 20100562 from Jilin Provincial Science and Technology Department, China.

References (35)

  • Y.Z. Xiang et al.

    Appl. Catal. A: Gen.

    (2010)
  • J. Ning et al.

    Catal. Commun.

    (2007)
  • Y.F. Hao et al.

    J. Mol. Catal. A: Chem.

    (2011)
  • F. Cardenas-Lizana et al.

    Appl. Catal. A: Gen.

    (2008)
  • X.C. Meng et al.

    J. Catal.

    (2009)
  • X.C. Meng et al.

    J. Catal.

    (2010)
  • F. Zhao et al.

    J. Catal.

    (2004)
  • S.H. Xie et al.

    Appl. Catal. A: Gen.

    (1999)
  • H.G. Manyar et al.

    J. Catal.

    (2009)
  • K. Takanabe et al.

    J. Catal.

    (2005)
  • Z.Y. Hou et al.

    Appl. Catal. A: Gen.

    (2003)
  • W.J. Wang et al.

    Appl. Catal. A: Gen.

    (1998)
  • H. Li et al.

    J. Mol. Catal. A: Chem.

    (2009)
  • G.X. Zhu et al.

    Carbon

    (2007)
  • V. Sunny et al.

    Carbon

    (2010)
  • S.W. Ho et al.

    J. Catal.

    (1998)
  • J. Xiong et al.

    Catal. Commun.

    (2007)
  • Cited by (0)

    View full text