Surface modification of multi-walled carbon nanotubes to produce a new bimetallic Fe/Mn catalyst for the aerobic oxidation of hydrocarbons

https://doi.org/10.1016/j.jiec.2018.09.044Get rights and content

Highlights

  • Easy preparation of a new family of bimetallic porphyrin-based nanohybrides as heterogenized catalyst was reported.

  • Investigation of the structure, morphology and thermal stability of the prepared nanohybride.

  • Aerobic oxidation of alkenes and alkanes in the absence of any reducing agent and/or additives was investigated.

  • Bimetallic catalyst exhibited higher catalytic activity compared with monometallic one.

  • Synergistic effects between the immobilized metalloporphyrins were observed.

Abstract

Fe/Mn bimetallic heterogenized catalyst [Fe/Mn(THPP)@MWCNT] was prepared by the simultaneous attachment of Mn and Fe porphyrins onto the surface of functionalized multi-walled carbon nanotubes. This new bimetallic nanohybrid displays a superior catalytic performance (81%) in comparison with their analogous monometallic counterparts (36% and 44% for Fe- and Mn-porphyrin respectively) in the oxidation of hydrocarbons with molecular oxygen as the sole oxidant. Experiments comparing bimetallic catalyst with equivalent physical mixtures of monometallic catalysts revealed the catalytic synergistic effect amongst metallic active centers onto the surface of the same nanotube, introducing a new class of bimetallic porphyrin-based catalysts for oxidation reactions.

Introduction

The synergistic effects of bimetallic systems play a critical role in the catalytic performance and energy storage of these hybrid materials, which have been of great interest to researchers since long years ago [1], [2], [3], [4], [5]. The scope of such studies usually has been limited to the supported precious metal nanoparticles or multimetallic clusters, omitting supported transition metal complexes [6], [7], [8]. Recently, various materials with different physical and chemical properties were applied as a solid support for preparing heterogenized catalysts to improve the efficiency of hybrid nanomaterials [9], [10], [11], [12], [13], [14]. Among these various materials, carbon-based materials as promising candidates for preparing hybrid nanomaterials display significant activity for catalytic systems [14], [15], [16], [17], [18]. A number of procedures are available to modify or functionalize the surface of nanostructured materials to improve their properties [19], [20], [21], [22].

Metalloporphyrins as the powerful and effective catalyst in oxidation reactions could be an appropriate candidate for immobilization on the solid supports to produce heterogenized catalysts [23], [24], [25], [26], [27]. According to the literature, the catalytic efficiency of immobilized porphyrins strongly depends on the active metal center of metalloporphyrins, which controls the activity, stability and selectivity of porphyrin-based catalysts [28], [29], [30]. We have recently prepared a number of immobilized metalloporphyrins as efficient catalysts in oxidation reactions [31], [32], [33]. In some cases thereof, we also compared the effect of active metal center on the catalytic activity of the heterogenized catalysts [34], [35]. By having different metal complexes attached onto the surface of the same support, the individual advantages of each active metal center would be added to the catalyst and a new class of bimetallic catalysts would be produced. Few reports available on porphyrin-based bimetallic systems merely cover twin-porphyrin or phosphine-porphyrin derivatives, but not immobilized porphyrins [36], [37].

Here, we report the easy preparation of a new family of bimetallic nanohybrid based on the immobilization of metalloporphyrins onto the surface of solid supports for the design of the advanced catalyst.

Section snippets

Physical measurements

FTIR spectra were recorded using an ABB Bomem: FTLA 200-100 spectrophotometer with potassium bromide pellets in the range 400–4000 cm−1. A Varian AA240 atomic absorption spectrometer was employed to determine the amount of metalloporphyrin complexes onto the solid support. The UV–Vis spectra were recorded by a Cam-Spec-M330 model in 2 mm path length quartz cell in the range of 200–800 nm. A scanning electron microscopy (SEM) was conducted using a EDf: oxford mat 50 (Tescan vega3, lab6 model,

Characterization of bimettalic nanohybrid

The new nanohybrid material was fully characterized by various microscopic and spectroscopic measurements and its catalytic efficiency was evaluated over aerobic oxidation of hydrocarbons without any reducing agent or co-catalyst.

Fourier infrared spectroscopy (FTIR) was used for initial characterization of the nanohybrid catalyst. Comparison of the FTIR spectra of MWCNTs and the prepared catalyst confirmed that the metalloporphyrins attached to the surface of MWCNTs by the esteric bond

Conclusions

In conclusion, we have introduced a simple method for the preparation of a new class of bimetallic nanohybrids by simultaneous attachment of metalloporphyrin onto the surface of functionalized multi-walled carbon nanotube via ester bonds. [Fe/Mn(THPP)@MWCNT] shows a superior catalytic activity (81%) with respect to their analogous monometallic counterparts (36% and 44% for Fe- and Mn-porphyrin respectively). This work shows the efficiency of bimetallic systems for the aerobic oxidation and

Acknowledgements

Financial support of this work by K.N. Toosi University of Technology Research Council and Iran National Science Foundation (INSF) under grant No. 96005913 are acknowledged.

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