Ind. Eng. Chem. Res. 2007, 46, 6449-6455
6449
Platinum Catalyst on Multiwalled Carbon Nanotubes for the Catalytic Wet Air Oxidation of Phenol G. Ovejero,* J. L. Sotelo, A. Rodrı´guez, C. Dı´az, R. Sanz, and J. Garcı´a Grupo de Cata´ lisis y Procesos de Separacio´ n (CyPS), Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, UniVersidad Complutense de Madrid, AVda. Complutense s/n, 28040 Madrid, Spain
Bare oxidized carbon nanotubes (CNTs) and carboxylated CNTs treated with sodium carbonate to generate carboxylate groups on the outer surface were used to prepare platinum catalysts by two different methods: incipient wetness and excess solution impregnation. The catalysts were characterized by N2 adsorption (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray fluorescence (XRF). These catalysts were tested in catalytic wet air oxidation of phenol aqueous solutions in a continuous trickle-bed reactor at 2.0 MPa of total pressure and temperatures of 160 and 200 °C. The phenol and TOC conversions obtained at steady state were above 94% and 80%, respectively, and the activity per catalyst weight was 30.0 mmolphenol‚gcat-1‚h-1 in steadystate conditions. This means the first use of CNT in catalytic wet air oxidation of phenol was in continuous trickle-bed reactors (CTBR). 1. Introduction Multiwalled carbon nanotubes (CNTs) have unique atomic structure (one-dimensional pore structure, rolled graphitic layers), very high aspect ratio, and extraordinary mechanical properties,1 which make these solids excellent catalysts.2 Thus, the properties and the chemical functionalization of CNTs have attracted much attention from the point of view of scientific interest and practical applications in sensors, separations, electronic devices, and gas storage.3-6 These properties have stimulated intensive research on the synthesis and modification of these novel carbon materials.7-9 Although recently several research groups have reported the direct functionalization of carbon nanotubes through the sidewalls,10,11 most of the functionalization work was still based on the use of surfacebounded carboxylic acid groups on the tubes. Solubilized carbon nanotubes were synthesized by the amidation reaction of longchain amine with carboxylic acid groups on the nanotubes by Chen et al.12 Riggs et al.13 have reported the functionalization of CNT nanotubes with aminopolymers through a similar reaction. Although the carboxylic acid groups provided a feasible way to modify CNTs, other functional groups such as hydroxyl groups were also useful in order to improve the CNTs chemistry as well as to explore novel self-assembled structures based on carbon nanotubes. Catalytic wet air oxidation (CWAO) has been investigated over the years on a variety of organic compounds using numerous catalysts with different results.14-18 Nowadays, the interest is focused on reducing the extreme operating conditions of wet air oxidation (WAO). Using nanotubes-based catalysts, intermediate refractory compounds, such as acetic acid and ammonia, are oxidized at much lower temperatures. Very few reports exist on the use of CNT as catalysts;1,19 they deal with oxidation of hydrogen sulfide, hydrogenation of nitrobenzene into aniline,19 and hydrogenation of cinnamaldehyde.20 Among the noble metal catalysts reported for liquid-phase oxidation, platinum-supported catalysts seem to be promising.21 * Corresponding author. Tel.: +34-91-394-4111. Fax: +34-91-3944114. E-mail:
[email protected].
Platinum catalysts were found to be effective during aqueousphase oxidation of low molecular weight organic carboxylic acids,16 ammonia,18 and alcohols.22 However, data about the application of platinum catalysts for CWO, especially for phenols, are still limited.23-25 Masende et al.26 reported that high conversion of phenol was achieved over graphite-supported platinum in a continuous-flow stirred-tank slurry reactor (CSTR); however, full conversion of the total organic carbon (TOC) was not obtained. This was attributed to the presence of side reactions, which lead to the formation of stable acids that do not readily degrade. As a result of the increasingly severe environmental regulations, novel selective and efficient oxidation catalysts and processes have been the subject of intensive research. In this context, the aim of the present paper is to investigate the effectiveness of multiwalled carbon nanotubes (CNTs) supported catalysts for liquid-phase oxidation of phenol in a continuous trickle-bed reactor. The influence of metal load, temperature, preparation method, and reuse of catalysts on the activity and stability has been studied, and a reaction mechanism has been proposed. 2. Experimental Section 2.1. Materials. The CNTs used as support were supplied from Sun Nanotech Co. Ltd. in Beijing and were prepared by chemical vapor deposition (CVD) of ethylene, using iron supported on alumina as catalyst.27 Phenol, platinum diammonium chloride PtCl4(NH4)2, and all other reagents used for analysis were analytical-reagent grade and were purchased from Sigma-Aldrich. Air from a cylinder with a minimum stated purity of 99.5% provided by Air Liquid S.A., Spain, was used for oxidation. 2.2. Preparation of CNT-C. A mixture of 0.033 g‚mL-1 of CNTs and HNO3 aqueous solution 5 M was refluxed for 3 h at 130 °C. Then, the mixture was filtered and washed with deionized water until the mother liquor pH reached the value of 7. The resulting material was dried overnight at 110 °C in a thermostated oven and crushed to powder. The preparation of the corresponding nanotubes sodium carboxylate salt (CNT-C) was accomplished by treatment of a
10.1021/ie070204p CCC: $37.00 © 2007 American Chemical Society Published on Web 08/30/2007
