Al,Cu-pillared clays as catalysts in environmental protection

https://doi.org/10.1016/j.cej.2009.05.004Get rights and content

Abstract

In order to perform water purification from toluene Al,Cu-pillared clay (PILC) was synthesized and tested as catalyst in the oxidation of toluene. Two different catalytic reactions at moderate temperatures were investigated: catalytic wet peroxide oxidation (CWPO) using Al,Cu-PILC and electrooxidation in acidic solution using Al,Cu-PILC based electrode. X-ray diffraction analysis confirmed that pillaring was successful. Chemical analysis confirmed the incorporation of Al3+ and Cu2+ species in the PILCs. The Cu2+ species were proven to be the active ones in both reactions. CWPO in the presence of Cu containing PILC resulted in efficient catalytic degradation of toluene. Steady state voltammograms for Al,Cu-PILC based electrode in acidic toluene solution showed broad anodic peak at about 1.7 V vs. Ag/AgCl ascribed to the electrooxidation of toluene. Another peak ascribed to toluene oxidation products occurred at 0.5 V. Anodic peak currents corresponding to these peaks linearly increased with the initial toluene concentration. Both CWPO and electrooxidation confirmed the possibility of use of the synthesized Al,Cu-PILC in the toluene oxidation and might be employed in the environmental protection.

Introduction

Toluene is a pollutant classified into the group of extremely dangerous compounds to the environment. Toluene is a common contaminant in waters in the vicinity of oil refineries. It is suggested by Environmental Protection Agency that the amount of toluene in drinking water should not exceed 1 ppm [1]. Therefore, preventing toluene from getting to water streams is an important task. The techniques usually adopted to achieve water purification from toluene are largely based on the use of phase separation methods and/or adsorption on active suspended materials, as well as biodegradation [2]. There are several drawbacks of these methods, including the inability of quantitative removal of pollutants or the fact that they are time consuming and effective only at low concentration levels [2], [3]. Oxidation of toluene into nontoxic carbon dioxide and water might be the optimal solution.

Catalytic wet peroxide oxidation (CWPO) appears to be a promising method for the removal of BTEX (which stands for benzene, toluene, ethylbenzene, o-, m- and p-xylene) from water under mild temperature and pressure conditions. CWPO is based on the presence of hydrogen peroxide as a source of highly reactive radicals generated on transition metal cation sites within employed catalysts [2], [4]. Possible leaching of active metal cations can be overcome if pillared interlayered clay (PILC) is used as a catalyst. Metal oxide PILCs represent a new class of materials that have found a wide range of potential applications in catalytic, adsorption and separation processes [5]. The CWPO of phenol using Al,Cu-PILCs at room temperature and atmospheric pressure was shown to be successful heterogeneous catalytic reaction [6], [7], [8]. According to literature this reaction has not been tested thoroughly in the case of degradation of less water soluble BTEX compounds, although the CWPO performance of Cu-doped alumina-pillared montmorillonite has been tested for toluene [9].

Catalysts in the form of electrode surfaces provide additional dimension of the electrode potential which can be used to control the catalyst reactivity and, in some cases, selectivity. Electrochemical oxidation of toluene has been performed on platinum and glassy carbon surfaces, by using different electrolytes [10], [11], [12]. Due to the electrolyte decomposition, toluene did not react when mixture of alcohol and sulfuric acid was used as electrolytic medium. However, it produces carbon dioxide and water when the reaction is carried out in sulfuric acid [13]. Many experimental results regarding electrooxidation of toluene on different surfaces in organic media have been published [14], while the same reaction in aqueous solution has been seldom reported [15], [16]. Treimer et al. have investigated Fe(III)-β-PbO2, Bi(V)-β-PbO2 and β-PbO2 electrodes for electrooxidation of toluene in acidic media [17]. Adsorption interaction of aromatic molecules at the Fe(III) sites is stronger than at the Bi(V) and Pb(IV) sites which results in faster reaction kinetics on the Fe(III)-β-PbO2 electrode. D’Elia et al. have tested vanadium pentoxide, titanium dioxide and cerium oxide as anodes for the electrooxidation of toluene. TiO2 showed some activity for the electrooxidation of toluene while CeO2 was totally inactive in this reaction. V2O5 was not active in this reaction under the experimental conditions [15], [16].

In this work Al,Cu-PILC was synthesized and tested as catalyst in the oxidation of toluene in order for water purification to be performed. The obtained PILC was employed as catalyst in the CWPO, and as electrode material in the electrocatalytic toluene oxidation in acidic solution. Al-PILC was synthesized and used for comparison purposes in order to prove catalytic activity of Cu species.

Section snippets

Materials

Starting material was domestic clay from Bogovina which had been previously characterized [18], [19]. The fraction of this clay with particle diameters ≤2 μm and cation exchange capacity (estimated by ammonium acetate method) of 765 mmol kg−1 was used in further experiments and denoted as raw clay. The raw clay was submitted to the Na-exchange procedure by repeated stirring with 1 M NaCl followed by filtering. Thus obtained filtration cake was rinsed with distilled water in order to remove NaCl and

XRD

According to X-ray diffraction patterns (Fig. 1) the following phases were identified in the investigated samples: smectite, quartz, feldspar and calcite [25].

Small hump between 19° and 30° 2θ attributed to X-ray amorphous matter was also observed. The (0 0 1) smectite peak around 2θ = 6° shifts during different stages of the pillaring process. The process of Na-exchange lowered corresponding basal spacing, d0 0 1, from 1.53 nm (2θ = 5.78°) for the starting clay, to 1.28 nm (2θ = 6.92°). The Na-exchanged

Conclusion

Al and Al,Cu-pillared clays were synthesized from bentonite clay. XRD analysis confirmed successfulness of the pillaring procedure. Chemical analysis confirmed the incorporation of Al3+ and catalytically active Cu2+ species in the PILCs. Leaching of these species was below 0.01 ppm. Al-PILC showed adsorption while Al,Cu-PILC showed efficient catalytic wet peroxide degradation of toluene. Voltammograms for the Al,Cu-PILC based electrode in acidic toluene containing solutions showed broad anodic

Acknowledgment

This work was supported by the Ministry of Science and Technological Development of the Republic of Serbia (Projects 166001B and 142019B).

References (32)

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