Catalytic complete oxidation of acetylene and propene over clay versus cordierite honeycomb monoliths without and with chemical vapor deposited cobalt oxide
Graphical abstract
Introduction
Volatile organic compounds (VOCs) are major contributors to the in- and outdoor air-quality deterioration [1], [2], [3]. Due to the progressive increase of their emissions, more strict legislations have been established and led to the development of various treatment technologies allowing higher efficiency and lower cost. The catalytic oxidation, which is one of the most widely used techniques, offers the possibility to operate efficiently at low temperatures (usually less than 500 °C) while avoiding the release of undesirable by-products, such as dioxins and NOX [4], [5]. The widely used catalysts fall broadly into two categories: supported noble metal-based catalysts and transition metal oxides such as chromium, cobalt, copper, nickel and manganese oxides [6], [7]. Although Pd and Pt catalysts are generally considered to be highly active, cobalt oxide (Co3O4) has been reported as one of the most efficient catalysts among transition metal oxides [8], [9], [10].
On the other hand, the physicochemical properties of the support catalysts have a determinant impact on the overall performance of the catalysts. Among the available support structures, the cordierite monolithic ceramic (2MgO–2Al2O3–5SiO2) is widely used due to its high thermal stability and plasticity [11], [12]. This material has been found to be very effective in a number of standard applications including automobile emission control and selective catalytic removal of NOX from exhaust gases [13], [14]. The usually used monolithic shapes of this material have a considerable advantage in terms of pressure drop as compared to pellet-shaped or powdered catalysts [15]. Besides the cordierite, some recent publications have reported that some natural clays might be easily extruded in diverse honeycomb monolithic shapes paving the way towards potential environmental and industrial applications [16], [17]. Clays are abundant, inexpensive, and environmental friendly, widely spread and naturally occurring materials. It is not surprising that they were used in pollution control and environmental protection as adsorbent for the removal of VOCs for a long period of time [18], [19]. More recently, clays have been studied as catalysts or supports for catalytically active phases in oxidation processes [20], [21]. However, their function is often associated with some particular modification or pre-treatment, such as pillaring and acid activation, as to enhance their effectiveness towards a specific catalytic reaction [22], [23].
Till date, in spite of the numerous studies covering the catalytic oxidation of VOCs over various modified clays [24], [25], there is no report dealing with deep oxidation of unsaturated hydrocarbons over natural unmodified clays or over Co3O4-supported on clay structures. Therefore, this paper aims to investigate the catalytic behavior of these novel structured materials in the catalytic oxidation of acetylene (C2H2) and propene (C3H6), chosen as representative compounds for unsaturated hydrocarbons in automotive exhaust gas [26], [27]. The catalytic performance is discussed relative to the cordierite supported Co3O4 that exhibits a considerable efficiency [28], [29].
Section snippets
Preparation of the catalysts
The honeycomb monoliths were made out of clay that was collected from a deposit in the northern region of Morocco. The raw clay was crushed and sieved to retain particles fraction diameter less than 120 μm, then extruded at the laboratory of Chemistry of Solids and Catalysis at University of Cadiz, Spain, following a procedure published elsewhere [30]. The obtained monoliths were cylinder-shaped with a diameter of 14.7 mm characterized by square open channels featuring a size of 1.2 mm and a cell
Chemical composition
Chemical analysis data (Table 1) obtained by X-ray fluorescence of the raw clay indicates the presence of silica, alumina (with Si/Al ratio of 2.5) and iron as major constituents, along with traces of sodium, calcium, zirconium, manganese, potassium, titanium, and magnesium oxides. It should be noted that the observed dull reddish color of the studied clay is associated with its high content of Fe2O3 which is typical of not only Moroccan clays [31], [32], but also several clays worldwide [33],
Conclusions
The results demonstrate that complete oxidation of unsaturated hydrocarbons such as C2H2 and C3H6 can be achieved over easily extruded clay monoliths below 360 °C and 460 °C, respectively, yielding a maximum CO concentrations which do not exceed 5% in the reaction products. The catalytic performance of the studied samples was tested under appropriate experimental conditions permitting to overcome the limitation of the contribution of mass-transfer and leading to reaction order of 0.66 and 0.91
Acknowledgements
The authors gratefully acknowledge the financial support of this research by the Bundesministerium für Bildung und Forschung within the project (BMBF 3681/and Project 10/010/Morocco/Germany). Mhamed Assebban would like to express his gratitude to the CNRST (Centre National de Recherche Scientifique et Technique Rabat, Morocco) for a research fellowship. The authors acknowledge also Prof. Hilario Vidal Muñoz and Prof. Jose Manuel Gatica Casas for clay monolith extrusion using the machine kindly
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Present address: Nanomaterials Research Unit, SAM Department, Centre des Recherche Public – Gabriel Lippmann, L-4422 Belvaux, Luxembourg.