Physico-chemical properties of mixed molybdenum and cerium oxides supported on silica–alumina and their use as catalysts in the thermal-catalytic cracking (TCC) of n-hexane

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Abstract

Mixed oxides, MoO3–CeO2, were being used as catalysts for the cracking (TCC) of liquid hydrocarbon feedstocks. The dispersion and interactions of MoO3, CeO2 and mixtures thereof impregnated into the silica–alumina surface were investigated using several techniques, which included X-ray diffraction (XRD) and laser Raman spectroscopy (LRS). The loadings and the chemical states of metal oxides incorporated separately had significant effects on the catalytic activities of the resulting monocomponent catalysts. Addition of cerium to molybdenum had a favorable effect on the production of light olefins in the TCC of n-hexane up to a certain level of cerium loading. In fact, high loadings of molybdenum and/or cerium favored the formation of aromatics, instead. The catalytic performance of the bicomponent catalysts also depended significantly on the incorporation methods. It was found that the co-impregnation of MoO3 and CeO2, which led to the highest production of light olefins, corresponded to the formation of (surface) cerium molybdate to the highest extent. On the other hand, the catalysts prepared by the two-step impregnation methods (sequential and reverse sequential impregnation) showed much lower catalytic performance due to low Mo–Ce interactions as suggested by an important segregation of the active phases, mostly MoO3. The sequence of catalytic performance (to the desired products, i.e. light olefins) fully coincided with that of the dispersion of molybdate species on the support surface.

Introduction

Light olefins and diolefins such as ethylene, propylene, butenes and butadienes are precursors of numerous plastic materials, synthetic fibers and synthetic rubbers. These precursors are mainly produced by several conventional petroleum processes that include steam cracking (SC), fluid catalytic cracking (FCC) and deep catalytic cracking (DCC) [1], [2]. However, these conventional processes suffer from several major drawbacks such as severe environmental problems, high energy cost and/or lack of catalyst long-term stability. Recently, a new emerging process known as thermal-catalytic cracking (TCC) has been developed, aiming at achieving higher combined yield of ethylene and propylene, but at lower energy consumption and lower greenhouse gases emission as well [3], [4], [5], [6], [7], [8], [9], [10], [11]. TCC is a combination of (mild) thermal cracking and catalytic cracking, the latter making use of unique hybrid or novel mesoporous mixed oxide catalysts. In the hybrid catalyst configuration, two components, microporous (zeolite type) and mesoporous (cocatalyst), were firmly bound to each other within a clay binder, such that a “pore continuum” effect was developed, allowing the rapid diffusion of reacting species from micropores to the mesopores or vice versa [8], [9], [10], [11]. Another developed version was the novel mesoporous mixed oxide catalysts, which is based on supported mixed molybdenum–cerium oxides [5], [6], [10].

In general, both molybdenum oxide and cerium oxide based catalysts have attracted the attention of many researchers, because of their numerous applications in various fields. For instance, molybdenum oxide based catalysts have been widely used in many important catalytic reactions [12], which include olefin metathesis [13], oxidation and ammoxidation reactions [14], [15], as well as hydrotreating [16]. Furthermore, cerium oxide forms an integral part of three-way catalysts (TWC) for automotive exhaust treatment [17] and serves as a promoter to the fluid catalytic cracking (FCC) catalysts [18]. The success of cerium oxide relies on its unique features that include its ability to release and store oxygen (high oxygen mobility) couples with the ability to shift easily between the reduced and oxidized states (Ce3+  Ce4+) (unique redox properties) [19], [20]. Nevertheless, to our knowledge, no work has been reported yet in literature on silica–alumina supported bicomponent MoO3–CeO2 system. Therefore, an attempt was made in this present study to shed some light into properties of this catalyst system. In the previous publication [10], we have laid the general features of the effect of doping cerium oxide into the supported molybdenum oxide catalyst. Cerium oxide, incorporated as a dopant, has a main catalytic effect, of increasing the selectivity to light olefins and diolefins while the production of BTX aromatics significantly decreases. In the present study, we are attempting to provide some basic insights into the physico-chemical properties and structure of silica–alumina supported monocomponent MoO3, CeO2 and bicomponent MoO3–CeO2 catalysts, and the effect of catalyst preparation. Therefore, a systematic study was undertaken on the characterization of silica–alumina supported monocomponent MoO3, CeO2 and bicomponent MoO3–CeO2 catalysts by various techniques such as Brunauer–Emmett–Teller (BET) surface area and pore volume, X-ray diffraction (XRD), Raman spectroscopy (LRS, a powerful tool to investigate supported metal oxides) and ammonia temperature-programmed desorption (surface acidity). The catalytic properties were evaluated during the TCC of n-hexane used herein as a model molecule for petroleum naphthas, and the results were discussed in relation with the structural properties.

Section snippets

Catalyst support

Commercial amorphous silica–alumina (SiAl) (Aldrich, catalyst support grade 135, SiO2 = 86 wt.%; Al2O3 = 14 wt.%) was used as a support. Prior to impregnation it was calcined at 650 °C for 3 h. This treatment results in silica–alumina with a surface area of 421 m2/g and a pore volume of 0.680 cm3/g, as measured with the BET technique.

MoO3/silica–alumina catalysts (MoO3/SiAl)

A series of MoO3/silica–alumina catalysts with MoO3 loadings in the range of 1.0–33.0 wt.% MoO3 were prepared by impregnation method. The required amount of ammonium

Monocomponent MoO3/SiAl and CeO2/SiAl catalysts

Mo oxides as metal oxides of Group VIB have several oxidation states, which may affect their catalytic activity, mostly when they are supported on “irreducible” oxides such as silica or silica–alumina. In its most stable oxidation state (+6), MoO3 also exhibits acidity of some strength and of both natures (Brönsted and Lewis) [23]. Surface structure of a supported metal oxide catalyst is of key importance for its catalytic properties. Several other physico-chemical parameters are also important

Conclusion

The loadings and the chemical states of metal oxides incorporated separately had significant effects on the catalytic activities of the resulting monocomponent catalysts. Addition of cerium to molybdenum had a favorable effect on the production of light olefins in the TCC of n-hexane up to a certain level of cerium loading. In fact, high loadings of molybdenum and/or cerium favored the formation of aromatics, instead. The catalytic performance of the bicomponent catalysts also depended

Acknowledgements

Financial supports from NSERC (Natural Science and Engineering Research Council of Canada) and Valeo Management are acknowledged.

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