Partial Oxidation of Methane Over Co-ZSM-5: Tuning the Oxygenate Selectivity by Altering the Preparation Route
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Partial Oxidation of Methane Over Co-ZSM-5: Tuning the Oxygenate Selectivity by Altering the Preparation Route Nadzeya V. Beznis • Bert M. Weckhuysen Johannes H. Bitter
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Received: 9 March 2009 / Accepted: 26 October 2009 / Published online: 10 November 2009 Ó The Author(s) 2009. This article is published with open access at Springerlink.com
Abstract For the first time the possibility to partially oxidize methane to methanol and formaldehyde at low temperature over Co-ZSM-5 using air is shown. The influence of the preparation method on the nature of the cobalt species is investigated. In addition, the catalytic activity and selectivity for methane oxidation as a function of the cobalt speciation is discussed. Based on UV–vis– NIR and FT-IR spectroscopy, H2-TPR, TEM and kinetic measurements it is concluded that cobalt in ion-exchange positions results mainly in the formation of formaldehyde, while larger Co-oxide particles prepared by impregnation result in the formation of methanol.
1 Introduction The direct conversion of methane to methanol and formaldehyde holds one of the greatest challenges in heterogeneous catalysis [1–4]. Methane is an important fossil feedstock and its direct partial oxidation to oxygenates is challenging for thermodynamic reasons. Currently, oxygenates (e.g. methanol) are prepared from methane via a multi-step process. First syn-gas (CO and H2) is generated followed by the oxygenate formation in a second step [5–7]. Though these processes are highly efficient, methanol synthesis following this route is only economically viable when performed on a large scale, which is the result of high capital costs for syn-gas production [8]. Obviously,
N. V. Beznis B. M. Weckhuysen J. H. Bitter (&) Inorganic Chemistry and Catalysis Group, Department of Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands e-mail: [email protected]
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a more convenient way for small-scale methanol production would be the direct synthesis of methanol from methane making use of air or oxygen. Fe-ZSM-5 is known to be active for this conversion although N2O was indispensable as oxidant [9, 10]. Due to the limited availability of N2O the use of molecular oxygen or air as an oxidant is more attractive. Recently, it was reported that Cu-ZSM-5 was able to perform the methane to methanol reaction. However, a preliminary calcination to introduce active oxygen is required. It is also necessary to mention here that the methanol formation is not continuous yet and an extraction procedure is needed [11, 12]. An initial screening study in our laboratory showed that especially Co-ZSM-5 displays intriguing catalytic behavior in the partial oxidation of methane to oxygenates. We will show in this work that Co-ZSM-5, preliminary calcined in air, can convert methane to methanol and/or formaldehyde. Co-ZSM-5 can be prepared by different methods: wet ion-exchange [13–15], solid-state ion-exchange [16, 17], incipient wetness impregnation [14, 17] and sublimation [17]
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