Research Article Determination of Optimum Conditions and the Kinetics of Methanol Oxidation In this study, the catalytic oxidation of methanol to formaldehyde was investi- gated in a laboratory-scale fixed-bed catalytic reactor, under a large number of different conditions. Iron-molybdate catalysts supported by silica or alumina with a molybdenium/iron (Mo/Fe) ratio of 1.5, 3 and 5 were studied for the gas phase reaction. In order to obtain the optimum conditions, six different tempera- tures in the range of 250–375 °C and three different space times of 50.63, 33.75 and 20.25 g/(mol/h) were investigated. After determining the optimum condi- tions for this reaction, experiments aimed at understanding the reaction kinetics, were carried out. These experiments were performed on the catalyst favoring the formation of formaldehyde, which has a (Mo/Fe) ratio of 5 on a silica support. Seven reaction models derived by the mechanisms cited in the literature were tested to elucidate the kinetics of the reaction and the surface reaction controlling model was found to be the most suitable reaction mechanism. Keywords: Fixed-bed catalytic reactor, Formaldehyde, Iron-molybdate catalyst, Kinetics of methanol oxidation, Methanol oxidation Received: July 7, 2009; revised: September 28, 2009; accepted: October 19, 2009 DOI: 10.1002/ceat.200900349 1 Introduction Methanol, one of the world’s most important chemical inter- mediates, is the starting material for the synthesis of various products such as formaldehyde [1]. Formaldehyde is a reactive molecule and the first in the series of aliphatic aldehydes [2] and is a base chemical of major industrial importance. In spite of fluctuations in the world economy, the growth of formalde- hyde production has been remarkably steady and is expected to continue [3]. Formaldehyde consumption has grown by 2–3 % per year on an average over the past two decades, due primarily to increasing demand in the construction sector for engineered wood products manufactured using formaldehyde- based resins [4]. The annual worldwide production capacity of formaldehyde now exceeds 25–27 million tons (calculated as 37 % solution) [5]. The main production process for formaldehyde is air oxida- tion of methanol with high conversion; by an exothermic reac- tion at atmospheric pressure and at a temperature of 300– 400 °C [6]. All industrial production of formaldehyde uses methanol and air as raw materials. Two technologies are avail- able, which are referred as the silver process and the oxide pro- cess, respectively [7]. Most of the newly built formaldehyde plants (more than 70 %) are based on the metal oxide catalyst (Fe-Mo) due to almost complete conversion of methanol (exceeding 99 %) as well as very high formaldehyde selectivity (95 %) and, most importantly, due to the imposition of strin- gent environmental regulations [3]. The most widely used cat- alyst consists of mixtures of iron-molybdate with molybdenum trioxide often modified by different additives. Its catalytic performance is highly satisfactory and practically unbeatable (98–100 % methanol conversion and 92–94 % selectivity to formaldehyde) [8]. Over the last decade several researchers have studied the kinetics of the partial oxidation of methanol to formaldehyde. Jiru et al. [9] suggested a redox mechanism similar to that ob- served for the Mars-van Krevelen type reaction mechanism for the description of the oxidation of aromatic hydrocarbons over V 2 O 5 . They found that the formaldehyde concentration affects the reaction rate, while the water concentration does not [4]. Santacesaria et al. studied the kinetics of the oxidation of methanol to formaldehyde over an iron-molybdate catalyst [10]. The ratio Mo/Fe of the employed catalyst was 2.5 and it contained small amounts of cobalt molybdate (1.8 % by weight of CoO). The results were described with a kinetic model de- rived on the basis of a redox mechanism partially hindered by the absorption of water [10]. McCormick et al. studied the oxi- dation of methane, methanol, formaldehyde and carbon mon- oxide over precipitated silica catalysts examined over a range of reactant and oxygen partial pressures [8]. The conversion Chem. Eng. Technol. 2010, 33, No. 1, 167–176 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Ayse Hilal Ulukardesler 1 Süheyda Atalay 1 Ferhan S. Atalay 1 1 Department of Chemical Engineering, Ege University, Bornova-Izmir, Turkey. – Correspondence: Dr. A. H. Ulukardesler (ahulukardesler@gmail.com), Department of Chemical Engineering, Ege University, 35100, Bornova- Izmir, Turkey. Methanol oxidation 167