Saliha Cetinyokus Kilicarslan, Meltem Dogan* and Timur Dogu Contribution of Pd Membrane to Dehydrogenation of Isobutane Over a New Mesoporous Cr/MCM-41 Catalyst DOI 10.1515/ijcre-2015-0031 Abstract: A chromium incorporated mesoporous silicate structured Cr/MCM-41 type catalyst was synthesized fol- lowing a one-pot hydrothermal route and tested in dehy- drogenation of isobutane to isobutene in a Pd membrane reactor. Characterization results of the catalyst proved that it had ordered pore structure with a narrow pore size distribution. This catalyst showed quite high activity for the dehydrogenation of isobutane. Membrane reactor tests performed at 823 K proved the advantages of in-situ removal of produced hydrogen from the reaction zone through the membrane, on isobutene yield. In fact, much higher isobutane conversions than the conversion values predicted from the equilibrium calculations were achieved at this temperature. However, at a higher tem- perature of 873 K, the Pd membrane itself also showed catalytic activity for the decomposition of isobutane and isobutene. As a result, lower isobutene selectivity values and quite high methane and propene selectivities were observed at this temperature. Cracking reactions also caused coke formation at 873 K, especially at high pres- sure differences across the membrane (70 kPa). Increase of pressure difference across the membrane caused fast removal of hydrogen from the reaction zone, which facili- tated coke formation due to cracking reactions. Keywords: isobutane dehydrogenation, Cr/MCM-41, Pd membrane reactor, coke formation 1 Introduction Isobutene is a valuable chemical, which has been widely used in the production of tert-ethers (such as MTBE, ETBE, etc.), butyl rubber and some other important chemicals. It is generally produced through dehydro- genation of isobutane, following different catalytic pro- cesses, in which catalyst deactivation due to coke formation is an important issue. Also, thermodynamic limitations of isobutane dehydrogenation require high temperature operation. Chromium-based catalysts are widely used to obtain isobutene by dehydrogenation of isobutane. Synthesis procedure and the type of support material of chromium based catalysts were reported to have significant effect on their performance in dehydrogenation of isobutane (Hakuli, Kytökivi, and Krause 2000; Ding et al. 2008; Wilson, Madhusudhan Rao, and Viswanath 2003; Korhonen, Airaksinen, and Krause 2006; Korhonen et al. 2007; Pak and Haller, 2001; Kilicarslan, Dogan, and Dogu 2013). Comparison of the structures of CrO x / alumina catalysts, which were prepared following an Atomic Layer Epitaxy (ALE) and impregnation techniques showed more homogeneous distribution of chromium on the catalyst surface, which was prepared by the ALE method (Hakuli, Kytökivi, and Krause 2000). Chromium based ceria, titania and ceria/zirconia supported catalysts were also tested in the literature for dehydrogenation of isobutane. Catalyst deactivation due to coke deposition was reported to take place due to adsorption of inter- mediates formed during reaction. Presence of CO 2 in the reaction medium was shown to reduce coke formation and helped to protect active chromium forms over the CrO x /active carbon catalysts (Ding et al. 2008). Silicate structured mesoporous catalysts supports, like MCM-41, MCM-48 and SBA-15, were generally con- sidered to be less prone to catalyst deactivation due to coke formation and they provide less diffusion resistance than the conventional microporous catalyst supports. Results reported for the characterization of Cr-MCM-41 and Cr-MCM-48 catalysts, which were prepared by the hydrothermal synthesis method, showed no change in the mesoporous structure after the reduction process of these materials (Pak and Haller, 2001). However, tetrahe- dral coordination of chrome turned to octahedral coordi- nation in the reducing atmosphere. Cr-MCM-41 catalysts were also synthesized by hydrothermal and impregnation *Corresponding author: Meltem Dogan, Chemical Engineering, Gazi University, 06570 Ankara, Turkey, E-mail: meltem@gazi.edu.tr Saliha Cetinyokus Kilicarslan, Chemical Engineering, Gazi University, 06570 Ankara, Turkey, E-mail: salihakilicarslan@gazi.edu.tr Timur Dogu, Chemical Engineering, Middle East Technical University, 06800 Ankara, Turkey, E-mail: tdogu@metu.edu.tr Int. J. Chem. React. Eng. 2016; 14(3): 727–736 Brought to you by | Cornell University Library Authenticated Download Date | 10/5/16 10:48 AM