Influence of support properties on the activity of 2Cr-Fe/MgO-MO2 catalysts (M = Ce, Zr, CeZr and Si) for the dehydrogenation of n-octane with CO2

Abstract

The influence of the support on catalytic activity and stability of supported 2Cr-Fe bimetallic catalysts for the CO₂-assisted dehydrogenation (DH) of n-octane has been investigated. Four MgO modified supports viz; MgO-CeO₂ (MgCe), MgO-ZrO₂ (MgZr), MgO-CeO₂-ZrO₂ (MgCeZr) and MgO-SiO₂ (MgSi) were synthesized by the sol-gel combustion technique. The supported catalysts were in turn prepared by vacuum impregnation and thereafter tested for the CO_2-assisted DH of n-octane. The catalysts were characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), N₂-physisorption, Raman spectroscopy, transmission electron microscopy (TEM), electron dispersive x-ray (EDX), temperature programmed desorption of CO₂ (CO₂-TPD), temperature programmed reduction and oxidation (H₂-TPR and CO₂-TPO), electron paramagnetic resonance (EPR) and thermal gravimetric analysis (TGA) techniques. Raman results showed that the CrO_x is stabilized as mono- and/or polynuclear Cr(VI) species over the 2Cr-Fe/MgCe catalyst, which are reduced to lower oxidation state species during the DH reaction. The 2Cr-Fe/MgZr, 2Cr-Fe/MgCeZr and 2Cr-Fe/MgSi catalysts stabilized the CrO_x as polymerized species, forming the more active Cr-O-Fe polymer units on the catalysts’ surface. XRD, TEM and EDX results showed that the ZrO₂-containing supports have smaller particles and stabilized the active metal oxides in a more dispersed amorphous state. The CO₂-TPO of the pre-reduced catalysts and EPR of the used catalysts indicated that the 2Cr-Fe/MgCeZr undergoes significant re-oxidation by CO₂ during the catalytic process. The 2Cr-Fe/MgCe was the least active, while the 2Cr-Fe/MgZr catalyst showed the best performance and stability over three regeneration cycles. Selectivity to C8 products (octenes and aromatics) was found to strongly depend on the surface basicity of the catalysts. Deactivation of the catalysts was found to follow first order kinetics and coke deposition was identified as the major cause.