Cr Incorporated MCM-41 Type Catalysts for Isobutane Dehydrogenation and Deactivation Mechanism

Cr incorporated MCM-41 type catalysts, which were prepared by direct hydrothermal synthesis and impregnation routes, were tested for dehydrogenation of isobutane. Among the surfactants (C17H38BrN, C19H42BrN, and C32H68BrN) used in the synthesis of MCM-41, C19H42BrN gave the best structure with ordered mesopores. Characterization results proved that chromium was very well dispersed within the MCM-41 lattice of the synthesized catalysts. In all of the synthesized catalysts, the presence of Cr6+ (2p3/2) in the form of chromates and Cr3+(2p1/2) in the form of CrO x or Cr2O3 was detected. All chromate types (mono, di, and poly) were observed in the catalyst synthesized hydrothermally according to the addition of metal solution simultaneously with silica source (CR_ALS). All characterization methods demonstrated that the highest amount of Cr6+ was present in this catalyst. Catalytic tests of the synthesized catalysts were carried out at 600 °C and at atmospheric pressure. The maximum conversion value (∼27%) was reached at 15 min on CR_ALS catalyst. No products other than isobutene and hydrogen were encountered at the reactor outlet. Results proved that coke deposition was negligible over the synthesized catalysts in isobutane dehydrogenation. It was shown that the monochromates were the most active phase among all types of chromates for isobutane dehydrogenation. Catalyst deactivation occurred when the tetrahedrally coordinated Cr(VI)O4 type of chromate was converted to inactive octahedrally coordinated Cr(III)O6 groups and Cr2O3 crystal phase.