Investigation of isobutane dehydrogenation on CrOx/Al2O3 catalyst in a membrane reactor

Isobutene is an important raw material for the production of additives that increase the octane number of gasoline and reduce emissions, some polymeric materials and pharmaceuticals. Isobutane dehydrogenation, which is widely used in the production of isobutene, is an equilibrium-limited endothermic reaction. In this study, it was aimed to overcome the equilibrium conversions by using membrane reactor. First, Al2O3 supported chromium-based catalysts were prepared by the impregnation technique. Characterization studies of catalysts were carried out by DR..........-UV-vis, N-2 adsorption/desorption, XRD and SEM/EDS analyses. Catalysts (6% Cr by mass) were prepared at three different values of the contact time - 24, 48, 72 h mixing times. The highest amount of monochromate was observed in the catalyst prepared during the mixing time of 48 h. Catalysts containing 6%, 8%, and 10% Cr by mass (mixing time: 48 h) were also synthesized. When the chromium concentration increased above 8%, a decrease in the amount of monochromate was observed. Then, reactions were performed with a catalyst containing 8% by mass of Cr, which was prepared in a mixing time of 48 h with the highest amount of monochromate. The reactions were carried out in a Pd based membrane reactor system with pure isobutane feed. The effect of reactor temperature was investigated at 550 ? and 600 ? (WHSV:0.3h(-1)). While no significant difference was observed between the conversion values at both reactor temperatures (?90%), isobutene selectivity values were higher (?85%) at 600 ?. It was observed that the high isobutene selectivity values determined at 600 ? were due to the high endothermic nature of the reaction. The reaction was also carried out by changing the weight hourly space velocity at 600 ? (WHSV:0.5h(-1)). Although the isobutane conversion values were close to each other (?90%), higher isobutene selectivity values (?90%) were determined at 0.3h(-1). The presence of less hydrogen in the environment decreased the formation of the hydrogenation reaction of isobutane. This situation enhanced an increase in isobutene selectivity values. The shift of the reaction towards the products also increased the selectivity of isobutene. It was observed that although the amount of carbon (3 times more) formed at high retention times caused negative effects such as a decrease in the surface area and closure of the pores, the active monochromate species preserved in the catalyst structure. It was determined that the formed amorphous carbon did not cause the rapid decreases in activity and selectivity. The results of the study showed that the equilibrium limitation could be overcome by removing hydrogen from the reactor medium (equilibrium conversions:73% at 600 ?, 53% at 550 ?). The results of the modeling studies carried out in the membrane reactor were found to be compatible with the experimental results.

Automated summary

In this study, membrane reactors were used to improve the equilibrium conversions in isobutene production. Chromium-based catalysts were prepared and higher chromate concentrations were observed with different mixing times. Higher isobutene selectivity values were observed at 600 ℃. The modeling studies in the membrane reactor were found to be compatible with the experimental results.