The Role of Chemical and Structural Changes on the Surface in Deactivation of Chromia-Alumina Catalysts in Dehydrogenation of Paraffinic Hydrocarbons

Deactivation Studies of the CrOx/Al2O3 Dehydrogenation Catalysts under Cyclic Redox Conditions

“…This type of CrOx/Al 2 O 3 catalyst deactivation is not typically observed in the field. To date there have been many articles in the literature discussing catalytic behavior of the CrOx/Al 2 O 3 catalyst; however, only a small number of studies focused on the catalyst deactivation especially induced by the changes of the catalyst properties. − It is well-accepted that the surface chromium species on the CrOx/Al 2 O 3 catalyst exist in two oxidation states as Cr 3+ and Cr 6+ , and the paraffin dehydrogenation activity is mainly associated with the redox and nonredox Cr 3+ active sites. ,, According to the literature, the CrOx/Al 2 O 3 catalyst deactivation in paraffin dehydrogenation has been attributed to the following factors: (1) the loss of Cr 3+ active sites by either forming inactive alumina-incorporated Cr 3+ or being converted to inaccessible Cr 3+ species, (2) the formation of solid solution of α-(Cr, Al) 2 O 3 , ,,, (3) the degradation of alumina phase, , (4) the change of catalyst porous texture, , and (5) the aggregation of Cr 2 O 3 . , For example, in the early 1970s, Bremer et al found that the irreversible deactivation of the CrOx/Al 2 O 3 catalyst within one cycle of reduction and oxidation was due to the formation of a solid solution of α-(Cr, Al) 2 O 3 in the catalyst at a reaction temperature over 650 °C and was independent of the coke deposition . Gitis et al reported that the reduction of the CrOx-Al 2 O 3 -K 2 O catalysts at 700 °C led to two types of deactivation: reversible and irreversible.…”

“…9−31 It is well-accepted that the surface chromium species on the CrOx/Al 2 O 3 catalyst exist in two oxidation states as Cr 3+ and Cr 6+ , and the paraffin dehydrogenation activity is mainly associated with the redox and nonredox Cr 3+ active sites. 11,24,29 According to the literature, the CrOx/ Al 2 O 3 catalyst deactivation in paraffin dehydrogenation has been attributed to the following factors: (1) the loss of Cr 3+ active sites by either forming inactive alumina-incorporated Cr 3+15 or being converted to inaccessible Cr 3+ species, 24 (2) the formation of solid solution of α-(Cr, Al) 2 O 3 , 16,17,19,26 (3) the degradation of alumina phase, 20,26 (4) the change of catalyst porous texture, 18,26 and (5) the aggregation of Cr 2 O 3 . 20 It was found that the concentration of α-Al 2 O 3 in the catalyst increased from zero in the fresh catalyst to 5.6% in the one-year aged sample and to 12.5% in the two-year aged sample, while the α-Cr 2 O 3 concentration decreased from 7.9% to 2.0% to 0.9% accordingly.…”

“…The former was associated with the decrease of chromium oxide surface area, and the latter was with irreversible changes of the catalyst porous texture as a whole 21. In addition, Sterligov et al studied the catalytic behavior of the CrOx/Al 2 O 3 catalyst with the additions of potassium, lithium, or niobium oxides in n-butane dehydrogenation at 550 °C for about 1000 redox cycles 19. Some interesting observations were made, including the significant decrease of Cr 6+ concentration in the catalyst, the decrease of the Cr 2p3/2 /Al 2s atomic ratio obtained by XPS, little reduction of the total catalyst surface area, and steady reduction of the catalyst activity.…”

“…18,19 For example, in the early 1970s, Bremer et al found that the irreversible deactivation of the CrOx/Al 2 O 3 catalyst within one cycle of reduction and oxidation was due to the formation of a solid solution of α-(Cr, Al) 2 O 3 in the catalyst at a reaction temperature over 650 °C and was independent of the coke deposition 17. Gitis et al reported that the reduction of the CrOx-Al 2 O 3 -K 2 O catalysts at 700 °C led to two types of deactivation: reversible and irreversible.…”