Effect of the K loading on effective activation energy of isobutane dehydrogenation over chromia/alumina catalysts

In this work, a series of the chromia/alumina catalysts with the same surface Cr concentration of 4.4 +/- 0.2 at. Cr/ nm(2) and different K loadings (0-3.9 wt.%) has been investigated to evaluate the effect of K on the effective activation energy of isobutane dehydrogenation at 520-550 degrees C. The results of the study show that at K loadings up to 2.0 wt.%, K preferably interacts with alumina and modifies an average dispersion of Cr(3+)Ox species in the reduced catalysts, thus leading to a growth of conversion and selectivity, but the number of direct contacts of K with active sites is too low to affect the effective activation energy of dehydrogenation. When K loading is higher than 2.0 wt.%, the alumina capacity for K is reached and K interacts directly with Cr(3+)Ox species. This causes an enlargement of effective activation energy with K loading. One possible explanation relies on the formation of a new type of active sites in Cr3+-O(H)-K+ with modified energetics of the reaction pathway or even with other reaction pathway. Another explanation is that potassium ions, being in contact with active sites, induce strong chemisorption of water, which poisons active sites, especially at low temperatures of the reaction.