THE ROLE OF THE DENSITY OF ACTIVE-SITES IN SKELETAL ISOMERIZATION OF N-BUTENE

Acidity of the catalysts plays a crucial role in the skeletal isomeris ation of n-butene. Very weakly acidic OH groups are not active, very s trongly acidic groups are active but they induce a too high extent of the undesired side reactions. This paper addresses the question to whi ch extent and in which way the selectivity can be manipulated by chang es on the catalyst. One can either vary the OH acid strength, by using different three-valent ions in the skeleton of, e.g., ZSM-5, or to ch ange the total number of OH groups, by variation in the Si/Al (Si/M)-r atio. Both ways are explored here and it is concluded that when a sele ctivity change is achieved, the dominating effect is that of the densi ty of the active sites. This is not a primary mechanistic effect but a n effect of variations in the apparent contact time on the extent of s ide reactions.