The role of shape selectivity and intrinsic selectivity of acidic sites of the catalysts in the skeletal isomerization ofn-butenes

Abstract

: The e †ect upon kinetic behaviour (activity and selectivity) of the physical properties (pore volume distribution) and acidic characteristics (nature of acidic sites and acidic strength distribution) of acidic catalysts in the skeletal isomerization of n-butenes has been studied. The catalysts studied are the following : c-alumina, alumina modiÐed by introduction of F, Cl and Br, silicaÈalumina, HZSM-5 zeolite-based catalyst and SAPO-34-based catalyst. It has been proven that the selectivity of acidic catalysts towards the skeletal isomerization of nbutenes is the result of a compromise between pore shape selectivity and intrinsic selectivity of the acidic sites of di †erent strength, this latter property being more signiÐcant. Strong acidic sites are needed for the skeletal isomerization, but sites which are too strong produce important amounts of byproducts. A chlorinated alumina is a highly active and selective catalyst for skeletal isomerization.

n- and Isoalkane Adsorption Mechanisms on Zeolite MCM-22

“…[1][2][3] So far, several shape-selectivity mechanisms have been elucidated and applications identified. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] In common molecular sieving, bulky molecules adsorb preferentially over bulkier species as a result of steric constraints imposed by the zeolites' framework, e.g., linear alkane molecules compared to branched isomers. [19][20][21][22][23] Besides this common adsorption competition based on molecular bulkiness, the reverse phenomenon can also occur, i.e., competitive adsorption in favor of the bulkier species.…”

“…So far, several shape-selectivity mechanisms have been elucidated and applications identified. − In common molecular sieving, bulky molecules adsorb preferentially over bulkier species as a result of steric constraints imposed by the zeolites' framework, e.g., linear alkane molecules compared to branched isomers. − Besides this common adsorption competition based on molecular bulkiness, the reverse phenomenon can also occur, i.e., competitive adsorption in favor of the bulkier species. Experiments revealed preferential adsorption of branched alkanes over their linear isomers on specific zeolites with tubular pore systems such as SAPO-5 and Mordenite. − Molecular simulations indicated that in such zeolites, depending on conditions, branched alkanes are preferentially adsorbed at the expense of linear isomers. − …”