Skeletal isomerization of n-butenes - II. Composition, mode of formation, and influence of coke deposits on the reaction mechanism

On a fresh HFER zeolite (Si/Al=13.8) at 623 K, the skeletal isomerization of n-butenes is accompanied by various reactions: disproportionation into propene and pentenes, coking, hydrogen transfer (in the decreasing order of significance). While there is a rapid deactivation of disproportionation, coking, and hydrogen transfer reactions, owing to the blockage of the pores by carbonaceous compounds (coke), an increase in the rate of isomerization is at first observed followed by a decrease at: long time-on-stream. At the maximum of isomerization only 10% of the pore volume remains accessible to nitrogen adsorbent. Moreover, IR spectroscopy shows that 75% of the OH groups are in interaction with coke molecules, the remaining 25% being inaccessible to ammonia and, hence, to the reactant, Coke was found to be composed of slightly condensed aromatics (with two to four aromatic rings) trapped at the intersections of the eight-and 10-membered-ring channels. Coke molecules result from the transformation of secondary benzenic products, slowly desorbed from the zeolite pores, through two different ways, (i) the classical route via alkylation, cyclization, and hydrogen transfer steps and (ii) dehydrogenative coupling. The large significance of this latter reaction in the formation of HFER coke molecules in comparison to the other zeolites, can be related to the proximity of adjacent channel intersections, To explain the initial increase in the rate of butene isomerization a new reaction process is proposed involving as active sites benzylic carbocations formed from coke molecules trapped in pores located near the outer surface of the crystallites (pore mouth catalysis). The deactivation of this reaction which occurs at long time-on-stream is shown to be due to the growth of the active coke molecules, (C) 1998 Academic Press.