MECHANISM OF SHORT-CHAIN ALKANE TRANSFORMATION OVER PROTONIC ZEOLITES- ALKYLATION, DISPROPORTIONATION AND AROMATIZATION

Ethane, propane and butanes can be transformed on large and average po re size protonic zeolites. It is shown that the reaction temperature d etermines both the mode of alkane activation on the acid sites (i.e. t he mode of carbocation formation) and the nature of their transformati on. At ambient temperature isobutane (and no other short-chain alkanes ) can be activated, but only in the presence of alkenes. This activati on occurs by hydride transfer to the carbenium ions resulting from alk ene adsorption on the protonic sites of the zeolite. The t-butyl carbe nium ions, formed from isobutane, alkylate the alkene molecules throug h a chain mechanism, At temperatures above 500 degrees C, pure C-2-C-4 alkanes can be transformed into aromatics, in particular on HMFI. The activation of alkanes occurs, like in superacid solutions, through pr otolysis of their C-H or C-C bonds with formation of hydrogen or alkan es and of carbenium ions which desorb as olefins, These olefins are tr ansformed into aromatic products through various reactions: oligomeriz ation-cracking, cyclization and hydrogen transfer. On HMFI, the protol ytic cleavage of C-H and C-C bonds is the limiting step of short-chain alkane aromatization. The association of gallium species to HMFI incr eases significantly the aromatization activity and selectivity of this zeolite and alkane aromatization occurs through a bifunctional scheme . At average temperatures, propane and butanes can be transformed thro ugh a dimerization-cracking process (disproportionation). In butane tr ansformation this process is responsible for the formation of propane and pentanes but also for butane isomerization. As is the case at high temperature the activation of alkanes occurs through protolysis. Howe ver this protolysis is only the initiation step of the carbenium ion c hain mechanism of disproportionation, for protolysis is much slower th an the hydride transfer from alkanes to the resulting carbenium ions. The reaction schemes of isobutane alkylation with 2-butene, of butane isomerization and of propane aromatization are described. The influenc e of the pore structure and of the acidity of the protonic zeolites on their activity and selectivity is discussed.