CONVERSION OF LIGHT ALKANES INTO AROMATIC-HYDROCARBONS .6. AROMATIZATION OF C2-C4 ALKANES ON H-ZSM-5 - REACTION-MECHANISMS

The conversion of ethane, propane, n-butane, isobutane and of the corresponding alkenes was studied at 530-degrees-C on a H-ZSM-5 zeolite (Si/Al = 40). Butanes transform about 4 times more rapidly than propane and 100 times more than ethane. From propane and butanes the primary products result from dehydrogenation and cracking, cracking being more rapid than dehydrogenation. Ethane undergoes mainly dehydrogenation and leads slowly to methane. It is suggested than these reactions occur through scission of carbonium ions formed by alkane protonation. The more stable the carbonium ion the higher the reaction rate. The olefinic primary products undergo very rapid reactions through carbenium ion intermediates: oligomerization, cyclization of the oligomers followed by aromatic formation through hydrogen transfer from naphthenes to light alkenes. Whatever the reactant, the limiting step of aromatization is the initial formation of alkenes.