Oligomerization reactions, Bronsted acid sites catalyzing

In the case of an oligomerization reaction catalyzed by Bronsted acid sites, the scheme reported in Fig. 1 must be modified as represented in Fig. 2a. 

According to this scheme, the first step of the reaction is the formation of a hydrogen-bonded precursor FH B, followed by the protonation of the monomer, leading to the formation of FBH". Examples of this type are shown in Section IV.B, where the oligomerization of unsaturated molecules in protonie zeolites is discussed. It is important that this first step is common to other reactions catalyzed by Bronsted acid sites. For example, in Section IV.A, the formation of methyl-substituted benzene carbocations as intermediate species involved in the MTO process in Hp zeolite is diseussed. 

If the polymerization reaction occurs in the presence of steric constraints, the activation energy associated with the monomer insertion grows with the progress of the reaction, in the same way as discussed for the oligomerization reactions catalyzed by Bronsted acid sites (Section LB and Fig. 2a), because the available space is progressively reduced. Under such conditions, the formation of polymeric species is limited, and small oligomeric species can become observable. An example of this situation is ethene polymerization on Cr/silicalite, in which the transition metal center is grafted to the internal surface of a cavity (Section VI.C.l). 

IV. Reactions Catalyzed by Bronsted Acid Sites The MTO Process and Oligomerization of Unsaturated Hydrocarbons... 

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