Oligomerization reactions, Bronsted acid sites

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... 

A pt, Fig. 2b) is expected to increase in the opposite direction (i.e., A pT(ethy-lac)< A pT(methylac)< AF pT(ac)), whereas the opposite is expected for the oligomerization rate. Therefore, the investigation of the oligomerization reaction of various acetylenic molecules on Bronsted acid sites offers the possibility of showing which kinds of species can be detected by FTIR spectroscopy, depending on the different values of the activation energies for proton transfer. 

Refs. [473,837,886,921]. Kustov et al. [922] published an interesting DRIFT spectroscopic study on ethene oligomerization, including a discussion of the role of Lewis- and Bronsted-acid sites and ethoxy groups as intermediates of this reaction. 

Alumina forms more and heavier coke than MgO but the latter deactivates faster. This is explained by the fact that on MgO primary aldol condensation reactions and formation of coke precursors species take place on the same basic sites. In contrast, alumina forms predominantly aromatic coke involving Bronsted OH groups that are inactive for aldol condensation reactions. The initial deactivation rate of acetone oligomerization on MgyAlOx oxides is essentially related to the surface basic properties. This is because the activity decline is significantly higher when coke poisons very active basic Mg-0 pairs than when eliminates moderately active acidic Al-O pairs. 

---