AB-INITIO MOLECULAR-ORBITAL CALCULATIONS OF THE PROTONATION REACTION OF PROPYLENE AND ISOBUTENE BY ACIDIC OH GROUPS OF ISOMORPHOUSLY SUBSTITUTED ZEOLITES
P. Viruelamartin et al., AB-INITIO MOLECULAR-ORBITAL CALCULATIONS OF THE PROTONATION REACTION OF PROPYLENE AND ISOBUTENE BY ACIDIC OH GROUPS OF ISOMORPHOUSLY SUBSTITUTED ZEOLITES, Journal of physical chemistry, 97(51), 1993, pp. 13713-13719
Ab initio molecular-orbital calculations using the 3-21G basis set are
performed to study the protonation reaction of propylene and isobuten
e by zeolite bridged hydroxyls ZOH, which are simulated by a cluster m
odel which consists of two Si tetrahedra and one Al tetrahedron. We ha
ve extended this study to include clusters with different compositions
by introducing B and Ga in T-III positions. The calculations show tha
t in all reactions the adsorption of the olefin molecule on;the acidic
OH group takes place, leading to a stable pi-complex with a structure
similar to those of the isolated olefin and clusters constituents. Th
e pi-complex is transformed into a zeolite-alkoxide of covalent charac
teristics. The surface alkoxides are the most stable structures with r
eaction energies in the range of -15 to -17 kcal/mol. These reactions
take place through transition states whose organic fragment has a geom
etry and charge distribution resembling those of the 2-propyl and tert
-butyl classical carbenium ions for zeolite-propylene and -isobutene,
respectively. The obtained activation energies (30-40 kcal/mol) are of
the same order of magnitude in all the reactions considered. The bifu
nctional mechanism of the reactions is rather complicated and implies
a concerted process involving the proton transfer from the zeolite tow
ard a carbon of the olefin double bond, and the simultaneous C-O, bond
formation at the adjacent oxygen on the zeolite structure. The reacti
on mechanism and the properties of the transition states are practical
ly the same, irrespective of the framework T-III atom and the increase
in propylene-isobutene branching. It is then proposed that the geomet
ric conformation of transition state can be more determinant than the
chemical composition of the zeolite. In this respect, the flexibility
of the zeolite structure plays an important role.