Heterocycles oligomerization in acidic zeolites: a UV-visible and IR study

FTIR spectroscopy shows that the first step of the interaction of pyrrole, furan and thiophene with H-beta and H-ZSM-5 zeolites is the formation of hydrogen bonded (precursor) species involving the Bronsted acid sites of the zeolite and the pi-electron system of the heterocyclic molecule. The precursors are then protonated to give monomeric BH(+) (where B = C(4)H(4)NH, C(4)H(4)O or C(4)H(4)S) species which, in the presence of excess B, can further react to form entrapped coloured B(n)H(+) oligomeric species. Characterization by means of in situ UV-Vis reflectance spectroscopy shows that the oligomeric products contain up to six conjugated double bonds and that they have carbocationic nature. However, the precise structure of the responsible species (particularly those absorbing in the visible) cannot be completely elucidated. The carbocationic character of the oligomers is also documented by the changes induced in the electronic spectra by adsorption of NH(3), i.e., of a base capable of extracting H(+) ions from B(n)H(+) to give NH(4)(+) and B(n). The role of intermolecular hydrogen transfer in the intrazeolitic oligomerization process is also evidenced and discussed.