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 = C4H4NH, C4H4O or C4H4S) species which, in the presence of excess B, can further react to form entrapped coloured BnH+ 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 NH3, i.e., of a base capable of extracting H+ ions from BnH+ to give NH4+ and B-n. The role of intermolecular hydrogen transfer in the intrazeolitic oligomerization process is also evidenced and discussed.
Heterocycles oligomerization in acidic zeolites: a UV-visible and IR study
SPOTO, Giuseppe;BORDIGA, Silvia;LAMBERTI, Carlo;SCARANO, Domenica;ZECCHINA, Adriano
1999-01-01
Abstract
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 = C4H4NH, C4H4O or C4H4S) species which, in the presence of excess B, can further react to form entrapped coloured BnH+ 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 NH3, i.e., of a base capable of extracting H+ ions from BnH+ to give NH4+ and B-n. The role of intermolecular hydrogen transfer in the intrazeolitic oligomerization process is also evidenced and discussed.
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