Propene oligomerization on H-mordenite: Hydrogen-bonding interaction, chain initiation, propagation and hydrogen transfer studied by temperature-programmed FTIR and UV-VIS spectroscopies

The oligomerization of propene on H-mordenite is the predominant process proceeding very rapidly already at room temperature. This process can be conveniently studied by fast FTIR spectroscopy. However due to the rapid blockage of the pore entrances caused by the growing chains, it is not possible to obtain a full sequence of spectra documenting the steps of the oligomerization process involving not only the catalytic centres at the pore entrances but also those located in inner positions. In this paper we demonstrate that, by changing the experimental conditions, i.e. by; using a temperature-programmed FTIR spectroscopy experiment, operating in the temperature range 100-300 K, the formation of the hydrogen-bonded precursor (at low temperature) and the successive oligomerization involving all the protonic centres can be followed without difficulty. It is demonstrated that under these conditions and as found in the H-ZSM-5/propene system, the oligomerization proceeds through three steeps: (i) formation of a short-lived hydrogen-bonded precursor by interaction of the alkene with both the external and internal acidic Bronsted sites; (ii) protonation of the hydrogen-bonded precursor and (iii) propagation of the chain via insertion of monomers. Although a comparison between H-ZSM5 and H-mordenite bronsted acidity (made on the basis of the downward shifts of v(OH) and v(C=C) stretching frequencies in the hydrogen-bonded precursor) shows virtually identical acidity, the protonation of propene proceeds faster on H-mordenite. A plausible explanation for this effect is discussed. While the main oligomerization process discussed so far leads to saturated chains, the parallel formation of minor amounts of unsaturated species can elude IR detection. It is demonstrated that UV-VIS spectroscopy is a very sensitive tool for the detection of unsaturated cationic species formed via a side reaction involving hydrogen transfer.