IR study of ethene and propene oligomerization on H-ZSM-5: hydrogen-bonded precursor formation, initiation and propagation mechanisms and structure of the entrapped oligomers
The oligomerization reaction of ethene and propene on H-ZSM-5 has been studied by fast FTIR spectroscopy. Oligomerization proceeds through: (i) formation of short-lived hydrogen-bonded precursors by interaction of the alkene with the internal acidic Brønsted sites, (ii) a protonation step and (iii) a chain-growth step. The relative strength of the hydrogen bonds in the ethene–OH and propene–OH π-complexes (precursors) is estimated on the basis of the downward shift of both the ν(OH) and ν(CC) frequencies (–389 and –11 cm–1 for ethene and –539 and –19 cm –1 for propene). For both molecules, the protonation of the precursors is the rate-determining step of the oligomerization process. The chain-growth mechanism and the structure of the entrapped oligomers are discussed on the basis of computer graphic and molecular dynamics simulations. Mainly linear of low branched products are formed whose length and structure is essentially determined by the steric hindrance imposed by the zeolitic framework.