Acetylene, methylacetylene and ethylacetylene polymerization on H-ZSM5: a spectroscopic study

Abstract

Acetylene, methylacetylene and ethylacetylene interact with the Brønsted acid sites of H-ZSM5 with the formation of hydrogen-bonded (precursor) species characterized by well defined IR properties. These precursors are then protonated to give intensely coloured carbocationic species. The speed of protonation is in the order: C₂H₅—CCH > CH₃—CCH > HCCH. Insertion of the monomer into the first protonation product, leads to an oligomeric species with carbocationic character. The IR and UV–VIS spectra of the carbocationic species are discussed in detail and found to correspond to those of analogous species generated in the homogeneous phase. The π-delocalization of the positive charge on the backbone is responsible for the peculiar and intense spectroscopic manifestations both in the IR and UV–VIS. The positive charge in the carbocationic species can be captured by bases (NH₃, pyridine) and subtracted to π-delocalization. This is accompanied by the disappearance of the spectroscopic manifestations associated with the positive charge and by the appearance of new absorptions associated with neutral oligomeric chains. The process can be fully reversed. The dimensions and shape of the oligomers are discussed in terms of the steric constraints imposed by the zeolitic framework, by means of computer graphics and molecular mechanics modelling.