ACETYLENE, METHYLACETYLENE AND ETHYLACETYLENE POLYMERIZATION ON H-ZSM5 - A SPECTROSCOPIC STUDY

Acetylene, methylacetylene and ethylacetylene interact with the Bronst ed acid sites of H-ZSM5 with the formation of hydrogen-bonded (precurs or) species characterized by well defined IR properties. These precurs ors are then protonated to give intensely coloured carbocationic speci es. The speed of protonation is in the order: C2H5-C=CH > CH3-C=CH > H C=CH. Insertion of the monomer into the first protonation product, lea ds to an oligomeric species with carbocationic character. The IR and U V-VIS spectra of the carbocationic species are discussed in detail and found to correspond to those of analogous species generated in the ho mogeneous phase. The pi-delocalization of the positive charge on the b ackbone is responsible for the peculiar and intense spectroscopic mani festations both in the IR and UV-VIS. The positive charge in the carbo cationic species can be captured by bases (NH3, pyridine) and subtract ed to pi-delocalization. This is accompanied by the disappearance of t he spectroscopic manifestations associated with the positive charge an d by the appearance of new absorptions associated with neutral oligome ric chains. The process can be fully reversed. The dimensions and shap e of the oligomers are discussed in terms of the steric constraints im posed by the zeolitic framework, by means of computer graphics and mol ecular mechanics modelling.