ACETYLENE POLYMERIZATION IN A H-ZSM-5 ZEOLITE

We have examined the acid-catalyzed reaction of acetylene in H-ZSM-5 zeolites in a batch reactor between 298 and 550 K, with acetylene pressures between 10 and 1000 Torr, in an attempt to form zeolite-encapsulated polyacetylene. The samples were then characterized by temperature-programmed desorption (TPD), thermogravimetric analysis (TGA), and transmission IR, reflectance UV-visible, and C-13 NMR spectroscopies. The reaction rate for acetylene was proportional to the zeolite Al concentration, indicating that reaction occurs at the Bronsted sites associated with the Al atoms in the zeolite framework. The reaction is activated and substantial rates were observed only above 425 K. Zeolite pore volumes decreased linearly with acetylene conversion, indicating that the products remain in the pore structure. TPD-TGA results showed that the products decomposed above approximately 550 K, a temperature similar to that observed for polyacetylene, with 70% of the hydrocarbons in the zeolite desorbing as simple aromatics and olefins. The IR spectra provided further evidence that reaction occurs at Bronsted sites in the zeolite but were inconclusive for identifying the hydrocarbon species. C-13 NMR showed the presence of substantial quantities of sp3 -hybridized C, implying that substantial defects probably exist in the oligmer chains. The hydrocarbon products in the zeolite also interact strongly with adsorbed bases, as demonstrated by the UV-visible spectra, which showed a broad band between 350 and 700 nm that decreased reversibly following adsorption of ammonia.