THE NATURE, STRUCTURE, AND COMPOSITION OF ADSORBED HYDROCARBON PRODUCTS OF AMBIENT-TEMPERATURE OLIGOMERIZATION OF ETHYLENE ON ACIDIC ZEOLITE H-ZSM-5

The nature, structure, and composition of hydrocarbon products that fo rm under oligomerization of ethylene on acidic zeolite H-ZSM-5 at room temperature (296 K) have been analyzed using a combination of in situ IR and C-13 NMR and ex situ C-13 NMR and GC-MS. It is shown that olig omers strongly retained by the zeolite framework represent a mixture o f hydrocarbons which composition ranges from C-5 to ca C14+ carbon ato ms. Adsorbed oligomers are bound to the oxygens of the zeolite framewo rk to form alkoxy species. Alkoxy species represent main adsorption st ate of the oligomers, but they are in equilibrium with olefins and car benium ions of the same composition, the concentration of both carbeni um ions and olefins seems to be beyond the sensitivity of both NMR and IR. The reactivity of the adsorbed oligomers is defined by the reacti vity of equilibrated carbenium ions which can easily interact with car bon monoxide and water to give a mixture of corresponding carboxylic a cids. Oligomeric alkoxides with linear hydrocarbon fragments prevail i n the mixture of the formed oligomers, The average length of the linea r oligomer chain includes approximately seven carbon atoms. Prevalence of the linear hydrocarbon fragments over branched ones in adsorbed ol igomers seems to be defined by both specific framework and dimensions of the channels of the zeolite H-ZSM-5. (C) 1998 Academic Press.