Reactions of butadiene in zeolite catalysts by in situ variable-temperature solid-state nuclear magnetic resonance spectrometry

The mechanisms by which butadiene oligomerizes in acidic zeolite catalysts leading to deactivation by pore blockage have been elucidated by using in situ {sup 13}C solid-state NMR spectroscopy with magic-angle spinning (MAS). Butadiene is found to oligomerize primarily by the 1,4-addition reaction upon adsorption at its melting point (164 K). Secondary reactions of the oligomers are strongly dependent upon the properties of the zeolite. The initially formed linear product undergoes cyclization reactions to form fused rings in zeolite HY, but isolated rings are formed in the smaller channels of zeolite HZSM-5. Branching reactions and/or 1,2 enchainment result in an appreciable methyl group content in the oligomers formed in zeolite HY. These results provide insight into the mechanisms by which oxide catalysts are deactivated by pore blockage. Neither carbenium ions nor any other reactive intermediates were observed spectroscopically, presumably because such intermediates reacted with either free butadiene or the oligomers so rapidly as to preclude a detectable steady state concentration.