n-Butene skeletal isomerization to isobutylene on shape selective catalysts. Ferrierite/ZSM-35

Article abstract of DOI:10.1021/j100023a022

Ferrierite/ZSM-35 is a shape selective zeolite catalyst for n-butene skeletal isomerization to isobutylene. Hydrothermal methods were used to synthesize well-crystallized ferrierite/ZSM-35 materials of different silicon to aliminum ratios and crystallinities in the presence of an organic template, pyrrolidine. Non-template-synthesized ferrierite was also studied. XRD, FTIR, SEM-EDX, NMR, ammonia TPD, pyridine chemisorption, and BET/pore size distribution methods were used to characterize these ferrierite materials. Ferrierite materials of high crystallinity contain more hydroxyl groups responsible for Bronsted acid sites and fewer Al species responsible for Lewis acid sites. Well-crystallized materials also have more perfect micropores. It is revealed that ferrierite materials of higher crystallinity are more shape selective for reaction of n-butene skeletal isomerization to isobutylene. Ferrierite materials of poorer crystallinity require more coke deposition to poison, block, and modify non-shape selective acid sites (external acid sites) in order to suppress side reactions such as butene dimerization followed by cracking. The presence of sodium ions in the zeolite is detrimental to isomerization reactions since they block the channels which have been partially blocked by the deposited coke. The preferential formation of cis-but-2-ene is observed and explained by a strong steric interaction between pore walls of small pore zeolites and methyl groups of the secondary butyl carbenium ions.