REACTIONS OF METHYLBUTYNOL ON ALKALI-EXCHANGED ZEOLITES - A LEWIS ACID-BASE SELECTIVITY STUDY

Alkali-exchanged zeolites have both Lewis acid and Lewis base centres. They provide an easy way to tune up the relative strengths of these two sites by controlling Si/Al ratio and electropositivity of the counter cation. These effects can be monitored both by FTIR and N1s XPS of chemisorbed pyrrole. A systematic study of the relationship between acid-base properties and the reactions of methylbutynol (MBOH) was performed over these alkali-exchanged zeolites. The results show that the Lewis base centres catalyze the cleavage reaction of MBOH, which produces acetone and acetylene, while the strong Lewis acid centres catalyze the dehydration of MBOH to 3-methyl-3-buten-1-yne (MBYNE). The strong basicity of alkali-exchanged X zeolites produces almost 100% selectivity to acetone and acetylene, whereas the strong Lewis acidity of alkali-exchanged ZSM-5 zeolites results in a high selectivity to MBYNE. Compared with the catalytic results over H-ZSM-5 zeolites, it is further concluded that the catalytic conversion of MBOH to 3-methyl-3-butene-2-one (MIPK) requires the presence of not Lewis acid but strong Bronsted acid sites. The reactions of MBOH over acid-base catalysts are considered as standard reactions for the characterization of acid and base centres. A correlation between these reactions and the FTIR, and XPS of pyrrole data will be discussed. The deactivation mechanism during MBOH conversion over alkali-exchanged zeolites is further discussed.