1-Butene Oligomerization in Brönsted Acidic Zeolites:  Mechanistic Insights from Low-Temperature in Situ FTIR Spectroscopy

1-Butene has been reacted over H-beta, H-MCM-22, and H-mordenite, starting at 77 or 300 K. The systems were followed in situ by FTIR spectroscopy. At 300 K, 1-butene oligomerized instantaneously, and mechanistic information about the initial interactions between the active sites and the olefin and the subsequent chain growth was not inferable. Mechanistic details about the oligomerization reactions were provided by adsorbing 1-butene at 77 K and slowly heating the catalyst wafers to 300 K, while collecting spectra continuously. 1-Butene forms transient H-bonded precursors with the acidic sites of the zeolites prior to oligomerization. The strength of the H-bonded precursors formed into the three frameworks seems to be very similar, suggesting equal acidities of the Brönsted sites. These adducts were observable only at the lowest temperatures, and their concentration soon reached a maximum when the temperature was raised and vanished as the oligomerization reactions proceeded. Among the three zeolite samples, the oligomeric chains appear to grow most extensively in the three-dimensional beta zeolite.