FT-IR Studies on Light Olefin Skeletal Isomerization Catalysis: III. Surface Acidity and Activity of Amorphous and Crystalline Catalysts Belonging to the SiO2–Al2O3System

The surface acidity of both amorphous and crystalline materials belonging to the SiO2–Al2O3system (i.e., amorphous silicas, amorphous silica–aluminas with different Al contents, HZSM5 and H-ferrierite (FER) zeolites, and silicated and pure γ-Al2O3) have been studied using the FT-IR spectroscopy of the surface hydroxy groups and of adsorbed acetonitrile (AN) and pyridine (Py). Lewis bonded and H-bonded species of AN have been observed, while a protonated form of adsorbed AN has been characterized on HZSM5. A molecular sieve effect has been found for Py adsorption on FER and HZSM5. The catalytic activity of these materials in the skeletal isomerization of 1-butene to isobutene has also been investigated. The Brønsted surface acidity scale found correlates well with the catalytic activity in 1-butene conversion, measured after 20–40 min time on stream. The selectivity to isobutene follows nearly an inverse trend, except for FER, which shows both higher activity and higher selectivity than silica–alumina, quite good resistance to coking, and selectivity increasing with time on stream. Comparative analysis of the behavior of these catalysts and of the spectrum of their surface OH groups gives new light on the structure of the acidic OHs on materials belonging to the SiO2–Al2O3system. The usefulness of zeolite and alumina-based catalysts for skeletal isomerization ofn-butene to isobutene is also briefly discussed.