Tungsten-Ferrierite Catalysts Prepared by Impregnation and Ion-Exchange: Characterization and Activity During the Skeletal Isomerization of Linear Butenes

Abstract

Tungsten-ferrierite catalysts were prepared by impregnation and ion-exchange. Potassium (KF), ammonium (AF), and protonic (HF) ferrierites were used as starting materials. Characterization of impregnated catalysts shows reduction peaks characteristic of tungsten species, whereas the absence of a peak in the corresponding profiles of the exchanged material indicates such species were incorporated into the framework. For impregnated materials, the tungsten species-surface interaction is not strong. Catalytic performance during the linear butene skeletal isomerization at 300 °C and at atmospheric pressure was measured. Exchanged materials reach both conversion and isobutene yield larger than the impregnated ones. Samples prepared from AF and HF practically display the same catalytic behavior at 5 min, being associated with the presence of strong acid sites. Tungsten-impregnated KF does not reach a high activity at a short time-on-stream (TOS), showing a particular by-product distribution related to the absence of strong acid sites. Even though exchanged samples starting from KF show the HF characteristic behavior, differences at 5 min appear. By-product distributions allow us to consider their formation from dimers and/or oligomers. At 5 min, the C₅ ⁺ fraction is the main one, following propene, propane, and butane in similar proportions. Over the “clean” surface, propene could be involved in other reactions considering its high reactivity. At a long TOS, both the C₅ ⁺ fraction and propane decrease, whereas propene increases, verifying the bimolecular mechanism.