Oligomerization of n-Butenes on the Surface of a Ferrierite Catalyst: Real-Time Monitoring in a Packed-Bed Reactor during Isomerization to Isobutene

Abstract

A packed-bed-configured oscillating balance reactor (OBR) was used to study the surface oligomerization reactions that accompany the isomerization of linear butenes to isobutene on a ferrierite (FER) catalyst. The modeling of the surface deposition process of high molecular weight species indicates that H-FER undergoes site blockage, followed by a pore blockage phase that sets in after ca. 70% of the total uptake is achieved. Prior to reaching saturation of the carbonaceous residue uptake, a significant fraction of selective sites for isobutene formation are lost. This suggests that such sites are unlikely to exist exclusively at the pore mouths of H-FER, as proposed earlier. In situ DRIFTS was also used to discriminate between oligomerization and true coke formation. Continuous mass spectrometric monitoring of the OBR product stream reveals that the production of octenes and/or higher oligomers goes through a maximum at short times on stream (i.e., before the zeolite pores reach saturation uptakes). This is consistent with the idea that bimolecular processes dominate the catalysis of fresh H-FER. In a temperature range representative of those normally used for reaction, the hydrocarbon residues formed on the surface of the catalyst upon a few hours of exposure to a 1-butene flow are more properly described as hydrogen-deficient high molecular weight species that do not reach the hard coke category.