Linear Butenes from Isobutene over H-Ferrierite: In Situ Studies Using an Oscillating Balance Reactor

The transformation of isobutene into linear butenes was studied by means of a packed-bed configured oscillating balance reactor (OBR), in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), and mass spectrometry. The zeolite surface becomes covered with oligomeric hydrocarbons within the first few minutes on stream. There are a number of key differences between the isobutene isomerization reaction and the reverse (n-butene-to-isobutene) transformation. First, the kinetically controlled regimes of these two reactions are roughly 70 K apart. The difference in apparent activation energies for the two processes (ca. 11 kcal/mol) suggests that the two reactions are likely to proceed via different kinetic pathways. Second, a higher degree of branching in the oligomeric species formed by acid-catalyzed polymerization of isobutene, relative to those from 1-butene, causes a smaller coke precursor critical uptake to effect the site-to-pore blockage transition. The “aging” (hydrogen loss and condensation leading to true coke) of such branched chains is slower than that of chains resulting from a 1-butene feed. The impact of intrazeolitic oligomeric hydrocarbons on both the rate of formation of coke precursors and the isobutene isomerization process is also discussed.