Tungsten promoted ammonium and potassium ferrierite: deactivation during the skeletal isomerization of linear butenes

Deactivation of tungsten promoted ferrierite during the skeletal isomerizat ion of 1-butene at 400 degreesC, atmospheric pressure and 0.15 atm 1-butene partial pressure was studied. Both potassium and ammonium ferrierites were impregnated with tungsten species using either tungstic acid or ammonium m etatungstate as precursors, reaching loadings between 1.4 and 7.3%. After t he tungsten addition on both ferrierite samples, neither the acid strength distribution nor the total acidity corresponding to the unpromoted material s change significantly. The strongest acid sites present on the ammonium fe rrierite with and without tungsten and absent on the tungsten promoted pota ssium ferrierite, are responsible for the side-reactions. Deactivation of t ungsten promoted ferrierites shows differences.. Ammonium ferrierite with a nd without tungsten reach similar carbon contents, being larger than the on es obtained on potassium ferrierite with and without tungsten. In all cases , the carbonaceous deposit shows both olefnic and aromatic species, the pro portion depending on the samples. Coke on tungsten promoted potassium ferri erite shows mainly an olefinic nature, while the deposit formed on tungsten promoted ammonium ferrierite has a more aromatic character. For the latter samples, the complete coke removal needs higher temperatures. The strength of acid sites determines not only the carbonaceous deposit amount but also its degree of condensation. The low isobutene selectivity at short time-on -stream (TOS) is avoided by starting the 1-butene feed with the catalytic b ed at 200 degreesC and then increasing temperature up to 400 degreesC. It c an be considered that a strong adsorption of reactant molecules takes place at low temperatures, thus deactivating the strongest acid sites. (C) 2001 Elsevier Science B.V. All rights reserved.