Porous Alumina-Supported Tungstated Zirconia Catalysts for Heptane Isomerization

Abstract

Novel WO₃–ZrO₂ (WZ) catalysts with palladium as an active metal on porous alumina supports of various phase compositions have been synthesized. Aluminas from Sasol molded in the form of extrudates (E) and spherical aluminas (S), in which the phase composition is presented by θ-Al₂O₃, δ-Al₂O₃, and α-Al₂O₃ alumina, have been used as supports. It has been shown that the phase composition of the support has a significant effect on the activity of supported Pd/WZ catalysts based on it. Upon transition from a set of θ- and δ-Al₂O₃ phases to the θ- and α-Al₂O₃ phase composition has led to an increase in the activity of the catalysts, as evidenced by a shift of 10–30°C in the temperature dependences of the heptane conversion to lower temperatures. The appearance of the α-Al₂O₃ phase is accompanied by a decrease in the specific surface area of the catalysts, which leads to an increase in the density of acid sites and, as a consequence, a change in activity. The Pd/WZ catalysts supported on S aluminas are characterized by a higher acidity (3.7–6.3 μmol/m²) as compared to the samples supported on E aluminas (2.8–3.6 µmol/m²). The high acidity of the Pd/WZ/S catalysts intensifies the heptane cracking side reactions to form gaseous C₁–C₄ hydrocarbons. In turn, the moderate acidity of the Pd/WZ/E catalysts contributes to a higher selectivity to heptane isomers (89.2–89.3% at a heptane conversion of 81.5–83.2%) as compared to the catalysts supported on S supports (isomerization selectivity of 84.9–85.6% at a heptane conversion of 80.4–81.4%).