Aluminum Fluoride‐Supported Platinum and Palladium as Highly Efficient Catalysts of n‐Pentane Hydroisomerization

Abstract

AbstractCatalytic isomerization of n‐pentane was studied over novel high surface area (HS) aluminum fluoride supported palladium and platinum catalysts, prepared by the nonaqueous fluorolytic sol–gel synthesis. HS‐AlF3 maintained well its nanoscopic, mesoporous, and highly acidic character at higher temperatures, up to 350 °C. Both Pt/HS‐AlF3 and Pd/HS‐AlF3 catalysts, characterized by very small metal particle sizes (≈2 nm) maintained very good activity, stability, and selectivity towards isomerization (up to nearly 100 %) at up to 350 °C. Very strong acidity of HS‐AlF3 is the prerequisite for isomerization but the presence of the metal is also essential to maintain a high conversion level. The effect of metal–acid balance in Pd/HS‐AlF3 catalysts, important for bifunctional isomerization, was tested by screening three differently metal‐loaded catalysts. The performance of the 0.5 wt % Pd/HS‐AlF3 appeared as good as that of 2.0 wt % Pd. The properties of the HS‐AlF3 support, catalyst precursors, and catalysts were characterized by XRD, TEM, H2 chemisorption, and FTIR photoacustic spectroscopy of adsorbed pyridine, and NH3 temperature programmed desorption. The first three methods allowed evaluating the state of the metal in the reduced catalysts. The suppressed chemisorption of H2 must result from different factors, such as carbon presence in Pd introduced during catalyst synthesis and activation, metal–acidic support interaction, and, partially, metal encapsulation in AlF3 during sol–gel synthesis. Lewis and Brønsted acidity was detected in activated catalysts and both types of acid sites were assumed to play a role in bifunctional isomerization. NH3 effectively blocks the strongest acid sites and drastically limits the overall catalytic performance. Introduction of water to the reaction system also degrades, but to a lesser extent, the Pd(Pt)/HS‐AlF3 catalyst performance, most probably because of poisoning of only the strongest Lewis acid sites.