Kinetic study of the selective hydrogenation of 3-hexyne over W-Pd/alumina catalysts

Low loaded W-Pd/alumina are relatively novel catalysts for performing the selective hydrogenation of alkynes, but there is scarce information on the working mechanism. This work studies the kinetics of the selective hydrogenation of 3-hexyne to (Z)-3-hexene over a low loaded W-Pd/alumina catalyst. Runs at different mild reaction conditions were used for fitting a set of Langmuir-Hinshelwood models. Semihydrogenation was the prevailing reaction path, leading selectively to (Z)-3-hexene>95%, as with classical Lindlar catalysts. Smaller amounts of (E)-3-hexene and negligible of n-hexane were detected. When considering a pseudo-homogeneous model, approximate orders in 3-hexyne and hydrogen were (2.5) and (-2.2), respectively. The latter value pointed to an important role of hydrogen chemisorption. Twelve kinetic models were fitted to the experimental data. A normal dissociative adsorption of hydrogen could not account for the high order in hydrogen, hence the adsorption of non-dissociated molecular hydrogen was also taken into account. Best fit model was the one considering adsorption of 3-hexyne as rate-limiting step, with molecular hydrogen acting as a competitor over Pdn+ sites, and with hydrogen being dissociated over other different sites: Pd delta-.