Kinetic study of the selective hydrogenation of 3‑hexyne

BETTI, CAROLINA; TORRES, GERARDO C.; MACCARRONE, MARíA J.; LEDERHOS, CECILIA R.; QUIROGA, MóNICA; YORI, JUAN C.; VERA, CARLOS R.

Reaction Kinetics, Mechanism and Catalysis

Springer Nature Switzerland AG

Año: 2019 vol. 127 p. 259 - 281

1878-5190

Low loaded W?Pd/alumina are relatively novel catalysts for performing the selectivehydrogenation 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 reactionconditions were used for fitting a set of Langmuir?Hinshelwood models. Semihydrogenationwas the prevailing reaction path, leading selectively to (Z)-3-hexene> 95%, as with classical Lindlar catalysts. Smaller amounts of (E)-3-hexene andnegligible of n-hexane were detected. When considering a pseudo-homogeneousmodel, 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 dissociativeadsorption of hydrogen could not account for the high order in hydrogen, hence theadsorption of non-dissociated molecular hydrogen was also taken into account. Bestfit model was the one considering adsorption of 3-hexyne as rate-limiting step, withmolecular hydrogen acting as a competitor over Pdn+ sites, and with hydrogen beingdissociated over other different sites: Pdä−.