Hydrogenation and hydrodesulfurization over sulfided ruthenium catalysts: I. Catalysts containing partial monolayers of adsorbed sulfur

Abstract

Thiophene hydrodesulfurization (HDS), 1-hexene hydrogenation, and 1-hexene isomerization have been investigated over sulfided Ru/γ-Al 2O 3 catalysts (1.8–10 nm crystallites) and CoMo/γ-Al 2O 3 catalysts using a differential microreactor operated at 101 kPa from 548 to 623 K. Catalysts were presulfided in 101 kPa of 10% H 2S/H 2 at T ≤ 673 K or in 101 kPa of 100% H 2S at T ≤ 523 K such that sulfided ruthenium catalysts retained ca. 0.25 monolayer of adsorbed sulfur. Structure-sensitive adsorption of sulfur was observed using microgravimetry and pulse oxygen chemisorption. Crystallites of 1.8 nm retained 0.1 monolayer of strongly bound sulfur while crystallites greater than 3.2 nm retained 0.25 monolayer. Sulfur adatoms produced virtual ruthenium cations with high isomerization activity while the structure-sensitive adsorption of sulfur provided for an “apparent structure sensitivity” in the hexene hydrogenation rate. This anomalous result could be reconciled by counting sulfur-free ruthenium atoms using pulse oxygen adsorption. Thiophene HDS over sulfided ruthenium catalysts proceeded via direct hydrogenolysis in the absence of tetrahydrothiophene formation. Ruthenium crystallites of 1.8 nm possessed specific activities 23 times greater than those found over 10-nm crystallites. The higher specific activity found over small ruthenium crystallites during HDS was attributed to the multiatomic ensemble requirements of the reaction, the high activity associated with a “clean metal” surface, and the greater number of “clean metal sites” found on smaller crystallites. Both 1-hexene hydrogenation and thiophene HDS were well represented by Langmuir-Hinshelwood rate expressions in which apparent activation energies, adsorption equilibrium constants, and heats of adsorption were found to depend on the ruthenium crystallite size. Sulfided Ru/Al 2O 3 (1.8-nm) catalysts provided ca. 13-fold higher 1-hexene hydrogenation rates than CoMo/Al 2O 3 catalysts when compared per square meter of active area, while specific 1-hexene hydrogenation rates were similar when compared per oxygen titratable site. Sulfided Ru/Al 2O 3 (1.8-nm) catalysts provided ca. 2-fold higher thiophene HDS rates than CoMo/Al 2O 3 catalysts when compared per square meter of active area, while specific thiophene HDS rates were ca. 7-fold higher over CoMo/Al 2O 3catalysts than over Ru/Al 2O 3 when compared per oxygen titratable site.