Hydrogenation, hydrodesulfurization, and hydrodenitrogenation for clean energy carriers

Hydrodefunctionalization and hydrogenation in the presence of sulfur (thiophenic) and basic nitrogen containing compounds are important reactions for the reduction of these compounds in fuels. Suitable catalysts are bifunctional consisting of a relatively acidic oxidic support, i.e., amorphous silica-alumina or zeolites, and a noble or base metal component such as Pt or Ni or alternatively on transition metal sulfide or phosphide. We explore the role of the support and the noble metal for the catalytic hydrodefunctionalization and hydrogenation combining kinetic measurements with in depth physicochemical characterization of the materials as well as investigate the sites and dynamic modification in molybdenum sulfide based materials.   

Active sites in hydrogenation

Using benzene and tetralin as model feed we have identified two types of active sites for the catalytic hydrogenation of the aromatic compound. On the metal particle adsorbed hydrogen and the aromatic molecule react in a Langmuir Hinshelwood type mechanism. In the second route the aromatic molecule adsorbed on sites at the perimeter of the metal particles reacts with atomic hydrogen from the metal particle. The pathway on Pt dominates in the case of clean feed leading to high catalytic activity. The acid base properties of the support induce marked metal support interactions via Lewis acid sites. It is not clear at present whether these variations in the metal properties influence the rates via the adsorption constants or via variations in the true energy of activation for the hydrogenation. In the presence of S-poisons the Pt-particles are hardly available for tetralin, thus, only the second hydrogenation route is available. Increasing concentrations of Brønsted acid sites increase the catalytic activity by offering additional sorption sites. In the presence of basic N-poisons, the acid sites and a fraction of the metal sites are blocked. The electronic effects are eliminated (equalized) and strong competition between tetralin and quinoline for the metal sites exists. The presence of both poisons decreases the hydrogenation rate of tetralin strongly and leads to continuous deactivation. This is caused by the fact that rates along both reaction routes are decreased and the concentration of hydrogen available for hydrogenation is strongly limited. Under all conditions, however, the S- and the N- poison are hydrogenated preferentially over tetralin.

Synergestic effects of Pt and Pd

The addition of Pd to Pt particles improves the stability of the catalysts in the presence of catalysts especially in the simultaneous presence of sulfur and nitrogen containing compounds. The presence of Pt on the surface of the mixed metal particles is mandatory to achieve high catalytic activity, as Pd is substantially less active than Pt. Electron transfer between Pd and Pt make the letter atoms more electron deficient than in pure Pt particles. In the presence of dibenzothiophene the catalytic activity is controlled primarily by the concentration of Brønsted acid sites on the support, while high metal dispersion is required to counteract the negative impact of quinoline, which is used as N containing base.

HDN on supported Ni-Mo sulfides

The hydrodenitrogenation of o-propylaniline on MoS2/γ‑Al2O3 and NiMoS/γ‑Al2O3 catalysts proceeds via two parallel routes, i.e., direct denitrogenation by C(sp2)‑N bond cleavage to form propylbenzene and hydrogenation of the phenyl ring to form propylcyclohexylamine, followed by C(sp3)‑N bond cleavage. Coordinatively unsaturated sites at the edges of the sulfide slabs are catalytically active for the direct denitrogenation. Dibenzothiophene decreases the direct denitrogenation rate, while it is mainly converted via direct desulfurization. Adding Ni2+ to MoS2 increases the CUS concentration and promotes the route via hydrogenation of the ring, but inhibits the direct denitrogenation, suggesting that Ni2+ is not involved in the active sites for the latter route. Catalytically active sites for the hydrogenation routes are SH groups at or near the edges of the sulfide slabs. The presence of DBT strongly increases the hydrogenation rates on NiMoS/γ‑Al2O3, increasing the electron density at the active sites due the electron pair donor properties of dibenzothiophene. The support does not induce a variation of the exposure of Ni of Mo sites on the sulfide slabs. All differences are related to variations in the degree of stacking.

Hydrogen activation on TMS catalysts

The activation of H2 and H2S on (Ni)MoS2/Al2O3 leads to the formation of SH groups with acid character able to protonate 2,6-dimethylpyridine. The variation in concentrations of SH groups induced by H2 and H2S adsorption shows that both molecules dissociate on coordinatively unsaturated cations and neighboring S2-. In typical materials studied one sulfur vacancy and four SH groups per 10 metal atoms exist at the active edges of MoS2 under the conditions studied. H2-D2 exchange shows that Ni increases the concentration of active surface hydrogen by up to 30 % by increasing the concentration of H2 and H2S chemisorbed.  

Related publications

Hydrogenation of tetralin on silica-alumina supported Pt catalysts I - Physicochemical characterization of the catalytic materials M..F. Williams, B. Fonfé, C. Sievers, A. Abraham, J.A. van Bokhoven, A. Jentys, J.A.R. van Veen and J.A. Lercher, J. Catalysis,  J. Catalysis, 251, 485 (2007).  

Hydrogenation of tetralin on silica-alumina supported Pt catalysts II Influence of the Support on the Catalytic Activity M.F. Williams, B. Fonfé, C. Woltz, A. Jentys, J.A.R. van Veen and J.A. Lercher, J. Catalysis, 251, 497 (2007).  

Hydrogenation of tetralin by silica-alumina supported Pt catalysts - Mechanistic aspects in the presence of sulfur and nitrogen containing poisons M.F. Williams, B. Fonfé, A. Jentys, C. Breitkopf, J.A.R. van Veen and J.A. Lercher, J. Phys. Chem. C., 114,  14532 (2010). 

Bimetallic Pt-Pd/silica-alumina hydrotreating catalysts  - Physicochemical characterization Y. Yu, B. Fonfé, A. Jentys, G. L. Haller, J. A. R van Veen, O. Y. Gutiérrez, J. A. Lercher, J. Catal. 292, 1 (2012).  

Bimetallic Pt-Pd/silica-alumina hydrotreating catalysts - Hydrogenation of tetralin in the presence of dibenzothiophene and quinoline Y. Yu, B. Fonfé, A. Jentys, G. L. Haller, J. A. R van Veen, O. Y. Gutiérrez, J. A. Lercher, J. Catal., 292, 13 (2012). 

Ring opening of 1,2,3,4-tetrahydroquinoline and decahydroquinoline on MoS2/γ-Al2O3 and NiMoS/γ-Al2O3 O. Y. Gutiérrez, A. Hrabar, J. Hein, Y. Yu, J. Han and J. A. Lercher, J. Catal., 295, 155 (2012).  

Tailoring silica-alumina supported Pt-Pd as poison tolerant catalysts for aromatics hydrogenation Y. Yu, A. Jentys, G. L. Haller, R. Colby, B. Kabius, O. Y. Gutiérrez, J. A. Lercher, J. Catal, 304, 135 (2013). 

γ‑Al2O3-supported and unsupported MoS2 and Ni‑MoS2 for hydrodenitrogenation of quinoline in the presence of dibenzothiophene J. Hein, A. Hrabar, A. Jentys, O. Y. Gutiérrez, J. A. Lercher, ChemCatChem, 6, 485 (2014)  

Simultaneous hydrodenitrogenation and hydrodesulfurization of dibenzothiophene and o-propylaniline on (Ni)MoS2 supported on mesoporous materials O. Y. Gutiérrez, S. Singh, E. Schachtl, E. Wuttke, J. Hein, and Johannes A. Lercher, ACS Catalysis, 4, 1487 (2014). 

Pathways for H2 activation on (Ni)-MoS2 catalysts E. Schachtl, E. Kondratieva, O. Y. Gutiérrez, J. A. Lercher, J. Phys. Chem. Lett., 6, 2929 (2015).