Molybdenum Dithiolene Complexes as Structural Models for the Active Sites of Molybdenum(IV) Sulfide Hydrodesulfurization Catalysts

Abstract

AbstractThe removal of sulfur (as H2S) from organosulfur species in petroleum feedstocks (hydrodesulfurization, HDS) is carried out on an enormous scale by using heterogeneous catalysts based on MoS2 (usually doped with Co). Partially hydrogenated thiophenes are postulated intermediates in the MoS2‐catalyzed hydrodesulfurization of thiophene. The present contribution describes new molecular models for the proposed active sites in HDS catalysis. The models are derived from a mixed‐ligand (push–pull) molybdenum trisdithiolene {[Mo(tfd)2(bdt)]; tfd = S2C2(CF3)2, bdt = S2C6H4}: selective intraligand alkyne binding converts the bdt group to a labile Mo‐chelating benzodithiin, which can be substituted with a variety of weak donor ligands. The complexes [Mo(tfd)2(dht)2] and [Mo(tfd)2(tht)2] (dht = 2,5‐dihydrothiophene; tht = tetrahydrothiophene) were synthesized and crystallographically characterized. The Mo(tfd)2 substructures closely resemble the presumed active site in MoS2 HDS catalysts. The coordination geometries at molybdenum are approximately trigonal prismatic, and the metal bears two strongly bound dithiolene (tfd) groups and two comparatively weakly bound thioether ligands (dht or tht). Competitive binding experiments establish that tht binds more strongly to the Mo center than dht (Keq = 6.5 ± 0.5). Preliminary reactivity studies reveal that [Mo(tfd)2(dht)2] decomposes to [Mo(tfd)3], thiophene, and unidentified species upon heating. Further, [Mo(tfd)2(tht)2] induces the isomerization of 1,4‐cyclohexadiene to 1,3‐cyclohexadiene at elevated temperatures.