HDS ACTIVITY OF CARBON-SUPPORTED NI-MO CATALYSTS DERIVED FROM THIOMOLYBDATE COMPLEXES

Activated-carbon (AC)-supported Ni-Mo sulfide catalysts were prepared employing ammonium thiomolybdate complexes and nickel salts as metal-s ulfide precursors and a commercial AC as support. Textural and chemica l characterization were carried out by XRD, TPR and BET surface-area m easurements. A well-defined NiI(NH3)(6)MoS4 phase was detected in Ni-M o/AC co-impregnated samples before pretreatments or catalytic reaction , but it disappeared or became too amorphous to be detected by XRD aft er prereduction or presulfiding. No relationship between the presence of the mixed phase in the precursor and the HDS activity was obvious. Although both types of pretreatments activated the samples for the hyd rodesulfurization (HDS) of thiophene, presulfiding always produced hig her activities than prereduction. The nickel salt used (chloride or ac etate) did not affect the activity, but the type of thiomolybdate empl oyed had a considerable influence on the HDS activity: The dimeric com plex (NH4)(2)Mo2S12. 2H(2)O was a better precursor than the monomer (N H4)(2)MoS4, while the trimer (NH4)(2)Mo3S13. 1.8H(2)O was the poorer. This is probably related to the different decomposition pathways that follow the complexes, as both, the monomer and dimer, decompose to MoS 2 through amorphous MoS3 while the trimer directly produces MoS2, pres umably more crystalline. The monomeric thiotungstate complex (NH4)(2)W S4 produced a more active Ni-W/AC catalyst than any of the Ni-Mo/AC sa mples measured, although for the unpromoted samples the situation was the reverse. Comparison with the alumina-supported catalyst suggests t hat the thiomolybdate derived samples should be further explored for H DS. (C) 1998 Elsevier Science B.V. All rights reserved.