Characterization and catalytic activity study of SBA-15-supported MoO3, MoS2, Ni or Co promoted MoS2 and Co promoted WS2
Hydrodesulfurization (HDS) catalysts supported on SBA-15 (6 and 9 nm) were prepared by thermal decomposition of (alkyl)ammonium thiomolybdate and ammonium thiotungstate using Co(Ni) acetate or Co(Ni) thiocarbamate (cobalt dimethyldithiocarbamate and nickel diethyldithiocarbamate) as the promoter atom sources. Prior to the catalytic test, the activations of the catalysts were carried out either under H2/N2 gas flow at 773 K (ex-situ activation) or during the catalytic tests (in-situ activation). The nature of the thiosalt precursor, the impregnation method as well as the activation mode show a strong influence on the MoS2 morphology, which is of great importance for the HDS catalytic activity. Despite the high MoS2 stacking, the ex-situ activated catalysts derived from ammonium thiomolybdate (ATM) at high Mo-loading exhibit higher HDS activity compared to a commercial-Al2O3-supported HDS catalyst. One important result of the studies is that the MoS2 stacking is not a prerequisite for a good HDS performance and also not a sign for the deactivation or lower HDS activity. The presence of large amounts of coordinatively unsaturated sites (CUS) could be responsible for the high HDS activity. The experiments clearly show that the ex-situ activation is not beneficial for the preparation of HDS active phases when a carbon-containing Mo source like tetramethyl-ammonium thiomolybdate (TMATM) is employed. During the activation process MoS2 particles with an unfavorable morphology are generated and big needle-like aggregates and closed shell structures are seen in the HRTEM pictures. The in-situ activation is found to be beneficial for the preparation of NiMo/SBA-15 catalysts with high activity. For CoMo/SBA-15 catalysts, the in-situ activation also results in an enhanced synergetic effect of Co by using TMATM. In this case, the improved HDS performance could be associated with the positive carbon effect. In contrast, the nature of the thiosalt precursor and the activation mode have only a negligible influence on the catalytic selectivity. MoO3 supported on SBA-15 was prepared by impregnation of SBA-15 with ammonium heptamolybdate followed by calcination at 773 K for 3 h in air. Interestingly, the presence of the hexagonal MoO3 modification was unequivocally identified by evaluation of the EXAFS data. In addition, the catalytic behavior of the hex-MoO3/SBA-15 in the propene oxidation reaction was investigated in the temperature range from 293 K to 773 K. The stabilized hex-MoO3-SBA-15 is shown to be capable of directly oxidizing propene to acrylic acid without the addition of further metal atoms as promoters. Intriguingly, the stabilized structure of hexagonal MoO3 supported on SBA-15 possesses an open channel structure similar to that of the highly active Mo5O14 or MoVNbTe catalysts. Hence, structural complexity of molybdenum oxides is sufficient to improve functional molybdenum sites for the activation of oxygen and alkene followed by selective oxidation of the alkene.