Alumina-supported Mo Catalysts Research Articles

Mo/γ-Al2O3 catalysts for the Biginelli reaction. Effect of Mo loading

Alumina supported Mo catalysts with varying metal loading (2–12wt%) were tested in the Biginelli reaction under solvent free conditions for synthesis of 3,4-dihydropyrimidin-2-(1H)-one (DHPM). The DHPM yield increased with Mo loading, going through a maximum for 7wt% Mo. Alumina supported Mo catalysts appeared to be highly stable and reusable materials for this reaction.A suite of characterization techniques that included X-ray diffraction, Fast Fourier Transform analysis and FTIR of adsorbed pyridine revealed that the DHPM formation is related to the existence of Brønsted acid sites that would be linked to the presence of Al2(MoO4)3 phase.

Influence of ageing on MoO3 formation in the preparation of alumina-supported Mo catalysts

Raman spectroscopy and XRD are used to study the preparation of alumina-supported Mo catalysts, with emphasis on the molecular level understanding of the influence of the ageing step (i.e. leaving the catalyst in contact with water after impregnation and before drying) on Mo speciation. For incipient wetness impregnation with heptamolybdate ((NH4)6Mo7O24), long oxide/water contact times (ageing) favor alumina dissolution and consequently increase the formation and crystallization of the resulting compound, (NH4)3[Al(OH)6Mo6O18] on Al2O3. Precipitation of this compound leads to the formation of MoO3 after calcination. On the contrary, minimizing the oxide/water contact time (and therefore alumina dissolution) by freeze-drying the system after impregnation inhibits the formation and crystallization of (NH4)3[Al(OH)6Mo6O18] and therefore drastically limits MoO3 formation after calcination.

Structures of molybdenum silica catalysts reduced by ethanol, and their relations to catalytic oxidation reactions: I. Structure changes of Mo/SiO 2 with gradual reduction by ethanol

Structural changes of silica supported molybdenum catalysts accompanied with reduction by ethanol were investigated with EXAFS and UV–VIS spectroscopies. Catalysts made from four different precursors were investigated, and it was demonstrated that the dependence on preparation method was small. Catalysts prepared by the reaction between surface hydroxide groups and organometallic complexes are called `fixed catalysts' and some researchers assert that fixed Mo/SiO 2 have uniform and specific molecular-like active sites, which remains controversial. It was found that what had been proposed as characteristics of the molecular-like active sites of the fixed catalysts were merely prejudiced interpretation of a few aspects in the structural changes common to Mo/SiO 2 in general. At least two distinctive states existed in the course of reduction. One has IVCT (intervalence charge transfer) absorption band and corresponds to shallow reductions. The other has 2.6 Å Mo–Mo bond and average oxidation state of it was estimated to be +5. Structure similar to the latter was found also on alumina supported Mo catalysts and can be considered thermodynamically stable.

Preparation of alumina supported Mo catalysts from molybdenum blue precursor

Alumina supported Mo catalysts were prepared by adsorption of molybdenum blue (MB) on two alumina supports. Sulfided catalysts were tested in HDN/HDS reactions of pyridine and thiophene and compared with catalysts prepared from ammonium heptamolybdate (AHM). Activities of catalysts prepared from MB were considerably higher in thiophene HDS and pyridine hydrogenation and lower in C−N bond hydrogenolysis.

Adsorption of oxovanadium(IV) porphyrins on oxides and molybdenum-containing catalysts

Spectroscopic analysis by Raman, diffuse reflectance spectroscopy and electron spin resonance of adsorbed oxovanadium(IV) porphyrins is reviewed. Two forms of donor-acceptor interactions are identified, namely coordination of the vanadyl group to eitner basic or acidic surface sites. In both cases additional interactions between the macrocycle and the surface seem to be operative. The mode of adsorption is strongly governed by the acid base properties of the adsorbent or catalyst surface. The primary adsorption sites on alumina-supported Mo-catalysts in both the oxide precursor and the sulfided state appear to be located on uncovered alumina support surface.

Hydrogen uptake by MoS 2 and sulfided alumina-supported Mo catalysts

Volumetric H 2-uptake measurements on polycrystalline MoS 2 (39 m 2g −1) have been carried out and uptake isotherms in the temperature range 423–573 K have been determined. The equilibrium stoichiometry at 500 mbar and 573 K was H 0.035MoS 2, and at 473 K the H Mo ratio increased to 0.051. Alumina-supported sulfided Mo catalysts provided H Mo ratios that were higher by approximately a factor of 4, indicating either a higher MoS 2 surface area, spillover of H atoms to the support, or both. Two possible mechanisms for H 2 dissociation are discussed, namely, heterolytic dissociation on MoS pairs and homolytic dissociation on disulfide (SS) 2− edge groups. The presence of the latter on MoS 2 is documented by Raman spectroscopy which shows the characteristic v(SS) mode at 529 cm −1. Although molecular analogs for H 2 activation on disulflde groups are known, it is not yet possible to make a clear distinction between the alternative H 2 dissociation routes.