Sulfided molybdena-alumina

Hydrodesulfurization uses sulfided cobalt/molybdena/alumina, or alternately with nickel and tungsten substituted for Co and Mo. 

It is obvious that there are many differences in results found and interpretations made on the state of molybdena-alumina catalysts, especially the sulfided state. Nevertheless, some common ground exists in these investigations, and we shall try to present a general picture of the state of these catalysts, pointing out areas of major disagreement that still persist. We shall limit our discussion to catalysts below 15% Mo and 4% Co, which encompasses the formulations of most commercial catalysts. 

Rather than survey all of the possible modifications that can be made to an alumina surface, we will focus on a subset involved in two different types of surface-catalyzed chemical reactions, namely, the partial oxidation of ethylene to ethylene oxide (EO) and hydrodesulfurization (HDS) processes. Both of these catalytic systems have functional points in common, in that alumina serves as a support (a-alumina for the EO process and 7-alumina for the HDS process) and alkali-metal salts serve as promoters for both reactions. To illustrate this commonality, this section will be divided into three parts (1) the adsorption of alkali-metal salts to 7-alumina, as reflected in the Rb and Cs solid-state NMR spectroscopy of these systems (2) the absorption of ethylene to silver supported on aluminas in the presence and absence of cesium salts, as followed by C NMR spectroscopy, and (3) the solid-state Mo NMR of fresh and reduced/ sulfided molybdena-alumina catalysts. 

Experiments with S labelled thiophene indicated that part of sulfur in cobalt promoted, previously sulfided molybdena-alumina is replaced by thiophene sulfur. 

The individual techniques used to characterize molybdena catalysts are now considered. Table II presents a listing of articles concerning the characterization of molybdena catalysts. Unless otherwise specified, we implicitly refer to Mo and/or Co supported on an activated alumina, commonly y-AlaOs. Most work has been done on the calcined (oxidized) state of the catalyst because of ease of sample handling. Reduced and sulfided catalysts are more difficult to work with since for meaningful results, exposure of these samples to air or moisture should be rigorously avoided. Therefore, sample transfer or special in situ treatment facilities must be provided. 

The most obvious choice to determine phases that may be present in the molybdena catalyst is XRD. Matching of diffraction lines obtained for the catalyst with those of pure bulk compounds gives unequivocal identification of phases present. This is one of the few techniques that yields positive results. The absence of matching diffraction lines, however, is not proof that the phase in question is not present in the catalyst. The XRD technique is limited to particle sizes of above approximately 40 A for oxides or sulfides, lower sized particles giving no discernible pattern over that of the broad alumina pattern. Thus, the presence of a highly dispersed phase, either as small crystallites or as a surface compound of several layers thickness will not be detected. Also, if the phase is highly disordered (amorphous), a sharp pattern will not be obtained, although some broad structure above that of the alumina may be detected. It is a moot point as to whether such a case is considered as a separate phase or a perturbation of the alumina structure. Ratnasamy et al. (11) have examined their CoMo/Al catalyst from the latter point of view, with particular emphasis on the effect of calcination temperature. 

In summary, the presence of a relatively strong Mo5+ signal found on A1203 catalysts but not on Si02 catalysts can be considered as additional evidence for an interaction between the molybdena and the alumina, which permits stabilization of the Mo5+ state in the reduced and sulfided catalyst. An H-containing specie seems a reasonable hypothesis for this state. The Co may be in tetrahedral or octahedral environment or both. 

In re-forming, molybdena on alumina is alternately subjected to oxidizing and reducing atmospheres which may contain sulfur compounds. To gain more basic information about the interactions of the catalyst with hydrogen, water vapor, hydrogen sulfide, sulfur dioxide, and sulfur trioxide, a series of adsorption studies were carried out. Various equilibrium conditions were calculated from thermodyuamic data (9) to interpret further the complex chemistry evidenced by these physicochemical studies. 

Equilibrium conditions for reactions between the catalyst and hydrogen, water vapor, hydrogen sulfide, and sulfur trioxide were determined. In these calculations interactions between the molybdena and the alumina support were not considered. 

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