Molybdena sulfided

Beavon [Beavon Sulfur Removal] Also called BSR. A process for removing residual sulfur compounds from the effluent gases from the Claus process. Catalytic hydrogenation over a cobalt/molybdena catalyst converts carbonyl sulfide, carbon disulfide, and other... 

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

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. 

Little definitive data are available on the sulfided state of CoMo/Al catalysts. Generally, higher sulfide levels are obtained than with comparable Mo/Al catalysts, but it is not certain whether this is due to sulfiding of the cobalt or additional sulfiding of the molybdena. 

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. 

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. 

Molybdena catalysts generally need to be activated by reduction or sulfidation in order to obtain an active catalyst for most reactions in which they are employed (except for oxidation-type reactions). Therefore, it is important to determine what changes occur in the state of the oxidized catalyst when it is subjected to these activation pretreatments. 

One of the more important uses of molybdena catalysts is in hydrodesulfurization processes. In operation, the catalyst is usually presulfided with hydrogen sulfide/hydrogen or other suitable sulfiding agents. Even when presulfiding is not employed, the catalysts become... 

Molybdena catalysts have been used for a large number of catalytic reactions—the literature is extensive in its use. We will limit our discussion to only the most common reactions occurring over reduced or sulfided catalyst. Furthermore, only those studies which attempt to relate catalyst activity to catalyst properties will be covered here. 

Increases Mo reduction. The molybdena catalyst reduces more in the presence of Co and activity depends on extent of reduction. As discussed earlier, there is considerable disagreement on the former point and the latter is unproved, especially for sulfided catalysts. 

The Alkazid process removes sulfur compounds from gas streams. All the sulfur compounds are first catalytically hydrogenated to hydrogen sulfide (H2S) using a cobalt/molybdena catalyst. The H2S is then absorbed in an aqueous solution of potassium salt of either methylamino propionic acid ( Alkazid M ), or dimethylamino acetic acid ( Alkazid DIK ). This solution is heated to regenerate the hydrogen sulfide as a concentrate. This concentrate is then treated by the Claus process to recover the sulfur1. 

Beavon Also called BSR [Beavon Sulfur Removal], A process for removing residual sulfur compounds from the effluent gases from the Claus process. Usually used in conjunction with other processes. Catalytic hydrogenation over a cobalt-molybdena catalyst converts carbonyl sulfide, carbon disulfide, and other organic sulfur compounds to hydrogen sulfide, which is then removed by the Stretford process. A variation (BSR/MDEA), intended for small plants, uses preliminary scrubbing with methyl diethanolamine. Developed by the Ralph M. Parsons Company and Union Oil Company of California in 1971. More than 100 plants were operating in 2000. See also SCOT. 

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. 

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. 

In all studies, increase of Scat was observed with increasing temperature up to 673 K. It was stated by the Tokyo Group (T Kabe and colleagues) that the maximal Scat was approached at 473 K on the non-promoted molybdena, whereas the maximal uptake by Co- and Ni-promoted MoOx/Al20 was reached only at 573 K and 673K.[ d7] j. concluded that molybdenum oxide was preferably sulfided in the lower temperature interval, whereas higher temperatures were necessary for sulfiding nickel and cobalt species. 

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

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