Sulfide, promotional effects

Promotional effects of sulfide can evidently be explained, because exposure of reduced metals Is Increased on reduced sulfided catalysts. The role of cobalt Is less clear. It Is normally not fully reduced. It apparently does not promote greater exposure of Mo In any form detected, either In the presence or absence of sulfide. On the contrary. It evidently only decreases the concentration of exposed Mo atoms, although, at concentrations typically used, most. Mo atoms are unaffected by Co. Either some property of Co alone or some local cooperative effect of adjacent Co and Mo must explain promotion. Simple mechanical mixtures will not give the synergism observed, however (1-4). 

Hydrogen sulfide promoted corrosion can be a serious problem (150) the best solution is prevention. Corrosion problems can be minimized by choice of the proper grades of steel or corrosion resistant alloys, usually containing chromium or nickel (150, 151) and avoiding generation of H S by sulfate reducing bacteria in situations where H S is not initially present. Cathodic protection of casing is often effective for wells less than 10,000 feet deep (150). 

The first stage of the synthesis involves the interaction of a nitro compound with sodium sulfide. When used alone, sodium sulfide is only slightly effective The reactions proceed slowly and the yields of mercaptanes are small. If elemental sulfur is added, the conversion accelerates markedly and the yield increases to 75-80%. The promoting effect of elemental sulfur can be easily explained by the radical-chain mechanism. The reaction starts with one-electron transfer from the nucleophile to the nitro compound further conversions resemble other chain ion-radical substitutions. 

Because of the importance of the promotion effect and because many of the central questions surrounding TMS catalysis are about promotion, it is valuable to review a history of the effect. The first reference to a catalyst based on molybdenum and cobalt sulfides capable of desulfurizing coal oils in the presence of hydrogen was a patent from I. G. Farben Industrie dated May 24, 1928 (5). Before this, M. Pier and his team at BASF (1924-1925)... 

It should be appreciated that in some instances, bulk MoS2 is unquestionably present on the sulfided catalyst, as proved by YRD analysis. Also, such catalysts are quite active and show Co promotional effects. In such cases, the monolayer model cannot obviously exclusively apply and either of the other models would be favored. However, even in this case, it is possible that an important fraction of the Mo could remain as a monolayer and exhibit catalytic activity. 

Promotional effects of Co or Ni are also construed in terms of a "remote control" theory proposed by Delmon and co-workers [3]. On the basis of the findings that catalytic synergies are generated even for physical mixtures of supported component sulfides, it is claimed in this theory that the catalytic, activity of Mo sulfides is enhanced by spillover hydrogen originally generated on highly dispersed Co or Ni promoter sulfides in the proximity. 

The effect of the additives was then investigated in more detail. Figure 2 shows the formation rate of products widi the sulfided Ca/Pd/SiOa with different atomic ratios of Ca to Pd at different reaction temperatures (613 K (a) mid 573 K (b)). The reaction pressure and gas-hourly space velocity were 5.1 MPa and 20 m (STP) kg-caf h, respectively. At 613 K, methanol fonnation rate increases sharply with the atomic ratios of Ca to Pd up to 0.5, and shows a plateau above this ratio. The formation rates of methane and CO2, on the other hand, show the maximum at Ca/Pd=0.5. At this ratio, the rate enhancement for methanol formation (4.5) was higher than that for methane formation (3.5). Thus, Ca additive could promote the formation of methanol rather than methane. It should also be noted here that sulfided Ca/SiO2 showed no activity for CO hydrogenation. At the lower reaction temperature, the rate enhancements for methanol and mediane formations were 5.5 and 2.5, respectively (Ca/Pd=0.5). The promoting effect of Ca additives on methanol formation was more notable at the lower reaction temperature. Similar results were obtained with supported sulfides modified with Mg and/or Y additives, whereas Cs and Mn additives hardly affected methanol formation irrespective of their contents. 

The importance of edge planes also arises in the industrially important promoted transition metal sulfide catalyst systems. It has been known for many years that the presence of a second metal such as Co or Ni to a M0S2 or WS2 catalyst leads to promotion (an increase in activity for HDS or hydrogenation in excess of the activity of the individual components) ( ). Promotion effects can easily be observed in supported or unsupported catalysts. The supported catalysts are currently the most important industrial catalysts, but the unsupported catalysts are easier to characterize and study. Unsupported, promoted catalysts have been prepared by many different methods, but one convenient way of preparing these catalysts is by applying the nonaqueous precipitation method described above. For example, for Co/Mo, appropriate mixtures of C0CI2 MoCl are reacted with Li2S in ethyl acetate ... 

The promotion effect of the CoMoS site appears to occur through increased hydrogen activation, which facilitates removal of sulfur atoms after cleavage of C-S bonds on exposed molybdenum ions/ Cobalt incorporated into the support may also act as a structural promoter by enhancing dispersion of the sulfided species. ... 

Recently, we observed the promotional effects of Pt added to the sulfided M0/AI2O3 catalyst in the HDS of thiophene and HDN of pyridine [4]. An interesting property of the promoted Pt-Mo(S)/Al203 catalyst was the high HDN activity, exceeding the activities of conventional CoMo and NiMo/Al203 catalysts. The Pt-Mo(S) system was deposited now on a mesoporous silica-... 

Gulkova and Zdrazil studied kinetics of the parallel HDS of thiophene and HDN of pyridine as model reactions for elucidating the effect of the Ni addition to W on AC. For this purpose, the fixed-bed continuous system was used at 2 MPa and 553 and 593 K. The rate constants from this study are summarized in Table 31. They confirmed the promoting effect of Ni on both HDS and HDN reactions. The rate constants shown in Table 29 for some noble-metal sulfides were determined under identical conditions. The effect of phosphorus on parallel HDS of thiophene and HDN of pyridine for catalysts in Table 7 was quantified by kinetic measurements conducted by Vasques et Some results from this study are shown in Table 32. 

The first description of a synergetic effect due to a mixed cobalt-molybdenum catalyst (oxides and sulfides) was in 1933 by Pease and Keithon (11) at the Princeton University. Their catalytic system was active for the HDS of a mixture of benzene and thiophene. However, difficulty in reproducing their results already pointed out the complexity of this promotion effect, highly dependent on the conditions of preparation and pretreatment and the experimental conditions. 

However, this model has been the subject of controversial discussions and some researchers argued against this remote control mechanism. Indeed, using emission Mossbauer spectroscopy (EMS), Wivel and co-workers (21) have shown that even when the CogSg phase is not detected, a promoter effect is observed by the addition of Co (Co/Mo < 0.4). Nevertheless, in any case, this model was very helpful in understanding metal sulfides activity and has clearly shown that the alumina support does not play any role in the promotion effect and that promotion resides uniquely at the interface of the two phases M0S2 and CogSg. 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

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