Desulfurisation catalysts

Juliao et al. recently disclosed an update on the use of chromium-based MOF catalysts in desulfurisation catalysts. In this work, a composite... 

Supported bimetallic systems containing molybdenum and a modifier such as Co, Ni... are frequently encountered in the literature, especially as desulfurisation catalysts. On the other hand, little attention has been devoted to Pd-Mo/oxide systems so far, despite their use for the CO + NO reaction by Halasz et al. [1] and several mentions as selective hydrogenation catalysts in the patent literature. 

Early attempts to control vanadium deactivation involved the addition of amorphous alumina to the cracking catalyst matrix. This was not particularly suitable because alumina increased coke formation and led to wider dispersion of nickel impurities. Hydrodesulfurization of FCC feeds is nseful it not only removes sulfhr, but the desulfurisation catalyst also adsotbs a significant proportion of the metal porphyrins. 

Komfeld, E. C.,J. Org. Chem., 1951, 16, 137 Raney nickel catalyst, containing appreciable amounts of the sulfide (after use to desulfurise thioamides) is rather pyrophoric. 

Studies on heterogeneous catalysts seem to invoke partial hydrogenation of thiophene prior to desulfurization [42] the catalysts are also active hydrogenation catalysts. Recently evidence for a facile and selective desulfurisation of partly hydrogenated thiophene has been reported, the reaction of 2,5-dihydrothiophene on (110) molybdenum surfaces (Figure 2.41) [43]. 

A catalytic example of C-S bond breakage in benzothiophene has been reported by Bianchini [47], A catalytic desulfurisation was not yet achieved at the time as this is thermodynamically not feasible at such mild temperatures because of the relative stability of metal sulfides formed. Bianchini used a water-soluble catalyst in a two-phase system of heptane-methanol/water mixtures in which the product 2-ethylthiophenol is extracted into the basic aqueous layer containing NaOH. Figure 2.43 gives the reaction scheme and the catalyst. The 16-electron species Na(sulfos)RhH is suggested to be the catalyst. Note that a hydrodesulfurisation has not yet been achieved in this reaction because a thiol is the product. Under more forcing conditions the formation of H2S has been observed for various systems. 

Molybdenum disulfide catalysts, promoted by cobalt or nickel, are used to remove organosulfur compoimds from crude petroleum by hydrogenolysis— hydrodesulfurisation or HDS [124,125,126], These compounds are imdesirable because they poison motor vehicle autoexhaust catalysts and bum to sulfur dioxide, an environmental pollutant. The most difficult compounds to desulfurise are sulfur heterocyclics thiophene, benzothiophene, dibenzothiophene and their methyl substituted derivatives. A typical reaction is the removal of sulfiir from thiophene ... 

Thiophene is the simplest sulfur heterocyclic that can be used as a model compound for desulfurisation. The earliest stage of the HDS process is, the adsorption of thiophene. The INS spectra of adsorbed thiophene [140] were obtained. The challenge is to interpret changes, if any, of the INS spectrum due to adsorption in terms of the interaction of thiophene with the catalyst. The INS spectra of thiophene complexes of known structure were used to aid in the interpretation of the INS spectrum of adsorbed thiophene. Fig. 7.35. 

The FT synthesis was conducted at temperatures between 180-200°C, slightly below the range considered to be of commercial interest at present. A process limitation related to the low operating pressure most of the reactors were run at atmospheric pressure. A few plants included reactors designed for operation at pressures of up to 10 bar. Ifie S5mthesis gas in these plants was desulfurised to ca 5 ppm sulfur to protect the FT catalyst (7). As a consequence, and as it is the case at present, all the FT products had very low sulfur contents. 

Catalytic reductions of the thiophene ring, or of substituents attached to it, are complicated by two factors poisoning of the catalyst, and the possibility of competing hydrogenolysis - reductive removal of sulfur, particularly with Raney nickel - indeed the use of thiophenes as templates on which to elaborate a structure, followed finally by desulfurisation, is an important synthetic strategy... 

Adiabatic pre-reforming of desulfurised diesel and jet fuels was demonstrated for commercial nickel catalysts at 480 °C and S/C ratios around 2.5 for 1000- and 500-h duration, respectively [81]. The feed was not completely sulfur-free, but still contained about 1 ppm sulfur and thus catalyst deactivation originated from sulfur poisoning and rather than from coke formation. 

Basically two strategies exist for the removal of sulfur compounds from hydrocarbons. They may be removed upstream or downstream of the reformer. For a higher sulfur content, as is present in mixtures of higher hydrocarbons such as heavy diesel or aviation fuel, the desulfurisation must be performed upstream of the reformer, otherwise the reforming catalyst suffers from sulfur poisoning. The operating temperature of the adsorption materials applied is usually around ambient or in a temperature range below 300 °C. Hydrodesulfurisation, which is applied on an industrial scale, is not suited to small mobile fuel processors, but could be used in stationary applications on the smaller scale. 

---