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This chapter focuses on the current research on chemistry of desulfurization reactions. Squires et al., 1980, have determined the potential of oxidation techniques for the removal of organic sulphur from coal by subjecting model compounds to the oxydesulphurization conditions used in the Ames process. The model compounds used represent the major types of sulfur compounds commonly believed to be present in coal. They are all in the divalent or sulfide oxidation state that is consistent with the generally reduced state of coal. The chapter presents the results of the oxydesulfurization of the model compounds. The amount of starting material recovered is an indication of the degree of reaction that the compound underwent. To assess the possibility that coal may act catalytically in oxidation, reactions of four compounds were carried out in the presence of coal. Compounds of the thiophene type and all sulphides except benzylic sulphides are inert to oxidation under the Ames oxydesulphurization conditions. Oxidation of benzylic sulfides is believed to occur by oxidation of the C atom in the CH2 group and not by oxidation of the S atom. To eliminate the possibility that the benzylic sulfides are, in fact, oxidized to either the sulfide or sulfone, which may then be responsible for the observed products, both benzyl phenyl sulfoxide and benzyl phenyl sulfone were subjected to the reaction conditions. The sulfoxide was converted partially to the sulfone, which did not undergo further reaction. The fact that sulphone is not a product of the oxidation of benzylic sulphides is evidence that the oxidation of the S atom does not occur.

Oxidation reactions

Squires et al. (1980) have determined the potential of oxidation techniques for the removal of organic sulphur from coal by subjecting model compounds to the oxydesulphurisation conditions used in the Ames process (Chang et al., 1980). The model compounds used, which are shown in Figure 7.1, represent the major types of sulphur compounds commonly believed to be present in coal. They are all in the divalent or sulphide oxidation state which is consistent with the generally reduced state of coal.

The results of the oxydesulphurisation of the model compounds are presented in Table 7.1. The amount of starting material recovered is an indication of the degree of reaction that the compound underwent. In order to assess the possibility that coal may act catalytically in the oxidation, reactions of four compounds were carried out in the presence of coal. These results are presented in Table 7.2. To take into account the decreased recovery expected due to adsorption of the model compounds onto coal, experiments using both oxygen and nitrogen were performed. The right hand column of Table 7.2 shows the yields of recovered starting material corrected for this adsorption.

The results presented in Tables 7.1 and 7.2 show that compounds of the thiophene type and all sulphides except benzylic sulphides are inert to oxidation under the Ames oxydesulphurisation conditions. These compounds represent the principal functional groups believed to be in coal. Thiophenol is readily oxidised to the diphenyl disulphide which under more severe conditions is oxidised to benzene sulphonic acid.

(7.1)

(7.2)

The oxidation of benzylic sulphides (benzyl methyl sulphide and benzyl phenyl sulphide) is believed to occur by the oxidation of the C atom in the CH2 group and not by oxidation of the S atom. To eliminate the possibility that the benzylic sulphides are, in fact, oxidised to either the sulphide or sulphone, which may then be responsible for the observed products, both benzyl phenyl sulphoxide and benzyl phenyl sulphone were subjected to the reaction conditions. The sulphoxide was converted partially to the sulphone which did not undergo further reaction. The fact that sulphone is not a product of the oxidation of benzylic sulphides (see Table 7.1) is evidence that the oxidation of the S atom does not occur. As further evidence that the oxidation takes place at the C atom, fluorene, which also contains a CH2 group, was found to be oxidised to fluorenone under similar reaction conditions.

(7.3)

Dibenzothiophene, did not react. The 55% yield of fluorenone is in good agreement with the yields obtained from the oxidation of the benzylic sulphides.

More recent work has involved subjecting sulphur-containing synthetic polymers and the pyridine extract of Iowa Lovilia coal to the Ames process conditions (Squires and Venier, 1981; Squires et al., 1981). The results obtained with three modified polystyrenes, shown in Table 7.3, add support to the model compound studies (Squires et al., 1981). Phenylthiomethyl and tolylthiomethyl polystyrenes are analogues of the model compound, benzyl phenyl sulphide. As expected, these polymers were reactive, but less so than the corresponding model compound. Polystyrene cross-linked by dibenzothiophene was unreactive, confirming the inertness of the dibenzothiophene moiety to oxydesulphurisation.

Perhaps the most complete evidence of the unreactive nature of some organic sulphur in coal to oxydesulphurisation has been obtained with a pyridine coal extract (Squires and Venier, 1981; Squires et al., 1981). All the sulphur in an extract is organically bound and hence the total sulphur analysis, which is quite accurate, is also the organic sulphur determination. The results in Table 7.4 show that oxydesulphurisation apparently does not reduce the organic sulphur content of the extract. However, some of the carbon in the extract is oxidised as evidenced by the increases in S/C ratio, the oxygen content and O/C ratio.

Warzinski et al. (1980) have studied the effect of the PETC oxydesulphurisation process on the following model sulphur compounds:

The reactions of the thiol and the sulphonic acid are similar to those reported by Squires et al. (1980). Compound 4, a sulphonated polystyrene copolymer also reacted in a manner similar to that reported by Squires et al. (1980) for organic compounds containing benzylic sulphides. Compound 5, polyphenyl sulphide, which contains no benzylic carbon was treated at 200°C with and without the presence of coal. In all cases the polymer was recovered (95%) unchanged.

The major disagreement between the work of Warzinski et al. (1980) and Squires et al. (1980) is in the reaction of benzothiophene. Squires et al. (1980) report an 87% recovery of the compound after subjection to the Ames oxydesulphurisation conditions. Warzinski et al. (1980) report 97% conversion of benzothiophene into 2-sulphobenzoic acid, benzoic acid, benzothiophene-1, 1-dioxide, and phenol, by the following proposed reaction:

It is unlikely that the differences in the process conditions (200°C, acidic solution, compared with 150°C and basic solution) would account for the disagreement. Oxidation of the S atom, as proposed by Warzinski et al. (1980), is unlikely in view of the work at Ames, reported above.

Attar (1980) has summarised the relative reactivities of sulphur containing species in coal to oxidation and reduction (Table 7.5).

The stability of the thiophenic sulphur and sulphides under oxydesulphurisation conditions may be more simply explained in terms of bond strengths. A strong bond exists between a S atom and a C atom within an aromatic ring. Indeed, nowhere in the oxidation of the model compounds shown in Table 7.1 is such a bond broken. Thiophene converts to diphenyl disulphide which is oxidised by the...