Hydrogenolysis of benzothiophenes

This extensive [(triphos)Ir] model chemistry demonstrates that all the necessary steps for the complete HDS of thiophene or for the hydrogenolysis of benzothiophene can be achieved without any need to previously saturate the heterocyclic ring, in agreement with one of the commonly invoked mechanisms in HDS catalysis - the desulfurization route - as discussed in Chapter 1. 

Scheme 8 Stepwise hydrogenolysis of benzothiophene on Ir-triphos complexes.

Scheme 12 The mechanism of homogeneous catalytic hydrogenolysis of benzothiophene.

Three independent rate equations are necessary one for the direct hydrogenolysis of benzothiophene on a sites, = /(x,p) one for its hydrogenation to dihydrobenzothiophene on x sites, = /(x,p) and one for the hydrogenolysis of the latter to ethylbenzene on a sites, = /(x,p), where X represents the vector of partial pressures and p the parameter vector. 

Direct hydrogenolysis of benzothiophene into styrene on o sites ... 

The reactions involving the hydrogenolysis of sulfur compounds encountered in hydroprocessing are exothermic and thermodynamically complete under ordinary operating conditions. The various molecules have very different reactivity, with mercaptan sulfur much easier to eliminate than thiophene sulfur or di-benzothiophene sulfur. 

HDS catalysts generally consist of (heterogeneous) Mo or W sulfides on alumina supports. However, Bianchini et al. described a two-step procedure for HDS of thiophenes by the hydrogenolysis of thiols, followed by the desulfurization of the thiols by applying their zwitterionic rhodium(I) complex, [Rh(sulphos((cod)] (see previous section) [17]. This complex is soluble in polar solvents, such as methanol and methanol-water mixtures, but not in hydrocarbons. Benzo[b]thiophene was chosen as substrate since it is one of the most difficult thiophene derivatives to degrade. Under the mild reaction conditions of the two-step process, the benzene rings of the (di)benzothiophenes were not affected. In the absence of a base, the double bond of benzo[b]thiophene was hydrogenated, while in the presence of a base (NaOH) 2-ethylthiophenolate was the major product (Scheme 1). 

Several more complex reactions such as the catalytic reforming of heptanes on Pt/Re/alumina were dealt with in terms of sets of rate equations of the Hougen-Watson type by Van Trimpont et al. [1986]. The hydrogenolysis of thiophene and benzothiophene on Co/Mo/alumina was studied along the same lines by Van Parijs et al. [1986a, b] and is also discussed in Examples 2.6.4.A and 2.7.2.2.A. 

Van Parijs et al. [1986] applied sequential discrimination in their experimental study of benzothiophene hydrogenolysis. The final rate equations were already given in Example 2.6.4.A. The efficiency of the sequential discrimination was remarkable. At 533 K, a total of seven experiments (four preliminary, three designed) were sufficient to reject 14 out of 16 rivals. Only one of the two remaining models was consistently the best at 513, 553, and 573 K also, after four, five and nine designed experiments, respectively. A total of 41 experiments sufficed to select the best among the postulated rate equations. The location of the settings of these experiments is shown in Fig. 2.7.2.2.A-1 of Example 2.7.2.2.A. 

When the models are nonlinear in the parameters, Xis replaced by J. An example of application of sequential design for optimal estimation is given by Van Parijs et al. [1986] in their study of benzothiophene hydrogenolysis. At 533 K the desired accuracy of the parameters of the equations given in Example 2.6.4.A was reached after four more experiments beyond the seven required for discriminating between the 16 rival models. In total, no more than 16 experiments were required for the four temperature levels that were investigated. 

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 ... 

When the integral method of kinetic analysis is applied, numerical integration of the continuity equations containing the rate equations is generally necessary for the comparison of the predicted and experimental responses for each experiment in each iteration cycle of the parameter estimation. Examples can be found in the work of De Pauw and Froment [1975] on n-pentane reforming in the presence of coke formation, in the work of Emig, Hofmann, and Friedrich [1972] on methanol oxidation, and in Example 2.6.4.A on benzothiophene hydrogenolysis. 

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