Aromatic ring, hydrogenation

It is believed that conventional HDS catalysts possess two very different types of catalytic sites which contribute to the different pathways described earlier. One induces the direct extraction of sulfur (kDo), and the other catalyzes aromatic ring hydrogenation (kHs,) CO- This topic is discussed in... 

The results presented in Figs. 12a and 12b provide one other piece of information. As discussed in more detail later, NiMoS /A Oj has a relatively high activity for aromatic ring hydrogenation. This indicates that the catalyst of choice for high-conversion HDS may change as PASCs become more refractory due to steric hindrance imposed by alkyl substituents. This is discussed in more detail in the next section. 

In the case of 4,6-DMDBT, it was possible to determine the rate constants for direct extraction of sulfur from the fully saturated sulfur-containing ring system (k0l) and for the secondary hydrogenation of the tetrahydro-dibenzothiophene intermediate (fcHs2)- As might be expected, the rate constants for direct sulfur extraction follow a clear trend in which A Dq < < d2 The reverse trend is observed in the aromatic ring hydrogenation rates, /cHs, > kHS, and kHPl > kw , which is consistent with the literature (see Fig. 10) (5, 35). 

As described in Section IV.B, dibenzothiophenes, when substituted in positions adjacent to the sulfur atom, have reduced activity for direct sulfur extraction. As a result, catalysts that promote aromatic ring hydrogenation offer another route to desulfurization, as the partially hydrogenated ring presents much less steric restrictions to adsorption via r -S type bonding (17,21) or to oxidative addition to form a metallathiabenzene intermediate, as discussed in Section IV.E.3. In addition, the metal-S coordination bond strength is increased by increasing the electron density on sulfur, and the C-S bonds in hydrothiophenes are much weaker. 

There are different methods to cleave benzyl ether bonds. The most common one is hydrogenolysis with palladium on carbon or platinum as catalysts under H2 atmosphere. The standard solvents are ethanol or ethyl acetate. Pd is the preferred and milder one, because the use of Pt at any rate results in aromatic ring hydrogenation. Also a number of methods have been developed in which hydrogen is generated in situ, e. g. from cyclo-hexene, -hexadiene or formic acid (see Chapter 7). 

In (b) and (d), the aromatic ring hydrogens accidentally have the same chemical shift. 

However, other results indicate that the P/Al catalyst has very low activity for C —N or C —C bond breaking 74, 81). A detailed kinetics study by Jian and Prins 81) showed that phosphorus addition decreases the C—N bond cleavage (rate constants k[ and k) in the reaction networks Figs. 33a and 33b) and the subsequent alkene hydrogenation (rate constant k() reactions in piperidine and DHQ HDN (see Table XIV). On the other hand, the presence of phosphorus increases aromatic ring hydrogenation of OPA... 

The hds of thiophen, benzothiophen, and dibenzothiophen and their methyl-substituted derivatives were compared in pulse experiments (623-723 K) over a sulphided CoO(5.6)-Mo03(11.2)/Al203 catalyst. Reactivities at 1 atm pressure were roughly the same but at hi er pressures, reactivity decreased with the number of rings. For benzothiophen, methyl substituents did not affect the reactivities, but for dibenzothiophen methyl substituents in the 4- or 4 - and 6- positions caused a decrease of desulphurization rate. Aromatic ring hydrogenation was not a prerequisite of C-S scission, which was the slow step. 

As soon as tin is introduced drastic changes of selectivity occur and no more aromatic ring hydrogenation is observed. 

Active catalysts for aromatic ring hydrogenation reactions of toluene, phenol, and anisole were prepared from [Rh5Pt(CO)i5] on an Amberlite anion-exchange resin.l 1 This catalyst did not work for aniline or nitrobenzene. 

Aromatic compounds have two characteristic types of hydrogens aromatic ring hydrogens (benzene ring hydrogens) and benzylic hydrogens (those attached to an adjacent carbon atom). 

Longer-range couplings, those that involve more than three bonds, are common in systems with al-lylic hydrogens (Section 5.7) or aromatic ring hydrogens (Section 5.11) and in compounds that are rigid bicyclic systems (Section 5.13). 

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