Kinetic modeling of the effect of H2S and of NH3 on toluene hydrogenation in the presence of a NiMo/Al2O3 hydrotreating catalyst. Discrimination between homolytic and heterolytic models
S. Blanchin et al., Kinetic modeling of the effect of H2S and of NH3 on toluene hydrogenation in the presence of a NiMo/Al2O3 hydrotreating catalyst. Discrimination between homolytic and heterolytic models, J PHYS CH A, 105(48), 2001, pp. 10860-10866
Toluene hydrogenation was studied within a wide range of hydrogen sulfide,
hydrogen, toluene, and ammonia partial pressures on a standard sulfided NiM
o/Al2O3 hydrotreating catalyst at 350 degreesC under a total pressure of 4.
9-9.8 MPa. The results showed a complex inhibiting effect of H2S on the hyd
rogenation activity with an order of reaction relative to H2S varying betwe
en -0.05 and -0.5. Unexpectedly, this inhibiting effect was enhanced by the
presence of ammonia. Several kinetic models based on the homolytic or hete
rolytic dissociation of hydrogen and hydrogen sulfide were investigated and
discriminated by using the CHEMKIN/SURFACE CHEMKIN II tool. The heterolyti
c dissociation was supposed to occur on centers composed of an unsaturated
Mo ion (on which hydride ions and organic molecules can adsorb) and of a su
lfur anion, host of the proton generated by the heterolytic dissociation of
hydrogen or hydrogen sulfide. It was concluded that toluene hydrogenation
was most likely to occur through a heterolytic mechanism starting with a hy
dride addition followed by a proton addition. In the range of H2S partial p
ressures investigated, the latter was the rate-determining step.