The exchange of thiophene with deuterium and reactions of
deuterium labelled propene and isobutene, followed by a combination
of mass spectrometry and microwave spectroscopy, have been used to
characterise the catalytic nature of a commercial CoO-MoO₃-Al₂O₃ (CMA)
catalyst. It was observed in this work that the oxide-CMA was partially
sulphided during the initial stages of ethanethiol hydrodesulphurization
(hds) or as a result of exposure to hydrogen sulphide. The results of
these exchange experiments indicated that the activity of partially
sulphided CMA. closely resembled that of unsupported molybdenum
disulphide (MoS₂) but was markedly different from that observed
Investigation of the hds of ethanethiol, hydrogenation of
ethene, hds of diethylsulphide and some relevant exchange reactions
with deuterium over oxide-CMA, partially sulphided CMA and unsupported
MoS₂ furnished a similar conclusion i.e. during hds, the partially
sulphided CMA may be described as MoS₂ supported on, and stabilized by,
alumina. Furthermore, by comparison of the relative rates of the various
reactions on partially sulphided-CMA and MoS₂, it was possible to establish,
in outline, a mechanistic scheme for the hds of ethanethiol: ethene,
produced by elimination of hydrogen sulphide, was the major product of
the hds reaction while ethane, involving hydrogenation, was produced in
much lesser quantities.
In contrast, during the decomposition of ethanol, the oxygen
analogue of ethanethiol, the CMA catalyst remained in the oxide form and
the decomposition was found to occur by two alternative routes i.e.
dehydrogenation to ethanal, which subsequently coupled to give butane
as a secondary decomposition product, or dehydration to diethylether.
The higher surface charge density of oxide-CMA relative to
partially sulphided CMA is believed to be the major contributor to the
differences between the two catalyst systems.