Table 5 Hydrodechlorination rate for selected single dechlorination
steps: T \ 573 K; W /F \ 85 g mol~1 h
Conclusions
The gas phase hydrodechlorination of chlorobenzene and 3-
Dechlorination step
103 R/mol g~1 h~1
chlorophenol on Ni/SiO proceeds via the electrophilic attack
2
of the adsorbed aromatic by surface dissociated hydrogen,
Chlorobenzene ] benzene
3-Chlorotoluene ] toluene
3-Chlorophenol ] phenol
5.6
8.6
9.4
3.0
possibly spillover hydronium ions, where both reactants do
not compete for adsorption sites on the catalyst surface.
Under reaction conditions, the catalyst surface is saturated
with HCl which is displaced by the incoming chloroaromatic
reactant. While the catalyst delivers an essentially time invari-
ant degree of chlorobenzene dechlorination with prolonged
and continuous use, the turnover of chlorophenol is accompa-
nied by an irreversible catalyst deactivation. The latter e†ect is
attributed to the inductive e†ect of the hydroxy substituent,
which results in a stronger interaction with the catalyst that
leads to an irreversible displacement of the electron density
from nickel. Contacting the catalyst with the chlorinated aro-
matic results in a dramatic loss of hydrogenation activity and
this can be linked to the observed disruption of the hydrogen
TPD proÐle.
1,4-Dichlorobenzene ] chlorobenzene
Hydrodechlorination mechanism
Based on the above, catalytic hydrodechlorination is con-
sidered to occur via the surface reaction between spillover
hydrogen and the adsorbed aromatic where both species do
not compete for the same adsorption site(s). An explicit
assignment of the aromatic adsorption site(s) is beyond the
scope of this report but work is at present under way, involv-
ing FTIR analysis of chloroaromatic adsorption on fresh and
used catalysts, as a Ðrst step to understanding the nature of
catalystÈchlorine interaction(s). The modiÐcations to the
hydrogen TPD proÐles are indicative of some form of
chlorineÈnickel interaction but this e†ect may be indirect,
occurring via the support, and the metal/silica interface may
well play a dominant role in the overall process. Hydro-
dechlorination has been viewed in terms of both
nucleophilic24,57 and electrophilic8,19,20 attack. One way of
distinguishing between these two possible mechanisms is to
vary the electron donor/acceptor properties of ring substit-
uents and examine the impact on dechlorination activity;
hydrodechlorination rates, under the same reaction condi-
tions, for a range of substituted aromatics are given in Table
5. The observed order of increasing dechlorination rate from
the strongly electron withdrawing chlorine substituent to the
Acknowledgements
EJS acknowledges partial Ðnancial support from the British
Council. AS wishes to thank Prof. Schulz-Eklo† and Prof.
Jaeger for the provision of experimental facilities and many
helpful discussions.
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