to strong adsorption of phenol (and maybe also of PCP) on
charcoal as previously observed in the electrocatalytic
hydrogenolysis of phenoxyphenol (31). This shows once more
the advantage of using alumina instead of activated charcoal
as support material for ECH in aqueous medium (31). A 3-fold
increase of the initial concentration of PCP (from 2.5 to 7.5
mmol L-1) had a beneficial effect on the efficiency of the
electrocatalytic hydogenolysis (about 77% current efficiency
in entry 4 compare to about 87% in entry 7). This is related
to an increase of the rate of the hydrogenation step (eq 4)
with substrate concentration as previously discussed (30).
In previous studies, it was found that increasing the current
density over a certain value, depending on the conditions
and the substrate, did cause a decrease in the ECH efficiency
due to an increase in the rate of the electrochemical
desorption of chemisorbed hydrogen (eq 5), the effect on
the rate of the hydrogenation step (eq 4) being less important
(28-30). For the electrocatalytic hydrogenolysis of PCP at
Pd/ C, an applied current of 25 mA was below the value where
the current has an effect on the efficiency since decreasing
the current to 10 mA had no influence (compare entries 4
and 8). The electrocatalytic hydrogenolysis was slightly less
efficient with ammonium hydroxide as supporting electrolyte
(entry 9: 67% yield of phenol, 86% material balance) than
with sodium hydroxide (entry 4: 77% yield of phenol, 95%
material balance). In contrast, dehydrochlorination of PCP
by catalytic hydrogenation was found much less efficient in
an ammonium hydroxide than in a sodium hydroxide
solution (17). Finally, increasing the temperature from 25 to
75 °C (see entries 3-5) favored hydrogenolysis (eq 2) over
hydrogen desorption (eqs 5 and(or) 6) as shown by the
increase in conversion rate (from 66% to 92%) and in yield
of phenol (from 60% to 92%). Hence and interestingly, at 75
°C, it was possible to reach complete hydrogenolysis of PCP
to phenol (98% yield) after 16 F mol-1 (61% current efficiency)
(entry 6).
Acknowledgments
Financial support from the Natural Sciences and Engineering
Council of Canada and the Fonds FCAR du Que´bec is
gratefully acknowledged.
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We have shown recently that Rh(5%)/ Al2O3 entrapped in
a RVC electrode is very efficient for the ECH of phenol in
aqueous alkali (31). Indeed, after 10 F mol-1 (corresponding
to the theoretical charge for complete hydrogenation of
phenol), all the phenol had been hydrogenated to cyclo-
hexanol (97% yield, 60% current efficiency). No hydrog-
enation occurred on the RVC electrode in the absence of
catalyst, showing that the ECH process does not take place
on vitreous carbon and that the phenolate anion is not re-
ducible by electronation in aqueous alkali as already
pointed out. In entries 10-12 of Table 1, it can be seen that
electrocatalytic hydrogenolysis of PCP to phenol is also
efficient on Rh/ Al2O3 and that the phenol is further elec-
trohydrogenated as expected from the previous studies.
The lower material balance at 50 °C (entry 12: 78%) than at
25 °C (entry 10: 100%) was due to the loss of cyclohex-
anol by evaporation during the electrolysis even if the
cell was equipped with a water-cooled condenser. Indeed,
when a 0.025 M solution of cyclohexanol was put in the
cell under the electrolysis conditions (except that no current
was applied), its concentration diminished gradually, and
there was none left after 12 h. There was no loss of
cyclohexanol at 25 °C. It is noteworthy that it was possible
to convert PCP completely to cyclohexanol (92% yield) with
some residual cyclohexanone (2%) at 25 °C with a charge
corresponding to 24 F mol-1 (about 62% current efficiency)
(entry 11).
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Chem. 1999, 77, 1225.
Received for review October 5, 1999. Revised manuscript
received January 10, 2000. Accepted January 10, 2000.
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