Catalysis Science & Technology
Paper
(R2 = 0.99) from linear regression of the remaining points. A
negative ρ value is indicative of electron flow away from the
aromatic ring in the rate determining step of the reaction.
Mizuno et al. applied the Hammett relationship to the
oxidative dehydrogenation of amines using Brown–Okamoto
σ constants,13,19 however we found a better relationship with
the traditional Hammett relationship. The better fit of the
results with the traditional Hammett σ constants compared
to the Brown–Okamoto σ constants suggests that positive
charge is not formed during the rate determining step.
Mizuno et al. found a much smaller ρ value (−0.154), indicat-
ing that under the conditions applied in our continuous
flow process, the substituents have a greater electronic effect
than in the batch process utilized by Mizuno et al. The dif-
ferences between ρ values and appropriate σ constants may
be due to a difference in rate determining steps of the batch
and continuous flow processes, or a result of variation in reac-
tion conditions including temperature, pressure and solvents
present.
presence of persistent surface OH groups. The band at 1645
cm−1 (assigned to an HOH bend due to the presence of water)
was no longer present after calcination, suggesting that all
physisorbed water was removed during calcination. The
observed IR spectra therefore agree with the postulation that
calcination at 350 °C in O2 leads to a less hydrated RuO2/
Al2O3 catalyst. Contributions to the spectra from carbonate
and carboxylate species were also reduced, with much
smaller blue shifted bands present at 1853, 1576 and 1460
cm−1, as seen in Fig. 9.
Upon exposure of the RuO2/Al2O3 catalyst to ethylamine
rapid changes were observed in the IR spectra, as shown by
Fig. 10. Within 1 min of exposure to ethylamine, IR absorp-
tion bands developed in the 2750–3100 cm−1 and 1200–1600
cm−1 regions. IR absorption bands in the 2750–3100 cm−1
region can be broadly assigned to C–H stretches,20 which
confirm the presence of ethylamine in the reaction cell. Spe-
cifically the peaks at 2968, 2942, 2885 and 2855 cm−1 can be
assigned to the CH3 symmetric stretch, CH2 symmetric
stretch, CH3 asymmetric stretch and CH2 asymmetric stretch,
respectively, indicating the presence of an ethyl group.25 IR
absorption bands in the complex 1200–1600 cm−1 region
include CH2 and CH3 bending vibrations, C–C stretches and
NH2 scissoring at 1550 cm−1.20,26,27
We propose that the rate determining step of the oxidative
dehydrogenation of amines in continuous flow involves the
removal of a hydrogen atom from the carbon atom adjacent
to the amine group, either by β-hydride elimination, in agree-
ment with Mizuno et al., or by another mechanism.
In comparison to the freshly calcined catalyst a reduction
in the intensity of the absorption bands at 1460 and 1853
cm−1 was observed in the IR spectra after 1 min of exposure
to ethylamine. This suggests that ethylamine replaced car-
bonate and carboxylate species that were present on the cata-
lyst surface prior to exposure to ethylamine.
After the RuO2/Al2O3 catalyst had been under reaction con-
ditions for 5 min, there were no absorption bands present in
the IR spectra indicative of the presence of an aldehyde or
nitrile compound. However, an imine may have been present
as a band was formed at 1588 cm−1 after 4.6 min under reac-
tion conditions, which can be assigned to a characteristic
imine CN stretching band.20 As no IR band that could be
attributed to an aldehyde was detected, we suggest that the
detected imine was an unsubstituted imine and intermediate
for the synthesis of acetonitrile.
3.2 In situ IR spectroscopy
The IR spectrum of the fresh RuO2/Al2O3 catalyst (Fig. 9) is
dominated by the presence of water and surface hydroxyls, as
shown by the presence of a broad band in the OH stretching
region (3700–3000 cm−1), which can be assigned to surface
hydroxyl groups and water.20–23 The presence of physisorbed
water is confirmed by the presence of a band that may be
assigned to an HOH bend at 1645 cm−1 21,22
Also present in
.
the spectrum of the fresh catalyst RuO2/Al2O3 are bands at
1401 and 1542 cm−1, which are probably due to the presence
of surface carbonate and carboxylate species formed due to
the adsorption of atmospheric CO2 on the catalyst surface.24
Calcination of the RuO2/Al2O3 catalyst significantly
reduced the OH contributions to the spectra, though a broad
OH stretching band remained at 3000–3750 cm−1, due to the
Fig. 10 IR spectra of RuO2/Al2O3 catalyst during exposure to aqueous
EtNH2 solution (70 wt%) in 20 ml min−1 O2 at 225 °C after 0, 0.9, 1.8,
2.8, 4.6 and 5.5 min.
Fig. 9 IR spectra of fresh RuO2/Al2O3 catalyst and catalyst calcined at
350 °C for 150 min in 20 ml min−1 O2.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.