20
T.F.S. Silva et al. / Catalysis Communications 96 (2017) 19–22
peroxide oxidants at room temperature (r.t.), catalyzed by Sn1
for longer reaction times (N6 h, see Fig. S2 of ESI) GC–MS analyses re-
vealed the presence of 1,2- and 1,4-cyclohexanediol. This formation of
side products of cyclohexane oxidation is believed to be the cause of
KA oil yield decrease observed for such longer reactions times.
Experiments under N2 atmosphere were also performed for the
−
xCoxO2 − δ (x = 0, 0.01 or 0.05) nanoparticles (NPs). The NPs synthesis
procedure and their structural, microstructural and optical characteriza-
tion have been described elsewhere [23].
2. Results and discussion
Sn0.95Co0.05O2 − δ nanocatalyst (Table 1, entry 13). The obtained overall
yield and TON values are similar to that obtained in air (compare entries
9 and 13, Table 1), suggesting only a slightly promoting effect of the air
atmosphere. Accordingly with less oxidant availability, the ratio cyclo-
hexanone/cyclohexanol decreased.
Control experiments performed in the absence of the SnO2 NPs with
any type of peroxide confirm the crucial role of the nanoparticles to ef-
ficiently catalyze the oxidation of cyclohexane (entries 17 to 20, Table
1). In fact, SnO2 NPs are believed to initially activate, not the alkane,
but the oxidant (Eqs.1 and 2) and form oxygen-centered radicals that
attack the alkane molecule generating the cyclohexyl radical Cy· (Eq. 3).
Fig. 1 shows a set of TEM and HRTEM micrographs of the undoped
and Co-doped SnO2 nanopowders samples. As previously reported
[23], all samples are composed of quasi-spherical single crystalline
nanoparticles whose dimensions follow a lognormal distribution with
mean grain sizes b 5 nm and the dopant element homogeneously dis-
tributed in the SnO2 matrix. In both undoped and Co-doped SnO2
HRTEM micrographs of Fig. 1, few nanoparticles showing the (110)
planes are delimited by dotted circles stressing that nanoparticles are
single crystalline domains.
The SnO2-based NPs can catalyze the oxidation of neat cyclohexane
with a peroxide: hydrogen peroxide (aq., 30%), ter-butyl hydroperoxide
(TBHP, aq. 70%), m-chloroperoxybenzoic acid (mCPBA, 77%) or urea hy-
drogen peroxide adduct (UHP) as depicted in Scheme 1. These new cat-
alytic systems operate at room temperature and in solvent-free
conditions. Cyclohexane is mainly oxidized to cyclohexyl hydroperox-
ide (CyOOH), cyclohexanol (CyOH) and cyclohexanone (Cy-H = O). Pre-
cise quantification of the oxidation products present in the reaction
mixture was performed by gas chromatography using Shul'pin's meth-
od [24–27] where cyclohexyl hydroperoxide was quantitatively con-
verted to cyclohexanol by reduction with triphenylphosphine. The
presence of the hydroperoxide was verified by additional GC injections
of the samples before the treatment with triphenylphosphine (see ex-
perimental details in the electronic supplementary information (ESI)).
Selected results corresponding to optimized experimental conditions
(6 h reaction time, r.t.) are given in Table 1.
Sn4þ
Sn2þ
þ
þ
2 ROOH → 2 ROOꢀ
2 ROOH → 2 ROꢀ
þ
2 Hþ
þ
Sn2þ
ð1Þ
þ
Sn4þ
þ
2 HO–
ð2Þ
ð3Þ
ROꢀ þ CyH→ ROH þ Cyꢀ
Cy· , in turn, reacts with dioxygen (Eq. 4) to form the cyclohexyl-
peroxyl radical CyOO· which gives rise to the cyclohexyl hydroperoxide
(CyOOH) (Eq. 5). The latter, in the presence of both reduced and oxi-
dized forms of the SnO2 nanocatalyst, decomposes (Eqs. 6–9) to cyclo-
hexanone and/or cyclohexanol.
Cyꢀ
CyOOꢀ
þ
O2 → CyOOꢀ
ROOH → CyOOH
Sn2þ → 2 CyOꢀ
ð4Þ
ð5Þ
þ
þ
ROOꢀ
2 HO–
SnO2-based NPs act as highly selective catalysts towards the forma-
tion of cyclohexanol and cyclohexanone, since no traces of by-products
were detected by GC–MS analysis of the final reaction mixtures for the
optimized conditions (6 h reaction time, see Fig. S2 of ESI). However,
2 CyOOH
þ
þ
þ
Sn4þ
ð6Þ
2 CyOOH þ Sn4þ→ 2 CyOOꢀ þ 2Hþ þ Sn2þ
ð7Þ
ð8Þ
ð9Þ
CyOꢀ
þ
CyH → CyOH
þ
Cyꢀ
2 CyOOꢀ þ →CyOH þ Cy‐H ¼ O þ O2
The strong inhibition effect (drop over 90%, entries 11 and 12, Table
1) observed when the reaction was carried out in the presence of either
the oxygen-radical trap Ph2NH or the carbon-radical trap CBrCl3, as well
as the involvement of the hydroperoxide CyOOH in the catalytic reac-
tion (compare entries 9 and 10, Table 1) suggests the involvement of
the above radical mechanism for the cyclohexane oxidation catalyzed
by the SnO2 NPs.
A maximum 15% yield of KA oil is achieved after 6 h reaction by using
TBHP oxidant and SnO2 nanopowder catalyst, with a turnover number
(TON, moles products/mol nanocatalyst) of 1.1 × 103 (see entry 1,
Table 1). This is a marked improved catalytic activity relative to other
nanocatalytic cyclohexane oxidation systems such as gold NPs support-
ed at carbon nanotubes (3.6% maximum yield, TON of 43 [28]).
The use of other peroxides in the present protocol leads to lower
yield and TON values (Table 1), following the trend mCPBA N H2O2
N UHP, as depicted in Fig. 2. This oxidant dependent behavior was also
found for other catalytic systems such as gold nanoparticles deposited
on oxide supports [29] or heterogenized metal complexes [30], with
TBHP exhibiting the highest oxidation efficiency. Further, replacement
of peroxides by dioxygen as oxidant agent was not well succeed,
allowing only a maximum of 0.6% total oxygenates yield (TON of 43).
Moreover, Co-doping of SnO2 nanoparticles leads to higher KA oil
yield and TON values, as Co/Sn ratio increases, up to 25% yield and
Fig. 1. TEM (up, left) and HRTEM (up, right) micrographs of undoped SnO2 NPs. In the
HRTEM micrograph four nanoparticles showing the (110) planes are delimited by
dotted circles. The corresponding [110] directions are also given. HRTEM micrograph
(down, left) of Sn0.99Co0.01O2
NPs and (down, right) HRTEM micrograph of
δ
−
TON of 1.9 × 103 (for Sn0.95Co0.05
lowing the above-mentioned trend in respect to the type of oxidant (Fig.
O2 − δ and TBHP, entry 10, Table 1), fol-
Sn0.95Co0.05O2
NPs. In both Co-doped SnO2 micrographs few nanoparticles are
δ
−
delimited by dotted circles stressing that nanoparticles are single crystalline domains.
Silva, Telma F.S.
