Initiator-free hydrocarbon oxidation using supported gold nanoparticles

Article abstract of DOI:10.1039/c4cy00088a

The oxidation of cyclic alkenes (cyclopentene, cyclohexene, cycloheptene and cyclooctene) has been carried out under green solvent-free conditions using ambient air as the oxidant gas with a carbon-supported gold catalyst. We show that radical initiators are only required for this reaction when the cyclic alkene contains a stabilizer molecule, such as 6-bis(t-butyl)-4-methylphenol (BHT) or 3-octadecyl-(3,5-di-t-butyl-4-hydroxyphenyl)-propanoate (irganox 1076). We show that either removing the stabilizer chemically or sourcing the cycloalkane without the stabilizer the oxidation, both with and without radical initiator, is identical. This indicates that the only function of the radical initiator is to counteract the stabilizer. We have extended this work to show that stabilizer-free internal linear alkenes can be effectively epoxidised with air using the carbon-supported gold catalyst. However, unfortunately linear α-alkenes still require the addition of an initiator to achieve reaction. This work demonstrates that removal of a sacrificial reagent from a reaction is possible which adheres to the principles of green chemistry. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00088a

Catalysis
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Technology
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Initiator-free hydrocarbon oxidation using
supported gold nanoparticles†
Cite this: Catal. Sci. Technol., 2014,
, 908
4
ab
a
ac
a
Hamed Alshammari, Peter J. Miedziak, Thomas E. Davies, David J. Willock,
Received 21st January 2014,
Accepted 22nd January 2014
a
a
David W. Knight and Graham J. Hutchings*
DOI: 10.1039/c4cy00088a
www.rsc.org/catalysis
The oxidation of cyclic alkenes (cyclopentene, cyclohexene,
cycloheptene and cyclooctene) has been carried out under green
solvent-free conditions using ambient air as the oxidant gas with a
carbon-supported gold catalyst. We show that radical initiators are
only required for this reaction when the cyclic alkene contains a
stabilizer molecule, such as 6-bis(t-butyl)-4-methylphenol (BHT) or
epoxide without the use of such undesirable reagents is
one of the major challenges in oxidation chemistry. Propene
is epoxidised industrially using H O as the oxidant, the
2 2
epoxidation of ethene using molecular oxygen is carried
6
,7
8
–11
out industrially using a silver catalyst
but higher alkenes
1
2
are generally oxidised using a radical initiator. The two
most common classes of initiators are organic peroxides and
3
-octadecyl-(3,5-di-t-butyl-4-hydroxyphenyl)-propanoate (irganox
1
3,14
1076). We show that either removing the stabilizer chemically or
azo compounds.
sourcing the cycloalkane without the stabilizer the oxidation, both
with and without radical initiator, is identical. This indicates that the
only function of the radical initiator is to counteract the stabilizer.
We have extended this work to show that stabilizer-free internal
linear alkenes can be effectively epoxidised with air using the
carbon-supported gold catalyst. However, unfortunately linear
α-alkenes still require the addition of an initiator to achieve reaction.
This work demonstrates that removal of a sacrificial reagent from a
reaction is possible which adheres to the principles of green chemistry.
Peroxides, such as tert-butyl hydroperoxide (TBHP) can
play important roles in the oxidation of alkenes, both as a
reaction initiator (in catalytic amounts) or as an oxidant
1
(in stoichiometric amounts). The use of peroxide as reaction
initiator in catalytic amounts and the use of air as oxidant have
1
been widely reported in the literature. Previously, we have
demonstrated that gold nanoparticles supported on graphite
15
are active for the solvent-free oxidation of cyclic alkenes.
However, a catalytic amount of an initiator (either H O or
2
2
TBHP (tert-butyl hydroperoxide)) was required to initiate
these reactions; no conversion was observed in the absence
of initiator, an effect which has been widely replicated in
Hydrocarbon oxidation reactions are important in the synthesis
of fine chemicals, conversions using oxygen from air represent
1,2
the greenest method of carrying out these reactions. Oxygen
is a diradical in the ground state which enables its ease of
activation in a number of radical reactions, making it possible
to employ molecular oxygen in a range of low temperature
1,16–19
the literature including several recent publications.
1
However, in our initial paper we did indicate that conversion
in the absence of a radical initiator was possible for cyclo-
hexene when much higher pressures of molecular oxygen
3,4
oxidations. However, at present many molecules and catalysts
are unreactive with molecular oxygen and therefore more reactive
oxidising agents containing a terminal oxidant are employed;
often these involve non-green stoichiometric oxygen sources
16
were used. A subsequent paper by Lambert and co-workers
confirmed that such initiator-free reactions are indeed feasible.
This has prompted us to study this effect in much greater
detail. In this present study, we have investigated the oxidation
of a range of cyclic alkenes, under solvent free conditions
and have demonstrated that these alkenes can be oxidized in
the absence of radical initiators if the stabilisers added by the
suppliers are first removed or indeed if stabiliser free-alkenes
are sourced.
5
such as permanganate or chromates. Selective electrophilic
addition of oxygen to an unactivated alkene to form an
a
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Park Place,
Cardiff, CF10 3AT, UK. E-mail: hutch@cf.ac.uk
b
Chemistry Department, Faculty of Science, Ha'il University, P.Box 2440,
Stabilisers such as 2,6-bis(t-butyl)-4-methylphenol (BHT) or
8
1451 Ha'il, Saudi Arabia
3
1
-octadecyl-(3,5-di-t-butyl-4-hydroxyphenyl)-propanoate (irganox
076) are typically added to some commercial alkenes as
c
Stephenson Institute for Renewable Energy, Chemistry Department,
The University of Liverpool, Crown Street, Liverpool, UK
stabilisers to prevent auto-oxidation. These stabilisers are added
in small quantities and are often not listed in the product
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00088a
908 | Catal. Sci. Technol., 2014, 4, 908–911
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information. Our initial studies investigated the epoxidation of
cis-cyclooctene in the absence of a radical initiator (TBHP) when
the stabiliser had been removed (see experimental for full details
of the method). The catalysts used in this study were prepared
by sol-immobilisation as previous studies have shown that
this methodology affords the most active catalysts. 1%Au/
graphite (1%Au/G) was used as catalyst and we have previously
reported extensive characterisation of these catalysts; the
metal particle size distribution was found to be in the range of
initiator does not enhance the conversion but instead simply
counteracts the effects of the stabiliser by reacting with it.
In view of these promising results, we tested two samples of
cyclohexene, firstly cyclohexene (as received) which contained
0.01% BHT as stabilizer, and secondly a sample that was
stabiliser-free. Table 1 also shows that, in the case of the
cyclohexene that contained the stabilizer, 8% conversion was
achieved when a small amount of TBHP was added as a radical
initiator. However, no cyclohexene oxidation was observed in
the absence of radical initiator. In contrast, the cyclohexene
which was inhibitor-free gave very similar activity and selectivity
in the absence or presence of TBHP. In a further set of
experiments, cyclopentene and cycloheptene were used as the
substrate (Table 1). Again, we found that if the cycloalkene
contained stabiliser, no reaction was observed in the absence
of a radical initiator but with a small amount of radical ini-
tiator the oxidation occurs. In contrast, the inhibitor-free
cycloalkene can be oxidised with almost the same activity and
selectivity either in presence or in absence of radical initiator.
We have also investigated α-alkenes under stabiliser-free
conditions; the results are shown in Table 2. For 1-octene
and 1-hexene the addition of the radical initiator was
required to achieve conversion even when these substrates
were stabiliser-free. However, in the case of cis-2-hexene and
cis-3-hexene, for which the alkene bond has a more similar
environment to the cyclic alkenes reported above, almost the
same conversion was achieved either in the presence or the
in absence of the radical initiator.
2
0
1
–12 nm with an average particle size of 4.7 nm and a median
2
1,22
size of 4 nm.
Reactions carried out in the absence of cata-
lyst, with and without stabiliser, are summarised in the ESI†
Table S1), Table 1 summarises the performance of the
catalyst for cis-cycloalkene oxidation, before and after the sta-
biliser removal treatment, using air as the O source and 1%
(
2
Au/G as catalyst. The results show that, before treatment
cis-cyclooctene, which contains irganox 1076 as a stabiliser,
exhibits 5.6% conversion with around 83% selectivity towards
the epoxide when tert-butyl hydroperoxide (TBHP) is used as
a radical initiator, the mechanism of the product formation
2
3
in these oxidation reactions has been discussed previously.
In contrast, in the absence of radical initiator (TBHP), no
cis-cyclooctene conversion was observed. This is consistent
with previous studies which confirm that a catalytic amount
1
,10,21
of initiator is necessary for this oxidation.
The experi-
ment was repeated after removal of the stabiliser, under the
same reaction conditions as were used without the treatment
and the conversion with and without TBHP is shown in Table 1.
It is interesting to note that 5.6% conversion, with 86% epox-
ide selectivity is achieved in the absence of radical initiator.
Furthermore, the addition of TBHP to the (treated) stabiliser
free cis-cyclooctene did not lead to any enhancement of the
conversion or epoxide selectivity indicating that the radical
Using these optimised conditions, time-on-line studies
were carried out for cyclohexene and cis-cyclooctene either
containing stabiliser or stabiliser-free. When the cycloalkene
contained stabiliser, a small amount of radical initiator (TBHP)
was added. The results, shown in Fig. 1, clearly indicate that,
Table 1 Comparison of the oxidation activity of cyclic alkenes using a 1%Au/graphite catalyst reaction conditions: substrate (5 ml), Au/G (0.06 g),
glass reactor, TBHP (0.032 × 10 mol where present), 24 h and atmospheric pressurea
−3
Selectivity (%)
Epoxide
d
Substrate
T (°C)
Stabiliser
TBHP
Conversion (%)
Alcohol
Ketone
Dimers
Cyclooctene
Cyclooctene
Cyclooctene
Cyclooctene
Cycloheptene
Cycloheptene
Cycloheptene
Cycloheptene
Cyclohexene
Cyclohexene
Cyclohexene
Cyclohexene
Cyclopentene
Cyclopentene
Cyclopentene
Cyclopentene
80
80
80
80
60
60
60
60
50
50
50
50
26
26
26
26
Present
Present
Removed
Removed
Present
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
5.6
0
5.3
5.6
9.1
0
10.1
9.4
8.2
0
8.9
8.7
4
83.7
—
86.8
86.7
18.4
—
18.2
18.4
6.4
—
6.3
6.3
12.7
—
10
—
7
7
41.8
—
43.5
43.3
41.4
—
46.0
41
37.6
—
5
—
5
5
39.7
—
38.4
38.3
50.5
—
47.0
44
23.7
—
—
—
—
—
—
—
—
—
—
—
—
—
19
—
13.5
22
b
b
Present
c
Nonec
None
Present
Present
c
Nonec
None
Present
Present
0
4.4
4.4
c
Nonec
12.4
12.5
43.6
36.2
23.9
22.4
None
a
b
c
Stabiliser removed by treatment.
Stabiliser removed by treatment method reported in ESI. Stabiliser-free alkenes purchased
d
(Aldrich). Dimers observed are 3-(cyclopent-2-en-1-yloxy)cyclopent-1-ene cyclopentanone, 3-[(2-cyclopenten-1-yl)oxy] as identified by GC-MS
and NMR. All selectivities are based on calculated mols of products.
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−
6
Table 2 Comparison of oxidation activity for α-alkenes (stabilizer-free),
in the presence/absence of TBHP and in the presence of a 1%Au/graphite
catalyst
it is clear that the addition of even a small amount (10 M)
of the stabiliser has a detrimental effect on the conversion
−
5
and a concentration of 3 × 10 M is sufficient to completely
quench the oxidation reaction for either alkene (Fig. 2a). This
concentration of stabiliser was subsequently used to demon-
strate the effect of the amount of TBHP on the conversion of
stabilised cycloalkenes and these results are shown in
Fig. 2b. For cyclopentene, if TBHP is added in equimolar
quantities to the stabiliser there is no conversion, in fact in
terms of TBHP over 10 times the concentration of that of
the stabiliser is required to show any improvement on the
negligible conversion that is observed. The reaction was
carried out with up to 33 times the concentration of TBHP
relative to the stabiliser which still only led to less than 50%
of the stabiliser-free conversion. For cyclohexene as soon as
any initiator was added there was an increase in conversion,
however we again did not observe the same conversion as
was achieved in the stabiliser free alkene despite the large
excess of initiator relative to the stabiliser. In summary, this
work emphasises the level of attention that needs to be
applied the source and exact content of substrates when they
are to be used for catalytic studies.
Conversion %
α-Alkene
(stabiliser free)
Temperature (°C)
With TBHP
Without TBHP
1
1
-Octene
-Hexene
70
40
40
40
2.9
1.2
1.2
9.6
0
0
0.9
11.4
cis-2-Hexene
cis-3-Hexene
We have shown that selective cyclic alkene oxidation can
be achieved under green, solvent-free conditions without the
addition of a radical initiator. We have demonstrated that
the only function of the radical initiator is to oxidise the
stabilisers present in the cyclic alkenes and the removal of a
sacrificial reactant from the process adheres to the principles
of green chemistry.
Fig. 1 Effect of reaction time for the conversion of cis-cyclooctene
at 80 °C and cyclohexene at 50 °C. Reaction conditions: 5 ml of
alkene, 0.06 g Au/G, glass reactor, TBHP (0.032 × 10 mol) and
−
3
atmospheric pressure. Cyclohexene containing stabiliser with TBHP
(●), stabiliser-free cyclohexene without TBHP (○), cis-cyclooctene
containing stabiliser with TBHP (♦) and cis-cyclooctene after the
removal of stabiliser without TBHP (◊).
Experimental section
Preparation of catalyst by using sol-immobilisation
1% Au was prepared by the sol-immobilisation method as
by removing the stabiliser, the activity is identical to that
when stabiliser and initiator are present at all times tested.
Finally, to determine the detrimental effect of the stabi-
liser (BHT) on the conversion of cyclopentene and cyclo-
hexene, we doped small quantities of BHT into stabiliser-free
cycloalkenes. The effects of this doping are shown in Fig. 2,
follows: an aqueous solution of HAuCl ·3H O (Johnson Matthey,
4
2
12.25 g in 1000 mL) was prepared. Polyvinyl alcohol (PVA)
(1 wt% solution, Aldrich, Mw = 10 000, 80% hydrolysed,
PVA/Au = 1.2 (by wt)) was added to the solution and stirred
with HAuCl ·3H O for 15 min. 0.1 M of NaBH (>96% Aldrich,
4
2
4
NaBH /Au (mol/mol = 5)) was freshly prepared and then
4
−
5
Fig. 2 a) The addition of radical initiator to cyclopentene containing BHT (3 × 10 M) and b) the addition of BHT to stabiliser free cyclopentene.
Reaction conditions: substrate (10 ml), catalyst (0.12 g), glass reactor, atmospheric pressure.
910 | Catal. Sci. Technol., 2014, 4, 908–911
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added to form a dark brown sol. The mixture was stirred for
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0 min and adjusted to pH = 1 by the drop wise addition of
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Typically, aqueous potassium hydroxide (20 ml, 3 M) was
added to cycloalkene (20 ml) under vigorous stirring for
1
0 min at room temperature. The two layers were separated
and aqueous potassium hydroxide (20 ml, 3 M) was added to
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Oxidation of alkenes: standard reaction conditions
Information on the alkenes source and stabiliser content is
detailed in Table S3.† Oxidation of alkenes using oxygen from
air was carried out in a glass reactor consisting of a 50 ml
round-bottomed flask fitted with a reflux condenser. Typically
a supported gold catalyst (0.12 g) was suspended in the alkene
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(10 ml) at the required temperature followed by the addition
of a small amount of radical initiator TBHP (when used).
The reaction mixture was stirred for the requisite time at
atmospheric pressure.
A sample of the reaction was analysed after the reaction
was completed. The analysis was conducted using gas chroma-
tography (GC) (Varian Star CP-3800) with a CP-Wax 52 column
and a flame ionization detector (FID). For each reported data
point, the GC analysis was run twice. Overall error, based on
repeat reactions was calculated to be 1.97%.
1
1
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2
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We would like to thank Hail University (Saudi Arabia) for
financial support.
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Catal. Sci. Technol., 2014, 4, 908–911 | 911