Aerobic oxidation of cyclohexane by gold-based catalysts: New mechanistic insight by thorough product analysis

Article abstract of DOI:10.1016/j.jcat.2009.12.003

The selective oxidation of cyclohexane to cyclohexanone and cyclohexanol with air over Au/Al₂O₃, Au/TiO₂, and Au/SBA-15 catalysts was investigated and compared with the industrial autoxidation process. In contradiction with the literature results, the Au-based catalysts did not exhibit excellent catalytic performance and the combined selectivity toward cyclohexanone and cyclohexanol decreased to ～70% with the increasing conversion above 5%. In addition, the observed product evolution and by-product evolution were typical for the autoxidation process, although a significant increase in adipic acid and CO₂ formation was observed. Additional oxidation experiments containing a radical scavenger completely inhibited the reaction and provided proof that the oxidation follows a radical-chain mechanism instead of a catalytic mechanism. This explains the low selectivity at increasing conversion. This important deviation from the literature results can be clarified by the complicated but necessary product analysis of both the gas and the liquid phases.

Full text of DOI:10.1016/j.jcat.2009.12.003

Journal of Catalysis 270 (2010) 16–25
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Aerobic oxidation of cyclohexane by gold-based catalysts: New mechanistic insight
by thorough product analysis
*
Bart P.C. Hereijgers, Bert M. Weckhuysen
Inorganic Chemistry and Catalysis Group, Debye Institute for NanoMaterials Science, Utrecht University, Sorbonnelaan 16, 3508 TB Utrecht, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
The selective oxidation of cyclohexane to cyclohexanone and cyclohexanol with air over Au/Al2O3,
Au/TiO2, and Au/SBA-15 catalysts was investigated and compared with the industrial autoxidation pro-
cess. In contradiction with the literature results, the Au-based catalysts did not exhibit excellent catalytic
performance and the combined selectivity toward cyclohexanone and cyclohexanol decreased to ꢀ70%
with the increasing conversion above 5%. In addition, the observed product evolution and by-product
evolution were typical for the autoxidation process, although a significant increase in adipic acid and
CO2 formation was observed. Additional oxidation experiments containing a radical scavenger com-
pletely inhibited the reaction and provided proof that the oxidation follows a radical-chain mechanism
instead of a catalytic mechanism. This explains the low selectivity at increasing conversion. This impor-
tant deviation from the literature results can be clarified by the complicated but necessary product anal-
ysis of both the gas and the liquid phases.
Received 30 October 2009
Revised 29 November 2009
Accepted 2 December 2009
Available online 29 December 2009
Keywords:
Gold
Selective oxidation
Autoxidation
Cyclohexane
KA-oil
Radical-chain mechanism
Heterogeneous catalysis
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
both processes differs, the autoxidation as well as the catalyzed
reaction is known to proceed via the complex radical-chain mech-
The liquid-phase aerobic oxidation of cyclohexane (CyH) is of
major importance for the production of cyclohexanone and cyclo-
hexanol (ketone/alcohol or KA-oil), the annual worldwide produc-
tion of which exceeds 106 tons [1,2]. Both oxidation products are
anism shown in reactions (1)–(5) [8]. The chain initiation is consid-
ered to proceed through the homolytic cleavage of cyclohexyl
hydroperoxide (CyOOH) according to reaction (1). Transition met-
als capable of undergoing a one-electron switch (e.g., Co2+, Mn2+
,
the main industrial precursors of, respectively,
e-caprolactam and
and Cr2+) are able to catalyze this initiation via a so-called Ha-
ber-Weiss cycle (Scheme 1) [3,9–12]. Once formed, the oxygen-
centered radicals can attack the CyH solvent via reaction (2) to
form cyclohexyl radicals. Via the rapid reaction (3) with molecular
O2 these radicals will form cyclohexyl peroxy radicals (CyOOꢀ). Cy-
OOꢀ is the main chain propagator causing the continuous formation
of new radicals as in reactions (3), (4). The formation of cyclohex-
anol (CyOH) and cyclohexanone (Cy@O) was long time attributed
to the chain termination by mutual destruction of two peroxy rad-
icals according to reaction (5) [2,3,8,13]. However, recent research
by Hermans et al. provided strong evidence that subsequent prop-
agation reactions of CyOOH within their Franck-Rabinowitch sol-
vent cage are responsible for the formation of both the ketone
and the alcohol [14,15].
adipic acid, building blocks of the nylon-6 and nylon-6,6 polymers
[1]. Commercially the aerobic oxidation process is operated at tem-
peratures of 415–435 K and 1–1.5 MPa pressure. In industry, the
conversion is kept at 4% to prevent the formation of excessive
amounts of by-products by over-oxidation of the relatively reactive
oxygenates, giving 70–85% selectivity toward KA-oil [1–3]. For the
production of e-caprolactam, the ketone is considered as the most
valuable product and consequently a high K/A-ratio is desired.
In essence there are two commercial processes exploited now-
adays, i.e., the non-catalytic autoxidation process and the catalyzed
process. In the first process, cyclohexyl hydroperoxide (CyOOH) is
formed non-catalytically, followed by a subsequent step convert-
ing CyOOH catalytically to KA-oil with typical K/A-ratio of 1.5
[4,5]. The second process makes use of a radical initiator, mostly
dissolved cobalt(II)naphthenate, to accelerate the process resulting
in a K/A-ratio 0.3–0.5 [3,6,7]. Although the product distribution in
CyOOH ! CyOꢀ þ ꢀOH
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
CyOꢀ þ CyH ! CyOH þ Cyꢀ
Cyꢀ þ O2 ! CyOOꢀ
CyOOꢀ þ CyH ! CyOOH þ Cyꢀ
2CyOOꢀ ! CyOH þ Cy ¼ O þ O2
* Corresponding author. Fax: +31 (0) 30 251 1027.
E-mail address: b.m.weckhuysen@uu.nl (B.M. Weckhuysen).
0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.12.003

B.P.C. Hereijgers, B.M. Weckhuysen / Journal of Catalysis 270 (2010) 16–25
17
CyOOH
CyO• + OH-
2.2. Catalyst characterization
Co2+
Co3+
TEM micrographs were obtained on a FEI Tecnai 20F Transmis-
sion electron microscope operated at 200 KeV equipped with a
Schottky Field Emission Gun (magnification range is 25ꢁ–700kꢁ)
and a FEI Tecnai 12 Transmission Electron Microscope operated
at 120 KeV (magnification range is 40ꢁ–700kꢁ). The gold particle
size distributions as determined from TEM represent at least 210
particles per catalyst material. Diffuse reflectance UV–vis-NIR
spectra were recorded on a Varian Cary 500 Scan spectrophotom-
eter. X-ray powder diffraction was performed on a Bruker AXS Ad-
CyOO• + H+
CyOOH
Scheme 1. Haber-Weiss cycle of the cobalt-catalyzed decomposition of cyclohexyl
hydroperoxide.
Especially at higher conversion (>5%), excessive by-product for-
mation is observed [6], which has for a long time been attributed to
the ring cleavage of cyclohexanone [3]. Recently Hermans et al.
proved that the hitherto overlooked propagation of cyclohexyl
hydroperoxide CyOOH into a cyclohexoxy radical (CyOꢀ) and subse-
vance D8 apparatus using CoK radiation (k = 1.78897 Å) operating
a
at 45 kV and 30 mV. N2-physisorption isotherms and specific sur-
face areas were measured on a Micromeretics Tristar 3000 ana-
lyzer. Fourier Transform Infrared (FT-IR) spectra were recorded in
transmission mode on a Perkin–Elmer System 2000 FT-IR spec-
trometer using the KBr technique, in the range of 4000–400 cmꢂ1
quent b-cleavage forming a ring-opened
x-formyl radical is the
most important origin of by-product formation [11,13]. The nature
of these by-products, e.g., peroxides, organic acids, and gasses,
complicates the analysis of the reaction mixture tremendously
[8]. Performing accurate selectivity calculations and obtaining a
complete mass balance are therefore not straightforward and is of-
ten underestimated [6].
with a resolution of 1 cmꢂ1
.
2.3. Cyclohexane oxidation experiments
Prior to conducting oxidation experiments, the reactor interior
was passivated by boiling with a saturated sodium pyrophosphate
(ABCR, 99%) solution for 4 h and thorough washing. The aerobic
catalytic oxidation experiments were carried out in a fed-batch
500 mL stainless steel autoclave reactor setup (SA182-F316, rated
up to 254 bar at 450 °C working conditions) by Autoclave Engi-
neers. The oxidation reactions were performed at 150 °C at 1.2–
1.5 MPa under continuous flow (900 sccm) of 8 vol% O2/N2 in order
to operate outside the flammability range of cyclohexane [31,32].
The gas flows were controlled by Brooks mass flow controllers
(type 5850i).
Typically 310 mL cyclohexane (241 g, 2.85 mol, Sigma–Aldrich,
99.5%) containing 1 mol% biphenyl (Acros, 99%) as internal stan-
dard was loaded in the reactor and the reactor was flushed with
nitrogen. The reactor was heated to a stable operation temperature
and subsequently O2 was introduced, defining t = 0. The effluent
gas was cooled in a reflux cooler to diminish the evaporation loss
of cyclohexane and volatile organics from the reactor. Remaining
gas-phase volatiles were collected in a cold trap. The effluent gas
stream was analyzed on-line by gas chromatography (GC) using a
Varian CP4900 Micro-GC, double channel on-line GC equipped
with porabond Q and 5MS molsieve column and TC detectors. To
monitor the O2 conversion and CO/CO2 formation the N2 signal
was used as internal standard.
Since radical chemistry plays the key role in by-product forma-
tion in cyclohexane oxidation, one should aim, in order to improve
the yield of the desired products, for a non-radical, catalytic con-
version of CyOOH to avoid the formation of CyOꢀ intermediates
[13]. Triggered by the successes of gold-based catalysts in a wide
variety of selective oxidation reactions [16–18], the use of nano-
structured gold catalysts for this particular oxidation reaction
was initiated by Suo et al. in 2004. Au/ZSM-5 and Au/MCM-41 cat-
alysts were found to be able to convert cyclohexane to KA-oil at
150 °C and 1 MPa oxygen pressure reaching 90+% selectivity at
conversion of 10–15% [19,20]. These promising results were con-
firmed by the publications coming from a variety of groups report-
ing similar results at similar conditions for, e.g., (modified) Au/
Al2O3 [21,22] and gold nano-particles supported on amorphous
Ti-doped SiO2 [23] and mesoporous silica materials [24–26]. In
combination with t-butyl hydroperoxide as initiator, Au/C was
found to exhibit a high selectivity toward K/A-oil formation only
at low conversion, decreasing significantly when the conversion in-
creased [27,28].
The results published justify a more thorough investigation of
the excellent catalytic performances of these supported gold cata-
lyst materials, especially since the product distribution during
reaction has received minor attention so far. In this regard, we
show in this work that the aforementioned results on gold-based
catalysts are obscured by the complicated product analysis. In fact,
the gold-based catalysts are relatively inert and the reaction path
follows the above-described radical-chain mechanism.
Liquid-phase aliquots were extracted from the reactor via a
manually operated sample valve equipped with a filter (5 lm)
and thermally quenched. Liquid-phase samples were collected
after flushing the tubing with approximately 5 mL of the reactor
content. Prior to the liquid-phase product analysis, the samples
were dissolved in pyridine (Fluka, 99%) and derivatized with an ex-
cess N-methyl N-trimethylsilyl trifluoroacetamide (MSTFA, ABCR,
97%). The silylated samples were analyzed using a Varian 430
2. Experimental methods
2.1. Catalyst preparation
GC, equipped with a VF-5 ms column (30 m, DF = 0.25 lm,
0.25 mm I.D.), CP-8400 autosampler, and FID detector, in split/
splitless injection mode. The injector temperature was set to
220 °C.
Supported gold nano-particles were prepared by deposition
precipitation on the different supports from a dilute HAuCl4
(Sigma–Aldrich, 99.99%) in HCl solution containing the exact
amount of gold to achieve a 1 wt% loading. Precipitation was per-
formed with a diluted ammonia solution (Merck, 25%) at a pH of
9.5 [29]. The support materials were obtained from a commercial
The product-specific FID response has been determined with re-
spect to biphenyl for all main products and by-products including
cyclohexanol, cyclohexanone, formic acid, acetic acid, butyric acid,
valeric acid, caprioc and hydroxy-caproic acid, glutaric acid, and
adipic acid as well as methanol, 1-butanol, 1-pentanol, and 1-hex-
anol. To determine the FID response factor for cyclohexyl hydro-
peroxide, the reaction samples were double analyzed, before and
after reducing CyOOH to cyclohexanol with triphenyl phosphine
(Acros, 99%). For the cobalt-catalyzed reaction cobalt–methylhex-
source; i.e.,
c
-Al2O3 (Engelhard, SBET = 230 m2/g, total pore vol-
ume = 0.76 cm3/g) and TiO2 (Degussa P25, SBET = 55 m2/g, total
pore volume = 0.19 cm3/g). The SBA-15 support (SBET = 696 m2/g,
total pore volume = 0.797 cm3/g) was home synthesized following
the procedure as described by Zhao et al. [30].

18
B.P.C. Hereijgers, B.M. Weckhuysen / Journal of Catalysis 270 (2010) 16–25
anoate (Sigma–Aldrich, 65 wt% Co2+ in mineral spirits) was used.
Radical inhibition experiments were performed using hydroqui-
none (ABCR, 99%) as inhibitor [33,34].
oxidation reaction is extended to 60 min [37]. A conversion of
0.19 mol% was obtained after 60 min of reaction after which the
reaction rate increased. Four percent conversion was reached only
after 184 min of reaction. It is obvious that, although the reactor is
not to be considered ‘‘inert”, the contribution of the reactor after
passivation to the oxidation rate is considerably low. Comparing
this to the gold-catalyzed reaction, it is clearly visible that the reac-
tion still exhibited an initiation period (30–45 min) but proceeded
significantly faster compared to the uncatalyzed reaction in the
passivated reactor. Therefore, we can truly address the activity of
the supported gold nano-particles in further experiments.
The product distribution and outlet oxygen concentration with
the conversion for cyclohexane autoxidation are presented in
Fig. 4. The results obtained on the Co2+-catalyzed reaction are in-
cluded as well for direct comparison with the industrial process.
Some reaction characteristics at the industrially applied conversion
of 4% cyclohexane are listed in Table 1 for comparison. For all oxi-
dation experiments during the initiation period a buildup of Cy-
OOH is observed, followed by a gradual loss in CyOOH selectivity,
caused by conversion into mainly CyOH and Cy@O. With increas-
ing conversion, selectivity toward Cy@O increased for all experi-
ments, however, the obtained K/A-ratios at 4% conversion range
from 0.43 (Co2+) to 1.23 (untreated reactor). When the conversion
is increased further above 5% the combined selectivity toward KA-
oil and CyOOH dropped significantly due to the formation of by-
products. In all cases the selectivity toward 6-hydroxy-caproic acid
is observed to increase and level off or even decrease subsequently.
Adipic acid formation was in general lower in the first period, but
increased at higher conversion. Also significant CO and CO2 forma-
tion is observed. The C6 group (defined as the unidentified species
with a retention time longer than caproic acid, so mostly C6-poly-
oxygenates), accounted for 5–10% of the products at 4% conversion,
although at higher conversion this increased to 10–20%. The forma-
tion of shorter acids (which generally form under oxygen starva-
tion) was low indicating that the reaction was not significantly
limited by the oxygen supply [13].
3. Results
3.1. Catalyst characterization
The catalyst materials under study were characterized by dif-
fuse reflectance UV–vis-NIR spectroscopy, TEM, X-ray powder dif-
fraction, and N2-physisorption. The presence of supported Au
nano-particles was confirmed by the distinctive pink/purple color
of the catalyst material after calcination. In Fig. 1 the UV–vis-NIR
absorption spectra of the catalysts are shown. The typical absorp-
tion band around 500–550 nm, caused by the surface plasmon
vibration of colloidal gold, is clearly visible [35]. TEM micrographs
of the catalyst materials and related particle size distributions, as
determined from TEM images are shown in Fig. 2. The gold parti-
cles are clearly visible and exhibit an average particle size of 4.7,
4.8, and 4.0 nm for Au/TiO2, Au/Al2O3, and Au/SBA-15, respectively.
The determined particle sizes are similar to the particle sizes re-
ported to form active cyclohexane oxidation catalyst materials
[21–23,25]. From the TEM micrographs of Au/SBA-15 it appeared
that the SBA-15 support was slightly damaged. Most likely this
originates from the Au deposition precipitation at pH = 9.5. This
was confirmed by an observed decrease in the SBET (553 m2/g)
and total pore volume (0.694 cm3/g) as determined from N2-phys-
isorption after Au deposition. Crystallinity of the catalyst materials
was verified by XRD. Only in the case of the Au/SBA-15 catalyst,
diffraction lines of gold particles could be observed at 2h values
of 44.8° and 52°. The XRD patterns of Au/Al2O3 and Au/TiO2
showed the broad diffraction lines characteristic for
c-Al2O3 and
the TiO2 rutile and anatase phases [36].
3.2. Catalytic testing
The product selectivity evolution versus CyH conversion for the
Au-catalyzed oxidation reactions are presented in Fig. 5. When
comparing these to the autoxidation results presented in Fig. 4
some differences are observed. However, in general, the oxidation
seemed to follow the same route leading to the same products and
by-products. One significant difference is that the SCyOOH at conver-
sions >2% was lower than that was observed for the cobalt-cata-
lyzed oxidation and autoxidation. This lower selectivity coincided
with the formation of Cy@O and CyOH immediately from the start
of the reaction. At low conversion (<2%) a minimum in SCy@O was
observed. One other remarkable is that, when employing a gold-
based catalyst CO2 formation was observed at lower conversion
than CO formation was observed and moreover, CO2 formation
was preferred over CO formation. This is contrary to observations
during the autoxidation as well as the cobalt-catalyzed oxidation.
The selectivities toward the other main by-products, 6-hydroxy-
caproic acid and adipic acid, are comparable through all experi-
ments, although the gold-based catalysts exhibited a relative high
adipic acid formation as compared to 6-hydroxy-caproic acid. In
total, over 40 different products were found in the final reaction
mixture. Reduction of liquid-phase samples with triphenyl phos-
phine revealed the presence of different peroxide species, e.g., 6-
hydroperoxo-caproic acid [38]. The presence of organic acids was
confirmed by the extraction of the final oxidation mixture with
NaOH (aq) prior to GC analysis.
To evaluate the inertness of the reactor interior (the stainless
steel vessel wall, impellor, thermo-well, cooling coil, and filtered
diptube) first of all blank oxidation experiments without catalyst
material were conducted. The results are displayed in Fig. 3. It is
observed that the autoxidation in the untreated reactor goes
through an initiation period of 40 min in which almost no conver-
sion is observed (0.16 mol%). Thereafter, the conversion increased
gradually to reach 4% after 104 min on stream. After passivation
of the reactor it is clearly observed that the initiation time of the
.
a
Abs
b
c
0
Interestingly, combining a gold-based catalyst with 1.5 ppm
dissolved Co2+ led to a performance more or less being the average
of the cobalt and the gold-catalyzed process. This is illustrated in
Fig. 5d for Au/SBA-15 combined with 1.5 ppm Co2+. The initiation
of the reaction was faster, reducing the time to reach 4% conversion
200
400
600
λ / nm
800
Fig. 1. Diffuse reflectance UV–vis-NIR spectra of (a) Au/TiO2, (b) Au/Al2O3, and (c)
Au/SBA-15.

B.P.C. Hereijgers, B.M. Weckhuysen / Journal of Catalysis 270 (2010) 16–25
19
50
40
30
20
10
0
1
3
5
8
12
Size (nm)
50
40
30
20
10
0
1
3
5
8
12
Size (nm)
50
40
30
20
10
0
1
3
5
8
12
Size (nm)
Fig. 2. TEM micrographs and particle size distributions of Au/TiO2 (a), Au/Al2O3 (b), and Au/SBA-15 (c). Particle size distributions represent at least 210 particles.
from 92 to 73 min. However, the observed rate was lower than that
when solely 1.5 ppm Co2+ was used. Remarkably, the product
distribution mimicked the gold-based catalyst results instead of
the results obtained with dissolved cobalt salt. Whereas the cobalt-
reaction (vide supra). This suggests that the oxidation product
and by-product formation is dominated by the less efficient initia-
tor, namely gold.
The evolution of the SK/A and the K/A-ratio with increasing
conversion for the different cyclohexane oxidation reactions is
presented in Fig. 6. For all gold-based catalysts under investiga-
tion the evolution of SK/A with increasing conversion followed
the same trend as is observed in the autoxidation reaction. The
combined selectivity toward the products dropped below 80% al-
ready at 3.5% conversion and decreased gradually with increasing
catalyzed reaction first shows a buildup of CyOOH (SCyOOH,initial
=
60%), in combination with a Au/SBA-15 catalyst a much lower
SCyOOH (35%) was observed favoring the initial formation of CyOH
(37%) and Cy@O (24%). Additionally, a relative high adipic acid: hy-
droxy-caproic selectivity ratio and significant CO2 formation were
observed. Both were shown to be typical for the gold-catalyzed

20
B.P.C. Hereijgers, B.M. Weckhuysen / Journal of Catalysis 270 (2010) 16–25
Table 1
25
20
15
10
5
Cyclohexane oxidation results for gold-based catalysts at 423 K and 1.2–1.5 MPa
referenced to the autoxidation and the cobalt-catalyzed oxidation reaction. t4% = time
passed until 4% conversion was reached. Selectivities and K/A-ratio are determined at
4% conversion. For the cobalt-catalyzed oxidation reaction 1.5 ppm Co2+ was used.
Catalyst
t4% (min)
Selectivity (%)
K/A-ratio
K
A
CyOOH
Untreated reactor
passivated reactor
Co2+
Au/SBA-15
Au/Al2O3
104
184
46
43
21
17
28
18
20
23
20
35
28
48
43
42
45
52
47
11
29
17
7
19
15
8
1.23
0.75
0.35
0.65
0.43
0.44
0.44
0.43
Au/SBA-15
Au/Al2O3
Au/TiO2
Co2+
Untreated
Passivated
CyH
C
92
115
121
73
Au/TiO2
Au/SBA-15 and Co2+
Au/TiO2 and Co2+
0
77
15
0
60
120
180
t / min
240
300
This was most probably caused by chromium present in the
stainless steel reactor surface which is known to catalyze the
dehydration of CyOOH into Cy@O [3,12,39]. Passivation of the
reactor resulted in a lower obtained K/A-ratio (<1) typical for
autoxidation [13,37,40]. In the cases of the catalyzed reactions,
lower K/A-ratios were observed, i.e., 0.34 and 0.65 for Co2+ and
Au/SBA-15, respectively. Note that the value of 0.34 for the co-
balt-catalyzed reaction is in good agreement with the literature
values for CyOOH decomposition in the presence of cobalt salts,
e.g., 0.45 for cobalt(II)methyl-hexanoate [3,7]. The influence of
Fig. 3. Conversion versus time on stream during cyclohexane autoxidation (open
symbols) and catalyzed oxidation (solid symbols) at 150 °C and 1.2–1.5 MPa.
conversion. Comparing this to the cobalt-catalyzed oxidation it is
clearly visible that the SK/A is more favorable in the latter case
since a stable selectivity of ꢀ80% is maintained when the CyH
conversion increases from 5% to 10%. In terms of K/A-ratio, the
autoxidation in the untreated reactor exhibited the best perfor-
mance, yielding a K/A-ratio increasing of 1.23 at 4% conversion.
10
a
10
5
Caproic
OH-Caproic
Adipic
Valeric
CO
Caproic
b
OH-Caproic
Adipic
5
Valeric
CO
CO2
CO2
0
0
100
100
P
P
80
60
40
20
0
80
60
40
20
0
S
S
0
5
10
15
20
25
30
0
5
10
15
20
25
30
CCyH / %
C
CyH / %
10
8
Caproic
OH-Caproic
Adipic
Valeric
CO
10
d
c
Untreated
Passivated
Co2+
5
CO2
6
0
100
p
]
4
80
60
40
20
0
S
2
[O
2
0
0
5
10
15
20
25
30
0
60
120
180
240
300
C
CyH / %
t / min
Fig. 4. Product selectivities versus conversion during cyclohexane autoxidation at 150 °C and 1.2–1.5 MPa pressure; untreated reactor (a), passivated reactor (b), Co2+ (c), and
outlet oxygen concentration (d). SCy@O (e), SCyOH (h), SCyOOH ), and SC (s).
(
D
6

B.P.C. Hereijgers, B.M. Weckhuysen / Journal of Catalysis 270 (2010) 16–25
21
Caproic
a
10
5
10
5
Caproic
OH-Caproic
Adipic
Valeric
CO
b
OH-Caproic
Adipic
Valeric
CO
CO2
CO2
0
0
100
100
CyOH
Cy=O
CyOOH
C6
CyOH
Cy=O
CyOOH
C6
p
p
80
60
40
20
0
80
60
40
20
0
S
S
0
2
4
6
8
10
12
14
16
18
0
2
4
6
8
10
12
14
16
18
CCyH / %
CCyH / %
10
5
c
d
10
5
0
0
100
100
p
p
80
60
40
20
0
S
80
60
40
20
0
S
0
2
4
6
8
10
12
14
16
18
0
2
4
6
8
10
12
14
16
18
CCyH / %
C
CyH / %
Fig. 5. Product selectivities versus conversion during gold-catalyzed cyclohexane oxidation at 150 °C and 1.2–1.5 MPa pressure; Au/Al2O3 (a), Au/TiO2 (b), Au/SBA-15 (c), and
Au/SBA-15 + 1.5 ppm Co2+ (d).
100
b
a
2
1
0
80
A
K/
S
KA/ratio
60
0
0
5
10
15
20
0
5
10
15
20
CCyH / %
CCyH / %
Fig. 6. K/A selectivity (a) and ratio (b) development with increasing cyclohexane conversion at 150 °C and 1.2–1.5 MPa. Autoxidation (j), Co2+ (ꢁ), Au/Al2O3 (s), Au/TiO2
and Au/SBA-15 (e).
(D),
adding 1.5 ppm Co2+ to the Au/SBA-15-catalyzed oxidation reac-
tion is illustrated in Fig. 7. When combining a gold-based catalyst
with Co2+, both the SK/A and the K/A-ratio became affected.
Whereas the selectivity at 4% conversion is improved by 5% in
the presence of cobalt, the K/A-ratio decreased significantly from
0.64 to 0.43 in the case of Au/SBA-15 combined with Co2+. In the
case of Au/TiO2 this effect was less apparent but unambiguously
present.

22
B.P.C. Hereijgers, B.M. Weckhuysen / Journal of Catalysis 270 (2010) 16–25
100
80
60
40
20
0
8
6
4
2
3
2
1
0
120
100
80
60
40
20
0
]
2
K/A
[O
S
KA/ratio
0
240
0
4
8
12
16
0
60
120
180
t / min
CCyH / %
Fig. 8. Cyclohexane oxidation over Au/SBA-15 at 150 °C and 1.0–2.0 MPa in the
presence of hydroquinone. CyH conversion (d), formation of Cy@O (e), CyOH (h)
Fig. 7. Influence of cobalt and Au on the SK/A (solid symbols) and K/A-ratio (open
symbols) evolution with cyclohexane conversion at 150 °C and 1.2–1.5 MPa. Co2+
(N), Au/SBA-15 (d), and Au/SBA-15 + Co2+ (j). 1.5 ppm Co2+ was used.
and CyOOH (D), and outlet O2 concentration (+). HQ was added at t = 110 and
150 min.
3.3. Radical inhibition experiments
autoxidation process in the selective oxidation of cyclohexane.
Based on TEM analysis we concluded that the Au-based catalysts
under study exhibited a similar particle size distribution as the cat-
alyst materials employed for cyclohexane oxidation as reported in
the open literature [21–23,25]. Moreover, the applied method of
gold deposition is the most widely used procedure and has been
proven to yield stable catalyst materials with a small average par-
ticle diameter [18,41] which are highly active in a wide variety of
oxidation reactions, e.g., propene epoxidation [29,42–44] and par-
tial oxidation of methanol [45]. Based on this we reason that the
catalytic performance of these catalyst materials in cyclohexane
oxidation is expected to be at least comparable to what is reported
in the literature. By performing catalytic test reactions, it was ob-
served that the Au-based catalysts are fairly slow in the conversion
of cyclohexane, even slower than the unpassivated stainless steel
reactor and the reaction exhibits an initiation period. All the main
by-products that were found in the gold-catalyzed oxidation pro-
cess are known to form in a typical cyclohexane autoxidation reac-
tion [8,13]. Moreover, the gold-based catalysts were found to be
less selective at increasing conversion as compared to the commer-
cial alternatives and only show an improvement of the K/A-ratio as
compared to using a Co2+ initiator. When conducting the oxidation
experiments in the presence of a radical scavenger the conversion
rate became nil.
By performing a thorough product analysis of the cyclohexane
oxidation reaction under industrially relevant conditions it was
found that gold-based catalysts do not establish high K/A selectiv-
ities (>90%) at high conversions (>10%) as was reported in the open
literature [19–25]. Although the gold-based catalysts clearly play a
role in directing the oxidation mechanism, it is obvious that the
reaction proceeds through a radical-chain mechanism. This is in
full agreement with the results published by Xu et al. showing that,
at 70 °C, the oxidation reaction over supported Au, Pd, and Pt cat-
alysts would only proceed when t-butyl hydroperoxide was ini-
tially added as radical initiator [28]. A radical-chain mechanism
directly explains the slow initiation of the oxidation reaction by
the gold-based catalysts. Since initiation proceeds through a Ha-
ber-Weis cycle, the initiation rate strongly depends on the redox
potential of the metal. In this regard, gold is not likely to be an effi-
cient initiator. Also, it was already reported earlier that gold cata-
lysts are inefficient in promoting hydrocarbon oxidation by oxygen
due to the lack of coordination of hydrocarbons with Lewis acid
sites [41]. Our findings in cyclohexane oxidation by Au-based
catalysts are completely in agreement with above-mentioned
In order to find additional proof for the hypothesis that the
gold-catalyzed oxidation reaction in fact follows a radical-chain
mechanism, the free-radical scavenger hydroquinone (HQ) was
added to the proceeding oxidation reaction. Hydroquinone can re-
act according to Scheme 2 scavenging oxygen-centered radicals
present, thus terminating the radical-chain [33]. The effect of HQ
addition on the product formation and O2 consumption is pre-
sented in Fig. 8. At 110 min on stream a small amount of HQ, dis-
solved in CyH, was added to the reactor and the outlet O2
concentration increased rapidly from 3.1% to 5.0% decreasing to
3% thereafter. At 150 min on stream when a conversion of 3.5%
was reached, a larger amount of HQ (0.5 g dissolved in a 1:1
CyH:acetone mixture) was added and the outlet oxygen concentra-
tion increased sharply to 7.5%, almost equal to the inlet concentra-
tion. Also the amount of products in the liquid phase remained
constant between 165 and 195 min on stream. It is evident that
the cyclohexane conversion was completely inhibited after the
addition of sufficient HQ to scavenge all free-radicals. The strong
yellow color of the liquid-phase samples after HQ addition indi-
cated the formation of benzoquinone, confirming the reaction as
described in Scheme 2. A second experiment beheld the addition
of HQ initially to the reactor. After 180 min on stream a CyH con-
version of 0.015 mol% was reached. Both experiments together
proof that the presence of free-radicals is required for both the ini-
tiation and propagation phases of the oxidation reaction.
4. Discussion
4.1. Discrepancies with the literature results
In this paper we have compared the performances of gold-based
catalysts with those of the classical dissolved cobalt salt and the
a
b
+ R
O
+ HO
HO
O
O
O
HO
O
OH
O
-
R
-
OH
H2O
Scheme 2. Mechanism of free-radical scavenging by hydroquinone. HQ reacting
with a hydroxy (a) or an alkoxy radical and (b) forming benzoquinone.

B.P.C. Hereijgers, B.M. Weckhuysen / Journal of Catalysis 270 (2010) 16–25
23
theories. The reason for this strong deviation from the literature re-
sults can be attributed to the complex but required product analy-
sis procedure as was described in the experimental section. In
what follows we will elaborate on these encountered
complications.
we will discuss some influences of the Au-based catalysts on the
oxidation mechanism.
4.2.1. CO versus CO2 formation
One significant difference in the autoxidation and cobalt-cata-
lyzed reaction is the high SCO compared to SCO. CO formation
2
(1) The experiments on gold-based catalysts as described in the
literature are performed in a closed batch reactor, which is
cooled down and depressurized before collecting the prod-
uct mixture for analysis. When the reaction mixture is
cooled to room temperature before extracting the liquid-
phase sample, solubility of various polar molecules (e.g., for-
mic acid, hydroxy-caproic acid, and adipic acid) in cyclohex-
ane decreases dramatically. This complicates the product
analysis in that respect that they adsorb on the heteroge-
neous catalyst and the reactor interior, and care has to be
taken to ensure that they are analyzed. Even the addition
of a solvent after reaction and stirring for 30 min are not suf-
ficient to extract all organics adsorbed on the catalyst.
Therefore it is necessary to extract the liquid samples when
they are still at reaction temperature and dissolve them in a
suitable solvent before analysis.
(2) The nature of the by-products causes complications during
analysis as well. The main important side product, CyOOH,
is thermally unstable and readily decomposes upon GC
injection giving approximately equal amounts of cyclohexa-
none and cyclohexanol [46]. To overcome this problem low
temperature injection, reduction with triphenyl phosphine,
and derivatization methods are reported [13,21,46,47]. Fur-
thermore, it is known that mono- and poly-functionalized
organic acids can strongly adsorb on the GC column during
analysis, causing broad and overlapping peaks, or in the
worst case, do not elute from the column at all. In very
few cases the volatility of organic acids is sufficient to allow
a direct GC analysis [48]. The high boiling point of, for exam-
ple, adipic acid (610 K) will cause the molecules to (partially)
crystallize already in the GC injector. At conversions >5%,
adipic acid itself accounts for 5–10% of all products and
might not reach the detector. All the above illustrates the
urge of derivatizing –OH-containing compounds into tri-
methylsilyl or alkyl esters prior to analysis. This makes the
molecules less polar, more volatile, and thermally stable
and therefore enables quantification by GC [47,49].
was proposed by Hermans et al. to originate from the slow decom-
position of CH3–(CH2)4–Cꢀ@O under oxygen starvation causing the
formation of monofunctionalized acids namely valeric acid and
butyric acid [13]. Indeed the formation of CO coincides with the
appearance of valeric acid and butyric acid. CO2 formation was in
the same publication ascribed to the unimolecular elimination of
CO2 from an acyloxy radical (ꢀOC(@O)–(CH2)4C(@O)OH) leading,
via propagation reactions similar to hydroxy-caproic acid propaga-
tion, to the formation of glutaric acid. The origin of the acyloxy rad-
ical was found in propagation reactions of hydroxy-caproic acid.
Indeed, when no Au-based catalyst is present in the reactor an
induction period for both CO2 and glutaric acid is observed, consis-
tent with the mechanism presented by Hermans et al. However,
when there is a gold-based catalyst present, a significant increase
in CO2 formation is observed and in addition CO2 formation starts
at an earlier stage in the reaction even before glutaric acid is ob-
served. This can be clarified by the selective oxidation of CO to
CO2 by the gold-based catalysts. These catalyst materials are
known to be very active for this particular reaction at the applied
temperature and again confirm that the catalyst materials under
study are indeed active catalysts [50,51].
4.2.2. Hydroxy-caproic acid and adipic acid formation
Here we address the observed differences between the forma-
tion of the two main oxidation by-products, hydroxy-caproic acid
and adipic acid, in a cobalt- and gold-catalyzed cyclohexane oxida-
tion. Hermans et al. identified hydroxy-caproic acid as the first
by-product in the cyclohexane autoxidation from which other
by-products are formed. Adipic acid formation starts with the
abstraction from the alcohol functionality of hydroxy-caproic acid
by CyOOꢀ, leaving an
-hydroxy-alkyl radical. This reaction was
aH
a
calculated to face a high energy barrier. Therefore, the radical con-
version of hydroxy-carpoic acid to adipic acid is slow and occurs on
the timescale of hours [13]. This explains the observed convex
shape in the evolution of hydroxy-carpoic acid as a function of
the total amount of by-products (Fig. 9a). Adipic acid formation
is initially lower, but evolves more or less linearly. This means that
for the cobalt-catalyzed reaction, hydroxy-caproic acid is initially
built up and subsequently converted slowly into other oxidation
products, mainly adipic acid. This is in full agreement with and ex-
plains the observed fast initial drop in SK/A and subsequent stabil-
ization at ꢀ80% between 5% and 10% conversion (Fig. 6).
(3) Significant amounts of gas-phase products (2–4% of the
reacting CyH) are formed, which will be lost when depres-
surizing the reactor. Thus, in order to obtain a complete car-
bon balance, the gas phase should be monitored on-line as
well.
(4) Due to the continuous gas feed and sample extraction there
will be a significant evaporation loss of cyclohexane during
reaction. A complete mass balance therefore is not possible
to obtain directly from GC analysis and the total conversion
thus has to be calculated from the formation of products. In
other words, the use of a suitable internal standard in the
reactor is inevitable.
In the case of the gold-catalyzed reaction (Fig. 10b), hydroxy-ca-
proic and adipic acids are observed initially in almost equal
amounts. This indicates that adipic acid here is rather a primary
by-product than a secondary product [52]. Since both by-products
are initially observed in equal amounts, both formation rates must
be of similar magnitude. This implies that the dominant by-prod-
uct formation mechanism differs when a heterogeneous catalyst
is added. In the case of the Au-catalyzed oxidation reaction up to
ꢀ5% conversion, a linear decrease in SK/A is observed (Fig. 6). This
can be directly ascribed to the observed enhanced hydroxy-caproic
acid and adipic acid formation.
Not considering the above-mentioned phenomena will unfortu-
nately lead to an unnoticed incomplete mass balance, and there-
fore misleading high selectivities.
4.2. Influence of gold-based catalysts on the oxidation mechanism
4.2.3. Combining Au and Co2+
As described in the results section, a combination of cobalt and
gold as catalyst led to a performance being more or less the aver-
age of the fast cobalt and the slow gold catalysts (Fig. 4). However,
there are two remarkable effects of adding a gold catalyst. First of
all, it is observed that the reaction using a gold-based catalyst as
We observed that the Au-based catalysts have a clear influence
on the radical-chain mechanism and can compete with 1.5 ppm
dissolved Co2+, which merely accelerated the initiation, leaving
the product distribution typical for gold. In the following section

24
B.P.C. Hereijgers, B.M. Weckhuysen / Journal of Catalysis 270 (2010) 16–25
100
80
60
40
20
0
140
a
b
120
100
80
60
40
20
0
0
100
200
300
400
0
100
200
300
400
500
Σ By-products / mM
Σ By-products / mM
Fig. 9. Evolution of by-products, hydroxy-caproic acid (solid symbols) and adipic acid (open symbols), as the function of the sum of all by-products formed in CyH oxidation
at 150 °C and 1.2–1.5 MPa over Co2+ (h) and Co2+ + TiO2 (e) (a), Au/TiO2 (s), and Au/TiO2 + Co2+
) (b).
(
D
a brown/yellow color after reaction (and even after dissolution of
the reactor content in excess acetone) also indicates strong adsorp-
tion of organics. FT-IR spectra recorded of the spent catalysts re-
vealed additional bands around 2850–2950, 1710, and 1460 cmꢂ1
resembling the spectra of hydroxy-caproic acid and adipic acid
(Fig. 10) confirming the posed hypothesis.
a
b
c
d
e
5. Conclusions
Tramsion
f
The catalytic oxidation of cyclohexane with molecular oxygen
over gold-based catalysts was compared with the classic autoxida-
tion and cobalt-catalyzed processes. The different gold-based cata-
lysts under study did not show significant deviations in catalytic
performance. It was found that the gold-based catalysts are not
efficient in initiating the reaction, which resulted in a long induc-
tion period. The products and by-products formed during the
gold-catalyzed oxidation of cyclohexane are typical for those found
in the autoxidation and cobalt-catalyzed oxidation processes,
although a higher product yield could be obtained with both pro-
cesses. The K/A-ratios over gold-based catalysts are slightly higher
compared to those obtained with cobalt, however, lower compared
to the autoxidation and not near the 1.5 obtained easily by
catalytic CyOOH decomposition. In fact, the gold-based catalysts
performed poorly compared to the autoxidation and the cobalt-
catalyzed cyclohexane oxidation in terms of selectivity, K/A-ratio,
and initiation rate. The low selectivity of the gold-based catalysts
was largely caused by the enhanced formation of hydroxy-caproic
and adipic acids. Addition of a free-radical scavenger to the reac-
tion slurry initially and at 3.5% conversion completely inhibited
the oxidation reaction. Summarizing, we have found the experi-
mental evidence that cyclohexane oxidation in the presence of
gold-based catalysts is not a catalytic process, but proceeds
through a radical-chain mechanism well known from autoxidation.
0
4000
3500
3000
2500
2000
1500
1000
ν / cm-1
Fig. 10. FT-IR spectra of Au/Al2O3 fresh (a) and spent (b) and Au/SBA-15 fresh (c)
and spent (d), hydroxy-caproic acid (e) and adipic acid (f).
well as 1.5 ppm Co2+ exhibits slower initiation compared to Co2+ on
its own. Here the gold-based catalysts seemed to function as an
inhibitor rather than a catalyst. Secondly, it was observed that
when both catalysts were combined, although the influence of co-
balt is obvious, the oxidation reaction more or less mimicked the
product selectivity characteristics of the gold-catalyzed reaction.
This is illustrated by the hydroxy-caproic evolution and adipic acid
evolution shown in Fig. 9b which are significantly enhanced in the
presence of a gold-based catalyst. Both observations suggest a
somehow strong interaction between the Au-based catalyst and
the propagating radical intermediates, products, or by-products.
Strong adsorption of species on the catalyst surface enables com-
petition between the gold particles and the faster Co2+-mediated
cage reactions. Additionally, adsorption of hydroxy-caproic acid
on the catalyst surface might facilitate the
aH-abstraction from hy-
droxy-caproic acid by CyOOꢀ, by stabilizing the
a-hydroxy-alkyl
radical product, thus enhancing the adipic acid formation rate. As
described gold-based catalysts will be able to dictate the product
and by-product formation. This is in agreement with the observed
relative low initial CyOOH selectivity favoring product formation.
The strong sorption of reaction intermediates was confirmed by
introducing 0.75 g TiO2 in the cobalt-catalyzed reaction. The evolu-
tion of hydroxy-caproic and adipic acid concentration followed the
trend as is observed for the gold-catalyzed reaction (Fig. 9a). This
proofs that adsorption of intermediate oxygenate species strongly
influenced the by-product formation. Coloring of the white TiO2 to
Acknowledgments
This research was performed with the financial support from
the ACTS/ASPECT program. Rudy Parton and Corinne Daguenet
(DSM Research, Geleen, the Netherlands) are kindly thanked for
many fruitful discussions. Ad Mens and Vincent Koot (Utrecht Uni-
versity) are acknowledged for their assistance in designing and
building the experimental setup. Cor van der Spek (Utrecht Univer-
sity) is thanked for the TEM analysis.

B.P.C. Hereijgers, B.M. Weckhuysen / Journal of Catalysis 270 (2010) 16–25
25
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