Cyclohexene esterification-hydrogenation for efficient production of cyclohexanol

Article abstract of DOI:10.1039/d0gc03020a

A novel process based on cyclohexene esterification-hydrogenation for the production of cyclohexanol, the key intermediate in the production of ε-caprolactam, was devised and validated for the first time. In this process, cyclohexene obtained from the partial hydrogenation of benzene is esterified with acetic acid to cyclohexyl acetate, followed by hydrogenation to cyclohexanol. The experimentally determined equilibrium conversion of cyclohexene for cyclohexene esterification at the stoichiometric ratio is always ≥68% in the temperature range of 333-373 K over the commercial Amberlyst 15 catalyst, which is substantially higher than that of cyclohexene hydration. The apparent activation energy (E_a) for the esterification of cyclohexene with acetic acid is 60.0 kJ mol^?1, which is lower than that of cyclohexene hydration. In the hydrogenation of cyclohexyl acetate to cyclohexanol, high conversion of 99.5% and high selectivity of 99.7% are obtained on the La-promoted Cu/ZnO/SiO₂catalyst prepared by the co-precipitation method. This process shows both a high overall atom economy of 99.4% comparable to that of the cyclohexene hydration process and a much higher catalytic efficiency than the phenol hydrogenation process. On the basis of the above fundamental works, a pilot-scale demonstration unit with a capacity of 8000 tonnes per annum was developed and operated smoothly for more than 1000 h with no indication of deactivation.

Full text of DOI:10.1039/d0gc03020a

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Cyclohexene esterification–hydrogenation for efficient production
of cyclohexanol†
Yunfeng Zhu,a,c Liang Gao,a Langyou Wen,a Baoning Zong,*a Hao Wang b and Minghua Qiao b
Received 00th January 20xx,
Accepted 00th January 20xx
A novel process based on cyclohexene esterification–hydrogenation for the production of cyclohexanol, the key
intermediate for the production of ε-caprolactam, was devised and validated for the first time. In this process, cyclohexene
obtained from the partial hydrogenation of benzene is esterified with acetic acid to cyclohexyl acetate, followed by
hydrogenation to cyclohexanol. The experimentally determined equilibrium conversion of cyclohexene for cyclohexene
esterification at the stoichiometric ratio is always 68% in the temperature range of 333–373 K over the commercial
Amberlyst 15 catalyst, which is substantially higher than that of cyclohexene hydration. The apparent activation energy (Ea)
for the esterification of cyclohexene with acetic acid is 60.0 kJ mol–1, also lower than that of cyclohexene hydration. For the
hydrogenation of cyclohexyl acetate to cyclohexanol, high conversion of 99.5% and high selectivity of 99.7% are obtained
on the La-promoted Cu/ZnO/SiO2 catalyst prepared by the co-precipitation method. This process shows both high overall
atom economy of 99.4% comparable to the cyclohexene hydration process and much higher catalytic efficiency than the
phenol hydrogenation process. On the basis of the above fundamental works, a pilot-scale demonstration unit with a
capacity of 8000 t/a was developed and operated smoothly for more than 1000 h with no indication of deactivation.
DOI: 10.1039/x0xx00000x
1. Introduction
ε-Caprolactam is the monomer of nylon-6 (polycaprolactam).
The huge demand across the industries, such as packaging,
electrical and electronics, consumer goods and appliances, and
automotive greatly increase the overall ε-caprolactam
consumption in the nylon-6 industry.1,2 The estimated market
value of ε-caprolactam is projected to reach $18.6 billion by
SCHEME
1 Three typical industrialized processes for the production of
cyclohexanol/cyclohexanone starting from benzene: (1) cyclohexane oxidation,
2019.3 The industrial manufacture of ε-caprolactam involves a (2) cyclohexene hydration, and (3) phenol hydrogenation.
multiple-step transformation of benzene to cyclohexanone
and/or cyclohexanolthe latter can be facilely converted to the
However, the existing industrial processes for the production
former by dehydrogenation, and the transformation of
cyclohexanone to ε-caprolactam.1 Cyclohexanol is also a key
precursor of adipic acid, one of the monomers for nylon-66
(poly(hexamethylene adipamide)).4 Thomas and Raja have
skilfully developed a green one-step process to bridge the gap
between cyclohexanone and ε-caprolactam by using redox and
acid sites co-existing on the metal-doped nanoporous
aluminophosphates (AlPOs) in tandem, thus avoiding the use of
corrosive oleum and sulfuric acid and the generation of
unwanted ammonium sulfate.5
of cyclohexanone/cyclohexanol is far from safe, efficient, or
atom-economic. Currently, there are three industrialized
processes for this purpose, namely, (i) cyclohexane oxidation,
(ii) cyclohexene hydration, and (iii) phenol hydrogenation
(Scheme 1). The first process, though prevailing, is restricted by
a low conversion of 4.5% to sustain a high selectivity of 93%
(Table S1†). In addition, the cyclohexane/air mixture is of high
explosion risk, which has resulted in one of the most
devastating accidents in the history of petrochemical industry.6
The second process was developed by Asahi Chemical in the late
1980s. Albeit cyclohexene hydration gives high cyclohexanol
selectivity of 99.3%, the reaction is of low efficiency due to the
poor miscibility of cyclohexene with water (200 ppm at 298 K7
and 500 ppm at 393 K simulated by Aspen Plus). Moreover, the
equilibrium conversion of cyclohexene hydration is only about
12.7%.8,9 For the third process, while phenol hydrogenation to
cyclohexanol can be highly selective,10 the production of phenol
is complicated, which involves the alkylation of benzene with
propylene, cumene oxidation to cumene hydroperoxide, and
a. State Key Laboratory of Catalytic Materials and Reaction Engineering, Research
Institute of Petroleum Processing, Sinopec, Beijing 100083, P. R. China.
b. Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis
and Innovative Materials, Fudan University, Shanghai 200438, P. R. China.
c.State Key Laboratory of Safety and Control for Chemicals, SINOPEC Research
Institute of Safety Engineering, Qingdao, Shandong, 266101, P. R. China.
E-mail: zongbn.ripp@sinopec.com.
†Electronic Supplementary Information (ESI) available: [Tables S1-4 and Figures S1-
7]. See DOI: 10.1039/x0xx00000x
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cleavage of cumene hydroperoxide to phenol.11 Furthermore,
Journal Name
Herein, we report a novel cyclohexene eVsiteewrAifrtiicclaetOionlnin–e
analogous to cyclohexane oxidation, cumene oxidation must be hydrogenation process for the produDctOioI:n10.o10f39c/yDc0loGhCe03x0a2n0oAl
kept at a low conversion of 25% and is of high explosion risk. (Scheme 2). In this process, cyclohexene obtained from the
Fig. 1 illustrates the overall atom economy and per pass yield partial hydrogenation of benzene20 is esterified with acetic acid
of cyclohexanol (including cyclohexanone for the first process) to cyclohexyl acetate. The latter is then hydrogenated to
via these three processes starting from benzene based on the cyclohexanol. Solid acid catalysts, such as silica-supported HPA
industrial and literature data compiled in Table S1†. According (HSiW) or H3PW12O40, sulfated zirconia, and ion-exchange
to Trost, atom economy is defined as how much of the reactants resins, have been used for the esterification of olefins with
end up in the product.12 Herein, the reactant refers to benzene, acids.21−23 Amberlyst 15 is a commercially available, strongly
and the product refers to cyclohexanol. The overall atom acidic macroreticular polymeric resin based on cross-linked
economy is in the order of 83.7% (Process 1) < 95.5% (Process styrene, which is one of the most frequently used ion-exchange
3) < 99.3% (Process 2). However, because of the bottlenecks resins in acid-catalysed reactions.21 In particular, Saha and
either in selectivity (Processes 1 and 3) or in thermodynamics Sharma reported that Amberlyst 15 was more active than the
(Process 2), the per pass yield of cyclohexanone/cyclohexanol is Amberlite IR-120 genular resin, Engelhard F-24 acid-treated
low and is in the order of 3.7% (Process 1) < 5.1% (Process 2) < clay, and homogeneous p-toluene sulphonic acid catalysts in
14.9% (Process 3). In industry, the cyclohexanol yield is cyclohexene esterification with formic acid.24 Chakrabarti and
improved by recycling the unreacted feedstocks, which Sharma found that Amberlyst 15 was more active than
increases the energy demand.
Engelhard F-24 in cyclohexene esterification with acetic acid.13
Aside from three industrial processes mentioned above, However, the thermodynamic parameters of the reaction of
other processes, including cyclohexene esterification– esterification of cyclohexene with acetic acid to cyclohexyl
transesterification and esterification–hydrolysis, have been acetate has not been tackled,13 and the thermodynamic
investigated to produce cyclohexanol. However, for the former properties of cyclohexyl acetate are not available in the
process, ether formation and olefin formation are common side literature.
reactions during transesterification.13–15 Besides, like
On the other hand, although the hydrogenation of ester to
esterification reactions, transesterification reactions are typical, alcohol is not thermodynamically limited and is usually of high
equilibrium limited reactions.15,16 These drawbacks greatly conversion and selectivity,25 the reaction of cyclohexyl acetate
increase the cost of product separation, thus inhibiting this hydrogenation to cyclohexanol and the corresponding
process from practical application. For the latter process, the hydrogenation catalyst have not been explored. In fact, the
hydrolysis step in particular is very complex due to multiple number of catalyst systems allowing the production of alcohols
reactions, phase splitting, and mismatch between the reaction from ester hydrogenation is limited. The catalysts of choice,
conditions and separation conditions. Furthermore, this process which allow the highly selective conversion of esters to alcohols,
is highly energy demanding.17,18 Hence, Freund and co-workers are mostly based on Cu.26–28 The Cu-based catalysts are also
concluded that unless a right catalyst can be developed, this more cost-effective than the noble metal-based catalysts for
process is economically not viable.19
this purpose,29 which is an important factor to be considered for
a reaction with significant industrial background. ZnO is capable
of dispersing and stabilizing Cu nanoparticles to replace the
conventional copper chromite catalysts that are hazardous to
the environment.26,30 SiO2 is anticipated to be a suitable
support, as it is not only able to disperse Cu,26 but also weakly
acidic in nature and will not lead to considerable undesired
intramolecular/intermolecular dehydration or isomerization of
the hydrogenation products.
In this work, we experimentally determined the equilibrium
conversions for cyclohexene esterification with acetic acid at
different molar ratios and temperatures. On the basis of these
experimental data, the thermodynamic parameters of the
reaction of cyclohexene esterification with acetic acid to
cyclohexyl acetate were derived for the first time. Then, we
measured the reaction rates of cyclohexene esterification at
various acetic acid/cyclohexene ratios and temperatures. In
conjugation with the thermodynamic parameters, we derived
the chemical activity-based kinetic equation. The Cu/ZnO/SiO2-
based catalysts were systematically screened for the
hydrogenation of cyclohexyl acetate to cyclohexanol, and the
reaction conditions were optimized. These experimental results
show that the novel cyclohexene esterification–hydrogenation
process devoid of the shortcomings of the existing industrial
Fig.
1 Comparison of the overall atom economy and per pass yield of
cyclohexanol/cyclohexanone of the current industrial processes and the novel
cyclohexene esterification–hydrogenation process from benzene.
SCHEME 2 The novel cyclohexene esterification–hydrogenation process for the
production of cyclohexanol starting from benzene.
2 | J. Name., 2012, 00, 1-3
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processes is highly promising to produce cyclohexanol in a safe, that the reaction has reached the steady state. TheVrieewaAcrttiicolenOrnalitnee
DOI: 10.1039/D0GC03020A
efficient, and green manner. The additional merit of the is calculated as follows:
F
r  C6 [(
W
xC6E
xC6  xC6E
xC6E
xC6  xC6E
cyclohexene esterification–hydrogenation process is that
ethanol is yielded in stoichiometry, which is widely used as
antiseptic, solvent, and fuel or fuel additive. Note that the direct
transformation of acetic acid to ethanol requires expensive Pt,
Rh, or Pd noble metal catalysts,31–33 and the conversion and
selectivity to ethanol are much lower than those of ester
hydrogenation.
)out (
)in]
in which C6 represents cyclohexene, while C6E represents the
resulting ester. x represents the molar fraction, which is
dimensionless. F is the total molar flow rate in mol h–1. And W
represents the catalyst mass with unit in kg. Both the
thermodynamic and kinetic studies were repeated at least
twice.
2. Experimental
2.3. Hydrogenation of cyclohexyl acetate
2.1. Chemicals
2.3.1.
Catalyst preparation
If not specified, the chemicals were of analytic grade. Benzene,
cyclohexene (A.R.), acetic acid (A.R.), Cu(NO3)2·3H2O,
Zn(NO3)2·6H2O, Na2SiO3·9H2O, and NaOH were purchased from
Sinopharm Chemical Reagent. Amberlyst 15 and La(NO3)3·6H2O
were purchased from J&K. The gases were purchased from
Beijing Haipu Gas.
The Cu/ZnO-based hydrogenation catalysts were prepared by
the co-precipitation method. In a typical synthesis, 1 mol of
Cu(NO3)2·3H2O, 1 mol of Zn(NO3)2·6H2O, and 2 mol of
Na2SiO3·9H2O were respectively dissolved in 500 mL of
deionized water. The solutions were simultaneously added
dropwise into a beaker in 1 h at room temperature under
mechanical stirring at 500 rpm. The formed precipitates were
aged at room temperature for 12 h. Then, the precipitates were
filtered and washed 10 times with deionized water. The filter
cake was dried at 383 K for 24 h and calcined at 623 K with a
heating rate of 5 K min–1 for 12 h. The resulting catalyst
precursor was pelletized, crushed, sieved to 30–50 meshes, and
denoted as Cu1Zn1Si2 with the subscripts representing their
nominal molar ratios during preparation.
The CuxZn1Si2 catalysts (x = 0, 0.5, 0.75, 1, 1.25, 1.5, and 2)
and CuyZn2–ySi2 catalysts (y = 0.5, 0.75, 1, 1.25, 1.5, 1.75, and 2)
were prepared by the procedures described above except for
the amounts of the copper and zinc salts used. The
Cu1Zn1Si2La0.1 catalyst was also prepared in a similar manner
except for the addition of 0.1 mol of La(NO3)3·6H2O during the
preparation.
2.2. Esterification of cyclohexene with acetic acid
2.2.1.
Thermodynamic study
Thermodynamic studies were carried out on a METLER TOLEDO
RC1 reaction calorimeter fitted with an MP06 medium-pressure
500 mL-capacity glass autoclave. Prior to the reaction, 100 g of
Amberlyst 15 were dried at 378 K for 24 h until the content of
water in the catalyst was below 1% as determined by the Karl
Fischer method. The autoclave containing the catalyst was
sealed and pressurized to 0.6 MPa with N2 for air tightness test.
Then, the autoclave was pressurized to 0.3 MPa, and
cyclohexene and acetic acid with a total weight of 200 g were
pumped into the autoclave with
a prescribed acetic
acid/cyclohexene molar ratio. After being heated to the desired
temperature (333–373 K) for a certain period, the liquid was
sampled and analysed gas chromatographically. The attainment
of the equilibrium of esterification was judged based on the
detection that the compositions of the liquid in two successive
analyses were identical.
2.3.2.
Catalytic testing
The activity test was conducted on a continuous flow high-
pressure unit equipped with a stainless-steel fixed-bed tubular
reactor with a length of 1.2 m and an inner diameter of 1 cm. In
a typical testing, 10 g of the as-prepared catalyst diluted with
quartz sand of the same size to 12 mL was loaded in the reactor.
Both sides of the catalyst bed were packed with quartz sand.
The catalyst was activated in H2 flow (200 mL min–1 and 3 MPa)
at 493 K for 24 h at a ramping rate of 10 K min−1. After being
cooled down to the reaction temperature of 483 K, cyclohexyl
acetate was pumped into the reactor at 5.0 g h–1 with the
H2/ester molar ratio of 30 and a system pressure of 5 MPa. The
reaction reached the steady state after 12 h on stream. Then,
the product was sampled every 12 h for at least three times, and
each sample was analysed three times gas chromatographically.
The catalysts were evaluated at least in duplicate.
2.2.2.
Kinetic study
The kinetic studies were conducted on a plug-flow fixed-bed
reactor. The experimental set-up is depicted in Fig. S1†. The
tubular reactor with the inner diameter of 4 mm and length of
10 cm was made of 316L stainless steel. Amberlyst 15 was dried
at 378 K for 24 h until the content of water in the catalyst was
below 1% as determined by the Karl Fischer method. Then, 1.0
g of the catalyst (dp < 0.5 mm) diluted with quartz sand (0.1 mm
< dp < 0.8 mm) to avoid temperature runaway was loaded in the
reaction zone. The remaining space was filled with quartz sand.
Prior to the reaction, the catalyst bed was further purged with
N2 at 373 K to remove water. After being cooled down to below
the reaction temperature, cyclohexene and acetic acid with a
prescribed molar ratio was continuously pumped into the
reactor at a flow rate 60 g h–1. Each study lasted for 2–3 h,
during which the effluent was sampled periodically to ensure
2.4. Product analysis
The esterification and hydrogenation products were analysed
on an Agilent 7890A gas chromatograph equipped with a flame
ionization detector (FID) and an HP PONA capillary column (50
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m  0.2 mm  0.5 μm). During the analysis, the flow rate of the the stoichiometric acetic acid/cyclohexene raVtieiow Aortficl1e,Otnhlinee
N2 carrier gas was 0.2 mL min–1. The flow rates of H2 and air cyclohexene conversion is 79.1% at 333 K, which decreases
DOI: 10.1039/D0GC03020A
were 30 mL min–1 and 400 mL min–1, respectively. The gradually to 68.0% at 373 K. When the acetic acid/cyclohexene
temperatures of the injector and FID were both 523 K. The initial r a t i o i s e l e v a t e d t o 3 , t h e
temperature of the column was set at 333 K, which was raised
to 473 K at the heating rate of 5 K min–1 and maintained for 15
min and further raised to 513 K at a heating rate of 5 K min–1
and maintained for 50 min.
On the basis of the chromatographic results, the conversion
of cyclohexene during esterification was calculated as:
Conv.C6 (%) = (nC6,0 – nC6,t)/nC6,0 × 100.
The selectivity to cyclohexyl acetate was calculated as:
SC6E (%) = nC6E,t/nC6,0 × 100.
The conversion of cyclohexyl acetate during hydrogenation
was calculated as:
Conv.C6E (%)= (nC6E,in –nC6E,out)/nC6E,in × 100.
The selectivity to cyclohexanol was calculated as:
SC6OH (%) = nC6OH,out/nC6E,in × 100,
in which C6OH represents cyclohexanol.
Fig. 2 (A) Effects of reaction temperature and acetic acid/cyclohexene molar ratio
on cyclohexene conversion in cyclohexene esterification, and (B) a plot of lnK
2.5. Characterization techniques
N2 physisorption was conducted at 77 K on a Micromeritics
Tristar 3000 apparatus. Prior to the measurements, the acid
resin catalyst was outgassed at 343 K for 8 h. The bulk
composition of the catalyst was determined by the inductively
coupled plasma-atomic emission spectroscopy (ICP–AES;
Thermo Elemental IRIS Intrepid). Exactly 200 mg of the catalyst
was added to 20 mL of 25 wt% HNO3 solution and heated to
boiling. Then, 20 mL of 40% of HF was added and stirred
vigorously. The solution was diluted to 200 mL with distilled
water for analysis. CO2 chemisorption was conducted on a
Micromeritics 2750 chemisorption apparatus equipped with a
thermal conductivity detector (TCD). The XRD pattern was
acquired on a Philips X’Pert Pro powder X-ray diffractometer
using Cu Kα radiation ( = 0.15418 nm). The tube voltage was
40 kV, and the current was 40 mA. The 2 angles were scanned
from 10 to 70o at 10o min−1. TEM characterization and elemental
mapping were conducted on an FEI TECNAI F20 G2 electron
microscope operated at 200 kV, to which a Gatan GIF 2001
spectrometer was attached. The energy filter (EF) images were
recorded under the EFTEM mode. The width of the EF slit was
10 eV, the GIF incidence aperture was 0.6 mm, and the 3-
window method was applied during elemental mapping
collection.
against 1/T.
cyclohexene conversion amounts to 93.5% at 333 K, which
decreases to 85.1% at 373 K. Nevertheless, the cyclohexene
conversion at the acetic acid/cyclohexene ratio of 1 at 373 K is
still much higher than the conversion of 12.7% via the route of
cyclohexene hydration.8 In addition, the selectivity to cyclohexyl
acetate remains as high as 99.7% at 343 K, while decreases
gradually to 95.5% at 373 K owing to the occurrence of the
oligomerization/isomerization of cyclohexene in the presence
of the acid catalyst (Fig. S2†). These by-products have also been
identified in cyclohexene hydration.7
Assuming that the enthalpy change (

Ho) and entropy change
(
So) are constant in the temperature range investigated, the
relationship between the reaction equilibrium constant (K) and
the reaction temperature can be expressed as:
lnK  So /R (Ho /R)(1/T)
On the basis of the data in Fig. 2A, we obtained a good linear
relationship between lnK and 1/T (Fig. 2B). From the intercept
and slope of the line, the following thermodynamic parameters
for the esterification of cyclohexene with acetic acid to
cyclohexyl acetate are derived:

Ho = –22860.8

2825.4 J mol–
1
,   
So = –42.8 8.0 J mol–1 K–1, and Go = –22860.8 + 42.8T J
mol–1 in the temperature range of 333–373 K. It should be noted
that the
Go for this reaction is far more negative than that for
3. Results and discussion
3.1. Esterification of cyclohexene with acetic acid

cyclohexene hydration (365 J mol–1 at 333 K and 7339 J mol–1 at
373 K, calculated by HSC5.0 software), unambiguously
confirming that esterification of cyclohexene with acetic acid is
thermodynamically more favourable than cyclohexene
hydration.
Fig. 3 shows the reaction rates of cyclohexene esterification
at various acetic acid/cyclohexene ratios and reaction
temperatures on a fixed-bed reactor. The effects of external
We experimentally determined the equilibrium conversion of
cyclohexene at different acetic acid/cyclohexene molar ratios
and temperatures using Amberlyst 15 as the catalyst. As
plotted in Fig. 2A, the equilibrium conversion of cyclohexene
decreases with the increase in the temperature, reflecting that
the esterification of cyclohexene with acetic acid is
exothermic. The cyclohexene conversion is improved when
increasing the acetic acid/cyclohexene ratio as expected. At
and internal diffusion have been eliminated (Fig. S3†). The
reaction temperature drastically affects the esterification rate
4 | J. Name., 2012, 00, 1-3
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of cyclohexene, which in general doubles with a 10 K- coefficient of least square fitting (R2) is 0.996. As compiled in
View Article Online
DOI: 10.1039/D0GC03020A
increment in the reaction temperature. Besides, the reaction Table S2†, the reaction parameters derived from the LHHW-
rate decreases with the increase in the acetic acid/cyclohexene type kinetics model show that the adsorption equilibrium
r
a
t
i
o
.
constant of cyclohexene is much smaller than that of acetic acid.
Moreover, the apparent activation energy (Ea) of the
esterification of cyclohexene with acetic acid is 60.0 kJ mol–1,
which is lower than the Ea of 71.2 kJ mol–1 for cyclohexene
hydration.34 Therefore, these experimental results substantiate
that the esterification of cyclohexene with acetic acid is both
thermodynamically and kinetically advantageous to
cyclohexene hydration.
3.2. Hydrogenation of cyclohexyl acetate to cyclohexanol
Since the hydrogenation of cyclohexyl acetate to cyclohexanol
has also not been explored so far, we prepared the Cu/ZnO/SiO2
catalysts for this purpose. Firstly, we systematically examined
the effects of the contents of Cu and the Cu/Zn molar ratios on
the catalytic performances of the Cu/ZnO/SiO2 catalysts in the
hydrogenation of cyclohexyl acetate. As summarized in Table 1,
over the Cu0Zn1Si2 catalyst without Cu, the hydrogenation
reaction did not occur. When increasing the content of Cu while
fixing the Zn: Si molar ratio at 1: 2, the conversion of cyclohexyl
acetate increased drastically, reaching a maximum over the
Cu1Zn1Si2 catalyst, manifesting that Cu is the active centre for
ester hydrogenation. When the content of Cu was further
increased, the conversion of cyclohexyl acetate declined
monotonically, which is attributed to the agglomeration of
excessive Cu. A similar effect of the content of Cu on the
hydrogenation activity has been observed on the Cu/ZnO/Al2O3
Fig. 3 Influence of the acetic acid/cyclohexene molar ratio on the esterification
rate of cyclohexene at various reaction temperatures. The curves are simulated
by the LHHW-type kinetics model.
It is plausible that cyclohexene adsorbs much more weakly than
acetic acid on Amberlyst 15, considering that acetic acid can
interact strongly with the sulfonic acid groups on Amberlyst 15
via hydrogen bonding. Hence, the adsorption of cyclohexene is
the key factor that limits the reaction. The weak adsorption of
cyclohexene with respect to acetic acid on the same active sites
is substantiated by the fitting results of the reaction data in Fig.
3 using the Langmuir–Hinshelwood–Hougen–Watson (LHHW)
type kinetics model with the chemical activity coefficients
derived from the thermodynamic data. The correlation
Table 1 The effects of preparation method, Cu content, Cu/Zn molar ratio, La promotion, and WHSV on the catalytic performances of the Cu/ZnO/SiO2 catalysts in the
hydrogenation of cyclohexyl acetate to cyclohexanol.
Reaction conditions
Item
Catalyst
Conv. (%)
Sel. (%)
H2/ester
(molar ratio)
T (K)
P (MPa)
WHSV (h–1)
Cu content
Cu0Zn1Si2
483
483
483
483
483
483
483
483
483
483
483
483
483
483
483
483
483
483
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.7
0
0
Cu0.5Zn1Si2
Cu0.75Zn1Si2
Cu1Zn1Si2
Cu1.25Zn1Si2
Cu1.5Zn1Si2
Cu2Zn1Si2
Cu0.5Zn1.5Si2
Cu0.75Zn1.25Si2
Cu1Zn1Si2
Cu1.25Zn0.75Si2
Cu1.5Zn0. 5Si2
Cu1.75Zn0.25Si2
Cu2Zn0Si2
88.2
93.3
99.5
98.8
93.5
87.2
93.8
95.2
99.5
98.6
94.3
92.2
86.1
99.5
99.5
99.5
99.3
97.4
98.1
98.3
97.9
96.9
97.3
97.3
95.9
98.3
97.1
96.9
97.3
92.5
99.7
98.3
99.7
99.8
Cu/Zn ratio
La promotion
WHSV
Cu1Zn1Si2La0.1
Cu1Zn1Si2
Cu1Zn1Si2La0.1
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483
483
5
5
30
30
0.9
1.1
99.1
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99.1
D9O8I:.210.1039/D0GC03020A
catalysts for the hydrogenation of ethyl acetate.35 For ester
hydrogenation over the Cu-based catalysts, it is generally
accepted that while Cu0 dissociatively adsorbs H2, Cu+ may act
as electrophilic or Lewis acid sites to polarize the C=O bond of
the ester group via the electron lone pair on oxygen, thus
improving the reactivity of the ester group.27 Excessive content
of Cu in the catalyst will lead to reduced Cu dispersion and
consequently less surface Cu0 and Cu+ species,27,36 which is
adverse to the conversion of cyclohexyl acetate. When altering
the Cu/Zn molar ratio while fixing the (Cu + Zn): Si molar ratio at
2: 2, the conversion of cyclohexyl acetate evolved in a volcanic
shape. At low Cu/Zn ratio, there was no sufficient active centres
for ester hydrogenation, while at high Cu/Zn ratio, there was no
sufficient ZnO to disperse Cu, and thus an optimal Cu/Zn molar
ratio occurred at 1: 1. An optimal Cu/Zn ratio was also identified
for the CuZn–SiO2 bimetallic catalysts in the hydrogenation of
methyl acetate.37 Among the Cu/ZnO/SiO2 catalysts
investigated, the Cu1Zn1Si2 catalyst displayed the highest
conversion of 99.5% and the highest cyclohexanol selectivity of
98.3% under identical reaction conditions of 483 K, 5.0 MPa,
H2/ester molar ratio of 30, and weight hourly space velocity
(WHSV) of 0.5 h–1. As anticipated, there were only trace
amounts of the acid-catalysed by-products including ethyl
acetate, cyclohexane, and cyclohexyl ether.
acetate when increasing the WHSV on the Cu1Zn1Si2La0.1
catalyst. The HRTEM image of the Cu1Zn1Si2La0.1 catalyst in Fig.
Fig. 4 (A) The XRD patterns of the Cu1Zn1Si2 and Cu1Zn1Si2La0.1 catalysts, (B) the
HRTEM image, (C) the TEM image and particle size distribution histogram, (D) the
HAADF–STEM image and the corresponding EDS mappings of (E) Si, (F) Cu, (G) Zn,
(H) La, and (I) the overlapping of these elements of the Cu1Zn1Si2La0.1 catalyst.
To further improve the selectivity, we modified the Cu1Zn1Si2
catalyst during the co-precipitation process by adding 0.1 molar
fraction of La relative to Cu (denoted as Cu1Zn1Si2La0.1). The
modification effects of lanthanum oxide have been reported to
be originated from several aspects, such as the formation of
basic active sites and the improvement of the surface area of
the catalyst.36,38 The basic physicochemical properties of the
Cu1Zn1Si2La0.1 and Cu1Zn1Si2 catalysts are presented in Table
4B shows only the Cu(111) lattice fringes with the interplanar
spacing of 2.08 Å, while the lattice fringes of ZnO were not
identified, which is consistent with the XRD result. The TEM
image and particle size distribution histogram of the
Cu1Zn1Si2La0.1 catalyst in Fig. 4C shows that the average particle
size of Cu is 15.8 nm, which agrees well with the crystallite size
derived from XRD.
S3†. As anticipated, lanthanum oxide enhanced the basicity of
Figs. 4D–4I present the scanning transmission electron
microscopic-energy dispersive spectroscopic (STEM–EDS)
images of the Cu1Zn1Si2La0.1 catalyst. Fig. 4D is the HAADF–STEM
image, Figs. 4E–4H are the corresponding EDS mappings of Si,
Cu, Zn, and La, and Fig. 4I is the overlapping of Figs. 4E–4H.
Comparison of Fig. 4D with Fig. 4F readily leads to the
conclusion that the bright particles in Fig. 4D are originated
from Cu. Comparison of Fig. 4G with Figs. 4D and 4F reveals that
Zn is located preferentially on or in vicinity to Cu rather than
distributed randomly on SiO2, signifying that ZnO interacts
strongly with Cu. This conclusion is additionally substantiated
by Fig. 4I. On the other hand, the distribution of La is
homogeneous and does not show such a preference as that of
Zn.
From the experimental results demonstrated above, we
figured out the overall atom economy of 99.4% and the yield of
cyclohexanol per pass of 34.6% for the novel cyclohexene
esterification–hydrogenation process, as also demonstrated in
Fig. 1. The former is comparable to that of the most selective
cyclohexene hydration process, whereas the latter is more than
twice of that of the most efficient phenol hydrogenation
the catalyst (Fig. S4† and Table S3†), which further suppressed
the occurrence of the acid-catalysed reactions during the
hydrogenation of cyclohexyl acetate, which improved the
cyclohexanol selectivity up to 99.7% (Fig. S5†), while did not
impose an adverse effect on the conversion (Table 1).
Moreover, increasing the WHSV from 0.5 to 1.1 h–1 did not
change significantly the conversion and selectivity over the
Cu1Zn1Si2La0.1 catalyst (Table 1).
According to Table S3†, the specific surface area (SBET) of the
Cu1Zn1Si2La0.1 catalyst is larger than that of the Cu1Zn1Si2
catalyst. The XRD patterns of the Cu1Zn1Si2 and Cu1Zn1Si2La0.1
catalysts in Fig. 4A show that aside from the broad peak at 2

of ~25o from amorphous SiO2, only the diffraction peaks due to
metallic Cu (fcc Cu, JCPDS 04-0836) were identified, indicating
that ZnO is in the amorphous state on both catalysts. The phase
relating to La on the Cu1Zn1Si2La0.1 catalyst was also not
discerned, which is attributed to its low loading and/or high
dispersion. The crystallite sizes of Cu calculated by the Scherrer
formula in terms of X-ray line broadening are 18.0 nm and 14.8
nm for the Cu1Zn1Si2 and Cu1Zn1Si2La0.1 catalysts, respectively,
which may account for the high conversion of cyclohexyl
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process. It is evident that the cyclohexene esterification– is always >99% during more than 1000 h on stream, giving rise
View Article Online
hydrogenation process is highly advantageous in producing to the conversion of 99.8% and the cyclDoOheI:x1a0.n10o3l9s/eDl0eGcCti0v3it0y20oAf
cyclohexanol in a safe, efficient, and green manner.
99.4% on average. In addition, the ethanol selectivity is 99.5%
on average during the whole reaction span.
3.3. Pilot-scale cyclohexene esterification–hydrogenation
4. Conclusions
We successfully demonstrated the feasibility of the novel
cyclohexene esterification–hydrogenation process for the
production of cyclohexanol, the intermediate for the
production of ε-caprolactam. The bench-scale studies verified
that cyclohexene esterification is thermodynamically and
kinetically more favourable than cyclohexene hydration. And
the Cu1Zn1Si2La0.1 catalyst afforded high conversion and
selectivity in the hydrogenation of cyclohexyl acetate to
cyclohexanol. The cyclohexene esterification–hydrogenation
process was operated smoothly in a long term on a pilot-scale
demonstration unit. Moreover, this process not only shows high
overall atom economy comparable to the cyclohexene
hydration process, but also exhibits much higher catalytic
efficiency than the phenol hydrogenation process. This work
elegantly bridges the gap between the Asahi’s process of
benzene partial hydrogenation to cyclohexene and Thomas and
Raja’s process of one-step transformation of cyclohexanone to
ε-caprolactam, thus completing the last technological puzzle for
Fig. 5 (A) The long-term evolutions of the conversion of cyclohexene and the
selectivity to cyclohexyl acetate in the esterification of cyclohexene with acetic
acid on the pilot-scale reactive distillation reactor. Reaction conditions: reboiler
duty of 1050 W, total reflux at ambient pressure, acetic acid feed at the top of the
column at 2.59 kg h–1, cyclohexene feed at the middle of the catalytic section at
3.54 kg h–1, and total catalyst loading of 2.65 kg, and (B) the long-term evolutions
of the conversion of cyclohexyl acetate and the selectivities to cyclohexanol and
ethanol in the hydrogenation of cyclohexyl acetate on the pilot-scale fixed-bed
reactor. Reaction conditions: temperature of 473 K, pressure of 6.2 MPa, H2 flow
rate of 100 L min–1, cyclohexyl acetate feed rate of 870 g h–1, and catalyst loading
of 1.0 kg.
the development of
a next-generation process for the
manufacture of nylon-6 from benzene in a safe, efficient, and
green manner.
Taking into account of the reaction characteristics of
cyclohexene esterification, we transferred this reaction onto a
reactive distillation reactor, which is capable of breaking
through the thermodynamic limitation imposed on the
conversion while retaining the high reaction rate for the
exothermic reaction.39 On conventional slurry-phase reactor or
fixed-bed reactor, the intrinsic conflict between the conversion
and reaction rate in cyclohexene esterification cannot be
disentangled. On the basis of the above experimental
thermodynamic and kinetic data and Aspen Plus simulations, a
reactive distillation reactor was built for cyclohexene
esterification, on which the operation parameters were further
experimentally optimized, which are out of the scope of the
present work. The optimized operation parameters for the
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Basic Research
Program of China (2012CB224800), the Natural Science
Foundation of China (20872035), and Technology Development
Program of SINOPEC, China (S411063).
reactive distillation reactor are presented in Table S4
corresponding temperature and composition profiles in the
reactive distillation reactor are illustrated in Fig. S6 . Then, we
established a pilot-scale demonstration unit with a capacity of
8000 t/a using the reactive distillation reactor for cyclohexene
esterification in conjunction with a fixed-bed reactor for ester
hydrogenation at Baling Petrochemical Company, SINOPEC (Fig.
†. And the
Notes and references
†
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S7
conversion is further improved to >99.2% with high selectivity
to cyclohexyl acetate of 99.0% (Fig. 5A). Moreover, the
†). On the reactive distillation reactor, the cyclohexene
~
reaction proceeds smoothly for more than 1000 h on stream
with no indication of deactivation.
The cyclohexyl acetate produced from the reactive distillation
reactor was directly fed into the successive fixed-bed reactor for
hydrogenation. Fig. 5B shows that the conversion of cyclohexyl
acetate is always close to 100%, and the cyclohexanol selectivity
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DOI: 10.1039/D0GC03020A
Graphical contents entry
Cyclohexene esterification–hydrogenation for efficient production of cyclohexanol
Yunfeng Zhu, Liang Gao, Langyou Wen, Baoning Zong,* Hao Wang, and Minghua Qiao
A novel cyclohexene esterification–hydrogenation process for green and efficient production of
cyclohexanol is proposed and successfully demonstrated.