Selective reduction of phenol derivatives to cyclohexanones in water under microwave irradiation

Article abstract of DOI:10.1039/c2nj20990j

Selective reduction of phenol to cyclohexanone over the Pd/C catalyst in the presence of a hydrogen source of HCOONa/H₂O has been studied. Surprisingly, phenol was transformed efficiently to cyclohexanone in an excellent yield of above 98% under microwave irradiation. The influence of some parameters like reaction temperature, time and amount of hydrogen donor, as well as the reaction pathway has been discussed. The combination of microwave irradiation and HCOONa/H₂O was certified to be effective for the reduction of phenol as well as its derivatives to their corresponding cyclohexanones. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj20990j

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PAPER
Selective reduction of phenol derivatives to cyclohexanones in water
under microwave irradiation
Haiyang Cheng,ab Ruixia Liu,a Qiang Wang,a Chaoyong Wu,a Yancun Yuab and
Fengyu Zhao*ab
Received (in Montpellier, France) 27th November 2011, Accepted 13th February 2012
DOI: 10.1039/c2nj20990j
Selective reduction of phenol to cyclohexanone over the Pd/C catalyst in the presence of a
hydrogen source of HCOONa/H2O has been studied. Surprisingly, phenol was transformed
efficiently to cyclohexanone in an excellent yield of above 98% under microwave irradiation.
The influence of some parameters like reaction temperature, time and amount of hydrogen donor,
as well as the reaction pathway has been discussed. The combination of microwave irradiation
and HCOONa/H2O was certified to be effective for the reduction of phenol as well as its
derivatives to their corresponding cyclohexanones.
catalysts under the optimized conditions.7–11 The gas phase
Introduction
phenol hydrogenation is usually performed at high tempera-
ture over supported Pd7–10 or Pt11 catalysts, it was reported
Water is the most abundant, cheap, safe, and environmentally
benign solvent in nature and the study of organic reaction in/
that the selectivity of cyclohexanone could be reached up to
95% over 5% Pt–0.4–4% Cr/C11 and nanotubular titanate
on water is an important theme of current research from the
point of green chemistry.1 Ordinarily, the solubility of organic
supported Pd catalysts9 at 200 1C. However, gas phase hydro-
genation suffers from some disadvantages like harsh reaction
substrates is the key factor for the performance of chemical
transformations in water. For example, the hydrogenation of
conditions and the catalyst deactivation induced by the
unavoidable coke deposition.12 Much interest has been
phenol could be carried out in the aqueous phase successfully
at a moderate temperature for that phenol is completely water
focused on the liquid phase phenol hydrogenation because
soluble at temperatures higher than 66 1C. But, the products of
savings.2,13,14 However, the selectivity of cyclohexanone often
cyclohexanone and cyclohexanol are only slightly soluble in
water under the reaction conditions, which may facilitate the
product separation from the aqueous phase.2 Cyclohexanone
can be further hydrogenated to cyclohexanol in the consecutive
is widely utilized in industrial production of caprolactam and
of the relatively mild conditions and the cost and energy
decreases with phenol conversion, because cyclohexanone
reaction. Thus, it is a great challenge to obtain a high selectivity
(495%) at an elevated phenol conversion (480%).13,15 Recently,
some effort has been made to improve the yield of cyclohexanone.
adipic acid, which are the monomers for manufacturing nylon-6,
nylon-6,6 and polyamide resins.3 Traditionally, cyclohexanone
is produced via the oxidation of cyclohexane and the hydro-
For example, Titirici et al. investigated the phenol hydrogenation
genation of phenol. The oxidation of cyclohexane requires high
using a hydrophilic carbon support palladium catalyst in the
temperature, and the yield of cyclohexanone is quite low
aqueous phase, and a 95% selectivity of cyclohexanone was
(o10%) due to the low conversion and the formation of some
the hydrophilic carbon support, it pushes the hydrophobic cyclo-
The hydrogenation of phenol is usually going through two
hexanone away from the surface of Pd nanoparticles, inhibiting its
different processes: (1) phenol is hydrogenated to cyclohexanol
obtained at 99% conversion of phenol, which was attributed to
byproducts like cyclohexanol and cyclohexyl hydroperoxide.4
further hydrogenation.16 Moreover, Han et al. reported that the
and then the produced cyclohexanol dehydrogenates to cyclo-
combination of Pd catalysts and solid Lewis acids was an efficient
hexanone at high temperatures;5,6 (2) phenol is selectively
catalytic system for the hydrogenation of phenol to cyclohexanone
hydrogenated to cyclohexanone directly over the suitable
in CH2Cl2 and compressed CO2, a selectivity of cyclohexanone
above 99.9% was obtained at the complete conversion of phenol.17
Most recently, Wang et al. researched the phenol hydrogenation
using mesoporous carbon nitrides supported palladium nano-
particles (Pd@mpg-C3N4) in the aqueous phase, and 499%
selectivity of cyclohexanone was obtained at 499% conver-
sion of the phenol under the mild conditions.18 Corma et al.
reported that Pd supported on hydroxyapatite, carbon and
a State Key Laboratory of Electroanalytical Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P. R. China. E-mail: zhaofy@ciac.jl.cn;
Fax: +86-431-8526-2410; Tel: +86-431-8526-2410
b Laboratory of Green Chemistry and Process, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun
130022, P. R. China
c
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alumina are chemoselective catalysts for the hydrogenation of
phenol to cyclohexanone.19 Furthermore, it was reported that
the selectivity to cyclohexanone could be improved in the
presence of dense phase carbon dioxide,12,17,20–22 because the
presence of CO2 could prohibit the further hydrogenation of
cyclohexanone to cyclohexanol.21
(TDX-01, 2 m) and identified with pure H2, O2, CO and CO2 to
confirm the production of hydrogen and CO during the reaction.
Hydrogenation of phenol with molecular hydrogen
The hydrogenation reaction with molecular hydrogen was
performed in a 50 ml Teflon-lined high-pressure stainless steel
batch reactor with oil heating. Phenol (0.625 mmol), 10% Pd/
C (5.3 mg), deionized water (2 ml) and H2 (2 MPa) were
introduced into the reactor. The reaction was performed at
80 1C with a stirring speed of 900 rpm, and the following
procedures are the same as described above.
In this work, a new method was developed for the selective
reduction of phenol and its derivatives with an alternative
hydrogen source of HCOONa/H2O under microwave irradia-
tion. That the microwave irradiation could promote some
organic reactions has been reported and well reviewed in the
literature.1 Under microwave irradiation, a,b-unsaturated
carbonyl compounds could be transformed to saturated carbonyl
compounds over the supported palladium catalyst, and 100%
chemoselectivity was achieved in the presence of hydrogen source
of MeOH/HCOOH/H2O (1: 2 : 3),23 and ketones could be
transformed to alcohols with a yield of 98% in the presence
of catalytic amounts of lithium isopropoxide and 2-propanol.24
Herein, phenol and its derivatives were reduced efficiently and
selectively to their corresponding cyclohexanones, and above
98% yield to cyclohexanone was achieved at the complete
conversion of phenol in the presence of HCOONa and H2O under
microwave irradiation. We offer a safe, facile, and environmentally
benign process for the production of cyclohexanones.
Results and discussion
HCOOH–NEt3 azeotropic mixture and HCOONa as alterna-
tives to molecular hydrogen have attracted more attention in
the catalytic transfer hydrogenations.25,26 Herein, the reduction
of phenol in aqueous solution was studied under the different
conditions, and the results are shown in Table 1. When the
reaction was carried out in an autoclave with oil heating and
molecular hydrogen as the reducing reagent, the selectivity to
cyclohexanone was lower than 80% and it decreased with
hydrogen pressure (entries 1 and 3), as well as it decreased with
conversion irrespective of the hydrogen pressure (entries 1–4).
By comparison, when HCOONa was used as the reducing
reagent, the selectivity to cyclohexanone reached about 98%
at a similar conversion (entries 3 and 6), however, the turnover
frequency (TOF) was much lower than that obtained using
molecular hydrogen (entries 1–6). It was assumed that the
hydrogen was produced from the formate and proton supplied
by water as described in the literature (eqn (1)),27,28 and then
the hydrogenation occurred through the produced hydrogen
molecules (eqn (2)). To confirm this, the reduction of phenol
with HCOONa/H2O as the hydrogen source was performed in
an open flask and refluxed with N2 flow, the result showed a
much low conversion (o1.0%, entry 7), it directly confirmed
that the transfer hydrogenation was insignificant, but the
hydrogenation should be more remarkable under the reaction
conditions. Moreover, hydrogen was detected in the reaction
product (gas phase) by TCD analysis. The above results
demonstrated that the reduction of phenol catalyzed with
the Pd/C catalyst in the presence of HCOONa in water was
going through hydrogen production (eqn (1)) and conventional
hydrogenation reaction (eqn (2)), rather than a hydrogen transfer
reaction. The concentration of hydrogen produced from organic
hydrogen donor HCOONa and water was lower compared to
the case of using hydrogen gas. Lower hydrogen concentration
may disfavor the step of hydrogenation of 1-hydroxycyclohexene
to cyclohexanol. Thus the relatively lower TOF and higher
selectivity to cyclohexanone were obtained (entries 1–6). In
addition, the effect of the microwave irradiation was checked
for the present reaction, as seen the reaction rate under micro-
wave irradiation was about 30 times higher than those under
conventional heating conditions (entries 5, 6, 8), furthermore, a
98% selectivity to cyclohexanone at the complete conversion of
phenol was achieved in the presence of HCOONa (entry 8).
Under microwave irradiation, the production of hydrogen via a
decomposition process (eq. (1)) could be improved remarkably,29
as well as the hydrogenation of phenol (eqn (2)) was accelerated
Experimental
Materials
HCOONaꢀ2H2O, diethyl ether, toluene and all the substrates
are of analytical grade and used without further purification.
5% Pd/C, 5% Pd/Al2O3, 10% Pd/C, 5% Pt/C, 5% Ru/C, and
5% Rh/C were purchased from WAKO. Gases of H2
(99.999%), O2 (99.9%), CO (99.999%) and CO2 (99.999%)
were used as delivered.
Reduction of phenol in the presence of HCOONa/H2O
The microwave-assisted reactions were performed in a 10 ml
quartz tube under microwave irradiation (Initiator, Biotage,
Sweden). In
a typical reaction, phenol (0.625 mmol),
HCOONaꢀ2H2O (1.875 mmol, 3 eq.), 10% Pd/C (5.3 mg),
and deionized water (2 ml) were added to the quartz tube.
Then, the reaction vessel was sealed and the reaction was
carried out under microwave irradiation at 80 1C with a stirring
speed of 900 rpm. After the reaction, the mixture was cooled and
extracted with diethyl ether and the resulted organic solution was
analyzed by a gas chromatograph (GC-shimadzu-2010, FID,
capillary column, Rtx-50 30 m ꢁ 0.25 mm ꢁ 0.25 mm) and
identified by gas chromatograph/mass spectrometry (GC/MS,
Agilent 5890). Toluene as the internal standard substance was
mixed to the sample in order to determine the conversion level and
selectivity. The conversion was calculated by moles of phenol
consumed divided by initial moles of phenol used, and the
selectivity was determined by moles of product i divided by total
moles of product obtained. Cyclohexanone and cyclohexanol are
the only reaction products observed under the experimental
conditions used, and the carbon balance values calculated from
phenol, cyclohexanone and cyclohexanol are around 100%. The
gas phase sample was collected and analyzed by a TCD detector
c
1086 New J. Chem., 2012, 36, 1085–1090
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Table 1 Results for the reduction of phenol under different conditions with 10% Pd/Ca
Selectivity (%)
Cyclohexanone
TOFb/minꢂ1
Cyclohexanol
Entry
Hydrogen source
Heating method
Time/min
Conv. (%)
1
2
3
4
0.2 MPa H2
0.2 MPa H2
2 MPa H2
2 MPa H2
HCOONa
HCOONa
HCOONa
HCOONa
CH
CH
CH
CH
CH
CH
CH
MW
60
90
45
90
120
300
300
15
58.1
84.3
66.0
100
32.2
63.6
1.0
11.4
11.1
17.3
—
3.2
2.5
79.4
69.8
65.1
14.7
99.8
98.1
100
20.6
30.2
34.9
85.3
0.2
1.9
—
2.0
5c
6c
7c,d
8c,e
0.04
—
100
98.0
a
Reaction conditions: phenol 0.625 mmol, 10% Pd/C 5.3 mg, substrate/catalyst = 125/1, H2O 2 ml, 80 1C. The reactions were performed in a
sealed Teflon-lined high-pressure stainless steel batch reactor (50 ml). CH: conventional heating method, MW: microwave irradiation. Turnover
b
frequency; which was given as the overall rate of phenol conversion normalized by the number of active sites over the specified time. The number of
active sites was calculated by (the metal dispersion) ꢁ (the total number of supported metal atoms). The Pd dispersion (D) was calculated from the
following equation: dispersion = 6(vm/am)/d, where vm and am are equal to 14.70 A3 and 7.93 A2, respectively, and the average diameter (d) of Pd
c
d
particles was determined to be 10.5 nm through the measurement from the TEM images. HCOONaꢀ2H2O 1.875 mmol. The reaction was
e
performed in an open flask and refluxed under N2 flow under the similar conditions. The reaction was performed in a sealed quartz tube (10 ml)
under microwave irradiation.
significantly. So the hydrogenation of phenol was performed
very fast and the high selectivity to cyclohexanone could be
obtained under microwave irradiation in the presence of
HCOONa and water. Herein, HCOONa was selected as the
hydrogen donor, and several parameters were discussed for the
reduction of phenol under the microwave irradiation.
ð1Þ
ð2Þ
The reaction temperature exhibited a large effect on the reaction
Fig. 2 Variation of conversion and selectivity with reaction time on
rate, with increasing temperature from 60 to 90 1C, the conversion
the reduction of phenol under microwave irradiation. Reaction
increased from 13.3 to 96.0% as shown in Fig. 1. When the
conditions: phenol 0.625 mmol, 10% Pd/C 5.3 mg, substrate/catalyst =
temperature was further enhanced up to 110 1C, the conversion
decreased, which may be attributed to the thermodynamic
125/1, HCOONaꢀ2H2O 1.875 mmol, H2O 2 ml, 80 1C. (’) Conversion of
phenol and selectivity to (J) cyclohexanone; (n) cyclohexanol.
limitations30 and/or to desorption of phenol and catalytically
active hydrogen from the catalyst surface at high temperature.31,32
The plot of phenol conversion and product selectivity versus
The selectivity to cyclohexanone changed slightly with the increase
reaction time was shown in Fig. 2. The reaction was very fast
in temperature under conditions used.
and cyclohexanone was produced as the predominant product
(99.8% in selectivity) until the conversion reached 86%, and
then cyclohexanone slowly transferred to cyclohexanol but the
selectivity was still above 98% at the complete conversion of
phenol after reaction for 15 min. Cyclohexanone and cyclo-
hexanol were produced throughout the reaction progress, but
1-hydroxycyclohexene was not detected. Based on the above
results and literature,13,33 a possible reaction pathway for
phenol reduction in the present system was discussed. According
to Scheme 1, 1-hydroxycyclohexene was formed as an inter-
mediate which might be easily converted into either the
cyclohexanone via isomerization (step II) or the cyclohexanol
via further hydrogenation (step III). The cyclohexanol should
not form from 1-hydroxycyclohexene (step III) for that the
selectivity to cyclohexanol was lower than 0.2% at the first
Fig. 1 Influence of temperature on the reduction of phenol under
7.5 min, but produced from the subsequent hydrogenation of
microwave irradiation. Reaction conditions: phenol 0.625 mmol, 10%
cyclohexanone (step IV) as its selectivity increased with a
Pd/C 5.3 mg, substrate/catalyst = 125/1, HCOONaꢀ2H2O 1.875 mmol,
consumption of cyclohexanone after the conversion of phenol
reached 86%. Step II that is isomerization of 1-hydroxycyclohexene
H2O 2 ml, 5 min. (’) Conversion of phenol and selectivity to (J)
cyclohexanone; (n) cyclohexanol.
c
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Table 3 Results of reduction of phenol with different catalysts under
microwave irradiationa
Selectivity (%)
Conversion TOF/
minꢂ1
Cyclohexanone Cyclohexanol
Entry Catalyst (%)
1
2
5% Pd/C 8.9
5% Pd/ 18.3
Al2O3
35.4
—
99.8
99.9
0.2
0.1
3
10% Pd/ 61.4
C
5% Pt/C ꢂ(84.1)
5% Ru/C ꢂ(39.8)
5% Rh/C 0.1 (76.9)
144.9
99.8
0.2
4b
5b
6b
—
—
—
ꢂ(38.1)
ꢂ(1.5)
ꢂ(61.9)
ꢂ(98.5)
ꢂ(34.2)
Scheme 1 Proposed route for hydrogenation of phenol.
100 (65.8)
a
to cyclohexanone should be much faster than step I that is
hydrogenation of phenol to 1-hydroxycyclohexene. However,
it should be noted that, when molecular hydrogen was used,
cyclohexanol was formed with a selectivity above 20% even at
conversion lower than 70% (entries 1 and 3 in Table 1),
suggesting that step III may happen, but step IV should be
much faster compared with the cases of HCOONa.
Reaction conditions: phenol 0.625 mmol, substrate/catalyst = 125/1,
b
HCOONaꢀ2H2O 1.875 mmol, H2O 2 ml, 5 min, 80 1C. The data
in parentheses are the results of reduction of phenol with 0.2 MPa H2
in 15 min.
The effect of the amount of HCOONa was checked and the
results are illustrated in Table 2. The selectivity to cyclohexanone
was independent of the amount of HCOONa, and the values
were above 98% at any alternative condition. But, the activity
increased with HCOONa amount (entries 1 and 5, 4 and 7, 6
and 8). The conversion of phenol was about 50% after reaction
for 60 min and it did not change with extending reaction time
when 1 eq. HCOONa was used (entries 2 and 3), because the
hydrogen donor was not enough to supply sufficient hydrogen
for the reaction. When the amount of HCOONa was increased
to 2 and 3 eq., the conversion reached 100% after reaction
proceeded for 30 and 15 min, respectively (entries 5 and 7).
It suggests that 2 eq. of HCOONa were required for supplying
enough hydrogen to reduce phenol to cyclohexanone from the
stoichiometry of the reaction.
Fig. 3 TEM images of (a) 5% Pd/C and (b) 10% Pd/C catalysts.
results were also reported that the TOF increased with decreasing
Pd crystallite size for the phenol hydrogenation in vapor phase
over the Pd/CeO2 catalyst.7 Moreover, Pd presented much higher
activity than those of Pt, Ru, Rh catalysts, as for 5% Ru/C, Pt/C
and Rh/C, they did not show any activity in the present catalytic
system with HCOONa (entries 4–6), even though Ru/C, Pt/C
and Rh/C were effective catalysts for the hydrogenation of
phenol with molecular hydrogen as shown in Table 3. It was
reported that the formate could decompose to produce hydrogen
over the Pd/C catalyst in water at reaction temperatures from
70 to 140 1C.27,28 However, such a catalytic activity may not
occur over the supported Pt, Ru and Rh catalysts under present
reaction conditions.
Under the microwave irradiation, the activities of several
noble metal catalysts were checked and compared. As per the
results listed in Table 3, the conversion was 8.9% and the
selectivity to cyclohexanone was 99.8% over 5% Pd/C, while
the conversion increased to 18.3% with a similar selectivity of
cyclohexanone over 5% Pd/Al2O3, but the conversion reached
up to 61.4% with a similar cyclohexanone selectivity over the
10% Pd/C catalyst, indicating that the support and Pd loading
had a large influence on the reaction rate. As shown in Fig. 3,
the average Pd particle size was about 17.7 nm for 5% Pd/C
and 10.5 nm for 10% Pd/C from the TEM images, suggesting
that the particle size affected the catalytic activity. The similar
The efficiency of the present catalytic system was examined
for several phenol derivatives under the similar reaction
conditions, and the results are shown in Table 4. As can be
seen, the ring hydrogenation of these substrates could be performed
to a certain extent within 40 min, and the corresponding
Table 2 Effect of the amount of HCOONa for reduction of phenol under microwave irradiationa
Selectivity (%)
Cyclohexanone
Cyclohexanol
Entry
Mole ratio of HCOONa/phenol
Time/min
Conversion (%)
1
2
3
4
5
6
7
8
1
1
1
2
2
3
3
4
30
60
90
15
30
5
35.9
49.3
49.3
75.7
99.3
61.4
100
99.9
99.8
99.5
99.8
99.2
99.8
98.0
99.4
0.1
0.2
0.5
0.2
0.8
0.2
2.0
0.6
15
5
68.8
a
Reaction conditions: phenol 0.625 mmol, 10% Pd/C 5.3 mg, substrate/catalyst = 125/1, H2O 2 ml, 80 1C.
c
1088 New J. Chem., 2012, 36, 1085–1090
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Table 4 Results of reduction of phenol derivatives under microwave irradiationa
Entry
Substrate
Time/min
Conversion (%)
Selectivity (%)
1
2
3
4
10
30
10
15
30.1
59.4
76.9
95.4
5
6
40
20
40
12.4
20.3
33.2
7
a
Reaction conditions: substrate 0.625 mmol, 10% Pd/C 5.3 mg, substrate/catalyst = 125/1, HCOONaꢀ2H2O 1.875 mmol, H2O 2 ml, 80 1C.
Table 5 Recycling of 10% Pd/C catalysta
cyclohexanones were produced as the main product with the
selectivity above 92% except for o-methoxyphenol (86.3%).
o-Methyl cyclohexanone was produced as the sole product
with 100% selectivity (entries 1 and 2), and p-methyl cyclo-
hexanone was produced with a selectivity of about 93%
(entries 3 and 4), and o- and p-methoxycyclohexanone were
produced with selectivity of about 86% and 97% from the
hydrogenation of o- and p-methoxyphenol, respectively
(entries 5–7). It was clear that the selectivity to cyclohexanones
was independent of the reaction conversion, but depended on
the molecular structure largely. The reaction rate and product
selectivity depended significantly on the chemical nature of the
substituent and its position relative to the ꢂOH group. Steric
effects resulted in a lower conversion for the substitution at
the ortho-position compared with the para-position (entries 1
and 3; 5 and 7). The effect of the substituent is consistent
with the Hammett parameters (s). The electron-releasing
substituents lowered the reactivity of phenol ring, and the
conversion of phenol with the MeO group (s: ꢂ0.27) was
much lower than that with the methyl group (s: ꢂ0.17) (entries
4 and 7). Moreover, the phenolic substrates with electron-
withdrawing group like o-chlorophenol and p-nitrophenol
have also been examined. But for o-chlorophenol, the hydro-
dechlorination of o-chlorophenol to phenol occurred firstly;
while for p-nitrophenol, p-aminophenol was mainly produced
under the reaction conditions.
Selectivity (%)
Cyclohexanone
Cyclohexanol
Run
Conversion (%)
1
2
3
76.3
37.7
32.9
99.2
99.4
99.5
0.8
0.6
0.5
a
Reaction conditions: phenol 0.625 mmol, 10% Pd/C 5.3 mg, substrate/
catalyst = 125/1, HCOONaꢀ2H2O 1.875 mmol, H2O 2 ml, 80 1C, 6 min.
washed three times and then introduced into the reactor with
H2O, and fresh phenol and HCOONaꢀ2H2O were charged into
the reactor again for the recycling runs. The activity decreased
obviously at the second run but decreased slightly at the third
run, the selectivity to cyclohexanone did not change and it was
higher than 99%. For the heterogeneous catalysis, metal
species leaching during the reaction is one problem for recycling.
In this work, Pd leaching was not detected in the filtrate by
ICP analysis (iCAP6300, Thermo USA, the detection limit was
0.1 ppm), but CO was detected in the gas phase of the reaction
product by TCD analysis and which might be responsible for
the deactivation. Baiker and co-workers reported that the
hydrogenation of carbonyl compounds over the Pt/C catalyst
in scCO2 provided CO as a side product, which might strongly
bind to the Pt metal on the catalyst surface, resulting in serious
catalyst deactivation.34,35 Bianchi and co-workers reported
that the FTIR spectra of the adsorbed CO species (linear
and bridged species) were formed on Pd/Al2O3, Pd/CeO2/
Al2O3, and Pd/La2O3/CeO2/Al2O3 at adsorption temperatures
The recycling of catalyst has been investigated and the
results are shown in Table 5. After the first run, the reaction
mixture was centrifuged, and then the catalyst was separated
from the liquid mixture by decantation. The catalyst was
c
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from 300 to 800 K.36 In this work, CO may be formed by the
thermal decomposition of HCOONa37 or the reduction of
CO2 produced from NaHCO3 and/or HCOONa.34,35
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Conclusions
In conclusion, a new environmentally benign and effective process
was developed for the reduction of phenol derivatives to cyclo-
hexanones. Under the assistance of microwave, the hydrogenation
of phenol was performed very fast and cyclohexanone was
produced with a yield of 498% in the presence of a hydrogen
source of HCOONa and water under the mild conditions. The
reduction of phenol with HCOONa in water was not going through
the hydrogen transfer reaction pathway, but the hydrogen produc-
tion and hydrogenation pathway. It is worth expecting a practical
utilization of the present process for phenol and its derivatives
transfer to their corresponding cyclohexanones in near future.
24 J. Ekstrom, J. Wettergren and H. Adolfsson, Adv. Synth. Catal.,
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