Highly selective Pd@mpg-C3N4 catalyst for phenol hydrogenation in aqueous phase

Article abstract of DOI:10.1039/c3ra41397g

The liquid phase selective hydrogenation of phenol to cyclohexanone has been investigated over polymeric mesoporous graphitic carbon nitride (mpg-C ₃N₄) supported Pd catalysts (Pd@mpg-C₃N ₄). This Pd@mpg-C₃

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Graphical Abstract
Pd supported on mesoporous graphitic carbon nitride (Pd@mpg-C3N4) could activate hydrogen
and follow with deuterium exchange; the catalyst has been successfully utilized as an active,
selective catalyst in the hydrogenation of phenol towards cyclohexanone.

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ARTICLE TYPE
Highly Selective Pd@mpg-C3N4 Catalyst for Phenol Hydrogenation in
Aqueous Phase
Yi Li, Xuan Xu, Pengfei Zhang, Yutong Gong, Haoran Li, Yong Wang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The liquid phase selective hydrogenation of phenol to cyclohexanone has been investigated over
polymeric mesoporous graphitic carbon nitride (mpgꢀC3N4) supported Pd catalysts (Pd@mpgꢀC3N4). This
Pd@mpgꢀC3N4 is shown to be highly active and promotes the selective formation of cyclohexanone
(~99%), an industrially important compound, in a “oneꢀstep” way. The effects of different variables like
10 solvents, reaction temperature, catalyst texture and catalyst preꢀtreatment are presented and discussed.
The different adsorption capacity of mpgꢀC3N4 between the phenol and cyclohexanone is suggested to
strongly promote the chemoselective reduction of phenol to cyclohexanone and avoid the deep
hydrogenation of cyclohexanone to cyclohexanol. The hydrogenation kinetics has been adequately
represented by a standard pseudoꢀfirstꢀorder approximation and isotopic tracing study and the transition
15 state calculation were applied for the discussion of the possible reaction mechanism.
coking during the reaction and involve the limitation of
circulation and further application.
Introduction
Liquid phase hydrogenation can be of great interest since the
50 reaction can be performed at relatively low temperatures.
Attempts have been made to use the Rh, Ru, Pt, and Pd based
catalysts to this end. As summarized in Table 1, the
cyclohexanone selectivity from liquid phase phenol
hydrogenation is strongly dependent on the type of the catalyst
55 and the properties of the catalyst support.25 It seems that Pd
catalysts usually demonstrate a higher selectivity of phenol
transformation to cyclohexanone as compared to Pt, Rh, Ru and
Ni catalysts. Activity of Pd catalysts can be controlled by varying
the support materials (MgO, Al2O3, activated carbon, TiO2,
60 MCMꢀ41, mpgꢀC3N4 etc) or introduction of other additives. As
carbon materials are mainly composed of carbon and even can be
made directly out of biomass, they are obviously the most
“sustainable” support materials for Pd nanoparticles. Indeed,
much interest has been focused on the liquid phase phenol
65 hydrogenation using Pd@carbon catalysts. The commercial
Pd@C catalysts can catalyze the hydrogenation reaction in the
liquid phase, but even if the required reaction temperature in a
batch process is lower than for gasꢀphase reactions, the selectivity
of phenol hydrogenation towards cyclohexanone remains poor,
70 which imposes further separation steps and strongly limits the
usefulness of these catalysts. Recently, some efforts have been
made to improve the performance of the Pd@carbon catalyst. It
was reported that the supercritical carbon dioxide could improve
the low catalytic efficiency of Pd@C, probably due to increasing
75 solubility of reactant gases or easy separation of catalyst and
products in the scꢀCO2 medium.6, 26, 27 Titirici et al. performed the
hydrogenation of phenol in aqueous phase to produce
cyclohexanone using a homeꢀmade hydrophilic carbon support
Hydrogenation of phenol to cyclohexanone has attracted much
attention both in industry and in the synthesisꢀoriented research
laboratory in recent years. The reason for this interest lies
20 principally in the fact that cyclohexanone is of great commercial
significance as a key raw material in the production of both
caprolactam for nylon 6 and adipic acid for nylon 66.1 The
manufacture of cyclohexanone has the alternative routes of the
oxidation of cyclohexane2ꢀ4 or hydrogenation of phenol.5 The
25 former route requires high temperatures and high pressures and
generates undesirable byproducts that lower the product yield and
complicate the recovery/separation steps.3,4 In the latter route,
phenol can be hydrogenated to cyclohexanone in a “oneꢀstep” or
a
“twoꢀsteps” process.6 The “twoꢀsteps” process involves
30 hydrogenation of phenol to cyclohexanol followed by
dehydrogenation to cyclohexanone. The “oneꢀstep” process of
selective reduction of phenol to cyclohexanone is certainly
preferable, as it avoids the endothermic dehydrogenation step and
obviously it is more economic. However, catalytic reduction of
35 phenol to cyclohexanone via “oneꢀstep” process remains
challenging. This is typically attributed to the high reaction
activity of cyclohexanone and it is difficult to avoid its deep
hydrogenation to cyclohexanol or other byproducts. According to
the literatures, one step hydrogenation of phenol can be
40 conducted either in gas or liquid phase.7,8 In the case of gas phase
hydrogenation, the reaction was generally carried out in a tubular
reactor at temperature of 120ꢀ300 oC over supported Pd catalysts,
such as Pd@Al2O3,9ꢀ11 Pd@MgO,9,12ꢀ14 Pd@SiO2,15 Pd@Ta2O5,16
Pd@CeO2,17 Pd@ZrO2,17 Pd@La2O3,10 Pd@hydrotalcite,18ꢀ20
45 Pd@HPS,21 Pd@TiO2,22 Pd@BETA,23 or Pd@C24 etc. However,
high temperatures usually cause catalyst deactivation due to
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Pd catalyst, and a 95% selectivity of cyclohexanone was obtained
Table 1. Comparative catalytic performance of different catalysts to
hydrogenate phenol
at 99% conversion of phenol.28 Recently, Zhao and coworkers
reported that the combination of microwave irradiation and
Pd@C catalyst was an efficient catalytic system for the
hydrogenation of phenol to cyclohexanone in the presence of a
hydrogen source of HCOONa.29 Moreover, Han and Jiang et al.
found a dualꢀsupported Pd@C Lewis acid catalyst in the
hydrogenation of phenol and attained both excellent conversion
and selectivity under mild conditions. 30
Catalyst
Catalyst
Temperature Time
Atmosphere
Conv. Sel.
53% 17%
amount
5
10
15
10MPa H2
+10MPa CO2
10MPa H2
5% Rh/C39 0.0228g
5% Ru/C39 0.0639g
5% Pd/C39 0.1400g
328K
328K
328K
328K
323K
323K
323K
373K
298K
323K
338K
323K
373K
333K
353K
Temperature
503K
503K
473K
453K
2h
2h
30%
3%
ꢀꢀꢀ
5%
54%
ꢀꢀꢀ
+10MPa CO2
10MPa H2
Our group and others have recently shown that an abundant
and easily accessible polymeric semiconductor, mpgꢀC3N4,
exhibits extreme chemical and thermal stability, can be
chemically shaped to a variety of nanostructures, and can be
directly used in heterogeneous catalysis or as catalyst support.2,
2h
+10MPa CO2
10MPa H2
5% Pt/C39
0.0780g
0.1g
2h
+10MPa CO2
4MPa H2
6
1% Rh/Al2O3
4h
35.1% 32.1%
98.4% 97.8%
100% 52.8%
99% 95%
99% 99%
97.5% 96.6%
99% 99%
99.9% 99.9%
100% 98%
39.7% 87.3%
50% 100%
Conv. Sel.
77% 98%
90% 82%
31ꢀ36
+12MPa CO2
4MPa H2
For example, very recently we developed a depositionꢀ
1%Pd/Alꢀ
MCMꢀ416
1%Rh/Alꢀ
MCMꢀ416
precipitation (DP) route to deposit Pd nanoparticles on a
mesoporous graphitic carbon nitride (mpgꢀC3N4) and found that
the asꢀmade Pd@mpgꢀC3N4 catalyst show a high activity for the
selective hydrogenation of phenol,31 nitriles35 and quinolones.36
20 Furthermore, the catalytic mechanism and reaction profile of
the Pd@mpgꢀC3N4 in the phenol hydrogenation, was not clear.
In this paper, a more convenient, environmentally friendly route
for the synthesis of the novel catalyst Pd@mpgꢀC3N4 through
ultrasonic deposition method was reported. The activity of the
25 catalyst in different reaction conditions was investigated in
detail, measured by the initial reaction rate. The possible
reaction mechanism was also discussed through the isotopic
tracing study and the transition state calculations.
0.1g
4h
+12MPa CO2
4MPa H2+12MPa
CO2
0.1g
4h
Pd@hydrophi
licꢀC28
50mg
20h
11h
0.75h
2h
1MPa H2
0.1MPa H2
1MPa H2
5 mol% to
phenol
Pd/MLꢀ10140
Pdꢀ
2.5wt% to
Au/MHSS41 phenol
Pd@mpgꢀ 5 mol% to
0.1MPa H2
1MPa H2
31
C3N4
phenol
5 mol% to
phenol
30
Pd/CꢀAlCl3
7h
Pd/Al2O3ꢀ 2.5 mol%
CWE42
Experimental section
0.83h
0.5MPa H2
2MPa H2
to phenol
0.5 mol%
to phenol
30 Catalysts Preparation
Pt NWsꢀ
43
Cyanamide, LudoxꢀHS 40, ammonium acid fluoride, and
palladium chloride were purchased from SigmaꢀAldrich. Phenol
and ethanol were obtained from Sinopharm Chemical Reagent
Co., Ltd., China. Pd@C was obtained from ThalesNano.
35 Pd@TiO2, Pd@CeO2, and Pd@γꢀAl2O3 were synthesized
according to the literatures.37
AlCl3
Ceꢀdoped
Pd(H) 44
0.1g Pd
1.5h
1MPa H2
Catalyst
amount
Feed
rate
Catalyst
H2/phenol
5.4 (mol ratio)
5.4 (mol ratio)
0.0135
mol/h
0.0135
mol/h
2.5
9
1%Pd/Al2O3
0.5g
0.5g
The support mesoporous carbon nitride was synthesized by
thermally induced selfꢀcondensation of cyanamide and the
mesoporous silica matrices (LudoxꢀHS 40) act as hard template 1%Pd/MgO9
40 to control morphological,38 which resulted in a yellow powder
22
1%Pd/TiO2
0.5g
4.5 (mol ratio) 79.6% 99%
with graphitic packing structure. In a typical procedure, 3 g
cyanamide and 7.5 g LudoxꢀHS 40 were mixed in aqueous
ml/h
0.0122
mol/h
6% Pd/C24
0.05g
4 (mol ratio)
>50% 100%
o
solutions. The mixture was heated at 80 C in an oil bath upon
stirring overnight to remove the water. The remaining white
45 composite was heated programmed to 550 oC for 4 h and hold for
another 4 h, in order to make sure the condensation of the
precursor into polymeric C3N4. Then NH4HF2 solution (4 M) was
selected to get rid of the silica template by stirring for 2 days. The
dispersion was then filtered; the solid is washed with water and
60
Table 2. Comparison of different metal loading routes of depositionꢀ
precipitation(DP) and ultrasonic deposition(US)
Particle Size
（TEM）/ nm
Particle Size
（XRD）/ nm
Loading
Efficiency
Route
Time / h
DP
US
50%
80%
3.5
3.8
8.0
8.3
15
1
o
50 ethanol. After filtering, the yellow compound was dried at 80 C
overnight in a vacuum oven.
A novel route to support the Pd nanoparticles on the mpgꢀC3N4
was developed in this work. Compared with previous depositionꢀ
precipitation (DP) way,31 the ultrasonic deposition (US) method
55 demonstrates quite a lot advantage (Table 2). The novel metal
supported route not only illustrates a higher noble metal loading
efficiency to make full use of the palladium precursor, but also
rapidly shortens the metal loading time from 15 h to 1 h with the
65 Pd nanoparticles size does not show much difference. (calculated
by Scherrer’s formula in XRD pattern and count nanoparticles in
TEM images.) Typically, 0.1 g support was dissolved in 10 mL
water under ultraꢀsound for 15 minutes. Then, 2 mL PdCl2 (1%
w/w) aqueous solution were added into the support solution for
2
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the impregnation, holding for another 15 minutes with ultrasound.
Then 5 mL NaBH4 solution (0.1% w/w) was added to this
The structures of the mpgꢀC3N4 support and the Pd@mpgꢀC3N4
catalyst were firstly investigated by Xꢀray diffraction analysis
suspension and the dispersed PdCl2 was reduced to Pd 60 (XRD). XRD patterns (Figure 1A) reveal the graphiteꢀlike
nanoparticles and deposited on the mpgꢀC3N4 immediately.
Finally, Pd@mpgꢀC3N4 was separated by filtration, washed with
water for several times and dried at 70 oC overnight. The loading
amount of Pd on mpgꢀC3N4 was then determined by Inductively
Coupled Plasma Atomic Emission Spectrometry (ICPꢀAES).
Characterizations of the Catalyst
packing of practically both products, showing the typical (002)
interlayerꢀstacking peak around 27.4 o, as is wellꢀknown for mpgꢀ
C3N4.31,38 However, compared with the parent mpgꢀC3N4
material, the (002) peak of Pd@mpgꢀC3N4 becomes less intense,
65 indicating disturbance of the graphitic structure after the loading
of Pd nanoparticles. The diffraction peaks at 40o and 46o in the
XRD patterns can be assigned to the (111) and (200) planes of Pd
metal particles, respectively, indicating the successfully loading
of palladium nanoparticles. The BET technique was applied to
5
10 The porous structure of mpgꢀC3N4 and the catalyst were
characterized by N2 adsorptionꢀdesorption isotherms at 77 K
o
using AUTOSORBꢀ1. Samples were out gassed at 150 C for 10
h until the pressure was less than 10ꢀ4 Pa, the surface area and 70 understand the textural properties of the catalysts and the
pore volume were obtained by the BET equation. Xꢀray powder
15 diffraction analysis (XRD) was conducted using Ultima TV Xꢀ
ray diffractometer with Cu Kα radiation (1.54 Å). Samples were
dried overnight at 80 oC prior to the characterization. The
isotherms are shown in Figure 1B. The N2 adsorptionꢀdesorption
isotherm curves reveal a moderate surface area of 123 m2/g and a
weak microporosity of the parent mpgꢀC3N4. After the
modification, the isotherm shape of Pd@mpgꢀC3N4 was
mesoporous materials were characterized by transmission 75 preserved, indicating that the Pd nanoparticles do not block or
electron microscopy (TEM), employing JEMꢀ1230 at an
20 accelerating voltage of 80 kV. The samples were suspended in
ethanol under ultraꢀsound for 1 h and then dispersed onto a holey
carbonꢀcoated Cu grid. The mean particle size presented in this
alter the pore system. The TEM image of Pd@mpgꢀC3N4 catalyst
is shown in Figure 2Aꢀ1. The Pd nanoparticles are well dispersed
on the support without any aggregation. Representative Pd
particle size distribution, based on TEM analysis, is presented in
study was based on a measurement of ca. 100 individual particles. 80 the histograms given in Figure 2Aꢀ2. The Pd@mpgꢀC3N4
Besides, the concentration of Palladium was measured by ICPꢀ
25 AES (IRIS Intrepid Ⅱ XSP, Thermo Fisher Scientific, USA).
Typical Procedure for the Phenol Hydrogenation and
Recycling
specimen exhibited a normal distribution with a mean size of 3.8
nm. According to the procedures described by Mahata9 and
Anderson45 et al, the dispersion (D), i.e., the fraction of exposed
Pd in the catalyst can roughly be calculated by the following
The hydrogenation reactions were carried out in a 25 mL roundꢀ 85 equation:
bottled flask with refluxing. Typically, phenol (0.585 mmol),
30 catalyst (25 mg, 4% mol relative to phenol), and 2 mL water were
introduced into the reactor in order. After H2 was charged into
the flask to remove the air for 3 times, the reaction was started
Accordingly, the D value of Pd@mpgꢀC3N4 catalyst was
determined to be 29.5%, which suggested that the Pd
nanoparticles in Pd@mpgꢀC3N4 have a large number of active
surface Pd atoms for the H2 chemisorption and activation.
with stirring at 1000 rpm under 1 bar of H2 after the desired 90 Therefore, a high catalytic activity of the Pd@mpgꢀC3N4 for
temperature was achieved. The reaction products were separated
35 and analyzed on GC (Shimadzu, GCꢀ2014) equipped with Rtxꢀ
1701 (0.25 mm i.d., 0.25 ꢁm film) and employing a flame
ionization detector. Decane was used as an internal standard to
calculate the concentration of the substrate and products. The GC
conditions for the analysis were as follows: Injection port
hydrogenation can be expected.
Fig. 1 XRD patterns (a) and Nitrogen adsorption/desorption isotherms (b) of
the mpgꢀC3N4 and Pd@mpgꢀC3N4
o
40 temperature 280 C and a carrier gas at a rate of 43 ml/min, the
temperature program used was 1 min at 60 oC, with a temperature
ramp of 20 oC/min up to 100 oC for 8 min and finally reaches the
temperature 250 oC for 1 min with the 80 oC /min gradient rate.
The reusability of Pd@mpgꢀC3N4 was tested for phenol
45 hydrogenation at 65 oC for 6 h. The catalyst used above was
recovered by centrifugation and the solid was washed with
ethanol for three times. The recycled catalyst was finally dried at
80 oC overnight. The slight loss of the catalyst was replenished by
new catalyst to keep the reaction conditions without any changes.
50 The conversion and selectivity are defined as follows:
95 Hydrogenation of Phenol
Generally, hydrogenation of phenol to cyclohexanone is
accompanied by the formation of cyclohexane, cyclohexene,
cyclohexanol, and cyclohexylcyclohexanone. In order to optimize
partial hydrogenation and achieve high cyclohexanone
100 selectivity, it has been typically necessary to sacrifice phenol
consumption rates as higher activities favor complete
hydrogenation to cyclohexanol or other byꢀproducts. The
Pd@mpgꢀC3N4 catalysts were tested for the hydrogenation of
phenol in aqueous media under mild conditions, as described in
105 Experiment Section.
55
Results and discussion
Catalyst characterization
Moreover, the catalysts prepared here were compared with the
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Aꢀ1
Aꢀ2
50nm
Bꢀ2
Bꢀ1
Fig. 3 Effect of different catalyst/substrate (mol %) on the rate of the
reaction for phenol hydrogenation under 1 bar H2
30
carried out at the higher stirring speed for the measurement of the
reaction kinetics (1000rpm). Then, the influence of different
catalyst amounts was investigated to prove the measurement of
the initial rates of reaction that could be applied in the reaction
50nm
35 conditions. As shown in Figure 3, a different ratio of Pd/phenol
(mol %) varying from 1.7 to 8.4 was found to affect the reaction
rate significantly. The straight line indicated that the reaction
exhibits a standard pseudoꢀfirstꢀorder rate on the weight of
catalyst or the phenol concentration, which is the same as those
40 reported by many other authors.15,30 A ratio of Pd/phenol at 4
mol% was chosen to study the reaction systematically and
investigate the other influence factors.
Fig. 2 TEM results for the fresh catalyst (A) and the 4 times reused
catalyst (B): 1. Tem image; 2.corresponding size distribution.
commercially available Pd@C, Pd@TiO2, Pd@MgO, Pd@CeO2,
and Pd@γꢀAl2O3, for the hydrogenation of phenol at 65 oC under
1 bar hydrogen pressure, and the results obtained are showed in
Table 3. Results from Table 3 clearly indicate that nature of the
support plays an important role in the activity and selectivity of
Pd catalysts for the phenol hydrogenation. The Pd@mpgꢀC3N4
5
The effect of calcination temperature
10 catalyst synthesized here gives better activity than the other
oxideꢀsupported Pd catalysts. The commercial Pd@C although
shows a well activity in phenol hydrogenation, the selectivity
towards cyclohexanone is very low (71%).
The kinetic curves of phenol hydrogenation catalyzed by the
45 catalyst with different preꢀtreatment temperatures are presented in
Figure 4. The ln(Co/Ct) vs. reaction time (t) curves were
according to the firstꢀorder reaction with
a equation of
Table 3 Catalytic results for different catalysts a
ln(Co/Ct)=kꢂt, and the slope of the liner relationship is used to
determine the rate constant. Co and Ct are the concentrations of
50 phenol before the reaction and at a reaction of time t,
respectively. As shown in Figure 4, the calcination temperature
was found to have a profound influence on the hydrogenation
activity of the Pd@mpgꢀC3N4 catalyst. The catalyst calcinated at
a low temperature (100 oC) showed good activity with a rate
55 constant of 0.212±0.007 (hꢀ1) at 65 oC and 1 bar hydrogen
pressure for 2 h. The activities of the catalyst decreased with
increasing calcination temperatures, for example, the catalyst
calcinated at 200 oC gave a much smaller rate constant of
0.07±0.001 (hꢀ1) under the same reaction conditions. This could
60 be attributed to the changes in composition and crystalline
structure of palladium species on support during the calcinations
in harsh conditions. 47
Entry
Catalyst
Pd@CeO2
Pd@MgO
Pd@TiO2
Pd@Al2O3
Pd@C
Time (h)
Conv. (%)b
Sel. (%)b
100
97
99
99
71
100
1
2
3
4
5
6
6
6
6
6
3.5
6
2
72
8
32
98
100
Pd@mpgꢀC3N4
a
Reaction conditions: phenol 0.585 mmol, Pd (4 mol% relative to
phenol), solvent 2 ml H2O, H2 1 bar, reaction temperature 65 oC.
b Determined by GC and GCꢀMS
Reaction kinetics
15 The reaction of phenol hydrogenation over Pd@mpgꢀC3N4 under
different conditions was then studied kinetically by measurement
of the initial reaction rates. There has, of course, been much prior
work concerning phenol hydrogenation using heterogeneous
system in vapor phase and,5,15,21 indeed, the method has also been
20 applied in the aqueous phase reaction.30 It is reported that the
standard methodologies could be applied46 as long as suitable
controls are in place to ensure that the reaction is conducted
under kinetically control. It was firstly established that there was
no difference in rates of reactions at the stirring speed of 500 and
25 1000 rpm, indicating that the mass transfer was not the rateꢀ
limiting under the reaction conditions; all reactions were then
65
70
75
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Fig. 4 Comparison of the activities of the Pd@mpgꢀC3N4 (r=1.0) catalysts
with different calcination temperature.
room temperature, thus requiring even no external heating,
55 yielding an economic and practical route. And hydrogenation
The effect of textural properties
The structural characteristics of three mpgꢀC3N4 materials used as
supports are reported in Table 4. As demonstrated in Table 4,
through the controlling of the amount of template LudoxꢀHS 40
used, the surface areas and mesopore volumes can be adjusted
accordingly. In all experiments, the product of phenol
hydrogenation is only cyclohexanone, and the selectivity is
5
60
10 almost 100%. The surface areas of the supports do not make any
difference in the selectivity toward cyclohexanone but it
dramatically affects the activity of the catalyst. The kinetics
curves of phenol hydrogenation to cyclohexanone in 3 h over the
three kinds of Pd@mpgꢀC3N4 with different texture properties are
15 represented in Figure 5. Determined from the slopes, the initial
rates of phenol hydrogenation over above catalysts were
0.084±0.001 (hꢀ1), 0.212±0.007 (hꢀ1), and 0.121±0.003 (hꢀ1),
respectively. It can be seen that Pd supported mpgꢀC3N4(r=1),
which has a medium surface area and pore size, showed
20 remarkably higher catalytic activity than the others. Therefore,
Pd@mpgꢀC3N4(r=1) was chosen in the subsequent experiments
aiming at investigating the effect of reaction temperature.
65
Fig. 6 The timeꢀactivity profile for selective hydrogenation phenol under
70 different reaction temperatures
reaction under mild conditions is extremely desirable in industry.
Attention was then turned to the effects on initial rates caused
by changes in the reaction temperature. The kinetic curves of
75 phenol hydrogenation under the reaction temperature from 30 to
o
80 C and associated Arrhenius plots were presented in Figure 7
and 8, respectively. As shown in Figure 7, to determine the
activation energy of the phenol hydrogenation, the data is
obtained at low conversions. It is evident that (correlation
80 coefficient>0.985) the pseudoꢀfirstꢀorder treatment serves as an
acceptable approximation of the reaction kinetics, forced to go
through the origin, the approximation will yield a straight line
plot relating to the rate constant k under different temperature, the
corresponding results are presented in Table 5. According to the
85 Van’t Hoff approximation rules, an increasing reaction
temperature for 10 K would accelerate the reaction with a higher
rate constant for 2ꢀ4 times. While, the rules could not be applied
in this system for the reaction under 353 K giving an initial rate
of 0.328 (hꢀ1), only about 7 times than that under 303 K (0.046(hꢀ
90 1)), and it can be better assessed from the Arrhenius plots to
understand the reaction. There is few investigation about the
activation energy (Ea) calculation for phenol hydrogenation in
aqueous phase as far as we know. Lercher and coꢀworkers
reported Ni/ZSMꢀ5 exhibits activation energy of 48 kJ/mol in
95 vapor phase hydrogenation recently,48 while the Pd@mpgꢀC3N4
in our work gave a lower activation energies of 35.9 kJ/mol for
phenol hydrogenation in aqueous media, which is consistent with
its high catalytic activity, suggesting an easily initiated reaction.
25
30
35
Table 4 Textural properties of carbon nitride materials synthesis by
controlling amount of template.
Entry
Sample a
SBETb (m2/g) PV c (cm3/g) PSd (nm)
1
2
3
mpgꢀC3N4(r=0.5)
mpgꢀC3N4(r=1.0)
mpgꢀC3N4(r=1.5)
116.4
124.2
158
0.3754
0.3884
0.4662
12.904
12.512
11.798
a mpgꢀC3N4(r=MSiO2/Mcyanamide); b BET surface area; c Pore Volume;
d Pore Size.
Fig. 5 Kinetics curves of phenol hydrogenation over the Pd@mpgꢀC3N4
40 with different texture properties
The effect of reaction temperature
100
The effect of reaction temperature on phenol hydrogenation was
then investigated, and the corresponding results were summarized
in Figure 6. As illustrated in Figure 6, variation of the reaction
45 temperature had a considerable effect on the cyclohexanone
yield. The higher temperature is beneficial for the activation of
the catalyst and substrate molecule. Conversion of 100% was
achieved with a selectivity exceeding 99% at 1 bar hydrogen
pressure and 80 oC for 3 h, and a TOF of ca. 33 hꢀ1 was obtained
50 under the same temperature. However, even at a temperature of
105
o
30 C, 100% conversion of phenol and 97% selectivity toward
cyclohexanone could be reached after a long reaction time of 38
h. It is worth nothing that the reaction could be conducted near
110
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Fig. 7 Preliminary kinetics curves of phenol hydrogenation under
revealed a well dispersion with a mean size of 3.9 nm, almost the
different reaction temperatures
5
55
60
65
10
Fig. 9 Reuse of the Pd@mpgꢀC3N4 (r=1) (A) and Pd@C (B) under 65oC.
Table 5 Ea calculation for aqueousꢀphase phenol hydrogenation reaction
same morphology as the fresh ones (Figure 2B) which strongly
indicated that no aggregation of the nano palladium particles
happened during the reaction process. The slight decrease activity
of mpgꢀC3N4 was probably due to minor metal leaching during
70 the recycle reaction. The reused catalyst was detected with a
metal concentration of 8.1 wt%, approximately 8% Pd leaching
for a recycle in average.
on Pd@mpgꢀC3N4
Entry
Temperature (K) Rate constant k (hꢀ1)
R2
Ea(kJ/mol)a
1
2
3
4
303
318
338
353
0.046
0.084
0.212
0.328
0.996
0.987
0.988
0.986
35.9
a
Determined from Arrhenius plots ( ln k= ꢀEa/RT + ln A)
Possible reaction mechanism
15 Fig. 8 Temperature dependence of the rate paremeter in the Arrhenius
coordinates for the Pd@mpgꢀC3N4
It has been proposed that the selectivity towards cyclohexanone
75 in phenol hydrogenation is mightily dependent on the adsorption
model between the catalyst and substrate. The interaction
between the benzene ring and the catalyst contributes to the
The large-scale transformation reaction
Taking into account the commercial significance of products, we
are interested if the catalyst is able to catalyze a largeꢀscale
20 transformation reaction. As shown in Scheme 1, we investigated
this possibility using phenol (10g, 0.11mol) as substrate in 40 mL
coplanar form adsorption, which is favorable for saturated
hydrogenation to cyclohexanol.12,
Yet, both our group and
44
80 others have demonstrated the efficient use of the NH and NH2
groups on the surface of carbon nitride for the effective
adsorption of phenol via OꢂꢂꢂHꢀN or OꢀHꢂꢂꢂN interactions
recently.31,49 In the typical reaction, the phenol molecules are
absorbed on both the outer surface of the graphitic carbon nitride
85 and the interior mesoporous structure of the catalyst. Thus, the
absorbed phenol molecules should exist in a nonplanar form
rather than in a coplanar form, leading to the formation of
cyclohexanone. Then the product would leave the surface of the
catalyst quickly for only a weak Hꢀbridge donor, avoiding the
90 further hydrogenation to cyclohexanol. To explain the high
selectivity toward cyclohexanone, one can assume, in a first
approximation, that the adsorption capacity for the phenol is
much higher than that of the cyclohexanone on the mpgꢀC3N4,
resulting to a larger intrinsic rate for the reduction of the phenol
95 on the Pd@mpgꢀC3N4 catalyst than that of the cyclohexanone. To
confirm this hypothesis, adsorption reaction of phenol and
cyclohexanone over mpgꢀC3N4 was investigated, and the
hydrogenation of phenol or cyclohexanone under the same
reaction conditions were carried out separately, typical procedure
100 run according to the literature.50 Indeed, difference can be clearly
observed in Figure 10, the mpgꢀC3N4 reaches saturation for the
absorption of cyclohexanone within 20 min and its absorption
capacity is approximately 5.7 mg/g, while the carbon nitride with
NH, NH2 groups exhibits a remarkable higher performance for
105 the adsorption of phenol than that of cyclohexanone with a higher
absorption capacity (~115 mg/g), almost 20 times than that of
cyclohexanone, involving the difference reaction activity between
these two substrates. As demonstrated in Table 6, upon treatment
o
water under 3 MPa H2 at 108 C. The phenol hydrogenation in
the presence of 250 mg catalyst provided 70% conversion and
99% selectivity. Worth of notice, in this case a TON of 1111 was
25 achieved after 4 h and the excellent selectivity to cyclohexanone
could become a potential industrial interest.
30
Scheme
1
Largeꢀscale transformation: Reaction of phenol to
35 cyclohexanone using Pd@mpgꢀC3N4 as a catalyst. TON was defined as
TON= [Moles of phenol converted] / [Moles of Pd × Pd dispersion]
The reusability of the catalyst
For the heterogeneous catalysis reaction, it is important to test the
40 reusability of the catalyst. Previous work has reported the activity
of Pd/C would decrease obviously at the second run in the phenol
hydrogenation.29 While, in our system, the Pd/C exhibited a
noticeable inactivation during the recycling, indeed only gave a
55% conversion at the 4th recycle (Figure 9). While, the
45 Pd@mpgꢀC3N4 was able to be reused at least 4 times without
significant loss of activity, for remaining at least 80% conversion,
which is a prerequisite for practical applications. It is important to
highlight that no changes regarding the selectivity to
cyclohexanone were observed compared to the fresh catalyst. The
50 TEM image of Pd@mpgꢀC3N4 catalyst after 4 times recycles
6
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o
of phenol with 1 bar H2 at 65 C for 6 h with water as solvent, a
100% conversion of phenol was obtained with a 100% selectivity
Fig. 11 Effect of solvent on the reduction of phenol. Reactions were
carried out at 65oC with 0.585mmol phenol, Pd@mpgꢀC3N4 (4 mol % Pd
toward cyclohexanone. On the contrast, in the case of the 50 relative to phenol) and 2 mL solvent under 1 bar hydrogen.
reduction of cyclohexanone to cyclohexanol, only 2%
5
cyclohexanone conversion was detected under the same
conditions, and even extending the reaction time to 12 h, only
4.2% conversion was achieved. Besides, the high temperature had
no significant effect for the cyclohexanone hydrogenation with
this catalyst and the yield of the cyclohexanone still remained
10 very low.
55
60
65
70
75
15
Table 6 Hydrogenation of the phenol or cyclohexanone with Pd@mpgꢀ
221.4 kJ/mol
85.5 kJ/mol
C3N4 under various conditionsa
Entry
Substrate
Phenol
Cyclohexanone
Cyclohexanone
Phenol
Temperature (oC)
Time(h) Conv. (%)
1
2
3
4
5
6
65
65
65
80
80
6
6
100
2
12
3
3
4.2
100
1.8
1.6
Cyclohexanone
Cyclohexanone
100
3
a
Reaction conditions: substrate 0.585mmol; Pd (4 mol% relative to
substrate); solvent 2mL H2O; pressure 0.1MPa H2
-46.6 kJ/mol
20
Fig. 10 Comparision of the phenol and cyclohexanone adsorption
capacity over mpgꢀC3N4
Fig. 12 Possible reaction mechanism and proton transfer of the transition
80 state.
It was found that the hydrogenation of phenol proceeded even
more effectively in terms of both conversion and selectivity in
To further illustrate the reaction mechanism, isotopic tracing
25 water than in organic solvents (for example toluene) in some
cases (Figure 11). Considering the hydrogenation of phenol
involves a proton transfer to form cyclohexanone (Figure 12), the
existed water molecules may accelerate this process. We then
calculated the proton transfer process with and without a water
30 molecule using density functional theory (DFT). Results based on
DFT calculations showed that the water molecule can
significantly lower the activation energy for the proton transfer
reaction compared to the reactions without a water molecule, and
in addition, can alter the preferred hydrogenation mechanism
35 (Figure 12). For example, without the assistance of water
molecule, the activation energy barrier of the proton transfer is
221.4 kJꢂmolꢀ1, however, the activation energy barrier decreases
to 85.5 kJꢂmolꢀ1 with the promoting of a water molecule.
study was introduced to the experiment. Typically, the deuterium
oxide (D2O) was used to replace water as the solvent under a
85 hydrogen pressure of 0.1 MPa. With an active hydrogen atom in
hydroxyl group, the phenol can exchange deuterium with the
D2O, which may lead to the formation of cyclohexanone
containing a deuterium atom with an ionic peak at 99 (Scheme 2).
Surprisingly, the GCꢀMS results demonstrated that several ionic
90 peaks with higher m/z belong to the cyclohexanone were also
detected (m/z=100, 101, 102, 103, 104 and 105) and these peaks
could be ascribed to the cyclohexanone with more deuterium
atoms (Figure 13). This phenomenon is attributed to the
resonance and the hydrogenꢀdeuteron exchange reaction in the
95 reaction system. Under the circumstance, resonance usually
occurs and cyclohexanone isomerizes to form enol, then active
hydrogen of the enol can exchange with deuterium of D2O. The
unstable enol would transfer to cyclohexanone with more than
one deuterium atom consequentially. However, only the 4
100 hydrogen atoms in the adjacent positions of the cyclohexanone
can be replaced by deuterium view resonance and proton
exchange, resulting a maximum ionic peak at 102 (Scheme 3),
which cannot explain ionic peaks with m/z values of 103 to 105.
According to the reaction procedure, the hydrogen molecule
105 could be activated by the palladium (the active component) firstly
40
45
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and then the benzene ring was partially hydrogenated to form
enol, it is probably that, the activated H atom from H2 by catalyst
exchanges with D in D2O, then at most 3 D atoms can locate on
the meta or contra positions via the following hydrogenation,
resulting ionic peaks with m/z=105 (Scheme 4). The total
description of the possible reaction mechanism including proton
exchange and hydrogen molecular activation was demonstrated in
Scheme 5. In a word, the isotopic tracing study helps to
understand this reaction route and proves the noble metal activate
5
10 the hydrogen through a new method
15
55 Fig. 13 Mass spectrum profiles of the isotopic tracing study reaction.
Scheme 2 Typically isotopic tracing study reaction: Deuterium oxide
used as the solvent.
20
Conclusions
In conclusion, an efficient and convenient ultrasonicꢀdispersion
method to deposit the palladium nanoparticles on mpgꢀC3N4 was
60 demonstrated. The asꢀmade Pd@mpgꢀC3N4 showed a highly
active and selective in the reaction of phenol hydrogenation to
cyclohexanone directly under mild conditions. The reaction
kinetics has been represented by a standard pseudoꢀfirstꢀorder
approximation and the influence factors of the phenol
65 hydrogenation were investigated in detail by the measurement of
the initial constant rate. Pd@mpgꢀC3N4 (r=1) with a low
temperature calcination and a suitable surface area exhibits good
activity and selectivity, providing an activation energy of 35.9
kJ/mol for the chemoselective reduction of phenol. The
70 mesoporous carbon nitride provided an extremely selective
adsorption for the phenol, rather than the cyclohexanone,
contributing to the highly selective phenol hydrogenation. Even
in the large scale reaction, the catalyst can also promote the
formation of cyclohexanone effectively, the catalyst has also
75 shown better reusability than Pd/C of a 4 times recycling without
25
Scheme 3 Deuterium atoms locate on the adjacent position of the
cyclohexanone through resonance and proton exchange route.
30
a
significant activity loss, which is of great industrially
application potential. Besides, it is confirmed that the green
solvent water could accelerate the reaction, attributing to a
smaller transition state free energy, according to the DFT
80 calculation. Isotopic tracing method was also introduced to help
to present the hydrogen activated by the metal completely and to
understand the mechanism of the hydrogenation which opens a
door to study the catalysis reaction mechanism.
35
40
45
Acknowledgement
85 Financial support from the Joint Petroleum and Petrochemical
Funds of the National Natural Science Foundation of China and
China National Petroleum Corporation (U1162124), the
Specialized Research Fund for the Doctoral Program of Higher
Education (J20130060), the Fundamental Research Funds for the
90 Central Universities, the Program for Zhejiang Leading Team of
S&T Innovation, and the Partner Group Program of the Zhejiang
University and the MaxꢀPlanck Society are greatly appreciated.
Scheme 4 Deuterium exchange between the solvent and the activated
hydrogen on metal catalyst
50
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5
10
15
20
25
35
40
45
50
55
Scheme 5 Possible reaction mechanism of phenol hydrogenation under
30 deuterium oxide
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