Transfer hydrogenation of phenol on supported Pd catalysts using formic acid as an alternative hydrogen source

Article abstract of DOI:10.1016/j.cattod.2014.02.039

Palladium nanoparticles with size smaller than 10 nm were loaded on several supports including activated carbon (AC), MIL-101, TiO₂, Al ₂O₃, and TiO₂-activated carbon composites (TiO₂-AC). These catal

Full text of DOI:10.1016/j.cattod.2014.02.039

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Transfer hydrogenation of phenol on supported Pd catalysts using
formic acid as an alternative hydrogen source
∗
Damin Zhang, Feiyang Ye, Teng Xue, Yejun Guan , Yi Meng Wang
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, North Zhongshan Road
3663, 200062 Shanghai, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium nanoparticles with size smaller than 10 nm were loaded on several supports including acti-
vated carbon (AC), MIL-101, TiO2, Al2O3, and TiO2-activated carbon composites (TiO2-AC). These catalysts
showed high activity in liquid phase hydrogenation of phenol. Their catalytic performances in trans-
fer hydrogenation of phenol with formic acid under mild conditions (T = 50 C and P < 5 bar) were also
tested for the purpose of in situ upgrading of bio-oil. The activity followed the trend of Pd/AC > Pd/TiO2-
Received 2 November 2013
Received in revised form 20 February 2014
Accepted 21 February 2014
Available online xxx
◦
AC > Pd/MIL-101 > Pd/TiO2 > Pd/Al2O3. When 400 ␮L of formic acid was introduced into 10 mL of 0.25 M
aqueous phenol solution, phenol can be fully hydrogenated on 200 mg of Pd/AC catalyst at 50 C within
Keywords:
Palladium
Formic acid
◦
4
h, with 80% selectivity to cyclohexanone. The high activity of Pd/AC catalyst in hydrogen transfer makes
it possible for in situ upgrading of bio-oil under mild conditions by converting unstable components (such
as HCOOH and phenol) to stable ones (mainly cyclohexanone) which can then be potentially introduced
into current refinery. In addition, the effect of formic acid on the hydrogenation activity of supported Pd
catalysts was also reported. Presence of formic acid significantly decreased the hydrogenation activity
of Pd supported on MIL-101 and oxides (TiO2 and Al2O3), probably due to the competitive adsorption of
molecular formic acid with phenol on a single Pd site.
Transfer hydrogenation
Phenol
Cyclohexanone
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
As for the mild hydrogenation, different catalysts such as Rh/C
9], Rh/C nanofiber [10], Pd/hydrophilic C [11], Ru/poly (N-vinyl-
[
The increasing global demand for energy and decreasing fos-
2-pyrrolidone) (PVP) [12], Pd/Mg and Pd/Fe [13], mesoporous
Ce-doped Pd [14], and Pd/C [15] have been studied. Cyclohexanone,
cyclohexanol, and/or their mixture can be efficiently produced
using hydrogen as reductant on these catalysts. Molecular hydro-
gen is well known as a very explosive gas in contact with air
and is therefore difficult to handle in large-scale under high pres-
sure. Moreover, hydrogen is becoming more expensive because
it is mainly produced from steam-reforming of fossil fuels [16].
Formic acid, a component in bio-oil, has been attracted much
interest in green chemistry because of its potential as a hydrogen-
carrier and also as a means of CO2 utilization [17,18]. Formic
acid is currently produced industrially by the hydration of carbon
monoxide as well as the hydrogenation of carbon dioxide [19].
As a hydrogen source, it has many advantages with regards to
handling, transportation, and storage, therefore has been used for
decades in organic synthesis via transfer hydrogenation [20–28].
Substrates include nitroarenes [25], aldehydes [26], olefins [27] and
hydrodechlorination [28]. Recently it has been also used in biomass
conversion [29], such as production of 2,5-dimethylfuran from HMF
sil fuel requires the urgent need to develop renewable energies,
especially transportation fuels. Regarding to the current energy sys-
tem, bio-oil, produced by the pyrolysis of wood or forest wastes,
is the most promising raw material to replace fossil fuels [1–3].
Bio-oil has been considered as a carbon-neutral resource because
there is no extra carbon emission during the carbon cycle. Addition-
ally it is more geographically dispersed than petroleum reserves.
Bio-oil contains large amount of water and unsaturated organic
compounds such as acids and phenolic moieties [3–7]. These highly
active and unstable organic compounds may undergo polymer-
ization catalyzed by the present organic acids even under mild
conditions. These disadvantages of bio-oil limit its usage in cur-
rent FCC process [8]. Mild hydrogenation can efficiently convert
the phenolic compounds to more stable and saturated ketones or
alcohols in economical and energy-efficient means.
[
[
30], hybrid diesel fuel from carbohydrates and petrochemicals
31], 1,2-propanediol from glycerol [32,33], trienoic and dienoic
∗ Corresponding author. Tel.: +86 2132530334; fax: +86 21 32530334.
E-mail address: yjguan@chem.ecnu.edu.cn (Y. Guan).
http://dx.doi.org/10.1016/j.cattod.2014.02.039
0
920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: D. Zhang, et al., Transfer hydrogenation of phenol on supported Pd catalysts using formic acid as an
alternative hydrogen source, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.039

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2
fatty acids from vegetable oils [34]. In most transfer hydrogena-
tion processes, aqueous formate solution was used as the hydrogen
donor and Pd/C was employed as the catalyst.
Pd
Pd/TiO2
We herein explored the catalytic activity of palladium cat-
alysts for transfer hydrogenation of phenol using formic acid
as reductant. The supports investigated included carbon, basic
oxides, and metal-organic framework, namely, TiO , Al O , acti-
Pd/TiO -AC
2
2
3
2
vated carbon (AC), MIL-101, and TiO -activated carbon composites
2
(
TiO -AC). A support-dependent reactivity of supported Pd catalyst
2
was observed, with Pd/AC being the most active one and Pd/Al O3
Pd/AC
2
being the worst one. This reactivity difference might be due to a
competitive adsorption effect between formic acid and phenol on
Pd nanoparticles.
Pd/MIL-101
2
. Experimental
3
0
35
40
45
50
55
60
65
70
o
2
.1. Preparation of TiO -AC support
2
2 θ ( )
Commercially available AC was dispersed in isopropanol, and
Fig. 1. XRD patterns of 6.3 wt.% Pd/TiO2, 4.2 wt.% Pd/TiO2-AC, 4.3 wt.% Pd/AC, and
then titanium isopropoxide was added dropwise in this solution
and stirred overnight. The solution was diluted by water, and then
filtered. The obtained solid was dried and calcined at 500 C under
4.9 wt.% Pd/MIL-101.
◦
with a DB-Wax capillary column (30 m length and 0.25 mm internal
diameter). To systematically investigate the hydrogenation mech-
anism in presence or absence of formic acid, various hydrogenation
procedures were conducted as following:
N2 for 3 h.
2.2. Preparation of Pd catalysts
The supported Pd catalysts were prepared by a deposition-
(
i) Hydrogenation with hydrogen. The reactor was charged with
a mixture of 10 mL of aqueous phenol solution and 100 mg of
reduction method. Typically, 4.24 mL of H PdCl solution (Pd:
2
4
1
1.8 mg/mL) was diluted into 50 mL. Then 0.95 g of vacuum-dried
supported Pd catalysts. It was then purged with H for five times
2
support was added into the resulting solution and thoroughly
stirred for 4 h. The final pH value of the mixture was further
adjusted to 9 by adding NaOH solution (1 M). Afterwards, aque-
ous hydrazine solution was added to the solution with vigorous
stirring. The precipitated Pd(OH)2 species was thus reduced. The
and then pressurized with 5 bar H . The reaction mixture was
2
◦
heated up to 50 C and held for 4 h.
(
ii) Hydrogenation with formic acid. The reactor was charged with a
mixture of 10 mL of aqueous phenol solution, 400 ␮L of formic
acid, and desired amount of Pd/AC catalyst (25, 50, 100, and
◦
solid catalyst was obtained by filtration and drying at 100 C under
2
00 mg). Then it was purged five times with N and pressurized
2
vacuum overnight.
◦
with 1, 3, or 5 bar N . The reactor was heated up to 50 C and
2
For comparison, two commercial 5.0 wt.% Pd catalysts, Pd/C
held for 4 h.
(
denoted as Pd/C-comm) and Pd/Al O3 were purchased from Alfa-
2
(
iii) Hydrogenation with hydrogen and formic acid. The reactor was
charged with a mixture of 10 mL of aqueous phenol solution,
Aesar and used in the obtained form.
2
00 ␮L of formic acid, and 100 mg of supported Pd catalysts. The
2
.3. Materials characterization
reactor was purged with H for five times and then pressurized
with 5 bar H . The reactor was heated up to 50 C and held for
4
2
◦
2
The powder X-ray diffraction (XRD) patterns were collected on
h.
a Rigaku Ultima IV X-ray diffractometer using Cu K␣ radiation
(
iv) Hydrogenation with hydrogen in presence of additives (acetic
acid and sodium formate). The reactor was charged with a mix-
ture of 10 mL of aqueous phenol solution, 100 mg of Pd/AC (or
Pd/Al2O3), and 200 ␮L of acetic acid (or 0.680 g sodium for-
(
ꢀ = 1.5405 A˚ ) operated at 35 kV and 25 mA. Transmission elec-
2
tron microscopy (TEM) images were taken on a FEI Tecnai G F30
microscope operating at 300 kV. The average Pd particle size (sur-
ꢀ
ꢀ
3
2
face mean diameter) was calculated by dTEM = (
n d /
n d )
i
i
i
i
mate). The reactor was purged with H2 for five times and then
pressurized with 5 bar H . The reactor was heated up to 50 C
and held for 4 h.
by measuring at least 100 particles. The amount of TiO on TiO /AC
2
2
◦
2
composite was also determined on a Mettler Toledo TGA/SDTA
8
◦
51e thermal analyzer at a heating rate of 10 C/min from room
◦
(v) Hydrogenation kinetics. A two-neck round-bottom flask was
charged with 100 mg catalysts, 10 mL of aqueous phenol solu-
temperature to 800 C in air. The Pd loading was determined by a
Thermo Elemental IRIS Intrepid II XSP inductively coupled plasma
emission spectrometer (ICP-AES). Desired amount of sample was
dissolved in 10 mL of aqua regia. The solution was heated to remove
the residual acid and diluted into a 50 mL of volumetric flask. The Pd
loading of self-made Pd/TiO , Pd/AC, Pd/TiO -AC, and Pd/MIL-101
tion and desired amount of formic acid or sodium formate. The
reaction mixture was heated up to 50 C under N with flow rate
of 30 mL/min. For comparison, the hydrogenation with flowing
hydrogen was also conducted under otherwise similar condi-
tions.
◦
2
2
2
is 6.3, 4.3, 4.2, and 4.9 wt.%, respectively.
2.4. Catalytic measurements
3. Results and discussion
No specific pretreatment was conducted prior to the reaction.
00 mg of as-obtained catalyst was mixed with 10 mL of aque-
3.1. Catalyst characterization
1
ous phenol solution (0.25 M) into a Teflon-lined (100 mL) stainless
steel batch reactor. Once the reactor was cooled to room tempera-
ture, the products were analyzed on a Shimadzu GC 2014 equipped
Fig. 1 shows the XRD patterns of as prepared Pd catalysts. Very
weak peaks of Pd centered at 41.1, 46.6 and 68.1 were observed
◦
on Pd/TiO , while no diffractions assigned to Pd was detected on
2
Please cite this article in press as: D. Zhang, et al., Transfer hydrogenation of phenol on supported Pd catalysts using formic acid as an
alternative hydrogen source, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.039

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Table 1
Activity of Pd catalysts for hydrogenation using molecular hydrogen.
Entry
Catalyst
Pd loading (wt.%)
Diameter (nm)
Conv. (%)
Sel. (%)
C
O
OH
34.6
5.7
19.7
11.5
1.6
1
2
3
4
5
6
Pd/AC
4.3
–a
4.9
6.3
–a
5.1
6.6
2.6
4.9
4.7
7.0
98.3
95.5
99.6
46.8
76.5
99.5
66.4
94.3
80.3
88.5
98.4
4.6
Pd/C-comm
Pd/MIL-101
Pd/TiO2
Pd/Al2O3
Pd/TiO2-AC
4.2
95.4
◦
Reaction conditions: 100 mg catalyst, 10 mL aqueous solution of phenol (0.25 M), 0.5 MPa H2, 50 C, 4 h.
a
5
wt.% (according to Alfa-Aesar).
Table 2
Hydrogen transfer of phenol with formic acid on supported Pd catalysts.
other catalysts such as AC, MIL-101, and TiO -AC. This result is con-
sistent with previous finding that the Pd diffraction is difficult to be
2
detected by XRD at low loading [35]. TiO -AC is a promising com-
Entry
Catalyst
Conv. (%)
Sel. (%)
2
posite used as support in phenol hydrogenation due to its tunable
surface property [36]. In this study, a mixture of anatase and rutile
C
O
OH
1
2
3
4
5
6
Pd/AC
65.6
56
37.9
24.7
18.1
47.3
96.3
98.6
98.2
99.2
99.4
96.4
3.7
1.4
1.8
0.8
0.6
3.6
phases was obtained for calcined TiO2 and TiO /AC materials [37].
2
Pd/C-comm
Pd/MIL-101
Pd/TiO2
Pd/Al2O3
Pd/TiO2-AC
The content of TiO2 on TiO /AC was determined to be 25 wt.% by
2
thermogravimetric analysis (not shown).
The particle size distributions of these Pd catalysts were char-
acterized by TEM and the representative HRTEM images are shown
in Fig. 2. Surface mean diameters as well as the standard devia-
tions were calculated and listed in Table 1. The average particle
sizes were 2.6 ± 0.5, 4.9 ± 1.4, 5.1 ± 1.6, 6.6 ± 2.1, 7.0 ± 2.3, and
Reaction conditions: 100 mg catalyst, 10 mL aqueous solution of phenol (0.25 M),
◦
0
.3 MPa N2, 400 ␮L formic acid, 50 C, 4 h.
4
.7 ± 0.9 nm for Pd/MIL-101, Pd/TiO , Pd/AC, Pd/C-comm, Pd/TiO -
2
2
AC, and Pd/Al O , respectively. It is noteworthy that very finely
Pd/TiO2 but lower than that of Pd/AC. In earlier findings [39], Pd/C
was found to be far more active than other supported catalysts such
as Pd/A12O3, Pd/kieselguhr, Pd/BaSO4, and Pd/CaCO3 in the transfer
hydrogenolysis of 4-chloroanisole with ammonium formate. The
conversion of phenol all declined compared with that in case of
H2 as reductant albeit excess amount of HCOOH was introduced.
Despite of the extraordinary high activity in phenol hydrogenation
with hydrogen, Pd/MIL-101 showed much low activity in transfer
hydrogenation. This low reactivity is quite unexpected since the
Pd/MIL-101 has been shown to be very active in formic decomposi-
tion for the production of molecular hydrogen [40]. This result may
point to a concerted reaction mechanism for transfer hydrogena-
tion on Pd catalysts. The aqueous phase transfer hydrogenation of
phenol using formic acid as hydrogen source proceeds through
two steps involving first decomposition of formic acid and sec-
ond hydrogenation of phenol. Two reactions might influence each
other and lead to a decrease in the overall-reaction rate. In fact
the formic acid dissociation can be inhibited or even completely
stopped by some hydrogen acceptor which is adsorbed in a com-
petitive manner onto a single Pd site on Pd catalysts [25]. In another
study, Bulushev and Ross [27] reported the enhanced vapour-phase
formic acid decomposition in the presence of olefins. Nevertheless,
an ideal catalyst for transfer hydrogenation of phenol should be
able to allow access of all the components to the active sites. AC is
of similar hydrophobic nature to the substrate, which may assist in
the adsorption of substrate onto the catalyst and, as a consequence,
result in substantial gains in reaction rates [39]. In this sense, Pd/AC
catalyst may give moderate adsorption strength of both phenol and
formic acid allowing for the best catalytic performance. It should be
noted that particle size and some other factors may also contribute
to the catalytic activity, which needs further study.
2
3
dispersed Pd nanoparticles were loaded on MIL-101 probably due
to its very high specific surface area and the phenyl-containing
framework. In this regard, high surface area activated carbon also
shows significant advantage in supporting Pd nanoparticles, with
7
7% of particles being smaller than 3 nm.
3.2. Catalytic activity
3.2.1. Phenol hydrogenation with molecular H2
As an initial experiment, the catalytic activities of these Pd
catalysts in phenol hydrogenation using molecular H2 as reduc-
tant were compared and the data are collected in Table 1. The
activities of Pd catalysts followed the trend Pd/MIL-101 > Pd/TiO2-
AC > Pd/AC > Pd/C-comm >Pd/Al O > Pd/TiO . In consistent with
2
3
2
previous reports that Pd/MIL-101 showed superior hydrogena-
tion activity to Pd nanoparticles supported on conventional metal
oxides owing to the narrowly distributed particles ca. 2.6 nm in size
[
38]. Under identical reaction conditions, Pd/AC catalyst exhibited
slightly higher activity than Pd/C-comm, which might be associated
with the property of carbon used. It should be noted that Pd/TiO2-
AC catalyst gave almost full conversion of phenol, but mainly to
cyclohexanol.
3.2.2. Hydrogen transfer with formic acid
Given the promising catalytic property of Pd catalysts
in hydrogenation, we then explored the transfer hydrogena-
tion activity with formic acid and the results are shown in
Table 2. In contrast with the trend in Table 1, a distinctly
different trend in reactivity was observed when formic acid
was employed in the hydrogenation process: Pd/AC > Pd/C-
comm > Pd/TiO -AC > Pd/MIL-101 > Pd/TiO > Pd/Al O . AC based
The plot of phenol conversion versus catalyst amount in trans-
fer hydrogenation is displayed in Fig. 3 with 400 ␮L of formic acid
2
2
2
3
Pd catalysts showed much better performance than inorganic metal
oxides supported counterparts, with up to 50% conversion of phe-
under 3 bar of N . It should be noted the transfer hydrogenation
2
readily took place under 1 bar of N , with phenol conversion of
2
nol. Commercial Pd/Al O3 exhibited the lowest activity with a
47.3% and formic acid selectivity to transfer hydrogenation of 30.3%
(Table S1, entry 1). Increasing N2 pressure to 3 bar led to a higher
phenol conversion (57.5%) and also higher formic acid selectiv-
ity (57.4%) (Table S1, entry 2). The economic efficiency of formic
2
phenol conversion of 18.1%. Similarly, Pd/TiO also gave a low con-
2
version of 24.7%. It is interesting to note that Pd/TiO -AC showed a
2
moderate conversion of 47.3%, which was much higher than that of
Please cite this article in press as: D. Zhang, et al., Transfer hydrogenation of phenol on supported Pd catalysts using formic acid as an
alternative hydrogen source, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.039

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Fig. 2. Representative TEM images and particle size distributions of (a) 4.3 wt.% Pd/AC, (b) 5.0 wt.% Pd/C-comm, (c) 4.2 wt.% Pd/TiO2-AC, (d) 4.9 wt.% Pd/MIL-101, (e) 6.3 wt.%
Pd/TiO2, and (f) 5.0 wt.% Pd/Al2O3.
acid utilization is essential to the practical application of this pro-
cess. Therefore a higher N2 pressure would be beneficial. Further
increase of the N2 pressure to 5 bar did not result in significant
increase of phenol conversion (55.4%), but a higher efficiency of
formic acid (77.2%) (Table S1, entry 3). Taking into account of these
results, we employed the N2 pressure of 3 bar in this study. The
conversion followed a linear relationship with the catalyst amount
in range 0–150 mg. The first-order dependence of the catalyst is
clearly observed from the plot, which has been widely reported for
other transfer hydrogenation processes [41,42]. Phenol conversion
increased to 90% with catalyst amount of 150 mg, and the selec-
tivity to cyclohexanone was 95%. Full conversion of phenol was
achieved when 200 mg Pd/AC catalyst was used, with 80% selec-
tivity to cyclohexanone. In a duplicate experiment to determine
the formic acid utilization efficiency, close to 91.5% selectivity of
cyclohexanone was observed (Table S1, entry 4). Another finding
of this experiment was that 54.5% of formic acid was converted,
with 89.6% of which participating in the transfer hydrogenation.
This result emphasizes the advantage of formic acid as a reduc-
tant in selective phenol reduction, which avoids the unselective
hydrogenation to cyclohexanol as observed using H2.
3.2.3. Effect of some additives: formic acid, acetic acid and
sodium formate
The decreased activity in transfer hydrogenation compared with
hydrogenation using H suggests that phenol may retard the disso-
2
ciation of formic acid in transfer hydrogenation. On the other hand,
formic acid may also inhibit the hydride transfer from Pd hydride
to phenol. Therefore we conducted the hydrogenation experiments
in molecular dihydrogen by adding tiny amount of formic acid
(200 ␮L) and the results are shown in Table 3. It is clear that the
activity of phenol hydrogenation decreased when formic acid was
introduced. Moreover, the activity loss varied with different sup-
ports. The activity of Pd/Al O3 decreased three times from 76.5%
2
(Table 1, entry 5) to 26.7% (Table 3, entry 5). However, there is no
obvious loss of phenol conversion on Pd/C-comm catalyst. In partic-
ular, for Pd/AC, the activity increased slightly from 98.3% (Table 1,
entry 2) to 99% (Table 3, entry 2). It is unlikely that formic acid
would affect the activity of H-H dissociation and Pd hydride forma-
tion in liquid phase [43]. Chase et al. [43] have followed the hydride
formation by in situ XAFS technique in liquid phase hydrogenation
in presence of inorganic acid, where phosphorus acid showed no
significant effect on PdHx formation.
Please cite this article in press as: D. Zhang, et al., Transfer hydrogenation of phenol on supported Pd catalysts using formic acid as an
alternative hydrogen source, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.039

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5
100
100
80
5
mmol FA
13 mmol FA
0 mmol SF
1 mmol SF
80
60
40
20
0
80
6
0
1
2
H
2
60
40
40
20
0
20
0
50
100
150
200
250
300
Time on stream (min)
Con.
Sel.
Fig. 4. Phenol conversion vs. time on stream of transfer hydrogenation of phenol
with formic acid (FA) and sodium formate (SF) on 4.3 wt.% Pd/AC. Reaction condi-
tions: 10 mL 0.25 M aqueous solution of phenol, 20 mL/min N2 at 1 atm, 50 C.
0
50
100
150
200
◦
Catalyst amount (mg)
9
5.9%. We speculate that acetic acid or formate group may strongly
Fig. 3. Effect of amount of 4.3 wt.% Pd/AC catalyst on the transfer hydrogenation
reaction. Reaction conditions: 10 mL 0.25 M aqueous solution of phenol, 0.3 MPa N2,
adsorb on the Pd nanoparticles of Pd/Al O3 and thus leading to a
2
decrease in activity, while this “poisoning effect” is not observed on
Pd/AC catalyst. This result further indicates that Pd/AC catalyst is
superior to other supported Pd catalysts in transfer hydrogenation.
◦
4
00 ␮L formic acid, 50 C, 4 h.
Table 3
Phenol hydrogenation activity on supported Pd catalysts with molecular hydrogen
in presence of formic acid.
3.2.4. Kinetics of transfer hydrogenation on Pd/AC
We further conducted the kinetic study of transfer hydrogena-
tion on Pd/AC using different amounts of formic acid and sodium
formate in a round-bottom flask at 1 atm under N2 flow. For com-
parison, the hydrogenation with molecular dihydrogen was also
performed. The phenol conversion and cyclohexanone selectiv-
ity as a function of the reaction time are displayed in Fig. 4.
Interestingly, regardless of the amount (5 mmol or 21 mmol) or
groups of reductants (HCOOH, HCOONa, or H2), similar activities
were observed under identical conditions in initial period. Previous
studies have suggested that the rate-determining step of transfer
hydrogenation is the hydride transfer on Pd surface [42]. Therefore
Entry
Catalyst
Conv. (%)
Sel. (%)
C
O
OH
29.3
4.4
4.4
1.6
1.5
1
2
3
4
5
6
Pd/AC
99
70.7
95.6
95.6
98.4
98.5
87.4
Pd/C-comm
Pd/MIL-101
Pd/TiO2
Pd/Al2O3
Pd/TiO2-AC
81.4
56.3
31.7
26.7
89.7
12.6
Reaction condition: 100 mg catalysts, 10 mL aqueous solution of phenol (0.25 M),
.5 MPa H2, 200 ␮L of formic acid, 50 C, 4 h.
◦
0
−
this result may also indicate that the H-H and C H (H COO ) dis-
sociation are both very fast on Pd/AC catalyst. Phenol conversion in
all transfer hydrogenation reached a plateau and could not reach
To further understand the role of the formic acid in control-
ling the activity and selectivity hydrogenation, we conducted the
hydrogenation in presence of two other additives, sodium formate
and acetic acid, instead of formic acid. The results are collected
in Table 4. For Pd/AC catalyst, the phenol conversion remained
unchanged in presence of sodium formate or acetic acid. Inter-
estingly, the selectivity of cyclohexanone increased from 66.4% to
1
00% albeit excess amount of atomic hydrogen was introduced in
all cases. This can be explained by that some atomic hydrogen were
combined and elapsed as gas phase molecular dihydrogen in the N2
flow and did not take part in the hydrogenation.
7
7.8% when adding acetic acid, whereas it dropped to 33.2% when
3
.2.5. Reusability of Pd/AC in transfer hydrogenation
The reusability of the Pd/AC catalysts was investigated and the
adding sodium formate which might be partially due to a higher
transfer hydrogenation activity of formate salts for C O bonds [44].
As for commercial Pd/Al O , the phenol conversion decreased in
results are shown in Fig. 5. The catalytic activity remained con-
stant in the first three runs. Afterwards a dramatic activity loss from
6
2
3
both cases while the selectivity to cyclohexanone kept the same as
5.8% to 28.6% was observed and then kept constant. A slight change
of the textural property of Pd/AC after 6 runs was observed (Fig.
2
Table 4
S1), with surface area being decreased from 1330 to 997 m /g and
3
Phenol hydrogenation with molecular hydrogen in presence of sodium formate and
acetic acid.
pore volume being decreased from 0.90 to 0.69 cm /g, respectively.
This result means that strong adsorption of organic compounds
(e.g. reactants, products, or polymerized intermediates) might take
place during the reaction course, which may partially block the
active sites. Two other reasons responsible for the deactivation
are the aggregation of Pd nanoparticles and/or the leaching of Pd
metal. First, the particle may aggregate due to a weak interaction
between Pd and carbon. TEM analysis suggested that the size of Pd
nanoparticles of the used catalyst increased to 8.3 nm after 6 runs
and particles smaller than 3 nm disappeared (Fig. 6). Second, the
leaching of Pd in acidic solution was also observed by the ICP-AES
analysis, with Pd loading being decreased from 4.3 to 2.1 wt.% after
Cat.
Additive
Conv. (%)
Sel. (%)
C
O
OH
33.6
66.8
22.2
1.6
Pd/AC
Pd/AC
Pd/AC
Pd/Al2O3
Pd/Al2O3
Pd/Al2O3
None
98.3
98.1
97.9
76.5
61.5
56.2
66.4
33.2
77.8
98.4
95.9
98.6
Sodium formate
Acetic acid
None
Sodium formate
Acetic acid
4.1
1.4
Reaction condition: 100 mg catalyst, 10 mL aqueous solution of phenol (0.25 M),
.5 MPa H2, 50 C, 4 h, 200 ␮L of CH3COOH or equal mole of HCOONa.
◦
0
Please cite this article in press as: D. Zhang, et al., Transfer hydrogenation of phenol on supported Pd catalysts using formic acid as an
alternative hydrogen source, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.039

G Model
CATTOD-8932; No. of Pages6
ARTICLE IN PRESS
D. Zhang et al. / Catalysis Today xxx (2014) xxx–xxx
Acknowledgements
6
Conv
Sel
100
This work was financially supported by the National Natural
Science Foundation of China (grant no. 21203065) and the Fun-
damental Research Funds for the Central Universities.
8
6
4
2
0
0
0
0
0
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2014.
0
2.039.
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Please cite this article in press as: D. Zhang, et al., Transfer hydrogenation of phenol on supported Pd catalysts using formic acid as an
alternative hydrogen source, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.039