Selective phenol hydrogenation in aqueous phase on Pd-based catalysts supported on hybrid TiO2-carbon materials

Article abstract of DOI:10.1016/j.apcata.2011.07.018

TiO₂-C materials have been prepared by solvothermal and slurry synthesis and the influence of carbon properties upon the catalytic activity and selectivity of Pd-based catalysts on phenol hydrogenation in aqueous phase under mild reaction condi

Full text of DOI:10.1016/j.apcata.2011.07.018

Applied Catalysis A: General 404 (2011) 103–112
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Selective phenol hydrogenation in aqueous phase on Pd-based catalysts
supported on hybrid TiO2-carbon materials
Juan Matos∗,1, Avelino Corma∗∗
Instituto de Tecnología Química, UPV-CSIC, Universidad Politécnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
TiO2-C materials have been prepared by solvothermal and slurry synthesis and the influence of car-
bon properties upon the catalytic activity and selectivity of Pd-based catalysts on phenol hydrogenation
in aqueous phase under mild reaction conditions has been studied. The nature of the catalysts can be
modified to direct the reaction either to cyclohexanol (∼100% yield) or to cyclohexanone (∼96% yield).
High selectivity to cyclohexanone is obtained with Pd on more polar TiO2-C supports, while when these
are transformed into hydrophobic TiO2-C supports the resultant catalyst becomes selective to cyclohex-
anol. Thus, product distribution during the selective phenol hydrogenation in aqueous phase is easily
controlled by controlling the functionalization of the hybrid support.
Received 13 April 2011
Received in revised form 11 July 2011
Accepted 14 July 2011
Available online 23 July 2011
Keywords:
Phenol hydrogenation
Selectivity
Pd-based catalysts
Hybrid TiO2-C
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
gas-phase phenol hydrogenation is usually performed at high tem-
peratures over supported Pd catalysts and different supports [5–8].
The design of industrial catalysts for superior performance in
certain applications and for the development of novel processes
requires a proper understanding of the catalyst structure and the
surface interactions that occur during reaction. One growing area in
heterogeneous catalysis is the study of support/active phase inter-
actions, since they can remarkably affect the catalytic behavior
of the final catalysts. A reaction in where the interplay between
support and metal active phase becomes determinant, is the cat-
alytic hydrogenation of phenol to cyclohexanone. This ketone is
of industrial interest for the production of caprolactam and adipic
acid [1]. The industrial production of cyclohexanone commonly
involves the oxidation of cyclohexane [2,3] or the hydrogenation
of phenol. The former route generates by-products that compli-
cate purification, and the yield of cyclohexanone is usually low. The
phenol hydrogenation route is undertaken through either two-step
or one-step processes. In the two-step procedure, phenol is first
hydrogenated to cyclohexanol, which is in turn dehydrogenated
to cyclohexanone. The one-step selective hydrogenation of phenol
to cyclohexanone is advantageous from an efficiency standpoint
[4], and the reaction can be conducted in gas or liquid phase. The
However, liquid-phase phenol hydrogenation can be of interest
since the reaction can be performed at relatively low tempera-
tures [9–15]. Different catalysts such as Rh/C [9], Rh/C nanofiber
[10], Pd/hydrophilic C [11], Ru/poly(N-vinyl-2-pyrrolidone) (PVP)
[12], Pd/Mg and Pd/Fe [13], mesoporous Ce-doped Pd [14], and Pd/C
[15] have been studied. However, the attainment of high selectivity
(>95%) at high phenol conversion (>80%) is a challenge [4] because
cyclohexanone can be further hydrogenated to cyclohexanol under
the reaction conditions [7,14]. Jiang et al. [4] proposed a mechanism
for the phenol hydrogenation and stabilization of cyclohexanone
over a dual supported Pd–Lewis acid catalyst. These authors found
that conversion of cyclohexanone to cyclohexanol catalyzed by
Pd/C–AlCl3 is much lower than with Pd/C indicating that the Lewis
acid suppresses the hydrogenation of cyclohexanone. It has been
reported working with hybrid TiO2-C materials that the different
functional groups on the surface of carbon strongly interacts with
TiO2 and this interaction induce different adsorption mechanism
of phenol [16–18], 4-chlorophenol [19–21], methylene blue [22]
and 2-propanol [23], with the corresponding differences in the
selectivity of products during oxidation reactions. Therefore, we
thought that TiO2-carbon composites could be suitable supports
of an active metal (Pd) to catalyze the hydrogenation of phenol to
cyclohexanone. Furthermore, by varying the surface properties of
the support it could be possible to adsorb phenol by the hydroxyl
groups avoiding in this way the formation of cyclohexanol. Thus, a
series of TiO2-carbon composites have been prepared in where the
carbon was generated through different routes changing the polar-
ity of the surface. When Pd is supported on TiO2-carbon composites,
∗
Corresponding author. Tel.: +58 212 5041166; fax: +58 212 5041166.
Corresponding author. Fax: +34 96 3877809.
E-mail addresses: jmatos@ivic.gob.ve, jmatoslale@gmail.com (J. Matos),
∗∗
acorma@itq.upv.es (A. Corma).
1
On leave from the Venezuelan Institute for Scientific Research, (IVIC), 20632,
Caracas 1020-A, Venezuela.
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.07.018

104
J. Matos, A. Corma / Applied Catalysis A: General 404 (2011) 103–112
Table 1
Summary of textural properties of supports employed for the phenol hydrogenation on Pd(1%, w/w)-based catalysts. Surface BET area (SBET), micropore area (S␮pore), micropore
volume (V␮pore), total pore volume (Vtot), mean pore diameter (Wpore).
SBET (m2 g−1
)
S
(m2 g−1
)
V
(cm3 g−1
␮pore
)
Vtot (cm3 g−1
)
Wpore (nm)
a
a
b
b
Support
␮pore
TiO2-P25
Fu-TiO2-C
Sac-TiO2-C
Fu-TiO2-C-C
Sac-TiO2-C-C
TiO2-ACCO
TiO2-ACN
TiO2-ACZnCl
TiO2-ACH
45
123
83
81
253
86
60
93
63
943
644
689
247
0
13
27
17
225
49
14
41
21
913
643
688
231
0.002
0.008
0.013
0.008
0.111
0.026
0.011
0.025
0.013
0.409
0.264
0.271
0.089
0.153
0.236
0.384
0.188
0.196
0.375
0.416
0.482
0.388
0.470
0.284
0.272
0.107
57.8
4.32
3.40
5.31
2.95
97.4
105.1
97.9
103.4
0.630
0.590
0.590
0.594
2
2
2
PO
3
4
ACCO
2
ACN
2
ACZnCl
ACH
2
PO
3
4
a
Obtained by HJ method.
Obtained by HK method.
b
it is possible to achieve close to 100% yield to cyclohexanone or to
cyclohexanol depending of the surface properties of the support.
methodology reported elsewhere [16,18,21,23] with the relative
amount TiO2:AC, expressed in mass, i.e. 10:1. In a typical synthesis,
250 mg TiO2-P25 with 25 mg AC in 80 mL of deionized H2O was vig-
orously stirred for 20 min at room temperature. After this time the
homogeneous slurry was filtered and dried by 2 h at 100 ◦C. Sam-
2. Experimental
ples prepared by this way were denoted as TiO2-ACCO TiO2-ACN
,
2.1. Materials
2
2
TiO2-ACZnCl , and TiO2-ACH
.
PO
3
4
2
Furfural, saccharose, titanium (IV) isopropoxide and phenol
were analytical grade and purchased from Sigma–Aldrich. Abso-
lute ethanol was purchased by VWR and used as supplied. TiO2-P25
(Degussa) was employed as a standard support for comparative
reasons. It consist of mainly anatase phase (ca. 70%), non-porous
polyhedral particles of ca. 30 nm mean size and BET surface area
2.4. Synthesis of Pd-supported catalysts
The synthesis of Pd-supported catalysts was performed by the
wetness impregnation method with PdCl2 as the source of Pd. In a
typical synthesis, 8.5 mg of PdCl2 was dissolved under vigorously
stirring in 4 mL of HCl aqueous solution (1 M). Once the PdCl2 was
fully dissolved, 0.5 g of the support was added under vigorously
stirring for about 20 min until the formation of homogenous slurry,
After this time, the slurry was heated at 80 ◦C for 30 min to obtain
incipient wetness. Finally, catalysts were dried under static air at
100 ◦C for 2 h. Table 1 showed a summary of textural properties of
the supports employed for the phenol hydrogenation on Pd(1%)-
based catalysts.
45 m2 g−1
.
2.2. Solvothermal synthesis of hybrid TiO2-C
In a typical synthesis [22], 1 g of furfural and 1 g of titanium
(IV) isopropoxide were dissolved in 18 mL of ethanol. The resulting
solution was sealed into a glass vial inside a Teflon-lined autoclave,
followed by solvothermal treatment at 175 ◦C overnight. The result-
ing brown solid was separated from the solution by filtration and
washed several times with ethanol. The material was dried under
static-air at 100 ◦C for 2 h and the resultant sample was denoted
Fu-TiO2-C. Same procedure was followed for the sample prepared
from saccharose and this solid was denoted Sac-TiO2-C. Carbonized
samples were obtained from these two samples by calcining the Fu-
TiO2-C and Sac-TiO2-C composites under N2 flow from ambient to
800 ◦C and then leaving this temperature by 5 h. The carbonized
samples were denoted Fu-TiO2-C-C and Sac-TiO2-C-C.
2.5. Characterization
Textural
characterization
was
performed
by
N2
adsorption–desorption isotherms at 77 K. The full isotherms
in the range of 4 × 10−3 to 84 kPa were measured in a Micromerit-
ics ASAP-2020. Surface area, micropore area and volume, and
pore diameters were obtained by Brunauer–Emmet–Teller (BET),
Harkins–Jura (HJ) and Horvath–Kawazoe (HK) methods, respec-
tively [25–27]. HJ [26] and HK [27] methods were employed
because in the present case TiO2-C materials can contain simul-
taneously slits and spherical pores. Fourier transform infrared
spectra of KBr wafers were recorded at room temperature in the
400–4000 cm−1 region with a Nicolet 205xB spectrophotometer,
equipped with a Data Station, at a spectral resolution of 1 cm−1
and accumulations of 128 scans. X-ray photoelectron spectroscopy
(XPS) measurements were performed with a SPECS spectrometer
equipped with a MCD-9 detector and using a non monochromatic
AlK␣ (1486.6 eV) X-ray source. Spectra were recorded using
analyzer pass energy of 50 V, an X-ray power of 50 W and under
an operating pressure of 10−9 mbar. Spectra treatment has been
performed using the CASA software. Binding energies (BE) were
referenced to C1s at 284.5 eV. Powder X-ray diffraction (XRD) pat-
terns were recorded in the range of 2ꢀ = 2–90◦ with a Cubix-PRO
diffractometer from PANalytical with CuK␣ (1.54056) radiation
and processed with the PW1877 program (Philips). Scanning
2.3. Activated carbon (AC) and TiO2-AC synthesis
Activated carbon (AC) powders were prepared from sawdust
of Tabebuia Pentaphyla wood by different methods to introduce
different texture and surface properties [24]. Physical activation
by gasification under CO2 flow at 800 ◦C and by pyrolysis under
N2 flow at 1000 ◦C by 1 h were firstly performed [19]. These AC
were denoted ACCO and ACN Chemical activation [20] was also
.
performed after impregnation2of the sawdust with 5% (w/w) aque-
ous solutions of ZnCl2 and H3PO4 followed by activation under
N2 flow at 450 ◦C for 1 h and finally extensively washed and
dried under static air at 100 ◦C for 2 h. These AC were denoted
as ACZnCl and ACH 4 . The synthetic conditions for the AC were
2
PO
3
2
selected because these AC showed the highest synergy effect
in presence of TiO2 during the catalytic photodegradation of 4-
chlorophenol [19,20]. TiO2-AC samples were obtained by a single

J. Matos, A. Corma / Applied Catalysis A: General 404 (2011) 103–112
105
electron microscopy (SEM) micrographs were collected in a JEOL
6300 microscope operating at 20 kV, 2 × 10−9 A beam current
˚
and15 mm as working distance. Samples were carbon coated to
eliminate charging effects.
Nanoparticle size and zeta potential measurements were con-
ducted using a Zetasizer Nano ZS (Malvern Instruments Ltd.). Dry
materials were re-suspended in deionized water at a concentra-
tion of 5 mg mL−1 and diffuse light scattering (DLS) measurements
were performed at 25 ◦C and 173◦ scattering angle. The mean
hydrodynamic diameter and zeta potentials were determined by
accumulative analysis. Surface pHPZC of bare activated carbon
materials and the binary and hybrids TiO2-C were obtained by the
drift method [19,20] by comparing the pH measured after 48 h stir-
ring (to achieve the equilibrium of charges) against the pH of initial
buffer solutions.
Thermogravimetric analysis (TGA) of the supports was per-
formed in a Perkin–Elmer TGA7 instrument. In order to be able
to quantitatively measure the amount of water occluded in the
samples, prior to the TGA, samples were kept for 24 h under an
atmosphere with a controlled humidity of ca. 33% (given by a
MgCl2·6H2O saturated solution kept at room temperature). TGA
were then performed under O2 flow (100 mL min−1) at a heating
rate of 10 ◦C min−1 from 25 to 600 ◦C.
2.6. Catalytic tests
A conic reinforced glass batch reactor (3.3 mL) was used to carry
out the liquid phase hydrogenation. Prior to the reaction, the cata-
lysts (0.0213 mmol Pd) were in situ reduced under H2 flow at 200 ◦C
for 2 h. After this time, and once the reactor is at room temperature,
phenol (0.426 mmol, 40 mg) was dissolved in 2 mL of water and
introduced into the reactor. The reactor was purged three times
with H2 and then pressurized with 5 bars H2. The reaction mixture
was heated up to 100 ◦C. The products were periodically analyzed
and identified by GC with an internal standard.
3. Results and discussion
3.1. Characterization of supports
Fig. 1. SEM images. (A) Fu-TiO2-C. (B) After carbonization, Fu-TiO2-C-C.
3.1.1. Texture
can be attributed to oxygen group removal from the surface of this
hybrid-material during carbonization [28]. Table 1 also shows an
Table 1 shows a summary of textural properties of supports
employed for the selective phenol hydrogenation on Pd(1%)-based
catalysts. This table contains the surface BET area (SBET), micropore
increase in the BET surface area from 83 m2 g−1 to 253 m2 g−1
,
a decrease in the total pore volume from 0.3844 cm3 g−1 to
0.1955 cm3 g−1 and a decrease from 3.4 nm to 3.0 nm of the mean
pore diameter for Sac-TiO2-C and Sac-TiO2-C-C, respectively. The
contribution of micropore area to total surface area was about 90%
in the Sac-TiO2-C-C indicating the activation of carbon framework
during carbonization, as reported elsewhere [29,30] for carbon
foams obtained from the controlled pyrolysis of saccharose.
area (S␮pore), micropore volume (V␮pore), total pore volume (Vtot
)
and mean pore diameter (Wpore) of the supports studied. Supple-
mentary material (Fig. S1A) shows the N2 adsorption–desorption
isotherm of Fu-TiO2-C and suggests that this hybrid material is
mesoporous following a type IV adsorption isotherm with an
important hysteresis loop. This is in agreement with the mod-
erate surface area of about 123 m2 g−1 and with the mean pore
width of about 4.3 nm (Table 1). It should be noted that a low
but not-negligible micropore contribution of about 10% of the sur-
face area was found for Fu-TiO2-C. By contrast, a type II adsorption
isotherm with a very small and vertical hysteresis can be observed
for Sac-TiO2-C, which is attributed to aggregation of nanoparticles
(Fig. S1B). This isotherm corresponds to a mesoporous material
with a low surface area ca. 83 m2 g−1 and with a mean pore width
ca. 3.4 nm (Table 1). Table 1 also shows that after carbonization
at 800 ◦C under N2 flow, the textural parameters of Fu-TiO2-C
and Sac-TiO2-C clearly changed. A decrease in BET surface area
(ca. 80 m2 g−1) and in total pore volume (0.1881 cm3 g−1) was
found for Fu-TiO2-C-C in comparison with values of Fu-TiO2-C
(123 m2 g−1 and 0.2362 cm3 g−1), while the mean pore diameter
changed from ca. 4.3 nm to 5.3 nm. Changes in textural properties
3.1.2. SEM images and TG analysis
Fig. 1 shows the SEM images of Fu-TiO2-C and Fu-TiO2-C-C. Both
samples are constituted by two different sizes of aggregated parti-
cles assembled into spheres. A mean particle size of about 4.1 ␮m
for the larger particles and between 0.2 and 0.4 ␮m for the smaller
ones was detected for Fu-TiO2-C support (Fig. 1A) while a mean
particle size of about 3.4 ␮m with smaller particles between 0.3
and 0.5 ␮m was detected for the carbonized support (Fig. 1B). Fig. 2
showed SEM images of Sac-TiO2-C and Sac-TiO2-C-C supports. In
this case, both samples are also constituted by aggregated particles
with two different sizes assembled into spheres. A mean parti-
cle size of about 3.4 ␮m for the larger particles and between 0.3
and 0.5 ␮m for the smaller ones was detected for Sac-TiO2-C sup-

106
J. Matos, A. Corma / Applied Catalysis A: General 404 (2011) 103–112
Table 2
Thermogravimetric analysis of supports studied in the hydrogenation of phenol on Pd-based catalysts. Conditions: ambient to 200 ◦C (10 ◦C min−1) under O2 flow
(100 mL min−1).
Support
Lost weight (%)30–110 ◦
C
Lost weight (%)110–200 ◦
C
Lost weight (%)30–200 ◦
C
Lost weight (%)30–600 ◦
C
TiO2-P25
Fu-TiO2-C
Sac-TiO2-C
Fu-TiO2-C-C
Sac-TiO2-C–C
TiO2-ACCO
TiO2-ACN
TiO2-ACZnCl
TiO2-ACH
0.9
2.7
4.6
0.6
1.2
0.6
0.5
2.9
3.0
1.3
1.0
3.8
3.9
0.5
0.7
1.8
0.4
1.0
0.5
0.3
0.5
0.4
0.2
0.3
0.5
0.6
1.4
3.4
6.4
1.0
2.2
1.1
0.8
3.4
3.4
1.5
1.3
4.3
4.5
2.7
27.7
58.9
6.1
35.9
5.8
3.2
7.4
9.4
2.5
2
2
2
PO
3
4
ACCO
2
ACN
2.0
4.5
6.0
2
ACZnCl
ACH
2
PO
3
4
Table 3
Summary of crystalline phase proportions of TiO2 in the different supports.
Solids
Anatase
Rutile
proportiona (%)
proportiona (%)
TiO2-P25c
Fu-TiO2-C
70
80
30
20
Fu-TiO2-C-carbonized
Sac-TiO2-C
78
22
b
b
Sac-TiO2-C-carbonized
47
53
a
Obtained from the quantification of all crystalline phases detected in the XRD
patterns.
b
XRD pattern from Fig. 3 suggests Anatase is the most important crystalline phase.
The Anatase/Rutile proportion remains the same that TiO2-P25 in the binary
c
materials: TiO2-ACCO , TiO2-ACN , TiO2-ACZnCl , TiO2-ACH
.
PO
4
2
2
3
2
Fig. 2. SEM images. (A) Sac-TiO2-C. (B) After carbonization, Sac-TiO2-C-C.
port (Fig. 2A) while a mean particle size of about 3.7 ␮m with the
smaller particles between 0.2 and 0.5 ␮m was detected for the car-
bonized support (Fig. 2B). SEM images of Sac-TiO2-C-C (Fig. 2B)
showed a higher proportion of smaller particles than for Sac-TiO2-
C (Fig. 2A). This can be associated with an important fragmentation
of the bigger particles of Sac-TiO2-C during carbonization, as sug-
gested by the TG analysis (Fig. S4) compiled in Table 2. A total
loss of about 59% (fifth column, Table 2) for the Sac-TiO2-C sup-
Fig. 3. XRD patterns of TiO2-P25, solvothermal and carbonized supports.

J. Matos, A. Corma / Applied Catalysis A: General 404 (2011) 103–112
107
1,4
A
B
1,0
0,8
0,6
0,4
0,2
0,0
1,2
1,0
0,8
0,6
0,4
0,2
0,0
4000 3500 3000 2500 2000 1500 1000 500
0
4000 3500 3000 2500 2000 1500 1000 500
0
-1
-1
Wavenumber (cm )
Wavenumber (cm )
C
D
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,5
0,4
0,3
0,2
0,1
0,0
4000 3500 3000 2500 2000 1500 1000 500
0
4000 3500 3000 2500 2000 1500 1000 500
0
-1
-1
Wavenumber (cm )
Wavenumber (cm )
Fig. 4. FTIR spectra of solvothermal TiO2-C supports before and after calcination at 800 ◦C. (A) Fu-TiO2-C, (B) Fu-TiO2-C-C and (C) Sac-TiO2-C and (D) Sac-TiO2-C-C.
carbonization [37]. XRD pattern of Sac-TiO2-C-C (Fig. 3) suggests
TiO2 is constituted by crystallites of smaller size than those of Fu-
TiO2-C-C in agreement with SEM images (Figs. 1B and 2B).
port indicates that some particles could be destroyed during the
carbonization, in agreement with the remarkable increase in
surface area from 83 m2 g−1 to 253 m2 g−1 (Table 1). From the
TG results in Table 2, two different types of TiO2-C supports
can be pointed out. Hydrophilic TiO2-C (Fu-TiO2-C, Sac-TiO2-C,
TiO2-ACZnCl and TiO2-ACH 4 ) with more than 3.4% (w/w) of
3.1.4. FTIR and XPS spectra
Fig. 4 presents the FTIR spectra of hybrid TiO2-C supports before
and after calcination at 800 ◦C. It can be seen from Fig. 4A and C
for Fu-TiO2-C and Sac-TiO2-C, respectively, that the OH bending
bands of aliphatic (1020 cm−1) and aromatic alcohols (3430 cm−1),
and the C O (1630 cm−1) vibration are the most important absorp-
tion bands. By contrast, almost no functional polar groups were
detected on Fu-TiO2-C-C and Sac-TiO2-C-C, Fig. 4B and D, respec-
tively. In other words, the hydrophilicity of hybrid TiO2-C samples
decrease after carbonization in agreement with TG results on water
adsorption–desorption. The present results are in line with those
reported by Titirici et al. [11] for hydrophilic carbon materials pre-
pared by solvothermal synthesis. The same trends were observed in
the case of the TiO2-AC binary supports (Supplementary Material,
Fig. S7). In short, acid functional groups such as carboxylic acid and
phenols practically disappeared when the activation temperature
PO
3
2
water evolved up to 200 ◦C, and hydrophobic materials (Fu-TiO2-
C-C, TiO2-ACCO and TiO2-ACN2 ) which in most of the cases adsorb
2
about 1% (w/w) water, with the exception of Sac-TiO2-C-C sample.
As will be presented below, the FTIR and XPS spectra will show that
the presence of acid and basic surface groups should be expected.
This in an important feature since it has been well establish that
the surface oxygen groups, which act as anchoring sites for metallic
precursors as well as for metals, commonly determine the proper-
ties of activated carbons as a catalyst support [16,17,24,31–35].
3.1.3. XRD patterns
Fig. 3 shows the XRD patterns of TiO2-P25, Fu-TiO2-C, Fu-TiO2-
C-C, Sac-TiO2-C, and Sac-TiO2-C-C, while in Table 3 a summary
of the crystalline phases present in TiO2 is given. XRD patterns
show that in both solvothermal materials, TiO2 in Fu-TiO2-C mainly
crystallizes as anatase [22,36] with an Anatase:Rutile ratio of 4:1
(Table 3), which is higher than that observed with TiO2-P25 of
about 2.3:1. After carbonization, the ratio Anatase to Rutile remains
practically unchanged (Table 3), but the crystallinity of TiO2 in
Fu-TiO2-C-C increases suggesting the presence of larger TiO2 crys-
tallites. In the case of Sac-TiO2-C, the XRD pattern showed that
carbon fully overlaps the TiO2 XRD peaks indicating the encapsu-
lation of TiO2 crystallites inside the carbon matrix. Fig. 3 shows an
important contribution of the Rutile phase with an Anatase:Rutile
of about 0.89 for Sac-TiO2-C-C, suggesting that a strong interac-
tion between TiO2 crystallites and C atoms has occurred during
of AC is high enough i.e. 800 ◦C or 1000 ◦C for ACCO and ACN
,
2
2
respectively, while at low activation temperature, the surface is
more hydrophilic [18,20,34,38,39].
Fig. 5 shows the XPS spectra in the region C1s of Fu-TiO2-C
(Fig. 5A) and Sac-TiO2-C (Fig. 5B). Data were referenced to C1s level
at 284.5 eV. Deconvolution of the C1s peak clearly indicates dif-
ferences in the surface functional groups [40]. Besides the peak
corresponding to the C–C bonding at about 284.5 eV [38], Fig. 5A
shows one peak at about 286.7 eV which can be attributed to
carbonyl groups (C O) indicating the hydrophilic nature of Fu-
TiO2-C hybrid support. XPS spectra in the region C1s of Sac-TiO2-C
(Fig. 5B) shows that besides the expected C–C bonding peak at about

108
J. Matos, A. Corma / Applied Catalysis A: General 404 (2011) 103–112
Table 4
A
Summary of kinetic results of phenol hydrogenation on Pd-based catalysts. Initial
activity, maxima phenol conversion (Phconv), selectivity of cyclohexanone (SC
and cyclohexanol (SOH) at maxima phenol conversion.
180000
160000
140000
120000
100000
80000
)
O
C1s
C-C
Catalyst
Pda (wt%) Activityb
(␮mol gPd−1 s−1
Phconv
(%)
SC
(%)
SOH
(%)
O
C=O
Unknown
Baseline
)
Pd/TiO2-P25
1.1
1.0
0.9
0.9
1.0
0.9
1.0
0.9
0.9
1.0
1.1
0.9
0.9
45.5
99
99
99
99
99
99
99
99
99
99
99
22c
6c
98
97
93
9
2
3
7
Pd/Fu-TiO2-C
Pd/Sac-TiO2-C
Pd/Fu-TiO2-C-C
Pd/Sac-TiO2-C-C
Pd/TiO2-ACCO
Pd/TiO2-ACN
71.0
40.3
156.2
60.8
62.1
48.3
91.1
62.2
52.8
23.7
91
92
76
81
4
11
37
54
34
7
8
24
19
96
89
63
46
66
93
2
2
Pd/TiO2-ACZnCl
Pd/TiO2-ACH
2
PO
3
4
60000
Pd/ACCO
2
Pd/ACN
2
Pd/ACZnCl
Pd/ACH
0.17
0.01
2
40000
PO
3
4
300
295
290
285
280
275
270
a
wt% of palladium measured by inductively coupled plasma (ICP) chemical anal-
Binding energy (eV)
ysis.
b
Phenol converted (␮mol) per Pd gram per second.
These catalysts never reached 99% phenol conversion
c
B
250000
200000
150000
100000
50000
C1s
C-C
C-OR, C-OH
Baseline
It has been recently claimed that Pd encapsulated on spe-
cial carbons [11] as well as on alumina [45] are very active and
cyclohexanone selective for the most hydrophilic samples. If we
compare the activity per atom of Pd (measured from initial rates) of
those reported catalysts with the activity of Pd/TiO2-P25 reported
here (Table 4), it can be seen that the activity of the former are
lower, but with a higher selectivity to cyclohexanone.
Following the reaction mechanism proposed in the literature
and a previous work [45], it becomes clear that it should be pos-
sible to increase activity and selectivity to cyclohexanone if we
could achieve a hybrid TiO2-C support with higher hydrophilicities.
From the different characterization techniques described above we
have seen that the composites prepared by solvothermal synthesis
are more hydrophilic than the corresponding carbonized samples.
Accordingly, one should see higher selectivity to cyclohexanone in
the non carbonized composites, as can be seen from the results pre-
sented in Table 4. A similar correlation is observed with activated
300
295
290
285
280
275
270
Binding energy (eV)
Fig. 5. XPS spectra in the region C1s of the solvothermal carbon materials. (A) Fu-
TiO2-C and (B) Sac-TiO2-C. Data were corrected to C 1s (284.5 eV).
carbons, being the most selective to cyclohexanone the Pd/ACZnCl
2
that combines high hydrophilicity, with the presence of a Lewis acid
(ZnCl2). Besides the higher selectivity obtained with composites
(Pd/Fu-TiO2-C and Pd/TiO2-ACZnCl2 ) their activity is also higher than
Pd/TiO2-P25 and the carbon based catalysts previously reported
[11,43]. On the other hand, it is remarkable that with the most
hydrophobic composite, i.e. Pd/Fu-TiO2-C-C, it is possible to achieve
practically 100% selectivity to cyclohexanol. It is then possible by
following different methods of preparation and activation of TiO2-
carbon composites to direct the phenol hydrogenation either to
cyclohexanone or cyclohexanol, being a key variable for catalyst
preparation, the polarity of the support.
284.3 eV, a broad peak at about 285.9 eV is found and attributed to
cyclic ethers (C–O–R) and bonding of aliphatic alcohols C–OH [38],
indicating the hydrophilic nature of the Sac-TiO2-C surface. This is
in agreement with the TG analysis discussed above.
3.2. Phenol hydrogenation
3.2.1. Activity and selectivity of Pd-based catalysts supported on
hybrid TiO2-C materials
Figs. 6 and 7 show the kinetic results of phenol hydrogenation on
Pd-based catalysts (conversion and yields of products) as a function
of reaction time. Meanwhile a summary of selected results, mainly,
the initial catalytic activity, maxima phenol conversion (Phconv),
selectivity of cyclohexanone (SC O) and cyclohexanol (SOH) are
given in Table 4.
The most common accepted mechanism for the hydrogena-
tion of phenol on supported Pd catalysts (see Fig. 8, which will
be discussed below) indicates that phenol is adsorbed and acti-
vated on the support while H2 is activated on the Pd and produces
the hydrogenation of the adsorbed phenol [41–44]. However,
the hydrogenation of phenol yields cyclohexanone and cyclohex-
anol depending that the adsorption of phenol occurs through the
hydroxyl or the aromatic ring respectively [41,42].
3.2.2. General discussion
From TG, FTIR, and XPS analysis it was suggested that the
present TiO2-C materials can be differentiated in hydrophilic and
less hydrophilic (or hydrophobic) ones. It was also inferred that car-
bonization at high temperatures introduce basic functional groups
which affects clearly the adsorption of polar molecules like water.
This influence is clear in Sac-TiO2-C-C support where in spite of
surface area of this material is about 3 times higher (Table 1)
than that of Sac-TiO2-C (without carbonization), Sac-TiO2-C-C has
less tendency for the adsorption of water molecules indicating
a hydrophobic behavior (Table 2). A similar trend is found for
the hybrid support Fu-TiO2-C-C which adsorbs less water than
that of non-carbonized sample and for the binary TiO2-ACCO and
2
TiO2-ACN supports which are composed with AC prepared at high
2

J. Matos, A. Corma / Applied Catalysis A: General 404 (2011) 103–112
109
Fig. 6. Kinetics of phenol hydrogenation on Pd/TiO2, Pd/Fu-TiO2-C and Pd/Fu-TiO2-C-C.
temperatures (800 ◦C for ACCO and 1000 ◦C for ACN2 ). In other
words, acidic groups on the su2rface decrease the hydrophobicity
of the carbon [16,24,34], leading to the accessibility of the sur-
face to polar or acid molecules as water or phenol, respectively,
while the less acidic surface groups increase the interaction of
Lewis acids as TiO2 with the support and, as a consequence, reduc-
ing the hydrophilic behavior of the later [34]. For example, it can
be seen from Table 2 that TiO2-ACCO and TiO2-ACN lost smaller
2
2
weight after heating up to 200 ◦C than that of TiO2 alone. By con-
trast, TiO2-ACZnCl and TiO2-ACH
lost higher weight than that of
PO
3
4
2
TiO2 alone. This result suggests that in presence of hydrophobic AC
such as ACCO and ACN , TiO2 reduce its hydrophilicity but in pres-
2
2

110
J. Matos, A. Corma / Applied Catalysis A: General 404 (2011) 103–112
Table 5
pHPZC of 7.4 and 7.1 for Fu-TiO2-C-C and Sac-TiO2-C-C in compari-
son of acidic surface pH found for the case of non-carbonized hybrid
samples with values of pHPZC equal to 5.8 and 5.0 for Fu-TiO2-C and
Sac-TiO2-C, respectively. In addition, it can be seen that the pres-
ence of basic AC lightly increase the pHPZC of TiO2 (6.8 against 6.5)
while in presence of acidic AC the surface pH of the binary materials
clearly decrease from 6.5 for the TiO2 alone to 6.2 and 6.0 for the
case of TiO2-ACZnCl and TiO2-ACH 4 , respectively. From pHPZC
Summary of surface pH (pHPZC), mean hydrodynamic diameter (D) and zeta potential
(Zpot) results on the different support studied in the phenol hydrogenation.
Support
pHPZC
D (nm)
Zpot (mV)
TiO2-P25
Fu-TiO2-C
Sac-TiO2-C
Fu-TiO2-C-C
Sac-TiO2-C-C
TiO2-ACCO
TiO2-ACN
TiO2-ACZnCl
TiO2-ACH
6.5
5.8
5.0
7.4
7.1
6.8
6.8
6.2
6.0
8.5
8.9
6.0
4.0
128.8 15.2
194.6 7.4
>1000
155.7 3.4
>1000
103.0 12.2
109.5 12.9
61.6 14.3
53.5 11.0
>1000
+26.1 0.3
−17.0 0.4
−16.5 0.2
−20.0 1.2
−18.9 1.4
−17.8 1.2
−25.6 1.9
−10.7 2.1
−12.1 0.8
−22.3 1.5
−30.5 2.8
−18.1 2.5
−15.3 2.0
PO
3
2
results it can be concluded that Fu-TiO2-C-C, Sac-TiO2-C-C, ACCO
2
2
and ACN are hydrophobic materials while Fu-TiO2-C, Sac-TiO2-C,
2
2
ACZnCl and ACH
are hydrophilic. In addition, the presence of
PO
4
2
3
2
PO
3
4
carbon clearly influences the polarity of the TiO2 as indicated the
change in the zeta potential showed in Table 5. In all the cases, it can
be seen that the presence of carbon stabilizes the electronic charge
of the TiO2 because the zeta potential clearly change from a posi-
tive charge of 26.1 to negative values. In other words, the polarity
of the carbon support influences the polarity of the TiO2.
ACCO
2
ACN
>1000
>1000
>1000
2
ACZnCl
ACH
2
PO
3
4
The pHPZC and zeta potential results are in line with the selec-
tivity results. Table 4 shows that most of Pd catalysts supported
on hybrid TiO2-C or binary TiO2-AC supports with hydrophilic sur-
face showed higher cyclohexanone/cyclohexanol ratios than that
obtained on Pd/TiO2-P25, while in the case of hybrid TiO2-C or
binary TiO2-AC supports with hydrophobic surface the cyclohex-
anone/cyclohexanol ratios are very low indicating that polarity of
the support remarkably influence the selectivity.
ence of hydrophilic AC such as ACZnCl and ACH
its hydrophilicity.
The pHPZC and zeta potential results compiled in Table 5 con-
firmed the above inferences. As expected, the higher the activation
or pyrolysis temperature employed in the synthesis of AC the higher
TiO2 increase
4
PO
3
2
the pHPZC (8.5 and 8.9 for ACCO and ACN2 , respectively) which
2
indicate the presence of basic functional groups on the surface of
ACCO and ACN2 . Concerning to AC prepared by chemical activation
2
Fig. 9 shows the correlation between cyclohexanone selectivity
at maxima phenol conversion as a function of the zeta poten-
tial (Table 5). It can be pointed out that hydrophilic carbons
lead to high selectivity of cyclohexanone while in presence of
hydrophobic carbon the main product is cyclohexanol (Table 4).
(ACZnCl and ACH 4 ), it can be seen that ACH
showed a very
PO
4
PO
3
acidic and lower pHPZC value (about 4.0) than3ACZnCl (about 6.0)
indicating that H3PO4 introduces more acidic groups o2n the AC sur-
face as reported elsewhere [20]. A similar trend was found for the
case of the carbonized at 800 ◦C hybrid TiO2-C materials with basic
2
Fig. 7. Kinetics of phenol hydrogenation on Pd/Sac-TiO2-C and Pd/Sac-TiO2-C-C.

J. Matos, A. Corma / Applied Catalysis A: General 404 (2011) 103–112
111
Fig. 8. Hydrogenation of phenol. (A) General reaction steps. (B) General pathway for selective hydrogenation of phenol to cyclohexanone.
Fig. 10. Changes in cyclohexanone selectivity as a function of the surface pH of
different series of supports.
anol via hydrogenation. The fast isomerization of cyclohexenol
to cyclohexanone is in agreement with the fact that this cyclic
unsaturated alcohol was not detected in the present work. It has
been reported that the mechanism of the hydrogenation of phe-
nol and the formation of cyclohexanone in the vapor phase on
Pd-based catalyst supported on acid Lewis supports [4] strongly
depend of the adsorption orientation of the phenol molecule, which
depends of the acidity and basicity of the medium or the support,
giving cyclohexanol and cyclohexanone, respectively. However, we
have found that hydrophilic and therefore acid supports leads to
high cyclohexanone selectivity while hydrophobic and therefore
basic support leads to high cyclohexanol selectivity in line with
results from Jiang et al. [4] and Chatterjee et al. [42] and this could
Fig. 9. Cyclohexanone selectivity (%) after 60 min reaction as a function of zeta
potential.
Therefore, the present results suggest that reaction follow a simi-
lar mechanistic pathway for the selective phenol hydrogenation to
cyclohexanone on Pd catalysts supported on hydrophilic materi-
als in agreeing with many works reported elsewhere [4,11,44,45].
It can be seen in Fig. 8A, the general reaction steps for the hydro-
genation of phenol [4], in a first step, the hydrogenation produces
cyclohexenol as intermediate product which readily isomerizes to
produce cyclohexanone, which in a second step gives cyclohex-

112
J. Matos, A. Corma / Applied Catalysis A: General 404 (2011) 103–112
be a consequence of a competition between the influence of tex-
tural and surface properties, mainly the polarity of the support.
Fig. 8B shows the general pathway for the selective hydrogena-
tion of phenol to cyclohexanone in aqueous phase. The pathway
showed in Fig. 8B is adapted from that reported by Jiang et al. [4].
In the present case, a co-planar adsorption orientation of phenol on
hydrophilic TiO2-C materials is considered in a first step. Herrmann
and Matos [16,17] have been reported that in aqueous phase phe-
nol is preferentially adsorbed with this orientation on hydrophilic
materials while is non-planar adsorbed on hydrophobic materials.
Then, in a second step, phenol diffuse to Pd nanoparticles where
the benzene ring is partially hydrogenated to cyclohexanol that
fastly isomerizes because one of the carbon in benzene tends to
become highly nucleophilic, inducing attack of the benzene ring by
an incipient electrophile (H+) from H2 chemi-dissociative adsorp-
tion on Pd particles producing the cyclohexanone. The inhibition of
the cyclohexanone hydrogenation is inhibited in presence of Lewis
acid support (TiO2-P25) or on hydrophilic carbon materials proba-
bly by a strong interaction of the cyclohexanone with the acid sites
[4] and because phenol is a strong H-bridge interacting system,
and therefore it is enriched in the pores of the hydrophilic support,
consequently, cyclohexanone is displaced (step 3) from the reac-
tive surface in line with previous results from Titirici et al. [11]. By
contrast, less hydrophilic or hydrophobic surface clearly induce the
hydrogenation of cyclohexanone to cyclohexanol indicated in the
step 2 of Fig. 8A.
Acknowledgement
Authors thank to José A. Gaona from ITQ-UPV for the technical
support.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.07.018.
References
[1] B.M. Nagaraja, V.S. Kumar, V. Shashikala, A.H. Padmasri, S.S. Reddy, B.D. Raju,
K.S. Rama, J. Mol. Catal. A: Chem. 223 (2004) 339–345.
[2] L.X. Xu, C.H. He, M.Q. Zhu, K.J. Wu, Y.L. Lai, Catal. Commun.
816–820.
[3] L. Liu, Y. Li, H. Wei, M. Dong, J. Wang, A.M.Z. Slawin, J. Li, J. Dong, R.E. Morris,
Angew. Chem. Int. Ed. 48 (2009) 2206–2209.
[4] H. Liu, T. Jiang, B. Han, S. Liang, Y. Zhou, Science 326 (2009) 1250–1252.
[5] S.G. Shore, E. Ding, C. Park, M.A. Keane, Catal. Commun. 3 (2002) 77–84.
[6] L.M. Sikhwivhilu, N.J. Coville, D. Naresh, K.V.R. Chary, V. Vishwanathan, Appl.
Catal. A: Gen. 324 (2007) 52–61.
[7] S. Scirè, S. Minicò, C. Crisafulli, Appl. Catal. A: Gen. 235 (2002) 21–31.
[8] P. Claus, H. Berndt, C. Mohr, J. Radnik, E.J. Shin, M.A. Keane, J. Catal. 192 (2000)
88–97.
[9] C.V. Rode, U.D. Joshi, O. Sato, M. Shirai, Chem. Commun. 196 (2003) 0–1961.
[10] H.J. Wang, F.Y. Zhao, S. Fujita, M. Arai, Catal. Commun. 9 (2008) 362–368.
[11] P. Makowski, R.D. Cakan, M. Antonietti, F. Goettmann, M.M. Titirici, Chem.
Commun. 99 (2008) 9–1001.
[12] F. Lu, J. Liu, J. Xu, Mater. Chem. Phys. 108 (2008) 369–374.
[13] J. Morales, R. Hutcheson, C. Noradoun, I.F. Cheng, Ind. Eng. Chem. Res. 41 (2002)
3071–3074.
9 (2008)
It can be seen from Fig. 10 that the selectivity to cyclohexanone
followed different trends if we compare the catalyst by series of
supports. For example, Pd supported on neat activated carbons
(Pd/AC) followed a monotonic decrease in the selectivity to cyclo-
hexanone as a function of the pHPZC of the AC (the higher the
pHPZC of AC the lower the selectivity to cyclohexanone). How-
ever, if Pd is supported on the binary TiO2-AC or on the hybrid
TiO2-C, the cyclohexanone selectivity goes through maxima selec-
tivities when pHPZC of these supports were close to the pHPZC of
TiO2 alone (about 6.5) and then remarkably decrease to very low
cyclohexanone selectivity. This suggests that the influence of the
surface polar groups of AC upon the selectivity is not same if AC
is in presence of TiO2. This could be attributed to changes is the
surface energy of both components (TiO2 and C) by means of an
intimated contact interface created between both solids as sug-
gested by Matos and Herrmann [16]. This change in the surface
energy could influence different distributions of polar groups at the
surface of the TiO2 to less polarity (more hydrophobic) explaining
why when Pd is supported on the binary or the hybrid support
with pHPZC close to neutral or basic the selectivity to cycloxanone
remarkably decrease in comparison of the cyclohexanone selectiv-
ity in presence of very basic AC which is clearly higher (Fig. 10).
Our efforts are aimed to demonstrate this suggestion by ab initio
computational estimations.
[14] H. Li, J. Liu, S. Xie, M. Qiao, W. Dai, Y. Lu, H. Li, Adv. Funct. Mater. 18 (2008)
3235–3241.
[15] E. Díaz, A.F. Mohedano, L. Calvo, M.A. Gilarranz, J.A. Casas, J.J. Rodríguez, Chem.
Eng. J. 13 (2007) 65–71.
[16] J. Matos, J. Laine, J.M. Herrmann, Appl. Catal. B: Environ. 18 (1998) 281–291.
[17] J. Matos, J. Laine, J.M. Herrmann, Carbon 37 (1999) 1870–1872.
[18] J. Matos, J. Laine, J.M. Herrmann, D. Uzcategui, J.L. Brito, Appl. Catal. B: Environ.
70 (2007) 461–469.
[19] T. Cordero, C. Duchamp, J.M. Chovelon, C. Ferronato, J. Matos, Appl. Catal. B:
Environ. 73 (2007) 227–235.
[20] T. Cordero, C. Duchamp, J.M. Chovelon, C. Ferronato, J. Matos, J. Photochem.
Photobiol. A: Chem. 191 (2007) 122–131.
[21] J. Matos, A. García, P.S. Poon, J. Mater. Sci. 45 (2010) 4934–4944.
[22] J. Matos, A. García, L. Zhao, M.M. Titirici, Appl. Catal. A: Gen. 390 (2010)
175–182.
[23] J. Matos, E. García-López, L. Palmisano, A. García, G. Marci, Appl. Catal. B: Envi-
ron. 99 (2010) 170–180.
[24] F. Rodríguez-Reinoso, Carbon 36 (1998) 159–175.
[25] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309–319.
[26] W.D. Harkins, G. Jura, J. Am. Chem. Soc. 66 (1944) 1366–1373.
[27] G. Horvath, K. Kawazoe, J. Chem. Eng. Jpn. 16 (1983) 470–475.
[28] M. Inagaki, K.C. Park, M. Endo, New Carbon Mater. 25 (2010) 409–420.
[29] J. Matos, M. Labady, A. Albornoz, J. Laine, J.L. Brito, J. Mater. Sci. 39 (2004)
3705–3716.
[30] J. Matos, M. Labady, A. Albornoz, J. Laine, J.L. Brito, J. Mol. Catal. A: Chem. 228
(2005) 189–194.
[31] E. Auer, A. Freund, J. Pietsch, T. Tacke, Appl. Catal. A: Gen. 173 (1998)
259–271.
[32] M.C. Roman-Martinez, D. Cazorla-Amoros, A. Linares-Solano, C. Salinas-
Martinez, Carbon 31 (1993) 895–902.
[33] F. Coloma, A. Sepulveda-Escribano, J.L.G. Fierro, F. Rodriguez-Reinoso, Appl.
Catal. A: Gen. 150 (1997) 165–183.
[34] A.E. Aksoylu, M. Madalena, A. Freitas, M. Fernando, R.R. Pereira, J.L. Figueiredo,
Carbon 39 (2001) 175–185.
4. Conclusions
[35] J. Matos, J. Laine, Appl. Catal. A: Gen. 241 (2003) 25–38.
[36] Y. Wang, G. Zhou, T. Li, W. Qiao, Y. Li, Catal. Commun. 10 (2009) 412–415.
[37] R. Koc, J.S. Folmer, J. Am. Ceram. Soc. 80 (1997) 952–956.
[38] W. Shen, Z. Li, Y. Liu, Rec. Pat. Chem. Eng. 1 (2008) 27–40.
[39] J. Aran˜a, J.M. Rodríguez, E. Tello, C. Garriga, O. González, J.A. Herrera, J. Pérez,
G. Colón, J.A. Navío, Appl. Catal. B: Environ. 44 (2003) 153–160.
[40] P. Andrew, M.A.P. Sherwood, Carbon 21 (1983) 53–59.
[41] S. Velu, M.P. Kapoor, S. Inagaki, K. Suzuki, Appl. Catal. A: Gen. 245 (2003)
317–331.
[42] M. Chatterjee, H. Kawanami, M. Sato, A. Chatterjee, T. Yokoyama, T. Suzuki, Adv.
Synth. Catal. 351 (2009) 1912–1924.
[43] S. Watanabe, V. Arunajatesan, Top. Catal. 53 (2010) 1150–1152.
[44] Y. Wang, J. Yao, H. Li, D. Su, M. Antonietti, J. Am. Chem. Soc. 133 (2011)
2362–2365.
TiO2-C materials were obtained by solvothermal and slurry
synthesis. Pd-based catalysts were prepared to study the selec-
tive phenol hydrogenation in aqueous phase under mild reaction
conditions. High selectivity to cyclohexanone is obtained with
Pd on more polar TiO2-C supports, while when these are trans-
formed into hydrophobic TiO2-C supports the resultant catalyst
becomes selective to cyclohexanol. Representative activities were
also found with Pd on the more hydrophobic C supports with
high selectivity to cyclohexanol. Thus, product distribution dur-
ing the selective phenol hydrogenation in aqueous phase is easily
controlled by controlling the functionalization of the hybrid sup-
port.
[45] Y.
Pérez,
M.
Fajardo,
A.
Corma,
Catal.
Commun.
(2011),
doi:10.1016/j.catcom.2011.03.026.