Preparation of TiO2 Nanotube Supported Pd for the Hydrogenation of 4-carboxy-benzaldehyde

Article abstract of DOI:10.1007/s10562-018-2469-2

Abstract: Mesoporous TiO₂ nanotube (TNT) with high surface area was synthesized and used to support palladium. The as prepared Pd/TNT was characterized by different physico-chemical techniques, such as XRD, SEM, TEM, N₂ physisorption, CO chemisorption and XPS. The catalytic behavior for the hydrogenation of 4-carboxy-benzaldehyde was evaluated and compared with that of a commercial Pd/C. It was observed that Pd species was highly dispersed on TNT at a nominal 0.5% loading, and mainly existed in a metallic state. In comparison with this commercial Pd/C, the Pd/TNT displayed a better catalytic performance for the conversion of 4-carboxy-benzaldehyde. At 0.6?MPa and 280?°C, the Pd/TNT tended to catalytically convert 4-carboxy-benzaldehyde into more stable benzoic acid via a decarbonylation process. Graphical Abstract: A Pd/TNT with high palladium dispersion was successfully prepared by the wet incipient impregnation using a high surface area TiO₂ nanotube support, and the fresh catalyst tended to catalytically convert 4-carboxy-benzaldehyde into more stable benzoic acid in the initial operation stage.

Full text of DOI:10.1007/s10562-018-2469-2

Catalysis Letters
https://doi.org/10.1007/s10562-018-2469-2
Preparation of TiO Nanotube Supported Pd for the Hydrogenation
2
of 4-carboxy-benzaldehyde
Jingwei Liu1 · Wenbin Du · Zezhuang Li · Aiwu Yang
1
1
1
Received: 27 April 2018 / Accepted: 19 June 2018
©
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Mesoporous TiO nanotube (TNT) with high surface area was synthesized and used to support palladium. The as prepared
2
Pd/TNT was characterized by different physico-chemical techniques, such as XRD, SEM, TEM, N physisorption, CO chem-
2
isorption and XPS. The catalytic behavior for the hydrogenation of 4-carboxy-benzaldehyde was evaluated and compared
with that of a commercial Pd/C. It was observed that Pd species was highly dispersed on TNT at a nominal 0.5% loading,
and mainly existed in a metallic state. In comparison with this commercial Pd/C, the Pd/TNT displayed a better catalytic
performance for the conversion of 4-carboxy-benzaldehyde. At 0.6 MPa and 280 °C, the Pd/TNT tended to catalytically
convert 4-carboxy-benzaldehyde into more stable benzoic acid via a decarbonylation process.
Graphical Abstract
A Pd/TNT with high palladium dispersion was successfully prepared by the wet incipient impregnation using a high surface
area TiO nanotube support, and the fresh catalyst tended to catalytically convert 4-carboxy-benzaldehyde into more stable
2
benzoic acid in the initial operation stage.
Keywords Mesoporous TiO nanotube · Pd/TNT · Dispersion · Hydrogenation of 4-carboxy-benzaldehyde · Benzoic acid
2
1
Introduction
*
Jingwei Liu
One dimensional nanostructured material is one of the most
promising supports for redox and acid/base reactions, owing
to their unique physical and chemical properties unlike
to their bulk counterparts. Among them, titanium oxide
jingweiliu@163.com
1
Research Institute, Sinopec Yangzi Petrochemical Co., Ltd.,
Nanjing 210048, China
Vol.:(0
1
123456789)
3

J. Liu et al.
nanotube (TNT) has high surface area, open mesoporous
channel, semiconductor bands gaps and strong electronic
synergetic interaction with active sites. Based on these
above merits, TNT can support and disperse rather large
amounts of active species, and also facilitate fast transporta-
tion of reagents and products. Hence, TNT has found wide
application in many catalytic processes, including selective
catalytic reduction of ammonia [1, 2], oxidation of carbon
monoxide [3, 4] and dibenzothiophene [5, 6], selective cata-
lytic oxidation of alcohols [7–10], double-bond migration
Degussa [21–23], respectively. Besides the single TiO sup-
2
port, Zhou et al. [24] found that titanium incorporated Pd/
CDC–SiC was significantly more active than unmodified
Pd/CDC–SiC toward the hydrogenation of 4-carboxyben-
zaldehyde, due to the increasing dispersion of Pd.
It is well known that a carrier with higher surface area
is beneficial for the dispersion of active sites, and further
improve catalytic reactivity. Inspired by the fact that conven-
tional titanium dioxide is a suitable support for the hydro-
genation of 4-CBA, we thought that TNT supported palla-
dium should display superior catalytic performance. Herein,
we report an effective and cost saving synthetic strategy of
TNT supported palladium materials, and the synthesized Pd/
[
11], water–gas shift reaction [12], hydroformylation [13,
1
4], hydrodesulphurization [15] and alkylation [16].
Terephthalic acid (TA) is a core aromatic starting mate-
rial to polyester fibers manufacture development, and
mostly produced by liquid phase aerobic oxidation of
TNT were characterized by N adsorption, X-ray diffrac-
2
tion (XRD), X-ray photoelectron microscopy (XPS), induc-
tively coupled plasma spectrometry (ICP), scanning electron
microscopy (SEM) and transmission electron microscopy
(TEM), CO chemisorption. Furthermore, their catalytic
activity are evaluated by means of hydrogenation of 4-CBA,
and a wide product distribution including 4-hydroxymethyl
benzoic acid (4-HMBA), terephthalic acid (TA), p-toluic
acid (PA) as well as benzoic acid (BA) is also studied at
different catalyst loadings.
p-xylene in the acetic acid medium using Co(OAc) and
2
Mn(OAc) homogeneous catalysts with a bromine pro-
2
moter, as described in a process commonly known as the
BP-AMOCO technology. In the oxidation process, some
intermediate oxidation impurities such as 4-carboxy-ben-
zaldehyde (4-CBA) and several colorful polyaromatics are
accompanied by terephthalic acid (TA). In order to obtain
purified terephthalic acid conforming to polymerization
grade standard, crude terephthalic acid must be dissolved
in hot water at 270–280 °C, and followed by hydrogenation
in a trickle bed reactor, where activated carbon supported
palladium species are demonstrated as an excellent catalyst
with good resistance to hot acid corrosion. In pursuit of
satisfactory catalytic performance of carbon based cata-
lysts, Menegazzo et al. observed that the Pd/C possessed
high metal dispersion and good reactivity at the pH of the
impregnating palladium precursor solution ranging from
2 Experimental Section
2.1 Catalysts Preparation
TNT support was fabricated according to our previous work
as reported elsewhere [9]. In a typical procedure, 0.9 g TiO
2
(Degussa P25) was dispersed in 30 ml of 10 M NaOH solu-
tion under vigorous stirring for 30 min. After then, the
mixture was transferred into a Teflon-lined stainless steel
autoclave and maintained at 130 °C in an oven for 24 h.
1
.5 to 2 [17]. Jhung et al. [18] incorporated inexpensive
ruthenium into Pd/C to prevent the sintering of Pd, and the
bimetallic catalyst displayed a higher activity and stabil-
ity than the single Pd catalyst. Zhu et al. [19] investigated
the effect on hydrogenation process on product distribu-
tion over a monolith based carbon fiber supported Pd, and
emphasized to present a consecutive reaction mechanism,
namely an intermediate product 4-hydroxymethyl benzoic
acid (4-HMBA) and another deep hydrogenation product
p-toluic acid (PA). Similar reaction characteristic was also
previously observed by Li et al. [20].Although mentioned
above, the activated carbon supported catalysts still suffer
from drawbacks such as low crush strength and abrasion
resistance, leading to the palladium leaching and contami-
nation of downstream products. In order to avoid without
the loss in reactivity, one support comprising titanium oxide
have been developed as an excellent substitute for carbon
supports in the past decades. For example, preparation of
titanium dioxide supported one and more metals belong-
ing to group VIII for the efficient removal of 4-CBA was
disclosed in patents, which were assigned to Amoco and
The as-synthesized paste was washed by 0.1 M HNO and
3
0.05 M HNO in a sequence. Subsequently, it was further
3
washed to neutral by distilled water at 45 °C. Finally, the
white paste obtained was dried at 40 °C overnight and then
calcined at 400 °C for 1 h.
A Pd/TNT catalyst with a nominal loading of 0.5 weight
percentage palladium was prepared by the wetness incipi-
ent impregnation. Firstly, TNT was added into the aqueous
solution containing an appropriate amount PdCl at pH of
2
2, and the slurry was dried at room temperature. Secondly,
a desired amount of n-butanol was added into the solid from
the first step, and then the second slurry was dried at 80 °C
and subjected to annealing at 220 °C for 1 h. Prior to the
catalytic test, Pd/TNT was reduced by H at 220 °C for 1 h
2
in order to obtain a high metal dispersion catalyst. The cor-
responding reduction samples were denoted as 0.5Pd/TNT,
where 0.5 refers to the approximate palladium weight per-
centage in the sample.
1
3

Preparation of TiO Nanotube Supported Pd for the Hydrogenation of 4-carboxy-benzaldehyde
2
2
.2 Characterization
as-prepared Pd/TNT and commercial Pd/C were crushed
into 300–400 mesh powder, and the agitation speed main-
tained at 800 rpm, according to our previous experiment
for Pd/C catalyst. Upon termination at some intervals, the
reactant and products were determined by an Agilent 1100
Series HPLC equipped with a Hypersil ODS C18 column
using acetonitrile-methanol-water ternary mixture as gradi-
ent elution mobile phase. The conversion of 4-CBA (T) with
selectivity to products (S) was expressed in the following
formula:
Powder X-ray diffraction (XRD) patterns were collected on
Shimadzu XRD-6000 diffractometer using Ni-filtered Cu Kα
radiation (λ=0.15418 nm), operated at 40 kV and 30 mA at
o
a scanning rate of 10 /min. The loading of palladium in Pd/
TNT was determined by inductively coupled plasma analysis
using a PerkinElmer Optima 2100DV.
The surface area, pore sized distribution and pore vol-
ume were determined from nitrogen adsorption–desorption
analysis at −196 °C using a Micromeritics ASAP 2020HD
analyzer. Prior to the nitrogen sorption, the sample was
C − Ci
0
T =
× 100%
C0
−
3
degassed to 10 Torr at 200 °C. Scanning electron micros-
copy images were obtained using a Hitachi S-4800 scan-
ning electron microscope. Transmission electron microscopy
(
TEM) was performed on a FEI Tecnai G2 F30 S-TWIN
microscope with a STEM HAADF detector under an accel-
erating voltage of 200 kV. The samples were dispersed in
acetone under ultrasonic conditions and deposited onto cop-
per grids coated with ultrathin carbon films. X-ray photo-
electron spectra (XPS) measurement was performed on a
PHI Quantera instrument equipped with a hemispherical
electron analyzer and an Al anode (Al Kα= 1486.6 eV).
The residual pressure in the spectrometer chamber during
−
8
data acquisition was 5 ×10 Pa. A value of 284.6 eV for
the binding energy of the main C1s component was used as
an internal calibration standard.
The dispersion of palladium was determined by CO chem-
isorption at 40 °C after reduction in hydrogen at 220 °C, on
a Micromeritics ASAP 2020HD analyzer. Considering the
effect of physical adsorption, a similar experimental pathway
was carried out in a sequence of first adsorption-evacuation-
second adsorption according to previous work. Briefly, the
first and second uptake was obtained, and the actual uptake
for the sample was calculated by subtracting the second
uptake from the first uptake. The dispersion of Pd can be
calculated by assuming an adsorption stoichiometry of one
CO molecular per surface palladium atom [25]. The mean
Pd crystallite size can also be calculated accordingly using
a sphere model. For a comparison, a commercial 0.5Pd/C
catalyst was used to determine the dispersion in the same
manner, where 0.5 was labeled as the approximate palladium
weight percentage, and the actual loading determined by
inductively coupled plasma was 0.496%.
2.3 Catalytic Test
The reaction of hydrogenation of 4-CBA was carried out in
a 100 ml Premex stirred autoclave reactor. In a typical run,
0
.2 g or 0.1 g catalyst, 0.7 g 4-CBA and 70 ml deionized
water were loaded and reacted at 0.6 MPa and 280 °C. In
order to eliminate diffusion effect as soon as possible, the
Fig. 1 XRD patterns of TNT (a) and 0.5Pd/TNT (b)
1
3

J. Liu et al.
2
1-1272) are observed on the TNT, in accordance with our
CHMBA
SHMBA
=
× 100%
previous result [9]. After loading of palladium followed by
reduction in hydrogen, the intensity of anatase reflection
peaks did not seem to change, and also no signals corre-
sponding to palladium species were noticeable, indicating
the high dispersion of palladium species on the TNT at
such a low Pd amount. Hanaor and Sorrell [26] reviewed
C − Ci
0
CTA
STA
SBA
SPA
=
=
=
× 100%
0
C − Ci
CBA
the incorporation of Pd into TiO brought about the phase
2
× 100%
× 100%
C − Ci
0
transformation from anatase to rutile. However, this phe-
nomenon was not occurred for the 0.5Pd/TNT, owing to low
Pd amount and low reduction temperature. Inductively cou-
pled plasma analysis showed that the actual weight percent-
age of Pd in the 0.5Pd/TNT is 0.482, lower than the nominal
palladium composition. Accordingly, it was calculated that
the atomic ratio of Ti to Pd is 275.
CPA
C − Ci
0
where, Ci, C
, C , C and C are represented as the
TA BA PA
HMBA
instantaneous molar concentration of 4-CBA, 4-HMBA, BA
and PA, respectively. C is the initial molar concentration
0
For the TNT, SEM image in Fig. 2a shows the presence
of messy bundles comprising many tiny rod-like filaments,
and the length ranged from tens to hundreds of nanometers.
After loading of Pd, the rod-like structure was still well pre-
served in Fig. 2b. For the TNT and its counterpart supported
catalyst, the tubular structure with 100–200 nm in length
and 10–15 nm in outside tubular diameter was further veri-
fied from TEM investigation in Fig. 2c, d. HAADF STEM
of 4-CBA.
3
Results and Discussion
Figure 1 shows the XRD patterns of the TNT and the 0.5Pd/
TNT. Five main characteristic peaks at 25.3°, 37.8°, 48.1°,
5
5.2° and 68.6° assigned to anatase (JCPDS references
Fig. 2 a Overview TNT SEM micrograph, and b overview 0.5Pd/TNT SEM micrograph, and c TNT TEM image, and d 0.5Pd/TNT TEM
image, and e HAADF STEM of 0.5Pd/TNT
1
3

Preparation of TiO Nanotube Supported Pd for the Hydrogenation of 4-carboxy-benzaldehyde
2
2
−1
3
−1
analysis (Fig. 2e) indicated the presence of highly dispersion
of Pd and the crystallite diameter between 1 and 4 nm.
Figure 3 gives the nitrogen adsorption–desorption iso-
therms and corresponding BJH pore size distributions from
desorption branch for the TNT and 0.5Pd/TNT samples, and
their textural parameters are listed in Table 1. For these two
samples, typical IV isotherms with a H3 hysteresis loop can
be observed, indicative of slit-shaped mesoporous structure.
As shown in Table 1, the TNT possesses a high surface area
260 m g with 1.31 cm g pore volume, which can be
attributed to the blocking or the partial collapse of nanotubu-
lar pore size due to palladium crystallite size. The difference
in tubular diameter determined from TEM investigation and
pore size from nitrogen adsorption was noticed, since the
latter adsorption method gave an average pore size including
nanotubular and nanotubular packing porosity.
The chemisorption of CO has been used to determine the
dispersion of noble metal on supports. Due to the fact of
strong metal support interaction and sintering agglomera-
tion, the actual number of accessible active sites is lower
than its theoretical value, where the ratio is usually defined
as metal dispersion. In order to avoid the weakly revers-
ible adsorption, i.e. physical adsorption, the obtained dif-
ference between first isotherm and second isotherm should
be an accurate amount reflecting real chemical adsorption,
as described in many previous works [27–30]. As seen in
Table 1 calculated from Fig. 4, the dispersion for the 0.5Pd/
TNT was 42.8%, and the mean Pd crystallite diameter based
on the spherical model was 2.67 nm, in accordance with the
TEM investigation. Therefore, it was reasonable that such
small palladium crystallite on TNT could not be detected by
the XRD technique. The 0.5 Pd/C catalyst used as a refer-
ence possessed a slightly lower dispersion of 40.2% with a
crystallite diameter of 2.78 nm.
2
−1
of 307 m g with an average pore width of 13.6 nm. Com-
paratively, the 0.5Pd/TNT has a rather lower surface area of
XPS was applied to measure the composition and chemi-
cal state of all elements on the surface of 0.5Pd/TNT.
Table 2 gives the data about the surface composition for
the 0.5Pd/TNT. The surface atomic ratio of Ti to Pd is 256,
which is slightly lower than the global ratio of Ti to Pd,
suggesting that the Pd species on the surface of TNT were
enriched. As shown in Fig. 5, XPS spectra due to the Pd
3
3
d
orbital was deconvoluted into two fitting peaks at ca.
5/2
35.0 and 336.9 eV, attributable to the binding energy of
metallic Pd and oxidative Pd or Pd species bearing a strong
metal-support interaction [31], respectively. The inspection
result revealed that the metallic Pd was predominant over
other oxidative states among these Pd species. Considering
that 0.5Pd/TNT was primarily reduced in hydrogen for 2 h
prior to the XPS analysis,it was as expected that most of Pd
species might be in its metallic state.
According to previous works, hydrogenation of 4-CBA
was usually regarded to follow the first order kinetics [17,
2
4]. Thus, the Napierian logarithm of the initial concentra-
tion of 4-CBA (C ) divided by the instantaneous concentra-
0
tion (C ) at a given time is a liner relationship with reaction
i
time, assuming that the apparent reaction rate constant is
invariable during the reaction period. Figure 6 displays the
results of hydrogenation of 4-CBA over the TNT, 0.5Pd/
TNT and 0.5Pd/C with time on stream at a catalyst loading
of 0.2 g. The TNT exhibited a very poor reactivity for the
hydrogenation of 4-CBA and the conversion is almost below
0.5%. In contrast, the conversion of 4-CBA was dramatically
Fig. 3 N adsorption–desorption isotherms and pore size distribution
2
from the desorption branches for TNT (a) and 0.5Pd/TNT (b)
1
3

J. Liu et al.
Table 1 Surface areas, pore
parameters, dispersion and
metal particle size
SBET (m g−1)
2
Sample
TNT
Pore size (nm)
Pore vol.
Dispersion (%)
Pd crystal-
lite size
3
−1
(
cm g
)
(
nm)
307
260
851
13.6
16.6
2.7
1.53
1.31
0.45
–a
42.8
40.2
–a
2.62
2.78
0.5Pd/TNT
.5Pd/C
0
aNot available
Fig. 4 CO chemisorption for
0
.5Pd/TNT (a) and 0.5Pd/C (b):
■
first adsorption, □ second
adsorption, ● difference adsorp-
tion
Table 2 Binding energies at
Pd 3d orbital level and surface
composition
Sample
Pd3d5/2 fitting peak location (eV)
Surface composition (%)
Metallic state
335.0
Oxidative state
336.9
Pd
O
Ti
C
0
.5Pd/TNT
0.1
47.9
25.6
26.4
enhanced over the 0.5Pd/TNT, due to the high dispersion
of Pd active sites. In spite of the relatively higher surface
area, the 0.5Pd/C was less active than the 0.5Pd/TNT, which
might be connected with the poor dispersion and large crys-
talline dimension. The corresponding apparent reaction rate
constants for TNT and 0.5Pd/TNT as well as 0.5Pd/C were
Table 3 gives the product distribution of 4-CBA hydro-
genation over the 0.5Pd/TNT after an hour under similar
condition, except for the different catalyst loading of 0.1
and 0.2 g. Earlier most studies reported that hydrogenation
of 4-CBA proceed in two consecutive steps to p-toluic acid
(PA) as a main goal product via 4-hydroxymethyl benzoic
acid (4-HMBA) as a main intermediate product [19, 20].
However, it is not the case in the present work and our initial
.9×10− , 1.23 and 1.12 h , respectively.
4
−1
1
1
3

Preparation of TiO Nanotube Supported Pd for the Hydrogenation of 4-carboxy-benzaldehyde
2
Fig. 5 XPS spectra of Pd 3d core level of 0.5Pd/TNT
operation stage of PTA industry plant, due to high catalytic
reactivity and trace amount of oxygen [32]. It is worth noting
that the 0.5Pd/TNT exhibited good decarbonylation perfor-
mance for the formation of benzoic acid studied in this work.
For example, in the case of 0.1 g catalyst loading, the main
product is 4-HMBA with 55.1% selectivity, while selectivity
to PA is only 4.8%. When the catalyst amount increased to
Fig. 6 Hydrogenation of 4-CBA over TNT, 0.5Pd/TNT and 0.5Pd/C
with time on stream in the case of 0.2 g catalyst loading
Table 3 Product distribution of 4-CBA hydrogenation over the 0.5Pd/
TNT catalyst with different loading, and a commercial 0.5Pd/C for
comparison at the same condition
Catalyst and loading (g) Conv. (%) Selectivity (%)
0
.2 g,most of 4-HMBA and PA was further decarbonylated
to benzoic acid, and the selectivity to benzoic acid reached
as high as 82.0 with 96.8% 4-CBA conversion. Compara-
tively, conversion of 4-CBA over the commercial 0.5Pd/C
was 91.1 with 75.9% selectivity to BA. When the 4-CBA
conversion reached about 96% over the 0.5Pd/C by prolong-
ing reaction time, the selectivity to PA and BA were 15.4
and 70.3%, while the selectivity to TA and 4-HBA decreased
to 1.7 and 12.6%. According to the accessible Pd amount
calculated in Table 1 and conversion of 4-CBA in Table 3,
TOF (turn over frequency) for 0.5Pd/TNT and 0.5Pd/C was
4-HMBA TA
BA
PA
0.5Pd/TNT, 0.2
0.5Pd/TNT, 0.1
0.5Pd/C, 0.2
96.8
74.3
91.1
13.4
55.1
14.8
3.7 82.0 0.9
14.9 25.2 4.8
2.5 75.9 6.8
4
Conclusions
Herein, a 0.5Pd/TNT was successfully prepared by the wet
−
1
incipient impregnation using a high surface area TiO nano-
1
125 and 1127 h , respectively. Although TOF for 0.5Pd/
2
tube support. It is evidenced that Pd species with 1–4 nm
crystallite size was highly dispersed and more enriched on
the surface of TNT. In terms of hydrogenation of 4-CBA, the
TNT was slightly lower than it for 0.5Pd/TNT, 0.5Pd/TNT
exhibited comparatively better hydrogenation conversion of
4
-CBA due to more accessible Pd number, further verifying
0
.5Pd/TNT exhibited superior catalytic performance than
the positive effect of good dispersion and small crystalline
size.
the 0.5Pd/C, and the corresponding apparent reaction rate
−
1
constants were 1.23 and 1.12 h , respectively. Over this
.5Pd/TNT, the 4-CBA was preferentially hydrogenated to
0
1
3

J. Liu et al.
an immediate 4-HMBA molecule, and subsequently decar-
bonylate to form a more stable benzoic acid molecule with
increasing catalyst loading.
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Acknowledgements We thank Dr. Yanwen Ma for help with SEM
investigations at Nanjing University of Posts and Telecommunica-
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of Jiangsu Province (BE2013101) is gratefully acknowledged.
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Compliance with Ethical Standards
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