Stick-like mesoporous titania loaded Pd as highly active and cost effective catalysts for hydrodebenzylation of hexabenzylhexaazaisowurtzitane (HBIW)

Article abstract of DOI:10.1016/j.mcat.2019.110556

Hydrodebenzylation of hexabenzylhexaazaisowurtzitane (HBIW) is one key step for the synthesis of High Energy Density Compound CL-20 (hexanitrohexaazaisowurtzitane). Design of a “high atom economy” debenzylation catalyst to promote the industry production

Full text of DOI:10.1016/j.mcat.2019.110556

Molecular Catalysis 477 (2019) 110556
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Stick-like mesoporous titania loaded Pd as highly active and cost effective
catalysts for hydrodebenzylation of hexabenzylhexaazaisowurtzitane
(
HBIW)
Shuang Liua,1, Fei Jia,1, Xiaoyu Li , Xiaoli Pan , Shuyuan Chen , Xuefei Wang , Yanhua Zhang ,
Yong Men
a
b
b
a
a
c
a,⁎
College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai, 201620, China
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China
b
c
Hebei University of Technology, Tianjin, 300401, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Hydrodebenzylation of hexabenzylhexaazaisowurtzitane (HBIW) is one key step for the synthesis of High Energy
Density Compound CL-20 (hexanitrohexaazaisowurtzitane). Design of a “high atom economy” debenzylation
catalyst to promote the industry production and massive use in the fields of aerospace and military has attracted
great attentions. Herein, we designed a catalyst of stick-like mesoporous titania supported ultrafine Pd nano-
particles with low loading 2 wt.% using a hydrothermal and then deposition-precipitation method. The supports
Catalytic hydrogenolysis
Debenzylation
CL-20
Pd catalyst
Titania
and catalysts were thoroughly characterized by N
microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Raman, ultraviolet–visible ab-
sorption (UV–vis), in situ diffuse reflectance infrared Fourier transform (DRIFT), H chemisorption to determine
2
physisorption, X-ray diffraction (XRD), scanning electron
2
their physical and chemical properties. The results revealed that stick-like titania (ST) treated at low temperature
2
showed rutile crystalline structure, large interplanar spacing (3.2394 Å) and surface area (53 m /g), small
average crystallite size (63 nm) with mesoporous. The Pd can deposit into the interstices and lattice of ST and
displayed ultrafine and even sub-nanometer sizes (average value 1.2 nm), and high dispersion. The Pd/ST-2
exhibited particularly high activity for hydrodebenzylation of HBIW. The product TADBIW was obtained in a
high turnover number (TON) 107, which is 2 ˜ 5 times higher than those of conventional Pd/C catalysts. This
work paves a promising way to develop highly efficient catalysts with low loading of Pd and high atomic
utilization for cost-effective synthesis of CL-20.
1. Introduction
environmental pollution caused by post-treatment of the massive used
Pd based catalysts. Meanwhile, the final deprotection in multi-step
Benzyl groups are often used to protect amine functionalities. The
synthetic studies often fails because of an unanticipated poor yield,
even if every required structural fragment is already constructed in the
protected target molecular, which always troubles organic synthetic
chemist. The catalysts play key roles to maximize product yield and
minimize usage amount of noble metal. Developing high performance
and economic catalysts for catalytic hydrodebenzylation has become a
great challenge in the synthesis of complex organic compounds.
High Energy Density Compounds (HEDC) have so many versatile
applications in the area of aerospace and arms and have attracted ex-
tensive attentions in recent years. Polycyclic nitramine hexanitrohex-
aazaisowurtzitane (HNIW, known as CL-20) is one of the most
corresponding deprotection are significant fundamental transforma-
tions in multi-step organic syntheses. The most common method of
deprotection is debenzylation under catalytic reduction conditions
2
using H . Heterogeneous Pd based catalysts are always massively ap-
plied in the process of debenzylation because of the advantages of good
stability and easy separation [1–5]. Meanwhile they are often used also
in the modern fine chemical industry [6–9]. However, low yield and
large usage of precious metal catalyst limits mass production. The cost-
effective processes are in urgent of development which can improve
utilization of precious metal resource and reduce economic costs and
⁎
Corresponding author.
E-mail address: men@sues.edu.cn (Y. Men).
As joint first-authors.
1
https://doi.org/10.1016/j.mcat.2019.110556
Received 15 April 2019; Received in revised form 2 August 2019; Accepted 3 August 2019
Available online 11 August 2019
2468-8231/ © 2019 Elsevier B.V. All rights reserved.

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
successful HEDC, which exhibits higher energy and density than
common those of monocyclic nitramines, e.g., cyclotetramethylene
tetranitramine (HMX) and cyclotrimethylene trinitramine (RDX)
compared with the microporous structure (< 2 nm) can facilitate fast
intraparticle molecular transfer of the substrate and product molecules
and promote dispersion of active metal owing to large pore size, pore
volume and high surface area [26–32]. On the basis of the advantages,
the mesoporous catalytic system can be expected to enhance the cata-
lytic performances and raise the atom utilization of the active metal.
Besides, metal oxide as support is an effective way to increase the
stability of active metal because of strong metal-support interaction and
maintain high catalytic activity [33–40]. In our previous reports, it was
found that the commercial titania P25 carried PdFe bimetal catalysts
displayed good performance with the yield 76% in the transformation
of HBIW [41]. In that work, the titania was nonporous and only
maintained the activity on the basis of high loading (6.5 wt.%) of Pd.
The atom economy of Pd is still low. In order to decrease loading of Pd
and heighten the atom utilization, we developed a strategy for syn-
thesizing ultrafine Pd nanoparticles with mesoporous titania (ST) as a
support in the study. The Pd/ST carried only 2 wt.% Pd and displayed
high turnover number (TON) which surpassed those of all the Pd based
catalyst reported. This work demonstrates low loading Pd and high
yield for debenzylation of HBIW and opens the possibility to develop
highly effective Pd over mesoporous metal oxide for organic synthesis
of CL-20 in the way of high atomic economy.
[
10–12]. On the basis of the superior performances, it has become the
key to break through the performances of the current propellants or
explosives in specific impulse, burning rate, ballistic and detonation
velocity. The industry synthesis of CL-20 includes four steps as shown in
Scheme S1, Electronic Supporting Information (ESI) [12–18]. The
conversion of the key intermediate hexabenzylhexaazaisowurtzitane
(
HBIW) to tetraacetyldibenzylhexaazaisowurtzitane (TADBIW) is the
major challenge of the CL-20 synthesis due to not only multiplex re-
action process but also the special features of HBIW [18–22]. The de-
benzylation of HBIW is a simultaneous hydrodebenzylation-acetylation
process during which the C–N bonds are hydrogenolyzed on the cata-
lysts followed by the acetylation of the forming amines with acetic
anhydride. The removing of all benzyl groups is essential in the con-
ditions including high temperature (30 °C–60 °C) and acid, such as
acylation reagent acetic anhydride, solvent acetic acid, additive HBr
from PhBr. However, HBIW has a complex molecular structure, which
contains two five-membered rings with four N-benzylation groups and a
six-membered ring with two N-benzylation groups, meanwhile, a CeC
bond with high tension connects the two five-membered rings in the
polycyclic caged organic compound. Due to the features, HBIW is un-
stable and decomposes easily in acid or heated conditions. In order to
avoid destruction of the cage, the debenzylation needs to be launched
and transformed quickly at low temperature (generally 15 °C–23 °C).
The better starting temperature can be controlled at 17 °C–19 °C by a
series of optimization experiments as shown in Scheme S2, ESI. The
2. Experimental
2.1. Catalyst preparation
intermediate
production
DATBIW
(diacetyltetrabenzylhex-
All reagents with A.R. grade were purchased and used without
further purification. The titania supports were synthesized by a co-
precipitation and then hydrothermal method according to the reports
aazaisowurtzitane) obtained by removing the two benzyls of HBIW can
be more stable and endure higher temperatures than HBIW. To accel-
erate the debenzylation of DATBIW, the heating can be carried out with
higher temperature (usually 34 °C–40 °C). In the current reports, high
efficiency of the transformation has to be guaranteed by very large
amount of noble metal, such as high loading of Pd, to supply enough
active sites [12,19,20]. Commercial Pd/C catalyst with high Pd loading
[42]. Typically, 0.55 mL TiCl
with 300 r/min, then 2 mL SrCl
4
was dropped on 10 g ice under stirring
(2.5 M) was mixed for 10 min. 20 mL
4
KOH (4 M) was added dropwise. After 10 min, the stirring speed in-
creased to 500 r/min and continued for 20 min. The white slurry was
obtained and transferred into 100 mL autoclave, which was sealed and
(
5 wt.%–10 wt.%) is commonly used. However, the low atom utilization
heated at 100 °C for 24 h. After cooling, the product SrTiO
washed with distilled water for several times and dried at 70 °C for 4 h.
0.5 g SrTiO was put into 50 mL autoclave equipped with a Teflon liner,
then 25 mL H O and 2.5 mL HCl was successively added into the au-
toclave. The autoclave was heated at 130 °C for 2.5 h under autogenous
pressure and naturally cooled to room temperature. For the mixture
taking from the autoclave, different treatment temperatures, freeze-
drying, 60 °C heating and 120 °C heating, were carried out for 10 h. The
corresponding powder finally received was marked as ST, ST(60) and
3
was filtered,
rate of Pd over the catalysts leads to high cost of synthesis and then
limits the mass-production and widespread application of CL-20. Ex-
ploring industrial available catalysts with high efficiency and low
loading of Pd, i.e., high atom economy, is urgently required for the
hydrodebenzylation of HBIW.
3
2
Pd supported catalysts for debenzylation of HBIW focused on the
use of different carbon carriers, e.g., various activated carbon, synthetic
carbon [13,16,18,21,23,24]. It was discovered that target product can
be hardly obtained when the loading of Pd was lower than 5 wt.%
under constant usage amount of catalysts. Maksimowski et al. [19]
prepared 10 wt.% Pd catalyst using Gryf-Skand Carbo Medicinalis ac-
tive carbon as support and studied deactivation of the catalyst in HBIW
hydrodebenzylation reaction. It was revealed that the aggregation of
palladium particles occurred and then decreased the catalytic perfor-
mance for debenzylation of HBIW. It is well known that gathering of Pd
often emerges due to the weak interaction between the carbon and
active metal. In order to make up for the loss of the active sites, more Pd
needs to be provided. Hence, it is important to enhance the interaction
between the support and Pd in order to stabilize the dispersion of Pd. In
addition, in our previous study it was found that common carbon with
the micropore (< 2 nm) as a carrier can form great Pd particles which
showed low utilization rate of Pd for HBIW debenzylation while me-
soporous carbon can reduce particle size of Pd and then improve the
activity [25]. It can be inferred that the microporous structure of the
common carbon support generated large Pd particles which reduced the
number of the active center and decreased the atom economy. There-
fore, it is important to enhance the pore structures of the support in
order to upgrade the Pd atom utilization for HBIW debenzylation.
It is known that the mesoporous framework with 2 nm–50 nm pores
ST(120). The sample T was prepared without SrCl
were the same as ST.
4
, and all other steps
Catalyst were synthesized through a deposition-precipitation (DP)
method. In a typical preparation, 500 mg of ST was mixed with 100 mL
of deionized water through a vigorous stirring to form a white sus-
pension. Next, calculated H
obtained by dissolving PdCl
2
PdCl
4
aqueous solution (10 mg Pd/mL,
2
from STREM with HCl solution) was
dripped into the suspension. The new suspension was continuously
stirred for 3 h. The pH value of the mixture was then carefully adjusted
to 9.5 by gradually introducing 10% NaOH solution, followed by vig-
orously stirring for another 5 h. The slurry was then filtered and wa-
shed. The filtrate was concentrated by a Rotary evaporator and then
tested by Inductively Coupled Plasma Atomic Emission Spectrometry
(ICP-AES, Varian 725-ES ICP). No Pd cation can be detected which
indicated all input Pd cation was loaded on the ST support. The catalyst
was dried at 60 °C in an oven overnight and termed as Pd/ST-x. x was
designated as the loading of Pd, 1 wt.%, 1.5 wt.%, 2 wt.% and 2.5 wt.%.
At the same time, Pd/ST(60)-2, Pd/ST(120)-2, and Pd/T-2 with 2 wt.%
Pd loading were acquired by the same process. The actual loading of Pd
was tested by ICP-AES.
2

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
2.2. Catalyst characterization
The nitrogen adsorption-desorption measurements were performed
at – 196 °C with a Micromeritics ASAP 2460 instrument. Prior to the
measurements, the samples were degassed at 298 K for 5 h. The specific
surface areas were calculated with BET equation and the average pore
diameters were estimated with desorption branches based on BJH
model. Field emission scanning electron microscopy (FESEM) experi-
ments were performed with a FEI Nova NanoSEM 450 electron micro-
scope operating at 15 kV. The X-ray diffraction (XRD) patterns were
recorded with a Bruker: D2 Phaser system, using Cu Kα mono-
chromatized radiation (λ = 0.1541 nm) combined with nickel filter at a
scan speed of 0.5 s/step, 30 kV and 10 mA. Laser Raman spectroscopic
measurements were performed on a Thermofisher DXR Raman micro-
scope system (Thermofisher Scientific Company). High resolution
transmission electron microscopy (HRTEM) images were recorded on a
JEM-2100 F microscope, operating at an accelerating voltage of 200 kV.
Aberration-corrected high-angle annular dark-field scanning transmis-
sion electron microscopy (HAADF-STEM) images were obtained on a
JEOL JEM-ARM200 F equipped with
a CEOS probe corrector.
Ultraviolet–visible absorption spectroscopy (UV–vis) was studied by
SHIMADZU UV-2450 to obtain the reflectance spectra of the powder
over a range of 200 nm–1000 nm. In situ diffuse reflectance infrared
Fourier transform (DRIFT) spectra were collected on a VERTEX 70 V
infrared spectrometer equipped with a mercury cadmium telluride
−1
(
MCT) detector at a resolution of 4 cm . The experiments were carried
out in a high temperature reaction chamber (HVC-DRP-5, Harrick)
equipped with ZnSe windows. Prior to the adsorption, the samples were
pretreated with He at 120 °C for 30 min. After the temperature de-
creased to 25 °C, the background spectrum was collected using 64
scans. Then the 3 vol.% CO/He was introduced for 30 min to the sam-
ples. The spectra were obtained after purging for 30 min with He. H
temperature-programmed reduction (H -TPR) was carried out on a
Micromeritics Autochem II 2920 apparatus equipped with cold trap.
2
2
1
00 mg of a sample was loaded into a U-shape quartz reactor and
−
1
purged with 30 mL min
Ar at 120 °C for 1 h to remove adsorbed
species. Then, after cooling to – 50 °C, the flowing gas was switched to a
0 vol.% H /Ar, and the catalyst was heated to 900 °C at a ramping rate
1
2
−1
of 10 °C min . Dispersion of Pd were tested through H
sorption with Micromeritics, Auto Chem II 2920 instrument. The
sample was pretreated at 50 °C for 1 h with 10 vol.% H /Ar and then the
gas was transferred into Ar at 60 °C for 30 min. Finally, the H pulses
were measured at 50 °C with 10 vol.% H /Ar.
2
pulse chemi-
2
2
2
Fig. 1. FESEM pictures of the support ST at different scales (a–c).
2.3. Catalytic performance test
solids subtracting that of the catalysts as reported [18,41].
All chemicals, dimethyl formamide (DMF), acetic anhydride (Ac
2
O),
(msolid − mcat)
MHBIW
MTADBIW
Yield=
×
× 100%
bromobenzene (PhBr) and glacial acetic acid, were purchased and used
as received. HBIW were synthesized by the cooperation factory.
The debenzylation of HBIW was carried out in a low pressure re-
action vessel (volume 150 mL) equipped with a water bath and heating
magnetic stirrer (500 rpm–600 rpm). Catalytic experiments were per-
formed according to a described procedure [25,41]. The reactants:
m
HBIW
TON can be calculated as follows:
(msolid − m ) × P
cat
MPd
mcat L
TON=
×
MTADBIW
Here msolid: the weight of the grey solid, g; mHBIW: the weight of the
2
0
.5 mL of DMF, 1.5 mL of Ac
2
O, 0.02 mL of PhBr, 1.0 g of HBIW and
reactant HBIW, g; mcat: the usage of the catalysts, g; MHBIW and MTADBIW
the molecular weight of HBIW and TADBIW with 708.95 mol/g and
16.60 mol/g respectively; MPd: the atomic weight of Pd, 106.42 mol/g;
:
.05 g of catalyst, were put into the reactor. Subsequently, the reactor
was purged three times with hydrogen. The reaction was then stirred
under 4 bar pressure of H at 17 °C–19 °C for 7 h and then 34 °C for 16 h.
5
2
P: the purity, %; L: the loading of Pd over the catalysts, %.
Filter and washing with ethanol for three times were carried out. The
filter cake was dried in oven at 40 °C and the grey solids containing
target product TADBIW and the catalysts can be obtained. The purity
3. Results and discussion
1
was analyzed by HPLC (ESI, Experimental). H NMR (400 MHz, DMSO)
δ: 7.39 (t, J =41.2 Hz, 10 H), 6.92 – 6.02 (m, 2 H), 5.70 – 5.13 (m, 4 H),
3.1. Support characterization
4
4
1
.27 – 3.77 (m, 4 H), 2.20 – 1.67 (m, 12 H) tested on the basis of Varian
00MR. Anal. Calcd. for TADBIW (C28
H
32
N
6
O
4
) : C, 65.10; H, 6.24; N,
FESEM of the support ST are shown in Fig. 1. It can be seen that ST
was consist of a stick-like structure of ∼20 nm x 200 nm. According to
the reports [42], these monodispersed titania NPs were formed via an
6.27. Found: C, 64.57; H, 6.25; N, 16.18 obtained by Elementar vario
EL III. The yield was calculated according to the weight of the grey
3

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
2
Fig. 2. a. N adsorption-desorption isotherms and the pore-size distributions of the supports (b. the whole ones and c. the enlarged ones).
oriented attachment (OA) mechanism and single crystalline was con-
firmed for each titania nanoparticle. Simultaneously, as shown in Fig.
S1a-b, ESI, similar shape can be seen for the sample ST(60) and
ST(120). However, the sample T exhibited a bulk structure, on the
surface of which many grains like irregular NPs gathered in Fig. S1c,
ESI.
Nitrogen adsorption-desorption isotherms of the supports, ST,
ST(60), ST(120), T, together with the corresponding pore-size dis-
tributions are shown in Fig. 2. The specific surface area, and pore
volume of these supports are listed in Table 1. The isotherms of ST,
ST(60) and ST(120) samples displayed type IV curves with a H3 hys-
teresis loop revealing a mesoporous structure, while the sample T
showed the isotherms with type I curves due to microporous with the
3
volume 0.03 cm /g. In addition, the sample T exhibited the largest
2
surface area 157 m /g, the distribution of pore with less than 2 nm and
3
0.06 cm /g mesoporous volume. In contrast, ST, ST(60) and ST(120)
showed no microporous volume, large mesoporous volume, uneven
pore size and gradually decreasing surface area. The distribution of
4

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
in sequence of ST (63 nm) < ST(60) (107 nm) < ST(120) (116 nm). It
was indicated that ST supports showed larger interplanar spacing and
smaller average crystallite size than others.
Table 1
Textural properties of the supports.
a
b
b
Sample
S
BET
V
micro
V
total
average crystallite
size
interplanar
spacing
Å
c
d
It is well known that the information depth of X-ray diffraction is on
the order of tens of micrometers, while Raman scattering tends to
provide information sensitive to the surface of the particles even in very
small quantity. Raman spectra were measured and are shown in Fig. 3B
in order to reveal and distinguish individual surface phases of titania. It
can be seen that the sample T presented weak peak at 667, 450, 269,
m2 g−1 cm3 g−1 cm3 g−1 nm
ST
ST(60)
ST(120) 32
T
53
42
0.00
0.00
0.00
0.03
0.11
0.08
0.06
0.09
63
3.2394
3.1960
3.1897
–
107
116
–
157
−
1
−1
1
90 cm . The peaks located at 450 and 190 cm can be attributed to
the rutile and anatase phases of titania, respectively [43,44]. Jeon et al.
45] found that H treatment can form Ti−OH on the surface of
rutile titania corresponding to the peaks at 284, 531, and 691 cm
a
b
surface area and.
pore volume of pores was obtained by N
2
adsorption–desorption at –
[
2 2
O
1
96 °C.
−
1
c
.
average crystallite size determined by XRD analysis.
calculated lattice constants from the strongest peak and crystal face (110).
d
Considering that the sample T was obtained with hydrothermal pro-
cessing and presented in amorphous morphology. It seems reasonable
to conclude that Ti−OH could appear on the surface of the sample T as
−1
suggested by the Raman peaks at 667 and 269 cm . For ST, ST(60)
and ST(120), four peaks (608 cm , 445 cm , 238 cm−1, 150 cm
−1
−1
−1
)
−1
−1
emerged. Three typical vibrational modes around 608 cm , 445 cm
−
1
and 238 cm
were assigned as rutile titania, thereby confirming that
the main titania crystal structure was rutile as revealed in XRD spectra;
−
1
the weak peak at 150 cm maybe induced by seldom anatase phase of
titania, and was attributed to OeTiOe bending type vibrations in ac-
cordance with the peaks as the reports [46]. Simultaneously, the gra-
dually stronger broaden peaks could explain the growth of crystal size
with increasing treatment temperature from ST to ST(120) well
matching the discovery of XRD as shown in inset of Fig. 3B. On the basis
of Raman scattering, it was suggested that the sample T contained the
both anatase and rutile phase with Ti−OH on the surface although the
bulk displayed amorphous morphology, while ST, ST(60) and ST(120)
mainly exhibited rutile phase and revealed the little presence of anatase
phase on the surface.
3.2. Catalytic performances
ST based catalyst with 2.5 wt.% Pd loading, Pd/ST-2.5, was studied
for hydrodebenzylation of HBIW as shown in Table 2. The purity of the
production TADBIW were detected by HPLC and listed in Table 2. As a
contrast, commercial 10 wt.% Pd/C with 50 wt.% water from Degussa
(
marked as D-Pd/C) and 5 wt.% Pd/C from Sigma (marked as S-Pd/C)
were investigated. The reaction introduced the same amount of dried
catalysts independently of Pd loading. It can be observed that com-
mercial D-Pd/C and S-Pd/C showed 76% and 74% yield with Pd/HBIW
5‰ and 2.5‰, respectively. Pd/ST-2.5 exhibited higher yield 82% and
used less Pd/HBIW equal to 1.25‰ than those of the two commercial
catalysts. High purity (92%∼95%) for all TADBIW can be obtained.
The addition of Pd over ST was further examined as presented in
Table 2. For every hydrodebenzylation reaction, the feeding amount of
catalysts was unchanged. As the loading of Pd decreased, the yield re-
duced from high yield to no production. It can be seen that high yield
can be maintained when the loading of Pd was not lower than 2 wt.%.
On the basis of 2 wt.% loading, the other supports, ST(60), ST(120)
and T, were tested and the results are listed in Table 2. No production
appeared using the sample Pd/T-2. Pd/ST(60)-2 and Pd/ST(120)-2
showed low yield 73% and 57%. The catalytic performance in the re-
ference currently reported are listed in Table 2. It was found that when
loading of Pd decreased to 4 wt.%, no products can be obtained unless a
large amount of catalysts were used [18,21,23]. For comparison,
turnover number (TON) can be calculated by TADBIW products divided
by input Pd. It was suggested that Pd/ST-2 catalysts showed the highest
TON with 107 than common commercial D-Pd/C with TON 21 and S-
Pd/C with TON 41 and all the reports listed in Table 2 including our
previous work.
Fig. 3. A. XRD and B. Raman spectra of the supports a. T, b. ST(120), c. ST(60),
d. ST.
pore focused on 3.6 nm and 10 nm for ST, and mainly centered on
32 nm for ST(60) and ST(120) which may be caused by crystal growing
in the process of synthesis.
XRD tests were carried out. As shown in Fig. 3A, the sample T
displayed no evident peak indicating amorphous morphology. Other
samples exhibited the intense diffraction peak which can be ascribed to
Rutile titania (PDF#21-1276). Noting that the peaks of the sample, ST,
ST(60) and ST(120), with treatment temperature increase transferred to
high angle shown as an inset in Fig. 3A, which may be caused by the
interplanar spacing and average crystallite size. Then it can be calcu-
lated based on the strongest peak and crystal face (110) listed in
Table 1. It can be found that the interplanar spacing decreased in the
order of ST (3.2394 Å) > ST(60) (3.1960 Å) > ST(120) (3.1897 Å)
while the average crystallite size according to Scherrer Equation grew
5

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
Table 2
Catalytic performances for HBIW debenzylation over Pd based catalysts.
Loading of Pdc (wt.%)
Cat./HBIW
g/g)
Pd/HBIW
(at.‰)
Y
(%)
d
TONe
molTADBIW/Pd
Purity /[ref.]
(%)
f
Entry
Sample
TADBIW
(
1
2
3
4
5
6
7
8
9
Pd/ST-2.5
2.1
10.0
5.0
2.1
2.1
2.1
1.9
1.4
1.0
20
10
6
0.05
0.10
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.02
0.10
0.10
0.1(0.2)
0.1
7.0
82
76
74
81
73
57
0
111
21
41
107
98
74
0
95
94
92
93
94
92
–
93
a
D-Pd/C
66.6
16.6
7.0
7.0
7.0
6.3
4.6
3.3
b
S-Pd/C
Pd/ST-2
Pd/ST(60)-2
Pd/ST(120)-2
Pd/T-2
Pd/ST-1.5
Pd/ST-1
31
0
61
0
–
1
1
0
1
Pd(OH)
2
/C
26.6
66.6
40.0
26.6(53.3)
40.0
333.1
133.1
66.6
173.2
26.6
13.3
20.0
10.2
–
679.5
672.8
339.7
40.0
66.6
26.6
66.6
13.1
86
86
82
0(76)
84
63
81
79
73
92
89
93
0
35
33
37
14
82
86
0
28
12
20
0(14)
21
1
6
11
4
33
64
46
0
87/ [24]
-/ [23]
Pd/Sibunit C
4
6
Pd/CFC C
Pearlman’s Pd(OH)
Degussa Pd/C
Pd/Carbo Medicinalis active carbon with H
Synthesized palladium
1
1
2
3
2
/charcoal
20
10
10
13
10
0.25
0.20
0.20
0.20
–
-/ [12]
-/ [19]
2
O 50 %
1
1
4
5
-/ [16]
96 [13]
97/ [13]
-/ [18]
Pd(OH)
2
/C
–
1
6
7
Pd(OH)
2
/C
9
0.034
0.034
1.01
0.51
1.01
4
.5
1
Undried (< 50 % H
Dried Pd(OH) /C (Pearlman's)
Dried Degussa-type El01 NE/W
Pd/Sibunit
2
O) Pd(OH)
2
/C (Pearlman's)
20
20
10
–
94/ [20]
89/ [20]
87/ [20]
78/ [20]
-/ [21]
2
0.4
0.5
0.3
20
12
0
0
.51
1
8
6
0.10
1
4
0
1
2
9
0
Pd/MC
PdFeTi
10
6.57
0.10
0.03
80
76
12
57
-/ [25]
99/ [41]
Commercial catalysts: Degussa 10 wt.% Pd/C with 50 wt.% water; b Sigma 5 wt.% Pd/C; c obtained by ICP-AES. d separated yields; e calculated by TADBIW products
a
divided input Pd; f tested by HPLC.
3
.3. Catalysts characterization
S3b, ESI and the enlarged ones in Fig. 4a-b. The pore focused on 3.6 nm
was changed. Specifically, the 3.6 nm pore disappeared with Pd loading
increased over ST (Fig. 4b). It can be suggested that the introduction of
Pd reduced the surface area and mesoporous volume, meanwhile the
microporous volume seemed unchanged, which implied that Pd can
precipitate preferentially in the edge or tunnel of the mesoporous
structure over the support.
XRD of the catalysts with different loading and supports are shown
in Fig. S5a-b, ESI. In all cases, the spectra were similar to those of the
supports, i.e., the XRD results did not demonstrate the appearance of
any crystalline Pd phase. It was suggested that loading Pd nanoparticles
to the supports formed either amorphism or small nanoparticles which
goes beyond the detection of XRD.
With Pd deposited over ST, ST(60), ST(120) and T, no evident dif-
ference can be observed for the nitrogen adsorption-desorption iso-
therms given in Fig. S2a-S3a, ESI, but the obvious contraction can be
revealed for total pore volume and surface area as displayed in Table 3.
Noting that the microporous volume was still constant after supporting
Pd over sample T. The pore-size distributions are showed in Fig. S2b-
Table 3
Physical and chemical properties of Pd based catalysts with different supports.
Sample
Pd/ST-2 Pd/ST(60)-
2
Pd/ST(120)-
2
Pd/T-2
TEM and HAADF-STEM pictures of Pd/ST-2 catalyst are showed in
Fig. 5A-B and Fig. 5C respectively. It can be seen that ultrasmall Pd NPs
(≤ 2 nm) and even sub-nanometer clusters (< 1 nm) can be formed as a
predominant presence and finely dispersed on the supports ST with
average diameter 1.21 nm. Meanwhile, it can be revealed that the
support ST was composed of dispersed titania NPs which was a stick-
like crystal with many channels and mesoporous combined with ni-
trogen physical adsorption tests. Similarly, HAADF-STEM images of Pd
based catalysts over ST(120) and T were displayed in Fig. 5D and E. It
can be observed that ST(120) as supports exhibited slightly increased
particles size with mean diameter 1.28 nm. In contrast, Pd particles
obviously grew in the support T with average particle size 1.86 nm. It
a
2
−1
S
V
V
BET (m
g
)
46
35
29
72
b
3
−1
micro (cm
g
)
0.00
0.10
1.85
0.00
0.06
1.53
0.00
0.05
1.41
0.03
0.05
0.98
total (cm3
b
g
−1)
c
2
H pulse chemisorption
3
−1
(
cm
g
)
d
Dispersion of Pd (%)
88
73
67
47
a
surface area and.
b
pore volume of pores less than 403.122 Å diameter at P/Po = 0.95 was
adsorption–desorption at – 196 °C.
pulse chemisorption.
based on the H pulse chemisorption and input of Pd in the process of
catalysts preparation.
obtained by N
2
c
H
2
d
2
6

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
Fig. 4. Pore-size distributions of a. Pd deposited over different supports and b. ST based catalysts with different Pd loading.
can be seen that particle size of Pd increased over the supports as fol-
lows: ST < ST(120) < T which was consistent with HBIW debenzyla-
tion.
ST-2, Pd/ST(120)-2 and Pd/T-2 markedly showed the absorption
spectra in the whole region compared with ST, ST(120) and T, which
could be ascribed to deposit Pd. Meanwhile the intensity of the ab-
sorption peak became stronger for Pd/T-2 than Pd/ST-2∼Pd/ST(120)-2
in the visible region. For the ultraviolet region, the absorbance peak
followed the order of Pd/ST-2 < Pd/ST(120)-2 < Pd/T-2. It was re-
ported that the extinction spectra were red-shifted with increasing
particle diameter of Pd [52]. The gradually grew Pd clusters were re-
sponsible for the strong absorption in the visible and ultraviolet region,
which was consistent with the statistics obtained by HAADF-STEM.
Fig. 8 presents the Raman spectrum of Pd based catalysts recorded
ST supported Pd catalyst (Pd/ST-2) was further analyzed by HRTEM
as shown in Fig. 6. The Pd was revealed to spread over the titania
surface as a thin layer with amorphism about 1 nm thick. Combined
with the preparation method of DP, it was indicated that the deposited
Pd could be amorphous. Noting that the interjunctions were composed
of fragmental lattice and variegated amorphous phase. In the area of
supports without Pd particles, a lattice fringe of 0.242 nm can be as-
cribed to the (210) plane of titania. For the fields of Pd appeared, the
enlarged lattice spaces can be clearly observed with 0.248 nm,
−
1
−1
in the range of 50 cm –1000 cm . For the sample T, after Pd de-
−
1
−1
0.249 nm and 0.257 nm in Fig. 6a, b and c. In addition, evident lattice
posited, the peak at 192 cm
of anatase phase, 452 cm
and
−
1
distortion can be observed in red frame of Fig. 6c. XPS spectra of the
catalyst Pd/ST-2 were measured and are shown in Fig. S6, ESI. As a
comparison, the XPS spectra of the catalysts Pd/ST(120)-2 and Pd/T-2
are given in Fig S6, ESI. The binding energy (BE) followed the order:
Pd/ST-2 > Pd/ST(120)-2 > Pd/T-2 for Pd 3d while Pd/ST-2 < Pd/
ST(120)-2 < Pd/T-2 for O 1s. The binding energy of Ti 2p unchanged
over the three samples. The shift of Pd 3d to higher BE positions sug-
663 cm of rutile phase disappeared as shown in Fig. 8a. Compared
with the supports, the peak at 150 cm
−
1
designated as anatase phase
gradually became weaken for Pd/ST-2 and Pd/ST(120)-2. Weaken or
even disappeared peaks can be contributed to Pd particles covered on
−1
the surface or phase transformation. Meanwhile the peak at 445 cm
ascribed to rutile phase contained redshift for loaded Pd catalysts Pd/
ST-2 and Pd/ST(120)-2 as revealed in Fig. 8b, nevertheless the supports
ST and ST(120) displayed the same peak shift in Fig. 3B of Raman
spectra. The shifting of the spectral peaks was demonstrated relative to
the oxygen stoichiometry [43]. According to redshift of the main peak,
it can be inferred that Pd precipitation induced the forming of oxygen
defects for Pd/ST-2 and Pd/ST(120)-2 in consistent with O 1s of XPS. As
implied by HRTEM and XPS, Pd deposited into the crystal interstices
and lattice of surface, and then can produce oxygen defects. Similarly,
Resasco et al. found that oxygen vacancies can be formed in Pd loaded
titania system [53]. Meanwhile it was reported that in the DP method,
because of better salt-support interaction, electron transfer would be
more facile from Pd to titania surface and consequently the electron
n+
gested the presence of electron-deficient Pd species (Pd ) for Pd/ST-2.
At the same time, the shift of O 1s to low BE positions implied low
concentration of surface species on the Pd/ST-2. It is well known that
4
+
4+
2+
Ti (0.605 Å), Pd (0.615 Å) and Pd (0.640 Å) have approximative
ionic radii [47]. It was reported that Pd can occupy interstices and then
distort the titania lattice [48]. Meanwhile it was proposed that the
nanocrystalline anatase and rutile had high surface energy and easily
released deformation of phase accompanying grain boundary sliding
and nanoscale cavitation [49,50], besides, pH and metal salt precursor
can promote salt-support interaction [51] and the lattice of titania can
be doped by Pd under a certain content [48]. Thus it can be inferred
that in the initial process of DP, metal Pd produced a strong interaction
with unstable edge and tunnel surface of titania, and then led to initial
Pd penetrated into interstices even lattice by pH and the strong salt-
support interaction. Subsequently sub-nanometer clusters and ultrafine
NPs arisen.
n+
deficient Pd (Pd ) species can be generated [51]. Thus it can be in-
n+
ferred that Pd/ST-2 can emerge the palladium–titanate species (Pd
-
oxygen vacancy (Vo)-Ti3+). ST(120) shaped better crystal structure
than ST as obtained by XRD and then facilitate Pd deposited into the
support, which was consistent with more Raman red shift of the main
peak. By comparison, the support T had amorphous structure and then
generated the disappearance rather than red or blue shift of the peaks.
UV–vis absorption spectra of ST, ST(120), T and corresponded Pd
catalysts with theoretical 2.0 wt.% loading are presented in Fig. 7. Pd/
7

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
Fig. 5. TEM images of Pd/ST-2 (A and B); HAADF-STEM pictures of the catalysts. C. Pd/ST-2, D. Pd/ST(120)-2, E. Pd/T-2.
Fig. 6. HRTEM pictures of Pd/ST-2 (F).
8

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
Fig. 7. UV–vis spectra of the samples ST, ST(120) and T with 2.0 wt.% Pd
loading.
Fig. 9. CO DRIFT spectra of Pd based catalysts with the different supports.
−
1
the shoulder associated with the band 2083 cm
implied that there
were two types of atop sites on Pd nanoparticles [55]. A weak ad-
−1
sorption band at 1869 cm
appeared, which could be assigned to the
CO adsorbed on 3-fold hollow sites, indicating seldom presence of large
Pd ensembles [54]. Pd/ST(120)-2 revealed similar peaks. The bands of
Pd/T-2 shifted to low wavenumber which explained larger particles
existed. The CO DRIFTS studies provided the evidence for the four ac-
tive states of Pd on ST.
The H
ST supports, the H
2
-TPR patterns of ST and Pd/ST-2 presented in Fig. 10a. For
consumption peaks occurred at high temperature
2
Fig. 8. Raman spectra of Pd based catalysts with the different supports.
The surface structure was further probed by CO-adsorbed IR tech-
nique. CO adsorption on Pd prefers the bridging or hollow sites much
more than atop sites, whereby the geometric structures of Pd surfaces
can be effectively probed. Fig. 9 exhibits the DRIFT spectra of CO ad-
sorption on the catalysts Pd/ST-2, Pd/ST(120)-2 and Pd/T-2. For Pd/
ST-2 catalyst, two main absorption bands centered at 1965 and
−1
2083 cm
were clearly distinguished. The former was stronger than
the latter and could be assigned to CO adsorbed on 2-fold bridging sites,
while the latter was broad with a shoulder and can be reasonably as-
cribed to CO linearly adsorbed on atop sites of Pd [54]. The presence of
2
Fig. 10. H -TPR profiles of Pd based catalysts with the different supports.
9

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
Fig. 11. Proposed catalytic mechanism of HBIW over Pd/ST-2.
4
+
4
49 °C and 542 °C, and attributed to the reduction of titania from Ti
particles accompanied with low dispersion can be observed. Rutile and
seldom anatase phase and mesoporous structure with disturbed edges
and tunnels surface appeared in the supports ST and ST(120), and then
promoted Pd depositing into the interstices and lattice through the pH
and strong interaction between metal salt and supports, which shaped
ultrafine and even sub-nanometer Pd nanoparticles with high disper-
sion. The crystal structure of ST(120) supports matured with treatment
temperatures increasing. Although the interaction and oxygen defects
can be strengthened in the process of Pd depositing, Pd nanoparticles
grew up on the smaller surface area of ST(120) and then exhibited
lower dispersion than ST. In a word, the particle size of Pd over the
supports followed the sequence: ST < ST(60) < ST(120) < T as
proved by HAADF-STEM and UV–vis, and the dispersion was followed
3
+
to Ti [51,53]. Two peaks may represent two types of titania. It is well
known that the anatase titania is less thermodynamically and less
structurally stable than rutile titania. On the basis of Raman spectra, it
can be implied that the peak at 449 °C and 542 °C may be ascribed to
that of anatase and rutile respectively. For Pd/ST-2 catalysts, an in-
teresting observation that can be made here is five H consumption
2
peaks appeared. It is generally accepted that fine bulk oxides of palla-
dium are easily reduced to metallic palladium (Pd ) at low temperature.
0
Combined with HAADF-STEM and CO DRIFT, the peak I at – 7 °C can be
evidently ascribed to the reduction of ultrafine Pd nanoparticles, and
the peak II at 100 °C associated with large particles in very small
amount. The theoretical H
87 μmol/g. The reduction of Pd oxides over Pd/ST-2 was incomplete
with the consumption of the peak I and II together 87 μmol/g. In the
temperature range of 200 °C–700 °C, three H consumption peaks (III-V)
2
consumption of Pd over Pd/ST-2 was
1
by Pd/ST-2 > Pd/ST(60)-2 > Pd/ST(120)-2 > Pd/T-2 according to H
2
pulse chemisorption, which explained the catalytic performance of
HBIW debenzylation. Then the catalytic mechanism was proposed and
2
was disclosed. It was reported that they may correspond to the reduc-
2
is shown in the Fig. 11 on the basis of H pulse chemisorption, HRTEM,
tion of palladium particles with various metal–support interactions
Raman and XPS. First, the ultrafine and even sub-nanometer Pd nano-
[
53]. Peak III was assigned to the reduction of titania in direct contact
particles were highly dispersed over mesoporous stick-like ST. The
n+
with Pd particles, Peak IV was attributed to the reduction of remote
titania that was not in direct contact with the Pd particles, and Peak V
was associated with the reduction of stable PdO species that were
strongly interact with the titania support, or the reduction of well-dis-
persed PdO species on titania support, or the reduction of palla-
dium–titanate species (Pd -oxygen vacancy (Vo)-Ti ). For Pd/ST-2
catalyst, all three peaks emerged. Presence of metal sites would be fa-
vored to the reduction of titania. Compared with the unloaded titania,
the reduction of titania in Pd/ST-2 catalysts can shift to lower tem-
peratures. According to XRD, HRTEM and Raman, peak III-V could be
assigned to the reduction of titania support in direct contact with Pd
particles and without direct contact with Pd particles, and the reduction
of palladium–titanate species (Pd -oxygen vacancy (Vo)-Ti ). In the
case of H -TPR, it was proved to form not only fine particles and strong
interaction but also palladium–titanate species with Pd -oxygen va-
cancy (Vo)-Ti
DRIFT. Similarly, H
electron-deficient Pd species (Pd ) can be formed by the strong in-
n+
teraction and the palladium–titanate species (Pd -oxygen vacancy
(Vo)-Ti3+) appeared, which can easily activate hydrogen. Second, the
dispersed Pd improved the adsorption of HBIW. Third, the activated
hydrogen attacked the bond of C–N from HBIW and finally, the hy-
drodebenzylation can be enhanced.
2
+
3+
4. Conclusions
Stick-like rutile titania with mesoporous were synthesized, on the
basis of which highly efficient Pd based catalysts were developed for
hydrodebenzylation of HBIW in this work. The preparation parameters
played key roles on the textural and structural properties of the titania.
The stick-like titania (ST) was received by low temperature treatment
and showed large interplanar spacing, small average crystallite size and
large mesoporous surface area. Pd can deposit into the interstices and
lattice in the process of DP and then generated ultrafine and even sub-
nanometer sizes with four kinds of states including palladium–titanate
n+
3+
2
n+
3
+
over Pd/ST-2 as revealed by HRTEM, Raman and CO
-TPR of Pd/ST(120)-2 and Pd/T-2 were tested and
2
shown in Fig. 10b. It can be seen that the reduction were shifted to
higher temperatures in the order of Pd/ST-2 < Pd/ST(120)-2 < Pd/T-
n+
3+
2
. The results were consistent with HAADF-STEM and CO DRIFT.
chemisorption amount of Pd based catalysts over the different
supports are shown in Table 3. On the basis of the H pulse chemi-
sorption, the dispersion of Pd can be calculated and are given in
Table 3. It can be seen that the H chemisorption amount and dispersion
species (Pd -oxygen vacancy (Vo)-Ti ), which were highly dis-
persed. Amorphous titania (T) obtained without Sr template showed
blocky bulk with many irregular NPs and exhibited microporous and
mesoporous structure and Ti−OH surface, meanwhile titania, ST(60)
and ST(120), treated by high temperature formed matured crystal and
mesoporous with small surface area. Then the particle size increased
and the dispersion decreased for Pd catalysts over T, ST(60) and
ST(120). Thus ST supported Pd catalysts with quite low loading 2 wt.%
and Pd/HBIW usage 1‰ exhibited the highest TON in hydro-
debenzylation of HBIW as compared to all the current reports and
H
2
2
2
of Pd increased by arrangement of the supports, T, ST(120) and ST.
Combined with XRD, HRTEM, Raman and physical adsorption, the
sample T formed microporous and amorphous crystal with Ti−OH on
the surface of anatase and rutile phase, on the basis of which large Pd
10

S. Liu, et al.
Molecular Catalysis 477 (2019) 110556
commercial Pd/C catalysts, as well as Pd based catalysts with ST(60),
ST(120) and T as supports.
Struc. Chem. 24 (2005) 204–208.
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[
2
Declaration of Competing Interest
The authors declare that they have no conflict of interest.
Acknowledgements
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Foundation of China (NO. 21503264 and 21503133), the Program for
Professor of Special Appointment (Eastern Scholar) at Shanghai
Institutions of Higher Learning, the Talent Program of Shanghai
University of Engineering Science, Science and Technology Commission
of Shanghai Municipality (18030501100), Shanghai Talent
Development Foundation (2017076) and Natural Science Foundation of
Hebei Province of China (B2017202182). The characterization is sup-
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[
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