Recyclable Pd-contained perovskite catalyst synthesized by a low temperature hydrothermal method for aerobic alcohol oxidation

Article abstract of DOI:10.1016/j.mcat.2018.04.023

We successfully prepared Pd-containing perovskite strontium titanate (Pd-STO) by a relatively low-temperature hydrothermal method (i.e., 373 K) without posterior calcination. The particle size and porosity of the STO perovskite could be tuned by changing the molar ratio of H₂O/NH₃ (e.g., 5.0, 12.5 and 25.0) during the preparation of the amorphous titania sources. The mesoporous contained Pd-STO(12.5) showed superior catalytic performance compared to that of the other Pd-STO(x) (x = H₂O/NH₃) perovskites for alcohol oxidation with molecular oxygen. Both the refluxing of the non-polar solvents and addition of molecular sieves enhanced the reaction yield significantly. The Pd-incorporated perovskite (Pd-STO) showed better recyclability compared with that of the impregnated perovskite (imp-Pd/STO).

Full text of DOI:10.1016/j.mcat.2018.04.023

Molecular Catalysis 453 (2018) 132–138
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Recyclable Pd-contained perovskite catalyst synthesized by a low
temperature hydrothermal method for aerobic alcohol oxidation
a,b
a
a
a
a,⁎
Leo Saputra , Takashi Kojima , Takayoshi Hara , Nobuyuki Ichikuni , Shogo Shimazu
a
Department of Applied Chemistry and Biotechnology, Graduate School of Science and Engineering, Chiba University, 1-33 Yayoi, Inage, Chiba, 263-8522, Japan
Department of Chemistry, Faculty of Mathematics and Natural Sciences, Riau University, Pekanbaru, 28293, Indonesia
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
We successfully prepared Pd-containing perovskite strontium titanate (Pd-STO) by a relatively low-temperature
Pd-contained perovskite
Intelligent catalyst
Hydrothermal synthesis
Aerobic alcohol oxidation
hydrothermal method (i.e., 373 K) without posterior calcination. The particle size and porosity of the STO
perovskite could be tuned by changing the molar ratio of H
paration of the amorphous titania sources. The mesoporous contained Pd-STO(12.5) showed superior catalytic
performance compared to that of the other Pd-STO(x) (x = H O/NH ) perovskites for alcohol oxidation with
2 3
O/NH (e.g., 5.0, 12.5 and 25.0) during the pre-
2
3
molecular oxygen. Both the refluxing of the non-polar solvents and addition of molecular sieves enhanced the
reaction yield significantly. The Pd-incorporated perovskite (Pd-STO) showed better recyclability compared with
that of the impregnated perovskite (imp-Pd/STO).
was very low (SBET ≤11 m g−1), as commonly observed for perovskite,
2
1. Introduction
and almost inactive for low-temperature alcohol oxidation (353 K,
The role of supports (e.g., carbon, silica and alumina) in hetero-
yield: 12%) [10].
geneous transition-metal catalysis is frequently crucial. The choice of an
appropriate support can enhance the performance of transition-metal
catalysts [1]. However, the recovery of the supported metal catalyst,
especially platinum group metals (pgms), is still a major problem due to
leaching or sintering [2,3]. This issue can be addressed using a pgm-
contained perovskite catalyst [1].
Unlike the abovementioned conventional methods, a hydrothermal
method has been rarely studied for Pd-contained perovskite fabrication
[11]. The significant advantages of this method are the controlled size
and morphology, low-temperature growth and no posterior calcination
[12]. In the present work, we successfully synthesized a Pd-STO per-
ovskite with tunable particle size and porosity by using a low-tem-
perature hydrothermal method (373 K) without any posterior calcina-
tion. The mesoporous-contained Pd-STO(12.5) showed the highest
catalytic activity for aerobic alcohol oxidation among the Pd-STO(x)
3
Perovskite ABO oxides (A = rare/alkaline earth, B = transition
metals) are low-cost materials with high thermal stability and con-
trollable physicochemical properties [4]. Appropriate changes in the
perovskite composition (i.e., partial substitution at B-sites with pgms)
can lead to various and interesting magnetic, (thermo)electric, piezo-
electric and catalytic properties [5]. The incorporation of a pgm into
the perovskite lattice can also enhance its reusability. For example, Pd
on perovskite could be reversibly reincorporated into the lattice, which
is referred to as an “intelligent catalyst” [6], thus maintaining high
dispersion and preventing leaching or aggregation, instead of on con-
ventional supports (Fig. 1).
2 3
(x = H O/NH ) materials. We showed that a refluxing of the non-polar
solvents (n-hexane, n-heptane and cyclohexane) and addition of mole-
cular sieves were required for obtaining the high yield (> 99%). The
incorporation of Pd into the perovskite enhanced its recyclability
compared with that of the impregnated perovskite.
2. Experimental
Pd-contained perovskite is usually prepared by conventional solid-
state [7], co-precipitation [8] and sol-gel (Pechini) [9] methods.
However, posterior calcination at a very high temperature (≥873 K) is
required in these conventional methods. In a previous report, we used a
molten-salt method (873 K) for Pd-contained perovskite fabrication
2.1. Chemicals
PdCl
2
·2NaCl·3H
2
O (95%), aqueous NH
3
(28%), 1-butanol (BuOH)
·8H O (90%), ethanol
(99%), acetonitrile (MeCN) (99.5%), Sr(OH)
(99.5%), KOH (85%), benzyl alcohol (99%), 1-phenylethanol (98%),
2
2
(
i.e., Pd-STO, STO = SrTiO
3
). The surface area of the resulting Pd-STO
benzoin (98%), 1-octanol (98%), α-cyclopropylbenzyl alcohol (99%), n-
⁎
Corresponding author.
E-mail address: shimazu@faculty.chiba-u.jp (S. Shimazu).
https://doi.org/10.1016/j.mcat.2018.04.023
Received 14 March 2018; Received in revised form 19 April 2018; Accepted 20 April 2018
2468-8231/ © 2018 Elsevier B.V. All rights reserved.

L. Saputra et al.
Molecular Catalysis 453 (2018) 132–138
Fig. 1. Self-regenerative Pd-contained perovskite as an “intelligent catalyst” [1,3].
a Rigaku Thermo plus Evo TG 8120 under a N
2
flow (250 mL min 1)
−
decane (99%), HNO
3
(60%) and molecular sieve (MS3A) pellets 3A 1/
−1
1
6 (pre-activated at 593 K for 6 h) were purchased from WAKO
Chemicals. Crotyl alcohol ( > 95%), 2-adamantanol ( > 98%), trans-
-hexenol ( > 95%), cyclooctanol ( > 98%), 4-tert-butylcyclohexanol
98%), and 2-octanol (98%) were purchased from TCI Chemicals. Ti
OBu) monomer (99%) was purchased from Kishida Chameleon
using Pt pans in the range of room temperature to 773 K (10 K min ).
ICP-OES measurements were performed using a Thermo Scientific iCAP
7000 Series equipped with an Autosampler ASX-260 CETAC (Pd:
340.4589 nm, K: 766.4904 nm, Sr: 407.7718 nm, Ti: 334.9411 nm). The
samples were dissolved in HCl 0.1 M prior to analysis.
2
(
(
4
Reagent. All solvents (WAKO: n-hexane (96%), cyclohexane (98%), n-
heptane (97%), toluene (99.5%), DMF (99.5%), 2-propanol (99.7%),
2.4. Catalytic test
1,4-dioxane (99.5%), TCI: trifluorotoluene) were distilled prior to use.
1
-Cyclohexylethanol (97%) and HCl (35%, for trace analysis grade)
Pd-STO (12.5 mg), 1-phenylethanol (25 mg, 0.204 mmol), n-decane
were purchased from Sigma Aldrich. Standard solutions (1000 ppm) of
Pd, K, Sr, and Ti were purchased from WAKO Chemicals.
as an internal standard and n-hexane (2 mL) as the solvent were placed
in a Schlenk flask equipped with reflux condenser, mechanical stirrer
and balloon. The reaction was carried out at approximately the reflux
2
.2. Preparation of the Pd-STO perovskite
temperature of n-hexane (342 K) under an O
4 h. In some cases, molecular sieves 3A (MS3A) were placed into the
reaction mixture.
2
atmosphere (0.1 MPa) for
2
The amorphous titania spheres (ATSs) were prepared by the direct
hydrolysis of Ti(OBu)
.7 g (5 mmol) of Ti(OBu)
lution A). A quantity of aqueous NH
solved in BuOH/MeCN (1:1 v/v) (Solution B). The molar ratio of H
was varied at 5.0, 12.5 and 25.0. The ATSs are denoted as ATS(x) with
x = H O/NH . The two solutions were preheated at 353 K for 10 min, then
mixed all together and stirred for 30 min. A white suspension was im-
mediately formed and precipitated. The white precipitate was rinsed with
ethanol and water and then dried at 348 K overnight.
4
, as described in our previous reports [13]. Briefly,
was dissolved in BuOH/MeCN (1:1 v/v) (So-
and distilled water were also dis-
O/NH
1
4
3
. Results and discussion
3
2
3
3.1. Characterization of the Pd-STO catalysts
2
3
All of the ATS have monodispersed spherical shapes, as shown in the
TEM (Fig. 2, a-c) and SEM (Fig. S3, a-c) images. Note that no pre-
cipitates were obtained in the absence of NH and/or H O. The average
3
2
particle sizes of ATS(5.0) and ATS(25.0) were approximately 0.5 μm,
Typically, the ATSs (2 mmol), Sr(OH)
lution (0.028 mmol) were poured into
2
·8H
a
2
O (2 mmol), and Pd so-
Teflon-lined autoclave
whereas the particle size of ATS(12.5) was approximately 1.5 μm. All of
2
the ATS contained microporous structures, as shown in the N isotherm
(
150 mL). Then, 20 mL of KOH 0.1 M and 80 mL of distilled water were
also poured into the autoclave. Depending on the ATS sources, the
perovskites were denoted as Pd-STO(x), with x = H O/NH . The mix-
plot (Fig. S2). Although the particle size of ATS(12.5) was relatively
larger than the others, the specific surface area (SBET) was significantly
higher (Table S1) due to the presence of meso- and/or macroporous
2
3
ture was homogenized under ultrasonic irradiation for 10 min. The
autoclave was placed into an oven for a hydrothermal reaction at 373 K
(
Fig. 2b, Fig. S3 b). These ATSs were then used for the preparation of
the Pd-contained perovskite.
After the hydrothermal reaction at 373 K, the particle size and shape
of the Pd-STO corresponded to the ATS sources (Fig. 2d-f). In addition
for 24 h. The obtained brown powder was rinsed with HNO
water, then dried at 348 K overnight. The chemical composition of the
Pd-STO(12.5) based on ICP-OES analysis was Pd1.0
.64 wt%). For comparison, a wetness impregnated catalyst (imp-Pd/
STO) was also prepared with equal Pd loading.
3
0.05 M and
K0.5Sr78.5Ti85.6 (Pd:
to the ATS sources, Pd-STO(12.5) also has the highest
S
BET
0
2 −1
(
17.3 m g ). This value was quite higher than the value resulting
2
−1
from the molten-salt method (≤11 m g ) [10]. Therefore, the H
2
O/
NH ratio during ATS preparation is crucial for tuning the particle size
3
2.3. Catalyst characterization
and porosity of the resulting Pd-STO perovskites. All of the ATS sources
could be used for perovskite formation, as shown in Fig. 3 c-e. The
The Pd-STO catalysts were characterized by a powder XRD MiniFlex
Rigaku instrument (Cu Kα, λ=1.5444 nm) with a Ni Kβ filter. The XRD
3
perovskite phase was confirmed and indexed to SrTiO (JCPDS card
35–734). The Pd-STO perovskite could be obtained after a hydro-
thermal reaction at 373 K (Fig. S4). To the best of our knowledge, this is
the lowest hydrothermal temperature for Pd-contained perovskite fab-
rication with tunable particle size and porosity. There were no other
phases observed, confirming the high purity of the resulting per-
ovskites. Additionally, we also used commercial TiO anatase and rutile
2
(not shown), but the perovskite phase was not formed, which proves the
necessity of an amorphous titania source.
−1
instrument operated at 40 kV and 15 mA with a scan rate of 10° min
and a scan step of 0.1°. SEM images were obtained using an SEM JEOL
JSM 6330F. N adsorption-desorption isotherms were obtained at 77 K
2
using a BELSORPmax instrument (BEL Japan). The samples were de-
gassed at 423 K for 2 h prior to the measurement. TEM images were
obtained using a Hitachi High-Tech H-7650 with an emissive gun, op-
erated at 150 kV. Thermogravimetric (TG) analyses were performed on
133

L. Saputra et al.
Molecular Catalysis 453 (2018) 132–138
Fig. 2. TEM images of (a) ATS(5.0), (b) ATS(12.5), (c) ATS(25.0), (d) Pd-STO(5.0), (e) Pd-STO(12.5) and (f) Pd-STO(25.0). Scale bar = 500 nm.
The necessity of pressure during the hydrothermal process was
confirmed, while the perovskite phase was not completely formed by
the reflux method at 373 K (Fig. 3b). Additionally, only TiO anatase
2
was obtained in the absence of the Sr source (Fig. 3a). No appreciable
shift in the perovskite reflections was observed by a small addition of
Pd, but the crystallinity was slightly decreased (Fig. S5). We could not
find any peak of PdO or Pd0 due to the low Pd feeding amount, very
small crystallite size, or overlapping of its 2θ position with the per-
ovskite (PdO 34° and Pd 40°) [14]. Even with a 10-fold increase in Pd,
the XRD patterns remained unchanged (Fig. S5). However, a small peak
of PdO appeared after calcination at 923 K for 6 h (but not for lower Pd
loaded samples).
Based on the ICP-OES analysis (Table S2), the amount of Pd in the Pd-
STO(12.5) was 0.06 mmol g−1 (0.64 wt%). This value is quite low for
detection using XRD or TEM. The ratio of Sr/Ti was approximately 1,
3
which agrees with the ABO perovskite formula. The Pd-impregnated
perovskite (imp-Pd/STO) showed an identical XRD pattern to the in-
corporated one. In TEM images with higher magnification (Fig. 4a), the
PdO or Pd particles could also not be seen due to the very well-dispersed
Fig. 3. Powder XRD patterns of (a) KTO(12.5), (b) Pd-STO(12.5) prepared by
reflux method (373 K), (c) Pd-STO(5.0), (d) Pd-STO(12.5) and (e) Pd-STO
0
3 2
(25.0). X = SrCO , O = TiO (anatase).
particles or the incorporation of Pd into the perovskite. However, the PdO
particles (2–5 nm) were clearly seen on the imp-Pd/STO (Fig. 4b).
Fig. 4. TEM images of (a) Pd-STO(12.5) and (b) imp-Pd/STO(12.5). Pd loading: 0.64 wt%. Scale bar = 50 nm.
134

L. Saputra et al.
Molecular Catalysis 453 (2018) 132–138
water can promote the formation of geminal diol (by the hydration of
aldehyde) which is subsequently dehydrogenated to acidic products.
Instead, Steinhoff et al. [16] reported that the MS3A did not alter the
Table 1
Catalyst screening for the Pd-STO perovskites.
influence of water but serves as a Brønsted base (Pd(OAc)
catalyst). In fact, the addition of external water inhibited the reaction
significantly (entry 5). Moreover, the addition of the base K CO
without MS3A) did not enhance the reaction at all (entry 6).
As expected, the calcined Pd-STO catalyst (923 K, 6 h, air) showed
2
/pyridine
2
3
(
Entry
Pd-STO(x)a
Conv./%
Yield/%b
TON
lower performance (Table 2, entry 7) due to a higher degree of crys-
tallization, similar to the catalyst prepared at a higher hydrothermal
1
2
3
Pd- STO(5.0)
Pd-STO(12.5)
Pd-STO(25.0)
Pd-STO(12.5)
STO(12.5)
31
63
54
36
0
31
63
54
36
0
85
172
147
98
temperature. Conversely, the reduction of the catalyst (573 K, 1 h, H
enhanced the reaction significantly (entry 8). The use of ethylene glycol
as a reducing agent) in the hydrothermal process also increased the
2
)
c
4
(
5
0
catalytic activity (not shown). This result is consistent with the previous
0
reports that suggested that Pd is the active phase [17,18]. However, a
Reaction conditions: 1-PE/Pd = 273; 1-PE, 25 mg; Pd-STO(x), 12.5 mg; n-
hexane, 2 mL; O , 0.1 MPa (balloon); 342 K; 24 h.
O]/[NH ].
The selectivity to AP is almost 100%. The Pd-STO(12.5) was prepared at
33 K (hydrothermal).
small amount of degradation products (e.g., decarbonylation) may be
produced due to the considerable initial activity of the pre-reduced
catalyst [17]. The reduction process at high temperature could also
2
a
x = [H
2
3
b
c
0
4
promote the sintering of Pd ; thus, it is more difficult to reincorporate
(
lessen the recyclability). For the as-prepared catalyst, smaller particles
0
3.2. Catalytic test
of Pd could be generated by the in situ reduction of the substrate al-
cohols [18].
The catalytic activity of the Pd-STO was first tested on the oxidation
of 1-phenylethanol (1-PE) to acetophenone (AP) (Table 1). The best
catalytic performance was shown by Pd-STO(12.5) with an AP yield of
3
.2.1. Solvent screening, reaction temperature and atmosphere
The use of an appropriate solvent was crucial for obtaining high
63% and a turnover number (TON = AP/Pd) of 172 (entry 2). Pd-STO
catalytic performance (Table 3). Under such conditions, the highest AP
yield ( > 99%) was obtained by using n-hexane as the solvent (entry
1
luene (entries 2–6) also exhibited higher AP yields than that of the polar
solvents (entries 7–12). The use of non-polar solvents ensures no
competition between the solvent and the substrate to adsorb onto the
catalyst surfaces. Additionally, leaching of the active phase, as usually
observed in highly polar solvents, can be minimized. A reaction at the
(
5.0) and Pd-STO(25.0) only demonstrated AP yields of 31% and 54%,
respectively. The presence of mesoporous or the higher surface area of
Pd-STO(12.5) is likely responsible for this higher performance; even the
particle size of the perovskite is much larger than that of the others
). Other non-polar solvents such as n-heptane, cyclohexane and to-
(
Fig. 2). Thus, further experiments were studied using the Pd-STO(12.5)
catalyst. STO itself did not show any catalytic activity under such
conditions. Pd-STO(12.5) as-prepared at a higher temperature (433 K)
exhibited an AP yield of only 36% due to the higher degree of crys-
tallization (i.e., drop in the surface area or porosity) (entry 4).
2
reflux temperature of n-hexane (342 K) was optimum for O to diffuse
into the solution and thus be available for reaction. Notably, a complete
conversion was also obtained of the reaction at the reflux temperatures
of n-heptane and cyclohexane (entry 3 and 5, respectively). The com-
plete conversion at 342 K was an exceptionally lower reaction tem-
perature compared with that of the previous work with an AP yield of
only 12% under such conditions (solvent: toluene) [10].
The Pd-STO perovskite showed catalytic activity even at tempera-
tures as low as 298 K (Table 4, entry 1). The activity was increased by
raising the reaction temperature and reached an optimum value at re-
flux conditions of 342 K (entry 4). At 323 K, we observed that the re-
duced catalyst also showed higher activity than the as-prepared catalyst
The as-prepared Pd-STO(12.5) catalyst demonstrated a maximum
AP yield of only 63% (Table 2, entry 1). Interestingly, > 99% of the
yield (TON up to 1070) could be obtained by the addition of molecular
sieves (MS3A) into the reaction system (entry 2 and 3). MS3A itself did
not show any catalytic activity under such conditions. We found that
MS3A after reaction contained more water than that of the blank ex-
periment (Fig. S6). This phenomenon is likely related to its ability to
absorb the water byproduct, thus inhibited the water absorption on the
perovskite catalyst surfaces (i.e., deactivation) (Fig. S8). In the absence
of MS3A, a small addition of water (0.03 g, 1.67 mmol) also decreased
the AP yield by 11%. Besson et al. [15] reported that the presence of
Table 3
Solvent screening.
Table 2
Effect of the catalyst pretreatments.
Entry
Solvent
Conv./%
Yield/%
Entry
Pd-STO(12.5)
Conv./%
Yield/%
TON
1
2
3
n-Hexane
> 99
44
> 99
67
> 99
46
16
15
36
26
31
> 99
44
> 99
67
> 99
46
16
15
36
26
31
n-Heptane
n-Heptane
Cyclohexane
Cyclohexane
Toluene
Trifluorotoluene
Acetonitrile
2-Propanol
Ethanol
1
As-prepared
As-prepared
As-prepared
As-prepared
As-prepared
As-prepared
Calcined
63
> 99
98
79
75
54
37
89
63
> 99
98
79
75
54
37
85
172
273
1070
216
205
147
101
232
a
a
2
b
4
5
3
b
c
4
d
6
7
8
9
5
e
6
7
8
Reduced
1
1
1
0
1
2
DMF
1,4-Dioxane
Reaction conditions: 1-PE/Pd = 273; 1-PE, 25 mg; Pd-STO(12.5), 12.5 mg; n-
hexane, 2 mL; O , 0.1 MPa (balloon); 342 K; 24 h. Addition of:
35
35
2
a
MS3A, 100 mg.
MS3A, 100 mg; 1-PE/Pd = 1,092.
MS3A, 50 mg.
MS3A, 100 mg; water, 0.25 mL.
2 3
K CO , 100 mg.
Reaction conditions: 1-PE/Pd = 273; 1-PE, 25 mg; Pd-STO (12.5), 12.5 mg; sol-
vent, 2 mL; O , 0.1 MPa (balloon); 342 K (n-hexane, refluxed); 24 h; MS3A,
2
b
c
100 mg.
d
e
a
Refluxed at 371 K.
Refluxed at 353 K.
b
135

L. Saputra et al.
Molecular Catalysis 453 (2018) 132–138
Table 4
Effect of the reaction temperature and atmosphere.
Entry
Temp./K
Conv./%
Yield/%
1
2
3
298
323
333
342 (refluxed)
383 (refluxed)
323
19
22
56
62(9)
49
36
19
22
56
62(9)
49
36
a
4
b
5
c
6
Reaction conditions: 1-PE/Pd = 273; 1-PE, 25 mg; Pd-STO(12.5), 12.5 mg; n-
hexane, 2 mL; O , 0.1 MPa (balloon); 4 h; MS3A, 100 mg.
2
a
The value in the parenthesis is of reaction under N
Solvent, toluene.
Reduced Pd-STO(12.5).
2
.
b
c
(
entry 6). Note that reaction under air (342 K, 24 h) demonstrated an
AP yield of only 82% compared with the > 99% for a reaction under
. The reaction did not proceed smoothly under N (entry 4, in the
parenthesis), which suggests the critical role of O . A reaction at a much
Fig. 5. Time profiles for the aerobic oxidation of 1-PE over Pd-STO(12.5) cat-
alyst. Reaction conditions: 1-PE/Pd = 273, 1-PE, 25 mg; Pd-STO(12.5), 12.5 mg;
n-hexane, 2 mL; O , 0.1 MPa (balloon); 342 K (refluxed); and MS3A, 100 mg.
2
O
2
2
2
higher temperature (383 K, toluene as solvent) also still exhibited a
high AP yield (entry 5), confirming the stability of the perovskite cat-
alyst.
Table 5
Substrate scopes for the secondary alcohols.
Since a small conversion still be observed under N
parenthesis)), we presume that O probably originated from the per-
ovskite. A gas consumption test revealed that O was consumed almost
stoichiometrically to the substrate at ca. 1:2 (Scheme 1). However, for
reactions under N , the gas volume did not significantly change (it
should be increased, if oxidant-free dehydrogenation occurs). Some
researchers reported that Pd has an oxidant-free dehydrogenation
ability. Unfortunately, under anaerobic conditions, the adsorbed hy-
drogen (or degradation products) was ineffectively removed from the
catalyst surface [17].
2
(Table 4, entry 4
Entry
1
Substrate
Product
Conv./%
> 99
Yield/%
> 99
(
2
2
2
3
42
42
2
> 99
> 99
4
80
80
3
.2.2. Time profile and hot filtration test
The time profile for the oxidation reaction over Pd-STO(12.5) is
5
6
22
11
22
11
shown in Fig. 5. An induction period was observed during the first hour
of the reaction time. This period is likely related to the reduction pro-
cess by the substrate alcohol to generate the active phase Pd . After-
wards, the reaction progressed in time and reached completion after
0
7
8
56
34
56
34
2
4 h. No further reaction was observed after hot filtration (no leaching
of the Pd species) that proved unequivocally that the reaction pro-
ceeded via heterogeneous catalysis. An ICP analysis of the filtrate after
hot filtration did not indicate Pd leaching. Additionally, the Pd content
of the reused catalyst remains the same (Table S2). In the absence of
molecular sieves (MS3A), the reaction rate was slightly lower especially
after a 3-h reaction time.
Reaction conditions: Substrate/Pd = 273; Pd-STO(12.5), 12.5 mg; n-hexane,
2 mL; O , 0.1 MPa (balloon); 342 K (refluxed); 24 h; MS3A, 100 mg.
2
almost 100%. We suggested that stability of the carbocation inter-
mediate is highly pivotal for obtaining the high yield. An aromatic ring
has ability to stabilize the adjacent carbocation over the resonance ef-
fect; thus the β-hydride elimination step is much easier. In contrast, the
substrates with alicyclic substituents such as cyclopropyl (entry 2) and
cyclohexyl (entry 4) were not smoothly converted. Lower conversions
were also observed for other secondary aliphatic/cyclic alcohols (entry
3
.2.3. Substrate scopes
Various secondary alcohols have been tested as
a substrate
(
(
Table 5). Mostly, aromatic secondary alcohols such as 1-phenylethanol
entry 1) and benzoin (entry 3) were easily oxidized compared with the
others. In all cases, the selectivity to the corresponding products is
5
–8).
Due to the lower carbocation stability, primary alcohols were harder
to oxidize (Table 6). The presence of an aromatic ring in benzyl alcohol
entry 1) leads to a higher yield, as mentioned above. The reduction
treatment and the addition of a hydrogen acceptor such as cyclohexene
entries 2 and 3, respectively) could enhance the reactions. The com-
plete conversion to benzaldehyde (entry 4) was obtained by the addi-
tion of K CO . By using TBHP as an oxidant, benzyl alcohol was ex-
(
(
2
3
clusively oxidized to benzoic acid (entry 6). Neither the reduced
catalyst nor the addition of a base could enhance the oxidation of the
primary aliphatic alcohols (entries 7–9). In addition to carbocation
stability, the primary aldehyde products may poison the catalyst by
Scheme 1. Aerobic and oxidant-free (anaerobic) dehydrogenation of alcohols
over the Pd-based catalyst.
136

L. Saputra et al.
Molecular Catalysis 453 (2018) 132–138
Table 6
Substrate scopes for the primary alcohols.
Entry
Substrate
Product
Conv./%
Yield/%
1
38
64
38
64
2a
3b
4c
5d
6e
7
55
55
> 99
0
> 99
0
> 99
7
0
7
8a
9c
7
7
7
7
Reaction conditions: Substrate/Pd = 273; Pd-STO(12.5), 12.5 mg; n-hexane,
mL; O , 0.1 MPa (balloon); 342 K (refluxed); 24 h; MS3A, 100 mg.
2
2
a
Reduced Pd-STO(12.5).
Addition of cyclohexene, 1 mol eq. to the substrate.
b
c
Fig. 7. Powder XRD patterns of Pd-STO(12.5): (a) fresh and (b) after the third
run of the catalytic tests.
Addition of K
2
CO
, 55 mg.
Addition of TBHP (70% in water), 0.5 mL.
3
, 55 mg.
d
e
2 3
Cat: K CO
3.2.5. Plausible mechanism
Based on the results and discussion above, we proposed a plausible
mechanism as shown in Scheme 2. The incorporated Pd is first reduced
by the alcohols and then moved onto the perovskite face. This process
produced aldehydes/ketones (Scheme S1); thus, under an oxygen-free
condition, a small amount of product could be detected (Table 4, entry
4). Meanwhile, Pd could promote oxidant-free dehydrogenation; how-
ever, the adsorbed byproducts could not be effectively removed in the
absence of oxygen [17]. This phenomenon was also observed by Ker-
strongly adsorbing on the metal surfaces [19].
3.2.4. Recyclability test
The Pd-STO catalyst could be easily separated from the reaction
mixture by centrifugation. After being rinsed with n-hexane and dried
under vacuum (at room temperature), the next reaction cycle was
performed, as described above. The Pd-STO(12.5) catalyst could be
recycled at least 4 times without significant loss of activity (Fig. 6). The
impregnated catalyst (imp-Pd/STO) showed poor recyclability which is
likely due to sintering (or leaching) of the active species (see: Fig. 4).
Therefore, the incorporation of Pd into the perovskite was critical to
maintaining its performance.
2 3
esszegi et al. [20] with 10.7% of conversion (Ar, Pd/Al O ) and by
Savara et al. [21] (N2, Pd/C). Grunwaldt et al. [17] proved by using in
0
situ XAS (X-ray absorption spectroscopy) that “Pd is the active phase
for aerobic dehydrogenation”. Interestingly, after being reoxidized by
O
2
, the Pd0 on the surface could be reversibly reincorporated into the
perovskite lattice, which will thus prevent leaching or sintering [6]. The
lack of a significant inhibition effect of a radical scavenger (TEMPO,
1 mol eq. to Pd) suggests that the reaction did not proceeded via a ra-
dical mechanism. Bürgi et al. [22] reported the formation of a Pd-alk-
oxide intermediate by in situ ATR spectroscopy analysis.
Hydrogen abstraction from the alcohol will produce adsorbed hy-
drogen while the oxidation state of Pd was increased. The adsorbed
hydrogen could also be used for alkenes hydrogenation. We proved that
aerobic dehydrogenation occurs by gas consumption testing as dis-
The perovskite phase remained unchanged, even after the third run
(
Fig. 7). In fact, perovskite is well known for its durability, especially at
high temperature, which is thus more applicable for industry [4]. The
high recyclability of the catalyst is likely due to the incorporation of Pd
into the perovskite, which prevents leaching or Pd aggregation (which
is commonly observed for supported Pd on conventional supports). This
reversible process occurs easily because small Pd particles could be
reduced or oxidized even at room temperature [14,18].
cussed above. The use of O
chemistry [23]. Some researchers proposed that the adsorbed hydrogen
could be oxidized by O to produce water. Oxygen could also remove
2
as an oxidant is highly desirable in green
2
the adsorbed decomposition byproducts, i.e., CO [17]. A considerable
increase in the reaction by the addition of molecular sieves suggests its
role to absorb the water byproduct. Finally, Pd could be reversibly re-
incorporated into the lattice, thus maintaining its high recyclability.
4. Conclusion
Pd-STO perovskites with a tunable particle size and porosity have
been successfully synthesized via low-temperature hydrothermal
method (373 K) without any posterior calcination. Porosity of the Pd-
STO perovskites play an important role for obtaining the high yield on
alcohol oxidation with molecular oxygen. Both refluxing of the non-
polar solvents and addition of molecular sieves enhanced the reaction
yield significantly. We demonstrated that the recyclability could be
maintained by the incorporation of Pd into the perovskites. We believe
this facile method could also be applied for the fabrication of other
transition metal-contained perovskite catalysts.
Fig. 6. Recyclability tests for Pd-STO(12.5) and imp-Pd/STO(12.5). Reaction
conditions: 1-PE/Pd = 273; 1-PE, 25 mg; catalyst, 12.5 mg; n-hexane, 2 mL; O
.1 MPa (balloon); 342 K (refluxed); 24 h; and MS3A, 100 mg.
2
,
0
137

L. Saputra et al.
Molecular Catalysis 453 (2018) 132–138
Scheme 2. A simplified mechanism for alcohol oxidation over the Pd-perovskite catalyst.
Acknowledgements
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