Development of reusable palladium catalysts supported on hydrogen titanate nanotubes for the Heck reaction

Article abstract of DOI:10.1016/j.jcat.2016.07.019

Palladium catalysts supported on hydrogen titanate nanotubes were prepared by adsorption of palladium(II) acetate from dichloromethane solutions. At low Pd(OAc)₂ loadings (up to 5?wt.%), the catalysts contained only highly dispersed chemisorbed

Full text of DOI:10.1016/j.jcat.2016.07.019

Journal of Catalysis 342 (2016) 138–150
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Development of reusable palladium catalysts supported on hydrogen
titanate nanotubes for the Heck reaction
c
Mark E. Martínez-Klimov a, Patricia Hernandez-Hipólito a, Tatiana E. Klimova b, , Dora A. Solís-Casados ,
⇑
Marcos Martínez-García a
a Instituto de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, Circuito Exterior, Coyoacán, Ciudad de México 04510, Mexico
b Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, Ciudad de México 04510, Mexico
c Centro Conjunto de Investigación en Química Sustentable, UAEM-UNAM, Km 14.5, Carretera Toluca-Atlacomulco, San Cayetano, Toluca, Estado de México 50200, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium catalysts supported on hydrogen titanate nanotubes were prepared by adsorption of palladium
(II) acetate from dichloromethane solutions. At low Pd(OAc)2 loadings (up to 5 wt.%), the catalysts con-
tained only highly dispersed chemisorbed Pd0 and Pd2+O species (XPS). Characteristic signals of Pd
(OAc)2 were not detected in these catalysts by FT-IR, FT-Raman, of XPS, indicating decomposition of
the precursor salt due to the strong metal–support interaction. An increase in the palladium loading
resulted in an increase in the proportion of Pd0 species and the appearance of some supported Pd
(AcO)2 (XPS). In the sample with the highest palladium loading (8.84 wt.% of Pd(OAc)2), formation of
small (ꢀ1.2–2.4 nm) palladium-containing clusters was detected on the support surface by HRTEM.
Catalytic activity was tested in the Heck reaction between 4-bromobenzaldehyde or 4-bromostyrene
and styrene. Only the E-isomers of the corresponding cross-coupled products were obtained. The activity
and TON numbers of the supported Pd catalysts were higher than those of the palladium(II) acetate under
homogeneous catalysis conditions. Strong palladium–support interaction resulted in the generation of
catalytically active Pd(0) species without additional reduction pretreatment of the catalyst. The possibil-
ity of reuse of the same catalysts in several catalytic cycles was tested. It was found that both the Pd load-
ing in the catalysts and the amount of water in the support have a strong influence on the catalyst’s
deactivation. A catalyst with stable activity, able to be recycled at least five times, was prepared with
Pd loading of 5 wt.% of Pd(AcO)2 using titanate nanotubes calcined at 350 °C for 2 h prior to the prepara-
tion of the catalyst.
Received 17 May 2016
Revised 23 July 2016
Accepted 25 July 2016
Keywords:
Palladium catalysts
Hydrogen titanate nanotubes
Cross-coupling Heck reaction
Catalytic activity
Deactivation
Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
reaction, giving good results. Nevertheless, homogeneous Heck
catalysis has some drawbacks, such as difficulty of catalyst separa-
Carbon–carbon bond formation is a general aim in organic
chemistry synthesis catalyzed by transition metals yielding valu-
able fine chemicals. The Heck coupling reaction is an effective
method for the C–C cross-coupling of aryl halides with aryl or alke-
nyl compounds, leading to a variety of alkenes with great pharma-
ceutical and industrial value [1–3]. The growing demand for fine
chemicals in the industries and increasingly stringent ecological
standards require development of new technologies for cross-
coupling reactions catalyzed by transition metal catalysts in order
to provide convenient synthetic routes for regio- and stereodefined
systems [4,5]. Different palladium catalysts, homogeneous and
heterogeneous, have been widely used for the cross-coupling Heck
tion and reuse after the reaction, as well as facile deactivation lead-
ing to limited catalyst lifetime. Because of this, in the past few
years, several attempts have been made to prepare active, stable,
and easy-to-handle and -separate palladium catalysts. Palladium
catalysts supported on different materials (carbon [5], metal oxides
[6], layered clays [7], zeolites [8], mesoporous molecular sieves [9],
carbon nanotubes [10], etc.) or encapsulated in large organic struc-
tures (dendrimers [11], block copolymer micelles [12], etc.) have
attracted much attention. The interest in them is due principally
to the fact that immobilization of Pd nanoparticles on the surface
of a solid material or inside an organic matrix makes it possible
to stabilize catalytically active well-dispersed metal species and
prevent their sintering. In [9], Pd-TMS11 catalysts with high Pd dis-
persion (25–32%) were prepared by the vapor grafting of a volatile
⇑
Corresponding author. Fax: +52 55 56225371.
E-mail address: klimova@unam.mx (T.E. Klimova).
organometallic complex [Pd(g-C5H5)-(g
3-C3H5)] onto the surface of
http://dx.doi.org/10.1016/j.jcat.2016.07.019
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
139
Nb-MCM-41 material previously degassed at 600 °C, followed by
the reduction of the catalyst in a stream of hydrogen at 350 °C
for 3 h. These catalysts showed high catalytic activity (TON num-
bers between 1000 and 5000) for the cross-coupling Heck reaction
of n-butyl acrylate with activated aryl substrate. However, the Pd-
TMS11 catalyst, recovered after the catalytic test, showed some
agglomeration of palladium and partial structural damage of the
Nb-MCM-41 support material [9]. In [10], poly(lactic acid) grafted
carbon nanotubes (f-CNTs) were used as a platform for in situ
deposition of Pd nanoparticles. The obtained catalysts were found
to be effective in the promotion of the Heck cross-coupling reac-
tion between aryl halides and n-butyl acrylate. The recycled
f-CNTs-Pd nanocatalyst showed stable activity in another three
reaction cycles. However, up to now, the search for new heteroge-
neous catalysts with well-dispersed and stable supported Pd spe-
cies remains an incomplete task.
In the present work, palladium catalysts supported on hydrogen
titanate nanotubes were prepared and tested in the cross-coupling
Heck reaction. Hydrogen titanate nanotubes are a novel and inten-
sively studied material that has attracted much attention in the
past decade because of its unique textural and physicochemical
properties [13–16]. This material found a variety of applications,
including catalysis [17,18] and photocatalysis [19,20]. Titanate
nanotubes have high specific surface area (ꢀ200–400 m2/g), inter-
nal tube diameter between 3 and ca. 10 nm, open mesoporous
morphology [21,22], and an absence of micropores that facilitate
transport of reagents and products during a catalytic reaction.
The high cation exchange capacity [14,23] of hydrogen titanate
nanotubes should enable high loading of an active catalyst with
even distribution and high dispersion. The semiconducting proper-
ties of titanate nanotubes [24,25] may result in a strong electronic
interaction between the support and a catalyst, which could be
beneficial for some catalytic reactions. Palladium catalysts sup-
ported on titanate nanotubes have already been prepared by an
ion-exchange technique [23,26], incipient wetness impregnation
[27], and photodeposition [28] and by hydrothermal synthesis of
titanate nanotubes with the simultaneous addition of a PdCl2 pre-
cursor [29]. In all these cases, water was used as a solvent. Palla-
dium catalysts with different characteristics were obtained
depending on the preparation method used and the palladium
loading. Here we report on the synthesis of palladium catalysts
supported on hydrogen titanate nanotubes by adsorption of Pd
(II) acetate from a nonaqueous solvent (dichloromethane). We
selected this method of preparation because of its simplicity and
the possibility of deposition of palladium species only by interac-
tion with the support. Catalysts were characterized and tested
in the Heck reactions between activated aryl substrates
(4-bromobenzaldehyde or 4-bromostyrene) and styrene.
of NaNT), filtered, washed with deionized water, and dried again
at 120 °C for 12 h. This procedure resulted in the formation of
hydrogen titanate nanotubes with low content of residual sodium
(below 0.1 wt.%), which will be denoted hereafter as NT. Part of the
synthesized NT material was calcined at 350 °C for 2 h (NTc sam-
ple). Palladium catalysts supported on the synthesized NT or NTc
were prepared by adsorption of Pd(II) acetate (Pd(OAc)2) from
dichloromethane solutions. Briefly, 1 g of the support was slurried
in 30 mL of CH2Cl2 solution of Pd(OAc)2 for 8 h at room tempera-
ture. After the first hour of this treatment, the white powder of
the support changed color to light brown, whose intensity
depended on the initial concentration of Pd(OAc)2. The catalysts
were filtered, washed with a small amount of CH2Cl2, and dried
at room temperature and then at 120 °C for 12 h. The remain-
ing solutions were analyzed by atomic absorption spectroscopy
(Perkin–Elmer) to determine the quantity of adsorbed palladium.
In addition, palladium loading in the dried catalysts was deter-
mined by SEM-EDX, giving good correlation between both tech-
niques. Four catalysts with nominal Pd(OAc)2 loadings between
2.5 and 10 wt.% were prepared using the NT support and two cat-
alysts using the NTc material. Hereinafter, the prepared catalysts
are designated as Pd(x)/NT or Pd(x)/NTc, where x is the theoretical
wt.% of Pd(OAc)2 in the catalyst.
2.2. Catalyst characterization
Obtained hydrogen titanate nanotubes (NT and NTc) and cata-
lysts were characterized by N2 physisorption, X-ray powder
diffraction (XRD), scanning electron microscopy (SEM-EDX), trans-
mission electron microscopy (TEM), thermogravimetric analysis
(TGA), and FT-IR, FT-Raman, and X-ray photoelectron spectroscopy
(XPS). Nitrogen adsorption–desorption isotherms were measured
with a Micromeritics ASAP 2020 automatic analyzer at liquid N2
temperature. Prior to the experiments, the samples were degassed
(p < 10À1 Pa) at 250 °C for 6 h. Specific surface areas (SBET) were cal-
culated by the BET method, the total pore volume (Vp) was deter-
mined by nitrogen adsorption at a relative pressure of 0.98, and
pore diameters (DP) and pore size distributions were obtained from
the adsorption isotherms by the BJH method. The X-ray powder
diffraction patterns of the synthesized samples were recorded at
room temperature from 3 to 80° (2h) on a Bruker D8 Advance
diffractometer, using Cu
Ka radiation (k = 1.5406 Å) and a
goniometer speed of 1° (2h) minÀ1. The chemical composition of
the synthesized materials was determined by SEM-EDX using a
JEOL 5900 LV microscope with OXFORD ISIS equipment. High-
resolution transmission electron microscopy (HRTEM) images
were recorded with a JEOL 2010 microscope operating at 200 kV
(resolving power 1.9 Å). The solids were ultrasonically dispersed
in heptane and the suspension was collected on carbon-coated
grids. HRTEM pictures were taken from different parts of the same
sample dispersed on the microscope grid. TGA analysis was used to
determine the amount of chemisorbed water in the synthesized
materials. TGA experiments were performed on a Mettler-Toledo
TGA/SDTA 851e in dry air flow (50 ml/min) in the temperature
range between 25 and 1000 °C with a heating rate of 10 °C/min.
FT-IR spectra were recorded on a Varian 640-IR spectrometer
equipped with a PIKE accessory. Micro-Raman spectra of the cata-
lysts were obtained using an HR LabRam 800 system equipped
with an Olympus BX40 confocal microscope. A Nd:YAG laser beam
(532 nm) was focused by a 50Â microscope objective onto an
2. Experimental
2.1. Support and catalyst preparation
Sodium titanate nanotubes were synthesized by an alkali
hydrothermal treatment following a procedure reported by Kasuga
et al. [13,30]. Commercial titanium dioxide (anatase, Aldrich,
54 m2/g) was used as the TiO2 source. In each synthesis, 10 g of
TiO2 was mixed with 150 ml of a 10 M alkali solution, followed
by hydrothermal treatment in a Teflon-lined autoclave at 140 °C
for 20 h under constant magnetic stirring. After the hydrothermal
reaction, the white powder of sodium titanate nanotubes (NaNT)
was filtered in vacuum, thoroughly washed with deionized water
to eliminate the excess of nonreacted NaOH, and dried at 120 °C
for 12 h. To decrease the sodium content, the dry NaNT was slur-
ried into a 0.1 M HCl solution for 2 h (100 mL of solution per 2 g
ꢁ1
lm diameter on the sample surface. The laser power at the
sample was regulated by a neutral density filter (OD = 1) to prevent
sample heating and structural changes induced in the sample. A
cooled CCD camera was used to record the spectra, usually aver-
aged for 100 accumulations in order to improve the signal-to-
noise ratio. All spectra were calibrated using the 521 cmÀ1 line of

140
M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
a silicon wafer. XPS measurements were performed with a JEOL
JPS-9200 instrument equipped with a Mg K radiation source
distributed open-ended nanotubular structures with 3–5 layered
walls can be observed. They are not always symmetric, with small
dispersion in inner and outer diameters and of different lengths.
The external diameters of the nanotubes were around 10–12 nm,
the internal pore size about 6–7 nm, and the lengths of the tubes
ranged from 50 to 200 nm. Observed layered structures of the tube
walls had interlayer spaces of ꢀ0.8 nm. This result is in line with
previous reports [13,30,31] and confirms successful preparation
of sodium and hydrogen titanate nanotubes.
a
(1253.6 eV). The spectrometer was operated at a pass energy of
10 eV with an X-ray power of 300 W. The base pressure in the ana-
lyzing chamber was maintained on the order of 1 Â 10À8 mbar. The
binding energy was determined using the carbon C (1s) line as a
reference with a binding energy of 284 eV. Quantification and
deconvolution were performed using the Gaussian functions of
the Origin 8.1 software.
The X-ray diffraction pattern of the synthesized NT material is
shown in Fig. S1 in the Supporting Information. In the diffrac-
togram, three main diffraction peaks were observed at 11.2, 24.4,
and 48.4° (2h). These reflections are typical of the monoclinic lay-
ered hydrogen trititanate phase H2Ti3O7 (JCPDS-ICDD Card
47-0561) [31], which forms the curved nanotubular walls, and
can be indexed to its (200), (110), and (020) crystal planes,
respectively. The signal at 11.2° (2h) is attributed to the interlayer
distance of hydrogen trititanate and is equal to 0.79 nm, which is in
good agreement with the HRTEM results described above. The
broadness of all observed diffraction signals indicates the poorly
crystalline nature of the synthesized titania nanotubes, which is
in line with previous reports for similar materials [32,33]. No
reflections attributable to TiO2 anatase used as a precursor were
detected, indicating that it was transformed into trititanate
nanotubes.
Nitrogen physisorption showed that the NT support had attrac-
tive textural characteristics: 260 m2/g specific surface area and
0.62 cm3/g total pore volume (Table 1). The corresponding N2
adsorption–desorption isotherm and pore volume distribution
are shown in Fig. S2 in the Supporting Information. The isotherm
was of type IV, according to the IUPAC classification [34], with a
noticeable hysteresis loop, which confirms the presence of meso-
and macropores. Pore size distribution presented a main pore con-
tribution at 7.3 nm, which corresponds well to the internal pore
size of titanate nanotubes observed by HRTEM, and two lower-
intensity signals at 3 and about 50 nm. The pores with a diameter
of 3 nm can indicate the presence of a small proportion of titanate
nanotubes with this internal size, whereas larger pores can be
attributed to the voids in the aggregation of the nanotubes [35].
Palladium catalysts supported on the above-described NT sup-
port were prepared by adsorption of Pd(OAc)2 from dichloro-
methane solution at room temperature (25 °C). The
corresponding adsorption isotherm is shown in Fig. 2. It can be
seen that at low Pd loadings, all palladium species are adsorbed
onto the support surface, the residual concentration of Pd(OAc)2
in the solution being less than 0.001 M (part I of the adsorption
2.3. General procedure for the Heck reaction
Styrene, 4-bromobenzaldehyde, 4-bromostyrene, triethylamine,
tri-(o-tolyl)phosphine (TOP), and N,N-dimethylformamide were
obtained from Sigma-Aldrich. Reactions were performed in a 250-
mL three-necked glass flask under reflux. In a typical experiment,
the flask was charged with 4-bromobenzaldehyde or 4-
bromostyrene (13.5 mmol), styrene (13.5 mmol), triethylamine
(10 mL), TOP (1.64 mmol), and N,N-dimethylformamide (25 mL).
The reaction mixture was heated to 150 °C and varied amounts of
the palladium catalyst (Pd(x)/NT or Pd(x)/NTc) was added. The
reaction was monitored by TLC. After completion of the reaction,
the catalysts were recovered by filtration and dried in air at room
temperature for 12 h and the reaction mixture was concentrated
under reduced pressure. Purification of the E/Z-isomers was carried
out by column chromatography with silica gel and a hexane/ethyl
acetate (8:2) solvent mixture, giving analytically pure products.
The obtained reaction products were characterized by 1H and 13C
NMR, IR, mass spectrometry, and elemental analysis (Fig. S6 and
product characterization in the Supporting Information). 1H and
13C NMR spectra were recorded on a Varian Unity 300 MHz with
tetramethylsilane (TMS) as an internal reference. Infrared (IR) spec-
tra were measured on a spectrophotometer FT-IR MAGNA 700.
Mass spectra were taken on a JEOL JMS AX505 HA instrument.
Turnover numbers (TON) were calculated as mol of obtained pro-
duct per mol of Pd, disregarding the oxidation state of palladium
species.
3. Results
3.1. Support and fresh catalysts characterization
Fig. 1 shows a TEM micrograph of the as-synthesized NaNT
material and the same material after washing with diluted
hydrochloric acid, during which Na+ cations were exchanged with
H+ resulting in the NT support. In both images, randomly
Fig. 1. HRTEM micrographs of as-synthesized NaNT (A) and NT (B) supports. Nanotubular structures of both materials shown can be seen.

M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
141
Table 1
Chemical composition and textural characteristics of NT and NTc supports and prepared palladium catalysts.
Sample
Theoretical loading
Pd(OAc)2 (wt.%)
Real Pd loadinga
Pd(OAc)2 (wt.%)
Textural characteristicsb
Atoms Pd/nm2
SBET (m2/g)
VP (cm3/g)
DP (nm)
NT support
Pd(2.5)/NT
Pd(5.0)/NT
Pd(7.5)/NT
Pd(10)/NT
NTc support
Pd(5.0)/NTc
Pd(10)/NTc
–
–
–
260
258
252
244
246
269
253
245
0.62
0.61
0.58
0.57
0.58
0.72
0.66
0.65
7.3
7.3
7.3
7.3
7.3
8.9
8.9
8.8
2.5
5.0
7.5
10.0
–
2.54
5.01
7.43
8.84
–
0.26
0.53
0.82
0.96
–
5.0
10.0
4.99
8.57
0.53
0.94
a
Determined by SEM-EDX.
b
SBET, specific surface area calculated by the BET method; VP, total pore volume; DP, pore diameter corresponding to the maximum of the pore size distribution calculated
from the desorption isotherm by the BJH method.
appeared. These results suggest the possibility of coexistence of
different types of Pd species adsorbed on the NT surface and that
their relative proportions can change depending on the palladium
loading.
For further characterization and catalytic activity evaluation,
we selected four Pd(x)/NT catalysts with nominal Pd(OAc)2 load-
ings of 2.5, 5.0, 7.5, and 10 wt.%. These freshly prepared samples
are shown in Fig. 3. After palladium acetate adsorption, the white
NT support changed color to light brown, the color becoming more
intense with increasing Pd content.
Table 1 shows the chemical composition of the prepared cata-
lysts determined by SEM-EDX analysis and their textural charac-
teristics. It can be observed that, as expected, the amount of
palladium in the catalysts was close to the theoretically expected
value at low Pd loadings (up to 5 wt.% of Pd(OAc)2), whereas at
higher loadings the real amount of Pd in the catalysts was smaller,
since some residual Pd species remained in the solution. For exam-
ple, the Pd(10)/NT catalyst had 8.84 wt.% of palladium as Pd(OAc)2
instead of the nominal 10 wt.% loading. Regarding textural charac-
teristics of the catalysts, adsorption of Pd(II) acetate almost did not
affect the textural characteristics of the NT support. Only a slight
decrease in the surface area and total pore volume was observed
after Pd incorporation, which can be attributed to an increase in
the materials’ density due to the adsorption of the Pd(OAc)2 pre-
cursor. A detailed analysis of the N2 adsorption–desorption iso-
therms and pore volume distributions of the catalysts (Fig. S2 in
the Supporting Information) showed that the adsorption of Pd
did not produce noticeable modifications in the characteristic
shape of the N2 adsorption–desorption isotherm of the NT support,
nor in its pore structure. Pore diameter maintained the same value
1.2
I
II
1.0
0.8
0.6
0.4
0.2
0.0
0
2
4
6
8
Ceq of Pd2+ (mmol/L)
Fig. 2. Isotherm of adsorption of palladium (II) acetate on hydrogen titanate
nanotubes in dichloromethane solutions at 25 °C. Part I of the isotherm corresponds
to irreversible adsorption of Pd species, whereas Part II corresponds to reversible
adsorption.
isotherm). In this part of the isotherm, Pd adsorption is irreversible
and seems to be chemical. On the other hand, an increase in the Pd
(OAc)2 concentration (part II of the isotherm) resulted in a further
increase in the amount of adsorbed palladium and the appearance
of some residual Pd2+ species in the solution. In this second part of
the isotherm, equilibrium is established between Pd2+ species in
the solution and the adsorbed ones, which evidences the presence
of some Pd species reversibly adsorbed on the NT surface, in addi-
tion to the chemisorbed ones. The above observation indicates the
possibility of different types of adsorbed Pd species in the first and
second parts of the isotherm. It is worth mentioning that previ-
ously, isotherms of similar shape were obtained for the adsorption
of Pd2+, Ru3+, and Au3+ species from aqueous solutions onto tita-
nate nanotubes [23,26]. The first part of the isotherms was attrib-
uted to the ion exchange of metal cations from solution with
protons of the hydrogen titanate nanotubes. In the present work,
we used Pd(II) acetate as the palladium source and a nonaqueous
solvent (dichloromethane). Since dichloromethane is a polar sol-
vent, the possibility of some ion exchange of Pd2+ species from
the solution with protons of the support cannot be completely
excluded. In addition, irreversible adsorption of Pd species in the
first part of the isotherm may be due to strong palladium–support
interaction leading to the appearance of some kind of chemisorbed
Pd species, whereas in the second part of the isotherm, in addition
to the chemisorbed Pd species, some physically adsorbed Pd
Fig. 3. NT support (a) and freshly prepared palladium catalysts: (b) Pd(2.5)/NT, (c)
Pd(5.0)/NT, (d) Pd(7.5)/NT, and (e) Pd(10)/NT. White color of the NT support
changed to light brown after adsorption of Pd(OAc)2.

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M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
(7.3 nm) in all Pd(x)/NT catalysts as in the corresponding NT sup-
port. X-ray diffraction patterns of the Pd(x)/NT catalysts (Fig. S1
in the Supporting Information) were similar to that of the NT sup-
port; only the reflections characteristic of the hydrogen trititanate
phase H2Ti3O7 (JCPDS-ICDD Card 47-0561) of the support were
observed. No reflections attributable to any Pd(II) or Pd(0) crys-
talline phase were detected, indicating good dispersion of the
deposited Pd species.
Further insight into the palladium dispersion in the prepared
catalysts was made by scanning electron microscopy (SEM-EDX)
and high-resolution transmission electron microscopy (HRTEM)
studies. Results obtained by SEM for the Pd(2.5)/NT catalyst are
shown in Fig. 4, whereas palladium maps of other catalysts can
be consulted in Fig. S3 in the Supporting Information. This study
also confirmed that palladium was highly dispersed and homoge-
neously distributed over the surface of the NT support.
Characterization of the prepared palladium catalysts by HRTEM
did not allow us to observe any kind of palladium species in the Pd
(2.5)/NT, Pd(5.0)/NT, and Pd(7.5)/NT catalysts, which may be due
to their very high dispersion. Only in the case of the Pd(10)/NT cat-
alyst with the highest palladium loading were very small (ꢀ1.2–
2.4 nm) Pd-containing clusters observed (Fig. 5).
Fig. 5. HRTEM micrograph of fresh Pd(10)/TNT catalyst. Pd-containing clusters are
indicated with arrows.
Summarizing all the above characterization results, it can be
stated that palladium catalysts with well-dispersed palladium spe-
cies were prepared by the adsorption method used in the present
work. In addition, it was assumed that different types of adsorbed
species (chemisorbed and physisorbed ones) could be formed.
Namely, at low Pd loadings, chemisorbed species were predomi-
nant, whereas at high metal loadings, some less strongly adsorbed
species appeared (part II of the Pd(OAc)2 adsorption isotherm).
Probably the latter are those that were observed as small
palladium-containing clusters by HRTEM. However, from all the
above results, it was not clear if Pd(II) acetate was adsorbed intact
onto the NT surface or if it suffered some transformations. In order
to answer this question, FT-IR, FT-Raman, and XPS characterization
of the prepared catalysts was performed.
appeared between 1600 and 1350 cmÀ1, which correspond to dif-
ferent vibration modes of the acetate group. Amplification of the
region 1900–1100 cmÀ1 is shown in Fig. 6A. There are three vibra-
tions for the acetate ligand in this region: asymmetric and sym-
metric stretching vibrations of the carboxylate (m(COO)asym and m
(COO)sym, respectively) and deformation vibrations for the methyl
group (d(CH3)sym). Since the acetate group can exist in different
coordination modes (bridging, bidentate, and monodentate) [38],
the exact assignment of bands for the carboxylate stretching vibra-
tions in acetate complexes sometimes is difficult. For comparison
purposes, the spectra of solid Pd(II) acetate and Pd(OAc)2 dissolved
in dichloromethane were also recorded (Fig. 6B).
The FT-IR spectrum of solid Pd(OAc)2, used as a palladium
source, is shown in Fig. 6B (spectrum a). It is well known that Pd
(II) acetate in the crystalline state exists as a trimer, Pd3(CH3COO)6,
with all acetate groups in bridging coordination (each acetate
group forms a symmetrical bridge between two Pd atoms) [38].
The FT-IR spectrum obtained for this compound showed three
bands located at 1595, 1416, and 1350 cmÀ1. According to the lit-
erature [38–40], the first band at 1595 cmÀ1 is assigned to the
Infrared spectra of the NT support and four Pd(x)/NT catalysts
with different Pd loadings are shown in Fig. S4A in the Supporting
Information. In the IR spectrum of the NT support, three main
bands were observed. The first broad band in the range 3200–
3500 cmÀ1 can be assigned to the stretching vibrations of adsorbed
water and Ti–OH surface hydroxyl groups,
m(OH) [36]. The high
intensity of this band can be attributed to high water content in
the NT support, which can vary between 17 and 20 wt.% [36].
Another band was observed at 1632 cmÀ1, which can be ascribed
to the molecular water bending mode [37]. The last intense band,
from about 1000 to 400 cmÀ1, is generally related to the Ti–O
and Ti–O–Ti skeletal frequency region. In the FT-IR spectra of all
Pd catalysts, the above-mentioned characteristic vibrations of the
NT support were observed, and in addition some new bands
asymmetric stretching vibrations
m
(COO)asym, whereas the second
(COO)sym
band, with a maximum at 1416 cmÀ1, corresponds to
m
with a contribution from d(CH3)sym (ꢀ1430 cmÀ1). A less intense
band at 1350 cmÀ1 is assigned to the symmetric scissoring vibra-
tion of the acetate methyl group. The frequency of asymmetric
and symmetric stretching vibrations depends on the coordination
of the acetate group and on the nature of the metallic atom. There-
Fig. 4. SEM image of Pd(2.5)/NT catalyst (A) and corresponding palladium map (B). Homogeneous distribution of palladium on the support can be observed.

M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
143
1555
1545 1450
1431
1595
B
A
1632
1416
(e)
(d)
(c)
(b)
1350
(b)
(a)
1614
(a)
1900 1800 1700 1600 1500 1400 1300 1200 1100
Wavenumber (cm-1)
1900 1800 1700 1600 1500 1400 1300 1200 1100
Wavenumber (cm-1)
Fig. 6. (A) IR spectra of (a) NT support, (b) Pd(2.5)/NT, (c) Pd(5.0)/NT, (d) Pd(7.5)/NT, and (e) Pd(10)/NT. (B) Some reference IR spectra: (a) Pd(OAc)2 solution in
dichloromethane and (b) Pd(OAc)2 trimer crystal.
fore, the differences (Dm) in the wavenumbers of carboxyl asym-
metric and symmetric stretch modes Dm (COO)asym (COO)sym
where the most striking aspect was the strong vibration at
338 cmÀ1 assigned to a Pd acetate breathing mode, whereby acet-
ate ligands stretch against Pd ions (m(Pd-OAc) in phase vibrations)
=
m
À
m
help in the identification of the coordination mode of acetates.
Thus, for the solid Pd(OAc)2, the value of Dm was equal to
179 cmÀ1, which is in line with the previously reported value of
184 cmÀ1 corresponding to acetate bridging two Pd atoms [38].
When Pd(OAc)2 was dissolved in dichloromethane, only a slight
[38]. The strength of this band was attributed to a high conven-
tional Raman polarizability. In the Raman spectra of the Pd(x)/NT
catalysts (Fig. S5), the above signal was not observed, which once
again confirms that the precursor’s Pd(II) acetate structure
was modified upon adsorption on the NT surface. In the
100–000 cmÀ1 region of the spectra, only four bands were
observed at 190, 269, 454, and 658 cmÀ1 which are characteristic
of the hydrogen form of titanate nanotubes [41].
shift of the m(COO)asym and m(COO)sym signals to higher wavenum-
bers was observed, probably because of solvation, indicating that a
trimeric Pd(II) acetate structure was maintained intact in the
CH2Cl2 solution. Regarding the spectra of Pd catalysts (Fig. 6A),
vibrations of the acetate group were observed at positions other
than those of the Pd(OAc)2 trimer, namely at about 1550 cmÀ1
The XPS analysis of Pd was performed to inquire into its chem-
ical state in the prepared Pd(x)/NT catalysts with different metal
loadings. The Pd3d spectrum of the starting Pd(II) acetate precur-
sor is also shown for comparison (Fig. 7a). For palladium acetate,
two peaks related to the spin–orbital doublet can be observed at
338.1 and 343.5 eV, corresponding to Pd3d5/2 and Pd 3d3/2, respec-
tively. Fig. 7b and c shows the Pd3d regions of the Pd(5.0)/NT and
Pd(10)/NT catalysts. In the spectrum of the first catalyst, it can be
clearly seen that the signals of Pd species were shifted to lower
binding energies than those of the Pd(OAc)2 precursor. The spec-
trum b can be deconvoluted into two components with Pd3d5/2
binding energies of 335.1 and 336.3 eV, which can be ascribed to
Pd(0) and Pd(II) in PdO, respectively. Regarding the spectrum of
the Pd(10)/NT catalyst with the highest palladium loading, three
components are present in the Pd3d5/2 region. Thus, in addition
to the above-mentioned signals at 335.1 and 336.3 eV, a signal at
338.3 eV can be observed, indicating the presence of Pd(II)
acetate-like species. The relative proportions of different types of
Pd species in the catalysts with different Pd contents is shown in
Table 2. It can be seen that at low Pd loading (5 wt.% of Pd
(OAc)2), Pd(II) as in PdO is predominant (77%), followed by reduced
Pd(0). An increase in the palladium loading (Pd(10)/NT catalyst)
resulted in the appearance of a small amount of Pd(II), similar to
that in Pd(II) acetate, and in an increase in the proportion of
reduced Pd(0) to 58%. These results are in line with a previous pub-
lication [26], where a similar increase in the proportion of Pd(0)
species was observed with an increase in the bulk Pd content in
the catalysts supported on titanate nanotubes and was attributed
to the fact that the Pd clusters were becoming more metallic at
higher loadings.
(m (m(COO)sym) and their intensi-
(COO)asym) and 1450-1430 cmÀ1
ties increased with Pd loading. This result indicates that the char-
acteristic trimeric structure of Pd(II) acetate was not preserved in
the catalysts. The value of Dm for the catalysts was between 95
and 124 cmÀ1, which can be ascribed to the presence of the biden-
tate acetate (Dm = 100 cmÀ1) [38] or acetate group bridging two Ti
atoms of the support (Dm around 110–155 cmÀ1) [40]. To confirm
the possibility of formation of similar acetate species in our cata-
lysts prepared by adsorption of Pd(OAc)2 on hydrogen titanate
nanotubes, we recorded the FT-IR spectrum of acetic acid impreg-
nated on the NT support (Fig. S4B, Supporting Information). In this
spectrum two bands were observed at 1545 and 1450 cmÀ1, simi-
lar to those detected in the IR spectra of Pd(2.5)/NT and Pd(5.0)/NT
catalysts (Fig. 6A). It seems that in the catalysts, these acetate spe-
cies are in direct interaction with the nanotubular support. An
increase in the Pd loading in the catalysts (Pd(7.5)/NT and Pd
(10)/NT samples) resulted in the appearance of the secondary
asymmetric and symmetric stretches at 1555 cmÀ1 and
1431 cmÀ1, respectively, which indicates that in these catalysts, a
new coordination mode of carboxylate binding appeared. Since
the intensity of these bands increases with the palladium loading
in the catalysts, it seems reasonable to assume that they corre-
spond to some bidentate acetate in coordination with Pd species,
such as Pd acetate dimer (with two bidentate acetate ligands coor-
dinated to one central Pd ion) or terminal Pd bidentate acetate spe-
cies. Therefore, the above results showed that the Pd(OAc)2 trimer
did not remain intact upon adsorption on the NT surface and suf-
fered some decomposition due to a strong interaction with the
support. In addition, the characteristics of the adsorbed acetate
species changed depending on the palladium loading in the
catalysts.
Summarizing the above FT-IR, FT-Raman, and XPS characteriza-
tion results, it can be concluded that different kinds of palladium
and acetate species are present in the Pd(x)/NT catalysts with dif-
ferent Pd loadings. It seems that at low Pd loadings (2.5 and 5.0 wt.%
of Pd(OAc)2), the precursor salt is decomposed upon adsorption,
leading to the presence of acetate species in interaction with the
The FT-Raman spectrum of the Pd(OAc)2 precursor is shown in
Fig. S5 (inset) in the Supporting Information. This spectrum is well
in line with a previous report for a palladium acetate trimer crystal

144
M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
low population (15%) in comparison with other types of Pd species
Pd (II)
300
200
100
0
or due to the fact that these Pd-acetate species are different from
those in the Pd(OAc)2 trimer used as a precursor, which leads to
the disappearance of the characteristic vibration at 338 cmÀ1. In
addition, it was observed that Pd(0) was present in all catalysts
and its proportion increased with Pd loading. No additional reduc-
tion treatment was applied to our catalysts after drying, which
indicates that the palladium reduction occurred during catalyst
preparation. Previously, it was reported that acetate itself or water
can function as reductants [42,43]. According to previous reports,
there are a number of factors (Pd dispersion, Pd distribution, its
oxidation state, water content in the catalysts, etc.) that affect cat-
alytic activity and stability of heterogeneous Pd catalysts in Heck
reactions. Therefore, the above differences in the characteristics
of the prepared Pd(x)/NT catalysts led us to expect different cat-
alytic performance in the selected Heck reactions.
3d 5/2
338.1
(a)
Pd (II)
3d 3/2
343.5
346
344
342
340
338
336
334
Binding energy (eV)
60
40
20
0
336.3
(b)
335.1
341.5
3.2. Catalytic activity
340.3
Palladium catalysts prepared in the present work were tested in
the Heck cross-coupling reaction of styrene and two aryl halides:
4-bromobenzaldehyde and 4-bromostyrene (Scheme 1). The struc-
ture of compounds 3 and 5 was confirmed by 1H and 13C NMR, IR
spectroscopy, mass spectrometry, and elemental analysis (Fig. S6
and product characterization results in the Supporting
Information).
344
342
340
338
336
334
332
Binding energy (eV)
The catalytic activity results and reaction conditions (reaction
time and amount of catalyst used) are summarized in Table 3. A
comparison was made with unsupported Pd(OAc)2, used fre-
quently as a catalyst for Heck reactions. It can be seen that all
tested catalysts were active for the Heck reaction of styrene and
4-bromobenzaldehyde (4-BrBA) or 4-bromostyrene (4-BrST). The
corresponding cross-coupled products were separated quantita-
tively, purified, and characterized. It was found that only the
E-isomers of compounds 3 and 5 were obtained. The amount of
the pure E-product finally obtained was used to calculate product
yield in each reaction. It can be seen in Table 3 that the reaction
between styrene and 4-bromobenzaldehyde performed with
0.49 mol.% of palladium(II) acetate in solution resulted in 79% pro-
duct yield at reaction time 24 h. The turnover number (TON) calcu-
lated for Pd(OAc)2 in the above reaction was 159. When the same
reaction was performed using the Pd(x)/NT catalysts, product 3
yields between 43% (Pd(2.5)/NT) and 98% (Pd(10)/NT) were
obtained. As expected, an increase in the product yield was
obtained with an increase in the Pd loading in the catalysts. It is
worth mentioning that in the experiments with the supported Pd
(x)/NT catalysts, the amount of Pd varied between 0.15 and
0.04 mol.%, which was 3–12 times less than in the reference reac-
tion performed with the dissolved Pd(OAc)2 mentioned above.
Therefore, it was possible to obtain high product yields using Pd
(x)/NT catalysts containing a smaller amount of Pd.
335.1
20
10
0
(c)
336.3
340.1
341.9
338.3
343.6
-10
344
342
340
338
336
334
332
Binding Energy (eV)
Fig. 7. XPS spectra of (a) Pd(OAc)2 used as a precursor and of (b) Pd(5.0)/NT and
(c) Pd(10)/NT catalysts.
Table 2
Binding energy (BE) and population of palladium species in different chemical states
for the selected fresh and used Pd(x)/NT catalysts.
Sample
BE Pd 3d5/2 (eV)
Pd2+ in Pd(OAc)2
338.1 (100%)
Pd2+ in PdO
–
Pd0
–
Pd(OAc)2 precursor
TON values obtained for the Pd(x)/NT catalysts varied between
1037 and 675, decreasing with increased Pd loading, which can be
attributed to some agglomeration of Pd species. In order to com-
pare the efficiency of Pd catalysts in the supported and dissolved
forms, an additional experiment was done using the same amount
of dissolved Pd(OAc)2 as in the Pd(5.0)/NT catalyst (0.08 mol.%). A
product yield of 37% was obtained in this case (TON equal to 450
vs. 827 for the Pd(5.0)/NT supported catalyst). It is noteworthy that
in the coupling of 4-bromobenzaldehyde with styrene under iden-
tical reaction conditions, TON values obtained for all Pd catalysts
supported on NT were higher than those obtained for dissolved
Pd(II) acetate, which demonstrates the higher efficiency of the sup-
ported catalysts in comparison with unsupported Pd(OAc)2.
To inquire into the effect of the reaction time, an additional
experiment was performed with the Pd(7.5)/NT catalyst, reducing
Fresh catalysts
Pd(5.0)/NT
Pd(10)/NT
–
336.3 (77%)
336.3 (27%)
335.1 (23%)
335.1 (58%)
338.3 (15%)
Used catalystsa
Pd(5.0)/NT (3C)
Pd(10)/NT (1C)
336.9 (47%)
335.0 (53%)
335.4 (100%)
a
Catalysts used in one (1C) or three (3C) catalytic cycles.
nanotubular titanate support, whereas palladium is present as Pd
(II) in PdO (major proportion) and reduced Pd(0). At high Pd con-
tents, the presence of some bidentate acetate species in coordina-
tion with Pd(II) was observed by FT-IR, which was confirmed by
XPS of the Pd3d region. The existence of these species was not
detected by FT-Raman spectroscopy, probably because of their

M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
145
Scheme 1. Synthesis of (E)-4-styrylbenzaldehyde 3 and (E)-1-styryl-4-vinylbenzene 5.
Table 3
Catalytic activity of Pd catalysts in Heck coupling reactions of styrene with aryl halides.a
Catalyst
ArXb
Reaction time (h)
Amount of catalyst
Catalyst (mg)
Product yield (%)c
TONd
Pd(OAc)2 (mg)
Pd (mol.%)
Pd (mmol)
Pd(OAc)2
Pd(OAc)2
4-BrBA
4-BrBA
4-BrBA
4-BrBA
4-BrBA
4-BrBA
4-BrBA
4-BrBA
4-BrBA
4-BrST
4-BrBA
24
24
24
24
24
3
24
0.75
24
18
24
15
2.5
50
50
50
50
50
25
50
50
50
15
0.49
0.08
0.04
0.08
0.12
0.12
0.15
0.14
0.08
0.08
0.15
0.067
0.011
0.006
0.011
0.017
0.017
0.020
0.010
0.011
0.011
0.020
79
37
43
68
96
94
98
29
64
80
100
159
450
1037
827
785
769
675
206
778
973
692
2.5
1.3
2.5
3.7
3.7
4.4
10
2.5
2.5
4.3
Pd(2.5)/NT
Pd(5.0)/NT
Pd(7.5)/NT
Pd(7.5)/NT
Pd(10)/NT
Pd(10)/NTe
Pd(5.0)/NTc
Pd(5.0)/NTc
Pd(10)/NTc
a
All reactions were carried out under nitrogen at 150 °C using aryl halide (13.5 mmol), styrene (13.5 mmol), triethylamine (72 mmol), dimethylformamide (25 mL), and
tri-(o-tolyl)phosphine (TOP, 1.64 mmol). The catalyst used, its amount, and the reaction time are shown in the table.
b
ArX, aryl halide substrate: 4-BrBA, 4-bromobenzaldehyde; 4-BrST, 4-bromostyrene.
Product yield was determined on the basis of the amount of coupling product separated and purified by column chromatography as (mol of coupling product)/(mol of
c
used reactant). Only E-isomers of coupling products were obtained in all cases.
d
TON = (mol of product)/(mol of catalyst).
Reaction performed without TOP addition.
e
the reaction time from 24 to 3 h. Only a small decrease in the pro-
duct yield (from 96% to 94%) was obtained, showing that it is pos-
sible to obtain high yields of product 3 at shorter times.
4-bromostyrene with styrene (product 5). The Pd(10)/NTc catalyst
gave 100% yield of the product 3 at 24 h reaction time, whereas
98% yield was obtained with the corresponding Pd(10)/NT sam-
ple. Therefore, catalytic activity of the Pd catalysts with similar
Pd loadings supported on uncalcined (NT) and calcined (NTc)
materials was found to be similar in the first catalytic cycle.
Finally, according to the mechanism proposed by Heck for
cross-coupling reactions of aryl halides and alkenes, Pd(0) is the
catalytically active species. This species can be formed from of Pd
(OAc)2 with the help of monophosphine ligands (Pd0Ln species)
[44], where L can be, for example, tri-(o-tolyl)phosphine (TOP),
used in the present work. In the case of heterogeneously catalyzed
reactions, reduction can be performed with hydrogen gas [9,45],
aqueous NaBH4 solution [26], or other reduction agents. XPS
results obtained for the Pd(x)/NT catalysts showed that all freshly
prepared samples had some amount of the reduced Pd(0) species
(Table 2). In order to inquire into the possibility of catalyzing the
Water content in the supported Pd/C catalysts was also men-
tioned as an important factor that has a strong influence on cat-
alytic activity in Heck coupling of bromobenzene with styrene
[43]. TGA analysis of the NT support used for the preparation of a
series of Pd(x)/NT catalysts with different metal loadings showed
that it had high water content (about 17.1 wt.%). To decrease this
content, a sample of the NT support was calcined at 350 °C for
2 h prior to catalyst preparation. The support obtained after this
thermal treatment (labeled as NTc) contained about 4.8 wt.% of
water and showed slightly increased textural characteristics (SBET
,
VP, and DP) relative to the starting NT support (Table 1). Evaluation
of the Pd(5.0)/NTc catalyst in the Heck reaction showed a yield of
the product 3 (64%) similar to that obtained previously with
the Pd(5.0)/NT sample and 80% yield in the coupling of

146
M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
Heck reaction using these Pd(0) species, an additional experiment
was performed in the absence of TOP (Table 3, entry 8), obtaining
29% yield of the product 3 at 0.75 h reaction time. After that time,
the catalyst turned black and lost activity.
(5.0)/NTc. These selected catalysts had similar Pd loadings (around
5 wt.% of Pd(OAc)2, Table 1), the difference between them being in
the amount of water in the support (17.1 wt.% in NT and 4.8 wt.% in
NTc) and their textural characteristics (Table 1). Results obtained
with these catalysts in recycling studies are shown in Table 4. After
each catalytic activity test, recovered catalyst was washed with
dichloromethane and dried in vacuum.
3.3. Characterization of the used catalysts and their reutilization
It can be seen in Table 4 that when the recycled Pd(5.0)/NT cat-
alyst was used for the second and third times, a noticeable
decrease in the product yield took place (from about 68% to 26%
and 13%, respectively). After three cycles, the catalyst maintained
its light brown color. No formation of Pd black was detected by
XRD and HRTEM. However, some agglomeration of the deposited
palladium species cannot be excluded. In addition, an increase in
the proportion of Pd(0) in the twice reused Pd(5.0)/NT catalyst
was detected by XPS (Fig. 11a and Table 2). The above could be
the main reasons for deactivation of the Pd catalysts supported
on NT material [26,42,43]. XRD characterization of the used Pd
(5.0)/NT catalyst showed that some Ti3O5 was formed in the NT
support after three catalytic cycles (Fig. 9A). The Pd(5.0) catalyst
supported on the NTc material with lower water content (4.8 wt.
%) showed almost stable catalytic activity in five catalytic cycles
(Table 4). Only a slight decrease in the product yield was observed
in the third and fourth recycles of the same catalyst, which can be
ascribed to a loss of catalyst during filtration. No formation of any
Pd black or crystalline Ti3O5 was found in this case (Fig. 9B). We
attribute this behavior to lower water content in the calcined nan-
otubular support. It is known that nanotubular titanate materials
contain a significant amount of water with different characteris-
tics: physisorbed and chemisorbed (interlayer and structural)
water [33]. Calcination of NT material at 350 °C for 2 h results in
the elimination of most of the physisorbed and part of the chemi-
sorbed (interlayer) water, whereas structural water remains in the
nanotubes. Our results show that an excess of water in the nan-
otubular titanate support has a negative effect on the catalytic
activity of recycled catalysts; meanwhile, the presence of a small
amount of structural water is necessary for the stabilization
of Pd catalysts. In order to prove the above supposition, a
reference Pd(5.0)/TiO2 catalyst was prepared using commer-
cial titania nanopowder as a support. This catalyst did not
have any water content. Activity evaluation in the reaction of
4-bromobenzaldehyde with styrene showed high product 3 yield
(71% under the same reaction conditions as in Table 4). However,
the catalyst separated after the reaction became completely black,
so its recycling was not possible.
After the catalytic activity tests, supported Pd catalysts were
separated from the reaction mixture by filtration. This easy separa-
tion offers the possibility of reutilization of the catalysts in several
catalytic cycles and quantitative recovery of palladium. It was
observed that Pd(x)/NT catalysts with high Pd loadings (x = 7.5
and 10) changed their color to black due to the precipitation of cat-
alytically inactive Pd black, whereas Pd(2.5)/NT, Pd(5.0)/NT, and Pd
(5.0)/NTc did not change their color (Fig. 8).
Formation of large (15–35 nm) metallic Pd particles in the used
Pd(10)/NT catalyst was also confirmed by powder XRD (Fig. 9A)
and TEM (Fig. 10). In addition, in the Pd(10)/NT (1C) sample, a
small signal at about 18° (2h) was observed, evidencing the forma-
tion of black titanium oxide with the formula Ti3O5, which indi-
cates that the NT support also suffered some changes during the
reaction. The XPS characterization of the Pd(10)/NT catalyst after
one reaction cycle showed only the presence of Pd(0) (Fig. 11
and Table 2).
Since only the catalysts with low palladium loadings main-
tained their light brown color after the reaction, we tested the pos-
sibility of recycling and reusing two of them: Pd(5.0)/NT and Pd
Fig. 8. Catalysts after one catalytic cycle: (a) Pd(2.5)/NT, (b) Pd(5.0)/NT, (c) Pd(7.5)/
NT, and (d) Pd(10)/NT. Used catalysts supported on calcined NTc material: (e) Pd
(5.0)/NTc after five catalytic cycles and (f) Pd(10)/NTc after three catalytic cycles.
Catalysts with low palladium loading (x = 2.5 and 5.0) maintained their color after
the Heck reaction, whereas catalysts with high palladium loading turned black or
gray.
NT support
Pd(2.5)/NT (1C)
Pd(5.0)/NT (3C)
Pd(10)/NT (1C)
NTcsupport
Pd(5.0)/NTc
Pd(5.0)/NTc(1C)
Pd(5.0)/NTc(5C)
B
A
(110)
(020)
*
+
*
*
0
0
20
40
60
80
20
40
60
80
2θ(degree)
2θ (degree)
Fig. 9. (A) XRD patterns of Pd(x)/NT catalysts after one (1C) or three (3C) catalytic cycles. ⁄ Palladium, syn. (ICDD card 00-005-0681); + titanium oxide Ti3O5 (ICDD card 04-
007-6635). (B) XRD patterns of fresh Pd(5.0)/NTc catalyst and after one (1C) or five (5C) catalytic cycles. (110) and (020) reflections of the hydrogen trititanate phase H2Ti3O7
(JCPDS-ICDD card 47-0561) of the NTc support are indicated.

M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
147
Fig. 10. TEM images of the Pd(10)/NT catalyst after one catalytic cycle. Dark particles of Pd black can be observed.
30
20
10
0
Table 4
Re-use of Pd catalysts supported on NT and NTc materials in Heck coupling of 4-
(a)
335
bromobenzaldehyde with styrene.a
336.9
Catalyst
Product 3 yield (%)
340.7
342.6
Fresh catalyst 1st reuse 2nd reuse 3rd reuse 4th reuse
Pd(5.0)/NT
Pd(5.0)/NTc 64
Pd(10)/NTc 100
68
26
62
65
13
64
21
–
53
–
–
56
–
a
Reaction conditions: nitrogen atmosphere, temperature 150 °C, 4-bromoben-
zaldehyde (13.5 mmol), styrene (13.5 mmol), triethylamine (72 mmol), dimethyl-
formamide (25 mL), and tri-(o-tolyl)phosphine (TOP, 1.64 mmol), 0.05 g of the
catalyst, 24 h reaction time.
344
342
340
338
336
334
332
Binding energy (eV)
nation of the NT support delays the process of agglomeration of Pd
species and formation of Pd black. Finally, from the above experi-
ments, it can be concluded that low Pd loading (up to 5 wt.% of
Pd(OAc)2) and the presence of a small amount of structural water
(about 5 wt.%) in the nanotubular support are the factors that pro-
vide stability to the supported Pd catalysts, opening up the possi-
bility of their recycling and reutilization.
80
60
40
20
0
335.4
(b)
340.9
4. Discussion
Nowadays, the development of active and stable heterogeneous
Pd catalysts for the Heck reaction is an actual task. These catalysts
have the advantage of easy separation from the reaction mixture
and therefore can be reused if they are not deactivated. Many
attempts have been made recently to find an appropriate catalytic
system. In many cases, highly active catalysts were obtained; how-
ever, their stability was not as good as desired. For example, in
work [43], Pd catalysts supported on activated carbon (Pd/C) were
used for Heck coupling of bromobenzene with styrene. The opti-
mization of the catalyst and reaction conditions allowed Pd/C
catalysts with very high activity in the above reaction (TON ꢁ 18,000).
However, the dramatic decrease in Pd dispersion during the reac-
tion resulted in a significant decrease in catalytic activity. Thus,
the conversion decreased from 92% (new catalyst) to 63% in the
first reuse and to 4% in the second reuse. Characterization of the
Pd/C catalyst before and after the reaction by TEM showed agglom-
eration of the Pd species after the reaction. In work [9], Heck olefi-
nation of aryl halides over Pd-TMS11 catalysts prepared by
palladium grafting on the Nb-MCM-41 material was studied. The
Pd-TMS11 catalyst recovered after three catalytic cycles in the
reaction between n-butyl acrylate and 1-bromo-4-nitrobenzene,
346
344
342
340
338
336
334
332
330
Binding Energy (eV)
Fig. 11. XPS spectra of (a) Pd(5.0)/NT catalyst after three catalytic cycles (3C) and
(b) Pd(10)/NT catalyst after one catalytic cycle (1C).
To inquire into the stability of the high-loading Pd catalysts
supported on calcined NTc material, the Pd(10)/NTc sample was
prepared and tested in recycling studies. This catalyst showed high
activity in the first catalytic test (100% product 3 yield at 24 h,
Table 3). However, its activity significantly decreased in the next
cycles (Table 4). In addition, the color of this catalyst changed to
gray after three catalytic cycles (Fig. 8f). Nevertheless, a compar-
ison of the deactivation processes of Pd(10)/NT and Pd(10)/NTc
samples shows that deactivation of the Pd(10) catalyst supported
on calcined NTc material was much slower than that of the
NT-supported analog, which deactivated completely after the first
catalytic test (see Fig. 8d). This result once again shows that calci-

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M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
with a total TON of 3000, showed in the XRD pattern some agglom-
eration of palladium, evidenced by two peaks of palladium
between 2h = 40° and 50°. In work [26], Pd catalysts supported
on titanate nanotubes were used for double-bond migration in
allylbenzene. In all reactions, the orange-brown catalysts became
completely black within the first 5 min. From the above examples,
it is clear that agglomeration of palladium species and formation of
Pd black are the main reasons for the deactivation of heteroge-
neous palladium catalysts. Similarly, formation of Pd black is a
well-known phenomenon for homogeneously catalyzed Heck reac-
tions [42]. Different approaches were used in homogeneous and
heterogeneous catalysis to stabilize Pd species against agglomera-
tion. In the present work, we tried to reach this aim by using strong
metal–support interaction between Pd species and the nanotubu-
lar hydrogen titanate support.
Following this idea, we selected the adsorption of Pd(II) acetate
on the support surface as the catalyst’s preparation method. This
would ensure the presence in the catalysts of only Pd species in
interaction with the titanate support. A variety of Pd(x)/NT cata-
lysts with different Pd loadings supported on NT material were
prepared. Catalytic activity tests and recycling experiments
showed that catalysts with low Pd loading (x = 2.5 and 5.0) were
more active (had higher TON values) and more stable than the
high-Pd-loading ones. This could be ascribed to good dispersion
and homogeneous distribution of Pd species in them. Further elim-
ination of physisorbed and interlayer water from titanate nan-
otubes by calcination resulted in the Pd(5.0)/NTc catalyst, which
showed almost constant catalytic activity in five catalytic cycles.
Therefore, we think that the proposed approach to stabilization
of Pd catalysts by metal–support interaction led to good results.
Strong metal–support interaction in the catalysts with low Pd
loadings (up to 5 wt.% Pd(II) acetate) was confirmed by the fact
that the starting Pd(OAc)2 precursor was decomposed after adsorp-
tion into Pd(II) species as PdO and Pd(0), whereas the acetate group
was coordinated to the Ti atoms of the support. In the high-loading
fresh Pd(10)/NT catalyst, in addition to Pd(II)O and Pd(0), some Pd
(II) species, as in Pd(OAc)2, were detected with an increase in the
proportion of reduced Pd(0). This catalyst showed a low TON value,
as well as low stability. This result is in line with previous observa-
tions [43], where high activity in the Heck coupling of bromoben-
zene with styrene was shown by catalysts with a low degree of Pd
reduction. According to our results, an increase in the proportion of
Pd(0) also has a negative effect on catalytic activity and stability,
since it promotes the formation of Pd black and increases the rate
of deactivation.
NaY zeolite [46]. In order to confirm that similar Pd leaching
could have taken place in the case of our catalysts supported
on nanotubular titanate in the Heck reaction between
4-bromobenzaldehyde and styrene at 150 °C, an additional exper-
iment was performed with the Pd(10)/NT catalyst. In this experi-
ment, samples of the reaction solution and the catalyst were
withdrawn at 30 min reaction time (after reaching a temperature
of 150 °C) and analyzed. The catalyst’s analysis by SEM-EDX
showed about a 30–37% decrease in the palladium loading. On
the other hand, chemical analysis of the filtered and evaporated
sample of the reaction solution (3 mL) by atomic absorption spec-
troscopy (AAS) showed the presence of palladium in the residue.
The amount of Pd in the reaction solution was estimated to be
about 20–22 ppm. The above results confirmed the possibility of
leaching of part of the Pd contained in the Pd(10)/NT catalyst into
the solution during the reaction. However, it was not clear if this
leaching was irreversible or the leached Pd species were read-
sorbed back onto the NT support’s surface. To answer this question,
we determined the Pd content in the catalysts of the Pd(x)/NT ser-
ies shown in Fig. 8a–d after use in one catalytic cycle. In all cases,
the Pd loading in the used catalysts was similar to that in the fresh
samples. For example, the Pd(10)/NT catalyst separated after the
end of the reaction (24 h reaction time) showed a Pd content cor-
responding to 8.47 wt.% as Pd(OAc)2, which is very close to the Pd
loading in the fresh catalyst (8.84 wt.%, Table 1). The above result
indicates that all Pd species leached from the Pd(10)/NT catalyst
during the reaction were redeposited onto the NT surface when
the reaction finished. Therefore, the possibility of irreversible pal-
ladium leaching during the reaction could not be considered as a
possible cause of deactivation. In addition, the reaction product
3, separated after the end of the reaction, was tested for the pres-
ence of Pd, giving a negative result. The above observations are
well in line with previously mentioned reports [43,46] and point
out the possibility of Pd leaching into the reaction solution during
the reaction and its redeposition onto the titanate support.
Regarding the product inhibition as another possible cause for
the deactivation of the Pd(x)/NT catalysts, we tested for the pres-
ence of the product 3 in the used catalysts, shown in Figs. 8a–d.
Since product 3 has a large conjugated system of
p bonds, it is a
highly fluorescent compound upon UV irradiation. No fluorescence
was detected in the used Pd(x)/NT catalysts when they were
exposed to UV light (333 nm). From the above, it seems that the
main reason for the deactivation of the studied Pd catalysts sup-
ported on hydrogen titanate nanotubes was agglomeration of Pd
species and formation of Pd black.
Regarding the mechanism of the homogeneous Heck reaction
[44], it is generally considered that Pd(0)Ln is a catalytically active
entity and the reaction starts by oxidative addition of aryl halide
(ArX) to Pd(0), which changes its oxidation state to 2+. When aryl
bromides are used, their oxidative addition to Pd(0) is slow and
this first step of the catalytic cycle becomes the rate-determining
step. As a consequence, most of the palladium is present in the
form of Pd(0), and if its concentration increases, agglomeration of
Pd(0) occurs faster, leading to the formation of palladium black
and the Heck reaction stops. Recently, it was proposed that when
heterogeneous Pd catalysts are used in the Heck reactions per-
formed at high temperature (120–160 °C) [42,43,46], leaching of
Pd species into the solution occurs during the reaction, and that
these dissolved Pd species function as a highly active homoge-
neous catalyst in the Heck reaction. However, after the reaction
was finished, less than 1 ppm of palladium was found in the solu-
tion, indicating that active palladium species reprecipitated onto
the support’s surface. Therefore, it was proposed that the solid cat-
alyst functions as a source of molecular palladium species in solu-
tion. Previously, the above behavior was observed for Pd catalysts
supported on different materials: carbon [43], titania, alumina, and
The explanation for the quick deactivation of the Pd(x)/NT cat-
alysts with high Pd loadings should be related to the amount of Pd
leached from the solid catalyst into the solution. This amount
should be larger at high Pd loading in the Pd(x)/NT catalysts
(x = 7.5 and 10), as well as due to the presence in these catalysts
of some reversibly adsorbed Pd(OAc)2-like species (part II of the
Pd acetate adsorption isotherm, Fig. 2) in weak interaction with
the support. An increase in the concentration of Pd species leached
into solution would accelerate the agglomeration of the Pd species
and the formation of Pd black. On the other hand, the stability of
the low-Pd-loading Pd(x)/NT catalysts could be due to the strong
interaction of the adsorbed Pd with the NT support and the low
proportion of Pd(0), both of which diminish the amount of leached
Pd(0) in the solution, decreasing Pd agglomeration and deactiva-
tion of the catalyst. In addition, the nanotubular titanate support
can also facilitate back precipitation of the reduced Pd(0) from
the solution and promote reoxidation of Pd(0) to Pd(II), which
was evidenced by the formation of the reduced Ti3O5 oxide
(Fig. 9A). Therefore, we consider that the participation of the nan-
otubular support in the stabilization of Pd2+ species on its surface
can be important for preventing the formation of Pd black and cat-

M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
149
lyst’s stability and deactivation through the formation of Pd black
was proposed. It considers both the catalyst’s Pd loading and the
possibility of leaching of the Pd species into the reaction solution
and their readsorption back, where the NT support helps to main-
tain a correct balance of different oxidation states of Pd, preventing
the formation of Pd black.
Acknowledgments
Financial support from DGAPA-UNAM, Mexico (Project PAPIIT
IN-113715) and CONACYT, Mexico (Project CB 220175), is grate-
fully acknowledged. The authors thank C. Salcedo Luna and I.
Puente Lee for technical assistance with powder XRD and electron
microscopy (SEM-EDX and HRTEM) characterizations, respectively.
We are also grateful to Luis Escobar-Alarcón for help with FT-
Raman characterization of the catalysts.
Scheme 2. Proposed mechanism for the action of the stable Pd(5.0)/NTc catalyst.
Palladium species participate in two cycles: (1) leaching of Pd2+ from the NTc
support into the solution and readsorption and (2) the Heck reaction catalytic cycle
taking place in the solution.
alyst deactivation. Finally, the amount of water in the nanotubular
hydrogen titanate is another important factor in the stability of the
supported Pd catalysts, the presence of a large amount of physi-
sorbed and interlayer water being undesired. Possibly, the pres-
ence of an excessive amount of water in the catalyst promotes
the reduction of Pd(II) species to Pd(0) which can be further trans-
formed into Pd black.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2016.07.019.
References
Results obtained in the present work show that both support
properties (water content) and Pd loading are important for the
stability of Pd catalysts supported on hydrogen titanate nanotubes.
In addition, the possibility of Pd leaching and readsorption during
the Heck reaction was proved. From the above results and consid-
ering that the main reason of the Pd catalysts’ deactivation is the
formation of Pd black, a possible reaction mechanism can be pro-
posed (Scheme 2). In this mechanism, the Heck reaction catalytic
cycle took place in the solution with the participation of Pd leached
from the solid catalyst. After leaching, dissolved Pd species can be
readsorbed again on the nanotubular support, and it seems that the
role of the support is to maintain a correct balance between Pd(0)
and Pd(II) species, to stabilize the catalyst and prevent an increase
in the proportion of reduced Pd(0) species, leading to the formation
of Pd black. To keep the catalyst stable, both cycles should work
together to maintain a small concentration of Pd species in the
reaction solution, thus preventing their agglomeration and the pre-
cipitation of Pd black. In addition, the higher catalytic activity of
the Pd catalysts supported on titanate nanotubes than of pure Pd
(II) acetate under homogeneous catalysis conditions indicates that
Pd species leached from the catalysts supported on titanate nan-
otubes are more active than those obtained from Pd(OAc)2 or that
Pd species deposited on the support’s surface (nonleached) can
also be highly active for catalyzing the Heck reaction. However,
these assumptions need further experimental confirmation and a
more detailed reaction mechanism and kinetic study.
[1] J. Tsuji, Palladium Reagent and Catalyst – lnnovations in Organic Synthesis,
Wiley, Chichester, UK, 1995.
[2] R.F. Heck, Palladium Reagents in Organic Synthesis, Academic Press, London,
1985.
[3] J. Tsuji, in: B.M. Trostand, I. Fleming (Eds.), Comprehensive Organic Synthesis,
vol. 7, Pergamon Press, Oxford, 1991.
[4] A. de Meijere, F. Meier, Angew. Chem. 106 (1994) 2473.
[5] N. Arul Dhas, H. Cohen, A. Gedanken, J. Phys. Chem. B 101 (1997) 6834.
[6] M. Andersson, E. Bakchinova, K. Jansson, M. Nygren, J. Mater. Chem. 9 (1999)
265.
[7] A.A. Mastalir, L. Turi, Z. Kiraly, I. Dekany, M. Bartok, Mol. Cryst. Liq. Cryst. Sci.
Technol. Sect. A 311 (1998) 333.
[8] D. Guillemot, M. Polisset-Thfoin, D. Bonnin, D. Bazin, J. Fraissard, J. Phys. IV 7
(1997) 931.
[9] C.P. Mehnert, D.W. Weaver, J.Y. Ying, J. Am. Chem. Soc. 120 (1998) 12289.
[10] G.M. Neelgund, A. Oki, Appl. Catal. A 399 (2011) 154.
[11] M. Zhao, R.M. Crooks, Angew. Chem. Int. Ed. 38 (1999) 364.
[12] A.B.R. Mayer, J.E. Mark, R.E. Morris, Polym. J. 30 (1998) 197.
[13] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998)
3160.
[14] X. Sun, Y. Li, Chem. Eur. J. 9 (2003) 2229.
[15] Z.Y. Yuan, B.L. Su, Colloids Surf. A Physicochem. Eng. Aspects 241 (2004) 173.
[16] C.C. Tsai, H. Teng, Chem. Mater. 16 (2004) 4352.
[17] C.H. Lin, S.H. Chien, J.H. Chao, C.Y. Sheu, Y.C. Cheng, Y.J. Huang, C.H. Tsai, Catal.
Lett. 80 (2002) 153.
[18] V. Idakiev, Z.-Y. Yuan, T. Tabakova, B.-L. Su, Appl. Catal. A 281 (2005) 149.
[19] C.H. Lin, C.H. Lee, J.H. Chao, C.Y. Kuo, Y.C. Cheng, W.N. Huang, H.W. Chang, Y.M.
Huang, M.K. Shih, Catal. Lett. 98 (2004) 61.
[20] J.C. Xua, M. Lua, X.Y. Guob, H.L. Lia, J. Mol. Catal. A 226 (2005) 123.
[21] J.J. Yang, Z.S. Jin, X.D. Wang, W. Li, J.W. Zhang, S.L. Zhang, X.Y. Guo, Z.J. Zhang,
Dalton Trans. 20 (2003) 3898.
[22] D.V. Bavykin, V.N. Parmon, A.A. Lapkin, F.C. Walsh, J. Mater. Chem. 14 (2004)
3370.
[23] D.V. Bavykin, A.A. Lapkin, P.K. Plucinski, L. Torrente-Murciano, J.M. Friedrich, F.
C. Walsh, Top. Catal. 39 (2006) 151.
[24] D.V. Bavykin, S.N. Gordeev, A.V. Moskalenko, A.A. Lapkin, F.C. Walsh, J. Phys.
Chem. B 109 (2005) 8565.
5. Conclusions
[25] J. Hong, J. Cao, J. Sun, H. Li, H. Chen, M. Wang, Chem. Phys. Lett. 380 (2003) 366.
[26] L. Torrente-Murciano, A.A. Lapkin, D.V. Bavykin, F.C. Walsh, K. Wilson, J. Catal.
245 (2007) 272.
[27] S.K. Mohapatra, N. Kondamudi, S. Banerjee, M. Misra, Langmuir 24 (2008)
11276.
[28] R. Camposeco, S. Castillo, I. Mejía-Centeno, J. Navarrete, J. Marín, Mater. Char.
95 (2014) 201.
[29] C.-H. Han, D.-W. Hong, I.-J. Kim, J. Gwak, S.-D. Han, K.C. Singh, Sens. Actuat. B
128 (2007) 320.
[30] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11 (1999)
1307.
[31] Q. Chen, G.H. Du, S. Zhang, L.-M. Peng, Acta Crystallogr. B 58 (2002) 587.
[32] R.A. Ortega-Domínguez, J.A. Mendoza-Nieto, P. Hernández-Hipólito, F.
Garrido-Sánchez, J. Escobar-Aguilar, S.A.I. Barri, D. Chadwick, T.E. Klimova, J.
Catal. 329 (2015) 457.
In the present work, Pd catalysts supported on hydrogen tita-
nate nanotubes were synthesized and characterized. It was found
that the deposited Pd species were well-dispersed and homoge-
neously distributed in all catalysts. The oxidation state of the sup-
ported Pd species depended on the Pd loading. In general, three
types of Pd were found in the catalysts, namely, Pd(II) in PdO
and Pd(0) at low Pd loadings (up to 5 wt.% of Pd(II) acetate) and
some Pd(II) in the Pd(OAc)2-like species at a higher Pd loading.
The effect of the water content in the titania nanotubes was also
tested. The Pd(5.0)/NTc catalyst containing about 5 wt.% of water
was found to be surprisingly stable, maintaining almost constant
activity in the Heck reaction of 4-bromobenzaldehyde and styrene
after five catalytic cycles. The possible explanation for the cata-
[33] E. Morgado Jr., M.A.S. de Abreu, G.T. Moure, B.A. Marinkovic, P.M. Jardim, A.S.
Araujo, Chem. Mater. 19 (2007) 665.

150
M.E. Martínez-Klimov et al. / Journal of Catalysis 342 (2016) 138–150
[34] G. Leofanti, M. Padovan, G. Tozzola, B. Venturelly, Catal. Today 41 (1998) 207.
[35] C.K. Lee, K.-S. Lin, C.-F. Wu, M.-D. Lyu, C.-C. Lo, Hazard. Mater. 150 (2008) 494.
[36] S. Mozia, E. Borowiak-Palén, J. Przepiórski, B. Grzmil, T. Tsumura, M. Toyoda, J.
Grzechulska-Damszel, A.W. Morawski, J. Phys. Chem. Solids 71 (2010) 263.
[37] A.J. Maira, J.M. Coronado, V. Augugliaro, K.L. Yeung, J.C. Conesa, J. Soria, J. Catal.
202 (2001) 413.
[38] D.D. Kragten, R.A. van Santen, M.K. Crawford, W.D. Provine, J.J. Lerou, Inorg.
Chem. 38 (1999) 331.
[39] K. Ito, H.J. Bernstein, Canad. J. Chem. 34 (1956) 170.
[40] S. Doeuff, Y. Dromzee, F. Taulelle, C. Sanchez, Inorg. Chem. 28 (1989) 4439.
´
ˇ ´
´
[41] A. Gajovic, I. Frišcic, M. Plodinec, D. Ivekovic, J. Mol. Struct. 924–926 (2009)
183.
[42] J.G. de Vries, Dalton Trans. (2006) 421.
[43] K. Köhler, R.G. Heidenreich, J.G.E. Krauter, J. Pietsch, Chem. Eur. J. 8 (2002) 622.
[44] H.A. Dieck, R.F. Heck, J. Am. Chem. Soc. 96 (1974) 1133.
[45] J.H. Ding, D.L. Gin, Chem. Mater. 12 (2000) 22.
[46] S.S. Pröckl, W. Kleist, M.A. Gruber, K. Köhler, Angew. Chem. Int. Ed. 43 (2004)
1881.