The effect of crystallinity on selectivity of palladium catalysts in hydrogenation of 2-ethyl-9,10-anthraquinone

Article abstract of DOI:10.1016/j.catcom.2020.106124

The catalytic properties of palladium crystallites obtained from various precursors were studied in the hydrogenation of 2-ethyl-9,10-anthraquinone. The highest yield of H₂O₂ (88percent) is achieved on unmodified Pd crystallites obtained from Pd(dba)₂ at 50°C. A sigmoidal relationship between the coherent domain size (CDS) of Pd particles and the content of anthrones, which are by-products of the eAQ transformation, is established. Small Pd clusters, along with the reduction of eAQ carbonyl groups, accelerate predominantly the hydrogenation of the eAQH₂ aromatic rings. The smallest contribution of the side processes is observed for a palladium catalyst with a CDS of 3–5 nm.

Full text of DOI:10.1016/j.catcom.2020.106124

CatalysisCommunications146(2020)106124
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
The effect of crystallinity on selectivity of palladium catalysts in
hydrogenation of 2-ethyl-9,10-anthraquinone
Lyudmila B. Belykh⁎, Nikita I. Skripov, Tatyana P. Sterenchuk, Fedor K. Schmidt
Department of Chemistry, Irkutsk State University, K. Marx, 1, 664003 Irkutsk, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
The catalytic properties of palladium crystallites obtained from various precursors were studied in the hydro-
genation of 2-ethyl-9,10-anthraquinone. The highest yield of H2O2 (88%) is achieved on unmodified Pd crys-
tallites obtained from Pd(dba)2 at 50°C. A sigmoidal relationship between the coherent domain size (CDS) of Pd
particles and the content of anthrones, which are by-products of the eAQ transformation, is established. Small Pd
clusters, along with the reduction of eAQ carbonyl groups, accelerate predominantly the hydrogenation of the
eAQH2 aromatic rings. The smallest contribution of the side processes is observed for a palladium catalyst with a
CDS of 3–5 nm.
Hydrogenation
Palladium
2-ethyl-9,10-anthraquinone
Hydrogen peroxide
1. Introduction
The main problem is that the hydrogenation of eAQ relates to
complex series–parallel reactions, in which the target product, 2-ethyl-
Hydrogen peroxide is an important chemical feedstock and reagent
and is widely employed as an oxidiser, bleacher, disinfectant, poly-
merisation initiator and cross-linking agent. The observed steady in-
crease in the global production of hydrogen peroxide, which reached
5.5 million tonnes/year in 2015 (calculated using 100% H2O2), is due
to the replacement of chlorine and other chlorinated bleaching agents
in paper production, as well as to substitution of chlorohydrin and
chlorine with Н2О2 in the industrial production of propylene oxide [1].
Currently, more than 99% of H2O2 is produced by the anthraquinone
method [2]. Despite the industrial application of the anthraquinone
method, the development of highly selective catalysts for hydrogena-
tion of 2-ethyl-9,10-anthraquinone to 2-ethyl-9,10-anthrahydroquinone
to improve profitability of H2O2 production remains an urgent research
challenge:
9,10-anthrahydroquinone, is further catalytically converted via two
main directions: hydrogenation of the aromatic rings and hydro-
genolysis of the C-OH bonds [3]. Among various products of eAQ
conversion, only 2-ethyl-5,6,7,8-tetrahydro-9,10-anthrahydroquinone
(Н4eAQН2) belongs to the “active quinones”, since it, like eAQH2, is
capable of oxidising with oxygen to H2O2 [4].
The known methods for improvement of palladium catalysts se-
lectivity involve the application of promoters and modifying agents
[5–8] or regulation of the texture or acid-base properties of the carrier
[9–11]. Meanwhile, the effects of the size, shape, and structural or-
dering of the active component of the catalyst on its catalytic properties
in the hydrogenation of eAQ still remain obscure. For example, ac-
cording to [12], the size effect in hydrogenation of 2-ethyl-9,10-an-
⁎ Corresponding author.
E-mail addresses: belykh@chem.isu.ru (L.B. Belykh), fkschmidt@chem.isu.ru (F.K. Schmidt).
https://doi.org/10.1016/j.catcom.2020.106124
Received 3 June 2020; Received in revised form 17 July 2020; Accepted 22 July 2020
Availableonline30July2020
1566-7367/©2020ElsevierB.V.Allrightsreserved.

L.B. Belykh, et al.
CatalysisCommunications146(2020)106124
thraquinone is associated only with the particle morphology. At the
same time, researchers reported [13,14] that the size of palladium
crystallites affects the yield of H2O2. Large Pd crystallites, especially
large agglomerates of colloidal palladium particles (540–600 nm), are
more active in hydrogenation of the aromatic ring of eAQH2 to
H4eAQH2, as well as in other undesirable reactions to afford decom-
position products. The palladium particles of 60–240 nm in size ensure
the highest yield of Н2О2 (80%), and, at the same time, they result in
the lowest conversion of eAQH2 to H4eAQH2 [13]. Preliminary ex-
periments revealed that the particle size can affect the type of side re-
action accelerated by palladium [15]. Star-like palladium crystallites of
filtrate and oxidised with atmospheric oxygen for 15–20 min. Then,
water (30 mL) was added, and upon stirring, H2O2 was extracted from
the organic layer (5 mL) into water (30 mL). The amount of H2O2 was
determined by titration of an aliquot (10 mL) of a pre-acidified aqueous
layer of KMnO4 (0.1 N), and the concentration of KMnO4 was mon-
itored using a primary standard of oxalic acid.
The hydrogenation of eAQ using other palladium precursors, (Pd
(OAc)2 and Pd(dba)2), was carried out similarly. The time of Pd-black
formation in hydrogen in toluene/1-octanol medium at 90°C for all Pd
precursors was 45 min. In addition, the formation of palladium particles
from Pd(OAc)2 and Pd(dba)2 was performed at 50°C for 5 min. No
experiments were carried out at 50°C with Pd(acac)2 as a precursor.
Due to the chelate binding of acac-ligands, Pd(acac)2 was not rapidly
reduced at temperatures below 70°C.
127
12 nm in size catalyse mainly the side hydrogenolysis of the C-
OH bond of the target 2-ethyl-9,10-anthrahydroquinone. Ziegler-type
systems Pd(acac)2 – AlEt3 are more active in the saturation of the
eAQH2 aromatic ring.
The composition of the conversion products of 2-ethyl-9,10-an-
thraquinone was analysed by GLC on a Chromatec-Crystal 5000.2
chromatograph (30 m long capillary column; phase - poly(5% di-
phenyl/95% dimethylpolysiloxane) - (BPX-5); flame ionisation detector
(DIP); temperature programming conditions: 160°C (3 min); 270°C
(20 min), heating rate 40°/min. In parallel, the structure of inter-
mediates and reaction products was determined on a GCMS-QP2010
Ultra Shimadzu gas chromatography-mass spectrometer (capillary
column GsBP·5MS, length 30 m, phase: poly(5% diphenyl/95% di-
methylpolysilphenylenesiloxane)).
The available literature data on the influence of particle size of the
active component and its structural ordering do not allow for clear
determination of the regularities, which can affect the selectivity of
eAQ hydrogenation. The establishment of these regularities is im-
portant not only for eAQ hydrogenation, but also for other hydro-
genation processes, which are accompanied by side hydrogenation of
the aromatic ring and/or hydrogenolysis of the C-OH bond (the
synthesis of drugs (vitamins A, B, E, K), biologically active additives,
and intermediates for the perfume industry [16,17]). In the present
paper, on the example of unmodified palladium particles, we de-
termined that CDS of Pd particles makes a major contribution to the
side processes of 2-ethyl-9,10-anthraquinone (eAQ) hydrogenation and
the yield of hydrogen peroxide.
2.2. Characterization techniques
In a quartz cuvette with an absorption layer of 0.1 cm, UV spectra
were recorded on an SF-2000 spectrometer (OKB-Spektr, Russia).
Next, TEM images were recorded on a Tecnai G2 (FEI, USA) electron
microscope with an accelerating voltage of 200 kV. A drop of the ob-
tained Pd-black suspension was applied onto a carbonized copper grid
(200 mesh) and dried at room temperature in a box under an inert
atmosphere. The images were recorded using a CCD camera (Soft
Imaging System, Germany). Then, the device was equipped with an
energy dispersive X-ray spectrometer (EDX, Phoenix) with a semi-
conductor Si(Li) detector. The parameters of particle in the images were
measured using the iTEM 5.0 and DigitalMicrographs 1.94.1613 soft-
ware. To analyze periodic structures and filtrate the images, the Fourier
methods, FFT (Fast Fourier Transformation) and IFFT (Inverse Fast
Fourier Transformation), were used. The average size was found by
treatment of a section containing at least 50–100 palladium particles.
Phase composition of the catalysts was determined with X-ray
powder diffraction analysis on XRD-7000 S (Shimadzu Co., Japan)
(CuKα radiation, Ni filter, λ = 1.5418 Å). The average size of the Pd
catalyst particles was calculated using the Scherrer formula A.1 (SI).
2. Experimental
2.1. General procedures
Solvents (toluene, 1-octanol) were purified by standard methods
[18]. Pd(acac)2 was synthesized by the procedure [19], and palladium
bis(dibenzylideneacetone) (Pd(dba)2) was obtained via the protocol
[20]. Additionally, Pd(OAc)2 (Merck, purity > 99.9%) was used
without additional purification.
Hydrogenation of 2-ethyl-9,10-anthraquinone was carried out in a
thermostated ‘duck’ type glass vessel at 50°C and an initial hydrogen
pressure of 2 atm in the presence of palladium catalysts, formed in situ.
The quantitative control of the conversion of the starting palladium
complexes (Pd(acac)2, Pd(dba)2) to palladium crystallites in a hydrogen
atmosphere was carried out by UV–Vis spectroscopy (Figs. A.1-A.2, SI).
The Pd(acac)2 concentration was calculated relative to the 330 nm
absorption band (ε330 = 10,630 L·cm−1·mol−1), while that of acet-
ylacetone (acacH) was computed relative to 290 nm (for acacH,
ε290
=
5000 L·cm−1·mol−1 and for Pd(acac)2, ε290
=
3. Results and discussion
3090 L·cm−1·mol−1). Pd(dba)2: 525 nm (d
→
d* transition,
ε525 = 6400 L·cm−1·mol−1); uncoordinated dibenzylideneacetone:
325 nm (n → π* transition, ε325 = 33,540 L·cm−1·mol−1).
Colloidal solutions and suspension of palladium in neutral media
represent the most convenient models to evaluate the main factors af-
fecting selectivity of the palladium catalysts in hydrogenation of 2-
ethyl-9,10-anthraquinone to active quinones. In this case, no need ex-
ists to take into account the interaction between the active component
and the carrier as well as the effects of mass transfer and internal dif-
fusion that may arise when using heterogeneous supported catalysts.
As a precursor, we employed oxygen-containing palladium (II)
compounds (Pd(acac)2, Pd(OAc)2), which are readily soluble in organic
solvents, and a palladium(0) complex (Pd(dba)2) containing labile di-
benzylideneacetone ligands. The reduction of oxygen-containing pal-
ladium (II) compounds with hydrogen and the conversion of the Pd
(dba)2 complex to palladium nanoparticles was carried out in toluene/
1-octanol medium at 50 and/or 90°C without introduction of the sta-
bilizing ligands. The results of 2-ethyl-9,10-anthraquinone hydrogena-
tion are presented in Table 1.
Preparation of the catalysts in situ. A solution of Pd(acac)2
(0.0152 g, 5 × 10−5 mol) in a mixture of solvents (toluene/1-octanol in
a 7:10 ratio) was reduced with hydrogen at 90 °C under pressure of
2 atm. for 45 min until the quantitative conversion of Pd(acac)2. The
formed black-brown suspension was cooled to 50°C under the same
hydrogen pressure, and
a solution of 2-ethyl-9,10-anthraquinone
(2.116 mmol, 0.4993 g) in toluene (3 mL) was added to the reaction
mixture with a syringe. Next, the hydrogenation was carried out under
vigorous stirring (500 rpm) that excluded the reaction occurrence in the
diffusion region (Table A.1, Fig. A.3, SI). The monitoring of eAQ hy-
drogenation was carried out by the absorption of hydrogen using a
manometer connected to the reaction vessel. After absorption of 1.2 or
1.3 mol H2·(eAQ mol)−1, samples were taken for analysis. For this
purpose, an aliquot of the reaction mixture was filtered from the cat-
alyst under an argon atmosphere. An aliquot (5 mL) was taken from the
The suspension of palladium formed in hydrogen (Table, entry 1, 5)
2

L.B. Belykh, et al.
CatalysisCommunications146(2020)106124
Table 1
catalysts synthesized at different temperatures from three palladium
precursors allowed the following regularities to be established.
According to TEM data, in hydrogen, in the presence of toluene/1-
octanol medium, the Pd(dba)2 complex gives fused aggregates of na-
noparticles, owing to a release of labile dibenzylideneacetone ligands
(Fig. 1, Fig. A.4, SI). With an increase in the temperature of catalyst
formation from 50 to 90 °C, the average size of the base particles
augments from 7 nm to 10–15 nm (Fig. 1, Fig. A.4, SI). Bright luminous
Effect of the precursor and temperature of palladium catalysts formation on the
yield of H2O2: СPd = 2.5 mmol L−1, νeAQ = 2.116 mmol, [eAQ]/[Pd] = 42.3,
the solvent – 1-octanol/toluene (10 ml: 10 ml), Т = 50°С, Р(Н2) = 2 atm.
№
Precursor
Ta, °С
Ab, min−1
Yield of Н2О2c, %
Productsd,%
Н4eAQ
Anthrones
1
2
3
4
5
Pd(acac)2
Pd(dba)2
Pd(dba)2
Pd(OAc)2
Pd(OAc)2
90
90
50
90
50
12.6
7.1
69.0
76.2
88.0
80.3
86.2
0.5
2.4
3.2
2.7
2.9
16.6
14.9
5.2
faces in dark-field images of sample
1 (precursor Pd(dba)2,
4.4
Tformation = 50°C) (Fig. A.4, SI) and sample 2 (precursor Pd(dba)2,
0.9
12.6
7.3
T
formation = 90 °C) (Fig. 1) indicate their crystalline nature.
13.0
The reduction of Pd(acac)2 with hydrogen at 90 °C delivers star-like
a
b
particles with a size of 127
12 nm (Fig. A.5, SI) [15]. The electron
Temperature of the catalyst formation.
diffraction images of all Pd samples display many concentrated rings,
indicating a large number of randomly oriented Pd crystallites (Fig. 1,
Figs. A.4–A.5, SI). Thus, all Pd samples obtained at 50 and 90°C from
different precursors are polycrystalline substances.
Activity is calculated from the initial hydrogen absorption rate
(mol min−1), referred to the amount of palladium precursor.
c
d
After the absorption of 1.2 mol Н2 (mol eAQ)−1
According to GC and GCMS.
.
The electron diffraction pattern of the catalyst formed from Pd
(acac)2 in hydrogen at 90°C also shows narrow Debye rings, evidencing
the polycrystalline nature of the sample (Fig. A.5, SI). However, the
diffraction pattern of the ray fragment of a single particle is a point
picture (Fig. 2). Consequently, the rays of star-like palladium particles
are close to the single crystals.
is almost comparable in activity with heterogeneous catalysts described
in the literature (for Pd/Al2O3, Pd/SiO2 and Pd/C, A = 12–16 min−1 at
T = 62 °C and PH2 = 1 atm [21].
The highest yield of H2O2 is observed for the catalyst obtained from
Pd(dba)2 at 50°C (Table, entry 3). When the temperature of the catalyst
formation increases from 50 to 90°C, the yield of H2O2 decreases from
88% to 76%, due to more pronounced contribution of the side hydro-
genolysis of the С-ОН bond of the target 2-ethyl-9,10-anthrahy-
droquinone. The share of anthrones increases almost threefold (Table,
entries 2–3). A high yield of anthrones is also ensured by a palladium
catalyst obtained from Pd(acac)2 at 90°C (Table, entry 1). For all stu-
died catalytic systems, anthrones are formed in higher amounts than
the products of aromatic ring hydrogenation. These results differ from
the data observed for supported palladium catalysts, for which the
constant of hydrogenation rate of the eAQH2 aromatic ring is by 5.9–6.6
times higher than that of hydrogenolysis rate of the C-OH bonds [3].
As a result of the reduction of palladium(II) compounds (Pd(acac)2,
Pd(OAc)2) with hydrogen, weak acids are formed and present in the
reaction medium: acetylacetone (acacH) and acetic acid (CH3COOH).
Decomposition of the Pd(dba)2 complex leads to the appearance of free
dibenzylideneacetone in the reaction system. Therefore, it cannot be
ruled out that acetylacetone, acetic acid, or dibenzylideneacetone are
adsorbed on the surface of Pd(0) particles and affect their catalytic
properties in eAQ hydrogenation. The close yields of H2O2 and the
selectivity values of palladium catalysts obtained from Pd(OAc)2 and Pd
(dba)2 at the same temperature (50 °C) refute the hypothesis of the
modifying effect of acacH, CH3COOH, dba and their influence on the
course of side processes.
X-ray diffraction patterns of Pd-black are consistent with the data of
dark-field TEM and electron diffraction and confirm the crystalline
nature of the catalyst samples (Figs. A.6–A.10, SI).
Previously, we found that the formation of structurally disordered
PdeP solid solutions increases the yield of Н2О2 from 69% for Pd-black
to 94–96% [7]. The high selectivity of amorphous PdeB particles in the
hydrogenation of eAQ was also noted by the authors [22]. As a result of
the formation of structurally disordered PdeP solid solutions, the pal-
ladium crystal lattice is distorted. The introduction of phosphorus also
leads to the appearance of a partial positive charge on palladium due to
electron density transfer. [7]. Both factors (or one of them) change the
properties of palladium catalysts in the hydrogenation of eAQ. An in-
dicator of the structural ordering of crystallites is CDS (the size of the
defect-free part of the crystal), because even a degree of disorientation
between adjacent cells of 1–2° violates the coherence of the scattered
beam [23]. Unlike CDS, TEM images provide size of the particles that
may consist of smaller intergrown (fused) crystallites or have defects.
Therefore, to establish the reasons for the change in the selectivity of
palladium catalysts obtained from different precursors at 50 and 90 оС,
the relationship between the yield of by-products and the CDS of pal-
ladium crystallites was considered.
The studied samples differ in the coherent domain size. With in-
creasing temperature and formation time, the CDS of palladium crys-
tallites increases from 7 nm to 11.4 nm for (Pd(dba)2, from 8.1 to
The TEM, electron diffraction, and XRD analysis of palladium
Fig. 1. Bright-field (a) and dark-field (b) TEM
images of sample 2 obtained from Pd(dba)2 at 90°C
in toluene/1-octanol medium. Image a shows elec-
tron diffraction: the interplanar distances of 2.290,
1.996, 1.408 and 1.203 are close to those of planes
111 (2.2458), 200 (1.94511), 220 (1.37537) and
311 (1.17295 А) of Pd crystallites (Pd # 00-046-
1043).
3

L.B. Belykh, et al.
CatalysisCommunications146(2020)106124
Fig. 2. Diffraction patterns of star-like palladium particles obtained by reduction of Pd(acac)2 with hydrogen at 90°C in toluene/1-octanol medium.
4. Conclusions
20
18
16
14
12
10
8
20
18
16
14
12
10
8
In conclusion, the catalytic properties of palladium crystallites,
obtained from three precursors at various temperatures, in the hydro-
genation of 2-ethyl-9,10-anthraquinone were studied. The results show
that CDS of Pd particles makes a major contribution to the side
processes of 2-ethyl-9,10-anthraquinone (eAQ) hydrogenation.
Researchers found that the CDS increases, when temperature of the
palladium crystallites formation grows from 50 to 90°C. The sigmoidal
dependence between the CDS of Pd particles and the amounts of by-
products of eAQH2 hydrogenolysis (anthrones) is established. Unlike
large crystallites, small palladium clusters reduce the carbonyl groups
of eAQ as well as accelerate, predominantly, hydrogenation of the
eAQH2 aromatic rings. The smallest contribution of side processes is
observed for a palladium catalyst with CDS of 3–5 nm.
2
1
6
6
4
4
2
2
0
0
Declaration of Competing Interest
0
2
4
6
8 10 12 14 16 18 20
CDS, nm
The authors declare that they have no conflict of interest.
Fig. 3. Dependence of the yield of 2-ethyl-5,6,7,8-tetrahydro-9,10-anthrahy-
droquinone (1) and anthrones (2) on CDS of Pd crystallites.
Acknowledgements
This work was supported by the Government Assignment for
Scientific Research from the Ministry of Science and Higher Education
of the Russian Federation (project no. FZZE-2020-0022). Electronic
images of the samples were obtained in the Baikal center of nano-
technologies.
12.4 nm for Pd(OAc)2 (Figs. A.6-A.8, SI). The CDS in the Pd(acac)2 - n
AlEt3 systems (n = 4, 6) is 2.5 and 1.3 nm, respectively (Figs. A.9-A.10,
SI). Ziegler systems accelerated, mainly, a side reaction - hydrogenation
of aromatic rings [15].
A sigmoidal dependence was observed between the concentration of
anthrones and the CDS values of Pd crystallites in the CDS range from
1.3 to 20 nm (Fig. 3). Meanwhile, size of the Pd particles crystallinity
region inversely depends on concentration of saturation products of the
eAQH2 aromatic rings. The sigmoidal dependence between the size of
crystallinity region of palladium samples and their reactivity in the
hydrogenolysis of C-OH bonds is consistent with the structural sensi-
tivity of this reaction and its occurrence at the Pd multiplet centers
located on the faces [12]. Since low-percent supported palladium cat-
alysts, used in hydrogenation of eAQ, contain small palladium crystal-
lites, their reactions are most often accompanied not by hydrogenolysis,
but by saturation of the eAQH2 aromatic rings.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2020.106124.
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