Chemoselective heterogeneous iridium catalyzed hydrogenation of cinnamalaniline

Article abstract of DOI:10.1039/d0cy01886d

Selective hydrogenation of unsaturated imines over heterogeneous catalysts is an ecologically feasible and effective way to produce commercially valuable saturated imines and unsaturated amines under mild conditions, avoiding the utilization of toxic halides. The liquid-phase hydrogenation of a model imine, cinnamalaniline, over Ir, Ru, Pd and Au catalysts was studied in polar protic (methanol, 2-propanol), polar aprotic (methyltert-butyl ether) and non-polar aprotic (toluene) solvents at 40-80 °C under atmospheric hydrogen pressure. Different metal oxides (Al₂O₃, ZrO₂, SiO₂) and carbon composites based on carbon nitrides synthesized by pyrolysis of ethylenediamine or melamine modified by KOH, HNO₃, NH₄Cl or TEOS (including template KIT-6), porous carbon (samples prepared by pyrolysis of sucrose, including template KIT-6) and mesoporous carbon Sibunit were applied to study the effect of the support. Among the tested metals, iridium exhibited the most promising catalytic performance in terms of hydrogenation activity and chemoselectivity towards unsaturated amine. The use of Ir on carbon nitrides (C₃N₄-NH₄Cl, C_xN_y-KIT-6) and amphoteric metal oxides (ZrO₂, Al₂O₃) in nonpolar aprotic toluene solvent provided the best selectivity towards unsaturated amine by minimizing oligomerization. Computational studies indicate that the selective hydrogenation of the CN group on Ir results from a favorable cinnamalaniline adsorption geometry at high surface coverage. Comparable heterogeneously catalyzed highly chemoselective hydrogenation of unsaturated imine into unsaturated amine under atmospheric hydrogen pressure and low temperatures has not been reported previously.

Full text of DOI:10.1039/d0cy01886d

Catalysis
Science &
Technology
View Article Online
View Journal
PAPER
Chemoselective heterogeneous iridium catalyzed
hydrogenation of cinnamalaniline†
Cite this: DOI: 10.1039/d0cy01886d
a
c
*
Risto Savela,
Nataliya D. Shcherban, b Marko M. Melander,
d
e
Igor Bezverkhyy,
Irina L. Simakova,
Otto Långvik, a Ekaterina Kholkina,f
Tamara Schindler,a Annabelle Krauβ,a Karoliina Honkala,
c
Dmitry Yu. Murzin f and Reko Leino
a
Selective hydrogenation of unsaturated imines over heterogeneous catalysts is an ecologically feasible and
effective way to produce commercially valuable saturated imines and unsaturated amines under mild
conditions, avoiding the utilization of toxic halides. The liquid-phase hydrogenation of a model imine,
cinnamalaniline, over Ir, Ru, Pd and Au catalysts was studied in polar protic (methanol, 2-propanol), polar
aprotic (methyl tert-butyl ether) and non-polar aprotic (toluene) solvents at 40–80 °C under atmospheric
hydrogen pressure. Different metal oxides (Al2O3, ZrO2, SiO2) and carbon composites based on carbon
nitrides synthesized by pyrolysis of ethylenediamine or melamine modified by KOH, HNO3, NH4Cl or TEOS
(including template KIT-6), porous carbon (samples prepared by pyrolysis of sucrose, including template
KIT-6) and mesoporous carbon Sibunit were applied to study the effect of the support. Among the tested
metals, iridium exhibited the most promising catalytic performance in terms of hydrogenation activity and
chemoselectivity towards unsaturated amine. The use of Ir on carbon nitrides (C3N4–NH4Cl, CxNy-KIT-6)
and amphoteric metal oxides (ZrO2, Al2O3) in nonpolar aprotic toluene solvent provided the best selectivity
towards unsaturated amine by minimizing oligomerization. Computational studies indicate that the
selective hydrogenation of the CN group on Ir results from a favorable cinnamalaniline adsorption
geometry at high surface coverage. Comparable heterogeneously catalyzed highly chemoselective
hydrogenation of unsaturated imine into unsaturated amine under atmospheric hydrogen pressure and low
temperatures has not been reported previously.
Received 25th September 2020,
Accepted 1st December 2020
DOI: 10.1039/d0cy01886d
rsc.li/catalysis
halides in combination with a suitable amine. Disadvantages
of this generally efficient and simple-to-operate method
Introduction
While several classical and homogeneously catalyzed
synthetic methods exist to form carbon nitrogen bonds to
produce alkyl or aryl substituted amines, the selective
heterogeneously catalyzed methods for unsaturated amine
synthesis are scarce. One of the most utilized classical
methods is the nucleophilic substitution reaction using alkyl
include the use of toxic and/or expensive organic halides and
the discharge of corroding organic salt waste.1 A number of
undesired side reactions may also take place, resulting in the
formation of higher amines. Alternatives to nucleophilic
substitution
include
e.g.
reductive
amination,2
hydroamination3 and alcohol amination via hydrogen
borrowing.4 Mechanistically, the final step in the reductive
amination and alcohol amination via hydrogen borrowing
involves reduction of the imine or enamine moieties formed,
which in the presence of other unsaturated bonds may result
in lower selectivity. Unsaturated amines are important
building blocks for production of several valuable compound
classes.2c,5 In particular, unsaturated terpene amines, readily
prepared from terpenes and amines by reductive amination,
have been used as intermediates of potential drugs for
neurological diseases.6
a Johan Gadolin Process Chemistry Centre, Laboratory of Molecular Science and
Technology, Organic Chemistry Research Group, Åbo Akademi University, FI-20500
Turku, Finland. E-mail: risto.savela@abo.fi
b L.V. Pisarzhevsky Institute of Physical Chemistry, National Academy of Sciences of
Ukraine, 31 pr. Nauky, Kiev, 03028, Ukraine
c Department of Chemistry, Nanoscience Center, University of Jyväskylä, P.O. Box
35, FI-40014 Jyväskylä, Finland
d Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université
de Bourgogne-Franche Comté, 9 Av. A. Savary, BP 47870, 21078, Dijon Cedex,
France
e Boreskov Institute of Catalysis, pr. Ak. Lavrentieva 5, 630090, Novosibirsk, Russia
f Johan Gadolin Process Chemistry Centre, Laboratory of Industrial Chemistry and
Reaction Engineering, Åbo Akademi University, Turku, Finland
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy01886d
Selective reduction of imines in the presence of olefins can
be achieved by stoichiometric amounts of a suitable hydride
source, such as sodium borohydride. While such hydride
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
reducing agents are convenient at a smaller scale, they suffer
from limited shelf life and excessive waste formation during
work up and purification. Thus, selective heterogeneous
hydrogenation methods would simplify and limit the amount of
waste formed in scaling up of the corresponding reduction
reactions.1 Various homogeneously7,8 and heterogeneously9
catalyzed hydrogenation methods for imine reduction have
been developed recently, with particular focus on
enantioselectivity.10 Yet, only a few of the homogeneously
catalyzed imine hydrogenation11 and hydrosilylation12 methods
developed to date, tolerate olefinic moieties in the substrate.
Similarly, selective heterogeneous hydrogenation13 and
hydrosilylation14 methods are scarce. Earlier, a series of studies
on selective synthesis of unsaturated amines through one-pot
monoterpene amination and controlled hydrogenation of
competitive CC and CN groups in the presence of gold
catalysts at 100–180 °C have been reported.15
hydrogenation to the fully saturated amine 4, as illustrated in
Scheme 1.
In this work, heterogeneous noble metal catalysts on
different support materials were investigated for selective
hydrogenation of an allylic imine. Based on the previous
reports on homogeneously and heterogeneously catalyzed
hydrogenation
and
hydrosilylation
of
imines,11–15
monometallic iridium, palladium, ruthenium and gold
catalysts were selected. Heterogeneous platinum catalysts
were excluded due to their earlier observed rapid deactivation
and poor recycling characteristics in the hydrogenation of
imines.18 Following the initial screening of the metal
catalysts, the influence of support and kinetics in
heterogeneously catalyzed hydrogenation of cinnamalaniline
was explored. Furthermore, DFT studies were utilized to
examine the hydrogenation mechanism and to explain the
experimentally observed regioselectivity towards unsaturated
amine 2 displayed by iridium-based heterogeneous catalysts.
While
homogeneously
catalyzed
selective
imine
hydrogenation reactions are plentiful, the main objective of
the present work was to develop a heterogeneous catalyst
system for chemoselective hydrogenation of imine CN
bonds in the presence of allylic CC bonds, at temperatures
below 100 °C and at atmospheric hydrogen pressure.
Optimally, this would allow removal of the catalyst by
filtration followed by simple purification procedure,
decreasing both the time and costs associated with chemical
transformations. Cinnamalaniline (1) (Scheme 1) was selected
as a model compound, being readily prepared in significant
quantities by condensation of cinnamaldehyde and aniline in
the presence of acid catalyst. A variety of aniline and
cinnamaldehyde derivatives are also commercially available,
that could be easily utilized to synthesize different allylic
imines, similar to compound 1, followed by hydrogenation to
the corresponding derivatives of compound 2. These types of
compounds could be further used in the synthesis of various
biologically active compounds, such as herbicides or
fungicides,16 or precursors for compounds with antiviral
properties.17 Also, the reactivity of unsaturated bonds in
Results and discussion
Catalyst characterization
The support structures were studied using XRD and the
results from XRD study of the phase composition of the
supports are presented in Fig. 1. All the synthesized carbon
nitride supports exhibit one main reflex (002) at 2θ ∼ 27.3–
27.6° (Fig. 1 left), which is characteristic of the interlayer
packaging of conjugated aromatic systems. These high-angle
reflexes correspond to an average interplanar distance d =
0.327–0.322 nm, similar to the (002) plane of graphitic
carbon nitride. The average crystallite sizes calculated from
the (002) broadening of the reflex by the Scherrer formula are
ca. 10 nm. Carbon supports C-KIT-6, CxNy-KIT-6 and C-micro
are X-ray amorphous due to their turbostratic structures,
which are characterized by a varying ordering degree of the
packing of graphite layers.
The resulting carbons have
a
spatially ordered
mesostructure corresponding to the initial mesoporous
molecular sieve (MMS). Thus, the XRD pattern (Fig. 1 right)
of the initial hard template KIT-6 is characterized by well-
separated hkl reflexes, which define a highly ordered cubic
structure with Ia3d symmetry type. Due to decrease of the
interplanar distances in carbon nitride obtained in KIT-6 (Cx-
compound
1
can mimic benzodiazepine derivative
precursors.11e Hydrogenation of cinnamalaniline may
proceed either via initial CN bond hydrogenation to
provide the unsaturated amine
2 or by –CC– bond
hydrogenation to saturated imine 3, followed by further
Scheme 1 The reaction scheme of cinnamalaniline hydrogenation.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
Fig. 1 X-ray diffraction patterns of the prepared supports: left: carbon nitride and microporous carbon; right: initial silica MMS KIT-6, CxNy-KIT-6
and C-KIT-6.
Ny-KIT-6), the peaks in the diffraction pattern are shifted
towards larger angles, compared to the initial MMS. The
decrease in the intensity of the reflexes in the diffraction
pattern of CxNy-KIT-6 indicates thickening of the walls in the
product compared to the KIT-6. Therefore, the samples
obtained after removal of the hard template replicate to a
certain extent the corresponding MMS.
Impregnation of carbon nitrides, carbon supports, as well
as metal oxides with H2IrCl6 followed by reduction with
hydrogen led to the formation of iridium nanoparticles with
an average diameter within the range of 1.3–1.7 nm (Fig. 2).
On alumina support, the average iridium nanoparticle size
was ca. 1 nm. The obtained Ir nanoparticles are mainly
uniformly distributed on the support surface.
Fig. 2 TEM images of the prepared supported Ir catalysts: from left to right. Above: Ir/C3N4–HNO3, Ir/C3N4–NH4Cl, Ir/CxNy-KIT-6; below: 3% Ir/
ZrO2, 4% Ir/Al2O3 and histogram of 4% Ir/Al2O3.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
Table 1 Textural characteristics of carbon nitride and carbon supports (N2, 77 K)
Sample
Vmicro, cm3 g−1
Dmicro, nm
Vmeso, cm3 g−1
Dmeso, nm
Smeso, m2 g−1
SBET, m2 g−1
Vsum, cm3 g−1
C3N4-bulk
C3N4–TEOS
C3N4–NH4Cl
C3N4–HNO3
C3N4-KOH
C-KIT-6
0
0
0
0
—
—
—
—
0.06
0.14
0.42
0.10
0.12
1.86
0.58
0
∼50
30 12
∼20
∼30
∼24
∼11
∼14
—
10
25
82
20
23
1300
440
2
10
28
84
22
25
1570
585
744
0.06
0.14
0.42
0.10
0.12
1.93
0.7
0
—
0.07
0.06
0.29
0.55
0.52
0.46
CxNy-KIT-6
C-micro
0.29
Nitrogen physisorption was used to determine the
microtextural characteristics of carbon nitride and carbon
supports. The surface areas and pore sizes of all
carbonaceous materials are presented in Table 1. Catalyst
supports C3N4–NH4Cl (84 m2 g−1), C-KIT-6 (1570 m2 g−1), Cx-
Ny-KIT-6 (585 m2 g−1), mesoporous C (Sibunit) (354 m2 g−1)
and sucrose-derived C-micro (744 m2 g−1) exhibit higher
specific surface area values compared to C3N4-bulk (10
m2 g−1) and C3N4–TEOS (28 m2 g−1). In case of the template
based samples, C-KIT-6 featured a three-fold higher specific
surface area compared to CxNy-KIT-6, with both KIT derived
samples displaying well-developed pore structures. Similar to
the KIT family, C3N4–NH4Cl material and mesoporous carbon
Sibunit exhibited a rather large mesopore volume with an
average pore diameter ranging from 11 to 20 nm, while
C-micro material contains only micropores with an average
diameter of 2 nm that could hamper the accessibility of the
metal active sites for bulky substrate molecules.
Details of the microtextural characteristics of both the fresh
and spent metal oxide supported iridium catalysts are given in
Table 2. Based on the measured pore volumes the materials
mainly exhibit mesoporous characteristics. Deposition of the
metal substantially influenced the surface area of the support,
which can be explained by blocking of micropores.
The basicity of carbon- and oxide-based supports was
measured using temperature-programmed desorption of CO2
(TPD CO2). In a typical TPD curve of carbon dioxide of the
synthesized C3N4 samples (Fig. 3), only one broad peak in the
TPD profiles can be found independent of the modification
type. The concentration of basic sites on similar materials
was reported to be ca. 40–60 μmol g−1.19 The peak
temperatures are all in the range of ca. 125–135 °C showing
that the base sites are of weak strength. The temperature
peak of carbon dioxide desorption at ca. 90–100 °C can be
attributed to CO2 physisorption.20 Data on the basicity of Ir/
ZrO2 and Ir/Al2O3 are provided in Table 3 illustrating that the
materials are mildly basic with the predominance of weak
basic sites.
Catalyst screening
Initially, alumina supported Ir, Pd, Au and Ru catalysts were
investigated for their catalytic activity and selectivity in the
cinnamalaniline (1) hydrogenation, followed by screening of
several support materials, such as different carbon and
carbon nitride materials, Al2O3, ZrO2, and SiO2. The initial
screening was performed at 40 °C and atmospheric hydrogen
pressure in methanol, which is a readily available, cheap and
often utilized solvent in hydrogenation reactions. The results
from catalyst screening are collected in Table 4.
According to GC analysis, and verification against
separately prepared compounds, the main products included:
the unsaturated amine, 1-aniline-3-phenyl-3-propene (2),
from CN bond hydrogenation; N-(3-phenylpropylidene)
benzamine (3), from CC bond hydrogenation; and N-(3-
phenylpropyl)aniline (4) from hydrogenation of both CC
and CN bonds. In addition, on the basis of the incomplete
mass balance of compounds 1–4, unknown high molecular
mass oligomerization products were formed, which could not
be detected by GC-FID and GC-MS methods.
Both alumina supported ruthenium and gold catalysts
exhibited poor conversion of compound
1 with barely
detectable amounts of the hydrogenation products 2–4
(Table 4). In contrast, the carbon supported ruthenium
catalyst reached a high conversion of 1, yielding mostly
unknown by-products and, consequently, was not
Table 2 Comparison of textural characteristics of metal oxide and carbon supported iridium catalysts (N2, 77 K)
Sample
Ir/ZrO2
Vmeso, cm3 g−1
Dmeso, nm
S
t-plot, m2 g−1
SBET, m2 g−1
VΣ, cm3 g−1
Fresh
Spent
Fresh
Spent
Fresh
Fresh
Spent
0.18
0.14
0.14
0.10
0.29
0.49
0.10
14
12
13
13
2–4
10
10
22
15
27
13
200
203
13
44 (106)a
34
0.18
0.14
0.14
0.10
0.29
0.13
0.10
Ir/Al2O3
54 (140)a
36
Ir/C
Ir/SiO2
221 (350)a
242 (250)a
36
a
In parenthesis surface area of the supports.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
found that, for alumina supported metals, conversion of
cinnamalaniline increases in the series: Ru ∼ Au ≪ Pd ∼ Ir,
whereas selectivity towards the unsaturated amine 2 adheres
to the following order: Ru ∼ Pd < Au ≪ Ir.
Preliminary metal screening showed that Ru, Pd and Au
catalysts were not effective for chemoselective hydrogenation
of the CN bond in compound 1 under the investigated
conditions, while Ir exhibited high catalytic hydrogenation
activity and chemoselectivity towards the unsaturated amine
2. Similar chemoselectivity has been observed for Ir catalysts
in the hydrogenation of multifunctional oxygenates, such as
citral21 and crotonaldehyde.22
Fig. 3 CO2-TPD profile of the prepared carbon nitride (C3N4-bulk).
Catalyst support screening – metal and silicon oxides
Next, the influence of support on the catalytic activity in the
chemoselective
cinnamalaniline
hydrogenation
was
Table 3 Concentration of basic sites on Ir/Al2O3 and Ir/ZrO2 catalyst
Concentration of basic sitesa μmol g−1
investigated for silica, zirconia and carbon-based Ir-catalysts
with the results summarized in Table 4. It was demonstrated
that all active Ir-catalysts are regioselective towards
compound 2, which can be explained by favorable adsorption
of the CN bond on pure iridium surface as shown by
computational studies below. Silica-supported iridium was
found to be completely inactive, while both alumina and
zirconia supported iridium catalysts exhibited high
conversions of cinnamalaniline to product 2 with overall
yields up to ∼50% based on GC-analysis. The small
Catalyst
Weak
Medium
Strong
Total
Ir/Al2O3
Ir/ZrO2
14.3
12.6
10.8
6.2
1.4
0.8
26.5
19.6
a
The basic sites were divided into three groups – weak (<750 K),
medium (750–1050 K) and strong (>1050 K) based on the ability to
desorb the probe molecule (CO2) from the surface of active sites.
differences in selectivity towards unsaturated amine
2
investigated further. Palladium on alumina showed a high
conversion of compound 1 and was initially selective towards
olefin bond hydrogenation yielding compound 3, followed by
further hydrogenation to the saturated amine 4. Similar to
the palladium catalyst, the alumina supported iridium
catalyst exhibited high conversion of compound 1 with high
selectivity towards the imine bond hydrogenation. It was
between alumina and zirconia supported catalysts can be
correlated with the metal oxide base strength, which in terms
of proton affinity was estimated to be similar for ZrO2 (900 kJ
mol−1) and Al2O3 (935 kJ mol−1).23 For iridium supported on
silica, alumina and zirconia, the conversion of
cinnamalaniline increases in the series: SiO2 ⋘ ZrO2
∼
Al2O3, for the last two supports the yields of the unsaturated
Table 4 Initial screening of Au, Pd, Ru and Ir catalysts in cinnamalaniline hydrogenation
GC-based yield
Conversion
of 1 (%)
Selectivity
Mass
balance
Catalyst
to 2a (%)
2
3
4
Pd/Al2O3
Ru/Al2O3
Ru/C
Au/Al2O3
Ir/Al2O3
Ir/ZrO2
Ir/SiO2
Ir/C
Ir/C3N4-bulk
Ir/C3N4–TEOS
Ir/C3N4–HNO3
Ir/C3N4–KOH
Ir/C3N4–NH4Cl
Ir/C-KIT-6
Ir/CxNy-KIT-6
Ir/C-micro
>99
15
97
0
0
0
—
—
—
3
44
48
—
11
21
18
6
16
45
28
39
—
32
—
6
50
—
5
—
16
12
—
—
3
3
2
4
10
5
82
75
14
87
60
60
—
25
74
77
91
81
70
44
57
86
15
20
44
48
—
13
42
41
35
41
51
30
39
0
2
>99
>99
0
88
50
44
17
39
88
—
—
—
2
—
—
—
—
4
94
>99
14
5
—
—
18
—
Compound 1 250 mg (1.21 mmol), methanol 59.5 mL, tetradecane 68 μL (0.26 mmol; introduced in 0.5 mL of toluene) and the corresponding
catalyst (1 mol%, for Ir and Au and 2 mol% for Ru and Pd) at 40 °C for 5 hours. a Selectivity (%) = [2]/([1]0 − [1]6h) × 100.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
amine
comparable.
2
and undetectable side products were also
Effects of the support characteristics on hydrogenation
selectivity
Based on the relative hydrogenation selectivity, compound 2
is the predominant hydrogenation product independent of
the iridium catalyst utilized (Fig. 4, middle). By also
considering the undesired side reactions, it can be seen that
application of the Ir/C3N4–NH4Cl catalyst reaches the highest
selectivity towards 2 at a moderate conversion level of
compound 1 but with a loss of selectivity as the reaction
progresses. In comparison, Ir/CxNy-KIT-6 catalyst was less
selective, yet maintaining the same level of selectivity until
full conversion of cinnamalaniline was achieved. Ir/C3N4-bulk
catalyst seems to exhibit very high selectivity (approx. 90%)
towards 2 at a very low conversion of cinnamalaniline, which
can be attributed at least partially to the detection/integration
limits of the instrumentation used to estimate the yield.
Hydrogenation selectivity profiles obtained with iridium
on strong alkali or silicate modified carbon nitride supports,
(Fig. 4, Ir/C3N4–KOH and Ir/C3N4–TEOS respectively) were
comparable to each other, but significantly below the
selectivity obtained with Ir/C3N4–NH4Cl. This suggests that
strongly basic supports can hinder the adsorption of the
imine bond (CN) on the iridium surface, whereas
moderately basic supports provide better adsorption of the
imine bond on the iridium surface leading to increased
selectivity towards compound 2.
Interestingly, for all iridium catalysts on modified carbon
nitride supports, the hydrogenation selectivity towards
compound 2 decreases with conversion, especially in the case
of Ir/C3N4–HNO3. The use of acidic supports (e.g., C3N4–
HNO3) leads to a significant decrease of selectivity towards
the unsaturated amine, which can tentatively be associated
with the subsequent olefin oligomerization over acid sites of
the support. Moreover, in comparison to iridium on alumina
or zirconia, the hydrogenation selectivity towards compound
2 was further decreased by faster subsequent –CC– bond
hydrogenation to yield the fully saturated compound 4. It is
worth to point out that the cinnamalaniline hydrogenation in
methanol over iridium catalysts seems to take place
Catalyst support screening – carbon based materials
Iridium supported on microporous carbon (Ir/C-micro)
displayed negligible activity, which is tentatively explained by
low accessibility of the bulky reagents to the active iridium
sites in the micropores. In comparison, the iridium on carbon
with
a
mesoporous support structure (Ir/C-KIT-6)
demonstrated 30% selectivity towards compound 2 and 5%
selectivity towards compound 3 at 94% conversion in 5 hours.
Depositing of the iridium particles on similarly prepared
carbon nitride-based supports resulted in improved selectivity
towards compound 2. For example, iridium supported on Cx-
Ny-KIT-6 achieved ca. 39% selectivity to compound 2 at full
conversion within the same reaction time. More prominent
nitrogen doping of the support further improved the selectivity
at the expense of activity, affording 42% selectivity towards
compound 2 at 50% conversion in the case of Ir/C3N4-bulk.
The highest catalytic activity for carbon nitride-based supports
was achieved with Ir/C3N4–NH4Cl, reaching up to 51%
selectivity to compound 2 with 88% conversion, comparable to
that of Ir/Al2O3 and Ir/ZrO2. The improvements in selectivity
can be attributed to the highly developed porous structure of
C3N4–NH4Cl compared to other C3N4 materials. Increase in
the catalytic activity of carbon nitride supported Ir catalysts (Ir/
C3N4–NH4Cl and Ir/CxNy-KIT-6), in comparison with Ir on
nitrogen-free carbon supports, results from their higher
basicity, as confirmed by CO2-TPD data (Fig. 3). Aside from the
textural and chemical properties, the role of different supports
in fine-tuning the selectivity to the compound 2 can be
attributed to the acid–base properties of the utilized support,
i.e., supports with mainly weak basic sites (Ir/C3N4–NH4Cl and
Ir/CxNy-KIT-6) can yield similar activity and selectivity as
amphoteric Ir/ZrO2 and Ir/Al2O3. At the same time, the overall
mass balance seems to be improved by limiting side reactions
due to narrow range of basic sites exhibited by the carbon
nitrides (Ir/C3N4–NH4Cl).
Fig. 4 Dependence of (left) selectivity and (middle) relative selectivity to the unsaturated amine 2 and (right) selectivity to the saturated imine 3 as
a function on cinnamalaniline 1 conversion. Reaction conditions: compound 1 250 mg (1.21 mmol), methanol 59.5 mL, tetradecane 68 μL (0.26
mmol; introduced in 0.5 mL of toluene), catalyst loading (1 mol%) at 40 °C.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Table 5 Effect of the solvent on the hydrogenation of cinnamalaniline
Conversion
Paper
GC-based yield
Mass
Catalyst
Solvent
of 1 (%)
2
3
4
balance
Ir/ZrO2
Methanol
Toluene
2-Propanol
MTBE
Methanol
Toluene
2-Propanol
MTBE
>99
81
94
44
>99
76
48
48
47
19
44
47
37
7
—
4
—
2
—
4
12
13
13
3
16
17
17
1
60
84
66
81
60
92
74
85
Ir/Al2O3
78
24
—
—
Compound 1 250 mg (1.21 mmol), solvent 59.5 mL, tetradecane (68 μL, 0.26 mmol; introduced in 0.5 mL of toluene) and catalyst (1 mol%) at
40 °C for 5 h.
exclusively via compound 2, excluding Ir/C3N4–NH4Cl and Ir/
C-KIT-6 catalysts (Fig. 4).
Ir/ZrO2 and Ir/Al2O3 catalysts were selected for this study, as
they displayed high conversion of cinnamalaniline and very
good selectivity towards the desired product 2. The results
are presented in Table 5. In protic solvents, both catalysts
exhibited very similar selectivities and overall mass balance,
while the conversion of cinnamalaniline in methanol was
somewhat higher than in 2-propanol. Application of the polar
aprotic MTBE resulted in a significantly lower conversion of
cinnamalaniline albeit at higher mass balance indicating a
need of proticity for polar solvents in order to obtain high
conversion and selectivity towards 2. The use of toluene as a
solvent significantly improved the mass balance at a slight
expense of reactivity. The solvent choice significantly
influences the rate of cinnamalaniline hydrogenation to
compound 2, followed by hydrogenation to compound 4. In
toluene, the hydrogenation rate is substantially slower than
in alcohol, as illustrated in Fig. 5 and 6 for Ir/Al2O3, due to
probably competitive adsorption of toluene and the substrate.
Generally, the influence of solvent nature on conversion and
selectivity could be very complex, including hydrogen
solubility, competitive adsorption, solvation of the transition
state and donation of protons.25 A detailed investigation of
the solvent effect is beyond the scope of the current study.
Analysis of the basicity of C3N4 supports modified with
KOH, TEOS, HNO3 and NH4Cl by CO2-TPD (Fig. 3) did not
result in any quantitative correlations between the support
basicity and selectivity towards unsaturated amine 2.
Nevertheless, a clearly different catalytic behavior in terms of
selectivity towards compound 2 was observed for iridium on
nitrogen-free carbon supports C and C-KIT, in comparison to
that of Ir/C3N4–NH4Cl, Ir/CxNy-KIT-6, Ir/ZrO2 and Ir/Al2O3
(Fig. 4). This indicates that the chemical nature of the
support is probably less important than the availability of
basic sites with an optimal strength on these supports.
Influence of the solvent and temperature
The selectivity towards hydrogenation of polyunsaturated
compounds containing polar unsaturated bonds generally
depends on the solvent polarity.24 Thus, the influence of the
solvent on hydrogenation of the CN bond vs. CC in
cinnamalaniline was investigated with two polar protic
solvents, methanol and 2-propanol, polar aprotic methyl
tert-butyl ether (MTBE) and one aprotic solvent toluene. Both
Fig. 5 Concentrations of compounds 1–4 vs. reaction time over 1 mol% Ir/Al2O3 catalyst in cinnamalaniline 1 hydrogenation using methanol (left)
and toluene (right) at 40 °C. Reaction conditions: compound 1 250 mg (1.21 mmol), corresponding solvent 59.5 mL, tetradecane (68 μL, 0.26
mmol; introduced in 0.5 mL of toluene) and the corresponding catalyst (1 mol%) at 40 °C for 5 h.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
Fig. 6 Selectivity towards compounds 2–4 vs. conversion of cinnamalaniline 1 over 1 mol% Ir/Al2O3 catalyst using methanol (left) and toluene
(right) at 40 °C. Reaction conditions: compound 1 250 mg (1.21 mmol), corresponding solvent 59.5 mL, tetradecane (68 μL, 0.26 mmol; introduced
in 0.5 mL of toluene) and the corresponding catalyst (1 mol%) at 40 °C for 5 h.
Next, influence of the reaction temperature on the
hydrogenation of cinnamalaniline was investigated. Based on
solvent screening, toluene was selected as the solvent and the
performance of both Ir/Al2O3 and Ir/ZrO2 catalysts was
evaluated at three different reaction temperatures (40, 60 and
80 °C). The temperature elevation resulted in similar increase
in hydrogenation rates for both catalysts (Table 6). The
concentration profiles for Ir/Al2O3 and Ir/ZrO2 (see ESI†) at
different temperatures suggest that an increase in the
reaction temperature did not have a significant effect on the
Ir/Al2O3 catalysts. While in methanol these carbon nitrides
supported catalysts exhibited somewhat better characteristics,
in toluene the amphoteric Ir/Al2O3 catalyst showed better
characteristics for selectivity and mass balance closure.
Comparison of the four selected catalysts is presented in
Table 7 with the concentration and selectivity profiles collected
into ESI.† The results of this work clearly demonstrate that
when searching for optimal cinnamalaniline hydrogenation
catalysts with high stability and selectivity, in addition to the
metal, the support and the solvent have a crucial role in fine-
tuning of the catalytic system.
selectivity towards
2 (Fig. 6 and 7). Notably, Ir/Al2O3
maintained a better overall mass balance than Ir/ZrO2 for
compounds 1–4 (Table 6). While the hydrogenation rate of
compound 2 is slightly higher at 80 °C compared to 40 and
60 °C, it appears that compound 4 is mainly formed by
hydrogenation of the imine bond of compound 3 (Fig. 7).
The activation energies for hydrogenation of cinnamalaniline
calculated from the data obtained from experiments at
different temperatures are 18.3 kJ mol−1 and 14.2 kJ mol−1 for
Ir/Al2O3 and Ir/Zr2O respectively, which are lower than the
values typically reported for hydrogenation reactions.26
Catalyst deactivation and side products
A brief investigation of deactivation of the catalyst and the
side reactions was carried out. Hot filtration of the catalyst
from the reaction mixture stopped the reaction (see ESI†).
This verifies that the hydrogenation proceeds on the
supported iridium particles which do not leach from the
support during the reaction. Recycling of the spent Ir/Al2O3
catalyst was also tested. While the cinnamalaniline
hydrogenation rate did not significantly decrease, selectivity
towards compound 2 was clearly influenced and the mass
balance of compounds 1–4 decreased significantly (see ESI†).
This may be due to deposition of oligomers into the pores of
On carbon composite supported catalysts, Ir/C3N4–NH4Cl
and Ir/CxNy-KIT-6, the hydrogenation of cinnamalaniline in
toluene at 60 °C resulted in similar improvements in the
selectivity and mass balance closure as observed for Ir/ZrO2 and
Table 6 Influence of the reaction temperature on the iridium catalyzed hydrogenation of cinnamalaniline in toluene over Ir/ZrO2 and Ir/Al2O3
GC-based yield
Temperature
Conversion
Mass
Catalyst
Ir/ZrO2
(°C)
of 1 (%)
2
3
4
balance
40
60
80
40
60
80
81
93
>99
76
92
48
47
53
47
56
55
4
13
21
27
17
27
35
84
77
80
92
91
90
2
—
4
3
—
Ir/Al2O3
>99
Compound 1 250 mg (1.21 mmol) was dissolved in toluene (60 mL containing tetradecane 68 μL, 0.26 mmol) and catalyst (1 mol%) at 40–80 °C
for 5 hours.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
Fig. 7 Right: Concentrations of compounds 1–4 vs. time in cinnamalaniline hydrogenation over Ir/Al2O3at 60 °C; and left: selectivity towards
compounds 2–4 vs. conversion of cinnamalaniline 1 at 60 °C. Reaction conditions: compound 1 250 mg (1.21 mmol), toluene (60 mL containing
tetradecane 68 μL, 0.26 mmol), catalyst loading (1 mol%), 5 h.
the utilized catalysts in line with the decrease in the surface
area of the spent catalysts (Table 2). Most likely, the balance
between the acidic/basic sites and activity of iridium per se is
crucial to achieve high hydrogenation selectivity. This type of
behavior has also been previously reported in the case of
heterogeneous cinnamaldehyde hydrogenation.27 Based on
the 1H-NMR-analysis, exposing cinnamalaniline to the fresh
catalyst in the absence of hydrogen leads into barely
detectable isomerization/oligomerization under the utilized
reaction conditions. In addition, hydrogenation of compound
2 to 4 proceeds without any side reactions. It is likely that
compound 3 is the main intermediate for the formation of
oligomeric compounds via the aldol condensation-type
reactions, via e.g. transimination with either compound 2 or
4, as hydrocinnamaldehyde (hydrolysed from compound 3)
has been shown to dimerize in the presence of metal oxides
and primary and secondary amines.28 A possible role of
compound 2 (and 4) was verified by mixing cinnamalaniline,
hydrocinnamaldehyde and 10 mol% of compound 2 for 4
incorporate cinnamalaniline to the oligomeric/polymeric
structures.
Computational studies on the regioselective hydrogenation of
the conjugated imine on iridium
The experimental results indicate that regioselective
hydrogenation of 1 is dictated by the nature of metal and, to
a minor degree, also by the catalyst support and the solvent.
Given that iridium was identified as the most regioselective
among the studied metals, our goal is to understand the
origin of Ir′s regioselectivity. For this, we performed density
functional theoretical calculations to elucidate the
mechanistic details controlling the selective hydrogenation of
1 on the most abundant Ir(111) surface. To reduce the
computational burden, the phenyl substituents were replaced
by vinyls, the smallest conjugated moieties and this part of a
reactant is not considered amenable towards hydrogenation.
1
Gas-phase reaction thermodynamics
hours at 40 °C followed by HPLC-SEC and H-NMR analysis.
The results obtained from size-exclusion chromatography are
similar to those obtained in solvent screening experiments,
i.e. protic polar solvents resulting in faster oligomerization
than aprotic solvents (see ESI† for the HPLC-SEC
chromatograms). Based on 1H-NMR analysis, oligomeric
structures are mainly generated from hydrocinnamaldehyde,
while extended heating (at least for 16 hours) can
First, we considered the thermodynamics for the formation
of the two main products, 2 and 3 and computed the
corresponding gas-phase reaction energies, which are ΔEr2
=
−58 kJ mol−1 and ΔEr3 = −69 kJ mol−1, respectively. This
means that formation of product 3 is thermodynamically
more favorable and a selective catalyst is required to form
product 2.
Table 7 Comparison of hydrogenation over the most active heterogeneous catalysts in toluene at 60 °C
GC-based yield
Conversion
Mass
Catalyst
of 1 (%)
2
3
4
balance
Ir/ZrO2
Ir/Al2O3
93
92
77
>99
47
56
36
50
2
3
5
21
27
22
29
77
91
86
79
Ir/C3N4–NH4Cl
Ir/CxNy-KIT-6a
—
Compound 1 250 mg (1.21 mmol), toluene (60 mL containing tetradecane 68 μL, 0.26 mmol) and catalyst (1 mol%) at 60 °C for 5 hours. a After
3 hours.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
Table 8 Low coverage adsorption energies of the reactant 1 in different geometries and 2 in its most stable geometry. CC and CN refer to bonding
to the Ir(111) surface from the CC and CN double bonds, respectively. Dark grey: carbon, blue: nitrogen, light grey: iridium, and pale pink: hydrogen
1
1
1
2
Conformation
Side view
CC and CN
CN
CC
C–N and CC
Eads (kJ mol−1
)
−150
−160
−60
−190
Adsorption energies
hydrogenated 3 are observed. However, from the CC/CN
bound geometry one would expect the product distribution to
be a mixture of 2, 3, and 4 as hydrogenation of the CN and
CC bonds are kinetically and thermodynamically similar as
shown in Fig. 8. Therefore, we need to identify a pathway
leading preferentially to 2 and possibly 4 without forming 3
at any point.
Initially, several adsorption geometries were studied for
reactant 1 and products 2 and 3 on Ir(111) and the most
stable optimized adsorption geometries together with the
corresponding adsorption energies are presented in Table 8.
To guarantee
a sufficiently unbiased probing of the
adsorption geometries, a thorough semi-global structure
search was performed using the minima-hopping algorithm
as detailed in the Computational details section. The reactant
1 was found to bind equally strongly from the one CN
bond or simultaneously from both CN/CC bonds. The
most stable adsorption geometry of 2 is bound to Ir(111)
from both the now hydrogenated C–N and the conjugated
CC bonds lying flat on the surface; all geometries with the
C–N tail detached from the surface are highly unstable (over
50 kJ mol−1 less stable) compared to the flat-lying geometry.
The flat-lying geometry is prone to further hydrogenation,
which is expected to be even easier than the first
hydrogenation of the CN double bond as the once
hydrogenated molecule is less conjugated than 1. We note
that Ir can also catalyze full hydrogenation to 4 as shown in
Tables 5–7 but only minor amounts of the CC
It has been previously shown that surface coverages are
crucial for determining the selectivity of metal-catalyzed
hydrogenation of conjugated organic molecules.29 This
together with the fact that we are able to find strongly
exothermic adsorption geometries for reactant 1 indicates
that the Ir catalyst surface has relatively high concentration
of adsorbed species and therefore the surface coverage effects
are considered. Table 9 summarizes the adsorption energies
and structures for the reactant at higher surface coverage.
The results show that adsorption of the reactant 1 is
energetically highly favorable even in the presence of another
reactant in its vicinity. Crucially, the CN group is bound
much stronger than the CC group and in all the high
coverage geometries the one or the other end of
1
spontaneously lifts up from the surface during structure
optimization. This makes the initial high-coverage geometry
Fig. 8 Left: Potential energy profile for the hydrogenation starting from the CC/NC geometry using gas-phase hydrogen as the reference. In
NHC–CC the N is hydrogenated first and in NCH–CC the C is hydrogenated first with the CC bond being away from the surface leading to
product 2. In CHC–NC and CCH–NC geometries the CC double bond is hydrogenated in different order with CN being away from the surface
and the pathways lead to product 3. Ads denotes adsorbed reactant 1 and Des refers to desorbed, gas-phase products while 1st H (2nd H) stands
for the number of added hydrogen. The dotted and thin solid lines indicate thermodynamic changes and barriers, respectively. Right: Transition
state structures for first hydrogenation along the NHC–CC (top) and CHC–NC (bottom) pathways. The reactive N and C are circulated with colors
corresponding to the energy profiles; the pink oval marks the vinyl group acting as a proxy for benzyl.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
Table 9 Average adsorption energies per molecule and binding geometries of two reactant 1 molecules on Ir(111) at high coverage. In geometries
named 2CC and 2CN, both reactant molecules are bound either from two CC bonds or two CN bonds, respectively. In the CC/CN
configuration, one reactant molecule is bound from the CC bond and the other one from the CN bond. The surface abundance is computed as the
Boltzmann weight at 40 °C
Above, 1 CC/CN
1, 2CC
1, 2CN
1, CC/CN
Geometry
Eads (kJ mol−1
Abundance
)
−80
−160
−120
1 × 10−14
1
1 ×10−7
susceptible to selective hydrogenation either from the CC
or the CN group. However, the geometry where 1 is bound
exclusively via the CN-bond (1, 2CN geometry in Table 8)
is significantly more stable than mixed binding with both
CC and CN (1, CC/CN) or exclusively CC-bound (1,
2CC) geometries. This indicates that at high surface
coverage the CN-bound geometry is clearly the most
abundant adsorbed reactant conformation as shown in
Table 9 – a pathway starting from this adsorption geometry is
likely to lead to 2 and even 4 while formation of 3 is avoided.
are no reactive carbon atoms close to the surface as shown in
Fig. 8 and Table 9. Our calculations indicate that the reactive
carbon atom can only approach the surface after N has been
hydrogenated. Therefore, the hydrogenation mechanism of
the complex imine 1 is different from the mechanism of
methylene imine (H2CNH) hydrogenation presented by
Pera-Titus and Sautet,30 where the C is hydrogenated before
N. For these reasons, we computed the first hydrogenation
barriers for N (NHC–CC in Fig. 8) and C (CHC–NC in Fig. 8),
which may determine product selectivity and lead to product
molecules 2 and 3, respectively. The barriers for these
competing hydrogenation steps are slightly different being
ΔE‡NHC–CC = 100 kJ mol−1 and ΔEC‡ HC–NC = 85 kJ mol−1
indicating a slight kinetic preference towards forming the
minor product 3 over the major product 2 on an Ir catalyst
but suggesting that both hydrogenations are feasible under
reaction conditions. This in turn means that if the reactant
favors an adsorption geometry, where both CN and CC
double bonds are sufficiently close to the Ir catalyst surface,
the product distribution includes both 2 and 3 species and
very likely also fully hydrogenated imine in similar
proportions.
As the experiments clearly demonstrate selectivity towards
2, the regioselectivity must be controlled by the reactant
adsorption energy and conformation at high-surface
coverages as has been shown previously for other selective
metal-catalyzed hydrogenations.29 This is indeed the case
here because the concentration of the CC bound reactant
configurations, giving product 3 on Ir(111), is very low due to
the substantial binding energy difference between NC and
CC bound geometries. This leads to very different
abundances (see Table 9) of reactant adsorption geometries.
We therefore ascribe the experimentally observed
regioselective formation of 2 on Ir to the high surface
coverage of CN-bound reactant geometry despite the fact
that thermodynamically and kinetically the formation of 3 is
slightly more favorable.
Hydrogenation thermodynamics and barriers
After considering the adsorption characteristics of the
reactant 1, the elementary kinetics and thermodynamics of
forming either
2 or 3 at high surface coverage were
addressed. Fig. 8 presents the thermodynamic analysis and
reveals that cinnamalaniline is preferentially hydrogenated at
the CC double bond (CCH and CHC in Fig. 8) leading to
formation of 3. The CN hydrogenation is also
thermodynamically favorable but desorption of the product 2
is more endothermic than that of product 3. Hence,
formation of the minor product 3 is more thermodynamically
favored also on the Ir surface.
Given the complexity of the system, entropy effects were
only qualitatively addressed for the adsorption and
desorption steps as detailed in the Materials and methods
section. Including the decrease (increase) in rotational and
translational degree of freedom during adsorption
(desorption) makes the adsorption energies more
endothermic by ∼66 kJ mol−1 and decreases the
thermodynamic desorption barriers by ∼65 kJ mol−1. Hence,
the adsorption is still highly exergonic but accounting for the
major entropic contributions enhances desorption while the
overall thermodynamics remain rather unchanged. Entropy
changes in elementary steps taking place solely on the
surface are considered minor.
As the elementary thermodynamic analysis is unable in
explaining the regioselective formation of the experimentally
observed major product on Ir, it is necessary to address the
reaction kinetics. In the high-coverage adsorption geometry
leading to product 2 (green and orange color in Fig. 8), there
Conclusions
Liquid-phase hydrogenation of cinnamalaniline in polar
(methanol, 2-propanol) and non-polar (toluene) solvents over
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
monometallic iridium, ruthenium, palladium and gold
catalysts with different carbon-based and metal oxide
supports at 40–80 °C and atmospheric hydrogen pressure
was studied. In addition to zirconia, silica, alumina and
microporous carbon supports, templated (silica KIT-6)
mesoporous carbon, Sibunit supports and carbon nitride
supports were investigated. The carbon nitrides were
prepared both as bulk and chemically modified materials,
using different pretreatments and templates followed by
iridium impregnation.
G1315B DAD-detector, 2× Jordi Gel DVB 500A (300 mm × 7.8
mm) columns and a 50 mm × 7.8 mm guard column. This
setup allows selective analyses of aromatic compounds. The
samples were analyzed in THF with one percent AcOH as
eluent at a flow rate of 0.8 mL min−1 with 35 min analysis
time/sample. The samples were dissolved in the eluent
solution to yield a concentration of 1 mg mL−1
.
Catalyst preparation
It was found that for alumina supported metals iridium
proved superior in terms of selectivity towards CN bond
hydrogenation. Both carbon nitride (Ir/C3N4–NH4Cl, Ir/CxNy-
KIT-6) and amphoteric metal oxide (Ir/ZrO2, Ir/Al2O3) based
catalysts provided the best regioselectivities towards
hydrogenation of the CN group, leading to the formation
Carbon nitride denoted as C3N4-bulk was prepared by bulk
pyrolysis of melamine. Melamine (5 g) was heated in air at 525
°C for 2 h to provide a yellow carbon nitride powder. For
synthesis of the C3N4–TEOS support, a mixture containing
melamine and the products of TEOS hydrolysis with the
melamine: TEOS molar ratio of 1.5 was subjected to thermal
treatment. To prepare this mixture, the desired amount of
melamine was dissolved in distilled water under heating,
followed by addition of the corresponding amount of TEOS,
stirring for 30 min and drying at 100 °C until complete
evaporation of the solvent. The C3N4–HNO3 material was
prepared from melamine precipitated from ethylene glycol
with 0.6 M nitric acid solution (HNO3 : melamine molar ratio =
6). For synthesis of C3N4–KOH, melamine treated with
potassium hydroxide was used. For this purpose, a solution of
melamine (1.6 g) and KOH (0.02 g) in distilled water (200 ml)
was stirred for 1 h at 90 °C and then dried at 100 °C until
complete evaporation of the solvent. The C3N4–NH4Cl material
was obtained by heat treatment of a mixture of melamine (1 g)
with ammonium chloride (6.37 g). All melamine derived
samples were heated under similar conditions (525 °C, 2 h).
The obtained materials were washed with distilled water, and
in the case of C3N4–TEOS, the silica component was removed
by treating the resulting powder with 15% HF solution.
To synthesize the C-KIT-6 support, a desired amount of
the initial silica mesoporous molecular sieve (MMS) KIT-6
(cubic symmetry) was mixed with an aqueous solution
containing sucrose and concentrated sulfuric acid (i.e., 1.25 g
and 0.14 g, respectively, per 1 g of MMS with the pore volume
of 1.3 cm3 g−1). The resulting suspension was first dried at
100 °C for 6 h, and then for additional 6 h at 160 °C. This
was followed by mixing again with an aqueous solution of
sucrose and sulfuric acid (0.8 g and 0.08 g, respectively, per 1
g of MMS with the same pore volume). After repeated
treatment under identical conditions (100 and 160 °C), the
obtained dark brown powder was heated (heating rate 5 °C
min−1) under argon to 900 °C and retained for 2.5 h. The
silica component was removed by treatment of the obtained
powder with 15% HF solution. The product was filtered,
washed with ethanol and dried at 100 °C.
of the unsaturated amine.
A non-polar aprotic solvent
significantly improved the mass balance closure at the
expense of reactivity, while an increase of the reaction
temperature did not affect selectivity to the unsaturated
amine. Based on DFT calculations, the regioselectivity of an
Ir catalyst is dictated by the reactant adsorption geometry
and substantially more favorable adsorption via the CN
bond leading to the experimentally observed product.
Experimental
General considerations. Commercial Ru/Al2O3 (5 wt%) and
Pd/Al2O3 (5 wt%) were purchased from Merck and
commercial 5% Ru/C with Ru cluster size of 2.5 nm and the
surface area 700 m2 g−1, were utilized after drying under
vacuum at 120 °C for 5 hours and then stored under argon.
Similar drying procedure was utilized for all catalysts. In case
of catalyst preparation reagent grade chemicals were utilized;
H2IrCl6 (Alfa Aesar, purity 99%), IrCl3 hydrate (TU 2625-067-
00196533-2002 OAO “V.N. Gulidov Krasnoyarsk factory of
nonferrous metals”, Krasnoyarsk), zirconia (Acros Organics,
SBET
= =
106 m2 g−1), γ-alumina (SBET 146 m2 g−1),
mesoporous carbon support Sibunit (SBET = 358 m2 g−1) and
silica (SBET = 378 m2 g−1). Chemicals and solvents utilized in
catalyst preparations (melamine, tetraethylorthosilicate
(TEOS), potassium hydroxide, ammonium chloride,
ethylenediamine, carbon tetrachloride, HF, ethanol, ethylene
glycol, sucrose, nitric acid, sulfuric acid) were dried, degassed
and stored in a glovebox.
Chemicals (reagent grade) and solvents (HPLC-grade)
utilized in hydrogenation experiments were bought from
commercial sources and utilized as is. Argon (99.996%) and
hydrogen (99.999%) gases (Linde Gas-AGA) were utilized.
Cinnamalaniline was prepared according to a literature
procedure31 and verified by NMR spectra measured on an
500 MHz NMR spectrometer. The measured NMR spectra
were calibrated using residual solvent signal as internal
standard.32 The molar weight profile was determined by high
pressure size-exclusion chromatography (HP-SEC) using an
For synthesis of CxNy-KIT-6, the initial silica KIT-6 (0.5 g)
was added to
a mixture of 70% aqueous solution of
ethylenediamine (2 ml) and carbon tetrachloride (1.5 ml) and
heated at 90 °C for 6 h under reflux. The resulting dark
brown mass was dried at 60 °C for 12 h and mixed again with
a mixture of 70% ethylenediamine solution (2 ml) and carbon
Agilent 1100 series HPLC instrument equipped with
a
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
tetrachloride (1.5 ml). The mixture was reheated at 90 °C for
6 h under reflux and dried. The obtained powder was then
heated under inert (argon) atmosphere to 600 °C (heating
rate 3 °C min−1) and kept at that temperature for 5 h. The
silica hard template KIT-6 was removed by treatment in HF
solution, the obtained product was then washed several times
with distilled water and ethanol, and finally dried at 100 °C.
For synthesis of the C-micro support, sucrose was kept in
an argon flow at 900 °C for 2.5 h.
mesopore volumes were obtained using the BJH (Barrett–
Joyner–Halenda) method. Micropore volume was evaluated by
Dubinin–Radushkevich method. Catalysts were pretreated
under vacuum (0.05 mbar) and heated to 180 °C for at least 7
h for moisture removal prior to measurements performed at
77 K. An exception was Ir/C, which pretreatment was done at
120 °C.
TEM images were obtained using the field emission TEM
JEM-2100F (JEOL) and JEM-2100 (JEOL) instruments with an
accelerating voltage of 200 kV. The samples were dispersed in
ethanol in an ultrasonic bath for 5 min, followed by
deposition of the suspension on a copper grid coated with a
carbon film.
Zirconia and alumina were precalcined in air at 500 °C for
2 hours before catalyst preparation.
To prepare Ir catalysts over C-micro, C3N4 family, KIT-6,
CxNy-KIT-6 and C-KIT-6 with 3 wt% metal loading a weighted
amount of the support was mixed with 0.5 M H2IrCl6
aqueous solution, stirred at room temperature overnight,
dried at 100 °C followed by reduction in flowing hydrogen at
300 °C for 2 h.
Basicity of the supports was determined by temperature
programmed desorption of CO2 using
a Micromeritics
Autochem 2910 equipment. Prior to adsorption of CO2, the
materials were heated to 400 °C (10 °C min−1) in a 10 ml
min−1 flow of helium and kept at this temperature for 60
min. Thereafter, the samples were cooled to ambient
temperature and CO2 was adsorbed for 30 min with a 50 ml
min−1 flow. After CO2 adsorption, the samples were flushed
with helium (20 ml min−1) for 30 min to remove physisorbed
CO2. The temperature programmed desorption was carried
out with a 10 °C min−1 heating rate until 600 °C recording
the TCD signal every second.
Incipient wetness impregnation with an aqueous solution
of IrCl3 hydrate was used for preparation of 3 wt% Ir/ZrO2, 3
wt% Ir/SiO2, 3 wt% Ir/C and 4 wt% Ir/Al2O3 followed by
drying at 110 °C for 17 h and reduction by molecular
hydrogen from room temperature up to 400 °C with a ramp
rate of
3
°C min−1, thereafter holding in hydrogen
atmosphere for 3 h to fully reduce IrCl3.
Au/Al2O3 (1 wt%) was prepared by deposition–precipitation
on alumina prepared by a sol–gel method33 using urea as the
precipitation agent. First, 4 g of the prepared alumina
support were added to 400 ml of aqueous solution of HAuCl4
(4.2 × 10−3 M) and urea (0.42 M). The initial pH of solution
was ca. 2. After vigorous stirring at 80 °C for 4 h, the
suspension was filtered and washed with ammonium
hydroxide (25 M) for 30 min. The pH of the solution after
stirring with ammonium hydroxide was ca. 10. Finally, the
sample was washed in water until pH 7, filtered and dried at
room temperature for 24 h.
Hydrogenation experiments of cinnamalaniline
All catalysts employed were heated to 150 °C under H2
atmosphere for 1 h before hydrogenation experiments. A four
necked glass reactor was purged with argon followed by
addition of cinnamalaniline (250 mg, 1.21 mmol) and solvent
(toluene, 2-propanol or methanol; 60 mL, containing
tetradecane 68 μL, 0.26 mmol; in the case of methanol, the
solution contained 0.5 mL of toluene in order to solve
tetradecane). A gas bubbler was introduced into the reactor and
the solution was purged with hydrogen for ca. 10 minutes. To
minimize any possibilities of detonation due to static discharge
the hydrogen atmosphere was switched to argon prior the
addition of the catalyst. The mixtures were heated to either 40
°C, 60 °C or 80 °C and hydrogen bubbling was continued
maintaining the H2 pressure of ca. 1 bar. The reaction was
followed by gas chromatograph (GC) with flame ionization
detector (FID), equipped with HP-5 column (30 m × 320 μm ×
0.25 μm), and He as the carrier gas, using the following
temperature program: injector 220 °C, oven T initial = 50 °C (4
min), rate 20 °C min−1, T final = 300 °C, hold 5 min.
Catalyst characterization
The phase composition of the catalysts was analyzed using
X-ray diffractometer D8 Advance (Bruker AXS) with CuKα-
radiation.
The porous structure characterization of carbon nitride
and other carbon based supports was performed by N2
physical adsorption at 77 K using Sorptomatic 1990, after
outgassing the samples at 473 K under vacuum for 4 h. The
total surface area, SBET, was calculated by the BET equation.34
The mesopore size was determined using the Barrett–Joyner–
Halenda method from the desorption branch of the
isotherm.35 The micropore size was calculated according to
the Horvath–Kawazoe equation.36
Selectivity towards compounds 2–4 was calculated by
dividing the GC-based yield of 2, 3 or 4 by converted
cinnamalaniline 1; while for calculation of relative selectivity:
the GC-based yield of 2 was divided by the combined GC-
based yields of the known products 2–4.
Measurements of the surface area and pore volume of the
supported Ir catalysts were performed by nitrogen
physisorption
using
MicroActive
3FlexTM
3500
Oligomerization experiments
(Micromeritics®). BET (Brunauer–Emmett–Teller) method
was applied for determination of the specific surface area.
External surface area was achieved using t-plot method. The
Into
3
mL vial with magnetic stirred was added
hydrocinnamaldehyde (34 mg, 0.25 mmol), cinnamalaniline
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
(52 mg, 0.25 mmol), N-(3-phenyl-2-propen-1-yl)-benzenamine
(3 mg, 0.013 mmol) and solvent (3 mL). The mixture was
heated to 40 °C and stirred for 4 hours, followed by analysis
of the reaction mixture with HP-SEC.
authors contributed in different extent in writing and editing
of the manuscript.
Conflicts of interest
There are no conflicts to declare.
Computational methods
All calculations were performed using the GPAW-1.5.1
software37 and ASE.37 The van der Waals corrected, surface-
specific BEEF-vdW functional38 was used. The grid spacing
was set to a standard value of 0.18 Å. The computed bulk
lattice constant is 3.91 Å which is in good agreement with
the experimental value of 3.84 Å. For the surface calculations
a 3 × 4 × 4 slab was used with the bottom two layers fixed
and a 3 × 2 × 1 k-point sampling was used. The geometry
optimizations were considered converged when the residual
force was below 0.05 eV Å−1. To simulate the liquid
environment, the SCMVD implicit solvent method39 with
water parameters was adopted to mimic the methanol
environment. To locate transition states the nudged-elastic
band40 with 5 intermediate images was utilized. For semi-
global optimization the minima hopping method41 was
applied. In this approach short molecular dynamics
simulations are run at a high temperature (1200 K) to escape
local minima. Subsequently local geometry optimization is
Acknowledgements
MMM and KH acknowledge funding by the Academy of
Finland through projects 307853 and 307623. The
computational studies were made possible by the Finnish
Grid and Cloud Infrastructure – FGCI. RS acknowledges
funding by the Academy of Finland through project 295364.
Synthesis of some catalysts (Ir on alumina, zirconia, silica,
Sibunit) was supported by RFBR Grant 18-53-45013 IND_a,
characterization (TEM, N2 physisorption, XRF) was supported
by Ministry of Science and Higher Education of the Russian
Federation. NDS acknowledges the support of the National
Academy of Sciences of Ukraine to the project “New effective
nanoscale catalysts for the production of valuable organic
compounds from bio raw materials and products of its
conversion” (no. 19/02-2020). Help of Lucas Lagerquist with
HP-SEC analysis is gratefully acknowledged.
performed. With this approach at least 20 minima around Notes and references
the geometries 1, 2, and 3 were studied resulting in a wide
search space for the conformers.
1 (a) G. W. Gribble, Chem. Soc. Rev., 1998, 27, 395–404; (b) M.
Periasamy and M. Thirumalaikumar, J. Organomet. Chem.,
2000, 609, 137–151; (c) M. V. N. de Souza and T. R. A.
Vasconcelos, Appl. Organomet. Chem., 2006, 20, 798–810.
2 (a) S. Gomez, J. A. Peters and T. Maschmeyer, Adv. Synth.
Catal., 2002, 344, 1037–1057; (b) A. A. Pletnev, Amines, in
Comprehensive Organic Transformations, ed. R. C. Larock, John
Wiley & Sons, Hoboken NJ, 3rd edn, 2018, vol. 2, pp. 1479–
1716; (c) O. I. Afanasyev, E. Kuchuk, D. L. Usanov and D.
Chusov, Chem. Rev., 2019, 119, 11857–11911.
3 (a) V. I. Isaeva and L. M. Kustov, Top. Catal., 2016, 59,
1196–1206; (b) C. Lepori and J. Hannedouche, Synthesis,
2017, 49, 1158–1167; (c) C. Michon, M.-A. Abadie, F. Medina
and F. Agbossou-Niedercorn, J. Organomet. Chem., 2017, 847,
13–27; (d) M. Patel, R. K. Saunthwal and A. K. Verma, Acc.
Chem. Res., 2017, 50, 240–254; (e) L. F. T. Novaes and J. C.
Pastre, Quim. Nova, 2017, 40, 932–945; ( f ) J. A. Gurak and
K. M. Engle, Synlett, 2017, 28, 2057–2065.
Given the complexity of the considered molecule the
entropic effects were not considered as this would require
extensive sampling of the vibrational and torsional modes as
well as 2D translation/rotational movement of the adsorbed
complexes for reliable results and is not within the scope of
the present study. The entropic effects would be most
significant for the adsorption and desorption. These
contributions can be approximated by removing/adding one
translational degree of freedom during adsorption/
desorption. For this, we computed the translational and
rotational energies and entropies within the 3D ideal gas
approximation for the solution phase molecules. For
adsorbed species, the 2D hindered rotor/translator
approximation with 50 kJ mol−1 barriers for both translation
and rotation were used to approximate energies and
entropies of the corresponding degrees of freedom. The
experimental temperature of 60 °C and 1 bar. With these
corrections, the adsorption free energies become weaker by
66 kJ mol−1 and the desorption more facile by 65 kJ mol−1.
4 (a) S. Bähn, S. Imm, L. Neubert, M. Zhang, H. Neumann and
M. Beller, ChemCatChem, 2011, 3, 1853–1864; (b) Q. Yang, Q.
Wang and Z. Yu, Chem. Soc. Rev., 2015, 44, 2305–2329; (c) A.
Corma, J. Navas and M. J. Sabater, Chem. Rev., 2018, 118,
1410–1459.
Author contributions
The work was originally conceptualized by RS, DYM, RL and
OL. Non-commercial catalysts were synthesized and
characterized by NDS, IB, EK and IS. RS and OL supervised
the hydrogenations experiments of Erasmus-interns TS and
AK. The hydrogenation experiments were finalized by RS.
MMM and KH devised the computational approach and
analyzed the results. MMM performed the calculations. All
5 (a) A. Stütz, Angew. Chem., Int. Ed. Engl., 1987, 26, 320–328; (b)
A. Shupak, I. Doweck, C. R. Gordon and O. Spitzer, Clin.
Pharmacol. Ther., 1994, 55, 670–680.
6 (a) A. A. Silver, R. D. Shytle, K. H. Sheehan, D. V. Sheehan, A.
Ramos and P. R. Sanberg, J. Am. Acad. Child Adolesc. Psychiatry,
2001, 40, 1103–1110; (b) T. G. Tolstikova, E. A. Morozova, A. V.
Pavlova, A. V. Bolkunov, M. P. Dolgikh, E. A. Koneva, K. P.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
Volcho, N. F. Salakhutdinov and G. A. Tolstikov, Dokl. Chem.,
2008, 422, 248–250; (c) D. I. Park, H. G. Kim, W. R. Jung, M. K.
Shin and K. L. Kim, Neuropharmacology, 2011, 61, 276–282; (d)
I. G. Kapitsa, E. V. Suslov, G. V. Teplov, D. V. Korchagina, N. I.
Komarova, K. P. Volcho, T. A. Voronina, A. I. Shevela and N. F.
Salakhutdinov, Pharm. Chem. J., 2012, 46, 263–265.
U. Sharma, P. K. Verma, N. Kumar and B. Singh, Adv. Synth.
Catal., 2012, 354, 870–878; (d) R. Kumar, E. Gravel, A. Hagège,
H. Li, D. Verma, I. N. N. Namboothiri and E. Doris,
ChemCatChem, 2013, 5, 3571–3575; (e) Y. Corre, W. Iali, M.
Hamdaoui, X. Trivelli, J.-P. Djukic, F. Agbossou-Niedercorn and
C. Michon, Catal. Sci. Technol., 2015, 5, 1452–1458; ( f ) R. J.
Maya, S. Poulose, J. John and R. L. Varma, Adv. Synth. Catal.,
2017, 359, 1177–1184.
7 For Reviews see: (a) D. Wang and D. Astruc, Chem. Rev.,
2015, 115(13), 6621–6686; (b) L. Alig, M. Fritz and S. Schneider,
Chem. Rev., 2019, 119, 2681–2751; (c) D. Wie and C. Darcel,
Chem. Rev., 2019, 119, 2550–2610.
13 (a) E. A. Artiukha, A. L. Nuzhdin, G. A. Bukhtiyarova, S. Y.
Zaytsev, P. E. Plyusnin, Y. V. Shubin and V. I. Bukhtiyarov,
Catal. Sci. Technol., 2015, 5, 4741–4745; (b) Q. Zhang, S.-S. Li,
M.-M. Zhu, Y.-M. Liu, H.-Y. He and Y. Cao, Green Chem.,
2016, 18, 2507–2513.
14 (a) B. S. Takale, S. M. Tao, X. Q. Yu, Z. J. Feng, T. Jin, M. Bao
and Y. Yamamoto, Org. Lett., 2014, 16, 2558–2561; (b) M.
Kumar, V. Bhatt, O. S. Nayal, S. Sharma, V. Kumar, M. S.
Thakur, N. Kumar, R. Bal, B. Singh and U. Sharma, Catal. Sci.
Technol., 2017, 7, 2857–2864.
15 (a) Y. S. Demidova, I. L. Simakova, M. Estrada, S. Beloshapkin,
E. V. Suslov, D. V. Korchagina, K. P. Volcho, N. F.
Salakhutdinov, A. V. Simakov and D. Y. Murzin, Appl. Catal., A,
2013, 464–465, 348–356; (b) Y. S. Demidova, E. V. Suslov, I. L.
Simakova, D. V. Korchagina, E. S. Mozhajcev, K. P. Volcho,
N. F. Salakhutdinov, A. Simakov and D. Y. Murzin, J. Mol.
Catal. A: Chem., 2017, 426, 60–67; (c) Y. S. Demidova, E. V.
Suslov, I. L. Simakova, K. P. Volcho, N. F. Salakhutdinov, A.
Simakov and D. Y. Murzin, Mol. Catal., 2017, 433, 414–419; (d)
Y. S. Demidova, E. V. Suslov, I. L. Simakova, D. V. Korchagina,
E. S. Mozhajcev, K. P. Volcho, N. F. Salakhutdinov, A. Simakov
and D. Y. Murzin, J. Catal., 2018, 360, 127–134.
16 (a) A. Stuetz, A. Georgopoulos, W. Granitzer, G. Petranyi and D.
Berney, J. Med. Chem., 1986, 29, 112–125; (b) R. K. Sharma, Y.
Monga, A. Puri and G. Gaba, Green Chem., 2013, 15, 2800–2809.
17 A. A. Voronov, K. A. Alekseeva, E. A. Ryzhkova, V. V. Zarubaev,
A. V. Galochkina, V. P. Zaytsev, M. S. Majik, S. G. Tilve, A. V.
Gurbanov and F. I. Zubkov, Tetrahedron Lett., 2018, 59,
1108–1111.
8 For recent Reports see: (a) S. Semwal and J. Choudhurry, ACS
Catal., 2016, 6, 2424–2428; (b) S. R. Flynn, O. J. Metters, I.
Manners and D. F. Wass, Organometallics, 2016, 35, 847–850;
(c) S. Semwal and J. Choudhurry, Angew. Chem., Int. Ed.,
2017, 56, 5556–5560; (d) M. Vellakkaran, K. Singh and D.
Banerjee, ACS Catal., 2017, 7, 8152–8158; (e) J. Chen, Z. Zhang,
B. Li, F. Li, Y. Wang, M. Zhao, I. D. Gridnev, T. Imamoto and
W. Zhang, Nat. Commun., 2018, 9, 5000; ( f ) H. Bauer, M.
Alonso, C. Färber, H. Elsen, J. Pahl, A. Causero, G. Ballman,
F. D. Proft and S. Haredr, Nat. Catal., 2018, 1, 40–47; (g) Y. Liu,
F. Chen, Y.-M. He, C. Li and Q.-H. Fan, Org. Biomol. Chem.,
2019, 17, 5099–5105.
9 (a) H. Li, Z. Dong, P. Wang, F. Zhang and J. Ma, React. Kinet.,
Mech. Catal., 2013, 108, 107–115; (b) M. Nasrollahzadeh, New J.
Chem., 2014, 38, 5544–5550; (c) T. Stemmler, F. A. Westerhaus,
A.-E. Surkus, M.-M. Pohl, K. Junge and M. Beller, Green Chem.,
2014, 16, 4535–4540; (d) N. M. Patil and B. M. Bhanage, Catal.
Today, 2015, 247, 182–189; (e) M. Bhardwaj, H. Sharma, S. Paul
and J. H. Clark, New J. Chem., 2016, 40, 4952–4961; ( f ) R. J.
Kalbasi and O. Mazaheri, New J. Chem., 2016, 40, 9627–9637;
(g) P. Ji, K. Manna, Z. Lin, A. Urban, F. X. Greene, G. Lan and
W. Lin, J. Am. Chem. Soc., 2016, 138, 12234–12242; (h) L. Jiang,
P. Zhou, Z. Zhang, Q. Chi and S. Jin, New J. Chem., 2017, 41,
11991–11997; (i) L. Jiang, P. Zhou, Z. Zhang, S. Jin and Q. Chi,
Ind. Eng. Chem. Res., 2017, 56, 12556–12565; ( j) H. Sharma, M.
Bhardwaj, M. Kour and S. Paul, Mol. Catal., 2017, 435, 58–68;
(k) M. N. Shaikh, M. A. Aziz, A. N. Kalanthoden, A. Helal, A. S.
Hakeem and M. Bououdina, Catal. Sci. Technol., 2018, 8,
4709–4717.
18 B. Kovács, R. Savela, K. Honkala, D. Y. Murzin, E. Forró, F.
Fülöp and R. Leino, ChemCatChem, 2018, 10, 2893–2899.
19 N. D. Shcherban, P. Mäki-Arvela, A. Aho, S. A. Sergiienko, P. S.
Yaremov, K. Eränen and D. Y. Murzin, Catal. Sci. Technol.,
2018, 8, 2928–2937.
20 J. Wu, W. Hua, Y. Yue and Z. Gao, Acta Phys.-Chim. Sin.,
2020, 36, 1904066(1–7).
21 H. Bernas, I. Simakova, P. Mäki-Arvela, I. P. Prosvirin, R. Leino
and D. Y. Murzin, Catal. Lett., 2012, 142, 690–697.
10 (a) J.-H. Xie, S.-F. Zhu and Q.-L. Zhou, Chem. Rev., 2011, 111,
1713–1760; (b) Z. Zhang, N. A. Butt and W. Zhang, Chem. Rev.,
2016, 116, 14769–14827; (c) C. S. G. Seo and R. H. Morris,
Organometallics, 2019, 38, 47–65.
11 (a) C. A. Willoughby and S. L. Buchwald, J. Am. Chem. Soc.,
1994, 116, 8952–8965; (b) Y. Misumi, H. Seino and Y. Mizobe,
J. Am. Chem. Soc., 2009, 131, 14636–14637; (c) H. Seino, Y.
Misumi, Y. Hojo and Y. Mizobe, Dalton Trans., 2010, 39,
3072–3082; (d) Q. Hu, Y. Hu, Y. Liu, Z. Zhang, Y. Liu and W.
Zhang, Chem. – Eur. J., 2017, 23, 1040–1043; (e) Z. Yang, Z.
Ding, F. Chen, Y.-M. He, N. Yang and Q.-H. Fan, Eur. J. Org.
Chem., 2017, 1973–1977; ( f ) Y.-N. Duan, X. Du, Z. Cui, Y. Zeng,
Y. Liu, T. Yang, J. Wen and X. Zhang, J. Am. Chem. Soc.,
2019, 141, 20424–20433.
22 I. L. Simakova, Y. S. Demidova, A. V. Simakov and D. Y.
Murzin, J. Sib. Fed. Univ., Chem., 2019, 12, 536–549.
23 V. N. Panchenko, E. A. Paukshtis, D. Y. Murzin and I. L.
Simakova, Ind. Eng. Chem. Res., 2017, 56, 13310–13321.
24 D. Y. Murzin, Catal. Sci. Technol., 2016, 6, 5700–5713.
25 E. Toukoniitty, P. Mäki-Arvela, J. Kuusisto, V. Nieminen, J.
Päivarinta, M. Hotokka, T. Salmi and D. Y. Murzin, J. Mol.
Catal. A: Chem., 2003, 192, 135–151.
12 (a) P. D. Pham, P. Bertus and S. Legoupy, Chem. Commun.,
2009, 6207–6209; (b) V. Kumar, S. Sharma, U. Sharma, B. Singh
and N. Kumar, Green Chem., 2012, 14, 3410–3414; (c) V. Kumar,
26 R. L. Augustine, Heterogeneous Catalysis for the Synthetic
Chemist, Marcel Dekker, New York, 1996.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
27 J. Barrault, A. Derouault, G. Courtois, J. M. Maissaint, J. C.
Dupin, C. Guimon, H. Martinez and E. Dumitriu, Appl. Catal.,
A, 2004, 262, 43–51.
28 (a) H. Hagiwara, J. Hamaya, T. Hoshi and C. Yokoyama,
Tetrahedron Lett., 2005, 46, 393–395; (b) K. A. Ostrowski, D.
Lichte, M. Stuck and A. J. Vorholt, Tetrahedron, 2016, 72,
592–598.
Linsvad, V. Haikola, H. A. Hansen, H. H. Kristoffersen, M.
Kuisma, A. H. Larsen, L. Lehtovaara, M. Ljungberg, O. Lopez-
Acevedo, P. G. Moses, J. Ojanen, T. Olsen, V. Petzold, N. A.
Romero, J. Stausholm-Møller, M. Strange, G. A. Tritsaris, M.
Vanin, M. Walter, B. Hammer, H. Häkkinen, G. K. H. Madsen,
R. M. Nieminen, J. K. Nørskov, M. Puska, T. T. Rantala, J.
Schiøtz, K. S. Thygesen and K. W. Jacobsen, J. Phys.: Condens.
Matter, 2010, 22, 253202; (c) A. H. Larsen, J. J. Mortensen, J.
Blomqvist, I. E. Castelli, R. Christensen, M. Dułak, J. Friis,
M. N. Groves, B. Hammer, C. Hargus, E. D. Hermes, P. C.
Jennigs, P. B. Jensen, J. Kermode, J. R. Kitchin, E. L. Kolsbjerg,
J. Kubal, K. Kaasberg, S. Lysgaard, J. B. Maronsson, T. Maxson,
T. Olsen, L. Pastewka, A. Peterson, C. Rostgaard, J. Schiøtz, O.
Schütt, M. Strange, K. S. Thygensen, T. Vegge, L. Vilhelmsen,
M. Walter, Z. Zeng and K. W. Jacobsen, J. Phys.: Condens.
Matter, 2017, 29, 273002.
29 S. Tuokko, P. M. Pihko and K. Honkala, Angew. Chem., Int. Ed.,
2016, 55, 1670–1674.
30 T. Wang, J. Ibañez, K. Wang, L. Fang, M. Sabbe, C. Michel, S.
Paul, M. Pera-Titus and P. Sautet, Nat. Catal., 2019, 2, 773–779.
31 H. Ghafuri and M. Roshani, RSC Adv., 2014, 4, 58280–58286.
32 G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb, A.
Nudelman, B. M. Stoltz, J. E. Bercaw and K. I. Goldberg,
Organometallics, 2010, 29, 2176–2179.
33 G. Perez, S. Fuentes, V. Petranovskii and A. Simakov, Catal.
Lett., 2006, 110, 53–60.
34 S. G. Gregg and K. S. W. Sing, Adsorption, surface area and
porosity, Acad. press, New York, 1982.
38 (a) M. Dion, H. Rydberg, E. Schröder, D. C. Langreth and B. I.
Lundqvist, Phys. Rev. Lett., 2004, 92, 246401; (b) J. Wellendorff,
K. T. Lundgaard, A. Møgelhøj, V. Petzold, D. D. Landis, J. K.
Nørskov, T. Bligaard and K. W. Jacobsen, Phys. Rev. B, 2012, 85,
235149.
35 E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc.,
1951, 73, 373–380.
36 G. Horváth and K. Kawazoe, J. Chem. Eng. Jpn., 1983, 16,
470–475.
37 (a) J. J. Mortensen, L. B. Hansen and K. W. Jacobsen, Phys. Rev.
B, 2005, 71, 035109; (b) J. Enkovaara, C. Rostgaard, J. J.
Mortensen, J. Chen, M. Dulak, L. Ferrighi, J. Gavnholt, C.
39 A. Held and M. Walter, J. Chem. Phys., 2014, 141, 174108.
40 (a) G. Henkelman, B. P. Uberuaga and H. Jónsson, J. Chem.
Phys., 2000, 113, 9901–9904; (b) G. Henkelman and H. Jónsson,
J. Chem. Phys., 2000, 113, 9978–9985.
41 A. A. Peterson, Top. Catal., 2014, 57, 40–53.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020