Gas phase transfer hydrogenation of α, β- unsaturated carbonyl compounds into saturated carbonyl compounds over supported Cu catalysts

Article abstract of DOI:10.1016/j.mcat.2019.110686

This work aims to produce hydrocinnamaldehyde via selective C[dbnd]C bond hydrogenation of cinnamaldehyde via transfer hydrogenation catalyzed by supported Cu catalysts. The catalytic activity and characterization results demonstrated that conversion of cinnamaldehyde and selectivity to hydrocinnamaldehyde is an integrated result of active Cu metal surface area and nature of support material. In the case of Cu/SiO₂ catalysts, the rate of formation of hydrocinnamaldehyde was linearly dependent on the active Cu metal surface area. A maximum rate of formation of hydrocinnamaldehyde 167.82 μmol.g^?1.s^-1 was obtained at an active Cu metal surface area of 6.1 m².g_cat^-1 over 20 wt. % Cu/SiO₂ catalyst. While in the case of other supported Cu catalysts (Cu/MgO, Cu/ZrO₂ and Cu/γ-Al₂O₃) along with the active Cu metal surface area, the surface acidity and basicity governed the selectivity of hydrocinnamaldehyde. The rate of formation of hydrocinnamaldehyde followed the trend Cu/SiO₂ >Cu/Al₂O₃ >Cu/MgO > Cu/ZrO₂. Unlike Cu/SiO₂ catalyst, the competitive adsorption of C[dbnd]C and CO[dbnd] bonds over these (Al₂O₃, MgO and ZrO₂) supported Cu catalysts altered the selectivity of hydrocinnamaldehyde. Cu/SiO₂ catalyst was superior in selective C[dbnd]C hydrogenation of α, β-unsaturated carbonyl compounds via transfer hydrogenation. Cyclohexenone, isophorone and crotonaldehyde were selectively converted into their saturated carbonyl compounds.

Full text of DOI:10.1016/j.mcat.2019.110686

Molecular Catalysis xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Gas phase transfer hydrogenation of α, β- unsaturated carbonyl compounds
into saturated carbonyl compounds over supported Cu catalysts
Saidulu Reddy Ka,b, Siva Sankar Enumula *, Kumara Swamy Koppadi ,
b,
b
b
b,
Seetha Rama Rao Kamaraju , David Raju Burri *
CSIR-Academy of Scientific and Innovative Research (CSIR-AcSIR), New Delhi, India
a
b
Catalysis & Fine Chemicals Department, CSIR-Indian Institute of Chemical Technology, Hyderabad, 500007, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
α, β- unsaturated carbonyl compound
Cinnamaldehyde
This work aims to produce hydrocinnamaldehyde via selective C]C bond hydrogenation of cinnamaldehyde via
transfer hydrogenation catalyzed by supported Cu catalysts. The catalytic activity and characterization results
demonstrated that conversion of cinnamaldehyde and selectivity to hydrocinnamaldehyde is an integrated result
Hydrocinnamaldehyde
of active Cu metal surface area and nature of support material. In the case of Cu/SiO
2
catalysts, the rate of
Cu/SiO
2
formation of hydrocinnamaldehyde was linearly dependent on the active Cu metal surface area. A maximum rate
Hydrogenation
−
1 -1
of formation of hydrocinnamaldehyde 167.82 μmol.g .s was obtained at an active Cu metal surface area of
2
-1
6
.1 m .gcat over 20 wt. % Cu/SiO
ZrO and Cu/γ-Al ) along with the active Cu metal surface area, the surface acidity and basicity governed the
selectivity of hydrocinnamaldehyde. The rate of formation of hydrocinnamaldehyde followed the trend Cu/
SiO > Cu/Al > Cu/MgO > Cu/ZrO . Unlike Cu/SiO catalyst, the competitive adsorption of C]C and
CO] bonds over these (Al , MgO and ZrO ) supported Cu catalysts altered the selectivity of hydro-
cinnamaldehyde. Cu/SiO
2
catalyst. While in the case of other supported Cu catalysts (Cu/MgO, Cu/
2
2 3
O
2
2
O
3
2
2
2
O
3
2
2
catalyst was superior in selective C]C hydrogenation of α, β-unsaturated carbonyl
compounds via transfer hydrogenation. Cyclohexenone, isophorone and crotonaldehyde were selectively con-
verted into their saturated carbonyl compounds.
Introduction
4
assisted by NaBH for hydrogenation of CALD to HCALD [7]. Cuong
et al. reported that the Pd supported on carbon nano fibers exhibited
better activity in HCALD synthesis than Pd/Charcoal due to the absence
of microporosity in carbon nanofibers [8]. Later, the same group re-
ported deposition of Pd nanoparticles on multi walled carbon nano
tubes (MWCNT) and proposed that strong metal support interactions
and absence of micropores are responsible for the higher selectivity to
C]C bond [9]. Recently, Pedro et al. have examined Pd decorated
composite material of MWCNT and activated carbon for selective hy-
The chemo selective hydrogenation of α, β-unsaturated carbonyls to
saturated carbonyls such as cinnamaldehyde (CALD) to hydro-
cinnamaldehyde (HCALD) is an imperative transformation in both the
academic and industrial research. Saturated carbonyl compounds are
versatile chemicals for biological applications. HCALD is a key chemical
intermediate in synthesis of drugs for AIDS and in other pharmaceu-
ticals [1–5]. Being a multifunctional molecule, selective hydrogenation
of CALD to a particular product is quite complex [6]. Most of the hy-
drogenations are targeted for the saturation of C]O rather than the
hydrogenation of thermodynamically favorable CC] bond. However,
the selective hydrogenation of C]C in α, β-unsaturated carbonyls can
only produce saturated carbonyls from unsaturated ones. Some of the
researchers have been reported the selective hydrogenation of C]C
bond of CALD to produce HCALD over noble metal and/or bimetallic
catalytic systems. Among the noble metals, Pd is the most active one in
selective hydrogenation of C]C of α, β-unsaturated carbonyls. Zhang
2
drogenation of CALD under pressurized H conditions [10]. Pd sup-
ported on modified carbon such as N-doped mesoporous and hier-
archically porous carbon have resulted much more selectivity towards
HCALD than their activated carbon catalysts [11,12]. Other supported
Pd catalysts like Pd/γ-Al
2 3 2 2 3
O [13], Pd/C [14], Pd/La O CO [15], Pd/
PPh -PEGPIILP [16] and Pd/ZIF-8 [17] also have successfully estab-
2
lished for hydrogenation of CALD to HCALD. In addition, Pt based
catalysts are also active for synthesis of HCALD from CALD. The ma-
jority of Pt catalytic systems are bimetallic catalytic systems which are
much more selective than their respective monometallic catalytic
2 2 3
et al. developed homogeneous PdCl –0.5Co(OAc) –PPh catalyst
⁎
Corresponding authors.
E-mail addresses: sivasankar.enumula@gmail.com (S.S. Enumula), david@iict.res.in (D.R. Burri).
https://doi.org/10.1016/j.mcat.2019.110686
Received 24 May 2019; Received in revised form 18 October 2019; Accepted 18 October 2019
2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Saidulu Reddy K, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110686

S.R. K, et al.
Molecular Catalysis xxx (xxxx) xxxx
systems. For instance, Pt–Ni/Carbon nanotubes [18], Pt-Ni/ZrO
Pt-Au/SiO [20] and Pt-Ru/Carbon nanofibers [21] are more selective
2
than their corresponding monometallic counterparts. Apart from noble
metal catalysts, Ni is also selective for C]C hydrogenation in CAL-
2
[19],
The content of present study involves investigation on the role of sup-
port in supported Cu catalysts with wide range of acidic and basic
oxides. Secondly correlation of Cu active metal surface area with the
catalytic activity and examination of this protocol on other α, β-un-
saturated carbonyls. Besides these, the reaction parameters such as
molar ratio of CALD to isopropanol, reaction temperature, WHSV
(Weight Hourly Space Velocity) etc. are optimized.
D.Linet.al. reported the promotional effect of Ir in Ni/TiO
hydrogenation of CALD to HCALD at 20 bar H pressure [22]. NiP/SiO
23], Ni/AC [24] catalysts are successfully optimized for the HCALD
2
for selective
2
2
[
2
synthesis under pressurized H in batch processes. Some of the re-
searchers have investigated bimetallic Cu catalysts for hydrogenation of
CALD. Ni-Cu/SiO [25], Cu-Mg-Al [26], Au–Cu/CeO [27] catalysts are
Experimental
2
2
found to be selective for synthesis of HCALD. Although afore mentioned
catalytic systems are selective for HCALD synthesis, there are difficul-
ties to be eliminated. Most of them are cost expensive noble metal
catalytic systems, operated at harsh reaction conditions (pressurized
Catalyst preparation
SiO
M/s. Sudchemi, India. Cu/SiO
0, 20 and 30 wt %) were prepared by wet impregnation method using
Cu (NO . 3H O (M/s. Sigma Aldrich USA) as Cu precursor. In a typical
method, an aqueous solution containing requisite amount of Cu pre-
cursor was added to SiO and the mixture was allowed to dry with
2
is purchased from M/s. Sigma Aldrich USA and γ-Al
2 3
O from
2
catalysts with various loadings of Cu (5,
2
with H ) and in batch processes need of further steps to separate pro-
1
duct and catalyst. Hence, there is a room for exploration of non-noble
metal catalytic systems for selective hydrogenation of CALD to HCALD
in continuous process under transfer hydrogenation conditions.
It is reported that Cu based catalytic systems are active in trans-
ferring hydrogen from alcohols to carbonyls. Recently, María et al.
stated that Cu-Mg-Al is effectively hydrogenated furfural to furfuryl
alcohol via transfer of hydrogen from isopropanol [28]. Long et al. in-
vestigated role of support in the catalytic transfer hydrogenation over
3
)
2
2
2
continuous stirring. The solid was dried in hot oven at 100 °C for 12 h
and then it was calcined in static air at 450 °C for 4 h. Some portion of
the solid samples was reduced in H
samples were labeled as xCu/SiO
2
flow at 280 °C for 3 h. The resultant
where x represents weight percen-
2
tage of Cu. In similar fashion, 20Cu/MgO, 20Cu/ZrO
Al were prepared. MgO and ZrO were prepared by precipitation
O and ZrOCl .8H O as precursors re-
2
and 20Cu/γ-
2 3
Cu/Al O and reported that amount and acid/base properties influence
2
O
3
2
the dehydrogenation of alcohol [29]. Earlier our group has patented the
coupling of endothermic dehydrogenation of Cyclohexanol with exo-
thermic hydrogenation of furfural in a hydrogen economy approach
over supported Cu catalysts in continuous process [30,31]. Cu is highly
active metal to dehydrogenate alcohol and subsequent transfer of the
hydrogen to unsaturated group in other molecule [32,33]. With these
preliminary understandings and on continuation of our research work
in selective hydrogenation of α, β-unsaturated carbonyls [34,35], the
present work has been undertaken to develop a non-noble metal Cu
based catalytic system for selective hydrogenation of CALD to HCALD
via transfer hydrogenation in continuous process (Scheme 1). There are
reports on the transfer hydrogenation of CALD over Zr based and other
metal oxide catalytic systems. But they produce cinnamyl alcohol
through C]O reduction via Meerwein–Ponndorf–Verley reduction
mechanism without disturbing C]C of CALD [36,37]. To the best of
our knowledge, still there are no reports regarding synthesis of HCALD
from CALD via transfer hydrogenation of C]C in continuous process.
method using Mg (NO
spectively.
3
)
2
6H
2
2
2
Catalyst characterization
The X-ray diffraction patterns were recorded on an UltimaIV (M/s.
Rigaku Corporation, Japan) operated at 40 kV and 40 mA equipped
with nickel-filtered Cu K radiation (λ=1.54056 Å). The detailed cata-
lyst characterization procedures are presented in Electron supplemen-
tary Information.
Activity test
The chemo-selective hydrogenation of CALD was performed in a
fixed bed down flow reactor (14 mm id 300 mm length) at atmospheric
conditions. In a typical reaction, 0.5 g of the catalyst was mixed with
equal amount of quartz particles and it was secured between two quartz
Scheme 1. Hydrogenation of cinnamaldehyde.
2

S.R. K, et al.
Molecular Catalysis xxx (xxxx) xxxx
plug wool. Prior to the reaction, the catalyst was reduced in flow of H
2
(
30 mL/min) at 280 °C for 3 h. Then the reaction temperature was fixed
2
and the carrier flow is switched to N (30 mL/min). The liquid feed with
required mole ratio of CALD and isopropanol by syringe feed pump (M/
s. B. Braun, Germany). The product mixture is collected in ice cold trap
and analyzed by FID equipped GC-7820 A (M/s. Agilent Technologies)
with HP–5 capillary column (30 m ×0.53 mm ×5.0 μm) and confirmed
by GC–MS, QP-2010 (M/s. Shimadzu Instruments, Japan) with EB–5
MS capillary column (30 m ×0.25 mm ×0.25 μm).
Results and discussion
2 2
In order to choose the optimum copper loading over SiO , Cu/SiO
catalysts were subjected to chemo selective hydrogenation of CALD to
HCALD and the results were presented in Fig. ES 1. At 5 wt% of copper
loading the conversion of CALD is 60% and selectivity to HCALD is
100%. With increase in copper loading the conversion of CALD is also
increased and complete conversion of CALD is obtained at 20 wt%
copper loading. Beyond this copper loading, the conversion of CALD is
declined. The decline in conversion of CALD at copper loadings higher
than 20 wt% is may be due to the increase in copper particles size in
Fig. 1. Powder XRD patterns of reduced supported Cu catalysts.
peaks at 2θ values of 24.2, 28.1, 31.4, 34.2 and 55.3° indicate the
presence of monoclinic ZrO phase (JCPDS- 37–1484). The diffraction
peaks at 2θ values of 46.1 and 66.8° corresponds to the crystalline
phase of γ-Al . Fig.ES2 shows the powder XRD patterns of reduced
Cu/SiO catalysts. In all the catalysts the broad diffraction signal at
3
0Cu/SiO
HCALD in all the cases is 100%. This demonstrates the excellence of
Cu/SiO catalysts in H -free chemo selective hydrogenation of CALD to
2
catalyst. It is interesting to mention that the selectivity to
2
2
2
2 3
O
HCALD. The chemo selective hydrogenation of CALD to HCALD was
examined over various supported Cu catalysts and results were shown
in Table 1. The schematic representation of hydrogenation of CALD is
2
around 2θ of 22° represents the amorphous silica. The diffraction peaks
at around 2θ of 43.5, 50.5 and 74.3° indicates the presence of metallic
Cu in agreement with JCPDS-0483.
The reducibility of copper oxide species in calcined supported
copper catalysts was investigated by temperature programmed reduc-
shown in Scheme 1. The catalytic activity of various Cu/SiO
was presented and discussed in Electronic Supplementary Information.
Among the supported Cu catalysts, 20Cu/SiO exhibited better catalytic
2
catalysts
2
performance with respect to CALD conversion and HCALD selectivity.
The conversion of CALD in all the supported Cu catalysts is above 70%,
tion (H
results were presented in Fig. 2. Except 20Cu/SiO
supported Cu catalysts show a single stage reduction of CuO species.
0Cu/ZrO , 20Cu/MgO and 20Cu/Al catalysts show reduction of
CuO species at Tmax in between 288–329 °C. It represents a single stage
2
-TPR) with H
2
/Ar mixture gas a probe gas and corresponding
2
catalyst, other
but a diverse product distribution is noticed except with 20Cu/SiO
catalyst. 20Cu/MgO, 20Cu/ZrO and 20Cu/Al catalysts showed
2
2
2 3
O
2
2
2 3
O
moderate selectivity of HCALD 53.51, 43.46 and 61.17% respectively.
In these catalysts, other products such as cinnamyl alcohol (CAOL),
allyl benzene (AB), propyl benzene (PB) and others are formed at the
expense of HCALD selectivity. The rate of formation of HCALD is higher
(0)
reduction of CuO species to Cu
2 2
i.e., CuO + H →Cu + H O. For
2
0Cu/MgO catalyst, the shift in reduction temperature to higher values
may be attributed to increased metal support interactions between Cu
and MgO through defect sites present in MgO [38]. The low tempera-
−
1 −1
(
167.82 μmol.g .s ) with 20Cu/SiO
catalysts. The rate of formation of HCALD over these supported cata-
lysts is in the order: 20Cu/SiO > 20Cu/Al > 20Cu/MgO > 20Cu/
ZrO . The results show discrepancy in the catalytic performance of
2
catalyst than other supported
ture reduction of copper oxide in 20Cu/SiO
represents the high reducibility of copper oxide over SiO
supports [39]. H -TPR profile of 20Cu/SiO catalyst shows two reduc-
2
and 20Cu/Al
2
O
3
catalysts
2
2 3
O
2
and Al O
2 3
2
2
2
supported Cu catalysts is presumably due to the unified effects of active
Cu metal dispersion and nature of support.
tion peaks at Tmax of 210 °C and 289 °C. The low reduction temperature
peak is attributed to well-dispersed CuO particles and the high reduc-
tion temperature peak is assigned to presence of bulk CuO species
The powder XRD patterns of reduced samples of supported Cu cat-
alysts were presented in Fig. 1. It could be observed from the Fig.1; all
the XRD patterns show diffraction peaks at 2θ values of 43.5, 50.6 and
[
40,41]. The reducible features of calcined SiO
2
supported catalysts are
evaluated by H -TPR and corresponding profiles were presented in Fig.
2
74.3° manifest the presence of metallic Cu which is in agreement with
ES3. All the silica supported copper catalysts shows two stage reduction
JCPDS-04836. No other Cu phases were observed from the XRD ana-
lysis. The average crystallite sizes of metallic Cu was estimated by
Scherer’s equation and presented in Table 1. For 20Cu/MgO sample, the
diffraction peaks at 2θ values of 36.8, 42.7, 62.2 and 78.5° confirm the
presence of MgO crystalline phase (JCPDS No. 4-829). The diffraction
(0)
of CuO species to Cu . It could be observed that there is a slight shift
in the first Tmax towards high temperature. It indicates the formation
large CuO crystallites with increase in copper loading. The physico-
chemical characteristics of the supported Cu catalysts were reported in
Table 1
Hydrogenation of CALD to HCALD over supported Cu catalysts.
HCALD μmol. g 1.s−1
−
TOF HCALD (s−1
)
Catalyst
Con. (%) CALD
Selectivity (%) HCALD CAOL AB PB Others
r
2
2
2
0Cu/MgO
0Cu/ZrO
0Cu/SiO
0Cu/Al
89
80
100
84
97
53.51
43.46
100
61.17
13.62
28.38
6.65
–
7.13
80.23
3.32
–
–
5.70
2.27
–
14.79
–
–
26.0
79.92
58.34
167.82
86.23
0.86
0.51
1.13
0.70
2
49.89
–
–
2.86
2
2
2
O
3
a
2
0Cu/SiO
2
Reaction Conditions: Catalyst amount: 500 mg, WHSV: 1 h−1, Mole ratio: 1:20 (CALD: Isopropanol), N
molecular H Reaction temperature: 260 °C, 1 atmospheric pressure. CALD-Cinnamaldehyde, HCALD-Hydrocinnamaldehyde, CAOL-Cinnamyl alcohol, AB-
Allylbenzene, PB-Propylbenzene, Others- Benzaldehyde, benzylbenzoate.
flow: 30 mL/min, Reaction temperature: 260 °C. a with
2
2
3

S.R. K, et al.
Molecular Catalysis xxx (xxxx) xxxx
Fig. 3. Relationship between active Cu metal surface area (SCu) and rate of
formation of HCALD with copper loading over SiO .
2
2
Fig. 2. H -TPR profiles of supported Cu catalysts.
Table 2.
The physico chemical features of Cu/SiO
in Table ES1. BET-surface areas of Cu/SiO
2
catalysts were presented
catalysts are decreasing
2
with increase in copper loading. In the same trend the dispersion of
copper metal is decreased. The active copper metal surface area is in-
creased with copper loading up to 20 wt% and reaches a maximum of
2
−1
6
.1 m .g.cat . Further increase in copper loading to 30 wt% results
decrease in active copper metal surface area may be due to the increase
in the copper particles size. A correlation between the active Cu metal
surface area (SCu) and rate of formation of HCALD with copper loading
was made and presented in Fig. 3. The rate of formation of HCALD is
Fig. 4. A correlation between active Cu metal surface area (SCu) and rate of
formation of HCALD over supported Cu catalysts.
strongly influenced by the active Cu metal surface area of the Cu/SiO
catalysts. maximum rate of formation of HCALD
2
A
−
1 −1
(
6
167.82 μmol.g .s ) is obtained at an active Cu metal surface area of
2
−1
2
−1
.1 m .g.cat . The active Cu metal surface area and rate of formation
SiO
2
(4.7 m . gcat ) (Fig.ES4) have similar active metal surface areas but
−1 −1
of HCALD are linearly dependent on copper loading. This trend is fol-
lowed up to 20 wt% copper loading on SiO . At 30 wt% of copper
the rate of formation of HCALD over 10Cu/SiO (123.37 μmol.g .s
)
2
2
is
nearly
two
fold
higher
than
20Cu/ZrO
2
catalyst
−1 −1
loading both the active Cu metal surface area and rate of formation of
HCALD show down fall due to increase in copper particle sizes.
The active Cu metal surface area of various supported Cu catalysts
was correlated with their catalytic activity and presented in Fig. 4.
(58.34 μmol.g .s ). These results suggested that the chemo selective
hydrogenation of CALD to HCALD over supported Cu catalysts is not
only dependent on the active copper metal surface of the catalysts but
also the nature of support which is responsible for the selectivity of the
product.
2
2
0Cu/SiO catalyst exhibits the highest active metal surface area
2
−1
(
(
6.1 m . gcat
)
and the highest rate of formation of HCALD
The acidity of the supported copper catalysts was evaluated by
167.82 μmol.g .s−1) in the series. 20Cu/ZrO
−
1
exhibits the lowest rate
2
temperature programmed desorption of ammonia (NH -TPD) which
3
of formation of HCLAD and 20Cu/MgO exhibits the lowest active metal
surface area in the series. The rate of formation of a product over a
catalyst represents the activity of the all the active sites present in that
allows to assess strength and amount of surface acidic sites presented in
the catalysts and the corresponding profiles were depicted in Fig. 5.
Based on the temperature at which adsorbed NH
desorb, the surface acidic sites are organized as weak (< 300 °C),
moderate (300–500 °C) and strong (> 500 °C) [42]. 20Cu/Al cata-
lyst shows a broad desorption peak in NH -TPD profile represents
3
molecules tend to
catalyst. 20Cu/SiO
2
catalyst with more number of active sites possesses
high copper metal surface area which in turn showed high rate of for-
2 3
O
mation. However, 20Cu/ZrO
MgO even though 20Cu/ZrO
2
catalyst exhibited lower rate than 20Cu/
has higher active metal surface area than
3
2
2 3
presence of wide range of acidic sites. It indicates 20Cu/Al O catalyst
possesses a majority of weak acidic sites along with moderate and
2
−1
20Cu/MgO catalyst. In addition, 20Cu/ZrO
2
(4.6 m . gcat ) and 10Cu/
Table 2
Physico-chemical characteristics of supported Cu catalysts.
ICP-AES (Wt aSBET (m2/
bD (%)
c
S
Cu (m2/
cat
d
A
Cu (m2/
)
e
Cu(0)
f
Cu(0)
gH
(mmol/g)
uptake
hAcidity (mmolNH
g)
iBasicity (mmol CO
g)
Catalyst
2
3
/
2
/
%)
g)
g
)
g
Cu
(nm)
(nm)
2
2
2
2
0Cu/SiO
0Cu/ZrO
0Cu/MgO
0Cu/Al
2
19.2
19.4
18.9
19.6
198
26
32
4.7
3.5
2.9
3.9
6.1
4.6
3.8
5.1
30.8
23.2
19.3
25.4
21.8
28.9
34.8
26.4
24.2
31.5
35.2
28.5
0.87
0.36
0.49
0.61
0.31
0.77
0.21
0.98
–
2
0.24
0.82
0.32
2
O
3
154
a
gas adsorption, bDispersion of Cu, Cu metal surface area, Cu metal surface area per gram of Cu, determined from N
c
d
e
BET surface area determined from N
2
2
O pulse
f
g
h
i
chemisorption, determined from XRD, From H
2
-TPR, From NH
3
-TPD, CO
2
-TPD.
4

S.R. K, et al.
Molecular Catalysis xxx (xxxx) xxxx
Fig. 7. Influence of support acidity/basicity on rate of formation of HCALD.
The nature of support in supported metal catalysts is influential in
determining the catalytic activity and selectivity. The catalytic activity
and selectivity of the supported Cu catalysts in chemo selective hy-
drogenation of CALD to HCALD is correlated with the surface acidity
and basicity of the catalysts and presented in Fig. 7. All the supported
Cu catalysts exhibit more than 80% conversion of CALD irrespective of
surface acidity and basicity but a diverse selectivity pattern of HCALD is
noticed. It clearly demonstrates the influence of surface acidic and basic
sites on product selectivity. CALD consists of two unsaturated func-
tional groups C]C and CO] so the hydrogenation is feasible in either
of the bonds or both also (as shown in Scheme 1). The surface acidity
and basicity directs the mode of adsorption of CALD over supported Cu
3
Fig. 5. NH -TPD profiles of supported Cu catalysts.
strong acidic sites. Despite of MgO is basic 20Cu/MgO catalyst pos-
sesses a little amount of moderate acidity. 20Cu/ZrO catalyst shows
2
two desorption peaks at 464 °C and 582 °C. The small desorption peak
at 464 °C represents small amount of moderate acidic sites and the high
temperature desorption peak at 582 °C demonstrates existence of major
amount of strong acidic sites in 20Cu/ZrO
shows a desorption peak in temperature region of 200–400 °C which
indicates 20Cu/SiO catalyst contains weak acidic sites.
The basicity of supported copper catalysts is investigated tempera-
ture programmed desorption of carbon dioxide (CO -TPD) and the re-
spective profiles were presented in Fig. 6.The CO -TPD profile of 20Cu/
Al catalyst exhibits desorption peaks in temperature region
50–380 °C indicating weak and moderate basic sites. 20Cu/MgO cat-
2 2
catalyst. 20Cu/SiO catalyst
2
2
catalysts. 20Cu/SiO catalyst with a little amount of weak acidic sites,
2
CALD is adsorbed in a direction to reduce only C]C bond without
disturbing CO] of CALD which results selective formation of HCALD.
While in other supported Cu catalysts having basicity and acidity there
is a competitive adsorption between C]O and CC] bonds. During this
competition, exclusive hydrogenation of C]C bond forms HCALD, ex-
clusive hydrogenation of C]O forms CAOL and hydrogenation of both
bonds forms hydrocinnamyl alcohol (HCAOL). MgO supported Cu cat-
alyst with basic sites exhibits 53.51 and 28.38% selectivity’s to HCALD
and CAOL respectively. Along with these products, allyl benzene (AB)
and other condensation products are also formed. AB is dehydration
product of HCAOL (as shown in Scheme 1) which supports the forma-
2
2 3
O
1
alyst is displaying a major desorption peaks at 328 °C along with other
small desorption peaks. 20Cu/MgO catalyst contains all the three types
of basic sites with high population of weak basic sites. There is no
significant desorption peak is observed for 20Cu/SiO
CO -TPD profile of 20Cu/ZrO catalyst exhibits a desorption peak at
high temperature ∼588 °C which represents existence of strong basic
sites in 20Cu/ZrO catalyst. The number of basic sites is higher in the
case of 20Cu/MgO catalyst compared to other catalysts.
2
catalyst. The
2
2
2
tion of HCAOL by hydrogenation of both the bonds. ZrO supported Cu
2
catalyst has both strong acidic and basic sites. 20Cu/ZrO
hibits 43.46, 6.65 and 49.89% selectivity’s to HCALD, CAOL and propyl
benzene (PB) respectively. 20Cu/ZrO promotes hydrogenation of C]O
in equal proportion to C]C bond and formed CAOL is further hydro-
genated to HCAOL. The presence of strong acidic sites in 20Cu/ZrO
favors hydrogenolysis of HCAOL to propyl benzene (PB). Al sup-
2
catalyst ex-
2
2
2 3
O
ported Cu catalyst has both weak acidic and basic sites also show a drop
in selectivity to HCALD. From these results it could be supposed that the
selectivity to HCALD via chemo selective transfer hydrogenation is
strongly dependent on nature of support in supported Cu catalysts.
The influence of reaction temperature on chemo selective transfer
hydrogenation of CALD to HCALD is examined over 20Cu/SiO catalyst
2
and the results were presented in Fig. 8. The study was done at tem-
perature ranging from 260 °C (near to the boiling point of CALD) to
305 °C. The formation of diverse products at different reaction tem-
peratures witnessed the influence of reaction temperature on the titled
reaction. At a reaction temperature of 260 °C, complete conversion of
CALD is obtained with 100% selectivity to HCALD. With increase in
reaction temperature to 275 °C, complete conversion of CALD is
achieved while the selectivity to HCALD is decreased to 89%. As long as
the reaction temperature increases the selectivity to HCALD is de-
creasing. At 305 °C, the least selectivity to HCALD 51% is obtained. The
2
Fig. 6. CO -TPD profiles of supported Cu catalysts.
5

S.R. K, et al.
Molecular Catalysis xxx (xxxx) xxxx
Fig. 10. Effect of mole ratio of CALD to isopropanol on chemo-selective hy-
Fig. 8. Effect of reaction temperature on chemo-selective hydrogenation of
CALD to HCALD.
drogenation of CALD to HCALD. Reaction Conditions: Catalyst amount: 500 mg,
−1
2
WHSV=1 h , N flow: 30 mL/min, Reaction temperature: 260 °C.
Reaction Conditions: Catalyst amount: 500 mg, WHSV=1 h−1, Mole ratio: 1:20
2
(CALD: Isopropanol), N flow: 30 mL/min,
The effect of mole ratio of CALD to isopropanol on catalytic per-
formance of 20Cu/SiO catalyst in chemo selective transfer hydro-
genation of CALD to HCALD is studied and results were presented in
Fig. 10. The conversion of CALD is increased with increase in moles of
isopropanol whereas the selectivity to HCALD is same in all the cases.
At a molar ratio of 1:10 and 1:15, the conversions of CALD are 32 and
2
raise in reaction temperature beyond 260 °C results the formation of
HCAOL, AB and PB at the expense of HCALD selectivity. The raise in
reaction temperature may activate the some of the formed HCALD
molecules to participate in further hydrogenation of its C]O bond to
produce HCAOL. Once the HCAOL is formed the acidity of 20Cu/SiO
2
73% respectively. With increase in mole ratio to 1:20 complete con-
catalyst is enough to perform hydrogenolysis and dehydration reactions
which produce PB and AB, respectively. Hence it could be suggested
that the optimum reaction temperature for this reaction under these
reaction conditions is 260 °C.
version of CALD takes place. Further increase in mole ratio doesn’t
influence the conversion of CALD and selectivity to HCALD. From these
results it could be suggested that, maximum conversion of CALD and
selectivity is achieved at molar ratio of 1:20 under these reaction
conditions. Under these reaction parameters, the influence of methanol
and ethanol instead of isopropanol was studied. With methanol the
conversion of CALD is 79% and the selectivity to HCALD is 78.2%. With
ethanol the conversion of CALD is 85% and the selectivity to HCALD is
86.6%. Among the investigated H-donors, isopropanol exhibited se-
lective formation of HCALD in gas phase transfer hydrogenation of
CALD.
The effect of amount of feed on catalytic performance of 20Cu/SiO
catalyst is examined and the results were presented in Fig. 9. At WHSV
2
−
1
(
Weight Hourly Space Velocity) of 1 h , complete conversion of CALD
is observed with 100% selectivity to HALD at1 h. With increase in
WHSV the conversion of CALD as well as selectivity to HCALD are
−1
gradually decreased. At WHSH of 2 h , the conversion of CALD is
−1
decreased to 94% and reaches a minimum of 58% at WHSV of 4 h
.
The high conversion at low WHSV is due to the reactant makes direct
contact with the catalyst surface where the number of reactant mole-
cules per active site is low. The decrease in activity with increase in
WHSV is may be due to the decrease in residence time of reactant
molecules over the catalyst active sites.
2
20Cu/SiO catalyst is subjected to time-on-stream study (TOS) for
chemo selective transfer hydrogenation of CALD to HCALD and results
were presented in Fig. 11. It shows that the yield of HCALD is declined
during time-on-stream study. The selectivity to HCALD is constant at
100% during the course of reaction while the conversion of CALD is
Fig. 9. Effect of amount of feed on chemo-selective hydrogenation of CALD to
HCALD. Reaction Conditions: Catalyst amount: 500 mg, Mole ratio: 1:20
Fig. 11. Time-On-Stream studies on various α, β-unsaturated carbonyl com-
pounds.
(
CALD: Isopropanol), N
2
flow: 30 mL/min, Reaction temperature: 260 °C.
6

S.R. K, et al.
Molecular Catalysis xxx (xxxx) xxxx
2 2
Fig. 12. TEM images of (a) Fresh 20Cu/SiO , (b) Spent 20Cu/SiO catalysts.
Fig. 13. Plausible reaction pathway for chemo-selective hydrogenation of CALD to HCALD over supported Cu catalysts.
Table 3
Reported catalytic systems for the selective hydrogenation of CALD to HCALD.
S. No. Catalyst Reaction Conditions
Con. of CALD (%)
Sel. to HCALD (%)
Ref.
01
02
03
04
05
06
07
08
09
10
11
12
PdCl
Pd/MWCNT
Pd/La CO
Pd/ZIF
PtNi/CNTs
PtNi/ZrO
PtAu/SiO
Ni–Ir/TiO
NiP/SiO
2
–0.5Co(OAc)
2
–PPh
3
Liquid phase, H
Liquid phase, H
Liquid phase, H
Liquid phase, H
Liquid phase, H
Liquid phase, H
Liquid phase, H
Liquid phase, H
Liquid phase, H
2
2
2
2
2
2
2
2
2
= 1 bar, T = 65 °C, t =5 h, ethanol
= 10 bar, T = 70 °C, t =0.5 h, 1,4-dioxane
= 1 bar, T = 80 °C, t =3 h, ethanol
= 20 bar, T = 40 °C, t =6 h, 2-propanol
= 20 bar, T = 70 °C, t =1.5 h, 2-propanol
= 20 bar, T = 70 °C, t =1.5 h, 2-propanol
= 10 bar, T = 180 °C, t =2 h, toluene
= 20 bar, T = 80 °C, t =2 h, 2-propanol
= 30 bar, T = 120 °C, t =2 h, cyclohexane
= 1 bar, T = 270 °C,
93
76
100
100
68
70
87
97
78
90
96
86
90
97
64
75
95
93
65
45
100
5
8
13
15
16
17
18
20
O
2 2
3
-NRs
2
2
2
2
21
23
24
Ni-Cu /SiO
Cu-Mg-Al
2
Vapor phase, H
Liquid phase, H
2
75
90
100
2
= 1 bar, T = 160 °C, t =7 h, propylene carbonate
20Cu/SiO
2
Vapor phase, isopropanol, N
2
= 1 bar, T = 260 °C
Present work
7

S.R. K, et al.
Molecular Catalysis xxx (xxxx) xxxx
decreased. The fresh and spent 20Cu/SiO
2
catalysts were analyzed by
Declaration of Competing Interest
TEM and the respective images and particle size distributions were
depicted in Fig. 12. From TEM images and particle size distributions, it
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
can be observed that the mean copper particle size in spent 20Cu/SiO
catalyst is increased from 21.6 nm (fresh 20Cu/SiO catalyst) to
8.4 nm. The decline in the catalytic activity of 20Cu/SiO catalyst is
2
2
2
2
due to increase in the copper particles size due to agglomeration. To
further investigate the present protocol on exclusive hydrogenation of
C = C in α, β unsaturated carbonyl compounds, various α, β un-
saturated carbonyl compounds like cyclohexenone, isophorone and
crotonaldehyde are tested at reaction temperatures 180, 220 and 110 °C
respectively. The selective hydrogenation of C = C in Cyclohexenone
yields cyclohexanone. The yield of cyclohexanone is decreased from
Acknowledgement
SRK, SSE and KSK thank University Grants Commission, New Delhi,
India for the award of fellowship.
CSIR-IICT Manuscript communication number: IICT/Pubs./2019/
193
100% to 73% during 6 h TOS. The selective hydrogenation of C = C in
Appendix A. Supplementary data
isophorone and crotonaldehyde yield 3, 3, 5-trimethylcyclohexanone
and butyraldehyde respectively. The yield of 3, 3, 5-trimethylcyclo-
hexanone and butyraldehyde are decreasing from 96 to 68% and 99 to
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.110686.
51% respectively. In all the cases the conversion of unsaturated car-
bonyl compound is decreased and selectivity to saturated carbonyl
compound is constant during the TOS. It shows the superiority of 20Cu/
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9