Ketone Hydrogenation by Using ZnO?Cu(OH)Cl/MCM-41 with a Splash of Water: An Environmentally Benign Approach

Article abstract of DOI:10.1002/asia.201901610

MCM-41-supported ZnO?Cu(OH)Cl nanoparticles were synthesized via an incipient wetness impregnation technique using zinc chloride and copper chloride salts as well as water at room temperature. The catalyst was characterized by powder X-ray diffraction (PXRD), infrared spectroscopy (IR), and TGA, whereas surface and morphological studies were performed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The above studies revealed the incorporation of metal species into the pores of MCM-41, leading to a decrease in surface area of the nanoparticles that was found to be 239.079 m²/g. The substituents attached to the ketone determine the rate of the reaction, and the utilization of the green solvent ‘water’ astonishingly completes the hydrogenation reaction in 45 minutes at 40 °C with 100% conversion and 100% selectivity as analyzed by gas chromatography-mass spectrometry. Hence, ZnO?Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and 6.39 wt% copper were demonstrated as an active catalyst for the reduction of ketones without using any gaseous hydrogen source making it highly efficient as well as environmentally and economically benign.

Full text of DOI:10.1002/asia.201901610

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Ketone hydrogenation by using ZnO-Cu(OH)Cl/MCM-41 with a
splash of water: an environmentally benign approach
Authors: Neha Choudhary, Topi Ghosh, and Shaikh M. Mobin
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Chemistry - An Asian Journal
10.1002/asia.201901610
Ketone Hydrogenation by using ZnO-Cu(OH)Cl/MCM-41
with a Splash of Water: an Environmentally Benign
Approach
Neha Choudharya
Topi Ghosh and Shaikh M. Mobin*
a
a,b,c
,
a
c
b
Discipline of Chemistry, Discipline of Metallurgy Engineering and Material Science and
Discipline for Biosciences and Bio-Medical Engineering, Indian Institute of Technology
Indore, Simrol, Khandwa Road, Indore 453552, India.
*
Email: xray@iiti.ac.in
Tel: +91 731 2438752
1
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Abstract:
MCM-41 supported ZnO-Cu(OH)Cl nanoparticles were synthesized via an incipient wetness
impregnation technique using zinc chloride and copper chloride salts using water at room
temperature. The catalyst was characterized by Powder X-ray diffraction (PXRD), Infrared
Spectroscopy (IR), and TGA, whereas surface and morphological studies were performed by
Scanning electron microscopy (SEM), Transmission electron microscopy (TEM). The above
studies revealed the incorporation of metal species into the pores of MCM-41, leading to
2
decrease in surface area of the nanoparticles found to be 239.079 m /g. The substituents
attached to the ketone determine the rate of the reaction, and the utilization of green solvent
‘water’ astonishingly completes the hydrogenation reaction in 45 minutes at 40°C with 100%
conversion and 100% selectivity as analyzed by Shimadzu GC-MS. Hence, ZnO-
Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and 6.39 wt% copper were deciphered
as an active catalyst for the reduction of ketones without using any gaseous hydrogen source
making it highly efficient and, environmentally and economically benign.
Keywords: Incipient wetness impregnation; Hydrogenation; ZnO-Cu(OH)Cl/MCM-41
nanocatalyst; Green solvent “water”; Recyclable catalyst.
2
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1
. Introduction
Nanoparticles as a catalyst have recently attracted the attention of the scientific community
because of their high selectivity, durability, recoverability with considerable economic
[
1]
benefits in manufacturing as an industrial commodity. Metal nanoparticles as catalysts are
popular owing to their high surface area properties, leading to enhanced interaction between
reactant and catalyst surface, hence high activities observed even under mild conditions.[2]
Nanocatalysts have various applications in numerous fields such as refinery, petrochemicals,
pharmaceuticals, chemicals, food processing.[3]-[4] The nanomaterial based on transition
metals is the center of attraction because these species can be used both as a catalyst and
catalyst support due to their activity and their activated surface.[5] The first row transition
metals are replacing the organometallic compounds due to their high stability and low cost.
Since they are not miscible in reaction mixture so we can say that nanoparticles bridge the
gap between homogeneous and heterogeneous catalysis.[6] Hence, after studying the
aforementioned advantages, we have synthesized the nanocatalyst ZnO-Cu(OH)Cl/MCM-41
nanoparticles with 2.46 wt% zinc and 6.39 wt% copper by using MCM-41 (Mobil
Corporation Matter no. 41) as support for zinc and copper oxides.
From previous reports, it deduced that individually ZnO and CuO have various
properties such as high electron mobility, high catalytic activity, high chemical and thermal
stability and non-toxicity, easy availability in nature as well as biocompatible,[7]-[8]-[9] while a
hybrid Copper-Zinc oxide catalyst possess application in steam forming[10]-[11], methanol
synthesis,[12]-[13]-[14] methanol reforming for fuel cell,[15] anodic material in lithium-ion
batteries[16] and CO fixation.[17] So, we have amalgamated them together and synthesized a
2
catalyst with zinc and copper supported on mesoporous silica MCM-41.
Out of various metal nanoparticle supports reported like MnO2,[18]-[19] alumina,[20]
graphene oxide[21] and cerium oxide,[22] silica MCM-41 as support pertaining its high surface
2
[23]
area (~1000 m /g) and pore diameter. Being a porous material, it is capable of providing
more active sites to the reactant and permitting both internal and external catalysis.[24] Hence,
the above properties make MCM-41 an appealing and a novel candidate to act as a support of
metal nanoparticles for catalysis. The previous reports has marked efficiency of MCM-41 as
support for catalysis of acetylene hydration,[25] oxidation of phenol solution,[26]
hydrohalogenation of chlorobenzene,[27] amination of glycerol,[28] formylation of amines,[29]
3
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Chemistry - An Asian Journal
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syngas methanation,[30] hydrosulfurization of dibenzothiophene,[31] toluene alkylation,[32]
oxidation of hydrocarbons,[33] hydrogenation[34]-[35] and also used as drug delivery[36], etc.
Herein, this work, the catalytic transfer hydrogenation and the effect of bulkiness on the
substrate, their amount of loading, and catalyst amount loading reported.
The principle of ‘green chemistry’, focusses on non-toxicity of the reaction solvent
along with recyclability and cost-effectiveness of the catalyst.[37] Water as a solvent
demonstrates distinctive activity. Hence, in this work water as “green solvent” used, which is
cheap, abundant, eco-friendly, and non-toxic for the reduction of ketones using synthesized
nanomaterial ZnO-Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and 6.39 wt%
copper along with sodium borohydride as reducing agent. Reduction using hydrogen sources
like water, methanol, isopropanol, which is known as transfer hydrogenation has attracted
attention owing to its secure handling and also safety concerns.[38] Although, sodium
borohydride reported for reduction of ketones and aldehydes to alcohols in organic
chemistry[39] with different catalyst. As previously reported that 1-Phenyl ethanol is used as a
flavoring agent, coloring agent in food additives, perfumery agent, and in pharmaceutical
industries.[40] We have designed and evaluated the reduction of acetophenone to 1-Phenyl
ethanol as model substrate using sodium borohydride and water as a solvent. [
-
41] [42]
4
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2
2
. Experimental
.1 Nanomaterial Synthesis via an incipient wetness impregnation technique
Materials used for synthesizing the nanoparticles were Mesoporous Silica MCM-41, Zinc
Chloride and Copper (ii) chloride (≥ 98%), Zinc Oxide nanopowder, and all reagent grade
ketones, heterocyclic compounds for the catalytic reaction were purchased from Merck.
For the preparation of bimetallic ZnO-Cu(OH)Cl catalyst supported on MCM-41,
firstly zinc chloride (ZnCl
2
) 3 g and copper chloride (CuCl ) 0.33 g were dissolved in 90 ml
2
deionized water with 9:1 weight% loading respectively and stir for 15 minutes. After that the
aqueous solution of metal salts was added dropwise to the MCM-41 (333 mg) with
continuous stirring at room temperature for 6 hours in the round bottom flask. The precipitate
was then centrifuged and dried at 100°C overnight, and hence, the synthesized catalyst named
as ZnO-Cu(OH)Cl/MCM-41 nanoparticles.
2
.2 General catalytic procedure of reduction of ketones
The reduction of ketones carried out in a 15 mL culture tube with stirring at a magnetic bar
stirrer at 40°C temperature. The ketone substrate (1 mmol), reducing agent NaBH (0.5-1
4
mmol), 3 mg of catalyst (ZnO-Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and
o
6
.39 wt% copper) and water (2 mL) stirred at 40 C for appropriate time(0.5-4 h). After 100%
conversion of the reactant, the reaction mixture was quenched with NH
4
Cl aqueous solution
to pH = 7 and then work up with ethyl acetate and further analyzed by Shimadzu GC-MS.
2
.3 Physio-chemical measurements
Powder x-ray diffraction (PXRD) of the nanoparticle performed using Cu Kα radiation (1.54
Å) using the Rigaku Smart Lab X-ray diffractometer. Fourier-transform infrared
spectroscopy (FT-IR) carried out using KBr pellets with a Bio-Rad FTS 3000MX instrument.
For analyzing thermal stability, the thermogravimetric analysis performed using TGA 4000,
Perkin Elmer using software PYRIS 6. To know the surface morphology, Scanning Electron
Microscopy (FE-SEM) and Field Emission Gun-Transmission electron microscopy (HR-
TEM, 300 kV) performed using Supra55 Zeiss and FEI Tecnai G2, F30 respectively. The
5
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Brunauer–Emmett–Teller (BET) surface area determined using N
2
and CO adsorption-
2
desorption measurements. XPS analysis of fresh and spent catalysts recorded using Auger
Electron Spectroscopy (AES) PHI 5000 Versa Prob II, FEI Inc. Identification of the products
of catalytic reactions carried out using Shimadzu GC-MS, QP2010 mass spectrometer.
6
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3
3
. Results and Discussion
.1. Characterization of nanoparticle
Synthesis of the catalyst is carried out via an incipient wetness impregnation technique as
shown below in Scheme 1:
Scheme 1 Synthesis of nanoparticle ZnO-Cu(OH)Cl/MCM-41.
The XRD patterns of ZnO-Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and 6.39
o
wt% copper catalyst is shown in Figure 1, in which a broad peak around 2θ = 23 , indicates
amorphous behavior of MCM-41 silica support [43]-[44] and the other diffraction peaks on 2θ =
o
o
1
#
6.16 , 32.28° and 39.64 representing the planes (100), (021) and (121) of Cu(OH)Cl (PDF-
51-0400) and the diffraction peak at 32.28° confirmed the ZnO formation and exactly
matched with JCPDS No. 36-1451.[45] This revealed the formation of ZnO-Cu(OH)Cl/MCM-
4
1 nanoparticles with 2.46 wt% zinc and 6.39 wt% copper. The average crystallite size of the
ZnO-Cu(OH)Cl/MCM-41 nanoparticles obtained 29 nm by using Scherrer’s equation.
7
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Figure 1. PXRD measurement data of ZnO-Cu(OH)Cl/MCM-41.
In the FT-IR spectrum, as shown in Figure 2, the absorption peak at 3566 cm-1
assigned to the stretching vibrations of –OH bond of silanol group of surface of supported
silica[46]-[26] and the absorption band at 1620 cm was because of bending vibration of H-OH
-1
bond[47], and 956 cm attributed to the asymmetric stretching vibration of Si-OH bond.
-1
[48]-[49]
-1
The absorption band at 802, 1087, and 1237 cm assigned to internal and external
asymmetric stretching of Si-O-Si of the SiO framework. The shift in absorption peaks was
2
may be because of the substitution of silicon by incorporated metals zinc and copper.[27] Due
to the introduction of zinc and copper the bond distance of Si-O increases and causes the
smaller wavenumbers.[27]
Thermogravimetric analysis (TGA) was performed to determine the thermal stability
of the catalyst (Figure 3). The major weight loss observed in three temperature regions : (i)
3
7 – 140 °C, (ii) 202 – 490 °C, and (iii) above 490 °C. The total weight of the sample is 4.336
mg and the weight loss in these three major regions is respectively 18.656 wt%, 8.659 wt%
and 20.023 wt%. The weight loss below 140 °C might be because of the removal of adsorbed
water and organic solvents.
8
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Figure 2. FT-IR spectrum of ZnO-Cu(OH)Cl/MCM-41.
The second significant weight loss was observed because of the removal of organic
species trapped inside the pores of supported silica. Beyond 490°C, the weight loss due to the
removal of hydroxyl groups linked to copper[43] and condensation of Si-OH groups of the
inner surface of pores of silica.[50]
Figure 3. TG analysis of ZnO-Cu(OH)Cl/MCM-41.
SEM analysis performed to study the surface morphology and the structure of the
nanoparticles, as shown in Figure 4. The presence of Zn, Cu, Si, and O elements was
9
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confirmed by EDAX and mapping (Figure S1 A-E). Moreover, the EDAX spectrum for
catalyst ZnO-Cu(OH)Cl/MCM-41 shows the presence of desired elements such as Zn, Cu, Si,
and O with 6.39 wt% of copper and 2.46 wt% of zinc present.
Figure 4. SEM analysis of catalyst ZnO-Cu(OH)Cl/MCM-41.
To understand the surface morphology and distribution of zinc and copper on the surface of
the catalyst, the TEM experiment was performed. As shown in all images, it is confirmed that
the metal nanoparticles are disseminated on the surface of MCM-41. The highly ordered
hexagonal channels of MCM-41 can easily be seen in Figure 5A, and B [51]-[52] and Figure 5C
indicates the incorporation of metal inside the pores which further corroborated by N
2
adsorption/desorption study which shows the decrease in surface area of MCM-41 in ZnO-
Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and 6.39 wt% copper.[53] It is also
validated that the MCM-41 is used as support for zinc and copper and after introducing the
metals, there is no change in the framework of MCM-41.[48]-[49] As shown in Figure 5D,
selected area electron diffraction (SAED) was performed which indicated the concentric rings
which show the crystalline nature of the catalyst ZnO-Cu(OH)Cl/MCM-41 nanoparticles with
2
.46 wt% zinc and 6.39 wt% copper.
1
0
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Figure 5. HRTEM analysis of catalyst at (A) 10 nm, (B) 50 nm and (C) 20 nm. (A), (B) in
the direction perpendicular to the pore axis, (C) in the direction of pores axis, and (D) SAED
images of nanoparticles at 5 1/nm.
Furthermore, the pore size and surface area of the catalyst was determined by the
nitrogen gas adsorption and desorption studies as shown in Figure 6A and B. The observed
adsorption-desorption isotherm was type IV which indicate the mesoporous behaviour of the
catalyst.[54]-[55] The surface area of ZnO-Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt%
zinc and 6.39 wt% copper calculated with the Brunauer–Emmett–Teller (BET) equation was
2
2
39.079 m /g, and pore volume is 0.743 cc/g and pore diameter 3.055 nm. The surface area
2
for MCM-41 is approximate 1000 m /g and the pore volume 0.98 cc/g and pore diameter 4
nm which decreases after incorporation of metal inside the pore of the MCM-41.
Incorporation of metal species blocked the pores of the supported silica MCM-41 and
reduced the specific surface area and pore volume.[56]-[57] The hysteresis curve indicates that
the metal species distributed all over the channels of MCM-41. The metal species were not
agglomerated at the opening of mesopores of MCM-41.[53]
1
1
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Figure 6. Surface area (A) and pore size (B) measurement by BET analysis.
The chemical composition and oxidation states of the prepared ZnO-
Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and 6.39 wt% copper were
investigated by X-ray photoelectron spectroscopy (XPS) analysis as shown in Figure 6. The
survey spectrum and high-resolution XPS of ZnO-Cu(OH)Cl/MCM-41 showed the existence
of Zn, Cu and O (Figure 7). The high-resolution XPS spectra of Zn 2p reveals the binding
energies of Zn 2p3/2 at about 1020.5 eV and Zn 2p1/2 center at 1043.7 eV, peak shape and
+
2
[58]-[59],
peak separation which is ~ 23eV revealed that presence of Zn in ZnO.
as shown in
Figure 7A. XPS spectra of Cu 2p showed the two peaks centered at 933 eV and 953 eV were
1
2
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assign to Cu 2p3/2 and Cu 2p1/2 present in Cu(OH)Cl in where Cu is in +2 oxidation state
shown in Figure 7B.[60] The O1s spectra at 530.4 eV attributed that O ions in the ZnO
lattice.[61]-[62]
2-
Figure 7. XPS analysis for ZnO-Cu(OH)Cl/MCM-41 (A) Zn 2p, (B) Cu 2p and (C) O 1s.
3
.2. Catalytic results
The catalytic transfer hydrogenation is a good choice of green chemistry to reduce the way to
reduce the ketones into alcohol without using molecular hydrogen. It is a less hazardous route
to reduce the ketones using cheap reducing agent sodium borohydride. The catalytic
1
3
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hydrogenation reaction was performed using sodium borohydride (NaBH
4
) as a reducing
°
agent in the presence of green solvent “water” at 40 C as shown in scheme 2.
The preliminary experiments were performed with acetophenone as model substrate.
Firstly, to know the effect of solvent on the model substrate, different types of solvents such
as water, methanol, acetonitrile, ethanol and DCM were used as shown in table 1. The
complete conversion of acetophenone into 1-Phenyl ethanol was observed with water,
methanol and acetonitrile in 1.5 h. The effect of solvents on the conversion of the ketone is in
order of polar protic solvents < polar aprotic < non-polar solvent.[40] Solvents like water,
methanol, acetonitrile have a high dielectric constant than dioxane and ethanol, which lead to
high conversion and selectivity.[27]
Scheme 2: General catalytic transfer hydrogenation of substituted acetophenones and
heterocyclic ketones by using ZnO-Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc
and 6.39 wt% copper, ZnCl
2
, CuCl
2
nanocatalyst with NaBH as a reducing agent.
4
For finalizing the solvent of the reaction, the reaction was carried out with these three
solvents that are water, methanol, and acetonitrile and as a result, water gave 100%
conversion and 100% selectivity within 0.75h, so further experiments were performed using
water as the solvent. The variation in conversion may be observed due to a higher dielectric
constant of water than methanol and acetonitrile.[27],[63]
Table 1. Effect of solvents on the formation of 1-Phenyl ethanol
1
4
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°
Entry
1
Solvent
Temperature ( C)
Conversion(%) Selectivity (%)
H
2
O
40
40
40
40
40
100
100
100
87
100
2
3
4
5
MeOH
CH CN
100
100
100
100
3
EtOH
Dioxane
49
Reaction condition : Substrate (acetophenone) = 1 mmol, catalyst (ZnO-Cu(OH)Cl/MCM-41
°
with) = 3 mg, NaBH
4
= 0.5 mmol, time = 1.5 h, temperature = 40 C, different solvent = 2
mL. Conversion and selectivity observed from Shimadzu - GCMS analysis.
Table 2. Time optimization for reduction of acetophenone to 1-phenylethan-1-ol
Entry
Time (min)
Conversion (%)
Selectivity (%)
1
2
3
4
15
30
28
46
100
100
45
60
100
100
100
100
Reaction condition : Substrate (acetophenone) = 1 mmol, catalyst (ZnO-Cu(OH)Cl/MCM-41)
°
=
3 mg, NaBH
4
= 0.5 mmol, H O = 2 mL, temperature = 40 C, time = (15-60) min.
2
Furthermore, the effect of reducing agent loading was performed and observed that
.5 mmol of NaBH is appropriate for complete conversion and selectivity. As shown in table
, when the amount of reducing agent increased from 0.4 mmol to 0.5 mmol, the conversion
0
3
4
and selectivity 100% obtained.
Table 3. Reducing agent amount loading for reduction of acetophenone
Entry
Reducing agent (mmol)
Conversion (%)
Selectivity (%)
1
2
0.25
0.3
28
40
100
100
1
5
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3
4
0.4
0.5
54
100
100
100
Reaction Condition : Substrate (acetophenone) = 1 mmol, catalyst (ZnO-Cu(OH)Cl/MCM-41)
o
=
3 mg, H
2
O = 2 mL, temperature = 40 C, time = 0.75 h, Reducing agent = 0.25-0.5 mmol.
Also, the optimization of the substrate amount loading on the catalytic reaction was
performed and observed that with increasing the loading, the reaction time increases, and
conversion decrease simultaneously.
Table 4. Substrate amount loading for reduction of acetophenone to 1-Phenyl ethanol
Entry
Substrate (mmol)
Conversion (%)
Selectivity (%)
1
2
3
4
0.5
1
100
100
100
100
1.5
2
27
18
100
100
Reaction condition: Substrate (acetophenone) = 0.5-2 mmol, NaBH
4
= 0.5 mmol, Catalyst
o
(
ZnO-Cu(OH)Cl/MCM-41) = 3 mg, H
2
O = 2 mL, temperature = 40 C, time = 0.75h .
Furthermore, To investigate the effect of Zn and Cu ions in catalytic reduction, we
performed the reaction using ZnCl , CuCl , ZnO as catalyst (Scheme 2, Table 5). Conversion
2
2
of acetophenone to the desired product 1- phenyl ethanol was observed 100% with the
catalyst ZnO-Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and 6.39 wt% copper (3
mg amount) with 100% selectivity. Conversion with CuCl2 and ZnO is 54% observed with
1
00% selectivity, whereas with ZnCl2, the conversion is 44% found. So far, the reaction
carried out without catalyst only 30% conversion observed.
Table 5. Catalyst screening for the reduction of acetophenone to 1-Phenyl ethanol
Entry
Catalyst
ZnCl
CuCl
Conversion (%)
Selectivity (%)
1
2
2
44
54
100
100
2
1
6
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3
4
5
ZnO (Purchsed)
54
100
30
100
100
100
ZnO-Cu(OH)Cl/MCM-41
-
Reaction condition Substrate (acetophenone) = 1 mmol, different catalyst = 3 mg, H O = 2
2
o
mL, NaBH
4
= 0.5 mmol, temperature = 40 C, time = 0.75h.
Further, we studied the scope of the reaction using different types of substrate, and
results are concluded in Table 6. Almost all the substrates were converted into a product
within 0.5 to 4 h with 100% selectivity and almost 100% conversion. There was observed no
effect of electron-withdrawing or electron releasing substituents on the reaction but there was
effect of bulkiness observed. For p-methyl and p-methoxy substituted acetophenone, there
was 90-100% conversion observed (Table 6, Entry 12, 13). We examined the effect of the
position of substituents on the phenyl ring of the acetophenone. The para-substituted
acetophenone gave more easily reaction in case of fluoro-, chloro- and bromo- substituents.
As we proceed the reaction with ortho- and meta- position substituted acetophenones, there
was effect of position of the substituent perceived in case of bromo- and it might be because
of the steric hindrance of the substituent group (Table 6, Entry 7-9).[64] For o-,m-,p- fluoro-
and chloro- substituents (Table 6, Entry 2-6) there was no significant difference observed in
conversion and selectivity. It estimated from the previous reports that might be bromo- on
ortho- position hinders the reaction site and after increasing reaction time, the conversion
found 86% which in the case of para-bromo was found 96%.[64] The effect of bulkiness was
perceived when we move from fluoro to iodo substituted acetophenone. For 1-(4-
iodophenyl)ethan-1-one, steric hindrance caused a drastic decrease in conversion in 24h
which is only 39% (Table 6, Entry 10).[65] It may be due to the internal catalysis phenomena
of porous material in which bulky substituent cannot pass through the pores of the catalyst.[66]
The same phenomena examined in the case of benzophenone that there is only 80%
conversion observed in 4h (Table 6, Entry 11). Next, to explore the activity of the catalyst,
the same reaction was performed with cyclic ketones such as cyclopentanone, cyclohexanone
and 2-methylcyclohexan-1-one and these all were converted 100% into products within 0.5 h
with 100% selectivity (Table 6, Entry 14-16). There was no effect observed of methyl
substituent on cyclohexenone. With heterocyclic compounds such as 1-(tetrahydrothiophen-
2
-yl)ethan-1-one, 100% conversion observed in 2h (Table 6, Entry 18). When we performed
1
7
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reaction with (E)-4-phenyl but-3-en-2-one and 2,3-dihydro-1H-inden-1-one, 100%
conversion observed with 100% selectivity (Table 6, Entry 17, 19). We have performed
reaction with amino- and hydroxy-substituted acetophenone but not considerable conversion
observed.[64]
Table 6. Substrate scope for reduction of ketones to corresponding alcohols
Entry
1
Substrate
Product
NaBH (eq.) Time(h) Con (%) Sel (%)
4
0
.5
0.75
1
100
100
99
100
100
100
2
3
1
1
2
4
5
6
1
1
1
2
2
2
100
96
100
100
100
88
1
8
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7
8
1
1
3.5
3
86
100
100
100
9
1
1
2
96
39
100
100
1
0
24
1
1
1
1
2
3
1
1
1
4
2
2
80
90
100
100
100
100
1
1
4
5
1
0.75
0.5
100
100
100
100
0.5
1
6
0.5
0.5
100
100
1
9
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Chemistry - An Asian Journal
10.1002/asia.201901610
1
7
1
3
100
100
1
1
8
9
1
1
2
3
100
100
100
100
Reaction conditions : Substrate 1 mmol, reducing agent (NaBH = 0.5-1 mmol), 3 mg catalyst
4
o
(
ZnO-Cu(OH)Cl/MCM-41), water (2 mL), temperature = 40 C, time = 0.5-24 h.
Generally, the hydrogenation reactions involve harsh reaction conditions such as high
temperature, molecular hydrogen as a hydrogen source, and use of expensive catalysts. Based
on different catalysts and substrates, it is challenging to compare the reduction of ketones.
However, to compare this work with some reported literature, we compile some of the
recently reported works in the hydrogenation of ketones [Table S1].
Based on our experimental result and available previous reports [
67] [68] [69]
-
- , the
plausible mechanism of reduction of ketones catalyzed by ZnO-Cu(OH)Cl/MCM-41
nanoparticles with 2.46 wt% zinc and 6.39 wt% copper in aqueous medium using NaBH
4
(
Figure S2) was proposed. Cao et al and Lai et al reported that the NaBH reacts with water
4
and hydride ion[
67] [70]
,
(step 1). This hydride ion adsorbs on the surface
and produces NaBO
2
of the catalyst and this hydride gave a very fast reduction of the ketone to alcohol (step 2 and
3
).
3
.3. Kinetic and recycle study –
The reduction of acetophenone was observed 100% in 0.75 h using 3 mg catalyst with water.
For the study, the kinetics of the reaction, periodically sample was taken by injection for GC
2
0
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Chemistry - An Asian Journal
10.1002/asia.201901610
analysis. The sample was taken on 15 min, 30 min, 45 min, and observed conversion were
respectively 28, 46, 100%. The kinetic study of the model reaction shown in Figure 8.
Figure 8. Kinetic study of the conversion of acetophenone to 1- Phenyl ethanol.
Reaction conditions : Substrate = 1 mmol, reducing agent (NaBH ) = 0.5 mmol, 3 mg catalyst
4
o
(
ZnO-Cu(OH)Cl/MCM-41), water (2 mL), = 40 C, time = 15-60 minutes.
The leaching test performed to know the reusability of the catalyst throughout the
catalytic reaction. For this test, after completion of reaction the catalyst was filtered and
separated to perform ICP-AES analysis with the supernatant and this result indicate no
considerable leaching of the catalyst (Table S2).
To know the reusability of the catalyst ZnO-Cu(OH)Cl/MCM-41 nanoparticles with
2
.46 wt% zinc and 6.39 wt% copper, we have performed the recycle study for the reduction
of the acetophenone to 1-Phenyl ethanol. After completion of the reaction one time, the
reaction mixture was centrifuged, and the catalyst was separated from the reaction mixture.[71]
Then dried at room temperature and again reuse this for the next cycle with the same reaction
condition, and we observed almost 100% conversion up to four cycles. After four cycles, the
conversion decreased to 89% which maybe because of the increase of carbon content, which
was trapped inside the pores of MCM-41 from the substrate.[72] The coke deposition also
confirmed by XPS analysis in which the carbon content increases when we compare both
fresh and spent catalyst samples. Hence, our synthesized nanoparticle is recyclable
2
1
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Chemistry - An Asian Journal
10.1002/asia.201901610
catalyst.[73],[74] There is reduction of copper observed after recycle by XPS analysis of spent
but this reduction of Cu(ii) into Cu(0) catalyse the reduction reaction and this Cu(0) again
oxidized to copper oxide in alkaline medium.[75],[76]
Reaction condition: Substrate 1 mmol, reducing agent (NaBH ) = 1 mmol, 3 mg catalyst
4
o
(
ZnO-Cu(OH)Cl/MCM-41), water (2 mL), temperature = 40 C.
4
. Conclusions
In conclusion, ZnO-Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and 6.39 wt%
copper is a heterogeneous catalyst with high surface area and stability for the reduction of
ketones, and it’s a very lesser amount (3 mg) is required for 100% conversion. The
nanomaterial ZnO-Cu(OH)Cl/MCM-41 nanoparticles with 2.46 wt% zinc and 6.39 wt%
copper was synthesized by wetness impregnation technique by using water as a solvent at
room temperature. HRTEM and BET studies proved the incorporation of zinc and copper
species on the surface of MCM-41 and inside the pores of MCM-41, which leads to decrease
in surface area of MCM-41 in metal nanoparticles ZnO-Cu(OH)Cl/MCM-41 nanoparticles
with 2.46 wt% zinc and 6.39 wt% copper. This catalyst was used the first time for reduction
of ketones by transfer hydrogenation using water as a solvent, provide the basis for
developing the new potential supports.
2
2
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Chemistry - An Asian Journal
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Acknowledgment
S.M.M. thanks SERB-DST, New Delhi (Project No. EMR/2016/001113), CSIR (project no.
1(2935)/18/EMR-II), New Delhi and IIT Indore for funding. N.C. is grateful to UGC, New
0
Delhi, and T.G. is grateful to MHRD, Government of India for research fellowship. We are
also thankful to Sophisticated Instrumentation Centre (SIC), IIT Indore for providing the
characterization facilities. We would also like to acknowledge IIT-Bombay, Sophisticated
Analytical Instrument Facility (SAIF) for HRTEM and ICP-AES analysis, and Advanced
Centre for Materials Science (ACMS) for XPS, IIT Kanpur. We would also like to
acknowledge Central Instrumentation Facility (CIF), IISER-Bhopal for TGA.
Conflict of interest
There is no conflict of interest.
Supporting information
EDAX, mapping of ZnO-Cu(OH)Cl/MCM-41 catalyst and GC-MS data of all the products
and ICP-AES can be found in supporting information.
2
3
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Chemistry - An Asian Journal
10.1002/asia.201901610
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