Cu(II)-Based Ionic Liquid Supported on SBA-15 Nanoparticles Catalyst for the Oxidation of Various Alcohols into Carboxylic Acids in the Presence of CO2

Article abstract of DOI:10.1007/s10562-021-03736-z

In this paper, we have produced carboxylic acids by the oxidation of various alcohols in the presence of CO₂ using SBA-15/IL supported Cu(II) (SBA-15/IL/Cu(II)) as nanocatalyst. The obtained products showed to have excellent yields by taking into account of SBA-15/IL/Cu(II) nanocatalyst. In addition, the analysis of EDX, SEM, TGA, TEM, XPS, and FT-IR showed the heterogeneous structure of SBA-15/IL/Cu (II) catalyst. It is determined that, after using SBA-15 excess, the catalytic stability of the system was enhanced. Moreover, hot filtration provided a full vision in the heterogeneous catalyst nature. The recycling as well as reuse of the catalyst were studied in cases of coupling reactions many times. Moreover, we have studied the mechanism of the coupling reactions. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03736-z

Catalysis Letters
https://doi.org/10.1007/s10562-021-03736-z
Cu(II)‑Based Ionic Liquid Supported on SBA‑15 Nanoparticles
Catalyst for the Oxidation of Various Alcohols into Carboxylic Acids
in the Presence of CO2
Qi Peng1 · Dejian Hou2 · Yanwu Chen1 · Litian Lin3 · Seyed Mohsen Sadeghzadeh4
Received: 17 March 2021 / Accepted: 8 July 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
In this paper, we have produced carboxylic acids by the oxidation of various alcohols in the presence of CO2 using SBA-15/
IL supported Cu(II) (SBA-15/IL/Cu(II)) as nanocatalyst. The obtained products showed to have excellent yields by taking
into account of SBA-15/IL/Cu(II) nanocatalyst. In addition, the analysis of EDX, SEM, TGA, TEM, XPS, and FT-IR showed
the heterogeneous structure of SBA-15/IL/Cu (II) catalyst. It is determined that, after using SBA-15 excess, the catalytic
stability of the system was enhanced. Moreover, hot fltration provided a full vision in the heterogeneous catalyst nature. The
recycling as well as reuse of the catalyst were studied in cases of coupling reactions many times. Moreover, we have studied
the mechanism of the coupling reactions.
Graphic Abstract
Cu(II)-based ionic liquid supported on SBA-15 nanoparticles
catalyst for the oxidation of various alcohols into carboxylic
acids in the presence of CO2
O O
Cl-
Cu
O
O
SiO
O
Si
N
N
N N
Cl-
O
O
O
O
O
OH
CO2
+
R
R
K2CO3
OH
Keywords Nanocatalyst · Green chemistry · Carboxylic acids · SBA-15 · Carbon dioxide
3
* Qi Peng
cheese_pp@126.com
Guangdong Province Key Laboratory of Rare Earth
Development and Application, Institute of Rare Metals,
Guangdong Academy of Sciences, Guangzhou 510651,
China
* Seyed Mohsen Sadeghzadeh
seyedmohsen.sadeghzadeh@gmail.com
4
Department of Chemistry, Faculty of Sciences, Neyshabur
Branch, Islamic Azad University, Neyshabur, Iran
1
College of Light Chemical Engineering and Materials,
Shunde Polytechnic, Foshan 528300, China
2
School of Materials Science and Engineering, Hanshan
Normal University, Chaozhou 521041, China
Vol.:(0123456789)
1 3

Q. Peng et al.
fammability, low vapour pressure. ILs have adjustable
structure and properties known as the most signifcant
characteristic of them. Many functional groups in cati-
ons/anions with diferent structures can be used to adjust
the physical and chemical characteristics of them. In den-
dritic ILs (DILs), the properties of dendrimers and ILs
are combine. DILs are widely utilized in the processes of
adsorption of heavy metal cations, catalysis and transport-
ers [30, 31].
1 Introduction
Diferent Cu(I) based heterogeneous catalysts has been
introduced like copper nanocluster [1], copper(I), a copper
manganese spinel oxide [2], zeolites [3], Cu2O and Cu/C
[4] on water [5]. In addition, the facet dependent catalytic
potential of Cu2O nanocrystals are proved to the regiose-
lective synthesis of focused 1,4-disubstituted 1,2,3-tria-
zoles in green media [6]. In the literature, lautens and his
group introduced the intramolecular Cu(I)‐catalyzed ter-
minated click–acylation domino reaction [7]. In the refer-
ence [8], researchers have investigated the use of copper
NPs as catalyst to produce 1,4-disubstituted 1,2,3-triazoles
in water. In the reference [9], researchers have investigated
the use of catalyst Cu(OAc)2 for 1,3-dipolar cycloaddi-
tion reactions by special focus on chelating ligands. In the
recent years, in order to obtain the appropriate 1,2,3-tria-
zole evoked the particular interest for Cu(II) catalysts,
researchers suggested Cu(II) acetate precatalysts and
Cu(II)-trenprecatalysts for the 1,3-dipolar cycloaddition
reactions [10, 11]. Despite several advances, scholars have
not yet proposed a reaction method that does not require
the use of explosive azides and is suitable for recyclable
catalysts, single step reactions, atom economy.
Synthesis of carboxylic acids from oxidation of alcohols
is a signifcant methods for the preparation of diferent fne
chemicals in the chemical industry, and has been kept a dif-
fcult task in the feld of green organic synthesis [32]. Stoichio-
metric, traditionally, expensive oxidants, and toxic, such as
CrO3 [33], and KMnO4 [34] have often been used to complete
this conversion, resulting in signifcant waste. Therefore, it is
highly desirable to develop an environmentally friendly new
oxidation scheme. Compared with traditional oxidants, the
non-toxic nature, cheap, and abundant of carbon dioxide has
brought great interest to its use as an accelerator in oxidation
reactions, including the oxidative dehydrogenation of alkanes,
the oxidation of alkyl aromatics, CH4, C–C bond formation
between aldehydes and the like [35]. Although metal–ligand
complexes have become a mature technology for the selective
catalytic oxidation of alcohols to aldehydes [36–38], there are
few reports on the oxidation of alcohols directly to carboxylic
acids.
Mesoporous silica materials can be used as a platform
for many applications in diagnosis, therapeutics and phar-
maceuticals [12]. SBA-15 as a mesoporous silica material,
is widely utilized as supports in the case of heterogene-
ous catalysis due to the dispersity of active sites. In addi-
tion, for increasing the efciency of mentioned materials,
one can cofunctionalize them by other functional groups
[13–15]. Moreover, SBA-15 can be utilized in wastewa-
ter purifcation, treatment and gas separation [16]. It is
mesopore-rich and showed to have considerable internal
surface area, high hydrothermal, plentiful surface hydroxyl
groups and appropriate mechanical stability. Currently, in
cases of catalyze reactions, a substantial number of cata-
lysts like ionic liquids (ILs) [17, 18] as well as transition
metal complexes [19, 20] were evaluated to be immobi-
lized on SBA-15. Moreover, these immobilized catalysts
demonstrated appropriate recyclability. SBA-15 is thought
to be proper choice for stably-supporting multi-site ILs due
to having mesoporous channel and plain loading [21–24].
In the process of catalysis synthesis, dendrimers are
widely utilized due to having unique dendritic efects,
abundant peripheral groups and high branched tridimen-
sional structures [25, 26]. The use of dendrimer catalyst
onto support materials was introduced as a new feld of
study and many researchers investigated it due to their
easy reusability and recoverability [27–29]. Ionic liquids
(ILs) are highly suggested in green chemistry because of
their unparalleled attributes such as good stability, low
Here, a novel and green approach is suggested to produce
SBA-15 nanoparticles (NPs), which is immobilized using ionic
liquid supported Cu(II) since IL/Cu(II) has a structure like
cage that utilized as an eco-friendly, inexpensive, non-toxic
ligand and highly reactive or catalyst to diferent ionic liquids.
The mentioned catalyst possessed many priorities such as good
activity, selectivity and easy separation for activating CO2 with
taking into account of mild conditions. In the present work, the
frst example of the application of SBA-15/IL/Cu (II) catalyst
is suggested for oxidation of various alcohols into carboxylic
acids (refer to Scheme 1).
2 Experimental
2.1 Materials and Methods
Chemical materials were purchased from Fluka and Merck in
high purity. Melting points were determined in open capillaries
using an Electrothermal 9100 apparatus and are uncorrected.
O
OH
+ CO2
SBA-15/IL/Cu(II)
K2CO3
R
R
OH
Scheme 1 Oxidation of various alcohols into carboxylic acids in the
presence of SBA-15/IL/Cu(II)
1 3

Cu(II)-Based Ionic Liquid Supported on SBA-15 Nanoparticles Catalyst for the Oxidation of…
FTIR spectra were recorded on a VERTEX 70 spectrometer
(Bruker) in the transmission modein spectroscopic grade KBr
pellets for all the powders. The particle size and structure of
nano particle was observed by using a Philips CM10 transmis-
sion electron microscope operating at 100 kV. Powder X-ray
difraction data were obtained using Bruker D8 Advance
model with Cu ka radition. The thermogravimetric analysis
(TGA) was carried out on a NETZSCH STA449F3 at a heating
rate of 10 °C min−1 under nitrogen. 1H and 13C NMR spectra
were recorded on a BRUKER DRX-300 AVANCE spectrom-
eter at 300.13 and 75.46 MHz, BRUKER DRX-400 AVANCE
spectrometer at 400.22 and 100.63 MHz, respectively. Elemen-
tal analyses for C, H, and N were performed using a Heraeus
CHN–O-Rapid analyzer. The purity determination of the prod-
ucts and reaction monitoring were accomplished by TLC on
silica gel polygram SILG/UV 254 plates. Mass spectra were
recorded on Shimadzu GCMS-QP5050 Mass Spectrometer.
SBA-15/3-Bromopropyl was fltered, cleansed twice with
deionized water, and dried at 90 °C for 12 h.
2.4 The Universal Approach to Provide SBA‑15/IL
NPs
The 4 g of SBA-15/3-Bromopropyl was suspended in 1-imi-
dazole (3.0 g), tetrahydrofuran (50.0 mL) was released, and
the blend was mixed at 70 °C for 20 h. SBA-15/Imidazole
(80 mg) was released into deionized water. EtOH blend (5:5
v/v, 80 mL) contained 2-chloroacetic acid (3.0 g), and the
reaction was performed at r.t. for 2.5 h. Upon separating the
solvent, the fnal blend was fltered. Then the fltrate was
vacuum-dried at 50 °C.
2.5 The Universal Approach to Provide SBA‑15/IL/
Cu (II)
2.2 General Approach for the Synthesis of SBA‑15
NPs
Cu(OAc)2 (0.15 g) was released to a blend of SBA-15/IL
(0.25 g) in 25 mL of ethanol and then mixed for 3 h at r.t.
Next, the ethanol was removed under decreased pressure and
the remnant was cleansed frst with Et2O, then with CH2Cl2:
Et2O (4:1 v/v), and fnally with Diethylether:Acetone (1:1
v/v) to give an orange solid.
SBA-15 is produced based on the reference [39]. Tetraethyl
orthosilicate (TEOS) is utilized as silica source and Pluronic
(P123) is utilized as template. Firstly, 8.0 g of P123 and a
constant amount of TEOS equal to 17.0 g are slowly added
to an aqueous solution of HCl (2 mol/L, 240 mL) along with
60 g of water. For a day, whole considered mixtures is stirred
under the temperature of 40 °C and after that was heated
to the temperature of 100 °C for another 24 h considering
static conditions. In the next step, the mesoporous material is
fltered, washed by water up to when no Cl− is not detected.
The result compound is dried under the temperature of 40 °C
considering vacuum condition. After that, the dried sample
is dissolved in methanol and sonicated under the tempera-
ture of 40 °C for 20 min, then was fltered and repeated for
three times. In the last step, the resulted compound is dried
under vacuum under the temperature of 40 °C for obtaining
SBA-15.
2.6 General Procedure for Oxidation of Alcohols
Aryl alcohols (1 mmol), SBA-15/IL/Cu (II) NPs (7 mg),
and K2CO3 (5 mmol) in 5 mL of DMSO were mixed in a
200 mL autoclave. Purifed twice with CO2 gas, pressur-
ized with 1.5 MPa of CO2, and then heated under 60 °C for
12 h. Upon completion of the reaction, the reaction mixture
was cooled to r.t. Remaining CO2 was carefully evacuated.
The progress of the reaction was monitored by TLC. EtOH
was released into the reaction blend and the SBA-15/IL/
Cu (II) was isolated. The solvent was then separated under
decreased pressure and the fnal carboxylic acids was puri-
fed by recrystallization employing ethyl acetate/n-hexane.
2.3 The Universal Approach to Provide
SBA‑15/3‑Bromopropyl NPs
3 Results and Discussion
The SBA-15 (3.0 g) was released into the solution of NaOH
(50 mL, 0.5 M), and the blend was refluxed for 3 h to
enhance hydrophilicity. Then, the resulting nanoparticles
were cleansed several times with H2O to neutralize the pH
of the deionized H2O and then vacuum-dried at 70 °C for
6 h. Next, 3-bromopropyltrimethoxysilane was supported
on the SBA-15 exterior layer to synthesis amino-modifed
SBA-15/3-bromopropyl. A blend of activated SBA-15 (4 g)
and 3-bromopropyltrimethoxysilane (7 mL) in 100 mL
ethanol was mixed at 100 °C for 17 h. In the last step,
The production of the SBA-15/IL/Cu (II) nanocatalysts
included many diferent stages (refer to Scheme 2). The
SBA-15/3-chloropropylsilane NPs (SBA-15) fbers showed
to have diferent Si–OH groups onto the surface. Hence,
we expected that the SBA-15 fbers can be simply func-
tionalized by 3-chloropropyltriethoxysilane for the for-
mation of SBA-15/3-chloropropylsilane NPs. After that,
we loaded imidazole as well as 2-chloro acetic acid in
SBA-15/IL NPs. The recyclable catalyst of SBA-15/IL
supported copper (II) is produced in aqueous solutions
1 3

Q. Peng et al.
O
Si
O
O
OEt
Si
EtO
EtO Si
EtO
Cl
Cl
Cl
EtO
OEt
OEt
O
Si
O
O
N
N
SBA-15
O
OH
O
Cl-
N
O
O
HO
Cl
O
O
Si
O
N
N
O
Si
N
N
Si
O
N
N
O
N
Si
O
HO
O
O
O
O
Cl-
Cu(OAc)2
O
O
Cl-
N
Cu
O
O
Si
O
N
N
O
N
Si
O
O
O
Cl-
O
Scheme 2 Preparation of SBA-15/IL/Cu(II) nanocatalyst
that demonstrates by easy availability of active sites along
with high catalytic activity is produced using a simple,
cost-efective approach.
As shown in Fig. 2, the mesoporous structure can be
clearly observed in the HAADF-STEM image. In the ele-
mental maps for the hybrid material, the colors for O, N, Si,
and Cu were uniformly dispersed and the adsorbed Cu(II)
was found in similar areas to the other elements. This con-
frmed that Cu(II) was adsorbed from the solution by the
grafted ligands and homogeneously distributed within the
inner channels of the SBA-15.
Moreover, SEM and TEM analysis are used to character-
ize the morphology and also pore structure of the SBA-15
and SBA-15/IL/Cu(II). Figure 1a and c shows the SEM and
TEM images of SBA-15. In addition, Fig. 1b and d shows the
SEM and TEM images of immobilized IL/Cu(II) catalysts.
As seen, the strip sharp and regular appearance of SBA-15
and hexagonal structure of SBA-15 is clear, demonstrating
the usual structure of SBA-15 [12]. Moreover, after loading
IL/Cu(II), the SBA-15 structure have not varied.
As can be seen in Fig. 3, the decomposition conduct
of the SBA-15 as well as IL/Cu(II) catalysts SBA-15/IL/
Cu(II) is compared for understanding the infuences of the
grafted dendritic IL molecules. In Fig. 3, the TGA analysis
1 3

Cu(II)-Based Ionic Liquid Supported on SBA-15 Nanoparticles Catalyst for the Oxidation of…
Fig. 1 SEM images of a SBA-15; and b SBA-15/IL/Cu(II) NPs; TEM images of c SBA-15; and d SBA-15/IL/Cu(II) NPs
Fig. 2 HAADF-STEM image and the corresponding multi-element (Si, O and Cu) EDS mapping images of SBA-AP with Cu(II) adsorption
1 3

Q. Peng et al.
Fig. 3 TGA diagram of SBA-
100
95
90
85
80
15/IL/Cu(II) NPs
0
100
200
300
400
500
600
700
800
T/oC
in the cases of SBA-15/IL/Cu(II) are shown. As seen in
the TGA curves, there are two processes of weight loss
that the frst process is determined among 30 and 100 °C
demonstrated small weight loss equal to 5.0% related to
the evaporation of physical adsorbed H2O from the sam-
ples. Second one is between 300 and 450 °C demonstrated
clear reduces by large weight loss of 22% related to the
lignin degradation.
The spectroscopy of FT-IR is utilized for detecting pos-
sible variations in the exterior layer of the produced catalyst
(refer to Fig. 4). As can be seen in Fig. 4a, in the spectrum
of FT-IR in the case of SBA-15, the bands of 1629, 1102,
Fig. 4 FTIR spectra of SBA-15
(a); SBA-15/IL/Cu(II) NPs (b)
1 3

Cu(II)-Based Ionic Liquid Supported on SBA-15 Nanoparticles Catalyst for the Oxidation of…
and 802 cm−1 stands for the tensile state of the water, which
is absorbed onto the exterior layer of solid, Si–O–Si vibra-
tions, and Si–OH, respectively. As can be seen in Fig. 4b,
there are many new absorption peaks demonstrated at 1702,
1060, and 854 cm−1 in comparison to the FT-IR spectrum of
lufa sponge revealed to the tensile vibration of Si–O bond as
well as the fexural vibrations of C=N bond onto the SBA-
15/IL exterior layer. In addition, the wide peak revealed at
1701 cm−1 is due to C = O tensile vibrations in the acetic
acid functional states. Bands at 1627 cm−1 are due to the
bending vibrations of N–H in the ammonium types. We have
found that IL is properly immobilized on the nanoparticle
of SBA-15.
the attendance of N 1 s additional proved that SBA-15 were
functionalized by utilizing the imidazole. In addition, the
existence of ions of Cl− known as the 2-chloroacetic acid
counter ion is characteristized using a sharp peak proved
copper moiety in the catalyst. In Fig. 6, in the catalyst, the
origins are demonstrated that determined using the analysis
of EDX. Figure 6 shows the EDX pattern of whole con-
sidered elements existed in SBA-15/IL/Cu(II) NPs such as
silicon, carbon, oxygen, nitrogen, copper and chlorine.
Table 1 illustrates that the produced SBA-15 possessed
similar properties as commercial the SBA-15 and SBA-15,
before and after grafting demonstrates a considerable large
diference. The typical IV-type containing the H1 hysteresis
loop is exhibited by using the BET data (Fig. 7). In cases of
SBA-15/IL/Cu(II), the surface areas of BET are determined
to be 337 m2 g−1; pore diameters are equal to 6.14 nm; and
pore volumes 0.54 cm3 g−1, respectively. Compared to pris-
tine SBA-15, the SBA-15/IL/Cu(II) nitrogen sorption analy-
sis has demonstrated a regular and uniform mesostructure
having a reduction of surface area, pore diameter and pore
volume parameters. The related pore volumes were con-
siderably decreased by the functionalization using Si-IL/
Cu(II). As seen, IL/Cu(II) are immobilized onto SBA-15
that is related to reducing surface area as well as total pore
volume, (refer to Table 1).
In the present paper, XPS analysis was utilized for inves-
tigating the chemical sections on the SBA-15/IL/Cu(II) NPs
level. In the case of considered catalyst, Fig. 5 shows XPS
scheme. There are the peaks of Si, C, O, Cl, N, and Cu and
The catalytic activity of the diferent kinds of SBA-15/
IL/Cu(II) are examined by oxidation of salicyl alcohol into
carboxylic acid in the presence of CO2 (refer to Table 2).
Table 1 Structural parameters of SBA-15, and SBA-15/IL/Cu(II)
materials determined from nitrogen sorption experiments
Catalysts
SBET (m2 g−1
)
Vt (cm3 g−1
)
DBJH (nm)
SBA-15
SBA-15/IL/Cu(II)
536
337
0.97
0.54
6.67
6.14
Fig. 5 XPS spectra of SBA-15/IL/Cu(II) NPs
Fig. 6 EDX spectra of SBA-15 (a); SBA-15/IL/Cu(II) NPs (b)
1 3

Q. Peng et al.
Table 2 Oxidation of salicyl alcohol by SBA-15/IL/Cu(II) NPs in dif-
ferent time, solvents, and bases
Entry Solvent
Base
Base (mmol) Time (h) Yield (%)a
1
2
3
4
5
6
7
8
Solvent-free K2CO3
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
5
3
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
14
12
10
8
–
–
–
–
–
–
–
92
17
62
23
46
32
31
–
EtOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
–
MeOH
i-PrOH
Dioxane
H2O
n-Hexane
DMSO
EtOAc
DMF
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
THF
CH3CN
CH2Cl2
CHCl3
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
Fig. 7 The N2 adsorption–desorption isotherms of SBA-15 and SBA-
15/IL/Cu(II)
CsF
Na2CO3
Et3N
–
59
–
The efect of diferent elements like a time, and solvent on
the model reaction and the impact of solvents on the oxi-
dation were examined (Table 2, rows 1–14). According to
the assessed outcomes of solvents, no quantity of the inter-
ested product was formed in the presence of polar protic
solvents, like isopropanol, methanol, and ethanol. Neverthe-
less, the efciency of the product was relatively moderate
in the attendance of polar aprotic solvents, such as EtOAc,
toluene, DMF, CHCl3, and CH2Cl2. Under standard condi-
tions, DMSO acts more efectively compared to other sol-
vents (Table 2, row 8). The results showed that K2CO3 is the
most efcient base for oxidation of salicyl alcohol. Under
optimal conditions, the reaction progress was monitored by
GC to detect the minimum time required in the attendance
of 10 mg of SBA-15/IL/Cu(II) NP. Results showed that the
excellent production efciency of carboxylic acid could be
achieved in 12 h (Table 2, row 25). The screening of the
quantity of K2CO3 depicted that 5.0 mmol was the premier
option (Table 2, entries 28 and 29). Enhancing the quantity
of base did not improved the results (Table 2, rows 25).
Catalyst loading is key determining element in the current
reaction (Fig. 8). the oxidation reaction had low efciency in
the absence of a catalyst. When 3–5 mg of SBA-15/IL/Cu(II)
NPs were added to the reaction, a moderate yield of oxida-
tion reaction was achieved. The best result was obtained in
the attendance of 7 mg of SBA-15/IL/Cu(II) NPs. Increas-
ing the amount of catalyst did not show any improvement in
the model reaction. No product was achieved in the nonat-
tendance of the catalyst. The efects of CO2 pressure in the
attendance of salicyl alcohol and SBA-15/IL/Cu(II) NPs for
12 h are depicted in Fig. 9. The catalyst blend showed the
highest efciency (92%) at a pressure of 1.5 Mpa. No spe-
cifc by-product was observed by GC in all the experiments
and the salicyl alcohol oxidation reaction was acquired with
NaOAc
KOH
–
34
60
77
–
92
92
86
64
92
71
K3PO4
Cs2CO3
tBuOK
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
12
12
Reaction conditions: salicyl alcohol (1 mmol), K2CO3 (5 mmol),
SBA-15/IL/Cu(II) NPs (10 mg), CO2 (2 MPa), solvent (15 mL),
under 80 °C
aIsolated yields
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
SBA-15/IL/Cu(II) (mg)
Fig. 8 Efect of increasing amount of SBA-15/IL/Cu(II) NPs on the
salicyl alcohol oxidation reaction
1 3

Cu(II)-Based Ionic Liquid Supported on SBA-15 Nanoparticles Catalyst for the Oxidation of…
100
formed after 12 h (Table 4, row 1). Moreover, no reaction
90
80
70
60
50
40
30
20
10
0
was perceived when SBA-15/IL as catalyst was employed
(Table 4, row 3). IL is not able to show good catalytic
activity under standard reactions. We compared the results
with many other similar catalysts. Due to these unfavora-
ble results, we continued research to increase efciency by
adding Cu (II) (Table 4, row 3). Our results show that the
reaction cycle is primarily catalyzed using Cu (II) complexes
in the SBA-15/IL nanostructure. Nanoparticles increase the
activity of the catalyst due to the increase in surface area
to volume, so they signifcantly increase the sensitivity
between the reactants and the catalyst and act as a homoge-
neous catalyst (Table 4, rows 3 and 4).
0
0.5
1
1.5
2
CO2 pressure (Mpa)
Fig. 9 Efect of CO2 pressure on yield of salicyl alcohol oxidation
On the basis of the above experiments, we proposed the
mechanism in scheme 3. Firstly, the formation of carbonate
ester is required to transform the hydroxyl group into a better
leaving group. We fnd that the key features are the forma-
tion of an intermediate carbonate ester followed by nucleo-
philic substitution by DMSO and fnally deprotonation and
proton shift which leads to the aldehyde intermediate. [40]
Furthermore, only CO2 was observed after the reaction but
no other reduced products of CO2 such as HCOO−CO or
HCOOH were found using neither in situ gas GC measure-
ments. [41, 42] That supported the role of DMSO as actual
oxidant. Eventually, the copper catalyst activates the inter-
mediate to be further oxidized to carboxylic acid.
reaction
100
90
80
70
60
50
40
30
20
10
0
30
40
50
60
70
However, as determined by ICP-MS, the Cu contents in
catalyst pre- and post- reaction was 1.2 and 1.1%, respec-
tively (Table 5). This indicates that a large amount of Cu
species leaching into solution are recaptured onto the fbers
of SBA-15 at the end of the reaction. The main concern is
the trace of other active metals (like Ni, Cd, Fe, Pd, and
Co) in each component of the reaction. It was essential to
perform ICP analysis of the reaction components. The con-
centration of Ni, Cd, Fe, Pd, and Co were very low that cast
any doubt (Table 6).
Temperature (oC)
Fig. 10 Efect of temperature on yield of salicyl alcohol oxidation
reaction
92% efciency. As seen in Fig. 10, the temperature of reac-
tion possessed a dramatic efect on the carboxylic acid yield.
The carboxylic acid yield can enhance by the enhancement
of reaction temperature ranging from 30 to 60 °C (92%,
60 °C) and levels of upon more enhancing temperature
equal to 92 °C.
In green chemistry, the catalyst reusability state is con-
sidered a signifcant property. Therefore, the reuse of the
SBA-15/IL/Cu(II) NPs was investigated with respect to the
optimal state of the synthesis of carboxylic acid. SBA-15/IL/
Cu(II) solid NPs were easily isolated from the liquid reaction
zone after a few seconds. The solvent can be used quickly
after cleaning. As Fig. 11 illustrates, the catalyst was recy-
cled for ten consecutive cycles. Product yield in the tenth
run was 89%, showing merely a 3% decrease in performance
compared to the fresh catalyst.
As can be seen in Table 3, in this novel one-pot approach,
many aryl alcohols are studied in cases of the substrate
scopes to produce carboxylic acids. Noted that, for all con-
sidered substrates, the catalyst is active. In addition, aryl
alcohols bearing a para-chloro group have been introduced
as proper substrates provided near-quantitative products.
Aryl alcohols and also the para-bromo or para-fuoro groups,
are additionally appropriate substrates. Whereas an electron-
donating group like methyl over the aryl group reduced the
product yield. Various aliphatic alcohols were also tested
and gave the corresponding products in good yield.
Furthermore, the heterogeneous essence of the cata-
lyst was comprehensively investigated. First, a hot fltra-
tion test was performed for the synthesis of carboxylic
acid under superior conditions and demonstrated that the
catalyst was removed at a yield of 48% (after 6 h). After
removal of the heterogeneous catalyst, it was found that
the free catalyst residues were relatively active, and the
For a deeper evaluation of the efciency of the catalyst,
various control experiments were conducted and the results
are shown in Table 4. The reaction performed deploying
SBA-15 showed that no amount of the carboxylic acid was
1 3

Q. Peng et al.
Table 3 Synthesis of carboxylic
acid derivatives in the
participation of SBA-15/IL/
Cu(II) NPs
Entry
1
Olefns
Products
Yield (%)a
92
OH
OH O
OH
OH
2
3
4
5
90
91
92
89
O
OH
OH
F
F
O
OH
OH
Cl
Cl
O
OH
OH
Br
Br
O
OH
OH
O2N
O2N
6
7
88
91
O
OH
OH
Me
Me
O
OH
OH
MeO
MeO
8
87
85
83
OH
OH
O
OH
9
O
OH
O
10
OH
OH
1 3

Cu(II)-Based Ionic Liquid Supported on SBA-15 Nanoparticles Catalyst for the Oxidation of…
Table 3 (continued)
Reaction conditions: alcohols (1 mmol), K2CO3 (5 mmol), SBA-15/IL/Cu(II) NPs (7 mg), CO2 (1.5 MPa),
DMSO (5 mL), under 60 °C
aIsolated yields
Table 4 Infuence of diferent catalysts for synthesis of carboxylic
conversion of 51% was achieved after 12 h of synthesis of
carboxylic acid. This depicted the heterogeneity of cata-
lyst during the reaction, with partial leaching. Eventually,
a Mercury-poisoning test was performed to confrm the
heterogeneous nature of the catalyst. Mercury (0) signif-
cantly attenuates the metal catalyst on the active outer
layer and calms its activity, confrming the heterogeneity
of catalyst. This test was accomplished on the mentioned
reaction model at optimal situations. After 6 h, about 300
molar mercury was released to the reaction compound and
stirred. After 12 h, no alteration was perceived in the poi-
soned catalyst. Figure 12 depicts the kinetics of the reac-
acid
Entry
Catalyst
Yield (%)a
1
2
3
4
SBA-15
–
–
92
91
SBA-15/IL
SBA-15/IL/Cu(II)
IL/Cu(II)
Reaction conditions: salicyl alcohol (1 mmol), K2CO3 (5 mmol),
SBA-15/IL/Cu(II) NPs (7 mg), CO2 (1.5 MPa), DMSO (5 mL), under
60 °C
aIsolated yield
O
S
O
S
HCO3
H
O
OH
H
CO2
DMSO
R
O
OH
R
H
R
O
S
B(base)
B
O
O
CO2 + H2O
BH
+
S
SBA-15/IL/Cu(II)
DMSO
HCO3
R
H
R
OH
Scheme 3 Proposed mechanism for SBA-15/IL/Cu(II) catalyzed oxidation of alcohols
Table 5 The loading amount of Cu in SBA-15/IL/Cu(II)
tion at the attendance of Hg (0). Negative experimental
outcomes illustrated that the SBA-15/IL/Cu(II) was het-
erogeneous and no signifcant copper leaching happened
during synthesis of carboxylic acid.
Entry
Catalyst
wt %
1.2
1
2
SBA-15/IL/Cu(II) NPs
SBA-15/IL/Cu(II) NPs after ten reuses 1.1
The SEM and TEM images analyzer showed the more
datain the case of the structure nanoparticles of SBA-15/
IL/Cu(II). As seen in Fig. 13a and b, the TEM and SEM
images for the new structure nanoparticles of SBA-15/IL/
Cu(II), and the 10th structure nanoparticles SBA-15/IL/
Cu(II) reused are demonstrated. The structure of tube-like
related to the catalyst can be still observed after being 10th
reused. The same structure between fresh nanoparticles
of SBA-15/IL/Cu(II) and SBA-15/IL/Cu(II) 10th reused,
observed to great power in recyclability. In addition, the
thermal stability of the reused SBA-15/IL/Cu(II) catalyst
after ten times recycling was not as good as that of the
fresh catalyst, which might be caused by the loss of IL/
Cu(II) in SBA-15 during recycling (Fig. 14). Fortunately,
it did not afect its practicability when used at 100 °C.
Table 6 Chemical composition
of the synthesis of carboxylic
acid using ICP
Entry
Element
Weight
percent
(%)
1
2
3
4
5
Fe
Co
Pd
Cd
Ni
<0.02
–
<0.01
–
<0.01
1 3

Q. Peng et al.
Fig. 11 Recyclability of the
100
90
80
70
60
50
40
30
20
10
0
catalyst
1
2
3
4
5
6
7
8
9
10
Reuse
4 Conclusions
In summary, a novel class of SBA-15 mesoporous silica
supported ionic liquid based on imidazole groups ligands
(SBA-15/IL). Then, the recyclable SBA-15/IL supported
copper (II) catalyst was synthesized that exhibited excellent
catalytic activity for synthesis of carboxylic acids by the
oxidation of various alcohols in excellent yields. The sur-
face area analysis studies of BET, FTIR, TEM, XPS, SEM,
TGA, ICP-MS and EDX proposed the functionalization of
IL and Cu (II) in the mesopores silica surface. Moreover, the
catalyst is determined to be recoverable and reusable. This
rational design in the case of single-site catalysts having
full utilization of each IL/Cu(II) active site and appropriate
recyclability and lower catalyst leaching can occur together
with the concepts of green chemistry. Hence, the investiga-
tion of SBA-15/IL/Cu(II) nanocatalyst binged an appropri-
ate platform for fabricating other nano catalyst that is very
efcient in diferent nano-catalyst based catalytic reactions.
Fig. 12 Leaching test for the catalyst of synthesis of carboxylic acid
Fig. 13 a TEM, and b FE-SEM
images of the recovered SBA-
15/IL/Cu(II) NPs after the 10 th
run for synthesis of carboxylic
acid
1 3

Cu(II)-Based Ionic Liquid Supported on SBA-15 Nanoparticles Catalyst for the Oxidation of…
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Acknowledgements This work was financially supported by the
Young Innovative Talents Project of Guangdong Provincial (No.
2020KQNCX242); the Innovative Team Program of Guangdong Prov-
ince (No. 2020KCXTD057); the Guangdong Basic and Applied Basic
Research Foundation (No. 2020A1515011188).
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