Au nanoparticle-immobilized L-cysteine-paired porous ionic copolymer as an efficient catalyst for additive-free oxidative coupling of alcohols and amines

Article abstract of DOI:10.1016/j.catcom.2019.105746

Herein, an L-cysteine-paired ionic copolymer (DVB-[MimLcy]_n) with mesoporosity was prepared by free radical copolymerization of divinylbenzene (DVB) and imidazolium ionic liquids (ILs), followed by anion-exchange with L-cysteine. Because of the rich functional groups of –NH₂, –SH, and –COO^– and the porous framework, the DVB-[MimLcy]₃ was revealed to be an ideal stabilizer for metal nanoparticles (NPs). Highly uniform dispersed small Au NPs (2–3 nm) immobilized on DVB-[MimLcy]₃ (Au_a/DVB-[MimLcy]₃) can act as an efficient heterogeneous catalyst for additive-free synthesis of imines through coupling of a broad range of alcohols and organic amines and can be easily recovered and steadily reused several times.

Full text of DOI:10.1016/j.catcom.2019.105746

Accepted Manuscript
Au nanoparticle-immobilized l-cysteine-paired porous ionic
copolymer as an efficient catalyst for additive-free oxidative
coupling of alcohols and amines
Shengyu Du, Chenjun Zhang, Yuchen Jiang, Pingping Jiang, Yan
Leng
PII:
DOI:
Article Number:
Reference:
S1566-7367(19)30208-0
https://doi.org/10.1016/j.catcom.2019.105746
105746
CATCOM 105746
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
23 April 2019
25 June 2019
25 June 2019
Please cite this article as: S. Du, C. Zhang, Y. Jiang, et al., Au nanoparticle-immobilized l-
cysteine-paired porous ionic copolymer as an efficient catalyst for additive-free oxidative
coupling of alcohols and amines, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.105746
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ACCEPTED MANUSCRIPT
Au Nanoparticle-Immobilized L-cysteine-paired Porous Ionic
Copolymer as an efficient catalyst for Additive-free Oxidative
Coupling of Alcohols and Amines
Shengyu Du, Chenjun Zhang, Yuchen Jiang, Pingping Jiang, Yan Leng*
a School of Chemical and Material Engineering, The Key Laboratory of Synthetic and Biological
Colloids, Jiangnan University, Wuxi, Jiangsu, 214122, China.
*Corresponding author. E-mail: yanleng@jiangnan.edu.cn
Abstract
Herein, an L-cysteine-paired ionic copolymer (DVB-[MimLcy]n) with mesoporosity was prepared
by free radical copolymerization of divinylbenzene (DVB) and imidazolium ionic liquids (ILs),
followed by anion-exchange with L-cysteine. Because of the rich functional groups of -NH2, -SH,
and -COO- and the porous framework, the DVB-[MimLcy]3 was revealed to be an ideal stabilizer
for metal nanoparticles (NPs). Highly uniform dispersed small Au NPs (2-3 nm) immobilized on
DVB-[MimLcy]3 (Aua/DVB-[MimLcy]3) can act as an efficient heterogeneous catalyst for
additive-free synthesis of imines through coupling of a broad range of alcohols and organic
amines and can be easily recovered and steadily reused several times.
Keywords: ionic copolymer; heterogeneous catalysts; metal nanoparticles; oxidative coupling
reaction
1. Introduction
Noble metals (such as Pd, Au, Rh, Ru, Ir, and so on) have been widely used as highly active and
excellent catalysts for various synthetic fields; typical examples include noble metal-based
homogeneous catalysts for oxidative coupling of alcohols and amines to synthesize imines [1-3].
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Despite the relatively high efficiency of these homogeneous systems, the separation and recovery
of the expensive and potentially toxic noble metal from used devices remain as pressing issues in
the development toward a more sustainable and environmentally friendly chemical industry. On
the other hand, heterogeneous catalysts are more favorable because of their facile recyclability and
easy compatibility, and extensive efforts have been focused on the development of heterogeneous
precious metal catalysts [4,5]. Presently, the most commonly used strategy is immobilization or
entrapping of precious metal on porous supports such as active carbon [6], molecular sieves [7],
metallic oxides [8], metal-organic frameworks (MOF) [9], and organic polymers [10]. However,
the majority of these catalysts often result in low activities and leaching of active sites because of
the weak interactions between metal and supports.
Ionic liquids (ILs), as tunable and environmentally friendly solvents, have been found to be
excellent media for stabilizing metal chelate-complexes and metal nanoparticles (NPs) by both
electronic and steric protection [11,12]. The possibility of modulating the physicochemical
properties of ILs by appropriate adjustment of their composition and structure enables the ILs to
present complementary properties in the field of metal species extraction and IL-mediated
catalysis [13,14]. During the past decades, catalysis by metal trapped in ILs that runs in
homogeneous/biphase/phase transfer modes has been demonstrated to be successful [15,16].
Nevertheless, ILs have some shortcomings such as high viscosity, high cost, difficulties in product
purification, recycling, and so on, which hinder their practical applications.
Ionic copolymers containing IL units have bifunctional mode of action as both ILs and
polymers [17,18]. In recent years, ionic copolymers have emerged as attractive materials for the
immobilization of active metal composites, which could solve the recycling problem and make it
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convenient to be applied in chemical industry [19,20]. The catalytic activities of these ionic
copolymer-supported metal catalysts strongly rely on the structural composition of ionic
copolymers, which can be designed to increase the metal–support interactions or to favor the
accessibility of the reagents to the active sites [21-23]. Despite the diverse composition structures
of cation/anion in ILs, most ionic copolymer-supported metal catalysts studied and reported thus
far are focused on the stability and coordination of cationic monomers and more rarely on anions.
Natural amino acids are an outstanding class of ligands in organocatalysis and organometallic
catalysis because of their abundant functional groups (amino group, carboxylic acid residue, and
various side groups) [24]. It is easy to obtain pure natural amino acids in large quantities at low
cost. Actually, the availability of amino acids as both anions and cations makes natural amino
acids desirable candidates to act as a platform for “task-specific” functional ILs [25,26].
Additionally, amino acid ILs have shown attractive potential for extraction of a range of metallic
species and offer an excellent environment for stabilizing metal species [27]. Accordingly, highly
dispersed metal species are expected to be immobilized on amino acid-based materials. In this
study, we initiate the application of L-cysteine-paired porous ionic copolymers for stabilization of
Au nanoparticles (NPs) and study their catalytic performance in oxidative coupling of alcohol and
amine to synthesize imines under additive-free conditions.
2. Experimental section
Please see Supporting Information (SI).
3. Results and discussion
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Scheme 1. Preparation of ionic copolymers DVB-[MimLcy]n
3.1 Synthesis and characterization of DVB-[MimLcy]n and Aux/DVB-[MimLcy]n
As shown in Scheme 1, bromopropylene was first reacted with vinyl imidazole to form
unsaturated imidazole IL monomer (MimBr). MimBr can undergo radical copolymerization
interaction with DVB using AIBN as the initiator to yield cross-linked porous ionic copolymer
DVB-[MimBr]n (n stands for the mass ratio of MimBr to DVB). To improve the copolymerization
efficiency, various solvents such as acetonitrile, ethyl acetate, methylbenzene, ethanol, and mixed
solvents were tested as the reaction media (Table S1, SI). An acetonitrile and methylbenzene
mixture solvent with a 3:1 volume ratio was found to be the optimum reaction media, which gives
93% copolymer yield. DVB-[MimBr]n was then treated with KOH solution, and the obtained
-OH-based ionic copolymer DVB-[MimOH]n was further anion-exchanged with Lcy to provide
the DVB-[MimLcy]n. SEM image of DVB-[MimLcy]3 reveals the flocculence structure, and the C,
N, O, and S elements are homogeneously dispersed on DVB-[MimLcy]3 as observed in the EDS
mapping images (Figure S1, SI).
The sample DVB-[MimLcy]n was then characterized by CHNS elemental analysis (Table S2,
SI), FT-IR, TG, and nitrogen-sorption analysis. In the FT-IR spectra (Figure S2, SI), the sample
displays bands at 2800-3200 cm−1 ascribed to C-H stretching vibrations for DVB and IL monomer
and bands at 1490 and 1155 cm−1 ascribed to C=N/C-N stretching vibrations for imidazole IL,
suggesting that the basic structures of monomers are preserved during cross-linking and
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anion-exchange processes. Compared with DVB-[MimBr]3 and DVB-[MimOH]3 (Figure S3, SI),
DVB-[MimLcy]3 exhibits two peaks at 1600 (-C=O) and 1401 cm-1 (-C-O) for -COO- groups, one
peak around 2599 cm−1 (-S-H) for -SH groups, and one peak at 840 cm−1 for wagging vibration
from the -NH2, indicating the successful introduction of Lcy anions onto the framework of ionic
copolymer. TG curves for DVB-[MimLcy]n show two decomposition steps (Figure S4, SI). The
first weight loss in the range of 250-400 °C is mostly due to the elimination of the Lcy anions. As
the ratio of MimLcy to DVB increased, the weight loss increased gradually from 10 wt% to 50
wt%. Further weight loss in the range of 400-550 °C can be attributed to the successive
degradation of polymeric backbones to H2O and CO2.
Figure 1. TEM images and the pertinent histograms with Au particle size distributions of (A)
Aua/DVB-[MimLcy]3, (B) Aub/DVB-[MimLcy]3
Three types of Au NPs immobilized on DVB-[MimLcy]3 were prepared by the adsorption
method (SI). Figure 1 and Figure S5 display the TEM images of the obtained catalysts
Aux/DVB-[MimLcy]3 (x = a, b, and c, indicates the Au particle size of 2-3, 8-9, and 10-12 nm,
respectively) and the pertinent histograms with the Au particle size distributions. It can be seen
that Au NPs adsorbed on this ionic copolymer display uniform distribution and the aggregates of
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Au NPs were not formed as a result of the drying step. The Au particle sizes of 2-3 nm, 8-9 nm,
and 10-12 nm and catalyst colors of brown, gray, and prunosus were observed for
Aua/DVB-[MimLcy]3, Aub/DVB-[MimLcy]3, and Auc/DVB-[MimLcy]3, respectively. The results
of Au loading, as determined by ICP-AES, are very close to the theoretical value 2 wt% for all the
three samples. The broad peak with 2θ values of 20o in the wide powder XRD pattern of
Aua/DVB-[MimLcy]3 can be readily attributed to the amorphous polymer, and two characteristic
peaks at 38.6o and 44.2o are indexed to (111) and (200) planes of the hexagonal phase of Au NPs,
respectively (Figure S6A, SI). Additionally, EDS elemental mapping demonstrated homogeneous
distribution of Au in the DVB-[MimLcy]3 (Figure S6B, SI).
(A)
(B)
1
10
Pore diameter dp (nm)
100
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p
o)
Figure 2. (A) FT-IR spectra of (a) DVB-[MimLcy]3 and (b) Aua/DVB-[MimLcy]3; (B) N2 sorption
isotherms and the BJH pore size distribution plots of Aua/DVB-[MimLcy]3.
Figure 2A shows a comparison of the FT-IR spectra of DVB-[MimLcy]3 and
Aua/DVB-[MimLcy]3. As shown in the figure, after the immobilization of Au NPs, the intensity of
the peaks at 2599 (-S-H) and 1600 cm-1 (-C=O) for Aua/DVB-[MimLcy]3 weakens, while the
intensity of peaks at 3400 and 840 cm-1 (-N-H) enhances. This implies that the strong adsorption
capability of DVB-[MimLcy]3 should be attributed to the -SH, -NH2, and -COO- groups, which
could form a strong coordination effect and electrostatic force with Au NPs. XPS analysis for
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Au/DVB-[MimLcy]3 was also performed (Figure S7, SI). The Au 4f XPS spectra can be resolved
into Au0 (83.36 and 87.04 eV) and Au+ (84.68 and 88.51 eV). The presence of Au+ is suggested to
arise from the strong interaction between Au and the support. In the S 2p spectrum, the signals at
162.5 and 163.8 eV can be assigned to the C-SH in DVB-[MimLcy]3. The position of these peaks
was shifted to lower energy than that of the common C-SH-containing compounds [28,29], which
further consolidate the strong interaction between Au and DVB-[MimLcy]3. The purple Au0
solution turned to colorless immediately after the addition of DVB-[MimLcy]3, also confirming
the strong interaction between the Au NPs and the support in Aua/DVB-[MimLcy]3 (Figure S8, SI).
The isothermal N2 adsorption−desorption analysis was performed to evaluate the porous feature of
Aua/DVB-[MimLcy]3 (Figure 2B). Aua/DVB-[MimLcy]3 provides typical type-IV isotherms at a
pressure region of 0.6 < P/P0 < 1, indicating the presence of mesoporosity. The BET surface area
for Aua/DVB-[MimLcy]3 is high, up to 335.4 cm2/g.
3.2 Oxidative coupling of alcohol and amine to form imines
Imines are of high importance as versatile intermediates in the synthesis of various
pharmaceuticals, fungicides, agricultural chemicals, and biological compounds [30]. Traditionally,
imines are synthesized by the acid- or noble metal-catalyzed condensation of aldehydes or ketones
with amines [31]. For environmentally benign and energy-saving purposes, the direct oxidative
coupling of alcohols and amines has been attracting increasing research interests. However, the
additives such as base are often required to achieve a high imine yield, which is unsatisfactory for
industrial production. Therefore, the development of the highly efficient and steadily recyclable
catalysts for the direct oxidative coupling of alcohols and amines under additive-free condition is
still a challenge. To the best of our knowledge, almost no additive-free heterogeneous catalytic
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systems of oxidative coupling of alcohols and amines have been reported.
The catalytic performances of Aux/DVB-[MimLcy]3 were first assessed in the oxidation
coupling of benzyl alcohol and benzylamine with TBHP by using toluene as the solvent under
base-free conditions; the results are listed in Table 1. Aua/DVB-[MimLcy]3 with Au NP particle
size of 2-3 nm presents the highest activity, providing 96% benzylamine conversion and 95%
benzyl alcohol conversion with 99% selectivity. The optimization of reaction time and TBHP
amount was investigated, and the results are shown in Tables S3 and S4 in SI.
Aub/DVB-[MimLcy]3 with Au NP particle size of 8-9 nm also exhibits a high benzylamine
conversion of 99% but offers a relatively low benzyl alcohol conversion of 72%. As the Au NPs
particle size increased to 10-12 nm, the benzylamine conversion and benzyl alcohol conversion
decreased to 86% and 58%, respectively. The catalytic activity of Aux/DVB-[MimLcy]3 in the
oxidation coupling reaction is higher or comparable to that of most of the previously reported
heterogeneous and homogeneous catalysts [32-34]. More importantly, the present reaction system
is much greener, that is, needless to add any additives.
Table 1. Catalytic behaviors of synthesized catalysts on the oxidative coupling of benzyl alcohol
and benzylamine.a
Benzylamine
Benzyl alcohol
Entry
Catalyst
Selectivity (%)
conversion (%) conversion (%)
1
2
3
4
Aua/DVB-[MimLcy]3
Aub/DVB-[MimLcy]3
Auc/DVB-[MimLcy]3
DVB-[MimLcy]3
96
99
86
34
95
72
58
28
99
98
95
92
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a Reaction conditions: benzyl alcohol (1 mmol), benzylamine (1 mmol), catalyst (0.02 g), TBHP
(100 µL); solvent, toluene (5 mL); temperature, 120 °C; time, 10 h. The byproduct was typically
benzaldehyde, and only negligible amount of phenylacetonitrile was detected.
It is well known that two possible competitive reactions can take place during the oxidative
coupling of benzylamine and benzyl alcohol [35]: (a) oxidation of benzyl alcohol to the aldehyde,
subsequently undergoing condensation with a benzylamine molecule to produce the imine; (b)
after the initial oxidative dehydrogenation of benzylamine, the dehydrogenation product
subsequently underwent condensation with a second molecule of benzylamine to produce the
imine (Scheme S1, SI). The catalytic results shown in Table 1 demonstrate that the above two
competitive reactions are significantly influenced by the particle size of Au. Both benzyl alcohol
and benzylamine reach very high conversions over Aua/DVB-[MimLcy]3; therefore, the reaction
can be assumed to proceed through path (a). The larger size of Au nanoparticles is not beneficial
for the conversion of benzyl alcohol to aldehyde, and thus, the reaction over Auc/DVB-[MimLcy]3
is implied to proceed mostly through path (b).
Considering that the presence of an organic functional group on a catalyst support may
contribute to the enhancement in catalysis, the catalytic performance of MimLcy was investigated.
As expected, MimLcy can catalyze the oxidative coupling of benzyl alcohol and benzylamine in
the absence of any metal active sites and displays 34% benezylamine conversion and 28% benzyl
alcohol conversion (Table 1, entry 4). However, MimBr has no activity in this reaction. Therefore,
it can be speculated that the weak basicity of Lcy anions in the catalyst should be responsible for
the excellent activity and selectivity of Aua/DVB-[MimLcy]3 in additive-free oxidative coupling
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reactions. Furthermore, contact angle (CA) measurement for Aua/DVB-[MimLcy]3 shows that
benzyl alcohol and benzylamine molecules can rapidly and almost completely spread on its
surface (Figure S9, SI). Thus, the excellent hydrophilic catalyst structure should have supplied a
unique accessible reaction microenvironment for the oxidative coupling reaction.
At the end of the reaction, the catalyst Aua/DVB-[MimLcy]3 can be separated by filtration,
washed with ethanol, and dried at 80 °C and then directly recharged into the next run for the
recycling test. The reaction was completely stopped after the catalyst was filtered off from the
reaction system (Figure S10, SI), indicating the heterogeneous nature of the present reaction
system. The results of a five-run recycling test shown in Figure S11 demonstrate that there is a
slight decline in conversion. After the fifth run, the conversions of benzyl amine and benzyl
alcohol were reduced to 80% and 72%, respectively. The TEM image of the reused catalyst
indicates the still uniformly dispersed Au NPs, but the particle size increased to 8-9 nm (Figure
S12, SI). Moreover, the specific surface area for the recovered catalyst was 205 m2/g, which is
slightly smaller than that of the fresh one (335 m2/g) (Figure S13, SI). These may be the reasons
for the slight loss of catalytic activity.
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Table 2. Aua/DVB-[MimLcy]3-catalyzed oxidative coupling of various alcohols and amines under
additive-free conditions.a
(88%, 87%) 99%, 42.86 h-1
(81%, 80%) 99%, 39.41 h-1
(84%, 81%) 96%, 39.90 h-1
(82%, 80%) 96%, 39.41 h-1
(84%, 83%) 98%, 40.89 h-1
(82%, 81%) 96%, 39.90 h-1
(83%, 81%) 96%, 39.90 h-1
(94%, 54%) 34%, 26.60 h-1
(83%, 82%) 98%, 40.39 h-1
(90%, 85%) 97%, 41.87 h-1
(95%, 43%) 77%, 21.18 h-1
(36%, 37%) 97%, 18.23 h-1
(34%, 37%) 97%, 18.23 h-1
(0%, 0%), -
(94%, 0%) 99%, -
a Reaction conditions: alcohol (1 mmol), amine (1 mmol), catalyst Aua/DVB-[MimLcy]3 (0.02 g),
o
TBHP (100 µL); solvent toluene (5 mL); temperature, 120 C; time 10 h; catalytic activity:
(benzylamine conversion%, benzyl alcohol conversion%) selectivity%, TOF Value.
To evaluate the scope of the reaction, we further tested the oxidative coupling of various
alcohols and amines using Aua/DVB-[MimLcy]3 as the catalyst without any additives. The results
are illustrated in Table 2. As shown in the table, good to excellent conversions could be obtained
in most of cases, and the type and location of substituent groups on the benzene ring have less
influence on the catalytic activity. However, relatively low activities are observed for the
secondary alcohol-contained oxidative coupling reactions. Particularly, benzhydrol fails to
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undergo oxidation and reacts with n-butylamine, and hence, no product is obtained. Moreover, in
the case of oxidative coupling of diphenylcarbinol with benzylamine, a high benzylamine
conversion of 94% is achieved, but the diphenylcarbinol conversion is almost zero. The low
catalytic activity of Aua/DVB-[MimLcy]3 for diphenylcarbinol-contained oxidative coupling
reactions can be attributed to the steric hindrance and limited diffusion of relatively bulkier
diphenylcarbinol, which may limit the interactions between substrates and active sites.
4. Conclusions
In conclusion, an L-cysteine-paired porous ionic copolymer (DVB-[MimLcy]3) was synthesized
by controlling the copolymerization of IL monomer and DVB. DVB-[MimLcy]3 with abundant
-SH, -NH2, and -COO- groups serves as an outstanding supporter and stabilizer for Au NPs. The
obtained typical catalyst Aua/DVB-[MimLcy]3 exhibited high activity and reusability in the
oxidative coupling of alcohols and amines to synthesize imines under additive-free conditions.
The narrowly distributed and highly dispersed Au NPs with small particle sizes of 2-3 nm,
together with the unique Lcy anions with weak basicity and hydrophilicity, account for the
excellent catalytic performance of Aua/DVB-[MimLcy]3 in additive-free oxidative coupling
reactions. This work provides a facile and effective strategy for the fabrication of immobilized
stable metal catalysts, which can be reasonably applied in more heterogeneous catalysis.
Acknowledgments
The authors thank the International Joint Research Laboratory for Biomass Conversion
Technology at Jiangnan University and MOE & SAFEA for the 111 Project (No. B13025) and the
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open fund of Jiangsu Key Laboratory for Biomass Energy and Material (No. JSBEM201906). The
authors thank the central laboratory of the School of Chemical and Material Engineering.
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Highlights
> Novel L-cysteine-paired porous ionic copolymer is prepared.
> Au-immobilized L-cysteine-paired copolymer can be used as a heterogeneous
catalyst.
> The catalyst gives high activity for additive-free oxidative coupling of alcohol and
amine.
> The catalyst can be easily recovered and reused.
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