Recyclable copper catalyst on chitosan for facile preparation of alkyl/aryl mixed phosphates via deaminated esterification between diphenylphosphoryl azides and aliphatic alcohols

Article abstract of DOI:10.1016/j.mcat.2020.111120

An efficient methodology was established for the preparation of alkyl/aryl mixed phosphates under aerobic conditions promoted by a easily recoverable heterogeneous catalyst. This highly active copper catalyst was obtained by directly mixing of copper precursor with chitosan solution, followed by a simple work-up process. In this catalytic system, different phosphoryl azides and aliphatic alcohols were employed to generate desired products in moderate to excellent yields with good functional group tolerance. This protocol provided a practical application of the chitosan supported heterogeneous copper catalyst towards a novel P–O bond formation.

Full text of DOI:10.1016/j.mcat.2020.111120

MolecularCatalysis494(2020)111120
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Recyclable copper catalyst on chitosan for facile preparation of alkyl/aryl
mixed phosphates via deaminated esterification between
diphenylphosphoryl azides and aliphatic alcohols
Lin-Yu Jiaoa,*, Ze Zhanga,1, Qian Honga,1, Zi-Hui Ninga, Shanshan Liub, Ming Suna,
Qingqing Haoa, Long Xua, Zhuo Lia, Xiao-Xun Maa,*
a School of Chemical Engineering, International Scientific and Technological Cooperation Base for Clean Utilization of Hydrocarbon Resources, Chemical Engineering
Research Center of the Ministry of Education for Advance Use Technology of Shanbei Energy, Shaanxi Research Center of Engineering Technology for Clean Coal
Conversion, Collaborative Innovation Center for Development of Energy and Chemical Industry in Northern Shaanxi, Northwest University, Xi'an, Shaanxi, 710069, PR
China
b Shaanxi Key Laboratory of Chemical Additives for Industry, College of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xi’an,
Shaanxi, 710021, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Copper catalysis
An efficient methodology was established for the preparation of alkyl/aryl mixed phosphates under aerobic
conditions promoted by a easily recoverable heterogeneous catalyst. This highly active copper catalyst was
obtained by directly mixing of copper precursor with chitosan solution, followed by a simple work-up process. In
this catalytic system, different phosphoryl azides and aliphatic alcohols were employed to generate desired
products in moderate to excellent yields with good functional group tolerance. This protocol provided a practical
application of the chitosan supported heterogeneous copper catalyst towards a novel PeO bond formation.
Chitosan supported catalyst
Phosphoryl azide
Aliphatic alcohol
Alkyl/aryl mixed phosphate
Esterification
1. Introduction
In the aforementioned transformations, some of phosphoric motifs,
reactants, as well as reagents are usually unstable, expensive, hard to
It has been recognized that phosphorus related organic compounds
are prominent motifs in biological systems and business applications
[1]. In the past decades, much efforts have been continuously devoted
not only to the basic research but also to the science and technology
achievements’ transformation. The introduction of phosphorus motifs
to the parent structures could provide diversified bonding properties
and polarization, hence their chemical and physical properties would
be substantially changed. Alkyl/aryl mixed phosphates, in particular,
are very important (sub)structures in fungicides, non-halogenated
flame retardants, pesticides, as well as pro-drugs [2], and several
known compounds exhibit bio-active characteristics [3]. However,
these compounds are very costly because they are rear in the nature.
As a consequence, in the last years several methodologies have been
fabricate in large scale, or even corrosive. Furthermore, most of these
transformations required harsh conditions with excess amount of ad-
ditives, which essentially limit their synthetic applications. Very re-
cently, our group have established a homogeneous catalytic protocol to
prepare desired alkyl/aryl mixed phosphates [9], in which diphenyl-
phosphoryl azide (DPPA) [10] was introduced as a novel and reactive
phosphoric reagent for the very first time. Although the high chemical
yield was achieved, the catalyst recyclability and the possible trace
mount of metal contaminant with product also restrict its application.
Hence, we are inspired to explore a highly efficient heterogeneous
catalytic system for this transformation.
In the past decades, heterogeneous catalysis with supported tran-
sition metals as active centers has become a prevailing fashion to realize
many organic transformations [11]. Importantly, compared with
homogeneous catalysis, it exhibits several advantages such as recover-
able, recyclability, and easily product separation, providing superior
platforms in the catalysis community. To achieve this goal, different
supports such as silica, polymers, carbon-based materials, zeolites etc
accomplished to synthesis alkyl/aryl mixed phosphates [4–8].
A
number of phosphorus containing compounds such as phosphorus
chloride [4], diphenyl phosphate [5], diphenyl phosphite [6], mixed
phosphite [7], as well as other substrates [8] were employed as starting
materials for the desired product preparation via different mechanisms.
⁎ Corresponding author.
E-mail addresses: lyjiao@nwu.edu.cn (L.-Y. Jiao), maxym@nwu.edu.cn (X.-X. Ma).
1 These authors contributed equally to this work.
https://doi.org/10.1016/j.mcat.2020.111120
Received 29 February 2020; Received in revised form 8 July 2020; Accepted 8 July 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

L.-Y. Jiao, et al.
MolecularCatalysis494(2020)111120
heterogeneous catalyst was observed via scanning electron microscope
(SEM, Carl Zeiss SIGMA). The X-ray photoelectron spectroscopy (XPS)
was conducted with a Thermo Fisher Scientific Nexsa™ instrument
equipped with an Al Kα X-ray radiation source (hν =1486.6 eV). The
binding energies (BE) were referenced to the C1s peak at 284.5 eV. Peak
fitting was performed by using XPSPEAK software with Shirley back-
ground. The actual metallic contents of catalysts was analyzed by in-
ductively coupled plasma-optical emission spectroscopy (ICP-OES) on
an Agillent 700DV instrument. 50 mg of sample was first digested (in a
mixture of HNO3, HF, and H2SO4) for 2 h. Then, aliquots of solution
were diluted to 250 mL using deionized water. The diluted samples
were introduced into the instrument and the area of the spectra ob-
tained were correlated to the concentration (thereby element content in
wt%) using a 5-point calibration curve in the range of 1–10 ppm.
Infrared (IR) spectra were recorded on a Frontier-PerkinElmer FT-IR
spectrophotometer equipped with an ATR unit with BaSO4 as a re-
ference and were reported in wavenumbers (cm–1). Analytical thin-
layer chromatography (TLC) was performed on silica gel GF254 glass
plates from Qingdao Haiyang Chemical Co. Lt. Flash column chroma-
tography was performed on silica gel (300–400 mesh) by Qingdao
Haiyang Chemical Co. Lt using the indicated solvents. 1H, 13C, and 31P
NMR spectra were recorded in CDCl3 on Bruker Avance III 400 MHz
instrument. High resolution mass spectrometry (HRMS) analyses were
performed on an Agilent 6460 instrument. Gas chromatography (GC)
was performed on a Fuli 9790II gas chromatography equipped with a
KB-5 capillary column (30 m ×0.32 mm, 0.25 μm film thickness) by
Fuli and high pure nitrogen as a carrier gas with flame ionization de-
tector.
Scheme 1. Chitosan supported palladium and copper catalysts in previous and
present transformations.
have been successively developed to immobilize the active species [12].
Among these achievements, polymers especially biopolymers are con-
sidered as environmentally friendly materials and have been widely
used in heterogeneous catalysis [13]. One of the most outstanding ex-
amples is chitosan (CS), which can be easily obtained and modified
through functionalization of naturally abundant chitin [14]. Ad-
ditionally, a variety of valuable chemicals and materials can be ac-
cessed from chitin/chitosan under the concept of "shell biorefinery",
which was proposed recently and gathered much attention immediately
afterwards [15]. Up to now, chitosan anchored with different transition
metals, for example palladium, titanium, rhodium, and copper have
exhibited high catalytic activities towards several kinds of bond for-
mations, such as CeB [16], CeC (Scheme 1a) [17], CeN (Scheme 1b)
[18], CeO [19], CeS (Scheme 1c) [20], and CeSi [21] bond con-
struction [22]. Significantly, transition metals immobilize on the chit-
osan to form stable compounds, and in some cases chitosan can even be
used as metal remover in aqueous solutions [23]. It is very meaningful
to extend the application of chitosan based heterogeneous catalyst to
novel organic transformations.
2.3. General procedure for the preparation of chitosan supported copper
catalysts
The chitosan supported copper catalysts were synthesized according
to the following procedure: 5 g of chitosan was suspended in 100 mL of
deionized water in a 250 mL round bottom flask. To this suspension,
1.0 g of CuSO4, Cu(OTf)2, CuCl, or Cu powder was added at 50 °C and
stirred for 3 h. After adsorption of the copper, the solid was precondi-
tioned by washing thoroughly to remove the unsupported copper salts.
The CS@copper catalysts were obtained by using
a centrifuge
(5000 rpm/5 min) and dried under vacuum at 50 °C for 12 h.
2.4. General procedure for the synthesis of alkyl/aryl mixed phosphates
A flame-dried Schlenk tube equipped with a magnetic stir bar was
successively charged with phosphoryl azides (0.50 mmol, 1.0 equiv.),
aliphatic alcohols (excess amount), CS@CuCl (30 mg, 10 mol% of [Cu]
loading), and Na2CO3 (26 mg, 0.25 mmol, 0.5 equiv.) in indicated sol-
vent or mixture (2.0 mL, 0.25 M). The reaction mixture was stirred at
the indicated temperature and time. The reaction was monitored by
TLC. After completion of the reaction, the reaction mixture was allowed
to cool to room temperature and filtered. The solid stuff was washed
with cyclohexane, dried in the oven, and collected to reuse for further
catalytic reactions. The filtrate was diluted with ethyl acetate (25 mL)
and washed with 1% HCl (aq, 5 mL) and H2O (5 mL). The organic phase
was dried over anhydrous Na2SO4 and concentrated under reduced
pressure. Purification by flash column chromatography on silica gel
using mixtures of petroleum ether and ethyl acetate as eluents afforded
the analytically pure product.
In continuation of our research topic on the development of syn-
thetic methodologies [9,24], and in combination with our interests in
heterogeneous catalysis, herein we wish to elaborate a chitosan sup-
ported cuprous chloride as an efficient and recyclable catalyst for the
preparation of a wide range of alkyl/aryl mixed phosphates deriving
from diphenylphosphoryl azides and aliphatic alcohols under aerobic
conditions (Scheme 1d).
2. Experimental part
2.1. Materials
All reagents and reactants were procured from Energy Chemical
(Shanghai) and used as received. Technical grade solvents for extrac-
tion and chromatography (ethyl acetate, petroleum ether, and diethyl
ether) were distilled prior to use and other solvents were used directly
without further purification.
3. Results and discussion
Initially, the optimization process started by using DPPA (1a) as
substrate and n-propyl alcohol (2a) as conventional nucleophile
(Table 1). In the reaction setup, different chitosan supported copper
salts were examined as heterogeneous catalysts. As illustrated, the
model starting material indeed underwent deaminated esterification to
2.2. Instrumentation methods
The X-ray diffraction (XRD) patterns were obtained by a powder X-
ray diffractometer (SmartLAB SE, Rigaku). The micromorphology of the
2

L.-Y. Jiao, et al.
MolecularCatalysis494(2020)111120
Table 1
Identification of the reaction conditions for the chitosan supported copper catalyst on the deaminated esterification of DPPAa.
Entry
Catalyst (x)
Additive
Solvent
T (oC)
t (h)
Conversion (%)b
Yield (%)c
1
2
3
4
5
6
7
8
CS@CuSO4 (10)
CS@Cu(OTf)2 (10)
CS@CuCl (10)
CS@Cu (10)
CS@CuCl (10)
CS@CuCl (10)
CS@CuCl (5)
CS@CuCl (10)
CS@CuCl (10)
CS@CuCl (10)
bare CS
e
n-PrOH
n-PrOH
n-PrOH
n-PrOH
n-PrOH
n-PrOH
n-PrOH
n-PrOH
40
40
40
40
60
80
80
80
80
80
80
80
80
24
24
24
24
24
24
40
24
24
24
24
24
24
< 5
8
10
< 5
31
90
76
79
94
87
trace
5
9
e
e
e
trace
28
84
74
71
90
81
17
90
74
e
e
e
NaOH
Na2CO3
NaHCO3
Na2CO3
Na2CO3
Na2CO3
9
n-PrOH
n-PrOH
n-PrOH
10
11
12
13
25
92
77
CS@CuCl (10)
CS@CuCl (10)
cyclohexane:n-PrOHd
CH2Cl2:n-PrOHd
a
Reaction conditions: DPPA (1a, 0.5 mmol, 1.0 equiv.), n-propyl alcohol (2a, excess amount), additive (0.25 mmol, 0.5 equiv.), catalyst (indicated amount),
tetracosane as internal standard, solvent (2.0 mL, 0.25 M) in a reaction tube under air atmosphere.
b
Determined by gas chromatography (GC) analysis.
Isolated yield through silica gel based flash column chromatography.
Volume ratio: 9:1.
c
d
form a new PeO bond as expected, although the desired product 3aa,
namely diphenyl propyl phosphate, was generated in very low con-
versions and yields at 40 °C (entries 1–4). It was shown that CS@CuCl
gave the best performance among these candidates (entry 3). The mo-
lecular structure was confirmed unambiguously through NMR (nuclear
magnetic resonance) and HRMS (high resolution mass spectroscopic)
analyses. Then several parameters such as reaction temperatures, ad-
ditives, and reaction mediums were investigated. It has been proved
that the reaction temperature played an important role in the reactivity.
Elevating temperature from 40 °C to 80 °C could provide essential im-
provement of the isolated yields (entries 3, 5, and 6). Notably, the
deaminated esterification underwent smoothly even with catalyst
loading reduced by half, albeit with a lower yield and much longer
reaction time (entry 7). Additives such as NaOH, Na2CO3, and NaHCO3
were evaluated to alter catalytic system, in which Na2CO3 obviously
exhibited positive effect thereof (entries 8–10). Interestingly, the bare
chitosan itself also showed low catalytic activity towards the model
reaction (entry 11), providing promising evidence in this topic. Con-
cerning the generality of our protocol and the increased solvent visc-
osity of other alcohols, we decided to introduce other solvent to form
mixed reaction media. Non-polar solvent for example cyclohexane
proved to be an ideal choice, giving rise to 90 % yield (entry 12).
Therefore, the optimal conditions was performed with the aid of 10 mol
% of CS@CuCl as catalyst, 0.5 equivalent of Na2CO3 as additive at
80 °C, under which either aliphatic alcohol alone or cyclohexane con-
taining reaction medium performed equally well (entries 9 and 12).
Importantly however, it was noteworthy that the isolated yields and
conversions correlated very well, we did not detect any other by-pro-
duct in this system with almost unreacted DPPA recovery (isolated after
purification by flash column chromatography on silica gel, not shown).
With the optimized catalytic conditions established, we were next
promoted to investigate various phosphoryl azides 1a–1k and aliphatic
alcohols 2a–2s with different substituents (Table 2). Not to our sur-
prise, moderate to excellent results were obtained for all the straight
chain fatty alcohols from C1 to C8 (2a–2c → 3aa–3ac, 2g → 3ag, 2i →
3ai, 2k–2m → 3ak–3am) (entries 1–3, 7, 9, and 11–13). Gratifyingly,
methanol (2b) and ethanol (2c) represented highly reactive nucleo-
philic reagents, providing full conversions and 90–92 % yields within
3 h (entries 2–3). Moreover, the fluorinated derivatives trifluoroethanol
(TFE, 2d) and hexafluoroisopropanol (HFIP, 2f) afforded corresponding
products in 84 % and 96 % yield, respectively (entries 3–6). When
cycloalkyls substituted alcohols such as cyclopentanol (2n) and cyclo-
hexanol (2o) were investigated, decreased results were obtained (en-
tries 14 and 15). It might be attributed to the steric hindrance effect on
the SN2 attack process. This hypothesis was convinced immediately
with less bulky substrates 2p and 2q containing at least one methylene
(CH2) group, and good results were realized though (entries 16 and 17).
Remarkably, this protocol was not applicable for either benzyl alcohol
(2r) or phenethyl alcohol (2s) bearing similar functional groups,
however, only traces amounts of products were observed in these aro-
matic substrates (entries 18 and 19).
Subsequently, we then turned our attention to examine different
phosphoryl azides under identical reaction conditions with methanol
(2b) as the oxygen source (Table 2). These reagents have been de-
monstrated to be effective under the heterogeneous catalysis, providing
methyl diaryl phosphate derivatives in moderate to excellent yields.
Generally, functional groups commonly existing in organic synthesis
such as methyl (1b–1d), other alkyl (1g and 1h), methoxyl (1i and 1j),
were well tolerated. However, bromo (1e and 1f) substituted substrates
led to reduced yields (entries 20–28). Compared with phosphoryl azides
bearing single substituent on the aromatic core, disubstituted starting
material at both 3- and 5-positions had performed equally well, re-
gardless of the electronic or steric properties (1k → 3ka) (entry 29).
Furthermore, this heterogeneous catalysis was proceeded success-
fully in gram scale reaction with 86 % yield (5.0 mmol of 1a was em-
ployed as starting material), further demonstrated the practicality and
scalability of our protocol (Table 2, entry 1).
Based on the aforementioned results, we then extended the alcohols
to more complicate glycerol derivative 2t and citronellol (2u) (Scheme
2). Compared with desired product 3au in 84 % yield, however, alcohol
2t with a dimethylketal group seemed unstable under basic conditions,
and only moderate result was achieved finally.
3

L.-Y. Jiao, et al.
MolecularCatalysis494(2020)111120
Table 2
Variation of phosphoryl azides substrate and alcohols reagent under the optimal catalytic conditions.a.
Entry
R1
R2
Solvent
Product
Yield (%)b
1
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
n-PrOH (2a)
Me (2b)
Et (2c)
CF3CH2 (2d)
i-PrOH (2e)
(CF3)2CH (2f)
n-Bu (2g)
i-Bu (2h)
n-Pen (2i)
i-Pen (2j)
n-Hex (2k)
n-propanol
methanol
ethanol
CF3CH2OH
i-propanol
(CF3)2CHOH
n-butanol
i-butanol
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
3aj
3ak
3al
3am
3an
3ao
3ap
90 (86)c
90
92
84
69
96
89
93
91
81
80
93
83
59
37
84
2d
3d
4
5
6
7
8
9
cyclohexane:n-pentanole
cyclohexane:i-pentanole
cyclohexane:n-hexanole
cyclohexane:n-heptanole
cyclohexane:n-octanole
cyclohexane:cyclopentanole
cyclohexane:cyclohexanole
cyclohexane:
10
11
12
13
14
15
16
n-Hep (2l)
n-Oct (2m)
cyclopentyl (2n)
cyclohexyl (2o)
cyclohexyl methyl (2p)
cyclohexylmethanole
cyclohexane:
17
H (1a)
cyclohexyl ethyl (2q)
3aq
87
cyclohexylethanole
cyclohexane:benzyl alcohole
cyclohexane:phenethyl alcohole
methanol
18
19
H (1a)
H (1a)
benzyl (2r)
phenethyl (2s)
Me (2b)
Me (2b)
Me (2b)
Me (2b)
Me (2b)
Me (2b)
Me (2b)
Me (2b)
Me (2b)
Me (2b)
3ar
3as
3bb
3cb
3db
3eb
3fb
3gb
3hb
3ib
3jb
trace
trace
91
89
89
32
64
92
86
20d
21d
22d
23d
24d
25d
26d
27d
28d
29d
2-Me (1b)
3-Me (1c)
4-Me (1d)
2-Br (1e)
4-Br (1f)
2-Et (1g)
2-i-Pr (1h)
3-OMe (1i)
4-OMe (1j)
3,5-Me (1k)
methanol
methanol
methanol
methanol
methanol
methanol
methanol
methanol
76
85
91
methanol
3kb
a
Reaction conditions: 1a–1k (0.5 mmol, 1.0 equiv.), 2a–2s (excess amount), CS@CuCl (0.05 mmol, 10 mol%), Na2CO3 (0.25 mmol, 0.5 equiv.) in solvent (2.0 mL,
0.25 M) at 80 °C for 24 h under air atmosphere.
b
Isolated yield through silica gel based flash column chromatography.
Yield in the bracket refers to the catalysis conducted in gram scale.
Reactions proceeded for 3 h.
Volume ratio: 9:1.
c
d
e
Scheme 2. Investigation the performance of glycerol derivative and citronellol
under the optimized conditions.
Fig. 1. XRD patterns of (a) chitosan; (b) CS@CuCl; and (c) recovered CS@CuCl
after three runs.
4

L.-Y. Jiao, et al.
MolecularCatalysis494(2020)111120
Fig. 2. SEM images of CS@CuCl. (a) before reaction; (b) recovered CS@CuCl after three runs.
The XRD patterns of the pure chitosan itself and supported hetero-
interacted with the copper [26]. In addition, the absorption bands at
1065 cm−1 (CeO groups) shifted to a lower frequency; suggested that
eOH groups also took part in the reaction (Fig. 3a). The peak at
2100–2000 cm-1 of the spent catalyst illustrated the azide group’s ex-
istence (Fig. 3b). It seems that traces amount of DPPA would remain on
the chitosan supporter after the recovery. It is worthy of note that there
are no obvious changes of the spectra for the recycled heterogeneous
catalyst (spectra with peaks could be found in the Figs. S1 and S2 in the
Supplementary Data).
geneous catalyst are illustrated in Fig. 1. The data revealed that the
related signal of copper did not show up. It was probably due to the fact
that the copper species was adsorbed on the eNH2 group of the sup-
porter as ions rather than as particles, and thus no XRD signal was
observed [20,25]. Furthermore, after the copper loading, the crystal-
linity of chitosan apparently decreased as indicated by XRD (Fig. 1a vs
1b), possibly due to the loosen chain arrangements with the in-
corporation of copper atoms.
A comparison of the surface morphology of chitosan supported
copper catalyst and recovered CS@CuCl after three runs is shown in
Fig. 2. The SEM images of the catalyst examined before and after the
heterogeneous catalytic application did not show significant changes in
the morphology, although the surface of the spent catalyst became
more smoother and the colour was getting lighter compared with the
fresh-prepared catalyst.
FT-IR spectra of fresh-prepared and recovered CS@CuCl catalyst are
illustrated in Fig. 3 and display a broad range of multiple strong ab-
sorption between 3600–3200 cm−1. Compared with pure chitosan (Fig.
S1), the wide peak at around 3424 cm−1, corresponding to the vibra-
tions of eOH and eNH2 groups, shifted to a lower frequency in the
fresh-prepared catalyst (Fig. 3a, for spectra with peaks, see Fig. S2);
indicated that these groups were involved in the complexation. The
absorption band at 1649 cm−1 and 1599 cm−1 assigned to the acet-
amide and amine group, respectively, disappeared in the supported
catalyst (Figs. S1 vs 3a/b) [23,26]. Interestingly, a new absorption band
at 1632 cm-1 appeared, which was probably the characteristic peak of
the chitosan-copper association, indicated that the “free” amine group
The loading amount of copper in the fresh-prepared and recovered
catalysts were determined by chemical analysis using ICP-OES and the
results are summarized in Table 3. After three cycles of catalysis, de-
cline in copper content from 9.46 % to 8.24 % (weight percentage) was
observed, indicating that the copper species were indeed slowly dis-
sociated from chitosan supporter either during the catalysis or re-
covered process [27], and this result might be responsible for the loss of
activity.
XPS technique is usually adopted with the objective to elucidate the
chemical states of various elements in the materials’ surface [28]. In
Fig. 4, it shows three sets of Cu 2p binding energies of high-resolution
XPS for fresh-prepared and used CS@CuCl catalyst. The survey spectra
showed obvious binding energy peaks of Cu 2p and clearly displayed
the co-existence of Cu2+ and Cu+ on the surface of both samples. In one
hand, the existence of Cu2+ could be identified by the peaks at 934.0 eV
(Cu 2p3/2) and 953.2 eV (Cu 2p1/2), respectively. In addition, the strong
Cu2+ satellites peaks located at both 943.4 eV and 963.2 eV further
allowed to conclude the existence of Cu2+ species. In the other hand,
the relative strong peaks at binding energies of 932.1 eV (Cu 2p3/2) and
951.6 eV (Cu 2p1/2) respectively confirmed the presence of the Cu+
species. It can be obviously concluded that the two spectra showed
highly overlapped curves with each other. It should be noticed that the
surface atomic ratio of Cu2+ to Cu+ exhibited very small changes from
0.18 for the fresh-prepared CS@CuCl (Fig. 4a) to 0.16 for recovered
catalyst after three runs (Fig. 4b).
In order to further study the application of this protocol, the reu-
sability of chitosan supported heterogeneous catalyst was discussed
(Fig. 5). The recycling studies on the indicated substrates revealed that
the catalytic activity was not deteriorated dramatically after three cy-
cles. However, it was not clear enough to explain the loss of activity
since the ICP-OES characterization only showed little evidence asso-
ciated with the copper species indeed leaching either during the cata-
lytic reaction or work-up procedure. Furthermore, we evaluated the
Table 3
Copper content for fresh-prepared and recovered catalyst.
Entry
Sample
Cu content (wt%)
1
2
Fresh-prepared CS@CuCl
CS@CuCl after 3 runs
9.46
8.24
Fig. 3. FT-IR spectra of CS@CuCl. (a) before reaction; (b) recovered CS@CuCl
after the third run.
5

L.-Y. Jiao, et al.
MolecularCatalysis494(2020)111120
Declaration of Competing Interest
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.
CRediT authorship contribution statement
Lin-Yu Jiao: Supervision, Project administration, Writing - review
&
editing. Ze Zhang: Investigation, Methodology. Qian Hong:
Methodology. Zi-Hui Ning: Methodology. Shanshan Liu: Supervision,
Visualization. Ming Sun: Formal analysis. Qingqing Hao: Formal
analysis. Long Xu: Formal analysis. Zhuo Li: Writing - review &
editing. Xiao-Xun Ma: Supervision.
Acknowledgments
Fig. 4. XPS spectra of CS@CuCl. (a) fresh-prepared; (b) recovered CS@CuCl
after the third run.
We are grateful to the National Key R&D Program of China
(2018YFB0604603), the National Natural Science Foundation of China
(NSFC: 21606182, 21901150), and the Natural Science Basic Research
Program of Shaanxi – Shaanxi Coal and Chemical Industry Joint
Foundation (Program No.: 2019JLM-20) for financial support.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111120.
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