Chemical fixation of CO2 to cyclic carbonate catalyzed by new environmental- friendly bifunctional bis-Β-cyclodextrin derivatives

Article abstract of DOI:10.1016/j.cattod.2018.06.020

In present work, three amino-bridged biomass bis-β-cyclodextrins (bis-β-CDs) and eight bifunctionalized bis-β-CDs derivatives were designed and synthesized as environmental friendly catalysts to fix CO₂. Both the bridged bis-β-CDs 1a-3a/potassium halide and haloalkane functionalized bis-β-CDs 1b-8b were poven to be efficient catalytic systems for the solvent-free coupling reaction of CO₂ and epoxides. After systematic investigation, the optimized conditions of (120 °C, 4 MPa, 0.14mol% 1a, 1.25mol% KI, 2 h) have been established for dual catalytic system 1a/KI and the excellent yield (94%) with high selectivity (99%) can be achieved for the cycloaddition of CO₂ and propylene oxide. Furthermore, after functionalization with alkyl halides, bis-β-CD derivative 1b-8b as sigle bifunctional catalyst exhibited better activity under milder optimal conditions of (110 °C, 2 MPa, 0.125mol% 1b and 4 h). Moreover, these bifunctional catalysts are also applicable to a variety of epoxides (including diepoxides) and good catalytic performances were abtained for producing the corresponding cyclic carbonates in most cases. Furthermore, this catalyst can be reused at least for five times without significant activity decrease. Finally, a possible mechanism including the activation of both epoxide and CO₂ was proposed based on the literatures and experimental results.

Full text of DOI:10.1016/j.cattod.2018.06.020

Accepted Manuscript
Title: Chemical fixation of CO2 to cyclic carbonate catalyzed
by new environmental- friendly bifunctional
bis-␤-cyclodextrin derivatives
Authors: Jing Peng, Sheng Wang, Hai-Jian Yang, Binru Ban,
Zidong Wei, Lihua Wang, Bo Lei
PII:
S0920-5861(18)30744-2
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.06.020
CATTOD 11509
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
27-1-2018
9-5-2018
14-6-2018
Please cite this article as: Peng J, Wang S, Yang H-Jian, Ban B, Wei Z,
Wang L, Bo L, Chemical fixation of CO2 to cyclic carbonate catalyzed by new
environmental- friendly bifunctional bis-␤-cyclodextrin derivatives, Catalysis Today
(2018), https://doi.org/10.1016/j.cattod.2018.06.020
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Chemical fixation of CO2 to cyclic carbonate catalyzed by new environmental- friendly
bifunctional bis-β-cyclodextrin derivatives
Jing Penga,b, Sheng Wanga, ǂ, Hai-Jian Yang*,a, Binru Bana, Zidong Wei*,b, Lihua Wanga, Bo Leia
aKey Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission &
Ministry of Education, Hubei Province, Key Laboratory of Analytical Chemistry of the State Ethnic
Affairs
Commission,
National Demonstration Center for Experimental Ethnopharmacology
Education, College of Chemistry and Materials Science, South-Central University for Nationalities,
Wuhan, 430074, P.R. China.
Tel./Fax: +86-27-67842752; Email addresses: yanghaijian@vip.sina.com
bThe State Key Laboratory of Power Transmission Equipment & System Security and New
Technology, College of Chemistry and Chemical Engineering, Chongqing university, Chongqing
400044, China.
Email addresses: zdwei@cqu.edu.cn
ǂ co-first author

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Graphic abstract
(OH)
NH
n
(OH)
6
HN
6
1a: n = 2
2a: n = 4
3a: n = 6
KI
+
(OH)
14
(OH)
14
R
O
O
R
CO
+
2
O
O
R
R
1b: n = 2, R = C H , X = I
4
9
+
NH
+
NH
2b: n = 2, R = C H , X = Br
4
9
(OH)
n
(OH)
6
6
3b: n = 2, R = C H , X = Cl
4
9
4b: n = 4, R = C H , X = I
X
4
9
X
5b: n = 6, R = C H , X = I
4
9
6b: n = 2, R = C H , X = I
2
5
(OH)
7b: n = 2, R = CH , X = I
14
(OH)
14
3
8b: n = 2, R = C H , X = I
8
17
Research Highlights:
 New environmental friendly bifunctional bis-β-CDs were synthesized for the cycloaddition of CO2
and epoxides.
 Excellent activity was achieved under milder optimal conditions.
 These bifunctional catalysts are applicable to a variety of epoxides including diepoxides.
 The catalyst can be reused at least for five times without significant activity decrease.
 A possible mechanism including the activation of both epoxide and CO2 was proposed.
2

Put your running title here: Elsevier General template for review
Abstract
In present work, three amino-bridged biomass bis-β-cyclodextrins (bis-β-CDs) and eight bifunctionalized
bis-β-CDs derivatives were designed and synthesized as environmental friendly catalysts to fix CO2. Both
the bridged bis-β-CDs 1a-3a/potassium halide and haloalkane functionalized bis-β-CDs 1b-8b were poven
to be efficient catalytic systems for the solvent-free coupling reaction of CO2 and epoxides. After systematic
o
investigation, the optimized conditions of (120 C, 4 MPa, 0.14mol% 1a, 1.25mol% KI, 2 h) have been
established for dual catalytic system 1a/KI and the excellent yield (94%) with high selectivity (99%) can
be achieved for the cycloaddition of CO2 and propylene oxide. Furthermore, after functionalization with
alkyl halides, bis-β-CD derivative 1b-8b as sigle bifunctional catalyst exhibited better activity under milder
optimal conditions of (110 oC, 2 MPa, 0.125mol% 1b and 4 h). Moreover, these bifunctional catalysts are
also applicable to a variety of epoxides (including diepoxides) and good catalytic performances were
abtained for producing the corresponding cyclic carbonates in most cases. Furthermore, this catalyst can be
reused at least for five times without significant activity decrease. Finally, a possible mechanism including
the activation of both epoxide and CO2 was proposed based on the literatures and experimental results.
Keywords: Chemical fixation of carbon dioxide; Bifunctional bis-β-cyclodextrins catalysts; Solvent-free;
Epoxide; Cyclic carbonate
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1. Introduction
As a renewable C1 source and greenhouse gas on the carpet for global warming, carbon dioxide (CO2)
has the advantages of being economical, abundant, nonflammable, non-toxic and capable of replacing fossil
fuel sources [1-3]. The coupling reaction between CO2 and epoxide is a 100% atom economical reaction
and environment-friendly because of the efficient resource utilization [4, 5]. The synthesis of cyclic
carbonates from pure CO2 or impure flue gas CO2 [6,7] and epoxides has attracted great interest both
industrially and academically that are extensively used as polar aprotic solvents, lithium batteries
electrolytes, fuel additives and chemical manufacture intermediates, et al [8-11]. Considering that a large
energy input is always required for almost all procedures of CO2 conversion [12, 13], appropriate and
efficient catalysts are necessary to facilitate the reaction.
During past decades, many catalytic systems have been developed for the synthesis of cyclic carbonates
from CO2 and epoxides, including Schiff base metal complexes [14-17], metal oxides, metalloporphyrins
[18-21], ionic liquids, functional polymers [22-27], N-heterocyclic carbenes (NHC), organocatalysts [28-
33], metal-organic frameworks (MOFs) [34-41], nanoparticles, mesoporous materials [44-48], biomass
[49-66], and so on. However, the real application of most kinds of above mentioned catalysts generally
suffers from low to moderate catalytic activity and selectivity. Besides, a co-solvent/additive and harsh
reaction conditions such as high pressure and/or high temperature are usually necessary in many cases.
Meanwhile, the inherent corrosion, toxicity and environmental problems associated with metallic cations
are also concerned from the viewpoint of green chemistry. Thus, it is highly desirable to develop efficient,
metal-free and environment-friendly catalytic system for the cycloaddition of carbon dioxide with epoxide
[49-50].
Among the reported catalysts, biomass-based eco-friendly catalysts have attracted much attentions due
to their green, nontoxic, full of hydroxyl groups to accelerate the cycloaddition reaction of CO2 with
epoxides through the hydrogen bond on the oxygen atom of the epoxides, and environmental-friendly
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characteristics. In this respect, betaine [53], lecithin [54], sugarcane bagasse [55], chitosan-based [56-60],
cellulose-based [61-65] and chitin-supported [66] compounds, β-cyclodextrin (β-CD) [67-68] were
reported as catalysts for the synthesis of cyclic carbonates from epoxides and CO2. As a natural hydrogen
bond donor, β-cyclodextrin (β-CD) is a well-known fascinating biopolymer (oligosaccharide) with
abundant hydroxyl groups and is cheap, stable, and readily available. Thus, β-CD is really a hot topic in
recent years and has been used in many reactions as catalyst component. Unfortunately, compared to the
use of other biological materials, the utilization of β-CD-based complexes for chemical fixation of CO2 is
rare. In 2008, Buxing Han’s research group has firstly proven that the dual system of β-CD and potassium
halide was an excellent catalyst system for the coupling reaction of CO2 with epoxides [67]. Their work
extended the application of β-CD to a new area, and more catalyts based on β-CD family were expected to
be studied. Unfortunately, since then, there was almost no paper were published in the area of cyclodextrin-
catalyzed cycloaddition reaction of CO2 with epoxides. Until 2017, Hou et al. [68] developed a ternary
system of DBU-based phenolates and their inclusion complexes with β-CD as catalyst for this reaction.
It was surprised to realize that no β-CD derivative was used for the fixation of CO2 so far, in spite of
various easy-to-handle ways to functionalize β-CD. We were intrigued by the catalytic performance of
environmental benign β-CD and want to find more one-component β-CD catalysts considering about the
easy-modification structure of β-CD. Based on the motif towards activating CO2 and epoxide, a Lewis acid
species and a nucleophile are required to build the catalyst of this coupling reaction. Indeed, many single
component catalysts which consisted of a metal centre and an anion were developed [69], such as the salen
scaffold catalysts developed by Melꢀndez [70] and charged metalloporphyrin polymers developed by Chen
[71]. In this work, the Lewis acidic metal center is replaced by hydroxy groups as hydrogen bond donors,
thus bis-β-CDs 1a-3a were derived from naturally available biomass and utilized as catalyst for the
cycloaddition reaction of CO2 and epoxides along with KI as a nucleophile. On that basis, several
ammonium functionalized bis-β-CDs 1b-8b were also synthesized and acted as one-component
bifunctional catalysts for this reaction in the absence of any additives and solvents. The systematical
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investigation of catalytic performances for both catalytic systems gave out the optimal catalytic conditions,
under which high cyclic carbonate yield and excellent selectivity can be achieved. To the best of our
knowledge, it is the first example to use the functionalized β-CD as one-component catalyst for CO2 fixation.
<Scheme 1>
2. Experimental section
2.1. Chemicals and analytical methods
The starting materials, β-CD, hexamethylenediamine were purchased from Sinopharm Chemical
Reagent Co.,Ltd, China; p-toluenesulfonyl chloride, butanediamine were purchased from Aladdin; Anion
exchange resin 1x8 200–400 mesh in Cl- form (Dowex®)was from Alfa Aesar chemicals Co., Ltd;
Ethanediamine was from Acros organics. Iodomethane was purchased from Xiya Reagent Reasearch center;
Iodoethane, Iodobutane was purchased from Shanghai Darui Fine Chemical Co., Ltd. All of the epoxides
we used were purchased from J&K Scientific Ltd. CO2 (99.99%) was provided by Sichuan Tianyi Science
& Technology Co., Ltd.
1H and 13C NMR spectroscopy was performed on a Bruker Al-400 MHz instrument using TMS as
internal standard. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 FT-IR
spectrometer in the range of 600-4000 cm-1 with the samples pressed into KBr. The melting points were
carried out on a X-4 digital microscopic melting point apparatus.
2.2. Preparation of bis-β-CDs 1a-3a and amino-functionalized bis-β-CDs 1b-8b
The 6-o-monotosyl-β-cyclodextrin (mono-Ts-β-CD) and amino-bridged bis-β-CD 1a-3a were
synthesized according to the literature (Scheme 2) [72] with quantitative yields. Compound 1a was known
and its structure was confirmed with 1H NMR only.
1a. 1H NMR (DMSO-d6): δ = 6.18 – 5.50 (m, 28H), 5.04 – 4.73 (m, 14H), 4.72 – 4.21 (m, 12H), 3.95 –
3.82 (m, 4H), 3.80 – 3.20 (m, 80H), 2.88 – 2.74 (m, 8H).
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2a. Mp. 218 – 220 oC. Selected IR peaks (KBr, cm-1): υ 3379, 2929, 1703, 1644, 1412, 1366, 1154, 1055,
1
1036, 941, 578. H NMR (DMSO-d6): δ = 6.43 – 5.57 (m, 28H), 5.16 – 4.78 (m, 14H), 4.77 – 4.22 (m,
12H), 3.98 – 3.89 (m, 4H), 3.81 – 3.25 (m, 80H), 2.76 – 2.66 (m, 8H), 1.64 – 1.50 (m, 4H). 13C NMR (101
MHz, DMSO) δ 102.36, 82.03, 73.50, 72.85, 72.48, 60.39, 51.65, 48.96, 30.40. GC-MS: m/z = 2321.82,
2320.82, 2322.83.
3a. Mp. 240 – 242 oC. Selected IR peaks (KBr, cm-1): υ 3376, 2928, 1703, 1658, 1411, 1364, 1153, 1057,
1
1036, 941, 578. H NMR (400 MHz, DMSO-d6) δ = 5.98 – 5.55 (m, 28H), 4.95 – 4.76 (m, 14H), 4.75 –
4.32 (m, 12H), 3.99 – 3.81 (m, 4H), 3.80 – 3.20 (m, 80H), 2.95 – 2.57 (m, 8H), 1.58 – 1.47 (m,4H), 1.46 –
1.35 (m, 4H). 13C NMR (101 MHz, DMSO-d6) δ = 102.44, 82.09, 73.53, 72.89, 72.49, 60.43, 49.04, 30.51,
29.42. GC-MS: m/z = 2349.85, 2348.85, 2350.86.
The ammonium-functionalized bis-β-CDs 1b-8b were synthesized using a modified literature procedure
(Scheme 2) [73]. In a 250 ml round bottom flask, bis-β-CD and excess of alkyl halide were mixed together
and stirred for 12 h at 80 oC. After the reactants cooling to room temperature, the product was precipitated
through adding an excess of acetone to the solution. After filtration and washing with ethanol for three
times, the product was dried under reduced pressure to obtain ammonium-functionalized bis-β-CDs 1b-8b.
1b. Mp. 214 – 216 oC. Selected IR peaks (KBr, cm-1): υ 3392, 2929, 1648, 1416, 1154, 1030, 942, 577.
1H NMR (DMSO-d6): δ = 6.08 – 5.59 (m, 28H), 5.00 – 4.77 (m, 14H), 4.70 – 4.40 (m, 12H), 4.01 – 3.86
(m, 4H), 3.76 – 3.24 (m, 80H), 2.88 – 2.60 (m, 8H), 0.94 – 0.75 (m, 6H). 13C NMR (101 MHz, DMSO-d6)
δ 102.39, 82.05, 73.50, 72.72, 72.44, 60.08, 51.55, 49.18, 30.79, 20.10, 14.08. GC-MS: m/z = 2661.74,
2660.74, 2662.74.
2b. Mp. 244–246 oC. Selected IR peaks (KBr, cm-1): υ 3379, 2928, 1655, 1413, 1155, 1033, 941, 579.
1H NMR (DMSO-d6): δ = 6.21 – 5.35 (m, 28H), 4.95 – 4.72 (m, 14H), 4.67 – 4.18 (m, 12H), 3.93 – 3.80
(m, 4H), 3.76 – 3.20 (m, 80H), 2.91 – 2.69 (m, 8H), 1.95 – 1.78 (m, 4H), 1.59 – 1.39 (m, 4H), 0.94 – 0.75
(m, 6H). 13C NMR (101 MHz, DMSO-d6) δ 102.24, 81.95, 73.51, 72.80, 72.45, 60.30, 56.43, 48.95, 30.44,
17.67, 13.99. GC-MS: m/z = 2567.77, 2566.76, 2568.77.
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3b. Mp. 278 – 279 oC. Selected IR peaks (KBr, cm-1): υ 3383, 2928, 1658, 1410, 1154, 1032, 941, 578.
1H NMR (DMSO-d6): δ = 6.03 – 5.54 (m, 28H), 4.92 – 4.78 (m, 14H), 4.71 – 4.26 (m, 12H), 3.97 – 3.80
(m, 4H), 3.75 – 3.20 (m, 80H), 2.71 – 2.69 (m, 8H), 0.98 – 0.80 (m, 6H). 13C NMR (101 MHz, DMSO-d6)
δ 102.27, 81.98, 73.54, 72.83, 72.50, 60.28, 56.41, 49.04, 30.54, 17.56, 10.90. GC-MS: m/z = 2477.87,
2476.87, 2479.87.
4b. Mp. 243 – 245 oC. Selected IR peaks (KBr, cm-1): υ 3376, 2929, 1646, 1414, 1154, 1031, 942, 580.
1H NMR (DMSO-d6): δ = 6.17 – 5.56 (m, 28H), 4.99 – 4.71 (m, 14H), 4.68 – 4.23 (m, 12H), 3.97 – 3.81
(m, 4H), 3.74 – 3.22 (m, 80H), 3.04 – 2.54 (m, 8H), 1.54 – 1.39 (m, 4H), 0.95 – 0.74 (m, 6H). 13C NMR
(101 MHz, DMSO-d6) δ 102.47, 82.08, 73.49, 72.75, 72.44, 60.45, 51.78, 45.18, 31.01, 20.86, 14.03. GC-
MS: m/z = 2704.80, 2703.79, 2705.80.
5b. Mp. 252 – 253 oC. Selected IR peaks (KBr, cm-1): υ 3385, 2929, 1644, 1416, 1154, 1031, 942, 580.
1H NMR (DMSO-d6): δ = 5.93 – 5.57 (m, 28H), 4.93 – 4.75 (m, 14H), 4.59 – 4.37 (m, 12H), 3.97 – 3.82
(m, 4H), 3.74 – 3.20 (m, 80H), 2.88 – 2.55 (m, 8H), 1.56 – 1.39 (m, 4H), 0.92 – 0.78 (m, 4H). 13C NMR
(101 MHz, DMSO-d6) δ 102.20, 81.83, 73.49, 72.75, 72.39, 60.32, 49.17, 31.04, 18.72, 14.21. GC-MS:
m/z = 2732.83, 2731.82, 2733.83.
6b. Mp. 218 – 220 oC. Selected IR peaks (KBr, cm-1): υ 3395, 2930, 1652, 1410, 1155, 1031, 942, 575.
1H NMR (DMSO-d6): δ =6.19 – 5.43 (m, 28H), 4.93 – 4.76 (m, 14H), 4.75 – 4.17 (m, 12H), 3.96 – 3.81
(m, 4H), 3.76 – 3.23 (m, 80H), 3.19 – 3.03 (m, 4H), 3.02 – 2.72 (m, 8H), 2.06 – 1.82 (m, 6H). 13C NMR
(101 MHz, DMSO-d6) δ 102.39, 82.14, 73.48, 72.83, 72.48, 60.33, 51.71, 49.12, 17.47. GC-MS: m/z =
2605.68, 2604.67, 2606.68.
7b. Mp. 209 – 210 oC. Selected IR peaks (KBr, cm-1): υ 3380, 2929, 1651, 1413, 1154, 1031, 943, 577.
1H NMR (DMSO-d6): δ =6.31 – 5.54 (m, 28H), 4.98 – 4.79 (m, 14H), 4.71 – 4.05 (m, 12H), 3.98 – 3.84
(m, 4H), 3.80 – 3.25 (m, 80H), 2.88 – 2.55 (m, 8H), 2.12 – 2.03 (m, 6H). 13C NMR (101 MHz, DMSO-d6)
δ 102.16, 81.95, 73.51, 72.81, 72.45, 71.00, 60.43, 53.38, 49.10, 17.48. GC-MS: m/z = 2576.64, 2577.65,
2578.65.
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8b. Mp. 221 – 223 oC. Selected IR peaks (KBr, cm-1): υ 3408, 2926, 1633, 1379, 1155, 1035, 942, 581.
1H NMR (DMSO-d6): δ =6.11-5.22 (m, 28H), 4.96 – 4.73 (m, 14H), 4.72 – 4.15 (m, 12H), 3.94 – 3.79 (m,
4H), 3.79 – 3.21 (m, 80H), 3.08 – 2.58 (m, 8H), 0.93 – 0.75 (m, 6H). 13C NMR (101 MHz, DMSO-d6) δ
102.28, 82.05, 73.50, 72.78, 72.44, 60.40, 51.70, 31.45, 31.10, 28.83, 28.70, 22.36, 14.17. GC-MS: m/z =
2773.87, 2772.86, 2774.87.
2.3. General procedure for cyclic carbonates synthesis from epoxides and CO2
All the cycloaddition reactions were carried out in a 250 mL stainless steel autoclave [18-19]. In a typical
reaction, desired amounts of catalyst and epoxide were charged into the reactor, CO2 was then introduced
into the clave at a desired pressure. The reactor was sealed and then immersed into oil bath at a suitable
temperature under stirring. After a certain time, the reactor was cooled down in an ice-water bath and the
remaining CO2 was released slowly. The yield and selectivity was determined by 1H NMR.
3. Results and Discussion
3.1. Effect of reaction parameters over 1a/KI
Recent progress has shown that the presence of a hydrogen bond could activate the epoxides and
remarkably enhance the cycloaddition process [74-85]. Inspired by Han et al. [67], the catalytic
performance of the multi-hydroxyl bis-β-CDs synthesized in this work were first tested for the coupling
reaction of CO2 with epoxides in the presence of KI as a nucleophile, and the results were depicted in Fig.
1. As can be seen in Fig. 1a, the propylene carbonate (PC) yield increased with the increasing CO2 pressure,
and slightly decreased after 4 MPa. Therefore, the optimal CO2 pressure for this reaction over 1a/KI was
chosen as 4 MPa.
<Fig. 1>
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The reaction temperature greatly affects both the PC yield and selectivity of this reaction. From Fig. 1b
it can be seen that the reaction temperature had positive influences on PC yield, but when the reaction
o
temperature was above 120 C, the amplification of PC yield was not obvious. Thus, the optimal
temperature for this reaction was chosen as 120 oC. To our delight, the temperature did not lower the PC
selectivity, since there was no significant mutation in selectivity as the temperature increasing.
Moreover, the effect of PC yield on the reaction time was also investigated at 120 oC and 4 MPa in Fig.
1c. The PC yield increased rapidly in the initial stage and held almost steady after 2 h. That is to say, nearly
all the propylene oxide (PO) could be converted within 2 h with > 99% PC selectivity. Therefore, 2 h was
the optimal reaction time for the conversion of PO.
3.2. Effect of reaction parameters over 1b
Motivated by the good catalytic performance of bis-β-CD 1a/KI system, the ammonium functionalized
bis-β-CD 1b was designed and utilized as single-component catalyst to avoid the using of additional co-
catalyst. As exhibited in Fig. 2, the variation tendencies of PC yields catalyzed by 1b was similar with the
ones catalyzed by 1a/KI. The influence of CO2 pressure on the PC synthesis over 1b is shown in Fig. 2a.
As can be seen, maximum PC yield was obtained at 3 MPa, further increasing the CO2 pressure would lead
the yield to diminish. This may due to the decrease in polarity of CO2 with increasing pressure, which
would reduce the solubility of reaction components in CO2 [86, 87]. Considering the fact that a 95% yield
under 2 MPa has been obtained and no big difference from the yield (96%) of 3 MPa, a moderate CO2
pressure 2 MPa was chosen for the following study.
<Fig. 2>
The effect of reaction temperature on the synthesis of PC is shown in Fig. 2b. The PC yield rose sharply
o
as temperature increased from 90 to 110 C, and then the PC yield kept almost constant with the further
o
increased temperature, indicating that 110 C was the optimized reaction temperature under the studied
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conditions. To our delight, low temperature did not appear to reduce the selectivity of PC, in view of low
temperature would favor for the production of polycarbonate.
Fig.2c depicts the dependence of PC yield on catalyst loading. It was found that the amount of catalyst
had a significant effect on the reaction. The results indicated that the increase of the catalyst loading under
a low catalyst amount level of (0.1–0.125) mol% resulted in a remarkable augment in PC yield, while the
further increasing of catalyst loading improved little PC yield. Therefore, 0.125 mol% was considered as
the optimal catalyst loading for the reaction.
The dependence of PC yield on reaction time was also investigated under identical reaction conditions
and illustrated in Fig. 2d. It can be observed that the PC yield increased with reaction time in the initial
stage, 96% PC yield was observed in 4 h. The strong rise in conversion observed after the third hour might
be attributed to the fact that the catalyst becomes more soluble in the increasingly formed carbonate with
stronger solvency than the epoxide precursor and the reaction rate increases. No further increase in the PC
yield was observed with prolonged reaction time. Therefore, a reaction time of 4 h was obtained as the
optimal choice for the synthesis of PC.
By comparing the optimal conditions of 1a/KI and 1b (120 oC, 2 h, 4 MPa, 0.14 mol% 1a, 1.25 mol%
KI vs 110 oC, 4 h, 2 MPa, 0.125 mol% 1b), it can be learnt that under close temperature, the two component
system 1a/KI required higher CO2 pressure to make catalysts fully contact with reactants, while the single-
component bifunctional catalyst 1b required longer reaction time to make substrate fully conversion.
Besides, the ammonium functionalized bis-β-CD catalyst loading was lower, and no additive was needed.
3.3. Effect of catalyst constitutions
With the optimized reaction conditions in hand, PC synthesis from PO and CO2 catalyzed by both bis-
β-CDs/potassium halide and functionalized bis-β-CDs was investigated under their respectively identical
solvent-free conditions. As shown in Table 1, all the catalysts showed excellent selectivity of PC, and
catalytic activities of various catalysts were related to their catalyst constitutions. No product was detected
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without catalyst under this reaction condition (entry 1). Bis-β-CDs with hydroxyl groups (1a-3a) or
potassium iodide (KI) can catalyze the reaction alone, but the catalytic activities are rather low (entries 2-
4, 12). The PC yield increased when combining 1a with nucleophile (entries 5, 8-11), especially the
nuclephiles with I-, i.e., KI and tetrabutyl ammonium iodide (TBAI) (entries 5 and 10). The surpassing
activity could be attributed to the synergistic effect of hydroxyl group and nucleophilic anion. Compared
the catalytic activity of 1a-3a (entries 5-7) under the same conditions in the presence of KI, it is found that
the PC yield almost unchanged. This result suggested that the alkyl chain length of bridge has little effect
on the activity.
In order to avoid the using of additional nucleophile, the functionalized bis-β-CDs 1b-8b were utilized
as bifunctional one-component catalysts for this reaction, which carried two leaving anions on the bridge
of each molecular. As expected, the catalytic activity of bifunctionalized bis-β-CDs 1b-8b was much higher
than non-functionalized bis-β-CDs 1a-3a (entries 2-4 vs 13-20). Among them, 5b presented better catalytic
activity, reaching 99% of PC yield and 99% of selectivity (entry 17). The order of the activity of halogen
anions was found to be I- > Br- > Cl- (entries 13-15), which is probably attributed to the nucleophilicity and
leaving ability of the anion [88-90]. This phenomenon was also observed in above results of potassium
halides (entries 5, 8 and 9). The alkyl chain length of the linkage also evidently affected the catalyst
activities. The results showed that the functionalized bis-β-CD with longer alkyl chain demonstrated higher
catalytic performance (n=2<n=4<n=6, entries 13, 16 and 17). Long alkyl chain may cause a weak
electrostatic interaction, which enhance the anion’s nucleophilicity [91]. Next, various secondary amine-
functionalized bis-β-CDs were tested, and the results suggested that the alkyl chain of amine had little effect
on the catalytic activity in a short chain range (entries 13, 17 and 18). But 8b exhibited a dramatic decline
of catalytic activity when the alkyl chain is long enough (entry 20), this can be ascribed to the steric
hindrance effect. As catalysts with simple alkyl chains showed excellent efficiencies, 1b was selected as
the benchmark structure for further study.
<Table 1>
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3.4. Cycloaddition of various epoxides and CO2
To survey the potential and general applicability of the synthesized functionalized bis-β-CD catalyst, the
cycloaddition reactions of CO2 with various epoxides catalyzed by bis-β-CDs/KI and bifunctional bis-β-
CD were investigated, and the results are depicted in Table 2 and Table 3. Both the binary catalytic system
1a-3a/KI and functionalized bis-β-CD catalyst 1b were found to be workable with various epoxides, which
have both electron-withdrawing and electron-donating substituents, to give the corresponding cyclic
carbonates in good yields and excellent selectivity. The good catalytic performance of binary catalytic
system bis-β-CDs/KI for a variety of substrates suggested that the catalyst with amino-bridged structure
was suitable for the coupling reaction, which ascribed to the activation of CO2 by the bridged -NH-. Detailed
discussion will be revealed in the mechanism part. Because of the ring steric hindrance, the conversion
yield of internal epoxide (cyclohexene oxide) was very low (Table 3, entry 4), and similar results were also
found in other reported literatures [92-95]. For those challenging epoxides with large steric hindrance,
longer reaction time was needed to convert (Table 2, entries 4-6 and Table 3, entries 3-11), because the
steric hindrance would obstruct the nucleophilic attack of the epoxide rather than its coordination to the
Lewis acid centre [96-99]. Despite the shorter reaction time, the binary catalytic system 1a-3a/KI required
higher CO2 pressure, temperature and catalyst loading. In general, the TOF values of 1a-3a/KI were lower
than bifunctionalized bis-β-CD catalyst 1b. It is also noteworthy that the bifunctionalized bis-β-CD catalyst
can also catalyze the diepoxides to produce corresponding bycyclic carbonates (entries 10 and 11), which
can be used as raw materials to synthesize non-isocyanate polyurethanes (NIPUs) without the use of toxic
phosgene or isocyanates via the reaction with polyfunctional primary amines [100,101].
<Table 2>
<Table 3>
3.5. Recyclability of ammonium functionalized bis-β-CD
The reusability of the bifunctional catalyst 1b was also investigated with epichlorohydrin (ECH) as the
13

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substrate at the optimized reaction conditions and the results are shown in Fig. 3. The catalyst could be
reused at least for five times and no dramatic decrease of the yield and selectivity was detected. It's worth
noting that, as the catalyst was partially soluble in the catalytic system, about 85% catalyst could be
recovered after each catalytic cycle. Thus in each cycle, ECH was reduced proportionally.
<Fig. 3>
3.6. Comparison of different biological catalytic systems
The catalytic activities of 1b and 1a/KI in the coupling reaction of CO2 and PO were compared with
other reported biological catalyst systems, and the results were listed in Table 4. Although the reaction
conditions adopted in every entry deviates from each other, a rough comparison is still feasible. Compared
with either the β-CD-based catalytic systems (entries 1 and 2) or other biological catalytic systems (entries
3-20) [102-104], both bis-β-CD catalytic system 1a/KI (entry 21) and ammonium functionalized bis-β-CD
catalyst 1b (entry 22) showed superior catalytic performance owing to employ milder reaction conditions
(or less catalyst loading) and higher TOF value. Particularly, compared with the recent reported similar
one-component catalyst cross-linked chitosan with a dicationic ionic liquid Ch-ILBr [105], our ammonium
functionalized bis-β-CD catalyst 1b (entry 19 vs 22) represented lower temperature, shorter reaction time
and higher TOF value. In general, the bis-β-CD based catalysts designed and used in present work are
excellent catalysts among the most efficient reported biological catalyst systems. To the best of our
knowledge, as bio-mass β-CD-based catalyst for the coupling of CO2 with epoxides, ammonium
functionalized bis-β-CDs studied in this work were used for the first time as a bifunctional catalyst rather
than a component of catalytic system to form cyclic carbonate. This provided a broadened thinking for the
design of β-CD-based bio-catalysts in future work.
<Table 4>
3.7. Proposed mechanism
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The two-component catalytic cycle of 1a/KI was referred to the synergetic catalytic mechanism reported
by Han et al [67]. By contrast, a possible reaction mechanism for the ammonium functionalized bis-β-CD
catalyst 1b was shown in Scheme 2, according to the reported literatures [106] and experimental results.
Firstly, the C-O bond of the epoxide was polarized by the hydrogen bond (-OH) interaction between the
hydroxyl group of 1b and the epoxide oxygen, which has been proved by some previous reports [107-111].
Meanwhile, CO2 was activated by bridged -NH- of the ammonium functionalized bis-β-CD. Subsequently,
the nucleophilic halide anion attacked the less hindered carbon atom of epoxide followed by ring opening
step to form an oxygen anion intermediate. Then, this oxygen anion intermediate reacted with activated
CO2 to generate a carbonate anion, following by an intramolecular ring-closure step to produce cyclic
carbonate and simultaneously regenerate the catalyst. Based on this bifunctional mechanism, the
cooperative effect between the electrophile, i.e., hydrogen bond and nucleophile, i.e., flexible halide anion
in one catalyst molecular could promote the coupling reaction effectively under additive/solvent-free
conditions [112,113]. Therefore, these bifunctional catalysts synthesized in this work might offer a potential
avenue to design efficient biomass-based catalysts for chemical fixation of CO2 without any co-catalyst.
< Scheme 2>
4. Conclusions
In summary, a series of ammonium functionalized bis-β-CD catalysts have been developed and
synthesized for the synthesis of cyclic carbonates from CO2 and epoxides under milder reaction conditions
without any cocatalyst or solvent. Both the bis-β-CD/KI and ammonium functionalized bis-β-CDs catalysts
can effectively activate epoxides through a synergistic effect of abundant hydroxyl groups of β-CD and the
halide anion of the extraneous nucleophile or functional group. These catalysts were derived from
environmentally friendly starting material β-CD that are green, biocompatible and non-toxic, which have
great potential in industrial application for the synthesis of cyclic carbonates.
15

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Acknowledgements
We are grateful to National Natural Science Foundation of China (No. 51073175), Natural Science
Foundation of Hubei province, P. R China (No. 2016CKB704) and the Fundamental Research Funds for
the Central Universities, South-Central University for Nationalities (CZP17066). Authors are also grateful
to Prof. Buxing Han for his valuable help and the financial support of Beijing National Laboratory for
Molecular Sciences (BNLMS).
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Figure Captions
Scheme 1. Coupling reaction of CO2 and PO catalyzed by bis-β-CD based catalysts.
Scheme 2. Synthesis of bis-β-CD (1a-3a) and amino-functional bis-β-CD (1b-8b) compounds.
Scheme 3. Proposed mechanism for the coupling reaction of CO2 with epoxide catalyzed by 1b.
Fig. 1. Effects of different reaction parameters on PC yield over 1a/KI.
Fig. 2. Effects of different reaction parameters on PC yield over 1b.
Fig. 3 Reuse of the catalyst.
Table 1 Reaction of CO2 with PO utilizing various catalysts.
Table 2 Cycloaddition between CO2 and various epoxides catalyzed by 1a-3a/KI
Table 3 Coupling reaction of CO2 with various epoxides catalyzed by 1b.
Table 4 Comparison of different biological catalytic systems for cycloaddition of CO2 to epoxides.
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(OH)
NH
n
(OH)
6
HN
6
1a: n = 2
2a: n = 4
3a: n = 6
KI
+
(OH)
14
(OH)
14
R
O
O
R
CO
+
2
O
O
R
R
1b: n = 2, R = C H , X = I
4
9
+
NH
+
NH
2b: n = 2, R = C H , X = Br
4
9
(OH)
n
(OH)
6
6
3b: n = 2, R = C H , X = Cl
4
9
4b: n = 4, R = C H , X = I
X
4
9
X
5b: n = 6, R = C H , X = I
4
9
6b: n = 2, R = C H , X = I
2
5
(OH)
7b: n = 2, R = CH , X = I
14
(OH)
14
3
8b: n = 2, R = C H , X = I
8
17
Scheme 1. Coupling reaction of CO2 and PO catalyzed by bis-β-CD based catalysts.
24

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OTs
(OH)6
(OH)6
OH
Cl
O
O
S
n
H2N
H2N
NMP, N2, 24h
(OH)14
(OH)14
R
R
+
+
NH
(OH)6
NH
n
(OH)6
n
(OH)6
(OH)6
HN
NH
R-X
OH-
X
X
(OH)14
(OH)14
(OH)14
(OH)14
1a: n = 2
2a: n = 4
3a: n = 6
1b: n = 2, R = C4H9, X = I
2b: n = 2, R = C4H9, X = Br
3b: n = 2, R = C4H9, X = Cl
4b: n = 4, R = C4H9, X = I
5b: n = 6, R = C4H9, X = I
6b: n = 2, R = C2H5, X = I
7b: n = 2, R = CH3, X = I
8b: n = 2, R = C8H17, X = I
Scheme 2. Synthesis of bis-β-CD (1a-3a) and amino-functional bis-β-CD compounds (1b-8b).
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O
R
O
X-
O
N+
CO2
O
R
H
R
OH
R
N+
H
R
N+
X-
O
R
H
O-
O
H
O
H
O
O
X-
R
R
N+
H
R
X-
+
N
O
H
O-
H
OH
O
O
R
X
R
X-
OH
O
HO
O
N+
X-
X-
=
*
*
H
OH
O
OH
NH NH
n
O
HO
OH
HO
7
R
R
7
Scheme 3. Proposed mechanism for the coupling reaction of CO2 with epoxide catalyzed by 1b.
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100
100
95
50
90
Yield
Selectivity
Yield
Selectivity
(b)
80
(a)
85
0
1
2
3
4
5
6
90
100
110
120
130
140
Temperature (oC)
CO2 pressure (MPa)
100
90
80
70
60
50
40
Yield
Selectivty
(c)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time (h)
Fig. 1. Effects of different reaction parameters on PC yield over 1a/KI. (a) Effects of CO2 pressure,
conditions: PO (5 mL, 71.5 mmol), 1a (0.14 mol%), KI (1.25 mol%), reaction temperature 120 oC, time 4
h. (b) Effects of reaction temperature, conditions: PO (5 mL, 71.5 mmol), 1a (0.14 mol%), KI (1.25 mol%),
CO2 pressure 4 MPa, time 2 h. (c) Effects of reaction time, conditions: PO (5 mL, 71.5 mmol), 1a (0.14
mol%), KI (1.25 mol%), reaction time 120 oC, CO2 pressure 4 MPa. The selectivities to products are all >
99 %. The effect of catalyst and co-catalyst molar ratio on the PC yield was demonstrated in Table S1,
supporting information.
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100
95
90
85
80
75
70
65
60
100
90
80
70
60
50
Yield
Yield
40
Selectivity
Selectivity
(b)
(a)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
90
100
110
120
130
140
150
Reaction temperature (oC）
CO pressure (MPa)
2
100
95
90
85
80
75
70
65
100
90
80
70
60
50
Yield
Yield
Selectivity
Selectivity
(d)
2.0
(c)
0.10
0.12
0.14
0.16
0.18
0.20
2.5
3.0
3.5
4.0
4.5
5.0
Catalyst concentration (%)
Time (h)
Fig. 2. Effects of different reaction parameters on PC yield over 1b. (a) Effects of CO2 pressure, conditions:
o
PO (5 mL, 71.5 mmol), 1b (0.167 mol%), reaction temperature 130 C, time 4 h. (b) Effects of reaction
temperature, conditions: PO (5 mL, 71.5 mmol), 1b (0.167 mol%), CO2 pressure 2 MPa, time 4 h. (c)
Effects of catalyst loading, conditions: PO (5 mL, 71.5 mmol), reaction time 110 oC, CO2 pressure 2 MPa,
time 4 h. (d) Effects of reaction time, conditions: PO (5 mL, 71.5 mmol), 1b (0.125 mol%), reaction time
110 oC, CO2 pressure 2 MPa. The selectivities to products are all > 99 %.
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100
80
60
40
20
0
1
2
3
4
5
Run
Fig. 3 Reuse of the catalyst. Reaction conditions: ECH (5 mL, 71.5mmol), 1b 0.33 mol%, CO2 pressure 2
MPa, reaction temperature 110 oC, reaction time 4 h.
29