Polymer- and Carbon Nanotube-Supported Heterogeneous Catalysts for the Synthesis of Carbamates from Halides, Amines, and CO2

Article abstract of DOI:10.1007/s10562-016-1738-1

Abstract: A series of tetraalkylammonium catalysts immobilized on polystyrene or carbon nanotubes were investigated in a reaction between alkyl halides, amines, and CO₂. The yield of carbamates was up to 85?% at 100?°C and 1?MPa of CO₂ after 3?h (first stage 25?°C, 1 h) in the presence of Cs₂CO₃ and DMF as a solvent. The best results were achieved in the case of large ammonium substituents (C₄ and C₆) in the catalyst, benzyl amine, and benzyl chloride as reagents. The catalysts retained their activity after at least five cycles when carefully dried between cycles. Both polymer and carbon nanotube supports were equally effective, but the latter were less prone to deactivation. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1738-1

Catal Lett
DOI 10.1007/s10562-016-1738-1
Polymer- and Carbon Nanotube-Supported Heterogeneous
Catalysts for the Synthesis of Carbamates from Halides, Amines,
and CO2
Tomasz Krawczyk1 Katarzyna Jasiak1 Aneta Kokolus1 Stefan Baj1
•
•
•
Received: 7 March 2016 / Accepted: 27 March 2016
Ó The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract A series of tetraalkylammonium catalysts
immobilized on polystyrene or carbon nanotubes were
investigated in a reaction between alkyl halides, amines,
and CO2. The yield of carbamates was up to 85 % at
100 °C and 1 MPa of CO2 after 3 h (first stage 25 °C, 1 h)
in the presence of Cs2CO3 and DMF as a solvent. The best
results were achieved in the case of large ammonium
substituents (C4 and C6) in the catalyst, benzyl amine, and
benzyl chloride as reagents. The catalysts retained their
activity after at least five cycles when carefully dried
between cycles. Both polymer and carbon nanotube sup-
ports were equally effective, but the latter were less prone
to deactivation.
1 Introduction
In recent years there has been a growing interest in the field
of carbon dioxide utilization. It has sparked off from the
worldwide effort to reduce the CO2 emission into the
atmosphere, as well as from its potential use as a renewable
C1 source [1]. Because of the relatively inert nature of CO2,
it is used in the industrial scale only for urea, carbonates,
and salicylic acid synthesis, but a number of other possible
applications of CO2 in the organic synthesis has been
proposed [2]. Promising results have been reported in the
case of the carbamate synthesis [3]. Carbamates are an
interesting group of compounds that are esters and amides
simultaneously. Various compounds incorporating carba-
mate groups have shown importance as agrochemicals,
antibacterial, antiviral, and anticancer agents [4]. Some
carbamates have also found various applications in the
organic synthesis [5]. Usually they are obtained in a
reaction between amines, phosgene, and alcohols, but
because of a highly toxic nature of phosgene safer routes
towards carbamates are in high demand. The most
notable examples include the urea alcoholysis [6], the
carbonate aminolysis [7, 8] or the urea-carbonate reaction
[9]. The starting materials for those syntheses can be
derived from CO2, but direct paths from CO2 towards
carbamate are available, too [10]. The simplest approaches
include alkylation of intermediate carbamic acid by a
halide [11–13].
Graphical Abstract
Keywords Immobilization Á CO2 valorization Á
Nanoparticles
The reaction has a wide scope and various amines or
halides can be used. The mechanism includes the formation
of a carbamic acid salt in the reaction between an amine
and CO2. In the second stage the salt reacts with a halide in
the presence of a base and a phase transfer (PT) catalysts
[13]. The flaw of this approach is that the halide is not
incorporated into the final product and halide containing
& Tomasz Krawczyk
tomasz.krawczyk@polsl.pl
1
Department of Chemical Organic Technology and
Petrochemistry, Faculty of Chemistry, Silesian University of
Technology, Krzywoustego 4, 44-100 Gliwice, Poland
123

T. Krawczyk et al.
waste is produced. The reported yields are usually mod-
erate (50–70 %) but can reach 98 % [12]. The process is
usually conducted at 25–100 °C, up to 8 MPa of CO2 in the
presence of Cs2CO3 and an ammonium catalyst in DMF
[12, 13]. Other strong bases, such as 1,8-diazabicycloun-
dec-7-ene (DBU) or guanidine, can also be used. The role
of a base is deprotonation of carbamic acid, which
improves its reactivity with the halide [14, 15].
bound tributylmethylammonium chloride: PS2-C4H9) [20]
and CNT-supported alkylammonium catalysts (CNT-C4H9)
have been reported previously [21].
2.2 Instruments
Chromatography was carried out using Waters Acquity
Ultra Performance Liquid Chromatography (UPLC) sys-
tem. HR-MS spectra were registered with a Waters Xevo
G2 QTof instrument and the ESI-MS source after chro-
matographic separation. Syntheses of carbamates were
conducted in a 100 mL stainless steel autoclave placed in a
thermostat (EasyMaxTM 102, Mettler Toledo).
Alcohols can also be employed as an alternative to
halides as alkylating agents [16, 17], but the reaction
conditions are harsher due to the low reactivity of alcohols.
It can be partially circumvented by the application of
Mistunobu’s reagent [18], or when reactive propargyl
alcohols and an appropriate catalytic system is used [19].
One of the trends in the development of catalytic sys-
tems for the carbamates synthesis, as well as in other types
of catalysis, is the immobilization of existing homogeneous
catalysts to obtain reusable, easily separable catalysts of
high activity. Among possible supports there are: polymers,
metal oxides, carbon nanomaterials, nanoparticles, and
many others. In this work we investigated immobilized
alkylammonium salts in the synthesis of carbamates from
halides.
2.3 Synthesis of Carbamates from CO2
An amine (2.85 mmol) and anhydrous DMF (15 mL) were
introduced into an autoclave. The autoclave was purged
twice with CO2 and the reaction was conducted at 25 °C
under 1 MPa of CO2. After 1 h, an appropriate amount of
the catalyst, cesium carbonate (9 mmol), and alkyl or
benzyl chloride (9 mmol) was added. The autoclave was
purged twice with CO2 and the mixture was stirred at
100 °C under 1 MPa of CO2 for 3 h. After cooling to room
temperature, the excess of CO2 was vented, and the catalyst
was separated by filtration. The post-reaction mixture was
poured into water (30 mL) and extracted with ethyl acetate
(3 9 30 mL). The organic layer was washed with brine
(30 mL) and water (2 9 30 mL). The organic layer was
dried over anhydrous magnesium sulfate, and evaporated to
give crude alkyl carbamate in the form of a yellow oil.
2 Experimental
2.1 Materials
Carbon dioxide of 99.5 % purity was obtained from SIAD.
Tributylmethylammonium chloride (TBMAC), polymer-
bound tributylmethylammonium chloride (PS1-C4H9)
200–400 mesh; Cl loading = 1.2 mmol g-1, polystyrene
(cross-linked with DVB) were purchased from Fluka.
Cesium carbonate (99.5 %), DBU (98 %), benzyl chloro-
formate (pure), 1-chlorobutane ([99 %), 2-chlorobutane
([99 %), benzyl chloride (99 %), choline chloride (99 %),
triethylamine (99 %), tributylamine (99 %) were supplied
by Acros Organics. Carbon nanotubes (CNT)
(6–9 nm 9 5 lm), tetrabutylammonium bromide (TBAB)
([99 %), trihexylamine (96 %) were provided by Sigma
Aldrich. Aniline, benzylamine, butylamine, diisobuty-
lamine, and other chemicals were bought from local
manufacturers. Merrifield’s resins, chloromethylated
polystyrenes PS2: 200–400 mesh, Cl loading:
2.25 mmol g-1, polystyrene 2 % cross-linked with DVB
(Sigma Aldrich), and PS3, 200–400 mesh, Cl loading:
2.5–3 mmol g-1, polystyrene 1 % cross-linked with DVB
was purchased from Acros Organics.
2.4 Quantitative Analysis of Products
The structure of all products was confirmed by HR-MS and
NMR. A quantitative analysis was performed with the
UPLC-PDA system. The mobile phase consisted of ace-
tonitrile (60 %) and water (40 %) delivered at
0.2 mL min-1. A BEH C18 1.7 lm 2.1 9 50 mm column
was kept at 40 °C. Injections of 0.2 lL of a solution of the
crude product (0.01 g in 10 mL of acetonitrile) were made
on the column to quantify all the products. Calculations
were based on external calibration lines established by
injecting standards of known purity. The retention time of
1 was typically 1.2–1.4 min; of 2 was 4.2–4.7 min, while 3
was eluted at 2.0 min. If pure standards were not available,
1H NMR spectra (in DMSO) of crude products with maleic
acid as an internal standard were used for quantification
purposes. Yields of products 1 and 2 were calculated based
on the initial quantity of the amine (2.85 mmol), while
yields of 3 were related to the initial quantity of benzyl
chloride (9 mmol).
The synthetic procedure and characterization of poly-
mer-supported catalysts (polymer-bound triethylmethy-
lammonium
chloride:
PS3-C2H5,
polymer-bound
trihexylmethylammonium chloride: PS3-C6H13, polymer-
123

Polymer- and Carbon Nanotube-Supported Heterogeneous Catalysts…
corresponding TONs, but only in the combination with a
low quantity of the catalyst. A detrimental effect of the
addition of DBU was also observed in the case of the PS1-
C4H9-catalysed reaction (Table 2), when it reduced the
yield of 1a two fold and the yields of both by-products, as
well.
3 Results and Discussion
The aim of this work was to asses the performance of
immobilized tetraalkylammonium salts as catalyst in the
carbamate synthesis. Two types of supports were used:
crosslinked polystyrene, and CNT. The general scheme of
the investigated reaction and the main impurities are shown
in Scheme 1. The mechanism of carbamate synthesis cat-
alyzed by quaternary ammonium salts was published ear-
lier [13].
The effect can be rationalized based on the general PT
catalysis mechanism [22]. It is likely that DBU acts pri-
marily as a base and has the strongest effect when com-
bined with K2CO3 (Table 1, entries 1, 2, 5, 6). A higher
catalyst content can offset insufficient basicity of the
environment (Table 1, entries 2, 3, 5, 8). It is likely that the
reactivity of carbamic acid and benzyl chloride determined
the yield when strong bases were present. Otherwise,
inadequate concentrations of ions or transfer of reagents
between phases were the limiting factors.
3.1 Reaction Conditions
Based on the literature data regarding the synthesis of
carbamates from CO2 and halides with the use of alky-
lammonium catalysts, we selected the pressure of
CO2 = 1 MPa and 100 °C as suitable conditions. The
relative impact of the base and the catalyst amount is
summarized in Table 1.
Another experiment that favors a PT-based explanation
of the observed results is the entry 7 of Table 2. The
presence of water had a negative impact on the yield of
carbamate 1a and the overall conversion of the amine
toward the CO2 derived products (the yield of 2a was more
than doubled comparing with the entry 3, the sum of 1a and
2a was 61 %). It can be a consequence of a less efficient
transfer of carbamic acid to the organic phase, because it is
more likely trapped in the aqueous Cs2CO3-rich phase.
The order of the reagent addition was also investigated.
If the reaction between benzylamine and CO2 was con-
ducted in the presence of a immobilized catalyst, Cs2CO3,
and benzyl chloride, the yield of 1a was low comparing
with the typical procedure, when the catalyst and the base
were added after saturation of the amine with CO2. One of
the possible reasons is partial alkylation of the amine to
tribenzylamine or tetrabenzylammonium salt when not
protected by the formation of carbamic acid.
The yield of benzyl benzylcarbamate (1a) increased
with the basicity of the carbonate (Cs2CO3 [ K2CO3). It
was also improved with the amount of the catalyst. How-
ever, at higher catalyst loading corresponding TONs were
lower comparing with 0.9 mmol of the catalyst. The
addition of DBU led to the increase of 1a yield and the
Scheme 1 Synthesis of carbamates from halides, amines and CO2.
R1, R2 = alkyl, benzyl, Ph, H. R3 = alkyl or benzyl. Product 3 is
formed only when R3 = benzyl
Table 1 Impact of the base and
the catalyst on the benzyl
benzylcarbamate synthesis
Entry
Bases
Catalyst
Type
Yielda (%)
TON
1a
Amount (mmol)
1a
2a
3a
1
2
3
4
5
6
7
8
9
K2CO3
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBMAC
TBAB
TBAB
0.9
0.9
9
5
2
1
2
4
5
4
3
3
8
1
0.2
1.1
0.1
0.1
1.2
1.9
0.1
0.2
0.2
K2CO3-DBU
K2CO3
35
37
37
37
60
16
75
69
13
10
14
6
K2CO3 DBU
Cs2CO3
9
0.9
0.9
9
Cs2CO3 DBU
Cs2CO3
6
3
Cs2CO3
9
20
10
Cs2CO3 DBU
9
Reaction conditions: benzylamine (2.85 mmol); anhydrous DMF (15 mL); Cs2CO3 (9 mmol); benzyl
chloride (9 mmol); DBU (1.3 mmol); reaction time = 1 h at 25 °C, 1 MPa of CO2, then 3 h at 100 °C,
1 MPa of CO2
a
Yields of 1a and 2a related to benzyl amine, yields of 2a related to benzyl chloride
123

T. Krawczyk et al.
Table 2 Yield of benzyl benzylcarbamate (1a) and by-products using various heterogeneous catalysts
Entry
Catalytic system (support-R)
Amount of catalyst (g)
Ammonium content (mmol g-1
)
Yieldd (%)
TON
1a
2a
3a
1a
1
2
3
4
5
6
7
8
PS3-C2H5
0.75
0.75
0.75
0.75
0.6
1.8
1.5
1.2
1.4
1.0
1.2
1.2
1.2
42
78
85
55
65
43
30
46
12
15
12
11
25
7
5
0.9
2.0
2.7
1.5
3.0
1.4
1.0
1.5
PS3-C6H13
PS1-C4H9
3
20
7
PS2-C4H9
CNT-C4H9
PS1-C4H9 DBUa
PS1-C4H9 H2Ob
PS1-C4H9
6
0.75
0.75
0.75c
2
31
5
1
24
Reaction conditions: benzylamine (2.85 mmol); anhydrous DMF (15 mL); Cs2CO3 (9 mmol); benzyl chloride (9 mmol) 1 h; 25 °C; 1 MPa of
CO2, then 3 h; 100 °C; 1 MPa of CO2. Crosslinking degree PS1, PS3 = 1 %, PS2 = 2 %
a
1.3 mmol of DBU;
b
1 g of H2O;
c
One-pot process;
d
Yields of 1a and 2a related to benzyl amine, yields of 2a related to benzyl chloride
3.2 Catalyst Structure
[24], which in turn diminishes their ability to enter the
organic phase.
The general structure of immobilized catalysts was shown
in Fig. 1. The materials were either commercial or were
synthesized according to previous reports [20, 21]. Both
types of catalysts were used as alternatives of homoge-
neous TBAB and the results were collected in Table 2.
Under homogeneous conditions the observed TON values
were not higher than 1.9 (Table 1), whereas immobiliza-
tion increased it by 50 % to reach 2.7–3.0, depending on
the support. In the case of a homogeneous system the
efficiency of tributylmethylammonium chloride (TBMAC)
was low comparing with tetrabutylammonium bromide (16
vs. 75 %, Table 1). Similarly, the heterogeneous catalysts
(Table 2) containing the triubutylammonium functionality
showed the best activity (85 % yield of 1a, TON = 3.0)
from amongst their alternatives containing three R = C2H5
(42 %, 0.9) or R = C6H13 groups (78 %, 2.0). The relative
order agrees with the observations made by other authors
studying various PT reactions. Typically, symmetrical
ammonium cations containing alkyl chains of 4 atoms are
the most efficient catalysts [23]. The relative efficiency of
various quaternary ammonium salts is also affected by the
solvent. It is due to the optimal balance between
hydrophilicity of small ammonium cations and their ability
to bind anions when the nitrogen atom is more accessible
A higher reactivity of immobilized PT catalysts was
observed in various systems [22]. Such behavior can be
explained on the ground of the PT catalysis mechanism.
Under PT conditions the rate of the reaction is strongly
affected by the partitioning of a quaternary ammonium salt
between the organic and aqueous phase or the interphase.
From this perspective the most efficient catalyst should
have a partitioning coefficient of 1 [25]. Such a situation is
not likely in the case of a homogeneous catalyst because of
different solvation energies in different solvents. As the
immobilized catalyst forms a third phase then it should be
well accessible by reagents that reside either in the organic
or aqueous phase.
Other authors rationalized the phenomenon on the
ground of increased lipophilicity of immobilized catalysts.
The explanation is based on the observation that quaternary
ammonium salts that are insoluble in organic solvents are
poor catalysts unless immobilized on lipophilic support
[26], such as polymers, but not on SiO2 [27].
Another important aspect that can contribute to the
increased efficiency of an immobilized catalyst is better
thermal stability of immobilized ammonium salts. Typi-
cally, they undergo Hoffman degradation above 100 °C,
which decreases the amount of the catalyst available. After
immobilization no apparent degradation occurs [20] at the
same conditions.
The increased yield observed for immobilized catalysts
can be partially attributed to better accessibility of car-
bamic acid when coordinated by ammonium cations on the
surface of the support comparing with a homogeneous
system. Such a phenomenon may also explain the fact that
Fig. 1 The structure of immobilized catalysts used. PS polystyrene
123

Polymer- and Carbon Nanotube-Supported Heterogeneous Catalysts…
the catalyst of a 2 % cross-linking degree was less effective
comparing with a 1 % counterpart (55 vs. 85 % yield of
1a).
3.3 Reaction Scope
The scope of the PS1-C4H9 catalyst was investigated with
various combinations of amines and halides. All the data
are collected in Table 3. The type of amine had a strong
effect on the yield of a carbamate. Only the primary and
benzyl amine gave good yields, while the aromatic and
secondary amine yielded only 3–6 % of the desired prod-
uct. The yield of the carbamate was also dependent on the
halide type, and among chlorides it increased in the order
of benzyl [ primary [ secondary, which reflects typical
reactivity in the nucleophilic substitution.
Fig. 2 Activity of recycled catalysts in benzylbenzycarbamate syn-
thesis. Reaction conditions: benzylamine (2.85 mmol); anhydrous
DMF (15 mL); Cs2CO3 (9 mmol); catalyst PS1-C4H9 (0.75 g) or
CNT-C4H9 (0.6 g) ; benzyl chloride (9 mmol) 1 h; 25 °C; 1 MPa of
CO2, then 3h; 100 °C; 1 MPa of CO2
3.4 Catalyst Recycle
fourth cycle). When the catalyst was left in a vacuum
desiccator for 2 weeks (over NaOH, 50 mbar) it allowed
for full regeneration of its catalytic activity. Comparing
with CNT support it indicates that the polymer is more
prone to water sorption and more difficult to be dried.
The possibility of reusing of the polymer-based and CNT-
based catalysts were investigated in the reaction of benzyl
amine and benzyl chloride. As shown in Fig. 2, it was
possible to reuse the same portion of the catalyst immo-
bilized on CNT in at least 5 reaction cycles. The catalyst
could be easily separated by filtration from the reaction
mixture. No special precautions were required besides
8–12 h drying in a desiccator. In each cycle a fresh portion
of Cs2CO3 was used. In the case of the polymer-supported
catalyst, the first and second reuse with the same portion of
Cs2CO3 yielded only 5 and 3 % of the carbamate when a
12-h drying in a desiccator was employed. The yield was
improved by the addition of fresh Cs2CO3 (45 % at the
4 Conclusions
A series of immobilized catalysts for the synthesis of car-
bamates from CO2 was showed to be active and reusable.
Comparing with homogeneous counterparts, the TON
value was 3–10 times higher and the yield of carbamates
using halides as reagents was up to 85 %. Comparing with
other similar systems our results are equally good or worse
by 10–20 % as many authors were able to reach 99 %
yields at least for the best performing reagents [6–13] at a
similar time, temperature, and CO2 pressure as used here,
but under homogeneous conditions.
Table 3 Yield of carbamates using various amines and catalytic
systems
Entry
Amine
R1
Halide
R3
Yielda (%)
TON
1
Partially responsible for the low yields is the consecu-
tive alkylation of the main product or the alkylation of the
amine before the carbamate formation. The former could
be limited by shortening of the reaction time and
improvement of the catalyst efficiency, while the latter by
increasing of the CO2 pressure.
R2
1
2
3
1
2
3
4
5
6
n-butyl
benzyl
phenyl
iso-butyl
benzyl
benzyl
H
benzyl
benzyl
benzyl
benzyl
n-butyl
2-butyl
63
85
6
12
12
0
20
20
3
1,7
2,2
0,2
0,1
2,0
0,8
H
H
iso-butyl
3
8
25
0
To a certain extend the yields could be improved by an
increase of the catalyst amount. In the case of the polymer
supported catalyst it was not possible because the swelling
prevented efficient stirring of the mixture. The problem
was not important in the case of the CNT-based catalyst.
Further investigations of immobilized catalysts should aim
at increasing the degree of functionalization of polymers or
CNT, as well as at other supports of high surface area.
H
H
77
30
0
0
0
Reaction conditions: amine (2.85 mmol); anhydrous DMF (15 mL);
Cs2CO3 (9 mmol); PS1-C4H9 (0.9 mmol); alkyl or benzyl chloride
(9 mmol); 1 h at 25 °C, 1 MPa of CO2 and then 3 h at 100 °C, 1 MPa
of CO2
a
Yields of 1 and 2 related to an amine, yields of 2 related to a
chloride
123

T. Krawczyk et al.
Acknowledgments This work was supported by the Polish National
Science Centre (NCN); Decision No.: 2011/03/B/ST8/06178. The co-
author of this paper, Katarzyna Jasiak, is a scholar under the project
‘‘DoktoRIS—Scholarship Program for Innovative Silesia’’ co-fi-
nanced by European Union under the European Social Fund.
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