Synthesis of dicationic ionic liquids and their application in the preparation of hierarchical zeolite Beta.

Abstract

Piperidine- and imidazole-based dicatoinic ionic liquids have been developed for the synthesis of zeolite Beta. Hierarchical Beta has a larger surface area and pore volume than conventional Beta. Beta derived from a dicationic ionic liquid exhibited remarkably higher catalytic activity than the conventional Beta. Experimental evidence and DFT calculations suggest that only a suitable conformation of such dicationic ionic liquids is able to form zeolite Beta (see scheme).

Full text

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DOI: 10.1002/chem.201102946
Synthesis of Dicationic Ionic Liquids and their Application in the Preparation
of Hierarchical Zeolite Beta
Rajkumar Kore,[a]Biswarup Satpati,[b]and Rajendra Srivastava*[a]
Zeolites are an important class of crystalline porous mate-
rials that have been used for numerous catalytic applications
because of their uniform channel size, strong acidity, high
thermal/hydrothermal stability, and unique molecular shape
selectivity.[1–3]Based on Si/Al ratios in their framework, zeo-
lites are classified into two categories: low-silica and high-
silica zeolites.[1,4]Low silica-zeolites such as Linde type (fau-
jasite zeolite A/X/Y) find applications in air separation, ion
adsorbents and manufacturing of detergent due to high ex-
change capacity.[5]High-silica zeolites are important as het-
erogeneous catalysts due to their strong Brønsted acidity,
shape selectivity and high thermal stability.[6,7]Among the
high silica zeolites, ZSM-5 and Beta rule the heterogeneous
catalysts market.
In recent years, zeolites with interconnected intra- or in-
tercrystalline mesoporosity have attracted much attention
due to improved diffusion, catalytic activity, selectivity, and
lifetime.[2,8–10]Several attempts have been made for the
preparation of nanosized zeolites from colloidal precursors
under mild hydrothermal conditions.[8–16]It includes meso-
pore generation with a hard template,[11,12]use of organosi-
lane surfactants as a soft template,[13–15]and dealumination
of zeolites for post synthetic generation of mesopores.[16,17]
Most of these studies have focused on the synthesis of nano-
crystalline ZSM-5.[11–15]Only a few efforts have been made
to prepare nanocrystalline Beta.[18–20]It was reported that hi-
erarchically porous Beta can be prepared by using 3,10-
diazoniabicyclo[10.2.2]hexadeca-12,14,15-triene-3,3,10,10-
tetramethyl-dichloride[18]or by using a suitable silylating
agent such as phenylaminopropyl-trimethoxysilane.[19]Very
recently, it was reported that nanosized Beta with various
particle sizes and high surface areas were synthesized with
tetrethylammonium hydroxide (TEAOH) as a structure-di-
recting agent in the presence of a cetyltrimethylammonium
(CTMA) surfactant such as CTMAX (X: Cl, Br, and OH)
using a dry-gel method.[20]Based on the literature survey
one can easily conclude that soft templates are more versa-
tile and enable us to prepare a variety of nanocrystalline
zeolites.
Ionic liquids (ILs) have been attracting considerable at-
tention in recent times due to their unique properties, such
as lack of measurable vapor pressure, non-flammability, cat-
alytic activity, and recyclability.[21–24]Recent research showed
that thermal degradation of monocationic ionic liquids
occurs at temperature below 473 K.[25,26]Organic reactions
at temperatures above 473 K requires ionic liquid with
higher thermal stability. Most recently, a series of geminal
dicationic ionic liquids with high thermal stability have been
prepared.[27,28]It may be noted that these geminal dicationic
ionic liquids have melting points in the range of 450–673 K
in contrast to monocationic ionic liquids (below 373 K).
Very few efforts have been made to prepare and utilize such
dicationic ionic liquids.[29]The objective of this study is to
prepare a variety of geminal dicationic ionic liquids and ex-
plore their potential in the synthesis of zeolites. In this
study, several piperidine and imidazole based geminal dicat-
ionic ionic liquids (Scheme 1 in the Supporting Information)
were prepared. These organic molecules are termed as dicat-
ionic ionic liquids (DCILs) because of the reason explained
elsewhere.[27,28]To the best of our knowledge, this is the first
report in which piperidine- and imidazole-based economical
DCILs were used as structure-directing agents (SDA) for
the synthesis of zeolite Beta using low cost sodium silicate
and aluminum sulphate.
In this study, DCILs were synthesized using multistep syn-
thetic route (Scheme 1 in the Supporting Information).
Cyclic and acyclic DCILs were prepared using commercially
available piperidne, 4,4’-trimethylenebis(1-methylpiperidine)
or imidazole as starting materials. The structure of these
ionic liquids was elucidated by using NMR spectroscopy, IR
spectrometry, and elemental analysis (see the Supporting In-
formation for details). Zeolite Beta was synthesized by
using DCILs as SDA under strong basic (pH?14) condi-
tions. No zeolite was obtained in the absence of DCILs and
the synthesis mixture remained transparent even after 10
days, in the absence of DCILs. When the DCIL was added,
the amorphous gel was converted to a crystalline zeolite
phase upon hydrothermal treatment at 443 K. The product
structure and the time required for zeolite crystallization de-
pends on DCILs structure when other synthesis parameters
such as hydrothermal temperature and gel composition were
[a] R. Kore, Dr. R. Srivastava
Department of Chemistry, Indian Institute of Technology Ropar
Rupnagar-140001, Punjab (India)
Fax: (+ +91)1881-223395
E-mail: rajendra@iitrpr.ac.in
[b] Dr. B. Satpati
Surface Physics Division, Saha Institute of Nuclear Physics
1/AF, Bidhannagar, Kolkata 700 064 (India)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102946.
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constant. Among all DCILs investigated, only DCIL4,
DCIL5, DCIL6, and DCIL9 were able to form zeolite Beta.
The addition of DCIL4 immediately caused gelation
under highly basic conditions (pH?14). Oragnosilicate com-
posite viscous gel was obtained within a minute after the ad-
dition of DCIL4. However, it may be noted that when tet-
raethylammonium (TEA+ +) bromide/hydroxide (a conven-
tional SDA for zeolite Beta) was used, no such immediate
gelation was observed. Elemental analysis reveals that the
initial gel phase (0 h) contains 16.5 wt% of DCIL4, whereas
the final zeolite product (24 h) contains 19.2 wt% of DCIL4
(based on dry weight). When zeolite Beta was synthesized
using conventional TEA+ +as the SDA, the initial gel phase
(0 h) contained only 2.5 wt% of TEA+ +and the final zeolite
phase contained 7.7 wt% TEA+ +. This clearly shows that
using TEA+ +as SDA, Na+ +was the predominant cation in
the initial gel, and this was replaced with TEA+ +after pro-
longed hydrothermal crystallization. The similar composi-
tion of DCIL4 in the initial phase and final zeolite phase
suggest that the initial phase consists of zeolite-like organ-
ized silicate species around DCIL4. Such a pre-organized
zeolite-like assembly is responsible for the facile synthesis of
zeolite Beta using DCIL4. The XRD pattern of samples col-
lected during the crystallization of zeolite Beta using DCIL4
is shown in Figure 1a. The XRD peak (at 7.88 8) for the 0 h
sample clearly demonstrates the presence of a pre-organized
zeolite-like assembly. A good quality zeolite Beta was ob-
tained in 24 h. The broad nature of the peak clearly demon-
strates that zeolite Beta is highly nanocrystalline in nature.
Beta synthesized using DCIL4 is termed Beta-DCIL4. TEM
images obtained of the zeolite phase after 0, 4, and 24 h of
hydrothermal treatment show that the mesoporous frame-
work was composed of zeolite nanocrystals of 14–15 nm in
diameter (Figure 2). This diameter is in agreement with the
crystallite size calculated from XRD by using Scherrer?s
equation. The N2adsorption study shows that Beta-DCIL4
exhibited type IV isotherm with a hysteresis loop corre-
sponding to capillary condensation in mesopores. Further-
more, the zeolite exhibited a broad distribution of mesopore
diameters (2.9 to 7.3 nm), which is consistent with the
random aggregation of nanosized crystals (Figure 3). Zeolite
Beta was obtained by using DCIL4 until Si/Al<100. It is
very interesting to note that when the synthesis was per-
formed without Al, zeolite MTW was obtained.
In contrast to a previous report (which states that those
SDA that only have a cyclic structure are suited for the for-
mation
DCIL5 was also able to form
zeolite Beta (here after repre-
sented as Beta-DCIL5) (Fig-
ure S1 in the Supporting Infor-
mation). However, the surface
area of Beta-DCIL5 was found
to be low when compared with
Beta-DCIL4
Figure 3) and its crystal size
was found to be the largest
(Figure 4, Table 1) among the
Beta samples prepared in this
study. It may be noted that it is
not possible to get pure zeolite
Beta phase using DCIL5 as
SDA when Si/Al>50. For Si/
Al>50, a mixture of zeolite
Beta and zeolite MTW was ob-
tained when DCIL5 was used
ofzeoliteBeta),[18]
(Table 1,
Figure 1. a) XRD patterns of product obtained during the crystallization of zeolite Beta using DCIL4 and
b) XRD patterns of Beta, Beta-DCIL4, and Beta-DCIL9 after 48 h of crystallization at 443 K.
Figure 2. TEM images of solid product collected during the crystallization
at 443 K of Beta synthesized using DCIL4 a) at 0 h, b) after 4 h, c) and
d) after 24 h. e) and f) represent the TEM images of Beta-DCIL9 after
48 h of crystallization at 443 K.
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as SDA. Pure silica zeolite Beta was obtained by using
DCIL6 (hereafter represented as Beta-DCIL6), which was
not possible with any other DCILs investigated in this study
(Figure S1 in the Supporting Information). The crystal size
of Beta-DCIL6 was found to be larger than Beta-DCIL4
but smaller than Beta-DCIL5 (Figure 4, Table 1). When zeo-
lite was synthesized by using DCIL6 with 50<Si/Al<100,
zeolite MTW was obtained. When the synthesis was per-
formed using DCIL6 with Si/Al<25, no zeolite phase was
obtained. MTW was obtained by the same organic SDA
used for Beta. The formation of MTW zeolite at low Al con-
tent in the present work is consistent with the reported liter-
ature, which states that MTW was thermodynamically fa-
vored under high-silica synthesis conditions.[30]
Success of the DCIL4 to DCIL6, encouraged us to devel-
op structurally homologous and cheap dicationic ionic liq-
uids for the synthesis of zeolite Beta. The cost of DCIL1 to
DCIL3 was very low but they did not produce zeolite Beta.
To develop cheap SDA, imidazole-based dicationic ionic liq-
uids, DCIL7 to DCIL9 were synthesized (Scheme 1 in the
Supporting Information). Among imidazole based DCILs,
only DCIL9 was able to form zeolite Beta (hereafter repre-
sented as Beta-DCIL9) (Figure 1b), whereas other
imidazole-based DCILs produced zeolite MTW.
The broad nature of the XRD peak of Beta-
DCIL9 and Beta-DCIL4 clearly shows that their
crystal sizes are smaller than conventional Beta
(Figure 1b, Table 1). Surface area and pore volume
of Beta-DCIL9 were found to be similar to that of
Beta-DCIL4 (Table 1, Figure 3). TEM analysis
showed that the mesoporous framework of Beta-
DCIL9 was composed of zeolite nanocrystals of
16–18 nm in diameter (Figure 2). Using DCIL7
and DCIL8, no zeolite phase was obtained when
the synthesis was performed with Si/Al<50. Zeo-
lite MTW was obtained using DCIL7 and DCIL8
when the synthesis was performed with Si/Al>50.
An interesting observation for Beta-DCIL9 is that it is a
polymorph C form of zeolite Beta (Figure 1b). Further
study in this direction is underway in our group to develop a
variety of SDA for the synthesis of various pure polymorphs
of Beta.
Zeolite Beta synthesized using diammonium DCILs had
high BET surface areas with large mesopore volumes, com-
pared with conventional Beta. Surface area and mesoporosi-
ty of the zeolites decreased in the following order: Beta-
DCIL4>Beta-DCIL9>Beta-DCIL6>Beta>Beta-DCIL5
(Table 1). It is reported that organic molecules that have a
strong interaction with the growing crystal surface can effec-
tively modulate the crystallization process of inorganic ma-
terials.[31,32]The synthesis of nanocrystalline zeolites using
DCILs indicates that the presence of two ammonium groups
in the same molecule could be the most important factor for
the generation of nanocrystalline morphologies and meso-
porosity.[33]It is highly possible that the diammonium type
DCILs are able to generate a large number of zeolite seeds
when compared with their monoammonium analogues
Figure 3. N2-adsorption isotherm of zeolite Beta synthesized in this study.
Inset shows pore size distribution. Beta-DCIL9 (&), Beta-DCIL4 (*),
Beta-DCIL6 (~), Beta (!), Beta-DCIL5 (^).
Table 1. Textural characteristics of Beta samples prepared using DCILs.
SampleSi/Al[a]
Average
crystal
size [nm][b]
Total
surface
area [m2g?1]
External
surface
area [m2g?1]
Total pore
volume
[ccg?1]
Beta
Beta-DCIL4
Beta-DCIL4
Beta-DCIL5
Beta-DCIL6
Beta-DCIL9
17.5
19.4
75
17.3
1
19.1
48–50
14–16
18–20
50–52
35–37
16–18
495
685
645
480
590
670
105
290
260
90
196
280
0.39
0.65
0.62
0.38
0.50
0.63
[a] Si/Al ratio was obtained by ICP analysis. [b] Approximately determined from
TEM images.
Figure 4. TEM images of a) Beta-DCIL4 (Si/Al=75), b) Beta-DCIL5,
and c) Beta-DCIL6 after crystallization at 443 K for 48 h.
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TEA+ +. This was confirmed from the XRD analysis of the
product obtained using DCIL4, which shows that a pre-or-
ganized zeolite-like assembly was formed during gelation
(0 h, before crystallization) (Figure 1a). This process re-
duced the nucleation time and further crystal growth took
place rapidly to form hierarchical zeolite Beta.
To understand why only certain types of SDA are able to
form zeolite Beta, the structure of DCILs were geometry
optimizedusing density functional
B3LYP with 6-31G basis set (Figure S2–S10 in the Support-
ing Information and (Table 2).[34]Very recently, it has been
methodemploying
reported that for the formation of zeolite Beta, the
molecular dimension of SDA should be less than
the pore channel of the 12-member ring of Beta
((pore dimensions: 6.6?6.7 ? for [100] and 5.6?
5.6 ? for [001]).[18]However, our results and previ-
ous report[35]clearly demonstrate that this may not
be the necessary condition for the formation of
zeolite Beta. The molecular dynamics simulation
indicated that conformational change by thermal
rotation and vibration is greatly suppressed, even
at elevated temperature (473 K), due to rigid cyclic
geometry. Such a limited conformational freedom
is known as an important prerequisite for the selec-
tive structure direction of zeolites.[36]The geometry-opti-
mized structure reveals that a piperidine chair conformation
is the appropriate conformation for SDA to form zeolite
Beta (Figures S5–S7 in the Supporting Information), where-
as a boat conformation is not suited for the formation of
Beta zeolite as observed in the case of DCIL1 to DCIL3
(Figures S2–S4 in the Supporting Information). The imida-
zole-containing SDA (DCIL7 and DCIL9) contain two imi-
dazole rings that are situated at different planes (Fig-
ures S8–S10 in the Supporting Information). Two imidazole
rings in DCIL7, DCIL8, and DCIL9 make angles of 58, 73,
and 1108 8, respectively. Based on the experimental and theo-
retical studies, one can conclude that zeolite Beta can be ob-
tained by using imidazole-containing cyclic SDA, when two
imidazole rings makes an angle of 1108 8. This further con-
firms that a suitable special orientation of heterocyclic rings
in DCILs is required for the formation of zeolite Beta.
As mentioned above, zeolites that have inter-/intracrystal-
line mesoporosity have attracted much attention in recent
years, due to interest in applications of catalysis and separa-
tion technologies. The short diffusion path length is an im-
portant structural parameter affecting molecular diffusion
and accordingly catalytic performances. In the present work,
the catalytic activity of micro-/mesoporous hierarchical Beta
zeolite was investigated in the acylation of ferrocene and
transesterification of vegetable oil with methanol to produce
biodiesel. Friedel–Crafts acylation of ferrocene with acetic
anhydride catalyzed by Beta leads almost exclusively to the
formationof monoacetylferrocene.
DCIL4 and Beta-DCIL9 were found to be remarkably more
active than conventional Beta (Table 3) in acylation and
transesterification reactions. Hierarchical Beta-DCIL4 and
Beta-DCIL9 have lower acidity than Beta (Table 3). Hence,
enhancement in the yield of the products can be correlated
with the large external surface area. This remarkable differ-
ence could be attributed to the enhanced molecular diffu-
sion through mesopores.
In conclusion, a variety of geminal dicationic ionic liquids
with cyclic and open structures were synthesized and utilized
for the synthesis of zeolite Beta. The synthesis cost for Beta
is very high due to the high cost for the preparation of or-
ganic SDAs in the hydroxide form. Structure-directing
HierarchicalBeta-
agents investigated in this study are economical compared
with existing structure-directing agents reported for the syn-
thesis of nanocrystalline zeolite Beta. Beta was synthesized
using economical structure-directing agents (in Br form)
with low cost sodium silicate as silica source and aluminum
sulphate as aluminum source. Experimental evidence and
DFT calculations suggest that only a suitable conformer of
SDA can form zeolite Beta. Zeolite Beta synthesized using
dicationic ionic liquids has a large surface area and pore
volume but slightly lower acidity than conventional Beta.
Beta derived from dicationic ionic liquids exhibited better
catalytic activity than the conventional Beta. This activity
difference can be ascribed to improved mass transport in
the mesoporous zeolite Beta compared with the convention-
al Beta.
Table 2. Physical and structural properties of DCILs investigated in this
study.
DCIL M.P
[K]
Dipole
moment
[Debye]
Energy
[a.u.]
Molecular
size [?]
DCIL1
DCIL2
DCIL3
DCIL4
DCIL5
DCIL6
DCIL7
DCIL8
DCIL9
549
563
545
0.17
3.27
1.04
4.16
6.12
4.01
1.29
5.52
0.82
?1047.34
?1279.59
?973.57
?1008.06
?1240.31
?934.27
?995.36
?1227.60
?921.58
16.34?9.14?5.89
13.84?5.80?5.30
13.99?6.84?4.63
12?5?6.8
16.49?8.3?6.82
12.16?8.1?4.78
10.46?7.6?4.62
21.0?7.76?5.23
10.88?8.24?3.82
>573
>573
>573
550
468
543
Table 3. Comparative catalytic study of zeolite Beta synthesized using DCILs in acyla-
tion and transesterification reactions.
EntryCatalyst [Si/Al]Total acidity
[mmolg?1]
Yield of 2-acetyl
ferrocene [%][a]
Yield of
biodiesel [%][b]
1
2
3
4
Beta-DCIL4 (19.4)
Beta-DCIL5 (17.3)
Beta-DCIL9 (19.1)
Beta (17.5)
2.51
2.78
2.46
2.76
47.2
22.8
45.3
23.5
25.3
7.7
23.4
8.5
[a] Reaction conditions: ferrocene (10 mmol); acetic anhydride (50 mmol); 1, 2-di-
chloroethane (5.0 mL); catalyst (100 mg); temperature: 423 K; reaction time: 4 h.
[b] Catalyst: 3 wt% of oil; oil (5 g); CH3OH (2.8 g); oil:CH3OH(mol)=1:15; tempera-
ture: 393 K; reaction time: 4 h. Biodiesel yield was estimated based on the isolated
yield of glycerol.
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Synthesis of Dicationic Ionic Liquids

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Experimental Section
Zeolite was synthesized using sodium-silicate (water glass) (Aldrich) as
the silica source and Al2(SO4)3·18H2O (Loba Chemicals) as the alumina
source. The silica source was diluted with a dilute NaOH solution, yield-
ing a sodium silicate solution with a molar ratio of 100SiO2:30Na2O. The
alumina source was dissolved in dilute sulfuric acid, to a molar ratio of
xAl2(SO4)3:1500H2O. The two solutions were mixed by vigorous stirring
at room temperature. The resultant mixture was a clear solution. When
DCIL was added dropwise into this solution, under vigorous stirring at
room temperature, the mixture gelled immediately with the addition of
theSDA. Themolar gelcomposition
30Na2O:xAl2O3:100SiO2:10DCIL:15H2SO4:6000H2O, in which the DCIL
was varied with DCIL1 to DCIL9. After aging for 6 h at room tempera-
ture, the mixture was transferred to a Teflon-coated stainless-steel auto-
clave and heated at 443 K with stirring. Precipitates were filtered, washed
with deionized water, dried at 383 K, and calcined at 823 K for 4 h under
flowing air. The calcined materials were designated as BEA-DCIL4,
Beta-DCIL5, Beta-DCIL6, Beta-DCIL9 according to DCIL used for the
syntheses. For catalytic applications, the calcined materials were ex-
changed with NH4
NH4
form of zeolite.
XRD patterns were recorded in the 2q range of 5–508 8 with a scan speed
of 28 8min?1on a PANalytical X?PERT PRO diffractometer using CuKara-
diation (l=0.1542 nm, 40 kV, 40 mA) and a proportional counter detec-
tor. Nitrogen adsorption measurement at 77 K was performed by Auto-
sorb-IQ Quantachrome Instruments volumetric adsorption analyzer.
Samples were out-gassed at 573 K for 2 h in the degas port of the adsorp-
tion apparatus. The specific surface area was determined by BET method
using the data points of P/P0in the range of about 0.05–0.3. The pore di-
ameter was estimated using the Barret–Joyner–Halenda (BJH) model.
The Transmission electron microscopy (TEM) investigation was carried
out using FEI, TECNAI G220, S-TWIN microscope operating at 200 kV,
equipped with a GATAN CCD camera. The calcined sample was dis-
persed in ethanol using ultrasonic bath, mounted on a carbon coated Cu
grid, dried, and used for TEM measurement. For TPD experiments, the
catalyst sample was pretreated in He (50 cm3min?1) at 873 K for 1 h.
After cooling down to 373 K, ammonia (partial pressure 100 Torr) was
passed through the samples for 1 h. Then, the sample was subsequently
flushed by He stream (50 cm3min?1) at 373 K for 1 h to remove physisor-
bed ammonia. The TPD experiments were carried out in the range of
373–873 K at a heating rate of 10 Kmin?1. The ammonia concentration in
the effluent was monitored by using a gold-plated, filament thermal con-
ductivity detector.
The catalytic activity of zeolite Beta was tested for acylation of ferrocene
with acetic anhydride and transesterification of soyabean oil with metha-
nol. For the acylation reaction, ferrocene (10 mmol), acetic anhydride
(50 mmol), 1,2-dichloroethane (5 mL) as solvent, and zeolite catalyst
(100 mg) were added to the reactor and the reaction was conducted at
423 K for 4 h. After the reaction, the autoclave was cooled to room tem-
perature immediately with cold water. The reaction mixture was filtered
and the catalyst was washed with acetone to remove all the products re-
maining on its surface. The organic portion was combined and dried. Re-
action mixture was poured into water and was extracted with ethyl ace-
tate. The solvent was evaporated and the product was purified using
column chromatography. In a typical transesterification reaction, vegeta-
ble oil (10 g), methanol (15 times excess), and catalyst (300 mg) were
added to a Teflon-lined steel autoclave. The reaction was conducted at
393 K for 4 h. Then, the autoclave was cooled to 298 K and the catalyst
was separated by centrifugation and filtration. The reaction mixture was
vacuum distilled and the excess methanol was recovered. Petroleum
ether (50 mL) was then added to the remaining contents. The esters and
oil readily mixed with petroleum ether. Glycerol remained as a separate
layer. It was separated and its yield was determined. Fatty acid alkyl
ester and unreacted oil were isolated by distillation (petroleum ether was
recovered). The composition of the fatty acid alkyl esters was determined
by a gas chromatograph.
was represented as
+ +three times using 1 molar NH4NO3solution. Then,
+ +exchanged-zeolite were calcined again at 823 K to obtain protonic
Acknowledgements
The authors thank the Department of Science and Technology, New
Delhi for financial assistance (DST grant SR/S1/PC/31/2009) and thank
Dr. T. J. Dhilip Kumar, Chemistry Department, IIT Ropar for providing
Gaussian 09 software. The authors are also thankful to Prof. M. K. Surap-
pa, Director IIT Ropar for his constant encouragement.
Keywords: acylation · biodiesel · ionic liquids · mesoporous
materials · zeolites
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Received: September 19, 2011
Published online: November 23, 2011
Chem. Eur. J. 2011, 17, 14360–14365? 2011 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim
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14365
COMMUNICATION
Synthesis of Dicationic Ionic Liquids