Molecular Assembly Line: Stepwise Hydrogenation of Multifunctional Substrates over Catalyst Mixtures

Article abstract of DOI:10.1002/cctc.201500988

The synthesis of many chemical products requires multiple steps. To simplify the overall synthesis by combining consecutive reaction steps, the concept of a molecular assembly line promises to yield high reaction rates and selectivities. In this study, mixtures of carbon nanotube (CNT)-supported catalysts were used to demonstrate the potential of this concept on hand of the hydrogenation of nitrobenzene via aniline to cyclohexylamine. Mixtures of Pt/CNT, having a high activity in nitrobenzene hydrogenation, and of Ru/CNT, highly selective for the hydrogenation of aniline to cyclohexylamine, provided high activity at constant high selectivity. Varying the Pt:Ru ratio to 5:95 was found to be the best case scenario, for which the rates of the two consecutive reaction steps are balanced. The use of catalyst mixtures rather than bimetallic catalysts avoids complex phenomena such as phase separation or migration of one of the metals to the surface of the bimetallic particles.

Full text of DOI:10.1002/cctc.201500988

DOI: 10.1002/cctc.201500988
Full Papers
Molecular Assembly Line: Stepwise Hydrogenation of
Multifunctional Substrates over Catalyst Mixtures
[
a]
Patrick Tomkins, Ewa Gebauer-Henke, and Thomas E. Müller*
The synthesis of many chemical products requires multiple
steps. To simplify the overall synthesis by combining consecu-
tive reaction steps, the concept of a “molecular assembly line”
promises to yield high reaction rates and selectivities. In this
study, mixtures of carbon nanotube (CNT)-supported catalysts
were used to demonstrate the potential of this concept on
hand of the hydrogenation of nitrobenzene via aniline to cy-
clohexylamine. Mixtures of Pt/CNT, having a high activity in ni-
trobenzene hydrogenation, and of Ru/CNT, highly selective for
the hydrogenation of aniline to cyclohexylamine, provided
high activity at constant high selectivity. Varying the Pt:Ru
ratio to 5:95 was found to be the best case scenario, for which
the rates of the two consecutive reaction steps are balanced.
The use of catalyst mixtures rather than bimetallic catalysts
avoids complex phenomena such as phase separation or mi-
gration of one of the metals to the surface of the bimetallic
particles.
Introduction
Multiple steps are often required to transform biomass and to
[
1]
synthesize valuable chemicals such as agrochemicals, phar-
[
2]
[3]
maceuticals, and biogenic platform chemicals. To improve
efficiency, there is a major incentive to seek “one-pot” synthe-
[
4]
[5]
ses with catalytic and noncatalytic systems. Special cases
are consecutive reactions catalyzed by solid catalysts. Several
reaction steps can be performed with the same catalyst. Cata-
lyst mixtures might also be employed. If using catalyst mix-
tures for the conversion of multifunctional molecules, the ad-
sorption strengths and, subsequently, the reaction rate of the
specific chemical moieties on the surface of the specific cata-
Scheme 1. Concept of using catalyst mixtures as a molecular assembly line
in consecutive hydrogenation reactions and requirements concerning the af-
finity of substrate, intermediate, and product for adsorption on the active
site of the catalyst.
[6]
lyst have to be well-balanced.
Thus, the current study explores the concept of using two
different metal catalysts, M1 and M2, in a one-pot synthesis for
the consecutive hydrogenation of two functional groups in
a single molecule (Scheme 1). We hypothesize that upon ap-
plying catalyst mixtures, the overall rate of hydrogenating sub-
H
the catalyst M1 ought to govern the conversion of A–B to A –
B, whereas the intrinsic selectivity of the catalyst M2 ought to
H
H
H
promote the conversion of A –B to A –B . Thus, the objective
is to ensure that, over the entire course of the reaction, opti-
mum concentrations of the respective reactants are present on
the catalyst surface to provide a high rate over both reaction
steps. By adjusting the relative amount of the two catalysts,
the rates of the two reaction steps can then be balanced.
An apt example of such a consecutive reaction involving the
reduction of several groups in the same molecule is the hydro-
genation of nitroaromatics to the corresponding cycloaliphatic
amines. The reaction typically proceeds via the corresponding
H
H
strate A–B to product A –B (H=hydrogenated) will be highest
if the catalysts are chosen in such a way that: on catalyst M1,
H
the substrate A–B reacts readily whereas the intermediate A –
B adsorbs weakly, and on catalyst M2, the substrate A–B ad-
H
sorbs weakly whereas the intermediate A –B reacts readily.
By separating the two catalytic functions, the driving idea
here is to avoid competition between substrate and intermedi-
ate for adsorption at the catalytically active centers on the sur-
face of the catalyst. Consequently, the intrinsic selectivity of
[
7]
aniline intermediates. In this case, the reducible groups are
chemically very different: Whereas the nitro group is consid-
ered to be “hard” (negative charge delocalized over two
oxygen atoms) in character, the aromatic ring is taken to be
[
a] P. Tomkins, Dr. E. Gebauer-Henke, Dr. T. E. Müller
CAT Catalytic Center
[
8]
RWTH Aachen University
Worringerweg 2, 52074 Aachen (Germany)
E-mail: Thomas.Mueller@catalyticcenter.rwth-aachen.de
“
soft” (not charged, delocalized p-system). Consequently, the
two chemical entities are frequently hydrogenated over differ-
ent catalysts. For instance, platinum (Pt) and palladium (Pd)
show a high activity and selectivity towards conversion of ni-
Supporting Information and ORCID(s) from the author(s) for this article
are available on the WWW under http://dx.doi.org/10.1002/
cctc.201500988.
[
7a,9]
troaromatics to anilines.
In contrast, ruthenium (Ru) and
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rhodium (Rh) are used for the conversion of anilines to cyclo-
revealed that small platinum particles with 1.5 nm mean diam-
eter were evenly distributed over the outer surface of the
carbon nanotubes giving rise to a BET surface area of
[
7,10]
hexylamines.
Hence, this study deals with the hydrogena-
tion of nitrobenzene over catalyst mixtures of Ru and Pt that
were supported separately on carbon nanotubes (CNT). The re-
spective results are compared to those obtained for the hydro-
genation of nitrobenzene over the corresponding single metal
catalysts as well as bimetallic alloy catalysts.
2
À1
213 m g . The actual ruthenium content in Ru/CNT was
3.1 wt%. TEM measurements revealed that small ruthenium
[
16]
particles with 1.5 nm mean diameter were evenly distributed
over the outer surface of the carbon nanotubes (Figure 1)
2
À1
giving rise to a BET surface area of 210 m g .
Results and Discussion
The hydrogenation of nitrobenzene (NB) was studied as
a model for a consecutive hydrogenation of two chemical moi-
eties in a one-pot system [Eq. (1)]. Here, nitrobenzene yields
aniline (AN) as reaction intermediate which, in turn, is hydro-
[7]
genated to cyclohexylamine (CA).
To demonstrate the concept, platinum (highly shielded d
electrons) was chosen as the metal for which selective hydro-
[9]
genation of nitrobenzene to aniline is known. Ruthenium
highly polarizable) was chosen as the metal that is known to
(
[
7a]
readily hydrogenate the aromatic ring of aniline. Notably,
metallic Pt and Ru have a similar atomic radius of 137.3 and
[8,11]
1
32.5 pm, respectively.
Owing to their different position in
Figure 1. Particle size distribution of Pt/CNT (blue) and Ru/CNT (red) and
the period table, the two metals differ, however, significantly in
their static average electric dipole polarizability (6.5 and 9.6
representative TEM images.
À24
3
[12]
1
0
cm for Pt and Ru, respectively).
Nitrobenzene hydrogenation over the parent Pt/CNT and
Ru/CNT catalysts
Catalyst synthesis and characterization
In a first step, the hydrogenation of NB over the parent Pt/CNT
and Ru/CNT catalysts was monitored as a function of time.
Pt/CNT: After the reactor was pressurized with hydrogen,
a short initiation period was observed (Figure 2, left). There-
Carbon nanotubes were chosen as well-defined support. To
[13]
anchor the metal particles
on the surface, oxidic groups
were generated by prior treatment of the CNT in refluxing
[
14]
À1
nitric acid. In the next step, a metal precursor was dispersed
evenly on the surface of the CNT by the deposition–precipita-
after, NB was consumed rapidly (À43.5 mol (mol min) ). AN
NB
Pt
was formed and, after another short time delay, CA and dicy-
clohexylamine (DA) were formed in a ratio of 86:14. Upon pro-
longed reaction times, the AN concentration remained con-
[
15]
tion method, followed by reduction to the metal with mo-
lecular hydrogen. Before the hydrogenation reactions, all cata-
lysts were characterized extensively (see the Supporting Infor-
mation) The actual platinum content in Pt/CNT was 2.4 wt%.
Measurements using transmission electron microscopy (TEM)
[
7a]
stant.
Ru/CNT: The hydrogenation of NB over Ru/CNT was consid-
erably different. After pressurizing the reactor with hydrogen,
Figure 2. Comparison of the time–concentration profiles of NB hydrogenation over Pt/CNT–Ru/CNT 5:95 (middle), Pt/CNT (left), and Ru/CNT (right); for Pt/
CNT, the signal of CA in the gas chromatogram overlapped with that of a non-identified compound; DA=dicyclohexylamine.
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À1
the consumption of NB commenced (À10.4 molNB (mol min) )
Pt
after a short initiation period, and AN was formed as the initial
product (Figure 2, right). The AN concentration reached a maxi-
mum (xAN =0.38) at 18 min and decreased thereafter at a rate
À1
of À2.4 mol (mol min) . In a consecutive reaction, CA and
AN
Pt
DA were formed in a ratio of 92:8. The reaction was finished
after 68 min (<1% residual aniline).
Thus, over Pt/CNT, known for its high selectivity towards
[9]
AN, the hydrogenation of NB proceeded fast and reduction
of the aromatic ring in AN was not observed once all NB had
been consumed. By contrast, over Ru/CNT the hydrogenation
of NB proceeded more slowly and reduction of the aromatic
ring in AN was observed.
Figure 3. Comparison of the rates of AN formation and the rate of AN con-
sumption for different Ru/CNT:Pt/CNT ratios.
Nitrobenzene hydrogenation using Pt/CNT:Ru/CNT catalyst
mixtures
in the presence of platinum (ꢀ23 min) compared to that for
the parent Ru/CNT catalyst (34 min). Thus, a 5:95 ratio provides
the best situation of approximately equal rates of NB and AN
consumption. This is reasonable as Pt/CNT provides a much
higher rate for NB hydrogenation than Ru/CNT for either NB or
AN hydrogenation (see below). The ideal situation of a molecu-
lar assembly line involves similar rates for NB and AN hydroge-
nation. Consequently, a small amount of Pt/CNT is adequate to
tune the performance of Ru/CNT for the hydrogenation of AN
towards CA.
In the second step, the hydrogenation of NB over mixtures of
the Pt/CNT and Ru/CNT catalysts was followed as a function of
time. For a 5:95 (n :n ) mixture Pt/CNT:Ru/CNT the consump-
Pt Ru
tion of NB commenced at a high rate (À20.6 mol (mol(Ru+
NB
À1
min) ) shortly after the reactor had been pressurized with
Pt)
hydrogen (Figure 2, middle). Similar to Ru/CNT, AN was formed
as the initial product. The AN concentration reached a maxi-
mum (xAN =0.44) after 10 min and decreased thereafter at a sur-
À1
prisingly high rate (À18.0 molAN (mol(Ru+Pt) min) ) to provide
Closer inspection of the time–concentration profiles reveals
that also the rate of aniline hydrogenation increased in the
presence of Pt/CNT compared that for the parent Ru/CNT cata-
lyst. This is readily explained by the formation of hydrogen-
bonded aggregates between nitro groups and amine
CA and DA in a ratio of 92:8. The reaction was complete after
2
4 min.
Thus, over Pt/CNT:Ru/CNT 5:95 the initiation period was
shortened, and the hydrogenation of NB proceeded twice as
fast as that for Ru/CNT. In particular, the AN formed in the first
step was consumed rapidly. This shows that all relevant reac-
tion rates were increased for the catalyst mixture relative to
the parent Ru/CNT catalyst. These observations fully agree with
the proposed model of a molecular assembly line. Here, the
conversion with Pt/CNT proceeds at a high rate for the hydro-
genation of NB towards AN and helps to produce sufficient AN
for the subsequent step.
[
7a,18]
groups.
This specific interaction enables the aromatic ring
of aniline to adsorb at the metal surface in the presence of
other coordinating groups leading to accelerated hydrogena-
tion. The addition of a small amount of Pt/CNT can shift the
relative concentrations of AN and NB to generate an ideal reac-
tant mixture in situ, thereby explaining the unexpectedly high
[
7a]
rate of AN conversion.
The observations concerning the final selectivities to CA ob-
tained for the reactions with varying metal ratios can be divid-
ed into two groups. In the presence of Ru/CNT, the selectivity
towards CA was high (>92%, Figure 4). This clearly indicates
that there is no dependency of the selectivity on the Pt:Ru
ratio. Only the parent Pt/CNT employed as a single catalyst
Variation of the Pt to Ru ratio
Based on the assumption that aniline produced over Pt/CNT
(
M1) is then handed over to the Ru/CNT catalyst (M2) for hy-
drogenation of the aromatic ring, the rate of NB consumption
[
17]
needs to be balanced with the rate of AN consumption. We
therefore studied the effect of varying the Pt-to-Ru ratio in the
Pt/CNT:Ru/CNT system (Figure 3). Upon decreasing the ratio to
2
0:80, the rate of NB consumption increased to
À1
À25.5 mol (mol(Pt+Ru) min) whereas the rate of AN consump-
NB
À1
tion decreased to À7.2 mol (mol(Pt+Ru) min) . The time re-
AN
quired to achieve full conversion remained equal (24 min to
<
1% residual aniline). Similarly, a ratio of 40:60 provided
À1
a rate of NB consumption of À19.4 molNB (mol(Ru+Pt) min)
whereas the rate of AN consumption decreased to À5.0 mo-
À1
l
(mol
min) . The time required to achieve full conver-
AN
(Ru+Pt)
sion was also slightly longer (30 min to <1% residual aniline).
Figure 4. Comparison of the final selectivities for catalyst mixtures with dif-
Notably, the time to full NB conversion shortened significantly
ferent Ru/CNT:Pt/CNT ratios (left) and bimetallic catalysts (right).
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showed a high selectivity towards AN (see below). Previously,
we showed that most of the secondary amine (the major by-
product if Ru/CNT is present) is formed during the hydrogena-
ized water (550 mL), and Pt(NH ) (NO ) (1.46 g, 3.77 mmol), and
3 4 3 2
stirred under Ar at 908C for 22 h. The reaction mixture was filtered
and the filter cake washed with little water. The product was dried
À1
[
7a]
in a stream of argon (100 mLmin ) at 1208C for 2 h and subse-
tion of AN. Assuming that Pt/CNT mainly yields AN for fur-
ther hydrogenation over Ru/CNT, it is clear that the final selec-
tivity is determined by Ru/CNT.
À1
quently reduced in a stream of hydrogen (100 mLmin ) at 2008C
for 1 h. Ru/CNT was prepared in an identical way using the follow-
ing quantities: treated CNT (19.3 g), urea (2.77 g, 46.0 mmol), de-
ionized water (600 mL), and Ru(NO)(NO ) (OH) (62.86 g of solution
In comparison, the selectivities to CA in a series of bimetallic
Ru–Pt/CNT alloy catalysts (determined after 60 min reaction
time) decreased rapidly at higher Pt loadings. For a 45:55
3
x
y
in dilute nitric acid, ꢂ1.5 wt% Ru) instead of Pt(NH ) (NO ) .
3
4
3 2
(
nPt:nRu) ratio in Pt/Ru/CNT only part of the aniline (68%) was
converted and CA and DA were formed in a ratio of 70:30.
Catalyst characterization
Over the Ru/Pt/CNT catalyst with 80:20 (n :n ) ratio only little
Pt Ru
Before the hydrogenation experiments, all catalysts were character-
ized (Supporting Information). Catalyst ratios refer to the molar
aniline (16%) was converted. Thus, the trend in selectivity was
markedly different in the series of catalyst mixtures and the
series of bimetallic alloy catalysts. Notably, the use of catalysts
mixtures improves the overall effectivity of the catalyst system,
amounts of metal n :n and relate to the nominal catalyst compo-
Pt Ru
sitions.
[
19]
because phase segregation
and migration of one of the
[20]
metals to the surface is avoided.
Hydrogenation experiments using catalyst mixtures
Hydrogenation reactions were performed in a 200 mL stainless-
steel autoclave equipped with a gas entrainment stirrer, heating
mantel, and sampling valve. The autoclave was charged with nitro-
benzene (2.50 g, 20.3 mmol), THF (120 mL), catalyst (0.41–0.79 g,
see the Supporting Information) and internal standard (dodecane,
Conclusions
By combining consecutive reactions steps, the concept of
a “molecular assembly line” was explored. Using single metal
catalysts in the hydrogenation of nitrobenzene to cyclohexyla-
mine, carbon-nanotube-supported platinum catalyst (Pt/CNT)
yielded aniline (AN) at a particularly high rate, whereas CNT-
supported ruthenium catalyst (Ru/CNT) provided AN as a reac-
tion intermediate, which was hydrogenated in a consecutive
reaction to cyclohexylamine (CA). Investigating the per-
formance of catalyst mixtures in consecutive reactions revealed
that the addition of a small amount of Pt/CNT to Ru/CNT led
to a significant increase in the rates of nitrobenzene (NB) and
1
.30 g). The mixture was heated to the reaction temperature
(
1408C), and the reaction was started by pressurizing the autoclave
with hydrogen to 100 bar. Samples of the liquid phase were taken
during the reaction for analysis by GC. Concentrations are given as
the molar fraction of the particular substance i normalized to the
initial concentration of the substrate (c/c
100 mol%).
i
substrate,t=0
Rates of reaction (r) were calculated at 50% of the maximum con-
i
centration of compound i by fitting the time–concentration
[21]
diagrams with a five-parameter logistic function.
AN conversion. At a 5:95 (n :n ) ratio, the two rates were
Pt Ru
Acknowledgements
nearly equal, which is highly favorable for such consecutive re-
actions where the final product is desired. The selectivity to-
wards CA (ꢁ92%) remained high as long as Ru was present.
By varying the Pt:Ru ratio, it was shown that a respective 5:95
Cleopatra Herwartz (GFE) for TEM measurements, Xaver Hecht
TUM) for measuring nitrogen physisorption isotherms. The
(
contribution of Sebastian Schrçder to preparing and testing the
bimetallic catalysts is highly acknowledged.
(
nPt:nRu) mixture of these catalysts provided the highest rate for
AN consumption. This balances the rate of NB hydrogenation
to the subsequent hydrogenation of AN over Ru/CNT. By using
mixtures of catalysts, which are optimal for one of the reac-
tions, greatly enhanced reaction rates can be obtained in con-
secutive reactions compared to the single catalysts. The use of
a small amount of the noble-metal-based Pt/CNT catalyst in
combination with the abundant ruthenium-based Ru/CNT cata-
lyst results in a more sustainable catalyst system.
Keywords: adsorption · cooperative effects · platinum ·
ruthenium · hydrogenation
[1] a) A. K. William, L. N. Davis, R. D. Michael, R. G. Joseph, J. M. Eric, O.
Nailah, C. N. Simon, J. P. Daniel, H. U. Steven, B. W. Gerald, N. Z. Ronald
in Agrochemical Discovery, Vol. 774, American Chemical Society, Wash-
ington D.C., 2000, pp. 205–213; b) A. Guan, C. Liu, X. Yang, M. Dekeys-
er, Chem. Rev. 2014, 114, 7079–7107.
[
2] a) K. C. Nicolaou, Z. Yang, J. J. Liu, H. Ueno, P. G. Nantermet, R. K. Guy,
C. F. Claiborne, J. Renaud, E. A. Couladouros, K. Paulvannan, E. J. Soren-
sen, Nature 1994, 367, 630–634; b) K. Tatsuta, S. Hosokawa, Chem. Rev.
Experimental Section
Catalyst preparation
2005, 105, 4707–4729.
[
[
3] F. M. A. Geilen, B. Engendahl, A. Harwardt, W. Marquardt, J. Klankermay-
er, W. Leitner, Angew. Chem. Int. Ed. 2010, 49, 5510–5514; Angew. Chem.
The CNT-supported catalysts were prepared by the deposition–pre-
cipitation method. Carbon nanotubes (BAYTUBES C 150 P, 20 g)
were suspended and heated at reflux for 2 h in nitric acid (65%,
[7a]
2010, 122, 5642–5646.
4] a) K. M. Koeller, C.-H. Wong, Chem. Rev. 2000, 100, 4465–4494; b) M. J.
Climent, A. Corma, S. Iborra, Chem. Rev. 2011, 111, 1072–1133; c) Y. Gu,
Green Chem. 2012, 14, 2091–2128.
1
50 mL). Subsequently, the CNT were removed by filtration and
washed with deionized water until the eluent had a neutral pH
value; then they were dried. For preparing Pt/CNT, the treated CNT
[5] M. Wang, Z. Yang, Z. Song, Q. Wang, J. Heterocycl. Chem. 2015, 52, 907–
(
14.0 g) were re-suspended in a solution of urea (0.572 g), deion-
910.
ChemCatChem 2016, 8, 546 – 550
www.chemcatchem.org
549
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
[
[
6] a) E. Gebauer-Henke, W. Leitner, A. Prokofieva, H. Vogt, T. E. Müller,
Catal. Sci. Technol. 2015, 5, 2539–2548; b) P. Schärringer, T. E. Müller, A.
Jentys, J. A. Lercher, J. Catal. 2009, 263, 34–41.
[16] see also a) J. Julis, M. Hçlscher, W. Leitner, Green Chem. 2010, 12, 1634–
1639; b) S. K. Karmee, L. Greiner, A. Kraynov, T. E. Müller, B. Niemeijer, W.
Leitner, Chem. Commun. 2010, 46, 6705–6707.
[17] G. Emig, E. Klemm, Technische Chemie: Einführung in die chemische Reak-
tionstechnik, Springer, Heidelberg, 2005.
7] a) P. Tomkins, E. Gebauer-Henke, W. Leitner, T. E. Müller, ACS Catal. 2014,
4, 203–209; b) S. G. Oh, V. Mishra, J. K. Cho, B.-J. Kim, H. S. Kim, Y.-W.
Suh, H. Lee, H. S. Park, Y. J. Kim, Catal. Commun. 2014, 43, 79–83.
8] J. C. Slater, J. Chem. Phys. 1964, 41, 3199.
9] a) C.-H. Li, Z.-X. Yu, K.-F. Yao, S.-f. Ji, J. Liang, J. Mol. Catal. A 2005, 226,
[18] E. Gebauer-Henke, P. Tomkins, W. Leitner, T. E. Müller, ChemCatChem
2014, 6, 2910–2917.
[
[
[19] a) B. N. Wanjala, J. Luo, R. Loukrakpam, B. Fang, D. Mott, P. N. Njoki, M.
Engelhard, H. R. Naslund, J. K. Wu, L. Wang, O. Malis, C.-J. Zhong, Chem.
Mater. 2010, 22, 4282–4294; b) A. Guerrero-Ruiz, B. Bachiller-Baeza, P.
Ferreira-Aparicio, I. Rodríguez-Ramos, J. Catal. 1997, 171, 374–382;
c) F. H. B. Lima, J. F. R. de Castro, E. A. Ticianelli, J. Power Sources 2006,
161, 806–812; d) A.-Q. Wang, C.-M. Chang, C.-Y. Mou, J. Phys. Chem. B
2005, 109, 18860–18867.
1
01–105; b) Z. Sun, Y. Zhao, Y. Xie, R. Tao, H. Zhang, C. Huang, Z. Liu,
Green Chem. 2010, 12, 1007–1011.
10] a) A. Kraynov, E. Gebauer, W. Leitner, T. E. Müller, ISJAEE 2010, 84, 37–
4; b) H. Greenfield, J. Org. Chem. 1964, 29, 3082–3084.
[
[
[
[
[
[
4
11] A. F. Holleman, E. Wiberg, N. Wiberg, Lehrbuch der anorganischen
Chemie, de Gruyter, Berlin/New York, 1995.
12] T. M. Miller in Handbook of chemistry and physics, Vol. 87th (Ed.: D. Lide),
CRC, Boca Raton, FL, 2006–2007, pp. 10–193 to 110–195.
13] A. Kraynov, T. E. Müller in Applications of Ionic Liquids in Science and
Technology (Ed.: S. Handy), Intech, Rijeka, 2011, pp. 235–260.
14] V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I.
Kallitsis, C. Galiotis, Carbon 2008, 46, 833–840.
[20] J. Liu, W. Liu, Q. Sun, S. Wang, K. Sun, J. Schwank, R. Wang, Chem.
Commun. 2014, 50, 1804–1807.
[21] P. G. Gottschalk, J. R. Dunn, Anal. Biochem. 2005, 343, 54–65.
15] I. Chorkendorff, J. W. Niemantsverdriet, Concepts of Modern Catalysis
and Kinetics, Wiley-VCH, Weinheim, 2007.
Received: November 9, 2015
Published online on December 14, 2015
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