Platinum functionalized multiwall carbon nanotube composites as recyclable catalyst for highly efficient asymmetric hydrogenation of methyl pyruvate

Article abstract of DOI:10.1039/c5ra21790c

Platinum functionalized carbon materials such as carbon fibres, graphene, MWNTs (multiwalled carbon nanotubes) and activated carbon were used as heterogeneous catalytic systems for asymmetric hydrogenation of α-ketoester i.e. methyl pyruvate using cinchonidine (CD) as a chiral modifier. Interestingly, the MWNTs exhibited excellent enantioselectivity (>99% ee) and conversion (99%) in comparison to other Pt/C systems due to their high surface area. Furthermore, in the case of Pt/MWNTs, Pt nanoparticles are found to be uniformly dispersed and bound to the MWNTs acting like a single atom catalyst. Time-dependent nuclear magnetic resonance (NMR) studies, cyclic voltammetry (CV) and diffuse reflectance spectroscopy (DRS) have been carried out to study substrate-modifier-catalyst interactions. Recyclability of the catalyst was also tested up to ten cycles without losing any significant catalytic activity.

Full text of DOI:10.1039/c5ra21790c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Platinum functionalized multiwall carbon nanotube
composites as recyclable catalyst for highly
efficient asymmetric hydrogenation of methyl
pyruvate†
Cite this: RSC Adv., 2015, 5, 102481
Poonam Sharma and Rakesh K. Sharma*
Platinum functionalized carbon materials such as carbon fibres, graphene, MWNTs (multiwalled carbon
nanotubes) and activated carbon were used as heterogeneous catalytic systems for asymmetric
hydrogenation of a-ketoester i.e. methyl pyruvate using cinchonidine (CD) as a chiral modifier.
Interestingly, the MWNTs exhibited excellent enantioselectivity (>99% ee) and conversion (99%) in
comparison to other Pt/C systems due to their high surface area. Furthermore, in the case of Pt/MWNTs,
Pt nanoparticles are found to be uniformly dispersed and bound to the MWNTs acting like a single atom
catalyst. Time-dependent nuclear magnetic resonance (NMR) studies, cyclic voltammetry (CV) and
diffuse reflectance spectroscopy (DRS) have been carried out to study substrate–modifier–catalyst
interactions. Recyclability of the catalyst was also tested up to ten cycles without losing any significant
catalytic activity.
Received 19th October 2015
Accepted 23rd November 2015
DOI: 10.1039/c5ra21790c
www.rsc.org/advances
Asymmetric homogenous catalysts have been widely used in
various organic transformations due to their high reactivity and
Metal nanoparticle/carbon composite (M/C) systems are of enantioselectivity. Homogeneous catalysis has an advantage
great interest as they blend the exclusive properties of carbon of molecular reaction phenomena and ease of characterization
Introduction
12
1
materials with metal nanoparticles. Carbon based materials of the reaction intermediates. Despite various advantages,
such as MWNTs, carbon bers (CF), activated carbon (AC) and separation and purication of product as well as recycling of the
13–19
graphene are some of the key nominees due to their catalysts make it less environmentally benign.
Strategic
commercial availability and ease of chemical modication. development of asymmetric heterogeneous catalysts is a viable
MWNTs and graphene particularly prove to be an excellent option to overcome such issues by offering innate practical
20
choice due to their extraordinary properties including high advantages over homogeneous catalyst.
specic surface area, electrical conductivity, good chemical
Various studies on asymmetric hydrogenation incorporated
and thermal stability etc. These materials can be used as the use of conventional supporting materials such as Pt loaded
2
–5
20–25
composites with other materials or loaded with metals via on alumina (Al
2
O
3
), silica (SiO
2
) and titanium oxide (TiO
2
).
chemical deposition. Since pristine carbon materials are The Orito reaction where chirally modied Pt/Al
O
2 3
has seen
2
6,34
chemically inactive, chemical tuning for the attachment of a resurgence of activity because of its synthetic use,
and
6
metal nanoparticles for catalytic applications is essential.
considered as standard catalyst for asymmetric hydrogenation
26–35
These composites are used in variety of applications, as an of a-functionalized carbonyls.
The chirally modied
example, as counter electrode in dye sensitized solar cell commercial Pt/Al
2
O
3
catalyst has been used extensively for wide
7
8
(
DSSC), dehydrogenation of ammonia borane and as an range of carbonyl substrates with high optical and chemical
9
33
electrocatalyst support for direct methanol fuel cells. M/C conversion. Impressive results with ee's higher than 95% were
composites form a highly stable system that is unreactive in obtained with synthetically modied nanoparticulated plat-
many acid and basic media, which do not promote any side inum supported alumina (Pt/g-Al O ) for asymmetric hydroge-
2 3
1
0
36,37
reactions. The M/C systems are believed to serve as nano- nation for ethyl and methyl pyruvates.
11
reactors for catalytic conversion.
Major advancement in the non-metal oxide supported
heterogeneous asymmetric catalyst was rstly reported by
11
Li and coworkers
who developed chirally modied
Department of Chemistry, Indian Institute of Technology Jodhpur, Old Residency Road, Pt-nanoparticle supported on multi-walled carbon nanotubes.
Ratanada, Jodhpur, Rajasthan-342011, India. E-mail: rks@iitj.ac.in
These Pt/MWNTs were found to be highly active and lead to
highly enantioselective hydrogenation for various a-ketoesters.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra21790c
1
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 102481–102487 | 102481

View Article Online
RSC Advances
Paper
Interestingly, high enantioselectivities and TOF were obtained chromatogram was recorded on WATERS, Ireland. Chromato-
when Pt nanoparticles are encapsulated in nanotubes than on graphic resolutions of the enantiomers were carried out on
the surface. The high catalytic activity of Pt encapsulated CNT a normal phase CHIRALPAK ID (Lot No. ID00CE-PL014) type
ꢁ
credited to nano-channel like behaviour that leads to greater chiral column under isocratic and isothermal (at 30 C) condi-
11
interactive enrichment of substrate and modier at Pt surface.
tions using respective mixture of n-hexane/iso-propanol as the
This study motivated us to examine all other carbon based mobile phase. Catalytic asymmetric hydrogenation reaction was
materials including MWNTs as support matrix for Pt based carried out in a Berghof HR-100 high pressure vessel equipped
heterogeneous catalysis with optimal enantioselectivity at with a Teon container. Specic rotation was measured from
multigram scale. Herein, we report the improved synthesis of Rudolph polarimeter (APII/2W), purchased from USA.
highly dispersed platinum nanoparticles loaded on function-
alized carbon materials (MWNTs, CF and graphene) for hydro- Preparation of metal loaded carbon materials (M/C)
genation of methyl pyruvate (representative example) using
a chiral modier. Heterogenisation is achieved by the coordi-
A typical preparation experiment for Pt/MWNTs has been
carried out as described below and similar procedure was used
for other carbon materials. First, the activation process was
nation between the chiral modier and the Pt functionalized
38–40
carbon surface.
We have carried out a brief and in-depth
carried out in which MWNTs (10 g) were reuxed under
study on asymmetric hydrogenation of methyl pyruvate using
Pt/C catalytic systems for achieving better insight in such
transformations.
ꢁ
constant stirring in HNO
3
(500 mL, 68 wt%) at 140 C, and
extracted by centrifugation at 3000 rpm. MWNTs were washed
several times with water followed by ethanol. Resultant MWNTs
ꢁ
were dried at 80 C for 4 h under argon.
The functionalized MWNTs were immersed in an ethanolic
solution under stirring and sonication simultaneously. Plat-
inum metal salt (5%, H PtCl ) dissolved in degassed ethanol
Experimental
Materials
2
6
Chloroplatinic acid (37.50% Pt basis) and MWNTs (>95% and added to CNT solution slowly via syringe pump under
carbon) were purchased from Aldrich. Graphene, methyl pyru- hydrogen bubbling for a period of 12 h. The solution was stirred
vate (98% assay) and cinchonidine (99% total base) was received for another 24 h at room temperature and nally heated at
ꢁ
from Alfa Aesar. Carbon ber (Auro carbon industry), acetic acid 100 C for 12 h. Metal nanoparticles were tinted on carbon
(Fisher scientic, 99.9%), platinum on activated carbon (across, surface, the platinum loaded MWCNT were isolated via centri-
5
%
Pt) and ethanol (commercial grade) were obtained fugation, followed by washing with deionised degassed water
ꢁ
commercially. High purity dry hydrogen and argon (99.999%) and drying at 80 C for 16 h under argon (Fig. 1(b)).
gas was used from local resources. BUCHI Glass Oven B-585
Kugelrohr was used to purify the reaction products.
Asymmetric hydrogenation of methyl pyruvate
Pt loaded catalysts were pre-treated in tubular furnace under
ꢁ
Characterization
hydrogen ow for 3 h at 300 C, prior to reaction. For reaction,
Pt/C (300 mg), chiral modier (CD, 40 mg, 0.13 mmol), and
acetic acid (15 mL, 23.7 mmol) were premixed in Teon lined
stainless steel vessel under hydrogen and aer 10 minutes
methyl pyruvate (0.75 mL, 8.3 mmol) was introduced. The
reaction mixture was purged with hydrogen 2–3 times to remove
the air and lled under the desired pressure (10–80 bar; Caution
of pressurized hydrogen!) followed by stirring for 4 h. Subse-
quently, the reaction mixture was centrifuged to obtain the
crude product. The product was puried via bulb to bulb
An X-ray diffractometer, D8 advance (Bruker, U.S.A.) using Cu
ꢀ
Ka
1
(l ¼ 1.54056 A) as a radiation source was used to ascertain
the quality and crystalline nature of carbon materials with tube
current and voltage of 40 mA and 40 kV, respectively.
Morphology of M/C was determined using scanning electron
microscopy (SEM, EVO18 Ziess) with an accelerating voltage of
20 keV. The composite was pasted on silver tape for imaging.
Transmission electron microscopy (TEM) was carried out in
2
a FEI Tecnai-G T20. For TEM study, the catalysts were
1
distillation and analysed by H NMR and HPLC. The % yield
was calculated by weighing the nally obtained puried
product, using balance (Mettlar Toledo-ML204) with accuracy of
dispersed in ethanol using tip ultrasonication. The samples
were prepared by placing a droplet of suspension on to a copper
grid and dried in air. Diffuse reectance spectroscopy
measurement was carried on Varian Cary 4000 in the range of
0.1 mg. The reaction was scaled up to 10 g successfully.
2
00–800 nm. The electrochemical activity of Pt/MWNTs catalyst
(
1
R)-(+)-Methyl lactate
was analysed in oxygen reduction reaction by cyclic voltamme-
try (CH Instruments, electrochemical workstation). Enantioen-
H NMR (CDCl
3
, 500 MHz) d: 1.41 (d, 3H, J ¼ 7.05 Hz, CH
3
), 2.93
), 4.29
, 125 MHz): 20.0,
riched samples were characterized by IR, UV, NMR, HPLC and (d, 1H, J ¼ 5 Hz OH, D
polarimeter. Fourier-transform infrared spectra (FTIR) were (q, 1H, J ¼ 6.94 Hz, CH) ppm; C NMR (CDCl
recorded on a Vertex 70v spectrometer (Bruker) in the range of 52.2, 66.7, 175.7 ppm; (see ESI Fig. S12–S16†), [a]
2
O exchangeable), 3.79 (s, 3H, CH
3
1
3
3
ꢁ
0 C
2
D
¼ +8.9
00–4000 cm . UV-vis spectroscopy data were recorded on (c 1.5, 1, 4-dioxane); IR (KBr) 3346, 2975, 1710, 1379, 1266,
ꢀ
1
41
4
ꢀ
1
Varian Cary 4000 in the range of 200–800 nm. Nuclear magnetic 1084, 1052, 831 cm (see ESI Fig. S9†). Optical purity was
resonance spectra ( H NMR) were recorded on a Bruker 500 determined by HPLC (220 nm); Daicel Chiral Cell CHIRALPAK
1
spectrometer operating at 500 MHz in CDCl
3
.
HPLC ID 0.46 cm/25 cm; hexane/iso-propanol/Na
2
B
2
O
7
¼ 90/10/0.1;
102482 | RSC Adv., 2015, 5, 102481–102487
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 1 (a) Process for surface activation and functionalization of carbon materials (b) preparation of highly dispersed Pt nanoparticles on carbon
materials.
ꢀ
1

ow rate ¼ 1.0 mL min ; retention time: 2.9 min for shown in Fig. 2. The XRD pattern for functionalized carbon
ꢁ
ꢁ
(
R)-(+)-methyl lactate, 3.4 min for (S)-(ꢀ)-methyl lactate (see ESI shows an intense peak at 25.90 along with other peaks at 42.70
ꢁ
Fig. S5–S8†). UV-vis (CH COOH); 210–250 nm (see ESI Fig. S10 and 53.50 corresponding to the (002), (100), (004) reections,
3
and S11†).
Results and discussion
A simple process adopted for surface activation and function-
alisation using HNO
in Fig. 1(a). This process introduces carboxyl (–COOH), hydroxyl
–OH) and carbonyl (–C]O) groups on to the surface of pristine
3
as oxidizing agent is shown schematically
(
42
carbon materials. These functional groups play a vital role in
facilitating the binding, embedding or stacking of nano-
42–45
particles on the surface of carbon based materials.
The Pt
nanoparticles are formed on the activated surface from the
highly diluted H PtCl solution in presence of hydrogen gas as
2
6
reducing agent. Functionalized carbon support and Pt metal
Fig. 2 XRD spectra of (a) Pt loaded carbon materials (b) functionalized
loaded on carbon materials were characterized using XRD as carbon materials.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 102481–102487 | 102483

View Article Online
RSC Advances
Paper
Fig. 3 SEM and TEM image of Pt nanoparticles loaded on carbon materials surface (a, d, g) Pt/CF (b, e, h) Pt/graphene (c, f, i) Pt/MWNT.
respectively. While the Pt loading results in three major peaks at sites due to higher surface area. The higher magnication TEM
ꢁ
ꢁ
ꢁ
3
9.68 , 46.4 and 67.7 corresponding to diffraction from the images in Fig. 3(g)–(i) provide further information about the
(111), (200), and (220) planes of the face-centred cubic (FCC) shape and allocation of nanoparticles on the surface of various
lattice of Pt metal. The (100) related to carbon disappears aer carbon materials. The Pt nanoparticles on MWNTs are partic-
the loading of Pt nanoparticles. However, the XRD analysis ularly interesting as the nanoparticles are seen encapsulated on
conrms that the carbon material retain its structural planes the outer as well as inner surface of the MWNTs (see ESI Fig. S1a
even aer functionalisation. The morphology, shape, size and and b†). The TEM image in Fig. 3(i) shows Pt nanoparticles
distribution of nanoparticles on CF, graphene and MWNTs distinctly dispersed at few nm length scales like a single atomic
were characterized by SEM and TEM imaging. As seen from SEM system. This provides low coordination with quantum size
26
images in Fig. 3(a)–(c), the Pt nanoparticles uniformly cover the effect and also prevents metal leaching. The EDX analysis
surface of carbon support system in all the three cases. The TEM shows that 4.1 ꢂ 0.6 wt% of Pt loading takes place upon
images in Fig. 3(d)–(f) show that the nanoparticles are extremely repeated synthesis of composite. (see ESI Fig. S3 and S4†). To
small in the size range of 2–20 nm. The nanoparticles are well optimize the reaction conditions, the hydrogenation of methyl
dispersed in case of graphene and MWNTs as compared to pyruvate (0.8 mmol) was carried out in acetic acid (2.3 mL) using
carbon bres. Graphene and MWNTs provide more reduction commercially available 5% Pt/AC (see ESI Fig. S2†) at room
temperature by varying the pressure (Table 1). Highest product
yield was obtained at 70 bar pressure. In absence of chiral
Table 1 Optimization of reaction conditions using commercial Pt modier, racemic mixtures of methyl lactate were obtained,
a
loaded activated carbon (Pt/AC) for hydrogenation of methyl pyruvate
where in presence of CD, the enantiomeric product was sepa-
rated and analysed using HPLC. Based on these observations,
asymmetric catalytic hydrogenation of methyl pyruvate was
studied using different catalysts under the optimized reaction
Table 2 Asymmetric hydrogenation of methyl pyruvate using various
a
catalytic systems
a
S. No.
Pressure (bar)
Yield (%)
b
S. No.
Catalyst
Yield (%)
ee (%)
1
2
3
4
5
6
7
8
10
20
30
40
50
60
70
80
60
80
90
92
94
96
97
90
1
2
3
4
Pt/AC
97
87
75
99
31
91
89
>99
Pt/graphene
Pt/carbon bre
Pt/MWNT
a
Pt loading for all catalyst is 5 wt%. Reaction condition: methyl pyruvate/
acetic acid/CD/hydrogen pressure/time/temperature: 0.75 mL/15 mL/
4
0
mg/70 bar/4 h/RT. Determined by HPLC analysis using
a
b
Reaction was carried out using 5% Pt/AC (4 mg) methyl pyruvate a CHIRALPAK ID chiral column. The optical yield was expressed as
(
0.8 mmol) and acetic acid (2.3 mL) for 4 h at room temperature. the enantiomeric excess (ee) of R-(+) enantiomer% ee ¼ ([R] ꢀ [S])/([R]
Racemic mixtures of the products were obtained.
+ [S]) ꢃ 100 (see ESI Fig. S5–S8).
102484 | RSC Adv., 2015, 5, 102481–102487
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
substrate and hydrogen are absorbed concertedly on the Pt
4
6–51
surface which provokes the enantioselectivity.
The in situ
1
H NMR was recorded in toluene-d8 as reaction progresses to
understand the substrate–modier interactions (see ESI
Fig. S17†). We have observed signicant changes in chemical
0
0
shi for H2, H6, H9, H5 and H8 protons when CD and methyl
pyruvate are combined. H2 and H6 proton goes to downeld
due to hydrogen bonding between the adsorbed CD modier
quinuclidine N and O atom of the methyl pyruvate (see ESI
Fig. S17d†). Proton H9 also showed downeld shi, see
Fig. 5(a) and (b). This chemical shi of proton (H9) shows
interaction between OH group of CD and O atom of methyl
pyruvate. Thus, overall NMR study suggested that besides
aliphatic part of CD, the aromatic ring (quinoline) is also
involved in the formation of the methyl pyruvate–CD interac-
Fig. 4 Intermediate structure of asymmetric hydrogenation reaction
of methyl pyruvate on catalyst surface.
4
7,48
tion via p-electrons.
The NMR studies also support p–p
communication between conjugated double bond of aromatic
rings of CD (Fig. 5). Fig. 5(c) and (d) shows chemical shi of
conditions (Table 2). Among these catalytic systems, the Pt/
MWNTs catalyst proved to be the most effective in terms of
product yield and enantioselectivity i.e. >99% ee, which is
highest for supported catalytic asymmetric hydrogenation of
methyl pyruvate (see ESI Table S2†). The specic rotation of the
product was determined using a polarimeter, which conrmed
the enantioselectivity reduction to R-(+)-methyl lactate.
0
0
0
H5 and H8 proton where it was observed that H8 proton goes
0
to downeld and H5 proton goes to upeld. The former
interaction (quinuclidine N and O atom of methyl pyruvate) is
stronger than the p–p interaction mentioned above, however,
the p–p interaction might be sufficient to alter the chemical
shi recorded in NMR with time (see ESI Table S1†). Both type
of interactions help to control the mode of adsorption of the
substrate and chiral induction. Thus, cinchonidine behaves
Substrate–modier–catalyst interaction
like
a bidentated ligand and form substrate–modier
While exploring the possible mechanism of the asymmetric
hydrogenation (Fig. 4), it is generally believed that CD,
complex.
Fig. 5 NMR studies of (a) NMR spectra for H9 proton and corresponding (b) chemical shift plotted with respect to reaction time. (c) NMR spectra
0
0
for H5 & H8 proton and corresponding (d) chemical shift versus reaction time.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 102481–102487 | 102485

View Article Online
RSC Advances
Paper
a
Table 3 Recyclability of catalyst (Pt/MWNT)
b
Entry (cycle)
Yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
98.5
98.3
96.4
95.9
94.4
94.3
94.1
93.9
93.3
93.0
>99
99
>97
97
97
96
96
96
94
94
1
0
a
Fig. 6 DRS spectrum of (a) CD (b) Pt/CD/MWNT (c) Pt/MWNT.
Pt loading for all catalysts is 5 wt%. Reaction condition: methyl
pyruvate/acetic acid/CD/hydrogen pressure/time/temperature: 0.75 mL/
b
1
5 mL/40 mg/70 bar/4 h/RT. Determined by HPLC analysis using
a CHIRALPAK ID chiral column.
UV-DRS spectra of CD, Pt/CD/MWNTs, and Pt/MWNTs are
shown in Fig. 6. As observed, CD produces a steep absorption
edge at about 350 nm with poor absorption. Whereas,
Pt/MWNTs shows a sharp absorption edge at 375 nm with
enhanced absorption in visible region at 608 nm and 669 nm
due to localized surface plasmon resonance (LSPR) associated
with platinum nanoparticles. In the presence of CD, the UV-DRS
spectrum of Pt/MWNTs/CD shows reduced absorption due to
the bonding of Pt nanoparticles with CD. Pt/MWNTs is observed
to exhibit high absorption among all three Pt/C composites (see
ESI Fig. S18†).
Recyclability of catalyst
Heterogeneous catalysts are more stable, which signify its
54
importance in organic synthesis. In present case, the stability
of Pt/MWNTs as catalyst was tested by recycling process, where
the catalyst was separated out by centrifugation and washed
with acetic acid aer each reaction cycle. Fresh reactant and
modier were added to the regenerated catalyst and steps were
repeated for 10 consecutive cycles. The regenerated catalyst
provided low enantioselectivity in absence of modier. A rela-
tively constant conversion and enantioselectivity was obtained
in all cycles (Table 3). The HRTEM images of recycled catalyst
(aer ten cycles) revealed individuality of catalytic reaction
centre that secured high enantioselectivity (See ESI Fig S19†).
Adsorption of CD on Pt surface has been investigated by
cyclic voltammetry (Fig. 7). The CV measurement of the CD
ꢀ
4
ꢀ1
(
10
compared to Pt/MWNTs measured in pure 0.5 mol L
This shows that CD adsorption take place on the Pt surface and
2 4
mol L ) in H SO exhibited decrease in current as
ꢀ
1
2 4
H SO .
52,53
inhibits the participation of Pt metal in redox process.
Current observations manifested that 1 : 1 complex of
Conclusion
substrate/modier absorb on Pt surface to make asymmetric
49,50
hydrogenation reaction successful.
Various interactions In conclusion, asymmetric heterogeneous hydrogenation of
brought the stereogenic centre of CD close enough to the methyl pyruvate is a representative example of a-ketoester has
substrate to achieve high enantioselectivity of 99%. A pictorial been studied using Pt/C as catalytic system. The Pt/MWNTs was
hypothesis of intermediate is shown in Fig. 4, where a favour- found to be most efficient and provided highest enantiose-
able transition state with both stereogenic centres of CD is lectivity. The supremacy of Pt/MWNTs is attributed to the high
being proposed.
absorption of CD/substrate on atomically dispersed Pt nano-
particles loaded on nanochannels of MWNTs that maximized
metal utilization. While NMR studies elucidated 1 : 1 substrate
modier complex, cyclic voltammetry suggested irreversible
adsorption of CD on Pt surface. The catalyst is reusable without
any signicant loss of activity even aer ten cycles. Further-
more, signicant attempts have being made to understand the
mechanism and kinetics of such reactions using NMR. Current
study provides an opportunity to develop large scale industrially
viable asymmetric hydrogenation catalysts.
Acknowledgements
We thank Indian Institute of Technology Jodhpur, India for
a studentship (to PS), and DST-RFBR (Indo-Russian bilateral
project INT/RFBR/P-134) for nancial support and Dr Ritu
Gupta (IIT Jodhpur) for useful discussions. Dedicated to Prof.
Fig. 7 Cyclic voltammetry of Pt/MWNT and CD adsorb Pt/MWNT in
H SO solution.
2 4
102486 | RSC Adv., 2015, 5, 102481–102487
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Ashoka G. Samuelson for his teaching and guidance on the 28 E. Talas, J. L. Margitfalvi and O. Egyed, J. Catal., 2009, 266,
occasion of his 60th birthday.
191–198.
2
9 P. B. Wells and R. P. K. Wells, in Chiral Catalyst Immobilization
and Recycling, ed. D. E. de Vos, I. F. J. Vankelecom and P. A.
Jacobs, Wiley–VCH, Weinheim, 2000, p. 123.
References
1
M. Titirici, R. J. White, N. Brun and V. L. Budarin, Chem. Soc. 30 D. Y. Murzin, P. Maki-Arvela, E. Toukoniitty and T. Salmi,
Rev., 2015, 44, 250. Catal. Rev.: Sci. Eng., 2005, 47, 175.
D. M. Sun, C. Liu, W. C. Ren and H. M. Cheng, Small, 2013, 9, 31 M. Bart ´o k, Curr. Org. Chem., 2006, 10, 1533.
2
1
188.
32 J. L. Margitfalvi and E. Talas, in Asymmetric hydrogenation of
activated ketones, ed. J. J. Spivey and K. M. Dooley, RSC,
Cambridge, 2010, vol. 22, pp. 144–278.
33 M. Studer, H. U. Blaser and C. Exner, Adv. Synth. Catal., 2003,
345, 45–65 and references cited therein.
3
4
D. Janas and K. K. Koziol, Nanoscale, 2014, 6, 3037.
G. Xiong, C. Meng, R. G. Reifenberger, P. P. Irazoqui and
T. S. Fisher, Energy Technol., 2014, 2, 897–905.
5
6
7
M. Fraga, M. Mendes and E. Jordao, J. Mol. Catal. A: Chem.,
2
002, 179, 243–251.
34 Y. Orito, S. Imai and S. Niwa, Collected papers of the 43rd
Catalysis Forum, Japan, 1978, p. 30.
35 S. Bhaduri, G. K. Lahiri, P. Munshi and D. Mukesh, Catal.
Lett., 2000, 65, 61–66.
B. Wu, D. Hu, Y. Kuang, B. Liu, X. Zhang and J. Chen, Angew.
Chem., Int. Ed., 2009, 48, 4751–4754.
H. Guoa, Y. Zhua, W. Lia, H. Zhenga, K. Wua, K. Dinga,
B. Ruana, A. Hagfeldtb, T. Mac and M. Wua, Electrochim. 36 X. Li, X. You, P. Ying, J. Xiao and C. Li, Top. Catal., 2003, 25,
Acta, 2015, 176, 997–1000.
63–70.
W. Chen, J. Ji, X. Feng, X. Duan, G. Qian, P. Li, X. Zhou, D. Chen 37 P. McMorn and G. J. Hutchings, Chem. Soc. Rev., 2004, 33,
and W. Yuan, J. Am. Chem. Soc., 2014, 136, 16736–16739.
108–122.
S. H. Hsieha, M. C. Hsub, W. L. Liua and W. J. Chen, Appl. 38 J. M. Fraile, J. I. Garcia, J. A. Mayoral and T. Tarnai,
Surf. Sci., 2013, 277, 223–230. Tetrahedron: Asymmetry, 1998, 9, 3997–4008.
0 R. S. Oosthuizen and V. O. Nyamori, Platinum Met. Rev., 39 T. P. Yoon and E. N. Jacobsen, Science, 2003, 299, 1691–1693.
8
9
1
1
1
1
1
1
1
2
011, 55, 154–169.
40 R. K. Sharma and A. G. Samuelson, J. Chem. Sci., 2006, 118,
569–573.
41 C. J. Abraham, D. H. Paull, T. Bekele, M. T. Scerba,
T. Dudding and T. Lectka, J. Am. Chem. Soc., 2008, 130,
17085–17094.
1 Z. Chen, Z. Guan, M. Li, Q. Yang and C. Li, Angew. Chem., Int.
Ed., 2011, 50, 4913–4917.
2 J. Pritchard, A. G. Filonenko, R. van Putten, E. J. M. Hensen
and E. A. Pidko, Chem. Soc. Rev., 2015, 44, 3808–3833.
3 B. Minder, T. Mallat, K. Pickel, K. Steiner and A. Baiker, 42 M. von Arx, N. Dummer, D. J. Willock, S. H. Taylor,
Catal. Lett., 1995, 34, 1–9.
4 M. Berthod, G. Mignani, G. Woodward and M. Lemaire,
Chem. Rev., 2005, 105, 1801–1836.
R. P. Wells, P. B. Wells and G. J. Hutchings, Chem.
Commun., 2003, 48, 1926–1927.
43 B. Satishkumar, E. M. Vogl, A. Govindaraj and C. Rao, J. Phys.
D: Appl. Phys., 1996, 29, 3173–3176.
5 T. Mallat, E. Orglmeister and A. Baiker, Chem. Rev., 2007,
1
07, 4863–4890.
6 G. Szollosi, B. Herm ´a n, F. F u¨ l ¨o p and M. Bart ´o k, J. Catal.,
010, 276, 259–267.
7 M. Bart ´o k, Chem. Rev., 2010, 110, 1663–1705.
44 S. Tsang, Y. Chen, P. Harris and M. Green, Nature, 1994, 372,
159–162.
45 E. Dujardin, T. Ebbesen, H. Hiura and K. Tanigaki, Science,
1994, 265, 1850–1852.
2
1
1
8 Enantioselective Organocatalysis, ed. P. I. Dalko, Wiley-VCH, 46 M. Garland and H. U. Blaser, J. Am. Chem. Soc., 1990, 112,
Weinheim, 2007. 7048–7050.
9 Handbook of Asymmetric Heterogeneous Catalysis, ed. K. Ding 47 T. A. Martinek, T. Varga, F. Fulop and M. Bartok, J. Catal.,
and Y. Uozumi, Wiley-VCH, Weinheim, 2008.
2007, 246, 266–276.
0 J. Hong, I. Lee and F. Zaera, Catal. Sci. Technol., 2015, 5, 680– 48 A. Baiker, J. Mol. Catal. A: Chem., 2000, 163, 205.
1
2
2
6
89.
49 O. Schwalm, B. Minder, J. Weber and A. Baiker, Catal. Lett.,
1994, 23, 271.
1 A. Wilhelmus, H. Vermeer, A. Fulford, P. Johnston and
P. B. Wellset, J. Chem. Soc., Chem. Commun., 1993, 1053– 50 K. E. Simons, P. A. Meheux, S. P. Griffiths, I. M. Sutherland,
1
054.
2 H. U. Blaser and H. P. Jalett, Stud. Surf. Sci. Catal., 1993, 78,
39–146.
P. Johnston, P. B. Wells, A. F. Carley, M. K. Rajumon,
M. W. Roberts and A. Ibbotson, Recl. Trav. Chim. Pays-Bas,
1994, 113, 465.
2
2
2
1
3 H. U. Blaser, H. P. Jalett, M. Muller and M. Studer, Catal. 51 D. Ferri, T. Buergi and A. Baiker, J. Catal., 2002, 210, 160–
Today, 1997, 37, 441–463.
170.
4 H. U. Blaser, H. P. Jalett, W. Lottenbach and M. Studer, J. Am. 52 I. Bakos, S. Szabo, M. Bartok and E. Kalman, J. Electroanal.
Chem. Soc., 2000, 122, 12675–12682.
Chem., 2002, 532, 113.
2
2
2
5 J. Kohler and J. S. Bradley, Catal. Lett., 1997, 45, 203–208.
6 Y. Orito, S. Imai and S. Niwa, J. Chem. Soc. Jpn., 1979, 1118.
7 Y. Orito, S. Imai, S. Niwa and N. G. Hung, J. Synth. Org. 54 V. Morawsky, U. Prusse, L. Witte and K. D. Vorlop, Catal.
Chem., 1979, 37, 173. Commun., 2000, 1, 15–20.
53 S. Lavoie, M. A. Laliberte, I. Temprano and P. H. McBreen, J.
Am. Chem. Soc., 2006, 128, 7588–7593.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 102481–102487 | 102487