Urea-pyridine bridged periodic mesoporous organosilica: An efficient hydrogen-bond donating heterogeneous organocatalyst for Henry reaction

Article abstract of DOI:10.1016/j.jcat.2015.07.011

Hydrogen-bond donating (HBD) organocatalysts have been employed as an important class of catalysts for the activation of small molecules. However, the use of these catalysts was restricted on account of the self-aggregation of HBD-active species. In this work, a successful approach was developed to construct novel urea-pyridine bridged periodic mesoporous organosilica (PMO) to prevent the unwanted self-recognition and aggregation of urea-based catalytic center by spatial isolation, unleashing its HBD organocatalytic activity along with cooperative effect from the pyridine unit for efficient Henry reaction. The urea-pyridine bridged PMO catalyst presents high structural stability and high catalytic recyclability.

Full text of DOI:10.1016/j.jcat.2015.07.011

Journal of Catalysis 330 (2015) 129–134
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Journal of Catalysis
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Research Note
Urea–pyridine bridged periodic mesoporous organosilica: An efficient
hydrogen-bond donating heterogeneous organocatalyst for Henry
reaction
Parijat Borah a, John Mondal a, Yanli Zhao a,b,
⇑
a Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore
b School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrogen-bond donating (HBD) organocatalysts have been employed as an important class of catalysts
for the activation of small molecules. However, the use of these catalysts was restricted on account of
the self-aggregation of HBD-active species. In this work, a successful approach was developed to con-
struct novel urea–pyridine bridged periodic mesoporous organosilica (PMO) to prevent the unwanted
self-recognition and aggregation of urea-based catalytic center by spatial isolation, unleashing its HBD
organocatalytic activity along with cooperative effect from the pyridine unit for efficient Henry reaction.
The urea–pyridine bridged PMO catalyst presents high structural stability and high catalytic recyclability.
Ó 2015 Elsevier Inc. All rights reserved.
Received 1 April 2015
Revised 3 June 2015
Accepted 4 July 2015
Keywords:
Henry reaction
Hydrogen-bond donating
Periodic mesoporous organosilica
Organocatalysts
Urea
1. Introduction
turnover efficiency, hindering organocatalytic reactions in
large-scale synthesis [9].
Electrophile activation by hydrogen-bond donors has emerged
as a new catalytic strategy in synthetic organic chemistry including
enantioselective catalysis [1–3]. Among various types of
organocatalysis, hydrogen-bond donating (HBD) catalysis is a spe-
cial class of mimetic alternative to Lewis acid activation, which
could aid in accelerating nucleophilic attack [4–7]. Urea and
thiourea have the potential to be used as double hydrogen-bond
donors in organocatalysis. However, the HBD catalytic capability
of urea is greatly restricted due to its high propensity to undergo
self-dimerization attributed by its oxygen to accept a hydrogen
bond in liquid phase homogeneous catalysis [4–8]. This unwanted
self-assembly of urea significantly lowers its solubility, resulting in
a potential decrease in catalytic reactivity. On the other hand, more
soluble thiourea analogue rules out the possibility of self-assembly
through strong intermolecular hydrogen-bonding interactions.
But, thiourea-based catalysts are often accompanied by poor ther-
mal stability, which limits their applicability in a broad range [8].
In addition, such HBD catalysts are associated with relatively low
The development of new heterogeneous organocatalysts other
than metal-based heterogeneous catalysts has attracted a lot of
attention owing to their several advantages including the robust-
ness, low toxicity without metal-leaching problem during the reac-
tions, inertness to moisture and oxygen, and simple handling and
storage capability in the vicinity of green chemistry [10,11].
Recently, a few successful studies have been reported regarding
the incorporation of urea based HBD molecules onto the solid
architectures to overcome abovementioned problems. Among
these examples, most of the solid HBD catalysts were prepared
by the post-modification of the surfaces of various solid supports
such as porous organic polymers (POPs) [12], mesoporous silica
[13,14], and metal-organic frameworks (MOFs) [15]. However,
low stability due to the possibility of structural collapse [16] and
the presence of metal ions in the frameworks [17] make MOFs less
efficient than other counterparts as the solid supports in heteroge-
neous HBD catalysis. In addition, the introduction of the urea mole-
cules onto the solid supports by the post-grafting method may still
have the possibility of catalytic quenching on account of the dimer-
ization with neighboring urea units on the surface. Furthermore,
the post-grafting method often generates unwanted hindrance
inside the pore channels of the support materials, making the dif-
fusion of the reaction substrates more difficult [18]. In order to
address the problems associated with urea-based organocatalysts,
⇑
Corresponding author at: Division of Chemistry and Biological Chemistry,
School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, Singapore 637371, Singapore.
E-mail address: zhaoyanli@ntu.edu.sg (Y. Zhao).
http://dx.doi.org/10.1016/j.jcat.2015.07.011
0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

130
P. Borah et al. / Journal of Catalysis 330 (2015) 129–134
we designed a novel periodic mesoporous organosilica (PMO)
incorporated with the urea–pyridine group as the organic bridge
on the pore walls of mesoporous structure. Through the prepara-
tion, we could achieve the hybrid PMO material with high density
of organic active groups, while restricting the anchored urea unit
from unwanted dimerization in order to unleash its great potential
in the HBD catalysis.
2. Experimental
All experimental details are displayed in Supplementary
information (SI).
3. Results and discussion
In order to prepare a PMO for catalytic applications, it is desir-
able to have a high degree of functionality within PMO by choosing
an organic precursor with at least two terminal alkoxysilanes
[19–21]. In the current work, an essential organic precursor (3)
with two urea linkages was synthesized (Scheme 1) through the
condensation between the amino group of compound 1 and the
isocyanato unit of compound 2. Although the synthesis of this
molecule has already been reported [22], it has not been utilized
for the preparation of PMO. By using 3 as the organic bridge, a
novel PMO was synthesized through the co-condensation [23,24]
with tetraethyl orthosilicate (TEOS) using cetyltrimethylammo-
nium bromide (CTAB) as structure-directing agent under basic con-
ditions (see SI for synthetic details). The obtained 3-incorporated
PMO was denoted as Urea/PMO for further uses.
The FT-IR spectrum (Fig. 1) of Urea/PMO displays many charac-
teristic peaks for various functionalities such as C@O and CAN
stretching from the urea–pyridine unit in the range of 1600–
1400 cmÀ1 and CAH stretching at 2950–2850 cmÀ1, providing
clear evidence of the successful integration of precursor 3 into
the organosilica framework. The spectrum (Fig. 1b) also shows
characteristic peaks of the SiAO stretching at 1084 and
1231 cmÀ1 and the SiAC stretching at 962 cmÀ1, indicating the for-
mation of silica network [25,26]. The predominant band ranging
from 1670 cmÀ1 to 1640 cmÀ1 was attributed to the bending fre-
quency of surface adsorbed H2O molecules, which unfortunately
masked few other signals from the urea–pyridine unit. Although
we pretreated the Urea/PMO at 120 °C under vacuum of 10À6 mbar,
we were still unable to remove the surface adsorbed water com-
pletely. Furthermore, a broad band around 3426 cmÀ1 was attribu-
ted to the OAH stretching vibration of hydrogen-bonded silanols
on PMO.
Fig. 1. FT-IR spectra of (a) compound 3 and (b) Urea/PMO.
characteristic signals (Fig. 2a) were also observed from the recov-
ered Urea/PMO_U catalyst after 6 consecutive cycles of catalysis,
reflecting good stability of the organic moiety within the silica
framework even after prolonged catalytic usage. 29Si CP MAS
NMR spectrum of Urea/PMO (Fig. 2b) shows major signals centered
at À110 and À100 ppm as well as a small shoulder at À90 ppm,
which could be attributed to the silicon species of Q4 [(„SiO)4Si]
and Q3 [(„SiO)3Si(OR)] respectively [27,28], where R represents
Et or H. In addition to these signals, Urea/PMO also displays one
more peak at À64 ppm, corresponding to the T3 resonance from sil-
icon species attached to the organic group [ACH2ASi(OSi„)3].
Apart from T3 signal, the spectrum does not display any other sig-
nals corresponding to T0, T1 and T2 [27,28]. This is solid evidence in
support of complete integration of the organic precursor into the
silica framework through the hydrolysis of terminal alkoxysilanes.
The 29Si CP MAS NMR spectrum of Urea/PMO_U (Fig. 2b) is essen-
tially similar to that of Urea/PMO, revealing the stable urea–pyri-
dine unit in the polysiloxane network.
The isothermal N2 adsorption/desorption measurements were
carried out for Urea/PMO and the same catalyst (Urea/PMO_U)
after 6 consecutive cycles of catalysis in order to determine their
surface areas and pore size distributions (Table 1). Urea/PMO illus-
trates a typical type IV isotherm (Fig. S1 in the SI) under the lowest
pressure, which accounts for the presence of mesopores. The Bru
nauer–Emmett–Teller (BET) surface area of Urea/PMO was mea-
sured to be 951 m2 gÀ1. The pore size distribution indicates a nar-
row distribution of mesopores (Fig. S1 in the SI) with an average
pore diameter of 3.8 nm calculated by employing Barrett–Joyner–
Halenda (BJH) model. A slight decrease in the BET surface area
(892 m2 gÀ1) with no remarkable change in the isotherm pattern
was observed for reused Urea/PMO_U catalyst (Fig. S1 in the SI),
indicating the preservation of the mesopores after the catalysis.
The pore diameter was found to be 3.8 nm for reused
The 13C cross-polarization magic angle spinning (CP MAS) NMR
spectrum of Urea/PMO (Fig. 2a) exhibits strong signals ranging
from 10 to 43 ppm, which could be attributed to different aliphatic
primary and secondary carbon atoms from the organic bridging
unit (3). Strong signals from aromatic carbons on the pyridine ring
could be identified at ꢀ105 ppm as well as in the range from 140 to
160 ppm. In addition to these signals, one strong signal at
ꢀ152 ppm was assigned to the carbonyl carbon from the urea link-
age. These clear observations further support the successful incor-
poration of compound 3 into the silica framework. The same
Scheme 1. Schematic representation of the synthetic route for the preparation of organic silane precursor (3).

P. Borah et al. / Journal of Catalysis 330 (2015) 129–134
131
Fig. 2. (a) 13C CP MAS solid-state NMR spectra and (b) 29Si CP MAS solid-state NMR spectra of (i) Urea/PMO and (ii) Urea/PMO_U. Urea/PMO_U is the catalyst recovered after 6
cycles of catalysis.
Table 1
the basicity of pyridine unit in Urea/PMO as a cooperative catalyst,
since this catalyst is selective enough to facilitate only Henry reac-
Analytical, textural, and porosity data of different catalysts.
tion without any side reactions, and (3) to elevate the acidity of the
proton on the -carbon of nitroalkane. Due to the presence of urea
Entry
Catalyst
BET surface area
BJH pore
diameter (nm)
Content of 3
(m2 gÀ1
)
(mmol gÀ1
)
a
unit on the channel surface of Urea/PMO, nitroalkane can interact
with urea through hydrogen-bonding interaction, which may
1
2
Urea/PMO
Urea/PMO_U
951
892
3.8
3.8
0.640
0.634
enhance the acidity of the proton on the a-carbon to make subse-
quent deprotonation by pyridine unit easier. Consequently, the
deprotonated anion reacts with aldehydic carbon, where its elec-
trophilicity is enhanced due to the formation of two-point
hydrogen-bonding interactions through acidic NAH bonds of the
embedded urea unit with the aldehydic oxygen.
Urea/PMO_U catalyst, suggesting that no clogging of the meso-
pores took place after the catalysis. The elemental analysis (EA)
of PMO was performed to determine the amount of nitrogen (N)
in the material. Precursor 3 is the only source of N in the PMO,
and each mole of the precursor contains 5 moles of N, from which
we quantified the final loading of precursor 3 in Urea/PMO before
and after the catalysis. From the analysis, the loading of precursor 3
in Urea/PMO was found to be 0.640 mmol gÀ1 and the value
became 0.634 mmol gÀ1 in the case of Urea/PMO_U (Table 1).
Powder X-ray diffraction (XRD) pattern of Urea/PMO (Fig. 3a)
shows a sharp peak at 2h = 2.25° corresponding to (100) plane,
which is obvious evidence for well-ordered hexagonal structure
of Urea/PMO. The distance of d1 0 0 space is 3.9 nm calculated by
Brag’s law from the (100) position at 2h = 2.25°, which is consis-
tent with the BJH pore diameter. TEM image (Fig. 3b) of
Urea/PMO shows that the mesopores with diameter of 3–4 nm
are homogenously distributed throughout the sample. TEM image
(Fig. 3c) of the reused Urea/PMO_U further indicates the preserva-
tion of the mesopores after the catalysis. The mesopores with reg-
ular porosity could be clearly observed.
The feasibility of incorporating urea group into PMO to improve
the HBD catalytic ability in heterogeneous phase was evaluated by
carrying out Henry reaction as a model reaction. The products of
the Henry reaction, i.e., 2-nitroalkanols, are not only the represen-
tatives of CAC bond formation, but also important starting materi-
als for numerous syntheses [29,30]. However, selective synthesis of
2-nitroalkanols is often encountered by many challenges such as
the polymerization as well as the formation of aldol olefins and
Cannizzaro products. Nevertheless, a careful control of the basicity
of the catalysts as well as the activation of lowest unoccupied
molecular orbital (LUMO) of electrophiles can circumvent these
issues to achieve better yields of b-nitroalcohols. Thus, primary
motivations behind choosing this reaction to evaluate catalytic
activity of Urea/PMO are as follows: (1) to utilize urea as the
HBD catalyst to activate the LUMO of electrophile, (2) to exploit
A series of Henry reactions between 4-nitrobenzaldehyde and
nitromethane were carried out (Scheme 2) in order to evaluate
the catalytic efficacy of Urea/PMO. The results show that the
Urea/PMO catalyst is efficient enough to facilitate the Henry reac-
tion at 25 °C to deliver respective b-nitro alcohol product, i.e.,
2-nitro-1-(4-nitrophenyl)-ethanol, without using any additional
base. The catalytic conditions were optimized by varying the cata-
lyst loading (molar ratio of compound 3 in Urea/PMO with respect
to aldehyde) and the reaction time (Figs. S2 and S3 in the SI). Based
on the experimental results, 12 h of reaction time with 10 mol% of
catalyst loading at 25 °C using acetonitrile as solvent was chosen as
the optimum reaction conditions, which were used for the follow-
ing catalytic studies.
In a typical Henry reaction of 4-nitrobenzaldehyde and nitro-
methane under the optimal reaction conditions using 10 mol% of
Urea/PMO, the reaction yield was as high as 93%, whereas the same
reaction cannot proceed without any catalyst (Table 2, entries 1
and 2). Moreover, bare mesoporous silica material MCM-41 also
contains a lot of hydroxyl groups on the surface, which may have
the ability to interact with the substrates through
hydrogen-bonding interactions. However, this possibility was
ruled out based on experimental result obtained by using
MCM-41 as the catalyst (Table 2, entry 3). Compound 3 as the cat-
alyst also afforded corresponding b-nitro alcohol with yield of 23%
(Table 2, entry 4), whereas a marginal decrease of yield was
observed when a mixture of MCM-41 and 3 was used as the cata-
lyst (Table 2, entry 5). Since Henry reaction demands a base, this
model reaction was also carried out in the presence of pyridine
or triethylamine (Et3N) without using Urea/PMO. The results show
significantly low yield of product (Table 2, entries 6 and 7) as com-
pared to that of Urea/PMO as the catalyst. Even after adding pyri-
dine as the base along with MCM-41, no improvement in the

132
P. Borah et al. / Journal of Catalysis 330 (2015) 129–134
Fig. 3. (a) Powder XRD pattern of Urea/PMO. TEM images of (b) Urea/PMO and (c) Urea/PMO_U.
Scheme 2. Henry condensation reaction used as a test reaction for the evaluation of
Urea/PMO at 25 °C using acetonitrile as solvent.
catalytic efficacy was achieved (Table 2, entry 8). Similarly, only a
trace amount of yield was observed when urea or a mixture of
MCM-41 and urea was used as the catalyst (Table 2, entries 9
and 10). Furthermore, a mixture of MCM-41, pyridine and urea
with exact equivalent to Urea/PMO showed low catalytic efficacy
(Table 2, entry 11). Therefore, it is evident from these controlled
experiments that the prevention of the urea dimerization through
surface grafting method is essential to achieve high catalytic effi-
cacy in Urea/PMO. In addition, synergistic effect between the urea
and pyridine units of embedded compound 3 is also responsible for
the catalytic activity, which activates the LUMO of electrophile as
the base. Thus, Urea/PMO satisfies all the major expectations
abovementioned and serves as an efficient HBD catalyst with coop-
erative activities.
Fig. 4. Catalytic recycling experiments of Urea/PMO for the Henry reaction.
Reaction conditions: 4-nitrobenzaldehyde (0.683 mmol, 103 mg), CH3ANO2
(0.819 mmol, 50 mg), reaction time of 12 h, CH3CN (10 mL) as solvent, catalyst
loading of 10 mol%, and 25 °C.
without any significant catalytic deactivation (Fig. 4). The recov-
ered catalyst after the 6th cycle (Urea/PMO_U) was further charac-
terized thoroughly by various analytical techniques including 13C
and 29Si solid-state NMR, isothermal N2 adsorption/desorption
measurements, and TEM. These characterizations revealed that
Urea/PMO_U was essentially similar to its pristine form
Urea/PMO, demonstrating the good chemical and morphological
stability of Urea/PMO during the catalysis.
In order to evaluate the catalytic recyclability of Urea/PMO, we
performed catalytic cycles using recovered catalyst for the same
reaction (Scheme 2) under the optimized catalytic conditions. It
is evident from the results that Urea/PMO presents a tremendous
recyclability with a consistent conversion up to 6 catalytic cycles
Table 2
Catalytic activities of different catalysts for the Henry reaction in terms of the yield of 2-nitro-1-(4-nitrophenyl) ethanol. Reaction conditions: 4-nitrobenzaldehyde (0.683 mmol,
103 mg), CH3ANO2 (0.819 mmol, 50 mg), reaction time of 12 h, CH3CN (10 mL) as solvent, and 25 °C. TOF = turnover frequency (moles of substrate converted per mole of catalytic
active sites per hour).
Entry
Catalyst
Catalyst amount
Yield (%)
TOF (hÀ1
)
1
2
3
Urea/PMO
No catalyst
MCM-41
10 mol% (0.1 g)
–
0.1 g
93
0
0
0.78
–
–
4
5
6
7
8
9
10
11
3
10 mol%
0.1 g of MCM-42 + 10 mol% of 3
10 mol%
10 mol%
23
22
31
26
24
Trace
3
0.19
0.18
0.26
0.22
0.2
MCM-41 + 3
Et3N
Pyridine
MCM-41 + Pyridine
Urea
MCM-41 + Urea
MCM-41 + Pyridine + Urea
0.1 g of MCM-41 + 10 mol% of pyridine
20 mol%a
–
0.1 g of MCM-41 + 20 mol% of ureaa
0.1 g of MCM-41 + 10 mol% of pyridine + 20 mol% of ureaa
0.013
0.23
27
a
One molecule of compound 3 contains two urea linkages. In order to maintain the same content of urea within Urea/PMO and 3, 20 mol% of urea was used in the control
experiments.

P. Borah et al. / Journal of Catalysis 330 (2015) 129–134
133
Table 3
Catalytic activities of Urea/PMO for the Henry reaction using various substituted
benzaldehydes. Reaction conditions: substituted aromatic aldehydes (0.683 mmol),
CH3ANO2 (0.819 mmol, 50 mg), reaction time of 12 h, CH3CN (10 mL) as solvent,
catalyst loading of 10 mol%, and 25 °C.
No.
1
Aromatic aldehyde
b-Nitro alcohols
Yield (%)
93
2
3
4
81
79
76
Fig. 5. Catalytic activities of different solid basic catalysts for Henry reaction in
terms of the yield of 2-nitro-1-phenyl-ethanol. Reaction conditions: benzaldehyde
(0.683 mmol), CH3ANO2 (0.819 mmol), reaction time of 12 h, CH3CN (10 mL) as
solvent, catalyst loading of 0.1 g, and 25 °C.
undergo the Henry reaction with higher yield (Table 3, entry 1)
when compared to those substituents with weaker
electron-withdrawing ability (Table 3, entries 2–5). On the other
hand, the substituents with electron-donating ability could reduce
the reactivity of the aromatic aldehydes markedly, and the corre-
sponding yields are even lower than that of benzaldehyde
(Table 3, entries 6–10). In addition, a trend of higher reactivity
was also observed for ortho-substitution over para-substitution
on substituted benzaldehydes.
In order to demonstrate the HBD catalytic activity provided by
the urea unit of 3 embedded in Urea/PMO, three control reactions
were carried out using a 1:1 mixture of p-methoxybenzaldehyde
(S1) and p-nitrobenzaldehyde (S2) under three different catalytic
conditions (Table 4). Since S1 has an electron-donating group at
the para position, it requires a LUMO activation of the aldehyde
group, whereas the aldehyde group of S2 is activated due to the
presence of electron-withdrawing nitro group at the para position.
At first, the catalysis was performed using Urea/PMO as the cata-
lyst, leading to corresponding b-nitro alcohols for both S1 and S2
(Table 4, entry 1). When pyridine or Et3N was used as the catalyst
(Table 4, entries 2 and 3), S1 did not undergo the Henry reaction to
give corresponding product due to the absence of any catalyst for
the LUMO activation of electrophile through HBD. However, S2
as an activated aldehyde afforded corresponding b-nitro alcohol
with a low yield. Given that only urea group can activate elec-
trophile, these results strongly indicate that the urea group in
Urea/PMO successfully serves as the HBD catalyst to activate the
electrophile and facilitate the Henry reaction.
5
76
6
7
73
71
68
47
43
8
9
10
Furthermore, we examined the substrate scope of organocat-
alytic Henry reaction with Urea/PMO catalyst for a series of substi-
tuted benzaldehydes and nitromethane in acetonitrile at 25 °C for a
period of 12 h. In most of the cases, it was observed that the con-
densation reaction rate of the aromatic aldehydes attached with
electron-withdrawing groups was higher than that of the aromatic
aldehydes substituted with electron-donating groups (Table 3). In
general, the electron-withdrawing substituents on the phenyl ring
of aldehydes favor the reaction (Table 3, entries 1–5). Stronger
electron-withdrawing substituent facilitates aromatic aldehyde to
Taking Henry condensation between benzaldehyde and nitro-
methane as a model reaction, we also compared the activity of
Urea/PMO with a variety of solid basic catalysts such as MgO, neu-
tral Al2O3, and Amberlyst A21 under similar reaction conditions.
The results (Fig. 5) show that Urea/PMO catalyst is more active
than other catalysts owing to its cooperative catalytic activities.
4. Conclusions
Table 4
Catalytic activities of different catalysts for Henry reactions using a 1:1 mixture of p-
methoxybenzaldehyde (S1) and p-nitrobenzaldehyde (S2) in terms of the yield of
corresponding b-nitro alcohols. Reaction conditions: S1 + S2 (0.683 mmol,), CH3ANO2
(0.819 mmol), reaction time of 12 h, CH3CN (10 mL) as solvent, catalyst loading of
0.1 mol%, and 25 °C.
In conclusion, the HBD catalysts have already been proven to
serve as an important class of catalysts for the activation of small
molecules. However, the use of these catalysts was limited to
homogeneous liquid phase catalysis, restricting their further appli-
cations. In this work, we have demonstrated for the first time the
synthesis of PMO-based HBD catalyst (Urea/PMO), which shows
tremendous cooperative catalytic activities toward Henry reaction.
Through the immobilization of the urea–pyridine conjugate in
PMO, the self-dimerization of the urea unit has been prohibited,
thus enhancing its catalytic activity. Additional base is not needed,
Entry
p-OMe (S1):p-NO2 (S2)
Catalyst
Yield (%) of b-nitro
alcohol
p-OMe
p-NO2
1
2
3
1:1
1:1
1:1
Urea/PMO
Et3N
Pyridine
33
0
0
82
25
17

134
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Acknowledgments
This research is supported by the National Research Foundation
(NRF), Prime Minister’s Office, Singapore, under its NRF Fellowship
(NRF2009NRF-RF001-015) and Campus for Research Excellence
and Technological Enterprise (CREATE) Programme–Singapore
Peking University Research Centre for a Sustainable Low-Carbon
Future, as well as the NTU-A⁄Star Silicon Technologies Centre of
Excellence under Grant No. 11235100003.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2015.07.011.
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