Bi-functional NH2-MIL-101(Fe) for one-pot tandem photo-oxidation/Knoevenagel condensation between aromatic alcohols and active methylene compounds

Article abstract of DOI:10.1039/c4cy01464b

Tandem reactions, which enable multistep reactions in one pot, offer enormous economical advantages. For the first time, this manuscript reported that NH₂-MIL-101(Fe), an earth abundant Fe-containing MOF material, can catalyze efficiently the one-pot reaction between aromatic alcohols and active methylene compounds via a tandem photo-oxidation/Knoevenagel condensation under visible light and at room temperature. NH₂-MIL-101(Fe) acts as a photocatalyst for the oxidation of aromatic alcohols to aldehydes as well as a base catalyst for Knoevenagel condensation between the as-formed aldehydes and the active methylene compounds. The comparison of the reactions over NH₂-MIL-101(Fe) and another two MOFs (NH₂-UiO-66(Zr) and NH₂-MIL-125(Ti)) reveals that the strength of the basic sites in the MOFs influences significantly the efficiency of the tandem reaction. This study highlights the great potential of MOFs as multifunctional photocatalysts for one-pot tandem reactions.

Full text of DOI:10.1039/c4cy01464b

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Bi-functional NH -MIL-101IJFe) for one-pot
2
tandem photo-oxidation/Knoevenagel
Cite this: DOI: 10.1039/c4cy01464b
condensation between aromatic alcohols
and active methylene compounds†
Dengke Wang and Zhaohui Li*
Tandem reactions, which enable multistep reactions in one pot, offer enormous economical advantages.
2
For the first time, this manuscript reported that NH -MIL-101IJFe), an earth abundant Fe-containing MOF
material, can catalyze efficiently the one-pot reaction between aromatic alcohols and active methylene
compounds via a tandem photo-oxidation/Knoevenagel condensation under visible light and at room
temperature. NH
as well as a base catalyst for Knoevenagel condensation between the as-formed aldehydes and the active
methylene compounds. The comparison of the reactions over NH -MIL-101IJFe) and another two MOFs
IJNH -UiO-66IJZr) and NH -MIL-125IJTi)) reveals that the strength of the basic sites in the MOFs influences
2
-MIL-101IJFe) acts as a photocatalyst for the oxidation of aromatic alcohols to aldehydes
Received 8th November 2014,
Accepted 2nd December 2014
2
2
2
DOI: 10.1039/c4cy01464b
significantly the efficiency of the tandem reaction. This study highlights the great potential of MOFs as
multifunctional photocatalysts for one-pot tandem reactions.
www.rsc.org/catalysis
4
promising heterogeneous catalysts. Since the constituent,
topology and surface functionality of MOFs can be readily
Introduction
Tandem reactions, which enable multistep reactions in one
pot, have attracted a great deal of attention since they avoid
the isolation of intermediates and reduce the production of
tuned by varying the metal ions or the bridging organic
linkers, MOFs can be an interesting platform to engineer
molecular solids for multifunctional catalysis. By adopting
5
1
wastes, thus offering enormous economical advantages.
this strategy, bi/multi-functionalizations on MOFs have been
6
Multifunctional catalysts which contains different catalytically
active sites in a site-isolation manner to maintain their inde-
pendent function is usually required for an efficient one-pot
tandem reaction. Currently most of the already reported
one-pot tandem reactions involve homogeneous catalysts,
which usually suffer from product contamination and limited
recyclability. It is the current research focus to obtain multi-
functional solid catalysts with a controlled spatial arrange-
ment of a high-density yet continuous range of different
active sites for one-pot heterogeneous tandem reactions, yet it
achieved to catalyze one-pot tandem reactions. However,
almost all of the investigated tandem catalytic reactions can
only take effect at elevated temperature. Photocatalysis is a
unique heterogeneous catalysis, which can make use of solar
light as the energy source to achieve organic transformations
7
under mild conditions. Previous studies have revealed that
MOFs can be promising photocatalysts for a variety of redox
2
reactions, such as CO reduction, water reduction and photo-
8
oxidation. The coupling of the current MOF-based catalysis
with photocatalysis over multi-functional MOFs to achieve
some tandem reactions under mild conditions would be
appealing, yet this field is almost unexplored. Until recently,
Matsuoka et al. reported the one pot reaction between
benzyl alcohol and malononitrile to synthesize benzylidene
2
is still challenging.
Metal–organic frameworks (MOFs) are a class of crystalline
micro–mesoporous hybrid materials which have already
3
shown a variety of applications. Their inherent high specific
surface area and uniform but tunable cavities make MOFs
malononitrile over NH -UiO-66IJZr) via a coupled photooxida-
2
tion and Knoevenagel condensation. Unfortunately, the reac-
tion can only be achieved upon UV irradiation and when
6
f
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on
Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou
heated. Unlike UV light responsive Zr-containing MOFs, all
the Fe-based MOF materials already reported show intensive
light absorption in the visible light region due to the exis-
tence of extensive iron oxo-clusters, which enable them to
make effective utilization of solar energy. Moreover, iron is
350002, People's Republic of China. E-mail: zhaohuili1969@yahoo.com
†
Electronic supplementary information (ESI) available: Experimental details;
characterizations; figures of XRD, IR, BET and UV-vis; other catalytic reaction
data. See DOI: 10.1039/c4cy01464b
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an earth-abundant metal, which makes it extremely appeal-
ing for the application of Fe-containing MOFs in visible-light-
induced photocatalysis coupled one-pot tandem reactions.
In this manuscript, we reported a one-pot tandem photo-
oxidation/Knoevenagel condensation between benzyl alcohols
and active methylene compounds over visible light irradiated
mixture of benzyl alcohol (0.1 mmol) and trifluorotoluene/
acetonitrile IJB/A, 10 : 1) (3.0 mL) saturated with O was trans-
2
ferred into a 10 mL Schlenk tube containing 20 mg of
catalyst. The suspension was irradiated with a 300 W Xe arc
lamp (Beijing Perfectlight, PLS-SXE300c) with a UV cutoff fil-
ter (λ ≥ 420 nm). For the one-pot tandem reactions, a mixture
of aromatic alcohol (0.1 mmol) and active methylene com-
pounds (0.3 mmol) were reacted in B/A (10 : 1) (3.0 mL) satu-
NH -MIL-101IJFe) at room temperature. In addition to acting as
2
a photocatalyst for the oxidation of aromatic alcohols to alde-
2
hydes, NH -MIL-101IJFe) acts as a base catalyst for Knoevenagel
2
rated with O . After the reaction, the reaction mixture was
condensation between the as-formed aldehydes and the active
methylene compounds. The mechanism for the tandem reac-
tion over NH -MIL-101IJFe) was proposed. This study highlights
2
the great potential of MOFs as multifunctional photocatalysts
for one-pot tandem reactions.
filtered through a porous membrane (20 μm in diameter) and
the products were determined by GC (Shimadzu GC-2014)
equipped with a flame ionization detector using a HP-5
capillary column.
Results and discussion
Experimental section
Preparation
NH -MIL-101IJFe) was synthesized using 2-aminoterephthalic
2
2
acid (H ATA) as organic linker according to the procedures
9
All the reagents and solvents employed were commercially
available and used as supplied without further purification.
reported previously. The XRD patterns and FT-IR spectra
of the as-obtained product indicate that pure phase of
NH -MIL-101IJFe) was prepared following previously reported
NH -MIL-101IJFe) have been obtained (Fig. S1 and S2†).
The N2 adsorption/desorption analysis of the as-obtained
2
2
9
procedures. Typically, 0.675 g (2.497 mmol) of FeCl
3
Ĵ6H
2
O
was dissolved in 7.5 mL of DMF, and a solution of 0.225 g
1.242 mmol) of 2-aminoterephthalic acid in 7.5 mL of DMF
was added. After thermal treatment in a stainless steel auto-
clave for 24 h at 110 °C, the reaction product was recovered
by filtration and washed several times with DMF and metha-
nol. Then the obtained solid was finally dried overnight at
NH -MIL-101IJFe) reveals the BET specific surface area to be
2
2
−1
2 −1
(
2476 m g , comparable to that reported previously (2436 m g )
9
b
(Fig. S3†).
The UV-vis spectrum of the as-prepared
NH -MIL-101IJFe) shows the absorption edge extending to
2
around 750 nm, in agreement with its brown color (Fig. S4†).
Inspired by a previous report that photocatalytic oxidation
of benzyl alcohol to aldehyde can be achieved over visible
6
0 °C in an oven.
1
0
2
light irradiated NH -UiO-66IJZr), we first investigated the
Characterizations
visible-light-induced photocatalytic oxidation of benzyl alco-
hol over NH -MIL-101IJFe). Like that over NH -UiO-66IJZr),
benzyl alcohol can also be oxidized to benzyl aldehyde over
2
2
The as-prepared sample was characterized by X-ray diffrac-
tion (XRD) patterns on a D8 Advance X-ray diffractometer
irradiated NH -MIL-101IJFe) and the reaction medium played
2
(Bruker, Germany) using Cu Kα (λ = 1.5406 Å) radiation at a
an important role in this reaction (Table S1†). Among all the
solvents investigated, a mixed solvent containing benzo-
trifluoride and acetonitrile IJB/A) in a 10 : 1 ratio gave the best
performance by showing the highest conversion of benzyl
alcohol at 39% and a selectivity to benzyl aldehyde at 95%
after irradiation for 40 h (Table S1,† entry 4). Control experi-
voltage of 40 kV and 40 mA. XRD patterns were scanned over
the angular range of 5–25° (2θ) with a step size of 0.02°. The
IR experiments were carried out on a Nicolet 670 FT-IR
spectrometer. N2 adsorption/desorption experiments were
carried out on a Micrometrics ASAP 2020 M volumetric gas
sorption instrument. After the sample was degassed in a vac-
uum at 150 °C for 10 h, the nitrogen adsorption and desorp-
tion isotherms were measured at −196 °C. UV-vis diffuse
reflectance spectrum (UV-vis DRS) was obtained by a UV-vis
spectrophotometer (Varian Cary 500). Barium sulfate (BaSO4)
was used as a reference. In the temperature-programmed
desorption of CO (CO -TPD; Micromeritics Auto Chem 2920
ments in the absence of NH -MIL-101IJFe) or without light
2
irradiation did not give any detectable product, indicating
that the transformation of benzyl alcohol to benzyl aldehyde
was truly induced by the photocatalysis over NH -MIL-101IJFe)
2
(
Table S1,† entry 5 and 6).
Knoevenagel condensation between an active methylene
compound and a carbonyl group (CO) is a useful C–C bond
2
2
instrument), the catalyst was pretreated at 150 °C for 2 h.
CO was adsorbed at 40 °C for 1 h. The sample was, then,
coupling reaction for the preparation of several important
2
1
1
chemical intermediates. Previous results have shown that
flushed for 1 h to remove physical adsorbed CO . The TPD
2
NH -MIL-101IJFe) can act as a Brønsted base to catalyze the
2
was conducted from 40 °C to 250 °C at a heating rate of
−
1
Knoevenagel reaction between benzyl aldehyde and malononitrile
1
0 °C min . The sample was then kept at 250 °C for 2 min.
12
due to the existence of the amine functionality. We wondered
whether it is possible to couple the photocatalytic oxidation
of benzyl alcohol to benzyl aldehyde with the Knoevenagel
condensation between benzyl aldehyde and malononitrile
Catalytic reactions
The photocatalytic oxidation of benzyl alcohol to benzyl
aldehyde was performed as follows. In a typical process, a
over NH -MIL-101IJFe) in a one-pot reaction. Therefore the
2
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reaction between benzyl alcohol and malononitrile in B/A
substrates and reacted with malononitrile. Although the
transformations of all the aromatic alcohol derivatives to the
corresponding benzylidene malononitriles proceeded under
the reaction conditions, the different substituents on the
phenyl ring influenced the conversion ratio and suggested
the existence of an electronic effect. As compared with bare
benzyl alcohol, aromatic alcohols with electron-withdrawing
(10 : 1), the best reaction medium observed on the photo-
oxidation of benzyl alcohol, was investigated over visible light
irradiated NH -MIL-101IJFe). To our anticipation, the reaction
2
between benzyl alcohol with malononitrile over NH -MIL-101IJFe)
2
gave benzylidene malononitrile as the main product, indicating
the successful coupling of photo-oxidation with the Knoevenagel
condensation in a one pot tandem reaction. As shown in Fig. 1,
the formation of benzylidene malononitrile increased with the
irradiation time and after irradiation for 40 h, the conversion of
benzyl alcohol reached 88% with the yield of benzylidene
malononitrile at 72% (Fig. 1a and Table 1, entry 1). In addi-
tion to benzyl aldehyde (5%) and benzylidene malononitrile,
β-hydroxyl benzalmalononitrile with a yield of 9% was also
detected. It is interesting to observe that the conversion of
benzyl alcohol in the multi-component reaction is higher
than that observed over photo-oxidation alone (39%, Table S1,†
entry 4), indicating that the removal of the intermediate
substituents like –NO showed a significantly decreased con-
2
version ratio (24%, Table 1, entry 4), while aromatic alcohols
with para-substituted electron donating groups like –CH and
3
–OCH exhibited a slightly increased conversion ratio (90–93%,
3
Table 1, entry 5 and 6). The existence of the steric hindrance
on this reaction was also observed since aromatic alcohols with
a methyl group (–CH ) substituted at different positions of
3
phenyl ring showed reaction rates in the order of meta- <
ortho- < para-substituted (Table 1, entry 6–8). The reaction
between the benzyl alcohol with other organics containing
active methylene groups, like ethyl cyanoacetate or diethyl
(benzyl aldehyde) is beneficial to the photo-oxidation reac-
malonate, also occurred over NH -MIL-101IJFe) under visible
2
tion. Control experiments in absence of either NH -MIL-101IJFe)
2
light irradiations, but the performance for different sub-
strates varied. For the reaction between ethyl cyanoacetate
and benzyl alcohol, the reaction proceeded efficiently with
83% of benzyl alcohol converted in 40 h, and a relatively high
yield of 66% to ethyl (benzylidene)cyanoacetate was achieved
(Table 1, entry 9). On the contrary, the reaction between
benzyl alcohol and diethyl malonate did not proceed smoothly.
A low conversion of benzyl alcohol (18%) with an even lower
yield of diethyl benzylidene malonate (2.7%) was observed
(Table 1, entry 10). The different reactivity of the methylene
compounds can be well explained in terms of their different
or visible light irradiation under otherwise similar conditions
gave almost negligible products, indicating that the formation
of benzylidene malononitrile was truly induced by the irradi-
2
ated NH -MIL-101IJFe) (Table 1, entry 2 and 3). The heteroge-
neous nature of this reaction was confirmed by the filtrate
reaction. No further reaction occurred in the filtrate after the
solid catalyst was removed from the reaction system after
1
2 h irradiation (Fig. 1b). The recycling of the photocatalyst
for three runs showed no obvious decrease of the photocata-
lytic activity (Fig. 2). Besides this, the XRD, IR and N adsorption/
2
desorption isotherms of the photocatalyst after the reaction
acidity, which decreases in the order of malononitrile (pK =
a
did not show much change (Fig. S5–S7†). All these suggest
11.1) > ethyl cyanoacetate (pK
(pK
a
a
= 13.1) > diethyl malonate
= 16.4). Since the first step of the Knoevenagel conden-
sation involves the deprotonation of the active methylene
group, the higher pK value of the methylene group in diethyl
13
that NH
reaction.
2
-MIL-101IJFe) is stable and reusable during the
Having established that NH
2
-MIL-101IJFe) can catalyze the
a
one-pot reaction between benzyl alcohol and malononitrile,
we extend the study to various substrates, including different
alcohols and other active methylene compounds. First, a
series of substituted aromatic alcohols were selected as the
malonate (16.4) indicates that the deprotonation of diethyl
malonate is difficult, which leads to the low efficiency of the
formation of diethyl benzylidene malonate via Knoevenagel
1
4
condensation between benzyl aldehyde and diethyl malonate.
The one-pot reaction between aromatic alcohols and active
methylene compounds over NH -MIL-101IJFe) via tandem
2
photo-oxidation/Knoevenagel condensation can be well illus-
trated in the following Scheme 1. First, the aromatic alcohol
was photocatalytically oxidized to aldehyde over irradiated
2
NH -MIL-101IJFe). It is believed that the organic linker ATA
in NH -MIL-101IJFe) acts as an antenna to absorb visible
2
light. Upon irradiation, the excited ligand ATA can transfer
3
+
electrons to the Fe-oxo clusters, and Fe in the Fe–O clusters
2
+
−
is reduced to Fe , which can reduce O to form O ˙ radical
2
2
(
step a). In the meantime, the organic substrates adsorbed
on the surface of the MOF material can donate an electron
to form the carbonium ions (step b), which can further react
−
with the as-formed O
2
˙ radicals to form the aromatic aldehydes
Fig. 1 Conversion ratio of benzyl alcohol as a function of irradiation
(
step c). The confinement effect of the nanometer-sized cavities
time in a one-pot tandem reaction over: (a) NH
b) the reaction system after NH -MIL-101IJFe) was removed after irra-
diated for 12 h.
2
-MIL-101IJFe); and
−
in MOF material helps to stabilize the as-formed O2˙
(
2
radical and the reactive intermediates. Such a photo-oxidation
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2
Table 1 Reaction between substituted aromatic alcohols and active methylene compounds over NH -MIL-101IJFe)
Substrates
Yield (%)
Entry
1
Conv. of alcohol (%)
–H
–H
–H
–NO
p-OCH
p-CH
m-CH
o-CH
–H
R
2
R
2
R
2
R
2
R
2
R
2
R
2
R
2
R
2
R
3
R
2
,R
,R
,R
,R
,R
,R
,R
,R
3
3
3
3
3
3
3
3
= –CN
= –CN
= –CN
= –CN
= –CN
= –CN
= –CN
= –CN
88
0.9
0.8
24
93
90
80
82
83
5
—
—
3.7
8
5.5
7
6.1
5.4
72
—
—
20
76
76
63
66
66
a
c
2
3
b
4
5
6
7
8
9
2
3
3
3
3
= –CN
= –COOEt
10
–H
,R
3
= –COOEt
18
12
2.7
2 2
Reaction conditions: aromatic alcohols (0.1 mmol), methylene compounds (0.3 mmol), B/A (10 :1) (3 mL), NH -MIL-101IJFe) (20 mg), O (1 atm),
a
b
c
light irradiation (λ ≥ 420 nm), 40 h. Without visible light irradiation. No catalyst. “—” refers to no products, or negligible products were
detected.
mechanism has already been demonstrated previously over
eliminate one molecule of water to make an α,β-unsaturated
compound (step iv).
The reaction between benzyl alcohol and malononitrile
over other amine-functionalized photocatalytic active MOFs
10
irradiated NH
2
-UiO-66IJZr). Due to the existence of the basic
-MIL-101IJFe), the as-formed aromatic alde-
–
NH site in NH
2
2
hydes can further react with active methylene compounds
via a Knoevenagel condensation. The first step in this
process involves the deprotonation of the active methylene
group under the assistance of the basic –NH2 site in NH2-
MIL-101IJFe) to form a carbonanion (step i). The as-formed
carbon anion can make a nucleophilic attack on the carbonyl
carbon atoms of the aromatic aldehydes (step ii). The as-formed
intermediate can take up one proton from the catalyst to
form the β-hydroxyl compound, and the catalyst is regene-
rated (step iii). Finally, the β-hydroxyl compound can
2 2
like NH -MIL-125IJTi) and NH -UiO-66IJZr) under visible light
irradiations was also investigated and the results are shown
in Table 2. Although the reaction between benzyl alcohol and
malononitrile also led to the formation of benzylidene
malononitrile over all three MOFs, both the conversion
ratio of the benzyl alcohol and the yield of benzylidene
malononitrile varied significantly among these MOFs. The
yield of benzylidene malononitrile was much lower over
Fig. 2 Conversion of benzyl alcohol ( ) and yields to benzaldehyde
Scheme 1 Proposed mechanism for one-pot tandem photocatalytic
(
), benzylidene malononitrile ( ) in the one-pot tandem reaction
2
oxidation/Knoevenagel condensation over bi-functional NH -MIL-
over NH -MIL-101IJFe) for recycling use.
2
101IJFe).
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Table 2 One-pot tandem reaction over different MOFsa
Yield (%)
Entry
Catalyst
Conv. of alcohol (%)
Benzyl aldehyde
Benzylidene malononitrile
1
2
3
NH
NH
NH
2
-MIL-101IJFe)
2
-UiO-66IJZr)
2
-MIL-125IJTi)
88
38
47
5
29
39
72
4.6
3.3
a
Reaction conditions: benzyl alcohol (0.1 mmol), malononitrile (0.3 mmol), B/A (10 : 1) (3 mL), MOFs (20 mg), O
2
(1 atm), light irradiation
(λ ≥ 420 nm), 40 h.
irradiated NH -UiO-66IJZr) (4.6%) and NH -MIL-125IJTi) (3.3%)
and NH -MIL-125IJTi) also show big desorption peaks, corre-
2
2
2
under otherwise similar reaction conditions as compared
to that over NH -MIL-101IJFe) (72%). However, the photo-
sponding to desorption of CO2 adsorbed on a basic site
(–NH ). However, the desorption peak of NH -UiO-66IJZr)
2 2
2
catalytic oxidation of benzyl alcohol to form benzyl aldehyde
occurs at 214 °C (Fig. 3(b)), while that of NH -MIL-125IJTi)
2
over NH -UiO-66IJZr) (28%) and NH -MIL-125IJTi) (37%) was
occurs at 192 °C (Fig. 3(c)). Since the basic strength of the
2
2
comparable to that over irradiated NH
Table S2†). This indicates that the lower efficiency of the
one-pot photo-oxidation/Knoevenagel condensation over
NH -UiO-66IJZr) and NH -MIL-125IJTi) was induced by their
2
-MIL-101IJFe) (39%)
catalyst is correlated with the desorption temperature in the
(
CO -TPD profile, the lower desorption temperature observed
2
over NH -UiO-66IJZr) and NH -MIL-125IJTi) indicate that the
2
2
2
2
strength of the basic sites in these two MOFs are not as
lower efficiency in the Knoevenagel condensation, instead of
the lower activity for the photo-oxidation. This assumption is
confirmed by the control Knoevenagel reaction between benz-
aldehyde and malononitrile under otherwise similar conditions,
strong as that in NH -MIL-101IJFe). Therefore, the Knoevenagel
2
condensation over NH -UiO-66IJZr) and NH -MIL-125IJTi) is
2
2
2
not as efficient as that over NH -MIL-101IJFe).
which shows a similar activity order of NH -MIL-101IJFe) >
NH -Uio-66IJZr) > NH -MIL-125IJTi) (Fig. S8†).
2 2
2
Conclusion
Based on the proposed mechanism, the efficiency of
Knoevenagel condensation is mainly dependent on the
deprotonation of the active methylene group. Therefore, the
strength of the basic site in different MOF materials should
play an important role in the Knoevenagel reaction. To
explore the basic characteristic of the above three MOF mate-
In summary, NH -MIL-101IJFe) can catalyze the one-pot reac-
2
tion between aromatic alcohols and active methylene com-
pounds under visible light and at room temperature.
NH -MIL-101IJFe) acts as a photocatalyst for oxidation of aro-
2
matic alcohols to aldehydes as well as a base catalyst for
Knoevenagel condensation between the as-formed aromatic
aldehydes and the active methylene compounds. The effi-
ciency of the tandem reaction is significantly influenced by
the strength of the basic sites in the MOF materials. We
believe that the present study will stimulate intensive
research in developing MOF-based multifunctional photo-
catalysts for one-pot tandem reactions. Considering the versa-
tile structures of MOF materials and their facile modulations,
it is believed that more and more multifunctional MOF-based
photocatalysts for one-pot tandem reaction can be obtained.
rials, their CO -TPD profiles were investigated and the results
2
are shown in Fig. 3. TPD is a very useful technique for char-
acterizing chemically bonded surfaces or adsorbed species
and the CO -TPD profile has been used extensively in deter-
2
1
5
mining the basic strength of the catalyst. As shown in
Fig. 3(a), the CO -TPD profile of NH -MIL-101IJFe) shows
2
2
three peaks at Tmax of 118 °C, 217 °C and 240 °C. The desorp-
tion peak occuring at 217 °C can be ascribed to desorption of
CO adsorbed on the basic –NH site. As compared with the
2 2
CO -TPD file of NH -MIL-101IJFe), those of NH -UiO-66IJZr)
2
2
2
Acknowledgements
This work was supported by NSFC (21273035), 973 Programs
(
2014CB239303) and Specialized Research Fund for the
Doctoral Program of Higher Education (20123514110002).
Z. Li thanks the Award Program for Minjiang Scholar Profes-
sorship for financial support.
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2
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2 2
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