An [Fe-Fe]-hydrogenase mimic immobilized on MCM-41 for the photochemical production of hydrogen in pure water

Article abstract of DOI:10.1002/cjoc.201400083

A composite proton-reducing catalyst Hy@MCM-41 was created by using a hydrophobic [Fe-Fe]-hydrogenase mimic (Hy) incorporated into the K ⁺-exchanged molecular sieve MCM-41. Hy acts as the active site to generate H₂ by reducing H⁺, and MCM-41 provide a large surface area to maintain the dispersion of Hy and to prevent aggregation and precipitation. Hy@MCM-41 was successfully applied to the light-induced hydrogen production in pure water with an iridium compound [Ir(ppy)₂bpy]Cl and triethylamine (TEA) as the photosensitizer and the sacrificial electron donor, respectively, exhibiting good stability and catalytic activity. The present study provides a general strategy for the application of water-insoluble catalysts in pure water by using mesoporous molecular sieves. Copyright

Full text of DOI:10.1002/cjoc.201400083

FULL PAPER
DOI: 10.1002/cjoc.201400083
An [Fe-Fe]-Hydrogenase Mimic Immobilized on MCM-41 for the
Photochemical Production of Hydrogen in Pure Water
Wen Wang, Tianjun Yu,* Yi Zeng, Jinping Chen, and Yi Li*
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
A composite proton-reducing catalyst Hy@MCM-41 was created by using a hydrophobic [Fe-Fe]-hydrogenase
mimic (Hy) incorporated into the K＋-exchanged molecular sieve MCM-41. Hy acts as the active site to generate H2
by reducing H＋, and MCM-41 provide a large surface area to maintain the dispersion of Hy and to prevent aggrega-
tion and precipitation. Hy@MCM-41 was successfully applied to the light-induced hydrogen production in pure
water with an iridium compound [Ir(ppy)2bpy]Cl and triethylamine (TEA) as the photosensitizer and the sacrificial
electron donor, respectively, exhibiting good stability and catalytic activity. The present study provides a general
strategy for the application of water-insoluble catalysts in pure water by using mesoporous molecular sieves.
Keywords energy conversion, hydrogen, hydrogenase, MCM-41, immobilization, photochemistry
Introduction
substituent ligand PTA (PTA＝1,3,5-triaza-7-phospha-
adamantane).[11] Since then, other water-soluble groups,
such as di-N-acetylated phosphatriazaadamantane
(DAPTA) ligand,[12] ether chains,[13] sulfonate,[14-16]
functionalized sugars,[17] protein,[18] and peptide[19,20]
were also applied to enhance the water solubility of hy-
drogenase mimics. However, conventional synthetic
processes for water-soluble [Fe-Fe]-hydrogenase mim-
ics are labor-, time-, and energy-intensive. In compari-
son with the covalent method, the noncovalent methods
display advantages such as tunability, simplicity and
conveniency, for improving the solubility of hydro-
genase mimics in water. SDS,[21] cyclodextrin[22] and
metal-organic framework (MOF)[23] have been used to
realize the application of water-insoluble hydrogenase
mimics in water.
The ordered mesoporous molecular sieves display
advantages such as tunable pore diameters (2－30 nm),
narrow pore size distributions, high surface areas and
electrostatic microenvironments, which have been
widely used in ion exchange, separation, catalysis, sen-
sor, biological molecular isolation and purification.[24-35]
The mesoporous molecular sieves with suitable size are
able to accommodate the hydrogenase mimics, which
can further conduct hydrogen production in water. Fur-
thermore, the hydrogenase mimics immobilized into the
mesoporous molecular sieves should be easily separated
from the production system than from a micelle or a
cyclodextrin inclusion complex. Herein, we report a
[Fe-Fe]-hydrogenase mimic (Hy), which was incorpo-
rated into the ordered mesoporous K＋-exchanged mo-
The requirement to solve the energy crisis and the
climate change has stimulated new approaches to mimic
natural photosynthesis in the conversion and storage of
solar energy.[1-3] Of these approaches, the photochemi-
cal production of hydrogen from water is at the fore-
front. In nature, the interconversion of protons and hy-
drogen is efficiently catalyzed by metalloenzymes
known as hydrogenases, which exist in many microor-
ganisms and rely on metals abundant on earth.[4] Since
the biological structures and the functionalities of the
active sites of hydrogenases were proposed, chemists
have made tremendous efforts to hydrogenase mimics
for constructing hydrogen production systems.[5-9] Water
is believed to be an ideal solvent to construct these sys-
tems for many reasons including its non-toxic property
as well as economic advantages.[10] However, the proton
reduction conducted on the hydrogenase mimics has
been limited to organic solvents or a mixture of organic
solvents and water because of the inherent insolubility
of the majority of hydrogenase mimics in water. Devel-
oping new methods capable of realizing the functional-
ity of hydrogenase mimics under aqueous conditions is
of great importance for the application of hydrogenase
mimics.
Over the past decades, only a handful of water-
soluble hydrogenase mimics have been synthesized by
introducing hydrophilic groups to the hydrogenase
mimics. In 2004, Darensbourg and co-workers devel-
oped a water-soluble hydrogenase mimic by using a
*
E-mail: yili@mail.ipc.ac.cn; tianjun_yu@mail.ipc.ac.cn; Tel.: 0086-010-82543518
Received February 14, 2014; accepted March 25, 2014; published online April 22, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc. 201400083 or from the author.
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Photocatalytic H2 generation
lecular sieve MCM-41 to form an MCM-41-immobi-
lized [Fe-Fe]-hydrogenase mimic (Hy@MCM-41). The
studies on the photochemical production of hydrogen by
Hy@MCM-41 demonstrate that immobilization into
mesoporous molecular sieves is an effective way for
application of hydrophobic hydrogenase mimics in pure
water.
All experiments of photochemical production of H2
were performed in a Pyrex reactor with 10 mL sample
solution and a magnetic stir. The sample solutions were
purged with nitrogen for 30 min prior to irradiation. A
300 W Xe lamp was used as the visible light source
with filters cutting off the light below 400 nm and above
800 nm and a set of neutral density filters was used to
adjust the irradiation intensity. The amount of H2 was
identified and quantified with a gas chromatograph
equipped with a 5 Å molecular sieve column, N2 carrier
gas, and a thermal conductivity detector (TCD) with
methane as an internal standard. The response factor of
1.52 for H2/CH4 was determined by calibration with
known amounts of H2 and CH4 under the experimental
conditions.
Experimental
Materials
Reagents were purchased from Acros, Alfa Aesar, or
Beijing Chemicals and were used without further puri-
fication unless otherwise noted. Mesoporous molecular
sieve MCM-41 (mole ratio of SiO2/Al2O3≥20, BET≥
900 m2/g, d＝3.5 nm) was purchased from Xian Feng
Nano Company. HPLC-grade hexane was used for the
preparation of Hy@MCM-41. Milli-Q deionized water
(Millipore) was used in the light-driven hydrogen gen-
eration experiments and spectroscopy measurements.
Acetonitrile and tetrahydrofuran were dried and distilled
under N2 atmosphere prior to use.
Results and Discussion
Preparation and characterization of Hy@MCM-41
A simple hydrogenase mimic [{(μ-SCH2)2N(CH2-
C6H5)}Fe2(CO)5P(Pyr)3] (Hy) was synthesized accord-
ing to the literature.[36] The details of the synthesis and
the characterization of Hy are described in the Support-
ing Information. The structure of the Hy is depicted in
Figure 1, which was characterized by H NMR and IR
spectroscopy as well as mass spectrometry (see the
Supporting Information).
Instrumentation
1H NMR (400 MHz) spectra were obtained from a
Bruker Avance Π-400 spectrometer with tetramethylsi-
lane as an internal standard. Infrared spectra were re-
corded on a Nicolet NEXUS 670 FTIR spectrometer.
MS spectra were obtained on MALDI-TOF (matrix as-
sisted laser desorption ionization/time of flight) BI-
FLEX III (Bruker Inc.) spectrometer. Absorption spec-
tra were run on a Shimadzu UV-1601PC spectrometer.
UV-Vis reflectance spectra were recorded on a Shima-
dzu UV-VIS-3100 spectrophotometer. ICP analysis was
performed on a Varian 710-OES instrument. The analy-
sis of hydrogen production was carried out on a Shima-
dzu GC-2014 with a TCD detector.
1
Preparation and characterization of Hy@MCM-41
The K＋-exchanged molecular sieve MCM-41 (K-
MCM-41) was prepared by mixing silica-alumina mo-
lecular sieve MCM-41 (5 g) with a potassium oxalate
solution (0.25 mol/L, 50 mL) and stirring for 6 h at
room temperature (fresh potassium oxalate solution was
replaced each 2 h). The resulting white solid was fil-
tered, washed with water, dried and activated at 120 ℃
and 540 ℃ for 3 h, respectively. The following proce-
dure was used to prepare Hy@MCM-41 by incorporat-
ing Hy into K-MCM-41. A small amount of K-MCM-
41 (150.0 mg) was suspended in the hexane solution of
Hy (4.0×10−3 mol/L, 2 mL), which was stirred at am-
bient temperature overnight. The suspension was then
centrifuged and the precipitate was dried at 40 ℃ un-
der vacuum. The amount of Hy immobilized into
K-MCM-41 was determined by the absorbance change
at 340 nm (ε＝1.4×104 mol−1•L•cm−1) of the 100-fold
solution/supernatant dilutions before and after immobi-
lization and the ICP analysis.
Figure 1 Absorption spectra of the 100-fold dilutions of Hy in
hexane (a, 4.0×10−5 mol/L) and the supernatant after addition of
K-MCM-41 (150.0 mg) to 2 mL Hy solution (b). Inset: Schematic
illustration of the structure of Hy.
The K ＋ -exchanged MCM-41 (K-MCM-41) was
prepared according to a reported method by using po-
tassium oxalate as the K＋ source.[37] K-MCM-41 was
further used to immobilize Hy to give the
Hy@MCM-41 composite. The adsorption of Hy into
K-MCM-41 was conducted by addition of K-MCM-41
(150.0 mg) to the solution of Hy in hexane (2 mL, 4×
10−3 mol/L). The quantity of Hy immobilized into
K-MCM-41 was 43.0 μmol/g by evaluating the absorb-
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An [Fe-Fe]-Hydrogenase Mimic for the Photochemical Production of Hydrogen in Pure Water
ance change of the 100-fold Hy-solution/supernatant
dilutions before and after immobilization (Figure 1),
which is in agreement with the result obtained from the
ICP analysis (44.6 μmol/g). The loading of Hy into
K-MCM-41 makes the color of K-MCM-41 change
from white to orange (Figure 2a). The firmness of Hy
loaded into K-MCM-41 was examined by stirring the
suspension of Hy@MCM-41 in water at room tempera-
ture overnight. No leakage of Hy from Hy@MCM-41
was observed by analyzing absorption spectrum of the
supernatant, which indicated that Hy was tightly incor-
porated into K-MCM-41.
sults validate the reality of the immobilization of Hy
into K-MCM-41. The FTIR spectrum of Hy@MCM-41
is smoother than that of the Hy, which can be ascribed
to the bad light transmission of the K-MCM-41-adul-
terated KBr tablet.
The adsorption of Ps into K-MCM-41 was further
studied by comparing the absorption spectra of the
aqueous solution of Ir(ppy)2bpyCl (Ps, 2.5×10−4 mol/L)
and the supernatant after addition of Hy@MCM-41 (4.5
mg, Hy loading: 43.0 μmol/g) and K-MCM-41 (4.5 mg)
to 10 mL Ir(ppy)2bpyCl solution (Figure S5). Both su-
pernatants show typical absorption characteristics of
Ir(ppy)2bpyCl, but the absorption intensities are ca. 15%
lower than the corresponding aqueous solution of
Ir(ppy)2bpyCl. These indicate that Ps can be partially
adsorbed into K-MCM-41. Similar intensity of both
supernatants suggests that the adsorption of Hy into
K-MCM-41 with loading 43.0 μmol/g does not affect
further adsorption of Ps.
Application of Hy@MCM-41 to hydrogen produc-
tion
The catalytic activity of Hy@MCM-41 for light-
driven hydrogen production was examined by using
[Ir(ppy)2bpy]Cl and triethylamine (TEA) as the photo-
sensitizer (Ps) and the sacrificial electron donor, respec-
tively (Figure 3). Pure water was used to solubilize or
disperse all the components. Irradiation of the
Hy@MCM-41 catalytic system with visible light re-
sulted in the production of hydrogen. Control experi-
ments indicated that Ps, Hy and TEA are all essential
for production of H2, and the absence of any of them led
to no detectable photochemical production of H2.
Moreover, the molecular sieve K-MCM-41 is also es-
sential for the H2 production in pure water and there is
no detectable H2 production by using an aqueous dis-
persion of Hy when other conditions are identical. The
performance of the photocatalyst system was dramati-
cally affected by the reaction conditions, such as the
concentrations of Ps and TEA, and the Hy loading of
Hy@MCM-41.
Figure 2
(a) Absorption spectra of the suspensions of
Hy@MCM-41 (3.0 mg, loading: 75.2 µmol/g, blue line) in water,
Hy (4×10−5 mol/L, red line) in hexane, K-MCM-41 (3.0 mg,
black line) and Hy (3.0 mg, cyan line) in 5 mL water. Inset: pho-
tographs of K-MCM-41 (left) and Hy@MCM-41 (right). (b)
FTIR spectra of K-MCM-41 (black line), Hy (red line) and
Hy@MCM-41 (blue line).
The Hy@MCM-41 was characterized by UV-Vis,
DRS-UV-Vis and FTIR spectroscopy. The absorption
spectrum of the Hy@MCM-41 suspension in water is
similar to the sum of those of Hy and K-MCM-41 with
absorption band at 344 nm characteristics of Hy (Figure
2a). The diffuse reflectance spectrum of Hy@MCM-41
also exhibits the absorption band of Hy (see Figure S4
in the Supporting Information). Three prominent bands
of the CO stretching vibration at 2055, 2010, and 1976
cm−1 attributed to Hy are observed in the FTIR spectra
of Hy@MCM-41 (Figure 2b). The characterization re-
Figure 3 Schematic illustration of the photochemical produc-
tion of H2 by Hy@MCM-41.
Chin. J. Chem. 2014, 32, 479—484
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To optimize the experiment condition, light-induced
hydrogen production was conducted by using
Hy@MCM-41 with varied loading of Hy in pure water
at different concentrations of Ps and TEA upon the visi-
ble light irradiation (Figure 4). The influence of the Ps
concentration on the photoinduced H2 production was
examined first when all the other conditions were kept
identical (Figure 4a). The higher concentration favors
the hydrogen production. When the concentration of Ps
increases from 0.063 to 0.25 mmol/L, the amount of
hydrogen in initial 1 h increases from 6.1 to 64.2 μL.
The concentration of TEA also produces an effect on
the catalytic performance. The best catalytic activity of
the catalytic system was accomplished at the concentra-
tion of TEA to be 0.28 mol/L with same amount of
Hy@MCM-41 (4.5 mg Hy@MCM-41 with loading of
43.0 μmol/g) and same concentration of Ps (0.25
mmol/L) in pure water, giving the turnover number
(TON) calculated from the amount of H2 molecules
versus the number of Hy molecules to be 18.3. The less
catalytic activity of the catalytic system at higher and
lower concentration of TEA may be rationalized by the
decomposition of the photosensitizer and the lower
concentration of electron donor, respectively.[38] The
photochemical production of hydrogen by Hy@MCM-
41 with different loading was examined in pure water,
and the results are shown in Figures 4c and 4d. Keeping
the optimal conditions of [Ps]＝0.25 mmol/L and [TEA]
＝0.28 mol/L, an obvious improvement in photochemi-
cal H2 production was observed when the Hy loading of
Hy@MCM-41 was successively increased from 13.5 to
119.5 μmol/g. However, the activity of Hy@MCM-41
decreases when the loading is increased. The growth of
H2 evolution in initial 10 min vs. the loading (13.5－
119.5 μmol/g) shows a parabolic-like trend when all the
other conditions are kept identical. The less catalytic
activity of the catalytic system at higher Hy loading of
Hy@MCM-41 may be attributed to the gradual decom-
position of Hy because Hy can absorb light during the
illumination.[5]
In order to figure out the reason for the cease of hy-
drogen production, parallel experiments were carried
Figure 4 Influence of the concentrations of Ps and TEA, and the Hy loading of Hy@MCM-41 on the photoinduced H2 evolution. (a) H2
evolution with various concentrations of Ps in pure water (10 mL). [Hy@MCM-41]＝4.5 mg (loading: 43.0 μmol/g), [TEA]＝0.28 mol/L.
(b) H2 evolution with various concentrations of TEA in pure water (10 mL). [Hy@MCM-41]＝4.5 mg (loading: 43.0 μmol/g), [Ps]＝0.25
mmol/L. (c) Photochemical evolution of H2 catalyzed by Hy@MCM-41 with different loading in pure water (10 mL). [Ps]＝0.25 mmol/L,
[TEA]＝0.28 mol/L, [Hy@MCM-41]＝4.5 mg. (d) H2 evolution vs. the Hy loading of Hy@MCM-41 in initial 10 min. [Ps]＝0.25
mmol/L, [TEA]＝0.28 mol/L, [Hy@MCM-41]＝4.5 mg.
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An [Fe-Fe]-Hydrogenase Mimic for the Photochemical Production of Hydrogen in Pure Water
out after the H2 evolution ceased (ca. 160 min). The
deactivation of the system might be caused by the de-
composition of Ps and/or the Hy@MCM-41 catalyst
because a huge excess amount of TEA exists in the
photocatalyst system. The experiment with re-addition
of Hy@MCM-41 (4.2 mg, loading: 119.5 μmol/g) to the
system showed no more hydrogen production under
irradiation and another experiment with re-addition of
Ps (1.25 μmol) demonstrated a recovered activity for the
photochemical H2 generation (Figure 5), which indi-
cated that the deactivation of the catalytic system is
mainly due to the decomposition of Ps. Moreover, the
decomposition of Hy@MCM-41 also accounts for the
decrease in the H2 production, because re-addition of Ps
does not fully recover the catalytic activity of the sys-
tem for H2 production.
Foundation of China (Nos. 21302196, 21173245,
21233011, 21004072, and 21273258), 973 Program
(Nos. 2013CB834703, 2013CB834505, 2010CB934500)
and the Solar Energy Initiative of the Chinese Academy
of Sciences is gratefully acknowledged.
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Figure 5 The plot of H2 production from 10 mL aqueous solu-
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Conclusions
A hydrophobic [Fe-Fe] hydrogenase mimic Hy was
immobilized into the mesoporous molecular sieve
K-MCM-41 to form a composite Hy@MCM-41. The
immobilized hydrogenase mimic has been successfully
applied to the photochemical production of hydrogen in
pure water by using [Ir(ppy)2bpy]Cl as the photosensi-
tizer and TEA as the sacrificial electron donor. The re-
sults of photochemical production of hydrogen demon-
strate that the immobilization of Hy into mesoporous
molecular sieve is a way to disperse hydrophobic hy-
drogenase mimics in water. The present study provides
a general strategy for applying hydrophobic artificial
catalysts in pure water by using mesoporous molecular
sieves.
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Financial support from the National Natural Science
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