Fe-Based Metal-Organic Frameworks for Highly Selective Photocatalytic Benzene Hydroxylation to Phenol

Article abstract of DOI:10.1021/acscatal.5b01949

Phenol is one of the most important chemicals in industry. One-step selective benzene hydroxylation is an attractive yet challenging method for phenol production, especially when such a reaction can be driven by solar energy. Herein, we reported that a highly selective benzene hydroxylation to phenol can be achieved over two Fe-based metal-organic frameworks [MIL-100(Fe) and MIL-68(Fe)] under visible light irradiations using hydrogen peroxide (H₂O₂) as an oxidant. An optimal benzene conversion of 30.6% was achieved with a H₂O₂:benzene ratio of 3:4 over MIL-100(Fe) after 24 h irradiations. ESR results and the kinetic studies suggested that a successful coupling of the photocatalysis of Fe-O clusters in Fe-based metal-organic frameworks (MOFs) with a Fenton-like route is involved in this benzene hydroxylation process. The comparison of the reaction over MIL-100(Fe) and MIL-68(Fe) reveals that the structure of MOFs significantly influences the photocatalytic efficiency. Because the composition and the structure of MOFs are highly tunable, this study highlights the great potential of using Fe-based MOFs for photocatalytic benzene hydroxylation to form phenol, which may result in an economical, sustainable, and thus green process for phenol production.

Full text of DOI:10.1021/acscatal.5b01949

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Article
Fe-based Metal-Organic Frameworks (MOFs) for Highly
Selective Photocatalytic Benzene Hydroxylation to Phenol
Dengke Wang, Mengtao Wang, and Zhaohui Li
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01949 • Publication Date (Web): 12 Oct 2015
Downloaded from http://pubs.acs.org on October 14, 2015
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Fe-based Metal-Organic Frameworks (MOFs) for Highly Selective
Photocatalytic Benzene Hydroxylation to Phenol
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Dengke Wang, Mengtao Wang, Zhaohui Li*
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on
Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002,
People’s Republic of China
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Abstract
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Phenol is one of the most important chemicals in industry. Oneꢀstep selective
benzene hydroxylation is an attractive yet challenging method for phenol production,
especially when such a reaction can be driven by solar energy. Herein, we reported
that a highly selective benzene hydroxylation to phenol can be achieved over two
Feꢀbased metalꢀorganic frameworks (MILꢀ100(Fe) and MILꢀ68(Fe)) under visible
light irradiations using hydrogen peroxide (H2O2) as an oxidant. An optimum benzene
conversion of 30.6% was achieved with a H2O2/benzene ratio of 3:4 over MILꢀ100(Fe)
after 24 h irradiations. ESR results and the kinetic studies suggested that a successful
coupling of the photocatalysis of FeꢀO clusters in Feꢀbased MOFs with a Fentonꢀlike
route is involved in this benzene hydroxylation process. The comparison of the
reaction over MILꢀ100(Fe) and MILꢀ68(Fe) reveals that the structure of MOFs
significantly influences the photocatalytic efficiency. Since the composition and the
structure of MOFs are highly tunable, this study highlights the great potential of using
Feꢀbased MOFs for photocatalytic benzene hydroxylation to form phenol, which may
result in an economical, sustainable and thus green process for phenol production.
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Keywords: benzene hydroxylation; phenol; photocatalysis; Feꢀbased metalꢀorganic
frameworks; H2O2
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Introduction
Phenol is one of the most important chemicals in industry.1 Currently, phenol
production is based upon the threeꢀstep cumene process, which involves a hazardous
cumene hydroperoxide intermediate and produces nearly equimolar amounts of
acetone as coꢀproduct.1,2 An attractive route in phenol production is the direct
oneꢀstep selective hydroxylation of benzene using environmentally friendly oxidants
such as O2, N2O and H2O2.3 In particular, the direct hydroxylation of benzene to form
phenol with H2O2 has attracted extensive recent research attention since this process is
green and clean, producing only water as a byproduct. However, the direct
introduction of hydroxyl functionality into benzene is challenging. Almost all the
developed catalytic systems require elevated temperature and suffer from low
conversion due to the notoriously low reactivity of aromatic C−H bonds.4 Moreover,
increased reactivity of phenol relative to benzene makes the selective oxidation of
benzene to phenol very difficult, especially under heating conditions.5
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The utilization of solar light to drive chemical reactions is extremely appealing.6
Especially, the selective hydroxylation of benzene to phenol via photocatalysis has
attracted increasing research interest.7 A few photocatalysts, including homogeneous
and metalꢀdoped heterogeneous catalysts, have already been reported on the selective
hydroxylation of benzene to phenol.8 However, the performance of the currently
developed photocatalytic systems for the benzene hydroxylation reaction is far from
satisfactory. In addition to low benzene conversion and inferior phenol selectivity,
homogeneous photocatalysts are difficult to be separated from the reaction system,
while the leaching of the active species from metalꢀdoped heterogeneous catalysts is
unavoidable. The development of new type of photocatalysts with superior
performance for highly selective hydroxylation of benzene is important, yet it is
challenging.
Metalꢀorganic frameworks (MOFs) are a class of crystalline microꢀmesoporous
hybrid materials which have already shown a variety of applications.9 Previous
studies have revealed that MOFs can be described as an array of selfꢀassembled
molecular catalysts. The light irradiations on the MOFs can lead to the generation of
electrons and holes which can take part in the redox reactions in close vicinity even
though their mobility is low as compared with those in semiconductors.10 This, in
combination with the inherent large surface areas, uniform but tunable cavities of
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MOFs and their tailorꢀable chemistry, make MOFs to be promising photocatalysts.
Actually, although studies on the photocatalysis based on MOFs is still in its infancy
stage, the applications of MOFs in photocatalytic CO2 reduction,11 organic
transformations,12 hydrogen evolutions,13 as well as pollutants degradations14 have
already been demonstrated. Among all the reported MOFꢀbased photocatalysts,
Feꢀcontaining MOFs are extremely attractive since Fe is an earthꢀabundant element
and Feꢀbased MOFs show intensive absorption in visible light region due to the
existence of ironꢀoxo (FeꢀO) clusters.11a,14 In addition, the activation of H2O2 to
radicals (·OH) via Fentonꢀlike route over Feꢀbased MOFs as well as Feꢀcontaining
complexes have been documented.15 Given the coupling of photocatalysis based on
the FeꢀO clusters with activation of H2O2 over Feꢀbased MOFs is successful, the
Feꢀbased MOFs would be promising photocatalysts for benzene hydroxylation to
produce phenol.
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Herein, we report the photocatalytic benzene hydroxylation to phenol with H2O2
over two waterꢀstable Feꢀbased MOFs, MILꢀ100(Fe) and MILꢀ68(Fe). Factors
influencing the performance were investigated and a possible photocatalytic
mechanism of benzene hydroxylation over Feꢀbased MOFs was proposed. This is the
first demonstration that MOF material can be used for photocatalytic benzene
hydroxylation and highlights the great potential of developing MOFs as photocatalyst
for organic synthesis.
Results and discussion
MILꢀ100(Fe), a threeꢀdimensional porous MOF material, was first chosen in the
photocatalytic benzene hydroxylation reaction. MILꢀ100(Fe) is built up from
supertetrahedra consisting of trimers of FeO6 octahedra sharing a common vertex
ꢁ3ꢀO, which delimits two types of mesoporous cages (Figure 1a).16 The preparation of
MILꢀ100(Fe) followed a modified procedure reported previously.16b The XRD
patterns and the N2 adsorption/desorption isotherms of the asꢀprepared product
confirm that pure phase of MILꢀ100(Fe) with high quality has been obtained (Figure
2a and S1). The UVꢀvis spectrum of MILꢀ100(Fe) shows an absorption edge
extending to around 600 nm, in agreement with its orange color (Figure 2b).
The benzene hydroxylation reaction was initially carried out in acetonitrile
(CH3CN) with a H2O2/benzene ratio to be 1:2 under visible light irradiations (Table 1,
entry 1). It was found that MILꢀ100(Fe) can effectively hydroxylate benzene to give
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phenol in the presence of H2O2 as oxidant. The conversion of benzene reached 10.3%
and a high selectivity to phenol (>99%) was achieved after irradiated for 8 h. No
overꢀoxidation products such as diphenol or quinone derivatives were detected.
Control experiments carried out in absence of MILꢀ100(Fe) or without light
irradiations did not give any detectable products, suggesting that the hydroxylation of
benzene to phenol over MILꢀ100(Fe) is really induced by photocatalysis (Table 1,
entry 2 and 3).
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Solvent played an important role in this reaction. As shown in Table 1 (entry 4ꢀ7),
the benzene conversion ratio over MILꢀ100(Fe) as well as its selectivity to phenol
varied in different solvents. Among all the solvents investigated, a mixed solvent
containing CH3CN/H2O (v/v =1:1) gave the best performance by showing the highest
benzene conversion ratio at 13.6% without sacrificing the selectivity to phenol (98%)
(Table 1, entry 7). It is obvious that the reaction carried out in the mixed solvent is
more effective in benzene hydroxylation in view of both the benzene conversion and
phenol production as compared with that in CH3CN or H2O only. An explanation to
its superior performance in this biphasic system is that the benzene hydroxylation
occurs in the aqueous phase in the cavity of the MOFs since the large inner surface
area of MILꢀ100(Fe) is hydrophilic and is easy for the aqueous phase component to
diffuse into the cavities (Figure S2).17a Moreover, the presence of suitable amount of
CH3CN not only increases the concentration of benzene in the aqueous phase which is
favorable for the aqueous benzene hydroxylation reaction, but also can extract phenol
produced into organic phase to minimize the overꢀoxidation of phenol by reducing the
contact between phenol and catalyst. Such a promoting effect played by the mixed
solvent was also observed previously in the benzene hydroxylation with H2O2 over
some Feꢀcontaining homogeneous complexes.17b
Since the ratio of H2O2/benzene can influence the efficiency of the benzene
hydroxylation, the performance of MILꢀ100(Fe) for the benzene hydroxylation was
investigated in different H2O2/benzene ratio. When H2O2/benzene ratio increased
from 1:2 to 3:2, the conversion of benzene in 8 h increased from the original 13.6% to
22.5%, but a slightly reduced selectivity to phenol from the original 98% to 92% was
also observed (Table 1, entry 7ꢀ10). The reduced selectivity in the presence of
excessive H2O2 with a H2O2/benzene ratio at 3:2 was mainly due to the hydroxylation
of phenol by H2O2 to form hydroquinone at ca. 5% yield as well as the oxidation of
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phenol to form pꢀbenzoquinone at ca. 2.3% yield. This phenomenon was also
observed in benzene hydroxylation catalyzed by TSꢀ1.18 However, no byproducts
were detected over the reactions in other H2O2/benzene ratio. A H2O2/benzene ratio at
3:4 gave a benzene conversion of 20.1%, a value only slightly lower than that
observed over a H2O2/benzene ratio of 3:2 (22.5%), but can still maintain a high
selectivity (98%) to phenol production (Table 1, entry 8). Therefore, a H2O2/benzene
ratio of 3:4 was used in the following experiments. Figure 3a shows the
timeꢀdependent conversion of benzene during the hydroxylation reaction. The
conversion of benzene increased with the reaction time and, due to the depletion of
H2O2, reached a maximum benzene conversion of 30.6% after 24h illumination. The
addition of H2O2 into the system can recover the reaction (Figure S3).
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The current photocatalytic system showed superior performance in terms of both
benzene conversion and selectivity to phenol as compared with those reported
previously, like FeꢀgꢀC3N4/SBAꢀ15 (11.9%), FeꢀCN/TSꢀ1 (10.4%) and
Au/Ti0.98V0.02O2 (18.0%).19 Unlike the above mentioned photocatalytic systems which
require heating or UV light irradiations, only visible light is necessary for the current
system. Moreover, H2O2 efficiency in the current photocatalytic system (26%) is
higher as compared with the above mentioned systems (Table S1), such as
FeꢀgꢀC3N4/SBAꢀ15 (20.7%), FeꢀCN/TSꢀ1 (18%) and Au/Ti0.98V0.02O2 (10.7%).
No further conversion of benzene occurred when MILꢀ100(Fe) was removed
from the reaction system at 4 h (Figure 3b). The ICP analysis on the filtrate revealed
no detectable Fe, indicating that the benzene hydroxylation is truly induced by
heterogeneous MILꢀ100(Fe). Cycling test showed that there was no obvious loss of
the photocatalytic activity after three runs of reactions (Figure 4). Besides these, the
XRD and N2 adsorption of the photocatalyst after the reaction did not show much
change (Figure S4 and S5). All these suggest that MILꢀ100(Fe) can hydroxylate
benzene to produce phenol under visible light and is stable during the photocatalytic
reaction.
Furthermore, it was found that the experiment can be scaled up without obvious
decrease of the performance. A comparable benzene conversion (19.8%) was still
observed in 8h when the reaction was scaled up by a factor of 10 (Table 1, entry 11).
To gain insight into the reaction mechanism, the kinetic isotope effect (KIE) was
examined. The KIE was defined as kH/kD, in which kH and kD referred to the rate
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constant for the benzene hydroxylation reaction carried out over benzene and
benzeneꢀd6, respectively. A plot of lnCt/C0 (C0 and Ct are the concentration of benzene
or benezeneꢀd6 at 0 and t time, respectively) versus time was obtained (Figure 5). The
KIE value was determined to be 1.67, which was close to the value (1.7) for
Fentonꢀtype hydroxylation.20 This suggests that a Fentonꢀtype reaction is likely to be
involved in the photocatalytic benzene hydroxylation over MILꢀ100(Fe). The benzene
hydroxylation to produce phenol by H2O2 is generally believed to be proceeded via an
oxygenation pathway induced by the inꢀsitu formed ·OH.7 The ESR spectrum of the
irradiated reaction system containing MILꢀ100(Fe), benzene and H2O2 in the presence
of 5,5ꢀdimethylꢀ1ꢀpyrroline Nꢀoxide (DMPO) as a trapping agent shows four typical
signals for the DMPOꢀ·OH adduct and thus confirmed the formation of ·OH radicals
during the photocatalytic reaction (Figure 6a). Moreover, the addition of ethanol,
a ·OH radical scavenger, into the catalytic system completely inhibited the benzene
hydroxylation and no phenol was detected (Figure 3c). This gives another
confirmation that benzene hydroxylation over MILꢀ100(Fe) truly follows a ·OH
radical mediated oxygenation pathway. It was proposed that the ·OH radical in this
system can be generated via the reduction of H2O2 by Fe2+ formed over the irradiated
MILꢀ100(Fe), similar to that observed in the homogeneous Feꢀcontaining complex via
a Fentonꢀlike reaction.15b Actually, the ESR signal corresponding to Fe3+ observed in a
suspension of MILꢀ100(Fe) and benzene was quenched when visible light was
irradiated, implying that upon the light irradiation, Fe3+ was reduced to generate Fe2+
(Figure 6b). A similar reduction of Fe3+ to form Fe2+ upon light irradiation was
previously observed over MILꢀ101(Fe), another Feꢀbased MOFs, and was found to be
responsible for its photocatalytic CO2 reduction.11a
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Based on the above observations, the mechanism for photocatalytic benzene
hydroxylation to phenol over MILꢀ100(Fe) was proposed (scheme 1). First, when
irradiated, an excited charge separation state occurs in MILꢀ100(Fe) by transferring
electrons from O2ꢀ to Fe3+ in the FeꢀO clusters and Fe3+ is thus reduced to Fe2+ (step
ⅰ). The asꢀformed Fe2+ can reduce H2O2 to generate active ·OH via a Fentonꢀlike
reaction, while itself was oxidized back to Fe3+ (step ⅱ). The ·OH radical can attack
benzene ring to form hydroxycyclohexadienyl radical (step ⅲ). The release of the
proton and electron from the hydroxycyclohexadienyl radical leads to the formation
of phenol and regenerates the Feꢀbased MOF to fulfill the photocatalytic cycle (step
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ⅳ).
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To further explore how the structure of the MOFs can influence its photocatalytic
performance for benzene hydroxylation, MILꢀ68(Fe), another waterꢀstable Feꢀbased
MOFs, was chosen and its photocatalytic activity for benzene hydroxylation was also
investigated. Unlike MILꢀ100(Fe), MILꢀ68(Fe) is assembled from the infinite straight
chains of cornerꢀsharing FeO4(OH)2 octahedra connected through a terephthalate
linker, defining two types of oneꢀdimensional channels (Figure 1b).21 The XRD
patterns and N2 adsorption/desorption of asꢀsynthesized MILꢀ68(Fe) confirmed that
MILꢀ68(Fe) with high quality was obtained (Figure S6). Although benzene was also
hydroxylated to form phenol over the irradiated MILꢀ68(Fe) (Table 1, entry 12), a
lower benzene conversion (14%) and phenol selectivity (90%) were observed over
MILꢀ68(Fe) as compared with those over MILꢀ100(Fe) (benzene conversion, 20.1%,
selectivity to phenol 98%). It is generally known that the light absorption by the
photocatalyst to generate the charge carriers is the initial step for photocatalysis. The
comparison of the DRS spectra of MILꢀ100(Fe) and MILꢀ68(Fe) reveals that
MILꢀ100(Fe) shows much intensive absorption in the visible light region (Figure 2b).
Therefore, an enhanced light absorption of MILꢀ100(Fe) as compared with
MILꢀ68(Fe) may contribute to its superior photocatalytic performance for benzene
hydroxylation. Besides this, previous studies on the catalytic activities for aerobic
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oxidation of alcohols over
a
trinuclear ruthenium carboxylate catalyst
Ru3O(O2CCH3)6(H2O)3, revealed that framework containing ꢁ3ꢀO played an
important role in its catalytic performance. It was believed that the existence of ꢀ3ꢀO
could promote the formation of a pronounced electronic delocalized state in Ru3O
cluster, which could help the electron transfer from metal ion to oxidant (O2) to form
active species (O2ꢀꢀ).22,23 Since MILꢀ100(Fe) also contains ꢁ3ꢀO in its framework, the
formation of the electronic delocalized state in Fe3O cluster can also promote the
electron transfer from the metal center to the substrate. Actually, in addition to
showing higher activity for benzene hydroxylation to form phenol, MILꢀ100(Fe) also
exhibited improved performance in the oxidation of benzyl alcohol. The conversion of
benzyl alcohol to benzaldehyde over MILꢀ100(Fe) (44.4%) was much higher than that
over MILꢀ68(Fe) (32.7%) in 12 h. These facts indicated that structures of MOFs
indeed affect the catalytic performance. Since the composition, structure of MOFs as
well as its light absorption capability can be facilely tailorable, an enhanced
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photocatalytic performance for benzene hydroxylation over the Feꢀbased MOFs can
be expected via a judicious selection of the organic ligand to form an optimum MOF
structure.
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Conclusions
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In conclusion, we demonstrated that MILꢀ100(Fe) and MILꢀ68(Fe), two
Feꢀbased MOFs, can selectively hydroxylate benzene to produce phenol under visible
light irradiations using aqueous H2O2 as oxidant. This study highlights Feꢀbased
MOFs to be promising photocatalysts for benzene hydroxylation, which may result in
an economical, sustainable and thus green process for phenol production. Considering
the diversified MOF structure due to the availability of different varieties of organic
linkers as well as the versatile coordination chemistry of the metal cations, we
believed that more efficient Feꢀbased MOF photocatalysts for selective benzene
hydroxylation to produce phenol can be developed.
Experimental Sections
Materials. 1,3,5ꢀBenzenetricarboxylic acid were provided by J&K Scientific Co.
1,4ꢀBenzenedicarboxylic acid were provided by Aladdin Co. Iron(III) nitrate
nonahydrate and iron (III) chloride hexahydrate were provided by Shanghai Chemical
Reagent Co. 5,5ꢀDimethylꢀ1ꢀpyrroline Nꢀoxide (DMPO) were provided by TCI Co.
All the reagents were analytical grade and used without further purifications.
Synthesis. MILꢀ100(Fe) was prepared following the procedure described in the
literature with slight modification.16b Typically, iron(III) nitrate nonahydrate (484 mg,
1.2 mmol) and 1,3,5ꢀbenzenetricarboxylic acid (210 mg, 1.0 mmol) were dissolved in
deionized water (5 mL). Then, the resulting solution was stirred and transferred to a
Teflon autoclave liner and sealed to heat at 180 °C for 12 h. After a period time, the
autoclave was cooled down to room temperature in air. The obtained yellow solid was
recovered by filtration and washed with deionized water and methanol for several
times. The synthesized MILꢀ100(Fe) was finally dried overnight at 60 °C in oven.
MILꢀ68(Fe) was prepared according to the method reported in the literature.15a
Typically, a mixture of iron (III) chloride hexahydrate (1.2 mmol, 324 mg),
1,4ꢀbenzenedicarboxylic acid (2.4 mmol, 798 mg), hydrofluoric acid (5 mol/L, 0.12
mL), and hydrous hydrochloride (1 mol/L, 0.12 mL) was dissolved in
N,Nꢀdimethylformamide (18 mL) in a Teflonꢀlined autoclave. After sonication to be a
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homogeneous solution, the mixture was heated at 100 °C for 120 h. After a period
time, the autoclave was cooled down to room temperature in air. The obtained solid
was recovered by filtration and washed with N,Nꢀdimethylformamide and methanol
for several times. The synthesized MILꢀ68(Fe) was finally dried overnight at 60 °C in
oven.
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Characterizations. The asꢀprepared sample was characterized by Xꢀray diffraction
(XRD) patterns on a D8 Advance Xꢀray diffractometer (Bruker, Germany) using Cu
Kα (λ= 1.5406 Å) radiation at a voltage of 40 kV and 40 mA. XRD patterns were
scanned over the angular range of 5ꢀ30° (2θ) with a step size of 0.02°. N2
adsorption/desorption experiments were measured with an ASAP 2020 M
(Micromeritics Instrument Corp). After the sample was degassed in a vacuum at
150°C for 10 h, the nitrogen adsorption and desorption isotherms were measured at
ꢀ196°C. UVꢀvis diffuse reflectance spectrum (UVꢀvis DRS) was obtained by a UVꢀvis
spectrophotometer (Varian Cary 500). Barium sulfate (BaSO4) was used as a reference.
The ESR spectra were performed on a Bruker A300 ESR spectrometer. Fe content in
filtrate was determined using Inductively Coupled Plasma Optical Emission
Spectroscopy (ICPꢀOES, PerkinꢀElmer, OPTIMA 8000).
Photocatalytic reaction. The photocatalytic benzene hydroxylation was performed as
follows. In a typical catalytic process, a mixture of acetonitrile (2 ml), water (2 ml)
and benzene (44ꢁL, 0.5 mmol) was transferred into a 10 mL schlenk tube containing
10 mg of catalyst. The resulting biphasic system was stirred with dropwise addition of
hydrogen peroxide (30 wt%, 39ꢁL, 0.38mmol) and irradiated with a 300 W Xe lamp
with a UVꢀcut filter to remove light with wavelengths less than 420 nm and an IRꢀcut
filter to remove all wavelengths longer than 800 nm (Figure S7). After the reaction,
the whole system was cooled down with iceꢀwater, followed by the addition of
iceꢀcooled ethanol (2 mL) to quench the reaction, which turn the biphasic system to a
singleꢀphase system. The products of the reaction were analyzed by a gas
chromatograph (Shimadzu GCꢀ2014) and GCꢀMS with toluene as the internal
standard. H2O2 concentration was determined by colorimetric titration method based
on the formation of a yellow colored Ti(IV)ꢀH2O2 complex. The reaction solution
(210 ꢁL) was added to Ti(SO4)2 solution (25 mmol/L, 5 mL) and was diluted to 50
mL. The resultant solution was analyzed using a UV/Vis spectrophotometer and the
adsorption peaks at 430 nm for Ti(IV)ꢀH2O2 complex were monitored.
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Upscaling experiment was conducted under the similar conditions in a homeꢀmade
reactor. Briefly, a mixture of acetonitrile (20 ml), water (20 ml) and benzene (440ꢁL,
5 mmol) was transferred into the homeꢀmade reactor containing 100 mg of catalyst.
The resulting biphasic system was stirred with dropwise addition of hydrogen
peroxide (30 wt%, 390ꢁL, 3.8mmol) and irradiated with a 300 W Xe lamp with a
UVꢀcut filter to remove light with wavelengths less than 420 nm and an IRꢀcut filter
to remove all wavelengths longer than 800 nm. After the reaction, the process of
treatment and products detection are similar to the previous reported.
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The labeling experiments were performed over benzene and benzeneꢀd6, respectively,
under standard reaction conditions. The kinetic isotope effect (KIE) is defined as
kH/kD, in which kH and kD is the rate constant for the benzene hydroxylation reaction
carried out over benzene and benzeneꢀd6 respectively.
Acknowledgements
This work was supported by 973 Program (2014CB239303), NSFC (21273035),
Specialized Research Fund for the Doctoral Program of Higher Education
(20123514110002) and Independent Research Project of State Key Laboratory of
Photocatalysis on Energy and Environment (NO. 2014A03). Z. Li also thanks the
Award Program for Minjiang Scholar Professorship for financial support.
Supporting Information Available: Detailed experimental protocols and additional
characterization data are included in the supporting information. This material is
available free of charge via the Internet at http://pubs.acs.org.
Author Information
Corresponding Author
∗ Author to whom all correspondences should be addressed.
Eꢀmail: zhaohuili1969@yahoo.com, Tel (Fax): 86ꢀ591ꢀ83779260
Notes
The authors declare no competing financial interest.
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References
5
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7
8
(1) Weber, M.; Weber, M.; KleineꢀBoymann, M. Phenol. Ullmann’s Encyclopedia
of Industrial Chemistry, WileyꢀVCH Verlag GmbH & Co. KGaA, Weinheim,
2004; Vol. 26, p 503.
9
(2) Hock, H.; Lang, S. Ber. Dtsch. Chem. Ges. 1944, 77, 257ꢀ264.
10
11
12
13
14
15
16
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19
20
21
22
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29
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47
48
49
50
51
52
53
54
55
56
57
58
59
60
(3) (a) Acharyya, S. S.; Ghosh, S.; Tiwari, R.; Pendem, C.; Sasaki, T.; Bal, R. ACS
Catal. 2015, 5, 2850ꢀ2858. (b) Navarro, R.; LopezꢀPedrajas, S.; Luna, D.;
Marinas, J. M.; Bautista, F. M. Appl. Catal. A-Gen. 2014, 474, 272ꢀ279. (c)
Morimoto, Y.; Bunno, S.; Fujieda, N.; Sugimoto, H.; Itoh, S. J. Am. Chem.
Soc. 2015, 137, 5867ꢀ5870.
(4) Herrerías, C. I.; Yao, X. Q.; Li, Z. P.; Li, C. ꢀJ. Chem. Rev. 2007, 107,
2546ꢀ2562.
(5) Piera, J.; Bäckvall, J. ꢀE. Angew. Chem. Int. Ed. 2008, 47, 3506ꢀ3523.
(6) (a) Lang, X. J.; Chen, X. D.; Zhao, J. C. Chem. Soc. Rev. 2014, 43, 473ꢀ486. (b)
Tong, H.; Ouyang, S. X.; Bi, Y. P.; Umezawa, N.; Oshikiri, M.; Ye, J. H. Adv.
Mater. 2012, 24, 229ꢀ251.
(7) Brillas, E.; Sirés, I.; Oturan, M. A. Chem. Rev. 2009, 109, 6570ꢀ6631.
(8) (a) Ohkubo, K.; Kobayashi, T.; Fukuzumi, S. Angew. Chem. Int. Ed. 2011, 50,
8652ꢀ8655. (b) Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. J. Am. Chem. Soc.
2013, 135, 5368ꢀ5371. (c) Devaraji, P.; Sathu, N. K.; Gopinath, C. S. ACS Catal.
2014, 4, 2844ꢀ2853.
(9) (a) Zhu, Q. ꢀL.; Xu, Q. Chem. Soc. Rev. 2014, 43, 5468ꢀ5512. (b) Cui, Y. J.; Yue,
Y. F.; Qian, G. D.; Chen, B. L. Chem. Rev. 2012, 112, 1126ꢀ1162. (c) Zhang, T.;
Lin, W. B. Chem. Soc. Rev. 2014, 43, 5982ꢀ5993. (d) Gao, W. ꢀY.; Chrzanowski,
M.; Ma, S. Q. Chem. Soc. Rev. 2014, 43, 5841ꢀ5866. (e) Zhou, H. ꢀC.;
Kitagawa, S. Chem. Soc. Rev. 2014, 43, 5415ꢀ5418. (f) Xuan, W. M.; Zhu, C. F.;
Liu, Y.; Cui, Y. Chem. Soc. Rev. 2012, 41, 1677ꢀ1695.
(10) (a) Alvaro, M.; Carbonell, E.; Ferrer, B.; Llabrés i Xamena, F. X.; Garcia, H.
Chem. Eur. J. 2007, 13, 5106ꢀ5112. (b) Tachikawa, T.; Choi, J. R.; Fujitsuka,
M.; Majima, T. J. Phys. Chem. C 2008, 112, 14090ꢀ14101. (c) Wang, J. ꢀL.;
Wang, C.; Lin, W. B. ACS Catal. 2012, 2, 2630ꢀ2640. (d) Nasalevich, M. A.;
Goesten, M. G.; Savenije, T. J.; Kapteijn, F.; Gascon, J. Chem. Commun. 2013,
49, 10575ꢀ10577. (e) Nasalevich, M. A.; Veen, M. van der; Kapteijn, F.; Gascon,
J. CrystEngComm 2014, 16, 4919ꢀ4926. (f) Nasalevich, M. A.; Becker, R.;
RamosꢀFernandez, E. V.; Castellanos, S.; Veber, S. L.; Fedin, M. V.; Kapteijn, F.;
Reek, J. N. H.; Vlugt, J. I. van der; Gascon, J. Energy Environ. Sci. 2015, 8,
364ꢀ375.
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4
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6
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8
9
(11) (a) Wang, D. K.; Huang, R. K.; Liu, W. J.; Sun, D. R.; Li, Z. H. ACS Catal.
2014, 4, 4254ꢀ4260. (b) Fu, Y. H.; Sun, D. R.; Chen, Y. J.; Huang, R. K.; Ding,
Z. X.; Fu, X. Z.; Li, Z. H. Angew. Chem. Int. Ed. 2012, 51, 3364ꢀ3367. (c) Sun,
D. R.; Fu, Y. H.; Liu, W. J.; Ye, L.; Wang, D. K.; Yang, L.; Fu, X. Z.; Li, Z. H.
Chem. Eur. J. 2013, 19, 14279ꢀ14285.
10
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44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(12) (a) Wang, D. K.; Li, Z. H. Catal. Sci. Technol. 2015, 5, 1623ꢀ1628. (b) Sun, D.
R.; Ye, L.; Li, Z. H. Appl. Catal. B: Environ. 2015, 164, 428ꢀ432. (c) Manna, K.;
Zhang, T.; Greene, F. X.; Lin, W. B. J. Am. Chem. Soc. 2015, 137, 2665ꢀ2673.
(13) (a) Silva, C. G.; Luz, I.; Llabrés i Xamena, F. X.; Corma, A.; García, H. Chem.
Eur. J. 2010, 16, 11133ꢀ11138. (b) Horiuchi, Y.; Toyao, T.; Saito, M.;
Mochizuki, K.; Iwata, M.; Higashimura, H.; Anpo, M.; Matsuoka, M. J. Phys.
Chem. C 2012, 116, 20848ꢀ20853.
(14) Laurier, K. G. M.; Vermoortele, F.; Ameloot, R.; De Vos, D. E.; Hofkens, J.;
Roeffaers, M. B. J. J. Am. Chem. Soc. 2013, 135, 14488ꢀ14491.
(15) (a) Zhang, J. ꢀW.; Zhang, H. ꢀT.; Du, Z. ꢀY.; Wang, X. Q.; Yu, S. ꢀH.; Jiang, H.
ꢀL. Chem. Commun. 2014, 50, 1092ꢀ1094. (b) Ma, J. H.; Song, W. J.; Chen, C.
C.; Ma, W. H.; Zhao, J. C.; Tang, Y. L. Environ. Sci. Technol. 2005, 39,
5810ꢀ5815.
(16) (a) Horcajada, P.; Surblé, S.; Serre,; Hong, D. ꢀY.; Seo, Y. ꢀK.; Chang, J. ꢀS.;
Grenèche, J. ꢀM.; Margiolaki, I.; Férey, G. Chem. Commun. 2007, 2820ꢀ2822.
(b) Seo, Y. ꢀK.; Yoon, J. W.; Lee, J. S.; Lee, U. ꢀH.; Hwang, Y. K.; Jun, C. ꢀH.;
Horcajada, P.; Serre, C.; Chang, J. ꢀS. Micropor. Mesopor. Mater. 2012, 157,
137ꢀ145.
(17) (a) Zhang, P. F.; Gong, Y. T.; Li, H. R.; Chen, Z. R.; Wang, Y. RSC Adv. 2013, 3,
5121ꢀ5126. (b) Bianchi, D.; Bortolo, R.; Tassinari, R.; Ricci, M.; Vignola, R.
Angew. Chem. Int. Ed. 2000, 39, 4321ꢀ4323.
(18) (a) Barbera, D.; Cavani, F.; Alessandro, T. D; Fornasari, G.; Guidetti, S.; Aloise,
A.; Giordano, G.; Piumetti, M.; Bonelli, B.; Zanzottera, C. J. Catal. 2010, 275,
158ꢀ169. (b) Neidig, M. L.; Hirsekorn, K. F. Catal. Commun. 2011, 12,
480ꢀ484.
(19) (a) Chen, X. F.; Zhang, J. S.; Fu, X. Z.; Antonietti, M.; Wang, X. C. J. Am.
Chem. Soc. 2009, 131, 11658ꢀ11659. (b) Ye, X. J.; Cui, Y. J.; Qiu, X. Q.; Wang,
X. C. Appl. Catal. B: Environ. 2014, 152ꢀ153, 383ꢀ389. (c) Devaraji, P.; Sathu,
N. K.; Gopinath, C. S. ACS Catal. 2014, 4, 2844ꢀ2853.
(20) Augusti, R.; Dias, A. O.; Rocha, L. L.; Lago, R. M. J. Phys. Chem. A 1998, 102,
10723ꢀ10727.
(21) Fateeva, A.; Horcajada, P.; Devic, T.; Serre, C.; Marrot, J.; Grenèche, J. ꢀM.;
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Morcrette, M.; Tarascon, J. ꢀM.; Maurin, G.; Férey, G. Eur. J. Inorg. Chem. 2010,
3789ꢀ3794.
6
7
(22) Bilgrien, C.; Davis, S.; Drago, R. S. J. Am. Chem. Soc. 1987, 109, 3786ꢀ3787.
(23) (a) Toma, H. E.; Araki, K.; Alexiou, A. D. P.; Nikolaou, S.; Dovidauskas, S.
Coordin. Chem. Rev. 2001, 219ꢀ221, 187ꢀ234. (b) Toma, H. E.; Cipriano, C. J.
Electroanal. Chem. 1989, 263, 313ꢀ322.
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(b)
b
FeO4(OH)2
octahedra
a
c
Figure 1 (a) Topological view of MILꢀ100 with MTNꢀtype zeolitic architecture; (b)
View of the structure of MILꢀ68 involving two types (hexagonal and trigonal) of
channels running through the cꢀaxis.
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asꢀsynthesized
calculated
(b)
MILꢀ100(Fe)
MILꢀ68(Fe)
Figure 2 (a) XRD patterns of MILꢀ100(Fe); (b) Uvꢀvis spectra of MILꢀ100(Fe) and
MILꢀ68(Fe).
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(b)
(c)
Figure 3 Conversion ratio of benzene as a function of irradiation time in
hydroxylation reaction over (a) MILꢀ100(Fe); (b) reaction system after MILꢀ100(Fe)
was removed after irradiated for 4 h; (c) 1 mL ethanol was added to the reaction
system.
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1st run
2nd run
3rd run
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Figure 4 The cycling use of MILꢀ100(Fe) for photocatalytic benzene hydroxylation.
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R2=0.999
benzeneꢀd6
(kD=0.012 minꢀ1)
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R1=0.997
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Figure 5 Kinetics studies for the benzene hydroxylation reaction.
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(b)
in the dark
after light
irradiation
Figure 6 (a) The DMPO spinꢀtrapping ESR spectra for the ·OH radical in the
presence of MILꢀ100(Fe) under visible light irradiation (λ≥420nm); (b) ESR spectra
of a mixture of MILꢀ100(Fe) and benzene in the dark and upon irradiations.
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Table 1 Photocatalytic benzene hydroxylation to form phenol over Feꢀbased MOFs.
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H2O2/
benzene
1:2
Yield/
ꢁmol
51.5
ꢀ
Entry
Catalyst
Solvent
Conv./%
Sel./%
1
2 [a]
3 [b]
4
MILꢀ100(Fe)
CH3CN
CH3CN
CH3CN
acetone
DMF
10.3
ꢀ [d]
ꢀ
>99
ꢀ
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1:2
MILꢀ100(Fe)
MILꢀ100(Fe)
MILꢀ100(Fe)
MILꢀ100(Fe)
1:2
ꢀ
ꢀ
1:2
2.4
3.3
8.3
99
76
85
11.9
12.5
35.5
5
1:2
6
H2O
1:2
CH3CN/H2O
(1:1)
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8
MILꢀ100(Fe)
MILꢀ100(Fe)
MILꢀ100(Fe)
MILꢀ100(Fe)
1:2
3:4
2:2
3:2
3:4
3:4
13.6
20.1
21.7
22.5
19.8
14
98
98
96
92
98
90
66.5
98.5
104
103.5
97
CH3CN/H2O
(1:1)
CH3CN/H2O
(1:1)
9
CH3CN/H2O
(1:1)
10
CH3CN/H2O
(1:1)
11 [c] MILꢀ100(Fe)
12 MILꢀ68(Fe)
CH3CN/H2O
(1:1)
63
Reaction conditions: benzene (0.5 mmol), H2O2, photocatalyst (10 mg), solvent (4
mL), light irradiation (λ ≥ 420 nm), 8 h. [a] Without photocatalyst. [b] Without light
irradiation. [c] The reaction was scaled up by 10 times. [d] “ꢀ” refers to no or
negligible products were detected.
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Scheme 1 Possible reaction mechanism for the photocatalytic benzene hydroxylation
over MILꢀ100(Fe).
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