Photocatalytic Activation of C-H Bonds by Spatially Controlled Chlorine and Titanium on the Silicate Layer

Article abstract of DOI:10.1021/acscatal.9b01284

Owing to the economic and environmental benefits, photocatalytic organic transformation via C-H activation is a crucially important, while still challenging area. We report a photocatalytic process for partial cyclohexane oxidation with enhanced catalytic activity that exploits chlorine and titanium atoms spatially arranged on a silica substrate. Silica-based catalysts bearing grafted titanium that effectively interacts with photochemically adsorbed chlorine were prepared. The catalysts were synthesized by reacting a layered silicate with titanium(IV) acetylacetonate and subsequent chlorination treatment. Characterization by a variety of techniques revealed that the organic ligand of the titanium species that is grafted onto the silicate layer surface is converted into chlorine while maintaining its coordination state. However, upon subsequent hydration, the chlorine existed as HCl. We investigated the performance of the catalyst for the production of cyclohexanol and cyclohexanone from cyclohexane with molecular oxygen under solar light irradiation. The chlorinated layered silicate photocatalyst exhibited a remarkably high production rate (0.7 mmol g^-1 h^-1) and large amount of product formed (0.8 mmol), while the overoxidation of cyclohexane was inhibited. The enhanced catalytic ability could be explained by the presence of a radical mediator (chlorine) located at a position neighboring an isolated titanium atom. The high surface density of these active species (both titanium and chlorine) on the silicate layer provided suitable electron transfer to form chlorine radicals.

Full text of DOI:10.1021/acscatal.9b01284

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Article
Photocatalytic Activation of C-H Bonds by Spatially
Controlled Chlorine and Titanium on the Silicate Layer
Nao Tsunoji, Hidechika Nishida, Yusuke Ide, Kenji Komaguchi, Shinjiro
Hayakawa, Yuya Yagenji, Masahiro Sadakane, and Tsuneji Sano
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01284 • Publication Date (Web): 13 May 2019
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Photocatalytic Activation of C-H Bonds by Spatially
Controlled Chlorine and Titanium on the Silicate
Layer
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Nao Tsunoji* , Hidechika Nishida , Yusuke Ide , Kenji Komaguchi , Shinjiro Hayakawa , Yuya
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Yagenji , Masahiro Sadakane and Tsuneji Sano .
†Graduate School of Engineering, Department of Applied Chemistry, Hiroshima University, 1-4-
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Kagamiyama, Higashi-Hiroshima 739-8527, Japan
‡International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials
Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
ABSTRACT
Owing to the economic and environmental benefits, photocatalytic organic transformation via C-
H activation is a crucially important, while still challenging area. We report photocatalytic process
for partial cyclohexane oxidation with enhanced catalytic activity that exploits chlorine and
titanium atoms spatially arranged on a silica substrate. Silica-based catalysts bearing grafted
titanium that effectively interacts with photochemically adsorbed chlorine were prepared. The
catalysts were synthesized by reacting a layered silicate with titanium(IV) acetylacetonate and
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subsequent chlorination treatment. Characterization by a variety of techniques revealed that the
organic ligand of the titanium species that is grafted onto the silicate layer surface is converted
into chlorine while maintaining its coordination state. However, upon subsequent hydration, the
chlorine existed as HCl. We investigated the performance of the catalyst for the production of
cyclohexanol and cyclohexanone from cyclohexane with molecular oxygen under solar light
irradiation. The chlorinated layered silicate photocatalyst exhibited a remarkably high production
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rate (0.7 mmol g h ) and large amount of product formed (0.8 mmol), while the over-oxidation
of cyclohexane was inhibited. The enhanced catalytic ability could be explained by the presence
of a radical mediator (chlorine) located at a position neighboring an isolated titanium atom. The
high surface density of these active species (both titanium and chlorine) on the silicate layer
provided suitable electron transfer to form chlorine radicals.
KEYWORDS. Photocatalysis, C-H activation, cyclohexane oxidation, KA oil, titanosilicate,
chlorine, layered silicate, partial oxidation.
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INTRODUCTION
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The efficient activation and transformation of C(sp )-H bonds in alkanes to produce higher-
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value products remains one of the biggest challenges in modern chemistry. Such bonds
are thermodynamically strong and kinetically inert, and molecular oxygen is an unreactive
oxidant. Therefore, traditional catalytic strategies for their conversion require the use of
expensive and aggressive reagents and/or severe reaction condition.4-7 Heterogeneous
photocatalysis has become regarded as a promising alternative strategy in recent years since
it can be used to activate stable alkanes, even in mild conditions, using solar energy
directly.8-12 Photocatalysis systems with various catalytic components (transition-metal
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oxides,13-19 metal-oxide-loaded silica,20-23 bismuth-containing semiconductors, cadmium
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sulfide,25-27 g-C N ,
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and hybrid-composites thereof30-34) and reaction conditions (e.g.,
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light wavelength and intensity,35-36 solvent,36-38 and oxidant31, 39) have been studied to
optimize catalytic performance. Nonetheless, as the lack of efficient photocatalysts for
scale-up production of target organics remains the bottleneck for industrialization, more
precise catalyst design strategies that can efficiently harness photons for organic
transformations are highly desirable.
In the current study, we selected two inorganic elements (chlorine and titanium) and
developed highly efficient photo-driven organic transformations by accurate control of
their spatial distribution. In typical photo-driven activations, photo-generated electron-hole
pairs and/or aggressive radicals (superoxide radicals, etc.) are the initiators of C-H
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cleavage. However, such species are often highly oxidative and exhibit low selectivity for
the target product by promoting over-oxidation. Although the role of chlorine during the
photocatalytic system have been attracted attention,40-41 recently, Yuan et al. reported that
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chlorine radicals generated on the surface of a titanium-oxide-based photocatalyst can
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activate primary hydrocarbons and effectively produce partially oxidized products. The
authors proposed that chemisorbed chlorine is trapped by photogenerated holes to produce
chlorine radicals. The specific radical generates alkyl radicals that can subsequently yield
partially oxidized organics through reactions with molecular oxygen. However, in this
system, over-oxidation can proceed through the consumption of electron-hole pairs by
molecular oxygen and the hydrocarbon, resulting in low catalytic efficiency for the partially
oxidized product. Therefore, the specific radical mediator (chlorine) should be located at a
position neighboring the photoexcitation center to ensure more efficient electron transfer
between them.
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In this study, we focused on grafted titanium complex having a tunable coordination
state. Generally, such isolated transition-metal species can be realized in porous silica
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networks, where they have specific tetrahedral coordination states exhibiting highly
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selective redox properties in the presence of oxidants and/or under photoirradiation.
In addition, because their catalytic activities are strongly dependent on the local
environment around the isolated active species, their coordination design has been
investigated using both bottom-up22, 46-47 and post-modification techniques.23, 48-53
However, as such species are presented in rigid three-dimensional (sometimes non-
ordered) silica networks, molecular-level design of their chemical environments is not
typically possible. Furthermore, because of the formation of unusual tetrahedrally
coordinated species due to the isolation of the transition metals within the silica matrix, it
is difficult to obtain the respective isolated sites with high surface densities.
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Recently, we developed a rational design strategy for realizing isolated metal species on
layered silicates. Layered silicates composed of anionic silicate sheets with interlayer
cations are a kind of layered material whose interlayer surfaces are covered with silanol
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(
SiOH) and silanolate (SiO ) groups with crystallographically defined arrangements and
bond angles. We found that isolated titanium species with uniformly tetrahedral
coordination states and high surface densities are provided by the grafting of titanium(IV)
acetylacetonate onto the surface of a Hiroshima University silicate (HUS, Figure 1).54-57 In
addition, the interlayer accessibilities and corresponding catalytic activities of such layered
catalysts can be enhanced by interlayer expansion58-59 and pore creation.55, 60-61
As shown in Figure 1, further remarkable features of this material are the specific
coordination environment of isolated species linked by two Ti−O−Si bridges and the
presence of exchangeable ligands (acetylacetonate or hydroxyl groups).
In this study, we attempted to incorporate chlorine at locations neighboring isolated titanium
atoms by ligand exchange, yielding a chlorinated layered silicate catalyst. The C-H activation
ability of the catalyst was evaluated in the photocatalytic oxidation of cyclohexane into its partially
oxidized products cyclohexanone and cyclohexanol, which are important intermediates in the
manufacture of nylon polymers.62-63 We also employed a chlorine-incorporating zeolite material
(TS-1) and titanium oxide (P25) for comparison in order to assess the influence of the spatial
distribution of the elements on the efficiency of the partial oxidation reaction.
EXPERIMENTAL
Catalyst preparation
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Synthesis of layered silicate precursor The layered silicate HUS-7
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Si O (OH) ·4[C H N]·11.85[H O]) was synthesized by hydrothermal treatment of an
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aqueous
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benzyltrimethylammonium hydroxide (BTMAOH, 40 wt%, Tokyo Chemical Ind. Co., Ltd.
TCI), Japan), and sodium hydroxide (>99%, High Purity Chemicals Inc., Japan). The
starting gel with the chemical composition SiO : 0.2(BTMAOH) : 0.2NaOH : 5.5H O, was
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transferred to a Teflon-lined stainless steel autoclave, and hydrothermal heating treatment
was performed at 125 °C for 7 d under rotation conditions (10 rpm) in the synthesis
apparatus (DRM-420DA, Hiro Company, Japan). The solid product was separated by
centrifugation, washed with distilled water, and dried at 70 °C overnight, yielding the as-
synthesized HUS-7.
In order to graft the bulky titanium complex, the interlayer space of HUS-7 was
expanded by ion-exchange with a long-chain alkylammonium species. The layered silicate
HUS-7 (20 g) was dispersed in an aqueous solution (400 mL) containing
hexadecyltrimethylammonium bromide (70 g, >98%, C TMABr, TCI, Japan). The
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mixture was stirred at 60 °C for 24 h and then centrifuged to remove the supernatant. The
C TMABr-containing solution was added to the separated solid and the ion-exchange
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treatment was repeated another two times. The solid finally obtained was washed with a
mixture of water and ethanol (50:50, v/v) and dried at 70 °C overnight, yielding the
hexadecyltrimethylammonium (C TMA)-exchanged layered silicate precursor (C TMA
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HUS-7).
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Titanium and chlorine incorporation Grafting of titanium onto the surface of the layered
silicate was performed by the reaction of C TMA HUS-7 with titanium(IV)
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acetylacetonate (Ti(acac) ) followed by the removal of C TMA by washing. C TMA
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HUS-7 (1.0 g) was mixed with a toluene solution (200 mL) containing Ti(acac) (6.02 g,
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ca. 63% in propanol, TCI Japan). After ultrasonification treatment for 1 h, the mixture was
stirred at room temperature for 1 d and the solvent was removed by evaporation. Acetic
acid (100 mL, 99.7%, Wako Pure Chemical Industries, Ltd.) was added to the obtained
slurry and the mixture was stirred for another 1 d. The solid was separated by
centrifugation, washed with a mixture of water and ethanol (50:50, v/v), and dried at 70 °C
overnight, yielding the titanium-grafted layered silicate (Ti-HUS).
Ti-HUS (0.5 g) was mixed with aqueous HCl (50 mL) at a predetermined concentration
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0.1–11.5 mol L ) and the mixture was stirred for 24 h under photoirradiation using > 300
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nm light at a power of 1,000 W m ). The solid was isolated by filtration and dried at room
temperature for 1.5 h, yielding chlorine-incorporating layered silicate (xCl/Ti-HUS, where
x denotes the chlorine/titanium (Cl/Ti) molar ratio). Chlorine incorporation was also
performed with the reference catalysts using zeolite (TS-1, ARC-TS1CL, Catalysis Society
of Japan) and titanium oxide (P25, NIPPON AEROSIL) samples.
Characterization
Powder X-ray diffraction (XRD) patterns of the solid products were collected using a
powder X-ray diffractometer (Bruker D8 ADVANCE) with graphite-monochromatized Cu
Kα radiation at 40 kV and 40 mA.
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Si/Ti and Cl/Ti ratios were determined by inductively coupled plasma optical emission
spectroscopy (ICP-OES, Seiko SPS7000) and the Mohr titration.
Crystal size and morphology were investigated using a Hitachi S-4800 scanning electron
microscope (SEM) coupled with an energy dispersive X-ray (EDX) analyzer.
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C and Si magic angle spinning (MAS) NMR spectra were recorded at 150.88 MHz
and 119.17 MHz, respectively, on a Varian 600PS solid NMR spectrometer using a 6-mm
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diameter zirconia rotor at 7 kHz. The Si spectra were acquired using 6.2 μs pulses, a 30 s
recycle delay, and 1,000 scans. Hexamethylbenzene and 3-(trimethylsilyl)propionic-
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,2,3,3-d acid sodium salt were used as chemical shift references for C and Si MAS
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NMR, respectively. H- C cross polarized (CP) MAS NMR spectra were measured with a
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0° pulse length of 5.6 µs, and a cycle delay time of 5 s.
Nitrogen adsorption isotherms were obtained at -196 °C using a conventional volumetric
apparatus (BELSORP-max, Bel Japan). The samples (≈50 mg) were heated at 50 °C for 3
h under evacuation conditions prior to adsorption measurements.
UV-Vis diffuse reflectance spectra were recorded on a UV-Vis spectrometer (UV-
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600Plus, Shimadzu Co., Japan) with a bandwidth of 8 nm and a scan speed of 900 nm
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min .
Elemental analysis was carried out using a PerkinElmer 2400 II CHN analyzer at the
Natural Science Center for Basic Research and Development (N-BARD), Hiroshima
University.
Photocatalytic tests
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Photocatalytic oxidation reactions were carried out under photoirradiation with a solar
simulator (San-Ei Electric Co., Ltd.) in a closed stainless steel container equipped with a
75-mL Pyrex glass vessel containing a mixture of catalyst (30 mg) and cyclohexane (2 mL)
in acetonitrile (18 mL). The air in the headspace was used as an oxygen source. The
container was placed ≈30 cm away from the light source and irradiated under 1 solar (1,000
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Wm ) light at 42 °C for 0.5–48 h with shaking. CO and organic compounds were
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quantitatively determined by GC-TCD (GC-8A) and GC-FID (GC-2014), respectively.
Referring to previous reports,17,
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selectivity for the partially oxidized product
cyclohexanol (CHAol) and cyclohexanone (CHAone) was calculated using the following
equation: [CHAone + CHAol]/[CHAol + CHAone + CHACl + (1/6) CO ] × 100, where
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CHACl indicates cyclohexane chloride.
Action spectra were obtained using monochromatic light irradiation in the wavelength
range 300–450 nm using a diffraction grating-type illuminator (SM-25M, JASCO Co.)
equipped with a 150 W Xenon lamp. The wavelength-dependent apparent quantum yields
were calculated as the ratio of the electrical charge number consumed and the incident
photons, taking into account that 4 electrons are required according to the stoichiometry of
the reactions (i.e., 2C H + O → 2C H OH and C H + O → C H O + H O).
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XAFS analysis
The X-ray absorption near edge structure (XANES) spectra were measured at room
temperature on the HiSOR beam line BL-11 with a Si(1 1 1) monochromator at Hiroshima
University. The energy range, step size, and counting time were 4,950–5,010 eV, 0.5 eV
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per step, and 5.0 s for Ti K-edge and 2810–2840 eV, 0.25 eV per step, and 20 s for Cl K-
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ESR analysis
Electron Spin Resonance (ESR) measurements were conducted using a JEOL JES RE-1X
2+
spectrometer (X-band). The magnetic field was calibrated with a Mn /MgO marker as an external
standard reference. The sample powder (≈60 mg) was placed in a Suprasil ESR tube (5 mm
external diameter) together with 1.0 mL of benzene containing the radical trapping agent N-tert-
butyl-α-phenylnitrone (PBN, 5 mg, TCI, Japan). ESR measurements were performed under
photoirradiation using a 500 W Xe lamp at λ > 330 nm at room temperature. The settings for the
ESR spectrometer were as follows: center field = 330 mT, modulation frequency = 100 kHz, and
modulation width = 0.2×1 mT.
RESULTS AND DISCUSSION
Chlorination of titanium species on the surface of HUS-7 layered silicate
First, the titanium complex was immobilized on the surface of the layered precursor C TMA
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HUS-7 to form Ti-HUS. Then, chlorine incorporation was performed, yielding xCl/TiHUS (where
x denotes the Cl/Ti atom ratio). Table 1 summarizes the pore characteristics and elemental analysis
results for the obtained samples. Upon HCl treatment, the amounts of organic ligand and titanium
species in the xCl/TiHUS decrease, as reflected by their acac/Ti and Si/Ti ratios, and the
2
-1
xCl/TiHUS materials exhibit higher Brunauer-Emmett-Teller (BET) areas (≈100–200 m g ) than
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-1
that of Ti-HUS (<100 m g ) before HCl treatment. By carefully changing the HCl concentration,
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we could control the Cl/Ti ratio of the sample in a range 0.5–1.7. For the zeolite (TS-1) and
titanium oxide (P25) samples, chlorine is incorporated while their BET areas barely change.
Comparing the Si/Ti ratios of the samples, the titanium contents in the xCl/TiHUS materials are
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- to 5-times higher than that of TS-1. The high Ti contents are due to the high density of reactive
silanol and silanolate groups on the surface of the layered silicate HUS-7.57, 59
As shown in the XRD patterns of all the layered silicate samples (Figure 2, left), a
diffraction peak at 2θ = 10.4°, attributed to the in-plane (200) of the original silicate layer
of the HUS, is clearly observed. This confirms that the structure of the silicate framework
is maintained upon titanium and chlorine incorporation. In contrast, Ti-HUS and xCl/Ti
HUS have shorter d001 spacings than that of C TMA-HUS-7, indicating a decrease in
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interlayer distance caused by the release of interlayer molecules (organic cations and
ligands). In the UV-Vis spectra (Figure 2, right), the sample without titanium species (C T
1
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MA-HUS-7) show virtually no absorbance, whereas Ti-HUS and xCl/Ti HUS exhibit
absorption in the range ≈200–500 nm. The 320–450 nm absorption is attributable to the π–
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π* transition in the acetylacetonate ligand, and the 220 nm absorption arises from ligand-
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to-metal charge-transfer (LMCT) by the tetrahedrally coordinated titanium species. With
increasing Cl/Ti ratio, the intensity of the peak attributed to organic ligands gradually
decreases. However, a new absorption band appears at ≈500 nm for samples with high Cl/Ti
ratios. This increase in visible-region optical absorption was also observed in a previous
study of chlorine-doped titanium-oxide-based materials prepared by the direct HCl
impregnation.67
1
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The results of H- C cross-polarized (CP) magic angle spinning (MAS) NMR (Figure 3, left)
reveal that the layered precursor C TMA-HUS-7 exhibits resonances assigned to the bulky
1
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interlayer molecule C TMA. The peaks disappear after titanium incorporation and resonances
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assignable to acetylacetonate ligands in the titanium complex are confirmed in both Ti-HUS and
1.1Cl/Ti-HUS. This replacement of bulky organic molecules with small ones during post-
modification is consistent with the XRD results and elemental analysis (No detection of N atoms).
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In the Si MAS NMR spectrum of C TMA-HUS-7 (Figure 3, right), the five resonance peaks are
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attributed to crystallographically different framework silicon atoms and are assignable to Q3
−
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57, 59
[
HOSi(OSi) or OSi(OSi) ] (-95–-103 ppm) and Q [Si(OSi) ] (-105–-120 ppm) structures.
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For Ti-HUS and 1.1Cl/Ti-HUS, the Q /Q structure peak intensity ratios (21:79 and 23:77,
respectively) are different to that for C TMA-HUS-7 (37:67). This strongly indicates covalent
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bond formation between the guest molecules (Ti(acac) ) and the reactive surface silanol groups of
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the layered silicates.56, 59 Considering (1) the distance between the oxygens in silanol and silanolate
(ca. 2.4 Å) (2) the amounts of disappeared organic ligand (almost equal to that of the chlorine
incorporated, Table 1), and (3) the amount of surface silanol groups lost upon Ti grafting (≈2.3 per
titanium atom), we concluded that Ti(acac) is grafted in a dipodal fashion via two Ti−O−Si
4
bridges from the layer surface (Figure 1) and that, in the chlorine-containing sample, the chlorine
is exchanged with the organic ligand. Figure 4 shows the XRD patterns and UV-Vis spectra of
untreated and treated zeolite (TS-1). The Cl/Ti ratio of the samples does not affect the XRD
patterns or UV-Vis spectra, indicating that no changes in their framework structure or
photoabsorption properties occur. We also evaluated the titanium oxide (P25) sample and
confirmed that there are no differences in its crystallinity and titanium state before and after
chlorination treatment (Figure S1).
Photocatalytic performance
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First, using the catalysts with isolated titanium species (Ti-HUS and TS-1), the
photooxidation of cyclohexane was performed and we compared the influence of the spatial
distributions of Cl and Ti on catalytic performance. We confirmed that the oxidation
reaction does not proceed under dark conditions or without catalyst. Figure 5 shows the
amount of product formed as a function of photoirradiation time over titanium-incorporated
zeolite and layered silicate samples (Ti-HUS, 1.1Cl/Ti-HUS, TS-1, and 1.2Cl/TS-1). For
all the catalysts, the total amounts of partially oxidized products (CHAol and CHAone)
increases with increasing irradiation time, while the amounts of over-oxidized product
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(
CO ) formed are relatively small (below 12 µmol). The total amounts of CHAol and
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CHAone are 3.3 µmol for Ti-HUS and 0.8 µmol for TS-1 after 6 h, and both catalysts form
hardly any CO , resulting in high product selectivity (99.2 and near 100 %, respectively).
2
This confirms the high catalytic selectivity of the isolated metal species loaded in silica.21,
56
In contrast, considering the low BET area and high titanium content in the layered silicate
sample compared to TS-1 samples, the higher amount of product fromed for Ti-HUS is due
to its high surface density (high content) of titanium species, as previously reported.56
Upon catalyst chlorination, the product amounts dramatically improve. 1.1Cl/Ti-HUS
and 1.2Cl/TS-1 produce a total of 125.6 and 22.5 µmol of CHAol and CHAone upon 6 h
of irradiation, respectively. Although small amounts of CO and chlorinated hydrocarbon
2
(cyclohexane chloride, CHACl) are formed, 1.1Cl/Ti-HUS and 1.2Cl/TS-1 still exhibit
high selectivity (96 and ≈100 %, respectively). In addition, IR measurement of the catalysts
after catalyst use could not detect the characteristics signal attributed to coordination
complexes with CO , suggesting its low-contribution to product selectivity. Although the
2
main products over 1.1Cl/Ti-HUS were partially-oxidized products (CHAol and CHAone),
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the radical reaction likely induces the formation of chlorinated product and dimerization of
cyclohexane. In the previous chlorine radical mediated system, Yuan et al. achieved the
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selective transformation of hydrocarbons into their partially-oxidized product. They
proposed that by a large excess of the alkane, most of the chlorine radicals are consumed
to form alkyl radicals and stable HCl at the solid-liquid interface. Then, owing to the
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presence of an ample O supply, the possibility of the reaction by chlorine radicals and
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alkyl radicals to form alkyl halides and coupling products is suppressed. Considering the
amount of oxygen and cyclohexane remained in the reaction system, we expect that similar
selective oxidation route proceeded in the present reaction system.
-1
-1
The total production rate of CHAol and CHAone over 1.1Cl/Ti-HUS (698 µmol g h )
is remarkably high compared to those of existing photocatalytic systems that employ
molecular oxygen as an oxidant.13, 23-24, 36 ꢀFurthermore, layered silicate exhibits a higher
degree of activity improvement upon chlorination (≈40-fold increase) than zeolite (≈20-
fold) despite the similar resultant Cl/Ti ratio, clearly demonstrating the effective
enhancement of catalytic performance possible by this strategy. Thus, these findings imply
that the combination of high surface density and suitable atom distribution endows layered
silicate with dramatically improved catalytic performance.
To gain insight into the effect of chlorination, turnover number (TON, i.e., CHAol and
CHAone formation per titanium in the catalyst) was plotted against Cl/Ti ratio for the
zeolite- and layered silicate-based catalysts, as shown in Figure 6. For both the zeolite- and
layered silicate-based catalysts, TON increases almost linearly with increasing Cl/Ti ratio.
Ti-HUS exhibits higher TON values than TS-1, especially at the high Cl/Ti ratios,
indicating that chlorine catalyzes photocatalytic cycles more efficiently on the layered
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silicate material. However, as shown in Table 1, chlorination of Ti-HUS also improves its
surface area. Thus, there is a possibility that enhanced surface area cause the TON increase.
Therefore, we normalized the TON value by BET area for Ti-HUS samples (TON*), which
is also plotted in Figure 6 (right). When the Cl/Ti ratio is ≈0.5, TON* increases sharply but
begins to plateau at a Cl/Ti ratio of 1.0. Therefore, the high original TON value observed
for the high Cl/Ti ratio is attributed to improved surface area, which may promote diffusion
of hydrocarbon molecules and/or active species on the surface of the layered silicate.
Furthermore, the trend in the TON* vs. Cl/Ti plot at high Cl/Ti ratios strongly indicates
that one chlorine atom per titanium is enough to efficiently enhance catalytic activity,
further confirming the effect of the titanium and chlorine configuration in the layered
silicate base catalyst.
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To investigate the coordination environments in Ti-HUS and TS-1, K-edge X-ray
absorption spectroscopy (XAS) measurement of titanium and chlorine was performed on
the catalysts before catalytic use (Figure 7). The titanium K-edge for TS-1 presents a very
sharp pre-edge peak at an energy of approximately 4,968 eV, attributable to transitions
from the 1s to the 3d level in isolated titanium atoms surrounded by four oxygen atoms.68
In contrast, although the pre-edge peak is broadened in the layered silicate sample, there is
hardly any change in the X-ray Absorption Near Edge Structure (XANES) spectrum upon
chlorination. This confirms that chlorination does not affect the coordination environment
of the titanium atoms in either sample. The broad pre-edge for Ti-HUS could possibly result
from the asymmetric coordination environment of titanium, which is composed of organic
(acac) and inorganic (SiO) ligands. A similar broadening of the pre-edge peak was also
59
observed in a previous study of titanium-complex immobilization on layered silicate. For
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the Cl K-edge, no difference in the spectra of the chlorinated zeolite and layered silicate is
observed. The spectra show a very broad second absorption at ≈2,840 eV below the broad
main absorption at 2,826 eV composed of two partially resolved peaks. A similar trend in
the XANES spectra has been observed for Cl anions in aqueous solution.69-70 Considering
the differences in titanium coordination state between TS-1 and Ti-HUS as well as the loss
of organic ligands in Ti-HUS upon chlorination, it appears that the organic ligands on the
silicate-immobilized titanium are first exchanged for chlorine. Then, during the separation
and drying process of catalyst preparation, hydration by moisture occurs due to the low
water resistance of Ti-Cl bonds, resulting in chlorine existing as a hydrated HCl in both
samples.
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As stated above, the local environment for chlorine is largely the same for TS-1 and
HUS, while the layered silicate samples exhibit remarkably enhanced catalytic activity
upon chlorination. Therefore, to investigate the influence of chlorine incorporation on
optical properties, quantum yields were measured under monochromatic light irradiation.
Figure 8a shows the quantum yields of cyclohexane oxidation under different light
wavelengths (action spectra). We also display the corresponding UV-vis spectra of the
samples. The intensity of the UV-vis spectra was normalized the quantum yield at 300 nm.
As shown in the spectra comparing the samples before and after chlorination, chlorination
improves the quantum yield in the long-wavelength range, and the chlorinated layered
silicate exhibits a high quantum yield (≈8%) at 350 nm. The UV-vis spectra shapes in both
TS-1 and 1.2Cl/TS-1 resembled to that of the quantum yield plot of bare TS-1. This strongly
indicate that the photo activity in both TS-1 samples were mainly derived from photo-
excitation titanium site (not chlorine). In contrast, clear correspondence relation between
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the light adsorption and action spectra could not be observed in the layered silicate samples.
This is due to the overlapped visible-range adsorption arising from the organic ligand.
However, taking into account the fact that the layered silicate sample bearing isolated
titanium with exchangeable ligands shows increased absorption in the visible-light region
upon chlorination (Figure 2), whereas the titanium fully surrounded by SiO bonds in the
zeolite does not (Figure 4), the enhanced absorption and corresponding high quantum yield
for 1.1Cl/Ti-HUS considered to be attributed to electron transfer from chlorine to titanium.
Therefore, to detect the photogenerated chlorine radicals directly, we performed ESI
measurements under photoirradiation in the presence of the radical trap reagent N-tert-
butyl-α-phenylnitrone (PBN) (Figure 8b). For the reference sample without catalyst (only
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PBN), spin adduct attributed to PBN is observed as three signals, whereas 1.1Cl/Ti-HUS
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shows a triplet-quartet splitting corresponding to the Cl-PBN radical adduct, confirming
the formation of chlorine radicals upon photoirradiation. In contrast, 1.2Cl/TS-1 hardly
shows such signal.
For a more quantitative evaluation, we measured the direct formation of chlorinated
hydrocarbon in the absence of oxidant. During the photocatalytic oxidation of cyclohexane via
chlorine radicals with a large excess of the alkane, most of the photogenerated chlorine radicals
are consumed to form alkyl radicals. Furthermore, owing to the presence of an ample oxygen
supply, subsequent oxidation of alkyl radicals proceeds readily and the possibility of the reaction
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between chlorine radicals and alkyl radicals to form alkyl halides is lessened. In contrast, when
the reaction is performed in the absence of oxygen, which inhibits the general oxidation route, the
formation of halide is promoted and the amount of halide formed directly reflects chlorine-radical-
formation efficiency. Figure 8c shows the amounts of cyclohexane chloride (CHA-Cl) formed over
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1
.2Cl/TS-1 and 1.1Cl/Ti-HUS in an inert (argon) atmosphere. Performing the reaction under argon
prevented oxidation by oxygen while promoting formation of the halide.
The amount of CHA-Cl over 1.1Cl/Ti-HUS (8.6 μmol) is approximately 7-times than
over 1.2Cl/TS-1 (1.3 μmol). Although this difference should be due to the different chlorine
contents of the catalysts, when we compared the formation of CHA-Cl per chlorine atom
in the catalyst (i.e., Cl-atom efficiency in Figure 8c), the 1.1Cl/Ti-HUS catalyst also shows
a higher value. By also taking into account the structural and catalytic differences between
the zeolite and layered silicate samples discussed above, it may be concluded that the
suitable spatial distribution of chlorine and titanium, which allows efficient electron
transfer between them, as well as the high surface density of active (Cl and Ti) species,
play important roles in the high photocatalytic activity of chlorinated Ti-HUS. Moreover,
when we used HCl instead of the catalyst under typical oxidation conditions, cyclohexane
oxidation did not proceed catalytically (TON = 0.2), confirming that the chlorine
immobilized the solid surface is effective. In addition, the halide product is relatively stable
under photoirradiation and conversion into the oxidized product is negligible.
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In contrast, to get insight into the influence of other halides, we incorporated bromine
into the Ti-HUS by using hydrobromic acid instead of HCl and investigated the catalytic
property. The obtained brominated catalyst exhibited catalytic activity superior to that of
Ti-HUS although the value was lower than that of chlorinated one. This indicates the
possibility of the efficient radical mediated system by using various halides, whereas the
selection of suitable kinds of halide would be important.
Finally, we optimized the catalytic reaction conditions to further increase the total yield
of CHAol and CHAone, and the activity was compared to that of a conventional titanium-
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oxide-based photocatalyst (P25) (Figure 9). We performed the reaction under oxygen at
high pressure (300 kPa) in the presence of a large amount of catalyst (60 mg) and with a
longer irradiation time (48 h). Titanium oxide (P25) exhibits lower selectivity because of
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the generation of a large amount of CO (502 μmol). However, after chlorination, the total
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amounts of CHAol and CHAone formed increase to 460 μmol and the formation of the
over-oxidized product is decreased to 262 μmol. This strongly indicates that mainly
chlorine radicals act as catalytically active species for selective oxidation, as previously
36
reported. Ti-HUS yields a small amount of CO (19.3 μmol) while the formed amounts
2
of CHAol and CHAone is also low (41.5 μmol). However, for 1.1Cl/Ti-HUS, a remarkably
large amount of product (748 μmol) is attained while maintaining a lower CO amount (231
2
μmol) than that over titanium-oxide-based catalysts.
To the best of our knowledge, the activity of 1.1Cl/Ti-HUS (≈0.8 mmol of product formed) is
the highest ever reported for heterogeneous photocatalytic partial cyclohexane oxidation with
molecular oxygen under visible and/or solar light irradiation.20,
22-23, 36, 56, 73
During the
photocatalytic reaction over titanium oxide, the partially oxidized products (CHAol and CHAone)
are decomposed to CO by photogenerated holes and O radicals, whereas the titanium oxide
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isolated in the porous material selectively promotes partial oxidation.23, 56 Thus, the inhibited
formation of CO over 1.1Cl/Ti-HUS arises from the original catalytic properties of the isolated
2
titanium preventing subsequent oxidation. Moreover, comparing the degrees of enhancement upon
chlorination for Ti-HUS and P25 reveals that the performance of titanium species isolated on the
layered silicate material is more effectively improved by chlorination, which promotes efficient
electron transfer for the titanium, allowing more efficient photocatalytic partial oxidation of
cyclohexane.
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To study the catalytic stability, we separated the solid catalysts (1.1Cl/Ti-HUS) from the
reaction mixtures after a reaction time of 48 h and investigated the reusability in the same
optimized condition. The reused catalyst exhibited the activity similar to Ti-HUS. The
amount of product formed were 43.5 μmol for CHAol and CHAone and 29.4 μmol for CO2.
Taking into account that the reused sample exhibited no change in the amount of titanium
while there was no Cl atom as evidenced by elemental analysis, the decreased catalytic
performance is due to the relatively weak interaction of HCl incorporated on the silicate
surface. In fact, the Cl in the catalyst could be removed by washing with water. However,
when we again treated the reused catalyst with hydrochloric acid, the high product yield
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(412 μmol of partial oxidized product and 99 μmol of CO ) was attained although the value
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could not reach the result in the first run, confirming the regeneration of the catalyst. This
implies that the chlorine exists as radical mediator on the solid-liquid interface as reported
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previously. However, as shown above, only the presence of the hydrochloric acid in the
reaction system hardly proceeded the oxidation. Removal of chlorinated catalyst (1.1Cl/Ti-
HUS) during the reaction inhibited the production of oxidized product. Therefore, we
considered that the high catalytic performance in 1.1Cl/Ti-HUS is caused by co-existing of
titanium that can effectively activate Cl to produce radical mediator under photo-
irradiation. For building more efficient production system, precise surface design to
maintain the chlorine or co-catalyst (chlorine) supplying system would be desirable.
CONCLUSIONS
We have reported the synthesis of layered silicate (HUS)-based photocatalysts featuring isolated
titanium species grafted onto the silicate layer that can efficiently interact with chlorine atoms. The
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catalysts thus obtained show better photocatalytic activities for partial cyclohexane oxidation with
molecular oxygen under solar-light irradiation than that of any corresponding previously reported
heterogeneous photocatalyst system. It should be noted that the materials in the present system
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(
zeolite, layered silicate, and titanium oxide) are composed of inexpensive and abundant elements
Cl, Ti, and Si), but that suitable spatial distribution of the radical mediator and photoabsorber can
(
improve the absolute value of the product formed. Even today, the efficiencies of photocatalytic
organic transformations still require improvement before industrialization. Thus, further design
optimization and development of this catalytic system (for example, in terms of the use of visible
light energy and/or new reaction routes), will be key for achieving more efficient photocatalytic
chemical conversions.
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
Nao Tsunoji, tel.: +81-82-424-7606, e-mail: tnao7373@hiroshima-u.ac.jp
Supporting Information. Characteristics of titanium oxide samples. This material is available
free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENT
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This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI
Grant Number 17K17906 and the Center for Functional Nano Oxide at Hiroshima University.
XAS experiments were performed at HiSOR (proposal nos. 18BG008).
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1. Fukuda, M.; Tsunoji, N.; Yagenji, Y.; Ide, Y.; Hayakawa, S.; Sadakane, M.; Sano, T.
Highly Active and Selective Ti-Incorporated Porous Silica Catalysts Derived From Grafting
of Titanium(IV)Acetylacetonate. J. Mater. Chem. A 2015, 3, 15280-15291.
2. Bellussi, G.; Perego, C. Industrial Catalytic Aspects of the Synthesis of Monomers for
Nylon Production. Cattech 2000, 4, 4-16.
63. Schuchardt, U.; Cardoso, D.; Sercheli, R.; Pereira, R.; Da Cruz, R. S.; Guerreiro, M. C.;
Mandelli, D.; Spinacé, E. V.; Pires, E. L. Cyclohexane Oxidation Continues to be a
Challenge. Appl. Catal. A: Gen. 2001, 211, 1-17.
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4. Sclafani, A.; Herrmann, J. Comparison of The Photoelectronic and Photocatalytic Activities
of Various Anatase and Rutile Forms of Titania in Pure Liquid Organic Phases and in
Aqueous Solutions. J. Phys. Chem. 1996, 100, 13655-13661.
65. Tada, H.; Jin, Q.; Nishijima, H.; Yamamoto, H.; Fujishima, M.; Okuoka, S.; Hattori, T.;
Sumida, Y.; Kobayashi, H. Titanium(IV) Dioxide Surface-Modified With Iron Oxide as a
Visible Light Photocatalyst. Angew. Chem. Int. Ed. Engl. 2011, 50, 3501-5.
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6. De la Peña, O. S.; Capel‐Sanchez, M.; Blanco‐Brieva, G.; Campos‐Martin, J. M.; Fierro, J.
L. The Usefulness of Time‐Dependent Density Functional Theory to Describe the
Electronic Spectra of Ti‐Containing Catalysts. Angew. Chem. Int. Ed. 2003, 42, 5851-5854.
67. Yuan, R.; Chen, T.; Fei, E.; Lin, J.; Ding, Z.; Long, J.; Zhang, Z.; Fu, X.; Liu, P.; Wu, L.
Surface Chlorination of TiO -Based Photocatalysts: a Way to Remarkably Improve
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Photocatalytic Activity in Both UV and Visible Region. ACS Catal. 2011, 1, 200-206.
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8. Thomas, J. M.; Sankar, G. The Role of Synchrotron-Based Studies in the Elucidation and
Design of Active Sites in Titanium− Silica Epoxidation Catalysts. Acc. Chem. Res. 2001,
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9. Jürgensen, A. X-Ray Absorption Spectroscopy of Aqueous Solutions in the X-Ray Energy
Region from 1800 to 3700 eV. Can. J. Chem. 2009, 87, 601-611.
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Table and Figures
Table 1. Characteristics of Ti-containing catalysts before and
after hydrochloric acid treatment
a
BET area
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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9
0
1
2
3
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9
0
Sample
Si/Tib acac/Tib
Cl/Tib
2
-1
[m g ]
Ti-HUS
0.5Cl/Ti-HUS
0
1
1
TS-1
0
0.6Cl/TS-1
1.2Cl/TS-1
P25
84
6.1
10.1
10.8
12.0
19.7
46.5
54.5
52.9
65.4
-
1.32
0.80
0.77
0.64
0
0.5
0.8
1.1
1.7
-
0.5
0.6
1.2
-
198
113
114
186
474
478
441
448
41
.8Cl/Ti-HUS
.1Cl/Ti-HUS
.7Cl/Ti-HUS
1.02
-
-
-
-
-
-
.5Cl/TS-1
Cl/P25
48
-
0.02
a
b
Determined using the BET method. Determined using ICP,
CHN elemental analysis, and Mohr titration.
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Figure 1. HUS-7 silicate framework viewed along the (a) c-axis, (b) b-axis, and (c) a-axis. The
dotted squares indicate the repeating structural unit containing a silanol and a silanolate group. (d)
Ti grafting and chlorination on the HUS-7silicate layer.
Figure 2. (left) XRD patterns and (right) UV-Vis spectra of layered silicate samples before and
after Ti grafting and chlorination. Interlayer expanded HUS-7 ion-exchanged with
hexadecyltrimethylammonium (C TMA-HUS-7), Ti-grafted HUS-7 (Ti-HUS), and Ti-HUS after
1
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hydrochloric acid treatment (xCl/Ti-HUS, where x denotes the number of Cl atoms per grafted Ti
atom).
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Figure 3. (left) C CP and (right) Si MAS NMR spectra of layered silicate samples before and
after Ti grafting and Cl incorporation.
Figure 4. XRD patterns and UV-Vis spectra of Ti-containing zeolite (TS-1) and TS-1 after
hydrochloric acid treatment (xCl/TS-1, where x denotes number of incorporated Cl atoms per Ti
atom in the catalyst).
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