Photocatalytic Chemical CO2 Fixation by Cu-BDC Nanosheet?Macroporous-Mesoporous-TiO2 under Mild Conditions

Article abstract of DOI:10.1021/acscatal.9b02553

Growing greenhouse gas CO₂ is driving the research on the chemical fixation of CO₂. Here, the organic reaction of CO₂ with benzyl halogen for chemical CO₂ fixation under ambient conditions with irradiation of ultraviolet light is successfully catalyzed by Cu-BDC nanosheet?macroporous-mesoporous-TiO₂ (Cu-BDC?macro-meso-TiO₂), which shows high photocatalytic activity for both benzyl chloride and bromide reacting with CO₂. Meanwhile, the prepared Cu-BDC?macro-meso-TiO₂ possesses a three-scale porous structure, including macropores, mesopores, and micropores. In the uniform hierarchical structure, the microporous Cu-BDC nanosheet is confined in the macropore of macroporous-mesoporous-TiO₂, while the ordered mesoporous structure is in the macroporous walls. This multilevel porous distribution can significantly improve the active surface areas and mass transfer efficiency of Cu-BDC?macro-meso-TiO₂. Therefore, this finding has opened a field of research on photocatalytic chemical CO₂ fixation.

Full text of DOI:10.1021/acscatal.9b02553

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Article
Photocatalytic Chemical CO2 Fixation by Cu-BDC
Nanosheet@Macroporous-Mesoporous-TiO2 under Mild Condition
Zhenxing Li, Junmei Chu, Dong Meng, Yangyang Wen, Xiaofei Xing, He
Miao, Mingliang Hu, Chengcheng Yu, Zhiting Wei, Yang Yang, and Yongle Li
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02553 • Publication Date (Web): 09 Aug 2019
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Photocatalytic Chemical CO Fixation by Cu-BDC
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Nanosheet@Macroporous-Mesoporous-TiO under
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Mild Condition
Zhenxing Li,*, †, # Junmei Chu,†, # Dong Meng, Yangyang Wen, Xiaofei Xing, Miao
‡
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‡
*,§
He, Mingliang Hu, Chengcheng Yu, Zhiting Wei, Yang Yang, and Yongle Li
†
State Key Laboratory of Heavy Oil Processing, College of New Energy and Materials,
Beijing Key Laboratory of Biogas Upgrading Utilization, China University of Petroleum
(Beijing), Beijing 102249, China
‡
Department of Materials Science and Engineering, University of California, Los
Angeles, California 90095, United States.
§Department of Physics, International Center for Quantum and Molecular Structures,
and Shanghai Key Laboratory of High Temperature Superconductors, Shanghai
University, Shanghai 200444, China
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ABSTRACT：The growing greenhouse gas CO is driving the research on the chemical
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fixation of CO . Here, the organic reaction of CO with benzyl halogen for chemical CO
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fixation under ambient conditions with the irradiation of ultraviolet (UV) light is
successfully catalyzed by Cu-BDC nanosheet@macroporous-mesoporous-TiO (Cu-
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BDC@macro-meso-TiO ), which shows high photocatalytic activity for both benzyl
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chloride and bromide reacting with CO . Meanwhile, the prepared Cu-BDC@macro-
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meso-TiO possess three-scale porous structure, including macropores, mesopores and
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micropores. In the uniformly hierarchical structure, the microporous Cu-BDC nanosheet
is confined in the macropore of the macroporous-mesoporous-TiO , while the ordered
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mesoporous structure is in the macroporous walls. This multi-level porous distribution
can significantly improve the active surface areas and mass transfer efficiency of Cu-
BDC@macro-meso-TiO . Therefore, this finding has opened a field of research on
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photocatalytic chemical CO fixation.
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KEYWORDS
：
Cu-BDC nanosheet, macroporous-mesoporous-TiO2, CO2,
carboxylation, ultraviolet light
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. INTRODUCTION
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With the rapid development of industrialization, the emission of carbon dioxide
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has been continually increasing,3
(CO ), the major component of greenhouse gases,
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and this issue inevitably poses a serious threat to the environment. Therefore, how to
effectively promote the capture and conversion of CO has become the focus of green
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chemistry in recent years.5,6 Meanwhile, CO is also a ubiquitous, low-costing and
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renewable C1 resource,7,8 and the catalytic transformation of CO into high-value fine
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chemicals can be regarded as a promising strategy to utilize CO2.9,10 However, the
inherent thermodynamic stability and kinetic inertness hinder the chemical synthesis of
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12-
CO , because CO is located in the most stable valence state in carbon derivatives.
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At present, chemical fixation of CO is limited by the strict reaction conditions, for
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instance, the sensitive reagent of noncatalytic processes,15,16 high-costing noble metal
catalysts and organic ligands,17 long reaction time,18 or extremely high pressure.19
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Accordingly, an inexpensive, benign and environment-friendly strategy for chemical
20,21
fixation of CO is imperiously desirable for the capture and conversion of CO .
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Recently, metal-organic frameworks (MOF), a sort of coordination polymers formed
by self-assembly of transition metal ions and organic ligands, have aroused much
attention in the field of chemical fixation of CO by the virtue of isolated catalytic sites,
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large pore volume, ultrahigh surface areas, great reusability, etc.22-25 Compared with the
traditional catalysts, the MOF materials exhibit an high absorption ability and abundant
catalytic sites for CO2.26,27 Therefore, the MOF is the ideal catalyst for the high-
efficiency chemical fixation of CO . More recently, photocatalysis reduction of CO has
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motivated much interest, since the reaction process is renewable and environmental-
friendly.28,29 Titanium dioxide (TiO ) is the main catalyst in the field of photocatalysis
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relying on the outstanding photochemical activity, good chemical stability and controlled
morphology,30,31 and however, the poor absorption capability and photo-generated
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carriers easy to recombine hinder the further application in the fixation of CO . In order
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to combine the strengths of MOF and TiO in the field of chemical fixation of CO , the
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TiO /MOF composites offer the opportunity.
In 2014, Fareed and Shigeyoshi
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reported the Ni-catalyzed carboxylation of the C(sp )-Cl bond with CO in the presence
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of MgCl , and MgCl played a crucial part in stabilization of a Ni -CO adduct. Barbara
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et al reported a simple preparation strategy of a hybrid catalyst, namely TiO /HKUST-1,
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and the composite TiO /HKUST-1 containing TiO anatase nanoparticles (NPs) and
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HKUST-1 was used as a progressive material for reducing CO to CH under solar light
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irradiation. However, the photocatalysis of organic reaction with CO using TiO /MOF
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composites as catalysts is rarely reported.
Herein, we synthesized the three-scale porous Cu-BDC nanosheet@macroporous-
mesoporous-TiO2 composite (Cu-BDC@macro-meso-TiO ), which the Cu-BDC
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nanosheets were successfully composited with macroporous-mesoporous-TiO . The
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macroporous, mesoporous and microporous structures were integrated in one
framework, and furthermore, all of them were highly ordered. The mesoporous structure
was in the macroporous walls, whereas the microporous Cu-BDC nanosheet was
confined in the macropore of the macroporous-mesoporous-TiO (macro-meso-TiO ).
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The Cu-BDC@macro-meso-TiO can be applied to catalyze the organic reaction of CO
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with benzyl halogen for chemical CO fixation under ambient conditions with the
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irradiation of ultraviolet (UV) light. Compared to the previous work (Scheme S1)37-39, Cu-
BDC@macro-meso-TiO showed high photocatalytic activity for both benzyl chloride
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and benzyl bromide reacting with CO and the reaction condition was more convenient.
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This finding has opened up a whole new field of research on photocatalysis chemical
fixation of CO2.
2. RESULTS AND DISCUSSION
Figure 1a illustrated the formation of Cu-BDC@macro-meso-TiO . The metal ions
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(Cu ) and ligand (terephthalic acid) were dissolved into ethanol to form a
homogeneous solution under stirring. During this process, the macro-meso-TiO was
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used as a template, and then the Cu-BDC nanosheet was growing in the macroporous
structure of macro-meso-TiO to form the Cu-BDC@macro-meso-TiO . The morphology
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of the above used PS microspheres can be observed from the scanning electron
microscopy (SEM) image (Figure S1), and the average diameter of PS microspheres
were 455 nm. The macro-meso-TiO was shown in Figure S2 and the well-ordered
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macroporous size was 400 nm (Figure S2a), which was a little smaller than that of PS
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microspheres due to the shrinkage in the calcination process to remove the PS
microspheres. The transmission electron microscopy (TEM) image of macro-meso-TiO2
presented the well-organized mesoporous structure on the macroporous walls (Figure
S2b), the mesoporous diameter was 5 nm. The morphological feature and structure of
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the Cu-BDC@macro-meso-TiO were also showed in Figure 1. In Figure 1b, SEM
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image showed the highly regular macropores assigned to a classic face centered cubic
(fcc) close-packed arrangement and the macroporous diameter was 400 nm, which was
consistent with the as-synthesized macro-meso-TiO . The high-magnification SEM
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(HRSEM, Figure 1c) further demonstrated the interconnected macroporous structure,
and the Cu-BDC nanosheets were in the macropores. While, for as-prepared Cu-
BDC@pure TiO , the SEM image (Figure S3) presented Cu-BDC and pure TiO grew
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sepatately. As the TEM image showed (Figure 1d), the Cu-BDC nanosheets were
almost filled with the macropores, strongly indicating the microporous Cu-BDC
nanosheets were successfully loaded in the macropores of macro-meso-TiO . The
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mesoporous diameter was 5.1 nm, which was well match with the macro-meso-TiO2,
while the thickness of Cu-BDC nanosheet was 3.6 nm, which was much less than the
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pure Cu-BDC (5 μm, Figure S4). The Cu-BDC nanosheet can be further confirmed and
the thickness distributions can be obtained from the atomic force microscopy (AFM) of
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Cu-BDC@macro-meso-TiO (Figure S5), and the thickness of Cu-BDC nanosheet was
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2-3 nm. Futhermore, it was worth noting that the confined effect of macro-meso-TiO2
played a vital role in the formation of Cu-BDC nanosheets. The high degree of
crystallinity of Cu-BDC@macro-meso-TiO was indicated by the high-resolution TEM
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(HRTEM) image (Figure 1e). The fringe spacings of 0.35 nm and 0.24 nm were
corresponding to the (101) and the (103) crystal planes of TiO , while the fringe
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spacings of 0.26 nm was attributed to the crystal Cu-BDC. The crystalline structure can
be also confirmed by polycrystalline diffraction rings in the selected area electron
diffraction (SAED) pattern (inset of Figure 1e). The elemental mapping images (Figure
1f) exhibited the uniform distribution of Ti, O, Cu elements throughout the whole
macroporous wall of Cu-BDC@macro-meso-TiO , confirming that the Cu-BDC was
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well-uniformly loaded on the macro-meso-TiO and the corresponding ratio of Ti and Cu
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was about 1.8 :1 according to the EDX analysis (Table S1).
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Figure 1. (a) The schematic illustration about the synthetic process of Cu-BDC@macro-
meso-TiO . (b) SEM image of the prepared Cu-BDC@macro-meso-TiO . (c) High-
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of the prepared FeCoNi-MOF@TiO2 (e) HRTEM image of the prepared Cu-
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BDC@macro-meso-TiO (the selective area electron diffraction pattern was shown in
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the inset). (f) EDX element mappings of the prepared Cu-BDC@macro-meso-TiO2.
The highly ordered multistage pore structure and the high degree of crystallinity of
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Cu-BDC@macro-meso-TiO were further analyzed by the small-angle X-ray diffraction
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(SAXRD) and wide-angle X-ray diffraction (WAXRD). The SAXRD plot of Cu-
BDC@macro-meso-TiO (Figure 2a) certified the representative hexagonal (p6mm)
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mesophase from the main peak at 2θ = 1.04° and other small peaks at 2θ = 1.78° and
2.08°, which can be classified as (100), (110) and (200) planes, respectively, and the
relevant d spacing was 8.4 nm, which was consistent with the TEM result. The SAXRD
plot of macro-meso-TiO (Figure S6) showed the same pattern as Cu-BDC@macro-
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meso-TiO . Thus the highly ordered pore structure of both macro-meso-TiO (Figure
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S6) and Cu-BDC@macro-meso-TiO (Figure 2a) can be confirmed. The WAXRD
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patterns of the simulated Cu-BDC, pure Cu-BDC, macro-meso-TiO , and Cu-
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BDC@macro-meso-TiO were displayed in Figure 2b. The diffraction peaks of Cu-
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BDC@macro-meso-TiO (black line) are distributed in the 2θ = 10.32°, 12.24°, 17.08°,
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25.36°, 37.96°, 48.10°, 54.39°, 62.72°, 69.81°, 75.31° and 82.69°. Among these peaks,
2θ = 10.32°, 12.24° and 17.08° were agreed well with pure Cu-BDC (green line) and
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simulated Cu-BDC (red line), and the 10.32° and 17.08° were accorded with the (110)
40
and (20-1) crystallographic planes, respectively, which confirmed the high crystallinity
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of pure Cu-BDC nanosheet. The rest of peaks were assigned into the anatase TiO2
(JCPDS card No. 21-1272) and corresponded to the (101), (004), (200), (105), (204),
(
220), (215) and (224) planes of anatase TiO , which coincided with the WAXRD pattern
2
of macro-meso-TiO (blue line). In the WAXRD image of the prepared Cu-BDC@pure
2
TiO (Figure S7), and both the diffraction peaks of Cu-BDC and pure TiO were
2
2
observed. Furthermore, the diffraction peaks of Cu-BDC in the WAXRD pattern of Cu-
BDC@macro-meso-TiO were obviously broader than that of pure Cu-BDC and
2
simulated Cu-BDC, which further confirmed that the Cu-BDC nanosheet was obtained.
Figure 2. (a) SAXRD curve of the prepared ordered Cu-BDC@macro-meso-TiO . (b)
2
WAXRD curves of simulated Cu-BDC, pure Cu-BDC, macro-meso-TiO and Cu-
2
BDC@macro-meso-TiO2.
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As expected, the Fourier-transform infrared spectroscopy (FTIR) spectrum also
characterized the main building block of the Cu-BDC@macro-meso-TiO (Figure S8),
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which was consistent with the previous result of XRD. The bands at 935 and 3600-2500
-1
cm were assigned to the bending and stretching frequencies of the O-H group of
-1
-1
ethanol or the water. The peaks at 1686 cm and 1575 cm were attributed to the
symmetric and asymmetric stretching vibrations of the carboxylate groups in Cu-BDC
-1
nanosheets. The band at 1510 cm was the stretching vibrations of benzene and the
-1 41
characteristic stretch peak of Cu-O appeared at 710 cm . Apart from the peaks of Cu-
-1
42
BDC, other peaks at 1140, 1114, and 1014 cm were belonged to the Ti-O-C bond,
-1
and the weak absorption peak at about 454 cm was assigned to the typical vibration of
Ti-O-Ti in macro-meso-TiO2.43
In order to investigate deeply the pore structure of Cu-BDC@macro-meso-TiO , the
2
N adsorption-desorption isotherms were carried out at 77 k (Figure 3). For the Cu-
2
BDC@macro-meso-TiO2 (Figure 3a), the N2 adsorption-desorption curve of Cu-
BDC@macro-meso-TiO was cited as a hybrid of type I and type IV isotherm according
2
to the IUPAC classification.44 The isotherm curve showed the steep trend at low
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pressure implying the feature of micropores and the light rise and the hysteresis loop at
middle relative pressure implying the feature of mesopores.45 The surface area of the
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Brunauer−Emmet−Teller (BET) model was calculated as 420.7 m /g. The
Barrett−Joyner−Halenda (BJH) and nonlocal density functional theory (NLDFT)
methods were applied to depict pore size dispersion of Cu-BDC@macro-meso-TiO2
(Figure 3b). As can be seen, there were a strong peak at 0.67 nm and a broad peak at 5
nm from the pore size distribution based on the NLDFT model. The pore size of 5 nm
was also clearly observed in the inset of Figure 3b based on the BJH model. The N2
adsorption-desorption isotherm of macro-meso-TiO was exhibited in Figure 3c. The
2
isotherm curve for macro-meso-TiO appeared a typical Ⅳ isotherm with a hysteresis
2
loop in the relative scope of 0.4-0.9. The pore diameter of 5.0 nm was presented in the
pore size distribution based on the BJH method (Figure 3d) and the BET surface area
2
was only 272.6 m /g, implying that the addition of Cu-BDC was beneficial to increase
the BET surface area of macro-meso-TiO . Based on above the experimental result, the
2
highly ordered macroporous, mesoporous and microporous structures were integrated
in the Cu-BDC@macro-meso-TiO2.
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Figure 3. (a) N adsorption-desorption isotherm and (b) pore size distribution analysis
2
based on the NLDFT method for Cu-BDC@macro-meso-TiO (the inset in (b) was the
2
corresponding BJH pattern). (c) N adsorption-desorption isotherm and (d) pore size
2
distribution analysis based on the BJH method for macro-meso-TiO2.
The CO2 adsorption performance of Cu-BDC@macro-meso-TiO2 was also
investigated at 273 K up to a pressure of 1 atm (Figure S9a). Comparing with the pure
Cu-BDC, the Cu-BDC@macro-meso-TiO emerged the higher CO adsorption capacity
2
2
3
of 13.6 cm /g. It can be further calculated that the CO adsorption amount of Cu-BDC in
2
3
the composite Cu-BDC@macro-meso-TiO was 34.9 cm /g according to the EDX
2
3
analysis (Table S1), which was three times more than pure Cu-BDC (11.2 cm /g).
Remarkably, the apparent difference in the uptake amount for CO was mainly
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attributed to the single-layer nanosheet structure of Cu-BDC in the composite Cu-
BDC@macro-meso-TiO , which greatly facilitated the uptake capacity for CO , while the
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morphology was stacked nanosheet structure in pure Cu-BDC. Undoubtedly, the high
stability of Cu-BDC@macro-meso-TiO with high CO adsorption capacity performance
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offered a promising candidate for the chemical fixation of CO . Additionally, the
2
interaction between Cu-BDC@macro-meso-TiO and CO was explored by in situ
2
2
diffuse reflectance infrared Fourier transform (DRIFT) characterization at the room
temperature (Figure S9b). During the process of CO adsorption, the Cu-BDC@macro-
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-1
meso-TiO emerged an intense peak at 2337 cm (black line), which was belong to the
2
ν3 mode of the CO asymmetric stretching. Compared with the ν3 mode frequency in
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-1
the gas phase (2349 cm ), the spectrum of CO adsorption showed an obvious red shift
2
resulting from the formation of H-bonds between CO and the -OH groups of Cu-
2
BDC@macro-meso-TiO2.46 The other bands at the range of 1800-1330 cm-1 were
47
ascribed to the interaction between CO and -OH groups. When the DRIFT spectra
2
-1
were conducted with Ar purging instead, the distinct peak at 2337 cm decreased over
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time. Whereas, the intensities of peaks among the 1800-1300 cm did not slowed down,
suggesting a strong interaction between CO and Cu-BDC@macro-meso-TiO .
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For further studying the elemental state in the surface of Cu-BDC@macro-meso-
TiO , X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical
2
composition and valence state. For macro-meso-TiO , the elements of Ti, O and C
2
appeared in the wide-scan XPS spectrum (Figure S10a), and the Ti 2p spectrum
(
Figure S10b) showed the specific peaks of Ti 2p3/2 and Ti 2p1/2 occurred at 464.3 and
458.6 eV, respectively, indicating that the existence of Ti4+ in the macro-meso-TiO2.48
For pure Cu-BDC, the elements of Cu, O and C emerged in the wide-scan XPS
spectrum (Figure S10c), and the main spectrum occurred at 954.5, 934.6 and 943.5 eV
(
Figure S10d), which assigned to the Cu 2p , Cu 2p , and typical satellite,
1/2
3/2
2+ 49
respectively, indicating the oxidation state of Cu in pure Cu-BDC was Cu . In order to
investigate whether the elements state changed after the combination of macro-meso-
TiO and Cu-BDC, we further performed XPS analysis on the Cu-BDC@macro-meso-
2
TiO . The presence of Ti, O, Cu elements were obtained in the wide-scan XPS
2
spectrum of the Cu-BDC@macro-meso-TiO (Figure 4a), indicating that the Cu-BDC
2
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was successfully composited with macro-meso-TiO . The values of Ti 2p and Ti 2p1/2
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were separately lied at 464.9 and 459.3 eV (Figure 4b),50 suggesting the existence of
Ti4+ in Cu-BDC@macro-meso-TiO2.51 The XPS profile of O 1s emerged two peaks
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Figure 4c), which were belong to the characteristic peaks of O-H (531.3 eV) linkage51
(
and the bonding of oxygen atoms and metals (528.5 eV).52,53 The Cu 2p photoelectron
peak (Figure 4d) showed two main peaks at 933.2 and 953.2 eV for Cu 2p3/2 and Cu
2
p , respectively, and an obvious satellite peak at 941.5 eV, which was assigned to the
1/2
Cu2+ oxidation state in the Cu-BDC@macro-meso-TiO .54 Compared with the XPS
2
spectra of pure material before recombination, the main peaks of Ti4+ and Cu2+ in the
composite Cu-BDC@macro-meso-TiO2 slightly shifts, indicating that Cu-BDC
successfully grew on the macro-meso-TiO2.
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Figure 4. (a) Wide-scan XPS spectrum of the prepared Cu-BDC@macro-meso-TiO . (b)
2
Ti 2p XPS spectrum of the prepared Cu-BDC@macro-meso-TiO . (c) O 1s XPS
2
spectrum of the prepared Cu-BDC@macro-meso-TiO . (d) Cu 2p XPS spectrum of the
2
prepared Cu-BDC@macro-meso-TiO2.
To learn about the thermodynamic stability of Cu-BDC@macro-meso-TiO , the
2
thermal gravimetric analyses (TGA) of the pure Cu-BDC and Cu-BDC@macro-meso-
TiO was measured. As shown in Figure 5a, the sharp degradation of pure Cu-BDC
2
arised at the stage of 300 - 360 ℃ and the slight degradation produced at the 360 - 500
℃ , and the total wight loss of pure Cu-BDC achieved 48 %, while the main
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decomposition trace of Cu-BDC@macro-meso-TiO arised at 300 - 450 ℃ and the total
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improvement of thermal stability of the Cu-BDC. The UV-vis spectra of the Cu-
BDC@macro-meso-TiO , macro-meso-TiO and Cu-BDC were all analyzed. Both the
2
2
macro-meso-TiO and Cu-BDC@macro-meso-TiO had a significant absorption at 323
2
2
nm (Figure 5b) while the pure Cu-BDC showed no absorption at 323 nm (Figure S11),
declaring the Cu-BDC of Cu-BDC@macro-meso-TiO had no effect on the absorption
2
region of macro-meso-TiO and providing a useful guidance on the wavelength of UV
2
light for the following the catalytic reaction. The UV-vis spectra of the prepared Cu-
BDC@pure TiO was also measured in Figure S12, and Cu-BDC had no effect on the
2
UV absorption of TiO2.
Figure 5. (a) TGA curves of the prepared Cu-BDC@macro-meso-TiO and pure Cu-
2
BDC. (b) UV-vis spectras of the Cu-BDC@macro-meso-TiO and macro-meso-TiO .
2
2
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Catalytic Reaction
In order to study the photocatalytic activity of the Cu-BDC@macro-meso-TiO for
2
chemical fixation of CO , the carboxylation of benzyl halide with CO catalyzed by the
2
2
Cu-BDC@macro-meso-TiO was investigated at atmospheric pressure under the UV
2
light (365 nm). Considering that the ratio of macro-meso-TiO and Cu-BDC of the
2
composite Cu-BDC@macro-meso-TiO2 may affect the catalytic activity on the
carboxylation of benzyl halide with CO , the different ratios were explored when the
2
carboxylation of benzyl chloride 1a with CO was chosen as the model substrate. The
2
maximum yield of 2a was obtained from the molar ratio of macro-meso-TiO and Cu-
2
BDC was 1:1 (Table S2).
The carboxylation of benzyl chloride 1a with CO was also chosen as model
2
substrate to optimize the reaction conditions including additive agents and bases. As
shown in the Table S3, CsF, Cs CO , K CO and Na CO were used as bases (entry 1-
2
3
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), and CsF showed the best yield of 72% among four bases. As for additive agents
serving as the co-catalyst, the yield of TBAI (entry 1, 72 %) was better than TBAB (entry
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6, 60 %). From the entry 5 and 7, the base and additive agent were essential for the
carboxylation of benzyl halide with CO . It was noted that the reaction did not work in
2
the absence of UV light (entry 8, 0%), indicating this reaction was photocatalytical
process.
Based on the above optimized conditions, the feasibility and substrate scope for the
carboxylation of benzyl halide with CO were analyzed using the Cu-BDC@macro-
2
meso-TiO as catalyst. In the case of the benzyl chloride (Table 1), the catalytical
2
reaction worked well for both the electron-donating (1b−e) and electron-withdrawing (1f−
h) functional groups of benzyl chloride, the target products 2 of carboxylic acid ester
were obtained in good isolated yields (57 % - 82%). In particular, the isolated yield of p-
tert-butylbenzyl formate 2e was up to 82 %. Moreover, the position of electron-donating
and electron-withdrawing functional groups, whether ortho- (1b, 1f), meta- (1c, 1g) or
para-position (1d, 1h) on the chloromethyl phenyl did not affect the yield obviously and
fitted well with this reaction. It was worth mentioning that the benzyl bromide was also
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obtained at the good yield under the same reaction condition whether the benzyl
bromide species carried an electron-withdrawing group or an electron-donating group
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(Table 1, 58 % - 83 %). Differently, the highest yield came out at o-fluorobenzyl formate
of 83 % (entry 14), and the yields of benzyl bromide bearing electron-withdrawing group
were slightly more than that of benzyl chloride with electron-withdrawing group.
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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
Table 1. Carboxylation of benzyl chloride with different functional groups a
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
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Yield of 3 b
Entry
1
1
2
Cl
Cl
Cl
O
O
72%
1a
2a
O
O
O
Me
Me
2
3
4
5
6
7
64%
57%
74%
82%
70%
64%
1b
2b
O
Me
Me
1c
2c
2d
Cl
O
O
Me
Me
1
d
e
Cl
O
O
Me
Me
Me
Me
Me
Me
1
2e
Cl
Cl
O
O
O
F
F
1f
2f
O
F
F
1
g
h
2g
2h
Cl
O
O
F
F
8
9
76%
68%
1
Br
O
O
1i
2a
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Br
Me
O
O
O
Me
10
71%
58%
75%
80%
83%
78%
72%
1j
2b
Br
O
11
12
13
14
15
16
Me
Me
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
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1k
2c
2d
Br
O
O
Me
Me
1l
Br
O
O
Me
Me
Me
Me
Me
Me
1
m
2e
Br
Br
O
O
O
F
F
1
n
2f
O
F
F
1
1
o
2g
2h
Br
O
O
F
F
p
a
General condition: benzyl halide (1 mmol), Cat. (Cu-BDC@macro-meso-TiO , 50 mg),
2
Mn powder (1 equiv), CsF (1 equiv), TBAI (1 equiv), CO (balloon), DMF (5 mL), UV
2
b
lamp (365 nm). Isolated yield.
For the purpose of looking into the catalytical mechanism, Cu-BDC@macro-meso-
TiO , Cu-BDC, macro-meso-TiO , Cu(NO ) and CuCl were used as the control
2
2
3 2
catalysts (Table S4), and the carboxylation of benzyl chloride 1a with CO was also
2
used to evaluate the catalytic performance at the mild condition of atmospheric pressure
under the UV light (365 nm). The yield of Cu-BDC@macro-meso-TiO (72%) was much
2
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2
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8
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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
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4
4
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4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
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more than the other control catalysts and almost twice than that of Cu-BDC (39%),
whereas the macro-meso-TiO (entry 3) and Cu(NO ) (entry 4) were no yield at all,
2
3 2
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
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
indicating that the absorption process of CO occurred in the Cu-BDC and the Cu-BDC
2
was the active site. The Cu-BDC@pure TiO was also used as the control catalyst
2
(
entry 7) and the low yield explained that hierarchical structure of macro-meso-TiO was
2
much more conducive to electronic transfer than pure TiO . In entry 5, the CuCl showed
2
a yield of 36 %, which provided a favorable guidance on the reaction mechanism. As
shown in the Table S5, no CO reduced product was formed without the benzyl halide in
2
the reaction, suggesting that the CO was not directly reduced by the loaded Cu-BDC
2
and the intermidiate containing 1a contributed to active CO in the reaction. The above
2
results indicated that the electrons of macro-meso-TiO were excited and easily
2
transferred to the loaded Cu-BDC after absorbing the UV light, and the Cu-BDC was the
catalytical active site. Based on the control experiments, the possible mechanism was
displayed in Scheme 1. In the atmosphere of carbon dioxide (CO ), CO will be firstly
2
2
adsorbed onto the Cu-BDC. Then the electrons of macro-meso-TiO were excited under
2
the irradiation of UV and transferred to the Cu-BDC to reduce the Cu2+ to obtain the
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+
Cu . The complex A was formed through the oxidative addition between the benzyl
chloride (1a) and the reduced catalyst. Stimulated by the instability of the complex A
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
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-
and the solvation of the DMF, then the Cl of the complex A was replaced by CO
2
absorbed by Cu-BDC to form the intermediate B. The product 2a was liberated through
the reductive elimination of the intermediate B.
Scheme 1. Possible mechanism for the carboxylation of benzyl halide with CO2.
In order to verify the existence of the photoinduced charge-tranfer of Cu-
BDC@macro-meso-TiO mentioned in the mechanism, the photocurrent response
2
(
Figure S13a) under UV light was also to investigate the excitation and transfer of
photoinduced charge carriers of Cu-BDC@macro-meso-TiO . The stable photocurrent
2
of Cu-BDC@macro-meso-TiO could be obviously observed under the irradiation of UV
2
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8
9
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
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4
4
4
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5
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light, which revealed that the UV light facilitated the separation of electron-hole pairs,
and photogenerated electrons were the key factor for the chemical fixation of CO with
2
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
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
benzyl halides. Combined with the electrochemical impedance spectroscopy (EIS) of
Cu-BDC@macro-meso-TiO (Figure S13b), the semicircle with UV light was smaller than
2
that of without UV in the Nyquist plots, which suggested that the smaller charge-transfer
resistance was beneficial to the transport and separation of photogenerated electron-hole pairs55
of Cu-BDC@macro-meso-TiO under the irradiation of UV light and further confirmed the
2
importance of UV light in the photocatalytic chemical CO2 fixation. The
photoluminescence emission spectrum (PL) of macro-meso-TiO and Cu-BDC@macro-
2
meso-TiO further confirmed the electron transfer from macro-meso-TiO to Cu-BDC
2
2
nanosheets in Figure S14a. Both the PL spectra were excited by wavelength of 320 nm
according to the UV-vis spectra. The macro-meso-TiO showed obvious emission peak
2
at 370 nm, while the emission peak was not observed in Cu-BDC@macro-meso-TiO2,
indicating that the electron on the excited state of macro-meso-TiO transferred to the
2
loaded Cu-BDC after absorbing UV. Moreover, the fitted time-resolved PL decay curves
of Cu-BDC@macro-meso-TiO and macro-meso-TiO presented the different trends
2
2
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(
Figure S14b), and the lifetime of macro-meso-TiO (2.09 ns) was obviously longer than
2
that of Cu-BDC@macro-meso-TiO (1.67 ns), which further confirmed the electron
2
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
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
transfer between macro-meso-TiO and Cu-BDC.
2
The validity of the mechanism circle in the scheme 1 was deeply assessed by the
FTIR spectra and DFT calculations. As the Figure 6 showed, the FTIR spectra of benzyl
chloride (1a), the mixture of benzyl chloride and Cu-BDC@macro-meso-TiO catalyst
2
(cat.), and the mixture of benzyl chloride, Cu-BDC@macro-meso-TiO catalyst, Mn
2
power, CsF and TBAI, were measured. The infrared absorption at 1265 cm−1 was
56
attributed to the chloride functionality (CH -Cl) stretched vibration. It was found that
2
the red shift of CH -Cl functional group appeared when the Cu-BDC@macro-meso-TiO
2
2
was added (red line), implying the coordination of benzyl chloride with Cu-BDC@macro-
meso-TiO . The FTIR spectrum of the reaction mixture (blue line) before the UV
2
radiation showed the same shift as that of mixture of benzyl chloride and Cu-
BDC@macro-meso-TiO , indicating that the additives in the reaction had no effect on
2
the coordination process of benzyl chloride with Cu-BDC@macro-meso-TiO . Then the
2
potential energy profiles for each step in the mechanism circle including 1a→A, A’→B’
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