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Bi2MoO6 composite as efficient photocatalyst for aerobic oxidation of amines to imines
Journal Pre-proofs Full Length Article Fabrication of Mo2C-QDs/C/Bi2MoO6 composite as efficient photocatalyst for aerobic oxidation of amines to imines Ling-Hu Meng, Xi-Ping Tan, Lang Chen, Sheng Shen, Jun-Kang Guo, ChakTong Au, Shuang-Feng Yin PII: DOI: Reference:
S0169-4332(20)33234-7 https://doi.org/10.1016/j.apsusc.2020.148476 APSUSC 148476
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
6 September 2020 3 November 2020 12 November 2020
Please cite this article as: L-H. Meng, X-P. Tan, L. Chen, S. Shen, J-K. Guo, C-T. Au, S-F. Yin, Fabrication of Mo2C-QDs/C/Bi2MoO6 composite as efficient photocatalyst for aerobic oxidation of amines to imines, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.148476
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Fabrication of Mo2C-QDs/C/Bi2MoO6 composite as efficient photocatalyst for aerobic oxidation of amines to imines Ling-Hu Menga,†, Xi-Ping Tana,†, Lang Chen*a, Sheng Shen*a, Jun-Kang Guoa, ChakTong Aub, and Shuang-Feng Yin*a a
State Key Laboratory of Chemo/Biosensing and Chemometrics, Provincial Hunan
Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions, Advanced Catalytic Engineering Research Center of the Ministry of Education, College of Chemistry and Chemical Engineering, Hunan University, Lushan South Road, Changsha 410082, Hunan, China b
College of Chemical Engineering, Fuzhou University, Fuzhou, 350002, Fujian,
China. † The
authors contributed equally.
* E-mail: [email protected] (L. Chen), [email protected] (S. Shen), [email protected] (S.F. Yin)
Abstract: As an efficient but mild oxidation species, O2•– generated from surface O2 and photo-excited electrons plays a key role in photocatalytic reactions. To improve photocatalytic activity, it is essential to facilitate the separation and transfer efficiency of photogenerated charge carriers. In this work, Mo2C-QDs/C, an alternative of noble metals, was used as an efficient and economical cocatalyst. It was added to Bi2MoO6 nanosheet to generate Mo2C-QDs/C/Bi2MoO6 (MCBMO) composites by a facile hydrothermal method. Under visible light irradiation, the carbon layer and the Mo2CQDs acts as electronic conductor and electron trapping agent to promote the transfer of photogenerated electrons for enhanced generation of O2•–. The MCBMO composites show boosted photocatalytic performance in the oxidative coupling of benzylamine to N-benzylbenzaldimine with excellent recyclability. Under the optimized conditions, benzylamine conversion over the optimized MCBMO-3 composite is up to 92% whereas that over pristine Bi2MoO6 is limited to 53%. The MCBMO-3 composite even outperforms Ag/Bi2MoO6 and Pt/Bi2MoO6. Based on the results of ESR and active species trapping studies, it is confirmed that O2•– and h+ are indispensable in the photocatalytic reaction, and a possible reaction mechanism has been proposed. Keywords: Mo2C quantum dots; Bi2MoO6; photocatalysis; oxidative coupling of amine; imine
1 Introduction Imines are versatile organic intermediates in organic synthesis, which are commonly used as building blocks for many pharmaceutical and organic-dye molecules [1]. Traditionally, imines are prepared through conventional oxidation of amines, which have disadvantages such as the need of noble-metal catalysts as well as the use of oxidants that are adverse to the environment [2, 3]. As clean and sustainable energy source, sunlight is widely utilized in solar-cell technology and in the field of photocatalysis [4-6]. An emerging way of sunlight utilization is organic synthesis based on photocatalysis over semiconductor materials. For example, the oxidation of amines to imines can be photocatalytically conducted under green and mild conditions with high selectivity [7]. As a highly active but mild oxidizing species, O2•– is regarded as important active species in the photocatalytic oxidation of amines to imines [8]. It is generated through the combination of a surface O2 molecule with a photo-excited electron that has migrated to the surface from the bulk. To improve the generation of O2•–, strategies have been developed to separate the photogenerated charge carriers, and to promote the transfer of excited electrons and holes to the surface of catalysts [9]. The introduction of a co-catalyst is one of the most effective methods for achieving high performance. Commonly, noble metals such as Pt, Pd and Au are excellent cocatalysts but the cost of using them is prohibitively high [10]. Hence it is urgent to develop a non-noble metal-based co-catalyst that is with comparative activity.
Mo2C is an important member of transition metal carbides (TMCs). With electronic structure similar to that of Pt [11], it has been regarded as a promising candidate to replace noble metals. Mo2C has been studied as a substitute for noble metals in hydrogenation, hydrodesulfurization, and hydrodenitrogenation reactions as well as in electrochemical applications [12-15]. As far as we know Mo2C has not been reported as a co-catalyst in the photocatalytic oxidation of amines to imines with O2 being used as oxidant. Recently, semiconductors such as TiO2, Nb2O5, MOF, WO3, C3N4, HNb3O8 or their composites have been reported as photocatalysts to prepare imines from amines [1624]. However, as a result of poor light absorption and serious recombination of charge carriers, their catalytic activity under visible light is unsatisfactory. Lately, in view of its suitable band position and visible-light response, bismuth molybdate (Bi2MoO6) has been studied for the partial oxidation of hydrocarbons into value-added organics [25-26]. Cheng’s group demonstrated that Pt/Bi2MoO6 was highly efficient for the photocatalytic selective oxidation of ethylbenzene [27]. Furthermore, ultrathin Bi2MoO6, Bi-self-doped Bi2MoO6-Bi2Mo3O12 and TiO2/Bi2MoO6 were found to show excellent selectivity as well as recyclability for the selective oxidation of aromatic alkanes under visible light irradiation [28-30]. Moreover, over carbon-dots-Bi2MoO6 Srivastava’s group observed good yield of N-benzylbenzaldimine from benzylamine [31]. It was pointed out that O2•– is indispensable in the partial oxidation of toluene, which is a reaction similar to the oxidative coupling of amine to imine. Based on the
above background, it is envisaged that Bi2MoO6 is a potential photocatalyst for the production of imines from amines. In the present study, we uniformly loaded a carbon layer 10‒15 nm in thickness with Mo2C quantum dots (denoted herein as Mo2C-QDs/C co-catalyst) by high temperature treatment. Then, Mo2C-QDs/C was combined with Bi2MoO6 nanosheets to form Mo2C-QDs/C/Bi2MoO6 (MCBMO) composites through hydrothermal method. The physicochemical properties of the materials were characterized by techniques such as XRD, SEM, TEM, and XPS. We evaluated the MCBMO composites for the selective oxidation of benzylamine as well as its derivatives under visible light irradiation. As an efficient electronic conductor, Mo2C-QDs/C not only accelerates the transfer of excited electrons from the conduction band of Bi2MoO6 to the catalyst surface for the generation of O2•–, but also efficiently separates the e--h+ pairs of Bi2MoO6 as a result.
2 Experimental section 2.1 Preparation All reagents were commercially available and used without further purification. The preparation of MCBMO was divided into two steps: First, synthesis of Mo2CQDs/C, and second combination of Mo2C-QDs/C with Bi2MoO6. 2.2 Synthesis of Mo2C-QDs/C MoO3 nanorods as precursor were prepared according to the method reported by Chen et al [32]. Typically, 2.8 g of (NH4)6Mo7O24·4H2O was dissolved in 80 mL of a
mixed solution of 65% HNO3 and deionized H2O in 1:5 volume ratio. Then the mixture was transferred into a 100 mL Teflon-lined autoclave and kept at 200 °C for 20 h. After cooling, the product was collected by centrifugation and washed with water and ethanol for several times before overnight drying at 80 °C. Then, 150 mg of the obtained MoO3 nanorods (proven by SEM and XRD analyses, Figs. S1a and S2a of Electronic Supplementary Information, ESIƗ) was added into 30 mL of deionized H2O in a 100 mL glass bottle. After the mixture was subject to ultrasonic treatment for 30 min, 25 mg of dopamine hydrochloride and 40 mL of absolute ethanol were added. The resulted solution was stirred for 5 min, followed by fast injection of 0.3 mL of aqueous NH3 (30 wt%). After reaction for 2.5 h under gentle stirring at room temperature, the precipitate was collected by centrifugation and washed with ethanol and dried at 30 °C in a vacuum oven overnight to get Mo-polydopamine nanosheets (proven by SEM and XRD analyses, Figs. S1b and S2b, ESIƗ). Finally, the assynthesized Mo-polydopamine precursor was heated to 750 °C with a ramp rate of 5 °C min‒1 and annealed at this temperature under Ar for 16 h to obtain Mo2C-QDs/C (Fig S2c, ESIƗ). 2.3 Synthesis of MCBMO composites Mo2C-QDs/C (1 mg) was added to 80 mL of deionized water and subject to ultrasonication for 3 h. Then 2 mmol Bi(NO3)3·5H2O was added and the mixture was stirred for 30 min, followed by the addition of 1 mmol Na2MoO4·2H2O and 0.15 g of hexadecyl trimethyl ammonium bromide (CTAB). The as-resulted mixture was stirred for 30 min, and transferred into a 100 mL Teflon-lined autoclave and kept at 180 °C
for 12 h. The obtained precipitation was thoroughly washed with deionized water, ethanol, and dried at 80 °C overnight to obtain Mo2C-QDs/C/Bi2MoO6 (denoted hereinafter as MCBMO). The samples with different amounts of Mo2C-QDs/C (10, 20, 30, 40 and 50 mg) are labelled as MCBMO-1, MCBMO-2, MCBMO-3, MCBMO-4 and MCBMO-5, respectively. The surface content of Mo2C-QDs/C in the MCBMO-3 was estimated using the XPS results as showed in Table S1 (ESIƗ). That the calculated content of Mo2C-QDs/C is slightly higher than that of the nominal value could be attributed to the fact that XPS is a surface sensitive analytical technique. 2.4 Characterization A Bruker D8 Advance X-ray diffractometer was used to characterize the crystal phase of samples (monochromatized Cu Kα radiation, λ=0.154 06 nm). The morphology and microstructure of samples were studied over a Hitachi S-4800 scanning electron microscope and a JEM-2100F transmission electron microscope. Xray photoelectron spectroscopic data of Bi, Mo, C, and O as well as the XPS valance band of prepared samples were acquired with a VG Multilab 2000 instrument. Light absorption capacity was studied using Cary-100 spectrophotometer using BaSO4 as reference. Photocurrent was measured at the CHI660B electrochemical workstation using a standard three-electrode system. In the system, platinum wire, standard calomel in saturated potassium chloride and the prepared catalyst was used as counter, reference and working electrode, respectively. The working electrode was immersed in a sodium sulfate electrolyte (0.2 M) and irradiated with visible light. To prepare the
working electrode, 20 mg of photocatalyst was dissolved in 0.5 mL of naphthol under grinding conditions. The suspension was then dipped on a 1×2 cm2 fluorine-doped tin oxide glass electrode, and the conductive glass was dried at 50 °C for 0.5 h. The electron-spin resonance (ESR) experiment was conducted to investigate the O2•– species in a photocatalytic process. Typically, 50 mg of a catalyst was dispersed in 1 mL of benzylamine, and 200 μL of DMPO was added to trap O2•–, and then the catalyst was irradiated with visible light. 2.5 Photocatalytic activity To evaluate the catalytic performance of the photocatalysts, we used the oxidation of benzylamine to N-benzylbenzaldimine at atmospheric pressure and room temperature as a model reaction. Normally, the reaction we carried out in a twonecked flask containing 50 mg of catalyst, 0.5 mmol of benzylamine and 3 mL of acetonitrile as solvent. The evaluation was carried out under visible-light irradiation (300 W Xe lamp, PLS-SXE 300C, Perfectlight, λ ≥400 nm) in a flow of O2 (3 mL·min−1). After the reaction, biphenyl was added as an internal standard, and then the catalyst was separated from the reaction solution by centrifugation (10 000 rm for 2 min). The product was monitored using a SHIMADZU Gas Chromatograph. The mass balance was estimated on the basis of total mass before and after the reaction, and then the conversion and selectivity were calculated.
3 Results and discussion
Fig. 1 reveals that Mo2C-QDs is of β-phase (JCPDS card No. 31-0787) and Bi2MoO6 is with orthorhombic structure (JCPDS card No. 21-0102) [33, 34]. The XRD patterns of the MCBMO composites show the diffraction peaks of Bi2MoO6 [34], but not the diffraction peaks of Mo2C, which may be ascribed to the low content and/or high distribution of Mo2C in the composites [35]. With the increase of Mo2CQDs/C content, there is no change in Bi2MoO6 crystallinity.
Fig. 1. XRD patterns of Mo2C-QDs/C and MCBMO.
The presence of Bi, Mo, O, and C on the surface of MCBMO-3 was confirmed by X-ray photoelectron spectroscopy (XPS) (Fig. 2a). Fig. 2b shows that the Bi 4f7/2 and 4f5/2 peak of Bi2MoO6 is at 164.2 eV and 158.9 eV binding energy, respectively [34], while those of MCBMO-3 are slightly shifted towards higher binding energies (164.4 eV and 159.1 eV). Fig. 2c shows that the binding energy of the Mo 3d5/2 and Mo 3d3/2 peak of Bi2MoO6 is around 232.4 eV and 235.5 eV, respectively, indicating the presence of Mo6+ species [34]. In the case of Mo2C-QDs/C, the Mo 3d profile can be split into three sets of 3d5/2 and 3d3/2 peaks. The peaks at 228.9 eV and 232.1 eV are assigned to Mo2+. Those at 229.4 eV and 232.7 eV to Mo4+, while those at 232.7 eV
and 235.8 eV to Mo6+ [12,36,37]. The results suggest the co-presence of Mo2+, Mo4+ and Mo6+ in Mo2C-QDs/C, and the existence of Mo4+ and Mo6+ is plausibly a result of Mo2C oxidation during the preparation process. For the MCBMO-3 composite, the Mo 3d5/2 and Mo 3d3/2 peaks at 232.5 eV and 235.6 eV belong to Bi2MoO6, while those at 228.9 eV and 232.1 eV to Mo2+ of Mo2C. The disappearance of the peaks corresponding to the Mo4+ and Mo6+ species may be due to the transformation of MoO2 and MoO3 to molybdates during the hydrothermal process, and molybdates could be lost during the washing process [37]. As shown in Fig. 2d, the C 1s peak at 284.8 eV could be assigned to carbon (C–C). The peak at 285.6 eV of Bi2MoO6 belongs to C–O, while the peaks at 283.8, 285.8, and 288.8 eV of Mo2C-QDs/C belong to C–Mo, C–O and –O–C=O bonds, respectively [38]. The C 1s peaks at 283.8, 285.7 and 288.7 eV of MCBMO-3 belong to C–Mo, C–O and –O–C=O, respectively. Compared to the pristine Bi2MoO6 and Mo2C-QDs/C, the Bi 4f and Mo 3d peaks of Bi3+ and Mo2+ species of MCBMO-3 slightly shift to higher and lower binding energies, respectively, indicating the transfer of electrons from Bi2MoO6 to Mo2C [35].
Fig. 2. (a) XPS survey spectrum of MCBMO-3; and (b) Bi 4f, (c) Mo 3d and (d) C 1s spectra of Bi2MoO6, Mo2C-QDs/C and MCBMO-3 samples.
The morphologies of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3 display layered structures with thickness ranging from 10 to 15 nm (Fig. S3, ESIƗ), and the thickness of MCBMO-3 is the lowest. The TEM images of Mo2C-QDs/C (Fig. 3a) confirm the sheet-like structure which is with nanoparticles (3‒5 nm in diameter) uniformly distributed on the surfaces. In the HRTEM image (Fig. 3b), the lattice plane distance of 0.23 nm is consistent with the (101) planes of β-Mo2C [33,38]. The results provide evidence for the successful deposition of Mo2C quantum dots on the surface of carbon layers. Despite that the images of the particles on the surfaces are not well-resolved (owing to the mingling of Bi2MoO6 nanosheets with Mo2C-QDs/C layers), lattice
fringes with distances of 0.23 nm and 0.27 nm, corresponding to (101) planes of βMo2C and (200) planes of Bi2MoO6, respectively, can be observed [26,33,38]. In Fig. S4 a (ESIƗ), the energy-dispersive X-ray (EDX) peaks of Mo, Bi, O and C elements are clearly detected over the MCBMO-3 sample, further confirming the presence of Mo2C-QDs/C on the Bi2MoO6 nanosheets. In addition, the results of elemental mapping reveal uniform dispersion of Bi, Mo, O and C in the composite (Figs. S4 b–f, ESIƗ). The above results confirm the successful preparation of the MCBMO photocatalysts. The formation of the sheet-like structures of Mo2C-QDs/C and MCBMO-3 may be related to the following process: When NH3·H2O was injected into the solution containing MoO3 nanorods and dopamine hydrochloride during MCBMO-3 preparation, the MoO3 nanorods gradually dissolved in the basic environment to release MoO42-, which combines with dopamine hydrochloride to generate Mo-polydopamine nanosheets; and upon calcination at 750 °C under Ar atmosphere, Mo2C-QDs/C is formed with the nanosheet morphology retained. As for the MCBMO composite, the Mo2C-QDs/C nanosheets act as structure directing agent, and in the hydrothermal process Bi2MoO6 nanosheets are in situ grown on the surface of Mo2C-QDs/C nanosheets, leading to the construction of Mo2C-QDs/C/Bi2MoO6 heterojunctions.
Fig. 3. TEM and HRTEM images of (a,b) Mo2C-QDs/C, (c,d) MCBMO-3.
The optical properties of the photocatalysts were studied using UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The absorption edge of the MCBMO composites is located at about 500 nm (Fig. 4a), signifying outstanding visible light response. Across the MCBMO samples with the increase of Mo2C content, there is no obvious change of absorption edge, but compared to Bi2MoO6 there is significant increase of visible-light absorption intensity at 500–800 nm. This phenomenon proves that the MCBMO heterojunctions absorb more visible light. It was observed that with the increase of Mo2C-QDs/C content, the colour of the composites gradually darkens. The excellent visible-light absorption capacity of the MCBMO heterojunctions would be beneficial for photocatalytic performance. According to the Kubelka–Munk equation: αhν=A(hν − Eg)n, where α is the absorption coefficient, hν is energy and A is a constant. The band gap energy of a semiconductor can be estimated via
transformation of its UV–vis DRS into a Tauc plot [39,40]. Both Bi2MoO6 and Mo2CQDs/C are direct-transition semiconductors, so n = 1/2 [41, 42]. The Eg value can be estimated by extrapolating the straight portion of (Ahν)2–(hν) plot to the A = 0 point. As revealed in Fig. 4b−d, the band gap energy of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3 is 1.41 eV, 2.53 eV and 2.54 eV, respectively. In addition, the relative potential of valence band (VB) was obtained by XPS VB spectra (Fig. S5 a, b and c) of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3, and the estimated value is 0.80 eV, 1.53 eV and 1.61 eV, respectively [43]. So, the conduction band edges of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3 is ‒0.61 eV, ‒1.00 eV and ‒0.93 eV, respectively, indicating sufficient thermodynamic driving force for O2 reduction to ·O2‒ (O2/·O2‒ potential is ‒0.28 V vs. NHE) [44,45].
Fig. 4 (a) UV-vis DRS, (b-d) (αhν)2 vs. hν of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3.
The Brunauer-Emmett-Teller (BET) specific surface areas and pore size distributions of the Mo2C-QDs/C, Bi2MoO6 and MCBMO-3 samples were determined by N2 adsorption-desorption isotherms. As shown in Fig. 5a, the samples exhibit typical IV isotherms each with a type H3 hysteresis loop from P/P0 0.5 to 0.95, indicating the presence of mesopores and macropores according to IUPAC classification. The MCBMO-3 composite is larger than Bi2MoO6 but smaller than Mo2C-QDs/C in specific surface area as well as in total pore volume (inset of Fig. 5a). Among the composites, BCBMO-3 is the highest in specific surface area and total pore volume (Table S2, ESIƗ), and hence is the most capable of providing surface active sites for adsorption and reaction [35]. To the best of our knowledge, MCBMO3 is superior to the reported Bi2MoO6-based photocatalysts in terms of specific surface area (Table S3, ESIƗ). We performed photoelectrochemical measurements to examine the ability of the catalysts for the separation of photogenerated charge carriers. Photocurrent response property of the samples is illustrated in Fig. 5b. The photocurrent intensity of MCBMO-3 is dramatically higher than that of pristine Bi2MoO6 nanosheets as well as those of the other MCBMO composites. We further examined the ability of the prepared materials for the separation of photo-generated carriers by fluorescence (PL) spectroscopy. The weaker the PL intensity, the better the separation effect on photogenerated carriers [46,47]. As can be seen from Fig. 5c, the introduction of Mo2CQDs/C can significantly inhibit the electron-hole recombination of Bi2MoO6. The results indicate that among the MCBMO samples separation efficiency of charge
carriers is the highest over MCBMO-3. The charge separation and collection efficiency at the interface can be illustrated by the spectra collected in electrochemical impedance spectroscopic (EIS) measurements. As displayed in Fig. 5d, compared with Bi2MoO6, the MCBMO photocatalysts display Nyquist plots of smaller diameter, indicating lower electronic resistance of the latter [48‒50]. It is clear that the MCBMO-3 photocatalyst exhibits the smallest diameter. The ability for the separation of photogenerated charge carriers follows the order: MCBMO-3 > MCBMO-2 > MCBMO-4 > MCBMO-5 > MCBMO-1 > Bi2MoO6. Apparently, the coupling of Mo2C-QDs/C with Bi2MoO6 promotes the transfer of excited electron, and inhibits charge recombination, which is beneficial for the enhancement of photocatalytic activity.
Fig. 5 (a) N2 adsorption-desorption isotherms and specific surface area, (b) transient photocurrent response, (c) PL spectra and (d) EIS plots of the prepared samples.
Photocatalytic activity of the photocatalysts was evaluated in the selective oxidation of benzylamine using O2 as oxidant under visible light irradiation (Fig. 6a). All samples display high selectivity (higher than 99%) to N-benzylidenebenzylamine, but the conversion of benzylamine varies. Among them, MCBMO-3 is the highest in photocatalytic activity, showing benzylamine conversion as high as 92%, while Bi2MoO6 is the poorest showing a conversion of only 53%. The results indicate that Mo2C-QDs/C is an efficient co-catalyst of Bi2MoO6 for the photocatalytic oxidation of benzylamine. Nonetheless, with further rise of Mo2C-QDs/C content, benzylamine conversion decreases. It is plausible that Mo2C-QDs/C in excess would result in Bi2MoO6 being covered and the incident light being blocked. As no catalytic activity was detected over pristine Mo2C, the outstanding photocatalytic activity of MCBMO3 may be related to its high separation efficiency of charge carriers as well as high specific surface area among the MCBMO samples. We studied the reusability of MCBMO-3 in five cycles of reaction having the catalyst recovered by simple centrifugation and reused without any treatment. The decrease of activity across the five runs is little (Fig. 6b), indicating good reusability of MCBMO-3. Effect of solvent on the catalytic efficiency was also studied (Table S4, ESIƗ). When acetonitrile is used as solvent, there is high conversion (92%) and selectivity (99%). Despite little change of N-benzylidenebenzylamine selectivity, there is significant decrease of benzylamine conversion in the cases of ethyl acetate, dichloromethane, and DMF (to 53%, 38%, and 46%, respectively), each of which is higher than acetonitrile in polarity. The decreases of conversion can be attributed to
the competitive adsorption between solvent and substrate molecules which can lead to slow reaction rate [23,51]. Considering the polarity effect of the tested solvents, acetonitrile was chosen as medium for the condensation reaction in the following studies. As reference, the commonly used cocatalysts, viz., Ag and Pt loaded Bi2MoO6 were studied under the same reaction conditions. Clearly, MCBMO-3 shows much higher activity than do 0.6% Pt/Bi2MoO6 and 2.5% Ag/Bi2MoO6, suggesting Mo2C-QDs/C is an efficient co-catalyst for this reaction (Table S4, ESIƗ). We also compared the photocatalytic performance of MCBMO-3 for oxidative coupling of benzylamine under the optimized conditions with the reported photocatalysts in references (Table S5, ESIƗ). The photocatalytic activity of MCBMO-3 is better than most of the reported photocatalyst at the same or similar conditions to our knowledge.
Fig. 6 (a) Photocatalytic performance of prepared catalysts in the oxidation of benzylamine to Nbenzylidenebenzylamine, (b) reusability of MCBMO-3. Reaction conditions: amine (0.5 mmol), catalyst (50 mg), acetonitrile (3 mL), O2 flow rate (3 mL min-1), time (6 h), visible-light irradiation (λ >400 nm, 300 W Xe lamp).
Table 1. Oxidation coupling of various amines over MCBMO-3.
NH2
R
Entry
MCBMO-3
6 h, O2 nm
Substrate
Product
NH2
1
N Cl
Cl
Cl
N
R
NH2
Cl
Cl
N
Cl
Cl
Br
Br
N
Br
Br
NH2
O
61
>99
83
>99
79
>99
94
>99
86
>99
96
>99
O
N
7 O
NH2
>99
Br
N
NH2
8
63
Br
NH2
O
>99
N
NH2
O
69
Cl
4
6
Sel. (%)
Cl
NH2
5
Conv. (%)
N
2
3
R
O
N
Reaction conditions: amine (0.5 mmol), catalyst (50 mg), acetonitrile (3 mL), O2 flow rate (3 mL min-1), time (6 h), visible-light irradiation (λ >400 nm, 300 W Xe lamp).
Selective oxidation of benzylamine derivative to the corresponding imines was tested to study the universality of the protocol over the optimized catalyst. As listed in Table 1, when an electron withdrawing group (-Br or -Cl) is attached to the benzene ring, the conversion is in the range of 61%−83% (Entries 1−5), while the attachment of an electron-donating substituents (-CH3 or -CH3O) results in enhanced conversion
in the range of 86%−96% (Entries 6−8). It could be seen that the oxidation of amines with electron-donating substituents proceeds much more efficiently than that of amines with electron-withdrawing substituents. Because there is conversion discrepancy across the substituents, it is clear that the reaction is under the electronic effect of substituents [51]. Moreover, the higher activity of para-substituted substrates relative to the meta and ortho isomers (ortho < meta < para) reveals the presence of steric effect (Table 1) [52]. The efficient generation of a wide range of imines over the MCBMO-3 photocatalyst suggests that the protocol of the present study has high potential for the synthesis of value-added imines from amines at mild conditions. It is generally recognized that the photocatalytic oxidation of amines to the corresponding imines undergoes a free-radical-mediated process [53]. In order to better explore the reaction mechanism, we carried out quenching experiments. As shown in Fig. 7a, with the addition of TEMPO (tetra-methylpiperidine N-oxide) which is a scavenger for all kinds of radicals, the reaction was almost completely terminated. When BQ (benzoquinone) was added to capture superoxide radicals, the conversion was reduced from 92% to 29%, indicating that superoxide radicals play an important role in the reaction [21]. When TBA (tert-butyl alcohol) was added to capture ·OH, the conversion was slightly reduced to 79%. When KSO (K2S2O8) and AO (ammonium oxalate) is incorporated into the reaction system to capture photogenerated electrons and holes, the conversion becomes 42% and 56%, respectively, suggesting that both electrons and holes play a key role in this reaction. To explore the oxidation mechanism of this reaction, an ESR spin-trapping
experiment using 5,5’-dimethyl-1-pyrroline N-oxide (DMPO) to trap the reactive oxygen species produced during light illumination was conducted (Fig. 7b) [54]. Four characteristic signals of superoxide radicals were detected, proving the formation of O2•–, and the signal intensity of superoxide radicals over MCBMO-3 is obviously higher than that over Bi2MoO6. It is because MCBMO-3 has stronger ability than Bi2MoO6 in the separation of photo-generated charge carriers, and more O2•– is produced over the former under light irradiation. To get more information on the mechanism of the photocatalytic reaction, the relative oxidative rates of parasubstituted benzylamines (MeO, Me, H, Cl, and CF3 groups) were explored [19]. A reasonable near-linear relation between lg(kx/kh) values and the Brown-Okamoto constant (σ+) was obtained for the oxidation of para-substituted benzylamines (Fig. S6, ESIƗ), indicating that the reaction occurs via the carbocation intermediate species.
Fig. 7 (a) Quenching experiments using different radical scavengers, (b) ESR spectra of superoxide radical species trapped by DMPO over BMO, MCBMO-3 under visible light irradiation and that over MCBMO-3 in the dark.
Based on the above results, a possible reaction mechanism is proposed (Scheme 1). Photo-excited electrons and holes are generated in the Mo2C-QDs/C/Bi2MoO6 composites upon light irradiation. The excited electrons transfer from Bi2MoO6 to Mo2C-QDs/C along the carbon layer or the Mo2C/Bi2MoO6 interface while the holes remain on the valance band of Bi2MoO6. The excited electrons on Mo2C-QDs combine with O2 to form superoxide radicals. In the steps of imine formation from amine [19], amine loses an electron to produce a carbocationic-radical-type intermediate. Then O2•– abstracts a proton and a hydrogen atom from the intermediate to generate the corresponding imine. Subsequently, the superoxy group reacts with the amine cation to form an intermediate imine and an H2O2 molecule. As show in Fig. S7 (ESIƗ), the H2O2 intermediate in the reaction system was detected by the spectrophotometric N,N-Diethyl-p-phenylenediamine sulfate (DPD) method [55,43]. The positively charged hole cooperates with imines, making it more prone to nucleophilic attack by amine for the formation of an aminal. Then with the assistance of a hole, the amino group undergoes ammonia elimination to generate the target product [19].
Scheme 1. Proposed mechanism for aerobic oxidative coupling of amines.
4 Conclusions In summary, we uniformly embedded Mo2C quantum dots on a carbon layer by high temperature calcination. The prepared Mo2C-QDs/C nanosheet (as a co-catalyst) was hydrothermally coupled with Bi2MoO6 nanosheets to generate the Mo2CQDs/C/Bi2MoO6 composite. Under visible light irradiation, the MCBMO-3 composite exhibits photocatalytic activity much superior to that of pristine Bi2MoO6 nanosheets in the selective oxidation of benzylamine to N-benzylidenebenzylamine using O2 as oxidant. The conversion over the former reaches 92%, while that over the latter 53%. The excellent performance can be attributed to (i) the metallic nature of Mo2C, (ii) presence of conductive carbon layer between Mo2C-QDs and Bi2MoO6, (iii) high specific surface area of the composite, (iv) efficient transfer of excited electrons from Bi2MoO6 to Mo2C-QDs/C, and (v) high generation of superoxide radicals on the Mo2C-QDs/C/Bi2MoO6 composites. It is expected that the results of this work can
serve as useful guidelines for the design and fabrication of noble-metal-free cocatalyst for selective oxidation of amines to the corresponding imines.
Acknowledgements This project was financially supported by the National Natural Science Foundation of China (Grants 21725602, 21776064, 21671062 and 21975069), the Innovative Research Groups of Hunan Province (Grant 2019JJ10001), and the Science and Technology Planning Project of Hunan Province (Grant 2019RS3010). C. T. Au thanks HNU for an adjunct professorship.
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Graphical Abstract Mo2C-QDs/C/Bi2MoO6 composite with carbon layer and Mo2C-QDs acting as electronic conductor & electron trapping agent was prepared by a facile hydrothermal method. It shows photocatalytic performance higher than that of pristine Bi2MoO6 in the oxidative coupling of amines to imines under visible light.
Highlights Mo2C-QDs/C/Bi2MoO6 composite was prepared by a facile hydrothermal method Carbon layer and Mo2C-QDs in the composite acting as electronic conductor & electron trapping agent It shows boosting activity and stability for selective oxidation of amines to imines
L. Chen and L. Meng designed the study and wrote the paper. X. Tan performed some of the experiments. J. Guo analyzed the data. S. Shen, C. Au, S. Yin analyzed the data and polished the paper.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0169-4332(20)33234-7 https://doi.org/10.1016/j.apsusc.2020.148476 APSUSC 148476
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
6 September 2020 3 November 2020 12 November 2020
Please cite this article as: L-H. Meng, X-P. Tan, L. Chen, S. Shen, J-K. Guo, C-T. Au, S-F. Yin, Fabrication of Mo2C-QDs/C/Bi2MoO6 composite as efficient photocatalyst for aerobic oxidation of amines to imines, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.148476
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Fabrication of Mo2C-QDs/C/Bi2MoO6 composite as efficient photocatalyst for aerobic oxidation of amines to imines Ling-Hu Menga,†, Xi-Ping Tana,†, Lang Chen*a, Sheng Shen*a, Jun-Kang Guoa, ChakTong Aub, and Shuang-Feng Yin*a a
State Key Laboratory of Chemo/Biosensing and Chemometrics, Provincial Hunan
Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions, Advanced Catalytic Engineering Research Center of the Ministry of Education, College of Chemistry and Chemical Engineering, Hunan University, Lushan South Road, Changsha 410082, Hunan, China b
College of Chemical Engineering, Fuzhou University, Fuzhou, 350002, Fujian,
China. † The
authors contributed equally.
* E-mail: [email protected] (L. Chen), [email protected] (S. Shen), [email protected] (S.F. Yin)
Abstract: As an efficient but mild oxidation species, O2•– generated from surface O2 and photo-excited electrons plays a key role in photocatalytic reactions. To improve photocatalytic activity, it is essential to facilitate the separation and transfer efficiency of photogenerated charge carriers. In this work, Mo2C-QDs/C, an alternative of noble metals, was used as an efficient and economical cocatalyst. It was added to Bi2MoO6 nanosheet to generate Mo2C-QDs/C/Bi2MoO6 (MCBMO) composites by a facile hydrothermal method. Under visible light irradiation, the carbon layer and the Mo2CQDs acts as electronic conductor and electron trapping agent to promote the transfer of photogenerated electrons for enhanced generation of O2•–. The MCBMO composites show boosted photocatalytic performance in the oxidative coupling of benzylamine to N-benzylbenzaldimine with excellent recyclability. Under the optimized conditions, benzylamine conversion over the optimized MCBMO-3 composite is up to 92% whereas that over pristine Bi2MoO6 is limited to 53%. The MCBMO-3 composite even outperforms Ag/Bi2MoO6 and Pt/Bi2MoO6. Based on the results of ESR and active species trapping studies, it is confirmed that O2•– and h+ are indispensable in the photocatalytic reaction, and a possible reaction mechanism has been proposed. Keywords: Mo2C quantum dots; Bi2MoO6; photocatalysis; oxidative coupling of amine; imine
1 Introduction Imines are versatile organic intermediates in organic synthesis, which are commonly used as building blocks for many pharmaceutical and organic-dye molecules [1]. Traditionally, imines are prepared through conventional oxidation of amines, which have disadvantages such as the need of noble-metal catalysts as well as the use of oxidants that are adverse to the environment [2, 3]. As clean and sustainable energy source, sunlight is widely utilized in solar-cell technology and in the field of photocatalysis [4-6]. An emerging way of sunlight utilization is organic synthesis based on photocatalysis over semiconductor materials. For example, the oxidation of amines to imines can be photocatalytically conducted under green and mild conditions with high selectivity [7]. As a highly active but mild oxidizing species, O2•– is regarded as important active species in the photocatalytic oxidation of amines to imines [8]. It is generated through the combination of a surface O2 molecule with a photo-excited electron that has migrated to the surface from the bulk. To improve the generation of O2•–, strategies have been developed to separate the photogenerated charge carriers, and to promote the transfer of excited electrons and holes to the surface of catalysts [9]. The introduction of a co-catalyst is one of the most effective methods for achieving high performance. Commonly, noble metals such as Pt, Pd and Au are excellent cocatalysts but the cost of using them is prohibitively high [10]. Hence it is urgent to develop a non-noble metal-based co-catalyst that is with comparative activity.
Mo2C is an important member of transition metal carbides (TMCs). With electronic structure similar to that of Pt [11], it has been regarded as a promising candidate to replace noble metals. Mo2C has been studied as a substitute for noble metals in hydrogenation, hydrodesulfurization, and hydrodenitrogenation reactions as well as in electrochemical applications [12-15]. As far as we know Mo2C has not been reported as a co-catalyst in the photocatalytic oxidation of amines to imines with O2 being used as oxidant. Recently, semiconductors such as TiO2, Nb2O5, MOF, WO3, C3N4, HNb3O8 or their composites have been reported as photocatalysts to prepare imines from amines [1624]. However, as a result of poor light absorption and serious recombination of charge carriers, their catalytic activity under visible light is unsatisfactory. Lately, in view of its suitable band position and visible-light response, bismuth molybdate (Bi2MoO6) has been studied for the partial oxidation of hydrocarbons into value-added organics [25-26]. Cheng’s group demonstrated that Pt/Bi2MoO6 was highly efficient for the photocatalytic selective oxidation of ethylbenzene [27]. Furthermore, ultrathin Bi2MoO6, Bi-self-doped Bi2MoO6-Bi2Mo3O12 and TiO2/Bi2MoO6 were found to show excellent selectivity as well as recyclability for the selective oxidation of aromatic alkanes under visible light irradiation [28-30]. Moreover, over carbon-dots-Bi2MoO6 Srivastava’s group observed good yield of N-benzylbenzaldimine from benzylamine [31]. It was pointed out that O2•– is indispensable in the partial oxidation of toluene, which is a reaction similar to the oxidative coupling of amine to imine. Based on the
above background, it is envisaged that Bi2MoO6 is a potential photocatalyst for the production of imines from amines. In the present study, we uniformly loaded a carbon layer 10‒15 nm in thickness with Mo2C quantum dots (denoted herein as Mo2C-QDs/C co-catalyst) by high temperature treatment. Then, Mo2C-QDs/C was combined with Bi2MoO6 nanosheets to form Mo2C-QDs/C/Bi2MoO6 (MCBMO) composites through hydrothermal method. The physicochemical properties of the materials were characterized by techniques such as XRD, SEM, TEM, and XPS. We evaluated the MCBMO composites for the selective oxidation of benzylamine as well as its derivatives under visible light irradiation. As an efficient electronic conductor, Mo2C-QDs/C not only accelerates the transfer of excited electrons from the conduction band of Bi2MoO6 to the catalyst surface for the generation of O2•–, but also efficiently separates the e--h+ pairs of Bi2MoO6 as a result.
2 Experimental section 2.1 Preparation All reagents were commercially available and used without further purification. The preparation of MCBMO was divided into two steps: First, synthesis of Mo2CQDs/C, and second combination of Mo2C-QDs/C with Bi2MoO6. 2.2 Synthesis of Mo2C-QDs/C MoO3 nanorods as precursor were prepared according to the method reported by Chen et al [32]. Typically, 2.8 g of (NH4)6Mo7O24·4H2O was dissolved in 80 mL of a
mixed solution of 65% HNO3 and deionized H2O in 1:5 volume ratio. Then the mixture was transferred into a 100 mL Teflon-lined autoclave and kept at 200 °C for 20 h. After cooling, the product was collected by centrifugation and washed with water and ethanol for several times before overnight drying at 80 °C. Then, 150 mg of the obtained MoO3 nanorods (proven by SEM and XRD analyses, Figs. S1a and S2a of Electronic Supplementary Information, ESIƗ) was added into 30 mL of deionized H2O in a 100 mL glass bottle. After the mixture was subject to ultrasonic treatment for 30 min, 25 mg of dopamine hydrochloride and 40 mL of absolute ethanol were added. The resulted solution was stirred for 5 min, followed by fast injection of 0.3 mL of aqueous NH3 (30 wt%). After reaction for 2.5 h under gentle stirring at room temperature, the precipitate was collected by centrifugation and washed with ethanol and dried at 30 °C in a vacuum oven overnight to get Mo-polydopamine nanosheets (proven by SEM and XRD analyses, Figs. S1b and S2b, ESIƗ). Finally, the assynthesized Mo-polydopamine precursor was heated to 750 °C with a ramp rate of 5 °C min‒1 and annealed at this temperature under Ar for 16 h to obtain Mo2C-QDs/C (Fig S2c, ESIƗ). 2.3 Synthesis of MCBMO composites Mo2C-QDs/C (1 mg) was added to 80 mL of deionized water and subject to ultrasonication for 3 h. Then 2 mmol Bi(NO3)3·5H2O was added and the mixture was stirred for 30 min, followed by the addition of 1 mmol Na2MoO4·2H2O and 0.15 g of hexadecyl trimethyl ammonium bromide (CTAB). The as-resulted mixture was stirred for 30 min, and transferred into a 100 mL Teflon-lined autoclave and kept at 180 °C
for 12 h. The obtained precipitation was thoroughly washed with deionized water, ethanol, and dried at 80 °C overnight to obtain Mo2C-QDs/C/Bi2MoO6 (denoted hereinafter as MCBMO). The samples with different amounts of Mo2C-QDs/C (10, 20, 30, 40 and 50 mg) are labelled as MCBMO-1, MCBMO-2, MCBMO-3, MCBMO-4 and MCBMO-5, respectively. The surface content of Mo2C-QDs/C in the MCBMO-3 was estimated using the XPS results as showed in Table S1 (ESIƗ). That the calculated content of Mo2C-QDs/C is slightly higher than that of the nominal value could be attributed to the fact that XPS is a surface sensitive analytical technique. 2.4 Characterization A Bruker D8 Advance X-ray diffractometer was used to characterize the crystal phase of samples (monochromatized Cu Kα radiation, λ=0.154 06 nm). The morphology and microstructure of samples were studied over a Hitachi S-4800 scanning electron microscope and a JEM-2100F transmission electron microscope. Xray photoelectron spectroscopic data of Bi, Mo, C, and O as well as the XPS valance band of prepared samples were acquired with a VG Multilab 2000 instrument. Light absorption capacity was studied using Cary-100 spectrophotometer using BaSO4 as reference. Photocurrent was measured at the CHI660B electrochemical workstation using a standard three-electrode system. In the system, platinum wire, standard calomel in saturated potassium chloride and the prepared catalyst was used as counter, reference and working electrode, respectively. The working electrode was immersed in a sodium sulfate electrolyte (0.2 M) and irradiated with visible light. To prepare the
working electrode, 20 mg of photocatalyst was dissolved in 0.5 mL of naphthol under grinding conditions. The suspension was then dipped on a 1×2 cm2 fluorine-doped tin oxide glass electrode, and the conductive glass was dried at 50 °C for 0.5 h. The electron-spin resonance (ESR) experiment was conducted to investigate the O2•– species in a photocatalytic process. Typically, 50 mg of a catalyst was dispersed in 1 mL of benzylamine, and 200 μL of DMPO was added to trap O2•–, and then the catalyst was irradiated with visible light. 2.5 Photocatalytic activity To evaluate the catalytic performance of the photocatalysts, we used the oxidation of benzylamine to N-benzylbenzaldimine at atmospheric pressure and room temperature as a model reaction. Normally, the reaction we carried out in a twonecked flask containing 50 mg of catalyst, 0.5 mmol of benzylamine and 3 mL of acetonitrile as solvent. The evaluation was carried out under visible-light irradiation (300 W Xe lamp, PLS-SXE 300C, Perfectlight, λ ≥400 nm) in a flow of O2 (3 mL·min−1). After the reaction, biphenyl was added as an internal standard, and then the catalyst was separated from the reaction solution by centrifugation (10 000 rm for 2 min). The product was monitored using a SHIMADZU Gas Chromatograph. The mass balance was estimated on the basis of total mass before and after the reaction, and then the conversion and selectivity were calculated.
3 Results and discussion
Fig. 1 reveals that Mo2C-QDs is of β-phase (JCPDS card No. 31-0787) and Bi2MoO6 is with orthorhombic structure (JCPDS card No. 21-0102) [33, 34]. The XRD patterns of the MCBMO composites show the diffraction peaks of Bi2MoO6 [34], but not the diffraction peaks of Mo2C, which may be ascribed to the low content and/or high distribution of Mo2C in the composites [35]. With the increase of Mo2CQDs/C content, there is no change in Bi2MoO6 crystallinity.
Fig. 1. XRD patterns of Mo2C-QDs/C and MCBMO.
The presence of Bi, Mo, O, and C on the surface of MCBMO-3 was confirmed by X-ray photoelectron spectroscopy (XPS) (Fig. 2a). Fig. 2b shows that the Bi 4f7/2 and 4f5/2 peak of Bi2MoO6 is at 164.2 eV and 158.9 eV binding energy, respectively [34], while those of MCBMO-3 are slightly shifted towards higher binding energies (164.4 eV and 159.1 eV). Fig. 2c shows that the binding energy of the Mo 3d5/2 and Mo 3d3/2 peak of Bi2MoO6 is around 232.4 eV and 235.5 eV, respectively, indicating the presence of Mo6+ species [34]. In the case of Mo2C-QDs/C, the Mo 3d profile can be split into three sets of 3d5/2 and 3d3/2 peaks. The peaks at 228.9 eV and 232.1 eV are assigned to Mo2+. Those at 229.4 eV and 232.7 eV to Mo4+, while those at 232.7 eV
and 235.8 eV to Mo6+ [12,36,37]. The results suggest the co-presence of Mo2+, Mo4+ and Mo6+ in Mo2C-QDs/C, and the existence of Mo4+ and Mo6+ is plausibly a result of Mo2C oxidation during the preparation process. For the MCBMO-3 composite, the Mo 3d5/2 and Mo 3d3/2 peaks at 232.5 eV and 235.6 eV belong to Bi2MoO6, while those at 228.9 eV and 232.1 eV to Mo2+ of Mo2C. The disappearance of the peaks corresponding to the Mo4+ and Mo6+ species may be due to the transformation of MoO2 and MoO3 to molybdates during the hydrothermal process, and molybdates could be lost during the washing process [37]. As shown in Fig. 2d, the C 1s peak at 284.8 eV could be assigned to carbon (C–C). The peak at 285.6 eV of Bi2MoO6 belongs to C–O, while the peaks at 283.8, 285.8, and 288.8 eV of Mo2C-QDs/C belong to C–Mo, C–O and –O–C=O bonds, respectively [38]. The C 1s peaks at 283.8, 285.7 and 288.7 eV of MCBMO-3 belong to C–Mo, C–O and –O–C=O, respectively. Compared to the pristine Bi2MoO6 and Mo2C-QDs/C, the Bi 4f and Mo 3d peaks of Bi3+ and Mo2+ species of MCBMO-3 slightly shift to higher and lower binding energies, respectively, indicating the transfer of electrons from Bi2MoO6 to Mo2C [35].
Fig. 2. (a) XPS survey spectrum of MCBMO-3; and (b) Bi 4f, (c) Mo 3d and (d) C 1s spectra of Bi2MoO6, Mo2C-QDs/C and MCBMO-3 samples.
The morphologies of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3 display layered structures with thickness ranging from 10 to 15 nm (Fig. S3, ESIƗ), and the thickness of MCBMO-3 is the lowest. The TEM images of Mo2C-QDs/C (Fig. 3a) confirm the sheet-like structure which is with nanoparticles (3‒5 nm in diameter) uniformly distributed on the surfaces. In the HRTEM image (Fig. 3b), the lattice plane distance of 0.23 nm is consistent with the (101) planes of β-Mo2C [33,38]. The results provide evidence for the successful deposition of Mo2C quantum dots on the surface of carbon layers. Despite that the images of the particles on the surfaces are not well-resolved (owing to the mingling of Bi2MoO6 nanosheets with Mo2C-QDs/C layers), lattice
fringes with distances of 0.23 nm and 0.27 nm, corresponding to (101) planes of βMo2C and (200) planes of Bi2MoO6, respectively, can be observed [26,33,38]. In Fig. S4 a (ESIƗ), the energy-dispersive X-ray (EDX) peaks of Mo, Bi, O and C elements are clearly detected over the MCBMO-3 sample, further confirming the presence of Mo2C-QDs/C on the Bi2MoO6 nanosheets. In addition, the results of elemental mapping reveal uniform dispersion of Bi, Mo, O and C in the composite (Figs. S4 b–f, ESIƗ). The above results confirm the successful preparation of the MCBMO photocatalysts. The formation of the sheet-like structures of Mo2C-QDs/C and MCBMO-3 may be related to the following process: When NH3·H2O was injected into the solution containing MoO3 nanorods and dopamine hydrochloride during MCBMO-3 preparation, the MoO3 nanorods gradually dissolved in the basic environment to release MoO42-, which combines with dopamine hydrochloride to generate Mo-polydopamine nanosheets; and upon calcination at 750 °C under Ar atmosphere, Mo2C-QDs/C is formed with the nanosheet morphology retained. As for the MCBMO composite, the Mo2C-QDs/C nanosheets act as structure directing agent, and in the hydrothermal process Bi2MoO6 nanosheets are in situ grown on the surface of Mo2C-QDs/C nanosheets, leading to the construction of Mo2C-QDs/C/Bi2MoO6 heterojunctions.
Fig. 3. TEM and HRTEM images of (a,b) Mo2C-QDs/C, (c,d) MCBMO-3.
The optical properties of the photocatalysts were studied using UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The absorption edge of the MCBMO composites is located at about 500 nm (Fig. 4a), signifying outstanding visible light response. Across the MCBMO samples with the increase of Mo2C content, there is no obvious change of absorption edge, but compared to Bi2MoO6 there is significant increase of visible-light absorption intensity at 500–800 nm. This phenomenon proves that the MCBMO heterojunctions absorb more visible light. It was observed that with the increase of Mo2C-QDs/C content, the colour of the composites gradually darkens. The excellent visible-light absorption capacity of the MCBMO heterojunctions would be beneficial for photocatalytic performance. According to the Kubelka–Munk equation: αhν=A(hν − Eg)n, where α is the absorption coefficient, hν is energy and A is a constant. The band gap energy of a semiconductor can be estimated via
transformation of its UV–vis DRS into a Tauc plot [39,40]. Both Bi2MoO6 and Mo2CQDs/C are direct-transition semiconductors, so n = 1/2 [41, 42]. The Eg value can be estimated by extrapolating the straight portion of (Ahν)2–(hν) plot to the A = 0 point. As revealed in Fig. 4b−d, the band gap energy of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3 is 1.41 eV, 2.53 eV and 2.54 eV, respectively. In addition, the relative potential of valence band (VB) was obtained by XPS VB spectra (Fig. S5 a, b and c) of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3, and the estimated value is 0.80 eV, 1.53 eV and 1.61 eV, respectively [43]. So, the conduction band edges of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3 is ‒0.61 eV, ‒1.00 eV and ‒0.93 eV, respectively, indicating sufficient thermodynamic driving force for O2 reduction to ·O2‒ (O2/·O2‒ potential is ‒0.28 V vs. NHE) [44,45].
Fig. 4 (a) UV-vis DRS, (b-d) (αhν)2 vs. hν of Mo2C-QDs/C, Bi2MoO6 and MCBMO-3.
The Brunauer-Emmett-Teller (BET) specific surface areas and pore size distributions of the Mo2C-QDs/C, Bi2MoO6 and MCBMO-3 samples were determined by N2 adsorption-desorption isotherms. As shown in Fig. 5a, the samples exhibit typical IV isotherms each with a type H3 hysteresis loop from P/P0 0.5 to 0.95, indicating the presence of mesopores and macropores according to IUPAC classification. The MCBMO-3 composite is larger than Bi2MoO6 but smaller than Mo2C-QDs/C in specific surface area as well as in total pore volume (inset of Fig. 5a). Among the composites, BCBMO-3 is the highest in specific surface area and total pore volume (Table S2, ESIƗ), and hence is the most capable of providing surface active sites for adsorption and reaction [35]. To the best of our knowledge, MCBMO3 is superior to the reported Bi2MoO6-based photocatalysts in terms of specific surface area (Table S3, ESIƗ). We performed photoelectrochemical measurements to examine the ability of the catalysts for the separation of photogenerated charge carriers. Photocurrent response property of the samples is illustrated in Fig. 5b. The photocurrent intensity of MCBMO-3 is dramatically higher than that of pristine Bi2MoO6 nanosheets as well as those of the other MCBMO composites. We further examined the ability of the prepared materials for the separation of photo-generated carriers by fluorescence (PL) spectroscopy. The weaker the PL intensity, the better the separation effect on photogenerated carriers [46,47]. As can be seen from Fig. 5c, the introduction of Mo2CQDs/C can significantly inhibit the electron-hole recombination of Bi2MoO6. The results indicate that among the MCBMO samples separation efficiency of charge
carriers is the highest over MCBMO-3. The charge separation and collection efficiency at the interface can be illustrated by the spectra collected in electrochemical impedance spectroscopic (EIS) measurements. As displayed in Fig. 5d, compared with Bi2MoO6, the MCBMO photocatalysts display Nyquist plots of smaller diameter, indicating lower electronic resistance of the latter [48‒50]. It is clear that the MCBMO-3 photocatalyst exhibits the smallest diameter. The ability for the separation of photogenerated charge carriers follows the order: MCBMO-3 > MCBMO-2 > MCBMO-4 > MCBMO-5 > MCBMO-1 > Bi2MoO6. Apparently, the coupling of Mo2C-QDs/C with Bi2MoO6 promotes the transfer of excited electron, and inhibits charge recombination, which is beneficial for the enhancement of photocatalytic activity.
Fig. 5 (a) N2 adsorption-desorption isotherms and specific surface area, (b) transient photocurrent response, (c) PL spectra and (d) EIS plots of the prepared samples.
Photocatalytic activity of the photocatalysts was evaluated in the selective oxidation of benzylamine using O2 as oxidant under visible light irradiation (Fig. 6a). All samples display high selectivity (higher than 99%) to N-benzylidenebenzylamine, but the conversion of benzylamine varies. Among them, MCBMO-3 is the highest in photocatalytic activity, showing benzylamine conversion as high as 92%, while Bi2MoO6 is the poorest showing a conversion of only 53%. The results indicate that Mo2C-QDs/C is an efficient co-catalyst of Bi2MoO6 for the photocatalytic oxidation of benzylamine. Nonetheless, with further rise of Mo2C-QDs/C content, benzylamine conversion decreases. It is plausible that Mo2C-QDs/C in excess would result in Bi2MoO6 being covered and the incident light being blocked. As no catalytic activity was detected over pristine Mo2C, the outstanding photocatalytic activity of MCBMO3 may be related to its high separation efficiency of charge carriers as well as high specific surface area among the MCBMO samples. We studied the reusability of MCBMO-3 in five cycles of reaction having the catalyst recovered by simple centrifugation and reused without any treatment. The decrease of activity across the five runs is little (Fig. 6b), indicating good reusability of MCBMO-3. Effect of solvent on the catalytic efficiency was also studied (Table S4, ESIƗ). When acetonitrile is used as solvent, there is high conversion (92%) and selectivity (99%). Despite little change of N-benzylidenebenzylamine selectivity, there is significant decrease of benzylamine conversion in the cases of ethyl acetate, dichloromethane, and DMF (to 53%, 38%, and 46%, respectively), each of which is higher than acetonitrile in polarity. The decreases of conversion can be attributed to
the competitive adsorption between solvent and substrate molecules which can lead to slow reaction rate [23,51]. Considering the polarity effect of the tested solvents, acetonitrile was chosen as medium for the condensation reaction in the following studies. As reference, the commonly used cocatalysts, viz., Ag and Pt loaded Bi2MoO6 were studied under the same reaction conditions. Clearly, MCBMO-3 shows much higher activity than do 0.6% Pt/Bi2MoO6 and 2.5% Ag/Bi2MoO6, suggesting Mo2C-QDs/C is an efficient co-catalyst for this reaction (Table S4, ESIƗ). We also compared the photocatalytic performance of MCBMO-3 for oxidative coupling of benzylamine under the optimized conditions with the reported photocatalysts in references (Table S5, ESIƗ). The photocatalytic activity of MCBMO-3 is better than most of the reported photocatalyst at the same or similar conditions to our knowledge.
Fig. 6 (a) Photocatalytic performance of prepared catalysts in the oxidation of benzylamine to Nbenzylidenebenzylamine, (b) reusability of MCBMO-3. Reaction conditions: amine (0.5 mmol), catalyst (50 mg), acetonitrile (3 mL), O2 flow rate (3 mL min-1), time (6 h), visible-light irradiation (λ >400 nm, 300 W Xe lamp).
Table 1. Oxidation coupling of various amines over MCBMO-3.
NH2
R
Entry
MCBMO-3
6 h, O2 nm
Substrate
Product
NH2
1
N Cl
Cl
Cl
N
R
NH2
Cl
Cl
N
Cl
Cl
Br
Br
N
Br
Br
NH2
O
61
>99
83
>99
79
>99
94
>99
86
>99
96
>99
O
N
7 O
NH2
>99
Br
N
NH2
8
63
Br
NH2
O
>99
N
NH2
O
69
Cl
4
6
Sel. (%)
Cl
NH2
5
Conv. (%)
N
2
3
R
O
N
Reaction conditions: amine (0.5 mmol), catalyst (50 mg), acetonitrile (3 mL), O2 flow rate (3 mL min-1), time (6 h), visible-light irradiation (λ >400 nm, 300 W Xe lamp).
Selective oxidation of benzylamine derivative to the corresponding imines was tested to study the universality of the protocol over the optimized catalyst. As listed in Table 1, when an electron withdrawing group (-Br or -Cl) is attached to the benzene ring, the conversion is in the range of 61%−83% (Entries 1−5), while the attachment of an electron-donating substituents (-CH3 or -CH3O) results in enhanced conversion
in the range of 86%−96% (Entries 6−8). It could be seen that the oxidation of amines with electron-donating substituents proceeds much more efficiently than that of amines with electron-withdrawing substituents. Because there is conversion discrepancy across the substituents, it is clear that the reaction is under the electronic effect of substituents [51]. Moreover, the higher activity of para-substituted substrates relative to the meta and ortho isomers (ortho < meta < para) reveals the presence of steric effect (Table 1) [52]. The efficient generation of a wide range of imines over the MCBMO-3 photocatalyst suggests that the protocol of the present study has high potential for the synthesis of value-added imines from amines at mild conditions. It is generally recognized that the photocatalytic oxidation of amines to the corresponding imines undergoes a free-radical-mediated process [53]. In order to better explore the reaction mechanism, we carried out quenching experiments. As shown in Fig. 7a, with the addition of TEMPO (tetra-methylpiperidine N-oxide) which is a scavenger for all kinds of radicals, the reaction was almost completely terminated. When BQ (benzoquinone) was added to capture superoxide radicals, the conversion was reduced from 92% to 29%, indicating that superoxide radicals play an important role in the reaction [21]. When TBA (tert-butyl alcohol) was added to capture ·OH, the conversion was slightly reduced to 79%. When KSO (K2S2O8) and AO (ammonium oxalate) is incorporated into the reaction system to capture photogenerated electrons and holes, the conversion becomes 42% and 56%, respectively, suggesting that both electrons and holes play a key role in this reaction. To explore the oxidation mechanism of this reaction, an ESR spin-trapping
experiment using 5,5’-dimethyl-1-pyrroline N-oxide (DMPO) to trap the reactive oxygen species produced during light illumination was conducted (Fig. 7b) [54]. Four characteristic signals of superoxide radicals were detected, proving the formation of O2•–, and the signal intensity of superoxide radicals over MCBMO-3 is obviously higher than that over Bi2MoO6. It is because MCBMO-3 has stronger ability than Bi2MoO6 in the separation of photo-generated charge carriers, and more O2•– is produced over the former under light irradiation. To get more information on the mechanism of the photocatalytic reaction, the relative oxidative rates of parasubstituted benzylamines (MeO, Me, H, Cl, and CF3 groups) were explored [19]. A reasonable near-linear relation between lg(kx/kh) values and the Brown-Okamoto constant (σ+) was obtained for the oxidation of para-substituted benzylamines (Fig. S6, ESIƗ), indicating that the reaction occurs via the carbocation intermediate species.
Fig. 7 (a) Quenching experiments using different radical scavengers, (b) ESR spectra of superoxide radical species trapped by DMPO over BMO, MCBMO-3 under visible light irradiation and that over MCBMO-3 in the dark.
Based on the above results, a possible reaction mechanism is proposed (Scheme 1). Photo-excited electrons and holes are generated in the Mo2C-QDs/C/Bi2MoO6 composites upon light irradiation. The excited electrons transfer from Bi2MoO6 to Mo2C-QDs/C along the carbon layer or the Mo2C/Bi2MoO6 interface while the holes remain on the valance band of Bi2MoO6. The excited electrons on Mo2C-QDs combine with O2 to form superoxide radicals. In the steps of imine formation from amine [19], amine loses an electron to produce a carbocationic-radical-type intermediate. Then O2•– abstracts a proton and a hydrogen atom from the intermediate to generate the corresponding imine. Subsequently, the superoxy group reacts with the amine cation to form an intermediate imine and an H2O2 molecule. As show in Fig. S7 (ESIƗ), the H2O2 intermediate in the reaction system was detected by the spectrophotometric N,N-Diethyl-p-phenylenediamine sulfate (DPD) method [55,43]. The positively charged hole cooperates with imines, making it more prone to nucleophilic attack by amine for the formation of an aminal. Then with the assistance of a hole, the amino group undergoes ammonia elimination to generate the target product [19].
Scheme 1. Proposed mechanism for aerobic oxidative coupling of amines.
4 Conclusions In summary, we uniformly embedded Mo2C quantum dots on a carbon layer by high temperature calcination. The prepared Mo2C-QDs/C nanosheet (as a co-catalyst) was hydrothermally coupled with Bi2MoO6 nanosheets to generate the Mo2CQDs/C/Bi2MoO6 composite. Under visible light irradiation, the MCBMO-3 composite exhibits photocatalytic activity much superior to that of pristine Bi2MoO6 nanosheets in the selective oxidation of benzylamine to N-benzylidenebenzylamine using O2 as oxidant. The conversion over the former reaches 92%, while that over the latter 53%. The excellent performance can be attributed to (i) the metallic nature of Mo2C, (ii) presence of conductive carbon layer between Mo2C-QDs and Bi2MoO6, (iii) high specific surface area of the composite, (iv) efficient transfer of excited electrons from Bi2MoO6 to Mo2C-QDs/C, and (v) high generation of superoxide radicals on the Mo2C-QDs/C/Bi2MoO6 composites. It is expected that the results of this work can
serve as useful guidelines for the design and fabrication of noble-metal-free cocatalyst for selective oxidation of amines to the corresponding imines.
Acknowledgements This project was financially supported by the National Natural Science Foundation of China (Grants 21725602, 21776064, 21671062 and 21975069), the Innovative Research Groups of Hunan Province (Grant 2019JJ10001), and the Science and Technology Planning Project of Hunan Province (Grant 2019RS3010). C. T. Au thanks HNU for an adjunct professorship.
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Graphical Abstract Mo2C-QDs/C/Bi2MoO6 composite with carbon layer and Mo2C-QDs acting as electronic conductor & electron trapping agent was prepared by a facile hydrothermal method. It shows photocatalytic performance higher than that of pristine Bi2MoO6 in the oxidative coupling of amines to imines under visible light.
Highlights Mo2C-QDs/C/Bi2MoO6 composite was prepared by a facile hydrothermal method Carbon layer and Mo2C-QDs in the composite acting as electronic conductor & electron trapping agent It shows boosting activity and stability for selective oxidation of amines to imines
L. Chen and L. Meng designed the study and wrote the paper. X. Tan performed some of the experiments. J. Guo analyzed the data. S. Shen, C. Au, S. Yin analyzed the data and polished the paper.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: