Visible-Light-Driven Photocatalytic Coupling of Neat Benzylamine over a Bi-Ellagate Metal–Organic Framework

Selective aerobic oxidation of benzylamine to N,N-benzylidenebenzylamine was achieved using a bismuth ellagate (Bi-ellagate) metal–organic framework (MOF) under simulated visible light irradiation. The bismuth ellagate photocatalyst was characterized using several spectroscopic techniques: powder X-ray diffraction (PXRD), diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and nitrogen sorption measurements. Product formation was confirmed using 1H-NMR, 13C-NMR, and FTIR. The photocatalytic performance of Bi-ellagate was studied for the first time, which exhibits a band gap value of 2.62 eV, endowing it with a high photocatalytic activity under visible light irradiation. The reaction product, N,N-benzylidenebenzylamine, was selectively obtained with a high conversion yield of ∼96% under solvent-free conditions compared to other control experiments. The Bi-ellagate photocatalyst was recovered and reused four times without any significant loss in its activity, which provides an eco-friendly, low-cost, recyclable, and efficient photocatalyst for potential photocatalytic applications.


INTRODUCTION
The development of new routes to clean, sustainable, ecofriendly, and inexpensive energy sources is an inevitable challenge. 1 Photocatalysis is one promising technology in which new appropriate materials or systems are explored and investigated. 2 In this regard, solar energy is captured and used via redox reactions for practical applications such as water splitting, 3,4 contaminant decomposition, 5,6 CO 2 photocatalytic reduction, 7,8 or CO 2 utilization. 9,10 The importance of photocatalysis lies on its ability to offer green and sustainable pathways for future development, 1,11 as well as the direct conversion of solar light without causing pollution to the environment. 12 However, some minor concerns are still associated with traditional photocatalysis as it has low efficiency, 13 poor recyclability, a slow rate of electron transport, and limited absorption of visible light. 14 For that, a wide range of attempted modification methods have been done to extend the light absorption range and reduce electron/hole recombination. 15,16 Such methods include ion doping, 17,18 heterojunctions, 19,20 composite fabrication, 21,22 and noble metal deposition. 23,24 Due to their chemical diversity and their robustness, inorganic semiconductors are an interesting class of photocatalysts compared to organic ones. Nonetheless, semiconductors with a distinguished photocatalytic activity may form upon electronic excitation during light-induced photocatalytic process. 25 Semiconductors including metals such as bismuth, 26,27 titanium, 28,29 or manganese 9,30 have been tremendously used as active photocatalysts for different applications. Among these, bismuth-based materials such as bismuth vanadate (BiVO 4 ), 31,32 bismuth oxycarbonate (Bi 2 O 2 CO 3 ), 33 and bismuth oxyhalides (BiOX, X = Cl, Br, and I), 27,34 have a superior photocatalytic activity under visible light irradiation, which is attributed to the narrow band gap energy (<3.0 eV) caused by the distributed electrons in the VB. 35 Another class of currently advanced materials used as photocatalysts are metal−organic frameworks (MOFs). MOFs are favored on account of their excellent structure stability, high surface area, defined diverse structure, and high tunable porosity, 36,37 which make them potential candidates for photocatalysis. 38−40 Particularly, bismuth-based organic frameworks (Bi-MOFs) are new attractive materials in the photocatalysis field. The combined properties of both bismuth metal and MOFs are found to highly induce photocatalytic performance. 41 Although there are limited publications on Bi-MOF photocatalysts, they have recently gained a lot of attention due to their diverse structure and features. 42 As an example, bismuth-based porphyrin MOF photocatalysts exhibited 97% conversion with qualitative reaction selectivity during sulfide oxidation. 43 Moreover, high photodegradation activity of rhodamine B was achieved using Bi-MOFs possessing different linkers as reported by Nguyen et al. 44 Furthermore, reported Bi-MOFs constructed from 1,3,5-benzenetricarboxylic acid (H 3 BTC) 45 and pyridine-2,6-dicarboxylic acid (H 2 PDC) 46 linkers were used for the photodegradation of methyl orange and other organic dyes.
Considering the above, we have chosen an MOF containing bismuth and an ellagate moiety to study its photocatalytic activity. The Bi-ellagate MOF was chosen for its high thermal and chemical stability, which was studied over a variety of media. 47 Moreover, the light-harvesting properties of bismuthcontaining compounds encourage us to investigate the photocatalytic properties of such materials. 48 Herein, we report the use of Bi-ellagate as an active photocatalyst for the oxidative coupling of benzylamine to N,N-benzylidenebenzylamine under simulated visible light irradiation.

Materials and General
Procedures. Bismuth(III) acetate (C 2 H 4 BiO 2 ), ellagic acid hydrate (C 14 H 8 O 9 ), benzylamine (C 7 H 9 N), glacial acetic acid, acetonitrile (ACN), and deuterated chloroform (CDCl 3 ) were purchased from Sigma-Aldrich and used as received. Powder X-ray diffraction (PXRD) was recorded using a Rigaku MiniFlex benchtop Xray diffractometer with a Cu Kα radiation tube (λ = 1.542 Å) running at 40 kV over the range of 3−50°(2θ) and a rate of 2°C min −1 . Over the range of 4000 to 500 cm −1 (512 scan average), the FTIR spectrum was obtained using ATR-IR spectroscopy (Agilent Technologies Cary 600 series FTIR spectrometer) where the background was first collected using a potassium bromide pellet and the oily product was sandwiched between two KBr disks. A Shimadzu UV-3600 spectrophotometer was used to perform UV−vis diffuse reflectance spectroscopy (DRS) of Bi-ellagate over the range of 200 to 800 nm in which barium sulfate was first used to record the baseline. The sample surface morphology and elemental composition were analyzed using Quattro ESEM scanning electron microscopy (SEM) instrument equipped with an energy-dispersive X-ray spectroscopy (EDX) detector, which were operated at high vacuum at a 30 kV accelerating voltage. To confirm the identity of the reaction product, proton nuclear magnetic resonance ( 1 H-NMR) and 13 C-NMR spectra were obtained using a Varian 400 MHz spectrometer in chloroformd as the solvent. Finally, in an aluminum pan, the activated sample was contained, and thermogravimetric analysis (TGA) was performed on it using a Mettler Toledo TGA 2 analyzer. The surface area and porosity were examined using nitrogen sorption experiments at 77 K. The amount of gas adsorbed (cm 2 ·g −1 ) was shown by the N 2 adsorption−desorption isotherm as a function of relative pressure (P/P 0 ), where P is the N 2 equilibrium pressure and P 0 is the saturated vapor pressure at 77 K. The activated sample was first placed under vacuum for 3 h and heated at 353 K. Transmission electron microscopy (TEM) images were recorded on a high-resolution transmission electron microscope (HR-TEM) (FEI Tecnai TF-20) with a field emission gun of 200 kV.

Synthesis of Bi-Ellagate.
According to a previously reported procedure, 47 Bi-ellagate was typically synthesized heterogeneously in a borosilicate glass beaker at room temperature. In the beaker, 0.150 g (0.495 mmol) of ellagic acid and 0.380 g of bismuth acetate (0.984 mmol) were added to a mixture of deionized water (15 mL) and 6% acetic acid (15 mL, prepared from glacial acetic acid). After adjusting the pH of the suspension solution to around 2.3, the reaction was left stirring for 48 h using PTFE stir bars. The solvent was then decanted, and the resulting solid powder was washed several times with deionized water and acetone in which it was centrifuged at 4000 rpm for 5 min after each wash. The produced Bi-ellagate was dried and activated at 120°C under vacuum for 6 h.
2.3. Photocatalytic Activity. The photocatalytic reaction was carried out in 25 mL round-bottom flask connected to a condenser under a normal atmosphere. An optimum of 5.0 mg (0.006 mmol) of Bi-ellagate was added to 51 μL (0.50 mmol) of benzylamine and stirred for 24 h while being subjected to irradiation from a 400 W halogen lamp. After the reaction was complete, the photocatalyst was washed with ACN and removed by syringe filtration. The product was purified by simple column chromatography using silica gel with ethyl acetate and hexane (1:2) to remove any possible impurities or byproducts. The solvent that contained the dissolved product was evaporated using a rotary evaporator. Finally, the isolated product (N-benzylidenebenzylamine) was confirmed using 1 H-NMR, 13 C-NMR, and FTIR. All control experiments were carried out following the same procedure with required changes in some variables as outlined in Table 1.

Characterization of Bi-Ellagate.
The phase structure and purity of the activated Bi-ellagate were confirmed by comparison of the obtained PXRD diffraction pattern to the simulated one (Figure 1), which was evidence of the successful synthesis of Bi-ellagate. The tetragonal lattice of Bi-ellagate was identified with a P4 2 /n space group with cell parameters of a = b = 18.62 Å and c = 5.55 Å. 47 The morphology of the Bi-ellagate framework was studied before and after the activation (Figure 2). The pre-activated sample had an undefined shape, which can be attributed to the presence of solvent molecules. Nonetheless, after Bi-ellagate was evacuated and activated at 120°C, columnar crystals were aggregated similar to the previously reported morphology ( Figure 2B). 47 Furthermore, EDX analysis revealed the expected elemental composition of the sample in which bismuth, carbon, and oxygen were present as they were the only elements in the framework ( Figure S1 and Table S1, Supporting Information). In agreement with the SEM images, the microscopic observations acquired by TEM (Figure 3) show elongated polydispersed crystals when the sample was dry (activated).
FTIR was used to confirm the binding mode and the coordination interaction between ellagic acid and bismuth in the solid state as shown in Figure 4. Comparing Bi-ellagate IR absorption (red spectrum) with the absorption of the linker (blue spectrum), a clear shift in the −OH band is observed from 3155 cm −1 (ellagic acid) to 3453 cm −1 (Bi-ellagate), which suggests the occurrence of a coordination interaction. Moreover, the IR band at 1517 cm −1 (labeled in green) in ellagic acid has disappeared in the Bi-ellagate MOF, which strongly confirms the chelation of bismuth through the hydroxyl group site of ellagic acid. The single-crystal data of Bi-ellagate reported by Grape et al. also supports this finding. 47 Other absorption bands such as the carboxylic band (1722 cm −1 vs 1696 cm −1 ) was slightly affected, which further confirms the binding site at the −OH group.
The thermal analysis of Bi-ellagate was conducted under a nitrogen atmosphere between room temperature and 600°C ( Figure S2, Supporting Information). The first drop in weight was a 12% weight loss of the material, which is attributed to water molecules from the framework pores. This was followed by the second drop, i.e., a dramatic decrease between 330 and 400°C (60% weight loss), which was observed because of the decomposition of the organic linker. The decomposition of Biellagate resulted with the formation of bismuth oxide (Bi 2 O 3 ) from the remaining 40% mass. The physical change of the sample color from pale yellow to dark yellow/orange was also evidence of the MOF thermal decomposition into oxides.
From the relationship between the reflected light intensity and wavelength, the Tauc plot method 19 was applied using the DRS data to calculate the band gap of Bi-ellagate ( Figure 5B). The diffuse reflectance data ( Figure 5A) shows a peak starting from 476.8 nm with light absorption occurring in the visible region. Estimated from a direct allowed transition, a linear fit line was extrapolated on the x axis in which the intercept was the band gap value. As shown in Figure 5B, Bi-ellagate possess a band gap of 2.6 eV (λ ≈ 476.8 nm), which makes it an active photocatalyst in the visible region.
According to Figure 6, which shows the N 2 adsorption− desorption isotherm, Bi-ellagate exhibits a type I isotherm, which is typical of a microporous material. The Brunauer− Emmett−Teller (BET) surface area was determined to be 26 m 2 g −1 , and the Langmuir surface area was 143 m 2 g −1 . Biellagate has a pore volume of 0.08 cm 3 g −1 and an average pore size of 7.7 Å.

Bi-Ellagate Photocatalytic Activity.
The oxidative coupling reaction of benzylamine was selected as a model photocatalytic reaction to investigate the photocatalytic   Upon the coupling of another benzylamine, the peak at 8.4 ppm is the most deshielded signal, which is also due to the generated double bond. Comparing the 1 H-NMR spectra of the starting material and the product, the −CH 2 protons of benzylamine that usually appear at 3.9 ppm are shown downfield at 4.9 ppm in N-benzylidenebenzylamine, indicating that it experienced a different environment, which further proves the coupling step.
In agreement with that, the 13 C-NMR data show a peak at δ C = 65 ppm corresponding to the environment of the two protons that are connected to −CN, which normally appears at a lower ppm value in benzylamine. Moreover, the carbon with the highest shift at δ C = 162 ppm corresponds to the product's characteristic peak of the N�C bond. On the other hand, the proton signals at the aromatic region in the benzene rings of the product show a higher ppm value associated with the effect of the new environment. Having the expected shifts along with the integration of each peak, with no starting material present or byproducts, we were able to confirm a complete reaction conversion yield of 96%. The FTIR spectrum of the product was then recorded and compared to the benzylamine spectrum from a database (NIST Standard Reference Database 35) to provide more evidence of the completion of the reaction over Bi-ellagate. The characteristic band of the −NH stretch for the primary  amine (−NH 2 ) is known to appear as a doublet between 3200 and 3500 cm −1 (symmetric and antisymmetric modes) in benzylamine. Upon coupling, this band has disappeared and a new band at 1643 cm −1 has appeared corresponding to the double bond of C�N and the characteristic benzene ring bands are clearly shown between 1650 and 1800 cm −1 ( Figure  S4, Supporting Information).
To further investigate other reaction conditions, various control experiments were conducted as summarized in Table 1 and percent yields were calculated as explained in Section V in the Supporting Information. The highest yield of 96% of the N-benzylidenebenzylamine product was obtained in an open air and solvent-free environment, while 73% (entry II) was obtained when the reaction was completed using ACN as a solvent. This gives Bi-ellagate an additional advantage to fully proceed the reaction without the need for any solvent, as using solvents is essential for most catalysts reported for this reaction. 49 Almost all other control experiments have shown no conversion yields in the absence of either light (entries III and V) or the photocatalyst (entries V and VI), which strongly supports the high photocatalytic activity of Bi-ellagate under visible light. Studies on the photocatalyst loading effect on the conversion yield were also carried out using higher and lower photocatalyst loadings (2.5 and 10 mg) (entries VII and VIII). It was clearly apparent that the photocatalyst loading has a significant effect on the conversion yield as 78% product yield was obtained using a photocatalyst loading of 2.5 mg. However, 10 mg of the photocatalyst achieved an almost similar yield as 5 mg of the catalyst (97 and 96%, respectively), which shows that using this amount of the photocatalyst as a target is enough to enhance the photocatalytic performance of the photocatalyst.
In addition, the stability of Bi-ellagate was tested through recycling experiments under identical conditions described earlier. Before being used for the next cycle, Bi-ellagate was first separated by centrifugation, flushed with ACN, and dried in an oven at 100°C for 2 h in which the solid Bi-ellagate was then checked and subjected to PXRD. Bi-ellagate maintained excellent photocatalytic performance with the expected product yields for four consecutive cycles ( Figure S5, Supporting Information). The PXRD results (Figure 7) showed barely any difference before and after the reaction of each cycle, suggesting that Bi-ellagate is a highly stable and recyclable photocatalyst for such a reaction.
The oxidative coupling of benzylamine over Bi-ellagate is likely to proceed through an O 2 •− mediated pathway, which is proposed in Scheme 1. Upon irradiation by visible light, the photoexcited electron of the photocatalyst rises to the conduction band (CB), in which it then reacts with atmospheric molecular oxygen to produce the O 2 •− radical, which was previously confirmed using electron spin resonance (EPR) measurements. 50−52 The adsorbed benzylamine is also oxidized by the photoinduced hole in the valence band (VB) to form an aminium radical cation intermediate. This radical cation then undergoes hydrogen-atom abstraction by the O 2 •− radical to produce phenylmethanimine (Ph−CH 2 −NH 2 ),   therefore producing Ph−CH�NH and H 2 O 2 . 51 The further dissociation of H 2 O 2 to •OH radicals acts as supporting material for benzylamine oxidation to the produced imine, which finally reacts with another free benzylamine molecule that eventually produces the N-benzylidenebenzylamine product liberating ammonia. 50 The present work is the first report on the optical properties of Bi-ellagate, a photocatalyst that has been found to be active in in the visible region. Thus, the photocatalytic activity of a Biellagate MOF was studied for the first time since it had been first synthesized by Grape et al. Moreover, an important aspect of this coupling reaction involves using a bismuth-based MOF as a photocatalyst that can provide high conversion yields under neat conditions and using a small amount of the photocatalyst. In addition to the amine self-coupling reaction, Bi-ellagate was planned to be investigated as a photocatalyst for CO 2 cycloaddition to various epoxides in which percent activities were to be compared. Despite its robust and high stability, Bi-ellagate surprisingly failed to fulfill this role as a photocatalyst as it dissolved completely in various epoxides, resulting in a baffling pattern as revealed by PXRD. The benzylamine coupling reaction was then selected as a model reaction instead to highlight the photocatalytic potential of Biellagate MOF. This present work paves the way for further investigations with different amine derivatives, which will show the catalyst's uniqueness in photocatalytic dimerization reaction.

CONCLUSIONS
Bi-ellagate was successfully used as a photocatalyst for the photocatalytic coupling of benzylamine to N-benzylidenebenzylamine. The band gap studies indicated the high potential of Bi-ellagate to work as a photocatalyst in the visible region. Rather than the traditional hydrothermal catalytic reactions that are usually done under high temperature and pressure, Biellagate showed an efficient photocatalytic performance under mild reaction conditions with high photocatalytic activity and stability. The present study offers a simple, green, sustainable, and inexpensive method for utilizing solar energy for multiple applications that aim on reducing fossil fuel depletion.