Controllable Assembly of Vanadium-Containing Polyoxoniobate-Based Materials and Their Electrocatalytic Activity for Selective Benzyl Alcohol Oxidation

During the controllable synthesis of two vanadium-containing Keggin-type polyoxoniobates (PONbs), [Ni(en)2]5[PNb12O40(VO)5](OH)5·18H2O (1) and [Ni(en)3]5[PNb12O40(VO)2]∙17H2O (2, en = ethylenediamine) are realized by changing the vanadium source and hydrothermal temperature. Compounds 1 and 2 have been thoroughly characterized by single-crystal X-ray diffraction analysis, FT-IR spectra, X-ray photoelectron spectrum (XPS), powder X-ray diffraction (PXRD), etc. Compound 1 contains a penta-capped Keggin-type polyoxoniobate {PNb12O40(VO)5}, which is connected by adjacent [Ni(en)2]2+ units into a three-dimensional (3D) organic-inorganic framework, representing the first nickel complexes connected vanadoniobate-based 3D material. Compound 2 is a discrete di-capped Keggin-type polyoxoniobate {PNb12O40(VO)2} with [Ni(en)3]2+ units as counter cations. Compounds 1 and 2 have poor solubility in common solvents and can keep stable in the pH range of 4 to 14. Notably, both 1 and 2 as electrode materials are active for the selective oxidation of benzyl alcohol to benzaldehyde. Under ambient conditions without adding an alkaline additive, compound 1 as a noble metal free electrocatalyst can achieve 92% conversion of benzyl alcohol, giving a Faraday efficiency of 93%; comparatively, 2 converted 79% of the substrate with a Faraday efficiency of 84%. The control experiments indicate that both the alkaline polyoxoniobate cluster and the capped vanadium atoms play an important role during the electrocatalytic oxidation process.


Introduction
Polyoxoniobates (PONbs), as a unique branch of polyoxometalates (POMs), have drawn widespread attention in the past few decades due to their diverse structures and multiple applications in catalysis, nuclear-waste treatment, and virology [1,2]. Nevertheless, compared with other POM members, such as polyoxotungstates, polyoxomolybdates, and polyoxovanadates, the development of PONbs is relatively slow due to the lack of soluble Nb precursors, and their low reactivity and narrow working pH range [3]. Recently, great progress has been made in isopolyoxoniobates and some large clusters, such as {Nb 27 12 } have been reported [4][5][6][7]. In 2002, the first Keggin-type PONb {(Ti 2 O 2 )SiNb 12 O 40 } was successfully synthesized by Nyman et al., marking the beginning of heteropolyoxoniobate chemistry [8]. After that, a series of heteropolyoxoniobates were reported and the Keggin-type {XNb 12 O 40 } n− (X = Si, Ge, P) are the most extensively studied [9,10].
In the periodic table, Nb and V are neighbors with similar ionic radius and electronegativity, and their hydrolysis and condensation can be performed under alkaline conditions. Inspired by these similarities, in 2011, we synthesized the first Keggin-type vanadium-containing PONb {VNb 12 O 40 (VO) 2 } stabilized by Cu complex units [11]. Since then, a series of vanadium-containing Keggin-type PONbs and their derivatives have been reported, including {XNb 12  where the introduced V acts as central, capping, or/and substituted atoms [12][13][14][15][16][17][18][19][20][21][22][23]. Notably, most of the clusters were modified by metal-complex units. The use of the metal-complex unit not only contributes to the isolation of novel Keggin-type PONbs, but can also link the discrete PONb clusters into extended structures. In general, most transition metals tend to hydrolyze rapidly into precipitation under alkaline conditions, and thereby the coexistence of transition metal ions with basic PONbs is a challenge [3]. As the Cu ion can tolerate the alkaline synthesis conditions combining with its Jahn-Teller effect, Cu-complexes are the dominated metal organic units in the synthesis of PONb-based hybrids [24][25][26][27][28]. In contrast, Ni-complexes were seldom used and the extended structure based on V-containing PONb hybrids and a Ni-complex is rare.
Compared with other POM members, the catalytic properties of PONbs are not extensively explored. Due to their Brønsted basicity, PONbs have been used to promote the hydrolysis of chemical warfare agents [29,30]. The introduction of V endows basic PONb clusters with interesting redox properties. For example, a double-anion cluster {PNb 12 O 40 (VO) 2 (V 4 O 12 ) 2 } was successfully prepared in our group, which can effectively promote the basic hydrolysis of the nerve agent simulant and the oxidative decontamination of the sulfur mustard simulant [31]. Then, we found that the organic-inorganic hybrids based on {PNb 12 O 40 (VO) 2 } were active for the selective oxidation of benzyl-alkanes to ketone [14]. Our investigation indicates that the V atoms of {V 5 Nb 23 O 80 } and {V 6 Nb 23 O 81 } play a key role in the selective oxidation of the sulfur mustard simulant [32]. Owing to their fast and reversible electron transfer behavior, POMs are also a kind of promising electrocatalyst [33][34][35]. Recently, the covalent triazine framework immobilized {PMo 10 V 2 O 40 } shows excellent activities in the electrocatalytic oxidation of benzyl alcohols and ethylbenzene [36,37]. However, the electrocatalytic activity of vanadium-containing PONbs is nearly unexplored.
Herein, we report the controllable synthesis and structural characterization of two vanadium-containing Keggin-  [10,[38][39][40][41]. Other starting chemicals and solvents were purchased from commercial source and used without further purification. IR spectra in KBr pellets were collected in the range of 400-4000 cm −1 using a Bruker FT-IR spectrometer (Leipzig, Germany). Thermogravimetric analyses (TGA) of the compounds were performed using a LABSYS EVO device (Setaram Inc., Lyon, France) from room temperature to 800 • C under N 2 atmosphere. Elemental analyses (Nb, V, Ni, P) were measured on a Thermo ICP atomic emission spectrometer (Waltham, MA, USA); C, H, N were performed on an ElementarVario EL cube Elmer CHN elemental analyzer (Langenselbold, Germany). X-ray photoelectron spectrum (XPS) analysis were measured on a Thermo ESCALAB 250 spectrometer using Al Kα radiation as the X-ray source (1486.7 eV). Powder X-ray diffraction (PXRD) data were obtained on SHI-MADZU XRD-6000 X-ray diffractometer (Kyoto, Japan) with Cu Kα radiation (λ = 1.54 Å; 2θ = 5-50 • ). Gas chromatograph analyses were detected on a Shimadzu GC-2014C instrument with an FID detector equipped with an HP 5 ms capillary column. The hydrogen was detected by a Techcomp GC-9700 gas chromatograph (Shanghai, China) with a 5 Å molecular sieve column (2 m × 2 mm) and a thermal conductivity detector (TCD). Temperatureprogrammed chemisorption of carbon dioxide (CO 2 -TPD) was performed on a PCA-1200 temperature-programmed chemisorption instrument. The UV-vis spectra were measured on a UV-2600 (Builder, Beijing, China). Electrochemical surface area experiments were measured on a CHI660E electrochemical workstation (CH Instruments, Shanghai, China). Other electrochemical experiments were performed on an Ivium-OctoStat30 multi-channel electrochemical workstation (Eindhoven, The Netherlands).

Synthesis of [Ni(en) 3 ] 5 [PNb 12 O 40 (VO) 2 ]·17H 2 O (2)
Compound 2 was synthesized by a similar procedure to that of 1, but without adding NaVO 3 . The pH value of the mixture was adjusted to 10.50 using 2 M NaOH and transferred to a Teflon-lined autoclave (23 mL), kept at 140 • C for 72 h, and then slowly cooled to room temperature. Brown block crystals of 2 were isolated in about 21.8% yield (based on Nb). Anal

Preparation of Working Electrode
Grinded crystal samples of compound 1 or 2 (10 mg) and acetylene black (3 mg) were dispersed uniformly in isopropanol (0.5 mL) containing 5 wt% Nafion under ultrasonic conditions. A total of 50 µL of the suspension was drop-cast onto a piece of carbon cloth (1 cm 2 ) and then dried slowly at room temperature.

Cyclic Voltammetry Experiments
Cyclic voltammetry experiments were performed in acetonitrile (10 mL) containing supporting electrolyte LiClO 4 (1.0 mmol) and benzyl alcohol (0.5 mmol) under ambient conditions, and the scan rate was kept at 40 mV s −1 . The cyclic voltammetry tests of 1 and 2 were performed using a three-electrode setup: carbon cloth modified by 1 or 2 as the working electrode, platinum plate electrode as the counter electrode, and Ag/Ag + as the reference electrode.

Electrochemical Surface Area Experiments
Electrochemical surface areas of compounds 1 and 2 were estimated by the capacitance of the double layer C dl , which were determined by cyclic voltammetry tests [42]. For the cyclic voltammetry tests of 1 and 2, glassy carbon electrode (3 mm diameter) was dripped with 5 µL isopropanol suspension of 1 or 2, and served as the working electrode. The potential window was 0.01-0.13 V vs. Ag/Ag + , where no Faradaic processes occur. The scan rates were 10 mV s −1 , 20 mV s −1 , 30 mV s −1 , 40 mV s −1 , 50 mV s −1 , 60 mV s −1 , 70 mV s −1 , 80 mV s −1 , 90 mV s −1 , and 100 mV s −1 . The C dl was calculated by plotting the relationship between ∆j and scanning rate at 0.07 V (∆j = j a − j c , j a , and j c represent the current densities of the anode and cathode, respectively), and the slope of the image is twice that of C dl .

Controlled Potential Electrolysis Experiments
Bulk electrolysis experiments were performed in an undivided cell using a threeelectrode setup with carbon cloth modified by catalysts as the working electrode, platinum plate electrode as the counter electrode, and Ag/Ag + electrode as the reference electrode. A mixture of acetonitrile (10 mL) containing supporting electrolyte LiClO 4 (1.0 mmol) and benzyl alcohol (0.5 mmol) was added to the undivided cell with applied potential of 1.6 V vs. Ag/Ag + . After the electrolysis experiment, biphenyl was added to the reaction solution as internal standard, and then the product was quantitatively detected by GC. For the recycle test, carbon cloth modified by compound 1 was washed three times with acetonitrile and ethyl alcohol, and dried for the next cycle.

Synthesis and Structure
Compound 1 was prepared by the hydrothermal reaction of K 7 HNb 6 O 19 ·13H 2 O, VOPO 4 ·2H 2 O, NaVO 3 , Ni(CH 3 COO) 2 ·4H 2 O, and en at 160 • C. The single-crystal structural analysis (Table S1) (Table S6). The oxidation state of the V atoms was further confirmed by XPS measurement. In the XPS spectrum of 1 ( Figure S5), the peaks at 523.2 eV and 516.0 eV are attributable to V 4+ 2p 1/2 and V 4+ 2p 3/2 , respectively.  (Table S7). We systematically explored the factors that influence the synthesis of 1 and 2. It is found that the used en can effectively protect Ni 2+ from hydrolysis under alkaline conditions. When 1,2-diaminopropane or 1,3-diaminopropane was used instead of en, the amount of precipitation was obtained. In addition, control experiments show that temperature and vanadium source play important roles in the synthesis of 1 and 2. Following a procedure similar to that of 1, compound 2 was obtained by removing NaVO 3 at 140 • C. In addition, compound 2 cannot be obtained by lowering the hydrothermal temperature or varying the ratio of VOPO 4 to NaVO 3 in the synthesis of 1. Therefore, we speculate that the evaluated hydrothermal temperature and the use of NaVO 3 might increase the number of vanadium caps in the PONb cluster, and meanwhile the terminal O atoms of the Keggin-type {PNb 12 } would be activated by introducing additional vanadyl caps. As a result, two-capped {PNb 12 O 40 (VO) 2 } was isolated as a discrete cluster with the Ni-complex as counter cations, and five-capped {PNb 12 O 40 (VO) 5 } gave rise to a 3D framework by using the Ni-complex as linker.
The IR spectra of 1 and 2 ( Figure S2 The phase purity of 1 and 2 was confirmed by PXRD ( Figure S3), where the collected diffraction peaks match well with the simulated ones. Compounds 1 and 2 are nearly insoluble in water and common organic solvents, such as CH 2 Cl 2 , THF, CH 3 COCH 3 , CH 3 CN, and DMF ( Figures S8 and S9). Therefore, we tested their pH stability in aqueous solution modified by PXRD ( Figure 3) and IR spectra ( Figure S7). As shown in Figure 3, 1 and 2 remained stable in the pH range of 4-14 after soaking for 24 h and began to decompose when the solution pH was 3. In addition, the crystals of 1 and 2 can keep their structure integrity after heating in organic solvent in the temperature range of 40-80 • C ( Figures S10 and S11).

Electrocatalytic Selective Oxidation of Benzyl Alcohol
The selective oxidation of alcohols to aldehydes is one of the important organic transformations [43,44]. Compared with the traditional oxidation processes, the electrochemical oxidation provides an efficient and sustainable alternative [45,46]. Driven by electricity, alcohols can be oxidized on the anodic electrode under ambient conditions with hydrogen released from the cathodic electrode. Although some electrocatalysts have been developed in the anodic oxidation of alcohols, the selective oxidation of alcohols to the corresponding aldehydes remains a challenge, and in the reported system, the reaction activity significantly relies on the addition of alkaline additives [36,47,48]. Therefore, it is necessary to develop efficient and cost-effective electrode materials to realize the selective oxidation of alcohols under alkaline additive free conditions.
Considering that V-containing PONbs 1 and 2 both have both Brønsted basicity and redox activity, we investigate the electrocatalytic activities of 1 and 2 using the selective oxidation of benzyl alcohol (BA) to benzaldehyde as a model reaction. The electrocatalytic activity of 1 and 2 was first evaluated by the cyclic voltammetry (CV) method, which was performed in an acetonitrile solution containing LiClO 4 and BA with a carbon cloth modified by 1 or 2 as the working electrode. As shown in Figure 4, for 1 and 2, the addition of BA leads to the significant increase in the anodic peak currents, indicating that the two compounds have a fast electrocatalytic response to the oxidation of BA. Notably, the anodic peak current of 1 is obviously higher than that of 2, revealing that 1 has better electrocatalytic performance than 2. The electrocatalytic activities of 1 and 2 were further verified by bulk electrolysis experiments performed in an undivided cell using 1 or 2 modified carbon cloth as the working electrode. As shown in Figure 5a and Table 1, both 1 and 2 are active for the selective oxidation of BA. Under ambient conditions, 92% of BA was converted by 1 in 6 h at the potential of 1.6 V vs. Ag/Ag + and the selectivity for benzaldehyde reached 95%, giving the Faradaic efficiency (FE) of 93% (Table 1, entry 2). In addition, a trace amount of N-benzylacetamide as the only by-product was detected ( Figures S12 and S14). Under the otherwise identical conditions, the catalytic activity of 2 (conversion: 79%, selectivity: 90%, FE: 84%, Table 1, entry 3) is lower than that of 1. During the electrolysis process, hydrogen was released on the counter electrode ( Figure S13).   To investigate the influence of the Ni-complex unit, PONb, and the capped V of 1 and 2 on the electrocatalytic selective oxidation of BA, the following control experiments were carried out (Table 1, entries 4-8). As shown in Table 1, entry 4, the electrocatalytic activity of the Ni-complex unit is negligible, because the catalytic performance of Ni(en) 3 Table 1, entry 6). The above results indicate that PONbs contribute to the conversion of BA because basic PONbs might facilitate the dehydrogenation oxidation of BA. Therefore, the temperatureprogrammed desorption of the carbon dioxide (CO 2 -TPD) measurement for 1 and 2 was performed, where the desorption peaks at 152 • C for 1 and 148 • C for 2 corresponding to the weak base site were observed, respectively ( Figure S15). In addition, polyoxovanadate, K 6 [V 10 O 28 ], can convert 83% of the substrate (Table 1, entry 7), but its selectivity (76%) is lower than that of the V-containing PONb 1 or 2. As shown in Table 1, entry 8, the catalytic activity of the bicapped Keggin-type [N(CH 3 ) 4 ] 9 [PNb 12 O 40 (VO) 2 ] (conversion: 74%, selectivity: 89%) is similar to that of 2 (conversion: 79%, selectivity: 90%). The control experiments above show that both the PONb cluster and the V caps contribute to the enhancement of BA oxidation. Then, we speculate that the different catalytic activity of 1 and 2 is mainly caused by their different number of V caps. This is further confirmed by the electrochemical surface area (ECSA) measurement: the ECSA of 1 (4.0 mF·cm −2 ) with five V caps is higher than that of 2 (3.3 mF·cm −2 ) with two V caps (Figures 5b and S16).
To explore the optimal reaction conditions, we systematically investigated the influences of electrolyte, solvent, applied potential, and catalyst dosage on the electrocatalytic selective oxidation of BA by 1. As shown in Figure 6a, compared with other types of supporting electrolytes, 1 exhibits excellent catalytic performance by using LiClO 4 . Meanwhile, it is found that acetonitrile with excellent conductivity exhibits a better performance than that of acetone, tetrahydrofuran, and N,N-dimethylformamide (Figure 6b). When the applied potential was increased from 1.4 to 1.6 V vs. Ag/Ag + , the conversion of BA was increased from 16% to 92%, but when it reached 1.7 V vs. Ag/Ag + , the selectivity decreased to 79%, although 98% of the BA was converted (Figure 6c). Therefore, 1.6 V vs. Ag/Ag + is the optimal potential. As shown in Figure 6d, the best catalytic performance was achieved by using 1.0 mg 1. After that, the catalytic activity was not further improved by increasing the catalyst dosage.
Moreover, the recyclability and stability of 1 were evaluated. As shown in Figure 7a, the catalytic activity of 1 is basically maintained after four cycles. There is no obvious change observed in the IR spectra of 1 before and after the reaction ( Figure S17), revealing that compound 1 is basically stable after the recycle test. We compared the XPS spectra of 1 before and after the recycle. As shown in Figure 7b, in the V 2p region, the peaks at 523.2 eV and 516.0 eV are basically unchanged, indicating that the oxidation state of V remains +4 in 1. Meanwhile, to verify the heterogeneity of 1, the reaction solution was tested by ICP-OES (detection limit ca. 1 ppm) and no Nb, V, or Ni was detected, indicating that there is no catalyst leaching during the electrocatalytic process. In addition, no characteristic absorption of V-containing PONb is detected by the UV-vis spectrum ( Figure S18). In order to explore the possible mechanism of the electrocatalytic selective oxidation of BA by 1, we performed free radical trapping experiments. Oxygen radical scavenger, diphenylamine, and hydroxyl radical scavenger, tert-butanol, were added to the reaction system, respectively. As shown in Table S8, the oxidation of BA was significantly inhibited after the addition of diphenylamine. Therefore, we speculate that a free radical process was involved in the electrocatalytic oxidation of BA. Based on the experimental results, a plausible reaction mechanism was proposed ( Figure S19). First, the electrocatalyst 1 in the reduced state (1-Red) is oxidized to its oxidized state (1-Ox) at the anode under constant potential. Then, the hydroxyl group of BA might be activated by the surface bridging O of the Keggin-type PONb cluster due to its Brønsted basicity [49,50]. After that, the BA is oxidized by 1-Ox through a −1e − /−1H + process, generating the oxygen radical species (PhCH 2 O • ). PhCH 2 O • is further oxidized to benzaldehyde through another −1e − /−1H + process, and meanwhile, 1-Ox is reduced to 1-Red, releasing protons to complete a catalytic cycle. The released protons are reduced at the cathode to produce H 2 .

Conclusions
In summary, two novel vanadium-containing Keggin-type PONbs modified by a Ni-complex have been successfully synthesized by controlling temperature and the vanadium source. In 1, the five-capped {PNb 12 3 ] 2+ as counter cations. Compounds 1 and 2 as organic-inorganic hybrid materials exhibit good pH stability in aqueous solution and thermal stability in organic solvent. Importantly, un-der alkaline additive free conditions, compounds 1 and 2 are highly active for the selective oxidation of BA. The control experiments show that both the Brønsted basicity of PONb and the redox activity of the V caps play an important role in the electrocatalytic process. This study not only enriches the structure data base of vanadium-containing PONbs but also extends their catalytic application.
Supplementary Materials: CCDC 2164660 (1), 2164659 (2) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac. uk/conts/retrieving.html, accessed on 13 April 2022. The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules27092862/s1, Table S1: Crystal data and structure refinement for 1 and 2; Tables S2-S5: Selected bond lengths and bond angles for 1 and 2; Tables S6 and S7: BVS results of Ni and V atoms for 1 and 2; Table S8: Electrocatalytic oxidation of benzyl alcohol (BA) catalyzed by 1 in the presence of radical scavengers; Figure S1: Digital photographs of 1 and 2; Figure S2: the IR spectra of 1 and 2; Figure S3: the PXRD patterns of 1 and 2; Figure S4: The TG curves of 1 and 2; Figure S5: The XPS spectra for Ni(2p) and V(2p) in 1; Figure S6: The XPS spectra for Ni(2p) and V(2p) in 2; Figure S7: The IR spectra of 1 and 2 after being soaked in aqueous solutions with different pH values for 24 h; Figures S8 and S9: The PXRD patterns and IR spectra of 1 and 2 after being soaked in different solvents; Figures S10 and S11: The PXRD patterns and IR spectra of 1 and 2 after heating in acetonitrile for 2 h; Figure S12: Gas chromatogram of the benzyl alcohol oxidation by 1; Figure S13: The gas chromatograph of benzyl alcohol oxidation before and after reaction; Figure S14: GC-MS spectrum of the by-product; Figure S15: CO 2 -TPD for 1 and 2; Figure S16: CV curves of 1 and 2 at different scan rates; Figure S17: The IR spectra of 1 after the recycle test; Figure S18: UV-vis spectra of postreaction solution, BA, benzyl aldehyde, and PNb 12 V 2 ; Figure S19: Proposed mechanism for the electrocatalytic selective oxidation of BA to benzaldehyde by 1.  Acknowledgments: The authors thank the Analysis and Testing Center, Beijing Institute of Technology, for device preparation and characterization.