Tetrabutyl Ammonium Salts of Keggin-Type Vanadium-Substituted Phosphomolybdates and Phosphotungstates for Selective Aerobic Catalytic Oxidation of Benzyl Alcohol

Abstract

A series of tetrabutyl ammonium (TBA) salts of V-included Keggin-type polyoxoanions with W (TBA₄PW₁₁V₁O₄₀ and TBA₅PW₁₀V₂O₄₀) and Mo (TBA₄PMo₁₁V₁O₄₀ and TBA₅PMo₁₀V₂O₄₀) as addenda atoms were prepared using a hydrothermal method. These synthesized materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), UV-Vis diffuse reflectance (DRS UV-Vis), thermogravimetric analysis (TGA), CHN elemental analysis (EA), inductively coupled plasma spectrometry (ICP-MS), and N₂ physisorption techniques to assess their physicochemical/textural properties and correlate them with their catalytic performances. According to FT-IR and DRS UV-Vis, (PV_XW(Mo)_12−XO₄₀)^(3+X)− anions are the main species present in the TBA salts. Additionally, CHN-EA and ICP-MS revealed that the desired stoichiometry was obtained. Their catalytic activities in the liquid-phase aerobic oxidation of benzyl alcohol to benzaldehyde were studied at 5 bar of O₂ at 170 °C. Independently of the addenda atom nature, the catalytic activity increased with the number of V in the Keggin anion structure. For both series of catalysts, TBA salts of polyoxometalates with the highest V-substitution degree (TBA₅PMo₁₀V₂O₄₀ and TBA₅PW₁₀V₂O₄₀) showed higher activity. The maximum benzyl alcohol conversions obtained were 93% and 97% using (TBA)₅PMo₁₀V₂O₄₀ and (TBA)₅PW₁₀V₂O₄₀ as catalysts, respectively. In all the cases, the selectivity toward benzaldehyde was higher than 99%.

1. Introduction

Selective alcohol oxidation is among the most important of oxidation reactions. These reactions are widely used in the chemical industry and in the academic world because carbonyl compounds are important intermediates in chemical synthesis [1]. Conventional processes for alcohol oxidation use some environmentally toxic solvents (e.g., chlorinated) and generally involve harmful inorganic oxidants, such as permanganates, chromates, and peroxides in stoichiometric quantities [2]. Although conventional processes are a viable alternative for the oxidation of alcohols, it is a great challenge to develop selective processes to obtain the desired product. For the reasons stated above, it is imperative to develop new catalytic systems that are eco-friendly and cost-effective for selective alcohol oxidation.

Over the past few years, many catalytic systems have been developed and used for the catalytic oxidation of benzyl alcohol to benzaldehyde, which is a valuable building block for the pharmaceutical, alimentary, and cosmetic industries [3]. To conduct this reaction, vapor-phase and liquid-phase reactions have been studied. Vapor-phase benzyl alcohol catalytic oxidation requires high temperatures, which results in high energy consumption and leads to low selectivity toward benzaldehyde, together with catalyst deactivation [4]. Therefore, liquid-phase oxidation of benzyl alcohol to benzaldehyde is preferable because it can overcome these drawbacks. The choice of oxidant determines the practicability and efficiency of the catalytic systems [5]. In liquid-phase alcohol oxidation, the aerobic system (O₂) is widely used as an oxidant because it has been shown to be an ideal oxidant for catalytic oxidation of these substrates [6,7]. Additionally, molecular oxygen O₂ has been regarded as the greenest oxidant because it is eco-friendly, inexpensive, easy to handle, and atom efficient, and water is the only byproduct [5].

In recent years, many catalysts have been studied, among which polyoxometalates (POMs) have attracted attention and been revealed to be very active for alcohol oxidation [4]. POMs are inorganic nano-sized anionic transition metal oxide anions with different types of structural architectures [8]. These aggregates consist of d⁰ metal addenda atoms (usually Mo, W, Nb, etc.) and oxygen arranged in octahedral units (MO₆) [9]. POMs with Keggin structures have been intensively studied as green oxidation catalysts for both homogeneous and heterogeneous systems because this catalyst shows both acid and redox properties [10]. The most important characteristic is that their thermal stability and their acidic and redox properties can be tuned by modifying their molecular compositions [11].

Several studies have demonstrated that the catalytic redox activity in Keggin-type polyoxometalates can be enhanced via the substitution of V in place of the Mo or W addenda atom in their primary structure [12,13]. This effect can be explained by considering that M⁺⁶ substitution via V⁺⁵ increases the POM oxidation potential due to the higher reducibility of vanadium [14]. The role of V substitution in the POM primary structure has been reported in toluene to phenol oxidation, [15], toluene to benzyl alcohol and aldehyde [3], and benzyl alcohol to aldehyde [4]. Frenzel et al. [12] studied the oxidation of sulfides to sulfones using Keggin-type silicopolyoxotungsto compounds (K₅(SiV_XW₁₁O₄₀) with a variable (1,2) vanadium content and reported that vanadium incorporation improves the catalytic activity under mild reaction conditions.

The main role of countercations in POM chemistry is controlling solubility via the electrostatic stabilization of polyoxoanions [16]. Modification of POMs with organic units has been applied as an effective strategy to improve their catalytic activity, recovery, and reuse. Many organic countercations have been studied for the modulation of POM solubility, such as tetramethyl (TMA), tetrapropyl (TPA), and tetrabutyl (TBA) ammonium salts. Tetrabutyl ammonium cations have been used for many years for POM precipitation into organic solvents [16], showing excellent performance in terms of catalytic oxidation reactions [17].

The points discussed thus far provide background information on the role of vanadium content in the oxidation properties and the catalytic activity of tetraalkylammonium salts of phosphomolybdates and phosphotungstates in catalytic systems.

The aim of this work is the synthesis and characterization of two series of Keggin-type phosphomolybdo (PMoV) and phosphotungstovanadates (PWV) with two different Mo and W replacement degrees (X = 1 and 2) of tetrabutylammonium (TBA) salts, to be used as bulk catalysts in the heterogeneous liquid-phase aerobic oxidation of benzyl alcohol to benzaldehyde.

2. Results and Discussion

2.1. Materials Characterization

The color of (TBA)₃PW, (TBA)₄PWV and (TBA)₅PWV₂ samples are shown in Figure 1. In the W-series, the V-unsubstituted (TBA)₃PW (a) shows a white color, whilst the V-substituted (TBA)₄PWV (b) and (TBA)₅PWV₂ (c) colorations are yellow and light orange, respectively. Regarding the Mo-series, a similar color change was observed when increasing the amount of V included. This color change is due to the increasing vanadium inclusion in the primary structure of the Keggin-type heteropolyanions. The orange color comes from the intense coloration that V has with an oxidation state of V⁺⁵. The colors obtained for these compounds are in accordance with the reported literature [18,19,20,21].

2.2. Characterization

The results of elemental analysis of the materials synthesized in the present study and determined by ICP-MS are shown in

Table A1. Metal analysis of pure and substituted Mo and W catalysts determined by ICP-MS.

Catalyst	M a (wt%) Exp. b	V (wt%) Exp.	M a (wt%) Calc. c	V (wt%) Calc.	M a mmol Exp.	V mmol Exp.	M a/V Ratio Exp.	M a/V Ratio Calc.
(TBA)3PMo	43.7	-	45.1	-	4.6	-	-	-
(TBA)4PMoV	39.1	1.9	38.4	1.8	4.1	0.4	10.9	11.0
(TBA)5PMoV2	35.1	3.9	32.6	3.5	3.7	0.8	4.8	5.0
(TBA)3PW	60.8	-	61.2	-	3.4	-	-	-
(TBA)4PWV	56.7	1.5	54.5	1.4	3.1	0.3	10.5	11.0
(TBA)5PWV2	50.1	2.8	48.1	2.7	2.7	0.5	5.0	5.0

^a M = Mo or W; ^b experimental values; ^c calculated values as anhydrous salts.

. The results obtained show that such anions have been successfully synthesized following the procedure described above.

The ICP-MS values are in good agreement with the calculated theoretical ones; this fact confirms that the desired Mo(W)/V ratios were obtained. The results of the CHN elemental analysis of the synthesized materials are summarized in

Table A2. CHN analysis of the pure and substituted Mo and W catalysts by elemental analysis.

Catalyst	C (wt%)	H (wt%)	N (wt%)	Cation (mmol × 103) a	Anion (mmol × 103) b	Cation/Anion Exp. c	Cation/Anion Theory d
(TBA)3PMo	23.1	4.4	1.6	3.0	1.0	3.1	3
(TBA)4PMoV	25.2	4.7	1.8	2.0	0.5	3.9	4
(TBA)5PMoV2	29.1	5.3	2.0	2.0	0.4	4.9	5
(TBA)3PW	16.1	2.9	1.0	2.1	0.7	3.0	3
(TBA)4PWV	17.7	4.5	1.3	2.5	0.6	4.4	4
(TBA)5PWV2	20.6	3.9	1.5	1.6	0.3	4.8	5

^a cation corresponds to a TBA salt; ^b anion corresponds to a (PMo_(12−x)V_x O₄₀)^(3+x)− or (PW_(12−x)V_xO₄₀)^(3+x)− moiety; ^c experimental value; ^d theoretical values calculated as anhydrous salts.

. The elemental analysis confirms that the expected cation/anion ratios for the (TBA)₃PW(Mo), (TBA)₄PW(Mo)V, and (TBA)₅PW(Mo)V₂ materials (3, 4, and 5 respectively) were successfully obtained.

Infrared spectra of the (TBA)₃PMo, (TBA)₄PMoV, (TBA)₅PMoV₂, (TBA)₃PW, (TBA)₄PWV, and (TBA)₅PWV₂ organic salts are shown in Figure 2. These spectra show their main absorption bands in the 800–1100 cm⁻¹ region (which is characteristic of the Keggin structure) and are in agreement with those previously reported in the literature [13,22] for substituted structures. Moreover, the FT-IR spectra exhibit the characteristic vibration bands of organic cation alkyl chains at 1381 and 1470 cm⁻¹ (assigned to the vibration of the C-H bonds) and the band at 1633 cm⁻¹ (assigned to the vibration of H₃O⁺ ions).

Additionally, the not-shown vibration bands at 3440, 2960, and 2875 cm⁻¹ are attributed to H₃O⁺ (the former) and C-H (the latter two).

On the other hand, the FT-IR spectra also show that the maximum of main Keggin band structure shifts to lower frequencies with an increasing V substitution degree in the heteropolyanions (

Table A3. Characteristic FT-IR bands of the pure and substituted Mo and W catalysts.

Catalyst	ν P-Oa (cm−1)	ν Mo-Od (cm−1)	ν Mo-Ob-Mo (cm−1)	ν Mo-Oc-Mo (cm−1)
(TBA)3PMo	1064	960	880	809
(TBA)4PMoV	1061	959	879	806
(TBA)5PMoV2	1060	954	877	805
(TBA)3PW	1081	981	892	816
(TBA)4PWV	1068	968	889	813
(TBA)5PWV2	1067	967	889	812

). These results confirm vanadium atom inclusion in the primary Keggin structure and agree with previous reports [23,24].

The Keggin-type heteropolyanions and their salts display a characteristic XRD diffraction peaks group at the 2θ values between 5° and 10° [13,25]. These characteristic peaks related to the H₃PMo₁₂O₄₀ (H₃PMo) Keggin structure appears at 6.5 (111), 8.2 (200), and 10.7 (220) of 2θ (hkl) (JCPDS File 01-070-0059). Similar values of 6.9 (010), 8.8 (200), and 9.0 (002) of 2θ (hkl) were reported for H₃PW₁₂O₄₀ (H₃PW) heteropolyacid (JCPDS File 00-050-0655).

For the TBA-synthesized solids—(TBA)₃PMo, (TBA)₄PMoV, and (TBA)₅PMoV₂ and their analogous W-based materials (TBA)₃PW, (TBA)₄PWV, and (TBA)₅PWV₂—the XRD patterns show both groups of characteristic diffraction peaks as previously described (Figure 3).

In the case of (TBA)₃PMo, the characteristic diffraction peaks related to the Keggin structure are located at 7.1, 8.7, and 10.6 of 2θ. These 2θ values are slightly different from those ascribed to the parent heteropolyacid due to the replacement of H⁺ by TBA⁺. However, in the case of (TBA)₄PMoV and (TBA)₅PMoV₂, the increment in the TBA⁺/anion ratio leads to major differences in peak position and intensity [26,27]. An analogous behavior was observed for the W-series.

The DRS spectra of the Mo-series (TBA)₃PMo, (TBA)₄PMoV, and (TBA)₅PMoV₂ organic salts are shown in Figure 4. It has been previously reported that the bands assigned to the charge transfer from bridging or terminal O 2p to W 5d (M-O-M and M-Od, respectively) in the absorption spectrum of unreduced polyanions appear in the range of 200–550 nm [13,18]. The metal M (W, Mo) linked to terminal O has strong double bond character and generates a charge transfer transition in the region of higher energy (210–230 nm). In contrast, the transfer from bridging O 2p to M 5d is observed in the lower energy region (240–550 nm) [28,29].

All of the DRS spectra in Figure 4 display the characteristic bands corresponding to M=O (210–230 nm) and Mo-O-Mo (240–550 nm) electron transfer. The W-series solids (TBA)₃PW, (TBA)₄PWV, and (TBA)₅PWV₂ exhibit the same behavior. The absorption edge energy (E_g) in the UV-Vis spectrum reflects the energy required for ligand-to-metal charge transfer (LMCT) [18].

Table 1. Absorption edge energy of the pure and substituted Mo and W catalysts.

Catalyst	Absorption Edge (nm)
(TBA)3PMo	495
(TBA)4PMoV	544
(TBA)5PMoV2	569
(TBA)3PW	356
(TBA)4PWV	493
(TBA)5PWV2	597

shows the absorption edge energy values calculated from the intersection of the extrapolated spectrum with the abscissa axis in the descending part of the curve [30,31].

A continuous diminution of E_g values in the order E_g PM > E_g PMV > E_g PMV₂ can be observed. The E_g value decreases in parallel with the increment vanadium atoms in the Keggin anion. The absorption edge energy corresponds to the energy required for the transfer of an electron from the higher energy occupied orbital (HOMO) to the lower energy unoccupied orbit (LUMO) [32]. In Keggin-type POMs, the HOMO is composed of 2p orbitals of the O bridges, while the LUMO is a mixture of d orbitals of the metallic centers of the structure and the 2p orbitals of the neighboring oxygen atoms [33]. These differences in terms of energy have been related to the oxidation potential; therefore, the smaller the energy difference between the HOMO and LUMO is, the greater the wavelength, and the POM will be more easily reduced [29,31]. The (TBA)₃PMo and (TBA)₃PW structures display higher E_g values for each series, and they decrease in parallel with an increase in the number of vanadium atoms in the Keggin anion. The substitution of an M atom in the Keggin structure does not affect the HOMO energy level because these orbitals are mainly composed of O 2p orbitals. However, the replacement of M by different elements affects the LUMO energy level since they are derived from the d orbitals of M. This indicates that the modification of the LUMO energy level is responsible for the different absorption edge energies of PMV and PMV₂ with respect to PM (where M is Mo or W). This result is in line with those previously reported by Barteau et al. [29], showing that V-containing POMs have enhanced redox properties because V stabilizes the LUMO energy level relative to the unsubstituted PM. Furthermore, the redshift of the absorption bands with the increase in V atoms is responsible for the color change observed in the solids (Figure 1).

The TGA analyses up to 240 °C showed no thermal degradation of the V-substituted materials. TGA of the synthesized materials showed a reduction in weight in the range 240–350 °C, corresponding to the organic TBA cation decomposition through a Hoffman’s elimination reaction [18,34] (Figure A1). At temperatures higher than 400 °C, the Keggin anion decomposition begins via the elimination of acidic protons in the form of structural water, as reported in the literature [35,36]. These results allowed us to use the synthesized materials in the aerobic oxidation reaction of benzyl alcohol at temperatures lower than 240 °C.

The N₂ adsorption isotherms obtained for the materials are presented in Figure 5, and the main textural property data are summarized in

Table 2. Textural properties of the pure and substituted Mo and W catalysts.

Catalyst	SBET (m²/g)
(TBA)3PMo	3
(TBA)4PMoV	9
(TBA)5PMoV2	14
(TBA)3PW	5
(TBA)4PWV	8
(TBA)5PWV2	14

. The figure shows the main features of type II IUPAC isotherms and the characteristics of non-porous or macro-porous materials [37,38]. The prepared materials display rather low BET area values (in the range of the experimental error ˂ 10 m²g⁻¹), which are in accordance with values reported previously in the literature [21]. It was determined that in both series (Mo and W), the higher the amount of vanadium included, the higher the BET area obtained (Table 2). This effect was reported by Hua et al. [39] and was attributed to an expansion of the latter due to the increase in the alkyl chain length of the countercation, implying that it is more accessible to the substrates. In our case, we could associate it with a higher number of organic cations resulting from a higher number of vanadium atoms [40,41]. The SEM images of the Mo-series (TBA)₃PMo, (TBA)₄PMoV, and (TBA)₅PMoV₂ organic salts are shown in Figure 6. In these images, we can observe clear differences in surface morphology between the unsubstituted and V-substituted materials. In the W-series (TBA)₃PW, (TBA)₄PWV, and (TBA)₅PWV₂, the same trend was observed. These differences result from the number of organic cations. In addition, it was found that the particles of the unsubstituted materials in each series (TBA)₃PMo, (TBA)₃PW show a more compact arrangement than the particles of the V-substituted ones. This implies that although the crystals are non-porous, the changes in their aggregation state of the V-substituted materials are responsible for the BET surface area changes.

2.3. Catalytic Benzyl Alcohol Oxidation

In all the liquid-phase aerobic catalytic oxidation of benzyl alcohol reactions, the selectivity was >99%. An example chromatogram is presented in Figure 7, where benzaldehyde is the only reaction product.

The liquid-phase aerobic catalytic oxidation of benzyl alcohol to benzaldehyde conversion results are shown in Figure 8a and Figure A2) (at 4 h of reaction and selectivity at a 100% conversion level) for the Mo and W materials.

The (TBA)₃PMo catalyst shows a total conversion of 52% and increases up to 67% for (TBA)₄PMoV, achieving a total conversion of 93% for the largest substituted material (TBA)₅PMoV₂. In the W-series, the (TBA)₃PW catalyst exhibits a low conversion (27%). The conversion increases to 82% and 97% in the presence of (TBA)₄PWV and (TBA)₅PWV_2, respectively. It is important to note that the control begins to show conversions after 180 min, reaching a maximum of 18% after 240 min. This may be due to the over-oxidation of the benzyl alcohol; this phenomenon was described by Sankar et al. [42]. Observed rate constants k for benzyl alcohol oxidation are also shown in Figure 8b. Results show that in each series (Mo or W), the observed rate constants become larger as the V-substitution [43] in the solid increases, and all the reactions appear to follow first-order kinetics (Figure A3).

Based on the results presented above, we can determine that the catalytic activity in the oxidation of benzyl alcohol was significantly improved by introducing V into the (TBA)₃PMo and (TBA)₃PW structure. The conversions of (TBA)₄PMV and (TBA)₅PMV₂ were much higher than those obtained for the V-free phosphomolybdates and phosphotungstates, which implies that V atoms play an important role in the studied reaction [44]. Likewise, the conversions obtained for the inclusion of two V atoms were higher than those obtained for a single V atom. This monotonic increase was previously reported by Li et al. [45] in the oxidation of arenes, and by Yajima et al. [5] in the oxidation of thioamides.

It can be also observed that for some reactions, an appreciable conversion takes place after an induction period. In the Mo-series, (TBA)₃PMo shows an induction period of approximately 30 min, (TBA)₄PMoV shows 20 min, and (TBA)₅PMoV₂ (with the largest inclusion of V) shows no induction time. Regarding the W-series, we noticed that for V-unsubstituted (TBA)₃PW and mono-substituted (TBA)₄PWV, the same induction period of 40 min was observed. No induction time was observed for di-substituted (TBA)₅PWV₂, as in the Mo-series. Therefore, the induction time tends to decrease with increasing vanadium substitution; this effect should be analyzed. Nomiya et al. [46] studied the induction period in benzene oxidation via V-substituted silicotungstates and found a relationship between this and the formation of vanadium active species in the polyoxotungstate structure. Other authors have reported that this induction period is associated with the formation time of catalytically active species in oxidation reactions of both alcohols (cyclohexanol) [47,48] and olefins [49]. According to Huang et al. [50], in a study of alcohol aerobic oxidation by V-substituted polyoxomolybdates, VO₂⁺ could be the catalytic active species present under reaction conditions [51].

In summary, shortening the induction time and incrementing the maximum conversion upon the addition of V in the Keggin structure were observed for (TBA)₃PMo and (TBA)₃PW. This effect could be attributed to the phenomenon of the higher the number of vanadium atoms, the higher the concentration and the faster the formation of these species. The net effect of this is a decrease in the induction period and an increase in the maximum conversion rates concerning the unsubstituted forms. Karcz et al. [52], experimentally and via DFT calculations studied the replacement of Co in place of the metal addenda in 12-tungstophosphate and 12-molybdophosphate salts for the aerobic oxidation of cyclooctane, obtaining results that are in accordance as ours.

Moreover, the role of countercations has also been related to the occurrence of an induction time. Guerin et al. [53] analyzed the relationship between the induction times and the use of organic salts as cations in phosphomolybdate salts for the oxidation of cyclooctene, reporting a long induction time (120–240 min) when using TBA salts as cations. This was attributed to the initial mass transport limitations for the oxidant. It is interesting to note that V inclusion leads to an increased TBA as a countercation due to charge compensation. Despite the expected mass transfer limitations, an opposite effect occur; this phenomenon may occur because the magnitude of the effect of V inclusion on the concentration of active species is greater than the effect of the countercation in terms of catalytic activity. Another study [39] analyzed the role of TBA as a countercation in the oxidation of olefins, in which an induction period was observed. It was proposed that the length and structure of the alkyl chains of the countercation play a crucial role in the catalytic performance of POM through the interactions that can be established between these chains and the substrate by means of van der Waals forces (more specifically, in the adsorption and reaction of the substrates) [54]. Another study with TBA salts of (PW₁₀V₂O₄₀)⁵⁻ and (PMo₁₁VO₄₀)⁴⁻ showed that, despite favoring the occurrence of the induction time, the TBA fulfills the important function of providing structural stability in the oxidation reactions and retaining the original structure of the polyoxoanion for a longer reaction time [46].

The products obtained in the catalytic oxidation of benzyl alcohol depends on the oxidation ability of the catalyst. Thus, if the nature of the catalyst is acidic, the reaction product is diphenylethane (a dimer of benzyl alcohol); if the nature of the material is redox, benzaldehyde is favored [11]. From the latter, we can conclude that liquid-phase aerobic catalytic oxidation of benzyl alcohol with the materials synthesized in this work occurs via a redox pathway. The selectivity values are comparable with those obtained by Abdenatanzi et al. [10] for polyoxotungstate immobilized in ionic liquids, although higher maximum conversions were obtained in our case. Therefore, for the synthesized pure and substituted Mo and W catalysts, the obtained results reveal that they are very selective in benzyl alcohol oxidation toward benzaldehyde, without forming over-oxidized products, the Mo-based catalysts showing slightly shorter induction times than their W-based counterparts. Regarding selectivity, no appreciable differences were observed. Similar results were obtained in the literature [53] for cyclooctene oxidation by butylpiridinium salts of PW₁₂O₄₀ and PMo₁₂O₄₀.

2.4. Catalyst Reuse

Reusability is a valuable characteristic of a solid heterogeneous catalyst. For the catalyst reuse study, we used one of the catalysts that presented the highest conversions [(TBA)₅PMoV₂] for liquid-phase aerobic catalytic oxidation of benzyl alcohol reaction. To evaluate the reusability of (TBA)₅PMoV₂, the catalyst was recovered and reused in the same reaction conditions. The results obtained for the three reuses at 4 h, 5 h, 6 h, and 7 h are summarized in Figure 9.

From these results, we can conclude that conversion values obtained when reusing the (TBA)₅PMoV₂ catalyst for three consecutive times are quite similar to those obtained when using a fresh catalyst. In all the cases, the selectivity values achieved were higher than 99%.

The FT-IR spectrum and the XRD diffractogram of the (TBA)₅PMoV₂ reused three times (Figure 10a,b, respectively) reveal that slight differences in the XRD diagram and in the FT-IR spectrum were detected, compared to the fresh sample. The slight differences observed between the diffraction patterns of fresh and reused POM may correspond to the loss and/or modification of crystalline phases. However, since these phases are not catalytically active, there are no appreciable differences in the conversion values in catalyst reuse. The SEM micrograph of the reused solid (Figure 10c) shows that the surface morphology of the catalyst does not undergo appreciable changes after the reuse experiments, which reinforces the information obtained by FT-IR and XRD.

The excellent reusability observed in these catalysts agrees with the catalytic mechanism reported in [11]. The slower step is based on the abstraction of the benzyl proton by the adjacent oxygen, and the subsequent rearrangement of the electrons results in the subsequent formation of the carbonyl group with the bridging oxygen. In this step, the desorption of the benzaldehyde molecule generates an oxygen vacancy, leaving the catalyst partially reduced. The last step is the reoxidation of the catalyst with molecular oxygen [55,56]. Thus, it is possible to suggest that V-substituted Mo- and W-type polyoxometalates have similar catalytic behavior, which would explain the high reusability.

2.5. Leaching Study

The benzyl alcohol conversion (1.5%) obtained in the leaching test (see Experimental 3.7) was comparable to the control experiment value at an equivalent time. These experiments proved that no detectable leaching of the catalytically active species into the solvent took place, allowing us to discard homogeneous phase reactions.

Finally, for comparative purposes,

Table 3. Catalytic performance in the oxidation of benzyl alcohol for various catalysts.

Catalyst	Solvent	Oxidant	Time (h)	Temperature (°C)	Conversion (%)	Selectivity a (%)	Ref.
Au-Pd	MeOH	H2O2	0.5	50	11.3	>85	[57]
Pd/CaSUP	-	H2O2 (30%)	8	80	88.0	89	[58]
NiOx-CuOx/SBA-15	Organic	-	5	90	88.9	76	[59]
[TMGHA]H0.6PW2.4	H2O	H2O2 (30%)	6	90	97.9	93	[60]
Ce-Pt/SBA-15	AcN	TBHP	7	90	98.8	99	[61]
A-CDQs/W	H2O	H2O2	0.05	25	98.0	93	[62]
(TBA)5PMoV2	MeOH:H2O	O2	4	170	93	>99	This work

^a Selectivity towards benzaldehyde.

summarizes some recently reported catalytic systems employed in the catalytic oxidation of benzyl alcohol. From these figures, we can assume that our catalytic system is competitive.

3. Materials and Methods

3.1. Synthesis of Heteropolycompounds [N(Butyl)₄]₃[PMo₁₂O₄₀]-TBA₃PMo and [N(Butyl)₄]₃[PW₁₂O₄₀]-TBA₃PW

Commercial heteropolyacids H₃PMo₁₂O₄₀ (99.9%, Merck, Darmstadt, Germany) and H₃PW₁₂O₄₀ (99.9%, Merck, Darmstadt, Germany) were precipitated as their corresponding organic salts of tetrabutylammonium (TBABr, 99.5%, Merck, Darmstadt, Germany) according to a previously reported procedure [63].

3.2. Synthesis of [N(Butyl)₄]_(3+x)[PV_xMo_12−xO₄₀] with x = 1 (TBA)₄PMoV and x = 2 (TBA)₅PMoV₂

The TBA salts of the Keggin-type phosphomolybdovanadates [PMo₁₁VO₄₀]⁴⁻ (PMoV) and [PMo₁₀V₂O₄₀]⁵⁻ (PMoV₂) were prepared using a hydrothermal process [13,64]. In a typical synthesis for the V-mono-substituted type, a stoichiometric mixture of 0.01 mol H₃PO₄ (85%, Merck, Darmstadt, Germany), 0.005 mol V₂O₅ (99%, Merck, Darmstadt, Germany), and 0.11 mol MoO₃ (99,5%, Merck, Darmstadt, Germany) was suspended in 150 mL of distilled water. The mixture was then heated and stirred at 80 °C and 800 rpm on a hotplate for 4 h. After cooling to room temperature and removing the insoluble molybdates and vanadates via vacuum filtration, the heteropolyacid solution was again heated to 60 °C, and tetrabutylammonium bromide (0.012 mol) was added. Precipitation of the organic salt was observed immediately (at 60 °C). The obtained organic salt was filtered at room temperature and dried under a vacuum to a constant weight (±0.002 g). For the synthesis of the V-di-substituted type, 0.01 mol of V₂O₅ was used.

3.3. Synthesis of [N(Butyl)₄]_(3+x)[PV_xW_12−xO₄₀] with x = 1 (TBA)₄PWV and x = 2 (TBA)₅PWV₂)

Phosphotungstovanadates [PW₁₁VO₄₀]⁴⁻ (PWV) and [PW₁₀V₂O₄₀]⁵⁻ (PWV₂) were also prepared using a hydrothermal process [18,25]. The first step was to prepare a stock solution of vanadium (V) by mixing 0.25 mol NH₄VO₃ (99%, Merck, Germany) with 0.50 mol NaOH (99.5%, Merck, Germany) in 500 mL of distilled water. Then, 0.0025 mol NaH₂PO₄·12H₂O (99.9%, Merck, Germany) was added to an aqueous solution of 0.019 mol Na₂WO₄·2H₂O (99%, Merck, Germany) with continuous stirring, and the pH was adjusted to 2.8 with HCl (36% v/v, Merck, Germany) dropwise, with 3.75 (PWV) or 15.0 mL (PWV₂) of the vanadium stock solution added. This mixture was then heated at 80 °C for 4 h on a hot plate. The final obtained solution was cooled and filtered to remove any insoluble salts. Finally, it was heated to 60 °C and tetrabutylammonium bromide (0.012 mol) was added under continuous stirring; precipitation of the organic salt was observed immediately (at 60 °C). The organic salts obtained were finally filtered at room temperature and dried under a vacuum to a constant weight.

3.4. Characterization

The XRD patterns were recorded at room temperature in a D8 ADVANCE (Bruker, Karlsruhe, Germany) diffractometer with Bragg–Brentano geometry using a LYNXEYE XE-T detector and copper Kα radiation (λ = 1.5406 Å) for a 2θ range of 5–80° (step = 0.015°, time per step = 0.15 s). The X-ray source was operated with a tension of 40 kV and a current of 30 mA. The FT-IR spectra were recorded using a Nicolet Nexus 470 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) in the 400–4000 cm⁻¹ wavenumber range at room temperature, and KBr tablets were used. The DRS UV-Vis spectra were recorded on a UV-3600 Plus UV-Vis-NIR Spectrophotometer (Shimadzu, Kyoto, Japan) with a Harrick single-beam Praying Mantis Diffuse Reflectance collection system. The spectra were then recorded at room temperature in the 190–1000 nm range. A Spectralon^® Diffuse Reflectance Standard was used to measure the background spectra. The DRS UV-Vis spectra were background corrected and normalized by the most intense absorption band, and the Kubelka–Munk function was used to display the data. A TGA analysis was performed on a TG 209F1 Iris (NETZSCH, Selb, Germany) thermogravimetric analyzer under an N₂ atmosphere in the temperature range of 25–550 °C using 10–25 mg of the solid and a heating rate of 10 °C min⁻¹. The CHN elemental analysis was performed on an EA 2400 series II (Perkin Elmer, Waltham, MA, USA); measurements were carried out with samples of 10–25 mg in weight. Inductively coupled plasma mass spectrometry (ICP-MS) measurements for Mo, W, and V determination in the materials were performed using an iCAP Q series ICP-MS (Thermo Scientific, Waltham, MA, USA) spectrometer. Specific areas, the pore volume, and the pore diameter were evaluated via the Brunauer–Emmet–Teller (BET) method using the nitrogen adsorption isotherms obtained on a ASAP 2010 (Micromeritics Instrument Corporation, Norcross, GA, USA) apparatus at −196 °C in the relative pressure range of 0.01–0.99 (P/P₀). The samples were previously pre-treated at 150 °C for 3 h. The specific surface area (SSA) was calculated from the adsorption isotherm branch using the BET method.

3.5. Catalytic Benzyl Alcohol Oxidation

The catalytic activity of the obtained solids was examined using benzyl alcohol as the reaction substrate. The tests were performed in an O₂ atmosphere in a semi-batch Parr-type reactor with a methanol–water solvent (90:10 v/v) at 170 °C for 4 h. In a typical reaction, 1 g (0.01 mol) of benzyl alcohol, 100 mg of the catalyst, and 20 mL of the solvent were added to the reactor. Subsequently, in order to avoid contamination with other gases, after the reactor was sealed, it was purged three times with O₂ (99.5%, Linde, Santiago, Chile) and pressurized with 5 bar of the same gas. Finally, the pressurized reactor was heated to 170 °C (10 °C min⁻¹ heating ramp), and the O₂ pressure was adjusted to 5 bar. The selected agitation speed was 600 rpm to avoid undesirable diffusion control during the catalytic performance evaluation. The study was then conducted for all the catalysts by taking samples at different reaction times for 4 h. All the reaction products were in line with the commercial standards. The reaction products were identified and quantified by using a FLEXAR^TM HLPC-DAD (Perkin Elmer Inc., Waltham, MA, USA) with a mobile phase composition of 0.1% glacial acetic acid (A) and acetonitrile (B) with a flow rate of 1.0 mL min⁻¹. The column used was an Inertsil C18 ODS-3 (250 mm × 4.6 mm, 5 μm) (GL Sciences Inc., Torrance, CA, USA), and the HPLC-DAD detection wavelength was 254 nm. Finally, the reaction products were confirmed by GC-MS using a model 7890A GC system (Agilent Technologies Inc., Wilmington, DE, USA) interfaced to a single quadrupole mass-selective detector 5975C (triple-axis detector).

The conversion for benzyl alcohol (X_BzOH) and the selectivity toward the products (S_product) were calculated by using calibration curves as follows:

$${\mathrm{X}}_{\mathrm{BzOH}}(\%)=\frac{{\left[\mathrm{BzOH}\right]}_{\mathrm{i}}-{\left[\mathrm{BzOH}\right]}_{\mathrm{t}}}{{\left[\mathrm{BzOH}\right]}_{\mathrm{i}}}\times 100$$

$${\mathrm{S}}_{\mathrm{product}}(\%)=\frac{ {\left[\mathrm{product}\right]}_{\mathrm{t}}}{{\left[\mathrm{BzOH}\right]}_{\mathrm{i}}-{\left[\mathrm{BzOH}\right]}_{\mathrm{t}}}\times 100$$

The control experiment was performed with the same as the typical reaction mentioned above, but without any catalyst addition.

3.6. Catalyst Reuse

The reusability of the catalysts was evaluated in three consecutive runs, using the same reaction conditions used in benzyl alcohol oxidation. The catalyst was isolated from the reaction mixture after each run, washed twice with ethanol (2 mL), dried under vacuum at 25 °C until constant weight, and then reused in a new experiment. For reusability experiments, the conversion and selectivity were evaluated at 4, 5, 6, and 7 h with the purpose of evaluating the reproducibility of the experiment. The solid obtained at the end of the experiments was characterized by XRD, FT-IR and SEM.

3.7. Leaching Studies

To assess the possibility of catalyst leaching to the solvent, ruling it out it or not, an additional experiment was carried out. In this experiment, 100 mg of the catalyst and 20 mL of the solvent were added to the reactor and heated to 170 °C. The O₂ pressure was adjusted to 5 bar and the reaction time was 3 h. Subsequently, the solid was separated by filtration and the solvent was used in a new reaction with 0.01 mol of benzyl alcohol without catalyst addition, under the same conditions used in the benzyl alcohol oxidation test (Section 3.5).

4. Conclusions

In this work, a series of tetrabutyl ammonium (TBA) salts of V-included Keggin-type polyoxoanions with W (TBA₄PW₁₁V₁O₄₀ and TBA₅PW₁₀V₂O₄₀) and Mo (TBA₄PMo₁₁V₁O₄₀ and TBA₅PMo₁₀V₂O₄₀) as addenda atoms were prepared and characterized by XRD, FT-IR, and DRS UV-Vis techniques.

The catalytic performance in the selective liquid-phase aerobic oxidation of benzyl alcohol, using O₂ as a green oxidant agent, indicates that in both the Mo and W series, the replacement of addenda atoms by V enhances the catalytic activity with respect to non-substituted atoms. Under the reaction conditions used, the di-substituted (TBA)₅PWV₂ and (TBA)₅PMoV₂ exhibited higher catalytic activity for the benzyl alcohol oxidation, with total conversions of 97% and 93%, respectively, and a selectivity towards benzaldehyde > 99%. These catalysts also proved to be reusable under these reaction conditions. The findings of this study on the effect of V-inclusion polyoxotugsto- and polyoxomolybdovanadates with Keggin structures in the catalytic oxidation of benzyl alcohol will lead to a better understanding of the mechanisms involved in this reaction.