Synthesis and Characterization of Insoluble Vanadium-containing Keggin-type Phosphomolybdates with nBu₄N⁺ Counterion: Recyclable Catalysts for 5-Hydroxymethylfurfural Oxidation

Introduction

Experimental

Synthesis of H_3+xPMo_12-xV_xO₄₀ solutions

The synthesis of pure H_3+xPMo_12-xV_xO₄₀ solutions with x of 1–4 was carried out in three steps, including activation of V₂O₅ by an ecological acid-free peroxide method (Eq. (2)), preliminary formation of P-Mo Keggin framework and insertion of activated vanadium(V) atoms into the Keggin structure with obtaining a mixed-addenda anion.

The synthesis was carried out as follows: a sample of V₂O₅ was dissolved in distilled water (V¹_H2O) pre-cooled to 4–6°C. Solution of 30% H₂O₂ cooled to a similar temperature was gradually added to the resulting suspension upon continuous stirring, maintaining the temperature of the mixture below 15°C until complete V₂O₅ dissolving. The dark-orange solution of 0.0175 М H₆V₁₀O₂₈ obtained was then stirred at room temperature until total O₂ evolving and stabilized by Н₃РО₄ adding (V¹_H3PO4) according to Eq. (3). Note that this procedure, first applied in 1995 [21], increases the stability of the H₆V₁₀O₂₈ solution up to several days, preventing the precipitation of a gel-like V₂O₅·nH₂O and facilitating the large-scale synthesis of HPA-x solutions. The total volume of 85% H₃PO₄ was preliminary diluted to 2.5 M concentration and divided into 2 portions in accordance with the volume required to obtain a 0.0125 M solution of H₉PV₁₄O₄₂.

At the next stage, a sample of MoO₃ and the remaining amount of Н₃РО₄ (V²_H3PO4) were introduced into a separate beaker with distilled water (V²_H2O) and heated above 80°C. The solution of H₉PV₁₄O₄₂ was added in portions to the suspension with stirring immediately after the appearance of a yellow colour. The resulting solution, after combining all components, was evaporated to the required volume and cooled to room temperature. The loadings of the required components (Table 1) were calculated using Eq. (4).

$$5V_2{O}_5+2H_2{O}_2+H_{2}O \to H_6{V}_{10}{O}_{28}+O_2$$

(2)

$$1.4 H_6{V}_{10}{O}_{28}+H_3{PO}_4 \leftrightarrow H_9{PV}_{14}{O}_{42}+1.2 H_{2}O$$

(3)

$$(12-x){\mathrm{M}\mathrm{o}\mathrm{O}}_3+0.5x{\mathrm{V}}_2{\mathrm{O}}_5+{\mathrm{H}}_3{\mathrm{P}\mathrm{O}}_4+0.5x{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_{3+x}{\mathrm{P}\mathrm{M}\mathrm{o}}_{12-x}{\mathrm{V}}_x{\mathrm{O}}_{40}$$

(4)

Table 1. Components to synthesize 200 mL of 0.2 M HPA-x solution.

Table 1. Components to synthesize 200 mL of 0.2 M HPA-x solution.

Solution	Abbrev.	V2O5		MoO3		V1H3PO4	V2H3PO4	30% H2O2	V1H2O	V2H2O
		[mol]	[g]	[mol]	[g]	[mL]	[mL]	[mol]	[L]	[L]
H4PMo11VO40	HPA-1	0.02		0.44	63.3	1.16	14.84	0.3	0.23	0.79
H5PMo10V2O40	HPA-2	0.04		0.40	57.6	2.28	13.72	0.6	0.46	0.72
H6PMo9V3O40	HPA-3	0.06		0.36	51.8	3.44	12.56	0.9	0.69	0.65
H7PMo8V4O40	HPA-4	0.08		0.32	46.1	4.57	11.43	1.2	0.91	0.58

Results and Discussion

IR-ATR investigations

In general, the formation of mixed [РМo_12-хV_хО₄₀]^(3+x)– anions can be considered as the replacement of Mo^VI atoms with V^V in the [РМo₁₂О₄₀]^3– heteropolyanion, the structure of which can be represented as a central PO₄ tetrahedron surrounded by twelve MoO₆ octahedrons assembled into four Mo₃O₁₃ triads [22]. As a consequence, such anion contain twelve quasi-linear Mo-O_b-Mo bonds between octahedrons from different Mo₃O₁₃ triads, twelve Mo-O_c-Mo bonds between octahedrons within Mo₃O₁₃ sets, four P-O_a-{Mo₃O₁₃} bonds between central heteroatom and triads, and twelve terminal bonds Mo=O_d. This leads to the formation of characteristic vibrations of the Keggin anions in the range of 700–1100 cm^–1, which makes IR spectroscopy a convenient method for their characterization.

According to Figure 2, the spectra of HPA-x (x = 1–4) contain four recognized strong absorption peaks in the fingerprint region of the Keggin polyoxoanions, which could be attributed to the fundamental P-O_a, Mo-O_d, Mo-O_b-Mo, and Mo-O_c-Mo asymmetric stretching vibrations. Compared with the IR spectrum of unsubstituted H₃PMo₁₂O₄₀ acid [23], the obtained vibrations in mixed P-Mo-V patterns did not reveal a significant difference, suggesting the formation of the Keggin anions at preparing HPAs-x via “peroxide” technique. The shoulder observed for P-O_a-{Mo₃O₁₃} vibrations can be explained by the replacement of Mo atoms by V ones in the framework of HP-anions with formation of Mo-O-V linkages [24], its presence becoming more noticeable with raising substitution content. Vanadium insertion also affects on the charge and symmetry of anions, causing a slight shift of ν_as(Mo-O_d) and ν_as(Mo-O_c-Mo) stretching vibrations and a band of ν_as(Mo-O_b-Mo) to lower and higher wavenumbers, correspondingly, compared to vanadium-free acid. The same effect was previously observed in the literature [8,25], confirming the retaining of the primary structure during the formation of mixed P-Mo-V anions. The similarity of (nBu₄N)_2.5+xH_0.5PMo_12-xV_xO₄₀ spectra at 1100–700 cm^–1 region with those of corresponding model acids strongly confirms the retention of the primary structure during the precipitation.

Expanded spectra of the prepared acids (Figure 3(a)) contain two additional bands at 3340 and 1604 cm^–1 that can be attributed to hydrogen bonds in crystal water [26]. Insertion of nBu₄N⁺ cations gives rise to new bands at 1481, 1468, and 1381 cm^–1, as well as a set of peaks in the region 3000–2800 cm^–1 (Figure 3(b)), which originate from asymmetric and/or symmetric bending and stretching vibrations of C–H, –CH₂–, and –CH₃ groups, respectively [27]. The peaks at 1153 and 1003 cm^–1 can be ascribed to C–N stretching vibrations, whereas other characteristic peaks of nBu₄N⁺ cation appear at 1000–600 cm^–1 and overlap with polyanion fingerprint domain. Nevertheless, it allows supposing that nBu₄N⁺ cation also maintains its structure in the obtained salts without degradation. The registered spectra are in good agreement with those described in the literature for sodium salts of HPAs-x with x of 1–3 [8,28].

Figure 2. IR-ATR spectra of (nBu₄N)_2.5+xH_0.5PMo_12-xV_xO₄₀ acidic salts and corresponding H_3+xPMo_12-xV_xO₄₀ acids with x of 1 (a), 2 (b), 3 (c), and 4 (d) in fingerprint region of the Keggin anion.

Figure 2. IR-ATR spectra of (nBu₄N)_2.5+xH_0.5PMo_12-xV_xO₄₀ acidic salts and corresponding H_3+xPMo_12-xV_xO₄₀ acids with x of 1 (a), 2 (b), 3 (c), and 4 (d) in fingerprint region of the Keggin anion.

Figure 3. Expanded IR-ATR spectra of (a) H_3+xPMo_12-xV_xO₄₀ acids and (b) (nBu₄N)_2.5+xH_0.5PMo_12-xV_xO₄₀ acidic salts with x of 1–4.

Figure 3. Expanded IR-ATR spectra of (a) H_3+xPMo_12-xV_xO₄₀ acids and (b) (nBu₄N)_2.5+xH_0.5PMo_12-xV_xO₄₀ acidic salts with x of 1–4.

Investigation of textural characteristics

The textural properties of the samples obtained were characterized by low-temperature adsorption-desorption of nitrogen at 77.4 K (Figure 5, Table 2).

According to Figure 5(a), the HPAs-x have type II isotherms according to the IUPAC classification [31], characteristic of disperse macroporous and nonporous materials, with H3-shaped hysteresis loops usually attributed to slit-like materials. The adsorption isotherms of TBA_2.5+xHPA-x acidic salts (Figure 5(b)) are S-shaped curves of the same type with a feebly marked capillary condensation hysteresis loops shifted to the region of Р/Р₀ > 0.85, which suggests the presence of mesopores. All samples possess a low specific surface area in the range of 2–13 m²·g^–1 with its values decreasing with passing from parental acids to the corresponding nBu₄N⁺ salts as well as with increasing vanadium(V) content (Table 2).

Figure 5. N₂ adsorption-desorption isotherms of (a) H_3+xPMo_12-xV_xO₄₀ acids and (b) (nBu₄N)_2.5+xH_0.5PMo_12-xV_xO₄₀ acidic salts with x of 1–4.

Figure 5. N₂ adsorption-desorption isotherms of (a) H_3+xPMo_12-xV_xO₄₀ acids and (b) (nBu₄N)_2.5+xH_0.5PMo_12-xV_xO₄₀ acidic salts with x of 1–4.

Thermal analysis

Figure 6 and Figure 7 depict the TG/DTG/DSC profiles of the parental acids and prepared nBu₄N⁺ acidic salts, allowing one to estimate their thermal stability and degree of hydration.

According to Figure 6, the parental acids exhibit a continuous stepwise loss of weight in the range of room temperature (r.t.) to 419–458°C. The first weight loss occurs at heating the samples from r.t. up to 59–93°C, with a minimum on the DTG curves at 51–70°C. This stage can be assigned to the release of physisorbed water, with its duration and observed weight loss decreasing as x increases. The second stage proceeds to 89–144°C, with the detected loss of mass increasing in the range of 3.3–11.0% with rising charge of heteropoly anion. It can be attributed to the liberation of zeolitic water. The next thermal event is characterized by a weight damage of 2.0–5.1 wt % and occurs to 165–195°C. This can be ascribed to the elimination of crystallization (hydrogen-bonded to the acidic protons) water to form an anhydrous compound. The subsequent alteration can be ascribed to the loss of structural water (acidic protons) and the beginning of destruction of the Keggin framework with its total collapse to structure-forming oxides near 419–458°C (according to the exothermic peak on the DSC curves). The displacement of these temperatures to the low-temperature region compared to vanadium-free H₃PMo₁₂O₄₀ acid (500°C) [8] seems to be due to reducing symmetry of the Keggin anions at the insertion of vanadium(V) atoms.

Figure 6. TG/DTG/DSC profiles of H_3+xPMo_12-xV_xO₄₀ acids with x of 1 (a), 2 (b), 3 (c), and 4 (d).

Figure 6. TG/DTG/DSC profiles of H_3+xPMo_12-xV_xO₄₀ acids with x of 1 (a), 2 (b), 3 (c), and 4 (d).

According to Figure 7, the TG curves of TBA_2.5+xHPA-x acidic salts have a relatively constant value to 185–210°C, after which they continuously decrease up to 430–440°C. The observed decrease in the TG curves is reflected by a large and broad endothermic peak on the DTG curves, which contains from 2 to 4 marked minima depending on the salt composition. This stepwise loss of weight can be assigned to the decomposition and removal of nBu₄N⁺ cations, as well as to the same processes discussed above for the parental acids. Additional peaks are present in the DSC profiles up to 215°C, indicating minor intermediate rearrangements, presumably due to three-dimensional nBu₄N⁺ cations. It can be concluded that the prepared salts are stable up to at least 370°C without loss of nBu₄N⁺ cations and degradation of the Keggin anions. It is obvious that the replacement of H⁺ ions with nBu₄N⁺ cations leads to an increase in thermal stability.

Figure 7. TG/DTG/DSC profiles of (nBu₄N)_2.5+xH_0.5PMo_12-xV_xO₄₀ salts with x of 1 (a), 2 (b), 3 (c), and 4 (d).

Figure 7. TG/DTG/DSC profiles of (nBu₄N)_2.5+xH_0.5PMo_12-xV_xO₄₀ salts with x of 1 (a), 2 (b), 3 (c), and 4 (d).

Catalytic tests

The catalytic activity of the obtained salts was studied in the oxidation reaction of 5-HMF as a model substrate. This reaction was disclosed to result in a preferred production of DFF with some formation of 5-formyl-2-furancarboxylic (FFCA), levulinic (LA), and formic (FA) acids as by-products. The influence of several parameters on 5-HMF conversion and DFF selectivity, such as temperature, substrate-to-catalyst molar ratio, and reaction time, was investigated and presented in Figure 8.

The first series of experiments was aimed at assessing the effect of temperature (Figure 8(a)). It was found that increasing the reaction temperature in the range of 50–95°C had a positive action on substrate conversion, raising it from 35 to 70%. The use of TBA_6.5HPA-4 salt leads to slightly superior values compared to TBA_5.5HPA-3, evidencing that the content of vanadium has a beneficial effect on the transformation of 5-HMF. Besides higher conversion, TBA_6.5HPA-4 catalyst also provides the greater yield toward the desired product (DFF), which some rises with increasing temperature. Selectivity of products changes differently that can be associated with varying in the contribution of processes of DFF overoxidation and 5-HMF hydration. The data obtained show that a temperature of 80°C provides the best relationship between the conversion of 5-HMF and the selectivity of DFF relative to other products.

Changing the reaction time in the range of 0.5–3 h (Figure 8(b)) showed that although the substrate conversion increased over time for both catalysts, the selectivity of the products progressed various. The best yield of the target product for TBA_6.5HPA-4 is achieved after 1.5 hours and then practically does not change, while for TBA_5.5HPA-3 a similar yield value is observed only after 2 hours.

Estimating the influence of catalyst loading showed the predominant formation of DFF in the entire studied range, with an increase in catalyst content leading to a considerable raise of 5-HMF conversion (Figure 8(c)). It was found that the yields of FA and LA reduced monotonically with increasing catalyst amount, which indicates rising the rate of the target reaction and predominating oxidation process over the hydration of 5-HMF. On the other hand, the application of high catalyst loading resulted in increasing DFF overoxidation with the formation of 2,5-furandicarboxylic acid (FDCA) as another by-product.

Figure 8. Effect of reaction temperature (a), process time (b), and catalyst loading (c) on 5-HMF conversion and product selectivity (a) or DFF yield (b, c). Reaction conditions: (a) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (10 mol%) in H₂O (15 mL), 3 h, 0.1 MPa O₂; (b) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (10 mol%) in H₂O (15 mL), 80°C, 0.1 MPa O₂; (c) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (10–50 mol%) in H₂O (15 mL), 80°C, 0.1 MPa O₂, 1.5 h (for TBA_6.5HPA-4) or 2 h (for TBA_5.5HPA-3).

Figure 8. Effect of reaction temperature (a), process time (b), and catalyst loading (c) on 5-HMF conversion and product selectivity (a) or DFF yield (b, c). Reaction conditions: (a) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (10 mol%) in H₂O (15 mL), 3 h, 0.1 MPa O₂; (b) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (10 mol%) in H₂O (15 mL), 80°C, 0.1 MPa O₂; (c) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (10–50 mol%) in H₂O (15 mL), 80°C, 0.1 MPa O₂, 1.5 h (for TBA_6.5HPA-4) or 2 h (for TBA_5.5HPA-3).

According to the data obtained, total substrate conversion and DFF selectivity of 91.2% are achieved in oxygen atmosphere (0.1 MPa) for 1.5 h at 80°C in the presence of TBA_6.5HPA-4 catalyst at its loading of 30 mol %. The content of by-products is 2.6, 2.3 and 3.8% for LA, FA, and FFCA, respectively. Similar result in DFF yield of 90.9% can be obtained in the presence of TBA_5.5HPA-3 catalyst, but this requires the application of a larger catalyst load (40 mol %) and an increase in reaction time to 2 h. The reached conversion and selectivity values are close to those for state-of-the-art solid catalysts studied in the oxidation of 5-HMF under similar conditions with water as a solvent [32].

Investigation of catalyst reusability

The reusability of TBA_2.5+xHPA-x (x = 3, 4) was investigated in 5 cycles of 5-HMF oxidation (Fig. (9)). After each reaction, the spent catalyst was separated by centrifugation and calcined at 150°C for 3 h in oxygen atmosphere. Both catalysts were found to demonstrate high stability and retention of catalytic activity during repeated application with the yield spread below 1.3%. The XRD patterns and IR spectra of the spent catalysts after 5 runs were identical to those of the samples after threefold hydrothermal treatment.

Figure 9. The change of DFF yield during 5 runs in the presence of TBA_6.5HPA-4 (a) and TBA_5.5HPA-3 (b). Conditions: (a) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (30 mol%) in H₂O (15 mL), 80°C, 1.5 h, 0.1 MPa O₂; (b) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (40 mol%) in H₂O (15 mL), 80°C, 2 h, 0.1 MPa O_2.

Figure 9. The change of DFF yield during 5 runs in the presence of TBA_6.5HPA-4 (a) and TBA_5.5HPA-3 (b). Conditions: (a) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (30 mol%) in H₂O (15 mL), 80°C, 1.5 h, 0.1 MPa O₂; (b) 5-HMF (0.39 mmol) in H₂O (5 mL), catalyst (40 mol%) in H₂O (15 mL), 80°C, 2 h, 0.1 MPa O_2.

Conclusions

References

1. Zhizhina, E.G. and Odyakov, V.F., Appl. Catal. A: Gen., 2009, vol. 358, pp. 254–258. [CrossRef]
2. Heravi, M.M. and Bamoharram, F.F., Heteropolyacids as Highly Efficient and Green Catalysts Applied in Organic Transformations; Elsevier, 2022, p. 61. [CrossRef]
3. Singh, R., Shah, A.A., Potter, A., Clarkson, B., Creeth, A., Downs, C., and Walsh, F.C., J. Power Sources, 2012, vol. 201, p. 159–163. [CrossRef]
4. Clauβnitzer, J., Bertleff, B., Korth, W., Albert, J., Wasserscheid, P., and Jess, A., Chem. Eng. Technol., 2020, vol. 43, no. 3, pp. 465–475. [CrossRef]
5. Ma, S., Bao, W., Liu, B., Zhang, C., Wang, C., Liu, Y., Guo, H., Pan, Y., Sun, D., and Lu, Y., Appl. Surf. Sci., 2022, vol. 606, art. 154781. [CrossRef]
6. Woźniak Budych, M.J., Staszak, K., Bajek, A., Pniewski, F., Jastrząb, R., Staszak, M., Tylkowski, B., and Wieszczycka, K., Coord. Chem. Rev., 2023, vol. 493, art. 215306. [CrossRef]
7. Li, J., Xu, J., Li, X., Gao, W., Wang, L., Wu, L., Lee, M., and Li, W., Soft Matter., 2016, vol. 12, pp. 5572–5580. [CrossRef]
8. Vilanculo, C.B., da Silva, M.J., Rodrigues, A.A., Ferreira, S.O., and da Silva, R.C., RSC Adv., 2021, vol. 11, pp. 24072–24085. [CrossRef]
9. Tsigdinos, G.A. and Hallada, C.J., Inorg. Chem., 1968, vol. 7, no. 3, pp. 437–441. [CrossRef]
10. Odyakov, V.F. and Matveev, K.I., SU Patent 1782934 A1, 1992.
11. van Putten, R.-J., van der Waal, J.C., de Jong, E., Rasrendra, C.B., Heeres, H.J., and de Vries, J.G., Chem. Rev., 2013, vol. 113, no. 3, pp. 1499–1597. [CrossRef]
12. Delidovich, I., Hausoul, P.J.C., Deng, L., Pfutzenreuter, R., Rose, M., and Palkovits, R., Chem. Rev., 2016, vol. 116, no. 3, pp. 1540–1599. [CrossRef]
13. Amarasekara, A.S., Green, D., and Williams, L.D., Eur. Polym. J., 2009, vol. 45, no. 2, pp. 595–598. [CrossRef]
14. Ma, J., Wang, M., Du, Z., Chen, C., Gao, J., and Xu, J., Polym. Chem., 2012, vol. 3, pp. 2346–2349. [CrossRef]
15. Richter, D.T. and Lash, T.D., Tetrahedron Lett., 1999, vol. 40, no. 37, pp. 6735–6738. [CrossRef]
16. del Poeta, M., Schell, W.A., Dykstra, C.C., Jones, S., Tidwell, R.R., Czarny, A., Bajic, M., Kumar, A., Boykin, D., and Perfect, J.R., Antimicrob. Agents Chemother., 1998, vol. 42, no. 10, pp. 2495–2502. [CrossRef]
17. Hopkins, K.T., Wilson, W.D., Bender, B.C., McCurdy, D.R., Hall, J.E., Tidwell, R.R., Kumar, A., Bajic, M., and Boykin, D.W., J Med. Chem., 1998, vol. 41, no. 20, pp. 3872–3878. [CrossRef]
18. Kozhevnikov I.V. Catalysts for fine chemical synthesis. Vol. 2. Catalysis by polyoxometalates, Chichester: John Wiley & Sons Ltd., 2002.
19. Tian, J., Fang, C., Cheng, M., Wang, X., Chemical Engineering and Technology, 2011, vol. 34, no. 3, pp. 482–486. [CrossRef]
20. Gromov, N.V., Medvedeva, T.B., Taran, O.P., Timofeeva, M.N., Parmon, V.N., Catalysis in Industry, 2021. vol. 13, no. 1, pp. 73–80. [CrossRef]
21. Matveev, K.I., Odjakov, V.F., Zhizhina, E.G., Motornyj, Ju.A., and Lange, S.A., RU Patent 1669109 C, 1995.
22. Pope, M.T., Heteropoly and Isopoly Oxometalates, Berlin: Springer-Verlag, 1983.
23. Javidi, J., Esmaeilpour, M., Rahiminezhad, Z., and Dodeji, F.M., J. Clust. Sci., 2014, vol. 25, no. 6, p. 1511. [CrossRef]
24. Rocchiccioli-Deltcheff, C. and Fournier, M., J. Chem. Soc. Faraday Trans., 1991, vol. 87, pp. 3913–3920. [CrossRef]
25. Huang, T., Tian, N., Wu, Q., Yan, Y., and Yan, W., Mater. Chem. Phys., 2015, vol. 165, pp. 34–38. [CrossRef]
26. Vakulenko, A., Dobrovolsky, Y., Leonova, L., Karelin, A., Kolesnikova, A., and Bukun, N., Solid State Ion., 2000, vol. 136, pp. 285–290. [CrossRef]
27. Sutjahja, I.M., Wonorahardjo, S., and Wonorahardjo, S., Inorganics, 2020, vol. 8, no. 9, art. 51. [CrossRef]
28. Boudjema, S., Rabah, H., and Choukchou-Braham, A., Acta. Phys. Pol. A, 2017, vol. 132, pp. 469–472. [CrossRef]
29. Liu, Y., Zhu, L., Tang, J., Liu, M., Cheng, R., and Hu, C., ChemSusChem, 2014, vol. 7, no. 12, pp. 3541–3547. [CrossRef]
30. Zhang, J., Tang, Y., Li, G., and Hu, C., Appl. Catal. A: Gen., 2005, vol. 278, no. 2, pp. 251–261. [CrossRef]
31. Thommes, M., Kaneko, K., Neimark, A.V., Olivier, J.P., Rodriguez-Reinoso, F., Rouquerol, J., and Sing, K.S.W., Pure Appl. Chem., 2015, vol. 87, no. 9–10, pp. 1051–1069. [CrossRef]
32. Ventura, M., Dibenedetto, A., and Aresta, M., Inorganica Chim. Acta, 2018, vol. 470, pp. 11–21. [CrossRef]