Polyoxometallic catalysts played an important role in the transformation of biomass or organic substrates under acidic and redox conditions [1-4]. Compared to their homogeneous catalysis, heterogeneous POMs showed most important advantages as easy separation and being recycled. But the barrier in mass transferring for solid POMs and organic substrates especial biomass feedstocks might decrease the reaction rates, which the catalytic activity of heterogeneous POMs could not be comparable to that of homogeneous ones in most cases [5]. Amphiphilic POMs containing surfactant cations and polyoxometallic anions had been developed to overcome the difficulty in mass transferring, which showed significant enhancement in cellulose conversion in water/solvent biphase and oxidative desulfurization in H₂O₂/alkane biphase [6-8]. However, such surfactant-POMs were often separated from the reaction mixture using high-speed centrifugation, which might cause some loss of the catalysts during the repeated experiment. Pickering emulsions are another emulsions containing micro- or nano-size solid particles, which could stabilize biphasic systems to perform reactions between immiscible substrates and catalysts [9-13]. Pickering interfacial catalysis (PIC) is not only able to alleviate the difficulty in mass-transfer, but also achieve more efficient separation and recovery of catalysts and products compared to traditionally surfactant ones. On this concept, polyoxometalate PIC system is of great value to fulfill the organic transformation in water/oil biphase. By now, there were some reports on Pickering assisted catalysis (PAC) system containing POMs and Pickering emulsion [12,14-16]. Among PAC, POMs were found to present good activity in oxidation of olefines, organosulfur and alcohols and in cyclization of citronellal as well. Homogeneous POMs acted as catalysts in combination with non-catalytic Pickering emulsion particles as stabilized agents. POM PIC systems are desirable due to their direct use as catalytically active particles as well as to stabilize the emulsion in water/oil biphase.

Herein, a new series of PIC of POMs/SiO₂(C₈/C₈NH₂ with molar ratio as 1:1) was fabricated through depositing (NH₄)₅H₆PMo₄V₈O₄₀ (PMo₄V₈) on hydrophobized SiO₂(C₈/C₈NH₂) nanoparticles. In our previous study, POMs containing V⁵⁺ and Mo⁶⁺ presented higher activity in 5-hydroxymethylfurfural (5-HMF) oxidation to 2, 5-diformylfuran (DFF) under O₂ pressure or atmospheric pressure [17-21]. And the catalytic activity was improved as increase the components of vanadium in H_nPMo_12-nV_nO₄₀. Therefore, PMo₄V₈ was a good choice for achieving the oxidation of 5-HMF to DFF under at atmospheric pressure. And these materials acted as Pickering interfacial catalysts in a H₂O/MIBK (MIBK is methyl isobutyl ketone) biphase, which showed significantly enhancement in catalytic activity in 5-HMF oxidation compared to PMo₄V₈ and PMo₄V₈ on hydrophilic silica. Meanwhile, such PIC enhanced the aerobic oxidation rate in H₂O/MIBK compared to those in single phase of H₂O or MIBK.

A series of PIC containing PMo₄V₈ was prepared according to the following procedure. The experimental details were given in Supporting information: (1) Hydrophobized silica nanoparticles SiO₂(C₈/C₈NH₂). The commercial silica nanoparticles was condensed by N-[3-[trimethoxy silicon-based]propyl]ethylenediamine and octyl triethoxy silane with molar ratio of 1:1; (2) Depositing PMo₄V₈ on hydrophobic SiO₂(C₈/C₈NH₂) nanoparticles. Sonicating or stirring of the mixture of PMo₄V₈ and SiO₂(C₈/C₈NH₂) hydrophobic nanoparticles (with weigh ratio = 3:7) for about 24 h at room temperature. The solid was collected to be washed with water to separate unloaded PMo₄V₈, while the POM Pickering interfacial catalyst PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) PIC (30 represented the loading amount of PMo₄V₈ on SiO₂) was fabricated (Scheme 1). The samples with different amount of PMo₄V₈ were synthesized using the same method defined as PMo₄V₈ (10, 20, 25 and 35)/SiO₂(C₈/C₈NH₂).

It was found that stable Pickering emulsions were formed in the mixture of PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) and H₂O/MIBK (Fig. 1). Meanwhile, the emulsifying ability of PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) PIC depended on the variety of POM loading amount. This might be attributed to the difference in their hydrophobicity and hydrophilicity on the surface. SiO₂ is a kind of hydrophilic nanoparticle, which could not form stabilized emulsion in H₂O/MIBK system. PMo₄V₈ dissolves in H₂O without any organic groups to form Pickering emulsion system. SiO₂(C₈/C₈NH₂) nanoparticles were altered its hydrophilicity into hydrophobicity on silica surface by functionalized with organic group, which could form Pickering emulsion in H₂O/MIBK biphase. The loading amount of PMo₄V₈ also influenced the hydrophobicity of PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) nanoparticles, while the water angles decreased as increasing POM loading amount. Nevertheless, all PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) showed the ability in emulsifying the mixture of H₂O and MIBK. The different length of alkyl group on SiO₂ nanoparticle were changed as six and ten, which the water angles were 74.4° and 99.1° corresponding to SiO₂(C₆/C₈NH₂) and SiO₂(C₁₀/C₈NH₂). This indicated that the length of alkyl chain influenced the amphiphilicity as well as the emulsion ability of silica particles.

PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) hybrids were well characterized using elementary analysis (Table S1 in Supporting information), FT-IR, XRD, XPS, SEM, TEM, and ³¹P MAS NMR (Figs. S1-S6 in Supporting information). These results indicated that PMo₄V₈ molecules deposited on the surface of SiO₂(C₈/C₈NH₂) nanoparticles in single molecular model, which some interaction between polyoxometallic anion and NH₂⁺ in silica occurred to prevent the leaching of PMo₄V₈ from SiO₂(C₈/C₈NH₂) during the reaction. The XPS was used to determine the valence state and component of PMo₄V₈(30)/SiO₂(C₈/C₈NH₂), which the valence states of Mo and V were +6 and +5, respectively (Fig. S3). The mean particle size for PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) were about 58.8 nm (Fig. S4), similar with that of SiO₂ and SiO₂(C₈/C₈NH₂) nanoparticles.

In the previous report, oxidation of 5-HMF to DFF was almost all achieved in single organic solvent as dimethyl sulfoxide (DMSO). However, the separation of DFF from DMSO was rather difficult to confirm the high yield of DFF. Meanwhile, production of DFF in water was rather difficult due to the instability of DFF in water and the multifunctional groups in 5-HMF [22-24]. MIBK was reported to be another solvent being used in oxidation of 5-HMF into DFF [25,26], which is immiscible with water to form biphase. Therefore, MIBK was selected here to be an organic phase to check the effect of POM PIC in aerobic oxidation of 5-HMF. Meanwhile, the biphase catalytic system containing H₂O/MIBK was also found to be more efficient in biomass conversion coupling the transformation and extraction in one unit [24]. Herein, the aerobic oxidation of 5-HMF was firstly done in single solvents of H₂O and MIBK, and H₂O/MIBK biphase upon PMo₄V₈, PMo₄V₈(30)/SiO₂(C₀/C₈NH₂), PMo₄V₈(30)/SiO₂(C₈/2C₈NH₂), PMo₄V₈(30)/SiO₂ (C₈/C₈NH₂) and PMo₄V₈(30)/SiO₂(2C₈/C₈NH₂) at 90 ℃ for 10 h (Fig. 2). A normal homogeneous reaction system presented 52.2% and 32.6% conversion of 5-HMF with 7.34% and 24.4% yields of DFF upon PMo₄V₈ in H₂O and MIBK. To obtain DFF from 5-HMF was hard to be explored upon PMo₄V₈ either in H₂O nor in MIBK. Therefore, mixture of H₂O and MIBK might give rise to some enhancement in efficiency for 5-HMF oxidation. In H₂O/MIBK biphase, PMo₄V₈ and 5-HMF dissolved both in H₂O and MIBK, which acted as a homogeneous catalyst to increase the 5-HMF conversion to 49.9%. But the DFF yield was not higher as expected. PMo₄V₈(30)/SiO₂(C₀/C₈NH₂) without alkyl groups, the conversion decreased due to the mass-transfer between 5-HMF, O₂ and solid catalyst in H₂O, MIBK, and H₂O/MIBK. The selectivity to DFF all increased being attributed to the protection of yield DFF by alkylamino groups on SiO₂(C₀/C₈NH₂) from being over-oxidized. PMo₄V₈(30)/SiO₂(C₀/C₈NH₂) with only hydrophilicity could not emulsify the mixture of H₂O and MIBK, which only acted as a heterogeneous catalyst. A remarkably high conversion and yield were achieved on PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) with 81.8% conversion and 73.7% yield in H₂O/MIBK compared to those on PMo₄V₈ and PMo₄V₈(30)/SiO₂(C₀/C₈NH₂). As mentioned above, PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) had a strong ability to emulsify the mixture of H₂O/MIBK to form stable water-in-oil (or oil-in-water system) [27]. PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) located on the interface of H₂O and MIBK, providing a stable and highly viscous Pickering emulsion and leading to the combination of emulsification and catalytic properties. In addition, the amount of alkyl groups and amino groups on the surface of SiO₂ nanoparticles influenced the amphiphilicity and hence the activity. From the water contact angles, decreasing amino group loading amount gave rise to the increase in their hydrophobicity 98.6° and 67.2° for PMo₄V₈(30)/SiO₂(2C₈/C₈NH₂) and PMo₄V₈(30)/SiO₂(C₈/2C₈NH₂), respectively. The conversion of 5-HMF decreased with 16.4% and 39.1% compared to PMo₄V₈(30)/SiO₂(C₈/C₈NH₂), indicating that stronger hydrophobicity or hydrophilicity of POM PIC did not favor for 5-HMF oxidation. A similar trend was observed using PMo₄V₈(30)/SiO₂(C₆/C₈NH₂) and PMo₄V₈(30)/SiO₂(C₁₀/C₈NH₂) as catalysts, which 51.6% and 63.7% conversion were obtained. These results suggested that PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) had suitable hydrophilic and hydrophobic properties, which can form a stable and highly viscous Pickering emulsion to promote the oxidation rate.

The reaction rate for aerobic oxidation of 5-HMF upon PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) was slow as 2.2 × 10⁻³ mmol L⁻¹ min⁻¹ at first 5 min with formation of the large emulsion droplets (Fig. 3). Prolonging the reaction time might increase the reaction rate to 3.4 × 10⁻³ and 3.6 × 10⁻³ mmol L⁻¹ min⁻¹ at 15 min and 30 min, while the sizes of the emulsion droplets also became smaller. This result further emphasized the enhancement in catalytic activity of amphiphilic PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) as an interfacial catalyst by stabilizing H₂O/MIBK Pickering emulsion. Another reason for enhancement of the reaction rate might be contributed to the enrichment of 5-HMF around PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) around the PI catalyst. Fig. 3 gave a rapid increase in 5-HMF absorption intensity at the initial stage of < 20 min, which became equilibrium after 30 min. The adsorption of 5-HMF by PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) was also determined by IR spectroscopy (Fig. S7 in Supporting information). The shifts for the vibration of ν_as (Mo-O_d) and ν_as (V-O_c-V) belonging to PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) were found, while a vibration of -OH group appeared. It indicated that the adsorption of 5-HMF by PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) occurred through the hydrogen bond between -OH group in 5-HMF and oxygen in PMo₄V₈¹¹⁻ anion.

The amphiphilicity of PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) could be modulated by the ratio between PMo₄V₈ to SiO₂(C₈/C₈NH₂) nanoparticles (Fig. S8 in Supporting information). It can be seen that increasing PMo₄V₈ loading amount gave rise to the decrease in their hydrophobicity. The different TOF values upon PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) were found to be 65.3 h⁻¹ for PMo₄V₈(10)/SiO₂(C₈/C₈NH₂) > 53.1 h⁻¹ for PMo₄V₈(20)/SiO₂(C₈/C₈NH₂) > 50.1 h⁻¹ for PMo₄V₈(25)/SiO₂(C₈/C₈NH₂) > 49.1 h⁻¹ for PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) > 39.7 h⁻¹ for PMo₄V₈(35)/SiO₂(C₈/C₈NH₂) (TOF = [5-HMF (mg) × Con. (%)]/[actual amount of PMo₄V₈ (mg) × reaction time (h)]). These TOF values were coherent with their hydrophobicity and their stabilizing ability for Pickering emulsion of H₂O and MIBK. The loading amount of PMo₄V₈ as 30 wt% was indeed most active with 81.8% conversion of 5-HMF among all hybrids, which was contributed to the high density of active sites and suitable water contact angles. DFF yields depended on the amount of PMo₄V₈ on SiO₂(C₈/C₈NH₂) nanoparticles, which 73.7% of DFF was obtained at 90 ℃ for 10 h for 30 wt% loading amount of PMo₄V₈ on SiO₂(C₈/C₈NH₂).

It reported that the emulsion volume could be fracted into three-layer containing oil, emulsion and water, which the droplet sizes were greatly affected by the ratio of oil to H₂O in the reaction mixture [10,28]. The R_o/w (volume ratio of H₂O to MIBK) indeed influenced the performance of PMo₄V₈(30)/SiO₂(C₈/C₈NH₂), which highest activity was delivered at R_w/o = 3:5 water-in-oil system (w/o) (Fig. S9 in Supporting information). At ratio of H₂O to MIBK from 8:0 to 5:3, an oil-in-water emulsion formed giving rise to increase in 5-HMF conversion but decrease in DFF yields. Meanwhile, enhancement of oil usage led to formation of water-in-oil emulsion, which presented a decrease in conversion and increase in DFF yields. The deep study on the influence of R_w/o would be done in the future. By now, 73.7% yield of DFF at 81.8% conversion was explored upon PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) nanoparticles in H₂O/MIBK at 90 ℃ for 10 h. This result confirmed the superiority of PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) as stabilizing agent and catalytic sites in aerobic oxidation of 5-HMF to DFF with high selectivity to 90.1% under lower temperature and atmospheric pressure of O₂. In addition, the effect of stirring speed on Pickering emulsion system was also studied (Fig. S10 in Supporting information). The results showed that the higher stirring speed resulted in the higher conversion rate. The influence of solvents was checked using toluene and p-chlorotoluene, which 11.5% and 17.2% yield of DFF at 39.0% and 43.9% conversion were found. The variety for these might be contributed to the existence of C=O in MIBK to link with -OH group in 5-HMF, hence to speed its conversion.

Finally, PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) nanoparticles showed a good stability and a long duration. After each cycle, the solid catalyst was decanted to the bottle of the reactor, which was washed with MIBK to isolate 5-HMF and DFF. The catalyst did not show any significant loss the activity and no leaching of PMo₄V₈ from SiO₂(C₈/C₈NH₂) being found after 8 runs (Fig. 4, Figs. S11 and S12 in Supporting information). DFF was detected mainly in MIBK phase, which was easily to be collected through distillation. The structure, components and morphology of reused PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) nanoparticles were characterized without any changes, determining its long duration and high stability (Figs. S13 and S14 in Supporting information).

PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) could also be used as a surface catalyst in aerobic oxidation of 5-HMF in DMSO system. The activity of PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) was also satisfied in 5-HMF oxidation in DMSO system (Fig. S15 in Supporting information), which 90.1% yield of DFF was obtained at 97.3% conversion of 5-HMF under the optimal reaction condition as 126 mg of 5-HMF and 70 mg catalysts at 130 ℃ for 10 h in 8 mL of DMSO with 10 mL/min of O₂. This might be almost the highest results using atmospheric pressure of O₂ by now. As results, PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) was determined to be a good candidate for production of DFF from 5-HMF in DMSO system as a surface solid catalyst. Moreover, PMo₄V₈(30)/SiO₂(C₀/C₈NH₂), PMo₄V₈(30)/SiO₂(C₈/2C₈NH₂), PMo₄V₈(30)/SiO₂(2C₈/C₈NH₂) also catalyzed 5-HMF transformation into DFF in DMSO system, which no significant difference in their activity due to the strong polarity of DMSO leveling the activity of these surface solid catalysts.

In summary, efficient Pickering interfacial catalyst PMo₄V₈(n)/SiO₂(C₈/C₈NH₂) had been constructed with a suitable balancing of the ratios between hydrophobic and hydrophilic groups, as well as PMo₄V₈ and SiO₂(C₈/C₈NH₂) on hydrophilic silica nanoparticles. PMo₄V₈(30)/SiO₂(C₈/C₈NH₂) was the most active one for the aerobic oxidation of 5-HMF into DFF in H₂O/MIBK biphase via the stabilizing Pickering emulsion. The efficiency in 5-HMF oxidation was significantly improved in H₂O/MIBK biphase in comparison of single H₂O or MIBK phase acting as Pickering interfacial catalyst, which explored the 81.8% conversion of 5-HMF to 73.7% yield of DFF at 90 ℃ for 10 h under atmospheric O₂. The solid catalyst was easily separated by centrifugation to be reused at least 8 cycles without significant loss of the activity. The produced DFF was mainly detected in MIBK, which could be easily collected through distillation. Such POM PIC system showed a prospective development concerning the heterogeneity of homogeneous POMs and catalysis in H₂O/solvent biphase.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was supported by National Natural Science Foundation of China (No. 51978134) and Jilin Provincial Science and Technology Department (No. 20210203205SF).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.062.