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. 2020 Sep 1;5(36):22808–22815. doi: 10.1021/acsomega.0c01921

Facile Synthesis and Synergetic Interaction of VPO/β-SiC Composites toward Solvent-Free Oxidation of Methanol to Formaldehyde

Gopa Mishra †, Gobinda C Behera ‡, Saroj Kumar Singh §, Kulamani Parida ∥,*
PMCID: PMC7495449  PMID: 32954129

Abstract

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Composite materials have revealed remarkable activities in various catalytic applications. However, choosing an appropriate material to enhance the catalytic activity and stability is a major challenge in the field of catalysis. In this article, we reported vanadium phosphorus oxide (VPO)/β-SiC as a stable composite material with good catalytic activity. VPO/β-SiC composite materials with different compositions were fabricated by the impregnation technique to investigate the catalytic activity and stability of these materials in liquid-phase reactions. The physiochemical characteristics of the prepared catalysts were analyzed by several spectroscopic methods. The catalytic activities of VPO/β-SiC composites were studied in a solvent-free oxidation of methanol using tert-butyl hydroperoxide (TBHP) as an oxidant. The reaction conditions were optimized by changing various reaction parameters. Under optimized reaction conditions, the 10 wt % VPO/β-SiC composite showed 100% conversion with 89.8% selectivity to formaldehyde.

Introduction

Fabrication of supporting and composite materials is a key strategy for enhancing various physical and chemical properties of catalysts. Nowadays, researchers are paying more attention toward supporting and composite materials for excellent chemical reactivity and stability. In this regard, various catalytic applications have been run over different supported catalysts.1−5 Selective oxidation of methanol over supported transition metal oxide catalysts is an important chemical process for formaldehyde (FA) production.6−13 Because of the economic and industrial significance of formaldehyde, the partial oxidation of methanol has been carried out widely using different heterogeneous catalysts such as CR@Ag/TiO2-nf,6 V2O5/SiO2,7 Fe2O3-MoO3,8 V-Mg-O,9 Sn-Mo-O,10 RuO2/Al2O3,11 Pd/Al2O3,12 and C-supported Pt.13 Generally, carbon, alumina, and silica are used as support for the oxidation reaction in commercial catalysts.14,15 However, low thermal conductivity of alumina and chemical reactivity of silica cause deactivation of the catalyst.16 Molybdenum has also not shown favorable applications in spite of its superior chemical activity as the reaction always produces various undesired byproducts.17 Carbon individually is also not used as a supporting material because it oxidizes quickly at a temperature of 650 °C.18,19 Hence, a new type of supporting material is needed that has similar benefits to those of alumina or silica along with high thermal conductivity and chemical inertness to avoid the issues such as loss of active phase and formation of undesired byproducts and to sustain the integrity of the catalyst.

To meet this challenge, we developed a vanadium phosphorus oxide (VPO)/β-SiC composite material. It is well known that β-silicon carbide (β-SiC) is covalently bonded with carbon and silicon atoms in a tetrahedral form, prepared at a temperature between 1400 and 1700 °C. β-SiC does not react with any acids, alkalis, or molten salts up to 800 °C. It also has very good physicochemical properties such as high resistance toward oxidation, high thermal conductivity, chemical inertness, and high mechanical strength, which are essential properties required for a good supporting material for the preparation of a catalyst.18,19

Because of the combination of these properties, β-SiC can be suitable as a heterogeneous catalyst promoter in various highly exothermic or endothermic reactions, i.e., in strong acidic or basic solutions.20 VPO materials are well known as heterogeneous catalysts for selective oxidation of n-butane to maleic anhydride.21−24 Recently, these materials have shown some significant activities in various liquid-phase organic transformation reactions.25−35 Selective oxidation of methanol to formaldehyde is an important strategy for fine chemical synthesis. Previously, our group has worked on gas-phase methanol oxidation over Al and WO3-VPO.34,36 Here, we have studied the catalytic activity of the VPO/β-SiC composite toward liquid-phase oxidation of methanol.

For this purpose, a series of VPO/β-SiC composites have been prepared by the impregnation technique and characterize by various spectroscopic techniques. The activity of the synthesized material has been studied in the methanol oxidation reaction. It has been found that the 10 wt % VPO/β-SiC composite shows superior catalytic activity to other VPO composite materials. Optimization of the conditions of reaction parameters such as the effect of reaction temperature, time, and molar ratio over the catalyst was carried out.

Results and Discussion

Figure 1 describes the X-ray diffraction (XRD) patterns of (I) VPO/β-SiC composites with varying amounts of VPO contents along with raw β-SiC and (II) pure VPO (calcined VOHPO4·0.5H2O) along with precursor VOHPO4·0.5H2O for comparison study. The highly intense peaks at 2θ = 35.78, 41.52, 60.13, 71.895, and 75.60° in Figure 1Ia have been assigned to the cubic phase of β-SiC, and the diffraction patterns are consistent with the standard JCPDS card (PDF # 02-1050). Figure 1IIb shows the intense peaks of precursor VOHPO4·0.5H2O at 2θ = 15.6, 19.7, 24.5, 27.4, 28.9, 30.4, 32.1, and 33.7°, which have been well matched with the reported literature.33,34,37 After calcinations, shifting of peaks toward the lower angle region has been observed for pure VPO (Figure 1IIa) at 2θ = 12.67, 17.31, 21.09, 25.42, and 29.35° corresponding to the reflectance pattern of VOPO4. This is because of the transformation of VOHPO4·0.5H2O to VOPO4 after calcinations.33,34,37 It has been observed that the diffraction peaks of both VPO and β-SiC are sharp and well-defined, which confirms that the neat samples are polycrystalline in nature. Apart from the neat sample, the VPO/β-SiC composite contains a two-phase composition, i.e., cubic mesoporous VPO and cubic phase of β-SiC with high degree of crystallinity. The diffraction peaks at 2θ = 12.67 and 29.35° start to appear and gradually intensify with an increasing amount of VPO from 5 to 15 wt % in VPO/β-SiC composite materials (Figure 1Ib–d). Furthermore, the diffraction peaks of β-SiC in the VPO/β-SiC composite get shifted to a lower end in comparison to neat β-SiC (Figure 1 II). The peak shifting shows that VPO is active over β-SiC.20,42

Figure 1.

Figure 1

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XRD patterns of (I) (a) β-SiC, (b) 5 wt % VPO/β-SiC, (c) 10 wt % VPO/β-SiC, and (d) 15 wt % VPO/β-SiC; (II) (a) pure VPO (calcined VOHPO4·0.5H2O) and (b) precursor VOHPO4·0.5H2O.

Figure 2 describes the Fourier transform infrared (FTIR) spectra of the VPO/β-SiC composite with varying amounts of the VPO content along with raw β-SiC and VPO for comparison study. The entire prepared sample shows a sharp band in the wavenumber range of 4000–400 cm–1. The IR band at 960–700 cm–1 corresponds to the stretching mode of Si–C.38 Apart from raw β-SiC, the bands at 3400 and 1642 cm–1 correspond to the symmetric stretching mode and bending vibration mode of absorbed water and hydroxyl groups.38,39 The infrared spectra of all of the catalysts apart from raw β-SiC in the range of 1230–900 cm–1 correlate with stretching modes of P–O and V = O groups.34 The band appearing at 652–500 cm–1 is assigned to the deformation vibration of the O–P–O group of the phosphate tetrahedral form.34 The band at 974 cm–1 corresponds to symmetric stretching vibrations of V4+ = O groups.34 Apart from these, the band appearing at 1045 cm–1 is attributed to symmetric stretching vibrations of PO3 groups and the bands at 1102 and 1230 cm–1 is assigned to the asymmetric stretching vibration of PO3 groups.34 It has been observed that after VPO loading over SiC the band at 840 cm–1 becomes more intense with a large peak width in the VPO/β-SiC composite. The peak shifting confirms that the VPO cluster is homogeneously dispersed on the β-SiC surface.

Figure 2.

Figure 2

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FTIR spectroscopy of (a) β-SiC, (b) 5 wt % VPO/β-SiC, (c) 10 wt % VPO/β-SiC, (d) 15 wt % VPO/β-SiC, and (e) VPO.

The UV–vis diffuse reflectance spectroscopy (DRS) spectra of the VPO/β-SiC composite with varying amounts of the VPO content along with raw β-SiC and VPO for comparison study are described in Figure 3. The presence of V4+ species is confirmed by the absorption band at 650 nm, whereas the absorption band at 450 nm is attributed to the V5+ species (VOPO4) of VPO crystallites and the absorption band at 564 nm is corroborated to β-SiC crystallites.20,40 Different V–P–O phases with V ions in different oxidation states have been found in neat VPO and VPO/β-SiC composites. Both V4+ and V5+ phases are required for the catalyst to show high activity and selectivity.41 The band gap energy of the catalyst is calculated using UV–vis DRS. The literature survey shows that β-SiC is an indirect-band-gap semiconductor.42,43 The Tauc plot (i.e., hνvs (α × hν)1/2) is used to calculate the band gap of the semiconductor and is found to be 2.19 eV. The calculated energy band gap value is well consistent with a previously reported value of 2.2 eV.42−44 More dispersion of VPO on the β-SiC surface results in a continuous red shift in the absorption edge (Figure 3b–d). The shifting of absorption edge due to different light absorption properties of VPO confirms the well dispersion of VPO over the β-SiC matrix.

Figure 3.

Figure 3

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Diffuse reflectance UV–vis spectra of (a) β-SiC, (b) 5 wt % VPO/β-SiC, (c) 10 wt % VPO/β-SiC, (d) 15 wt % VPO/β-SiC, and (e) VPO.

Figure 4 describes the X-ray photoemission spectra of neat β-SiC, VPO, and the VPO/β-SiC composite with varying amounts of the VPO content. The C 1s peak (Figure 4A) of pure β-SiC shows the characteristic binding energy at 282.88 eV assigned to the C–Si bond in the SiC lattice.20,42Figure 4B shows the Si 2p peak of neat β-SiC at 100.8 eV assigned to Si+4 of the Si–C bond.45 Because of controversial interpretations of the V 2p3/2 peak as well as contradictory binding energy values for Vn+ reported in the literature, the VOPO4 phase structure observed through UV–vis spectra is used to calibrate the binding energy of the reference V 2p3/2 peak. As shown in Figure 4C, the photoelectron peak of V 2p3/2 at 517.3 eV is assigned to the V5+ state and the O 1s peak at 531.7 eV is assigned to the O2– state in oxides.33,34,46−48 The photoelectron peak of P 2p at 134 eV corresponds to the + 5 oxidation state of VPO (Figure 4D).33,34 The shift to an initially lower and then toward a higher binding energy of the C 1s, Si 2p, V 2p3/2, O 1s, and P 2p peaks (Figure 4A–D) with increasing VPO loading may reflect a change from the VPO cluster to VPO monolayer deposition or a change in the overlayer–support interaction in the VPO/β-SiC composite.

Figure 4.

Figure 4

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X-ray photoelectron spectra (XPS) of (A) C 1s, (B) Si 2p, (C) V 2p, O 1s, and (D) Si 2p of β-SiC, raw VPO, and different wt % VPO/β-SiC catalysts.

Figure 5 describes transmission electron microscopy (TEM) images, selected-area electron diffraction (SAED) patterns, and energy-dispersive X-ray (EDX) spectra of neat β-SiC and VPO and VPO/β-SiC composites with different concentrations of VPO from 5 to 15 wt %. It is observed from Figure 5a that the shape and size of β-SiC particles are nonuniform. Some β-SiC particles are found to be platelet- and cube-like structures with size varying from 30 to 80 nm. The circular-patterned SAED image (Figure 5a) demonstrates that β-SiC particles are cubic polycrystalline having d values of 0.254, 0.153, and 0.131 nm, respectively. These values are well consistent with the standard XRD pattern JCPDS card of β-SiC (PDF#02-1050). The EDX spectrum of β-SiC (Figure 5a) confirms the presence of Si and C elements. Only the Cu peak is found that comes from the carbon-coated Cu grid used in TEM. The contamination during the sample preparation for TEM analysis may result in the oxygen peak of hydroxyl groups. Figure 5b shows the TEM image of VPO particles that are semispherical and in close contact with each other to form a cluster morphology. The corresponding SAED pattern (Figure 5b) indicates that nanoparticles are polycrystalline in nature. EDX analysis (Figure 5b) predicts that nanoparticles are composed of V, P, and O. The morphologies of VPO/β-SiC composites with varying amounts of VPO from 5 to 15 wt % are shown in Figure 5c–e. The SAED pattern (Figure 5b–e) describes that VPO/β-SiC composites are polycrystalline in nature with the existence of both VPO and β-SiC phases. Further, the EDX analysis supports the presence of C, O, Si, V, and P in VPO/β-SiC composites. The above analysis confirms the VPO loading over the β-SiC surface and concurrence of both the phases in all VPO/β-SiC composites.

Figure 5.

Figure 5

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Transmission electron microscopy images, selected-area electron diffraction patterns, and energy-dispersive X-ray spectra of (a) β-SiC, (b) VPO, (c) 5 wt % VPO/β-SiC, (d) 10 wt % VPO/β-SiC, and (e) 15 wt % VPO/β-SiC.

High-resolution TEM (HR-TEM) representation of 10 wt % VPO/β-SiC composites is described in Figure 6. The micrograph shows a well-defined lattice fringe with separation d = 0.31 nm of the (001) plane correlating with the VPO phase and the fringe separation with d = 0.25 nm of (111) planes consistent with the cubic phase of SiC. Selected-area electron diffraction (SAED) patterns observed in Figure 5d also agree with it, which shows the presence of VPO and β-SiC phases. This clearly indicates the well dispersion of VPO over the β-SiC surface.

Figure 6.

Figure 6

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HR-TEM image of the 10 wt % VPO/β-SiC composite.

Textural properties of all of the synthesized samples are studied by N2 adsorption/desorption at 77 K, and the results are given in Table 1. The surface area and pore volume of β-SiC are found to be 31.59 m2 g–1 and 0.45 cm3 g–1, respectively. The surface area of the VPO/β-SiC catalyst gradually decreases with the increase of the VPO percentage. The VPO cluster deposition blocks some of the pores of the β-SiC surface, making the pores unavailable for N2 adsorption, which causes a decrease in the surface area of VPO/β-SiC composites.

Table 1. Textural Properties of All of the Synthesized Catalysts.

catalyst surface area (m2 g–1) pore size (Å) pore volume (cm3 g–1)
β-SiC 31.59 519.2 0.45
VPO 14 100.0 0.05
5 wt % VPO/β-SiC 31.05 508.8 0.35
10 wt % VPO/β-SiC 28.27 490.6 0.27
15 wt % VPO/β-SiC 24.61 487.4 0.25
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Figure 7 describes field emission scanning electron microscopy (FESEM) with EDX analysis of β-SiC, VPO, and VPO/β-SiC composites with varying amounts of VPO from 5 to 15 wt %. Neat β-SiC powder forms platelet-shaped aggregates, whereas neat VPO forms rose-like flowery morphology. At the lower concentration of VPO in VPO/β-SiC composites, there is slight formation of β-SiC nanoplates along with VPO aggregates. Since the VPO concentration is increased in the catalysts, the β-SiC surface is homogeneously dispersed with VPO, which is clearly visible in the micrographs. The EDX analysis of neat β-SiC powder confirms the presence of Si and C elements, whereas neat VPO confirms the presence of V, P, and O elements. EDX analysis of the catalysts confirms that the elements present are Si, C, V, P, and O. The oxygen peak of hydroxyl groups found in the spectrum of β-SiC is due to the contamination while sample preparation was carried out. The micrographs depicted in Figure 7c–e reveal that VPO particles are homogeneously sprinkled over the β-SiC surface. VPO and β-SiC do not significantly affect the individual morphology. EDX analysis confirms the coexistence of both VPO and β-SiC in VPO/β-SiC composites. As they are in close contact, the interatomic interactions between VPO and β-SiC confirm that VPO is well dispersed over the β-SiC surface.

Figure 7.

Figure 7

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FESEM images and EDX analyses of (a) β-SiC, (b) VPO (c) 5 wt % VPO/β-SiC, (d) 10 wt % VPO/β-SiC, and (e) 15 wt % VPO/β-SiC catalysts.

The study of the catalytic activities of β-SiC- and VPO/β-SiC-based catalysts has been carried out for selective oxidation of methanol in a solvent-free medium. At the start of our investigation, we just tested the activity of our VPO/β-SiC composite materials, and the results are summarized in Table 2. All of the materials show a remarkable catalytic activity, giving 100% conversion. However, in the case of formaldehyde (FA), 10 wt % VPO/β-SiC gave the highest selectivity of 76.4% compared to other materials. The increase in VPO wt % decreases the selectivity, and this may presumably be due to the formation of another byproduct dimethyl ether (DME). It is observed that no methyl formate (MF) was formed during the reaction, which signifies that the materials are very selective toward formaldehyde production without over-oxidation. The mechanism of the reaction is not studied fully. However, this may follow the dissociative adsorption of methanol on the surface of the VPO/β-SiC catalyst to form surface methoxo-vanadium (V) centers. The oxidation of activated methoxo ligands and their elimination as formaldehyde ensure the reduction of the catalyst.34

Table 2. Comparison of Catalytic Activities of VPO/β-SiC Materialsa.

entry catalyst conversion (%) formaldehyde sel. (%)
1 without catalyst 3 0.1
2 β-SiC 61 5
3 VPO 87.2 51.5
4 5 wt % VPO/β-SiC 100 62.3
5 10 wt % VPO/β-SiC 100 76.4
6 15 wt % VPO/β-SiC 100 70
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a

Conditions: Amount of catalyst, 0.05 g; Temp., 80 °C; time, 6 h; oxidant, tert-butyl hydroperoxide (TBHP); methanol/oxidant ratio, 1:1.

Figure 8 illustrates the influence of variation in molar ratios of substrates to the oxidant in the oxidation reaction over the 10 wt % VPO/β-SiC composite. A remarkable conversion (100%) has been achieved by all of the molar ratios of the substrate to the oxidant except for 0.5:1 (98.6% conversion and 55% selectivity). When the molar ratio of the substrate to the oxidant has been increased to 1:1, the selectivity has been changed to 76.4%. However, no marginal change in conversion is observed. At the substrate-to-oxidant molar ratio 2:1, a selectivity of 89.8% has been achieved toward formaldehyde formation. When the substrate-to-oxidant molar ratio is increased to 3:1, no further increase in selectivity is observed. Hence, the substrate-to-oxidant molar ratio 2:1 has been used as the optimum for the rest of the parameter studies.

Figure 8.

Figure 8

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Influence of the substrate-to-oxidant molar ratio. Conditions: Catalyst, 0.05 g; Temp., 80 °C; time, 6 h.

Figure 9 illustrates the results of methanol oxidation at various temperatures (40–100 °C). It has been observed that oxidation of methanol is greatly influenced by varying the reaction temperature. The conversion of methanol is found to be 86% at 40 °C with formaldehyde selectivity of 70%. With the increase in temperature, the conversion increases to 100%. However, the formaldehyde selectivity has been found to be highest (89.8%) at 80 °C. A further increase in temperature to 100 °C results in lower formaldehyde selectivity (77.8%). This is due to the over-oxidation of the substrate at higher temperature to another byproduct, methyl formate. Hence, 80 °C has been taken as the optimum reaction temperature for other reaction parameter studies.

Figure 9.

Figure 9

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Influence of reaction temperature. Conditions: Catalyst, 0.05 g; methanol-to-TBHP ratio, 2:1; time, 6 h.

Figure 10 illustrates the influence of reaction period on the oxidation of methanol. Interestingly, the conversion has been found to be 100% in all entries except for 2 h, which showed a little lower conversion of 98%. However, the highest formaldehyde selectivity (89.8%) has been achieved when the reaction is run for 6 h. A further increase in the reaction period decreases the formaldehyde selectivity. This is due to the over-oxidation of the substrate to other byproducts. Hence, the reaction period of 6 h has been taken as the optimum value to achieve a high degree of formaldehyde selectivity.

Figure 10.

Figure 10

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Influence of reaction period on methanol oxidation. Conditions: Catalyst, 0.05 g; Temp., 80 °C; methanol-to-TBHP ratio, 2:1.

The heterogeneity of VPO/β-SiC materials has been assessed through a hot filtration test. Throughout the path of reaction, the catalyst in the solid form has been separated from the reaction mixture by filtration following a conversion of 30%. The filtrate thus obtained has been again run under the same reaction conditions. After the completion of reaction, the product has been tested by offline gas chromatography. There is no alteration found in the conversion of methanol. No loss of catalyst components throughout the course of reaction confirms the heterogeneous nature of the catalyst. Further, the liquid product was tested by atomic absorption spectroscopy (AAS) for elemental analysis, which confirms a very insignificant leaching of vanadium (<1 ppm) during the reaction. This stability of the catalyst has a connection with XRD results.

The VPO/β-SiC composite showed its capability of reusability for four consecutive batch oxidation reactions (Table 3). The solid catalyst has been separated from the reaction mixture by centrifugation and then washed properly several times using acetone. The solid thus obtained has been reused as a fresh catalyst in the next reaction under the same conditions. No considerable change has been observed in the performance of catalyst after 24 h. The stabilization of conversion has been observed in a range between 99 and 100%, while formaldehyde selectivity remained fairly the same. The used VPO/β-SiC sample has also been analyzed by XPS and FESEM, and the results are found to be the same as the original. The photoelectron peak of V 2p3/2 is found at 517.0 eV, which is assigned to the V5+ state, and the O 1s peak at 533.4 eV is assigned to the O2– state in oxides (Figure S1). Also, the surface morphology of the used sample did not change (Figure S2). This confirms the synergetic stability of the VPO/β-SiC catalyst. Being a highly chemically inert material, β-silicon carbide can withstand prolonged contact with aggressive media without any structural integrity modification. Moreover, it also possesses high oxidative resistance, which allows it to be fully utilized for medium temperature reactions in the presence of oxygen.

Table 3. Reusability Test of VPO/β-SiCa.

no. of cycles methanol conv. (%) formaldehyde sel. (%)
1 100 89.8
2 100 89.8
3 100 89.8
4 99 89.8
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a

Conditions: Amount of catalyst, 0.05 g; Temp., 80 °C; time, 6 h; oxidant/TBHP and methanol/oxidant ratios, 2:1.

Conclusions

In conclusion, VPO/β-SiC composites were successfully synthesized by the wetness impregnation method for catalyzing methanol oxidation in a solvent-free liquid-phase medium. In addition, XRD, FTIR, UV–vis DRS, HR-TEM, FESEM, Brunauer–Emmett–Teller (BET), XPS, and other characterization methods were used to confirm the dispersion and interaction of VPO over the β-SiC surface. The growth of collaborating interlinkage between β-SiC and VPO clusters, because of the existence of oxygen molecules relocating across the extended V–P–O network, smoothens the reaction and enhances the formaldehyde production. Among the entire prepared composites, the 10 wt % VPO/β-SiC composite has shown remarkable catalytic activity, giving 100% conversion with 89.8% product selectivity. The results demonstrate that the obtained catalyst after being used for four repeated cycles is still stable and shows high performance for catalytic application in methanol oxidation. This fascinating potential and stability of the VPO/β-SiC composite encourage its further use in other oxidation reactions.

Experimental Section

The β-SiC powder in a nano form was commercially supplied by SRL (50 nm, 98%). For further purification, the powder sample was heated at 650 °C for about 2 h to remove volatile impurities and properly washed with HCl (1:1), HNO3 (1:2), and HF (40%) for removal of impurities.

The VPO precursor preparation method was as follows: V2O5 (5.0 g, purity 99%, Strem) and o-H3PO4 (30 mL, purity 85%, Aldrich) were digested in deionized water (120 mL) for 16 h. The yellow solid recovered was vacuum-filtered and washed in cold water (100 mL) followed by acetone (100 mL). The purified sample was dried in air (110 °C, 24 h) to get dihydrate VOPO4·2H2O. Further, the dihydrate (4 g) was refluxed in isobutanol solvent (80 mL) for about 21 h and the obtained product was vacuum-filtered, dried at 110 °C (16 h), digested in deionized water (9 mL H2O/solid (g)) for about 2 h, filtered in hot conditions, and air-dried at 110 °C for 16 h.

The synthesis of different wt % VPO/β-SiC materials was carried out by the impregnation method taking water as a solvent. Initially, the required amount of VPO was dissolved in 40 mL of distilled water. Then, the mixture was stirred for some time and the required amount of β-SiC was added to the solution. The stirring was continued at low temperature until the solution was completely evaporated. The obtained residue was dried for 8 h at 120 °C followed by calcination at 400 °C for 5 h. Various compositions of VPO (5, 10, and 15 wt %)-supported β-SiC composites were synthesized by adopting the above procedure.

The phase identification and structural analysis of synthesized materials were carried out by the XRD technique (Philips PANalytical PW 3040/60) using Cu Kα radiation with 2θ varying from 20 to 80°.

The Bruker-Alpha (Eco-ATR) FTIR spectrometer was used with a spectral range of 500–4000 cm–1 to study the bonding and structural analysis of synthesized materials. For FTIR spectroscopy analysis, pellets were prepared by mixing KBr with the catalyst by applying 50 kg cm–2 pressure.

The optical absorbance study of synthesized materials was carried out in the 200–800 nm spectral region by UV–visible diffuse reflectance spectroscopy (UV–vis DRS) (Varian, Australia, model EL 96043181), and boric acid was used as a reflectance standard.

The electronic states of Si, C, V, P, and O were determined by XPS (X-ray photoelectron spectroscopy) (Kratos Axis 165 with a dual-anode (Mg and Al)) using a Mg Ka source. C 1s (C 1s = 284.9 eV) is taken as a reference to calibrate the binding energy values. The charge of the sample was balanced using charge neutralization of 2 eV. Binding energy shifts were completely reproducible within the uncertainty of ±0.1 eV.

The shape, size, and composition of β-SiC and VPO/β-SiC composites were determined by a TEM attached with an energy-dispersive system (EDS) (FEI, TECNAIG2 20, TWIN).

The surface area and pore volume distributions of the synthesized composite materials were determined by N2 adsorption at 77 K (Micromeritics, ASAP 2010). Before the measurement of surface area, the sample was evacuated at 110 °C for about 2 h to remove the moisture present.

The structural and morphological properties of prepared composite materials were studied by the FESEM (ZEISS 55) analyzer.

The catalytic application of synthesized catalysts was studied for the methanol oxidation reaction in a double-necked glass batch reactor (100 mL) attached with a condenser and a magnetic stirrer, by taking 0.05 g of the catalyst and methanol (Merck, 98%) and TBHP (Merck, 99.8%) in a 2:1 molar ratio. The contents were then refluxed gently at 80 °C for 6 h. It took 20 min to reach 80 °C temperature. The product was filtered after the completion of the reaction and analyzed by GC (Shimadzu, GC-17A) using a capillary column (ZB-1; length, 30 m; ID, 0.5 nm; and film thickness, 3.0 l) in flame ionization detector (FID) mode. The temperature gradient was linearly ramped at 10 °C min–1 from 37 to 90 °C. The injector and detector temperature was set at 90 °C. Other gas parameters were as follows: H2 flow rate = 40.0 mL min–1, air flow rate = 400.0 mL min–1, and N2 flow rate = 30.0 mL min–1.

Acknowledgments

All of the authors are grateful to the Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India, management for financial support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c01921.

  • XPS and FESEM analyses of used 10 wt % VPO/β-SiC (PDF)

Author Present Address

⊥ Chemical Division, National Test House (Western Region), Andheri (E), Mumbai 400093, Maharashtra, India.

The authors declare no competing financial interest.

Supplementary Material

ao0c01921_si_001.pdf (169.2KB, pdf)

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