Selective Oxidation of Glycerol with Hydrogen Peroxide Using Silica-Encapsulated Heteropolyacid Catalyst

1.   Introduction

Due to the depletion of fossil fuels and the improvement of awareness of environmental protection, biodiesel resources have been receiving extensive worldwide attention and are being rapidly flourished because of their excellent renewability, environmental performance, and innocuous advantages ^1-3. Biodiesel is the best known in biofuels, obtained through the transesterification between vegetable oils and short chain alcohols with a catalytic process ⁴. The glycerol in an amount of ca. 10% (w, mass fraction) of the overall biodiesel production is generated as a by-product in this process, which has been identified as an important platform compounds for producing high value chemicals ⁵. Representative reaction pathways for the glycerol valorization contain hydrodeoxygenation, hydrogenation, hydrogenolysis, oxidation and dehydration etc. Among these various utilization approaches, the glycerol oxidation is highly attractive, which can provide C1 products such as formic acid (FA), C2 products such as glycolic acids (GCA) and oxalic acid (OA), and many C3 products such as tartronic acid, glyceraldehyde, glyceric acid, and dihydroxyacetone. These products are all high-added chemical intermediates and are still being obtained through expensive processes or any other way ^6-8. Among the routes for the utilization of these chemicals, the increasing interest in efficient FA production aims at its application as hydrogen carrier ^{9, 10}. As FA owns 4.4% hydrogen content and some well-known systems could decompose it under mild conditions to hydrogen and carbon dioxide ^11-13. Additionally, FA is also a convenient source of C1 raw material for the chemical synthesis. For example, the efficient hydrogenation/ disproportionation of formic acid (FA) to methanol by using iridium catalysts has been reported recently ¹⁴.

Over the past decades, the selective oxidation of glycerol has been conducted to obtain valuable products with molecular oxygen. The seminal studies have founded that supported gold catalyst have high selectivity and conversion for glycerol oxidation ^15-17. In addition, the aerobic oxidation of glycerol has been widely studied using noble metal such as Pt ^{18, 19}, Pd ²⁰ etc. In these catalytic systems, the noble metals play a key role in glycerol oxidation and the catalytic performance depends on the basicity of the reaction medium. However, due to its basicity, it produces a lot of organic acid salts and thus the mixture after reaction needs further neutralization and acidification so as to obtain the target products. In addition to its basicity, the other disadvantage is that the supported noble metal catalysts are rather expensive for the further practical application and also tend to lose activity rapidly due to the oxidation of metal surface and thus result in the poisoning of the active sites ²¹. Therefore, the design of other catalyst systems was becoming a hot point in the current research field.

On the other hand, hydrogen peroxide has been widely used as an oxidant in organic oxidation reactions owing to its environmentally friendly nature in that the degradation products of hydrogen peroxide are only oxygen and water ²². Hydrogen peroxide is strong oxidant which can oxidize a broad variety of inorganic and organic substrates in liquid-phase reactions under very mild reaction conditions ²³. Hence, the catalytic oxidation of glycerol has been carried out by using the hydrogen peroxide as an oxidant recently ^24-26.

The catalyst acidity could play a very important role in accelerating the hydrolysis of glycerol molecules ^{27, 28}. The Keggin type heteropolyacids (HPAs) have attracted increasing attention due to their strong Bronsted acidity and environmental friendliness ^29-31. Meanwhile, HPAs also showed excellent redox properties ³². Their low surface area (< 10 m²·g^-1) and high solubility in polar solvents limit its potential catalytic application. For example, HPAs have been employed as efficient catalysts in aqueous media for the oxidation of glycerol into FA, but the separation of homogenous HPA catalysts was tedious due to miscibility of HPAs and FA with water media ³³.

Generally, the heterogeneous catalysts have some particular advantages over homogeneous ones, for example, the solid catalysts can be removed easily from the reaction media and thus recycled. From this point of view, solid catalysts usually represent a better option in liquid phase reaction. On the basis of discuss above, in this work, we attempted to immobilize HPA (H₄PMo₁₁VO₄₀) within silica matrix via a sol-gel method and the sequential silylation technique, which offered an appropriate hydrophobicity and also shielding of HPA, suppressing the leaching of HPA and but allowing penetration of reactants and products through porous silica during the aqueous reaction. It was found that the as-obtained catalyst exhibited excellent activity and reusability for glycerol oxidation using H₂O₂ as an oxidant under very mild conditions.

2.   Experimental

2.1   Materials

Disodium hydrogen phosphate, sodium metavanadate, sulfuric acid, sodium molybdenum oxide, ethyl ether, dimethyl sulfoxide, tetraethyl orthosilicate, acetonitrile, glycerol, toluene, dimethyldichlorosilane were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai. Distilled water used in this work was produced by our own laboratory. High purity N₂ (99.999%) was supplied by ShangNong Gas Factory. All other chemicals (analytical grade) were from Sino pharm Chemical. Toluene (analytical grade) was dried by using the standard methods.

2.2   Catalyst preparation

2.2.1   Preparation of H₄PMo₁₁VO₄₀ (HPA)

In a typical synthesis route, 0.71 g of disodium hydrogen phosphate was dissolved in 10 mL of water, and then 0.61 g of sodium metavanadate that had been dissolved by boiling in 10 mL of water was mixed with the above solution under stirring. The cooled mixture was acidified with 0.5 mL of concentrated sulfuric acid to a red color. 11.26 g of sodium molybdenum was dissolved in 20 mL of water, and added to this mixture. Finally, 8.5 mL of concentrated sulfuric acid was slowly dropped into the previously prepared aqueous solution with vigorous stirring. In the process, the dark red color changed to a lighter red and the pH of the solution was 1–2. The water solution was cooled and the heteropolyacid extracted with 40 mL of ethyl ether ³⁴. In the extraction process, it will be divided into three layers and the heteropoly etherate was present in the middle layer. After separation, a stream of air was passed through the heteropoly etherate layer to free it of ether, which offered the orange solid powder as HPA (3.70 g).

2.2.2   Preparation of SiO₂-encapsulated HPA catalyst

Absolute ethanol (10 mL) was mixed with distilled water (0.12 g), then add tetraethyl orthosilicate (1.6 mL) into the mixture. Allowed the solution standing for thirty minutes and followed by adding dimethyl sulfoxide solution (4 mL) containing HPA (0.1 g); in the end, added 4 mL of NH₃·H₂O (28%). After 2 h, the as-obtained gel was dried. The sample was designated as HPA@SiO₂ sample. Additionally, silica gel (SiO₂) was prepared by the same method besides HPA was not added.

2.2.3   The silylation of HPA@SiO₂

Surface silylation of silica was carried out using dimethyldichlorosilane as the silylation agents. The procedures for surface modification of silica with the agents were similar to that in the literature ³⁵. 0.8 mmol of dimethyldichlorosilane was dissolved in 10.0 mL of dried toluene. 0.2 g of dried HPA@SiO₂ composite was dispersed in the above solution. After stirring for 20 min at 80 ℃ under a N₂ atmosphere, the solid was isolated through filtrated, thoroughly washed with toluene for several times. The obtained product was dried for 12 h at 100 ℃ under vacuum. Then, the dried sample followed by calcination at 250 ℃ for 1 h in a nitrogen reduction furnace prior to reaction. This sample was named as HPA@SiO₂-S-N₂. The elemental analysis gave that the actual loading of HPA was about 13%. The as-synthesized silica gel (SiO₂) support was impregnated conveniently with HPA to afford 13% HPA/SiO₂ catalyst.

2.3   Catalyst characterization

XRD analysis of types samples were performed in the 2θ range of 10°–80° on an D/MAX 2550 VB/PC instrument (Rigaku Corporation, Japan) using a graphite crystal a monochromator. The textural properties from N₂ adsorption isotherms were obtained on NOVA 2200e equipment (Quantachrome, USA). The surface area was obtained from the isotherms in the relative pressure range of 0.0–0.35. Pore volume was determined at p/p₀ of 0.99. The inductively coupled plasmaatomic emission spectroscopy ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) analysis was carried out on a ICP-710ES instrument (Varian Company, Germany). The sample was putted in a nickel crucible mixed with some sodium hydroxide and then putted into muffle at 500 ℃ for 30 min to destroy the structure of sample, followed by dissolving with water. FT-IR spectra were recorded from pressed KBr pellets at room temperature on a Magna550 (Nicolet, USA). A spectrum of dry KBr was also recorded as background. The thermal stability of catalysts was determined by TGA method (heating rate: 10 ℃·min^-1; N₂ flow, 100 mL·min^-1) using Perkin Elmer Pyris Diamond Analyser (Netzsch, Germany). The SEM images were performed on JSM electron microscopes (JEOL JSM-6360LV, Japan). TEM was performed in a JEM 2010 instrument (JEOL Corporation, Japan) operating at 200 kV with a nominal resolution of 0.25 nm. The samples for TEM were prepared by dropping the aqueous solutions containing the NPs onto the carbon-coated Cu grids. Water contact angle (CA) was carried out by the Contact Angle Meter (CA100C, Innuo Company, Shanghai, China) using the droplet profile as a method. The CA was determined using a tangent placed at the intersection of the liquid and solid. A water droplet with a volume of 2 μL was dispensed by a piezo doser onto each sample disk.

The amount of acid sites of the different samples was obtained with temperature programmed desorption of ammonia (NH₃-TPD). About 0.15 g of catalyst was loaded in a quartz tube. Prior to each test, the sample was pre-treated in He at 300 ℃ for 1 h, cooled to 50 ℃ to remove surface water. Then, the sample was maintain at 50 ℃ for 1 h and saturated with a 10% NH₃-in-N₂ mixture, and then flushed by He for 1 h to remove physically adsorbed ammonia. Then, the sample was heated to 400 ℃ at a heating rate of 10 ℃·min^-1 in the same flow of He. The profiles of desorption were recorded using a thermal conductivity detector (TCD, Vodo, Zhejiang, China).

2.4   Catalytic reaction

The oxidation of glycerol was carried out as follows. A mixture of glycerol (1.25 mmol), acetonitrile (0.90 mL), 30% aqueous H₂O₂ (6.25 mmol), and the HPA@SiO₂-S-N₂ catalysts (0.06 g) were placed in a 25 mL Schlenk flask equipped with a magnetic stirrer and then stirred at 70 ℃ for 12 h. When the reaction was finished, the mixtures were filtrated by centrifugation. The resulting supernatant liquid was diluted 5 times with distilled water and analyzed by HPLC using LC-100 chromatograph equipped with a Aminex HPX-87H Column (300 × 7.8 mm, Bio-Rad Company, USA) used with a solution of 0.005 mol·L^-1 H₂SO₄ (0.5 mL·min^-1) as the eluent at 55 ℃. The recovered catalyst washed with ethanol and toluene for several times and dried in a vacuum at 60 ℃ for recycling tests. The conversion of glycerol and selectivity towards products were calculated as follows:

Hot filtration experiment for the HPA@SiO₂-S-N₂ catalyst was performed with two parallel experiments. The procedures were given by the following procedures: a mixture of glycerol (1.25 mmol), acetonitrile (0.88 mL), 30% aqueous H₂O₂ (6.25 mmol), and 0.06 g catalyst was placed in a 25 mL Schlenk flask equipped with a magnetic stirrer and condenser, followed by stirring at 70 ℃. Two parallel experiments were performed without or with separating the catalyst from the hot reaction mixture after 8 h ³⁶. Hot filtration experiment for the HPA/SiO₂ catalyst was carried out in a similar method except that the catalyst was isolated after 50 min.

3.   Results and discussion

3.1   Catalyst characterization

In this work, the HPA was introduced conveniently into the silica by a sol-gel procedure, followed by the silylation approach. The preparation route has been depicted briefly in Scheme 1.

Scheme 1

Scheme 1.  The preparation approach of the silica-encapsulated HPA catalyst

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The obtained samples were first characterized by FT-IR. A small band observed at 1635 cm^-1 and a broad band at 3420 cm^-1 was due to absorbed water in Fig. 1a–e. The pure HPA showed IR spectra with the specific bands of the Keggin structure as shown in Fig. 1a, which showed band at 1062 cm^-1 (v_as P―Oa), 961 cm^-1 (v_as Mo＝O_d), 867 cm^-1 (v_as Mo―O_b―Mo), and 777 cm^-1 (v_as Mo―Oc―Mo), 591 cm^-1 (δ P―O), respectively. These absorptions are constant with the Keggin structure of the heteropolyanion ³⁷. As shown in Fig. 1b, SiO₂ showed the vibration band at 1095 cm^-1, 950 cm^-1, 800 cm^-1 and 584 cm^-1, which can be assigned to v_as (Si―O―Si), v (Si―O), v_s (Si―O―Si) and δ (Si―O―Si) bonds, recpectively ³⁸. After the HPA was introduced into the silica matrix, the specific bands of the Keggin structure showed a slight change, as compared with that of the parent HPA. It was noted that bands of HPA/SiO₂ and HPA@SiO₂-S-N₂ catalyst in the 1200–400 cm^-1 region were completely or partially overlapped by that of SiO₂. For example, the band assigned to the P―O asymmetric stretching vibration at 1062 cm^-1 of HPA is completely overlapped by the strong band at 1095 cm^-1 of SiO₂ (Fig. 1c–e), while the two strong bands appeared in the spectra of HPA/SiO₂ and HPA@ SiO₂-S-N₂ appeared at 950 and 800 cm^-1, as a result of the overlapping of the absorption bands of silica and HPA. However, the asymmetric stretching vibration band at 863 cm^-1 that assigned to Mo―O_b―Mo vibration, which was not found in SiO₂, but it indeed appeared in SiO₂-encapsulated/supported HPA catalysts (Fig. 1c–e). Additionally, a well defined band appeared at 2965 cm^-1 after silylation in Fig. 1d–e can be assigned to C―H stretching vibration in surface ―CH₃ groups ³⁹. All these results indicated clearly that HPA has been embedded within silica matrix and also the silylation was successful by dimethyldichlorosilane.

Figure 1

Figure 1.  FT-IR spectra of (a) HPA; (b) SiO₂; (c) HPA/SiO₂ catalyst; (d) the fresh HPA@SiO₂-S-N₂ catalyst; (e) the HPA@SiO₂-S-N₂ catalyst after reused for 5 times

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The XRD patterns of the obtained catalysts were depicted in Fig. 2. The most intensive reflections of bulk HPAs appear in the following range: 7°–10°, 17°–23° and 26°–32° (Fig. 2a), which are characteristic of the Keggin structure⁴⁰, while HPA/SiO₂ and HPA@SiO₂-S-N₂ catalysts did not give obvious diffraction peak and only displayed a large and flat diffraction peak appeared at 2θ= 15°–35°, which was assigned to amorphous silica (Fig. 2a–2e), which evidenced that bulk HPA did not form but HPA was highly dispersed on in the SiO₂ support.

Figure 2

Figure 2.  X-ray diffraction patterns of (a) HPA; (b) SiO₂; (c) HPA/SiO₂ catalyst; (d) the fresh HPA@SiO₂-S-N₂ catalyst; (e) the HPA@SiO₂-S-N₂ catalyst after reused for 5 times

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The porous properties of the catalysts were measured by the N₂ adsorption-desorption isotherms method (Fig. S1). As shown in Table 1, the BET surface area of pure SiO₂ was 287 m²·g^-1 (Table 1, entry 1), the encapsulation of HPA into the silica matrix resulted in a lower surface area (Table 1, entries 3–5). But their surface areas are still higher than that of the catalyst prepared by impregnation (Table 1, entry 2). Interestingly, the encapsulation of HPA into silica matrix could result in the increase of pore diameter (Table S1, entries 1 vs. 2 and 3), possibly arising from the effect of bulky HPA molecules. In the next, the hydrophobicity of the catalysts has been investigated by the measurement of contact angle (CA). The SiO₂ and HPA/SiO₂ was completely hydrophilic and the contact angle was close to 0° (Table 1, entries 1 and 2). However, SiO₂-encapsulated HPA catalyst (HPA@SiO₂) showed slightly stronger hydrophibicity, as compared with that of SiO₂ and HPA/SiO₂ samples likely due to the difference in surface morphology. Especially, the silylated catalyst even gave a larger CA (Table 1, entry 4). As a result, the silylation procedure can increase obviously the hydrophobicity of the catalysts. Furthermore, the pore diameter reduced rationally after silylation, as compared with that of the patent catalyst (Table S1, entries 2 vs. 3).

Table 1

Table 1.  Physicochemical properties and catalytic performances of the oxidation of glycerol using different catalysts ^a

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Entry	Catalyst	Surface area/(m2·g-1)	CA/(°)	Conversion/%b	Selectivity/% b
					FA	GCA	others
1	SiO2	287	0	0	—	—	—
2	HPA/SiO2	175	0	76(5)	66(45)	27(14)	7(41)
3	HPA@SiO2	261	96	61(28)	68(54)	27(22)	5(24)
4	HPA@SiO2-S-N2	245	134	34(51)	70(74)	27(22)	3(4)
5c	HPA@SiO2-S-N2	245	134	40	67	—	33
a Reaction conditions: 1.25 mmol of glycerol, 6.25 mmol of 30% aqueous H2O2, 0.88 mL CH3CN, 0.06 g catalyst, T = 70 ℃, t = 12 h. FA = formic acid; GCA = glycolic acid; b The values in the parentheses represented the conversion or selectivity in the second run. c GCA as a substrate. The carbon mass balance for the glycerol oxidation was normally more than 85%

Subsequently, the surface morphology of catalysts was examined by SEM. As shown in Fig. 3, the SEM images showed that the structure of SiO₂ is spherical (Fig. 3a and d), and HPA/SiO₂ catalyst maintained the morphology of the SiO₂ but we can see some particles on the surface (Figs. 3b and e). When the HPA was encapsulated into the silica by a sol-gel procedure, the SEM images showed that the structure of HPA@SiO₂ catalyst is spherical but showed rougher surface (Fig. 3c and f). After silylation, the images of HPA@SiO₂-S-N₂ (Fig. 3g and i) catalyst were similar to that of HPA@SiO₂ catalyst. Interestingly, the HPA encapsulation into SiO₂ through the sol-gel method can reduce the size of SiO₂ particles significantly and the silylation did not influence the surface morphology. As discussed above, the encapsulation of HPA into SiO₂ (HPA@SiO₂) can result in stronger hydrophobicity, as compared with that of HPA/SiO₂, which could be attributed to the smaller particle size and the different surface morphology ^{41, 42}.

Figure 3

Figure 3.  SEM images of (a, d) SiO₂; (b, e) HPA/SiO₂; (c, f) HPA@SiO₂; (g, i) the fresh HPA@SiO₂-S-N₂ catalyst; (h, j) HPA@SiO₂-S-N₂ catalyst after reused for 5 times

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In the next, the TEM images showed that the structure of HPA/SiO₂ is spherical with about 150 nm diameter (Fig. 4a), while the structure of HPA@SiO₂-S-N₂ catalyst is spherical with about 12 nm diameter Fig. 5g, which was in well agreement with that of SEM images (Fig. 3b vs. 3g). Energy-dispersive X-ray spectrometry (EDS) elemental mapping analysis (Figs. 4 and 5) validated the immobilization of Mo species had a tendency to be distributed around silica particles on HPA/SiO₂ catalyst (Fig. 4e and f) but highly uniform dispersity within HPA@SiO₂-S-N₂ catalyst (Fig. 5e and f).

Figure 4

Figure 4.  TEM images of HPA/SiO₂ (a), and elemental mapping images of Si (b), O (c), Mo (d), Si and Mo (e), O and Mo (f)

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Figure 5

Figure 5.  TEM image of HPA@SiO₂-S-N₂ (a, g), and elemental mapping images of Si (b), O (c), Mo (d), Si and Mo (e), O and Mo (f)

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The temperature-programmed desorption of ammonia (NH₃-TPD) is a useful method to provide general information about the strength distribution of acid sites of solid acids. According to the desorption temperature of the absorbed ammonia, the strength of solid acid sites are formally assigned as weak and strong. The NH₃-TPD curves of the samples were shown in Fig. 6. The NH₃-TPD patterns of both HPA@SiO₂-S-N₂ and HPA@SiO₂ catalysts showed only a desorption peaks in similar temperature regions (50–200 ℃). On the other hand, HPA/SiO₂ samples exhibited an over-lapping peak including two peaks around 200 ℃ and 340 ℃, respectively ⁴³. This indicated that HPA/SiO₂ showed a stronger surface acidity than that of HPA@SiO₂-S-N₂ and HPA@SiO₂, which was understandable because HPA was mainly loaded on surface silica for HPA/SiO₂, leading to more accessible adsorption of NH₃ molecules, as compared with that of silica-encapsulated HPA catalysts.

Figure 6

Figure 6.  NH₃-TPD profiles of the (a) HPA@SiO₂-S-N₂ catalyst; (b) HPA@SiO₂; (c) HPA/SiO₂ catalyst

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The thermal stability of the catalysts was investigated by TGA. As shown in Fig. S2, both SiO₂ and HPA@SiO₂-S-N₂ showed a slight weight loss below 120 ℃, which could be attributed to the removal of the adsorbed H₂O on the surface of SiO₂ and the HPA@SiO₂-S-N₂. Increasing temperature from 120 ℃ to 700 ℃, it was seen that SiO₂ only afforded a slight weight loss from 98% to 95%, which could be assigned to the increase of siloxane bridges (Si―O―Si) because of the condensation of isolated silanol groups ⁴⁴. The HPA@SiO₂-S-N₂ catalyst gave a weight loss from 95% to 85.5% in the range of 300° to 700°, which was mainly attributed to the removal of gradual degradation of methyl on the surface of SiO₂.

3.2   Catalytic performance

The as-prepared catalysts have been tested for the glycerol oxidation reaction. As shown in Table 1, except SiO₂ other catalysts were all active for the oxidation of glycerol reaction (Table 1, entries 2–4). However, they exhibited different catalytic activity although the loading amounts of the HPA were same. The fresh HPA/SiO₂ catalyst showed high catalytic performance for glycerol oxidation, but HPA on catalyst surface is not stable and was visually leached into reaction media after reaction, resulting in yellow effluent and thus giving very poor activity in the second run (Table 1, entry 2). The HPA@SiO₂ catalyst showed some improvement for catalyst recyclability, but the obvious decrease of activity was still observed in the second run (Table 1, entry 3). However, after the HPA@SiO₂ sample was silylated and then calcined at 250 ℃ under the N₂ flow, the as-obtained catalyst HPA@SiO₂-S-N₂ can be recyclable and even the catalytic activity became better in the second run (Table 1, entry 4), which might be attributed obviously to the strong hydrophobicity and reducing pore sizes, resulting in shielding of HPA, suppressing the leaching of HPA and but allowing penetration of reactants and products through porous silica during the oxidation reaction. Obviously, the higher catalytic activity of fresh HPA/SiO₂ could be related with these easy accessible strong acidic sites, as presented at NH₃-TPD spectra (Fig. 6).

The effects of reaction condition (temperature, reaction time, amounts of catalyst) were investigated over the HPA@SiO₂-S-N₂ catalyst. Firstly, the effect of the reaction temperature was shown in Fig. 7a, which indicated that the reaction temperature had a crucial impact on the reaction. It was found that the conversion of glycerol increased continuously and reached a maximum but that decreased above 70 ℃, resulting from the decomposition of H₂O₂ under higher temperature. Additionally, the selectivity to FA kept unchanged but the selectivity to GCA decreases above 70 ℃, indicating that GCA could be converted into FA under higher temperature. The parallel experiment with GCA as a substrate show ed that GCA indeed can be oxidized into FA in 40% of conversion and 67% selectivity under the same conditions (Table 1, entry 5).

Figure 7

Figure 7.  Dependence of glycerol conversion and selectivity on the reaction condition over HPA@SiO₂-S-N₂catalyst

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The effect of reaction time was obtained from Fig. 7b. Under the condition of 70 ℃, the conversion of glycerol had a gradual increase within the first 5 h and then had a rapidly increment between 5 h and 10 h. However, longer reaction time could result in the decrease of FA and GCA selectivity. This can be explained that FA and GCA can be degraded further into CO₂, which has been confirmed by GC-MS (7890A-5975C, Agilent, USA), showing that the gas samples indeed contained CO₂, but no CO was detected from formic acid decomposition. Hence, the optimal reaction time is 12 h. Last, the influence of amounts of catalyst on the oxidation reaction of glycerol was also examined at 70 ℃ for 12 h. As shown in Fig. 7c, the conversion of glycerol was increased with the increase of catalyst until 0.06 g, in which can eliminate the influence of diffusion. The following reaction has been performed according to the conditions above.

Because acidity site has a crucial effect on catalytic activity, the impact of different acid additives on catalytic activity has been evaluated for the glycerol oxidation with H₂O₂. The results were listed in Table 2, which illustrated that Lewis acids used in this work almost did not influence the conversion of glycerol at all (Table 2, entries 2–4), but Bronsted acid additives can improve the conversion of glycerol significantly (Table 2, entries 5–9). Among of them, CF₃SO₃H is the most active acid from either conversion, or selectivity to FA (Table 2, entry 9). The control experiments indicated that when only acid additives were employed for glycerol oxidation, most of acids only offered low activity for glycerol conversion (Table S2, entries 1–7). CF₃SO₃H gave moderate activity (Table S2, entry 8), but the combination of CF₃SO₃H and HPA@SiO₂-S-N₂ catalyst exhibited much higher catalytic performance (Table 2, entry 9). These results proved unambiguously that strong Bronsted acid played a critical role in enhancing the activity and selectivity of glycerol oxidation over HPA@SiO₂-S-N₂ catalyst.

Table 2

Table 2.  Different acid additives for the oxidation of glycerol reaction over HPA@SiO₂-S-N₂ catalyst^a

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Entry	Additives	Conversion/%	Selectivity/%
			FA	GCA	others
1	none	34	70	27	3
2	AlCl3	33	63	28	9
3	ZnCl2	34	66	19	15
4	InCl3	37	69	23	8
5	HC1	46	72	17	11
6	p-CH3(C6H4)SO3H	42	81	18	1
7	H2SO4	45	56	14	30
8	(CF3SO2)2NH	55	63	13	24
9	CF3SO3H	52	81	17	2
a Reaction conditions: 1.25 mmol of glycerol, 6.25 mmol of 30% aqueous H2O2, 0.88 mL CH3CN, 0.06 g catalyst, acid = 0.5 mmol. T = 70 ℃, t = 12 h. FA = formic acid; GCA = glycolic acid. The carbon mass balance for the glycerol oxidation was normally more than 90%.

On the base of the results above, HPA@SiO₂-S-N₂ catalyst showed the highest catalytic performance. Hence, HPA@SiO₂-S-N₂ catalyst, was employed for the oxidation of the other biomass platform molecules including ethylene glycol, glucose, sorbitol, fructose, xylitol and 1, 2-propanediol. As shown in Table 3, ethylene glycol, glucose and sorbitol had a low reactivity (Table 3, entries 1–3), while fructose, 1, 2-propanediol and xylitol had a relatively high conversion (Table 3, entries 4–6) in the absence of any acid additive. When CF₃SO₃H was added as an additive, all biomass-derived molecules afforded higher conversion, although the selectivity to FA was variable (Table 3, entries 1–6). For example, the conversion of fructose improved from 60% to 100% and selectivity to FA from 48% to 62% due to the addition of CF₃SO₃H. It was noting that although only CF₃SO₃H afforded the moderate conversion (Table S3) for the oxidation of the other biomass platform molecules in the absence of th e HPA@SiO₂-S-N₂ catalyst, but the activity was much lower that of the combined CF₃SO₃H and HPA@SiO₂-S-N₂ catalysts (Table 3, entries 1–6). These results indicated that Bronsted acid additives can enhance conversion of other biomass-derived molecules through acid hydrolysis for this reaction ²⁸.

Table 3

Table 3.  Different substrates for the oxidation reaction catalyzed by HPA@SiO₂-S-N₂ catalyst ^a

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Entry	Substrates	Conversion/%	Selectiviy/%
			FA	GCA	IS	LA	AA	GA	others
1	Ethylene glycol	31 (59)	15(81)	47(0)	—	0(0)	0(5)	0(0)	37(14)
2	Glucose	25 (78)	67(51)	0(12)	—	0(0)	0(3)	0(0)	33 (34)
3	Sorbitol	29 (72)	57 (53)	0(9)	26(2)	0(0)	0(0)	0(14)	17(22)
4	Fructose	60(100)	48 (62)	17(31)	—	7(0)	0(2)	0(0)	28 (5)
5	1, 2-Propanediol	54 (76)	45 (44)	23(2)	—	0(0)	0 (45)	0(0)	32(9)
6	Xylitol	48(81)	54 (67)	0(10)	—	0(0)	25(5)	8(12)	13 (6)
a Reaction conditions: 1.25 mmol of glycerol, 6.25 mmol of 30% aqueous H2O2, 0.88 mL CH3CN, 0.06 g catalyst, T = 70 ℃, t = 12 h. FA=formic acid; GCA = glycolic acid; IS = isosorbide; LA= lactic acid; AA = acetic acid; GA= glyoxylic acid. The values in the parenthesis denoted the conversion or selectivity with CF3SO3H (0.5 mmol) as an additive

3.3   Catalyst stability

The reusability of the HPA@SiO₂-S-N₂ catalysts was examined under the optimized conditions (Fig. 8). After reaction, the solid catalyst was recovered conveniently by simple centrifugation, followed by washing with ethanol and toluene for several times. And then it was dried at 60 ℃ under the vacuum. Then the recovered catalyst was reused for the next recycle. At first, the reusability of HPA@SiO₂-S-N₂ catalyst was evaluated. It was found that the catalyst could be reused for at least five times with high conversion and selectivity to FA. It is worth noting that the conversion in the second run is even higher than that of the initial activity but remained unchanged as starting from the second recycle while the selectivity of the product remained the same (Fig. 8a). In contrast, after strong Bronsted acid CF₃SO₃H was added to the reaction system, the conversion of glycerol and selectivity to formic acid kept high and did not change obviously during the consecutive five recycle (Fig. 8b). These results strongly suggested that strong Bronsted acid, as well as redox sites is important for glycerol activation, and HPA@SiO₂-S-N₂ catalyst could expose more active sites after the first run, and thereby increasing contact area of the catalyst and substrates, which can be proved by a loss of hydrophobicity arising from a partial removal of methyl group of silica surface by oxidation reaction (Fig. S3c vs. d).

Figure 8

Figure 8.  Recyclability of (a) HPA@SiO₂-S-N₂ catalyst without CF₃SO₃H as an additive; (b) HPA@SiO₂-S-N₂ catalyst with CF₃SO₃H (0.5 mmol) as an additive

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The spent catalyst has also been examined for the possible structural changes. The characteristic vibration bands of heteropolyanions with the Keggin structure at 1095, 950, 863, 800 and 584 cm^-1 were observed in FT-IR spectrum of the used HPA@SiO₂-S-N₂ (Fig. 1d and e), indicating that the structure of the heteropolyanion remains intact. The XRD pattern of the reused catalyst was also in agreement with that of fresh one (Fig. 2d and e). The surface morphology of the reused catalyst was shown in Fig. 3h and j, which indicated that the catalyst was still spherical and the morphology of the catalyst did not undergo obvious changes after reaction. The spent HPA@SiO₂-S-N₂ catalyst still showed hydrophobicity (CA = 114°), although the water CA decreased slightly, as compared with that of the fresh one.

According to the above discussion, there came the idea that whether the present catalytic reaction went through a heterogeneous way or not. Thus, the fast hot filtration at the reaction temperature was employed to examine the filtrate for activity. For HPA@SiO₂-S-N₂ catalyst, as shown in Fig. 9a, no further glycerol was converted once the catalyst was removed, which evidenced unambiguously that the catalytically active species was not present in the homogeneous filtrate. Finally, the possible leaching of HPA into media after the catalytic reaction was determined by ICP-AES, which showed that no P, Mo and V species were found in effluents under our reaction conditions. This result proved that the catalytic reaction proceeded over HPA@SiO₂-S-N₂ catalyst by a heterogeneous mechanism. However, for HPA/SiO₂ catalyst, as shown in Fig. 9b, because of the leaching of heteropolyacid active species into filtrate, there was still further glycerol was converted in the filtrate after the catalyst was removed.

Figure 9

Figure 9.  Hot filtration experiments for the glycerol oxidation with H₂O₂ over (a) HPA@SiO₂-S-N₂ catalyst; (b) HPA/SiO₂ catalyst

Solid square points (●): without isolating catalyst; Solid triangle points (▲): with isolating catalyst and then reaction in the filtrate

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4.   Conclusions

In this work, we employed a sol-gel method and sequential silylation technique to encapsulate HPA into silica to afford an insoluble and easily separable heterogeneous catalyst. The as-prepared HPA@SiO₂-S-N₂ catalyst could be reused for at least five times with high conversion and showed no HPA leaching. The silylation procedure obviously increased the hydrophobicity but reduced the pore sizes, which led to high leach-resistance of HPAs during the consecutive reaction runs. Under the best reaction conditions, the conversion of the glycerol can be reached 50% and the selectivity to formic acid was 70% and to glycolic acid 27% by using H₂O₂ as an oxidant, respectively. In addition, the addition of strong Bronsted acid into the reaction system can improve the catalytic activity significantly, which indicated that Bronsted acid sites have an important impact on activating glycerol molecule during the glycerol oxidation reaction.

Supporting Information: available free of charge via the internet at http://www.whxb.pku.edu.cn.

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