One pot oxidative esterification of benzaldehyde over a supported Cs-salt of mono nickel substituted phosphotungstate

Abstract

As a cleaner alternative to traditional two steps procedures, the one pot oxidative esterification of benzaldehyde to methyl ester was carried out over a supported Cs salt of mono nickel substituted phosphotungstate (CsPW₁₁Ni). CsPW₁₁Ni was supported on ZrO₂ and characterized using various physico-chemical techniques. The influence of reaction parameters, such as molar ratio of substrate to H₂O₂, amount of catalyst, reaction time, and reaction temperature, on the oxidative esterification reaction was investigated. Moreover, the catalyst could be recovered and reused up to three cycles without significant loss in selectivity. The present heterogeneous catalytic system was found to be efficient not only in terms of activity (63%) but also in selectivity (79%) of the desired product.

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Introduction

Esters are valuable chemical products and they have widespread applications such as in the fragrance industry, and as flavoring agents, solvent extractants, diluents and intermediates. The traditional two step synthesis of esters from aldehydes generally suffers from the generation of a vast amount waste, by-products and use of toxic reagents.¹ From the viewpoint of demand, as well as the significance of acidic and oxidation reactions, it would be more beneficial to develop bifunctional catalytic systems for single step oxidative esterification reactions. To date, considerable efforts have been devoted to the development of direct synthetic methods for esters from the oxidative esterification of aldehydes with alcohols using different catalysts based on gold-nickel oxide (AuNiO_x),² TS-1,³ gold nanoparticles Au/TiO₂,⁴ Pb and Mg doping in Al₂O₃-supported Pd,⁵ the ionic liquid BmimBF₄,⁶ and manganese phthalocyanine immobilized on silica gel.⁷ In the same context, polyoxometalate (POMs) based materials have been well explored for this reaction.^8–10 Recently, a subgroup of POMs, which are transition metal substituted polyoxometalates (TMSPOMs), are gaining remarkable attention in the field of catalysis.^11–14 Among the various TMSPOMs ([XW₁₁M(L)O₃₉]⁵⁻; X = P, Si; M = transition metal), the nickel-substituted POMs are of considerable interest because of their redox properties and variable oxidation states, which make them important for various catalytic processes.^15–17

In 2004, Y. Yang et al. prepared TiO₂-APS-PW₁₁M (M = Ni/Co) and explored its catalytic performance in organochlorine pesticides and dyes.¹⁸ The preparation and detailed characterization of Na₅[PW₁₁O₃₉M] (M = Ni²⁺, Co²⁺, Cu²⁺, or Zn²⁺) supported on carbon was carried out by Pizzio and co-workers in 2007.¹⁹ They also evaluated the catalytic activity of the synthesized materials for isopropyl alcohol dehydration. The synthesis and photocatalytic activity of amine-functionalized mesoporous silica/anatase titania impregnated with TMSPOMs ([M(H₂O)PW₁₁O₃₉]⁵⁻-APS-TiO₂, M = Co/Ni) was reported by Guo et al. in 2007.²⁰ Hu and co-workers in 2009, carried out the study of K_10−nXⁿ⁺MW₁₁O₃₉-Schiff-SBA-15, (X = P/Si, M = Co/Ni/Cu/Mn) and its catalytic behaviour was evaluated for the oxidation of styrene to benzaldehyde.²¹ In 2012, Li et al. prepared MCM-41 incorporated (PW₁₁O₃₉M1)⁵⁻(M1–POM, M1 = Ni²⁺, Co²⁺ or Cu²⁺) and its catalytic performance in esterification was investigated.²² Recently, R. A. Frenzel et al. reported the use of transition metal-modified polyoxometalates [PW₁₁O₃₉M(H₂O)]⁵⁻, where M = Ni²⁺, Co²⁺, Cu²⁺ or Zn²⁺ supported on carbon as a catalyst in 2-(methylthio)-benzothiazole sulfoxidation.²³ Thus, the literature survey shows that there are no reports on the oxidative esterification of aldehyde, which is an important industrial organic transformation, using supported Ni substituted phosphotungstate.

Recently, we reported the one pot oxidative esterification over the Cs salt of mono nickel substituted phosphotungstate (CsPW₁₁Ni), and although the catalyst was homogeneous, it could be recycled upto two cycles without any degradation and modification of its structure and therefore as an extension of that study, we explored heterogeneous behavior of this catalyst by supporting it onto zirconia. As we mentioned in our previous report,²⁴ we have been working on the same catalyst for six months and succeeded to develop a heterogeneous catalyst comprising CsPW₁₁Ni and hydrous zirconia.

Thus, in the present study, we report the synthesis of a Cs salt of mono nickel substituted phosphotungstate supported onto zirconia and its characterization, as well as catalytic activity for oxidative esterification of benzaldehyde to methyl benzoate was evaluated. Different reaction parameters, including percentage loading of catalyst, mole ratio, amount of catalyst, time, temperature, and quantity of methanol, were optimized for better results. The catalyst was recycled and regenerated upto three cycles.

Experimental

Materials

All chemicals used were of A.R. grade. ZrOCl₂·8H₂O (Loba Chemie), 12-tungstophosphoric acid (H₃PW₁₂O₄₀), NaOH, NiCl₂·6H₂O, CsCl, liq. NH₃, C₆H₅CHO, CH₃OH, 30% H₂O₂, CH₂Cl₂ were obtained from Merck and used as received.

Synthesis of the support, ZrO₂

Hydrous zirconia was synthesized using the same method reported by us earlier.²⁵ In the typical procedure, an aqueous ammonia solution was added to an aqueous solution of ZrOCl₂·8H₂O up to pH 8.5, aged at 100 °C over a water bath for 1 h, filtered, washed with conductivity water until chloride free water was obtained and dried at 100 °C for 10 h. The obtained material is designated as ZrO₂.

Synthesis of Cs salt of mono nickel substituted phosphotungstate²⁴

H₃PW₁₂O₄₀ (2.88 g, 1 mmol) was dissolved in water (10 mL) and the pH of the solution was adjusted to 4.8 using an NaOH solution. NiCl₂·6H₂O (0.237 g, 1 mmol) dissolved in a minimum amount of water was mixed with the abovementioned hot solution. The final pH was adjusted to 4.8 and heated at 80 °C with stirring for 2 h and filtered hot to which a saturated solution of CsCl was added. The obtained light green crystals were filtered, air dried and designated as CsPW₁₁Ni.

Synthesis of Cs salt of mono nickel substituted phosphotungstate supported on zirconia

A series of catalysts containing 10–40% of CsPW₁₁Ni supported on ZrO₂ was synthesized using the impregnation method. ZrO₂ (1 g) was impregnated with an aqueous solution of CsPW₁₁Ni (0.1/10–0.4/40 g mL⁻¹ of double distilled water) and dried at 100 °C for 10 h. The resulting materials were designated as 10% PW₁₁Ni/ZrO₂, 20% PW₁₁Ni/ZrO₂, 30% PW₁₁Ni/ZrO₂, and 40% PW₁₁Ni/ZrO₂.

Characterization

The acidity of the catalyst was determined via n-butyl amine and potentiometric tests. Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo Star SW 7.01 up to 600 °C. Adsorption–desorption isotherms were performed on a Micromeritics ASAP 2010 surface area analyzer at −196 °C. Specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method. Fourier transform infrared (FT-IR) spectroscopy was carried out using KBr pellets on a Perkin Elmer instrument. Fourier transform Raman (FT-Raman) spectra were recorded on an FT-Raman spectrophotometer (Bruker FRA 106). Powder X-ray diffraction (powder XRD) was carried out using a Philips diffractometer (PW-1830). Electron spin resonance (ESR) spectra were obtained on a Varian E-line Century series X-band ESR spectrometer at low temperature and scanned from 2000 to 3200 gauss.

Catalyst acidity

n-Butyl amine acidity by titration. A 0.025 M solution of n-butyl amine in toluene was prepared to assess the total acidity of the catalyst.²⁶ 0.25 g catalyst was suspended in 0.025 M n-butyl amine solution for 24 h and the excess base was titrated against trichloroacetic acid using neutral red as the indicator. This test obtains the total acidity of the material.

Acidic site determination using potentiometric titration. The type of acidic sites was investigated by employing potentiometric titration with 0.05 N n-butylamine, which helps in computing different acidic sites.²⁷ 0.5 g of catalyst sample was suspended in 50 mL acetonitrile and the mixture was aged at 25 °C. 0.05 N n-butylamine (0.5 mL) in acetonitrile solution was added in equal time periods and the potential (mV) was recorded.

Catalytic evaluation. The reaction of benzaldehyde (0.01 mol) with H₂O₂ (0.03 mol) and methanol was carried out in a 100 mL batch reactor fitted with a double walled air condenser, magnetic stirrer, and guard tube. The reaction mixture was refluxed at 80 °C for 6 h. Then, the product was extracted with dichloromethane and the obtained products were analyzed on a gas chromatograph (Shimadzu-2014) using a capillary column (RTX-5). Each analysis was carried out three times and the associated percentage conversion values were found to be in the range of ±1.5% using the standard deviation of error analysis, which were calculated using eqn (1).

Leaching test. The leaching of active species from the support makes a catalyst unattractive and unproductive, thus it is necessary to examine the stability, as well as leaching of CsPW₁₁Ni from the support. Polyoxometalates are quantitatively characterized by their heteropoly blue color, which is visible when they react with a mild reducing agent such as ascorbic acid. In the present study, a reported method was used to determine the leaching of CsPW₁₁Ni from the support.²⁸ The obtained results were further confirmed by atomic absorption spectroscopy (AAS).

Results and discussion

Characterization of catalyst

The leaching test showed the absence of a blue color, which indicates that there was no leaching of CsPW₁₁Ni from the support into the reaction medium. Furthermore, AAS analysis did not show the presence of any metal content of CsPW₁₁Ni and this suggests the absence of leaching or if any metal was present, it was below the detection limit, which corresponds to less than 1 ppm.

The total acidity of the catalysts, 10–40% CsPW₁₁Ni loaded ZrO₂, was evaluated via the n-butyl amine titration method and the results are presented in Table 1. It can be observed from Table 1 that with an increase in the percentage loading, the total acidity increases up to 30% loading. Further increase in the percentage loading leads to a decrease in the acidity. A decrease in the acidic value can be attributed to the blocking of acidic sites due to the higher percentage loading.

Table 1 n-Butyl amine acidity values

Catalyst	Acidic sites mmol of n-butyl amine/g
ZrO2	0.62
10% PW11Ni/ZrO2	0.68
20% PW11Ni/ZrO2	0.72
30% PW11Ni/ZrO2	0.80
40% PW11Ni/ZrO2	0.75

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The types and strength of the acidic sites were further obtained via a potentiometric titration method. The strength of the acidic sites in terms of initial electrode potential is shown in Table 2.

Table 2 Acidity determined by potentiometric titration

Catalyst	Acidic strength Ei (mV)	Types of acidic sites (meq. g^−1)			Total no. of acidic sites
		Very strong	Strong	Weak
ZrO2	53	0	0.5	0.8	1.3
CsPW11Ni	50	0	0.7	1.7	2.4
30% PW11/ZrO2	58	0	0.5	1.0	1.5
10% PW11Ni/ZrO2	55	0	1.2	1.6	2.8
20% PW11Ni/ZrO2	70	0	1.2	1.8	3.0
30% PW11Ni/ZrO2	110	0.2	1.3	2.2	3.7
40% PW11Ni/ZrO2	75	0	1.4	2.5	3.9

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It can be observed from the Table 2 that the incorporation of CsPW₁₁Ni increases the strength of the acidic sites of the catalysts to a great extent. It is interesting to note that 30% PW₁₁Ni/ZrO₂ shows the highest acidic strength. For the 40% loaded catalyst the total number of acidic sites is higher, however, acidic strength decreases due to the blocking of the acidic sites at a higher loading. Therefore, 30% PW₁₁Ni/ZrO₂ was selected for detailed characterization studies.

The TGA of 30% PW₁₁Ni/ZrO₂ (Fig. 1) shows an initial weight loss of 8.8% upto 180 °C, indicating the loss of adsorbed water molecules. Furthermore, about 2.2% weight loss upto 300 °C was observed due to the loss of water of crystallization. In addition to this, no significant weight loss was observed upto 550 °C, which suggests the high thermal stability of the synthesized catalyst.

Fig. 1 TGA of 30% PW₁₁Ni/ZrO₂.

The BET surface area of 30% PW₁₁Ni/ZrO₂ was found to be 229 m² g⁻¹, whereas that of zirconia is 170 m² g⁻¹.²⁵ The surface area of 30% PW₁₁Ni/ZrO₂ was observed to be higher as compared to that of the support zirconia due to the support of PW₁₁Ni, as expected. The pore size distribution curve (Fig. 2) exhibits an average pore diameter of 30.9 Å (3.09 nm).

Fig. 2 Nitrogen adsorption–desorption isotherm and pore size distribution of 30% PW₁₁Ni/ZrO₂.

The FT-IR spectrum of ZrO₂ (Table 3) shows broad bands in the region of 3400, 1600 and 1370, and 600 cm⁻¹ which are attributed to O–H asymmetric stretches, H–O–H and O–H–O bending, and Zr–O–H bending, respectively.²⁵ The FT-IR spectrum of CsPW₁₁Ni (Table 3) exhibits bands at 1062; 961; and 885; and 810 cm⁻¹, which correspond to P–O, W=O, and W–O–W asymmetric stretching frequencies. Moreover, a band at 490 cm⁻¹ is observed, which is attributed to Ni–O vibration, and this indicates the inclusion of the metal ion into the Keggin framework. In the case of CsPW₁₁Ni the splitting value (Δν) for P–O is 0 cm⁻¹, which is in good agreement with the literature.²⁹

Table 3 FT-IR data of ZrO₂, CsPW₁₁Ni and 30% PW₁₁Ni/ZrO₂

Catalyst	Frequency (cm^−1)
	O–H	H–O–H, O–H–O	Zr–O–H	P–O	W=O	W–O–W	Ni–O
ZrO2	3400	1600, 1370	600	—	—	—	—
CsPW11Ni	3447	1624	—	1062	961	885, 810	490
30% PW11Ni/ZrO2	3255	1624, 1375	682	1064	938	852	453

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The FT-IR spectrum of 30% PW₁₁Ni/ZrO₂ shows similar bands as CsPW₁₁Ni at 1064; 938; 852; and 453; cm⁻¹ which correspond to P–O; W=O; W–O–W and Ni–O, respectively (Fig. 4). In addition, the peaks at 3255; 1624 and 1375; and 682 cm⁻¹ are attributed to O–H stretches, H–O–H and O–H–O bending, and Zr–OH bending, respectively. The shift in the band positions of the supported catalyst as compared to CsPW₁₁Ni is due to the interaction of the terminal oxygen of CsPW₁₁Ni with hydrogen from the surface –OH of zirconia. Thus, it can be observed that the characteristic bands of the active species in the supported catalyst are retained even after the heterogenization.

The Raman spectrum of ZrO₂ (Fig. 3) shows broad peaks in the region from 100 to 800 cm⁻¹, which are associated with the long-range disordering arrangement in the amorphous state.³⁰ The Raman spectrum of CsPW₁₁Ni (Fig. 3) shows bands at 993, 976, 895, 506, and 225 cm⁻¹, which correspond to ν_s (W=O_d), ν_as (W–O_d), ν_as (W–O_b–W), ν_s (W–O_c–W), and ν_s (W–O_a), respectively. An additional band in the range of 400–500 cm⁻¹ is attributed to Ni–O stretching and this confirms the presence of nickel in CsPW₁₁Ni. The spectrum of 30% PW₁₁Ni/ZrO₂ displays bands at 992, 975, 942, 522, and 213, which correspond to ν_s (W=O_d), ν_as (W–O_d), ν_as (W–O_b–W), ν_s (W–O_c–W), and ν_s (W–O_a), respectively. The presence of all the bands of PW₁₁Ni in 30% PW₁₁Ni/ZrO₂ confirms that the structure of CsPW₁₁Ni remains intact even after supporting onto ZrO₂. The significant shift observed in the bands is due to the interaction of CsPW₁₁Ni with the surface hydroxyl groups of ZrO₂.

Fig. 3 Raman Spectra of (a) ZrO₂, (b) CsPW₁₁Ni and (c) 30% PW₁₁Ni/ZrO₂.

The powder XRD pattern of CsPW₁₁Ni is shown in Fig. 4. The peaks ranging from 15° to 30° 2θ indicate that the presence of the characteristic peaks of the parent Keggin ion are shifted in the case of CsPW₁₁Ni due to the substitution of the metal ion into the lacuna. The diffraction peaks corresponding to crystalline phase CsPW₁₁Ni are absent in the XRD pattern of 30% PW₁₁Ni/ZrO₂, which confirms the good dispersion of PW₁₁Ni onto the support.

Fig. 4 Powder XRD pattern of (a) CsPW₁₁Ni, (b) ZrO₂ and (c) 30% PW₁₁Ni/ZrO₂.

Full range (3200–2000 G) X-band liquid nitrogen temperature ESR spectra for CsPW₁₁Ni (Fig. 5a) and 30% PW₁₁Ni/ZrO₂ (Fig. 5b) were also recorded. The obtained g value for CsPW₁₁Ni (g ∼ 2.07) is in good agreement with the reported value,³¹ which indicates the presence of Ni(II) in an octahedral or distorted octahedral environment. Similarly, the ESR spectra of 30% PW₁₁Ni/ZrO₂ shows a signal at g ∼ 2.0, which confirms the presence of undegraded CsPM₁₁Ni on the surface of ZrO₂. In other words, the Keggin structure remains unaltered after it was supported on ZrO₂.

Fig. 5 ESR spectra of (a) CsPW₁₁Ni and (b) 30% PW₁₁Ni/ZrO₂.

Catalytic activity

To evaluate the efficiency of the catalyst for the direct conversion of aldehydes into their corresponding esters, the reaction of benzaldehyde with methanol in the presence of hydrogen peroxide as an oxidant was carried out, as shown in Scheme 1. The effect of different reaction variables, such as benzaldehyde/H₂O₂ mole ratio, amount of catalyst, reaction time and temperature, was studied to optimize the conditions for maximum conversion.

Scheme 1 Oxidative esterification of benzaldehyde.

Effect of percentage loading

The reaction was carried out by varying the percentage loading of the active species (10–40%) onto the support with 10 mg of catalyst for 6 h at 80 °C. The results are presented in Fig. 6, which shows that conversion and selectivity increases with an increase in percentage loading from 10% to 30% PW₁₁Ni/ZrO₂.

Fig. 6 Mole ratio (benzaldehyde/H₂O₂) (1 : 3), amount of catalyst (10 mg), temperature (80 °C), time (6 h), and volume of methanol (5 mL).

However, further increase in loading from 30% to 40% results in a decrease in conversion and selectivity. This may be due to the faster decomposition of H₂O₂ in the presence of excess active centers in the case of 40% PW₁₁Ni/ZrO₂, which results in a decrease in conversion of benzaldehyde. The obtained results may be due to the blocking of acidic sites in the catalyst (Tables 1 and 2). Thus, the 30% PW₁₁Ni/ZrO₂ catalyst was selected for detailed catalytic study.

Effect of mole ratio

The effect of H₂O₂ on catalytic activity was studied by varying the mole ratio of benzaldehyde : H₂O₂ from 1 : 1 to 1 : 4. As observed in Fig. 7, with an increase in the mole ratio from 1 : 1 to 1 : 3, conversion and selectivity increased simultaneously.

Fig. 7 Amount of catalyst (10 mg), temperature (80 °C), time (6 h), and volume of methanol (5 mL).

This increase in conversion may be due to the increase in concentration of H₂O₂. A decrease in the conversion and selectivity is observed in the case of 1 : 4. Therefore, the 1 : 3 mole ratio of benzaldehyde : H₂O₂ was optimized, which gave a 63% conversion and 79% ester.

Effect of amount of 30% PW₁₁Ni/ZrO₂

To determine the optimum amount of catalyst, the reaction was investigated using five different masses of catalyst, keeping the other parameters fixed (1 : 3 mole ratio of benzaldehyde to H₂O₂ for 6 h at 80 °C and 5 mL of methanol) and the result is presented in Fig. 8.

Fig. 8 Mole ratio (benzaldehyde/H₂O₂) (1 : 3), temperature (80 °C), time (6 h), and volume of methanol (5 mL).

It can be observed from Fig. 8 that the conversion initially increases with an increase in the amount of 30% PW₁₁Ni/ZrO₂ from 7.5 mg to 10 mg. However, upon an increase in the catalyst amount from 10 mg, a decrease in conversion, as well as ester selectivity was observed. This decrease in conversion with an increase in the amount of catalyst is due to the rapid unproductive decomposition of H₂O₂ in the presence of excess amount of catalyst. To confirm the obtained results, the decomposition of H₂O₂ (SI-1) with different amounts of catalyst was studied and the results are presented in Table S1.† From Table S1,† it is clear that on increasing the catalyst amount, the percentage decomposition of H₂O₂ increases in the same time. The obtained result suggests that the best utilization of H₂O₂ was obtained in the case of 10 mg of catalyst, whereas an amount greater than 10 mg results in the unproductive decomposition of H₂O₂. Moreover, the rapid decomposition of H₂O₂ generates relatively excess moles of water and this results in decrease in the selectivity of the esterification process, which is a reversible process in presence of water. The reaction carried out using 10 mg of catalyst obtained 63% conversion and 79% ester selectivity, which is quite good with such a minimal amount of catalyst.

Effect of temperature

An increase in temperature resulted in an increase in conversion, as observed in the case of 60 °C and 80 °C. As observed from Fig. 9, at 60 °C good selectivity is observed. In the case of 80 °C, it obtained better conversion and selectivity when compared with that of the other varied temperatures. In the case of 90 °C, conversion and ester selectivity decreased. This may be due to the fast thermal and catalytic decomposition of H₂O₂ at elevated temperatures. Thus, as a result, 80 °C was selected as the optimum reaction temperature.

Fig. 9 Mole ratio (benzaldehyde/H₂O₂) (1 : 3), amount of catalyst (10 mg), time (6 h), and volume of methanol (5 mL).

Effect of time

For the optimization of reaction time (Fig. 10), the reaction was carried out at various time durations, which resulted in good conversion in the case of 6 h as compared to the other cases. As shown in Fig. 10, with an increase in reaction time, conversion increased proficiently along with selectivity. However, as the reaction time was increased no significant change in conversion and ester selectivity was observed, which is due to attainment of equilibrium in the reaction. As a result, the reaction was optimized for 6 h for better conversion and selectivity.

Fig. 10 Mole ratio (benzaldehyde/H₂O₂) (1 : 3), amount of catalyst (10 mg), temperature (80 °C), and volume of methanol (5 mL).

Effect of methanol quantity

The effect of methanol volume was also studied for the optimization of the reaction (Table 4). With 2 mL methanol, 67% conversion and 35% ester selectivity was observed.

Table 4 Effect of methanol quantity^a

Methanol quantity	% conv.	% sel.
		Ester	Acid
a Mole ratio benzaldehyde to H2O2 1 : 3; reaction temperature, 80 °C; and reaction time, 6 h.
2 mL	67	35	65
5 mL	63	79	21
8 mL	56.3	74.5	25.5

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However, when the volume of methanol was increased to 5 mL, better conversion and selectivity was obtained. Furthermore, it was observed that with an increase in the volume of methanol there was a decrease in conversion, which may be because of the lower availability of hydrogen peroxide for the reaction and dilution of the reaction with an increase in the volume of methanol. Thus, 5 mL methanol was selected as the optimized parameter for the reaction.

Control experiment

The control experiment for the present catalytic system was carried under optimized conditions and results are shown in Table 5. Oxidative esterification of benzaldehyde was carried out without employing any of the catalysts, which obtained a very negligible conversion (3%) and thus proving the need for an efficient catalyst to speed up the reaction. When the supported lacunary counterpart, 30% PW₁₁/ZrO₂, was employed as the catalyst, 56.3% conversion with 61.9% ester selectivity was obtained. On comparing this result with the present catalytic system, it is clearly shown that the 30% PW₁₁Ni/ZrO₂ is a better catalyst in terms of both activity and selectivity. The control experiment was also carried out with unsupported CsPW₁₁Ni, which shows 59.4% conversion with 62.3% ester selectivity. Furthermore, by comparing the conversion and selectivity for homogeneous, as well as supported systems, it can be observed that the 30% PW₁₁Ni/ZrO₂ yields a relatively abundant amount of ester. The obtained data from the control experiments revealed the catalytic activity of the supported catalyst, which is a good source to accelerate and precede the reaction to the desired level. Upon comparison of the activity and selectivity of all the catalysts, 30% PW₁₁Ni/ZrO₂ was found to be the best catalyst, because it obtains excellent conversion and selectivity with a very low catalyst amount.

Table 5 Control experiments

Entry	% conv.	% sel.
		Ester	Acid
a Mole ratio, 1 : 3; reaction temperature, 80 °C; catalyst amount, 10 mg (a = 2.3 mg); reaction time, 6 h; volume of methanol, 5 mL.
Without catalyst	3.0	—	100
30% PW11/ZrO2	56.3	61.9	38.1
CsPW11Ni^a	59.4	62.3	37.7
30% PW11Ni/ZrO2	63.0	79.0	21.0

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Thus, the optimum conditions are mole ratio of benzaldehyde to H₂O₂ (1 : 3), catalyst amount 10 mg, temperature 80 °C, time 6 h, and methanol volume 5 mL, and the resulting turnover number (TON) is 9690.

Heterogeneity test

Accurate evidence of heterogeneity can be gained only by filtering the catalysts before completion of the reaction and analyzing the filtrate for percentage conversion.³² This test was performed by filtering the catalyst from the reaction mixture at 80 °C after 3 h of the reaction and the filtrate was allowed to react further.

The reaction mixture and the filtrate (after 6 h) were analysed via gas chromatography (Table 6). No significant change in the percentage conversion indicates the role of the catalyst in obtaining good yield and selectivity of the desired product. The present catalyst falls into category C,³²i.e. the active species does not leach and the observed catalysis is truly heterogeneous in nature.

Table 6 Heterogeneity test

Reaction time	% conv.	% ester	% acid
3 h	49.8	74.5	25.4
6 h	49.6	73.9	26.0

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Regeneration of the catalyst

The catalyst was regenerated to examine its stability and recycled for investigating its activity. The catalyst after the reaction was separated from the reaction mixture by simple centrifugation, washed using dichloromethane and then dried at 100 °C.

Characterization of the regenerated catalyst

The regenerated catalyst was further characterized using powder XRD to confirm the retention of the structure of the catalyst.

The reused catalyst was characterized via powder XRD, as shown in Fig. 11. There is no appreciable change in the XRD pattern of fresh 30% PW₁₁Ni/ZrO₂ and recycled R-30% PW₁₁Ni/ZrO₂, which indicates that the material remains unchanged even after regeneration.

Fig. 11 Powder XRD of (a) 30% PW₁₁Ni/ZrO₂ and (b) R-30% PW₁₁Ni/ZrO₂.

Catalytic activity of recycled catalyst

Recycling data (Table 7) shows almost the same conversion for all the catalysts. However, a remarkable decrease in the selectivity of the ester was observed, especially for the first cycle. Subsequently, no significant change in selectivity was observed for subsequent cycles. The observed trend in selectivity can be explained on the basis of the acidity of the catalyst. From the Table 7, it is clearly observed that for the first cycle, the total number of acidic sites decreases considerably as compared to the fresh catalyst, which is responsible for the drastic decrease in selectivity of the ester. However, the total number of acidic sites for the successive recycled catalyst remained constant and as a result no significant change in ester selectivity was observed.

Table 7 Recycling study of 30% PW₁₁Ni/ZrO₂^a

Cycle	Total no. of acidic sites (meq. g^−1)	% conv.	% selectivity		Turn over number (TON)	Turn over frequency (TOF) h^−1
			Ester	Acid
a Reaction conditions: mole ratio, 1 : 3; reaction temperature, 80 °C; catalyst amount, 10 mg; reaction time, 6 h; and volume of methanol, 5 mL.
Fresh	3.7	63	79	21	9690	1615
1	3.2	63	62	38	9636	1606
2	3.2	63	64	36	9587	1598
3	3.1	61	63	37	9364	1560

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Conclusion

Herein, we report a one pot oxidative esterification for the direct conversion of benzaldehyde to methyl benzoate over a heterogeneous catalyst. The superiority of the catalyst is displayed by its good conversion and excellent selectivity for the methyl benzoate with high TON (9690). The advantages of using a recyclable Ni based catalyst under mild reaction conditions instead of more expensive metals makes this methodology interesting from an economic and an ecological point of view.

Acknowledgements

We are thankful to Department of Science and Technology (DST), Project. No. SR/S1/IC-36/2012, New Delhi, for the financial assistance. Ms. Pravya Prakashan is thankful to the same for the award of a research fellowship. We are also thankful to Department of Chemistry, The Maharaja Sayajirao University of Baroda, for N₂ adsorption–desorption analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04362c

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