Rajesh Sadasivan and
Anjali Patel*
Polyoxometalates & Catalysis Laboratory, Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadooara, Gujarat, India. E-mail: anjali.patel-chem@msubaroda.ac.in
First published on 3rd September 2019
A heterogeneous catalyst comprising mono-copper substituted phosphotungstate and hydrous zirconia was synthesized using wet impregnation method, characterized by various physico-chemical techniques and evaluated for solvent-free oxidation of styrene using TBHP as oxidant. Various reaction parameters like time, catalyst amount, amount of TBHP and temperature were optimized with focus on optimum selectivity of styrene-oxide. Further, the catalytic activity was compared with that of unfunctionalized PW11Cu to understand the role of the support. Finally, the role of each component of the reaction was clearly elucidated by a detailed kinetic study of the reaction using both the catalysts.
The greatest disadvantage of catalysts in bulk form is their low surface area, which reduces the efficiency of the catalyst.19 Moreover, homogeneous catalysts always have the drawback of limited reusability as they cannot be easily be recycled. In order to overcome these drawbacks, anchoring of catalytic materials on various supports via dative, covalent, acid–basic, or electrostatic interactions has emerged as an upcoming research area.20–23 Amongst all materials, polyoxometalates (POMs) anchored to various supports have received increasing attention as they can undergo fast reversible multi-electron redox transformations under mild conditions. Use of different supports like mesoporous silica, zirconia, zeolites, and alumina to immobilize POMs have been explored for oxidation of styrene.
In 2008, Tangestaninejad et al. synthesized di-vanadium substituted phosphomolybdate supported on mesoporous MCM-41 and used the same for oxidation of various cyclic and aromatic alkenes to their respective aldehydes and ketones under UV radiations.24 In the same year, Guo et al. synthesized periodic mesoporous composite catalysts, [(n-C4H9)4N]4[γ-SiW10O34(H2O)2]/SBA-15], and used for epoxidation of styrene.25 In 2009, various transition metal substituted polyoxotungstates, K10−nXn+MW11O39 (X = P/Si, M = Co/Ni/Cu/Mn), were supported on Schiff base modified SBA-15 by Guo et al. These were further used for selective oxidation of styrene to benzaldehyde.26 Later, in 2010, Kholdeeeva et al. synthesized MIL-101 supported phosphotungstates [(PW4O24)3− and (PW12O40)3−] and used them as sustainable and reusable catalysts for selective epoxidation of styrene.27 In 2013, Balula et al. reported the synthesis of SiW11Co and SiW11Fe immobilized onto an amine functionalized SBA-15 and investigated their catalytic property for oxidation of styrene and geraniol.28 In the following year, the same group reported the synthesis of a zinc substituted polyoxometalate, [PW11Zn-(H2O)O39]5− (PW11Zn), and encapsulation into silica nanoparticles using a cross-linked organic–inorganic core. They further used the same as a versatile catalyst for the oxidation of styrene.29 Selective oxidation of styrene to epoxystyrene was carried out by Zhang et al. in the same year using phosphomolybdic acid supported on 1-triethoxysilylpropyl-3-methylimidazoliumchloride IL modified MCM-41.30 Our group has worked extensively in this field wherein, a wide range of POMs have been anchored on various metal-oxide as well as silica based supports and used for styrene oxidation. Parent as well as mono-lacunary phosphotungstates, -molybdates and silicotungstates anchored to zirconia, alumina, mesoporous silica and zeolites were used for oxidation of styrene with green oxidants like hydrogen peroxide and molecular oxygen.31–37
A literature survey showed that there are no reports available where copper substituted phosphotungstate is supported on zirconia. Further, all the above mentioned reports have used either H2O2 or molecular O2 as oxidants. No accounts are available where tert-butyl hydroperoxide (TBHP) has been used as the oxidant for oxidation of styrene, despite it being an equally green and environmentally benign oxidant.
In the present work, we have anchored mono-copper substituted POM to hydrous zirconia by wet impregnation technique and carried out characterization of the supported catalyst by various physico-chemical techniques. We report a comparative study of the catalytic activity of both, the supported as well as unsupported catalysts for the oxidation of styrene using TBHP as the oxidant and discuss the role of the support. Leaching and heterogeneity tests as well as recycle studies have been carried out. Finally, the kinetics of the reaction using both catalysts has also been studied in detail.
In order to find out the optimum amount of loading, a preliminary reaction was carried out at varied % loading of active species (10–40%) and the results are presented in Fig. 1. It is seen that with increase in loading amount up to 30%, there is a steady increase in conversion, as expected. But it is interesting to note a decrease in selectivity of benzaldehyde with a steady increase in selectivity of styrene-oxide. Beyond 30% loading, there is a decrease in % conversion of styrene, which may be attributed to blocking of catalytic sites due to excess loading.
It is well known that the completion of the reaction to give benzaldehyde depends on the acidity of the catalyst. In the present case, the acidity of the catalyst decreases with increase in % loading of active species. As a result, the selectivity of the intermediate, i.e., styrene-oxide tends to increase. Keeping in mind the importance of epoxide in the chemical industry, 30% PW11Cu/ZrO2 was optimized and further characterizations as well as optimizations have been carried out using 30% PW11Cu/ZrO2 designated as only PW11Cu/ZrO2.
TGA of PW11Cu/ZrO2 (Fig. 2) shows initial weight loss of 4.4% up to 150 °C, which is attributed to adsorbed water. Further weight loss of 7.2% is noticed between 180–350 °C because of water of crystallization. No other substantial weight loss indicates that the synthesized material is stable up to 550 °C.
The FT-IR spectra of ZrO2, PW11Cu and PW11Cu/ZrO2 are shown in Fig. 3. ZrO2 shows bands at 3365, 1625, 1396 and 608 cm−1 which are characteristic of asymmetric O–H stretching, H–O–H and O–H–O bending and Zr–O–H bending vibrations respectively.
PW11Cu exhibits bands at 1103 and 1060 cm−1 corresponding to P–O stretching, 964 cm−1 corresponding to WO stretching, 887 and 810 cm−1 corresponding to W–O–W stretching and 516 cm−1 corresponding to Cu–O stretching vibrations respectively. The FT-IR spectrum of PW11Cu/ZrO2 shows characteristic bands of both PW11Cu and ZrO2. Bands at 1103 and 1064 cm−1, 952 cm−1 and 813 cm−1 corresponding to P–O, W
O and W–O–W stretching vibrations are similar to PW11Cu. In addition, broad bands at 3417, 1631 and 1402 cm−1 correspond to O–H stretching, H–O–H and O–H–O bending respectively are similar to that of ZrO2. The slight shifts observed in frequencies of supported catalyst is attributed to the chemical interaction of PW11Cu with the support. However, the Cu–O vibration band is not visible, due to overlapping with the Zr–O–H band and hence confirmed by FT-Raman.
Fig. 4 shows the FT-Raman spectra of PW11Cu as well as PW11Cu/ZrO2. ZrO2 shows a number of broad peaks from 100–800 cm−1 associated with long range amorphous state disordering.41 The Raman spectra of PW11Cu shows peaks at 996 cm−1 corresponding to WO symmetric stretch; 985, 215 and 153 cm−1 corresponding to W–O symmetric stretch; and 967 and 946 cm−1 corresponding to W–O–W symmetric stretch respectively. Further, an additional peak at 483 cm−1 is incorporation of copper in the lacuna of PW11. Similarly, Raman spectra of PW11Cu/ZrO2 shows peaks at 985 cm−1 corresponding to W
O symmetric stretch, 972 and 218 cm−1 corresponding to W–O symmetric stretch, and 964 cm−1 corresponding to W–O–W symmetric stretch respectively. A slight shift as well as decrease in intensity, along with absence of some peaks may be because of the interaction of PW11Cu with the support. Thus, FT-IR and FT-Raman confirm that PW11Cu remains intact even after impregnation on to the support.
X-band liquid nitrogen temperature ESR spectra of unsupported PW11Cu, ZrO2 and PW11Cu/ZrO2 are presented in Fig. 5. PW11Cu gives a four line hyperfine spectrum with g = 2.0883 and g = 2.4031, typically found in Cu(II) complexes.39 PW11Cu/ZrO2 retains the hyperfine spectrum of PW11Cu, indicating that copper remains in 2+ oxidation state and the environment around Cu(II) stays intact even after impregnation on to the support. However, the decrease in intensity observed may be due to interaction of PW11Cu with the support.
The H2-TPR spectra of PW11Cu and PW11Cu/ZrO2 are shown in Fig. 6. Romanelli et al. obtained two peaks in the TPR pattern of sodium salt of PW11Cu with maxima at 666 °C and 960 °C which have been assigned to reduction of species after anion decomposition.42
In the present case, similar maxima are obtained at 592 °C and 819 °C, which is attributed to formation of WO3 species.43 The decrease in reduction temperature is attributed to increase in the consumption of H2 gas due to Cs counter cation.44 The lowering of reduction temperature may also be an indication of enhanced oxidation ability.
The H2-TPR spectra of PW11Cu/ZrO2 has a peak with lesser intensity and shows maxima at 518 °C, while the second peak gets overlapped with the reduction peaks of ZrO2, which fall in the temperature range of 700–900 °C.45 The decrease in intensity as well as reduction temperature may be due to chemical interaction of [PW11Cu]5− anions with the support.42 This is further confirmed by 31P MAS NMR.
31P NMR is an important tool to understand the environment around phosphorus in polyoxometalates as well as the interaction of the anion with support.46 The 31P MAS NMR of PW11Cu and PW11Cu/ZrO2 are presented in Fig. 7. The peak at −10.42 in case of the supported material arises because of the PO4 unit of PW11Cu. The slight downfield shift may be attributed to the hydrogen bonding between the terminal oxygen of the POM and –OH groups of zirconia. The broad peak at −2.35 is attributed to the formation of [Zr–OH2]+[Cs4(PW11CuO39)]− species.47–49 This may be explained as follows. Loading of PW11Cu on to ZrO2 is carried out by wet impregnation at 100 °C. Dehydration of water molecules of zirconia occurs, which is followed by subsequent entrapment of PW11Cu anions within the network of zirconia. Further, due to the acidic medium,
Zr–OH gets protonated to form [
Zr–OH2]+, which acts as a cation for the anionic (PW11CuO39)5− species. As a result, a chemical bond formation occurs between the two as opposed to simple physisorption. This, along with hydrogen bonding, ensures that PW11Cu does not leach during the catalytic reaction and the catalyst remains intact. Intensity of both the peaks in NMR indicates that while majority of PW11Cu form ionic pairs with zirconia, some interact only by hydrogen bonds.
The wide angle powder XRD of unsupported PW11Cu, ZrO2 and PW11Cu/ZrO2 are presented in Fig. 8. PW11Cu shows sharp peaks from 20–30 degrees 2θ characteristic to the Keggin structure, with a slight shift due to incorporation of copper. This is further confirmed by sharp peaks at 48 degrees 2θ attributed to Cu(II).39 The absence of crystalline peaks in case of PW11Cu/ZrO2 indicates complete dispersion of PW11Cu on to the support.
The BET surface area of ZrO2 and PW11Cu/ZrO2 are shown in Table 1. It is clearly seen that the surface area of PW11Cu/ZrO2 (218 m2 g−1) is greater than that of zirconia (170 m2 g−1), because of the supporting, which is as expected. An average pore diameter of 38.25 Å was obtained from the pore size distribution curve (Fig. 9).
Material | Surface area (m2 g−1) |
---|---|
ZrO2 | 170 |
PW11Cu/ZrO2 | 218 |
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Fig. 9 (a) N2 adsorption–desorption isotherm and (b) pore size distribution curve of PW11Cu/ZrO2 and (insets) pure ZrO2. |
The reaction was then carried out at different catalyst amounts and the results are presented in table 10b. It can be seen that with increase in catalyst amount from 5 mg to 15 mg, there is a steady increase in conversion of styrene. However, the selectivity of benzaldehyde initially decreases up to 10 mg and then increases, while the selectivity of styrene oxide initially increases and then decreases. This may be because with increase in catalyst amount, the reaction tends to move forward towards completion. Beyond 15 mg, the increase in conversion as well as selectivity of benzaldehyde is negligible. Hence, 15 mg was optimized as the catalyst amount.
The amount of TBHP was then varied to study the effect of oxidant and the results are presented in Fig. 10c. With increase in TBHP amount from 1 mL to 2.5 mL, there is an increase conversion and selectivity of benzaldehyde along with decrease in selectivity for styrene-oxide, which is the expected trend. Excess of TBHP would lead to increase in oxygen concentration, and this would lead to further oxidation of benzaldehyde to benzoic acid, as seen in case of 2.5 mL TBHP. Hence, 2 mL TBHP was considered optimum for the reaction.
Finally, the reaction was carried at different temperatures and the results are presented in Fig. 10d. When the temperature was increased from 50 °C to 60 °C, there is an increase in conversion as well as selectivity of styrene-oxide. But on increasing temperature beyond 60 °C, there is a significant decrease in the selectivity of benzaldehyde as well as styrene-oxide. This may be due to the degradation of TBHP at higher temperatures, thereby resulting in polymerization of styrene. Hence, temperature was optimized at 60 °C.
Next, reaction time was optimized keeping all other parameters constant (Fig. 11b). As the time is increased from 8 h to 16 h, steady increase in conversion is observed, along with selectivity of styrene-oxide. Further increase in reaction time shows no significant change in either conversion or selectivity of the required product. Therefore, reaction time was optimized at 16 h.
In order to optimized amount of oxidant, the reaction was carried out by adding different quantities of TBHP (Fig. 11c). With increase in TBHP amount from 1 mL to 2 mL, there is a steady increase in conversionof styrene, with increase in styrene-oxide selectivity and decrease in that of benzaldehyde. On further increase in TBHP amount to 2.7 mL, conversion increases, but selectivity of the epoxide decreases, with an increase in selectivity of benzaldehyde. Hence, 2 mL TBHP was considered optimum for the reaction.
Finally, the reaction temperature was optimized by carrying out the reaction at different temperatures (Fig. 11d). On increasing the temperature from 50 °C to 60 °C, there is increase in conversion and styrene oxide selectivity. But further increase in temperature results in significant decrease of selectivity of both epoxide as well as aldehyde, with formation of unwanted by-products in the form of polymers. This is due to decomposition of TBHP at higher temperatures. Therefore, reaction temperature was optimized at 60 °C.
The reaction conditions for oxidation of styrene using both catalysts were optimized as follows: (I) for PW11Cu: catalyst amount – 15 mg; reaction time – 16 h; TBHP – 2 mL; reaction temperature – 60 °C; (II) for PW11Cu/ZrO2: catalyst amount – 25 mg (active species – 6.25 mg); reaction time – 16 h; TBHP – 2 mL; reaction temperature – 60 °C. It is necessary to bring to notice that in the present systems, optimization has been carried out keeping in priority the selectivity of styrene-oxide. These conditions may be varied by the chemist depending on the requirement of the products.
To check the heterogeneity of PW11Cu, the reaction was first run for 12 h, after which the catalyst was filtered out from the reaction mixture. The reaction was then allowed to proceed further up to 16 h. Similar set of experiments were carried out for PW11Cu/ZrO2. No change in the conversion as well as selectivity of the products (Table 2) indicates that the both catalysts are truly heterogeneous in nature.
Catalyst | % conversion | % selectivity | Turn over number (TON) | |
---|---|---|---|---|
Benzaldehyde | Styrene-oxide | |||
a Catalyst amount – 15 mg; TBHP – 2 mL; temperature – 60 °C; time – 16 h.b Catalyst amount –6.25 mg; TBHP – 2 mL; temperature – 60 °C; time – 16 h.c Catalyst amount – 25 mg (active amount of PW11Cu-6.25 mg); TBHP – 2 mL; temperature – 60 °C; time – 16 h. | ||||
aPW11Cu | 44 | 63 | 21 | 1102 |
bPW11Cu | 21 | 67 | 20 | 1102 |
cPW11Cu/ZrO2 | 44 | 44 | 37 | 2309 |
It is observed that the conversion for both, supported as well as unsupported catalyst under optimized conditions is the same. However, it is interesting to note that in case of PW11Cu/ZrO2, the active species present is less than half the amount to that of PW11Cu alone. This is the superiority of the supported catalyst.
The total number of acidic sites of the support, PW11Cu and PW11Cu/ZrO2 were calculated by potentiometry and the results are presented in Table 4. There is a phenomenal increase in the overall acidic strength in case of supported catalyst compared to the individual materials, and hence the higher catalytic activity. This is further confirmed by a detailed kinetic study of the reaction using PW11Cu as well as PW11Cu/ZrO2 as catalysts.
Material | Acidic strength (mV) | Types of acidic sites | Total no. of acidic sites | ||
---|---|---|---|---|---|
Very strong | Strong | Weak | |||
ZrO2 | 53 | 0 | 0.5 | 0.8 | 1.3 |
PW11Cu | 20 | 0 | 0.1 | 0.3 | 0.4 |
PW11Cu/ZrO2 | 140 | 0.1 | 0.1 | 1.0 | 1.2 |
![]() | (1) |
Alternatively, an experiment was then carried out keeping the concentration of styrene as well as TBHP the same. Eqn (2) establishes a relation between the concentration and time.
![]() | (2) |
The effect of reaction rate with respect to catalyst concentration was studied wherein rate constants were plotted at different catalyst concentrations and shown in Fig. 14. When the concentration of PW11Cu is increased from 1.545 × 10−3 mmol to 4.573 × 10−3 mmol, there is a linear increase in rate of the reaction. Similarly, increase in concentration of active species in PW11Cu/ZrO2 from 9.15 × 10−4 mmol to 4.573 × 10−3 mmol, also shows linearity. This indicates that the reaction follows first order kinetics with respect to catalyst concentration as well.50,51
k = A![]() | (3) |
As seen clearly, the present system is a two phase system, and hence it is necessary to clarify whether the reaction is truly governed by a chemical step or it is merely limited to mass transfer or diffusion. Usually, mass transfer limited reactions show Ea in the range of 10–15 kJ mol−1, as opposed to a true chemical reaction which shows Ea > 25 kJ mol−1.50,52 Significantly higher activation energy of 64.81 kJ mol−1 for PW11Cu and 40.31 kJ mol−1 in case of PW11Cu/ZrO2 indicates that the reaction is truly governed by a chemical step and also that both the catalysts have been exploited to its maximum capacity. Interestingly, lower activation energy of PW11Cu/ZrO2 compared to PW11Cu also showcases the superiority of the supported system over unsupported one. Thus, the conclusions obtained from kinetic studies fittingly support that of catalysis that PW11Cu/ZrO2 is a better catalytic system than PW11Cu.
2,6-Di-tert-butyl-4-methyl phenol was used as the radical scavenger to gain an idea on the mechanism and know the intermediate in the reaction for both the systems. The radical scavenger was added 12 h after the reaction was started and then the reaction was allowed to proceed to completion, and the results are present in Table 5. No significant change in the conversion and selectivity in both the cases, indicates that the intermediate formed is a radical. The slight increase in conversion and selectivity of benzaldehyde may be due to presence of the radical intermediate after immediate addition of scavenger, as in the present case, the oxidant used is TBHP, which is a well-known radical initiator. This confirms that the reaction proceeds via radical mechanism, which is as expected.53
![]() | ||
Fig. 16 Recycle study for (a) PW11Cu (catalyst amount: 15 mg; temp: 60 °C; TBHP: 2 mL) and (b) PW11Cu/ZrO2 (catalyst amount: 25 mg; temp: 60 °C; TBHP: 2 mL). |
In both the cases, same conversion and selectivity for the products are obtained even after multiple cycles indicating that the catalyst remains stable during the course of the reaction and that it can be reused for multiple times. A slight decrease in conversion in the third cycle may be due to loss of the catalyst during the recovery.
The FT-IR spectrum of Rec. PW11Cu (Table 6) shows characteristic bands at 1103, 1056, 964, 887 and 813 cm−1 corresponding to P–O, WO, and W–O–W stretching frequencies respectively. The Cu–O band can be clearly seen at 483 cm−1. No significant shifting of bands clearly indicates that the catalyst does not degrade and the structural morphology of the complex remains unchanged during the course of the reaction. Similarly, Rec. PW11Cu/ZrO2 shows bands at 1101, 1057, 960 cm−1 corresponding to P–O and W
O frequencies, and 3417, 1665 and 1413 cm−1 respectively for O–H, H–O–H and O–H–O frequencies of ZrO2. The slight decrease in the sharpness of the bands in recycled catalyst may be due to impurities that remain on the catalyst after recycling.
Catalyst | FT-IR frequencies (cm−1) | ||||||
---|---|---|---|---|---|---|---|
P–O | W![]() |
W–O–W | Cu–O | O–H | H–O–H | O–H–O | |
PW11Cu | 1103 | 964 | 887 | 489 | — | — | — |
1061 | 810 | ||||||
Rec. PW11Cu | 1103 | 964 | 887 | 483 | — | — | — |
1056 | 813 | ||||||
PW11Cu/ZrO2 | 1103 | 952 | 813 | — | 3417 | 1631 | 1402 |
1064 | |||||||
Rec. PW11Cu/ZrO2 | 1101 | 960 | — | — | 3417 | 1665 | 1413 |
1057 |
The FT-Raman spectra of fresh and recycled PW11Cu/ZrO2 (Fig. 18) shows that all the characteristic peaks of the fresh catalyst are retained in the recycled one. Thus, FT-IR and FT-Raman spectra indicate that the structure remained intact even after regeneration.
The wide angle powder XRD patterns of fresh and recycled PW11Cu/ZrO2 are shown in Fig. 19. No appreciable change in the powder XRD of regenerated catalyst compared to the fresh one indicates that the structure remains unaffected after the regeneration of catalyst.
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