Solvent free oxidation of ethylbenzene over Ce-BTC MOF

2.1 Synthesis of the catalyst

Ce-BTC was synthesized via a solvothermal reaction in a typical synthesis run. Thus, 6 mmol (1.26 g) BTC and 10 mmol (4.34 g) cerium nitrate hydrate were dissolved in 60 mL DMF. Then, the mixture was sealed in a 100 mL Teflon-lined stainless steel autoclave and heated at 130 °C for 24 h. After cooling to room temperature naturally, the product was filtered, washed with DMF, and vacuum-dried at 50 °C (Scheme S1).

2.2 Characterization of the catalyst

Powder XRD patterns were recorded using a Rigaku D/Max 2200+Ultima diffractometer with Cu-Kα radiation (λ = 0.154 nm). Diffraction data were recorded in the 2θ range 5–30° at a step of 0.02°/s. TGA was performed using a Scinco TGA N-1000 thermo gravimetric (TG) analyzer. The S_BET and pore volume were measured with a BEL-Belsorp II volumetric adsorption analyzer. Prior to each adsorption measurement, the samples were evacuated under vacuum (p < 10⁻⁵ mbar) at 105 °C for 6 h. SEM images were captured on a JEOL JSM 5600 scanning electron microscope. Ultraviolet–visible diffuse reflectance (DRS UV–vis) spectra were recorded on a SCINCO Neosys 2000 spectrometer. Fourier transform infrared (FT-IR) spectra were recorded at room temperature with 4 cm⁻¹ resolution between 4000 and 400 cm⁻¹ with an FT-IR spectrometer (Nicolet IR 200). The electron spin resonance (ESR) spectrum was recorded on a JES-TE200 (JEOL, Japan) instrument with a microwave frequency of 9.4 Hz.

2.3 Catalytic reaction

The catalytic activity and selectivity of the oxidation of ethylbenzene (EB) were studied in solvent-free conditions. For this, we followed a method reported in prior studies (Raju et al., 2008) with some modifications. In a typical run, 5 mL EB and 0.10 g catalysts were taken in a 50 mL round bottom flask with a water-cooled reflux condenser and an O₂ gas inlet. The mixture was heated at a constant temperature and with continuous O₂ gas flow of 5.5 mL/min with stirring for 6 h. The evaporated EB becomes condensed with the aid of an attached water-cooled reflex condenser, so only excess O₂ after the reaction will escape through the needle attached over the condensed plug (the reaction set-up is shown in Supplementary file Fig. S1). Then, the reaction mixture was cooled to room temperature and filtered to remove the catalyst. The recovered catalyst was recycled by washing it with ethanol and then drying it. The reaction products were analyzed via GC (Agilent, HP-5 column, 50 m × 320 μm × 0.52 μm and FID detector).

3.1 Characterization of catalyst

The results of the powder XRD are shown in Fig. 1(A). The peaks that appeared at 8.4, 10.6, and 18.1 (2θ) revealed that the crystallinity of the Ce-BTC obtained was good and comparable to that reported in prior literature (Nazmul et al., 2010). The peaks in XRD of Ce-BTC synthesized here are similar to that of the Ce-BTC in the earlier reports (Almáši et al., 2010). The SEM image (Fig. 1(B)) showed that the Ce-BTC catalyst displayed needle-shaped particles, 10–20 μm in size, and with smaller broken particles (Nazmul et al., 2010; Luo et al., 2008).

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Figure 1 (A) XRD pattern of Ce-BTC, (B) SEM image of Ce-BTC.

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Fig. 2(A) shows the N₂ adsorption–desorption isotherms of Ce-BTC, with a type I isotherm for micropore materials with 778.5 m² g⁻¹ of surface area and 0.3346 m³ g⁻¹ of pore volume. Fig. 2(B) shows the TGA result of Ce-BTC. There was a minor weight loss at 100 °C, due to loss of physically adsorbed and lattice coordinated water, further a slow weight loss was observed until 300 °C which may be due to the solvent DMF and water coordinated with the pores of MOF and the next major weight loss occurred at around 350 °C, due to the decomposition of the Ce-BTC framework. Thus, the catalyst is stable up to about 350 °C, which is similar with that of Almáši et al. report (2010) on Ce-MOF. (Almáši, M., Zeleňák, V., Císařová, I., A. Bell. A novel cerium metal-organic framework. http://photon-science.desy.de/annual_report/files/2010/20101138.pdf. (accessed on 15/12/2013)).

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Figure 2 (A) Nitrogen adsorption–desorption isotherms of Ce-BTC, (B) TGA curve of Ce-BTC.

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The FT-IR spectrum of the synthesized Ce-BTC is shown in Fig. 3(A). The intense band observed at around 3450 cm⁻¹ was assigned to the OH stretching of water molecules, which may be physically adsorbed or bonded (coordinated). Water peaks can also be observed even when the product was dried at 150 °C for 1 h, which is due to KBr pellet preparation in an atmosphere of air, so the MOF may capture water molecules during the process. Bands for —OCO— asymmetric and symmetric stretching were observed at 1644 and 1373 cm⁻¹. C⚌C stretching of the aromatic rings appeared at 1587 and 1450 cm⁻¹. Additionally, low intensity bands of Ce-O stretching vibrations were seen near 400 and 700 cm⁻¹ (Preetha and Janardanan, 2012).

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Figure 3 (A) FT-IR spectra of (a) Ce-BTC, (b) Ce-BTC after reaction, (B) DRS-UV spectra of (a) Ce-BTC, (b) Ce-BTC after reaction.

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DRS-UV spectra of the catalyst are shown in Fig. 3(B). The peak at 250 nm is due to the Ce³⁺ ← O²⁻ transition, which is needed for oxidation. The absence of a band at 297 nm demonstrates that there is no Ce⁴⁺ ← O²⁻ charge transfer complex formed in the framework, as was found in an earlier report (Rao and Sahu, 2001). The broad envelope peak at 1.97 g in ESR spectra (Fig. 4(A)) of Ce-BTC also gives an evidence of the presence of a Ce³⁺ phase with an oxygen-adsorbed condition, matching that in a previous study on Ce-doped metal composites and other catalysts (Subbiah et al., 2011).

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Figure 4 (A) ESR spectrum of Ce-BTC, (B) EB conversion and AP selectivity over Ce-BTC at various temperatures.

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3.2 Catalytic activity studies

The oxidation of EB was carried out at 160 °C using Ce-BTC as catalyst under solvent-free condition. As one of the reactant is liquid i.e. EB which was acted here as the medium of reaction. For comparison, the reaction was also carried out in the absence of a catalyst. The major expected product of EB oxidation was acetophenone (AP), with a small amount of benzaldehyde (BZ) and benzoic acid (BA), as shown in Table 1. Only 5.01% conversion was achieved without a catalyst, while in the presence of the catalyst both EB conversion and AP selectivity increased. In particular, AP selectivity reached 90.26%. Fig. 4(B) confirms the effect of temperature on the reactions; it shows that the conversion of EB and selectivity of AP increased from 130 to 160 °C, and then a slight decrease was observed at a temperature of 180 °C. Thus, to obtain a higher conversion and selectivity toward AP, 160 °C was chosen as an appropriate reaction temperature for the oxidation of EB over Ce–BTC.

From Fig. 5(A), it is evident that EB conversion increased by increasing the time of the reaction from 6 to 20 h and then reaching a steady state. The maximum conversion of 68.75% was achieved at 20 h. The selectivity of AP also increased with an increase in reaction time. However, the difference was not as large as that found with respect to conversion. As is indicated in Fig. 5(A), over 90% selectivity was achieved at all selected reaction times, and it reached a maximum value of 95.27% at 20 h. The results showed that even at a short reaction time, high product selectivity (>90%) was achieved, while the conversion was more dependent on reaction time. However, from an industrial point of view, conversion and selectivity are equally important. Thus, 20 h was chosen as the optimum reaction time. The conversion and selectivity achieved were less than those reported using Ce-AlPO-5 (Subbiah et al., 2011). The lower conversion is likely due to the reaction here, which was in an atmosphere of oxygen over the liquid phase. The conversion may increase if the reaction was carried out by bubbling the oxygen into the liquid; Ce³⁺ sites on the catalyst may act more efficiently in the conversion. Similarly, the decrease in the selectivity compared to that found in earlier reports may be due to the retention of the intermediate 1-phenylethanol over the catalyst for a longer period of time. This restricts the diffusion of a new entrant (ethylbenzene) into the pores, leading to exhaustive oxidation of the intermediate into benzaldehyde and benzoic acid. These intermediates were not observed in earlier reports.

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Figure 5 (A) EB conversion and AP selectivity over Ce-BTC at various reaction times, (B) EB conversion and AP selectivity over Ce-BTC at various catalyst weights.

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As shown in Fig. 5(B), both the conversion of EB and the selectivity of AP increased with an increase in the catalyst quantity, from 0.01 to 0.07 g, and thereafter it decreased. The low conversion and selectivity at lower catalyst weight were caused by insufficient catalyst active sites, while the conversion and selectivity decreased when the catalyst weight was more than 0.07 g and were likely due to the abundant amount of nascent oxygen released from the active sites, which ultimately proceeded to vigorously oxidize and produce products other than AP. When 0.07 g of catalyst was loaded, the conversion and selectivity were 84.99% and 95.63%, respectively. As reported earlier, a plausible mechanism for the reaction is given in Scheme 1 (Subbiah et al., 2011). With respect to the mechanism, in the first step, the Ce³⁺ site will adsorb the O₂ and then form a metal hydroperoxide by abstracting the proton of the —CH₂— group of ethylbenzene where, compared with the methyl proton, the methylene proton is more susceptible to oxidation due to the overcrowding of CH₃ versus —CH₂—. Further, the methyl group rotates more than —CH₂—, and this restricted rotation by benzene makes it more susceptible to oxidation, as reported earlier. In the next step, the 1-phenylethanol formed will be attacked by atomic oxygen produced by the Ce³⁺ site, causing the expelling of hydrogen in the form of water and producing AP. If the intermediate 1-phenylethanol resides over the catalyst for a long time, it may cause extensive oxidation to other products, such as benzaldehyde and benzoic acid. An earlier report showed a high selectivity for AP due to the use of air as the oxidant where the O₂ quantity is much lower; thus, a slower reaction will proceed to give the desired product with higher conversion (Subbiah et al., 2011). In the present work, we studied the oxidation using pure oxygen. There is more reactive O₂ available here, and the quantity of intermediate formed will rapidly be converted into other products, leading to a small decrease in selectivity.

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Scheme 1 Possible reaction mechanism for the oxidation of EB over Ce-BTC.

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The next notable property related to using a heterogeneous catalyst is the reusability of that catalyst. Thus, in the present study the catalyst recovered from the first cycle was subjected to two subsequent runs. The results obtained (Table 2) demonstrated that the catalyst activity remained unchanged, but the pore volume was slightly reduced (0.1062 m³ g⁻¹) from the initial catalyst. This may be due to the presence of small amounts of reactant or product AP (the boiling point of which is 202 °C, which is higher than the catalyst activation temperature) formed during the reaction, indicating that active Ce³⁺ sites responsible for oxidation are present on the surface and not inside the pores as expected. This can be further confirmed from the decrease in peak intensity in DRS-UV spectra (Fig. 3(B)) of Ce-BTC. The decrease in intensity may be due to the presence of AP near the Ce³⁺ site which mask/interfere with the intensity of light absorbance by the Ce³⁺ site. With respect to this mechanism, further investigation is now underway in lab.