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Selective oxidation of ethylbenzene over CeAlPO-5
Applied Catalysis A: General 407 (2011) 76–84
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Selective oxidation of ethylbenzene over CeAlPO-5 Subbiah Devika, Muthiahpillai Palanichamy, Velayutham Murugesan ∗ Department of Chemistry, Anna University, Chennai 600025, India
a r t i c l e
i n f o
Article history: Received 9 June 2011 Received in revised form 16 August 2011 Accepted 17 August 2011 Available online 23 August 2011 Keywords: CeAlPO-5 Hydrothermal synthesis Fluoride medium Ethylbenzene Oxidation Acetophenone
a b s t r a c t CeAlPO-5(Al/Ce = 25, 50, 75, 100 and 125) molecular sieves were synthesized hydrothermally in fluoride medium and their structures were confirmed from an XRD analysis. The incorporation of Ce3+ into the framework was verified by XRD and DRS-UV–Vis analyses. The surface areas were in the range of 202–215 m2 g−1 . The ESR spectrum revealed the presence of adsorbed oxygen which was evident from the peak at g = 2.0. The catalytic activity of molecular sieves was tested in the vapour phase oxidation of ethylbenzene between 150 and 250 ◦ C. CeAlPO-5(25) showed higher conversion than other catalysts. The selectivity to acetophenone was above 90% at all the temperatures. The selective oxidation of methylene group was ascribed to slow rotation of the ethyl group compared to its methyl group at the reaction temperature. Cerium was not leached out in CeAlPO-5(25) even after activation of the spent catalyst at 550 ◦ C. The time on stream study also indicated the absence of catalyst deactivation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ceria is a versatile catalyst for the oxidation of many organic substrates [1–6]. Since ceria sinters at high temperatures, its activity declines with increase in the number of cycles [7]. Hence, free ceria or supported ceria catalysts may not be suitable for long time use. This problem, however, can be remedied by framework incorporation of cerium into the molecular sieves. Zhao et al. [8] reported the incorporation of cerium into the framework of AlPO5 molecular sieve. Its application as a catalyst in the liquid phase oxidation of cyclohexane gave only 13% conversion [8]. In cerium incorporated MCM-48 molecular sieves, tiny crystallites of ceria were shown to be formed due to leaching at high cerium content [9]. Catalytic activity of cerium incorporated MCM-41 was evaluated in the oxidation of cyclohexane [10–12]. Araujo et al. [13] reported enhancement of surface area as a result of cerium substitution in AlPO-11. Although cerium can exist in +3 or +4 oxidation state, it is the former which is active in the oxidation of –CH2 groups of organics in air. Ce3+ in the framework can adsorb oxygen from air and activate it for oxidation. This adsorption is favoured by the unpaired electron in Ce3+ . The magnetic field associated with Ce3+ and oxygen may be the cause for the chemisorption of the latter on the former. Ce4+ may not be active for –CH2 oxidation, as it cannot chemisorb oxygen. The framework substitution of cerium in the molecular sieves can combine the high activity and selectivity of homogeneous
∗ Corresponding author. Tel.: +91 44 22357023/8645; fax: +91 44 2220066/1213. E-mail address: v [email protected] (V. Murugesan). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.08.023
catalysts with ease of recovery and recycling, which is a characteristic of heterogeneous catalysts. The high surface area is an additional advantage acquired by framework incorporation of cerium into AlPO-5. With the advantages of framework incorporation of cerium into the molecular sieves, the present study synthesized CeAlPO-5 with different Al/Ce ratios in fluoride medium and evaluated their catalytic activity in the vapour phase oxidation of ethylbenzene in air. The selective oxidation of ethylbenzene to acetophenone is an important reaction as the product is the raw material for the production of perfumes, pharmaceuticals, resins and alcohol [14]. It is also an important route to utilize ethylbenzene effectively in the xylene stream of petrochemical industry. Liquid phase oxidation of ethylbenzene to acetophenone by oxygen or air using homogeneous transition metal (viz. Co, Mn, Cu, or Fe) compounds as catalysts was reported [15,16]. The oxidation of ethylbenzene with hydrogen peroxide (H2 O2 ) catalyzed by 8-quinolinolato manganese (III) complexes in water–acetone medium containing ammonium acetate and acetic acid as solvents resulted in ethylbenzene conversion of 26.1% and yield of 65% under optimum conditions [17]. Kanjina and Trakarnpruk [18] studied the oxidation of ethylbenzene over Co-substituted heteropolytungstate catalyst using H2 O2 green oxidant and acetonitrile as solvent. The reaction products were acetophenone (93%) and 1-phenylethanol. The selective oxidation of ethylbenzene to acetophenone was carried out using mixed metal oxide catalysts and t-butyl hydroperoxide (TBHP) as oxidant at 130 ◦ C. In this reaction, 87% ethylbenzene conversion and 92% selectivity to acetophenone were reported [19]. Oxidation of ethylbenzene using TBHP as oxidant was widely studied [20–23]. Highly efficient solvent-free oxidation of ethylbenzene
S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
using TBHP as oxygen source at 80 ◦ C over manganese nanocatalysts was reported with high catalytic activity and selectivity to acetophenone (93%) [24]. Parida and Dash [25] studied the liquid phase oxidation of ethylbenzene using TBHP as oxidant under mild reaction conditions at a temperature of 80 ◦ C with 57.7% ethylbenzene conversion and selectivity to acetophenone (82.2%) and benzaldehyde (18%). Vetrivel and Pandurangan [26,27] studied the vapour phase oxidation of ethylbenzene over Mn and Co containing MCM-41 molecular sieves in air and reported acetophenone as the major oxidation product. This literature review reveals that cerium incorporated AlPO-5 molecular sieves have not been tested for the vapour phase oxidation of ethylbenzene in air. 2. Experimental 2.1. Preparation of the catalysts Aluminium isopropoxide (Merck), aluminium hydroxide (Merck), orthophosphoric acid (Merck), cerium nitrate hexahydrate (Fluka), hydrofluoric acid (Merck) and triethylamine (Merck) were used in the synthesis of CeAlPO-5. Ethylbenzene (Merck) was used as the substrate for oxidation. The procedure adopted by Zhao et al. [8] was slightly modified and used for the synthesis of CeAlPO-5 molecular sieves. The modifications involved high crystallization time and varying amount of isopropyl alcohol content derived from the aluminium source (aluminium isopropoxide). The following gel composition was used for the synthesis: xCe(NO3 )3 ·6H2 O:1.0Al2 O3 :1.3P2 O5 :1.6TEA:1.3HF:425H2 O. Aluminium isopropoxide (7.151 g) was soaked in 50 ml deionized water for 24 h and stirred for 2 h. Orthophosphoric acid (4.9 g), triethylamine (2.8 g) and cerium nitrate hexahydrate (0.60 g) were dissolved in appropriate amount of water and stirred for 2 h at 30 ◦ C. The aluminium precursor was then added to the above mixture and stirred for 2 h. Dilute hydrofluoric acid (0.93 g) was then added and the mixture was stirred again for another 2 h. Finally, the gel was transferred to a Teflon-lined stainless steel autoclave and heated at 180 ◦ C for 6 h under static condition. After quenching the hot autoclave in cold water, the crystallized product was recovered by filtration, dried in an oven at 110 ◦ C and calcined at 550 ◦ C in air for 5 h at a heating rate of 1 ◦ C min−1 . The same procedure was repeated for the synthesis of AlPO-5 without cerium. CeAlPO-5(25) without alcohol was also synthesized using Al(OH)3 as the source for aluminium for the purpose of comparison and designated as CeAlPO-5(25)(WA). 2.2. Catalyst characterization The X-ray diffraction (XRD) patterns of synthesized materials were recorded on a PANalytical X’pert PRO diffractometer equipped ˚ as the radiation source and a liquid nitrowith Cu K␣ (1.54 A) gen cooled germanium solid state detector. The samples were scanned from 5◦ to 40◦ (2Â) in steps of 0.02◦ with a count time of 5 s at each point. Fourier transform infrared (FT-IR) spectra of the samples were recorded in the range of 4000–400 cm−1 on an FT-IR spectrometer (Nicolet, Avatar 360) using KBr pellet technique. Thermogravimetric analysis (TGA) was carried out on an SDTQ-600 WATERS system under nitrogen atmosphere in the temperature range of 30–1000 ◦ C at a heating rate of 10 ◦ C min−1 . Thermogravimetric analysis was also carried out in air atmosphere in the temperature range of 30–1000 ◦ C at a heating rate of 10 ◦ C min−1 on an SDTQ-600 V8.3 Build 101 model system. The diffuse reflectance ultraviolet–visible spectra (DRS-UV–Vis) were recorded using JASCO V-550 instrument. The morphology of the samples was examined by scanning electron microscopy (SEM). The sample was suspended in methanol
77
Table 1 Effect of alcohol content and dielectric constant on the crystallinity of CeAlPO5(25)(WA) and CeAlPO-5(25). Catalyst
Alcohol (%)
Dielectric constant (k)
Crystallinity (%)
CeAlPO-5(25)(WA)
– 4.26 8.52
88.0000 83.3337 79.4602
100.00 115.77 120.16
12.78 17.04 21.3
76.3373 72.6362 69.0188
178.59 139.17 123.55
CeAlPO-5(25)a WA, without alcohol. a Al/Ce = 25.
and the specimen stub dipped into it and removed. Evaporation of methanol left the sample evenly dispersed on the stub. The sample was coated with gold by ion sputtering for 2 min and the SEM image was recorded using HITACHI-S-3400N instrument. The Al/Ce ratios of the samples were determined by energy dispersive X-ray analyzer (EDAX) housed in the same unit. X-ray photoelectron spectrum (XPS) was performed on a Thermo Multilab 2000 using monochrome Al K␣ as the excitation source. The electron spin resonance (ESR) spectrum was carried out on a Bruker EMX plus instrument with a microwave frequency of 9.859461 Hz and modulation frequency of 100 kHz. Surface area measurement was carried out by nitrogen adsorption at 77 K on an ASAP-2020 porosimeter from Micromeritics Corporation (Norcross, GA, USA). Before nitrogen adsorption–desorption measurement, each sample was degassed for 3 h at 250 ◦ C under vacuum (10−5 mbar) in the degas port of the adsorption analyzer. 2.3. Catalytic studies The vapour phase oxidation of ethylbenzene was carried out in a fixed bed vertical downward flow glass reactor of internal diameter 2 cm. About 0.5 g of a catalyst was placed at the center of the reactor supported on either side with a thin layer of quartz wool and ceramic beads. The reactor was heated to the requisite temperature using a temperature programmed furnace. The catalyst was activated before the reaction at 500 ◦ C for 6 h in a controlled stream of carbon dioxide free air. The reactant was fed into the reactor using a syringe infusion pump. The products, collected in the receiver flask, were analyzed in a gas chromatograph (Shimadzu 17A) equipped with a DB-5 capillary column (30 m × 0.25 mm × 0.25 m) and flame ionization detector. The products were also identified using GC–MS (Perkin Elmer Clarus 500) with helium as the carrier gas at a flow rate of 1 ml min−1 . 3. Results and discussion 3.1. XRD The XRD pattern of CeAlPO-5, synthesized using aluminium hydroxide as the precursor for aluminium in fluoride medium, is shown in Fig. 1A(a). It exhibited the characteristic peaks of AlPO5 [28]. The XRD pattern of CeAlPO-5, synthesized with aluminium isopropoxide as the source for aluminium, is depicted in Fig. 1A(b) which also exhibited the characteristic peaks of AlPO-5. There were no extra peaks in both the patterns other than the peaks due to AlPO-5, thus confirming the absence of free cerium based phases. The crystallinity of the materials was calculated by considering the intense peaks at 2Â = 7.4◦ , 19.7◦ , 20.9◦ and 22.3◦ and the values are presented in Table 1. For comparison, the crystallinity of CeAlPO-5(25)(WA) synthesized without alcohol was taken as the reference (crystallinity = 100%). The crystallinity increased by 15%, when aluminium isopropoxide was used as the source for
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S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
Table 2 Lattice parameters of as-synthesized and calcined AlPO-5 and CeAlPO-5 samples and their BET results. Catalyst
As-synthesized
Calcined
a=b AlPO-5 CeAlPO-5(25) CeAlPO-5(50) CeAlPO-5(75) CeAlPO-5(100) CeAlPO-5(125)
c
13.7584 13.6373 13.6772 13.7012 13.7324 13.7096
± ± ± ± ± ±
0.0750 0.0493 0.0037 0.0063 0.0556 0.0488
8.4818 8.3856 8.4846 8.5481 8.4819 8.4579
± ± ± ± ± ±
0.1446 0.0551 0.0163 0.0430 0.1022 0.0876
˚ d(1 0 0) (A)
a=b
11.80 11.73 11.83 11.81 11.79 11.76
13.1561 13.8004 13.8312 13.8860 13.8972 13.8900
aluminium. Since the procedure adopted in the synthesis of both the materials differed only in the use of alcohol, the decrease in dielectric constant of the medium was inferred to increase the crystallinity of the samples. In order to optimize the amount of alcohol for good synthesis, its content (therefore dielectric constant) in the synthesis medium was gradually varied, and the synthesized samples were subsequently analyzed by XRD. All of them showed an increase in the intensity of peaks due to (1 0 0), (2 1 0), (0 0 2) and (2 1 1). The crystallinity of the materials was also calculated and presented in Table 1. The crystallinity increased with increase in alcohol content upto 12.78% and above which it decreased, thus suggesting a non-linear dependence of crystallinity on dielectric
A
Calcined c ± ± ± ± ± ±
0.3631 0.0629 0.0813 0.1148 0.0980 0.1080
8.4652 8.1179 8.4443 8.4172 8.3544 8.3632
± ± ± ± ± ±
0.5995 1.3042 0.1630 0.1929 0.2464 0.2789
˚ d(1 0 0) (A)
BET surface area (m2 g−1 )
Pore volume (cm3 g−1 )
11.35 11.88 11.92 11.95 11.94 11.95
202 215 210 208 205 205
0.22 0.28 0.27 0.26 0.25 0.24
constant. Since alcohol (12.78%) mediated synthesis gave higher crystallinity than others, the same content of the precursor of aluminium was used in the synthesis of CeAlPO-5 with Al/Ce ratios 50, 75, 100 and 125 employing the same procedure. The XRD patterns of as-synthesized and calcined samples are presented in Fig. 1A and B. All of them showed an increase in the intensity of the peaks with expected high crystallinity. The lattice parameters were also calculated and presented in Table 2. The values of dspacing of CeAlPO-5 were higher than AlPO-5. It also supported framework incorporation of cerium. The increase was due to a large size of cerium compared to aluminium, but a linear variation was not obtained: the d-spacing of CeAlPO-5(25) was lower than that of the other CeAlPO-5 molecular sieves with varying Al/Ce ratios.
3.2. FT-IR
Intensity (a.u.)
(f) (e) (d) (c) (b) (a) 5
10
15
20
25
30
35
40
2θ (degree)
The FT-IR spectrum of calcined CeAlPO-5(25) is shown in Fig. 2a. The intense broad band between 2500 and 4000 cm−1 was due to –OH stretching vibration of water. The corresponding bending vibration occurred at 1667 cm−1 . The absence of peaks due to –CH2 – vibrations just below 3000 cm−1 confirmed the complete absence of template. The asymmetric and symmetric stretching vibrations of Al–O–P occurred between 800 and 1500 cm−1 . The corresponding bending vibration occurred at 735 cm−1 . Similar types of vibrations were also observed in Ln–AlPO-5 (Ln = La, Ce, Sm, Dy, Y, Gd) [28]. The FT-IR spectra of calcined CeAlPO-5(50, 75, 100 and 125) are presented in the same figure (Fig. 2b–e). All of them displayed similar characteristics as that of Fig. 2a, thus establishing the absence of template.
(e) (d) (c) (b) (a)
Transmittance (a.u.)
Intensity (a.u.)
B
(f) (e) (d) (c) (b) (a) 5
10
15
20
25
30
35
40
2θ (degree) Fig. 1. (A) XRD patterns of as-synthesized (a) CeAlPO-5(25)(WA), (b) CeAlPO-5(25), (c) CeAlPO-5(50), (d) CeAlPO-5(75), (e) CeAlPO-5(100) and (f) CeAlPO-5(125). (B) XRD patterns of calcined (a) CeAlPO-5(25)(WA), (b) CeAlPO-5(25),(c) CeAlPO-5(50), (d) CeAlPO-5(75), (e) CeAlPO-5(100) and (f) CeAlPO-5(125).
4000
3500
3000
2500
2000
1500
1000
500
Wave number (cm-1) Fig. 2. FT-IR spectra of calcined (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO5(75), (d) CeAlPO-5(100) and (e) CeAlPO-5(125).
S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
79
4
102 100
100
96
Weight (%)
90
(d) (b) (e)
85
2
94 92
(b)
90
1
Δ W /Δ T ( %/°C )
(c)
Weight (%)
3
98
95
88 80 86
(a)
0 (a)
84
75
82 200
400
600
800
100
1000
200
400
500
600
700
-1 800
Temperature (°C)
Temperature (ºC) Fig. 3. TGA results of as-synthesized CeAlPO-5 samples (N2 atmosphere) (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO-5(75), (d) CeAlPO-5(100), and (e) CeAlPO-5(125).
300
Fig. 4. (a) TGA and (b) DTG results of calcined CeAlPO-5(25) (air atmosphere).
the analysis, thus supporting again the chemisorption of oxygen only during calcination. 3.3. TGA 3.4. BET studies The nitrogen adsorption–desorption isotherms of calcined AlPO-5 and CeAlPO-5 molecular sieves are depicted in Fig. 6. All of them exhibited type III adsorption. Similar isotherms were also reported in the literature [28]. Since hysteresis was not noticed in the adsorption–desorption isotherms of all the catalysts, the absence of mesopores in these catalysts was established. The BET surface area and pore volume derived from adsorption isotherms are presented in Table 2. Cerium incorporated AlPO-5 molecular sieves showed slightly higher surface area than the parent AlPO-5 which supported the framework entry of cerium. As cerium possesses larger ionic radius than aluminium, such variation in surface area is expected. The pore volume of CeAlPO-5 molecular sieves was also slightly higher than AlPO-5 [28]. This also supported the incorporation of cerium into the framework. Hence, increase of surface area and pore volume, and the variation of lattice parameters supported incorporation of cerium into the framework.
2.5
2.0
1.5
ΔT (°C/mg)
The thermogram of as-synthesized CeAlPO-5(25) is shown in Fig. 3a. The initial weight loss below 200 ◦ C was due to desorption of water. The weight loss between 250 and 450 ◦ C was due to loss of template. A sharp weight loss at 607 ◦ C was ascribed to condensation of defective –OH groups. The total weight loss was about 15% in which 3% was assigned to desorption of water and the rest 12% to loss of template. The thermogram of as-synthesized CeAlPO-5(50) (Fig. 3b) showed similar features as that of CeAlPO-5(25). Desorption of water occurred between 30 and 200 ◦ C, and the removal of template between 200 and 500 ◦ C. The weight loss at 568, 650 and 844 ◦ C derived from the DTG traces (figure not shown) was due to –OH group condensation. The thermogram of as-synthesized CeAlPO-5(75) (Fig. 3c) showed a weight loss due to desorption of water from 30 to 200 ◦ C. But the template decomposed in two stages between 200 and 500 ◦ C. The weight loss due to condensation of –OH groups occurred at 590 ◦ C. This is attributed due to uptake of oxygen, but it is not desorbed. Hence oxygen adsorption is irreversible. In the thermogram of as-synthesized CeAlPO-5(100) (Fig. 3d), the weight loss below 200 ◦ C was due to loss of water and that between 200 and 455 ◦ C was due to template. A slight increase in weight was observed at 590 ◦ C. This is attributed to chemisorption of nitrogen [29]. The thermogram of as-synthesized CeAlPO-5(125) is shown in Fig. 3e. The weight loss below 200 ◦ C was due to desorption of water. The loss of template occurred between 200 and 500 ◦ C. A series of weight increase were observed above 500 ◦ C similar to that of CeAlPO-5(100). Hence, they were assigned to adsorption of nitrogen. The TGA and DTG curves of CeAlPO-5(25) calcined in air were recorded in order to establish the temperature at which chemisorption of oxygen occurred. It is also important to decide the temperature range over which the vapour phase oxidation of alkyl aromatics can be carried out. The results are illustrated in Fig. 4. There was no weight increase in these curves. However, the presence of oxygen was verified from the ESR spectrum (Fig. 10) of CeAlPO-5(25) as discussed below. Hence, Ce3+ might chemisorb oxygen during calcination itself. The TGA results of other catalysts (figure not shown) also showed similar behaviour. Hence, oxygen chemisorption occurred in all the catalysts only during calcination. The DTA results of calcined CeAlPO-5, shown in Fig. 5, also confirmed the absence of oxygen uptake during
1.0
0.5
0.0
-0.5
100
200
300
400
500
600
700
Temperature (ºC) Fig. 5. DTA results of calcined CeAlPO-5(25) (air atmosphere).
800
S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
(a)
Absorbance (a.u.)
Volume of N2 adsorbed (a.u.)
80
(f) (e) (d) (c)
(b) (c) (d)
(b) (a)
0.0
0.2
0.4
0.6
(e)
0.8
1.0
200
Relative pressure (p/po)
300
400
500
600
700
800
Wavelength (cm-1)
Fig. 6. Nitrogen adsorption–desorption isotherms of (a) AlPO-5, (b) CeAlPO-5(25), (c) CeAlPO-5(50), (d) CeAlPO-5(75), (e) CeAlPO-5(100) and (f) CeAlPO-5(125).
Fig. 8. DRS-UV–Vis spectra of calcined (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO-5(75), (d) CeAlPO-5(100) and (e) CeAlPO-5(125).
3.5. DRS-UV–Vis The DRS-UV–Vis spectrum of as-synthesized CeAlPO-5(25) (Fig. 7a) showed an intense broad absorption close to 250 nm. This is assigned to O2− → Ce3+ charge transfer transition [28]. It was reported that CeO2 semiconductor exhibited a maximum absorption around 370 nm [30] for band gap excitation, but such absorption maximum was absent in the spectrum. The DRS-UV–Vis spectra of as-synthesized CeAlPO-5(50, 75, 100 and 125) depicted in Fig. 7(b–e) also showed an intense O2− → Ce3+ charge transfer absorption maximum. This established the incorporation of cerium entirely in the framework. In addition, the absorbance maximum due to framework cerium gradually decreased with increase in Al/Ce ratios. The DRS-UV–Vis spectrum of calcined CeAlPO-5(25) is shown in Fig. 8a. The absorbance maximum of framework Ce3+ was very much shifted towards longer wavelength. There were also shoulders close to charge transfer band of framework Ce3+ . This observation revealed different environment for Ce3+ in CeAlPO5(25). In other words, the 4-oxidic sites around the framework cerium may not be equidistant in all Ce3+ sites. This may arise if cerium sites are located at different regions. Further, there is no evidence for the presence of Ce4+ sites [30]. The DRS-UV–Vis
spectra of calcined CeAlPO-5(50, 75, 100 and 125) are also shown in the same figure (Fig. 8b–e). All the spectra displayed features similar to calcined CeAlPO-5(25). Hence, it was established that free CeO2 [30] and Ce2 O3 [31] were absent in all of them. In other words, cerium was not leached out from the framework of CeAlPO-5 during calcination.
3.6. SEM The SEM images of calcined CeAlPO-5(25), CeAlPO-5(50), CeAlPO-5(75), CeAlPO-5(100) and CeAlPO-5(125) are shown in Fig. 9(a–e). The images displayed large hexagonal rods with tiny crystallites on their surface [8,28]. The amount of tiny crystallites decreased with increase in Al/Ce ratios. The presence of tiny crystallites indicated insufficient crystallization time and slow crystallization process. This is attributed to the incorporation of ˚ cerium in the framework. As the ionic radius of cerium Ce3+ (1.37 A) ˚ its incorporation into the frameis larger than that of Al3+ (0.53 A), work is difficult in comparison to aluminium. In addition, the large size of cerium could slow down the rate of reaction of its hydroxide with phosphate. This might be the cause for the appearance of tiny crystallites on the surface of hexagonal rods. This analysis also established the difficulty in the incorporation of metal ions of larger size into the framework of AlPO-5 during crystallization.
Absorbance (a.u.)
3.7. ESR
(a) (b) (c) (d) (e)
200
300
400
500
600
700
800
Wavelength (cm-1) Fig. 7. DRS-UV–Vis spectra of as-synthesized (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO-5(75), (d) CeAlPO-5(100) and (e) CeAlPO-5(125).
The ESR spectrum of calcined CeAlPO-5(25) is shown in Fig. 10. The broad envelope at g = 2.0 is due to chemisorbed oxygen on cerium [32]. The chemisorbed oxygen carries an unpaired electron for which an ESR signal was observed. The broad unsymmetrical signal with g value between 1.1 and 2.4 indicated highly unsymmetrical environment of Ce3+ . As a result of chemisorption, Ce3+ is not oxidized to Ce4+ since there was no evidence for Ce4+ in the DRS-UV–Vis spectra. Other signals were also observed above g = 2.4. The extra peaks in the ESR spectrum with high g values could be ascribed to Ce3+ with different environments as discussed in the DRS-UV–Vis study. It could lead to either expansion or contraction of 4f orbitals by which g values could be altered. Although the nuclear spin quantum of cerium is 5/2, hyperfine lines were completely masked.
S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
81
Fig. 9. SEM images of calcined (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO-5(75), (d) CeAlPO-5(100) and (e) CeAlPO-5(125).
3.8. XPS The XPS spectrum of CeAlPO-5(25) is shown in Fig. 11. The features depicted in the figure were similar to those reported by Zhao et al. [8]. There was no evidence for the presence of Ce4+ sites which was also confirmed from DRS-UV–Vis and ESR analyses. Hence, Ce3+ alone might be present in the framework. The first peak between 878 and 890 eV was due to Ce3d5/2 state and the other peak between 895 and 910 eV was due to Ce3d3/2 state [33]. 3.9. Catalytic oxidation 3.9.1. Effect of temperature The vapour phase oxidation of ethylbenzene was carried over CeAlPO-5 catalysts at 150, 175, 200, 225 and 250 ◦ C with an air flow rate of 17 ml min−1 and WHSV of ethylbenzene 3.34 h−1 . The major product was acetophenone. The conversion of
ethylbenzene and product selectivity over all the catalysts are shown in Figs. 12 and 13. A comparison of results revealed that CeAlPO-5(25) showed high conversion at 175 ◦ C. The reaction is suggested to proceed as shown in the reaction (Scheme 1). The scheme was supported by occurrence of the reaction only in the presence of air and catalyst between 150 and 250 ◦ C. It was also supported by the absence of conversion in nitrogen. The selectivity to acetophenone was more than 90% at all temperatures. Raju et al. [34] reported acetophenone as the major product in the oxidation of ethylbenzene over nickel supported on USY, hydroxyapatite (HAp), SBA-15 and SiO2 . They also reported small amount of 1-phenylethanol, benzaldehyde and phenyl acetaldehyde at 150 ◦ C. Thermal conversion in the absence of catalyst was less than 1%, but significantly close to 300 ◦ C. The exclusive formation of acetophenone and the absence of 2-phenylacetaldehyde can be rationalized from the following discussion. Though methyl group carries more number of
1.5x10
7
5
1.0x10
1.5x10
5
6
5.0x10
1.4x10 1.4x10
Counts/s
Intensity
0.0 6
-5.0x10
1.3x10 1.3x10
5
Ce3d 3/2 5
Ce5d 3/2
5
5
7
-1.0x10
1.2x10
7
1.1x10
-1.5x10
1.1x10
7
5
5
5
-2.0x10
2
4
6
8
10
g factor Fig. 10. ESR spectrum of calcined CeAlPO-5(25).
12
14
920
910
900
890
880
Binding energy (eV) Fig. 11. XPS spectrum of calcined CeAlPO-5(25).
870
82
S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
.
O2 Ce
3+
O2
CH
3+
Ce
CH3
O H
O
+ O
Ce
3+
OH + 2-Phenylethanol Acetophenone
+
O
CH CH3
+
3+
Ce
C CH3
OH
OH Fast -H2O
.
C
CH3
3+
Ce
OH
+
Ce3+
Scheme 1. Possible pathway for the oxidation of ethylbenzene to acetophenone.
H
H
100
Ethylbenzene conversion (%)
90
H
80
H
70 60 50 40 CeAlPO-5(25) CeAlPO-5(50) CeAlPO-5(75) CeAlPO-5(100) CeAlPO-5(125)
30 20 10 0
160
180
200 220 Temperature (°C)
240
Fig. 12. Effect of temperature on ethylbenzene conversion.
hydrogen than the methylene group, it was not oxidized. Hence the methyl group might be rotating more rapidly than the methylene group and hence it was not attacked by oxygen as shown below. The existence of barrier to rotation about Csp2 –Csp3 was already reported [35]. Hence, the hydrogen of methylene group was more readily available for oxidation than that of methyl group.
100
Acetophenone selectivity (%)
95 90 85 80 75 70
CeAlPO-5(25) CeAlPO-5(50) CeAlPO-5(75) CeAlPO-5(100) CeAlPO-5(125)
65 60 55 50
140
160
180 200 220 Temperature (°C)
240
Fig. 13. Effect of temperature on product selectivity.
260
H
The distant chemisorbed oxygen abstracts a hydrogen from the methylene group of ethylbenzene and forms phenyl ethyl radical and metal hydroperoxide. The free radical rapidly reacts with metal hydroperoxide to form 1-phenylethanol. The local magnetic field of cerium could be thought to retain the paramagnetic phenyl ethyl radical until it is hydroxylated. The resulting 1phenylethanol is rapidly acted upon by atomic oxygen to form the product radical as shown in the reaction (Scheme 1). The oxidation of 1-phenylethanol might be more rapid than ethylbenzene, as the rotation of –CHOH group is slower than the –CH2 – group. The radical expelled hydrogen atom which reacted with Ce-OH group to form water and acetophenone. Since phenyl ethyl radical is rapidly oxidized to 1-phenylethanol and then to acetophenone, the selectivity to acetophenone can be replaced instead of selectivity to phenyl ethyl radical. Since phenyl ethyl radical was not isolated and characterized, the selectivity to acetophenone was used in the discussion rather than that of phenyl ethyl radical. The reaction was carried out only between 150 and 250 ◦ C, as oxidation took place even without catalyst at 300 ◦ C. The results of oxidation of ethylbenzene over other catalysts are also presented in Figs. 12 and 13. The conversion decreased with increase in Al/Ce ratio, thus elucidating the dependence of conversion on cerium content in the catalysts. The selectivity to acetophenone was above 90% over all the catalysts. Based on the conversion, CeAlPO-5(25) was found to be more active than others. When WHSV was increased from 3.45 to 5.12 h−1 , the increase in conversion was only 1% (Table 3). However, further increase in WHSV decreased the conversion. Similarly the selectivity to acetophenone also increased at 5.12 h−1 compared to 3.45 h−1 . The decrease in conversion at 6.81 and 8.47 h−1 may be due to molecular crowding around the active sites by which many reactant molecules could diffuse out without oxidation. In order to establish the role of Ce3+ in CeAlPO-5, the reaction was also carried out using freshly prepared ceria (CeO2 ) catalyst under the same reaction conditions. CeO2 was prepared by calcination of cerium nitrate hexahydrate at 550 ◦ C in air for 12 h. It was characterized by XRD (Fig. 14) and the pattern showed peaks characteristic of CeO2 [36]. The study of the activity of CeO2 in the oxidation of ethylbenzene revealed absence of conversion. Hence, the oxidation of ethylbenzene to acetophenone was established due to Ce3+ in CeAlPO-5. As this process requires oxygen for oxidation, only Ce3+ can adsorb and activate oxygen, and Ce4+ is incapable of doing the same. Though CeO2 was proved to be absent in the synthesized catalyst, even if present, it is incapable of activating oxygen in the air for the oxidation of ethylbenzene.
S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
83
Table 3 Effect of WHSV on ethylbenzene conversion and product selectivity. Catalyst
Flow rate of the feed (ml h−1 )
WHSV (h−1 )
Ethylbenzene conversion (%)
CeAlPO-5(25)
2 3 4 5
3.45 5.12 6.81 8.47
95 96 90 81
Product selectivity (%) Acetophenone
Others
91.2 97.5 91.8 98.1
8.8 2.5 8.2 1.9
Reaction condition: Temperature 175 ◦ C; catalyst weight 0.5 g; air flow rate 17 ml min−1 .
Table 5 Effect of time on stream on ethylbenzene conversion and product selectivity.
111
600
400 220
300
311 200
200
400
222
0 30
Ethylbenzene conversion (%)
1 2 3 4 5 6
96 95 96 96 97 95
Product selectivity (%)
Acetophenone
Others
98.1 98.2 93.6 91.8 95.1 97.3
1.9 1.8 6.4 8.2 4.9 2.7
Reaction condition: Catalyst CeAlPO-5(25); temperature 175 ◦ C; catalyst weight 0.5 g; WHSV 3.45 h−1 ; air flow rate 13 ml min−1 .
100
20
Time (h)
331 420
Intensity (cps)
500
40
50
60
70
80
2θ (degree) Fig. 14. XRD pattern of CeO2 .
3.9.2. Effect of air flow rate The effect of air flow rate on ethylbenzene conversion and product selectivity was studied at 16, 15, 14 and 13 ml min−1 over CeAlPO-5(25) at 175 ◦ C with a WHSV of 3.45 h−1 and the results are presented in Table 4. The results of air flow rate of 17 ml min−1 are also presented in the same table for the purpose of comparison. The conversion remained the same although the flow rate of air decreased. Hence, a slight change in the flow rate of air may not affect the conversion significantly. Though the selectivity to acetophenone slightly increased with decrease in flow rate, the selectivity was above 90% in all the flow rates. Hence, the mechanism of oxidation is suggested to remain the same irrespective of temperature, feed rate and flow rate of air. 3.9.3. Effect of time on stream The effect of time on stream on the ethylbenzene conversion and product selectivity was studied for 6 h over CeAlPO-5(25) at 175 ◦ C with an air flow rate of 13 ml min−1 and the results are presented in Table 5. The conversion remained the same but the selectivity showed slight variation (all the values are above 90%) throughout the time on stream illustrating the existence of the same Table 4 Effect of air flow rate on ethylbenzene conversion and product selectivity. Air flow rate (ml min−1 )
Ethylbenzene conversion (%)
17 16 15 14 13
95 96 94 95 96
Product selectivity (%)
Acetophenone
Others
91.2 97.6 98.3 96.1 98.1
8.8 2.4 1.7 3.9 1.9
Reaction condition: Catalyst CeAlPO-5(25); temperature 175 ◦ C; catalyst weight 0.5 g; WHSV 3.45 h−1 ; feed rate 3 ml h−1 .
mechanism in the oxidation. Acetophenone was identified as the main product and 2-phenylethanol as one of the minor products. The other products (<2%) were not identified. The time on stream also proved the stability of catalyst and absence of coke formation. The amount of feed was nearly equal to the amount of product formed which also established the absence of coke formation. The activity of the catalyst was tested for three cycles and the tests gave similar results of ethylbenzene conversion and acetophenone selectivity. The spent catalyst was also characterized by XRD after calcination. The pattern showed the same features as that of the fresh catalyst and hence, leaching of cerium from the framework was ruled out. The activity of the catalyst was also tested once again. The ethylbenzene conversion and acetophenone selectivity were the same. 4. Conclusions CeAlPO-5 molecular sieves were synthesized in fluoride medium and the framework incorporation of cerium was established by the results of XRD and DRS-UV–Vis. The oxidation state of cerium (Ce3+ ) was confirmed by the results of ESR and XPS. Free CeO2 and Ce2 O3 phases were absent. The presence of adsorbed oxygen on cerium was established by the results of ESR study. Retaining of oxygen without desorption from 30 to 1000 ◦ C was confirmed by the results of TGA. Hence, CeAlPO-5 molecular sieves could be active catalysts for oxidation in the same temperature range. The study of ethylbenzene oxidation over CeAlPO-5 molecular sieves resulted in selective oxidation to acetophenone. The selective oxidation of methylene group to keto group was ascribed to energy barrier for free rotation between the aromatic ring and the ethyl group. The increase of Ce3+ content in the samples increased the oxidative conversion of ethylbenzene. As ethylbenzene conversion and selectivity to acetophenone were above 90%, CeAlPO-5 molecular sieves could be convenient catalysts for industrial applications. Acknowledgements The authors gratefully acknowledge the financial support from the Department of Science and Technology (DST) (Sanction No. SR/S1/PC-10/2009), Government of India, New Delhi,
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Selective oxidation of ethylbenzene over CeAlPO-5 Subbiah Devika, Muthiahpillai Palanichamy, Velayutham Murugesan ∗ Department of Chemistry, Anna University, Chennai 600025, India
a r t i c l e
i n f o
Article history: Received 9 June 2011 Received in revised form 16 August 2011 Accepted 17 August 2011 Available online 23 August 2011 Keywords: CeAlPO-5 Hydrothermal synthesis Fluoride medium Ethylbenzene Oxidation Acetophenone
a b s t r a c t CeAlPO-5(Al/Ce = 25, 50, 75, 100 and 125) molecular sieves were synthesized hydrothermally in fluoride medium and their structures were confirmed from an XRD analysis. The incorporation of Ce3+ into the framework was verified by XRD and DRS-UV–Vis analyses. The surface areas were in the range of 202–215 m2 g−1 . The ESR spectrum revealed the presence of adsorbed oxygen which was evident from the peak at g = 2.0. The catalytic activity of molecular sieves was tested in the vapour phase oxidation of ethylbenzene between 150 and 250 ◦ C. CeAlPO-5(25) showed higher conversion than other catalysts. The selectivity to acetophenone was above 90% at all the temperatures. The selective oxidation of methylene group was ascribed to slow rotation of the ethyl group compared to its methyl group at the reaction temperature. Cerium was not leached out in CeAlPO-5(25) even after activation of the spent catalyst at 550 ◦ C. The time on stream study also indicated the absence of catalyst deactivation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ceria is a versatile catalyst for the oxidation of many organic substrates [1–6]. Since ceria sinters at high temperatures, its activity declines with increase in the number of cycles [7]. Hence, free ceria or supported ceria catalysts may not be suitable for long time use. This problem, however, can be remedied by framework incorporation of cerium into the molecular sieves. Zhao et al. [8] reported the incorporation of cerium into the framework of AlPO5 molecular sieve. Its application as a catalyst in the liquid phase oxidation of cyclohexane gave only 13% conversion [8]. In cerium incorporated MCM-48 molecular sieves, tiny crystallites of ceria were shown to be formed due to leaching at high cerium content [9]. Catalytic activity of cerium incorporated MCM-41 was evaluated in the oxidation of cyclohexane [10–12]. Araujo et al. [13] reported enhancement of surface area as a result of cerium substitution in AlPO-11. Although cerium can exist in +3 or +4 oxidation state, it is the former which is active in the oxidation of –CH2 groups of organics in air. Ce3+ in the framework can adsorb oxygen from air and activate it for oxidation. This adsorption is favoured by the unpaired electron in Ce3+ . The magnetic field associated with Ce3+ and oxygen may be the cause for the chemisorption of the latter on the former. Ce4+ may not be active for –CH2 oxidation, as it cannot chemisorb oxygen. The framework substitution of cerium in the molecular sieves can combine the high activity and selectivity of homogeneous
∗ Corresponding author. Tel.: +91 44 22357023/8645; fax: +91 44 2220066/1213. E-mail address: v [email protected] (V. Murugesan). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.08.023
catalysts with ease of recovery and recycling, which is a characteristic of heterogeneous catalysts. The high surface area is an additional advantage acquired by framework incorporation of cerium into AlPO-5. With the advantages of framework incorporation of cerium into the molecular sieves, the present study synthesized CeAlPO-5 with different Al/Ce ratios in fluoride medium and evaluated their catalytic activity in the vapour phase oxidation of ethylbenzene in air. The selective oxidation of ethylbenzene to acetophenone is an important reaction as the product is the raw material for the production of perfumes, pharmaceuticals, resins and alcohol [14]. It is also an important route to utilize ethylbenzene effectively in the xylene stream of petrochemical industry. Liquid phase oxidation of ethylbenzene to acetophenone by oxygen or air using homogeneous transition metal (viz. Co, Mn, Cu, or Fe) compounds as catalysts was reported [15,16]. The oxidation of ethylbenzene with hydrogen peroxide (H2 O2 ) catalyzed by 8-quinolinolato manganese (III) complexes in water–acetone medium containing ammonium acetate and acetic acid as solvents resulted in ethylbenzene conversion of 26.1% and yield of 65% under optimum conditions [17]. Kanjina and Trakarnpruk [18] studied the oxidation of ethylbenzene over Co-substituted heteropolytungstate catalyst using H2 O2 green oxidant and acetonitrile as solvent. The reaction products were acetophenone (93%) and 1-phenylethanol. The selective oxidation of ethylbenzene to acetophenone was carried out using mixed metal oxide catalysts and t-butyl hydroperoxide (TBHP) as oxidant at 130 ◦ C. In this reaction, 87% ethylbenzene conversion and 92% selectivity to acetophenone were reported [19]. Oxidation of ethylbenzene using TBHP as oxidant was widely studied [20–23]. Highly efficient solvent-free oxidation of ethylbenzene
S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
using TBHP as oxygen source at 80 ◦ C over manganese nanocatalysts was reported with high catalytic activity and selectivity to acetophenone (93%) [24]. Parida and Dash [25] studied the liquid phase oxidation of ethylbenzene using TBHP as oxidant under mild reaction conditions at a temperature of 80 ◦ C with 57.7% ethylbenzene conversion and selectivity to acetophenone (82.2%) and benzaldehyde (18%). Vetrivel and Pandurangan [26,27] studied the vapour phase oxidation of ethylbenzene over Mn and Co containing MCM-41 molecular sieves in air and reported acetophenone as the major oxidation product. This literature review reveals that cerium incorporated AlPO-5 molecular sieves have not been tested for the vapour phase oxidation of ethylbenzene in air. 2. Experimental 2.1. Preparation of the catalysts Aluminium isopropoxide (Merck), aluminium hydroxide (Merck), orthophosphoric acid (Merck), cerium nitrate hexahydrate (Fluka), hydrofluoric acid (Merck) and triethylamine (Merck) were used in the synthesis of CeAlPO-5. Ethylbenzene (Merck) was used as the substrate for oxidation. The procedure adopted by Zhao et al. [8] was slightly modified and used for the synthesis of CeAlPO-5 molecular sieves. The modifications involved high crystallization time and varying amount of isopropyl alcohol content derived from the aluminium source (aluminium isopropoxide). The following gel composition was used for the synthesis: xCe(NO3 )3 ·6H2 O:1.0Al2 O3 :1.3P2 O5 :1.6TEA:1.3HF:425H2 O. Aluminium isopropoxide (7.151 g) was soaked in 50 ml deionized water for 24 h and stirred for 2 h. Orthophosphoric acid (4.9 g), triethylamine (2.8 g) and cerium nitrate hexahydrate (0.60 g) were dissolved in appropriate amount of water and stirred for 2 h at 30 ◦ C. The aluminium precursor was then added to the above mixture and stirred for 2 h. Dilute hydrofluoric acid (0.93 g) was then added and the mixture was stirred again for another 2 h. Finally, the gel was transferred to a Teflon-lined stainless steel autoclave and heated at 180 ◦ C for 6 h under static condition. After quenching the hot autoclave in cold water, the crystallized product was recovered by filtration, dried in an oven at 110 ◦ C and calcined at 550 ◦ C in air for 5 h at a heating rate of 1 ◦ C min−1 . The same procedure was repeated for the synthesis of AlPO-5 without cerium. CeAlPO-5(25) without alcohol was also synthesized using Al(OH)3 as the source for aluminium for the purpose of comparison and designated as CeAlPO-5(25)(WA). 2.2. Catalyst characterization The X-ray diffraction (XRD) patterns of synthesized materials were recorded on a PANalytical X’pert PRO diffractometer equipped ˚ as the radiation source and a liquid nitrowith Cu K␣ (1.54 A) gen cooled germanium solid state detector. The samples were scanned from 5◦ to 40◦ (2Â) in steps of 0.02◦ with a count time of 5 s at each point. Fourier transform infrared (FT-IR) spectra of the samples were recorded in the range of 4000–400 cm−1 on an FT-IR spectrometer (Nicolet, Avatar 360) using KBr pellet technique. Thermogravimetric analysis (TGA) was carried out on an SDTQ-600 WATERS system under nitrogen atmosphere in the temperature range of 30–1000 ◦ C at a heating rate of 10 ◦ C min−1 . Thermogravimetric analysis was also carried out in air atmosphere in the temperature range of 30–1000 ◦ C at a heating rate of 10 ◦ C min−1 on an SDTQ-600 V8.3 Build 101 model system. The diffuse reflectance ultraviolet–visible spectra (DRS-UV–Vis) were recorded using JASCO V-550 instrument. The morphology of the samples was examined by scanning electron microscopy (SEM). The sample was suspended in methanol
77
Table 1 Effect of alcohol content and dielectric constant on the crystallinity of CeAlPO5(25)(WA) and CeAlPO-5(25). Catalyst
Alcohol (%)
Dielectric constant (k)
Crystallinity (%)
CeAlPO-5(25)(WA)
– 4.26 8.52
88.0000 83.3337 79.4602
100.00 115.77 120.16
12.78 17.04 21.3
76.3373 72.6362 69.0188
178.59 139.17 123.55
CeAlPO-5(25)a WA, without alcohol. a Al/Ce = 25.
and the specimen stub dipped into it and removed. Evaporation of methanol left the sample evenly dispersed on the stub. The sample was coated with gold by ion sputtering for 2 min and the SEM image was recorded using HITACHI-S-3400N instrument. The Al/Ce ratios of the samples were determined by energy dispersive X-ray analyzer (EDAX) housed in the same unit. X-ray photoelectron spectrum (XPS) was performed on a Thermo Multilab 2000 using monochrome Al K␣ as the excitation source. The electron spin resonance (ESR) spectrum was carried out on a Bruker EMX plus instrument with a microwave frequency of 9.859461 Hz and modulation frequency of 100 kHz. Surface area measurement was carried out by nitrogen adsorption at 77 K on an ASAP-2020 porosimeter from Micromeritics Corporation (Norcross, GA, USA). Before nitrogen adsorption–desorption measurement, each sample was degassed for 3 h at 250 ◦ C under vacuum (10−5 mbar) in the degas port of the adsorption analyzer. 2.3. Catalytic studies The vapour phase oxidation of ethylbenzene was carried out in a fixed bed vertical downward flow glass reactor of internal diameter 2 cm. About 0.5 g of a catalyst was placed at the center of the reactor supported on either side with a thin layer of quartz wool and ceramic beads. The reactor was heated to the requisite temperature using a temperature programmed furnace. The catalyst was activated before the reaction at 500 ◦ C for 6 h in a controlled stream of carbon dioxide free air. The reactant was fed into the reactor using a syringe infusion pump. The products, collected in the receiver flask, were analyzed in a gas chromatograph (Shimadzu 17A) equipped with a DB-5 capillary column (30 m × 0.25 mm × 0.25 m) and flame ionization detector. The products were also identified using GC–MS (Perkin Elmer Clarus 500) with helium as the carrier gas at a flow rate of 1 ml min−1 . 3. Results and discussion 3.1. XRD The XRD pattern of CeAlPO-5, synthesized using aluminium hydroxide as the precursor for aluminium in fluoride medium, is shown in Fig. 1A(a). It exhibited the characteristic peaks of AlPO5 [28]. The XRD pattern of CeAlPO-5, synthesized with aluminium isopropoxide as the source for aluminium, is depicted in Fig. 1A(b) which also exhibited the characteristic peaks of AlPO-5. There were no extra peaks in both the patterns other than the peaks due to AlPO-5, thus confirming the absence of free cerium based phases. The crystallinity of the materials was calculated by considering the intense peaks at 2Â = 7.4◦ , 19.7◦ , 20.9◦ and 22.3◦ and the values are presented in Table 1. For comparison, the crystallinity of CeAlPO-5(25)(WA) synthesized without alcohol was taken as the reference (crystallinity = 100%). The crystallinity increased by 15%, when aluminium isopropoxide was used as the source for
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S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
Table 2 Lattice parameters of as-synthesized and calcined AlPO-5 and CeAlPO-5 samples and their BET results. Catalyst
As-synthesized
Calcined
a=b AlPO-5 CeAlPO-5(25) CeAlPO-5(50) CeAlPO-5(75) CeAlPO-5(100) CeAlPO-5(125)
c
13.7584 13.6373 13.6772 13.7012 13.7324 13.7096
± ± ± ± ± ±
0.0750 0.0493 0.0037 0.0063 0.0556 0.0488
8.4818 8.3856 8.4846 8.5481 8.4819 8.4579
± ± ± ± ± ±
0.1446 0.0551 0.0163 0.0430 0.1022 0.0876
˚ d(1 0 0) (A)
a=b
11.80 11.73 11.83 11.81 11.79 11.76
13.1561 13.8004 13.8312 13.8860 13.8972 13.8900
aluminium. Since the procedure adopted in the synthesis of both the materials differed only in the use of alcohol, the decrease in dielectric constant of the medium was inferred to increase the crystallinity of the samples. In order to optimize the amount of alcohol for good synthesis, its content (therefore dielectric constant) in the synthesis medium was gradually varied, and the synthesized samples were subsequently analyzed by XRD. All of them showed an increase in the intensity of peaks due to (1 0 0), (2 1 0), (0 0 2) and (2 1 1). The crystallinity of the materials was also calculated and presented in Table 1. The crystallinity increased with increase in alcohol content upto 12.78% and above which it decreased, thus suggesting a non-linear dependence of crystallinity on dielectric
A
Calcined c ± ± ± ± ± ±
0.3631 0.0629 0.0813 0.1148 0.0980 0.1080
8.4652 8.1179 8.4443 8.4172 8.3544 8.3632
± ± ± ± ± ±
0.5995 1.3042 0.1630 0.1929 0.2464 0.2789
˚ d(1 0 0) (A)
BET surface area (m2 g−1 )
Pore volume (cm3 g−1 )
11.35 11.88 11.92 11.95 11.94 11.95
202 215 210 208 205 205
0.22 0.28 0.27 0.26 0.25 0.24
constant. Since alcohol (12.78%) mediated synthesis gave higher crystallinity than others, the same content of the precursor of aluminium was used in the synthesis of CeAlPO-5 with Al/Ce ratios 50, 75, 100 and 125 employing the same procedure. The XRD patterns of as-synthesized and calcined samples are presented in Fig. 1A and B. All of them showed an increase in the intensity of the peaks with expected high crystallinity. The lattice parameters were also calculated and presented in Table 2. The values of dspacing of CeAlPO-5 were higher than AlPO-5. It also supported framework incorporation of cerium. The increase was due to a large size of cerium compared to aluminium, but a linear variation was not obtained: the d-spacing of CeAlPO-5(25) was lower than that of the other CeAlPO-5 molecular sieves with varying Al/Ce ratios.
3.2. FT-IR
Intensity (a.u.)
(f) (e) (d) (c) (b) (a) 5
10
15
20
25
30
35
40
2θ (degree)
The FT-IR spectrum of calcined CeAlPO-5(25) is shown in Fig. 2a. The intense broad band between 2500 and 4000 cm−1 was due to –OH stretching vibration of water. The corresponding bending vibration occurred at 1667 cm−1 . The absence of peaks due to –CH2 – vibrations just below 3000 cm−1 confirmed the complete absence of template. The asymmetric and symmetric stretching vibrations of Al–O–P occurred between 800 and 1500 cm−1 . The corresponding bending vibration occurred at 735 cm−1 . Similar types of vibrations were also observed in Ln–AlPO-5 (Ln = La, Ce, Sm, Dy, Y, Gd) [28]. The FT-IR spectra of calcined CeAlPO-5(50, 75, 100 and 125) are presented in the same figure (Fig. 2b–e). All of them displayed similar characteristics as that of Fig. 2a, thus establishing the absence of template.
(e) (d) (c) (b) (a)
Transmittance (a.u.)
Intensity (a.u.)
B
(f) (e) (d) (c) (b) (a) 5
10
15
20
25
30
35
40
2θ (degree) Fig. 1. (A) XRD patterns of as-synthesized (a) CeAlPO-5(25)(WA), (b) CeAlPO-5(25), (c) CeAlPO-5(50), (d) CeAlPO-5(75), (e) CeAlPO-5(100) and (f) CeAlPO-5(125). (B) XRD patterns of calcined (a) CeAlPO-5(25)(WA), (b) CeAlPO-5(25),(c) CeAlPO-5(50), (d) CeAlPO-5(75), (e) CeAlPO-5(100) and (f) CeAlPO-5(125).
4000
3500
3000
2500
2000
1500
1000
500
Wave number (cm-1) Fig. 2. FT-IR spectra of calcined (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO5(75), (d) CeAlPO-5(100) and (e) CeAlPO-5(125).
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79
4
102 100
100
96
Weight (%)
90
(d) (b) (e)
85
2
94 92
(b)
90
1
Δ W /Δ T ( %/°C )
(c)
Weight (%)
3
98
95
88 80 86
(a)
0 (a)
84
75
82 200
400
600
800
100
1000
200
400
500
600
700
-1 800
Temperature (°C)
Temperature (ºC) Fig. 3. TGA results of as-synthesized CeAlPO-5 samples (N2 atmosphere) (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO-5(75), (d) CeAlPO-5(100), and (e) CeAlPO-5(125).
300
Fig. 4. (a) TGA and (b) DTG results of calcined CeAlPO-5(25) (air atmosphere).
the analysis, thus supporting again the chemisorption of oxygen only during calcination. 3.3. TGA 3.4. BET studies The nitrogen adsorption–desorption isotherms of calcined AlPO-5 and CeAlPO-5 molecular sieves are depicted in Fig. 6. All of them exhibited type III adsorption. Similar isotherms were also reported in the literature [28]. Since hysteresis was not noticed in the adsorption–desorption isotherms of all the catalysts, the absence of mesopores in these catalysts was established. The BET surface area and pore volume derived from adsorption isotherms are presented in Table 2. Cerium incorporated AlPO-5 molecular sieves showed slightly higher surface area than the parent AlPO-5 which supported the framework entry of cerium. As cerium possesses larger ionic radius than aluminium, such variation in surface area is expected. The pore volume of CeAlPO-5 molecular sieves was also slightly higher than AlPO-5 [28]. This also supported the incorporation of cerium into the framework. Hence, increase of surface area and pore volume, and the variation of lattice parameters supported incorporation of cerium into the framework.
2.5
2.0
1.5
ΔT (°C/mg)
The thermogram of as-synthesized CeAlPO-5(25) is shown in Fig. 3a. The initial weight loss below 200 ◦ C was due to desorption of water. The weight loss between 250 and 450 ◦ C was due to loss of template. A sharp weight loss at 607 ◦ C was ascribed to condensation of defective –OH groups. The total weight loss was about 15% in which 3% was assigned to desorption of water and the rest 12% to loss of template. The thermogram of as-synthesized CeAlPO-5(50) (Fig. 3b) showed similar features as that of CeAlPO-5(25). Desorption of water occurred between 30 and 200 ◦ C, and the removal of template between 200 and 500 ◦ C. The weight loss at 568, 650 and 844 ◦ C derived from the DTG traces (figure not shown) was due to –OH group condensation. The thermogram of as-synthesized CeAlPO-5(75) (Fig. 3c) showed a weight loss due to desorption of water from 30 to 200 ◦ C. But the template decomposed in two stages between 200 and 500 ◦ C. The weight loss due to condensation of –OH groups occurred at 590 ◦ C. This is attributed due to uptake of oxygen, but it is not desorbed. Hence oxygen adsorption is irreversible. In the thermogram of as-synthesized CeAlPO-5(100) (Fig. 3d), the weight loss below 200 ◦ C was due to loss of water and that between 200 and 455 ◦ C was due to template. A slight increase in weight was observed at 590 ◦ C. This is attributed to chemisorption of nitrogen [29]. The thermogram of as-synthesized CeAlPO-5(125) is shown in Fig. 3e. The weight loss below 200 ◦ C was due to desorption of water. The loss of template occurred between 200 and 500 ◦ C. A series of weight increase were observed above 500 ◦ C similar to that of CeAlPO-5(100). Hence, they were assigned to adsorption of nitrogen. The TGA and DTG curves of CeAlPO-5(25) calcined in air were recorded in order to establish the temperature at which chemisorption of oxygen occurred. It is also important to decide the temperature range over which the vapour phase oxidation of alkyl aromatics can be carried out. The results are illustrated in Fig. 4. There was no weight increase in these curves. However, the presence of oxygen was verified from the ESR spectrum (Fig. 10) of CeAlPO-5(25) as discussed below. Hence, Ce3+ might chemisorb oxygen during calcination itself. The TGA results of other catalysts (figure not shown) also showed similar behaviour. Hence, oxygen chemisorption occurred in all the catalysts only during calcination. The DTA results of calcined CeAlPO-5, shown in Fig. 5, also confirmed the absence of oxygen uptake during
1.0
0.5
0.0
-0.5
100
200
300
400
500
600
700
Temperature (ºC) Fig. 5. DTA results of calcined CeAlPO-5(25) (air atmosphere).
800
S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
(a)
Absorbance (a.u.)
Volume of N2 adsorbed (a.u.)
80
(f) (e) (d) (c)
(b) (c) (d)
(b) (a)
0.0
0.2
0.4
0.6
(e)
0.8
1.0
200
Relative pressure (p/po)
300
400
500
600
700
800
Wavelength (cm-1)
Fig. 6. Nitrogen adsorption–desorption isotherms of (a) AlPO-5, (b) CeAlPO-5(25), (c) CeAlPO-5(50), (d) CeAlPO-5(75), (e) CeAlPO-5(100) and (f) CeAlPO-5(125).
Fig. 8. DRS-UV–Vis spectra of calcined (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO-5(75), (d) CeAlPO-5(100) and (e) CeAlPO-5(125).
3.5. DRS-UV–Vis The DRS-UV–Vis spectrum of as-synthesized CeAlPO-5(25) (Fig. 7a) showed an intense broad absorption close to 250 nm. This is assigned to O2− → Ce3+ charge transfer transition [28]. It was reported that CeO2 semiconductor exhibited a maximum absorption around 370 nm [30] for band gap excitation, but such absorption maximum was absent in the spectrum. The DRS-UV–Vis spectra of as-synthesized CeAlPO-5(50, 75, 100 and 125) depicted in Fig. 7(b–e) also showed an intense O2− → Ce3+ charge transfer absorption maximum. This established the incorporation of cerium entirely in the framework. In addition, the absorbance maximum due to framework cerium gradually decreased with increase in Al/Ce ratios. The DRS-UV–Vis spectrum of calcined CeAlPO-5(25) is shown in Fig. 8a. The absorbance maximum of framework Ce3+ was very much shifted towards longer wavelength. There were also shoulders close to charge transfer band of framework Ce3+ . This observation revealed different environment for Ce3+ in CeAlPO5(25). In other words, the 4-oxidic sites around the framework cerium may not be equidistant in all Ce3+ sites. This may arise if cerium sites are located at different regions. Further, there is no evidence for the presence of Ce4+ sites [30]. The DRS-UV–Vis
spectra of calcined CeAlPO-5(50, 75, 100 and 125) are also shown in the same figure (Fig. 8b–e). All the spectra displayed features similar to calcined CeAlPO-5(25). Hence, it was established that free CeO2 [30] and Ce2 O3 [31] were absent in all of them. In other words, cerium was not leached out from the framework of CeAlPO-5 during calcination.
3.6. SEM The SEM images of calcined CeAlPO-5(25), CeAlPO-5(50), CeAlPO-5(75), CeAlPO-5(100) and CeAlPO-5(125) are shown in Fig. 9(a–e). The images displayed large hexagonal rods with tiny crystallites on their surface [8,28]. The amount of tiny crystallites decreased with increase in Al/Ce ratios. The presence of tiny crystallites indicated insufficient crystallization time and slow crystallization process. This is attributed to the incorporation of ˚ cerium in the framework. As the ionic radius of cerium Ce3+ (1.37 A) ˚ its incorporation into the frameis larger than that of Al3+ (0.53 A), work is difficult in comparison to aluminium. In addition, the large size of cerium could slow down the rate of reaction of its hydroxide with phosphate. This might be the cause for the appearance of tiny crystallites on the surface of hexagonal rods. This analysis also established the difficulty in the incorporation of metal ions of larger size into the framework of AlPO-5 during crystallization.
Absorbance (a.u.)
3.7. ESR
(a) (b) (c) (d) (e)
200
300
400
500
600
700
800
Wavelength (cm-1) Fig. 7. DRS-UV–Vis spectra of as-synthesized (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO-5(75), (d) CeAlPO-5(100) and (e) CeAlPO-5(125).
The ESR spectrum of calcined CeAlPO-5(25) is shown in Fig. 10. The broad envelope at g = 2.0 is due to chemisorbed oxygen on cerium [32]. The chemisorbed oxygen carries an unpaired electron for which an ESR signal was observed. The broad unsymmetrical signal with g value between 1.1 and 2.4 indicated highly unsymmetrical environment of Ce3+ . As a result of chemisorption, Ce3+ is not oxidized to Ce4+ since there was no evidence for Ce4+ in the DRS-UV–Vis spectra. Other signals were also observed above g = 2.4. The extra peaks in the ESR spectrum with high g values could be ascribed to Ce3+ with different environments as discussed in the DRS-UV–Vis study. It could lead to either expansion or contraction of 4f orbitals by which g values could be altered. Although the nuclear spin quantum of cerium is 5/2, hyperfine lines were completely masked.
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Fig. 9. SEM images of calcined (a) CeAlPO-5(25), (b) CeAlPO-5(50), (c) CeAlPO-5(75), (d) CeAlPO-5(100) and (e) CeAlPO-5(125).
3.8. XPS The XPS spectrum of CeAlPO-5(25) is shown in Fig. 11. The features depicted in the figure were similar to those reported by Zhao et al. [8]. There was no evidence for the presence of Ce4+ sites which was also confirmed from DRS-UV–Vis and ESR analyses. Hence, Ce3+ alone might be present in the framework. The first peak between 878 and 890 eV was due to Ce3d5/2 state and the other peak between 895 and 910 eV was due to Ce3d3/2 state [33]. 3.9. Catalytic oxidation 3.9.1. Effect of temperature The vapour phase oxidation of ethylbenzene was carried over CeAlPO-5 catalysts at 150, 175, 200, 225 and 250 ◦ C with an air flow rate of 17 ml min−1 and WHSV of ethylbenzene 3.34 h−1 . The major product was acetophenone. The conversion of
ethylbenzene and product selectivity over all the catalysts are shown in Figs. 12 and 13. A comparison of results revealed that CeAlPO-5(25) showed high conversion at 175 ◦ C. The reaction is suggested to proceed as shown in the reaction (Scheme 1). The scheme was supported by occurrence of the reaction only in the presence of air and catalyst between 150 and 250 ◦ C. It was also supported by the absence of conversion in nitrogen. The selectivity to acetophenone was more than 90% at all temperatures. Raju et al. [34] reported acetophenone as the major product in the oxidation of ethylbenzene over nickel supported on USY, hydroxyapatite (HAp), SBA-15 and SiO2 . They also reported small amount of 1-phenylethanol, benzaldehyde and phenyl acetaldehyde at 150 ◦ C. Thermal conversion in the absence of catalyst was less than 1%, but significantly close to 300 ◦ C. The exclusive formation of acetophenone and the absence of 2-phenylacetaldehyde can be rationalized from the following discussion. Though methyl group carries more number of
1.5x10
7
5
1.0x10
1.5x10
5
6
5.0x10
1.4x10 1.4x10
Counts/s
Intensity
0.0 6
-5.0x10
1.3x10 1.3x10
5
Ce3d 3/2 5
Ce5d 3/2
5
5
7
-1.0x10
1.2x10
7
1.1x10
-1.5x10
1.1x10
7
5
5
5
-2.0x10
2
4
6
8
10
g factor Fig. 10. ESR spectrum of calcined CeAlPO-5(25).
12
14
920
910
900
890
880
Binding energy (eV) Fig. 11. XPS spectrum of calcined CeAlPO-5(25).
870
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S. Devika et al. / Applied Catalysis A: General 407 (2011) 76–84
.
O2 Ce
3+
O2
CH
3+
Ce
CH3
O H
O
+ O
Ce
3+
OH + 2-Phenylethanol Acetophenone
+
O
CH CH3
+
3+
Ce
C CH3
OH
OH Fast -H2O
.
C
CH3
3+
Ce
OH
+
Ce3+
Scheme 1. Possible pathway for the oxidation of ethylbenzene to acetophenone.
H
H
100
Ethylbenzene conversion (%)
90
H
80
H
70 60 50 40 CeAlPO-5(25) CeAlPO-5(50) CeAlPO-5(75) CeAlPO-5(100) CeAlPO-5(125)
30 20 10 0
160
180
200 220 Temperature (°C)
240
Fig. 12. Effect of temperature on ethylbenzene conversion.
hydrogen than the methylene group, it was not oxidized. Hence the methyl group might be rotating more rapidly than the methylene group and hence it was not attacked by oxygen as shown below. The existence of barrier to rotation about Csp2 –Csp3 was already reported [35]. Hence, the hydrogen of methylene group was more readily available for oxidation than that of methyl group.
100
Acetophenone selectivity (%)
95 90 85 80 75 70
CeAlPO-5(25) CeAlPO-5(50) CeAlPO-5(75) CeAlPO-5(100) CeAlPO-5(125)
65 60 55 50
140
160
180 200 220 Temperature (°C)
240
Fig. 13. Effect of temperature on product selectivity.
260
H
The distant chemisorbed oxygen abstracts a hydrogen from the methylene group of ethylbenzene and forms phenyl ethyl radical and metal hydroperoxide. The free radical rapidly reacts with metal hydroperoxide to form 1-phenylethanol. The local magnetic field of cerium could be thought to retain the paramagnetic phenyl ethyl radical until it is hydroxylated. The resulting 1phenylethanol is rapidly acted upon by atomic oxygen to form the product radical as shown in the reaction (Scheme 1). The oxidation of 1-phenylethanol might be more rapid than ethylbenzene, as the rotation of –CHOH group is slower than the –CH2 – group. The radical expelled hydrogen atom which reacted with Ce-OH group to form water and acetophenone. Since phenyl ethyl radical is rapidly oxidized to 1-phenylethanol and then to acetophenone, the selectivity to acetophenone can be replaced instead of selectivity to phenyl ethyl radical. Since phenyl ethyl radical was not isolated and characterized, the selectivity to acetophenone was used in the discussion rather than that of phenyl ethyl radical. The reaction was carried out only between 150 and 250 ◦ C, as oxidation took place even without catalyst at 300 ◦ C. The results of oxidation of ethylbenzene over other catalysts are also presented in Figs. 12 and 13. The conversion decreased with increase in Al/Ce ratio, thus elucidating the dependence of conversion on cerium content in the catalysts. The selectivity to acetophenone was above 90% over all the catalysts. Based on the conversion, CeAlPO-5(25) was found to be more active than others. When WHSV was increased from 3.45 to 5.12 h−1 , the increase in conversion was only 1% (Table 3). However, further increase in WHSV decreased the conversion. Similarly the selectivity to acetophenone also increased at 5.12 h−1 compared to 3.45 h−1 . The decrease in conversion at 6.81 and 8.47 h−1 may be due to molecular crowding around the active sites by which many reactant molecules could diffuse out without oxidation. In order to establish the role of Ce3+ in CeAlPO-5, the reaction was also carried out using freshly prepared ceria (CeO2 ) catalyst under the same reaction conditions. CeO2 was prepared by calcination of cerium nitrate hexahydrate at 550 ◦ C in air for 12 h. It was characterized by XRD (Fig. 14) and the pattern showed peaks characteristic of CeO2 [36]. The study of the activity of CeO2 in the oxidation of ethylbenzene revealed absence of conversion. Hence, the oxidation of ethylbenzene to acetophenone was established due to Ce3+ in CeAlPO-5. As this process requires oxygen for oxidation, only Ce3+ can adsorb and activate oxygen, and Ce4+ is incapable of doing the same. Though CeO2 was proved to be absent in the synthesized catalyst, even if present, it is incapable of activating oxygen in the air for the oxidation of ethylbenzene.
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Table 3 Effect of WHSV on ethylbenzene conversion and product selectivity. Catalyst
Flow rate of the feed (ml h−1 )
WHSV (h−1 )
Ethylbenzene conversion (%)
CeAlPO-5(25)
2 3 4 5
3.45 5.12 6.81 8.47
95 96 90 81
Product selectivity (%) Acetophenone
Others
91.2 97.5 91.8 98.1
8.8 2.5 8.2 1.9
Reaction condition: Temperature 175 ◦ C; catalyst weight 0.5 g; air flow rate 17 ml min−1 .
Table 5 Effect of time on stream on ethylbenzene conversion and product selectivity.
111
600
400 220
300
311 200
200
400
222
0 30
Ethylbenzene conversion (%)
1 2 3 4 5 6
96 95 96 96 97 95
Product selectivity (%)
Acetophenone
Others
98.1 98.2 93.6 91.8 95.1 97.3
1.9 1.8 6.4 8.2 4.9 2.7
Reaction condition: Catalyst CeAlPO-5(25); temperature 175 ◦ C; catalyst weight 0.5 g; WHSV 3.45 h−1 ; air flow rate 13 ml min−1 .
100
20
Time (h)
331 420
Intensity (cps)
500
40
50
60
70
80
2θ (degree) Fig. 14. XRD pattern of CeO2 .
3.9.2. Effect of air flow rate The effect of air flow rate on ethylbenzene conversion and product selectivity was studied at 16, 15, 14 and 13 ml min−1 over CeAlPO-5(25) at 175 ◦ C with a WHSV of 3.45 h−1 and the results are presented in Table 4. The results of air flow rate of 17 ml min−1 are also presented in the same table for the purpose of comparison. The conversion remained the same although the flow rate of air decreased. Hence, a slight change in the flow rate of air may not affect the conversion significantly. Though the selectivity to acetophenone slightly increased with decrease in flow rate, the selectivity was above 90% in all the flow rates. Hence, the mechanism of oxidation is suggested to remain the same irrespective of temperature, feed rate and flow rate of air. 3.9.3. Effect of time on stream The effect of time on stream on the ethylbenzene conversion and product selectivity was studied for 6 h over CeAlPO-5(25) at 175 ◦ C with an air flow rate of 13 ml min−1 and the results are presented in Table 5. The conversion remained the same but the selectivity showed slight variation (all the values are above 90%) throughout the time on stream illustrating the existence of the same Table 4 Effect of air flow rate on ethylbenzene conversion and product selectivity. Air flow rate (ml min−1 )
Ethylbenzene conversion (%)
17 16 15 14 13
95 96 94 95 96
Product selectivity (%)
Acetophenone
Others
91.2 97.6 98.3 96.1 98.1
8.8 2.4 1.7 3.9 1.9
Reaction condition: Catalyst CeAlPO-5(25); temperature 175 ◦ C; catalyst weight 0.5 g; WHSV 3.45 h−1 ; feed rate 3 ml h−1 .
mechanism in the oxidation. Acetophenone was identified as the main product and 2-phenylethanol as one of the minor products. The other products (<2%) were not identified. The time on stream also proved the stability of catalyst and absence of coke formation. The amount of feed was nearly equal to the amount of product formed which also established the absence of coke formation. The activity of the catalyst was tested for three cycles and the tests gave similar results of ethylbenzene conversion and acetophenone selectivity. The spent catalyst was also characterized by XRD after calcination. The pattern showed the same features as that of the fresh catalyst and hence, leaching of cerium from the framework was ruled out. The activity of the catalyst was also tested once again. The ethylbenzene conversion and acetophenone selectivity were the same. 4. Conclusions CeAlPO-5 molecular sieves were synthesized in fluoride medium and the framework incorporation of cerium was established by the results of XRD and DRS-UV–Vis. The oxidation state of cerium (Ce3+ ) was confirmed by the results of ESR and XPS. Free CeO2 and Ce2 O3 phases were absent. The presence of adsorbed oxygen on cerium was established by the results of ESR study. Retaining of oxygen without desorption from 30 to 1000 ◦ C was confirmed by the results of TGA. Hence, CeAlPO-5 molecular sieves could be active catalysts for oxidation in the same temperature range. The study of ethylbenzene oxidation over CeAlPO-5 molecular sieves resulted in selective oxidation to acetophenone. The selective oxidation of methylene group to keto group was ascribed to energy barrier for free rotation between the aromatic ring and the ethyl group. The increase of Ce3+ content in the samples increased the oxidative conversion of ethylbenzene. As ethylbenzene conversion and selectivity to acetophenone were above 90%, CeAlPO-5 molecular sieves could be convenient catalysts for industrial applications. Acknowledgements The authors gratefully acknowledge the financial support from the Department of Science and Technology (DST) (Sanction No. SR/S1/PC-10/2009), Government of India, New Delhi,
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