КИНЕТИКА И КАТАЛИЗ, 2011, том 52, № 6, с. 844-848
UDC 541.128:546.183'78-325
HIGHLY ACTIVE AND REUSABLE CATALYST FOR FRIES REARRANGEMENT
OF PHENYL ACETATE
© 2011 L. S. Roselin1, R. Selvin1, *, P. Aneesh1, M. Bououdina2, 3, S. Krishnaswamy1
department of Chemical and Materials Engineering, Lunghwa University of Science and Technology, Taiwan 2Nanotechnology Center, University of Bahrain, Kingdom of Bahrain 3Department of Physics, College of Science, University of Bahrain, Kingdom of Bahrain *E-mail: rosilda@mail.lhu.edu.tw Received 02.12.2010
Silica-12-tungstophosphoric acid core-shell nanoparticles were prepared by sol-gel method followed by steaming. The catalytic activity of fresh and steamed catalysts was studied in Fries rearrangement of phenyl acetate. The reaction parameters, such as catalyst loading and reaction temperature, were optimized. The structural properties of the prepared catalysts were analyzed by X-ray diffraction and transmission electron microscopy techniques. The nature and strength of acid sites in the catalysts were analyzed by pyridine adsorption followed by infrared spectroscopy and differential scanning calorimetry measurements. The XRD and TEM analyses confirm the formation of silica-12-tungstophosphoric acid core-shell nanoparticles during steaming process. Acidity measurement indicates that both fresh and steamed catalyst samples carry weak acid sites and Bronsted acid sites. In addition, the steaming of heteropoly acid contained silica enhances the strength of Bronsted acid sites. The catalytic activity of fresh as well as steamed catalysts in liquid-phase Fries rearrangement showed that the steam treated sample exhibits higher conversion and selectivity to the desired product compared to the fresh catalyst sample. The higher activity of steam treated catalysts has been explained in terms of surface acidity of the catalysts. Reusability of the steamed catalyst shows that there is no appreciable change either in the conversion rate or product selectivity.
The Fries rearrangement of phenyl acetate leads to hydroxyacetophenones, which are valuable precursors in the pharmaceutical industry [1]. Traditionally, the Fries rearrangement is conducted with stoichiometric amounts of Lewis acid (e.g., AlCl3) or mineral acid (e.g., H2SO4 or HF) and generates large amounts of inorganic salts as by-products. Therefore, the environmental impact ofthese processes makes highly desirable the development of new technologies that employ heterogeneous, reusable catalyst, and of reactants that generate more environmentally friendly co-products [2]. Consequently a variety of solid acids, particularly zeolites, have been studied in the Fries rearrangement both in the liquid and vapor phases [3—5]. As found recently, Keggin-type heteropoly acids (HPAs) are highly active solid acid catalysts for Fries rearrangement in liquid phase [6—8]. These catalysts are much more active than zeolites and can be separated and reused. However, the conversion is still poor. Thus, there still exists scope to develop better catalysts which would catalyze the Fries rearrangement with excellent conversion and selectivity at comparatively low temperatures.
Статья публикуется на английском языке в авторском варианте.
This paper reports an efficient catalytic Fries rearrangement of phenyl acetate using 12-tungstophos-phoric acid included in silica nanoparticles. The effects of steam treatment, the amount of catalyst and the temperature on the conversion rate were investigated.
Experimental Chemicals
Tetraethoxysilane (TEOS), dodecatungstophos-phoric acid (DTP), dodecane and phenyl acetate were commercial samples from "Merck" and were used without further purification.
The silica—12-tungstophosphoric acid core-shell nanoparticles catalyst (25 wt % DTP/SiO2) was prepared by sol-gel method followed by steaming. In a typical procedure, DTP (2.5 g) was dissolved in deion-ized water (10 ml). TEOS mixed with ethanol (26 g TEOS and 10 g EtOH) was dropped into the above solution under vigorous stirring. Upon addition of TEOS and ethanol mixture, the sol was subjected evaporation at 70°C under vacuum and during concentration the sol turned into a transparent viscous gum-like liquid. After continuous heating this became a transparent sticky solid, which finally transformed into transparent sugar-like cubes. The transparency of
HIGHLY ACTIVE AND REUSABLE CATALYST FOR FRIES REARRANGEMENT
845
29, degrees
Fig. 1. XRD patterns of 25 wt % DTP/SiO2 samples before
(1) and after (2) steaming at 150°C for 6 h.
the solid implies that they were composed of nanometer-sized particles. The solid powder was dried in an air oven at 120°C for 6 h. The dried solid powder was then steamed at 150°C for 6 h.
Characterization
Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku 2000 diffractometer using Cu^a radiation (X = 1.5418 A) in the range 29 from 5 to 45° at a scan rate of 2°/min with a step size of 0.04°.
Morphological observations of the silica-12-tung-stophosphoric acid core-shell nanoparticles were carried out using transmission electron microscope (TEM) JEM-2010 at 200 kV.
Vapor phase adsorption of pyridine as probe molecule [9] was adopted to find out the acidity of catalyst samples. The nature of acid sites was characterized on the basis of IR spectral data. The nature and strength
of acidic sites were determined by differential scanning calorimetric (DSC) measurements.
Catalytic Activity Measurements
The rearrangement of phenyl acetate was carried out in liquid phase over the silica—12-tungstophos-phoric acid core-shell nanopaticles at 150°C under nitrogen atmosphere in a 100 ml glass reactor equipped with a condenser and a magnetic stirrer. In typical experiment, the reaction mixture consisted of phenyl acetate (50 mmol) in dodecane was taken with a total liquid volume of 30 ml. The freshly activated catalyst (0.5 g) was then added to the flask, which is heated at constant temperature of 150°C in oil bath and stirred magnetically. This operation was conducted under nitrogen atmosphere for 2 h. The progress of the reaction was monitored by gas chromatographic analysis using SE-30 columns and flame ionization detector. The composition of products was confirmed by gas chromatography—mass spectroscopy method.
RESULTS AND DISCUSSION XRD
The XRD patterns of 25 wt % DTP/SiO2 sample before and after steaming are shown in Fig. 1. It is clear that XRD pattern of the sample is very similar to that of pure silica except presence of a broad peak located at 29 range of 3—8°. This broad peak located at very low angles can be attributed to the scattering effects due to the presence of very fine particles, i.e. at the nanoscale. The XRD result confirms the absence of free DTP particles before and after steaming.
TEM
The TEM micrographs of 25 wt % DTP/SiO2 sample before and after steaming are presented in Fig. 2. As seen in the micrographs, steaming leads to an increase of the DTP average particle's size: during steaming, the DTP particles were dissolved in the steam and then
Fig. 2. TEM images of 25 wt % DTP/SiO2 samples before (a) and after (b) steaming at 150°C for 6 h. КИНЕТИКА И КАТАЛИЗ том 52 № 6 2011
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ROSELIN h gp.
Table 1. Conversion and product selectivity in Fries rearrangement of phenyl acetate over fresh and steamed 25 wt % DTP/SiO2
OCOCH3
OH
Catalyst Conver- Selectivity, %
sion, % PhOH 2HAP 4HAP 4AAP
Fresh <5 64.0 7.9 7.1 21.0
Steamed 59.8 51.1 21.0 12.3 15.6
Catalyst IR frequency, cm-1 AH, J/g
weak acid sites Br0nsted sites
HPA before 1490 1639 300
steaming 1543
HPA after 1487 1644 708.6
steaming 1556
16 14
12
,0
an6
r Tr
4
2
0
\
\
y
1400 1450
1500 1550 1600 Wavenumber, cm-1
1650 1700
Fig. 3. FT-IR spectra of pyridine adsorbed on 25 wt % DTP/SiO2 samples before (1) and after (2) steaming at 150°C for 6 h.
COCH3
+
phenyl acetate
OH
Note. Catalyst weight 0.5 g, temperature 150°C, phenyl acetate 50 mmol, reactiontime 2 h.
Table 2. DSC and FT-IR data for pyridine adsorption on catalysts
COCH3 4HAP
2HAP
OCOCH3 OH
COCH3 4AAP
phenol
coated on the surface of the silica nanoparticles, therefore forming SiO2-DTP core-shell nanoparti-cles as final product.
Catalyst Activity
Fries rearrangement of phenyl acetate was studied using SiO2-DTP core-shell nanoparticles. The products of the rearrangement were found to be 2- and 4-hy-droxyacetophenones (2HAP and 4HAP, respectively), 4-acetoxyacetophenone (4AAP) and phenol (PhOH):
Freese et al. [10] have suggested the mechanism of Fries rearrangement of phenyl acetate over H-form of Y- and ZSM-5 zeolites catalysts. It was deduced that the para-product is formed via an ionic species (ben-zylium ion) by an intermolecular reaction whereas the ortho-product is formed by an intramolecular mechanism.
The influence of steam treatment on the conversion of phenyl acetate and the product selectivity is presented in Table 1. The fresh 25 wt % DTP/SiO2 catalyst is not much active in Fries rearrangement of phe-nyl acetate (<5% conversion). However, steam treatment of 25 wt % DTP/SiO2 (150°C, 6 h) tremendously increased its catalytic activity (59.8% conversion). In addition, the selectivity of HAPs has also been found to be increased. This can be explained by surface acidity of both catalysts. Silica contained het-eropoly acid carries weak acid sites and Br0nsted acid sites (Table 2). Steaming of heteropoly acid contained silica enhanced the strength of Br0nsted acid sites and increased the heat of desorption of pyridine from 300 to 708 J/g. It is supported by the IR analysis of the py-ridine adsorbed samples, where 2 absorption bands located at 1639 and 1644 cm-1 are observed (Fig. 3), which indicate the presence of strong Br0nsted acid sites. The increase in activity of the catalyst after steaming can also be explained on the basis of the FT-IR data. The broad peak observed about 3382 cm-1 corresponding to silanol groups (Fig. 4a
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