VIETNAM JOURNAL OF CHEMISTRY Vol. 49(4) 517-521 AUGUST 2011
OXIDATION OF STYRENE OVER TiOz/DI LINH CLAY CATALYSTS
Nguyen Tien Thao*, Trinh Dang Tuan
Faculty of Chemistry, Hanoi University of Science, (VNU) Received 21 February 2011
Abstract
Ti02 is loaded on the Di Linh clay and used as a catalyst for the liquid oxidation of styrene. All catalysts are characterized by several physical techniques (XRD, FTIR, SEM). The oxidation of styrene is carried out at mild conditions (40-60°C, atmospheric pressure, H2O2 oxidant). The reaction data show that Ti02 species serve as active centers for the oxidation of styrene to benzaldehyde. Styrene conversion and product selectivity are strongly dependent on the TiOi loadings and reaction conditions. The benzaldehyde selectivity may reach up to 89% at a styrene conversion of 55% under typical conditions.
1. INTRODUCTION 2. EXPERIMENTAL
The oxidation reaction of unsaturated hydrocarbons to oxygenated compounds such as aldehydes and ketones is an important process in chemical industry [1]. The process can be performed over both homogeneous and heterogeneous catalysts [1 3]. However, the major drawbacks of homogeneous catalytic practices always require the further steps for catalyst recovery or separation [1, 2]. In many cases, catalyst separation from the reaction mixture leads to an increased process cost [3]. Therefore, another attractive route for catalytic oxidation, is the use of a solid catalyst. Indeed, the heterogeneous catalysts have several advantages such as recyclable catalyst and using environmentally friendly reagent including molecular oxygen, air, and hydrogen peoxide as the oxidants [3 6]. The reaction can be carried out under mild conditions (room temperature, atmospheric pressure...) [5 7]. Another important target is possible to achieve the highest possible yield and thus to minimize the additional costs of separation and waste removals. Among several well- publicized-catalysts such as methyltrioxorhenium [8], peroxovanadium [9], nanospinel MgFe304 [10], V-Ag-Ni-0 mixed oxides [11]; Ti02-based systems [5, 12] are known as one of the most promising catalysts for the selective oxidation of styrene to desired products.
The aim of this work is to deal with the preparation of Ti02 supported on Di Linh clay used as catalyst for the oxidative conversion of styrene to benzaldehyde (BzH) in the presence of H2O2 oxidant.
2.1. Catalyst preparation
Di Linh elay was pretreated in order to improve the content of montmorillonite according to the Ref [13]. Then, the pretreated elay was impregnated with Ti(0'C3H7)4 solution [5]. For example, the pretreated clay was added into a 200 mL-beaker containing 100 mL solution of isopropanol solvent at room temperature. Then, a certain amount of Ti(0'C3H7)4 was introduced into the same beaker. The suspension was stirred until the complete evaporation of solvent at room temperature. The obtained solid was dried, ground and then calcined at 250°C in air. A reference catalyst, designated as Ref sample, was prepared by physically mixing 5 wt% Ti02 and 95 wt% clay without any further modification.
2.2. Catalyst characterization
X-ray diffraction spectra of solids were collected on a D8ADVACE diffraetometer with CuKa (A- = 1.54056 A) radiation operating 40 mA. FT-IR spectra were recorded on a Shimadzu IR 8101 spectrophotometer; the specimens were ground with KBr and pressed into thin wafers. The scanning range was from 4000 to 400 cm'' with a resolution of 4 cm"' The scanning electron microscopy (SEM) microphotographs were obtained in a JEOS JSM- 5410 LV
2.3. Catalytic performance
The epoxidation of styrene with H2O2 solution
VJC, Vol. 49(4), 2011
was carried out in a batch reactor. In a typical run, 100 mmol of styrene, 50 ml of solvent and 0.3 grams of catalyst were added into a 100-ml three-necked flask. The flask was stirred for 5 min, and then immersed in a water bath. Then reaction was started by addition of hydrogen peroxide (30% aq.) via a burette at a controlled rate of 1 mL/min. The mixture was stirred vigorously by a magnetic stirrer and heated to the desired temperature. After the reaction, the mixture was quenched to room temperature and then catalyst was filtered off The filtrate was quantitatively analyzed by a GC-MS (HP-6890 Plus). The product selectivity was determined by combining the analytical measurements and the calculation; i.e. the moles of all the components inclusive of organic reaetant and products in the reaction mixture were directly quantified using toluene as an internal standard, then conversion of styrene (mol%) and selectivity (%) of each product were calculated accordingly.
3. RESULTS AND DISCUSSION 3.1. Catalytic characteristics
Four catalysts are prepared with Ti02 loadings from 5 to 10 vvt.%. The XRD spectra of the samples are displayed in figure 1 [3, 5, 7]. X-ray diffraetograph of the elay (not shown here) appears some reflection peaks at 20 of 7.5° (d = 12.10 A), 19.8° (d = 4.50 A) and 26.8° (d = 3.4 A), 36.6° (2.58
A), 61.4 (1.50 A) characterizing the presence of montmorillonite phase [13]. Also, these peaks clearly observed on the impregnated clays (Fig. 1).
No reflections of Ti02 phase are detectable at lower loadings (< 7 wt%) Ti02) duet to either the formation of a high dispersion of tiny Ti02 particles on the clay surface or the sensibility limit of XRD method.
When the Ti02 content increases to 7 10 wt%, XRD pattern appears some week signals at 26 of 21, 24, and 30°, corresponding to the presence of Ti02 anatase. This observation indicates that an increased Ti02 loading results in the formation of aggregates of Ti02 on the support [5, 7]. The presence of Ti02/clay can be drawn from IR spectra. Indeed, FT- IR spectra of Ti02/clay samples are characterized with a specific band titanium or silicon bond in the framework (Fig. 2a).
The peaks at 3625, 3412 and 1637 cm' are attributed to the structural hydroxyl groups in the clay mineral layer and water molecules in the interlayer spaces [13, 14]. A strong band at 1032 cm' ' with a shoulder about 1250 cm"' corresponds to the stretching vibrations of Si-O-Si in the clay. Also, the bending and stretching vibrations of Si-0 and Al-0 in the tetrahedral framework are observed at 526 490 cm"' It is noted that the appearance of shoulder around 930 - 970 c m ' is assigned to the Ti-O-Si vibrations, in accordance with the data reported by Ref [14]. Accordingly, Ti02 may interact with Si- OH groups in clay to form Si-O-Ti bonds [4], alter the catalyst surface and affect the catalytic activity.
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Figure 1: XRD patterns of 5, 7, 10 wt%Ti02/Di Linh clays (M: montmorillonite)
However, SEM image indicates no significant spaces between the particles, which may give a change in the morphology of catalyst surface, higher external surface area [13, 14].
Figure 2b depicts the representative SEM
photograph of 7.5 wt.% Ti02/clay. SEM image 3.2. Oxidation of styrene indicates that the catalyst consists of different
shaped-particles. The average diameters vary from The liquid oxidation of styrene with 30% H2O2 hundred nanometers to micrometers. There are many solution is carried out over all prepared catalysts.
VJC, Vol. 49(4), 2011
Figure 3 displays the catalytic performance of all samples at 55"C. It is noted that only traces of benzaldehyde product are produced in the absence of catalyst (blank test) and no significant amount of BzH (1 3%) is formed on the reference sample at conversion of 5 10%. In contrast, the catalytic activity dramatically increases while the reaction is performed on a set of TiOi-impregnated samples, presumably suggesting that Ti-species may also interact with the Si-011 groups. Furthermore, these Ti species are believed to be active sites for the oxidation of st\roiie to benzaldehyde. The conversion of styrene increases from 10 to 64% as increasing TiO^ loadings from 0 to 10%. An increased st\rcne comersion also facilitates the secondary reactions, which make a decreased the
Oxidation of styrene over...
selectivity to benzaldehyde. Indeed, the desired product selectivity (BzH) shows an opposite trend.
Benzaldehyde selectivity obtains a maximum value on sample 5 wt%) Ti02/elay. A decrease in selectivity at a higher Ti02 loading may correlate with a higher conversion of styrene to undesired products due to the secondary reactions [3, 5].
Furthermore, at a higher loading, Ti02 may yield several Ti02 anatase aggregates that lead to be less selective for the formation of BzH. This assumption is strongly supported by reaction products tested different conditions. Figure 4 presents catalytic activity in the reaction temperature range of 35 65°C. Under similar reaction conditions, three catalysts exhibit similar profiles of conversion and product distribution.
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Figure J: Effect of TiO? loadings on the catalytic activity at 55°C over 5-10 wt%Ti02/clay In overall, the styrene conversion is proportional
to the Ti02 contents in whole range of reaction temperatures (Fig. 4a). The reaetant conversion decreases as the order of 10 wt% > 7 wt% Ti02 > 5 wt% TiO^/ciay while the profile of selectivity to desired product passes through a maximal temperature of 45-55°C (Fig. 4b). At a higher
reaction temperature, the benzaldehyde may be further converted to other oxygenates possibly due to the occurrence of secondary reactions [3,5].
Analysis of products on GC-MS indicates the presence of styrene epoxide, benzyl alcohol, benzoic acid in addition to the main product of benzaldehyde (table 1) [3, 5, 11].
VJC, Vol. 49(4), 201 Nguyen Tien Thao, et aL
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Figure 4: Conversion (a) and product selectivity (b) at different reaction temperatures The highest selectivity to benzaldehyde obtained
on 5 wt% Ti02/clay is explained by the formation of highly dispersed Ti02 on support as compared with the other catalysts. This is in accordance with the XRD data. Figure 4b also indicates that the optimal
reaction temperature is about 50 55°C. Therefore, this temperature range is chosen to investigate the stability of 5 wt%) Ti02/clay. The reaction has tested for 6 hours at 55°C and the data are collected in table 1.
Table 1: Stability of Ti02 catalyst in the oxidation of styrene over 5 wt% Ti02/clay
Reaction time, h 2
4 6
Styrene conversion, % 55.88
58.20 70.59
Selectivity, % Benzaldehyde
89.1 84.6 57.1
Other products 15.40 42.90 With increasing reaction time, the conversion
slightly increases after 2 hour-reaction, but significantly goes up for 4 hours. As a result, selectivity to BzH decreases from 89 to 57 % after 6 hours- on-time. An increased selectivity to byproducts (table 1) can be explained by the over- oxidation of styrene to other products and the further oxidation of benzaldehyde formed in a batch reactor with increasing reaction time.
4. CONCLUSIONS
Three catalysts are prepared with different loadings of Ti02. The presence of Ti02 on support is confirmed by XRD and FT-IR spectra. The reflection lines of Ti02 is only observed at a high Ti02 content (> 7 wt%), demonstrating a high dispersion of Ti02 on the elay support. All samples show a good activity in the liquid oxidation of styrene. The selectivity to benzaldehyde is rather high over impregnated samples, especially the higher dispersion of Ti02 gives rise to better selectivity to BzH. At the rather high conversion (60%) of styrene, the selectivity to benzaldehyde can
achieve about 89%). An optimal reaction temperature was found around 50 55°C. The catalyst still remains the activity after 6 hours at 55°C.
Acknowledgements: The present work was financially supported by Nanofosted under Project
Na 104.99-2011.50.
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Corresponding author: Nguyen Tien Thao
Faculty of Chemistry, Hanoi University of Science, (VNU) 19 Le Thanh Tong, Hoan Kiem, Hanoi, Vietnam
Email: nguyentienthao(^gmaiI.com
