106 CHAPTER 4 LIQUID PHASE AEROBIC OXIDATION OF ETHYLBENZENE OVER PrAlPO-5 4.1 INTRODUCTION Selective catalytic oxidation of alkyl aromatics is a viable technology to functionalize saturated and unsaturated hydrocarbons (Wentzel et al 2000).The benzylic oxidation of alkyl aromatics is considered to be one of the important catalytic reactions for the preparation of corresponding carbonyl compound of the reactant as they are used in the synthesis of fine chemicals and pharmaceuticals (Lu et al 2010). Parida& Dash (2009) studied the liquid phase oxidation of ethylbenzene using TBHP as oxidant under mild reaction conditions at a temperature of 80 C. They reported 57.7% ethylbenzene conversion and selectivity to acetophenone (82.2%) and benzaldehyde (18%).The catalytic oxidation of ethylbenzene over Cosubstituted heteropolytungstate catalyst using H 2 O 2 oxidant with acetonitrile as solventgave acetophenone (93%) and 1-phenylethanol (Kanjina&Trakarnpruk 2010).Kanjina&Trakarnpruk (2011) reported the selective oxidation of ethylbenzene to acetophenone using tertbutylhydroperoxide (TBHP) as oxidant at 130 C in the presence of mixed metal oxide catalysts. The reaction showed 87% ethylbenzene conversion and 92% selectivity to acetophenone. Vanadia supported on ceria catalysts were used in the liquid phase oxidation of ethylbenzene using H 2 O 2 oxidant (Radhika et al 2007).
107 Acetophenone was found to be the major product along with 2-hydroxyacetophenone as minor product.the sintering at high temperature and leaching of metal ions are serious drawbacks of transition metal oxide based catalysts (Yu et al 2006). Organic peroxides, hydrogen peroxide and molecular oxygen are cost effective oxidants for the oxidation of alkyl aromatics in the presence of a suitable catalytic system. However, organic peroxides are not environmentally benign as they leave large volume of organic waste. Hydrogen peroxide decomposedrapidly above 70 C, as a consequence the formation of water in the reaction mixture decreased the activity of the catalyst by decreasing the interaction between the substrate and the catalyst surface. Molecular oxygen (air) is the cheapest and clean oxidizing agent. Perkas et al (2001) reported the aerobic oxidation of cyclohexane over mesoporous iron-titania catalyst. The reaction showed 90% selectivity to cyclohexanol. Rajuet al (2008) reported supported Ni catalysts for the aerobic oxidation of ethylbenzene. They concluded that Ni based systems were active for the sidechain oxidation of ethylbenzene and the formation of products was anchored by acidity of the catalysts. Selective oxidation of ethylbenzene catalyzed by fluorinated metalloporphyrins with molecular oxygen (Li et al 2007) gave 94% acetophenone with a turnover number of 2719 under mild conditions. Solvent plays an important role in the liquid phase reactions (Mal &Ramasamy 1996, Jha et al 2006). However, the use of solvent also led to environmental problems. Guo et al (2003) reported solvent free aerobic oxidation of cyclohexene. Tusar et al (2011) reported solvent free oxidation of alkyl aromatics to aromatic ketones using molecular oxygen. Zhan et al (2007), Tian et al (2004), Araujo et al (2003) and Devika et al (2012) reported a variety of metal incorporated AlPO-5 molecular sieves for the selective oxidation of organic molecules. Devika et al (2011) interpreted the paramagnetic behavior of Ce 3+ ions in CeAlPO-5 molecular sieves and
108 oxygen chemisorption behavior in the selective oxidation of ethylbenzene. In the lanthanide family, praseodymium the successor of cerium has two lone pair electrons in the 4f shell which couldexhibit better interaction with molecular oxygen than cerium. Since Pr 3+ and Pr 4+ are paramagnetic in nature, they can activate molecular oxygen and thus facilitate the oxidationof ethylbenzene. The framework substitution of praseodymium in the molecular sieves can combine the high activity and selectivity of homogeneous catalysts with ease of recovery and recycling, which are characteristics of heterogeneous catalysts. The high surface area is an additional advantage acquired by framework incorporation of praseodymium into AlPO-5. Further,the weak and moderate acid sitescreated in PrAlPO-5 framework aided side chain oxidation rather than ring hydroxylation (Reddy et al 1993, Mal et al 1995, Chen et al 1996, Selvam&Singh 1995, Chen &Sheldon 1995). Keeping in mind the advantages of framework incorporation of praseodymium into AlPO molecular sieves, in the present study PrAlPO-5 with different (Al+P)/Pr ratios in fluoride medium were synthesized and evaluated their catalytic activity in the liquid phase aerobic oxidation of ethylbenzene. 4.2 CATALYTIC STUDIES Solvent free liquid phase aerobic oxidation of ethylbenzene was carried over PrAlPO-5 molecular sieves in the temperature range 60-140 o C.Acetophenone was found to be the major product (>90%) and 1- phenylethanol, 2-phenylethanol, phenyl acetaldehyde and phenyl acetic acid as minor products (Scheme 4.1).
109 Scheme 4.1Aerobic oxidation of ethylbenzene The influence of reaction parameters such as temperature, reaction time, substituents and Pr content was also studied. The plausible mechanism for the reaction is depicted in Scheme 4.2. In this mechanism, Pr 3+ is oxidized to Pr 4+ by the chemisorbed oxygen. This then abstracts a hydrogen from the methylene group of ethylbenzene, thus forming metal hydroperoxide and phenyl ethyl radical. The formation of 1-phenylethanol is attributed to the reaction between metal hydroperoxide and phenyl ethyl radical. Further, the oxygen radical present in Pr 4+ abstracts a hydrogen from 1-phenylethanol to form a tertiary radical which eliminates a molecule of water thus resulting acetophenone. The reaction was carried out between 60 and 140 o C. There was practically no reaction in this temperature range in the absence of catalyst. Further, the reaction did not proceed in nitrogen atmosphere instead of air atmosphere. This supported the proposed reaction mechanism.
110 Scheme 4.2 Possible pathway for the oxidation of ethylbenzene to acetophenone 4.2.1 Effect of Temperature Liquid phase oxidation of ethylbenzene in the presence of air was carried over PrAlPO-5 (25, 50, 75 and 100) catalysts in the temperature range 60-140 o C. The experimental results are presented in Table 4.1. The ethyl benzene conversion and acetophenone selectivity were found to be maximum at 120 o C. When the reaction temperature was increased above 120 o C, there was no significant improvement in the selectivity of 1-phenylethanol whereas selectivity to others increased appreciably. The acetophenone selectivity also decreased considerably above this reaction temperature. 2-Phenylethanol, phenyl acetaldehyde and phenyl acetic acid were formed at 140 o C due to activation of methyl group. Since the formation of acetophenone from 1-phenylethanol is a rapid reaction, the formation of 1-phenyl ethane-1,2-diol is ruled out. The competition of additional reaction forming phenyl acetic acid
111
112 suppressed 1-phenylethanol formation, and this decreased the selectivity to acetophenone. Thus, ethylbenzene conversion remained the same but the selectivity to acetophenone decreased over PrAlPO-5 catalysts above 120 o C. 4.2.2 Effect of (Al+P)/Pr Ratios PrAlPO-5 with different (Al+P)/Pr ratios viz., 25, 50, 75 and 100 were used for the aerobic oxidation of ethylbenzene and the results are presented in Table 4.1. PrAlPO-5 with (Al+P)/Pr ratio 25 showed slightly higher selectivity to acetophenonethan others at 120 C. Since Pr content in this catalyst was high, it could rapidly converted 1-phenylethanol to acetophenone. The decrease of 1-phenylethanol selectivity also supported this view. As 1-phenylethanol is a neutral compound, and there is also an appreciable steric hindrance for adsorption on Pr sites through its OH group, it can rapidly diffuse out. Hence,increase of framework Pr content in AlPO-5 could rapidly converted 1-phenylethanol to acetophenone. This study revealed the dependence of acetophenone selectivity on the framework Pr content in AlPO-5. 4.2.3 Effect of Reaction Time The influence of reaction time on conversion and selectivity was studied between 1 and 10 h over PrAlPO-5 (25) at 120 o C and the results are depicted in Figure 4.1. The percentage conversion of ethyl benzene increased from 1 to 6 h reaction time. Though ethylbenzene conversion remained the same upto 10 h, the selectivity to acetophenone decreased. The concentration of acetophenone was found to be maximum after 6 h reaction time. It was presumed that a small amount of acetophenone adsorbed on the acid sites further oxidized to benzoic acid (Chumbhale et al 2005). Since the selectivity to acetophenone was found to be maximum at 6 h reaction time, this was chosen as the optimum condition.
113 Figure 4.1 Effect of reaction time on ethyl benzene oxidation 4.2.4 Effect of Substituents Aerobic oxidation of ethylbenzene over PrAlPO-5 (25) at 120 o C yielded acetophenone as the major product along with 1-phenylethanol as minor product. The various substituents in the aromatic ring ofethylbenzenechanged the electron density around the benzylic hydrogen atom. As stated already the reaction proceeded via hydrogen abstraction mechanism. The free radical formed in the proposed mechanism could be stabilized either by electron releasing or electron withdrawing substituent. Hence, the electron density around the benzylic hydrogen may not alter the selectivity to form the respective carbonyl compound. The benzylic oxidation of various substituted ethylbenzenes was attempted and the results are presented in Table 4.2. All the substituted compounds exhibited almost similar selectivity (> 90 %) under the given reaction conditions. Since the free radical formed at benzylic carbon is resonance stabilized by both electron
114 releasing and electron withdrawing substituents in the para position, this catalyst is found to be suitable for the oxidation of substituted ethylbenzene compounds. Table 4.2 Effect of substituents on benzylic oxidation S.No. R 1 R 2 Conversion (%) a Selectivity (%) Major product 1. CH 3 H 95 95 Acetophenone 2. CH 3 Cl 94 94 4-Chloroacetophenone 3. CH 3 Br 94 95 4-Bromoacetophenone 4. CH 3 F 93 95 4-Fluoroacetophenone 5. CH 3 I 91 93 4-Iodoacetophenone 6. CH 3 NO 2 90 94 4-Nitroacetophenone 7. H NO 2 91 93 4-Nitrobenzaldehyde 8. CH 3 OCH 3 94 93 4-Methoxyacetophenone 9. H H 94 93 Benzaldehyde 10. H OCH 3 94 94 4-Methoxybenzaldehyde 11. C 6 H 5 H 96 95 Benzophenone a Determined by GC-MS
115 4.2.5 Catalyst Recycling In order to address the problem of leaching of Pr from AlPO-5, 5 mg of fresh catalyst was dissolved in aqua regia and elemental composition was analyzed using ICP-OES. The catalyst recovered from the reaction mixture after 2 h reaction time was washed with 5% dilute nitric acid, dried and dissolved completely in aqua regia. The elemental composition of this solution was also performed using ICP-OES. The results clearly showed that praseodymium content in the catalyst before and after the reaction remained almost the same. Hence it was concluded that praseodymium remained intact in AlPO-5 and well incorporated in the framework. The recovered catalyst after the reaction was washed well with ether and dried at 200 o C. The recycled catalyst was used in the reaction for five times under the same reaction conditions. It was found that the catalyst showed similar activity up to five reaction cycles. 4.3 CONCLUSION The hydrothermal synthesis of PrAlPO-5 with different (Al+P)/Pr ratios was successfully accomplished in the fluoride medium. The ESR study confirmed the presence of chemisorbed oxygen on the catalyst, which concluded that this catalyst is found suitable for oxidation reactions. The TPR study revealed the absence of free Pr 2 O 3 and PrO 2 which confirmed that the oxidation reaction proceeded via chemisorbed oxygen in the aerobic oxidation of ethylbenzene. The decrease of (Al+P)/Pr ratio increased the conversion and selectivity. This correlated the dependence of Pr content in AlPO-5 and reactivity. It is also concluded that the weak and moderately strong acid sites created by the framework incorporation of praseodymium in AlPO-5 favored the side chain oxidation rather than ring hydroxylation. The electron density in the aromatic ring did not influence the selectivity to acetophenone. This study also concluded that ethylbenzene and different substituted ethylbenzenes can be effectively oxidized using molecular oxygen as oxidant over PrAlPO-5 at 120 o C.