CHAPTER 7 SELECTIVE OXIDATION OF ETHYL BENZENE

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1 104 CHAPTER 7 SELECTIVE OXIDATION OF ETHYL BENZENE 7.1 INTRODUCTION Aromatic ketones such as acetophenone are important intermediates for the synthesis of drugs and pharmaceuticals (Choudhary et al 2004). Traditionally such ketones were derived by the oxidation of appropriate methylene groups attached to the aromatic ring using stoichiometric quantities of oxidizing agent likes KMnO 4 and KCr 2 O 7 (Cullis et al 1955 and Clark et al 1989). An alternative route is Friedel Crafts acylation of aromatics using appropriate acid halides or anhydrides in stoichiometric amounts to catalysts. All the above methods are hazardous and yield wastes which demand disposal. Hence it is important to develop solid redox catalysts to perform such oxidation in an eco friendly manner. In addition, hydrogen peroxide and ter-butyl hydro peroxide were also used as the oxidizing agents. Such peroxides are also hazardous. Hence designing heterogeneous catalysts which can use air as the oxidant is advantageous. Chromium containing catalysts were used for the oxidation of ethyl benzene (Sakthivel et al 2001 and Dapurkar et al 2003). Jha et al (2006) reported Ti, V and Cr containing MCM-41 material for the oxidation of ethyl benzene but with peroxide as the oxidizing agent in liquid phase. Devika et al (2011) reported oxidation of ethyl benzene over Ce-AlPO-5 molecular sieves. A high ethyl benzene conversion and acetophenone selectivity were noted. Since cerium is incorporated in the

2 105 frame work of AlPO-5 one can also expect incorporation of the same in silicalite zeolites. For oxidation presence of aluminium is not desired, instead of zeolite, Slicalite-1 molecular sieve could be chosen for the framework incorporation of cerium. Such catalyst could carry the characteristics of the selectivity of homogeneous catalysts, and recyclability of heterogeneous catalysts, and shape selectivity of medium pore zeolites. Hence in the present study it was first attempted to framework incorporation of cerium in Silicalite-1and carryout vapour phase oxidation of ethyl benzene using air as the oxidant. Incorporation of cerium in the framework of Silicalite-1 could create acid sites like aluminium. Since such acid sites can actively take part in isomerisation and disproportionation, the running of vapour phase oxidation demands low temperature. 7.2 CATALYTIC STUDIES The oxidation of ethyl benzene over cerium silicalite molecular sieves was tested in air flow rate of 10 ml/min and reactant feed 2mL/h. The main product was acetophenone confirmed by GC-MS and the conversion of ethyl benzene and selectivity of product are presented in Table Effect of Temperature The increase in conversion of ethyl benzene with increased in temperature over Ce-Silicalite (50). Coke formation was not noticed at all temperatures. Cerium with its adsorbed oxygen obstract hydrogen atom from the methlyene group of ethyl benzene to form ethyl benzene radical is shown in the reaction Scheme 7.1. Transfer of hydroxyl group from Ce-OOH to ethyl benzene radical gives 1-phenyl ethanol. Subsequent oxidation of 1-phenyl ethanol to acetophenone involves abstraction of hydrogen formation carbon that contains OH group estimation of hydrogen atom forms acetophenone. The released hydrogen combines with CeOH to form water.

3 106 O 2 CH 3 OOH + CH - CH 3 Ethylbenzene radical OH + OH 1 Phenylethanol CH 3 Ce Si O H 3 C H 2 O + + O Acetophenone Subsequent Oxidation Scheme 7.1 Oxygen chemisorption and ethylbenzene oxidation to acetophenone The sequential oxidation of ethyl benzene to 1 phenyl ethanol and then to acetophenone might occur on single cerium site, since the selectivity to acetophenone was higher than 95 %. If 1-phenyl ethanol is oxidized on other sites, then its selectivity will be high, as it can also diffuse out without oxidation. Table 7.1 Effect of temperature on ethylbenzene conversion and acetophenone selectivity Catalysts (Ce/Si) Temperature ( o C) EB conversion Selectivity (%) (Wt %) Acetophenone Others Reaction conditions: Catalyst.Wt = 0.3 g; Air flow rate 8 ml/min; WHSV=2.888 h -1

4 107 Oxidation of methyl group of ethyl benzene was not noticed at any temperature. As it might be rapidly rotating is not acted on adsorbed oxygen. In contrast ethyl group might not be rapidly rotating as the energy barrier between sp 2 and sp 3 carbons is high. The high energy barrier for this bond was also reported in the literature (Schaefer et al 1994). As oxidation of ethyl benzene occurred at 350 C without catalyst, the reaction was carried out below this temperature. The oxidation of ethyl benzene over Ce-Silicalite-1(75 and 100) was also carried out under the same condition as that of Ce-Silicalite-1 (50) and the results are presented in the same Table 7.1. The results also showed increase of ethyl benzene conversion with increasing temperature but the values were less than Ce-Silicalite-1 (50) due to reduced amount of cerium content. The selectivity to acetophenone was also above 90 % with very low selectivity to others. Based on ethyl benzene conversion product selectivity 300 C was chosen optimum one and Ce-Silicalite-1 (50) as the more active catalyst than others Effect of WHSV The effect of the WHSV on ethyl benzene conversion was studied over Ce-Silicalite-1 (50) at 300 C. The EB conversion and product selectivity are presented in Table 7.2. The EB conversion was incecreased with increase in the WHSV of feed. As the flow rate of air was same, increased in WHSV of the feed resulted in high conversion. It was due to increase in the concentration of feed close towards the availability of active sites. Most of the reactants will be diffusing out easily when the WHSV was high. The selectivity was higher than 90 % at all WHSV.

5 108 Table 7.2 Effect of WHSV on ethylbenzene conversion Catalysts (Ce/Si) 50 WHSV (h -1 ) EB conversion (Wt %) Selectivity (%) Acetophenone Others Reaction conditions: Catalyst.Wt = 0.3 g; Temp = 300 C, Air flow rate 8 ml/min Effect of Air Flow Rate The effect of flow rate of air was studied at 6,8,10, 12 and 14 ml/h. The results are presented in Table 7.3. The conversion of ethyl benzene decreased with the increase in air flow rate above 10 ml h -1, but increased at the low flow rate due to high contact time increased conversion. The selectivity of the products was above 90 % at all flow rates. Table 7.3 Effect of air flow rate on vapour phase oxidation of ethyl benzene conversion Catalyst Si/Ce Si/Ce(50) Air Flow rate (ml/min) EB conversion Selectivity (%) (Wt %) Acetophenone Others Reaction conditions: Catalyst Ce-Silicallite-1 Si/Ce = 50, Temperature = 300 C, Air flow rate; 8 ml/min, WHSV = h -1

6 Effect of Time on Stream The effect on time on stream on ethyl benzene conversion and the product selectivity was studied at 300 C for 6 h. The results are presented in Table 7.4. The conversion remained steady for the entire period of time on study. It confirmed the absence of catalyst deactivation by coke formation. The selectivity of product also remained as the same throughout the reactions. The study also conformed absence of leaching of cerium. The spent catalyst was also analyzed by DRS-UV, but free CeO 2 was not formed. Hence cerium might be strongly bound to the silicallite-1 frame work. Table 7.4 Effect of time on stream on ethylbenzene conversion and acetophenone selectivity Catalyst (Ce/Si) Time on stream (h) EB conversion Selectivity (%) (Wt %) Acetophenone Others Reaction conditions: Catalyst. Wt = 0.3g, Temperature = 300 C, Air flow rate; 8 ml/ min, WHSV = CONCLUSION Ce-Silicalite-1 molecular sieves were first synthesized with Silicalite 1 structure in fluoride medium. It was difficult to synthesize of Ce-Silicalite-1 (25), and only higher Si/Ce ratios were favored. Frame work

7 110 incorporation of cerium in Silicalite-1 frame work was established by XRD and DRS-UV. The ESR spectroscopy showed the presence of Ce 3+, and adsorbed oxygen at g value 2.0. As Ce is in +3 oxidation state, there might be Brönsted acid sites to balance the negative charge on Ce like in zeolite. At higher temperate the unpaired electron in 4f shell of Ce might be transferred to adsorbed oxygen, but it was proved in ESR study. As there was no cracking of ethyl benzene, presence of Brönsted acid sites was ruled out. Neither oxygen adsorption nor desorption was noticed during the thermo gravimetric analysis. Hence oxygen adsorption was established to occur even during the calcinations. Based on the absence of desorption oxygen between 30 and 1000 C, as it was established that the suitability of catalyst for oxidation in the above temperature range. The oxidation of ethyl benzene over Ce-Silicalite-1 (50) gave high ethyl benzene conversion (>50%) and high selectivity to acetophenone (>90%). Hence for the oxidation of ethyl benzene in industries Ce-Silicalite-1 molecular sieves could be convenient, stable and eco friendly catalyst.

CHAPTER 6 SELECTIVE OXIDATION OF DIPHEYLMETHANE TO BENZOPHENONE

110 CHAPTER 6 SELECTIVE OXIDATION OF DIPHEYLMETHANE TO BENZOPHENONE 6.1 INTRODUCTION Oxidation of diphenylmethane (DPM) to benzophenone is an industrially important reaction as the product benzophenone