CHAPTER 6 SELECTIVE OXIDATION OF DIPHEYLMETHANE TO BENZOPHENONE

Transcription

1 110 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 is used as an intermediate in the synthesis of perfumes, photoinitiators, drugs and pharmaceuticals (Bauer et al 1990). Generally, benzophenone has been prepared by Friedel-Crafts acylation of benzene with benzoyl chloride in the presence of a Lewis acid catalyst (Khadilkar and Borkar 1997, Jacob et al 1999, Bezouhanova 2002, Patil et al 2002 and Tagawa et al 2004). This process is homogeneous, hazardous, corrosive, and the catalyst cannot be recovered easily for recycling. Further, the catalyst is deactivated and it can not be easily disposed off. Manganese(III) Schiff base complex was an efficient homogeneous catalyst for the oxidation of DPM with 30% H 2 O 2 in acetonitrile under ambient condition and it resulted in 69% benzophenone (Mardani and Golchoubian 2006). Since it is a homogeneous process, separation of the product from the reaction mixture is difficult. It is, therefore, of great practical interest to develop an efficient heterogeneous, easily separable, reusable and eco-friendly catalyst for the production of aromatic ketones. In contrast to benzoylation of benzene, oxidation of DPM over heterogeneous catalyst is an eco-friendly alternative for industrial production of benzophenone.

2 111 The survey of literature revealed several reports on the oxidation of DPM using different heterogeneous catalysts. Clark et al (1989) reported the first heterogeneous catalysed oxidation of DPM to benzophenone over alumina supported chromium and manganese. Shaabani et al (2002) reported solvent free oxidation of DPM over KMnO 4 supported montmorillonite K10 with good yield of benzophenone. But the process is hazardous. The selective oxidation of DPM over KMnO 4 impregnated on alumina under microwave irradiation in dry media was studied by Oussaid and Loupy (1997). This process is not only hazardous but also not suitable for long term use. Although sodium chlorite (NaClO 2 ) was effectively used as a selective catalyst for DPM oxidation either in combination with tert-butylhydroperoxide (TBHP) in stoichiometric quantity or with N-hydroxyphthalimide (Silvestre and Salvador 2007), this process is also hazardous. Mn-MCM-41 catalysts were also reported for DPM oxidation using air as the oxidant. However, the yield of benzophenone was low (Caps and Tsang 2000). Low DPM conversion (12-15%) and poor selectivity was observed over metal incorporated M-MCM-41 (where M is Ti, V and Cr) using H 2 O 2 as oxidant and acetonitrile as the solvent (Jha et al 2006). The direct oxidation of DPM to benzophenone was also studied over cobalt doped MCM-41 (Co/MCM-41) and cobalt doped mesoporous TiO 2 (Co/MTiO 2 ) (Tang et al 2008). Chang et al (2005) reported that cobalt doped MCM-41 (Co/MCM-41) was a highly selective catalyst for the direct oxidation of DPM to benzophenone employing H 2 O 2 as the oxidant in acetic acid. It was reported that MnO 4 exchanged Mg-Al-hydrotalcite showed good selectivity in the oxidation of DPM (Choudhary et al 2004). Kishore and Rodrigues (2009) carried out liquid phase oxidation of DPM over ternary hydrotalcites with TBHP as oxidant using different solvents, and they reported 95% DPM conversion with 100% selectivity to benzophenone. However, TBHP is a sacrificial and hazardous oxidant. Thus the literature

3 112 revealed the use of solvents or hazardous materials for the oxidation of DPM. Impregnated ceria was examined for the oxidation of alkyl aromatics (De Klein et al 1986, Molander 1992). Though the catalyst is active, its sintering nature at high temperature is a drawback (Yu et al 2006). The problem of sintering can be circumvented by incorporation of cerium into the framework of porous molecular sieves. In addition, such framework incorporated catalysts can combine the high selective characteristics of homogeneous catalysts as well as the recovery and recyclability of heterogeneous catalysts. Hence the present study prompted the synthesis of AlPO-5 and its catalytic evaluation in the oxidation of DPM in air. 6.2 CATALYTIC STUDIES Vapour phase oxidation of diphenylmethane (DPM) was studied over AlPO-5(25), AlPO-5(50), AlPO-5(75), AlPO-5(100) and AlPO-5(125) catalysts between 250 and 325 ºC. The reaction parameters were optimised in order to get maximum conversion and high selectivity to benzophenone. The effects of temperature, feed rate, air flow rate and time on stream on conversion and selectivity of products are discussed in the following sections Effect of Temperature The vapour phase oxidation of diphenylmethane (DPM) over AlPO-5(25) was carried out at 250, 275, 300 and 325 ºC with an air flow rate of 7 mlmin -1 and feed rate 2 mlh -1. Since air oxidation of DPM occurred significantly at 350 ºC, the catalytic oxidation was studied only between 250 and 325 ºC. The product analysis confirmed benzophenone as the major product and diphenylmethanol as the only minor product. The results of DPM conversion and product selectivity are shown in Figures 6.1 and 6.2 respectively.

4 DPM conversion (%) AlPO-5(25) AlPO-5(50) AlPO-5(75) AlPO-5(100) AlPO-5(125) Reaction temperature ( C) Figure 6.1 Effect of temperature on DPM conversion Benzophenone selectivity (%) AlPO-5(25) AlPO-5(50) AlPO-5(75) AlPO-5(100) AlPO-5(125) Reaction temperature ( C) Figure 6.2 Effect of temperature on benzophenone selectivity The conversion increased with increase in temperature. Since benzophenone was the major product, its precursor, viz., diphenylmethanol might be rapidly oxidised to the final product as soon as it was formed. The sequence of reactions in the oxidation is illustrated in Scheme 6.1.

5 O2 O2 3+ CH + O O H 4+ OH +. O O Fast -H2O HO C + OH 4+ Benzophenone Scheme 6.1 Plausible pathway for the oxidation of diphenylmethane Oxygen is chemisorbed on 3+ sites of AlPO-5 molecular sieve. When DPM passes close to the chemisorbed oxygen on 3+ sites in AlPO-5, the distant oxygen abstracts a hydrogen from the CH 2 group of DPM to form diphenylmethyl radical. The metal hydroperoxide rapidly transfers its OH group to diphenylmethyl radical to form diphenylmethanol. The resulting metal oxy radical rapidly abstracts the hydrogen of diphenylmethanol to form diphenylmethanol radical, and the latter radical rapidly releases a hydrogen atom to form benzophenone. The metal hydroxy group and the hydrogen atom combine to form water. Hence this oxidation is a selective one. The selectivity to benzophenone was above 99% and it remained the same with increase in temperature. The results of oxidation over AlPO-5(50, 75, 100 and 125) also exhibited nearly similar trend as that of AlPO-5(25). But over these catalysts, the conversion was lower than that of AlPO-5(25) at each

6 115 temperature. Hence, the density of active sites might be the main parameter to control conversion. The order of catalytic activity followed the order of cerium content in the catalysts. All other catalysts showed high selectivity to benzophenone as that of AlPO-5(25). Although the cerium content in these catalysts was less than AlPO-5(25), the same selectivity to benzophenone was confirmed. This revealed the sequence of oxidation of DPM to diphenylmethanol and then to benzophenone on the same chemisorbed oxygen molecule. Based on the DPM conversion and product selectivity, AlPO-5(25) was proved to be more active than AlPO-5(50, 75, 100 and 125) catalysts. The optimum temperature for maximum conversion with high selectivity was found to be 325 ºC Effect of Feed Rate The results of effect of feed rate on conversion and product selectivity are presented in Table 6.1. The conversion decreased with increase in feed rate. But the selectivity to benzophenone was above 90% irrespective of the feed rate. Hence, 2 mlh -1 was chosen as the better one than the others. The same selectivity to benzophenone irrespective of different feed rate established the oxidation of diphenylmethanol on the same chemisorbed oxygen from which it was formed. Hence slight increase in feed rate might not have significant influence on the oxidation of DPM to benzophenone on the same oxygen molecule. As the air flow rate is the main parameter to control oxidation, the feed rate might not have significant influence on the selectivity of the products. The decrease in conversion with increase in feed rate might be due to increase in concentration of the feed around the active sites by which most of the feed might escape without being oxidised. In other words, oxidation of diphenylmethanol to benzophenone is a fast process.

7 116 Table 6.1 Effect of feed rate on DPM conversion and product selectivity Feed rate (mlh -1 ) WHSV (h -1 ) DPM conversion (%) Product selectivity (%) Benzophenone Diphenylmethanol Reaction condition: Catalyst AlPO-5(25); catalyst weight 0.5 g; temperature 325 ºC; air flow rate 6 mlmin -1 The reaction was also carried out by using freshly prepared ceria (O 2 ) catalyst under the same conditions. O 2 was prepared by calcination of cerium nitrate hexahydrate at 550 ºC in air for 12 h (Chapter 3, section 3.1). The oxidation of DPM over O 2 with air as oxidant revealed almost negligible conversion. Hence, the oxidation of DPM was established due to 3+ sites present in AlPO-5(25). As this process required oxygen for oxidation, 3+ could adsorb and activate oxygen whereas 4+ is incapable of adsorption of oxygen. Though O 2 was proved absent in the synthesised catalysts, even if present, it might be incapable of activating oxygen for the oxidation of DPM Effect of Air flow Rate The effect of air flow rate on conversion and product selectivity was studied over AlPO-5 molecular sieves and the results are presented in Table 6.2. The conversion of DPM increased when the flow rate of air increased from 5 to 6 mlmin -1 and decreased thereafter. Hence, 6 mlmin -1 was chosen as the optimum one. But the selectivity to benzophenone remained the same irrespective of the air flow rate, which confirmed the use of both oxygen atoms of the same chemisorbed molecule for the oxidation as explained above. It was also verified from the study that the rate of oxidation of DPM to benzophenone might be faster than air flow rate.

8 117 Table 6.2 Effect of air flow rate on DPM conversion and product Air flow rate (mlmin -1 ) selectivity DPM conversion (%) Product selectivity (%) Benzophenone Diphenylmethanol Reaction condition: Catalyst AlPO-5(25); catalyst weight 0.5 g; temperature 325 ºC; feed rate 2 mlh Effect of Time on Stream The effect of time on stream on DPM conversion and product selectivity was tested for 6 h over AlPO-5(25) at 325 ºC with an air flow rate of 6 mlmin -1 and the results are presented in Table 6.3. The conversion and product selectivity remained the same throughout the time on stream, thus confirmed the absence of leaching of 3+ sites from the framework. Further, the sequence of reactions might be same during the entire period of time on stream. Since selectivity to benzophenone remained the same, the same mechanism of oxidation might operate throughout the time on stream. Table 6.3 Effect of time on stream on DPM conversion and benzophenone selectivity Time DPM conversion Product selectivity (%) (h) (%) Benzophenone Diphenylmethanol Reaction condition: Catalyst AlPO-5(25); catalyst weight 0.5 g; ; temperature 325 ºC; feed rate 2 mlh -1 ; air flow rate 6 mlmin -1

9 118 This study also proved the stability of catalysts 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 spent catalyst was characterised by XRD after calcination. The pattern showed similar features as that of the fresh catalyst and hence leaching of 3+ from the framework was ruled out. The catalyst was activated after use at 500 ºC and then reused for the reaction. It was found that the catalyst was not only stable but also yielded nearly the same conversion and selectivity for three cycles. 6.3 CONCLUSION The synthesis of AlPO-5 molecular sieves in fluoride medium was successful. The characterisation revealed the absence of non-framework ceria in all the catalysts. The framework incorporation of cerium was established by XRD and DRS-UV-vis studies. Oxidation via activation of oxygen adsorption was evident from this study. The ESR study proved the absence of oxidation of 3+ to 4+ at room temperature, although adsorption of oxygen on 3+ was established. Single-site AlPO-5 molecular sieves were effective catalyst for selective oxidation of diphenylmethane to benzophenone. Though active cerium site isolation is a requirement for selective oxidation, the magnetic field of cerium sites and free radicals produced during oxidation were also suggested to play a major role in the selective oxidation. In addition, the free rotation across the phenyl and methyl carbon bond was also suggested as a key factor for selective oxidation of diphenylmethane. The time on stream study revealed the stability and reusability of the catalysts.