CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 GENERAL Catalysts are indispensable in chemical industries. They increase reaction rate and decrease activation energy. They play a major role in efficient utilization of raw materials, eliminating wastes and avoiding use of hazardous solvents and reagents. So, they reduce E-factor (defined as the mass ratio of waste to desired product), and enhance atom efficiency (defined as the ratio of molecular weight of desired product to sum of the molecular weight of all the products). All these features of catalysts are made realistic, as they replace hazardous catalysts such as mineral acids (H 2 SO 4, H 3 PO 4 etc.), Lewis acids (AlCl 3, BCl 3 etc.) and bases (KOH, NaOH) with recyclable solid acids and bases. Solid acids, such as zeolites, acidic clays and related materials, have many advantages in this respect (Barton et al 1979, Vazquez et al 2000). They are heterogeneous, non-corrosive, recoverable and easy to handle. Solid acid catalysts are, in principle, applicable to a plethora of acid-promoted processes in organic synthesis. Solid acids are crucial to the petrochemical industry. Catalytic reforming, cracking, alkylation, isomerisation and conversion of methanol into olefins are some of the most important processes aided by solid acid catalysts such as zeolites.

2 2 One of the major problems related to the use of heterogeneous catalysts is the loss of catalyst activity and/or selectivity with time on stream. The changes produced by catalyst deactivation play a significant role, although disadvantageous for a large number of important industrial processes. There is no scientific approach which allows catalyst deactivation to be entirely eliminated (Petrov and Kumbilieva 2006). Hughes (1984) reported catalyst deactivation due to (i) poisoning of catalysts by impurities present in the reaction mixture, (ii) sintering of the catalyst, and (iii) strong adsorption of initial reactants, products or intermediates that arise in the succession of elementary steps involved in the reaction mechanism. Heterogeneous catalysis is a key phenomenon in many fields of modern technology. Many heterogeneous catalysts have been employed in the treatment of environmental pollution (Ertl et al 1999), production of fine chemicals and energy storage, and conversion (Ertl et al 1997, Thomas and Thomas 1997). Many of the heterogeneous catalysts are porous. According to the IUPAC definition, porous inorganic materials can be grouped into three major classes based on their pore diameter (Ramaswamy 2000) (Table 1.1).

3 3 Table 1.1 Classification of porous materials Microporous Mesoporous Macroporous Materials Pore Structure Size (nm) Materials Pore Structure Size (nm) Materials Size (nm) Zeolitas Faujasites (X, Y),, MOR, ZSM- 5etc. Small Medium MCM-41 MCM-48 SBA-15 SBA-1 1 and 3 dimen Porous gel Porous glass > 50 Silica moleular - sieves Silicalite Metallo silicates Medium Large Phosphate based - molecular sieves AlPO, SAPO Medium Large In the following sections zeolites, their classifications and catalytic applications are discussed. This is followed by an elaborate discussion on ZSM ZEOLITES Zeolites are hydrated, micro porous, crystalline aluminosilicates built with silicon and aluminium tetrahedra. As silicon is tetravalent and aluminium is trivalent, the framework carries a negative charge balanced by alkali cations or protons. Their general formula is M x/n ((AlO 2 ) x (SiO 2 ) y )zh 2 O where M is an extra-framework cation that balances the anionic charge of the framework. Since 1960 zeolites have been applied in an increasing number of catalytic processes. The interstitial spaces or channels formed by the crystalline network enable zeolites to be used as molecular sieves in separation processes. Their high surface area, provided by their open structure, gives them very good sorption properties. Their open structure

4 4 permits in turn accommodating a wide variety of cations such as Na +, K +, Ca 2+, Mg 2+ and other ions to balance the framework negative charge. These ions are loosely held and so can be readily exchanged for others in a contact solution. If such cations are exchanged by protons, then the zeolites become solid acids with catalytic properties. It is because of these properties zeolites have become an important group of inorganic materials finding extensive applications in industries as sorbents, ion-exchangers and catalysts (Corma 1995). Up to now, 136 different structures have been reported, of which about 40 are naturally occurring, and the rest has been synthesized in laboratory. The naturally occurring zeolites are of limited value because (i) they always contain undesired impurity phase, (ii) their chemical composition varies from one deposit to another and even from one stratum to another in the same deposit, and (iii) nature does not optimize their properties for catalytic applications. It was only with the advent of synthetic zeolites that this class of porous materials began to play a key role in catalysis. A land mark event was the introduction of synthetic faujasites (zeolites X and Y) on an industrial scale in fluid catalytic cracking of heavy petroleum distillates, which is the most important chemical process worldwide. In the period after 1962, zeolite catalysts rapidly conquered additional processes in the fields of petroleum refining and basic petrochemistry. The most important processes are hydrocracking of heavy petroleum distillates (Scherzer and Gruia 1996), octane number enhancement of light gasoline by isomerisation, disproportionation of toluene into benzene and xylenes, and isomerisation of xylenes. It is well known that zeolites are usually used as solid acid catalysts (Corma 1995). In the beginning of 1990s, zeolites were used as base catalysts in their ion-exchanged and impregnated forms (Weitkamp et al 2001).

5 5 Altogether catalysis is the single most important application of zeolites in terms of financial market size with an estimated market value around 1 billion US dollars per year (Naber et al 1994). The main advantages of zeolites compared to conventional solid acids include the following. i) Well-defined crystalline structures differing in channel diameters, geometry and dimensionality. ii) High surface area. iii) Precisely defined inner void volume. iv) Ability to absorb and transform molecules in the inner volume. v) Isomorphous substitution of some trivalent cations into the silicate framework enable to tuning the strength and concentration of the acid sites. vi) Shape selectivity, given by the ratio of the kinetic diameters of the reactants, intermediates and products to the dimensions of the channels. vii) Environmental tolerance. viii) Thermal and hydrothermal stability Classification of Zeolites Zeolites are classified on the basis of their (i) chemical composition based on SiO 2 /Al 2 O 3 ratio (Table 1.2), and (ii) pore diameter (Table 1.3).

6 6 Table 1.2 Classification of zeolites based on the chemical composition Low Silica Si/Al = 2 to 3 Classification of zeolites Medium silica Si/Al = 4 to 10 A, X type zeolites Y, L, MOR, Omega type zeolites High silica Si/Al > 10 ZSM-5, ZSM-11 type zeolites Table 1.3 Classification of zeolites based on the pore opening Type of molecular size Small pore Structure type code Largest pore dimension (nm) Number of Si/Al tetrahedral in pore cross-section Zeolite A LTA 0.41 (circular) 8 Erionite ERI 0.36 x Medium pore ZSM-5 MFI 0.53 x ZSM-11 MEL 0.53 x Large pore Zeolite X and Y (faujasite) FAU 0.74 (circular) 12 Mordenite MOR 0.65 x Acidity in Zeolite The zeolites carry weak, medium, and strong acid sites and each set of acid sites requires protons of particular acidity. Reactions catalyzed by weak acid sites are also catalysed by medium and strong acid sites. But those reactions which are catalysed exclusively by strong acid sites are not

7 7 catalyzed by weak acid sites. Reactions catalysed by weak, medium and strong acid sites of zeolites are briefly outlined below. Though the defective silanol groups in zeolites are believe to be catalytically inactive. They were reported in vapour phase beckmann rearrangement of cylcohexonone oxime to caprolactum (Sato et al 1993). Formation of acetals by the reaction of carbonyl compound and alcohols is catalysed by weak acid sites of acid zeolites (Corma et al 1990). Conversion of aldehyldes to the corresponding 1, 1-diacetates, was catalyzed by medium acid sites of ZSM-5 (Joshi et al 1993). Skeletal isomerisation of n alkanes is also promoted by medium acid sites of zeolites (Gruver et al 1996). Electrophilic additions of alcohols or water to olefin are catalyzed by strong acid sites. Gas phase synthesis of methyl tertiary butyl ether was reported to be catalysed by strong acid sites of zeolites (Kogelvauer et al 1994). Transformation of -pinene, limonene, and -terpene was catalysed by strong acid sites of USY, H-mordenite and ZSM-5 (Chatteriee et al 1995). Selectivity of isomerisation of 1 butene to isobutene over MCM-22 was strongly improved by strong acid sites of zeolites (Asenisi et al 1994). Isomerisation of secondary butyl benzene, cymene, and other alkyl arens were also due to strong acid sites of zeolites (Artizzir et al 1994 and Ceika et al 1994) Applications of Zeolites As zeolites have inbuilt pore system, approaching molecular size, they can discriminate molecules based on size. This leads to the term molecular sieve coined by McBain (1932). The molecular sieving property of zeolites is used in industrial separation.

8 8 Zeolites can perform filtering, odor removal and gas absorption tasks. The most well known use for zeolites is in water softeners. Calcium in water can cause it to be hard and capable of forming scum and other problems. Zeolites charged with much less damaging sodium ions can allow hard water to pass through its structure and exchange calcium for sodium ions. This process is reversible. In a similar way zeolites can adsorb ions and molecules and thus act as a filter for odor control, toxin removal and as a chemical sieve. Zeolite catalysts has widely used in the processing of petroleum and in the production of various petrochemicals. Reactions such as cracking, hydrocracking, alkylation, dealkylation, trans-alkylation, isomerisation, polymerisation, addition, disproportionation and other acid catalyzed reactions can be performed with the aid of zeolites. Both natural and synthetic zeolites are known to be active for reactions of these kinds. 1.3 ZSM-5 ZEOLITE Among the zeolites, the medium pore ZSM-5 is important as it is extensively used in industries. The following section is a concise account of structure of ZSM-5, synthesis and catalytic applications Structure of ZSM-5 ZSM-5 (Zeolite Sqcony Mobil-5) was first made by Argauer and Landolt (1972). It has a type of zeolite built from the pentasil unit. ZSM-5 has two types of channels, both of which have 10-membered ring openings (Figure 1.1). One channel system is sinusoidal and has a nearly circular ( Å) cross section. The other channel system has elliptical openings ( Å). These are straight and perpendicular to the first system. The cavity at the intersection of the channels is about 9 Å in diameter (Figure 1.1).

9 9 It is the only zeolite with high silica to alumina ratio (Scherzer and Gruia 1996). Sinusoidal (5.4 x 5.6 Å) Straight Straight ( Å) (5.2 x 5.8 Å) a) Side view of channel structure b) Top view of channels Figure 1.1 Structure of ZSM-5 It has been suggested (D de ek et al 2000) that aluminium tetrahedra in ZSM-5 zeolites are predominantly (70-80%) located in the channel intersections of diameter 9 Å. Generally the synthesis of ZSM-5 is carried out with tetrapropylammonium compounds. The corresponding cations are located in the channel intersection and thus after their decomposition protonic sites are formed in such places. It was reported (D de ek et al 2002) that synthesis of ZSM-5 without tetrapropylammonium cations with only sodium ions as counter ions also led to similar distribution of aluminium. Therefore, it can be assumed that the active sites are situated predominantly in the channel intersection in ZSM-5. So, the major portion of catalytic transformations in ZSM-5 occurs mainly in the channel intersections. 1.4 SYNTHESIS OF ZSM-5 In most cases, the synthesis of ZSM-5 is achieved by hydrothermal process. The crystallisation process and final products are sensitively

10 10 dependent on the composition of source materials, temperature, time, templates and other initial conditions of the reaction system Template Free Synthesis of ZSM-5 Grose and Flanigen (1981) first reported the synthesis of organic-free ZSM-5 with and without seeds after h of reaction at 200 ºC. Shiralkar and Clearfield (1989) investigated the compositional constraints in the synthesis of pentasil molecular sieves, including aluminosilicate zeolites, without organic templates from the gel composition of a SiO 2 : Al 2 O 3 : bna 2 O : 1500 H 2 O, in the temperature range C, where a = 20, b = When a < 30, the system crystallized into mordenite as a major phase along with traces of ZSM-5. At SiO 2 /Al 2 O 3 60, the contribution from -quartz increased coexisting with ZSM-5 and mordenite. Some hydrous silicon (IV) oxide was also present as an impurity phase. Almost 100% pure -quartz was obtained, when the gel contained no added alumina. At a composition of a = 40 and b = , pure ZSM-5 crystallized with the occlusion of Na + in excess of charge compensation in the zeolite framework. As the Na 2 O content was increased in the gel, ZSM-5 product was lower in framework silica-alumina ratio probably due to greater solubility of silica. Schwieger et al (1989) also reported the synthesis of organic free ZSM-5 using narrow SiO 2 /Al 2 O 3 molar ratio of and reaction time of h. Li et al (2003) synthesized template-free nano sized ZSM-5 seeds from commercially available ZSM-5 powder. By use of these seeds, thin and hydrophilic ZSM-5 zeolite membranes were prepared on the outer surface of a porous -alumina tube in a clear solution free from organic template. The membranes showed high thermal stability to withstand pretreatment at 400 C. Kim et al (2004) synthesized MFI type high silica ZSM-5 with well-defined crystal morphology in the absence of organic template within a

11 11 short reaction time under stirring at 200 rpm and 190 C. The results clearly showed the structure directing as well as a charge balancing role of Na +. Nanosized ZSM-5 zeolite was successfully synthesized by Cheng et al (2008) at 180 C for 24 h after stirring for 5 h and aging of 24 h in the synthesis mixture with the molar composition of 12Na 2 O : 100SiO 2 : 2Al 2 O 3 : 2500H 2 O. Na + ion was reported to play the structure directing role in place of organic template. Synthesis temperature, time, concentration, agitation and aging were important to prepare nano sized ZSM-5 zeolite. ZSM-5 was synthesized by template-free method using sodium silicate as a silica source and the content of Na 2 O in mother liquid was controlled by H 2 SO 4 (Kang et al 2009). The effect of Na 2 SO 4 generated by added H 2 SO 4, hydrothermal crystallisation was carried out with two types of mother liquids having low and high Na 2 SO 4 content at 170 C. High crystalline, isometrical shaped, uniform 1-2 m sized ZSM-5 with relative crystallinity around 100% could be obtained successfully from the mother liquids having the composition (11-21)Na 2 O : ( ) Na 2 SO 4 : 100SiO 2 : 2Al 2 O 3 : 4000H 2 O Template Synthesis of ZSM-5 The properties of ZSM-5 zeolite crystallized from the reaction mixture of 10Na 2 O: Al 2 O 3 : 60SiO 2 : 3000H 2 O:10 Organic (Organic = tetrapropylammonium bromide, hexane-1,6-diol, hexane-1,6-diamine or piperazine) at 150 C were compared with those of organic free ZSM-5 by Araya and Lowe (1986). It was shown that organic species had a marked effect on the composition and excess cation content of the product. Ali et al (2003) prepared the hydrogel from sodium silicate and aluminium sulphate with silicon-to-aluminium molar ratios in the range and transformed it into ZSM-5 zeolite using tetrapropylammonium bromide template at ph 10. The crystallisation of gel having Si/Al molar ratio of 10 was achieved with difficulty and represented lower limit of Si/Al ratio, while the gel with Si/Al

12 12 molar ratio of 9 could not be crystallized even after several attempts. The catalytic activity of synthesized zeolites was evaluated at 70, 80, 90 and 100 C in a flow reactor. For the reaction of methanol and isobutene to produce methyl t- butyl ether, the activity increased with decreasing Si/Al molar ratio of the zeolite and with increasing reaction temperature. These zeolites showed methyl t-butyl ether selectivity in the range of 90-98% which decreased with increase in reaction temperature. Nayak and Moffat (1990) synthesized ZSM-5 zeolites of various silicon-to-aluminium (Si/Al) ratios with tetrapropylammonium bromide as templating agent and one sample (Si/Al = 36) prepared by use of ammonia in place of tetrapropylammonium bromide. Cracking activity of 1-hexene, 1-heptene and 1-octene was tested. The activities and selectivities were found to be similar for the catalysts prepared with the two templating agents. In addition, the cracking activity of zeolite was shown decrease with increasing content of Si/Al ratio approximately The ease of alkenes cracking was found to be in the order: octenes > heptenes > hexenes. The cracking of alkenes resulted mainly in the production of C 3, C 4 and C 5 alkenes. Fouad et al (2006) synthesized nanosized ZSM-5 zeolite from precursor mixtures containing different templates, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide and tetrabutylammonium hydroxide and different template/silica mole ratios viz., 0.215, 0.322, 0.43 and Physico-chemical investigations showed that the products obtained by different type of templates and different template/silica mole ratios were ZSM-5 phase. The as-synthesised ZSM-5 sample prepared by using tetrapropyl ammonium hydroxide template and template/silica mole ratio at 230 C for 45 h had the highest crystallinity. It was found that the average crystallite size increased in the

13 13 following order: tetramethyl < tetrabutyl < tetraethyl < tetrapropyl, and surface area increased in the following order: TMAOH < TEAOH < TBAOH < TPAOH Synthesis of ZSM-5 from Environmental Waste ZSM-5 zeolite was also synthesised from lignite fly ash. Chareonpanich et al (2004) synthesised ZSM-5 with varying SiO 2 /Al 2 O 3 mole ratio (20-100) in the presence of tetrapropylammonium bromide. Their results revealed that SiO 2 /Al 2 O 3 mole ratio in the raw fly ash is too low to synthesise ZSM-5 zeolite (SiO 2 /Al 2 O 3 mole ratio = 2.8) while a maximum yield of 43 wt% was obtained with SiO 2 /Al 2 O 3 mole ratio of 40. Rice husk (RH) consists of about 40% cellulose, 30% lignin group and 20% silica. When RH is burnt, approximately one fifth of the original weight is obtained as a by-product. Rice husk ash (RHA) contains over 80% silica. Hence, RH is an excellent source of silica (Vempati et al 1995, Chandrasekhar et al 2003). A simple synthetic route was demonstrated by the efficient production of ZSM-5 zeolite by Kordatos et al (2008) Synthesis of ZSM-5 in Fluoride Medium Though ZSM-5 has been synthesized with different conditions and sources of silica, in order to further extend its contribution to catalysis, new synthesis routes that offer controlled morphology, surface acidity, crystallinity, large particle size and high surface areas are always important. An unprecedented discovery in zeolite synthesis is the replacement of hydroxide ion mineraliser by fluoride. The latter facilitates synthesis of zeolites even in acidic medium (Flanigen and Patton 1978). Synthesis of microporous aluminosilicates, aluminophosphates and gallophosphates in

14 14 fluoride medium was reported by Guth et al (1984), Guth et al (1989), Guth et al (1992) and Kessler et al (1994). Several advantages have been reported for fluoride mediated synthesis. Fluoride medium construct crystals of large size with fewer defects and high surface area (Guth et al 1986, Mostowicz et al 1994, Nigro et al 1997, Schüth and Schmidt 2002). Fluoride favours fewer metastable phases that facilitate formation of a desired zeolite structure (Axon and Klinowski 1990, Guth et al 1992, Camblor et al 1999). Fluoride aids direct formation of NH 4 ZSM-5 in acidic medium, which can be directly calcined to H-ZSM-5 without resorting to ion-exchange of Na-form to get H-form. But there are disadvantages like occlusion of fluoride inside the cages (George and Catlow 1995, Camblor et al 1999, Fyfe et al 2001) which reduces the acidity of proton (Louis and Kiwi-Minsker 2004) and reduced transport of aluminium from the gel to the framework due to formation of AlF x species (Gougeon et al 2001). Aiello et al (1999) studied the influence of NH + 4, Na +, K + and Cs + on the transport of aluminium from the gel to the framework during the synthesis of H-ZSM-5 in fluoride medium. It was reported that potassium as the most effective one for aluminium transport and ammonium the least effective one. Even though the alkali ions facilitate aluminium transport to framework during synthesis, it retains alkali ions as charge-compensating ion for framework negative charge. It is a major disadvantage, as it demands the hectic ion-exchange of the product zeolite after synthesis just like synthesis in alkaline medium. As reduced aluminium transport is due to complexation of aluminium by fluoride ions, incorporation of an additional co-complexant to aluminium to compete with fluoride in the gel medium was thought to be a cognizant solution to solve such problem in comparison to the use of alkali ions. In the present investigation phosphate was used as the co-complexant, and its property was verified in our lab.

15 CATALYTIC APPLICATIONS OF ZSM-5 ZSM-5 is one of the most important zeolites due to its application in many fields like separation of gases or liquids (Dyer 1988, van Bekkum et al 1994), synthesis of fine chemicals (Hölderich and van Bekkum 1991), one step phenol production (Hölderich and van Bekkum 1991, Häfele et al 1997, Louis et al 2001), in space research (Ghobarkar et al 1999) and essentially as a solid acid catalyst (Corma 1995, Mota et al 1997, Chen 2001). It has gained increasing importance as high potential catalyst in a number of commercially important processes (Guth et al 1986). It showed high resistance to deactivation by coke deposition than other commercial zeolites, which has been related to absence of large cavities in the pore structure and its low concentration of acid sites (Mostowicz et al 1994, Nigro et al 1997). A brief account of catalytic applications is delineated below. 1.6 SHAPE SELECTIVE CATALYSIS Small, uniform intracrystalline cavities and pores are present in zeolite catalysts. If the overwhelming majority of the catalytic sites are confined within this pore structure and if the pores are small, the fate of reactant molecules and the probability of forming product molecules are determined mostly by molecular dimension and configuration. Only molecules whose dimensions are less than the critical size can enter the pores, have access to internal catalytic sites and react there. Furthermore, only molecules which can leave, appear in the final product. Bulkier molecules will react or bulkier products will form only on the relatively few catalytic sites on the external surface of the zeolite crystals. Shape selective catalysis was first reported by Weisz and Frilette (1960) and subsequently many applications appeared in the literature. Csicsery (1984) reviewed shape

16 16 selective catalysis in zeolites, and Derouane (1980) discussed shape selective catalysis with ZSM-5 zeolites. Shape selective alkylation of naphthalene to methylnaphthalene on ZSM-5 zeolite catalysts was reported by Fraenkel et al (1986). 1.7 TYPES OF SHAPE SELECTIVITY Reactant selectivity is observed when only one portion of the reactant molecules can pass through the catalyst pores. The remaining molecules are too large to diffuse through the pores. The cage or window effect is a special case of reactant selectivity in which certain molecules react at a rate different from most other molecules because their length matches the length of a sieve cavity. Product selectivity occurs when, among all these product species formed within the pores, only those with the proper dimensions can diffuse out and appear as observed products. Bulky products, if formed, are either converted to less bulky molecules or eventually deactivate the catalyst by blocking the pores. Restricted transition state selectivity occurs when certain reactions are prevented because the corresponding transition state would require more space than available in the cavities, and reactions requiring smaller transition states proceed unhindered. Figure 1.2 represents the three types of selectivities.

17 Figure 1.2 Schematic representation of the types of shape selectivity 17

18 SHAPE SELECTIVE CATALYTIC REACTIONS Particle size and crystallinity of the material are important for shape selective catalytic reaction. The uniform and narrow particle size distribution have much significant role in suppress further isomerisation and enhance the diffusion rate of selective desire product. In the following sub sections, fluoride media synthesized of ZSM-5 zeolites and Ce-Silicalite-1 catalysts crystallinity exclusive for p-selective importance of catalytic reactions studies are illustrated Isopropylation of Toluene Alkylation of toluene with isopropyl alcohol to cymenes is an industrially important reaction. Cymenes, especially p- and m- iosomers, are important starting materials for the production of a range of intermediates and products such as cresols, fragrances, pharmaceuticals, herdicides, heat transfer media etc (Ito et al 1973, Welstead Jr 1978, Derfer et al 1978). For p-selective isopropylation over ZSM-5 zeolite has been mainly focused because of its unique properties of medium pore size. A verity of Friedel- Crafts catalysts, such as FeCl 3, AlCl 3 and BF 3 were used for toluene isopropylation. However over these catalysts the proportion of undesired o-isomers was up to 5% and multi alkylation cannot be prevented. As these catalysts are homogeneous, corrosive and demand waste disposal, solid acid catalysts such as zeolites could be advantageous. The most preferred isopropylation requires low o-cymene content, since it is difficult to oxidize and inhibit the oxidation of other isomers. p-selective isopropylation of toluene is important and can be achieved with ZSM-5, as its channel diameters are closed to the kinetic diameters of aromatic molecules. Wichterlová et al (1994, 1996) reported isopropylation of toluene over Y, Beta, Mordinite, ZSM-12 and medium pore MFI silicallites, aluminium and iron in the framework. Parikh and Subrahmaniyum (1992) reported isopropylation of toluene over ZSM-5

19 19 mordinite, Y and Beta. Valtierra et al (1997) reported isopropylation of toluene over MCM-41/ -alumina catalyst, and found that the isopropyl toluene fraction mainly contain para isomer. It was accounted in terms of few acid sides in MCM-41 which allowed easy diffusion of p-cymene. Perego and ingallina reported isopropylation of toluene using propylene over microporous and mesopores catalyst in liquid phase. -Zeolite was reported better than others. Savida and Pandurangan (2004) reported isopropylation of toluene over zeolites and MCM-41 molecular sieves. Isopropyl acetate was used as the alkylating agent. The conditions were optimized for high selectivity to p-cymene. ZSM-5 showed high p-cymene selectivity at low contact times. In aluminum, gallium and iron incorporated MFI structures p-cymene selectivity increased with decrease in acidity. Ghiaci et al (2006) reported isopropylation of toluene over phosphoric acid modified ZSM-5. They showed enhanced selectivity to p-product. The coke deposits at the pore mouth and/or within the catalyst reduced the effective channel dimensions resulting of tighter fit in molecules and final discrimination between the cymene isomers. Barman et al (2005) was studied alkylation of toluene with isopropyl alcohol over Ce-exchanged Na x zeolite. The product mixture contained both p- and m-isomers, but there was no o-cymene. Antony raj et al (2006) reported, toluene isopropylation with isopropyl alcohol over AEL and AFI molecular sieves. The effect of cocking on the catalytic activity, catalyst pore size and product selectivity was discussed. Isopropylation of toluene was studied over microporous and meso porous solid acids, and factors such as pore size, acid strength, density of acid sites and coke depositions were considered for p-selective alkylation. Among the catalysts the medium pore zeolite ZSM-5 could be advantageous for p-selective isopropylation of toluene, as it palaces high constrain to

20 20 diffusion for m- and o-isomers which could be formed in the channel intersections. Though pore size reduction of ZSM-5 zeolite can favor p-selective alkylation, conversion could be significantly lower. Presence of external acid sites on it is an additional disadvantage to suppress p-selectivity. Coke deposition can favor p-selectivity but conversion is again suppressed. In this context, it could be advantageous, if ZSM-5 synthesized in fluoride medium is used for p-selective alkylation. As discussed above fluoride medium offers ZSM-5 with high crystallinity and large crystal size. Both factors have strong influence on p-selective alkylation. Hence in the present investigation it was planned to study isopropylation of toluene over ZSM-5 synthesized in fluoride medium. For p-selective isopropylation non porous catalysts, large pore zeolites and meso porous materials are not advantages, though they were used as catalysts. For p-selective alkylation medium pore ZSM-5 catalyst is the ultimate one for industrial convenience. In ZSM-5 catalyst synthesized in alkaline medium external acid sites are unavoidable. As these sites promote isomerisation of p-isopropyl to m-isopropyl toluene blocking of active sites is very much important. In contrast to such ZSM-5 catalysts, the one synthesized in fluoride medium is to be advantageous, as its crystallinity is very much higher than others. Since high crystallinity leads to minimum planting of external acid sites, it could be a convenient catalyst for p-selective isopropylation of toluene. An added advantage is the crystal size also large by which p-selectivity can be enhanced. Hence ZSM-5 synthesized in fluoride medium may not require pore size reduction for p-selective alkylation. In that case conversion over virgin ZSM-5 should be higher than those ZSM-5 with reduced pore size. So, in this study it was planned to investigate ethylation of benzene over ZSM-5 catalyst, synthesized in fluoride medium.

21 Disproportionation of Ethylbenzene Disproprotionation of ethyl benzene is commercially an important process for production of valuable diethyl benzene (DEB). Ethylbenzene disproportionation is accepted by the Catalysis Commission of the IZA as a standard reaction for acidity characterization (Vos et al 2002). Brönsted acid sites were seated for catalytic activity in disproprotionation of ethylbenzene (EB) and toluene (Benesi et al 1967, Wang et al 1972, Karge et al 1982, Karge et al 1983). DEB is used for the production of polyesters after oxidation (Benesi et al 1983), as a solvent (Swift et al 1995), photo developer (Colvin et al 1986), anti-oxidant etc. Divinyl benzene derived from DEB is an important precursor for cross linking agents in the production of polymers. p-deb is a desorbent for parex process of UOP (Swift et al 1995). Kaeding and Co workers (1981, 1985) was reported disproportionation of EB to p- DEB over boron and phosphorous modified ZSM-5 zeolite. The selectivity of p- isomers was close to 100 %. It was attributed to higher diffusivity of p- isomer than the others. Antony Raj et al (2007) studied disproportionation of ethylbenzene over Mn AlPO-5 molecular sieve. p-deb was the major product. The disproportionation of EB is an interesting model reaction, was studied by Karge et al (1982, 1983), Weitkamp et al (1986). Using this test reaction, the authors were able to obtain information about the number of active acid sites in zeolites (Karge et al 1982, 1983) and to discriminate between large (12-member ring) and medium-size (10-member ring) microporous zeolites (Weitkamp et al 1986). Mechanistic studies of alkyl group transfer in the alkyl benzene disproportionation have been firstly carried out in liquid acids prepared by a mixture of hydrogen fluoride and boron trifluoride (Mccaulay 1953). According to these investigations, the alkyl group maintained it configuration

22 22 while transferring from one ring to the other at room temperature via a transition state containing a partial bond between the alkyl group and second alkyl benzene. The experimental results showed that ethylbenzene was disproportionated to ca. 90% via this pathway at below the room temperature (Scheme 1.1) (Mccaulay 1953). However, the protonated alkylbenzene was split into a benzene molecule and an alkyl carbenium ion maximum within the temperatures between 0 and 66 C. This carbenium ion alkylated second alkyl benzene to complete the disproportionation (Santilli et al 1986) higher at 0 C. At the same time, some highly reactive alkyl carbenium ions reacted further by polymerization or hydride abstraction. The experiments demonstrated that the alkyl benzene disproportionation by this mechanism was accompanied by side-reactions leading to complex products (Mccaulay et al 1953). The migration of methyl groups in xylene was much slower than that of ethyl groups in EB (Lien el Al 1953). In a solution of excess HF-BF 3, no conversion of xylenes occurred at 60 C, but the EB almost completely disproportionated at 0 C (Lien el Al 1953). CH 3 CH 3 F B F F + at 0 o C 90 % Conversion + Scheme 1.1 Schematic representation of disproportionation of ethylbenzene over BF 3 catalyst H 3 C For p-selective disproportionation of ethyl benzene ZSM-5 synthesized in alkaline medium might not be much advantageous, as it promotes isomerisation with its external acid sites. Such acid sites are unavoidable as crystallinity was not high. Though disproprotionation was studied over large pore zeolites and zeotype molecular sieves, such materials

23 23 might not be good for p-selective disproportionation due to large pore size. p-selective disproportionation could be run over pore size optimized ZSM-5, but conversion could be drastically reduced. In this context ZSM-5 synthesized in fluoride medium could be the ultimate choice for p-selective disproportionation without isomerisation to m-product. Fluoride medium offers ZSM-5 with high crystallinity, hence external acids that promote isomerisation of p-product are reduced. It also offers large crystal size, hence disproportionation might be highly p-selective. It also avoids pore size optimization. Hence in the present study it was one of the main objectives to synthesis ZSM-5 in fluoride medium for disproportionation of EB Ethylation of Benzene Ethyl benzene is an important raw material for the manufacture of styrene which is the monomer poly styrene. It is also used as a solvent and for the production of dyes (Odedario et al 2010). Conventionally ethylbenzene is produced by ethylation of benzene with ethylene, using homogeneous catalysts such as aluminium chloride or phosphoric acid. These catalysts are hazarder s corrosive and demand waste disposal (Weikamp et al 1999, Perego et al 2002). In this context ethylation of benzene with ethanol over solid acid catalyst is advantageous. The reaction was attempted over ZSM-5, and it is now in commercial practice (Mobil Badger process) for production of ethylbenzene (Sridevi et al 2001). The use of ZSM-5 offers an environmentally friendly route to ethylation of benzene and the possibility of achieving superior product selectivity through pore size control. Chandawar et al (1982) reported a significant improvement in ethyl benzene selectivity by modification of ZSM-5 with phosphorous and boron. Levesque and Dao (1989) also studied the same reaction using stream treated ZSM-5 zeolite. They reported that ethyl benzene selectivity increased at high benzene to ethanol molar ratio. For ethylation of benzene ethylene and

24 24 ethanol have been largely used, but an additional reagent diethyl carbonate was also used as an novel reagent for ethylation (Wei at al 2003, Zhang et al 2005, 2007). Li et al reported ethylation of benzene using diethyl carbonate over ZSM-5 with high ethylbenzene selectivity Gao et al (2009). Gao et al (2009) studied the same reaction over zinc incorporated ZSM-5. There was an increase in ethyl benzene selectivity due to small crystal size. Raj et al (2006) studied the same reaction over Mn AlPO-5 catalyst using the molar ratio (1:1). Ethyl benzene selectivity was high at 400 C. In the present investigation ethylation of benzene was attempted with the interest of maximizing p- diethylbenzene (p-deb), which is also an important commercial material as discussed in the preceding section. p-deb can be obtained by shape selective ethylation of EB over ZSM-5. Since benzene is surplus in pertro chemical industries, its consecutive alkylation to p-deb over ZSM-5 cannot be a difficult task. It is the mole ratio of benzene to ethanol which is important in this reaction. The mole ratio, 1:1, 2:1 or any ratios with high benzene content can always lead high selectivity to EB, but in all other ratios DEB isomers could be predominant. The selectivity of p-deb can be enhanced with shape selective catalyst like ZSM-5 again isomerisation of p-deb to m-deb can be avoided using an ZSM-5 catalyst synthesized in fluoride medium, because it has high crystallinity with negligible amount of external acid sites. As the crystal size of ZSM-5 is also large, it may not require pore size optimization for p-selective alkylation. When the virgin ZSM-5 catalyst synthesized in fluoride medium is used there is no need for pore size optimization for high selectivity. Based on such ideas in the present study it was planned to investigate ethylation of EB with ethanol over ZSM-5 synthesized in fluoride medium.

25 Oxidation of Ethyl Benzene Catalytic oxidation of alkyl aromatics is commercially important because oxidized aromatics are commercially more valued than the raw materials. Aromatic ketones such as acetophenone are important intermediates for the synthesis of drugs and pharmaceuticals (Choudhary et al 2004). Acetophenone can be obtained by acylation of benzene, but for this reaction either acetic anhydride or acetyl chloride is to be the reagents. Acetic anhydride is the banded material in India, and acetyl chloride is a hazardous reagent. In addition for acylation corrosive Friedel-Crafts catalysts such as AlCl 3 are to be employed in stoichiometric amount. Hence instead of acylation of benzene, ethyl benzene oxidation to acetophenone could be treated as a convenient substitute. For the oxidation of ethylbenzene stoichiometric quantities of oxidizing agents like KMnO 4 or KCr 2 O 7 were used for catalytic purpose, but both are hazardous (Cullis et al 1955 and Clark et al 1989). Recently oxidation of ethyl benzene to acetophenone was attempted over cerium AlPO-5 (Devika et al 2011). They reported high conversion and selective oxidation for alkylation of alkyl aromatics. Cerium incorporated molecular sieves is advantageous as the active sites are isolated and exhibit uniform activity and selectivity. Ceria as such can be used as a catalyst but high temperature oxidation it s sintering one is unavoidable. In this context framework substituted cerium based catalysts are important, as they carry the characteristics of both homogeneous and heterogeneous catalysts. Since cerium enters the frame work of AlPO-5 it is logical to think its entry into ZSM-5 zeolite. But in this study silicalite-1 with MFI structure was chosen to incorporate cerium in the frame work. Such Ce-Silicalite-1 molecular sieves could be a novel catalyst for oxidation of alkyl aromatics. Hence in the

26 26 present study it was planned to investigate vapor phase oxidation of ethylbenzene to acetophenone over Ce-Silicalite-1 synthesized in fluoride medium Oxidation of p-xylene Oxidation of p-xylene to teripthalic acid, p-toluic acid or p-tolaldehyde is important as every product has commercial significance. Catalytic oxidation of p-xylene to teripthalic acid was reported by Chakrabarty et al (2007). Cobalt (III) cubane cluster was used as the catalyst for oxidation. Di-oxygen was used as the oxidizing agent favor to p-toluic acid and terephthalic acid were formed with p-toluic acid in excess. In most cases cobalt (II) salts and complexes were used as catalysts for p-xylene oxidation to terephthalic acid. Large induction period was reported for this reaction (Partenheimer 1995). Gas phase oxidation of alkyl aromatics Ratnasamy (2001) reported selective oxidation of p-xylene to terephthalic acid by trinuclear oxo-bridged cluster complexes of cobalt and manganese in presence of oxygen. Jacob et al reported oxidation of p- xylene over zeolite encapsulated copper and manganese using t-butylperoxide at low temperatures. The major products were toluic acid, tolayl alcohol, tolaldehyde. The catalyst was recoverable for reuse. Devika et al (2011) reported ethyl benzene oxidation to acetophenone over Ce-AlPO-5 molecular sieves using air as an oxidizing agent. They confirmed frame work incorporation of cerium in AlPO 5 and selective oxidation with high ethyl benzene conversion. Based on such reports in the present study was made an effort to incorporate cerium in Silicalite-1 molecular sieve with MFI structure. It was also attempted to study oxidation of p-xylene over Ce-Silicalite-1 molecular sieve. It possesses medium pore size and p-xylene can easily

27 27 diffuse into it and make it accessible to the active frame work cerium sites. This catalyst was first time synthesized and used for the shape selective oxidation of p-xylene. 1.9 SCOPE AND OBJECTIVES The scope of present investigation was to hydrothermally synthesize (i) ZSM-5 in fluoride medium and study their catalytic activity for vapour phase isopropylation of benzene, disproportionation of ethyl benzene and ethylation of toluene, and (ii) Ce-Silicalite-1 in fluoride medium and study their catalytic activity for vapour phase oxidation of ethyl benzene and p-xylene. The major objectives of the investigation were as follows: Hydrothermal synthesis of ZSM-5 (50, 75 and 100) in fluoride medium at 170 C using tetraethyl orthosilicate, aluminium sulphate, ammonium fluoride, phosphoric acid and tetrapropyl ammonium bromide. Characterization of the as-synthesized materials using XRD, TGA and FTIR. Calcination of the as-synthesized materials at 650 C to expel the template Characterization of the calcined materials using XRD, FTIR, TGA, BET, SEM, EDS and TPD (ammonia). The study of isopropylation of toluene using isopropyl alcohol, ethylation of benzene using ethanol and disproportionation of ethylbenzene in the vapor phase over the above characterized catalysts.

28 28 Study of the effect of temperature, WHSV on conversion and product selectivity. Study of the effect of time on stream on conversion and product selectivity. Hydrothermal synthesis of Ce-Silicalite-1 molecular sieves (50, 75 and 100) in fluoride medium using tetraethyl orthosilicate, cerium nitrate, phosphoric acid, tetra propyl ammonium bromide. Characterisation of the as-synthesized materials using XRD, FTIR, TGA, DRSUV. Calcination of the as-synthesized Ce-Silicalite-1 catalyst at 650 C to expel the template. Characterisation of the calcined materials using XRD, FTIR, TGA, ESR, BET, SEM, XPS and EDS. Study of the vapor phase oxidation of p-xylene and ethyl benzene in air over Ce-Silicalite-1. Study of the effect of temperature, feed rate and air flow rate on conversion and product selectivity. Study of the effect of time on stream on conversion and product selectivity. Hydrothermal synthesis of Na-ZSM-5 (50, 100 and 150) zeolites in the absence of template using tetraethyl orthosilicate, aluminium sulphate in alkaline medium. Characterisation of the as-synthesized material using XRD, SEM, BET, EDS, TGA and FTIR. Study of vapour phase isopropylation of toluene, disproportionation of ethyl benzene and ethylation of benzene over it for comparison.