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Page 1-21.FH10 CHAPTER I GENERAL INTRODUCTION CHAPTER I GENERAL INTRODUCTION 1.1 Alkylation Reactions of Aromatic Hydrocarbons: The importance of alkylation in organic preparations was realized as early as 1877 with the reaction of benzene and amyl chloride, to produce amyl benzene, by Charles Friedel and James Mason Crafts-^. This was apparently the first typical alkylation reaction and came to be known as the Friedel-Crafts alkylation reaction. Friedel-Crafts reactions now find a number of industrial applications such as m the manufacture of high octane gasoline, synthetic rubber, plastics, synthetic fibers, synthetic detergents etc. Besides its commercial importance, the area of organic chemistry dealing with this reaction encompasses classic examples of some of the most interesting aspects of modern organic chemistry: electrophilic aromatic substitution, carbocation formation and rearrangement. In the Friedel-Crafts reaction, the alkylating or acylating agent and the catalyst, such as aluminium chloride and hydrogen chloride, react to form either carbonium or acylium ion or complex. The ion or complex then attacks the aromatic ring^. Thus in the alkylation of olefins in the presence of the catalyst, the following reactions take place. R-CH=CHo+ AICI^ + HCI R-CH-CH3AlC I4 CH-CH3 r -1- + R-CH-CH3AICI4 AlCl ^ +AICI4 + HCl The aromatic ring to which the olefin gets attached may be that of benzene, substituted benzene or more complicated ring systems like naphthalene or anthracene. Friedel-Crafts reactions are complicated by the rearrangement of the attacking agent and in some cases of the aromatic starting materials . There is a tendency for a carbonium ion formed during the reaction, to rearrange to a stable secondary or tertiary carbonium ion. Although, the alkylating agent involved in the discovery of the Friedel-Crafts reaction was the alkyl halide, many other alkylating agents ha^f* been used. Today, the most commonly used alkylating agent is the alkene. Other alkylating agents which can be used for the reaction are alcohol, ethers and esters^'^. 1.2 Transalkylation Reactions of Aromatic Hydrocarbons: Another feature of Friedel-Crafts alkylation reaction is polyalkylation. Since alkyl groups activate the aromatic ring towards further attack, there is a marked tendency for polysubstitution during the alkylation. This affects the yield of the monoalkyl product. It was shown that alkyl groups could be transferred from one aromatic ring to another by the catalytic effect of aluminium chloride and hydrogen chloride^. For example, when ethylbenzene was heated with the catalyst, a mixture of benzene, ethylbenzene, diethylbenzene (mainly m- and p- isomers), and higher boiling material was produced. C2H5 C2H5 4- fOV-C2H5 Transfer of alkyl groups from one to another, similar or dissimilar aromatic rings, in the presence of an acid catalyst, is known as the transalkylation (or disproportionation) reaction. Industrially, this reaction is valuable as some of the low valued products, like polyalkylbenzenes, can be converted to their monosubstituted homologues having higher demands and values e.g. transalkylation of diethylbenzenes to ethylbenzene. 1.3 Catalysts for Alkylation and Transalkylation Reactions: Friedel-Crafts alkylation or transalkylation is an acid catalyzed reaction. Both Bronsted and Lewis acids can catalyze these reactions^'-'. For the liquid phase alkylation reaction, anhydrous aluminium chloride is preferred as a catalyst, although a co-catalyst or promoter like hydrogen chloride is usually needed to obtain efficient alkylation. In the vapor-phase alkylation, the most commonly used catalyst is phosphoric acid supported on kieselguhr (also known as solid phosphoric acid (SPA) catalyst). Phosphoric acid in the presence of small amount of water, alkylate benzene effectively to products like ethylbenzene, isopropylbenzene etc. Besides these two catalysts, various other catalysts have been used for the alkylation reaction of aromatics^" . These include weak acidic metal halides (Lewis acids) such as AICI3.CH3NO2, BF3, FeCl3, TiCl^, SnCl^ or ZnCl2; protonic acids (Bronsted acids) such as BF3, HF, H2SO4, polyphosphoric acid (PPA); solid super acids such as perfluorinated sulphonic acid Nafion-H; and inorganic acidic oxides such as phosphorous pentoxide on alumina. Most of the catalysts mentioned earlier have to be used in the liquid phase. The use of such catalysts is often associated with problems of corrosion, toxicity and effluent pollution. This makes it desirable to replace homogeneous catalysts by heterogeneous catalysts. Furthermore, the gas phase also offers advantages in the field of process technology . Solid acid catalysts such as zeolites are free of environmental and corrosion problems. Further, the combination of acidity and shape selectivity in the case of zeolites makes them potential catalysts for different Friedel-Crafts alkylation and transalkylation reactions. Some of the important and interesting organic reactions which were not feasible because of low activity, selectivity and/or service life of the catalyst, can now be commercially exploited using zeolites. 1.4 Zeolites as Alkylation and Transalkylation Catalysts: Zeolites are crystalline alumino-silicates with a framework based on extensive three dimensional network of SiO^ and AlO^ tetrahedral building blocks^. The tetrahedral co-ordination permits a variety of ring structures, which further link to form the cage or channel structures, that give rise to the molecular discriminatory nature of the zeolites. The isomorphous substitution of Si by Al gives rise to a net negative charge, compensated by cations, which are present in voids and channels of zeolites. The cations are generally from group I and II, although other metals, nonmetals, and organic cations may also be used. They are quite mobile and can be exchanged to varying degrees with other cations. All zeolites that are significant for catalytic and adsorbent applications can be classified by the number of T-atoms (T = Si or Al) that define the pore openings^' . There are only three types of pore openings known todate in the aluminosilicate zeolite system; they are descriptively referred to as the 8- membered ring (small pore), 10-membered ring (medium pore) and 12-membered ring (large pore) zeolites. The size and shape of the pore opening is determined by factors such as (1) configuration of the T and O atoms relative to each other, (2) silica to alumina ratio, (3) size and location of the cation, (4) temperature and (5) framework structure of the zeolites. 1.5 Active Sites in Zeolites: The reactivity of molecular sieve zeolites as catalysts is determined by active sites provided by the imbalance in the charge between the silicon and aluminium ions in the framework. Thus, each aluminium atom present within the framework constitutes an active site^. Classical Bronsted and Lewis acid models have been used to classify active sites in zeolites. Bronsted acidity arises in the zeolite when the cation balancing the anionic framework charge is a proton (H"*") . Differences in acidic strength between different zeolites is often related to the T-O-T bond angles and lengths, and crystal resonance energy . ® H H 0. Si Al ^ /' / \ 0/ \ 0 0/ ^ Bronsted Acidity A trigonally co-ordinated aluminium atom, which acts as an electron acceptor, behaves as a Lewis acid. Higher temperature (>500°C) can result in conversion of the Bronsted acid sites to Lewis acid sites by dehydroxylation^. Dealumination by hydrothermal treatment of zeolites has also been found to produce a variety of cations and neutral species which function as Lewis acids • These cations also induce activity on nearby Bronsted sites. Strong electrical field in the small pore zeolites (including medium pore ZSM-5), arising from the presence of various charged species causes large energy gradients with in the molecular sieve, pores. They also may affect activity and selectivity of the catalyst^-*-. 1.6 Hydrothermal Synthesis of Zeolites: Natural zeolites occur in the cavities of volcanic or metamorphic rocks, and on a much larger scale, sedimentary tufi deposits-^''. The formation conditions for these natural deposits are quite mild, with typical temperatures in the range of 70 to 350°C and they get accumulated over geological time scale. In typical modern procedure for synthesizing high silica zeolites, a solution of alumina and an organic base, such as tetra- aTcylammonium hydroxide, are mixed intimately with a sol or solution of the silica component^'^ '-"--^'-^'^. This highly alkaline mixture forms a thick gel that crystallizes in a few hours when maintained at or above 100°C, at hydrothermal conditions. Synthetic zeolites thus obtained by hydrothermal treatment are metastable phases. Changes in the nature and properties of these phases may be induced by many factors that affect the nucleation and crystal growth rates of the competing zeolite phases. This can determine the nature of the framework that finally crystallizes. Basic understanding of the crystallization mechanism is limited and largely empirical. Three variables have major influence on the zeolite crystallization; the gross composition of reaction mixture, temperature and time ' ' . Components of the reaction mixture that influence final crystalline material are silica to alumina ratio, hydroxide content of the gel, presence of inorganic cations, organic additives etc. Influence of all these components are interrelated. 1.7 Zeolite Structural Characterization: Zeolites are crystalline and
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