INTRODUCTION TO CATALYSIS AND CATALYSTS Since ancient times, fermentation (bio-catalytic) processes were used to produce wine/beer, alcohol, etc. In the 18" century Berzelius, Davy and Faraday among others, laid the basis of modern catalysis. Indeed, catalysis has tremendous influence on the human activities such as economic development. environment preservation, major influence on societal progress. Catalysis and catalysts play a primary role as a technology in great part of fertilizers, pharmaceuticals, petroleum & energy sectors. More than 90% of all molecules of current and alternative transportation fuels have passed over on atleast one catalyst and some 80% of chemical products are manufactured with the aid of catalysts. ‘The word catalysis (Catalyze) was coined by Berzilius in 1835. It means (cata-dissolution; lysis-set free) those substances which dissolve to make a chemical reaction possible (st fre) which would not otherwise take place. The Chinese term Tsoo Mei is more interesting which means a marriage broker. A trace of catalyst is sufficient to produce great change, without itself being changed. It was Ostwald who showed that a catalyst changes the velocity of a chemical reaction without itself appearing in the end products and has no effect on the positcion of the equilibrium. The change in rate can be both positive as well as negative. In the first case they are known as positive catalysts and in the second case they are known as inhibitors, Sometimes either heat evolved in a chemical reaction or one of the product formed is sufficient to cause the acceleration, in that case the phenomenon is known as auto catalysi: and the reverse is known as auto inhibition, Catalysis is a core and critical technology to many industries. The catalysts used in petroleum refining are essentially acidic as well as bifunctional in nature such as cracking/ hydrocracking, isomerization, etherification, alkylation, hydrotreating, hydrogenation, reforming, etc. In basic chemicals. ammonia synthesis, nitric acid synthesis by ammonia oxidation. sulphuric acid synthesis, methanol, aromatics and recently olefins. In petrochemistry. synthesis of intermediate chemicals and polymers involve cataysis. In removal/conversion of pollutants from the emissions of stationary and mobile combustors, and in the production of fine and speciality chemicals for synthesis of high value intermediates and active compounds. Catalysis is a field, encompassed with many disciplines from material science to chemical engineering. 158IMPACT OF CATALYSIS ON THE SOCIETY Chemicals and fuels have become a key factor in every sphere of human life nutrition, trans- portation. medicine, consumer products, etc. Today. a state of the art chemical and liquid fuels production thus forms the vital basis for a successful economy. Catalysis and catalysts are playing a pivotal role by producing many vital chemicals as mentioned above which are used for most of our needs and have major influence on the progress of the world. Catalysis is critical to two of the largest industries (petroleum and chemicals processing) and also a vital component of the critical technologies identified for the future research. Another clear benefit from catalysis is the application of catalysts to improve our environment. Catalysis plays a fundamental role in the economy, environment and the public health of the world. The total worldwide market for catalysts for petroleum, petro-chemical, chemical and en- vironmental applications was estimated to be more than $6.6 billion with a growth rate of 6.7% upto 1998, The average value of catalysts as a percentage of the product produced is conservatively estimated at 02%. From these two numbers, one can conclude that the worldwide value of products produced via catalytic technology is in excess of $1 trillion per year. The total value of fuels and chemicals derived through catalysis in US corresponds to 17% of its GNP and the value addition throughout the world is estimated as $2.4 trillion. For example, a single pound of refinery catalyst can produce 1.200-2.000 Ibs of gasoline. Clearly, catalysis is a technology of major financial importance, In petroleum refining, zeolite based cracking catalysts are dominating today and their use has resulted in savings of ~400 million barrels/year which is equal to ~$ 7 billion/year (if the crude available at $ 17/bbl) for the same amount of gasoline produced with other catalyst systems. Catalysts and catalyst development therefore, form one of the cornerstones of successful chemical production. Catalysis has become one of the most powerful tools of the current manufacturing industry during the twentieth century and the developments are shown in Table 5.1. Table 5.1 Historical Summary of the Development of Industrial Processes Based on Catalysis ‘Year Process Catalysts 1750 H,SO, Tead chamber process NOMNO: 1870 ‘SO; oxidation Pr 1880 Deacon process (Cl, from HCD ZaClyCuCh; 1885 Claus process (HS and SO, 10 Sy Bauxite 1900 Fat hydrogenation Ni Methane from Syngas Ni i Fe Upgrading coal liquids ws, 1910 ‘Ammonia synthesis (Haber-Bosch) FelK NH, oxidation to nitric acid PL ‘Methanol synthesis (high pressure) Za Cr oxide Fischer-Tropsch synthesis Promoted Fe, Co 1920 SO, oxidation V,0ySi0, Acetaldehyde from acetylene ig" /HySO, Catalytic cracking (fixed bed. Hloudry) Clays Enhylene epoxidation Ag Polyvinyl chloride Peroxide 1930 Polyethylene (low density. ICI) Peroxide Oxidation of benzene to maleic anhydride ‘Supported V,0, Alkylation HF/H;SO, (Contd)Year Process Catalysts Hydroformylation, Alkene to aldehyde Co Catalytic Reforming (gasoline) Pt Cyclohexane oxidation (nylon 66) Co 1940 Benzene hydrogenation to cyclohexane Ni,Pt Synthetic rubber, SBR Li, peroxide BNR Peroxide Butyl rubber Al Polymethlene (high density), Ziegler-Natta Ti Phillips cr Polypropylene. Ziegler-Natta Ti Polybutadiene. Ziegler-Natta Ti Hydrodesulfurization (HDS) Co, Mo sulfides 1950 Naphthalene oxidation to phthallic anhydride V, Mo oxides Ethylene oxidation to acetaldehyde Pd, Cu p-Xylene oxidation to terephthallic acid Co, Mn Ethylene oligomerization Co Hydrotreating of naphtha Co-Mo/ Al, Butene oxidation to maleic anhydride V,P oxides ACN (ammoxidation of propylene-Sohio) Bi, Mo oxides Propylene oxidation to acroleirvacrylic acid Bi, Mo oxides Xylene hydroisomerization Pc Propylene metathesis W, Mo, Re Adiponitrile (butadiene hydrocyanation) Ni 1960 Improved reforming catalysts Pt, Ref Al;O} Improved cracking catalysts Zeolites Acetic acid from MeOH (carbonylation) Co Vinyl chloride via ethylene oxychlorination Cu chloride Ethylene oxidation to vinyl acetate PdiCu ‘0-Xylene oxidation to phthallic anhydride V, Ti oxides Propylene oxidation to propylene oxide Mo Hydrocracking Ni-W/AI,0) HT water-gas shift process Fe,0y/Cr,0Mg0 LT water-gas shift process CuO/ZnO! Al,O, 1970 ‘Methanol synthesis (low pressure, ICI) Cu-Zn-Al oxide Acetic acid (MeOH Carbonylation, low pressure process, Monsanto) Rh Improved process for xylene isomerization Zeolite a-Alkenes via ethylene oligomerization! isomerization’ metathesis (SHOP) Ni, Mo Improved hydroformylation Rh Auto exhaust gas catalysts PURh L-DOPA (Monsanto) Rh Cyclooctenamer (metathesis) w Hydroisomerization PuZeolite Selective Reduction of NO (with NH) V,0STIO> 1980 Gasoline from MeOH process (Mobil) Zeolite Vinyl acetate (Ethylene-acetic acid) Pa Methyl acetate (carbonylation) Rh Methyl acrylate via t-butanol oxidation Mo oxides Improved coal liquefaction Co, Mo sulfides (Contd)Year Catalysts Co Light alknaes to aromatics Ga-ZSM-5 1990 Polyketone (from CO and Ethylene) Pd Deep desulfurization of gas oil Co-Mo/A/203, with high surface area and acid amount Aromatization of LPG or light naphtha Metallosilicate catalysts (Z-forming) Aromatization of C,, Cs raffinate or Modified ZSM-5 zeolite LPG/C,. Cs fraction of FCC gasoline catalyst (Alpha process) ZrCl,-EtAICl;-Et, Al- Olefin manufacturing by ethylene organic P/S ligand oligomerization catalyst Methyl formate process for MMA Production of dimethyl carbonate from cucl acetone Conversion of phenol to hydroquinone and Ti-silicalite catechol Isomerization of but-I- ene to 2- H+ = ferrierite, H+ - methylpropene Theta-t (acidic zeolites) Isomerization of oxime of cyclohexanone to silico- alumino- caprolactam phosphate molecular sieve (SAPO -11) Ammoxidation of cyclohexanone to its Ti-silicate oxime using H,0; Production of acrylamide from vinyl cyanide immobilized nitrile hydratase Complete combustion of natural gas (at ca, noble metals and/or 1300°C) mixed oxides ‘Sweetening of natural gas by selective mixed oxides oxidation of H,S to S Oxidation of benzene to phenol via zeolites cyclohexane Methanol to light alkenes silico ~ alumino — phosphate molecular sieve Alkene oligomerization zeolite Decomposition of hypochlorite NiO Dehydration of alkanols hetropolyacid salts Conversion of toluene to toluene cis-1, 2- Pseudomonas putida dithdroxy- 3-methylbenzene Production of 2, 6-disopropyinaphthalene acidic zeolite (modemite) using propene as alkylating agent “The data refer to activities of a pilot plant scale at least DEFINITION OF CATALYTIC ACTIVITY The catalyst is defined as a substance that changes the kinetics but not the thermodynamics of achemical reaction or a catalyst is a substance or mixture of substances which increases the rate of a chemical reaction by providing an alternative quicker reaction path without modifying the thermodynamic factors and the catalyst remains unaltered at the end of thecatalytic process. The conceptual framework for kinetics of catalytic reaction was provided by the introduction of the activated reaction theory which is shown in Fig. 5.1. The role of catalyst is to alter the reaction mechanism so that the thermal pathway that requires a high activation energy is transformed te two or more steps, each with a lower activation energy steps. ‘As on today the heterogeneous catalytic reaction can be divided into at least five distinct steps: (i) Diffusion of the reactants to the catalyst (ii) The formation of adsorption complex (reactant surface) (ii) The chemical change on the surface (iv) The decomposition of adsorption complex (product-surface ) (v) Diffusion of reaction products from the catalyst The activation step which has the highest activation energy determines the rate of reaction. | Activated state for gas reaction Activated state for surface ction Potential energy Gaseous products Adsorbed products Fig. 5.1 Potential-energy curves for a reaction proceeding homogeneously (full curves) and on a surface (dotted lines) id-catalyzed reaction can be expressed as In general. the rate of any gas-solid or liquid-s the product of the apparent rate coefficient k and a pressure-(or concentration) dependent term rate = kf (p) where p; is the partial pressure of the reactant I. The rate coefficients for the overall catalytic reaction may incorporate the rate coefficients of many of the elementary reaction steps that precede the rate- determining step. For several reasons, this rate coefficient will change on the prevailing conditions of the reaction (temperature, pressure, surface concentrations, ete.) and it is operationally convenient to use the Arhenius equation k= A’ exp (-E/RT)where A’ is a temperature-independent pre-exponential factor and £” is the apparent activation energy. The concentration of reactant at the catalyst surface is in general temperature~ dependent. The more convenient way of defining catalytic activity is the use of the concept of the turnover frequency or turnover number. The turnover frequency (often designated TOF) is simply the number of times n that the overall catalytic reaction in question takes place per catalytic site per unit time for a fixed set of reaction conditions (temperature, pressure or concentrations, reactant ratio, extent of reaction). In other words, _ No.of molecules of a given product (No. of active sites) x (Times) TOF The influencing properties of a catalyst which determine its selection are given below: i Activity The activity can be expressed either in terms of rate (moles of product per volume of catalyst per time) or of turnover numbers (moles product produced per mole of catalyst or active site). The higher the activity, the higher the productivity and require lesser reactor volume and milder processing conditions can be used. ii, Selectivity The selectivity will be expressed as the moles of desired product produced per mole of reactant converted. Higher selectivities are preferred to avoid the loss of reactants, costs required for separation, purification, etc iii, Life Time Expressed in terms of years, amount of chemical produced per amount of catalyst 01 the number of turnovers observed before it die: TYPES OF CATALYSIS The catalysis is classified on the basis of the state of catalyst and type of catalytic processing. Catalysis is mainly divided into heterogeneous and homogeneous types based on the phases involved in the process. Processes based on heterogeneous catalysis are predominantly used in fossil fuels conversion and bulk chemicals production. Homogeneous catalysts are exzensively used in the chemical intermediates, fine and speciality chemicals production. Homogeneous Catalysis ‘The reactants, the products, the catalysts are in the same phase. Most of the liquid phase catalytic processes belongs to this group of catalysis, The catalysts are soluble acids, bases. salts and organometallic complexes. The main advantages of homogeneous catalysis can be summaiized in the following way: — The utilization of almost all the active sites/molecules of the catalyst in the conversion — Higher selectivities such as stereo, regio, selectivities can be obtained — The easier control of the temperature for highly exothermic reactions — Milder conditions and higher electivities A number of commercial processes based on metal complexes or liquid acids are producing a large number of chemicals and chemical intermediates. Some of these processes are the conversion of propenc to epoxide/alcohol, cumene to phenol, cyclohexane to cyclo hexanone, cyclohexanone to adipic acid, p-xylene to terepthalic acid, etc. Apart from these chemicals, 1 large number of fine chemicals and chemical intermediates are produced with homogeneous catalysts. The noted examples are the carbonylation of methanol and hydroformylation of propene.The homogencous catalysts and catalysis have some serious disadvantages in comparison with heterogeneous catalysis. The disadvantages are: — The expensive procedures of separation and catalyst recovery — Serious problems of corrosion — Expensive treatments of liquid wastes generated during separation. regeneration and recycling of catalyst — The possibility of contamination of products with toxic catalysts — The existence of gas-liquid mass-transfer limitations in cases where one of the reac- tants is a gas (H>, 02) Homogeneous catalysts have advantages due to in built-in selectivity advantages and because of its promise to utilize metals upto 100% efficiency especially for noble metals. However, the catalyst recycling is a major hurdle and the research efforts are continuing to combine the advantages of both homogeneous and heterogencous catalysis, the field of heterogenizing homogeneous transition metal based catalysts developed. ‘she mode of heterogenizing homogeneous catalysts were shown in Fig. 5.2(a) and (b). The expected advantages for heterogenizing the homogeneous catalysts are many and some of these are given below: © ease of separation of catalyst from the product * molecular dispersion and accessibility of nearly all metal atoms achievement of high selectivity © potential for ligand tailoring © mild reaction conditions © use of conventional equipment such as packed bed or fluidized bed reactors A number of different approaches developed to support homogeneous metal complex catalysts with traditional inorganic supports or organic polymeric supports in different reaction environments: i. supported liquid phase catalysis heterogenized metal complexes iii. porous material entrapped complexes, and iv. use of membrane filtration and use of oligomeric complexes ‘Support t Y Inorganic Organic (polymer) | | | | _ t Y Y Y 1 Y Physical Chemical bond Entray in pores Physical ‘Chemical bond Entrapped it adsorption (covalent, lonic) (zeolites, pillared adsorption (covalent, ionic) pores ‘on support to support clays) of support. on support to support of support Fig. 5.2(a) How to heterogenize complexes+Licy \ scot nf 4 ZC iM OX pone, € 4 04 oOo ome 6 Cae + CIPPhg +KPPh f f te fuer ey 04a o<)>- PPh, oC )- CH,PPh, P-Mortifiold's resin Fig, 5.2(b) Functionalization of organic polymer Polymerization Catalysis The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE), polypropylene (PP) and polystyrene (PS) has grown into a large scale industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. Most of the PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkyl aluminium chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most of the titanium based PE catalysts are supported on MgCl. Ziegler- Natta invented the polymerization catalysts and commercialized in late 50s. The aluminium alkyl-titanium chloride catalysts originally employed in the high pressure synthesis of polyethylene, polypropylene, styrene and other plastics were modified and/or replaced by heterogeneous catalysts such as chromia on alumina and a new low-pressure synthesis was developed. Metallocene Catalysts One of the most exciting developments in polyolefin polymers production in recent years has centered on the development of commercial homogeneous comprising a metal atom sandwiched between parallel planar cyclopentadienyl groups. Some of the representative examples of metallocenes were shown in Fig. 5.3. These single site catalysts produce olefin polymers with properties that are different when compared with traditional thermoplastics. The key discovery by Kaminsky and Sinn that methylaluminoxane (MAO, {MeAIO],) in conjunction with Cp,TiMe, and ‘Cp,ZrCl, afforded extremely active catalysts for PE and atactic PP. The most valuable feature of single-site catalysts is the ability to logically control the structure of the polymer from the design of the catalyst. The most commonly used families of single-site catalysts are based on metal complexes shown in the figure. The polymer prop- erties were strongly influenced by the catalyst structure. The bis-cyclopentdieny! based or metallocene, single-site catalysts are generally activated with MAO in a relatively large molar amounts. The catalytic activity is also a function of the transition metal co-ordinated. The most commonly used metals for olefins polymerization are zirconium, titanium and hafnium and their order of activity is Hf > Ti > Zr. These catalyst systems have extremely high activity (~40 000 kg polyethylene/g.metal/ hour). There are several features that distinguish metallocene catalysts from other systems.21%, i S2em on Nad Wy \ CS LPP R oy LLOPE, HDPE Hoechst Exxon-Mitsui Potroleum Fina aPP Exxon Eastman Kodak Chisso Alpha-OLEFINS ‘Sumitomo Shell 4B wey /y cy Meesi | Tix, y T% MesC LUPE sP8. Fina-Mitsui Toatsu Ethylene Dow- Hoschet Styrene Copal Idemnitsu Fig. 5.3 New generation metallocene-based polymerization catalysts They can polymerize vinyl monomers regardless of the monomer’s molecular weight or stress hindrance, they produce very uniform polymers of narrow molecular weight distribution and they can polymerize a-olefins with very high stereo-regularity to give isotactic or syndistactic polymers. In this way these new generation catalysts are robust, highly selective and processes based on these catalysts are clean and energy efficient. Heterogeneous Catalysis The catalysis which operates with different phases of reactants, products and catalysts is known as heterogeneous catalysis. Catalyst used in heterogeneous catalysis is a composite material characterized by the relative amount of several components, its shape, its size. its pore size distribution and its surface area, Usually. the catalysts are solids and the reactions occur mainly in gas/vapour or liquid phases, The catalysts are either inorganic solids such as oxide supported metals, bulk metal oxides, sulfides, chlorides or organic solids such as resins, These catalysts can be used as a powder in slurry reactors with solvents, as pellets in tricle. bed or fixed bed reactors and as small particles to be used in fluidized bed reactors, The main advantages of heterogeneous catalysis is its robustness in separation of catalysts from reactants and products, the elimination of corrosion problems and liquid waste streams. The main disadvantages are the difficulty in controlling the temperature, the mass transfer limitations, etc.Heterogeneous catalysts are more widely used in industry and efforts are continuing to heterogenize the homogencous catalysts too, mainly to avoid the separation and corrosion problems. The main components of a catalyst used in heterogeneous catalysis are the active sites, physical promoters (surface area stabilizers/structural stabilizers) as well as chemical promotors (modifies the activity and selectivity), supports. CATALYST SUPPORT The support materials in heterogeneous catalysis plays multiple roles and it is the main component of the composite catalyst. The generally used catalyst support materials are aluminas (5, ce, % n). silica, zirconia, titania, active carbon mixed oxides, cordeitite, ete. The main functions of support are — to reduce the amount of expensive active species or optimum utilization of costly met- als such as noble metals — to provide the base, high surface area and optimum pore size di metal/oxide species — to increase the mechanical resistance of the catalyst composite — to create bifunctional or polyfunctionalities by introducing new functionalities — to increase the heat exchange capacity of the catalyst composite (SiC) — to stabilize the active metal/metal oxide species to remain as small particle sizes during the commercial operations — to provide/retain the different valence and coordination states for active sites ribution to active The selection of support is based on certain desirable characteristics. The main characteristics of the support materials should have the (i) inertness, (ii) desirable mechanical properties (attrition resistance, hardness, etc.), (iii) stability under reaction and regeneration conditions, (iv) surface area (mainly depends on application and conditions of application), (v) porosity, and (vi) low cost and easy accessibility. Variety of materials mainly noble metals, transition metals and mixed metal oxides are used as heterogeneous catalysts for various reactions such as oxidation, hydrogenation, reduction, pollution control, reforming, hydroprocessing, etc. Among these. the zeolites are the most robust and efficient catalysts for many applications from pollutants conversion to photochemical transformations. Other classes of materials which have interesting catalytic properties are heteropoly acids/salts and clays. These materials also can be transformed into multifunctional catalytic materials for various applications like zeolites or other zeotype materials. Zeolites Zeolites are the three-dimensionai crystalline mixed oxides with well-defined regular pore openings and large surface areas. Many studies on zeolite minerals have been carried since their discovery. Moreover, in the late 1940s the preparation of synthetic zeolites was the start of a large range of zeolitic materials reflecting complete range of framework substituted tectosilicate including phosphates. The naturally occurring zeolites are mainly crystalline aluminosilicates containing metal ions and these are represented by the chemical formula given below. x/2M"[(Al0;), (SiO;), WH; Ion exchange and adsorption properties were recognized as the special properties of zeolites. Because of absorption characteristics of these materials to absorb the molecules of relatively low size over molecules of larger size formed the basis for the intcoduction of the term ‘molecular sieves’.Microporous aluminosilicate catalysts possessing structures are particularly well suited for the conversion of hydrocarbons, certain oxygenates and other species into useful products. They are the solids possessing three dimensional (3D) surface, replete with cages and channels, which may or may not intersect. Linking the pores and distributed in a more or less spacially uniform fashion throughout their bulk are the ‘active sites’ which are bridging hydroxyl groups as shown in Fig. 5.4. Structure I of the active site represents a Bronsted acid site, Interaction of water with it results in structure II, although a possible alternative is the H,O* complex, shown as structure III. The first generation of zeolites (1940-1950) reflects the zeolites with low silica to alumina ratios (Si0,/Al,0, $10), prepared from a crystallization of reactive gels with alkali and alkaline earth metal hydroxides. Such zeolites are characterized by high ion exchange capacity, an extremely hydrophilic surface and many acid sites with a wide variety of acid strength, The zeolites belong to this class are zeolite A, X, Y, chabazite, erionite and mordenite. ‘Structure II Structure Ill Fig. 5.4 Structure of active sites in zeolites The second generation of zeolites were prepared by using all possible quarternary ammonium ions as templates and resulted in many new materials with a new structure type \d different chemical compositions (SiO,/Al;0,>20). ZSM ~ 5 is the most popular member in this class of zeolites. The extensive size of organic amines as structure directing templates -ore filling agents coupled with a new gel chemistry resulted in discovery of a third generation zolites containing AP* and P* as lattice atoms, Some of novel zeolites with extra large pores recently synthesized are shown in Table 5 Table. 5.2. Summary of Extra Large Pore Materials Material ‘Year Mainframework Ring size Pore size [A*] reported composition (atoms) VPI-S 1988 ‘IPO, 18 13 AIPO,8 1990 APO, 4 <10 Cloverite 1991 GaPO, 20 <0 JDF-20 1992 AIPO, 20 te)Material Year Mainframework Ring size Pore size [A*] reported composition (O atoms) ULM-S 1994 GaPO, 16 NR [4] AIMcPO—B 1995 AL(CH;PO)); 18 6 YPA-SNS - 3 1995 Sn,S, 32{b] NR not named 1996 V30;(PO4); 16 NR ULM-16 1996 GaPO, 16 NR UTD-1 1996 SiO, 14 10 ULM-15 1997 FePO, 16 NR lal In A; proven by adsorption [b] Contains a total of 32 atoms [cl Collapse upon activation Id] NR: not reported Zeolites are normally solid acids of crystalline alumino silicates after the exchange of metal ions with protons. The promising characteristics of zeolites or zeotype materials as catalysts are due to their well-defined crystalline structure, high internat surface area, uniform pores, good thermal stability, ability to adsorb and concentrate the hydrocarbons as well as the functionalized hydrocarbons selectively. They can be prepared acidic, bifunctional and polyfunctional catalysts even by exchange methods. The unique properties of zeolite frameworks are causing unusually high catalytic activity and selectivity has added a new dimension to the field of heterogeneous catalysis and catalytic process engineering. The acid sites are accessible through framework channels and windows with different sizes. Several types of zeolites are known in the nature such as fauzasite, A, mordenite, etc., and many are synthesized (ZSM — 5, B - zeolite). These materials possess different shapes and sizes of channel openings. SHAPE SELECTIVITY Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects: (i) formation of an otherwise possible products blocked because it cannot fit into the pores and (ii) formation of the product is blocked not by (i) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (i) is involved in zeolite catalyzed reactions which favour a para disubstituted benzene aver the ortho and meta. The low rate of deactivation observed in some reactions of hydrocarbons on some zeolites has been ascribed to (ii) inhibition of bimolecular steps forming coke. Low alumina zeolites such as ZSM-5 have played a crucial role in the synthesis of variety of neo novel zeolites. The particular use of shape selectivity of ZSM-S is in the methanol to gasoline (MTG) process, LPG to aromatics (Ga-ZSM-5) and many other applications are to be mentioned in this context. A material of the same ZSM-S crystal structure but containing about 1% of structural Ti without aluminium in the framework has unique selective oxidation characteristics converts hydrocarbons with shape selective way. In the recent past, a variety of zeolites with transition metal ions in the framework are reported in the literature. These zeolites have promising characteristics to replace a large number of waste generating stoichiometric reagents. HETEROPOLY COMPOUNDS Heteropoly anions are polymeric oxoanions (polyoxometallates) formed by condensation ofoxoanions. The term heteropoly (HPA) compound is used for acid forms and the salts. Other HPA related compounds are organic and organometallic complexes of polyanions. Heteropoly anions are composed of oxides of addenda atoms (V. Nb, Mo, W. ete.) and heteroatoms (P. Si, etc.) which are shown in the Table 5.3. Table 5.3 Heteropolyanions Known addenda atoms (C1) & heteroatoms (0) incorporated in heteropolyanions w@inlwlwiciwiwlu mek | ct ee | fm a | no | | The heteropoly acid compounds are useful as catalysts because of the following charac- teristics: (i) Acidic properties and oxidizing ability are systematically controllable Gi) Polyanions are well defined oxide clusters which are promising candidates for catalyst design at molecular level (ii) Unique reaction environment, such as the pseudo liquid phase and suitable for possible useful coordination sites. ‘The structures of heteropolyanions and their respective salt are shown in Fig. 5.5. (b) () @ counter cation; _ Cs*, NH, etc. BOA ‘ig.5.5 Structure of heteropoly acid/salt (a) Primary structure-Keggin structure, PW,,0,) ; (b) Secondary structure, HjPW}2049 (c) Example of a tertiary structure, a primary particle of Cs;PW1204oHeteropoly acids are very strong bronsted acids with redox characteristics and their acidity is due to the large size of the polyanion having a low and delocalizable surface charge density. Some HPAs in solid state are thermally stable and suitable for high temperature applications. The thermal stability is in the order of HyPW,:0j > HySiW;2O4 > HyPMoy:Oqo > H,SiMo,,O,9 and can be enhanced by the formation of appropriate salts. The redox and acidic characteristics can be controlled by the formation of salts. Clays and Pillared Interlayered Compounds In the history of acid catalysis, clays have played an important role in the past. Acid leaching of natural smectite clays led to amorphous aluminosilicates with large acid surface able to initiate the carbocation chemistry which has so many diverse applications in catalysis. Clays are aluminosilicates with a two- dimen- sional or layered structure including the com- mon sheet 2: 1 alumino- and magnesium-sili- cates (montmorillonite, hectorite, micas, varmiculites) and | : | minerals (kaolinites, chlorites). These materials swell in water and other polar solvents upto a point where there remains no mutual interaction between the clay sheets. After dehydration below 120°C, the clay can be restored in its original form/ state. These materials swell in water and other polar solvents upto a point where there remains no mutual interaction between the clay sheets. After dehydration below 120°C, the clay can be restored in its original form/ state. The main disadvantages of the clays are their instability in severe operating conditions. The clays pillaring with various metal ions such as alumina, zirconia, titania stabilize the structure and generate more available active sites for different catalytic applications. Uni/Bi/Polyfunctional Catalysts Fig. 6.6 Basic structure of smectite clay The catalysts are also divided into unifunetional, bifunctional or multifunctional bs reaction and the desired catalytic functionalities for conversion with desired select catalysts with unifunctional active sites are required for hydration, dehydration, alkylation, halogenation, etherification of hydrocarbons. In the above reactions, acid sites of liquid acids, zeolites, heteropoly acids, metal halides, clays and other materials with acidic nature play a critical role, Bifunctional catalysts play an important role in isomerization and auto exhaust catalysis. Noble metals on chlorided alumina or noble metal on acidic zeolite are the important catalysts used for isomerization of hydrocarbons to increase the octane number of gasoline. In three-way conversion of auto exhaust emissions by oxidation of hydrocarbons and PuPd and reduction of NOx with hydrocarbons on Rh sites are the popular examples.The first polyfunctional catalyst developed was a reforming catalyst. The main reactions which occur in the reforming process are the skeletal isomerization of n-paraffins and the transformation of cycloparaffins to aromatics. The reforming catalyst is constituted of 1.22% Pt, 0.01% Re, 97.29 g-alumina and 1.5% HCI. The reforming mechanism can be illustrated by using n-heptane isomerization as an example: n-heptene «— n-heptane—isoheptene <— isoheptane ‘The key-features of this reaction scheme and of the designed catalyst are the following: 1. Acid catalysts are not active in paraffin isomerization, but are active in skeletal isomer- ization of olefins. In order to transform the paraffin to an olefin, a dehydrogenation component is therefore introduced together with the acid catalyst. In a following step, the olefin is isomerized by the acid function of the catalyst. High temperatures are necessary in order to favour the first step: however, at high tem- perature, cracking reactions become important. In addition, low temperatures and also H, pressure are necessary to decrease the amount of residual olefins in the product. These contradictions in the optimal reaction conditions of the several steps are over- come by choosing a very active catalyst, for the first and final step, driving the dehydro- genation-hydrogenation reaction at equilibrium. The overall rate-determining step must be the acid one. 4. The presence of HCI is necessary to increase the acidity of the alumina, while the presence of Re as promoter of Pt is necessary to decrease the hydrogenolysis reaction » OXIDATION CATALYSIS One more example of polyfunctional catalysts are the oxidation catalysts. Partial oxidation catalysts are an important part of the metallic oxide catalysts which are composed of acidie, basic, total oxidation and partial oxidation catalysts. The main property of such partial oxida- tion catalysts is to contain redox couple elements, i.e., to involve redox type mechanism for the reaction. A general mechanism is presented in Fig. 5.1 and is designated as Mars and van Krevelen mechanism since, 1958. In such a mechanism a cationic redox couple, good electron transfer and lattice oxygen anions ability are concerned. The redox couple could correspond to the same or different cations able to change their oxidation state as Fe2*/Fe?*,V**/V5+, cr'*/Cr** Mo** /Mo™ Cu*/Cu*, etc. Electron conductivity of material and oxygen anion mobility are necessary to allow the reaction to occur favourably. A general mechanism for partial oxidation and ammoxidation of propylene is presented in Fig 5.2 and schematized in scheme 1. It involves an H atom abstraction to yield an anionic allyl intermediate, a redox step to yield a cationic allyl species, then lattice oxygen insertion and at last a second H atom abstraction, For butane oxidation to maleic anhydride the mechanism involves 8 H atoms abstraction, 3 O atoms insertion and 13 electrons transfer on vanadyl pyrophosphate catalyst. The question which we have to face is to determine the active site to insure the reaction and also what is (are) the oxygen (s) incorporated into the hydrocarbon molecule: Latice oxygen adsorbed dissociated oxygen, adsorbed molecular or 1-Oxo oxygen? Role of acid-base and redox sites (i) Acid-base step: CH,=CH—CH; + 0? S$ —> [CH,—CH—CH,] + OHSGi) Redox step: [CH,—CH—CH,] + 2M" —> [CH,—CH—CH,]* 42M" (ii) Nucleophillic addition of O*- ions (acid base with O* transfer; Guttman): [CH,—CH—CH,]* +07 S$ — Ch,=CH—CH,—O" S (iv) Acid-base: CH;=CH—CH,0-S + 0” —> CH,==CH—CH—O*S + OHS. (v) Redox: S + 2M" —> CH,=CH—CHO + 2M"'* CH,=CH—CH— (vi) Redox: amtrit W202 ongnt 4 Few polyfunctional catalysts are known, but it is very likely that in many other types of reactions different kinds of functional groups are indeed working. The concept of polyfunctionality can help in the design of new types of catalysts. Polyfunctionality can be obtained by (1) simply mixing two different active components. (2) by supporting one active component onto a material with different active functionality (i.e., a dehydrogenating com- pound onto an acid support), or (3) by creating catalysts which have different types of active sites in their structure. Basic Catalysts Basic catalysts are constituted of KOH, K on carbon, NaOH on alumina, or of basic oxides such as MgO, CaO, ZnO. The basic catalysts are not extensively used in industry because they deactivate by exposure to the atmosphere (H,). (CO,). In comparison to acid catalysts, basic catalysts catalyze other types of reaction and exhibit a peculiar selectivity for some reactions. The first step in the mechanism of a base-catalyzed reaction is the formation of a carboanion through the interaction of the organic substrate with the catalyst surface. The main differences in reactivity between basic and acid catalysts are shown below: 1. Isomerization of double bonds Basic catalysts isomerize olefins forming the cis iso- mer; in addition they do not possess cracking activity. 2. Dehydration Basic catalysts dehydrate secondary alcohols to primary olefins, while acid catalysts produce mixtures of primary and secondary olefins. 3. Alkylation Basic catalysts alkylate alkylaromatic compounds in side chains and not in the aromatic rings. 4. Aldolic condensation and diels alder reaction These are typical reactions catalyzed by basic catalysts. 5. Polymerization Basic catalysts catalyze the formation of polymers with very high mo- lecular weights. CATALYSTS FOR THE TRANSFORMATION OF CO ‘The most important catalytic reactions of CO transformation are: 1. Synthesis of methanol from CO and H,, with Cu/Zn/AVO based catalysts. 2. Synthesis of long-chain paraffins from CO and H, with supported Fe catalysts. 3. Synthesis of acetic acid from methanol and CO, with Rh complexes.4, Synthesis of aldehydes from CO/H, and olefins, with Rh or Co complexes. 5. Co +H,O. to produce H, and CO, with Cu/Zn/AVO based catalysts. AUTO-EXHAUST CATALYSIS Stimulated largely by legislation in California, the car industry began seriously to seek ways of minimizing pollution from auto-exhaust in the late 1960s. The US Clean Air Act of 1970 led gradually to the introduction of catalytic mufflers (auto-exhaust catalysts), the initial objective being to reduce the emission of CO and un-bumt hydrocarbons (C, H,). The so- called three-way catalyst has been in use since 1979. Steps for such types of catalysts use is being made mandatory by supreme court of India recently. Its name reflects the simultaneous treatment by the catalyst of the two reducing pollutants, CO and C, H,. and the oxdizing pollutant, oxides of nitrogen NO, Three-way catalysis is possible provided the fuel/air ratio, termed A, in the gas mixture is stoichiometric, At 2 < | the activity for NO reduction is high. but not for the oxidation of CO and C,H,. At 2>1 the reverse is the case. Hence. a special sensor-governed and electronic control system has been developed in motor vehicles that are filled with auto-exhaust catalysts so as to guarantee the desired gas composition. Indeed the control system is rather more critical than the catalyst itself. which contains thodium and platinum as key constituents. An auto-exhaust catalytic converter typically contains 1-2 ¢ of platinum, and 0.2-0.3 of rhodium. Serious efforts are continuing to develop the catalysts which will effectively convert both the reduction of NOx and oxidation of HCs/CO in lean conditions. Copper and promoted copper on zeolites are identified as promising catalysts and further work is continuing all over the world. Further efforts are continuing to develop effective catalysts which decomposes the NOx into N3 and O;. Catalysts in Electrochemistry and Photoelectrochemistry The efficient production of fuels from inexpensive precursors by utilization of solar energy with cheap and stable chemical systems is nowadays the target of much pure and applied research. Desirable reactions are. typically, the reduction of water to hydrogen and of CO, to methanol. each driven by the absorption of light. Since absorption of light creates electron- hole pairs-a “hole” is simply that which is left behind in an orbital or band of orbitals when an electron is promoted to higher energy—the fuel producing reaction must be accompanied by an oxidation reaction, Ideally, this oxidation reaction should consume a plentiful materia). e.g., water, thereby generating O, or, alternatively, produce a chemical of commercial valus such as Cl; from Cl ions. There is an analogy here with photosynthesis, which employs light absorption to produce vital organic materials and Qs. A number of photochemical schemes have been formulated with the aim to harness solar energy. Tc te effective. it is necessary to engineer solids with band gaps. i.e.. energy separations of the highest filled and lowest unfilled orbitals of around 2 eV, so as to take good advantage of the solar spectrum. The engineering of semiconductor solids that absorb light leads us, in turn, to design ‘dual- function catalysts’ of a kind different from those discussed, where we considered hydrocarbon reforming. What is meant here is that there should be a semiconductor catalyst possessing the appropriate electronic properties so that electrons and holes can be used to effect reduction at one electrode (say colloidal platinum) and oxidation at another (Colloidal Ruthenium dioxide, RuO,). Extending the ideas of the Swiss worker Gratzel. the entire catalyst “capsule” or ‘microcapsule’, in which all three components echere. would then be so arranged so as to effect continuous photocleavage of water. The princip’ alsa used int the solar-dclean-up of environmentally harmful chemical by-products, especially chlorohydrocarbons, using titanium dioxide (TiO,) as a photocatalyst. Insofar as destruction of C, and Cy chlorohydrocarbons are concerned, however, catalytic combustion with platinum-group metals is still the preferred option. In essence, catalysis is the vital subject of importance which can provide the efficient solutions for effective conversion of natural gas, coal, solar energy. biomass into usable products, The catalysis have a major role in increasing the efficiency of the processes for conventional liquid transportation fuels/clean fuels and alternative fuels for conventional vehicles as well as non-conventional vehicles fitted with solar cells/fuel cells. Catalysts are essential in protection of environment and processes for prevention of pollutants formation during manufacturing. The world with zero emissions and processes with high selectivities and low energy processing will have to be well integrated with efficient catalysts, The momentum in understanding the intricacies of catalysts and processes based on catalysts is increasing day by day. More efforts will be continued in the 21st century to design the better catalysts for sustainable development of the world. PRE-TREATED STOICHIOMETRIC VANADIUM (PSV) CATALYST PSV is a Zeigler type transition metal catalyst. It is prepared by reacting SV catalyst with (i-Bu),A|-tetrahydro furan (THF) complex (1 M in n-heptane) at a mole ratio of 2:1 (i-Bu)s ALV (0.1M vanadium concentration) for 20 hours at room temperature. When (i-Bu), Al is alone used PSV is aged only 0.08 hour before using. The PSV catalyst prepared with (i-Bu); AL-THF gives a chocolate brown dispersion and can be separated by centrifuge into heptane soluble and heptane insoluble. X-ray studies show isobutyl is attached to aluminium, excessive heat treatment decreases its catalytic activity. Other modification with ethyl, benzyl or triethyl amine can be carried out ina similar way. Caring Agents Muira, Soola and Masaki have contributed review articles on curing agents. The most widely used compounds as curing agents are those that contain active hydrogen, e.g., amines, acid anhydrides, and Lewis acids, etc, Table. 6.4 Accelerating Effect of Various Additives on Cure of DGEBA/Imidazoline Blend Additive Peak Gel time, minutes (at 5.6% unless noted) exotherm, °c With additive Without additive Acids: Formic 130 88 243 Acetic 127 ut 243 Butyric 124 112 243 ‘Caprice 128 134 237 Lactic (crude) 127 104 237 Oxalic, dihydrate 1 9 wt Maleic 109 130 19t Benzoic 128 101 268 Phthalic rt) 179 191 (Condy270 LeXtWOOK OF rulymers—pasi¢ Concepts ‘Additive Peak Geltime, minutes: (at 5.6% unless noted) exotherm. °c With additive Without additive Salicylic 134 m 191 p-Toluenesulfonic, monohydrate 140 59 23 Alcohols and phenols: Water, 5% 159 47 192 Methanol 127 112 m2 Propane-1,2-diol 130 123 242 Polyethylene glycol, mot. wi. 300, 118, 180 237 Glycerol 125 uu 237 Phenol 130 98 212 Resorcinol 135 ar 268 Amides and sulfonamides: ‘Acetamide 129 100 196 Dimethylformamide a1 285 218 106 233 196 Urea, 2% 133 195 300 Fatty polyamide, 5% 137 15 300 Salicylamide 135 85 191 Benzenesulfonamide 149 95 266 N-Ethyl-o-and p-toluenesulfonamides (crude mixturey 132 128 lo Hydroxyamine Dicthanolamine 135 8 237 Triethanolamine 134 7 243 Trithydroxymethyljaminomethane 128 n7 191 ‘o-Aminophenol 133 n2 268 p-Aminophenol 128 110 268 ‘m-Diethylaminophenol 125 143 237 Tetracthanolammonium hydroxide solution 135 14 237 Miscellaneous compounds: Toluene 83 31s 260 Xylene 87 323 250 Dioxane 84 320 260 Salicylaldehyde 7 207 237 Methylethyl ketone 62 390 260 Ethyl acetate "4 365 260 Ethylene sulfite 1a 160 250 Acetonitrile 104 280 250 Nitromethane 149 235 260 Nitrobenzene 95 265 260 Note: Maleic acid, phthalic acid, acetamide, and urea were incompletely dissolved. The following additives may have suffered partial decomposition or reaction with curing agent: trihydroxymethyl) aminomethane, oxalic acid dihydrate, maleic acid, phthalic acid, ethylene sulfite, and nitromethane. ALIPHATIC DIAMINES H,N(CH,)nNH, CH; Polymethylene diamines H,N(CH,), baw,CH; CH; H,N (CH,),0(CH,CH,0),, (CH;), NH HAN cH, CHINE, ete, Hy Polyether diamines Branched-chain polymethylene diamines Linear and Branched Aliphatic Polyamines H,N(CH,),NH(CH,),.NH, CH,(CH,),(CH,CH,NH),CH,NH, Dicthylenctriamine (1 = 2) Substituted polyamines Iminobispropylamine (n = 3) Bis(hexamethylene) triamine (n = 6) H,N{(CH),NH},(CH3),NH» {CH;(CH3),]2N(CH),NH> Triethylenetetramine (n = 2) Dimethylaminopropylaminc (1 = 0) Tetraethylenepentamine ( = 3) Diethylaminopropylamine (1 = 1) Pentaethylenehexamine (n = 4) CH, H,N(CH;):NH(CH;),0H | H, N(CH, )N(CH);NH2 Aminoethylethanolamine JMethyliminobispropylamine NH, RNHCH,CHH, Hy Substituted 1, 2-diamines (R= isopropyl, phenyl, etc). ALICYCLIC POLYAMINES CH HCV /\_ | — NH» Hy — 1H ww ns CNH: H,NCH,CH)—N $ NI CH. Menthane diamine N-Aminoethylpiperazine NH, HAN NH HC. / Ss So HyC* \—< ‘CH,NH 1, 3 Diaminocyclohexane Isophoronediamine ALIPHATIC AMINES CONTAINING AROMATIC GROUPS, CH,NH, CHNH; y av Aca LAL, AK ~~~ CH,NH; cl &4 cl CH,NH, m-Xylylenediamine Tetrachloro-p-xylenediamineLinear and Branched Aliphatic Amines (a) Polyether Diamines The polyether diamines are synthesysed either by reacting ammonia with ethylene or propyl- ene glycol or by reacting amines with halohydrin ethers. The typical polyether diamine is diethyl glycol bis-propylamine having a molecular weight of 210 with 4 active hydrogens. It is used as a curing agent to the extent of 28 phr. & 66°. (b) Ethylene Diamine (EDA) Poly Methylene Diamine (PMDA) Aliphatic amines are prepared by several methods such as reduction of corresponding nitro compounds or direct reaction of ammonia on hali CICH,CH,Cl + 4NH, —> H,NCH,CH,NH, + 2NH,Cl BrCH,CH,Br + 2NH,; —> H;NCH,CH,NH, + 2HBr ethylene dibromide ethylene diamines But the most important process is the catalytic hydrogenation of corresponding nitriles either in vapour phase or liquid phase. Gaseous hydrogen is used in liquid phase. Nitriles are produced by any one of the following processes: (i) Amminlysis of carboxylic acids (ii) Reaction of alkylhalides with alkalimetal cyanides Gili) Cycloparaffins are reacted with ammonia using suitable catalysts. EDA (bp 116°) gives a short pot life and the reaction products are brittle. It cures the epoxy resin at the room temperature but complete cure can be effected in 1 to 2 hours at 150°C. The branched chain polymethylene diamine is used in casting and has a tg 47. (c) Diethylene Triamine (DETA) DETA is a low viscosity liquid which forms in contact with air at room temperature. DETA having five active hydrogen atoms per molecule is normally used in amounts 8-12 p hr. When used less than I p hr. DT decreases. Curing with glycidyt ether resins is complete in 24 hours at 25°C. By post curing, it for 1 to 2 hours at 65°C to 95°C the properties very much improve. DETA is used with epoxidized olefins to provide long pot life systems, DETA has also been used as curing agent with aromatic glycidyl ether epoxy resins for casting, laminates, adhesives and baking type solution coatings. (d) Iminobis-propyl Amine Iminobis-propyl amine is best used as LO p hr with diglycidy! ether of bisphenol A tc obtain best mechanical properties. (e) Bis (Hexa Methylene) Triamine Bis (hexa methylene) triamine has an unpleasant odour and must be dissolved in the resin at 50°C, It provides an extended pot life with epoxy resins together with a room temperature cure. (P Triethylene Tetramine (TETA) TETA is a widely used curing agent with diglycidyl ether resins. It give products with improved mechanical strength and good chemical resistance with epoxidized polybutadiene and gives a long pot life at room temperature and requires a cure time of 2 to 4 hours at 150°. It has a lower vapour pressure.(g) Tetra Ethylene Pentamine (TEPA) TEPA is the most widely used curing agent for solution coating based on diglycidyl ether bisphenol A resin (mw 950). TEPA behaves as curing agent in a way similar to TETA but its concentration with epoxidized phenol formaldehyde novolac effect the DT. (h) Diethyl Amino Propylamine (DEAPA) DEAPA contains tertiary amino groups. It possesses only two active hydrogens which serve to bind the amine into the cured system. It is used for specific purposes as it has a lower cure rate and a much longer pot life. Itis used at 4 to 8 p hr. Cycloaliphatic amines These amines may contain primary, secondary. or tertiary amino groups. The most common compounds of this group used are methane diamine, N-methyl piperizine, N - B hydroxyl ethyl piperizine, piperdine, pyrrolidine, isophorondiamine, bis (p-amino cyclo hexyl) methane, 2, 6 diamine cyclo hexanol, and 1, 3 diamino cyclo hexane. Smily synthesized several cycloaliphatic amines. Methane Diamine (MDA) MDA is used with aromatic glycidyl ether resins to promote long pot life with fast. moderate cure cycles from 2 hours at 80°C to 30 minutes at 130°C, When it is post cured at 160°C the DT is about 160. Piperizine Piperizine is prepared by the dimerization of ethylamine in presence of an acid. Itis solid with mp 104°C and bp 140°C. Piperidine is obtained either by the reaction of sodium and absolute alcohol on pyridine or by the catalytic hydrogenation with raney nickel of gluconamide. It has a boiling point of 105°C. Pyrolidine is prepared by the reduction of pyrrole or ethylene cyanide CH,—-C=N CH,—~CH . +4H, —— | “NH CH-C=N CH >—CH,~ CH=CH 1%, CH)—CH2 i a NH cece “N Naatohot CH)—cH, “ It is a liquid boiling at 89°C. Although epoxy resins are somewhat more expensive than the conventional resins and are used in coating materials, now they are used 50% of their total production in coating. N-Amino Ethyl Piperizine (AEP) AEP has a tertiary amino group in the cyclic ring which takes part in catalytic curing very sluggishly. Itis a clear, high boiling liquid. It forms an adduct with butyl glycidyl ether which does not affect any curing even for 4 hours at 120°C. In small quantities, it does not cure completely at room temperature, but in large quantities (20 - 22 p hr ) the curing rate of the reaction exceeds that of aliphatic polyamines. It provides a DT of 110°C and much better impact strength of the castings.Isophorone Diamine (IPD) UNH LH) —CH Isophorone diamine (IPD) C=C (CH) “CH»—CH” ‘NH? and bis (4-amino-3 methylcyclohexyl) methane caromin C 260. ACH _CHLCH, CHCH,CH < "CHNH, “ cH,CH, ~ “CH,CH Me (PACM-20) are colourless liquids of low viscosity. They readily mix with epoxy resins and do not stain the skin. They are issued at a concentration of 33 p hr and require a temperature of 150°C for 2-4 hours for curing. Aliphatic Amines Containing Aromatic Groups m-xylene diamine and tetrachloro-p-xylene diamine are the representative compounds belonging to the above class. m-xylene diamine provides excellent heat cures with DGE resins. Tetrachloro-p-xylene diamine imparts flame resistance to the cured resin, Di (mi- xylene) triamine and the reaction products of phenoplasts, phenols with epoxy resins are the other substances. Similarly the reaction products of ethylene diamine with bromo- and chloro methyl phenyl ethers are also used as curing agent. Amine Adducts Amine adducts are room temperature curing agents and are formed by reacting an excess quantity of aliphatic amines with epoxy containing materials. Amine adducts are generally preferred because they are less volatile and provide performance advantage. NHC. They possess milder odour, lower volatility, less critical mixing ratio, less tendency to produce curing agent blush under humid conditions and less tendency to corrode metal containers. They also provide improved pot life, resin compatibility, faster or slower cure and lower dermatitic potential. maximum resistance to solvents, acids, and other corrosive chemicals. Initial flexibility and retention of these properties is adequate for most uses over rigid and semi-rigid substances. There are two types of adducts: (1) Based on a liquid epoxy resin. (2) Based on a solid epoxy resin. In either case, there is an increase in molecular weight, thus reducing its vapour pressure, volatility and odour. Amine Adducts with Mono and Diepoxides The aliphatic polyamines can be adducted to one of the following: (a) Monofunctional glycidyl ether (b) Pentachloropheny! glycidyl ether (c) The reaction product of epichlorohydrin with bisphenol A containing phenolic hydroxyl() Epoxidized olefins from unsaturated fatty acid glycrides (e) DGEBA and its higher molecular weight compounds The general reaction is H OH Zo. ' ? H,NRNH; + R'CH—CH, ——> H)NRNCH,CHR’ Amine Resin Adduct Formulation epoxy resin 46% DETA 52% phenol 2% (a) Liquid resin DETA is charged at 21°C to a closed reaction vessel which should be clean and dry. The kettle is provided with an agitator, heating coils and cooling coils. The temperature is raised to 77°C with moderate agitation. Epoxy resin is slowly charged to the kettle and the endothermic reaction is controlled by regulating the addition of resin to maintain the temperature between 74°C-99°C during the addition and reaction steps. After the completion of the reaction, the temperature is cooled to 54°C for an additional 15 minutes. The material is filtered. 4 phr phenol is added to accelerate the cure in thin films. This product is clear amber coloured with a viscosity of 7000 — 8000 cps at room temperature. This is used at 25 phr in DGEBA. The exhotherm reaches to 250°C when the reaction is carried in small masses and the pot life is 20 minutes at room temperature. (b) Solid resin ‘The formulation is DETA 12.6% Epoxy resin (high mw) 46.7% Toluene 10.7% n-butyl alcohol (NBA) 21.4% Glycolether 2.6% The required amount of epoxy resin, toluene, NBA and glycol ether or styrene oxide are charged to a closed kettle with a condenser, heating and cooling coils and an agitator. After the ingredients are thoroughly blended, DETA is rapidly added with constant agitation. The temperature will rise to 71°C and the reaction mixture is maintained at 71°C for 2 hours after which it is allowed to cool to 57°C- 60°C. The material is filtered. It gives a clear light coloured solution with a viscosity of 400 cps at room temperature. The contains 42% non-volatile and has an equivalent weight of 200. sol Wide variety of primary amines like hydrazine are employed in producing these resin adducts. Similarly other resins like DGE of polyoxypropylene is adducted with aliphatic polyamines.Ethylene and Propylene Oxide Adducts Ethylene or propylene oxide adducts of amines are obtained by the reaction with amines in presence of water. Depending upon the ratio of reactants and the reaction conditions various mono or bis-hydroxyalkyl derivatives like N-hydroxy diethylene triamine, N, N'-Bis (hydroxy ethyl) diethylene triamine, N (2-hydroxypropyl) ethylene diamine, are produced. 52% DETA is warmed upto 76°C and 46% by weight resin is added. The reaction is exothermic and the solution is maintained between 75°C and 100°C for 2 hours. 2% phenol is then added and the solution is cooled to 55°C at the end of the reaction. Adduct is obtained after filtration. This adduct has a viscosity of 70 - 80 poises and is used at a concentration of 25 phr. DETA reacts with propylene oxide as follows: oO a H,0 HN(CH3))NH(CHy))NH + CHjCH—CH, "=> CHyCHCH2NH(CH3)NH(CHy)2 Kcess DETA Propylene oxide and H)NCH;CHCH2(0 CH2CH3),NE Polyoxypropylene diamine + CHy—CH) ——~ HO(CH,))NH(CH,)NH(CH,);NHCH,CE ° H}N(CH2))NH(CH2)2NI DETA Ethylene oxide and HN(CH)NH(CH)3NHCH3CHO Hydroxyethyl Diethylene Triamine Normally, it contains 15% bis and 85% monohydroxyethy! diethylene triamine. It has a viscosity of 200 to 500 centi poises at room temperature. Ata concentration of 20 p hr the pot life of the resin is 18 minutes. It obtains a maximum DT of 90°C at a concentration of 20 p hr. Glycidyl Adducts of Amine DGEBA is reacted with excess of DETA at lower temperatures. It is cooled and vigourously stirred to avoid gelation. °. OH fo. | H)NRNH2 + R'CH—CH, H2NRNCH;CHR’ Amine Acrylonitrile Adduct ‘The reaction of amines with acrylonitrile yields curing agents with reduced activity. In this reaction amines add to the double bond of acrylonitrile H H,NRNH, + CH;—CH—C==N —> ruven CH,CH,C==N —s —CH:CHC=SN_, Polyamine Acrylonitrile | HOA Necch,cH,N an CH,CH,C=NNormally DETA is used with acrylonitrile but cyano ethylated hexamethylene diamine is a flexibilizing co-curing agent. This used at 22 phr to reduce the viscosity of the mix from 30— 40 centipoises to 100 to 800 centi- poises, These adducts are useful in casting; the vapour pressure is also reduced from 0.2 mm Hg at 25°C to less than 0.1 mm Hg at 25°C. Schiff’s Bases and Ketone Blocked Polyamines Amines may be condensed with aldehydes to produce Schiff's bases and with ketones to produce alkylnitro alkyldene compounds also known as ketamines, 2 moles of ketone on. reaction with diethyl amine give a compound in which both the primary amines are blocked. H H | I R,C=0 + H, NCH,CH,NCH,CH,NH, —> R,C=C—CH,CH,NCH,CH,N=CR, + H,0 Imine on reaction with phenyl glycidylether gives RyC—= NCH;CH.NCH,CHN—=CRz CH,CHOHCH;0 ~(O) Although such compounds when reacted with DETA, TETA or phenyl glycidyl ether are more non-reactive, but on hydrolysis with moisture C==N bond hydrolyses to regenerate primary amines to provide cure and ketone which diffuses to the surface and evaporates. ‘These are used at about 25 to 30 p hr, which provide pot life of 8 hours. These are used in high solid surface coating system. Normally in coating formulation, a mixture of ketone amine is used instead of condensate. Acid Salts Primary Amine Complexes Amines on being reacted with metallic salts of organic or inorganic acids give products of long pot life and cure at about 120°C. Inorganic acid salts of primary amines can be dissociated with water. Acid salts of amine resin adducts are used as curing agents in fabric treatment. Diester and hydrochloric acid salts of monoethanol amine act as curing agent. The acid salts of primary amines are used in leaking type of coatings. Amine Blends It is useful to use a blend of two amines to obtain properties between the two. DETA may be blended with a fatty mono-amine CH, (CH,)n NH; (n is from 8 to 22 ) to obtain a flexible cured product. A primary amine on being blended with a secondary amine like ethylene diaminodi -O- cresol to incorporate a phenolic group in the cure structure. TETA blended with triethanol amine lengthens the pot life. Ammonia and ethylene glycol give a blend of amines for use as epoxy curing agent. Aromatic Primary Amines (MPDA) m-phenylene diamine is prepared by the partial reduction of m-dinitro benzene with alcoholic ammonium sulphide or aniline which on nitration in presence of strong sulphuric acid gives n-nitroaniline which on reduction gives m-phenyiene diamine.184 Textbook of Polymers—Basic Concepts MPDA is a colourless crystalline solid with a molecular weight of 108 and a melting point of 63°C. On exposure to air, it darkens due to oxidation. It possesses four active hydrogens and is used at 14.5 p hr. It remains liquid at room temperature, when it is heated to 80°C and then super cooled slowly to room temperature. Solid MPDA is incorporated in epoxy resin by two methods: (1) MPDA is melted at 65°C and resin is also warmed to 65°. The two components readily blend together. (2) The resin is heated to 80°C and HPDH is added slowly, stirring the melt continuously. In either case, considerable irritating fumes are given out. The mixture is gelled at 85°C for 2 hours and then postcured at 150°C. Epoxidized novolac resin with a functionality of 3.5 when cured with MPDA gives at DT of 200°C. Epoxidized polybutadiene, cured with MPDA gives properties similar to those obtained by curing with TETA. Using resorcinol as an accelerator it cures in 4 hours at 150°C. It partially cures to give a B stage resin which flows on application of heat. It is used in glass cloth aminates at 120°C — 150°C. It is also used in filament wet winding applications. The B- stage systems in combination with fillers are used as stick solders and moulding compounds and castings. It imparts high strength and high degree of chemical and heat resistance. Methylene Dianiline (MDA) Methylene dianiline is a light brown solid of molecular weight 198 and a melting point of 89°C, It possesses four active hydrogens and is used at a concentration of 26 phr. Its concentration effects the DT of the product. It is heated to 100°C and mixed with equal parts of the resin at 100°C and the mixture is added to the remainder part of the resin at the room temperature. Then its pot life is 20 hours at 25°C. In larger proportions, the material is allowed to set overnight at room temperature. With epoxidized novolac resins and MDA higher cure temperatures are required with epoxidized olefin resins and MDA, it requires an accelerator. It is used in laminated products which are cured in 120°C to 180°C range. Its unreinforced castings are somewhat brittle. 4, 4' Diaminodiphenyl Sulphone (DADPS) DADPS is a free flowing reddish to yellow powder of molecular weight 256 and a melting point 175°C. Its concentration also effects the DT. It is employed in hot melt system at 25°C. It provides the best resistance strength after prolonged exposure at elevated temperature. It is used as a curing agent for wet and dry lay up laminating. It is slightly skin irritant and contact with the fumes should be avoided.