Properties of Zeolites as Cataystics

Properties of Zeolites as Cataystics1.0 INTRODUCTION TO ZEOLITESZeolites atomic number 18 vapourous aluminosilicates, composed of TO4 tetrahedra (T = Si, Al) with O atoms connecting neighbouring tetrahedral, that view as pores and cavities of molecular dimensions (Breck, 1974). Many occur as cancel minerals, besides it is the celluloid varieties which ar among the most widely practice sessiond sorbents, catalysts and ion- re-sentencing materials in the world (Barrer, 1982).The transmit be large enough to allow the passage of guest species. In the hyd gaitd phases, dehydration occurs at temperatures broadly below about cdC and is largely reversible. The fashion model may be interrupted by (OH, F) groups these occupy a tetrahedron apex that is not sh ard with adjacent tetrahedra. Zeolites ar different from other porous hydrates, as they retain their structural oneness upon loss of urine.The Structure Commission of the International Zeolite Association identifies each material with a three-letter mnemonic code (Baerlocher et al., 2001) e.g. Amicite- GSI Faujasite- FAU etc.1.1 IMPORTANCE OF GREEN PROCESSESIn the chemic substance substance industry, the acceptability of a summons is not only governed by cost and yield but in terms of eco-friendliness and pollution abatement. Choosing a more(prenominal) than efficient catalytic route has greatly improved the power of chemical functiones.Green chemistry has been defined as the design of chemical products and does in order to clip or eliminate the generation of raging substances (Armor, 1999). The principles of green chemistry listed by Armor (1999) employs future approaches to revolutionary chemical processes. It includes efficient use of raw materials, energy efficiency, use of biodegradable products and other subtle features.2.0 HISTORICAL DEVELOPMENT OF ZEOLITES2.1 NATURAL ZEOLITESResearch in the line of merchandise of zeolite acquaintance and technology made its first steps with innat e(p) zeolites and was mostly foc utilise on native zeolites until the beginning of the 1950s. The history of zeolites began in 1756 when Swedish mineralogist A.F. Cronstedt discovered the first zeolite mineral, stilbite when theatreing its apparent properties discovered its strange behavior upon heating although in that location is no certain proof of its identity. The term zeolite was coined from cardinal Greek words, zeo (to boil) and lithos (stone). On the contrary, the first zeolite, chabazite, described by Bosch DAntic in 1792 has puzzle out evidence in literature. Several other zeolites were discovered in the following years and around 1850, only about 20 zeolite graphic symbols were musical themeed in mineralogy books, including analcime, brewsterite, chabazite, edingtonite, epistilbite, faujasite, gismondine, gmelinite, harmotome, heulandite, laumontite, levyne, mesolite, natrolite, phillipsite, scolecite, stilbite, and thomsonite. Starting from the middle of the 19 th century until about 1975, there was a moderate increment in the number of zeolites discovered (about one new type every 6-7 years) and a clear acceleration in the pull through twenty five-thirty years. About 40 rude(a) zeolites are cognize (Tschernich, 1992). close zeolites known to occur in nature are of lower Si/Al ratios, since organic fertiliser structure-directing agents necessary for physical composition of siliceous zeolites are absent. some cadences natural zeolites are found as large single vitreous silicas, though are very difficult to make in the laboratory. The catalytic natural process of natural zeolites is limited by their impurities and low surface areas.However, interests in natural zeolites shifted towards zeolite entailment and synthetic materials, as they offered a series of advantages such as wider versatility, more open mannikins( for adsorption and catalysis),and quality in constitution and chemistry. As a proceeds, research on natural zeolites, was brinyly devoted to ion transpose process which was discovered around 1850 (Thompson, 1850 Way, 1850). Few years later, Eichhorn observed that chabazite and natrolite be breakd as reversible ion exchangers. In the early decades of the 20th century, ion exchange selectivity of a variety of zeolites for peculiar cations, e.g., ammonium was per peeed (Barrer, 1950) and starting from the end of the 1950s, found uses in various sectors of environmental relevance, e.g., treatment of wastewaters and soil rebuilding and remediation. The most upstart frontier in the application of natural zeolites is in the field of feeling eruditions.One of the drawbacks of natural zeolite research for application purposes is due(p) to the limited availability of zeolite as it is a precious mineral, compared to the synthetic counterparts which could be mass produced at a lower cost (Colella, 2005).2.1.1 FormationThe pathway of natural zeolite blueprintation is similar to the laboratory implicatio n of zeolite. Zeolite nucleation, crystallisation and crystal growth comeback place as a result of relax to fast locomotive cooling system of warm to hot magmas(of volcanic origin), which are basic, oversaturated in silicate and aluminate species and contain alcalescent and/or alkali-earth cations.hot fluid + volcanic ash oversaturated basic magma zeolite crystals radical + gel)The magma is obtained via hydrolysis of the original glassy material and is responsible for the tetrahedral coordination of aluminium and together with silicon. The main factors responsible for the structural formation are temperature, chemistry of the ash and the chemical composition of the resulting solution. Gel is formed along the process but is even not directly connected to nucleation and growth, as there is evidence that zeolite nuclei form from the oversaturated solution at the glass shards / solution interface (Aiello et al., 1980).Temperature and time are two factors which differentiate natura l zeolitisation from laboratory implication.2.1.2 Physico-chemical propertiesi.Cation exchange The ion exchange properties of natural zeolites depend on their chemistry which is generally in terms of selectivity. Selectivity depends on the frame escape topology, ion size and shape, charge density on the anionic mannequin, ion valence and electrolyte assimilation in the aqueous phase (Barrer et al., 1978).ii.Reactions with alkalis Oncein alkaline environments, zeolites exit unstable as they tend to transform, similarlyto glassy systems, into more stable phases, usually into other framework silicates (Goto and Sand, 1988). The interaction of zeolite-rich materials with Ca(OH)2 give rise to calcium silicates and aluminates, which upon hydration are able to chasten in both aerial and aqueous environments. This behaviour makes them to be known as pozzolanic materialsThermal properties Heating of zeolite powder induces physical and chemical changes, which have been shown to include w ater loss (which causes expansion on heating), decomposition and foul up evolution, phase inflection, structure breakdown, re-crystallisation, melting etc (Colella, 1998). This property enables zeolite tuff stones to dis lam headspring behaved sound-proofing and heat insulation and serve as good building materials. Depending on zeolite nature, chemical composition and rock constitution, the tuff expands as a result of quick heating at temperatures of 1250C or higher up, inadvertently followed by a rapid quenching to room temperature.2.2 SYNTHETIC ZEOLITESEarly work could be traced back to the claimed tax deduction of levynite by St Cl breezee Deville in 1862 as there were no reliable methods for fully identifying and characterising the products. The origin of zeolite tax deduction however, evolved from the work of Richard Barrer and Robert Milton which commenced in the late 1940s. The first synthetic zeolite unknown as a natural mineral later found to have the KFI structure ( Baerlocher et al., 2001 ) was discovered by Barrer when investigating the conversion of known mineral phases under the action of knock-down(prenominal) salt solutions at fairly high temperatures (ca. 170-270 C). Robert Milton was the first person to use freshly precipitated aluminosilicate gels to carry out reactions under milder conditions. This led to the discovery of zeolites A and X (Milton et al., 1989). Initially, the synthesis of zeolites required the use of only inorganic reactants but was however expanded in 1961 to include quaternary ammonium cations trail to the discovery of silica-rich phases (high-silica zeolites). Subsequently, more synthetic zeolites have been discovered (Baerlocher et al., 2001), as well as zeolite-like or zeolite-related materials (Szostak, 1989) known as zeotypes- re leaveed by microporous alumino- and gallo phosphates (AlPO4s and GaPO4s) and titanosilicates.Studies on understanding zeolite synthesis have continued to be carried out upto the pres ent day (Table 1). This has been due to discoveries of new materials, advances in synthetic procedures, innovations in divinatory modelling methods and, especially, by the development of new techniques for the investigation of reaction mechanisms and the picture of products.Table 1 Evolution of materials development in the zeolite field slump Si/Al zeolites (1-1.5)A, XIntermediate Si/Al zeolites (f2-5) A)cancel zeolites erionite, chabazite, clinoptilolite,MordeniteSynthetic zeolites Y, L, large-pore mordenite, omega juicy Si/Al zeolites (10-100)By thermochemical framework modification super silicious variants of Y, mordenite, erioniteBy direct synthesis ZSM-5, silicon oxideteSilica molecular sievessilicaliteSource Flanigen (1980)2.2.1 Mechanism of Hydro caloric tax deductionExperimental observations of a typical hydrothermal zeolite synthesisDue to its chemical re application and low cost, amorphous and oxide-like Si and Al which make up the microporous framework are mixed with a cation source usually, in a basic water-based medium. The resulting aqueous mixture is so heated in a sealed autoclave at above 100C allowing the reactants to remain amorphous for sometime (induction period) aft(prenominal) which gossamer zeolites are detected (Figure 2). Gradually, an approximately equal mass of zeolite crystals which is recovered by filtration, washing and drying replaces all the amorphous materials (Cundy and Cox, 2005).The bond type created in the crystalline zeolite product (e.g. zeolite A or ZSM-5) which contains Si-O-Al linkages is similar to that present in its harbinger oxides, therefore the enthalpy change is not great. This process reduces nucleation rates, thereby forming large crystals.Reactivity of the gel, temperature and pH affect the rate of zeolite formation as an enlarge in pH and temperature leads to increase in the rate of formation of zeolite crystals. In their mother liquors, the zeolitic phases are metastable, thereby transforming the initial zeolite into an undesired thermodynamically more stable phase (Ullmann, 2002).2.2.3 Synthesis from clay mineralsKaolin and metakaolin (calcining kaolin at 500-700C) are two big ashess employ for the production of the zeolites NaA, NaX, and NaY (Breck, 1974 Barrer, 1978) because ring-binder-free extrudates and granules which offer advantages in adsorption technology are produced.2Al2Si2O5(OH)42Al2Si2O7+4H2OKaolin MetakaolinDepending on the zeolite, the clay is shaped and, SiO2and seed crystals are added and while in the preformed shape, the zeolite crystallises. Alternatively, zeolite is formed when the binder component of metakaolin undergoes hydrothermal treatment with sodium hydroxide solution (Goytisolo et al., 1973 Chi and Hoffman, 1977). utilise ultrasonic radiation, reaction rate is enhanced and there is energy rescue and lower production cost due to lower temperatures. This process is less(prenominal) often used as it could cause odor of the product due to impu rities present in clay e.g. iron2.2.2 Industrial Zeolite SynthesisZeolite synthesis is an extremely broad area of research and due to differences in the readiness of each zeolite type, two representative zeolite types, TPA-ZSM-5 and zeolite Na-A, are chosen for a more detailed presentation of the synthesis Table 2 (Jansen, 2001).Table 2 Synthesis mixtures, physical chemical properties of the representative zeolitesMolar oxide ratioNa-ATPA-ZSM-5SiO211Al2O30.5Na2O10.16H2O1749TPA2O0.3T (C) one hundred fiftyPhysical chemical substance propertiesPore arrangements3D, cages connected via windows2D, intersecting channelsBronsted activitylowHighAffinityhydrophilicHydrophobicPore lot (cm3/g)0.370.18Source Jansen (2001)The composition of zeolite product can be expressed by the cation type and its overall Si/Al ratio. In the preparation of zeolite, nucleation is the rate determining step which is influenced by a range of factors dependent on the temperature of the reaction mixture.Low Temp erature Reaction Mixture Here, the reaction mixture is disposed(p) at low temperature, At high pH, condensation occurs when the nucleophilic deprotonated silanol group on monomeric neutral species is attacked (Figure 5). The panellingity of the silanol group depends on the number and type of substituents on the silicon-atom (Jansen, 2001).Temperature raise of the reaction mixture from High Temperature Reaction Mixture At this temperature, zeolites are formed from amorphous material which involves, reorganisation of the low temperature synthesis mixture, nucleation and precipitancy (crystallisation). During the induction period, gel and species in solution rearrange from a continuous changing phase of monomers and clusters which disappears through hydrolysis and condensation, in which nucleation occurs (Jansen, 2001). The process particles become stable and nuclei forms, followed by crystallisation which could occur in metastable solid, highly dispersed or dense gel forms.Product q uality, reaction time and yield influence efficient production of zeolites by optimising their composition.2.2.2 Secondary Synthesis MethodsCatalytic or adsorbent properties that cannot be achieved by direct synthesis utilise post-synthesis (secondary) treatments to increase catalytic activity, shape selectivity or porosity and thermal/hydrothermal stability. Dealumination and ion exchange are used to carry out these modifications.DealuminationThe zeolite structure is selectively dealuminated by acid solutions, washing out aluminium out of the crystal, as was observed for zeolite A. However, for higher silica containing materials (clinoptilolite), a fully decationated structure is produced after continuous acid treatment. The metal ion is replaced with H3O+ followed by (Al+3 + H3O+) removal, generating a hydroxyl nest.Aluminium is removed from the framework but not the crystal by hydrothermal dealumination. The heterogeneity in the concentration of the framework and non-framework of aluminium depends on the type of modification used. Hydrothermal treatment causes the amorphous aluminium to collect on the crystal surface which through fluorosilicate treatment can reduce aluminium centred acid sites. Often, a secondary pore system is generated and hydroxyl nests can be annealed. In order to enhance the catalytic properties as well as stability, silicon, aluminium and other metal ions are introduced into the framework (Szostak, 2001). Other methods of producing thermally and hydrothermally stable cracking catalysts include use of EDTA, SiCl4 drying up, and (NH4)2SiF6.Acid mediated dealumination process via aluminium extraction and generation of hydroxyl nest (Szostak, 2001)Ion ExchangeThis is an grave technique in pore-size engineering for the production of zeolitic adsorbents (Breck, 1974). Ion exchange used in the production of Brnsted acid sites has major importance in the synthesis of solid acid catalysts (Ullmann, 2002). Ion exchange can be achieved also , for certain intermediate-silica and high-silica zeolites (e.g., mordenite) by treatment with mineral acids although involves the risk of dealuminating the zeolite framework (McDaniel and Maher, 1976). An indirect route via an ion exchange with ammonium salt solutions must be followed, producing the ammonium form calcined at ca. 400C to liberate ammonia water and give the hydrogen form (Ullmann, 2002). When cations to be exchanged are positioned inaccessible cages, a sieve set is produced.pH is an important factor in ion exchanging of highly charged transformation metal ions in order to prevent metal hydroxide precipitation especially at low pH.2.3 CHARACTERISATION OF ZEOLITESIn order to look out the relationships between the physical and physicochemical as well as sorptive and catalytic properties of zeolites, it is important to know the structural, chemical and catalytic characteristics of zeolites. Several standard techniques are employed in zeolite characterisation. The most common of which is X-ray diffraction used in determining the structure and purity of zeolites. Others include x-ray fluorescence spectroscopy (XRF) or atomic absorption spectrometry, used to analyze elemental composition, sorption analysis to study the pore system, IR-spectroscopy, typically using adsorbed probe molecules to characterize the acid sites, examine electron microscopy (SEM), for determining the size and morphology of zeolite crystallites, high-resolution transmission electron microscopy (HRTEM), nuclear magnetic resonance (NMR) spectroscopy, temperature programme desorption (TPD) and many others (Schth, 2005).3.0 GENERAL APPLICATIONS OF ZEOLITESZeolites are used primarily in 3 major applications ion-exchange, adsorbents, and catalysts. earthy zeolites correspond an important subroutine in absolute majority mineral applications.Adsorbent applications super acid adsorbent applications focus on removal of small polar molecules and bulk withdrawals, by more aluminous zeolites and based on molecular sieving processes individually (Table 3).Table 3 Zeolite commercial applications as adsorbentsPurificationBulk separationsDrying natural gas (including LNG), cracking gas (ethylene plants), refrigerantNormal/iso-paraffin separation, Xylene separationCO2 removal natural gas, flue gas (CO2 + N2) cryogenic air separation plantsOlefin separation, Separation of organic solventsPollution abatement removal of Hg, NOx, SOSeparation of amino acids, n-nitrosoaminesSweetening of natural gas and turn petroleum gasSeparation of CO2, SO2, NH3Source Flanigen (1980).Catalyst applicationsZeolites have the superior use in catalytic cracking. They also play a role in hydroisomerisation, hydrocracking and remindfuls processing. The strong acidity of zeolites plays a role in hydrocarbon processing. Asides this, they are finding increasing use in synthesis of fine chemicals and organic intermediates in isomerisation reactions, nucleophilic substitution and addition et c.Table 4 Zeolite applications in CatalysisInorganic reactions H2S oxidation, NO decrement of NH3, CO oxidation, reductionHydrocarbon conversion Alkylation, CrackingOrganic reactions Aromatization (C4 hydrocarbons), Aromatics (disproportionation, hydroalkylation, hydrogenation, hydroxylation, nitration, etc.)DehydrationEpoxidationBeckman rearrangement(cyclohexanone to caprolactam)Methanol to gasolinechlorofluorocarbon decompositionShape-selective reformingSource Flanigen (1980) Galarneau et al (2001).Ion-exchange applicationsZeolite properties are directly exploited in several applications such as in the detergent industry, where zeolites are used for water softening or building, animal food supplementation and in the treatment of wastewater (Townsend and Coker, 2001). Zeolite A has selectivity for Ca2+, thereby providing a unique advantage. Also, natural zeolites can be used to remove of Cs+ and Sr 2+radioisotopes through ion-exchange (Payra and Dutta, 2003).Table 5 Applications a nd advantages of Ion-exchangeApplicationsAdvantageMetals removal and recoveryHigh selectivities for various metalsRemoval of Cs+ and Sr2+Stable to ionizing radiationDetergent constructor zeolite A, zeolite X (ZB-100, ZB-300)Remove Ca2+ and Mg2+ by selective exchange, no environmental paradoxIon exchange fertilizersExchange with plant nutrients such as NH4+ and K+ with slow release in soilSource Flanigen (1980)Other ApplicationsZeolites also play important roles in health-related applications (such as antibacterial agents, vaccine adjuvants, drug delivery, elevate formation, biosensors and enzyme mimetics), oil refining, and petrochemical processes. Zeolite powders are used for odor removal and as waxy additives. Zeolitic membranes offer the possibility of organic transformations and separations coupled into one unit (Payra and Dutta, 2003).3.1 ZEOLITES AND THE purlieuNearly all applications of zeolites are driven by environmental concerns, from alter toxic (nuclear) wastes, to treatment of wastewater, thereby reducing pollution. Zeolites have now been used to replace harmful phosphate builders in powder detergents due to water pollution risks. Zeolite catalysts help to save energy as they make chemical processes more efficient, minimising un-necessary waste and by-products. When used as solid catalysts and redox catalysts/sorbents, they reduce the need for corrosive liquid acids and remove atmospheric pollutants, (such as engine exhaust gases and ozone-depleting CFCs) respectively (Bell, 2001). In wastewater, zeolites (clinoptilolite, mordenite) are used to remove ammonia and ammonium ions (Townsend and Coker, 2001), as well as heavy metal cations and transition metals.3.2 ZEOLITE CATALYSTS IN GREEN CHEMISTRYZeolite catalysts have contributed to the design and synthesis of unexampled materials and development of new methodologies in organic synthesis, displacing the conventional and waste generating reagents thereby maximising atom utilization and reduc ing waste generated (E-factor).Zeolites play an important role in acid-catalyzed reactions such as acylation, alkylation, isomerisation and condensation, cyclisation and electrophilic aromatic substitution.Acylation of aromatic substrates used in fine chemicals manufacture although has proven unsuccessful in less reactive aromatic compounds due to adsorption imbalance, unless performed in vapor phase using H-ZSM-5 (Singh and Pandey, 1997).4.0 CONCLUSIONDue to the role zeolites play mainly as catalysts in the environment as well as in chemical industry, the efficiency of the zeolite catalysts has been greatly improved. The yield and selectivity of the zeolite process is quantitative and in addition, reduces energy requirements, capital costs and complexity of equipments.Over the years, the synthesis process of zeolites have encompassed the principles of green chemistry as described in the report which has included waste prevention, energy efficiency, fewer environmental impacts, safe r solvents, renewable materials, process intensification, catalysis and reduction in capital cost.Though present techniques have the appearance _or_ semblance to apply some of the principles of green chemistry, further research is still being employed to improve the overall process.3.0 REFERENCESAiello, R., Colella, C., Casey, D. G. and Sand, L.B. 1980. In L.V.C. Rees, ed. Proc. 5th Int. Conf. on Zeolites. Heyden Son, London, U.K. pp. 49.Armor, J. N. employ Catalysis A 189 (1999) 153-162.Baerlocher, C., Meier, W.M., Holson, D. 2001. Atlas of Zeolite Framework Types. 5th ed. Amsterdam Elsevier.Barrer, R. 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