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IUPAC have divided the definition of Porous Materials into three different categories. Microporous (pore size < 2nm), Mesoporous (2 - 50nm) and Macroporous (> 50nm). The most frequently used term is Nanoporous, which coveres a wide range of pore size (1 - 100nm) (K.S.W. Sing 1985. From Akira. T. Ferdi. S ref 1). Therefore all the terms described before would come under this definition of nanometer materials. There are many kinds of porous materials which come under the nanoporous range such as (pillared) clays, anodic alumina, carbon nanotubes and related carbon porous carbons (Akira. T. Ferdi. S). The best known member's amongst the family of micropores are Zeolites, which have a narrow and uniform micropore size distribution due to their crystallography defined pore system.
Zeolites are solid crystalline compounds with micropores. The pores on zeolite surface extends into the inside of zeolite effectively creating large surface area.( Atkins. P. Paula. J. D).Natural zeolites have been known for almost 250 years the Swedish scientist Axel Fredrik cronstedt has been accredited with inventing the name zeolites in 1756 (Weitkemp,J). The naturally occurring zeolites are less valuable than the synthetic zeolites. The impurities in the structure of naturally occurring zeolites and the variation in chemical composition of deposits make the less specific hence less suited catalytic application. In early 1950s Barrer and Milton have made great advances in synthetic zeolites, the catalytic properties of these class of zeolite had made it possible for an increase in yield of fuel and increased the revenue for petroleum industry by at least several billion US dollars per year. An example of these zeolites are faujasites x and y used in fluid catalytic cracking (FCC) (Weitkemp, J).
The building unit in zeolites consist of SiO2 and AlO2 tetrahedral which are linked to one another through doubly bridging oxygen atoms. The Al atoms in the structure create a negative charge. These excess charges are balanced by exchangeable cations. The zeolite structure has within it channels that run throughout it, these channels have dimension of 0.2 to 1 nm. Inside these channel there are water molecules and cations. The frame work of ZSM-5 contains a primary pentasil unit arranged in a novel configuration of tetrahedral linked to form secondary building unit consisting of eight five membered rings. These units join through edges to form chains which extend along the z-axis. The chains are connected to form sheets and the sheets are linked across mirror plane forming four and six membered rings leading to three dimensional framework structure of ZSM-5. (Narayanan,S. Deshpande, K). Because of the dimensions of the channels zeolites are selective in their catalytic and sieving properties (Atkins, P. Paula, J. D).
Fig: 1 Structures of four Zeolites showing their micropore systems and dimensions.
The acidity of zeolites ZSM-5 arises for the fact that it contains Al3+ in its framework. In zeolites ZSM-5 there is more silicon then aluminium. The aluminium site in the zeolites is strongly acidic and can therefore react with base or compensating ion is a proton (Atkins et, al)
Fig.2 Acid sites in zeolites (Narayanan,S. Deshpande, K)
1.3 application of zeolite
1.3.1 Petrochemical Industry - Heterogeneous Catalysis
Heterogeneous catalysis is defined as catalysis where the phase of the catalyst differs to the phase of the reactants. This reaction takes place inside the internal cavities of the zeolite. This is exploited in many industries such as the petrochemical industry, catalytic cracking of long unsaturated chains cracking them via adsorbtion.
The oil industry is a super rich industry where small improvements in efficiency would lead to major increase in profit. In the case of cracking, zeolite has proven to be 10,000 times more active than conventional catalysts. Zeolites are shape-selective have surface-acidity and have a large surface area, this makes them good to be used in the petrol refining industry (Weitkemp, J). When the absorbate is trapped inside the pore of the zeolite the steric and electrostatic effects occur affecting its bonding structure as described by Zhen. S, Seff. K, (2000).
Zeolites can also be used to break long chains of hydrocarbons to unsaturated carbon chains. Proton accepting sites (the oxygen from the Alumino-Silicate structure) break a bond within the chain taking hydrogen and leaving a carbocation resulting in cracking of the long chain, forming branched isomers which are more efficient in combusting efficiently in engines.
1.3.2 Nuclear Industry
Zeolites microporous ability to capture some ions while allowing others to roam freely due to its shape selectivity (the diameter of its entrance and stereochemistry of the molecule) allows fission products to be efficiently removed from nuclear waste and permanently trapped. Mineral properties of zeolites enable them to be very robust and resilient to radiation due to its alumino-silicate structure.
Once the micropores are filled with fission products and trapped, the zeolite waste can be compressed to become extremely durable closing the pores and trapping the waste within a solid block. This waste from significantly reduces the hazard of radiation as compared to other forms of nuclear waste disposal. (Osmanlioglu. A, E)
Commercial application of zeolites is vast and amongst those is that of Commercial Washing Powder. The loose bound structure of zeolites allow for free migration of cations when in aqueous solution readily exchanged for other types of metals. The preference of a given zeolite among available cations can be due to ion sieving or due to a competition between the zeolite phase and aqueous phase for the cations that are present. Most zeolites are synthesised with sodium ions in the pores, if another ion is required the sodium must be exchanged by placing the zeolite in a solution of the ion. This is done in water softening, where sodium and potassium prefer to exchange out of the zeolite, being replaced by calcium and magnesium ions from the water (Weitkemp, J). However Robert Dryfe stated that 'Simply trying to replace the sodium with hydrogen ions by washing the zeolite in an acid is difficult because the acid attacks the aluminium in the zeolite framework, effectively dissolving its structure'. It's evident that in water the reaction is feasible without compromising the structure if the zeolite, whereas in acid that isn't the case. (Simon Hadlington)
1.3.4 Molecular sieving and Selectivity
Molecular sieving industry has been widened and refined by the use of zeolites as it opens a wide range of sieving possibilities of gases and liquids of varying shapes and sizes. Zeolites possess a strong electrostatic field within its internal opening of the surface. This is a cause of its negative charge of the structure with a counter-action from cations in the zeolite. The cations in the zeolite can be exchanged to determine the pore size and adsorption characteristics. The exchange of cations (Ion exchange) can be used as a form of sieve for specific purposes. The selectivity of the adsorption is dependent on the size and shape of the molecule as well as the physical molecular properties. The ability to adjust the pores sizes to specificity enables for molecules of different sizes that are smaller than pore sizes to be adsorbed while eliminating larger molecules from entrance. The zeolite molecular sieve is ideal for high temperatures, as it's able to maintain its high capacity even at high temperatures. This enables the zeolite molecular sieve to be an optimal product to be used as a sieve in relatively high temperatures.
Fig: 4 The shape selectivity of zeolites illustrating the rejection of reactance and product.
1.4.1 Types of adsorption
In adsorption, molecules of one phase are present in higher concentration at the surface of the second phase. There are two way in with molecules are adsorbed in to surface physisorption and chemisorptions. in physisorption the molecule interact with surface by Van Dar Waal forces the energy released when physisorption occurs is similar to that released when condensation take place. The small energy involved in physisorption is insufficient to break bonds therefore the molecule retains its identity and might only be distorted. In chemisorptions the interaction between the surface and the molecule is by the formation of usually covalent bond. The entropy of chemisorptions is much lager the that in physisorption the molecules are held much closer in comparison and because of the energy involved the molecular fragmentation take place thus molecule doesn't retain its identity ( Atkins, P. Paula, J .D).
1.4.2 Langmuir isotherm and Freundlich Adsorption Isotherm
Langmuir adsorption isotherm was developed by Irving Langmuir in 1916 an American chemist. Langmuir isotherm describes the relationship between adsorbate and adsorbent. Adsorption on solid substrate in which adsorption occurs for molecule is at equilibrium with the solid at constant temperature. This means that adsorbed molecules undergo desorption and the point at which adsorption and desorption are equal equilibrium is established. Numbers of assumption were made Langmuir adsorption isotherm.
Aniline standards were made up from a known aniline working standard solution (0.01w/v). From this a series of dilutions were prepared for 10 different standards (0.0005, 0.0007, 0.0009, 0.0010, 0.0015, 0.0020, 0.0025 0.0030 and 0.0035 w/v). The standard solutions were analysed using Uv-vis spectroscopy and their absorptions were recorded. Using the data a calibration graph was plotted, of absorption against concentration.
A series of aniline with known concentrations (0.0005, 0.0007, 0.0009, 0.0010, 0.0015, 0.0020, 0.0025 0.0030 and 0.0035) was prepared from the working standard. These solutions were analysed using uv-vis at 230nm, their absorptions were recorded. 0.1g of zeolite zsm-5 was weighed for each solution, these were placed in the beakers. The solutions were then added to the beaker and left for 1 hour to reach equilibrium on a magnetic stirrer at room temperature (20C°). The solutions are put in test tubes and placed in a Centrifuge for 20min at 6000 rpm. The separated solution was put in 1cm quartz cuvette and analysed using UV-vis. The above procedure was repeated with the same solution but the temperature was kept at 0-1°C by using water cool overnight for the preparation of the solutions and keeping the solution in ice for the duration of the experiment.
The data obtained from the uv-vis was summarised in table 1. For the purpose of analysis, the data needed to be made clear, in a manner that showed trends. This cannot be obtained from tables thus the data was presented pictorially, which reveals the trends. This should indicate the relationships between the parameters, to analyse and make presentation of the data in numerical descriptive and graphical description method, which included maximum, minimum, mean, standard deviation, standard errors, bar graphs and combination graphs.
The technique used to investigate the ability of zeolite ZSM-5 to remove aniline from aquatic solutions was Uv-vis spectroscopy. Uv-vis spectroscopy is routinely as analytical technique. Uv-vis spectroscopy is an absorption spectroscopy technique which measures the absorbed electromagnetic radiation by molecules. The energy of a microscopic system, such as an atom and molecules are quantised this means that only certain energy levels are allowed. The energy of molecules is made up of its electronic, rotational and vibrational transitions giving rise to molecular band spectra. The difference between these energy levels gives the frequency of absorbed electromagnetic radiation (photon). This information is used to determine the quantities of substance. Spectrophotometer consists of a light source which is continues source that emits all wavelengths of region being used. The most common used light source in visible spectrometer is filament lamp and in the Uv region the source is deuterium lamp (100-400 nm).Monochromator