Synthesis of Zeolite A from By-Product of Aluminum Etching Process: Effects of Reaction Temperature and Reaction Time on Pore Volume

Problem statement: Increasing of the carbon dioxide (CO2) concentration in the atmosphere creates a global warming. Zeolite is used as CO2 absorbent. Approach: By-products from aluminum etching process, which carry a large amount of Al2O3 content, are useable as raw materials for synthesis zeolites type A using hydrogel process. A pore volume of zeolites is the key to achieve high CO2 absorption capacity. In this study, two-factor factorial experiments with 3 replicates and α = 0.05 have been conducted to examine the influence of reaction temperature and reaction time on the pore volume and to determine the process conditions for containing the highest pore volume. Three levels of the reaction temperature of 75, 85 and 95°C and three levels of the reaction time of 1, 2 and 3 h (s), while the stirring speed is controlled at 300 rpm, are selected for the experiment. The mole ratios of the starting reactants are fixed at 2 of SiO2/Al2O3, 2 of Na2O/ Al2O3 and 85 of H2O/Na2O. Analysis of surface area is performed by the BJH method and the DR method for analysis of gas adsorptiondesorption using nitrogen. Results: The results found that there was significant interaction between the reaction temperature and the reaction time and at 95°C of 3 hours gave the highest pore volume, resulting in 0.0205cc g of the BJH method and 0.0065 cc g in the DR method. Besides, the CO2 absorption test is also performed with the zeolite synthesized at 95°C. Conclusion: Zeolite syntheses of by product from alkaline etching of the aluminum industry have a major analysis agree well with those of reference zeolite A. Adsorption test with CO2, yield the best result with ZA31, with the capacity of 0.1023 ccg.


INTRODUCTION
The properties and uses of zeolites are being explored in many scientific disciplines: modern inorganic chemistry, physical chemistry, colloid chemistry, biochemistry, mineralogy, geology, surface chemistry, oceanography, crystallography, catalysis and in all types of chemical engineering process technology. The wide variety of applications includes separation and recovery of normal paraffin hydrocarbons, catalysis of hydrocarbon reactions, drying of refrigerants, separation of air components, carrying catalysts in the curing of plastics and rubber, recovering radioactive ions from radioactive waste solutions, removing carbon dioxide and sulfur compounds from natural gas, sampling air at high altitudes and removal of atmospheric pollutants such as sulfur dioxide (Breck, 1984).
Commercial adsorbents which exhibit ultraporosity and which are generally used for the separation of gas and vapor mixtures include the activated carbons, activated clays, inorganic gels such as silica gel and activated alumina and the crystalline aluminosilicate zeolites. Zeolite molecular sieves have pores of uniform size (3-10 Å) which are uniquely determined by the unit structure of the crystal. These pores will completely exclude molecules which are larger than their diameter.
Zeolites are crystalline, hydrated aluminosilicates of group I and group II elements (as formed in nature or synthesized), in particular, sodium, potassium, magnesium, calcium, strontium and barium. Structurally the zeolites are "framework" aluminosilicates which are based on an infinitely extending three-dimensional network of AlO 4 and SiO 4 tetrahedra linked to each other by sharing all of the oxygen's. The chemical formula of zeolite is best expressed for the crystallographic unit cell as: M 2/n O.Al 2 O 3. xSiO 2. yH 2 O (Breck, 1984). In this oxide formula, x is generally equal to or greater than 2 since AlO 4 tetrahedra are joined only to SiO 4 tetrahedra, n is the cation valence. The framework contains channels and interconnected voids which are occupied by the cation and water molecules as shows in Fig. 1 (Auerbach et al., 2003).
Hydrogel process of zeolites is generally synthesized from sodium aluminosilicate gel prepared from various silica and alumina ratio as raw material (Breck, 1984). The gels are crystallized in a closed hydrothermal system at temperatures varying generally from room temperature to about 175°C. The alkali metals form soluble hydroxides, aluminates and silicates. These materials are well suited for the preparation of homogenous mixture. The gel preparation and crystallization is represented schematically using the Na 2 O-Al 2 O 3 -SiO 2 -H 2 O (zeolite A) system as shows in Fig. 2 (Breck, 1984).
The gel is probably produced by the copolymerization of the individual silicate and aluminates species by a condensation-polymerization mechanism. The gel composition and structure appear to be controlled by the size and structure of the polymerizing species.
The etching process in the aluminum profile is etched by sodium hydroxide solution which produces an aluminum hydroxide (Al(OH) 3 ) as by-product, which carry a large amount of Al 2 O 3 content. It produces a low wholesale price aluminum hydroxide (Al(OH) 3 ) normally further used for producing alum (Al 2 (SO 4 ) 3 ).
Thus, higher valued added product such as zeolite should be considered as in this research.
This research focuses on reaction temperature and reaction time as controlling factor for synthesis of zeolite A and also on its pore volume. This study aims to achieving the following: (i) synthesis of zeolite A by hydrogel process using raw material from the byproduct of aluminium etching process, (ii) analysis of surface area is performed by the BJH method and the DR method for analysis of gas adsorption-desorption using nitrogen., (iii) CO 2 adsorption capacity.

Materials:
Synthetic zeolite was synthesized by hydrogel process, using as sodium metasilicate (from Ajax Finechem, lab grade), sodium hydroxide (from Carlo Erba, lab grade) and by-product from aluminum etching process.

Synthesis of zeolites:
Preparing of the reactant sodium silicate solution; weigh sodium metasilicte about 42 g, dissolve in water at temperature 50°C to make the mole ratio of reactants of 2 (SiO 2 /Al 2 O 3 = 2, Na 2 O/Al 2 O 3 = 2). Preparing of sodium aluminates solution; weigh byproduct from aluminum etching process about 10.7525 g to make the mole ratio of reactants of 2 (SiO 2 /Al 2 O 3 = 2, Na 2 O/Al 2 O 3 = 2), dissolve into water 240 cm 3 and add 30 cm 3 of 50% NaOH at temperature 60°C for 30 min before filtrating . The hydrogel process of the synthetic was carried out between sodium silicate solution and sodium aluminates solution at the range of temperature from 75-95°C in stirrer tank reactor, temperature controlled by water bath with fixed stirring speed at 300 rpm for 1-3 h. The experimental design was reported in Table 1, with the crystallization products obtained by filtrating, washed with deionized water and dried in an air oven at 100°C for 16 h.

Characterization:
The chemical composition of byproduct, referenced Zeolite A (ZA) and synthetic  Fig. 3: Schematic of a wavelength dispersive X-Ray fluorescence spectrometer (Sibilia, 1996) zeolites were determined by X-ray fluorescent spectroscopy (Philips PW-2404) is, a non-destructive method for qualitative and quantitative analysis of elemental composition for a wide range of materials. The method is extremely useful because of the ease of sample preparation. For a qualitative elemental analysis, the sample can be run "as received" with virtually no sample preparation. The sample can be in any shape (powder, pellet, film, fiber, or chunk of material) and state (liquid, solid, or suspension). The sample can be several centimeters in diameter or weigh as little as a few milligrams. XRF is based on the photoelectric effect. When an atom is irradiated with highly energetic photons, an electron from one of the inner shells may be ejected. As the vacant place is filled by an electron from an outer shell, a photon, whose energy is characteristic of the atom, is released. This radiation is called the fluorescent radiation and each element has its own set of characteristics emission or XRF lines. The intensity and the energy of these lines are measured using a spectrometer.
The spectrometer depends on the method used to separate the energies of the emitted radiation. In the wavelength dispersive X-Ray Fluorescence (WDXF) spectrometer, a crystal grating is used to separate the energies and the intensities are measured by a conventional X-ray detector (Fig. 3) (Sibilia, 1996).
The surface areas of various samples were determined by gas adsorption and desorption (Micromeritics ASAP 2010). The specific surface areas of the samples were calculated using the Brunauer-Emmett-Teller (BET) single-point method. Approximately 0.01-0.03 g of the sample was placed in the sample cell, heated to 623 K and held at that temperature for 6 h under a N 2 /He flow. The sample was then cooled to room temperature and dipped into liquid nitrogen. After the adsorption of nitrogen reached equilibrium, the sample cell was then dipped into a water bath at room temperature. The amount of nitrogen desorped was measured by a gas chromatograph.
The surface area of powdered or porous solid can be calculated from the volume of gas absorbed onto the surface of the solid. In general, solids adsorb gases weakly bound due to Van der Waal's forces only, to cause sufficient gas to be absorbed for surface area measurement. The volume of gas absorbed increases with increasing pressure. The physical absorption of gases by solids increases with decreasing temperature and with increasing pressure. The process is exothermic, i.e., energy is released. The investigative procedure has first to establish what is known as an absorption (or desorption) isotherm. This, quite simply, is a measure of the molar quantity of gas n (or standard volume V a , or general quantity q) taken up, or released, at a constant temperature usually T by an initially clean solid surface as a function of gas pressure P. Most frequently the test is conducted at a low temperature, usually that of liquid nitrogen (LN 2 ) at its boiling point (77.35 K at one atmospheric pressure) (Sibilia, 1996).
Convention has established that the quantity of gas absorbed is expressed as its volume at standard conditions of temperature and pressure (0°C and 760 torr and signified by STP), while the pressure is expressed as a relative pressure. This is the actual gas pressure P, divided by the vapor pressure P 0 of the adsorbing gas (called the adsorptive prior to adsorption and adsorbate after ward) at the temperature at which the test is conducted. All analyses first must establish information in the form of quantity adsorbed (or desorbed) Vs. pressure. Therefore the measurements requirement of the highest quality cannot be overemphasized. These data are gathered at one temperature. Plots of V a as the ordinate against P/P 0 as the abscissa reveal much about the structure of the adsorbing material (called the absorbent) simply from their shape.
The Dubinin-Radushkevich method (DR), while the concept of an adsorbed layer of gas on the walls of large pores is valid, is not necessarily applicable when the pores are a little more than one gas molecule wide as in micro pores. This led Dubinin and Radushkevich to develop an empirical equation applicable to volume filling, in microporous carbons. Their theory incorporates earlier work by Polanyi, in regard to the adsorption potential A (Webb, 1997). Where: (1) and expresses the differential molar work of adsorption. In this expression, R is the universal gas constant, T the absolute temperature, P 0 /P the reciprocal relative pressure and ΔG the change in the Gibbs free energy. The Dubinin-Radushkevich (D-R) (Paul) equation has the form: Where: W = The quantity, adsorbed at relative pressure P/P 0 and temperature T, W 0 = The limiting micropore volume E 0 = The characteristic energy of adsorption for the reference vapor, benzene and β the affinity coefficient This has a value of unity when the adsorptive is the reference vapor. Substituting Eq. 3 for Eq. 3, then extracting the logarithm gives (Webb, 1997): Some data sets from experimental isotherms, when plotted as ln W against ln 2 (P 0 /P), produce straight lines over a range of gas pressures. When a straight line is achieved, its slope is the value of -(RT/βE) 2 and the yaxis intercept is lnW 0 .W 0 , being a gas volume, must be converted to its liquid volume to characterize appropriately the total micro porosity. Using nitrogen gas also produces straight lines on micro porous solids.
BJH method, the procedure for calculating pore size distribution using the Kelvin equation, involves an imaginary emptying of condensed adsorptive in the pores in a stepwise manner, as the relative pressure is likewise decreased. The mathematics of the technique is equally applicable, whether following the adsorption branch of the isotherm downward from high to low pressure, or following the desorption branch. In either case, the condition must be set arbitrarily where all pores are to be considered filled. Typically this is taken to be at about 99.5% relative pressure (P/P 0 = 0.995). The calculation method follows generally that described hence called the BJH method. As an example, it is convenient to imagine the isotherm as a series of steps downward of equal relative pressure, say 0.95, 0.90, 0.85. The amount of adsorptive lost in each step (after conversion from gas volume at standard conditions to a liquid volume) represents the core volumes of pores emptied in that step (Webb, 1997): Where: P * = The critical condensation pressure γ = The liquid surface tension ν = The molar volume of condensed adsorptive θ = The contact angle between the solid and condensed phase (taken to be zero when the adsorptive is nitrogen, hence cosθ = 1) r m = The mean radius of curvature of the liquid meniscus and P * /P 0 , R and T as used previously It follows when the condensate meniscus is concave (θ<90°), which capillary condensation will proceed in the pores of radius r m . This will happen as long as the adsorptive pressure is greater than the critical condensation and can or cannot occur at a given relative pressure. Now from Eq. 4 (Webb, 1997), with the substitution of (r-t) for r m , a value of r is calculated. At this point another choice needs to be made, with regard to the shape of the pore. For example, if a straight cylindrical pore model is chosen, substituting the value of? r m and the volume of pores of size, r m . This will happen into the general equation for the volume of a right circular cylinder (volume = πr 2 L) allows resolution of the total length of pores. From this we can calculated the wall area of the pores using the menstruation equation, area = 2πrL. All of the above was only for the first step down the isotherm. A pore distribution of radius (or diameter), area and volume will not have been achieved, until the process is completed.
The CO 2 adsorption using small column test equipment was conducted on the adsorbents of the synthesized zeolite. Adsorbent (1g) of the synthesized zeolite was placed in a u-tube stainless steel column 1 cm diameter, using CO 2 (99.5%) as test gas with N 2 as carrier with the total flow rate 15 cm 3 min −1 . A Gas Chromatograph (GC) was used to analyze the concentration of CO 2 in the inlet and outlet gases.

Design of experiments:
In this study, we conducted two-factor factorial experiments with 3 replicates and α = 0.05 in order to examine the influence of reaction temperature and reaction time on the pore volume and to determine the process conditions for containing the highest pore volume. We used three levels of the reaction temperature of 75°C, 85 and 95°C and three levels of the reaction time of 1, 2 and 3 h (sec) , while the stirring speed is controlled at 300 rpm,. The mole ratios of the starting reactants are fixed at 2 of SiO 2 /Al 2 O 3 , 2 of Na 2 O/Al 2 O 3 and 85 of H 2 O/Na 2 O. Analysis of surface area is performed by the BJH method and the DR method for analysis of gas adsorption-desorption using nitrogen.

Chemical characterization:
The chemical composition of the aluminum etching by-product consisted of the following oxides expressed as percentage by weight; Al 2 O 3 , 92.17 %; Na 2 O, 6.03 %; SiO 2 , 0.47 % (Hussaro et al., 2008). It carries a large amount of Al 2 O 3 content which is useable as raw materials for synthesis of zeolites. Si and Na compositions could be adjusted by the addition of sodium metasilicate.
The results of the mole ratio and the chemical composition in the referenced zeolite A (ZA) and synthetic zeolites were reported in Table 2. These results show that the mole ratio of Na 2 O/Al 2 O 3 and SiO 2 /Al 2 O 3 of ZA and synthetic zeolites are affected by reaction temperature and reaction time in the range between 75-95°C and 1-3 h respectively. The above results indicate that the highest reaction temperature and time in the range (ZA31, 95°C in 3 h) resulted in the successful synthesis of zeolite A. The compositions were compared with the reference zeolite A in Table 2. The increasing reaction temperature and reaction time ensured the better value of the mole ratio of Na 2 O/Al 2 O 3 (ZA = 1.05), ZA31 = 1.07) and SiO 2 /Al 2 O 3 (ZA = 2, ZA31 = 2.10), which is the indices of the zeolite A framework.
The results of the mean of Pore Volume (PV) from three methods are reported in Table 3. By varying two variables simultaneously and obtaining multiple measurements under the same experimental conditions, the experimental responds are randomized.  Mole ratio of samples  ---------------------------------------------------   The normality of the respond data is assumed. They are analyzed by using SPSS statistical software. The results of applying the analysis of variance are shown in Table 4. All three ANOVA tables show a significant interaction between reaction temperature and reaction time on the pore volume (Sig.<0.05).
The results of gas adsorption studies conducted utilizing a gas of 99.5 % CO 2 with 99.9 % N 2 carrier on samples were shown in Table 5. Carbon dioxide (CO 2 ) adsorption for ZA31, ZA21 and ZA11 showed that the amount of CO 2 adsorbed increased progressively with reaction time of synthesis zeolite. The total amount of CO 2 adsorbed followed the order of ZA31>ZA21>ZA11.
The capacity of CO 2 adsorbed by ZA was 0.1183 cc g −1 . While that of ZA31 was 0.1023 cc g −1 , which was better agreed with ZA as compared with other sample (Table 5). These results show that the synthetic zeolite could absorb the CO 2 , but the adsorption capacity of reaction time these result confirmed by pore volume analysis, showing the increase pore volume with increased reaction temperature and reaction time.