Project report

2.5.1.3 Dual-phase Transition Mechanism

This mechanism was proposed as a compromise between the two mechanisms earlier discussed. It was later believed with more advanced detection technique that zeolite formation can proceed utilizing the two mechanisms. This strongly depends on the condition such as the source material. ZSM-5 is also a typical example of zeolite that followed both solid gel formation and liquid phase mechanism based on the nature of silicon soured used. (Kovo, 2010)

2.5.2 Influence of Various Factors on Zeolite Crystal Growth

2.5.2.1 Aging of the Reaction mixture

Ageing is defined as the gap in time between the formation of aluminosilicate gel and crystallization.(Kovo, 2010). It is well known that the low-temperature aging of aluminosilicate gel precursor markedly influences the course of zeolite crystallization at the appropriate temperature. The primary effects of gel aging are shortening of the ‘‘induction period’’ of crystallization, acceleration of the crystallization process, and lowering of the crystal size. (Auerbach et al., 2003). In contrast to the independency of the crystal growth rate on the aging of hydrogels, a zeolite gel is defined as a hydrous metal aluminosilicate prepared from aqueous solution, reactive solids, colloidal sols, or reactive aluminosilicates such as the residue structure of meta-kaolin and glasses, the growth rate of silicalite and zeolite A crystallized from clear (alumino)silicate solutions considerably depends on the aging of the reaction mixture. The increase in growth rates with aging time is probably an indication that nuclei had agglomerated and the growth rate measured by quasi-elastic light scattering was the apparent rate of growth of the agglomerate, which was higher because of the increased surface area. This phenomenon probably does not occur in the gel systems because the amorphous gel suspends and isolates the crystallites until very near to the end of the process when settling of macroscopic single crystals occurs. (Auerbach et al., 2003; Farag and Zhang, 2012).

Once the gel is made, it can either be heated straight away, or left to age at room temperature prior to use. The ageing process is said to provide a period for nuclei to form, which are the building blocks of the final crystals. Traditionally nucleation takes place in the initial heat up period of the gel to reaction temperature. One parameter that can alter the heating duration required is whether or not a gel has been aged prior to use. It was found that when a zeolite A gel was left for 24 hours, or had 10% by weight of aged gel added, the time taken for crystallization to occur was reduced from 10 minutes to 1 minute.
2.5.2.2 Crystallization Temperature

Crystallization temperature is one of most frequently studied crystallization condition that influences the kinetics of crystal growth of zeolites. Measurements of the kinetics of the crystal growth of zeoliteA, analcime, hydroxysodalite, Dodecasil, faujasites, mordenite, omega, silicalite-1, and ZSM-5 as a function of crystallization temperature have shown that in all cases the crystal growth rate increased with the crystallization temperature in accordance with the Arrhenius law, that is,

Equation 2.1

where Kg is the rate constant of linear crystal growth at the reaction temperature T, R = 8.3143 J K^-1 mol^-1 is the gas constant, T is absolute temperature, A is the appropriate constant, and Ea(g) is the activation energy of the crystal growth process. Hence, in accordance with Eq. (1), the activation energy of the crystal growth process may be determined as the slope of the 1n Kg vs. 1/T straight line , that is,

Equation 2.2 (Auerbach et al., 2003)

2.5.2.3 Crystallization time

Crystallization time is also an important parameter that controls the type of final zeolite product obtained. The key objective during the synthesis of zeolite is to use the minimum time to prepare the desired zeolite material. Therefore, optimization of crystallization time depends absolutely on the choice of other parameters such as crystallization temperature, concentration of mineraliser and seeding. In most cases, increase in such parameters will ultimately reduce the induction time and indeed the crystallization time.

(Dyer,1988) stated the two ways the effect of time play a part in the formation of zeolite from reacting species or gels. An induction period during which the reaction mixture is held near ambient temperature prior to raising the crystallization temperature often optimizes zeolite yield (as in X and Y synthesis). Different zeolites crystallize from one reaction mixture at different times. This second time element is because all zeolite are metastable species whereby the initial zeolite formed is an open structure which with time are transformed into more dense or closed structure following Ostwald’s law of successive transformation. An example is the conversion of Philipsite (an open structure) to clinoptilolite( a less open structure) and finally to dense and most stable analcime. Similarly mordenite can also transform to analcime. (Kovo, 2010).
2.5.2.4 Alkalinity of Crystallizing System

The alkalinity in the synthesis batch is one of the most important parameters for control of the crystallization of zeolites. The increase in alkalinity causes an increase in the crystallization

rate via an increase in the crystal growth rate and/or nucleation consequent to an increasing concentration of reactive silicate, aluminate, and aluminosilicate species in the liquid phase of the crystallizing system. The increase in the concentration of the reactive silicate, aluminate, and aluminosilicate species in the liquid phase of the crystallizing system with increasing alkalinity of the reaction mixture (hydrogel) is caused by the more rapid increase in the solubility Sg of amorphous (alumino)silicate precursor than the increase in the solubility Sz of crystallized zeolite(s) with increasing alkalinity A (i.e., Sg/Sz increases with increasing A). (Auerbach et al., 2003).

2.5.2.5 Dilution of Crystallizing System

Following a general principle that the rate of crystal growth is proportional to the concentration

of reactants, expressed by the concentration function f (C), that is,

dL/dt_c= kg f (C) Equation 2.3

it is not unexpected that dilution of crystallizing system (e.g., an increase of water content) causes a decrease of the concentration of reactive species in the liquid phase, and thus a decrease of the crystal growth rate. It was found that the growth rates for all faces of silicalite-1 crystals crystallized at 150°C from reaction mixture 0.1TPABr/0.05Na₂O/ SiO₂:xH₂O decreased with an increase of the ratio x = H₂O/SiO₂ (increased dilution), although the dependence of the growth rate was slightly different for each face. (Auerbach et al., 2003).


2.6 PRODUCT CHARACTERIZATION

2.6.1 IR Analysis

Figure 7 is the IR spectrum of a typical synthesized zeolite sample. Based on (Flanigen et al., 1971), the peak of transmittance frequency of 1020.80 cm-1 corresponds to internal tetrahedron asymmetrical stretch vibration found in zeolites, 574.19 cm-1 peak is related to the presence of double ring in the framework structure of zeolites while the 720.88 cm-1 represent the tetrahedral atom. These peaks suggest the synthesized samples to be zeolite in the class of zeolitic A, X or Y due to auspicious double ring vibrational peak (Atta et al., 2007)

Fig 2.10: IR Analysis Spectrum

2.6.2 Scanning Electron Microscope (SEM)

Shape and size of NaY particles laboratory synthesized by different procedures were studied by SEM and images with a scale bar of 10 μm displayed in Figure 8.The SEM images (Figure 8 a & b) show, the solid product contained a mixture of

multi-faced spherules crystals with an ice hockey shape with different particle diameter along with round amorphous particles. Since some particles apparently connected with other particles, the particle size distribution was expected to be large. Comparison of SEM image of NaY zeolite synthesized from kaolin ( Figure 8c) with those of NaY synthesized via modified hydrothermal crystallization procedure and homoionic sodium form zeolite NaY respectively (Figure8 a&b), indicates that there were minimal differences between the morphology, crystal shapes and sizes. It can generally be suggested that the framework of homoionic sodium form NaY zeolite is not widely affected, via treatment with NaCl, albeit the sodium bonds are broken down inside its atomic structure (Matti and Surchi, 2014)

Figure 2.11: SEM of Kaolin

2.6.3 X-Ray Florescence

The structural formulae, the bulk chemical composition Si/Al of laboratory synthesized samples were determined using XRF analysis. The resulting SiO₂, Al₂O₃, Na₂O and H₂O contents and unit cell composition for different laboratory synthesized zeolites were summarized in Table 2and Table 3. The results of XRF analysis show that the major components of laboratory synthesized zeolites are SiO₂, Al₂O₃, Na₂O and H₂O, along with a small quantity of CaO and trace amounts of K₂O, Fe₂O₃, P₂O₅ and MgO. The data in table 2 indicate that there is an small degree of correlation between the results obtained from EDS analysis in the amount of silicon, aluminum, sodium and oxygen for each sample, with that calculated from XRF analysis (Matti and Surchi, 2014).

Table 2.6 The chemical content based on XRF analysis laboratory synthesized zeolite samples at 30°C. (Matti and Surchi, 2014).

2.6.4 XRD Analysis:

The XRD pattern represented in Figure 2.12 shows successively synthesis of zeolite Y, zeolite X and an unnamed zeolitic phase not picked by the equipment. Along with these, anatase and quartz were also observed which could have originated from kaolin clay, indicating the inefficiency in either pretreatment or dealumination procedure. As shown in the footnote of Figure 2, titanium oxide, quartz, zeolites X and Y and other unnamed 2 crystalline phases were identified. Anatase (TiO ) and 2 quartz (SiO ) identified as part of the crystalline phases are undesired by-products as residual reactants from the metakaolin (Atta et al., 2007)

Fig 2.12: XRD Analysis of a Typical Synthesized Zeolite Sample (Atta et al., 2007)
2.7 REVIEW OF PREVIOUS WORK DONE ON SYNTHESIS OF ZEOLITE

2.7.1 Synthesis of Faujasite Zeolites from Kankara Kaolin Clay by (Atta et al.,2007)

The synthesis of zeolite X from locally available Kankara kaolin clay sourced in Nigeria was done by Atta et al. The beneficiated kaolin was converted to metakaolin at 600°C. Subsequently, the highly reactive metakaolin was leached with sulphuric acid to achieve the required silica-alumina ratio for zeolite X synthesis. Infrared spectra analysis suggest the presence of zeolite framework structure, hence the further confirmatory characterization by x-ray diffraction method. Analysis by x-ray diffraction revealed a composite crystalline phase consisting of zeolites X (32-36%) and Y (21-25%), unnamed zeolite (<1%), quartz (2-5%) and anatase (2-5%). In addition, an amorphous phase (>20%) was found present in the synthesised zeolite. Furthermore, ion exchange capacity of the synthesised samples was found to be 4.72- 4.94 meq/g. The structure was also observed to be stable up to a temperature of about 600°C.

2.7.2 Making zeolite A membrane from kaolin by electrophoresis by (Mohammadi and Pak, 2002)

Mohammadi and Pak worked on process of synthesis of kaolin based zeolite A membrane by electrophoresis. They used kaolin from Zenooz mine, Marand, Iran. Tubular module of kaolin was prepared by electrophoretic deposition on a cylindrical electrode. Voltage and current in electrophoresis method was 10 V and 0.25 A, respectively. Required time for preparation of a module with o:d: ¼ 10 mm (i:d: ¼ 8 mm) was 2 min. The density of suspended clay was 1.5 g/cm3. Increasing voltage causes gas generation from anode. Lower voltage needs more time for the module to be prepared with desirable diameter. After preparation, module was removed from anode, dried at 80°C and then calcined at 700°C for 3 h. Finally, it was placed in an autoclave with 14% sodium hydroxide solution. A thin zeolite A membrane is formed on the kaolin tube. Then, the tube was washed with distillated water and dried over night at 60°C. For drying temperatures of more than 60°C, the membrane was separated from the support. This membrane was used for ethanol/water separation. The highest separating factor of 25.14 was measured using the membrane. _ 2002 Elsevier Science Inc. All rights reserved. Zeolite A membrane prepared from pure kaolin was cracked due to its water adsorption. A 30% c-alumina module which was soaked in zeolite solution showed a maximum selectivity of about 25.14 which is not very high, however, its strength was very improved. Calcination of kaolin at a temperature of 1050°C caused a membrane with a separation factor of 19.13 to be formed. Further research to enhanced strength and separation factor of membrane by optimising calcination temperature, using kaolin with low impurity and adding stabilizing materials is being undertaken.

2.7.3 In-situ Synthesis of NaY Zeolite with Coal-Based Kaolin by (Liu et al., 2003)

NaY zeolites were in-situ synthesized from coal-based kaolin via the hydrothermal method. The effects of various factors on the structure of the samples were extensively investigated. The samples were characterized by N2 adsorption, XRD, IR and DTG-DTA methods, and the results show that the crystallization temperature and amount of added water play an important role in the formation of the zeo-lite structure. The 4A and P zeolites are the competitive phase present in the resulting product. However, NaY zeolites with a higher relative crystallinity, excluding impure crystals and the well hydrothermal stability, can be synthesized from coal-based kaolin. These zeolites possess a larger surface area and a narrow pore size distribution, and this means that optimization of this process might result in a commercial route to synthesize NaY zeolites from coal-based kaolin.

Coal-based kaolin with little quartz is a promising material for NaY zeolite synthesis. The pure NaY zeolites were in-situ prepared by activation treatment using hydroxide sodium and crystallization hydrothermal synthesis under the proper synthesis conditions. The crystallization temperature and pH of the solution play an important role in the formation of the zeolite prepared from coal-bases kaolin. There is apparent competition among 4A,

NaP and NaY zeolites during the synthesis from coal- based kaolin. 4A and NaY zeolites are the metastable pattern and NaP is the relative stable one. Thus, it is important to control the reaction parameters to obtain the desired zeolite. Pure NaY with a high relative crystallinity can be prepared from kaolin. The samples possess a larger specific area, sharper pore distribution and good hydrothermal stability.

2.7.4 Preparation of Zeolite Y Catalyst for Petroleum Cracking by (Mu and Mya, 2008)

It is concluded that the mabisan clay used as raw material in this experiment is the kaolinite clay. The zeolite Y with SiO₂/Al₂O₃ molar ratio of 3.53 was prepared from kaolin by activation treatment using sodium hydroxide and hydrothermal crystallization under the proper condition. The optimum parameters were molar composition of 6SiO₂:Al₂O₃:9Na₂O:249H₂O, ageing at 50˚C for 24hrs and crystallizing at 100˚C for 48 hrs. There is an apparent competition between zeolite Y and P during the zeolite preparation. It is concluded that the ageing temperature is an important parameter to reduce the formation zeolite P. To obtain the higher SiO₂/Al₂O₃ molar ratio of zeolite, the prepared zeolite NaY was dealuminated by exchanging sodium ion by ammonium and hydrogen ions. The SiO₂/Al₂O₃ molar ratio was increased to 3.8 and 4.6. Their ion exchange capacities are 0.8169 and 1.463. It is concluded that the ion exchanging by hydrogen ion can increase higher SiO₂/Al₂O₃ molar ratio than that by ammonium ion. The ion exchanging was decreased the peak intensity of zeolite. The catalytic activities of prepared zeolites were investigated by laboratory scale simplified fixed-bed cracking unit. Their activities were determined by product (gasoline) selectivity. It is concluded that the activity of catalyst HY is the highest among NaY, NH₄Y and HY.

CHAPTER THREE

MATERIALS AND METHODOLOGY

3.1 MATERIALS

3.1.1 Kaolin Clay

The kaolin clay used in this research work was procured from Okpella village, Edo State Nigeria through the “Freedom Group Mining Company.’’ The kaolin clay is termed ‘white and off-white raw Okpella kaolin’ due to its mixed white and off-white appearance. The feasibility of the original Okpella kaolin as an alumina and silica source for zeolite synthesis was investigated through the X-ray fluorescence (XRF), where the elemental composition of the Okpella kaolin was determined. The raw Okpella kaolin was then refined and then followed by drying and dehydroxylation/metakaolinization as explained in the literature and finally the synthesis was carried out using the external silica source under the best synthesis conditions. Characterization of the synthesized zeolite was carried out using the XRD, XRF, SEM, BET, and FTIR spectrophotometer.

Figure 3.1: Raw white and off-white Okpella kaolin



Fig3.2: Project group and supervisor at kaolin mining site at Okpella



Fig 3.3: Project students, supervisor and plant operator at kaolin processing plant, Okpella.

3.2 REAGENTS AND APPARATUS

The following reagents used in the synthesis of zeolite Y were of analytical grade and a product of Qualikems Laboratory

• 
  Sodium Hydroxide Pellets (99% w/w) Qualikems Laboratory Reagents
  
• 
  Sodium Metasilicate (99% w/w) Qualikems Laboratory Reagents
  
• 
  De-ionised water
  

Apparatus used in the experimental process include simple laboratory glassware such as measuring cylinders, beakers, test tube, as well as the oven, timer(stop watch), stirrer, samples bottles, plastic buckets, crucibles, electric weighing scale, and the muffle furnace (Searchtech Instrument); Model = SX-5-12.
3.3 EXPERIMENTAL TECHNIQUES

3.3.1 Preliminary Characterization of Raw Okpella Kaolin

The raw Okpella kaolin representative sample of 50kg was taken from Okpella in Edo state from the Freedom Group Mining Company. The initial analysis of the raw sample was carried out with the aid of x-ray fluorescence (XRF) at the National Geosciences Research Laboratory (NGRL) Kaduna. This was done to know the elemental composition or constituent mineral and determine the impurities present.

3.3.2 Refining of Raw Okpella Kaolin

The preliminary analysis of the raw Okpella Kaolin as determined using the XRF showed that the raw Okpella kaolin clay contains impurities such as quartz, iron, titanium, zinc, copper, calcium etc in percentage oxides which are accessory minerals because they do not exhibit plasticity thus impede the application of zeolites in catalysis because of their abrasivity (Kovo, 2010). This associated minerals requires removal or purification hence size separation based on differences in particles size and density as typified by Stokes law of settling particles in solution is used for the refining process. The refining process for Okpella kaolin clay is shown in the figure 3.1 below

Raw Kaolin Crushing Preliminary

(Size Reduction) Characterization

Decanting of Refining of Raw Okpella

the Supernatant Kaolin in Settling Tank

Settling of Oven Dying at Refined Okpella Kaolin

Kaolin Particles 45^oC for 59hrs

Figure 3.4 Process Flow Chart of the Refining of Okpella Kaolin

The experimental procedure for the refining of the raw Okpella kaolin is presented using the sequential steps below.

60g of raw Okpella kaolin was measured using electric weighing scale. The deionised water was poured into 5 measuring cylinders of 34cm height and 1000ml by volume. A portion of the measured deionised water was poured into a beaker where the 60g of raw kaolin was poured into it to obtain a sample of slurry. The kaolin slurry was then poured into the second portion of the deionised water in the measuring cylinder used as settling tanks. The size separation was achieved by allowing free settling of clay suspension in the measuring cylinder of 34cm height. A clay suspension was created from a mixture of clay and deionised water, settling time of heavier component (quartz) was determined from Stoke law equation based on the density of particle and solvent (water), the viscosity of solvent (water) and the diameter of the particle of 20µm which was chosen base on the analysis and personal discussion with my supervisor, Engr. Okolie, J.I. and the temperature of the surrounding environment and height of the settling tanks. Using the Stokes’ law of sedimentation, the settling time was calculated to be 12 minute, 57 seconds. At the end of the settling time, the heavier (coarse) component of the clay settled naturally at the base of the settling tubes while the kaolin sample (lighter fraction) remains as supernatant. The supernatant was carefully decanted into a white transparent plastic bucket and was covered and allowed to settle for 24 hours. After 24 hours, a settled fine clay sample was obtained after decanting the suspended deionised water, and was transferred into another transparent plastic bucket. The process was repeated severally in order to obtain enough samples of the refined kaolin for other experimental stages. The settled fine kaolin clay were dried using the oven for 59 hours at 45^oC and were packed into sample bottles for the next experimental stage.



Figure 3.5: Settling tanks for refining kaolin
3.3.3 Dehydroxylation/ Metakaolinization of Refined Dried Okpella Kaolin

The refined Okpella kaolin was converted to metakaolin which will be used for the development of zeolite Y and this was achieved using the following sequential steps;

A muffle furnace (SEARCHTECH INTRUMENTS) Model SX-5-12 was initially heated to a temperature of 550˚C. During the process of heating the muffle furnace to 550˚C, 10g each of refined dried Okpella kaolin was measured into different ceramic crucibles as container, and immediately the furnace attained the temperature of 550˚C, the crucibles containing 10g each of refined dried Okpella kaolin were loaded into the furnace and were allowed to heat for 5minutes, 10minutes, 15minutes, 30minutes, 60minutes and 90minutes respectively. At the end of each heating time the crucibles containing the refined kaolin were removed and placed in desiccators to cool. The weight of the refined kaolin was noted before and after dehydroxylation and at the end, the mass loss was calculated and tabulated as presented in table 4.2. The dehydroxylation/metakaolinization was carried out at different temperature(s) of 600˚C, 650˚C, 700˚C, 750˚C respectively for 5, 10, 15, 30, 60, 90 minutes heating time and at 900^oC for 30 minute heating time. From the calculated mass loss, the heating time of 60minute at 650˚C was chosen based on a careful analysis in the variation of mass loss of the metakaolin and the process was repeated severally at 60minutes heating time for 650˚C to get enough sample of metakaolin. The essence is to dehydroxylate the refined Okpella kaolin to form an activated X-ray amorphous material called metakaolin. The dehydroxylated kaolin were packed into small sample bottles and labelled in preparation for synthesis.