Zeolites

SYNTHESIS OF ZEOLITES

ZEOLITE SYNTHESIS Zeolite and aluminophosphate micro porous materials are made hydrothermally. - reactants are heated up in water (100-250 C) For An aluminosilicate Zeolite, Silica source : cabosil,sodium silicate Alumina Source : high surface area aluminum oxyhydroxide, sodium aluminates,Al3+ salts. Base (p H ~ 12) : Alkali metal hydroxide,quarternary ammonium hydroxide,etc., Template : Organic Cation, Hydrated metal ion.

Zeolite formation is a kinetically controlled process. In zeolite synthesis, the reaction is stopped when the thermodynamically meta-stable zeolite has formed. Extended reaction time at high temperature and/or high pressure usually results in dense phases.

HYDROTHERMAL SYNTHETIC TECHNIQUES Hydrothermal synthetic technique is the basic route for zeolite synthesis. Refers High temperature and high pressure(>100 C ,>1bar). Techniques used for Preparation of, - Important inorganic materials - Super ionic conductors - Chemical sensors - Electronically conducting solids - complex oxide ceramics and fluorides - magnetic materials -luminescence phosphors

FEATURES OF HYDROTHERMAL REACTIONS High reactivity of reactants Easy control of solution or interface reactions Formation of metastable phases Unique condensed phases Reduced Air pollution Low energy consumption

Hydrothermal conditions (High T and High P) - Accelerate the reaction rate among the complex ions and make stronger the hydroxylation reaction. - Can Promote the reactivity of reactants with low solubility at ambient temperature. - Viscosity of water decreases with T .Therefore the mobility of molecules or ions in water under HC is enhanced significantly. - Water plays several roles under HC, -Acting as a solvent -Changing the chemical Physical properties of reactants and products. -Accelerating the reaction -participating in the rxn in some cases -Transferring the pressure

Contd., In zeolite synthesis, the reaction is stopped when the thermodynamically meta-stable zeolite has formed. Extended reaction time at high temperature and/or high pressure usually results in dense phases (Quartz not good). Most Aluminosilicate zeolite probably obtained below 100 C in alkaline solutions. (Low Si/Al ratio) In order to reduce the rxn time (For High Si/Al ratio) and control crystallite sizes, morphologies, and compositions- synthesis performed at above 100 C under autogeneous pressure in autoclaves.

Steps involved in the experiment.

IMPORTANT SYNTHESIS PARAMETERS Gel composition Order of mixing Gel aging Seeding Temperature ramp Reaction time

FACTORS AFFECTING ZEOLITE SYNTHESIS Batch Composition Si and Al Sources Si/Al ratio Alkalinity H2O Content Inorganic Cations Organic Templates Solvents Crystallization temperature and time Aging -Effect of aging on crystallization kinetics - Effect of aging on the nucleation kinetics Stirring Seeding

Following steps as mechanism: Precipitation of gel phase Dissolution of Zeolite Nucleation of Zeolite Continued crystallization and crystal growth of zeolite Dissolution of initial meta stable phase Nucleation of more stable or meta stable phase or phases. Continued crystallization and crystal growth of the new phases. Crystalline phase while initial crystals are dissolved. Dissolution of metastable phase Crystallization and crystal growth of final crystalline phase

For zeolites with a low Si/Al ratio, they could be obtained at temperatures below 100°C in alkaline solutions (usually in CSTR). However, in order to reduce the reaction time (especially for zeolites with a high Si/Al ratio) and to control crystallite sizes, morphologies, and compositions, syntheses are performed at temperatures above 100°C under autogeneous pressure in autoclaves

The composition of the reaction mixture ( gel) and the kind of precursor materials used in the reaction mixture are very important parameters. They determine the properties of the resulting material, like for example its structure, morphology, particle sizes, particle size distribution, homogeneity of elements within the crystallites and many more. Typical precursor materials are:

Importance of templates The true template effect, where the zeolite is formed around the template molecule which determines the pore topology due to its own shape. A pore filling effect, where the template stabilizes the micropores of the zeolite by filling them and, thus, preventing a collapse of the pores A pH-stabilizing effect, due to the functionality of the template molecules.

Importance of templates

Crystallization is most crucial step involved in formation of zeolite crystals. Principal factors affecting the rate are gross composition, temperature of atmosphere and time to which it allowed to reside in atmosphere. Concentration of Hydroxyl ion on other hand plays a vital role.

Zeolite formation Crystallization is the principle stage of zeolite formation represents schematics of zeolite formation. Formation of zeolite: Molecular Approach.

The role of quaternary directing agents Small individual alkyl chain length quaternary directing agents generate the formation of microporous solids Long alkyl chain length quaternary directing agents self-assemble to supramolecular Species which can generate the formation of mesoporous molecular sieves 9

ZSM -5 Synthesis The synthesis involves three different solutions. The first solution is the source of alumina, sodium ions, and hydroxide ions. The second solution has the tetrapropylammonium cation that acts as a templating agent. The third solution is the source of silica. Reagents required are : Sodium Hydroxide, Aluminum Sulfate, Sulfuric Acid. Silicic Acid Tetrapropylammonium Bromide,n-propylamine

Synthesis of ZSM-5/SAPO-11composite

Add the first solution to this beaker. Typical Recipe for synthesis of ZSM-5 Weigh out about 0.510 g of sodium hydroxide pellets and finely grind them. Place it in a 250 ml beaker and add 1.01 g of tetra propyl ammonium bromide with 2.01 g of silicic acid. Mix with 5.0 ml distilled water, then add 1.0 ml of n-propylamine and mix again Place 1.0 ml of 1 M solution of Al2(SO4)3 along with about 0.05 ml of conc. H2SO4 in a separate 50 ml beaker. Add the first solution to this beaker. Add distilled water to raise the volume to about 25 ml, and mix the solution (26 ml total volume) on a stir plate for ten minutes.

Transfer the solution to the Parr "Bomb" and seal it. Heat the sample to 160°C and hold there for 44 hours. Cooling to attain room tempertaure. Remove a small sample for x-ray analysis check if it matches with expected one. Filter the rest of the reactant in a Buchner funnel with fine filter paper (541 grade). Wash it and then dry for 20 minutes on the filter paper.

Calcine the sample to remove the organic cation at 5000. Any sodium ions remaining in the zeolite will now be ion exchanged for protons to fully convert the zeolite to the acid form. The acid hydrogen form of the compound is prepared by transferring the oven-dried compound to a tube furnace. Heat the ammonium zeolite for 3 hours to ensure the thermal decomposition of the NH4+ ions. Zeolite will turn from a white to brown/black to an off-white color over the course of reaction.

Since, higher concentration of hydroxyl ion concludes alkaline nature and the obtained crystalline phase might be distinguished with that of low hydroxyl counterparts. Increase in hydroxyl ions dissolves silica, which is responsible for unit cell framework for zeolites. At constant SAR and fixed time for crystallization, increase of hydroxyl ions favours formation of mordenite over ZSM-5 Crystallization duration also plays critical role in formation of particular phase of zeolite. For first hour of reaction meta kaolin is obtained later on followed by zeolite A .

Furthermore, Lewis acidity can be caused by cations within the pores Substitution of trivalent atoms such as Ga3+,Fe3+, B3+ modifies acidity within the zeolite From, dissociation energy calculations an outcome came as bridging hydroxyl groups are more acidic than terminal hydroxyl groups The results were also confirmed by the lower vibrational frequency of bridging hydroxyl groups

Post – SYNTHESIS STEPS If template molecules are used during the synthesis, a calcination process has to follow the synthesis (generally 450 – 600 deg C) For application as acidic catalyst, a zeolite has to be transformed into a protonated form, achieved by exchanging the sodium in the zeolite by ammonia and successive calcination by which the NH4+ is transformed into H+ Important post – synthesis steps include: Calcination (removal of water and template) Ammonium exchange +calcination ( transformation into H+ form) Dealumination ( steaming, acid leaching) Catalyst forming ( extruder, pelletizer).

Summary of Possible Synthetic Strategies for M41S Materials Type of interaction Surfactant Inorganic precursor Notation Examples Cationic + Anionic S+-----I- M41S, M-MCM-41, 48 Ionic (Direct pathways) Anionic + Cationic S------I+ M-M41S, Cationic + Cationic S+ X-I+ SBA, APM Ionic (Mediated pathways ) Anionic + Anionic S- M+ I- Metal Oxides Neutral + Amine Neutral HMS Hydrogen bonding (Neutral ) S0-----I0 Neutral + Polymer Neutral SBA S0-----I0 Covalent Neutral + Neutral TMS S-----I 10

Preparation of ZEOLITE Y Catalyst for Petroleum Cracking Kaolin is fused by the addition of sodium hydroxide (kaolin/NaOH=1/1.5 by wt.) at 850˚C for three hours. Ten grams of fused kaolin powder and 12.67 grams of sodium silicate are dispersed in 150 ml of deionized water under constant stirring for 1 hour The slurry with a molar composition of 6SiO2:Al2O3:9Na2O:249H2O and the pH range of (13-14) is aged at various temperature for a desired period to form gel slurry. Then the gel slurry is transferred into the polyethylene container to hydrothermally crystallize. Crystallization is carried out at about 100˚C. Subsequently, the resultant precipitate is separated from the mother liquor by filtration. The crystalline mass is then washed with deionized water until a pH range of (9-12) and dried at 100˚C for 16 hours.

Role of kaolin From SEM imaging the following was the chemical composition of the kaolin: The major component of kaolin is Al2 (Si2O5) (OH4), doped with a small quantities of Fe and trace amounts of Ca and Mg. In addition,very little quartz phase exits. The Si-O or Al-O structures in kaolin are inactive which is difficult to directly synthesize zeolites. So, kaolin is pre-activated to change this inert structure. To activate the kaolin, it was treated with sodium hydroxide at 850˚C for 3 hours

Effect of water volume added on zeolite formation The amount of water exerts a strong influence on the structure of the zeolites prepared from kaolin. When the water volume is below 10 ml and above 30 ml, the samples have amorphous patterns. This reveals that too much (or) too little water may inhibit the formation of zeolite from kaolin. Product C is the desired one.

Effect of Ageing Time on Zeolite Formation The table below shows the effect of Ageing Time on Zeolite Formation: The proper ageing time was 24 hours for the formation of zeolite Y with higher intensity. At the optimum point, the yield percent of product was 48% and SiO2/Al2O3 molar ratio was 3.3. The desired sample was C-3

Effect of Crystallization Time on Zeolite Formation The crystallization temperature 100 deg C The table below shows the effect of crystallization time: At an optimal point, the molar ratio of SiO2/Al2O3 is 3.3 and yield percent of 48% , found to be maximum, the crystallization time was 2 days. Beyond this point, the molar ratio has decreased. C – 3 -2 is the desired sample.

Effect of Ageing Temperature on Zeolite Formation The optimum ageing temperature is 50˚C at which the resulting zeolite has SiO2/Al2O3 molar ratio of 3.53 and yield percent of 43%. According to XRD pattern obtained, the zeolite has octahedral crystal. Prepared zeolite at optimum ageing temperature has the proper crystallinity.

Rapid synthesis of ZSM-5 zeolite catalyst for amination of ethanolamine The reactants used are: water glass (Na2O%= 23%, SiO2=77%), Al2(SO4)3⋅18H2O, H2SO4, ethylenediamine and deionized water. For the pretreatments, the water glass is first aged at 160 °C with stirring for 8 hours. Then appropriate amount of treated water glass and ethylenediamine (EDA) is added to the deionized water to yield solution A. A is rapidly added with rigorous stirring to solution B prepared by resolving Al2(SO4)3⋅18H2O and H2SO4 in deionized water. The mixed solution is stirred for about 15 minutes to get a homogeneous gel-mixture. .

It is then transferred to stainless-steel autoclaves lined with PTFE It is then transferred to stainless-steel autoclaves lined with PTFE. The sealed autoclaves is placed in an air oven already maintained at the desired temperature (180 deg C) At the end of crystallization (nearly 33 hours), the autoclave is cooled to room temperature. The crystalline product was separated by filtration and washed with deionized water. The product was then dried at 110 °C for 12 hours and calcined at 500 °C for 8 hours. The XRD pattern of sample indicates that highly crystallized ZSM-5 zeolite is formed with-out impurities. The characteristic peak indicates typical MFI-type structure The morphology of the crystals is lath-shaped

Applications of Zeolites Ion-exchange Water pollutants (Natural zeolites ) Clinoptilolite is successfully employed for removing Ammonium from municipal sewages Ammonia is air stripped in regeneration and absorbed by sulphuric acid to give ammonium sulphate, a kind of fertilizer An advantage of zeolites for elevated resistance compared to organic resins for wastewaters of nuclear industry has been investigated

Adsorption Most of the applications are based on water vapour adsorption Based upon such properties zeolites are used as drying media for air, sour natural gas and other artificial gas streams Water adsorption-desorption cycles chabazite, clinoptilolite gave some good inferences in air conditioning and refrigeration Gas separation technologies are also under investigation

Catalysis : Petroleum Refining and Petrochemistry Due to shape selectivity and acidic sites provided by zeolites they have found their dominant applications in petroleum industry Since, most of the refining and petrochemical conversions are acid catalyzed zeolites provide them necessary locations Conversion of methane to aromatics using Mo/H-ZSM-5, Biooil upgradation etc.

Important catalytic processes involving zeolites Starting Material Zeolite Products Catalytic cracking Crude oil faujasite Gasoline, heating oil Hydrocracking Crude oil + H2 Kerosene Dewaxing Middle distillate ZSM-5, mordenite Lubricants Benzene alkylation Benzene, ethene ZSM-5 Styrene Toluene disproportionation Toluene Xylene, benzene Xylene isomerization Isomer mixture P-xylene MTG Methanol Gasoline MTO Olefins Intermediate products Diverse Acidic and bifunctional zeolite Chemical raw materials SCR process Power station flue gases mordenite NOx-free off-gases

Aromatization of short C2-C4 alkanes into aromatics mainly Benzene Toluene Xylene developed by BP/UOP involves Bifunctional Ga/HZSM-5 Beyond these isomerizations of linear butenes were also investigated fro various zeolites A very interesting kinetics resulted in this conversion over various ring opening zeolites because the unwanted oligomerization transformation was suppressed in medium pore zeolites giving higher higher i-Butene selectivity

Membrane Zeolites The principal reason that zeolites based membrane systems have attracted researchers is their excellent ability to give Angstorm ranged separations However, due to this they need frequent regeneration but it is justified as they give pure separations. Table 5 gives us a brief idea on membrane zeolites.

Zeolites: Membrane Applications Zeolite Type Oxygen numbers at ring opening Channel dimensionality Pore opening width A (max.) General applications CHA SAPO-34, SSZ-13 8 3D 3.8 Gas separations LTA 4.4 Pervaporation, Membrane Reactors FER ZSM-35 10 2D 5.4 Pervaporation, Gas separations MEL ZSM-11 Pervaporation MOR 12 1D 7.0