BAHIR DAR INSTITUTE OF TECHNOLOGY

SCHOOL OF CHEMICAL AND FOOD ENGINEERING

PREPARATION AND CHARACTERIZATION OF ZEOLITE USING SILICAFROM RICE HUSK ASH

PREPARED BY:-MULUGETA ADUGNATESFAYE ALAMIREW

SUMMITED TO:-

NIGUS GABBIYE (PHD)

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Acknowledgment

.We wish to express our sincere thanks and deep gratitude to our teacher Dr.Nigus

Gabbiye for his invaluable guidance, constant encouragement, inspiration and assistant

throughout this project.

Our great appreciation is also given to Ato Addis for all his help especially in performing

the ICP analysis and DSC analysis. We would also like to acknowledge laboratory

assistances in the research and organic laboratory, where the majority of the experiment

work was completed.

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TABLE OF CONTENT

1. Introduction...................................................................................... 6

1.1 Introduction to the zeolites catalyst .................................................................... 6

1.2 Catalyst and catalysis ........................................................................................... 6

2. Literature Review of the Zeolite Catalysts .......................................... 9

2.1 History of zeolites ................................................................................................ 9

2.2 Uses of zeolites .................................................................................................. 10

2.3 Structures of zeolites ......................................................................................... 11

2.4 Catalytic properties of zeolites ........................................................................... 13

2.4.1 Catalytic activity of zeolites ............................................................................... 13

2.4.2 Catalytic selectivity of zeolites........................................................................... 15

3. Limitations....................................................................................... 18

4. Objective of project ......................................................................... 19

5. Methodology ................................................................................... 20

5.1 introduction of zeolite preparation ......................................................................... 20

5.2 Experimental work .................................................................................................. 21

5.2.1 Extraction of silcon dioxide from rice husk .......................................................... 21

5.2.2 Preparation of the zeolites catalyst ..................................................................... 23

5.2.3 Preparation of zinc impregnated zeolite.............................................................. 26

6. characterization of prepared zeolite type y catalyst ......................... 29

6.1 inductively coupled plasma (ICP) analysis ........................................................... 29

6.2 Differential scanning calorimetry analyses ............................................................. 30

6.3 Point of zero charge ................................................................................................ 32

6.4 PH ............................................................................................................................ 33

6.5 Performance evalution of prepared zeolite catalyst .............................................. 34

7. Conclusion ....................................................................................... 36

8 recommendations............................................................................. 37

Reference ............................................................................................ 38

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LIST OF FIGUREFigure 2.1 Tetrahedral units for the zeolite structures................................ 12Figure 2.2 Formation of the hydroxyl bridge in the zeolite framework.......14Figure 2.3 Reversible formations of the classical Lewis and Brönsted acidsites ............................................................................................................14Figure 2.4 Reactant selectivity ...................................................................16Figure 2.4 (B) Product selectivity ................................................................ 17Figure 2.5 (C) Restricted transition-state selectivity ..................................17Figure 5.1. Flow Chart of the Synthesis of Faujasite Type Y–Zeolite Catalyst...................................................................................................................25

LIST OF PICTURES

Picture 1: prepared Sodium Aluminate......................................................18Picture 2: washed rice husk with distilled water.........................................22Pic3: washed rice husk before boiled .........................................................22Pic5: dried in oven for about 24 hr .............................................................22Figure:-6 prepared NaYzeolite ....................................................................26Pic: 7 impregnated zeolites with zinc solutions ..........................................28Picture:8 zinc promoted zeolites after calcination......................................28

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Abstract

This project was conducted how to prepare and characterize the NaY –type zeoliteprepared locally from rice husk (which considered as a type of agricultural waste that isdifficult to discard) which is used as source of silca, and also the experiment studied onthe removal of one divalent zinc [Zn+2] ions from solutions by wet impregnation processusing our prepared NaY-type zeolites as an adsorbent material. We try to characterize thecatalyst by ICP techniques to know about the composition of Aluminum , silicon andzinic impregnated to this support. The Si/Al ratio is around 2.28. we also conduct DSC toknow the specific heat, enthalpy & some physical characterization like pH of ourcatalyst.

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1. Introduction

1.1 Introduction to the zeolites catalyst

This project is conducted to study the preparation and characterization of zeolites

from silicon dioxide that is extracted from rice husk, which is often considered as a solid

waste from rice milling, contains approximately 70% of organic compounds and 30% of

hydrate silica (SiO2). In general, the rice husk can be used as a cheap energy source

through combustion, generate heat or electric power or for other purposes as low value

material such as adsorption of heavy metals, synthesis of different types of zeolites and

also to produce metallurgical silicon.

Zeolites as a type of porous material have become important for catalytic

processing, either in the cracking of crude oil distillate for fuel manufacture or in the

conversion of crude oil fractions to gasoline in the presence of hydrogen in hydro

cracking processes. In chemical and petrochemical industries, zeolites have the ability to

act as catalysts for the chemical reactions, which occur inside the internal channels, and

to modify the products. Thus, the petroleum industry is the most significant consumer

of zeolites-based catalysts [7]. Adding the catalyst diminishes decomposition

temperature and promotes decomposition speed, hence makes a chemical process more

efficient and reduces pollution by saving energy while minimizing unnecessary products

and by-products.

Nevertheless zeolites have special catalytic properties; only a few types of zeolites

have the required physical and chemical specifications to act as catalysts, namely acidity,

thermal stability and pores sizes large enough to allow reactant molecules ready access to

their surface. Of these zeolites, the Y-type is still the main cracking component of

today’s catalyst, which presents as an excellent model for stud [9].

1.2 Catalyst and catalysis

Generally, the process by which a catalyst affects a reaction, speeds up or slows

down, is called catalysis, and a catalyst can be either heterogeneous or homogeneous

with bio-catalysts (enzymatic) are often seen as a separate group. A catalyst is defined

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as any substance “organic, synthetic, or metal” that works to accelerate a chemical

reaction by reducing its activation energy (Ea) without affecting in any way the

possibilities for this reaction within a chemical system. In fact, these possibilities

described in terms of thermodynamics by means of the Gibbs free energy of the

materials involved in the reaction – the reaction proceeds spontaneously if ∆G < 0 and

vice versa. However, heterogeneous catalysis is the common attractive method for

activating reactions that are thermodynamically possible but which occur at a very slow

rate because of their chemical kinetics.

Since heterogeneous catalysis is a surface “interface phenomenon”, the catalysts

need a large surface area often expressed in m2.g-1 to provide a sufficiently high

activity. The highly porous structures of zeolites, containing a three dimensional

network of channels, make them ideal as industrial catalysts, especially as it is possible

to modify their porosities and activities to selectivity minimize as much as possible the

formation of by-products. Therefore, this kind of catalyst is utilized to realize a

maximum conversion of the feedstock by increasing the rate of reaction. In addition, the

catalyst must has a good thermal stability (i.e. temperature at which it decomposes upon

heating at a constant rate) and is highly resistance to chemical agents during catalysis [5,

4, 3].

Moreover, zeolite catalyst has a high diffusivity – an important physical property

required for a commercially successful operation, which characterizes the ability of

fluids to diffuse throughout the zeolite structure. As such, the mass transfer of the

reactants to the active sites is increased, making possible the use of higher space

velocities of hydrocarbons often expressed in time-1 and lower residence times in the

reactor chamber. Additionally, most recent zeolitic or molecular sieve catalysts, as

they are also known, have a good hardness and are able to resist attrition and abrasion,

meaning that each catalyst particle is able to hold its shape. The choice of catalyst for

any specific purpose is depending on the operating conditions, feedstock, product

demands and the cost of process [6].

Theoretically, the overall reaction that takes place throughout the active sites on

the catalyst’s surface follow a five-step mechanism ,

These stages are: –

1- Thermal decomposition, generally after the transport of reactants from the

homogeneous phase (i.e gaseous or liquid) to the zeolite surface.

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2- Primary catalytic reaction following absorption of reactants on specific

sites of the surface so as to produce the intermediate chemisorbed species.

3- Secondary reactions between primary products in the sorbed phase.

4- Desorption of the products from the sorbed phase to release the sites.

5- Removal of the products from the catalyst surface into the homogeneous

phase, and accumulation of polymerizable products from further reaction

by their adsorption on the surface of the catalyst as coke.

Catalyst poisoning is a problem in all reactions, but the generation of coke as a by-

product is the most significant problem. The deposition of coke by occupying active

catalytic sites leads to reduce catalyst activity, thereby reducing the products yield

with deactivation occurring in two discrete ways: 1) pore blockage which prevents

the access of reactant molecules to the whole segments of zeolite pores, and 2) site

coverage caused by poisoning the zeolite acid sites. The acidity of zeolite is generated

by aluminium ions, which can be present in the zeolite framework or as extra

framework aluminium (EFAl) species. It is possible to increase the catalyst life by

means of decreasing its sensitivity to the effects of coking, however a proper

regeneration treatment is required to burn off all the coke in an oxygen rich dry

atmosphere [10].

.

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2. Literature Review of the Zeolite Catalysts

2.1 History of zeolites

Previously, the content of rice husk ash at different combustion temperatures has

been studied. The white ash that was obtained from combustion is generally 10-15% of

the total dry weight of rice husk. The water content may affect the combustion

temperature and the rice husk that has been treated with hot-water and some steam-

explosion processes give a lower level of metallic impurities. When the rice husk is

leached with mineral acid and calcined in oven, white powder rice husk silica (RHS) is

obtained. The RHS with high silica purity is suitable as a silica source for the production

of inorganic materials such as silicon carbide and silicon nitride. In a research field

related to catalysis, RHS was used as a silica source for the synthesis of micro-porous

materials such as zeolites and meso-porous silica. Rice husk was successfully used as a

silica source for the synthesis of type Y-zeolite in sodium form (NaY). In this study, a

low coast agricultural waste, which is Rice Husk was used as a raw material to synthesis

type Y-zeolite and the using of this prepared zeolite in the removal of zinc ion (Zn+2)

from solution. The remaining samples of type Y-zeolite after treating with aqueous

solutions containing zinc ion (Zn+2) was tested as a zinc promoted type Y-zeolite

catalyst and compared this catalyst with normally type Y-zeolite catalyst prepared from

rice husk only (without treatment with zinc ion (Zn+2).

The history of zeolites began in 1756 when the first zeolite mineral was discovered.

Zeolites originate in cavities inside rocks, as they are produced due to chemical

reactions within the volcanic magma. Axel F. Cronstedt, a Swedish mineralogist who

derived the term from two classical Greek words “zeo” and “lithos”, which mean, “to

boil” and “a stone”, first used the word “zeolite”. In addition, the name “boiling stone”

was also used because of the bubbles that zeolites release when heated in blowpipes

under high temperatures [13].

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Some of the more fundamental properties of zeolites were established over a period

of 130 years beginning in 1840, when Damour reported that zeolites had undergone

reversible dehydration with no apparent change in the transparency of the crystal form,

and in 1858 Eichhorn observed the reversibility of ion exchange on zeolite minerals. In

1930, Taylor and Pauling determined the crystal structure of the zeolites and showed

evidence of the presence of cavities in these structures; soon after in 1932 McBain

established the term “molecular sieve” to describe the porous solid materials, and the

ability of zeolite structure to act as sieves on a molecular scale. In 1945 Barrer – the

father of zeolite science in the United Kingdom; reported the first classification of

zeolite minerals based on the size and the rate of molecules absorbed: rapidly, slowly

or not significantly at room temperature. In the early 1950s, Milton and Breck

discovered the commercially vital synthetic zeolites A, P, X and Y. These zeolites

were synthesized from readily available raw materials. At that time, only aluminium-

rich zeolites could be synthesised. In 1967, Wadlinger and his co-workers introduced

the first silica- rich forms of zeolite beta (BEA). To date over 197 zeolite framework

types with an array of physical and chemical properties have been synthesized. Infact,

these are used to great effect in a wide range of industrial processes and it is always

important to know the specific type of zeolite one is using in order to assure that it

is appropriate for one's need [11,9].

2.2 Uses of zeolites

The larger-pore zeolite structures of type (Y) are often used in catalytic cracking

and hydro cracking processes in the petroleum industry, and are also used in catalytic

degradation of polymer wastes for recycling processes, which gives rise to an

increase in the recovery yield of gasoline-range hydrocarbons as the elementary

compositions between plastics and petroleum fractions are similar [6].

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In contrast, the smaller-pore zeolite structures of type (A) are employed to

selectively separate small molecules e.g.; H2S, H2O from organic molecules, and is

often used in the separation of light normal from branched paraffins, because the

latter (iso-alkanes) are slightly larger than the former. The type (A) zeolite may also

be used to aid the elimination of traces of sulphur compounds. Other industrial

applications of zeolites include the removal of potentially harmful organics or ions

from water, where natural zeolites are employed as ion exchangers, aiding the removal

of NH4+ from wastewater, and also as builders in Laundry detergents to remove Ca2+

and Mg2+,here by softening the washing liquid.In addition, they are used as absorption

agents, and in membrane synthesis, and soil treatment processes for agriculture, and

also as modifiers in electrochemical processes, as well as in the nuclear industry or the

removal of radioactive species. Given the wide spread uses of zeolites, it is important to

consider the implications on health, and as such selective zeolites have been certified as

safe for human consumption – this includes e.g. Clinoptilolite (HEU), which is the most

abundant natural zeolite used as a good adsorbent in sulfar dioxide, SO2, removal [11,8].

2.3 Structures of zeolites

Zeolites are generally defined as crystalline alumina-silicates, and may be found as

natural minerals that are extensively mined in many parts of the world; however most

pure zeolites used in industrial processes are produced synthetically. Commonly, the

alumina-silicate framework of zeolites consists of alumina (AlO4)5- and silica (SiO4)

4-

tetrahedral units and their corners link all of these tetrahedral units together. Since

silicon has a valance of four and aluminum a valance of only three, the AlO4

tetrahedron carries a net negative charge. Accordingly, a positive extra-framework cation

such as sodium (Na+) is incorporated as a charge counter-balance, and that gives the

zeolite its ion exchange characteristic. Furthermore, Lowenstein’s rule states that (four

Si atoms can surround each Al atom, while up to four Al atoms can surround the Si

atom), with oxygen bridges joining the Al and Si atoms such that no two aluminum

atoms bond to the same oxygen atom. This accounts for the fact that the zeolite LTA has

the lowest possible value of the Si/Al ratio [5].

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With the oxygen atoms distributed throughout the zeolite network, a rich variety

of beautiful three-dimensional structures may be found. The two types of tetrahedral

units can be arranged in a variety of ways and presented in different ratios – including

the faujasite type zeolite, for instance, which is characterised by interconnected voids

bounded by supercages with a diameter of 1.3 nm that can host cations and water

molecule. As such water moves freely inside the structure, but the zeolite framework

remains rigid. Figure 2.1 shows a tetrahedral structure that consists of a Si or Al atom.

The oxygen at each tetrahedral corner is connected with another tetrahedron by straight

lines that schematically represent the T-O-T bridges. These tetrahedrons are called

primary-building units (PBUs) and combine to shape the secondary- building units or

SBUs that give rise to the unique topology. As soon as SBUs are linked together, the

sodalite like in zeolite Y or any other geometrical shapes can

.

Figure 2.1 Tetrahedral units for the zeolite structures [5].

Since zeolites are microporous structures, each zeolite topology has a typical poreopening, dependent on the size of the oxygen ring that defines the pore (i.e. the size ofthe SBU). Thus, a description of a zeolite structure always relates with a description ofthe pore openings and the dimensionality of the channel system within. Their uniqueporous properties make zeolite consumption increases to a global market of severalmillion tonnes per annum.

According to the international union of pure and applied chemistry (IUPAC), the

classification is as follows; Micropores: dp ≤ 2 nm, Mesopores: 2 nm, < dp ≤ 50 nm

and Macropores: dp > 50 nm, with dp being the pore diameter. In these pores, the

dissolved organic molecules with appropriate sizes to fit into the catalyst pores are

adsorbed during the reaction.

As a general rule, zeolites structures have important properties and these

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properties are summarized in the following points :

Stability of the crystal structure when dehydrated (i.e. the removal

of water from the zeolite crystals) – this is common of many types

of zeolites with dehydration occurring at temperatures below 400 °C.

Adsorption of gases, vapour and other molecules inside the

microporous channels, because they are large enough to allow the

passage of guest species. In additional to a large void volume, a low

density and uniform molecular sized channels characterize the

majority types of the zeolite materials.

A variety of other physical properties such as electrical

conductivity, cation exchange and catalytic properties [5, 4]

2.4 Catalytic properties of zeolites

Zeolites can operate both as ion-exchange materials and also reversible

adsorption systems for water or small organic molecules, with a potential capacity

of more than 25% of the framework weight; however the two most significant

properties for zeolites are acidity and porosity. The acidity of a zeolite is usually

responsible for the catalytic activity of catalysts, whilst the porosity is responsible

for the catalytic selectivity during the reactions. These catalytic properties can be

modified to provide enhanced flexibility across a range of applications [14].

2.4.1 Catalytic activity of zeolites

Zeolites are mostly employed as acid catalysts, with the catalytic activities

of zeolites attributed to the generation of strong acidic sites on their surfaces.

Electron pair acceptors or Lewis acid sites (L) and proton donor or Brönsted acid

sites (B), are both found in zeolites with the former resulting from the rupturing of

hydroxyl bridges between aluminium and silicon atoms in the framework, and the

latter resulting from the hydroxyl bridge that forms as shown in Figure 2.2. The

Brönsted acid site is formed when the negatively chared aluminium framework is

Counter-balanced by proton (H+), such that it is necessary to replace the cations

present in the freshly synthesized zeolite with protons, for instance by substitution

of sodium ion (Na+) with an ammonium ion (NH+4). A high temperature calcining

process is then required to drive off the ammonia and leave a protonated form of

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the zeolite. In other circumstances where the zeolite is not protonated, a trigonally

coordinated Al-atom possessing a vacant orbital is produced that can accept an

electron pair and acts as a Lewis acid site [1,8].

Figure 2.2 Formation of the hydroxyl bridge in the zeolite framework [1].

Steaming modification may be used to increase the lattice Si/Al ratio of a zeolite by

means of removing different fractions of framework Al-atoms, where heating of

Brönsted acid sites causes dehydroxylation with the formation of an electron acceptor

“Lewis acid sites” at high temperatures, and fixation of water leads to some of the (L)

sites changing into (B) sites as shown in the Figure2.3. The rate of change is

proportional to the temperature used in the process – it increases with the increasing of

temperature [10].

Figure 2.3 Reversible formations of the classical Lewis and Brönsted acidsites [10-11].

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This process is followed by the ejection of aluminium species (AlO+) from the lattice

positions into cationic positions. Alternatively, water vapour used during the steaming

process can provide support to the oxygen atoms within the framework by increasing

their abilities to bond with the migrating silica species from other parts of the crystal and

causing the formation of new Si-O-Si bonds, in order to re-occupy the created vacancies

by these silicon atoms – such a structure tends to shrink under stabilization. The activity

of a zeolite catalyst may be defined by: (a) the strength of acidity, (b) the acid sites

density, and (c) the accessibility of the bridging hydroxyl groups, which act as Brönsted

acid sites. Undoubtedly, a decrease in the number of Al-atoms in the framework “high

Si/Al ratio” causes a decrease in the density of Brönsted acidity of a zeolite, but may

also increase the single acid site “proton donor” strength. By decreasing the Al content,

the charge density of anions “hydroxyl groups within the framework” decreases and

leads to less intense interaction of OH-groups whitin the framework, thereby increasing

the ease of proton transfer from the surface site to the adsorbed base. Thus, the overall

catalytic activity of a zeolite can be enhanced [10, 11].

It should be noted that stronger Brönsted acidic sites are present in highly

crystalline zeolite structures and that such structures have greater activity than the non-

crystalline type with same chemical composition. The impact of this is that

crystallization time is considered as a major parameter in the hydrothermal synthesis

process.

Whilst it would be expected that stronger acid sites would be foremost in the

catalyst selection process, this is not always true – especially in the cases of processes

such as cracking or hydrocracking where weak interactions are as a rule preferred,

assuming there is sufficient strength to catalyze the reaction. This is because the use of

zeolite catalyst with much stronger acid sites leads to easy deactivation it due to rapid

deposition of coke or poisoning with impurities [10].

2.4.2 Catalytic selectivity of zeolites

A catalyzed chemical reaction frequently takes place within the zeolite pores,

internal channels or cavities, and therefore there are size restrictions on the reactants,

products, or transition states intermediates. The maximum free pore diameters must thus

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significantly influence the shape-selectivity phenomenon.

Usually, shape selective catalysis is applied either to increase yields of a preferred

product or to hinder undesirable reactions, and the desire for precise control over

selectivity means that the heterogeneous catalysis is more favourable than the

homogeneous one for cracking reactions, since the pore size depends on the type of

cation present within the zeolite framework – e.g. a monovalent cation such as

potassium or sodium reduces the actual pore size of zeolite-A to below 0.4 nm.

However, the pores enlarge slightly at higher temperatures, which can then allow the

diffusion of molecules into or out-of the channel systems throughout the reaction. Whilst

there are many factors impacting shape selectivity, the zeolite frameworks may be

modified for specific applications, for instance, the uniform micropores in Y-type

zeolites provide excellent catalytic selectivity opportunities, as the faujasite type zeolites

have a regular opening large enough to accommodate molecules commonly found in gas

or oil refining operations [1, 13].

The desire to increase the porous properties of more siliceous zeolites has led to the

development of high surface area mesoporous materials, with extra- porosity of zeolites

such as ZSM-5 being created by desilication methodology. This involves the removal of

silicon from the framework, accordingly decreasing the lattice Si/Al ratio.

Weisz and Csiscery have shown that zeolite shape-selectivity can be divided into three

main categories, described with mechanisms shown in Figure 2.4 A, B and C:

A- Reactant selectivity: This arises when some of the reactant molecules

are too large to enter the zeolite channel system and products are only

formed from those molecules that are able to diffuse through the

Catalyst pores.

Figure 2.4 Reactant selectivity [1].

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B- Product selectivity: This arises when some of the product molecules

created inside the channel systems are too large to transport out of the

zeolite structure. They either deactivate the catalyst or are converted

by cracking to less bulky molecules, which then escape from catalyst

pores.

Figure 2.4 (B) Product selectivity [1] .

.

C- Restricted transition-state selectivity: This arises when some transition state

molecules are too large to form in the zeolite channels or cavities because those

molecules would require more space than available.

Both reactant and product molecules are prevented from dispersing through the

pores and only the possible product molecules from the

transition states are produced in the void space.

Figure 2.5 (C) Restricted transition-state selectivity [1]

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3. LimitationsThe first problem we encounter is Unavailability of chemicals like Sodium Aluminate

Oxide (NaAlO2) which is used as a raw material for preparation of feed stock gel. But we

try to prepare this chemical in our laboratory from Sodium Hydroxide and Aluminium

oxide.

Chemical reaction

2NaOH(s) +Al2O3(s) 2NaAlO2(s) + H2O(l)

By using stochiometric balance

80 gm NaOH=102 gm Al2O3 80gm NaOH=164 gm NaAlO2

10 gm NaOH=? 10 gm NaOH=?

Therefore 10 gm of NaOH is reacting with 12.75 gm of Al2O3 and gives 20.50 gm of NaAlO2.The above reaction is takes place using magnetic stirrer at room temperature. The reactionis exothermic. After the reaction takes place we separate the product from water byfiltration, and then dry it at room temperature.

Picture 1: prepared Sodium Aluminate

Secondly Unavailability of chemicals like n-heptane which is used for performance

evaluation of our catalyst as catalytic cracking materials. But we try to use hexane as

catalytic cracking materials.

H2O

NaAlO2

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4. Objective of projectThis project is essentially focused on the synthesis and characterization of Y-type zeolite

catalytic properties for the cracking of a long chain hydrocarbon into its derivative

components on a lab scale,. In view of that, the aim of the present study can be

summarized as follows;

General objective

Synthesis of Na-Y zeolites form from silicon dioxide which extracted from rice

husk and aluminum source

Specific objective

Analysis of the synthesized Y-samples by means of the most common

characterization techniques (i.e. ICP, DSC, TGA and BET) to investigate their

properties and establish the correlations between the achieved results from these

characterizations.

Impregnate the synthesized NaY zeolites with zinc

The final aim of the present study was catalytic cracking of n-heptanes (nC7) over

the selected Y-catalysts in order to assess the effectiveness of the catalyst...

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5. Methodology

5.1 introduction of zeolite preparation

In this project we have gathered and analyzed data from different sources. The

major data are collected by doing experiments, different books, observation we were

give conclusion, recommendation and possible solution that we understood from the

information samples.

In general zeolites can be synthesized from a reaction mixture containing silica,

alumina, alkali hydroxide and water. The nucleation and crystal growth are the two

most essential steps in zeolites crystallization, with nucleation taking place during the

induction period within complex chemical reactions. In fact, the induction time is a point

on the crystallization curve, where the conversion of amorphous material into crystalline

product begins

For the duration of the induction period, reorganization of the amorphous alumina-

silica “intermediate” gel takes place, and a number of small crystalline nuclei are formed

as the zeolites synthesis mixture is heated. The nucleation mechanisms in liquid-solid

systems can be divided into primary and secondary nucleation stages, where the former

is an important part in the zeolite synthesis system, and is itself divided into

homogeneous and heterogeneous nucleation. Homogeneous nucleation occurs only

within the solution, and after the amorphous gel “extraneous materials” appears, the

interface between the gel phase and liquid phase playing a significant part for the

heterogeneous nucleation to take place. Briefly, the amorphous materials in the

primary amorphous phase “initial gel” develops during heating and convert into the

secondary amorphous phase “equilibrated gel”, nuclei gradually form and are

transformed to the crystalline zeolite product.

In addition, the rate of crystallization mechanism may be increased by the use of

elevated temperature and aging (i.e. adding seed crystals to a crystallization system),

where a combination of the two leads to proceed the mechanism more quicker than in

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the non-aged case, due to a significant increase in the available

Surface area for crystal growth and nucleation of new crystals [12, 15]

5.2 Experimental workIn order to prepare zeolite we followed the following pre preparation steps

5.2.1 Extraction of silcon dioxide from rice huskMaterials needed

Teflon Baker Pyrex Baker Balance Magnetic stirrer droplet heater Oven furnace Sulphuric acid Sodium hydroxide Aluminium oxide Sodium hydroxide Zinc chloride Sodium Aluminate Oxide Husk Distilled water

The procedure that we follow to extract silcon dioxide from rice husk were as follows

Rice husk was collected from Woreta fields in the Southern of Gonder.

The husk was washed three times with distilled water. Excess distilled water was

used to remove the soluble materials present in the rice husk bringing from field.

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Picture 2: washed rice husk with distilled water

Boiled to remove color and other fine impurities may be found in the rice husk

Pic3: washed rice husk before boiled Pic4: color removal afterboiled

And then dried at 105°C for 24 hours. When the rice husk was heated at 105°C

in an oven, most of the water had been removed from the rice husk while the

second major mass loss of about 45-65% was attributed to the breakdown of

cellulose constituent char, which is a carbonaceous residue.

Pic5: dried in oven for about 24 hr

The rice husk was treated with 10% sulfuric acid (H2SO4) for 24 hours for

preliminary removing all impurities. Until now we are try to remove the impurities.

. Dry rice husk were sieved to eliminate residual rice and clay particles and also

They were well washed with double distilled water, filtered, dried in air, and

calcined at 750°C for 6 h

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12 g of calcined were then subjected for dissolution in sodium hydroxide NaOH

(4 M) followed by boiling at 90°C for 12 h.

Concentrated hydrochloric acid (HCl (37%)) was then added to the

aforementioned base dissolved rice hask for complete precipitation. So the rice

husk were filtered, washed with excess distilled water to be freeing from

chloride ions and finally dried in an oven at 120°C for 6 h

At this stage all hydrated silcon dioxide wich was found in rice husk is extracted

5.2.2 Preparation of the zeolites catalyst

In general zeolites can be synthesized from a reaction mixture containing

silica, alumina, alkali hydroxide and water, So, Faujasite type Y–zeolites could

be synthesized using silicon dioxide which is extracted from rice husk as a silica

source.

Seed gel preparation

A 500 ml Teflon beaker containing a magnetic stirrer was washed with

deionized water.

Sodium hydroxide of 1.6616 g was added slowly to deionized water

and stir until clear and homogenous solution appeared for about 5

minutes.

The aqueous solution of sodium hydroxide was ready for the

preparation of seed gel. The gel was prepared according to the

following molar chemical composition

1.67 Na2O:0.1 Al2O3: 1 SiO2: 5 H2O

Feed stock gel preparation

Two milliliter aqueous solution of sodium hydroxide was added to 0.7515g

sodium aluminate oxide until a homogenous mixture was formed.

1.5361g rice husk was added separately to 5.5 ml sodium hydroxide aqueous

until homogenously mixed.

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Both of the preparations were heated under vigorous stirring to obtain a

homogenous mixture.

The sample was aged for 24 hours at room temperature in the Teflon bottle.

The aluminate and silicate solutions were mixed together in the polypropylene

beaker, subsequently stirred for 2 hours with the purpose of making it

completely homogenized.

This combined solution was used as the feed stock gel. The flow chart of the process is

shown in figure 5.1

25

Figure 5.1. Flow Chart of the Synthesis of Faujasite Type Y–Zeolite Catalyst

26

Figure:-6 prepared NaYzeolite

5.2.3 Preparation of zinc impregnated zeoliteWe impregnate our prepared sample zeolite in order to know the ability to adsorb

zinc ions from solutions that contains zinc even though this experiment is done in

adsorption units.Zinc impregnated zeolites is used as catalyst for cracking of n-alkanes

hydrocarbons especially for n-heptanes

27

We impregnate our prepared zeolite with zinc ion as follows

the experiments were carried out using simulated synthesis aqueous

solutions of (Zn+2)ions.1000 mg/l stock solution of (Zn+2) ions was

prepared by dissolving suitable amount of zinc sulfate (ZnSO4) in one

liter of double distilled water

All solutions using in the experiments were prepared by diluting the

stock solution with distilled water to the desired concentrations for the

experimental work of this investigation. The (Zn+2) ions concentrations

were measured using ICP

Then we were impregnate half of the prepared zeolite using prepared

solutions as shown on the picture below\

28

Pic: 7 impregnated zeolites with zinc solutionsAfter impregnation we calcined the sample at 300 OC

Picture:8 zinc promoted zeolites after calcination

zeolite

Zinc solutions

29

6. characterization of prepared zeolite type y catalystIn general, the characterization of a zeolite catalyst has to provide information

about structure and morphology, the chemical composition, the ability to sorb and retain

molecules and the ability to chemically convert these molecules. Information on the

structural, chemical and catalytic characteristics of zeolites is essential for deriving

relations between their chemical and physicochemical properties on the one side and the

sorptive and catalytic properties on the other. Such relations are of high importance, as

they allow the rational development of sorbents, catalyst and advanced structural

materials. In this project, zeolite was synthesized from silcon dioxide of rice husk. The

main uses are as an adsorbent material to adsorb divalent zinc (Zn+2) ions from

simulated aqueous solution and use the remaining samples as a catalyst for n-

heptane isomerization, thus only characterizations with respect to these applications

are being dealt with in depth. There are many characterization techniques but the

important ones in this study are inductive coupled plasma (ICP), Differential

scanning calorimetry (DSC) and determination of BET surface area and pore volume of

prepared zeolite catalysts.

6.1 inductively coupled plasma (ICP) analysis

In order to identify and/or determine the concentration of the atomic and molecular

species present in a chemical composition, an analytical spectroscopy technique can

commonly be used such as ICP-AES – inductively coupled plasma, atomic emission

spectrometry. It is fundamentally a type of emission spectroscopy widely employed to

detect the traces of metals within the sample, which can identify each element from the

wavelength of its electro- magnetic radiation. The atoms or the molecules in accordance

with their electronic structures frequently emit certain wavelengths of photons when

transmitting from an excited state to a lower energy state. As a result, raising the

intensity of this emission refers to an increase of concentration of metal within the

sample. Consequently, the composition of the sample can be determined.

The procedure that we followed for the analysis is as follows

30

Determination of silcon concentration

Before the analysis with the ICP proceed we had prepared the following samples

Prepared of standard solution using 100gm of silica gel in 1L of distilled water

Prepared our samples using sodium hydroxide in order to digest 10gm of zeolite

samples.

So the ICP result showed as the concentration of silcon in the samples is [Si] = 5

571.30 X mg/l

Determination of Aluminium concentration

Prepared of standard solution using 100gm of Aluminium nitrate in 1L of

distilled water

Prepared our samples using hydrochloric acid in order to digest 10gm of

zeolite samples.

So the ICP result showed as the concentration of Aluminium in the samples is

[Al] =2 438.665X mg/l

So from above result si/Al ratio is 2.28

Determination of zinc concentration

Prepared of standard solution using 100gm of zinc sulphate in 1L of distilled

water

Prepared our samples using Sulphuric acid in order to digest 10gm of zeolite

samples.

So the ICP result showed as the concentration of zincin the samples is [Zn] = 887.71 X

mg/l

6.2 Differential scanning calorimetry analysesThe word calorimeter is derived from the Latin word calore, which means heat, and

Calorimetry is a science deals with the heat of chemical reactions or heat capacity

measurements from physical changes. In the project, differential scanning calorimetry

DSC is used to investigate the enthalpies of transitions during the calcination step. The

DSC-instrument can mostly be utilized to investigate the thermo physical properties of

polymers and to study oxidation or other chemical reactions.

31

Procedure used in analyses is as follows

About 10 mg of zeolite sample and 10mg of zinc impregnated sample was charged

into a hermetically sealed aluminium sample pan and weighed before and after the seal –

with a second pan and lid used as a reference chamber. The sample and the reference

were then placed on the heat flux dish “thermoelectric disc”, which can generate a tightly

controlled heat flux. (Heat flux (q/t) can be expressed in terms of change of heat (q) vs.

change of time (t) , and both the sample and reference are maintained at the same

temperature (T) and heating rate (∆T/t) during the experiment. Thus, whether the heat

capacity (Cp) of the sample (i.e. the amount of heat required to raise the temperature of

the sample by one degree, J.°C-1) or the enthalpy (∆H) can be calculated as follows :

(q/t)/(ΔT)= q/ΔT= CP=ʃdq= ʃCPdT=ΔH

Generally, more or less heat must flow to the sample inside the DSC-instrument

depending on whether the applied process is exothermic or endothermic. The enthalpy

is expressed by the following equation , with instrument error in DSC typically ± (0.5 -

1) ºC:

H K .A (3.16)

Where:-∆H is the enthalpy of transition (J.g-1), which measures the heatcontent. Changes of state or phase of matter are also accompaniedby enthalpy changes, and if ∆H is positive, the reaction isendothermic such as in a melting process – heat is absorbed by thesystem. In contrast, if ∆H is negative, the reaction is exothermic suchas in a freezing process – heat is desorbed from the system.K is the calorimetric constant, which is actually varies frominstrument to instrument and can be determined by analyzing awell-characterised sample with known enthalpies of transition (i.e.well-defined heat capacity over the range of temperatures). Indium(In), a very soft metal as a reference with a value of K about1.0780 was used for this purpose, andA is the area under the peaks that are reflected in the DSC plot [11, 4,10].

What we had seen from the result DSC result is the enthalpy, the specific

heat flow of the NaY zeolite is better than zinc promoted zeolites.

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6.3 Point of zero chargeThe zero point of charge is a fundamental description of a mineral surface, and is more

or less the point where the total concentration of surface anionic sites is equal to the total

concentration of surface cationic sites, and most (but not all) of the sites are as the neutral

hydroxide.

At pH values above the ZPC, the surface has a net negative or anionic charge, and the

surface would participate in cation attraction, and cation exchange reactions.At pH values

below the ZPC, the surface has a net positive charge, and the surface will attract anions,

and participate in anion exchange reactions

We followed the following procedure in order to know point of zero charge

Mix 0.2gm of our sample with 0.2gm of sodium nitrate in 30ml of distilled water

Then titrate our sample with sodium hydroxide and nitric acid

Finally plot a graph PH versus change in PH.

∆pH=pH measured-pH initial

pH 1.21 1.34 1.41 1.76 1.99 10.83 11.17 11.4 11.70∆pH -9.19 9.06 -8.99 -8.64 -8.41 0.43 0.77 1 1.3

Table 1: point of zero charge of zeolite catalyst.

Graph 1: point of zero charge of zeolite catalyst.

-10

-8

-6

-4

-2

0

2

1 1.21 1.34 1.41 1.76 1.99 10.83 11.17 11.7 11.7

∆pH

∆pH

33

pH 1.16 1.30 1.47 1.67 2.05 11.52 11.87 12.10 12.25 12.35∆pH -8.78 -8.64 8.47 -8.27 -7.89 1.58 1.93 2.16 2.31 2.41

Table 2: point of zero charge of Zinc impregnated catalyst..

Graph 2: point of zero charge of Zinc impregnated catalyst.

So from graph point of zero charge of zeolite is and for impregnated zink also______

6.4 PHThe pH scale measures how acidic or basic a substance is. It ranges from 0 to 14. A

pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic.

We checkered the PH of our sample as follows

Dissolve 10mg of our sample in distilled water

Then we measure the PH

The PH meters showed as

PH of zeolites is around 10.40 and PH of zinc impregnated is around 9.94

-10

-8

-6

-4

-2

0

2

4

1 1.16 1.3 1.47 1.67 2 2.05 11 11.52 11.87 12 12.1 12.25 12.35

∆pH

∆pH

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6.5 Performance evalution of prepared zeolite catalystWe try to evaluate our sample activity performance in the following two methods

1. Activity Test of Synthesized Type NaY– Zeolite

The activity test of our sampled is

The activity of our sample Na Y–zeolite prepared was studied by

applying removal of (Zn+2) ions.

The (Zn+2) ions removal from solution was carried out in a laboratory

by using of wet impregnated. Then the outlet samples collected and

tested by inductive coupled plasma equipment to find the remaining

concentration of (Zn+2) ions.

. (Zn+2) ions removal was calculated from the equation:

[Zn+2]0 – [Zn+2]R%= _____________

[Zn+2]0

Where: [Zn+2]o and[Zn+2] are initial and residual divalent zinc (Zn+2)ion concentration

and where R is removal respectively from ICP result

[Zn+2]0 =12803.04mg/L and [Zn+2]0 – [Zn+2] = 887.71 mg/Concentration

adsorbed zinc by zeolite

R%= 887.71/12803.04

R%== 0.695%

Even though the result is very small it showed as our prepared zeolite sampled has an ability to

2. Activity Test of zinc promoted Synthesized Type Y– ZeoliteThe zinc promoted type Y zeolite catalyst activity was studied by applying n - heptane

catalytic cracking reaction.

But we couldn’t work this activity test because of the following reasons

Since the catalytic cracking reaction is takes place under high temperature

between 4000c-5000c and the experiments of catalytic cracking were performed in

an experimental fluidized bed unit. The unit consists of n-heptane storage tank, gas

35

flow meter, dosing pump, evaporator, condenser/separator, cooler with appropriate

control, and power supply box. due to these difficulties we couldntt work

experiments in our laboratory

gas chromatography device (GC) that is found in our research lab is not

worked due to an availability of standard solutions since A sample of

gaseous product after cracking was collected and then analyzed by gas

chromatography device (GC),

36

7. ConclusionZeolites as a type of porous material have become important for catalytic

processing. Nevertheless zeolites have special catalytic properties; only a few types of

zeolites have the required physical and chemical specifications to act as catalysts,

namely acidity, thermal stability and pores sizes large enough to allow reactant

molecules ready access to their surface. In general zeolites can be synthesized from

a reaction mixture containing silica, alumina, alkali hydroxide and water, So, zeolites

could be synthesized using silicon dioxide which is extracted from rice husk as a silica

source. Then we impregnate our prepared sample zeolite in order to know the ability to

adsorb zinc ions from solutions that contains zinc. So the ICP result showed as the

concentration of zinc in the samples is [Zn] = 887.71 X mg/l, concentration of

Aluminium in the samples is [Al] =2 438.665X mg/l and concentration of silcon in the

samples is [Si] = 5 571.30 X mg/l. the Si/Al ratio is 2.28.

We also characterize the specific heat and enthalpy of catalyst support and Zn

impregnated catalyst by using DSC.

37

8 recommendationsIn the future, in order to modify the catalytic properties of zeolite-Y catalysts.

Investigation of the catalytic performance of the nheptane cracking reaction at 450 ºC ofzeolite catalysts produced and deactivation time. Characterization techniques like BET,TGA and GC will be performed to know more about the physical and chemicalcharacteristics of zeolite and to modify it. Some chemicals are not easily available, so onehave to collect all necessary chemicals before doing his/her experiment. The Si/Al ratiocan be modified to better catalyst property by adjusting it.Post-synthesis modification ofzeolites may be achieved using techniques such as de-alumination or de-silication andhave been developed in an attempt to improve several operational properties

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