An Experimental Enquiry into The Growth of Mordenite Nanocrystals Sans Seed

Addition

Mohammad Hassnain 3/1/17 Material Science and

Nanotechnology

1

Abstract

The importance of porous materials with very small crystal diameter is immensely increased

because of their large range of applications in our daily life. Mordenite falls in the category

which is a microporous, synthetic zeolite with an ideal unit cell composition of Na8. (AlO2)8.

(SiO2)40. nH2O or Na8Al8Si40O96.nH2O and a lot of work is going on its synthesis according to

its application.

But we have mainly focused on Mordenite synthesis in nanosize without adding seed and

incorporating its effect on Mordenite morphology by comparing with standard Mordenite.

Synthesis of Mordenite nanocrystals was mainly divided into three steps. The first step covered

the procedure for preparation of gel without adding seed. The gel is then converted into raw

Mordenite under hydrothermal conditions in the second step. Finally, in third step raw Mordenite

product is recovered into pure Mordenite crystals by applying washing with distilled water and

drying techniques. The effect of sans adding seed and distilled water in the sample is then

studied with the help of X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Vast range of laboratorial and industrial applications of Mordenite are summarized into three

major uses. These includes the use of Mordenite as a catalyst, as an adsorbent and as an ion

exchanging sieve. Other uses of Mordenite are also discussed.

2

Table of Contents

Chapter 1 ......................................................................................................................................... 3

Introduction ................................................................................................................................. 3

1.1 Definition of Zeolite: ......................................................................................................... 3

1.2 Mordenite definition by type and classifications ............................................................... 4

1.3 Structure............................................................................................................................. 4

1.4 Types and properties of zeolites: ....................................................................................... 7

1.5 Classifications .................................................................................................................... 7

1.6 Occurrences: ...................................................................................................................... 8

1.7 Mordenite and Price Group/economy: .............................................................................. 9

Chapter 2 ....................................................................................................................................... 10

Synthesis.................................................................................................................................... 10

2.1 Chemical Reactions and their conditions: ....................................................................... 10

2.2 Procedure: ........................................................................................................................ 10

2.3 Flow Sheet ....................................................................................................................... 13

2.4 Results and Discussions: ................................................................................................. 14

Chapter 3 ....................................................................................................................................... 17

Applications .............................................................................................................................. 17

3.1 Catalysis: ......................................................................................................................... 17

3.2 Adsorption: ...................................................................................................................... 17

3.3 Ion Exchange: .................................................................................................................. 18

3.4 Other Uses: ...................................................................................................................... 18

References ..................................................................................................................................... 19

Bibliography ................................................................................................................................. 20

3

Chapter 1

Introduction

Since the last century, material chemistry is not only a very broad subject to study and research

but also represents an extremely important and applicable field to everyday life.

Scientists are now focusing on research, synthesis and development of ‘Sponge-like’ porous

material which is one of the most important class of material chemistry.

These materials are also well known as ‘Molecular sieves’ in the literature.

The term ‘Molecular sieve’ is generally used for the materials which have small pores and which

may not necessarily be uniform. The examples of molecular sieves include zeolites, porous

glasses or certain type of carbons.

Since the activity and effectiveness of porous materials is enhanced as the amount of surface area

increases, Thus the most important factor in these materials is the surface area. That is why

nowadays the study and research of these materials is largely dominated by development and

enhancement of pores of these materials.

One of these materials is known as zeolite and a large amount of work is going on its

development.

1.1 Definition of Zeolite:

"A zeolite is an aluminosilicate with a tetrahedral framework structure enclosing cavities

occupied by large cations and water molecules, both of which have considerable freedom

of movement, permitting cation exchange and reversible dehydration".

Chemically zeolites have molecular formula of M ·nAlO2·SiO2· H2O.

Where M is the charge compensating metal (It may be sodium, potassium or calcium), n and

represents the number of moles of AlO2 and SiO2 respectively and must be greater than n

(since according to Lowenstein rule, Al-O-Al bond is not permitted in the zeolite or AlO2

tetrahedral never share same oxygen ion in the framework thus Al-O-Al is not permitted in a

zeolite) and is the number of moles of water.

There are six main classes of silicate minerals if divided mineralogically and aluminosilacate

belongs to one of its class known as tectosilicates.

Tectosilicates represents a three dimensional infinite structure of SiO2 and AlO2 in which oxygen

ion is shared by neighboring tetrahedral as shown below.

4

Fig 1.1: Aluminosilicate framework of zeolites [1]

The zeolite group of minerals are obtained when tectosilicates are subdivided into several

groups.

Cations such as sodium, calcium or potassium neutralizes the negative charge incorporated by

aluminum as AlO2 tetrahedral in the above structure of aluminosilicate. The arrangement and

position of silicon and aluminum in the structure is determined by Lowenstein rule as discussed

earlier.

The ions of silicon, aluminum and oxygen in the aluminosilicate framework constitute the radii

of 0.39Ao, 0.57Ao and 1.35Ao respectively. Thus the four surrounding oxygen ions form a cavity

due to dense tetrahedral geometry in which both silicon and aluminum can neatly fit.

A three dimensional aluminosilicate skeleton having cages is formed by a crystallographic

arrangement of SiO2 and AlO2 tetrahedral. These cages are then connected through windows and

as a result a pore system is formed with a diameter depending upon the type of molecular sieve.

These pores are filled with water in case of inactivated molecular sieve and upon driving the

water out of the sieve, a highly porous crystal is obtained which can adsorb any guest molecule

but the guest molecule must be small enough to enter the pore system. At low aluminum content

or at silica to alumina ratio higher than 3, molecular sieves are stable in aqueous solution in a PH

range of 5 to 12 and can withstand stronger acid solutions and strong heating without structural

collapse.

Desorption of adsorbed molecule is possible by increasing the temperature, lowering the

pressure, or by washing for the displacement by another molecule.

1.2 Mordenite definition by type and classifications

Mordenite is a microporous, synthetic zeolite with an ideal unit cell composition of Na8.

(AlO2)8. (SiO2)40. nH2O or Na8Al8Si40O96.nH2O and a structure refined in the cmcm space

group.

Where the value of n is reported as 24 in some books.

1.3 Structure

The dimensions of sodium Mordenite unit cell is described as:

5

a: 18.121 Ao

b: 20.517 Ao

c: 7.544 Ao

The most common structure of Mordenite is needle like orthorhombic crystal with c direction

elongation.

The morphology of Mordenite can be discussed by two pore channels of micro pore system of

Mordenite:

1)The pore channel which runs parallel to the c-axis (6.7 × 7.0 Ao). This is elliptical in shape.

2) The pore channel which runs parallel to the b-axis (2.6 × 5.7 Ao).

Model based on skeletal tetrahedral with silicon and aluminum in the centers is shown below.

In this model, twelve oxygen ions form the elliptical windows which give access to each pore

section.

Fig 1.2: Structure of Modernite [2]

Line drawn showing wall of pore and four side pocket only. In this structure all large diameter

pores run parallel. Eight membered rings having a free diameter of 2.2 Ao are interconnected by

these large diameter pores. Thus it is assumed generally that except very small molecule like

water can be able to diffuse through these pores. Pockets with four sides and two on each side

are formed by eight membered rings having a minimum of 3.9Ao as a free diameter. However,

6

natural Mordenite shown in figure 1.3 with an effective pore diameter of 4.0Ao or higher are

synthesized to widen the pores by chemical treatment thus can adsorb large molecule too.

Fig 1.3: Structure of natural Modernite [3]

7

1.4 Types and properties of zeolites:

Although many types of zeolites have been synthesized and their properties are discussed in the

literature, however only six most important zeolites are discussed through a table shown as

follows:

Table 1.1: Properties of Zeolites [4]

Types of sieve Origin

Natural/Synt

hetic

Typical

Si/Al ratio

No. of O-ions

forming

windows

Apertures as

obtained from

X-ray data

assuming an

oxygen radius

of 1.35 Ao

(Ao)

Water

adsorption

Capacity

(gram of

water/100gra

ms of

activated

sieve)

A

Chabazite

Erionite

Mordenite

X

Y

Synthetic

Natural

Natural

Synthetic

Synthetic

Synthetic

1

2

3

5

1.2

2.5

8

8

8

12

12

12

4.1

3.6x3.7

3.6x5.2

6.7x7.0

7.4

7.4

29

30

21

15

33

33

In this report we will mainly discuss about Mordenite zeolite.

Most Mordenites are synthetic in origin with a typical silicon to aluminum (Si/Al) ratio of 5. But

Mordenite with silicon to aluminum (Si/Al) ratio of about 8 is prepared in this report. This ratio

can be varied through chemical treatment depending upon need. As discussed in structure that

number of oxygen ions forming windows is 12. Synthetic Mordenite has a minimum aperture of

6.7Ao and maximum aperture of 7.0Ao. This data is obtained from X-ray data while assuming the

radius of oxygen as 1.35Ao. The water adsorbent capacity of Mordenite lies in between zeolite

Erionite and zeolite X that is 15 grams of water adsorbed per 100 grams of Mordenite.

1.5 Classifications

1.5.1 Classification on the basis of diameter of pore opening:

This classification of porous material is given by the International Union of Pure and Applied

Chemistry (IUPAC) depending upon the diameter of pore opening of porous material.

According to this classifications, porous materials are divided into three categories:

1- Microporous Materials: Materials having pore diameter less than 20Ao.

2- Mesoporous Materials: Materials with pore diameter between 20Ao-500Ao.

8

3- Macroporous Materials: Materials having pore diameter greater than 500Ao.

Mordenite zeolite falls in the category of microporous materials, as most of the natural and

synthetic Mordenite zeolites have average pore size less than 20Ao.

1.5.2 Classification on the basis of origin:

In various books and literature porous materials are also divided on the basis of origin.

According to this division porous zeolites are categories into:

1- Natural Zeolites

2- Synthetic Zeolites

As discussed in the properties of porous materials or molecular sieves that Mordenite zeolite

have synthetic origin. However natural deposits of Mordenite zeolite are also found in form of

composites having Mordenite mixed with other naturally occurring zeolites such as AW 300

which is the mixture of clinoptilolite and Mordenite.

1.6 Occurrences:

1.6.1 Natural Occurrence:

Natural deposits of Mordenite is found in basalt in composite form. Some natural traces of

Mordenite are also found in volcanic tuff in United States and in New Zealand, Japan, East

Africa and Canada. A 5×106 tons of natural deposits of Mordenite have been found with some

non-zeolite impurities in united states. Union Carbide Corporation and the Norton Company

supply natural molecular sieves including Mordenite to meet the requirements. Also several

mineral houses provide various types of molecular sieves including Mordenite on demand.

1.6.2 Synthetic Sources:

Synthetic zeolites are prepared and can be found from the following industries:

Industry Name Location

Union Carbide Corporation (Linde Division) United States

The Norton Company United States

The Davison Chemical Company United States

The Farbenfabrik Bayer West Germany

The Ceca Company and Pechiney France

Peter Spence and Sons Ltd England

Uetikon Switzerland

9

Synthetic molecular sieves including Mordenite zeolites are also manufactured in Eastern Europe

and in some territories of Russian Federation.

1.7 Mordenite and Price Group/economy:

The price of Mordenite is dependent on various factors including pore size, diameter, type of

charge compensating metal and the quality of manufacturing company. Generally, the price

range of most common synthetic sieve is from 5.6 US dollars (PRs 587.048) to 33.6 US dollars

(PRs 3522.288) per kg.

10

Chapter 2

Synthesis

There are many methods developed for the preparation of Mordenite zeolite depending upon the

type of transition metal and solvent used, crystal size, crystal shape and applications.

We have adopted a method which ensures the minimum crystals size and maximum crystallinity

of Mordenite zeolite from a source [5]. But the Mordenite we have prepared in this report has

following changes:

In this method we have used distilled water instead of double deionized water (DDW).

Mordenite seed is not added in the sample.

The procedure covering all the aspects of synthesis of Mordenite zeolite including chemical

reaction and its conditions, procedure, schematic diagram, explanation and product recovery is

discussed below:

2.1 Chemical Reactions and their conditions:

The chemical reactions involved for the synthesis of Mordenite zeolite is as follows:

SiO2(S) + 2NaOH(aq) → Na2SiO3(aq) + H2O(liq) Beaker A (Overnight Stirring at Ambient Conditions)

2Al(s) + 2NaOH(aq) + 2H2O(liq) → 2NaAlO2(aq) + 3H2(g)↑Beaker B (Overnight Stirring at Ambient Conditions)

Na2SiO3(aq) + NaAlO2(aq) → AlNaSi2O6.nH2O Beaker C (One Hour Stirring at Ambient Conditions)

AlNaSi2O6.nH2O ↔ Na8Al8Si40O96.nH2O Autoclave (Hydrothermal Conditions:150oC and 24 hours Stirring)

2.2 Procedure:

Synthesis of Mordenite is divided into three major sections:

2.2.1 Step 1

Preparation of Gel:

In this step two beakers have been taken with names as beaker A and beaker B. Mordenite with

the smallest crystal diameter was optimized to have following compositions of material in beaker

A and beaker B.

Beaker A:

Firstly 10.00 grams of distilled water was taken and then 0.84 grams of sodium hydroxide

(NaOH) and 5.60 grams of silica gel (SiO2) were added in the beaker. The beaker was covered

11

with aluminum foil and was stirred overnight with the help of magnetic stirrer assembly.

Sodium silicate was formed in this beaker at the end.

Beaker B:

At the same time solution B was prepared by taking 0.70 grams of sodium hydroxide(NaOH) and

0.30 grams of aluminum (Al) in 5.00 grams of distilled water. The solution was also covered

with aluminum foil and allowed to stir overnight. But in this case aluminum foil must be opened

by making small holes in it to vent off hydrogen gas. In this beaker sodium aluminate was

formed at the end.

Beaker C:

Solution C was made by adding solution B in solution A. But Just before mixing, 12.00 grams

and 16.80 grams of distilled water were added in beaker A and beaker B respectively.

The resultant solution is then allowed to stir for about 1 hour and a gel type mixture is formed at

the end.

2.2.2 Step 2

Nucleation of Mordenite:

A gel type sodium aluminum silicate solution made in beaker C was then shifted in Teflon lined

autoclave.

The autoclave was then shifted in conventional oven to give favorable environment for proper

nucleation of Mordenite crystals without seed under hydrothermal conditions

(Temperature:150oC and residence time:24 hours).

Important Note:

Safety must be considered while using autoclave under such hydrothermal conditions.

Autoclave must be Teflon lined to avoid corrosion due to highly basic solution.

Pressure in autoclave was calculated and material of construction and volume of

autoclave was then selected.

2.2.3 Step 3

Recovery of Mordenite crystals:

Resultant solution with impurities was then centrifuged in which the crystals and clumps

of Mordenite were separated on the basis of density from water containing impurities like

NaOH, traces of aluminum and compounds of sodium.

Crystals of Mordenite in the form of clumps with some entrained water was filtered with

ordinary filter paper.

12

The product was then washed with distilled water. Washing is necessary for optimizing

PH of product below 9, as the entrained water contains major quantity of NaOH. This

process was time consuming and took about three hours.

Then the resultant washed product was dried in conventional oven, which was operated at

100oC. Drying process was carried out overnight for proper drying.

The dried Mordenite crystals from oven were cooled at room temperature to obtain fine

crystals of Mordenite with required crystal diameter in nanometers.

13

2.3 Flow Sheet

14

2.4 Results and Discussions:

The crystals of Mordenite are characterized by using the techniques of X-ray diffraction (XRD)

and scanning electron microscopy (SEM). The results are discussed below.

2.4.1 X-ray diffraction (XRD)

The X-ray diffraction technique is widely used for determining the crystal structure and for phase

identification.

Fig 2.2: XRD pattern of: (a)Referred Mordenite [5] (b) Standard Mordenite [5] (c) Mordenite

Prepared as per procedure in this report.

As shown above the XRD results of Mordenite prepared according to the procedure adopted in

this report almost matches the referred and standard XRD results of Mordenite found in the

referred source [6]. The variation in peaks shows some extra impurities in the sample and the

irregularities and noises show that the sample has some amorphous characteristics. This is

because the seed is not added in the preparation of Mordenite and thus the crystals are less likely

to be made. Peaks at 2theta of almost 14o,20o,22o,24o, 27o,29o and 36o shows the crystalline

15

nature of sample. Peaks are not so much clear and abrupt as in referred and standard XRD results

which shows less nucleation rate and crystal growth.

The average diameter of the pore davg can be obtained from Bragg’s Equation as:

2davg sinθavg = mλ

Where,

m= Any Positive Integer = 1 (in this case for example)

λ= Wavelength of X-rays = 1.54×10-10 meters

θavg = 13.232o (obtained from experimental peak data of XRD)

Putting the values in above equation we have:

davg= Average diameter of the pore = 33.646nm

2.4.2 Scanning Electron Microscopy (SEM)

Another technique adopted for observing the visual characteristics of Mordenite crystals or its

morphology is the scanning electron microscopy (SEM). The SEM results of prepared Mordenite

conjugated with standard and reference Mordenite are shown in figure 2.3.

Figure 2.3 (a) elaborates the needle like morphology of Mordenite crystals at an optical zoom of

500nm. And in figure 2.3 (c), at an optical zoom of 50µm although the structure of crystals is not

clear and sharp but roughly it can be seen that the whole figure contains very small irregular

crystals bonded with small impurities and large size clumps. In figure 2.3 (b) somehow a clearer

image can be seen at an optical zoom of 10µm with more details of structure. As seed is not

added in the prepared sample so it results in an irregular crystal growth. Distilled water is used

for sample preparation instead of double deionized water (DDW) so impurities might be present

in the sample.

16

Fig 2.3: SEM result of: (a) Standard Mordenite [6] , (b) and (c) Mordenite Prepared at an

optical zoom of 10 µm and 50 µm respectively.

17

Chapter 3

Applications

A wide variety of applications of Mordenite have been reported on laboratory and industrial

scale. Since the effectiveness of Mordenite is directly dependent on its surface area and pore size

so its microscopic study has revealed its tremendous applications in various fields. These

applications include its advantages in nanochemistry, electrochemistry, photochemistry, super

molecular catalysis and optoelectronics. These applications can be summarized as follows:

3.1 Catalysis:

Mordenite is widely used as a catalyst for various important reactions like alkylation,

hydrocracking, dewaxing, hydro isomerization, production of dimethyl amines and reforming

due to its high acidic and thermal stability.

Catalytic activity can be enhanced by making the pore size more and more smaller. As the pore

size is decreased, the Mordenite zeolite give more surface and contact area for the reactants.

Thus it helps to decrease the activation energy and results in fast rate of reaction. Therefore, due

to high activity and high stability, Mordenite is the best replacement with conventional cracking

catalyst. That is why in United States more than ninety percent (90%) refinery industry uses

Mordenite and other same type of molecular sieves.

3.2 Adsorption:

The nano-sized Mordenite is used as an adsorbent due to less diffusional limitations. As the size

of crystal is decreased, the surface area is increased and the reactant species will now diffuse

more. So as the size of Mordenite is decreased, its adsorption capacity is enhanced.

Thus an excellent separation can be achievable as the small molecules are selectively entered in

the pores with less diffusional limitations.

Therefore, it is widely used in laboratory as well as in industry for the adsorptive separation of

gases or liquid mixtures. For example, it is used for selective adsorption of branched and

unbranched paraffins to upgrade certain petrol fractions and for the exhaustive drying of gases

and liquids

It is also used in industry for the adsorption of permanent reactive, volatile and harmful

components from gases. For example, the adsorption of hydrogen sulfide (H2S) from natural gas

and removal of carbon dioxide (CO2) from exhaust gasses. Mordenite, in all the adsorption

applications, acts as an inert carrier which can be recovered by heating and by displacement of

another adsorbent such as water.

18

3.3 Ion Exchange:

Mordenite is used as a high capacity ion exchanging sieve in various industries and plants. The

major applications include the recovery of radioactive ions from the waste stream and the

purification of water by ion exchange method.

3.4 Other Uses:

Other uses include the wide applications in nonlinear optics, semiconductors and chemical

sensors. It is also used in the production of thin film fibers and self-standing membrane.

19

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[2] D.P.Roelofsen, Molecular Sieve Zeolites-Properties and Applications in Organic Synthesis,

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[3] D.P.Roelofsen, Molecular Sieve Zeolites-Properties and Applications in Organic Synthesis,

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[4] D.P.Roelofsen, Molecular Sieve Zeolites-Properties and Applications in Organic Synthesis,

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[5] B. O.Hincapie, "Synthesis of mordenite nanocrystals," Microporous and Mesoporous

Materials, p. 21, 2003.

[6] B. O.Hincapie, "Synthesis of mordenite nanocrystals," Microporous and Mesoporous

Materials, p. 23, 2003.

20

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