Characterization and applications of nanoporous materials...Nomenclature BET Brunauer, Emmet, Teller CBU Composite Building Units COD Crystallography Open Database EDX Energy-dispersive

Characterization and applications

of nanoporous materials

Gonçalo José Dias Antunes

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisors:

Prof. Carlos Manuel Faria de Barros Henriques

Dr. Vladimir L. Zholobenko

Examination Committee

Chairperson: Prof. Henrique Anibal Santos de Matos

Supervisor: Prof. Carlos Manuel Faria de Barros Henriques

Member of the Committee: Prof. José Manuel Félix Madeira Lopes

November 2018

1

Acknowledgements

First and foremost I would like to thank my family for their continuous support, specially my

brother Duarte for fomenting my scientific interest.

I would also like to thank my supervisor at Keele University, Dr. Vladimir Zholobenko, for his

continuous support and availability to help both in this project and in my integration in Keele University.

A huge thanks to prof. Carlos Henriques as well for providing this opportunity and his wise counselling

both in this thesis and previous works.

I want to thank everyone in Birchall Centre for making me feel at home. Special thanks to Aqeel

who guided me through a lot of the lab work. Big thanks to Cátia and Martin who were always there

when I needed help, and Isabel, Ma, Emma and Matt (as well as the aforementioned) for all the moments

in and out of the office.

Back in Portugal, I want to thank Sara for supporting me throughout the last 5 years, Constança

for being the best goddaughter I could wish for. Another big thanks for the people who worked with me

across multiple labs and projects, especially Ricardo and Patrícia for all the times we worked and

laughed together, and my final project group: Maria, Rita and Adriana for being the most quirky and

unique team I’ve ever worked with. Last but not least, my flatmates Francisco and Sérgio, always

present in one way or another.

2

Abstract

Zeolites are widely used as a heterogeneous catalysts in a plethora of chemical processes.

One of the limitations of zeolites as catalysts is the accessibility of their active sites, since the

micropores where those are located hinder the diffusion rate of both reactant and product.

There have been multiple approaches to this issue in the last decades, mainly the synthesis of

frameworks with larger pores and the synthesis of mesoporous materials. The synthesis of zeolites

with mazzite and Linde Type L frameworks was attempted with the end goal of creating hierarchical

zeolites.

The synthesis of the mazzite framework was carried out with tetramethylammonium as a

structure directing agent, and a pure mazzite phase was never achieved. The synthesis of the Linde

Type L framework was carried out without an organic structure directing agent. The resulting samples

were analysed using Powder X-Ray Diffraction, Fourier Transform Infrared Spectroscopy and

Scanning Electron Microscopy. Time constraints due to equipment availability and the issues during

the mazzite synthesis made it impossible to complete a hierarchical zeolite.

Keywords: micropores; mesopores; hierarchical zeolites; mazzite; ltl

3

Resumo

Os zeólitos são utilizados frequentemente na catálise heterogénea em diferentes aplicações.

Uma das limitações destes é a acessibilidade dos seus centros activos, uma vez que a maioria destes

se encontram no interior de microporos o que dificulta a velocidade de difusão quer de produtos quer

de reagentes.

Várias tentativas de minimizar este problema têm sido tentadas nas últimas décadas,

principalmente a síntese de estruturas com poros mais largos e a síntese de materiais mesoporosos.

Neste trabalho foi tentada a síntese de zeólitos com as estruturas da mazzite e Linde Type L, com

objectivo final de criar zeólitos hierárquicos.

A síntese da mazzite foi feita utilizando tetrametilamónia como director de estrutura, não

tendo sido obtida uma fase pura de mazzite. A síntese do zeólito Linde Type L não utilizou compostos

orgânicos como orientador de estrutura. As amostras resultantes foram analisadas por Difração de

Raio X, Espectroscopia no Infravermelho por Transformada de Fourier, e Microscopia Eletrónica de

Varrimento. As dificuldades na síntese de mazzite aliadas à disponibilidade de equipamento e

constrangimento do calendário impossibilitaram a finalização de um zeólito hierárquico.

Palavras-chave: microporos; mesoporos; zeólitos hierárquicos; mazzite; ltl

4

Table of Contents Acknowledgements ........................................................................................................................... 1

Abstract ................................................................................................................................................. 2

Resumo ................................................................................................................................................. 3

List of figures ...................................................................................................................................... 5

List of tables ........................................................................................................................................ 6

Nomenclature ...................................................................................................................................... 7

Introduction .......................................................................................................................................... 8

Zeolites and zeotypes ................................................................................................................... 8

Hierarchical Zeolites .................................................................................................................... 10

Mazzite and Linde Type L structures ...................................................................................... 12

Characterization ............................................................................................................................ 15

Physical Model .............................................................................................................................. 16

Experimental ...................................................................................................................................... 18

Synthesis ........................................................................................................................................ 18

Calcination ..................................................................................................................................... 20

Ion exchange ................................................................................................................................. 21

X-Ray Diffraction .......................................................................................................................... 22

Fourier Transform Infrared Spectroscopy ............................................................................. 23

Scanning Electron Microscopy ................................................................................................. 24

Results and Discussion .................................................................................................................. 25

Synthesis ........................................................................................................................................ 25

XRD .................................................................................................................................................. 26

FTIR .................................................................................................................................................. 42

SEM-EDX ......................................................................................................................................... 48

Conclusions and Future Work ...................................................................................................... 49

References ......................................................................................................................................... 50

5

List of figures Figure 1 - Silanol bonds responsible by Brönsted acidity in zeolites [42] ............................................... 8

Figure 2 - Mazzite (a) and LTL (b) structures viewed from [001] .......................................................... 12

Figure 3 - Mazzite (a) and LTL (b) structures viewed from [100] .......................................................... 13

Figure 4 - Physical model of LTL ............................................................................................................ 16




Figure 8 – FTIR spectrometer ................................................................................................................ 23

Figure 9 - Simulated XRD pattern of MAZ [43] ...................................................................................... 26

Figure 10 – Simulated XRD Pattern of LTL [43] ..................................................................................... 27

Figure 11 - XRD Pattern for sample 2 before ion exchange .................................................................. 28

Figure 12 - XRD Pattern for Sample2 after ion exchange ...................................................................... 28

Figure 13- Comparison between the sample before (black) and after ion exchange (blue) ................ 29

Figure 14 - XRD Pattern for Sample 2 matched with theoretical MAZ peaks ....................................... 30

Figure 15 - Overlap of the Pattern for Sample 2 and the Simulated Pattern ........................................ 30

Figure 16 - Comparison of the Pattern from Sample 2 with COD ......................................................... 31

Figure 17 - XRD Pattern for sample 3 .................................................................................................... 32

Figure 18 – XRD Pattern for Sample 3 matched with theoretical MAZ peaks ...................................... 32


Figure 20 - Comparison of the Pattern from Sample 3 with COD ......................................................... 33

Figure 21 - Comparison between the regular sample (black) and the one with carbon nanotubes

(blue) ..................................................................................................................................................... 34

Figure 22 - Comparison between the mesoporous material before (black) and after calcination (blue)

............................................................................................................................................................... 35

Figure 23 - XRD Pattern for Sample 4 .................................................................................................... 36


Figure 25 - Overlap of the Pattern from Sample 4 and the Simulated Pattern .................................... 37

Figure 26 - XRD Pattern for Sample 5 before calcination ...................................................................... 37

Figure 27 - XRD Pattern for Sample 5 after calcination......................................................................... 38


Figure 29 – Overlap of the Pattern from Sample 5 and the Simulated Pattern .................................... 39

Figure 30 – Comparison of the Pattern from Sample 5 with COD ........................................................ 39

Figure 31 - XRD Pattern for Sample 6 (KLTL) ......................................................................................... 40


Figure 33 - FTIR Spectra for Sample 2 ................................................................................................... 42

Figure 34 - FTIR Spectra for sample 2 showcasing Py adsorption ......................................................... 43

Figure 35 - FTIR Spectra for sample 3 .................................................................................................... 43

Figure 36 - FTIR Spectra for sample 3 after Py adsorption .................................................................... 43

Figure 37 - Subtraction spectra for sample 3 ........................................................................................ 44

Figure 38 - FTIR Spectra for Sample 4 exchanged with amm. Nitrate .................................................. 44

Figure 39 - FTIR Spectra for Sample 4 exchanged with amm. Acetate ................................................. 45

Figure 40 - FTIR Spectra of Sample 4 with both ion exchanges ............................................................ 45

Figure 41 - Subtraction of spectra for sample 4 exchanged with amm. nitrate ................................... 45

Figure 42 – Subtraction of Spectra for sample 4 exchanged with amm. Acetate ................................. 46

Figure 43 - FTIR Spectra for sample 6 .................................................................................................... 46

Figure 44 - FTIR spectra for sample 6 after Py adsorption .................................................................... 47

Figure 45 - Subtraction of the spectra for sample 6 before and after Py. adsorption .......................... 47

6

List of tables

Table 1 - Molar ratios for mazzite synthesis ......................................................................................... 14

Table 2 – Relevant molar ratios of the first two synthesis attempts .................................................... 18

Table 3 - Relevant molar ratios for the latter attempts ........................................................................ 19

Table 4 – Atomic percentages in the zeolite samples ........................................................................... 48

Table 5 – Bulk composition ratios in the zeolite samples ..................................................................... 48

7

Nomenclature

BET Brunauer, Emmet, Teller

CBU Composite Building Units

COD Crystallography Open Database

EDX Energy-dispersive X-ray Spectroscopy

FAU Faujasite

FTIR Fourier-Transform Infrared Spectroscopy

gme Gmelnite

LTL Linde Type-L

MAZ Mazzite

MOR Mordenite

NMR Nuclear Magnetic Ressonance

OSDA Organic Structure Directing Agent

Py Pyridine

SDA Structure Directing Agent

SEM Scanning Electron Microscopy

TPD Temperature Programmed Desorption

XRD X-ray Diffraction

8

Introduction

Zeolites and zeotypes

Zeolites are three-dimensional, crystalline silicates and aluminosilicates, comprised by

tetrahedrally coordinated silica and aluminium connected by oxygen atoms. When other metals exist

within the structure, they’re referred as zeotypes instead. [1] [2]

Zeolites are widely used in different applications, from catalysis to adsorption, due to their

chemical properties and unique porous structures that allow different zeolite structures to be applied to

each need. The substitution of Si on the SiO4 tetrahedrons by Al3+ confers the structure a negative

charge, which is then compensated by cations inside the pore channels to neutralise this negative

charge in the structure. These cations can be exchanged with H+ in order to obtain the acidic form of

zeolites, allowing them to be used as catalysts in reactions like isomerization and cracking. At the same

time, they can be impregnated with other metals to serve as metallic catalysts and bifunctional catalysts

in cases where both sites are used. Zeolites with higher Si/Al ratio are usually more hydrophobic and

stronger acids, while zeolites with lower Si/Al are usually less hydrophobic and have a higher

concentration of cations to balance the extra negative charge in the framework. Typically silanol and

bridged OH groups are responsible for Brönsted acidity, while negatively charged extra-framework

aluminium contributes for Lewis acidity. [1] [2] [3] [4] [5]

Figure 1 - Silanol bonds responsible by Brönsted acidity in zeolites [42]

The traditional method for zeolite production is hydrothermal synthesis, with reaction time and

temperature varying greatly depending on the intended structure. Most structures also require a

Structure Directing Agent (SDA), often an organic compound (OSDA), to be added during the

hydrothermal process. Recent studies have been carried out on zeolite synthesis in non-aqueous

synthesis. Non-aqueous synthesis aren’t necessarily water-free, but use organic solvents, usually

alcohols while adding small amounts of water to direct the reaction. In recent years, computer models

have made it possible to try and find new SDA for specific structures, with said models allowing to

rationally design specific SDA that fit the structure, reducing the amount of time and trials necessary to

find suitable compounds for this role. [6] [7] [8] [9]

Zeolites, when synthesised, are not in their acid form, as the negative charge from the structure

isn’t being balanced by protons, but by cations present in the reaction medium like sodium or potassium,

depending on what species are present in the synthesis medium. To obtain the acid form, it’s necessary

to ion exchange it to its H-form. This is usually done by exchanging it with NH4+, then thermally degrading

it to form H+. [2] In cases where OSDA were used, it is also necessary to remove these from the zeolite

9

pores before performing the ion exchange. The most frequent method is calcination, where the OSDA

is burned in controlled temperature and gas flow, and removed from the pores. This step requires

knowledge of the structure’s thermal stability, as it is possible to cause a collapse of the zeolite structure

if the calcination temperature is too high. [2] [7] [9] [10]

Other synthesis strategy is the conversion of other aluminosilicates to zeolites. The source

material can be amorphous or a different structure of zeolite, and processes vary from the usage of the

typical SDA that are used for the regular hydrothermal synthesis of the intended structure, to using a

seed of the intended structure placed in the reaction medium. Okubo et al have studied the relation

between the structures of the source and final structures, reaching the conclusion that this conversion

is possible as long as the structures share a single Composite Building Unit (CBU). It is also possible to

convert a mixture of different zeolites that share CBU between themselves to a structure that only shares

CBU with one of those structures, as shown by Honda et al. [10] [24]

While the narrow pore systems that define zeolites can be useful for selectivity due to steric

confinement of the reactants and products in certain reactions, the same narrow pores pose a problem

to the diffusion of these compounds, which can hinder the reaction rates due to diffusion limitations. [9]

[11] [12]

There have been two main approaches to this issue, synthesizing structures with larger pore

diameters, like AlPO8, and introducing a secondary mesopore system, creating hierarchical zeolites

with both a micro and mesopore system intertwined. The reduction of crystal sizes can also improve the

diffusion in zeolites, as it lowers the diffusion path of molecules inside the pores. [9-17]

The aperture size of a zeolite pore is connected to the amount of tetrahedrally coordinated

atoms (T-atoms) that form the ring of their opening. Zeolites don’t typically form openings bigger than

12-member rings, but the same isn’t true with zeotypes, which can go up to 20-rings. It is worth noting

that the ring diameter can change between rings with the same number of T-atoms, like cacoxenite and

cloverite which both have a 20-ring but with apertures of 14.2 Å and 13.2 Å, respectively. It is also worth

noting many of these materials aren’t thermally stable at higher temperatures, mak ing them unsuitable

for catalytic purposes. [17]

Hierarchical zeolites are zeolites with a secondary porosity system. Pore systems are

characterized by their diameter, with pores considered micropores between 0.2nm and 2nm, mesopores

between 2nm and 50nm and macro pores between 50nm and 100nm. By having both a micro and

mesoporous system, the accessibility to the acid and/or metallic sites inside the pores is increased, and

the diffusion path inside the micropores is also shortened, leading to increased diffusion. [9-17]

The goal of this work is to synthesize and characterize hierarchical zeolites of the Mazzite and

Linde Type-L structures. Details on these structures are mentioned further into this introduction.

10

Hierarchical Zeolites

There are multiple strategies to create hierarchical zeolites, with these strategies being divided

in top-down and bottom-up approaches. Top-down strategies focus on the modification of pre-

synthesized zeolites, while bottom-up strategies aim to synthesize hierarchical zeolites without the

need of post synthesis demetallation. Bottom-up strategies include hard templating, soft templating,

assembly of nanosized zeolites and zeolization of preformed solids, while top-down includes

dealumination, desilication, recrystalization and surfactant templated crystal rearrangement. Pérez-

Ramírez et al defined a hierarchy factor to maximize in hierarchical zeolite design, given by the

formula (1):

𝐻𝐹 =𝑉𝑚𝑖𝑐𝑟𝑜

𝑉𝑡𝑜𝑡𝑎𝑙

×𝑆𝑚𝑒𝑠𝑜

𝑆𝐵𝐸𝑇

(1)

This factor is maximized when the mesopore surface area increases without losses of

micropore volume, this way a zeolite with high hierarchy factor is one that retains its microporosity

despite gaining a secondary pore structure. [9][11] [12] [16] [17] [18] [39]

Hard templating consists on the synthesis of a zeolite in the porosity of a solid structured

template material, using solid compounds like carbon black, carbon nanotubes, pyrolized wood,

CaCO3 nanoparticles, and others as templates. The template then needs to be removed, usually by

calcination. [9] [12] [16] [17]

Soft templating utilizes templates like cationic surfactants, cationic polymers, polymeric

aerogels, starch and bacteria. The usage of SDA and soft templates can sometimes result in high

amounts of amorphous material formed, however these templates can in some cases be developed to

act as both mesopore template and SDA, removing that issue.[9] [12] [16] [17] [39]

Zeolization of mesoporous materials consists in the conversion of mesoporous amorphous

silicates or aluminosilicates into zeolites, usually recurring to SDA. [9] [12] [16] [17] [39]

On the top-down approaches, dealumination is the removal of some of the aluminium from the

zeolite framework. This can be achieved by post synthesis calcination, steaming, acid leaching, or

chemical treatments. Since dealumination increases the Si/Al ratio, it can also be used to increase the

strength of the acid sites. [9] [12-18] [39]

Desilication is the removal of some silica from the framework, by selective hydrolysis of the Si-

O-Si bonds in alkaline medium, usually NaOH. Bulkier bases can be used for selective removal of

silica, as the bulkier bases will be unable to enter the pores and removing silica from the inside of

these. The key factors for this process are concentration, temperature and time on the medium, and

the Si/Al ratio of the zeolite. The negative charge of the framework aluminium repels the OH-,

protecting the silanol groups from being attacked. For this reason, desilication is more effective for

mesopore formation in dealuminated zeolites. The ideal Si/Al ratios for desilication are usually

between 25 and 50, with ratios lower than 20 being too resistant to the attack on the Si-O bonds, and

11

ratios higher than 50 being too susceptible to it, leading to excessive and unselective removal. Crystal

defects like stacking faults can also favour the desilication mechanism. [9] [12-17] [39]

The surfactant templated top-down approaches are similar to desilication but use weaker

bases and milder conditions, as well as surfactants to induce mesopore formation, similar to the soft

templating bottom-up strategy. Despite the similarities with desilication, there is no silica loss in these

reactions, leaving the Si/Al ratio untouched. [9] [12] [16] [17]

12

Mazzite and Linde Type L structures

The Mazzite structure (MAZ) consists stacked columns of gmelnite cavities, forming 12-ring

channels in between those columns. The opening of these pores is 7.4 Å between opposing oxygen

atoms. This structure was first synthesised in the 1960s by Union Carbide and given the name zeolite

Ω. Mobil also synthesized the same structure with a slightly higher Si/Al ratio and named it ZSM-4. In

1972, the natural counterpart of these structures was discovered in basaltic lavas in France, and the

structure is now known by this name. [19] [20] [27] [40]

The Linde Type L (LTL) structure has a similar shape when compared with MAZ, but instead

of gmelnite, the structure is made by columns of cancrinite cages and double six-membered rings,

forming the characteristic 12-channel of ltl cages. The 12-member cage on LTL is wider than the one

in MAZ, being able to fit a 10.01 Å sphere inside it versus 8.09 Å in the MAZ, however the pore

opening is slightly smaller at 7.1 Å. Zeolite L was first synthesised in 1965 by Union Carbide and has

no known natural counterpart. [21] [22]

A side by side comparison of these structures can be visualized in Figure 2. As visible on this

figure, the two structures are identical when seen from [001]. However, when seen from [100] like in

Figure 3. the difference between the 12-channel formed between the gme cavities and the ltl cage can

easily be seen.

Figure 2 - Mazzite (a) and LTL (b) structures viewed from [001]

13

Figure 3 - Mazzite (a) and LTL (b) structures viewed from [100]

Despite the structural similarities, the uses for these two structures are widely different, with

MAZ performing well in acid catalysis and LTL in basic catalysis. MAZ isn’t widely used, but has been

studied as a catalyst for acid and bifunctional catalysis, in cracking, toluene conversion, hydration, and

particularly successful in isomerization where a sample of dealuminated Pt-HMAZ outperformed a

commercial Pt-MOR catalyst in a study carried out by Guisnet and coworkers. MAZ has been reported

to be more selective to dibranched isomer formation when compared to FAU and MOR catalysts, both

widely used, and dealuminated MAZ samples have been reported to retain ammonia at temperatures

up to 773K. Despite these promising signs, MAZ isn’t widely used commercially, one of the

motivations of this work is to try to understand the reason behind that apparent contradiction. On the

other hand, LTL performs well as a catalyst in reforming, aromatization and dehydrogenation

reactions, with authors somewhat divided between the key factor for its success being due to its shape

or its basic characteristics inhibiting acid sites.[18] [23] [26] [28] [32] [33-35]

The synthesis of these two structures is also wildly different. The hydrothermal synthesis of

LTL is quite simple, with potassium ions acting as SDA and therefore not needing the addition of

OSDA. Typically, the LTL obtained this way has a Si/Al ratio between 3 and 6, with alternative

14

synthesis methods being able to form LTL at lower ratios. The crystallization usually occurs in 2 days

or less, at temperatures between 100ºC and 200ºC. [21] [33-35]

The same cannot be said about the MAZ synthesis. The zeolite omega synthesis has been

plagued with secondary phases from the start, with common contaminates including gmelnite, offretite,

zeolite P, sodalite, analcime, philipsite and gismondine. The original and most common used SDA for

MAZ synthesis is tetramethylammonium (TMA), usually in either hydroxide or bromide forms. Sodalite

formation tens do increase with TMA concentration, while analcime formation increases with Na

concentration. Si/Al ratios are between 2.5 and 10 in the original patents. Crystallization temperatures

are usually between 80ºC and 200ºC, but usually below 150ºC. Synthesis times vary greatly with

temperature and composition, with crystallization occurring between 1 and 40 days. Some procedures

use a seeding solution that requires aging before the crystallization starts. At temperatures below

135ºC the crystallization starts by forming a faujasite phase that is then converted to MAZ. At higher

temperatures this behaviour has not been noticed. This behaviour, as well as the frequent

contaminant phases comes from the fact that mazzite is one of several metastable phases in systems

containing TMA. Frequent formation of other phases has led to an interest in research for other SDA

to produce MAZ, with compounds like dioxane, diquaternary alkylammonium ions, piperazine and

crown ethers being tested with varying degrees of success. Considerable research has also been into

alternative synthesis methods like magadiite conversion, zeolite conversion and non-aqueous

synthesis. [7-8] [10] [18] [23-32] [40]

The molar ratios in which MAZ synthesis is ideal [40] are resumed in Table 1

Table 1 - Molar ratios for mazzite synthesis

Species Minimum Maximum

𝑵𝒂𝟐𝑶 + 𝑻𝑴𝑨𝟐𝑶

𝑺𝒊𝑶𝟐

0.3 0.5

𝑺𝒊𝟐𝑶

𝑨𝒍𝟐𝑶𝟑

8.0 20.0

𝑻𝑴𝑨𝟐𝑶

𝑻𝑴𝑨𝟐𝑶 + 𝑵𝒂𝟐𝑶 0.0 0.2

𝑯𝟐𝑶

𝑻𝑴𝑨𝟐𝑶 + 𝑵𝒂𝟐𝑶 15.0 40.0

15

Characterization

There are many different analytical methods for zeolite characterization, including Powder X-

Ray Diffraction (XRD), Fourier Transformed Infrared Spectroscopy (FTIR), Scanning Electron

Microscopy (SEM), Gas Adsorption, Microcalorimetry, Nuclear Magnetic Ressonance (NMR), and

others. [1-6]

The XRD technique is based on detecting the diffraction gratings of X-ray wavelengths caused

by the structure and composition of the sample. The equipment generates X-ray waves in a cathode

ray tube, which are then filtered on a monochromator to generate monochromatic radiation, which is

then directed towards the sample. The interference generated by the sample is then detected and

processed, and the sample is scanned across a range of angles to obtain every possible diffraction

direction. Conversion of the diffraction peaks to d-spacings allows identification of the structure, as

every mineral has unique d-spacings. The identification is usually done by comparing the results with

references. [36]

FTIR probes the vibrational properties of a sample, allowing the identification of functional

groups that can be associated with characteristic bands. In the case of zeolites, this method can be

used to distinguish acid sites from non-acidic OH groups, as well as identify the accessibility and

strength of different acid sites by using probe molecules that adsorb on these. [37]

Zeolites are pressed into disks, then go through an activation stage at high temperature. After

activation and cooling down, the spectra can be collected with the cell in vacuum. Acid sites can be

studied by injecting probe molecules like ammonia into the cell, wait for the zeolite to adsorb, collect

the spectrum and compare it with the original spectrum. After that, the acid strength can be tested by

gradually increasing the temperature and collecting multiple spectra at different temperatures, as

stronger acid sites can retain bases at higher temperatures. Different probe molecules can be used for

different studies, with bulkier molecules like pyridine being used for accessibility studies. It is also

possible to determine the number of acid sites by using ammonia additions and their spectra as a

titration. [18] [23]

Another widely used method is gas adsorption. The two most used for physical adsorption are

volumetric (or manometric), where the volume adsorbed or gas pressure are measured, and

gravimetric, where the sample is weighted in a microbalance and pressure is also measured.

Gravimetric adsorption is most convenient at room temperature, so since the most commonly used

gases are argon and nitrogen, both having very low boiling points, this method isn’t as used as the

manometric. For many years, nitrogen adsorption at 77K was considered the standard measure of

surface area, however argon has been gaining popularity as it doesn’t exhibit specific interactions with

functional groups, making it faster to adsorb. Carbon dioxide is also occasionally used to access even

smaller micropores, as its kinetic diameter is slightly lower at 0.33nm versus 0.34 and 0.36 for argon

and nitrogen respectively. The obtained data is then used in adsorption models, typically Brunauer-

Emmett-Teller (BET). [38]

16

Physical Model

On the side of the experimental work, a physical model of the LTL structure was built, using

Cochranes of Oxford Minit Molecular Models, connected by pieces of lab grade tubing. The finished

model can be seen in figures 4 to 7.

Figure 4 - Physical model of LTL


17



18

Experimental

Synthesis

The synthesis mixtures were prepared using TMABr (Sigma Aldrich, 99%), TMAOH (Alfa Aesar,

99%, pentahydrate), NaOH (Fisher Scientific, 99%), KOH (Fisher Scientific, 99%), alumina (Sigma

Aldrich, 99%), aluminium sulphate (Sigma Aldrich, 99%) and colloidal silica (Ludox, 30wt% and 40wt%).

The first attempt at mazzite synthesis were based on Vaughan and Strohmaier’s contributions

in the book Verified Syntheses of Zeolitic Materials. [41] This procedure uses a seeding solution that

should be aged before the crystallization step. The seeding solution was produced in excess, to use in

further attempts with longer ageing.

The seeding solution was prepared by mixing 1.872g of alumina, 9.06g of sodium hydroxide

and 16.90g of water, and microwave this mixture at 150ºC for 30 minutes. Then 7.01g of the solution

obtained in the previous step was mixed with 8.34g of colloidal silica and 4.39g of water to finalize the

seeding solution.

After 24h of ageing, three other solutions were made: an alumina solution containing 14.07g of

water, 5.55g of sodium hydroxide and 7.07 of alumina, that was microwaved at 150ºC for 30 minutes to

dissolve the alumina; a solution of 5.65g of TMA bromide in 7.08g of water; and a solution of 4.23g of

aluminium sulphate in 7.07g of water. The crystallization batch was obtained by mixing 13.40g of the

first solution, 1.98g of the seeding solution, the remaining two solutions, 60.48g of sodium silicate and

4.90g of water. This mixture was left to cristalize for 5 days at 100ºC, and the final product was separated

by centrifugation.

A second attempt with this method was made, with the seeding solution ageing 20 days and a

crystallization time of 7 days. The rest of the procedure was similar, with minor variations in the amounts

weighted, and composition differences of the final mixture can be seen on Table 2.

Table 2 – Relevant molar ratios of the first two synthesis attempts

Species First Second


𝑺𝒊𝑶𝟐

0.433 0.387

𝑺𝒊𝟐𝑶


9.08 10.26


𝑻𝑴𝑨𝟐𝑶 + 𝑵𝒂𝟐𝑶 0.16 0.16

𝑯𝟐𝑶

𝑻𝑴𝑨𝟐𝑶 + 𝑵𝒂𝟐𝑶 31.4 31.3

Further crystallization attempts were based on Flanigen’s patent, with this method not requiring

either seeding solution or ageing, and using TMA hydroxide instead of bromide. [40]

19

This mixture was prepared by mixing 15.27g of water, 1.98g of alumina and 3.18g of sodium

hydroxide and heating it in an oven at 180ºC overnight. The heating was done in an oven overnight

instead of in a microwave in 30 minutes like in the previous attempts due to an equipment failure with

the microwave. 25.00g of silica (ludox30) and 3.64g of TMA hydroxide were then mixed with the solution,

and crystallization was performed at 100ºC over 20 days.

Another attempt was made based on this method with a different approach, opting to use higher

temperature and a shorter crystallization time. For this attempt, 3.90g of alumina, 6.37g of sodium

hydroxide and 30.2g of water were first mixed and put in an oven at 180ºC overnight, and then mixed

with 7.26g of TMA hydroxide and 50.01g of silica (Ludox 30). Crystallization was performed at 180ºC

for 4 days.

An attempt at a hierarchical version was also done, adding carbon nanotubes to the synthesis

batch and letting that sit for a day before starting crystallization. For this attempt, 10.4g of a previously

prepared solution of alumina and sodium hydroxide was used. To this solution 1.85g of TMAOH and

12.75g of silica (Ludox30) were added. After mixing, 0.64g of carbon nanotubes were added. The molar

ratios of these mixtures can be seen in table 3.

Table 3 - Relevant molar ratios for the latter attempts

Species First Second Mesoporous


𝑺𝒊𝑶𝟐

0.72 0.72 0.72

𝑺𝒊𝟐𝑶


9.34 9.49 9.49


𝑻𝑴𝑨𝟐𝑶 + 𝑵𝒂𝟐𝑶 0.12 0.12 .12

𝑯𝟐𝑶

𝑻𝑴𝑨𝟐𝑶 + 𝑵𝒂𝟐𝑶 22.24 22.21 22.19

The LTL synthesis was successful on its first attempt. The method was based on the original

patent by Union Carbide. [21] First, 2.01g of alumina, 8.98g of potassium hydroxide, 6.07g of sodium

hydroxide and 77.00g of water were mixed, and left in an oven at 180ºC overnight. The following day,

58.00g of silica (Ludox40) were added, and the mixture was left ageing for 24h. Crystallization was

performed for 24h at 175ºC. 2.34g of this zeolite were produced, despite the purging of a quarter of the

synthesis mixture due to limitations of the autoclave.

20

Calcination

The calcination was done in two steps, the first in nitrogen at a lower temperature, and the

second, more aggressive, in oxygen at higher temperature.

The early samples were calcined in nitrogen at 400ºC for an hour, then temperature was lowered

to 300ºC, the gas switched from nitrogen to oxygen, and the second step of calcination done at 500ºC

for 16h. The samples were then let cooling down until room temperature.

Latter samples were calcined using the same method and timing, but with the temperatures

increased to 450ºC and 550ºC respectively. The LTL sample wasn’t calcined as it didn’t require any

OSDA, so there was no point in calcining it to remove organic compounds.

21

Ion exchange

The ion exchange was done using ammonium nitrate (analaR, 98%) and ammonium acetate

(GPR, 97%).

The first sample wasn’t exchanged, the second was exchanged using 5M solutions of ammonia

nitrate. Latter samples were exchanged using 1M solutions of ammonia nitrate or ammonia acetate.

Every ion exchange was done three times per sample, with the zeolites being separated by

centrifugation, washed with hot water, and centrifuged again between treatment, and centrifuged and

left to dry at 100ºC overnight after the treatment. All ion exchange treaments were performed at room

temperature and atmospheric pressure.

22

X-Ray Diffraction

Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer

with CuKα radiation at 40kV and 40mA over the 2-theta angle range of 5-60º. The crystalline phases

were matched by comparing the XRD patterns of the catalysts with those reported in the literature, both

manually with simulated XRD patterns, and using the Crystallography Open Database (COD).

Samples were not subjected to any treatment before performing this analysis.

23

Fourier Transform Infrared Spectroscopy

FTIR spectra were collected using a Protege 460 spectrometer, with a high sensitivity MCT

detector, with a spectral range from 600 to 7000 cm-1.

Spectra were collected before and after pyridine adsorption, to study the accessibility of the acid

sites of the samples. Temperature Programmed Desorption (TPD) can be used to study properties of

the synthesized zeolites like the acidic strength of different sites in the porous system. However, this

analysis was not performed since a pure MAZ sample was not obtained and acid sites on LTL are not

relevant for the reactions that use it as catalyst.

Samples were prepared by pressing the powder between 1-3 tons to form disks. These disks

then go through an activation step at 450ºC, to remove adsorbed water and other impurities that might

interfere with the measurements.

Figure 8 – FTIR spectrometer

24

Scanning Electron Microscopy

SEM data was collected with a Hitachi TM3000, attached to a Bruker EDX system allowing the

determination of bulk composition via Energy-dispersive X-ray spectroscopy (EDX).

This equipment didn’t have enough magnification to identify the shape of individual crystals,

but the EDX system made it possible to obtain bulk Si/Al ratios.

The samples were prepared by forming disks like the ones used in FTIR, glue them into a

sample holder with an appropriate glue and finally carbon coat them to avoid powder to escape into

the vacuum system.

25

Results and Discussion

Synthesis

Six different samples were synthesized, with the first five being failed attempts at obtaining a

pure MAZ phase, and the sixth a successful attempt at synthesizing pure LTL.

The synthesis of mazzite is known to be tricky, but the difficulties in obtaining a pure sample

were higher than expected. To obtain a pure phase, further experiments would be required, preferably

running multiple simultaneous attempts in similar conditions in a way that a single parameter can be

varied to obtain comparable results. Ideally, the alumina dissolution steps should be performed on a

microwave unit, as this allows much quicker results versus a regular oven where the solution needs to

stay overnight for complete dissolution. This was the case on the early synthesis attempts but an

equipment failure made it so that latter attempts required that extra waiting time. Gas Adsorption was

also not performed due to equipment malfunction

There was also some insistence in the conventional synthesis methods, which in hindsight was

likely a mistake, since zeolite conversion is also a viable synthesis approach that would have been

interesting to study and compare with conventional hydrothermal synthesis.

The LTL synthesis, simpler than MAZ, was finally attempted to illustrate how such similar

structures can have such different properties and difficulty in the synthesis process. It was however too

late to attempt a bottom-up approach at hierarchical LTL synthesis, and there wasn’t enough material

to attempt a top-down, as only 2.34g of pure LTL was obtained and steaming and/or acid leaching

required larger amounts.

26

XRD

XRD results were compared with the data from the Collection of Simulated XRD Powder

Patterns for Zeolites, 4th edition. Said simulated patterns are shown in figures 9 and 10. A peak analysis

in the Origin software was run on these to find characteristic peaks to match with the experimental data.

Figure 9 - Simulated XRD pattern of MAZ [43]

27

Figure 10 – Simulated XRD Pattern of LTL [43]

The first attempt at synthesising Mazzite was unsuccessful, forming a mostly amorphous

material. The XRD for this sample had no relevant peaks so no other characterization was performed

for this sample.

The second sample was crystalline, and its pattern was compared both with the data from the

book, and with the COD. This sample was subjected to harsher conditions in its ion exchange, so XRD

patterns were collected before and after that treatment. Figures 11 to 13 show the patterns before and

after ion exchange, as well as an overlap of the two.

28

Figure 11 - XRD Pattern for sample 2 before ion exchange

Figure 12 - XRD Pattern for Sample2 after ion exchange

29

Figure 13- Comparison between the sample before (black) and after ion exchange (blue)

As it can be seen, there was no alteration to the structure during the ion exchange. Both spectra

were then matched with the references, and the spectra for the ion exchanged sample was compared

with the COD database. The comparison with a MAZ reference shows that while the characteristic peaks

are present, there are also other peaks as strong as the ones that match the MAZ phase, indicating the

presence of a secondary phase. The COD comparison didn’t match mazzite, finding chabazite,

pyrophillite, andalusite and mullite instead.

30

Figure 14 - XRD Pattern for Sample 2 matched with theoretical MAZ peaks

Figure 15 - Overlap of the Pattern for Sample 2 and the Simulated Pattern

31

Figure 16 - Comparison of the Pattern from Sample 2 with COD

The third sample was also compared with both references too, but no XRD were made after ion

exchange since the previous comparison shows that this doesn’t affect the crystalline structure. As it

can be seen in figures 17 to 20 the peaks found in Origin are present, but there are extra peaks around

14º and after 30º. Comparison with the COD matches chabazite, silimanite, andalusite and mullite, and

no mazzite again.

32

Figure 17 - XRD Pattern for sample 3

Figure 18 – XRD Pattern for Sample 3 matched with theoretical MAZ peaks

33


Figure 20 - Comparison of the Pattern from Sample 3 with COD

34

The fourth and fifth samples share the same crystallization conditions, but the fifth sample was

crystallized with carbon nanotubes as a mesopore template. Figures 21 and 22 show comparison

between the XRD data of the 3, first between the regular and the mesoporous sample, and then between

the mesoporous sample before and after calcination. There are no significant differences between the

spectra of the regular and mesoporous version, meaning the microporous structure isn’t altered between

the two. There are a few peaks that don’t match Mazzite, most notably the most intense peak at 24.33.

After calcination, most peaks intensity diminished, which might mean loss of crystallinity during

calcination. This could have happened since the calcination of this sample didn’t go as smoothly as the

previous, with a lot of carbonaceous material forming and just a small amount of sample being properly

calcined. This might have been caused by an increase of temperature due to the burning of nanotubes.

The mesoporous sample spectra was compared with the COD, and the same results as the third sample

were obtained, with mazzite not being identified, and chabazite, andalusite, silimanite and mullite being

all matched. These comparisons as well as each isolated pattern can be seen in figures 24 to 30.

Figure 21 - Comparison between the regular sample (black) and the one with carbon nanotubes (blue)

35

Figure 22 - Comparison between the mesoporous material before (black) and after calcination (blue)

36

Figure 23 - XRD Pattern for Sample 4


37

Figure 25 - Overlap of the Pattern from Sample 4 and the Simulated Pattern

Figure 26 - XRD Pattern for Sample 5 before calcination

38

Figure 27 - XRD Pattern for Sample 5 after calcination


39

Figure 29 – Overlap of the Pattern from Sample 5 and the Simulated Pattern

Figure 30 – Comparison of the Pattern from Sample 5 with COD

The LTL sample wasn’t run on COD, as it perfectly matched the reference. The peaks were

identified using the same procedure in Origin, however this graph isn’t displayed due to visual clutter

40

from the amount of peak numbers. The experimental spectra and its matching with reference can be

seen in figures 31 and 32.

Figure 31 - XRD Pattern for Sample 6 (KLTL)

41


42

FTIR

Mazzite is known to have two distinct bands for OH groups, Si-OH at 3744 cm-1, with a shoulder

at 3735 cm-1, and Si-OH-Al at 3600 cm-1. Furthermore, pyridine adsorbed in Mazzite forms bands at

1636 and 1542 cm-1 when pyridinium ions form on Brönsted acid sites, and 1622 and 1454 cm-1 when

pyridine is coordinated on Lewis acid sites. Samples without pyridine adsorbed show bands at 1496 and

1462 cm-1. [18] [23]

While the XRD data for Sample 2 looked promising, the FTIR data brings expectations down,

with no relevant peaks on the 3600-3700 cm-1 region, characteristic of silanol and bridged OH groups.

The absence of these groups means the absence of acid sites in the structure, which are required for

the adsorption experiments. There is also a small peak around the 1450-1500 cm-1 region that may

indicate the presence of a small quantity of leftover SDA. For this reason, the calcination temperature

was increased in further experiments. Figure 33 shows the spectra before and after pyridine adsorption,

as well as the subtraction of the two, illustrating a small drop around the 3600-3700 cm-1 region, while

Figure 34 shows the same spectra in a different range, illustrating a couple small peaks increasing after

pyridine adsorption in the regions between 1450-1500 cm-1, indicating a very slight amount of pyridine

adsorption, consistent with the data from the other range

Figure 33 - FTIR Spectra for Sample 2

43

Figure 34 - FTIR Spectra for sample 2 showcasing Py adsorption

The results for the third sample are better, with clear peaks around 3750 cm-1 and in the 3600-

3650 cm-1 region, however this second peak is broader than it should, indicating multiple non-equivalent

tetrahedral atom positions. There is also a band around 1485 cm-1. Subtraction spectra is consistent

with some pyridine adsorption, but the majority of the acid sites were left untouched

Figure 35 - FTIR Spectra for sample 3

Figure 36 - FTIR Spectra for sample 3 after Py adsorption

44

Figure 37 - Subtraction spectra for sample 3

For sample 4, two different ion exchanges were performed, one with ammonium nitrate and

another with ammonium acetate. The sample that was exchanged with ammonium nitrate has one of

its OH bands at 3550 cm-1, while the sample that was exchanged with ammonium acetate has both

that band and a shoulder at 3625 cm-1, which suggests is coherent with the sample having two

different phases, with one blocking the other’s pores, as nitrate is slightly bulkier than acetate, making

it harder for it to diffuse through. This difference can easily be seen in Figure 40. In the 1400-1700 cm-

1 region there is no significant difference between the two exchanges

Figure 38 - FTIR Spectra for Sample 4 exchanged with amm. Nitrate

45

Figure 39 - FTIR Spectra for Sample 4 exchanged with amm. Acetate

Figure 40 - FTIR Spectra of Sample 4 with both ion exchanges

Figure 41 - Subtraction of spectra for sample 4 exchanged with amm. nitrate

46

Figure 42 – Subtraction of Spectra for sample 4 exchanged with amm. Acetate

There is no FTIR data for sample 5, as its calcination didn’t provide enough clean material to

press into a disk.

Because LTL isn’t used as an acid catalyst, the FTIR data available in literature for this zeolite

usually covers its K and Na forms instead of H form. This means there is no literature to do a direct

comparison with sample 6. The sample has a peak in at 3745 cm-1, a band in the 3620-3660 cm-1 region

and two very broad bands between 2200 and 3600 cm-1. After pyridine adsorption, the band at 2650-

3600 cm-1 changes drastically, revealing multiple peaks between these wavenumbers. The peak at 3745

cm-1 decreases considerably and the band at 3620-3660 cm-1 almost disappears. This means pyridine

is adsorbing on the acid sites of the zeolite.

The adsorption gave origin to two peaks at 1455 cm-1 and 1490 cm-1 and three bands, one

between 1520 and 1560 cm-1 and two between 1590 and 1650 cm-1.

Figure 43 - FTIR Spectra for sample 6

47

Figure 44 - FTIR spectra for sample 6 after Py adsorption

Figure 45 - Subtraction of the spectra for sample 6 before and after Py. adsorption

48

SEM-EDX

As mentioned in the previous section, this SEM equipment does not have enough magnification

to reveal the shape of the crystals. However, the EDX couple to this equipment allows determination of

bulk Si/Al ratios, as well as confirmation if there are any leftover metal cations after ion exchange.

The early mazzite attempt samples have a near one to one ratio of sodium and aluminium, while

latter attempts start much lower at around 60% sodium to aluminium. This fact, combined with the higher

bulk Si/Al ratio of the latter attempts suggests the presence extra-framework aluminium, as the

stoichiometry of framework aluminium and monovalent sodium is 1:1. The LTL sample has close to a

one to one ratio of potassium to aluminium, and a silica to aluminium ratio close to its theoretical value

[22], suggesting low extra-framework aluminium.

It is also worth noting that the metallic ion removal during ion exchange is substancially lower in

LTL compared to the remaining samples.

The data collected with this technique can be seen in Tables 4 and 5:

Table 4 – Atomic percentages in the zeolite samples

Sample Ion

exchanged

Si (%) Al (%) Na (%) K (%)

2 No 22.08 8.71 8.81 -

2 Yes 23.42 9.80 2.15 -

3 No 22.27 6.81 4.74 -

4 No 28.66 8.83 5.14 -

4 Yes 24.62 8.29 0.67 -

6 No 19.93 6.94 0.43 6.80

6 Yes 23.50 7.90 0.12 3.44

Table 5 – Bulk composition ratios in the zeolite samples

Sample Ion exchanged Si/Al Na/Al K/Al

2 No 2.54 1.01 -

2 Yes 2.39 0.22 -

3 No 3.17 0.63 -

4 No 3.25 0.58 -

4 Yes 2.97 0.08 -

6 No 2.87 0.06 0.98

6 Yes 2.97 0.02 0.44

49

Conclusions and Future Work

Mazzite is a curious case among zeolites, where despite multiple literature showing its

advantages as a catalysts versus widely used structures like faujasite or mordenite, its use isn’t

widespread. On top of that, while literature on mazzite synthesis, characterization and uses exists,

there is a much smaller amount of it compared to these popular structures. Throughout this work, the

difficulties of synthesising pure mazzite were evident, with results ranging from amorphous material, to

materials that match the XRD pattern quite well but can’t adsorb probe molecules. Detected phases in

the XRD patterns include chabazite, silimanite, mullite, and andalusite, the last two of which are not

zeolites. The failure to produce pure, high quality mazzite hints that one of the reasons this structure

isn’t widely used despite its great acidic properties can be the difficulty of the synthesis process.

Some of the FTIR results suggest that some of the SDA was retained in the structure. That,

allied to the inability to diffuse pyridine, suggests a blockage on the channels, possibly caused by the

presence of secondary phases. This corroborates the presence of non-zeolitical aluminosilicates. The

fact that the sample that was ion exchanged using ammonium acetate had a broader OH band than

the equivalent sample exchanged using ammonium nitrate also hints at a partial blockage of the pore

system.

For these reasons, further optimization of the synthesis process is required, and alternative

processes like zeolite conversion seem to have more potential than the traditional hydrothermal

process for this particular structure. It is worth noting that despite the favourable properties, zeolite

conversion can start from zeolites like faujasite that are just marginally inferior for the intended use of

mazzite, which may make its conversion pointless. A better optimization effort would require more time

and a larger amount of autoclaves and ovens available, in order to be able to conduct multiple

experiments with different synthesis conditions simultaneously.

Further work should be made on the approach to hierarchical mazzite as well. If the zeolite

conversion does become that standard method for producing this phase, optimization of the conditions

for dealumination and desilication will be key on producing high quality hierarchical mazzite, with its

Si/Al ratio tuned accordingly for the intended catalytic effect.

LTL zeolite was a simple synthesis compared to mazzite, despite the structures being visually

similar. It is also interesting to note that unlike MAZ, it performs better as a basic catalyst, so its

potential uses are in processes like aldol or Knoevenagel condensation or aromatization of linear

compounds. All these processes can use quite bulky reactants and form bulky products, making the

optimization of hierarchical LTL zeolites for specific reactions a potentially interesting project. Further

work should be done on the effect of Si/Al ratios on each reaction, as well as the stability of

mesoporous LTL obtained from different processes.

50

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