lab final write up

Practical G: Synthesis and characterisation of zeolitic materialsLearning Objective
To synthesize crystalline functional materials and to apply analytical techniques to
determine/characterise their structure. To understand how some parameters affect the structure and
final properties of zeolite materials. To learn modern techniques applied in the synthesis of crystalline
inorganic compounds and to interpret X-ray diffraction patterns/ATR-IR spectra.
Relates To Course Material
•
•
Crystallography and X-ray diffraction in CHEM0013 (BS lecture notes) and CHEM0014 (JKC
lecture notes).
In depth XRD analysis (CHEM0025).
Introduction
Zeolites are microporous crystalline solids with well-defined structures. Generally, they contain
silicon, aluminium and oxygen in their framework and cations, water and/or other molecules within
their pores. Many occur naturally as minerals and are extensively mined in many parts of the world.
Others are synthetic, and are made commercially for specific uses, or produced by research scientists
trying to understand more about their chemistry. Because of their unique porous properties, zeolites
are used in a variety of applications with a global market of several million tonnes per annum. In the
western world, major uses are in petrochemical cracking, ion-exchange (water softening and
purification), and in the separation and removal of gases and solvents. Other applications are in
agriculture, animal husbandry and construction. They are often also referred to as molecular sieves.
Framework materials
A defining feature of zeolites is that their frameworks are made up of 4-connected networks of atoms.
One way of thinking about this is in terms of tetrahedra, with a silicon atom in the middle and oxygen
atoms at the corners. These tetrahedra can then link together by their corners (see illustration below)
to from a rich variety of beautiful structures.
The framework structure may contain linked cages, cavities or channels, which are of the right size to
allow small molecules to enter – i.e. the limiting pore sizes are roughly between 3 and 10 Å in diameter.
In all, over 130 different framework structures are now known. In addition to having silicon or
aluminium as the tetrahedral atom, other compositions have also been synthesised, including a
category of what are known as ‘zeotype’ materials like the microporous aluminophosphates, more
commonly known as ALPOs.
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Adsorption and separation
The shape-selective properties of zeolites are also the basis for
their use in molecular adsorption. The ability preferentially to
adsorb certain molecules, while excluding others, has opened
up a wide range of molecular sieving applications. Sometimes
it is simply a matter of the size and shape of pores controlling
access into the zeolite. In other cases different types of
molecule enter the zeolite, but some diffuse through the
channels more quickly, leaving others stuck behind, as in the
purification of para-xylene by silicalite.
The shape of para-xylene means
Cation-containing zeolites are extensively used as desiccants that it can diffuse freely in the
due to their high affinity for water, and also find application in channels of silicalite.
gas separation, where molecules are differentiated on the
basis of their electrostatic interactions with the metal ions. Conversely, hydrophobic silica zeolites
preferentially absorb organic solvents. Zeolites can thus separate molecules based on differences of
size, shape and polarity
Catalysis
Zeolites have the ability to act as catalysts for chemical reactions which take place within the internal
cavities. An important class of reactions is that catalysed by hydrogen-exchanged zeolites, whose
framework-bound protons give rise to very high acidity. This is exploited in many organic reactions,
including crude oil cracking, isomerisation and fuel synthesis. Zeolites can also serve as oxidation or
reduction catalysts, often after metals have been introduced into the framework. Examples are the
use of titanium ZSM-5 in the production of caprolactam, and copper zeolites in NOx reduction.
Underpinning all these types of reaction is the unique microporous
nature of zeolites, where the shape and size of a particular pore system
exerts a steric influence on the reaction, controlling the access of
reactants and products. Thus, zeolites are often said to act as shape
selective catalysts. Increasingly, attention has focused on fine-tuning the
properties of zeolite catalysts in order to carry out very specific syntheses
of high-value chemicals e.g. pharmaceuticals and cosmetics.
Typical application: xylenes in zeolites
The industrial separation of para-xylene from C8 aromatics (p-, o-, m-xylenes and ethylbenzene) is an
important step in the large-scale synthesis of petrochemicals. Xylenes are used as industrial solvents
or intermediates for many derivatives. The para isomer is the most interesting, it is at the basis of the
production of polyester films and fibres. Each derivative must be produced with a high purity for
further use in synthesis. Classical methods of separation, e.g. distillation and crystallisation, are not
efficient or economically attractive and the processes now in operation are based on zeolites of
faujasite structure with barium as compensating cation. The industrial processes work in the liquid
phase at a temperature of ~ 150°C.
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The
shape
and
dimensions of the pores
within a zeolite allow for
a wide range of shape
selective reactivity (i.e.
preferential formation of
p-xylene over o-, and mforms) and separation.
Ion exchange
The loosely bound nature of extra-framework metal ions (such as in zeolite NaA, right) means that
they are often readily exchanged for other types of metal when in aqueous solution. This is exploited
in a major way in water softening, where alkali metals such as sodium or potassium prefer to exchange
out of the zeolite, being replaced by the “hard” calcium and magnesium ions from the water. Many
commercial washing powders thus contain substantial amounts of zeolite. Commercial wastewater
containing heavy metals, and nuclear effluents containing radioactive isotopes can also be cleaned up
using such zeolites.
Zeolites and the environment
Zeolites contribute to a cleaner, safer environment in a great
number of ways. In fact, nearly every application of zeolites has
been driven by environmental concerns, or plays a significant role in
reducing toxic waste and energy consumption.
In powder detergents, zeolites replaced harmful phosphate
builders, now banned in many parts of the world because of water
pollution risks. Catalysts, by definition, make a chemical process
more efficient, thus saving energy and indirectly reducing pollution.
Moreover, processes can be carried out in fewer steps, minimising
unnecessary waste and by-products. As solid acids, zeolites reduce
Sodium Zeolite A, used as a
the need for corrosive
water softener in detergents.
liquid acids, and as redox
catalysts and sorbents, they can remove atmospheric
pollutants, such as engine exhaust gases and ozone-depleting
CFCs. Zeolites can also be used to separate harmful organics
from water, and in removing heavy metal ions, including
those produced by nuclear fission, from water.
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Zeolite Synthesis
The development of materials chemistry over the past 5 decades, fuelled by the enormous importance
of new materials to industry, has led to the design and synthesis of increasingly sophisticated
structures in microporous materials. Some key breakthroughs are listed below:
The basic philosophy behind the synthesis of two of these material types is represented graphically
below:
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High alumina containing materials (prepared in the 1960s)
(low temperature and pressure) eg Zeolite X and A.
Medium of an alkali
hydroxide solution
Aluminium Source
Silicon Source
Gel
~ 100 °C
Zeolite
Faujasite (also known as zeolite X or Y) with its
large 12- membered supercage at 7.4 A.
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High silica containing materials (prepared in the 1970s)
Made under high temperature and pressure. Examples include ZSM-5, ZSM-11 etc.
Medium of an alkali hydroxide
solution and an organic template
Aluminium Source
Silicon Source
Gel
~ 100-200 °C
Zeolite
The critical component of this route is the presence of an organic molecule that is necessary for the
framework structure to form. The shape and size of the chosen template influences the structure of
the channels and / or pores within the inorganic framework. Post-synthesis calcination at high
temperature removes the template, leaving the open pores available for use.
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Objectives
•
•
•
To prepare synthesis gels and apply hydrothermal methods in the synthesis of two zeolite
materials.
To recover the product and to characterise the samples using analytical techniques such as X-ray
powder diffraction and solid-state ATR-IR.
To use the characterisation techniques to determine the nature of the recovered products and
their properties.
Safety Assessment
Aluminium hydroxide
Hazards: Moderate irritant.
First Aid: Wash with copious amounts of water for at least 15 minutes.
Sodium Aluminate
Hazards: Moderate irritant.
First Aid: Wash with copious amounts of water for at least 15 minutes.
Sodium Hydroxide
Hazards: Corrosive. Causes eye and skin burns. Do not get on skin or in eyes. Do not ingest or inhale.
First Aid: Wash with copious amounts of water for at least 15 minutes.
Sodium Silicate
Hazards: Moderate irritant.
First Aid: Wash with copious amounts of water for at least 15 minutes.
Tetraethylorthosilicate
Hazards: Corrosive. Flammable. Causes eye and skin burns. Do not get on skin or in eyes. Do not
ingest or inhale.
First Aid: Wash with copious amounts of water for at least 15 minutes.
Tetrapropylammonium hydroxide
Hazards: Moderate irritant.
First Aid: Wash with copious amounts of water for at least 15 minutes. In the case of eye
contamination, seek immediate medical attention.
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Hydrothermal synthesis:
Hydrothermal synthesis is reliant on water pressure at high temperature. You should have watched
the autoclave assembly video on moodle:
https://moodle.ucl.ac.uk/mod/lti/view.php?id=3356117
The TEFLON liners should be filled only with half the volume for safe operations. You should show
a demonstrator once filled, and a demonstrator should be present during the assembly.
The stainless steel autoclaves should be handled with heat protective gloves and due care since they
will be at high temperature when removing from the oven. The stainless steel autoclave should only
be opened after it reaches room temperature and if, it is warm, water-aided cooling should be
carried out for at least 30 minutes.
Pre-Lab Tasks
Task 1: Safety Assessment
•
•
Complete your COSHH and risk assessments.
Complete the pre-lab safety quiz on Moodle.
Task 2: Pre-lab preparation
•
Find the following references: i) P. Morales-Pacheco et al. J. Phys. Chem. C. (2007), 111,2368,
ii) supplementary information from J. Ruiz-Martinez et al. Angew. Chemie Int. Ed. (2013), 52,
5983. It will be useful.
•
For zeolite ZSM-5 synthesis, Write the formula and calculate the mass of tetrapropylammonium
hydroxide present to make 3ml of solution. IMPORTANT : tetrapropylammonium hydroxide is
typically available as a 20 % solution. Therefore, for example, 3 ml ≠ 3.036 g
•
Write a balanced chemical equation and determine the moles of:
(i)
Silica and ethanol obtained from the hydrolysis of 1.5 g of tetraethyl orthosilicate.
(ii)
Silica and sodium oxide obtained from the hydrolysis of 7 g sodium orthosilicate.
•
Determine the wt. and moles of alumina, water and sodium oxide obtained from 0.05 g of sodium
aluminate (corresponding to a composition of 53 wt.% of alumina, 9 wt.% of water and 38 wt.%
of sodium oxide)
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Task 3: Pre-Lab Questions
1.
Zeolite ZSM-5 has 96 T atoms per unit cell. Determine the number of Al atoms per unit cell for a
Si/Al ratio of 15. Calculate the number of tetrapropylammonium cations needed for charge
compensation.
2.
Consider what important information can be extracted from the X-ray diffraction patterns of the
samples? Representative XRD patterns of the zeolites can be found in the suggested database.
Use the following links for the two zeolites you will be making.
For Zeolite X – https://europe.iza-structure.org/IZA-SC/pow_pat.php?STC=FAU&ID=FAU_2
For ZSM-5 – https://europe.iza-structure.org/IZA-SC/pow_pat.php?STC=MFI&ID=MFI_0
3.
Consider what important information can be extracted from the IR spectra of the zeolite samples?
Reference IR spectra can be easily found online. For instance you may find articles similar to
these:.
Naikoo et al. RSC Adv., 2016, 6, 99202–99210 https://doi.org/10.1039/C6RA19708F for Zeolite X
Narayanan et al., J. Porous Materials 23, 741 DOI: 10.1007/s10934-016-0129-8 for ZSM-5
Importance of Characterisation by XRD and IR spectroscopy.
Zeolite synthesis is sensitive to the stoichiometric ratio of Al and Si in the solution. Secondary phases
(including other zeolite topologies, non-porous phases) will be present after synthesis if the reactant
solution(s) deviate strongly from what is described in the experimental procedure. A similar problem
rises if the reagents are not properly dissolved or mixed. Consult the reference XRD patterns in the
zeolite structure database (http://europe.iza-structure.org/IZA-SC/ftc_table.php ) on conclusion of
the synthesis work to determine the phase purity of the zeolite samples synthesised.
ATR-IR spectra will be recorded using the ATR-IR spectrometer available in the lab. About 30 mg of
sample should be sufficient for a sample measurement. A reference spectrum for
tetrapropylammonium
hydroxide
can
be
found
here:
https://sdbs.db.aist.go.jp/sdbs/cgibin/direct_frame_top.cgi
Task 4 : Lab Notebook
•
This task to be completed alongside your two-day experiment
The procedures you carried out should be recorded in your lab book. Ensure that the lab book is
signed by a demonstrator before you leave the lab on Day 2 as this will be marked later. If the
signature of the demonstrator does not appear, this part will not be marked.
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Practical
Schedule
NOTE : This practical is to be conducted individually.
This practical contains overnight syntheses and requires you to use equipment at specified times.
Consequently, you must ensure you follow the schedule below to successfully complete the lab; there
is no room in the schedule for any additional overnight experiments. You must perform the XRD
sample preparation on the specified day, and the appropriate diffractometers will not be available at
other times.
Day 1: Materials synthesis: Dedicated to the synthesis of both zeolite X and zeolite ZSM-5 using
hydrothermal methods. You will prepare precursor gel and introduce them in the autoclave.
** Due to limited space in the fume hood, we recommend that one half of the cohort start with the
Zeolite X synthesis and other half of the cohort make ZSM-5 gel, which has to be handled in the
fume hood. Once the first gel is inserted into the autoclave, the other synthesis can be started. The
assignment will be done on Day 1 – consult a senior demonstrator on site. All the autoclaves should
be in the respective oven by 4 PM**
Synthetic work involving organic reagents should be performed inside fume hoods , with the only
exception of the hydrothermal vessel assembly and treatment in a dedicated oven. For any overnight
reactions you need to ensure that an orange overnight form, signed by a senior demonstrator is in
place, visible to safety officers.
Day 2: Materials recovery and characterisation: Dedicated to the recovery and drying of both
products before preparing the samples for submission for XRD and measurements of ATR-IR spectra.
Important Notes
General: Any synthesis involving organic reagents (example: tetrapropyl ammonium hydroxide)
should be carried out in the fume hood. Loading gel in the autoclave can be carried out in your working
bench. Instructional videos for several of the procedures used in this lab (autoclave usage, and XRD
sample preparation) are available on Moodle under the tab “Useful Videos and Resources”. You
should watch these videos before you start the lab.
Timings: To successfully complete this lab, which involves an overnight step, you must ensure that you
complete the specific tasks at the correct time. You may also only use the diffractometers at the
specified times.
Autoclaves: When you sign out an autoclave from stores, you will be required to confirm that you have
watched the video of autoclave usage on Moodle. If you have not watched it, you must ask a
demonstrator to go through it with you before you will be given an autoclave. A demonstrator must
be present when you assemble the autoclave before placing it in the oven.
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Ovens: All ovens have sample sign in/out sheets on them. Please use these so that we know what is in
the ovens and can keep track of samples. To place/remove samples in the vacuum oven, ask a
demonstrator for assistance.
XRD: you must watch the video of PXRD sample preparation on Moodle
https://moodle.ucl.ac.uk/mod/resource/view.php?id=3037412 You must prepare the PXRD samples
in the Day 2 session.
Day 1:
Preparation of Zeolite precursor gels
Hydrothermal synthesis of Zeolite X
For the preparation of the zeolite precursor gel you need to make two solutions. One containing the
Al precursor and the other the Si precursor. Use plastic beakers for zeolite X synthesis as silica
solutions etch Pyrex glassware. Cover the beakers containing the solutions with a watchglass to
prevent any contamination.
Solution A: Add 2.5 g of NaOH to 2.5 mL of distilled water in a plastic beaker. Stir the mixture using a
magnetic stirrer until all solids are dissolved completely. Add 2.43 g Al(OH)3 and heat up to around 85
°C while stirring with a magnetic stirrer bar until all Al(OH)3 is fully dissolved. This may take up to 30
minutes. To avoid loss of water, you may use a watch glass to cover the plastic beaker. (Care must be
taken not heat beyond 75 oC on the hot plate as the plastic beaker may start melting). Once the
aluminium compound is dissolved, cool down to 25 °C. Add a further 5.05 g water.
Solution B: In a plastic beaker, dissolve 2.96 g NaOH in 30.6 g water. Stir, whilst adding 11 g sodium
silicate solution. Stir until content is well mixed.
Solution C: In a plastic beaker, take 5 g of Solution A, add a further 30.6 g water and 2.96 g NaOH. Stir
until fully dissolved. Add Solution B and stir for 30 minutes.
Take note of the pH of the gel at this point using pH indicator paper. If the hydrothermal synthesis in
the following steps goes well this pH should remain the same.
The resultant synthesis gel for the zeolite should possess the following chemical formula:
Na86Al86Si106O384:wH2O (w ~ 260)
For the hydrothermal treatment of the gel, the solution needs to be transferred to an autoclave.
Collect one of the 23 ml autoclaves from stores
CAUTION: when placing the solution into the autoclave, ensure that the Teflon liner is filled to a
maximum of 50% , ~12.5 ml of the total volume of 25 ml. Show this to a demonstrator and also make
sure you seal the autoclave following the instructions and in presence of a demonstrator. The
demonstrator must confirm that that the autoclave is assembled and closed properly. Place it in the
assigned oven that will be heated at 90oC for 16 hours (overnight). You will be able to recover and
process your sample in the next lab session.
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– Collect one of the 23 ml autoclaves from stores. Autoclave must be handled and assembled with care
to ensure safety: this assembly operation should be done in presence of a demonstrator.
Any remaining solution should be discarded safely in a non-chlorinated solvent disposal vessel.
Hydrothermal synthesis of Zeolite ZSM-5
Crystallisation of ZSM-5 zeolite is carried out in static conditions at 180 °C in PTFE-lined stainless steel
23ml autoclaves. The organic structure-directing agent employed is tetrapropylammonium hydroxide
(1M solution). Tetraethyl orthosilicate, sodium aluminate and water are used as reagents. The final
gel composition is 8.1 TPAOH: x Na2O : y Al2O3 : 25 SiO2 : 600 H2O, where x and y are similar in the
range of 0.32 ≤ x ≤ 0.84 and 0.32 ≤ y ≤ 0.84..
This composition is obtained by mixing the appropriate amounts of sodium aluminate and water in a
plastic beaker, followed by the addition of the silica source and the structure-directing agent. Note
that according to the gel molar composition, the amount of sodium aluminate should be kept between
0.0335 and 0.0900 g, corresponding to a composition of 28.3 wt.% of Al and 28.1 wt.% of Na. The
mixture is stirred for 60 minutes. The solution is then placed in an autoclave and heated at 180 °C for
12 to 16 h.
– Collect one of the 23 ml autoclaves from stores. Autoclave must be handled and assembled with care
to ensure safety: this assembly operation should be done in presence of a demonstrator.
– CAUTION: when placing the solution into the autoclave, ensure that the vessel is not filled more than
approximately half-way (12 ml maximum). Close the autoclave and ensure it is tightened. Confirm with
a demonstrator that the autoclave is assembled and closed properly. – You will be able to recover and
process your sample in the next lab session.
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Day 2:
Zeolite washing and drying
Solid recovery: At the start of the following lab session (or once the oven cycle is finished and the
oven has reached room temperature), take both the autoclaves out of the respective ovens. Cool
them down further using cold water for at least 30 minutes (if warm or hot). Once the autoclave is
cold, open it with care. Remove the PTFE vessel and weigh it out and open the lid. The total weight
before and after hydrothermal synthesis should be approximately the same. Open the lid and measure
the pH of the liquid phase using a pH indicator stick, the product should have settled to the bottom of
the vessel. Extract the solid from the slurry by centrifuge process. Gently pour the liquid into a
centrifuge tube and add distilled water to the PTFE vessel, pouring the washings in to a centrifuge
tube. Carry out centrifuge process and decant the supernatant liquid. Rinse the solid multiple times
with distilled water, ideally 3 times. Collect the filtrate using a spatula and place it in a watchglass.
Place the dish in an oven at 120 °C to dry the powder for a minimum period of 3 h. Once this procedure
is completed, the dried powder can be collected from the oven and processed for XRD and ATR-IR
measurement.
Record the mass of product recovered. Estimate the percentage yield of the synthesis.
The procedures you carried out should be recorded in the lab book. Ensure that the lab book is
signed by a demonstrator before you leave the lab on Day 2 as this will be marked later. If the
signature(s) of the demonstrator do not appear, this part will not be marked.
Characterisation of recovered zeolite powders
The powder samples produced will be submitted for XRD analysis and ATR-IR measurements. To
prepare the zeolite samples for PXRD using Cu X-ray radiation, powdered samples are placed between
two thin plastic films and placed in the circular sample holder. Sample holders are collected from the
stores after entering your name in the computer system. Sample preparation is identical to the
procedure used for either LFP or HA synthesis 1. If in doubt consult a demonstrator for the procedure.
Samples need to be taken to room 313 for PXRD data collection; please notify a demonstrator when
your sample is ready. Demonstrators will gather and escort small groups of students to room 313 and
explain the main features of the diffractometers employed.
For recording IR spectra, take the oven-dried samples to the ATR-IR machine and sprinkle a sufficient
amount of zeolite powder (< 30 mg) onto the sample mount. Close the sample holder and record IR spectrum. Re-record the spectrum on the same sample after leaving to stand at room temperature for 30 minutes. Are there any differences between the IR spectra recorded? After receiving your PXRD data (this will be later than Day 2): PXRD data will be provided as downloadable files on Moodle with dataset number (as recorded on the instrument) and student name(s). These files can be loaded directly into Excel and instructions are available on Moodle for simple data inspection and comparison against standard sample data. As described previously, use the International Zeolite Association (IZA) database to compare your data with the calculated patterns in the database: https://europe.iza-structure.org/IZA-SC/ftc_table.php 13 i. Generate X-ray diffraction patterns for the two zeolites that you have identified and print off a copy. ii. Record IR spectra of the zeolites after drying and after exposure to air over 30 minutes Task 5: Data Presentation • Record the mass of product recovered. Estimate the percentage yield of the syntheses. Include these data in the final report. • Attach the PXRD patterns for all samples synthesized. Aim to produce pictures of good quality (details are available on Moodle). Comment on the quality of data. • Attach your ATR-IR spectra for zeolite X and ZSM-5. Comment on the quality of data. Task 6: Experimental Evaluation Based on the data and observations you present in your lab notebook and as part of task 5, discuss how well the experiment has worked. Use the following guidelines for writing this evaluation. • Comment on the yield you have obtained • Compare the PXRD with the reference patterns from the database (see the links provided). Do the measured XRD pattern confirm the synthesis of the target zeolite (zeolite X and ZSM-5) compounds? Is there any evidence of amorphous or secondary crystalline phases? Mark the peaks corresponding to any secondary phase in your XRD patterns. Also mark crystallographic Miller indices for at least 5 strong reflections • Compare your ATR-IR spectra for zeolite X and ZSM-5. Assign the observed bands to the appropriate vibrational modes of the template/zeolite present in each of the samples. Comment on the differences in the spectra between the two samples. The detailed assessment framework for this task is given below 14 Assessment Scheme (Illustrative) TASK 5: Outcome of the experiment 15 0/4 1/4 2/4 3/4 4/4 XRD patterns of synthesised zeolite Not present. XRD patterns are given. Details of synthesis attempts of zeolite X and ZSM-5 are given. Axes not labelled correctly. In addition to Criterion 1: Axes labelled correctly . No discussions on (i) phase ID and (ii) amorphous content. No indexing is presented In addition to Criterion 3 Identification of additional phases and/or comments on the presence/absence of amorphous content in the sample. inclusive of indexing lines for the respective zeolites present IR spectra of synthesised zeolite Not present. IR spectrum presented. Results of synthesis attempts of zeolite X/ZSM-5 are given. Axes not labelled correctly In addition to Criterion 1, Axes labelled correctly No attempt was made to assign IR bands In addition to Criterion 2 Identification of additional phases and/or comments on the presence/absence of amorphous content in the sample. No Indexing is given Or Indexing is given but phase identification is incomplete In addition to Criterion 2, Incorrectly identified bands in the IR spectra for zeolite/template. Quality of products synthesised Not present. Poor (very low yield) Good (lower yield than expected) and explanation is given Excellent (product synthesised in excellent yield), explanation was not satisfactory Excellent (product synthesised in excellent yield) satisfactory explanation is given In addition to Criterion 3, correctly identified bands in the IR spectra for zeolite/template. TASK 6 : Experimental Evaluation 0/4 1/4 2/4 3/4 4/4 Not present. Minimal effort in presenting results without any discussion Some attempt at discussion of need for template and hydrothermal methods. Good attempt at discussion of need for template and hydrothermal methods. Comprehensive and correct discussion of the data. Quality based XRD and IR results are poor. All labelled properly. Quality based XRD and IR results are good. All labelled properly. No clear discussion on the differences between zeolites 16 No clear discussion on the differences between zeolites Quality based XRD and IR results are good. All labelled properly. Details of the differences between zeolites are present.