2 nd FEZA School Paris, 1-2 September, 2008

transcript

Eni Refining & Marketing Division

2nd FEZA SchoolParis, 1-2 September, 2008

Structural Characterization of Zeolites and Structural Characterization of Zeolites and Related Materials by X-Ray Powder DiffractionRelated Materials by X-Ray Powder Diffraction

Roberto Millini, Stefano Zanardi

TOPICS

• X-RAYS

• X-RAY POWDER DIFFRACTION

• METHODS

PHASE IDENTIFICATION

PATTERN INDEXING

UNIT CELL PARAMETERS REFINEMENT

CRYSTALLINITY

CRYSTALLITE SIZE

• CONCLUSIONS

What are X-rays?

Electromagnetic radiation with wavelength, Electromagnetic radiation with wavelength, , in the region 0.01 , in the region 0.01 – 100 Å– 100 Å

In the electromagnetic spectrum, X-rays are placed between UV In the electromagnetic spectrum, X-rays are placed between UV and γ-radiationsand γ-radiations

RADIO MICROWAVE IR VISIBLE UV X-RAY γ-RAY

5·109 1·104 500 250 0.5 5·10-41·107

λ (nm)

2.48·10-7 0.124 2.48 4.96 2480 2.48·1061.24·10-4

E (eV)

Production of X-rays

HV source

X-rays

evacuated tube

heated W filament

electrons

Only 1% of the energyOnly 1% of the energyproduces X-rays!produces X-rays!

99% is lost as heat99% is lost as heat

Photon Energy (keV)

Kβλ = 0.184374 Å

λ = 0.2090100 Å

Bremsstrahlung(80 – 90%)

Characteristic X-rays(10 – 20%)

The X-ray spectrum of W

Emax = Ee- (87 keV)

X-ray diffraction

Scattering occurs when there is a perfectly elastic collision among photons and electrons: the photons change their direction without any transfer of energy

If the scatterers (atoms) are arranged in an ordered manner (crystal) and the distances among them are similar to the wavelength of the photons, the phase relationship becomes periodic and interference diffraction effects are observed at various angles.

X-raysX-rays InterferenceInterference

X-ray diffractionThe Bragg’s lawThe Bragg’s law

The difference in path between the waves scattered in B and D is equal to

AB+BC = 2dsinθ

If AB+BC is equal to a multiple of λ, the two waves combine themself with maximum positive interference; therefore:

nλ = 2dsinθ

the fundamental relationship in crystallography, known as Bragg equation

X-ray diffractionsingle crystal vs. powdersingle crystal vs. powder

X-rays

integration

X-ray powder diffraction (XRD)

XRD pattern

InstrumentationBragg-Brentano diffractometerBragg-Brentano diffractometer

S = X-ray source

DS = divergence slit

SP = sample

RS = receiving slit

D = detectorθ

DS = X-ray source

DS = divergence slit

SP = sample

RS = receiving slit

D = detector

AS = antidivergence slit

s1, s2 = Soller slits

The XRD pattern

peakanisotropy

intensityI = k · Lp · P · A · F2

position23.13° 2θ, d = 3.845 Å

Information contained in the XRD pattern

Background

Scattering fromsample-holder, air, …

Amorphous phase,disorder, …

Incoherent scattering(Compton, TDS, …)

Sample

Information contained in the XRD pattern

Position

Lattice parametersSpace group

Qualitative phase analysisPhase purityThermal expansionCompressibilityPhase change

Reflections

Intensity

Crystal structure:Atomic positionsOccupancyThermal factorsTextureCrystallinity

Quantitative phase analysis

Profile

InstrumentalSample

Crystallite sizeStressStrain

ZeolitesFramework types vs. materialsFramework types vs. materials

Each open 4-connected 3D net, with (approximate) AB2 composition, where A is a tetrahedrally connected atom and B is any 2-connected atom, constitutes a framework typeframework type, which is defined by a 3-letter code assigned by the IZA Structure Commission

“The 3-letter codes describe and define the network of the corner sharing tetrahedrally coordinated framework atoms … [and] should not be confused or equated to actual materialsmaterials.”

“The framework types do not depend on composition, distribution of the T-atoms, cell dimensions or symmetry.”

Several materials may possess Several materials may possess the same framework typethe same framework type

ZeolitesPeculiar propertiesPeculiar properties

• Variable composition of the framework (e.g., Si, Ge, Si/Al, Si/B, Si/Ga, Si/Ge, Si/Ti, Al/P, Si/Al/P)

• Variable stoichiometry (e.g. Si/Al = 1 – ∞)

• Variation of the nature and concentration of the extra-framework species (inorganic cations and/or organic species)

Each change of the basic structure Each change of the basic structure produces a new materialproduces a new material

All these phenomena induce the change of:

• the dimensions of the unit cell, hence the positions of the Bragg reflections

• the intensities of the reflections

SAMPLE

XRD characterization

INDEXINGIDENTIFICATION

FRAMEWORKCOMPOSITION

CRYSTALLINITY

CRYSTALLITESIZE

STRUCTUREDETERMINATION

STRUCTUREREFINEMENT

XRD NEW PHASE

XRD characterizationPhase identificationPhase identification

Each crystalline phase is characterized by a XRD pattern constituted by a set of reflections with well-defined positions (2θ (°) or d (Å)) and

relative intensities (I/I0·100)

The XRD pattern is the fingerprint of the crystalline phase

INPUT DATA

A list of 2θ (or d) – relative intensities [(I/I0)·100] of the reflections

METHODS

• Automated search in databases: the PDF2 (Powder Diffraction File, by ICDD) contains some 200,000 measured and calculated patterns

• Atlas of Zeolite Framework Types: the Structure Commission of IZA periodically publishes the Atlas of the Zeolite Framework TypesAtlas of the Zeolite Framework Types and a Collection of Simulated XRD Powder Patterns for ZeolitesCollection of Simulated XRD Powder Patterns for Zeolites; all the information are available on the web (http://www.iza-structure.org/databases/), with the possibility to simulate the XRD pattern with custom-defined parameters

• Search on the open and patent literature: the “last chance” when the other methods fail

IF THE SEARCH IS UNSUCCESSFUL, WE ARE IN THE PRESENCE IF THE SEARCH IS UNSUCCESSFUL, WE ARE IN THE PRESENCE OF A NEW CRYSTALLINE PHASEOF A NEW CRYSTALLINE PHASE

XRD characterizationThe PDF2 fileThe PDF2 file

Automated search on PDF2 database of a complex mixture of zeolite phases

1. The XRD pattern

2. Definition of the background

3. Peak search

4. Identification of Phase 1

The phase composition (framework and/or extraframework species) influences positions and relative intensities of the reflections, making sometimes difficult the automated phase identification

ERB-1 (B-containing MWW)

as-synthesizedNH4

+-exchanged

intercalated with: quinuclidineethylenglycoli-PrOH

R. Millini et al., Microporous Mat., 1995

as-synthesized

calcined

ERB-1 (B-containing MWW)

R. Millini et al., Microporous Mater., 1995

Indexing the XRD pattern

The structural characterization of an unknown crystalline phase firstly requires the determination of the unit cellunit cell and of the symmetry

elements associated to one of the 230 space groups230 space groups

The indexing process tries to find the solution to the relation:

ddhklhkl = f( = f(h, k, l, a, b, c,h, k, l, a, b, c, αα, , ββ, , γγ))

The form of the equation depends on the crystal system:

from the simple cubic system:

d*d*22hkl hkl = (= (hh22 + + kk22 + + ll22)a*)a*22

… to the complex triclinic system:

d*d*22hkl hkl ==hh22a*a*22++kk22bb*2*2++ll22c*c*22+2+2hkhka*b*cosa*b*cosγγ*+2*+2hlhla*c*cosa*c*cosββ*+2*+2klklb*c*cosb*c*cosαα**

Indexing the XRD patternThe cubic systemThe cubic system

h k l d(obs) d(calc) res(d) 2T.obs 2T.calc res(2T) 1 2 2 0 8.63321 8.63721 -0.00400 10.238 10.233 0.005 2 3 1 1 7.36358 7.36584 -0.00226 12.009 12.005 0.004 3 3 3 1 5.60587 5.60457 0.00130 15.795 15.799 -0.004 4 5 1 1 4.70244 4.70150 0.00094 18.855 18.859 -0.004 5 4 4 0 4.31898 4.31861 0.00037 20.547 20.549 -0.002 6 6 2 0 3.86310 3.86268 0.00042 23.003 23.005 -0.003 7 5 3 3 3.72596 3.72550 0.00046 23.862 23.865 -0.003 8 5 5 1 3.42090 3.42085 0.00005 26.025 26.026 -0.000 9 6 4 2 3.26457 3.26456 0.00001 27.295 27.295 -0.00010 6 6 0 2.87871 2.87907 -0.00036 31.040 31.036 0.00411 5 5 5 2.82066 2.82090 -0.00024 31.696 31.693 0.003

a = 24.4297(23) Å

V = 14579.9(41) Å3

a = dhkl · (h2 + k2 + l2)1/2

Laboratory XRDλ = 1.54178 Å

Synchrotronλ = 1.1528 Å

Indexing the XRD patternA lower symmetry case: ERS-7 (ESV)A lower symmetry case: ERS-7 (ESV)

R. Millini et al., Proc. 12th IZA, 1999

The program TREORTREOR was used for indexing the complex XRD pattern.

The input is simple:

• the d (or 2θ) values of the first 20 – 30 lines

• the maximum UC volume (negative if all the systems should be checked, otherwise only the cubic, tetragonal, orthorhombic and hexagonal are considered)

• the maximum β angle for monoclinc system

• some specific input parameters if more information are available from other sources

The output consists of a number of possible solutions, all characterized

by specific figure of merits

The consistency of the best solution should be checked

1 or more unindexed reflections indicate the presence of impurities or that the solution is not reliable

Once a reliable UC is found, the possible space groups are searched through the inspection of the systematic absences, i.e. the classes

of reflections absent for symmetry

The following systematic extinctions were detected:

h00: h = 2n+1 0k0: k = 2n+1 00l: l = 2n+1

hk0: h = 2n+1 0kl: k+l = 2n+1

possible space groups:

Pn21a or Pnma

Indexing the XRD patternProblemsProblems

• Diffractometer and sample. The experimental setup should be accurately checked and the sample accurately prepared

• Data collection strategy. The results are strongly related to the accuracy in the determination of d (or 2θ); requiring all the first 20 – 30 lines, those located in the low-angle region (usually present in the XRD patterns of zeolites) are more critical to measure

• Overlap of the reflections. As the UC dimensions increase and the symmetry decreases the number of reflections increases; therefore, high-resolution powder diffraction data are necessary

• Phase purity. The presence of a second phase (even in trace amounts) makes difficult the indexing process; the reflections of the second phase (if unknown) can be identified by inspecting other samples synthesized in a similar way.

Unit cell parameters refinement

The accurate determination of the UC parameters is important because they depend on the chemical composition of zeolites. In fact:

• Zeolites can be synthesized in a wide Si/Al rangewide Si/Al range or it can be modulated by post-synthesis treatments (e.g. dealumination by steaming)

• The framework composition can be varied by isomorphous isomorphous substitutionsubstitution, i.e. by replacing (at least partially) Al and/or Si by other trivalent (e.g. B, Ga, Fe) and tetravalent (e.g. Ge, Ti) elements

The determination of the real framework The determination of the real framework composition is important because from it depend composition is important because from it depend

the properties of the materialthe properties of the material

Unit cell parameters refinement

Different analytical (e.g. Cs+-exchange) and spectroscopic (e.g. MAS NMR, FT IR) techniques have been proposed but XRD proved to be, in many cases, the most effective

The XRD methods are based on the observation that:

the incorporation of a heteroatom (i.e. an element different from Si) the incorporation of a heteroatom (i.e. an element different from Si) in the framework produces an expansion or a contraction of the UC in the framework produces an expansion or a contraction of the UC parameters, depending on its size respect to Si (provided that no parameters, depending on its size respect to Si (provided that no changes of the T-O-T angles occur)changes of the T-O-T angles occur)

Unit cell parameters refinementLeast-squares fit on the interplanar spacings of selected reflectionsLeast-squares fit on the interplanar spacings of selected reflections

The computer programs based on this classical approach minimize the sum of the squares of the quantity:

Q(hkl)obs - Q(hkl)calc

where:

Q(hkl) = 1/d2 = 4(sin2θ)/λ2

Input data (minimal):

• hkl indices and corresponding d (Å) or 2θ (°) for a certain number of reflections

Output:

• UC parameters and volume with the associated e.d.s.’s

• calculated d and/or 2θ and the difference respect to the experimental value(s) (for each reflection)

Unit cell parameters refinementLeast-squares fit on the interplanar spacings of selected reflectionsLeast-squares fit on the interplanar spacings of selected reflections

The method is easy and can be used even when the crystal structure of the phase under investigation is unknown; however, the reliability of the results depends on the complexity of the XRD pattern and on the quality of the input data

The main problems arise when:

• a non-strictly monochromatic radiation is used (e.g., CuKα1/CuKα2)

• the reflections are affected by severe overlapping phenomena

• the geometry of the diffractometer is not accurately adjusted (angular shift)

• the sample is not accurately prepared (sample displacement)

The use of a reference material (e.g. Si SRM 640b) as an external or, better, internal standard is suggested. In this way, the measured 2θ values can be corrected by the Δ2θ shifts measured on the reflections of the standard

Unit cell parameters refinementFull-profile fitting methodsFull-profile fitting methods

The use of full-profile fitting procedures has to be preferred when possible, namely when reliable structural information are available for the phase(s) under investigation

The goal of these methods is the reproduction of the experimental XRD pattern through the appropriate parametrization and refinement of the structural and instrumental parameters

On this concept is based the well known:

Rietveld MethodRietveld Method

The Rietveld Method

• Developed in the late years 1960s by H. M. Rietveld for refining neutron powder diffraction data

• At the end of years 1970s, it was extended to the refinement of XRD pattern

It is not a method for solving the crystal structure of a given phase but only for the refinement of a reasonable structural

model derived from other sources

During the least-squares refinement, the function minimized is:

R = Σiwi(YiO – YiC)2

where:

YiO and YiC are the observed and calculated intensities at step i

wi the weight assigned at each step and generally equal to 1/YiO

The Rietveld MethodThe refinement involves the variation of:

Scale factor

Instrumental parameters

(Wavelenght)

(Polarization)

Angular shift

Background intensities

Peak-profile coefficients

FWHM vs 2θ

Peak asymmetry

(Wavelenght)

(Polarization)

Angular shift

Background intensities

Peak-profile coefficients

FWHM vs 2θ

Peak asymmetry

Structural parameters

a, b, c, α, β, γ

Atomic coordinates

Site occupancy

Thermal factors

a, b, c, α, β, γ

Atomic coordinates

Site occupancy

Thermal factors

Correction parameters

Primary extinction

Surface adsorption

Preferred orientation

Sample displacement

Primary extinction

Surface adsorption

Preferred orientation

Sample displacement

The Rietveld MethodApplicationsApplications

• Structure refinementStructure refinement

• Accurate determination of UC parametersAccurate determination of UC parameters

• Quantitative phase analysis (including Quantitative phase analysis (including quantification of the amorphous phase)quantification of the amorphous phase)

The Rietveld MethodStructure refinementStructure refinement

Rough structural model required, produced by applying different strategies:

Direct methods, Patterson, …

Identification of an isostructural phase with known structure

Use of difference Fourier methods to investigate phases of known structure

Trial & Error methods

Computer modeling techniques

EMS-2: Na2K2Sn2Si10O26·6H2O isostructural with the mineral natrolemoynite: Na4Zr2Si10O26·9H2O

S. Zanardi et al., Microporous Mesoporous Mater., 2007

Location of hexamethonium dications in EU-1 (EUO)Model built by molecular modeling

R. Millini et al., Microporous Mesoporous Mater., 2001

The Rietveld MethodQuantitative phase analysisQuantitative phase analysis

PHASEConc. (wt%)

EXP. FOUND

CaSO4·2H2O 28.1 26.7

CaSO4·0.5H2O 4.3 5.3

CaSO4 6.6 6.1

α-Al2O3 2 2.5

CaCO3 (calcite) 45 45

SiO2 (quartz) 4 2

CaC2O4·H2O 10 12.3

Standardless quantitative phase analysis is possible even on relatively complex mixtures of crystalline phases

R. Millini, unpublished results

The Rietveld MethodDetermination of UC parametersDetermination of UC parameters

The application of the Rietveld Method is preferred when the determination of the UC parameters should be performed on complex XRD patterns, provided that an accurate structural model is available

The Rietveld programs take into account (and can refine):

• the use of non-strictly monochromatic radiation (e.g., CuKα1/CuKα2)

• severe overlapping phenomena of the reflections

• the geometry of the diffractometer (angular shift)

• moderate sample displacement deriving from a non-optimal preparation of the sample

It is not necessary to use an internal standard, but the data collection strategy should be accurately designed in terms of: 2θ range, step size, counting time

Unit cell parameters refinementCase study: assessing Ti and B incorporation in the silica frameworkCase study: assessing Ti and B incorporation in the silica framework

BOR-CAcid Catalyst

TS-1Oxidation Catalyst

Incorporation of Ti in: MFI (TS-1), MFI/MEL (TS-2, TS-3)

Incorporation of B in: RTH (BOR-A), BETA (BOR-B), MFI (BOR-C), MFI/MEL (BOR-D), MWW (ERB-1), EUO, LEV, MTW, ANA

Incorporation of B in MFI framework

The contraction of the UC parameters is expected when the small B3+ ions are incorporated in the zeolite framework

To unambiguously assess the incorporation of the heteroatom, the UC parameters of samples with increasing B3+ content should be accurately determined

PROBLEM

The XRD pattern of the orthorhombic MFI-type zeolites is very complex (it contains 500+ reflections below 50°2θ). Only a few single reflections can be used for the least-squares refinement of the UC parameters

Incorporation of B in MFI framework

The experiments confirmed that the UC parameters linearly decrease as the B3+ content increases

Vx = VSi – VSi[1 – (dB3/dSi

HYPOTHESIS

The contraction of the UC volume is due only to the smaller dimensions of the [BO4] tetrahedron respect to [SiO4] and no change of the T-O-T angles occurs:

VSi = 5345.5 Å3, dSi = 1.61 Å (typical Si-O bond length in zeolites), dB = 1.46 Å (mean tetrahedral B-O bond length in the mineral reedmergnerite, NaBSi3O8):

Vx = 5345.5 – 1359.2xM. Taramasso et al., Proc. 5th IZA, 1980

Incorporation of Ti in MFI framework

The expansion of the UC parameters is expected when the large Ti4+ ions are incorporated in the zeolite framework

UC parameters and volume were firstly determined by least-squares fit on the interplanar spacings of selected reflections

G. Perego et al., Proc. 7th IZA, 1987

The data produced by the least-squares fitting procedure are scattered from the regression curve, because the severe overlap of some reflections made difficult the accurate determination of the peak positions

A significant improvement of the quality of the data are expected by applying the Rietveld Method

• Low angular region excluded because of the high asymmetry of the reflections

• High angular region excluded because of the very low intensitiy and the excessive overlap of the reflections

R. Millini et al., J. Catal., 1992

The experiments confirmed that the UC parameters linearly increase as the Ti4+ content increases

Vx = VSi – VSi[1 – (dTi3/dSi

The expansion of the UC volume is due only to the larger dimensions of the

[TiO4] tetrahedron respect to [SiO4] and no change of the T-O-T angles occurs:

VSi = 5339.4 Å3, dSi = 1.61 Å (typical Si-O bond length in zeolites),

Vx = 5339.4 + 2110.4x

dTi = 1.79 Å

Tetrahedral Ti-O bond lengths BaTiO3 in the range 1.63 – 1.82 Å

Incorporation of Ti in MFI frameworkThe same method was applied on high-resolution synchrotron powder diffraction patterns collected on samples treated at 400 K under vacuum and sealed in capillaries under vacuum

C. Lamberti et al., J. Catal., 1999

Laboratory data

Synchrotron data

Incorporation of Ti in MFI framework• Determination of the Ti content in the framework with an accuracy

of 2 – 3 %

• Quantification of the extraframework Ti species (e.g. anatase, SiO2-TiO2 glassy phases, …)

• Determination of the maximum Ti content in MFI framework (2.5 atoms%)

G. Perego et al., Molecular Sieves –Science and Technology Vol. 1, 1998

Determination of the crystallinity

Useful for determining:

• the kinetics of crystallization of a given phase• the stability of a phase after thermal/hydrothermal treatments• the variations eventually occurred on zeolite catalysts

CR = [Σ(I)/Σ(I0)]·100

20 21 22 23 24 25

2-Theta [°]

20 21 22 23 24 25

2-Theta [°]

20 21 22 23 24 25

2-Theta [°]

Determination of the crystallinityThe method is easy to apply but:

• the crystallinity values are non absolute, being relative to the reference sample

• it may give wrong or unrealistic results if not correctly applied

In particular:

• the composition (framework and extraframework) of the reference and the unknown samples should be similar

• preferred orientation phenomena should be avoided

• the data collection strategy should be suitably selected

• the intensity data should be corrected for the decay of the intensity of the X-ray beam (measured by an external reference intensity standard)

If even one of these conditions is not respected, the If even one of these conditions is not respected, the results are meaninglessresults are meaningless

Determination of the crystallinity

Same framework structure

Different framework and extraframework composition

Different relative intensitiesof the reflections

Same extraframework composition

Different framework composition

Slightly different framework structure

Determination of the crystallite sizeThe term crystallitecrystallite is intended as coherent scattering domaincoherent scattering domain

It may not correspond to a geometrically well-defined particle, because it can be composed by two or more coherent scattering domains deriving from the presence of defects, fractures, …

Electron microscopy techniques (SEM, TEM) are useful for determining the particle size but, in many cases, the aggregation of the crystallites may render difficult the correct evaluation of the their size

1000 Å

Determination of the crystallite size

The breadth of a reflection is due to instrumental and sample factors

The instrumental breadthinstrumental breadth is that characterizing the reflections of the XRD pattern collected on infinite perfect crystals; it depends on the type and geometry of the diffractometer

The sample factorssample factors include: crystallite size, presence of defects (stacking faults, dislocations), microstrains due to the presence of inclusions incompatible with the crystalline lattice, the fluctuation of stoichiometry among different domains, surface relaxation typical of nanosized materials

If the breadth of the reflection is due to size effects only, the crystallite size D can be computed with the Scherrer equation:

D = K·λ/(β·cosθ)

where the constant K ~ 0.9, β is the FWHM of the reflection

Dhkl = 0.9·λ/(β·cosθhkl)

D is usually referred to a given hkl reflection:

It is common practice to consider the effective value of β as:

β = (B2 – b2)1/2

where B is the measured FWHM of the hkl reflection and b the corresponding instrumental breadth

In the case of zeolites, the presence of defects is probably the main cause affecting the correct evaluation of the crystallite size

The Scherrer equation is useful not for determining the absolute crystallite size but for evaluating its relative variations

1000 Å1000 Å

Crystallinity and crystallite sizeCase study: thermal stability of zeolite Beta catalystCase study: thermal stability of zeolite Beta catalyst

Polimeri Europa uses a zeolite Beta catalyst in its cumene and ethylbenzene technologies, based on the direct alkylation of benzene with propylene and ethylene, respectively

It is important to determine the thermal stability (in terms of loss of crystallinity and framework dealumination) of the catalyst for better defining the regeneration conditions

POLYMORPH A POLYMORPH B

Tetragonal, P4122a 12.5, c 26.4 Å

Monoclinic, C2/ca b 12.5√2, c 14.4 Å

114°J.M. Newsam et al., Proc. R. Soc. London, 1988.

The zeolite Beta structure

Newsam et al., Proc. R. Soc. London A 420 (1988) 375.

Polymorph A 50%

Polymorph B 50%

The zeolite Beta structure

[1] Perez-Pariente et al., Appl. Catal. 31 (1987) 35; J. Catal. 124 (1990) 217.

[2] Liu et at., J. Catal. 132 (1991) 432.

Thermal stability of zeolite Beta is controversial:

Tmax 550°C [1]

Tmax < 760°C with limited dealumination and structural collapse [2]

HH++-BETA-BETAHH++-BETA-BETA

CharacterizationCharacterizationCharacterizationCharacterization

650°C650°C650°C650°C 750°C750°C750°C750°C 850°C850°C850°C850°C 900°C900°C900°C900°C

Thermal stability of zeolite Beta

Calcinations: 5 hrs in air

Thermal stability of zeolite Beta

008 600

Complete breakdown of the structure: > 850°C

Loss of crystallinity: < 20% at 850°C

Progressive decrease of the average crystallite size, more pronounced when computed on the sharp 008 reflection

Is it really a size effect?Is it really a size effect?R. Millini et al., Proc. 14th IZC, 2004

Thermal stability of zeolite BetaAssessing effective framework compositionAssessing effective framework composition

Vx = VSi – VSi[1 – (dAl3/dSi

Vx = 4076.8 Å3 (experimental)

dAl = 1.75 Å

dSi = 1.61 Å

x = 0.071 (from NH3 titration, 0.091 from elemental analysis)

VSi = 3996 Å3

Indices of sharp reflections according to the tetragonal model

of polymorph A

Thermal stability of zeolite BetaAssessing effective framework compositionAssessing effective framework composition

Known VSi, dAl and dSi, from the experimental Vx value the x molar

fraction of Al in the zeolite Beta framework is computed

Elementalanalysis

NH3 titration

The progressive dealumination of the framework produces The progressive dealumination of the framework produces structural defects, which also contribute to the broadening of structural defects, which also contribute to the broadening of

the reflectionsthe reflections

Conclusive remarks

XRD techniques are very powerful, allowing the accurate structural characterization of polycrystalline samples to be performed

As for all the other analytical, spectroscopic, …, techniques the achievement of reliable results depends both on the skills of the researcher and on the availability of high quality experimental data

Standard laboratory instruments are sufficient for solving most of the structural problems

The achievement of reliable results strongly depends on the accurate setup of the diffractometer, on the appropriate preparation of the sample and on the use of the most suitable data collection strategy

DON’T WASTE YOUR TIME ON BAD DATA

Structure determinationThe knowledge of the crystal structure of a material is fundamental

for understanding and even predicting its properties

Usually determined by single crystal X-ray diffraction, if specimens of suitable dimensions (> 50 μm for standard laboratory diffractometers, > 5 μm when operating with synchrotron radiation) are available Zeolites usually crystallize in form of powder composed by very small crystallites, even with submicronic dimensions

X-ray powder diffraction data only are available

2 m 100 nm

Structure determination from XRD data

Reciprocal spacemethods

Direct spacemethods

All require chemical and basic structural information:

UC parameters and space group

Chemical composition (elemental analysis)

Framework density (helium pycnometry)

Tetrahedra per UC (n = (V ρ)/(Mw 1.6603))

Independent T-atoms (e.g. 29Si MAS NMR)

Structure determination from XRD dataBasic information: the case of ERS-7Basic information: the case of ERS-7

Chemical composition:

Na0.04R0.08(Si0.89Al0.11)O2

Total density: 2.04 g·cm-3

R + H2O = 15.5 wt% (TGA)

Na = 1.2 wt% (AA)

Density: 1.70 g·cm-3

Unit cell volume: 2821 Å3

48.1 T-sites/unit cell

6 to 12 independent T-sites

Primitive orthorhombic cell

a = 9.81, b = 12.50, c = 23.01 Å

Space group: Pna21 or Pnma

No significant SHG signal suggests Pnma

5 10 15 20 25 30 35 40

2-Theta [°]

= 1.1528 Å

INDEXING (TREOR90)

Structure determination from XRD dataReciprocal space methodsReciprocal space methods

The methods are those used for structure solution from single crystal X-ray diffraction data: Patterson function, heavy-atom method, isomorphous replacement, anomalous dispersion, direct methods

The intensities of all the reflections in the XRD pattern are extracted by using automatic profile fitting programs and the structure factors are calculated and used as input data for structure solution programs

The main problems of these approaches (very successful for single crystal data) are related to:

• the uncertainties in the intensity values when severe overlapping of the reflections occurs

• the data set is considerably smaller than that obtained from single crystal

Structure determination from XRD dataReciprocal space methodsReciprocal space methods

The direct methods approach was used for determining the structure of some zeolites, including, for instance:

ITQ-12 (ITW): C2/m, 3 T-atoms, V = 1354 Å3

Yang X.B. et al. J. Am. Chem. Soc., 126, 10409 (2004)

ITQ-22 (IWW): Pbam, 16-T atoms, V = 6737 Å3

Corma A. et al. Nature Materials, 2, 493 (2003)

MCM-35 (MTF): C2/m, 6-T atoms, V = 2121 Å3

Barrett P.A. et al. Chem. Mater., 11, 2919 (1999)

The wider application of the reciprocal space methods is somewhat limited by the complexity of the XRD pattern

Structure determination from XRD dataDirect space methodsDirect space methods

When the classical crystallographic approaches fail, a starting structural model has to be built up by:

the identification of an isostructural material with known crystal structure (es. EMS-2, the synthetic Sn-counterpart of natrolemoynite, a rare microporous zirconium silicate)

the use of difference Fourier methods to investigate derivatives of known phases(location of adsorbed molecules in zeolite pores)

model building (trial & error)(es. UMZ-5 (UFI), SSZ-59 (SFN), MCM-22 (MWW), …)

computer modeling techniques(automated model building schemes, such as simulated annealing or tempering, global optimization algorithms, FOCUS, …)

Energy (keV)

X-ray are produced through two different mechanisms:

1. Bremsstrahlung (braking radiation)

Emax = Ee-

X-ray are produced through two different mechanisms:

2. Characteristic X-ray radiation

Energy (keV)

.) KβKα

80 - 90%

10 - 20%

Production of X-raysThe K The K spectrumspectrum of Cu of Cu

K(1s) 8979

L1(2s)

L2(2p1/2)L3(2p3/2)

M1(3s)

M2(3p)

M3(3d)

952933

0 (eV)

Kα1 Kα2 Kβ

E = c·h/λ

Kα1 = 1.54056 Å

Kα2 = 1.54439 Å

Kβ = 1.39222 Å

Fundamental crystallographic data

• Unit Cell (UC): the smallest part of the crystal which maintains the properties of the crystal itself; the entire crystal can be constructed by translating the UC along the three directions. It is defined by the unit cell parameters: the lengths of the sides [a, b, c] and the angles [α, β, γ]

• Crystal system

• Space group

2 nd FEZA School Paris, 1-2 September, 2008

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