structural characterization of zeolites and related ... · eni refining & marketing division 1...

Eni Refining & Marketing Division 1

2nd FEZA SchoolParis, 1-2 September, 2008

Structural Characterization of Zeolites and Related 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, , in the region 0.01– 100 Å

In the electromagnetic spectrum, X-rays are placed between UVand 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

anode

heated W filament

electrons

Only 1% of the energyproduces X-rays!

99% is lost as heat


Photon Energy (keV)

In

ten

sit

y (

co

un

ts

10

3)

Kβλ = 0.184374 Å

Kα1

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 amongphotons and electrons: the photons change their direction withoutany 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 thephotons, the phase relationship becomes periodic and interferencediffraction effects are observed at various angles.

X-rays Interference


d

λ

X-ray diffractionThe Bragg’s law

A C

B

θ

D

The difference in path betweenthe waves scattered in B and D isequal to

AB+BC = 2dsinθ

If AB+BC is equal to a multiple ofλ, the two waves combinethemself with maximum positiveinterference; therefore:

nλ = 2dsinθ

the fundamental relationship incrystallography, known as Braggequation


X-ray diffractionsingle crystal vs. powder

X-rays


integration

X-ray powder diffraction (XRD)

XRD pattern


InstrumentationBragg-Brentano diffractometer

Ss1

s2DS

SP

RS

AS

D

S = X-ray source

DS = divergence slit

SP = sample

RS = receiving slit

D = detectorθ

2θ

SDS

SP

RS

DS = X-ray source

DS = divergence slit

SP = sample

RS = receiving slit

D = detector

AS = antidivergence slit

s1, s2 = Soller slits


The XRD pattern

Kα1

Kα2

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 phaseanalysis

Profile

InstrumentalSample

Crystallite sizeStressStrain


ZeolitesFramework 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-connectedatom, constitutes a framework type, which is defined by a 3-lettercode assigned by the IZA Structure Commission

“The 3-letter codes describe and define the network of the cornersharing tetrahedrally coordinated framework atoms … [and] shouldnot be confused or equated to actual materials.”

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

Several materials may possess the same framework type


ZeolitesPeculiar 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 organicspecies)

Each change of the basic structure produces a new material

All these phenomena induce the change of:

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

• the intensities of the reflections


SAMPLE

XRD characterization

INDEXINGIDENTIFICATION

FRAMEWORKCOMPOSITION

CRYSTALLINITY

CRYSTALLITESIZE

STRUCTUREDETERMINATION

STRUCTUREREFINEMENT

XRD NEW PHASE

KN

OW

N P

HA

SE


XRD characterizationPhase 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 IZAperiodically publishes the Atlas of the Zeolite Framework Types anda Collection of Simulated XRD Powder Patterns for Zeolites; all theinformation are available on the web (http://www.iza-structure.org/databases/), with the possibility to simulate the XRDpattern with custom-defined parameters

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

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


XRD characterizationThe 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-synthesized

NH4+-exchanged

intercalated with:quinuclidine

ethylenglycol

i-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 cell and of the symmetry

elements associated to one of the 230 space groups

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

dhkl = f(h, k, l, a, b, c, α, β, γ)

The form of the equation depends on the crystal system:

from the simple cubic system:

d*2hkl = (h2 + k2 + l2)a*2

… to the complex triclinic system:

d*2hkl =h2a*2+k2b*2+l2c*2+2hka*b*cosγ*+2hla*c*cosβ*+2klb*c*cosα*


Indexing the XRD patternThe 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 0.0052 3 1 1 7.36358 7.36584 -0.00226 12.009 12.005 0.0043 3 3 1 5.60587 5.60457 0.00130 15.795 15.799 -0.0044 5 1 1 4.70244 4.70150 0.00094 18.855 18.859 -0.0045 4 4 0 4.31898 4.31861 0.00037 20.547 20.549 -0.0026 6 2 0 3.86310 3.86268 0.00042 23.003 23.005 -0.0037 5 3 3 3.72596 3.72550 0.00046 23.862 23.865 -0.0038 5 5 1 3.42090 3.42085 0.00005 26.025 26.026 -0.0009 6 4 2 3.26457 3.26456 0.00001 27.295 27.295 -0.000

10 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 XRD

λ = 1.54178 Å

Synchrotron

λ = 1.1528 Å

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

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



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

The input is simple:

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

• the maximum UC volume (negative if allthe systems should be checked, otherwiseonly the cubic, tetragonal, orthorhombicand hexagonal are considered)

• the maximum β angle for monoclincsystem

• some specific input parameters if moreinformation are available from othersources



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

FOMs



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 patternProblems

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

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

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

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


Unit cell parameters refinement

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

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

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

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

properties of the material


Unit cell parameters refinement

Different analytical (e.g. Cs+-exchange) and spectroscopic (e.g. MASNMR, FT IR) techniques have been proposed but XRD proved to be, inmany 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)in the framework produces an expansion or a contraction of the UCparameters, depending on its size respect to Si (provided that nochanges of the T-O-T angles occur)


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

The computer programs based on this classical approach minimize thesum 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 numberof reflections

Output:

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

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


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

The method is easy and can be used even when the crystal structureof the phase under investigation is unknown; however, the reliabilityof the results depends on the complexity of the XRD pattern and onthe 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 2θvalues can be corrected by the Δ2θ shifts measured on the reflectionsof the standard


Unit cell parameters refinementFull-profile fitting methods

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

The goal of these methods is the reproduction of theexperimental XRD pattern through the appropriateparametrization and refinement of the structural andinstrumental parameters

On this concept is based the well known:

Rietveld Method


The Rietveld Method

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

• At the end of years 1970s, it was extended to the refinement ofXRD 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 Method

The refinement involves the variation of:

Scale factor

Instrumental parameters

(Wavelenght)

(Polarization)

Angular shift

Backgroundintensities

Peak-profilecoefficients

FWHM vs 2θ

Peakasymmetry

Structural parameters

a, b, c, α, β, γ

Atomiccoordinates

Siteoccupancy

Thermalfactors

Correction parameters

Primaryextinction

Surfaceadsorption

Preferredorientation

Sampledisplacement


The Rietveld MethodApplications

• Structure refinement

• Accurate determination of UC parameters

• Quantitative phase analysis (includingquantification of the amorphous phase)


The Rietveld MethodStructure refinement

Rough structural model required, produced by applying differentstrategies:

Direct methods, Patterson, …

Identification of an isostructural phase with knownstructure

Use of difference Fourier methods to investigate phases ofknown structure

Trial & Error methods

Computer modeling techniques



K

Na

W

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 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 relativelycomplex mixtures of crystalline phases

R. Millini, unpublished results


The Rietveld MethodDetermination of UC parameters

The application of the Rietveld Method is preferred when thedetermination of the UC parameters should be performed on complexXRD 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-optimalpreparation of the sample

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


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

MFI

B

BOR-CAcid Catalyst

Ti

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 UCparameters of samples with increasing B3+ content should beaccurately determined

PROBLEM

The XRD pattern of theorthorhombic MFI-type zeolites isvery complex (it contains 500+reflections below 50°2θ). Only afew single reflections can be usedfor the least-squares refinement ofthe UC parameters


Incorporation of B in MFI framework

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

Vx = VSi – VSi[1 – (dB3/dSi

3)]x

HYPOTHESIS

The contraction of the UC volume is dueonly to the smaller dimensions of the[BO4] tetrahedron respect to [SiO4] andno change of the T-O-T angles occurs:

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

Vx = 5345.5 – 1359.2x

M. 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-squaresfit on the interplanar spacings of selected reflections



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



The data produced by the least-squares fitting procedure arescattered from the regression curve, because the severe overlap ofsome reflections made difficult the accurate determination of the peakpositions

A significant improvement of the quality of the data are expected byapplying the Rietveld Method

• Low angular region excludedbecause of the high asymmetryof the reflections

• High angular region excludedbecause of the very lowintensitiy and the excessiveoverlap of the reflections

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



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

Vx = VSi – VSi[1 – (dTi3/dSi

3)]x

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-Obond length in zeolites),

Vx = 5339.4 + 2110.4x

dTi = 1.79 Å

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




The same method was applied on high-resolution synchrotron powderdiffraction patterns collected on samples treated at 400 K undervacuum and sealed in capillaries under vacuum

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

Laboratory data

Synchrotron data




• Determination of the Ti content in the framework with an accuracyof 2 – 3 %

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

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

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 [°]



The method is easy to apply but:

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

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

In particular:

• the composition (framework and extraframework) of the referenceand 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 intensityof the X-ray beam (measured by an external reference intensitystandard)

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



Same framework structure

but

Different framework and extraframework composition

Different relative intensitiesof the reflections

Same extraframework composition

but

Different framework composition

Slightly different framework structure


Determination of the crystallite size

The term crystallite is intended as coherent scattering domain

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

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

1000 Å



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

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

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

If the breadth of the reflection is due to size effects only, thecrystallite 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 thecorresponding instrumental breadth

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

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



1000 Å1000 Å


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

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

It is important to determine the thermal stability (in terms of loss ofcrystallinity and framework dealumination) of the catalyst for betterdefining 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]

H+-BETA

Characterization

650°C 750°C 850°C 900°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 pronouncedwhen computed on the sharp 008 reflection

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


Thermal stability of zeolite BetaAssessing effective framework composition

Vx = VSi – VSi[1 – (dAl3/dSi

3)]x

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 composition

Known VSi, dAl and dSi, from the experimental Vx value the x molarfraction of Al in the zeolite Beta framework is computed

Elementalanalysis

0.091

NH3 titration

0.071

0.068

0.065

0.056

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

the reflections


Conclusive remarks

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

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

Standard laboratory instruments are sufficient for solving most ofthe structural problems

The achievement of reliable results strongly depends on theaccurate setup of the diffractometer, on the appropriatepreparation of the sample and on the use of the most suitable datacollection strategy

DON’T WASTE YOUR TIME ON BAD DATA


Structure determination

The 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 ofsuitable 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 smallcrystallites, 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-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 methods

The methods are those used for structure solution from single crystalX-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 extractedby using automatic profile fitting programs and the structure factorsare calculated and used as input data for structure solution programs

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

• the uncertainties in the intensity values when severe overlappingof the reflections occurs

• the data set is considerably smaller than that obtained from singlecrystal


Structure determination from XRD dataReciprocal space methods

The direct methods approach was used for determining the structureof 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 somewhatlimited by the complexity of the XRD pattern

http://izasc.ethz.ch/fmi/xsl/IZA-SC/ftc_main_image.xsl?-db=Atlas_main&-lay=fw&STC=IWW&-find%20


Structure determination from XRD dataDirect space methods

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

the identification of an isostructural material with known crystalstructure(es. EMS-2, the synthetic Sn-counterpart of natrolemoynite, a raremicroporous zirconium silicate)

the use of difference Fourier methods to investigate derivatives ofknown 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 annealingor tempering, global optimization algorithms, FOCUS, …)


Energy (keV)

In

ten

sit

y (

a.u

.)


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)

In

ten

sit

y (

a.u

.)

KβKα

K

L

M

N

Kβ

Kα

80 - 90%

10 - 20%


Production of X-raysThe K spectrum of Cu

K(1s) 8979

L1(2s)

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

M1(3s)

M2(3p)

M3(3d)

1097

952933

122

76

0 (eV)

Kα1 Kα2 Kβ

E = c·h/λ

Kα1 = 1.54056 Å

Kα2 = 1.54439 Å

Kβ = 1.39222 Å


c

aO

b

βγ

α

Fundamental crystallographic data

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

• Crystal system

• Space group

structural characterization of zeolites and related ... · eni refining & marketing division 1...

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