Download presentation

Presentation is loading. Please wait.

Published byNathanael Dede Modified about 1 year ago

1
Eni Refining & Marketing Division 1 2 nd FEZA School Paris, 1-2 September, 2008 Structural Characterization of Zeolites and Related Materials by X-Ray Powder Diffraction Roberto Millini, Stefano Zanardi

2
Eni Refining & Marketing Division 2 TOPICS X-RAYS X-RAY POWDER DIFFRACTION METHODS PHASE IDENTIFICATION PATTERN INDEXING UNIT CELL PARAMETERS REFINEMENT CRYSTALLINITY CRYSTALLITE SIZE CONCLUSIONS

3
Eni Refining & Marketing Division 3 What are X-rays? Electromagnetic radiation with wavelength,, in the region 0.01 – 100 Å In the electromagnetic spectrum, X-rays are placed between UV and γ-radiations RADIOMICROWAVEIR VISIBLE UVX-RAYγ-RAY 5·10 9 1· · ·10 7 λ (nm) 2.48· · ·10 -4 E (eV)

4
Eni Refining & Marketing Division 4 Production of X-rays HV source X-rays evacuated tube anode heated W filament electrons Only 1% of the energy produces X-rays! 99% is lost as heat

5
Eni Refining & Marketing Division 5 Photon Energy (keV) Intensity (counts 10 3 ) Kβ λ = Å Kα 1 λ = Å Bremsstrahlung (80 – 90%) Characteristic X-rays (10 – 20%) The X-ray spectrum of W E max = E e- (87 keV)

6
Eni Refining & Marketing Division 6 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-raysInterference

7
Eni Refining & Marketing Division 7 d λ X-ray diffraction The Bragg’s law AC B θ D 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

8
Eni Refining & Marketing Division 8 X-ray diffraction single crystal vs. powder X-rays

9
Eni Refining & Marketing Division 9 integration X-ray powder diffraction (XRD) XRD pattern

10
Eni Refining & Marketing Division 10 Instrumentation Bragg-Brentano diffractometer S s1 s2 DS SP RS AS D S = X-ray source DS = divergence slit SP = sample RS = receiving slit D = detector θ 2θ2θ S DS SP RS D S = X-ray source DS = divergence slit SP = sample RS = receiving slit D = detector AS = antidivergence slit s1, s2 = Soller slits

11
Eni Refining & Marketing Division 11 The XRD pattern Kα1Kα1 Kα2Kα2 peak anisotropy intensity I = k · L p · P · A · F 2 position 23.13° 2θ, d = Å

12
Eni Refining & Marketing Division 12 Information contained in the XRD pattern Background Scattering from sample-holder, air, … Amorphous phase, disorder, … Incoherent scattering (Compton, TDS, …) Sample

13
Eni Refining & Marketing Division 13 Information contained in the XRD pattern Position Lattice parameters Space group Qualitative phase analysis Phase purity Thermal expansion Compressibility Phase change Reflections Intensity Crystal structure: Atomic positions Occupancy Thermal factors Texture Crystallinity Quantitative phase analysis Profile InstrumentalSample Crystallite size Stress Strain

14
Eni Refining & Marketing Division 14 Zeolites Framework types vs. materials framework type Each open 4-connected 3D net, with (approximate) AB 2 composition, where A is a tetrahedrally connected atom and B is any 2-connected atom, constitutes a framework type, which is defined by a 3-letter code assigned by the IZA Structure Commission materials “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 materials.” “The framework types do not depend on composition, distribution of the T-atoms, cell dimensions or symmetry.” Several materials may possess the same framework type

15
Eni Refining & Marketing Division 15 Zeolites Peculiar 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 produces 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

16
Eni Refining & Marketing Division 16 SAMPLE XRD characterization INDEXINGIDENTIFICATION FRAMEWORK COMPOSITION CRYSTALLINITY CRYSTALLITE SIZE STRUCTURE DETERMINATION STRUCTURE REFINEMENT XRDNEW PHASE KNOWN PHASE

17
Eni Refining & Marketing Division 17 XRD characterization Phase 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/I 0 ·100) The XRD pattern is the fingerprint of the crystalline phase

18
Eni Refining & Marketing Division 18 XRD characterization Phase identification INPUT DATA A list of 2θ (or d) – relative intensities [(I/I 0 )·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 the Zeolite Framework Types Collection of Simulated XRD Powder Patterns for ZeolitesAtlas of Zeolite Framework Types: the Structure Commission of IZA periodically publishes the Atlas of the Zeolite Framework Types and a Collection 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 OF A NEW CRYSTALLINE PHASE

19
Eni Refining & Marketing Division 19 XRD characterization The PDF2 file

20
Eni Refining & Marketing Division 20 XRD characterization Phase identification 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 5.Identification of Phase 2 6.Identification of Phase 3 7.Identification of Phase 4

21
Eni Refining & Marketing Division 21 XRD characterization Phase identification The phase composition (framework and/or extraframework species) influences positions and relative intensities of the reflections, making sometimes difficult the automated phase identification

22
Eni Refining & Marketing Division 22 ERB-1 (B-containing MWW) XRD characterization Phase identification as-synthesized NH 4 + -exchanged intercalated with: quinuclidine ethylenglycol i-PrOH R. Millini et al., Microporous Mat., 1995

23
Eni Refining & Marketing Division 23 as-synthesized calcined ERB-1 (B-containing MWW) XRD characterization Phase identification R. Millini et al., Microporous Mater., 1995

24
Eni Refining & Marketing Division 24 Indexing the XRD pattern unit cell 230 space groups 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: d hkl = 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* 2 hkl = (h 2 + k 2 + l 2 )a* 2 … to the complex triclinic system: d* 2 hkl =h 2 a* 2 +k 2 b *2 +l 2 c* 2 +2hka*b*cosγ*+2hla*c*cosβ*+2klb*c*cosα*

25
Eni Refining & Marketing Division 25 Indexing the XRD pattern The cubic system h k l d(obs) d(calc) res(d) 2T.obs 2T.calc res(2T) a = (23) Å V = (41) Å 3 a = d hkl · (h 2 + k 2 + l 2 ) 1/2

26
Eni Refining & Marketing Division 26 Laboratory XRD λ = Å Synchrotron λ = Å Indexing the XRD pattern A lower symmetry case: ERS-7 (ESV) R. Millini et al., Proc. 12 th IZA, 1999

27
Eni Refining & Marketing Division 27 Indexing the XRD pattern A lower symmetry case: ERS-7 (ESV) TREOR The program TREOR 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

28
Eni Refining & Marketing Division 28 Indexing the XRD pattern A lower symmetry case: ERS-7 (ESV) 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

29
Eni Refining & Marketing Division 29 Indexing the XRD pattern A lower symmetry case: ERS-7 (ESV) 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+10k0:k = 2n+100l:l = 2n+1 hk0:h = 2n+10kl:k+l = 2n+1 possible space groups: Pn2 1 a or Pnma

30
Eni Refining & Marketing Division 30 Indexing the XRD patternProblems 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.

31
Eni Refining & Marketing Division 31 Unit cell parameters refinement The accurate determination of the UC parameters is important because they depend on the chemical composition of zeolites. In fact: wide Si/Al rangeZeolites can be synthesized in a wide Si/Al range or it can be modulated by post-synthesis treatments (e.g. dealumination by steaming) isomorphous substitutionThe framework composition can be varied by isomorphous substitution, 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 composition is important because from it depend the properties of the material

32
Eni Refining & Marketing Division 32 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) in the framework produces an expansion or a contraction of the UC parameters, depending on its size respect to Si (provided that no changes of the T-O-T angles occur)

33
Eni Refining & Marketing Division 33 Unit cell parameters refinement Least-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/d 2 = 4(sin 2 θ)/λ 2 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)

34
Eni Refining & Marketing Division 34 Unit cell parameters refinement Least-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., CuKα 1 /CuKα 2 ) 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

35
Eni Refining & Marketing Division 35 Unit cell parameters refinement Full-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 Method

36
Eni Refining & Marketing Division 36 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 = Σ i w i (Y iO – Y iC ) 2 where: Y iO and Y iC are the observed and calculated intensities at step i w i the weight assigned at each step and generally equal to 1/Y iO

37
Eni Refining & Marketing Division 37 The Rietveld Method The 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

38
Eni Refining & Marketing Division 38 The Rietveld MethodApplications Structure refinementStructure refinement Accurate determination of UC parametersAccurate determination of UC parameters Quantitative phase analysis (including quantification of the amorphous phase)Quantitative phase analysis (including quantification of the amorphous phase)

39
Eni Refining & Marketing Division 39 The Rietveld Method Structure 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

40
Eni Refining & Marketing Division 40 The Rietveld Method Structure refinement K Na W EMS-2: Na 2 K 2 Sn 2 Si 10 O 26 ·6H 2 O isostructural with the mineral natrolemoynite: Na 4 Zr 2 Si 10 O 26 ·9H 2 O S. Zanardi et al., Microporous Mesoporous Mater., 2007

41
Eni Refining & Marketing Division 41 The Rietveld Method Structure refinement Location of hexamethonium dications in EU-1 (EUO) Model built by molecular modeling R. Millini et al., Microporous Mesoporous Mater., 2001

42
Eni Refining & Marketing Division 42 The Rietveld Method Quantitative phase analysis PHASE Conc. (wt%) EXP.FOUND CaSO 4 ·2H 2 O CaSO 4 ·0.5H 2 O CaSO α-Al 2 O CaCO 3 (calcite)45 SiO 2 (quartz)42 CaC 2 O 4 ·H 2 O Standardless quantitative phase analysis is possible even on relatively complex mixtures of crystalline phases R. Millini, unpublished results

43
Eni Refining & Marketing Division 43 The Rietveld Method Determination 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., CuKα 1 /CuKα 2 ) 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

44
Eni Refining & Marketing Division 44 Unit cell parameters refinement Case study: assessing Ti and B incorporation in the silica framework MFI B BOR-C Acid Catalyst Ti TS-1 Oxidation 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

45
Eni Refining & Marketing Division 45 Incorporation of B in MFI framework The contraction of the UC parameters is expected when the small B 3+ ions are incorporated in the zeolite framework To unambiguously assess the incorporation of the heteroatom, the UC parameters of samples with increasing B 3+ 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

46
Eni Refining & Marketing Division 46 Incorporation of B in MFI framework The experiments confirmed that the UC parameters linearly decrease as the B 3+ content increases V x = V Si – V Si [1 – (d B 3 /d Si 3 )]x HYPOTHESIS The contraction of the UC volume is due only to the smaller dimensions of the [BO 4 ] tetrahedron respect to [SiO 4 ] and no change of the T-O-T angles occurs: V Si = Å 3, d Si = 1.61 Å (typical Si-O bond length in zeolites), d B = 1.46 Å (mean tetrahedral B-O bond length in the mineral reedmergnerite, NaBSi 3 O 8 ): V x = – x M. Taramasso et al., Proc. 5 th IZA, 1980

47
Eni Refining & Marketing Division 47 Incorporation of Ti in MFI framework The expansion of the UC parameters is expected when the large Ti 4+ 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

48
Eni Refining & Marketing Division 48 Incorporation of Ti in MFI framework G. Perego et al., Proc. 7 th IZA, 1987

49
Eni Refining & Marketing Division 49 Incorporation of Ti in MFI framework 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

50
Eni Refining & Marketing Division 50 Incorporation of Ti in MFI framework R. Millini et al., J. Catal., 1992

51
Eni Refining & Marketing Division 51 Incorporation of Ti in MFI framework The experiments confirmed that the UC parameters linearly increase as the Ti 4+ content increases V x = V Si – V Si [1 – (d Ti 3 /d Si 3 )]x The expansion of the UC volume is due only to the larger dimensions of the [TiO 4 ] tetrahedron respect to [SiO 4 ] and no change of the T-O-T angles occurs: V Si = Å 3, d Si = 1.61 Å (typical Si-O bond length in zeolites), V x = x d Ti = 1.79 Å Tetrahedral Ti-O bond lengths BaTiO 3 in the range 1.63 – 1.82 Å R. Millini et al., J. Catal., 1992

52
Eni Refining & Marketing Division 52 Incorporation of Ti in MFI framework The 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 R. Millini et al., J. Catal., 1992

53
Eni Refining & Marketing Division 53 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, SiO 2 - TiO 2 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

54
Eni Refining & Marketing Division 54 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(I 0 )]·100

55
Eni Refining & Marketing Division 55 Determination of the crystallinity The 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 results are meaningless

56
Eni Refining & Marketing Division 56 Determination of the crystallinity Same framework structure but Different framework and extraframework composition Different relative intensities of the reflections Same extraframework composition but Different framework composition Slightly different framework structure

57
Eni Refining & Marketing Division 57 Determination of the crystallite size crystallitecoherent scattering domain 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 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

58
Eni Refining & Marketing Division 58 Determination of the crystallite size The breadth of a reflection is due to instrumental and sample factors instrumental breadth The instrumental 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 sample factors The sample 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

59
Eni Refining & Marketing Division 59 Determination of the crystallite size D hkl = 0.9·λ/(β·cosθ hkl ) D is usually referred to a given hkl reflection: It is common practice to consider the effective value of β as: β = (B 2 – b 2 ) 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

60
Eni Refining & Marketing Division 60 Determination of the crystallite size 1000 Å

61
Eni Refining & Marketing Division 61 Crystallinity and crystallite size Case 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

62
Eni Refining & Marketing Division 62 POLYMORPH A POLYMORPH B Tetragonal, P a 12.5, c 26.4 Å Monoclinic, C2/c a b 12.5√2, c 14.4 Å 114° J.M. Newsam et al., Proc. R. Soc. London, The zeolite Beta structure

63
Eni Refining & Marketing Division 63 Newsam et al., Proc. R. Soc. London A 420 (1988) 375. Polymorph A 50% Polymorph B 50% The zeolite Beta structure

64
Eni Refining & Marketing Division 64 [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: T max 550°C [1] T max < 760°C with limited dealumination and structural collapse [2] H + -BETA CharacterizationCharacterization 650°C650°C750°C750°C850°C850°C900°C900°C Thermal stability of zeolite Beta Calcinations: 5 hrs in air

65
Eni Refining & Marketing Division 65 Thermal stability of zeolite Beta 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? R. Millini et al., Proc. 14 th IZC, 2004

66
Eni Refining & Marketing Division 66 Thermal stability of zeolite Beta Assessing effective framework composition V x = V Si – V Si [1 – (d Al 3 /d Si 3 )]x V x = Å 3 (experimental) d Al = 1.75 Å d Si = 1.61 Å x = (from NH 3 titration, from elemental analysis) V Si = 3996 Å 3 Indices of sharp reflections according to the tetragonal model of polymorph A

67
Eni Refining & Marketing Division 67 Thermal stability of zeolite Beta Assessing effective framework composition Known V Si, d Al and d Si, from the experimental V x value the x molar fraction of Al in the zeolite Beta framework is computed Elemental analysis NH 3 titration The progressive dealumination of the framework produces structural defects, which also contribute to the broadening of the reflections

68
Eni Refining & Marketing Division 68 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

69
Eni Refining & Marketing Division 69 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 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

70
Eni Refining & Marketing Division 70 Structure determination from XRD data Reciprocal space methods Direct space methods 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 )) Independent T-atoms (e.g. 29 Si MAS NMR)

71
Eni Refining & Marketing Division 71 Structure determination from XRD data Basic information: the case of ERS-7 Chemical composition: Na 0.04 R 0.08 (Si 0.89 Al 0.11 )O 2 Total density: 2.04 g·cm -3 R + H 2 O = 15.5 wt% (TGA) Na = 1.2 wt% (AA) Density: 1.70 g·cm -3 Unit cell volume: 2821 Å T-sites/unit cell 6 to 12 independent T-sites Primitive orthorhombic cell a = 9.81, b = 12.50, c = Å Space group: Pna2 1 or Pnma No significant SHG signal suggests Pnma Theta [°] = Å INDEXING (TREOR90)

72
Eni Refining & Marketing Division 72 Structure determination from XRD data Reciprocal 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

73
Eni Refining & Marketing Division 73 Structure determination from XRD data Reciprocal 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 Å 3 Yang X.B. et al. J. Am. Chem. Soc., 126, (2004) ITQ-22 (IWW): Pbam, 16-T atoms, V = 6737 Å 3 Corma A. et al. Nature Materials, 2, 493 (2003) MCM-35 (MTF): C2/m, 6-T atoms, V = 2121 Å 3 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

74
Eni Refining & Marketing Division 74 Structure determination from XRD data Direct 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, …)

75
Eni Refining & Marketing Division 75 Energy (keV) Intensity (a.u.) Production of X-rays X-ray are produced through two different mechanisms: 1. Bremsstrahlung (braking radiation) E max = E e-

76
Eni Refining & Marketing Division 76 Production of X-rays X-ray are produced through two different mechanisms: 2. Characteristic X-ray radiation Energy (keV) Intensity (a.u.) KβKβ KαKα K L M N KβKβ KαKα % %

77
Eni Refining & Marketing Division 77 Production of X-rays The K spectrum of Cu K(1s) 8979 L 1 (2s) L 2 (2p 1/2 ) L 3 (2p 3/2 ) M 1 (3s) M 2 (3p) M 3 (3d) (eV) Kα1Kα1 Kα2Kα2 KβKβ E = c·h/λ Kα 1 = Å Kα 2 = Å Kβ= Å

78
Eni Refining & Marketing Division 78 c a O b β γ α 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

Similar presentations

© 2016 SlidePlayer.com Inc.

All rights reserved.

Ads by Google