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

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. TOPICS. X-RAYS X-RAY POWDER DIFFRACTION METHODS PHASE IDENTIFICATION PATTERN INDEXING

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

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

  2. TOPICS • X-RAYS • X-RAY POWDER DIFFRACTION • METHODS • PHASE IDENTIFICATION • PATTERN INDEXING • UNIT CELL PARAMETERS REFINEMENT • CRYSTALLINITY • CRYSTALLITE SIZE • CONCLUSIONS

  3. What are X-rays? VISIBLE RADIO MICROWAVE IR UV X-RAY γ-RAY λ (nm) 5·109 1·107 1·104 500 250 0.5 5·10-4 E (eV) 2.48·10-7 1.24·10-4 0.124 2.48 4.96 2480 2.48·106 Electromagnetic radiation with wavelength, , in the region 0.01 – 100 Å In the electromagnetic spectrum, X-rays are placed between UV and γ-radiations

  4. Production of X-rays evacuated tube heated W filament electrons anode - + + - HV source Only 1% of the energy produces X-rays! 99% is lost as heat X-rays

  5. The X-ray spectrum of W Intensity (counts  103) Emax = Ee- (87 keV) Photon Energy (keV) Kα1 λ = 0.2090100 Å Characteristic X-rays (10 – 20%) Kβ λ = 0.184374 Å Bremsstrahlung (80 – 90%)

  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-rays Interference

  7. X-ray diffraction The Bragg’s law λ D A C d θ B 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. X-ray diffraction single crystal vs. powder X-rays

  9. X-ray powder diffraction (XRD) XRD pattern integration

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

  11. The XRD pattern intensity I = k · Lp · P · A · F2 Kα1 peak anisotropy Kα2 position 23.13° 2θ, d = 3.845 Å

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

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

  14. Zeolites Framework 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 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 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. 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) • 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 Each change of the basic structure produces a new material

  16. XRD characterization XRD NEW PHASE SAMPLE INDEXING FRAMEWORK COMPOSITION IDENTIFICATION STRUCTURE DETERMINATION KNOWN PHASE STRUCTURE REFINEMENT CRYSTALLINITY CRYSTALLITE SIZE DATABASE

  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/I0·100) The XRD pattern is the fingerprint of the crystalline phase

  18. XRD characterization Phase identification 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 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. XRD characterization The PDF2 file

  20. XRD characterization Phase identification Automated search on PDF2 database of a complex mixture of zeolite phases • The XRD pattern • Definition of the background • Peak search • Identification of Phase 1 • Identification of Phase 2 • Identification of Phase 3 • Identification of Phase 4

  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. XRD characterization Phase identification ERB-1 (B-containing MWW) intercalated with: quinuclidine ethylenglycol i-PrOH NH4+-exchanged as-synthesized R. Millini et al., Microporous Mat., 1995

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

  24. 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*

  25. Indexing the XRD pattern The 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 -0.000 10 6 6 0 2.87871 2.87907 -0.00036 31.040 31.036 0.004 11 5 5 5 2.82066 2.82090 -0.00024 31.696 31.693 0.003 a = dhkl · (h2+ k2 + l2)1/2 a = 24.4297(23) Å V = 14579.9(41) Å3

  26. Indexing the XRD pattern A lower symmetry case: ERS-7 (ESV) Laboratory XRD λ = 1.54178 Å Synchrotron λ = 1.1528 Å R. Millini et al., Proc. 12th IZA, 1999

  27. Indexing the XRD pattern A lower symmetry case: ERS-7 (ESV) • 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. Indexing the XRD pattern A lower symmetry case: ERS-7 (ESV) FOMs 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

  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+1 0k0: k = 2n+1 00l: l = 2n+1 hk0: h = 2n+1 0kl: k+l = 2n+1 possible space groups: Pn21aorPnma

  30. Indexing the XRD pattern Problems • 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. 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 range or it can be modulated by post-synthesis treatments (e.g. dealumination by steaming) • The 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. 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. 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/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)

  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: • the geometry of the diffractometer is not accurately adjusted (angular shift) • the sample is not accurately prepared (sample displacement) • a non-strictly monochromatic radiation is used (e.g., CuKα1/CuKα2) • the reflections are affected by severe overlapping phenomena 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. 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. 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

  37. The Rietveld Method The refinement involves the variation of: Scale factor Instrumental parameters Structural parameters Correction parameters (Wavelenght) (Polarization) Angular shift Background intensities Peak-profile coefficients FWHM vs 2θ Peak asymmetry a, b, c, α, β, γ Atomic coordinates Site occupancy Thermal factors Primary extinction Surface adsorption Preferred orientation Sample displacement

  38. The Rietveld Method Applications • Structure refinement • Quantitative phase analysis (including quantification of the amorphous phase) • Accurate determination of UC parameters

  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. The Rietveld Method Structure refinement W K Na EMS-2 : Na2K2Sn2Si10O26·6H2O isostructural with the mineral natrolemoynite: Na4Zr2Si10O26·9H2O S. Zanardi et al., Microporous Mesoporous Mater., 2007

  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. The Rietveld Method Quantitative phase analysis Standardless quantitative phase analysis is possible even on relatively complex mixtures of crystalline phases R. Millini, unpublished results

  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 geometry of the diffractometer (angular shift) • moderate sample displacement deriving from a non-optimal preparation of the sample • the use of non-strictly monochromatic radiation (e.g., CuKα1/CuKα2) • severe overlapping phenomena of the reflections 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. Unit cell parameters refinement Case study: assessing Ti and B incorporation in the silica framework Ti B MFI TS-1 Oxidation Catalyst BOR-C Acid 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. 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

  46. Incorporation of B in MFI framework The experiments confirmed that the UC parameters linearly decrease as the B3+ content increases 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: Vx = VSi – VSi[1 – (dB3/dSi3)]x 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 – 1359.2x M. Taramasso et al., Proc. 5th IZA, 1980

  47. 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

  48. Incorporation of Ti in MFI framework G. Perego et al., Proc. 7th IZA, 1987

  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. Incorporation of Ti in MFI framework R. Millini et al., J. Catal., 1992

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