# Presentation: 071063

## Structural Refinement of Disordered Zeolites using the PDF Method

**María Martínez, Raúl F. Lobo**
**Department of Chemical Engineering**
**University of Delaware**
**Inmaculada Peral**
**Center for Neutron Research**
**NIST**
**Thomas Proffen**
**Los Alamos Neutron Science Center**
**LANL**
**ACNS**
**June 6, 2004**

## Zeolites

**C****rystalline aluminosilicates** with a 4-connected **tetrahedral framework** structure enclosing **cavities **occupied by **large ions and water **molecules, both of which have considerable freedom of movement, permitting ion exchange and reversible dehydration (Smith, 1988)
**Most important class of solid acids**
**Important Environmental Applications**
**Catalytic Converters (Diesel and Gasoline)**
**DeNOx Catalyst in Power Plants**
**VOC Removal**
**Structure-property relation require detailed atomic information**

## Zeolite Mordenite

## Why so much interest in Zeolites?

**Well-defined molecular structure**
- Establish structure-property relationships
- Prediction of properties from structure
**BUT...**
*Disorder is an inescapable  fact on zeolitic materials*
- Structure-Property relations are not always simple

## Disorder in Zeolites

**Framework Substitution**
- Other atoms besides Si
**Extra-Framework Cations**
- Partial occupancies, multiple cations
**Adsorbed Species**
- No translational symmetry
- Lower symmetry than framework
**Framework Disorder**
- Static
- Dynamic
**Opportunity for the PDF method**

## Previous Application of PDF Method to Zeolites

- Cs atoms in Siliceous Zeolites
- Petkov, V. et al., PRL, 2002
- Coordination of CHCl3 to Acid Sites
- Eckert, J. et al, JACS, 2002

## Static Disorder

**Two-Dimensional Translational Symmetry**
- Disordered stacking of ordered layers
- ABC-family of Zeolites
**One-Dimensional Translational Symmetry**
- Disordered arrangement of ordered ‘tubes’
- ZSM-48, SSZ-31 and others
**Zero-Dimensions**
***Description must be Statistical***

## Building ZSM-48 Using 1D Units

## ZSM-48: Experiment and Simulation

- Experiment
- Anis. Broadening
- Isotropic Broad

-  =0.2 =0.1 =0.8

## Objectives

- Determine the suitability of the PDF method to investigate the local structure in selected zeolite systems
- Use zeolite beta as a test case
- Zeolite beta:
- 9 Si and 16 O atoms in asymmetric unit

## Zeolite Beta

- Important alkylation catalyst
- 12-ring (~7.5 Å) 3-D Pore system
- Intergrowth of two polytypes

**Polytype A**

**Polytype B**

## Polytype B

## XRD of Beta

**Simulations**
**with DiFFAX**

## Periodic Building Unit of Beta

- Basic building element is the Periodic Building UnitPerBU
- Both polytes A and B can be described from the same PerBU

## 29Si NMR Spectra of Zeolite Beta

- Siliceous Beta (Reported)

- Camblor et al. Chem.Comm. 1996

- Highly resolved spectra are obtained
- 9-T sites are distinguished
- PerBU of Polytypes A and B should be the same

## Sample Preparation

- From B-Beta
- B removed in acid media
- T-vacancies are not healed
- From Zn-Beta
- Highly hydrophobic sample
- No defects (by NMR)
- Preferred sample

## 29Si NMR Spectra of Zeolite Beta

- Siliceous Beta (Reported)

- Our Sample

- Camblor et al. Chem.Comm. 1996

## Scattering Measurements

- Neutron PDF
- GLAD, IPNS (good statistics 20 Å-1)
- NPD, LANL (good statistics up to 30 Å -1)
- X-ray PDF
- APS, =0.1573 Å
- Data treatment with ATLAS, PDFGetX and PDFGetN
- Refinement with PDFFit

## Simulated PDF of polytypes of Zeolite Beta (as reported)

## NPDF Beta: Energy Minimization

- After energy minimization (GULP) the PDF of both polytypes is nearly identical

- Comparison of experimental measurement and minimized structure match well

## NPDF vs. XPDF

- NPDF was not sufficient
- XPDF is better but gives distorted tetrahedra
- Need to use scattering length ratios at Q=4.4 Å-1fSi/fO=2.2 gives good fit of first two peaks

## Progress of RefinementPolytype A

- Comparison of starting and final models after refinement
- Initial Rw=33.7%
- Final Rw=20.3%

## Refinement of Polytype B

- Comparison of starting and final models after refinement
- Initial Rw=30% (XPDF)
- Final Rw=20.0 %
- Final Rw=20.8% (NPDF)

## Refinement of the NPDF

## Refined bond distances

|  |  | <Si-O> (Å) |  (Å) | <O-Si-O>(o) |  (o) |
| --- | --- | --- | --- | --- | --- |
| Polytype A | Original | 1.616 | 0.0001 | 109.47 | 0.01 |
|  | Refined | 1.609 | 0.0164 | 109.43 | 3.91 |
| Polytype B | Original  | 1.616 | 0.0015 | 109.47 | 0.08 |
|  | Refined | 1.609 | 0.0240 | 109.41 | 3.82 |

## Conclusions

- The PDF method can be used for the refinement of the PerBUs of zeolite structures provided:
- Sample is compositionally simple
- Both Neutron and X-ray data sets are refined sequentially (or simultaneously)
- Complexity of the PerBU is about 9-T atoms or less

## Application of PDF to Dynamic Disorder

- Zeolite Chabazite
- Large negativethermal expansioncoefficient
- Diffraction suggestsSi-O-Si linkages arecontracting
- Others suggest rockingof rigid tetraherda

## Chabazite: RT Experiment and Model

## PDF of Chabazite

- Data consistent with rocking of rigid tetrahedra in sample
- Mechanim of contraction still unknown

## On going studies...

- Use Rigid-Unid-Modes (RUMs) to analyze the motion of tetrahedra in the zeolite
- Based on the RUMs, predict most stable configurations and determine if these are consistent with PDF data

## Acknowledgments

- APS, IPS, LANL
- Sarvjit Shastri
- Chris Benmore
- Valeri Petkov
- Thomas Proffen
- Sven Vogel
- NSF for funding