Combining structure modeling and electron microscopy to determine complex zeolite framework structures.

Zeolites are a family of open-framework aluminosilicates that are widely used in fields as diverse as catalysis, adsorption, and ion exchange. Although all zeolite frameworks consist of tetrahedrally coordinated atoms such as Si and Al (T atoms) bridged by O atoms, the different ways in which they can be connected lead to zeolite frameworks with wide structural diversity. To fully exploit their unusual functions and features, it is essential that their structures be understood on an atomic level. They are typically prepared under hydrothermal conditions and usually crystallize in polycrystalline form (crystallites with a volume of less than 100 mm), so conventional single-crystal methods of structure analysis cannot be applied. In the past decades, many zeolite structures have been solved by using X-ray powder diffraction techniques, high-resolution transmission electron microscopy (HRTEM) data, and, very recently, a combination of both. An alternative approach to zeolite structure determination is to use computer-assisted model building based on prior knowledge such as chemical composition, symmetry, coordination geometry, and connectivity. The well-established T O distances, O-T-O angles, and TO-T angles are the fundamental constraints used in all structure-modeling procedures. However, with only the information on bonding geometry, complex zeolite structures, especially those with more than ten Tatoms in the asymmetric unit, have proved to be difficult to solve by modeling, because of the computational overhead caused by the large number of degrees of freedom. One way to tackle this problem is to introduce additional, experimentally derived, structural information into the computational modeling procedure. It seemed natural to concentrate on the porosity of a zeolite for this purpose, as that is the key structural difference between zeolites and other silicate framework materials. Furthermore, many studies have demonstrated that such information can easily be extracted from HRTEM images. Here we present a Monte Carlo model-building method for determining the structures of complex zeolite frameworks. In contrast to other model-building procedures, this approach uses structural information derived from one or more HRTEM images as an additional constraint in the computational modeling procedure. A term that evaluates the agreement between the structure models generated and the HRTEM image has been added to the cost function for structure simulation. To the best of our knowledge, this is the first time that HRTEM images have been used directly in real-space Monte Carlo simulation. Our results show that by using just one HRTEM image as a constraint, the number of degrees of freedom for solving the structure-modeling problem can be reduced to a practically manageable level, even for the most complex zeolite structures. The power of this approach is demonstrated with the solutions of the structures of IM-5 ([Si288O576]) and TNU-9 ([Si192O384]), the two most complex zeolite structures known. Zeolite IM-5 ([Si288O576]) was first synthesized in 1998, [17]

[1]  David Olson,et al.  Atlas of Zeolite Framework Types , 2007 .

[2]  S. Hovmöller,et al.  Structure of the Polycrystalline Zeolite Catalyst IM-5 Solved by Enhanced Charge Flipping , 2007, Science.

[3]  Scott M Woodley,et al.  Engineering microporous architectures: combining evolutionary algorithms with predefined exclusion zones. , 2007, Physical chemistry chemical physics : PCCP.

[4]  Christian Baerlocher,et al.  Complex zeolite structure solved by combining powder diffraction and electron microscopy , 2006, Nature.

[5]  C. Gilmore,et al.  Electron crystallography of zeolites--the MWW family as a test of direct 3D structure determination. , 2005, Acta crystallographica. Section A, Foundations of crystallography.

[6]  Jihong Yu,et al.  Design of Chiral Zeolite Frameworks with Specified Channels through Constrained Assembly of Atoms , 2005 .

[7]  Yi Li,et al.  Design of Zeolite Frameworks with Defined Pore Geometry through Constrained Assembly of Atoms , 2003 .

[8]  Kenji Hiraga,et al.  Framework Determination of a Polytype of Zeolite Beta by Using Electron Crystallography , 2002 .

[9]  L. McCusker,et al.  The application of structure envelopes in structure determination from powder diffraction data , 2002 .

[10]  O. Terasaki,et al.  Structural elucidation of microporous and mesoporous catalysts and molecular sieves by high-resolution electron microscopy. , 2001, Accounts of chemical research.

[11]  Mark E. Davis,et al.  Electron Diffraction Structure Solution of a Nanocrystalline Zeolite at Atomic Resolution , 1999 .

[12]  Lynne B. McCusker,et al.  Zeolite structure determination from powder diffraction data: applications of the FOCUS method , 1999 .

[13]  M. Deem,et al.  A biased Monte Carlo scheme for zeolite structure solution , 1998, cond-mat/9809085.

[14]  Lynne B. McCusker,et al.  Powder Diffraction Data and Crystal Chemical Information Combined in an Automated Structure Determination Procedure for Zeolites , 1997 .

[15]  L. McCusker,et al.  Using a structure envelope to facilitate structure solution from powder diffraction data , 1997 .

[16]  Michael W. Deem,et al.  Framework crystal structure solution by simulated annealing : test application to known zeolite structures , 1992 .

[17]  J. Pannetier,et al.  Prediction of crystal structures from crystal chemistry rules by simulated annealing , 1990, Nature.

[18]  J. M. Newsam,et al.  Determination of 4-connected framework crystal structures by simulated annealing , 1989, Nature.

[19]  C. D. Gelatt,et al.  Optimization by Simulated Annealing , 1983, Science.

[20]  J. Thomas,et al.  Zeolitic structures as revealed by high-resolution electron microscopy , 1980, Nature.

[21]  W. M. Meier,et al.  The topology of three-dimensional 4-connected nets: Classification of zeolite framework types using coordination sequences , 1979 .

[22]  A. Burton.  Structure solution of zeolites from powder diffraction data , 2004 .