Research Guides

Department of Chemistry University of Oxford

Dr Asel Sartbaeva

My principal research interests are flexibility in framework structures and interactions between this internal flexibility and interstitial atoms and molecules. These structures include zeolite frameworks, manganite and cuprite perovskite materials, and glasses. I use a combination of direct experimental investigation (neutron or X-ray diffraction with PDF and Rietveld analyses) and theoretical modelling and geometric simulation for the interpretation of results.

Zeolite work

Zeolites have the ability to act as catalysts for chemical reactions which take place within the internal cavities. Hydrogen-exchanged zeolites, whose framework-bound protons give rise to very high acidity, are exploited in many organic reactions, including crude oil cracking, isomerisation and fuel synthesis. Such industrial importance leads to a race for researchers to synthesize new frameworks with specific pore geometries. An interesting question in zeolites, which I have explored by geometric modeling, is whether the global (average) structure permits individual SiO4 units to be perfectly tetrahedral or whether local distortions are inevitable. This is very important because the local structures of pores and cages of zeolites may affect properties like catalytic reactions and diffusion. I found that all real (natural and synthesized) zeolites possess a unique property - a ‘flexibility window’ (Figure 1) - a range of densities over which the framework structure is perfectible (work done at Arizona State University with M.F. Thorpe). Hypothetical zeolite structures which possess such a window are potential candidates for synthesis. This is a significant criterion when searching among candidate hypothetical structures for synthesis.

Figure 1: Models of faujasite zeolite structure with a pure silica composition at different densities, viewed down the cubic 110 direction. FAU catalysts are used industrially for petroleum cracking. The upper half of the picture shows the rigid tetrahedral units, with small spheres representing the corner-sharing oxygen anions. The lower part shows the oxygen anions drawn with their nominal van der Waals radius of 1.35A. The animation shows the range of densities over which the tetrahedral units can, theoretically, be made geometrically perfect- the flexibility window.

This internal flexibility, which zeolites possess, has only recently started to be appreciated by the scientific comunity. My research is aimed, firstly, at understanding and, secondly, exploiting this flexibility for the purpose of storage of small molecules in zeolites. This can be applied for hydrogen storage, for example, where current hydrogen storage systems involve dangerous quantities of compressed gas.

Silicates

As a PhD student at Cambridge University (Supervisor – Prof. Simon Redfern), I worked on experimental determination of ionic transport through the structural domain walls in silicates. The effect of structure (which can be domain boundaries, impurities, structural defects or grain boundaries) and structural phase transitions on ionic diffusion is not well understood. I studied the effect of twin domain walls and structural phase transition on Li ion diffusion in silicates. I used neutron diffraction and Rietveld analysis for structure determination of beta-eucryptite (LiAlSiO4) and observed a cooperative motion of Li ions. 6Li and 7Li NMR measurements let me quantify Li motion and calculate the diffusion coefficients. Dielectric measurements helped me to observe the difference of ionic AC conductivity between the high-temperature (beta) quartz and low-temperature (alpha) quartz. My ion and electron microprobe data indicated that there was no difference in the amount of Li moved through high-temperature phase quartz (with no domain walls) and low-temperature phase (lots of domain walls) (Figure 2). My DC transport measurements showed a large increase in conductivity and transport at the quartz phase transition. Finally, geometric analysis helped me to reconcile all these data and conclude that thermal motion of oxygen atoms in the high-temperature phase and low-frequency motions of SiO4 tetrahedra at lower temperatures, where domain walls form, blurred the distinction between the domain walls and the bulk. This showed me for the first time, how internal flexibility of the structure affects the transport of atoms through the channels.

Figure 2: Geometric simulation of Li+ ion (purple) moving through a beta-quartz channel. Si atoms are blue, Al atom is brown and Oxygen atoms are red. As the ion moves through the channel the tetrahedra of the framework tilt and rotate to accommodate the motion.

Zeolites and guest molecules by simulations

The size and shape selectivity which zeolite molecular sieves exhibit toward organic molecules are central to their numerous applications. Of particular interest for me are the cases where the flexibility of the framework is necessary for motion of guest molecules through the channels and cages. This is significant not only for uptake and transport, but may also illuminate the role of structure directing agents in zeolite synthesis. Simulations which allow framework flexibility and retain steric detail may provide information that is not accessible by conventional MD simulation, in which zeolite frameworks are usually held rigid and guest molecules are represented with a united atom model. I propose to study guest molecule motion in zeolites using geometric simulation. This is already established as a successful simulation approach for silicate frameworks and also for flexible biomolecules. With this method I can obtain information on steric host-guest interactions and develop an understanding of the effect of framework flexibility on guest molecule motion and vice versa. It will allow me to perform a wide-ranging survey of different zeolite frameworks and guest molecules. Although the method is not thermodynamically detailed, it will provide information on the effect of geometry on relative mobilities. Another interesting question is where guest molecules induce local distortions of the framework. Most experimental and theoretical studies on zeolites are concerned with how guest molecules move through the framework, without looking at the changes in the framework. To fully understand the interactions between the guest molecule and the zeolite framework which give rise to the size and shape selectivity, with the ultimate goal of theoretically modeling these systems in a reliable manner, it is necessary to do a systematic experimental investigation of the host-guest complexes. I propose to combine this with theoretical modeling and fast simulation – rigidity analysis and geometric simulations. Of particular interest will be cases where the size of the guest molecule is comparable or even larger than the width of a pore aperture in the crystal structure; framework flexibility leading to variations in the size and shape of apertures, relative to the crystal structure, will then be critically important in determining mobility and uptake for the molecule. 

Seel A.G., Sartbaeva A., Rammirez-Cuesta A.J., Edwards P.P.`Inelastic neutron scattering of Na-zeolite A with in situ ammoniation: an examination of initial coordination' Physical Chemistry Chemical Physics, 12, 33, 9661-9666, 2010.

Sartbaeva A., Wells S.A., Sommariva M., Lodge M.M.J., Jones M.O., Ramirez-Cuesta A.J., Li G. and Edwards P.P. `Formation of crystalline Sodium Hydride nanoparticles encapsulated within an amorphous framework', Journal of Cluster Science, 21, 543-549, 2010.

Lei M., de Gra A.M.R., Thorpe M.F., Wells S.A., Sartbaeva A. `Uncovering the intrinsic geometry from the atomic pair distribution function of nanomaterials', Physical Review B, 80, 024118, 2009.


Gatta G.D., Sartbaeva A. and Wells S.A.`Compression behaviour and flexibility window of the analcime-like feldspathoids: experimental and theoretical ndings' European Journal of Mineralogy, 21, 571-580, 2009.

Sartbaeva A., Kuznetsov V., Wells S.A. and Edwards P.P. `Hydrogen nexus in a sustainable energy future', Energy and Environmental Science, 1, 79-85, 2008.


Sartbaeva A., Gatta G.D. and Wells S.A. `Flexibility window controls compression behavior in analcine zeolite framework' Europhysics Letters, 83, 26002, 2008.


Bozin E.S., Sartbaeva A., Zheng H., Wells S.A., Mitchell J.F., Pro en Th., Thorpe M.F. and Billinge S.J.L. `Structure of CaMnO3 in the range 10K-550K from neutron time-of-ight total scattering' Journal of Physics and Chemistry of Solids, 69, 2146-2150, 2008.

Sartbaeva A., Wells S.A., Thorpe M.F., Bozin E. and Billinge S.J.L. `Quadrupolar Ordering revealed from Scattering and Geometrical Modelling' Physical Review Letters, 99, 155503, 2007.


Sartbaeva A., Wells S.A., Huerta A. and Thorpe M.F. `Infuence of local structural variability on the Intermediate phase in network glasses' Physical Review B, 75, 224204, 2007.


Sartbaeva A., Wells S.A., Treacy M.M.J. and Thorpe M.F. `The fexibility window in zeolites' Nature Materials, 5, No. 12, 962-965, 2006. Highlighted in Nature Materials, 5, No. 12, 931-932, 2006.


Sartbaeva A., Wells S.A., Thorpe M.F., Bozin E. and Billinge S.J.L. `Geometric modeling of perovskite framework with Jahn-Teller distortions: application to the cubic manganites' Physical Review Letters, 97, 065501, 2006.