N I S T Center for Neutron Research
Accomplishments and Opportunities 2001
E T S - 4: Maxwell's Zeolite
Scottish physicist James Clerk Maxwell (1831–1879) discussed a mythical device that could separate molecules by temperature, a concept now known as Maxwell’s demon. Maxwell used this idea to explore how sorting molecules by size would violate the second law of thermodynamics. If one did have a Maxwell’s demon, a more useful task for it would likely be to separate molecules by size. This would allow mixtures to be separated into pure substances, and would facilitate removal of dangerous contaminants from valuable materials. In some cases, molecular separations can be performed using zeolites and similar materials. Zeolites are composed of linked tetrahedral SiO4 and AlO4 species, arranged in rings to create a framework of molecule-sized cavities. Related zeolitic materials may use other tetrahedral atoms to build the framework. Molecules can access these cavities or pores, under two conditions: if they can pass through the appropriate rings in the framework and these rings are not blocked by other species in the material, such as cations. If the ring size in a material matches the needs for a particular chemical separation, the material may be appropriate. Chemical modifications, such as ion exchange or changing the framework composition can sometimes influence the pore size. So, occasionally it is possible to modify a zeolite to a particular separation. However, this does not allow what Maxwell might envision, where the pore size can be adjusted over a wide range to suit the desired process, in effect a Maxwell’s demon for size separations. This exact capability has now been demonstrated in material E T S - 4 (Refer to Reference 1). E T S - 4 can be processed in such a way so that the pore sizes can be tuned for particular separations.
Graphics Caption FIGURE 1. Size of the eight-ring pore opening in E T S - 4 as a function of dehydration temperature.
Graphics Caption TABLE 1. Eight-ring opening as a function of dehydration temperature. Distances are the pore-openings separating van der Waals radii between opposite O5 (D1), O1 (D2) and O2 (D3) atom pairs (Refer to FIGURE 2).
Graphics Caption FIGURE 2. Atom and pore-opening distance labeling scheme in the eight-ring opening of E T S - 4.
Graphics Caption FIGURE 3. Adsorption of selected molecules as a function of dehydration temperature.
E T S - 4 is a titanosilicate material invented by Engelhard in the 1980s (Refer to Reference 2). E T S - 4 differs from most zeolitic materials in a number of ways. Chemically, it is an oxide of Ti and Si, while the majority of zeolitic materials are oxides of Al and Si. Structurally, the material is composed of both tetrahedral TiO4 and octahedral TiO6 units, as well as SiO4 units. Finally, most zeolitic materials hydrate and dehydrate reversibly or are destroyed during dehydration. E T S - 4, at least in some ion-exchanged forms, can be induced to dehydrate irreversibly and without destruction of the pores. Dehydration reduces the average pore opening size. Thus it is possible to tune the zeolite to accept molecules of a particular size by changing the level of dehydration. The temperature to which the material is exposed in turn dictates the amount of dehydration. In 1999, the Advanced Technology Program funded Engelhard to exploit this effect, which Engelhard calls the Molecular Gate®, to develop methods for separating oxygen from air, a process with useful applications for prevention of pollution as well as medical and other industrial applications.
The mechanism behind the Molecular Gate® effect was demonstrated crystallographically using data from the B T - 1 neutron powder diffractometer. Pores in E T S - 4 must be accessed via a ring of eight Si and Ti atoms, each linked by oxygen atoms. This ring is commonly called an eight-ring, despite the fact that it is actually composed of sixteen atoms. In Figure 1, the size of this eight-ring is shown as a function of dehydration temperature. The actual size of molecules that can be admitted via this opening are dictated by the van der Waals radii of the O atoms in the eight-ring and the van der Waals dimensions of the molecule to be admitted. Since accurate determination of oxygen atom siting is needed, and since E T S - 4 does not form single crystals, neutron powder diffraction measurements are the only way to obtain accurate measurements of the ring openings as a function of dehydration temperature. Measurements of the eight-ring opening from Rietveld refinements are shown in Table 1, relative to the distances diagrammed in Figure 2.
The proof that dehydration indeed does change the pore access is demonstrated by adsorption isotherms, shown in Figure 3. The separation shown in the top of this figure, between O2 and N2, shows how precisely the pore sizes can be adjusted. The difference in van der Waals radius between oxygen and nitrogen is only 0.1 Å.
References
[1] S. M. Kuznicki, V. A. Bell, S. Nair, H. W. Hillhouse,
R. M. Jacubinas, C. M. Braunbarth, B. H. Toby, and M. Tsapatsis,
Nature 412, 720 (2001).
[2] S. M. Kuznicki, US Patent 4938939 (1990).
Authors
S. M. Kuznicki, V. A. Bell
and R. M. Jacubinas
Strategic Technology Group
Engelhard Corporation
Iselin, NJ 08830
S. Nair, H. W. Hillhouse,
C. M. Braunbarth and M. Tsapatsis
Department of Chemical Engineering,
University of Massachusetts
Amherst, MA 01003
B. H. Toby
N I S T Center for Neutron Research
National Institute of Standards and Technology
Gaithersburg, MD 20899-8562