Research in the Clearfield Group
Our research is grouped around several major themes: metal phosphonate chemistry, metal organic frameworks (MOF's), solid state chemistry applied to nuclear waste remediation, surface reactions of Zirconium Phosphate, and polymer nanocomposites.
Phosphonic acids have the general formula R-PO3H2 where R can be almost any organic moeity. They are more complex than carboxylic acids in that they have two replaceable protons and three oxygen coordination sites. We have produced dozens of phosphonates of mono-, di, tri- and tetravalent metals and systematized their overall chemistry. An important component of these metal phosphonates are those with porous framework structures (MOFs) prepared by hydrothermal and solvothermal methods. Structure solutions are obtained by both single crystal and powder X-ray diffraction methods. We are attempting to design compounds with uniform pore sizes and to impart special properties such as hydrophobic or hydrophilic character, ion exchange and complexing behavior, optical and catalytic activity. Many of these compounds are nanodimensional requiring a range of physical measurements to characterize them.
Above Left: Crystal structure of Cu2(O3PC6H4OC6H4PO3). The CuO5 square pyramids (shaded) share an edge and the rings are oriented at right angles to each other.
Above Right: A schematic drawing of how micropores may form in the metal biphenylbis(phosphonates). The horizontal boxes represent inorganic layers MO6 (M = Zr, Sn(IV)) which are crosslinked by the biphenyl groups. R = F- or OH-. The layers are thought to grow at different rates to produce pores.
Kirumakki, S.; Samarajeewa, S.; Harwell, R.; Mukherjee, A.; Herber, R.H.; Clearfield, A., Sn(IV) phosphonates as catalysts in solvent-free Baeyer-Villiger oxidations using H2O2. Chem. Commun. 5556-5558 (2008).
Clearfield, A., Unconventional metal organic frameworks: porous cross-linked phosphonates. Dalton Trans. 6089-6102 (2008).
Kirumakki, S.; Huang, J.; Subbiah, A.; Yao, J.; Rowland, A.; Smith, B.; Mukherjee, A.; Samarajeewa, S.; Clearfield, A., Tin(IV) phosphonates: porous nanoparticles and pillared materials. J. Mater. Chem. 19, 2593-2603 (2009).
Surface Functionalization of α-Zirconium Phosphate
Zirconium Phosphate, Zr(O3POH)2, is a layered compound in which the Zr ions are octahedrally coordinated by three oxygen atoms from each of the monohydrogen phosphate groups. The P-OH groups fill the interlayer space, forming a rectangular array on both sides of the layer. We have found that a large number of reactive molecules, such as silanes, epoxides, and PEGS, will bind to the phosphate surface. In this way we can functionalize the surface for drug delivery, catalysis, click chemistry, and other purposes. We can also utilize these techniques to synthesize Janus particles, which have different surface groups on opposite sides of the layer.
Sun, L.; O'reilly, J.Y.; Kong, D.; Su, J.Y.; Boo, W.J.; Sue, H.J.; Clearfield, A., The effect of guest molecular architecture and host crystallinity upon the mechanism of the intercalation reaction. J. Colloid Interface Sci. 333, 503-509 (2009).
Diaz, Agustin; David, Amanda; Perez, Riviam; Gonzalez, Millie L.; Baez, Adriana; Wark, Stacey E.; Zhang, Paul; Clearfield, Abraham; Colon, Jorge L. Nanoencapsulation of Insulin into Zirconium Phosphate for Oral Delivery Applications. Biomacromolecules (2010), 11(9), 2465-2470.
Nuclear Waste Remediation
One of the most compelling environmental problems facing the United States is the remediation of enormous stocks of nuclear waste that exists throughout the land. The most critical waste is that accumulated as a result of our nuclear weapons programs. This waste arose mainly as byproducts of the processes utilized for the separation of uranium and plutonium and is contained in large steel tanks at Hanford and Savannah River. The goal is to utilize sorbents that are resistant to either strong acids or bases to remove the highly radioactive species for immobilization in glass and storage underground. We are designing sorbents to remove radioactive Cs+, Sr2+ and some actinides from the several mixed wastes. The sorbent compounds are titanium silicates, titanates, pyrochlores and amorphous oxides. We are licensed to use several isotopes, Cs-137, Sr-90, U-238, but actinides are dealt with at Savannah River National Laboratory. To determine the mechanism of the ion exchange processes, in-situ studies are carried out at the Brookhaven synchrotron, NSLS. The origin and control of ion selectivity allows the targeted ions to be removed from mixtures very rich in Na+ and K+.
Above: Top view (down the c-axis) of sodium titanium silicate showing the clusters of four Ti-O6 octahedra (yellow) bridged by orange silicate groups with red oxygens. The tunnels are filled with Na+ (green) and water molecules (red). The green Na+ on top of the orange tetrahedra symbolizes the Na+ ions sandwiched between silicate groups within the framework.
Celestian, Aaron J.; Parise, John B.; Clearfield, Abraham. Crystal Growth and Ion Exchange in Titanium Silicates in Handbook of Crystal Growth, Springer-Verlag, Berlin 2010, pp1637-1662.
Clearfield, Abraham. Seizing the caesium. Nature Chemistry (2010), 2(3), 161-162.
Celestian, A.J.; Kubicki, J.D.; Hanson, J.; Clearfield, A.; Parise, J.B., The Mechanism Responsible for Extraordinary Cs Ion Selectivity in Crystalline Silicotitanate. J. Am. Chem. Soc. 130, 11689-11694 (2008).
A great body of research exists in which clays have been exfoliated into single layers and inserted into polymers. These nanocomposites exhibit physical properties such as increased modulus and reduced permeability. A major problem is the inability in many cases to make the clay compatible with the polymer.
Furthermore, the composition of clays is variable. Our idea is to synthesize layered materials that can be readily functionalized with organic groups so as to be pure, uniform and compatible with the polymer of choice. The electron micrograph shows the dispersion of a zirconium phosphate in an epoxy polymer. It is seen that the layered phosphate is well distributed throughout the polymer. We collaborate with Professor H.J. Sue, a polymer engineer in our mechanical engineering department in the construction of the polymer composites and measurement of their resultant physical properties.
Above: Transmission electron micrograph of α-Zirconium Phosphate/epoxy polymer at high magnification showing the dispersion of the layers throughout the polymer.
Liu, J.; Boo, W.J.; Clearfield, A.; Sue, H.J., Intercalation and Exfoliation: A Review on Morphology of Polymer Nanocomposites Reinforced by Inorganic Layer Structures. Mater. Manuf. Processes 21, 143-151 (2006).
Sun, L.; Liu, J.; Kirumakki, S.R.; Schwerdtfeger, E.D.; Howell, R.J.; Al-Bahily, K.; Miller, S.A.; Clearfield, A.; Sue, H.-J., Polypropylene Nanocomposites Based on Designed Synthetic Nanoplatelets. Chem. Mater. 21, 1154-1161 (2009).
Our work is supported by the National Science Foundation, the Department of Energy, and the Robert A. Welch Foundation.