Group Members

About Dr. Clearfield

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Our research is grouped around several major themes; metal phosphonate chemistry, metal organic frameworks (MOF's), porous mixed oxides, solid state chemistry applied to nuclear waste remediation, and polymer nanocomposites.

Metal Phosphonates: 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, as given below for mixed oxides.

Ball and stick representation of the crystal structure of Cu₂ (O₃PC₆H₄OC₆H₄PO₃).

The CuO₅ square pyramids (shaded) share an edge and the rings are oriented at right angles to each other.

Selected References:

    A. Subbiah, D. Pyle, A. Rowland, J. Huang, R. A. Narayanan, P. Thiyagarajan, J. Zoń, and A. Clearfield; A Family of Microporous Materials Formed by Sn(IV)Phosphonate Nanoparticles, J. Amer. Chem. Soc. 127, 10826-10827 (2005).

    D. Kong, J. Zoń, and A. Clearfield; Crystal Engineered Three-Dimensional Hydrogen-Bonding Networks Built with 1,3,5-Benzenetri(phosphonic acid) and Bipyridine Synthons, Cryst. Growth & Design Vol. 5 (5), 1767-1773 (2005).

   M. M. Gómez-Alcántara, A. Cabeza, L. Moreno-Real, M.A.G. Aranda, and A. Clearfield; Microporous Aluminum Bisphosphonates, Micropor. Mesopor. Mater. 88, 293-303 (2006).

   D. Kong, J. Zon, J. McBee, A. Clearfield; Rational Design and Synthesis of Porous Organic-Inorganic Hybrid Frameworks Constructed by 1,3,5-Benzenetriphosphonic Acid and Pyridine Synthons, Inorg. Chem. 45, 977-986, (2006).

Mixed Oxides: Another group of porous materials are prepared from mixtures of oxides consisting of silica, alumina together with transition metal oxides.  The pores are controlled to be larger than those of zeolites, but smaller than mesoporous oxides.  Because these composites are amorphous, we are utilizing techniques such as small angle neutron scattering (SANS), extended X-ray absorption edge fine structure (EXAFS) and atomic pair distribution functions (PDF) and unusual NMR procedures to characterize these materials.  These compounds exhibit unusual magnetic phenomena and interesting catalytic behavior.

Nitrogen adsorption-desorption isotherms of porous ZnO-Al₂O₃-SiO₂ powders as a function of the n-alkylamine chain length.

Samples were calcined at 500°C for 9 hours.

Selected References:

  B.G. Shpeizer, V.I. Bakhmutov, A. Clearfield; Supermicroporous alumina-silica zinc oxides, Micropor. Mesopor. Mater. 90, 81-86 (2006).

  Vladimir Bakhmoutov, Boris G. Shpeizer and Abraham Clearfield; Solid-state NMR spectra of paramagnetic silica-based materials: observation of ²⁹Si and ²⁷Al nuclei in the first coordination spheres of manganese ions. Magn. Reson. Chem., 44, 861-867 (2006).

 Vladimir I. Bakhmutov, Boris G. Shpeizer and Abraham Clearfield; Structure of a paramagnetic supermicroporous silica-based material via a multinuclear solid-state NMR monitoring, Magn. Reson. Chem., 45, 118-122 (2007).

  A. R. Oki, Q. Xu, B. Shpeizer, A. Clearfield, X. Qiu, S. Kirumakki, Shane Tichy; Synthesis, characterization, and activity in cyclohexene epoxidation of mesoporous TiO₂-SiO₂ mixed oxides. Catalysis Communications 8, 950-956 (2007).

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⁺. 

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 (reproduced from ref. 11).

Selected References:

  A.J. Celestian, D.G. Medvedev, A. Tripathi, J.B. Parise, A. Clearfield; Optimizing synthesis of Na₂Ti₂SiO₇•2H₂O (Na-CST) and ion exchange pathways for Cs_0.4H_1.6Ti₂SiO₇•H₂O (Cs-CST) determined from in situ synchrotron X-ray powder diffraction, Nucl. Instrum. Methods. Phys. Res. B 238, 61-69 (2005).

  A. Clearfield, A. Tripathi, D. Medvedev, A.J. Celestian, J.B. Parise; In situ type study of hydrothermally prepared titanates and silicotitanates, J. Mater. Sci. 41, 1325-1333 (2006).

Polymer Nanocomposites: 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. 

  Transmission electron micrograph of α-Zirconium Phosphate/epoxy polymer at high magnification showing the dispersion of the layers throughout the polymer.

Selected References:

  J. Liu, W.-J. Boo, A. Clearfield and H.-J. Sue; Intercalation and Exfoliation: A Review on Morphology of Polymer Nanocomposites Reinforced by Inorganic Layer Structures, Mater. Manuf. Processes 20, 143-151 (2006).

  W.-J. Boo, L. Sun, G. L. Warren, E. Moghbelli, H. Pham, A. Clearfield, H.J. Sue; Effect of nanoplatelet aspect ratio on mechanical properties of epoxy nanocomposites., Science Direct, Polymer 48, 1075-1082 (2007).