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Topological Analysis of Void Space in Phosphate Frameworks: Assessing Storage Properties for the Environmentally Important Guest Molecules and Ions: CO2, H2O, UO2, PuO2, U, Pu, Sr2+, Cs+, CH4, and H2

Cramer, Alisha J., Cole, Jacqueline M.
ACS sustainable chemistry 2016 v.4 no.8 pp. 4094-4112
adsorbents, adsorption, alternative fuels, carbon dioxide, cesium, crystal structure, energy, hydrogen, industry, ions, methane, phosphates, sanitation, strontium, surveys, temperature, thermal stability, topology, wastes
The entrapment of environmentally important materials to enable containment of polluting wastes from industry or energy production, storage of alternative fuels, or water sanitation, is of vital and immediate importance. Many of these materials are small molecules or ions that can be encapsulated via their adsorption into framework structures to create a host–guest complex. This is an ever-growing field of study and, as such, the search for more suitable porous materials for environmental applications is fundamental to progress. However, many industrial areas that require the use of adsorbents are fraught with practical challenges, such as high temperatures, rapid gas expansion, radioactivity, or repetitive gas cycling, that the host material must withstand. Inorganic phosphates have a proven history of rigid structures, thermal stability, and are suspected to possess good resistance to radiation over geologic time scales. Furthermore, various experimental studies have established their ability to adsorb small molecules, such as water. In light of this, all known crystal structures of phosphate frameworks with meta- (P₃O₉) or ultra- (P₅O₁₄) stoichiometries are combined in a data-mining survey together with all theoretically possible structures of LnₐPbOc (where a, b, c are any integer, and Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, or Tm) that are statistically likely to form. Topological patterns within these framework structures are used to assess their suitability for hosting a variety of small guest molecules or ions that are important for environmental applications: CO₂, H₂O, UO₂, PuO₂, U, Pu, Sr²⁺, Cs⁺, CH₄, and H₂. A range of viable phosphate-based host–guest complexes are identified from this data-mining and pattern-based structural analysis. Therein, distinct topological preferences for hosting such guests are found, and metaphosphate stoichiometries are generally preferred over ultraphosphate configurations.