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

Abstract

The identification of inorganic materials, which are able to encapsulate environmentally important small molecules or ions via host–guest interactions, is crucial for the design and development of next-generation energy sources and for storing environmental waste. Especially sought after are molecular sponges with the ability to incorporate CO₂, gas pollutants, or nuclear waste materials such as UO₂ and PuO₂ oxides or U, Pu, Sr²⁺, or Cs⁺ ions. Porous framework structures promise very attractive prospects for applications in environmental technologies, if they are able to incorporate CH₄ for biogas energy applications or to store H₂, which is important for fuel cells, e.g., in the automotive industry. All of these applications should benefit from the host being resistant to extreme conditions such as heat, nuclear radiation, rapid gas expansion, or wear and tear from heavy gas cycling. As inorganic tungstates are well known for their thermal stability and their rigid open-framework networks, the potential of Na₂O–Al₂O₃–WO₃ and Na₂O–WO₃ phases for such applications was evaluated. To this end, all known experimentally determined crystal structures with the stoichiometric formula M_aM′_bW_cO_d (M = any element) are surveyed together with all corresponding theoretically calculated Na_aAl_bW_cO_d and Na_<i>x</i>W_<i>y</i>O_<i>z</i> structures that are statistically likely to form. Network descriptors that categorize these host structures are used to reveal topological patterns in the hosts, including the nature of porous cages, which are able to accommodate a certain type of guest; this leads to the classification of preferential structure types for a given environmental storage application. Crystal structures of two new tungstates NaAlW₂O₈ (<b>1</b>) and NaAlW₃O₁₁ (<b>2</b>) and one updated structure determination of Na₂W₂O₇ (<b>3</b>) are also presented from in-house X-ray diffraction studies, and their potential merits for environmental applications are assessed against those of this larger data-sourced survey. Overall, results show that tungstate structures with three-nodal topologies are most frequently able to accommodate CH₄ or H₂, while CO₂ appears to be captured by a wide range of nodal structure types. The computationally generated host structures appear systematically smaller than the experimentally determined structures. For the structures of <b>1</b> and <b>2</b>, potential applications in nuclear waste storage seem feasible