Isostructural mesoporous ionic crystals as a tunable platform for acid catalysis

Polyoxocationic antimony oxide cluster with acidic protons

The success and continued expansion of research on metal-oxo clusters owe largely to their structural richness and wide range of functions. However, while most of them known to date are negatively charged polyoxometalates, there is only a handful of cationic ones, much less functional ones. Here, we show an all-inorganic hydroxyiodide [H_10.7Sb_32.1O₄₄][H_2.1Sb_2.1I₈O₆][Sb_0.76I₆]₂·25H₂O (HSbOI), forming a face-centered cubic structure with cationic Sb₃₂O₄₄ clusters and two types of anionic clusters in its interstitial spaces. Although it is submicrometer in size, electron diffraction tomography of HSbOI allowed the construction of the initial structural model, followed by powder Rietveld refinement to reach the final structure. The cationic cluster is characterized by the presence of acidic protons on its surface due to substantial Sb³⁺ deficiencies, which enables HSbOI to serve as an excellent solid acid catalyst. These results open up a frontier for the exploration and functionalization of cationic metal-oxo clusters containing heavy main group elements.

Oxygen Evolution Reaction Driven by Charge Transfer from a Cr Complex to Co-Containing Polyoxometalate in a Porous Ionic Crystal

Considerable efforts have been devoted to developing oxygen evolution reaction (OER) catalysts based on transition metal oxides. Polyoxometalates (POMs) can be regarded as model compounds of transition metal oxides, and cobalt-containing POMs (Co-POMs) have received significant interest as candidates. Nanocomposites based on Co-POMs have been reported to show high OER activities due to synergistic effects among the components; however, the role of each component is unclear due to its complex structure. Herein, we utilize porous ionic crystals (PICs) based on Co-POMs, which enable a composition-structure-function relationship to be established to understand the origin of the synergistic catalysis. Specifically, a Keggin-type POM [α-CoW₁₂O₄₀]^6- and a Cr complex [Cr₃O(OOCCH₂CN)₆(H₂O)₃]⁺ are implemented as PIC building blocks for the OER under nonbasic conditions. The potentially OER-active but highly soluble [α-CoW₁₂O₄₀]^6- was successfully anchored in the crystalline PIC matrix via Coulomb interactions and hydrogen bonding induced by polar cyano groups of the Cr complex. The PIC exhibits efficient and sustained OER catalytic activity, while each building block is inactive. The Tafel slope of the linear sweep voltammetry curve and the relatively large kinetic isotope effect value suggest that elementary steps closely related to the OER rate involve single-electron and proton transfer reactions. Electrochemical and spectroscopic studies clearly show that the synergistic catalysis originates from the charge transfer from the Cr complex to [α-CoW₁₂O₄₀]^6-; the increased electron density of [α-CoW₁₂O₄₀]^6- may increase its basicity and accelerate proton abstraction as well as enhance electron transfer to stabilize the reaction intermediates adsorbed on [α-CoW₁₂O₄₀]^6-.

Basicity of Isostructural Porous Ionic Crystals Composed of Nb/Ta-Substituted Keggin-Type Polyoxotungstates

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW12O40]5– (BW12), [SiW11NbO40]5– (SiW11Nb), and [SiW11TaO40]5– (SiW11Ta)) are synthesized. Experimental and theoretical characterizations...