The understanding of structure–activity relationships at the atomic level has played a profound role in heterogeneous catalysis, providing valuable insights into designing suitable heterogeneous catalysts. However, uncovering the detailed roles of how such active species’ structures affect their catalytic performance remains a challenge owing to the lack of direct structural information on a specific active species. Herein, we deposited molybdenum­(VI), an active species in oxidation reactions, on the Zr6 node of a mesoporous zirconium-based metal–organic framework (MOF) NU-1200, using solvothermal deposition in MOFs (SIM). Due to the high crystallinity of the NU-1200 support, the precise structure of the resulting molybdenum catalyst, Mo-NU-1200, was characterized through single-crystal X-ray diffraction (SCXRD). Two distinct anchoring modes of the molybdenum species were observed: one mode (Mo1), displaying an octahedral geometry, coordinated to the node through one terminal oxygen atom and the other mode (Mo2) coordinated to two adjacent Zr6 node oxygen atoms in a tetrahedral geometry. To investigate the role of base in the catalytic activity of these Mo centers, we assessed the activity of Mo-NU-1200 for the aerobic oxidation of 4-methoxybenzyl alcohol as a model reaction. The results revealed that Mo-NU-1200 exhibited remarkably higher catalytic reactivity under base-free conditions, while the presence of base inhibited the catalytic reactivity of this species. SCXRD studies revealed that the molybdenum binding motifs (structures of the supported metal on the Zr6 node in the MOF) changed over the course of the reactions. Following the oxidation without base, both pristine coordination modes (Mo1 and Mo2) evolved into a new coordination mode (Mo3), in which the molybdenum atom coordinated to two adjacent oxygen atoms from the Zr6 node in an octahedral geometry, while in the presence of base, the pristine Mo1 coordination mode evolved entirely into the pristine Mo2. This study demonstrates the direct observation of an active species’ structural evolution from metal installation to subsequent catalytic reaction. As a result, these subtle structural changes in catalyst binding motifs led to distinct differences in catalytic activities, providing a compelling strategy for elucidating structure–activity relationships.

Solid supports are crucial in heterogeneous catalysis due to their profound effects on catalytic activity and selectivity. However, elucidating the specific effects arising from such supports remains challenging. We selected a series of metal− organic frameworks (MOFs) with 8-connected Zr 6 nodes as supports to deposit molybdenum(VI) onto to study the effects of pore environment and topology on the resulting Mo-supported catalysts. As characterized by X-ray absorption spectroscopy (XAS) and single-crystal X-ray diffraction (SCXRD), we modulated the chemical environments of the deposited Mo species. For Mo-NU-1000, the Mo species monodentately bound to the Zr 6 nodes were anchored in the microporous c-pore, but for Mo-NU-1008 they were bound in the mesopore of Mo-NU-1008. Both monodentate and bidentate modes were found in the mesopore of Mo-NU-1200. Cyclohexene epoxidation with H 2 O 2 was probed to evaluate the support effect on catalytic activity and to unveil the resulting structure−activity relationships. SCXRD and XAS studies demonstrated the atomically precise structural differences of the Mo binding motifs over the course of cyclohexene epoxidation. No apparent structural change was observed for Mo-NU-1000, whereas the monodentate mode of Mo species in Mo-NU-1008 and the monodentate and bidentate Mo species in Mo-NU-1200 evolved to a new bidentate mode bound between two adjacent oxygen atoms from the Zr 6 node. This work demonstrates the great advantage of using MOF supports for constructing heterogeneous catalysts with modulated chemical environments of an active species and elucidating structure−activity relationships in the resulting reactions.

Identifying mass transfer limitations is imperative for the practical application of nanoporous solids in adsorptive separations and catalysis. In particular, metal–organic frameworks (MOFs) with a staggering assortment of unique pore architectures and chemical binding sites are one class of materials where understanding structure–property relationships can facilitate material design. Here, we performed volumetric physisorption measurements and collected n-hexane adsorption isotherms of nine Zr-MOFs with unique pore architectures, textural properties, and crystal sizes. We collected measurements on a commercially available adsorption instrument and used a generalized mass transfer model that includes intracrystalline diffusion as well as a possible mass transfer resistance at the crystal boundary. The results indicate that uptake rates in all of the MOFs considered here are limited by mass transfer through crystallite surfaces. Moreover, the severity and guest concentration dependence of these surface resistances differ for each MOF. The identification of surface permeability as the rate-limiting mass transfer process within MOFs will aid the design of next-generation adsorbents and catalysts.

Developing energy-efficient alternatives for propylene (C 3 H 6 ) and propane (C 3 H 8 ) separation is of great significance and challenge in the petrochemical industry. Herein, we report the rational design of a new yttrium-based ultramicroporous metal-organic framework (MOF) comprised of 12-connected hexanuclear [Y 6 (OH) 8 (COO) 12 ] 2À cluster and 5-(3,5-dicarboxybenzoylamino)isophthalic acid (H 4 dbai) with ftw topology. It possesses a suitable pore window size and a relatively large pore volume for molecular sieving separation of C 3 H 8 from C 3 H 6 with a high C 3 H 6 capacity. At 298 K and 100 kPa, the adsorption capacity of C 3 H 6 was 2.57 mmol/g, which is the highest among the reported C 3 H 6 /C 3 H 8 molecular sieving MOF adsorbents. The molecular simulation revealed that the steric hindrance effect together with the electrostatic interaction of the oxygen sites in the window resulted in the molecular sieving separation of C 3 H 6 /C 3 H 8 . The breakthrough experiments confirmed its excellent separation performance under dynamic conditions to produce high purity (97.1%) of C 3 H 6 with a working adsorption capacity of 1.75 mmol/g.

Reticular chemistry allows for the rational assembly of metal−organic frameworks (MOFs) with designed structures and desirable functionalities for advanced applications. However, it remains challenging to construct multi-component MOFs with unprecedented complexity and control through insertion of secondary or ternary linkers. Herein, we demonstrate that a Zr-based MOF, NU-600 with a (4,6)-connected she topology, has been judiciously selected to employ a linker installation strategy to precisely insert two linear linkers with different lengths into two crystallographically distinct pockets in a onepot, de novo reaction. We reveal that the hydrolytic stability of these linker-inserted MOFs can be remarkably reinforced by increasing the Zr 6 node connectivity, while maintaining comparable water uptake capacity and pore-filling pressure as the pristine NU-600. Furthermore, introducing hydrophilic −OH groups into the linear linker backbones to construct multivariate MOFs can effectively shift the porefilling step to lower partial pressures. This methodology demonstrates a powerful strategy to reinforce the structural stability of other MOF frameworks by increasing the connectivity of metal nodes, capable of encouraging developments in fundamental sciences and practical applications.