Xitong Chen

Cooperative Crystallization of Heterometallic Indium-Chromium Metal-Organic Polyhedra and Their Fast Proton Conductivity.

Metal-organic polyhedra (MOPs) or frameworks (MOFs) based on Cr(3+) are notoriously difficult to synthesize, especially as crystals large enough to be suitable for characterization of the structure or properties. It is now shown that the co-existence of In(3+) and Cr(3+) induces a rapid crystal growth of large single crystals of heterometallic In-Cr-MOPs with the [M8L12] (M=In/Cr, L=dinegative 4,5-imidazole-dicarboxylate) cubane-like structure. With a high concentration of protons from 12 carboxyl groups decorating every edge of the cube and an extensive H-bonded network between cubes and surrounding H2O molecules, the newly synthesized In-Cr-MOPs exhibit an exceptionally high proton conductivity (up to 5.8×10(-2) S cm(-1) at 22.5 °C and 98% relative humidity, single crystal).

Selective Ion Exchange and Photocatalysis by Zeolite-Like Semiconducting Chalcogenide

The development of novel photocatalysts usually centers on features such as band structures, various nano-, micro-, or macro-forms, and composites in efforts to tune their light absorption and charge separation efficiency. In comparison, the selectivity of photocatalysts with respect to features of reactants such as size and charge has received much less attention, in part due to the difficulty in designing semiconducting photocatalysts with uniform pore size. Here, we use crystalline porous chalcogenides as a platform to probe reactant selectivity in photocatalytic processes. The 3-in-1 integration of high surface area, uniform porosity, and favorable band structures in such chalcogenides makes them excellent candidates for efficient and selective photocatalytic processes. We show that their photocatalytic activity and selectivity are closely related to their differing affinity and selectivity for different guest species. In particular, unlike common solid-state photocatalysts with neutral framework, the anionic nature of the porous chalcogenide framework used here endows them with a high degree of selectivity for cationic species in both guest exchange and closely coupled photocatalytic transformation of such guests. Another interesting discovery is the observation of an unusual ion exchange process involving a transient state of over-saturation of exchanged ions, which can be explained by a transition from an initially kinetically controlled process to a subsequent thermodynamically controlled one. This work is part of ongoing efforts to contribute to the development of a new generation of crystalline porous photocatalysts with custom-designed selectivity for various reactants or products.

Cation-Exchanged Zeolitic Chalcogenides for CO2 Adsorption

We report here the intrinsic advantages of a special family of porous chalcogenides for CO2 adsorption in terms of high selectivity of CO2/N2, large uptake capacity, and robust structure due to their first-ever unique integration of the chalcogen-soft surface, high porosity, all-inorganic crystalline framework, and the tunable charge-to-volume ratio of exchangeable cations. Although tuning the CO2 adsorption properties via the type of exchangeable cations has been well-studied in oxides and MOFs, little is known about the effects of inorganic exchangeable cations in porous chalcogenides, in part because ion exchange in chalcogenides can be very sluggish and incomplete due to their soft character. We have demonstrated that, through a methodological change to progressively tune the host-guest interactions, both facile and nearly complete ion exchange can be accomplished. Herein, a series of cation-exchanged zeolitic chalcogenides (denoted as M@RWY) were studied for the first time for CO2 adsorption. Samples were prepared through a sequential ion-exchange strategy, and Cs+-, Rb+-, and K+-exchanged samples demonstrated excellent CO2 adsorption performance. Particularly, K@RWY has the superior CO2/N2 selectivity with the N2 adsorption even undetected at either 298 or 273 K. It also has the large uptake of 6.3 mmol/g (141 cm3/g) at 273 K and 1 atm with an isosteric heat of 35-41 kJ mol-1, the best among known porous chalcogenides. Moreover, it permits a facile regeneration and exhibits an excellent recyclability, as shown by the multicycling adsorption experiments. Notably, K@RWY also demonstrates a strong tolerance toward water.

Integrating Zeolite-Type Chalcogenide with Titanium Dioxide Nanowires for Enhanced Photoelectrochemical Activity

Developing photoanodes with efficient visible-light harvesting and excellent charge separation still remains a key challenge in photoelectrochemical water splitting. Here zeolite-type chalcogenide CPM-121 is integrated with TiO2 nanowires to form a heterostructured photoanode, in which crystalline CPM-121 particles serve as a visible light absorber and TiO2 nanowires serve as an electron conductor. Owing to the small band gap of chalcogenides, the hybrid electrode demonstrates obvious absorption in visible-light range. Electrochemical impedance spectroscopy (EIS) shows that electron transport in the hybrid electrode has been significantly facilitated due to the heterojunction formation. A >3-fold increase in photocurrent is observed on the hybrid electrode under visible-light illumination when it is used as a photoanode in a neutral electrolyte without sacrificial agents. This study opens up a new avenue to explore the potential applications of crystalline porous chalcogenide materials for solar-energy conversion in photoelectrochemistry.

CCDC 1827988: Experimental Crystal Structure Determination

Related Article: Xiang Zhao, Edward T. Nguyen, Anh Hong, Xitong Chen, Pingyun Feng, Xianhui Bu|2018|Angew.Chem.,Int.Ed.|57|7101|doi:10.1002/anie.201802911

Enabling Homochirality and Hydrothermal Stability in Zn4O-Based Porous Crystals

The [Zn4O]6+ cluster is well-known to form the archetypal MOF-5 topology with dicarboxylate ligands. Here we report two new materials (CPM-300 and -301) that show dramatic alteration of topological and chemical behaviors of [Zn4O]6+ clusters. In CPM-300, [Zn4O]6+ untypically forms the MIL-88/MOF-235 type framework with a small pentane-ring-based chiral dicarboxylate. In contrast, in CPM-301, when mediated by [Zn9(btz)12]6+ clusters (btz = benzotriazolate), the MOF-5 topology is regenerated with the same chiral ligand, albeit with alternating [Zn4O]6+ and [Zn9(btz)12]6+ clusters. Importantly, both CPM-300 and CPM-301 are homochiral, hydrothermally stable in boiling water and alcohol, and thermally stable to 440 °C or higher. It is concluded that small methyl groups on the chiral ligand is sufficiently powerful to shield [Zn4O]6+ clusters from degradation by water, even at high temperatures. These results reveal a promising platform for the development of a new class of cluster-based homochiral and hydrothermally stable porous materials.

Charge- and Size-Complementary Multimetal-Induced Morphology and Phase Control in Zeolite-Type Metal Chalcogenides

Zeolite-type chalcogenides are desirable due to their integration between porosity and semiconductivity. CPM-120, with super-sodalite topology (Zeolite Structure Code: RWY), is among the few zeolite-type chalcogenides with permanent porosity, and is the only chalcogenide with a zeolite code. Importantly, the RWY-type has evolved into a platform for studying properties of porous chalcogenides. Yet so far, few studies have been made to probe the effects of synthetic parameters and framework compositions on this platform. Here, we probe the effects of the third metal type (Ga3+ , In3+ , Cd2+ , and Sn4+ ) on the Zn2+ /Ge4+ /S2- platform. We find that charge-complementary and size-compatible Ga3+ leads to the synthesis of CPM-120-ZnGaGeS which is the first trimetallic zeolite-type chalcogenide, with improved crystal morphology and reproducibility. We also find that charge-compatible and size-complementary cations (Cd2+ or Sn4+ ) can induce an abrupt phase transition from super-sodalite to super-diamond, also with unprecedented trimetallic T2 clusters. For In3+ , which is dual-complementary (charge and size), a gradual phase transition is observed with increasing In3+ amount. Furthermore, by controlling the cluster composition, tunable band gaps can be realized. These materials show promising properties such as high CO2 uptake (4.32 mmol cm-3 , 298 K, 1 bar) and high photocatalytic activity.

Homo-Helical Rod Packing as a Path Toward the Highest Density of Guest-Binding Metal Sites in Metal-Organic Frameworks

In porous materials, metal sites with coordinate solvents offer opportunities for many applications, especially those promoted by host-guest chemistry, but such sites are especially hard to create for Li-based materials, because unlike transition metals, lithium does not usually possess a high-enough coordination number for both framework construction and guest binding. This challenge is addressed by mimicking the functional group ratio and metal-to-ligand charge ratio in MOF-74. A family of rod-packing lithium-organic frameworks (CPM-47, CPM-48, and CPM-49) were obtained. These materials exhibit an extremely high density of guest-binding lithium sites. Also unusual is the homo-helical rod-packing in the CPM series, as compared to the hetero-helical rod packing by helices of opposite handedness in MOF-74. This work demonstrates new chemical and structural possibilities in developing a record-setting high density of guest-binding metal sites in inorganic-organic porous materials.