Cassandra T. Buru

Postsynthetic Incorporation of a Singlet Oxygen Photosensitizer in a Metal-Organic Framework for Fast and Selective Oxidative Detoxification of Sulfur Mustard

A fullerene-based photosensitizer is incorporated postsynthetically into a Zr6 -based MOF, NU-1000, for enhanced singlet oxygen production. The structural organic linkers in the MOF platform also act as photosensitizers which contribute to the overall generation of singlet oxygen from the material under UV irradiation. The singlet oxygen generated by the MOF/fullerene material is shown to oxidize sulfur mustard selectively to the less toxic bis(2-chloroethyl)sulfoxide with a half-life of only 11 min.

Increased Electrical Conductivity in a Mesoporous Metal–Organic Framework Featuring Metallacarboranes Guests

Nickel(IV) bis(dicarbollide) is incorporated in a zirconium-based metal-organic framework (MOF), NU-1000, to create an electrically conductive MOF with mesoporosity. All the nickel bis(dicarbollide) units are located as guest molecules in the microporous channels of NU-1000, which permits the further incorporation of other active species in the remaining mesopores. For demonstration, manganese oxide is installed on the nodes of the electrically conductive MOF. The electrochemically addressable fraction and specific capacitance of the manganese oxide in the conductive framework are more than 10 times higher than those of the manganese oxide in the parent MOF.

Atomistic Approach toward Selective Photocatalytic Oxidation of a Mustard-Gas Simulant: A Case Study with Heavy-Chalcogen-Containing PCN-57 Analogues

Here we describe the synthesis of two Zr-based benzothiadiazole- and benzoselenadiazole-containing metal-organic frameworks (MOFs) for the selective photocatalytic oxidation of the mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES). The photophysical properties of the linkers and MOFs are characterized by steady-state absorption and emission, time-resolved emission, and ultrafast transient absorption spectroscopy. The benzoselenadiazole-containing MOF shows superior catalytic activity compared to that containing benzothiadiazole with a half-life of 3.5 min for CEES oxidation to nontoxic 2-chloroethyl ethyl sulfoxide (CEESO). Transient absorption spectroscopy performed on the benzoselenadiazole linker reveals the presence of a triplet excited state, which decays with a lifetime of 9.4 μs, resulting in the generation of singlet oxygen for photocatalysis. This study demonstrates the effect of heavy chalcogen substitution within a porous framework for the modulation of photocatalytic activity.

Single-Atom-Based Vanadium Oxide Catalysts Supported on Metal–Organic Frameworks: Selective Alcohol Oxidation and Structure–Activity Relationship

We report the syntheses, structures, and oxidation catalytic activities of a single-atom-based vanadium oxide incorporated in two highly crystalline MOFs, Hf-MOF-808 and Zr-NU-1000. These vanadium catalysts were introduced by a postsynthetic metalation, and the resulting materials (Hf-MOF-808-V and Zr-NU-1000-V) were thoroughly characterized through a combination of analytic and spectroscopic techniques including single-crystal X-ray crystallography. Their catalytic properties were investigated using the oxidation of 4-methoxybenzyl alcohol under an oxygen atmosphere as a model reaction. Crystallographic and variable-temperature spectroscopic studies revealed that the incorporated vanadium in Hf-MOF-808-V changes position with heat, which led to improved catalytic activity.

Pushing the Limits on Metal–Organic Frameworks as a Catalyst Support: NU-1000 Supported Tungsten Catalysts for o-Xylene Isomerization and Disproportionation

Acid-catalyzed skeletal C-C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Brønsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal-organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), 31P NMR, N2 physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WO x clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000-supported catalyst were comparable to a control WO x-ZrO2, but the NU-1000 composite material was unusually active toward the transmethylation pathway that requires two adjacent active sites in a confined pore, as created when PTA is confined in NU-1000. This work shows the promise of metal-organic framework topologies in giving access to unique reactivity, even for aggressive reactions such as hydrocarbon isomerization.

Thermally induced migration of a polyoxometalate within a metal–organic framework and its catalytic effects

The polyoxometalate (POM), H3PW12O40, was postsynthetically incorporated into the metal–organic framework (MOF), NU-1000. The POM@MOF composite, PW12@NU-1000, was activated under mild conditions, resulting in a material whose diffraction pattern and spectroscopic properties differ from the same material heated at elevated temperatures. These discrepancies, corroborated by difference envelope density analyses, were attributed to the POM residing either in the mesoporous or microporous channels of NU-1000. As a testament to the importance of catalyst accessibility, the POMs locational change also induced a change in the composites rate and selectivity toward oxidizing 2-chloroethyl ethyl sulfide.

Revisiting the structural homogeneity of NU-1000, a Zr-based metal–organic framework

Synthesis and activation of phase-pure and defect-free metal–organic frameworks (MOFs) are essential for establishing accurate structure–property relationships. Primarily suffering from missing linker and/or node defects, Zr6-based MOFs can have polymorphs, structures with the identical linker and node but different connectivity, which can create multiple phases in a sample that complicate the characterization. Here, we report the synthesis of phase-pure NU-1000, a mesoporous Zr6-based MOF that typically contains a significant secondary phase within the individual crystallites. Large biomolecules and smaller inorganic molecules have been installed in NU-1000 as probes to verify the near elimination of the microporous secondary-phase. Obtaining structurally homogenous MOFs will assist the design of new materials with distinct structural features.