Karen L. Mulfort

Supercritical Processing as a Route to High Internal Surface Areas and Permanent Microporosity in Metal−Organic Framework Materials

Careful processing of four representative metal-organic framework (MOF) materials with liquid and supercritical carbon dioxide (ScD) leads to substantial, or in some cases spectacular (up to 1200%), increases in gas-accessible surface area. Maximization of surface area is key to the optimization of MOFs for many potential applications. Preliminary evidence points to inhibition of mesopore collapse, and therefore micropore accessibility, as the basis for the extraordinarily efficacious outcome of ScD-based activation.

Post-Synthesis Alkoxide Formation Within Metal#Organic Framework Materials: A Strategy for Incorporating Highly Coordinatively Unsaturated Metal Ions

A new noncatenated metal-organic framework containing pendant alcohol functionalities was synthesized. The alcohols were then post-synthetically converted to either lithium or magnesium alkoxides, with the incorporated metals anchored far from nodes or carboxylate functionalities. The metal alkoxide sites can be obtained stoichiometrically while maintaining the permanent porosity and large surface area of the parent hydroxylated material. The incorporated metal ions are found to induce an unusual pattern of binding energetics for H(2): isosteric heats of adsorption increase, rather than decrease, with increasing H(2) loading. Additionally, at 1 atm and 77 K, uptake (at least with low Li(+) loading) is increased by two hydrogen molecules per Li(+).

An Interpenetrated Framework Material with Hysteretic CO2 Uptake

A new, twofold interpenetrated metal-organic framework (MOF) material has been synthesized that demonstrates dramatic steps in the adsorption and hysteresis in the desorption of CO(2). Measurement of the structure by powder X-ray diffraction (PXRD) and pair distribution function (PDF) analysis indicates that structural changes upon CO(2) sorption most likely involve the interpenetrated frameworks moving with respect to each other.

Photodriven Charge Separation Dynamics in CdSe/ZnS Core/Shell Quantum Dot/Cobaloxime Hybrid for Efficient Hydrogen Production

Photodriven charge-transfer dynamics and catalytic properties have been investigated for a hybrid system containing CdSe/ZnS core/shell quantum dots (QDs) and surface-bound molecular cobaloxime catalysts. The electron transfer from light-excited QDs to cobaloxime, revealed by optical transient absorption spectroscopy, takes place with an average time constant of 105 ps, followed a much slower charge recombination process with a time constant of ≫3 ns. More interestingly, we also observed photocatalytic hydrogen generation by this QD/cobaloxime hybrid system, with >10,000 turnovers of H(2) per QD in 10 h, using triethanolamine as a sacrificial electron donor. These results suggest that QD/cobaloxime hybrids succeed in coupling single-photon events with multielectron redox catalytic reactions, and such systems could have potential applications in long-lived artificial photosynthetic devices for fuel generation from sunlight.

Alkali Metal Cation Effects on Hydrogen Uptake and Binding in Metal-Organic Frameworks

A 2-fold interwoven metal-organic framework has been chemically reduced and doped with Li(+), Na(+), and K(+). At low pressures and temperatures, the reduced and doped materials exhibit enhanced H2 uptakeup to 65% higher than for the neutral framework. Notably, at similar doping levels, H2 binding is strongest with Li(+) and decreases as Li(+) > Na(+) > K(+). However, the uptake increases in the opposite order. We attribute the behavior to structural changes accompanying framework reduction.

Cavity-Tailored, Self-Sorting Supramolecular Catalytic Boxes for Selective Oxidation

Using a steric self-sorting strategy, the assembly of highly ordered and rigid supramolecular boxes possessing catalytic properties has been achieved in one step. The formation of these assemblies, comprising up to 18 porphyrin centers, was readily confirmed by solution X-ray scattering in conjunction with fluorescent spectroscopy. Size-selective and enantioselective oxidation catalysis were both demonstrated.

A Zn-based, pillared paddlewheel MOF containing free carboxylic acids via covalent post-synthesis elaboration

A Zn-based, mixed-ligand (pillared paddlewheel), metal-organic framework (MOF) has been covalently and quantitatively decorated with free carboxylic acids to demonstrate the utility of covalent post-synthesis modification in the construction of otherwise inaccessible carboxy-functionalized MOFs.

Nature-Driven Photochemistry for Catalytic Solar Hydrogen Production: A Photosystem I–Transition Metal Catalyst Hybrid

Solar energy conversion of water into the environmentally clean fuel hydrogen offers one of the best long-term solutions for meeting future energy demands. Nature provides highly evolved, finely tuned molecular machinery for solar energy conversion that exquisitely manages photon capture and conversion processes to drive oxygenic water-splitting and carbon fixation. Herein, we use one of Natures specialized energy-converters, the Photosystem I (PSI) protein, to drive hydrogen production from a synthetic molecular catalyst comprised of inexpensive, earth-abundant materials. PSI and a cobaloxime catalyst self-assemble, and the resultant complex rapidly produces hydrogen in aqueous solution upon exposure to visible light. This work establishes a strategy for enhancing photosynthetic efficiency for solar fuel production by augmenting natural photosynthetic systems with synthetically tunable abiotic catalysts.

Framework reduction and alkali-metal doping of a triply catenating metal-organic framework enhances and then diminishes H2 uptake.

A permanently microporous metal-organic framework compound with the formula Zn(2)(NDC)(2)(diPyTz) (NDC = 2,6-naphthalenedicarboxylate, diPyTz = di-3,6-(4-pyridyl)-1,2,4,5-tetrazine) has been synthesized. The compound, which features a triply catenating, pillared-paddlewheel structure, was designed to be easily chemically reduced (diPyTz sites) by appropriate channel permeants. Reduction was achieved by using the naphthalenide anion, with the accompanying metal cation (Li(+), Na(+) or K(+)) serving to dope the compound in extraframework fashion. H(2) uptake at 1 atm and 77 K increases from 1.12 wt % for the neutral material to 1.45, 1.60, and 1.51 wt % for the Li(+)-, Na(+)-, and K(+)-doped materials, respectively. The isosteric heats of adsorption are similar for all four versions of the material despite the large uptake enhancements for the reduced versions. Nitrogen isotherms were also measured in order to provide insight into the mechanisms of uptake enhancement. The primary mechanism is believed to be dopant-facilitated displacement of catenated frameworks by sorbed H(2). More extensive cation doping decreases the H(2) loading.

An Interpenetrated Framework Material with Hysteretic CO[subscript 2] Uptake

A new, twofold interpenetrated metal-organic framework (MOF) material has been synthesized that demonstrates dramatic steps in the adsorption and hysteresis in the desorption of CO(2). Measurement of the structure by powder X-ray diffraction (PXRD) and pair distribution function (PDF) analysis indicates that structural changes upon CO(2) sorption most likely involve the interpenetrated frameworks moving with respect to each other.