Michael J. Katz

A facile synthesis of UiO-66, UiO-67 and their derivatives

A scalable, reproducible method of synthesizing UiO-66- and UiO-67-type MOFs, entailing the addition of HCl to the reaction mixture, has been investigated. The new protocol requires a fraction of the time of previously reported procedures, yields exceptional porosities, and works with a range of linkers.

Destruction of chemical warfare agents using metal–organic frameworks

Chemical warfare agents containing phosphonate ester bonds are among the most toxic chemicals known to mankind. Recent global military events, such as the conflict and disarmament in Syria, have brought into focus the need to find effective strategies for the rapid destruction of these banned chemicals. Solutions are needed for immediate personal protection (for example, the filtration and catalytic destruction of airborne versions of agents), bulk destruction of chemical weapon stockpiles, protection (via coating) of clothing, equipment and buildings, and containment of agent spills. Solid heterogeneous materials such as modified activated carbon or metal oxides exhibit many desirable characteristics for the destruction of chemical warfare agents. However, low sorptive capacities, low effective active site loadings, deactivation of the active site, slow degradation kinetics, and/or a lack of tailorability offer significant room for improvement in these materials. Here, we report a carefully chosen metal-organic framework (MOF) material featuring high porosity and exceptional chemical stability that is extraordinarily effective for the degradation of nerve agents and their simulants. Experimental and computational evidence points to Lewis-acidic Zr(IV) ions as the active sites and to their superb accessibility as a defining element of their efficacy.

Remnant PbI2, an unforeseen necessity in high-efficiency hybrid perovskite-based solar cells?a)

Perovskite-containing solar cells were fabricated in a two-step procedure in which PbI2 is deposited via spin-coating and subsequently converted to the CH3NH3PbI3 perovskite by dipping in a solution of CH3NH3I. By varying the dipping time from 5 s to 2 h, we observe that the device performance shows an unexpectedly remarkable trend. At dipping times below 15 min the current density and voltage of the device are enhanced from 10.1 mA/cm2 and 933 mV (5 s) to 15.1 mA/cm2 and 1036 mV (15 min). However, upon further conversion, the current density decreases to 9.7 mA/cm2 and 846 mV after 2 h. Based on X-ray diffraction data, we determined that remnant PbI2 is always present in these devices. Work function and dark current measurements showed that the remnant PbI2 has a beneficial effect and acts as a blocking layer between the TiO2 semiconductor and the perovskite itself reducing the probability of back electron transfer (charge recombination). Furthermore, we find that increased dipping time leads to an increase in the size of perovskite crystals at the perovskite-hole-transporting material interface. Overall, approximately 15 min dipping time (∼2% unconverted PbI2) is necessary for achieving optimal device efficiency.

Simple and compelling biomimetic metal-organic framework catalyst for the degradation of nerve agent simulants.

Inspired by biology, in which a bimetallic hydroxide-bridged zinc(II)-containing enzyme is utilized to catalytically hydrolyze phosphate ester bonds, the utility of a zirconium(IV)-cluster-containing metal-organic framework as a catalyst for the methanolysis and hydrolysis of phosphate-based nerve agent simulants was examined. The combination of the strong Lewis-acidic Zr(IV) and bridging hydroxide anions led to ultrafast half-lives for these solvolysis reactions. This is especially remarkable considering that the actual catalyst loading was a mere 0.045 % as a result of the surface-only catalysis observed.

High Efficiency Adsorption and Removal of Selenate and Selenite from Water Using Metal−Organic Frameworks

A series of zirconium-based, metal-organic frameworks (MOFs) were tested for their ability to adsorb and remove selenate and selenite anions from aqueous solutions. MOFs were tested for adsorption capacity and uptake time at different concentrations. NU-1000 was shown to have the highest adsorption capacity, and fastest uptake rates for both selenate and selenite, of all zirconium-based MOFs studied here. Herein, the mechanism of selenate and selenite adsorption on NU-1000 is explored to determine the important features that make NU-1000 a superior adsorbent for this application.

Turning on catalysis: incorporation of a hydrogen-bond-donating squaramide moiety into a Zr metal-organic framework.

Herein, we demonstrate that the incorporation of an acidic hydrogen-bond-donating squaramide moiety into a porous UiO-67 metal-organic framework (MOF) derivative leads to dramatic acceleration of the biorelevant Friedel-Crafts reaction between indole and β-nitrostyrene. In comparison, it is shown that free squaramide derivatives, not incorporated into MOF architectures, have no catalytic activity. Additionally, using the UiO-67 template, we were able to perform a direct comparison of catalytic activity with that of the less acidic urea-based analogue. This is the first demonstration of the functionalization of a heterogeneous framework with an acidic squaramide derivative.

Directed Growth of Electroactive Metal-Organic Framework Thin Films Using Electrophoretic Deposition

Electrophoretic deposition (EPD) is used to assemble metal-organic framework (MOF) materials in nano- and micro-particulate, thin-film form. The flexibility of the method is demonstrated by the successful deposition of 4 types of MOFs: NU-1000, UiO-66, HKUST-1, and Al-MIL-53. Additionally, EPD is used to pattern the growth of NU-1000 thin films that exhibit full electrochemical activity.

Are Zr6-based MOFs water stable? Linker hydrolysis vs. capillary-force-driven channel collapse

Metal-organic frameworks (MOFs) built up from Zr6-based nodes and multi-topic carboxylate linkers have attracted attention due to their favourable thermal and chemical stability. However, the hydrolytic stability of some of these Zr6-based MOFs has recently been questioned. Herein we demonstrate that two Zr6-based frameworks, namely UiO-67 and NU-1000, are stable towards linker hydrolysis in H2O, but collapse during activation from H2O. Importantly, this framework collapse can be overcome by utilizing solvent-exchange to solvents exhibiting lower capillary forces such as acetone.

A UiO-66 analogue with uncoordinated carboxylic acids for the broad-spectrum removal of toxic chemicals

Zirconium-based metal–organic frameworks (MOFs) are of great importance as sorbents due to their increased chemical and thermal stability when compared to other MOF families. Here we report a novel analogue of UiO-66 modified with oxalic acid via solvent-assisted ligand incorporation. This analogue has the ability to remove ammonia, cyanogen chloride, sulphur dioxide, nitrogen dioxide and octane at levels greater than or equal to the base UiO-66. We report here the highest known capacities exhibited by a MOF for SO2 and NO2, at pressures less than 0.10 bar and at room temperature, by UiO-66-ox. Furthermore, we show here the importance of the secondary building unit of the MOF in the removal of ammonia and cyanogen chloride.

Effects of Adsorbed Pyridine Derivatives and Ultrathin Atomic-Layer-Deposited Alumina Coatings on the Conduction Band-Edge Energy of TiO2 and on Redox-Shuttle-Derived Dark Currents

Both the adsorption of t-butylpyridine and the atomic-layer deposition of ultrathin conformal coatings of insulators (such as alumina) are known to boost open-circuit photovoltages substantially for dye-sensitized solar cells. One attractive interpretation is that these modifiers significantly shift the conduction-edge energy of the electrode, thereby shifting the onset potential for dark current arising from the interception of injected electrons by solution-phase redox shuttle components such as Co(phenanthroline)(3)(3+) and triiodide. For standard, high-area, nanoporous photoelectrodes, band-edge energies are difficult to measure directly. In contrast, for flat electrodes they are readily accessible from Mott-Schottky analyses of impedance data. Using such electrodes (specifically TiO(2)), we find that neither organic nor inorganic electrode-surface modifiers shift the conduction-band-edge energy sufficiently to account fully for the beneficial effects on electrode behavior (i.e., the suppression of dark current). Additional experiments reveal that the efficacy of ultrathin coatings of Al(2)O(3) arises chiefly from the passivation of redox-catalytic surface states. In contrast, adsorbed t-butylpyridine appears to suppress dark currents mainly by physically blocking access of shuttle molecules to the electrode surface. Studies with other derivatives of pyridine, including sterically and/or electronically diverse derivatives, show that heterocycle adsorption and the concomitant suppression of dark current does not require the coordination of surface Ti(IV) or Al(III) atoms. Notably, the favorable (i.e., negative) shifts in onset potential for the flow of dark current engendered by organic and inorganic surface modifiers are additive. Furthermore, they appear to be largely insensitive to the identity of shuttle molecules.