Bryan J. Schindler

The effect of water adsorption on the structure of the carboxylate containing metal–organic frameworks Cu-BTC, Mg-MOF-74, and UiO-66

Metal–organic frameworks (MOFs) with metal–carboxylate bonds, including Cu-BTC (HKUST-1), Mg-MOF-74 (Mg/DOBDC), and UiO-66, have been shown to have varying degrees of water stability. The three MOFs in this study are three of the most highly studied MOFs in the literature. We investigate here how each MOF degrades at several temperature and humidity conditions over the course of 28 days. At conditions of 90% relative humidity (RH) and 25 °C, water uptake for Cu-BTC is shown to be higher than at 90% RH and 40 °C, causing the degradation of the inner structure of Cu-BTC to occur more readily at the lower temperature. However the external surfaces of Cu-BTC degrade more readily, as shown through SEM images, at conditions of 90% RH and 40 °C. Mg-MOF-74 has a nearly complete loss of surface area after just one day of exposure to each of the conditions studied, however the PXRD patterns show only a change in the [100] peak. We offer here a novel mechanism for the degradation of Mg-MOF-74, involving a 6-coordinate Mg intermediate, which leaves the 1-dimensional channels of Mg-MOF-74 intact. Furthermore, we conclude that UiO-66 is stable to each of the aging conditions for the full 28 days of this study.

Metal-catalyzed graphitic nanostructures as sorbents for vapor-phase ammonia

Activated carbons have long been used as substrates for the filtration of vapor-phase molecules, often with metal salts or oxides added to improve their sorption capacities for specific agents, but their real-world performance and applicability may be hindered by such factors as long-term stability and complex processing. En route to a new class of carbon-based sorbents, we have developed solid-state synthetic methods to produce bulk carbonaceous solids based on the pyrolysis of thermoset solids containing low concentrations (<1 wt%) of metal precursors (based on either Ni, Fe, or Co) that decompose in situ to catalyze the formation of a complex graphitic nanostructure. Selective combustion of residual amorphous carbon from the pyrolyzed solid generates a mesoporous network that facilitates diffusional transport of gas-phase molecules to the interior surfaces of the solid, and also converts the entrained metals to their respective metal oxide forms. We examine the ammonia-sorption properties of a series of these graphitic nanostructured compositions, and demonstrate that ammonia uptake is primarily determined by the type of residual metal oxide, with the Co-containing carbonaceous solid providing the best ammonia-sorption capacity (1.76 mol kg−1). Thermal reduction of the Co-containing material drastically decreases its ammonia-sorption capacity, showing that the oxide form (in this case Co3O4) of the entrained metal nanoparticles is most active for ammonia filtration. The effects of the carbon–oxygen functionalities on the nanostructured graphitic surfaces for ammonia sorption are also discussed.

Evaluation of a robust, diimide-based, porous organic polymer (POP) as a high-capacity sorbent for representative chemical threats

A previously described porous organic polymer (NU-POP-1) was evaluated against four representative chemical threats: ammonia, cyanogen chloride, sulfur dioxide, and octane. Ammonia, cyanogen chloride, and sulfur dioxide are examples of toxic industrial chemicals (TICs) spanning the range from highly basic to strong-acid forming substances, while octane is used to assess physical adsorption capacity. Experiments were carried out using a microbreakthrough test apparatus, which measures the adsorption capacity at saturation and gives an indication of the strength of adsorption. The NU-POP-1 material exhibited substantial removal capabilities against the majority of the toxic chemicals, with capacities as high as or better than an activated, impregnated carbon. The ability to remove the highly volatile toxic chemicals ammonia and cyanogen chloride was intriguing, as these chemicals typically require reactive moieities for removal. The present work presents a benchmark for toxic chemical removal, and future work will focus on incorporating functional groups targeting the toxic chemicals of interest.

Bottom-Up Synthesis of Anatase Nanoparticles with Graphene Domains

Using alizarin and titanium isopropoxide, we have succeeded in preparing a hybrid form of nanostructured graphene-TiO2 following a bottom-up synthetic approach. This novel graphene-based composite offers a practical alternative to synthesizing photocatalytically active materials with maximized graphene-TiO2 interface. The molecular precursor alizarin was chosen because it efficiently binds to TiO2 through the hydroxyl groups and already possesses the graphene building block through its anthracene basis. XPS and Raman spectroscopy proved that the calcined material contained majority sp(2)-hybridized carbon that formed graphene-like clusters. XRD data showed the integrated structures maintained their anatase crystallography, therefore preserving the materials properties without going through phase transitions to rutile. The enhanced graphene and TiO2 interface was confirmed using DFT computational techniques. The photocatalytic activity of the graphene-TiO2 materials was demonstrated through degradation of methylene blue.

Porphyrin-embedded organosilicate materials for ammonia adsorption

This study describes the application of porphyrin-embedded porous organosilicate materials to the adsorption of ammonia gas. Organosilicate scaffolds were synthesized through a surfactant-templating process combined with a phase separation technique. The structure offers a macro-textured scaffold to facilitate flow through the sorbent material and provide enhanced access to the available surface area provided by a combination of micro- and mesopores distributed over a range of sizes. The materials were grafted post-synthesis to provide sites for covalent immobilization of porphyrins. These porphyrins were utilized for incorporation of metal sites into the organosilicate materials. The removal of ammonia was evaluated for a number of materials incorporating copper metalloporphyrins of varied structure at varied loading levels. Results have been compared to removal of ammonia by a carbon material. Copper deuteroporphyrin IX bis-ethylene glycol provided the strongest interactions with ammonia. High loading levels of this porphyrin within the sorbent structure showed increasing evidence of stacking and did not improve the performance of the material.

A fiber optic, ultraviolet light-emitting diode-based, two wavelength fluorometer for monitoring reactive adsorption

Construction and use of an ultraviolet light-emitting diode-based fluorometer for measuring photoluminescence (PL) from powder samples with a fiber optic probe is described. Fluorescence at two wavelengths is detected by miniature photomultiplier tubes, each equipped with a different band pass filter, whose outputs are analyzed by a microprocessor. Photoluminescent metal oxides and hydroxides, and other semiconducting nanoparticles, often undergo changes in their emission spectra upon exposure to reactive gases, and the ratio of the PL intensities at two wavelengths is diagnostic of adsorption. Use of this instrument for reactive gas sensing and gas filtration applications is illustrated by measuring changes in the PL ratio for zirconium hydroxide and zinc oxide particles upon exposure to air containing low concentrations of sulfur dioxide.