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Metal-Organic Frameworks: Synthesis Of S-Block / F-Block Materials And A New Method In Thin Film Synthesis


Many priority pollutants listed by the U.S. Environmental Protection Agency (EPA) found in wastewater are anionic in charge and are difficult to remediate. This is either due to high mobility in wastewater, radioactivity, or being in the presence of other inorganic oxyanions (carbonate, nitrate, etc.) in much higher concentrations that would impede immobilization. Several anions that are EPA priority pollutants include: perchlorate, pertechnetate, arsenate, arsenite, and chromate. In order to remediate these pollutants, highly selective cationic materials must be made in order to exchange them out of wastewater.

Anion exchange resins are the current standard material used to remediate anionic pollutants. These inexpensive polymers have poor selectivity towards these pollutants in the presence of other anions and are very unstable in different chemical and thermal environments. Inorganic extended materials show much promise in addressing the shortcomings of anion exchange resins but many of the available materials are neutral or anionic in charge (eg. zeolites), thus rendering them ineffective towards efficient anion uptake capacities. Layered double hydroxides (LDHs) are a class of materials that are cationic in charge and have been extensively studied for their anion uptake capacities from wastewater. However, LDHs have a high propensity to uptake carbonate, even after calcining the material to remove the anion from the interlamellar space. This severely diminishes the potential remediation of other anions in wastewater since carbonate is present in the parts per thousand.

The use of s-block metals in synthesizing metal-organic frameworks (MOFs) has been relatively unexplored. By exploring potential MOF structures using barium, higher coordination geometries are possible and may lead to unique structural environments that could prove successful in efficient anion exchange. We have successfully synthesized two barium-containing MOFs with formulas Ba2F2[O3SC2H4SO3] (denoted SLUG-13) and Ba[O3SC2H4SO3] (denoted SLUG-14). Both structures are 2D layered materials with inorganic sheets pillared by 1,2-ethanedisulfonate ligands that were characterized with powder X-ray diffraction (PXRD). Thermal analysis showed that these materials are stable up to 325 °C and 375 °C for SLUG-13 and SLUG-14, respectively. Although they did not show any activity in anion exchange, SLUG-13 showed promising catalytic activity in acid catalyzed ketal formations.

Little has been studied on the use of f-block metals in MOFs using alkanedisulfonates as the organic component. We have shown the first isoreticular synthesis of three structures with formulas Nd2(OH)4(OH2)2[O3SC2H4SO3] (denoted SLUG-28), Nd2(OH)4(OH2)2[O3SC3H6SO3] (denoted SLUG-29), and Nd2(OH)4(OH2)2[O3SC4H8SO3] (denoted SLUG-30). Powder X-ray diffraction analysis showed that their structures are all composed of the same neodymium oxohydroxo layers pillared by alkanedisulfonates of increasing carbon length. Thermal analysis of the three structures showed similar decomposition characteristics. These structures are unique examples of the use of sulfonates with rare earth metals and further expand the class of f-block MOFs.

MOF thin films have been of tremendous interest because of their applications in optical coatings, heterogeneous catalysis, chemical separations, and sensor devices. Recently, there have been reports of their semiconducting properties and their potential applications in photovoltaic cells. Most current methods of fabricating a MOF film involve tedious procedures and expensive components, none of which have been done on a conductive substrate to further explore their semiconducting capabilities. We have developed a novel methodology in the facile synthesis of four isoreticular metal-organic framework (IRMOF) thin films on a conductive substrate using ZnO nanowires as a template. The method allows for the growth of IRMOF single crystals on a bed of nanowires with excellent coverage of the substrate and without using unstable self-assembled monolayers as the nucleation source. We have also used microwaves to dramatically reduce the synthesis time of these films to produce IRMOF films of high crystallinity in 10 minutes.

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