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Open Access Publications from the University of California

Synthesis of Cationic Extended Frameworks for Anion-Based Applications

  • Author(s): Fei, Honghan
  • Advisor(s): Oliver, Scott RJ
  • et al.

Many of the metal pollutants listed as priorities by the EPA (U.S. Environmental Protection Agency) occur in water as their oxo-hydroxo anionic forms (e.g. perchlorate, chromate, selenite, etc.). Radioactive technetium (Tc-99) in the form of soluble pertechnetate (TcO4−) is highly problematic in low-activity waste (LAW) to separate the nuclear waste into primary solids. Its easy leakage from glass after vitrification does not meet long-term storage performance assessment requirements. LAW also contains other non-radioactive inorganic and organic species [e.g. carbonate (CO32−), nitrate (NO3−), etc.] that may interfere with immobilizing radioactive species in solid-state ion-exchange materials. Chromate is another problematic anion for vitrification because it weakens the integrity of the waste glass by forming spinels; such particles can also obstruct the glass flow within the melter during vitrification.

There is an extensive class of purely inorganic extended materials and hybrid inorganic-organic extended frameworks. However, the majority of this group of materials occurs to bear a neutral or negative charge on their extended framework (e.g. zeolites). Layered double hydroxides (LDHs) are a class of well-studied isostructural cationic materials, and have been extensively studied in anion exchange. This group of materials, however, has limited capacity as evidenced by adsorption titration and isotherms. They also display low selectivity towards anion pollutants, especially in the presence of carbonate.

Exploration of transition metals and lower p block metals lead to synthesis of cationic inorganic materials and cationic metal-organic frameworks (MOFs). Ag(I) based cationic extended frameworks with α,ω-alkanedisulfonate as anionic SDAs has successfully been synthesized with the formula as following, Ag2(4,4' bipy)2 (O3SCH2CH2SO3)*4H2O (SLUG-21). The unbound ethanedisulfonate anions display effective anion pollutant trapping on permanganate (MnO4−) and perrhenate (ReO4−) with high adsorption capacity and selectivity. These two anions are chosen for anion exchange study owing to the same group as pertechnetate (TcO4−). SLUG-21 displayed its adsorption capacity in record levels over all previous materials with 292 mg/g and 602 mg/g, respectively, for permanganate and perrhenate. These values are over five times compared to the conventional layered double hydroxides (LDHs) under the same condition. We further investigated the mechanism of these exceptional high adsorption capacities in view of crystallography. In addition to exceptional high adsorption capacity, SLUG-21 displayed excellent selectivity towards anionic pollutants over non-toxic anions. One hundred fold excess of nitrate or carbonate do not interfere with SLUG-21 trapping permanganate and perrhenate. The favorable trend of anions to be intercalated in SLUG-21 is as following: MnO4−> ReO4−> ClO4−> CrO42−> NO3−> CO32−, with the toxic pollutants topping the list.

We are also successful in the synthesis of cationic inorganic layered materials, which displayed higher thermal and chemical stability than cationic MOFs. Our approach focuses on the use of anionic SDAs and excluding any potential cationic SDAs. The first copper-based cationic layered extended framework Cu4(OH)6(O3SCH2CH2SO3)* 2H2O (SLUG-26) was hydrothermally synthesized. Inorganic connectivity (Cu-O-Cu) construct a cationic 2-D extended layer [Cu4(OH)6]2+ with ethanedisulfonate weakly bounding between adjacent layers. This material display rich intercalation chemistry with different α,ω-alkanedicarboxylate anions. The d-spacing between cationic cuprate layer can be tuned from 7.6 Å with intercalating malonate (−O2CCH2CO2−) to 11.1 Å with glutarate (−O2C(CH2)3CO2−) separating layers. Besides organic anions, SLUG-26 also showed exchange capabilities upon inorganic anion pollutants, exhibiting five times higher adsorption capacity for permanganate than LDHs.

The complete exchange of the interlamellar anions of a 2-D cationic inorganic material was demonstrated. The α,ω-alkanedisulfonates were exchanged for α,ω-alkanedicarboxylates, leading to two new cationic materials with the same [Pb2F2]2+ layered architecture. Both were solved by single crystal X-ray diffraction and the transformation also followed by in-situ optical microscopy and ex-situ powder X ray diffraction. This report represents a rare example of metal-organic framework displaying highly efficient and complete replacement of its anionic organic linker while retaining the original extended inorganic layer. It also opens up further possibilities for introducing other anions or abatement of problematic anions such as pharmaceuticals and their metabolites.

A rare example of an extended nickel oxide open framework with succinate capping the channels was synthesized. A honeycomb-like layer of 14-membered rings centered in the (-111) plane are connected by vertex-sharing NiO6 octahedra and water resides in the channels. The structure is the second example of an extended hybrid containing 3-D Ni-O-Ni connectivity and was structurally characterized by single-crystal and powder X-ray diffraction. The material displays excellent chemical stability in aqueous solution from pH ~ 1 to 13 and thermal stability to ~ 375 °C as evidenced by thermogravimetic analysis coupled mass spectroscopy. The Ni2+ ions order ferromagnetically order below Tc = 5.1 K, and anisotropic exchange interactions lead to a field-induced metamagnetic transition and spin-glass-like dependence on cooling conditions in magnetic field.

[Sb6O7][(SO4)2] (SLUG-34) consists of a very unusual 1-D antimony oxide chain four Sb atoms wide, with unprotonated sulfate between the chains. The material can be synthesized in high yield and pure phase and was characterized by both powder and single-crystal X-ray diffraction. The entirely inorganic nature of SLUG-34 along with infinite 1-D Sb-O-Sb connectivity results in high thermal stability and chemical resistance. SLUG-34 is thermally stable to ca. 500 °C as evidenced by in-situ variable temperature thermodiffraction as well as thermogravimetric analysis. Unlike the basic nature of layered double hydroxides (which are the only well-studied class of cationic inorganic materials), SLUG-34 is chemically stable in aqueous acidic conditions. This opens up the possibility for synthesis of other non-LDH type cationic inorganic materials with potential host-guest applications based on their extraframework anions.

A facile and inexpensive approach to fabricate "nanospider" TiO2 thin films was demonstrated with not only an amazing morphology but highly efficient water splitting to produce hydrogen. Our method employs benzene-swollen poly(ethylene glycol) as a sacrificial organic polymer to template the semiconductor thin film. The synthesized TiO2 thin films are highly crystalline with optimized particle and channel size to enhance the liquid-semiconductor junction interaction. This enhanced contact area leads to more than twice the water splitting performance than conventional P25 thin films. In addition, the nanospider thin films also outperform P25 films in the photodegradation of toxic organics.

An inexpensive method using solvent-swollen poly(methyl methacrylate) as a sacrificial template for mesoporous titanium oxide thin films was investigated with tunable meso/nano morphology. The conversion efficiency reaches 4.2 % despite using a solid state electrolyte, which circumvents the longevity issues of liquid electrolytes. The cells show a large short-circuit photocurrent density of 7.98 mA, open-circuit voltage of 0.78 V and maximum conversion efficiency of 4.2 % under air-mass 1.5 global illumination. At higher titania precursor ratios, nanodisk particles are formed, increasing light scattering and doubling the efficiency over our previous reports. The tunability of the semiconductor morphology and all solid-state nature of the cells makes the method a viable alternative to existing solar cell technology.

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