Kun-Yi Andrew Lin

Magnetic cobalt–graphene nanocomposite derived from self-assembly of MOFs with graphene oxide as an activator for peroxymonosulfate

While metal organic frameworks (MOFs) have been extensively explored as a platform for developing porous metal oxides, another intriguing direction is to use MOFs as precursors to prepare carbonaceous materials. By simple one-step carbonization, MOFs can be turned into promising hierarchical carbon materials. Such a technique can be also used to convert MOF-based composites to carbon-based composites with task-specific functionality different from that of the precursors. In this study, this strategy is adopted to prepare a magnetic cobalt–graphene (MCG) nanocomposite by carbonizing a self-assembly of a cobalt-based MOF, ZIF-67, and graphene oxide (GO). The preparation of MCG represents a simple alternative route to synthesize magnetic graphene materials and graphene-supported cobalt materials. By combining cobalt and reduced graphene oxide (RGO), the as-prepared MCG can be an effective catalyst to activate peroxymonosulfate (PMS) in the advanced oxidation process. Thus, the activation capability of MCG is evaluated by decolorizing acid yellow (AY) dye in water. MCG exhibited an enhanced catalytic activity to activate PMS compared to carbonized ZIF-67 because RGO also activated PMS and improved electron transport ability. The kinetics of the decolorization of AY (10 mg L−1) was 0.0119 min−1 with PMS = 200 mg L−1 and MCG = 500 mg L−1. The activation energy of the decolorization using PMS activated by MCG was found to be 12 kJ mol−1. Factors influencing the PMS activation were also investigated including temperature, pH, UV, ultrasonication and inhibitors. To evaluate the long-term catalytic activity of MCG, a 50-cycle decolorization test was performed and the regeneration efficiency remained at 97.6% over 50 cycles, showing its stable and effective catalytic activity. These features make MCG a promising catalyst to activate PMS.

Effects of Bonding Types and Functional Groups on CO2 Capture using Novel Multiphase Systems of Liquid-like Nanoparticle Organic Hybrid Materials

Novel liquid-like nanoparticle organic hybrid materials (NOHMs) which possess unique features including negligible vapor pressure and a high degree of tunability were synthesized and their physical and chemical properties as well as CO(2) capture capacities were investigated. NOHMs can be classified based on the synthesis methods involving different bonding types, the existence of linkers, and the addition of task-specific functional groups including amines for CO(2) capture. As a canopy of polymeric chains was grafted onto the nanoparticle cores, the thermal stability of the resulting NOHMs was improved. In order to isolate the entropy effect during CO(2) capture, NOHMs were first prepared using polymers that do not contain functional groups with strong chemical affinity toward CO(2). However, it was found that even ether groups on the polymeric canopy contributed to CO(2) capture in NOHMs via Lewis acid-base interactions, although this effect was insignificant compared to the effect of task-specific functional groups such as amine. In all cases, a higher partial pressure of CO(2) was more favorable for CO(2) capture, while a higher temperature caused an adverse effect. Multicyclic CO(2) capture tests confirmed superior recyclability of NOHMs and NOHMs also showed a higher selectivity toward CO(2) over N(2)O, O(2) and N(2).

Adsorption of fluoride to UiO-66-NH2 in water: Stability, kinetic, isotherm and thermodynamic studies.

To provide safe drinking water, fluoride in water must be removed and adsorption processes appear to be the most widely used method. Metal organic frameworks (MOFs) represent a new class of adsorbents that have been used in various adsorption applications. To study the adsorption mechanism of fluoride to MOFs in water and obtain related adsorption parameters, we synthesized a zirconium-based MOF with a primary amine group on its ligand, named UiO-66-NH2. The kinetics, adsorption isotherm and thermodynamics of fluoride adsorption to UiO-66-NH2 were investigated. The crystalline structure of UiO-66-NH2 remained intact and the local structure of zirconium in UiO-66-NH2 did not change significantly after being exposed to fluoride. The kinetics of the fluoride adsorption in UiO-66-NH2 could be well represented by the pseudo second order rate law. The enthalpy of the adsorption indicates that the F(-) adsorption to UiO-66-NH2 was classified as a physical adsorption. However, the comparison between the adsorption capacities of UiO-66-NH2 and UiO-66 suggests that the fluoride adsorption to UiO-66-NH2 might primarily involve a strong interaction between F(-) and the metal site. The fluoride adsorption capacity of UiO-66-NH2 was found to decrease when pH>7. While the presence of chloride/bromide ions did not noticeably change the adsorption capacity of UiO-66-NH2, the ionic surfactants slightly affected the adsorption capacity of UiO-66-NH2. These findings provide insights to further optimize the adsorption process for removal of fluoride using zirconium-based MOFs.

Investigation of CO2 capture mechanisms of liquid-like nanoparticle organic hybrid materials via structural characterization

Nanoparticle organic hybrid materials (NOHMs) have been recently developed that comprise an oligomeric or polymeric canopy tethered to surface-modified nanoparticles via ionic or covalent bonds. It has already been shown that the tunable nature of the grafted polymeric canopy allows for enhanced CO(2) capture capacity and selectivity via the enthalpic intermolecular interactions between CO(2) and the task-specific functional groups, such as amines. Interestingly, for the same amount of CO(2) loading NOHMs have also exhibited significantly different swelling behavior compared to that of the corresponding polymers, indicating a potential structural effect during CO(2) capture. If the frustrated canopy species favor spontaneous ordering due to steric and/or entropic effects, the inorganic cores of NOHMs could be organized into unusual structural arrangements. Likewise, the introduction of small gaseous molecules such as CO(2) could reduce the free energy of the frustrated canopy. This entropic effect, the result of unique structural nature, could allow NOHMs to capture CO(2) more effectively. In order to isolate the entropic effect, NOHMs were synthesized without the task-specific functional groups. The relationship between their structural conformation and the underlying mechanisms for the CO(2) absorption behavior were investigated by employing NMR and ATR FT-IR spectroscopies. The results provide fundamental information needed for evaluating and developing novel liquid-like CO(2) capture materials and give useful insights for designing and synthesizing NOHMs for more effective CO(2) capture.

Iron-based metal organic framework, MIL-88A, as a heterogeneous persulfate catalyst for decolorization of Rhodamine B in water

While Metal Organic Frameworks (MOFs) have been extensively investigated as photocatalysts to eliminate toxic pollutants in water, studies using MOFs as chemical oxidative catalysts to degrade contaminants are still limited. MOFs used as catalysts for chemical oxidation reactions were prepared in N,N-dimethylformamide (DMF), a potential carcinogenic solvent. If such MOFs are not well activated and have not properly undergone solvent exchange, DMF can still be encapsulated inside MOFs, leading to secondary pollution. Considering the essence of wastewater treatment and pollutant reduction, DMF-free MOFs which still exhibit the catalytic activity to activate oxidants should be developed. Thus, we selected an iron-based MOF, MIL-88A, which can be prepared using Fe3+ with fumaric acid just in water. The as-synthesized MIL-88A is evaluated as the heterogeneous catalyst to activate persulfate for the decolorization of Rhodamine B (RB) dye. Iron oxide clusters (i.e., Fe2O3) form within MIL-88A through the coordination of Fe3+ and fumaric acid. Fe3+ of Fe2O3 is expected to induce the generation of persulfate radicals which in turn lead to the formation of sulfate radicals to decolorize RB dye. Factors influencing the activation of persulfate and RB decolorization were examined including persulfate dosage, MIL-88A loading, temperature, pH, UV and US irradiation as well as inhibitors. MIL-88A was assessed for the multiple-cycle activation of persulfate without additional regeneration of spent MIL-88A. These features make MIL-88A an effective and recyclable heterogeneous catalyst for the activation of persulfate.

Prussian blue analogue derived magnetic carbon/cobalt/iron nanocomposite as an efficient and recyclable catalyst for activation of peroxymonosulfate

A Prussian blue analogue, cobalt hexacyanoferrate Co3[Fe(CN)6]2, was used for the first time to prepare a magnetic carbon/cobalt/iron (MCCI) nanocomposite via one-step carbonization of Co3[Fe(CN)6]2. The resulting MCCI consisted of evenly-distributed cobalt and cobalt ferrite in a porous carbonaceous matrix, making it an attractive magnetic heterogeneous catalyst for activating peroxymonosulfate (PMS). As Rhodamine B (RhB) degradation was adopted as a model test for evaluating activation capability of MCCI, factors influencing RhB degradation were thoroughly examined, including MCCI and PMS dosages, temperature, pH, salt and radical scavengers. A higher MCCI dosage noticeably facilitated the degradation kinetics, whereas insufficient PMS dosage led to ineffective degradation. RhB degradation by MCCI-activated PMS was much more favorable at high temperatures and under neutral conditions. The presence of high concentration of salt slightly interfered with RhB degradation by MCCI-activated PMS. Through examining effects of radical scavengers, RhB degradation by MCCI-activated PMS can be primarily attributed to sulfate radicals instead of a combination of sulfate and hydroxyl radicals. Compared to Co3O4, a typical catalyst for PMS activation, MCCI also exhibited a higher catalytic activity for activating PMS. In addition, MCCI was proven as a durable and recyclable catalyst for activating PMS over multiple cycles without efficiency loss and significant changes of chemical characteristics. These features demonstrate that MCCI, simply prepared from a one-step carbonization of Co3[Fe(CN)6]2 is a promising heterogeneous catalyst for activating PMS to degrade organic pollutants.

Magnetic iron/carbon nanorods derived from a metal organic framework as an efficient heterogeneous catalyst for the chemical oxidation process in water

By one-step carbonization, metal organic frameworks (MOFs) can be conveniently turned into hierarchical hybrid materials which exhibit versatile functionalities. Even though theoretically all MOFs can be carbonized to yield carbon-based hybrids, iron-based MOFs seem to be suitable precursors owing to the abundance and low-toxicity of iron. While many iron-based MOFs have been developed, most of these MOFs are synthesized using DMF, a carcinogenic solvent. Thus, an iron-based MOF, MIL-88A, appears to be an ideal precursor because it can be prepared just in water. MIL-88A-derived carbonaceous material consists of iron oxide nanoparticles and porous carbon, making it a magnetic porous support/adsorbent. However, its iron content and porosity in fact can enable it to be a promising heterogeneous and magnetic catalyst for chemical oxidation, which, however, has not been investigated. Herein, MIL-88A was used to prepare a magnetic iron/carbon nanorod (MICN). The MICN was characterized and then evaluated as a heterogeneous catalyst to activate oxidants, including hydrogen peroxide and sodium persulfate to decolorize Rhodamine B (RB) dye in water. While RB could not be removed via the adsorption to MICN, and degradation by oxidants, the combination of MICN and oxidants successfully decolorized RB owing to the iron content of MICN which activated peroxide and persulfate through a Fenton-like reaction. In addition, MICN loading was found to be a more critical factor than the oxidant dosage for RB decolorization. Elevated temperature also improved the RB decolorization, whereas basic conditions were not favorable for MICN-activated oxidative processes. Ultrasonication represented a useful external facilitator to enhance the decolorization while the addition of ascorbic acid greatly inhibited the activation process. The recyclability of MICN was also demonstrated, showing that MICN could be reused for multiple cycles without regeneration treatment. These features enable MICN to be an effective and easy-to-recover chemical oxidative catalyst.

α-Sulfur as a metal-free catalyst to activate peroxymonosulfate under visible light irradiation for decolorization

While transition metals have been frequently used to activate peroxymonosulfate (PMS) for chemical oxidation reactions, recently metal-free activation of PMS has also drawn great attention considering that no metal is required and the environmental impact can be minimized. In this study, orthorhombic α-sulfur (α-S), for the first time, is employed as a metal-free photocatalyst to activate PMS under visible (vis) light irradiation. To study the activation of PMS by the α-S/vis process, decolorization of rhodamine B (RhB) dye is selected as a model reaction. Parameters affecting the decolorization were investigated, including α-S loading, PMS dosage, temperature, pH, salt and inhibitors. The decolorization using PMS activated by α-S/vis was much faster than the self-activation of PMS. A higher α-S loading also facilitated the activation of PMS; however, over-loading of α-S led to the shielding effect, thereby decreasing the decolorization extent. Higher PMS dosages and temperatures were both preferable for the decolorization. While the decolorization was improved under acidic conditions, the activation of PMS was hindered under alkaline conditions. When high concentrations of NaCl were added to RhB solutions, the decolorization extent still remained almost the same. Electron paramagnetic resonance (EPR) spectroscopic analysis was performed to probe into the mechanism of PMS activated by the α-S/vis process. The α-S/vis process was found to be recyclable and stable over multiple cycles, even though α-S did not undergo any regeneration treatments. Considering these features, the α-S/vis process appears to be a promising and environmentally friendly process to activate PMS for chemical oxidation reactions.

Multi-functional MOF-derived magnetic carbon sponge

Carbon sponges/foams are important supports for developing various functional devices. While post modifications are commonly adopted to decorate carbon sponges with certain fine particles, one-step carbonization of 3D organic templates assembled with micro/nanoscale components appears to be a rapid, convenient and scalable technique to obtain functional carbon sponges. By selecting specific micro/nanoscale components, certain chemical and morphological properties of functional carbon sponges can be designed and obtained. Here, we demonstrate this concept by carbonizing a self-assembly of metal organic frameworks and melamine sponge to yield a MOF-derived magnetic carbon sponge (MCS), which exhibits macroporous properties from carbon sponge, and nanoscale porosity and catalytic sites derived from the immobilized MOFs. As the cobalt-based ZIF-67 was selected as a representative MOF in this study, the resulting magnetic carbon sponge exhibited promising capabilities for separation of floating oil from water, removal of oil droplets from oil/water emulsions, environmental catalysis and catalytic H2 production. We anticipate that this preparation strategy can be expanded by adopting various types of MOFs and organic templates to develop versatile 3D hierarchical carbon materials for various applications.

Efficient demulsification of oil-in-water emulsions using a zeolitic imidazolate framework: Adsorptive removal of oil droplets from water.

To demulsify oil-in-water (O/W) emulsions, a zinc-based zeolitic imidazolate framework (ZIF-8) was employed for the first time to remove oil droplets from water. ZIF-8 exhibits a high surface area and positive surface charges, making it a suitable adsorbent to adsorb negatively-charged oil droplets. Adsorption behaviors of oil droplets to ZIF-8 were studied by analyzing the adsorption kinetics and isotherm with theoretical models. The activation energy of adsorption of oil droplets to ZIF-8 was determined as 24.1kJmol(-1). The Langmuir-Freundlich (L-F) model was found to be most applicable to interpret the isotherm data and the predicated maximum adsorption capacity of ZIF-8 can reach 6633mgg(-1), revealing a promising capability of ZIF-8 for demulsification. Factors influencing the adsorption of oil droplets to ZIF-8 were investigated including temperature, pH, salt and surfactants. The adsorption capacity of ZIF-8 for oil was improved at elevated temperatures, whereas alkaline condition was unfavorable for the adsorption of oil droplets due to the electrostatic repulsion at high pH. The adsorption capacity of ZIF-8 remained similar in the presence of NaCl but it was reduced in the presence of surfactants. ZIF-8 was regenerated by a simple ethanol-washing method; the regenerated ZIF-8 exhibited more than 85% of regeneration efficiency over six cycles. Its crystalline structure also remained intact after the regeneration. These characteristics indicate that ZIF-8 can be a promising and effective adsorbent to remove oil droplets for demulsification of O/W emulsions.