MOF/graphite oxide hybrid materials: exploring the new concept of adsorbents and catalysts

Two types of metal-organic framework (MOF)/graphite oxide hybrid materials were prepared. One is based on a zinc-containing, MOF-5 and the other on a copper-containing HKUST-1. The materials are characterized by X-ray diffraction, sorption of nitrogen, thermal analyses, Fourier Transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). Their features are compared to the ones of the parent materials. The water stability and ammonia adsorption capacity of the hybrid materials were also evaluated. It was found that the latter compounds exhibit features similar to the ones of the parent MOF. In most cases, their porosity increased compared to the one calculated considering the physical mixture of MOF and GO. This new porosity likely located between the two components of the hybrid materials is responsible for the enhanced ammonia adsorption capacity of the compounds. However, for both the zinc-based and the copper-based materials (MOFs and hybrid materials), a collapse of the framework was observed as a result of ammonia adsorption. This collapse is caused by the interactions of ammonia with the metallic centers of MOFs either by hydrogen bonding (zinc-based materials) or coordination and subsequent complexation (copper-based materials). Whereas the MOF-5 based compounds collapse in presence of humidity, the copper-based materials are stable.     

Synthesis of hollow ellipsoidal silica nanostructures using a wet-chemical etching approach

We have utilized wet-chemical etching of ellipsoidal silica nanoparticles (ESNs) to form silica nanoshells of a range of elliptical morphologies, with the thicknesses of the ellipsoidal silica nanoshells (ESSs) controlled through variation of synthesis conditions. A mechanism has been proposed to explain how the nanoshells are formed, and we demonstrate that the porosity of the silica ellipsoid plays a role in generating silica shells. Our self-templated, wet-etching approach is an attractive alternate procedure to the approaches presently in existence for the following reasons: (i) it is a facile, one-step process that directly produces ellipsoidal silica nanoshells, while overcoming barriers (such as requirement of removing a solid-core template seed) utilized in many reported chemical etching studies; (ii) it results in ellipsoidal silica nanostructures with dimension less than 100 nm; (iii) with an appropriate etchant, the roughness of the silica shells can be well-controlled; and (iv) it results in tunable, uniform size particles with controllable shell thicknesses. Moreover, the silica materials with the unique structures might be adjusted to meet practical application requirements.     

Exploring the coordination chemistry of MOF-graphite oxide composites and their applications as adsorbents

Metal-organic frameworks (MOFs), besides being porous materials exhibit a very rich chemistry, which can be used for the synthesis of composites and/or the reactive adsorption of toxic gases. In this study, composites of MOFs (MOF-5, HKUST-1 or MIL-100(Fe)) and a graphitic compound (graphite or graphite oxide, GO) were synthesized and tested for the removal of NH(3), H(2)S and NO(2) under ambient conditions. The materials were characterized before and after exposure to the target gases by X-ray diffraction, thermogravimetric analysis, N(2) sorption measurement and FT-IR spectroscopy. The results indicate that strong chemical bonds exist between the MOF and GO as a result of the coordination between the GO oxygen groups and the MOFs' metallic centers. Depending on the structure of the MOF, such interactions induce the formation of a new pore space in the interface between the carbon layers and the MOF units, which enhances the physical adsorption capacity of the toxic gases. When unsaturated metallic sites are present in the MOFs, the target gases are also adsorbed via coordination to these centers. Further reaction with the framework leads to the formation of complexes. This is accompanied by the collapse of the MOF structure.     

Photooxidation of dibenzothiophene on TiO(2)/hectorite thin films layered catalyst

A new titanium(IV) oxide-hectorite nanofilm photocatalyst was prepared on quartz slides. It was evaluated in the photooxidation of dibenzothiophene (DBT) in nonpolar organic solution (tetradecane), as a model for diesel fuel. A removal regimen was developed consisting of catalytic photooxidation followed by adsorption of products on silica gel. Photooxidation of DBT was performed with and without catalyst, at 254 and 300 nm. Comparison was made with a commercially available TiO(2) catalyst, Degussa P25. The catalyst was analyzed by nitrogen adsorption, XRD, SEM, and TGA-DTA. DBT concentrations were measured by HPLC and UV spectrophotometry. Preliminary qualititative analysis of products was performed by UV and HPLC. Results indicated that the outlined process was effective in reducing sulfur levels to below 10 ppm sulfur.     

Hydrogen Sulfide Adsorption on MOFs and MOF/Graphite Oxide Composites

Composites of a copper‐based metal‐organic framework (MOF) and graphite oxide (GO) were tested for hydrogen sulfide removal at ambient conditions. In order to understand the mechanisms of adsorption, the initial and exhausted samples were analyzed by various techniques including X‐ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analyses, and sorption of nitrogen. Compared to the parent materials, an enhancement in hydrogen sulfide adsorption was found. It was the result of physical adsorption of water and H₂S in the pore space formed at the interface between the MOF units and the graphene layers where the dispersive forces are the strongest. Besides physisorption, reactive adsorption was found as the main mechanism of retention. H₂S molecules bind to the copper centers of the MOF. They progressively react with the MOF units resulting in the formation of copper sulfide. This leads to the collapse of the MOF structure. Water enhances adsorption in the composites as it allows the dissolution of hydrogen sulfide.     

S掺杂纳米多孔碳的光催化活性:表面化学和孔隙率的重要性(英文)

This minireview summarizes our recent findings on the photoactivity of S-doped nanoporous carbons. The materials were either synthesized from the sulfur-containing polymers or obtained by heat treatment of commercial carbon with hydrogen sulfide. Their surface was extensively charac terized from the points of view of its surface chemistry, porosity, morphology, and electronic properties. The carbons showed enhanced activity towards oxidation of arsine and removal of diben-zothiophenes from model diesel fuel. The latter were oxidized to various oxygen containing intermediates and the cleavage of C–C bonds in aromatic ring was detected when carbon with adsorbed species was exposed to UV or visible light. Irradiation resulted in generation of photocurrent in a broad range of wavelength. The presence of sulfur led to the reduction of oxygen and contributed to an increased capacitive performance. We link these effects to the presence of reduced sulfur in the small pores which enhances the dispersive interactions via inducing a positive charge to carbon atoms, to sulfur in oxygenated forms which contribute to Faradaic reactions and increase the polar interactions, and to the hydrophobicity of a surface in small pores where oxygen can be reduced by excited electrons from chromophoric-like sulfur containing groups.     

Reactive adsorption of penicillin on activated carbons

A series of activated carbons with varied surface chemistry, obtained by wet oxidation and thermal treatment, was used for the removal of penicillin from low concentration aqueous solution. It was found that the carbon surface chemistry favors the degradation of the antibiotic, giving rise to various intermediates detected both in solution and in the adsorbed phase (deposited with the pore structure of the activated carbons). The confinement of penicillin molecules entrapped in the nanopores of activated carbons of acidic nature accelerates their degradation compared to that one in the bulk solution, which can be linked the strong local pH fall inside the pores. Degradation also takes place in activated carbons of basic pH, although the nature and partition of the intermediates formed differ from those in the acidic carbons. In both cases most of the breakdown products do not present therapeutic activity.     

On the adsorption/oxidation of hydrogen sulfide on activated carbons at ambient temperatures

Activated carbons of various origins (bituminous coal, wood, coconut shells, and peat) were studied as adsorbents of hydrogen sulfide. Before the experiments the surface of the adsorbents was characterized by using the sorption of nitrogen, Boehm and potentiometric titrations, thermal analysis, and FTIR. The adsorbents were chosen to differ in their surface areas, pore volumes, and surface acidities. To broaden the spectrum of surface acidity, carbons were oxidized by using nitric acid and ammonium persulfate. After hydrogen sulfide adsorption the species present on the surface were analyzed using thermal analysis, ion chromatography, and elemental analysis. The H(2)S breakthrough capacity tests showed that the performances of different carbons differ significantly. For a good performance of carbons as hydrogen sulfide adsorbents a proper combination of surface chemistry of carbon and porosity is needed. It was demonstrated that a more acidic environment promotes the formation of sulfur oxides and sulfuric acid despite yielding small H(2)S removal capacities. On the other hand, a basic environment favors the formation of elemental sulfur (sulfur radicals) and yields high capacities. The presence of a sufficient amount of water preadsorbed on the carbon surface to facilitate dissociation also plays an important role in the process of H(2)S adsorption/oxidation. The results showed that there is a critical value in carbon surface acidity, which when exceeded results in a negligible hydrogen sulfide breakthrough capacity. This is consistent with the mechanism of H(2)S adsorption on unmodified carbons, where the rate-limiting step is the reaction of adsorbed hydrogen sulfide ion with dissociatively adsorbed oxygen. When the acidity is expressed as pH, its value should be higher than 5 to ensure the effective removal of hydrogen sulfide from the gas phase. Study of carbon regeneration using water washing and heat treatment showed that the adsorbents can be regenerated to about 40% of their initial capacity.     

Template-free synthesis of silica ellipsoids

We have created ellipsoidal or spherical morphologies of silica particles in a template-free scheme that involves controlling surface tension forces through selected volume ratios of a water/oil micellar system.