Yongde Xia

Zeolitic imidazolate framework materials: recent progress in synthesis and applications

Zeolitic imidazolate frameworks (ZIFs) represent a new and special class of metal organic frameworks comprised of imidazolate linkers and metal ions, with structures similar to conventional aluminosilicate zeolites. Their intrinsic porous characteristics, abundant functionalities as well as exceptional thermal and chemical stabilities have led to a wide range of potential applications for various ZIF materials. Explosive research activities ranging from synthesis approaches to attractive applications of ZIFs have emerged in this rapidly developing field in the past 5 years. In this review, the development and recent progress towards different synthesis strategies to generate both powder and membrane/film-based ZIF materials are analysed and summarised. Their attractive and potential applications in gas separation, catalysis, sensing and electronic devices, and drug delivery in the past years are discussed and reviewed. In addition, the prospects and potential new development of ZIF materials are presented.

Hydrogen Storage in High Surface Area Carbons: Experimental Demonstration of the Effects of Nitrogen Doping

The influence of nitrogen doping on the hydrogen uptake and storage capacity of high surface area carbon materials is presented in this report. To generate suitable study materials, we have exploited the relationship between synthesis conditions and textural properties of zeolite-templated carbons to generate a range of high surface area carbons with similar pore size distribution but which are either N-doped or N-free. For N-doped carbons, the nitrogen content was kept within a narrow range of between 4.7 and 7.7 wt %. The carbon materials, irrespective of whether they were doped or not, exhibited high surface area (1900-3700 m(2)/g) and pore volume (0.99 and 1.88 cm(3)/g), a micropore surface area of 1500-2800 m(2)/g, and a micropore volume of 0.65-1.24 cm(3)/g. The hydrogen uptake varied between 4.1 and 6.9 wt %. We present experimental data that indicates that the effect of N-doping on hydrogen uptake is only apparent when related to the surface area and pore volume associated with micropores rather than total porosity. Furthermore, by considering the isosteric heat of hydrogen adsorption and excess hydrogen uptake on N-free or N-doped carbons, it is shown that N-doping can be beneficial at lower coverage (low hydrogen uptake) but is detrimental at higher coverage (higher hydrogen uptake). The findings are consistent with previous theoretical predictions on the effect of N-doping of carbon on hydrogen uptake. The findings, therefore, add new insights that are useful for the development of carbon materials with enhanced hydrogen storage capacity.

Porous carbon-based materials for hydrogen storage: advancement and challenges

The development of highly efficient hydrogen storage materials is one of the main challenges that must be tackled in a widely expected hydrogen economy. Physisorption in porous materials with high surface areas and chemisorption in hydrides are the two main options for solid state hydrogen storage, and both options possess their inherent advantages and drawbacks. In this work, recent progress towards porous carbon-based materials for hydrogen storage is analyzed and reviewed. The hydrogen storage performance of plain porous carbons, metal-supported porous carbons and porous carbons confined hydrides is summarized. Some strategies for effectively controlling the hydrogen storage capacity and tuning the hydrogen adsorption enthalpy for porous carbon materials via appropriate manipulation of surface area, pore volume and pore size are discussed in detail. The new development of porous carbon-based materials for hydrogen storage is particularly emphasized.

Hollow spheres of crystalline porous metal oxides: A generalized synthesis route via nanocasting with mesoporous carbon hollow shells

Hollow spheres and shells of crystalline porous metal oxides have been nanocast using hollow spheres of mesoporous carbon as hard template. The metal oxides are fabricated from alkoxide precursors within the pore channels of the carbon templates. Remarkably, only one infiltration cycle was required to introduce (metal alkoxide) molecular precursors into the pores of the carbon templates. Removal of the carbon by calcination (at 500–600 °C) results in porous metal oxides with predominantly hollow sphere morphology, thus demonstrating the replication of the hollow sphere morphology from carbon to metal oxide. The metal oxides (titania, zirconia, alumina and magnesia) exhibit highly crystalline frameworks and relatively high surface area. The surface area is particularly high for alumina (γ-Al2O3, 212 m2 g−1) and titania (anatase, 100 m2 g−1). Mixed (MgO–Al2O3) or binary (MgTiO3) metal oxides with relatively well formed hollow sphere morphology and high surface area (154 m2 g−1 for MgTiO3 and 322 m2 g−1 for MgO–Al2O3) may also be nanocast.

Cobalt sulfide/N,S codoped porous carbon core-shell nanocomposites as superior bifunctional electrocatalysts for oxygen reduction and evolution reactions.

Exploring highly-efficient and low-cost bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) in the renewable energy area has gained momentum but still remains a significant challenge. Here we present a simple but efficient method that utilizes ZIF-67 as the precursor and template for the one-step generation of homogeneous dispersed cobalt sulfide/N,S-codoped porous carbon nanocomposites as high-performance electrocatalysts. Due to the favourable molecular-like structural features and uniform dispersed active sites in the precursor, the resulting nanocomposites, possessing a unique core-shell structure, high porosity, homogeneous dispersion of active components together with N and S-doping effects, not only show excellent electrocatalytic activity towards ORR with the high onset potential (around -0.04 V vs.-0.02 V for the benchmark Pt/C catalyst) and four-electron pathway and OER with a small overpotential of 0.47 V for 10 mA cm(-2) current density, but also exhibit superior stability (92%) to the commercial Pt/C catalyst (74%) in ORR and promising OER stability (80%) with good methanol tolerance. Our findings suggest that the transition metal sulfide-porous carbon nanocomposites derived from the one-step simultaneous sulfurization and carbonization of zeolitic imidazolate frameworks are excellent alternative bifunctional electrocatalysts towards ORR and OER in the next generation of energy storage and conversion technologies.

Finite-size and surface effects on the glass transition of liquid toluene confined in cylindrical mesopores

Some of the most regular porous silicates (MCM-41 and SBA-15), with several different pore diameters from 2.4 to 8.7 nm, are used to study the van der Waals fragile liquid toluene in confined geometry. We measure two major macroscopic signatures of a glass transition, i.e., a discontinuous change in the heat capacity and in the thermal expansion, by adiabatic calorimetry and neutron scattering experiments. A nontrivial size dependence of the glass transition features, most notably a nonmonotonic variation of the mean glass transition temperature, is observed. The range of the glass transition is found extremely broad. This supports the notion of competition between surface boundary conditions and cutoff or finite-size effects.

On the synthesis and characterization of ZSM-5/MCM-48 aluminosilicate composite materials

Composite micro/mesoporous ZSM-5/MCM-48 materials have been prepared using a simple two step crystallisation process. The synthesis process involved the assembly of precursor zeolite species, containing ZSM-5 units at various stages of crystallisation, into a mesostructured material. By varying the time allowed for the crystallisation of the precursor zeolite species (between 2 and 8 hours) it was possible to modify the composition of the composite materials (i.e., zeolite/MCM-48 ratio). Furthermore, it was possible to obtain relatively pure forms of both end members (ZSM-5 and MCM-48) from the precursor zeolite species after appropriate aging. The resulting composite materials were characterized by powder XRD, nitrogen sorption, IR spectroscopy, TGA, SEM and TEM and found to exhibit varying levels of zeolitisation. The ZSM-5 present in the composite materials was made up of nanosized zeolite crystallites. The textural properties of the composite materials were, in some cases, comparable to conventional mesoporous materials. The acidity and hydrothermal stability of the composite materials was found to be dependent on the extent of zeolitisation, i.e., ZSM-5/MCM-48 ratio.

New catalyst of SO 4 2− /Al2O3–ZrO2 for n-butane isomerization

The catalytic behavior of Al-promoted sulfated zirconia for n-butane isomerization at low temperature in the absence of H2 and at high temperature in the presence of H2 was studied. The addition of Al enhances the activity and stability of the catalysts for reaction at 250°C and in the presence of H2 significantly. After on stream for 120 h, the n-butane conversion of the catalyst containing 3 mol% Al2O3 keeps steadily at 88% of its equilibrium conversion and no observable trend of further deactivation has been observed. The difference in behavior of the promoted and unpromoted catalysts at low and high temperature is associated with a change of reaction mechanism from bimolecular to monomolecular. Experimental evidence is presented to show that the promoting effect of Al is different from that of the transition metals. Microcalorimetric measurements of NH3 adsorption on catalysts reveal that the remarkable activity and stability of the Al-promoted catalysts are caused by an enhancement in the number of acid sites effective for the isomerization reaction.

Are mesoporous silicas and aluminosilicas assembled from zeolite seeds inherently hydrothermally stable? Comparative evaluation of MCM-48 materials assembled from zeolite seeds

Well ordered cubic (MCM-48) mesoporous silica and aluminosilica materials were assembled from zeolite seeds (ZSM-5, Beta or silicalite-1) via a two-step crystallization process and their hydrothermal stability in steam and boiling water was investigated using a variety of techniques including powder XRD, nitrogen sorption studies, elemental analysis and monitoring of acid content. The presence of zeolite seeds within the framework of the resulting mesoporous materials was inferred from infrared spectroscopy and thermogravimetric analysis (TGA) studies. The proton exchanged forms of MCM-48 materials assembled from zeolite seeds (ZSM-5 or Beta) were found to exhibit hydrothermal stability which is much higher than that of conventional (direct mixed-gel synthesized) Al-MCM-48 but comparable or lower than that of equivalent Al-grafted MCM-48 materials. A purely siliceous MCM-48 material assembled from silicalite-1 zeolite seeds was found to exhibit much lower hydrothermal stability. The hydrothermal stability of Al-containing materials was in all cases (regardless of whether they contained zeolite seeds) higher than that of pure silica MCM-48 assembled from silicalite-1 zeolite seeds. This suggests that mesoporous materials assembled from zeolite seeds are not inherently hydrothermally stable. However, Al-MCM-48 materials assembled from zeolite seeds exhibited the highest acidity.

Synthesis of mesoporous silica hollow spheres in supercritical CO2/water systems

The synthesis of hollow silica spheres with mesopore wall structures via CO2-in-water emulsion templating in the presence of non-ionic PEO-PPO-PEO block copolymer surfactants as mesostructure-directing templates has been investigated under various conditions. The successful synthesis of hollow spheres strongly depends on the pressure and density of CO2. The mesoporosity (pore size) and morphology of the silica hollow spheres can be controlled by varying the operating CO2 pressure. Other types of surfactants, such as long chain alkylammonium ions, may also be used to template the porosity of the shells.