Selectivity

Abstract: This study considered the potential of a natural Australian zeolite, clinoptilolite, to remove ammonium from water Ammonium-exchange capacity and rates of adsorption are critical to the assessment of the feasibility of the zeolite for application to continuous wastewater treatment A laboratory study was undertaken, using pure solutions, to investigate the equilibria and kinetic characteristics of ammonium exchange in the zeolite Binary equilibrium experiments provided information on the adsorption characteristics of the zeolite in terms of ammonia capacity at varying solution concentrations These experiments also revealed that the highest ammonium removal efficiency was achieved when the zeolite's exchange sites were converted to the sodium form Multicomponent equilibrium experiments were carried out to determine the effects of competing cations on the ammonium-exchange capacity of the zeolite The laboratory study indicated the zeolite's selectivity for ammonium ions over other cations typically pre

Abstract: Membranes have the potential to substantially reduce energy consumption of industrial chemical separations, but their implementation has been limited owing to a performance upper bound—the trade-off between permeability and selectivity. Although recent developments of highly permeable polymer membranes have advanced the upper bounds for various gas pairs, these polymers typically exhibit limited selectivity. We report a class of hydrocarbon ladder polymers that can achieve both high selectivity and high permeability in membrane separations for many industrially relevant gas mixtures. Additionally, their corresponding films exhibit desirable mechanical and thermal properties. Tuning of the ladder polymer backbone configuration was found to have a profound effect on separation performance and aging behavior. Description When aging is an asset Separations achieved using membrane processes offer the prospect of reducing the energy costs of industrial-scale chemical separations, but innovative membrane materials are required to achieve the combinations of flux and selectivity. Lai et al. developed a set of ladder polymers prepared using catalytic arene-norbornene annulation polymerization that incorporate fluorene and dihydrophenanthrene units (see the Perspective by Budd). Upon physical aging, these polymers contort in a way that enhances their size-sieving capabilities, as demonstrated for separation between methane and carbon dioxide mixes and hydrogen and methane. —MSL Aging of ladder polymer membranes can tune their structure, leading to enhanced separation of industrially important gas pairs.

Abstract: The unique performances of supported rhenium oxide catalysts for the selective oxidation of methanol to methylal (dimethoxymethane) [3CH3OH + 1/2O2 → CH2(OCH3)2 + 2H2O] were examined in a fixed-bed flow reactor and in a pulse reactor. Rhenium oxides supported on α-Fe2O3, γ-Fe2O3, and V2O5 among many oxide supports showed high activities (15−49% conversion) and selectivities (90−94% selectivity) for catalytic methylal synthesis at 513 K. These Re catalysts are the first to show good performances for one-stage methylal synthesis from methanol. The catalysts were characterized by XRD, XPS, TPR, TPD, and FTIR spectroscopy to provide insight into the active Re species relevant to the selective oxidation of methanol to methylal. Rhenium oxides on α-Fe2O3 were dispersed as tetrahedral ReO4 species up to one monolayer at 0.7 wt % Re loading. It was suggested that active Re−Fe−O mixed-oxide clusters were formed above 1 wt % Re. ReO2 crystallites were also observed for the Re/α-Fe2O3 samples with Re loadings above ...