Search

Abstract: The present work describes the adsorption and separation of xylene isomers by ZIF-8. Although the formal pore diameter of ZIF-8 is much smaller than the molecular diameter of the xylene isomers, ZIF-8 is able to separate the isomers by molecular sieving. A structural study indicates that the diffusion of the xylenes into the pore structure of ZIF-8 happens via a transitory deformation of the pore aperture which is based on a tilt of the imidazolate linkers, followed by a return to the initial conformation. The rate of adsorption depends on the size of the isomer, i.e. it decreases from para- to meta- and to ortho-xylene. The separation of the xylene isomers is good in the gas phase. In liquid phase breakthrough experiments, the quality of the separation is deteriorated. Moreover, as expected for a separation based on molecular sieving, para-xylene cannot be well separated from ethyl-benzene. [Copyright &y& Elsevier]

Abstract: The crystal chemistry of divalent metal N,N′-piperazinebis(methylenephosphonates) of the STA-12 family, (M2(H2O)2(O3PCH2NC4H8NCH2PO3)·xH2O, M=Mg, Mn, Fe, Co, Ni) is compared. The different metal analogues are isostructural in the hydrated forms, possessing symmetry, but their reversible dehydration behaviour and resultant porosity are strongly dependent on the metal cation. Whereas the Co and Ni forms change symmetry to triclinic upon dehydration, giving permanent porosity to N2 of 0.14cm3 g−1 and 0.27cm3 g−1, respectively, the Mn and Fe forms remain rhombohedral but exhibit a strong decrease in unit cell volume (e.g. 23% for STA-12(Mn)). Structure determination of dehydrated STA-12(Mn) indicates a topotactic transformation with a change in coordination of the phosphonate O atoms. Negligible permanent porosity is observed in either dehydrated STA-12(Mn) or (Fe), suggesting the presence of non-crystallographic pore blocking. Dehydration of STA-12(Mg) results in loss of some long range order, preventing structural determination of the fully dehydrated form, but does give appreciable permanent porosity for N2 of 0.20cm3 g−1. Infrared spectroscopy (and for STA-12(Mg) solid-state NMR spectroscopy) have been used to follow the changes upon dehydration. Applications of the most porous and stable STA-12 structure, the Ni form, have also been investigated. CO2 adsorption selectivity over CH4 and CO has been measured via analysis of breakthrough curves, and a Porous Layer Open Tubular (PLOT) gas chromatographic column has been prepared and used to separate a mixture of low molecular weight alkanes. [Copyright &y& Elsevier]