Journal
MICROPOROUS AND MESOPOROUS MATERIALS
Volume 157, Issue -, Pages 3-17Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.micromeso.2011.12.003
Keywords
Phosphonate MOFs; Dehydration behaviour; Adsorption; Breakthrough curves; GC PLOT column
Categories
Funding
- EPSRC [EP/F008384/01]
- DFG [STO 643/2]
- EPSRC [EP/E041825/1, EP/F008384/1] Funding Source: UKRI
- Engineering and Physical Sciences Research Council [EP/F008384/1, EP/J501542/1, EP/E041825/1] Funding Source: researchfish
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The crystal chemistry of divalent metal N,N'-piperazinebis(methylenephosphonates) of the STA-12 family, (M-2(H2O)(2)(O3PCH2NC4H8NCH2PO3)center dot xH(2)O, M = Mg, Mn, Fe, Co, Ni) is compared. The different metal analogues are isostructural in the hydrated forms, possessing 123 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 N-2 of 0.14 cm(3) g(-1) and 0.27 cm(3) 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 N-2 of 0.20 cm(3) 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. (C) 2011 Elsevier Inc. All rights reserved.
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