From non-porous crystalline to amorphous microporous metal(IV) bisphosphonates
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Porous layered hybrid materials have been prepared by the reaction of organo-bisphosphonate ligands, 4-(4′-phosphonophenoxy)phenylphosphonic, 4,4′-biphenylenbisphosphonic and phenylphosphonic acids, with metal(IV) cations (Zr and Sn). Crystalline Zr(IV) and Sn(IV) layered bisphosphonates were also prepared, which were non-porous. The amorphous M(IV) bisphosphonates showed variable compositions and textural properties ranging from mainly mesoporous to highly microporous solids with BET surface areas varying from 300 to 480 m2 g-1, micropore volumes ranging 0.10-0.20 cm3/g, and narrow porous size distributions for some materials. N2 isotherms suggest that Sn(IV) derivatives show a comparatively higher micropore contribution than the Zr(IV) analogous at least for the ether-bisphosphonate hybrids. Sn(IV) bisphosphonates exhibit high microporosities without the need of using harmful DMSO as solvent. If ether-bisphosphonic acid is partially replaced by less expensive phenylphosphonic ligand, porous products are also obtained. 31P and 17F MAS NMR and XPS data revealed the presence of hydrogen-phosphonate groups and small (F-, Cl- and OH-) anions, which act as spacer ligands within the inorganic layers, in these hybrid materials. The complexity of the inorganic layers is higher for the Sn(IV) bisphosphonates likely due to the larger amount of small bridging anions including fluorides. It is suggested that the presence of these small inorganic ligands may be a key factor influencing both, the interaction of the inorganic layer with the bisphosphonate groups, which bridge the inorganic layers, and the generation of internal voids within a given inorganic layer. Preliminary studies of gases adsorption (H2 and NO) have been carried out for selected Sn(IV) bisphosphonates. The H2 adsorption capacity at 77 K and 1 bar was low, 0.26 wt%, but the NO adsorption capacity at ∼1 bar and 298 K was relatively high, 4.2 wt%. Moreover, the hysteresis in the NO isotherms is indicative of partial strong irreversible adsorption of NO. © 2008 Elsevier Inc. All rights reserved.
author list (cited authors)
Cabeza, A., del Mar Gómez-Alcántara, M., Olivera-Pastor, P., Sobrados, I., Sanz, J., Xiao, B. o., ... Aranda, M.