Evaluation of the microporosity of pillared [Fe(CN)(6)]-MgAl-LDHs

Layered double hydroxides (LDHs) containing Mg2+ and Al3+ in the basic laye rs and NO3- as an interlayer anion were synthesized by the method of coprec ipitation (pH 10). By changing the Mg2+/Al3+ ratio (1.5-4.5), the charge de nsity on the (NO3)-MgAl-LDH sheets was varied. After pillaring with Fe(CN)( 3)(6-), which was based on an anion exchange process, the interlayer space became accessible. This was reflected in the large created surface areas an d micropore volumes. The applied models for the calculation of the micropor e size distributions (Maes-Zhu-Vansant and Horvath-Kawazoe) gave matching r esults, revealing narrow distributions for all the samples, with the majori ty of the pores smaller than 0.71 nm. A correlation was found between the M g2+/Al3+ ratio and the resulting microporosity after pillaring. The optimal ratio was situated around 3.3, resulting in a pillared [Fe(CN)(6)]-MgAl-LD H with a Langmuir surface area of 499 m(2)/g and a micropore volume between 0.158 ml/g (mu PVmin) and 0.177 ml/g (mu PVmax). As an alternative, direct coprecipitation of the pillared LDHs was evaluated. This one-step mechanis m proved to be a method producing similar results. Taking all this into con sideration, one can conclude that hexacyanoferrate(III) complexes form idea l anionic pillars for the creation of microporous layered double hydroxides . (C) 1999 Elsevier Science B.V. All rights reserved.