Microporous PILCs — Synthesis, pillaring mechanism and selective cation exchange

Abstract

Based on a 3-dimensionally ordered, synthetic Cs-hectorite (Cs 0.56[Mg 2.44Li 0.56]<Si 4> O 10F 2) a truly microporous pillared inter-layered clay (PILC) was obtained by intercalation of diprotonated DABCO molecules (DABCO = 1,4-diazabicyclo[2.2.2]-octane). Not only is the stacking order retained upon pillaring ( C 2/ m (no. 12); a = 5.247(1) Å, b = 9.083(2) Å, c = 14.581(5) Å and β = 96.799°), additionally, a 2-dimensional long range order of the pillars in the interlamellar space was observed. Indexing proved this 2-dimensional superstructure to be commensurable with the host ( a ⁎ ≈ 3 × a = 15.731(3) Å, b ⁎ ≈ b = 9.090(2) Å). This in turn suggests that it is not the electrostatic repulsion between the positively charged pillars but instead the host–guest-fit that determines the lateral arrangement of pillars in the interlamellar space. The large crystal sizes, which this PILC comes with, allowed to verify that intercalation of the organic pillars follows the ring mechanism in accordance with what has been proposed some years ago by Weiss et al. (1970). Noteworthy, upon intercalation, the acidic protons undergo a fast dynamic exchange between the pillars, water molecules present in the micropores, and framework atoms of the host. Consequently, a considerable amount of the open space between the pillars is actually occupied by H 3O + and H 2O, which cannot be removed completely by drying. Interestingly, a selective partial cation exchange of these acidic protons by Na + could be achieved. Thus, pillaring with 2H-DABCO affords a well ordered, Brønsted-acidic, microporous material with a narrow pore size distribution.