The Largely Unknown Class of Microporous Hybrid Materials: Clays Pillared by Molecules

Abstract

AbstractThe review summarizes recent progress in the field of synthetic clay minerals pillared with organic or metal complex cations (PILCs). We briefly introduce the field of such PILCs and discuss the weaknesses of the state of the art characterization chart of PILCs. When PILCs are made from nanoscopic host materials like natural clay minerals, charge heterogeneity accompanied by interstratification, the turbostratic disorder in the stacking and the influence of the large external surface weaken essential analytical results. Turning to coarse‐grained, well ordered clay minerals synthesized from the melt at temperatures above 1000 K removes all these obstacles and allows to present consistent data underlining the validity of the appealing pillaring concept: Refinement of PILC structures gives information on the pillar‐host interaction and the orientation of pillars in the interlayer space. Two‐dimensional superstructure reflections give direct evidence of the well‐ordered lateral arrangement of pillars. And most importantly, analysis of chemical composition and physisorption isotherms are not corrupted by contributions of large external surfaces and this allows for predicting micropore volume and widths from pillar/host ratios in combination with pillar size/shape. Moreover, recent progress allows fine‐tuning the charge density of the host lattice post‐synthesis. Such deliberate and fine‐graded alteration of the pillar density in turn permits adjusting the pore size in steps as small as 0.1 Å to a given adsorbate and thus will pave the way to maximize adsorption enthalpies and to improve selectivity. This feature is unique to PILCs and is not available to MOFs whose porosity can be adjusted only in steps no smaller than the order of chemical bond lengths. Rational design of microporous hybrid materials with full control over size, shape, and chemical nature of micropores certainly represents the strength and great potential of the pillaring approach.