Overview of intentional formation of paired heteroatom sites in zeolite frameworks

Abstract

Functional sites in close proximity are known to exhibit unique and/or different behavior compared to isolated sites, and thus, their intentional construction in synthetic materials has been investigated in the field of material chemistry. Zeolites, crystalline and porous metallosilicates, are attractive target materials due to their tunability via isomorphous substitution of heteroatoms into frameworks as well as their broad applications as catalysts and ion exchangers. To date, the intentional formation of juxtaposed functional sites, so-called paired heteroatom sites represented by M–O–(Si–O)_x–M (x = 1 or 2, M = Al or Ga) atomic arrangements, in various zeolite frameworks has been achieved mainly for Al; very recently, the juxtaposition of Ga has also been demonstrated. This highlight overviews the intentional formation of paired heteroatom sites by classifying reported works into three categories on the basis of each of the employed synthetic approaches: the use of appropriate sources of Si and Al and order of addition of substances on the basis of their screening; cooperative charge compensation by bulky and small positively charged structure-directing agents; and transcription of pre-formed paired species from a precursor to a zeolite framework. The typical characterization techniques (i.e., solid-state ²⁹Si nuclear magnetic resonance spectroscopy and the ion-exchange method using Co²⁺ as a probe species) and reported applications of paired heteroatom sites are also summarized and described.