Pillared Clays and Ion-Exchanged Pillared Clays as Gas Adsorbents and as Catalysts for Selective Catalytic Reduction of No

Abstract

Pillared interlayered clays (PILC), or pillared clays, are two-dimensional layer materials prepared by exchanging the charge-compensating cations (e.g. Na+, K+ and Ca2+) between the swelling phyllosilicate clay layers with larger inorganic hydroxy cations, which are polymeric or oligomeric hydroxy metal cations formed by hydrolysis of metal salts. Upon heating, the metal hydroxy cations undergo dehydration and dehydroxylation, forming stable metal oxide clusters which act as pillars keeping the silicate layers separated and creating interlayer spacing (gallery spaces) of molecular dimensions. Pillared clay development started in the mid 1950s by Barrer and co-workers1,2 who synthesized high surface area materials by pillaring montmorillonite clay with cations of N(CH3)4 and N(C2H5)4. However, these materials have low thermal and hydrothermal stabilities i.e., up to ca. 250°C for the decomposition temperature of interlayer cations, and are therefore of limited use as adsorbents and catalysts. Much interest and research have, since the 1970s, been directed toward the synthesis of pillared clays with high thermal and hydrothermal stabilities. The most promising ones for use as adsorbents and catalysts are as follows; Al-PILC3-l8, Zr-PILC4,5,19-23, Cr-PILC6,24-28, Fe-PILC29-35, Ti-PILC36-38, Ni-PILC39 and Si-PILC40-41. The choices of hydroxy cations are not limited to those mentioned; in fact any metal oxide or salt that forms polynuclear species upon hydrolysis42 can be inserted as pillars, and all layered clays of the abundant phyllosilicate family as well as other layered clays can be used as the hosts16,43-49. In order to increase the thermal and hydrothermal stabilities and the catalytic activities of pillared clays, mixed pillars have been synthesized using mixed hydroxycations obtained by isomorphous substitution of mvalent cations (e.g. Ga3+, Ce3+, Fe3+, Cr3+) with Al3+ in the AI polyoxocation12,50-52. In addition, positively charged sol