Pillared Clays and Layered Silicates

Pillared clays (PILC) have been reported to show catalytic properties comparable to zeolites in several reactions, and therefore to have comparable acidities [107]. 

The preparation and properties of smectites pillared with A1 hydroxy oligomers have been extensively investigated. Most of the work has been performed on montmoriUonite and hectorite, which have been intercalated with a great variety of pillaring agents and oUgocations. 

Intercalation of hydroxy silicoaluminum compounds increases the thermal stability and the acidity of the pillared clays. 

The distributions of acid strengths of various pillared clays obtained by calorimetry of NH3 adsorption showed that Al-PILC displays high acidity, with a small number of sites as strong as those measured on zeolites. Acidity should be directly related to the environment of the A1 ions in the pillars, which could be modified by the preparation. The adsorption of NH3 on pillared beidellite showed the presence of many strong acid sites (160 kJ mol ) [108]. 

A natural clay has been pillared with mixed solutions containing both A1 and Fe, Ti or Cr. The intercalation-generated solids distribution of acid strengths measured by calorimetric adsorption of ammonia is comparable to that of zeoUtes. The surfaces appear as heterogeneous and show initial adsorption heats close to 150-160 kJ moT if one excludes the first point of the differential heat versus coverage curves, which is much higher (=190kJ mol ) [109]. 

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