Sorption and molecular sieve properties of clays and their importance as catalysts

Abstract

Kandites, smectites and vermiculites all form interlamellar inclusion complexes. A comparison has been made between sorption in zeolites, clay minerals and clathrates. In smectites and vermiculites sorption isotherms reflect in contour and behaviour three situations. Kandites show only the first of these, in which the water-free crystals are used and the interlamellar cations, if any, are small inorganic ions (for example Na + , Ca 2+ ). The guest molecules are polar and penetration of the host occurs with swelling after a threshold pressure or activity is reached. The isotherms may show one or more steps and hysteresis between sorption and desorption cycles is normal. In the second situation all the interlamellar space is filled by long chain organic cations. If a potential guest is imbibed there is further expansion of the host and isotherms sometimes resemble in contour those obtained when benzene is imbibed by rubber. Cohesive energy densities of the interlamellar regions relative to those of the potential guest species strongly influence the interlamellar uptake and the selectivity. In the third situation the clay lamellae are propped apart permanently by cations that do not fill all interlamellar space and are chosen to give vertical free distances between sheets and horizontal free distances between adjacent cations which are of molecular dimensions. The resultant porous crystals behave like zeolite molecular sieves both in the contours of isotherms for the interlamellar part of the sorption and in the molecule sieving effects that have been obtained. In addition suitably modified or natural clay minerals can catalyse many kinds of reaction. These include cracking, isomerization, dimerization, and oligomerization, redox reactions and hydrogen transfer, hydration and dehydration, esterification, lactonization, etherification, conversion of primary to secondary amines and reaction between guest and interlamellar organic cations. Some comments on and examples of certain of these processes are given.