Inclusion complexes of faujasite with paraffins and permanent gases

Abstract

An investigation has been made of occlusion of O 2 , A and N 2 , and of CH 4 , C 2 H 6 , C 3 H 8 , n - and iso -C 4 H 10 , n -, iso - and neo -C 5 H 12 , n -C 6 H 14 , n -C 7 H 16 , n -C 8 H 18 and iso -C 8 H 18 within the crystal lattice of dehydrated synthetic faujasite (Na 2 O. Al 2 O 3 . 2·67SiO 2 . m H 2 O). The three permanent gases are selectively occluded in the order of their quadrupole moments (N 2 > O 2 , A). This selectivity permits the separation of N 2 and O 2 , as a result not of molecular sieve action but of different sorption affinities. The crystals of faujasite occlude nearly half their own volume of all the hydrocarbons, expressed as cm 3 of liquid, whatever the molecular volume of the occluded species. O 2 , N 2 and A also fill about the same intersti­tial volume. Thus all interstitial positions are equally accessible to small and to very large molecules and so must be of substantial dimensions. The numbers of molecules per unit cell, of 24·8 Å edge, range from 254 (H 2 O), 149 (A) down to 22 ( iso -octane). On the basis of a crystal structure of polyhedra, each built from tetrahedra and linked with each other in fourfold co-ordination, one may satisfy the crystal symmetry and unit cell dimensions. There are then eight very large cages per unit cell, interconnected through wide windows. Each of these cages is therefore able to contain a large number of molecules (e. g. 32 water molecules to 2·8 iso -octane molecules), and has a free diameter of ~ 12 Å. Some properties of the sorbed molecules have been determined. Over the earlier part of the isotherm, plots of p/v against p may have linear sections. As the interstitial volume in the crystals becomes more nearly saturated with occluded molecules the behaviour resembles that of a liquid filling a limited volume. This is shown in several ways, including the apparent thermal expansion of sorbed molecules. Evaluation of differential heats of sorption for various hydrocarbons has shown clear relations between the heat and the number of carbon atoms in the molecule, with modifications for isomeric structures; and between isosteric heats and heats of liquefaction of the liquid sorbates. Standard entropies of sorption and partial molar entropies of sorbed molecules have also been derived.