Effect of gas phase modification of analcime zeolite composite membrane on separation of surfactant by ultrafiltration

Abstract

Ceramic–analcime zeolite composite membranes have been synthesized by hydrothermal crystallization of zeolite over clay supports. The zeolite layer is characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and pore size distribution determined using the bubble point technique. The XRD pattern of the zeolite is found to match with JCPDS file #19-1180 which is an analcime-o zeolite of molecular formula NaAlSi 2O 6·H 2O having orthorhombic crystal structure with lattice parameters: a=13.72 Å, b=13.714 Å and c=13.714 Å. In this paper, we report a gas phase nitration scheme, in which we show through the elemental analysis that the modification of entire matrix (and not limited to pores and channels) occurs. The nitrated zeolite was reduced to aminated zeolite membrane by reacting it hydrazine hydrate. Separation of surfactant (CPC) was performed using these three types (unmodified, nitrated and aminated) of membranes and it showed about 300% increase in the retention of the surfactant after its modification. Its hydrophilic nature also increases as shown by the increase in the permeate flux. In order to find the reason for this enhanced performance, structural analysis of the modified membranes was carried out. The XRD patterns of these were found to be identical and they did not match with those of any of the compounds given in the JCPDS files. The patterns were therefore indexed, using first principle, to find their crystal structure and it was found that the structure changed from orthorhombic for unmodified to tetragonal geometry for the modified zeolites. This also caused about 10% increase in the unit cell volume of the modified zeolites. Anion exchange capacity and the elemental analysis showed that the nitration occurred over the entire zeolite matrix (not restricted to the pores and channels) and on an average about one amine group was present in every second formula unit of the zeolite. This extensive presence of nitrate or amine group in the zeolite matrix makes the modified zeolite membrane highly hydrophilic and may be responsible for the increase in the retention of the surfactant and permeate flux for the modified membranes.