Preparation and characterization of a thin continuous faujasite membrane on tubular porous mullite support

The progressive increase of VOC emissions, the growing information of their hazardous nature, and the resulting increasingly more restrictive environmental regulations in the industrialized countries have induced the development of different methods for VOCs abatement. Membrane separation process is a good alternative to gain this purpose. In this paper, a high quality faujasite membrane was prepared. Tubular porous mullite supports were fabricated from Kaolin clay by a temperature-programmed calcination treatment, following a leaching treatment. As observed in SEM photographs, the porosity of the support increased during the leaching treatment. The NaX zeolite powders were prepared hydrothermally and characterized with XRD and SEM. The best crystalline powder and the corresponding starting gel were respectively used for seeding treatment and hydrothermal crystallization of a thin, continuous and fully covering X-type zeolitic top layer on outer surface of the cylindrical support. The membrane was characterized with XRD and SEM.

It was shown that the mullite support porosity increased as a result of the leaching treatment using the caustic solution, because the free silica in initial support structure was removed by caustic. The support was porous enough to be coated with a top layer. A high degree of crystallinity of a NaX-type zeolite without any crystal phase impurity was also indicated. As a result, the crystallization was entirely performed using a starting gel with the Si/Al ratio of 1.25 in 10 h and there is not any other crystallization process in parallel during the hydrothermal treatment. On the other hand, the powder prepared using the other synthesis gel, Si/Al = 1.5, was not found to be faujasite. Therefore, it seems that the Si/Al ratio in starting gel should be less than 1.5 for faujasite synthesis. The membrane prepared using the porous mullite support, the selected hydrothermal method (363 K and autopressure for 10 h) and the selected synthesis gel (Si/Al = 1.25) had a highly continuous and fully covering coating of NaX-zeolite crystals as a top layer. The top layer thickness was found to be about 3.7 µm. The NaX-type zeolite membrane can be used for VOCs/water separations.