A study of DDR-type zeolite crystals and membranes

Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2007.A zeolite membrane consists of a homogenous layer of intergrown zeolite crystals synthesized on the surface of a ceramic support. DDR zeolites consist of three types of window connected silica oxide cages that build up the rhombohedral DDR structure and have pore apertures of 0.36 x 0.44nm. Membranes (formed by a hydrothermal synthesis process) are gaining an important place in chemical technology as they are able to selectively control the permeation rate of chemical species that pass through it. The DDR membrane has been shown to separate CO2 (g) from CH4 (g). The objective of this research was to acquire a better understanding of the properties of DDR zeolites in combination with ceramic membranes. Therefore investigations were initially carried out on the manufacture of ceramic membranes using Alcoa powder in conjunction with research on the DDR-type zeolite crystals, as the hydrothermal synthesis period of the DDR crystals is a lengthy 25 days. The aim of this investigation was to acquire a better understanding of DDR zeolites through optimization of the experimental procedure. This thesis investigated the coating of the DDR-type zeolite on to a ceramic membrane which has been accomplished only once before. The manufacture of ceramic membranes was investigated using a cheaper source of a-alumina powder (Alcoa powder). The first treatment of pH flotation was optimized at a loading mass of 200g Alcoa powder suspended in a pH2 solution of HNO3 with APMA as the dispersing agent. As a result of this acid treatment, SEM analysis showed that the fine particles remained in suspension and the heavier particles settled. Secondly, a novel fractionation process using the heavy particles that had settled in the pH flotation showed optimum separation results at linear velocities of 5ml/min and 15ml/min using a 75g loading mass of Alcoa powder. Thereafter, centrifugal casting of the fractions was carried out to produce asymmetrical tubular green casts. Finally, a programme of sintering allowed for strengthening and transformation of the green cast into a ceramic support. To synthesize a DDR membrane, the crystals firstly had to be made and used as seeds to accelerate the growth of a DDR membrane. It was found that when the water concentration was decreased from 11240 moles to 7838 moles of water, homogenous crystals (1.4m) of well defined morphology were obtained. During the hydrothermal synthesis of a DDR membrane various factors were investigated. Results showed that a support refluxed in HN03 (aq) had improved zeolite attachment when compared to the pre-treatment of sonification. A seeding mass of 0.008g and a ten fold increase to 0.08g did not show a difference in the amount of coverage of the support with DDR crystals. When the seeding techniques of immersion and centrifugation were used, the same homogenous, but inadequate seed coverage was seen. Irrespective of the synthesis parameters investigated, a gel was consecutively produced on the support after hydrothermal synthesis. Only when the hydrothermal synthesis period was increased from 48 hours to 96 hours, some crystallization occurred. This investigation on the manufacture of ceramic support was partly successful in that a cheaper Alcoa ceramic membrane was reached, although repeatability was poor. SEM and XRD analysis confirmed the size and purity of DDR crystals after using an optimized synthetic procedure. Information from this thesis lays the foundation for the successful synthesis of a DDR membrane, as it has provided valuable information to direct the future research on this topic.Master