Selection of Zeolite Materials

Various zeolites have been studied as the dispersed phase in the mixed-matrix membranes. Zeolite performance in the zeolite/polymer mixed-matrix membrane is determined by several key characteristics including pore size, pore dimension, framework structure, chemical composition (e.g., Si/Al ratio and cations), crystal morphology and crystal (or particle) size. These characteristics of zeolites are summarized in Chapter 6. 

Mixed-matrix membranes comprising small-pore zeolite or small-pore non-zeolitic molecular sieve materials will combine the solution-diffusion separation mechanism of the polymer material with the molecular sieving mechanism of the zeolites. The small-pore zeolite or non-zeolitic molecular sieve materials in the mixed-matrix membranes are capable of separating mixtures of molecular species 

Small-pore zeolite Nu-6(2) has a NSI-type structure and two different types of eight-membered-ring channels with limiting dimensions of 2.4 and 3.2 A [54]. Gorgojo and coworkers developed mixed-matrix membranes using Nu-6(2) as the dispersed zeolite phase and polysulfone Udel as the continuous organic polymer phase [55]. These mixed-matrix membranes showed remarkably enhanced H2/ CH4 selectivity compared to the bare polysulfone membrane. The H2/CH4 selectivity increased from 13 for the bare polysulfone membrane to 398 for the Nu-6(2)/ polysulfone mixed-matrix membranes. This superior performance of the Nu-6(2)/ polysulfone mixed-matrix membranes is attributed to the molecular sieving role played by the selected Nu-6(2) zeoHte phase in the membranes. 

The chemical composihons of the zeolites such as Si/Al ratio and the type of cation can significantly affect the performance of the zeolite/polymer mixed-matrix membranes. MiUer and coworkers discovered that low silica-to-alumina molar ratio non-zeolitic smaU-pore molecular sieves could be properly dispersed within a continuous polymer phase to form a mixed-matrix membrane without defects. The resulting mixed-matrix membranes exhibited more than 10% increase in selectivity relative to the corresponding pure polymer membranes for CO2/CH4, O2/N2 and CO2/N2 separations [48]. Recently, Li and coworkers proposed a new ion exchange treatment approach to change the physical and chemical adsorption properties of the penetrants in the zeolites that are used as the dispersed phase in the mixed-matrix membranes [56]. It was demonstrated that mixed-matrix membranes prepared from the AgA or CuA zeolite and polyethersulfone showed increased CO2/CH4 selectivity compared to the neat polyethersulfone membrane. They proposed that the selectivity enhancement is due to the reversible reaction between CO2 and the noble metal ions in zeolite A and the formation of a 7i-bonded complex. 

Zeolites used for the preparation of mixed-matrix membranes not only should have suitable pore size to allow selective permeation of a particular molecular component, but also should have appropriate particle size in the nanometer range 

---