FAU zeolite membranes synthesized using nanoseeds - Separation mechanism and optimization for the pervaporation dehydration of various organic solvents

Membrane-based pervaporation (PV) is an energy-efficient separation technology for liquid molecular mixtures, which is an increasingly important function in chemical industries. This work accomplished the first synthesis of high-performance FAU (NaX type) zeolite membranes on alpha-alumina tubular supports via the secondary growth method using 50 nm nanoseeds. The effects that the synthesis parameters exerted on the crystal structures, morphologies, and PV performance of FAU membranes were systematically investigated. The nanosized seeds showed high levels of activity that promoted the rapid growth of the zeolite layer. In addition, either extending the crystallization time or raising the temperature readily caused trans-crystallization to form NaP impurity zeolites, which decreased the membrane flux and separation factor. When synthesis conditions were optimized, a high-quality and pure-phase FAU membrane was obtained and tested for the permeation of single gases and for the PV dehydration of various organic solvents (methanol, ethanol, n-propanol, iso-propanol, and tert-butanol). The relationships between the single-gas permeance, PV permeance, and polarity index were investigated using a similar size range of permeate molecules (0.289-0.5061 nm). The results show that gas permeance exhibits Knudsen selectivity. The PV permeance, however, correlated well with the polarity index of the permeate, which indicates that PV dehydration through the FAU membrane is controlled by an adsorption-diffusion mechanism. Moreover, the correlation provides a new tool for predicting PV performance. Compared with studies using FAU membranes, the as-synthesized FAU membrane features good reproducibility, excellent stability, and high PV performance, which suggests the great potential of FAU membranes for use in industrial organic solvent dehydration.