Abstract
NaA zeolite membrane is an ideal hydrophilic candidate for organic dehydrations; however, its instability in salt solutions limits its application in industries as the membrane intactness was greatly affected due to the replacement of cation ions. In order to explore the relationship between the structural variation and the cation types, the obtained NaA zeolite membranes were treated by various monovalent and divalent cations like Ag+, K+, Li+, NH4+, Zn2+, Mg2+, Ba2+ and Ca2+. The obtained membranes were subsequently characterized by contact angle, scanning electron microscopy (SEM), pervaporation (PV), and vapor permeation (VP). The results showed that all of the hydrophilicities of the exchanged membrane were reduced, and the membrane performance varied with cation charges and sizes. For the monovalent cations, the membrane performance was largely determined by the cation sizes, where the membrane remained intact. On the contrary, for the divalent cation treatments, the membrane separation was generally reduced due to the presence of cation vacancies, resulting in some unbalanced stresses between the dispersive interaction and electrostatic forces, thereby damaging the membrane intactness. In the end, a set of gas permeation experiments were conducted for the two selected cation-treated membranes (K+ and Ag+) using H2, CO2, N2 and CH4, and a much higher decreasing percentage (90% for K+) occurred in comparison with the permeation drop (10%) in the PV dehydration, suggesting that the vaporization resistance of phase changing for the PV process was more influential than the water vapor transport in the pore channel.
Highlights
IntroductionFuel ethanol is considered as a promising alternative to the conventionally used fossils due to its significant advantages in terms of environment protection, energy intensity, and resource abundancy [1,2,3]
Fuel ethanol is considered as a promising alternative to the conventionally used fossils due to its significant advantages in terms of environment protection, energy intensity, and resource abundancy [1,2,3].At present, microbial fermentations are the mainstream route that is used in the industry to produce fuel ethanol; by-products and water generated during the fermenting activities [4] should be removed simultaneously in order to maintain the production efficiency
Different cation-exchanged NaA zeolite membranes were successfully derived on hollow fibers, using monovalent and divalent salts
Summary
Fuel ethanol is considered as a promising alternative to the conventionally used fossils due to its significant advantages in terms of environment protection, energy intensity, and resource abundancy [1,2,3]. In the past two decades, many types of zeolites have been fabricated as the supported zeolite membranes [7,9], including LTA [10,11], FAU [12], T [13], MOR [14] and MFI [15,16,17] Among these candidates, The tubular NaA zeolite membrane has been widely applied in pharmaceutical and food industries for solvent recoveries [18]. It is necessary to scientifically and systematically explore the relationship of separation variations with cation sizes and charges using both PV dehydration and gas permeation techniques, so as to provide guidance in order to broaden the industrial application of hollow fibered supported NaA zeolite membranes. The PV decrement for several cation treatments was compared with that for the gas permeation and vapor permeation (VP) tests to evaluate the importance of the surface hydrophilicity and the vaporization resistance of the NaA zeolite membranes for water molecules
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