Synthesis, ethanol dehydration and thermal stability of NaA zeolite/alumina composite membranes with narrow non-zeolitic pores and thin intermediate layer

Abstract

NaA zeolite/alumina composite membranes were prepared by the secondary growth process using a nanometer-scale seed, preparatory to investigation of their ethanol dehydration behavior. Single-gas permeation data for hydrogen, nitrogen, n-C 4H 10 and SF 6 indicated that the prepared membranes have relatively narrow non-zeolitic pores. The water flux sharply decreased in an existence of alcohol. The phenomenon could be explained by the ethanol blockage in the α-cage with window of 4.2 Å, being activated by the hydrogen bond between adsorbed water and ethanol. In dehydrating 95 wt.% ethanol at 70 °C, the prepared membranes showed a high water/ethanol separation factor (∼10,000) and a low flux (∼0.03 kg/m 2 h). It announced that at the pervaporation condition, the water flux through the zeolitic pores is ignorable. In nine membranes among 14 total prepared, well-developed cracks were observed in the formed zeolite layers before or during a dehydration test at 70 °C. The cracks were induced by the mismatch of the thermal expansion coefficients of the NaA zeolite layer (negative expansion) and the porous alumina support (positive expansion). The cracked NaA zeolite membranes showed a low water/ethanol separation factor (30–300) and a high total flux (1–10 kg/m 2 h) in the pervaporation condition. By a simple thermal stress calculation, it was established that the zeolite membranes with a clear phase boundary between the LTA layer and α-alumina support is highly vulnerable to thermal crack formation during the heating.