The crystallization of defect-free, acid-proof film microstructure is consistently pursued for making hydrophilic pervaperation (PV) membranes popular in process industries. We show that a novel modified secondary growth method can fabricate high-PV-performance, excellent-acid-stability mordenite membranes by eliminating grain boundary defects, dramatically by optimizing film microstructure. This method consists of growing the assembled nanocrystals of an ultrathin seed layer to a well-intergrown film directly on a macroporous support at a relatively low crystallization temperature (150 degrees C) by avoiding events that lead to an acid nonresistant film microstructure, such as the formation of Al-rich film surface and grain boundaries. Fluoride anions are used as mordenite crystal structure modifiers to optimize distribution of aluminum atoms in zeolitic film, so grain boundaries can be kept intact for long-term acidic corrosion. The polycrystalline films are ultrathin (3-4 mu m), and oriented with different Si/Al ratios (5.6-20.1). Comparison with conventional synthesized membranes shows that these microstructurally optimized membranes have superior PV performance and acid stability for the dehydration of high-concentration acetic acid (AcOH) mixtures. (c) 2012 Elsevier B.V. All rights reserved.