Fabrication And Applications Of Oriented MFI Zeolite Membranes

Zeolite membranes have advantages such as precisely defined pore structure,controllable catalysis-based characteristics and high stability, which could enable applications in gas or liquid separations, catalytic membrane reactors, photoelectric materials, and host for chemical sensors. MFI zeolite crystal (ZSM-5) has a two -dimensional system of intersecting channels with subnanometer pore size (0.55nm). Because of its unique molecular sieving effect and selectively catalytic performances, MFI zeolite membrane can be used for separation of many industrially important compounds and catalysis of the partial oxidation of C1 to C6 hydrocarbons. Thus, MFI zeolite membranes are considered as one of the most potential zeolite membranes. Thanks to its anisotropic pore structure, the oriented nanometer-sized pore network of constituent microcrystals in a MFI zeolite membrane can significant- ly affect its molecular sieving effect, and permeation and catalytic performances, which are crucial for practical applications in various fields. In recent years, therefore, there has been growing interest in the preparation and applications of oriented MFI zeolite membranes.The core of this thesis is to prepare the oriented pure-silica MFI and the oriented transition metal hetero-atom Fe-MFI zeolite membranes. Much attention has been paid to the relation between the nanometer-sized zeolitic pore network in a MFI zeolite membrane and its permeation and catalytic performances. The effect of the microstructure and chemistry of the porous substrate surface on the growth of zeolite film was firstly invesitigated in detail, which confirmed that the orientation and microtructure of Si-MFI zeolite films onα-Al2O3 supported silica-zirconia layers were dominantly controlled by the chemical nature of the substrate surface. The abundance of organic functional groups could be successfully seeded onto the substrate surface by modification, where the functional groups served as the structure-directing matrix to induce the orientation and growth of the zeolite crystals with their b-axes perpendicular to the substrate surface.In the present study, we firstly fabricated a close-packed b-oriented monolayer Si-MFI film supported by a thin chitosan film, and provided the direct, specific high-resolution TEM and high-press DSC evidence to illustrate the true evolution of zeolite films during in-situ hydrothermal synthesis. This approach gives us a unique opportunity to explain the self-assembly mechanism of pure-silica MFI crystals on the surface of the chitosan film during in-situ crystallization.Based on the formation mechanism of zeolite membrane, a novel twice seeded growth synthesis procedure was proposed to fabricate oriented zeolite films onα-Al2O3 substrates directly. On the randomly oriented silicalite-1 seed layers, thin, continuous and h0h-oriented silicalite-1 membranes with an extremely high Si/Al ratio could be prepared at 150℃, and thick, dense and c-oriented silicalite-1 membranes could be obtained at 180℃. It was found that Fe³⁺ ions in the synthesis solution could be favourable to the intergrowth of MFI zeolite crystals along the in-plane direction. We have fabricated h0h-oriented and b-oriented Fe-silicalite-1 or Fe-ZSM-5 membranes on randomly oriented and b-oriented silicalite-1 or Fe-ZSM-5 seed layers, respectively.A unique low-temperature calcination technique was adopted in this thesis, which could quite efficient to restrain the formation of open grain boundaries and cracks during the template removal. This method can dramatically improve the pervaporation performance of Fe-silicalite-1 membranes for ethanol/water mixtures. Compared with the b-oriented membrane, the h0h-oriented Fe-silicalite-1 membrane was more suitable to the separation of ethanol/water mixtures. It could exceed the ethanol/water azeotropic point, directly concentrate the 5 wt.% ethanol/water fermentation broths to over 96 wt.% bio ethanol, and the ethanol/water separation factor could reach to as high as 530 at 60oC.In order to investigate the advantage of catalytic membrane reactions, the highly h0h-oriented Fe-ZSM-5 membranes were utilized in a modified interphase membrane reactor. The hydroxylation of phenol catalyzed by Fe-ZSM-5 using H2O2 as oxidant was investigated in the traditional slurry reactor and the interphase membrane reactor, respectively. Compared with the catalytic performance of Fe-ZSM-5 crystal grains loaded in the traditional slurry reactor, the use of the h0h-oriented Fe-ZSM-5 membrane in the interphase membrane reactor gives rise to a pretty high conversion for phenol and 100% selectivity for catechol and hydroquinone.