(198o) Controllable Manipulation of Continuous AFI Membranes with Distinctive Microstructures on Macroporous Alpha-Alumina Substrates

Zeolite
and molecular sieve membranes with uniform and molecular-sized pores have
excellent thermal, mechanical and chemical stabilities, and they have acquired
potential applications as separation membranes and membrane catalysts. Besides,
new application areas such as low-dielectric constant materials as well as
anti-corrosion and anti-microbial coatings have been developed. The
microstructure of zeolite and molecular sieve membranes including continuity, crystal
orientation and thickness is of great significance in actual applications. However,
it is still a challenging issue to manipulate the microstructure of zeolite
membranes today.

The
availability of continuous and well-defined AFI-type molecular sieve membranes (AFI
membranes) would trigger their applications in innovative materials,
microreactors and even microelectronic devices. Currently, the synthesis of AFI
membranes has been focused on controlling the crystal orientation to achieve c-oriented
channels perpendicular to the substrate surface, leading to the uniform
residence time of involved molecules across diffusion channels. Up to present,
there have been several routes for fabricating AFI arrays, films and membranes
with a preferred orientation, including electric-field-driven assembly,
geometry-confined growth, epitaxial growth, manipulation of synthesis
conditions, microwave-enhanced growth, and induced growth at the substrate
surface. It should be noted that the majority of AFI membranes reported to date
were prepared on dense substrates such as silicon wafers or glass plates that would
not be usable in separation and catalysis processes. Herein, we wish to manipulate
the microstructure of AFI membranes in the in-situ crystallization. As such,
the continuous and dense AFI membranes of single- and double-layered structure
covered with either highly c-oriented hexagonal prisms or spherical
agglomerates could be synthesized at will. There are not relevant reports yet.

A well-designed manipulation strategy is proposed to produce
continuous AFI membranes with four different microstructures over porous alpha-alumina
substrates, as shown in Figure 1. A double-layer and highly c-oriented
AFI membrane of hexagonal prisms is obtained when a thin layer of medium
molecular weight (MMW) chitosan is employed as the structure-directing matrix
together with aluminum isopropoxide (AIP) as the Al source, it can be
transformed to a single-layer and highly c-oriented AFI membrane of hexagonal
prisms if the structure-directing matrix is replaced by a thin layer of low
molecular weight (LMW) chitosan. When the Al source is changed to
pseudo-boehmite, the single-layer AFI membrane is composed of highly ordered
spherical agglomerates of small crystals instead of vertical hexagonal prisms;
while the membrane will turn to the double-layer AFI membrane of highly-ordered
crystal agglomerates if a thin layer of MMW chitosan is used once again and
keeping pseudo-boehmite as the Al source.

Figure
1. Schematic diagram
of manipulating microstructure of AFI membranes using a thin chitosan layer of different
molecular weight and different Al sources.

The
existence of a thin chitosan layer ensures the formation of the continuous and
well-intergrown AFI membrane of either highly c-oriented hexagonal prisms or
highly-ordered crystal agglomerates. The coverage, denseness and orientation of
all synthesized AFI membranes can be further optimized by adjusting the water
content in the precursor solution. It has been demonstrated that the
manipulation method established in this study is rather reliable with pretty
high reproducibility.

Acknowledgements

This
work was supported by the National Natural Science Foundation of China under a Key-Program
Grant (21136008).