Controllable Growth of Zeolitic Imidazolate Framework Composite Membranes for Gas Separation
Creators: name: Ezzatollah Shamsaei
Publisher: name: Figshare
Year: 2017
Subjects: subject: 90499 Chemical Engineering not elsewhere classified
subject: FOS: Chemical engineering

description: As an emerging class
of hybrid organic-inorganic nanoporous material, metal organic frameworks
(MOFs) with tunable pore size and chemistry are very attractive for integration
into membranes and thin films for gas separation applications. Zeolitic
imidazolate frameworks (ZIFs), a subfamily of MOFs, are known for their
permanent porosity and exceptional chemical and thermal stability. Among a number
of available ZIF materials, ZIF-8 is particularly interesting owing to its
relative ease of synthesis as well as its great potential in separating small
gas molecules. However, the progress on the fabrication of ZIF-8 membranes with
satisfactory gas separation performance is very limited and there is no report
of ZIF membranes being used in industrial scale so far. Therefore, development
of simple and more effective methods to fabricate high quality ZIF molecular
sieving membranes with high gas selectivity is still required. The new
processing approaches require the advantages of being rapid, reproducible,
scalable, and economically and environmentally viable while simultaneously
producing high quality ZIF membranes. <br>
   The ultimate goal of this PhD research program is to address
challenges that hinder the facile synthesis of supported-ZIF membranes in a
reproducible and scalable manner. In this thesis, three new approaches are
demonstrated to potentially address these challenges. First, a novel scalable
strategy of using vapor phase to chemically modify the polymer support for ZIF
membrane fabrication is developed. Such surface modification enabled fast
formation of a continuous ZIF-8 ultrathin layer after only 3 minutes. The
resulting ZIF-8 membranes exhibited exceptional H<sub>2</sub> permeance as high as 2.05
×10<sup>-6</sup> mol m<sup>-2</sup> s<sup>-1</sup> Pa<sup>-1</sup> with high H<sub>2</sub>/N<sub>2 </sub>and H<sub>2</sub>/CO<sub>2</sub> selectivities (9.7 and 12.8,
respectively). Next, based on the chemical vapor modification, a simple,
effective, and environmentally friendly method is described for the fabrication
of high-quality ZIF-8 membranes with controllable positioning on a polymer
substrate in aqueous solution. The ZIF-8 membrane exhibited a propylene
permeance of 1.50 × 10<sup>–8</sup> mol m<sup>–2</sup> s<sup>–1</sup> Pa<sup>−1</sup> and excellent selective permeation
properties; after post heat-treatment, the membrane showed ideal selectivities
of C<sub>3</sub>H<sub>6</sub>/C<sub>3</sub>H<sub>8</sub> and H<sub>2</sub>/C<sub>3</sub>H<sub>8 </sub>as high as 27.8 and 2259, respectively. The new
synthesis approach holds promise for further development of the fabrication of
high-quality polymer-supported ZIF membranes for practical separation
applications. Finally, a new concept for the use of one-dimensional material
(e.g. CNT) as nano-scaffolds and pseudo-seeds for the fabrication of molecular
sieving membranes supported on a porous substrate is introduced. To demonstrate
the potential for universal applicability of the proposed pseudo-seeding and
nano-scaffolding method, ZIF-8/CNTs membranes were prepared on both polymeric
and inorganic substrates. At 25 °C and 1 bar, the ideal separation
selectivities of H<sub>2</sub>/CO<sub>2</sub>, H<sub>2</sub>/N<sub>2</sub>, H<sub>2</sub>/CH<sub>4</sub>, H<sub>2</sub>/C<sub>3</sub>H<sub>6</sub>, and H<sub>2</sub>/C<sub>3</sub>H<sub>8</sub> are 14, 18, 35,
52.4 and 950.1, respectively, with H<sub>2</sub> permeance as high as 2.87 × 10<sup>−5</sup> mol m<sup>−2</sup>
s<sup>−1</sup> Pa<sup>−1</sup>. This high hydrogen permselectivity combined with its mechanically
reinforced structure shows that the ZIF-8/CNT membrane is a promising candidate
for hydrogen separation and purification. Finally, it is anticipated that the
novel strategies developed in this research may be further developed for the
fabrication of other MOF and zeolite molecular sieve membranes.