(407g) Gas Permeation and Separation in Zsm-5 Micromembrane Unit

The last few years
have seen increasing interest on microchemical systems including micromixers microreactors
and microseparators. The smart, integrated microchemical systems are expected
to bring into realization a distributed, on site and on demand production
network for high value added products in the form of miniature factories and
micro-pharmacies. Such microchemical devices will enable the rapid
implementation of new synthesis and process technologies for more efficient
production under environmentally responsible and safe conditions. It has been
acknowledged that microscale separation is one of the core technologies that
needed to be developed for microchemical system. Most separation processes can
benefit directly from the large surface area-to-volume ratio that can be
obtained in a microseparator. In fact, extraction and membrane separation have
been successfully miniaturized. It is expected that membrane will play an
important role in microscale separation.

Zeolites and
molecular sieves are ideal materials for micromembranes. Zeolites are capable of separating molecules by their size,
shape and polarity. Close boiling compounds, isomers and azeotropes were
successfully separated using zeolite membranes. The crystalline zeolites also
have excellent mechanical strength and thermal stability, and are resistant to
most acids, bases and organic solvents. This work describe the successfully
fabrication of freestanding zeolite micromembranes on silicon substrate. The
procedure involves pre-fabrication of support structure, followed by selective
seeding and growth of oriented zeolite films. Low temperature template removal
using ozone avoids the crack and defect formations associated with high
temperature treatment methods. The zeolite micromembranes were tested for
single gas permeation using permanent gases and hydrocarbon vapors. Excellent permselectivity
with evidence of molecular sieving was observed from the experiments. The
thickness and composition (i.e., Si/Al ratio) of ZSM-5 affects both the
permeation and permselective of the membrane for the various gases. Binary
(i.e., C₃/N₂, C₄/N₂), ternary (H₂/CH₄/CO₂)
and quaternary (H₂/CH₄/CO₂/CO) separation
experiments were also conducted to demonstrate that the zeolite micromembranes
are efficient separator.