(695g) Synthesis, Characterization and Separation Property of Zeolite Membrane for Steam Separation

Membrane
technologies will offer us great opportunities to reduce energy demands
revolutionary in future chemical and petroleum industries.  Among emerging
membrane technologies, great attention has been paid to zeolites as membrane
materials.  Zeolites, microporous aluminosilicates crystals, have been
widely used as adsorbents, catalysts, and ion exchangers.  In addition to
their high thermal, chemical, and structural stability, zeolite micropore
systems can uniquely recognize molecules by molecular sieving and/or
preferential adsorption.

In commercial
applications, a membrane to remove steam selectivelyfrom syngas above 200^oC seems to be promising for
a membrane extractor inreactions such as
Fischer-Tropsch synthesis and methanol synthesis, where water is a by-product. 
These syngas reactions (C1 chemistry) require further technical development
considering future growth in alternate fuel sectors, driven by both strategic
and environmental considerations.

In the 2006 AIChE annual
meeting, we reported that a ZSM-5 type zeolite membrane highly permeated and
separated steam from hydrogen above 200^oC.  This study aimed at
clarifying correlation among synthesis condition, membrane structure, and
separation property of ZSM-5 membranes.    

ZSM-5 membranes
were prepared on the outer surface of porous alumina tubular support by a
secondary-growth method. The molar composition of a
synthesis mixturewas 10Na₂O: 0.15Al₂O₃:
36SiO₂: 1200H₂O.  Hydrothermal treatment was carried
out at 180°C with prolonged synthesis time from 6 to 18 h.  Sort
of cations in the zeolite membranes were controlled by ion exchange. Membrane structures were characterized by
FE-SEM and permporometry.  Separation properties of membranes were
evaluated by water/hydrogen separation in the temperature range of 150-250^oC. 

Permporometry measurements
suggest that voids among crystal boundaries were filled after the crystallization
for 12 h and that remaining inter-crystal pathways readily lead to a deterioration
of water/hydrogen separation performance.  The sizes of pores and voids
were tentatively estimated using the Kelvin equation.  Gas permeation via
pores with Kelvin diameter of larger than 0.55 nm was not detected in
permporometry measurements through a Na-exchanged ZSM-5 (Na-ZSM-5) membrane
with a separation factor of water/hydrogen = 74 (at 150^oC and
partial pressures of water/hydrogen = 10/91 kPa).  On the other hand, 3-7 % of gas permeation was via pores with the
Kelvin diameter of 0.55-1.2 nm through a Na-ZSM-5
membrane with separation factor of water/hydrogen = 26, while the voids of larger
than 1.2 nm was not detected. 

Separation
performance of water/hydrogen mixture was influenced by Na cation occluded in
the micropores of zeolite.  With increasing partial pressure of water, hydrogen
permeation through Na-ZSM-5 membrane was effectively blocked, whereas such
hindrance of hydrogen permeation with water was difficult to observe with H⁺-exchanged
ZSM-5 (H-ZSM-5) membrane at higher temperature.  Adsorption experiments revealed that the
amount of water adsorbed on Na-ZSM-5 was much larger than that on H-ZSM-5. 
When comparing hydrogen
permeation through Na-ZSM-5 and H-ZSM-5 membranes at the same loading of water, the Na-ZSM-5
membrane showed more remarkable inhibition of hydrogen permeation than the H-ZSM-5
membrane.  Differences in permeation and separation properties between
Na-ZSM-5 and H-ZSM-5 membranes were more significant at higher
temperature.