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Internal Surface Modification of MFI-Type Zeolite Membranes for High
Selectivity and High Flux for Hydrogen
Zhong Tang and Junhang Dong*
Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio 45221
Tina M. Nenoff
Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico 87185
Received February 7, 2009. Revised Manuscript Received March 18, 2009
MFI-type zeolite membranes were modified by depositing molecular silica at a small number of active sites in the
internal surface by in situ catalytic cracking of silane precursor. The limited silica deposition reduced the effective size of
the zeolitic channels that dramatically enhanced the H
2
selectivity without causing a large increase in H
2
transport
resistance. The modified zeolite membrane achieved an extraordinary H
2
/CO
2
permselectivity of 141 with a high H
2
permeance of 3.96 Â 10
-7
mol/m
2
3
s
3
Pa at 723 K. The effect of pore modification on the gas transport behavior was
studied on the basis of single gas permeation data.
The recent search for high-temperature hydrogen (H
2
)-perm-
selective membranes has been largely driven by the idea to
produce H
2
with simultaneous CO
2
capture through a single-step
water gas shift (WGS) of fossil fuel- and biomass-derived syngas.
The highly siliceous zeolite membranes are attracting growing
interest because of their necessary sulfur tolerance and hydro-
thermal stability not possessed by other candidates such as the
palladium alloy and amorphous silica membranes.
1,2
Currently,
the main challenge for the zeolite membranes is the incom-
patibility between selectivity and flux for H
2
separation from
the complex gas mixtures involved in the catalytic reaction
systems.
2
The main components in gas streams from hydrogen produc-
tion by catalytic conversion of biomass and fossil fuels include H
2
,
CO
2
, CO, CH
4
,H
2
O, and common impurity H
2
S. These small
gases are essentially nonadsorbing in the siliceous zeolites at high
temperature (HT) (>573 K). Therefore, HT H
2
separation
through the zeolite membranes must rely on the differentiation
of molecular diffusivity and/or the size exclusion effect depending
on the ratio (λ) of the molecular kinetic diameter (d
k
) to the
membrane pore size (d
p
) (i.e. λ =d
k
/d
p
).
3-6
In recent years, the MFI-type zeolite membranes with a large
Si/Al ratio and the all-silica DDR-type zeolite membranes have
been particularly investigated for HT H
2
separation.
7-11
The
primary mass-transport channels in MFI-type zeolites have an
effective diameter of 0.56 nm, which offers high selectivity by size
discrimination for critically sized molecules such as xylene iso-
mers.
12
However, the transport of H
2
, CO
2
, and other small gases
in the MFI zeolitic pores is dominated by activated gaseous
diffusion
4,10
resulting in high permeance but low selectivity for
H
2
. The DDR-type zeolite has a pore structure of cages connected
by small windows with an effective size of 0.4 nm. However, the
DDR-type zeolite membrane also exhibited low H
2
selectivity
with high permeance because of the mesoscale intercrystalline
spaces inevitably existing in the polycrystalline film.
8
Modifications of the DDR and MFI types of zeolite mem-
branes have been reported in an attempt to enhance the H
2
selectivity. The DDR-type membranes were modified by counter-
diffusion chemical vapor deposition (CVD) of silica using tetra-
ethyl orthosilicate (TEOS) as a precursor to reduce the size of the
intercrystalline pores.
8,10
The MFI-type zeolite membranes were
modified by catalytic thermal cracking of preadsorbed methyl-
diethoxysilane (MDES) to deposit molecular silica in the intra-
crystalline pores and intercrystalline spaces.
11,13
The MDES mol-
ecule is nearly linear with a kinetic size of 0.4 nm  0.91 nm, which
is small enough to enter the zeolitic pores (d
p
= 0.56 nm). TEOS
and tetramethoxyl silane (TMOS) are common precursors for the
modification of MFI zeolite external surfaces and intercrystalline
pores in membranes by the CVD method because these molecules
are too large to enter the intracrystalline MFI zeolite pores.
14,15
The TMOS and TEOS molecules are nearly spherical with large
sizes of 0.89 and 0.96 nm, respectively.
These membrane modifications resulted in significant increases
in HT H
2
/CO
2
selectivity but caused unacceptable losses of H
2
permeance of about an order of magnitude. Figure 1 presents the
*Corresponding author. Phone: (513) 556-3992. Fax: (513) 556-3474.
E-mail: junhang.dong@uc.edu.
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Published on Web 4/6/2009
© 2009 American Chemical Society
DOI: 10.1021/la900474y Langmuir 2009, 25(9), 4848–4852 4848