Preparation of carbon molecular sieve membranes on porous...

Details Preparation of carbon molecular sieve membranes on porous... Preparation of carbon molecular sieve membranes on porous...

2002-03-28

2004-05-04

Beck, Shrive P. (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Pretreatment of substrate or post-treatment of coated substrate

C427S002300, C427S533000, C427S577000, C427S245000, C427S249200, C427S255230, C427S255280, C118S7230AN, C095S045000, C095S285000, C096S011000

Reexamination Certificate

active

06730364

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to a process for preparing a carbon molecular sieve membrane, and in particular for preparing a supported carbon molecular sieve membrane.
BACKGROUND OF THE INVENTION
Currently, carbon molecular sieve membranes are prepared by subjecting a thermoset polymer to pyrolysis or calcination at a high temperature in an inert gas or vacuum environment, thereby releasing volatile gases, such as H
2
O, CO, CO
2
, CH
4
, HCN, N
2
and H
2
etc., and forming an amorphous carbon membrane having a pore size of several microns to several angstroms. The pore size is closely related to the material of the polymer and the conditions of pyrolysis. Published researches on the carbon molecular sieve membranes are discussed in the following:
(1) Bird and Trimm used polyfurfuryl alcohol (PFA) to prepare non-supported and supported carbon molecular sieve membranes. Due to the occurrence of shrinkage during pyrolysis, a continuous carbon separation membrane could not be produced [P. L. Trimm et al., Carbon, 21(3), 177, 1983].
(2) Koresh and Soffer have done a very systematic research on carbon molecular sieve membranes [J. E. Koresh and A. Soffer, Sep. Sci. Technol., 18 (1983) 723; J. E. Koresh and A. Soffer, Sep. Sci. Technol., 23 (1987) 973]. A hollow fibrous polymer membrane was calcined at a medium temperature (800-950° C.) in nitrogen or an inert gas, thereby forming a carbon molecular sieve membrane. The selectivity of He to O
2
was 8, and the selectivity of He to N
2
was 20. In particular, the permeance of He reached 3×10
−7
mol.m
−2
.s
−1
.Pa
−1
, which was tens to hundreds times higher than that of a polymer membrane.
(3) Linkov et al. produced a three-zoned asymmetrical carbon membrane by subjecting a polyacrylonitrile, (PAN)-based hollow fibrous precursor to a thermal oxidation stabilization and a carbonization in an inert atmosphere [V. M. Linkov, R. D. Sanderson and E. P. Jacobs, J. Membrane Sci., 95 (1994) 93]. The intermediate zone had longitudinal voids with a length 5-15 &mgr;m and a diameter 3-7 &mgr;m. The inner layer had voids with a pore size of 3-5 &mgr;m. The outermost layer is a denser layer of 0.1-0.4 &mgr;m. Subsequently, a mixture gas of TiCl
4
and CH
4
was subjected to a gas phase pyrolysis to grow a TiC membrane on said hollow carbon fiber, which was then subjected to a high temperature oxidation in order to reduce the pore size on the outermost layer to less than 90 nm. In 1994, a combined magnetron sputtering and ion beam technique was used to coat a diamond-like carbon (DLC) membrane on the abovementioned fiber. The results indicated that a low sputtering rate could form a continuous membrane which fully covered the original voids. However, this composite membrane (without receiving a further carbonization) still had a Knudsen diffusion mechanism in transporting gas, and not a molecular sieve mechanism.
(4) Yamada et al. produced a carbon molecular sieve *membrane by subjecting a polyimide (PI) to a carbonization [Y. Yamada, et al., Carbon, 30, 719, 1992]. The produced membrane had an oxygen-to-nitrogen separation ratio of 4.6 and had a molecular sieve mechanism.
(5) Damle et al. used various materials and methods to perform various surface treatments on a commercial carbon membrane having a pore size of 0.2-1.0 &mgr;m:
dip-coating a polymer of polyacrylonitrile (PAN), polyfurfuryl alcohol (PFA), phenol-formaldehyde resin (PF) or cellulose precursors, on the carbon membrane;
using a plasma polymerization to coat PAN on the carbon membrane;
coating a solution of a PFA resin monomer on the carbon membrane, and adding a catalyst for an in-situ polymerization;
using high temperature pyrolysis to decompose propylene into tiny carbon particles to deposit on the carbon membrane substrate [A. S. Damle at al., Gas Separation & Purification8(3), 137, 1994]. After the abovementioned processing, said membrane was subjected to high temperature carbonization in order to improve the properties of the carbon membrane. The results indicated that the permeance was reduced by all the abovementioned processing. Besides, except in-situ polymerization, the processing had no significant improvement in selectivity. Although the in-situ polymerization process slightly improved the selectivity, the transport mechanism was in Knudsen diffusion range without involving molecular sieve effect.
(6) Collins and Yin used a DC sputtering technique to coat a diamond-like carbon (DLC) on a silicon substrate, and carbonizing the coated substrate by a vacuum baking [Y. Yin and R. E. Collins, Carbon, 31 (1993) 1333]. A QCM (quartz crystal microbalance) was used to measure the absorption of benzene and 2,2-dimethylbutene in said carbon membrane. It was found that the absorption of benzene (5.2 Å) was at least ten times greater than the absorption of 2,2-dimethylbutene (6.0 Å). After the high temperature treatment, said diamon-like carbon membrane formed extremely fine pores on the membrane to have very conspicuous molecular sieve functions and could separate molecules of slightly different sizes. Furthermore, the porosity and the pore size distribution of said carbon membrane were affected by the sputtering conditions (e.g. composition of the gas, bias voltage of the target, etc.) and the conditions of high temperature treatments of diamond-like membrane. Therefore, the molecular sieve function could also be varied.
Meanwhile, many people have investigated silica-based molecular sieve membranes. The published results on this topic are outlined in the following:
(1) Gavalas et al. first successfully used a thermal CVD to effectively reduce the macropores of a Vycor glass substrate, by introducing SiH
4
and O
2
separately from both sides of the substrate in order to carry out reactions within the pores to form a molecular sieving SiO
2
membrane. The selectivity of H
2
to N
2
reached 1000; and the permeance of H
2
at 450° C. was 10
−8
mol.m
−2
.s
−1
.Pa
−1
. They also introduced SiCl
4
and O
2
into the pores from the same side of said glass substrate for reaction. The reaction temperature was 600-800° C.
(2) Yan et al. used a porous &agr;-alumina tube as a substrate, which was first subjected to a boehimite solution dip-coating, followed by pyrolysis, thereby forming an &ggr;-alumina membrane on the outside of the tube [S. Yan, H. Maeda, K. Kusakabe, S. Morooka and Y. Akiyama, Ind. Eng. Chem. Res., 33 (1994) 2096]. Then, tetraethylorthosilicate (TEOS) was used as a feed in thermal CVD to deposit SiO
2
membranes. The permeance of H
2
at 600° C. was 10
−7
mol.m
−2
.s
−1
.Pa
−1
, and the selectivity of H
2
to N
2
reached 1000.
A SiO
2
membrane could only withstand a temperature up to 500° C., and the separation of H
2
often requires a higher operating temperature. Furthermore, the SiO
2
membrane could only separate a mixture of small molecules (such as H
2
, He etc.) and other gases. Therefore, many researchers proposed the membranes containing Si—C or Si—O—C structure. Such the structure could withstand a temperature up to 1200° C. and separate gases at a higher temperature. Furthermore, the pore size could be controlled to separate a mixture of gases with similar molecule sizes (e.g. O
2
, N
2
having a difference of 0.2 Å).
(1) Tsay, Dah-Shyang et al. produced a SiC molecular sieve by adding 5% of alumina into a raw material; sintering the material into a porous tube; coating the inner wall of the tube with a SiC having a particle size of 30 nm and containing 2% of alumina; sintering said tube into an asymmetrical tube; filling the tube with a polydimethylsilane solution; and subjecting the tube to a thermal treatment, a curing and a pyrolysis. A separation membrane, which was pyrolyzed at 300° C., had a H
2
permeance of 10
−7
mol.m
−2
.s
−1
.Pa
−1
at 200° C., and a H
2
/N
2
selectivity of up to 100. A separation membrane, which was pyr

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