Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2001-09-21
2003-01-21
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S045000, C095S054000, C096S004000, C096S014000
Reexamination Certificate
active
06508860
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to mixed matrix membranes for gas separation incorporating a molecular sieve dispersed in a polymeric continuous phase, and more specifically, to such mixed matrix gas separation membranes in which the molecular sieve has been treated with a monofunctional silane compound.
BACKGROUND OF THE INVENTION
The use of selectively gas permeable membranes to separate the components of gas mixtures is a well developed and commercially very important art. Such membranes are traditionally composed of a homogeneous, usually polymeric composition through which the components to be separated from the mixture are able to travel at different rates under a given set of driving force conditions, e.g. transmembrane pressure and concentration gradients.
A relatively recent advance in this field utilizes mixed matrix membranes. Such membranes are characterized by a heterogeneous, active gas separation layer comprising a dispersed phase of discrete particles in a continuous phase of a polymeric material. The dispersed phase particles are microporous materials that have discriminating adsorbent properties for certain size molecules. Chemical compounds of suitable size can selectively migrate through the pores of the dispersed phase particles. In a gas separation involving a mixed matrix membrane, the dispersed phase material is selected to provide separation characteristics which improve the permeability and/or selectivity performance relative to that of an exclusively continuous phase polymeric material membrane.
Previous research on mixed matrix membranes with enhanced selectivity has suggested the use of zeolites for the dispersed phase particles. Some types of zeolites identified for use in mixed matrix membranes include: MFI type, such as silicalite, LTA types 4A and 5A, and FAU types X and Y with various substituted cations. Further understanding of conventional use of zeolites in mixed matrix membranes may be understood with reference to the following patents:
U.S. Pat. No. 5,127,925 (Kulprathipanja et al.) describes a process for separating a first gas component from a feed gas mixture calling for contacting the mixture with a mixed matrix membrane of an organic polymer having an adsorbent incorporated therein. The adsorbent material is selected from among zeolites such as crystalline aluminosilicates, silicalite, inorganic oxides, activated carbon or ion exchange resin.
U.S. Pat. No. 4,925,562 (te Hennepe et al.) discloses a pervaporation process which employs a membrane comprising an elastomeric polymer matrix containing zeolite. In a preferred embodiment, the elastomeric polymer matrix is silicone rubber comprising a polysiloxane. Preferred zeolites include hydrophobic aluminosilicates with a high Si/Al ratio. Silicalite is disclosed as a suitable zeolite.
Although this technology has steadily advanced, it is a persistent general shortcoming of gas separation membranes that usually either high transmembrane flux or high selectivity can be obtained at sacrifice of the other. The advent of mixed matrix membranes provided the ability to better design a membrane to achieve optimum performance by using two permeable materials with different permeability and selectivity characteristics. However, the heterogeneous nature of mixed matrix membranes also introduces certain problems which have prevented this technology from fully surmounting the aforementioned shortcoming. In particular, unless there is affinity between the dispersed phase and continuous phase materials, voids can occur at the interface during membrane fabrication and use. The migrating components can travel through the voids and thereby avoid passing through the selectively permeable materials. This contributes to poor separation performance.
U.S. Pat. No. 4,925,459 (Rojey et al.) discloses a gas separation membrane comprising an active layer including particles of a selective solid dispersed in a continuous non-porous and non-elastomeric polymer phase and a porous support. The patent teaches that zeolites are particularly adapted for use in the dispersed phase. It also states that the dispersed phase can be pretreated with a binding agent such as a silane before mixing it with polymer solution. A theory has now been formulated that certain adhesive agents, including some silanes, can actually interfere with transport of the selectively permeating species through the discrete phase particles. For example, it is suggested that the adhesive can block entry into the pores or surround the particles with a permeability inhibiting barrier layer. These or other mechanisms may be responsible for reducing mixed matrix membrane separation performance. Thus it remains highly desirable to provide a mixed matrix gas separation membrane having molecular sieve dispersed in a continuous polymer matrix which can produce the combination of higher permeability and selectivity than has heretofore been attainable.
SUMMARY OF THE INVENTION
The present invention now provides a mixed matrix gas separation membrane comprising particles of a treated molecular sieve dispersed in a continuous phase consisting essentially of a matrix polymer, in which the treated molecular sieve comprises the reaction product of a molecular sieve and a monofunctional organosilicon compound having the formula Si(XYR
1
R
2
) in which X is a displaceable radical, Y is a linking group, and R
1
and R
2
are each independently any radical other than a displaceable radical.
There is also provided a process for making a mixed matrix gas separation membrane having an active layer of particles of a treated molecular sieve dispersed in a continuous phase consisting essentially of a matrix polymer, comprising the steps of,
(A) providing a monofunctional organosilicon compound having the formula Si(XYR
1
R
2
) in which X is a displaceable radical, Y is a linking group, and R
1
and R
2
are each independently any radical other than a displaceable radical,
(B) reacting a molecular sieve with the monofunctional organosilicon compound at a temperature and for a duration effective to bond the molecular sieve to the Si atom of the monofunctional organosilicon compound by displacing the radical X, thereby forming the treated molecular sieve,
(C) blending the product of the preceding step with an amount of the matrix polymer effective to obtain a blend having a proportion of about 5-50 weight parts molecular sieve per 100 weight parts polymer, and
(D) forming a membrane structure from the blend.
In another aspect, this invention provides a process for making a mixed matrix gas separation membrane as described immediately above in which after the reacting (B) step further comprises the step of sizing the treated molecular sieve with the polymer comprising reacting a linking group of the treated molecular sieve to the matrix polymer.
There is yet further provided a process for separating component gases of a gas mixture comprising the steps of
(A) providing a mixed matrix gas separation membrane comprising an active layer of a treated molecular sieve dispersed in a continuous phase of a matrix polymer in which the treated molecular sieve comprises the reaction product of a (i) molecular sieve through which at least one component gas in the mixture is preferentially gas permeable relative to another component gas, with (ii) a monofunctional organosilicon compound having the formula Si(XYR
1
R
2
) in which X is a displaceable radical, Y is a linking group, and R
1
and R
2
are each independently any radical other than a displaceable radical,
(B) contacting the gas mixture on one side of the membrane to cause the component gases to selectively permeate the membrane, and
(C) removing from the opposite side of the membrane a permeate gas composition enriched in concentration of the at least one component gas.
DETAILED DESCRIPTION
This invention pertains to mixed matrix membranes. By “mixed matrix” is meant that the membrane has a selectively gas permeable layer which comprises a continuous phase of a polymeric material and discrete particles of adsorbent m
Corbin David R.
Hasse David J.
Kulkarni Sudhir S.
Patel Aspi N.
L'Air Liquide - Societe Anonyme a'Directoire et Consei
Lew Jeffrey C.
Russell Linda K.
Spitzer Robert H.
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