The present invention involves a new type of poly(amidoamine) (PAMAM) dendrimer-cross-linked polyimide membranes and methods for making and using these membranes. The PAMAM-cross-linked polyimide membranes described in the current invention are prepared by cross-linking of asymmetric aromatic polyimide membranes using PAMAM dendrimer as the cross-linking agent.
This invention relates to a new type of poly(amidoamine) dendrimer-cross-linked polyimide membranes with high permeance and high selectivity for separations and more particularly for natural gas upgrading.
Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Polymeric membranes have been proven to operate successfully in industrial gas separations such as separation of nitrogen from air and separation of carbon dioxide from natural gas.
Commercially available polymer membranes, such as cellulose acetate, polyimide, and polysulfone membranes formed by phase inversion and solvent exchange methods have an asymmetric integrally skinned membrane structure. See U.S. Pat. No. 3,133,132. Such membranes are characterized by a thin, dense, selectively semipermeable surface “skin” and a less dense void-containing (or porous), non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the “skin.” However, fabrication of defect-free high selectivity asymmetric integrally skinned membranes is difficult. The presence of nanopores or defects in the skin layer reduces the membrane selectivity. One approach to reduce or eliminate the nanopores or defects in the skin layer of the asymmetric membranes has been the fabrication of an asymmetric membrane comprising a relatively porous and substantial void-containing selective “parent” membrane such as polysulfone or cellulose acetate that would have selectivity were it not porous, wherein the parent membrane is coated with a material such as a polysiloxane, a silicone rubber, or a UV-curable epoxysilicone in occluding contact with the porous parent membrane, the coating filling surface pores and other imperfections comprising voids (see U.S. Pat. No. 4,230,463; U.S. Pat. No. 4,877,528; U.S. Pat. No. 6,368,382).
In order to combine high selectivity and high permeability together with high thermal stability, new high-performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole were developed. These new polymeric membrane materials have shown promising properties for separation of gas pairs like CO2/CH4, O2/N2, H2/CH4, and C3H6/C3H8. However, current polymeric membrane materials have reached a limit in their productivity-selectivity trade-off relationship. In addition, gas separation processes based on glassy polymer membranes frequently suffer from plasticization of the stiff polymer matrix by the sorbed penetrating molecules such as CO2 or C3H6. Plasticization of the polymer is exhibited by swelling of the membrane structure and by a significant increase in the permeances of all components in the feed and decrease of selectivity occurring above the plasticization pressure when the feed gas mixture contains condensable gases. Plasticization is particularly an issue for gas fields containing high CO2 concentrations and for systems requiring two-stage membrane separation.
U.S. 2005/0268783 A1 disclosed chemically cross-linked polyimide hollow fiber membranes prepared from a monoesterified polymer followed by final cross-linking after hollow fiber formation.
U.S. Pat. No. 4,931,182 and U.S. Pat. No. 7,485,173 disclosed physically cross-linked polyimide membranes via UV radiation. The cross-linked membranes showed improved selectivities for gas separations. However, it is hard to control the cross-linking degree of the thin selective layer of the asymmetric gas separation membranes using UV radiation technique, which will result in very low permeances although the selectivities are normally very high.
Therefore, it is still highly desirable to prepare commercially viable high selectivity asymmetric membranes for separations.
The present invention discloses a new type of poly(amidoamine) (PAMAM) dendrimer-cross-linked polyimide membranes and methods for making and using these membranes.
A new type of poly(amidoamine) (PAMAM) dendrimer-cross-linked polyimide membranes with high selectivities for gas separations has been made.
The present invention generally relates to gas separation membranes and, more particularly, to high selectivity poly(amidoamine) (PAMAM) dendrimer-cross-linked polyimide membranes for gas separations. The poly(amidoamine) (PAMAM) dendrimer-cross-linked polyimide membranes with high selectivities described in the current invention were prepared from asymmetric aromatic polyimide membranes by chemical cross-linking using PAMAM dendrimer as the cross-linking agent (
Cross-linking of asymmetric aromatic polyimide membranes by PAMAM dendrimer reduces polyimide polymer chain flexibility, which often results in greater differences in diffusivities between molecules of different sizes. The diffusion differences will allow greater selectivities, but reduce permeances. The PAMAM-cross-linked polyimide membranes have improved plasticization resistance and enhanced chemical stability compared to the un-cross-linked polyimide membranes.
The invention provides a process for separating at least one gas from a mixture of gases using the new PAMAM-cross-linked polyimide membranes with high selectivities described herein, the process comprising: (a) providing a PAMAM-cross-linked polyimide membrane described in the present invention which is permeable to said at least one gas; (b) contacting the mixture on one side of the PAMAM-cross-linked polyimide membrane to cause said at least one gas to permeate the membrane; and (c) removing from the opposite side of the membrane a permeate gas composition comprising a portion of said at least one gas which permeated said membrane.
The new PAMAM-cross-linked polyimide membranes with high selectivities are not only suitable for a variety of liquid, gas, and vapor separations such as desalination of water by reverse osmosis, non-aqueous liquid separation such as deep desulfurization of gasoline and diesel fuels, ethanol/water separations, pervaporation dehydration of aqueous/organic mixtures, CO2/CH4, CO2/N2, H2/CH4, O2/N2, H2S/CH4, olefin/paraffin, iso/normal paraffins separations, and other light gas mixture separations, but also can be used for other applications such as for catalysis and fuel cell applications.
a shows the polymer structure used in the examples.
b shows the poly(amidoamine) dendrimer structure and the values of n in the dendrimer structure.
The following examples are provided to illustrate one or more embodiments of the invention, but the invention is not limited to these embodiments. Numerous variations can be made to the following examples that lie within the scope of the invention.
A 1 wt % PAMAM 0.0 cross-linking solution was prepared by mixing 0.56 g of poly(amidoamine) generation 0.0 (PAMAM 0.0) dendrimer solution (62.35 wt % PAMAM 0.0 in methanol) and 34.44 g of DI water. A low selectivity, high permeance, porous asymmetric flat sheet poly(3,3′,4,4′ -diphenylsulfone tetracarboxylic dianhydride-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) (DSDA-TMMDA) polyimide membrane with CO2 permeance of 640 GPU and CO2/CH4 selectivity of 1.72 at 50° C. with a 10% CO2 and 90% CH4 mixed gas feed and the feed at 791 kPa (100 psig) was prepared for the cross-linking study. The skin layer surface of the DSDA-TMMDA membrane was contacted with the 1 wt % PAMAM 0.0 cross-linking solution for 1 min. The resulting membrane was then dried at 70° C. for 1 hour.
The surface of the PAMAM 0.0-cross-linked DDSDA-TMMDA membrane was dip coated with a 5 wt % RTV615A/615B silicone rubber solution. The coated membrane was dried inside a hood at room temperature for 30 min and then dried at 70° C. for 1 hour. The 5 wt % RTV615A/615B silicone rubber solution was prepared from 0.9 g of RTV615A, 0.1 g of RTV615B and 19 g of hexane. The dried PAMAM 0.0 cross-linked DSDA-TMMDA polyimide membrane (abbreviated as PI-PAMAM-0.01) was cut into 7.6 cm diameter circles for permeation testing.
A 2 wt % PAMAM 0.0 cross-linking solution was prepared by mixing 2.25 g of poly(amidoamine) generation 0.0 (PAMAM 0.0) dendrimer solution (62.35 wt % PAMAM 0.0 in methanol) and 67.75 g of DI water. A low selectivity, high permeance, porous asymmetric flat sheet poly(3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) (DSDA-TMMDA) polyimide membrane with CO2 permeance of 640 GPU and CO2/CH4 selectivity of 1.72 at 50° C. with a 10% CO2 and 90% CH4 mixed gas feed and the feed at 791 kPa (100 psig) was prepared for the cross-linking study. The skin layer surface of the DSDA-TMMDA membrane was contacted with the 2 wt % PAMAM 0.0 cross-linking solution for 5 min. The resulting membrane was then dried at 70° C. for 1 hour.
The surface of the PAMAM 0.0-cross-linked DDSDA-TMMDA membrane was dip coated with a 5 wt % RTV615A/615B silicone rubber solution. The coated membrane was dried inside a hood at room temperature for 30 min and then dried at 70° C. for 1 hour. The 5 wt % RTV615A/615B silicone rubber solution was prepared from 0.9 g of RTV615A, 0.1 g of RTV615B and 19 g of hexane. The dried PAMAM 0.0 cross-linked DSDA-TMMDA polyimide membrane (abbreviated as PI-PAMAM-0.02) was cut into 7.6 cm diameter circles for permeation testing.
The surface of a low selectivity, high permeance, porous asymmetric flat sheet poly(3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) (DSDA-TMMDA) polyimide membrane with CO2 permeance of 640 GPU and CO2/CH4 selectivity of 1.72 at 50° C. with a 10% CO2 and 90% CH4 mixed gas feed and the feed at 791 kPa (100 psig) was dip coated with a 5 wt % RTV615A/615B silicone rubber solution. The coated membrane was dried inside a hood at room temperature for 30 min and then dried at 70° C. for 1 hour. The 5 wt % RTV615A/615B silicone rubber solution was prepared from 0.9 g of RTV615A, 0.1 g of RTV615B and 19 g of hexane. The dried RTV615A/RTV615B coated DSDA-TMMDA polyimide membrane (abbreviated as PI-0.05) was cut into 7.6 cm diameter circles for permeation testing.
The PI-PAMAM-0.01, PI-PAMAM-0.02, and PI-0.05Si membranes prepared in Examples 1-3 were tested for CO2/CH4 separation at 50° C. under 6996 kPa (1000 psig) mixed gas feed pressure with 10% CO2 in the feed. The results in the following Table show that both the new PAMAM cross-linked membranes PI-PAMAM-0.01 and PI-PAMAM-0.02 have significantly higher CO2/CH4 selectivity than the un-cross-linked PI-0.05Si membrane. The CO2 permeances of the PAMAM cross-linked membranes are higher than 82 GPU (5 A.U.) although they are lower than that of the un-cross-linked PI-0.05Si membrane.
aTested at 50° C. under 6996 kPa (1000 psig) mixed gas pressure, 10% CO2; 1 GPU = 7.5 × 10−9 m3 (STP)/m2 s (kPa)