CARBON MOLECULAR SIEVE (CMS) HOLLOW FIBER MEMBRANES AND PREPARATION THEREOF FROM PRE-OXIDIZED POLYIMIDES

Abstract

Prepare a carbon molecular sieve membrane from a polyimide (e.g., a 6FDA/BPDA-DAM polyimide) that has a glass transition temperature of at least 400° C. and includes a bridged phenyl compound for separation of hydrogen and ethylene from one another whether present as a pure mixture of hydrogen and ethylene or as components of a cracked gas. Preparation comprises two sequential steps a) and b). In step a), place a membrane fabricated from defect-free fibers of the polyimide in contact with an oxygen-containing atmosphere under conditions of time and temperature sufficient to produce a pre-oxidized and pre-carbonized polymeric membrane that is insoluble in hot (110 C) n-methylpyrolidone and at least substantially free of substructure collapse. In step b) pyrolyze the pre-oxidized and pre-carbonized membrane in the presence of a purge gas under conditions of time and temperature sufficient to yield a carbon molecular sieve membrane that has at least one of a hydrogen permeance and a hydrogen/ethylene selectivity greater than that of a carbon molecular sieve membrane prepared from the same membrane using only pyrolysis as in step b).

Claims

1. A process for preparing a carbon molecular sieve membrane from a polyimide for separation of hydrogen and ethylene from one another comprising sequential steps as follows: a) placing a membrane fabricated from fibers of a polyimide selected from a group consisting of ((5,5′-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis -1,3-isobenofurandione (6FDA), 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA), 2,4,6-trimethyl-1,3-phenylene diamine (DAM)) (61.sup.4DA/BPDA-DAM) polyimides and polyimides that i) have a glass transition temperature (T.sub.g) of at least 400° C. and ii) include a bridged phenyl compound in contact with an oxygen-containing atmosphere or gas at a temperature within a range of from greater than 300° C. to less than 400° C. and for a time within that temperature range of from greater than or equal to five minutes up to 200 hours to produce a pre-oxidized and pre-carbonized polymeric membrane that is insoluble in hot (110° C.) n-methylpyrolidone and substantially free of substructure collapse; and b) pyrolyzing the pre-oxidized and pre-carbonized polymeric membrane in the presence of a purge gas at a temperature within a range of from 450° C. to 1000° C. and for a time within a range of from 1 minute to 100 hours, to yield a carbon molecular sieve membrane that has at least one of a hydrogen permeance and a hydrogen/ethylene selectivity greater than that of a carbon molecular sieve membrane prepared from the same membrane using only pyrolysis as in step b).

2. The process of claim 1, wherein the oxygen-containing atmosphere or gas is selected from pure oxygen and air.

3. The process of claim 1 or claim 2, wherein the purge gas comprises argon or another inert gas and, optionally, a trace amount of oxygen.

4. The process of claim 1 or claim 2, wherein time within the temperature range is from greater than or equal to 30 minutes to two hours.

5. The process of claim 1, wherein the temperature for step a) is within a range of from 340° C. to 380° C.

6. The process of claim 1 or claim 2, wherein the time for step b. is within a range of from two hours to eight hours.

7. The process of claim 1 or claim 2, wherein the temperature for step b) is within a range of from 500° C. to 800° C.

8. The process of claim 1 or claim 2, wherein the hydrogen permeance for the carbon molecular sieve prepared using steps a) and b) is 10 percent greater than the hydrogen permeance of the carbon molecular sieve using only step b).

9. The process of any of claim 1 or claim 2, wherein the polyimide is 6FDA/BPDA-DAM and 6FDA and BPDA are present in a molar ratio within a range of from 10:1 to 1:10.

10. The process of claim 9 wherein the molar ratio is 1:1.

Description

EXAMPLE (EX) 1, COMPARATIVE EXAMPLE (CEX) A AND CEX B

[0024] Form defect-free Matrimid, 6FDA-DAM, and 6FDA/BPDA-DAM precursor fibers via a dry-jet/wet-quench fiber spinning process. The fiber spinning apparatus and conditions are described by Liren Xu et al., in “Olefins-selective asymmetric carbon molecular sieve hollow fiber membranes for hybrid membrane-distillation processes for olefin/paraffin separations”, Journal of Membrane Science 423-424 (2012), pages 314-323. In that process, first dry the polymer precursor overnight in a vacuum oven operating at a set point temperature of 110° C. to remove moisture and residual organics. Prepare a spinning dope by forming a visually homogeneous polymer solution (Matrimid™=26.2 wt % polymer, 53 wt % N-methyl-2-pyrrolidone (NMP), 14.9 wt % ethanol (EtOH), and 5.9 wt % tetrahydrofuran (THF); 6FDA-DAM=22 wt % polymer, 43 wt % NMP, 25 wt % EtOH, 10 wt % THF; and 6FDA/BPDA-DAM=25 wt % polymer, 43 wt % NMP, 22 wt % EtOH, 10 wt % THF) by placing a Qorpak® glass bottle sealed with a Teflon® cap on a roller at room temperature. Place the polymer solution in a 500 milliliter (mL) syringe pump and allow it to degas overnight. Coextrude the spinning dope and bore fluid via syringe pumps through a spinneret into an air gap, filtering both through in-line filtration means between delivery pumps and the fiber spinneret, and then into a water quench bath before resulting fibers are taken up using a rotating polyethylene drum after passing over several Teflon™ guides. After collecting the fibers from the take up drum, rinse the fibers in at least four separate water baths over a course of 48 hours, then solvent exchange the fibers in metal tube containers with three separate 20 min methanol baths followed by three separate 20 min hexane baths. Dry the solvent-exchange fibers under vacuum at ˜110° C. for ˜1 hr.

[0025] Treat defect-free precursor Matrimid fibers (T.sub.g of 305° C.) (CEx A), defect-free 6FDA-DAM polyimide fibers (CEx B) (T.sub.g of 395° C.), and defect free 6FDA/BPDA-DAM polyimide fibers (Ex 1) (T.sub.g of 424° C.) in air at temperatures shown in Table 1 below to form preoxidized fibers. Use Scanning Electron Microscopy (SEM) to check preoxidized fiber morphology to determine whether there is evidence of porous layer collapse (yes (Y) or no (N)) and immerse preoxidized fibers in hot (110° C.) n-methylpyrolidone (NMP) to evaluate their solubility (yes (Y) or no (N)). Solubility is an indication of polymer crosslinking, with a decrease in solubility suggesting a corresponding increase in degree of polymer crosslinking and, conversely, an increase in solubility suggesting a corresponding decrease in degree of polymer crosslinking. Show results in Table 1 below.

TABLE-US-00001 TABLE 1 Effect of treatment temperature on pre-oxidation 300° C. for 1 hour 350° C. for 1 hour 400° C. for 1 hour Polymer Solubility Collapse Solubility Collapse Solubility Collapse CEx A N Y N Y N Y Ex 1 Y N N N N Y CEx B Y N Partial Y N Y

[0026] The data in Table 1 show that pre-oxidation of 6FDA/BPDA-DAM fibers in air at 350° C. for one hour provides sufficient crosslinking to render such fibers insoluble in hot NMP while avoiding porous layer densification (also known as “porous substructure collapse”). Pre-oxidation in air at 300° C. for one hour does not provide enough crosslinking as evidenced by preoxidized fiber solubility. Pre-oxidation of 6FDA/BPDA-DAM fibers in air at 400° C. for one hour leads to porous substructure collapse. In other words, when pre-oxidizing 6FDA/BPDA-DAM fibers in air for one hour, the temperature at which pre-oxidation occurs must be greater than 300° C. but less than 400° C. By way of contrast, pre-oxidation in air for one hour does not provide both insolubility (sufficient crosslinking) and structure stability (no porous substructure collapse) at any temperature within a range of from 300° C. to 400° C. for either Matrimid fibers (CEx A where substructure collapse occurs at all temperatures) or 6FDA-DAM fibers (CEx B where one gets either insolubility or no substructure collapse, but not both).

[0027] Prepare membranes from the preoxidized 6FDA/BPDA-DAM fibers of Ex 1 as well as 6FDA/BPDA-DAM fibers that have no pre-oxidation treatment Subject the membranes to pyrolysis conditions as shown in Table 2 below and evaluate the pyrolyzed membranes for pure gas permeation of hydrogen ((P.sub.H2/l) in GPU) and selectivity (permeance of hydrogen versus ethane ((P.sub.H2/l)/(P.sub.C2H4/l)).

[0028] In evaluating pyrolyzed membranes for pure gas permeation properties, pot the membranes into hollow fiber membrane testing modules. Collect pure gas hydrogen and ethane permeation data by exposing the membrane upstream side to a pressure of 66.7 pounds per square inch absolute (psia) (459,880 pascals) while starting with a downstream side pressure at 16.7 psia (115,142 pascals) in a constant pressure gas permeation system (argon sweep). Measure the flow rate of permeate and sweep gas mixture by a flowmeter and the composition by a gas chromatograph (GC). Use the flow rate and composition to calculate gas permeance and selectivity.

[0029] Two properties may be used to evaluate separation performance of a membrane material: its “permeability”, a measure of the membrane's intrinsic productivity; and its “selectivity,” a measure of the membrane's separation efficiency. One typically determines “permeability” in Barrer (1 Barrer=10.sup.−10 [cm.sup.3 (STP) cm]/[cm.sup.2 s cmHg], calculated as the flux (n.sub.i) divided by the partial pressure difference between the membrane upstream and downstream (Δp.sub.i), and multiplied by the thickness of the membrane (l).

[00001]$P_i=\frac{n_i.Math.l}{\Delta .Math..Math.p_i}$

[0030] Another term, “permeance,” is defined herein as productivity of asymmetric hollow fiber membranes. It is typically measured in GPU and determined by dividing permeability by effective membrane separation layer thickness.

[00002]$\left(\frac{P_i}{l}\right)=\frac{n_i}{\Delta .Math..Math.p_i}$

[0031] Finally, “selectivity” is defined herein as the ability of one gas's permeability through the membrane or permeance relative to the same property of another gas. It is measured as a unitless ratio.

[00003]${\propto }_{i/j}.Math.=\frac{P_i}{P_j}=\frac{\left(P_i/l\right)}{\left(P_j/l\right)}$

TABLE-US-00002 TABLE 2 Transport properties of CMS membranes prepared from 6FDA/BPDA-DAM, and pre-oxidized 6FDA/BPDA-DAM Pyrolysis (P/l).sub.H2 Precursor T (° C.) (GPU) α(H2/CH4) Pre-oxidized 6FDA/ 550 1061 ± 42  10.4 ± 2.3 BPDA-DAM 6FDA/BPDA-DAM (no 550 212 ± 59  8.2 ± 1.8 pre-oxidation) Pre-oxidized 6FDA/ 675 273 ± 11 25.1 ± 2.4 BPDA-DAM 6FDA/BPDA-DAM (no 675 113 ± 16 17.4 ± 0.1 pre-oxidation)

[0032] The data in Table 2 show that preoxidized 6FDA/BPDA-DAM fibers produces CMS fiber membranes with better H.sub.2/CH.sub.4 separation performance than membranes fabricated from 6FDA/BPDA-DAM fibers that have no pre-oxidation treatment.