BROMINATEB POL Y(2,6-BIPHENYL-1,4-PHEN YLENE OXIBE) ANB ITS NANOCOMPOSITES AS MEMBRANES FOR CO ₂ SEPARATIONS

Background of the Invention The invention relates generally to brominated derivatives of poly(2,6-dimethyl-l ,4- phenylene oxide) and, more specifically to membranes formed of the new compounds that have improved mechanical and CO ₂ -separation properties.

Poly(2,6-dimethyl-l,4-phenylene oxide) (PPO for short) is known to be a membrane material that has high CO ₂ affinity, and hence high CO ₂ permeability (Chowdhury, G.; Kruczek, B.; Matsuura, T. (Eds.), Polyphenylene Oxide and Modified Polyphenylene Oxide

Membranes; Gas, Vapor, and Liquid Separation, Kluwer Academic, 2001, 105-145; He, Z.;

Pinnau, L; Morisato, A. Desalination 2002, 146, 11-15; Chowdhury, G.; Vujosevic, R.;

Matsuura, T.; Laverty, B. J. Appl. Polym. Sd. 2000, 77, 1137-1143; Hamad, F.; Khulbe, K.

C; Matsuura, T. J. Memb. Sd. 2005, 256, 29-37; Khulbe, K. C; Chowdhury, G.; Matsuura, T.; Lamarche, G. J. Memb. Sd. 1997, 123, 9-15). PPO properties can be improved by chemical modification. For example, Story et al. reported that substituting the aromatic ring with bromine groups can increase the CO ₂ permeability as much as 2.5 times without sacrificing its selectively (Story, B. J.; Koros, W. J. J. Memb. Sd. 1992, 67, 191-210).

Hamad et al. further improved the CO ₂ selectivity relative to CH ₄ by introducing a sulfonic acid group to the brominated PPO at the ring position (Hamad, F.; Matsuura, T. J. Memb.

Sc/. 2005, 253, 183-189).

One of the PPO derivatives that has not been explored yet as a CO ₂ membrane material is poly(2,6-diphenyl-l,4-phenylene oxide) (DPPPO). DPPPO was synthesized by

Hay et al. in the 1960's (Hay, A. S. Macromolecules 1969, 2, 107-108). DPPPO easily crystallizes with a Tm of about 470 ⁰ C, which is near its decomposition temperature, which will adversely impact its processing and mechanical properties (Yang, H.; Hay, A. S. J.

Polym. Sd. 1993, 31, 1261-73). Preferred embodiments of the present invention are focused on other PPO modifications, such as brominated DPPPO and nanoparticle-containing

DPPPO.

Summary of the Invention

We synthesized and characterized a new brominated derivative of DPPPO, BDPPPO for short, and its silica nanocomposite, and compared its CO ₂ membrane properties against PPO and DPPPO. The new BDPPPO membranes exhibit better mechanical and CO ₂ - separation properties. For example, they exhibit higher CO ₂ permeability (about 40% higher relative to DPPPO and about 250% higher relative to PPO) and higher CO ₂ /N ₂ selectivity, referred to as permselectivity (about 75% higher relative to DPPPO and about 90% relative to PPO). Furthermore, a mixture of BDPPPO and silica (SiO ₂ ) nanoparticles is demonstrated to form a compatible nanocomposite that exhibits superior separation properties. For example, a BDPPPO-silica nanocomposite containing 20%wt 10 nm silica particles can further improve the CO ₂ permeability by about 170% relative to plain BDPPPO without changing the permselectivity much.

Brief Description of the Drawings Fig. 1 is a chart of ¹ H-NMR spectra of (a)PPO, (b)DPPPO, and (c) BDPPPO.

Fig. 2 is a graphical representation of the DSC curves of (a)PPO, (b) DPPPO, and (c) BDPPPO.

Fig. 3 is a graphical representation of the gas permeability of BDPPPO/silica nanocomposite membrane as a function of the silica concentration at 10-psig feed pressure and room temperature.

Fig. 4 is a graphical representation of the gas selectivity of the BDPPPO/silica nanocomposite membrane as a function of the silicia concentration at 10-psig feed pressure and room temperature.

Detailed Description of Preferred Embodiments

Experimental Approach

Chemicals: PPO (Mn -25000, polydispersity -2.0), 2,6-diphenylphenol (98%),

N,MW',N'-tetramethylethylenediamine (TMEDA, 99%), bromine (Br ₂ , 99.5+%), chloroform

(CHCl ₃ , 99.8%), methanol (99.8+%), ethanol (99.5%+), 1,2-dichlorobenzene (99%), anhydrous hydrazine (98%), silicon dioxide nanopowder (SiO ₂ , 10 nm, 99.5%) were purchased from Aldrich and used as received. Copper(I) chloride (CuCl, 93.2%), which was

purchased from J. T. Baker Chemical Co., was stirred with glacial acetic acid, filtered, washed with ethanol, and dried.

Synthesis of DPPPO: Synthesis of DPPPO was carried out according to the method of Hay. In a typical procedure, 0.041 g of CuCl, 0.031 g of TMEDA, 2 g of anhydrous magnesium sulfate, and 35 ml of 1,2-dichlorobenzene was added to a 100 ml flask. The flask was placed in an oil bath at 65 ⁰ C, stirred, and saturated with oxygen for 10 min. When the solution turned green, a solution of 5 g of 2,6-diphenylphenol in 40 ml of 1,2- dichlorobenzene was added slowly for 20 min. At that point, the reaction solution became dark red. The reaction continued for 24 h. When the reaction was complete, several drops of anhydrous hydrazine were added to reduce the diphenoquinone byproducts. The inorganic solids were then removed by filtration. The polymer was precipitated by adding the solution dropwise to 400 ml of methanol containing several drops of hydrazine. After stirring for several hours, the precipitated polymer was collected by filtration, then redissolved in 40 mL of chloroform, and precipitated in 400 mL of methanol. The polymer was again filtered, dried in a vacuum oven at 80 ⁰ C for 24 h, and characterized: 3 g, [17] = 0.58, Mn -150000.

Synthesis of BDPPPO: Five g of DPPPO and 50 ml of CHCl ₃ was stirred in a 100 ml flask. A solution of 10 ml of bromine in 10 ml of chloroform was added dropwise to the mixture over a 30-minute period. The mixture maintained a dark red color throughout the bromination reaction. An argon purge was maintained to remove HBr released from the solution. After stirring at room temperature for 1 hour, the polymer was precipitated in 800 ml of mechanically stirred ethanol, filtered, and dried under vacuum at room temperature. Its total yield was 7 g.

Characterization: ¹ H-NMR analyses of 2% w/w solutions of the PPO, DPPPO and BDPPPO samples in deuterated choloroform were made using a Bruker Advance DRX-400 spectrometer. The glass transition temperatures (Tg) were determined using a differential scanning calorimeter, TA Instruments, model QPlO, at a heating rate of 20 °C/min. All tests were repeated at least twice to ensure reproducibility.

Gas permeation testing: Membranes of PPO, DPPPO and BDPPPO were cast on a glass plate at room temperature from 3% w/w solutions of chloroform. A constant-volume variable-pressure apparatus was used for testing gas permeation. All experiments were preformed at 22 ⁰ C and 21 psi feed pressure.

Results and Discussion

The PPO derivatives synthesized for this study are shown in Scheme 1. DPPPO is a white powder and BDPPPO is a yellow powder.

Scheme 1. Polymer structures

The ¹ H-NMR spectra of PPO, DPPPO and BDPPPO are shown in Fig.l. The DSC curves of PPO, DPPPO and BDPPPO are shown in Fig.2. The Tg 's of PPO, DPPPO and BDPPPO are 216 ⁰ C, 235 ⁰ C, and 275 ⁰ C, respectively, obtained by differential scanning calorimetry (DSC). Fig. 2 suggests that DPPPO is crystalline, with a melting point (7m) of 470 ⁰ C, while BDPPPO is amorphous.

PPO, DPPPO and BDPPPO are found to form good membranes by casting 3% w/w solutions on glass plates at room temperature. As shown in Table 1, among these three polymeric membranes, the BDPPPO membrane has the highest CO ₂ permeability and the highest CO ₂ /N ₂ permselectivity. The DPPPO membrane, on the other hand, has the lowest CO ₂ permeability due to its high crystallinity. Relative to PPO, the BDPPPO permselectivity is two times higher and its permeability is about 40% higher. Relative to DPPPO, the BDPPPO permeability and permselectivity are much higher, 3.5 and 1.7 times, respectively.

PPO-silica, DPPPO-silica and BDPPPO-silica membranes were cast by mixing the polymers and 20% wt/wt of 10 nm SiO ₂ nanoparticles in a solvent. The PPO-silica and DPPPO-silica membranes are found to be heterogeneous, that is phase separated, which leads to poor mechanical properties. By contrast, the BDPPPO-silica membrane is found to be uniform, which leads to very good mechanical properties. Its CO ₂ /N ₂ permeation data given

in Table 1 suggest that the silica nanoparticles further improve the CO ₂ permeability by about 170% relative to plain BDPPPO without changing the permselectivity much.

Table 1. Results of CO?/N? separation.

Polymer CO ₂ N ₂ CO ₂ /N ₂ membrane permeability permeability permselectivity

(Barrer) (Barrer)

PPO 66.0 4.5 14.7

DPPPO 26.0 1.6 16.3

BDPPPO 90.2 3.2 28.2

BDPPPO/silica 240.0 8.2 29.3

Chloroform solutions of PPO, DPPO, and BDPPPO were mixed with 9, 17, and 23 wt-% of 10 and 30 nm-silica nanoparticles (NPs) and used to cast membranes. Such nanocomposite BDPPPO/silica membranes are more homogeneous than DMPPO/silica and DPPPO/silica membranes, and remain flexible up to about 23 wt-% of silica in the membranes. The permeability and slectivity (ideal selectivity, or permselectivity) of the BDPPPO/silica membranes as a function of the silica weight percentage and silica nanoparticle size are illustrated in Figs. 3-4. The permeabilities of all the gases increase with increasing the silica concentration. The Pco ₂ of the BDPPPO/10-nm-silica membrane is 177 Barrer at 9 wt-% of silica and reaches 436 Barrer at 23 wt-%, about 5.6 times that of the pure BDPPPO membrane, while selectivity remains unchanged.

In Fig. 4, the CO _Z /N ₂ and CO2/CH4 selectivities in BDPPPO/silica membranes remain almost the same as those for the pure BDPPPO membranes, which shows that silica enhances the gas permeability without deteriorating the membrane selectivity.

The foregoing description and drawings comprise illustrative embodiments of the present inventions. The foregoing embodiments and the methods described herein may vary based on the ability, experience, and preference of those skilled in the art. Merely listing the steps of the method in a certain order does not constitute any limitation on the order of the steps of the method. The foregoing description and drawings merely explain and illustrate the invention, and the invention is not limited thereto, except insofar as the claims are so limited. Those skilled in the art who have the disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention.