reactive polymer membranes for ethylene/ethane separation

ELSEVIER Journal of Membrane Science 136 (1997) 111-120

journal of MEMBRANE

SCIENCE

Reactive polymer membranes for ethylene/ethane separation

A. Sungpet, J.D. Way*, EM. Thoen, J.R. Dorgan Chemical Engineering and Petroleum Refining Department, Colorado School of Mines, Golden, CO 80401, USA

Received 31 March 1997; received in revised form 13 June 1997; accepted 13 June 1997

Abstract

Olefin/paraffin separations by distillation are highly energy intensive. Facilitated transport, or reactive membranes have long been investigated as an alternative and/or complementary separation technology to conventional distillation. However, stability problems associated with facilitated transport membranes have been the primary obstacle in the development of commercial FT processes. In this paper, we report the development of a polymer membrane containing silver(I) ion that facilitates the transport of ethylene in the absence of solvent. Blends of ionically conductive and electrically conductive polymers were found to have the appropriate electronic environment to allow reaction of silver(I) ion and ethylene. The results for the ethylene/ethane separation were obtained with composite membranes of silver(I)-form Nation ® and 2 wt% poly(pyrrole). Permeation measurements were performed with ethylene/ethane mixtures at total feed pressures ranging from 760 to 1900 mmHg and at temperatures of 40°C to 70°C. Pure gas permeability measurements were obtained at a total feed pressure of 1900 mmHg and temperatures of 30°C and 40°C. Ethylene/ethane separation factors with the silver(I)-form Nafion-poly(pyrrole) composite membranes increased from 8 to 15 as temperature decreased. Ethylene permeabilities increased from 0.2 to 1 Barter over the temperature range of 30°C to 70°C. An ethylene/ethane mixed gas permeability ratio of about 2 was observed with non-reactive proton-form Nafion-poly(pyrrole) composite membranes. Ethylene permeation measurements as a function of membrane thickness suggested that the facilitated transport of ethylene approached the reaction-limited regime at membrane thickness of 5 ktm. The complexation between ethylene molecules and silver(I) ions in Nafion-poly(pyrrole) composite membrane was observed with FTIR spectroscopy.

Keywords: Composite membranes; PTIR spectroscopy; Facilitated transport; Gas separations; Ion-exchange membranes; Olefin/paraffin separations

1. Introduct ion

Separation processes in the petroleum and petro- chemical industries are highly energy intensive. Among these processes, the separation of olefins by distillation consumes the most energy, 0.12 quad/year (1 quad -- a trillion billion BTU) [1]. The primary

*Corresponding author. Tel.: +1-303-273-3519; fax: +1-303- 273-3730; e-mail: [email protected].

0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PI1 S0376-7388(97)00 1 5 8-0

reason for this very large energy consumption is that light olefins such as ethylene and propylene are among the top ten chemicals produced annually in the U.S. [2]. Consequently, there is an enormous economic incentive to develop alternative separation processes with lower energy consumption. Chemically specific separations such as facilitated transport membranes for olefin/paraffin separations offer an intriguing alternative to distillation. Given appropriate performance and stability, membrane processes could potentially

112 A. Sungpet et al./Journal of Membrane Science 136 (1997) 111-120

replace distillation processes to reduce energy consumption or allow more efficient membrane-distillation hybrid processes. However, despite extensive research on olefin/paraftin separations by facilitated transport membranes, this technology has not been applied to industrial applications. The primary problems are a lack of both membrane and carrier stability. Both immobilized liquid membranes (ILMs) and solvent swollen ion exchange membrane supports (IEMs) require that the feed gas be saturated with solvent to prevent the membranes from drying out. In the case of silver(I) form-IEMs, without solvent present, our group and others [3-10] have observed no facilitation of olefins. The feed gas can be saturated with solvent in the laboratory work; however, the costs of scaling-up for the production for polymer grade olefins are high [9].

A solution to this problem is the development of reactive polymer membranes that do not require solvent for the carrier to react with the olefin penetrant. This paper describes the synthesis and application of a new class supports for facilitated transport membranes: blends of ionically and electrically conductive polymers.

2. Olefin-silver(1) ion complexation

The interaction of oletins with transition metals has been known for over a century [[11] and references therein]. The most widely accepted model to explain the structure of bonding was proposed by Dewar [12] and Chatt et al. [ 13]. This model implies that the metal ion is located symmetrically above the plane of the olefin. Beverwijk et al. [14] discussed the complexation reaction of olefins with silver(I) ion in detail.

The olefin-silver(I) ion complex results from two interactions. The oletin acts as an or-electron donor with the empty 5 s orbital of silver(I) ion. The silver(I) ion also acts as a donor. The fully filled shell 4 d of silver(I) ion backdonates its nonbonding electrons to the empty 7r* molecular orbital of olefin. The donation of 7r-electrons to the silver(I) ion is a cr-bond and the back donation from d-orbital of the silver(I) ion to the olefin is a n-bond.

Basch [15] calculated the electronic structure of the ethylene-silver(I) ion complex by using nonempirical self-consistent field theory in an extended Gaussian

orbital basis set for a top-complex geometry corresponding to Dewar's original two-way donor acceptor model. The calculation showed that the charge of silver(I) ion in ethylene-silver(I) ion complex was more negative than the charge of the free silver(I) ion. This suggested that the e-bond is stronger than the n-bond. This concept was confirmed by the orbital population analysis. The most highly mixed molecular orbital was the one made up of primarily the 7r orbital of ethylene. This molecular orbital has only 6.5% of silver(I) 5 s atomic orbital character. Therefore, the polarization effects induced by the positive charge on the silver(I) ion had a great effect on the electronic rearrangement of the ethylene.

Antonio et al. [16] studied the coordination of silver(I) ion in several ionomer membranes, including Naton, for facilitated olefin transport. They found that a silver(I) ion in the membrane coordinated to three oxygen atoms. Within the membrane, the coordinating oxygen atoms are from water molecules and/or the membrane matrix. In addition, the bonding between water molecules and silver(I) ion was weak since silver(I) ion had a relatively low affinity with oxygen donor ligands [17]. Antonio et al. suggested that the weak interaction between silver(l) ion and the oxygen atoms enhanced the olefin-silver(I) ion interaction.

Therefore, the electronic environment of silver(l) ion in the membrane is very important for complexation of ethylene-silver(l) ion to occur. Most likely, the bond between silver(I) ion and the counter ion, e.g. the sulfonate groups of the perfluorosulfonated ion exchange membrane, is not a favorable electronic environment due to the strong interaction between silver(I) ion and the counter ion. The 5 s orbital of silver(l) ion is occupied by electrons from the counter ion. This prevents the silver(l) ion from forming a complex with an olefin and would explain why no facilitation is observed for silver(I)-form Nation in the absence of solvent.

To fabricate a solid reactive polymer membrane selective for olefins, the electronic environment of the silver(I) ions in the membrane must be suitably mod- ified. Silver(I) ion must be shielded from the counter ion. The synthesis of weakly coordinating anions for silver(I) ion was reported [18]. The Schiff base macrocyclic complexes of silver(l) ion can be synthesized [19-22]. However, incorporating these compounds into a polymer film is not an easy task. The use of

A. Sungpet et al./Journal of Membrane Science 136 (1997) 111-120 113

H H H N N N

Fig. 1. Bipolaron distortion on a poly(pyrrole) chain.

a polymer that can shield the silver(I) ion from the counter ion is another possibility. We have found that oxidized poly(pyrrole), an electrically conducting polymer, is capable of changing the local electronic environment between the silver(l) ion and the counter ion, allowing silver(I) ion to form a complex with ethylene. The chemical structure of oxidized poly- (pyrrole) is shown in Fig. 1 [23].

Oxidized poly(pyrrole) contains positive charges and requires the presence of anions as counter ions. When incorporated into a Nation film, the sulfonate groups in Nation serve as counter ions for the positive charges of oxidized poly(pyrrole). Polymerization of pyrrole into Nation has been reported by many researchers [24-28]. However, we have developed another synthesis procedure which is less complex.

Our proposed chemical structures of silver(I)-form Nafion-poly(pyrrole) are shown in Fig. 2.

The first hypothetical chemical structure is shown in Fig. 2(A). The localized positive charges on a poly- (pyrrole) chain share the negative charges from the sulfonated groups of Nation with silver(l) ions. This allows silver(l) ion to have a slightly positive charge. For the hypothetical chemical structure shown in Fig. 2(B), the silver(I) ions are localized on the poly- (pyrrole) chain but still have low mobilities due to the interaction with sulfonate groups. In both cases, the 5 s orbital of silver(I) ion is not totally occupied by the electrons from a sulfonate group, allowing silver(I) ion to accept 7r-electrons from an olefin.

Another factor that affects the transport of olefin through the membrane is the kinetic stability of the complex, primarily the rate of ligand-exchange. The olefin-silver(I) ion complexation must not be kineti- cally inert. Silver(I) ion must exchange its ligands very rapidly, otherwise, the transport of olefin through the membrane will be dominated by the diffusion of uncomplexed olefin. The factors that may influence the reaction rate of the complex are the nature of leaving ligand, the nature of entering ligand, and the

SO3 Ag'*" SO3Ag SO3" Age'`. SO3Ag

" t

(A)

SO" SO 3. SO3Ag ! 3 SO3Ag

(B)

Fig. 2. Hypothetical chemical structures of silver(I)-form Nation- poly(pyrrole) composite membrane.

nature of the other ligands attached to the metal atom, etc. [29]. In the silver(I)-form water-swollen membrane, the HOMO (highest occupied molecular orbital) of the water molecule can interact with the LUMO (lowest unoccupied molecular orbital) of silver(I) ion, the 5 s orbital, forming a water-silver(I) ion complex. This suggests that water in the membrane may influence the kinetics of olefin-silver(I) ion complexation as well.

In this present work, transport measurements of ethylene and ethane through the Nafion-poly(pyrrole) composite membranes are reported. The effects of pressure, temperature and membrane thickness on the membrane performance are investigated. FTIR spectroscopy experiments are performed with various types of membranes, which are exposed to ethylene, to verify the coordination of ethylene and silver(I) ion in the membranes.


3. Experimental

3.1. Materials

Nation solution, 5 wt% of Nation EW 1100 in a mixture of lower aliphatic alcohols and water, was purchased from Aldrich (Milwaukee, WI). Pyrrole, 98%, was purchased from Aldrich and used without further purification. Aqueous hydrogen peroxide 30 wt% in water and silver nitrate, 99+%, were certified A.C.S. grade. 1-Propanol and 95% ethanol were reagent grade. Purified and deionized water was used in all experiments. The ethylene and ethane purities were 99.5% and 99.0%, respectively. Helium, 99.999% purity, was used as the flow system sweep gas and the carrier gas for gas chromatography. The compressed gases were used without further purification. Potassium bromide, 99+%, FTIR grade, was used in the FTIR spectroscopy experiments.

3.2. Preparation of Nafion-poly(pyrrole) composite membranes

The solvent was allowed to evaporate from the desired amount of Nation solution at room temperature, resulting in a H+-form Nation film. The film was redissolved in a mixture of equal volumes of 1-propanol and ethanol by using an ultrasonic bath. The solution was then cast on a clean glass plate. Solvent was allowed to evaporate at room temperature. The clear film was annealed at 200°C for 3 h and allowed to cool slowly to room temperature. The film was removed from the glass plate by immersing in water at room temperature. To polymerize pyrrole into a Nation film, a solution containing 0.012 M pyrrole and 0.0073 M hydrogen peroxide was prepared. Then, the Nation film was immersed in the mixture for about 10 min. A deep green-black film of the Nafion-poly- (pyrrole) composite polymer was obtained. The film was rinsed with water and allowed to dry at room temperature. This film was a proton-form membrane. To obtain a silver(I)-form membrane, the film was immersed in 1 M silver nitrate solution for 6 h. The composite film was allowed to dry at room temperature before mounting in the diffusion cell prior to transport measurements. The amount of poly(pyrrole) was obtained by weighing the dry proton-form membranes before and after pyrrole polymerization. The

amount of silver(I) ion in a silver(I)-form membrane was presumed to be the weight gained after silver(I)- exchange. The composite film was found to contain ca. 2 wt% poly(pyrrole) and 7 wt% silver(l) ion. The color of poly(pyrrole) in a composite film comprised of more than 2 wt% poly(pyrrole) changed from deep green-black to dark yellow after the film was immersed in 1 M silver nitrate solution, indicating the partial reduction of poly(pyrrole). This film showed lower ethylene permeability and ethylene/ ethane permeability ratio than a film which had all poly(pyrrole) in the oxidized state. Furthermore, some silver(0) was observed in a silver(I)-form film with a very high poly(pyrrole) loading. As a result, the amount of poly(pyrrole) in the composite films used in the present work was controlled to be about 2 wt%.

3.3. Transport measurement

The membrane cell and gas flow system described previously [30] were used in the present work without humidifiers and driers. The composite film was mounted into a membrane cell which was immersed in a water bath for temperature control. The flow configuration in the membrane cell was cross flow. Pure ethylene and ethane were mixed in a gas flow system to obtain the desired feed composition. The binary feed mixture of between 40 to 60 mole% ethylene in ethane was used as the feed gas unless otherwise specified. The total pressure on the feed side was varied from 760 to 1900 mmHg while the total pressure on the sweep side was kept constant at 760 mmHg throughout all experiments. Helium was used as the sweep gas. The feed and sweep pressures were controlled by back pressure regulators. The permeate and retentate streams from the membrane cell were analyzed by gas chromatography with a thermal conductivity detector.

3.4. FTIR spectroscopy measurements

All FTIR measurements were carried out on a Bio- Rad Frs-40 FFIR spectrometer equipped with a liquid-nitrogen-cooled, narrow-band-pass mercury cadmium telluride (MCT) detector. The spectra were collected in absorbance mode using 128 scans at a resolution of 2 cm 1. A stainless steel chamber was used in conjunction with the Harrick Scientific


Praying Mantis Diffuse Reflection Attachment (DRA). The window material of the dome was zinc sulfide. The membranes were placed on potassium bromide powder to increase the collected radiation from the membranes. The temperature of the sample cup, filled with potassium bromide, was kept at 30°C for every measurement. Before ethylene was intro- duced to the sample cup, the spectra of the films were collected and used as the backgrounds. Ethylene pressure in the sample cup was maintained at about 3700 mmHg by a back pressure regulator. This pressure was much higher than the highest ethylene partial pressure in the transport measurements, 1900 mmHg. This was because the intensity of the signal obtained at 1900 mmHg was very weak. The films were exposed to ethylene for 1 h before the final spectra were collected. Beyond this point, there were no significant changes in the spectra.

4. Results and discussion

4.1. Proton-form Nafion-poly(pyrrole) composite membrane

To observe the performance of a non-reactive film, a proton-form Nation-poly(pyrrole) composite film with a thickness of 5 gm was used to determine the ethylene and ethane permeabilities. Within the experimental error, ethylene and ethane permeabilities were independent of the feed hydrocarbon partial pressures. A proton-form Nation-poly(pyrrole) composite film with a thickness of 10 ~tm also showed similar results. Average permeabilities of ethylene and ethane at various temperatures are shown in Table 1.

The separation factors for the proton-form composite membrane are similar to those measured for the

Table 1 Average ethylene and ethane permeabilities of proton-form Nafion-poly(pyrrole) composite film at various temperatures

Temperature Ethylene Ethane Permeability (°C) permeability permeability ratio

(Barrer) (Barrer)

40 0.0564 0.0256 2.20 50 0.0805 0.0354 2.27 60 0.1490 0.0697 2.13 70 0.2170 0.1010 2.15

potassium(I)-form commercial Nation films [10]. Therefore, addition of the poly(pyrrole) does not enhance the ethylene transport across the membrane and the transport mechanism is likely to be solution- diffusion.

4.2. Silver(I)-form Nafion-poly(pyrrole) composite membrane

A silver(I)-form Nation-poly(pyrrole) composite membrane, 6 gm thick, was used to investigate the ethylene/ethane separation. The mixture permeabilities measured were reproducible after the membrane was stored in the dark in air for two weeks. Ethane permeabilities of silver(I)-form Nafion-poly(pyrrole) as a function of ethane partial pressures at various temperatures are shown in Fig. 3.

Ethane permeabilities were basically the same as those observed with proton-form membranes. They were constant at each temperature and increased as the temperature was raised, which was presumably due to the increase in the diffusion coefficient of ethane in the membrane. The permeability of pure ethane through the membrane at 40°C was about the same as those obtained from the mixed gas experiments. This suggests that the effect of ethane-ethylene interactions on penetrant transport was very small. Mixture permea-

u~

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JJrlrlirJIIiiIl~i~l

• • •

t l l l l i l l l l

5 ~ 1~0

[ I I I I

1500

m

A

÷

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2000 Ethane Partial Pressure (mm Hg)

Fig. 3. Ethane permeabilities as a function of ethane partial pressures at various temperatures: (+) 30°C (pure ethane), (A) 40°C (pure ethane), (A) 40°C, ( 0 ) 50°C, ( I ) 60°C, ( 0 ) 70°C.


1 . 0 0 '

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/k

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t l l l r l l l J r l ~ l l S l t l

500 1000 1500 2000 Ethylene Partial Pressure (ram Hg)

Fig. 4. Ethylene permeabilities as a function of ethylene partial pressures at various temperatures: (+) 30°C (pure ethylene), (A) 40°C (pure ethylene), (A) 40°C, (O) 50°C, (m) 60°C, ( 0 ) 70°C.

16.00

O -~

14.00 --

i _ 12.00 --

_

~ 10.00 --

_

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0

1~iIllil(lJllrl~l~.

÷

A A~

• •

I l l l l l l l l l ~ l l ) L l l l

500 1000 1500 Ethylene Partial Pressure (ram Hg)

E

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Fig. 5. Ethylene/ethane permeability ratios as a function of ethylene partial pressures at various temperatures: (+) 30°C (Ideal), (/k) 40°C (Ideal), (A) 40°C, (O) 50°C, (m) 60°c, ( 0 ) 70°c.

tion experiments at 30°C could not be performed because the concentrations of the permeating gases were below the detection limit of the thermal conductivity detector used in the gas chromatograph.

Ethylene permeabilities, observed with the same membrane, as a function of ethylene partial pressures at various temperatures are shown in Fig. 4.

Ethylene permeabilities were about 4 to 5 times higher than those obtained from proton-form membranes. This suggests that the presence of silver(I) ion increased the ethylene permeability due to the well known olefin-silver(I) ion complexation reaction. However, at a specific temperature, ethylene permeabilities were essentially constant as ethylene partial pressure increased. In the case of reaction rate limited facilitated transport, the permeability of the facilitated (reactive) penetrant often decreases as its partial pressure in the feed gas increases due to the well known carrier saturation phenomenon [31 ]. It is possible that over the range of ethylene partial pressure studied, the membranes may have already been in the carrier saturation regime.

The ethylene/ethane permeability ratios, or separation factors, obtained from silver(I)-form membrane, are shown in Fig. 5.

The ethylene/ethane permeability ratio at low temperature was larger than that of high temperature. This

is reasonable because the olefin-silver(I) ion complexation is an exothermic reaction. Low temperatures are more favorable for the formation of the olefin- silver(I) ion complex. Another factor responsible for the facilitated transport of olefin across a membrane is the equilibrium constant of the complexation. The enthalpy of the olefin-silver(I) ion complex formation is negative, resulting in an increase of the equilibrium constant as temperature decreases. Furthermore, the diffusion of both ethane and uncomplexed ethylene are accelerated as the temperature increases. This promotes the transport by the solution--diffusion pathway which is less selective to ethylene. The ideal permeability ratios were obtained from the pure gas measurements.

The ethylene permeability and ethylene/ethane permeability ratios of the silver(I)-form Nafion-poly- (pyrrole) composite membrane were lower than those measured in our laboratory for silver(I)-form water- swollen Nation [30]. There may be several reasons for these differences. First, the silver(I) ions that are affected by the presence of poly(pyrrole) may be less than the total amount of silver(I) ion in the membrane. In other words, the Nafion-poly(pyrrole) composite membrane may have fewer reactive carriers that can form the complex with ethylene than the water-swollen Nation does. Second, as mentioned earlier, the

A. Sungpet et al. /Journal of Membrane Science 136 (1997) 111-120 1 1 7

kinetic stability of the complex is very important. Water molecules in the water-swollen membrane may substitute for ethylene in the ethylene-silver(l) ion complex as the exchanging ligands, allowing ethylene to rapidly diffuse through the membrane. On the contrary, poly(pyrrole) does not have this ability. Ethylene could complex with silver(I) ion for a longer period of time, causing the simple diffusion transport mechanism to dominate. Finally, the mobilities of silver(l) ions, ethylene and ethylene- silver(I) ions has a great effect on ethylene permeability and the ethylene/ethane permeability ratio if the transport mechanism involves the diffusion of solutes, carriers and complexes. In this case, the carrier, silver(I) ion, forms a complex with a solute, an ethylene molecule, and the complex diffuses toward the sweep side. The complexed ethylene molecule may be released from the present silver(I) ion and form a complex with another silver(I) ion. The former silver(l) ion diffuses back to the feed side and forms a complex with another ethylene molecule. Therefore, the mobilities of ethylene, silver(I) ions and ethylene- silver(I) ion enhances the facilitated diffusion. How- ever, in a dry film, the mobility of these species may be limited. Facilitated transport largely depends on the diffusion of an ethylene molecule between two com- plexing sites, silver(I) ions, along the polymer chains [32]. As a result, the facilitation effect is reduced without water present.

For industrial application, a very thin membrane is desirable to obtain high fluxes. However, the mathe- matical model derived by Noble [32] for fixed carder membranes shows that the facilitation factor, the ratio of the solute flux with carrier present to the solute diffusion flux, decreases as the inverse Damkohler number increases. The inverse Damkohler number is the ratio of the characteristic reverse reaction time to the characteristic diffusion time defined as: DAB/kr L2 where DAB is the effective diffusion coefficient of olefin-silver(I) ion complex, kr is the reverse rate coefficient of the complexation reaction, and L is the membrane thickness. Therefore, this number increases as the membrane becomes thinner. The facilitated mass transport mechanism approaches the reaction-limited regime when the inverse Dam- kohler number is large, causing a decrease in the facilitation factor and ethylene/ethane permeability ratio. Fig. 6 shows the ethylene/ethane permeability

O .=~

12.00

1 1 . 0 0 - -

1 0 . 0 0 - -

9 . 0 0 - -

8 . 0 0

4

[ q t I I

I I r I

6 8 10 Membrane Thickness (lain)

Fig. 6. Ethylene/ethane permeability ratios as a function of membrane thickness at various temperatures: (O) 50 °C, (m) 60°C, ( 0 ) 70°C.

ratios as a function of membrane thickness at various temperatures. The transport measurements were performed at a total feed pressure of 1900 mmHg with a 50% ethylene and 50% ethane mixture on the feed side.

Within the experimental error, ethylene/ethane permeability ratios were constant at a given temperature. The experiment with a 3 ~tm membrane at the same experimental conditions was not successful since the membrane failed due to operation with a total pressure difference between feed and sweep streams. However, the permeation measurement with a 3 ~tm membrane was successful when the total pressures on feed and sweep sides were kept to be the same at 760 mmHg. At a temperature of 50°C and about 50% ethylene and 50% ethane on the feed side, the ethylene/ethane permeability ratio was 9.64. This permeability ratio was lower than the value obtained with a 6 ~tm film of 11.25 at the same temperature. The transport measurement with the 6 ktm film was performed at a slightly higher ethylene partial pressure of 470 mmHg compared to an ethylene partial pressure of 360 mmHg in the experiment with the 3 ~tm film. However, the ethylene/ethane permeability ratios of the 6 ~tm membrane were constant at a value of about 11.3 over a range of ethylene partial pressures of 470 to 914 mmHg, as shown in Fig. 5. Presumably, the ethylene/ethane

118 A. Sungpet et al. /Journal of Membrane Science 136 (1997) 111-120

permeability ratio of a 6 ~tm membrane obtained at 360 mmHg would be about 11.3 as well. Therefore, the difference in permeability ratios of the 3 and 6 ~tm films can be assumed to be significant. The reason for the lower ethylene/ethane permeability ratio was a decrease in the ethylene permeability while ethane permeability remained approximately constant. This observation suggests that the ethylene flux via the facilitated transport pathway as well as the facilitation factor should decrease as the thickness of the membrane decreases. This is because the cartier is less utilized as the membrane becomes thinner. Therefore, decreasing the membrane thickness results in facilitated transport in the reaction-limited regime.

4.3. FTIR spectroscopy

An ethylene molecule has a C=C stretching frequency of 1623 cm -1, which is infrared inactive [33]. When an ethylene molecule coordinates with a silver(I) ion, the C=C vibrational mode becomes obser- vable due to the change in symmetry of the ethylene molecule. Upon complexation in silver(I)-aqueous solution, the weakening of the ethylene double bond results in the shift of the double bond stretching vibration to a lower frequency. A lowering in the C=C stretching frequency by 73 cm -~ in solution was reported [33]. In this work, the FFIR spectroscopy experiments were performed with three types of membranes which were exposed to ethylene. The spectra are shown in Fig. 7.

The intensity of the signal obtained from Diffuse Reflection Attachment (DRA) method is not linearly related to the concentration of the sample [34]. The signal has to be calculated according to the Kubelka- Munk theory and the Kubelka-Munk unit is used instead of absorbance. An infrared absorption band at 1542.77 cm -1 was observed with silver(I)-form Nafion-poly(pyrrole) composite film. The experiment with a proton-form film was done to verify that this absorption band was not from the interaction between the polymer film and ethylene. This spectral feature was not found with dry silver(I)-form Nation, which corresponds to the fact that the silver(I) ions in Nation membrane are not capable of forming the complex with ethylene in the absence of water. Therefore, this spectral band can be assigned to the C=C stretch. A shift of about 80 cm -1 suggests that ethylene

0.04 t I

0.03

0.01

0.00 t ] i

1600.00 1550.00 1500.00

Wavenumber (era "l)

Fig. 7. Diffuse reflectance FTIR spectroscopy of various membrane exposed to 3700 mmHg of ethylene at 30°C: ( ) silver(I)- form Nafion-poly(pyrrole) membrane, (- - -) proton-form Nafion- poly(pyrrole) membrane, ( - - - - - ) Silver(I)-form Nation membrane.

molecules have relatively strong interaction with silver(l) ions. Correspondingly, this is an indication that poly(pyrrole) greatly modifies the electronic environment of silver(I) ion to be suitable for forming a complex with ethylene.

5 . C o n c l u s i o n s

There is a great incentive to replace the highly energy intensive distillation process for the olefin- paraffin separation with alternate separation processes with lower operating costs. Membrane or membrane- distillation hybrid processes are possible candidates. In the present work, the ethylene/ethane separation using Nafion-poly(pyrrole) composite membranes was reported. The membranes were tested with mixtures of ethylene and ethane at the total feed pressures ranging from 760 to 1900 mmHg. The experiments were done at increasing temperatures from 40°C to 70°C. Pure gas permeability measurements were performed at a total feed pressure of 1900 mmHg and temperatures of 30°C and 40°C. The silver(I)-form Nafion-poly(pyrrole) composite membranes had ethylene/ethane permeability ratios in the range of 8 to 15


with the ethylene permeabilities ranging from 0.2 to 1 Barrer. The permeance values ranged from 300 to 2000 Barrer cm -1 over the same range of conditions.

The ethylene/ethane permeability ratio of about 2 was observed with proton-form Nafion-poly(pyrrole)

composite membranes. The 3 ~tm silver(I)-form membrane showed lower ethylene permeability and ethylene/ethane permeability ratio compared to the results obtained from the thicker membranes. This suggests the facilitated transport of ethylene approaches the reaction-limited regime as the membrane becomes thinner. Finally, FTIR spectroscopy experiments confirmed the complexation between ethylene molecules and silver(I) ions in Nafion-poly(pyrrole) composite membrane.

6. Future work

Although solid reactive membranes selective for ethylene were successfully made, the performance of the membrane needs to be improved. Studies that lead to a better understanding of thermodynamic and kinetic stabilities of ethylene-silver(I) ion complex are essential. Athayde et al. [35] has theoretically shown that the facilitated flux of olefins can be

increased by application of an alternating electric field. The use of a membrane that has higher carrier

loading together with an AC field applied to the membrane may improve the ethylene permeability and selectivity of the membrane. To improve the process economics, the materials used to prepare the reactive membranes should be inexpensive. The membrane costs may be reduced by using other polyelectrolytes which are less expensive than Nation. Studies of other systems such as propylene/propane or cyclohexene/cyclohexane by these membranes are in progress. Furthermore, the performance of an ion- exchange resin incorporated with poly(pyrrole) and silver(I) ion as an olefin adsorbent will be investigated.

Acknowledgements

The authors gratefully acknowledge support from The Government of Thailand for A. Sungpet's grad- uate fellowship. J.D.W. is supported by the Depart- ment of Energy Office of Basic Energy Sciences,

Department of Chemical Sciences under Grant # DE-FG03-93ER14363. Acknowledgment is made to the Donors of the Petroleum Research Fund, adminis- tered by the American Chemical Society, for the partial support of this research by Grant # 23187- G7. We would like to thank Dr. Andrew M. Herring for his help with the FTIR experiments.

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[4] O.I. Eriksen, E. Aksnes and I.M. Dahl, Facilitated Transport of Ethene Through Nation Membranes. Part I. Water Swollen Membranes, J. Membrane Sci., 85 (1993) 89.

[5] O.I. Eriksen, E. Aksnes and I.M. Dahl, Facilitated Transport of Ethene through Nation Membranes. Part II. Glycerine Treated, Water Swollen Membranes, J. Membrane Sci., 85 (1993) 99.

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reactive polymer membranes for ethylene/ethane separation

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