DESALINATION

ELSEVIER Desalination 145 (2002) 359-364 www.elsevier.com/locate/desal

Propylene/propane separation with copolyimides containing benzo- 15-crown-5-ether to incorporate silver ions

Sandra HelJ*, Gunter Scharfenberger, Claudia Staudt-Bickel, Riidiger N. Lichtenthaler

Applied Thermodynamics, Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

Tel. + 49 (6221) 544256; Fax + 49 (6221) 544255; email: Sandra. hess@urz. uni-heidelberg.de

Received 1 February 2002; accepted 21 March 2002

Abstract

A 6FDA (4,4‘-hexafluoro isopropylidene diphthalic anhydride) based copolyimide has been synthesized containing the diamines DABA (3,Sdiamino benzoic acid) and a 15-crown-Sdiamine to separate propylene/ propane mixtures. With the carboxylic acid group of DABA crosslinking the polymer chains is possible leading to a suppression of plasticization of the polymer. Thereby the loss of separation performance at higher pressures of membranes made from such a polymer can be avoided. The crown ether group allows to incorporate silver ions by complexation in the membrane polymer. As a consequence facilitated transport of oleflns is possible. Pure gas permeation and sorption experiments at 35°C and feed pressures up to 8 bar have been carried out with propylene and propane using non-crosslinked and EG-crosslinked copolyimide membranes, the latter ones with and without silver ions.

Keywords: Copolyimide membranes; Crown ether; Facilitated transport; Pure gas permeation; Pure gas sorption

1. Introduction

In the chemical industry propylene is an im- portant substance for a large number of chemical

*Corresponding author.

synthesis, but its main application is the pro- duction of polypropylene. In Europe, for example, 14.3 M tons of propylene were produced in the year 2000 and 53.3% of that were used in the synthesis of polypropylene [I].

Presented at the international Congress on Membranes and Membrane Processes (ICON), Toulouse, France, July 7-12, 2002.

00 1 l-91 64/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO0 II-9 164(02)00436-8

360 S. Hej et al. /Desalination I45 (2002) 359-364

Propylene is produced in refinery processes as a byproduct in the ethylene production, or by dehydrogenation of propane. In all cases a propylene/propane mixture occurs which needs further treatment in order to obtain the pure substances. For the separation of this mixture cryogenic distillation is used which is very ex- pensive due to the necessity of low temperatures and high pressures. Therefore great interest exists in the development of more economical separation processes. Membrane-based gas separation would be a promising alternative to the conventional methods, but so far membrane materials having such good separation charac- teristics which make industrial application of the process economical, are not available.

2. Selection of the polymer structure

Most of the commercially available mem- brane polymers show plasticization effects when exposed to hydrocarbon mixtures resulting in a loss of separation performance. Plasticization is caused by the high solubility of such components in the polymer. As a consequence an increase in the free volume and in the segmental mobility of the polymer chains occurs leading to a strong increase in permeability usually combined with a decrease in selectivity. A possibility to suppress this un-desirable effect is crosslinking the polymer chains, which has been shown successfully for copolyimides used in the separation of COJCH4 [2]. In the present work copolyimides containing carboxylic acid groups are used which allow crosslinking.

A method to enhance selectivity of the olefin/paraffin separation is the facilitated trans- port of olefins by metal ions like Ag+ incorp- orated in the membrane material. In contrast to the paraffin the olefin is able to form a rever- sible complex with the silver ion. As a con- sequence solubility of olefins in the membrane polymer increases and furthermore facilitated transport leads to an increase of their perme- ation rate. Olefins pass the membrane due to the solution-diffusion mechanism as well as by

facilitated transport while paraffins permeate only according to the solution-diffusion mech- anism.

Two different types of facilitated transport are possible shown schematically in Fig. 1; the olefin transport with mobile carriers and the transport with fixed-site carriers. In the first case the olefin/silver complexes formed at the feed-side boundary of the membrane permeate due to their concentration gradient and after de- complexation at the permeate side the silver ions permeate back and the circle starts again. In the second case silver ions are fixed in the polymer and olefin molecules permeate the membrane by “hopping” from one fixed silver ion to another.

The polymers developed in this work are of the type of fixed-site carriers. In order to immo- bilize silver ions in the membrane, crown ether

(a)

f- &t Ag”

Copolyimide

Fig. 1. Mechanisms of facilitated transport: a) mobile Ag’-carrier; b) fixed-site Agf-carriers.

S. HeJ et al. /Desalination 145 (2002) 359-364 361

groups have been introduced into the polymer backbone. Crown ethers are able to form strong complexes with light metal ions showing com- plexation selectivity. Li’, for example, has an ionic diameter of 136 pm and forms strong com- plexes with a 12-crown-4-ether which has a cavity with the diameter of 120-150 pm [3]. Silver ions have a greater ionic diameter of 252 pm. In this case complexation occurs with a 15-crown-5-ether with a diameter of the cavity of 170-220 pm. As the crown ether cavity is a little bit smaller than the ionic diameter Ag’ sits above the crown ether cavity. Nevertheless the complexation is strong enough to immobilize the silver ions.

3. Polymer synthesis and membrane forma- tion

The crosslinkable copolyimides are synthe- sized by polycondensation (Fig. 2) of the dian-

hydride 6FDA (4,4’-hexafluoro isopropylidene diphthalic anhydride) with the diamines 6FpDA

(4,4’-hexafluoro isopropylidene dianiline), 15-

crown-5 (4’,4”(5”)-diamino-dibenzo-15-crown- 5) and DABA (3,5-diamino benzoic acid) con- taining a carboxylic acid group where cross- linking with a diol is possible. To obtain a cross-linked polymer the crosslinkable copolyi- mide is dissolved in dimethyl acetamide and after adding the crosslinking agent EG (ethylene

COOH GFDA 6FpDA 15crown-5

chemical imidization

DABA

crosslinkable ~FDA-~F~DA/~~-c~ow~-~/DABA 4:4:1

+ Ho4/oH - H,O

crosslinked GFDA-GFpDM 5-crown-5/DABA 4:4:1

Fig. 2. Synthesis of an EG-crosslinked copolyimide.

362 S. He/ et al. /Desalination 145 (2002) 359-364

glycol) the solution is casted onto a plane glass. After evaporating the solvent the mem-brane is heated at a temperature of 150°C to remove the water set free during the cross-linking reaction.

To incorporate silver ions into the mem- brane the EG-crosslinked polymer is immersed into a solution of AgN03 in a 50:50 methanol water mixture for 24 h. The membrane is dried in the dark at a temperature of 80°C under vacuum.

Depending on the concentration of the various monomers in the mixture prepared for the synthesis copolymers can be synthesized containing different numbers of the various chemical groups. In particular the final degree of crosslinking can be monitored by changing the number of carboxyclic groups/polymer molecule. In this work the copolymer 6FDA- 6FpDA/l5-crown-S/DABA 4:4: 1 has been in- vestigated which according to the ratio 4:4:1 of the monomers has a crosslinkable group in each ninth polymer unit.

m non-crosslinked, propane 0 non-crosslinked, propylene v EG-crosslinked, propane v EG-crosslinked. propylene

m n n n m

0 2 4 6 a 10

Feed Pressure [bar]

4. Experimental results

Pure gas permeation experiments have been carried out with non-crosslinked and with EG- crosslinked 6FDA-6FpDA/l5-crown-S/DABA 4:4:1 membranes as well as with EG-cross- linked membranes containing silver ions. Ad- ditionally pure gas sorption experiments have been performed with all of these membranes using a microbalance apparatus.

4. I. Gas permeation experiments

Pure gas permeation experiments were carried out at a temperature of 35’C with vari- ous feed pressures pF and permeate pressures pp between 15 and 30 mbar. As shown in Fig. 3 the permeability of propylene is always higher than the permeability of propane. This was to be

expected as due to its n-electrons propylene is more polar than propane and therefore it has a higher affinity to the polar copolyimides. Table 1 shows the permeabilities of propylene and

-I 2 4 6 8 10

Feed Pressure [bar]

Fig. 3. Pure gas permeability of the 4:4: l-membrane as a function of/ for propylene and propane at T = 35°C and pl’ ~30 mbar.

S. He/3 et al. /Desalination I45 (2002) 359-364 363

Table 1 Pure gas permeability P, and ideal separation factors cqdeal (T = 35” C, p’ = 6 bar, pp <30 mbar)

Polymer P Propane, bamr PPropy~ene, bmer @ideal

6FDA-6FpDA/I 5-crown-S/DABA 4:4: 1 non-crosslinked 0.085 0.285 3.4

6FDA-6FpDA/l5-crown-S/DABA 4:4: 1 EG-crosslinked 0.593 0.767 1.3 6FDA-6FpDA/I 5-crown-S/DABA 4:4: 1 EG-crosslinked with An+ 0.049 0.324 6.6

propane as well as the ideal separation factors at a feed pressure of 6 bar.

Crosslinking the polymer with EG leads to an increase of permeability while selectivity de creases compared to the non-crosslinked one, because introduction of the crosslinker increases the spatial distance between the polymer chains leading to an increase in free volume. Incorporating silver ions into the cavity of the crown ethers of the EG-crosslinked copolyimide results in a decrease of the permeability compared to the one of the EG-crosslinked po- lymer without silver ions due to the decrease of free volume. The silver ions block the cavity of the crown ether and as a consequence the gases are hindered on their way through the mem- brane. The transport of propane happens by the

V

Feed Pressure [bar]

solution-diffusion mechanism, because propane is not able to complex with the silver ions. However, the silver ions form reversible com- plexes with propylene. Therefore in addition to the solution-diffusion mechanism propylene permeates the membrane also by “hopping” from one silver ion to another. This results in a better selectivity compared to the EG-cross- linked membrane without silver ions.

4.2. Gas sorption experiments

Pure gas sorption experiments have been carried out at a temperature of 35°C and vari- ous pressures. The results are shown in Fig. 4.

In all cases propylene is the preferentially absorbed component. Crosslinking reduces the

30

n non-crosslinked, propane 0 non-crosslinked, propylene

; r EG-crosslinked, propane . u EG-crosslinked, propylene

0 2 4 6 8

Pressure [bar]

Fig. 4. Pure gas sorption of the 4:4: 1 -membrane as a function of pressure for propylene and propane at T= 35°C.

S. HeJ et al. /Desalination 145 (2002) 359-364

solubility because the overall polarity of the polymer is reduced due to the decrease of “free” carboxylic acid groups. At low pressures the sorption of propane is almost the same for the non-crosslinked and the EG-crosslinked copo- lyimide but at higher pressures the sorption of the non-crosslinked polymer is larger.

The sorption of propylene of the EG-cross- linked polymer containing silver ions is larger than the one without the ions. The reason for the larger propylene sorption is the formation of complexes between the silver ions and the ole- fin. Also for propane there is a slightly larger sorption obtained for the EG-crosslinked poly- mer containing silver ions although complex formation is not possible. The reason might be that the polarity of the crown ether groups is lowered in the presence of silver ions, because the electrons of the oxygen atoms in the ether groups are used for the complexation with the ions. As a consequence the interaction of the non-polar propane with the crown ether groups is stronger in the presence of Ag+ -ions.

5. Conclusions

The following conclusions can be drawn from this work:

Crosslinking of the polymer chains with EG increases permeability of propylene as well as of propane while selectivity decreases. Incorporating silver ions into the EG- crosslinked membrane leads to a de-crease of permeability correlated with an increase of selectivity. The sorption of propylene and propane of the EG-crosslinked membrane is lower than that of the non-crosslinked one. The sorption of propylene and propane increases by incorporating silver ions in the EG-crosslinked membrane.

For an industrial application the permeability of the investigated polymers is too low. To increase permeability of the membrane mono- mers with bulky groups should be used to syn- thesize copolymers with a larger free volume.

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