Incorporation of zeolites in polyimide matrices

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1521

Incorporation of zeolites in polyimide matrices P. Sysela, M. Fry(,ovfia, R. Hobzovfi~, V. Krystlb, P. Hrabfinekb, B. Bernauer b, L. Brabec c and M. K o ~ i ~ c aDepartment of Polymers, Institute of Chemical Technology, 166 28 Prague 6, Czech Republic bDepartment of Inorganic Technology, Institute of Chemical Technology, 166 28 Prague 6, Czech Republic r Heyrovslc~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague 8, 182 23 Czech Republic

3-Aminopropyltriethoxysilane terminated polyimide precursors (polyamic acids) and Silicalite-1 were used to prepare zeolite-filled polyimide films. The accessibility of the pores of the zeolite built in both non-treated and treated polyimide matrices was studied using sorption of iodine from the vapour phase. Transport properties of the filled films were investigated using small probe molecules of hydrogen and methane.

1. INTRODUCTION Polymeric membranes have been successfully applied in numerous processes of gas separation. Principal requirements of the membrane technologies are on increasing permeability at sufficiently high selectivity. In this respect pure polymeric membranes reached their limit [ 1]. A promissing route to membranes of improved permeabilities consists in incorporation of microporous materials into polymers. Thus, the incorporation of zeolites into a rubbery polymeric membrane may result in an improvement of its properties both in separation of gases and pervaporation [2,3]. Lower selectivies have been achieved by incorporation of zeolites into glassy polymers. One reason of this result was generation of voids at the polymer zeolite interface [4-6]. This drawback may, however, be overcompensated in many respects with outstanding mechanical and chemical stability of some high-performance glassy polymers at elevated temperatures. Aromatic polyimides (PI) exhibit very good chemical, mechanical and dielectric stability at temperatures up to 200-250~ They are mostly used in (micro)electronics, aviation industry, aerospace investigation and as polymeric separation membranes [7]. Non-porous polyimide membranes show high separation factors for separation of

1522 mixtures of permanent gases but low permeability for both permanent gases and organic vapours [8]. Asymmetric membranes [9], hollow-fiber membranes [10] and non-porous membranes [ 11] based on polyimides, poly(ether-imide)s and poly(amideimide)s crosslinked with poly(ethylene adipate), respectively, were successfully employed in separation of organic vapours from permanent gases. The aim of the present work is to examine feasibility of Silicalite-1 - PI composites which would exhibit (i) a sufficient interracial adhesion of phases, (ii) accessibility of zeolitic phase for sorbing molecules, (iii) an enhanced flow of species at reasonable selectivities.

2. EXPERIMENTAL 2.1. Chemicals

Pyromellitic dianhydride (PMDA) and 4,4"-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) were heated to 180~ overnight in a vacuum before use. 4,4"Oxydianiline (ODA) (all Aldrich, Czech Republic) and p-aminopropyltriethoxysilane (APTES) (ABCR, Germany) were used as received. N-Methyl-2-pyrrolidone (NMP) (Merck, Czech Republic) and N,N-dimethylformamide (DMF) (Aldrich) were distilled under vacuum over phosphorus pentoxide and stored in an inert atmosphere. 2.2. Procedures

Polyimide precursors, polyamic acids (PAA), were prepared in a 250 ml two-necked flask equipped with a magnetic stirrer and a nitrogen inlet/outlet. PAA based on a dianhydride and ODA with uncontrolled number average molecular weight (M.) were prepared by the reaction of equimolar amounts of the dianhydride and the diamine in NMP or DMF (solid content 15 wt.%) at room temperature for 24 h [7]. A typical example of the Pl-aminopropyltriethoxysilane terminated PAA (6FDAODA) with Mn = 10000 g mol is as follows: 6FDA was dissolved in NMP or DMF and the terminating agent (APTES) was added to the reaction mixture and allowed to react with 6FDA for 2 h. 6FDA was then added and the reaction (solid content 15 wt.%) was allowed to proceed at room temperature for 24 h. Zeolite crystals were prepared according to the protocol by Kornatowski [12]. They were S ilicalite-l-90~ with S i/A1 ratio cca 350 and the lenght Lc cca 190 ~tm. The most crystals are of prismatic form with a minimum crosses and rosettes. The template (tetrapropylammonium bromide) was removed in a single stage calcination process in the flow of air (60 ml min l ) using the heating programme applied to a shallow bed of crystals. ZSM-5 was supplied by PQ Zeolites (Conteka) and NaY by VURUP Bratislava. Membranes were prepared by dispersion of the zeolite in PAA solution in NMP or DMF (by stirring for 2 h) and subsequent casting of the mixture on a Teflon substrate.

1523 After solvent removal the films were heated in subsequent steps up to 230~ for 2 h. The content of zeolites in the films was 10 wt.%.

2.3.

Instrumental techniques

Sorption experiments using Iodine Indicator Technique (involving light microscopy) [13,14] were performed with crystals embedded in polyimide matrix. Fine iodine particles placed at the beginning of the experiment into the cell represented the source of iodine vapour. The sorption kinetics was monitored by taking coloured photographs of the filled films at different contact times. The photographs provided information on uniformity of colouring process and the time interval necessary for reaching a limiting intensity of crystal colouring. Optical observations of sorption by transmission light microscopy were done with a microscope Peraval Interphako (Carl Zeiss Jena) coupled with a digital camera Nikon Coolpix 950. A sorbent loaded with volatile species was examined in a closed glass cell with a total thickness of about 2.5 mm (including the glass) and an internal space of a cylindrical shape (15 mm in diameter and 1.2 mm in depth). All the observations were performed at room temperature under air. The permeability and selectivity of the composite membranes were investigated using Wicke-Kallenbach cell and small probe molecules as hydrogen and methane [151.

3. RESULTS AND DISCUSSION

3.1. System selection The characterization of the zeolites tested in this study is given in Table 1. Distribution of both ZSM-5 and NaY inclusions in polyimide films was strongly nonuniform (based on observations by light microscopy - crystal aglomerates form dark patterns) probably due to their hydrophilic character. The most uniform distribution of zeolitic inclusions was obtained with Silicalite-1 crystals (Figure 1). We were not successful in the preparation of self-standing PI-zeolite films with PI

Table 1 Characterization of zeolites

Silicalite- 1 ZSM-5 NaY

Crystal length

Pore size

(~m)

(nm)

190 1-3 1

0.5 0.5 0.8

Si / A 1

350 25 2

Hydrophilicity

very low middle high

1524 based on PMDA and ODA (both with uncontrolled or controlled Mn). The most probable reason of miscarriage was PI (PMDA-ODA) chain rigidity. On the contrary, PI-zeolite films based on 6FDA and ODA (both with uncontrolled or controlled M,) were self-standing up to 50 wt.% zeolite content. All composites based on PAA (6FDA-ODA) with uncontrolled Mn (see Figure 2 for its preparation) and zeolites summarized in Table 1 exhibited very high permeabilities and very low selectivities. One reason of this result was undoubtedly a generation of interfacial voids at the polymer-zeolite interface. This phenomenon is typical for glassy polymer- zeolite combination [4].

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Figure 1. Light micrographs of PI (6FDA-ODA)-zeolite films based on p-aminopropyltriethoxysilane terminated PAA (6FDA-ODA) with theoretical M, = 10000 o g mol 1 and 10 wt,~ of a) Silicalite-1, b) ZSM-5 and c) NaY

1525

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Figure 2. Preparation of PI (6FDA-ODA) with uncontrolled Mn To improve the adhesion between the matrix and the zeolite we prepared PAA (6FDA-ODA) with controlled Mn terminated with APTES (see Figure 3 for its preparation). The theoretical Mn was 10000 g mo1-1. The imidization was accompanied with the formation of Si-O-Si bonds between the polymer matrix and the zeolite surface bearing hydroxyl groups (Figure 4). The silylation of the zeolite with APTES was studied as a tool to improve zeolite incorporation in the polyimide matrix [6].

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1526

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Figure 4. Formation of chemical bonds between polymer matrix and zeolite surface

The permeabilities of hydrogen and methane of the filled PI prepared in the presence of APTES were very low. The untreated PI (6FDA-ODA)-Silicalite-1 film was tested so far. The permeability coefficient of methane was 1.2x10 q5 mol mqslPa "l and selectivity for the mixture hydrogen/methane amounted to 23. It has been also found that the zeolite channels in such filled films were practically inaccessible for iodine.

3.2. Zeolite channel accessibility The procedures to improve the zeolite channel accessibility were evaluated via iodine sorption. The PI films based on PAA (6FDA-ODA) with Mn = 10000 g mol 1 terminated with APTES and filled with 10 wt.% Silicalite-1 were used in the present study. The iodine sorption was enhanced by the following operations: a) N-methyl-2-pyrrolidone (b.p. 202~ was substituted by a lower-boiling N,Ndimethylformamide (b.p. 153~ during the PAA preparation (Figure 5a) b) Silicalite-1 was soaked with low boiling solvent before incorporation to PAA solution (Figure 5b) c) filled film was evacuated at elevated temperatures up to 200~ for 48 h (Figure

5e) d) PI layers which cover zeolite inclusions at either membrane sides were removed by a mechanical treatment (grinding) (Figure 5 d) They are compared with the pure Silicalite-1 (Figure e) and the sample prepared from untreated sample using NMP (Figure 5 f).

1527

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Figure 5. Light micrographs of PI (6FDA-ODA) (10000 g mol "1, APTES terminated) S ilicalite-1 films prepared under various conditions or pure Silicalite-1 (see the above text for the specification, contact time of iodine with samples was of 3 h)

This work was supported by the grant CEZ:MSM 223100002 and by Grant Agency of the Academy of Sciences of the Czech Republic via Grant No. 4040901 and A 1040101.

1528 REFERENCES

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