Synthesis of highly c-oriented ZIF-69 membranes by secondary growth and their gas permeation properties

Journal of Membrane Science 379 (2011) 46–51

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Synthesis of highly c-oriented ZIF-69 membranes by secondary growth and their gas permeation properties Yunyang Liu, Gaofeng Zeng, Yichang Pan, Zhiping Lai ∗ Division of Chemical and Life Sciences and Engineering, Center of Advanced Membranes and Porous Materials, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 29 March 2011 Received in revised form 14 May 2011 Accepted 19 May 2011 Available online 14 June 2011 Keywords: ZIF-69 membranes Secondary growth Orientation Gas permeation Separation

a b s t r a c t A seeded growth procedure was successfully developed to synthesize highly c-oriented and wellintergrown zeolitic imidazolate framework-69 (ZIF-69) membranes on porous ␣-alumina substrates. The synthesis conditions were optimized both for seed preparation and for secondary growth. For seeding, a facile method was developed to prepare smaller and flat ZIF-69 microcrystals in order to make thin and coriented seed layers. While for secondary growth, a synthesis condition that favored the growth along the c-direction was chosen in order to form highly c-oriented ZIF-69 membranes after growth. As a result, the majority of ZIF-69 grains inside the membrane have their straight channels along the crystallographic caxis aligned perpendicularly to the substrate surface. Such alignment was confirmed by both XRD and pole figure analysis. The mixture-gas separation studies that were carried out at room temperature and 1 atm gave separation factors of 6.3, 5.0, 4.6 for CO2 /N2 , CO2 /CO and CO2 /CH4 respectively, and a permeance of ∼1.0 × 10−7 mol m−2 s−1 Pa−1 for CO2 in almost all mixtures. Both the separation factor and permeance were better than the performance of the ZIF-69 membranes prepared by the in situ solvothermal method due to improvement in the membrane microstructure by the seeded growth method. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Zeolitic imdazolate frameworks (ZIF) are a new class of hybrid inorganic–organic crystalline porous materials and have captured wide attentions due to their well-defined, adjustable and highly open porous framework structures [1–5]. ZIF-based membranes are expected to offer unique opportunities for advanced applications such as separations, membrane reactors and sensors. Several types of ZIF membranes have been reported recently which include ZIF-7, ZIF-8, ZIF-22, ZIF-69 and ZIF-90. Permeation studies on these membranes have shown promising performance for gas separations [6–12]. Different methods have been developed to synthesize ZIF membranes, such as in situ crystallization, seeded growth, stepwise layer-by-layer growth and modification of substrate by SAM (self-assembled organic monolayer) [13–19]. Some of these methods are adapted from the membrane synthesis of other ordered porous materials such as zeolites. However, the synthesis procedures developed in some of these pioneer studies are quite complicated. For example, some procedures required a complicated

∗ Corresponding author at: Chemical and Biological Engineering, Al-Jazri Building (Building 4), Room 4218, PO Box #1537, 4700 King Abdullah University of Science and Technology, Thuwal, 23955-6900, Saudi Arabia. Tel.: +966 2 8082408. E-mail address: [email protected] (Z. Lai). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.05.041

pre-treatment of the support surface in order to achieve continuous membranes [20]. The synthesis of continuous and thin ZIF membranes still remains a challenge. In this study we target to extend the seeded growth method to the synthesis of ZIF-69 membranes. ZIF-69 has a zeolite GME topology that has 12-membered ring (MR) straight channels along the c-axis and 8 MR channels along the a- and b-axes. The pore size along the c-axis is about 0.78 nm. To get better gas permeation performance, one would expect the optimal microstructure of ZIF-69 membranes should be thin, compact and most importantly, c-oriented, i.e. all of the straight channels aligned perpendicularly to the support surface. In our previous work [9] we have synthesized ZIF-69 membranes by the in situ solvothermal method. The membrane showed preferred permeation of CO2 over other simple gases such as H2 , N2 , and CO. The results indicated the potential applications of ZIF-69 membranes for CO2 capture and sequestration. Theoretical simulation also showed that ZIF-69 had higher adsorption capacity and diffusivity than other types of ZIFs for CO2 , particularly the capacity increased with increasing pressure [21]. However, although the membrane made by the in situ solvothermal method showed certain degree of c-orientation, significant amount of mis-orientation were observed due to the nature of the in situ crystallization method. Seeded growth is another important method, which has been widely used in synthesis of high performance inorganic zeolite

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Fig. 1. (a) SEM image of ZIF-69 crystals synthesized by using zinc nitrate as precusor. (b) SEM image of ZIF-69 seeds coated on ␣-alumina substrate. (c) XRD patterns for (i) ZIF-69 seeds as-synthesized, (ii) ZIF-69 powder synthesized by solvothermal method, (iii) ZIF-69 powder by simulation from Mercury Software (Cambridge Crystallographic Data Centre). (d) N2 adsorption isotherms of ZIF-69 seeds at 77 K.

membranes [22–25]. However very few cases have been reported for ZIF membranes. The seeded growth method normally includes two steps. The first step is to apply a layer of seeds on a support surface. In the second step the seeded support is exposed to a synthesis solution to carry out secondary growth. Interstitial gaps among seeds are closed-up during the secondary growth by growth of seeds, and finally a continuous membrane is formed. The notable advantage of the seeded growth method is that it can systematically control the membrane orientation, which can be achieved either by the van der drift competitive growth theory [26] or by preserving the orientation from the seed layer [24]. Other membrane properties such as membrane thickness and grain boundary structure can also be optimized by the seeded growth method, which in many cases have led to better membrane performance [25,27–30]. Hence, the target of this study is to use the seeded growth method to improve the degree of orientation of ZIF-69 membranes, and in consequence, the membrane performance. 2. Experimental 2.1. Preparation of ZIF-69 seeds A mixture of 2-nitroimidazole (0.4071 g, 3.6 mmol) and 5chlorobenzimidazole (0.5493 g, 3.6 mmol) was dissolved in 36 mL of N,N-dimethylformamide (DMF). Then 9 mL of zinc acetate dihydrate solution (0.2 M, in DMF) was added dropwise into the above solution under vigorous stirring at room temperature. A white

cloudy precipitation can be obtained immediately. After addition, the reaction solution was kept stirred for another 48 h. The ZIF69 powder was collected by centrifugation and washed thoroughly with fresh DMF three times to remove any unreacted reactants. After dried for 24 h at 100 ◦ C under vacuum, 0.56 g of ZIF-69 seeds was obtained. Finally, the ZIF-69 seeds were dispersed in 56 mL of DMF under ultrasound to form a ZIF-69 colloid (1 wt.%) for the next coating step. 2.2. Synthesis of ZIF-69 membranes on porous ˛-alumina substrate by secondary growth Porous ␣-alumina discs (2 mm thickness and 22 mm diameter) were used as support, which were homemade from high purity alumina powder (Baikowski, CR-6). The detailed preparation procedure can be found elsewhere [24]. One side of the support was smoothed by polishing with 1200 grit sandpaper. The disc was cleaned with DI water and dried at 200 ◦ C for 6 h and then cooled down to room temperature before use. After dip-coating in ZIF-69 colloid for 3 s, the seeded ␣-alumina disc was dried at 100 ◦ C for 2 h and cooled down to room temperature. The seeded disc was placed horizontally into a 45 mL Teflon-lined autoclave with the seeded surface facing upward. A reaction solution was prepared by dissolving 0.136 g of 2-nitroimidazole (98%, Sinopharm Chemical Reagent Pte Ltd), 0.183 g of 5-chlorobenzimidazole (96%, Aldrich) and 0.178 g Zn(NO3 )2 ·6H2 O (99%, Fluka) subsequently in 12 mL N,Ndimethylformamide (DMF, 99.5%, Merck). The reaction solution

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Fig. 2. (a) Top view of ZIF-69 membrane by secondary growth. (b) Cross section view of ZIF-69 membrane by secondary growth. (c) XRD patterns for (i) ZIF-69 seeded ␣-alumina substrate, (ii) ZIF-69 powder by simulation from Mercury Software (Cambridge Crystallographic Data Centre), (iii) Highly oriented ZIF-69 membrane by secondary growth in this study. * are peaks from ␣-alumina substrate.

was loaded into the Teflon-lined autoclave. The solution was sealed and heated to 100 ◦ C for 72 h. After synthesis the autoclave was cooled down to room temperature. Then the disc was taken out and rinsed three times with fresh methanol to remove any solvent on the surface, and then immersed in 30 mL fresh methanol for another 12 h to exchange the solvent that is occluded inside the channels of ZIF-69. Finally, the disc was vacuum dried at 60 ◦ C for 24 h.

2.3. Characterization X-ray diffraction (XRD) patterns were measured from a Bruker D8 Advance X-ray Diffractometer with Cu K␣ radiation. Pole figure analysis was taken by a thin film XRD (PANalytial X’Pert PRO PW3040/60). Scanning electron microscopy (SEM) images were obtained from a scanning electron microscope (JEOL JSM-6390LA) operating at 15 kV. SEM samples were first stuck to a SEM holder by carbon tape, and then sputter-coated with a thin layer of gold–palladium alloy to increase conductivity. Physisorption of nitrogen at 77 K was carried out on a Quantachrome Autosorb-1C static volumetric instrument. Prior to the measurement the sample was degassed at 150 ◦ C for 24 h until the residual pressure was below 1 × 10−4 Torr. Single-gas permeation was carried out by the vacuum permeation method. When switching from one gas to the other, the sample was evacuated for at least 12 h. The separations of equi-molar CO2 /N2 , CO2 /CO and CO2 /CH4 mixture were conducted by the Wicke-Kallenbach method [31]. The feed flow rate of the mixture gas was 100 cm3 /min, and the helium flow rate on the sweep side was 50 cm3 /min. The permeation area of the membrane is ∼1 × 10−4 m2 . The total pressure on both sides was maintained at 1 atm. The gas composition was analyzed by a gas

chromatograph (Agilent 7890). All the permeation measurements were repeated on at least 3 different samples.

3. Results and discussion 3.1. Manipulate the crystal morphology of seeds ZIF-69 was first synthesized by Yaghi et al. [1]. The initial synthesis recipe used zinc nitrate as zinc precursor. Fig. 1a shows a typical crystal morphology obtained after this recipe. It has a shape of hexagonal prism elongated along the c-axis, indicating the growth rate along the c-axis is much bigger than that along the other two axes. The crystal size is typically bigger than 10 ␮m. These crystals are not suitable to be used as seeds. An apparent reason is that the size is too big. It will not be possible to make thin films out of these crystals. Hence the first challenge in this study is to reduce the particle size of ZIF-69 crystals. Intensive experimental efforts have been made trying to reduce the crystal size by varying synthesis conditions that normally lead to reduction of crystal size, such as increasing the synthesis temperature and concentration, or reducing synthesis time, but all of these efforts were not successful. Only when we replaced zinc nitrate with zinc acetate as zinc precursors, then the crystal size could be significantly reduced below 1 micrometer. Fig. 1b–d shows the SEM, XRD and BET characterization data of the ZIF-69 crystals synthesized by this modified recipe. The SEM image shows that all ZIF-69 seeds appear as a flat hexagonal prism with double hexagonal pyramid heads. The size is in the range of 200–800 nm. From the SEM one can also see that when these seeds deposite on a flat support surface, most of them will have the hexagonal face (c-face) heading upwards,

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although the pyramid heads may provent them to be fully vertical. This self-alignment along the c-axis should be very helpful for the formation of c-oriented membrane during secondary growth. The XRD pattern has low background signals and all the peak positions are consistent with that of the simulated one or with that synthesized by the solvothermal method [1,9] indicating a pure phase of ZIF-69 with high crystallinity was obtained. The N2 adsorption isotherm exhibits type I isotherm and gives a Langmuir surface area of 1412 m2 /g. The micropore volume is around 0.437 cm3 /g calculated by Dubinin–Radushkevich method. 3.2. The characterization of ZIF-69 membrane According to the van der drift competitive growth model [26] in order to prepare c-oriented ZIF-69 membranes the secondary growth condition should favor the growth along the c-axis. In this context the synthesis condition for seed synthesis will not be suitable for the secondary growth. Instead, the initial synthesis recipe reported by Yaghi et al. would fulfill the requirement. Fig. 2a and b shows the typical SEM images in topview and in cross-section view of the ZIF-69 membranes obtained by secondary growth. The topview SEM image shows the membrane is well intergrown with smooth membrane surface. The average grain size inside the membrane is around 10 ␮m. The cross-section view reveals that the ZIF-69 membrane is composed of tightly intergrown crystals highly oriented to the porous alumina support. The membrane thickness is around 40 ␮m. Fig. 2c shows the XRD patterns for (i) ZIF-69 seeded ␣-alumina support, (ii) ZIF-69 powder by simulation from Mercury© Software, (iii) ZIF-69 membrane by secondary growth. The XRD pattern of the membrane agrees well with the simulated XRD pattern in term of the peak positions, indicating that pure phase of ZIF-69 was obtained. However, the relative peak intensity is significantly different. The peak at a 2 angle of 9.1◦ which is assigned to the (0 0 2) plane of ZIF-69 crystals is much higher than other peaks, indicating that the membrane is highly c-oriented. Hence, the facets observed in the topview image are mainly (0 0 1) facet. Compared to the ZIF-69 membrane synthesized by in situ solvothermal method [9] the membrane surface synthesized by secondary growth is much smoother. All the grains are highly aligned along the c-axis. While significant amount of mis-orientations can be observed in the ZIF69 membranes synthesized by in situ solvothermal method (Fig. 2a in Ref. [9]). The crystallographic preferential oritentation (CPO) indexing method [32,33], which is based on the comparison between the observed XRD diffractogram with that of a randomly oriented powder, can be used to estimate the orientation of a membrane. The CPO002/100 index is calculated by using the below formula:



CPO002/100 =

I002 I100

 M

−

 I002  

 I002  I100

I100

R

R

where I is the integrated intensity of the corresponding reflection at the indicated peak. M represents ZIF-69 membrane and R is the simulation of a randomly oriented ZIF-69 powder. The CPO index in this study, obtained using the (0 0 2) and (1 0 0) reflections, is approximately 277 which indicates a strong “c” out of plane orientation. Similarly, the CPO index using (0 0 2) and (1 0 1) reflections is around 473. Therefore, both CPO values imply that the membrane is highly c-oriented. The membrane orientation was further studied by pole figure analysis. Fig. 3 shows the (0 0 2) pole figure of the ZIF-69 membrane. It has maximum intensity at 0◦ tilt angle, which indicates that most of the (0 0 2) planes in the membrane are parallel to the support surface.

Fig. 3. (0 0 2) pole figure of a ZIF-69 membrane.

3.3. Gas permeation Fig. 4a shows the dependence of the single gas permeance of CO2 , N2 , CO and CH4 upon trans-membrane pressure drop. The permeance of CO2 remains almost constant with increased feed pressure, while the permeances of N2 , CO and CH4 decrease, indicating that viscous flow through defects can be neglected and the ZIF-69 membrane is primary defect-free. Generally, the permeance of CO2 should be lower than those of N2 , CO and CH4 if all gases follow the Knudsen diffusion mechanism. However, in this study the permeance of CO2 is higher than those of N2 , CO and CH4 . The possible reason is the permeation behavior of CO2 is controlled by surface diffusion, which attribute to the strong selectively adsorption of ZIF-69 for CO2 [1]. The permeances of CO2 , N2 , CO and CH4 are 23.6 ± 1.5 × 10−9 , 10.6 ± 0.8 × 10−9 , 8.2 ± 0.4 × 10−9 and 8.6 ± 0.5 × 10−9 mol m−2 s−1 Pa−1 at 298 K under 1 bar, respectively. Fig. 4b shows the ideal selectivities of a ZIF-69 membrane as a function of trans-membrane pressure drop at 298 K. The ideal selectivities of CO2 /N2 , CO2 /CO and CO2 /CH4 are 2.2, 2.9 and 2.7, respectively. These selectivities increase with increased feed pressure. Fig. 5 shows the binary permeation data for CO2 /N2 , CO2 /CO and CO2 /CH4 . The binary mixture permeation was conduct by the Wicke-Kallenbach mode and the permeation setup was described in detail elsewhere [31]. The feeding gases contained 1:1 volume ratio of gas mixture. Helium was used as the sweep gas on the permeation side. The total pressure on both feed side and permeation side was maintained at 1 atm. The permeation gases were analyzed by GC. Fig. 5 shows that for all three systems steady-state

50

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200

CO2 N2 CH4 CO

150

Permeance (10

100 50

a

0 1.0

1.5

2.0

2.5

3.0

Ideal Selectivity (IS)

5

-10

2

mol/m s.Pa)

250

CO2/CO CO2/CH4 CO2/N2

4

3

2

b 1.0

1.5

2.0

2.5

3.0

Transmembrane Pressure Drop (bar)

Transmembrane Pressure Drop (bar)

Fig. 4. (a) Single gas permeances of CO2 , N2 , CH4 and CO through a ZIF-69 membrane as a function of transmembrane pressure drop at 298 K. (b) The ideal selectivities of CO2 /CO, CO2 /CH4 , and CO2 /N2 for a ZIF-69 membrane as a function of transmembrane pressure drop at 298 K.

Separation Factor

7 6 5 4

CO2/N2 CO2/CO CO2/CH4

3 2 0

5

10

15

20

25

30

35

Test Time (hours) Fig. 5. Separation factors of mixtrue gas CO2 /CO, CO2 /CH4 , and CO2 /N2 (50% molar each) as a function of test time for ZIF-69 membrane at 298 K.

can be reached in very short time. The steady-state separation factors of CO2 /N2 , CO2 /CO and CO2 /CH4 are around 6.3, 5.0 and 4.6, respectively. Table 1 shows the comparisons of the single and binarymixture data for CO2 /N2 , CO2 /CO and CO2 /CH4 , respectively. It reveals that both the permeance and selectivity of CO2 in mixture permeations are higher than those in single-component permeation. For example, the ideal selectivity and permeance of CO2 /N2 are around 2.2 and 23.6 ± 1.5 × 10−9 mol m−2 s−1 Pa−1 , Table 1 Single and binary gas (50% molar each) permeances and separation factors for ZIF-69 membranes at 298 K. Deviation was calculated based on 3 samples. Permeance (10−9 mol m−2 s−1 Pa−1 ) Single gas CO2 N2 CO CH4 Binary gas CO2 (50 vol.% in N2 ) N2 (50 vol.% in CO2 ) CO2 (50 vol.% in CO) CO (50 vol.% in CO2 ) CO2 (50 vol.% in CH4 ) CH4 (50 vol.% in CO2 )

23.6 10.6 8.2 8.6

± ± ± ±

1.5 0.8 0.4 0.5

103.4 16.3 103.1 20.5 102.3 22.4

± ± ± ± ± ±

2.2 0.7 1.4 1.1 1.6 0.8

CO2 ideal selectivity

CO2 separation factor

– 2.2 2.9 2.7

respectively, while in mixture, the corresponding values are 6.3 and 103.4 ± 2.2 × 10−9 mol m−2 s−1 Pa−1 , respectively. The reason should be due to selective adsorption of CO2 , which enhanced the transport rate of CO2 . Another interesting finding is that the CO2 permeance in all the three binary-mixture systems are almost constant, ca. 1.0 × 10−7 mol m−2 s−1 Pa−1 . Compared to the ZIF-69 membranes that was made by in situ solvothermal method reported in our previous report [9] improvement in both selectivity and permeance was observed in the current study. For example, under the same permeation conditions and similar membrane thickness the selectivity for CO2 /CO system was increased from 3.5 to 5.0, while the permeance from 3.6 × 10−8 mol m−2 s−1 Pa−1 to ∼1.0 × 10−7 mol m−2 s−1 Pa−1 , almost two times higher. Both improvements are due to improvement in membrane microstructure, such as higher extend of c-orientation and tighter grain boundary structures. 4. Conclusions In summary, we have successfully synthesized highly c-oriented and well-intergrown ZIF-69 membranes on porous ␣-alumina substrates by seeded growth method using the sub-micrometer sized ZIF-69 crystals as seeds. The single-gas permeation experiments indicated that N2, CO and CH4 mainly followed the Knudsen diffusion mechanism whereas CO2 was dominated by surface diffusion due to the adsorption affinity of ZIF-69. The separations of equimolar mixture gas of CO2 /N2 , CO2 /CO and CO2 /CH4 were studied in the Wicke-Kallenbach mode and measured by gas chromatograph (GC). The separation factors of CO2 /N2 , CO2 /CO and CO2 /CH4 through ZIF-69 membrane were 6.3, 5.0 and 4.6 with a permeance of ∼1.0 × 10−7 mol m−2 s−1 Pa−1 for CO2 at room temperature. Compared to ZIF-69 membranes synthesized by the in situ crystallization methods, the highly c-oriented ZIF-69 membrane has better selectivity and higher permeance. Acknowledgement We acknowledge the financial support from KAUST distribution fund 31500/4000000066.

6.3 5.0 4.6

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