www.sciencemag.org/content/346/6215/1356/suppl/DC1

Supplementary Materials for

Metal-organic framework nanosheets as building blocks for molecular sieving

membranes

Yuan Peng, Yanshuo Li,* Yujie Ban Hua Jin, Wenmei Jiao, Xinlei Liu, Weishen Yang* *Corresponding author. E-mail: leeys@dicp.ac.cn (Y.L.); yangws@dicp.ac.cn (W.Y.)

Published 12 December 2014, Science 346, 1356 (2014) DOI: 10.1126/science.1254227

This PDF file includes: Materials and Methods

Figs. S1 to S19

Tables S1 and S2 References

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Materials and Methods

1. Solvothermal synthesis of Zn2(bim)4

Solution A: 0.612 g ZnCl2 (Sigma-Aldrich, ≥98%) was dissolved in 23 mL N,N-dimethylformamide (DMF, Bodi, AR) and stirred for 20 min. Solution B: 0.354 g benzimidazole (bim) was dissolved in 23 mL DMF with 0.22 g diethylamine (DEA, Bodi, AR) and stirred for 20 min. Solution B was added to solution A under stirring. The molar ratio of Zn2+/bim/DEA/DMF was 1.5:1:1:200. Subsequently, the reaction solution was transferred to a Teflon-lined stainless steel autoclave and kept at 130 °C for 42 h. The resultant white product was washed with methanol (Bodi, AR) and dried in an air oven at 50 °C overnight. The yield is around 6% based on bim.

2. Synthesis of Zn2(bim)4 via hydrothermal transformation of ZIF-7 nanoparticles

ZIF-7 nanoparticles were synthesized using a modified protocol according to the report by Li et al. (26). 1000 mL DMF was added to a solid mixture of 3.025 g Zn(NO3)·6H2O (Aldrich, 98%) and 7.695 g benzimidazole (Aldrich, 98%). The molar ratio of Zn2+/bim/DMF = 0.154:1:200. After stirring for 1 h, the solution was kept statically at room temperature for 72 h. The ZIF-7 nanoparticles were recovered by centrifugation and thoroughly washed with methanol. The wet product was dried at 50 °C overnight and then dried at 120 °C for 48 h in a vacuum oven. The yield of ZIF-7 is about 55% based on zinc.

For hydrothermal transformation, the obtained ZIF-7 nanocrystals were dispersed in distilled water at a concentration of 0.5 wt % and then boiled and refluxed at 100 °C for 24 h. The turbid dispersion was filtered and washed with distilled water and methanol, and then dried at 50 °C overnight. The XRD characterization indicated that the product was layered Zn2(bim)4. The yield of Zn2(bim)4 is about 73% based on ZIF-7.

3. Exfoliation of Zn2(bim)4 precursors to Zn2(bim)4 nanosheets

In a typical process, 0.025 g Zn2(bim)4 precursor was dispersed in 100 mL of a mixed solvent of methanol and n-propanol (V: V=1: 1) and sealed in a 150 mL ball milling jar. The jar was fixed in a ball mill (PM400, Retsch Co.), and the mixture underwent wet ball milling at a steady speed of 60 rpm for 1 h. It should be noted that the jar counter-rotated every 15 s. Further exfoliation was performed by treating the mixture in ultrasonic bath (Branson, Emerson Co.) for 30 min, which was diluted 2.5 times before ultrasonic treatment. A colloidal suspension of Zn2(bim)4 nanosheets was obtained after sedimentation of larger unexfoliated particles for at least one week. The concentration of the suspension was measured by weighing the residual white powder after evaporating the solvent in a vacuum oven. Exfoliation using other types of solvents, or exfoliation without pretreatment using wet ball-milling, was also performed in this study. The exfoliation rate is about 15%.

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4. Preparation of powdered Zn2(bim)4 nanosheets

A colloidal suspension of Zn2(bim)4 nanosheets in p-xylene was prepared following a similar process as described above, except that p-xylene was used as the solvent. Powdered Zn2(bim)4 nanosheets were obtained by removing the p-xylene solvent via freeze drying in a LGJ-18C vacuum freeze drier (SHKY Co.) for more than 72 h. We used p-xylene for this preparation because of its nonpolar nature (avoiding re-stacking of the nanosheets) and its considerably higher melt point (13.2 °C) compared with methanol (-97.6 °C). The powdered Zn2(bim)4 nanosheets were used for N2 adsorption, TG and FT-IR characterization.

Pretreatment of α-Al2O3 supports

Asymmetric α-Al2O3 disks (Inocermic) were used as supports in this study. The disk has a diameter of 18 mm and thickness of 1 mm. The pore size of the top layer is 70 nm. The disks were first washed in distilled water at 60 °C for 3 h and then transferred into acetone and washed for 2 h at 30 °C. Then, the α-Al2O3 disks were dried overnight at 50 °C.

5. Preparation of MSN membranes by hot-drop coating

The α-Al2O3 support was pre-heated to a certain temperature on a heating plate (homemade, Fig. S9). A certain amount of colloid dispersion of Zn2(bim)4 MSNs was then deposited on the support surface dropwise using a syringe. The coated α-Al2O3 disk was dried at the same temperature as that of the coating procedure. Different support surface temperatures from room temperature to 200 °C, and different drop volumes of Zn2(bim)4 MSNs from 1 to 15 mL, were investigated in the current study, as listed in Table S1.

6. Preparation of MSN membranes by filtration

The α-Al2O3 support was placed into a filtration setup. 25 mL colloid dispersion of Zn2(bim)4 MSNs was vacuum filtered through the sealed support. The vacuum degree was maintained at approximately 0.015 MPa (absolute pressure). The as-obtained membranes were dried at room temperature for 1 h and transferred in an air oven and dried overnight at 50 °C.

7. Powder X-ray diffraction (XRD) characterization and simulation

The XRD measurements were performed on Rigaku D/MAX 2500/PC with Cu Kα radiation (λ=0.154 nm at 40 kV and 200 mA). The data were recorded from 2° to 40° with a scan speed of 5 °/min. The simulated powder XRD pattern of Zn2(bim)4 layered precursor was collected with Diamond software [Ref. No. 675375, Cambridge Crystallographic Data Centre (CCDC)], as shown in Fig.1C.

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The variable temperature in situ X-ray diffraction study of the Zn2(bim)4 layered precursor was performed on Rigaku D/MAX 2500/PC with Cu Kα radiation (λ=0.154 nm at 40 kV and 200 mA), with a flow of N2 and a heating/cooling rate of 10 °C/min from room temperature to 300 °C, and then to room temperature, noting that there was 4 min for equilibration before collecting the X-ray diffraction data at each temperature. For the measurements, the Zn2(bim)4 layered precursor was compressed into a 1.3 mm-thick disk with a diameter of 13 mm. Fig. S2 illustrates that there was no obvious change in the in-plane structure of Zn2(bim)4 layered precursor. The inter-lamellar spacing along the c-axis, i.e., the d-spacing of (002) peak, increased by about 0.1977 Å as the temperature increased from room temperature to 300 °C. The expansion d-spacing is reversible under the decomposition temperature.

8. Transmission electron microscope (TEM) characterization and simulation

A Tecnai G2 Spirit (FEI Co.) operated at 120kV and a JEM-2100 (JEOL Co.) operated at 200kV were used for the TEM images. To prepare samples for TEM analysis, 20-60 drops of an MSN dispersion were coated on copper-supported ultrathin carbon films (200 mesh, Zhongjingkeyi Co.) and dried overnight. The SAED pattern of few-layered Zn2(bim)4 nanosheets perpendicular to the c-axis, as shown in Fig. 2B, was simulated with the SingleCrystal software.

9. Atomic force microscopy (AFM) characterization

The MSNs samples for AFM measurements were prepared by placing a drop of the Zn2(bim)4 MSNs dispersion on a cleaned silicon wafer and drying at room temperature. The images were obtained using a MultiMode 3D AFM (Bruker Co.) operated in tapping mode. All the images were recorded using 1- 10 Ωcm phosphorous (n) doped Si cantilevers (MPP-11100-10, Bruker Co.). The spring constants were 40 N/m and the resonant frequencies were 300 kHz. Additionally, the image analysis was performed with the Nanoscope TM systems from Digital Instruments Veeco Metrology Group. Zn2(bim)4 nanosheets with large area or multi-layered structure were also observed (Fig. S5).

10. Scanning electron microscopy (SEM) characterization

The SEM images were recorded on a Quanta 200 FEG instrument (FEI Co.). To reduce charging effects, the samples were sputtered with gold for 50 s before characterization. For the cross-section images of the membranes, the membranes were first wetted with methanol and then simply fractured and adhered to the side face of a metal block.

11. N2 adsorption characterization

N2 adsorption-desorption isotherms were collected on a Quantachrome Autosorb Automated Gas Sorption instrument at 77 K.

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12. Thermogravimetry (TG) measurement

For the TG measurements, the original Zn2(bim)4 layered precursor and the powdered Zn2(bim)4 nanosheets were heated with a heating rate of 10 °C/min from 40 °C to 800 °C on a Netzsch STA 449F3 instrument under a flow of dry Air with a volumetric flow rate of 20 mL/min.

13. Fourier Transform Infrared Spectroscopy (FTIR) characterization

Infrared spectra were recorded on a Nicolet 6700 Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR, Thermo Scientific Co.) spectrophotometer. The spectra backgrounds were recorded 32 times, and then, the spectra of the samples were recorded 32 times.

14. X-ray photoelectron spectroscopy (XPS) characterization

Information regarding the surface elements of the membranes was collected on a Thermo ESCALAB 250Xi X-ray photoelectron spectrometer equipped with a monochromatic Al Kα X-ray source. The sample was fabricated by hot-drop coating an α-Al2O3 disk at 120 °C with 15 mL of an MSN suspension.

15. Gas separation tests of the MSN membranes

The obtained membranes were sealed in Wicke-Kallenbach cells to measure their separation performance for H2/CO2 gas mixture (see Fig. S12). To avoid direct contact of silicone O-ring with the membrane surface, which might damage the MSN layer, the edge of the membrane disk was masked with a silicone rubber pad-coated aluminum gasket, exposing a 5-mm-diameter hole in the center of the membrane. The mixed feed flow rates were constant with a total volumetric flow rate of 100 mL/min, with 50 mL/min of each gas (1:1 mixture), regulated by mass flow controllers (MFCs). Argon, a relatively bulky molecule, was used as a sweep gas to minimize the influence of back-diffusion of the sweeping gas to the feed side. Additionally, the use of argon makes our separation results comparable with those measured on the GO membranes (12), where argon was applied as the sweep gas. The sweep gas flow rate was 50-100 mL/min for the sake of eliminating concentration polarization in the permeate side since the permeance of the MSN membranes was several thousand GPUs. For single gas permeation tests, the feed flow rates were constant at 100 mL/min. There was no pressure drop between the sides of the membranes, in order to prevent any distortion of the MSN layers. The separation factor is defined as the molar ratio of H2 to CO2 in the permeate over the molar ratio in the feed.

Gas permeation measurements at different temperatures were conducted using a heating tape (BIH102080LG, BriskHeat Co.) and a temperature controller (Yudian Co.) with a heating rate of 1°C/min. MSNs membranes were balanced at every measured temperature for more than 1 h.

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Gas permeance is calculated by the following equation:

i ii

i i

N FPA P P

= =∆ ∆

where Pi is the permeance of component i (mol m-2 s-1 Pa-1), Ni is the permeate rate of component i (mol s-1), A is the effective membrane area (m2), and Pi is the transmembrane pressure difference of component i (Pa). Permeance can also be expressed as pressure normalized flux. Fi is the flux of component i (mol m-2 s-1).

In this paper, gas permeance is given in unit of GPU (Gas Permeation Unit). 10 2 1 11 GPU=3.3928 10 mol m s Pa− − − −×

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Fig. S1. Zn2(bim)4 layered precursors synthesized via hydrothermal transformation route. SEM images and XRD patterns of (A) original ZIF-7 nanocrystals, and (B) Zn2(bim)4 layered precursors. The SOD structure of ZIF-7 consists of six-membered rings (indicated by purple circle) and four-membered rings (indicated by green circle). It could be possible that the coordination bonds in the six-membered rings were broken (indicated by red lines marked with red X) during the hydrothermal treatment and the four-membered rings fused together, resulting in 2D Zn2(bim)4 layers. The Zn2(bim)4 layers stacked along c-axis as a result of hydrophobic forces and formed the layered precursors.

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Fig. S2 Variable temperature in situ XRD patterns of Zn2(bim)4 layered precursor. The symbol + represents the heating stage and the symbol - represents the cooling stage.

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Fig. S3 Typical TEM images of Zn2(bim)4 nanosheets exfoliated in different solvents. (A) Methanol. (B) Ethanol. (C) 1-Propanol. (D) Isopropanol. (E) Isobutanol. (F) Dimethoxyethane. (G) 1-Hexane. (H) 1-Heptane. (I) p-Xylene. The table shows the surface tension, viscosity values, and exfoliation effects of the solvents.

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Fig. S4 TEM images of Zn2(bim)4 nanosheets. The four images were taken from different batches. Images A, B and C used mixed solvent of methanol and n-propanol. Imgaes D used methanol as the solvent.

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Fig. S5 AFM images of Zn2(bim)4 nanosheets with different layers of lamellas and height profiles of the nanosheets along the black lines marked on the images. (A) Single layer with a thickness of 1.04 nm, marked by two blue triangles. (B) Three layers containing two steps, marked by an orange and two blue triangles; thickness of the first step (single layer) is 1.15 nm, and the second one (two layers) is 2.01 nm. (C) Three layers with a total thickness of 3.64 nm.

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Fig. S6 FTIR-ATR spectra of pristine Zn2(bim)4 layered precursors and Zn2(bim)4 nanosheets.

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Fig. S7 TG curves of pristine Zn2(bim)4 layered precursors and Zn2(bim)4 nanosheets.

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Fig. S8 XRD pattern of Zn2(bim)4 MSN membrane prepared via vacuum filtration method. The peak at 2θ of 9° corresponds to the (002) reflection of layered Zn2(bim)4 precursor. Peaks marked with dots correspond to the diffraction peaks of α-Al2O3 support.

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Fig. S9 Illustration of the hot-drop coating apparatus.

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Fig. S10 Low-magnification SEM images of (A, B) top view and (C, D) cross-sectional view of an MSN membrane on α-Al2O3 supports.

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Fig. S11 (A) Zn 2p and (B) Al 2p XPS spectra of an MSN membrane prepared via hot-drop coating at 120 °C.

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Fig. S12 Illustration of gas separation measurement apparatus (Wicke-Kallenbach type). MFC: Mass flow controller (D07-12A/ZM, Beijing Sevenstar Electronics Co.). GC: Gas chromatograph (7890A). A TDX-01 column was used for mixed gas and single gas permeation tests of H2, CO2, CH4 and C2H6; An Agilent 5A column and an Agilent HQ column in series were used for N2 permeation test. For all the gases except C2H6, TCD detector was used for analysis. FID detector was used for C2H6 analysis.

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Fig. S13 XRD patterns of the membrane prepared via drop coating at 60 °C and room temperature. The peaks marked with dots correspond to the diffraction peaks of α-Al2O3 support. The peaks marked with triangle correspond to the (002) reflection of layered Zn2(bim)4 precursor. The peaks marked with asterisk correspond to the reflections of expanded stacked lamellas (which might occur during slow evaporation of the solvent) with d-spacing of 3.62 and 1.81 nm, respectively.

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Fig. S14 H2/CO2 selectivity vs. drop volume for 9 membranes prepared via hot drop coating at 120 °C. Detailed descriptions of these membranes are given in Table S2. Correlations between selectivity and drop volume were examined by linear regression analysis. Pearson’s correlation coefficient is less than 0.297 (< 0.3), indicating that there is little or no association between H2/CO2 selectivities and drop volume.

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Fig. S15 Single gas permeation of an MSN membrane prepared via hot-drop coating at 120 °C.

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Fig. S16 Arrhenius temperature dependence of H2 and CO2 permeances for an MSN membrane prepared via hot-drop coating at 120 °C.

The temperature dependence of gas permeation can be stated by Arrhenius equation:

,exp(- )act ii i

EP A

R T=

⋅

, 1ln - act ii

EP a

R T= ⋅

where Pi is the gas permeance of component i, Ai represents for the pre-exponential factor of component i, Eact, i is the apparent activation energy of component i, R is the ideal gas constant (8.314 J mol-1 K-1) and T is the absolute temperature (K). A plot of ln (Pi) versus 1/T gives a straight line, whose slope is used to calculate Eact, i.

The Eact, H2 is about 6.5 kJ/mol, and Eact, CO2 is about 18.3 kJ/mol.

The apparent activation energy is an association of diffusion activation energy and adsorption heat.

act diff adsE E H= −∆ The adsorption heat of H2 and CO2 on ZIF materials is ~5 kJ·mol-1 (29) and ~30 kJ·mol-1

(30), respectively. The difusion activation energy Ediff is therefore estimated to be 11.5 kJ·mol-1 and 48.3 kJ·mol-1 for H2 and CO2, respectively, indicating activated diffusion for both H2 and CO2.

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Fig. S17 Effect of feed composition on H2, CO2 permeance and H2/CO2 selectivity for an MSN membrane prepared via hot-drop coating at 120 °C.

When the CO2 molar fraction in the feed was higher than 0.5, CO2 adsorption has influence on the H2 permeation to some extent. It is possible that competitive adsorption CO2 on the external surface of Zn2(bim)4 nanosheet membrane affects the external surface diffusion of H2 to the pores of the nanosheets [i.e., F2.J in the model of gas transport in zeolite crystal membranes proposed by Barrer (31)]. In the whole composition range, the H2/CO2 selectivity is always higher than 90.

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Fig. S18 Comparison of Zn2(bim)4 MSN membranes with polymeric membranes and molecular sieve membranes for H2/CO2 mixture. Nine MSN membranes prepared via hot-drop coating at 120 °C (Table S2) are shown in this figure. The black solid line represents the 2008 upper bound of polymeric membranes for H2/CO2 (1). Permeability is converted to permeance assuming membrane thickness of 0.1 μm. The black dashed line represents the 2010 upper bound of microporous inorganic membranes for separation of H2/CO2 mixtures (33). Several new membranes reported after 2010 are also presented in this figure. The green area represents the membrane performance targets space (H2 permeance > 1,000 GPUs, H2/CO2 selectivity > 60) for integrated gasification combined cycle with 90% CO2 capture (40).The table below explains the test conditions of the literature data points and this work.

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Fig. S19 The upper figure is a summary of the long term test of an MSN membrane. The membrane was firstly tested in a heating and cooling cycle (~140 h) for investigating the activted transport of H2 and CO2 through the membrane. After being stored in a desiccator for ~80 days, the membrane was tested for single gas permeation for 104 h. The membrane was then heated to 150 °C, and exposed to an equimolar H2/CO2 feed containing ~4 mol. % steam. The hydrothermal stability test lasted for 120 h. After that, the membrane was heated up to 200 °C, and tested for 50 h before cooling down to room temperature. During the test for more than 400 h in total, the membrane shows good stability and reversibility upon temperature cycling. The lower figure represents for hydrothermal stability of the membrane at 150 °C. No degradation in membrane performance was observed within the 120 h test.

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Table S1. Preparation parameters and H2/CO2 separation performances of the 15 membranes reported in Fig. 3.

Linear regression for estimating the correlation between H2 permeance and H2/CO2 selectivity

Intercept Slope Statistics Value Standard Error Value Standard Error Pearson’s r Adj. R-Square

D 0 -- 0.07972 0.00661 0.95507 0.90588

One Way ANOVA for examining the influence of

coating temperature on H2/CO2 selectivity

Sum of Squares df Mean Square F Sig. Between groups 40685.067 4 10171.267 3.189 0.062 Within groups 31894.667 10 3189.467

Total 72579.733 14

ANOVA analysis finds a significant effect of coating temperature on H2/CO2 selectivity at 10% level (0.062<0.1).

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Table S2. Preparation parameters and H2/CO2 separation performances of the 9 membranes reported in Fig. S14 and S15.

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