1

Aqueous phase synthesis of ZIF-8 membrane with

controllable location on an asymmetrically porous

polymer substrate

Ezzatollah Shamsaei,† Xiaocheng Lin,† Ze-Xian Low,†‡ Zahra Abbasi,† Yaoxin Hu,† Jefferson Zhe

Liu,§ and Huanting Wang*†.

†Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia. ‡

§Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria

3800, Australia.

KEYWORDS: metal−organic framework membranes, ZIF-8, contra-diffusion, chemical vapor

modification, gas separation.

ABSTRACT

In this study, we have demonstrated a simple, scalable, and environmentally friendly route for

controllable fabrication of continuous, well-intergrown ZIF-8 on a flexible polymer substrate via

contra-diffusion method in conjunction with chemical vapor modification of the polymer surface.

The combined chemical vapor modification and contra-diffusion method resulted in controlled

formation of a thin, defect-free and robust ZIF-8 layer on one side of the support in aqueous

solution at room temperature. The ZIF-8 membrane exhibited propylene permeance of 1.50×10-8

mol m-2 s-1 Pa and excellent selective permeation properties; after post heat-treatment, the

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membrane showed ideal selectivities of C3H6/C3H8 and H2/C3H8 as high as 27.8 and 2259,

respectively. The new synthesis approach holds a promise for further development for the

fabrication of high-quality polymer-supported ZIF membranes for practical separation

applications.

1. INTRODUCTION

Zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic frameworks (MOFs), are

porous crystalline hybrid materials consisting of imidazolate ligands (Im) bridging tetrahedral

metal ions (e.g., Zn, Co).1 They closely resemble the topologies of zeolites, due to the M-Im-M

(M = Zn, Co) bond angle of 145°, which is close to the T-O-T (T = Al, Si, P) angle (140-170°) in

zeolites.2-3 ZIFs show properties that combine the attractive features of both MOFs and zeolites

such as tunable pore size and chemistry, large internal surface area and relatively good thermal

and chemical stability.4-5 These properties make ZIFs excellent candidates for the fabrication of

molecular sieving membranes for gas separation.6-8 ZIF-8 membranes, for example, have been

reported to be capable of molecularly discriminating propylene (~4.0 Å) from propane (~ 4.3 Å)

since the effective pore aperture size of ZIF-8 falls in the range of 4.0-4.2 Å (larger than its

crystallographic value of 3.4 Å, owing to the swaying effect of the ligands).2, 9-11

ZIFs have been widely used to fabricate the so-called mixed matrix membranes (MMMs,

consisting of pre-synthesized ZIF particles dispersed in a polymeric matrix) to afford a solution to

go beyond the Robeson's upper-bound trade-off limit of the polymeric membranes.6, 12-15 While

MMMs have been shown to enhance the permeation properties of polymeric membranes, further

enhancements were made using in-situ synthesized ZIF membranes.16 For example, ZIF-8

supported membranes prepared by Pan et al.10 showed superior propylene/propane permselectivity

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(permeability of propylene up to 200 barrers and a propylene/propane separation factor up to 50)

compared to those of ZIF-8 MMMs, as for instance the ZIF-8/ 6FDA–DAM polyimide (a

propylene permeability of 56.2 Barrer and propylene/propane ideal selectivity of 31.0) reported by

Zhang et al.15To date, several synthesis methods have been reported for the formation of ZIF films

on various substrates.6, 17 In particular, polymer-supported ZIF membranes are of great interest as

they potentially combine the advantages of both polymer membranes (e.g. easy processing and

low cost) and ZIFs (e.g. high selectivity). In principle, the growth of ZIF films on flexible

polymeric substrates can be easily achieved due to favorable chemical interaction between the

polymer and the organic ligand of ZIFs. Nagaraju et al.18 and Cacho-Bailo et al.19 grew ZIF-8 on

a porous polysulfone using in situ (direct) growth. However, although the in situ synthesis is a

simple method that allows for simultaneous nucleation, deposition and crystal growth, it is not

very effective in preparing continuous ZIF membranes due to limited heterogeneous nucleation

sites on the substrate.20 Alternatively, Ge at al.21 used secondary seeded growth to fabricate a

continuous ZIF-8 film on an asymmetrically porous polyethersulfone substrate. Secondary seeded

growth has been shown to effectively induce controlled ZIF growth on the polymer support, but

the resulting ZIF layer often suffers from weak adhesion to the support, leading to membrane

delamination. 22 The surface modification has been commonly used to functionalize the support,

thereby promoting heterogeneous nucleation and enhancing the ZIF-to-substrate adhesion

strength.23-25

Very recently, we successfully developed a new strategy of vapor phase modification to

introduce amine groups and reduce surface pore sizes of the polymer support; such surface

modification enabled fast formation of a continuous ZIF-8 ultrathin layer in the presence of

ammonium hydroxide (as a deprotonating agent) under sonication for only 3 minutes.23 However,

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the sonication-induced crystallization offers limited control over the ZIF-8 crystal sizes and

intergrowth, and thus membrane properties. Our group was one of the first groups to report contra-

diffusion synthesis, which self-limits growth of ZIF films on porous substrate, and has great

potential to offer better control in the membrane fabrication.11, 26-28 However, the growth of ZIF

films via contra diffusion method depends on the surface properties and porous structure of

support; the formation of ZIF layer on both sides of support or within porous channels of support

has been reported. 26-27 To achieve better control over the membrane position, Brown et al. recently

introduced an interfacial synthesis approach.29 The control over the membrane position relies on

employing an oil/ water system, in which crystals grow at the interfaces between the two

immiscible solvents. The resulting ZIF-8 membrane exhibited high gas separation performance

with H2/C3H8 and C3H6/C3H8 separation factors as high as 370 and 12, respectively.

In this work, we report a simple, effective and environmental friendly method for the fabrication

of high-quality ZIF-8 membrane with controllable location on a polymer substrate in aqueous

solution. Our synthesis method is based on contra-diffusion (CD) concept in conjunction with

chemical vapor modification (hereafter chemical vapor modification-contra diffusion method). A

flat sheet asymmetric30 bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO)

ultrafiltration membrane was prepared via phase inversion and employed as the support for

growing a thin ZIF-8 layer via contra diffusion after modification. We have selected BPPO for

ZIF-8 growth because of its outstanding membrane formation and mechanical properties as well

as excellent hydrolytic stability.31 It can also be easily functionalized and crosslinked due to the

abundant highly reactive –CH2Br groups. Using vapor-phase ethylenediamine (EDA), we have

previously shown that amine functional groups can be covalently attached selectively on the top

layer of the support without affecting the sublayer structure.23 The presence of the covalent link

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(amine groups) can be a driving factor for maintaining a high concentration of the metal ions

selectively near the support surface. When combined with the slow diffusion of the ligand in contra

diffusion process, the approach can lead to well-controlled crystal growth in the vicinity of the

support surface. This results in the formation of thin, defect-free and robust ZIF-8 layer on one

side of the support at room temperature without the addition of deprotonating agents, which has

proven to be challenging when using other reported synthesis procedures.11, 26, 32-33

2. MATERIALS AND METHODS

2.1. Chemicals. BPPO was kindly supplied by Tianwei Membrane Co. Ltd., Shandong of China.

Ethylenediamine (EDA, 99.5%),1-methyl-2-pyrrolidone (NMP, 99.5%), zinc acetate dihydrate

(Zn(CH3COO)2.2H2O, 98%) and 2-methylimidazole (Hmim, C4H6N2, 99%) were purchased from

Sigma-Aldrich, Australia and used as received. Methanol (absolute) was purchased from Merck,

Australia. The water used for the experiments was purified with a water purification system (Milli-

Q integral water purification system, Merck Millipore) with a resistivity of 18.2 MΩ/cm. Distilled

water was obtained from a laboratory water distillation still (Labglass Aqua III).

2.2. Sample Preparation. BPPO support ultrafiltration membranes were prepared via non-

solvent induced phase separation at room temperature. The casting solution was prepared by

dissolving 15 wt% of BPPO in NMP for 12 h with mechanical stirring at 200 rpm. The

homogenous solution was left to degas for 10 h before use. Subsequently, the solution was cast on

a clean glass plate using an adjustable micrometer film applicator (Paul N. Gardner Co., Inc. USA)

with a gap of 200 μm at room temperature (22 ± 2 °C) and immediately immersed in a coagulation

bath of deionized water. After peeling off from the glass plate, the membranes were removed from

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the bath, washed and kept in fresh deionized water (DI) for at least one day to thoroughly remove

the residual solvents.

The vapor-phase EDA modification was conducted in a custom-made container according to our

previous work.23 In brief, 20 mL of EDA was allowed to vaporize, and stabilized for 1 h. The

support membranes were quickly placed inside the containment with the top layer exposed and

suspended above the EDA solution. After surface modification at room temperature for 16 h, the

surface modified membranes were removed from the containment and immediately washed with

pure water to completely remove the residual EDA. The resultant membrane were denoted as

BPPO-EDA.

For preparation of the BPPO supported ZIF-8 membranes, the modified BPPO supporting

membrane was cut into 32 mm diameter discs, which were then mounted on a home-made setup

(Figure S1, Supporting Information), where the zinc acetate solution and Hmim solution were

separated by the supporting membrane. Zinc acetate solution was prepared by dissolving 0.09 g of

Zn (CH3COO)2.2H2O (0.5 mmol, Sigma–Aldrich) in 20 mL of deionized water, and Hmim

solution was prepared by adding 0.649 g of Hmim (8 mmol, Sigma–Aldrich) in 20 mL of

deionized water. The designed Hmim: Zn2+ molar ratios in the system was 16 and was kept

constant in our study. After crystallization at room temperature (22 ± 2 °C) for 60–120 min, the

membrane samples were taken out and rinsed with DI water several times. Finally, the composite

membranes were dried in ambient conditions for 24 h, followed by heating at 120-200 °C for 2 h

before tests. The resulting samples were denoted as ZIF-8-BPPO-EDA-t-T, where “t” and “T”

denote the crystallization time and heat treatment temperature, respectively.

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2.3. Characterization. Fourier Transform Infrared (FTIR) spectra of the membranes were

recorded using an attenuated total reflectance (ATR) FTIR (Perkin Elmer, USA) in the range of

400-4000 cm-1 at an average of 20 scans with a resolution of 4 cm-1. Scanning electron microscopy

(SEM; FEI Nova NanoSEM 450) with an X-ray detector (Bruker Nano GmbH, Germany) was

used for imaging the surface and cross-sectional morphologies of membranes. Energy-dispersive

X-ray spectroscopy (EDS) line-scan analysis of the membrane samples was conducted using EDX

equipped in Nova NanoSEM 450 (Quantax 400 X-ray analysis system, Bruker, USA). The

membranes were fractured in liquid nitrogen, fixed on stubs with double-sided carbon tape and

then sputter coated with roughly 2 nm iridium (Ir) layer to ensure good electrical conductivity. The

images were recorded at an accelerating voltage of 5 kV with different magnifications.

Thermogravimetric analyses (TGA) were carried out on a SETARAM (TGA 92) device from 30

to 800 °C at a heating rate of 10 °C min-1 under air flow. Powder X-ray diffraction (XRD) patterns

were measured using a Miniflex 600 diffractometer (Rigaku, Japan) with Cu Kα radiation (15 mA

and 40 kV) at a scan rate of 2° min-1 with a step size of 0.02°. The XRD studies were carried out

at room temperature.

The single gas permeation of composite membranes was measured using the pressure rise

method.34 The schematic of the single gas permeation setup is shown in Figure S2. To measure the

gas permeation flux, the composite membrane (16 mm diameter disc) was attached to a porous

stainless steel holder (pore size ~200 nm) using epoxy resin (Torr seal, Varian), and then placed

inside a larger Pyrex tube and connected to a sensitive pressure transducer (MKS 628B Baratron)

and a vacuum line. The effective remaining membrane area was 1 cm2. For each single gas

measurement, the pure single gas was fed to one side (feed) of the membrane while the other side

(permeate) of the membrane was under vacuum. Since the feed side was at ambient pressure, a

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pressure difference of 1 atm was maintained between the permeate side and the feed side during

permeation measurements. After allowing enough time to achieve a steady state conditions, the

permeate side was shut off from vacuum and the pressure buildup of the permeating gas was

measured by the pressure transducer and continuously recorded in a computer. To accomplish a

single test, the pressure was allowed to reach a few Torr. To repeat an analysis, the permeate side

was evacuated again and then shut off from vacuum so as to record the pressure rise. All the gas

permeation tests were performed at room temperature. The molar flow rate of the permeating gas

was calculated based on the recorded pressure. The permeance, Pi, of each gas was calculated

according to the following equation,

Pi= (V/RTAΔp) (dp/dt)

where, V is the volume of the permeate side that was obtained by calibration using a bubble

flowmeter (m3) , R is the ideal gas constant (m3 Pa K−1 mol−1), T is the temperature (K), ∆p is the

pressure difference across the membrane (Pa), A is the effective membrane area (m2), and dp/dt is

the rate of pressure rise in the permeate side (Pa/s). The ideal selectivity Sij is defined as the ratio

of the two permeances Pi and Pj. Permeation data are average values recorded from at least three

samples, which were prepared from different batches.

3. RESULTS AND DISCUSSION

Figure 1 illustrates the synthesis of dense and defect-free polymer supported ZIF-8 membrane

using chemical vapor modification-contra diffusion method. As illustrated in the figure (step (1)),

the surface chemistry and pore size of the top layer of the BPPO are modified by using EDA-

vapor. Substitution of bromide functional group with amine groups during EDA-vapor

modification was confirmed by FTIR (Figure 2). Upon modification, the peak at 586 cm-1 and 633

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cm-1 attributed to the benzyl bromide (–CH2Br) groups (C–Br stretching) almost disappear and a

new broad band in the range of 3100–3600 cm-1 emerges, which is attributed to the N–H stretching

and confirms the amination of BPPO. TGA results (Figure S3, Supporting Information) also show

that when the temperature is below 150 ºC, BPPO-EDA have more weight loss (~4%) than BPPO

due to higher moisture residual in the membrane as a result of the introduction of hydrophilic

amine groups. Additionally, the decomposition of EDA initiates at ~ 180 °C, which is higher than

the boiling point (117 °C) of the EDA, indicating that there is an interaction between the EDA

molecules and BPPO. Furthermore, SEM images (Figure 3 a, b) show an obvious decrease in the

size of the nanopores at the top surface of the membrane after EDA vapor-phase modification. The

reduction in the pore size of the support (from 25.5 to 15 nm) after its modification was further

confirmed by TOC analysis (see measurements of the support pore size in the Supporting

Information). The changes in the surface microstructure can be attributed to the partial cross-

linking effect of the EDA, since the crosslinking causes tightening of the polymer network which

reduces the pore size of the BPPO substrate. In addition, the final decomposition of the BPPO-

EDA in the TGA results (Figure S3) is much slower than BPPO, which indicates a higher thermal

stability due to partial crosslinking of the BPPO substrate. Note that partial crosslinking reduces

the flexibility of the polymer support, which is favorable for avoiding ZIF layer cracking.

The ZIF-8 membrane is formed on the pre-treated support by applying contra diffusion

synthesis, in which the metal precursor solution and ligand (Hmim) solution are separated by the

modified BPPO substrate (step (2) in Figure 1), at room temperature for various crystallization

times. As demonstrated in our previous study,23 the direct heterogeneous nucleation and growth

of a dense ZIF-8 layer on untreated BPPO surface was unsuccessful (Figure S4, Supporting

Information). In fact, due to the fast diffusion of zinc ions through the pores of the unmodified

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support, crystallization occurs constantly inside the support channels, where there can be a high

concentration of the reactant solutions, until the entire path through the ZIF-8 layer becomes

“plugged”. A similar phenomenon was observed by Hara et al. in preparing copper-benzene

tricarboxylate (Cu-BTC) or ZIF-8 membranes using porous α-alumina capillary substrate by

applying a typical contra diffusion method.28, 33

Figure 1. Schematic diagram of the preparation of a BPPO polymer supported ZIF-8 membrane

using chemical vapor modification and subsequent contra diffusion synthesis.

Figure 2. FTIR ATR spectra of the untreated BPPO support, BPPO modified with EDA-vapor

(BPPO-EDA), BPPO-EDA supported ZIF-8 layer (ZIF-8-BPPO-EDA), and ZIF-8 powder.

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After modification of the BPPO with EDA-vapor before contra diffusion synthesis, a compact

ZIF-8 layer was selectively formed on only one side (pre-treated side) of the support. Figure 3 and

4 show the SEM images and XRD patterns of the ZIF-8 membranes grown for different lengths of

time. As shown in Figure 3c and Figure 4, a large amount of ZIF-8 crystals with clear facets is

observed on the modified support even after 60 min of the contra diffusion synthesis at room

temperature. Nanopores of the skin layer of the support are still observable through inter-

crystalline gaps between the ZIF-8 crystals in the high magnification image (inset in Figure 3c).

With increasing the reaction time to 90 min, a dense and continuous ZIF-8 layer is formed on the

modified BPPO skin layer, as shown in Figure 3d. Eventually, after 120 min of reaction, a layer

of well intergrown ZIF-8 crystals with rhombic dodecahedron morphology and a thickness of

about 2 µm fully covered the support surface without any visible defects such as pinholes or cracks

(Figure 3e, f and Figure S5 in Supporting Information).There are only few studies reporting such

a thin continuous polycrystalline ZIF-8 film23-24, 35 and most of the membranes prepared by the

conventional in situ methods are too thick (in the range of tens of micrometers), showing lower

gas flux through the membranes.11, 36 The continuous thin ZIF-8 membranes remained unchanged

even with further growth, demonstrating the self-limiting crystal growth, in which the crystals

continue to grow only if the metal ions and the ligand molecules are in contact. Another important

observation is that unlike the conventional contra diffusion method in which the crystals grow

along the whole thickness of the support, ZIF-8 crystals can be observed only at the very outermost

section of the EDA-vapor-modified BPPO support (Figure 3f). Energy-dispersive X-ray

spectroscopy (EDS) line-scan analysis (Figure S6 in the Supporting Information) further

confirmed the presence of ZIF-8 within the support sublayer as zinc was detected up to about 1

µm underneath the support surface. This means that the heterogeneous nucleation and crystal

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growth occur on the skin layer of the pre-treated support, which is contrary to what was observed

for the untreated BPPO, where the nucleation and crystal growth happened all along the support

channels with a non-continuous film, if any, on the surface. As explained in our previous work,

EDA can simultaneously create amino groups, which can coordinate to the free zinc ions and

provides a large number of nucleation sites; and reduce the substrate pore size induced by its

crosslinking effect.23 In the present work, therefore, the reduction in the surface pore size and the

coordination interaction between the support surface and zinc ions can lead to a decrease of the

diffusion rate of Zn2+ and provide a relatively high precursor concentrations at the support surface

and restricting the reaction zone in this vicinity (Figure 1). This high precursor concentration and

the large number of previously formed nucleation sites result in the faster and thinner crystal

growth in the vicinity of the support skin layer as compared to the slow and undirected crystal

growth along the channels of untreated BPPO.

Figure 3. SEM images of untreated BPPO (a), vapor-phase-EDA-modified BPPO (BPPO-EDA)

(b), ZIF-8@BPPO-EDA grown for 60 min (c), 90 min (d), and 120 min (e, f).

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Figure 4. XRD patterns of ZIF-8@BPPO-EDA membranes as a function of growth time.

Figure 5 presents ZIF-8 membranes synthesized by contra diffusion (at the reaction time of 2h)

after modification of the BPPO via immersion of the polymer in 60% EDA aqueous solution at

room temperature for 3h. It is obvious that the crystals are grown along the whole thickness of the

polymer. As shown, ZIF-8 crystals not only block the polymer micro-channels but also grow

within the whole porous structure of the polymer as the interface between the ZIF-8 and the

polymer matrix is hardly distinguishable. This is because the immersion of the polymeric support

in the nucleophilic diamine solution can result in an extremely high degree of modification (a large

number of nucleation sites) within the bulk polymer and the subsequent formation of ZIF-8 crystals

when applying contra diffusion synthesis. This shows that crystal formation within the whole

polymer support is unavoidable when modifying the BPPO via solution immersion method.

Instead, as already shown, contra-diffusion method in conjunction with vapor phase modification

of the support offers more degrees of freedom in directing the formation of ZIF-8 membranes.

It is worth mentioning that growing MOFs inside the pores of the support can be very attractive

for molecular separations and such membranes have been shown to outperform mixed matrix

membranes (MMM) for organic solvent nanofiltration (OSN) applications, as demonstrated by

Livingston et al.16, 36-37 Although the resulting ZIF-8 membrane (Figure 5) was too brittle to allow

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permeation experiments, this work demonstrates a new methodology for in situ growth of ZIFs

predominantly inside the supports, which needs to be improved further for practical applicability.

Figure 5. SEM images of cross-section of ZIF-8 membrane synthesized by contra-diffusion (at

the reaction time of 2 h) after modification of the BPPO via immersion of the polymer in 60%

EDA aqueous solution at room temperature for 3 h.

To further evaluate the quality of the obtained ZIF-8 membranes, single-component gas

permeation experiments were conducted, and the results are summarized in Figure 6 and Table S1

(Supporting Information). In comparison, single gas permeation of the untreated BPPO and its

vapor phase modified counterpart were tested. Due to their large pores (pore size of 25.5 and 15

for untreated and treated supports, respectively), none of these membranes showed any obvious

gas selectivity. However, upon modification, H2 permeance was decreased by more than half when

compared to the untreated support. This is attributed to the reduced pore size of the support induced

by partial crosslinking effect of the EDA modification, which increases the support dimensional

stability and surface tightness. This also lessens the flexibility of the polymeric substrate, which is

beneficial for reducing ZIF layer cracking.24-25 Membranes started to display molecular sieve

performance, favoring the smaller molecules, with a moderate H2 permselectivity after 90 min of

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the crystal growth (H2/C3H8 ideal selectivity of 32.9 compared to Knudsen diffusion selectivity of

4.7). However, no clear C3H6/C3H8 ideal separation selectivity was observed at this point,

indicating the presence of grain boundary micro-defects. As crystallization time is extended, more

pronounced molecular sieving behavior with an obvious increase in the propylene/ propane ideal

selectivity can be observed which reaches as high as 16 after 120 min reaction time. It is worth

mentioning here that except for a few membranes,11, 29, 33, 38-39 majority of ZIF-8 membranes

reported so far have not shown any decent C3H6/ C3H8 selectivity.40 Figure 7 depicts C3H6/ C3H8

separation performance of ZIF-8 membranes developed in this study in comparison to those

reported in the literatures. As can be seen, our membranes not only overtake polymeric and ZIF-8

mixed-matrix membranes in terms of C3H6/C3H8 selectivity and C3H6 permeance they are also

amongst the best ZIF-8 membranes previously reported (Table S2, Supporting Information). For

instance, the high quality ZIF-8 membranes made in water-octanol system by interfacial

microfluidic membrane processing (IMMP) method could achieve H2/C3H8 ideal selectivity of

more than 600 and the permeance of H2 around 50 ×10-8 mol m-2 s-1 Pa-1.29 While in the current

study, the H2/C3H8 selectivity and H2 permeance are considerably enhanced, with H2/C3H8

selectivity of 833.3 and H2 permeance of 75×10-8 mol m-2 s-1 Pa-1. The enhancement in permeance

in this study is in agreement with the reduction in thickness of the ZIF-8 layer (~2 versus ~9 µm

in 29) and also the highly porous and asymmetric structure of the support, which minimizes the

overall hydraulic resistance of the permeate flow through the membrane structure; whereas the

enhanced selectivity is mainly due to the improved membrane quality, such as the well-structured

grain boundary and absence of pinholes or defects. Highly improved grain boundary structure, on

the other hand, can be related to the confinement effects of the crystals within the porous support

as the confinement can increase the compactness of the grain boundary structure.41 Another

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possible reason for the high quality grain boundary structure can be the well-intergrown ZIF-8

crystals with no preferred orientation as a result of the aqueous synthesis. It was found that water,

being less acidic, in comparison with organic solvents can more easily deprotonate the organic

ligand on the growing surface, leading to growth occurring in more directions, resulting in a better

crystals intergrowth and formation of denser ZIF membranes.38, 42 Attributed to the formation of

high heterogeneous nucleation density in the vicinity of the support surface, the EDA vapor

modification is helpful for the controlled synthesis of thin, defect-free and reproducible ZIF-8

membranes. In summary, the enhanced gas permeation properties strongly suggest that the

chemical vapor modification-contra diffusion method provides a new route for preparing high

quality ZIF-8 membranes having superior grain boundary structure as compared to those prepared

by other methods.

Figure 6. Single gas permeances (a) and ideal selectivities (b) as a function of kinetic diameter of

gas molecules of the ZIF-8 membranes grown for 120 min and activated at 120 C (ZIF8@BPPO-

EDA-120-120) and at 150 C (ZIF8@BPPO-EDA-120-150).

17

Figure 7. Comparison of C3H6 permeability and C3H6/C3H8 selectivity of the membranes in the

present work with previously reported membranes. Closed and open symbols indicate separation

data obtained from single and binary gas permeation analysis, respectively. Hexagon: inorganic

supported ZIF-8 membranes;10 pentagon: ZIF-8 mixed matrix membranes;15 triangle: polymer

membranes;43 circle: carbon membranes;44 star: polymer supported ZIF-8 membranes in this study.

Finally, we investigated the effects of activation temperature on the morphology and

performance of ZIF-8 membranes. Membranes (grown for 120 min) were further exposed to 150

and 200 ºC for 2 h under oxidative conditions (air). For the membranes activated at 150 ºC, a

compact well-intergrown ZIF-8 grains of rhombic dodecahedral shape with no defects (i.e.

pinholes or cracks) in the entire membrane surface can be observed (Figure S7 a,b, and c). A very

intimate contact between ZIF-8 and the support is also observed in the cross section view of the

membranes as the interface between ZIF-8 and the support is hardly distinguishable (Figure S7 c).

Figure 8 shows the room-temperature C3H6/C3H8 permeation properties of ZIF-8 membranes

activated at different temperatures. As shown in the figure, by increasing the activation

temperature from 120 to 150 ºC the C3H6/C3H8 ideal selectivity is significantly increased, with

minimal effect on the permeance. This unique behavior indicates that the ZIF-8@BPPO-EDA

membrane becomes even denser with probably more compact grain boundary structure when

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activated at a higher temperature. This positive result is likely attributed to the fact that the BPPO

and BPPO-EDA form crosslinking structure by heating (Scheme S1),45 which can further increase

the stability and tightness of the support. This was further supported by the 20 (±4) % reduction

observed in the hydrogen permeability of the BPPO-EDA substrate upon heating at 150 °C for 2

h. Since a fraction of ZIF-8 is formed within the support porous structure, it may subsequently

enhance the interfacial interaction between the ZIF-8 and the support and also increase the

compactness of the grain boundary structure, thereby minimizing the non-selective intercrystalline

diffusion and leading to improved separation performance. Thermally induced cross-linking

reaction was verified by FTIR result (Figure S8, Supporting Information), where bands attributed

to the benzyl bromide (CH2Br) and amine groups for BPPO and BPPO-EDA, respectively,

disappeared upon their thermal treatment at 150 ºC. Another possible reason for the observed

improvement in the membrane selectivity activated at a higher temperature can be the complete

removal of residual solvent molecules from the ZIF-8 layer.46 However, further analysis is required

to fully understand the effect of annealing temperature on the grain boundary and subsequent gas

performance. Further increasing the activation temperature up to 200 ºC results in a significant

increase in the propylene permeance (more than one order of magnitude) with a drop in C3H6/C3H8

ideal selectivity from 27.8 to 4.5, indicating the grain boundary structure of the membranes was

compromised. While the XRD patterns in Figure S9 indicate that the ZIF-8-BPPO-EDA samples

did not undergo obvious structural alterations in the studied temperature range compared to the

simulated ZIF-8 pattern, the FTIR shows a drop in the intensity of the Zn-N peak for the membrane

activated at 200 °C. The activation process at the elevated temperature caused some degradation

of the ZIF-8 layer (Figure S7 d, e, f). However, as can be seen from the cross sectional view (Figure

S7 f), the degradation was apparently restricted to the membrane surface, resulting in still

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reasonable C3H6/C3H8 ideal selectivity of 4.5 (compared to the Knudsen propylene/propane

selectivity of ∼1.02). Similar degradation behavior of ZIF-8 membranes upon activation process

at high temperatures were recently reported and was correlated to the corrosion action of water or

methanol on ZIF-8 grains.41, 47 These results indicate that the activation temperature plays a critical

role in determining the gas permeation properties of the ZIF-8 membranes. However, an elaborate

choice of activation conditions (temperature, duration, and environment) is required in order to

achieve ZIF-8 membranes with the best performance.

Figure 8. Room-temperature propylene/propane permeation properties of ZIF-8 membranes grown

for 120 min (ZIF-8@BPPO-EDA-120) as a function of activation temperatures.

It should be noted that the propylene/propane selectivity obtained in this study is the best ever

obtained for ZIF-8 membrane on polymeric supports, but it is still lower than those obtained with

inorganic-supported ones. For example, alumina-supported ZIF-8 membrane prepared by an in

situ counter-diffusion method11 exhibited both higher C3H6/C3H8 selectivity (∼50) and propylene

permeability (∼200 ×10-10 mol m-2 s-1 Pa-1) than the best membrane obtained in this work with the

C3H6/C3H8 selectivity and propylene permeability of 27.8 and 75×10-10 mol m-2 s-1 Pa-1,

respectively. However, since the contra diffusion method is dependent on the surface properties

and porous structure of support, the performance of the membranes could be further improved by

20

investigating the effect of the modification reaction condition. Optimizing activation conditions

could also further improve the performance of the ZIF-8 membranes.

In addition, it is worth mentioning that the synthesis procedure developed here offers a number

of advantages over those reported in the literature. First of all, almost all of previously reported

ZIF membranes were made by using organic solvents (e.g. methanol, dimethyl formamide,

octanol) and/or alkaline additives (e.g. sodium formate, ammonia) under non-ambient conditions

(i.e. high temperature, high pressure).11, 27, 48-49 In many other cases the use of seed crystals and a

long reaction time were unavoidable.26, 38, 50-51 The high quality ZIF-8 membranes in this study

were made in aqueous solution under ambient conditions (room temperature, atmospheric

pressure) in a relatively short period of time (less than 2 h), without any additives or seed crystals.

The synthesis process requires significantly smaller amounts of metal salt and organic ligand

reagents. For example, ∼ 41-90% savings in the usage of the reagents (per cm2 of permeable area)

could be achieved compared to ZIF-8 membranes fabricated by the microfluidic experimental

approach.35

4. CONCLUSIONS

In summary, we reported a novel strategy, contra-diffusion based synthesis in conjunction with

vapor modification, for room temperature synthesis of high-quality ZIF-8 membranes on an

asymmetric polymeric substrate in aqueous solution. The ZIF-8 membranes have shown excellent

gas permeation properties (e.g. propylene/propane ideal selectivity of 16 with propylene

permeance of 150×10-10 mol m-2 s-1 Pa-1), intensely indicating impressively enhanced membrane

microstructure (in particular enhanced grain boundary structure). More importantly, by increasing

the activation temperature from 120 to 150 ºC, the propylene/propane selectivity was further

21

increased (almost two-fold), without compromising the high permeance of propylene, indicating

the important role of thermal activation conditions (in particular activation temperature) in

microstructures of ZIF-8 membranes. With efficient synthesis conditions, the strategy developed

here provides an effective and environmentally friendly route for preparing high-quality ZIF

membranes on the surface of polymeric support.

ASSOCIATED CONTENT

Supporting Information

Measurements of the support pore size, photo of a home-made contra-diffusion cell, schematic

diagram of gas permeation set-up, TGA thermograms of BPPO and BPPO-EDA, SEM images of

ZIF-8 membranes grown by conventional CD method using untreated BPPO, SEM images of ZIF-

8@BPPO-EDA at different magnifications, EDS line scan across ZIF-8@BPPO-EDA, SEM

images of ZIF-8@BPPO membranes activated at different temperatures, FTIR spectra of the

BPPO, BPPO-EDA support after being heated at 150 °C, FTIR spectra and XRD patterns of ZIF-

8@BPPO-EDA membranes as a function of activation temperature (°C), Heat-induced cross-

linking reaction of BPPO substrate, Single gas permeances and ideal selectivities for the ZIF-

8@BPPO-EDA, Comparison of gas permeation properties of the ZIF-8@BPPO-EDA composite

membrane in this work with other ZIF-8 membranes in the literature. . This material is available

free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: huanting.wang@monash.edu; Tel: +61 39 9053449

Present Addresses

22

‡Centre for Advanced Separations Engineering, Department of Chemical Engineering, University

of Bath, Claverton Down, Bath BA2 7AY, United Kingdom.

Funding Sources

This work was supported by the Australian Research Council (Project No: DP140101591, and

FT100100192)

ACKNOWLEDGMENT

The authors acknowledge use of the facilities and the assistance of Kathryn Waldron at the

Monash Center for Electron Microscopy. Ezzatollah Shamsaei thanks Monash University for MGS

and FEIPRS scholarships.

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Table of Contents and Synopsis

This study provides a simple, effective and environmental friendly route for the selective

fabrication of ZIF-8 membrane on a flexible polymer substrate for enhanced gas separation. The

ZIF-8 membranes exhibited high propylene over propane selectivity.