Inorganic Chemistry Communications 48 (2014) 77–80

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Inorganic Chemistry Communications

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New membrane architecture: ZnO@ZIF-8 mixed matrix membraneexhibiting superb H2 permselectivity and excellent stability

Yaguang Liu a, Shaohui Li a, Xiongfu Zhang a,⁎, Haiou Liu a, Jieshan Qiu a, Yanshuo Li c, King Lun Yeung b

a State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR Chinab Department of Chemical and Biomolecular Engineering, the Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, Chinac State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China

⁎ Corresponding author.E-mail address: xfzhang@dlut.edu.cn (X. Zhang).

http://dx.doi.org/10.1016/j.inoche.2014.08.0231387-7003/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 July 2014Received in revised form 23 August 2014Accepted 28 August 2014Available online 29 August 2014

Keywords:Metal organic frameworksZnO nanorodsFilmMembraneGas separation

A new ZnO@ZIF-8 mixedmatrix membrane was achieved by in-situ growth of ZIF-8 crystals within well-alignedZnO nanorods supported on a porous α-Al2O3 ceramic tube. The ZnO nanorods act as both seed sites for ZIF-8growth and strut-like frameworks for the connection of themembranewith the alumina substrate. The resultingmembrane exhibits superb H2 selective permeation performance and excellent stability.

© 2014 Elsevier B.V. All rights reserved.

Metal-organic framework (MOF) structure and chemistry gave raisetomany unique and interesting properties that have promising applica-tions in chemical conversion, gas separation and storage [1,2]. MOFswith their well-defined pores and tunable chemistry are superb candi-dates for membranes [3–7]. Indeed, MOF membranes have been suc-cessfully prepared by different synthesis strategies [5,6]. However,low-defect MOF membranes remain a challenge due to the poor nucle-ation and growth of MOF thin films. Poor intergrowth and brittlenesscan also compromise membrane performance [8–11].

It is not uncommon in preparingMOFmembranes tomodify the sup-port in order to improve film deposition and growth [12–15]. Seededgrowth method has proven to be versatile for preparing a large varietyof MOF membranes [16–18]. High quality seeds and MOF nanoparticlesare commonly used, and polymers are often used to adsorb and assemblethe seeds on the support. The reactive seedingmethodwas developed toavoid the use of polymers that may interfere with the membrane trans-port. Membranes of ZIF-78 and ZIF-71 on ZnO disks and MIL-53 on alu-mina disks were prepared by the reactive seeding method for selectiveseparation [19–21].

Although great progress has been made for MOF preparation, indi-cating that metal oxides can seed and promote the formation of MOFstructure in many morphologies [22–24], to the best of our knowledge,there is no report on forming a continuousmembranewithin ZnOnano-rods for separation.

This work explores a new membrane architecture, ZnO@ZIF-8 hy-bridmembrane, whereinwell-aligned ZnOnanorods are used to anchorand reinforce the ZIF-8 metal-organic framework membrane on a po-rous ceramic support. ZIF-8 was nucleated uniformly on the surface ofthe nanorods, and induced to grow radially outward from the rods ina direction parallel to the support until it intergrownwith the neighbor-ing crystals forming amembrane. The scheme of themembrane processis illustrated in Fig. 1.

The newmembranewas prepared on hollow fiber tubes (HFT, 4mmo.d., 3 mm i.d.) of 100 nm pore size. Each membrane measured 60 mmin length. In brief, thewell-aligned ZnOnanorodswere grown seamless-ly from the support (Fig. 2a) by hydrothermal synthesis. For the sampleshown in Fig. 2a, a uniform growth was obtained over the entire sub-strate. Fig. 2b shows that the ZnO nanorods measure 3 ± 0.5 μm inlength and have a hexagonal cross-section of about 200 nm. A uniformnucleation on the surface of the ZnOnanorods results in a uniform radialgrowth of ZIF-8 by an in-situ synthesis from a synthesis solution with amolar composition of 2 HCOONa:1.0 ZnCl2:3 Hmim:360 EtOH for 6 h at353K. The ZnO@ZIF-8mixedmatrixmembrane architecturewas shownin Fig. 2c & d. Unlike conventional membranes, the enormous surfacearea of the nanorods provides a solid anchor for themembrane. Thenano-rods remained aftermembrane synthesis and can be seen from themem-brane cross-sections (cf. Fig. 2d), EDXS composition profile (cf. Fig. 2e)and X-ray diffraction (cf. Fig. 2f). The seamless intergrowth betweenneighboring ZIF-8 crystals is clearly evident in Fig. 2d and indeed, com-mon defects such as pinholes and cracks were absent. This could be thereason for the excellent gas permeation of these membranes.

Porous alumina

ZnO rod growth ZIF-8 growth

ZnO nanorods

Fig. 1. Preparation scheme for the ZnO@ZIF-8mixedmatrixmembrane on porousα-Al2O3

tube.

Table 1Results of single gas permeation forfivemembranes obtained fromdifferent batches at thesame condition.

Membrane H2 permeancea H2/CO2 H2/N2 H2/CH4

M1 32.86 5.8 13.1 16.6M2 41.19 5.6 13.6 15.2M3 97.27 5.4 10.1 11.5M4 52.22 5.4 12.4 15.3M5 39.02 6.5 13.5 15.8Average 41.32 5.8 13.2 15.7

a H2 permeance/∗10−9 mol·m−2·s−1·Pa−1. The average is calculated except for M3.

78 Y. Liu et al. / Inorganic Chemistry Communications 48 (2014) 77–80

Our previous work [25,26] has shown that ZIF-8 crystals mainlygrow on the upper ends of the ZnO nanorods, resulting in forming atype of strut-like ZIF-8 membrane through activation process, followedby regrowth. Here, an in-situ synthesis without activation, was used tomake ZIF-8 crystals mainly grow within the frameworks of the ZnOnanorods, thus leading to amixedmatrixmembranewith excellent sta-bility as the following permeation results.

Five membranes grown from separate synthesis batches were pre-pared and their gas permeances were measured and shown in Table 1.Four of the five membranes have comparable H2 permeance of (41 ±8) × 10−9 mol·m−2·s−1·Pa−1. All four membranes have very similarH2/N2, H2/CO2 and H2/CH4 permselectivities indicating goodmembranereproducibility.M3membrane has significantly higher fluxpossibly dueto defective end sealing. The gas permeance decreases rapidly withincreasing size of the diffusing gas molecule (i.e., H2, 0.29 nm; CO2,0.33 nm; N2, 0.36 nm; CH4, 0.38 nm). Framework flexibility and poredistortion in ZIF-8 explain the permeation of N2 and CH4 gasesthat are larger than ZIF-8 pores (ca. 0.34 nm). However, their highpermselectivities (i.e., H2/N2 = 13; H2/CH4 = 16) and the absence

d

c a

b

Fig. 2. SEM images of the samples: a, b: ZnO nanorods grown on the substrate; c, d: ZIF-8membcross-section; and f: XRD patterns of the samples (g: magnification of the membrane sample c

of viscous/Poiseuille flow suggest that the membranes are free of de-fects. Binary gas separation measurements were carried out for M1membrane at 303 K. The H2 permeance from H2:CO2, H2:N2 and H2:CH4 binary gas mixtures was 27.0 × 10−9 mol·m−2·s−1·Pa−1 at atransmembrane pressure of 0.05 MPa. Separation selectivities forH2/CO2, H2/N2 and H2/CH4 were respectively 3.2, 10.4 and 12.6, lowerthan the measured permselectivity values of 5.8, 13.1 and 16.6. This isnot unusual as diffusion inmixtures can often lead to lower flux and se-lectivities in porousmembranes. Furthermore, the sorption of gases candistort the ZIF-8 framework and change the motion of the organiclinkers altering the interaction and mobility of the gases in the pores[27].

The newmembrane architecture is suitable for thepreparation of ZIFmixedmatrixmembranes and has the advantages of greater rigidity andstability. Fig. 3 plots the results of single gas permeation experimentscarried out on M1 membrane. Measurements were made as the mem-brane was heated from 303 K to 423 K and cooled back to 303 K. Itcan be observed from the plot that the process is reversible. Tempera-ture has the least effect on H2/N2 and H2/CH4 permselectivities, whileH2/CO2 increases from 5.8 to 12.4 for 303 K to 423 K. This phenomenon

4 m

e

Zn

O

Al

Zn

10 20 30 40 50

5 10 15 20 25 30

*

2 (degree)

*

d

c

b

Inte

nsity

(a.u

.)

2 (degree)

a

g f

5 m

rane grownwithin the ZnO nanorods; e: EDX elemental analysis of the hybridmembrane).

0

5

10

15

20

423K

400300 350 300350

Perm

sele

ctiv

ity

T / K400

H2/CO2

H2/N2

H2/CH4

0

5

10

15

20

1

5

10

25

50

423KH2

N2

400400350 350 300

Perm

eana

nce/

10-9

mol

m-2

s-1Pa

-1

T / K300

CH4

CO2

1

5

10

25

50

Fig. 3. Single gas permeation as a function of temperature: permeance (left) and permselectivity (right).

79Y. Liu et al. / Inorganic Chemistry Communications 48 (2014) 77–80

is observed in different literatures [28,29] including our previous work[26]. The permeances of all gases decreased with increasing tempera-tures. This might be related to the changes in the energetics of C_Cbonds that govern the stretching of the methyl-imidazole ring andtherefore the size of the pore aperture. And this is also proved by thein-situ IR results at different temperatures (Fig. S11). The peaks at1350–1500 cm−1 are assigned to the stretching of two kinds of C_Cbonds of the methyl-imidazole ring and have an obvious change withtemperature, while there is no significant change for C_N bond at1584 cm−1. As shown in Fig. S12, both the C_C bonds (a and b) havethemost important effect on the aperture of ZIF-8. It is generally acceptedthat configurational diffusion is the predominant transport mechanism[26,30] as the size of the diffusing molecule (i.e., CO2) approaches thatof the pore opening (i.e., ZIF-8). Therefore, any perturbation in pore diam-eter caused by temperature or gas sorption is expected to cause a largechange in the gas permeation. This could explain the greater temperaturesensitivity of CO2 permeance, implying that the newmembrane architec-ture is free of defects.

In conclusion, a new ZnO@ZIF-8 mixed matrix membrane was suc-cessfully conceptualized and fabricated by in-situ growth of ZIF-8 crys-tals within well-aligned ZnO nanorods. The ZnO nanorods can act asboth nucleation sites and anchors for the formation of a dense and ro-bust ZnO@ZIF-8 mixed matrix membrane. The resulting membranepossesses highly selective H2 permeation from small gases (CO2, N2

and CH4) due to its molecular sieving function presented by the poreof themembrane. It is worth noting that the permeance of CO2 throughthemembrane obviously decreaseswith the temperature, implying thatthis membrane is free of defects. The present synthesis strategy mayopen up a new route for preparing new MOF membrane architectureand also be used to prepare metal oxide@MOF mixed matrix filmswhich can serve as possible semiconducting multifunction [31].

Acknowledgment

We gratefully acknowledge the financial support by the National Nat-ural Science Foundation of China (Nos. 21476039, 21076030, 21036006),the Natural Science Foundation of Liaoning Province (No. 201202027)and the Specialized Research Fund for the Doctoral Program of HigherEducation (No. 20130041110022).

Appendix A. Supplementary Data

Electronic Supplementary Information (ESI) available: The detailsof synthesis, characterization and gas permeation of the membrane.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2014.08.023.

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