Zeolitic imidazolate framework ZIF-8 films by ZnO to ZIF-8 conversion and their usage as seed layers for propylene-selective ZIF-8 membranes

https://doi.org/10.1016/j.jiec.2018.12.039Get rights and content

Highlights

  • ZIF-8 films with varying microstructures were prepared by ZnO to ZIF-8 conversion.

  • The ZIF-8 films were used as seed layers to synthesize supported ZIF-8 membranes.

  • The film microstructures significantly affected resultant membrane performances.

  • Membrane growth inside substrates improved mechanical strength of the resultant membranes.

Abstract

Supported ZIF-8 films with varying microstructures are prepared by ZnO to ZIF-8 conversion reaction in a 2-methylimidazole ligand solution. The grain size and continuity of the films are controlled by manipulating two critical conversion parameters such as solvents and their blends, and temperature. The ZIF-8 films with varying microstructures are used as seed layers to synthesize propylene-selective ZIF-8 membranes. The propylene/propane separation performances are greatly affected by the seed layer microstructures. Non-continuous seed layers with compactly packed ZIF-8 nanocrystals tend to yield more propylene-selective membranes than those grown from continuous seed layers with larger grain size likely due to improved grain boundary structure. Furthermore, the membranes are mechanically stronger than those grown from conventional seed layers, which are obtained by dip-coating ZIF-8 nanocrystals on substrates, possibly owing to the presence of mechanical interlocks between the membranes and porous substrate network.

Introduction

Zeolitic imidazolate frameworks (ZIFs), a subclass of porous crystalline metal–organic frameworks (MOFs), are composed of divalent nodes (e.g., Zn, Co, and Cd) bridged by imidazole or its derivative linkers [1]. ZIFs, which structurally resemble zeolites, have been an attractive subset of MOFs for numerous applications [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13] due to their relatively high thermal and chemical stabilities as compared to other MOFs [1]. The ZIFs are particularly interesting for gas separations due to their molecular-level pore aperture size [1], [13], [14], [15], [16]. ZIF-8, composed of zinc nodes interconnected by 2-methylimidazoles, is one of these materials most extensively studied [1], [17], [18]. Since the effective pore aperture size of ZIF-8 lies between the kinetic diameters of propylene and propane, it can distinguish these molecules based on molecular sieving [17], [18]. Motivated by the intrinsic separation potential of ZIF-8, there have been significant efforts to construct ZIF-8 membranes to realize energy-efficient membrane-based propylene/propane separation [7], [8], [19], [20], [21], [22], [23], [24], [25].

Polycrystalline ZIF membranes can be synthesized by either in-situ or secondary growths [23], [26], [27]. In general, the secondary growth method offers better control of membrane microstructures (e.g., thickness, orientation, and grain boundary structure) than the in-situ growth by decoupling nucleation and crystal growth by starting the synthesis with seeded substrates. Therefore, when the secondary growth method is chosen, achieving high quality seed layers (e.g., compact seed crystal packing and strong attachment to substrates) is one of the decisive parameters directly affecting final membrane performance [26], [28], in addition to optimizing growth and activation conditions.

Moving away from conventional seeding approaches (e.g., dip-coating and rubbing), innovative MOF/ZIF seeding techniques have recently been developed [28], [29], [30], [31], [32], [33], [34], [35], [36]. Representative examples are reactive seeding [29], [30], microwave-assisted seeding [28], [31], [32], layer-by-layer seeding [33], [34], and electrophoretic seeding [35], [36] methods that led to MOF/ZIF seed layers in a simple and reproducible manner with strong attachment and thus highly functional MOF/ZIF membranes. However, a majority of these reports focus only on introducing new seeding techniques [28], [29], [30], [31], [32], [33], [34], [35], [36], and the construction of seed layer structure-membrane property (e.g., performance and stability) relationships has been mostly neglected except a recent work by Ramu et al, which is important for both fundamental understanding and applications when secondary growth is chosen for polycrystalline membrane growth. Ramu et al. [32] produced ZIF-8 seed layers with various microstructures by microwave-assisted seeding and studied how the different seed layer microstructures influence separation properties of secondarily grown ZIF-8 membranes. However, in general, microwave synthesis is not easily accessible and scalable for research and commercial applications [37].

A direct metal oxide to MOF conversion process in the presence of ligand offered a simple route to obtain hierarchical metal oxide/MOF hybrid or MOF structures [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]. In this approach, MOF nucleation and growth are spatially controlled near the surface of metal oxide templates where metal oxide dissolution and MOF crystallization occur. Therefore, the restricted heterogeneous crystallization enables shaped MOF structures (e.g., nanorod, hierarchical porous structure, and film) resembling sacrificial metal oxide templates. Furthermore, as reported by Meckler et al. [43], MOF films with distinctively different microstructures seem to be easily obtainable by simply adjusting conversion parameters that affect metal oxide dissolution and crystallization kinetics as compared to other approaches (e.g., microwave synthesis) [32], [37]. There are also separate reports regarding the formation of MOF films and membranes using the conversion approach [24], [39], [40], [41], [42], [46], [47], [49], [50], [51], [52], [53] in which the majority reported about supported ZnO nanorod arrays or thin films to ZIF-8 transformation in the presence of 2-methylimidazole ligand. For instance, Zhang et al. [50] partially converted ZnO nanorod arrays into ZIF-8 under solvothermal conditions and used them as seed layers to construct mechanically strong and flexible ZIF-8 membranes with multiple ZnO anchorages on ceramic substrates. The similar approach was taken by Khaletskaya et al. [46] and Yu et al. [41] to synthesize continuous ZIF-8 films and membranes from sputter-coated ZnO layers, respectively. In addition, instead of achieving the conversion under solvothermal conditions, there were a few trials to implement the conversion in a vapor phase [24], [52], [53]. Li et al. [24] and Ma et al. [53] succeeded in synthesizing propylene-selective ZIF-8 membranes by directly transforming supported ZnO layers on polymeric hollow fibers and ceramic substrates, respectively to ZIF-8 in the presence of 2-methyimidazole vapor.

In this regard, here we attempt to prepare an array of supported ZIF-8 films with systematically varied microstructures using the simple metal oxide to conversion process and use the films as seed layers for propylene-selective ZIF-8 membranes to investigate seed layer structure-membrane property relationship.

Section snippets

Chemicals

Zinc nitrate hexahydrate (Sigma-Aldrich, 98%) and 2-methylimidazole (Sigma-Aldrich, 99%, hereafter 2-mIm) were used as metal and ligand sources, respectively. Dimethylformamide (Dae-Jung, 99.5%, hereafter DMF), methanol (Dae-Jung, 99.5%), and deionized (DI) water were used as solvents. The DI water was prepared by a deionized water system (Aqua Max ultra 360, Young-Lin). α-Al2O3 powder (Baikowski, CR6), poly(vinyl alcohol) (Dae-Jung, 12,000 g/mol, hereafter PVA), and nitric acid (Dae-Jung, 60%)

Results and discussion

α-Al2O3 supported ZnO layers were prepared by repeating dip-coating and drying processes with a ZnO (ca. 80–140 nm) slurry. Top and cross-sectional SEM images show that the ZnO layer with a thickness of ∼1 μm is free of visible cracks and pinhole defects, and a XRD pattern of the composite confirms the presence of each layer (Fig. 1). It should be noted that the ZnO layer was sintered after dip-coating to assure strong adhesion to the α-Al2O3 substrates, which is essential to localize ZnO to

Conclusions

In conclusion, we attempted to prepare supported ZIF-8 films with various microstructures via ZnO to ZIF-8 conversion. Proper selection of solvents and their mixtures combined with conversion temperatures enabled systematic variation of film microstructures in terms of film continuity and crystal grain size. When the films were used as seed layers for continuous ZIF-8 membranes, the different film microstructures dramatically influenced the final membrane qualities. First, the membranes were

Acknowledgements

This work was supported by the KIST Institutional Program (Project No. 2Z05460-18-076), the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant NRF-2016R1A2B4014805) and the Engineering Research Center of Excellence Program of the Korea Ministry of Science, ICT & Future Planning (MSIP)/National Research Foundation of Korea (NRF) (Grant NRF-2014R1A5A1009799).

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