One step in situ synthesis of supported zeolitic imidazolate framework ZIF-8 membranes: Role of sodium formate

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Abstract

Zeolitic imidazolate frameworks (ZIFs), a sub-class of metal–organic frameworks (MOFs), are noted for their remarkable thermal and chemical stability, tunable microporous channels, and tailorable physical/chemical properties. When synthesized as films, they hold great potentials for gas sensing, catalytic membrane reactor, and gas separation membrane applications. Here, we report one step in situ synthesis of ZIF-8 membranes on unmodified porous α-alumina supports in the presence of sodium formate. In this in situ method, sodium formate plays a critical role for formation of well-intergrown continuous ZIF-8 membranes. Sodium formate was found to enhance the heterogeneous nucleation of ZIF-8 crystals on alumina supports as well as to promote intergrowth of ZIF-8 crystals. It was confirmed that sodium formate reacts with zinc source to form zinc oxide layers on α-alumina supports, which in turn promote heterogeneous nucleation. ZIF-8 membranes show molecular sieving behavior, favoring smaller molecules. It was found that sodium formate promotes heterogeneous nucleation in other ZIF systems as well, leading to continuous films of ZIF-7, Zn(Im)2 (ZIF-61 analog), ZIF-90, and SIM-1.

Highlights

► ZIF-8 membranes were synthesized by one step in situ method using sodium formate. ► During in situ growth sodium formate reacts with zinc source to form zinc oxide layer on the support. ► Zinc oxide layer promotes heterogeneous nucleation and acts a secondary metal source. ► Sodium formate promotes heterogeneous nucleation for other ZIF systems yielding ZIF-7, ZIF-90, Zn(Im)2 (ZIF-61 analogue), SIM-1 films.

Introduction

Zeolitic imidazolate frameworks (ZIFs) are a sub-class of metal–organic frameworks (MOFs), comprising hybrid organic–inorganic moieties and exhibiting regular crystalline lattices with well-defined pore structures [1], [2], [3], [4], [5], [6], [7], [8], [9]. ZIFs consist of metal nodes coordinated to imidazolate-based ligands. The metal-linker-metal bond angle (∼145°) in ZIFs is comparable to the T–O–T bond angle in zeolites, thereby resulting in zeolite topologies. Their exceptional thermal and chemical stabilities coupled with microporous cavities [9] make them desirable candidates for gas sensors [10], catalytic membrane reactors [11], [12], and gas separation membranes [13], [14], [15], [16], [17], [18], [19]. As a result, synthesis of ZIF films and membranes has attracted a great deal of interest in recent years [20], [21].

Among other ZIF systems, ZIF-8 is of particular interest due to its robust synthesis protocol as well as its potentials in small-gas separations. ZIF-8 is comprised of Zn atoms interconnected with 2-methylimidazolate ligands, forming the sodalite (SOD) zeolite-like structure with large cavities (11.6 Å) and small pore apertures (3.4 Å) [9]. Diverse synthesis protocols for ZIF films and membranes have been reported [21] and can be classified into two categories: in situ growth [13] and secondary (seeded) growth [22], [23], [24]. Recently Pan et al. [24] synthesized ZIF-8 membranes using secondary growth method and showed excellent propylene/propane gas separation performance. However, secondary growth method requires seeding step in addition to solvothermal growth step. Increased number of steps adds to the complexity of the process, thereby potentially causing reproducibility issues [21]. ZIF-8 membranes have been synthesized using in situ method as well. To promote heterogeneous nucleation of ZIF-8 crystals, supports were often times modified [14] or more expensive supports such porous titania than commonly-used α-alumina were used [13].

Here we report one step in situ synthesis of ZIF-8 membranes on unmodified porous α-alumina supports in the presence of sodium formate. This approach is different from our previously reported technique [14] in which ligand-modified supports were used to promote heterogeneous nucleation. In this work, ZIF-8 membranes were prepared on as-prepared supports, thereby eliminating the need for support modification and simplifying the process. The focus of this work is to investigate the role of sodium formate in this one step in situ method. Besides the previously reported role of sodium formate as a deprotonator promoting the intergrowth of ZIF crystals [14], we identified a previously unknown role of sodium formate of promoting heterogeneous nucleation on α-alumina supports. One step in situ method was used to prepare continuous films of several other ZIFs including ZIF-7 [9], Zn(Im)2 (ZIF-61 analog) [25], [26], ZIF-90 [27], and SIM-1 [11], suggesting the general applicability of the method.

Section snippets

Chemicals

All the chemicals were used as received without further purification. Zinc chloride (ZnCl2 > 95%, Fisher Scientific) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O > 98%, Sigma–Aldrich) were used as zinc sources. 2-methylimidazole (m-Im) (C4H6N2 > 99%), benzimidazole (b-Im) (C7H6N2 > 98%), imidazole-2-carboxaldehyde (Ica) (C4H4N2O > 97%), 4-methyl-5-imidazolecarboxaldehyde (m-Ica) (C5H6N2O > 99%), imidazole (Im) (C3H4N2 > 99%), sodium formate (HCOONa > 99%) were purchased from Sigma–Aldrich. Methanol (99.8%) and N

ZIF-8 membrane fabrication using sodium formate

ZIF-8 membranes were synthesized using one step in situ method in the presence of sodium formate. ZIF-8 membranes are phase-pure (Fig. 1a) and well-intergrown with a thickness of about 25 μm (Fig. 1b). Previously, our group reported ZIF-8 membranes synthesized in situ by modifying alumina supports with ligands (m-Im) and followed by growing ZIF-8 crystals in the presence of sodium formate [14]. Though the growth conditions are similar (both in the presence of sodium formate), the grains of the

Conclusions

In conclusion, we synthesized continuous well-intergrown ZIF-8 membranes by a simple one step in situ growth method. During in situ growth, sodium formate along with zinc salts leads to formation of zinc oxide on α-alumina supports. This zinc oxide acts as a heterogeneous nucleation site as well as a secondary metal source. Once ZIF crystals are nucleated, sodium formate promotes the intergrowth of ZIF crystals by acting as a deprotonator (reported previously) leading to continuous

Acknowledgements

H.-K.J. acknowledges the financial supports from the National Science Foundation (CBET-0930079), the Department of Energy Advanced Research Project Agency-Energy (DE-AR0000073) and the Artie McFerrin Department of Chemical Engineering at Texas A&M University and Texas Engineering Experiment Station through a new faculty startup. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund (PRF-48884-DNI5) for partial support of this research. The FE-SEM

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