Synthesis of ZIF‐8 and ZIF‐67 by Steam‐Assisted Conversion and an Investigation of Their Tribological Behaviors

Abstract

Zeolitic imidazolate frameworks (ZIFs) are a new subclass of porous metal-organic frameworks (MOFs) which frequently have expanded zeolite topologies. [1-8] Most interestingly, some guest-free ZIFs have the large surface area and pore volume of classical MOFs; at the same time, they also have the high chemical and thermal stability of conventional zeolites. [4] Because of these combined and desirable features, ZIFs show great potential for many applications, especially for gas storage and separations. [5, 9-12] ZIF-8 (Zn(mim) 2 , mim = 2-methylimidazolate) and ZIF-67 (Co(mim) 2) are the most representative ZIF materials with a zeolite sod topology. The expanded sod framework exhibits intriguing features: a large sod cage (11.6 Š) is accessible through a narrow six-ring pore (3.4 Š). Moreover, ZIF-8 has a high thermal stability (550 8C in N 2) and large surface area (BET: 1630 m 2 g À1). [4] These features have made ZIF-8 the most established ZIF material and it has found a variety of impressive applications. [9, 11, 13-15] Thermally and chemically stable ZIF-8 and ZIF-67 have generally been synthesized by using DMF as an organic solvent that ultimately fills the pore space. [4] However, the guest molecule DMF is actually larger than the aperture of the sod cage, and could not be directly released. Their extensive applications require that the synthesis and activation of ZIF-8 and ZIF-67 samples are facile and environmentally friendly. Therefore, solvent-free (melting) synthesis (possible because of the low melting point of mim: m.p. 144 8C) or hydrothermal synthesis, have significant potential for the development of effective and environmentally benign routes to ZIF-8 and ZIF-67. Actually, in the absence of the organic solvent DMF, a eutectic mixture is formed upon heating a mixture of Zn-(OAc) 2 and mim below the melting point of mim (Supporting Information, Figure S1). The eutectic mixture was then kept for 24 h either at 120 8C or 150 8C. However, the powder X-ray diffraction (PXRD) analysis of the unwashed products showed that this solvent-free (melting) synthesis had not lead to any detectable ZIF phase (Supporting Information, Figure S2). Moreover, the unwashed products were found to be soluble upon washing with water, further indicating that the required crystallization had not occurred. We attempted a hydrothermal synthesis of ZIF-8 and ZIF-67. However, only two new compounds with the dense dia framework (referred to as dia(Zn): Zn(mim) 2 and dia(Co): Co(mim) 2) were formed from Zn(OAc) 2 or Co(OAc) 2 with excess mim (Figure 1, left). The structure of dia(Zn) was determined by single-crystal X-ray diffraction. Each Zn II ion is coordinated by four N atoms from bridging 2-methylimi-dazolate groups. A view along the b-axis of the monoclinic cell reveals 1D channel with chairlike hexagonal apertures, which intersect with other regular channels to form an infinite 3D framework that resembles the dia topology (Supporting Information, Figure S3). The dia(Co) analogue was identified as being isostructural to dia(Zn) on the basis of the PXRD pattern (Supporting Information, Figure S4). The porosities of dia(Zn) and dia(Co) could not be established by N 2 uptake owing to the small pore size (Supporting Information, Figure S7). The calculated density of dia(Zn) (1.58 g cm À3) is higher than that of other ZIFs (e.g. ZIF-8, 0.95 g cm À3), confirming, the two new compounds with the dia framework are quite dense. These results inspired us to devise a new path to exploit the structure-directing effect of H 2 O to prepare open and porous ZIF-8 and ZIF-67. A path (denoted as dry-gel conversion) that has been exploited in our laboratory, is the transformation of aluminosilicate gels into zeolites by contact with a vapor of volatile amines. [16] A similar method (denoted as steam-assisted conversion) for transforming a gel to a zeolite in H 2 O vapor has also been developed (Fig-ure 1, right). [17] Compared to most conventional hydro-solvo-thermal synthesis or solid-state transport synthesis, the difference in this method is due to the high concentration of the Figure 1. Schematic diagram of the reaction vessels for ZIFs synthesis. Left: hydrothermal synthesis (HTS), and right: the steam-assisted conversion (SAC) method; M = Zn, Co.