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][2][3][4][5][6][7][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][10][11][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 ) is accessible through a narrow six-ring pore (3.4 ). 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][14][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-methylimidazolate 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 identif...

Exceptionally Low Shear Modulus in a Prototypical Imidazole-Based Metal-Organic Framework

“…Second, while ZIFs based upon substituted imidazolates, like ZIF-8, are resistant to temperatureinduced amorphization, they can be irreversibly amorphized via ball milling [26] because extensive shearing induces framework distortion and collapse. Third, we can now explain the surprising result that a liquid lubricant containing ZIF-8 additives demonstrates excellent antiwear properties [27]; given our findings, this seems reasonable because ZIF-8's low shear resistance reduces friction at interfaces, thus promoting better wear protection. In conclusion, while the elastic properties of the prototypical MOF, ZIF-8, are indeed complex, a detailed understanding of its elasticity opens up new possibilities for tuning the physical properties for emerging applications.…”

Post‐Synthetic Anisotropic Wet‐Chemical Etching of Colloidal Sodalite ZIF Crystals

“…Indeed, N 2 sorption measurements (taken at 77 K up to 1 bar) of all the etched ZIF-8 and ZIF-67 crystals proved that they had consistent sorption capacity (Table 1) as that of the initial crystals (see Figure S10-S20). [9,15] In fact, the newly etched materials exhibited slightly higher porosity-a difference that we attributed to the creation of defects[16] during etching.…”

“…Indeed, N 2 sorption measurements (taken at 77 K up to 1 bar) of all the etched ZIF-8 and ZIF-67 crystals proved that they had consistent sorption capacity (Table 1) as that of the initial crystals (see Figure S10-S20). [9,15] In fact, the newly etched materials exhibited slightly higher porosity-a difference that we attributed to the creation of defects[16] during etching.In conclusion, we have demonstrated anisotropic etching of sodalite ZIF crystals. The etching solution in this process is acidified/basified XO, which serves to sequentially protonate the imidazolate linkers, break the coordination bonds and sequester the liberated metal ions.…”

Fine-scale tribological performance of zeolitic imidazolate framework (ZIF-8) based polymer nanocomposite membranes

“…18 Hitherto, there are only two reports associated with the wear behaviour of MOF-type materials. Shi et al 19 investigated the tribological behavior of ZIF-8 and ZIF-67 as additives in liquid lubricants, both of which appear to demonstrate promising anti-wear property.…”