A metal-organic framework with ultrahigh glass-forming ability

Abstract

Glass-forming ability (GFA) is the ability of a liquid to avoid crystallization during cooling. Metal-organic frameworks (MOFs) are a new class of glass formers (1-3), with hitherto unknown dynamic and thermodynamic properties. We report the discovery of a new series of tetrahedral glass systems, zeolitic imidazolate framework-62 (ZIF-62) [Zn(Im_2-x bIm _x )], which have ultrahigh GFA, superior to any other known glass formers. This ultrahigh GFA is evidenced by a high viscosity η (10⁵ Pa·s) at the melting temperature T_m, a large crystal-glass network density deficit (Δρ/ρ_g)_network, no crystallization in supercooled region on laboratory time scales, a low fragility (m = 23), an extremely high Poisson's ratio (ν = 0.45), and the highest T_g/T_m ratio (0.84) ever reported. T_m and T_g both increase with benzimidazolate (bIm) content but retain the same ultrahigh T_g/T_m ratio, owing to high steric hindrance and frustrated network dynamics and also to the unusually low enthalpy and entropy typical of the soft and flexible nature of MOFs. On the basis of these versatile properties, we explain the exceptional GFA of the ZIF-62 system.

Fig. 1. Structural units and calorimetry of ZIF-62.

(A) Similarity between tetrahedra in silicate glasses and ZIF-62 Im/bIm networks. (B) C_p and mass loss versus T, heated at 10 K min⁻¹, following desolvation to eventual melting at T_m = 708 K. (C) C_p upscans of ZIF-62 glass quenched from above T_m showing a clear glass transition (T_g = 595 K), yielding T_g/T_m (0.84). Inset: Optical image of a transparent MQ glass.

Fig. 2. ZIF-62 ultrahigh GFA, high viscosity at T_m, and crystal-glass structural evolution.

(A) XRD patterns of ZIF-62 glasses in argon at temperatures approaching T_m (0.88 < T/T_m < 0.92) for t_a = 24 hours. AU, arbitrary units. (B) Crystalline and glass pair distribution functions D(r) with Im geometry identifying peaks 1 to 6, which are replotted from the study of Bennett et al. (17). (C) Raman spectra of crystal and glass. Insets: Changes in nodes (Zn-N) and linkers (C-N). (D) Temperature dependence of η for bIm/(Im + bIm) = 0.125 liquid with MYEGA (Mauro-Yue-Ellison-Gupta-Allan) fit (25). A two-parallel plate oscillation technique was used to avoid high-temperature oxidation, the first η measured for any MOF liquid.

Fig. 3. Effects of linker substitution on microstructure and thermodynamic characteristics.

(A) bIm/(Im + bIm) ratios in both ZIF-62 crystal and glass from ¹H liquid NMR (see the Supplementary Materials) versus synthesized values. The crystal structure was checked through XRD patterns (fig. S7). (B) ¹H-¹³C CPMAS NMR spectra of crystalline samples of ZIF-62 with varying bIm/(Im + bIm) ratio highlighting bIm contributions [calculated shifts in black (see the Supplementary Materials)]. ppm, parts per million; expt., experiment; calc., calculation. (C) Increase in T_m and T_g with increasing bIm. Inset: Almost constant T_g/T_m for mixed linkers, compared to the 2/3 rule (–7). (D) Increases in ΔH_m and ΔS_m on fusion with increasing bIm.

Fig. 4. The ultrahigh T_g/T_m ratio of ZIF-62.

The comparison in T_g/T_m ratio between ZIF-62 and other types of glass-forming systems, including water (29, 30), oxide (33), metallic (34), and organic (35, 36)—good GFA typified by CPs, PMMA, B₂O₃, and SiO₂ in contrast with the ultrahigh GFA of the ZIF-62.