Water Promotes Melting of a Metal-Organic Framework

Abstract

Water is one of the most reactive and abundant molecules on Earth, and it is thus crucial to understand its reactivity with various material families. One of the big unknown questions is how water in liquid and vapor forms impact the fast-emerging class of metal-organic frameworks (MOFs). Here, we discover that high-pressure water vapor drastically modifies the structure and hence the dynamic, thermodynamic, and mechanical properties of MOF glasses. In detail, we find that an archetypical MOF (ZIF-62) is extremely sensitive to heat treatments performed at 460 °C and water vapor pressures up to ∼110 bar. Both the melting and glass transition temperatures decrease remarkably (by >100 °C), and simultaneously, hardness and Young's modulus increase by up to 100% under very mild treatment conditions (<20 bar of hydrothermal pressure). Structural analyses suggest water to partially coordinate to Zn in the form of a hydroxide ion by replacing a bridging imidazolate-based linker. The work provides insight into the role of hot-compressed water in influencing the structure and properties of MOF glasses and opens a new route for systematically changing the thermodynamics and kinetics of MOF liquids and thus altering the thermal and mechanical properties of the resulting MOF glasses.

Figure 1

(a) Calorimetric heating scans of ZIF-62 crystals mixed with varying amounts of water, corresponding to maximum water pressures of 1.0, 2.5, 11.6, 22.3, 51.5, and 111.1 bar. (b) Melting temperature (T_m) and glass transition temperature (T_g) as a function of the maximum hydrothermal pressure. Open crucibles are regular aluminum DSC (max internal pressure of 3 bar) pans, while high-pressure crucibles are capable of withstanding internal pressures up to 270 bar. Arrows indicate relevant y-axis. (c) Optical micrographs of the formed glasses, showing an increase of fluidity with increasing water pressure. The scalebar in each micrograph corresponds to 1 mm.

Figure 2

(a) Glass transition temperature (T_g) as a function of melting temperature (T_m) for various derivatives of ZIF-4 (green circles), ZIF-62 (red squares),^, and ZIF-76 (gray triangles)^, glasses as well as the present ZIF-62 glasses subjected to hydrothermal treatment (orange stars). (b) In situ pressure sensitivity of the glass transition temperature (dT_g/dP) as a function of the glass transition temperature for a range of organic,⁻ chalcogenide,^, and inorganic glasses, as well as the studied ZIF-62 system for the highest pressure case.

Figure 3

(a) Solid-state ⁶⁷Zn MAS NMR spectra of samples prepared at 1 and 111.1 bar of hydrothermal pressure along with a numerical simulation of the spectra. (b) Solid-state ¹H MAS NMR spectra of samples prepared at 1 and 111.1 bar of hydrothermal pressure. Inset shows the −2 to 6 ppm range, highlighting the signal from hydroxide (OH^–). The data in the main plot are shifted vertically for the sake of clarity. Coloring is similar to that in panel (a). The stars (*) in both panels (a) and (b) indicate spinning sidebands. (c) Ab initio MD simulated atomic snapshot of configuration of hydroxide coordinating to a Zn polyhedra together with both imidazolate and benzimidazolate. Zn is green, N blue, C orange, O red, and H white. All aromatic hydrogens are omitted for clarity. (d) Pair distribution function, G(r), of ZIF-62 glasses prepared under argon and water gas pressures ranging from 1 to 111.1 bar. An illustration of the internal distances in the zinc-imidazolate-zinc ZIF building block is shown as an inset. Distances are labeled in roman numerals and correlated with the shaded peaks in the G(r) data.

Figure 4

(a) Sketch of an indentation experiment. (b) Examples of residual indentation imprints on the surfaces of the samples experiencing a maximum pressure of 2.5 bar (left) and 111.1 bar (right) during the glass synthesis procedure (the indentation load was 200 mN). The white scalebar is 20 μm in length. (c,d) Indentation-based measurements of (c) Vickers hardness (H_V) and (d) Young’s modulus (E) of the studied ZIF-62 glasses at water gas pressures ranging from 1 to 111.1 bar. The H_V and E data are also compared with previous literature reports, shown with star symbols in panels (c) and (d), acquired using microscopic analysis of indents and ultrasonic echography,^, respectively, on ZIF-62 synthesized in open crucibles (i.e., P_max = 1 bar, star symbol). Dashed lines in panels (b), (c), and (d) are guides for the eye.