Polymorph Selection of Zeolitic Imidazolate Frameworks via Kinetic and Thermodynamic Control

Abstract

Exploring the kinetics of metal−organic framework crystallization is crucial in order to design novel synthesis methods benefiting from far-from-equilibrium conditions, such as flow, microfluidic, or gel-phase reactors. Herein we focus on zeolitic imidazolate frameworks (ZIFs) obtained in the reaction of 2-methylimidazolate and zinc ions as a model system. The room-temperature synthesis carried out in pure water solvent without the addition of any alkaline chemical yields a highly crystalline product with good conversion within minutes. The short-term kinetic characteristics were determined by a high-speed camera, the result of which was complemented by a UV−vis photometer based long-term investigation. At low concentrations and moderate linker excess, which facilitate sluggish coordination and precipitation, a fast colloid formation is followed by a delayed and remarkable recrystallization, which leads to the formation of a dense dia(Zn) polymorph of ZIF. Although this is the thermodynamically favored route, it should be avoided due to the nonporous structure of the product. The same stoichiometric excess of the linker ions together with higher concentrations gives rise to the production of the thermodynamically less stable polymorph of ZIF: i.e., ZIF-8 with a sodalite-like structure is obtained via kinetic control. Similar conclusions are drawn in the case of cobalt(II)-containing imidazolate frameworks, which highlights that kinetic control based polymorph selection might be achieved for other MOFs as well. Our kinetic results were supported by scanning electron microscopy, powder X-ray diffraction, nitrogen gas adsorption, and Fourier-transformed infrared and Raman spectroscopy.

XRD and Spectroscopic Investigations of ZIF—Microchannel Glass Plates Composites

“…As shown in [ 46 ], the strongest confirmation of the existence of the SOD phase in the complex structure of ZIF-8 can be demonstrated in the IR spectrum in the region of 640–800 cm −1 (see Figure S5a —our data, compared with Figure S5b and Table S2 from [ 46 ]). As seen in our data in Figure S5a , the IR spectrum of jn-01 powder showed peaks at 755, 706 and 672 cm −1 , which correlates with the literature peaks at 758, 702, and 676 cm −1 as shown in Table S2 , and related to the SOD polymorph [ 42 ]. In addition, a peak at 1179 cm −1 (corresponding to 1175 cm −1 in Table S2 for SOD) is also shown for sample jn-01 in Figure 2 a (and in Figure S5c ), as well as for samples jn-04 and jn-07 (ZIF-67).…”

“…Despite less noticeable changes for different polymorphs in the Raman spectrum for complex ZIFs, the Raman spectra of the powder sample jn-01 also confirmed the presence of the SOD structure, as shown in the [ 42 ] (see Figure S9 and Table S5 ), as well as in our spectra with the appearance of the 935 cm −1 band (together with the 953 cm −1 band) for the jn-01 powder sample. Table S5 [ 42 ] shows the presence of the SOD phase, in this case, can also be confirmed by the simultaneous absence of the band at 759 cm −1 .…”

“…Despite less noticeable changes for different polymorphs in the Raman spectrum for complex ZIFs, the Raman spectra of the powder sample jn-01 also confirmed the presence of the SOD structure, as shown in the [ 42 ] (see Figure S9 and Table S5 ), as well as in our spectra with the appearance of the 935 cm −1 band (together with the 953 cm −1 band) for the jn-01 powder sample. Table S5 [ 42 ] shows the presence of the SOD phase, in this case, can also be confirmed by the simultaneous absence of the band at 759 cm −1 . It should be noted that the Raman spectra, recorded for composites based on jn-01 powder and MCG plates (samples jn-02 and jn-03), demonstrated the presence of a peak at 933 cm −1 and a complete absence of the band at 759 cm −1 (see also Figure 7a,b below).…”

“…ZIF-67-1 demonstrated excellent agreement with the previously published ZIF-67 material with the main peaks at index as 7.9° (011), 10.6° (002), 12.9° (112), 14.7° (022), 16.5° (013), 18.4° (222), 22.0° (114), 24.6° (233), 25.9° (002), 26.4° (134), 29.6° (044), 31.2° (244), and 32.3° (235) [ 41 ]. This ZIF phase corresponds to the SOD (sodalite-like) topology with high surface area [ 42 ].…”

Understanding and controlling the nucleation and growth of metal–organic frameworks

“…By tuning the activation energy of the MOF through changing synthesis conditions, polymorphs of varying stabilities can be formed; thus polymorphs can be found at different energy minimas of a free energy landscape diagram. 61 Since polymorphs can display different crystal structures while retaining identical molecular composition, the structure-function relationship based on structural parameters can be investigated. 62,63 Polymorph transition can be induced post-synthesis through physical steps like additional heating, 64 pressure, or template mediation.…”