Early stage structural development of prototypical zeolitic imidazolate framework (ZIF) in solution

Abstract

Given the wide-ranging potential applications of metal organic frameworks (MOFs), an emerging imperative is to understand their formation with atomic scale precision. This will aid in designing syntheses for next-generation MOFs with enhanced properties and functionalities. Major challenges are to characterize the early-stage seeds, and the pathways to framework growth, which require synthesis coupled with in situ structural characterization sensitive to nanoscale structures in solution. Here we report measurements of an in situ synthesis of a prototypical MOF, ZIF-8, utilizing synchrotron X-ray atomic pair distribution function (PDF) analysis optimized for sensitivity to dilute species, complemented by mass spectrometry, electron microscopy, and density functional theory calculations. We observe that despite rapid formation of the crystalline product, a high concentration of Zn(2-MeIm) (2-MeIm = 2-methylimidazolate) initially forms and persists as stable clusters over long times. A secondary, amorphous phase also pervades during the synthesis, which has a structural similarity to the final ZIF-8 and may act as an intermediate to the final product.

The Chemistry of Nucleation: In Situ Pair Distribution Function Analysis of Secondary Building Units During UiO‐66 MOF Formation

“…However, SAXS cannot provide atomic level structural information, and the study only proposed the initial clusters as monodisperse homogeneous spheres, while the internal atomic structure of the cluster remained unknown. Recently, a pioneering in situ PDF study also revealed a high concentration of Zn(2‐MeIm) 4 cluster initially formed, and remained stable over long times before crystallization during ZIF formation . Thus, this present study provides a direct structural evidence of the atomic structure of MOF prenucleation clusters.…”

“…Recently,apioneering in situ PDF study also revealed ah igh concentrationo fZ n(2-MeIm) 4 cluster initially formed,a nd remained stable over long times before crystallization during ZIF formation. [18] Thus, this present study provides ad irect structural evidence of the atomics tructure of MOF prenucleation clusters. Figure 4a shows the evolution of the PDF with respect to time when heatinga t1 50 8C.…”

“…Previously, several studies have used SAXS, EXAFS, ESI‐MS or NMR spectroscopy to demonstrate the formation of multinuclear clusters prior to MOF crystallization ,. For example, in situ SAXS studies of ZIF‐8 have identified the spontaneous formation of clusters with approximate sizes of 20 Å, when mixing the precursor component solution .…”

“…[17] Recently,t he pioneeringw ork by Billinge et al combined multiple techniquess uch as in situ PDF,m ass spectrometry,a nd DFT calculations to reveal the early stage structural development of ap rototypical zeolitic imidazolate framework in solution. [18] These resultss howedt he promise of PDF analysiso f SBU structures, and inspiredt he present in situ study of the solvothermalformation of UiO-66. Since the SBUs in MOFs consist of metal-oxide clusters,w ec hose ar elativelyh eavy metal MOF system to generate strong X-ray scattering.…”

Control of Metal–Organic Framework Crystallization by Metastable Intermediate Pre‐equilibrium Species

“…[9,10] ZIF-8, [Zn(mIm) 2 ](mImH = 2-methylimidazole), shows potential for carbon capture,p ropene/propane separations,a nd dielectric applications [11] and is one of the few MOFs that have been investigated in detail as am odel system for MOF crystallization. [12][13][14][15][16][17][18][19] ZIF-8 nanoparticles were first synthesized by Cravillon et al in 2009, using excess mImH to deprotonate linkers and terminate particle surfaces. [12] Static light scattering revealed slow,c ontinuous nucleation, and fast crystal growth.…”