Kinetic control of metal-organic framework crystallization investigated by time-resolved in situ X-ray scattering.

Metal–organic frameworks (MOFs) are among the most sophisticated nanostructured solids: they often possess high surface areas and pore volumes, with the possibility of finetuning their chemical environment by either selecting the appropriate building blocks or by postsynthetic functionalization. For many frameworks, flexibility of the lattice allows them to undergo a significant transformation in solid state. All these features make MOFs a special class of solids with the potential of transcending many common limitations in different technological disciplines, such as ferromagnetism, semiconductivity, gas separation, storage, sensing, catalysis, drug delivery, or proton conductivity. However, the crystallization mechanism of these complex structures is far from understood. Notwithstanding the plethora of publications that present new MOFs, and the effectiveness of the high-throughput approach, serendipity still governs the synthesis of new structures. Understanding how these materials are assembled will ultimately enable the rational design of new generations of MOFs targeting specific desired topology and properties. Surprisingly, only a small number of crystallization studies on the synthesis of different prototypical MOFs have been reported to date, most notably using X-ray absorption, dynamic light scattering, atomic force spectroscopy, and X-ray diffraction. More recently, Millange and co-workers reported the first in situ diffraction study on the crystallization of different MOFs (CuBTC, MIL-53(Fe), and MOF-14) under hydrothermal conditions. In the latter, the authors emphasized the importance of in situ methods, the necessity of tackling more complex MOF systems, and the use of combined techniques that allow crystallization to be followed over several length scales. The diffraction data provides information about crystalline phases; however, important primary processes, such as reactions occurring in solution or gel formation stages and nucleation, cannot be directly probed. In this work, we report the first in situ combined smalland wide-angle scattering (SAXS/WAXS) study on the crystallization of two topical metal–organic frameworks synthesized from similar metal and organic precursors, NH2MIL-101(Al) and NH2-MIL-53(Al). [19] These two structures differ in the connectivity of the metal nodes and organic linkers: The former contains supertetrahedral (ST) building units formed by aminoterephthalate ligands and trimeric Al octahedral clusters, whereas the latter consists of AlO4(OH)2 octahedra connected by the same linker. X-ray scattering was shown to be an indispensible tool for studying the synthesis process of zeolites and zeotypes, mesoporous materials, nanoparticles and colloids, and interfaces: This approach is especially valuable when combined with other methods such as XRD, NMR, X-ray absorption, and Raman spectroscopy. The scattering patterns recorded during the formation of NH2-MIL-101(Al) at 403 K are shown in Figure 1a,b. Scattering at low Q (< 1 nm ) develops immediately with the start of the heating well before the formation of most Bragg peaks. The diffraction pattern that was obtained corresponds to the NH2-MIL-101 structure (Fd3̄m, cubic, a= 88.87 ; [18] Figure 1c). Low Q scattering is likely to be due to the formation of amorphous primary particles which subsequently assemble into the crystalline structures. Remarkably, the reflection at Q= 6.3 nm , corresponding to a d spacing of 9.7 , develops at first. It exhibits a high multiplicity factor of 72: 24 sets of {119} planes and 48 sets of {357} planes contribute to this reflection. From the FWHM of the reflections, the size of the first crystallites was estimated to be about 60 nm in size, reaching about 90 nm after 2500 s. Intensity at low Q remains constant during later phase of the synthesis owing to the cumulative scattering by particles of different sizes and due to scattering by imperfectness of the crystals (such as defects and voids). Figure 1d shows normalized crystallization curves produced by integration of the Bragg peak atQ= 2.4 nm 1 (plane 357) recorded at different temperatures. Analysis of the kinetic profiles was performed using the model developed by Gualtieri and applied for the MOF formation. This model (see SI) allows decoupling the nucleation and crystal [*] Dr. E. Stavitski National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 (USA) E-mail: [email protected]