Evolution of MOF single crystals

The mechanism of MOF crystal growth is fundamentally important to their design. In this issue of Chem, Deng and co-workers use an optical method to visualize and track the crystal interface during MOF growth. They derive a reaction kinetic equation for individual crystals of MOFs, thereby offering guidance to MOF synthesis. The mechanism of MOF crystal growth is fundamentally important to their design. In this issue of Chem, Deng and co-workers use an optical method to visualize and track the crystal interface during MOF growth. They derive a reaction kinetic equation for individual crystals of MOFs, thereby offering guidance to MOF synthesis. Reticulation of metal ions and charged organic units has led to ultraporous metal-organic framework (MOF) crystals.1Yaghi O.M. O’Keeffe M. Ockwig N.W. Chae H.K. Eddaoudi M. Kim J. Reticular synthesis and the design of new materials.Nature. 2003; 423: 705-714Crossref PubMed Scopus (8129) Google Scholar,2Férey G. Mellot-Draznieks C. Serre C. Millange F. Dutour J. Surblé S. Margiolaki I. A chromium terephthalate-based solid with unusually large pore volumes and surface area.Science. 2005; 309: 2040-2042Crossref PubMed Scopus (4244) Google Scholar The molecularly defined pores give rise to the utility of MOFs in a plethora of applications, such as CH4 storage,3Jiang J. Furukawa H. Zhang Y.-B. Yaghi O.M. High methane storage working capacity in metal−organic frameworks with acrylate links.J. Am. Chem. Soc. 2016; 138: 10244-10251Crossref PubMed Scopus (230) Google Scholar H2O capture from air,4Hanikel N. Pei X. Chheda S. Lyu H. Jeong W. Sauer J. Gagliardi L. Yaghi O.M. Evolution of water structures in metal-organic frameworks for improved atmospheric water harvesting.Science. 2021; 374: 454-459Crossref PubMed Scopus (138) Google Scholar and CO2 capture and conversion,5Choi K.M. Kim D. Rungtaweevoranit B. Trickett C.A. Barmanbek J.T.D. Alshammari A.S. Yang P. Yaghi O.M. Plasmon-enhanced photocatalytic CO2 conversion within metalorganic frameworks under visible light.J. Am. Chem. Soc. 2017; 139: 356-362Crossref PubMed Scopus (435) Google Scholar,6Jiang Z. Xu X.H. Ma Y.H. Cho H.S. Ding D. Wang C. Wu J. Oleynikov P. Jia M. Cheng J. et al.Filling metal-organic framework mesopores with TiO2 for CO2 photoreduction.Nature. 2020; 586: 549-554Crossref PubMed Scopus (339) Google Scholar making MOFs a new field in chemistry and materials science. Currently, the MOF family extends to more than 100,000 structures, some of which have already been produced at the ton scale. Although much progress in design and crystallization of MOFs is continually being made, understanding the mechanism for their formation remains less developed. The challenge is how to determine the impact of reaction conditions on the reticulation process and how to deconvolute each of these conditions to help understand their role in MOF crystal growth. Since the establishment of correlation between reaction rate and reactant concentration by Van’t Hoff, chemical kinetics studies played critical role in unraveling the mechanism of various chemical reactions. This classic tool is elegantly used in the study of MOF crystal growth as illustrated by Deng and co-workers in this issue of Chem.7Han J. He X. Liu J. Ming R. Lin M. Li H. Zhou X. Deng H. Determining factors in the growth of MOF single crystals unveiled by in situ interface imaging.Chem. 2022; 8: 1637-1657https://doi.org/10.1016/j.chempr.2022.03.006Abstract Full Text Full Text PDF Scopus (13) Google Scholar A flow cell was designed to offer precise control of temperature and concentration of the reactants, providing solid basis for systematically examining the crystal formation kinetics. In contrast to a sealed reaction system, the use of flow cell ensures the steady growth of MOF crystal and achieves a linear growth curve with nanometer accuracy. In this way, the reaction rate can be directly determined from the linear growth curve as the slope. The change of slope reflects the increase or decrease in the reaction rate when the temperature or the concentration of the reactant is varied systematically. An isolation method was applied, whereby only one variable is changed at a time. This allowed for the extraction of apparent reaction orders of metal ions and organic linkers separately. It is worth noting that an optical method is used in this work, in contrast to atomic force microscopy (AFM) or transmission electron microcopy (TEM) that would promise higher resolution.8Hosono N. Terashima A. Kusaka S. Matsuda R. Kitagawa S. Highly responsive nature of porous coordination polymer surfaces imaged by in situ atomic force microscopy.Nat. Chem. 2019; 11: 109-116Crossref PubMed Scopus (61) Google Scholar, 9Patterson J.P. Abellan P. Denny M.S. Park C. Browning N.D. Cohen S.M. Evans J.E. Gianneschi N.C. Observing the growth of metal-organic frameworks by in situ liquid cell transmission electron microscopy.J. Am. Chem. Soc. 2015; 137: 7322-7328Crossref PubMed Scopus (169) Google Scholar, 10Xing J. Schweighauser L. Okada S. Harano K. Nakamura E. Atomistic structures and dynamics of prenucleation clusters in MOF-2 and MOF-5 syntheses.Nat. Commun. 2019; 10: 3608Crossref PubMed Scopus (60) Google Scholar The special optic geometry of dark-field microscopy (DFM) beams the focus on the interface of the crystal rather than the bulk. This combined with the nondestructive nature of light and fast camera rate, 50 Hz, makes it ideally suited for the in situ tracking of interface during crystal growth in solution phase. A super-resolution/super-line technique is also developed to obtain the progression profile of interface with a trace resolution of 4 nm, sufficient to give an accurate reaction rate. The growth of individual crystals in the same camera view is monitored simultaneously, through which the reaction order of metal ion and organic linker, as well as activation energy, for each crystal are derived. Collective analysis of multiple crystals gave the data for several MOFs, each representing a different class of pores: Cu-MOF-74 (1D), Co-ZIF, Cu-MOF-2-BDC, Cu-MOF-2-NDC (2D), and HKUST-1 (3D). It is counterintuitive that, for all five MOFs, the reaction orders for metal ions and organic linkers are independent of the corresponding MOF chemical formulae; some are integers, but others are non-integers or negative. This means the conventional practice in MOF synthesis, where variations using the ratio of metal and linker in MOF structure, does not necessarily yield a suitable crystallization condition. The negative reaction order obtained for organic linker in the growth of HKUST-1 crystal is intriguing. It doesn’t mean that the addition of linkers inhibits crystal growth, but that excess linker will slow down the reaction, instead of accelerating it as commonly expected. The independence of the reaction rate from the chemical formula of MOF also indicates that the metal ions and organic linkers play different roles in the crystal growth process. With the experimental reaction orders in hand, the authors discussed the possible crystal growth mechanism as the most fascinating part of this work. A useful concept developed for the structure design of MOF is secondary building units (SBUs).1Yaghi O.M. O’Keeffe M. Ockwig N.W. Chae H.K. Eddaoudi M. Kim J. Reticular synthesis and the design of new materials.Nature. 2003; 423: 705-714Crossref PubMed Scopus (8129) Google Scholar The underlying geometry of the molecular building blocks and the directionality of their linkages in SBU dictate the overall topology of the resulting MOF. A recent study showed the presence of SBUs at the nucleation stage using TEM.10Xing J. Schweighauser L. Okada S. Harano K. Nakamura E. Atomistic structures and dynamics of prenucleation clusters in MOF-2 and MOF-5 syntheses.Nat. Commun. 2019; 10: 3608Crossref PubMed Scopus (60) Google Scholar However, a detailed picture of how the SBUs assemble at the crystal interface to grow into large single crystals remains unknown. The reaction orders measured by the current interface optical imaging work indicates that the formation of MOF crystal is unlikely to be an elementary reaction, but rather a multi-step process involving both the linkage and dissociation of the coordination bond between molecular building blocks. Taking 2D MOFs Cu-MOF-2-BDC and -NDC as examples, the possibilities of building block-by-building block addition and direct accumulation of Cu2L4 SBU (L represents the organic linker) on crystal interface are ruled out because neither of the derived reaction orders from the above scenarios matches with the experimental data. A new mechanism is proposed involving the formation of Cu2L4 SBU and its fragmentation into CuL2, giving a satisfactory match to the experimental reaction orders of both metal ion and organic linker (Figure 1). In this mechanism, the Cu2L4 SBU is a chemically more stable species, while the CuL2 growth unit is relatively more reactive and thus responsible for the MOF growth. The division of Cu2L4 is a critical step, providing fast equilibrium between these two species, hence generating sufficient CuL2 as the growth units for the following rate-determining step, where CuL2 combined with Cu2+ to join the active growth of MOF crystal interface (CuL)n. There surely are other possible mechanisms; this proposed new one offers the possibility that the SBU might be an important intermediate, but a different species might exist as the actual growth unit in the formation of MOF single crystal. Another interesting observation is the retreat and recovery of the crystal interface upon interruption, which can be associated with the accumulation and dissociation of the molecular building blocks. This further supports the multi-step nature of the crystal growth and points out the possible existence of a transition layer with structure and composition different from that of the crystal bulk (Figure 1). The stability of this reversible transition layer is also different from the crystal core, where the transition layer rather is dissolvable in the flow of pure solvent but not the crystal core. The thickness of the transition layer was measured and found to be dependent on the crystal size. It decreased as the crystal grew larger. The discovery of the transition layer shows not only that the temperature and reactant concentration are the determining factors for MOF crystal growth but also that the crystal interface itself is another critical one. This transition layer is reversible and stands as a bridge between the assembly of the molecular building blocks and the formation of crystalline bulk of MOF. The determining factors unveiled here fill in a missing piece in the crystal growth of MOFs. These studies not only help to elucidate the MOF crystal interface behavior at molecular level but are also likely to further guide the industrial production of MOFs. The nondestructive optical methods in combination with super-line technique developed in this study may very well prove suitable for the mechanistic examination of other important crystals in solution. The author declares no competing interests. Determining factors in the growth of MOF single crystals unveiled by in situ interface imagingHan et al.ChemMarch 28, 2022In BriefCrystal interface evolution is monitored by super-resolution technique during the growth of small MOF crystals into larger ones, in solution. The reaction orders for metal ion and organic linker are determined by isolation method and found to be independent from the corresponding MOF chemical formulas. This leads to the proposal of a new mechanism for MOF growth, involving the assembling and fragmentation of secondary building units, followed by fragment accumulation in a reversible transition layer at the interface of the MOF crystal. Full-Text PDF