Buffered Coordination Modulation as a Means of Controlling Crystal Morphology and Molecular Diffusion in an Anisotropic Metal–Organic Framework

Abstract

Significant advances have been made in the synthesis of chemically selective environments within metal-organic frameworks, yet materials development and industrial implementation have been hindered by the inability to predictively control crystallite size and shape. One common strategy to control crystal growth is the inclusion of coordination modulators, which are molecular species designed to compete with the linker for metal coordination during synthesis. However, these modulators can simultaneously alter the pH of the reaction solution, an effect that can also significantly influence crystal morphology. Herein, noncoordinating buffers are used to independently control reaction pH during metal-organic framework synthesis, enabling direct interrogation of the role of the coordinating species on crystal growth. We demonstrate the efficacy of this strategy in the synthesis of low-dispersity single-crystals of the framework Co2(dobdc) (dobdc 4− = 2,5-dioxido-1,4-benzenedicarboxylate) in a pH 7-buffered solution using cobalt(II) acetate as the metal source. Density functional theory calculations reveal the anisotropic nature of competitive acetate binding, and by using a series of cobalt(II) salts with carboxylate anions of varying coordination strength, it is possible to control crystal growth along the c-direction. Finally, we use zero length column chromatography to show that crystal morphology has a direct impact on guest diffusional path length for the industrially-important hydrocarbon m-xylene. Together, these results provide molecular-level insight into the use of modulators in governing crystallite morphology and a powerful strategy for the control of molecular diffusion rates within metal-organic frameworks. ASSOCIATED CONTENTSupporting Information. Experimental details, gas adsorption measurements, morphological indexing, DFT, additional crystal characterization, and ZLC details. This material is available free of charge via the Internet at http://pubs.acs.org.

Metal‐Organic Framework (MOF) Morphology Control by Design

“…In that study, cubic crystal morphology (six {100} facets) samples exhibit better catalytic performance than crystals with an octahedral morphology (eight {111} facets) [10] . Typically, MOF crystal morphology control studies include additives and/or altering the pH of the growth medium, and organic linker functionalization [7–19] . For instance, metal coordinating polymers have been used as additives to inhibit crystal growth along selected crystallographic directions and control MOF morphology [14,15] .…”

“…[10] Typically, MOF crystal morphology control studies include additives and/or altering the pH of the growth medium, and organic linker functionalization. [7][8][9][10][11][12][13][14][15][16][17][18][19] For instance, metal coordinating polymers have been used as additives to inhibit crystal growth along selected crystallographic directions and control MOF morphology. [14,15] In analogy to inorganic nanoparticles, [20,21] surfactants have also been used to generate new morphologies of MOFs.…”

Evaluating Solvothermal and Mechanochemical Routes towards the Metal–Organic Framework Mg2(m-dobdc)

“…Other methods previously reported for the preparation of Mg2(dobdc), such as employing Mg(OAc)2•4H2O as a basic Mg precursor, were tested as well. [22][23][24] Characterization of the produced solids by powder X-ray diffraction (PXRD) 25 validated that combining H4m-dobdc and Mg2(NO3)2•6H2O in 1:1 DMF:MeOH (0.03 M) at 120 °C for 48 h was optimal to yield highly crystalline Mg2(m-dobdc)-ST (Figure 3, SI Figure S4). This synthesis can be readily scaled to produce Mg2(m-dobdc)-ST on at least 0.5 g scale (Figure 3, SI Figure S4) and is reproducible as well (SI Figure S15).…”

Cooperative Assembly of 2D‐MOF Nanoplatelets into Hierarchical Carpets and Tubular Superstructures for Advanced Air Filtration

“…[7] The deliberate synthesis of anisotropically figured MOF crystals is not trivial. Nevertheless, many great advancements have been made in this field [8] during recent years mainly due to the increasing scientific interest in advanced synthetic 2D materials beyond graphene. [9,10] One subclass of MOFs, ideally suited for synthesis of materials anisotropic in shape, are intrinsic 2D MOFs, consisting of layers formed by coordination bonds and held together by weak intermolecular interactions.…”