Perspectives on the Influence of Crystal Size and Morphology on the Properties of Porous Framework Materials

Abstract

Miniaturization is a key aspect of materials science. Owing to the increase in quality experimental and computational tools available to researchers, it has become clear that the crystal size and morphology of porous framework materials, including metal-organic frameworks and covalent organic frameworks, play a vital role in defining the physicochemical behaviour of these materials. However, given the multiscale and multidisciplinary challenges associated with establishing how crystal size and morphology affect the structure and behaviour of a material–from local to global structural modifications and from static to dynamic effects–a comprehensive mechanistic understanding of size and morphology effects is missing. Herein, we provide our perspective on the current state-of-the-art of this topic, drawn from various complementary disciplines. From a fundamental point of view, we discuss how controlling the crystal size and morphology can alter the mechanical and adsorption properties of porous framework materials and how this can impact phase stability. Special attention is also given to the quest to develop new computational tools capable of modelling these multiscale effects. From a more applied point of view, given the recent progress in this research field, we highlight the importance of crystal size and morphology control in drug delivery. Moreover, we provide an outlook on how to advance each discussed field by size and morphology control, which would open new design opportunities for functional porous framework materials.

The importance of crystal size for breathing kinetics in MIL-53(Al)

“…31 In general, crystal size and morphology strongly influence flexibility in MOFs, showing general trend towards rigidification of the framework with reducing crystal size. 32 A similar trend was observed in MIL-53(Al) by Kundu et al 33 who designed the crystal size of MOF by controlled nucleation in water/DMF mixtures. Recently, size-dependent flexibility has been addressed using in silico techniques.…”

Computational Study of Two Three-Dimensional Co(II)-Based Metal–Organic Frameworks as Quercetin Anticancer Drug Carriers

“…The size and morphology of MOF particles have been controlled using different strategies, including change in solvent, modulator, temperature, reaction time, pH, metal to ligand ratio, microwave-assisted, and sonochemical synthetic techniques. Controlled modifications of the morphology and size of MOFs have been achieved through various methods in a wide range of applications including semiconductors, catalysts, gas storage, and separation. − The size and morphology of MOF particles influence physicochemical properties (material stability and adsorption and diffusion processes) and play a vital role in drug delivery by affecting the carrier biodistribution, drug release kinetics, cell internalization, and the clearance of the drug in complex biological media. − …”

Kinetics of Guest-Induced Structural Transitions in Metal–Organic-Framework MIL-53(Al)-NH2 Probed by High-Pressure Nuclear Magnetic Resonance

“…Beyond particle size, the morphology of a MOF impacts not only material physicochemical properties, but also pharmacokinetics. 11,15 An energetic perspective on correlations between crystal size and phase transition has been given by Sakata et al 16 For smaller ("downsized") crystals, the phase transition (e.g., from open pore to closed pore) is suppressed. Two possible mechanisms have been discussed: either thermodynamically, due to a too small free energy difference between the open and the closed pore state, or respectively kinetically, due to a too high activation barrier between the two states.…”

“…Two possible mechanisms have been discussed: either thermodynamically, due to a too small free energy difference between the open and the closed pore state, or respectively kinetically, due to a too high activation barrier between the two states. 11,16 To gain deeper understanding into transition mechanisms, the application of experimental techniques allowing to obtain local information will be very insightful. The main objectives of this work are to exploit NMR relaxometry as noninvasive tool to gain deeper insight into pore opening and closing processes in MOFs.…”

“…In the second scenario, there is a distribution of the critical pore opening pressures from crystal to crystal, due to, for example, a distribution of the crystal sizes as discussed in the literature. 11 In order to experimentally verify one of these mechanisms, we have performed so-called scanning experiments, in which the pressure reversal was done upon incomplete transformation of the sample. It has been discusssed earlier in the context of the first-order phase transitions that, in the latter case (scenario 2), the scanning behavior mimics the shapes of the boundary transitions.…”