Hierarchically Ordered Macro–Microporous Polyoxometalate-Based Metal–Organic Framework Single Crystals

Abstract

Facile construction of ordered macroporous polyoxometalate-based metal–organic frameworks (POM@MOFs) to break the intrinsic microporous restriction is significant but remains challenging. On one hand, the POMs introduced improve the structural stability and modify the pores of MOFs, e.g., introducing functional catalytic and adsorptive units. Meanwhile, the acidic POMs severely affect the nucleation and growth of the POM@MOFs, resulting in complicated synthesis and difficult assembly control. Herein, a general approach has been developed to fabricate ordered macroporous POM@MOF single crystals, involving close-packed polystyrene (PS) nanosphere templates. The artificially selected polar solvents exerting strong solvent effect with POMs weaken the affinity between POMs and metal ions, thereby effectively stabilizing the precursors from assembly before filling into the PS template interstices. The weak alkaline carboxylate used regulates the in situ nucleation and growth of POM@MOFs through deprotonation of the ligands as well as coordinating modulation, affording a series of hierarchically cuboctahedral POM@MOF single crystals with ordered macropores (ca. 180 nm) and intrinsic micropores after template removal. The ordered macroporous structure and thinned microporous skeleton markedly improve mass diffusion properties, while the integral single-crystal lattice retains superior stability.

Mechanistic Principles for Engineering Hierarchical Porous Metal–Organic Frameworks

“…In several special cases, the capillary condensation and capillary evaporation in mesopores are reversible and no hysteresis loops can be observed in gas physisorption isotherms. This phenomenon typically happens in materials that have pore sizes close to the micropore range or their pore size distributions are broad. , Illustrative examples have been found in macro-/microporous MOFs. , …”

“…(A) Comparison of catalytic oxidation of 2-chloroethyl ethyl sulfide between HP-MOF om -NENU-3a and microporous MOF cm -NENU-3a. Reproduced from ref . Copyright 2021 American Chemical Society.…”

“…21 In this work, the prerequisite to obtain ordered macro-/microporous ZIF-8 single crystals was the utilization of methanol/ammonia−water (1:1 v/v) as solvents for ZIF-8 crystallization within confined spaces, manipulating the crystallization rate of ZIF-8 in the mixed solvent. Following this strategy, ordered macro-/ microporous ZIF-67, 23 Mil-125, 52 and NENU-3a 53 were also fabricated for different applications, highlighting the general applicability of this strategy. More recently, Li et al achieved the synthesis of macro-/microporous ZIF-8 (or ZIF-67 or HKUST-1) single crystals in a three-dimensionally ordered macroporous PS replica (Figure 4F).…”

“…MOFs containing macropores typically have a type II or type III physisorption isotherm with abrupt gas adsorption at high P/P 0 , as observed in ordered macro-/microporous ZIF-8. 21 However, type I isotherms were also found in macro-/microporous NENU-3a 53 and MIL-125. 52 For these cases, the combination of physisorption and SEM/TEM is indispensable to determine the porosity of targeted MOFs.…”

Recent advances in nanoarchitectures of monocrystalline coordination polymers through confined assembly

“…The obtained HKUST-1 crystals contained bimodal or trimodal pores, which facilitated adsorption and enabled a fast Friedländer reaction. When the networks were formed by densely packed monodispersed polystyrene spheres, ordered macropores could be created inside ZIF-8 single crystals or other coordination polymers ( Figure 5 ) [ 117 – 118 ]. The inverse opal single crystals presented higher catalytic activity than microporous single crystals.…”