Crystal Facet Engineering of Copper-Based Metal–Organic Frameworks with Inorganic Modulators

Manipulating the exposed facets of metal–organic frameworks (MOFs) is of importance toward understanding their facet-dependent property in a variety of applications. Herein, we apply a novel inorganic competitive coordination strategy to control the growth orientation of copper-based MOFs (HKUST-1, MOF-14, and Cu-MOF-74) without sacrificing the pore accessibility and crystallinity. Through monitoring the reactant composition, we find that the competitive coordination induced by the added aluminium nitrate mainly affects the crystal growth stage rather than the nucleation stage. The kinetic study further reveals that Al3+ competes with Cu2+ to coordinate with ligands, restraining the growth rate of certain facets and resulting in the orientated growth of copper-based MOFs. Compared to the reduced pore accessibility of HKUST-1 crystals modulated by the organic modulation method, Al3+-modulated HKUST-1 displays a much larger surface area (>2200 m2/g) and more accessible Cu active sites. Hydroxylation of toluene was utilized as a model reaction to investigate the facet-catalytic activity for as-synthesized HKUST-1. The selectivity of the preferred product cresol increases with the morphology transformation of HKUST-1 from octahedron to cube.