Template-free synthesis to micro-meso-macroporous hierarchy in nanostructured MIL-101(Cr) with enhanced catalytic activi
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Published online 24 September 2020 | https://doi.org/10.1007/s40843-020-1435-3
Template-free synthesis to micro-meso-macroporous hierarchy in nanostructured MIL-101(Cr) with enhanced catalytic activity 1,5†
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Tian Zhao , Songhe Li , Yu-Xuan Xiao , Christoph Janiak , Ganggang Chang , Ge Tian and 1,2,3* Xiao-Yu Yang Hierarchical porous (HP) materials [1,2], having multimodal pore-size distribution, receive keen attention due to advantages offered by improved diffusion efficiency and mass transport. HP metal-organic frameworks (HPMOFs) is an emerging field focussing on delivering improved performance in catalysis [3,4], adsorption/ separation [5,6], sensorics [7] and energy storage [8,9], especially when bulky guest molecules are involved. Micropores usually offer high surface area while mesopores/ macropores will significantly enlarge the application as a host material to carry bulky anchoring molecular catalysts, large drug molecules and even nanomaterials. This is indeed an advantage for catalysis reaction or transformation without transportation limitations in a confined space. Some MOFs with the large pore-sizes of over 2 nm diameter are formally micro-/mesoporous in the defectfree crystalline state. Isoreticular expansion [10] allows synthesizing MOF materials with pore sizes of up to 10 nm [11]. Aside from those special cases of fragile and expensive MOFs, MOF materials of general importance could be synthesized in HP form using a templating method [12,13], post-synthetic modification/etching [14– 16], co-ligands/modulators [17] or kinetically controlled precipitation [18]. Most of the used methods are material specific, and the attainment of trimodal micro-/meso-/ macroporosity is still rarely reported. MIL-101(Cr) (MIL = Materials Institute Lavoisier) is
one of the few MOFs having real prospects for practical applications [19]. It possesses spacious pores (maximum diameter is ~34 Å, maximum entrance diameter is ~15 Å) 2 −1 and high surface area (up to ~4000 m g ). Due to kinetic inertness of Cr(III), MIL-101(Cr) is one of the most chemically and hydrothermally stable MOFs, particularly promising for catalysis at elevated temperatures [20]. The targeted micro-/meso-/macroporous hierarchical porosity is scarcely reported for MIL-101(Cr). In general, MIL-101 (Cr) is synthesized at relatively high temperature (220°C) and sensitive to change of conditions. The efficient softtemplating strategy is hardly applicable at such temperatures, which demand special surfactants [21]. The universally applicable hard-templating is one possibility for creating hierarchical porosity, and it was demonstrated that organic polymers could form composites with MIL-101(Cr) [22,23]. Nevertheless, the straightforward and economical template-free approach is much more attractive. The use of modulators, which are typically competing monodentate, terminal ligands, is a very general way of influencing the properties of MOF materials. Our screening of different modulators in the synthesis of MIL101(Cr) indicated a formatio
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