Controllable formation of meso- and macropores within metal–organic framework crystals via a citric acid modulator

Abstract

Citric acid (CA) is introduced to modulate the synthesis of octahedral HKUST-1 (also known as Cu₃(BTC)₂, BTC = benzene-1,3,5-tricarboxylate) particles with hierarchical micro, meso- and/or macropores (noted as HP-HKUST-1 particles) under mild conditions. The particle size of the resultant product could be tuned from 300 nm to 2.4 μm by varying the amount of CA added, solvents, reaction time and so on. The intrinsic microporous structure of HKUST-1 is maintained despite the strong chelation capability of CA towards Cu²⁺ while the larger pores’ size is in the range of 30 to 100 nm. The possible formation mechanism of these octahedral HP-HKUST-1 particles is proposed on the basis of our systematic investigations. The resulting micrometer HP-HKUST-1 particle is employed to encapsulate phosphomolybdic acid hydrate (HPMo), a catalyst for the methanolysis of styrene oxide. After encapsulation, a superior catalytic activity with a high conversion rate (99.6%) is achieved when the reaction is carried out at 40 °C for 20 min.