Tuning the gate-opening pressure and particle size distribution of the switchable metal-organic framework DUT-8(Ni) by controlled nucleation in a micromixer

Controlled nucleation in a micromixer and further crystal growth were used to synthesize Ni₂(2,6-ndc)₂dabco (2,6-ndc-2,6-naphthalenedicarboxylate, dabco-1,4-diazabicyclo[2.2.2]octane), also termed DUT-8(Ni) (DUT = Dresden University of Technology), with narrow particle size distribution in a range of a few nm to several μm. The crystal size was found to significantly affect the switching characteristics, in particular the gate opening pressure in nitrogen adsorption isotherms at 77 K for this highly porous and flexible network. Below a critical size of about 500 nm, a type Ia isotherm typical of rigid MOFs is observed, while above approximately 1000 nm a pronounced gating behaviour is detected, starting at p/p₀ = 0.2. With increasing crystal size this transition gate becomes steeper indicating a more uniform distribution of activation energies within the crystal ensemble. At an intermediate size (500-1000 nm), the DUT-8(Ni) crystals close during activation but cannot be reopened by nitrogen at 77 K possibly indicating monodomain switching.