Template-free synthesis of hierarchical nanocrystal UiO-66 and its adsorption thermodynamics for n-heptane and methyl cyclohexane

Abstract

Hierarchical nanocrystal UiO-66 was synthesized by a template-free double solvent-modulated crystallization method. The influences of the ammonium hydroxide (NH₄OH)/Zr and acetic acid (HAC)/Zr ratios and pH of reaction solution on the morphological and the porous properties of UiO-66 were investigated. The results show that the nanoscale grains of UiO-66 with the size of ∼20 nm and interparticle mesopores of ∼30 nm are obtained with the NH₄OH/Zr of 0.96–3.84, HAC/Zr of 50 and the pH from 1.68 to 2.70. The introduction of NH₄OH is favorable to the formation of Zr⁴⁺ oxo/hydroxo clusters and deprotonation of the ligands for accelerating the nucleation and crystallization, while HAC can effectively control the hydrolysis of zirconium chloride to avoid the formation of particles with low crystallinity. The equilibrium adsorption capacities of nHEP and MCH on UiO-66(2.88, 50) are higher than that on UiO-66-AS at 323–373 K, respectively. The effective diffusion coefficients of n-heptane/methyl cyclohexane on the hierarchical UiO-66 nanocrystals are about 5 and 18 times higher than that on microporous UiO-66 at 373 K and 0.05–6 kPa, respectively. The introduction of mesopores increases the adsorption performance of the adsorbent for alkanes and enhances mass transfer.