The relentless growth of metal-organic framework (MOF) chemistry is paralleled by the persistent urge to control the MOFs physical and chemical properties. While this control is mostly achieved by solvothermal syntheses, room temperature procedures stand out as more convenient and sustainable pathways for the production of MOF materials. Herein, a novel approach to control the crystal size and defect numbers of a dihydroxy-functionalized zirconium-based metal-organic framework (UiO-66(OH)2) at room temperature is reported. Through a reaction-diffusion method in a 1D system, zirconium salt was diffused into an agar gel matrix containing the organic linker to form nanocrystals of UiO-66(OH)2 with tailored structural features that include crystal size distribution, surface area, and defect number. By variation of the synthesis parameters of the system, hierarchical MOF nanocrystals with an average size ranging from 30 nm up to 270 nm and surface areas between 201 and 500 m2 g-1 were obtained in a one-pot synthetic route. To stress the importance of crystal size, morphology, and structural defects on the adsorption properties of UiO-66(OH)2, the adsorption capacity of the MOF toward methylene blue dye was tested with the largest and most defected crystals achieving the best performance of 202 mg/g. The distinctive structural characteristics including the hierarchical micromesoporous frameworks, the nanosized particles, and the highly defective crystals obtained by our synthesis procedure are deemed challenging through the conventional synthesis methods. This work paves the way for engineering MOF crystals with tunable physical and chemical properties, using a green synthesis procedure, for their advantageous use in many desirable applications.
Keywords: Crystal growth; Zr-MOFs; adsorption; defects control; metal−organic frameworks; methylene blue; reaction diffusion process.