Controlled Growth of Highly Defected Zirconium-Metal-Organic Frameworks via a Reaction-Diffusion System for Water Remediation

Abstract

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.

Figure 1

(a) Synthetic scheme and crystal structure of UiO-66(OH)₂ indicating the coordination of the zirconium clusters to the 2,5-dihydroxyterephthalic acid ligands. Zirconium clusters are represented in green, carbon atoms in gray, and oxygen atoms in red. Hydrogen atoms have been omitted for clarity. (b) A photograph profile of UiO-66(OH)₂ prepared in a test tube showing the diffusion of the outer electrolyte (zirconium nitrate) into a gel matrix made up of acetic acid, terephthalic acid derivative, and agar gel. Three zones were extracted from the reaction tube. (c) PXRD patterns of the MOF particles extracted from all zones along with their characteristic crystallographic planes. (d) SEM images of the MOF particles extracted from the three reaction zones. Inset histograms represent the MOF particle size distribution. Note that the SEM scale bar is 1 μm.

Figure 2

(a) TGA (left axis, solid lines) and DTG (right axis, dashed lines) curves of UiO-66(OH)₂ particles isolated from the three reaction zones. The bottom horizontal dashed brown line represents the lower end of the theoretical TGA weight-loss plateau, WL_final. The upper horizontal dashed brown line represents the upper end of the theoretical TGA weight-loss plateau, WLP_th. The vertical dashed brown line represents the temperature of the combustion of the linker, T_link. (b) Nitrogen adsorption–desorption isotherms recorded at 77 K for each zone extracted from UiO-66(OH)₂. -●- represents adsorption, and -○- represents desorption. (c) Correlation between zone number and defects, represented by surface area and zone number of UiO-66(OH)₂.

Figure 3

Effect of (a) concentration of the outer electrolyte, (b) concentration of the agar gel, and (c) type of gel on the particle size distribution of UiO-66(OH)₂ particles.

Figure 4

(a) The adsorption capacity of MB removal from water using UiO-66(OH)₂-Z and UiO-66(OH)₂-Sv at different concentrations. (b) Comparison between the defects number and the maximum adsorption capacity of UiO-66(OH)₂-Z and UiO-66(OH)₂-Sv. (c) 100 ppm MB dye uptake capacity onto all tested UiO-66(OH)₂ samples as a function of contact time. (d) UiO-66(OH)₂ samples after MB adsorption (5 min, 100 ppm) displaying the efficiency of the MOF prepared via the reaction–diffusion method in the dye removal from water.