Aqueous synthesis of highly adsorptive copper–gallic acid metal–organic framework

Abstract

A greener route to synthesize mesoporous copper–gallic acid metal–organic framework (CuGA MOF) than the conventional method using harmful DMF solvent was proposed in this study. Various synthesis attempts were conducted by modifying the synthesis conditions to produce CuGA MOF with comparable physical properties to a reference material (DMF-synthesized CuGA NMOF). The independent variables investigated include the molar ratio of NaOH to GA (1.1 to 4.4) and the synthesis temperature (30, 60, 90 °C). It was found that proper NaOH addition was crucial for suppressing the generation of copper oxide while maximizing the formation of CuGA MOF. On the other hand, the reaction temperature mainly affected the stability and adsorption potential of CuGA MOF. Reacting Cu, GA, and NaOH at a molar ratio of 1:1:2.2 and a temperature of 90 °C, produced mesoporous MOF (CuGA 90–2.2) with a surface area of 198.22 m2/g, a pore diameter of 8.6 nm, and a thermal stability of 219 °C. This MOF exhibited an excellent adsorption capacity for the removal of methylene blue (124.64 mg/g) and congo red (344.54 mg/g). The potential usage of CuGA 90–2.2 as a reusable adsorbent was demonstrated by its high adsorption efficiency (> 90%) after 5 adsorption–desorption cycles.

One-step synthesis of nitrogen-grafted copper-gallic acid for enhanced methylene blue removal

“…2 c). The PXRD spectra of N g -CuGA showed a close resemblance to the diffraction spectra of unmodified CuGA 22 , specifically at 2θ = 10.1, 13.2, 20.1, 28.0, 31.1, and 42.8°. N 2 adsorption/desorption analysis was performed to determine the surface area and pore properties of the N g -CuGA.…”

“…For comparison purposes, the reported adsorption capacity of MB on different metal-linker coordination adsorbents is listed in Table 4 . Compared with the CuGA MOF 22 , the adsorption capacity of MB by N g -CuGA was 53.09% higher, indicating the synergistic effect of the N-functional groups from Gly addition. Furthermore, the higher adsorption capacity of N g -CuGA compared to CuGA can be attributed to the immense pore volume and pore diameter, i.e., 0.56 cc/g and 23.25 nm for N g -CuGA and 0.43 cc/g, and 8.6 nm for CuGA.…”

“…Based on the BET calculation, the N g -CuGA has a surface area of 2.00 m 2 /g, with pore volume and average pore diameter of 0.6 cc/g and 23.3 nm, respectively. Compared to the non-grafted CuGA (surface area of 198 m 2 /g, pore volume of 0.4 cc/g, and average pore diameter of 8.6 nm) 22 , the N g -CuGA possess a larger pore size. This phenomenon might be occurred due to the incorporation of small-grafting molecules (i.e., Gly), which may strain the internal pores of CuGA, and eventually caused a significant reduction in the surface area of N g -CuGA 24 , 46 .…”

“…The adsorption was conducted at three different temperatures (30, 40, and 50 °C). After 24 h, the concentration of residual MB at the end of the adsorption was measured using a UV–Vis spectrophotometer (Shimadzu UV 2600) 22 . The adsorption data were plotted as versus and were fitted using two-parameter isotherm models (Langmuir, Freundlich, and Temkin) and three-parameter models (Sips and Redlich–Peterson).…”

“…However, as TA and PDA overdose is postulated to cause adverse effects on human health and the environment 20 , 21 ; therefore, GA was chosen in this work. Besides this toxicity issue, recently, our group reported the potential usage of CuGA as an adsorbent for dye removal 22 ; thus, functionalization of this particular MPN is expected to improve their adsorptivity.…”