Adsorption Capacity Of UiO-66 Research Articles

Heating-induced adsorption promoting the efficient removal of toluene by the metal-organic framework UiO-66 (Zr) under visible light

The removal of indoor/outdoor toluene by photocatalysis has drawn much attention due to its low energy consumption and easy availability. However, light inevitably generates heat, and pollutants desorb from catalysts as the temperature rises, which is not beneficial to degradation. Contrast to the frequently occurred phenomena, we firstly found that the adsorption capacity of UiO-66 (Zr) on toluene increased with increasing temperature as adsorption isotherms and in-situ Fourier transform infrared spectra (in-situ FTIR) showed. The optimum temperature was 30 °C. This stage in which adsorption capacity was positively correlated with temperature was called heating-induced adsorption, which achieved a toluene removal efficiency of 69.6 %. By density functional theory (DFT) calculations and changing the metal centers and organic ligands of UiO-66 (Zr) respectively, we disclosed that the heating-induced adsorption was mainly related to the π-π stacking interaction of MOF ligands and toluene. The analysis of samples before and after adsorption showed that the interaction between UiO-66 (Zr) and adsorbed toluene facilitated the charge transfer and prolonged the carrier lifetime, leading to the increase of hydroxyl radicals (•OH) in photocatalysis. Therefore, a synergistic effect between heating-induced adsorption and photocatalysis was proposed by analyzing the adsorption of toluene on UiO-66 (Zr) in detail. This work provided new viewpoint to understand the role of concomitant heat contributed to the adsorption and degradation of toluene during photocatalysis.

Acid promoted activity of UiO-66 as an efficient adsorbent for boron removal from aqueous solution

It is a meaningful study to develop adsorbents with high adsorption capacity and fast adsorption rate for the removal of specific ions in aqueous solution. This paper mainly studies the removal of boron in aqueous solution by UiO-66. Regular octahedral structured UiO-66(HAC) via acetic acid-promoted was synthesized by a hydrothermal method. The boron adsorption kinetics, isotherms, thermodynamics, mechanism, and recycling on UiO-66(HAC) were further investigated in batch adsorption process, which found that adsorption capacity of UiO-66(HAC) is enhanced by 40% compared with UiO-66. UiO-66(HAC) exhibits great adsorption performance of 79.43 mg·g−1 at 25 °C and the adsorption process reaches equilibrium rapidly in 2 h. The selective adsorption of boron by UiO-66(HAC) is due to the presence of a large amount of Zr-OH which can chemically adsorb with boron. Compared with traditional boron removal adsorbents, UiO-66(HAC) has higher boron adsorption capacity and is easy to regenerate. It is a promising boron removal adsorbent in aqueous solution.

Boosted capture of volatile organic compounds in adsorption capacity and selectivity by rationally exploiting defect-engineering of UiO-66(Zr)

The great challenge in capturing harmful volatile organic compounds (VOCs) using Metal-Organic-Frameworks (MOFs) materials still rely on rationally modifying its appreciable microporous character in hydrostable structure. Defected structure in MOFs might be very beneficial for creating expected engineering adsorbent. In this study, defect-engineering UiO-66 via imparting acetic acid (HAc) as an interfering agent was successfully fabricated by a simple hydrothermal method. The physicochemical properties of the obtained materials were intensively ascertained and identified through a series of applicable characterizations. By the proper participation of tuning concentration of HAc (HAc/TAc = 24, molar ratio), the obtained adsorbent UiO-66–1.0HAc was endowed with the highest adsorption capacity for benzene (367.13 mg g−1, 25 °C) in the known UiO-66 materials, which was 41.9% higher than that of the non-defected UiO-66. With a higher optimized added amount of HAc (HAc/TAc = 48, molar ratio), the adsorption capacity of UiO-66–2.0HAc to toluene achieved the highest capture values (410.21 mg g−1, 25 °C), which boosted 93% compared with the original counterpart. This demonstrated improvement was ascribable to the imparted structural defect-engineering derived from the missing-linkers of MOFs increasing the unsaturated adsorption site and concomitant enlarged structural properties of the resultant material. By identifying the DIH (Difference in adsorption heat), the obtained maximum adsorption selectivity of UiO-66–1.0HAc to toluene/water was further enhanced to 64.4, which was 3.7 times the original UiO-66, owing to the existed defect-engineering in adsorbents, indicating the potential differential competitive adsorption effect for benzene and toluene under humid conditions.

On the general water harvesting capability of metal-organic frameworks under well-defined climatic conditions

The ability of some highly porous materials to adsorb and desorb water vapor under atmospheric conditions without the use of external power sources could become a promising methodology for the capture and release of water in arid or desert regions of the world.Here, the use of some readily available metal-organic frameworks as water harvesting materials has been tested and compared with other porous materials. Water adsorption experiments were conducted at 25 °C and at a relative humidity (RH) of 40%, while the desorption process took place at 45 °C with a RH of 10%. These experimental conditions are in good accordance with the climatic conditions being present in the Sahara desert during the summer season.In all experiments, UiO-66 derived MOFs showed the best water harvesting efficiency, releasing up to 89% of the adsorbed water at 45 °C and 99% at 60 °C. Though, the adsorption capacity of UiO-66 is noticeably lower than that of more hydrophilic materials such as the MOF compound HKUST-1 and the zeolite A3. The latter, however, are only able to release water when they are heated up to at least 60 °C.Finally, the stability of the investigated MOFs after adsorption-desorption cycles have been confirmed through XRD and FT-IR analyses.

Adsorptive removal of methylchlorophenoxypropionic acid from water with a metal-organic framework

For the first time, the adsorptive removal of methylchlorophenoxypropionic acid (MCPP) from water, using a metal-organic framework (MOF), namely Zr-benzenedicarboxylate (UiO-66), was investigated to determine the applicability of MOFs in the removal of hazardous herbicides/pesticides from contaminated water. Compared with activated carbon, UiO-66 has a very high adsorption rate (kinetic constant ∼30 times that of activated carbon). This rapid adsorption is remarkable because the pore size of UiO-66 is smaller than that of activated carbon. Moreover, the adsorption capacity of UiO-66 is higher than that of activated carbon especially at low MCPP concentrations (∼7.5 times at 1ppm of MCPP). These rapid and high uptakes by UiO-66 suggest that there is a special mechanism for interactions between MCPP and UiO-66. Additionally, the adsorbent can be reused for adsorptive removal by washing the spent adsorbent with a simple solvent. MOFs such as UiO-66 are therefore potential adsorbents for use in the adsorptive removal of MCPP from contaminated water. A plausible adsorption mechanism is suggested based on the effects of pH on the zeta potential of the adsorbent and on adsorption. For the adsorption of MCPP by UiO-66, electrostatic and π–π interactions might both be important.