Method for preparing acid-resistant Zr-MOF material with selective adsorption of hexavalent chromium

Abstract

Disclosed is a method for preparing an acid-resistant Zr-metal-organic framework (Zr-MOF) material with selective adsorption of hexavalent chromium, which is specifically a method for preparing a MOF material with selective adsorption of hexavalent chromium. The present invention aims to solve the technical problems that the existing MOFs have poor water stability, is unstable in an aqueous solution, and therefore may structurally collapse. In addition, the micropore characteristics of most MOFs have affected the mass transfer rate of the adsorbate, which in turn limits the industrial application thereof. The MOF of the present invention has excellent water stability and acid resistance, showing a strong adaptability and great potential in treating mostly acidic industrial wastewater. As a new functional material for water treatment, the MOF has optimal selective adsorption capacity and a desirable adsorption capacity for hexavalent chromium in water, which improves the recycling and later resource utilization.

Claims

1. A method for preparing an acid-resistant Zr-metal-organic framework (Zr-MOF) material with selective adsorption of hexavalent chromium, wherein the method for preparing an acid-resistant Zr-MOF material with selective adsorption of hexavalent chromium is carried out according to the following steps: thoroughly mixing a carboxylic acid ligand containing a triazine group and a zirconium salt together in an organic solvent, adding formic acid thereto, then reacting same under closed conditions at 80-150 C. for 12-72 h, subjecting the product to solid-liquid separation, then drying the solid to obtain a colorless transparent single crystal which is the Zr-MOF; wherein the carboxylic acid ligand containing a triazine group is 2,4-bis(3,5-dicarboxyphenylamino)-6-ol triazine; a molar ratio of the carboxylic acid ligand containing a triazine group to the zirconium element in the zirconium salt is 1:(0.5-2); a ratio of the mole of the carboxylic acid ligand containing a triazine group to the volume of the formic acid is 1 mmol:(100-110 mL); a volume ratio of the formic acid to the organic solvent is 1:(0.5-1.5).

2. The method for preparing an acid-resistant Zr-MOF material with selective adsorption of hexavalent chromium according to claim 1, wherein the zirconium salt is one or a mixture of several of ZrCl.sub.4, ZrOCl.sub.2.8H.sub.2O, Zr(NO.sub.3).sub.4 and Zr(SO.sub.4).sub.2.

3. The method for preparing an acid-resistant Zr-MOF material with selective adsorption of hexavalent chromium according to claim 1, wherein the organic solvent is one or a mixture of several of C.sub.2H.sub.5OH, CH.sub.3OH, DMF, DMA and DMSO.

4. The method for preparing an acid-resistant Zr-MOF material with selective adsorption of hexavalent chromium according to claim 1, wherein the drying is conducted at 65 C.

5. The method for preparing an acid-resistant Zr-MOF material with selective adsorption of hexavalent chromium according to claim 1, wherein the solid-liquid separation is done by means of suction filtration.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a schematic diagram of the structure of a carboxylic acid ligand containing a triazine group in Experiment 1;

(2) FIG. 2 is a crystal morphology of the Zr-MOF prepared in Experiment 1 under a microscope with a 40 times magnification;

(3) FIG. 3 is an SEM of the Zr-MOF material prepared in Experiment 1;

(4) FIG. 4 is a TEM image of the Zr-MOF material prepared in Experiment 1;

(5) FIG. 5 is an XRD image of the Zr-MOF prepared in Experiment 1;

(6) FIG. 6 is an XRD image in Experiment 2; and

(7) FIG. 7 is a graph of the adsorption experimental data in Experiment 3.

DETAILED DESCRIPTION

(8) Specific Implementation 1: this implementation relates to a method for preparing an acid-resistant Zr-MOF material with selective adsorption of hexavalent chromium, and in particular, the method is carried out according to the following steps:

(9) thoroughly mixing a carboxylic acid ligand containing a triazine group and a zirconium salt together in an organic solvent, adding formic acid thereto, then reacting same under closed conditions at 80-150 C. for 12-72 h, subjecting the product to solid-liquid separation, then drying the solid to obtain a colorless transparent single crystal which is the Zr-MOF;

(10) where the carboxylic acid ligand containing a triazine group is 2,4-bis(3,5-dicarboxyphenylamino)-6-ol triazine (H.sub.4BDPO);

(11) a molar ratio of the carboxylic acid ligand containing a triazine group to the zirconium element in the zirconium salt is 1:(0.5-2);

(12) a ratio of the mole of the carboxylic acid ligand containing a triazine group to the volume of the formic acid is 1 mmol:(100-110 mL);

(13) a volume ratio of the formic acid to the organic solvent is 1:(0.5-1.5).

(14) Specific Implementation 2: this implementation is different from Specific Implementation 1 in that the zirconium salt is one or a mixture of several of ZrCl.sub.4, ZrOCl.sub.2.8H.sub.2O, Zr(NO.sub.3).sub.4 and Zr(SO.sub.4).sub.2. Others are the same as those in Specific Implementation 1.

(15) Specific Implementation 3: this implementation is different from Specific Implementation 1 or Specific Implementation 2 in that the organic solvent is one or a mixture of several of C.sub.2H.sub.5OH, CH.sub.3OH, DMF, DMA and DMSO. Others are the same as those in Specific Implementation 1 or 2.

(16) Specific Implementation 4: this specific implementation is different from one of Specific Implementations 1 to 3 in that the drying is conducted at 65 C. Others are the same as those in one of Specific Implementation 1 to 3.

(17) Specific Implementation 5: this implementation is different from Specific Implementation 4 in that the solid-liquid separation is done by means of suction filtration. Others are the same as those in Specific Implementation 4.

(18) The following experiments were used to verify the present invention:

(19) Experiment 1: this experiment is a method for preparing an acid-resistant Zr-MOF material with selective adsorption of hexavalent chromium, and the method was carried out according to the following steps:

(20) thoroughly mixing 135 mg of a carboxylic acid ligand containing a triazine group and 75 mg of anhydrous ZrCl.sub.4 together in 30 mL of DMF, adding 30 mL of formic acid thereto, then reacting same under closed conditions at 120 C. for 72 h, subjecting the product to suction filtration, then drying the filter cake to obtain a colorless transparent single crystal which is the Zr-MOF.

(21) where the carboxylic acid ligand containing a triazine group is 2,4-bis(3,5-dicarboxyphenylamino)-6-ol triazine (H.sub.4BDPO), and the structure thereof is shown in FIG. 1.

(22) FIG. 1 is a schematic diagram of the structure of a carboxylic acid ligand containing a triazine group in Experiment 1. This ligand not only introduces a triazine group into the MOF, but also constructs the MOF with a mesoporous property by means of its own long ligand property. The buffer pair provided on this ligand also allows the Zr-MOF prepared in Experiment 1 to have high resistance to acidic conditions.

(23) FIG. 2 is a crystal morphology of the Zr-MOF prepared in Experiment 1 under a microscope with a 40 times magnification, suggesting that the preparation method of this experiment is stable and efficient.

(24) FIG. 3 and FIG. 4 are the SEM and TEM images of the Zr-MOF material prepared in Experiment 1, respectively. It can be seen from the figures that the Zr-MOF is of a tubular structure with a pore size of 2-50 nm. Thus, the structure has microscope channels, which facilitates the acceleration of mass transfer.

(25) Experiment 2: this experiment is to evaluate the water stability and acid resistance of the Zr-MOF:

(26) two 100-mL beakers were prepared and 50 mL of deionized water was added to each beaker. The pHs of the aqueous solutions in the two beakers were adjusted with nitric acid to pH=7 and pH=1, respectively. Two 20 mg Zr-MOFs prepared in Experiment 1 were weighed in two beakers, respectively and soaked for one week. After one week, the material was separated and its XRD image was examined.

(27) FIG. 5 is an XRD image of the Zr-MOF prepared in Experiment 1. FIG. 6 is an XRD image, where curve 1 is the material after having been soaked for one week under the conditions of pH=1 in Experiment 1 and curve 2 is the material after having been soaked for one week under the conditions of pH=7 in Experiment 2. It can be seen that the XRD diffraction pattern basically remained unchanged, proving the excellent water stability and acid resistance of the Zr-MOF.

(28) Experiment 3: this experiment is to evaluate the adsorption capacity of the Zr-MOF:

(29) Nine 50-mL aqueous solutions of Sb.sup.5+, Sb.sup.3+, Bi.sup.3+, Co.sup.2+, Ni.sup.2+, Pb.sup.2+, Cu.sup.2+, and Cr.sup.6+ with a metal ion concentration of 200 mg/L were prepared, respectively, and placed in respective sample vials. 20 mg of the Zr-MOF material prepared in Experiment 1, as an adsorbent, was added to each vial. Then, all the vials were shaken at a constant temperature of 25 C. for 2 h. Finally, the heavy metal ion concentration of the supernatant in each sample in is examined and the adsorption capacity can be obtained by calculation.

(30) FIG. 7 is a graph of the adsorption experimental data in Experiment 3; where 1 is Sb.sup.5+, 2 is Sb.sup.3+, 3 is Bi.sup.3+, 4 is Co.sup.2+, 5 is Ni.sup.2+, 6 is Cd.sup.2+, 7 is Pb.sup.2+, 8 is Cu.sup.2+, 9 is Cr.sup.6+. It can be seen from the figure that the Zr-MOF material prepared in Experiment 1 not only has a higher adsorption capacity (113 mg/g) for Cr.sup.6+, but also has a strong selective adsorption. This feature shows that the Zr-MOF material prepared in Experiment 1 has a great potential in the adsorption treatment of Cr.sup.6+-containing water and the later recycling of Cr.