CN105107463A - Preparation of phosphotungstic acid composite material and experimental method for testing adsorbing performance of phosphotungstic acid composite material to methylene blue - Google Patents

Abstract

The invention discloses a preparation method of a phosphotungstic acid composite material and an experimental method for the adsorption performance of methylene blue. Using Dawson type H ₆ P ₂ W ₁₈ O ₆₂ and metal organic framework MOF-5 as raw materials, the adsorbent H ₆ is prepared by solvothermal P ₂ W ₁₈ O ₆₂ /MOF-5 composites were characterized by IR, XRD, SEM and other means. The adsorption properties of methylene blue (MB) in aqueous solution were studied, and the effects of initial pH, temperature, and different initial concentrations of MB on the adsorption capacity were discussed. Compared with the prior art, the present invention synthesizes the organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 by modifying MOF-5 with H ₆ P ₂ W ₁₈ O ₆₂ . The methylene blue (MB) solution has a better adsorption effect, which provides an application prospect for the liquid phase adsorption of MOFs materials, and has certain research significance.

Description

Preparation of Phosphotungstic Acid Composite Material and Experimental Method for Adsorption Performance of Methylene Blue

technical field

The invention relates to a metal experiment method, in particular to a method for preparing a MOF-5 loaded Dawson-type phosphotungstic acid composite material and an experiment method for methylene blue adsorption performance.

Background technique

Dyes are widely used in industries such as textiles, coatings, food technology, and hair dyes ^[1-8] . In the daily production process of these industries, a large amount of dye wastewater will inevitably be generated, resulting in dyes becoming the main component of wastewater. Dyestuffs have become the main source of pollution in major water systems in my country. Many dyes in the dye family are toxic and even carcinogenic to the human body, directly threatening human beings themselves. And with the diversification of dye components, the difficulty of its processing is gradually increasing. Finding a new treatment method for dye wastewater is an urgent problem to be solved at present.

Researchers at home and abroad have done a lot of exploration and innovation to solve the problem of dye wastewater treatment. Chemical, biological and physical treatment techniques such as chemical oxidation, calcination, biodegradation, separation, adsorption, etc. ^[9,10,11] are applied to dye wastewater treatment. Among them, adsorption is considered to be the most direct and effective method. It can not only adsorb dyes that are difficult to degrade in wastewater, but also recycle dyes to realize circular economy. The principle of adsorption is that the adsorbent has large pores and specific surface area, which can induce the dyes in the organic wastewater to enter the pores of the adsorbent or deposit on its surface, and then separate the adsorbent to achieve the purpose of purifying the wastewater.

Metal-organic frameworks (MOFs) are a class of porous materials with tunable pore size and large specific surface area, which are widely used in the fields of catalysis and adsorption. At present, the synthesis technology of MOF-5 is the most mature one. MOF-5, namely [Zn ₄ O(BDC) ₃ ], is a complex with a microporous structure formed by the bridge of Zn ₄ O through benzene rings (provided by ligand terephthalic acid (H ₂ BDC)). It has the advantages of huge specific surface area, high pore volume, adjustable pore structure, composition and functional design, and shows broad application prospects in gas storage, adsorption separation and catalysis. The preparation methods of MOFs materials mainly include hydrothermal method, microwave method, ultrasonic method, liquid phase diffusion method and so on. The most commonly used synthesis method for MOFs is the solvothermal (hydrothermal) method, that is, first add metal complex ions, ligands and appropriate solvents into the reactor, stir at room temperature for a period of time, and then transfer the reactor to a constant temperature drying oven for a period of reaction. time. After the reaction, cool to room temperature. The reacted solution is centrifugally separated, the adsorbent is washed several times with a solvent, and the adsorbent can be obtained after vacuum drying the washed adsorbent. The existing literature reports on the adsorption performance of MOF-5 mainly focus on gas phase adsorption, such as CO ₂ , CH ₄ , N ₂ O and N ₂ ^[12] . No literature has reported the adsorption of MOF-5 in the liquid phase.

Contents of the invention

The purpose of the present invention is to provide a preparation of phosphotungstic acid composite material and an experimental method for methylene blue adsorption performance in order to solve the above problems.

The present invention achieves the above object through the following technical solutions:

The present invention is characterized in that, comprises following method:

Experimental reagents: sodium tungstate [Na ₂ WO ₄ ·2H ₂ O], concentrated phosphoric acid, concentrated hydrochloric acid, anhydrous ether, zinc nitrate hexahydrate [Zn(NO ₃ ) ₂ ·6H ₂ O], terephthalic acid [H ₂ BDC], N,N-dimethylformamide, methylene blue, twice distilled water;

Preparation of metal organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5:

(1) Preparation of Dawson-type phosphotungstic acid: Add 35mL concentrated H ₃ PO ₄ (85% mass fraction) dropwise into 60mL aqueous solution dissolved with 50gNa ₂ WO ₄ ·2H ₂ O, heat to 120°C and keep reflux for 8h . After it is fully cooled, add excess hydrochloric acid for acidification, and then extract the acidified liquid with an equal volume of ether to obtain a mixture of ether phosphotungstic acid, heat it on low heat to evaporate the ether, and then dry it to obtain a light yellow solid , which is H ₆ P ₂ W ₁₈ O ₆₂ xH ₂ O(P ₂ W ₁₈ );

(2) Preparation of metal-organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 by hydrothermal method: 0.149g of Zn(NO ₃ ) ₂ ·6H ₂ O (0.5mmol) and 0.0166g of H ₂ BDC (0.1mmol) and 0.05g of H ₆ P ₂ W ₁₈ O ₆₂ were added to the reaction kettle, then 15mL N,N-dimethylformamide (DMF) was added, stirred at room temperature for 30min, and then the reaction kettle was transferred to a constant temperature drying oven The reaction was carried out for 21 hours, and the reaction temperature was 120°C. After the reaction, cool to room temperature. Centrifuge the reacted solution to separate the adsorbent, wash the adsorbent with DMF for 5-6 times, and dry the washed adsorbent in vacuum for 12 hours at a drying temperature of 80°C to obtain the adsorbent H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5;

Preparation of methylene blue standard solution:

Dilute the 100mg/L methylene blue dye solution into dilute solutions with mass concentrations of 2mg/L, 4mg/L, 6mg/L, 8mg/L, and 10mg/L respectively, and measure the absorbance of the MB solution at the maximum absorption wavelength (664nm) , draw the mass concentration-absorbance standard curve, the linear equation is: Y=0.2026X+0.0502, R ² =0.9967;

Metal-organic framework composite H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 adsorption methylene blue solution experiments:

A certain amount of metal-organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 was added to methylene blue solution to conduct adsorption experiments, and the effects of initial pH, concentration and temperature of MB on adsorption were investigated. The absorbance of the absorbing solution was measured after high-speed centrifugation, and the absorbing amount was calculated from the MB standard curve and the following formula.

${\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; v</span>}\mathrm{<span\; class="notranslate">\; 1000</span>}}{\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; 373.9</span>}}$

In the formula: c ₀ and c _e are the initial and equilibrium mass concentrations of MB (mg/L); v, m, q _e are the solution volume (L), adsorbent mass (g) and equilibrium adsorption capacity (μmol/ g).

The beneficial effects of the present invention are:

The present invention is a preparation of phosphotungstic acid composite material and an experimental method for methylene blue adsorption performance. Compared with the prior art, the present invention synthesizes an organic framework composite by modifying MOF-5 with H ₆ P ₂ W ₁₈ O ₆₂ Material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5, the modified material has a better adsorption effect on methylene blue (MB) solution, which provides application prospects for MOFs materials in liquid phase adsorption, and has certain research significance.

Description of drawings

Fig. 1 is the standard curve figure of methylene blue of the present invention;

Fig. 2 is the infrared spectrum of the present invention H ₆ P ₂ W ₁₈ O ₆₂ , MOF-5, H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5;

Fig. 3 is the XRD pattern of H ₆ P ₂ W ₁₈ O ₆₂ , MOF-5, H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 of the present invention;

Figure 4 is the TG diagram of MOF-5 of the present invention, H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5;

Fig. 5 is the N ₂ -adsorption-desorption curve and pore size distribution diagram of MOF-5 of the present invention, H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5;

Figure 6 is the SEM images of H ₆ P ₂ W ₁₈ O ₆₂ (a, b), MOF-5 (c) and H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 (d) of the present invention;

Figure 7 is a schematic diagram of the influence of pH on MB adsorption in the present invention;

Figure 8 is a schematic diagram of the influence of temperature on MB adsorption capacity in the present invention;

Figure 9 is a schematic diagram of the influence of the initial mass concentration of MB on the adsorption capacity of the present invention;

Figure 10 is a schematic diagram showing the comparison of adsorption effects between MOF-5 of the present invention and H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5;

Fig. 11 is the Langmuir isotherm adsorption line schematic diagram of methylene blue of the present invention;

Fig. 12 is a schematic diagram of pseudo-first-order kinetics under different concentrations of methylene blue of the present invention;

Fig. 13 is a schematic diagram of pseudo-second order kinetics at different concentrations of methylene blue in the present invention.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing:

The present invention is characterized in that, comprises following method:

Experimental reagents: sodium tungstate [Na ₂ WO ₄ ·2H ₂ O], concentrated phosphoric acid, concentrated hydrochloric acid, anhydrous ether, zinc nitrate hexahydrate [Zn(NO ₃ ) ₂ ·6H ₂ O], terephthalic acid [H ₂ BDC], N,N-dimethylformamide, methylene blue, twice distilled water;

Preparation of metal organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5:

(1) Preparation of Dawson-type phosphotungstic acid: Add 35mL concentrated H ₃ PO ₄ (85% mass fraction) dropwise into 60mL aqueous solution dissolved with 50gNa ₂ WO ₄ ·2H ₂ O, heat to 120°C and keep reflux for 8h . After it is fully cooled, add excess hydrochloric acid for acidification, and then extract the acidified liquid with an equal volume of ether to obtain a mixture of ether phosphotungstic acid, heat it on low heat to evaporate the ether, and then dry it to obtain a light yellow solid , which is H ₆ P ₂ W ₁₈ O ₆₂ xH ₂ O(P ₂ W ₁₈ );

(2) Preparation of metal-organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 by hydrothermal method: 0.149g of Zn(NO ₃ ) ₂ ·6H ₂ O (0.5mmol) and 0.0166g of H ₂ BDC (0.1mmol) and 0.05g of H ₆ P ₂ W ₁₈ O ₆₂ were added to the reaction kettle, then 15mL N,N-dimethylformamide (DMF) was added, stirred at room temperature for 30min, and then the reaction kettle was transferred to a constant temperature drying oven The reaction was carried out for 21 hours, and the reaction temperature was 120°C. After the reaction, cool to room temperature. Centrifuge the reacted solution to separate the adsorbent, wash the adsorbent with DMF for 5-6 times, and dry the washed adsorbent in vacuum for 12 hours at a drying temperature of 80°C to obtain the adsorbent H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5;

Preparation of methylene blue standard solution:

Dilute the 100mg/L methylene blue dye solution into dilute solutions with mass concentrations of 2mg/L, 4mg/L, 6mg/L, 8mg/L, and 10mg/L respectively, and measure the absorbance of the MB solution at the maximum absorption wavelength (664nm) , draw the mass concentration-absorbance standard curve (as shown in Figure 1), the linear equation is: Y=0.2026X+0.0502, R ² =0.9967;

Figure 1. Standard curve for methylene blue.

Metal-organic framework composite H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 adsorption methylene blue solution experiments:

A certain amount of metal-organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 was added to methylene blue solution to conduct adsorption experiments, and the effects of initial pH, concentration and temperature of MB on adsorption were investigated. The absorbance of the absorbing solution was measured after high-speed centrifugation, and the absorbing amount was calculated from the MB standard curve and the following formula.

${\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; v</span>}\mathrm{<span\; class="notranslate">\; 1000</span>}}{\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; 373.9</span>}}$

In the formula: c ₀ and c _e are the initial and equilibrium mass concentrations of MB (mg/L); v, m, q _e are the solution volume (L), adsorbent mass (g) and equilibrium adsorption capacity (μmol/ g).

Results and discussion

Characterization of Metal Organic Framework Composite H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5

IR analysis

Fig. 2 IR spectra of H ₆ P ₂ W ₁₈ O ₆₂ , MOF-5, H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5.

It can be seen from Figure 2 that the main absorption peaks of MOF-5 are 3428.6cm ^-1 , 1657.9cm ^-1 , 1601.8cm ^-1 , 1389.0cm ^-1 , 823.4cm ^-1 , 750.0cm ^-1 , while pure H ₆ P ₂ W The main absorption peaks of ₁₈ O ₆₂ are consistent with those reported in literature ^[12,14] . The absorption peaks of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 are 1673.2, 1601, 1384.3, 823.4, 750.0cm ^-1 , and besides retaining the basic skeleton characteristic absorption peaks of MOF-5, it also contains Dawson phosphotungstic acid 1093.3 The characteristic absorption peaks of cm ^-1 v _as (PO _a ), 968.8cm ^-1 v _as (W=O _d ), and 917.4cm ^-1 v (WO _b -W) indicate that H ₆ P ₂ W ₁₈ O ₆₂ Still maintaining the Dawson structure, phosphotungstic acid exists as a guest molecule in the rigid framework of MOF-5.

XRD analysis

Fig. 3 H ₆ P ₂ W ₁₈ O ₆₂ , MOF-5, XRD patterns of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5.

It can be seen from Figure 3 that the XRD diffraction peaks ^of H ₆ P ₂ W ₁₈ O ₆₂ are mainly concentrated in the two intervals of 2θ=7～10° and 14～20°. ^15] , and the addition of H ₆ P ₂ W ₁₈ O ₆₂ did not affect the structure of MOF-5, suggesting that MOF-5 has a good ordered structure. At the same time, no diffraction peak of H ₆ P ₂ W ₁₈ O ₆₂ was detected in H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5, indicating that H ₆ P ₂ W ₁₈ O ₆₂ was highly dispersed in the channels of MOF-5.

TG analysis

Figure 4 MOF-5, TG diagram of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5.

It can be seen from Figure 4 that MOF-5 and H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 lose weight gradually with the increase of temperature. From room temperature to 160°C is the process of losing guest molecules, mainly including: physical adsorption of water and crystal water; after 160°C is the stage of gradual collapse of the skeleton. Compared with MOF-5, H ₆ P ₂ W ₁₈ O ₆₂ /MOF- 5 The temperature required at this stage is higher, indicating that H ₆ P ₂ W ₁₈ O ₆₂ is loaded in the framework of MOF-5 or dispersed in the channels of MOF-5.

BET analysis

Fig. 5 Adsorption-desorption isotherms of pure framework MOF-5 and complexes.

It can be seen from Figure 5 that the adsorption-desorption isotherms of the pure framework MOF-5 and the composite belong to type IV, and the appearance of the hysteresis loop is due to capillary condensation, indicating that both substances belong to mesoporous materials. The pore size distribution diagram (inset of Figure 5) was analyzed by the BJH method, and the average pore diameters (as shown in Table 1) were 2.057nm and 3.451nm respectively, further proving that both are mesoporous materials. It can be seen from Table 1 that the specific surface area and pore volume of the two are not very large, and the specific surface area of the composite becomes smaller, indicating that the composite has fewer pores than pure MOF-5. Combining the comparison of the adsorption properties of the two, it further illustrates that the composite The adsorption of the species on MB is controlled by chemisorption.

Table 1 Structural parameters of MOF-5 and H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5

SEM analysis

Fig.6 SEM images of H ₆ P ₂ W ₁₈ O ₆₂ (a, b), MOF-5 (c) and H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 (d).

It can be seen from Figure 6 that H ₆ P ₂ W ₁₈ O ₆₂ has a rough spherical shape (6a) and a porous structure (6b), indicating that it is easy to be modified by doping. MOF-5 is a cube with a particle size of about 20 μm (6c). After H ₆ P ₂ W ₁₈ O ₆₂ is loaded on MOF-5, the morphology changes, and the composite presents spherical structures of different sizes (6d). It implies that H ₆ P ₂ W ₁₈ O ₆₂ is uniformly dispersed in the channels of MOF-5.

3. Study on Adsorption of Methylene Blue by 2H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5

Effect of pH of methylene blue solution on adsorption effect

Take 20 mg of adsorbent and shake it in 20 mL of 20 mg/L methylene blue solution to investigate the effect of the adsorbent on the adsorption of methylene blue solution at different pH.

Figure 7 Effect of pH on MB adsorption.

It can be seen from Figure 7 that the pH value of the solution significantly affects the adsorption capacity of the complex to methylene blue. The adsorption amount decreases with the increase of pH, low pH value is beneficial to the adsorption of methylene blue, and high pH inhibits the adsorption of MB. Adsorption is achieved by the electrostatic interaction between the adsorbent (H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5) and the adsorbate (MB) ^[16,17] , and the effect of pH is mainly manifested in the change of the adsorbate and the charge on the surface of the adsorbent, thus affecting the adsorption effect. After the Zeta potential test of the adsorbent, under the condition of stronger acidity, it is concluded that the surface of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 is more negatively charged, while the surface of methylene blue is positively charged, so a low pH value is beneficial Adsorption of methylene blue.

Effect of temperature on adsorption effect

Take 15mg of adsorbent, put it in 20mL of 30mg/L, 35mg/L, 40mg/L, 45mg/L, 50mg/L MB solution, adjust the pH to 2, and investigate the influence of temperature on adsorption.

Fig. 8 Effect of temperature on MB adsorption amount.

It can be seen from Figure 8 that the adsorption amount of MB by H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 increases from 137.68 μmol/g to 145.38 μmol/g with the increase of temperature, which indicates that the process of adsorption of methylene blue is an endothermic process . The increase in temperature is beneficial to the increase of the adsorption amount of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5. It may be that at higher temperature, methylene blue molecules become active, which promotes the adsorption of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5. The adsorption of methylene blue; on the other hand, the increase of temperature within a certain range has a certain effect on the hole expansion of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5, thereby increasing the adsorption capacity.

Effect of initial concentration on adsorption effect

Weigh 15mg of adsorbent, put it in 20mL of methylene blue solution with concentrations of 20mg/L, 25mg/L, 30mg/L, 35mg/L, 40mg/L, and 45mg/L, and adjust the pH=2 to shake, and measure the sample every 10min , to investigate the effect of the initial mass concentration of MB on the adsorption.

Figure 9 Effect of initial mass concentration of MB on adsorption capacity.

As shown in Fig. 9, the adsorption amount of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 increases with the increase of the initial concentration of MB. This is because the increase in the concentration of the methylene blue solution will increase the concentration difference between the H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 surface and the solution body, which will increase the driving force for adsorption and thus increase the adsorption amount.

3.2.4 Comparison of adsorption effects between MOF-5 and H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5

Take 15mg of MOF-5 and H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 respectively, put them in 20mL of methylene blue solution with a concentration of 30mg/L and adjust the pH=2 to shake, measure a sample every 10min, and investigate MOF-5 and H Comparison of the adsorption effect of ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 on methylene blue.

Figure 10 Comparison of the adsorption effects of MOF-5 and H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5.

It can be seen from Figure 10 that when the concentration of the methylene blue solution is 30 mg/L and the pH=2, the monolayer adsorption capacity of MOF-5 is much smaller than that of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5. It shows that the modification of MOF-5 by H ₆ P ₂ W ₁₈ O ₆₂ can make MOF-5 material used for liquid phase adsorption, and it is very meaningful to carry out this research.

Determination of Adsorption Isotherm Equation

On the basis of exploring the effect of temperature, we further investigated two isothermal adsorption models (Langmuir and Freundlich) for MB. Among them, the Langmuir linear equation ^[18] is as follows:

$\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}{{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}}$

q _e and q _m (mg/g) are the adsorption capacity and maximum monolayer adsorption capacity of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 at equilibrium, respectively, Ce(mg/L) is the concentration of methylene blue at equilibrium, K _L (L/mg) is Langmuir's adsorption constant. Plotting Ce against Ce/q _e (Figure 11), the linear relationship shows that the adsorption of methylene blue by H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 conforms to the Langmuir adsorption model, and the calculated parameters are shown in Table 2.

Figure 11 Langmuir adsorption isotherm of methylene blue.

The basic properties of the Langmuir isotherm adsorption can be expressed by the value of R _L ^[19]

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}}$

K _L (L/mg) is the adsorption constant of the Langmuir isotherm equation, C ₀ (mg/L) is the initial concentration of the dye.

R _L illustrates the adsorption capacity of the corresponding isotherm:

R _L > 1 is not conducive to adsorption

0<R _L <1 good adsorption

_RL = 0 irreversible adsorption

_RL = 1 linear adsorption

The _RL values of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 adsorbed methylene blue at 20°C, 30°C, and 40°C used in this paper are: 0.9998, 0.9997, and 0.9994, which proves that H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 is a good adsorption for methylene blue.

Freundlich adsorption is an empirical formula. The Freundlich adsorption isotherm equation is generally expressed as:

${\mathrm{<span\; class="notranslate">\; lnq</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; lnC</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; lm\; w</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}$

Among them, K _F (mol/g(L/mol) ^1/n ) roughly indicates the adsorption capacity of the adsorbent, and 1/n indicates the adsorption strength. The relevant parameters are calculated in Table 2.

The isotherm adsorption curve parameter of table 2 methylene blue

In Table 2, the relevant parameters of the Langmuir and Freundlich adsorption isotherms are calculated. It can be seen from the data in the table that the experimental data is more in line with the Langmuir isotherm adsorption model, and there are also reports ^[20] using the Langmuir isotherm adsorption model to explain the adsorption of dyes from dye solutions by adsorbents.

Determination of Adsorption Kinetics

In order to explore the control mechanism of the adsorption process, the experimental data were fitted by two kinetic models.

Pseudo-First Order Kinetic Model

The Langmuir equation is a simple kinetic adsorption analysis equation, which can be expressed by the following formula ^[21] :

$\frac{{\mathrm{<span\; class="notranslate">\; dq</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}}}{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; )</span>$

k _l is the H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 adsorption rate constant (min ^-1 ), q _e and q _t are the amount of dye adsorbed at equilibrium and time t (mg/g), respectively. When t=0, q _t =0 and t=t, q _t =q _t , then the above formula can be transformed into:

ln(q _e -q _t )＝lnq _e -k _I t

K _I can be obtained by plotting t against ln(q _e -q _t ), as shown in the figure:

Figure 12 Pseudo-first-order kinetics at different concentrations of methylene blue.

It can be seen from Figure 12 that the linear correlation coefficients of the adsorption pseudo-first-order kinetic model of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 at 20 mg/L and 30 mg/L are 0.8145 and 0.91 respectively, and the adsorption amount q _e is completely inconsistent with the adsorption amount q _t calculated by the first-order kinetic formula (see Table 3 for specific values), thus indicating that the adsorption of methylene blue by H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 does not conform to the pseudo-first-order kinetics Therefore, the adsorption process is not a diffusion-controlled phenomenon ^[22] .

Pseudo-second-order kinetic model

The linear equation of the pseudo-second-order kinetic model is:

$\frac{\mathrm{<span\; class="notranslate">\; t</span>}}{{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}\mathrm{<span\; class="notranslate">\; t</span>}$

k ₂ is also the H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 adsorption rate constant (min ^-1 ), q _e and q _t are the same as above

Figure 13 Pseudo-second-order kinetics at different concentrations of methylene blue

It can be seen from Fig. 13 that the adsorption amount of MB obtained in the experiment is consistent with the value of the theoretically calculated adsorption amount (Table 3), and the linear correlation coefficient (R ² ) of the obtained pseudo-second-order kinetic model is greater than 0.99, indicating that H ₆ The adsorption of MB by P ₂ W ₁₈ O ₆₂ /MOF-5 is a pseudo-second-order kinetic model controlled by chemical adsorption.

Table 3 MB pseudo-first-order kinetics and pseudo-second-order kinetics parameters at different concentrations

Thermodynamic parameters

At 293K, 303K and 313K, experiments were carried out on the adsorption of methylene blue on H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 to study the adsorption equilibrium constant (k _L ), Gibbs free energy (ΔG°), enthalpy change (ΔH° ) and entropy change (ΔS°), they can be calculated by the following formula:

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}}$

ΔG°=-RTlnK _L

Wherein, R is 8.3145Jmol ^-1 K ^-1 , and T is temperature (K).

According to the equation ΔH° and ΔS° can be calculated from the slope and intercept of the plot of 1/T versus lnk _L , respectively. The relevant thermodynamic parameters are shown in Table 4.

Table 4 Thermodynamic parameters of MB adsorption on H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5

From the data in Table 4, it can be seen that ΔG°<0 indicates that the adsorption is spontaneous; ΔH°>0 indicates that the process of dye adsorption is an endothermic process, which further confirmed the experimental results.

Comparison of adsorption effects of different adsorbents

Table 5 Comparison of adsorption capacities of different adsorbents for MB

Table 5 compares the maximum monolayer adsorption capacity of different adsorbents on MB. From the Q _e value in Table 5, it can be seen that the monolayer adsorption of MB by H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 is better than other adsorbents . Therefore, H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 is a good MB adsorbent with potential application value.

in conclusion

(1) The maximum monolayer adsorption capacity of H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 to methylene blue is 58.26 mg/g, which has practical industrial application value. Increasing temperature and decreasing pH are beneficial to the adsorption of methylene blue by H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5, and the thermodynamic parameters also prove this conclusion.

(2) The adsorption of MB by H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 conforms to the Langmuir isotherm adsorption model, and the adsorption process is fitted by a pseudo-second-order kinetic model. Kinetic experimental data ΔG°<0 indicates that the adsorption is spontaneous; ΔH°>0 indicates that the process of dye adsorption is an endothermic process.

The basic principles and main features of the present invention and the advantages of the present invention have been shown and described above. Those skilled in the industry should understand that the present invention is not limited by the above-mentioned examples. What are described in the above-mentioned examples and the description only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention also has various Variations and improvements, which fall within the scope of the claimed invention. The protection scope of the present invention is defined by the appended claims and their equivalents.

Claims (1)

1. A preparation of phosphotungstic acid composite material and an experimental method for methylene blue adsorption performance, characterized in that, comprising the following methods: Experimental reagents: sodium tungstate [Na ₂ WO ₄ ·2H ₂ O], concentrated phosphoric acid, concentrated hydrochloric acid, anhydrous ether, zinc nitrate hexahydrate [Zn(NO ₃ ) ₂ ·6H ₂ O], terephthalic acid [H ₂ BDC], N,N-dimethylformamide, methylene blue, twice distilled water; Preparation of metal organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5: (1) Preparation of Dawson-type phosphotungstic acid: Add 35mL concentrated H ₃ PO ₄ (85% mass fraction) dropwise to 60mL distilled water dissolved in 50gNa ₂ WO ₄ ·2H ₂ O, heat to 120°C and keep reflux for 8h After it is fully cooled, add excess hydrochloric acid for acidification, and then use an equal volume of ether to extract the acidified solution to obtain a mixture of ether phosphotungstic acid, heat it on low heat to evaporate the ether, and then dry it to obtain a light yellow solid. That is H ₆ P ₂ W ₁₈ O ₆₂ xH ₂ O(P ₂ W ₁₈ ); (2) Preparation of metal-organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 by hydrothermal method: 0.149g of Zn(NO ₃ ) ₂ ·6H ₂ O (0.5mmol) and 0.0166g of H ₂ BDC (0.1mmol) and 0.05g of H ₆ P ₂ W ₁₈ O ₆₂ were added to the reaction kettle, then 15mL N,N-dimethylformamide (DMF) was added, stirred at room temperature for 30min, and then the reaction kettle was transferred to a constant temperature drying oven React in medium for 21 hours, the reaction temperature is 120°C, after the reaction, cool to room temperature, centrifuge the reacted solution to separate the adsorbent, wash the adsorbent with DMF for 5-6 times, vacuum dry the washed adsorbent for 12 hours, dry The temperature is 80°C, and the adsorbent H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 can be obtained; Preparation of methylene blue standard solution: Dilute the 100mg/L methylene blue dye solution into dilute solutions with mass concentrations of 2mg/L, 4mg/L, 6mg/L, 8mg/L, and 10mg/L respectively, and measure the absorbance of the MB solution at the maximum absorption wavelength (664nm) , draw the mass concentration-absorbance standard curve, the linear equation is: Y=0.2026X+0.0502, R ² =0.9967; Metal-organic framework composite H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 adsorption methylene blue solution experiments: Add a certain amount of metal-organic framework composite material H ₆ P ₂ W ₁₈ O ₆₂ /MOF-5 to the methylene blue solution to conduct adsorption experiments to investigate the effect of initial pH, concentration and temperature of MB on the adsorption. The absorbance of the adsorption solution was measured after high-speed centrifugation , the adsorption amount was calculated from the MB standard curve and the following formula, ${\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; v</span>}\mathrm{<span\; class="notranslate">\; 1000</span>}}{\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; 373.9</span>}}$ In the formula: c ₀ and c _e are the initial and equilibrium mass concentrations of MB (mg/L); v, m, q _e are the solution volume (L), adsorbent mass (g) and equilibrium adsorption capacity (μmol/ g).