Synthesis of lithium slag-based ANA zeolite and molecular dynamics study of carbon dioxide adsorption performance

2.1. Raw materials and reagents

The lithium slag (LS) is provided by Guoqiang Mining Co., Ltd. Both sodium hydroxide particles and anhydrous ethanol from Tianjin Kmart Chemical Technology Co., Ltd. were determined to be analytically pure. Table 1 is the chemical composition of the lithium slag and Fig. 2 is the XRD pattern of the lithium slag. It can be seen that the lithium slag is mainly composed of quartz (SiO₂) and albite (Na₂O·Al₂O₃·6SiO₂), and the main elements in the lithium slag are silica and alumina, accounting for about 85 %.

2.2. Preparation of zeolite from lithium slag

The lithium slag was pretreated by the alkali fusion. The main steps are as follows: (1) First, lithium slag and sodium hydroxide particles with different weight ratios (0.5, 0.8, 1.5) were mixed by grinding. (2) Then the alkali-slag mixture was transferred to the crucible and alkali activation was carried out at fusion temperatures (200 °C, 350 °C, 500 °C) and fusion time (1 h, 2 h, 6 h). The experimental steps for hydrothermal synthesis reaction are as follows: (1) Transfer 3 g of the above alkali-activated product to a beaker containing 30 mL of deionized water and stir to obtain a mixture. (2) The mixture was transferred to an autoclave with Para-polyphenylene lining and heated at hydrothermal temperatures (150 °C, 180 °C, 220 °C) and hydrothermal time (2 h, 6 h, 8 h). (3) After the reaction, it was cooled to room temperature, filtered and dried, and finally the zeolite product was obtained. The synthesis conditions in Table 2.

Fig. 2XRD patterns of lithium slag

2.3. Simulation of zeolite adsorption performance based on GCMC method

In this paper, the Materials Studio of mainstream material simulation modeling software is used to study the adsorption performance of zeolite. The simulation steps are mainly divided into two steps: First, a zeolite simulation skeleton consistent with the XRD pattern of the experimental sample is constructed. Due to the silica-alumina ratio of ANA zeolite is between 1.82 and 2.2 [5], 1/3 of the Si atoms in the ANA all-silica zeolite framework are replaced by Al atoms and follows the Lowenstein rule [4]. Then the Na⁺ is introduced to balance the charge in the framework system. The Dmol3 module and the Reflex module are used to optimize the zeolite framework structure and simulate the XRD patterns, respectively.

Fig. 3Optimized ANA zeolite structure model and XRD

The results are shown in Fig. 3, and the positions of the peaks are basically consistent, indicating that the skeleton model is highly consistent with the structure of ANA zeolite. Second, the Sorption module is used to simulate the adsorption capacity and heat of ANA zeolite for carbon dioxide at different temperatures (10 °C, 20 °C, 30 °C) and atmospheric pressure (101.3 kPa). Ewald summation method was used with the accuracy of 0.01 kcal·mol^-1 to calculate the long-range electrostatic interactions. Additionally, using atom-based summation method with the cut off distance of 6.5 (Å)×10^-10m, van der Waals potential energy was calculated. The number of calculations for achieving the equilibrium was considered 1×10⁶ steps for equilibrium and 1×10⁶ steps for production. The duration of each timestep was 1 fs.

3.1. Effect of alkali fusion conditions on lithium slag

Three important conditions for alkali fusion: (1) alkali fusion temperature, (2) alkali fusion time, and (3) the weight ratio of NaOH to lithium slag (alkali-slag ratio) have some influence on Al₂O₃ and SiO₂ in the raw material lithium slag. Fig. 4(a) shows that at alkali-slag ratio of 1.5, alkali fusion time of 1h and alkali fusion temperature of 200 °C, 350 °C and 500 °C, respectively, a phase change occurs in the lithium slag of the raw material. As can be seen in the figure, most of the silicon dioxide in the lithium slag melts with sodium hydroxide and reacts to form NaAlSiO₂ at 200 °C. When the temperature rises to 350°C, the lithium slag reacts with sodium hydroxide to give sodium silicate and sodium aluminate under the conditions of alkali-slag ratio and alkali fusion time of 1.5 and 1 h, respectively. When the temperature reaches 500°C, the SiO₂ has completely dissolved into sodium aluminate and sodium silicate. Fig. 4(b) shows the lithium slag at an alkali-slag ratio of 1.5, alkali fusion temperature of 500 °C and an alkali fusion time of 1 h, 2 h and 6 h, respectively. When the alkali fusion time gradually increases, the characteristic crystallization peaks at $2\theta =$28° and 39° are enhanced, and the lithium slag is gradually activated to soluble aluminosilicate with increasing alkali fusion time at high temperature. Fig. 4(c) shows alkali fusion temperature of 500 °C, alkali fusion time of 1 h, and alkali-slag ratio of 0.5, 0.8 and 1.5, respectively. When the alkali-slag ratio is 0.5, sharp characteristic crystallization peaks are found at $2\theta =$27° and 33°, indicating that the SiO₂ in the lithium slag is not completely melted. At an alkali slag ratio of 1.5, the SiO₂ is completely melted and a large amount of soluble aluminosilicate is formed. From the above alkali fusion experiments, it is clear that the optimum alkali fusion conditions are alkali-slag ratio of 1.5, alkali fusion temperature of 500 °C, and alkali fusion time of 6 h.

3.2. Effect of hydrothermal conditions on zeolite

Fig. 5(a) is the X-ray diffraction pattern of the zeolite prepared under the optimum alkali fusion conditions at a hydrothermal time of 6 h and hydrothermal temperature of 150 °C, 180 °C and 220 °C, respectively. The black line represents the characteristic peak of the analcime (Na₂[AlSi₂O₆]₂-2H₂O, No: 41-1478) in the standard PDF. As can be seen in Fig. 5(a), at hydrothermal temperature of 150 °C, the intensity of the characteristic peak of ANA zeolite is greater, but at the same time there are impurity peaks near $2\theta =$15°, 43° and 30°. When the hydrothermal temperature is 220 °C, the characteristic peak of ANA zeolite is clearly visible, and there is no other impurity peak in the diagram. Fig. 5(b) shows the hydrothermal temperature is 150 °C and the hydrothermal time is 2 h, 6 h and 8 h, respectively. When the hydrothermal time is 2 h and 6 h, there are obvious characteristic peaks of ANA zeolite, but at the same time there are also different peaks near $2\theta =$15°, 43° and 30°. With the increase of hydrothermal time to 8 h, the characteristic peak of ANA zeolite crystallization in the product is obvious, and there is no other impurity peak in the graph.

XRD analysis shows that the product of this experiment is consistent with that of ANA zeolite. Table 3 shows the crystallinity of ANA zeolite prepared under different hydrothermal conditions. Under the optimum alkali fusion conditions, the alkali-activated lithium slag product was hydrothermally heated at 220 °C for 8 hours to obtain a cristobalite with crystallinity up to 96.3 %. Moreover, the crystallinity of the zeolite synthesized by the long-term hydrothermal reaction is about 85 % for 8 hours. However, too high hydrothermal temperature or too long hydrothermal time leads to energy waste. Therefore, the optimum hydrothermal temperature is 220 °C and the hydrothermal time is 6 h.

Fig. 4XRD patterns of the products at different alkali fusion condition

a) Influence of alkali fusion temperature

b) Influence of alkali fusion time

c) Influence of alkali slag ratio

Fig. 5XRD patterns of the products at different hydrothermal condition

a) Influence of hydrothermal temperature

b) Influence of hydrothermal time

3.3. Simulation results of carbon dioxide adsorption performance of ANA zeolite

The adsorption quantity in Table 4 indicates the instantaneous values at which the amount of adsorbed carbon dioxide reaches the maximum and minimum at a certain time of the simulation process. The average value of adsorption quantity can represent the strength of adsorption capacity, and the average value of isosteric adsorption heat is a standard to measure the strength of adsorption capacity between the adsorbent and adsorbate. The larger the value, the stronger the adsorption capacity and vice versa. From the table, it can be seen that as the temperature increases, the average adsorption amount of CO₂ on the zeolite skeleton gradually decreases. The reason is that the adsorption of carbon dioxide on the zeolite occurs by a physical adsorption mechanism. This adsorption is caused by the van der Waals force between the molecules and the interaction between the surfaces of the adsorbent, so it decreases with increasing temperature [6]. Moreover, the isosteric adsorption heat of CO₂ on ANA zeolite increases slightly with increasing temperature, which is also consistent with the trend of numerical change in the literature [7].