In the early stage, through screening three kinds of zeolite molecular sieves (4A, 5A, 13X) commonly used in industry, the best adsorbent 13X molecular sieves were selected. 13X molecular sieve samples were purchased from Sinopharm chemical reagent Co., LTD. (≥99.0%), molecular formula: Mx/m [(AlO₂)_x(SiO₂)_y]_zH₂O. All the other reagents were also purchased from Sinoprecipent Chemical Reagent Co., LTD except sodium hydroxide (Beijing Chemical Reagent Company) and potassium bromide (Belgium Acros). N₂ (99.999%, Beijing Praxair Practical Gas Co., LTD.), O₂ (99.999%, Beijing Zhaoge Gas Co., LTD.) and SO₂/N₂ (1%, Beijing Zhaoge Gas Co., LTD.).

A certain quantity of Cu(NO₃)₂·3H₂O(calculated by the amount of CuO loaded) was added to 20 mL deionized water to completely dissolve it. The beaker containing the solution was placed on a constant temperature magnetic stirrer, and the constant temperature magnetic stirrer was adjusted to 40˚C. A certain amount of molecular sieve was weighed and slowly added into the copper nitrate solution in the stirring process. After 1 h, the beaker was removed. After 12 h, the sample was placed in a centrifuge at 500 R/min to eliminate the interference of excess Cu²⁺. The prepared samples then were dried in an oven at 150˚C and roasted in Muffle furnace at 450˚C for two hours. Finally, CuO modified 13X molecular sieve modified adsorption material was obtained, hereinafter referred to as 13X-xwt%CuO (mass fraction of X-loaded CuO). The modified materials were characterized by scanning electron microscopy (SEM), specific surface area and pore structure (BET), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD).

Referring to the composition of industrial flue gas, the main gas components except dust were simulated in the laboratory. The main components of the simulated flue gas are SO₂, N₂, O₂, H₂O(g), and the four gas components (volume fraction) are set as follows: 0.3%, 83.7%, 6%, 10%. The total volume flow of simulated flue gas was 500 mL·min⁻¹, the temperature was 110˚C (actual outlet temperature of industrial flue gas), and the space speed (GHSV) was 1200 h⁻¹. The experimental setup was shown in Figure 1.

The accurately weighed modified adsorbent was loaded into the experimental device, and the pollutant gas was injected into the experimental device after the investigation conditions were preset. The exhaust gas components after passing through the adsorption bed are detected to investigate the adsorption situation of the bed. The exhaust gas components after passing through the adsorption bed are detected, and the saturation desulfurization value and penetration desulfurization value are taken as the evaluation indexes to investigate the adsorption situation of the bed. Sulfur capacity saturation value is the maximum adsorption mass of unit mass modified zeolite molecular sieve for SO₂.

This experimental study is to use CuO modified zeolite molecular sieve to adsorb SO₂ gas, so that it can meet the national emission standard-GB-13271-2014 boiler air pollutants emission standard. Therefore, the penetration point of SO₂ in the outlet gas is positioned at 200 ppm. In the analysis of experimental data, when the outlet SO₂ reaches the specified penetration point, the adsorption capacity of the adsorbent is used as the basis for evaluating the activity of the desulfurizer. The calculation formula of the penetration adsorption capacity is as follows:

q = 10 − 6 Q ∫ 0 t m ( C 0 − C 1 ) d t 22.4 G (1)

where q is the adsorption capacity of the adsorbent when the outlet SO₂ reaches the adsorption penetration point, mmol·g⁻¹; G is the adsorbent mass, g; t is the time for outlet SO₂ to reach the penetration point, min; C₀ is the Concentration of SO₂ at the air inlet, ppm; C₁ is the Concentration of SO₂ at the air outlet, ppm; Q is the Total gas flow, mL·min⁻¹.

Main equipment: Electronic balance (ML-104, Mettler-Toledu), Fourier transform infrared spectrometer (IS50, ThermoFisher), SO₂ analyzer (CLD62, Switzerland Eco Physics), Automatic surface area and aperture distribution analyzer (AutoSORB-IQ, Quantachrome), XPS (250XI, Thermo Fisher), XRD (Smartlab9kw, Rigaku) gas distribution system and related accessories.

The study of adsorption kinetics can not only estimate the kinetic adsorption rate of molecular sieve, but also speculate the adsorption mechanism. Some experts and scholars have analyzed the fluid structure and dynamics of different types of adsorbent materials, and discussed the kinetic diffusion behavior of adsorbent in adsorbent, as well as the diffusion and migration mechanism of gas in molecular sieve. In this experiment, the quasi-first-order and quasi-second-order kinetic models and the intra particle diffusion model equations were used to fit the SO₂ adsorption data of modified 13X-xwt%CuO molecular sieve, so as to analyze the removal mechanism of SO₂ by modified materials.

1) Fitting analysis of quasi-first-order and quasi-second-order adsorption kinetics models

Equation of quasi-first-order dynamic model [12]:

ln ( q e − q t ) = ln q e − k 1 t (2)

where q_e is the equilibrium adsorption capacity of SO₂, mg·g⁻¹; q_t is the adsorption capacity at reaction timet, mg·g⁻¹; k₁ is the quasi-first-order adsorption rate constant, min⁻¹.

The quasi-second-order adsorption kinetic model is established on the basis of Langmuir isothermal adsorption equation, which is commonly used to describe the chemisorption process with ion migration. If the adsorption process involves the formation of new chemical bonds, the adsorbent will be firmly bound to the surface of the adsorbent due to the role of chemical bonds. This model assumes that the adsorption rate in the adsorption process is linearly squared with the remaining adsorption active sites in the absorbent. Therefore, the quasi-second-order adsorption kinetic model can be expressed by the following equation [13]:

d q t d t = k 2 ( q e − q t ) 2 (3)

After integration, we can get:

q t = k 2 q e 2 t 1 + k 2 q e t (4)

where q_e is the equilibrium adsorption capacity of SO₂, mg·g⁻¹; q_t is the adsorption capacity at reaction time t, mg·g⁻¹; k₂ is the Quasi-second-order adsorption rate constant g·(mg·min)⁻¹.

2) Model equation for intra-particle diffusion

The kinetic model could not elucidate the diffusion mechanism and rate control steps of SO₂ adsorption on molecular sieve. Therefore, the Weber-Morris particle diffusion model was selected, which was proposed by Weber-Morris et al. on the basis of Fick’s second diffusion law. The model is mainly used to describe the adsorption process dominated by diffusion in pores, and the adsorption process can be analyzed according to the internal diffusion rate constant. If the fitted curve is linear with T^0.5, it indicates that particle diffusion is a way to adsorb SO₂. If the fitted curve with T^0.5 passes through the origin, it indicates that intra-particle diffusion is the only way for molecular sieve to adsorb SO₂. The adsorption rate equation of the intra-particle diffusion model is as follows [14]:

d q t d t = 0.5 k 1 t − 0.5 (5)

when t = 0, q_t = 0; Whent = t,q_t =q_t; After integration, we can get:

q t = k 1 t 0.5 + C (6)

where q_t is the Adsorption capacity of unit mass adsorbent for SO₂ at time t, mg·g⁻¹; k₁ is the internal diffusion rate constant, mg·g⁻¹·min^−0.5; C is the intercept, boundary layer thickness, mg·g⁻¹; T is the Reaction time, min.

The adsorption and penetration curves of 13X zeolite supported with different mass fractions of CuO are shown in Figure 2.

As can be seen from Figure 2, the time for 13X zeolite with different CuO loads to completely adsorb SO₂ from large to small is: 13X-3%WTCuO > blank sample > 13X-6%WTCuO > 13X-9%WTCuO, indicating that CuO, as an active component, can improve the adsorption capacity of SO₂ by zeolite in a certain range of loading capacity, and the penetration adsorption capacities are 37.55 mg·g⁻¹, 43.41 mg·g⁻¹, 35.19 mg·g⁻¹ and 22.53 mg·g⁻¹, respectively. The saturated adsorption capacities were 41.14 mg·g⁻¹, 49.27 mg·g⁻¹, 42.95 mg·g⁻¹ and 30.21 mg·g⁻¹, respectively. The amount of CuO loading had a great influence on the adsorption effect of modified 13X molecular sieve. It may be that the addition of CuO changes the specific surface area of molecular sieve, changes the pore structure, and increases the porosity. Cu²⁺ enters the pore and enhances the Coulomb force between molecules. At the adsorption temperature of 110˚C, physical adsorption dominates, and the conditions for chemisorption are insufficient.

Studies have shown that [7]: The richer the pore structure of the adsorption material, the larger the specific surface area, and the surface can provide a large number of adsorption active sites, which means that the adsorption capacity is higher. In this study, by means of CuO modification, the content and type of alkali metal cations in the pores of 13X molecular sieve were changed, and part of the crystal water in the pores was removed by roasting at high temperature, so as to change the pore size and type of zeolite and the pore size distribution and type of molecular sieve. And the content and type of alkali metal cations jointly control the adsorption process of SO₂ by 13x-xwt %CuO.

The adsorption effect of SO₂ by molecular sieve is related to the pore size and specific surface area of molecular sieve itself.

Figure 3 shows the comparison of SEM characterization of 13X molecular sieve blank sample, 13X-3%CuO and 13X-9%CuO. As can be seen the surface of 13X molecular sieve has bulges or depressions of different shapes and sizes distributed irregularly on the surface of the material, increasing the specific surface area of the material. The morphology of 13X-3%CuO (10 μm) and 13X-9%CuO (5 μm) is not significantly changed from that of 13X blank sample (10 μm).

Figure 4 shows the N₂ adsorption-desorption curve of loaded 3wt%Cuo-13x and blank sample. Figure 5 shows the pore size distribution diagram of loaded 13x-3wt%CuO and blank sample.

It can be seen from Figure 4 that the adsorption isotherms of 13X zeolite and 13X blank samples after CuO loading belong to type I adsorption isotherms. The characteristic is that when the relative pressure is very small, the curve shows a significant rise, and then the curve remains basically stable. This is also a characteristic of typical microporous materials. When P/P0 is 0.8, 13x-3wt %CuO shows insignificant hysteresis loop, which may be due to capillary agglutination in the molecular sieve. This phenomenon generally occurs in mesoporous pores, which indirectly proves that the load distribution of 3wt%CuO on 13X molecular sieve is relatively good and does not block the original pore of molecular sieve.

Combined with Figure 5, it can be seen that after the modification of 13X molecular sieve by CuO active component, the pore size distribution map becomes a symmetric peak. Compared with the untreated 13X molecular sieve, the pore size distribution is slightly changed, from the original two peak distribution concentrated between 0.4 - 0.8 nm and 1.0 - 1.3 nm to the uniform distribution between 0.5 - 0.9 nm. This indicates that the addition of CuO can change the original pore size of molecular sieve. In the process of CuO modification of 13X zeolite, the addition of Cu(NO₃)₂·3H₂O and the dissolution of NO 3 − and roasting at 450˚C for a long time resulted in the increase of the small pore size and the disappearance of the large pore size.

The area enclosed by the 13x-3wt %CuO curve is significantly larger than that of the blank sample, indicating that the pore volume increases, which proves that CuO does not block the pore.

Figure 5 shows the XPS map of 13X-3wt%CuO, so as to obtain the loading situation of CuO on 13X molecular sieve and the valence state of Cu element. Cu(NO₃)₂ decomposed into CuO and Cu₂O or both after roasting at high temperature. All elements except H and He are identified according to the positions of the characteristic spectral lines in the energy spectra measured by XPS. The content or relative concentration of reactive atoms can be obtained from the intensity of the photoelectron line (the area of the photoelectron peak) in the XPS energy spectrum.

According to the XPS total spectrum in Figure 6, the characteristic peaks of copper ions appeared on the surface of the modified 13X molecular sieve, which proved the existence of Cu ions. In order to further show that CuO is successfully loaded after 13X modification, the XPS map of Cu element is analyzed as follows: Figure 7 peak fitting is performed using the Gaussian/Laurentus equation minimum mean square error Shirley elimination method. It can be seen that it is composed of two peaks, which are CuO and Cu₂O peaks respectively. This indicates that CuO and Cu₂O are generated by Cu(NO₃)₂ decomposition at 450˚C. By comparing the area of CuO and Cu₂O peaks, CuO/Cu₂O ≈ 1.5 can be obtained. This indicates that Cu(NO₃)₂ is not completely transformed into CuO after roasting at 450˚C for 2 h.

1) Adsorption kinetic model fitting analysis

Figure 8 shows the fitting curves of CuO modified 13X-Xwt %CuO molecular sieve adsorption materials when the adsorption performance of SO₂ reaches the penetration point for quasi-first-level (a) and quasi-second-level (b) adsorption kinetic models. Table 1 shows the relevant fitting parameters and determination coefficient R².

It can be seen intuitively from Figure 8 that the quasi-second-order curve is basically a straight line, and the quasi-first-order curve approximately presents a free-fall curve with the increase of adsorption time. As can be seen in Table 1, the R² values of the quasi-first-order kinetic model are all smaller than those of the quasi-second-order kinetic model, and the R² values of the quasi-second-order kinetic model are all above 0.99 and close to 1, indicating that the quasi-second-order kinetic equation is more relevant and can better describe the adsorption process of SO₂ by CuO modified 13X-Xwt %CuO molecular sieve adsorption material. When the adsorption capacity of SO₂ exceeds a certain amount, the correlation of the quasi-second-order kinetic model becomes worse, indicating that the adsorption process of SO₂ by 13x-xwt %CuO is controlled by multiple factors.

2) Kinetic model fitting analysis of diffusion in particles

The kinetic model could not fully elucidate the diffusion mechanism and rate control steps of 13x-xwt%CuO for SO₂ adsorption. Therefore, the adsorption process of adsorbent on adsorbent is often described by Weber-Morris particle diffusion model.

Figure 9 and Table 2 show the results after fitting the kinetic model of diffusion in 13X-Xwt %CuO particles. The results show that QT and T^0.5 show broken lines, indicating that diffusion in particles is not the main control step of the rate. The adsorption process can be roughly divided into two stages: the first stage (from the beginning of air intake to the penetration point) is the rapid adsorption stage, which is characterized by the joint control process of surface adsorption and intra-particle diffusion rate, and the adsorption contribution rate is large, accounting for about 90%. This stage is also more consistent with the quasi-second-order adsorption kinetic model. The second stage (from the penetration point to adsorption saturation) is the control stage of the diffusion rate in particles, which is mainly microporous adsorption or chemisorption. The adsorption amount is small, and is positively correlated with the CuO attachment load, indicating that CuO modification of 13X modified molecular sieve adsorption material enhances the chemisorption performance of molecular sieve, and reduces the physical adsorption performance. At this stage, the correlation of quasi-second-order adsorption kinetics model becomes worse.

The fitting coefficient of determination R² of the two stages is above 0.90, and the R² of the first stage is greater than or close to 0.99. The results indicate that the diffusion kinetic model can better explain the adsorption process of SO₂ by modified molecular sieve. Since C in the formula is a constant that is not 0, the curve does not reach the origin, indicating that the removal of SO₂ is not completely adsorbed by the form of intra-particle diffusion, and a small amount of SO₂ is adsorbed by the modified molecular sieve by chemical adsorption. The internal diffusion rate constantk₁ is basically the same, indicating that CuO modification has little effect on the pore size distribution of 13x molecular sieve.

Combined with Figure 2, it can be concluded that the modification of 13X molecular sieve with CuO can improve the adsorption performance of the material for SO₂ when the CuO attachment load is about 3%. With the increase of the attachment load, the adsorption performance of the material for SO₂ decreases, but the chemical adsorption amount increases.