Study on Adsorption Performance of Fe-Modified ZIF-67 Bimetallic Organic Framework for Toluene

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Introduction
Volatile Organic Compounds (VOCs) are the most common pollutants emitted by industries such as petrochemicals, leather manufacturing, printing, automotive exhaust, and painting. Different countries or organizations have different definitions for VOCs [1]. According to the World Health Organization (WHO), VOCs refer to a variety of organic compounds with boiling points ranging from 50 to 260°C under normal atmospheric pressure [2]. In China, VOCs are defined as organic compounds with a saturated vapor pressure greater than 70.91 Pa at room temperature, standard atmospheric pressure of 101.3 kPa, boiling points below 50-260°C, and an initial boiling point equal to 250°C [3].
VOCs have a significant impact on human health and the natural environment, necessitating efficient and clean methods to handle VOCs generated in production and daily life [4]. Generally, based on different mechanisms for VOCs treatment, VOCs control technologies can be categorized into recovery techniques and decomposition techniques. Recovery techniques involve physically capturing and separating VOCs through methods such as adsorption using porous structures, gas dissolution, phase changes, and selective permeation [5]. Common recovery techniques include adsorption, absorption, condensation, and membrane separation. Decomposition techniques, on the other hand, chemically destroy the molecular structure of VOCs through combustion, catalysis, oxidation, and other methods to eliminate the impact of VOCs [6]. Major decomposition techniques include combustion, photocatalysis, plasma treatment, catalytic oxidation, thermal catalysis, and biodegradation [7].
Compared to traditional materials used for VOCs treatment, Metal-Organic Frameworks (MOFs) offer advantages such as large surface area, high adsorption and catalytic efficiency, and low pollution, making them highly valuable with broad applications and development prospects [8]. MOFs are a novel type of porous framework material formed by self-assembly of metal ions and organic ligands [9]. The preparation methods for MOFs mainly include hydrothermal (solvent-thermal) synthesis, diffusion method (room temperature and atmospheric pressure synthesis), microwave synthesis, ultrasonic synthesis, and mechanochemical synthesis [10].
Although MOFs research and development have been rapidly advancing in recent years, they are not as mature as traditional adsorbents [11]. In this study, a series of Fe-doped ZIF-67 with different Fe doping amount were prepared using a typical solvent-thermal synthesis method. Dynamic adsorption tests were conducted on toluene at room temperature to explore the influence of Fe doping on the adsorption performance of the materials.

Material Preparation Method
The Co(NO 3 ) 2 ꞏ6H 2 O crystal weighing 582 mg was dissolved in 10 ml of methanol solution and subjected to 20 minutes of stirring at 60°C in an ultrasound environment to ensure uniform dispersion and complete dissolution. The resulting methanol mixture containing dissolved Co(NO 3 ) 2 ꞏ6H 2 O is designated as solution A. Additionally, the 2-methylimidazole crystal weighing 1312 mg was measured and dissolved in 20 ml of methanol solution. It underwent 20 minutes of stirring at 60°C in an ultrasound environment to achieve uniform dispersion and complete dissolution. The resulting methanol mixture containing dissolved 2methylimidazole is designated as solution B. After the completion of stirring for solutions A and B, solution A was slowly poured into solution B, forming a purple mixed solution. The mixture was then placed in an ultrasound environment and stirred at 60°C for 20 minutes. After stirring, the mixture was left to stand at room temperature for 24 hours, resulting in the formation of purple crystals at the bottom of the container. The mixed solution was centrifuged for 15 minutes to remove the supernatant, yielding the purple crystals. The purple crystals were subjected to oscillation washing in a sonic cleaning apparatus with the addition of 50 ml of methanol solution. After completion of the washing process, the purple crystals were separated by centrifugation at 4000 rpm. This washing process was repeated four times. Subsequently, the purple crystals were placed in a blast drying oven and dried at 70 °C for 12 h. The final product obtained is referred to as ZIF-67 crystals and denoted as FCM0.
The preparation process for the FCMx series materials was essentially the same as that of ZIF-67 materials. Furthermore, a 10 ml methanol mixed solution containing 56.56 mg of Fe(NO3)3ꞏ9H2O (with a molar ratio of Fe metal ions of 7%) was prepared and referred to as solution C. After the completion of stirring for solutions A and B, solution C was added to the mixture. It underwent 20 minutes of stirring at 60°C in an ultrasound environment to achieve uniform dispersion and complete dissolution. The remaining steps followed the ZIF-67 preparation process, and the resulting samples were labeled as FCM7. Additionally, materials with Fe molar ratios of 14% and 21% were prepared and named FCM14 and FCM21, respectively.

VOCs Adsorption Testing
Prior to each test, the tested materials were placed in a vacuum drying oven and activated at 150 ℃ for 3 hours. The activated samples were then crushed and formed into small spherical pellets. Subsequently, 1 g of the 10-20 mesh sieved sample was loaded into a cylindrical quartz tube adsorption reactor with dimensions of 8×500 mm and a wall thickness of 1 mm. High-purity nitrogen gas was used as the carrier and protective gas, with the gas flow divided into two streams at the outlet of the gas cylinder. One stream of nitrogen gas, serving as the carrier gas, was introduced into the toluene solution and bubbled through the toluene bubbler, while the flow rate of the mixed gas was controlled using a flowmeter and eventually merged with the other stream of nitrogen gas, which served as the protective gas. High-purity oxygen gas was used as the reaction gas, and its flow rate was controlled by a flowmeter before merging with the nitrogen-toluene mixture. The entire gas mixture was then introduced into the quartz tube reactor. By adjusting the flow rate of the flowmeter, the toluene concentration was maintained at 300 ppm, and the total flow rate of the reaction was set at 400 ml/min. The temperature was controlled at 25 ℃, and the oxygen-to-nitrogen concentration ratio was maintained at 2:8 to simulate real-world application environments. The inlet and outlet concentrations of toluene were measured.

Adsorption Performance of FCMx for Toluene at Different Fe Doping Amounts
Under the conditions of an initial toluene concentration of approximately 300 ppm, a temperature of 25 ℃, and a total gas flow rate of 400 ml/min, Figures 1(a)-(d) represented the performance curves of FCM0, FCM7, FCM14, and FCM21 for toluene adsorption.
Through the four adsorption performance curves, it was evident that under different Fe doping conditions, the adsorption process of FCMx for toluene could be divided into four stages: the adsorption unsaturated stage, initial breakthrough stage, rapid breakthrough stage, and adsorption saturation stage.
In Figure 1(a), it could be observed that the adsorption process of FCM0 for toluene could be specifically categorized into the following four stages: Stage I, the adsorption unsaturated stage (0-1400 s, 0 ≤ C < 5% C 0 ). During this stage, toluene was essentially completely adsorbed by FCM0, and the adsorption rate of the material far exceeded the desorption rate, resulting in an outlet concentration of almost 0 ppm.
Stage II, the initial breakthrough stage (1400-1900 s, 5% C 0 ≤ C < C'max). FCM0 began to approach adsorption saturation, and the adsorption curve showed a slow rise. This was because the adsorption capacity for toluene gradually approached its maximum value, leading to a decline in adsorption performance. Furthermore, the desorption rate of toluene increased, resulting in an elevated concentration at the outlet. The calculation began at 5% of the inlet concentration C 0 .
Stage III, the rapid breakthrough stage (1900-4000 s, C'max ≤ C < C 0 ). The adsorption performance of FCM0 for toluene rapidly decreased during this stage, as the adsorption capacity for toluene approached its maximum value. The desorption of toluene increased significantly, leading to a rapid rise in toluene concentration at the outlet, up to the maximum concentration at the quartz tube inlet. The calculation began at the point where the first slope of the adsorption curve reached its maximum value, C'max.
Stage IV, the adsorption saturation stage (4000-6000 s, C = C 0 ). FCM0 had reached a dynamic equilibrium between adsorption and desorption, resulting in a stabilized toluene concentration at the outlet. Figures 1(b) and (c) could also be divided into the same four stages (I, II, III, and IV). With an increase in Fe doping level in the material, FCMx exhibited a trend of decreasing adsorption capacity, followed by an increase, and then a subsequent decrease for toluene. When the Fe doping level was 0%, toluene was completely absorbed by FCM0 and lasted for approximately 1400 s. As the adsorption capacity gradually decreased and reached the saturation stage, it lasted for about 2600 s. When the Fe doping level was 7%, toluene was adsorbed by FCM7 in Stage I for approximately 1200 s, with a slight increase in adsorption time. As the adsorption performance gradually declined and reached the saturation stage, Stages II and III together lasted for approximately 2500 s. When the Fe doping level was 14%, toluene was completely absorbed by FCM14 for approximately 1800 s, which was the longest duration among the four materials. As it transitioned from the adsorption stage to Stages II and III, it lasted for approximately 3200 s. Furthermore, when the Fe doping level continued to increase to 21%, the complete absorption time of toluene by FCM21 decreased to approximately 1500 s. When it reached Stages II and III of adsorption, the total duration was approximately 2800 s, slightly lower compared to FCM14, but still longer than FCM0. In terms of the total time from adsorption initiation to saturation, FCM0, FCM7, FCM14, and FCM21 lasted approximately 4000 s, 3700 s, 5000 s, and 4300 s, respectively. Therefore, the overall adsorption capacity of the FCMx series materials could be ranked as follows: FCM14 > FCM21 > FCM0 > FCM7. Figure 2 illustrated the proportional durations of each stage in the total measurement time. It was evident from the graph that as the Fe doping level increased, the durations of each stage underwent corresponding changes. FCM7 showed a decrease of approximately 5% in the proportion of total adsorption time compared to FCM0. When the Fe doping level was 14%, the proportion of saturation time was the highest, with an increase of approximately 21.6% compared to the lowest FCM7. Regarding the breakthrough stage, the duration increased from 43.3% for FCM0 to 50% for FCM14 under different Fe doping. Similarly, in the adsorption unsaturated stage, the proportion of stage time increased from 23.3% for FCM0 to 30% for FCM14. The increase in the proportion of time in this stage was relatively significant because the bimetallic organic framework structure underwent changes with variations in Fe doping, resulting in deformations in the crystal pore structure and alterations in the total adsorption capacity for toluene. It could be proposed that doping an appropriate amount of Fe in FCMx could modify the pore structure of FCMx, thereby affecting the adsorption performance of toluene and altering the breakthrough time of adsorption.

Adsorption Performance of FCMx for Toluene at Different Temperatures
Under conditions of approximately 300 ppm toluene concentration, a total gas flow rate of 400 ml/min, and temperatures of 25 ℃, 50 ℃, 100 ℃, and 150 ℃, Figures 3(a)-(b) presented the performance curves depicting adsorption performance of FCMx for toluene. The four adsorption performance curves clearly illustrated that FCM14 followed a four-stage adsorption process for toluene, which included the adsorption unsaturated stage, the initial breakthrough stage, the rapid breakthrough stage, and the saturation stage, regardless of the temperature variations. As the temperature increased, the time required for FCM14 to reach adsorption saturation diminished progressively. At ambient temperature, FCM14 achieved complete adsorption over a duration of approximately 1800 s, underwent the breakthrough stage for approximately 3200 s, and attained adsorption-desorption dynamic equilibrium after approximately 5000 s. When the temperature rose to 50 ℃, the duration of the complete adsorption stage decreased to approximately 1600 s, and the breakthrough time reduced to around 3150 s, with little variation compared to ambient temperature. Eventually, adsorption-dynamic equilibrium was established after approximately 4750 s of adsorption. This could be attributed to the intensified thermal motion of toluene molecules at higher temperatures, facilitating their desorption from the crystal pores and consequently reducing the time required to reach dynamic equilibrium. Furthermore, at 100 ℃, as molecular thermal motion intensified further, the duration of the complete adsorption stage further decreased to approximately 1500 s, and the breakthrough time shortened to around 3000 s. Ultimately, adsorption-dynamic equilibrium was achieved after approximately 4500 s. Upon reaching a temperature of 150 ℃, the heightened thermal motion of toluene molecules resulted in a reduced duration of approximately 1400 s for the complete adsorption stage and a breakthrough time of approximately 2900 s. Eventually, adsorption-saturation equilibrium was attained after approximately 4300 s. In summary, the ascending order of the time required for FCM14 to reach adsorption-desorption dynamic equilibrium was as follows: t 25℃ > t 50℃ > t 100℃ > t 150℃ . Fig. 3. The adsorption curves of the FCMx series materials at different temperatures. Figure 4 presented the duration and proportion of each adsorption stage. As the temperature increased, the time required for FCM14 to reach adsorption-desorption dynamic equilibrium continued to decrease. This indicated that toluene molecules became more active as the temperature rose, resulting in a decrease in the adsorption performance of FCM14. In the breakthrough stage, the escape rate of toluene molecules started to accelerate, with the duration decreasing from 3200 s at room temperature to 2900 s at 150 ℃. This suggested that FCM14 was influenced by the thermal motion of toluene molecules in high-temperature environments, leading to a certain decline in adsorption performance, although it still outperformed FCM0 in terms of adsorption effectiveness at room temperature.

Conclusions
The FCMx series materials were subjected to isothermal dynamic adsorption experiments with toluene at ambient temperature to investigate the adsorption performance and mechanisms. FCMx exhibited excellent adsorption performance.
(1) FCM14 with 14 mol% Fe doping in ZIF-67 resulted in the maximum enhancement of adsorption time and capacity, with a maximum adsorption time of approximately 5000 s.
(2) The adsorption capacity of FCM14 decreased by 22.9% with temperature increasing from 25 ℃ to 150 ℃, indicating that the adsorption of toluene on FCMx was significantly influenced by physical adsorption and a certain chemical adsorption.
(3) The adsorption performance of ZIF-67 could be improved by doping with an appropriate amount of Fe.