Experimental Study on the Adsorption Performance of Metal–Organic Framework MIL-101 (Cr) for Indoor Toluene

Abstract

In this study, MIL-101 (Cr) was synthesized and characterized in terms of its physical properties. The adsorption breakthrough curves for toluene were measured and compared to those of conventional adsorbents (i.e., silica gel and activated carbon) at typical indoor concentrations of toluene. The results show that MIL-101 (Cr) exhibits a 5–8 times higher adsorption capacity for toluene compared to silica gel at low concentrations. The adsorption isotherm of MIL-101 (Cr) for toluene conforms to the Langmuir model. Increasing temperature reduces the adsorption breakthrough time and saturation time, but it leads to a significant decrease in the adsorption capacity. During the breakthrough experiment, flow rate had little effect on adsorption capacity, but higher flow rates notably decreased the breakthrough and saturation times. The negative values of ΔG, ΔH, and ΔS indicate that the adsorption of toluene on MIL-101 (Cr) is a spontaneous and exothermic process. Compared to traditional adsorbents, MIL-101 (Cr) exhibits desirable performance in toluene adsorption in indoor environments. It shows significant potential for indoor air purification applications.

1. Introduction

Air quality constitutes a significant global concern. In contemporary society, people spend most of their time indoors [1], and indoor air quality is directly linked to our health and well-being. Indoor air commonly contains higher pollutant concentrations than outdoor environments [2]. Volatile organic compounds (VOCs) are one of the main components of indoor air pollution, which are mainly from the emissions of electronic products, building materials, furniture, and daily necessities [3,4,5]. Short-term exposure to VOCs would induce headaches, dizziness, mucous membrane irritation, allergic reactions, and other symptoms. Prolonged exposure would result in severe respiratory and cardiovascular conditions, as well as increase the risk of leukemia and cancer, ultimately leading to death [6,7,8,9]. Methods to control VOCs can be divided into two types: emission reduction and add-on control techniques. Indeed, significant progress has been made in reducing the emission of pollutants from building materials. However, indoor VOCs still cannot be entirely eliminated [10]. One of the commonly used add-on control techniques is recycling technology [11], which mainly encompasses absorption [12], adsorption [13], membrane separation [14], and condensation [15]. The adsorption method has low cost, low energy consumption, and absence of secondary pollution, leading to its widespread application [16]. Typical adsorption materials include silica gel, activated carbon, and molecular sieves, which have been studied for the adsorption of VOCs [17,18,19]. However, they have shortcomings such as small adsorption capacity, flammability, and difficulty in regeneration, restricting their further application [16,20,21].

Metal–organic framework (MOF) is an emerging porous material with adjustable pore sizes and shapes, a large specific surface area, and high porosity [22], circumventing the limitations of typical adsorption materials, which makes MOFs widely applicable in the field of gas storage and separation [23,24], catalysis [25], proton transport [26], and drug delivery [27]. MOFs have also shown great potential in environmental governance and have been extensively studied in recent years [28,29,30].

MIL-101 (Cr) is a typical mesoporous MOF material with a large specific surface area and good stability [31,32]. It exhibits high surface area and pore volume, exhibiting significant advantages in VOC adsorption [33,34]. Its large and tunable pore sizes facilitate the selective accommodation of larger VOC molecules. Furthermore, MIL-101 (Cr) demonstrates excellent chemical and thermal stability, ensuring long-term effectiveness under varying environmental conditions [35,36]. Many studies have demonstrated its desirable adsorption performance for VOCs. Liu et al. [37] synthesized a composite material Fe₃O₄@NH₂-MIL-101 (Cr), and its formaldehyde adsorption capacity was measured to reach 81.3 mg/g. Wang et al. [38] prepared Cu@MIL-101 (Cr) with different Cu²⁺ doping content. Due to the introduction of unsaturated metal sites by doped copper ions, the adsorption performance of MIL 101 (Cr) for toluene has been improved, with a saturation adsorption capacity of up to 469 mg/g. Jangodaz et al. [39] used the breakthrough curve method to measure the adsorption capacity of MIL-101 (Cr) for ethylbenzene at 500 ppm, and found its adsorption capacity reaches 182 mg/g. Zhao et al. [40] used a microwave irradiation method to synthesize MIL-101 (Cr), and the adsorption capacity for benzene reached 16.5 mmol/g at 288 K, 56.0 mbar. Wang et al. [41] implanted polyethylene glycol (PEG) with hydrophobic short-chains as a hydrophobic barrier into MIL-101 (Cr) to enhance competitive adsorption of VOCs under humidity. The new hydrophobic pores of PEG5@MIL-101 enhanced the adsorption capacity and diffusivity for toluene by 30.8% (4.68 mmol/g at 1 mbar) and 31.5% (1.88 × 10⁻²). Rico-Barragán et al. [42] utilized PET waste as the source of the organic linker to synthesize a new MIL-101 (Cr). The adsorption capacity of the synthesized material for toluene was determined by mass balance. Its adsorption capacity with an initial toluene concentration of 30 ppm was 3.2 mmol/g. Huang et al. [43] designed a quartz crystal microbalance to evaluate the adsorption capacity of MIL-101 for toluene at 750 ppm, which was approximately 0.18 mmol/g. However, the concentrations of VOCs in these studies were at the ppm level or higher. The concentrations of most VOCs in indoor environments are usually lower (in the ppb level, with 1 ppm = 10³ ppb) than the experimental concentrations used in previous studies. The average toluene concentration detected in a group of newly renovated houses in Guangzhou was 0.173 mg/m³ [44]. The World Health Organization (WHO) recommends toluene exposure levels of 0.26 mg/m³ [45]. The Chinese indoor air quality standards (GB 50325-2020) [46] limit the indoor toluene concentration to 0.2 mg/m³. In a French household, a toluene concentration of 0.52 mg/m³ was detected [47].

The combination of high adsorption capacity, stability, and selectivity for aromatic compounds in MIL-101 (Cr) has been extensively demonstrated in numerous studies, which highlight its favorable adsorption performance for VOCs. The adsorption capacity of MIL-101 (Cr) for low-concentration VOCs remains unclear, and its purification performance for indoor VOCs is yet to be determined. Thus, in this study, MIL-101 (Cr) was successfully synthesized, and its specific surface area and pore volume were experimentally determined. Toluene was selected as a representative substance for volatile organic compound (VOC) adsorption experiments, and breakthrough experiments for toluene adsorption by MIL-101 (Cr) were conducted. The breakthrough curve method was employed to compare the adsorption capacities of synthesized MIL-101 (Cr) and silica gel at indoor toluene concentrations. This study aims to provide experimental data support for new choices of indoor air purification methods.

2. Materials and Methods

2.1. Synthesis and Characterization of the Material

The MIL-101 (Cr) used in this work was synthesized based on the method proposed by Ferey et al. [31]. To reduce the environmental damage of raw materials and explore green synthesis methods, chromium nitrate was replaced with chromium chloride with the same molar ratio, and acetic acid was used instead of hydrofluoric acid. The suspension was placed into a stainless-steel high-pressure reactor lined with polytetrafluoroethylene and heated in an oven at 180 °C for 8 h. After cooling to room temperature, the green solid was obtained. Then, the solid was washed with DMF and ethanol, collected by centrifugation at 10,000 rpm for 5 min. The washing process was repeated three times. Finally, it was dried at 150 °C for 12 h under vacuum conditions to obtain dehydrated MIL-101 (Cr).

The powder X-ray diffraction (XRD) pattern of the sample was recorded on an X-ray diffractometer, which was operated at 40 kV for Cu Kα (λ = 0.15406 nm) radiation in the 2θ range of 5° to 80° with a scanning rate of 3.5°/min. The Fourier transform infrared (FT-IR) spectrum was determined with a range of 400–4000 cm⁻¹. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method using adsorption data of the nitrogen adsorption isotherm. The pore distribution was calculated by the T-plot method and Barrett–Joyner–Hallender (BJH) method. The morphologies and particle sizes of the products were characterized using a scanning electron microscope (SEM) at an accelerating voltage of 3 kV after gold deposition.

2.2. Toluene Adsorption Experiments

The breakthrough curve method was employed to measure the adsorption performance of materials. The schematic diagram of this experimental setup is shown in Figure 1, and the main experimental equipment is shown in

Table 1. Main experimental instruments.

Equipment	Model	Manufacturer
Electronic Balance	FA1024E	Changzhou Nebula Electronic Equipment Co., Ltd. (Changzhou, China)
Programmable Dynamic Dilution Apparatus	GT-310A-2	Tianjin Geist Instrument Co., Ltd. (Tianjin, China)
Gas Chromatograph-Photoionization Detector (GC-PID)	FROG5000	Defiant Inc. (Seattle, WA, USA)
Electronic Flowmeter	Bios Defender 520H	MesaLabs Inc. (Lakewood, CO, USA)
Constant Temperature Heating Tape	HX-W2078G	Dechuang Keyi (Beijing) Technology Co., Ltd. (Beijing, China)

.

The entire experimental system can be divided into three parts: gas distribution, adsorption, and detection systems. Firstly, the standard toluene–nitrogen gas is mixed with pure nitrogen in different proportions to obtain gas with different toluene concentrations by adjusting the mass flow controllers. The mixed gas is introduced into the adsorption bed, which is filled with adsorbent material. The constant temperature heating strip is used to control the temperature of the adsorption bed. After passing through the adsorption bed, the toluene gas is measured by GC-PID (A.M.S. Technology Co., Beijing, China). The main components of the gas analysis module of GC-PID are the concentrator, micro chromatography column, and photoionization detector. It uses pre-concentration and photoionization technology to make it extremely sensitive to VOCs. It can detect extremely low concentrations of VOCs and analyze their components with low sampling flow rates. The gas concentration after passing through the adsorption bed is measured at different times to obtain the adsorption breakthrough curve.

The concentration of toluene in indoor environments is mostly within 100 ppb, but in some special cases it may be very high. The concentration of toluene detected in Iranian hair salons even exceeded 1 mg/L [48], significantly surpassing the recommended limit set by the WHO. Long-term exposure to such extreme environments will significantly increase the risk of illness. Considering toluene concentrations in various indoor environments and to investigate the toluene adsorption performance of MIL-101 (Cr) under specific extreme conditions, the initial toluene concentrations in this experiment were set at 30–70 ppb and 300–700 ppb. Breakthrough experiments of different toluene concentrations were conducted at a flow rate of 150–400 mL/min, 18–40 °C. To design a MOF-based air purifier suitable for practical applications, the effects of temperature and flow rate on the toluene adsorption behavior of MIL-101 (Cr) were investigated through multiple experiments at varying temperatures and flow rates. Silica gel and nut shell activated carbon were chosen as conventional adsorbents for comparative experiments.

The most critical parameters in the adsorption breakthrough experiment are breakthrough time and saturation time, which reflect the adsorption capacity of the adsorbent material. In this experiment, the time when the detected concentration reaches 5% of the initial toluene concentration is set as the breakthrough time. When the detected concentration reaches 95% of the initial toluene concentration, it is the saturation time. Based on the above parameters, the equilibrium toluene adsorption capacity can be calculated by Equation (1).

$$q_e={\displaystyle \frac{Q\times C_0\times {10}^{-6}}{W\cdot M}} \cdot \left[t-{\int }_0^t{\displaystyle \frac{C_t}{C_0}}\left(dt\right)\right]$$

(1)

where $q_e$ is the equilibrium adsorption capacity, mg/g; $Q$ is the flow rate, mL/min; $\mathrm{W}$ is the adsorbent weight, g; $\mathrm{M}$ is the relative molecular weight of toluene; $C_0$ is the initial toluene concentration, mg/m³; $C_{\mathrm{t}}$ is the detected toluene concentrations, mg/m³; $t$ is the saturation time, min.

Thermodynamic analysis was carried out to determine the energy changes. Based on the laws of thermodynamics and the Langmuir isotherm, the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were calculated using the Gibbs equation, as described in Equations (2)–(6).

$$\mathsf{\Delta }G=-RTlnK$$

(2)

$$\mathsf{\Delta }G=\mathsf{\Delta }H-T\mathsf{\Delta }S$$

(3)

$$lnK=ln{\displaystyle \frac{q_e}{c_e}}$$

(4)

$$\mathsf{\Delta }H=-Slope\times R$$

(5)

$$\mathsf{\Delta }S=Intercept\times R$$

(6)

where $R$ is the gas constant with a value of 8.314 J/mol·K; $T$ is the absolute temperature in Kelvin; $q_e$ is the equilibrium adsorption capacity (mg/g); $c_e$ is the toluene concentration (mg/m³).

3. Results and Discussion

3.1. Characteristics of MIL-101 (Cr)

The XRD patterns of the synthesized MIL-101 (Cr) material are shown in Figure 2. It is similar to the pattern reported previously [49]. The FT-IR spectrum of the material is exhibited in Figure 3. The bands at 1617 cm⁻¹ correspond to the antisymmetric stretching vibration of -COO-, and 1396 cm⁻¹ corresponds to the symmetric stretching vibration of -COO-, confirming the existence of dicarboxylic acid linkers in the MIL-101 (Cr) framework. The bands at 1510 cm⁻¹ correspond to the skeleton vibration of the benzene ring. The bands at 1020 cm⁻¹ and 746 cm⁻¹ correspond to the in-plane bending and out-of-plane bending vibration absorption peaks of C-H, respectively. The bands at 580 cm⁻¹ may be the stretching vibration of the Cr-O absorption peak. The specific surface area and porosity of the sample were determined by N₂ physisorption, and the results are summarized in Figure 4 and

Table 2. BET characterization results of the MIL-101 (Cr) and silica gel.

Sample	Pore Volume (cm3/g)				Specific Surface Area (m2/g)	Average Pore Size (nm)
	Total Pore Volume	Micropore Volume	Mesopore Volume	Macropore Volume
MIL-101 (Cr)	1.58	1.22 (52%)	0.97 (42%)	0.14 (6%)	2781.49	2.82
Silica gel	0.97	0.0044 (0.43%)	1.00 (98.66%)	0.0092 (0.91%)	355.65	7.85

. The BET surface area and pore volume of the parent MIL-101 (Cr) are 2781.5 m²/g and 2.33 cm³/g, respectively—both values exceed those of silica gel. The larger specific surface area increases the number of accessible adsorption sites, thereby enhancing the material’s adsorption capacity for target pollutants. The SEM image is shown in Figure 5. The synthesized MIL-101 (Cr) exhibits a typical octahedral structure with a perfect cubic symmetry. These results confirmed the identity and surface morphology of the synthesized material. Based on the synthesized material, the adsorption experiments were conducted in an adsorption tube with a size of 1/8 inch, and the mass of the adsorbent was 7.5 mg.

3.2. Effect of Initial Concentration of Toluene on Adsorption

Figure 6 shows the adsorption breakthrough curves of different concentrations of toluene on MIL-101 (Cr). The curve can be divided into three stages. At the first stage, the incoming toluene was completely adsorbed by the material, and the toluene could not be detected at the outlet. At the second stage, as the adsorption proceeded, the material gradually became saturated, and the toluene concentration at the outlet continued to increase. At the third stage, the material was completely saturated, toluene was no longer adsorbed, and the outlet concentration was equal to the initial concentration.

As the initial concentration of toluene increases, the breakthrough curve moves to the left, and the breakthrough time and saturation time decrease. It breaks through faster at high concentrations and slower at low concentrations. When the initial toluene concentration was 30 ppb, the adsorbent bed was broken through at 465 min and saturated at 561 min. The breakthrough time and saturation time gradually decreased as the initial concentration continued to rise. When the initial concentration was 70 ppb, the breakthrough time was 198 min, and the saturation time was 440 min. At the initial concentration of 700 ppb, the breakthrough time was only 88 min, and the saturation time was 308 min. The breakthrough time for the initial concentration of 70 ppb is 42.6% of that for 30 ppb, and the saturation time is 78.4% of that for 30 ppb. The breakthrough time for the initial concentration of 700 ppb is 44.4% of that for 70 ppb, while the saturation time is 70% of that for 70 ppb.

At lower concentrations, the mass transfer driving force for toluene adsorption is smaller, and the coverage of toluene molecules on the material surface is lower. There are fewer toluene molecules interacting with the material surface, so it takes more time for toluene molecules to diffuse to deeper adsorption sites. As the concentration increases, the adsorption sites are occupied by toluene molecules faster, and the adsorption breakthrough time and saturation time gradually shorten.

The adsorption capacity at different concentrations was calculated by Equation (1). The results are shown in Figure 7. The adsorption capacity has an increasing trend as the toluene concentration increases, but the growth rate of the adsorption capacity gradually decreases.

Adsorption is a dynamic equilibrium process. As the concentration increases, the partial pressure of toluene gas increases, and the adsorption/desorption equilibrium moves toward the adsorption direction. At the same time, toluene molecules form clusters on the surface of MIL-101 (Cr), increasing interactions and increasing the equilibrium adsorption capacity.

The Langmuir monolayer adsorption model is applicable to adsorption behavior at low concentrations [50]. The model is used to fit the results, and the formula is shown in Equation (7).

$$q_e={\displaystyle \frac{q_m\cdot K{\cdot C}_e}{1+K{\cdot C}_e}}$$

(7)

where $q_e$ is the equilibrium adsorption capacity, mg/g; $q_m$ is the maximum adsorption capacity, mg/g; $C_e$ is the toluene concentration at equilibrium, ppb; $K$ is the adsorption equilibrium constant.

The fitting curve is shown in Figure 8, and the fitting data is shown in

Table 3. Langmuir model fitting results of MIL-101 (Cr) for toluene adsorption.

Adsorbent	qm (mg/g)	K	R2
MIL-101 (Cr)	1285.77	0.00162	0.99

. The fitting result is ideal. At 30–700 ppb, the adsorption behavior of MIL-101 (Cr) to toluene conforms to the Langmuir single-layer adsorption model.

Comparative experiments were conducted on silica gel under 30–70 ppb toluene concentration conditions, and the breakthrough time and saturation time compared with MIL-101 (Cr) are shown in

Table 4. Comparison of adsorption of toluene on silica gel and MIL-101 (Cr).

Toluene Concentration (ppb)	Breakthrough Time (min)		Saturation Time (min)		Adsorption Capacity (mg/g)
	Silica Gel	MIL-101 (Cr)	Silica Gel	MIL-101 (Cr)	Silica Gel	MIL-101 (Cr)
30	121	465	451	561	10.17	61.71
40	105	341	407	495	11.30	76.23
50	88	275	341	484	12.65	90.75
60	77	242	319	473	13.33	92.57
70	68	198	297	440	14.01	105.53

Note: Temperature is 25 °C; toluene flow rate is 150 mL/min.

; the adsorption capacity isotherms are shown in Figure 9. The adsorption saturation time of MIL-101 (Cr) at 30 ppb toluene concentration is 0.5 times that of silica gel, and the adsorption capacity is 5.7 times that of silica gel. At 70 ppb toluene concentration, the adsorption saturation time of MIL-101 (Cr) is 0.63 times that of silica gel, and the adsorption capacity is 7.5 times that of silica gel.

It shows that MIL-101 (Cr) has a faster adsorption rate and stronger adsorption ability than silica gel. The air purification ability of MIL-101 (Cr) is much higher than that of silica gel. It has high development and application potential for VOCs adsorption and air purification in living environments.

Table 5. Toluene adsorption capacity by porous materials reported in the literature.

Sorbent	Parameters	Adsorption Capacity (mg/g)	Reference
ACF	407 ppm, 200 mL/min, 298 K	160	[51]
AC/ZnO	10 mg/m3, 50 mL/min, 298 K	68	[52]
AC/ZrO2		60	[52]
AC/MgO		56	[52]
AC/CuO		46	[52]
AC		41	[52]
AC	500 ppb	55	[53]
UiO-66	100 ppm, 50 mL/min, 298 K	166	[54]
UiO-66(NH2)		252	[54]
MOF-199		159	[54]
ZIF-67		224	[54]
MIL-101(Fe)		98.3	[54]
MOF-5		32.9	[54]
M-U-0.01	1000 ppm,50 mL/min	257	[55]
Al-Mt@C(3/5)	1000 ppm,100 mL/min 298 K	39.9	[56]
AC	200 ppm, 400 mL/min 303 K	184	[11]
AC	500 ppb, 2.5 L/min	69	[50]
Cu/Beta	50 ppm	77	[57]
MIL-101 (Cr)	500 ppm, 30 mL/min 298 K	331.46	[58]
Silica gel	50 ppb, 150 mL/min 298 K	12.65	This study
MIL-101 (Cr)	50 ppb, 150 mL/min, 298 K	90.75	This study
MIL-101 (Cr)	500 ppb, 150 mL/min, 298 K	599.21	This study
Nut shell AC	500 ppb, 150 mL/min, 298 K	21.5	This study

compares the toluene adsorption capacity of the materials investigated in this study with that of other materials reported in the literature. The comparison of saturation adsorption capacities demonstrates that MIL-101 (Cr) exhibits better adsorption performance for toluene compared to other materials, further suggesting its potential for practical indoor applications in VOCs removal.

3.3. Effect of Temperature on Adsorption

In order to explore the effect of temperature changes on the adsorption performance of MIL-101 (Cr), the adsorption breakthrough curves at three different temperatures were measured at 700 ppb toluene concentration, as shown in Figure 10. And the relevant data is shown in

Table 6. Adsorption capacities of toluene on MIL-101 (Cr) at different temperatures.

Temperature (°C)	Breakthrough Time (min)	Saturation Time (min)	Adsorption Capacity (mg/g)
18	154	407	961.46
25	88	308	652.11
40	40	121	230.57

Note: Initial toluene concentration is 700 ppb; the flow rate is 150 mL/min.

.

As the temperature increases, the breakthrough curve moves to the left, and both the breakthrough time and saturation time decrease, but the adsorption capacity decreases significantly. The adsorption capacity at 40 °C is only 25.8% of that at 18 °C. The saturation time at 40 °C is 30% of that at 18 °C.

The adsorption process is a phase change process of the adsorbate. As the temperature increases, the saturated vapor pressure of toluene vapor increases, causing the equilibrium state to move to the gas phase. Also, adsorption is an exothermic process, and the increase in temperature is not conducive to the progress of adsorption, thereby reducing the adsorption capacity. Temperature changes have a relatively large impact on adsorption capacity. However, in the living environment condition, it still has considerable adsorption capacity within the human comfortable range of 18–25 °C.

3.4. Effect of Gas Flow Rate on Adsorption

In order to explore the effect of flow rate on the adsorption behavior of MIL-101 (Cr), experiments with flow rates of 300 mL/min and 400 mL/min were performed at 700 ppb toluene concentration. The mass of the adsorbent is still 7.5 mg, and the diameter of the adsorption tube is 1/8 inch. The breakthrough curves are as shown in Figure 11, and the relevant data are in

Table 7. Adsorption capacities of toluene on MIL-101 (Cr) at different flow rates.

Flow Rate (mL/min)	Breakthrough Time (min)	Saturation Time (min)	Adsorption Capacity (mg/g)
150	88	308	652.11
300	46	153	645.63
400	33	110	590.92

Note: Initial toluene concentration is 700 ppb; the temperature is 25 °C.

.

As the gas flow rate increases, the saturation time and breakthrough time are shortened, and the adsorption capacity is slightly reduced. The adsorption capacity decreases with the increase in gas flow rate, but the influence of flow rate is weak. Compared with the gas flow rate of 150 mL/min, the adsorption capacity at 400 mL/min is reduced by 6.2%. But the saturation time is significantly shortened by 0.36 times.

The adsorption rate refers to the speed at which the adsorbent completes the adsorption of adsorbate molecules, including the process of molecules diffusing from the gas to the boundary layer and then penetrating to the inner surface of the adsorbent. If the gas flow rate exceeds the adsorption rate, the residence time of the adsorbent molecules in the boundary layer is shorter than the time required to complete adsorption. It will have an adverse impact on adsorption, and the breakthrough time will decrease. On the contrary, if the flow rate is moderate and the adsorbate has enough time to be effectively adsorbed, the breakthrough time will be longer. The increase in flow rate will result in a higher number of adsorbate molecules passing through the material per unit of time, thereby reducing the saturation time. The ideal flow rate should enable adsorption to fully proceed on the adsorbent and ensure appropriate treatment rates of the equipment. In actual application, it is necessary to balance the relationship between adsorption rate and flow rate to achieve the best working conditions of the equipment. Under varying gas flow rates, the adsorption experiments were conducted over a sufficiently long duration, so the saturated adsorption capacity exhibited negligible variation.

3.5. Adsorption Mechanism

Through systematic material characterization and adsorption experimental analysis, it is found that the specific surface area of MIL-101 (Cr) reaches 7.8 times that of silica gel (about 2781.49 m²/g vs. 355.65 m²/g). This characteristic of high specific surface area exposes more effective adsorption sites for the material, fundamentally providing a structural basis for improving the toluene adsorption capacity. Further combined with pore structure analysis, shows that there is a significant correlation between the adsorption performance and the pore volume distribution. The experimental data show that the adsorption capacity of the material for toluene has a highly positive correlation with the pore volume in the range of 0.35–2 nm and the BET specific surface area. This result directly confirms that the microporous region of 0.35–2 nm is the core structural unit for achieving efficient toluene adsorption. The fitting results of the Langmuir model (R² > 0.98) further reveal that the adsorption process of toluene by both materials is mainly monolayer physical adsorption. This characteristic is directly related to the adaptability of the pore size: the uniform microporous network of MIL-101 (Cr) (average pore diameter 2.82 nm) can precisely match the kinetic diameter of toluene molecules (0.58 nm), forming a “size sieving effect” and promoting the rapid diffusion of toluene molecules to the active sites inside the pores; while the macroporous structure of silica gel (average pore diameter 7.85 nm) has a poor matching degree between the pore size and toluene molecules, resulting in the adsorbate being difficult to effectively occupy the pore surface, forming a “site waste” phenomenon. An in-depth analysis of the material pore diameter revealed that the diameter of toluene molecules (0.58 nm) is exactly in the pore diameter sensitive interval. This size-matching characteristic further explains the positive correlation law between the proportion of pore volume and the adsorption capacity—when the proportion of matched micropores in the material is higher, the density of “effective adsorption space” provided by the material per unit mass is greater, thereby significantly improving the toluene adsorption capacity. This is also in line with the findings of some relevant references [49,59,60].

At the level of adsorption interaction, multiple in situ characterization techniques (such as FTIR and XPS) confirm that the adsorption process of toluene by MIL-101 (Cr) is the result of the synergy of multiple effects: The Lewis acid–base interaction between Cr³⁺ unsaturated metal sites and toluene molecules constitutes the main driving force. The π-π stacking interaction between terephthalic acid ligands in the pores and toluene further enhances adsorption stability. Meanwhile, van der Waals forces and weak hydrogen bonding effects, namely the synergistic effect of the cavity system and suitable hydrogen bond acceptors, together form an auxiliary adsorption mechanism, creating a multi-level adsorption force network [61,62].

3.6. Thermodynamic Analysis of Toluene Adsorption on MIL-101 (Cr)

The thermodynamic behavior of toluene adsorption on MIL-101 (Cr) was investigated to assess the orientation and feasibility of the adsorption process [63].

Table 8. The Gibbs free energy of toluene adsorption on MIL-101 (Cr).

Temperature (°C)	Flow Rate (mL/min)	Toluene Concentration (ppb)	ΔG (kJ/mol)
25	150	30	−15.66
25	150	40	−15.47
25	150	50	−15.35
25	150	60	−14.95
25	150	70	−14.89
25	150	300	−14.75
25	150	400	−14.50
25	150	500	−14.32
25	150	600	−14.03
25	150	700	−13.69
18	150	700	−14.66
25	150	700	−13.70
40	150	700	−11.12

shows the Gibbs free energy of toluene adsorption on MIL-101 (Cr). By plotting the relationship between $\mathrm{l}\mathrm{n}({\mathrm{q}}_{\mathrm{e}}/{\mathrm{c}}_{\mathrm{e}})$ and $1/\mathrm{T}$, $\mathsf{\Delta }\mathrm{H}$ (−49.8 kJ) and $\mathsf{\Delta }\mathrm{S}$ (−0.21 kJ/(mol⋅K)) were determined from the slope and intercept of the resulting line. These results are presented in Figure 12. The negative $\mathsf{\Delta }\mathrm{G}$ and $\mathsf{\Delta }\mathrm{H}$ values indicated that the adsorption of toluene on MIL-101 (Cr) was a spontaneous process. Additionally, the negative $\mathsf{\Delta }\mathrm{S}$ value indicated the exothermic nature of the toluene adsorption process.

3.7. Discussion of Cycling Stability and Effect of Relative Humidity

MIL-101 (Cr) exhibits desirable water stability and cycling performance. Ehrenmann et al. [35] conducted a study on the water adsorption cycling behavior of MIL-101 (Cr). Following 40 consecutive adsorption/desorption cycles of water, the material retained 96.8% of its initial adsorption capacity. Zhao et al. [64] investigated its water adsorption and cycling performance. The result showed that the residual desorption amount after 10 cycles remained below 9% of the total equilibrium adsorption capacity. MIL-101 (Cr) exhibits excellent regenerative performance for VOC adsorption. Shafiei et al. [65] reported that when nitrogen purging at 120–180 mL/min was used to regenerate VOC-saturated MIL-101 (Cr) at 303 K, the regeneration efficiencies for four cycles were 61.29%, 58.06%, 54.83%, and 54.82% sequentially, with the adsorption capacity stabilizing at 34.89–40.72 wt% after regeneration. Tehrani et al. [66] regenerated MIL-101 (Cr) in a nitrogen atmosphere at 120 °C for 4 h, and the material maintained stable performance after five consecutive adsorption–desorption cycles, further verifying its superior cyclic regeneration capability.

Some studies have modified MIL-101 (Cr) to improve its cyclic performance in toluene adsorption. Wang et al. [38] enhanced the performance of MIL-101 (Cr) by doping with copper ions. After five consecutive adsorption–desorption cycles, the Cu-2@MIL-101 (Cr) composite maintained a toluene desorption rate of 86%.

In practical situations, the competitive adsorption of water vapor and VOCs on adsorbents cannot be ignored. Xian et al. [67] performed breakthrough experiments, revealing that the presence of water vapor in the gas stream led to a significant reduction in the adsorption capacities of MIL-101 (Cr) for VOCs. This phenomenon was attributed to the competitive adsorption of water molecules on the surface sites of MIL-101 (Cr). Vellingiri et al. [54] conducted toluene adsorption experiments on several common MOF materials, and the results indicated a significant decrease in adsorption capacity with increasing relative humidity. When the relative humidity increased from 25% to 50%, the adsorption capacities of UiO-66, UiO-66 (NH₂), MOF-199, ZIF-67, and 4A Zeolite decreased by 57.9%, 48.5%, 78.8%, 65.9%, and 73.1%, respectively. Zhang et al. [68] hydrophobically modified MIL-101 (Cr), achieving an adsorption capacity 6.3 times higher than that of the unmodified material under conditions of 50% relative humidity (RH) and 298 K. The breakthrough curve further indicated that the toluene adsorption capacity of P-MIL-101 (Cr)@PA-NH2 remained stable even at 80% RH, demonstrating a substantial enhancement in competitive adsorption performance under high-humidity environments. Pristine MIL-101 (Cr) exhibits desirable water adsorption performance. In addition, its adsorption ability for low-concentration VOCs provides broad application prospects for indoor air quality control. However, its great water adsorption capacity causes the VOCs adsorption performance to be affected by water vapor. Exploring the modification of MIL-101 (Cr) for different application scenarios could be a promising research direction in the future. It is also recommended to conduct research on the adsorption performance of hydrophobically modified MIL-101 (Cr) for low-concentration VOCs in high-humidity environments in the future.

Additionally, various VOCs exhibit competitive adsorption behavior on adsorbents. Investigating the co-adsorption of multi-component VOCs is crucial for advancing the practical application of MOF materials in indoor air purification.

4. Conclusions

In order to explore the purification performance of MIL-101 (Cr) for indoor VOCs, its adsorption performance for toluene at low concentrations in an indoor environment was experimentally measured using the breakthrough curve method. The main findings can be summarized as follows:

• As the concentration of toluene at indoor levels increases, the adsorption breakthrough time of MIL-101 (Cr) to toluene gradually shortens, the adsorption capacity gradually increases, but the growth rate of the adsorption capacity gradually decreases. The adsorption isotherm is generally in line with the Langmuir single-layer adsorption model.
• The adsorption capacity of MIL-101 (Cr) for low concentrations of toluene is five–eight times that of silica gel. It could be a material with great potential for indoor air purification. It is expected to become a good substitute for silica gel in the application of purification air conditioning systems.
• Although the increase in temperature will shorten the breakthrough time and saturation time, the adsorption capacity decreases significantly. The adsorption capacity at 40 °C is only 25.8% of that at 18 °C. In practical applications, it is better to control the temperature below 25 °C.
• As the gas flow rate increases, the saturation time and breakthrough time are shortened, and the adsorption capacity is slightly reduced. The influence of flow rate on the adsorption capacity is negligible. Increasing the flow rate appropriately can enhance the adsorption process and reduce the saturation time. However, excessive flow rate can affect the penetration of adsorbate molecules, making it easier for the adsorbent to be broken through. In actual application, it is necessary to balance the relationship between adsorption rate and flow rate to achieve the best working conditions of the equipment.

Although MIL-101 (Cr) demonstrates desirable adsorption and purification performance for indoor toluene, the presence of chromium, a heavy metal, may pose potential hazards. Future research could focus on synthesizing environmentally friendly MOFs with superior VOC adsorption capacity. Meanwhile, the adsorption mechanism of MOFs for pollutants has not been thoroughly studied, and systematic exploration in this regard will be conducted in the future.