Enhancement of Gaseous o-Xylene Elimination by Chlorosulfonic Acid-Modified H-Zeolite Socony Mobil-5

It is important to develop effective strategies for enhancing the removal capacity of aromatic volatile organic compounds (VOCs) by modifying conventional porous adsorbents. In this study, a novel HZSM-5 zeolite-supported sulfonic acid (ZSM−OSO3H) was prepared through ClSO3H modification in dichloromethane and employed for the elimination of gaseous o-xylene. The ClSO3H modification enables the bonding of −OSO3H groups onto the HZSM-5 support, achieving a loading of 8.25 mmol·g−1 and leading to a degradation in both crystallinity and textural structure. Within an active temperature range of 110–145 °C, ZSM−OSO3H can efficiently remove o-xylene through a novel reactive adsorption mechanism, exhibiting a removal rate exceeding 98% and reaching a maximum breakthrough adsorption capacity of 264.7 mg. The adsorbed o-xylene derivative is identified as 3,4-dimethylbenzenesulfonic acid. ZSM−OSO3H demonstrates superior adsorption performance for o-xylene along with excellent recyclability. These findings suggest that ClSO3H sulfonation offers a promising approach for modifying various types of zeolites to enhance both the elimination and resource conversion of aromatic VOCs.


Introduction
The emission of various aromatic volatile organic compounds (VOCs) from industrial activities poses a significant threat to human health and the atmospheric environment due to their high toxicity and carcinogenic properties [1][2][3][4].In order to address safety concerns and comply with more stringent regulations, various techniques have been developed for the removal of aromatic VOCs [5][6][7][8].Among them, adsorption was widely employed as an independent purification or pretreatment approach for capturing low-concentration aromatic VOCs (below 2500 mg•m −3 ) due to its operational simplicity and economic feasibility.However, the use of adsorbents with superior performance in capturing aromatic VOCs is a prerequisite [9][10][11].
A series of typical porous zeolites, such as ZSM-5, Y, X, and BEA, have been developed and applied for aromatic VOC adsorption [12,13].Compared with commercial carbonbased materials such as activated carbons [14], activated carbon fibers [15], and biochar [16], zeolites exhibited superior regeneration performance, excellent thermal stability, and incombustibility [17].More importantly, they also demonstrated a significant capacity or potential improvement in capturing aromatic VOCs.Furthermore, zeolites were crystalline aluminosilicate minerals characterized by a porous structure, and their adsorption performance is intricately linked to the distribution of pores and surface hydrophobicity.Therefore, optimizing pore distribution and surface modification are crucial strategies for effectively enhancing the adsorption capacity of aromatic VOCs [18][19][20][21].
The adsorption capacities and diffusion efficiencies of toluene were enhanced by the introduction of 5-10 nm mesopores on micro-porous ZSM-5 zeolite via NaOH etching [22].The ZSM-5/SBA-15 composites were proven to have stronger hydrophobicity and higher toluene adsorption capacity compared to ZSM-5 due to the introduction of all-siliceous SBA-15 and the construction of intracrystalline mesopores [23].In order to enhance the toluene adsorption capacity of zeolite under high relative humidity conditions, core-shell structured zeolite-hydrophobic organic polymer composites were synthesized [24].However, previous studies have primarily focused on regulating the physical adsorption between zeolite and aromatic VOCs.Therefore, it is necessary to explore the selection of appropriate reagents to facilitate stronger chemical adsorption of aromatic VOCs on the zeolite by modifying its surface.
Herein, chlorosulfonic acid (ClSO 3 H) was chosen to modify HZSM-5 (proton form zeolite Socony Mobil-5), namely the HZSM-5 zeolite-supported sulfonic acid (ZSM−OSO 3 H) was synthesized by conducting a sulfonation reaction between ClSO 3 H and the ample hydroxy groups present in HZSM-5 (Scheme 1).Then, the enhanced removal ability of ZSM−OSO 3 H under different process conditions was examined using o-xylene as a representative pollutant of aromatic VOCs.Various characterization methods were employed to analyze the ZSM−OSO 3 H and the adsorbed o-xylene derivative on its surface.This study presents an investigation into the feasibility of utilizing ZSM−OSO 3 H as a heterogeneous reactive adsorbent for the removal and recovery of o-xylene from gas streams, thereby offering a potential approach to modify other types of zeolites in order to enhance the elimination of VOCs.crystalline aluminosilicate minerals characterized by a porous structure, and their adsorption performance is intricately linked to the distribution of pores and surface hydrophobicity.Therefore, optimizing pore distribution and surface modification are crucial strategies for effectively enhancing the adsorption capacity of aromatic VOCs [18][19][20][21].
The adsorption capacities and diffusion efficiencies of toluene were enhanced by the introduction of 5-10 nm mesopores on micro−porous ZSM−5 zeolite via NaOH etching [22].The ZSM−5/SBA−15 composites were proven to have stronger hydrophobicity and higher toluene adsorption capacity compared to ZSM−5 due to the introduction of allsiliceous SBA−15 and the construction of intracrystalline mesopores [23].In order to enhance the toluene adsorption capacity of zeolite under high relative humidity conditions, core−shell structured zeolite−hydrophobic organic polymer composites were synthesized [24].However, previous studies have primarily focused on regulating the physical adsorption between zeolite and aromatic VOCs.Therefore, it is necessary to explore the selection of appropriate reagents to facilitate stronger chemical adsorption of aromatic VOCs on the zeolite by modifying its surface.
Herein, chlorosulfonic acid (ClSO3H) was chosen to modify HZSM−5 (proton form zeolite Socony Mobil−5), namely the HZSM−5 zeolite−supported sulfonic acid (ZSM−OSO3H) was synthesized by conducting a sulfonation reaction between ClSO3H and the ample hydroxy groups present in HZSM−5 (Scheme 1).Then, the enhanced removal ability of ZSM−OSO3H under different process conditions was examined using oxylene as a representative pollutant of aromatic VOCs.Various characterization methods were employed to analyze the ZSM−OSO3H and the adsorbed o-xylene derivative on its surface.This study presents an investigation into the feasibility of utilizing ZSM−OSO3H as a heterogeneous reactive adsorbent for the removal and recovery of o-xylene from gas streams, thereby offering a potential approach to modify other types of zeolites in order to enhance the elimination of VOCs.
Scheme 1. Schematic of the preparation for ZSM−OSO3H.

Structure and Basic Properties of Adsorbents
From Scheme 1, it can be observed that the sulfonic acid group of ZSM−OSO3H is anchored onto HZSM−5 through the sulfonation reaction of chlorosulfonic acid with its surface hydroxyl groups (eliminating HCl).After conducting an acid-base titration measurement, it was determined that the sulfonic acid loading was 8.25 mmol•g −1 , from which its mass percentage content was calculated to be 66.8%.This value closely aligns with the weight loss rate (68.2%) of ZSM−OSO3H within the testing temperature range of 50-300 °C (Figure 1a) [25,26].This suggests that the sulfonic acid groups were anchored on the Scheme 1. Schematic of the preparation for ZSM−OSO 3 H.

Structure and Basic Properties of Adsorbents
From Scheme 1, it can be observed that the sulfonic acid group of ZSM−OSO 3 H is anchored onto HZSM-5 through the sulfonation reaction of chlorosulfonic acid with its surface hydroxyl groups (eliminating HCl).After conducting an acid-base titration measurement, it was determined that the sulfonic acid loading was 8.25 mmol•g −1 , from which its mass percentage content was calculated to be 66.8%.This value closely aligns with the weight loss rate (68.2%) of ZSM−OSO 3 H within the testing temperature range of 50-300 • C (Figure 1a) [25,26].This suggests that the sulfonic acid groups were anchored on the HZSM-5, thereby improving its stability and reaction activity [26].The ZSM−OSO 3 H FTIR spectrum exhibited additional bands at 2610, 1278, 1006, 889, and 850 cm −1 compared to the HZSM-5 spectrum (Figure 1b), which were attributed to the presence of anchored sulfonic acid [26,27].The SEM images of HZSM-5 (Figure 1c) and ZSM−OSO 3 H (Figure 1d) Molecules 2024, 29, 3507 3 of 11 revealed a dispersed polyhedral morphology of the particles.The ClSO 3 H modification induced an increase in surface roughness and aggregation tendency for ZSM−OSO 3 H, which could be attributed to the etching effect of ClSO 3 H in CH 2 Cl 2 medium and the strong polarity (hydrophilicity) of anchored sulfonic acid groups.The etching and modification effects could be further confirmed through the EDS results (Figure 1e,f).In ZSM−OSO 3 H, the presence of sulfur indicated successful anchoring of sulfonic acid groups.Simultaneously, compared to HZSM-5, there was an increase in silicon content (from 28.3 atom.% to 35.1 atom.%) and a decrease in aluminum content (from 4.2 atom.% to 2.0 atom.%), which could be attributed to the minor leaching of Al 3+ ions from the zeolite skeleton.
distribution curve demonstrates an extremely low volume distribution (dv/dD), indicating an exceptionally low specific surface area (10 m 2 •g −1 ) and negligible pore volume (0.02 cm 3 •g −1 ), which might be attributed to the presence of anchored sulfonic acid group in the ZSM−OSO3H.These groups occupy the pore surfaces, resulting in blockage of the pore surfaces [25,26].Considering that zeolite's microporous network is composed of intersecting straight and sinusoidal channels (0.53 × 0.56 nm 2 and 0.51 × 0.55 nm 2 , respectively) [30], the selective anchoring of sulfonic acid groups in specific pores may significantly impact the adsorption and diffusion performance of o-xylene, implying that regulating the porosity of zeolite could be an effective strategy to improve the distribution and quantity of anchored sulfonic acid groups.

Enhanced Removal Performance of o-xylene on ZSM−OSO3H
The elimination performances of HZSM−5 and ZSM−OSO3H adsorbents were assessed through a series of o-xylene dynamic breakthrough tests conducted under various process conditions.The removal ratios of o-xylene were investigated as a function of temperature, ranging from 30 °C to 200 °C, using a continuous step−by−step heating manner (at a rate of 2 °C•min −1 ).The experiments were conducted with a bed height of 15 mm, a flow rate of 0.050 L•min −1 , and an inlet concentration of 7.5 mg•L −1 (Figure 2d).The removal ratio of HZSM−5, like conventional porous adsorbent, exhibited a significant decline with increasing temperature, while maintaining the higher removal rate (>75%) at lower temperatures (<50 °C) [31][32][33].This behavior could be attributed to the physical adsorption phenomenon resulting from its extensive surface area and pore volume [31,33].The elimination of ZSM−OSO3H for o-xylene, however, presented an atypical behavior.Specifically, the removal curve of o-xylene could be divided into three distinct regions: the activation region (<110 °C), the active region (110~145 °C), and the deactivation region (>145

Enhanced Removal Performance of o-Xylene on ZSM−OSO 3 H
The elimination performances of HZSM-5 and ZSM−OSO 3 H adsorbents were assessed through a series of o-xylene dynamic breakthrough tests conducted under various process conditions.The removal ratios of o-xylene were investigated as a function of temperature, ranging from 30 • C to 200 • C, using a continuous step−by−step heating manner (at a rate of 2 • C•min −1 ).The experiments were conducted with a bed height of 15 mm, a flow rate of 0.050 L•min −1 , and an inlet concentration of 7.5 mg•L −1 (Figure 2d).The removal ratio of HZSM-5, like conventional porous adsorbent, exhibited a significant decline with increasing temperature, while maintaining the higher removal rate (>75%) at lower temperatures (<50 • C) [31][32][33].This behavior could be attributed to the physical adsorption phenomenon resulting from its extensive surface area and pore volume [31,33].The elimination of ZSM−OSO 3 H for o-xylene, however, presented an atypical behavior.Specifically, the removal curve of o-xylene could be divided into three distinct regions: the activation region (<110 • C), the active region (110~145 • C), and the deactivation region (>145 • C).The removal efficiency of o-xylene by ZSM−OSO 3 H increased with the rising temperature in the activation region and decreased in the deactivation region.In the active region, a stable elimination of over 98% of o-xylene from the gaseous stream was observed, indicating a robust interaction between the anchored sulfonic acid group of ZSM−OSO 3 H and o-xylene within this temperature range, thereby enhancing its elimination effectiveness.The reaction at the gas-solid interface resulted in the removal of o-xylene (refer to Section 2.3 for details).Therefore, this process can be referred to as reactive adsorption.
Inside the active region, four discrete temperatures (110 • C, 120 • C, 130 • C, and 140 • C) were selected to experimentally record the isothermal breakthrough curves of ZSM−OSO 3 H for o-xylene under identical process conditions (a bed height of 15 mm, a flow rate of 0.050 L•min −1 , and an inlet concentration of 7.5 mg•L −1 ).The corresponding adsorption performance metrics are provided in detail in Table 1.For comparison purposes, the physical adsorption results of HZSM-5 for o-xylene at a temperature of 30 • C are also presented in Figure 3a.Clearly, the adsorption performance of ZSM−OSO 3 H was significantly superior to that of HZSM-5, particularly at the intermediate temperature of 120 • C. In comparison to HZSM-5's physical adsorption of o-xylene, at this optimized temperature, ZSM−OSO 3 H' t B extended by 18.4 min, Q B increased by 3.7 times, and Q m increased by 3.4 times.The removal efficiency of ZSM−OSO 3 H towards o-xylene initially increased and then decreased as the bed temperature rose within a specific activation temperature range.This temperature−responsive behavior differed from that of aromatic VOCs by conventional porous materials, which exhibited a gradual decline in performance with increasing bed temperature [34][35][36][37].However, while maintaining the bed temperature at 120 • C, variations in bed height, flow rate, and inlet concentration individually exhibited similar tendencies in terms of their impact on the removal efficiency of o-xylene as observed in conventional physical adsorption processes [35][36][37][38].Specifically, the increase in bed height (Figure 3b), coupled with the decrease in flow velocity (Figure 3c) and initial concentration (Figure 3d), led to an upward trend for both Q B and Q m values.Under the conditions with flow rate of 0.025 L•min −1 , initial concentration of 7.5 mg•L −1 , and bed height of 15 mm, the maximum Q B and Q m were determined to be 264.7 mg•g −1 and 456.4 mg•g −1 , respectively, showcasing the superiority of ZSM−OSO 3 H when compared with the physically adsorbing capacities of aromatic VOCs by common porous materials (Table 2).The enhancement effect might be attributed to the alterations in the interaction mechanism between o-xylene and ZSM−OSO 3 H induced by chlorosulfonic acid modification, rather than conventional enhancement of interface mass transfer.

o-Xylene Reactive Adsorption and Reusability of ZSM−OSO 3 H
The ethanol solvent was utilized to extract the adsorbed o-xylene derivative from the spent ZSM−OSO 3 H, followed by identification of the extracted derivative using the HPLC-MS methodology.The mass spectrum is presented in Figure 4a.In the negative ion mode, a prominent molecular ion peak at m/z = 185 was observed, indicating that the relative molecular mass of the adsorption product was 186.Therefore, a plausible mechanism for o-xylene elimination by the novel ZSM−OSO 3 H adsorbent has been proposed in combination with our previous research [25,26,44] (Scheme 2).The enhanced adsorption performance of o-xylene is achieved by undergoing a sulfonation reaction with the sulfonic acid group in ZSM−OSO 3 H, leading to its conversion into a non-volatile sulfonic acid derivative (m/z = 185), thereby facilitating its efficient removal from the gaseous stream.In this process, the sulfonic acid groups serve as active sites for the elimination of o-xylene, contrasting with the hydroxyl groups in unmodified HZSM-5 that function as ordinary adsorption sites.The sulfonic acid groups play a crucial role in this context.We propose that the surface reaction enthalpy between o-xylene and anchored sulfonic acid groups is more significant compared to its adsorption enthalpy with surface hydroxyl groups [45].Obtaining relevant enthalpy data through DFT calculations, molecular modeling, or experimental studies will contribute to a deeper understanding of the adsorption process in perspective studies [46][47][48].Additionally, the sulfonation reaction primarily involved electrophilic substitution, wherein electrophiles typically attacked the ortho-or para-position of the methyl group on the benzene ring due to their higher electron cloud density, rendering them more susceptible to electrophilic attack [49,50].Consequently, it could be inferred that the resulting sulfonic acid derivative was 3,4-dimethylbenzenesulfonic acid.

Materials and Chemicals
The HZSM−5 zeolite with a SiO2/Al2O3 ratio of 46 was purchased from Nankai University Catalyst Co., Ltd.(Tianjin, China).All chemicals, such as chlorosulfonic acid, oxylene, dichloromethane, potassium hydrogen phthalate, methanol, sodium hydroxide, and hydrochloric acid, were of analytical grade.The sulfonic acid group was simply incorporated into the HZSM−5 zeolite during the synthesis of ZSM−OSO3H.Specifically, the dichloromethane solution (10 mL) of chlorosulfonic acid (2.5 mL) was slowly added to a suspension of HZSM−5 in dichloromethane (3 g/15 mL) at ambient temperature and stirred for 3 h followed by alternating treatment of filtration and chloroform washing to give the sulfonated zeolite.The sulfonic acid loading of the ZSM−OSO3H was determined to be 8.25 mmol•g −1 using a classic acid-base titration method.

Adsorption Measurements
A custom-designed dynamic breakthrough test was developed to assess the performance of ZSM−OSO3H in removing o-xylene from gas streams.Prior to each test, the ZSM−OSO3H material was packed into a glass column with a diameter of 6 mm (φ6 mm), and the inlet concentration of o-xylene was adjusted to 7.5 mg•L −1 (C0).The flow rate was controlled at 0.05 L•min −1 using a flow meter, and the concentration of o-xylene was monitored using the flame ionization detector (FID) of a gas chromatograph (GC7900).The temperature range for the reaction between ZSM−OSO3H and o-xylene was determined through continuous step-by-step heating.The impact of typical process conditions, such as adsorption temperature, bed height, flow rate, and inlet concentration, on the adsorption capacity of ZSM−OSO3H for o-xylene was investigated through a breakthrough test.The removal ratios of o-xylene (Xo-x, %) and the breakthrough adsorption capacity (QB, mg•g −1 ) were calculated using Equations ( 1) and ( 2), respectively.fonic acid group in ZSM−OSO3H, leading to its conversion into a non-volatile sulfonic acid derivative (m/z = 185), thereby facilitating its efficient removal from the gaseous stream.
In this process, the sulfonic acid groups serve as active sites for the elimination of o-xylene, contrasting with the hydroxyl groups in unmodified HZSM−5 that function as ordinary adsorption sites.The sulfonic acid groups play a crucial role in this context.We propose that the surface reaction enthalpy between o-xylene and anchored sulfonic acid groups is more significant compared to its adsorption enthalpy with surface hydroxyl groups [45].
Obtaining relevant enthalpy data through DFT calculations, molecular modeling, or experimental studies will contribute to a deeper understanding of the adsorption process in perspective studies [46][47][48].Additionally, the sulfonation reaction primarily involved electrophilic substitution, wherein electrophiles typically attacked the ortho− or para−position of the methyl group on the benzene ring due to their higher electron cloud density, rendering them more susceptible to electrophilic attack [49,50].Consequently, it could be inferred that the resulting sulfonic acid derivative was 3,4-dimethylbenzenesulfonic acid.
After recovering the resulting 3,4-dimethylbenzenesulfonic acid from the spent ZSM−OSO3H through ethanol extraction, the HZSM−5 support was reclaimed.By subjecting it to chlorosulfonic acid modification once again in dichloromethane (3 g/15 mL) under ambient temperature, a new ZSM−OSO3H adsorbent was obtained and applied for subsequent reactive adsorption of o-xylene.After undergoing four cycles of adsorption, extraction, and re-sulfonation, Figure 4b illustrates the QB values of o-xylene for each treatment.The QB values for each ZSM−OSO3H adsorbent showed only minimal fluctuations between 171.2 mg•g −1 and 173.3 mg•g −1 compared to their initial use, thereby demonstrating the exceptional regeneration performance of the HZSM−5 support.To be more precise, the resulting adsorbed aryl sulfonic acid product could be recovered through solvent extraction, while the regenerated HZSM−5 zeolite could be reused for preparing a fresh batch of ZSM−OSO3H adsorbent, highlighting the advantages of this method in terms of both sorption efficiency and adherence to green chemistry principles.After recovering the resulting 3,4-dimethylbenzenesulfonic acid from the spent ZSM−OSO 3 H through ethanol extraction, the HZSM-5 support was reclaimed.By subjecting it to chlorosulfonic acid modification once again in dichloromethane (3 g/15 mL) under ambient temperature, a new ZSM−OSO 3 H adsorbent was obtained and applied for subsequent reactive adsorption of o-xylene.After undergoing four cycles of adsorption, extraction, and re-sulfonation, Figure 4b illustrates the Q B values of o-xylene for each treatment.The Q B values for each ZSM−OSO 3 H adsorbent showed only minimal fluctuations between 171.2 mg•g −1 and 173.3 mg•g −1 compared to their initial use, thereby demonstrating the exceptional regeneration performance of the HZSM-5 support.To be more precise, the resulting adsorbed aryl sulfonic acid product could be recovered through solvent extraction, while the regenerated HZSM-5 zeolite could be reused for preparing a fresh batch of ZSM−OSO 3 H adsorbent, highlighting the advantages of this method in terms of both sorption efficiency and adherence to green chemistry principles.

Materials and Chemicals
The HZSM-5 zeolite with a SiO 2 /Al 2 O 3 ratio of 46 was purchased from Nankai University Catalyst Co., Ltd.(Tianjin, China).All chemicals, such as chlorosulfonic acid, o-xylene, dichloromethane, potassium hydrogen phthalate, methanol, sodium hydroxide, and hydrochloric acid, were of analytical grade.The sulfonic acid group was simply incorporated into the HZSM-5 zeolite during the synthesis of ZSM−OSO 3 H.Specifically, the dichloromethane solution (10 mL) of chlorosulfonic acid (2.5 mL) was slowly added to a suspension of HZSM-5 in dichloromethane (3 g/15 mL) at ambient temperature and stirred for 3 h followed by alternating treatment of filtration and chloroform washing to give the sulfonated zeolite.The sulfonic acid loading of the ZSM−OSO 3 H was determined to be 8.25 mmol•g −1 using a classic acid-base titration method.

Adsorption Measurements
A custom-designed dynamic breakthrough test was developed to assess the performance of ZSM−OSO 3 H in removing o-xylene from gas streams.Prior to each test, the ZSM−OSO 3 H material was packed into a glass column with a diameter of 6 mm (φ6 mm), and the inlet concentration of o-xylene was adjusted to 7.5 mg•L −1 (C 0 ).The flow rate was controlled at 0.05 L•min −1 using a flow meter, and the concentration of o-xylene was monitored using the flame ionization detector (FID) of a gas chromatograph (GC7900).The temperature range for the reaction between ZSM−OSO 3 H and o-xylene was determined through continuous step-by-step heating.The impact of typical process conditions, such as adsorption temperature, bed height, flow rate, and inlet concentration, on the adsorption capacity of ZSM−OSO 3 H for o-xylene was investigated through a breakthrough test.The removal ratios of o-xylene (X o-x , %) and the breakthrough adsorption capacity (Q B , mg•g −1 ) were calculated using Equations ( 1) and (2), respectively.
where V q (m 3 •min −1 ) represents the gas flow rate; m (g) is the adsorbent mass; C in and C out (mg•L −1 ) denote the inlet and outlet o-xylene concentrations in the gas flow, respectively; t B (min) infers the breakthrough time, which is the time for the C out /C in ratio to reach 0.05.
To identify the adsorbed o-xylene derivative and reusability of the spent ZSM−OSO 3 H, it was repeatedly extracted with absolute ethanol until aliquots of the extracts spotted onto thin layer chromatography plates were no longer visualized under UV light [44].The residual solid was washed with distilled water and dried at 100 • C to recover HZSM-5 zeolite, and then the recovered HZSM-5 zeolite was converted again in dichloromethane (3 g/15 mL) under ambient temperature to the ZSM−OSO 3 H.The adsorbate was obtained by evaporating the extract [25,26].

Characterization
The morphology and chemical composition of the adsorbents were investigated using an S-4800 cold-field emission scanning electron microscope equipped with an INCA 350 energy dispersive spectrometer accessory (SEM-EDS; Hitachi, Tokyo, Japan).Fourier−transform infrared spectra (FTIR; KBr pellets) were recorded on a TENSOR 27 spectrometer (Bruker AXS, Karlsruhe, Germany).The N 2 adsorption/desorption isotherms of the materials were measured at 77 K using a Kubo X1000 automatic surface area and pore analyzer (Beijing Builder Co., Ltd., Beijing, China).Prior to testing, the materials were subjected to treatment at 100 • C for 1.5 h in an MD-300 preprocessor (Beijing Builder Co., Ltd., China).The specific surface area, pore volume, pore width, and pore size distribution were determined using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models.The thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses were conducted using the STA 449F 3TG/DSC analyzer (Netzsch, Germany).In this analysis, the samples were heated at a rate of 2.5 • C•min −1 from 50 to 300 • C under a nitrogen atmosphere.X-ray photoelectron spectroscopy (XPS) analysis was performed using an ESCALAB 250Xi spectrometer (Thermo Fisher, Dartford, UK) with monochromatic Al Kα radiation (hν = 1486.6eV).The X-ray diffraction (XRD) analysis of the adsorbent was carried out using a D8 Advance X-ray diffractometer operating with Cu Kα radiation (λ = 0.154 nm; Bruker AXS, Germany).The adsorbed o-xylene derivative was subjected to analysis using high-performance liquid chromatography−mass spectrometry (HPLC-MS) on an Agilent 1260 system (Agilent Technologies, Santa Clara, CA, USA).

Conclusions
The HZSM-5 zeolite-supported sulfonic acid (ZSM−OSO 3 H), with a loading of 8.25 mmol•g −1 , was synthesized via the ClSO 3 H sulfonation method in dichloromethane.The results obtained from TG/DSC, FTIR, SEM, XRD, and N 2 adsorption-desorption analyses confirmed the successful bonding of sulfonic acid groups onto the HZSM-5 support.However, it was found that the modification with ClSO 3 H led to a degradation in both crystallinity and texture structure.The dynamic breakthrough test results demonstrated that the ZSM−OSO 3 H adsorbent exhibited an active temperature range of 110-145 • C, effectively eliminating over 98% of o-xylene from the gaseous stream.The removal efficiency of ZSM−OSO 3 H towards o-xylene initially increased and then decreased as the bed temperature rose within this active temperature range, showcasing its superior adsorption performance compared to HZSM-5.Furthermore, increasing the bed height while decreasing flow velocity and initial concentration led to a simultaneous increase in both Q B and Q m values.Under the given conditions with a flow rate of 0.025 L•min −1 , initial concentration of 7.5 mg•L −1 , and bed height of 15 mm, the maximum Q B and Q m values were determined to be 264.7 mg•g −1 and 456.4 mg•g −1 , respectively.The HPLC-MS characterization revealed that the adsorbed o-xylene derivative was identified as 3,4-dimethylbenzenesulfonic acid.A plausible mechanism for o-xylene elimination by the novel ZSM−OSO 3 H adsorbent was proposed, i.e., the enhanced elimination of o-xylene was attributed to the sulfonation reaction of it with the sulfonic acid group in ZSM−OSO 3 H.Furthermore, the ZSM−OSO 3 H adsorbent demonstrated excellent recyclability and reusability over four consecutive cycles of adsorption/extraction/re-sulfonation.Therefore, ClSO 3 H sulfonation offers a promising approach for modifying various types of zeolites to enhance the elimination of VOCs.

Scheme 2 .Scheme 2 .
Scheme 2. Schematic diagram and adsorption mechanism of the reactive adsorption process for removing o-xylene in ZSM−OSO3H.Scheme 2. Schematic diagram and adsorption mechanism of the reactive adsorption process for removing o-xylene in ZSM−OSO 3 H.

Table 1 .
Adsorption parameters for o-xylene under different process conditions.

Table 1 .
Adsorption parameters for o-xylene under different process conditions.

Table 2 .
Evaluation of the adsorption capacity of diverse materials to o-xylene.

Table 2 .
Evaluation of the adsorption capacity of diverse materials to o-xylene.