Application of high silica zeolite ZSM-5 in a hybrid treatment process based on sequential adsorption and ozonation for VOCs elimination

A comprehensive review and perspective research in technology integration for the treatment of gaseous volatile organic compounds

Volatile organic compounds (VOCs) are harmful pollutants emitted from industrial processes. They pose a risk to human health and ecosystems, even at low concentrations. Controlling VOCs is crucial for good air quality. This review aims to provide a comprehensive understanding of the various methods used for controlling VOC abatement. The advancement of mono-functional treatment techniques, including recovery such as absorption, adsorption, condensation, and membrane separation, and destruction-based methods such as natural degradation methods, advanced oxidation processes, and reduction methods were discussed. Among these methods, advanced oxidation processes are considered the most effective for removing toxic VOCs, despite some drawbacks such as costly chemicals, rigorous reaction conditions, and the formation of secondary chemicals. Standalone technologies are generally not sufficient and do not perform satisfactorily for the removal of hazardous air pollutants due to the generation of innocuous end products. However, every integration technique complements superiority and overcomes the challenges of standalone technologies. For instance, by using catalytic oxidation, catalytic ozonation, non-thermal plasma, and photocatalysis pretreatments, the amount of bioaerosols released from the bioreactor can be significantly reduced, leading to effective conversion rates for non-polar compounds, and opening new perspectives towards promising techniques with countless benefits. Interestingly, the three-stage processes have shown efficient decomposition performance for polar VOCs, excellent recoverability for nonpolar VOCs, and promising potential applications in atmospheric purification. Furthermore, the review also reports on the evolution of mathematical and artificial neural network modeling for VOC removal performance. The article critically analyzes the synergistic effects and advantages of integration. The authors hope that this article will be helpful in deciding on the appropriate strategy for controlling interested VOCs.

Cooperative and Bifunctional Adsorbent-Catalyst Materials for In-situ VOCs Capture-Conversion

Volatile organic compounds (VOCs) are gases that are emitted into the air from products or processes and are major components of air pollution that significantly deteriorate air quality and seriously affect human health. Different types of metals, metal oxides, mixed-metal oxides, polymers, activated carbons, zeolites, metal-organic frameworks (MOFs) and mixed-matrixed materials have been developed and used as adsorbent or catalyst for diversified VOCs detection, removal, and destruction. In this comprehensive review, we first discuss the general classification of VOCs removal materials and processes and outline the historical development of bifunctional and cooperative adsorbent-catalyst materials for the removal of VOCs from air. Subsequently, particular attention is devoted to design of strategies for cooperative adsorbent-catalyst materials, along with detailed discussions on the latest advances on these bifunctional materials, reaction mechanisms, long-term stability, and regeneration for VOCs removal processes. Finally, challenges and future opportunities for the environmental implementation of these bifunctional materials are identified and outlined with the intent of providing insightful guidance on the design and fabrication of more efficient materials and systems for VOCs removal in the future.

Improving the adsorption performance of non-polar benzene vapor by using lignin-based activated carbon

Both indoor and outdoor contamination continually contain benzene vapor. It has primary concerns about long-term health risks to the living environment. Benzene is a crucial airborne pollutant in the environment due to its apparent acute toxicity, high volatility, and poor degradability. It is especially urgent to restrain benzene emissions due to the persistent concentration increase and stringent processes. Benzene adsorption is a highly efficient mechanism with low cost, low energy consumption, and a simple process. In this study, biomass-derived porous carbon materials (TCACs) were synthesized by pyrolysis activation combined with H3PO4, HNO3, and HCl. TCAC44 has the best activation conclusion, showing that surface area and pore volume were 1107 m2/g and 0.58 cm3/g treated with H3PO4 and so was chosen for subsequent benzene adsorption/desorption tests. The adsorption capacities of benzene for TCAC44 were increased from 58 mg/g for 35 °C + 95% RH to 121 mg/g for 25 °C + 15% RH and presented a higher adsorption capacity of benzene than TCAC101 and TCAC133. Otherwise, well recyclability of TCAC44 was revealed as the benzene adsorption capacity reductions were 22.49% after five adsorption-desorption cycles. Furthermore, the present study established the property-application relationships to promote and encourage future research on the newly synthesized innovative TCAC44 for benzene removal.

Effective Removal of Acetaldehyde Using Piperazine/Nitric Acid Co-Impregnated Bead-Type Activated Carbon

Acetaldehyde (CH₃CHO) in the atmosphere is associated with adverse health effects. Among the various options for use in removing CH₃CHO, adsorption is often employed because of its convenient application and economical processes, particularly when using activated carbon. In previous studies, the surface of activated carbon has been modified with amines to remove CH₃CHO from the atmosphere via adsorption. However, these materials are toxic and can have harmful effects on humans when the modified activated carbon is used in air-purifier filters. Therefore, in this study, a customized bead-type activated carbon (BAC) with surface modification options via amination was evaluated for removing CH₃CHO. Various amounts of non-toxic piperazine or piperazine/nitric acid were used in amination. Chemical and physical analyses of the surface-modified BAC samples were performed using Brunauer-Emmett-Teller measurements, elemental analyses, and Fourier transform infrared and X-ray photoelectron spectroscopy. The chemical structures on the surfaces of the modified BACs were analyzed in detail using X-ray absorption spectroscopy. The amine and carboxylic acid groups on the surfaces of the modified BACs are critical in CH₃CHO adsorption. Notably, piperazine amination decreased the pore size and volume of the modified BAC, but piperazine/nitric acid impregnation maintained the pore size and volume of the modified BAC. In terms of CH₃CHO adsorption, piperazine/nitric acid impregnation resulted in a superior performance, with greater chemical adsorption. The linkages between the amine and carboxylic acid groups may function differently in piperazine amination and piperazine/nitric acid treatment.

Removal of gaseous volatile organic compounds via vacuum ultraviolet photodegradation: Review and prospect

Volatile organic compounds (VOCs) have attracted much attention for decades as they are the precursors of photochemical smog and are harmful to the environment and human health. Vacuum ultraviolet (VUV) photodegradation is a simple and effective method to decompose VOCs (ranging from tens to hundreds of ppmV) without additional oxidants or catalysts in the air at atmospheric pressure. In this paper, we review the research progress of VOCs removal via VUV photodegradation. The fundamentals are outlined and the key operation factors for VOCs degradation, such as humidity, oxygen content, VOCs initial concentration, light intensity, and flow rate, are discussed. VUV photodegradation of VOCs mixture is elucidated. The application of VUV photodegradation in combination with ozone-assisted catalytic oxidation (OZCO) and photocatalytic oxidation (PCO) systems, and as the pre-treatment technique for biological purification are illustrated. Based on the summary, we propose the challenges of VUV photodegradation and perspectives for its future development.

Recent advances in the abatement of volatile organic compounds (VOCs) and chlorinated-VOCs by non-thermal plasma technology: A review

Most of the volatile organic compounds (VOCs) and especially the chlorinated volatile organic compounds (Cl-VOCs), are regarded as major pollutants due to their properties of volatility, diffusivity and toxicity which pose a significant threat to human health and the eco-environment. Catalytic degradation of VOCs and Cl-VOCs to harmless products is a promising approach to mitigate the issues caused by VOCs and Cl-VOCs. Non-thermal plasma (NTP) assisted catalysis is a promising technology for the efficient degradation of VOCs and Cl-VOCs with higher selectivity under relatively mild conditions compared with conventional thermal catalysis. This review summarises state-of-the-art research of the in plasma catalysis (IPC) of VOCs degradation from three major aspects including: (i) the design of catalysts, (ii) the strategies of deep catalytic degradation and by-products inhibition, and (iii) the fundamental research into mechanisms of NTP activated catalytic VOCs degradation. Particular attention is also given to Cl-VOCs due to their characteristic properties of higher stability and toxicity. The catalysts used for the degradation Cl-VOCs, chlorinated by-products formation and the degradation mechanism of Cl-VOCs are systematically reviewed in each chapter. Finally, a perspective on future challenges and opportunities in the development of NTP assisted VOCs catalytic degradation were discussed.

Promoting the Catalytic Ozonation of Toluene by Introducing SO42- into the α-MnO2/ZSM-5 Catalyst to Tune Both Oxygen Vacancies and Acid Sites

Mn-based catalysts hold the promise of practical applications in catalytic ozonation of toluene at room temperature, yet improvement of toluene conversion and CO_x selectivity remains challenging. Here, an innovative α-MnO₂/ZSM-5 catalyst modified with SO₄^2- was successfully prepared, and both characterizations and density functional theory (DFT) calculations showed that SO₄^2- introduction facilitated the formation of oxygen vacancies, Lewis and Brönsted acid sites, and active oxygen species and enhanced the adsorption ability of toluene on α-MnO₂/ZSM-5. Characterizations also showed that SO₄^2- introduction made the catalyst possess larger specific surface area, superior reducibility, and stronger surface acidity. As a result, α-MnO₂/ZSM-5 with a S/Mn molar ratio of 0.019 exhibited the best toluene conversion and CO_x selectivity, 87 and 94%, respectively, after the reaction for 8 h at 30 °C under an initial concentration of 5 ppm toluene and 45 ppm ozone, relative humidity of 45%, and space velocity of 32,000 h^-1, far superior to those of non-noble catalysts reported to date under comparable reaction conditions. The synergistic role of increased oxygen vacancies and acid sites of α-MnO₂/ZSM-5 modified with SO₄^2- resulted in excellent toluene conversion and CO_x selectivity. The findings represented a critical step toward the rational design and synthesis of highly efficient catalysts for catalytic ozonation of toluene.

Adsorption of acetone, ethyl acetate and toluene by beta zeolite/diatomite composites: preparation, characterization and adsorbability

The hierarchical porous composites (Beta/Dt) were prepared by secondary growth method using natural diatomite and beta zeolite. Moreover, XRD, SEM, and BET characterize the composite's composition, surface structure, and pore structure. The adsorbability of Beta/Dt was evaluated by adsorption of three common volatile organic compounds (VOCs) of the printing industry: acetone, ethyl acetate, and toluene. The results show that under the optimum preparation condition, the adsorption capacities of the three VOCs on Beta/Dt were about 3.5 times those of pure beta zeolite and 4.7-35.3 times those of diatomite, respectively. It indicates the synergistic adsorption effect between beta zeolite and diatomite. The superior adsorption capacity of Beta/Dt can be attributed to the suitable micropore size, the increase of the diffusion channels, and the chemical adsorption on modification diatomite. The adsorption of acetone, ethyl acetate, and toluene on Beta/Dt conformed to the pseudo-second-order kinetic model. In contrast, adsorption isotherms conformed to the Langmuir model, meaning that both physical and chemical adsorption occurred simultaneously during the adsorption process, and the adsorption belonged to the monolayer adsorption. The chemical adsorption mechanism can be ascribed to the nucleophilic reaction between the three VOCs (acetone, ethyl acetate, and toluene) and Beta/Dt with positive charges resulting from the modification diatomite. Furthermore, the composite could still keep more than 90% of the adsorption capacity of the original adsorbent after five regeneration cycles.

Application of MCM-48 with large specific surface area for VOCs elimination: synthesis and hydrophobic functionalization for highly efficient adsorption

MCM-48 molecular sieve with a large specific area (1470.87 m²/g) was hydrothermally synthesized for VOCs elimination by the adsorption method. The dynamic adsorption behaviors of toluene on this material were evaluated via breakthrough curves under both dry and wet conditions. A high toluene adsorption capacity of 171.13 mg/g was observed under dry conditions; however, in the presence of water vapor (20% RH), the adsorption capacity greatly decreased to 58.88 mg/g due to the competitive occupation of adsorption sites between water molecules and toluene molecules. To improve the affinity to toluene, functionalized MCM-48 materials were obtained by the co-condensation method and grafting method, respectively. It was found that co-M48(1:5)-100/48 sample by co-condensation method presents the highest dynamic adsorption capacity at both dry condition (194.62 mg/g) and 20% RH (122.42 mg/g), which has a significant advantage in the same type of adsorbent. This could be ascribed to the conjugated π-electrons effect between aromatic rings of phenyl groups uniformly distributed in MCM-48 skeleton and toluene molecules, which was qualitatively confirmed by FTIR. Moreover, cycle tests confirmed that this adsorbent possesses superior stability. The Yoon-Nelson model was successfully employed to describe the dynamic adsorption behavior of toluene over the organofunctionalized MCM-48 adsorbents, and the adsorption force of toluene was explained. Finally, a diagram describing the effect of different functionalization methods on the hydrophobicity and organophilicity of MCM-48 was given for a better understanding.

Effectiveness of toluene mineralization by gas-phase oxidation over Co(II)/SiO2 catalyst with ozone

The results of experimental study on effectiveness of gas-phase total oxidation of toluene towards carbon dioxide and water with the aid of ozone over Co(II)/SiO₂ catalyst are presented in this work. The main objective of the work was to determine ozone demand necessary for total mineralization of toluene at the temperature range of 40-100°C chosen to minimize catalyst poisoning by water. Complete mineralization of toluene was possible if sufficient ozone/toluene ratio was maintained in the gas supplied to the reactor. For ozone/toluene molar ratios less than 20 the extent of toluene mineralization increased with temperature up to a plateau starting at approximately 60°C, which was caused by ozone shortage. Stoichiometry of the total oxidation of toluene with ozone indicates that only one oxygen atom in the ozone molecule is used for the oxidation of toluene, to achieve complete mineralization. Experimentally determined ozone/toluene ratio (20-25) necessary for the total oxidation of toluene was larger than the theoretical one mostly due to ozone losses resulting from its 'unproductive' decomposition. At the range of lower values of mineralization rate, the toluene oxidation proceeds according to a more efficient mechanism, indicating less ozone demand being between 6 and 18 moles of ozone per mole of toluene. A possible mechanism of toluene oxidation was suggested. The mechanism involves the formation of •OH radicals, which may explain the effectiveness of Co(II)/SiO₂ catalyst in combination with ozone for the oxidation of toluene and other aromatic VOCs in a low-temperature process.

Potential applications of porous organic polymers as adsorbent for the adsorption of volatile organic compounds

Volatile organic compounds (VOCs) with high toxicity and carcinogenicity are emitted from kinds of industries, which endanger human health and the environment. Adsorption is a promising method for the treatment of VOCs due to its low cost and high efficiency. In recent years, activated carbons, zeolites, and mesoporous materials are widely used to remove VOCs because of their high specific surface area and abundant porosity. However, the hydrophilic nature and low desorption rate of those materials limit their commercial application. Furthermore, the adsorption capacities of VOCs still need to be improved. Porous organic polymers (POPs) with extremely high porosity, structural diversity, and hydrophobic have been considered as one of the most promising candidates for VOCs adsorption. This review generalized the superiority of POPs for VOCs adsorption compared to other porous materials and summarized the studies of VOCs adsorption on different types of POPs. Moreover, the mechanism of competitive adsorption between water and VOCs on the POPs was discussed. Finally, a concise outlook for utilizing POPs for VOCs adsorption was discussed, noting areas in which further work is needed to develop the next-generation POPs for practical applications.

Dynamic adsorption/desorption of p-xylene on nanomorphic MFI zeolites: Effect of zeolite crystal thickness and mesopore architecture

Zeolites have attracted great interest as an adsorbent for the removal of volatile organic compounds. However, they suffer from low adsorption capacities due to severe diffusion limitations. Here, the effects of zeolite thickness and mesopore architecture on dynamic adsorption of p-xylene have been examined with a number of MFI-type zeolites with different crystal thicknesses and mesopore openings (i.e. open mesopore, constricted mesopore), which were prepared via hydrothermal synthesis with various organic structure-directing agents and post-synthetic desilication. The results showed that the breakthrough time of MFI zeolite could be improved by more than 2.3 times by reducing the crystal thickness of zeolite to a single-unit-cell dimension (∼2 nm). The time improvement can be attributed to the short diffusion path length that results in easy access of p-xylene to intracrystalline micropores and a large external crystal surface area. In the case of mesopore openings, the presence of constricted mesopores caused the mass transfer of p-xylene into zeolite adsorbents to slow down while open mesopores did not. Furthermore, mesopore opening is an important factor for the desorption behavior of p-xylene. Adsorbed p-xylene by mesoporous zeolites could be desorbed at lower temperatures only when facile diffusion to the exterior through mesoporous channels was possible.

Enhanced toluene adsorption/desorption dynamic performances over modified USY zeolites after an aqueous ammonia treatment

The dynamic adsorption/desorption performances of modified hierarchical USY zeolites treated with an ammonia solution (NH₄OH) at different concentrations were investigated using gas-phase toluene as an indicator. The characterization results indicated that the ammonia treatment could result in the expansion of microporous channels and the formation of a mesoporous structure without evident decrease in crystallinity. The experiment results regarding dynamic adsorption/desorption performances revealed that the mass transfer resistance of modified USY adsorbents were greatly reduced treating with NH₄OH. Among the modified samples, the 0.1 mol L⁻¹ NH₄OH treated USY adsorbent exhibited large adsorptive capacity and highest desorption rate, which show good cyclic performance that could preserve its adsorbent capacity after 20 cycles. In contrast, pristine USY samples had lost around 28% of the initial adsorption capacity after 20 cycles. Moreover, the NaOH-treated sample showed great crystallinity decline compared to the NH₄OH-treated samples due to excessive silicon atom leaching from the USY framework, and had lower adsorption capacity under humid conditions. Therefore, NH₄OH-modified USY zeolites could be promising adsorbents for the adsorption/desorption process of volatile organic compounds (VOCs).

USY zeolites treated by aqueous ammonia (a simple method) shows enhanced and stabilized toluene adsorption/desorption dynamic performances.

Multiscale exploration of hydrocarbon adsorption and hopping through ZSM-5 channels - from Monte Carlo modelling to experiment

In this article the results of statistical MC modelling corroborated by the FT-IR spectroscopy and gravimetric adsorption studies of low aliphatic hydrocarbons in ZSM-5 (Si/Al = 28 or silicalite) are presented. The extension of the existing Dubbeldam's forcefield towards inclusion of the finite aluminium-containing zeolites is proposed and its applicability is verified experimentally for the sorption of linear and branched hydrocarbons. The FT-IR spectroscopy applicability to follow the kinetics of small hydrocarbon adsorption has been successfully verified by the application of the Crank solution for diffusion to spectroscopy derived results.

Toluene adsorption on porous Cu-BDC@OAC composite at various operating conditions: optimization by response surface methodology

The work presented here describes the synthesis of Cu-BDC MOF (BDC = 1,4-benzenedicarboxylate) based on oxidized activated carbon (microporous Cu-BDC@OAC composite) using an in situ method. The adsorbents (oxidized activated carbon (OAC), Cu-BDC and microporous Cu-BDC@OAC composite) were characterized by XRD, FTIR, SEM, EDS and BET techniques. Optimization of operating parameters affecting the efficiency of adsorption capacity, including adsorbent mass, flow rate, concentration, relative humidity and temperature, was carried out by central composite design (CCD) of the response surface methodology (RSM). An adsorbent mass of 60 mg, a flow rate of 90 mL min-1, the concentration of toluene (500 ppm), the relative humidity of 30% and a temperature of 26 °C were found to be the optimized process conditions. The maximum adsorption capacity for toluene onto Cu-BDC@OAC composite was 222.811 mg g-1, which increased by almost 12% and 50% compared with pure Cu-BDC and oxidized AC, respectively. The presence of micropores enhances the dynamic adsorption capacity of toluene. The regeneration of the composite was still up to 78% after three consecutive adsorption-desorption cycles. According to the obtained adsorbent parameters, microporous Cu-BDC@OAC was shown to be a promising adsorbent for the removal of volatile organic compounds.

A char-clay composite catalyst derived from spent bleaching earth for efficient ozonation of recalcitrants in water

Catalytic ozonation is an efficient process that can be utilized to degrade recalcitrant organics. Char-clay composite derived from refinery spent bleaching earth (SBE) is an economical and readily available catalyst that can be used during the ozonation treatment of recalcitrants in wastewater. Four catalysts of SBE-N₂-650, SBE-N₂-850, SBE-O₂-650, and SBE-O₂-850 were prepared by heating the SBE at 650 and 850 °C under N₂ or O₂ conditions. High surface OH sites in the SBE-N₂-650 and SBE-O₂-650 relative to the SBE-N₂-850 and SBE-O₂-850 resulted in an increase in catalytic activity. Additional carbon (C), that existed in the SBE-N₂-650 and SBE-N₂-850, had a positive effect on catalytic activity. The SBE-N₂-650 exhibited the highest activity among those prepared catalysts. During catalytic ozonation, the SBE-N₂-650 increased the mineralization rate of benzoic acid by 36% when compared with single ozonation. Molecular ozone was decomposed at the surface active sites on SBE-N₂-650, generating active •OH, •O₂^-, or ¹O₂ species. Gas and liquid products having calorific values that are generated during SBE-N₂-650 preparation can be further utilized. This study introduces a potential use of SBE for the ozonation treatment of recalcitrant wastewaters.

Mechanisms of 1,4-Dioxane Biodegradation and Adsorption by Bio-Zeolite in the Presence of Chlorinated Solvents: Experimental and Molecular Dynamics Simulation Studies

The use of bioaugmented zeolite (bio-zeolite) can be an effective technology for irreversibly removing recalcitrant organic pollutants in aqueous mixtures. Removal of 1,4-dioxane by a bio-zeolite (Pseudonocardia dioxanivorans CB1190-bioaugmented ZSM-5) in the presence of several chlorinated volatile organic compounds (CVOCs) was superior to removal by adsorption using abiotic zeolite. Mixtures containing 1,1-dichloroethene (1,1-DCE) were an exception, which completely inhibited the bio-zeolite system. Specific adsorption characteristics were studied using adsorption isotherms in single-solute and bisolute systems accompanied by Polanyi theory-based Dubinin-Astakhov (DA) modeling. Adsorption behavior was examined using characteristic energy (E_a/H) from modified DA models and molecular dynamics simulations. While the tight-fit of 1,4-dioxane in the hydrophobic channels of ZSM-5 appears to drive 1,4-dioxane adsorption, the greater hydrophobicity of trichloroethene and cis-1,2-dichloroethene cause them have a greater affinity over 1,4-dioxane for adsorption sites on the zeolite. 1,4-Dioxane was desorbed and displaced by CVOCs except 1,1-DCE because of its low E_a/H value, explaining why bio-zeolite only biodegraded 1,4-dioxane in 1,1-DCE-free CVOC mixtures. Understanding the adsorption mechanisms of solutes in complex mixtures is crucial for the implementation of sorption-based treatment technologies for the removal of complex contaminant mixtures from aquatic environments.

Ozonation and ozone-enhanced photocatalysis for VOC removal from air streams: Process optimization, synergy and mechanism assessment

The present work evaluates ozone driven processes (O₃, O₃/UVC, O₃/TiO₂/UVA) in the NETmix mili-photoreactor, as a cost-effective alternative for the removal of volatile organic compounds (VOCs) from air streams, using n-decane as a model pollutant. The network of channels and chambers of the mili-photoreactor was coated with a TiO₂-P25 thin film, resulting in a catalyst coated surface per reactor volume of 990 m² m^-3. Ozone and n-decane streams were fed to alternate chambers of the mili-photoreactor, promoting a good contact between O₃/n-decane/catalyst. Initially, direct reaction between n-decane and ozone (ozonation) was assessed for different O₃/n-decane (O₃/dec) feed molar ratios and total feed flow rates. Under the best conditions, ozonation process achieved total n-decane conversion (below the limit of detection), yielding a reaction rate (r_dec) of 6.8 μmol min^-1 or 6.7 mmol m^-3_reactor s^-1. However, the low reactivity of ozone with the degradation by-products resulted in a quite poor mineralization (~10%). For the O₃/UVC system, an increase on relative humidity from 7 to 40% slight improved the n-decane oxidation rate, mainly associated with the generation of HO from the reaction of active oxygen radicals (O) and water molecules. A strong synergistic effect was observed when coupling TiO₂/UVA photocatalysis with ozonation (O₃/TiO₂/UVA), enhancing substantially the mineralization of n-decane molecules up to 100% under O₃/dec feed molar ratio of 15, photonic flux of 2.67 ± 0.03 J s^-1 and a residence time of 2.0 s. Different reaction intermediates were detected for O₃, TiO₂/UVA and O₃/TiO₂/UVA oxidative systems, indicating the participation of different oxidant species (O₃, HO, O, etc.).

Adsorption/desorption kinetics and breakthrough of gaseous toluene for modified microporous-mesoporous UiO-66 metal organic framework

In this work, micro-mesoporous UiO-66 was successfully prepared with P123 (EO₂₀PO₇₀EO₂₀) as structure-directing agent by a simple solvothermal method. Adsorption/desorption kinetics of gaseous toluene over pristine UiO-66 and micro-mesoporous UiO-66 were investigated by breakthrough experiments, toluene vapor adsorption isotherm measurements and temperature programmed desorption (TPD) experiments. The interactions between toluene and UiO-66 samples were assessed through the Henry's law constant (K_H) and the isosteric adsorption heat (ΔH_ads). The micro-mesoporous UiO-66 crystal demonstrated 2.6 times toluene adsorption capacity of the pristine UiO-66 when the P123/Zr⁴⁺ molar ratio was 0.2. Results showed that micropore adsorption was the main adsorption process and the larger pores in micro-mesoporous UiO-66 increased molecular diffusion rate and reduced the mass transfer resistance. This result indicated that micro-mesoporous structures and defect sites had a positive effect on toluene molecules capture. The breakthrough times and the working capacities decreased with the increase of the relative humidity and adsorption temperature. A good thermal stability and reproducibility were revealed over the micro-mesoporous UiO-66 in this paper.

Synergetic degradation of VOCs by vacuum ultraviolet photolysis and catalytic ozonation over Mn-xCe/ZSM-5

Volatile organic compounds (VOCs) are one of the most important precursors to form the fine particulate matter and photochemical smog, and should be strictly controlled. Vacuum ultraviolet (VUV) photolysis has provided a facile and an effective way to remove VOCs due to its powerful oxidation capability under mild reaction conditions. However, VUV irradiation would generate ozone which brings about secondary pollution. In this study, ZSM-5 supported Mn-Ce mixed oxides (Mn-xCe/ZSM-5) were fabricated as efficient catalysts for ozone catalytic oxidation (OZCO) process, which were applied in combination with VUV photolysis to remove O₃ byproduct and simultaneously facilitate toluene oxidation. The results indicated that the Mn-3Ce/ZSM-5 catalyst considerably enhanced the catalytic degradation efficiency up to 93% for the gas-phase toluene, one of the hazardous VOCs. Meanwhile, almost all the O₃ by-product could be eliminated in the process. It was found that the strong interaction of the MnOCe bond and the variable chemical valence of Mn and Ce based species in the mixed oxides would tune the redox capacity of Mn-xCe /ZSM-5. An increase in surface Ce³⁺ species and surface density of oxygen vacancies would benefit the adsorption and catalytic transformation of O₃ which eventually form the reactive oxygen species over Mn-xCe/ZSM-5.

The Use of Zeolites for VOCs Abatement by Combining Non-Thermal Plasma, Adsorption, and/or Catalysis: A Review

Non-thermal plasma technique can be easily integrated with catalysis and adsorption for environmental applications such as volatile organic compound (VOC) abatement to overcome the shortcomings of individual techniques. This review attempts to give an overview of the literature about the application of zeolite as adsorbent and catalyst in combination with non-thermal plasma for VOC abatement in flue gas. The superior surface properties of zeolites in combination with its excellent catalytic properties obtained by metal loading make it an ideal packing material for adsorption plasma catalytic removal of VOCs. This work highlights the use of zeolites for cyclic adsorption plasma catalysis in order to reduce the energy cost to decompose per VOC molecule and to regenerate zeolites via plasma.

High-silica zeolites for adsorption of organic micro-pollutants in water treatment: A review

High-silica zeolites have been found to be effective adsorbents for the removal of organic micro-pollutants (OMPs) from impaired water, including various pharmaceuticals, personal care products, industrial chemicals, etc. In this review, the properties and fundamentals of high-silica zeolites are summarised. Recent research on mechanisms and efficiencies of OMP adsorption by high-silica zeolites are reviewed to assess the potential opportunities and challenges for the application of high-silica zeolites for OMP adsorption in water treatment. It is concluded that the adsorption capacities are well-related to surface hydrophobicity/hydrophilicity and structural features, e.g. micropore volume and pore size of high-silica zeolites, as well as the properties of OMPs. By using high-silica zeolites, the undesired competitive adsorption of background organic matter (BOM) in natural water could potentially be prevented. In addition, oxidative regeneration could be applied on-site to restore the adsorption capacity of zeolites for OMPs and prevent the toxic residues from re-entering the environment.

Behaviors and kinetics of toluene adsorption-desorption on activated carbons with varying pore structure

This work was undertaken to investigate the behaviors and kinetics of toluene adsorption and desorption on activated carbons with varying pore structure. Five kinds of activated carbon from different raw materials were selected. Adsorption isotherms and breakthrough curves for toluene were measured. Langmuir and Freundlich equations were fitted to the equilibrium data, and the Freundlich equation was more suitable for simulating toluene adsorption. The process consisted of monolayer, multilayer and partial active site adsorption types. The effect of the pore structure of the activated carbons on toluene adsorption capacity was investigated. The quasi-first-order model was more suitable for describing the process than the quasi-second-order model. The adsorption data was also modeled by the internal particle diffusion model and it was found that the adsorption process could be divided into three stages. In the external surface adsorption process, the rate depended on the specific surface area. During the particle diffusion stage, pore structure and volume were the main factors affecting adsorption rate. In the final equilibrium stage, the rate was determined by the ratio of meso- and macro-pores to total pore volume. The rate over the whole adsorption process was dominated by the toluene concentration. The desorption behavior of toluene on activated carbons was investigated, and the process was divided into heat and mass transfer parts corresponding to emission and diffusion mechanisms, respectively. Physical adsorption played the main role during the adsorption process.

Synergistic effect of UV pretreated Fe-ZSM-5 catalysts for heterogeneous catalytic complete oxidation of VOC: A technology development for sustainable use

In this work, the performance of benzene, toluene, ethylbenzene, and xylene (BTEX) removal and degradation from gas, air streams on UV pretreated Fe-ZSM-5 in a batch reactor at room temperature were studied. The Fe-ZSM-5 zeolite catalyst was prepared by hydrothermal reaction method. The influence of UV pre-irradiation time on the removal of BTEX were assessed by varying the time, ranging from 15min to 60smin. Then, sustainability of the activation of the catalyst resulted by UV pretreatment was studied by the four-cycle experiment with one time UV irradiation and after each cycle irradiation followed by BTEX removal after every cycle respectively. The results of BTEX removal depicted that 30min of UV pretreatment was sufficient for complete organics removal. The UV pretreatment effect on the catalytic oxidation and the stability of the catalyst were studied by modern instrumental techniques. The novelty of the process was the sustainable reuse of catalyst with persistent VOC removal, which works on the -adsorption-oxidation-regeneration-adsorption- cycle, which was confirmed by the characterization studies of the catalyst after four runs. The results revealed that the change in the structure, stability, morphology, and removal efficiency of the catalyst during the experiments was negligible. The VOC degradation mechanism studies showed that the oxidation occurs due to the formation of free radicals as well as hydroxyl ions, so named it heterogeneous photo-Fenton oxidation. The residual materials analysis showed the complete mineralization of VOC except small amount of acetone as oxidation product. Lastly, the kinetics of the VOC removal was studied.

Sintered metal fibers@carbon molecular sieve membrane (SMFs@CMSM) composites for the adsorptive removal of low concentration isopropanol

Structured fixed-bed adsorber based on SMF@CMSM adsorbents show enhanced mass transfer and 3 bed utilization efficiency.