Tuning the Hierarchical Pore Structure and the Metal Site in a Metal-Organic Framework Derivative to Unravel the Mechanism for the Adsorption of Different Volatile Organic Compounds

Catalytic oxidation of volatile organic compounds over supported noble metal and single atom catalysts: A review

Volatile organic compounds (VOCs) exhausted from industrial processes are the major atmospheric pollutants, which could destroy the ecological environment and make hazards to human health seriously. Catalytic oxidation is regarded as the most competitive strategy for the efficient elimination of low-concentration VOCs. Supported noble metal catalysts are preferred catalysts due to their excellent low-temperature catalytic activity. To further lower the cost of catalysts, single atom catalysts (SAC) have been fabricated and extensively studied for application in VOCs oxidation due to their 100 % atom-utilization efficiency and unique catalytic performance. In this review, we comprehensively summarize the recent advances in supported noble metal (e.g., Pt, Pd, Au, and Ag) catalysts and SAC for VOCs oxidation since 2015. Firstly, this paper focuses on some important influencing factors that affect the activity of supported noble metal catalysts, including particle size, valence state and dispersion of noble metals, properties of the support, metal oxide/ion modification, preparation method, and pretreatment conditions of catalysts. Secondly, we briefly summarize the catalytic performance of SAC for typical VOCs. Finally, we conclude the key influencing factors and provide the prospects and challenges of VOCs oxidation.

Emission and risk of atmospheric hexachlorobutadiene might be underestimated globally

Ambiguous emission of new pollutants is a fundamental obstacle of risk prevention, such as hexachlorobutadiene (HCBD). To quantify the HCBD emission in China, we probed the emission factors for main sources based on analysis of 847 gas samples from field experiments. Considering treatment efficiency, emissions of 362 kinds of sources were measured. Results showed that 15115.34 tons HCBD were emitted in 2019, among which more than half was contributed by agricultural sources (53.85 %) that were overlooked in ongoing HCBD regulation. More industrial sectors were recognized to produce and emit HCBD with the cumulative contribution being 29.35 %, such as industrial coating, plastic and rubber products manufacturing industry. Residential coal and transportation contributed 11.03 % and 5.5 % to total HCBD emission. Significant occupational health threats in the chemical raw material manufacturing industry indicated that low emission did not mean low risk, since emission of this industry was low. Assessment of population-weighted non-occupational risk revealed a higher risk for residents in Central and Eastern China. High detection rate of HCBD in agricultural and industrial sources implied a probably underestimated emission and impact over the globe. The conclusions highlight the importance of emission factor localization and source full-coverage in new pollutants regulation.

Tuning the pore structure and hydrophobicity to unravel the adsorption mechanism of polycyclic aromatic hydrocarbons onto carbon materials

Polycyclic aromatic hydrocarbons (PAHs) are widely distributed in water, soil, and air, posing significant risks to human health. Yet little is known about whether the large molecular size of PAHs could hinder the understanding of their adsorption dynamics and the universality of the adsorption mechanism remains unclear. Low-cost adsorbents with different pore structures and hydrophobic properties were prepared, and the adsorption process and feasible adsorption sites of PAHs were investigated. The results showed that the more developed pore structure promoted the diffusion of PAHs. Adsorbents with high affinity for PAHs obtained better performance due to the π-π interaction and the sieving of groove areas formed by folds, and the equilibrium adsorption capacity was 99.9 and 98.9 mg/g for NAP, 9.42 and 9.53 mg/g for PHE, 0.929 and 0.887 mg/g for PYR. Meanwhile, the pseudo second-order kinetic model and Freundlich isotherm model could fit the actual adsorption process. Moreover, XlogP3 or EHOMO emerged as good descriptors in QSAR. Hydrogen bond basicity and hexadecane-air partition coefficient of organic pollutants play a dominant role in the adsorption stage. This study lays a theoretical foundation for screening adsorbents and provides novel support to explore the adsorption mechanism of PAHs.

Tunable reconstruction of metal-organic frameworks for advanced electrocatalytic degradation of antibiotics: Key role of structural defects and pollutant properties

Although metal-organic frameworks (MOFs) catalysts are appealing for removing emerging contaminants, they are significantly restricted by the activity-stability trade-off effect. This study develops a facile and sustainable strategy to realize the synchronous promotion of stability and activity by utilizing the metastable property of UiO-66 with optimal defect concentration to in-situ reconstruct into more stable and active ZrOOH@UiO-66 heterojunction under working conditions. The crucial role of structural defects and pollutant properties in controlling MOFs reconstruction was unveiled by tracking dynamic structure evolution during electrocatalytic degradation of antibiotics. The mechanism is the selective oxidation of exposed metal active sites on defective UiO-66 during electrocatalysis, forming well-dispersed ZrOOH with rich oxygen vacancies that protected MOFs and reduced activation energy for •OH and •O2- radicals generation. Additionally, lower ionization potential and stronger adsorption of antibiotics restricted reconstruction of defective UiO-66 by inhibiting electron transfer and occupying reconstruction site. Besides, the reconstructed UiO-66 electrocatalytic membrane presented high stability, removing approximately 90 % of tetracycline with efficient self-cleaning and low energy consumption (0.4 mW•h/m3) in surface water remediation over 200 h. This strategy is also feasible for other carboxylate-based MOFs, which provides the guidance for MOFs-based catalysts in emerging contaminants removal from complex water matrices.

Light-driven directional decomposition of ammonia nitrogen coupled with proton migration for efficient hydrogen production

The targeted construction of coupled systems, utilizing the unique features of combined photocatalytic redox reactions, is an effective strategy for enhancing catalytic performance. However, the hydrogen source in photocatalytic ammonia-nitrogen oxidation-coupled systems and the multi-electron proton migration pathways in such synergistic systems remain rarely studied. In this study, MoS2/UiOS-MCS composites were synthesized for the photocatalytic conversion of ammonia-nitrogen wastewater into hydrogen. The edge hydrogen evolution effect of MoS2 and the type-II heterojunction formed between both UiO-66-(SH)2 and MnCdS effectively facilitated the separation and transfer of photogenerated charge carriers. The composites exhibited a synergistic hydrogen production rate of 787.42 μmol g-1 h-1, 22 times higher than that of pure MCS, and the nitrogen selectivity is 99.05 %. In situ EPR and controlled experiments identified •NH2 as the intermediate product; Isotope tracing experiments revealed that the protons in hydrogen primarily originate from water, while protons from ammonia nitrogen are transported from the oxidation site to the reduction site. The mechanisms of ammonia nitrogen oxidation, proton transport pathways between reaction sites, and their contributions to hydrogen evolution were elucidated, and new insights were provided for designing photocatalytic synergistic systems and optimizing photocatalyst performance.

UiO-66 with missing cluster defects for high-efficient extraction and enrichment of benzoylurea insecticides

The creation of defects in crystalline structures can tune metal-organic frameworks (MOFs) properties, such as improving their adsorptive and catalytic performance with producing more porosity and active sites. In this work, the bimetallic UiO-66 containing Zn and Zr was prepared. And then UiO-66 with missing cluster defects (UiO-66-1/3) were obtained by acid washing to remove the Zn nodes. UiO-66-1/3 was used as sorbent of dispersive solid-phase extraction (dSPE) to extract and enrich (BUs). Combination with high-performance liquid chromatography-diode array detector (HPLC-DAD) was developed to detect trace BUs in soil samples. Adsorption equilibrium can be reached in 3 min. The method possesses high enrichment factor (202-325), low detection limit (0.005-0.04 ng·mL-1), and wide linear range (0.02-200 ng·mL-1). In addition, the recovery rate of UiO-66-1/3 as an adsorbent was still higher than 95% after reused for 16 times. This work provides a new material for the enrichment and detection of benzoylurea insecticides in the environment.

Nanocellulose encapsulated nZVI@UiO-66-NH2 aerogel for high-efficiency p-chloronitrobenzene removal with selective reduction

A poriferous nZVI aerogel (nZVI@UiO-66-NH2/TCNF) was elaborately constructed by in-situ deposition of nZVI on UiO-66-NH2 and coupling with a bio-based TEMPO oxidized cellulose nanofiber (TCNF) substrate, followed by freeze-drying process for p-chloronitrobenzene (p-CNB) degradation. With degradation efficiency of above 85 % within 3 h under a wide pH range of 3-9, the nZVI@UiO-66-NH2/TCNF aerogel presented better p-CNB removal performance than other developed aerogels. Extended to 24 h, superior p-CNB removal performance (99.83 %) and 4-chloroaniline (p-CAN) selectivity (98.84 %) were successfully achieved. This could be attributed to 1) the facilitated mass transfer via concentration-gradient driving force with buffering and drag-reducing hydrated shear layer from porous channels of hydrophilic TCNF; 2) the enhanced adhesion of p-CNB onto UiO-66-NH2 and accelerated electron transfer by Fe-O-Zr bonds, synergistically improving the nitro- reduction of p-CNB using nZVI. This work pioneered a unique paradigm, providing nZVI with both solid bio-based moldability and highly-selective removal for the treatment of chloronitrobenzene containing wastewater.

A Concise Review on Porous Adsorbents for Benzene and Other Volatile Organic Compounds

Emissions of volatile organic compounds (VOCs) such as benzene, toluene, xylene, styrene, hexane, tetrachloroethylene, acetone, acetaldehyde, formaldehyde, isopropanol, etc., increase dramatically with accelerated industrialization and economic growth. Most VOCs cause serious environmental pollution and threaten human health due to their toxic and carcinogenic nature. Adsorption on porous materials is considered one of the most promising technologies for VOC removal due to its cost-effectiveness, operational flexibility, and low energy consumption. This review aims to provide a comprehensive understanding of VOC adsorption on various porous adsorbents and indicate future research directions in this field. It is focused on (i) the molecular characterization of structures, polarity, and boiling points of VOCs, (ii) the adsorption mechanisms and adsorption interactions in the physical, chemical, and competitive adsorption of VOCs on adsorbents, and (iii) the favorable characteristics of materials for VOCs adsorption. Porous adsorbents that would play an important role in the removal of benzene and other VOCs are presented in detail, including carbon-based materials (activated carbons, active carbon fibers, ordered mesoporous carbons, and graphene-based materials), metal-organic frameworks, covalent organic frameworks, zeolites, and siliceous adsorbents. Finally, the challenges and prospects related to the removal of VOCs via adsorption are pointed out.

Unveiling the Role of Hydrophobicity on Multilayer Carbon Nanosheets Enriched in sp2-Carbon for Toluene Adsorption under Humid Conditions

Carbon materials are regarded as a promising adsorbent for the adsorption of volatile organic compounds (VOCs). However, their adsorption behaviors are usually compromised at ambient conditions, attributed to the competitive VOCs adsorption with water vapor. In this study, we demonstrated that the selectivity for toluene than water of carbon can be effectively enhanced by introducing more sp2-carbon with two-dimensional nanosheets stacked. The multilayer carbon nanosheets enriched with sp2-carbon (CNS-MCA) exhibit a 151° H2O-contact angle, indicating hydrophobicity. Dynamic adsorption behaviors revealed that CNS-MCA retain 71% of their toluene adsorption capacity (91 mg/g) even at 60% relative humidity. Density functional theory (DFT) calculations, static adsorption studies, in situ Raman spectroscopy, and time-resolved in situ nuclear magnetic resonance (NMR) spectroscopy collectively indicate that toluene exhibits enhanced adsorption and selectivity due to π-π* interactions between its aromatic rings and the sp2-carbon. Conversely, water adsorption is attenuated, attributed to the reduced availability of surface-exposed hydrogen bonds associated with sp2-carbon and the inherent hydrophobic nature of multilayer graphene. This study extends a novel solution for the enhancement of VOCs adsorption under humid conditions.

Efficacious destruction of typical aromatic hydrocarbons over CoMn/Ni foam monolithic catalysts with boosted activity and water resistance

Developing cost-effective monolith catalyst with superior low-temperature activity is critical for oxidative efficacious removal of industrial volatile organic compounds (VOCs). However, the complexity of the industrial flue gas conditions demands the need for high moisture tolerance, which is challenging. Herein, CoMn-Metal Organic Framework (CoMn-MOF) was in situ grown on Ni foam (NiF) at room temperature to synthesize the cost-effective monolith catalyst. The optimized catalyst, Co1Mn1/NiF, exhibited excellent performance in toluene oxidation (T90 = 239 °C) due to the substitution of manganese into the cobalt lattice. This substitution weakened the Co-O bond strength, creating more oxygen vacancies and increasing the active oxygen species content. Additionally, experimentally and computationally evidence revealed that the mutual inhibiting effect of three typical aromatic hydrocarbons (benzene, toluene and m-xylene) over the Co1Mn1/NiF catalyst was attributed to the competitive adsorption occurring on the active site. Furthermore, the Co1Mn1/NiF catalyst also presents outstanding water resistance, particularly at a concentration of 3 vol%, where the activity is even enhanced. This was attributed to the lower water adsorption and dissociation energy derived from the interaction between the bimetals. Results demonstrate that the dissociation of water vapor enables more reactive oxygen species to participate in the reaction which reduces the formation of intermediates and facilitates the reaction. This investigation provides new insights into the preparation of oxygen vacancy-rich monolith catalysts with high water resistance for practical applications.

Interface Defects and Carrier Regulation in MOF-Derived Co3O4/In2O3 Composite Materials for Enhanced Selective Detection of HCHO

Gas sensors for real-time monitoring of low HCHO concentrations have promising applications in the field of health protection and air treatment, and this work reports a novel resistive gas sensor with high sensitivity and selectivity to HCHO. The MOF-derived hollow In2O3 was mixed with ZIF-67(Co) and calcined twice to obtain a hollow Co3O4/In2O3 (hereafter collectively termed MZO-6) composite enriched with oxygen vacancies, and tests such as XPS and EPR proved that the strong interfacial electronic coupling increased the oxygen vacancies. The gas-sensitive test results show that the hollow composite MZO-6 with abundant oxygen vacancies has a higher response value (11,003) to 10 ppm of HCHO and achieves a fast response/recovery time (11/181 s) for HCHO at a lower operating temperature (140 °C). The MZO-6 material significantly enhances the selectivity to HCHO and reduces the interference of common pollutant gases such as ethanol, acetone, and xylene. There is no significant fluctuation of resistance and response values in the 30-day long-term stability test, and the material has good stability. The synergistic effect of the heterostructure and oxygen vacancies altered the formaldehyde adsorption intermediate pathway and reduced the reaction activation energy, enhancing the HCHO responsiveness and selectivity of the MZO-6 material.