Competitive Adsorption of Methanol-Acetone on Surface Functionalization (-COOH, -OH, -NH2, and -SO3H): Grand Canonical Monte Carlo and Density Functional Theory Simulations

Competitive Adsorption of Trace Gases on Ice at Tropospheric Temperatures: A Grand Canonical Monte Carlo Simulation Study

The coadsorption of two atmospheric trace gases on ice is characterized by using, for the first time, grand canonical Monte Carlo (GCMC) simulations performed in conditions similar to those of the corresponding experiments. Adsorption isotherms are simulated at tropospheric temperatures by considering two different gas mixtures of 1-butanol and acetic acid molecules, and selectivity of the ice surface with respect to these species is interpreted at the molecular scale as resulting from a competition process between these molecules for being adsorbed at the ice surface. It is thus shown that the trapping of acetic acid molecules on ice is always favored with respect to that of 1-butanol at low pressures, corresponding to low coverage of the surface, whereas the adsorption of the acid species is significantly modified by the presence of the alcohol molecules in the saturated portion of the adsorption isotherm, in accordance with the experimental observations. The present GCMC simulations thus confirm that competitive adsorption effects have to be taken into consideration in real situations when gas mixtures present in the troposphere interact with the surface of ice particles.

Competitive adsorption characteristics of VOCs and water vapor by activated carbon prepared from Fe/N-doped pistachio shell

Various materials have been developed to capture volatile organic compounds (VOCs) to mitigate air pollution. However, sorbent materials with excellent resistance to water are rare. Here, several Fe/N-doped activated carbons (ACs) have been prepared to capture VOCs in humid environments. The ACs were analyzed by various characterization techniques, such as BET, SEM, XPS, XRD, FTIR, and Raman. The results showed that Fe/N doping resulted in the specific surface area of the ACs increasing by 500 to 1000 m2 g-1, the average pore size increasing to approximately 2 nm, improved mesoporous structure, higher graphitization, lower hydrophilicity, and polarity. The VOCs adsorption performance of the ACs was evaluated by static and dynamic adsorption experiments. The uptake of toluene and ethyl acetate by ACs was enhanced to 224 mg g-1 and 135 mg g-1, respectively. And ACs were able to maintain 70 to 80% VOCs adsorption capacity for VOCs at 80% relative humidity. Furthermore, the microscopic mechanisms were investigated by the grand canonical Monte Carlo method (GCMC). The highly graphitized structure and the N functional groups favored the VOC adsorption process and discouraged the adsorption of water vapor. This work affirmed the dominance of Fe/N-doped carbon, which will contribute to the evolution of water-resistant VOCs adsorbent materials.

Functional Group Regulated Ni/Ti3C2Tx (Tx = F, -OH) Holding Bimolecular Activation Tunnel for Enhanced Ammonia Borane Hydrolysis

Developing economical and efficient catalyst for hydrogen generation from ammonia borane (AB) hydrolysis is still a huge challenge. As an alternative strategy, the functional group regulation of metal nanoparticles (NPs)-based catalysts is believed to be capable of improving the catalytic activity. Herein, a series of Ni/Ti₃C₂T_x-Y (T_x = F, -OH; Y denotes etching time (d)) catalysts are synthesized and show remarkably enhanced catalytic activity on the hydrolysis of AB in contrast to the corresponding without regulating. The optimized Ni/Ti₃C₂T_x-4 with a turnover frequency (TOF) value of 161.0 min^-1 exhibits the highest catalytic activity among the non-noble monometallic-based catalyst. Experimental results and theory calculations demonstrate that the excellent catalytic activity benefits from the bimolecular activation channels formed by Ni NPs and Ti₃C₂T_x-Y. H₂O and AB molecules are activated simultaneously in the bimolecular activation tunnel. Bimolecular activation reduces the activation energy of AB hydrolysis, and hydrogen generation rate is promoted. This article provides a new approach to design effective catalysts and further supports the bimolecular activation model for the hydrolysis of AB.

Analysis of SO2 Physisorption by Edge-Functionalized Nanoporous Carbons Using Grand Canonical Monte Carlo Methods and Density Functional Theory: Implications for SO2 Removal

Nanoporous carbons (NPCs) are ideal materials for the dry process of flue gas desulfurization (FGD) due to their rich pore structure and high specific surface area. To study the effect of edge-functionalized NPCs on the physisorption mechanism of sulfur dioxide, different functional groups were embedded at the edge of NPCs, and the physisorption behavior was simulated using the grand canonical Monte Carlo method (GCMC) combined with density functional theory (DFT). The results indicated that the insertion of acidic oxygenous groups or basic nitrogenous groups into NPCs could enhance the physisorption of SO₂. The influence of edge functionalization on the pore structure of NPCs is also analyzed. To further explore the interaction in the adsorption process, the van der Waals (vdW) interaction and electrostatic interaction between the SO₂ molecule and the basic structural unit (BSU) were investigated. Simulated results showed that edge functionalization had limited influence on vdW interaction and did not significantly change the distribution characteristics of vdW interaction. According to the study on electrostatic interaction, edge functionalization was found to promote inhomogeneity of the surface charge of the adsorbent, enhance the polarity of the adsorbent, and thus enhance the physisorption capacity of SO₂. More importantly, we provide an idea for studying the difference in adsorption capacity caused by different functional groups connected to carbon adsorbents.

Highly-efficient PVDF adsorptive membrane filtration based on chitosan@CNTs-COOH simultaneous removal of anionic and cationic dyes

An adsorptive membrane filtration based on polyvinylidene fluoride (PVDF) with chitosan (CS) and carboxylated carbon nanotubes (CNTs-COOH) is prepared by method of phase conversion, and the PVDF-CS@CNTs-COOH membranes can effectively separate anionic and cationic dye wastewater. Compared to pure PVDF membranes, PVDF-CS@CNTs-COOH increases pure water flux from 36.39 (L·m^-2·h^-1) to 85.25 (L·m^-2·h^-1), an increase of nearly 230%. The membrane exhibits excellent rejection performance in the filtration of six types of dye wastewater. The modified membranes also performed well in terms of rejection of mixed anionic and cationic dyes and also have a high performance in recycling, with a flux of over 94% for both anionic and cationic dyes. In addition, the adsorption curve fitting results showed that the adsorption process was more consistent with the pseudo-second-order adsorption kinetic model and Langmuir mode.

Porous structured cotton-based ACF for the adsorption of benzen

Fibrous activated carbon has attracted emerging research interests due to its remarkable adsorption performance for volatile organic compounds (VOCs). Though this adsorption behavior for VOCs is closely related to the pore structure on the surface of activated carbon fiber (ACF), few researchers paid attentions to the influence of textural properties of this adsorption process. Especially, cotton-based activated carbon fiber (CACF) for adsorbing benzene pollutant is rarely reported. Herein, in order to develop a high-performance adsorbent for the removal of VOCs pollutants, this work studied the influence of textural properties of CACF on the adsorption of benzene. The results showed that the increase of carbonization temperature would lead to the reduction of mesopores but the increase of micropores for CACF; the embedment of phosphoric acid and its derivatives into the carbon layers contributed to the formation of pore structure for CACF; furthermore, specific surface area of CACF can also be enlarged by increasing the concentration of phosphoric acid. More importantly, it was found that the adsorption capacity of CACF for benzene was strongly dependent on the specific surface area and volume of micropores within CACF because micropores can provide more favorable binding sites. This adsorption process preferred to occur on the wall of micropores, then the accumulated benzene would slowly fill the pores. Interestingly, the decrease of pore size of micropores can unexpectedly improve the affinity of CACF to benzene on the contrary. This work provides a new strategy to develop porous structured ACF materials for the high-performance adsorption of VOCs.

Effect analysis of pore wall thickness, pore size, and functional group of activated carbon on adsorption behavior based on molecular simulation

To effectively investigate the influence of activated carbon on the adsorption of volatile organic compounds (VOCs), physical and chemical factors of activated carbon including pore wall thickness, pore size, and functional groups were studied using grand canonical Monte Carlo (GCMC) simulation. In addition, benzene and acetone were taken as two representative components of VOCs. Simulation results was presented by the changes in characteristics of benzene and acetone. The results show that at the saturated vapor pressure (P₀), the adsorption density hardly varies with the mentioned factors of activated carbon. Differently, the saturated adsorption capacity increases considerably with the rise of pore size or the reduction of pore wall thickness, and the rise of pore size also leads to a dramatic increase in adsorption layer and a subsequent fall in ordering. However, when the pressure is less than 0.001P₀, the monomolecular interaction energy and the isosteric heat are strengthened greatly with the addition of carboxyl and amino groups, while the threshold pressure shows an opposite change to the monomolecular interaction energy. In the meantime, the decrease of pore size or the increase of pore wall thickness will result in the same results. Findings in this paper can provide valuable insights into the microscopic mechanisms of the adsorption between activated carbon and VOCs.

On the chirality-dependent adsorption behavior of volatile organic compounds on carbon nanotubes

The capture and removal of volatile organic compounds (VOCs) have received extensive attention due to their toxicity and carcinogenicity. In order to extend the applications of carbon nanotubes (CNTs) in this field, a deep understanding of the interaction mechanism between VOCs and CNTs is crucial. In this article, molecular dynamics simulations are performed to systematically investigate the multi-molecule adsorption behavior of four representative VOC species on CNTs with a variety of chirality indices. Simulation results reveal that different VOC species exhibit significantly different adsorption preferences on CNTs. For both zigzag and armchair CNTs, the adsorption affinity is positively correlated with the hydrophobicity of VOC molecules and follows the order of toluene > ether > acetone > methanol. This adsorption preference is supported by the binding free energy calculations resulting from the umbrella sampling algorithm. Moreover, the adsorption affinity increases with the diameter of both zigzag and armchair CNTs. Furthermore, the effects of diameter become more significant for those VOC species possessing higher hydrophobicity. As for the effects of chirality, zigzag CNTs show greater adsorption affinity than armchair ones with similar diameters. However, simulation results also indicate that the adsorption affinity does not vary monotonically from zigzag to armchair orientations, leading to additional complexities of harvesting and elimination of VOC molecules in terms of CNTs. Results and data analysis presented in this work suggest that CNT chirality is an important factor for controlling the adsorption of harmful VOC molecules on CNT surfaces.

Adsorption of Volatile Organic Compounds (VOCs) on Oxygen-rich Porous Carbon Materials Obtained from Glucose/Potassium Oxalate

To investigate the effects of oxygen-containing functional groups on the adsorption of volatile organic compounds (VOCs) with different polarity, oxygen-rich porous carbon materials (OPCs) were synthesized by heat treatment of glucose/potassium oxalate material. The carbon material had a large specific surface area (1697 m²  g^-1 ) and a high oxygen content (18.95 at.%). OPC exhibited high adsorption capacity of toluene (309 mg g^-1 ) and methanol (447 mg g^-1 ). The specific surface area and total pore volume determined the adsorption capacity of toluene and methanol at the high-pressure range, while the oxygen-containing groups became the main factor affecting the methanol adsorption at the low-pressure range due to the hydrogen bond interaction through the density functional theory (DFT) calculations. This study provides an important hint for developing a novel O-doped adsorbent for the VOCs adsorption applications and analyzing the role of oxygen-containing groups in the VOCs adsorption under the low-pressure range.

Synthesis of Silanol-Rich MCM-48 with Mixed Surfactants and Their Application in Acetone Adsorption: Equilibrium, Kinetic, and Thermodynamic Studies

Mesoporous silica MCM-48 with rich silanol was prepared using polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) as mixed templates, and the dynamic adsorption performance of acetone was evaluated by testing breakthrough curves. The mixed micelle formed by CTAB and PVP, as well as the hydrogen bond between the carbonyl group of PVP and silanol group affected the condensation process of Si-OH group during the formation of mesoporous structure, resulting in the increase of Si-OH group number on the surface of MCM-48. Compared with MCM-48 synthesized by single template (CTAB), the acetone adsorption capacity of MCM-48 (1:3) synthesized by mixed templates (PVP:CTAB = 1:3) improved by 23.86%, which was attributed to the increase of silanol group amount and the decrease of pore size. In addition, Bangham model had the highest goodness of fit to describe the adsorption process among four kinetic models for the adsorbents, conforming to the mechanism of pore diffusion. The Langmuir and Freundlich models were used to fit the adsorption isotherm data, and the Freundlich model could better describe the adsorption of acetone. Freundlich model fitting results showed that MCM-48 with rich silanol had a strong affinity for acetone, and the adsorption of acetone on MCM-48 belonged to multilayer adsorption. The thermodynamic results showed that the adsorption of MCM-48 for acetone was physical adsorption, and the adsorption behavior was exothermic. This work provided insight into how the inherent properties of an adsorbent and environmental factors (including initial concentration and adsorption temperature) affected the adsorption performance of ketones, thus more ideas could be provided for the accurate design of adsorbents. Furthermore, silanol-rich MCM-48 synthesized by mixed templates is expected to be a promising adsorbent for acetone removal.

Insights into the Adsorption of VOCs on a Cobalt-Adeninate Metal-Organic Framework (Bio-MOF-11)

With increasingly severe air pollution brought by volatile organic compounds (VOCs), the search for efficient adsorbents toward VOC removal is of great significance. Herein, an adenine-based metal-organic framework, namely, bio-MOF-11 [Co₂(ad)₂(CH₃CO₂)₂·0.3EtOH·0.6H₂O, ad = adeninate], was synthesized via a facile method, and its VOC adsorption was reported for the first time. This novel bio-MOF-11 was investigated by employing four common VOCs (i.e., methanol, acetone, benzene, and toluene) as adsorbates. The saturated adsorption capacity of these targeted VOCs on bio-MOF-11 was estimated to be 0.73-3.57 mmol/g, following the order: toluene < benzene < acetone < methanol. Furthermore, with the adsorption temperature increasing from 288 to 308 K, the saturated adsorption capacity was reduced by 7.3-35.6%. It is worth noting that acetone adsorption is most sensitive to temperature ascribed to its low boiling point and strong polar nature. Meanwhile, owing to the molecular sieve effect, the adsorption capacity appears negatively correlated to the size of VOC molecules. Besides, the abundant exposed nitrogen atoms and amino groups in bio-MOF-11 cavities facilitate the adsorption of polar VOC molecules. This work promotes the fundamental understanding and practical application of bio-MOF for adsorptive removal of VOCs.