Adsorption–desorption behavior of gaseous formaldehyde on different porous Al2O3 materials

Porous alumina nanosheet-supported asymmetric platinum clusters for efficient diboration of alkynes

Precisely designing asymmetrical structures is an effective strategy to optimize the performance of metallic catalysts. Asymmetric Pt clusters were attached to defect-rich porous alumina nanosheets (Pt clu/dp-Al2O3) using a pyrolysis technique coupled with wet impregnation. These Pt-functionalized nanosheets feature a high concentration of active sites, demonstrating remarkable cycling performance and catalytic activity in alkyne diboration. The conversion yield and selectivity can reach up to 97% and 95%, correspondingly.

Nanostructured micro/mesoporous graphene: removal performance of volatile organic compounds

In this study, we demonstrate an integrated synthesis strategy, which is conducted by the thermochemical process, consisting of pre- and post-activation by thermal treatment and KOH activation for the reduction of graphite oxide. A large number of interconnected pore networks with a micro/mesoporous range were constructed on a framework of graphene layers with a specific surface area of up to 1261 m2 g-1. This suggests a synergistic effect of thermally exfoliated graphene oxide (TEGO) on the removal efficiency of volatile organic compounds by generating pore texture with aromatic adsorbates such as benzene, toluene, and o-xylene (denoted as BTX) from an inert gaseous stream concentration of 100 ppm. As a proof of concept, TEGO, as well as pre- and post-activated TEGO, were used as adsorbents in a self-designed BTX gas adsorption apparatus, which exhibited a high removal efficiency of up to 98 ± 2%. The distinctive structure of TEGO has a significant effect on removal performance, which will greatly facilitate the strategy of efficient VOC removal configurations.

Preparation of Mn-Fe Oxide by a Hydrolysis-Driven Redox Method and Its Application in Formaldehyde Oxidation

Homogeneous distribution of Mn-Fe oxides (xMn1Fe) with different Mn/Fe ratios was synthesized by a hydrolysis-driven redox method, and their catalytic activities in HCHO oxidation were investigated. The results showed that HCHO conversion was significantly improved after doping iron due to the synergistic effect between manganese and iron. The 5Mn1Fe catalyst exhibits excellent catalytic activity, achieving >90% HCHO conversion at 80 °C and nearly 100% conversion at 100 °C. The physicochemical properties of catalysts were characterized by BET, XRD, H2-TPR, O2-TPD, and XPS techniques. Experimental results revealed that the introduction of Fe into MnO x resulted in a large surface area, a high ratio of Mn4+, abundant lattice oxygen species and oxygen vacancy, and uniform distribution of Mn and Fe, thus facilitating the oxidation of HCHO to CO2 and H2O.

Zipper phosphorene as sensing element towards formaldehyde and acetaldehyde - A first-principles insight

We ascertained the structural stability of zipper phosphorene nanosheet (zP-NS) and studied the adsorption behaviour of toxic aldehyde compounds including formaldehyde (FD) and acetaldehyde (AD) on zP-NS based on first-principles calculation. Considerably, zP-NS reveal a semiconducting character with band gap of 1.35 eV. Especially, four distinct favourable adsorption positions including bridge-, hollow-, top- and valley-site of FD and AD vapours on zP-NS were investigated. Furthermore, the calculated binding-energy of prominent adsorption sites are observed to be in the scope of -0.143 eV to -0.411 eV advocating physisorption nature of the interaction of chief aldehydes on zP-NS. The overall outcomes recommend that zP-NS can be persuasively utilised as a chemical sensor for monitoring FD and AD molecules in indoor air environment.

An efficient strategy for the enhancement of adsorptivity of microporous carbons against gaseous formaldehyde: Surface modification with aminosilane adducts

In an effort to develop a cost-effective mitigation tool for volatile organic compounds, particularly formaldehyde (FA), microporous activated carbon (AC) was modified into three different forms of AC-1, AC-2, and AC-3 using a raw commercial AC product (AC-0). First, AC-1 and AC-2 were produced by the modification of AC-0 with N/S heteroatoms using identical mixture of dicyandiamide and thiourea precursors through either solvothermal (AC-1) or microwave-assisted calcination (AC-2) synthesis. Second, aminosilane-functionalized AC (AC-3) was prepared solvothermally using N-[3-(Trimethoxysilyl)propyl]ethylenediamine reagent. The relative adsorption performances for gaseous FA (1 ppm) in terms of 10% breakthrough volume (BTV10: L atm g^-1) at near-ambient conditions (25 °C and 1 atm) were AC-3 (132) > AC-2 (66.5) > AC-1 (14.2) > AC-0 (10.4). In a comparison based on partition coefficients (mole kg^-1 Pa^-1) at BTV10, AC-3 outperformed AC-0 by a factor of 214, while the adsorption performance of AC-2 was 36-times higher than AC-1. The enhanced performance of AC-2 over AC-1 reflected the effect of the microwave synthesis protocol on the improvement of surface chemistry (e.g., N/S doping) and texture (e.g., surface area and pore volume) of AC-based adsorbents as compared to conventional solvothermal method. Further, the prominent role of surface chemistry (e.g., relative to textural properties), as observed with the increases in the amount of doped functional elements (including N:C and silicon:C ratios), is supported by the apparent dependence of performance on the selected modification procedures. Based on kinetic and X-ray photoelectron spectroscopy analyses, the superiority of aminosilylated AC-3 can be attributed to a synergistic effect between physisorption (e.g., pore diffusion) and chemical interactions of the FA carbonyl (C=O) group with amine and silica functionalities (via Mannich coupling [Schiff base] and cycloaddition reaction mechanisms, respectively). This confirms the significance of surface chemistry, relative to pore diffusion, in achieving maximum adsorption of gaseous FA molecules.

Utilization of metal-organic frameworks for the adsorptive removal of an aliphatic aldehyde mixture in the gas phase

Considerable efforts have been undertaken in the domain of air quality management for the removal of hazardous volatile organic compounds, particularly carbonyl compounds (CCs). In this study, the competitive sorptive removal of six CCs (namely, formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde) was assessed using selected metal-organic frameworks (MOFs: MOF-5, MOF-199, UiO-66, and UiO-66-NH₂) and inexpensive commercial activated carbon as a reference sorbent. The sorption experiments were conducted using a mixture of the six CCs (formaldehyde and acetaldehyde at ∼1 Pa and propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde at ∼0.2 Pa) together with 15 Pa water and 2.6 Pa methanol in 1 bar nitrogen. For all of the carbonyl compounds other than formaldehyde, MOF-199 showed the best 10% breakthrough performance ranging from 34 L g^-1 and 0.14 mol kg^-1 Pa^-1 for acetaldehyde to 1870 L g^-1 and 7.6 mol kg^-1 Pa^-1 for isovaleraldehyde. Among all the sorbents tested, UiO-66-NH₂ exhibited the best 10% breakthrough performance metrics towards the lightest formaldehyde which remains to be one of the most difficult targets for sorptive removal (breakthrough volume: 285 L g^-1 and partition coefficient: 1.1 mol kg^-1 Pa^-1). Theoretical density functional theory (DFT)-based computations were also conducted to provide better insights into the adsorbate-adsorbent interactions. Accordingly, the magnitude of adsorption energy increased with an increase in the CC molar mass due to an enhancement in the synergetic interaction between C[double bond, length as m-dash]O groups (in adsorbate molecules) and the MOF active centers (open metallic centers and/or NH₂ functionality) as the adsorbent. Such interactions were observed to result in strong distortion of MOF structures. In contrast, weak van der Waals attraction between the hydrocarbon "tail" of CC molecules and MOF linkers were seen to play a stabilizing role for the sorbent structure. The presence of the NH₂ group in the MOF structure was suspected to play a key role in capturing lighter CCs, while such an effect was less prominent for heavier CCs. Overall, the results of this study provided a basis for the establishment of an effective strategy to enhance the sorption capacity of MOFs against diverse carbonyl species.

An efficient tool for the continuous monitoring on adsorption of sub-ppm level gaseous benzene using an automated analytical system based on thermal desorption-gas chromatography/mass spectrometry approach

It became an important task to effectively adsorb volatile organic compounds (VOCs) at or near real-world levels for efficient control of airborne pollution in ambient environments. Nonetheless, most studies carried out previously for the control of VOCs are confined to significantly polluted conditions (e.g., >100 ppm) that are far different from real-world or ambient conditions. To help acquire the meaningful data for the adsorptive removal of VOCs at near real-world levels, a new approach was designed and implemented to measure adsorption of gaseous benzene (as a representative or model VOC) at trace-level quantities (as low as 0.14 ng (0.43 ppb) for a 100 mL sample) using activated carbon (sieved to 212 μm mesh size) as a model sorbent. With the aid of a thermal desorption-gas chromatography/mass spectrometry system, the key adsorption performance metrics (such as 10% breakthrough volume (10% BTV) points: 10% as the key reference) were determined: 1018 L atm g^-1 at 0.1 ppm benzene with the corresponding partition coefficient of 3.85 mol kg^-1 Pa^-1. If the adsorption capacity values (at 10% BTV) are compared across the varying concentration levels of benzene, the maximum value of 1.07 mg g^-1 was observed at 1 ppm benzene (within the concentration range selected in this work). As such, it was possible to quantitatively assess the sorbate-sorbent interactions at significantly low concentrations of VOCs that actually prevail under the near real-world conditions.

Synthesis of mesoporous MFI zeolite via bacterial cellulose-derived carbon templating for fast adsorption of formaldehyde

Mesoporous ZSM-5 (MFI) zeolite was synthesized by using bacterial cellulose-derived activated carbon (BC-AC500) with a high surface area as a hard template. Different ratios of BC-AC500 and zeolite precursor gel were prepared in a Teflon-lined autoclave and crystallized at 180 °C for 48 h in a rotating oven. The physicochemical properties of the samples were characterized by x-ray diffraction (XRD), scanning/transmission electron microscopies (SEM/TEM), and N₂ physisorption techniques. It was found that the mesoporous ZSM-5 zeolites have a specific surface area of 184-190 m²/g, a high mesopore volume of 0.120-0.956 ml/g and a wide pore size distribution ranging from 5 to 100 nm with a maximum at approximately 25.3 nm. The successfully made mesoporous ZSM-5 was tested as an adsorbent for formaldehyde adsorption in batch mode. The mesoporous ZSM-5 zeolite made from bacterial cellulose-derived activated carbon showed significantly faster adsorption kinetics than conventional ZSM-5 (0.0081 vs. 0.0007 g/mg min, respectively). The prepared material has an adsorption capacity of 98 mg/g and is highly reusable. The reported mesoporous ZSM-5 zeolites can be deployed for the rapid removal of toxic organics from wastewater when urgently needed, e.g., under breakthrough conditions.

The retrograde adsorption phenomenon at the onset of breakthrough and its quantitation: An experimental case study for gaseous toluene on activated carbon surface

The adsorption dynamics of common solid sorbents against various pollutant species are yet poorly understood with respect to the retrograde phenomenon in which the relationship between breakthrough vs. pulled volume is characterized by an early unusual trend (initial increase followed by a decrease to a minimum) and by a normal trend of finally increasing to 100% (or equilibrium). If such trend is expressed in terms of the partition coefficient (PC), a reversed trend of adsorption processes becomes more evident. Retrograde has been previously observed in the initial breakthrough (<10%) isotherms in continuous flow gas-phase adsorption processes. However, retrograde has been neglected/overlooked and not discussed at all in the main stream literature even when it is explicitly observed from isotherm datasets. To properly describe the various aspects of such process, a stop-flow technique was developed to measure the adsorption isotherm of a model volatile organic compound (i.e., toluene in this study) onto a commercial low-cost sorbent (activated carbon: AC). Accordingly, a 10% breakthrough volume of 762 L atm g^-1 (corresponding adsorption capacity of 142 mg g^-1) was determined (at an inlet stream 5 Pa of toluene in 1 atm of N₂ and 5 mg of AC). This automated method was effective to generate a detailed breakthrough profile at high stream-flow rates (or high space velocities) to specifically detect the retrograde phenomenon at the breakthrough onset. This study offers a practical approach towards establishing an in-depth monitoring protocol for the rare retrograde phenomenon.

Adsorption properties of advanced functional materials against gaseous formaldehyde

Intense efforts have been made to eliminate toxic volatile organic compounds (VOCs) in indoor environments, especially formaldehyde (FA). In this study, the removal performances of gaseous FA using two metal-organic frameworks, MOF-5 and UiO-66-NH₂, and two covalent-organic polymers, CBAP-1 (EDA) and CBAP-1 (DETA), along with activated carbon as a conventional reference material, were evaluated. To assess the removal capacity of FA under near-ambient conditions, a series of adsorption experiments were conducted at its concentrations/partial pressures of both low (0.1-0.5 ppm/0.01-0.05 Pa) and high ranges (5-25 ppm/0.5-2.5 Pa). Among all tested materials at the high-pressure region ㅐ (e.g., at 2.5 ppm FA), a maximum adsorption capacity of 69.7 mg g^-1 was recorded by UiO-66-NH₂. Moreover, UiO-66-NH₂ also displayed the best 10% breakthrough volume (BTV10) of 534 L g^-1 (0.5 ppm FA) to 2963 L g^-1 (0.1 ppm FA). In contrast, at the high concentration test (at 5, 10, and 25 ppm FA), the maximum BTV10 values were observed as: 137 (UiO-66-NH₂), 144 (CBAP-1 (DETA)), and 36.8 L g^-1 (CBAP-1 (EDA)), respectively. The Langmuir isotherm model was observed to be a better fit of the adsorption data than the Freundlich model under most of the tested conditions. The superiority of UiO-66-NH₂ was attributed to the van der Waals interactions between the linkers (framework) and the hydrocarbon "tail" (FA) coupled with interactions between its open metal sites and the FA carbonyl groups. This study demonstrated the good potential of these advanced functional materials toward the practical removal of gaseous FA in indoor environments.

Competitive adsorption of gaseous aromatic hydrocarbons in a binary mixture on nanoporous covalent organic polymers at various partial pressures

Covalent-organic polymers (COPs) are recognized for their great potential for treating diverse pollutants via adsorption. In this study, the sorption behavior of benzene and toluene was investigated both individually and in a binary mixture against two types of COPs possessing different -NH₂ functionalities. Namely, the potential of COPs was tested against benzene and toluene in a low inlet partial pressure range (0.5-20 Pa) using carbonyl-incorporated aromatic polymer (CBAP)-1-based diethylenediamine (EDA) [CD] and ethylenetriamine (DETA) [CE]. The maximum adsorption capacity and breakthrough values of both COPs showed dynamic changes with increases in the partial pressures of benzene and toluene. The maximum adsorption capacities (A_max) of benzene (as the sole component in N₂ under atmospheric conditions) on CD and CE were in the range of 24-36 and 33-75 mg g^-1, respectively. In contrast, with benzene and toluene in a binary mixture, the benzene A_max decreased more than two-fold (range of 2.7-15 and 6-39 mg g^-1, respectively) due to competition with toluene for sorption sites. In contrast, the toluene A_max values remained consistent, reflecting its competitive dominance over benzene. The adsorption behavior of the targeted compounds (i.e., benzene and toluene) was explained by fitting the adsorption data by diverse isotherm models (e.g., Langmuir, Freundlich, Elovich, and Dubinin-Radushkevich). The current research would be helpful for acquiring a better understanding of the factors affecting competitive adsorption between different VOCs in relation to a given sorbent and across varying partial pressures.

A TiO2/C catalyst having biomimetic channels and extremely low Pt loading for formaldehyde oxidation

This study presents a TiO2/C hybrid material with biomimetic channels fabricated using a wood template. Repeated impregnations of pretreated wood chips in a Ti precursor were conducted, followed by calcination at 400-600 °C for 4 hours under a nitrogen atmosphere. The generated TiO2 nanocrystals were homogenously distributed inside a porous carbon framework. With an extremely low Pt catalyst loading (0.04-0.1 wt%), the obtained porous catalyst could effectively oxidize formaldehyde to CO2 and H2O even under room temperature (conv. ∼100%). Wood acted as both a structural template and reduction agent for Pt catalyst generation in sintering. Therefore, no post H2 reduction treatment for catalyst activation was required. The hierarchal channel structures, including 2-10 nm mesopores and 20 μm diameter channels, could be controlled by calcination temperature and atmosphere, which was confirmed by SEM and BET characterizations. Based on the abundant availability of wood templates and reduced cost for low Pt loading, this preparation method shows great potential for large-scale applications.

Efficient removal of gaseous formaldehyde in air using hierarchical titanate nanospheres with in situ amine functionalization

Amine-grafted titanate nanospheres are fabricated as efficient and recyclable adsorbents for formaldehyde removal.