A quantitation method for gaseous formaldehyde based on gas chromatography with metal–organic framework cold-trap sorbent as an effective alternative for HPLC-based standard protocol

Assessment of interfering/synergistic effects in the adsorption between polar and non-polar VOCs on a commercial biomass-based microporous carbon

This research has been carried out to investigate unique relationships in adsorption behavior between polar and non-polar volatile organic compounds (VOCs: formaldehyde (FA) versus toluene) using commercial macadamia nutshell (MNS)-based microporous activated carbon (i.e., Procarb-900: namely, P900). The breakthrough (BT) volume, adsorption capacity, and partition coefficient of P900 are estimated for 100 ppm FA as a single component and as a binary phase with 100 ppm toluene. The contrasting features of adsorption (such as interfering/synergistic relationships) for VOC mixtures with different polarities are accounted for in terms of interaction between the key variables (e.g., pore size distribution, adsorbent particle size, surface element compositions, and sorbent bed mass). Accordingly, the powdered P900 (0.212-0.6 mm: 150 mg) exhibits an adsorption capacity of 5.7 mg g-1 and a partition coefficient of 0.19 mol kg-1 Pa-1 for single-phase FA at the 10% BT level. Interestingly, its FA adsorption performance is synergistically improved in the presence of toluene (e.g., > 150%) in the early stage of adsorption (e.g., 10% BT), although their competition reduced its performance at 99% BT. The apparent synergistic trend in the early BT stage may possibly reflect diffusion resistance of the adsorbent (e.g., small particle size and developed ultra-micropore structure) and natural attributes of FA (e.g., low affinity and smaller kinetic diameter). The overall results of this study are expected to offer a better understanding of the mechanisms underlying the interactions between the mixed VOC system and microporous adsorbents.

Investigation on the source of VOCs emission from indoor construction materials using electronic sensors and TD-GC-MS

Indoor air quality (IAQ) is critical to the health and wellbeing of people. As the majority of people spend greater amounts of time indoors, either in office spaces or households, the level of air pollutants in such environments is critical. Building materials and furniture are known sources of air pollutants such as Volatile Organic Compounds (VOCs) and may be associated with discomfort, detrimental health of the occupants, etc. In this study, the VOCs found in a brand new office complex were monitored over a period of 6 months, with an emphasis on monitoring and quantifying harmful VOCs and identifying their emission source. Air samples were taken from a closed, unoccupied office space on a weekly basis and analysed using Thermal Desorption-Gas Chromatography-Mass Spectrometry (TD-GC-MS), while continuous monitoring of the air quality was performed using two commercially available IAQ sensors. To identify the source of the emitted VOCs, pieces of all construction material that were used in the office, including flooring, finished wall material, and adhesive glues, were removed, and placed in air-tight glass containers prior to analysis confirming that the source of VOCs is indeed the flooring. Identified compounds included mainly material origin VOCs such as BTEX (benzene, toluene, ethylbenzene, xylene) and styrene, but also common VOCs such as acetone and propan-2-ol. Of significant importance was the concentration of toluene that was found to be the most abundant VOC in both the flooring material and the indoor air.

Thermocatalytic oxidation of a binary mixture of formaldehyde and toluene at ambient levels by a titanium dioxide supported platinum catalyst

The thermocatalytic oxidative potential of various supported noble metal catalysts (SNMCs) is well-known for hazardous volatile organic compounds (VOCs), e.g., formaldehyde (FA) and toluene. However, little is known about SNMC performance against ambient VOC pollution with low concentration (subppm levels) relative to industrial effuluents with high concentrations (several hundred ppm). Here, the thermocatalytic oxidation performance of a titanium dioxide (TiO2)-supported platinum catalyst (Pt/TiO2) has been evaluated for a low-concentration binary mixture of FA and toluene at low temperatures and in the dark. A sample of TiO2 containing 1 wt% Pt with thermal reduction pre-treatment under hydrogen achieved 100 % conversion of FA (500 ppb) and toluene (100 ppb) at 130 °C and a gas hourly velocity of 59,701 h-1. Its catalytic activity was lowered by either a decrease in catalyst mass or an increase in VOC concentration, relative humidity, or flow rate. In situ diffuse reflectance infrared Fourier transform spectroscopy, density functional theory simulations, and molecular oxygen (O2) temperature-programmed desorption experiments were used to identify possible VOC oxidation pathways, reaction mechanisms, and associated surface phenomena. The present work is expected to offer insights into the utility of metal oxide-supported Pt catalysts for the low-temperature oxidative removal of gaseous VOCs in the dark, primarily for indoor air quality management.

Dual-ligand 3D lammelar chiral metal-organic framework for capillary electrochromatographic enantioseparations

Dual-ligand metal-organic frameworks (MOFs) based on tryptophan and camphoric acid were designed and synthesized as the stationary phase of the capillary electrochromatography (CEC) system. This CEC system showed significantly improved enantioseparation ability for nine drugs, compared with the single-ligand MOF stationary phase. Characterization methods such as N₂ adsorption-desorption isotherms and scanning electron microscopy proved that the dual-ligand MOFs possessed excellent 3D spatial structures (ligand ratio is 1:1) which ensured the enantioseparation capability of the CEC system. The influence of ligand types on the chiral selectivity of MOFs was explored using racemic phenylalaninol and its single enantiomers as models. When the chiral type of the ligands is consistent, the enantioseparation ability of the CEC system is better. The chromatographic conditions such as buffer concentration, buffer pH, organic solvent addition ratio, and applied voltage were optimized, and satisfactory repeatability and stability of the CEC system were verified. Additionally, the enantioseparation mechanism of the CEC system was discussed through adsorption kinetic experiments, adsorption isotherm fitting, and thermodynamics.

In Silico Screening of Metal-Organic Frameworks for Formaldehyde Capture with and without Humidity by Molecular Simulation

Capturing formaldehydes (HCHO) from indoor air with porous adsorbents still faces challenges due to their low capacity and poor selectivity. Metal-organic frameworks (MOFs) with tunable pore properties were regarded as promising adsorbents for HCHO removal. However, the water presence in humid air heavily influences the formaldehyde capture performance due to the competition adsorption. To find suitable MOFs for formaldehyde capture and explore the relationship between MOFs structure and performance both in dry air and humid air, we performed grand canonical Monte Carlo (GCMC) molecular simulations to obtain working capacity and selectivity that evaluated the HCHO capture performance of MOFs without humidity. The results reveal that small pore size (~5 Å) and moderate heat of adsorption (40-50 kJ/mol) are favored for HCHO capture without water. It was found that the structure with a 3D cage instead of a 2D channel benefits the HCHO adsorption. Atoms in these high-performing MOFs should possess relatively small charges, and large Lennard-jones parameters were also preferred. Furthermore, it was indicated that Henry's constant (K_H) can reflect the HCHO adsorption performance without humidity, in which the optimal range is 10^-2-10¹. Hence, Henry's constant selectivity of HCHO over water (SK_H HCHO/H₂O) and HCHO over mixture components (H₂O, N_2, and O₂) was obtained to screen MOFs at an 80% humidity condition. It was suggested that SK_H for the mixture component overestimates the influence of N₂ and O₂, in which the top structures absorb a quantity of water in GCMC simulation, while SK_H HCHO/H₂O can efficiently find high-performing MOFs for HCHO capture at humidity in low adsorption pressure. The ECATAT found in this work has 0.64 mol/kg working capacity, and barely adsorbs water during 0-1 bar, which is the promising candidate MOF for HCHO capture.

A novel room-temperature formaldehyde gas sensor based on walnut-like WO3 modification on Ni–graphene composites

Ternary composite with great modulation of electron transfers has attracted a lot of attention from the field of high-performance room-temperature (RT) gas sensing. Herein, walnut-like WO3-Ni–graphene ternary composites were successfully synthesized by the hydrothermal method for formaldehyde (HCHO) sensing at RT. The structural and morphological analyses were carried out by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). SEM and TEM studies confirmed that walnut-like WO3 nanostructures with an average size of 53 ± 23 nm were functionalized. The Raman and XPS results revealed that, due to the deformation of the O-W-O lattice, surface oxygen vacancies Ov and surface-adsorbed oxygen species Oc were present. The gas-sensing measurement shows that the response of the WO3-Ni-Gr composite (86.8%) was higher than that of the Ni-Gr composite (22.7%) for 500 ppm HCHO at RT. Gas-sensing enhancement can be attributed to a p-n heterojunction formation between WO3 and Ni-Gr, Oc, spill-over effect of Ni decoration, and a special walnut-like structure. Moreover, long term stability (%R = 61.41 ± 1.66) for 30 days and high selectivity in the presence of other gases against HCHO suggested that the proposed sensor could be an ideal candidate for future commercial HCHO-sensing in a real environment.