Novel application of sodium manganese oxide in removing acidic gases in ambient conditions.

In this study, we have demonstrated the application of sodium manganese oxide for the chemisorption of toxic acidic gases at room temperature. The fabricated alkali ceramic has Na0.4MnO2, Na2Mn3O7, and NaxMnO2 phases with a surface area of 2.6 m2 g-1. Na-Mn oxide was studied for oxidation of H2S, SO2, and NO2 gases in the concentration range of 100-500 ppm. The material exhibited a high uptake capacity of 7.13, 0.75, and 0.53 mmol g-1 for H2S, SO2, and NO2 in wet conditions, respectively. The material was reusable when regenerated simply by soaking the spent oxide in a NaOH-H2O2 solution. While the H2S chemisorption process was accompanied by sulfide, sulfur, and sulfate formation, the SO2 chemisorption process yielded only sulfate ions. The NO2 chemisorption process was accomplished by its conversion to nitrite and nitrate ions. Thus, the present work is one of the first reports on alkali ceramic utilization for room-temperature mineralization of acidic gases.

Impregnation of Activated Carbon with Organic Phase-Change Material.

In this study, we developed a thermal storage medium comprising porous activated carbon filled with organic phase-change materials (PCMs) that utilizes the latent heat of phase-change to absorb heat during heating and release heat during cooling. For the activated carbon, we used both charcoal-based powdered activated carbon (250-350 mesh) and granular activated carbon. The organic phase-change materials used in the experiments were dodecane, tridecane, tetradecane, and pentadecane. Material properties such as thermal conductivity, latent heat, and melting temperature range were evaluated experimentally and theoretically, with the results observed to be consistent. The cyclic thermal performance of the proposed medium was also evaluated. Notably, filling the activated carbon with a mixture of organic PCMs resulted in the highest temperature-moderating effect. The procedure and results presented in this study are expected to aid in further improvement in the performance of thermal storage media containing PCM where stable temperatures are required, including building heating and cooling.

A Novel Metal-Containing Mesoporous Silica Composite for the Decolorization of Rhodamine B: Effect of Metal Content on Structure and Performance.

A series of novel MnxFey@SiO2 (x,y = 1-20%) nanocomposites were synthesized for the first time via the sol-gel/combustion method with different content of precursors (Mn and Fe acetate salts). The effect of precursor content and ratio on physicochemical properties were observed by various characterization methods. Moreover, Rhodamine B (RhB) was chosen as the target pollutant to test the performance of these nanocomposites under a photocatalytic Fenton-like reaction. The results showed that the nanocomposite morphology improved by increasing Fe and Mn content. In this study, interesting behavior was observed in BET results which were different from the fact that increasing metal content can decrease the surface area. This study revealed that one metal could be more critical in controlling the properties than another. Moreover, the precursor ratio appears to have a more tangible effect on the surface area than the effect of precursor content. Among all synthesized nanocomposites, Mn1Fe5@SiO2 showed the highest surface area of 654.95 m2/g. At optimum batch conditions (temp = 25 °C, catalyst dosage = 1 g L-1, H2O2 = 75 mmolL-1, and initial RhB concentration = 50 mg L-1), complete removal (simultaneous adsorption/degradation) occurred using Mn1Fe5@SiO2 at neutral pH. This study showed that the designed nanomaterial could be used as a dual functional adsorbent/photocatalyst in different environmental applications.

Sustainable Removal of BTEX Gas Using Regenerated Metal Containing SiO2.

In the last decades, the removal of benzene, toluene, ethylbenzene, and xylene (BTEX) has been considered a major environmental crisis. In this study, two novel nanocomposite materials (Fe2O3/SiO2 and Fe2O3-Mn2O3/SiO2) that have regeneration ability by UV irradiation have been fabricated to remove BTEX at ambient temperature. This research revealed that both nanocomposites could remove more than 85% of the BTEX in the first cycle. The adsorption capacities followed the order of ethylbenzene > m-xylene > toluene > benzene as in the molecular weight order. The reusability test using UV irradiation showed that the performance of Fe2O3/SiO2 decreased drastically after the fifth cycle for benzene. On the other hand, when Mn is located in the nanocomposite structure, Fe2O3-Mn2O3/SiO2 could maintain its adsorption performance with more than 80% removal efficiency for all the BTEX for ten consecutive cycles. The difference in the reusability of the two nanocomposites is that the electron energy (from the valence band to the conduction band) for BTEX decomposition is changed due to the presence of manganese. This study provides a promising approach for designing an economical reusable nanomaterial, which can be used for VOC-contaminated indoor air.

Fabrication of Na0.4MnO2 Microrods for Room-Temperature Oxidation of Sulfurous Gases.

Phase pure Na0.4MnO2 microrods crystallized in the orthorhombic symmetry were fabricated for the wet oxidation of H2S and SO2 gases at room temperature. The material was found highly effective for the mineralization of low concentrations of acidic gases. The material was fully regenerable after soaking in a basic H2O2 solution.

Ternary metal oxide nanocomposite for room temperature H2S and SO2 gas removal in wet conditions.

A ternary Mn-Zn-Fe oxide nanocomposite was fabricated by a one-step coprecipitation method for the remotion of H2S and SO2 gases at room temperature. The nanocomposite has ZnO, MnO2, and ferrites with a surface area of 21.03 m2 g-1. The adsorbent was effective in mineralizing acidic sulfurous gases better in wet conditions. The material exhibited a maximum H2S and SO2 removal capacity of 1.31 and 0.49 mmol g-1, respectively, in the optimized experimental conditions. The spectroscopic analyses confirmed the formation of sulfide, sulfur, and sulfite as the mineralized products of H2S. Additionally, the nanocomposite could convert SO2 to sulfate as the sole oxidation by-product. The oxidation of these toxic gases was driven by the dissolution and dissociation of gas molecules in surface adsorbed water, followed by the redox behaviour of transition metal ions in the presence of molecular oxygen and water. Thus, the study presented a potential nanocomposite adsorbent for deep desulfurization applications.

Probing the origin and stability of bivalency in copper based porous coordination network and its application for H2S gas capture.

A bivalent Cu(I,II) metal-organic framework (MOF) based on the 4,4',4″-s-Triazine-2,4,6-triyl-tribenzoate linker was synthesized via a solvothermal method. The MOF possessed 43.8% of the Cu sites as Cu+ with a surface area of 1257 m2 g-1. The detailed spectroscopic analysis confirmed dimethylformamide (DMF) solvent as the reductant responsible for Cu+ sites in the synthesized MOF. The Cu+ sites were easily accessible and prone to oxidation in hot water or acidic gas environment. The MOF showed water-induced structural change, which could be partially recovered after soaking in DMF solvent. The synthesized MOF showed a high hydrogen sulfide (H2S) uptake capacity of 4.3 mmol g-1 at 298 K and an extremely low H2S pressure of 0.0005 bar. The adsorption capacity was the highest among Cu-based MOFs with PCN-6-M being regenerable, which made it useful for deep desulfurization applications. The adsorbed H2S was mineralized to sulfide, sulfur, and sulfates, mediated by the Cu+/Cu2+ redox cycle in the presence of adsorbed water and molecular oxygen. Thus, the study confirmed that DMF as a reductant is responsible for the origin of bivalency in PCN-6-M and possibly in other Cu-based MOFs reported in the literature. Also, the developed MOF could be a potential candidate for flue gas desulfurization and gas purification applications.

Role of Bimetallic Solutions in the Growth and Functionality of Cu-BTC Metal-Organic Framework.

Bimetallic solutions play a vital role in the growth and functionality of copper trimesate (Cu-BTC) metal-organic frameworks (MOFs). The effect of Ag+, Ca2+, Mn2+, Co2+, and Zn2+ on the growth of Cu-BTC was studied by fabricating M-Cu-BTC MOFs at room temperature using bimetallic M-Cu solutions. While Ag+ in the MOF had a rod-like morphology and surface properties, divalent cations deteriorated it. Moreover, unconventional Cu+ presence in the MOF formed a new building unit, which was confirmed in all the MOFs. Apart from Ag and Mn, no other MOF showed any presence of secondary cations in the structure. While Ag-Cu-BTC showed an improved H2S uptake capacity, other M-Cu-BTC MOFs had superior organic pollutant adsorption behavior. Thus, we have demonstrated that the physicochemical properties of Cu-BTC could be modified by growing it in bimetallic solutions.

Behavior of nitrogen and sulfur compounds in the rice husk pellet bioscrubber and its circulation water.

In this study, pellet-type biofilter media was developed with rice husk and applied in a wet scrubber system for odor removal. The lab-scale bioscrubber system was operated for 200 days to evaluate odorous gas removal (i.e., NH3, H2S, methyl mercaptan, and dimethyl sulfide), and the removal mechanism of odor gases was studied by analyzing the behavior of nitrogen and sulfur compounds in circulation water of bioscrubber system. The rice husk pellets supplied the organic carbon source and phosphoric acid necessary for microbial growth, allowing the system to continue successfully for 200 days without any maintenance technology. By analyzing the behavior of the nitrogen and sulfur compounds in the circulation water, we confirmed that the odor gas removal resulted from various mechanisms, including adsorption and biodegradation. Ammonia gas was absorbed by the rice husk pellets and accumulated in the circulation water as nitrite under conditions of sufficient alkalinity and above pH 7. Conversely, when the alkalinity and pH decreased, nitrite was rapidly converted to nitrate. However, H2S gas was oxidized to sulfate and continuously accumulated in the circulation water regardless of the pH and alkalinity. In addition, it was confirmed that the decrease in nitrate in the bioscrubber system was due to heterotrophic denitrification by the organic carbon source supply and autotrophic denitrification by sulfur gas. During the operation of the rice husk pellet bioscrubber for 8 months, under low solubility condition, more than 99% of NH3 and H2S were removed and about 85% of methyl mercaptan (MM) and dimethyl sulfide (DMS) were removed.

Metal-organic framework-derived NaMxOy adsorbents for low-temperature SO2 removal.

This work reported the fabrication of NaMxOy-type adsorbents from air calcination of (Na, M)-trimesate metal-organic frameworks. NaMnxOy (NMO) crystallized as disc-shaped microsheets, whereas NaCoxOy (NCO) crystallized as smooth microsheets with surface deposition of polyhedral nanoparticles. The oxides have a surface area of 1.90-2.56 m2 g-1. The synthesized adsorbents were studied for low-temperature SO2 removal in breakthrough studies. The maximum adsorption capacity of 46.8 mg g-1 was recorded for NMO at 70 °C. The adsorption capacity increased with the increasing temperature due to the chemisorptive nature of the adsorption process. The capacity increased with the increasing bed loading and decreasing flow rate due to the improved SO2 retention time. The elemental mapping confirmed the uniform distribution of sulfur species over the oxide surface. X-ray diffraction showed the absence of metal sulfate nanoparticles in the SO2-exposed samples. The X-ray photoelectron analysis confirmed the formation of surface sulfate and bisulfate. The formation of oxidized sulfur species was mediated by hydroxyl groups over NMO and lattice oxygen over NCO. Thus, the work demonstrated here is the first such report on the use of NaMxOy-type materials for SO2 mineralization.

From MOF-199 Microrods to CuO Nanoparticles for Room-Temperature Desulfurization: Regeneration and Repurposing Spent Adsorbents as Sustainable Approaches.

MOF-199 is one of the well-studied metal-organic frameworks (MOFs) for the capture of small gas molecules. In this study, we have investigated the thermal transformation of MOF-199 microrods to CuO nanoparticles by various microscopic and spectroscopic techniques. The growth of oxide was initiated by the formation of ∼2.5 nm particles at 200 °C, which ended up as CuO nanoparticles of ∼100-250 nm size at 550 °C. An intermediate presence of Cu2O along with CuO was recorded at 280 °C. The MOF and calcined products were tested for the room-temperature desulfurization process. MOF-199 showed the maximum adsorption capacity for H2S gas (77.1 mg g-1) among all adsorbents studied. Also, MOF-199 showed a better regeneration efficiency than the derived oxide. For a sustainable process, the exhausted adsorbents were used for the photocatalytic degradation of methylene blue. The exhausted materials showed better degradation efficiencies than the fresh materials. This study reports new sustainable approaches for MOF-199 application in air and water decontamination.

Metal organic framework derived NaCoxOy for room temperature hydrogen sulfide removal.

Novel NaCoxOy adsorbents were fabricated by air calcination of (Na,Co)-organic frameworks at 700 °C. The NaCoxOy crystallized as hexagonal microsheets of 100-200 nm thickness with the presence of some polyhedral nanocrystals. The surface area was in the range of 1.15-1.90 m2 g-1. X-ray photoelectron spectroscopy (XPS) analysis confirmed Co2+ and Co3+ sites in MOFs, which were preserved in NaCoxOy. The synthesized adsorbents were studied for room-temperature H2S removal in both dry and moist conditions. NaCoxOy adsorbents were found ~ 80 times better than the MOF precursors. The maximum adsorption capacity of 168.2 mg g-1 was recorded for a 500 ppm H2S concentration flowing at a rate of 0.1 L min-1. The adsorption capacity decreased in the moist condition due to the competitive nature of water molecules for the H2S-binding sites. The PXRD analysis predicted Co3S4, CoSO4, Co3O4, and Co(OH)2 in the H2S-exposed sample. The XPS analysis confirmed the formation of sulfide, sulfur, and sulfate as the products of H2S oxidation at room temperature. The work reported here is the first study on the use of NaCoxOy type materials for H2S remediation.

Fabrication of Zn-MOF/ZnO nanocomposites for room temperature H2S removal: Adsorption, regeneration, and mechanism.

Zn-MOF/ZnO nanocomposites with different organic linkers were fabricated by a rapid ultrasonication method using freshly prepared Zn(OH)2 precipitate. The high metal-to-ligand ratio led to the simultaneous formation of MOFs and ZnO nanoparticles in the MOFs. The surface area was in the range of 12-21 m2 g-1. The nanocomposites were tested for H2S adsorption at room temperature, where the maximum adsorption capacity of 14.2 mg g-1 was recorded for ZnBTC/ZnO in dry conditions. The spent adsorbents were regenerated using methanol and UV irradiation as individual and combined strategies. The successive effect of methanol and UV radiation led to an increased adsorption capacity in the second cycle. The spectroscopic investigation of spent ZnBDC/ZnO confirmed the chemisorption of H2S over Zn-sites via Zn2+-S2- interaction. The XPS analysis of regenerated ZnBDC/ZnO confirmed a decreased sulfur content and decreased Zn ionic character. The regeneration work in this study is one of the first attempts and could be extrapolated to well-studied Zn-MOFs like MOF-5 for the desulfurization process.

Terephthalate and trimesate metal-organic frameworks of Mn, Co, and Ni: exploring photostability by spectroscopy.

We report a rapid synthesis for the fabrication of terephthalate and trimesate metal-organic frameworks (MOFs) of Mn, Co, and Ni by ultrasonication of organic linkers with freshly prepared metal hydroxides. The MOFs were characterized by various microscopic and spectroscopic techniques to understand their structural, functional, and optical properties. MOFs with low bandgap energy (1.88-2.73 eV) showed strong absorbance in the UV-visible range. MOFs were exposed to UV irradiation for 40 h to understand their photostability. The MOFs showed decreased surface area and porosity with CoBTC as an exception. PXRD was less convincing for exploring functional changes in the UV-irradiated MOFs. XPS predicted changes in the oxidation states of metal nodes, the degradation of the organic linkers, and decarboxylation process in many of the transition MOFs. The study predicted terephthalate-based MOFs as more photostable than corresponding trimesate-based MOFs. This study is one of the first attempts in exploring photostability of MOFs with Mn, Co, and Ni as nodes.

Chemisorption of hydrogen sulfide over copper-based metal-organic frameworks: methanol and UV-assisted regeneration.

Three copper-based metal-organic frameworks (MOFs) with different organic linkers were synthesized for the removal of H2S gas at room temperature. The synthesized MOFs were characterized by microscopic and spectroscopic techniques to understand their structural, functional, and optical properties. The H2S adsorption capacity of MOFs calculated by column studies followed the trend: 105.6 mg g-1 (CuBDC) > 27.1 mg g-1 (CuBTC) > 1.3 mg g-1 (CuBDC-N) in dry conditions. The adsorption capacity increased in moist conditions due to an easy dissolution and dissociation of H2S in a film of water. X-ray photoelectron spectroscopy confirmed the presence of sulfur bound to Cu-sites and sulfate ions. The spent MOFs were regenerated by the successive effect of methanol and low power UV-C radiation. The regenerated CuBTC showed an exceptionally high adsorption capacity of 95.6 mg g-1 in the second cycle, which was linked to the reactivation of Cu-sites and improved surface area and porosity. The regeneration process developed in this study is a cost-effective method to recycle chemisorbed MOFs without compromising with their structural and functional integrity.

Correction: Terephthalate and trimesate metal-organic frameworks of Mn, Co, and Ni: exploring photostability by spectroscopy.

[This corrects the article DOI: 10.1039/D1RA00181G.].

A novel one-step synthesis of Ce/Mn/Fe mixed metal oxide nanocomposites for oxidative removal of hydrogen sulfide at room temperature.

In this study, CeO2/Fe2O3, CeO2/Mn2O3, and CeO2/Mn2O3/Fe2O3 nanocomposites were synthesized by the calcination of molten salt solutions. The microscopic images confirmed polyhedral nanocrystals of 10-20 nm size, clustered to form nanospheres. The elemental mapping confirmed the uniform distribution of transition metal oxides in the CeO2 matrix. The X-ray diffraction analysis confirmed the phase purity of metal oxides in nanocomposites. The surface area of nanocomposites was in the range of 16-21 m2 g-1. X-ray photoelectron spectroscopy confirmed 25-28% of Ce3+ ions in the CeO2 of nanocomposites. These nanocomposites were tested for the removal of hydrogen sulfide gas at room temperature. The maximum adsorption capacity of 28.3 mg g-1 was recorded for CeO2/Mn2O3/Fe2O3 with 500 ppm of H2S gas and 0.2 L min-1 of flow rate. The adsorption mechanism probed by X-ray photoelectron spectroscopy showed the presence of sulfate as the only species formed from the oxidation of H2S, which was further confirmed by ion chromatography. Thus, the study reports room-temperature oxidation of H2S over mixed metal composites, which were synthesized by a novel one-step approach.

UV-activated adsorbents as novel materials for enhanced removal of malodorous gases.

The performance of UV-activated Fe2O3-containing ZSM-5 (UFZ5) and silica (UFS) as adsorbents for the removal of low concentration of volatile organic sulfur compounds (VOSCs) was investigated in ambient conditions. A ∼99.9% and ∼89.2% removal of 10 ppm DMDS was observed for UFZ5 and UFS, respectively, due to a higher proportion of iron in UFZ5. The N2 adsorption isotherm confirmed an unpredictable increase in the surface area and pore volume of adsorbents after the fifth adsorption-oxidation cycle, which made adsorbents highly reusable for multiple cycles. The spectroscopic analysis predicted an increase in the hydroxyl density along with the resistance of catalytic sites against sulfur deactivation, which favored the adsorption-oxidation process. The adsorption capacity of UFZ5 remained in the range of 0.12-0.22, 0.26-0.48, 0.40-0.75 mg g-1 for methyl mercaptan (MM), dimethyl sulfide (DMS), and dimethyl disulfide (DMDS), respectively which were significantly higher than those calculated for UFS. The analysis confirmed SO2, CO, CH3OH, H2O, and elemental S as by-products. The UFZ5 was found competent and robust for the treatment of VOSCs-contaminated indoor air.

Photocatalytic Degradation of Organic Pollutants over MFe2O4 (M = Co, Ni, Cu, Zn) Nanoparticles at Neutral pH.

In this study, we report a surfactant-mediated synthesis of ferrites (MFe2O4: M = Co, Ni, Cu, Zn) using the co-precipitation-oxidation method. The band gap calculated from UV-Visible diffuse reflectance spectra were found in the range of 1.11-1.81 eV. These ferrite nanocatalysts were studied for the photocatalytic degradation of multiple organic dyes in a 32 W UV-C/H2O2 system. All the four ferrites showed an excellent dye degradation rate in the range of 2.065-2.417 min-1 at neutral pH. In the optimized condition, NiF was found to degrade 89%, 92%, 93%, and 78% of methylene blue, methyl orange, bromo green, and methyl red, respectively within 1 min of UV-irradiation. A 40% TOC removal was recorded after 5 min of degradation reaction, which increased to 60% after 50 min. Mechanism elucidated by scavenger studies and fluorescence spectroscopy revealed that •OH and holes were the primary reactive radicals responsible for the degradation process. Ferrite photocatalysts showed an insignificant performance loss in seven consecutive cycles. The photocatalyst was found efficient in the presence of a high concentration of salts. Thus, it was concluded that these photocatalysts are highly suitable for the remediation of dye-contaminated wastewater.

Structural variations and generation of binding sites in Fe-loaded ZSM-5 and silica under the effect of UV-irradiation and their role in enhanced BTEX abatement from gas streams.

Here, we have investigated the effect of UV-pretreatment on the physicochemical properties of an adsorbent. The UV-irradiated Fe-incorporated ZSM-5 (UV-FZ5) showed structural and chemical changes arising due to UV-cleaving of framework bonds resulting in a decreased crystallinity and change in the local environment of Fe species. More visible modifications were observed for UV-irradiated iron-containing silica (UV-Fe/AS) where silica network reconfiguration, increased hydroxyl density, and change in the Fe coordination were estimated. A 0.5-81.2% increase in BTEX adsorption was recorded for UV-irradiated adsorbents. These hiked performances were attributed to the increased pore size, increased hydroxyl density, and formation of newer isolated Fe3+ framework species. For FZ5, adsorption occurred via size-selective diffusion followed by hydrogen bonding and cation-pi interaction, whereas, for Fe/AS, diffusion was followed by cation-pi interactions. Moreover, adsorbents retained their adsorption capacity for multiple cycles and were found economically suitable for treating VOCs-contaminated air.