Humid Gas Streams Research Articles

Rubbery organic frameworks (ROFs) toward ultrapermeable CO2-selective membranes.

The capture of CO2 is of high interest in our society representing an essential tool to mitigate man-made climate warming. Membrane technology applied for CO2 capture offers several advantages in terms of energy savings, simple operation, and easy scale-up. Glassy membranes are associated with low gas permeability that negatively affect on their industrial implementation. Oppositely, rubbery membranes offer high permeability, but their selectivity is low. Here we report rubbery organic frameworks (ROFs) combining the high permeability of soft matrices with the high sieving selectivity of molecular frameworks. The best performing membranes provide a CO2/N2 selectivity up to 104 with a CO2 permeability up to 1000 Barrer, representing relevant performances for industrial implementation. Water vapors have a positive effect on CO2 permeability, and the CO2/N2 selectivity is higher than in dry conditions, as most of CO2 gas emissions are present in fully humidified gas streams. The synergetic high permeability/selectivity performances are superior to that observed with current state-of-the-art polymeric membranes.

The directions of enhanced activated carbon fibers for iodine capture from humid gas streams: Synergistic mechanisms of microporosity, hydrophobicity, and nitrogen sites

Activated carbon fiber (ACF) is widely used as a critical adsorbent for removing gaseous radioiodine produced during spent fuel reprocessing. However, conventional ACF exhibits a broad pore distribution and high moisture affinity, significantly diminishing their effectiveness in capturing iodine under humid conditions. In this study, we endowed bare ACF with the dual functions of preferentially I2-matched pores (93.62 % microporosity) and hydrophobicity (water contact angle of 132.1°) using a facile method of depositing polydivinylbenzene (PDVB) nanoparticles, overcoming the limitations of traditional modification methods that have a single target. We successfully prepared amine-functionalized hydrophobic ACF (ACF-PDA@PDVB-HMTA) with a high specific surface area (1392 m2/g) by introducing iodine-affinity reactive groups (NH2, CN, and OH). ACF-PDA@PDVB-HMTA exhibited a groundbreaking static adsorption capacity of 1886 ± 56 mg/g for I2 at 96 % relative humidity. Notably, its dynamic iodine adsorption on iodine-water vapor mixed stream was 1873.78 mg/g, which is 66.31 % higher than that of the original ACF (1126.7 mg/g) at real-life conditions of 75 °C and 50 % relative humidity, underscoring the synergistic “1 + 1 + 1 = 3” effect of the three mechanisms. The scalable application of the spray-deposited PDVB approach to other porous carbon materials also yielded exemplary modification results. This synergistic strategy provides a promising solution for improving the overall I2 capture.

Constructing oxygen vacancy-enriched Fe3O4@MnO2 core–shell nanoplates for highly efficient catalytic oxidation of H2S in blast furnace gas

The rational design of catalyst structures holds paramount significance for realizing efficient catalytic reactions. In this study, an Fe3O4@MnO2 core–shell catalyst was successfully synthesized using a facile one-pot method and applied in the selective oxidation of H2S in blast furnace gas. The introduction of the MnO2 shell suppressed the further growth and aggregation of Fe species, leading to outstanding catalytic stability. Simultaneously, abundant oxygen vacancies facilitated the adsorption and activation of oxygen, thereby enhancing H2S oxidation. Experimental results revealed that the Fe3O4@MnO2 catalyst (Fe/Mn = 4:1) displayed 100 % H2S conversion and 95.7 % sulfur selectivity at 150 °C in a humid gas stream containing O2 and CO. Remarkably, no discernible decline in catalytic activity was observed after 170 h of operation, indicating the promising industrial prospects of this catalyst. This study provides an effective strategy for designing efficient and stable desulfurization catalysts.

Bio-engineering of carbon adsorbents to capture CO2 from industrial sources: The cement case

Pine cone leaves (PCL) and pine kernel shells (PKS), abundant by-products of the Spanish food industry, were selected as precursors for activated carbons (ACs) to adsorb CO2 selectively at industrial post-combustion capture conditions. The goal was to maximize the development of narrow microporosity in the final carbons to boost the CO2 adsorption capacity. We have designed kinetics and equilibrium of adsorption experiments with CO2/H2O/N2 on a selected AC derived from PCL in a thermogravimetric analyzer (TGA) at 50 °C and three partial pressures of CO2 to prove the suitability to capture CO2 from industrial off-gases. When humid flue gas streams were tested, competitive adsorption of CO2 and H2O occurred; however, the difference in the uptake rates favored CO2 adsorption in the early stages. The joint CO2 + H2O uptake was around 2 mmol g−1 at 50 °C in humid conditions, where CO2 reached the equilibrium uptake at the corresponding partial pressure for 15 and 32 vol% CO2 in the feed stream. Moreover, the dynamic performance was addressed by cyclic adsorption–desorption experiments representing different industrial post-combustion capture scenarios in a lab-scale fixed-bed rig. The selected AC showed a stable performance in adsorption-regeneration cycles and very remarkable CO2 capture capacity under dry conditions (up to 1.08 mmol g−1 at 50 ˚C for 30 vol% CO2). Kinetics analysis also supported the faster adsorption of CO2 under cement flue gas conditions.

Highly efficient iodine capture by hydrophobic bismuth-based chrysotile membrane from humid gas streams

Because of their non-toxicity and low cost, bismuth-based materials have received considerable attention for the capture of gaseous iodine, especially for the removal of radioactive iodine in nuclear post-processing plants. In the actual operation conditions of reprocessing plant, the presence of water vapor must be considered as an important factor because it will significantly affect the iodine capturing efficiency for most iodine-capturing materials. To address this problem, we successfully synthesized a hydrophobic bismuth-based chrysotile membrane, which was modified by the silane coupling agent KH570. The chrysotile fiber was chosen to provide rich reactive sites for bismuth loading, and the bismuth was chosen due to its high affinity to iodine. The hydrophobic surface endowed by KH570 acts as a ‘protecting layer’ when water vapor exists during iodine adsorption. The maximum of iodine capture efficiency of the prepared membrane reached 483.37 mg/g in air, and 443.04 mg/g under 75 % relative humidity. Comparing hydrophobic membrane with one that non-modified by KH570, the hydrophobic membrane increases about 54 % iodine adsorption capacity in humid condition. This study provides a certain reference value for the application of bismuth-based materials in iodine removal under humid conditions.

Evaluating the performance of Cr-Soc-MOF Super-Adsorbents for CO2 capture from flue gas under humid condition through molecular simulation

One of the major criticisms for gas separation by utilizing Metal Organic Framework (MOF) porous material for all classes of adsorbents is their very poor performance under humidified gas streams. Very few MOFs show better CO2 separation from N2 mixture in the presence of moisture. Conversely, plenty of MOFs demonstrated good CO2 capture from dry gas streams. In order to explore the potential characteristics of gas separation under humid conditions by coordinatively unsaturated metal organic framework (CUS-MOF) with square octahedral (Soc) topology were exercised single component (CO2, N2) adsorption and co-adsorption (CO2/N2) in the presence of moisture at 298 K with pressure ranging from 0 to 10 bar. Herein, we investigated five exceptional Cr-Soc-MOFs similar to the experimentally established iso-structural topology by differing the polynuclear aromatic ring size and N-heteroatom to their pore wall. The specific interaction of guest molecules to the coordinatively unsaturated metal site (CUS), i.e., rigid μ3-oxygen-centered tri-nuclear metal carboxylate, of Cr-Soc-MOF-n series were identified with Density Functional Theory (DFT) calculations, this energy profile was used to derive specific force field. Further, Grand canonical Monte Carlo (GCMC) simulation was employed with DFT-derived force field to identify suitable CO2 capture for Cr-Soc-MOFs in the presence of moisture. Interestingly, the Cr-Soc-MOFs with larger pore volume were retained not only CO2 uptake in the range of 23 – 35 wt% but also selectivity about 20 – 50 range even up to 70% of RH. Importantly, we introduced a mathematical tool to theoretically analyze the dynamic co-adsorption in the form of breakthrough curve measurement from CO2/N2 simulated co-adsorption by mimicking the real flue gas condition, also working capacity (ΔN), Regenerability (R%) and Adsorbent Performance Indicator (API) of Cr-Soc-MOF-n series were investigated. The CO2 separation performances of Cr-Soc-MOF-n series suggested that the polynuclear aromatic ring to their pore wall withstand CO2 uptake in the presence of humidity rather than N-heteroatom decorated Cr-Soc-MOF due to its high pore volume and the accommodation of extra aromatic ring in their pore channels.

Separation of isoprene from biologically-derived gas streams

ABSTRACT Renewable organic precursors, including olefinic compounds such as isoprene, have attracted interest from the polymer and pharmaceutical industries. Biologically-derived processes can generate these target compounds; however, their gaseous product streams are complex mixtures of condensable organic vapors (COVs), water vapor, carbon dioxide (CO2), and/or nitrogen (N2). Because COVs, CO2 and water vapor are known to alter polymer membranes, mixed gas separations data at ambient and elevated temperatures are limited. This study focused on two classes of polymer membranes, glassy [polyetherimide (Ultem®)] and a rubbery [polydimethylsiloxane (PDMS)] with results indicating that isoprene separation is possible in humidified gas environment (2–4 vol% water). Gas permeabilities of these membranes did not noticeably change in the presence of humidity; however, the selectivity of these membranes was significantly lower compared to their performance under dry conditions. The role of water vapor in gas transport was derived from the energy of activation of permeation (Ep) for PDMS and Ultem® from 30–80°C in humidified mixed gas streams. For both polymers, Ep data shows a slight decrease in selectivity with the other gases (hydrogen, N2, CO2, and methane) at elevated temperatures in the presence of water vapor. Thus, these COVs separations are feasible with polymer membranes in the presence of humidified gas streams, even in the case of glassy and rubbery membranes in series.

Silicone tube humidity generator

Abstract. We describe the model and construction of a two-flow (or divided-flow) humidity generator, developed at LNE-Cnam, that uses mass flow controllers to mix a stream of dry gas with a stream of humid gas saturated at 28 ∘C. It can generate a wide range of humidity, with mole fractions in the range of 0.7×10-6<x<9000×10-6, without using low temperature or high pressure. This range is suitable for calibrating balloon-borne instruments that measure humidity in the stratosphere, where x∼5×10-6. The generator's novel feature is a saturator that comprises 5 m of silicone tubing immersed in water. Water enters the humid gas stream by diffusing through the wall of the tubing until the gas stream flowing through the tubing is saturated. This design provides a simple, low-cost humidity generator with an accuracy that is acceptable for many applications. The key requirement is that the tubing be long enough to ensure saturation so that the saturator's output is independent of the dimensions and permeability of the tube. A length of only a few meters was sufficient because the tube was made of silicone; other common polymers have permeabilities that are 1000 times smaller. We verified the model of the transition from unsaturated flow to saturated flow by measuring the humidity while using three tube lengths, two of which were too short for saturation. As a more complete test, we used the generator as a primary device after correcting the calibrations of the mass flow controllers that determined the mixing ratio. At mole fractions of 50×10-6<x<5000×10-6, the generator's output mole fraction xgen agreed to within 1 % with the value xcm measured by a calibrated chilled-mirror hygrometer; in other words, their ratio fell in the range xgen/xcm=1.00±0.01. At smaller mole fractions, their differences fell in the range of xgen-xcm=±1×10-6.

Chemical Degradation of Fuel Cell Membranes Subjected to Mechanical Stresses in a Hydrogen Peroxide Vapor Test

Two synergistic pathways for membrane degradation by combined mechanical and chemical stressors have been summarized by Kusoglu and Weber [1]. In a commonly observed pathway, mechanical stressors increase chemical degradation through the increasing gas crossover by mechanically induced macroscopic formation and growth of defects. In another proposed synergistic pathway, mechanical stress can directly enhance the chemical stressor at the nanoscale without increased gas crossover through defect formation. Up to date, very little evidence for the latter pathway have been reported in the literature, notably in works by Yoon et al. [2] and Kusoglu et al. [3] in ex-situ tests subjecting membrane to tension and compression, respectively, in Fenton’s solution. More recently, Robert et al. [4] reported enhanced fluoride emission rates of Nafion XL and NR-211 membranes by a 5 MPa cyclic compressive stress in a serpentine cell with H2O2 or Fenton’s solution.In this study, we have conducted experiments with a goal to correlate the membrane chemical degradation with various degrees of tensile stresses typically found in operating fuel cells [5]. To chemically degrade a membrane while applying tensile stresses, we utilized an ex-situ hydrogen peroxide vapor test that has been proven robust and useful in the investigation of the effect of temperature, RH, iron contamination, chemical mitigants, and chemical end chain modification on membrane chemical degradation [6-8]. Our experimental setup included four cells that can be tested in parallel under the same conditions. Each test cell is composed of a polysulfone plate on one side and a Teflon plate on the other. Both plates have a square 50 cm2 serpentine flowfield composing 2-mm wide channels and 2.125-mm wide lands. The channel depth is 1 mm. Membrane was compressed between the two flowfield plates with matching land/channel patterns. Tensile stress of the membrane across the channel was induced by a differential pressure from the N2 stream of higher pressure on the polysulfone side toward the Teflon side where a humidified gas stream of N2 and 30 ppm H2O2 flowed. The test cells were placed in a convection oven to maintain constant temperature during the test. The N2/H2O2 exhaust stream then flew into a chiller where the condensed solution was collected for subsequent fluoride concentration measurement.Two types of membranes were investigated in this study. One is the commercially available 25-µm Nafion® NR-211 without a chemical stabilizing additive and the other, a 12-µm ePTFE-supported PFSA membrane doped with cerium as a chemical stabilizer [9]. Tests were conducted with various constant differential pressures ranging from 0 to 100 kPa under 90°C and 30% RH. It should be noted that the NR-211 membrane ruptured in tests at and above 150 kPa, suggesting that our tests have covered a wide range of tensile stress from zero up to the mechanical strength limit of Nafion NR-211. Figure 1 shows the fluoride emission rate (FER) measured from 8 to 94 hours for both membranes under various pressures. It is seen that the FER of N211 increases with time but the 12-µm ePTFE-supported PFSA membrane is about 10 times lower while remaining relatively constant through the test. The lower FERs in the 12-µm PFSA membrane reflects the effectiveness of cerium as a chemical stabilizer. Importantly, there is no observable correlation between FER and applied differential pressures. In fact, all FERs for each membrane type are within the test-to-test variation from our experience with the H2O2 vapor test. The result in this study is contrary to those by Yoon, Kusoglu, and Robert, suggesting that more work on this subject is still required in the future. References Kusoglu and Weber, J. Phys. Chem. Lett., 6, 4547 (2015)Yoon et al., ECS Trans., 33(1), 907 (2010).Kusoglu et al., ECS Electrochem. Lett., 3, F33 (2014).Robert et al., J. Power Sources, 476, 228662 (2020).Lai et al., ASME J. Fuel Cell Sci. Tech., 6 (2), 0210021 (2009).Coms et al., ECS Transactions, 50 (2) 907-918 (2012)Xu et al., 220th ECS Meeting, Boston, MA, Oct. 13, 2011Coms et al., ECS Transactions, 64 (3) 389-402 (2014) https://www.hydrogen.energy.gov/pdfs/review18/fc156_kumaraguru_2018_o.pdf Figure 1

A flat-plate spiral-channeled membrane heat exchanger for methane dehumidification: Comparison of kraft paper and thin-film composite membrane

Membrane heat exchangers were used for moisture and heat recovery from a hot and humid natural gas stream in indirect contact with an air stream. A plate membrane module with spiral channels on each side was designed to configure gas and air streams in co-current and counter-current flow. Two types of membranes, a commercial kraft paper, and a laboratory-made thin-film composite membrane, were used as the module core. The thermal efficiency and the water recovery from the humid gas were calculated and compared under different operating conditions. At a unit flow ratio of air/gas, the greatest effectiveness was achieved at a methane flow of 100 cm3/min, but water recovery increased at higher gas flows. For the counterflow configuration, an air/methane flow ratio of 2 was sufficient to achieve the highest total effectiveness and water recovery; however, larger air/gas ratios were beneficial to improve heat and moisture transfer for the co-current configuration. The highest water recovery by the kraft paper and composite membrane was achieved (approximately 15 mgH2O/cm2.h) for the input gas with the highest humidity. The dehumidification performance of kraft paper as a porous and hydrophilic membrane was slightly better than that of the composite membrane containing a dense hydrophilic polymer thin film in most cases. A relatively low heat transfer coefficient was found for the module, which was increased by using higher gas flow rates at higher humidity conditions.

Structural investigation of cobalt oxide seeded silica xerogels under harsh hydrothermal condition

This work reports the hydrothermal treatment effect on cobalt silica-seeded xerogels under harsh conditions at 550 °C for 100 h in a humid gas stream containing 25 mol% H2O(v). A series of two seeded xerogels were prepared by two distinct methods, namely, xerogel seeding (XG) and sol–gel seeding (SG). Post-hydrothermal structural analysis showed type I sorption isotherms for both seeded xerogel, associated with microporous domains after harsh hydrothermal treatment. However, the SG series loaded with ≥25 mol% Co resulted in the formation of hysteresis, a characteristic of mesoporous silica. As a consequence, the SG series resulted in higher pore volume and surface area for higher Co loading than the XG series. Interestingly, surface area and pore volume retention (ratio of prior over post-hydrothermal treatment) increased with Co loading. For instance, seeded xerogels (0 mol% Co) resulted in ~45% retention whilst those containing 40 mol% Co reached structural retention values up to 70% (surface area) and 80% (pore volume). Further analysis of the pore size distribution showed densification for the pore size ranges of <14 and 17–28 A, though pore size enlargement in the pore ranges of 14–17 and 28–387 A. A mechanistic model is proposed indicating that cobalt oxide particles confer a region of protection to the adjacent silica structure that opposes densification.

A metal-OH group modification strategy to prepare highly-hydrophobic MIL-53-Al for efficient acetone capture under humid conditions

A series of highly-hydrophobic MIL-53-Al (MIL = Materials of Institut Lavoisier) frameworks synthesized via decoration of the Al-OH groups by alkyl phosphonic acid were developed as adsorbents for removing acetone from humid gas streams. The newly prepared materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), N2 adsorption-desorption and thermogravimetric analysis (TGA). Their adsorption behaviors toward acetone vapor under dry and wet conditions were studied subsequently. Results showed that alkyl phosphonic acid was successfully grafted into MIL-53-Al skeleton through coordinating interaction with Al3+ generating MIL-53-Al@Cx (x = 12, 14, 18). The MIL-53-Al@Cx exhibited similar crystal structure and thermal stability to parent MIL-53-Al. Furthermore, the modified materials showed significantly enhanced hydrophobicity. The water vapor uptake of MIL-53-Al@C14 decreased by 72.55% at 75% relative humidity (RH). Dynamic adsorption experiments demonstrated that water vapor had almost no effect on the acetone adsorption performance of MIL-53-Al@C14. Under the condition of 90% RH, the acetone adsorption capacity of MIL-53-Al@C14 was 102.98% higher than that of MIL-53-Al. Notably, MIL-53-Al@C14 presented excellent adsorption reversibility and regeneration performance in 10 adsorption-desorption cycles. Taken together, the strategy of metal-OH group modification is an attractive way to improve the acetone adsorption performance over metal-organic frameworks (MOFs) under humid conditions. Besides, MIL-53-Al@C14 would be deemed as a promising candidate for capturing acetone in high moisture environment.

Preferential SOx Adsorption in Mg-MOF-74 from a Humid Acid Gas Stream.

The preferential adsorption of SOx versus water in Mg-MOF-74 from a humid SOx gas stream has been investigated via materials studies and nuclear magnetic resonance (NMR). Mg-MOF-74 has been synthesized and subsequently loaded simultaneously with water vapor and SOx (62-96 ppm) in an adsorption chamber at room temperature over a time period of 4 days with a sample taken every 24 h. Each sample was analyzed by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA)-mass spectrometry, and scanning electron microscopy-energy-dispersive spectroscopy. The metal-organic framework (MOF) showed retained crystallinity and peak intensity in PXRD, and after 2 days, it showed no obvious degradation to the structure. Use of multiple techniques, including TGA, identified 10% by weight of SOx species, specifically H2S and SO2, within the MOF. 1H solid-state NMR shows a substantial reduction of H2O when SOx is present, which is consistent with SOx preferentially binding to the oxophilic metal site of the framework. After 14 weeks aging, the sulfur remains present in the three-dimensional MOF, with only half being desorbed after 23 weeks in air.

CoFe-based layered double hydroxide for high removal capacity of hydrogen sulfide under high humid gas stream

Although the adsorption method was considered as the most realistic way in the removal of gaseous hydrogen sulfide (H2S), finding an efficient sorbent at low temperature with the high humid condition is still desired. A layered double hydroxide (LDH) of CoFe bimetal was prepared by co-precipitation of bivalent cobalt and trivalent iron and applied to H2S gas stream treatment. CoFe-LDH was optimally synthesized under 3:1 of Co/Fe molar ratio, pH 10 of a mixing solution of precipitating reagents, and 60 °C of aging temperature, which allowed high breakthrough capacity (0.209 g H2S/g) in H2S removal. XPS and spectral analyses on spent CoFe-LDH proved that the H2S removal proceeded through two reaction processes: formed intermediate HS− from dissociated H2S which finally replaced with the hydroxide on the metal surface; and formed HS− through carbonate counter ion at interlayer. This resulted in the formation of CO2 as a gaseous product with CoFeS2 and CoFeSO4 as solid products after the treatment of H2S. Environmental application evidenced the prepared CoFe-LDH more efficient in high humid (90% relative humidity (RH) at room temperature) condition and even at high concentrated H2S source (3333 ppm).

Falling-film thermosyphons: Application to water harvesting from humid gas streams

Water harvesting from humid gas streams is central to fresh-water production, desalination, and particulate removal from combustion gas streams. Two-phase thermosyphons are well suited for these applications due to their very low thermal resistance, but have several operating limits at the high heat flows required for such applications. This work introduces the falling-film thermosyphon for large-scale condensation applications. Working fluid is pumped to the top of the evaporator to provide the evaporating liquid film on the inner tube wall, rather than by vapor condensation. The flooding, dry-out and pool-boiling limits are eliminated, resulting in significantly higher maximum heat fluxes for the same physical evaporator size. Additionally, the condenser no longer needs to be located vertically above the evaporator, which allows for a standard steam condenser to be used. A model for the condensation process in humid air was developed that estimates the fluid and heat transfer and is confirmed experimentally. Other benefits include the use of a high-thermal-conductivity polymer material for the evaporator section to minimize corrosion, and the ability to impose a temperature boundary condition on the evaporator, which is made possible due to the elimination of a liquid pool.

(Invited) Using Computational High-Throughput Screening and Machine Learning for the Data Design of High Performance Nanoporous Materials

Metal organic frameworks (MOFs) are a diverse class of materials built from organic and inorganic building units that can self assemble to form nanoporous frameworks. The hallmark feature of MOFs is the potential to tune the materials for any given application, which arises from the seemingly endless number of combinations of building units one can construct the materials from. To overcome this combinatorial design challenge we have developed a number of in silico screening tools and applied it to the development of new materials for gas separation processes such as post-combustion CO2 capture. First we have constructed a database of millions hypothetical materials using a novel structure spawning algorithm in which MOF structures can be generated from any topological periodic net (~1800 are experimentally known). Using molecular simulation techniques, we have screened the hypothetical materials for their gas adsorption properties, which provides the locations of the guest-host binding sites in each material. From the best performing materials, a similarity analysis was performed on over 100,000 binding sites to determine if the best performing materials share any common features in their binding sites. For the application of post-combustion CO2 capture, we found that two aromatic rings separated by 7.2 Ang provide an ideal, hydrophobic binding pocket that selectively adsorbs CO2. Our experimental collaborators were then able to synthesize 2 new MOFs with the targeted binding pockets and experimentally confirm that CO2 binds in them as predicted. To date these MOFs are amongst the best materials for selective CO2 capture under realistic humid combustion gas streams. Time permitting, we will also present how we have used deep learning models of nanoporous materials to rapidly predict the low-pressure adsorption properties of MOFs. Under these conditions, the chemistry of the pores is important and the geometric features such as the pore size, are not good predictors of the performance of the materials. Using a distance-between-features descriptor that accounts for the local chemistry of the materials, we have been able to develop accurate models that can predict the adsorption properties of nanoporous materials. Models were trained and tested on large, diverse databases of over 330,000 MOFs giving correlation R2 values on the test sets of >0.95, while geometric features provide R2 values of 0.71.

An overview on trace CO2 removal by advanced physisorbent materials

This review paper focuses on various gas processing technologies and materials that efficiently capture trace levels of carbon dioxide (CO2). Fundamental separation mechanisms such as absorption, adsorption, and distillation technology are presented. Liquid amine-based carbon capture (C-capture) technologies have been in existence for over half a century, however, liquid amine capture relies upon chemical reactions and is energy-intensive. Liquid amines are thus not economically viable for broad deployment and offer little room for innovation. Innovative C-capture technologies must improve both the environmental footprint and cost-effectiveness. As a promising alternative, physisorbents have many advantages including considerably lower regeneration energy. Generally, existing classes of physisorbent materials, such as metal-organic frameworks (MOFs) and zeolites are selective toward C-capture. However, their selectivity is currently not high enough to remove trace levels (e.g., ~1%) of CO2 from various natural gas process streams. This review summarizes the current advancements in physisorbent materials for CO2 capture. Here, key performance parameters needed to select the most suitable candidate are highlighted. Furthermore, this review discusses the scope for the development of better performing CO2 selective physisorbents from both environmental and economic perspectives. In addition, hybrid ultra microporous materials (HUMs), characterized mainly by ultra-micro pores (<0.7 nm), are discussed in reference to C-capture. Various characteristics of HUMs result in high selectivity and applicability in difficult separations such as the gas sweetening and C-capture from complex humid mixed gas streams.

Capture of nitrogen dioxide and conversion to nitric acid in a porous metal-organic framework.

Air pollution by nitrogen oxides, NOx, is a major problem, and new capture and abatement technologies are urgently required. Here, we report a metal-organic framework (Manchester Framework Material 520 (MFM-520)) that can efficiently confine dimers of NO2, which results in a high adsorption capacity of 4.2 mmol g-1 (298 K, 0.01 bar) with full reversibility and no loss of capacity over 125 cycles. Treatment of NO2@MFM-520 with water in air leads to a quantitative conversion of the captured NO2 into HNO3, an important feedstock for fertilizer production, and fully regenerates MFM-520. The confinement of N2O4 inside nanopores was established at a molecular level, and the dynamic breakthrough experiments using both dry and humid NO2 gas streams verify the excellent stability and selectivity of MFM-520 and confirm its potential for precious-metal-free deNOx technologies.

Improving efficiency of PVDF membranes for recovering water from humidified gas streams through membrane condenser

Membrane condenser (MC) is a novel and promising operation to recover evaporated water from highly humid gas discharged from industrial production. By means of hydrophobic membrane surface, water vapor condensed on membrane retention side, finally be collected. The aim of this paper is to enhance the recovery efficiency of poly(vinylidene fluoride) (PVDF) membrane in MC process. Significantly, coating three kinds of Hyflon ADs onto PVDF membranes effectively improved hydrophobicity of original ones. Among them, Hyflon AD40L/PVDF membrane showed the best membrane and MC performance, the LEPw and water contact angle of which reached 0.43 MPa and 126°, respectively. In terms of dehydration performance in MC, by coated with Hyflon AD40L, water recovery was increased from 7.92% to 18.85%, and condensate water flow (0.35 kg·m−2·h−1) was twice that of pristine ones (0.15 kg·m−2·h−1). These results indicate that Hyflon AD composite membrane is a promising candidate for MC process with high efficiency.

Alkylamine-Integrated Metal-Organic Framework-Based Waveguide Sensors for Efficient Detection of Carbon Dioxide from Humid Gas Streams.

Metal-organic framework (MOF)-based chemical sensors have recently been demonstrated to be highly selective, sensitive, and reversible for CO2 sensing across a range of platforms including optical fiber and surface acoustic wave-based sensors. However, interference of water molecules is a primary issue in CO2 sensing systems based upon MOF layers due to cross-sensitivity, stability of MOF-based materials in humid conditions, and associated baseline drift over the lifetime of sensors. Herein, we develop a simple approach of alleviating the negative effect of water vapor to the optical fiber sensor by using alkylamine (i.e., oleylamine) to form a protective hydrophobic layer on the surface of MOFs for improving water stability.Alkylamine-modification of a MOF-coatedoptical fiber sensor provides a reversible and stable sensing response to a wide range of CO2 concentrations while also enhancing the CO2 sensitivity of the sensor under wet conditions. The FT-IR and breakthrough studies on the oleylamine-modified MOF confirm that the water vapor does not adversely impact the intrinsic CO2 sorption capacities. Thus, this simple stratrgy for enhancing the CO2/H2O selectivity in the MOF sorbent could also be useful for improving CO2 capture/separation performance in flue gas stream.