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Adsorption is a promising technology for simultaneously capturing nitrogen oxides (NO x ) from flue gases and recycling NO 2 as a profitable chemical, for which a robust and efficient adsorbent provides the key step for success in practical applications. This work reports the enhancement of NO x adsorption performances with less cost of desorption energy on Cu-ZSM-5 zeolites prepared by a facile and rapid (690 s) modification method, the incipient-wetness impregnation coupled with microwave drying (IM). In comparisons to H-ZSM-5, Na-ZSM-5 and conventionally liquid-phase ion-exchanged counterparts under sub-1000 ppm NO x feed concentrations and room temperature, the IM sample renders a record NO x adsorption capacity (q t,NOx ) of 0.878 mmol/g from dry gas stream on zeolites, and an applicable q t,NOx of 0.1 mmol/g from wet gas stream with a proper copper loading (2.1 wt%). The temperature programmed desorption of NO x on the optimal IM sample saturated with NO x from wet gas stream exhibit primary peak temperature lower than reported Cu-ZSM-5 and significant NO 2 proportion (72.6 %) in desorbed NO x . Deeper insights into advantageous NO x oxidative adsorption over the properly-loaded Cu-ZSM-5 in terms of diverse adsorbate states and competitiveness towards H 2 O were gained, showing IM method a promising sorbent improvement strategy for practical use., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Adsorption is a potential technology that is expected to meet NOx ultra-low emission standards and achieve the recovery of NO2. In this study, the adsorption/desorption behavior of NOx with competitive gases (e.g., H2O(g) and CO2) was studied on MFI zeolites with different Si/Al ratios and under different relative humidity (0~90% RH). Sample characterization of self-synthesizing zeolites was conducted by means of X-ray diffraction, Ar adsorption-desorption, and field emission scanning electron microscopy. The results showed that low-silica HZSM-5(35) showed the highest NOx adsorption capacity of 297.8 μmol/g (RH = 0) and 35.4 μmol/g (RH = 90%) compared to that of other adsorbents, and the efficiency loss factor of NOx adsorption capacity at 90%RH ranged from 85.3% to 88.1%. A water-resistance strategy was proposed for NOx multicomponent competitive adsorption combined with dynamic breakthrough tests and static water vapor adsorption. The presence of 14% O2 and lower adsorption temperature (25 °C) favored NOx adsorption, while higher CO2 concentrations (~10.5%) had less effect. The roll-up factor (η) was positively correlated with lower Si/Al ratios and higher H2O(g) concentrations. Unlike Silicalite-1, HZSM-5(35) exhibited an acceptable industrial desorption temperature window of NO2 (255~265 °C). This paper aims to provide a theoretical guideline for the rational selection of NOx adsorbents for practical applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Comparisons of NO x adsorption and desorption among series of zeolites are conducted. • Effects of zeolite structures, cations, SiO 2 /Al 2 O 3 ratios on NO x sorption are shown. • H-typed 10-membered-ring straight-channel zeolite is selected as preferred sorbent. • Importance of kinetics of NO oxidation-NO 2 physisorption process is revealed. • Accessibility of Brønsted acid sites that drives NO-NO 2 catalytic cycle is discussed. Adsorption is a promising technology for deep purification of nitrogen oxides (NO x) from flue gases and simultaneously recycling NO 2 as an economically valuable chemical. Zeolite as a robust NO x adsorbent provides the key step for success in practical applications, of which the screening remains challenging regarding the NO-NO 2 oxidation and adsorption combined process. In this work, experimental evaluations on practical NO x sorption performances on series of zeolites were conducted. Effects of the zeolite typological structure, compensating cation, and SiO 2 /Al 2 O 3 ratio on deep purification efficacy, water resistance, and regenerability at varying humidity were obtained, revealing the importance of kinetics of the NO oxidation and NO 2 physisorption consecutive process. The H-typed 10-membered-ring straight-channel zeolites (MFI and FER) were demonstrated to be the preferred NO x adsorbents qualified for industrial needs, as compared to other caged-channel, hybrid-channel, larger-sized straight-channel, or Na-typed counterparts. Deeper insights into the key role of Brønsted acid sites (BAS) in driving the NO-NO 2 catalytic cycle and promoting the adsorption competitiveness of NO x over water vapor were revealed based on in-situ diffuse reflectance Fourier transform infrared spectroscopy characterizations. The MFI zeolite with improved performance at the low-silica version could benefit from the efficient utilization of BAS within the highly accessible medium-sized straight channel, while the FAU zeolite (representative caged-channel zeolite) requiring appropriate SiO 2 /Al 2 O 3 ratio for optimal BAS utilization may suffer from the limited site accessibility. The findings clarify the relationships between NO x oxidative adsorption performances and zeolite properties, which provides a fundamental perspective and scientific screening strategy for NO x adsorbents. [ABSTRACT FROM AUTHOR]

Adsorption as an air purification method is widely employed to remove volatile organic compounds with low concentration using zeolites. In this study, ZSM-5 and LSX zeolites modified by alkali metal cations (Li+, Na+ and K+) were used as adsorbents for the abatement of toluene. The adsorption behaviors of toluene onto adsorbents with different pore sizes and cation contents were evaluated systematically based on the experiments of adsorption equilibrium, kinetic diffusion and desorption. Besides, Langmuir and Freundlich model, constant-pattern wave propagation model and Kissinger method were applied to fit with the above experimental data. The adsorption amount followed the sequence of Li-LSX > Na-LSX > K-LSX > Li-ZSM-5 > Na-ZSM-5 > K-ZSM-5 obtained from equilibrium isotherms. LSX zeolites showed twice adsorption capacity on unit volume compared to ZSM-5. The mechanisms of steric hindrances and electrostatic interactions controlling the diffusion rate under high and low concentrations were proposed, respectively. At relatively low loadings of toluene, the diffusion resistances were primarily affected by electrostatic interaction due to preferential adsorption of toluene on cationic sites. While at relatively high loadings, the diffusion resistance was mainly restricted by steric effect because of saturated cationic sites. The strength of adsorbate-adsorbent interactions was assessed by temperature programmed desorption experiments. It showed that both adsorption sites and microporous walls contributed to the desorption energy. [ABSTRACT FROM AUTHOR]

Abstract Adsorption thermodynamics of N 2 and O 2 on two most commonly used zeolites (5A and Li-LSX) under plateau special conditions of wide ranges of temperature (238–298 K) and pressure (0–300 kPa) for pressure swing adsorption (PSA) oxygen production were studied based on molecular simulations and theoretical model fitting. Adsorption isotherms for each adsorbate-adsorbent pair at different temperatures were fitted by Langmuir, Freundlich and Sips models, obtaining equilibrium parameters including adsorption capacity, strength, heterogeneity and N 2 /O 2 selectivity. Thermodynamic parameters such as Gibb's free energy, enthalpy, and entropy changes were numerically derived, and isosteric adsorption heats and distributions of potential energy were simulated. The results suggest that the adsorption capacity and affinity follow the order of N 2 -Li-LSX > N 2 -5A > O 2 -5A > O 2 -Li-LSX, leading to higher N 2 /O 2 selectivity on Li-LSX compared to 5A. For Li-LSX, N 2 /O 2 selectivity increases significantly with temperature above 273 K due to the difference in adsorption thermodynamics of N 2 and O 2 varying with temperature in terms of different adsorption heterogeneities. For 5A, the homogeneous distributions of N 2 and O 2 as well as the wide scopes of affinities from Na+ and Ca2+ exhibit N 2 /O 2 selectivity with negligible temperature dependence, which is beneficial for stable oxygen production with a fixed PSA process under large day-and-night temperature difference. This study provides theoretical basis for obtaining an optimal balance between efficiency and stability of oxygen production by using a proper mixture of Li-LSX and 5A. Highlights • Adsorption equilibrium of N 2 and O 2 on zeolites at plateau conditions are obtained. • N 2 /O 2 selectivity is higher with greater temperature dependence on Li-LSX than on 5A. • Adsorption capacity and affinity follow the order of N 2 -Li-LSX > N 2 -5A > O 2 -5A > O 2 -Li-LSX. • Advantage of thermodynamic stability of 5A over wide temperature range is discussed. [ABSTRACT FROM AUTHOR]

The kinetics of the thermal desorption of CO2 adsorbed on zeolite 13X were obtained using a differential thermogravimetric analyser under two different carrier gas conditions. The varying heating rates were set as 8, 12, 16, and 20 K min−1, respectively. The desorption activation energy of the physisorption sites for this experiment evaluated by an integral method without prediction of the reaction order ranged from 12.15 to 14.12 kJ mol−1 (CO2 as the carrier gas) and 43.32 to 50.42 kJ mol−1 (Ar as the carrier gas), respectively. The desorption activation energy of the chemisorption sites ranged from 57.95 to 58.53 kJ mol−1 (CO2 as the carrier gas) and 74.02 to 79.92 kJ mol−1 (Ar as the carrier gas), respectively. [ABSTRACT FROM AUTHOR]

Adsorption is a promising technology for capturing nitrogen oxides (NO x) from humid flue gases, for which as the preferred NO x adsorbent, zeolite with an appropriate silica to alumina (Si/Al) ratio provides the key step for success in practical applications. In this work, the effects of Si/Al ratio, temperature, relative humidity (RH), and feed gas (NO x -H 2 O(g)–CO 2 –O 2 -N 2) compositions on NO x sorption behavior on HZSM-5 zeolites were systematically investigated, based on a series of characterizations, fixed-bed adsorption and temperature programmed desorption (TPD) tests. The low-silica HZSM-5(25) showed the best performance (326.6 μmol/g and 39.2 μmol/g at dry and 90%RH) as compared to others with higher Si/Al ratios at each RH, in which the roll-up effect was positively correlated with the Si/Al ratios and humidity. O 2 enhanced NO x adsorption and reached an optimal adsorption capacity at the concentration of 14%, while CO 2 had little effect on NO x adsorption. The decrease in temperature increased NO x adsorption capacity on HZSM-5(25), benefitting from NO 2 physisorption as well as satisfactory NO oxidation within the active adsorbent structure. The facile thermal desorption of NO x and significant NO–NO 2 conversion on HZSM-5(25) was exhibited by bimodal TPD peaks at 343–353K and 533–543K respective for NO and NO 2 , in contrast to those on Silicalite-1. A potential "water-resistant" strategy via the enhancement of NO x adsorption competitiveness was revealed, providing a theoretical guideline for NO x adsorbent screening in industrial applications. [Display omitted] • Influential rules of multiple factors on NO x adsorption are systematically explored. • Low-silica HZSM-5 shows the best NO x adsorption performance at each condition. • Contributions of NO oxidation and NO 2 physisorption at varying temperatures are shown. • Greater regenerability of NO x from NO x -H 2 O(g)–CO 2 –O 2 -N 2 feed on HZSM-5 is demonstrated. • A "water-resistant" strategy for enhancing NO x adsorption competitiveness is proposed. [ABSTRACT FROM AUTHOR]

• Binary adsorption of N 2 and O 2 on FAU zeolites at plateau conditions is obtained. • Water interacting with zeolites via hydrogen bond competes with N 2 and O 2 adsorption. • Effect of water content is greater than that of temperature change on air separation. • Trade-off between N 2 /O 2 selectivity and performance stability is addressed. • X zeolite with Si/Al close to 1.5 is recommended for PSA O 2 production in plateau. Adsorption thermodynamics of N 2 and O 2 on faujasite (FAU) zeolites with different Si/Al molar ratios under plateau conditions of wide ranges of temperature and humidity for pressure swing adsorption (PSA) oxygen production were studied based on validated molecular simulations. Binary adsorption isotherms and N 2 /O 2 selectivity on each adsorbent were obtained. The results suggest that dehydrated feed air with very low dew point of 213 K could still lead to ~10 wt% water adsorption capacity in FAU zeolites. The effect of water content was larger than that of temperature change on N 2 /O 2 separation performance, causing reductions in N 2 /O 2 selectivity of 50.9% and affinities of 15.6% and 10.6% for N 2 and O 2 at 238 K, and those of 45.7%, 13.7% and 11.1% at 298 K on FAU-1.1, respectively. The strong interaction of water with zeolites via hydrogen bond was shown to decrease the pore volume by 25.61%, occupy the cationic sites, and thermodynamically form competitive adsorption over N 2 and O 2. The increase in Si/Al molar ratio suppressed the negative impact of water and temperature change but sacrificed the N 2 /O 2 selectivity. To reach an optimal balance between the air separation performance and the adsorbent robustness, X zeolites with Si/Al molar ratio close to 1.5 would be recommended for PSA O 2 production under the plateau special condition. [ABSTRACT FROM AUTHOR]

[Display omitted] • Development of oxygen generation from air over zeolite by PSA was reviewed in this work. • Mechanisms of O 2 separation from N 2 over zeolite by adsorption were discussed. • Requirements for zeolite adsorbents to improve the O 2 purity were analyzed. • Effect of impurities in air on the nitrogen–oxygen separation was concluded. The adsorptive generation of oxygen from air has become increasingly important in the chemical and medical industries due to the growing demand for O 2 with high purity. Among the commonly used three methods of cryogenic distillation, pressure swing adsorption (PSA) and membrane separation for N 2 /O 2 separation, PSA has received much attention due to the advantages of small volume, low management cost, and short start-up time, etc. Zeolites, with their unique pore structure, highly polarized pore environment, and abundant ion exchange sites, are widely employed as an efficient adsorbent in N 2 /O 2 separation. Hence, in this work, a review about the application of zeolites in O 2 generation via PSA was performed. In detail, the common PSA techniques applied for O 2 generation were summarized and compared from both the characteristics and application scenarios firstly. Then, the main mechanisms of O 2 separated from N 2 in air over zeolite by adsorption were summarized from three aspects of the steric effect, kinetic effect and equilibrium effect. Based on above, the requirements for silica-alumina zeolite adsorbents to improve the O 2 purity clarified from skeleton topology, extra-framework cations as well as physical & chemical properties. Moreover, the operation conditions and impurities having a significant effect on the product O 2 quality were discussed. Finally, both the challenges and prospects of air separation and O 2 generation are proposed based on existing research results. [ABSTRACT FROM AUTHOR]

Highlights • LSX containing minimum Li and Ag were prepared and evaluated for air separation. • ion exchange procedures were designed to miniemize Li+ on inaccessible sites. • The mixed-cation LSX were compared against pure Li-LSX for air separation by PSA. • The mixed cation zeolites show promising performance for replacing Li-LSX. Abstract Mixed-cation LiSr-LSX, AgCa-LSX and AgSr-LSX containing minimum lithium and silver were prepared by exchanging small fractions of Li+ into Sr-LSX and Ag+ into Sr-LSX and Ca-LSX. To avoid diffusion of the Li+ into the inaccessible sites, the LiSr-LSX sample was exposed to a specifically derived dehydration condition. Similarly, the mixed-silver cation samples were treated to the selected condition post-ion-exchange to allow the Ag+ to migrate into SII*. Strong evidences were provided that significant fractions of the exchanged Li+ and Ag+ remained in SIII and SII* respectively after comparisons of the N 2 /O 2 adsorption isotherms and isosteric heats of adsorption of the mixed-cation and single-cation samples. Furthermore, the mixed-cation LiSr-LSX, AgCa-LSX and AgSr-LSX samples were compared against the single-cation Li-LSX sample based on their O 2 productivity performance via a PSA simulation model. [ABSTRACT FROM AUTHOR]

Highlights • Sr-LSX zeolite shows comparable or higher N 2 adsorption capacity than Li-LSX depending on pretreatment. • Sr-LSX matches the O 2 productivity performance with Li-LSX from PSA simulation. • Sr-LSX zeolite is a promising replacement for Li-LSX. Abstract Due to the surging demand of lithium for lithium-ion batteries, the price of lithium has seen a steep rise and is projected to maintain the trend for the foreseeable future. With Li-LSX being the sorbent of choice in pressure swing adsorption systems for air separation, it is imperative to develop efficient adsorbents with significantly lower lithium content (mixed-cations zeolites) or completely replace it with abundant and low-cost alkali-earth metal cations such as Sr2+. This article presents promising results for pure Sr-LSX zeolite for O 2 productivity performance via PSA and the possibility of completely replacing Li-LSX with Sr-LSX. [ABSTRACT FROM AUTHOR]

In this study we report on a vacuum swing adsorption process and adsorbent which allows simultaneous production of high purity methane and high purity carbon dioxide from a biogas feedstock. This is a departure from conventional biogas purification processes which focus on production of high purity methane while venting the lower purity methane/carbon dioxide stream. Using NaUSY as the adsorbent, and based on experimental and simulation data, we are able to produce product purities of 98% CH 4 and 95% CO 2 in the two product streams, respectively, for an energy consumption of 0.23 GJ/tCO 2 from a feed containing 50/50% CH 4 /CO 2 . [ABSTRACT FROM AUTHOR]

[Display omitted] • Separation mechanisms for CO2/CO mixture were studied systematically by experiments and simulations. • The NaX zeolites showed low adsorption heat and high separation coefficients on CO2/CO. • The CaA zeolites showed excellent separation stability under different ratios for CO2/CO mixture. • The above separation performances were revealed by the number, distribution and charge of cations and characteristics of gas adsorption complexes. Adsorption and recovery of CO 2 from industrial gases are of great environmental significance in the field of environmental protection with reducing greenhouse gas emissions and increasing CO concentration. A comprehensive investigation with thermodynamics and kinetics experiments of CO 2 and CO on NaX and CaA zeolites were studied and the separation mechanism explained by molecular simulation methods. The experiments show that the NaX zeolites have significant differences on CO 2 and CO adsorption amounts leading the high CO 2 /CO selectivity, while the CaA zeolites exhibit separation stability under various partial pressures of mixed gas, which is supported by the standard deviation results. The theoretical simulations verify the above-mentioned experimental phenomena and provide a visual mechanism explanation for adsorption selectivity from the aspect of systematic interaction between cations and gas molecules. Compared with NaX, the same number and homogeneous distributions of Ca2+ and Na+ in the CaA provide stable separation performance. The strong electrostatic interactions between divalent Ca2+ and gas molecules especially CO make differences in the order of adsorption capacity and adsorption heat. Deeper insights into the cations-gas adsorption complexes revealed that the significant differences in the types of adsorption complexes for CO and CO 2 in NaX contribute to the improvement of the adsorption selectivity. [ABSTRACT FROM AUTHOR]