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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 aim of this work was to reduce/minimize Li in Li-LSX by replacing the 70% Li+ cations in Li-LSX that are bonded to the interior or inaccessible sites which are not used for adsorption. Thus, mixed-cation LiCa-LSX containing minimum lithium were prepared by exchanging small fractions of Li+ into Ca-LSX, followed by dehydration under mild conditions to avoid migration/equilibration of Li cations. Comparisons of adsorption isotherms of N2/O2 and heats of adsorption for the LiCa-LSX samples with that for pure-cation Li-LSX and Ca-LSX provided strong evidence that significant amounts of these Li cations indeed remained on the exposed sites (SIII). The mixed-cation LiCa-LSX samples were compared against the pure-cation Ca-LSX and Li-LSX based on their performance for oxygen production by PSA, via model simulation. The results showed that the mixed-cation LiCa-LSX samples yielded significantly higher O2 product productivities at the same product purity and recovery than their pure-cation precursor (Ca-LSX). © 2017 American Institute of Chemical Engineers AIChE J, 64: 406-415, 2018 [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Cu-ZSM-5 is prepared by incipient-wetness impregnation coupled with microwave drying. • A record NO x adsorption capacity at sub-1000 ppm dry gas feed on zeolites is obtained. • Applicable NO x adsorption capacity at wet feed is gained on properly-loaded Cu-ZSM-5. • Lower NO x desorption temperature and higher NO 2 proportion in desorbed gas are shown. • Advantageous NO x oxidative adsorption competing with H 2 O on Cu-ZSM-5 is revealed. [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]

Highlights • Adsorption of CH 4 , N 2 and O 2 on carbon molecular sieves (CMSs) were studied. • CH 4 /N 2 /O 2 separation performances on CMSs were evaluated in equilibrium and kinetics. • A selectivity for practical selection of kinetic separation adsorbent was developed. • A proper existence of microporosity of CMS would benefit practical methane upgrading. Abstract Upgrading coal mine methane (CMM) with adsorption method is an effective way to enhance the energy utilizing efficiency and reduce greenhouse gas emissions. In this work, adsorption equilibrium and kinetics of CH 4 , N 2 and O 2 , as typical components in CMM, on three carbon molecular sieves (CMSs) and the corresponding separation performances were studied. The equilibrium and kinetic selectivity were obtained from adsorption isotherms fitted with the Langmuir model and uptake curves fitted with the micropore-diffusion model, respectively. O 2 with the highest diffusion time constant among three gases suggests the advantage of CMS-based kinetic separation in deoxygenation of CMM and the critical role that N 2 /CH 4 kinetic separation plays in CH 4 enrichment. CMS-1 with the highest microporosity has the largest adsorption capacities, and CMS-3 with a proper average pore size has the greatest N 2 /CH 4 kinetic selectivity. The actual separation performances shown by CH 4 /N 2 /O 2 breakthrough curves according to the principle for practical CMM upgrading processes exhibit the order of CMS-1 > CMS-3 > CMS-2 which is inconsistent with that of the conventional N 2 /CH 4 selectivity highly due to the underestimation of effects of N 2 equilibrium adsorption relative to N 2 kinetic selection without covering specific adsorption range corresponding to practical working capacity region. A modified selectivity with consideration of actual N 2 amount adsorbed was therefore developed, showing the correct performance order and the potential to be used as a practical adsorbent selection parameter. An advanced CMS materials design with proper existence of microporosity would benefit practical CMM upgrading in terms of reaching an optimal balance between adsorption equilibrium and kinetics. [ABSTRACT FROM AUTHOR]

Adsorptive removal of gas phase low concentration macromolecular organic component, represented by naphthalene, from the enclosed space using ordered mesoporous carbon (OMC) has been studied by molecular simulation and experimental investigation. The simulation results indicated that both adsorption capacity and adsorption stability of the OMCs for naphthalene decreased with the increase of pore sizes from 2 nm to 8 nm. Characterizations showed that the prepared OMCs had the pore structure similar to the simulated OMCs except for the rough surface. In particular, the adsorption performance of the prepared OMCs was significantly lower than that of the simulated OMCs when pore size was 2 nm and 3 nm, which was attributed to the rough inner surface of these adsorbents, blocking the narrow pore channels and significantly reducing the pore volume. OMC with pore size of 4 nm had the highest adsorption amount for naphthalene. The co-adsorption experiments in the presence of both naphthalene and toluene, acetone or water showed the adsorption performance of OMCs for naphthalene were almost unaffected by the presence of low concentration toluene and acetone, as well as high relative humidity. [Display omitted] • Adsorption of naphthalene by OMCs were investigated by simulation and experiment. • Realistic molecular models for OMCs were constructed by "delete atom" method. • The impact of pore structure on the adsorption of naphthalene was revealed. • OMC with pore size of 4 nm has the highest adsorption capacity for naphthalene. • OMCs show excellent adsorption selectivity and good moisture resistance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Adsorption of toluene and naphthalene by OMS were investigated by simulation. • The adsorption selectivity of toluene and naphthalene was predicted by IAST. • The mechanism of selective adsorption was elucidated at molecular level. • Adsorbents with the desired pore size were prepared by template synthesis method. • The experimental results are in good agreement with the simulation results. In order to ensure regeneration property of adsorption materials in the air pollution purification system of the airtight cabin, such as submarine and space station, it is necessary to selectively separate semi-volatile organic compounds whose desorption is difficult. For this reason, the effects of pore size on the adsorption capacity and adsorption selectivity of toluene and naphthalene were investigated by both molecular simulation and experimental investigation. The ordered porous silica-based adsorbents with pore sizes ranging from 1.0 nm to 2.8 nm were constructed by simulation and were prepared by template synthesis method in this study. Both simulation and experiment show that the adsorption capacity of toluene and naphthalene firstly increased and then decreased with the increase of the pore size of adsorbents from 1.0 nm to 2.8 nm, and the pore diameter (1.3 nm) corresponding to the maximum adsorption amount of toluene was smaller than that (2.3 nm) of naphthalene. Simulation results show that the adsorption energy and adsorption stability of naphthalene are higher than those of toluene, and binary-component co-adsorption experiments also testified that toluene molecules could be replaced by naphthalene molecules. The optimum pore size for selective adsorption of naphthalene is 2.3 nm. [ABSTRACT FROM AUTHOR]

• Influence of coexisting nanoparticles on toluene adsorption was studied. • Competition between gas and particles in interaction with zeolites was addressed. • Reduction of toluene adsorption capacity by particles was up to 16.0 % on K-ZSM-5. • Reduction of toluene adsorption rate by particles was up to 20.3 % on Li-ZSM-5. • Adsorption site occupation and steric hindrance could be caused by nanoparticles. Adsorption as one of the most important air cleaning methods has been extensively applied during which the coexisting airborne nanoparticles (NPs) with sizes close to adsorbent pore sizes could inevitably influence gas adsorption processes. In this work, the influence of sub-20 nm NPs on toluene adsorption on ZSM-5 zeolites exchanged with different cations (Li+, Na+ and K+) were studied based on gas-and-particle coexisting adsorption/filtration tests. Affinities for both toluene and NPs on adsorbents follow Li-ZSM-5 > Na-ZSM-5 > K-ZSM-5 regarding the orders of charge density, pore size, and internal and external specific surface areas. The toluene adsorption was shown to be impaired by coexisting NPs from perspectives of thermodynamics and kinetics. For Li-ZSM-5, Na-ZSM-5 and K-ZSM-5, significant relative reductions of 10.4 %, 10.5 % and 16.0 % in toluene adsorption capacity at the lower feed concentration, and of 20.3 %, 15.2 % and 2.3 % in mass transfer coefficient at the higher feed concentration were observed, respectively. The influential mechanisms regarding competitiveness between toluene and NPs in interaction with cationic and porous surfaces were accordingly proposed, which are of practical significance for selecting robust adsorbents under realistic harsh air conditions. [ABSTRACT FROM AUTHOR]