CO2 adsorption on zeolite X/activated carbon composites

Two series of zeolite X/activated carbon composites with different ratios of zeolite and activated carbon were prepared through a combination process of CO₂ activation of the mixtures of elutrilithe and pitch and subsequent hydrothermal crystallization in alkaline solution. An additional surface modification was achieved in diluted NH₄Cl solution. CO₂ and N₂ uptakes on the composites before and after modification were determined for pressures up to 101?kPa at 273 and 298?K, respectively. Langmuir-Freundlich and Toth adsorption models were used to describe the adsorption isotherms of CO₂ and the corresponding heats of adsorption were estimated with the Clausius-Clapeyron equation. Both before and after modification, all composites exhibited a remarkable preferential adsorption of CO₂ compared to N₂, with the modified composites showing a higher adsorption selectivity to CO₂ over N₂ than the unmodified composites. With an increasing ratio of zeolite in the composites, adsorption capacity and adsorption heat of CO₂ on the composites increased simultaneously. Lower adsorption heat was observed both before and after modification especially at the low-loading region and when there was less energetic heterogeneity on the surface of the modified composites. The results may be attributed to the elimination of strong basic sites on the modified composites, which is favorable for desorption of CO₂ on the adsorbents and application in pressure swing adsorption processes.     

CO2 dynamic adsorption/desorption on zeolite 5A studied by 13C magnetic resonance imaging

We present the first 13C magnetic resonance imaging study of CO2 transient adsorption/desorption processes in a zeolite 5A column. CO2 transient concentration profiles were measured with a centric scan spin-echo single point imaging technique. The adsorption wave profiles were determined under flow conditions, with the results analyzed by the Bohart-Adams model. The model adequately accounts for the spatial and the temporal behavior of CO2 in the column. CO2 adsorption rate constants were calculated from the fit. Desorption profiles were acquired by blowing a helium stream through a zeolite 5A column saturated with CO2. An asymmetry between the adsorption and desorption profiles is readily apparent. A linear relationship between the CO2 condensed phase concentration and square root of time was observed.     

Diffusion mechanism of CO2 in 13X zeolite beads

A systematic study of the diffusion mechanism of CO₂ in commercial 13X zeolite beads is presented. In order to gain a complete understanding of the diffusion process of CO₂, kinetic measurements with a zero length column (ZLC) system and a volumetric apparatus have been carried out. The ZLC experiments were carried out on a single bead of zeolite 13X at 38 °C at a partial pressure of CO₂ of 0.1 bar, conditions representative of post-combustion capture. Experiments with different carrier gases clearly show that the diffusion process is controlled by the transport inside the macropores. Volumetric measurements using a Quantachrome Autosorb system were carried out at different concentrations. These experiments are without a carrier gas and the low pressure measurements show clearly Knudsen diffusion control in both the uptake cell and the bead macropores. At increasing CO₂ concentrations the transport mechanism shifts from Knudsen diffusion in the macropores to a completely heat limited process. Both sets of experiments are consistent with independent measurements of bead void fraction and tortuosity and confirm that under the range of conditions that are typical of a carbon capture process the system is controlled by macropore diffusion mechanisms.     

Unconventional, highly selective CO2 adsorption in zeolite SSZ-13

Low-pressure adsorption of carbon dioxide and nitrogen was studied in both acidic and copper-exchanged forms of SSZ-13, a zeolite containing an 8-ring window. Under ideal conditions for industrial separations of CO(2) from N(2), the ideal adsorbed solution theory selectivity is >70 in each compound. For low gas coverage, the isosteric heat of adsorption for CO(2) was found to be 33.1 and 34.0 kJ/mol for Cu- and H-SSZ-13, respectively. From in situ neutron powder diffraction measurements, we ascribe the CO(2) over N(2) selectivity to differences in binding sites for the two gases, where the primary CO(2) binding site is located in the center of the 8-membered-ring pore window. This CO(2) binding mode, which has important implications for use of zeolites in separations, has not been observed before and is rationalized and discussed relative to the high selectivity for CO(2) over N(2) in SSZ-13 and other zeolites containing 8-ring windows.     

Near-critical adsorption of CO2 on 13X zeolite and N2O on silica gel: lack of evidence of critical phenomena

The excess adsorption of CO₂ on 13X zeolite and of N₂O on silica gel has been studied at high pressure using a magnetic suspension balance, i.e. a gravimetric method. Recently, a detailed study on the density distribution in the measuring cell of the magnetic suspension balance showed that a proper approach to thermostatting the unit should be used in order to obtain reliable and accurate excess adsorption measurements. This is particularly important in the vicinity of the critical point of the fluid, where the density is strongly dependent on pressure and temperature. In the past, several effects were observed in our laboratory when measuring near-critical adsorption on 13X zeolite and on silica gel, namely critical adsorption and critical depletion. In the present study, these effects have been checked using the balance in the new thermostatting configuration, and the conclusion can be drawn that the accuracy of the measurement is not sufficient to prove that they indeed occur. More accurate adsorption data for the two systems have been measured and reported.     

High-pressure adsorption of CO2 on NaY zeolite and model prediction of adsorption isotherms

Supercritical carbon dioxide is an efficient solvent for adsorptive separations because it can potentially be used as both the carrier solvent for adsorption and the desorbent for regeneration. Recent results have demonstrated an anomalous peak or "hump" in the adsorption isotherm near the bulk critical point when the adsorption isotherm is plotted as a function of bulk density. This work presents new data for the adsorption and desorption of carbon dioxide in the near-critical region on a crystalline, well-structured adsorbent (NaY zeolite). The results indicate a strong affinity for CO(2) as well as a significant hump near the critical point. The lattice model previously developed by Aranovich and Donohue is applied to analyze the adsorption.     

Water adsorption on zeolite 13X: comparison of the two methods based on mass spectrometry and thermogravimetry

Two different experimental methods have been used for studying equilibrium adsorption of water vapour on zeolite 13X based on thermogravimetry and a novel technique using mass spectrometry. Good agreement can be found between experimental data of the adsorption isobars from these two methods. Also the isosteric heat of adsorption of this system has been determined from the equilibrium data. Water adsorption has been measured under a variety of operation conditions of the cooling systems, i.e. pressures from 12.28 to 73.84 mbar and temperatures from 50 to 230 °C.     

Pure and binary adsorption isotherms of ethylene and ethane on zeolite 5A

Pure and binary adsorption equilibrium data of ethylene and ethane on zeolite 5A were collected with a volumetric method for the temperature range 283 K to 323 K and pressure up to 950 kPa. The applicability of the binary adsorption prediction by the vacancy solution theory (VST) was investigated. Further individual adsorption and selectivity were obtained by VST prediction. According to the experimental results, zeolite 5A has a high adsorption capacity and selectivity for ethylene in the ethylene/ethane system. VST predicts that ethylene selectivity increases with pressure; it also shows that the amount of ethylene separated by zeolite 5A increases as the temperature decreases at a specified pressure.     

Adsorption of CO2 on nitrogen-enriched activated carbon and zeolite 13X

Adsorption may be a potentially attractive alternative to capturing CO₂ from stationary sources in the context of Carbon Capture and Sequestration (CCS) technologies. Activated carbon and zeolites are state-of-art adsorbents which may be used for CO₂ adsorption, however physisorption alone tends to be insignificant at high temperatures. In the present work, commercial adsorbents have been impregnated with monoethanolamine (MEA) and triethanolamine (TEA) in order to investigate the effect of the modified surface chemistry on CO₂ adsorption, especially above room temperature. Adsorption isotherms for CO₂, N₂ and CH₄ were measured in a gravimetrically system in the pressure range of UHV to 10 bar, at 298 and 348 K for activated carbon and zeolite 13X supports. The adsorbed concentration of CO₂ was significantly higher than those of CH₄ and N₂ for both adsorbents in the whole pressure range studied, zeolite 13X showing a remarkable affinity for CO₂ at very low pressures. However, at 348 K, the adsorbed concentration of CO₂ decreases significantly. The supports impregnated with concentrated amine solutions and dried in air suffered a detrimental effect on the textural properties, although CO₂ uptake became much less susceptible to temperature increase. Impregnations carried out with dilute solution followed by drying in inert atmosphere yielded materials with very similar textural characteristics as compared to the parent support. CO₂ isotherms in such materials showed a significant change with similar capacities at 348 K as compared to the original support at 298 K in the case of activated carbons. The impregnated zeolite showed a decrease in adsorbed phase concentration in low pressures for a given temperature, but the adsorbed amount also seemed to be less affected by temperature. These results are promising and indicate that CO₂ adsorption may be enhanced despite high process temperatures (e.g. 348 K), if convenient impregnation and drying methods are applied.     

Thermodynamics of hydrogen adsorption on the zeolite Li-ZSM-5

Thermodynamic characterisation of the adsorption process (at a low temperature) of dihydrogen on the zeolite Li-ZSM-5 was carried out by means of variable-temperature infrared spectroscopy, with the simultaneous measurement of temperature and equilibrium pressure. Adsorption renders the H–H stretching mode infrared active, at 4092 cm⁻¹. The standard adsorption enthalpy and entropy resulted to be ΔH⁰=−6.5(±0.5) kJmol⁻¹ and ΔS⁰=−90(±5) Jmol⁻¹ K⁻¹, respectively. The adsorption enthalpy is significantly larger than the liquefaction heat, and this fact renders Li-ZSM-5 a potential cryoadsorbent for hydrogen storage.     

H2 adsorption on Fe/ZSM-5 zeolite: a theoretical approach

The hydrogen reduction of the red-ox Fe/ZSM-5 zeolite is an essential step for catalyst preparation. In this letter, various adsorption modes of the H(2) molecules on the Fe(III)/ZSM-5 zeolite were first explored by density functional theory, with their exact configurations provided. The adsorption energies revealed that the two configurations with Fe(III) at the sextet states are the main ones, consistent with the results of inelastic neutron scattering (INS) experiments. On such basis, the vibrational and orbital analyses were made, which will be valuable for the future studies on the Fe/ZSM-5 systems.     

Polyfuran/zeolite LTA composites and adsorption properties

Composites of polyfuran (PFu) with LTA type (3A, 4A, 5A) zeolites were prepared via chemical oxidative polymerization of furan (Fu) in the presence of a dispersion of zeolites (powder) in ACN solvent using anhydrous FeCl₃ oxidant at an ambient temperature. The composites were characterized by Fourier transform infrared spectroscopic (FTIR) analysis. FTIR results showed that the composites of 3A zeolite with the smallest pore size did not indicate absorption for benzoyl chloride due to surface structural of OH groups. Thermogravimetric analysis revealed the stability order as: 3A > PFu/3A > PFu, 4A > PFu/4A > PFu, 5A > PFu/5A > PFu. Scanning electron microscopic analysis and X-ray diffraction analysis indicated the formation of polyfuran on zeolite structures. Conductivity values of samples are in the range of semi conductors.     

Chromium (VI) ion adsorption features of chitosan film and its chitosan/zeolite conjugate 13X film

This research evaluated the importance of the adsorption properties of chitosan a chitosan/zeolite conjugate film for the removal of Cr(VI) ions from solutions in the 5-260 mg/L concentration range, when the pH was adjusted to 4.0 and 6.0. The uptake capacities of the films formed by chitosan and by the chitosan/zeolite conjugate were calculated by mass balance. The equilibrium isotherms were fitted to the Langmuir, Freundlich and Redlich-Peterson models. The chitosan film seems to be a good sorbent for Cr(VI) at pH 4, but its physical instability suggests the need for a more resilient support. Due to this fact zeolite was added to the chitosan matrix in solution and a chitosan/zeolite (CS/Zeo) film was thus formed. The solubility of the film and the characterization of the different matrices by FTIR, TGA and X-Ray showed that a cross-linked structure was formed between the chitosan and zeolite and the solubility of the film increased. In this study, the low manufacturing cost of the CS/Zeo matrix, the good uptake of Cr(VI) at acidic pH (17.28 mg/g) and the non desorption of Cr(VI) from the film in water suggests this combination should be tested in industrial environment.     

Interpenetrating and pseudo-interpenetrating polymer networks of polyethylacrylate and zeolite 13X

Free radical polymerization of liquid ethylacrylate in the presence of zeolite 13X yielded PIPNs without crosslinker and IPNs if the crosslinker ethylene glycol dimethacrylate was present. We studied these materials both unextracted as well as partially extracted with a variety of solvents using DSC, SEM as well as Small Angle X-ray Scattering (SAXS). These studies suggest that in the composites polyethylacrylate chains entered the internal pores of the zeolite. These chains had an extended state and did not exhibit a bulk glass transition, a similar behavior to that previously reported for polystyrene/zeolite 13X composite. © 1996 John Wiley & Sons, Inc.     

Heats of adsorption of CO2 on high-silicon zeolites ZSM-5 and silicalite

The heat of adsorption of C0₂on NaZSM-5 at zero occupancy is 50.0 kJ/mole. The differential heats have two linearly descending segments, corresponding to the formation of two types of adsorption complexes with one or two C0₂ molecules, on the average. The heat of adsorption on silicalite coincides with the heat of adsorption of CO₂ on the noncationic segment of the NaZSM-5 zeolite structure (28–29 kJ/mole). The adsorbate-adsorbate interaction forces are not evident on the zeolites up to 1.5 mmole/g occupancy. The isotherms for the adsorption of C0₂ on zeolite NaZSM-5 and silicalite at 303 K in the occupancy region of 0–1.5 and 0–0.5 mmole/g are completely described by VMOT equations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2636–2638, November, 1989.     

Kinetics and equilibrium of adsorption on 4A zeolite

Adsorption isotherms for Ar, 0₂, N₂, CO, CO₂, CH₄, and C₂H₆ on 4A zeolite at three or more temperatures were determined. An adsorption equation based on a 2-dimensional virial equation in terms of integer powers of the reciprocal of (A - σ) was shown to fit the equilibrium data accurately with three constants for C₂H₆ and two constants for other gases. Here A is the area per molecule and σ is the area of the molecule in a close-packed situation.Rates of adsorption and desorption of Ar, N₂, CO, CH₄, and C₂H₆ on 4A zeolite were determined over ranges of temperature in which the rate was moderately fast. Electron microscopy showed that the particles were cubes, and their size-distribution was determined. The conventional Fick's law rate equation for cubes was used to produce a generalized rate curve for the particle size distribution of the adsorbent. This curve was applied to the last 20% of the rate curve to obtain a diffusivity that could be related to the final amount adsorbed. This procedure also avoids the initial rapid portion of the adsorption, in which large variations of adsorbent temperature from that of the bath often occur.The diffusivities increased with amount adsorbed by a small extent for Ar and CH₄ and by larger amounts for N₂, CO, and C₂H₆. The activation energy for diffusion, as well as the heat of adsorption, were nearly independent of amount adsorbed for Ar and CH₄, but these quantities decreased substantially with coverage for N₂, CO, and C₂H₆. The dependence upon amount adsorbed of diffusivity and activation energy seemed related to the shape of the adsorption isotherm; those for Ar and CH₄ were nearly linear, whereas isotherms for the other gases had large curvatures. The activation energy for diffusion varied with coverage in the same way as heat of adsorption.     

Heats of adsorption of methanol and ethanol on high-silicon ZSM-5 zeolite

The heats of adsorption of lower alcohols on NaZSM-5 have a stepwise appearance and each step corresponds to the stoichiometric formation of adsorption complexes of Na⁺ ions with from one to four alcohol molecules. All the adsorption complexes are located at zeolite channel intersections, while the alkyl groups enter these channels. The heats of adsorption of alcohols on NaZSM in the region of the formation of adsorption complexes with cations markedly exceed the heats of adsorption on silicalite, while on the noncationic part of the NaZSM-5 structure, they are identical to the heats of adsorption on silicalite. The mean molar integral adsorption entropies of alcohols are significantly less than the entropy of the liquid. The adsorbed molecules are in a solidlike state. The isotherms for the adsorption of alcohols on NaZSM-5 are completely described by VMOT equations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2633–2635, November, 1989.     

Structural studies on thorium(IV)-titriplex IV-interacted 13X zeolite

A new zeolite derivative has been prepared by interacting Th(IV) and complexone in HCl medium and neutralising with aqueous ammonia. The amorphous and siliceous derivative showed a total mass loss of 29.25% in TG for stepwise dehydration, dehydroxylation and decomposition. Heats of reaction were 1167.6 J g^–1 at 88.7°C and 75.167 J g^–1 at 492.5°C for loss of volatile components and decomposition respectively.²⁹Si and²⁷Al MAS NMR spectra as well as XRD data of the derivative before and after calcination indicate presence of both four-coordinated and six-coordinated Al in varying ratios and the total loss of crystallinity.The author is grateful to Dr. Alan Dyer of the Department of Chemistry and Applied Chemistry, Salford University and to Dr. Jacek Klinowski of the Department of Chemistry, Cambridge University for providing the thermal data and NMR spectra respectively. He also acknowledges the comments received from Dr. N. Suryaprakash of the Sophisticated Instrument Facility, I.I.Sc., Bangalore on the²⁷Al and²⁹Si spectra of the derivatives.     

Luminescence spectra and adsorption characteristics of naphthalene on various metal-substituted forms of type X zeolite

The low-temperature luminescence spectra of samples obtained after adsorption of naphthalene by metal-substituted type X zeolites are examined. In zeolites of the first group (LiNaX, NaX, KNaX, RbNaX, CsNaX, MgNaX, CaNaX, ZnNaX, SrNaX, and BaNaX), the luminescence spectra of all the systems studied are largely the same. In the analysis of these spectra, naphthalene molecules which are physically sorbed and those bound by donor-acceptor interaction are distinguished. In forming the adsorbed charge-transfer complex (CTC), the naphthalene molecules behave as electron donors. The elements of the zeolite lattice serve as the acceptors. With zeolites of the first group, oxidation of the anphthalene, occurring in the adsorbed phase on contact with air, is observed spectroscopically. In constrast, zeolites of the second group (CoNaX, NiNaX, CuNaX, AgNaX, and CdNaX) are characterized by individual spectra due to the formation of a CTC between the naphthalene molecule and the metal present in the lattice. None of the five systems displayed oxidation of the naphthalene on contact with air.In conclusion, we wish to express our deep appreciation to V. L. Broude for fruitful discussions while considering the experimental results.