The nature of cationic adsorption sites in alkaline zeolites—single, dual and multiple cation sites (original) (raw)
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RSC Adv., 2015
Development of zeolite based adsorbents with high adsorption capacity and selectivity is the key requirement for efficient and economic separation processes. However, less attention has been given so far towards understanding the mechanism of adsorption on the zeolites. In the present study adsorption of carbon monoxide, methane and nitrogen on zeolite-X exchanged with magnesium, calcium, strontium and barium cations was carried out using a volumetric gas adsorption method. Calcium, strontium, and barium ion exchanged zeolite-X shows increase in carbon monoxide, methane and nitrogen adsorption capacity. Strontium exchanged zeolite-X shows carbon monoxide adsorption capacity of 28.4 molecules per unit cell and calcium exchanged zeolite-X shows methane and nitrogen adsorption capacity of 18.8 and 13.8 molecules per unit cell, respectively at 303 K and 760 mm Hg pressure, maximum among the alkaline earth metal ion exchanged zeolite-X samples. However, barium exchanged zeolite-X shows methane/nitrogen selectivity of 1.78, maximum among the studied samples.
Carbon monoxide adsorption on low-silica zeolites—from single to dual and to multiple cation sites
Physical Chemistry Chemical Physics, 2007
Infrared spectra of CO adsorbed on the Al-rich Na-A zeolite were analysed by using a combined theoretical and experimental approach, showing that such spectra cannot be interpreted by assigning each IR band to CO interacting with a specific type of single cation site. This concept, which usually works well for highsilica zeolites, should not be uncritically extended to Al-rich zeolites that are crowded with cations in configurations which lead to preferential formation of CO adsorption complexes involving more than one cation site.
Adsorption of CO 2 , CH 4 , and H 2 O in Zeolite ZSM-5 Studied Using In Situ ATR-FTIR Spectroscopy
The Journal of Physical Chemistry C, 2013
Biogas and natural gas are interesting fuels with high H/C ratio. However, these gases frequently contain carbon dioxide and water which lowers the heat value of the gas and may induce corrosion. Therefore, the development of more efficient processes, such as membrane processes and improved adsorbents, for the separation of carbon dioxide and water from biogas and natural gas is of great importance. Zeolite ZSM-5 membranes are promising for this separation which is controlled by the adsorption and diffusion of the different species in the zeolite. Multicomponent adsorption data are therefore required for development of new membrane and adsorption processes. In the present work, the adsorption of water, carbon dioxide, and methane in a Na-ZSM-5 zeolite film at various temperatures was studied by in situ Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy for the first time. Adsorption isotherms were retrieved from the experimental data and the Langmuir model fitted the isotherms very well. Limiting heat of adsorption was determined from the Henryś law regime and the values determined agreed well with previously reported data. A few experiments were conducted with multicomponent mixtures and the experimentally determined amounts adsorbed were compared with values predicted by the Ideal Adsorbed Solution Theory (IAST). It was found that for the binary mixture of carbon dioxide and methane there was good agreement between the experimental values and those predicted by the IAST. However, when water was also introduced, the IAST could not fully capture the adsorption behavior of the multicomponent mixture, probably because the adsorbed phase is not ideal. These findings are in line with previous reports for adsorption in zeolites. The multicomponent adsorption behavior of this system will be further investigated in forthcoming work.
On the mechanism of adsorption and separation of CO2 on LTA zeolites: An IR investigation
Vibrational Spectroscopy, 2008
The adsorption of CO 2 on the zeolites 3A (K-LTA), 4A (Na-LTA) and 5A (Ca,Na-LTA), all with Si/Al a.r. = 1, has been investigated by IR spectroscopy. CO 2 adsorption on 3A zeolite is mostly limited at the external surface, both in the form of linear molecular species and of carbonatelike species. In the case of 4A and 5A zeolites, which are applied in the industry for CO 2 separation, the adsorption of both linear molecular species and carbonate species occurs mostly in the cavities. The adsorption in the form of carbonates is definitely stronger than that of linear molecules. Much more carbonate-like species are formed on 4A than on 5A zeolite. This is explained with the partial poisoning of the cations of 5A zeolite in the form of calcium hydroxyl and carbonate species, which are already present in the sample after activation. The relevance of the participation of framework oxygen species in adsorption in metal exchanged zeolites is shown. #
Microporous and Mesoporous Materials, 2019
Adsorption and co-adsorption of propane and propene were investigated by combination of calorimetric-volumetric measurements with breakthrough experiments on alkali metal exchanged (Li, Na-, K-) FER zeolites with different Si/Al ratio. Effect of cation type and their concentration in the zeolite on adsorption selectivity was evaluated and discussed. Based on determined adsorption heats at zero coverage limit, the bridged complexes of propene were confirmed only in the case of K-FER zeolites. Presence of bridged complexes between two nearby K + cations in dual cationic sites, described in details previously (Rubes et al. J. Phys. Chem. C 122 (2018) 6128), results in significant increase in adsorption selectivity while adsorption capacity of the zeolite is not influenced and is comparable with other M-FER zeolites. On the other hand, Na-FER zeolites exhibited the lowest propene/propane selectivity (at least six-times lower than K-FER-8.6 zeolite).
Modeling of Adsorption Properties of Zeolites: Correlation with the Structure
The Journal of Physical Chemistry B, 1997
The adsorption of N 2 and CO in Na X-zeolites has been studied for different framework structures and extraframework cation distributions. To this aim, the cation-molecule system modeling one site has been embedded in a set of external point charges which simulate the zeolite environment of the site and has been treated quantum chemically, using a method based on density functional theory. This procedure has been applied to the 64 cationic sites accessible for adsorption in a crystal unit cell of an ideal X-zeolite with a Si/Al ratio equal to 1. These calculations have shown that only a few cations are favorable for initial adsorption and that those cations are always of type III(III′). Their efficiency depends both on the framework geometry and on their location in the supercages. The analysis of the quantum chemical results in terms of a classical description involving electrostatic and induction interaction energies with the framework has led to the conclusion that the direction of the electric field vector created by the zeolite in the supercages is an important factor determining the zeolite adsorption properties.
Systematic Measurements of CH4 and CO2 Adsorption Isotherms on Cation-Exchanged Zeolites 13X
Journal of Chemical & Engineering Data, 2019
Experimental data for adsorption of pure carbon dioxide, methane, and nitrogen on zeolite 13X granules at different temperatures (288.15-318.15 K) and pressure up to 20 bar are reported. The cation of adsorbent is exchanged with H+, Li+, and Cu2+, and the adsorption of pure gases is measured. The equilibrium adsorption isotherms of gases are performed with a static volumetric adsorption instrument, which was designed and built. The results show that the adsorption capacity of carbon dioxide is higher than that of methane and nitrogen and that the cation exchange improved the adsorption capacity of pure gases. The LiX adsorbent has the highest adsorption capacity among the studied adsorbents. However, the relative selectivity of carbon dioxide over methane for zeolite 13X has the highest value of 30.48. The CuX adsorbent shows the highest selectivity for carbon dioxide over nitrogen. The adsorption isotherms for all of the pure gases are fitted successfully with the Toth model.