Combined volumetric, infrared spectroscopic and theoretical investigation of CO2 adsorption on Na-A zeolite (original) (raw)
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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. #
A computational study of CO2, N2, and CH4 adsorption in zeolites
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The adsorption properties of CO 2 , N 2 and CH 4 in all-silica zeolites were studied using molecular simulations. Adsorption isotherms for single components in MFI were both measured and computed showing good agreement. In addition simulations in other all silica structures were performed for a wide range of pressures and temperatures and for single components as well as binary and ternary mixtures with varying bulk compositions. The adsorption selectivity was analyzed for mixtures with bulk composition of 50:50 CO 2 /CH 4 , 50:50 CO 2 /N 2 , 10:90 CO 2 /N 2 and 5:90:5 CO 2 /N 2 /CH 4 in MFI, MOR, ISV, ITE, CHA and DDR showing high selectivity of adsorption of CO 2 over N 2 and CH 4 that varies with the type of crystal and with the mixture bulk composition.
Physical Chemistry Chemical Physics, 1999
Carbon monoxide was found to adsorb, at room temperature, on Na-ZSM-5. IR spectra of adsorbed CO showed two main bands at 2176 and 2112 cm~1, which were assigned to the CwO stretching mode of Na`É É ÉCO and Na`É É ÉOC adducts, respectively. The complex structure of these bands suggested that cation sites in Na-ZSM-5 are not all identical. Further proof of the presence of slightly di †erent cation sites was obtained by IR spectroscopy of adsorbed carbon dioxide. Microcalorimetry of adsorbed CO, in conjunction with IR spectroscopy and volumetric adsorption data, allowed a detailed thermodynamic study to be carried out on the CO/Na-ZSM-5 system. CO adsorption was found to follow a Langmuir-type isotherm. The (exothermic) adsorption process showed a di †erential heat of adsorption of ca. 27 kJ mol~1. The enthalpy change in the formation of Na`É É ÉCO and Na`É É ÉOC species was found to be *H¡ \ [28 and *H¡ \ [24 kJ mol~1, respectively, while the value of T *S¡ \ [41 kJ mol~1 was derived for both adsorption modes.
Physical Chemistry Chemical Physics, 1999
Adsorption of CO on NaZSM-5 zeolite was investigated at temperatures in the range 300È470 K and at pressures of 5È500 Torr using FTIR spectroscopy. The e †ect of exchanging the charge balancing cation in NaZSM-5 with a proton or calcium was evaluated. Data were also collected on NaY, CaY and CaX zeolites for comparison. We detected the development of six distinct CwO stretching bands with maxima at around 2111, 2130, 2146, 2160, 2176 and 2194 cm~1 during the adsorption of CO on NaZSM-5 zeolite at ambient temperatures. This was accompanied by the appearance of a prominent band at 2356 cm~1 and weak shoulder bands at frequencies around 2336, 2340, 2370 and 2380 cm~1 in the region of All the l(CO) bands and l 3 CO 2. also the bands in the region of exhibited similar behaviour as a function of adsorbate pressure, l 3 CO 2 evacuation, rise in sample temperature, and the exchange of charge balancing cation. For instance, the intensity of all the CwO stretching bands showed a similar growth behaviour with increasing adsorbate pressure, though the extent of this growth was di †erent for the individual IR bands. Similarly, these bands were removed simultaneously on evacuation. Furthermore, while all the vibrational bands in the l(CO) region showed a uniform isotopic shift corresponding to a frequency ratio l(13C/12C) of ca. 0.977 and l(18O/16O) of 0.976 for the adsorption of 13C16O and 12C18O, respectively, the bands in the region showed a red l 3 (CO 2) shift l(13C/12C) of 0.972 with 13CO and an isotopic shift corresponding to 16O12C18O on 12C18O adsorption. No shift in l(OH) bands was observed after CO adsorption under the conditions of this study. The results thus indicate that the individual zeolitic surface sites e.g., the Al3`sites, Bronsted acid sites or the charge balancing cations, may not participate directly in the bonding of CO molecules at room temperature or above. Instead, the cage e †ect of zeolites plays an important role. The data are interpreted to suggest the formation of weakly bonded clusters of CO and molecules, occluded in the zeolitic cages and stabilized under the cationic CO 2 Ðeld.
Combined DFT/CC and IR spectroscopic studies on carbon dioxide adsorption on the zeolite H-FER
Energy & Environmental Science, 2009
Adsorption of carbon dioxide on H-FER zeolite (Si:Al ¼ 8:1) was investigated by means of a combined methodology involving variable-temperature infrared spectroscopy and DFT/CC calculations on periodic zeolite models. The experimentally found value of adsorption enthalpy was DH 0 ¼ À30 kJ mol À1. According to calculations, adsorption complexes on isolated Si(OH)Al Brønsted acid sites (single sites) involve an adsorption enthalpy in the range of À33 to À36 kJ mol À1 , about half of which is due to weak intermolecular interactions between CO 2 and the zeolite framework. Calculations show clearly the significant role played by weak intermolecular interactions; adsorption enthalpies calculated with standard GGA type exchange-correlation functionals are about 13 kJ mol À1 underestimated with respect to experimental results. Good agreement was also found between calculated and experimentally observed stretching frequencies for these complexes. Calculations revealed that CO 2 adsorption complexes involving two neighbouring Brønsted acid sites (dual sites) can be formed, provided that the dual site has the required geometry. However, no clear evidence of CO 2 adsorption complexes on dual sites was experimentally found.
Energy & Fuels, 2003
We present a molecular model for the adsorption of CO 2 , N 2 , H 2 , and their mixtures in dehydrated zeolite Na-4A. The interatomic potentials for this model were developed by comparing the results of grand canonical Monte Carlo (GCMC) simulations of single-component adsorption at room temperature with experimental measurements. GCMC simulation is also used to assess the adsorption selectivity of CO 2 /N 2 and CO 2 /H 2 mixtures, as a function of temperature and gasphase composition. At room temperature, Na-4A is strongly selective for CO 2 over both N 2 and H 2 , although this selectivity decreases slightly as the gas-phase pressure increases. Ideal adsorbed solution theory is shown to give accurate predictions of the adsorption selectivity at low CO 2 partial pressures, provided that a functional form that accurately describes the CO 2 singlecomponent isotherm is used. The adsorption properties of CO 2 /N 2 mixtures in Na-4A are compared to the same mixtures in silicalite.
Advances in principal factors influencing carbon dioxide adsorption on zeolites
Science and Technology of Advanced Materials, 2008
We report the advances in the principal structural and experimental factors that might influence the carbon dioxide (CO 2 ) adsorption on natural and synthetic zeolites. The CO 2 adsorption is principally govern by the inclusion of exchangeable cations (countercations) within the cavities of zeolites, which induce basicity and an electric field, two key parameters for CO 2 adsorption. More specifically, these two parameters vary with diverse factors including the nature, distribution and number of exchangeable cations. The structure of framework also determines CO 2 adsorption on zeolites by influencing the basicity and electric field in their cavities. In fact, the basicity and electric field usually vary inversely with the Si/Al ratio. Furthermore, the CO 2 adsorption might be limited by the size of pores within zeolites and by the carbonates formation during the CO 2 chemisorption. The polarity of molecules adsorbed on zeolites represents a very important factor that influences their interaction with the electric field. The adsorbates that have the most great quadrupole moment such as the CO 2 , might interact strongly with the electric field of zeolites and this favors their adsorption. The pressure, temperature and presence of water seem to be the most important experimental conditions that influence the adsorption of CO 2 . The CO 2 adsorption increases with the gas phase pressure and decreases with the rise of temperature. The presence of water significantly decreases adsorption capacity of cationic zeolites by decreasing strength and heterogeneity of the electric field and by favoring the formation of bicarbonates. The optimization of the zeolites structural characteristics and the experimental conditions might enhance substantially their CO 2 adsorption capacity and thereby might give rise to the excellent adsorbents that may be used to capturing the industrial emissions of CO 2 .