Variable‐Temperature Infrared Spectroscopy Studies on the Thermodynamics of CO Adsorption on the Zeolite Ca–Y (original) (raw)
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The Journal of Physical Chemistry B, 1998
were observed during the room-temperature adsorption of CO over NaY zeolite. This was accompanied by the appearance of a prominent band at 2356 cm-1 along with the sidebands at 2336, 2342-2348, and 2364-2370 cm-1 in the ν 3 region of CO 2 vibrations. The intensity and the frequency of these bands showed monotonic changes on evacuation, temperature rise, and exchange of Na + ions with a proton or a group IIA cation and also on using the isotopically labeled CO. For instance, the adsorption of isotopic 13 C 16 O and 12 C 18 O gases gave rise to a uniform red shift in all the major ν(CO) bands, corresponding to a frequency ratio of ν(13 CO)/ν(12 CO) ≈ 0.978 and ν(C 18 O)/ν(C 16 O) ≈ 0.976. Similarly, all the asymmetric stretching bands of CO 2 showed a uniform isotopic shift corresponding to ν(13 C)/ν(12 C) ≈ 0.971. Furthermore, the intensity of most of the above-mentioned bands in ν(CO) and ν 3 (CO 2) regions exhibited an exponential type growth behavior with increasing adsorbate pressure, though the extent of this growth was different for individual vibrations. No significant change was, however, observed in the frequency and the intensity of the hydroxyl region bands on adsorption of CO under experimental conditions of this study. Arguments are presented to show that the observations of this study are not in harmony with the reported modes of direct CO adsorption at the specific zeolitic sites such as the charge-balancing cations, Al 3+ sites, or the Bronsted acid sites. Instead, the formation of the weakly held (CO) n and (CO 2) n molecular clusters occluded in the zeolitic cavities and stabilized under the cation field is in conformity with the data of this study.
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. #
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.
J. Chem. Soc., Faraday Trans., 1992
The physical adsorption of CO at low temperature on HY zeolite leads to the formation of three types of physi sorbed CO, depending on the CO pressure. At low CO pressure, CO primarily interacts with HF hydroxyls through hydrogen bonding and gives rise to Vco at 2176 cm-1 (species A). Simultaneously, the v0H vibration of the perturbed hydroxyls is shifted by 298 cm-1 toward lower wavenumbers, which provides a measurement of the acid strength of the protons. The saturation of hydroxyl sites is followed by the formation of more weakly physisorbed CO (species C) exhibiting Vco at 2160 cm-1, tentatively ascribed to CO adsorbed on framework oxygens acting as basic sites. At higher CO pressures, a liquid-like CO phase (species B, Vco = 2140 cm-1) forms extensively, exhibiting hindered rotational behaviour. The progressive formation of OH-CO complexes is accompanied by frequency shifts of all v0H and Vco vibrations, whether the corresponding species are directly involved or not in the CO adsorption. In particular, LF hydroxyls, although not interacting with physisorbed CO, experience coverage-dependent downward shifts. These phenomena are demonstrated to correlate better with CO-induced effects than with intrinsic acid strength heterogeneity.
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.
Two Coordination Modes of CO in Zeolites: A Temperature-Dependent Equilibrium
Angewandte Chemie International Edition, 1998
Besides their wide application as solid acid catalysts in the petrochemical industry, zeolites are currently being investigated as catalysts for the manufacture of fine chemicals. New directions include the incorporation of redox metals and chiral centers either in the zeolite framework or as encaged extraframework species, which can lead to highly selective biomimetic solid catalysts. Intrazeolite (catalytic) chemical processes are mediated, inter alia, by strong electrostatic fields acting inside the zeolite channels and by spatial restrictions imposed by the dimensions and topology of the intracrystalline void space. Electrostatic fields, created mainly by extraframework (exchangeable) cations, contribute to the formation of internal adducts with adsorbed molecules. The electron distributions of these molecules are thereby significantly modified, and this can lead ultimately to new reactivity patterns. Precise characterization of such intrazeolite species is a prime requirement to understanding chemical reactivity. Detailed knowledge of the interaction of CO with zeolites is also of considerable interest with regard to zeolite characterization, since CO is the most widely used probe molecule for IR spectroscopic investigations, including determination of acidity and intrazeolite electrostatic fields. We report here on a variable-temperature (77 to 303 K) FT-IR study of CO adsorbed on Na-ZSM-5, which is a medium-pore zeolite having the MFI-type structure. It is shown that CO can coordinate to Na ions in two different modes: either through the C end or through the O end. Both adducts are in a temperaturedependent equilibrium which follows the vant Hoff relationship with a DH À value of 3.8 kJ mol À1 ; the C-bonded adduct is the more stable species.
Microporous and Mesoporous Materials, 2011
Adsorption isotherms of carbon dioxide on the Na-A zeolite were measured in the temperature range from 273 to 333 K. Henry's constants and isosteric adsorption heats calculated from the temperature dependence of CO 2 isotherms were combined with results of the variable temperature IR spectroscopy to obtain a spectroscopic signature of adsorption complexes and corresponding thermodynamic characteristics. The adsorption of CO 2 in Na-A zeolite was also investigated computationally, using a periodic density functional model improved for the proper description of dispersion interactions. Based on a good agreement between experimental and calculated adsorption heats and shifts of asymmetric CO 2 stretching frequency due to adsorption the following model of CO 2 /Na-A system is proposed: (i) CO 2 adsorption complexes formed at a low coverage involves interaction of CO 2 with three Na + cations in the LTA supercage; they are characterized by the IR band at 2349 cm À1. (ii) CO 2 adsorption complexes at higher coverages are formed between a pair of Na + cations; they are characterized by the IR band centered at 2360 cm À1. Almost half of the interaction energy between CO 2 and Na-A is due to the dispersion interaction.