Isotherms and IR spectra were measured for CO2-MZSM-5 (M = Li +, Na +, K +, Rb +, Cs +) adsorption systems. The observed adsorption characteristics were well approximated by a physical adsorption where the van der Waals force and an electrostatic interaction force are operating: the degree of irreversible adsorption (chemisorption) was less than 10% of the total adsorption. Although the adsorbed CO2 molecule interacts with both the cation and the pore wall, only the CO2-cation interaction can convert the IR inactive u1 vibration into an IR active state. The adsorption model proposed enables us to calculate the initial heat of adsorption as well as the molecular orientation angle of CO2 against the cation site.
Infrared spectra of methane adsorbed by five kinds of ion-exchanged zeolites have been measured, and the results have been compared with adsorption characteristics obtained from the adsorption isotherms measured separately. The shifts of the IR u1 peak position (Aul), the IR absorption coefficient (A), and also the isosteric heat of adsorption (q,?) have been found to be intimately related to the ionic radius of the cation exchanged. Furthermore, the electric field ( E ) of the cationic site has been found to decrease according to the sequence Li+ > Na+ > K+ > Rb+ > Cs+. From these results, it has been postulated that the interaction between the cationic site and a methane molecule plays a dominarit role in the adsorption. In addition, the IR and adsorption data have revealed that there are two kinds of adsorption sites over the zeolite surface, i.e., the silicalite-like site (site 1) and the cationic site (site 2). The difference between the adsorption energy for site 1 and for site 2 has been estimated to be qs2 -20.92 kJ/mol, which has been almost identical with the chemical potential difference evaluated from IR data.
The effect of silyl substituents in diphenylprolinol silyl ether catalysts was investigated. Mechanistically, reactions catalyzed by diphenylprolinol silyl ether can be categorized into three types: two that involve an iminium ion intermediate, such as for the Michael-type reaction (type A) and the cycloaddition reaction (type B), and one that proceeds via an enamine intermediate (type C). In the Michael-type reaction via iminium ions (type A), excellent enantioselectivity is realized when the catalyst with a bulky silyl moiety is employed, in which efficient shielding of a diastereotopic face of the iminium ion is directed by the bulky silyl moiety. In the cycloaddition reaction of iminium ions (type B) and reactions via enamines (type C), excellent enantioselectivity is obtained even when the silyl group is less bulky and, in this case, too much bulk reduces the reaction rate. In other cases, the yield increases when diphenylprolinol silyl ethers with bulky substituents are employed, presumably by suppressing side reactions between the nucleophilic catalyst and the reagent. The conformational behaviors of the iminium and enamine species have been determined by theoretical calculations. These data explain the effect of the bulkiness of the silyl substituent on the enantioselectivity and reactivity of the catalysts.
Carbon monoxide adsorption on alkali metal ion-exchanged ZSM-5 zeolites was investigated by IR spectroscopy. The IR peak positions and intensities of adsorbed CO, and the change in the spectra with temperature are discussed. The IR spectra for physisorbed CO over ZSM-5 zeolites were assigned to the oriented species on a cation, such as M+–C–O and M+–O–C, and the loosely adsorbed CO in the ZSM-5 pore.
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