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.
Amounts of nitrogen adsorbed by ion-exchanged ZSM-5 zeolites (H+, Li+, Na+, K+, Rb+, and Cs+) and silicalite have been measured under various conditions; pressure=6.7×102–6.7×104 Pa, temperature=196–273 K. IR spectra of adsorbed nitrogen have also been measured at 226 K under various nitrogen pressures (1.3×102–4.0×104 Pa). The adsorption capacity, the specific surface area, and the heat of adsorption have been evaluated from the adsorption data. These quantities coupled with the IR peak shifts observed have revealed that the nitrogen molecule adsorbed on a small and strong cationic site (Li+, Na+) has an orientation in which the longer molecular axis lies in parallel with the direction of the electric field of the cation site. It has also been revealed that the nitrogen molecule adsorbed on a large (K+, Rb+, Cs+) or weak (H+) cationic site takes on no special orientation. The electric field evaluated by analyzing the IR intensity is consistent with the adsorption model obtained.
Adsorption of methane has been measured under a wide range of temperature (-77 to +75 "C) and pressure (0.0007-10 MPa) using five kinds of molecular sieves (ZSM-BA, -B, -C, Zeolon-900H, and MS-13X) as adsorbents. The amounts of nitrogen adsorption have also been measured for comparison. The experimental data have been utilized in determining a reasonable generalization method. It has been demonstrated that the use of the initial heat of adsorption qoij (i, adsorbate; j, adsorbent) as a parameter that characterizes the gas-solid system enables us to generalize the adsorption. The generalized adsorption equation obtained is [In (W/ W0)li. = F(ci4/biJ), where W is the volume of the adsorbed phase, Wo is the saturation volume of the adsorbedphase, F is a generalized adsorption function, ti is an adsorption potential, and pij is an affinity coefficient defined by the heat of adsorption ratio (qoiJ/ho*,s); the subscripts * and indicate the reference adsorbate and the reference adsorbent, respectively. IntroductionAlthough studies of the physical adsorption of gases by solid adsorbents have a long history, intensive studies on the same subject are still continuing. Several re~earchersl-~ have recently published results of theoretical treatments on adsorption, and the vacant solution theory of Dabber et al.4>5 has been shown to be applicable for a number of adsorption systems. Sircar and Myers6 have developed a method of correlation of adsorption data on a heterogeneous adsorbent. Experiments of Findenegg et al.' for the Kr adsorption onto porous adsorbents have been extended
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