Hysteresis loops frameworks

The differences in the shape of isotherms have been attributed to the formation of secondary pores during framework dealumination. The hysteresis loops observed were attributed to capillary condensation in the secondary pores. 

Figure 5 presents the N2 adsorption/desorption isotherms for calcined (650 °C) MSU-G silicas assembled from C H2 +,NH(CH2)2NH2 surfactants with n = 10, 12, and 14. The inset to the figure provides the framework pore distributions. The maxima in the Horvath-Kawazoe pore size distributions increase in the order 2.7, 3.2, 4.0 nm as the surfactant chain length increases. The textural porosity evident from the hysteresis loop at P/P0... 

Important trends of double-mesopore structural development resulting from the water-treatment are revealed in Figure 2 by the N2 adsorption-desorption isotherms and the corresponding BJH pore size distribution based on the desorption branch for the representative samples mentioned above. The isotherm of the normally synthesized DMS simple shows a typical irreversible type IV adsorption isotherm with two separate, well-expressed HI hysteresis loops as defined by lUPAC at relative pressures p/po of 0.2-0.45 and that of 0.8-1.0, respectively. The first condensation step on the isotherm at p/pQ=0.2-0.45 is similar to that for usual MCM-41 materials, however, obviously, this inflection at higher relative pressures differs completely from that of previously-synthesized mesoporous materials in the aspect of their effects on the mesoporous frameworks of the product, namely, this material is of a clear double mesopore size distribution. After 1 day of postsynthesis hydrothermal treatment, the properties of the samples changed dramatically. Compared with the normally synthesized DMS sample, the water-treated sample at 373K shows more steep adsorption steps at 0.25-0.4p/po and 0.8-1.0p/po, respectively, suggesting that double-... 

Important trends in N2 isotherm when the PS beads are used as a physical template are shown in Table 1 and Fig. 2. In Table 1, PI is the alumina prepared without any templates, P2 is prepared without ]4iysical template (PS bead), P3 is prepared without chemical template (stearic acid), and P4 is prepared with all templates. For above 10 nm of pore size and spherical pore system, the Barrett-Joyner-Halenda (BJH) method underestimates the characteristics for spherical pores, while the Broekhoff-de Boer-Frenkel-Halsey-Hill (BdB-FHH) model is more accurate than the BJH model at the range 10-100 nm [13]. Therefore, the pore size distribution between 1 and 10 nm and between 10 and 100 nm obtained from the BJH model and BdB-FHH model on the desorption branch of nitrogen isotherm, respectively. N2 isotherm of P2 has typical type IV and hysteresis loop, while that of P3 shows reduced hysteresis loop at P/Po ca. 0.5 and sharp lifting-up hysteresis loop at P/Po > 0.8. This sharp inflection implies a change in the texture, namely, textural macro-porosity [4,14]. It should be noted that P3 shows only macropore due to the PS bead-free from alumina framework. 

During desorption, as the relative vapor pressure is reduced, pore solids in which capillary condensation occurs often show a hysteresis loop. The simplest interpretation of this phenomenon is given by the ink-bottle model (6). In the framework of this model (Fig. 12) the adsorption and desorption processes are controlled, respectively, by the void and neck sizes. Thus, desorption from a given pore occurs at a lower pressure than adsorption. 

Fig. 16 Views and magnetic properties of the open-framework coordination polymers based on cobalt imidazolates. (a) Evacuated [Co(im)J-0 5Ch. (b) [Co(im)j4- (c) [Co(im)j]j. (d) [Co5(im)j 0.4Mb]j. (e) Plots of temperature dependence of ac susceptibility obtained at 0.1 Oe field for [Co(im)J 0.5Ch. A peak appears at 15 K, which is indicative of the ferromagnetic character of this compound, (f) Plot of the thermal evolution of the for [CoCiml lj at 10 kOe field. The inset shows the hysteresis loop at 1.8 K ( ) and 5.0 K (o). (Co, blue O, red C, brown N, violet H, white. Solvent guest molecules in (d) are represented as spheres) Reprinted from ref 168 [Tian YQ, Cai C-X, Ren X-M, Duan C-Y, Xu Y, Gao S, You X-Z, The SUica-Like Extended Polymorphism of Cobalt(ll) Imidazolate Three-Dimensional Erameworks X-ray Single-Crystal Structures and Magnetic Properties (2003) Chem Eur 1 9 5673]. Copyright WUey-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

The adsorption and desorption isotherms of water vapor are drawn at 25°C for dealuminated HY zeolites upon framework Si/Al ratio. The isotherms are compared to that of the parent NaY zeolite. The isotherm changes in shape from the type I to the type IV with an hysteresis loop changing from the type H4 to the type H2, as increases the Si/Al ratio. The POLANYI-DUBININ theory is used to determine the micropore volume accessible to water. It decreases with increasing Si/Al ratios, down to zero at a Si/Al ratio of 35. Such a result is accounted by the adsorption on the hydrophilic centers which are the cations (H ) associated with the structural aluminium ions, each cation being coordinated by 8H2O. 

Which theoretically have neutral frameworks, show more rapid upswings at lower p/p than the carbons, possibly due in part to the presence of -OH groups at defect sites. Furthermore, VPI-5 is unusual in that it has a clear step at p/p = 0.02-0.06. Low pressure hysteresis was observed for each of these adsorbents, a small hysteresis loop was found even with the narrowest molecular sieve carbons. For the microporous carbons, it was noted that the hysteresis loop broadens as the pore width increased indicating that the high-pressure hysteresis is related to the process of capillary condensation. Again VPI-5 is unusual as it has a double hysteresis loop which closes at p/p - 0.07 the loop at lower p/p is associated with the step and cannot be explained by capillary condensation. 

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