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  • Title: A method for the estimation of pore anisotropy in porous solids.
    Author: Pomonis PJ, Armatas GS.
    Journal: Langmuir; 2004 Aug 03; 20(16):6719-26. PubMed ID: 15274577.
    Abstract:
    In this work a method for the estimation of pore anisotropy, b, in porous solids is suggested. The methodology is based on the pore size distribution and the surface area distribution, both calculated from trivial N2 adsorption-desorption isotherms. The materials used for testing the method were six MCM-Alx solids in which the ordered pore structure (for x = 0) was gradually destroyed by the introduction of Al atoms (x = 5, 10, 15, 20, 50) into the solids. Additionally, four silicas having random porosity were examined, in which the surface of the parent material SiO2 (pure silica) was gradually functionalized with organosilicate groups of various lengths (triple bond Si-H, triple bond Si-CH2OH, triple bond Si-(CH2)3OH) in order to block a variable amount of pores. As pore anisotropy, the ratio bi = Li/Di is defined where Li and Di are the length and the diameter of each group of pores i filled at a particular partial pressure (Pi/P0). The ratio of the surface area Si over the pore volume Vi, at each particular pressure (Pi/P0), is then expressed as Si3/Vi2 = 16 pi Nibi = 16 pi lambdai, where Nibi is the number of pores having anisotropy bi which are filled at each pressure (Pi/P0) and lambdai is the total anisotropy of all the pores Ni belonging to the group i of pores. Then plot of lambdai vs (Pi/P0) provides a clear picture of the variation of the total pore anisotropy lambdai as the partial pressure (Pi/P0) increases. For the functionalized silicas there appears a continuous drop of lambdai as partial pressure (Pi/P0) increases, a fact indicating that both Ni) and bi are continuously diminished. In contrast, for the MCM-Alx materials a sudden kink of lambdai appears at the partial pressure where the well-defined mesopores are filled up, a fact indicating that at this point Ni and/or bi is large. The kink disappears as the ordered porosity is destroyed by increasing the x doping in MCM-Alx. The pore anisotropy bi of each group i of pores is then estimated using the expression (Si3/Vi2) = 8 pi NiriSi and plotting log(lambdai) vs log ri. From those plots, the values of si can be found and therefore the values of bi = 0.5riSi are next defined. In the MCM-Alx materials the maximum pore anisotropy b is very high (bi approximately 250) for x = 0. Then as mesoporosity is destroyed by increasing x, the maximum b values drop gradually to b approximately 11 (x = 5), b approximately 8 (x = 10), and b approximately 3 (x = 15). For x = 20 and x = 50, the maximum b obtains values equal to unity. The same phenomena, although less profound, are also observed for the functionalized silicas, where the anisotropy b is altered by the process of functionalization and from bi approximately 0.5 for the nonfunctionalized or bi approximately 0.9 for the solid functionalized with Si-H groups drops to b = 0.3 and b = 0.2 for the solid functionalized with triple bond Si-(CH2)OH and triple bond Si-(CH2)3OH, respectively. A correlation factor F is suggested in cases where the pore model departs from the cylindrical geometry.
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