Molecular Simulation of Adsorption and Transport in Hierarchical Porous Materials
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- 作者：Benoit Coasne ; Anne Galarneau ; Corine Gerardin ; Fran莽ois Fajula ; Fran莽ois Villemot
- 刊名：Langmuir
- 出版年：2013
- 出版时间：June 25, 2013
- 年：2013
- 卷：29
- 期：25
- 页码：7864-7875
- 全文大小：595K
- 年卷期：v.29,no.25(June 25, 2013)
- ISSN：1520-5827
文摘
Adsorption and transport in hierarchical porous solids with micro- (1 nm) and mesoporosities (>2 nm) are investigated by molecular simulation. Two models of hierarchical solids are considered: microporous materials in which mesopores are carved out (model A) and mesoporous materials in which microporous nanoparticles are inserted (model B). Adsorption isotherms for model A can be described as a linear combination of the adsorption isotherms for pure mesoporous and microporous solids. In contrast, adsorption in model B departs from adsorption in pure microporous and mesoporous solids; the inserted microporous particles act as defects, which help nucleate the liquid phase within the mesopore and shift capillary condensation toward lower pressures. As far as transport under a pressure gradient is concerned, the flux in hierarchical materials consisting of microporous solids in which mesopores are carved out obeys the Navier鈥揝tokes equation so that Darcy鈥檚 law is verified within the mesopore. Moreover, the flow in such materials is larger than in a single mesopore, due to the transfer between micropores and mesopores. This nonzero velocity at the mesopore surface implies that transport in such hierarchical materials involves slippage at the mesopore surface, although the adsorbate has a strong affinity for the surface. In contrast to model A, flux in model B is smaller than in a single mesopore, as the nanoparticles act as constrictions that hinder transport. By a subtle effect arising from fast transport in the mesopores, the presence of mesopores increases the number of molecules in the microporosity in hierarchical materials and, hence, decreases the flow in the micropores (due to mass conservation). As a result, we do not observe faster diffusion in the micropores of hierarchical materials upon flow but slower diffusion, which increases the contact time between the adsorbate and the surface of the microporosity.