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Kleitz F.,Laval University | Berube F.,Laval University | Guillet-Nicolas R.,Laval University | Yang C.-M.,National Tsing Hua University | Thommes M.,Quantachrome Instruments
Journal of Physical Chemistry C | Year: 2010

In order to investigate the details of the process of pore condensation and hysteresis mechanisms in three-dimensional (3-D) pore networks, we performed a systematic study of the adsorption and pore condensation behavior of N 2 (77.4 K) and Ar (77.4 and 87.3 K) in a 3-D ordered pore system, i.e., cubic Ia3̄d mesoporous KIT-6 silica materials with mode pore diameters ranging from ca. 5 nm up to 11 nm. KIT-6 silica is a porous material composed of two intertwined mesoporous subnetworks similar as in MCM-48, but this material can be prepared with much larger mean pore diameters. Accurate pore size analysis was performed by X-ray diffraction modeling and by state-of the art application of nonlocal density functional theory (NLDFT) on N 2 (77.4 K) and Ar (87.3 K) sorption data. Furthermore, our data suggest that the width of the adsorption/desorption hysteresis loop observed for 3-D KIT-6 silica can be narrower as compared to that of pseudo-one-dimensional SBA-15 silica of the same pore size (i.e., in the pore diameter range from 6 to 8 nm). This specific behavior correlates well with the existence of the highly interconnected 3-D pore network of the KIT-6 material. Moreover, the results of our investigations are also consistent with previous observations that the SBA-15 pore system becomes more and more interconnected with increasing aging temperatures, i.e., SBA-15 changes from being a material with a pseudo-one-dimensional mesopore system to a material exhibiting a three-dimensional pore system resembling KIT-6 silica. These results provide new insights into the effects of pore interconnectivity on pore condensation and hysteresis behavior in both KIT-6 and SBA-15 silica materials and enable a more thorough understanding of the pore structure and textural properties of these materials. © 2010 American Chemical Society.

Thommes M.,Quantachrome Instruments | Mitchell S.,Institute for Chemical and Bioengineering | Perez-Ramirez J.,Institute for Chemical and Bioengineering
Journal of Physical Chemistry C | Year: 2012

Advanced physicochemical characterization of the pore structure of hierarchical zeolites, including the precise knowledge of pore size, interconnectivity, and surface properties, is crucial in order to interpret their superior performance in catalytic and adsorption applications. Postsynthetic mesopore formation by alkaline treatment of zeolites leads to simultaneous compositional changes due to the selective dissolution of silicon. By careful tuning of these effects through subsequent acid treatment, the Si/Al ratio can be restored to that of the parent zeolite. We evaluate the application of argon (87.3 K) and water (298.5 K) adsorption to assess the porous properties of mesoporous ZSM-5 zeolites with equivalent porosity, but differing composition. An accurate and combined micro/mesopore size analysis is obtained by applying NLDFT (nonlocal density functional theory) analysis on the argon isotherms. The argon adsorption data clearly reveal the two different relative pressure regions of micro- and mesopore filling of hierarchical zeolites. In contrast, the two filling regions overlap upon adsorption of water due to the hydrophobic nature of the ZSM-5 micropores and the much more hydrophilic nature of the mesopores. This indicates that water adsorption is sensitive to the Si/Al ratio, the distribution of aluminum species in the zeolite, and to the presence of polar groups on the mesopore surface. Thus, further insights into the surface and structural properties of the pores in hierarchical zeolites prepared by desilication can be gained. Based on our results, we put forward a hydrophilicity index capable of identifying differences in surface chemistry between distinct porous materials, and also between the micro- and mesopores present within hierarchically structured nanoporous materials. © 2012 American Chemical Society.

Min B.-H.,Inha University | Jeong E.-Y.,Inha University | Thommes M.,Quantachrome Instruments | Park S.-E.,Inha University
Chemical Communications | Year: 2011

Plugged mesoporous SBA-15 having a 2-D hexagonal pore structure could be directly synthesized under acidic conditions using P123 as a supramolecular template, sodium metasilicate and alcoholamines. The use of alcoholamines seemed to play roles as capturing agents for silica sources that could form internal porous plugs. © The Royal Society of Chemistry 2011.

Rasmussen C.J.,Rutgers University | Vishnyakov A.,Rutgers University | Thommes M.,Quantachrome Instruments | Smarsly B.M.,Justus Liebig University | And 2 more authors.
Langmuir | Year: 2010

We studied cavitation in metastable fluids drawing on the example of liquid nitrogen confined to spheroidal pores of specially prepared well-characterized mesoporous silica materials with mean pore diameters ranging from ∼6 to ∼35 nm. Cavitation was monitored in the process of evaporation/desorption from fully saturated samples with gradually decreasing vapor pressure at the isothermal conditions. The onset of cavitation was displayed by a sharp step on the desorption isotherm. Wefound that the vapor pressure at the onset of cavitation depended on the pore size for the samples with pores smaller than ∼11 nm and remained practically unchanged for the samples with larger pores. We suggest that the observed independence of the cavitation pressure on the size of confinement indicates that the conditions of bubble nucleation in pores larger than ∼11 nm approach the nucleation conditions in the bulk metastable liquid. To test this hypothesis and to evaluate the nucleation barriers, we performed grand canonical and gauge cell Monte Carlo simulations of nitrogen adsorption and desorption in spherical silica pores ranging from 5.5 to 10 nm in diameter. Simulated and experimental adsorption isotherms were in good agreement. Exploiting the correlation between the experimental cavitation pressure and the simulated nucleation barrier, we found that the nucleation barrier increased almost linearly from ∼40 to ∼70 kBT in the range of pores from ∼6 to ∼11 nm, and varied in diapason of 70-75 k BT in larger pores, up to 35 nm. We constructed the dependence of the nucleation barrier on the vapor pressure, which asymptotically approaches the predictions of the classical nucleation theory for the metastable bulk liquid at larger relative pressures (>0.6). Our findings suggest that there is a limit to the influence of the confinement on the onset of cavitation, and thus, cavitation of nanoconfined fluids may be employed to explore cavitation in macroscopic systems. © 2010 American Chemical Society.

Silvestre-Albero J.,Instituto Universitario Of Materiales | Silvestre-Albero A.,Instituto Universitario Of Materiales | Rodriguez-Reinoso F.,Instituto Universitario Of Materiales | Thommes M.,Quantachrome Instruments
Carbon | Year: 2012

In order to get more insight into the characterization of nanoporous carbons by gas adsorption, the use of different probe molecules has been compared. A series of activated carbons with ranging porosity (burn-off) have been prepared from olive stones using CO 2 as activating agent and characterized using nitrogen and argon adsorption at low temperature (77.4 K for N 2 and 87.3 K for Ar) together with CO 2 adsorption at 273 K and immersion calorimetry into liquids of different molecular dimensions. Experimental results show that argon adsorption in narrow carbon micropores takes place at a higher relative pressure compared to nitrogen due to a weaker effective adsorption potential (lower strength of dispersion forces), including the absence of specific interactions of argon with the adsorbent surface. We show further that application of advanced theoretical approaches based on the density functional theory (NLDFT and QSDFT) provides an accurate description of the pore-size distribution (PSD). The PSD obtained from the argon adsorption data at 87.3 K is in good agreement with immersion calorimetry measurements. Our results demonstrate that argon adsorption at 87.3 K in combination with the application of advanced DFT methods (e.g. QSDFT) allows for a reliable characterization of the narrow microporosity in highly heterogeneous activated carbons. © 2011 Elsevier Ltd. All rights reserved.

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