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Probing Adsorption, Pore Condensation, and Hysteresis Behavior of Pure Fluids in Three-Dimensional Cubic Mesoporous KIT-6 Silica Freddy Kleitz,*,† Franc¸ois Be´rube´,† Re´my Guillet-Nicolas,† Chia-Min Yang,‡ and Matthias Thommes*,§ Canada Research Chair on Functional Nanostructured Materials, Department of Chemistry, UniVersite´ LaVal, Quebec G1V 0A6, Canada, Department of Chemistry, National Tsing-Hua UniVersity, Hsinchu, Taiwan, and Quantachrome Instruments, 1900 Corporate DriVe, Boynton Beach, Florida 33426 ReceiVed: October 13, 2009; ReVised Manuscript ReceiVed: March 19, 2010
In order to investigate the details of the process of pore condensation and hysteresis mechanisms in threedimensional (3-D) pore networks, we performed a systematic study of the adsorption and pore condensation behavior of N2 (77.4 K) and Ar (77.4 and 87.3 K) in a 3-D ordered pore system, i.e., cubic Ia3jd 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 N2 (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 pseudoone-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. Introduction During the past decade, significant progress was achieved in the understanding of the sorption and phase behavior of pure fluids confined in materials consisting of single pores1 (e.g., in materials such as MCM-412 and SBA-153). However, the details of the process of pore condensation and the hysteresis mechanisms in three-dimensional (3-D) pore networks are still a matter of investigation. This is partly due to fact that until recently only materials with disordered pore networks, such as silica gels, porous Vycor, and controlled pore glass (CPG), were readily available for such studies.1 In connection to that, recent synthesis progress enabled the development of a novel type of large-pore mesoporous silica material exhibiting a highly ordered cubic Ia3jd mesopore structure. This so-called KIT-6 silica was synthesized using a blend of a poly(alkylene oxide) triblock copolymer (Pluronic P123, EO20PO70EO20) and 1-butanol (BuOH) acting as structure-directing agents under mild acidic conditions.4 This new mesostructured silica material is consisting of two interwoven mesoporous subnetworks quite similar as in the case of MCM-48,2 but it may be prepared with much larger mean pore diameters.4-7 In addition to promising applications * To whom correspondence should be addressed: Freddy Kleitz, tel +1 418 656 7812, fax +1 418 656 7916, e-mail
[email protected].; Matthias Thommes, tel +1 561 731 4999, fax +1 561 732 9888, e-mail
[email protected]. † Universite´ Laval. ‡ National Tsing-Hua University. § Quantachrome Instruments.
in catalysis,8,9 separation,10 biomolecule adsorption,11 and materials synthesis based on nanocasting methods,4,12-17 this family of silica materials has also great potential to serve as a model system to study the details of adsorption, pore condensation, and hysteresis behavior in ordered mesopore networks. Two principal factors determine the hysteretic behavior in porous materials: (i) hysteresis on the level of a single pore of a given shape (i.e., independent pore model) and (ii) cooperative effects due to the specifics of connectivity of the pore network. On the pore level, adsorption hysteresis is considered as an intrinsic property of the vapor-liquid phase transition in a finite volume system. A classical scenario of capillary condensation implies that the vapor-liquid transition is delayed due to the existence of metastable adsorption films and hindered nucleation of liquid bridges.1,18 In open uniform cylindrical pores of finite length, however, metastabilities occur only on the adsorption branch. Indeed, in an open pore filled by liquid-like condensate, the liquid-vapor interface is already present, and evaporation occurs without nucleation, via a receding meniscus. Consequently, the desorption process is associated with the equilibrium vapor-liquid transition, and it follows that the desorption branch can be used for reliable pore size analysis. This mechanism of adsorption hysteresis is dominant in ordered mesoporous materials with large uniform cylindrical pores (MCM-41 with pores larger than ca. 3-4 nm and SBA-15). Typically, one observes a hysteresis loop of type H119 (see Figure 1). Meanwhile, modern, microscopic approaches based on density
10.1021/jp909836v 2010 American Chemical Society Published on Web 05/03/2010
Three-Dimensional Pore Networks
Figure 1. Capillary condensation/evaporation in a single pore exhibiting regular slit or cylindrical pore geometry.
functional theory and molecular simulations are able to qualitatively and, to some extend, quantitatively predict the pore condensation and hysteresis behavior of fluids in such highly ordered model materials.1,20 Modern nonlocal density functional theory (NLDFT) approaches21 are capable of correlating the location of the vaporlike spinodal as well as the position of the equilibrium liquid-vapor transition with the pore size.22 This allows the pore size distribution to be obtained from the adsorption branch of a hysteretic isotherm by applying the kernel of (metastable) NLDFT adsorption isotherm.22 If the hysteresis is caused solely by the delayed condensation effect, the pore sizes calculated from the adsorption branch (by applying the kernel of metastable adsorption isotherms) and the pore sizes calculated from the desorption branch (by applying the kernel of equilibrium NLDFT isotherms) must thus be in agreement. Such an agreement was indeed found for MCM-41 and also for some SBA-15 samples.22,23 In contrast, hysteresis in pore networks is a much more complex phenomenon and various factors play a crucial role. Network models for the description of hysteresis have been developed for disordered adsorbents,24 which attribute hysteresis to the so-called pore blocking effect. This effect is expected to occur if a pore has access to the external surface only through a narrower neck, as in an ink-bottle pore.25 The wide body of an ink-bottle pore is filled at a vapor pressure, which corresponds to the delayed condensation, and remains filled during desorption until the narrow neck empties first at a lower vapor pressure. Thus, in a network of ink-bottle pores, evaporation of the capillary condensate is obstructed by the pore necks. The vapor pressure, at which a pore body empties, depends on the size of the necks, the connectivity of the network, and the state of the neighboring pores. The pore network empties when the relative pressure is below a characteristic percolation threshold associated with the onset of a continuous cluster of pores open to the surface. In such a case, the desorption branch of the hysteresis loop is significantly steeper than the adsorption branch, which results in a triangular hysteresis loop of type H2 according to the IUPAC classification.19 Adsorption/desorption mechanisms were recently revisited with the aid of model porous materials, which contained well-defined ink-bottle pores (e.g., SBA-16,3,26 FDU-1,27 hierarchically ordered porous materials,28 etc.). Theoretical and experimental studies have revealed that if the neck diameter is smaller than a critical size (estimated to be ca. 5 nm for nitrogen at 77.4 K, assuming that the neck can be considered as a cylindrical pore), the mechanism of desorption from the larger region involves cavitation (i.e., the spontaneous nucleation of gas bubbles). Thus, for a given adsorptive and temperature, the neck diameter of an ink-bottle pore determines the mechanism of evaporation from the pore body (as described above). However, such pore blocking/ percolation or cavitation effects do not play a role in the ordered
J. Phys. Chem. C, Vol. 114, No. 20, 2010 9345 3-D networks of MCM-48 and KIT-6 silica; i.e., adsorption isotherms reveal here perfect type H1 adsorption hysteresis,1,5 which indicates that the pore channels do not exhibit constrictions which would otherwise lead to pronounced deviation from the type H1 hysteresis due to pore blocking/percolation effects and thus would give rise to type H2 hysteresis. Yet, the question here is whether the pore condensation and hysteresis behavior in such materials can still be fully described within the independent pore model. In order to address this matter, we first synthesized a series of reference mesoporous KIT-6 silica materials which were obtained by applying different aging temperatures during synthesis. We then performed a systematic study of the adsorption, pore condensation, and hysteresis behavior of nitrogen and argon in this series of KIT-6 silica samples, exhibiting mean pore sizes ranging from 5 to 11 nm, and compared these with the behavior in the respective mesoporous SBA-15 silica materials. Accurate pore size analysis is performed by application of modern nonlocal density functional theory (NLDFT) on nitrogen (77.4 K) and argon (87.3 K) sorption data and supplemented by X-ray diffraction modeling. Furthermore, the isotherms reveal that the width of the adsorption/desorption hysteresis loop of 3-D KIT-6 silica can be narrower as compared to that of pseudo-one-dimensional SBA15 silica of same pore size. Our results provide new insights into pore condensation and hysteresis behavior of fluids confined in three-dimensional pore networks and will permit a more thorough understanding of the pore structure and textural properties of KIT-6 and SBA-15 silica materials. Experimental Section Materials. High-quality mesoporous KIT-6 silica materials were obtained in large quantities following the method reported by Kleitz et al.4,5 Briefly, 9.0 g of Pluronic P123 (EO20PO70EO20, Sigma-Aldrich) was dissolved in 325 g of distilled water and 17.40 g of HCl (37%) under vigorous stirring. After complete dissolution, 9.0 g of 1-butanol (BuOH, Aldrich, 99%) was added. The mixture was left under stirring at 35 °C for 1 h, after which 19.35 g of tetraethoxysilane (TEOS, Acros, 99%) was added to the homogeneous clear solution. The synthesis is carried out in a closed polypropylene bottle. The molar composition of the starting reaction mixture is TEOS/P123/HCl/ H2O/BuOH ) 1/0.017/1.9/195/1.31. This mixture was left under stirring at 35 °C for 24 h, followed by an aging step, alternatively at 50, 80, 100, or 130 °C for 24 h under static conditions (this process is referred to as hydrothermal treatment). The resulting solid products were then filtered and dried for 48 h at 95 °C. For template removal, the as-synthesized silica powders were first shortly slurried in an ethanol-HCl mixture and subsequently calcined at 550 °C for 2 h. Alternatively, the copolymer template was also removed via hot sulfuric acid treatment (48 wt % in H2O) followed by a calcination at 250 °C according the procedure reported by Yang et al.29 KIT-6 samples aged at different temperatures are designated KIT-6-y, where y stands for the aging temperature. For comparison purpose, 2-D hexagonal SBA-15 samples were synthesized following the method proposed by Choi et al.30 This synthesis of SBA-15 is briefly described as follows: 13.9 g of Pluronic P123 was dissolved in 252 g of distilled water and 7.7 g of HCl (37%). After complete dissolution, 25.0 g of TEOS was added at once. The mixture was left under stirring at 35 °C for 24 h, followed by hydrothermal treatment at 100 °C (this temperature may be changed as the previous case of KIT-6) for 24 h under static
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conditions. The solid product was then filtered, dried, and finally calcined at 550 °C as reported. Characterization. Laboratory powder X-ray diffractograms of the calcined samples were recorded on a Stoe STADI P θ-θ X-ray diffractometer in reflection geometry (Bragg-Brentano) using Cu KR1+2 radiation with secondary monochromator and a scintillation detector (MPI fu¨r Kohlenforschung, Mulheim an der Ruhr, Germany). The high-resolution XRD patterns were collected at the BESSY synchrotron line in Berlin, Germany. Powder XRD structure modeling was performed by using the continuous density function (CDF) technique developed by Solovyov et al.31 in which the density distribution in the materials is simulated by a flexible analytical continuous function having adjustable parameters. High-resolution nitrogen (77.4 K) and argon (77.4 and 87.3 K) adsorption/desorption isotherm measurements were performed with an Autosorb-1MP adsorption instrument (Quantachrome Instruments) in a relative pressure range from 1 × 10-6 to 1. Prior to measurement, the samples were outgassed at 150 °C overnight under turbomolecular pump vacuum. The Brunauer-Emmett-Teller (BET) equation was used to calculate the surface area SBET from adsorption data obtained at P/P0 between 0.05 and 0.2. Pore size analysis was performed by applying proper nonlocal density functional theory (NLDFT) methods with either N2 or Ar as the adsorptive, as implemented in Quantachrome’s data reduction software version 1.55. Results and Discussion Synthesis and Structural Characterization of KIT-6 Silica. The synthesis method that we employed to prepare KIT-6 silica is based on using 1-butanol as a costructure-directing agent mixed with P123 under mild acidic aqueous conditions. TEOS is used as the silicon source. After addition of TEOS to the synthesis mixture which is held precisely at 35 °C in a closed polypropylene bottle, a precipitate forms within 3 h under vigorous stirring. This mixture is left under stirring 24 h in total and then aged for 24 h at a given temperature (ranging from 50 to 130 °C), after which the solid is recovered, dried, and finally calcined under air. It is worth mentioning that this method offers some advantages over other procedures reported for the preparation of large pore cubic Ia3jd silica.32 In particular, these conditions afford excellent reproducibility, high structural order, mesophase purity, and easy scale-up. Most importantly, both the addition of 1-butanol (BuOH) and the use of low acid concentrations (120 °C), main mesopores >9 nm, wall thickness 2 nm, no micropores. Scheme adapted from Galarneau et al. (ref 45).
than that in a system consisting of independent mesopores (e.g., SBA-15) of a similar size (see Figure 8a). In agreement with this are earlier experimental results obtained for MCM-48 silica compared to MCM-41.48a Although the desorption branches of the KIT-6 and SBA-15 samples shown in Figure 8a totally overlap, a smaller hysteresis loop for KIT-6 might also be explained if one would assume that in reality the mean pore size of KIT-6 is smaller than that of SBA-15 (this would mean that methods based on the single pore model are in a strict sense not applicable to pore networks). However, such an assumption is not supported by the excellent agreement observed for KIT-6 between XRD pore size analysis and NLDFT pore size obtained from the desorption branch, which reflects the equilibrium vapor-liquid transition (as discussed before). This is also in line with an earlier work46b in which it was shown that the NLDFT method allows one to obtain correctly the mean pore size of MCM-48 from reversible nitrogen sorption measurements. Furthermore, one has to keep in mind that the SBA-15 samples exhibiting 3-D mesopore connectivity (samples aged at high temperatures, i.e., >100-120 °C, with pore diameter >8-9 nm) show an identical hysteresis behavior as KIT-6, in contrast to SBA-15 silica with a pseudo-one-dimensional mesopore structure (standard SBA-15 silica3 aged at temperatures e100 °C). This again would suggest that the effect of interconnectivity is related to the observed reduction of the width of hysteresis, supporting the idea that initiated condensation is the underlying cause of the observed smaller width of hysteresis (i.e., metastability associated with pore condensation is less pronounced in an interconnected system as compared to a system with independent single pores).
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J. Phys. Chem. C, Vol. 114, No. 20, 2010 9353 width of hysteresis loop. However, when the connections between the channels are mesoporous, and are of the same or similar size as the main pore channels,7 one observes a reduction in the width of the hysteresis loop (as compared to a system of independent pores) in line with the possibility of initiated capillary condensation (as discussed before). Hence, the situation depicted then in Figure 13c corresponds to an interconnected mesopore system with well-defined pores (without constrictions) as can be found for materials such as MCM-48 silica, KIT-6 silica, and the large-pore SBA-15 materials prepared with high aging temperatures (e.g., g130 °C). Conclusions
Figure 13. Simplified schemes representing pore condensation and hysteresis (type H1) behavior in single pore systems and ordered pore networks: (a) true independent cylindrical pore system (e.g., MCM41-type); (b) interconnected cylindrical channels, with bridges of too small sizes to influence hysteresis behavior (standard SBA-15 conditions, e.g., aging temperature
