Diffusion Properties of Hexane in Pseudomorphic MCM-41

Article pubs.acs.org/JPCC

Diffusion Properties of Hexane in Pseudomorphic MCM-41 Mesoporous Silicas Explored by Pulsed Field Gradient NMR Ziad Adem,† Flavien Guenneau,*,† Marie-Anne Springuel-Huet,† Antoine Gédéon,† Julien Iapichella,‡ Thomas Cacciaguerra,‡ and Anne Galarneau‡ †

UMR 7574, Chimie de la Matière Condensée de Paris, UPMC Univ Paris 06 and CNRS, F-75005, Paris, France UMR 5253 CNRS/UM2/ENSCM/UM1, ENSCM, Institut Charles Gerhardt Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France

‡

S Supporting Information *

ABSTRACT: Pulsed field gradient (PFG) NMR is a powerful tool to examine diffusion of adsorbates in porous systems. The use of mesoporous silicas with uniform particle sizes allowed us to demonstrate the possibilities of this technique. In particular, we confirmed that, in the Mitra mathematical approach of diffusion, the surface-to-volume ratio is related to the geometry of the whole particle and not of a single pore. Hexane diffusion measured by PFG-NMR was efficient to study innovative materials like pseudomorphic MCM-41 mesoporous silicas presenting different pore topologies. The thorough analysis of the diffusion data allows monitoring the extension of the restricted diffusion domain. This method gives quantitative information on diffusion processes in bimodal pore systems and permits to gain insight into the internal structure of the pseudomorphic materials at different synthesis times. For a simple pore geometry, it is observed that the diffusion coefficient increases with the pore size. However, when materials possess a bimodal pore system (as for the intermediate materials of the pseudomorphic transformation), the diffusion can either decrease or increase depending on the connectivity of the secondary large mesopores with the main mesoporous channels. By PFG-NMR it was possible to detect the rearrangement of the mesoporous network of MCM-41 with synthesis time and to confirm the time necessary for the ordered mesoporous channels of MCM-41 to run through the whole particle. This type of measurement can nicely complement usual characterization techniques (N2 adsorption, SEM, TEM, etc.) in order to give a better picture of diverse porous materials.

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INTRODUCTION Ordered mesoporous silicas such as micelle-templated silicas1−4 (MTS) possess unique textural properties in addition to their high surface area: narrow mesopore size distribution, controlled pore size, and connectivity make them particularly suitable for chromatographic applications (size exclusion chromatography, high performance liquid chromatography (HPLC), and capillary gas chromatography). In these applications, the particle morphology has to be tailored at the micrometer scale with a homogeneous distribution of particle size to ensure a good packing of the columns and a better efficiency of the stationary phase. The preparation of ordered mesoporous silicas of MCM-415 or MCM-486 type structure in the form of discrete monodisperse spheres with tunable particle sizes7 has been achieved through the concept of pseudomorphic transformation.6−10 Pseudomorphism is well-known in the mineral world. It enables the preparation of a mineral with a morphology not related to its crystallographic symmetry group. The resulting mineral takes the outward crystal habit of a different mineral. This principle occurs at nonconstant matter content, by using a mineralization solution that © 2012 American Chemical Society

exchanges anions (or cations) with an existing (preshaped) solid body and allows the new structure to precipitate while maintaining the existing morphology. The concept of pseudomorphic transformation has been successfully applied to amorphous preshaped silica particles6−10 and monoliths11 to produce MTS with the same morphology, using alkaline solution to dissolve the silica and reprecipitate it around surfactant micelles into the ordered MTS structures. This concept uses a kinetic control of the dissolution of the silica habit (particle or monolith) and its reprecipitation in the presence of a surfactant. The entire dissolution/reprecipitation process occurs inside the habit and transforms a particle or a monolith of amorphous silica into an ordered mesoporous one. Spherical particles of 5, 10, 50, and 800 μm of MTS have been obtained according to this procedure.7 MTS with particle sizes of 5 and 10 μm have been successfully used as chromatography supports in HPLC,7,8,10 a very demanding application in terms Received: November 3, 2011 Revised: May 20, 2012 Published: May 21, 2012 13749

dx.doi.org/10.1021/jp210577t | J. Phys. Chem. C 2012, 116, 13749−13759

The Journal of Physical Chemistry C

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Table 1. Textural Properties of Silica Particles and Their Pseudomorphic Transformation into MCM-41 as a Function of Synthesis Time sample

synthesis time

synthesis temp (°C)

Nucleosil Nuc-6h Nuc-1d Nuc-4d LiChrospher LiC-1d LiC-3d LiC-4d Sylopol Syl-6h Syl-16h Syl-4d Syl-6d

0 6h 1 day 4 days 0 1 day 3 days 4 days 0 6h 16 h 4 days 6 days

0 90 115 115 0 115 115 115 0 115 115 115 115

particle diam (μm)

V (mL/g)

SBET (m2/g)

DBdB (nm)

± ± ± ± ± ± ± ± ± ± ± ± ±

1.10 0.63−0.99 0.91 1.01 0.77 0.70 0.75 0.90 1.65 0.71 0.89 0.86 0.80

310 650 1030 1140 750 831 936 1030 329 962 1063 1033 870

18.0 3.6−28.0 3.6 3.9 6.0 3.6 3.6 3.9 18.0 3.7 3.7 3.7 3.7

4.5 4.5 4.4 4.4 10 10 10 10 50 50 50 50 50

1.5 1.5 1.5 1.5 4 4 4 4 10 10 10 10 10

Figure 1. (left) Schematic representation of the stirring system used for pseudomorphic transformation without shearing. (right) SEM pictures of initial parent silica and their corresponding MCM-41 pseudomorphs.

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of particle size and morphology, which demonstrated that homogeneity in mesopore size and high connectivity enhance mass transfer leading to higher column efficiency at higher linear velocity, suitable for fast chromatography. Considering the foreseen applications, the mass transfer properties of these materials are of considerable importance. Pulsed field gradient (PFG) NMR has proved to be a successful tool to study the diffusion of various molecules in microporous materials.12 Later on, M41S materials have also been studied by this technique.13−15 Since then, several papers were devoted to the characterization of the diffusion mechanisms inside MCM41 mesopores.16−24 Recently PFG-NMR has been employed to investigate more complex porous materials, presenting both micro- and mesoporosity25 or consisting of zeolites intergrowth.26 The aim of this work is to show that this technique can be used to characterize the diffusion properties and the porous network of pseudomorphic MCM-41 at different synthesis stages.

EXPERIMENTAL METHODS

Materials. MCM-41 type spheres were synthesized by reacting various preshaped amorphous porous silicas in alkaline solutions containing the surfactant. The solutions were prepared from cetyltrimethylammonium bromide (CTAB, Aldrich), NaOH (SDS), and deionized water. The different sources of silica used in these pseudomorphic synthesis consisted of different porous silicas such as Sylopol (renamed recently Davicat 1700 by the supplier Grace Davison) featuring spheroidal particles of 50 μm and spherical bead materials traditionally used as chromatographic supports: Nucleosil 100-5 (5 μm) (Macherey-Nagel) and LiChrospher 60 (10 μm). The textural characteristics of the precursor silicas and their pseudomorphs as a function of synthesis time and temperature are given in Table 1. The reaction mixtures were prepared in stainless-steel reactors at 55 °C under stirring with an endless screw stirrer (Figure 1) to get a homogeneous solution without 13750



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adsorption capacity of n-hexane in the MCM-41 mesopores of the solid. Diffusion Measurements by PFG NMR. For diffusion studies we chose to adsorb an amount of hexane corresponding to 40% of the total adsorption capacity of the mesopores. The desired adsorbate quantity was obtained by introducing the required pressure in a calibrated volume of the adsorption apparatus. This amount was subsequently trapped in the NMR sample tube containing the solid by freezing. Before PFG-NMR measurements, samples were kept at room temperature for at least one night to guarantee total adsorption equilibrium. The PFG experiments were run on a 300 MHz Bruker DSX spectrometer equipped with a Diff30 probe delivering a maximum gradient of 12 T m−1. Following our previous results on diffusion in crystals of NaX zeolites,29 the bipolar 13-interval pulse sequence30 was chosen to avoid any effect arising from the presence of internal field gradients. The sequence is

shearing to avoid particle damage. The silica source was added to the solution to give gel mixtures with the molar composition 1 SiO2/0.10 CTAB/0.25 NaOH/50 H2O, and the slurry was stirred for 1 h at 55 °C and 6 h at 90 °C. Gels were then autoclaved at 115 °C for various synthesis times, i.e., between 1 and 6 days, under static conditions. The materials were recovered by filtration, washed with water, oven-dried at 80 °C for 24 h, and calcined under air at 550 °C for 8 h. Characterization. Nitrogen adsorption/desorption isotherms of materials were measured at 77 K using a Micromeretics ASAP 2010 instrument. Before measurements, samples were outgassed at 250 °C until stable static vacuum of 3 × 10−3 Torr was reached. Mesopore diameters were calculated from the desorption branch of the nitrogen isotherms by the Broekhoff and de Boer (BdB) method,27 which has been shown to provide reliable results for MCM-41 materials.28 Particle morphology was studied using a Hitachi S-4500 I scanning electron microscope (SEM). Structural evolution of the silica particles during the pseudomorphic synthesis was followed using transmission electron microscopy (TEM). TEM micrographs were recorded in transmission mode on a JEOL 1200 EX II microscope operating at 120 kV. The silica particles were trapped in a resin (LR White) and cut into slices (70 nm thick) by ultramicrotomy. Before adsorption, samples were dehydrated at 400 °C under vacuum. Hexane adsorption isotherms were measured at 25 °C using a homemade adsorption equipment. As an example, the isotherms of the completely transformed materials, LiC-4d and Syl-4d, are given in Figure 2, but all isotherms have the same

Figure 3. 13-interval bipolar spin echo sequence.

schematically presented in Figure 3. In this case, the echo attenuation, Ψ = I/I0, takes the form (adapted from ref 30) ⎡ ⎛ τ δ ⎞⎤ Ψ = exp⎢ −γ 2δ 2g 2Deff ⎜Δ + − ⎟⎥ ⎝ ⎣ 4 6 ⎠⎦

(1)

where Deff is the effective diffusion coefficient, Δ is the observation time, i.e. the time interval between the gradient pulses, γ is the gyromagnetic ratio, δ denotes the duration of the gradient pulses, and g is the gradient strength. τ is the time interval between the last radio-frequency pulse and the spin echo (echo time). The pulse duration, δ, was set to 200 μs and the echo time τ to 5.6 ms. The observation time was varied from 6 to 12 ms. The intensity of the applied gradients was set between 0 and 12 T m−1. The duration of the π/2 radiofrequency pulse was 10 μs. All the experiments were performed at 25 °C Typically, Deff is obtained from the slope of the echo attenuation curves ln Ψ = f(g2) which correspond to the linear transformation of eq 1. For molecules diffusing in a homogeneous medium during Δ, Deff is invariable and represents the genuine diffusion coefficient D0. In the presence of restricted diffusion molecules are, at least partially, reflected by the diffusion barriers, and Deff decreases with the observation time. For a totally reflecting interface, following the mathematical treatment of Mitra et al.,31−33 the first-order approximation leads to a linear dependence of Deff with Δ1/2:

Figure 2. Hexane adsorption isotherms of LiC-4d (squares) and Syl4d (triangles) pseudomorphic materials.

shape. In the Henry region (P < 3 kPa), the two solids adsorb the same quantity. This is in agreement with the comparable BET surface area for the two solids. For pressures around 3− 3.5 kPa, n-hexane condenses in the MCM-41 mesopores. At higher pressure, the two isotherms exhibit a plateau corresponding to the complete filling of these mesopores with liquid alkane. One can notice that, in the pressure range investigated (0−12.2 kPa), only the MCM-41-type mesopores are filled with n-hexane. In particular, the larger secondary pores existing in Syl-4d are not filled at P = 12.2 kPa. The pressure has to come near the saturation vapor pressure of nhexane (P0 = 20 kPa) to observe the condensation of n-hexane in very large pores. As a consequence, the extrapolation of the linear part of the plateau to the ordinate axis gives the total

Deff (Δ)/D0 = 1 −

S D0Δ 9 πV 4

(2)

where S is the surface area and V the volume of the restricting domain. 13751



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Figure 4. Nitrogen adsorption/desorption isotherms at 77 K of Nucleosil 100, LiChrospher 60, Sylopol, and their pseudomorphs as a function of synthesis time.

Figure 5. TEM pictures of slices of initial silica particles of Nucleosil 100.

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RESULTS AND DISCUSSION According to mineralogists, a phase transformation is termed pseudomorphic when it does not modify the macroscopic morphology of a material.34−36 We have introduced the term in the field of mesostructured materials to define the transformation of amorphous silica beads into silica−surfactant mesostructures with preservation of the shape and size of the beads. The process of self-assembly between the surfactant molecules and the silicates occurs inside the voids of the nanoparticles composing the silica micrometric particle. The process is controlled by the dissolution of the amorphous silica nanoparticles, which provides the inorganic component to the hybrid mesostructure and requires that the pore volume of the parent silica source is large enough to accommodate the volume expansion resulting from the phase transformation into MTS. SEM images illustrate the morphology preservation characteristic of the pseudomorphic process (Figure 1). The pseudomorphic transformation of the three silica sources has been followed as a function of time by nitrogen sorption at 77 K (Table 1, Figure 4, and Figure S1) and by X-ray diffraction (Figure S2). After few minutes at 90 °C, the signature of the initial porosity of the parent silica materials has disappeared and is replaced by a bimodal porosity. In addition to the mesostructured porosity centered at 3.8 nm, the nitrogen

adsorption/desorption data of the early products reveal the presence of pores larger than those present in the parent silica. For Nuc-6 h and Syl-6h (obtained from Nucleosil and Sylopol after 6 h of synthesis) the secondary pore sizes are centered at about 30 and 40 nm, respectively. For LiC-1d (obtained from LiChrospher after 1 day of synthesis), these large pores are not clearly evidenced by the nitrogen isotherm (no vertical hysteresis loop), but their presence is reflected by the horizontal hysteresis loop for 0.43 < p/p0 < 1. This type of hysteresis loop is also observed for Nuc-6 h in addition to the vertical hysteresis loop. It is characteristic of large pores embedded inside the particles and connected to the exterior through the smaller MCM-41 mesopores. Therefore, nitrogen condensed in the pores during adsorption is desorbed through the pores of MCM-41 by a cavitation process which occurs classically at a pressure of p/p0 = 0.47. For Syl-6h, this type of horizontal hysteresis does not exist; the vertical hysteresis loop at high pressure (p/p0 > 0.9) is characteristic of the presence of large secondary pores directly connected to the exterior of the particles. In this case nitrogen desorption follows the retraction of the meniscus (Kelvin law) without any cavitation. With increasing synthesis time the signature of this secondary porosity progressively disappears. Depending on the silica source, it takes several days (1 day for Nucleosil, 4 days for LiChrospher and Sylopol) to obtain a nitrogen isotherm of 13752



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Figure 6. TEM pictures of a slice of particle of complete pseudomorphic transformation into MCM-41 type material of Nucleosil 100 (Nuc-1d). Inset: enlargement of the picture.

MCM-41 type material with only the typical micelle-templated pores at 3.8 nm (sharp step in adsorption at p/p0 = 0.37). The pseudomorphic transformation is complete at that time. All final MCM-41 type materials feature the same textural properties: 1000 m2 g−1, 0.9 mL g−1, 3.8 nm pore diameter. The pseudomorphic transformation is a slow and progressive process. The total transformation of the Nucleosil particle is confirmed by comparing TEM of the entire particle of Nucleosil (Figure 5) and its pseudomorph after 1 day, Nuc1d (Figure 6). It is to notice that marks visible every ∼300 nm on the particles are due to the ultramicrotomy knife and are not related to the material texture. Nuc-1d reveals a homogeneous mesostructure of worm-like type in the whole particle after 1 day of pseudomorphic transformation (Figure 6). For LiChrospher, after 1 day of pseudomorphic transformation, TEM of LiC-1d (Figure 7) shows a homogeneous particle, with additional large secondary pores of 30−40 nm (Figure 8) homogeneously distributed in the whole particle. This confirms the assumption suggested by nitrogen isotherm of the presence of larger pores in addition to textural MCM-41 pores for LiC1d. After 4 days of pseudomorphic transformation, TEM of LiC-4d does not show this secondary porosity anymore, and the whole particle is homogeneously formed of MCM-41-like porosity having a worm-like porosity arrangement (Figure 8). With increasing synthesis time, the secondary porosity is progressively replaced by the mesostructured porosity as exemplified in Figure 8. For Sylopol, TEM images suggest a different mechanism of pseudomorphic transformation compared to Nucleosil and LiChrospher silicas. The large secondary porosity (∼40 nm) leading to a hysteresis loop at high pressure in nitrogen isotherm is clearly observed into the TEM picture of Syl-6h (Figure 9). But after 6 days of pseudomorphic transformation, instead of disappearing and being transformed into mesostructured porosity, this secondary porosity increases in size. Since Sylopol silica features an average particle size of 50 μm, it is difficult to observe by TEM the whole particle and

Figure 7. TEM pictures of a slice of particle of pseudomorphs of LiChrospher 60 after 1 day (LiC-1d) and 4 days (LiC-4d) of synthesis.

the mesoporosity at the same time. In some of the smallest particles of Syl-6d (Figure 10), the inside of the particle has been dissolved giving rise to hollow particles with a gradient of density of a mesostructured phase. The network is denser at the border of the particle and becomes less and less dense when progressing toward the hollow center of the particle. In summary, a schematic representation of the different pseudomorphic transformation processes is proposed in Figure 11. In the fast initial step of the transformation, the dissolution process of the primary nanoparticles and the reprecipitation into mesostructured silica of MCM-41 type in their interparticular voids leads to a particle with two porosities homogeneously distributed in the whole particles: an ordered one due to the surfactant at 3.8 nm and a disordered one around 30−40 nm. In the slower second step, the restructuration of the initial MCM-41 type material with 13753



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Figure 8. Enlargement of TEM pictures of a slice of particle of pseudomorphs of LiChrospher 60 after 1 day (LiC-1d), 3 days (LiC-3d), and 4 days (LiC-4d) of synthesis. Inset: enlargement of the picture of LiC-4d.

Figure 9. TEM pictures of a slice of particle of pseudomorphs of Sylopol after 6 h (Syl-6h) and 6 days (Syl-6d) of synthesis. Inset: enlargement of the picture of Syl-6d.

silicates in solution. In the case of Sylopol (large particle size), the initial pore volume is higher than the final pore volume of MCM-41, and the subsequent evolution proceeds differently. The created secondary porosity is not embedded inside the particle and can communicate with the exterior. In this case, only the second redissolution process seems to occur, and silicates migrate to the exterior of the particle instead of

thick walls occurs and leads to MCM-41 type materials with thinner walls featuring larger pore volume and surface area (Table 1). This slow secondary evolution consists in the filling of the secondary porosity until the complete transformation of the particles into MCM-41 with thinner walls. This corresponds to an equilibrium between the porous particle and the external solution media, controlled essentially by the pH and the 13754



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Figure 10. TEM pictures of a slice of particle of pseudomorph of Sylopol after 6 days (Syl-6d) of synthesis.

purely in the intraparticle space during the whole NMR pulse sequence. On this regard PFG NMR measurements benefit from the large size of the pseudomorphic particles. For all diffusion experiments n-hexane, featuring a kinetic diameter of 0.43 nm, was chosen as a probe molecule. Before exploring the pseudomorphs, the parent silicas were first examined. For Nucleosil possessing a 5 μm particle size, the n-hexane diffusion is too fast considering the experimental observation times attainable in PFG-NMR (more than 6 ms in our case). Consequently, pseudomorphs of Nucleosil could not be studied by this method since most of the diffusing molecules leave the intraparticle space during the measurement. This leads to a fast signal attenuation essentially characteristic of long-range diffusion in the interparticle space. For LiChrospher and its MCM-41 pseudomorphs (LiC-1d and LiC-4d), possessing 10 μm particle size, the spin echo attenuation ln Ψ = f(g2) does not follow the simple expression of eq 1 (see for example Figure 12A corresponding to LiC-4d). Indeed, for molecules diffusing in several domains, the signal attenuation is described by a superposition of exponentials:

Figure 11. Schematic representation of pseudomorphic transformation with time of synthesis depending on the initial silica parent particles.

reprecipitating with the surrounding surfactants. For large particles, the CTAB diffusion may not be fast enough to reach the interior of the particle before the precipitation of the mesostructured silica occurs. Therefore, the silica in the center of the particle interacts with fewer surfactant molecules. It is consequently less stabilized by the surfactant and migrates in the solution, creating hollow spheres. The pseudomorphic synthesis is well apprehended by this combined analysis of nitrogen isotherms and TEM micrographs of the whole particles. However, since these materials are suitable for chromatographic or catalytic supports, an important question is also the role that the type of porosity (pore size, pore connectivity, multimodal pore structure) plays in the mass transfer of molecules inside the particles. To complement their characterization and to quantify their diffusion properties, the PFG NMR technique was applied to the different silica particles obtained by pseudomorphic synthesis. The diffusion of guest molecules followed by PFG NMR became a powerful method for the exploration of many porous materials. In order to quantify the diffusing properties of porous media, one must be able to observe some molecules diffusing

⎡

n

Ψ(Δ) =

⎛

∑ pi exp⎢⎣−(γδg )2 Di⎝⎜Δ + i=1

τ δ ⎞⎤ − ⎟⎥ 4 6 ⎠⎦

(3)

where pi denotes the relative number of molecules having a diffusion coefficient Di. From Figure 12A one can suspect two distinct components, i.e. n = 2 in eq 3. Since the diffusion rates appear to be sufficiently separated, two independent linear relationships were used to describe the diffusion data. The initial steep decay is attributed to molecules leaving the mesoporous particles and diffusing rapidly in the interparticle space. The second part of the attenuation represents the diffusion of n-hexane strictly confined in the mesoporous network. The linear analysis allows us to calculate the associated effective intraparticle diffusion coefficient Deff. The measured diffusivities exhibit a dependence on the observation time characteristic of restricted diffusion (Figure 12B). Such a behavior has already been observed for a hexane/MCM-41 system with uncontrolled shape and particle size (7 μm on average).15,20,21 An analysis of their data by the 13755



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× 10−10 m2 s−1) fairly due to the larger pore size of LiChrospher with pores of 6 and 3.9 nm, respectively. However, the diffusion of hexane in the intermediate pseudomorph LiC-1d (Dintra = 3.5 × 10−10 m2 s−1) featuring a double mesoporosity (3.6 nm and secondary pores of 40 nm) is also slower than diffusion in LiChrospher and in pure MCM41 phase, whereas larger pores exist in the particle. These findings confirm the results obtained by chromatography using Van Deemter curves as diffusion analysis, which revealed easier mass transfers in porous materials with larger and more homogeneous pores.7,10 In LiC-1d there is a succession of small mesopores (3.6 nm) connected to cavities (40 nm), which is apparently not suitable for a fast diffusion even if these two types of pores are homogeneously distributed in the whole particle. The S/V ratios obtained from the analysis of the PFG data according to eq 2 allow us to follow the extent of MCM-41 structured region inside the particles as a function of synthesis time. If one makes the crude assumption of spherical domains of restricted diffusion, and takes into account the porosity ϕ of the particles, the surface-to-volume ratio becomes (S/V)PFG = 3/(ϕRPFG). The ϕ parameter is deduced from the total pore volume V measured by N2 adsorption and the density ρS of amorphous silica (2.2 g cm−3): ϕ = (ρS − ρ)/ρS with 1/ρ = 1/ ρS + V. The RPFG value obtained for the starting amorphous silica is in fair agreement with the measured mean radius of the particles (Tables 1 and 2). For LiChrospher (10 μm particle size) after 1 day of pseudomorphic transformation, the diffusion coefficient decreases by almost a factor of 2, which could be easily explained by the presence of the combined larger and smaller pores, as mentioned above. But more remarkably, the RPFG value is also divided by 2, confirming that some restrictions appear in the internal structure of the particles at the early stages of the pseudomorphic transformation. However, no precise significance can be assigned to the value of RPFG due to the very simplistic assumptions used to describe the geometry of the domains of restricted diffusion. For the complete pseudomorphic transformation into MCM-41 type porosity (LiC-4d), the long-range order of the material is nicely confirmed by PFG measurements since RPFG coincides again with the radius of the particle, within the experimental error. The same analysis as for LiChrospher was applied to obtain the genuine intraparticle diffusion coefficient of hexane in Sylopol (Table 3). The value of 50 × 10−10 m2 s−1 is 10 times higher than in LiChrospher, which can be easily explained by the larger pore of Sylopol (18 nm compared to 6 nm). The deduced radius RPFG (23.1 μm) for Sylopol is in accordance with the diameter of the particle (50 μm). This nicely confirms that the outer surface is the source of the observed restricted diffusion in particles of mesoporous materials. For MCM-41 pseudomorphs of Sylopol (Syl-6h and Syl-4d), the attenuation curves (Figure 13A) exhibit a more complex behavior than for LiChrospher and require a three-component nonlinear fit (eq 3 with n = 3) of the whole data in order to extract all diffusion coefficients. This “three region” approximation can appear as quite simplistic since the TEM pictures show that the system is clearly more complex, and one should only consider it as a canvas used to describe the evolution of Sylopol pseudomorphic materials with synthesis time. Apart from the very fast component attributed to the molecules leaving the particles, two distinct effective intraparticle diffusion coefficients Deff(1) and Deff(2) are obtained. From their dependence on the observation time (Figure 13B,C) one can obtain the genuine intraparticle diffusion coefficient for the fast

Figure 12. Spin-echo attenuation curve for Δ = 9.5 ms (A) and effective intraparticle diffusivities (B) for n-hexane adsorbed in the pseudomorph of LiChrospher 60 after 4 days of synthesis (LiC-4d).

Mitra formalism leads to a surface-to-volume ratio (S/V) which is 3 orders of magnitude lower than expected from N2adsorption measurements. The authors attributed the restriction to the curvature of the channels in the spiral particles resulting from the acidic synthesis of MCM-41, and therefore the S/V ratio is not related to the mesopore diameter, but rather to the particle dimensions. For our materials the MCM41 type porosity is obtained from a basic synthesis, and the mesoporous channels do not show any detectable curvature. The restricted diffusion of hexane molecules revealed by the PFG experiments can originate from the interface between the intra- and interparticle volumes which acts as a partially reflecting boundary. More exactly, for the molecules leading to the second part of the attenuation curves this interface acts as a totally reflecting boundary, and Deff should follow eq 2. Thus, the genuine diffusion coefficient, D0, is obtained from a linear fit of the curves Deff = f(Δ1/2). It corresponds to diffusion of hexane molecules in the interior of the porous particles and is generally referred as Dintra, the intraparticle diffusion coefficient. For LiChrospher and its MCM-41 pseudomorphs similar diffusion coefficients are obtained (Table 2) with values between 3.5 × 10−10 and 5.9 × 10−10 m2 s−1. By comparing the diffusion coefficients, it appears that diffusion is slightly faster or easier in the parent LiChrospher (Dintra = 5.9 × 10−10 m2 s−1) than in its MCM-41 pseudomorph LiC-4d (Dintra = 4.1 Table 2. Intraparticle Diffusion Coefficient (Dintra) and Radius of the Restricted Diffusion Domains (RPFG) for 10 μm Diameter Silica Particles LiChrospher 60 and Its Pseudomorphic Transformation into MCM-41 as a Function of Synthesis Time

samples

pore diam (nm)

secondary pore diam (nm)

porosity ϕ

Dintra (m2 s−1)

RPFG (μm)

LiChrospher 60 LiC-1d LiC-4d

6.0 3.6 3.9

no 40 no

0.63 0.61 0.66

5.9 × 10−10 3.5 × 10−10 4.1 × 10−10

5.8 3.9 6.4 13756



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Table 3. Intraparticle Diffusion Coefficient (Dintra) and Radius of the Restricted Diffusion Domains (RPFG) for 50 μm Diameter Silica Particles Sylopol and Its Pseudomorphic Transformation into MCM-41 as a Function of Synthesis Time samples Sylopol Syl-6h Syl-4d

pore diam (nm) 18.0 3.7 3.7

secondary pore diam (nm) no 20−100 >100

porosity ϕ

Dintra (1) (m2 s−1) −10

50 × 10 90 × 10−10 87 × 10−10

0.78 0.61 0.64

RPFG (1) (μm) 23.1 19.9 18.1

Dintra (2) (m2

−1

no 15 × 10−10 8.3 × 10−10

)

RPFG (2) (μm) no 7.6 7.2

large pore systems there is no evolution of the diffusion parameters for pore diameters above 20 nm. In the case of Sylopol pseudomorphic transformation, our PFG NMR data do not seem to be sensitive to the appearance of large micrometer sized pores. Hexane molecules passing through this region probably contribute to the fast decay component. Also, the RPFG(1) radius settles around 18 μm even after 4 days. For hexane diffusion in MCM-41 pores (region 2), the intraparticle diffusion Dintra(2) decreases from 15 × 10−10 to 8.3 × 10−10 m2 s−1 after 4 days of pseudomorphic transformation. It seems that the denser part of the mesoporous region is growing during synthesis via the same phenomenon as the one described before, namely the transformation of MCM41 with thick walls into MCM-41 with thinner walls, leading to an occupation of a larger volume of the particle corona. MCM41 type porosity formed at the end of the transformation, in equilibrium with the solution, has the same characteristics as the porosity of the other materials coming from smaller particle size. It possesses a pore volume around 0.9 mL g−1, a surface area around 1000 m2 g−1, and a pore size centered on 3.8 nm (Table 1). The estimated RPFG(2) radius remains the same around 7 μm after 4 days. Again, RPFG has no quantitative significance because of simplistic geometrical assumptions but could correspond in this last case to the corona thickness of MCM-41 type porosity in the hollow spheres. PFG NMR has allowed to show that the lack of homogeneity of mesopores in comparison to cylindrical mesopores leads to a slightly lower diffusion (as observed for LiC-1d presenting pores of 3.6 nm (MCM-41) and embedded cavities of 40 nm (3.5 × 10−10 instead 4.1 × 10−10 m2 s−1)). Such cavities increase the tortuosity of the materials by creating some “stagnation zones” in the pore in comparison to straight channels. In opposite for a material presenting pores of MCM-41 (3.6 nm) and connected pores of 20−100 nm directly connected to the exterior of the particles (as observed for Syl-4d or 6d), the “stagnation zones” are not present, and the secondary large pores will help to enhance the diffusion in the mesopore (from 4.1 × 10−10 to 8 × 10−10 or 15 × 10−10 m2 s−1) as “highways” connected “main roads” in opposite to “main roads” containing “traffic circles” (Figure 14). This observation is a proof that diffusion in porous system is controlled by the smaller pores in the material (as large pore only gives a diffusion coefficient larger than 50 × 10−10 m2 s−1) and that enhancement of diffusion is possible if secondary larger pores are created in the materials but only if they are not embeded in the structure (characterized by a horizontal hysteresis loop in nitrogen sorption isotherm), which means that they are connected to the exterior of the particles (characterized by a vertical hysteresis loop in nitrogen sorption isotherm). The diffusion of hexane followed by PFG NMR is a powerful tool as it allows to highlight and quantify the difference in diffusion for different bimodal materials and show that hierarchical materials (Figure 14c) lead to a real increase in diffusion rates, which is not the case for every bimodal materials. It will depend on the type of secondary larger pores.

Figure 13. Spin-echo attenuation curve for Δ = 9 ms (A) and effective intraparticle diffusivities in region 1 (B) and region 2 (C) for n-hexane adsorbed in the pseudomorph of Sylopol after 4 days (Syl-4d) of synthesis.

component Dintra(1) (90 × 10−10 m2 s−1) comparable to the one of Sylopol (50 × 10−10 m2 s−1) and the slower component Dintra(2) (15 × 10−10 and 8 × 10−10 m2 s−1 for Syl-6h and Syl4d, respectively). These two distinct diffusions are attributed to two different regions (regions 1 and 2) inside the particles. The value of (8−15) × 10−10 m2 s−1 for Dintra(2) is in fair agreement with our results on Licrospher MCM-41 pseudomorphs and with previously published values for the same adsorbate/ adsorbent system (hexane in MCM-41).15,20,21 Therefore, region 2 corresponds to a MCM-41 type porosity. Region 1 is consequently related to the larger secondary pores formed during the pseudomorphic transformation. For instance, pores between 20 and 100 nm, observed in TEM pictures of Syl-6h, ultimately lead to hollow particles for Syl-4d, with secondary pores larger than 100 nm. The diffusion characteristics of the secondary porosity in Sylopol MCM-41 pseudomorphs, namely Dintra(1), remain unchanged even if their pore size has increased with the synthesis time. It appears that for hexane adsorbed in 13757



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diffusing molecules occurs, at least partially, at the interface between the inside and the outside of the particles but also between regions with different pore structures. Consequently, it was possible to detect the reorganization of the mesoporous network with the increase of the synthesis time. For materials obtained from Lichrospher 60, our findings confirm that, after 4 days of synthesis, the channels run through the whole particle. On the contrary, the pseudomorphic transformation of Sylopol never leads to homogeneous particles, some secondary mesoporosity being evidenced by the diffusion measurements even after 4 days. PFG NMR results can nicely complement other characterization techniques (N2 adsorption, SEM/TEM, etc.) in order to predict and quantify diffusion properties in hierarchical and complex porous materials such as the newly developed mesoporous zeolites.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 14. Schematic representation of MCM-41 materials (3.8 nm pores) presenting (a) no secondary pores, (b) secondary pores in the form of cavities (20 nm), (c) secondary pores (20 nm) connected to the exterior of the particles, and (d) cylindrical 20 nm pores as well as their corresponding diffusion.

Figures S1−S8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail fl[email protected]; Tel +33 1 44 27 36 27; Fax +33 1 44 27 15 04.

CONCLUSION Hexane diffusion measured by PFG NMR is a powerful tool to examine diffusion in porous system. The study of silicas with different particle sizes and different pore sizes and their MCM41 pseudomorphs as a function of time allowed us to demonstrate the possibilities of this technique. It was confirmed that for monomodal porosities the diffusion coefficient increases when the pore size increases: pore diameters of 3.8, 6.0, and 18 nm lead to diffusion coefficients of 4.1 × 10−10, 5.9 × 10−10, and 50 × 10−10 m2 s−1, respectively. The intermediary products of the pseudomorphic transformation feature larger secondary pores than the entirely mesostructured ones and can also be used as models of pore morphology to understand diffusion of molecules. For particles with ordered pores of 3.8 nm, the presence of larger (∼30 nm) pores embedded in their channels leads to some pore blocking and the diffusion coefficient of hexane decreases to 3.5 × 10−10 m2 s−1. This type of secondary pores is visible in nitrogen adsorption/desorption isotherms in the form of a horizontal hysteresis loop at p/p0 > 0.43, where the desorption branch parallels the adsorption branch. Conversely, the presence of larger secondary pores interconnected with the 3.8 nm channels and directly connected to the exterior of the particles increases the molecules diffusion up to 15 × 10−10 m2 s−1. This type of secondary pores is characterized by a vertical hysteresis loop at high pressure (p/p0 > 0.9) in nitrogen isotherms. On the one hand, this study proves that MCM-41 materials are good model materials to understand and verify physical theories. The pseudomorphic MCM-41 materials allow us to show that diffusion in hierarchical materials is controlled by diffusion in the smallest pores. To facilitate diffusion in such systems, it is necessary to add secondary larger mesopores connected to the exterior of the particles. On the other hand, this paper clearly shows the efficiency of PFG NMR to study innovative materials like pseudomorphic mesoporous silicas. This method gives not only information on the diffusion properties but also permits to gain insight into the internal structure of these materials at different stages of synthesis. Indeed, it was clearly evidenced that reflection of

Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; Mc Cullen, S. B.; Higgins, J. B. J. Am. Chem. Soc. 1992, 114, 10834−43. (2) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024−6036. (3) Di Renzo, F.; Galarneau, A.; Trens, P.; Fajula, F. In Handbook of Porous Solids; Schüth, F., Sing, K., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 2002; pp 1311−1395. (4) Sun, Q.; Vrieling, E. G.; Van Santen, R. A.; Sommerdijk, N. A. J. M. Curr. Opin. Solid State Mater. Sci. 2004, 8, 111−120. (5) Martin, T.; Galarneau, A.; Di Renzo, F.; Fajula, F.; Plee, D. Angew. Chem., Int. Ed. 2002, 41, 2590−2592. (6) Petitto, C.; Galarneau, A.; Driole, M.-F.; Chiche, B.; Alonso, B.; Di Renzo, F.; Fajula, F. Chem. Mater. 2005, 17, 2120−2130. (7) Galarneau, A.; Iapichella, J.; Bonhomme, K.; Di Renzo, F.; Kooyman, P.; Terasaki, O.; Fajula, F. Adv. Funct. Mater. 2006, 16, 1657−1667. (8) Martin, T.; Galarneau, A.; Di Renzo, F.; Brunel, D.; Fajula, F.; Heinisch, S.; Cretier, G.; Rocca, J.-L. Chem. Mater. 2004, 16, 1725− 1731. (9) Lefevre, B.; Galarneau, A.; Iapichella, J.; Petitto, C.; Renzo, F. D.; Fajula, F.; Bayram-Hahn, Z.; Skudas, R.; Unger, K. Chem. Mater. 2005, 17, 601−607. (10) Galarneau, A.; Iapichella, J.; Brunel, D.; Fajula, F.; BayramHahn, Z.; Unger, K.; Puy, G.; Demesmay, C.; Rocca, J. L. J. Sep. Sci. 2006, 29, 844−855. (11) Babin, J.; Iapichella, J.; Lefevre, B.; Biolley, C.; Bellat, J.-P.; Fajula, F.; Galarneau, A. New J. Chem. 2007, 31, 1907−1917. (12) Kärger, J.; Ruthven, D. M. Diffusion in Zeolites and Other Microporous Solids; John Wiley & Sons, Inc.: New York, 1992. (13) Kärger, J. GIT Spez. Chromatogr. 1997, 17, 26−28. (14) Kärger, J.; Krause, C.; Schafer, H. Fortschr.-Ber. VDI, Reihe 3 1998, 555, 104−119. (15) Hansen, E. W.; Courivaud, F.; Karlsson, A.; Kolboe, S.; Stöcker, M. Microporous Mesoporous Mater. 1998, 22, 309−320. (16) Valiullin, R.; Dvoyashkin, M.; Kortunov, P.; Krause, C.; Kärger, J. J. Chem. Phys. 2007, 126.

13758



Article

(17) Krause, C.; Stallmach, F.; Hoenicke, D.; Spange, S.; Kärger, J. Adsorption 2003, 9, 235−241. (18) Kärger, J.; Stallmach, F.; Vasenkov, S. Magn. Reson. Imaging 2003, 21, 185−191. (19) Matthae, F. P.; Basler, W. D.; Lechert, H. Stud. Surf. Sci. Catal. 1998, 117, 301−308. (20) Courivaud, F.; Hansen, E. W.; Kolboe, S.; Karlsson, A.; Stöcker, M. Microporous Mesoporous Mater. 2000, 37, 223−232. (21) Courivaud, F.; Hansen, E. W.; Karlsson, A.; Kolboe, S.; Stöcker, M. Microporous Mesoporous Mater. 2000, 35−36, 327−339. (22) Stallmach, F.; Kär ger, J.; Krause, C.; Jeschke, M.; Oberhagemann, U. J. Am. Chem. Soc. 2000, 122, 9237−9242. (23) Gjerdaker, L.; Aksnes, D. W.; Sørland, G. H.; Stöcker, M. Microporous Mesoporous Mater. 2001, 42, 89−96. (24) Stallmach, F.; Graser, A.; Kärger, J.; Krause, C.; Jeschke, M.; Oberhagemann, U.; Spange, S. Microporous Mesoporous Mater. 2001, 44−45, 745−753. (25) Furtado, F.; Galvosas, P.; Gonçalves, M.; Kopinke, F.-D.; Naumov, S.; Rodriguez-Reinoso, F.; Roland, U.; Valiullin, R.; Kärger, J. J. Am. Chem. Soc. 2011, 133, 2437−2443. (26) Menjoge, A.; Bradley, S. A.; Galloway, D. B.; Low, J. J.; Prabhakar, S.; Vasenkov, S. Microporous Mesoporous Mater. 2010, 135, 30−36. (27) Broekhoff, J. C. P.; De Boer, J. H. J. Catal. 1968, 10, 153−165. (28) Galarneau, A.; Desplantier, D.; Dutartre, R.; Di Renzo, F. Microporous Mesoporous Mater. 1999, 27, 297−308. (29) Adem, Z.; Guenneau, F.; Springuel-Huet, M.-A.; Gédéon, A. Microporous Mesoporous Mater. 2008, 114, 337−342. (30) Cotts, R. M.; Hoch, M. J. R.; Sun, T.; Markert, J. T. J. Magn. Reson. 1989, 83, 252−266. (31) Mitra, P. P.; Sen, P. N.; Schwartz, L. M.; Ledoussal, P. Phys. Rev. Lett. 1992, 68, 3555−3558. (32) Mitra, P. P.; Sen, P. N.; Schwartz, L. M. Phys. Rev. B 1993, 47, 8565−8574. (33) Latour, L. L.; Mitra, P. P.; Kleinberg, R. L.; Sotak, C. H. J. Magn. Reson., Ser. A 1993, 101, 342−346. (34) Sinkankas, J. Mineralogy; Van Nostrand Reinhold: New York, 1964. (35) Chesterman, C. W. National Audubon Society Field Guide to North American Rocks and Minerals; Alfred A. Knopf: New York, 1979. (36) Garcia, G. G. Bocamina 1996, 2, 38.

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Diffusion Properties of Hexane in Pseudomorphic MCM-41

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