Characterization of mesoporous silica and its pseudomorphically transformed derivative by gas and liquid adsorption

Microporous and Mesoporous Materials 102 (2007) 111–121 www.elsevier.com/locate/micromeso

Characterization of mesoporous silica and its pseudomorphically transformed derivative by gas and liquid adsorption Julien Iapichella b, Juan-Miguel Meneses a, Isabelle Beurroies a, Renaud Denoyel Zo¨fre Bayram-Hahn c, Klaus Unger c, Anne Galarneau b a

a,*

,

MADIREL, UMR 6121 CNRS/Universite´ de Provence, Centre Saint-Je´roˆme, Avenue Escadrille-Normandie-Niemen, 13397 Marseille Cedex 20, France b Laboratoire de Mate´riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS/ENSCM/UM1, Institut Charles Gerhardt FR 1878, ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France c Institut fu¨r Anorganische Chemie und Analytische Chemie, Johannes-Gutenberg-University of Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany Received 24 October 2006; received in revised form 8 December 2006; accepted 13 December 2006 Available online 21 December 2006

Abstract Pseudomorphism is a term introduced by mineralogist to describe phase transformation that does not change the shape of a material. Pseudomorphic process, assisted by surfactants, for mesoporous pre-shaped silica particles allows to narrow the mesopore size distribution, to increase the surface area and the pore volume without changing the initial shape of silica particles. The textural, chemical and mechanical comparisons between commercial silica LiChrospher 60 (Merck) and its pseudomorphic transformation at constant particle size and morphology opens a unique opportunity to understand the effects of such mesoporosity transformation and judge the performance of both types of materials. On a mechanical point of view, commercial silica and its pseudomorphic transformation are stable until 40 MPa, which is enough to resist to chromatographic column packing. Liquid chromatographic tests reveal that column efficiency and mass transfer properties are maintained during the pseudomorphic transformation. Adsorption from polar probes and thermogravimetric measurements show that the surface chemistry is strongly modified by the pseudomorphic transformation, which is not pseudochemical. The surfactants use to generate well defined mesoporosity induct in one hand a low silanol density and in the other hand a more homogeneous silanol distribution on the silica surface, which opens the route towards a better control of surface functionalization. Moreover, grafted-mesoporous silicas exhibit an extremely high mechanical stability (>280 MPa). Pseudomorphic-silicas are remarkable candidates for applications needing morphology and surface chemistry control.  2006 Elsevier Inc. All rights reserved. Keywords: Pore size; Surface chemistry; Surface modification; Mechanical stability; Chemical stability

1. Introduction Designed adsorbents demand defined properties such as an ordered pore structure, a narrow pore size distribution, an adjusted surface chemistry and an optimum morphology to address successfully specific separation problems. Micelle-Templated Silicas (MTS) feature unique textural properties thanks to their uniform pore size distribution with tuneable sizes in the range of mesopores between 2 *

Corresponding author. E-mail address: [email protected] (R. Denoyel).

1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.12.026

and 20 nm [1–4]. This family of mesostructured silicas have been proposed as stationary phases for various chromatographic applications including gas chromatography [5] and liquid chromatography (HPLC) [6–16]. It was anticipated that their high specific surface area and high pore volume would improve the performances as compared with traditional supports. However, particle shape and size as well as homodispersity play a decisive role in determining column efficiency because particle size scattering can seriously affect separation performance. Galarneau and coworkers [17–21] have developed a new template assisted method to transform amorphous mesoporous silicas into ordered

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materials maintaining the spherical morphology of the parent silica. It is based on the concept of pseudomorphic synthesis and allows a control of morphology from the nano to the microscale. The preparation of discrete spheres without agglomeration makes these supports suitable for column packing and they have been successively used in HPLC in fast separation process [15,16]. In order to better understand the properties of these novel pseudomorphic materials a number of characterization methods in the gas phase and in solution were applied to monitor the changes of physico-chemical properties employing a commercial silica adsorbent as parent material: LiChrospher 60 from Merck with a nominal pore diameter of 6 nm. Pore size distribution and organisation, surface chemistry, hydrolysis stability, mechanical stability, separation performances are compared. The comparison between the amorphous material and its ordered derivative at constant particle size and morphology opens a unique opportunity to judge the performance of both types of materials. 2. Experimental section 2.1. Materials The Micelle-Templated Silica material, named MTS1, was synthesized by reacting the pre-shaped amorphous porous silica, LiChrospher 60 (10 lm) (Merck KGaA, Germany), in alkaline solution containing the surfactant in the presence of swelling agent. The solution was prepared as followed: 33.2 g NaOH (SDS), 400 g deionized water, 60.54 g 1,3,5-trimethylbenzene (TMB, Aldrich), 117.6 g of a low cost surfactant named Noramium MS 50 (CECA ATO), 1336 g deionized water. The reaction mixture was prepared in stainless-steel reactor at 25 C under stirring until a clear solution was obtained. Noramium MS 50 has the following weight composition: 20% water, 30% isopropanol, 50% alkyltrimethyl ammonium chloride containing 30% C16TACl, 70% C18TACl. The silica source (LiChrospher 60) (155.56 g) was then added to the alkaline solution under stirring. The gel mixture features the molar composition: 1 SiO2/0.32 NaOH/0.021 C16TACl/0.047 C18TACl/0.19 TMB/37.7 H2O/0.23 isopropanol. After 1 h stirring at 20 C, the mixture was stirred for 4 h at 100 C. The slurry was then filtered and washed with 6 L of water. The samples were then dried at 100 C overnight and calcined at 550 C for 8 h under air flow. 2.2. Functionalization of silicas LiChrospher 60 and its pseudomorphic derivative MTS1 were functionalized with octyl chains according to an already described procedure [22]. Chlorodimethyloctyl silane was added to a stirred suspension of outgassed silica (180 C for 12 h) in anhydrous refluxing toluene. Five molecules of grafting agent and 5 molecules of pyridine per nm2 of dried silica were added (which corresponds to

6.11 and 8.37 mmol of grafting agent per g of dried silica for LiChrospher 60 and MTS1, respectively). The mixtures were then stirred for 15 h under reflux at 120 C under nitrogen flow. The powders were filtered and washed successively with toluene, acetone, acetone/water, acetone, chloroform, and ether. The samples were dried under vacuum at 120 C overnight. 2.3. Characterization The particle morphology was achieved using a Hitachi S-4500 I scanning electron microscope (SEM). The particle size distribution was determined by laser diffraction (Mastersizer 2000 from Malvern Instrument): 200 mg of sample was added to 10 mL of acetone, ultrasonicated for 30 min, dried, and dispersed into 700 mL of water for analysis. Nitrogen sorption isotherms at 77 K where determined with an ASAP2010 apparatus from Micromeritics. Before measurement, the samples where outgassed under vacuum at 250 C for silicas and 120 C for grafted-silicas for at least 12 h. The BET equation was applied to determine the surface area. Average pore diameters have been evaluated from the nitrogen desorption branch according to the Broekhoff and De Boer (BdB) method, as this method has been demonstrated to be one of the more accurate method to evaluate pore size [23]. The mechanical stability of the samples were determined by preparing different pellets of 8 mm diameter and 1 mm thickness under different pressure with a press classically used in FTIR. Mercury porosimetry experiments were carried out with Micromeritics Autopore 9220 equipment. Samples were evacuated at room temperature. For all samples at least two intrusion– extrusion cycles were carried out. A contact angle of 130 was used to calculate the pore size distribution by the Washburn equation. High resolution thermogravimetry under nitrogen gas flow was used in order to calculate the density of hydroxyls groups existing on the surface of the samples. Assuming that, after outgassing at 150 C, most OH groups are still on the surface, it is possible to derive their amount from the mass loss between 150 and 850 C. The TG analyses were achieved by a TGA Q500 apparatus from TA Instruments. The high-resolution program was followed from 30 to 850 C. 2.4. Gas and liquid adsorption methods LiChrospher 60 and its pseudomorphic MTS1 were evaluated in the separation of diethylphtalate/dibutylphtalate (Acros, 99% purity) using a HPLC 1100 HP system monitored by the Chemstation software (Agilent Technology). The chromatographic columns (125 mm · 4 mm) were prepared with the different materials by the slurry method. The silicas beads (1.5 g) were first ultrasonicated with 30 mL 1,4-dioxane during 30 min and then packed in the column under 15 MPa. Around 0.8 g of stationary phase was contained in the columns. Heptane and 1,4-dioxane were HPLC grade (Carlo Erba) and used as mobile

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phase with varying ratio in order to adjust the retention factors, toluene was used as non-retained solute. HPLC measurements were performed at room temperature. Pore size distributions were also evaluated by inverse size exclusion chromatography (ISEC): polystyrene standards (purchased from PSS Standards, Mainz, Germany) of various molecular weights were dissolved in 1,4-dioxan (HPLC grade, Riedel-de Hae¨n) at a concentration of 0.01 mg/mL. The adsorption isotherms of alpha-tocopherol and terbutyl-phtalate on the two materials were determined by the solution depletion method (amount adsorbed is derived from the variation of concentration due to adsorption) in static conditions at 25 C. The sample is placed into a glass tube that is closed with a septum. The system is heated at 120 C during 3 h under nitrogen dry flow at 5 mL min1. After treatment the weight loss is calculated from difference between final and initial weight of the tube. With a glass syringe, around 15 mL of heptane previously dried with molecular sieve (5 A) are added in the tube through the septum. The system is stirred under dry nitrogen flow. A stock solution is added step by step in the tube. When the equilibrium time is reached after each step, a volume around 100 lL is taken from the supernatant (stirring is stopped) for UV analysis thus allowing the equilibrium concentration to be determined. The adsorption enthalpies of the selected molecule are determined with a Tian-Calvet type microcalorimeter already described [24]. Like for the adsorption isotherm procedure, the stock solution is added step by step to a stirred suspension of silica samples in heptane. Dried solvent and solution are used and the calorimetric cell is maintained under argon. At each step, the measured heat has a contribution of both dilution and sorption phenomena. The dilution contribution was measured by the same procedure but without solid inside the cell. It is subtracted from the measured heat to get the adsorption enthalpy. The integral adsorption enthalpy was obtained by dividing the cumulative heat by the total adsorbed amount. All adsorption experiments are done at 25 C. Adsorption isotherms of water vapour at 25 C were determined with a home made apparatus based on a symmetrical commercial vacuum microbalance from Setaram [25]. The samples were preliminary evacuated under vacuum at 120 C for 12 h. 3. Results and discussion Pseudomorphism is a term introduced by mineralogists [26] to describe phase transformations that does not change the shape of a material. In the present case, the transformation of amorphous silica into a MTS material is based on a progressive dissolution-reprecipitation process that takes place inside the pores of the parent silica beads. The reprecipitation of the silica in presence of surfactant had to be faster than the dissolution of silica in basic media to insure the formation of MTS material inside the bead and not in the surrounding solution. Adjustment of synthetic condi-

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tions is needed to control the kinetic of silica dissolution and MTS reprecipitation. The pseudomorphic transformation can be described as a progressive process occurring inside a bead, in which a first poorly ordered mesostructure is formed and then gradually rearranges into well-defined ordered domains via short distance mass transfer phenomena [17–21]. The process occurs at non-constant matter content: the final product contains around 70% of silica introduced in the synthesis, the rest being dissolved in the solution. In the following lines, we compare the two silica samples, LiChrospher 60 (named LiChro 60) and its pseudomorphic MTS1 under different aspects that are important in separation applications. 3.1. Morphology, particle size Morphology and particle size has important consequences on the column packing and separation efficiency. The conservation of the spherical shape of the particles as well as the absence of aggregation, which is an important quality for column packing, is clearly shown by the SEM images in Fig. 1. The particle size distributions determined by laser diffraction are also very similar. As a consequence, one can expect that a column filled by either LiChro 60 or MTS1 will contain the same number of particles. This allows calculating the expected ratio of mass sample in a column from their specific pore volume given by the sorption isotherms data (Table 1) with the following equation: mMTS q  V p ðLiÞ þ 1 ¼ q  V p ðMTSÞ þ 1 mLi

ð1Þ

where mMTS and mLi are the mass of MTS and LiChro 60, respectively, filling the same column, Vp(i) is the specific porous volume of sample i and q the silica density (equal as 2.2 for amorphous silica). Therefore the mass of MTS1 present in the column is inferior to the mass of LiChro 60 with mMTS/mLi = 0.56, which result is in accordance with silica yield in pseudomorphic transformation around 60–70%. This is an important point as the separation performance will be related to the surface developed per column volume and not per gram. In a recent paper, it has been shown that the retention factor was correlated to the surface area per particle [16], which is equivalent to the surface area per column volume in the case of samples with similar particle size and packing density. The ratio of surface areas per column for the two silicas can be deduced from Eq. (1): r¼

AMTS  mMTS AMTS  ðq  V p ðLiÞ þ 1Þ ¼ ALi  ðq  V p ðMTSÞ þ 1Þ ALi  mLi

ð2Þ

where AMTS and ALi are the specific surface areas of MTS1 and LiChro 60, respectively. The surface area developed in a column of MTS1 is proportional to its specific surface area, but it also inversely proportional to its pore volume. Therefore the parent silica LiChro 60 exhibits a larger

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Fig. 1. SEM images at two magnifications and particle size distribution determined by laser diffraction of pseudomorphic transformation of LiChrospher 60 (plain line) into Micelle-Templated Silica MTS1 (dotted line) maintaining the spherical morphology.

surface area per column compare to MTS1, with r = 0.76. This is also mainly due to the exceptional surface area (750 m2/g) compared to conventional commercial silicas for chromatography, which exhibit specific surface area around 300–400 m2/g. To obtain a larger surface area per column for pseudomorphic MTS, the parent silica should have a specific surface area inferior to 565 m2/g, which is usually the case for the majority of commercial silicas. The choice of LiChro 60 as benchmark material was to really compared pseudomorphic MTS with the highest grade silicas for chromatography. Surface area, porosity, pore size distribution. Nitrogen adsorption–desorption isotherms of LiChro 60 and MTS1 are presented in Fig. 2. The larger amount of nitro-

gen adsorbed at all relative pressure for the pseudomorphic MTS1 indicates an increase of specific surface area and especially of specific pore volume (Table 1). The BET specific surface areas reaches 1009 m2/g, accordingly to specific surface area obtained for classical MTS synthesis using alkylammonium surfactants [1–3]. The specific mesopore volumes calculated at the end of the pore filling increases from 0.77 mL/g for LiChro 60 to 1.72 mL/g for its pseudomorphic MTS1, in accordance to MTS synthesized using swelling agent and alkylammonium surfactants [27,28]. The pore size distributions (Fig. 3) determined from the nitrogen desorption branch and by using BdB method show that LiChro 60 and its pseudomorphic derivative have very similar pore diameters around 6 nm. This is due to the choice of surfactant and the amount of swelling agent [27,28]. In previous studies [17–21], it was shown that starting from the same parent silica several pseudomorphic MTS materials may be prepared with various mean pore size as well as with different pore arrangement (hexagonal or cubic networks). The similar pore size of the two present materials permits to understand the textural, mechanical and chemical effects induct by the pseudomorphic transfor-

Fig. 2. Nitrogen adsorption–desorption isotherms at 77 K of LiChrospher 60 (black) and its pseudomorphic transformation MTS1 (white).

Fig. 3. Pore size distribution of LiChrospher 60 (plain line) and its pseudomorphic material MTS1 (dashed line) determined by BdB method.

Table 1 Features of parent silica LiChrospher 60 and its pseudomorphic MTS1: specific pore volume (V), specific surface area As, pore diameter D (nm) determined by BdB method and particles diameter (Dp) Silicas

V (mL/g)

As (m2/g)

D (nm)

Dp (lm)

LiChro 60 MTS1

0.77 1.72

750 1009

6.0 6.8

10 ± 7 10 ± 7

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mation. The pore size distributions are slightly different in term of dispersion. The MTS1 distribution is a little bit narrower than that the LiChro 60, despite of the low cost surfactant composed of different alkyl chain length (30% C16TACl, 70% C18TACl). This kind of narrow pore size distribution for MTS1 is in accordance with the synthesis conditions used where MCM-41 type sample is expected (hexagonal arrangement of hexagonal pores) [27,28]. Even if XRD and TEM signatures [23b,28] of hexagonal structure for large-pore MTS samples is not as clearly defined as for small-pores MCM-41 samples (4 nm and below), nitrogen adsorption isotherms (Fig. 2) and pore size distributions (Fig. 3) reveal that pseudomorphic process has completely modified the mesoporous type. Calculating the mean pore diameter by the Gurvitch equation D = 4 Vp/As valid for cylindrical pores, where As is the specific surface area of the samples, gives 4.0 and 6.8 nm for LiChro 60 and MTS1, respectively. The better agreement of this value with that calculated by the BdB method for the MTS1 sample is in accordance to a pore shape modification leading to cylinders during the pseudomorphic transformation. From the data of gas adsorption it is possible to calculate the ratio r given by Eq. (2). The value, r = 0.76, shows that, in the present case, the pseudomorph transformation leads to a decrease of surface area per column due to the fact that the surface area measurement (Table 1) is obtained in m2 per grams and not per volume of material. Further developing Eq. (2) by introducing the geometric relation between surface area, pore volume and pore radius for cylinders A = 2V/R, one gets:   RLi  q þ V p 1ðLiÞ   r¼ ð3Þ 1 RMTS  q þ V p ðMTSÞ The interesting information brought by this Eq. (3) is that a pseudomorphic transformation that increases the pore volume at constant pore radius should give a ratio r larger than 1, which is not observed here. The reason may be that the assumption of pores with a cylindrical

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shape is a two crude approximation notably for the parent silica sample. This observation confirms the difference of pore shape between the two samples. The pore size distributions of the two samples were also obtained by inverse size exclusion chromatography (Fig. 4, Table 2). The mean pore sizes obtained by ISEC are 7.8 and 10.4 nm for LiChro 60 and MTS1, respectively, which are larger values than 6 and 6.8 nm obtained by gas adsorption results, respectively, but are in reasonable agreement if one considers that the two approaches are totally different. More important is to notice that by both methods the pore size distribution is narrower in the case of MTS1 (Table 2). The samples were also analyzed by mercury porosimetry (Fig. 5). The higher intrusion pressure applies to fill the pores was 410 MPa. The mesopores volume found by mercury intrusion was 0.60 and 1.39 mL/g for LiChro 60 and MTS1 (Table 3), respectively, slightly lower (80%) but in good accordance with values found by gas adsorption (0.77 and 1.72 mL/g, respectively). The difference could be due to the presence of smaller pores (inferior to 3 nm) that cannot be filled by mercury at 410 MPa or to too short equilibrium time between successive mercury intrusion. The first assumption is less probable as for MTS1 the nitrogen isotherm close at p/p0 = 0.55 corresponding to the smaller pore size around 5.8 nm. The pore size is determined using the Washburn–Laplace law for cylindrical pore: D ðnmÞ ¼ 4  103 c cos Hpint where pint is the intrusion pressure, H = 130 and c = 0.485 N m1. The steepest slope in the mesopore range shows the narrowest pore size distribution of MTS1. The pore diameter determined by mercury intrusion is 5.7 and 6.6 nm for LiChro 60 and MTS1, respectively, which is equal to the data of nitrogen adsorption. After applying the pressure of 410 MPa, strong mercury retention was observed for both samples LiChro 60 and MTS1 due to structure

Fig. 4. (a) Partition coefficients obtained by ISEC for LiChrospher 60 (black lozenge) and MTS1 (empty circle). (b) Pore size distribution of each materials obtained by inverse size exclusion chromatography.

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Table 2 ISEC analysis: mean pore size and pore distribution width Silicas

Mean pore size (nm)

Half height distribution width (nm)

LiChro 60 MTS1

7.86 10.40

1.25 0.56

breaking as revealed by their mechanical stability study in the next paragraph (Fig. 6). In order to improve their mechanical stability, both silicas were modified with octyl chain by the procedure described in the experimental part. The comparison between mercury intrusion–extrusion curves before and after grafting is presented in Fig. 5 for each sample. Thanks to the grafting process, the intrusion–extrusion curves of both silicas were reversible in the mesoporous range, revealing also a narrower pore size distribution for grafted-MTS1 sample. In the low pressure range (i.e. Washburn pore diameters above 1 lm), the mercury porosimetry graph is characteristic of the interparticle domain (Fig. 5). The good superimposition of curves for MTS1 and LiChro 60 in this region confirm the conservation of particle size and morphology of the pseudomorphic transformation. The interparticular void of 3 lm is in good agreement with a good packing

of spherical particles of 10 lm (void diameter being around 30% of the particle size for a dense packing). Mercury porosimetry analyses confirm that pseudomorphic synthesis allow keeping the same particles morphology and improving the pore size distribution at the same time. Mechanical stability. The comparison of the two mercury porosimetry of native MTS1 and LiChro 60 in Fig. 5 indicates that MTS material is more fragile due to the formation of mesopores with thinner walls due to its higher specific surface area. It is well known that large-pore mesoporous ordered silicas are rather fragile on a mechanical point of view and notably in the case of MCM-41 type structures, especially for pore size larger than 6 nm [29–32]. To determine materials limitations undergo high pressures, we conducted a systematic comparison of the mechanical stability of the two samples LiChro 60 and MTS1. Pellets were prepared at various pressures with a press used to prepare samples for infrared analysis. The evolution of nitrogen adsorption–desorption isotherms is followed as a function of pressure treatment (Fig. 6, Table 4). As expected the amount adsorbed at the end of pore condensation decreases as the pressure increases whereas the steps corresponding either to adsorption or desorption are shifted towards lower pressures. The hysteresis shapes are

Fig. 5. Mercury intrusion–extrusion curves of native and grafted-LiChrospher 60 and native and grafted-MTS1.

J. Iapichella et al. / Microporous and Mesoporous Materials 102 (2007) 111–121 Table 3 Mesopore diameter (D), interparticular pore diameter (Dp), mesopore volume (V) and interparticular volume (Vp) obtained by mercury porosimetry for native and grafted silicas Silicas

V (mL/g)

D (nm)

Vp (mL/g)

Dp (lm)

LiChro 60 MTS1 Grafted-LiChro60 Grafted-MTS1

0.60 1.39 0.37 0.72

5.7 6.6 5.8 6.0

0.74 1.75 0.63 1.11

3.0 3.5 2.7 3.0

also apparently modified. In the case of MTS1, calculations of the mean pore size by the BdB method show that ˚ and D (adsorption) = 70 A ˚ for D (desorption) = 68 A ˚ and D (adsorpMTS1, whereas D (desorption) = 53 A ˚ for grafted MTS1. It means that the apparent tion) = 56 A enlargement of hysteresis may be due to the range of pore size which is analysed here: on a linear scale for p/p0, a decrease of pore size leads to an apparent increase of hysteresis width (provided it is far enough from the critical size where hysteresis vanishes). In the case of Lichrosphere 60, the adsorption isotherm modification by grafting is more pronounced: in that case the decrease of pore size by grafting may lead to some pore blocking effect or to the appearance of pores that are connected to the outside through pores that are smaller than the critical size. For all samples the main effect of pressure treatment is a decrease of both the total pore volume and the mean pore

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size. Native MTS1 sample is stable until 40 MPa, which is enough to resist to a chromatographic column packing, but the native pseudomorphic transform is more sensitive to pressure treatment than the native silica. MTS1 has lost 65% of its pore volume for an applied pressure of 270 MPa, whereas LiChro 60 has lost only 30% of its pore volume (Fig. 7). Nevertheless, the pore volume and the surface area of MTS1 are still higher than the ones of LiChro 60 after the compression of 270 MPa. The adsorption in the low pressure range (below p/p0 = 0.4) is less affected by the pressure treatment than the domain of capillary condensation. It means that the surface area of the sample is less modified than the pore volume. After a high pressure applied on the materials, the entire materials surface was again accessible to be adsorbed by nitrogen whereas the pore volume was reduced. After surface modification by octyl chains, both samples are only slightly modified by the pressure treatment. The pore volume decreases a little bit (less than 6%) and the surface area is unchanged. It means that surface modification is a way to stabilize MTS silica in view of applications at high pressure. The samples can be used in reverse phase chromatography as already shown [15]. Surface chemistry. The chemical behaviour of silica in many applications can be directly related to the type and surface density of silanols. To compare the surface chemistry of the two samples LiChro 60 and MTS1, the density of

Fig. 6. Evolution of nitrogen adsorption–desorption isotherms at 77 K in function of applied pressure: (a) 0, (b) 40, (c) 80, (d) 160, (e) 270 MPa, for native and grafted-LiChrospher 60 and for native and grafted-MTS1.

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Table 4 Main characteristics of samples deduced from N2 adsorption desorption methods: pore volume V (mL/g), surface area As (m2/g), pore diameter D (nm) obtained by the BdB method and pressure P applied (MPa) Samples

Surface chemistry

V (mL/g)

LiChro 60

Native silica

0.85 0.77 0.54 0.37 0.37 0.35 1.75 1.46 0.64 0.83 0.79 0.78

Grafted silica

MTS1

Native silica

Grafted silica

As (m2/g)

D (nm)

p (MPa)

737 701 588 262 256 250

6 4.2 4.5 4.8 4.9 4.8

0 80 270 0 80 270

1009 872 714 425 387 387

6.8 6.6 4.6 5.4 5.3 5.2

0 80 270 0 80 270

a

OH groups was determined by thermogravimetry. Moreover, in order to get more insight directly on the reactivity of the samples, the affinity of polar probes for the surface was deduced from adsorption isotherms in both liquid and vapour phase. In Fig. 8a, the adsorption isotherms of water on the two samples are presented. There is an interesting result at low pressure range: the specific amount adsorbed of water is higher for LiChro 60 than for MTS1 indicated a more hydrophobic character of the MTS sample. In Table 5, the adsorbed amounts are reported to the surface area, which increases the difference of hydrophilic character between the two samples. The LiChro 60 sample is more hydrophilic than MTS1 but does not have itself a high hydrophilic character. Indeed at a relative pressure of p/p0 = 0.2, the surface density of water is 100 lg m2, whereas a monolayer of water would correspond to a surface concentration around 280 lg m2 if a cross section area of around 0.1 nm2 is assumed for water molecule [33]. The difference of hydrophilic character seems to be related to the OH density of the two samples (Table 5). It is 2.0 OH groups per nm2 for MTS1 against 3.0 for LiChro 60 (values determined by thermogravimetry,

Weigth loss % mg

b 100 98 96 94 92

MTS1 Si60

90 88 86 0

100

200

300

400

500

600

700

800

900

Temperature/°C Fig. 8. (a) Adsorption of water by gravimetry of LiChrospher 60 (plain line) and its pseudomorphic MTS1 (dashed line), (b) high resolution thermogravimetry of both samples.

Fig. 8b). These values are estimated from the weight loss between two temperatures (150 C and 800 C) but are only an approximate value of the real amount of accessible silanol groups. If in the present case the BET equation is applied to water adsorption to determine the amount of water adsorbed at monolayer and assuming that each water molecule interact approximatively with one silanol group, one gets 3.6 OH/nm2 and 2.0 OH nm2 for LiChrospher 60 and MTS1, respectively. This is in reasonable

Fig. 7. Decrease of the pore volume (V/V0), with V0 being the initial pore volume, in function of applied pressure for the native and grafted-LiChrospher 60, and for the native and grafted-MTS1.

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Table 5 Evaluation of the hydrophilic character: amount of water adsorbed (mg.g1), surface concentration of water (lg m2) and –OH groups (molecules nm2) Samples

LiChrospher 60 MTS1

Water at p/p0 = 0.2 Adsorbed (mg g1)

Surface concentration (lg m2)

–OH groups (molecules nm2)

74

102

3.0

40

38

2.0

Surface concentration/µmol.m

-2

agreement with OH contents determined by TG. Another important point is shown by Fig. 8a: the adsorption of water is not reversible for MTS1. A second cycle shows that water affinity increases at low pressure but most of the pore volume is lost. After contact with water there is a rehydroxylation of the MTS surface, which is more or less destructive of the structure as already observed [34]. This rehydroxylation seems to occur after pore filling by capillary condensation. In fact after synthesis and calcination, MTS samples have a low density of OH groups on the surface, which can lead to new surfaces applications, such as a more homogeneous functionalization of the silica surface or a higher loading of proteins by grafting compared to conventional silicas [35]. The surfactants use to generate well defined mesoporosity inducts in one hand a low density of OH groups and in the other hand a better OH groups distribution on the surface, which can permit a surface grafting more homogeneous. Tocopherol and terbutyl-phtalate are suitable molecules to compare the surface polarity of the two samples and have been adsorbed from a non-polar solvent. Practically the same molecule (alkyl-phtalate) will be then used for chromatographic tests in the next paragraph. In Figs. 9 and 10, the adsorption isotherms of tocopherol from heptane onto LiChro 60 and MTS1 are presented together with adsorption enthalpies. The affinity is higher for LiChro 60 as shown both by the initial slope of the adsorption isotherms and the adsorption enthalpy. At saturation the surface concentration is slightly higher on MTS1. The 1.0

Lichro 60

0.6 0.4 0.2 0.0 0.000

0.002

0.004

0.006

0.008

same behaviour was observed for terbutyl-phtalate adsorption. This may be due to the narrowest pore size distribution of MTS1: curvature effects may be predominant in the case of the LiChro 60 sample that contains a significant amount of small pores (see Fig. 3). The highest enthalpies of adsorption observed for LiChro 60 confirm the highest polarity of this surface. These results confirm the hydrophobic character of the MTS1 sample and these results give an approach about the behaviour of tertbutyl-phtalate adsorption, which provides more details regarding the alkyl-phtalate separation and mass transfer properties. Mass transfer properties. The comparison of the samples by chromatographic tests is here limited to mass transfer properties as evidenced by plate height (H) versus linear velocity (u), (H/u) plots [36]. In a recent paper, retention factors were compared for a set of silica and pseudomorphs and correlated to the surface area per particle [16]. Columns were prepared with MTS1 and LiChro 60 to be tested in normal phase HPLC for the separation of dibutyl- and diethyl-phthalates. The column efficiency is evaluated from H/u curves (Fig. 11). The Knox equation was applied to determine the mass transfer term C [37]: H ¼ A þ B=u þ Cu

MTS1

0.8

Fig. 10. Enthalpy of displacement of tocopherol from heptane onto LiChrospher 60 (black lozenge) and MTS1 (empty circle).

0.010

Equilibrium concentration/mol.Kg -1

Fig. 9. Adsorption isotherm of tocopherol from heptane onto LiChrospher 60 (black square) and MTS1 (empty square).

where A, Eddy diffusion term, expresses the convective mixing in the interstitial voids and depends on the quality of the column packing; B accounts for longitudinal diffusion and controls H at low linear velocity; C is the mass transfer term, which expresses the mass transfer kinetics between the mobile phase and the stationary phase of the column. The H values at the minimum are 46.3 and 52.8 lm for LiChro 60, respectively (Fig. 11 and Table 6). The C values are 170 and 171 ms for LiChro 60 and MTS1, respectively. The same C value for the two samples shows a ‘‘memory effect’’ of mass transfer properties between pseudomorphic MTS materials and its parent silica. In a recent paper, it was observed that for a given structure,

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Fig. 11. Plate height (H) versus linear velocity (u) curves of LiChrospher 60 (black lozenge) and MTS1 (empty circle). The slope of each fitted curves (plain line) corresponds to the C parameter of Knox equation of each materials.

Table 6 Mass transfer C (ms) and plate height H (lm) determined with Knox equation in the H versus u curves Samples

Mass transfer term C (ms)

Plate height H (lm)

LiChrospher 60 MTS1

170 171

46.3 52.8

C decreases with an increase of pore size (better mass transfer) and that for equivalent pore size, LiChro 60 sample ranges between MCM-41 type structure and MCM-48 type structure [16,21]. This is a logical intermediate behaviour between hexagonal and cubic symmetry since one may consider that its connectivity is between that of infinite channel (hexagonal phase) and highly connected pores (cubic structure). In fact, the two principal features responsible for improved mass transfer properties are a larger mesopore size and higher connectivity. Thanks to the pseudomorphic process, the mesoporous domain of the LiChro 60 was completely modified by narrowing the pore size distribution and the mass transfer properties of the parent material preserved in the pseudomorph material. The connectivity is not strongly affected because the length of one-dimensional channels is probably small as compared to the particle size. 4. Conclusion The physical chemistry characteristics of commercial silica and its pseudomorphic transform have been compared. The Micelle-Templated Silica obtained by pseudomorphic process has a pore size similar to that of its parent material; the surface area and pore volume are largely increased without changing the initial shape of silica particles. Chromatographic tests show that mass transfer properties, which depend on particle size, pore size and connectivity, are maintained. Adsorption from polar probes and thermogravimetric measurements show that the surface

chemistry is strongly modified by the pseudomorphic transformation which is not pseudo-chemical. In fact, the surfactants use to generate well defined mesoporosity inducts low density of OH groups and a better OH groups distribution on the surface, which can permit a more homogeneous surface grafting. Mercury porosimetry and the studies of mechanical stability have shown that mesoporous silica materials was incredibly more stable under high pressure thanks to grafting procedure. That is why the pseudomorphic material kept a higher pore volume and a higher surface area compared to its parent silica in spite of the high pressure applied. Adsorption of water and tocopherol have shown that the MTS sample is more hydrophobic compared to its parent silica LiChrospher 60, which can lead to new surfaces applications. Acknowledgments Authors thank the European Commission for funding this work under the GROWTH-INORGPORE program (project G5RD-CT-2000-00317) and Dr. Dominique Plee for his assistance in large quantities material synthesis. References [1] J.S. Beck, J.C.W.J. Vartuli, L. Roth, M.E.C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc 114 (1992) 10834. [2] D. Zhao, P. Yang, Q. Huo, B.F. Chmelka, G.D. Stucky, Curr. Opin. Solid State Mater. Sci. 3 (1998) 111. [3] F. Di Renzo, A. Galarneau, P. Trens, F. Fajula, in: F. Schu¨th, K. Sing, J. Weitkamp (Eds.), Handbook of Porous Materials, Wiley- VCH, 2002, p. 1311. [4] Q. Sun, E.G. Vrieling, R.A. van Santen, N.A.J.M. Sommerdijk, Curr. Opin. Solid State Mater. Sci. 8 (2004) 111. [5] M. Razimondo, G. Perez, N. Sinibaldi, A. DeStefanis, A.A.G. Tomlinson, Chem. Commun. (1997) 1343. [6] A. Kurganov, K.K. Unger, T. Iasseva, J. Chromatogr. A 753 (1996) 177. [7] P. De Phillips, A.M. Lenhoff, J. Chromatogr. A 883 (2000) 39. [8] T. Nassivera, A.G. Eklund, C.C. Landry, J. Chromatogr. A 973 (2002) 97. [9] M. Gru¨n, A.A. Kurganov, S. Schacht, F. Schu¨th, K.K. Unger, J. Chromatogr. A 740 (1996) 1. [10] K.W. Gallis, J.T. Araujo, K.J. Duff, J.G. Moore, C.C. Landry, Adv. Mater. 11 (1999) 1452. [11] C. Thoelen, K. van de Walle, I.F. Vankelcom, P.A. Jacobs, Chem. Commun. (1999) 1841. [12] C. Thoelen, C. Paul, I.F. Vankelcom, P.A. Jacobs, Tetrahedron. Assym. 11 (2000) 4819. [13] K.W. Gallis, A.G. Eklund, S.T. Jull, J.T. Araujo, J.G. Moore, C.C. Landry, Stud. Surf. Sci. Catal. 129 (2000) 747. [14] J.W. Zhao, F. Gao, Y.L. Fu, W. Jin, P.Y. Yang, D.Y. Zhao, Chem. Commun. (2002) 752. [15] T. Martin, A. Galarneau, F. Di Renzo, D. Brunel, F. Fajula, S. Heinisch, G. Cretier, J.L. Rocca, Chem. Mater. 16 (2004) 1725. [16] A. Galarneau, J. Iapichella, D. Brunel, F. Fajula, Z. Bayram-Hahn, K.K. Unger, G. Puy, C. Demesnay, J.-L. Rocca, J. Sep. Sci. 29 (2006) 844. [17] T. Martin, A. Galarneau, F. Di Renzo, F. Fajula, D. Plee, Angew. Chem. Int. Ed. 41 (2002) 2590. [18] A. Galarneau, J. Iapichella, C. Petitto, F. Di Renzo, F. Fajula, Mater. Res. Soc. Symp. Proc. 847 (2005) 345.

J. Iapichella et al. / Microporous and Mesoporous Materials 102 (2007) 111–121 [19] B. Lefevre, A. Galarneau, J. Iapichella, C. Petitto, F. Di Renzo, F. Fajula, Z. Bayram-Hahn, R. Skudas, K.K. Unger, Chem. Mater. 17 (2005) 601. [20] C. Petitto, A. Galarneau, M-F. Driole, B. Chiche, B. Alonso, F. Di Renzo, F. Fajula, Chem. Mater. 15 (2005) 2120. [21] A. Galarneau, J. Iapichella, K. Bonhomme, F. Di Renzo, P. Kooyman, O. Terasaki, F. Fajula, Adv. Funct. Mater. 16 (2006) 1657. [22] M. Martin, A. Galarneau, D. Brunel, V. Izard, V. Hulea, A.C. Blanc, S. Abramson, F. Di Renzo, F. Fajula, Stud. Surf. Sci. Catal. 135 (2001) 4621. [23] (a) J.C.P. Broekhoff, J.H. de Boer, J. Catal. 10 (1968) 377; (b) A. Galarneau, D. Desplantier, R. Dutartre, F. Di Renzo, Micropor. Mesopor. Mater. 27 (1999) 297. [24] R. Denoyel, F. Giordano, J. Rouquerol, Colloids Surf. 76 (1993) 141. [25] J. Rouquerol, L. Davy, Thermochim. Acta 24 (1978) 391. [26] J. Sinkankas, Mineralogy, Van Nostrand Reinhold, New York, 1964, p85. [27] D. Desplantier-Giscard, A. Galarneau, F. Di Renzo, F. Fajula, Stud. Surf. Sci. Catal. 135 (2001) 1105.

121

[28] M.F. Ottaviani, A. Moscatelli, D. Desplantier-Giscard, F. Di Renzo, P.J. Kooyman, B. Alonso, A. Galarneau, J. Phys. Chem. B 108 (2004) 12123. [29] M. Hartmann, C. Bischof, J. Phys. Chem. B 103 (1999) 6230. [30] V. Gusev, X.Yu. Feng, Z. Bu, G.L. Haller, J.A. O’Brien, J. Phys. Chem. 100 (1996) 1985. [31] D. Desplantier-Giscard, O. Collart, A. Galarneau, P. Van Der Voort, F. Di Renzo, F. Fajula, Stud. Surf. Sci. Catal. 129 (2000) 665. [32] D. Desplantier-Giscard, A. Galarneau, F. Di Renzo, F. Fajula, Mater. Sci. Eng. C 23 (2003) 727. [33] F. Rouquerol, J. Rouquerol, K.S.W. Sing, Adsorption by Powders and Porous Solids, Academic Press, London, 1999. [34] M. Ribeiro Carrott, M.L. Estevao Candeias, A.J. Carrott, P.J.M. Unger, Langmuir 15 (1999) 8895. [35] G. Renard, M. Mureseanu, A. Galarneau, D.A. Lerner, D. Brunel, New J. Chem. 29 (2005) 912. [36] J.J. VanDeemter, F.J. Zuiderweg, A. Kinkenberg, Chem. Eng. Sci. 5 (1956) 271. [37] G.J. Kennedy, J.H. Knox, J. Chrom. Sci. 10 (1972) 549.