Entrance Size Analysis of Silica Materials with Cagelike Pore

Article pubs.acs.org/JPCC

Entrance Size Analysis of Silica Materials with Cagelike Pore Structure by Thermoporometry Tamara M. Eggenhuisen,†,§ Gonzalo Prieto,†,§ Herre Talsma,‡ Krijn P. de Jong,† and Petra E. de Jongh*,† †

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht, The Netherlands Pharmaceutics, Utrecht Institute for Pharmaceutical Science, Utrecht University, Utrecht, The Netherlands

‡

S Supporting Information *

ABSTRACT: Ordered mesoporous materials offer many potential applications in catalysis, chromatography, and drug delivery. If the pores form cages rather than straight channels, the entrance size to these cages is a crucial parameter but notoriously difficult to asses. For example, classical physisorption techniques are limited by forced closure of the hysteresis loop. Here we apply thermoporometry by differential scanning calorimetry (DSC) of confined water to quantify the entrance sizes in a series of mesoporous silica materials with cagelike pore structure, i.e. SBA-16 and FDU12. With DSC, entrance sizes of materials with cages of up to 15 nm were determined which could not be assessed by nitrogen physisorption and were validated by argon physisorption at 77 K. For cage sizes exceeding 15 nm, entrance sizes were quantified by thermoporometry that were inaccessible by either of the physisorption techniques. In addition, the size distribution of the widest-entrance path toward the cages was determined by applying a step-equilibration method with DSC, providing essential structural information beyond the limits of physisorption with N2 or Ar at 77 K. We show that thermoporometry extends the range of classical physisorption techniques and is a powerful tool for the determination of entrance sizes in mesoporous materials.

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INTRODUCTION Ordered mesoporous materials are used for many applications ranging from catalysis,1 catalyst supports,2,3 drug delivery,4 separation, or as a template for the synthesis of other mesoporous materials via nanocasting.5−7 They are also used as catalyst (model) supports to study the effect of catalyst preparation on the nanoparticle dispersion and distribution.8−13 Silica materials with cagelike pore structure form a unique class within the ordered mesoporous materials family that possess a 3-dimensional highly ordered arrangement of spherical cages interconnected by 6 up to 12 windows, with varying cage size, entrance size, and symmetry depending on the synthesis conditions applied.14−17 They are of particular interest as catalyst supports, as the necks help to confine the catalytically active phase to the cage, while the 3-dimensional connectivity provides transport for substrates and products. They are applied to reduce sintering of supported metal nanoparticles18,19 or prevent leaching of homogeneous catalysts.20,21 The entrance and cage size are crucial structural parameters. Although cage sizes can be derived from gas physisorption,22 extracting entrance sizes has remained a challenge. Different approaches have been employed, such as advanced electron microscopy,23−25 diffraction and scattering techniques,26−29 synthesis of negative replicas,16,30 size selective adsorption,31 and anchoring organic moieties to the surface with gradually increasing molecular sizes.32−34 Nevertheless, all these methods © 2012 American Chemical Society

involve specific material requirements and have limited applicability. Gas physisorption is a common analysis technique to analyze mesoporosity and does not require structural ordering of the material.35−37 The adsorption isotherm is used to determine the cage size, while the entrance size can be extracted from the desorption branch as long as evaporation is controlled by the pore restriction.16,24 The use of the desorption isotherm is limited due to the cavitation effect or forced closure of the hysteresis loop, but using alternative adsorptives such as argon or krypton at different temperatures stretches the forced closure of the hysteresis loop.38−40 For example, argon physisorption at 77 K has shown to provide a modest extension toward entrance sizes of ∼4 nm as compared to the limit of ∼5 nm in classical nitrogen physisorption.34,41−43 Alternative approaches use consecutive adsorption of H2O and N2,44,45 quantification of intrawall porosity from the adsorption isotherm,17,46 and pore size distribution analysis with a combined cylindrical and spherical pore model.27,47 Thermoporometry by 1H NMR (also referred to as cryoporosimetry)48 or differential scanning calorimetry (DSC)49 of confined water is an alternative technique to characterize mesoporosity. The melting point of a material Received: July 16, 2012 Revised: September 20, 2012 Published: October 4, 2012 23383

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inside a mesopore depends on the pore size, similar to capillary adsorption in gas sorption.10,50−53 Freezing of a liquid in a mesopore shows similarities to gas desorption and contains information on pore connectivity or entrance sizes.52,54,55 Earlier reports of thermoporometry by DSC on mesoporous materials with cagelike pore structure showed that the freezing of water confined within the cages was dominated by homogeneous nucleation of ice at ∼−39 °C.56−60 Similar to cavitation in gas desorption, homogeneous nucleation of ice sets a lower limit for the entrance size range that can be analyzed. Although the limits of nitrogen and argon physisorption have been discussed extensively,32,39,61−63 the sensitivity of the freezing of water measured by DSC toward entrance and cages size has not been reported in detail. Here we explore thermoporometry by DSC of confined water for entrance size analysis of mesoporous cage materials. A series of SBA-1634,64,65 and FDU-1224 silica materials were synthesized with a range of entrance sizes such that the desorption isotherms in nitrogen physisorption varied between pore restriction-controlled and full cavitation. The quantification of cage and entrance sizes obtained by DSC are validated by physisorption with N2 and Ar at 77 K, for those materials where data are available. To allow a fair comparison between the techniques, entrance sizes were derived from the desorption branches of N2 and Ar using the same the BJH-based approach, 38,66 and a freezing temperature−pore radius calibration for DSC was determined based on the BJH-pore radii of reference silica materials with ordered cylindrical pores. To demonstrate the advantage of thermoporometry, the cage and entrance sizes of FDU-12 with cage diameters larger than 15 nm were analyzed that were inaccessible by both physisorption techniques. Finally, in order to exploit the full potential of DSC, a new stepwise scanning method was developed to derive the size distribution of the widest entrance path to the cages.

Table 1. Synthesis Conditions for SBA-16 and FDU-12 with Varying Cage and Neck Sizes hydrothermal treatment sample

synthesis gel

T (°C)

time (h)

SBA1-60-24 SBA1-100/24 SBA2-90/48 SBA2-120/24_II SBA2-120/24_I SBA2-120/24 SBA2-100/72 FDU-35/24 FDU-60/48 FDU-100/48 FDU-110/48 FDU-120/48 FDU-130/48

(1) (1) (2) (2) (2) (2) (2) (3) (3) (3) (3) (3) (3)

60 100 90 120 120 120 100 35 60 100 110 120 130

24 24 48 24 24 24 72 24 48 48 48 48 48

additional heat treatment

800 °C, 3 h 650 °C, 3 h

dropwise under fast magnetic stirring, and the gel was aged in an oven at 35 °C for 20 h under static conditions. The mixture was further treated for 24−72 h at 90−120 °C. The temperature and duration of the last hydrothermal treatment were adjusted to obtain SBA-16 silica materials with varying cage and neck sizes. The different materials are denoted by their gel composition (2) and hydrothermal treatment temperature (T) and time in hours (t): SBA2-T/t. A series of large cage FDU-12 silica materials were synthesized employing 1,3,5-trimethylbenzene (TMB) as swelling agent according to the procedure reported by Yu et al.24 Pluronic F127 and KCl were dissolved in HCl/H2O at room temperature. TMB was subsequently added and the mixture stirred at 14 °C for 24 h. Then TEOS was added dropwise under fast magnetic stirring, and the mixture further stirred at 14 °C for 20 h. The final synthesis gel molar composition was: 0.004 F127:1.7 KCl:0.93 TMB:1.0 TEOS:6.1 HCl:157 H2O labeled as (3) in Table 1. Subsequently, the mixture was submitted to a final thermal treatment at a temperature of 35−130 °C for 24−48 h in an oven. The temperature and duration of this treatment were adjusted to obtain FDU-12 silicas with different cage and neck sizes. FDU12 materials are denoted by their hydrothermal treatment temperature (T) and time in hours (t), FDU-T/t. In all cases, sealed polypropylene bottles were used for hydrothermal treatments at temperatures below 90 °C, while for thermal treatments at higher temperatures the gel was transferred to Teflon-lined steel autoclaves. After the hydrothermal treatment, the white solid was filtered, extensively washed with double deionized water, and dried at 120 °C for 10 h. Finally, the product was calcined at 540 °C in a muffle oven in order to remove the copolymer template. An additional thermal treatment was applied to selected calcined silicas in order to controllably shrink the porous structure.65 This additional thermal treatment was performed in static air at a temperature of 650 or 800 °C for 3 h. Structural Characterization. To perform transmission electron microscopy, the samples were embedded in an epoxy resin (Epofix, EMS) and cured at room temperature for 72 h. The embedded materials were then cut into ultrathin sections with 100 nm nominal thickness using a Diatome 35° diamond knife mounted on a Reichert-Jung Ultracut E microtome, and

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EXPERIMENTAL SECTION Synthesis of Ordered Mesoporous Silicas. Block copolymers Pluronic F127 (EO106PO70EO106) and Pluronic P123 (EO20PO70EO20) from Sigma-Aldrich, 1-butanol (p.a., Acros), 1,3,5-trimethylbenzene (99%, Acros), KCl (p.a., Acros), and tetraethyl ortosilicate (TEOS, ≥99%, Sigma-Aldrich) were used as received. The synthesis conditions employed for the individual SBA-16 and FDU-12 samples are summarized in Table 1. SBA-16 silica materials with small cages were synthesized following the procedure reported by Kim et al.34 The synthesis gel had the following molar composition: 0.0016 P123:0.0037 F127:1.0 TEOS:4.4 HCl:140 H2O labeled as (1) in Table 1. The copolymers were dissolved at room temperature in HCl/ H2O. TEOS was subsequently added dropwise under fast magnetic stirring, and the gel was aged in an oven at 35 °C for 20 h under static conditions. The mixture was treated at 60 or 100 °C for 24 h. The different materials are denoted by their gel composition (1) and hydrothermal treatment temperature (T) and time in hours (t): SBA1-T/t. Large cage SBA-16 silica materials were prepared using 1butanol (BuOH) as a swelling agent at low acid concentrations as described by Kleitz et al.64 A synthesis gel with the following molar ratios was used: 0.0035 F127:1.79 BuOH:1.0 TEOS:0.91 HCl:120 H2O labeled as (2) in Table 1. After dissolving the block copolymer in HCl/H2O, 1-butanol was added, and the mixture was stirred at 35 °C for 1 h. Then, TEOS was added 23384

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⎛ ⎞0.5085 0.1605 ⎜ ⎟⎟ t(p /p0 ) [nm] = ⎜ ⎝ (0.1156 − log( −0.06199 + p/p0 ) ⎠

the sections were collected on a carbon-coated Ni grid (200 mesh). Transmission electron micrographs were acquired using a Tecnai 12 microscope (FEI Co.) operated at 120 keV. The volume averaged cage diameter was determined over ca. 50 mesocages on different micrographs of ∼100 nm thick ultramicrotomed slices. The cages were distinguished from the silica pore walls by image thresholding, and the individual cage diameters were determined from the area-equivalent circle. Low-angle X-ray diffraction patterns (LA-XRD) were recorded for the calcined silica mesostructures with a BrukerNonius D8 Advance X-ray diffractometer using Co Kα12 radiation (λ = 1.790 26 Å). The diffraction patterns were recorded in the 2θ range of 0.7°−2.5° using a scanning step size 0.08° and acquisition time 140 s step−1. Porosity Characterization. N2 and Ar physisorption isotherms were measured at −196 °C (77 K) (Micromeritics, TriStar 3000). This temperature is below the triple point of Ar (87.8 K). Two different values for the saturation pressure (P0) at 77 K are commonly encountered in the literature, both that corresponding to vapor in equilibrium with solid bulk Ar and that of vapor in equilibrium with pore-confined supercooled liquid Ar.67 Here, the saturation pressure was measured periodically during the acquisition of the Ar adsorption isotherms using the built-in module of the adsorption apparatus, as reported elsewhere.31,65 The measured P0 of 26.7−27.1 kPa corresponds to the Ar solid−vapor equilibrium. Prior to the measurements, samples (50−100 mg) were dried under flowing N2 at 250 °C for 12 h. The average cage size was defined as the maximum value in the pore size distribution (PSD) derived from the physisorption results by applying the Barrett−Joyner−Halenda (BJH) model modified according to the Kruk−Jaroniec−Sayari (KJS) correction (eq 1) and the empirical form of the Harkins−Jura equation as thickness reference curve (eq 2), to the adsorption branch.66,66 r(p /p0 ) [nm] =

−0.959 + t(p /p0 ) + 0.3 0.434 log(p/p0 )

⎛ ⎞0.3968 60.65 ⎟⎟ t(p /p0 ) [nm] = 0.1⎜⎜ ⎝ 0.03071 − log(p/p0 ) ⎠

(4)

The average entrance sizes were defined as the peak maximum in the thus obtained pore size distributions from N2 and Ar desorption. Thermoporometry was performed by analysis of the freezing and melting behavior of water confined in mesoporous silica materials using differential scanning calorimetry (DSC, TA Instruments, Q2000). The temperature and heat flow were calibrated using a certified indium sample. Measurements were performed in the range of −70 to 25 °C under a flow of 50 mL min−1 N2. 3−5 mg of silica were mixed with a 2-fold excess of H2O (Millipore, R = 18 MΩ·cm) based on the pore volume determined by N2 physisorption. The impregnated sample was hermetically sealed in an aluminum pan (40 μL, Tzero, TA Instruments) to prevent water evaporation during the measurement. A heating/cooling rate of 1 °C min−1 and 5 min isothermal periods at −70 °C were used for all measurements. The melting and freezing peaks were integrated using linear baselines in Universal Analysis 2000 software (TA Instruments). To derive an empirical temperature−pore radius relation, the onset temperatures (Tons) for freezing and melting of water/ice confined in ordered mesoporous silica materials with cylindrical pores (MCM-41 and SBA-15) of radii ranging from 1.6 to 6.3 nm were determined and fitted to a simplified Gibbs− Thomson equation, eq 5: ΔT = 0 °C − Tons =

CGT = −

2γslνlT0 ΔHf

(5)

where r represents the pore radius in nm, CGT a constant in °C nm, and tnf a correction for a nonfreezing layer in nm.49,69 The radii of the pores were independently assessed by N2 physisorption at −196 °C. For cylindrical mesopores, cavitation does not occur. The BJH formalism modified according to eqs 1 and 2 provides an accurate determination of the pore size.63 Therefore, in order to calibrate the DSC method, the melting point onset temperature (Tm) and the freezing point onset temperature (Tf) were related to pore sizes of the cylindrical mesoporous silica derived as the maximum in the PSD calculated from the N2 adsorption isotherm with the BJH model.66 For Tf, a good fit was obtained excluding the correction for a nonfreezing layer. Table 2 lists the fitting results, while the experimental data and corresponding fits are included in the Supporting Information (Figures S1 and S2). To derive the entrance and cage sizes, the onset freezing and melting temperatures were determined with a heating/cooling rate of 1 °C min−1 in the presence of extraporous ice by first

(1)

(2)

where r represents the pore radius, p/p0 the relative pressure, and t the statistical film thickness. The size distribution of the widest entrance path to the cages was derived from the N2 desorption branch by using the BJH model modified according to the Kruk−Jaroniec−Sayari (KJS) correction and the Harkins−Jura thickness equation. It was also calculated from the Ar desorption branch using the BJH model with empirically established corrections as published by Kruk et al. (eq 3).68 r(p /p0 ) [nm] =

CGT , r − tnf

Table 2. Gibbs−Thomson Fitting Parameters for the Melting Point Depression (Tm) and Freezing Point Depression (Tf) for H2O Confined in Cylindrical Ordered Mesoporous Silica Using the Mean Pore Radii Derived from N2 Physisorption Determined with the BJH Model

−0.5393 + t(p /p0 ) + 0.343 log(0.8259p /p0 ) (3)

The statistical thickness reference curve reported therein was obtained using a macroporous silica (LiChrospher Si-1000)68 and was found to be accurately represented in the relative pressure range of 0.1−0.95 by the empirical formula displayed in eq 4. 23385

Gibbs−Thomson fitting parameters

Tm (°C)

Tf (°C)

CGT (°C nm) tnf (nm)

53.1 0.67

80.0 0

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Figure 1. TEM micrographs for SBA-16 silicas (A) SBA1-60/24, (B, C) SBA2-90/48, FDU-12 silicas (D, E) FDU-60/48 and (F) FDU-120/48 along the (100) or the (111) crystallographic planes as indicated. Insets show corresponding Fourier diffractograms.

Figure 2. DSC thermograms recorded during heating at 1 °C min−1 of (A) selected SBA-16 and (B) selected FDU-12 silica materials impregnated with a 2-fold excess of H2O as compared to the pore volume; thermograms are offset for clarity.

cooling the sample to −70 °C and heating to −5 °C (for SBA16 materials) or −3 °C (for FDU-12 materials) and averaged over three independent measurements for each material. The standard error was ≤0.04 °C. A scanning method with different starting temperatures was used to determine the size distribution of the widest entrance path to the cages. Impregnated samples were equilibrated for 10 min at a given temperature between −45 and −10 °C. The first measurement started with equilibrium at −45 °C and the next at a slightly higher temperature, until a series of measurements had been performed scanning the whole temperature region between −45 and −10 °C with 1 or 0.5 °C intervals. After each equilibration at a given temperature the sample was heated to −5 °C (for SBA-16 materials) or −3 °C (for FDU-12 materials). The melting enthalpy released by ice confined in the cages after equilibration at a certain temperature (ΔH(Teq)) was compared to the melting enthalpy of the total amount of pore-confined ice, i.e., that released during the heating step after cooling to −70 °C (ΔH−70). The fraction of ΔH−70 released after each equilibration temperature (Teq) corresponded to the fraction of pore volume accessible through entrances with such a size that had allowed freezing of confined water at the equilibration temperature (V(Teq)):

V (Teq) =

ΔH(Teq) ΔH −70

Vtot

(6)

The total pore volume (Vtot) was determined with DSC following a procedure described earlier, using the melting enthalpy of extraporous ice and thermogravimetric analysis.50 The differential pore volume was obtained by differentiating with respect to the equilibrium temperature. Finally, the equilibration temperature was correlated to the entrance size by the Gibbs−Thomson equation using the parameters given in Table 2, leading to the entrance size distribution expressed as differential pore volume as a function of the entrance size.

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RESULTS Structural Characterization of SBA-16 and FDU-12 Silicas. Thermoporometry for silica materials with cage pore structure was evaluated on a series of SBA-16 and FDU-12 materials. SBA-16 materials display a body-centered cubic ordering of cages with 8 connecting entrances (Im3m symmetry).23 In FDU-12 (Fm3m), cages display a face-centered cubic close-packing and are connected to 12 nearest neighbors.24 Here, SBA-16 with cages ranging from 5−9 nm as well as some ultralarge cage FDU-12 materials (13−22 nm) 23386

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were used. Transmission electron microscopy (TEM) confirmed large ordered porosity domains for selected SBA-16 and FDU-12 materials (Figure 1). The views recorded along the (100) and (111) crystallographic directions agree with the cubic Im3m and Fm3m symmetries for the SBA-16 and FDU-12 samples, respectively. For the SBA-16 materials, the use of 1butanol as a swelling agent as well as the increase in the hydrothermal treatment temperature and time led to an increase in the cage diameter and a concurrent decrease in the wall thickness (cf. SBA1-60/24 in Figure 1A and SBA2-90/ 48 in Figure 1B,C). Similar trends were observed for the series of FDU-12 materials (Figure 1D−F), for which an increased hydrothermal treatment temperature from 60 °C (FDU-60/48) to 120 °C (FDU-120/48) led to an enlargement of the cage diameter at the expense of the wall thickness. Long-range structural ordering was confirmed by low-angle X-ray diffraction (Figure S3). For the SBA-16 materials, the XRD patterns were in agreement with the expected the Im3m symmetry. However, the (200) and (220) diffraction lines were difficult to observe for materials with the largest unit cells.46 For the ultralarge cage FDU-12 silica materials, the very large unit cell prevented recording of the (110) diffraction peak and indexing of higher diffractions. Cage Size Analysis by Thermoporometry. The depressed melting and freezing point of confined water was determined with 0.04 °C accuracy using differential scanning calorimetry. Analogous to adsorption in physisorption, the depressed melting point of confined water is related to the cage size, while freezing is correlated to the entrance size. We first discuss the use of thermoporometry by DSC of pore-confined water to evaluate the cage sizes of the SBA-16 and FDU-12 series. Figure 2 shows the melting of water impregnated in selected SBA-16 and FDU-12 materials. The DSC thermograms recorded during heating showed single, narrow melting peaks, confirming uniform cage sizes. Only for SBA2-100/72 a bimodal melting peak was observed, which agrees with the observation of a two-step adsorption in nitrogen physisorption (vide inf ra). The depressed melting point is inversely related to the cage diameter according to the Gibbs−Thomson equation.49 For the selected SBA-16 materials shown in Figure 2A, the onset melting point varied between −16.3 and −26.4 °C as a result of the varying cage diameters (see also Table 3). The additional heat treatment of SBA2-100/24_I and SBA2100/24_II was expected to cause cage shrinkage, and this was indeed reflected by the lower melting points as compared to SBA2-100/24. For FDU-12, roughly two groups of materials are found, i.e., those displaying smaller cages with melting onset temperatures around −9 °C and those having larger cages, showing melting point onsets around −7 °C. Nevertheless, as can be seen in Table 3, the small differences in cage sizes within each group of FDU-12 materials still result in significantly distinguishable melting point onsets, highlighting the exceptional accuracy of DSC. The cage sizes calculated by DSC, N2 physisorption, and TEM are listed in Table 3. Determination of cage sizes by TEM is inherently subject to a certain inaccuracy since it implies the use of 2D projections of multiple overlaying cages under the assumption of spherical geometry. Shrinkage and shear forces due to ultramicrotomy may also affect the obtained value. Nonetheless, a remarkably good agreement was found between the cage sizes derived from TEM for selected materials and those determined by N2 physisorption, with relative deviations lower than 17%. The correlation of the cage diameters derived

Table 3. Cage Size Analysis for Selected SBA-16 and FDU-12 Silica Materials cage diameter (nm) sample

onset melting point (°C)a

DSCb

N2-physc

TEMd

SBA1-60/24 SBA1-100/24 SBA2-90/48 SBA2-120/24_II SBA2-120/24_I SBA2-120/24 SBA2-100/72 FDU-35/24 FDU-60/48 FDU-100/48 FDU-110/48 FDU-120/48 FDU-130/48

−26.4 −21.8 −17.5 −18.1 −17.0 −16.1 −16.3 −9.5 −9.1 −6.8 −6.8 −6.6 −6.4

5.4 6.2 7.4 7.2 7.6 7.9 7.8 12.5 13.0 16.9 16.9 17.4 17.9

5.4 6.3 7.8 7.5 7.9 8.5 8.5 13.5 13.6 17.6 19.6 21.8 21.8

4.5 ± 0.4 7.5 ± 0.6

14.2 ± 0.5

21.4 ± 1.3

Melting point determined by DSC (heating rate 1 °C min−1). bCage size determined from the onset melting point by DSC using the Gibbs−Thomson equation with fitting parameters listed in Table 2. c Cage size determined from adsorption isotherm using the BJH model. d Volume averaged cage diameter and standard deviation. a

from DSC and N2 physisorption is illustrated in Figure 3. The cage sizes determined by both techniques are in fair agreement

Figure 3. Parity plot for cage sizes of selected SBA-16 and FDU-12 silicas as determined by N2 physisorption and calculated from the onset temperature for melting of confined water determined by DSC. Line is added as a guide to the eye.

for sizes below ca. 18 nm. It has been reported earlier that the melting depression is independent of the pore shape,57,59 confirming the validity of cylindrical mesoporous materials as reference. However, as seen in Figure 3, for FDU-12 materials with cage sizes above 18 nm, the cage diameters obtained by DSC are systematically lower, up to 20%, than those estimated based on N2 physisorption. This deviation might result from a pore shape effect on the melting point depression for very large cages, as the KJS-BJH method was initially derived for smaller (