Structure and morphology of propylthiol-functionalised mesoporous silicas templated by non-ionic triblock copolymers

Microporous and Mesoporous Materials 79 (2005) 241–252 www.elsevier.com/locate/micromeso

Structure and morphology of propylthiol-functionalised mesoporous silicas templated by non-ionic triblock copolymers Robert P. Hodgkins a, Alfonso E. Garcia-Bennett b, Paul A. Wright a

a,*

School of Chemistry, University of St. Andrews, Purdie Building, North Haugh, St. Andrews, Fife KY16 9ST, UK b Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691 Stockholm, Sweden Received 20 August 2004; accepted 19 October 2004 Available online 7 January 2005

Abstract The effect of the co-condensation of 3-mercaptopropyltriethoxysilane (MPTES) in the preparation of mesoporous silicas has been examined for levels of MPTES of 0–10 mol% (based on silicon) in reported syntheses using the block co-polymers P123 and F127. In syntheses using P123, SBA-15 (p6mm) forms with 0–5 mol% MPTES whereas addition of 7 mol% of the silane results in highly ordered Ia3d silica. Using F127, synthesis in the absence of MPTES gives the reported cubic Fm3m FDU-12 structure, based on the cubic close packing of surfactant micelles, whereas addition of MPTES results in the introduction of a high concentration of stacking defects in the close packing sequences and changes in particle morphology. At 5 mol% MPTES, tapered hexagonal prismatic particles with nano-domains of the hexagonally close packed structure (space group P63/mmc) are obtained. The accessible thiol contents in the members of these two series of solids have been determined using EllmanÕs reagent (5,5 0 -dithiobis-(2-nitrobenzoic acid)): values of 50–70% of the thiols incorporated in the solid have been measured as being accessible to this reactant. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Mesoporous solids; SBA-15; FDU-12; Thiol contents; EllmanÕs reagent

1. Introduction The ease of preparation of functionalised mesoporous silicas [1] with structures related to those mesoporous silicas synthesised previously [2–5] has suggested potential optical applications [6], or use in ion exchange, catalysis [7] and gas separation [8]. The functional groups can be used in the selective removal of cations from solution, or to act as tethers for catalytic complexes or as catalysts themselves in acid or base-catalysed reactions. Among the groups that can be included are carboxylic acids (via cyano-groups) [9,10], amines

*

Corresponding author. Tel.: +44 1334 463793; fax: +44 1334 463808. E-mail address: [email protected] (P.A. Wright). 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.10.036

[11] and alkylammonium groups [12]. Thiol groups can also be incorporated [13] and used to synthesize carbon replicas of the silicas [14]. These groups can be included by post-synthetic grafting or by co-condensation during synthesis [1]. In post-synthetic grafting, a pre-calcined mesoporous silica, partially re-hydrated to generate surface hydroxyls, is reacted with the appropriate alkoxysiloxane, whereas co-condensation involves the addition of both tetraethylorthosilicate (TEOS) and the functionalised siloxane (EtO)3–Si–X to the synthesis mixture. The relative strengths of these approaches have been described previously [1]: for the in situ method the approach is a one-pot method to prepare functionalised material, whereas the post-synthetic modification ensures location of the functional groups at the silica surfaces. We have previously prepared functionalised mesoporous silicas to act as enzyme supports and adsorbents

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for size selective uptake of proteins [13,15]. In these studies we prepared SBA-15 samples functionalised with mercaptopropyl, cyanopropyl and chloropropyl groups. The most promising results for supporting the protease enzyme trypsin were obtained for SBA-15 functionalised in situ by propylthiol groups. The same material acts as a molecular sieve for proteins, displaying irreversible and size selective adsorption. Other groups have also shown interest in the functionalisation of mesoporous solids as precursors to supported sulphonic acids [16]. The thiol groups can be oxidised to sulphonic acid groups by using aqueous H2O2 as the oxidising agent for use in catalysis. During our attempted syntheses of thiol-functionalised SBA-15 we noticed that inclusion of thiol groups at around 7% (based on SiO2) resulted in the formation of mesoporous silica with the cubic Ia 3d structure [17]. The group of Zhao et al. reported that incorporation of either 3-mercaptopropyltrimethoxysilane, MPTMS or aromatic co-solvents (benzene, methylbenzene, etc.) to the preparation gives rise to the formation of the extra large pore cubic material, which they named FDU-5 [18]. Furthermore Wang et al. have shown that the large pore Ia 3d structure can be prepared by addition of triethoxyvinylsilane to the mixture at a concentration of 20% [19]. In this paper we detail the effect of the addition of variable amounts of 3-mercaptopropyltriethoxysilane (MPTES) on the synthesis of large pore mesoporous materials using the non-ionic surfactant, Pluronic P123 (EO20PO70EO20) and describe the properties of the resultant solids. We also describe the effects of adding MPTES to the synthesis of silicas using the non-ionic surfactant, Pluronic F127 (EO106PO70EO106). The synthesis and characterisation of the face centred (Fm3m) cubic structure in the pure silica structure using this surfactant has been reported independently by Zhao and coworkers [20] and Ryoo and coworkers [21]. These materials, the structures of which are based on condensation of silica around a cubic close packed array of spherical micelles, are similar to the much smaller pore versions of ÔmesocageÕ materials (SBA-2 [22,23], STAC-1 [24] and STA10 [25]) prepared using the dicationic Gemini surfactant CH3(CH2)15N(CH3)2(CH2)3N(CH3)3Br2, or the slightly larger pore SBA-12, prepared using the non-ionic surfactant, Brij 76 (C18H37(EO)10) [26]. Che et al. have

shown that it is also possible to prepare the fully ordered hexagonal end member of the series of structures based on close packed arrays of micelles, space group P63/ mmc, with cage size similar to that of SBA-2, using the cationic surfactant cetyltrimethylammonium and well defined concentrations of sulphate counterions [27]. High resolution electron microscopy revealed frequent stacking faults and intergrowths in SBA-2 and SBA-12, typical of close packed systems. Similar microstructures (but with larger unit cell and pore sizes) have also been observed in the pure silica form of FDU-1, prepared with the block copolymer B50-6600 (EO39– BO47–EO39) [28]. Here we show the co-condensation of 3-mercaptopropyltriethoxysilane on syntheses of mesostructured silicas using F127 results in changes to the microstructure and morphology of the mesoporous phase that forms. In addition, for all the thiol-containing mesoporous solids, we report the results of the use of EllmanÕs reagent, 5,5 0 -dithiobis-(2-nitrobenzoic acid), to quantify the fraction of included thiol groups [29] that are accessible to adsorbed molecules. This analysis is based on the method proposed by Badyal et al. [30] for the quantification of thiol groups on macroporous polymer resins. Knowing the concentration of the accessible (rather than total) thiol groups is of importance when using them to functionalise further the solid or in assessing the specific activity of thiol sites in adsorption and catalytic processes.

2. Experimental 2.1. Synthesis of mercaptopropyl-functionalised silicas using P123 Organically-modified molecular sieves were prepared by the co-condensation of trialkoxysilanes and tetraethylorthosilicate. The general procedure adopted was similar to that reported for SBA-15 [5]. The non-ionic triblock copolymer Pluronic P123 (EO20–PO70–EO20, MWav. 5750; BASF) was used as a template and tetraethylorthosilicate (TEOS, 98% Aldrich) was the source of the majority of the silica. 3-mercaptopropyltriethoxysilane (MPTES), (EtO)3Si(CH2)3SH (95%, Avocado) was added at concentrations of 0–10 mol% (based on sil-

O2N S HO2C

S

CO2H

RSH

S

CO2H

HS

CO2-

RS

H+ NO2

NO2 Scheme 1.

NO2

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Table 1 Elemental analyses of extracted propylthiol-functionalised mesoporous silicas templated with P123 and F127 Non-ionic surfactant

SH loading (based on SiO2) (%)

C content (%)

H content (%)

S content (%)

P123 P123 P123 P123 F127 F127 F127 F127

2 5 7a 7b 2 5 7 10

11.58 14.05 11.21 14.44 19.93 20.11 12.00 20.38

2.65 3.04 2.47 3.26 3.78 3.98 2.16 4.04

0.81 1.84 2.49c, 2.51c 2.57 0.68 1.52 2.67 3.03

a b c

313 K hydrolysis step. 323 K hydrolysis step. Repeat values.

Fig. 2. XRD of extracted SBA-15-SH-7% showing mesophase product as a function of hydrolysis temperature. From bottom to top: SBA-15SH-7%-313 K; SBA-15-SH-7%-323 K; SBA-15-SH-7%-333 K.

0

1

2

2 Theta /

3

4

5

o

Fig. 1. X-ray diffraction patterns of propyl-thiol functionalised mesoporous silica templated by P123. Diffractograms are offset for clarity. From bottom to top: SBA-15-pure silica; 2% added MPTES (represented as SH); 5% SH; 7% SH and 10% SH.

ica). Molar compositions of the sol–gels were (1  x)Si(OEt)4:x(EtO)3Si(CH2)3SH:0.017P123:2.9HCl: 200H2O, 0 6 x 6 0.10. The surfactant was dissolved in the acidic media and stirred at 313–333 K. TEOS and MPTES were added simultaneously and the mixture stirred for 24 h to allow the onset of hydrolysis and the mesostructure formation to take place. Finally the mixture was transferred to a Teflon bottle and hydrothermal treatment applied at 373 K for 48 h enabling further condensation of the silica framework. The resulting solids were filtered, air dried and rendered porous by either calcination (N2 823 K, 4 h; O2 823 K, 4 h) or solvent extraction (3 washings: refluxing with EtOH 50 ml/g, 8 h).

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Fig. 3. SEM micrographs of mesoporous silica templated by P123 with varying thiol loadings: (a) 2% SH; (b) 5% SH; (c) 7% SH and (d) 10% SH.

For comparison, experiments were also performed in which similar concentrations of phenyl-triethoxysilane, cyanopropyltriethoxysilane or chloropropyltriethoxysilane were used in the synthesis in place of MPTES. 2.2. Synthesis of mercaptopropyl-functionalised silicas using F127 The synthetic route was adapted from the published method by Zhao and coworkers [20] for the pure silica mesoporous solid FDU-12. The non-ionic triblock copolymer Pluronic F127 (Prill form; EO106PO70EO106 Mrav. 12,600; BASF) is used as the template and TEOS (98% Aldrich) as the silica precursor for the formation of FDU-12 in acidic media using KCl and 1,3,5-trimethylbenzene (TMB) as additives. MPTES (95% Avocado) was incorporated between 0–10 mol% (based on SiO2). Molar compositions of the sol–gels were (1  x) Si(OEt)4:x(EtO)3Si(CH 2)3SH:0.004F127:6.08HCl:141 H2O:1.70KCl:0.42TMB, 0 6 x 6 0.10. The surfactant and additives were dissolved in acidic solution and stirred at 313 K for 24 h before the simultaneous addition of TEOS/MPTES. The resulting mixture was stirred at 313 K for 24 h and transferred to a Teflon bottle under static conditions to allow hydrothermal treatment at 373 K for 72 h. The resulting white solid was filtered, air dried and the template removed either by calcination or solvent extraction as for the samples prepared with P123.

2.3. Quantifying the accessibility of thiols within mesoporous silica To determine the accessibility of the SH groups included during the synthesis, extracted propyl-thiol functionalised mesoporous silicas were analysed using EllmanÕs reagent. The method has been adapted from Badyal et al. [30]: it can be used quantitatively because the disulphide within 5,5 0 -dithiobis(2-nitrobenzoic acid) (EllmanÕs reagent; 99% Aldrich) reacts with the propyl-thiol groups bound to the silica framework to form an anion which exhibits a strong yellow colour absorbing at 412 nm (Scheme 1). In a typical analysis, EllmanÕs reagent, 0.1 g, in excess (250lmol) is dissolved in 25ml MeOH. A known amount of mesoporous solid (based on the expected moles of SH present) was added to a 100 ml volumetric flask with 20ml MeOH. EllmanÕs solution was added together with 500ll of N,N-diisopropylethylamine (DIPEA; 99% Aldrich) and the flask was shaken for 30min. The solid was then filtered off and the filtrate transferred to a 250 ml volumetric flask. The solid was washed with MeOH and the filtrate added to the 250 ml volumetric flask before bringing the volume to 250ml with MeOH. A 10ml aliquot was taken and 90 ml MeOH added to reduce further the concentration for UV analysis. The values are compared against those from the reaction of known amounts of cysteine in methanol. Typically between 50lmol and 200 lmol of SH are quantified in this way.

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Fig. 4. HRTEM micrographs of P123 templated extracted propyl-thiol functionalised mesoporous silica. Micrographs (a) and (b) show the pores perpendicular to the beam for the pure siliceous and 2% thiol loading respectively. Micrograph (c) is taken along the [1 1 1] direction of the (poorly ordered) Ia 3d structure for 5% thiol loading, (d) shows the ordered cubic Ia3d structure along [1 1 1] for 7% a sample with thiol loading—with corresponding Fourier simulated electron diffraction pattern inset and (e) shows poorly ordered Ia3d structure along the [3 1 1] zone axis (10% thiol sample).

Characterisation. Powder X-ray diffraction (XRD) patterns of the extracted and calcined mesoporous mate˚ ). rials were obtained using Cu Ka radiation (k = 1.5418 A Porosity measurements were obtained using a IGA-II

series automated gravimetric analyser. Samples were degassed at 393 K for 4 h before setting the dry mass and data collection. The equilibrium adsorption values were those calculated by mathematical analysis of the

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Fig. 5. HRTEM micrographs of P123 templated silica with 7% SH loading varying the hydrolysis temperature from (a) 333 K (pores perpendicular to the incident beam with hexagonal p6mm symmetry); (b) 323 K (viewing the [1 1 0] zone axis of cubic symmetry) and (c) 313 K (viewing along [1 1 1] of Ia 3d).

Table 2 Unit cell parameters of selected extracted and calcined mesoporous silica templated by P123 and functionalised with thiol by diffraction and microscopy ˚) ˚) Material XRD (A TEM (A SiO2 pure siliceous-calcined SiO2 2% SH-extracted SiO2 5% SH-extracted

ahex = 164 ahex = 111 ahex = 100

SiO2 SiO2 SiO2 SiO2 SiO2

acub = 235 acub = 208 acub = 235 ahex = 116 acub = 221

7% SH 313 K-extracted 7% SH 313 K-calcined 7% SH 323 K-extracted 7% SH 333 K-extracted 10% SH-extracted

ahex = 137 ahex = 119 ahex = 102a acub = 227a acub = 230 – acub = 230 ahex = 103 acub = 205

a Both hexagonal and cubic phases observed by TEM. Only the hexagonal lattice value is extracted from the XRD pattern.

asymptotic increase or decrease of weight within a 1 h time period. Specific surface areas are calculated using the Brunauer–Emmett–Teller (BET) model on the type IV isotherms in the region applicable to the derivation of the BET equation. Pore size distributions are calcu-

lated using the de Boer model on the adsorption branches. Scanning electron microscopy micrographs were obtained via a JEOL SEM-5600 microscope and high resolution transmission electron microscopy (HRTEM) micrographs using a JEOL JEM-2011 electron microscope operating at 200 keV. Samples were ground before being dispersed in acetone and then deposited onto a holey carbon film, supported on a copper grid. UV/Visible absorption spectra were recorded on a PerkinElmer Lambda 35 UV/Vis spectrometer using a matching pair of quartz cuvettes (pathlength 1 cm). The spectra have a starting wavelength of 650 nm and a finishing wavelength of 360 nm with a scan speed of 60 nm/min.

3. Results and discussion Elemental analyses of silicas templated by P123 and F127 surfactants and functionalised with different levels of the thiol-containing siloxane are given in

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3.1. Syntheses using P123 surfactant

1400

1200

Volume cm-3 (STP)

1000

800

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400

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0 0.00

0.20

0.40

0.60

0.80

1.00

P/Po Pure siliceous

2% SH

5% SH

7% SH

10% SH

1600

1400

1200

Volume cm-3 (STP)

1000

800

600

400

200

0 0.00

247

0.20

0.40

0.60

0.80

1.00

P/Po Pure siliceous

2% SH

5% SH

7% SH

10% SH

Fig. 6. N2 adsorption and desorption isotherms (77 K) of (bottom) calcined and (top) extracted propyl-thiol functionalised mesoporous solids templated with P123 surfactant. (Successive samples offset by 250 cm3 (STP)/g for clarity.)

Table 1. These indicate that the thiol-functionalised siloxane groups are incorporated into the silica products in ratios close to those in the synthesis gels, except for the 10% loading with F127 surfactant, where the efficiency of incorporation of the thiol groups is lower.

The addition of 0–10% MPTES gives a series of mesoporous silicas, the XRD patterns of which (Fig. 1) reveal the highly ordered p6mm structure for siliceous solids and those prepared with 2% of added MPTES, which show clear secondary peaks at 1.5–2.0° 2H, expected for the (1 1) and (2 0) reflections. A lower degree of long range order, with no observable secondary peaks, is observed with 5% and 10% added thiol. At 7% thiol, however, the X-ray pattern is characteristic of that expected for silica with Ia3d symmetry, with a shoulder on the first, major peak, and additional diffracted intensity at 2° 2H. The effect of temperature of the hydrolysis step is shown in Fig. 2: performing the entire synthesis at 333 K results in a lower degree of long range order and synthesis of the hexagonal, rather than the cubic phase. This may reflect the change with temperature of the relative stabilities of the silica-coated micelles that give rise to the cubic and hexagonal phases. Scanning electron microscopy of the solids (Fig. 3a– d) shows an evolution in the particle morphology with increasing thiol content. Particles of solids with a 2% thiol loading, which show the SBA-15 X-ray diffraction pattern, have elongated, worm-like morphology similar to that of the pure silica analogue, whereas solids of higher thiol content are made up of spherical particles 1–10 lm in diameter. The change in morphology between 0% and 2% to 5% and 7% thiol concentrations (worm-like to spherical) is consistent with the different symmetries of the products. SBA-15 is known to occur with worm-like morphology [31] whereas MCM-48 (cubic Ia3d) is observed with spherical morphology [32]. Transmission electron microscopy confirms the difference in the product phase that occurs upon increasing the thiol content of the preparations (Fig. 4). The pure silica and 2% thiol samples show the typical hexagonal SBA-15 pore structure. The 5% thiol sample, more poorly ordered by XRD, shows small domains of both the hexagonal and cubic phases, suggesting that this concentration of thiol is at the boundary between the two phasesÕ formation fields. No particles were found that included both phases. The 7% MPTES-containing material prepared following hydrolysis at 313 K shows well defined Ia3d cubic structure. The effect was found to be reproducible: samples with SH contents of 2.49%, 2.51% and 2.57% (Table 1) all show the cubic structure by TEM. Increasing the hydrolysis to 333 K in temperature results in a disordered hexagonal p6mm structure as shown in Fig. 5. The 10% thiol sample is generally more poorly ordered, much of it without clearly defined hexagonal or cubic ordered regions, although some small particles of the cubic phase can be observed (Fig. 4e). Details of the unit cell size from selected extracted and calcined material, as determined

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Table 3 Porosity measurements of thiol-functionalised mesoporous silica template by P123 Material

Specific surface area BETa (m2/g)

˚) Pore size modeb (A

Total N2 uptake cm3 (STP) g1

SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 SiO2

587 793 534 812 490 791 602 784 437 264 710

116 137 143 120 114 90 75 82 105 63 58

632 735 576 688 472 600 520 600 344 192 416

pure siliceous-extracted pure siliceous-calcined 2% SH-extracted 2% SH-calcined 5% SH-extracted 5% SH-calcined 7% SH 313 K-extracted 7% SH 313 K-calcined 7% SH 333 K-extracted 10% SH-extracted 10% SH-calcined

a

BET surface area calculated within 0.05–0.4 P/P0. Pore size distribution calculated on the adsorption branch of the isotherm using the de Boer model. Taken as the maximum in the pore size distribution curve. b

from the XRD and TEM data, are given in Table 2. This includes two values for the 5% sample, as two phases were observed by TEM. Nitrogen adsorption isotherms of extracted and calcined samples (Fig. 6) are typically type IV and display well defined hysteresis. Specific surface areas calculated from BET and pore size distribution measured on the adsorption branch using the de Boer model are given in Table 3. Pure silica and 2% thiol samples show similar high porosity and well-defined adsorption, with the 5% sample displaying a more poorly defined capillary condensation step and lower porosity. The cubic Ia3d (7% SH) sample shows high porosity and well defined capillary condensation, corresponding to the high degree of order in these samples, whereas increasing the thiol content to 10% results in much reduced adsorption capacity, in line with the structural data. The nitrogen isotherms of the 10% thiol sample show a lower and less well-defined hysteresis loop than materials with lower thiol contents, whereas the microporosity (particularly when considering the calcined samples) is similar for all the samples. This loss of mesoporosity in the 10% thiol sample must result from the incorporated micelles being more poorly ordered and less in volume. Inclusion of the related siloxane, MPTMS at higher levels has previously been shown to result in similar disruption of the surfactant aggregates in a related system [33]. There is an increase in adsorption capacity for all samples upon calcination, because residual surfactant remains after extraction. Most of the increase in capacity is in the micropore region, indicating that much of the residual surfactant after extraction is in micropores in the structure, as reported for similar SBA-15 materials [34]. In order to understand better the role of the added functionalised siloxane in controlling the product selectivity, organically-modified silicas were also prepared under identical conditions in the presence of phenyl-, cyanopropyl- and chloropropyltriethoxysilanes. Transmission electron microscopy of the samples reveals that

the cubic Ia3d structure is obtained in the presence of 5% added phenyl-functionalised siloxane, but not at any concentrations for the other additives (Supplementary data—S1). Furthermore, in previous work the cubic Ia3d structure has been prepared with the pure silica composition by the addition of aromatics [18] or butanol [35] to the synthesis mixture. Indeed, we have found that it is possible in a straightforward manner to obtain thiol-functionalised cubic Ia3d silicas with 2% thiol contents by adding MPTES to syntheses of this kind that contain butanol (not shown). It appears that adding thiol-, vinyl- [10] and phenylsiloxanes or butanol or aromatics compounds to the mix affects the phase selectivity of the mesophase in a similar way, giving the Ia3d structure at specific reactant concentrations (the amount of added MPTES required to give rise to the cubic phase under the conditions reported here (7%) is similar to that reported by Zhao and coworkers [18] for the MPTMS additive). Such molecules as these are expected to become concentrated within or at the surface of the surfactant micelle. Since the Ia3d structure possesses a lower surface curvature than the p6mm structure, which remains the phase that forms if chloropropyl- or cyanopropyl-siloxanes are added, the Ia3d phase formation is thought to result from the effect of the additives described above on the geometry of the silicate-covered surfactant micellar phase that forms. The exact mechanism by which this modification of the micelle geometry occurs remains an open question [36,37]. 3.2. Syntheses using F127 surfactant A series of mesoporous solids was prepared using the F127 surfactant with added MPTES. N2 adsorption isotherms (Fig. 7) at 77 K of the extracted and calcined materials reveal that the samples with 0–7% added MPTES possess well defined mesoporosity and large surface areas, whereas the sample containing 10%

R.P. Hodgkins et al. / Microporous and Mesoporous Materials 79 (2005) 241–252 1400

1200

Volume cm-3 (STP)

1000

800

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400

200

0 0.00

0.20

0.40

0.60

0.80

1.00

P/Po Pure siliceous

2% SH

5% SH

7% SH

10% SH

1600

1400

Volume cm -3 (STP)

1200

1000

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600

400

200

0 0.00

0.20

0.40

0.60

0.80

1.00

P/Po Pure siliceous

2% SH

5% SH

7% SH

10% SH

Fig. 7. N2 adsorption and desorption isotherms (77 K) of (bottom) calcined and (top) extracted propyl-thiol functionalised solids templated with F127 surfactant. (Successive samples offset by 250 cm3 (STP)/g for clarity.)

MPTES shows only a small capillary condensation step and hysteresis loop. The inflexion points of the capillary condensation steps of the adsorption branches have P/P0 values of between 0.75–0.86 for extracted and calcined samples. Scanning electron micrographs reveal clear changes in the particle morphology (Fig. 8a–e). In the pure silica

249

materials the shapes are not very well defined, with both rounded and faceted particles present. Increasing the thiol content to 2% results in more hexagonal prismatic morphology, and by 5% the particles have a clearly defined hexagonal prismatic geometry, tapering towards the basal faces. The 7% thiol-containing sample consist of hexagonal plates, with much reduced length to cross sectional area ratio and at 10% no clear particle morphology is observed. The origin of the changes in shape become apparent from the HRTEM images. For the pure silica sample the well defined Fm3m structure described for FDU12 is observed. Fig. 9a and b show micrographs taken down the [0 0 1] and [1 1 0] axes of the Fm3m unit cell, where the a cell dimension is 27 nm. Describing this in terms of the stacking sequence of close packed spheres, this approximates to the ideal ABCABC stacking sequence. For the 5% SH material it was thought that the hexagonal morphology might result from an ordered hexagonally close packed version of the close packed array, since the particle morphology approaches that reported [27] for the ordered smaller pore material prepared with cetyltrimethylammonium and sulphate counter ion. Such an ordered arrangement (ABAB) has also been observed previously in small domains in SBA-12. However, examination of the electron micrographs taken parallel to the plane of close packing show this is not the case. The larger, hexagonal prismatic particles display stacking sequences that include twin planes, stacking faults as well as nano-domains of both cubic (ABC) and hexagonal (ABAB) sequences (Fig. 9c– e). For some smaller particles the cubic close packing is retained. Nevertheless, the inclusion of the MPTES in the synthesis, as is the case with the P123 syntheses, does have an important effect on the structure and morphology of the resultant silica phase. Unlike the p6mm to Ia3d phase transition, there is no obvious change in curvature in the Fm3m to P63/mmc transition, if the structures correspond to ideal close packing, since the difference lies in the stacking sequence rather than the local structure. We are currently investigating whether careful control of the conditions in this system can give rise to the perfect hexagonal P63/mmc structure. 3.3. Quantification of the accessible thiol content As described in the introduction, EllmanÕs reagent was used to quantify the accessible thiol for the silicas containing 2–7% thiol that are prepared with P123 or F127 surfactants (Table 4). Nitrogen adsorption isotherms of samples analysed in this way show typical Type IV isotherms (Supplementary data—S2), indicating that the pore structure is not degraded by the analysis. The analyses indicate that at least 50–70% of the added thiol groups are accessible to the EllmanÕs reagent

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Fig. 8. SEM micrographs of mesoporous silica templated with F127. (a) Pure silica; (b) 2% thiol loading show an unclear well-defined morphology; (c) 5% thiol-loading showing hexagonal prisms; (d) 7% thiol-loading show hexagonal plates and (e) 10% thiol-loading show no defined morphology.

for these solids, suggesting that at least this fraction is attached to the internal surfaces of the solids. The results suggest that a larger fraction of the thiol groups are accessible in the p6mm and Ia 3d silicas (58–72%) than in the Fm3m/P63/mmc materials (50–60%) and are therefore available for applications such as heavy metal extraction from solution or, through further reaction, as sulphonic acids for acid catalysis.

4. Conclusions The addition of 3-mercaptopropyltriethoxysilane to the reported syntheses of mesoporous silicas using the triblock copolymers P123 and F127 results in changes

to the pore structure and particle morphology of the silica frameworks that form. For the structures templated by P123, addition of MPTES at a level of 7% results in the formation of a well defined cubic Ia3d phase, similar to the phase FDU-5 reported elsewhere. The range of thiol concentrations that form this solid without additional additives is narrow: increasing the thiol content to 10% or increasing the hydrolysis temperature by 20 K results in loss of long range order. However, solids with Ia3d symmetry and lower levels of thiol functionalisation can be prepared by the addition of more hydrophobic additives such as butanol. Co-condensing 5 mol% phenyl-functionalised siloxane in the preparation also results in the formation of the cubic Ia3d silica, but adding cyano-

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Fig. 9. HRTEM micrographs of F127 templated mesoporous silica with corresponding Fourier simulated electron diffraction pattern inset: (a) and (b) pure siliceous FDU-12 viewed down the [0 0 1] and [1 1 0] zone axes, respectively. Analysis of the 5% thiol-loaded sample is given in micrographs (c)–(e), (c) shows cubic (ABC) stacking without structural defects. (d) The stacking sequence CABCBCABCACBABC, which includes two twin planes and a stacking fault. (e) A hexagonal close packed domain with the stacking sequence BCBCBCBCB.

propyl- or chloropropyl-functionalised siloxanes does not. In the case of F127-templated materials, which in the absence of thiol give the cubic Fm3m structure (FDU12), addition of thiol results in changes in the stacking

sequences of the close packed micelles. This takes the form of frequent twinning and stacking faults and in some cases nano-domains of the hexagonal close packed arrangement. The intergrowth structure results in well defined particle morphologies that may either be tapered

252

R.P. Hodgkins et al. / Microporous and Mesoporous Materials 79 (2005) 241–252

Table 4 Quantification of thiol-groups via EllmanÕs reagent for extracted P123 and F127 templated solids Samplea

SH loading (based on SiO2)

% SH accessibility

P123 P123 P123 F127 F127 F127

2% 5% 7%b 2% 5% 7%

58 72 68 50 56 60

a b

All samples extracted. Hydrolysis temperature: 313 K.

hexagonal prisms or hexagonal plates. It should be possible to adjust the synthesis conditions to prepare for the first time the fully ordered large pore hexagonal P63/ mmc mesoporous solid. For all these thiol functionalised solids, the use of a modified analysis method using EllmanÕs reagent is possible without degradation of the pore structure and indicates that at least 50–70% of the included thiol groups are accessible to small molecules. Acknowledgment We gratefully acknowledge Johnson Matthey and the EPSRC for funding (RPH) and Sylvia Williamson for assistance in collecting the N2 adsorption data. Dr. T. Sen (UMIST) is thanked for the low-angle XRD data. BASF, Mount Olive, NJ, USA are thanked for their kind supply of Pluronic surfactants.

Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.micromeso.2004.10.036.

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