Mesopore constrictions derived from the substitutionally co-packed SBA-15

Microporous and Mesoporous Materials 129 (2010) 179–188

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Mesopore constrictions derived from the substitutionally co-packed SBA-15 Lan Chen a,*, Wen-Hua Zhang a, Ju Xu a, David A. Tanner b, Michael A. Morris a,c,** a

Department of Chemistry, University College Cork, Cork, Ireland Department of Manufacturing and Operations Engineering and Materials and Surface Science Institute (MSSI), University of Limerick, Limerick, Ireland c Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland b

a r t i c l e

i n f o

Article history: Received 27 March 2009 Received in revised form 2 September 2009 Accepted 19 September 2009 Available online 6 October 2009 Keywords: Mesoporous silica Substitutionally co-packed Bimodal SBA-15

a b s t r a c t A highly ordered, substitutionally co-packed, bimodal hexagonal mesoporous silica, ORK-1, with a dual mesopore/constriction size distribution has been synthesized successfully by the co-packing of two different copolymer micelles (amphiphilic triblock copolymer systems) in strong acid media. The materials have a similar but constricted channels (as shown by transmission electron microscopy, powder X-ray diffraction, TGA and N2 adsorption isotherm (BET)) to those of SBA-15 materials. However, real-time UV–Vis absorbance spectra confirm the strongest intra-micellar interaction of two types of copolymer molecules in a binary system occurs when the molar content of these copolymers are equal, i.e. P123:P65 = 0.5:0.5. The stronger the interaction between these copolymers, the shorter the gelation (packing) time is. Both the size and the distribution of the mesopores/constrictions can be adjusted by varying the molar ratio between the surfactants. The size distribution of pores ‘templated’ by the surfactant micelles has been proved to depend on the relative concentrations of the two block copolymers. In particular, the higher the concentration fraction of one of the block copolymers in the solution the larger is the micelles formed there from and so the greater the diameter of the derived pore size in the bimodal porous structure. The lower the concentration fraction of the block copolymer, the smaller the micelles (associated with that copolymer) and the smaller the pore diameter. A mechanism for the formation of these materials is discussed. Evidence suggests that these interacted copolymer surfactants have significant influence on their sol–gel properties and the final mesostrucures. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Since the discovery of the first M41S family of ordered mesoporous materials (OMMs) in the 1990s [1,2], they have been intensively investigated to the present day. Various new families of OMM have been reported and among these, SBA-15 is extensively researched because of its larger pore size and higher thermal stability than the M41S family [3,4]. Based on their structural motifs, the relatively large pore diameter (compared to microporous materials) and their high surface area OMMs have found many applications in different fields such as catalysis [5–10], optics [11], electrochemistry [12,13], environment [14] and sensors [15,16]. Mesoporous materials with uneven channels or bimodal pores (i.e. containing two distinct pore diameters) are especially attractive to scientists in the area of sorption and catalysis. This is because of the possibility of improving mass transport through the solids, enhancing size-selective reactions and creating highly size selective membranes. Recently, various types of hierarchically (macro–micro [17,18], macro–meso [19,20], meso–micro [21,22] * Corresponding author. Tel.: +353 21 4902911; fax: +353 21 4274097. ** Corresponding author. E-mail addresses: [email protected] (L. Chen), [email protected] (M.A. Morris). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.09.014

and meso–meso [23–29]) porous materials have been synthesized. The most common approach to making bimodal systems is to combine the concept of templating (surfactant micelle formation around which an inorganic precursor condenses) with inter-particulate or inter-granular porosity. Van der Voort et al. have previously reported the synthesis of plugged hexagonal templated silica (PHTS) by a mediated SBA-15 preparation procedure in very high Si/P123 ratios, which is a typical hierarchical micro-mesoporous materials with the significant increase in the microporous volume and pore wall thickness [30–32]. However, the distribution of these micropores accompanied around the mesopores is random and seems to have no correlation with that of the mesopores. A few papers have also reported the synthesis of the bimodal mesoporous silica by using template mixtures [29,33,28,34,35], but they either form disordered bimodal mesopore arrangements or ordered mono-modal mesopore with small size template molecular systems embedded in larger entities. It should also be stressed that although the systems are described in the literature as hierarchical the distribution of pore sizes is, again arbitrary and uncorrelated. Here, we have developed a facile route to make highly ordered SBA-15 with tunable mesopores/constrictions. The substitutionally co-packed ordered (SCO) hexagonal mesoporous silica (ORK-1)

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(100)

have been synthesized using two amphiphilic triblock poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (EO–PO– EO) copolymers with the different length as templates. In this case the copolymers were Pluronic P123 (EO20PO70EO20) and Pluronic P65 (EO20PO30EO20) available from BASF.

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2. Experimental 2.1. Materials and synthesis

XRD patterns were recorded on a Phillips Xpert MPD diffractometer using CuKa radiation at 40 kV of voltage. N2 isotherms were measured using a Micromeritics Gemini 2375 volumetric analyzer at 77 K and all samples were degassed at 393 K for 4 h prior to analysis. Pore size distributions and pore volumes (for both micropores and mesopores) were calculated using the BJH (also t-plot) model and the pore surface area were determined by the BET method. While they may be considered not accurate enough for the quantification of the microporous volume and surface area, they remain most readily accessible, especially, for the systematical comparison under the comparable conditions. In any case, they give the self-consistent results on the parameters discussed herein. TEM images were recorded on a JEOL 2011 microscope operated at 200 kV. Real-time UV–Vis spectra was recorded on a Cary 50 UV–Visible spectrophotometer where the wavelength ranges from 800 to 300 nm and the scanning time is 5 h at a interval of 1 min. The signal starts to be collected as soon as the copolymer micelle solution mixes with TEOS at room temperature. The thermogravimetric analysis (TGA) was run in a platinum crucible in a Setaram Labsys TGA-DTA-DSC instrument with nitrogen flowing at a RAMP of 10 °C min 1. 3. Results and discussion In order to understand the nature of the SCO mesoporous system, a serial of parallel syntheses were carried out as shown in Fig. 1. They are the typical preparations for mesoporous silica with pure Pluronic P65, indicated as ‘1’ and typical SBA-15 as ‘2’, or the mixture of two products in dry state (‘3’, two calcined samples ‘1’

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2.2. Characterization

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Pluronic P65 (Mav = 3400), EO20PO30EO20 (BASF); Pluronic P123 (Mav = 5800), EO20PO70EO20 (BASF); tetraethyl orthosilicate (TEOS) (Aldrich); Hydrochloric acid, 37.5 wt.% (Fluka). ORK-1 was prepared by a conventional sol–gel process in strong acidic condition [4,3]. In a typical preparation (ORK-1(0.5:0.5) where 0.5:0.5 indicates the molar ratio of P123:P65), 1 g of P123 was first dissolved in 63 ml of water, followed by addition of 12 ml of 37.5 wt.% HCl solution under magnetic stirring for 1 h. 0.59 g of P65 was then added into this solution, which was stirred for another 4 h. Then, 5 ml of TEOS was added into this solution which was further stirred for 20 h. All of the above processes were performed at 40 °C. Finally, the mixture was transferred into a Teflon-lined autoclave and aged hydrothermally at 95 or 105 °C for 48 h for different samples. The as-synthesized white solid was recovered by filtration and washing with distilled water for 3–5 times then air-dried at 105 °C overnight. To obtain the final SCO mesoporous silica, the samples were heated to 500 °C from RT at a RAMP of 5 °C min 1 and kept at 500 °C for 10 h under ambient atmosphere. The synthesis procedure is the same to that of ORK-1 (0.5:0.5) and in order to synthesis SBA-15 and SBA-15(2Si) 100% and 50% of the equivalent mole of pure P123 were used, respectively, instead of the surfactant amount used in the above process while all other conditions were kept same.

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Fig. 1. Low angle PXRD patterns of mesoporous silica calcined at 500 °C which are prior made at 105 °C from (1) pure Pluronic P65 micelles; (2) pure Pluronic P123 micelles; (3) a dry mixture of the calcined products from (1) to (2) by weight ratio of 1:1; (4) a wet mixture of the uncalcined products from ‘1’ and ‘2’ which was prepared by the equivalent mole of P65 and P123 (molar ratio of 1:1) respectively in the same condition and stirred for 20 h at 40 °C and then mixed quickly and kept statically at 105 °C for a further 48 h and (5) ORK-1(0.5:0.5).

and ‘2’ were mixed by wt.% of 1:1) or wet state (‘4’, two copolymerTEOS mixtures ‘1’ and ‘2’ with same moles of copolymers were mixed after stirred at 40 °C for 20 h, respectively). Low angle PXRD patterns for calcined ORK-1 (‘5’) show three intense diffraction peaks in the 2h range of 0.8–2° that can be indexed as (1 0 0), (1 1 0) and (2 0 0) reflections associated with a pore arrangement of 2D hexagonal p6 mm symmetry, similar to that of pure hexagonal phase (SBA-15) [3,4]. The intense (1 0 0) reflection peak indicates a d-spacing of 99.8 Å, corresponding to a large unit cell parameter (a = 115.24 Å). The mixture of two phases either before the hydrothermal treatment (‘4’) or after the calcination (‘3’) gives less ordered XRD patterns compared with the pure hexagonal phase (‘2’) and the SCO one (‘5’) and have different profile as shown in Fig. 1. The absence of (2 1 0), (3 0 0) and (2 2 0) higher lattice plane indices diffraction peaks in ORK-1 (‘5’) rather than in pure hexagonal phase (‘2’) denotes the loss of short range order caused by the presence of mesopores/constrictions in an array. SI Fig. 1 (see Supporting Information) shows that the P123 micelles substituted by 50% of P65 in molar content give rise to a bit larger interpore distance than that in a typical SBA-15 while the pure P123 micelle with 50% less in its molar content yields a more smaller interpore distance. N2 adsorption isotherms in Fig. 2a of calcined ORK-1 (‘5’) display a type-IV mesoporous curve shape with a readily observed binary capillary condensation step at P/P0 = 0.65–0.75 (large step) and P/P0 = 0.55–0.60 (smaller step). The material has a H1-type hysteresis loop typical of large-pore mesoporous materials with cylindrical channels [3]. The presence of two resolved capillary condensation steps shows that ORK-1 (0.5:0.5) has a bimodal mesopore/constriction distribution and this is clearly seen in the BJH pore size distribution (PSD) data in Fig. 2b. The mesopore diameter is determined from the adsorption isotherm (‘5’) at about 7.8 nm while the constriction size is equal to or larger than 4.7 nm. The larger mesopores can be tentatively assigned as being primarily due to P123 and the smaller constrictions to P65 as is consistent

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Fig. 2. N2 sorption isotherms (a) and pore size distribution (PSD) curves (b, determined by Barrett–Joyner–Halenda (BJH) on the adsorption branches) of mesoporous silica calcined at 500 °C which are prior made at 105 °C from (1) pure Pluronic P65 micelles; (2) pure Pluronic P123 micelles; (3) a dry mixture of the calcined products from (1) to (2) by weight ratio of 1:1; (4) a wet mixture of the uncalcined products from (1) to (2) which was prepared by the equivalent mole of P65 and P123 (molar ratio of 1:1) respectively in the same condition and stirred for 20 h at 40 °C and then mixed quickly and kept statically at 105 °C for a further 48 h and (5) ORK-1(0.5:0.5).

with their respective hydrophobic block (PO moiety) sizes. As might be expected, ORK-1 might be the mixture of P123-derived hexagonal phase and P65-derived amorphous phase. But either the XRD patterns or the N2 sorption isotherms of ORK-1 (‘5’) are different with other pure products and their (‘1’, ‘2’, ‘3’ and ‘4’) as shown in Figs. 1 and 2. The big pores of 30 nm in ‘4’ (mixed after gelation) are different with other mixtures with the same composition like ‘3’ (mixed after calcination) and ‘5’ (mixed before gelation) indicating that the formation of large inter-particle pores is probably contributed to the soft agglomeration of ‘wet’ particles (in the scale of hundreds of nanometers) after gelation. The additional delay between P/P0 = 0.45–0.5 in the N2 desorption branch of ORK-1 (‘5’) implies the formation of pores with constriction structure which is quite different with any single phase or the combination of two separated phases after gelation/calcination and is quite significant on the PSD plot based on the sorption behavior as shown in SI Figs. 2 and 3 (see Supporting Information). A smaller ‘lattice’ constant (SI Fig. 1) and a similar pore size (SI Fig. 2) indicate the formation of a more denser mesopore wall in SBA-15(2Si) system which is consistent with a dramatically decreased micropore area/volume (55.2 m2 g/0.023 cm3 g) 2 compared to SBA-15 (140.7 m g/0.065 cm3 g) and ORK-1(0.5:0.5) (142.1 m2 g/0.068 cm3 g) materials. Both the ratio of the micropore volume to the total single point adsorption total pore volume and the percentage of the constricted pore volume in the cumulative total pore volume in ORK-1 (0.5:0.5) system are higher than those in SBA-15 and are above 2.6 times higher than those in SBA-15(2Si) system as listed in SI Table 1. A smaller single point adsorption total pore volume combining with a larger micropore volume in ORK-1 (0.5:0.5) compared to SBA-15 and SBA-15(2Si) shows the formation of the micelles with the smaller size and uneven external surfaces as shown in SI Table 1. TEM images (Fig. 3) of calcined ORK-1 (0.5:0.5) (and typical of other samples) show well-ordered hexagonal arrays of mesopores with 1D channels that are similar to SBA-15 system [3]. The cell parameter (a) estimated from the TEM images in Fig. 3a is approximately 118 Å, which is in close agreement with the value calculated from PXRD data (115 Å). The channel arrays with an average channel-to-channel separation of 92 Å were clearly seen in Fig. 3b and the uneven wall fringes can sometimes be seen.

However, the constricted mesopores with coned depth of field are seen directly in Fig. 3c and the difference in their projective area in Fig. 3d implies the existence of the constricted pores. The image analysis of Fig. 3d supports the description of the system as having a bimodal distribution of mesopores/constrictions and this is shown explicitly in SI Fig. 4 (see Supporting Information). Two separated bands in the pore size distribution centered on 5.4 nm and 6.9 nm were observed. These values differ a little from the BJH calculated values in Fig. 2b but this can be explained by many factors such as the aberrancy in the circularity of pores and pores with constrictions/expansions. However, the combination of the BJH data and the TEM observations provide strong evidence for the bimodal distribution of mesopores/constrictions in the calcined ORK-1 (0.5:0.5). By varying the ratio of P123:P65 used in the syntheses we can probe the changes in the distribution of the constricted pores within the prepared materials. As the P123:P65 ratio is changed there are systematic changes in the sorption isotherms and derived PSD curves as shown in Fig. 4. As the relative amount of P123 increases, the step-wise increase in the volume adsorbed (in both N2 adsorption and desorption branches) associated with capillary condensation moves to higher P/P0 values. Further, in the adsorption curves the region of capillary condensation is made up of two components in the range of P123:P65 = 0.4:0.6–0.8:0.2 and suggestive of a bimodal mesopores/constrictions distribution. Outside of this range only a single capillary condensation step (i.e. a mono-modal mesopore distribution) exists. The BJH curves from the desorption branches described in Fig. 4b show these changes explicitly. As the P123:P65 ratio is increased (i.e. more P123) there is a monotonic increase in the average pore size. Bimodal mesopores/constrictions size distributions are clearly seen in Fig. 4b when the molar fraction of P123 varies from 0.4 to 0.8. From the adsorption isotherms it can also be seen (most easily in the Supporting Information SI Fig. 5) that the total adsorbed volume increases linearly with the increase of the P123 fraction. It is reasonable to conclude that the increase in adsorbed volume is due to the increased concentration of mesopores which derive from the P123 surfactant. SI Fig. 6 (see Supporting Information) provides evidence that the specific desorbed volumes of N2 due to the presence of

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Fig. 3. TEM images of ORK-1(0.5:0.5) prepared at 105 °C and calcined at 500 °C viewed in the [0 0 1] direction (a) and its corresponding Fourier Transformation (inset), viewed in the [1 1 0] direction (b) and its corresponding FT (inset), in the dark (c) and the bitmapped mode (d).

P123-derived mesopores (Mm) and P65-derived constrictions (Mc) increase with the molar fractions of the corresponding Pluronic copolymers. Furthermore, the relative amount of Mm to Mc also follows the similar trend and increases as the molar ratio of P123/P65 increases (see Supporting Information SI Fig. 6). Generally, good structural order in the products produced is observed suggesting that surfactant ‘mixing’ does not have a deleterious effect on structure. This is witnessed by the PXRD profiles (Fig. 5) which show intense features as well as clear (1 1 0) and (2 0 0) reflections as ORK1 (0.5:0.5). The loss of good mesostructural order was seen for those samples prepared from 100% P65. PXRDs display only a broad ill-defined peak at about 1.4° 2h and all other discriminable diffractions peaks disappear. It can also be seen that there is a systematic shift of the (1 0 0) reflection to lower 2h with increasing P123 content. This is consistent with an increased pore-to-pore distance associated with the larger micelles of this surfactant. The mixed micelle structures in the bimodal synthesis are described as P123-dominated and P65-dominated which are referred as d-P123 and d-P65 for simplicity. The mechanism by which these unusual materials are made may be studied by analysis of the assynthesized samples. N2 sorption isotherms and PSD data of assynthesized ORK-1(0.5:0.5) are shown in SI Fig. 7 (see Supporting Information). These are similar to the data from the calcined version described in Fig. 2. The magnitude of the volumes adsorbed

in the isotherms, the pore volumes estimated and the presence of two well separated PSD (in both adsorbed or desorbed branches) testify to very significant amounts of surfactant removal prior to high temperature calcination. The size of the larger pore determined at 65 Å and the pore to pore spacing determined by PXRD confirm that the silica wall is significantly thicker than in the calcined version (37 Å) at about 54 Å. It is important to state that the micellar rods that template the pores do contain mixtures of P123 and P65 but the solubility of the surfactants in each other is limited. This is clear because the large pore sizes formed are smaller than that of the P123 surfactant alone and vice versa (see Supporting Information SI Fig. 7). It is also consistent with the observation of mono-modal pore size distributions in similar mixtures prepared in different conditions [36]. One the other hand, the direct evidence for what copolymer components these materials contain comes from TGA data (see Supporting Information SI Fig. 8). The pictures show that the weight loss behavior is different from sample to sample upon heating under nitrogen flowing. ‘2P0.5’ (ORK-1 (0.5:0.5)) shows totally different TGA curve with materials prepared from either pure P123 in normal or higher Si:P123 ratio. The weight loss is low in the samples of ‘2Si0.5’ and ‘2P0.0’ above 270 °C showing the narrow openings/pores and dense walls in these systems. The similarity of the TGA profiles between ‘2P1.0’ (SBA-15) and ‘2P0.5’ (ORK-1

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(0.5:0.5)) shows the similar pore structure (channel) while the slower weight loss above 270 °C for ORK-1 (0.5:0.5) than that for SBA-15 confirms the existence of the small constrictions in their channels. The TGA plot superposed from 70% ‘2P1.0’ and 30% ‘2P0.0’ (in molar ratio) systems fits well with that of ORK-1 (0.5:0.5) which shows slight different copolymer composition in the original micelles (P123:P65 = 0.5:0.5) and the final uncalcined product (P123:P65 = 0.7:0.3) as shown in SI Fig. 8. As supposed, ORK-1 (0.5:0.5) may consist of a hexagonal phase (dominated by the d-P123 micelles in a diluted concentration, i.e. a increasing Si:P123 ratio) and an amorphous phase (dominated by the d-P65

micelles) which should show a mixing TGA behavior of 50% ‘2Si0.5’ and 50% ‘2P0.0’. However, the ORK-1 (0.5:0.5) profile as indicated by the dash line in SI Fig. 8 shows a superposed profile of ‘2P1.0’ and ‘2P0.0’ and excludes the possibility that ORK-1 (0.5:0.5) is a mixture of the ‘diluted’ hexagonal (‘2Si0.5’) and amorphous phases (‘2P0.0’). The gelation point in silica sol–gel chemistry can be determined by the transition of the solution color from transparent to opaque (turbidity). The gelation time determined by naked eyes is usually not accurate enough due to the observation heavily depends on the observer’s experience and response. Bagshaw [37,38], Prouzet [39], Goldfarb [40] and co-workers have previously investigated the formation mechanism of mesoporous silica templated by a single nonionic surfactant and the interactions between the PEO coronas of the micelles and silicate oligomers using different advanced technologies [37–42]. But no similar studies have been done on the intra-micelle interactions in the binary copolymer systems, in other words, the influence on the sol–gel chemistry of a micelle– silicate system in the presence or absence of a second copolymer surfactant is still not clear. Herein, this preliminary study on the intra-micelle interactions in a binary copolymer system was carried out using real-time UV–Vis absorbance spectrophotometer associating with other technologies. As we know, this is the first report on the technology used in silica sol–gel chemistry. As shown in Fig. 6, all the formation processes can be divided into three stages, the hydrolysis stage, which contains partial filling of the PEO corona by silicate oligomers (blue area), the assembly stage (transition between the blue and the red area) and the condensation stage (red area). No turbidity was observed in the first stage and the absorbance intensity for all binary copolymer systems (left column) increases sharply after 26 min (the assembly point) of the mixing of TEOS and the copolymer solution, which is 14 min longer than the pure P123 copolymer (in the right column) system. The strongest absorbance starts from the 40th minute (the condensation stage) for the binary systems which corresponds the growth of the porous silica particle aggregates. The strongest absorbance advances by 15 min in the pure P123 systems (right column). All absorbance decrease obviously after 90 min for both pure and binary copolymer systems. In most cases, the solution changes totally

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Si:P123 1.00:0.10 1.00:0.25 1.00:0.50 1.00:0.75 1.00:0.90

Abs. (a.u.)

clear during the analyzed time frame of 5 h. No noticeable change in absorbance can be detected by UV–Vis spectrometer in TEOS– water–HCl system without the copolymer over 24 h. So the occurrence of the turbidity can be reasonably considered as the result of the interaction between the copolymer micelles and the hydrolyzed TEOS species. The turbidity, therefore, corresponds to the packing (self-assembly) and condensation of these micelle-silicate systems. Prouzet et al. also confirm the formation of the stable, isolated micellar hybrid with a three-layered structure (i.e. the alkyl core, the first PEO shell and the second silica shell) and the strong interaction between the PEO moiety and the hydrolysed silicate oligomer in the nonionic PEO surfactant–TEOS–H2O system by the combining use of small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) technology [39]. Two distinct steps are also observed in this system, i.e. the assembly step and the subsequent condensation step which is consistent with our real-time UV–Vis absorbance results. Prouzet and co-workers have reported the effective interaction only occurs between the PEO moieties and the silicate oligomers and the PEO concentration is directly proportional to the surfactant content in a pure copolymer system. Different copolymers have different micellar properties even that they possess the same number of PEO units, such as critical micellar concentration, gelation point, condensation time and final structure. Therefore, here we tend to use the effective micellar concentration rather than the surfactant (PEO) concentration to describe the sol–gel chemistry behaviour in a binary system. Considering the difficulty of quantitative studies on the composition of a binary system, only qualitative explanation is made in this paper. Compared with the binary micelles, either pure P123 or pure P65 micelles result in the similar but delayed gelation time indicating that the binary system can not be simply attributed to the mixture of two separated phases since the turbidity observed in the binary system with any ratio of P123 to P65 is significantly later than that of the corresponding pure copolymer system as shown in Fig. 7a. On one side, the gelation time seems to be determined by the molar ratio of Si to micelle (note that it is not the ratio of Si to surfactant, in some cases, the same mole of surfactant does not naturally result in the same micellar concentration in the presence of other surfactants). Diluted copolymer solution contains fewer but looser (larger in the total volume) micelles and favors a faster but longer gelation as shown in Fig. 7b. Goldfarb and coworkers have reported the effect of the Si/surfactant ratio on the sol–gel chemistry and they claim higher silicon content favors longer polymerization, although the initial assembly step is shorter [40], which coincides with our results as shown in the right panel of Fig. 6. On the other side, the gelation time can also be affected by the relative content of two copolymers in a binary system. The bigger the relative content of a copolymer in the solution, the slower the gelation occurs. The faster gelations happen in a binary system only if the molar content of two copolymers in the solution approximates, i.e. P123:P65 = 0.5:0.5. A short gelation time can, therefore, be contributed to the strong (substitutional) interact between two types of copolymer molecules. For example, d-P123 binary micelles yield similar gelation behavior (steep slope) just like the pure P123 micelles while d-P65 binary micelles do another way just like the pure P65 ones (less steep slope) as shown in Fig. 7b. The shifted interval on the gelation for d-P123 micelles with different concentration is almost systematically invariable between those in the presence and absence of P65 copolymer in Fig. 7b, which also shows the second copolymer has significant influence on the main component. Combining with the larger micropore and constricted pore volume percentage of the ORK-1 (0.5:0.5) materials, a convincing conclusion could be drawn on that the minor component copolymer has significant influence on the major one, where a novel mesopore structure can, therefore, be produced.

P123:P65 0.00:1.00 0.10:0.90 0.25:0.75 0.50:0.50 0.75:0.25 0.90:0.10 1.00:0.00 3

6

9 12 15 18 21 24 27 30 33 36 39 42 45 48

Time (min.) Fig. 7. Gelation time for binary P123–P65 mixture micelle system (a) the binary P123–P65 and (b) the corresponding pure P123 micelle system. All absorbance data were collected under a fixed wavelength of 300 nm.

On the evidence presented here we can tentatively describe the formation mechanism of the present bimodal mesopore/constriction structures as follows. In an appropriate composition range the amphiphilic block copolymer surfactants initially form sphere shaped micelle aggregates in aqueous solution in the absence of the silicate species. However, in a binary system, any pure micelles comprising of same surfactant molecules are not stable in dynamics and the surfactant molecule exchange among these micelles is inevitable under the experimental conditions. Some P65 molecules enter into a pure P123 micelle to form a d-P123 micelle while the substituted P123 molecules may be embedded into a pure P65 micelle to form a d-P65 micelle. A new equilibrium will, therefore, be reached in a short time (in a few minutes). As a result, two types of pure micelles are blended to some extent to form two new types of mixed micelles and the composition for each type of newly formed micelles is almost same, i.e. the P123/P65 ratio is largely equal for each type of mixed micelles. The mixed surfactant micelles only have significant stability when the distribution coefficient of the minority to the majority surfactant in all mixed micelles is largely equal to the molar ratio of two surfactant content. Immediately, these mixed micelles cooperatively interact with the hydrolyzed species of TEOS, silicate oligomer, to form spherical micelle–silicate complexes with different hydraulic diameter at low tempera-

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tures (35–80 °C) in the first stage (<30 min after the addition of TEOS). The driving force for the silica layer growth on the outer surfaces of a spherical complex is the ratio of Si/PEO around the micelle. As observed by Prouzet et al. [39], the number of the accessible PEO units for a given corona space (in the range of the hydraulic diameter of a micelle) is large in the initial a few minutes after the addition of TEOS, therefore, the ratio of Si/PEO is small and the silica condensation on the PEO coronas is slow. The number of the accessible PEO units start to decrease with the partial adsorption of the silicate species on them and it results in a slow increase of the Si/PEO ratio in the first 26 min (which is proved both in our synthesis and in the typical SBA-15 preparation [42,41]), where the solution outside the micelles is considered as a Si reservoir which is largely kept invariable in this stage. It is, therefore, a reaction (polymerization) controlled process in this stage. The Si/PEO ratio increases sharply to infinite because the available PEO units reach nearly to zero when the PEO corona is completely filled by the silicate species. Thereafter, the self-assembly process occurs as soon as the growth of the silica layer is infinitely accelerated and the transformation of spherical micelles into elongated spherical or cylindrical micelles in the same time. This process is usually initiated by a sharp change of the solution from colorless to turbidity as confirmed in both Refs. [36,39,40] and our experiments and it looks like sudden packing of the spherical micelle–silica complexes. As suggested by Imperor-Clerc et al. [42,41], the hexagonal packing (assembly) of cylindrical hybrid micelles starts after 26 min which is consistent with our results. Finally, the condensation occurs in the third stage with the growth of the porous silica particles which are previously formed in the second stage (assembly stage) and eventually, the condensation process slows down by the decrease of available silicate oligomers around the formed particles where the process is controlled by the diffusion rate rather than the polymerization rate of the available silicate species. The condensation process ends after about 5 h as soon as the silicate species in the solution are completely depleted and the solution becomes totally clear as observed in Fig. 6. Considering the decrease of the PEO unit number in the reference pure

P123–TEOS systems (diluted), their Si/PEO ratios are higher than the corresponding binary system and result in a shorter initial stage but a longer condensation stage as shown in Figs. 6 and 7b. As we know, the micellization ability of the pure P65 molecules is weaker than that of the pure P123 molecules and, therefore, the d-P65 micells have a looser aggregate, i.e. a smaller molecule (PEO) number in a micelle which contributes a higher Si/PEO ratio and a faster growth of the silica layers outside the micelles. A schematic of the as-synthesized mesostructured micelle–silicate system is shown in Fig. 8. The ideal arrangement of a SCO hexagonal unit cell by equivalent mole of two copolymers, e.g. P123:P65 = 0.5:0.5 is indicated in Fig. 8, in fact, the channels are not ideal straight and they consist of a serial of narrowed and widened constrictions at intervals (along [1 1 0] view in Fig. 8) and the conical pores are seen along [0 0 1] direction as illustrated in Fig. 8 or seen in Fig. 3c. Such interval arrangement is derived from d-P123 and d-P65 micelles respectively. Two obvious condensation steps can be seen on both the adsorption and desorption branches when the numbers of mesopores and constrictions along the radial direction of a continuous pore are comparative as it is shown in the case of P123/P65 = 0.5:0.5. However, the second step disappears on the adsorption branch but it is still seen on the desorption branch in the cases of P123/P65 – 0.5:0.5 as shown in Fig. 4a. That is because constrictions of a continuous channel are easy to differ them with the channel with additional small delays on their desorption branches due to the well-known capillary condensation while they are not able to discern such indistinct difference by their adsorption behavior on the same conditions. P123 tends to form stable micelles and the existence of a small quantity of P123 in P65 micelles improves the stability of the P65 micelles and yields a sharp mono-modal behavior as shown in Fig. 4b (P123/P65 = 0.3:0.7). 0.4 and 0.8 of P123 in P65 are the lower and upper limits respectively for the formation of the bimodal mesoporous silica. Small pores or the constriction parts of a continuous channel are derived from d-P65 (P123/P65 < 0.3:0.7) micelles which are almost invariable in different synthesis while larger pores or the channels are derived from d-P123 micelles which have

d-P123 micelle-silicate composite

[110] view

[001] view

EO20PO70EO20 (P123) EO20PO30EO20 (P65) d-P65 micelle-silicate composite

O Si O O

O n

Fig. 8. Schematic representation for the formation of bimodal mesopores/constrictions of ORK-1 and the relevant structured units involved.

L. Chen et al. / Microporous and Mesoporous Materials 129 (2010) 179–188

P65:P123 0.2:0.8 0.4:0.6 0.5:0.5 0.6:0.4

Fraction of P123

0.0:1.0

Fraction of P65

Distribution probability of micelles

d-P123 micelle number

0.8:0.2 1.0:0.0 d-P65 micelle number Micelle size (nm)

Fig. 9. Relationship between the probabilities of MSDs and relative content of P123/P65 in aqueous solution.

a quite wide range of PSDs depending on the relative content of P123 to P65 (0.8:0.2 > P123/P65 > 0.4:0.6). Jaroniec [32] and Van der Voort [43,30] et al. have reported the synthesis of PHTS (plugged hexagonal template silica) with the increase in silica/surfactant ratio which resembles to some extent the SBA-15(2Si) materials by the appearance of nitrogen sorption. But the percentage of the micropore volume and the constricted pore volume in ORK-1(0.5:0.5) is significantly higher than those in SBA-15(2Si) indicating the formation of more loose walls in ORK-1(0.5:0.5) materials than those in SBA-15(2Si). Moreover, real-time UV–Vis absorbance spectra shows the distinct sol–gel behavior for ORK-1(0.5:0.5) and SBA-15(2Si) systems supporting the strong interaction between the different copolymer molecules. Furthermore, TEM gives the direct evidence on the abnormal mesopore structure in ORK-1 system. The important feature of the synthesis of ORK-1 is the co-packing of these two distinct micellar arrangements which provide the template for the hexagonally SCO unit cell depicted schematically in Fig. 8. The occurrence probability for d-P123 and d-P65 micelles in a unit cell is principally determined by the fractional amount of each surfactant present. Further, the higher the content of one type of micelle in the solution is, the larger the distribution probability of that micelles in the bimodal solid. In a perfect hexagonal unit cell of ORK-1, the number of lattice positions is invariable, and the increased concentration of one micelle type (d-P123 or d-P65) is necessarily accompanied by a decrease in the concentration of the other micelle. Thus, the solid can be described covariant as the amounts of P123 micelles and P65 micelles in a unit cell are oppositely correlated. It should also be remembered that as well as a changing molar ratio of two types of surfactants there must also be a variation in the distribution coefficient of one surfactant in another in the micelles as the pore size of the larger and smaller pore changes with relative surfactant concentration in Fig. 4b. Fig. 9 shows a cartoon illustrating how the distribution behavior of each surfactant and its size vary with composition. 4. Conclusion SCO hexagonal mesoporous silica system, ORK-1, with covariant bimodal mesopore/constriction size distribution, has been synthesized successfully by the co-packing of two types of amphiphilic triblock Pluronic copolymer micelles, P123 and P65 in strong acid

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media. It is also the first attempt to investigate the mesopore constrictions derived from the substitutionally co-packed SBA-15. These syntheses not only provide us with a novel SCO mesoporous material but also help to detail how multi-size micelles co-pack into an ordered array with covariant probability distributions of multi-size mesopores. The distribution of surfactants in the micelles is complex being dependent on surfactant concentration but not showing a statistical distribution of surfactant molecules in the micelles. N2 sorption, TEM, low angle XRD, TGA and realtime UV–Vis spectra suggests the significant difference in the sol–gel properties (gelation point) derived from the important energetic or structural intra-micellar interactions or the organic– inorganic interaction between surfactant and the silicate species. We suggest that this bimodal porous solid is potentially important for applications in catalysis, separation, the petroleum industry and environmental technologies where size change is important. It may also have a number of applications in the area of nanotechnology as guest–host materials where providing differing pore sizes could provide a broader spectrum of properties. Acknowledgments This work is supported by SFI (Science Foundation of Ireland, the Grant No. is 03/IN3/I1375). We appreciate Prof. Mietek Jaroniec with his benefit discussion on the N2 sorption behaviors and Dr. Wynette Redington for her thermogravimetric analysis. Appendix A. Supplementary material Pore size distribution analysis; nitrogen sorption isotherm; X-ray low angle diffraction; thermogravimetric analysis and other additional information. This materials is available free of charge via the Internet. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso. 2009.09.014. References [1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, 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) 10834–10843. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [3] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548–552. [4] D.Y. Zhao, Q.S. Huo, J.L. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024–6036. [5] J.G. Yu, J.C. Yu, M.K.P. Leung, W.K. Ho, B. Cheng, X.J. Zhao, J.C. Zhao, J. Catal. 217 (2003) 69–78. [6] Y. Zhang, X. Shi, J.M. Kim, D. Wu, Y. Sun, S.Y. Peng, Catal. Today 93–95 (2004) 615–618. [7] S. Perathoner, P. Lanzafame, R. Passalacqua, G. Centi, R. Schlogl, D.S. Su, Microporous Mesoporous Mater. 90 (2006) 347–361. [8] M.N. Timofeeva, S.H. Jhung, Y.K. Hwang, D.K. Kim, V.N. Panchenko, M.S. MeIgunov, Y.A. Chesalov, J.S. Chang, Appl. Catal. A 317 (2007) 1–10. [9] J.G. Yu, G.H. Wang, B. Cheng, M.H. Zhou, Appl. Catal. B 69 (2007) 171–180. [10] X.H. Zhao, X.L. Wang, J. Mol. Catal. A 261 (2007) 225–231. [11] B.J. Scott, G. Wirnsberger, G.D. Stucky, Chem. Mater. 13 (2001) 3140–3150. [12] A. Cremonesi, D. Bersani, P.P. Lottici, Y. Djaoued, R. Bruning, Thin Solid Films 515 (2006) 1500–1505. [13] Y.G. Guo, Y.S. Hu, J. Maier, Chem. Commun. (2006) 2783–2785. [14] A. Sayari, S. Hamoudi, Y. Yang, Chem. Mater. 17 (2005) 212–216. [15] Y. Shimizu, A. Jono, T. Hyodo, M. Egashira, Sens. Actuators B 108 (2005) 56– 61. [16] A. Palaniappan, X.D. Su, F.E.H. Tay, J. Electroceram. 16 (2006) 503–505. [17] B.T. Holland, L. Abrams, A. Stein, J. Am. Chem. Soc. 121 (1999) 4308–4309. [18] L.M. Huang, Z.B. Wang, J.Y. Sun, L. Miao, Q.Z. Li, Y.S. Yan, D.Y. Zhao, J. Am. Chem. Soc. 122 (2000) 3530–3531. [19] P.D. Yang, T. Deng, D.Y. Zhao, P.Y. Feng, D. Pine, B.F. Chmelka, G.M. Whitesides, G.D. Stucky, Science 282 (1998) 2244–2246. [20] T. Sen, G.J.T. Tiddy, J.L. Casci, M.W. Anderson, Angew. Chem. Int. Ed. 42 (2003) 4649–4653. [21] C.J.H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt, A. Carlsson, J. Am. Chem. Soc. 122 (2000) 7116–7117.

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