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Meso-cellular silica foams, macro-cellular silica foams and mesoporous solids: a study of emulsion-mediated synthesis
Microporous and Mesoporous Materials 78 (2005) 255–263 www.elsevier.com/locate/micromeso
Meso-cellular silica foams, macro-cellular silica foams and mesoporous solids: a study of emulsion-mediated synthesis T. Sen a, G.J.T. Tiddy b, J.L. Casci c, M.W. Anderson a
a,*
Department of Chemistry, Centre for Microporous Materials, UMIST, Manchester M60 1QD, UK b Department of Chemical Engineering, UMIST, Manchester M60 1QD, UK c Synetix, P.O. Box 1, Billingham, Cleveland TS23 1LB, UK Received 16 June 2004; received in revised form 17 September 2004; accepted 19 September 2004 Available online 8 December 2004
Abstract A simple room temperature synthesis of a series of porous silica materials from pure mesoporous solid to meso-cellular foams to macro-cellular foams using an emulsion containing surfactant, water, oil or polystyrene spheres is reported. A mesoporous solid is obtained using either a low oil concentration with slow stirring or synthesis at alkaline pH. Faster stirring at low oil concentration produced meso-cellular silica foam. A series of macro and mesoporous silica composites are obtained with various wall thickness and interconnectivity using intermediate to high oil concentration. An amorphous solid was formed in an acidic pH using polystyrene spheres as droplets whereas an ordered macroporous solid with mesoporous walls was formed at alkaline pH. The formation of such structures is explained based on the rate of various processes, e.g. hydrolysis of silica, precipitation as micelles, condensation of silica, creaming or coalescence of oil or polystyrene. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Macro- and meso-cellular silica foams; Emulsion; Creaming; Worm-hole; Mesoporous
1. Introduction Porous materials are well known for a variety of uses from molecular sieves to catalysis [1]. Introduction of a range of pore sizes from macro (>50 nm) to meso (0.2–50 nm) to micro (<0.2 nm) in an ordered way is important for specific applications, e.g. shape selective catalysis [2] and bio-molecular sieving [3]. Most synthetic methods to produce hierarchical structures are laborious multi-step processes and consequently there is an impetus for low cost, one-pot, room temperature syntheses. One such method was first published by Stucky and co-workers [4] using an emulsion method
*
Corresponding author. Tel.: +44 161 200 4517; fax: +44 161 200 4559. E-mail address: [email protected] (M.W. Anderson). 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.09.022
to synthesise mesoporous silica. Imhof and Pine [5] has reported the synthesis of ordered macroporous materials using an emulsion method. Subsequently, various authors published the synthesis of such ordered macroporous materials using a monodispersed polystyrene monolith as template [6]. Most of the methods produce an ordered macroporous solid without meso or microporosity. Introduction of micro or mesoporosity or both in a macroporous structure is also reported recently using a combination of organic quaternary amine/surfactant and polystyrene spheres as template in a one-pot synthesis [7–10]. A room temperature synthesis of mesoporous silica materials using a silica gel containing a surfactant is also reported [11]. Stucky and co-workers [12] described the synthesis of mesocellular silica foams (MCFÕs) during the preparation of SBA type materials using mesitylene as a swelling agent. All these methods are specific for the synthesis
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of materials of one kind. Recently we have reported [13] the synthesis of meso-cellular foams (MCFÕs) along with UMIST macro-cellular foams (UMCFÕs) using an emulsion method without proper understanding of the formation mechanism. Herein we report the one-pot synthesis and the formation mechanism of a range of porous silica materials by emulsion-mediated method using oil or polystyrene droplets as a dispersed phase in a continuous water phase.
2. Experimental 2.1. Synthesis An aqueous solution was used as a continuous phase and mesitylene or a monodisperse polystyrene suspension was used as a dispersed phase. Cetyl trimethyl ammonium bromide (CTAB) was used as surfactant and tetraethyl orthosilicate (TEOS) as a silica source. The gel composition (wt%) of different syntheses is presented in Table 1. First, CTAB was dissolved in water under stirring, then the required amount of hydrochloric acid or sodium hydroxide was added to this stirred solution to give a colourless solution. Finally, a mixture of mesitylene and tetraethyl orthosilicate was added slowly for a period of 30 min under stirring at a fixed rpm. During addition, the mixture turned opaque. The whole mixture was then stirred for 1 h using a mechanical stirrer (stainless steel, three blades, propeller shaped). After settling, two layers were observed in all synthetic batches: a thick white layer floated on top; a transparent pale blue solution below. When the gel was stirred at 300 rpm (sample 1i) a white precipitate was observed at the bottom of the jar whereas at 800 rpm (sample 1ii) an additional thick top layer (sample 1) was also formed. When polystyrene was used as a dispersed Table 1 Gel composition and condition of various synthesis batchesa Batch no.
Dispersed phase
pH condition
Gel composition (TEOS:CTAB: Mesitylene/polystyrene: Water:HCl/NaOH), wt%
1ib 1ii 2 3 4 5 6 7 8 9 10
Mesitylene ,, ,, ,, ,, ,, ,, ,, ,, Polystyrene Polystyrene
Acidic (0.3) ,, ,, ,, ,, ,, ,, Neutral (6.9) Basic (11.4) Basic (12.3) Acidic (0.2)
3.6:4.2:2.0:87.4:2.8 ,, 3.6:4.2:15.2:74.2:2.8 3.6:4.2:21.2:68.2:2.8 3.6:4.2:26.4:63.0:2.8 3.6:4.2:31.0:58.4:2.8 3.6:4.2:39.5:49.9:2.8 3.6:4.2:31.0:58.4:0 3.6:4.2:31.0:58.4:3.9 5.9:6.8:5.2:81.3:0.8 5.4:6.1:4.8:80.1:3.6
a b
800 rpm. 300 rpm Stirring speed.
phase (batches 9 and 10), the synthesis route is as follows: 4.74 g of CTAB was dissolved in 30 g of water in alkali (0.55 g NaOH) or acid (8 ml, 35% HCl). 30 g of polystyrene suspension PS-B6 (312 nm, 12% solid in suspension) was added to the CTAB solution and stirred at 800 rpm for 5 min to make a homogeneous solution. The whole solution was milky in colour. 4.12 g of tetraethyl orthosilicate (TEOS) was added drop wise for 15 min to the mixture. The whole mixture was then stirred for another 1 h before transfer to a glass jar. The mixture was kept at room temperature over night and two layers were observed the following day: a bottom solid layer and a top clear solution. White solids were filtered out either from the bottom or from the top layer. The separated solids were washed with de-ionised water several times and dried at 40 °C for 6 h. The dried materials were white in colour. These dried materials (batches 1–8) were calcined at 650 °C for 10 h in air. Temperature of the furnace was increased at a rate of 1 °C/min. In the case of batches 9–10, the polystyrene template was initially removed by toluene extraction at RT for 48 h before calcined at 550 °C for 10 h in air. The calcined materials were white in colour. 2.2. Characterisation X-ray diffractograms were recorded on a XDS 2000 Scintag machine using CuKa radiation with a scan rate of 1°/min. Simultaneous TG/DTA analyses of the assynthesised and toluene treated materials were performed on an automatic instrument (TGA-DTA VI.1B TA Inst 2000). The thermograms of the samples were recorded under air (air flow ffi16 ml/min) with a heating rate of 5 °C/min. The morphology and pore structure were characterised by scanning electron microscopy (SEM). SEM micrographs were recorded on a Philips XL30 with field-emission gun. Macropores were characterised by Hg porosimetry using a Micromeritics Autopore IV instrument. The mesopores and the surface area of the materials were characterised by N2 adsorption using a Micromeritics ASAP2010 instrument. The nature of mesophases and their ordering was further characterised by transmission electron microscopy (TEM) on a Phillips CM20 200 kV. The specimen for analysis was prepared using a few mg of sample crushed and dispersed in acetone and a drop was placed onto a carboncoated copper grid. 29Si NMR of various samples was performed on a Bruker MSL 400 spectrometer with single pulse (SP) and cross polarisation (CP) with magicangle spinning, 4 kHz, at a Larmor frequency of 79.474 MHZ. TMS was used as a reference for SP and Q8M8 for CP MAS experiments. The flip angle was 7.5 ls (p/2) and the relaxation delay was 100 s for SP MAS NMR. The 29Si CP MAS experiments were performed using a flip angle 7.5 ls (p/2), CP mixing time 2 ms and relaxation delay of 10 s.
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3. Results and discussion X-ray diffractograms are shown in Fig. 1 for a variety of uncalcined samples. They all exhibit low angle reflections characteristic of ordering at the mesoscale. Typically two groups of lines are observed both corresponding to lamellar phases. The first is characterised by two broad reflections in the region approximately 3.4–4.0 nm and 1.7–2.0 nm. The second is characterised by two sharp reflections at 2.7 and 1.35 nm. The former pairs of lines is characteristic of a mesoporous lamellar silicate phase and the later pair of lines is characteristic of a pure lamellar surfactant phase. The ratio of silicate and surfactant phases depends strongly upon the synthetic conditions as does the precise d-spacing of the silicate mesophase. The pure surfactant phase displays two different mesophases with slightly different d-spacing
Fig. 1. XRD diffraction pattern of various uncalcined samples: (a) 1i, (b) 1ii, (c) 4, (d) 7, (e) 8, (f) 9.
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both of which are present in sample 4 (Fig. 1c). After calcination (XRD not shown) the pure surfactant phase completely disappears and the silicate phase displays only one reflection with a slight contraction of d-spacing. This suggests that whilst ordering remains at the mesoscale this ordering is less than that of a lamellar phase. In other words, the mesophase becomes more disordered upon calcination. In order to optimise the calcination condition, we performed TG-DTA analysis (Fig. 2) for two different samples synthesised using oil (sample 5, Fig. 2a) and polystyrene (sample 9, Fig. 2b) as the dispersed phase. Sample 5 displays two exothermic weight losses: 40% up to T = 280 °C; and 20% above 280 °C. There is virtually no weight loss above 480 °C. The low temperature weight loss is partly due to loss of mesitylene and the high temperature weight loss due to the loss of CTAB and remaining mesitylene. The presence of a weak endothermic peak at 100 °C in both samples is probably due to the loss of adsorbed water. The presence of exothermic peaks (Fig. 2b) is due to the burning of polystyrene and CTAB. The steady TG pattern is observed above 550 °C. The total weight loss for sample 9 due to the burning of polystyrene and CTAB is nearly 80%. The macroporous nature of these materials is easily observed by scanning electron microscope (Fig. 3). Sample 1i exhibited a SEM image of a nonporous structure with shell morphology and sample 1ii exhibited a spherical structure with cauliflower morphology. The top layer of sample 1ii exhibited similar cauliflower morphology along with macroporous structure (Fig. 3c). Samples 2, 3, 4, 5 and 6 all exhibited a macroporous structure where the pores are not well ordered and ranges from a few hundred nm to a few micron in size. The thickest walls were formed in sample 2 and the thinnest in sample 6. Sample 5 exhibited a unique strut-like structure. The cells of the strut structure are approximately 3 lm with interconnecting windows with size approximately 1–2 lm (Fig. 3g and h). This strut-like macroporous structure we name UMIST macro-cellular foams (UMCF) [13]. Sample 6 exhibits a vesicle-like structure with vesicle sizes approximately 5–10 lm (Fig. 3i). The sample synthesised at neutral pH exhibits a porous structure ranging from 100–300 nm with no ordering of pores (Fig. 3j). The sample synthesised in basic medium using mesitylene exhibits no porosity with a mixture of various morphologies (spheres, rhombohedral, cylindrical etc., Fig. 3k). In contrast, using monodispersed polystyrene spheres under basic condition (sample 9), exhibits an ordered macroporous structure of pores of size 200 nm with a wall thickness of 80 nm. This corresponds well with the size of the polystyrene spheres (312 nm) used as a dispersed phase. Synthesis at acidic pH (batch 10) exhibits an agglomerated form of ellipsoids (Fig. 3l).
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Fig. 2. TG–DTA pattern of sample 5 (a) and sample 9 (b).
The TEM in Fig. 4 does not show any long range ordering of the mesoporosity consistent with the single reflection in the X-ray diffraction pattern of calcined samples. The structure is most probably similar to a disordered worm-hole morphology [14]. The presence of mesopores is confirmed by nitrogen adsorption measurements and the results of three characteristics samples e.g. meso-cellular foams (sample 1ii), UMIST macro-cellular foams (sample 5) and ordered macroporous silica (sample 9) are presented in Fig. 5. All these samples exhibits a nitrogen adsorption isotherm of type IV with a hysteresis in the region of 0.4 < p/p0 < 1, which is a direct evidence of the presence of mesopores. The mesopores in the different samples however are different as evidenced by the type of hysteresis. Sample 1ii exhibits a weak hysteresis indicating a small mesopores in comparison to sample 5 (macrocellular foams) and sample 9 (ordered macroporous silica). The meso and macro-cellular foams exhibit type H2 hysteresis and ordered macroporous solid exhibit type H4 hysteresis. Type H2 hysteresis is attributed to a difference between the condensation and evaporation processes occurring in pores with narrow necks and wide bodies (‘‘ink-bottle’’ pores). This is consistent with the previous report [12] on meso-cellular foams formed with windows and cells. Type H4 is often associated with narrow slit-like pores as might be expected to ensure from a lamellar precursor. The monolayer uptake (p/p0 < 0.1) of meso- and macro-cellular foams (samples 1ii and 5) is nearly the same but the multi-layer uptake is completely different as the hysteresis loops are different. The monolayer uptake is much lower in case of ordered macroporous solid (sample 9). The BET surface area calculated based on monolayer uptake is presented in Table 2. The high surface area indicates that despite the lamellar nature of the precursor phase the porosity does not collapse upon calcination. This is quite different to the behaviour of MCM-50 [15].
The BJH desorption pore size distribution is broad for samples 5 and 9 (macro-cellular foams and macroporous solid) whereas sample 1ii (meso-cellular foams) exhibited a narrow bimodal size distribution. The mesopore sizes and volume of these materials are also presented in Table 2. The size and distribution of macropores are determined using Hg porosimetry (Fig. 6). Sample 1ii exhibited no peaks of pore size above 50 nm (0.05 lm) (Fig. 6a) indicating the absence of macroporosity. The two peaks centered at approximately 10 and 30 nm (0.01 and 0.03 lm) correspond to windows and cell structure of meso-cellular foams. Samples 5 and 9 exhibit a unimodal pore size distribution (Fig. 6b and c). The broad peak centered at 1 lm corresponds to the presence of windows of broad size distribution. The narrow size distribution centered at 200 (0.2 lm) nm of sample 9 corresponds to uniform macropores. The macropore sizes from Hg porosimetry results correspond well to the value obtained from SEM. The nature of the silicon environment of the pure mesoporous solid (sample 1i), the meso-cellular foams (sample 1ii) and the macro-cellular foams (sample 5) were characterised by 29Si solid state MAS NMR with and without cross-polarisation, CP, see Fig. 7. It is clearly observed from single-pulse, SP, spectra that there are three distinct peaks (chemical shift approximately 90, 100 and 110 ppm) due to the presence of Q2, Q3 and Q4 Si species in uncalcined samples [16]. The ratio of (Q4/Q2 + Q3) as a measure of the degree of condensation is presented in Table 2. In the uncalcined state the meso-cellular foams exhibit a lower degree of condensation than the macro-cellular foams. Also upon calcination the meso-cellular foams condense further whereas the macro-cellular foams retain their degree of condensation. The cross polarisation experiment give an indication of how close the Q4 silicon is to the proton pool associated with the Q3 and Q2 Si-species and also water bound near these latter species. Although it is important not to draw too many conclusions from such
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Fig. 3. SEM micrographs of various calcined samples: 1i (a), 1ii (b), 1(c), 2(d), 3(e), 4(f), 5(g,h), 6(i), 7(j), 8(k), 9(l), 10(m).
an experiment the following is noteworthy. The Q4 resonance under CP for the meso-cellular foams has significant intensity which reduces upon calcination. The most likely explanation being that there are Q2 and Q3 species distributed within the walls of these materials. Conversely, for the macro-cellular foams the Q4 resonance under CP is very small both in the uncalcined and calcined state which suggests that the Q2 and Q3 species are predominantly at the periphery of the wall. The formation of different porous structures is illustrated in Figs. 8 and 9. An emulsion is defined as a macro-
scopically homogeneous mixture of two (or more) kinds of immiscible liquids. Typical immiscible liquids are oil in water (O/W), in which oil droplets are dispersed in a continuous water phase. The other is water in oil (W/O), which has a continuous oil phase containing water droplets. An emulsion is not stable in a thermodynamic sense because of its high interfacial energy (area) between the two phases of water and oil. The emulsions, therefore, will separate into two bulk phases over a period of time to minimise the interfacial area. Therefore, the stability of emulsions is defined
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Fig. 4. TEM image of sample 5 in two different magnification.
Fig. 5. Nitrogen adsorption and desorption results. Sample 1ii (a: isotherm, b: pore size distribution), sample 5 (c: isotherm, d: pore size distribution), sample 9 (e: isotherm, f: pore size distribution).
Table 2 Characterization results of various samples Sample
1i 1ii 5 9
NMR Q4/(Q2 + Q3)
1.13 (uncal), 0.39 (cal) 1.12 (uncal), 0.6 (cal) 1.69 (uncal), 1.69 (cal)
N2 adsorption SBET (m2/g)
V (ml/g)
d (nm)
1131 871 796 254
0.75 0.59 0.61 0.42
2.7 2.6 & 3.9 3.1
SBET: BET surface area, V: mesopore volume, d: mesopore diameter.
kinetically, which means that the rate of separation is slow in stable emulsions. The separation of an emulsion proceeds in two steps (Fig. 8B and C). In the first step,
the emulsion droplets float up (in the case of mesitylene) or settle down (in the case of polystyrene) owing to gravity (Fig. 8B). In the case of O/W emulsions, oil droplets float up and the water phase is separated out at the bottom of the vessel. This is a process called creaming during which the droplets do not change in size. The second step (Fig. 8C) is the coalescence of emulsion droplets. In this step, the size of emulsion droplets becomes larger and larger, until finally bulk separation takes place. To avoid the creaming phenomenon, one has to make the emulsion particles small enough (<1 lm) to be dispersed by Brownian motion. The other method is to make the dispersion medium viscous enough to prevent the parti-
Fig. 6. Hg porosimetry results. Incremental pore size distribution of sample 1ii (a), sample 5 (b) and sample 9 (c).
T. Sen et al. / Microporous and Mesoporous Materials 78 (2005) 255–263
Fig. 7. 29Si single-pulse MAS (left column) and 1H–29Si CPMAS NMR (right column): uncalcined 1i (a), calcined 1i (b), uncalcined 1ii (c) calcined 1ii (d), uncalcined 5 (e) and calcined 5 (f).
cles from floating up or settling down. Surfactants play a significant role for the stability of an emulsion. To
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obtain stable emulsions without coalescence, surfactant molecules must adsorb efficiently at the interface between the water and oil phases. The surfactant end with hydrophilic nature dissolves into the water phase and the end with hydrophobic nature dissolves into the oil phase. The surfactant molecules may not adsorb efficiently at either interface. Consequently, the hydrophilic and hydrophobic nature must be balanced in the surfactant molecule for it to work well as a good emulsifier. At low oil concentration the surfactant molecules forms micelles where oil droplets acts as a swelling agent. The TEOS will hydrolyse and condense and then bind to the swollen micelles so that the ultimate material is either a mesoporous silica or a meso-cellular foam. Here the silica precipitates out in step A (Fig. 8) before the oil starts to cream (step B, Fig. 8). In the presence of a large quantity of oil silica binding occurs at step B or step C. If silica binds in step B (Fig. 8), it will produce ordered macroporous structures with or without interconnectivity (Fig. 9) depending on the rate of creaming and the rate of silica binding. Windows will form if the rate of creaming is faster than the rate of hydrolysis and vice versa. In the case of polystyrene spheres as a dispersed phase, the hydrolysis and condensation of TEOS around polystyrene spheres is faster than the polystyrene spheres can cream to the bottom. Sample 9 is formed by silica binding at step B. In the case of mesitylene as a dispersed phase, the rate of coalescence is faster than the rate of hydrolysis and condensation of TEOS, hence the materials have a disordered macrostructure. This demonstrates that silica binding at step C (Fig. 8) is the dominating process for samples 2–6. Synthesis at alkaline pH using mesitylene as a dispersed phase or acidic pH using polystyrene as a dispersed phase produces materials without macroporosity due the fact that the hydrolysis and condensation of TEOS is much faster than the rate of creaming or coalescence i.e. an
Fig. 8. Diagrammatic representation of the creaming process and associated formation of silica superstructures.
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Fig. 9. Formation mechanism describing the creation of windows within the superstructures.
inhomogeneous distribution of silica particles in a creamed (step B) or coalescence (step C) phase. For synthesis at neutral pH, the rate of hydrolysis and condensation of silica is nearly the same as creaming, hence, the coalescence step is less probable resulting in macroporosity with a smaller pore diameter. Stirring at 300 rpm produces a mesoporous solid whereas 800 rpm produces a meso-cellular foam in step A (Fig. 8), perhaps due to a different dispersion of oil in the emulsion.
4. Conclusions Mesoporous silica, meso-cellular silica foams (MCFÕs), macro-cellular silica foams (UMCFÕs) and ordered macroporous silica were prepared in a one pot
synthesis at room temperature using a very simple emulsion route. Pure mesoporous silica was obtained using a low oil concentration with slow stirring, whereas mesocellular silica foams were formed with faster stirring. Synthesis using intermediate to high oil concentration produced macroporous solids with various pore sizes and wall thickness. Synthesis at a specific oil concentration (31.5%), produced a strut-like macro-cellular silica foams (UMCFs) with interconnectivity between the cells. Increasing the pH from acidic to neutral to basic, the macroporous structure starts to disappear and forms a pure mesoporous solid. Unlike mesitylene as a dispersed phase, polystyrene spheres produce a well ordered macroporous solid without interconnectivity at basic pH and an amorphous solid at acidic pH. Mesocellular foams are formed through a phase transition
T. Sen et al. / Microporous and Mesoporous Materials 78 (2005) 255–263
as reported [12] when mesitylene acts as a swelling agent on the CTAB micelles whereas macro-cellular foams are formed due to the creaming/coalescence of mesitylene or polystyrene.
Acknowledgement We are grateful to ICI Synetix for financial support and to Noreen Hanif for recording scanning electron micrographs.
References [1] M.E. Davis, Accounts. Chem. Res. 26 (1993) 111. [2] S.M. Csicsery, Zeolites 4 (1984) 202. [3] J.W. Zhao, F. Gao, Y.L. Fu, W. Jin, P.Y. Yang, D.Y. Zhao, Chem. Commun. (2002) 752. [4] S. Schacht, Q. Huo, I.G. VoigtMartin, G.D. Stucky, F. Schu¨th, Science 273 (1996) 768. [5] A. Imhof, J. Pine, Nature 389 (1997) 948.
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[6] O.D. Velev, T.A. Jede, R.F. Lobo, A.M. Lenhoff, Nature 389 (1997) 448. [7] B.T. Holland, L. Abrams, A. Stein, J. Am. Chem. Soc. 121 (1999) 4308. [8] C. Danumah, S. Vaudreuil, L. Bonneviot, M. Bousmina, S. Giasson, S. Kaliaguine, Micropor. Mesopor. Mater. 44 (2001) 241. [9] T. Sen, J.L. Casci, G.J.T. Tiddy, M.W. Anderson, Angew. Chem. Int. Ed. En. 42 (2003) 4649. [10] T. Sen, G.J.T. Tiddy, J.L. Casci, M.W. Anderson, Chem. Mater. 16 (2004) 2044. [11] Q.S. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996) 1147. [12] P.S. Winkel, W.W. Lukens, P. Yang, D.I. Margolese, J.S. Lettow, Y.J. Ying, G.D. Stucky, Chem. Mater. 12 (2000) 686. [13] T. Sen, G.J.T. Tiddy, J.L. Casci, M.W. Anderson, Chem. Commun. (2003) 2182. [14] T.R. Pauly, Y. Liu, T.J. Pinnavaia, S.J.L. Billinge, T.P. Ricker, J. Am. Chem. Soc. 121 (1999) 8835. [15] 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. [16] G. Engelhardt, D. Michel, Solid State NMR of Zeolites and Silicates, Wiley, New York, 1987.
Meso-cellular silica foams, macro-cellular silica foams and mesoporous solids: a study of emulsion-mediated synthesis T. Sen a, G.J.T. Tiddy b, J.L. Casci c, M.W. Anderson a
a,*
Department of Chemistry, Centre for Microporous Materials, UMIST, Manchester M60 1QD, UK b Department of Chemical Engineering, UMIST, Manchester M60 1QD, UK c Synetix, P.O. Box 1, Billingham, Cleveland TS23 1LB, UK Received 16 June 2004; received in revised form 17 September 2004; accepted 19 September 2004 Available online 8 December 2004
Abstract A simple room temperature synthesis of a series of porous silica materials from pure mesoporous solid to meso-cellular foams to macro-cellular foams using an emulsion containing surfactant, water, oil or polystyrene spheres is reported. A mesoporous solid is obtained using either a low oil concentration with slow stirring or synthesis at alkaline pH. Faster stirring at low oil concentration produced meso-cellular silica foam. A series of macro and mesoporous silica composites are obtained with various wall thickness and interconnectivity using intermediate to high oil concentration. An amorphous solid was formed in an acidic pH using polystyrene spheres as droplets whereas an ordered macroporous solid with mesoporous walls was formed at alkaline pH. The formation of such structures is explained based on the rate of various processes, e.g. hydrolysis of silica, precipitation as micelles, condensation of silica, creaming or coalescence of oil or polystyrene. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Macro- and meso-cellular silica foams; Emulsion; Creaming; Worm-hole; Mesoporous
1. Introduction Porous materials are well known for a variety of uses from molecular sieves to catalysis [1]. Introduction of a range of pore sizes from macro (>50 nm) to meso (0.2–50 nm) to micro (<0.2 nm) in an ordered way is important for specific applications, e.g. shape selective catalysis [2] and bio-molecular sieving [3]. Most synthetic methods to produce hierarchical structures are laborious multi-step processes and consequently there is an impetus for low cost, one-pot, room temperature syntheses. One such method was first published by Stucky and co-workers [4] using an emulsion method
*
Corresponding author. Tel.: +44 161 200 4517; fax: +44 161 200 4559. E-mail address: [email protected] (M.W. Anderson). 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.09.022
to synthesise mesoporous silica. Imhof and Pine [5] has reported the synthesis of ordered macroporous materials using an emulsion method. Subsequently, various authors published the synthesis of such ordered macroporous materials using a monodispersed polystyrene monolith as template [6]. Most of the methods produce an ordered macroporous solid without meso or microporosity. Introduction of micro or mesoporosity or both in a macroporous structure is also reported recently using a combination of organic quaternary amine/surfactant and polystyrene spheres as template in a one-pot synthesis [7–10]. A room temperature synthesis of mesoporous silica materials using a silica gel containing a surfactant is also reported [11]. Stucky and co-workers [12] described the synthesis of mesocellular silica foams (MCFÕs) during the preparation of SBA type materials using mesitylene as a swelling agent. All these methods are specific for the synthesis
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T. Sen et al. / Microporous and Mesoporous Materials 78 (2005) 255–263
of materials of one kind. Recently we have reported [13] the synthesis of meso-cellular foams (MCFÕs) along with UMIST macro-cellular foams (UMCFÕs) using an emulsion method without proper understanding of the formation mechanism. Herein we report the one-pot synthesis and the formation mechanism of a range of porous silica materials by emulsion-mediated method using oil or polystyrene droplets as a dispersed phase in a continuous water phase.
2. Experimental 2.1. Synthesis An aqueous solution was used as a continuous phase and mesitylene or a monodisperse polystyrene suspension was used as a dispersed phase. Cetyl trimethyl ammonium bromide (CTAB) was used as surfactant and tetraethyl orthosilicate (TEOS) as a silica source. The gel composition (wt%) of different syntheses is presented in Table 1. First, CTAB was dissolved in water under stirring, then the required amount of hydrochloric acid or sodium hydroxide was added to this stirred solution to give a colourless solution. Finally, a mixture of mesitylene and tetraethyl orthosilicate was added slowly for a period of 30 min under stirring at a fixed rpm. During addition, the mixture turned opaque. The whole mixture was then stirred for 1 h using a mechanical stirrer (stainless steel, three blades, propeller shaped). After settling, two layers were observed in all synthetic batches: a thick white layer floated on top; a transparent pale blue solution below. When the gel was stirred at 300 rpm (sample 1i) a white precipitate was observed at the bottom of the jar whereas at 800 rpm (sample 1ii) an additional thick top layer (sample 1) was also formed. When polystyrene was used as a dispersed Table 1 Gel composition and condition of various synthesis batchesa Batch no.
Dispersed phase
pH condition
Gel composition (TEOS:CTAB: Mesitylene/polystyrene: Water:HCl/NaOH), wt%
1ib 1ii 2 3 4 5 6 7 8 9 10
Mesitylene ,, ,, ,, ,, ,, ,, ,, ,, Polystyrene Polystyrene
Acidic (0.3) ,, ,, ,, ,, ,, ,, Neutral (6.9) Basic (11.4) Basic (12.3) Acidic (0.2)
3.6:4.2:2.0:87.4:2.8 ,, 3.6:4.2:15.2:74.2:2.8 3.6:4.2:21.2:68.2:2.8 3.6:4.2:26.4:63.0:2.8 3.6:4.2:31.0:58.4:2.8 3.6:4.2:39.5:49.9:2.8 3.6:4.2:31.0:58.4:0 3.6:4.2:31.0:58.4:3.9 5.9:6.8:5.2:81.3:0.8 5.4:6.1:4.8:80.1:3.6
a b
800 rpm. 300 rpm Stirring speed.
phase (batches 9 and 10), the synthesis route is as follows: 4.74 g of CTAB was dissolved in 30 g of water in alkali (0.55 g NaOH) or acid (8 ml, 35% HCl). 30 g of polystyrene suspension PS-B6 (312 nm, 12% solid in suspension) was added to the CTAB solution and stirred at 800 rpm for 5 min to make a homogeneous solution. The whole solution was milky in colour. 4.12 g of tetraethyl orthosilicate (TEOS) was added drop wise for 15 min to the mixture. The whole mixture was then stirred for another 1 h before transfer to a glass jar. The mixture was kept at room temperature over night and two layers were observed the following day: a bottom solid layer and a top clear solution. White solids were filtered out either from the bottom or from the top layer. The separated solids were washed with de-ionised water several times and dried at 40 °C for 6 h. The dried materials were white in colour. These dried materials (batches 1–8) were calcined at 650 °C for 10 h in air. Temperature of the furnace was increased at a rate of 1 °C/min. In the case of batches 9–10, the polystyrene template was initially removed by toluene extraction at RT for 48 h before calcined at 550 °C for 10 h in air. The calcined materials were white in colour. 2.2. Characterisation X-ray diffractograms were recorded on a XDS 2000 Scintag machine using CuKa radiation with a scan rate of 1°/min. Simultaneous TG/DTA analyses of the assynthesised and toluene treated materials were performed on an automatic instrument (TGA-DTA VI.1B TA Inst 2000). The thermograms of the samples were recorded under air (air flow ffi16 ml/min) with a heating rate of 5 °C/min. The morphology and pore structure were characterised by scanning electron microscopy (SEM). SEM micrographs were recorded on a Philips XL30 with field-emission gun. Macropores were characterised by Hg porosimetry using a Micromeritics Autopore IV instrument. The mesopores and the surface area of the materials were characterised by N2 adsorption using a Micromeritics ASAP2010 instrument. The nature of mesophases and their ordering was further characterised by transmission electron microscopy (TEM) on a Phillips CM20 200 kV. The specimen for analysis was prepared using a few mg of sample crushed and dispersed in acetone and a drop was placed onto a carboncoated copper grid. 29Si NMR of various samples was performed on a Bruker MSL 400 spectrometer with single pulse (SP) and cross polarisation (CP) with magicangle spinning, 4 kHz, at a Larmor frequency of 79.474 MHZ. TMS was used as a reference for SP and Q8M8 for CP MAS experiments. The flip angle was 7.5 ls (p/2) and the relaxation delay was 100 s for SP MAS NMR. The 29Si CP MAS experiments were performed using a flip angle 7.5 ls (p/2), CP mixing time 2 ms and relaxation delay of 10 s.
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3. Results and discussion X-ray diffractograms are shown in Fig. 1 for a variety of uncalcined samples. They all exhibit low angle reflections characteristic of ordering at the mesoscale. Typically two groups of lines are observed both corresponding to lamellar phases. The first is characterised by two broad reflections in the region approximately 3.4–4.0 nm and 1.7–2.0 nm. The second is characterised by two sharp reflections at 2.7 and 1.35 nm. The former pairs of lines is characteristic of a mesoporous lamellar silicate phase and the later pair of lines is characteristic of a pure lamellar surfactant phase. The ratio of silicate and surfactant phases depends strongly upon the synthetic conditions as does the precise d-spacing of the silicate mesophase. The pure surfactant phase displays two different mesophases with slightly different d-spacing
Fig. 1. XRD diffraction pattern of various uncalcined samples: (a) 1i, (b) 1ii, (c) 4, (d) 7, (e) 8, (f) 9.
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both of which are present in sample 4 (Fig. 1c). After calcination (XRD not shown) the pure surfactant phase completely disappears and the silicate phase displays only one reflection with a slight contraction of d-spacing. This suggests that whilst ordering remains at the mesoscale this ordering is less than that of a lamellar phase. In other words, the mesophase becomes more disordered upon calcination. In order to optimise the calcination condition, we performed TG-DTA analysis (Fig. 2) for two different samples synthesised using oil (sample 5, Fig. 2a) and polystyrene (sample 9, Fig. 2b) as the dispersed phase. Sample 5 displays two exothermic weight losses: 40% up to T = 280 °C; and 20% above 280 °C. There is virtually no weight loss above 480 °C. The low temperature weight loss is partly due to loss of mesitylene and the high temperature weight loss due to the loss of CTAB and remaining mesitylene. The presence of a weak endothermic peak at 100 °C in both samples is probably due to the loss of adsorbed water. The presence of exothermic peaks (Fig. 2b) is due to the burning of polystyrene and CTAB. The steady TG pattern is observed above 550 °C. The total weight loss for sample 9 due to the burning of polystyrene and CTAB is nearly 80%. The macroporous nature of these materials is easily observed by scanning electron microscope (Fig. 3). Sample 1i exhibited a SEM image of a nonporous structure with shell morphology and sample 1ii exhibited a spherical structure with cauliflower morphology. The top layer of sample 1ii exhibited similar cauliflower morphology along with macroporous structure (Fig. 3c). Samples 2, 3, 4, 5 and 6 all exhibited a macroporous structure where the pores are not well ordered and ranges from a few hundred nm to a few micron in size. The thickest walls were formed in sample 2 and the thinnest in sample 6. Sample 5 exhibited a unique strut-like structure. The cells of the strut structure are approximately 3 lm with interconnecting windows with size approximately 1–2 lm (Fig. 3g and h). This strut-like macroporous structure we name UMIST macro-cellular foams (UMCF) [13]. Sample 6 exhibits a vesicle-like structure with vesicle sizes approximately 5–10 lm (Fig. 3i). The sample synthesised at neutral pH exhibits a porous structure ranging from 100–300 nm with no ordering of pores (Fig. 3j). The sample synthesised in basic medium using mesitylene exhibits no porosity with a mixture of various morphologies (spheres, rhombohedral, cylindrical etc., Fig. 3k). In contrast, using monodispersed polystyrene spheres under basic condition (sample 9), exhibits an ordered macroporous structure of pores of size 200 nm with a wall thickness of 80 nm. This corresponds well with the size of the polystyrene spheres (312 nm) used as a dispersed phase. Synthesis at acidic pH (batch 10) exhibits an agglomerated form of ellipsoids (Fig. 3l).
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Fig. 2. TG–DTA pattern of sample 5 (a) and sample 9 (b).
The TEM in Fig. 4 does not show any long range ordering of the mesoporosity consistent with the single reflection in the X-ray diffraction pattern of calcined samples. The structure is most probably similar to a disordered worm-hole morphology [14]. The presence of mesopores is confirmed by nitrogen adsorption measurements and the results of three characteristics samples e.g. meso-cellular foams (sample 1ii), UMIST macro-cellular foams (sample 5) and ordered macroporous silica (sample 9) are presented in Fig. 5. All these samples exhibits a nitrogen adsorption isotherm of type IV with a hysteresis in the region of 0.4 < p/p0 < 1, which is a direct evidence of the presence of mesopores. The mesopores in the different samples however are different as evidenced by the type of hysteresis. Sample 1ii exhibits a weak hysteresis indicating a small mesopores in comparison to sample 5 (macrocellular foams) and sample 9 (ordered macroporous silica). The meso and macro-cellular foams exhibit type H2 hysteresis and ordered macroporous solid exhibit type H4 hysteresis. Type H2 hysteresis is attributed to a difference between the condensation and evaporation processes occurring in pores with narrow necks and wide bodies (‘‘ink-bottle’’ pores). This is consistent with the previous report [12] on meso-cellular foams formed with windows and cells. Type H4 is often associated with narrow slit-like pores as might be expected to ensure from a lamellar precursor. The monolayer uptake (p/p0 < 0.1) of meso- and macro-cellular foams (samples 1ii and 5) is nearly the same but the multi-layer uptake is completely different as the hysteresis loops are different. The monolayer uptake is much lower in case of ordered macroporous solid (sample 9). The BET surface area calculated based on monolayer uptake is presented in Table 2. The high surface area indicates that despite the lamellar nature of the precursor phase the porosity does not collapse upon calcination. This is quite different to the behaviour of MCM-50 [15].
The BJH desorption pore size distribution is broad for samples 5 and 9 (macro-cellular foams and macroporous solid) whereas sample 1ii (meso-cellular foams) exhibited a narrow bimodal size distribution. The mesopore sizes and volume of these materials are also presented in Table 2. The size and distribution of macropores are determined using Hg porosimetry (Fig. 6). Sample 1ii exhibited no peaks of pore size above 50 nm (0.05 lm) (Fig. 6a) indicating the absence of macroporosity. The two peaks centered at approximately 10 and 30 nm (0.01 and 0.03 lm) correspond to windows and cell structure of meso-cellular foams. Samples 5 and 9 exhibit a unimodal pore size distribution (Fig. 6b and c). The broad peak centered at 1 lm corresponds to the presence of windows of broad size distribution. The narrow size distribution centered at 200 (0.2 lm) nm of sample 9 corresponds to uniform macropores. The macropore sizes from Hg porosimetry results correspond well to the value obtained from SEM. The nature of the silicon environment of the pure mesoporous solid (sample 1i), the meso-cellular foams (sample 1ii) and the macro-cellular foams (sample 5) were characterised by 29Si solid state MAS NMR with and without cross-polarisation, CP, see Fig. 7. It is clearly observed from single-pulse, SP, spectra that there are three distinct peaks (chemical shift approximately 90, 100 and 110 ppm) due to the presence of Q2, Q3 and Q4 Si species in uncalcined samples [16]. The ratio of (Q4/Q2 + Q3) as a measure of the degree of condensation is presented in Table 2. In the uncalcined state the meso-cellular foams exhibit a lower degree of condensation than the macro-cellular foams. Also upon calcination the meso-cellular foams condense further whereas the macro-cellular foams retain their degree of condensation. The cross polarisation experiment give an indication of how close the Q4 silicon is to the proton pool associated with the Q3 and Q2 Si-species and also water bound near these latter species. Although it is important not to draw too many conclusions from such
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Fig. 3. SEM micrographs of various calcined samples: 1i (a), 1ii (b), 1(c), 2(d), 3(e), 4(f), 5(g,h), 6(i), 7(j), 8(k), 9(l), 10(m).
an experiment the following is noteworthy. The Q4 resonance under CP for the meso-cellular foams has significant intensity which reduces upon calcination. The most likely explanation being that there are Q2 and Q3 species distributed within the walls of these materials. Conversely, for the macro-cellular foams the Q4 resonance under CP is very small both in the uncalcined and calcined state which suggests that the Q2 and Q3 species are predominantly at the periphery of the wall. The formation of different porous structures is illustrated in Figs. 8 and 9. An emulsion is defined as a macro-
scopically homogeneous mixture of two (or more) kinds of immiscible liquids. Typical immiscible liquids are oil in water (O/W), in which oil droplets are dispersed in a continuous water phase. The other is water in oil (W/O), which has a continuous oil phase containing water droplets. An emulsion is not stable in a thermodynamic sense because of its high interfacial energy (area) between the two phases of water and oil. The emulsions, therefore, will separate into two bulk phases over a period of time to minimise the interfacial area. Therefore, the stability of emulsions is defined
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Fig. 4. TEM image of sample 5 in two different magnification.
Fig. 5. Nitrogen adsorption and desorption results. Sample 1ii (a: isotherm, b: pore size distribution), sample 5 (c: isotherm, d: pore size distribution), sample 9 (e: isotherm, f: pore size distribution).
Table 2 Characterization results of various samples Sample
1i 1ii 5 9
NMR Q4/(Q2 + Q3)
1.13 (uncal), 0.39 (cal) 1.12 (uncal), 0.6 (cal) 1.69 (uncal), 1.69 (cal)
N2 adsorption SBET (m2/g)
V (ml/g)
d (nm)
1131 871 796 254
0.75 0.59 0.61 0.42
2.7 2.6 & 3.9 3.1
SBET: BET surface area, V: mesopore volume, d: mesopore diameter.
kinetically, which means that the rate of separation is slow in stable emulsions. The separation of an emulsion proceeds in two steps (Fig. 8B and C). In the first step,
the emulsion droplets float up (in the case of mesitylene) or settle down (in the case of polystyrene) owing to gravity (Fig. 8B). In the case of O/W emulsions, oil droplets float up and the water phase is separated out at the bottom of the vessel. This is a process called creaming during which the droplets do not change in size. The second step (Fig. 8C) is the coalescence of emulsion droplets. In this step, the size of emulsion droplets becomes larger and larger, until finally bulk separation takes place. To avoid the creaming phenomenon, one has to make the emulsion particles small enough (<1 lm) to be dispersed by Brownian motion. The other method is to make the dispersion medium viscous enough to prevent the parti-
Fig. 6. Hg porosimetry results. Incremental pore size distribution of sample 1ii (a), sample 5 (b) and sample 9 (c).
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Fig. 7. 29Si single-pulse MAS (left column) and 1H–29Si CPMAS NMR (right column): uncalcined 1i (a), calcined 1i (b), uncalcined 1ii (c) calcined 1ii (d), uncalcined 5 (e) and calcined 5 (f).
cles from floating up or settling down. Surfactants play a significant role for the stability of an emulsion. To
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obtain stable emulsions without coalescence, surfactant molecules must adsorb efficiently at the interface between the water and oil phases. The surfactant end with hydrophilic nature dissolves into the water phase and the end with hydrophobic nature dissolves into the oil phase. The surfactant molecules may not adsorb efficiently at either interface. Consequently, the hydrophilic and hydrophobic nature must be balanced in the surfactant molecule for it to work well as a good emulsifier. At low oil concentration the surfactant molecules forms micelles where oil droplets acts as a swelling agent. The TEOS will hydrolyse and condense and then bind to the swollen micelles so that the ultimate material is either a mesoporous silica or a meso-cellular foam. Here the silica precipitates out in step A (Fig. 8) before the oil starts to cream (step B, Fig. 8). In the presence of a large quantity of oil silica binding occurs at step B or step C. If silica binds in step B (Fig. 8), it will produce ordered macroporous structures with or without interconnectivity (Fig. 9) depending on the rate of creaming and the rate of silica binding. Windows will form if the rate of creaming is faster than the rate of hydrolysis and vice versa. In the case of polystyrene spheres as a dispersed phase, the hydrolysis and condensation of TEOS around polystyrene spheres is faster than the polystyrene spheres can cream to the bottom. Sample 9 is formed by silica binding at step B. In the case of mesitylene as a dispersed phase, the rate of coalescence is faster than the rate of hydrolysis and condensation of TEOS, hence the materials have a disordered macrostructure. This demonstrates that silica binding at step C (Fig. 8) is the dominating process for samples 2–6. Synthesis at alkaline pH using mesitylene as a dispersed phase or acidic pH using polystyrene as a dispersed phase produces materials without macroporosity due the fact that the hydrolysis and condensation of TEOS is much faster than the rate of creaming or coalescence i.e. an
Fig. 8. Diagrammatic representation of the creaming process and associated formation of silica superstructures.
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Fig. 9. Formation mechanism describing the creation of windows within the superstructures.
inhomogeneous distribution of silica particles in a creamed (step B) or coalescence (step C) phase. For synthesis at neutral pH, the rate of hydrolysis and condensation of silica is nearly the same as creaming, hence, the coalescence step is less probable resulting in macroporosity with a smaller pore diameter. Stirring at 300 rpm produces a mesoporous solid whereas 800 rpm produces a meso-cellular foam in step A (Fig. 8), perhaps due to a different dispersion of oil in the emulsion.
4. Conclusions Mesoporous silica, meso-cellular silica foams (MCFÕs), macro-cellular silica foams (UMCFÕs) and ordered macroporous silica were prepared in a one pot
synthesis at room temperature using a very simple emulsion route. Pure mesoporous silica was obtained using a low oil concentration with slow stirring, whereas mesocellular silica foams were formed with faster stirring. Synthesis using intermediate to high oil concentration produced macroporous solids with various pore sizes and wall thickness. Synthesis at a specific oil concentration (31.5%), produced a strut-like macro-cellular silica foams (UMCFs) with interconnectivity between the cells. Increasing the pH from acidic to neutral to basic, the macroporous structure starts to disappear and forms a pure mesoporous solid. Unlike mesitylene as a dispersed phase, polystyrene spheres produce a well ordered macroporous solid without interconnectivity at basic pH and an amorphous solid at acidic pH. Mesocellular foams are formed through a phase transition
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as reported [12] when mesitylene acts as a swelling agent on the CTAB micelles whereas macro-cellular foams are formed due to the creaming/coalescence of mesitylene or polystyrene.
Acknowledgement We are grateful to ICI Synetix for financial support and to Noreen Hanif for recording scanning electron micrographs.
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