J. Phys. Chem. C 2009, 113, 21283–21292
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Micropore Characterization of Mesocellular Foam and Hybrid Organic Functional Mesocellular Foam Materials Sasha Boskovic,†,‡ Anita J. Hill,§ Terry W. Turney,| Geoffrey W. Stevens,† Michelle L. Gee,‡ and Andrea J. O’Connor*,† Department of Chemical and Biomolecular Engineering and School of Chemistry, Particulate Fluids Processing Centre, The UniVersity of Melbourne, Victoria, 3010, Australia, Commonwealth Scientific and Industrial Research Organisation (CSIRO) Materials Science and Engineering, PriVate Bag 33, Clayton South MDC, Victoria, 3169, Australia, and ARC Centre for Green Chemistry, Monash UniVersity, Victoria, 3800, Australia ReceiVed: May 14, 2009; ReVised Manuscript ReceiVed: NoVember 1, 2009
The micro- and mesoporous structures of microemulsion templated mesocellular foam (MCF) silica materials aged at different temperatures were investigated by using nitrogen sorption Rs-comparison plots and positron annihilation lifetime spectroscopy (PALS). The impact of surface amino functionalization with (3aminopropyl)triethoxysilane (APTES) on these structures was also studied. The nitrogen sorption Rs-comparison plots show that the micropore volume of the MCF silica can be controlled by varying the synthesis aging temperature such that the higher the aging temperature, the smaller the micropore volume. The corresponding PALS data showed that this effect of temperature on micropore volume is a direct consequence of a temperaturedependent micropore population, with more micropores in the MCF material aged at lower temperature. After functionalization of the silica MCF materials with APTES, the BET surface area was seen to have reduced substantially, while maintaining the characteristic mesopore sizes. To quantitatively determine the effect on the micropore volumes of the hybrid APTES functional materials, a macorporous amorphous silica material was similarly functionalized with APTES and a high resolution nitrogen sorption isotherm was recorded at 77 K. For the hybrid APTES functionalized MCF silica materials, the nitrogen sorption Rs-comparison plots revealed that the materials were no longer microporous. This result was confirmed via PALS completed in both nitrogen and air environments, which indicated that the micropores present in the silica materials prior to functionalization were blocked and replaced by a closed microporosity associated with the APTES layer formed within the mesopores. This study has shown the highly complementary nature of the results obtained from standard nitrogen sorption measurements and PALS. Additionally, the high-resolution nitrogen sorption comparison plot for the APTES functionalized macroporous silica is reported, which is applicable to the study of other meso- and microporous APTES functionalized materials. 1. Introduction Mesoporous molecular sieves such as the M41S family of materials have attracted huge research activity in materials templated by surfactant micellar systems since their discovery in the early 1990s.1 These developments have opened up the possibility of applications exploiting the large uniform pore sizes for such as bioseparations and enzyme encapsulation.2,3 The desire for the access and kinetics provided by larger-pore materials has resulted in an increase in the use of amphiphilic block copolymers rather than simple surfactants as templates for mesoporous molecular sieves.4-8 SBA-15 has probably been the most commonly studied mesoporous material templated by amphiphilic block copolymers.7 It was initially thought to possess a highly ordered hexagonal mesoporous structure, analogous to MCM-41 but with larger pores, up to 30 nm (although this has been since revised to 6-12 nm)9 and BET surface areas larger than 800 m2/g.10 * Corresponding author. Tel: + 61 3 8344 8962; Fax: + 61 3 8344 4153; E-mail:
[email protected]. † Department of Chemical and Biomolecular Engineering, The University of Melbourne. ‡ School of Chemistry, The Unversity of Melbourne. § Commonwealth Scientific and Industrial Research Organisation. | ARC Centre for Green Chemistry.
However it has since been confirmed by many different studies and authors that it also contains micropores or smaller mesopores.11-18 A recent study reported the progression of this porosity as a function of synthesis temperature.11 At lower synthesis temperatures (35-60 °C), samples showed very high micropore volumes, as determined by argon sorption, but transmission electron microscopy of platinum replicas did not show bridges replicated from small interconnecting mesopores. For samples synthesized at higher temperatures (100 °C), platinum replicas did show interconnecting small mesopores. Samples synthesized at even higher temperatures (130 °C) showed no microporosity but more of the interconnecting smaller mesopores. This study led the authors to conclude that at the lower temperatures, only very small micropores are present, estimated to be around 1 nm or less from argon sorption, and as the synthesis temperature is increased, these small micropores begin to collapse and bring about the interconnecting mesoporosity. For materials templated by true liquid crystal templating of amphiphilic block copolymers, some recent studies have focused on the use of chord length distributions from small-angle X-ray scattering intensity profiles recorded to characterize the micropores.12,16 The chord length distribution provides a statisti-
10.1021/jp904495e 2009 American Chemical Society Published on Web 12/01/2009
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J. Phys. Chem. C, Vol. 113, No. 51, 2009
cal description of the distances connecting a phase boundary in a two-phase system with sharp phase boundaries;16 however, difficulties can arise from overlap of the micropores and pore walls.12 For materials templated with polystyrene-block-poly(ethylene oxide)-block copolymers,12 micropore diameters of around 1 nm were determined via this technique, with some variation seen depending upon the ratio of the two blocks in the copolymer, whereas for materials templated with n-alkypoly(ethylene oxide)-block copolymers, a similar size of around 1 nm was reported.16 As for SBA-15, their presence was attributed to the hydrated poly(ethylene oxide) component of the block copolymer. It is now widely accepted that the high surface areas obtained for materials templated from amphiphilic block copolymers are due to micropores as well as mesopores and that these micropores are due to the poly(ethylene oxide) components being embedded in the inorganic framework during the templating process.14,17 Indeed, it is these remarkable porous structures that have both mesopores and interconnecting smaller mesopores and micropores that have opened up a whole new area in templating carbon mesoporous materials and replicas.19 It is desirable to functionalize these types of silica for many potential applications, including biomolecular separation and immobilization.20-22 Amino groups are commonly used to impart ion-exchange capacity to such materials for techniques such as ion-exchange chromatography. They can also be used for the immobilization of biomolecules by using the amino group as a tether to attach other chemical species and thus immobilize biomolecules on the surface.23 Amino functionalization of silica can be achieved by using aminopropylsilanes as silanating agents.24 Nonaqueous solventphase reactions are commonly used with reagents such as APTES ((3-aminopropyl)triethoxysilane), whereby the silica is refluxed in a dry solvent under an inert atmosphere.25 Some water is required for this reaction to occur, but the presence of water can also result in the formation of polymerized products on the material surface.26 It is likely that this type of functionalization with polymerized products will have a significant effect on microporosity. For the characterization of the microporous nature of templated mesoporous materials, nitrogen-sorption comparison plots are a commonly used technique.27,28 Positron-annihilation lifetime spectroscopy (PALS) is a less widely known technique that has been developed as a powerful tool over the past few decades. It enables the detection and quantification of materialfree volume, ranging from defects on the atomic scale to freevolume elements in polymers and porosity in microporous and mesoporous amorphous and crystalline solids.29,30 PALS uses positrons (e+) generated by an unstable isotope, typically 22Na, the birth of which is detected by the simultaneous release of a 1.28 MeV gamma ray. The positrons enter the porous material under investigation and reach a thermalizd state through elastic and inelastic collisions within a few picoseconds, which is small compared to the total positron lifetime. Some of the positrons form positronium, an electron-positron bound state, which can exist as either orthopositronium where the spins of the electron and positron are parallel or parapositronium where the spins are anti parallel. The positron will eventually annihilate with an electron, and its mass will be converted to two 0.511 MeV γ rays (detected as its death).29 The positron or positronium lifetimes are longer when they are localized in regions of lower electron density, such as pores and voids. In a vacuum, the orthopositronium lifetime is 142 ns, but in a porous material with a certain electron density, the orthop-
Boskovic et al. ositronium (oPs) will annihilate by pick off with an electron from the solid with the opposite spin, with the reduction in the lifetime being a function of the electron density and thus of the pore size. The intensity of this lifetime in the PALS spectra reflects the relative number of pores.31,32 It is worth noting that the PALS technique is able to probe the porosity considered to be closed to adsorptives such as nitrogen because of two features: (i) oPs has a Bohr radius of 0.53 Å or a van der Waals radius of 1.3 Å; therefore, pores too small for N2 are accessible to oPs and (ii) positrons are injected into the mesocellular material where oPs is subsequently formed; therefore, oPs can probe internal pores that are closed to the external surface (and hence closed to N2 adsorption). PALS spectra consist of various exponential components, each of them due to a specific positron-annihilation mode. The shortest lifetime can be attributed to parapositronium selfannihilation (∼0.125 ns), the second lifetime can be attributed to free- and trapped-positron annihilation (∼0.3-0.9 ns), and the longest lifetime or lifetimes can be attributed to orthopositronium pickoff annihilation.29 The average lifetimes (τ) and intensities (I) of the different components are obtained by applying nonlinear fit routines to the obtained spectra as a weighted sum of discrete exponentials: k+1
N(t) )
I
∑ τii e(-λ t)
(1)
i
i)1
where λi is the decay rate, the reciprocal of the lifetime (τi). The conversion of the lifetime to a pore size is often carried out via the quantum mechanical model of Tao and Eldrup:33
(
λ)21+
R 1 2πR + sin R + ∆R 2π R + ∆R
)
(2)
where R is the radius of the potential well or the pore radius and ∆R has been found empirically to be 0.17 nm.32 The model of Tao and Eldrup is limited to pores of a few nanometres or less, and the conversion of longer lifetimes associated with larger pores is the subject of much recent and ongoing work.33-36 For the lifetimes measured in this work, we have used eq 2 to extract assumed spherical-pore sizes for micropores for comparative purposes; however, the exact geometry of the micropores is not known. A few studies have used PALS for the characterization of templated mesoporous materials, with most publications focused on the determination of the primary mesopore sizes in materials such as the M41S family of mesoporous materials.37-42 PALS has also been used to determine micropore sizes in SBA-15, with small micropores of 0.52-0.64 nm diameter observed, depending on the synthesis aging temperature.18,43 One study also reported the presence of larger micropores of 1.8 ( 0.5 nm diameter in SBA-15 samples aged at 80-130 °C, following similar trends to those seen when using platinum replicas.11,18 It has been shown that the number of micropores in both SBA15 and the related microemulsion templated mesocellular foam (MCF) decreases as the aging temperature is increased.18,30 More recent work has also used PALS to characterize a range of templated mesoporous materials such as SBA-15, MCF, and KIT-6 together with corresponding carbon replicas.44 In this study, we report on the use of PALS and nitrogen sorption Rs-comparison plots to probe the micropores present in MCF silica materials and hybrid MCF silica materials formed via a post-synthesis functionalization with (3-aminopropyl)triethoxysilane. MCF is obtained via the addition of 1,3,5trimethylbenzene (TMB) to the synthesis of SBA-15.45 In this case, the TMB and the amphiphilic block copolymer form a
Micropore Characterization of MCF microemulsion, from which the MCF is templated.46 MCF is composed of large spherical cells, 22-42 nm in diameter, that are interconnected by windows of 10 nm in diameter to create a continual 3-dimenisonal pore system.45 The spherical cell diameter of these materials can be controlled by increasing the TMB/block copolymer ratio, which increases the microemulsion droplet size, whereas the addition of ammonium fluoride increases the window diameter from 10 to 20 nm without affecting the spherical cell size.47 2. Materials and Methods 2.1. Materials. Pluronic triblock copolymer poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123, BASF), TMB (Ajax, 99%), ammonium fluoride (Sigma-Aldrich, 97%), tetraethoxysilane (TEOS, Fluka, 99%), 3-aminopropyl)triethoxysilane (APTES, Sigma-Aldrich, 99%), isopropanol (Ajax Finechem), and Nucleosil-1000-7 macorporous amorphous silica (Macherey-Nagel) were used as received. Hydrochloric acid (1.6 M) was made from a 35% w/w solution (Ajax) with distilled water. Toluene (BDH) was initially dried on 4 Å molecular sieves (Ajax Finechem), distilled over Na metal (BDH), and stored on 4 Å molecular sieves prior to use. Nitrogen used was high purity (BOC Gases). 2.2. MCF Synthesis. The MCF synthesis method used was based on a previously reported method,45,48,49 with a TMB/P123 ratio of 2.5 and fluoride. Initially, 4 g of P123 was dissolved in 150 mL of 1.6 M HCl in a beaker with vigorous stirring. The resulting solution was then heated to 37-40 °C prior to the addition of 11.4 mL of TMB and 46 mg of NH4F. The mixture was stirred for a further 30 min followed by the addition of 8.8 g of TEOS. After 20 h at 37-40 °C with vigorous stirring, the mixture was then transferred to a PTFE coated autoclave and aged under static conditions. Two MCF samples were prepared from such mixtures; MCF-100 was aged at 100 °C for 24 h, and MCF-60 was aged at 60 °C for 24 h. After cooling, the mixture was filtered on a Buchner funnel and allowed to dry for a further 2 days, after which it was calcined at 500 °C in air for 8 h to remove the organic template. MCF samples were stored in a desiccator post calcination prior to further use. 2.3. Amino Functionalization. MCF-60 and MCF-100 were functionalized with APTES in a solvent phase reaction via a method previously reported for MCM-41 mesoporous silica.25 In the functionalization reaction, 2.0 g of MCF was dispersed in 200 mL of dry toluene in a round-bottom flask under nitrogen, which was followed by the addition of 16 mL of APTES. The mixture was then refluxed for 18 h. After cooling to ambient temperature, the sample was filtered on a Buchner funnel, washed with dry toluene and isopropanol, heated overnight in a vacuum oven at 100 °C, and stored in a desiccator prior to further use. After functionalization, the samples are referred to as NH2-MCF-100 and NH2-MCF-60. Details of the chemical nature and characterization of post-synthesis amino-functionalized templated mesoporous materials have been reported previously.21,24 For the functionalization of the Nucleosil-1000-7 macroporous amorphous silica with APTES, the functionalization was completed similarly to MCF; however, the amount of APTES used was scaled down to 0.7 mL. This was to account for the significantly reduced surface area of the Nucleosil-1000-7 material. After functionalization, the material is referred to as NH2-Nucleosil-1000-7. 2.4. Nitrogen Sorption. Nitrogen-sorption isotherms were recorded at 77 K by using a Micromeritics ASAP 2000 gassorption analyzer, except for the high-resolution isotherms which
J. Phys. Chem. C, Vol. 113, No. 51, 2009 21285 were recorded on a Micromeritics ASAP 2010 gas-sorption analyzer. Prior to analysis, the MCF samples were degassed overnight at room temperature under vacuum (
