Direct Synthesis and Accessibility of Amine-Functionalized

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Direct Synthesis and Accessibility of Amine-Functionalized Mesoporous Silica Templated Using Fluorinated Surfactants Gifty Osei-Prempeh,† Hans-Joachim Lehmler,‡ Stephen E. Rankin,§ and Barbara L. Knutson*,§ †

Department of Chemical Engineering, West Virginia University Institute of Technology, Montgomery, West Virginia 25136, United States ‡ Department of Occupational and Environmental Health, College of Public Health, University of Iowa, Iowa City, Iowa 52242-5000, United States § Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506-0046, United States ABSTRACT: 3-Aminopropyl functionalized silica is synthesized directly using cationic fluorinated surfactants (C6F13C2H2NC5H5Cl (HFOPC) and C8F17C2H2NC5H5Cl (HFDePC)) and a hydrocarbon surfactant (C16H33N(CH3)3Br (CTAB)) as templates. The resulting 3-aminopropyl functionalized silica has a two-dimensional hexagonal pore structure, with greater order and surface area in the CTAB-templated material than the fluorinated-surfactant-templated materials. Greater amine incorporation is achieved using CTAB (1.44 mmol/g), with the least amine incorporation being observed in the HFDePCtemplated material (0.92 mmol/g). The incorporation of fluorescein isothiocyanate (FITC) is used to qualitatively probe the accessibility of the amine groups. The reaction of benzaldehyde with the amine groups results in the incorporation of benzaldehyde, which is consistent with the higher accessibility of amines in fluorocarbon-surfactant-templated silica. The interaction of dry CO2 with amines results in higher CO2 sorbed in 3-aminopropyl-functionalized silica than nonfunctionalized silica, with greater amine accessibility being observed in the fluorocarbon-templated silica. The incorporation of perfluorooctyl in 3-aminopropylfunctionalized silica reduces the physisorption and CO2amine interaction.

’ INTRODUCTION The synthesis of organic functionalized nanoporous silica is an active research area, because of the widespread application of these high-surface-area materials in sensing, catalysis, and separation. Nanoporous silica is characterized by very large surface areas, large pore volumes, and narrow pore size distributions. The pore size and structure can be tailored for specific applications through the choice of pore template and synthesis conditions. Surfactant-templated synthesis of porous silica is based on the solgel process, in which precursors such as alkoxysilanes are hydrolyzed and condensed in the presence of a structure directing agent (template). Various pore structures (disordered, hexagonal, cubic, and lamellar) can be obtained as a function of synthesis conditions, chain length, and structure of the surfactant template.1 Since the discovery of the M41S family of materials2 various organic functional groups have been incorporated into mesoporous silica by means of post-synthesis grafting of functional groups on preformed silica or the direct co-condensation of a mixture of tetraalkoxysilane and organic functionalized alkoxysilane (direct synthesis).3 Synthesis of functionalized nanoporous silica by cocondensation results in high functional group incorporation4 and uniform distribution of functional groups in the material. Amine-functionalized silica materials have been widely synthesized; the high reactivity of amines provides various applications for these materials. Nanoporous silicas functionalized with amine groups have been applied in the removal of metals from environmental waste,5,6 gas separation,79 chromatography,10 catalysis,11,12 and drug release.13 Model systems that can be used to probe amine accessibility in amine-functionalized silica are the r 2011 American Chemical Society

application areas where access to the amine groups can be qualitatively or quantitatively determined. For example, in environmental remediation the accessibility of the amine group is demonstrated by the adsorption capacity of the functionalized silica material for toxic metals in aqueous media.14 Protein adsorption has also been investigated to assess the amine group accessibility in amine-functionalized silica.15 Most recently, CO2 adsorption on amine-functionalized mesoporous silica has been investigated to aid in the design of ideal CO2 capture adsorbents.1623 Accessibility of amine groups by fluorescein isothiocyanate (FITC) in amine-functionalized silica has also been employed, especially in evaluating the effect of steric hindrance, as a function of pore size and dimension.24 CO2 capture is a potentially sensitive means to probe the extent of amine functionalization of microporous and mesoporous silica while minimizing the steric effects associated with probe molecules such as FITC. In addition, CO2 adsorption is sensitive to the density of amine functionalization and the surface properties (available OH groups and adsorbed water). The reaction of amine with small molecules, such as benzaldehyde, provides a measure of accessibility without the problems of steric hindrance in FITC incorporation or the close proximity of the amines required in CO2 sorption. Presently, the synthesis of amine-functionalized mesoporous silica mainly utilizes traditional hydrocarbon cationic surfactants Received: July 6, 2010 Accepted: March 8, 2011 Revised: February 28, 2011 Published: March 25, 2011 5510

dx.doi.org/10.1021/ie101313t | Ind. Eng. Chem. Res. 2011, 50, 5510–5522

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(e.g., CTAB),25 anionic surfactants (e.g., sodium dodecylsulfate, SDS),26 or nonionic surfactants27 and block copolymers28 as templates. Fluorinated-surfactant templates for amine-functionalized materials are yet to be investigated. Fluorinated surfactants possess higher hydrophobicity than hydrocarbon surfactants, which results in lower surface tensions and critical micelle concentrations.29 These properties allow for the formation of small micelles and a broad range of nanoscale selfassembled structures30,31 that are more stable and better organized32 than their hydrocarbon analogues. Cationic fluorinated surfactants (a series of perfluoroalkylpyridinium chloride) have recently been demonstrated as templates for the synthesis of nanoporous silica3337 with varied particle morphology (irregular, spherical, elongated) and pore structures (disordered, hexagonal, mesh phase). The morphology and pore structure are a function of synthesis conditions, surfactant structure, and chain length. The high hydrophobicity of fluorocarbon materials may allow better incorporation of hydrophobic functional groups during direct synthesis. Our research team has investigated the direct synthesis of vinyl,38 perfluorooctyl, perfluorodecyl, and n-decyl-functionalized nanoporous silica using fluorinated-surfactant templates (HFOPC and HFDePC).39,40 The type of functional group (hydrocarbon or fluorocarbon) and functional group chain length affect pore size, order, and surface area of the functionalized silica. The incorporation of a vinyl group, which is short and, therefore, is incorporated in the palisade region of the surfactant micelle, led to a substantial decrease in pore size, relative to the nonfunctionalized silica, while maintaining high surface area.38 In contrast, minimal pore size reduction was observed when fluorocarbon and hydrocarbon silica precursors were incorporated in templated silica in the presence of cationic fluorinated and hydrocarbon surfactants.39,40 These longer chain tails (perfluorooctyl, perfluorodecyl, n-decyl) of alkoxysilane precursors most likely acted as co-surfactants, inserting directly into the micelle during self-assembly.41 This study examines the incorporation of a hydrophilic reactive functional group (amine) by direct synthesis during the templating of silica by cationic fluorinated surfactants. The 3-aminopropyl group of the silica precursor is a short-chain, neutral (in basic synthesis medium), hydrophilic group that is expected to be incorporated at the micellar/water interface.25,42,43 Hence, we hypothesize that the 3-aminopropyl group should be accessible after surfactant extraction. The degree of 3-aminopropyl incorporation and materials properties of 3-aminopropyl functionalized mesoporous silica are investigated as a function of the type of surfactant template (hydrocarbon or fluorinated) and synthesis conditions (addition of ethanol). CO2 sorption, as well as the reactivity of a fluorescent probe molecule and benzaldehyde, are used to examine the accessibility and surface properties of 3-aminopropyl functionalized silica. Amine groups react reversibly with CO2, forming carbamate in the absence of water (eq 1) and bicarbonate (or carbonate) if water is present (eq 2):16 CO2 þ 2RNH2 T RNHCOO þ HCO3 þ

ð1Þ

CO2 þ H2 O þ RNH2 T RNH3 þ þ HCO3

ð2Þ

The analysis of CO2 sorption under dry conditions will provide information on the proximity of the surface amine groups, since two amine molecules are required to react with

Figure 1. Structures of surfactants: (A) tridecafluoro-1,1,2,2-tetrahydro-octyl pyridinium chloride (HFOPC), (B) heptadecafluoro-1,1,2,2tetrahydro-decyl pyridinium chloride (HFDePC), and (C) cetyltrimethylammonium bromide (CTAB).

one molecule of CO2, according to eq 1. While CO2 sorption is used in this investigation to probe the accessibility and reactivity of the amine group in the functionalized ceramics, the potential commercial interest in solid adsorbents for CO2 capture and storage is significant.17 We also test the hypothesis that incorporating a hydrophobic functional group, such as a fluorocarbon, in addition to the reactive amine group in the silica, will minimize nonspecific surface interactions, isolating the effect of the amine in further applications of the functionalized silica. This technique has previously been investigated4446 by patterning the post-synthesis amine functionalization on a silica surface to obtain functional groups that are uniformly distributed and isolated. Although low loading of the amine group was achieved, compared to direct synthesis, all of the incorporated amine sites were found to react uniformly.44 Thus, the amine accessibility is compared for 3-aminopropyl-functionalized and bifunctionalized (3-aminopropyl and tridecafluoro-1,1,2,2-tetrahydrooctyl) nanoporous silica formed by direct synthesis.

’ MATERIALS AND METHODS Tetraethoxysilane (TEOS, with purity of 99%), 3-aminopropyltriethoxysilane (APTES, 95% purity) and tridecafluoro1,1,2,2-tetrahydro-octyltriethoxysilane (Rf6h2TES, 95% purity) were purchased from Gelest, Inc. CTAB (Figure 1C) was obtained from Sigma with 99% purity. The cationic fluorinated surfactant templates used were C6F13C2H4C5H5NCl (tridecafluoro-1,1,2,2-tetrahydro-octyl pyridinium chloride) and C8F17C2H4C5H5NCl (heptadecafluoro-1,1,2,2-tetrahydrodecyl pyridinium chloride), labeled as HFOPC and HFDePC, respectively (see Figures 1A and 1B). The surfactants were synthesized as previously described.38 Deionized ultrafiltered water (DIUFW) was purchased from Fisher Scientific. All solvents were analytical grade. In the tables and figures in this paper, nonfunctionalized materials were labeled simply as their surfactant templates (i.e., CTAB, HFOPC, and HFDePC). CTAB-templated material is commonly referred to as MCM-41 for hexagonal-pore-structured silica. The 3-aminopropyl functionalized mesoporous silica 5511

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Scheme 1. Reaction of Fluorescein Isothiocyanate (FITC) with 3-Aminopropyl-Functionalized Silica

was synthesized using a 10:1 molar ratio of TEOS to APTES and are labeled using their surfactant template: CTAB, HFOPC, and HFDePC. The prefix AP corresponds to 3-aminopropyl functionalized material: AP-CTAB, AP-HFOPC, and AP-HFDePC. HFDePC-templated materials were synthesized in a homogeneous water/ethanol solution, where the addition of ethanol facilitated surfactant dissolution. The bifunctionalized silica materials were synthesized by utilizing a 10:2.5:1 molar ratio of TEOS:Rf6h2TES:APTES and were labeled as AP-Rf6h2-CTAB, AP-Rf6h2-HFOPC, and AP-Rf6h2-HFDePC, respectively. The materials were prepared by first adding the surfactant to deionized ultrafiltered water and stirring the mixture for 5 min. NH4OH (catalyst, 28%30% solution, from Mallinckrodt) was then added with continuous stirring of the mixture for another 10 min, after which TEOS (and Rf6h2TES) was slowly added. For aminopropyl functionalized silica, APTES was added to this mixture after 20 s. APTES hydrolyzes faster than TEOS and Rf6h2TES, because it is more soluble in an aqueous medium; therefore, a short delay time is required before adding APTES to the solution. The nonfunctionalized silica materials were synthesized using an identical procedure, without the addition of a functionalized precursor. The mixture was left to age under stirring at room temperature for 24 h. The molar ratio of the reactants used in the synthesis was 186 DIUFW:0.184 CTAB (0.082 HFOPC):5.73 NH4OH:1 TEOS and 0.1 APTES or (0.1 APTES: 0.25 Rf6h2TES) (for functionalized silica); and 136 DIW:64 ethanol:0.197 HFDePC:18.4 NH4OH:1 TEOS and 0.1 APTES or (0.1 APTES: 0.25 Rf6h2TES) (for functionalized silica). The synthesis procedure is based on the room-temperature synthesis of Kumar et al.47 The mixture was vacuum-filtered after the aging period and left to dry in a vacuum oven at 40 °C for 24 h. An ethanol/HCl solution of 150 mL of ethanol and 5 g of aqueous HCl was used for the extraction of the surfactant for each batch of materials synthesized. Extraction was accomplished by stirring the dried sample in acidic ethanol for 24 h. This extraction process was repeated once. The amine functionalized materials were further washed with ethanol and ethanol/ NH4OH solution to neutralize any possible protonation of the amine groups after the acidic ethanol wash.

’ MATERIALS CHARACTERIZATION Nitrogen adsorption measurements were performed using a Micromeritics Tristar 3000 system. The materials were degassed at 150 °C under flowing nitrogen gas for 4 h before performing the adsorption analysis. Fourier transform infrared (FTIR) analysis was performed to verify surfactant removal and functional group incorporation.

A small amount (∼1 wt %) of the silica sample was pressed with KBr matrix and analyzed using a Thermo Nicolet Nexus 470 FTIR system. Carbon, hydrogen, and nitrogen (CHN) elemental analysis was performed using a LECO CHN-2000 elemental analyzer under flowing oxygen. Powder X-ray diffraction (XRD) patterns were obtained using a Siemens 5000 diffractometer operating with Cu KR radiation (wavelength of 1.54098 Å) and a graphite monochromator. Transmission electron microscopy (TEM) images of the materials, after surfactant extraction, were observed and recorded with a JEOL 2000FX transmission electron microscope. The TEM samples were prepared by moving a lacey carbon TEM grid (Ted Pella, Inc.) through the dry sample, thereby allowing the smallest particles to be mounted on the grid. The accessibility of the aminopropyl groups was determined qualitatively through the attachment of FITC to the aminopropyl group (see Scheme 1), resulting in a fluorescent material. The silica sample (0.2 g) was added to a 10-mL solution of FITC in an ethanol/NH4OH mixture (6.89 103 mmol/L concentration of FITC) of pH 9. The mixture was stirred overnight, vacuum-filtered, and washed with more ethanol for 24 h to remove unreacted FITC. The samples were mounted on a glass slide and viewed with a Leica TCS NT SP Laser Scanning Confocal Microscope to confirm FITC incorporation by observing the fluorescence of FITC using an excitation wavelength range of 400500 nm. An HP 8453 UVvis spectrometer was used to quantify the rate of incorporation of FITC into the functionalized silica by observing the depletion of FITC in the ethanol/NH4OH solution. Ten milligrams of silica sample was added to 1 mL FITC/ethanol/NH4OH solution having an initial absorbance of ∼1 at a wavelength of 502 nm. The timedependent depletion of FTIC from solution was observed over a 6-h period. The solution was centrifuged for 1520 min before the UV absorbance of the FITC in solution was measured. To measure the amine accessibility to benzaldehyde, 100 mg of silica sample was added to 0.7 mL of benzaldehyde solution (from a stock solution of 10 mL of ethanol, 2 mmol of benzaldehyde, and 2 drops of 12.1 N HCl solution). The mixture was heated to 50 °C for 1 h and then allowed to sit at room temperature overnight. The mixture was then centrifuged and the clear solution decanted. The settled powder was washed with copious ethanol for 24 h. This wash cycle was repeated twice. The samples were filtered and allowed to dry overnight under ambient conditions. The samples with and without benzaldehyde incorporation was analyzed by TGA (MettlerToledo, Model TGA/SDTA851e) to determine the amount of organic group incorporated. FTIR was also performed to confirm the formation 5512


Industrial & Engineering Chemistry Research of imine as a result of amine reaction with the benzaldehyde (see Scheme 2). CO2 sorption analysis was performed using a Netzsch STA 449 CTA thermogravimetric analyzer. Silica samples (1020 mg) were placed in an alumina crucible and loaded onto the TGA instrument. Initially, the temperature was ramped to 110 °C in the presence of N2 gas flowing at 20 mL/s to remove adsorbed water. The system was maintained under these conditions for 5 min and then ramped to 50 °C and maintained there for an hour to gain thermal stability. The gas was then switched to CO2 (99% purity) and the system temperature was maintained until the CO2 adsorption was constant (∼20 min). FTIR analyses of the CO2 adsorbed silica samples were performed following CO2 adsorption to confirm the reaction of CO2 with the amine group. Approximately 15 min after the samples were taken out of the TGA a small amount (∼1 wt %) Scheme 2. Reaction of Benzaldehyde with 3-Aminopropyl Functionalized Silica

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was pressed with KBr matrix and analyzed using a Thermo Nicolet Nexus Model 470 FT-IR system.

’ RESULTS AND DISCUSSION Chemical Analysis of Functional Group Incorporation. Fourier transform infrared (FTIR) spectroscopy analysis, performed on the functionalized and nonfunctionalized materials, demonstrates successful removal of surfactant templates and also provides evidence of functional group incorporation (Figure 2). The unextracted CTAB template material (AP-CTAB-A; suffix A denotes unextracted material) displays peaks at ∼2850 cm1 (symmetric stretching of CH2) and ∼2920 cm1 (antisymmetric stretching of CH2), because of the presence of CTAB surfactant and the incorporated 3-aminopropyl group (Figure 2A). These peaks completely disappear in the FTIR profile of nonfunctionalized silica (CTAB) and are greatly reduced in that of AP-CTAB after surfactant extraction. In the fluorocarbon-surfactant-templated 3-aminopropyl functionalized silica materials, small peaks are evident, at ∼2850 cm1 and ∼2920 cm1, before and after extraction (see Figure 2B). The peak due to the symmetric stretching of CF2, observed at ∼1145 cm1 in unextracted silica (AP-HFOPC-A), is attributed to the presence of a fluoro-surfactant template, and disappears after surfactant extraction (see Figure 2B).

Figure 2. FTIR plots of (A) CTAB-templated AP silica materials; (B) HFOPC-templated AP-silica materials; (C) AP-HFDePC; and (D) HFDePC templated AP-Rf6h2-silica. The suffix “-A” denotes unextracted samples (samples containing the surfactant template). 5513



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Figure 3. Powder X-ray diffraction plots of nonfunctionalized, amine-functionalized, and bifunctionalized (amine and fluorocarbon) silica materials templated with (A) CTAB, (B) HFOPC, and (C) HFDePC.

A magnified image of the FTIR bands observed for APHFDePC in the range of 1250 cm1 to 3050 cm1 is provided in Figure 2C. This figure highlights the peaks due to the presence of 3-aminopropyl group: the HCH scissoring vibration (1407 cm1, ∼1488 cm1), the NH or HCH bend (1520 cm1), and the HCH asymmetric and symmetric stretch (triple peaks observed at ∼2812, 2850, and ∼2927 cm1). These peaks were also observed for AP-CTAB and AP-HFOPC. In the bifunctionalized silica, the peaks at 1145 cm1 and 1211 cm1 (all due to CF2 vibrations; see Figure 2D) are still present after surfactant extraction and suggest the incorporated fluorocarbon functional group. Characteristic silica peaks are observed in both the functionalized and nonfunctionalized materials (see Figure 2). These peaks appear at 460 and 1100 cm1 and as a shoulder at 1200 cm1 (SiOSi vibrations)); at 950 cm1 (SiOH stretch); and as peaks due to adsorbed water (1650 cm 1) and OH stretching (due to silanols (SiOH) and adsorbed water) at 3400 cm1.48,49 Carbonhydrogennitrogen (CHN) elemental analyses were performed to quantify the degree of amine incorporation. CHN elemental analyses of the samples suggest traces of nitrogen (N) present in the nonfunctionalized silica (see Table 1). This contribution is most likely from residual surfactant and is estimated to be 510 mol % of the nitrogen observed in the amine-functionalized samples. Solvent extraction, which is a preferred method for removing the template of organic-functionalized mesoporous silica, more efficiently removes the surfactant template when the materials are aged at room temperature (data not shown). However, the slightly elevated temperatures of aging in this study (40 °C) are required to maintain the ordered structure of the pores. AP-CTAB had the highest incorporation of elemental nitrogen (i.e., 3-aminopropyl group) of the 3-aminopropyl functionalized silicas, corresponding to a maximum incorporation of 1.44 mmol/g. Surprisingly, a lower amine content is observed in AP-HFDePC (0.92 mmol/g), which

Table 1. Elemental Nitrogen Analysis of the Nonfunctionalized and Amine-Functionalized Material sample

theoretical nitrogen

nitrogen content from elemental

content (mmol/g)

analysis (mmol/g)a

CTAB

0.11 ((0.01)

HFOPC

0.11 ((0.01)

HFDePC

0.05 ((0.01)

AP-CTAB

1.39

1.44 ((0.01)

AP-HFOPC

1.39

1.04 ((0.02)

AP-

1.39

0.92 ((0.01)

AP-Rf6h2-

0.58

0.63 ((0.01)

CTAB AP- Rf6h2-

0.58

0.68 ((0.02)

0.58

0.53 ((0.01)

HFDePC

HFOPC AP-

Rf6h2-

HFDePC a

The range of nitrogen content in parentheses is based on duplicated analyses.

was synthesized in a homogeneous water/ethanol solution. Previous investigations have shown that fluorocarbon- and hydrocarbon-functionalized silica synthesized using a HFDePC template in a homogeneous water/ethanol solution has a higher functional group incorporation than CTAB and HFOPC templates. 39,40 The measured amine contents of the 3-aminopropyl functionalized silica are similar to (for the CTAB template) or lower than (for the fluorocarbon surfactant template) the theoretical 3-aminopropyl content, based on the ratio of TEOS:APTES used in the direct synthesis of the materials (1.39 mmol/g). This theoretical organic content is based on complete hydrolysis and siloxane bond formation 5514



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Figure 4. Sample TEM images of (A) AP-CTAB, (B) AP-HFOPC, (C) AP-HFDePC, (D) AP-Rf6h2-CTAB, (E) AP-Rf6h2-HFOPC, and (F) AP-Rf6h2HFDePC.

(i.e., 100% yield). The theoretical 3-aminopropyl content in the bifunctionalized silica (0.58 mmol/g) is lower than the measured values for AP-Rf6h2-CTAB (0.63 mmol/g) and AP-Rf6h2HFOPC (0.68 mmol/g). This is possibly due to residual surfactants or lower yield from the hydrolysis and condensation of the TEOS precursor. Bifunctionalized material synthesized in ethanol/water (AP-Rf6h2-HFDePC) has the lowest 3-aminopropyl incorporation of the bifunctional material synthesized, similar to the trend observed in the 3-aminopropyl-functionalized silica. Pore Structure, Size, and Order of Synthesized Materials. Powder X-ray diffraction (XRD) was performed to investigate the pore structure and order of the materials following surfactant extraction. A two-dimensional (2-D) hexagonal pore structure was observed for all of the nonfunctionalized materials (CTAB, HFOPC, and HFDePC), as interpreted from the presence of (100), (110), and (200) reflections (see Figure 3). CTABtemplated materials maintain their well-ordered 2-D structures during the incorporation of amines by direct synthesis, as indicated by the presence of the characteristic (100), (110), and (200) reflections (see Figure 3A). Transmission electron microscopy (TEM) images of AP-CTAB confirm a 2-D hexagonal pore structure (observed as uniform channels in Figure 4A).

In contrast, XRD of the 3-aminopropyl silica synthesized using the fluorinated templates (AP-HFOPC and AP-HFDePC) resulted in one broad (100) reflection. Corresponding TEM images (Figures 4B and 4C) show cylindrical pores with no apparent order. The pores in the TEM image seem to be interconnected and have a wormhole-like appearance. The XRD patterns and TEM images suggest differences in material properties, due the synthesis in a water/ethanol solution. The XRD pattern of AP-HFDePC has a slightly more intense (100) reflection than that of AP-HFOPC; however, both samples display a wormhole-like pore structure in the TEM images (see Figures 4B and 4C). AP-HFDePC is comprised of spherical particles (Figure 4C), which is characteristic of materials synthesized in a water/ethanol solution. The high solubility of the precursor in the water/ethanol synthesis medium promotes good dispersion and controlled precipitation of small oligomers, which contributes to the formation of particles with a spherical morphology.50,51 The degree of order in the silica mesostructure is dependent on the effect of incorporated organic group on the surfactant assembly and the relative condensation of silica around the micelles.52 Previous investigation suggested that the self-assembly 5515



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Table 2. Materials Textural Properties of Nonfunctionalized, Amine-Functionalized, and Bifunctionalized (AmineFluorocarbon) Silica, as a Function of the Surfactant Template (100) interplanar total surface area, external surface area, mesopore volume, pore diameter, spacing, d100 nm

Stotal (m2/g)

CTAB HFOPC

4.01 2.87

995 811

HFDePC

3.29

739

AP-CTAB

4.34

352

AP-HFOPC

3.15

176

AP-HFDePC

3.20

203

AP-Rf6h2-CTAB

4.01

520

AP- Rf6h2-HFOPC

2.99

369

AP- Rf6h2-HFDePC Rf6h2-CTAB

3.46 3.94

285 568

Rf6h2-HFOPC

2.87

675

Rf6h2-HFDePC

3.37

803

material

a

unit-cell

pore-wall

Vp (cm3/g)

dp (nm)a

0.76 0.28

3.67 2.60

4.63 3.31

0.96 0.71

41.0

0.40

2.77

3.80

1.03

52.0

0.27

3.04

5.01

1.97

0.03

2.39

3.64

1.25

26.0

0.12

2.50

3.70

1.20

15.0

0.25

2.56

4.63

2.07

0.10

2.39

3.45

0.88

0.06 0.21

2.41 2.57

3.99 4.55

1.58 1.98

0.20

2.41

3.31

0.90

0.23

2.03

3.89

1.86

Sexternal (m2/g) 106 323

111

117 15.0 12.6 242 21.5

parameter, a0 (nm) thickness, tpw (nm)

Pore diameter (size) obtained using the BJH method with a modified Kelvin equation.

of 3-aminopropyltriethoxysilane ((C2H5O)3SiC3H6NH2) with CTAB does not favor incorporation into the surfactant micelles43 as well as that with hydrophobic functionalized silanes. The presence of the NH2 group (uncharged in basic medium)53 in the functionalized precursor can allow the 3-aminopropyl chain to be randomly oriented, even becoming part of the silica matrix,43 which might lead to a reduction in long-range order. The effect of the self-assembly of 3-aminopropyl precursor with the surfactants is more pronounced in the presence of the fluorinated surfactants (silica materials with less long-range order than that of CTAB template materials), leading to disordered pores. Use of TEOS:APTES ratios of 6:1 and 4:1 resulted in completely disordered silica materials for the fluorocarbonsurfactant templates, with no observable XRD reflections. Yokoi et al.25 also observed a decrease in the order of 3-aminopropylfunctionalized silica with increases in the APTES:TEOS ratio, using CTAB as a template and synthesis in a basic medium. They observed an ordered pore structure for materials synthesized with a TEOS:APTES ratio of 1:10 and disordered materials at a TEOS:APTES ratio of 1:5; no silica precipitation occurred for TEOS:APTES ratios of 7:10 and higher. Hence, there seems to be a limit on how much amino-silica precursor can be used for the direct synthesis of amine-functionalized silica. The (100) interplanar spacing√(d100) is used to calculate the unit-cell parameter [a0 = d100(2/ 3)], which is comprised of the pore size and the pore-wall thickness.54 The d100 spacings of APCTAB and AP-HFOPC increase by ∼0.33 and ∼0.38 nm, respectively (see Table 2), compared to that of the corresponding nonfunctionalized silica. An increase in (100) interplanar spacing was also observed previously for CTAB-templated 3-aminopropyl-functionalized silica.14 This is proposed to be due to the minimal hydrophobic interaction between the uncharged hydrophilic C3H6NH2 group and the hydrophobic core of the surfactant aggregate, which does not allow molecules of the organic group to be deeply drawn into the surfactant aggregate core and results in a corresponding increase in pore wall thickness.25 In contrast, the d100 spacing of the functionalized silica synthesized in an ethanol/water solution, APHFDePC, decreases slightly. Incorporating a hydrophobic group in addition to the reactive 3-aminopropyl group may minimize interactions between the

silanols on the silica surface and potential adsorbate, reactant, or probe molecules. The pore order of functionalized silica improves with the incorporation of Rf6h2, in addition to the AP functional group, in silica materials templated with CTAB and HFOPC. The intensities of the (110) and (200) reflections of AP-Rf6h2-CTAB and the (100) reflection of AP-Rf6h2-HFOPC increase, relative to those of the nonfluorinated functional material (see Figure 3). However, a decrease in intensity of the (100) reflection (decrease in order) is observed for AP-Rf6h2HFDePC, compared to AP-HFDePC. TEM images confirm the 2-D hexagonal pore structure (seen as channels) for AP-Rf6h2-CTAB and AP-Rf6h2-HFOPC and the disordered pore structure for AP-Rf6 h2-HFDePC (see Figures 4DF), consistent with the XRD patterns. Despite the poor long-range order of the pores in AP-Rf6h2-HFDePC, the pores do seem to have a net radial orientation in Figure 4F (most clearly visible at the particle edge). Type IV nitrogen adsorption isotherms (typical for mesoporous materials)55 are observed for all silica materials (see Figure 5). The incorporation of the functional groups significantly reduces the sorption capacity of all templated materials. The inflection points in the isotherms move to lower relative pressures after functionalization, corresponding to smaller pore sizes. Analysis of the isotherms using the KJS method, which is based on the modified Kelvin equation,5658 reveals a corresponding reduction in pore size (dp), pore volume (Vp), and total surface area (St) (see Table 2 and Figure 5). However, the between pore wall thickness (tpw, denoted as the difference √ the hexagonal cell parameter (a0 = 2d100/ 3) and the pore size (dp)), increases for all functionalized silica materials (see Table 2). Both the decrease in pore size and the increase in pore wall thickness with functionalization are greatest for AP-CTAB, relative to the fluorinated templated material. Amine incorporation results in an increase in pore wall thickness of 1.01 nm for AP-CTAB (tpw = 1.97), relative to nonfunctionalized silica (tpw = 0.96). This suggests more condensed silica with the aggregated CTAB micelle interface during synthesis; this is consistent with the higher degree of amine incorporation from elemental analysis. The pore wall thickness follows the same trend as the degree 5516



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Figure 5. Nitrogen adsorption isotherm and pore size distribution of (A) CTAB-templated materials (B) HFOPC-templated materials, and C) HFDePC-templated materials.

of incorporation from elemental analysis (AP-CTAB > APHFOPC > AP-HFDePC). Accessibility of the Amine Functional Group. The use of FITC incorporation, benzaldehyde reactivity, and CO2 adsorption analyses in this paper spans the range of probe molecules from a bulky fluorescent molecule (FITC) to a much smaller gaseous molecule (CO2) in accessing the incorporated 3-aminopropyl group. In addition, the effects of steric hindrance and surface amine proximity on binding of probe molecules in the silica materials is further investigated using the one-to-one reaction of accessible surface amine groups with benzaldehyde. FITC incorporation in the functionalized silica, as demonstrated in fluorescence images obtained by confocal microscopy (shown in Figure 6), confirms the accessibility of the 3-aminopropyl group. The exposure of FITC to nonfunctionalized silica did not result in fluorescence (not shown), which accounts for the fact

that the observed FITC incorporation is not due to entrapment as a result of collapsed pores. While the observed FTIC fluorescence may be due to attachment of the bulky FTIC molecule primarily to surface amine groups, imaging of the various depths in the particles using confocal microscopy shows a distribution of the FITC fluorescence throughout the silica particles (not shown). Accessibility of amine groups by FITC molecules was qualitatively analyzed by the rate and extent of depletion of FITC from solution (Figure 7). The extent of incorporation is normalized by the weight of sample (10 mg) added to the starting solution. The initial rate and extent of FITC incorporation after more than 5 h is lowest for AP-HFOPC, relative to AP-CTAB and AP-HFDePC, which have similar rates. This is consistent with the degree of fluorescence of the silica samples, following reaction with FITC, in which some of the AP-HFOPC silica particles are nonfluorescent. The smaller pore size in 5517



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Figure 6. Confocal microscopy images of (A) AP-CTAB, (B) AP-HFOPC, and (C) AP-HFDePC, after samples were reacted with FITC in an ethanol/ NH4OH mixture (6.89 103 mmol/L concentration of FITC) of pH 9, washed with ethanol and dried.

Table 3. Benzaldehyde Incoporation in Amine-Functionalized Silica Amount of Benzaldehyde Incorporated

Figure 7. Solution phase concentration of FITC during incorporation into amine functionalized silica. Absorbance values are normalized by weight of samples (10 mg). Initial concentration of FTIC in solution is 6.89 103 mmol/L.

AP-HFOPC (2.39 nm), compared to that in AP-CTAB (3.04 nm), could limit the accessibility of the amines by FITC (maximum global dimension = 1.55 nm). For AP-HFDePC (pore size of 2.50 nm), the possible radial orientation36,38 of the pores due to synthesis in a homogeneous medium could contribute to the high accessibility rate (similar to AP-CTAB), even though AP-HFDePC has low amine incorporation and a relatively smaller pore size. Accessibility of the amine group in the bifunctionalized silica materials is reduced, relative to the 3-aminopropyl functionalized materials. The lower FITC depletion rate (Figure 7) observed in the bifunctionalized material is in part due to normalization by weight of added sample. The fluorocarbon group contributes significantly to the weight of the bifunctionalized silica (∼ 25%). Similar initial rates are seen in the bifunctionalized silica across templates, but the final extent of FITC incorporation is slightly less for AP-Rf6h2-HFDePC, compared to AP-Rf6h2-CTAB and AP-Rf6h2-HFOPC. The lower amine content and the higher incorporation of fluorocarbons could account for the low FITC incorporation in AP-Rf6h2-HFDePC. Benzaldehyde reactivity with the amine group was analyzed for the amine-functionalized silica to examine the accessibility of a probe molecule that reacts in a 1:1 ratio with amines. Assuming a one-to-one reaction, the maximum amount of benzaldehyde to be incorporated is 1.44 mmol/g in AP-CTAB, 1.04 mmol/g in AP-HFOPC, and 0.97 mmol/g in AP-HFDePC, based on the molar incorporation of amines in the respective silica materials.

mmol/g

μg/m2

AP-CTAB AP-HFOPC

0.610 ((0.05) 0.663 ((0.04)

184 ((16) 400 ((22)

AP-HFDePC

0.469 ((0.03)

245 ((16)

However, AP-CTAB shows 42% of the maximum amount of benzaldehyde incorporation (see Table 3). The efficiency of benzaldahyde incorporation is greater in the fluorinated-surfactant template material (for AP-HFOPC, 63% of amines reacted; and for AP-HFDePC, 51% of amines reacted). Surprisingly, APHFOPC, which has the smaller surface area and pore size and an intermediate level of organic group incorporation, shows much greater accessibility of amines toward benzaldehyde. The increased benzaldehyde incorporation in AP-CTAB per gram of material, compared to AP-HFDePC, is most likely due to the much higher amine content in AP-CTAB. AP-HFDePC shows more accessible amine per unit of total surface area. FTIR spectra of AP-HFDePC and AP-HFDePC reacted with benzaldehyde are shown in Figure 8. The region between 1300 cm1 and 1800 cm1 is expanded for better peak observation. The band due to imine formation (CdN vibration) is expected to fall between 1620 cm1 and 1690 cm1.59 However, this peak could not be observed, because of possible overlap with the adsorbed water band at 1650 cm1. However, a band at 1408 cm1 appears after the benzaldehyde reaction. A similar peak at 1459 cm1 has been observed in formaldehyde solution of benzaldehyde and also aniline reacted with benzaldehyde.60 Although there is a peak shift (possibly due to solvent effects) in the benzaldehydeamine silica sample, the presence of this band confirms the incorporation of benzaldehyde in the silica materials. Characterization of Functionalized Synthesized Silica by CO2 Capture. Dry CO2 sorption analysis was performed at 50 °C with a TGA instrument, using both nonfunctionalized and amine-functionalized porous silica templated using the three surfactants (CTAB, HFOPC, HFDePC). CO2 sorption increased gradually (as measured by an increase in sample weight) for all porous silica materials, as a result of CO2 uptake. A steady state in the uptake of dry CO2 is reached within 15 min. FTIR analysis of the silica samples performed after CO2 adsorption analysis with the TGA, confirms the reaction of the aminopropyl functional groups with CO2. Figure 9 provides an FTIR spectrum 5518



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(taken 15 min after CO2 adsorption) of AP-HFDePC (before and after pure CO2 capture) and HFDePC (after CO2 capture). Exposure to CO2 results in the appearance of an intense peak at a

Figure 8. FTIR of AP-HFDePC before reaction with benzaldehyde (trace a) and after reaction with benzaldehyde (trace b).

Figure 9. FTIR of AP-HFDePC before and 15 min after CO2 adsorption. The dotted lines indicate the wavenumbers for the asymmetric CO stretch.

wavenumber of 1411 cm1 for AP-HFDePC. The peak is attributed to the asymmetric CO stretch of ammonium carbamate in dry CO2 adsorption.61 This peak is not observed in nonfunctionalized silica after CO2 adsorption (Figure 9). Table 4 compares the steady-state values of sorbed CO2 on nonfunctionalized and amine-functionalized porous silica templated using CTAB, HFOPC, and HFDePC. Incorporation of the amine functional group results in a greater amount of CO2 sorbed for all surfactant-templated materials. The nonfunctionalized materials, upon which CO2 capture occurs solely by physisorption, show greater sorption capacity on the CTABtemplated silica (18 mg/gadsorbent) compared to 12.2 mg/ gadsorbent on HFOPC and 14.4 mg/gadsorbent on HFDePC. The higher CO2 sorption capacity per gram of CTAB-templated materials is attributed to its higher surface area, compared to that of the fluorocarbon-surfactant-templated materials. However, AP-HFOPC silica adsorbed 21.7 mg/gadsorbent of pure CO2 at steady state, compared to 19.2 mg/gadsorbent captured by AP-HFDePC and 19.2 mg/gadsorbent captured by AP-CTAB. In our investigation, a dry CO2 sorption capacity of at least 30.6 mg/g-adsorbent due to carbamate formation reaction (involving two amines in close proximity and one CO2 molecule), is possible for the amine-functionalized material synthesized at a TEOS:APTES ratio of 10:1 (based on a theoretical 3-aminopropyl content of 1.39 mmol/g-adsorbent). This capacity does not include the additional adsorption of CO2 due to the physisorption sites that are present on the silica surfaces. While the amount of CO2 adsorbed on the amine-functionalized silica is lower than the calculated theoretical contribution due to CO2amine interactions alone, the increased CO2 sorption with amine functionalization is a clear indicator of the accessibility of the amines. The CO2 adsorption capacity per area is greatest for APHFOPC (which possesses the lowest surface area), followed by AP-HFDePC and then AP-CTAB. This trend, comparable to the amount of benzaldehyde incorporated, is the reverse of the surface area trend, suggesting a higher surface density of amine in the fluorocarbon-templated silica. The normalized values of the number of moles of CO2 adsorbed per mole of N incorporated in the amine-functionalized materials are also higher for AP-HFOPC (0.41), followed by AP-HFDePC (0.38), and then AP-CTAB (0.21) (see Table 3). Thus, more of the incorporated amines in the fluorocarbon-templated silica are accessible. Fluorocarbons are considered to be CO2-philic,6264 and their incorporation in ceramic (e.g., TiO2 and γ-Al2O3) membranes

Table 4. Capacity of Synthesized Silica for Dry CO2 Amount CO2 sorbed 50 °C

*

Normalized CO2/N ratio

mg/gadsorbent

μg/m2

mol (CO2)/mol (N)

CTAB

18.0 ( 3.1

18.1 ( 3.1

0

AP-CTAB

19.2 ( 2.3

54.5 ( 6.4

0.21

HFOPC

12.2 ( 2.4

15.0 ( 3.0

0

AP-HFOPC

21.7 ( 3.5

123 ( 19.0

0.41

HFDePC

14.4 ( 2.1

19.5 ( 2.8

0

AP-HFDePC

19.2 ( 1.4

94.8 ( 6.7

0.39

Rf6h2-HFDePC

4.10 ( 0.8

5.16 ( 1.0

0

AP-Rf6h2-HFDePC

5.30 ( 0.7

18.5 ( 2.5

0.16

Normalized ((CO2)/N) ratio = [(amount of CO2adsorbed (mol/m2)) (avg amount of CO2 adsorbed on non-amine-functionalized silica (mol/ m2))]/(moles of N incorporated in silica (mol/m2)). *

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Industrial & Engineering Chemistry Research has been previously investigated, with a goal of enhancing CO2 separation from a mixed gas stream.65 In this investigation, incorporation of fluorocarbon functional group in the aminefunctionalized porous silica has a marked, but unexpected, effect on the surface adsorption of CO2. Although the surface areas of both Rf6h2-HFDePC and bifunctionalized AP-Rf6h2-HFDePC silica are larger than that of AP-HFDePC, the CO2 adsorption capacities of the Rf6h2 incorporated silicas are significantly lower per gram (see Table 4). The amount of CO2 adsorbed by Rf6h2HFDePC is 4.10 mg/g and 5.30 mg/g adsorbed by AP-Rf6h2HFDePC, compared to an average value of 14.87 mg/g adsorbed by nonfunctionalized silica. This is possibly due to the weak molecular interactions that are associated with the fluorocarbon group, which hinders the physisorption of CO2 on the fluorocarbon-functionalized silica. While CO2 has a considerable level of solubility in bulk fluorocarbons, some of this may be due to the large free volume of fluorocarbons, which would be expected to play less of a role in solid-supported films. When the amount of adsorbed CO2 is normalized by the surface area, AP-Rf6h2HFDePC displays the expected higher CO2 capacity (18.5 ( 2.5 μg/m2), because of the presence of the amine group, compared with Rf6h2-HFDePC (5.16 ( 1.0 μg/m2). However, a normalized CO2/N molar ratio of 0.16 is measured for APRf6h2-HFDePC, compared to 0.39 for AP-HFDePC. This implies that the incorporated fluorocarbon functional groups acted as spacers, which render most of the amine groups incapable of reacting with CO2, since two amine groups in close proximity are required for carbamate formation under dry conditions.

’ CONCLUSIONS 3-Aminopropyl mesoporous silica with two-dimensional hexagonal (prepared using the CTAB template) or disordered (prepared using fluorocarbon-surfactant templates, HFOPC and HFDePC) pore structure have been synthesized by direct (“one-pot”) synthesis. Functionalized silica materials with thicker pore walls are obtained, because of the presence of the aminefunctionalized precursor during self-assembly. The degree of amine incorporation follows the same trend as the thickness of the pore wall of the functionalized silica; AP-CTAB, which has the highest amine incorporation, also has a larger increase in pore wall thickness. The incorporation of a fluorocarbon functional group, in addition to the amine group, in bifunctionalized silica, results in improved pore order, with the exception of AP-Rf6h2HFDePC, which has reduced order, because of high fluorocarbon incorporation. The 3-aminopropyl group is readily accessible in all amine-functionalized silica, as demonstrated using FITC as a probe molecule and benzaldehyde. The reaction of benzaldehyde with the amine silica materials suggests higher accessibility of the amines in fluorocarbon-templated materials than in CTAB-templated materials. Also, as expected, CO2 sorption increases (on a surface area basis) in amine-functionalized silica, because of the reaction of CO2 with surface amine groups. Fluorocarbon (HFOPC)-template amine silica with the lowest surface area results in the highest CO2 sorbed, in a trend comparable to benzaldehyde incorporation. This is possibly due to the greater accessibility of the amine groups. Fluorocarbon incorporation, in addition to the reactive amine group, in the bifunctionalized material reduced the physisorption of CO2 and indicated possible isolation of the amine group, which, although not advantageous for carbamate formation, suggests the potential

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for using fluorocarbon functionalization to eliminate nonspecific binding.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This material is based on work sponsored by the National Science Foundation (under Grant No. DMR-0210517), the USDA Biomass Research and Development Initiative Grant (No. 68-3A75-7-608), and the Department of Chemical Engineering, WVU Institute of Technology. We thank Dr. John Selegue (Department of Chemistry, University of Kentucky), Dr. Richard Eitel (Department of Chemical and Materials Engineering, University of Kentucky) for use of the TGA instrument for CO2 adsorption analysis, and Ms. Margaret Grider (Center of Applied Energy Research, University of Kentucky) for CHN elemental analysis. ’ REFERENCES (1) Pinnavaia, T. J.; Thorpe, M. F. Access in Nanoporous Materials; Fundamental Materials Research Series; Kluwer Academic Publishers: New York, 2002. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenkert, J. L. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates. J. Am. Chem. Soc. 1992, 114, 10834. (3) Wight, A. P.; Davis, M. E. Design and Preparation of OrganicInorganic Hybrid Catalysts. Chem. Rev. 2002, 102, 3589. (4) Lim, M. H.; Stein, A. Comparative Studies of Grafting and Direct Syntheses of InorganicOrganic Hybrid Mesoporous Materials. Chem. Mater. 1999, 11, 3285. (5) Walcarius, A.; Etienne, M.; Lebeau, B. Rate of Access to the Binding Sites in Organically Modified Silicates. 2. Ordered Mesoporous Silicas Grafted with Amine or Thiol Groups. Chem. Mater. 2003, 15, 2161. (6) Bowe, C. A.; Poore, D. D.; Benson, R. F.; Martin, D. F. Extraction of Heavy Metals by Amines Adsorbed onto Silica Gel. J. Environ. Sci. Health, Part A 2003, 38 (11), 2653. (7) Kim, S.; Ida, J.; Guliants, V. V.; Lin, J. Y. S. Tailoring Pore Properties of MCM-48 Silica for Selective Adsorption of CO2. J. Phys. Chem. B 2005, 109, 6287. (8) Im, H.-J.; Yang, Y.; Allain, L. R.; Barnes, C. E.; Dai, S.; Xue, Z. Funtionalized Sol-Gels for Selective Copper(II) Separation. Environ. Sci. Technol. 2000, 34, 2209. (9) Huang, H. Y.; Yang, R. T.; Chinn, D.; Munson, C. L. AmineGrafted MCM-48 and Silica Xerogel as Superior Sorbents for Acidic Gas Removal from Natural Gas. Ind. Eng. Chem. Res. 2003, 42, 2427. (10) Zhu, G.; Yang, Q.; Jiang, D.; Yang, J.; Zhang, L.; Li, Y.; Li, C. Synthesis of Bifunctionalized Mesoporous Organosilica Spheres for High-performance Liquid Chromatography. J. Chromatogr. A 2006, 1103, 257. (11) Wang, X.; Tseng, Y.-H.; Chan, J. C. C.; Cheng, S. Catalytic Applications of Aminopropylated Mesoporous Silica Prepared by a Template-free Route in Flavanones Synthesis. J. Catal. 2005, 233, 266. (12) McKittrick, M. W.; Yu, K.; Jones, C. W. Effect of metallation protocol on the preparation and performance of silica-immobilized Ti CGC-inspired ethylene polymerization catalysts. J. Mol. Catal A: Chem. 2005, 237, 26. (13) Wang, G.; Otuonye, A. N.; Blair, E. A.; Denton, K.; Tao, Z.; Asefa, T. Functionalized mesoporous materials for adsorption and 5520


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Direct Synthesis and Accessibility of Amine-Functionalized

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