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Microporous and Mesoporous Materials 85 (2005) 16–24

Fluorinated surfactant templating of vinyl-functionalizednanoporous silica

Gifty Osei-Prempeh a, Hans-Joachim Lehmler b, Barbara L. Knutson a,Stephen E. Rankin a,*

a Chemical and Materials Engineering Department, University of Kentucky, Lexington, KY 40506-0046, USAb Department of Occupational and Environmental Health, University of Iowa, College of Public Health, Iowa City, IA 52242-5000, USA

Received 15 December 2004; received in revised form 31 May 2005; accepted 14 June 2005Available online 27 July 2005

Abstract

Ordered nanoporous vinyl-functionalized silica materials are synthesized by the �one-pot� (direct) synthesis method using threecationic fluorinated surfactants (C6F13C2H4NC5H5Cl, C8F17C2H4NC5H5Cl and C10F21C2H4NC5H5Cl) as templates in basic med-ium under ambient conditions. The materials obtained possess lower degrees of 2D hexagonal closed packed ordering of the poreswhen compared to CTAB templated materials. Increases in vinyl content lead to reductions in pore size as well as material order. Inspite of having smaller pores, bromination experiments suggest that the vinyl groups are more accessible in the fluoro-surfactanttemplated materials. Although confinement of the vinyl groups to the palisade region of the fluorinated micelle templates may con-tribute to this increase in accessibility, reduced pore order in the fluorinated templated materials may also improve the accessibility,as observed here for intentionally disordered CTAB-templated materials. Cationic fluorocarbon surfactants not only have thepotential to be templating agents, but also present the possibility of re-organizing the alignment of organic functional groups inthe pores of organic–inorganic hybrids obtained by direct synthesis.� 2005 Elsevier Inc. All rights reserved.

Keywords: Mesoporous silica; Vinyl; Fluorinated surfactants

1. Introduction

The discovery of ordered mesoporous silica preparedby surfactant templating in the 1990s [1] has led to theinvestigation of different templating agents (cationic, an-ionic and non-ionic surfactants, and block-copolymers)[2,3] and functional group incorporation. Tailoring thesurface of mesoporous silica materials has a wide rangeof applications including catalysis, separation and sens-ing. Functional groups such as metals [4–7], organiccompounds [8–13], and biomolecules [14,15] have been

1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.micromeso.2005.06.014

* Corresponding author. Tel.: +1 8592 579799; fax: +1 8593 231929.E-mail addresses: hans-joachim-lehmler@uiowa.edu (H.-J. Lehm-

ler), bknutson@engr.uky.edu (B.L. Knutson), srankin@engr.uky.edu(S.E. Rankin).

incorporated into these ordered porous silicas for appli-cations ranging from heavy metal removal [9,16,17] todrug delivery [18].

The incorporation of organic functional groups intoporous silica to form organic–inorganic hybrid materialscan be achieved by several methods. Post-synthesisgrafting techniques involve adsorption of the organicprecursor species into the pores of the silica followedby covalent bond attachment of the functional groupto the silica support. Alternative incorporation methodsare the ship-in-bottle technique (piece-by-piece con-struction of the organic material within the pores ofthe silica) [8] and the �one-pot� synthesis technique,which involves the direct co-condensation of the silicaprecursor and a functionalized precursor [8,13,19,20].In contrast to post-synthesis grafting, the direct

G. Osei-Prempeh et al. / Microporous and Mesoporous Materials 85 (2005) 16–24 17

synthesis method results in a high functional grouploading [13]. However, a less ordered pore structure isusually observed in the materials as the functional grouploading increases [13,21]. Vinyl group incorporation intoMCM-41 (2D hexagonal ordered mesoporous silica)templated with cetyltrimethylammonium bromide(CTAB) has been investigated as a model system for di-rect synthesis [13,21].

Conventional surfactants for the synthesis of meso-porous silica are hydrocarbon based [22,23]. The effectof the template structure has been explored, and well-or-dered materials with pore sizes in the mesoporous range(2.7 nm and greater) have been obtained using these tem-plates [24]. Only recently, researchers have begun to alsoinvestigate the use of fluorinated surfactants for the tem-plating of structured nanoporous silica. Blin and col-leagues [25] have investigated the use of the non-ionicfluorinated surfactant F(CF2)8C2H4(OC2H4)9OH in thepreparation of silica with an ordered 2D hexagonal porestructure. Our group has demonstrated the base-cata-lyzed precipitation of silica particles using cationicfluorinated surfactants, 1H,1H,2H,2H-perfluoroalkylpy-ridinium chlorides, as templates [26–28]. 2D hexagonalcylindrical structures with narrow pore size distributions(2.6 nm pores) were obtained using 1H,1H,2H,2H-per-fluorooctylpyridinium chloride (C6F13C2H4NC5H5Cl)as the templating agent [26]. A similar surfactant withtwo more fluorocarbon units in the tail (C8F17C2H4

NC5H5Cl) was found to form elongated nanoparticleswith mesh-phase slit pores oriented normal to the longaxis of the particles [27]. An increase in surfactant taillength produces an increase in pore size analogous to thatobserved for hydrocarbon templates. In addition, porestructure changes are also observed from wormhole-liketo hexagonal close packed cylinders to a mesh phase.

These studies have confirmed some of the advantagesof fluorinated surfactants over their hydrocarbon ana-logues for the tailoring of mesoporous silica. Thereplacement of the hydrogen atoms in the surfactant tailby larger, more electronegative fluorine atoms increasesthe volume of the surfactant chain due to the larger areaoccupied by the –CF2– group. The increased hydropho-bicity of the fluorinated surfactants results in lower sur-face tensions and critical micelle concentrations [29],indicative of their ease of self-assembly. These propertiesallow micelle templating of unusually small pores inmaterials [28]. The fluorinated surfactants generallyform a broader range of nanoscale structures than theirhydrocarbon analogues [30,31]. These mesophases aremore stable, better organized and rigid [32].

This work extends the use of fluorinated surfactant aspore templates to organically modified ceramic materi-als formed by �one-pot� synthesis. The successful synthe-sis of vinyl-functionalized ordered porous silica usingfluorinated surfactants (C6F13C2H4NC5H5Cl (FSPCl-1), C8F17C2H4NC5H5Cl (FSPCl-2) and C10F21C2H4NC5

H5Cl (FSPCl-3)) as templates is reported. The structureand textural properties of the materials are analyzed andcompared to those of CTAB templated materials. Theability of the lipophobic fluorocarbon templates to alterthe accessibility of the vinyl functional group is exam-ined by means of a simple bromination reaction.

2. Materials and methods

2.1. Surfactant synthesis

Vinyl-functionalized porous silica materials were syn-thesized using the three partially fluorinated surfactanttemplates and, for comparison, CTAB, a hydrocarbonanalogue. CTAB was obtained from Sigma with 99%purity. The fluorinated surfactants differ in the alkylchain associated with the fluorine and have the generalformula CnF2n+1C2H4NC5H5Cl (1H,1H,2H,2H-perflu-oroalkylpyridinium chloride). The surfactants are la-beled FSPCl-1, FSPCl-2 and FSPCl-3 for n = 6, 8 and10, respectively. The synthesis procedure of the surfac-tants is given elsewhere [28,33].

2.2. Materials synthesis

Tetraethoxysilane (TEOS, 99% purity) was obtainedfrom Fluka Chemika and vinyltriethoxysilane (VTES,97% purity) was purchased from Sigma–Aldrich. Vi-nyl-functionalized MCM-41 samples were synthesizedusing CTAB as template. The vinyl-modified sampleswere labeled v4-MCM-41 and v6-MCM-41, where thenumber following the ‘‘v’’ in the prefix indicates theTEOS:VTES ratio (4:1 and 6:1, respectively). The syn-thesis procedure is based on the room-temperature syn-thesis of Kumar et al. [22]. The work of Stein et al. [13]suggested a TEOS:VTES ratio of 4:1 as the limit forobtaining ordered materials. A less ordered materialv4-MCM-41d was synthesized with CTAB:TEOS ratioof 0.12, based on investigations by Bore et al. [34], wherea CTAB:TEOS ratio approaching a limit of 0.09 (in anon-functionalized material) gives disordered materials.The partially fluorinated surfactant templates were usedto synthesize v4-UKA8, v6-UKA8, v4-UKHn and v6-UKHn where n corresponds to the total alkyl chainlength and v4 and v6 have the same meaning as previ-ously stated. The letters A and H after UK correspondto aqueous and homogeneous ethanol/water synthesismedium, respectively. Two hundred proof ethanol (Aa-per Alcohol and Chemical Company) was used as sol-vent and de-ionized ultra-filtered water (DIUFW) wasobtained from Fisher Scientific. The molar ratios ofthe reactants used in the synthesis of all the materialsare given in Table 1. The ratio of vinyl to Si in the syn-thesis solution is 0.20 for v4 samples and 0.14 for v6samples.

Table 1Molar ratios of reactants used in material synthesis

Material DIUFWa Ethanol Surfactant NH3 TEOSa VTESa

MCM-41 149 0.15 2.65 1v4-MCM-41 186 0.21 (CTAB) 3.31 1 0.25v4-MCM-41d 186 0.12 3.31 1 0.25v6-MCM-41 186 0.21 (CTAB) 3.31 1 0.17UKA8 149 0.058 (FSPCl-1)b 2.65 1v4-UKA8 186 0.073 (FSPCl-1) 3.31 1 0.25v6-UKA8 197 0.077 (FSPCl-1) 3.31 1 0.17UKH10 137 64 0.28 (FSPCl-2)b 9.66 1v4-UKH10 137 64 0.28 (FSPCl-2) 9.66 1 0.25v6-UKH10 137 64 0.28 (FSPCl-2) 9.66 1 0.17UKH12 137 64 0.07 (FSPCl-3)b 9.66 1v4-UKH12 137 64 0.07 (FSPCl-3) 9.66 1 0.25v6-UKH12 137 64 0.07 (FSPCl-3) 9.66 1 0.17

a DIUFW: de-ionized ultra filtered water, TEOS: tetraethoxysilane, VTES: vinyl-triethoxysilane.b FSPCl-1: C6F13C2H4NC5H5Cl, FSPCl-2: C8F17C2H4NC5H5Cl, FSPCl-3: C10F21C2H4NC5H5Cl.

18 G. Osei-Prempeh et al. / Microporous and Mesoporous Materials 85 (2005) 16–24

The samples were synthesized by first adding the sur-factant to DIUFW (or DIUFW/ethanol homogeneoussolution) and stirring the mixture for 5 min. NH3 (cata-lyst, 28–30% solution from Malinckrodt) was thenadded with continuous stirring. After 10 min of stirring,TEOS or a mixture of TEOS and VTES, in the desiredproportions, was slowly added. The mixture was leftto age under stirring at room temperature for 24 h (2 hfor synthesis in homogenous medium to match the con-ditions used previously by Tan et al. [28]). The mixturewas vacuum-filtered after the aging period and left toair-dry for 24 h. An ethanol/HCl solution of 150 mlethanol and 5 g aqueous HCl was used for the extractionof the surfactant, which was accomplished by stirringthe sample in solution for 24 h. This extraction processwas repeated once. Concentrated HCl of 37.3% assay(Fisher Scientific) was used for the extraction.

2.3. Materials characterization

Nitrogen adsorption measurements were carried outusing a Micromeritics Tristar 3000 for the materialsdetermined to be mesoporous, and a MicromeriticsASAP 2010 for those samples that were microporous.The materials were degassed at 150 �C using flowingnitrogen gas for 4 h before performing the adsorptionanalysis.

Fourier Transform Infrared (FTIR) analysis was per-formed to verify surfactant removal and vinyl incorpo-ration. A small amount (approximately 1 wt%) of thesample was pressed with KBr matrix and analyzed usinga Thermo Nicolet Nexus 470 FT-IR analyzer.

Powder X-ray diffraction patterns were obtainedusing Siemens 5000 diffractometer operating withCu Ka radiation of wavelength 1.54098 A and a graph-ite monochromator.

Sample TEM images of the materials, after surfactantextraction, were observed and recorded with a JEOL

2000FX transmission electron microscope. The TEMsamples were mounted on lacey carbon TEM grids(Ted Pella Inc.) from an acetone suspension.

Accessibility of the vinyl group was analyzed bymeans of a bromination reaction („Si–CH@CH2 +Br2 ! „Si–CHBr–CH2Br). As the reaction proceeds,the UV absorbance at 412 nm is reduced due to the con-sumption of Br2. Following the method of Stein�s group[13], 5 mg of material was added to 2.5 ml of brominesolution (bromine in dichloromethane). The solutionin a 3 ml cuvette had an initial UV absorbance of 1 atthe wavelength of 412 nm. The solution with the samplewas shaken and allowed to sit for 15–30 min after whichit was centrifuged for 15 min for the particles to settle,and the absorbance of the solution measured. This wasdone for a period of 4.5 h, taking absorbance readingsabout every 45 min. Duplicate bromination experimentswere conducted for each sample.

3. Results and discussion

3.1. FTIR analysis

FTIR analysis confirms the presence of vinyl group inthe materials after extraction of the surfactant. Com-plete surfactant extraction was observed by the disap-pearance of the surfactant peaks at 2854 cm�1 and2924 cm�1 (for CTAB templated materials, due to C–H vibrations in the CTAB chain [37]) and 1493 cm�1

(fluoro-surfactant templated materials), and a reductionin the peak at 1416 cm�1 for all materials (results notshown). The peak at 1493 cm�1 is attributed to thepresence of pyridinium moiety in the fluorinatedsurfactants [35]. The presence of –CH@CH2 in thevinyl-functionalized materials contributes to the peakobserved at 1416 cm�1, due to the symmetric deforma-tion of the =CH2 group [36]. This peak was also present

2 8R

elat

ive

Inte

nsity

A

(100)

(110) (200)

(210)

MCM-41

v6-MCM-41

v4-MCM-41

v4-MCM-41d

Rel

ativ

e In

tens

ity

(100)

(110)

(200) (100)

(100)

B

UKA8

v6-UKA8

v4-UKA8

Rel

ativ

e In

tens

ity

Rel

ativ

e In

tens

ity

C

(100)

(110)(200)

UKH10

v6-UKH10

v4-UKH10

D

(100)

UKH12

v6-UKH12

v4-UKH12

3 4 5 6 7 2 83 4 5 6 7

2 83 4 5 6 7 2 83 4 5 6 7

Fig. 1. Powder X-ray diffraction patterns of (A) CTAB templatematerials, (B) FSPCl-1 template materials, (C) FSPCl-2 templatematerials and (D) FSPCl-3 template materials. The plots have beenoffset vertically.

G. Osei-Prempeh et al. / Microporous and Mesoporous Materials 85 (2005) 16–24 19

in non-functionalized materials prior to extraction ofeither surfactant, but it was observed to have a higherabsorbance value in the vinyl-functionalized materials.This suggests a contribution from –CH– stretching tothe peak. This peak disappeared from the non-function-alized samples after extraction, so its presence aftersurfactant extraction in the functionalized materialsindicates successful incorporation of the vinyl group.

3.2. Powder X-ray diffraction analysis

The pore order of the materials was investigated byperforming powder X-ray diffraction (Fig. 1). Most ofthe non-functionalized materials display X-ray patternssuggesting a 2D hexagonal structure by the presence ofat least three reflecting planes that can be indexed to thisstructure, (100), (110), and (200). The hexagonal struc-ture observed in the XRD reflections is consistent withthe observation of Stein�s and Ozin�s groups for theincorporation of vinyl into CTAB template silica[13,21]. The fluorinated surfactant-templated materialsdisplay a (100) reflection that is broader when com-pared to the CTAB templated materials. The samplesnot showing evidence of significant long-range ordering(Fig. 1D) are the non-functionalized material (UKH12)and functionalized material (v6-UKH12) templated bythe longest chain fluorinated surfactant. In addition tobroad (100) reflections, the XRD patterns of severalof the vinyl-functionalized materials (v4-UKA8, v6-UKA8, v6-UKH10, v4-UKH12) exhibit only fainttraces of the (110) and the (200) reflections. The broad(100) reflection and missing higher order reflectionsshow that the fluorinated surfactant templated sampleshave lower degrees of order as compared to the CTABtemplate materials. This could be due to smaller ordereddomains in these materials.

The (100) reflection for all the fluorinated surfactanttemplated materials occurred at higher 2-h values withcorresponding smaller d100-spacing, indicating pore sizeswhich are smaller than the CTAB template materials.This is attributed to the longer chain length of CTABrelative to the fluorinated templates, leading to largerpores. Non-functionalized fluorinated surfactant tem-plated materials, UKA8 and UKH10, display more orderthan their corresponding vinyl-functionalized samples,and the order decreases with increasing vinyl content(Fig. 1B and C). These observations are consistent withprevious evidence that an increase in vinyl functionaliza-tion reduces long-range pore order in CTAB templatesilica [13,21]. In contrast, the materials obtained fromthe longest chain fluorinated surfactant (the 12-carbonchain surfactant, FSPCl-3) display evidence of order inthe materials synthesized at highest vinyl content (v4-UKH12), but not in the non-functionalized material(UKH12) or in the presence of a reduced ratio of thevinyl precursor (v6-UKH12). This unexpected observa-

tion may be due to a structure transition towards amore-curved aggregate (cylindrical structure), creatinga more stable pore structure, but we were unable to di-rectly test this hypothesis.

To aid in the interpretation of accessibility studies, aless ordered version of v4-MCM-41 was synthesized.

20 G. Osei-Prempeh et al. / Microporous and Mesoporous Materials 85 (2005) 16–24

v4-MCM-41d (where d denotes disordered) has similardegree of pore order as the fluorinated surfactant tem-plate functionalized materials (Fig. 1).

3.3. Transmission electron microscopy

Examples of transmission electron microscopy(TEM) images of the vinyl-functionalized materialsare presented in Fig. 2, for the systems v4-MCM-41,v6-UKA8 with inverse FFT (fast Fourier transform),v6-UKH10 and v4-UKH12. The images are consistentwith the XRD results. The TEM image of v4-MCM-41 (Fig. 2A) shows very well ordered 2D hexagonal porestructure whilst the images for the fluorosurfactant tem-plated materials display partially ordered 2D hexagonalstructure. In most of the samples, the pores are ran-domly oriented in small close-packed domains, but insample v6-UKH10 (Fig. 2C) the pores are oriented radi-ally towards the edges of spherical particles. The mor-phology and pore orientation are consistent with whatwas observed in the non-functionalized materials tem-plated by this fluorinated surfactant in aqueous ethanol[28].

3.4. Nitrogen adsorption analysis

Analysis of the pore size, specific surface area andpore volume of the materials was performed by meansof nitrogen adsorption analysis. All the CTAB tem-plated materials display type IV isotherms (mesoporous

Fig. 2. TEM images of (A) v4-MCM-41, (B) v6-U

materials) (Fig. 3), consistent with previous observationsfor CTAB templated non-functionalized [1] and func-tionalized [13,21] materials. UKA8, UKH10 and v6-UKH10, also display type IV isotherms (Fig. 3). Theinflection point on the type IV isotherms indicates mes-opore filling, which occurs at lower relative pressures inthe fluorinated surfactant-templated materials suggest-ing smaller pores. Functionalized and non-functional-ized samples prepared with the 12-carbon chainsurfactant (UKH12, v6-UKH12 and v4-UKH12) havetype IV isotherms with maximum sorption capacitiesat about 150 cm3/g STP, which is much lower whencompared to all the other mesoporous materials synthe-sized in this study (results not shown). This is consistentwith the absence of long-range order observed by XRD.The isotherms display large hysteresis at relative pres-sures above 0.5 due to adsorption between particles orin macropores.

The adsorption data for the mesoporous sampleswere analyzed by the KJS method (BJH method witha modified Kelvin equation) [39–41] which makes useof the high resolution as-plot method. The resulting poresize distributions of the mesoporous materials, based onthe modified Kelvin equation [41], are shown in Fig. 4.A summary of the textural properties of all synthesizedmaterials is given in Table 2.

Type I isotherms, characteristic of microporousmaterials [38], were observed for the vinyl-functional-ized material synthesized from the 8-carbon fluorinatedsurfactant (v4-UKA8 and v6-UKA8) and the function-

KA8, (C) v6-UKH10 and (D) v4-UKH12.

0

500

1000

1500

2000

2500

0.0 0.2 0.4 0.6 0.8 1.0

Relative Pressure, P/Po

Volu

me

Adso

rbed

, cm

3 /g

STP

a

bc

d

e

f

g

h

i

j

Fig. 3. Nitrogen adsorption isotherms of (a) v4-MCM-41d, (b) v4-MCM-41, (c) v6-MCM-41, (d) MCM-41, (e) v4-UKH10, (f) v6-UKH10, (g) UKH10, (h) v4-UKA8, (i) v6-UKA8, (j) UKA8. Opensymbols are for adsorption and filled symbols are for desorption.

Pore Size, nm

2

dV/d

D, c

m3 /g

-nm

0.2

0.4

0.6

0.8

0.0

1.0MCM-41

v4-M

CM

-41

v4-M

CM

-41d

v6-M

CM

-41

Pore Size, nm

1

UKA8

Pore Size, nm

UKH10

v6-U

KH

10

3 4 5 2 3 4 5 1 2 3 4 5

Fig. 4. Pore size distributions calculated using the BJH method with amodified Kelvin equation [41].

G. Osei-Prempeh et al. / Microporous and Mesoporous Materials 85 (2005) 16–24 21

alized material synthesized at the highest vinyl content(v4-UKH10) from the 10-carbon fluorinated surfactant.Physical properties reported of the microporous materi-als, v6-UKA8, v4-UKA8 and v4-UKH10, are based onthe methods of Dubinin and Kaganer [38] (Table 2). Thepore size of v4-UKH10 may not be estimated well bythis analysis. The d100-spacing obtained for v4-UKH10suggests that (if the wall thicknesses are similar in these

Table 2Summary of the textural properties of all the materials

Sample d100 (nm) St (m2/g)

MCM-41 3.84 995v6-MCM-41 3.74 450v4-MCM-41 3.87 997v4-MCM-41d 3.80 752UKA8 2.87 811v6-UKA8 2.76 865v4-UKA8 2.72 943UKH10 3.20 739v6-UKH10 2.86 717v4-UKH10 2.78 834UKH12 – 286v6-UKH12 – 524v4-UKH12 3.46 635

St is the specific surface area in m2/g, vm is the mesopore/micropore volume inmethod with a modified Kelvin equation for mesopores and the Dubinin–Kag[1].

materials) the pore size should have been close to that ofv6-UKA8, which is between 1.8 and 1.9 nm. The smallpore size of v4-UKH10 determined from the Dubinin–Kaganer method may be the result of an apparentoverestimation of the surface area and a concomitantunderestimation of the pore volume.

As summarized in Table 2, the incorporation of vinylin the samples obtained using the 8-carbon chain fluori-nated surfactant (FSPCl-1) shifts the pore size from2.56 nm (mesopore) for UKA8 to 1.86 nm (micropore)for v6-UKA8 (lower vinyl content; TEOS:VTES of6:1) and 1.66 nm (micropore) for v4-UKA8 (higher vinylcontent; TEOS:VTES of 4:1). The reduction in pore sizewith increasing vinyl content is consistent with theobservation of Jaroniec and Ozin�s group [21] who

vm (cm3/g) dp (nm) a0 (nm)

0.76 3.66 4.430.20 2.58 4.320.69 3.25 4.470.50 2.91 4.390.28 2.60 3.310.31 1.86 3.190.33 1.66 3.140.40 2.78 3.700.38 2.44 3.300.30 1.45 3.210.14 3.22 –0.19 2.96 –0.27 2.69 4.00

cm3/g and dp is the pore diameter (size) in nm measured using the BJHaner method for micropores. a0 is hexagonal cell parameter (2d100/3

1/2)

Time, min

0 50 100 150 200

Time, min

0 50 100 150 200

Abso

rban

ce a

t 412

nm

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0v4-MCM-41dv4-MCM-41v4-UKA8v4-UKH10v4-UKH12

A

Abso

rban

ce a

t 412

nm

0.2

0.4

0.6

0.8

1.0

v6-MCM-41v6-UKA8v6-UKH10v6-UKH12

B

Fig. 5. Bromination reaction plots of (A) samples prepared with 4TEOS: 1 VTES and (B) samples prepared with 6 TEOS: 1 VTES.

22 G. Osei-Prempeh et al. / Microporous and Mesoporous Materials 85 (2005) 16–24

showed that increase in vinyl content for CTAB tem-plate materials reduces the pores from mesopores tomicropores. Similarly, the 10-carbon chain fluoro-sur-factant (FSPCl-2) template materials exhibit a pore sizetrend of 2.78 nm, 2.34 nm to 1.45 nm for UKH10, v6-UKH10 and v4-UKH10 respectively.

3.5. Vinyl accessibility analysis

Bromination reactions were performed on the vinyl-functionalized materials to investigate the accessibilityof vinyl group within the pores of the materials [13].The accessibility correlates with the decreased absor-bance of the reactant bromine in solution with respectto time, as measured for v4- (TEOS:VTES of 4:1) func-tionalized samples (Fig. 5A) and the v6- (TEOS:VTESof 6:1) functionalized samples (Fig. 5B). The absorbancevalues are normalized with the weight of material addedto the bromine solution. A control experiment (notshown) was performed on non-functionalized samples,MCM-41 and UKA8, to investigate if other factors suchas adsorption of bromine atoms on the surface contrib-ute to accessibility measurement. The absorbance of thesolutions in the control experiments was unchanged over4 h. In the functionalized materials, absorbance dropssignificantly over the first 50 min of the reaction dueto the bromination of the most accessible vinyl groups,those outside the pores and near the pore openings.After the first 50 min there is a gradual decease in absor-bance due to bromination of the vinyl groups inside thepores. After approximately 150 min, the absorbanceapproaches a constant value in the functionalizedmaterials.

The accessibility of the vinyl groups in the CTAB-templated material (v4-MCM-41), based on the finalvalues, is lower than that for all the other v4-sampleswhich were synthesized with fluorinated surfactants(Fig. 5A). v4-UKH10 (radial pore structure) and v4-UKH12 show similar accessibilities, which was greaterthan the accessibility in v4-UKA8 (Fig. 5A). The resultsfor the v6-samples also suggest that there are feweraccessible vinyl groups in v6-MCM-41 relative to thefluorinated samples synthesized at an identical ratio offunctionalized precursor (Fig. 5B). Surprisingly, v6-UKH10 displays the highest accessibility, followed byv6-UKA8 (Fig. 5B) in spite of the lower vinyl contentduring the synthesis relative to the v4-materials.

The high accessibilities observed in the fluorinatedsurfactant template materials may be due to the in-creased attraction of the vinyl pendant group to thesurface of the micelles during the synthesis of the fluori-nated surfactant template materials. Alternatively, theaccessibility of the vinyl-functionalized group may beimproved by a lower degree of long-range order in thefluorinated surfactant-templated materials. Accessibilitywithin disordered materials has been observed to be

higher than in ordered materials [42]. In the case ofwell-ordered materials, accessibility may be reduced bysteric hindrance of attached groups at the pore open-ings. This possibility was further investigated bysynthesizing a less-ordered CTAB-templated material(v4-MCM-41d) with pore order similar to the fluori-nated template materials. The accessibility within thismaterial is about 100% greater than in the ordered

G. Osei-Prempeh et al. / Microporous and Mesoporous Materials 85 (2005) 16–24 23

v4-MCM-41 after 200 min of bromination (absorbanceapproaching a constant value; Fig. 5A). Thus higheraccessibilities in v4-UKH10 and v6-UKH10 could bedue to their decreased ordered pore structure when com-pared to v4-UKA8, v6-UKA8 and v4-UKH12.

4. Conclusions

Partially fluorinated cationic surfactants have beensuccessfully used to prepare ordered mesoporous silicafunctionalized with organic vinyl. This is the first dem-onstration of fluorinated surfactant templating to pro-duce organic functionalized silica by direct synthesis.Adsorption data indicate that the pore sizes decreasewith an increase in vinyl content irrespective of the sur-factant chain length. However, the size of the pores in allfluorinated surfactant-templated materials is smallerthan that of the CTAB-templated material, for a givenlevel of functionalization. From the powder X-ray dif-fraction analysis, the order of the pores in the materialsobtained from C6F13C2H4NC5H5Cl and C8F17C2H4

NC5H5Cl templates decreases as the vinyl content in-creases, while the opposite effect was observed in theC10F21C2H4NC5H5Cl template materials. In spite of adecrease in pore size of the fluorinated surfactant-tem-plated materials (some even being microporous), the vi-nyl accessibility was shown to be higher than in orderedCTAB template materials. The greater accessibility inv4-MCM-41d, relative to the more ordered v4-MCM-41, shows that the long-range pore order strongly affectsthe accessibility of the functional groups within thepores. This study begins to address the unusual potentialto alter the accessibility of the organic functional groupon the basis of the high hydrophobicity and lipophobic-ity of the tail of the fluoro-surfactant template.

Acknowledgements

This material is based on work sponsored by theNational Science Foundation under grant numberDMR-0210517 and Kentucky Science and EngineeringFoundation grant number KSEF-159-RDE-001. Wethank Dr. Rodney Andrews and Rolando Gonzalez ofthe Center for Applied Energy Research for performingthe nitrogen adsorption analysis of the microporoussamples.

References

[1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge,K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B.McCullen, J.B. Higgins, J.L. Schlenkert, J. Am. Chem. Soc. 114(1992) 10834.

[2] P. Selvam, S.K. Bhatia, C.G. Sonwane, Ind. Eng. Chem. Res. 40(2001) 3237.

[3] S. Han, J. Xu, W. Hou, X. Yu, Y. Wang, J. Phys. Chem. B 108(2004) 15043.

[4] Z.M. Michalska, L. Rogalski, K. Rozga-Wijas, J. Chojnowski,W. Fortuniak, M. Scibiorek, J. Mol. Catal. A: Chem. 208 (2004)187.

[5] J. El Haskouri, S. Cabrera, C.J. Gomez-Garcia, C. Guillem, J.Latorre, A. Beltran, D. Beltran, M.D. Marcos, P. Amoros, Chem.Mater. 16 (2004) 2805.

[6] A.C. Sorensen, B.L. Fuller, A.G. Eklund, C.C. Landry, Chem.Mater. 16 (2004) 2157.

[7] X. Guo, M. Lai, Y. Kong, W. Ding, Q. Yan, C.T.P. Au,Langmuir 20 (2004) 2879.

[8] A.P. Wight, M.E. Davis, Chem. Rev. 102 (2002) 3589.[9] J. Aguado, J.M. Arsuaga, A. Arencibia, Ind. Eng. Chem. Res. 44

(2005) 3665.[10] W.J. Hunks, G.A. Ozin, Chem. Mater. 16 (2004) 5465.[11] Y.Q. Wang, C.M. Yang, B. Zibrowius, B. Spliethoff, M. Linden,

F. Schuth, Chem. Mater. 15 (2003) 5029.[12] A. Walcarius, C. Delacote, Chem. Mater. 15 (2003) 4181.[13] M.H. Lim, A. Stein, Chem. Mater. 11 (1999) 3285.[14] M. Qhobosheane, S. Santra, P. Zhang, W. Tan, Analyst 126

(2001) 1274.[15] J. Andersson, J. Rosenholm, S. Areva, M. Linden, Chem. Mater.

16 (2004) 4160.[16] L. Bois, A. Bonhomme, A. Ribes, B. Pais, G. Raffin, F. Tessier,

Colloids Surf. A 221 (2003) 221.[17] A. Bibby, L. Mercier, Chem. Mater. 14 (2002) 1591.[18] C.-Y. Lai, B.G. Trewyn, D.M. Jeftinija, K. Jeftinija, S. Xu, S.

Jeftinija, V.S.-Y. Lin, J. Am. Chem. Soc. 125 (2003) 4451.[19] Yan Jun Gong, Zhi Hong Li, Dong Wu, Yu Han Sun, Feng

Deng, Qing Luo, Yong Yue, Micropor. Mesopor. Mater. 49(2001) 95.

[20] A. Doyle, B.K. Hodnett, Micropor. Mesopor. Mater. 58 (2003)255.

[21] M. Kruk, T. Asefa, M. Jaroniec, G.A. Ozin, J. Am. Chem. Soc.124 (2002) 6383.

[22] D. Kumar, K. Schumacher, C. Fresne von Hohenesche, M. Grun,K.K. Unger, Colloids Surf. A 187–188 (2001) 109.

[23] L. Sierra, J.-L. Guth, Micropor. Mesopor. Mater. 27 (1999)243.

[24] R. Ryoo, I.-S. Park, S. Jun, C.W. Lee, M. Kruk, M. Jaroniec, J.Am. Chem. Soc. 123 (2001) 1650.

[25] J.L. Blin, P. Lesieur, M.J. Stebe, Langmuir 20 (2004) 491.[26] S.E. Rankin, B. Tan, H.J. Lehmler, K.P. Hindman, B.L. Knutson,

Micropor. Mesopor. Mater. 73 (2004) 197.[27] B. Tan, A. Dozier, H.J. Lehmler, B.L. Knutson, S.E. Rankin,

Langmuir 20 (2004) 6981.[28] B. Tan, H.J. Lehmler, S.M. Vyas, B.L. Knutson, S.E. Rankin,

Chem. Mater. 17 (2005) 916.[29] K. Kubo, Y. Moroi, K. Nomura, Y. Abe, T. Takahashi,

Langmuir 18 (2002) 8770.[30] Ke Wang, G. Oradd, M. Almgren, T. Asakawa, B. Bergenstahl,

Langmuir 16 (2000) 1042.[31] P. Kekicheff, G.J.T. Tiddy, J. Phys. Chem. 93 (1989) 2520.[32] F. Giuleri, M.P. Krafft, Thin Solid Films 284–285 (1996) 195.[33] T. Asakawa, H. Hisamatsu, S. Miyagishi, Langmuir 11 (1995)

478.[34] M.T. Bore, S.B. Rathod, T.L. Ward, A.K. Datye, Langmuir 19

(2003) 256.[35] C. Otero Arean, M. Rodriguez Delgado, V. Montouillout, J.C.

Lavalley, C. Fernandez, J.J. Cuart Pascual, J.B. Parra, Micropor.Mesopor. Mater. 67 (2004) 259.

[36] F.R. Dollish, W.G. Fateley, F.F. Bentley, Characteristic RamanFrequencies of Organic Compounds, John Wiley and Sons Inc.,1974.

24 G. Osei-Prempeh et al. / Microporous and Mesoporous Materials 85 (2005) 16–24

[37] K.P. Weidkamp, C.A. Hacker, M.P. Schwartz, X. Cao, R.M.Tromp, R.J. Hamers, J. Phys. Chem. B 107 (2003) 11142.

[38] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity,Academic Press, London, 1967.

[39] M. Kruk, M. Jaroneic, A. Sayari, Langmuir 13 (1997) 6267.

[40] M. Jaroneic, M. Kruk, J.P. Olivier, Langmuir 15 (1999) 5410.[41] M. Kruk, M. Jaroneic, Chem. Mater. 13 (2001) 3169.[42] R. Ryoo, J.M. Kim, C.H. Ko, C.H. Shin, J. Phys. Chem. 100

(1996) 17718.