Model System for Solid-State NMR Study on Co-condensation

Sep 23, 2009 - Yu-Chi Pan,† Hao-Yiang Wu,‡ Chia-Chun Kao,† Hsien-Ming Kao,*,† ... Science and Engineering, Mingdao UniVersity, 369 Wen-Hwa Roa...
0 downloads 0 Views 2MB Size
Download PDF
J. Phys. Chem. C 2009, 113, 18251–18258

18251

Model System for Solid-State NMR Study on Co-condensation Behavior of Silicon Precursors in Periodic Mesoporous Organosilicas Yu-Chi Pan,† Hao-Yiang Wu,‡ Chia-Chun Kao,† Hsien-Ming Kao,*,† Yaw-Nan Shieh,§ George T. K. Fey,| Jen-Hsuan Chang,† and Hui-Hsu Gavin Tsai*,† Department of Chemistry, National Central UniVersity, Chung-Li, Taiwan 32054, Republic of China, Department of Neurological Surgery, Tri-SerVice General Hospital, National Defense Medical Center, 325, Section 2, Cheng-Kung Road, Nei-Hu Dist, Taipei 11490, Taiwan, Republic of China, Department of Materials Science and Engineering, Mingdao UniVersity, 369 Wen-Hwa Road, Peetow, ChangHua, Taiwan 52345, Republic of China, and Department of Chemical and Materials Engineering, National Central UniVersity, Chung-Li, Taiwan 32054, Republic of China ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: September 7, 2009

Ordered periodic mesoporous organosilicas containing different fractions of benzene groups in the silica framework, based on the cubic SBA-1 mesostructure (Pm3jn mesophase), were synthesized with the directsynthesis route via co-condensation of tetraethoxysilane (TEOS) and 1,4-bis(triethoxysilyl)benzene (BTEB) under acidic conditions using cetyltriethylammonium bromide as a structure-directing agent. A significantly large amount of TEOS, up to 70 mol % based on silica, can be incorporated into the silica wall without degrading the structural integrity of the materials. By optimization of the synthesis compositions, the resulting materials exhibited much higher surface areas (up to 1210 m2/g) and larger pore volumes (up to 0.64 cm3/g) as compared to the previous analogue, which only exhibited a surface area of 381 m2/g and a pore volume of 0.11 cm3/g. Two-dimensional (2D) 29Si{1H} heteronuclear correlation (HETCOR) NMR spectra, acquired as a function of contact time, provided direct spectroscopic evidence that a single mesophase with various Q (from TEOS) and T silicon species (from BTEB) located randomly within the pore walls via co-condensation of BTEB and TEOS at a molecular level. Such information is often not achievable by other characterization techniques. The 1H-29Si distance information between phenylene protons and nearby T3 silicon atoms obtained from density functional theory calculations is also in good agreement with the observations of 2D 29Si{1H} HETCOR NMR experiments. Introduction A new class of mesoporous silica materials, designated as periodic mesoporous organosilicas (PMOs), has recently been synthesized via surfactant templating to integrate organic moieties within the silica wall by using organosilane precursors with two trialkoxysilyl groups bridged by an organic group, represented by the general formula (R′O)3Si-R-Si(R′O)3.1 Extensive research has been conducted in order to include a variety of bridging spacer groups and to control the mesoporous materials with various mesostructures with a number of surfactants.2–4 By varying the organic spacer groups of the organosilane precursors, the chemical and physical properties of PMOs can be tuned in designated ways for potential applications in different fields such as catalysis and adsorption. For example, Inagaki et al. reported a successful synthesis of periodic mesoporous benzene-silica with crystallike pore walls.3a They also showed that sulfonation of the bridging phenylene groups in the pore walls can be achieved for application as solid-acid catalysts and electrolytes for fuel cells.3d,e However, only limited numbers of PMOs can be directly used for catalysis because of the lack of catalytic active * Corresponding author. Fax: +886-3-4227664. E-mail: hmkao@ cc.ncu.edu.tw (H.-M.K.). † Department of Chemistry, National Central University. ‡ National Defense Medical Center. § Mingdao University. | Department of Chemical and Materials Engineering, National Central University.

sites in most cases. A good way to overcome this problem is to introduce two or more organic functional groups via cocondensation of different silicon precursors to achieve the desired functionality and selectivity for PMOs. When multiple silicon precursors are used for the synthesis of bifunctionalized PMOs, a fundamental and key issue is whether a highly dispersed multiphase system (i.e., phase separation) or a single mesophase with all organic spacers located randomly within the pore walls is formed. The formation of a single PMO mesophase with the different types of organic spacers simultaneously present in the silica framework is often assumed without any direct spectral evidence. This is because in most cases the characterization techniques employed, such as powder X-ray diffraction (XRD), multinuclear 1-D solidstate NMR, transmission electronic microscopy (TEM), thermogravimetric analysis (TGA), and nitrogen adsorption, often cannot provide a clear-cut answer to this issue. Recently, in the case of the hexagonal PMO materials synthesized by using ethane and ethylene-bridged silsesquioxane precursors, Treuherz and Khimyak5 have used 1H-13C and 1H-29Si cross-polarization magic angle spinning (CPMAS) NMR to specifically address this issue, namely, whether both organic spacers are located within the same “bifunctional” mesophase or in different, highly dispersed mesophases. The delicate analysis results of the crosspolarization (CP) kinetics data seem to indicate that both organic functionalities are heterogeneously distributed in the same PMO structure. By contrast, Yang and Sayari have carefully examined the powder XRD patterns of the materials prepared by using

10.1021/jp905626g CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

18252

J. Phys. Chem. C, Vol. 113, No. 42, 2009

phenylene and biphenylene silica precursors and concluded that their data were consistent with either phase separation or island formation.6 Among the various types of mesostructures, the cubic SBA-1 (Pm3jn) structure exhibits a unique cage-type structure with open windows.7 A general synthesis condition for the formation of SBA-1 is based on the S+X-I+ pathway, where S, X, and I correspond to surfactant, halide, and inorganic species, respectively, in an acidic medium at 0 °C, using cetyltriethylammonium bromide (CTEABr), a surfactant with a large head group, as the structure-directing agent. As revealed by 29Si MAS NMR,8 SBA-1 exhibits a very low cross-linking degree of silica framework due to the low synthesis temperature conditions. As a result, the as-synthesized SBA-1 material is not stable toward washing with water and thus imposes severe limitations on their further practical applications. The stability of SBA-1 can be improved by optimizing the synthesis compositions and conditions as in, for example, introduction of some additives such as sugarlike molecules9 and organosilanes10 as another silicon source in the reaction mixture. Alternatively, incorporating the benzene functionality into the SBA-1 mesostructure will allow the silica framework to be more hydrophobic, leading to a more stable structure for further potential applications. Recently, Goto and Inagaki reported successful syntheses of phenylene-silica hybrid materials with three-dimensional cage structures by using 1,4-bis(triethoxysilyl)benzene (BTEB) as the sole silicon source with a number of surfactants such as hexadecyltrimethylammonium bromide (CTMABr), hexadecylpyridinium chloride, CTEABr, and triblock copolymer F88.11 However, the obtained material (denoted as Ph-HMMc-3) prepared by using CTEABr as the structure-directing agent exhibited very poor stability since only one broad XRD peak, instead of three well-resolved (200), (210), and (220) peaks as in the case of normal cubic Pm3jn mesophases, was observed after the solvent extraction process. As a result, this material only exhibited a low surface area of 381 m2/g and a small pore volume of 0.11 cm3/g.11 However, the exact reason for poor stability of Ph-HMMc-3 was not given. The organic functional groups may not necessarily all be incorporated into the same mesophase when there are big differences in the hydrolysis rates between the matrix precursor (e.g., TEOS) and the functional silanes. If the rate of hydrolysis of a given organosilane is not nearly the same as that of TEOS, then it is possible that the organosilane introduced is not fully incorporated into the matrix when TEOS condenses around the surfactant micelles during the formation of the ordered mesoporous organosilicas. If the organosilane hydrolyzes faster, then it is likely to undergo self-condensation reactions, instead of co-condensation with TEOS, and this will influence the nature of the condensed phases formed; most possibly phase separation may occur. It is of great interest to understand that both organic spacers are located within the same “bifunctional” mesophase or in different, highly dispersed mesophases while incorporating other functional groups into benzene-silica. The silicon precursors containing other functional groups often exhibit similar 29Si NMR chemical shifts to those of BTEB. Under these circumstances, 29Si solid-state NMR should be considered to yield ambiguous results, because the location of silicon species belonging to the different organosilanes cannot be directly inferred from the chemical shift assignment to different silicon species. A powerful technique to provide detailed structural information regarding molecular and interfacial environments is 29 Si{1H} heteronuclear correlation (HETCOR) NMR spectroscopy, which makes use of correlating the chemical shift of

Pan et al. protons in the materials with the nearby 29Si species via their respective heteronuclear 1H-29Si dipole-dipole couplings. Such couplings depend strongly on the respective separations of the nuclei involved, which allows the spatial proximities between the protons and silicon nuclei present at the pore surface to be probed. Such HETCOR experiments have been employed by Vega12 to study the 1H-29Si correlation between silanol groups in zeolites and have been recently applied to probe the organic-inorganic interfacial structure in mesoporous materials.13–16 In the present study, therefore, we have chosen BTEB and TEOS as our baseline model system for understanding the cocondensation behavior of silicon precursors for the formation of PMOs since these two silicon precursors give Q and T silicon species with distinct 29Si chemical shifts. Considering the possibility of phase separation associated with the incorporation of organic functional groups, it turns out that the 2D 29Si{1H} HETCOR NMR will be more advantageous to establish the correlations between various silicon species. This prompted us to conduct a detailed 2D 29Si{1H} HETCOR NMR characterization of the benzene-silica based on the SBA-1 mesostructure as a function of TEOS contents. By correlating the 1H NMR signals of BTEB with the 29Si Q species from the condensation of TEOS through 2D 29Si{1H} HETCOR NMR, whether BTEB and TEOS co-condense at a molecular level to form a single mesophase can be revealed. Experimental Section Materials Preparation. Benzene-silicas based on the SBA-1 mesostructure were synthesized via co-condensation of BTEB (Aldrich) and a conventional silicon source TEOS (Aldrich) using CTEABr as the structure-directing agent under acidic conditions. The synthesis procedure described earlier was slightly modified.11 Since the previously reported procedures led to a material with poor stability, the synthesis procedure was first optimized by varying the BTEB/CTEABr ratio in the present study. The obtained material exhibited much intense XRD diffraction peaks when the BTEB/CTEABr ratio was 2 (Figure S1, Supporting Information). Therefore, the total amount of silicon sources, namely, BTEB and TEOS, to the surfactant was fixed at 2. In a typical synthesis, TEOS and BTEB with a molar ratio systematically varied from BTEB/(BTEB + TEOS) ) 100% to 0% were premixed. An aqueous HCl solution containing the surfactant CTEABr, synthesized following a previously published procedure,7 was stirred for 30 min to obtain a homogeneous solution. Then the mixture of TEOS and BTEB was added into the solution and stirred for 1 h at 0 °C. The reaction mixture was then treated at 45 °C for another 24 h. The resultant white precipitate was filtered without washing and then dried at 60 °C overnight. The molar composition of the reaction mixture was 1 CTEABr:2 (TEOS + BTEB):140 HCl: 2400 H2O. The materials obtained were denoted as Bz-SBA-x, where x is the mol % of BTEB/(BTEB + TEOS) in the initial gel mixture. The template was removed from the as-synthesized material by the solvent extraction process. A suspension of 0.5 g of as-synthesized sample was stirred in a solution of 3 g of HCl (36 wt %) in 100 mL of ethanol at 50 °C for 6 h to remove the organic templates. Characterization. Powder X-ray diffraction (XRD) patterns were collected on the Wiggler-A beamline (λ ) 0.133 367 nm) at the National Synchrotron Radiation Research Center in Taiwan. N2 adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 analyzer. The templateextracted Bz-SBA-x samples were degassed at 180 °C for several hours before measurements. The specific surface areas were

Co-condensation of Si Precursors in PMOs

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18253

obtained by the Brunauer-Emmett-Teller (BET) method in the relative pressure range of P/P0 ) 0.05-0.3, and the pore size distribution was analyzed from the adsorption branch of the isotherm by the Barrett-Joyner-Halenda (BJH) method. Pore volumes were obtained from the volumes of N2 adsorbed at P/P0 ) 0.95 or in the vicinity. Thermogravimetric analysis (TGA) was conducted on a Perkin-Elmer TGA 7 apparatus at a heating rate of 10 °C/min under an air flow. Solid-state 1H and 29Si magic angle spinning (MAS) and 13C CPMAS NMR spectra were recorded on a Varian Infinityplus500 NMR spectrometer, which is equipped with 7.5 and 4 mm Chemagnetics T3 probes. The Larmor frequencies for observing 1 H, 13C, and 29Si spins are 498.5, 125.0, and 99.7 MHz, respectively. One-pulse experiments under conditions of a π/6 pulse of 2 µs and a long recycle delay of 300 s were employed to collect the 29Si MAS NMR spectra in a quantitative way. The 1H, 13C, and 29Si chemical shifts were quoted to tetramethylsilane (TMS), as an external reference, at 0.0 ppm. 2D 29Si{1H} HETCOR NMR experiments, an extended version of the standard cross-polarization (CP) sequence, were recorded without 1H homonuclear decoupling during the t1 evolution time. Basically, the pulse sequence is composed of an initial time period (t1) during which the 1H magnetization evolves, followed by a CP step with a variable contact time that transfers polarization to the 29Si spins close to the 1H spins. The magnetization of the 29Si spins is then detected under highpower proton decoupling (rf strength ν1 ) 65 kHz) during t2. Phase-sensitive detection was accomplished by the timeproportional phase incrementation (TPPI) phase-cycling method.17 1

1

2D H- H exchange NMR experiments were performed with a NOESY-type sequence with a three 90° pulse. After the initial excitation, an additional π/2 pulse is applied after the evolution time (t1) to store the 1H magnetization along the z-axis. Then, magnetization is exchanged between homonuclear dipolarcoupled proton species during a mixing time (tmix) period. The mixing time period determines the extent of proton spin diffusion. For a given adequate mixing time, the 1H-1H exchange sequence produces homonuclear correlated spectra giving rise to off-diagonal intensities at positions where nuclei undergo chemical exchange or spin diffusion. The off-diagonal intensities depend strongly on the 1H-1H distances. Therefore, the 1H-1H separation can be probed by varying the mixing time. For 1H MAS, 1H-1H exchange, and 29Si{1H} HETCOR NMR experiments, the template-extracted samples were dehydrated at 100 °C for 8 h before NMR measurements. The sample was tightly packed into the rotor in a glovebox under a dry nitrogen atmosphere to avoid rehydration. Density Functional Theory Calculations. To further investigate whether the T and Q monomers are dispersed or selfaggregated in solution, the models of T3-T3-Q4-Q4 and T3-Q4-T3-Q4 motifs were constructed and energy minimized using density functional theory (DFT). Each peripheral Si atom of these two motifs was capped and saturated with hydrogen atoms. The T3-Q4-T3-Q4 motif represents the highly dispersed condition, whereas the T3-T3-Q4-Q4 motif is used to model the situation of island formation. DFT calculations within the suite of Gaussian 03 programs18 were performed by using the B3LYP density functional, Becke’s three-parameter exchange functional,19 and the Lee-Yang-Parr gradient-corrected correlation20 functional with moderate-sized 6-31G(d,p) basis set.21

Figure 1. Powder XRD patterns of (A) as-synthesized and (B) template-extracted Bz-SBA-x samples, where x ) (a) 100, (b) 90, (c) 70, (d) 50, (e) 30, (f) 10, and (g) 0 is the mol % of BTEB/(BTEB + TEOS) in the initial gel mixture.

Results and Discussion Structural Ordering as a Function of TEOS Contents. The XRD patterns of the as-synthesized and surfactant-free PMO materials synthesized using different molar ratios of BTEB and TEOS precursors are shown in Figure 1. All the as-synthesized samples showed three well-resolved XRD diffraction peaks in the region of 2θ ) 1.5°-3°, which can be indexed to the (200), (210), and (211) diffractions characteristic of the cubic Pm3jn mesostructure. After solvent extraction for template removal, the intensities of the XRD diffractions became worse for the samples with less BTEB contents, suggesting that the stability of Bz-SBA-x was strongly related to the BTEB contents in the sample. Unlike the 2D p6mm mesostructures, a series of regularly spaced diffraction peaks was not observed at a wider angle (2θ > 5°). Only two broad diffractions were observed for the Bz-SBA-100, and their intensities became worse as TEOS was incorporated (Figure S2, Supporting Information). Notice that in most of earlier reports22–25 PMOs were synthesized with multiple silicon precursors in the presence of nonionic surfactants under acidic conditions where molecular-scale order is difficult to achieve. In contrast to the materials prepared under basic conditions, the presence of a very broad peak centered around 2θ ) 12° with d-spacing of 0.8 nm due to periodicity within the pore walls indicates that all the Bz-SBA-x materials have very low periodicity in the pore walls. Wang et al. reported that periodic mesoporous benzene-silicas synthesized using nonionic alkyl poly(oxyethylene) surfactant in acidic media showed an ordered structure with two periodicities of 1.2 and 0.8 nm in the walls, which were only observed as very lowcontrast diffraction spots in the electron diffraction pattern.26 However, we observed only a broad XRD peak due to the 0.8 nm periodicity of phenylene groups in the pore walls. A similar XRD peak broadening due to the periodicity was also observed for mesoporous benzene-silicas with 2D-hexagonal pores synthesized under acidic conditions.27,28 These results suggested

18254

J. Phys. Chem. C, Vol. 113, No. 42, 2009

Pan et al.

Figure 2. Representative N2 adsorption-desorption isotherms of template-extracted Bz-SBA-x samples, where x ) (a) 100, (b) 70, (c) 30, and (d) 0 is the mol % of BTEB/(BTEB + TEOS) in the initial gel mixture. The isotherms in black and red represent the adsorption and desorption isotherms, respectively. The pore size distribution curves are shown in the insets.

TABLE 1: Textural Properties of the Template-Extracted Bz-SBA-x Samples Synthesized with Various BTEB/(BTEB + TEOS) Ratios x

d210 (nm)

a0 (nm)a

100 70 50 30 0

3.78 (3.94)d 3.67 (3.90) 3.60 (3.90) 3.54 (3.94) 3.06 (3.75)

8.45 (8.81) 8.21 (8.72) 8.06 (8.72) 7.91 (8.81) 6.85 (8.38)

ABET (m2/g)b Vp (cm3/g)c 1210 961 1082 901 878

0.64 0.52 0.58 0.51 0.48

pore size (nm) 2.4 2.5 2.5 2.7 2.7

a Lattice parameters a0 were calculated based on the formula a0 ) 5jd210. b ABET: BET surface area. c Vp: pore volume. d The numbers in parentheses were obtained from the as-synthesized samples.

that the synthesis in acidic media did not promote molecularscale periodicity in the pore walls. Structural Properties. The typical N2 adsorption-desorption isotherms of the template-extracted Bz-SBA-x samples under investigation are shown in Figure 2, and the structural properties are also listed in Table 1. All the isotherms are of type IV according to the IUPAC classification and showed a significant uptake of N2 at P/P0 ) 0.10-0.25. As seen in Table 1, these samples exhibited a pore size of around 2.4-2.7 nm, a BET surface area in the range of 878-1210 m2/g, and a pore volume in the range of 0.48-0.64 cm3/g. The BET specific surface area of the materials increased with increasing BTEB concentrations in the initial gel. Larger shrinkage of structural lattice after surfactant removal was observed with decreasing BTEB contents in the materials. When the composition of BTEB:CTEABr:HCl: H2O ) 1:0.4:112:1400 was used as reported in the literature,11 the resultant material only exhibited a surface area of 381 m2/g and a pore volume of 0.11 cm3/g. Therefore, the synthesis composition is critical in the formation of well-ordered cubic benzene-silicas under acidic conditions. Multinuclear NMR. Multinuclear solid-state NMR experiments including 13C CPMAS and 1H and 29Si MAS NMR were first performed on the template-extracted Bz-SBA-x samples

Figure 3. (a) 13C CPMAS and (b) 1H MAS NMR spectra of templateextracted Bz-SBA-30. Asterisks denote spinning sidebands.

in order to verify whether both BTEB and TEOS are incorporated into the silica framework. Figure 3a displays the 13C CPMAS NMR spectrum of Bz-SBA-30. A major resonance peak at 133 ppm, associated with a large spinning sideband manifold due to the 13C chemical shift anisotropy, was ascribed to the Si-C6H4-Si carbon atoms. It is worth noting that the 13 C CPMAS NMR spectrum exhibited no significant peaks in the range of 10-30 ppm, which gave a clear indication that virtually all the surfactant was removed from the samples during the template extraction process. Two major 1H NMR peaks at 7.2 and 1.8 ppm, corresponding to the phenylene protons and Si-OH groups, respectively, were observed in the 1H MAS NMR spectrum (Figure 3b). Figure 4 displays the 29Si MAS NMR spectra of the templateextracted samples. Six major signals around -94, -103, and -113 ppm, corresponding to Q2 (Si(OSi)2(OH)2), Q3 (Si(OSi)3(OH)), and Q4 (Si(OSi)4) species, and around -63, -72, and -81 ppm for T1 (C-Si(OH)2(OSi)), T2 (C-Si(OH)(OSi)2), and T3 (C-Si(OSi)3) species, respectively, were observed for all the samples under investigation. The observation of T groups confirms the presence of benzene moieties inside the silica framework. Meanwhile, the intensity of T groups increases with increasing BTEB amounts in the initial reaction mixture. This clearly indicates that the incorporated functional groups in the mesoporous silica materials are proportional to the BTEB contents added into the synthesis mixture. However, the T3/T2 peak intensity ratio is lower than that observed for the sample prepared under basic conditions,3a but comparable to the sample prepared under acidic conditions.3f Since the distinct T signals in the 29Si MAS NMR spectra are relatively well-resolved, the relative intensities of Qn and Tm NMR signals can be obtained by deconvolution of the spectra and then integrating them to allow a quantitative measure of the

Co-condensation of Si Precursors in PMOs

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18255

Figure 4. 29Si MAS NMR spectra of template-extracted Bz-SBA-x, where x ) (a) 70, (b) 50, and (c) 30. The dashed lines represent the components used for the spectral deconvolution.

TABLE 2: 29Si MAS NMR Results of the TemplateExtracted Bz-SBA-x Samples Synthesized with Various BTEB/(BTEB + TEOS) Ratios x 30 50 70

T1 (%) T2 (%) T3 (%) Q2 (%) Q3 (%) Q4 (%) 2.1 9.5 8.9

24.1 38.0 41.7

21.2 20.1 36.8

1.7 3.0 0.6

21.4 15.2 6.0

28.9 14.2 6.0

T/(T + Q) (%) 48.0(46.2)a 67.6(66.7) 87.4(82.4)

a The numbers in parentheses were estimated from the synthesis composition.

incorporation degree of the organic moiety. The ratios of Tm/ (Tm + Qn) thus obtained (Table 2) are in good agreement with those expected from the initial composition of the reaction mixture. From 29Si MAS NMR analysis, the relative amounts of T (from BTEB) and Q (from TEOS) within the resultant material are determined, but not their spatial proximity. If these two silicon precursors form separate phases, the 29Si MAS NMR analysis will give the same results. The 2D 29Si{1H} HETCOR NMR method (shown below) can resolve this issue by providing the information about the spatial proximity, which is probed by the CP step, between the T and Q groups. Thermal Stability. The results from thermogravimetric analyses of two selected as-synthesized Bz-SBA-x are shown in Figure 5. Both samples showed two main weight-loss regions. The first weight loss was due to decomposition of surfactant. Interestingly, two decomposition temperatures for the decomposition of the surfactant CTEABr were observed at 238 and 287 °C and at 244 and 291 °C for Bz-SBA-70 and Bz-SBA30, respectively. The additional decomposition at a higher temperature might result from the decomposition of surfactant molecules in different mesophases due to the phase separation of the Bz-SBA-x material. The possibility of phase separation can be excluded since similar two decomposition temperatures were also observed for Bz-SBA-100, in which there is no TEOS incorporated. The silica framework is composed of Si-C6H4-Si

Figure 5. TGA and DTA curves of as-synthesized (a) Bz-SBA-70 and (b) Bz-SBA-30.

and Si-O-Si fragments. The two decomposition temperatures can be ascribed to the possible different interactions between the surfactant molecules and the silica framework. The second weight loss around 540-555 °C is related to the decomposition of phenylene fragments in the pore walls and further condensation of silanols. This result suggests that the organosilica framework structure of the materials is preserved up to 550 °C, similar to the mesoporous benzene-silicas derived from CTMABr under basic conditions.3 Co-condensation Behavior of BTEB and TEOS. 2D 29 Si{1H} HETCOR NMR experiments were utilized to address the issue of a highly dispersed multiphase system or a single mesophase with all organic spacers located randomly within the pore walls in the mesoporous wall. Figure 6 displays the contour plot of the 2D 29Si{1H} HETCOR NMR spectra, acquired with different contact times (i.e., 1 and 3 ms), for the template-extracted Bz-SBA-30 material. It is advantageous for such HETCOR NMR experiments to monitor the correlations between spatially adjacent species as a function of contact time. As shown in Figure 6a, both the Q3 and T2 species are correlated strongly to the Si-OH protons resonance at 1.8 ppm since both silicon species themselves bear Si-OH groups. As expected, the broad peak at 7.2 ppm is correlated with all the T silicon species, but not with the Q silicon species at a short contact time of 1 ms. By increasing the contact time from 1 to 3 ms, which allows the probing of spatial proximity in longer distances, correlations between the T groups and Q groups can be clearly observed (Figure 6b). A similar spectral phenomenon has been observed for Bz-SBA-10 and Bz-SBA-70 (Figure 7). The T3 groups also show some correlations with the 1H peak at 1.8 ppm at a contact time of 3 ms, which predominantly resulted from either the Q3 species and/or the T2 species. The possibilities of the Q2 and T1 species are tentatively excluded because of their low concentrations. The 1H peak at 3.5 ppm may result

18256

J. Phys. Chem. C, Vol. 113, No. 42, 2009

Pan et al.

Figure 7. 2D 29Si{1H} HETCOR NMR spectra of (a) Bz-SBA-10 and (b) Bz-SBA-70, acquired with a contact time of 3 ms at a spinning speed of 5 kHz. Figure 6. 2D 29Si{1H} HETCOR NMR spectra of Bz-SBA-30, acquired with a contact time of (a) 1 and (b) 3 ms at a spinning speed of 5 kHz.

from small amounts of water or the residual surfactant molecules. For the case of Bz-SBA-100 (Figure S3), only the correlation peaks between the 29Si and 1H signals of the T groups were observed since there are no Q groups in this material. The relative intensities of the correlation peaks between the T and Q groups in a series of Bz-SBA-x (x ) 10, 30, and 70) confirm that a single mesophase with all organic spacers located randomly within the pore walls in the mesoporous wall is formed. Figure 8 shows the 2D 1H-1H exchange NMR spectra acquired at mixing times of 10 and 20 ms. With a short mixing time of 10 ms, only small cross peaks were observed. These cross peaks became more intense when the mixing time was increased to 20 ms. The cross peaks were not observable at a mixing time shorter than 7 ms. Therefore, the contribution from the 1H spin diffusion to the observation of the correlation peaks in the 29Si{1H} HETCOR NMR spectra is negligible at a contact time of 3 ms. The great advantage of such 2D 29Si{1H} HETCOR NMR is that co-condensation of BTEB and TEOS can be directly confirmed without recruiting a detailed and complex CP dynamics analysis. Since clear correlation peaks between the T and Q species were observed in the 29Si{1H} HETCOR NMR spectra, we can concluded that BTEB and TEOS co-condense at a molecular level instead of forming separate phases. When BTEB and TEOS form separate phases,

the observation of correlation between them is not possible because the distance between them will be at least over 1 nm, which is too long for a NMR correlation signal to be observed. Density Functional Theory Calculations. To explore the possible compositions for the various Si units, DFT calculations were performed on the two proposed framework compositions, the T3-T3-Q4-Q4 and T3-Q4-T3-Q4 motifs, which present the cases of island formation and homogeneous condensation, respectively. Figure 9 shows that the distances of the Si atom on the Q4 species in the first sphere to the nearest phenylene hydrogen atoms on T3 are nearly the same for both motifs (distances of 4.02 and 4.03 Å in the T3-T3-Q4-Q4 and T3-Q4-T3-Q4 motifs, respectively). In contrast, the distances of the Si atom on the Q4 species in the second sphere to the nearest phenylene hydrogen atoms on T3 for the T3-T3-Q4-Q4 motif is 7.01 Å. The shorter 1H-29Si distance (4.02 Å) of the former Q4 species to the T3 species will allow the signal of 1 H-29Si correlation to be observed when the CP contact time is increased from 1 to 3 ms, while it is too far for such correlations to be observed for the further Q4 species (1H-29Si distances of 7.01 Å). This indicates that, in the T3-Q4-T3-Q4 motif, the Si atom on the Q4 species will correlate with the hydrogen atoms in the two nearest benzene rings on T3 and result in a stronger 1H-29Si correlation signal in the 2D 29Si{1H} HETCOR NMR spectrum. By contrast, for the T3-T3-Q4-Q4 motif, the Si atom on the Q4 species will correlate with the hydrogen atoms in the only one nearest benzene ring on T3 and result in a much weaker 1H-29Si correlation signal. The distance

Co-condensation of Si Precursors in PMOs

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18257

Figure 9. DFT-optimized structures of (a) T3-T3-Q4-Q4 and (b) T3-Q4-T3-Q4 motifs. For clarity, the hydrogen atoms are ignored except for the ones in the benzene ring.

Figure 8. 2D 1H-1H exchange NMR spectra of Bz-SBA-30, acquired with a mixing time of (a) 10 ms and (b) 20 ms.

information obtained from DFT calculations is in good agreement with the observations of 2D 29Si{1H} HETCOR NMR experiments. If the major product of co-condensation of TEOS and BTEB is the island formation, the low concentrations of the interfacial T3-Q4 connectivities cannot make significant contributions to the 1H-29Si correlations as observed in the 2D 29 Si{1H} HETCOR NMR experiments (Figure 6b). Interestingly, the results of B3LYP/6-31G(d,p) calculations show that the energies of these two motifs are nearly identical. The T3-T3-Q4-Q4 motif is only 0.02 kcal/mol lower in energy than that of the T3-Q4-T3-Q4 motif. This result is not surprising because the chemical connectivities are identical for both motifs and the rigid phenylene ring will not allow extra stabilization energy for any of these two motifs. However, the homogeneous combination of the T3-Q4-T3-Q4 motif will gain more favorable mixing entropy than that of T3-T3-Q4-Q4. Taken together, the combined results of 29Si{1H} HETCOR NMR and density functional theory calculations suggest that the favorable framework composition in TEOS-incorporated periodic mesoporous benzene-silicas is the T3-Q4-T3-Q4 motif, in which the T and Q species are homogeneously distributed in the same silica framework. Conclusions Ordered and stable hybrid periodic mesoporous organosilicas with Pm3jn cubic symmetry have been successfully synthesized

via co-condensation of two silica precursors, namely, BTEB and TEOS, in varying molar ratios in the presence of CTEABr as a structure-directing agent under acidic conditions. To preserve the structural ordering of the resultant materials, the maximum TEOS content that can be incorporated into the silica wall cannot be over 70 mol % based on silica. Two-dimensional 29 Si{1H} heteronuclear correlation NMR provided the direct spectral evidence of a single mesophase with all the T and Q silicon species located randomly within the pore walls via cocondensation of BTEB and TEOS at a molecular level. Acknowledgment. The financial support of this work by the National Science Council of Taiwan is gratefully acknowledged. Supporting Information Available: XRD patterns (Figures S1 and S2) and 2D 29Si{1H} HETCOR NMR spectrum of BzSBA-100 (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611. (b) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (c) Yoshina-Ishii, C.; Asefa, T.; Coombs, N.; MacLachlan, M. J.; Ozin, G. A. Chem. Commun. 1999, 2539. (d) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302. (2) (a) Temtsin, G.; Asefa, T.; Bittner, S.; Ozin, G. A. J. Mater. Chem. 2001, 11, 3202. (b) Guan, S.; Inagaki, S.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 5660. (c) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403. (d) Asefa, T.; Kruk, M.; MacLachlan, M. J.; Coombs, N.; Grondey, H.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2001, 123, 8520. (e) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. J. Phys. Chem. B 2001, 105, 9935.

18258

J. Phys. Chem. C, Vol. 113, No. 42, 2009

(3) (a) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304. (b) Kapoor, M. P.; Yang, Q.; Inagaki, S. Chem. Mater. 2004, 16, 1209. (c) Kapoor, M. P.; Yang, Q.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 15176. (d) Okamoto, K.; Kapoor, M. P.; Inagaki, S. Chem. Commun. 2005, 1423. (e) Yang, Q.; Kapoor, M. P.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 9694. (f) Goto, Y.; Inagaki, S. Chem. Commun. 2002, 2410. (g) Hamoudi, S.; Kaliaguine, S. Chem. Commun. 2002, 2118. (h) Sayari, A.; Yang, Y. Chem. Commun. 2002, 2582. (i) Wang, W.; Zhou, W.; Sayari, A. Chem. Mater. 2003, 15, 4886. (j) Morell, J.; Wolter, G.; Fro¨ba, M. Chem. Mater. 2005, 17, 804. (k) Olkhovyk, O.; Jaroneic, M. J. Am. Chem. Soc. 2005, 127, 60. (4) (a) Lu, Y.; Fan, H.; Doke, N.; Loy, D. A.; Assinik, R. A.; LaVan, D. A.; Brinker, C. J. J. Am. Chem. Soc. 2000, 122, 5258. (b) Fan, H.; Lu, Y.; Stump, A.; Reed, S. T.; Bear, T.; Schunk, R.; Perez-Luna, V.; Lopez, G. P.; Brinker, C. J. Nature 2000, 405, 56. (c) Rogers, J. A.; Bao, Z.; Makhija, A.; Braum, P. AdV. Mater. 1999, 11, 741. (d) McInal, M. D.; Scott, J.; Mercier, L.; Kooyman, P. J. Chem. Commun. 2001, 2282. (e) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151. (5) Treuherz, B. A.; Khimyak, Y. Z. Microporous Mesoporous Mater. 2007, 106, 236. (6) Yang, Y.; Sayari, A. Chem. Mater. 2008, 20, 2980. (7) (a) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (b) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (c) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (8) Kim, M. J.; Ryoo, R. Chem. Mater. 1999, 11, 487. (9) (a) Kao, H.-M.; Ting, C.-C.; Chiang, A. S. T.; Teng, C.-C.; Chen, C.-H. Chem. Commun. 2005, 1058. (b) Kao, H.-M.; Cheng, C.-C.; Ting, C.-C.; Hwang, L.-Y. J. Mater. Chem. 2005, 15, 2989. (10) (a) Kao, H.-M.; Wu, J.-D.; Cheng, C -C. Microporous Mesoporous Mater. 2006, 88, 319. (b) Kao, H.-M.; Shen, T.-Y.; Wu, J.-D.; Lee, L.-P. Microporous Mesoporous Mater. 2008, 110, 461. (c) Pan, Y.-C.; Wu, H.Y.; Jheng, G.-L.; Tsai, H.-H. G. J. Phys. Chem. C 2009, 113, 2690. (11) Goto, Y.; Inagaki, S. Microporous Mesoporous Mater. 2006, 89, 103. (12) Vega, A. J. J. Am. Chem. Soc. 1988, 110, 1049. (13) Janicke, M. T.; Landry, C. C.; Christiansen, S. C.; Kumar, D.; Stucky, G. D.; Chmelka, B. F. J. Am. Chem. Soc. 1998, 120, 6940. (14) Melosh, N. A.; Lipic, P.; Bates, F. S.; Wudl, F.; Stucky, G. D.; Fredrickson, G. H.; Chmelka, B. F. Macromolecules 1999, 32, 4332.

Pan et al. (15) Christiansen, S. C.; Zhao, D.; Janicke, M. T.; Landry, C. C.; Stucky, G. D.; Chmelka, B. F. J. Am. Chem. Soc. 2001, 123, 4519. (16) Baccile, N.; Laurent, G.; Bonhomme, C.; Innocenzi, P.; Babonneau, F. Chem. Mater. 2007, 19, 1343. (17) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid State NMR and Polymers; Academic Press: New York, 1994; p 478. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.02; Gaussian Inc.: Wallingford, CT, 2004. (19) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (20) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (21) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081. (22) Burleigh, M. C.; Jayasundera, S.; Spector, M. S.; Thomas, C. W.; Markowitz, M. A.; Gaber, B. P. Chem. Mater. 2004, 16, 3. (23) Morell, J.; Gu¨ngerich, M.; Wolter, G.; Jiao, J.; Hunger, M.; Klar, P. J.; Fro¨ba, M. J. Mater. Chem. 2006, 16, 2809. (24) Cho, E.-B.; Kim, D.; Jaroniec, M. Langmuir 2007, 23, 11844. (25) Goto, Y.; Okamoto, K.; Inagaki, S. Bull. Chem. Soc. Jpn. 2005, 78, 932. (26) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Asefa, T.; Coombs, N.; Ozin, G. A. Chem. Mater. 2001, 14, 1903. (27) Goto, Y.; Inagaki, S. Chem. Commun. 2002, 2410. (28) Kapoor, M. P.; Setoyama, N.; Yang, Q.; Ohashi, M.; Inagaki, S. Langmuir 2005, 21, 443.

JP905626G