Functionalization of the Internal Surface of Pure-Silica MFI Zeolite with

Feb 20, 2008 - These results are in systematic agreement with theoretical estimates that consider both internal and external surface functionalization...
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J. Phys. Chem. C 2008, 112, 3543-3551

3543

Functionalization of the Internal Surface of Pure-Silica MFI Zeolite with Aliphatic Alcohols Chil-Hung Cheng,† Tae-Hyun Bae,† Benjamin A. McCool,‡ Ronald R. Chance,†,§ Sankar Nair,*,† and Christopher W. Jones*,† School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe NW, Atlanta, Georgia 30332-0100, ExxonMobil Process Research Laboratories, and ExxonMobil Research and Engineering, 1545 Route 22 East, Annandale, New Jersey 08801 ReceiVed: October 9, 2007; In Final Form: December 20, 2007

The functionalization of the internal surface of pure-silica MFI zeolite using aliphatic alcohols (1-butanol and 1-hexanol) is reported. Calcined MFI nanoparticles (50, 100, 200, and 500 nm in size) are treated with neat 1-butanol and 1-hexanol under reflux conditions. The 1-butanol loadings on 200 and 500 nm particles are determined to be 0.7 mmol/g SiO2 by thermogravimetric analysis (TGA) and are similar to the tetrapropylammonium (TPA) template content of the as-made nanoparticles. 13C cross-polarization magicangle spinning (CP-MAS) NMR and TGA data suggest that the observed 1-butanol loading is strongly correlated with the concentration of internal silanol defect sites. In addition, significantly higher 1-butanol loadings on 50 nm (1.1 mmol/g SiO2) and 100 nm (0.93 mmol/g SiO2) MFI nanoparticles reflect the concurrent functionalization of silanols on the external surfaces of the nanoparticles. These results are in systematic agreement with theoretical estimates that consider both internal and external surface functionalization sites. MFI nanoparticles (50, 100, and 300 nm) treated with 1-hexanol result in 1.34, 1.28, and 1.14 mmol/g SiO2 of 1-hexanol content, respectively, levels that are higher than expected from consideration of the results of butanol treatment. These higher organic loadings of 1-hexanol may imply the existence of additional physisorbed 1-hexanol molecules trapped between other 1-hexanol molecules within the zeolite micropores and/or the formation of dimeric/oligomeric complexes by hydrophobic interactions between the hexyl groups. 13C/29Si MAS and CP-MAS NMR investigations suggest that the organic groups are covalently bonded to the internal silanol defect sites, consistent with previous work on the chemisorption of methanol in MFI.

Introduction Zeolites, possessing micropores of diameter 96%, Alfa Aesar), hexanes (ACS grade, EMD), 1-butanol (ACS grade, >99.4%, J.T. Baker), and 1-hexanol (reagent grade, >98%, Aldrich). The alcohols were further dried through prescribed drying procedures.41 Cab-OSil M-5 was a gift from Cabot Corporation. Zeolite Synthesis. The procedures published by Schoeman were used to synthesize pure-silica MFI nanoparticles.42,43 TEOS was used as the silica source, and TPAOH was used as the structure-directing agent (SDA). The molar ratio of the precursor solution was 1 TEOS/0.36 TPAOH/x H2O. After the mixture was aged at room temperature for 24 h, it was placed under hydrothermal reaction conditions for the designated synthesis duration. For example, to synthesize 500 nm pure-silica MFI particles (with x ) 180), 25 g of TEOS was added dropwise into 21.53 g of TPAOH solution while stirring. The mixture was stirred vigorously for 1 h and became completely transparent. To this mixture, 368.12 g of deionized water was added to the transparent solution, and it was kept on stirring vigorously for another 24 h. The resulting mixture was transferred to a 700 mL autoclave reactor (HR-700, Berghof Inc.) to perform

Cheng et al. the hydrothermal reaction. The temperature was 150 °C, and the synthesis duration was set as 48 h. The resulting suspension was centrifuged at 8000 rpm for 20 min (Fisher accuSpin model 400 equipped with a six-place 45° fixed-angle rotor), and the supernatant was discarded. The solid was redispersed into deionized water by sonication and was centrifuged afterward. This centrifugation-sonication cycle was repeated until the pH of the supernatant was below 8. The redispersed nanoparticles were dried at room temperature. This as-synthesized material was calcined at 550 °C for 8 h in air. To synthesize 200 nm silicalite particles, the water content of the precursor solution (x) was set to 20, the hydrothermal reaction temperature was 150 °C, and the synthesis duration was 96 h. To synthesize 100 nm silicalite particles, the water content of the precursor solution (x) was set to 20, the hydrothermal reaction temperature was 95 °C, and the synthesis duration was 48 h. To synthesize defectfree pure-silica MFI crystals, a fluoride-mediated synthesis route was followed.44,45 First, 0.81 g of TPABr and 0.058 g of NH4F were dissolved in 13.44 g of DI water. The mixture was stirred at room temperature for 10 min. Then, 2.24 g of Cab-O-Sil was added to the TPABr-NH4F-H2O mixture, and the mixture was blended manually for 10 min to obtain a homogeneous gel. This gel was transferred to a 20 mL autoclave. The reactor was placed in a synthesis oven and heated at 180 °C statically for 14 days. The resulting solids were washed with DI water several times after the autoclave was cooled down to room temperature. Surface Treatment with 1-Butanol and 1-Hexanol. All the alcohol treatments were performed under neat conditions as illustrated by the following example. In a 100 mL round-bottom flask, 0.5 g of calcined or as-synthesized MFI nanoparticles (500, 200, or 100 nm) was degassed at 200 °C for 6 h on a Schlenk line (∼15 mTorr). The degassed MFI nanoparticles were transferred into a glovebox, where approximately 10 g of dried 1-butanol was added to the MFI particles in a roundbottom flask sealed with a valve. The whole setup was brought out of the glovebox and connected to a distillation head (ChemGlass Inc. CG-1237-01) that had been dried at 120 °C for 30 min in an oven and then purged with argon for several minutes on a Schlenk line. The round-bottom flask was immersed in a silicone oil bath (108-115 °C) for 48 h. The resulting material was centrifuged and redispersed in hexane under sonication. The washing-redispersion cycle was repeated three times. The redispersed particles were first dried by rotovaporation and then degassed on a Schlenk line at room temperature for 2 h before performing TGA characterization. All the 1-hexanol treatments were performed using a similar procedure as that described above for 1-butanol. Characterization. Scanning electron microscope (SEM) images of the zeolite crystals were taken on a Hitachi S-800 operating at 13 kV. Powder X-ray diffraction (XRD) patterns were obtained on a Philips X’pert diffractometer equipped with X’celerator using Cu KR radiation. Samples were analyzed over a range of 4-55° 2θ with a step size of 0.02°. Thermogravimetric analysis was performed on a Netzsch STA409. Samples were heated under a nitrogen-diluted air stream from 30 to 900 °C at a rate of 10 °C/min. The organic loading was estimated from the weight loss within the range of 240-420 °C for 1-butanol-treated samples, within the range of 250-520 °C for 1-hexanol-treated samples, and within the range of 340-540 °C for as-made samples. 1H/29Si MAS and 29Si cross-polarization magic-angle spinning (CP-MAS) NMR measurements were performed on a Bruker DSX300 spectrometer operating at 59.64 MHz with a spinning rate of 5 kHz. Spectra were acquired using a 7 mm probe with ZrO2 rotors and a 10 s recycle delay.

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Figure 1. SEM images of pure-silica MFI nanoparticles: (a) 500, (b) 200, (c) 100, and (d) 50 nm. The scale bars in (a), (b), and (c) are 500 nm. The scale bar in (d) is 200 nm.

Chemical shifts were referenced to 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt. 13C CP-MAS spectra were recorded at 38.45 MHz on a Bruker DSX300 with a spinning rate of 5 kHz. FT-IR/FT-Raman spectra were obtained on a Bruker 66 v/s/FRA-106. Pelletized pure KBr was used as the background and was subsequently subtracted from the spectrum of the pelletized sample and KBr mixture. A total of 1024 scans was acquired per spectrum. Nitrogen adsorption measurements were performed on a Micromeritics ASAP 2020 micropore analyzer. About 0.1-0.2 g of sample was degassed under vacuum at 200 °C for 18 h. The measurements were conducted in a liquid nitrogen bath at 77 K. The micropore volume was determined by the t-plot method. Results and Discussion Zeolite Nanoparticle Size and Structure Characterization. To investigate zeolite crystal size effects (e.g., the ratio of internal and external surface sites) on the functionalization results, several nanoparticle sizes were synthesized by altering the water content, synthesis duration, and synthesis temperature as described in the Experimental Section (Figure 1 shows four SEM images). The 50 and 100 nm particles did not display a distinct morphology and are best described as spherical. However, larger particle sizes in the 200-500 nm range begin to exhibit a more well-defined prismatic morphology. Thus, the particle sizes were reported as the length of the prismatic crystals. Powder XRD patterns (Figure 2, top) identify all the nanoparticle samples as possessing the MFI structure. Although not the focus of this report, a closer examination of the roomtemperature XRD patterns (particularly in the 22-26° 2θ range) shows that the fine structure of the MFI crystals depends on particle size. Specifically, the structure changes from that of twinned monoclinic domains in larger crystals to an orthorhombic structure in smaller crystals. This is evinced, for

Figure 2. (Top) Powder XRD patterns of pure-silica MFI nanoparticles with 2θ range of 6-55°: (a) 500, (b) 200, and (c) 100 nm. (Bottom) The traces of powder XRD patterns of pure-silica MFI nanoparticles with 2θ range of 22-26° where the transition of crystal structure between monoclinic and orthorhombic phases is indicated (ref 46). The dotted rectangular area around the 24.2-24.8° 2θ range displays the transition from orthorhombic phase (singlet peak) to monoclinic phase (doublet peak).

example, by the convergence of the doublet peak observed between 24.2° and 24.8° 2θ in larger crystals into a single peak

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at 24.5° 2θ in the smaller crystals (Figure 2, bottom). Separate detailed studies (not shown here) using a large number of synthesized MFI crystal sizes indicated a well-defined phase transition occurring at a particle size between 400 and 500 nm. The ferroelastic monoclinic-orthorhombic phase transition in MFI has been extensively studied and is well-known to occur40,46-48 by application of temperature or external stress. However, its induction by particle size changes has not been reported previously. Size-dependent phase transitions are known in ferroelectric materials.49 Nitrogen physisorption data of the calcined pure-silica MFI samples show micropore volumes around 0.15 cm3/g, consistent with previous results.50-52 The micropore volumes are determined using the first plateau of the hysteresis occurring within the relative pressure range of 0.1-0.2. The micropore volume is determined as 0.19 cm3/g when the physisorption is analyzed using the second plateau of the hysteresis.50,53 The hysteresis in the low relative pressure range is attributed to the phase transition of adsorbates.50 However, the hysteresis becomes less pronounced as the particle size decreases.54 The external specific surface area extracted from the physisorption data decreases (as expected) with increasing particle size. TABLE 1: Organic Loading (mmol/g SiO2) of Pure-Silica MFI under Different Treatment Conditions as Measured by TGA sample treatment

organic weight loss temp (°C)

organic group

100 nm 200 nm 500 nm

as-made particles 340-520 TPA 0.72 calcined particles 280-400 butyl 0.91 treated with 1-butanola as-made particles 280-520 TPA and butyl 0.83 treated with 1-butanol

0.72 0.75

0.7 0.69

0.80

0.74

a Note: For the 1-butanol-treated samples, the reported organic loadings were measured after the samples were treated under vacuum for 2 h at 200 °C. As mentioned in the Results and Discussion section, the reported organic loadings are essentially the same as those measured after the samples were treated under vacuum for 2 h at room temperature.

1-Butanol and 1-Hexanol Treatment and Characterizations. TGA was used to quantify the alcohol content of the functionalized materials. The as-made pure-silica MFI nanoparticles of the corresponding particle size were first used in control experiments. As shown in Table 1 for three different particle sizes, the temperature range of the organic weight losses on the as-made materials is 340-520 °C and the tetrapropylammonium (TPA) content is about 0.7 mmol/g SiO2. The weight loss temperature range is in agreement with published results.55 The weight loss corresponds to ∼3.6 TPA cations per unit cell, close to the theoretical value of 4 TPA cations per unit cell in the as-made MFI structure.53 For the calcined particles treated with 1-butanol, the temperature range of the organic weight loss is 240-420 °C, and the amount of weight loss changes with the particle size. Two separate 1-butanoltreated samples were subjected to vacuum for 2 h at room temperature and 200 °C, respectively, and were found to retain essentially the same organic loading. The organic loading decreases steadily with increasing particle size and stabilizes at about 0.7 mmol/g SiO2 for the largest (500 nm) particles. This loading is almost identical to the TPA content of the corresponding as-made sample. The MFI crystal structure of the nanoparticles is not disturbed after the alcohol treatment

(based on powder XRD patterns, see the Supporting Information) but there are changes in the relative intensities of some peaks, as expected. For example, the relative intensities I(011)/ I(051) and I(200)/I(051) decrease after being treated with 1-butanol. Similar results are also observed in the XRD patterns of TPA-containing pure-silica MFI nanoparticles before and after calcination. For 100 nm particles, the butanol content is 0.91 mmol/g SiO2, 27% higher than the TPA loading. This additional organic loading becomes more prominent when the particle size is further decreased to 50 nm, where we observed a butanol loading of 1.10 mmol/g SiO2. These results clearly suggest that the alcohol loading due to functionalization of the external surface becomes more significant as the particle size decreases. To further examine this hypothesis, we performed the 1-butanol treatment on the asmade materials and measured the organic weight losses. Given the measured negligible micropore volume of as-made puresilica MFI, we can assume that 1-butanol molecules cannot access the internal surface due to the presence of TPA cations in the pore structure. Hence, 1-butanol can only react with the silanol groups on the outer surface of the as-made zeolite particles. From Table 1, it is clear that the difference in total organic content between the as-made samples and the as-made 1-butanol-treated samples is large for the smaller particle sizes and systematically decreases with increasing particle size. For the large crystal sizes this difference is quite small (about 0.04 mmol/g), implying that almost all the 1-butanol groups are functionalizing the internal sites in calcined samples. This result also corroborates the hypothesis that 1-butanol molecules functionalize both the internal and external surface sites according to their particle-size-dependent relative concentrations. Thus, the sum total of the above data supports the following two assertions. First, the TPA and butanol contents of the asmade and alcohol-treated materials match the concentration of the internal silanol defect sites.56 In particular, there is approximately one internal silanol defect site per TPA cation, the defect sites are located at the intersections of the straight and sinusoidal channels as known from previous work, and each internal defect site is subsequently functionalized with approximately one butanol molecule. Second, since the as-made materials have inaccessible micropores due to the pore blocking of TPA cations, one can resolve the organic loadings on the internal defect sites and on the outer surface silanol groups by comparing the results from different particle sizes. The above assertions are expressed more quantitatively in Figure 3, which compares the TGA data to theoretical estimates for the internal and external surface butanol loadings. Since the concentration of the internal defect sites and the 1-butanol loadings of pure-silica MFI nanoparticles are correlated, we can predict the particle size effect on the 1-butanol loading. We assume the particles are approximately spherical. The density of calcined pure-silica MFI is 1.78 g/cm3, and its micropore volume was measured previously as 0.184 cm3/g.51 Given the liquid density of 1-butanol (0.81 g/cm3), the maximum theoretical 1-butanol loading within the micropores can be estimated as 2 mmol/g SiO2, assuming that the micropores are filled with liquid-like 1-butanol. This loading is independent of the particle size and also represents an approximate upper bound shown as line a in Figure 3. The concentration of internal defect sites, where the internal silanol groups are located, was calculated based on the theoretical TPA cation concentration, which is 4 TPA cations per unit cell.56 These assumptions lead to a (particle-size-independent) theoretical estimate of 0.70 mmol/g SiO2 of chemisorbed 1-butanol loading in the internal sites

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Figure 3. Comparison of theoretical calculations and experimental measurements of the 1-butanol loading on pure-silica MFI nanoparticles. Line a is the upper-bound liquid-like 1-butanol loading within the micropores; line b is the theoretical concentration of internal silanol defect sites (0.7 mmol/g SiO2); curve c is the theoretical concentration of external silanol groups each functionalized with one 1-butanol molecule; curve d is the total theoretical 1-butanol loading and is the sum of (b) and (c). The measured data of calcined samples treated with 1-butanol in Table 1 are represented by the circular symbols, whereas the measured data of the 0 days column in Table 3 are represented by the square symbols. The measured data from 50 nm particles treated with 1-butanol (mentioned in the text) is represented by the triangle symbol.

(Figure 3, line b). To quantify the loading on external sites, we assumed an external surface silanol density of 4 Si-OH/nm2.57,58 If all the external sites are successfully functionalized, the theoretical estimate for 1-butanol loading on the external surface is given by the (particle-size-dependent) curve (c) in Figure 3. Curve (d) in Figure 3 represents the total theoretical 1-butanol loading which is the sum of curve (c) and line (b). The measured total loading as a function of the particle size is also plotted in Figure 3, and closely follows the theoretical predictions. TABLE 2: Organic Loading (mmol/g SiO2) of Pure-Silica MFI Treated with 1-Hexanol as Measured by TGA particle size (nm)

organic weight loss temp (°C)

organic loading

50 100 300

250-520 250-520 250-520

1.34 1.28 1.14

In addition, we studied the particle size dependence of 1-hexanol treatment. As shown in Table 2, the 1-hexanol loading shows a similar particle size dependence as that of 1-butanol. However, a key difference is that the loadings are considerably higher than those using 1-butanol. There may be several possible explanations for this, including the possible existence of physisorbed molecules in the channels of MFI which are trapped due to the bulky hexyl groups of the molecules bound to the defect sites, or the formation of dimeric/oligomeric complexes due to hydrophobic interactions between long hexyl groups. The lower end (250 °C) of the weight loss temperature range is somewhat below that for 1-butanol (280 °C) and may indicate the loss of physisorbed 1-hexanol molecules. The higher end of the weight loss temperature range for 1-hexanol (520 °C) overlaps (and even exceeds) that for 1-butanol (400 °C), indicating that a significant number of 1-hexanols are coordinated to the silanol defect sites. More quantitative characterization of 1-hexanol-treated samples is complicated by their more complex structure. Further studies are warranted. MAS and CP-MAS NMR Characterization. To study in more detail the chemical environment of the organic molecules in the zeolite structure, we chose to employ the 1-butanol-

Figure 4. 13C CP-MAS NMR spectra of 500 nm pure-silica MFI nanoparticles (a) calcined and treated with 1-butanol; (b) as-made and treated with 1-butanol; (c) as-made.

functionalized materials in our NMR investigation. We expect that the 1-hexanol-functionalized materials lead to qualitatively similar results that may, however, be more difficult to interpret because of the comparatively higher complexity of these hybrid materials. The 1H MAS NMR spectrum of calcined 500 nm pure-silica MFI particles (Supporting Information) indicates the presence of physisorbed water molecules (chemical shift at 4.2 ppm)59 and hydrogen-bonded silanol groups (around 8 ppm)28 in the framework. The presence of hydrogen-bonded silanol groups could be attributed to the presence of silanol nests reported in pure-silica zeolite frameworks.60,61 After 1-butanol treatment, the 1H MAS NMR spectrum of the MFI particles indicates the presence of organic groups in the framework (0.9 ppm)28 as well as isolated silanol groups (around 4 ppm).28 13C CP-MAS NMR measurements were made on calcined 500 nm particles treated with 1-butanol, as-made 500 nm particles treated with 1-butanol, and as-made TPA-containing 500 nm particles (Figure 4). The use of large particles allows us to minimize the relative contribution from external surface sites to the NMR signal. The spectrum in Figure 4a shows chemical shifts at 63, 35, 20, and 13 ppm, corresponding to the R, β, γ, and δ carbons of the 1-butanol molecule, respectively.28 The spectrum in Figure 4c shows chemical shifts at 63, 16, and 11 ppm, corresponding to R, β, and γ carbons of the TPA cations in the as-made sample.62 The splitting of the peak corresponding to the γ carbon is due to the interactions between these carbon atoms with the two types of channels in MFI structure.62 The spectrum in Figure 4b shows that the chemical shifts of the as-made TPA-containing material treated with 1-butanol are identical to those of the untreated as-made material in Figure 4c, and hence, there is negligible signal from butyl groups on the external surface. These results provide clear evidence that, in the calcined 500 nm particles, almost all the 0.7 mmol/g SiO2 of 1-butanol loading is in micropores.

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Figure 5. (Top) Powder XRD patterns of as-made defect-free puresilica MFI crystals (black) and calcined defect-free pure-silica MFI crystals (gray). The inset is an SEM image of a large defect-free puresilica MFI crystal. The scale bar is 20 µm. (Bottom) 29Si CP-MAS NMR of as-made defect-free pure-silica MFI crystals.

To assess the hypothesis that the 1-butanol molecules are located at silanol defect sites in the MFI pores, we synthesized silanol-defect-free pure-silica MFI crystals via the fluoridemediated synthesis route44,63 and performed 1-butanol treatment on these crystals after calcination. Figure 5 shows the SEM image, powder XRD pattern and 29Si CP-MAS NMR spectrum of the as-made defect-free MFI particles. The crystal has a prismatic shape with a length of about 60 µm so that external surface effects are negligible. The XRD pattern shows that the structure is MFI. The 29Si CP-MAS NMR spectrum shows that the structure contains only Q4 silicon atoms within the framework and that there is negligible concentration of silanol defect sites (Supporting Information). The multiple peaks within the Q4 range are caused by distinct TO4 sites within the MFI structure.44 TGA results show that the TPA content of the asmade defect-free crystals is 0.71 mmol/g SiO2. After treating the calcined defect-free MFI sample with 1-butanol, only small weight losses were observed within the temperature range of 280-420 °C (∼0.04 mmol/g SiO2). This result clearly suggests that the 1-butanol molecules functionalizing the MFI particles (made by the conventional route) are coordinated with the silanol defect sites. The possibility of the alcohol hydrolyzing the SiO-Si bridges of the MFI framework is very low under the current reaction conditions. 1H-29Si cross-polarization experiments generate enhanced signals from silicon nuclei bonded to OH groups, since they

Cheng et al. are located in close proximity to protons.64,65 This can give further information regarding the nature of binding of the butanol molecules, e.g., whether they form covalent Si-O-C bridges by condensation with silanol groups or whether they only form a strongly hydrogen-bonded complex with the silanol defect. In the literature, the formation of Si-O-C bonding on MFI nanoparticles was studied by methanol treatment and characterized by FT-IR and 13C CP-MAS NMR.34-36,66 As the concentration of methanol in the structure increased, the IR adsorption intensity of isolated silanol group (3800-3600 cm-1) decreased while that of the C-H stretching band (3000-2800 cm-1) increased simultaneously. In 13C CP-MAS NMR spectra, the formation of methoxy groups was supported by the appearance of chemical shifts at 48 and 49 ppm: the former caused by isolated methoxy groups, whereas the latter is indicative of hydrogen-bonded methoxy groups with nearby silanol groups. It is noteworthy that 29Si MAS NMR has rarely been applied to verify the nature of the bonding. Similar results were observed for mesoporous materials treated with various aliphatic alcohols,25,27,28,32,33,67 wherein 13C CP-MAS NMR was intensely applied to verify the presence of alkoxy groups on the surface of materials by the detection of a peak at 63 ppm from R carbon of alcohols. As we have already performed 13C CP-MAS NMR to verify the presence of 1-butanol molecules on MFI nanoparticles, we pursued this issue by using 29Si MAS and 29Si CP-MAS NMR techniques. To quantify the CP-MAS NMR signal, the peak intensity should be measured with an optimal contact time so that the magnetization can be effectively transferred from protons to silicon atoms.68,69 Test experiments indicated an optimal contact time of 5 ms to obtain spectra that allowed semiquantitative comparisons of the Q3/Q4 population ratios in the samples. The 29Si MAS and CP-MAS NMR spectra of calcined 200 nm particles before and after 1-butanol treatment are shown in Figure 6. The Q3/Q4 ratio derived from the 29Si MAS NMR spectrum is about 0.06 irrespective of the 1-butanol treatment. Thus, no information on the nature of the organic-zeolite bonding could be ascertained from these spectra. On the other hand, the Q3/Q4 ratio of the 29Si CP-MAS NMR spectrum from the untreated samples is much higher (>5), as expected due to the enhancement of the Q3 signal with cross-polarization. This ratio falls sharply to 0.56 after the 1-butanol treatment. These results strongly support the hypothesis that the 1-butanol molecules are covalently bonded to the MFI framework via a Si-O-R bond. The large reduction in the CP signal from the Q3 groups suggests a reduction in the proton concentration due to condensation with the silanol groups and loss of water, rather than the formation of a hydrogen-bonded complex. The reduction in Q3 signal is also consistent with the fact that the 1H29Si cross-polarization signal falls off rapidly as the sixth power of the distance between the Si nuclei and the nearest protons.70 It would thus be expected to decrease in intensity (as is indeed observed by us) when water molecules physisorbed at the defect sites are physically displaced by alkoxy (-O-R) groups and the silanol groups are chemically replaced by Si-O-R linkages. Similar results were obtained for 500 nm butanol-treated particles (Supporting Information). The observed Q3/Q4 ratio of 0.06 (corresponding to about 6 silanol groups per unit cell) is rather lower than the value of about 0.2 (corresponding to 16 silanol groups per unit cell) reported in some MFI materials that contain “silanol nest” defects.68 These are thought to be a result of missing Si atoms in the lattice, each such missing atom leading to a “nest” of four silanol groups belonging to the four surrounding Si atoms

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J. Phys. Chem. C, Vol. 112, No. 10, 2008 3549 TABLE 3: Organic Content of Alcohol-Treated Samples Exposed to High-Vacuum and High-temperature Environments particle size (nm) 500 200 100 100

treatment reflux with 1-butanol for 48 h reflux with 1-butanol for 48 h reflux with 1-butanol for 48 h reflux with 1-hexanol for 48 h

room temperature and 10 mTorr 0 days 2 days

200 °C and 10 mTorr 2 days

0.72

0.69

0.67

0.77

0.79

0.77

0.93

0.87

0.73

1.12

1.13

1.07

additional 2 days. The 1-butanol loadings after each stage were measured by TGA, and the results are summarized in Table 3. The 1-butanol loadings on the calcined particles (Tables 1 and 3) are comparable, indicating that the results are reproducible, as also illustrated in Figure 3. The organic loadings on 500 and 200 nm particles are slightly decreased (7% for 500 nm particles and 3% for 200 nm particles). For 100 nm particles, the organic loading decreased from 0.93 to 0.73 mmol/g SiO2. These results are interpreted as being consistent with the hypothesis that the remaining butoxy groups (∼0.7 mmol/g SiO2) are covalently bonded to the internal silanol defect sites. In the case of the 100 nm particles, the results also imply that the butoxy groups on the external surface were easily removed by a combined vacuum and thermal treatment so that the final organic loading corresponds to the value expected from only the presence of the functionalized internal surface sites. It is also interesting to note that 1-hexanol-treated 100 nm particles also demonstrate a good thermal stability based on the similar organic loadings measured after each treatment step, as shown in Table 3. However, we did not observe a substantial decrease in the organic loading after heating 1-hexanol-treated particles at 200 °C under vacuum for 2 days. The reason for this behavior is still unclear; however, it is consistent with our aforementioned suggestion regarding the formation of dimeric/oligomeric complexes or 1-hexanol molecules trapped in the MFI channels. Figure 6. (Top) 29Si MAS and (bottom) CP-MAS NMR spectra of 200 nm pure-silica MFI crystals with and without 1-butanol treatment: (a) 200 nm pure-silica MFI particles; (b) 200 nm pure-silica MFI particles treated with 1-butanol.

TABLE 4: Micropore Volumes and Organic Loadings of Alcohol-Treated Samples particle size (nm) 200

29Si

after calcination. On the other hand, our MAS NMR spectrum more closely resembles that reported by other workers,69 who interpret the defects in terms of uncondensed silanols. There is debate68-70 regarding the precise nature and concentration of the internal silanol defects, e.g., whether they mainly arise from silanol nests or from uncondensed silanols resulting from local disorder in the lattice, and whether or not they can be annealed upon heating/calcination thereby reducing the number of Q3 Si atoms. In the present study, we are specifically concerned with functionalizing MFI materials by exploiting the fact that these defects involve silanol groups, irrespective of their origin and precise structure. Thermal Stability and Micropore Volume Characterizations. The binding strength of the alkoxy groups was further tested by maintaining the samples under vacuum (10 mTorr), at room temperature for 2 days and then at 200 °C for an

100

treatment reflux with 1-butanol for 48 h reflux with 1-hexanol for 48 h

organic loading (mmol/g SiO2)

micropore Volume (cm3/g)a

0.7

0.115

1.03

0.075

a The measured micropore volume of calcined pure-silica MFI particles before R-OH treatment is 0.146 cm3/g.

With the hypothesis that alcohol molecules are covalently bonded to the internal defect sites within micropores, smaller micropore volumes of alcohol-treated samples should be expected in comparison to untreated calcined MFI. As shown in Table 4, the organic loading for 1-butanol-treated 200 nm particles after N2 physisorption is 0.7 mmol/g SiO2, or about 30% of the available pore volume. The micropore volume of the 1-butanol-treated sample is 0.115 cm3/g, which is about 21% smaller than that measured for the blank nanoparticles (0.146

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SCHEME 1: Aliphatic Alcohol Treatment on MFI Crystalsa

a (Top) 1-Butanol; (bottom) 1-hexanol; the gray circles indicate the internal silanol defect sites; the question mark shows the hypothesized dimeric/oligomeric 1-hexanol complex.

cm3/g). Measurements of p-xylene uptake yield a 30% decrease in pore volume for the 1-butanol-treated samples. For 1-hexanoltreated 100 nm particles, the organic loading after N2 physisorption is 1.03 mmol/g SiO2 and the micropore volume further decreases to 0.075 cm3/g compared with that of the 1-butanoltreated sample, resulting from the longer aliphatic chain length of 1-hexanol molecules. The micropore volumes obtained from the aliphatic alcohol treated pure-silica MFI particles obey the expected trends and support our assumption that the functionalization occurs primarily within the zeolite micropores (Figure 7). We have also made preliminary water adsorption measurements on the 1-butanol-functionalized materials, which show a 50% decrease in water uptake (in comparison to calcined MFI crystals) at room temperature and relative vapor pressures below 0.2. At higher pressures, the water uptake was progressively increased due to slow hydrolysis of the Si-O-C bonds and loss of the alcohol molecules from the material. More detailed water uptake studies on 1-butanol- and 1-hexanol-treated samples are underway to fully determine the degree of lowhumidity operation that may be required for application of these materials. In the present paper, we have focused on combining

Figure 7. Nitrogen physisorption on (a) 1-butanol-treated and (b) calcined 200 nm pure-silica MFI nanoparticles. The solid curve and the dashed curve represent the adsorption branches; empty squares and empty diamonds represent the desorption branches.

TGA, 13C/29Si MAS NMR, 29Si CP-MAS NMR, and N2 physisorption measurements to clearly demonstrate that aliphatic alcohol molecules can be covalently bonded to almost exclusively the internal silanol defect sites or to both the internal silanol defect sites and external silanol groups, as shown in Scheme 1. Conclusions The micropores of pure-silica MFI nanoparticles were successfully functionalized with 1-butanol and 1-hexanol by reflux treatment (“azeotropic distillation”) under neat conditions. This reaction condition is much milder than that used for previous functionalization studies with methanol. The 1-butanol-functionalized material was selected for detailed quantitative studies employing a range of MFI particle sizes. The combined interpretation of the characterization data from TGA, 13C/29Si CP-MAS NMR, and N2 physisorption, as well as theoretical estimates, supports the assertion that the 1-butanol molecules are mostly located within micropores and are covalently attached to the internal silanol defect sites. The 1-butanol loadings within the micropores are independent of the particle size, whereas significant 1-butanol loadings on the external surface can be observed when the particle size is smaller than 100 nm. The results for 1-hexanol-functionalized MFI are qualitatively similar. The functionalized materials also exhibit good thermal stability under vacuum. The successful functionalization with sizable organic groups makes the pore structure of the resulting hybrid material considerably different from the original zeolite. The current technique therefore shows potential as a way to create tunable zeolite materials with new functionalities for separations, catalysis, and other applications. Acknowledgment. This work was sponsored in part by ExxonMobil Research and Engineering. The authors gratefully acknowledge Johannes Leisen (Georgia Tech) for assistance with NMR measurements, Michael Hershkowitz and Prashant Vasudevan (Georgia Tech) for assistance with zeolite synthesis, Sebastian Reyes (ExxonMobil) for xylene sorption measure-

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