Surface-Selective Solution NMR Studies of Functionalized Zeolite

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Letter pubs.acs.org/JPCL

Surface-Selective Solution NMR Studies of Functionalized Zeolite Nanoparticles Yulia Tataurova, Michael J. Sealy, Russell G. Larsen, and Sarah C. Larsen* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States S Supporting Information *

ABSTRACT: The surface chemistry of zeolite nanoparticles functionalized with the organosilane aminopropyldimethylmethoxysilane (APDMMS) was selectively probed using solution 1H NMR spectroscopy. The use of solution NMR spectroscopy results in high-resolution NMR spectra, and the technique is selective for protons on the surface organic functional groups due to their motional averaging in solution. In this study, 1H solution NMR spectroscopy was used to investigate the interface of the organic functional groups of APDMMS-functionalized silicalite nanoparticles (∼35 nm) in D2O. The pKa for the amine group of APDMMS-functionalized silicalite nanoparticles in D2O was determined using an NMR−pH titration method based on the variation in the proton chemical shift for the alkyl group protons closest to the amine group with pH. The resulting NMR spectra demonstrate the sensitivity of solution NMR spectroscopy to the electronic environment and structure of the surface functional groups. SECTION: Surfaces, Interfaces, Catalysis

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there are no reports of using these methods to study functionalized porous nanomaterials, such as zeolites or mesoporous silica. In porous aluminosilicate and silicate materials, it is particularly advantageous to be able to differentiate surface and bulk proton signals. These solution NMR methods applied to nanomaterial systems also have great potential for studies of environmental or biological interfaces involving nanoparticle surface processes. Porous nanomaterials,18−22 such as zeolites and mesoporous silica, have emerged as nanomaterials with new properties and many potential applications, in areas such as environmental catalysis,23 drug delivery,24,25 imaging,26−30 and other biomedical applications.31,32 While the large internal surface area of these materials has traditionally been exploited for applications in catalysis and ion-exchange, porous nanoparticles also have large external surfaces that can be functionalized for specific applications in biomedicine33−35 or adsorption.36−38 Characterization of the surface structure of the functionalized zeolite nanoparticles is critical in developing an understanding of the surface chemistry and the environmental and biological interfaces that result from applications of these materials. Information about surface structure and composition can also be used to design functionalized nanomaterials with specific applications. Nanocrystalline silicalite (∼35 nm crystal size), the purely siliceous form of the zeolite ZSM-5 with the MFI structure, was synthesized,22,39 functionalized40 with aminopropyldimethylmethoxysilane (APDMMS), and characterized by powder X-ray

uclear magnetic resonance (NMR) spectroscopy is a widely used analytical technique for investigating a broad range of chemical systems. Solution NMR methods are routinely employed to identify organic products of synthetic reactions and to probe structure in large biomolecules. When solution samples are not readily available, solid-state magic angle spinning (MAS) NMR spectroscopy provides structural insights for a range of solid systems, but the resolution is generally reduced relative to that of solution-phase NMR spectroscopy. Despite the decreased resolution, solid-state MAS NMR spectroscopy is often the NMR technique of choice for studying solid-phase nanomaterials. A useful strategy in nanotechnology is the functionalization of nanomaterials to tailor the properties for specific applications. Many characterization methods for functionalized nanomaterials present challenges associated with differentiating surface and bulk chemical species. Solution NMR techniques have been used to selectively probe the surface structure and composition of functionalized nanoparticles in colloidal solutions.1−6 The fast rotational motion of the organic functional groups on the surface of the nanoparticles in solution reduces the line broadening to the extent that NMR spectra of the surface functional groups can be observed using solution NMR techniques. For example, solution NMR studies of functionalized gold nanoparticles have been used to elucidate the organic substituents and ligand exchange reactions on gold surfaces.2,3,7−10 Not only does solution NMR provide excellent spectral resolution, but it is also selective for surface protons because bulk protons are not motionally averaged and therefore not observed in solution NMR spectroscopy. Solution NMR techniques have been used to study functionalized gold,5,11−13 semiconductors,10,14−16 and metal oxide nanoparticles,17 but © 2012 American Chemical Society

Received: December 1, 2011 Accepted: January 10, 2012 Published: January 20, 2012 425

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diffraction (XRD), scanning electron microscopy (SEM), zetapotential, thermal gravimetric analysis (TGA), and nitrogen adsorption isotherms. The external surface area of the assynthesized silicalite-1 (with the organic template still in the pores) was 91 m2/g. The external surface area was used to estimate the size of the silicalite crystals as 35 nm according to a previously derived formula,39 x = 3214/Sext, where x is the silicalite-1 crystal size in nm and Sext is the measured external specific surface area in m2/g assuming cubic crystals. The size was confirmed by SEM images. The characterization data are provided as Supporting Information. The functionalization of silicalite-1 with APDMMS is shown schematically in Figure 1. The organic functional group loading

Figure 2. Proton MAS NMR spectra (at pH = 7) of (A) silicalite-1 in D2O, (B) APDMMS-functionalized silicalite-1, and (C) APDMMS in D2O. The peak at ∼2.2 ppm is due to acetone, which was added as an NMR chemical shift reference compound.

of the organic functional groups on the nanoparticle surface. Only the surface protons are observed in the solution spectrum because the protons located in the bulk nanoparticle are not free to rotate in solution, and thus, the resonances are too broad to be observed in solution NMR spectra. The amine protons are not observed due to hydrogen bonding and exchange with the solvent. Table 2 lists the proton NMR spectral assignments. The top spectrum in Figure 2C is for APDMMS in D2O, and proton resonances due to the protons are observed in the NMR spectrum, as labeled. The proton signal from the methoxy group is observed at 3.34 ppm in the spectrum of APDMMS in D2O but is absent from the spectrum of APDMMS-functionalized silicalite because it is the leaving group in the surface-functionalization reaction. The spin−lattice relaxation rate (T1) was also measured for APDMMS and APDMMS-functionalized silicalite. The measured T1 values were 1.9 s for APDMMS and 0.8 s for APDMMS-functionalized silicalite. The T1 decreased for the surface-bound functional group, as expected due to the decrease in motion relative to the free APDMMS. Previously, Rivas-Cardon and Shantz measured T1 values for alkyltripropylammonium silica mixtures and found that T1 decreased with increasing silica concentrations in the mixtures of alkyltripropylammonium cations and silica.42−44 The proton chemical shifts for APDMMS vary with pH, with the protons closest to the amine group exhibiting the largest shifts. Protonation typically takes place on the time scale of diffusion; therefore, in the NMR spectrum, the resonance observed will be a weighted average of the chemical shifts for the protonated and nonprotonated forms. NMR−pH titrations have been reported in solution NMR studies such that the change in the chemical shift has been used to calculate the pKa value.45−49 The protons closest to the amine functionality in APDMMS experience different electronic environments depending on whether the amine is neutral or protonated. The variation in chemical shift with pH can be used to determine the pKa according to the following relationship

Figure 1. Reaction scheme depicting the reaction of the surface silanol groups with the APDMMS to prepare APDMMS-functionalized silicalite-1.

and % coverage were determined from TGA and are listed in Table 1. The 44% surface coverage was calculated assuming a surface density of 4 SiOH groups/nm2.41 Table 1. pKa Values Calculated from Fits to Proton NMR Data Collected As a Function of pH sample APDMMS in D2O APDMMSsilicalite-D2O

organic loading (mmol/g)a

%surface coverageb

pKa (from proton NMR data)c 10.55 (0.01)

0.27

44

10.51 (0.01)

a

Organic loading was calculated from TGA data using the organic weight loss in the range of 200−400 °C. b% Surface coverage was calculated by assuming a SiOH density of 4 SiOH/nm2. cThe pKa was calculated from eq 1 using δ(NH3+ form) = 2.98 ppm and δ(NH2 form) = 2.59 ppm and the experimental data. 1

H NMR spectroscopy was used to characterize the APDMMS-functionalized silicalite-1. For comparison, 1H NMR spectra were obtained for APDMMS in D2O and unfunctionalized silicalite. The solution proton NMR spectra of silicalite, APDMMS-functionalized silicalite, and APDMMS, all in D2O at pH ≈ 7, are shown in Figure 2A−C, respectively. The proton NMR spectrum of unfunctionalized silicalite is shown in Figure 2A, and only the solvent D2O peak is observed. No signals due to bulk or surface protons are observed for silicalite in D2O presumably due to the lack of free rotation for surface hydroxyl groups. The proton NMR spectrum of APDMMS-functionalized silicalite (Figure 2B) includes peaks due to the protons in the functional group, as labeled on the inset molecule. The proton NMR spectrum for the surface groups of APDDMSfunctionalized silicalite are observed due to the free rotation

δobs = 426

δ L + δ LH+ × 10 pKa − pH 1 + 10 pKa − pH

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Table 2. Proton Chemical Shift Assignments at pH = 7 in D2O resonance

chemical shift, ppm-APDMMS (relative integrated area)

chemical shift, ppm-APDMMS-silicalite (relative integrated area)

1 2 3 4 5

0.65 (1.0) 1.71 (0.99) 2.98 (1.0) 3.34 (1.2) 0.15 (3.1)

0.65 (1.0) 1.71 (1.0) 2.98 (1.0) 0.15 (2.9)

multiplicity

assignment

triplet quintet triplet singlet singlet

Si−CH2−CH2−CH2−NH2 Si−CH2−CH2−CH2−NH2 Si−CH2−CH2−CH2−NH2 −Si−OCH3 Si(CH3)2

hexadecyl amide at the air/water interface was found to be ∼0.5 pKa units lower than the pKa for related amides in solution. The lowering of the pKa was attributed to an increase in electrostatic repulsion at the interface and therefore a favoring of the neutral form of the molecule at the interface according to the following equilibrium. However, the amide group was near the surface, in contrast to the system reported here in which the APDMMS covalently binds to the surface through the silane end of the molecule. Therefore, in our study, the pKa does not change appreciably for the surface-bound APDMMS relative to the solution APDMMS. This is interpreted as an indication that the surface functional groups are oriented sufficiently far from the surface such that the electrostatic influence of the surface is negligible with respect to stabilizing the protonated or nonprotonated forms of the functional group. These results suggests that the properties of the covalently attached functional group on silicalite with respect to adsorption of an environmental contaminant or biological molecules can be tailored based on the solution-phase reactivity of the functional group. Furthermore, these results indicate that future NMR studies of the functional group interface with environmental contaminants or biological molecules will provide insight into the molecular structure of the interface.

where δL is the chemical shift of the basic form and δLH+ is the chemical shift of the protonated form. δobs is the measured chemical shift.45 The experimental chemical shift obtained as a function of pH can be fit to this functional form using a nonlinear least-squares method to obtain the fitted value of the pKa. Using this method, the pKa’s for the amine group of APDMMS in solution and APDMMS on the surface of silicalite both in D2O can be determined from the proton chemical shift. For the determination of the pKa, the protons on C3 will be used. The variation of the proton chemical shift of C3 versus pH is shown in Figure 3 along with the nonlinear least-squares

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EXPERIMENTAL SECTION Silicalite-1 Synthesis and Functionalization. Nanocrystalline silicalite-1 was synthesized according to the procedure described previously.13 The silicalite-1 was characterized by powder XRD (Siemens D5000 X-ray diffractometer with Cu Kα and a nickel filter) to determine the crystallinity. The surface area of the as-synthesized silicalite-1 was measured using the BET method on a Nova 1200 nitrogen adsorption instrument (Quantachrome). Approximately 100 mg of silicalite was dried overnight at 120 °C under vacuum. A seven-point BET isotherm was obtained, and the specific surface area was calculated for the sample. The silicalite was calcined at 600 °C for 12 h. The external surface of calcined silicalite (35 nm) was covalently modified with 6 g of APDMMS in 30 mL of toluene. A functionalized silicalite sample was prepared by refluxing for 48 h at 125 °C. Then, the reaction mixture was centrifuged, washed with water three times, and dried overnight at 80 °C. The functionalized silicalite-1 samples were characterized by 29 Si solid NMR and TGA to assess the functional group loadings (see the Supporting Information). Samples were prepared for proton solution NMR experiments by dispersing approximately 10 mg of functionalized silicalite-1 in 0.6 mL of D2O. Acetone was added as a chemical shift standard. The pH was adjusted by adding NaOH or HNO3 to the NMR sample tube. The pH was measured before and after the NMR experiment using a Corning 320 pH meter, and the average of the two values was used in subsequent data analysis. The pH reading obtained before and after each NMR

Figure 3. Proton NMR titration curve for proton 3 of APDMMS in D2O (solid circles) and APDMMS-functionalized silicalite-1 (open triangles). The solid lines represent nonlinear least-squares fits to the experimental data using a monoprotic titration model.

fits to eq 1 with δ(NH3+) = 2.98 ppm and δ(NH2) = 2.59 ppm. The fitted pKa values and corresponding errors are listed in Table 1. The pKa for APDMMS in D2O is 10.51(±0.01) compared to 10.55 (±0.01) for APDMMS-functionalized silicalite. The reported errors are the statistical fitting errors; however, a slightly larger error is expected due to uncertainty in the measured pH, which is estimated to be ∼0.1 pH units. Taking this into account, these results indicate that the pKa for the surface functional group and the free amine functional group are the same within experimental error. The pKa’s of functional groups at interfaces have been measured using many different experimental techniques, including contact angle measurements,50 titration of the surface,51 and sum frequency generation (SFG) spectroscopy.52 In these studies, the pKa was typically found to be lower at the interface relative to the solution, and this was attributed to increased electrostatic repulsion at the interface.51 In a study by Kumar and Oliver, amides at the air/water interface were investigated by proton NMR spectroscopy.51 The pKa for the 427

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experiment differed by