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PFG NMR Investigations of TPA-TMA-Silica Mixtures Xiang Li and Daniel F. Shantz* Department of Chemical Engineering, Texas A&M University, 3122 TAMU, College Station, Texas, 77843-3122, United States
bS Supporting Information ABSTRACT:
Pulsed-field gradient (PFG) NMR studies of tetrapropylammonium (TPA)-tetramethylammonium (TMA)-silica mixtures are presented, and the effect of TMA as a foreign ion on the TPA-silica nanoparticle interactions before and after heating has been studied. Dynamic light scattering (DLS) results suggest that silica nanoparticles in these TPA-TMA systems grow via a ripening mechanism for the first 24 h of heating. PFG NMR of mixtures before heating show that TMA can effectively displace TPA from the nanoparticle surface. The binding isotherms of TPA at room temperature obtained via PFG NMR can be described by Langmuir isotherms, and indicate a decrease in the adsorbed amount of TPA upon addition of TMA. PFG NMR also shows a systematic increase in the self-diffusion coefficient of TPA in both the mixed TPA-TMA systems and pure TPA systems with heating time, indicating an increased amount of TPA in solution upon heating. By contrast, a much smaller amount of TMA is observed to desorb from the nanoparticles upon heating. These results point to the desorption of TPA from the nanoparticles being a kinetically controlled process. The apparent desorption rate constants were calculated from fitting the desorbed amount of TPA with time via a pseudosecond-order kinetic model. This analysis show the rate of TPA desorption in TPA-TMA mixtures increases with increasing TMA content, whereas for pure TPA mixtures the rate of TPA desorption is much less sensitive to the TPA concentration.
’ INTRODUCTION Zeolite crystallization mediated by organic molecules has been intensely studied due to the importance of zeolites in catalysis, adsorption, and ion-exchange processes. A particular case is the synthesis of pure-silica ZSM-5 (silicalite-1) from a solution mixture containing a silica source (tetraethyl orthosilicate (TEOS)), a structure-directing agent (tetrapropylammonium hydroxide (TPA)) and water at relatively low temperatures ( 99%) was purchased from Fluka. Sodium hydroxide (NaOH, 99%) was purchased from BDH Chemicals. Ethanol (CH3CH2OH, > 99.5%) was purchased from Acros Organic. Deuterium oxide (D2O, 99.96% D) and sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) were purchased from Cambridge Isotopes and used as received. Sample Preparation. TMA-TPA-silica mixtures were synthesized as follows. NaOH was dissolved in deuterated water. After complete dissolution of the sodium hydroxide, TEOS was added to give a mixture composition of 9NaOH:40SiO2:9500D2O:160EtOH. This solution was stirred for at least 12 h to allow for full hydrolysis of TEOS and is hereafter denoted as C3. This mixture was chosen as it has been previously studied.21,22 The desired amount of TPABr and/or TMABr was added to the C3 solution to provide mixed organocation-silica mixtures with molar ratio of yTPABr: xTMABr (x = 0.125-1.0; for a given value of x, y was varied between 0.125-1.25). These mixtures are denoted as yTPA/ xTMA/C3. In these mixtures, sodium hydroxide was used to set the solution initial pH when varying the TAA cation concentration, and thus control the nanoparticle formation. To investigate the effect of hydrothermal treatment, TMA-TPA-silica mixtures were placed in screw cap Teflon containers and heated at 90 °C. Aliquots were taken from the heated mixtures that were quenched at different time intervals. All samples used for NMR measurements were allowed to equilibrate for a minimum of 12 h before measuring. In addition, 0.5TPA/xTMA/C3 (x = 0, 0.5, 1.0, 1.5) samples were prepared as described above (but in protonated water) and heated at 90 °C for 2 weeks. The collected solids were removed from their mother liquid by centrifugation at a rate of 13 000 rpm, washed with distilled water until the pH was between 7 and 8, and then dried overnight at 40 °C before being subjected to further characterization. Analytical. Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku MiniFlex II (Cu KR radiation) in reflection mode from 2θ = 5 to 40° with a step size of 0.02° and 2 s per step. Thermogravimetric analyses (TGAs) were performed using a NETZSCH TG 209 with a heating rate of 5 K/min from 298 to 973 K under mixed O2/N2 environment. The O2 and N2 flow rates were 10 and 15 mL/min, respectively. Fieldemission scanning electron microscopy (FESEM) measurements were performed with a JEOL JSM-7500 microscope operating at 5 kV.
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NMR Measurements. All NMR experiments were performed on a Varian INOVA 500 MHz spectrometer equipped with a 5 mm broadband indirect detection probe and a z gradient coil (up to 32 G/cm). The temperature was regulated at 25 °C for all experiments, and the temperature calibration was performed using methanol. Chemical shifts are reported relative to an internal reference of DSS at 0 ppm. Approximately 500 μL of mixture was added to the 5-mm tube (Wilmad Labglass). The samples were allowed to thermally equilibrate for at least 15 min prior to analysis. 1 H PFG-NMR experiments were carried out with static samples using the bipolar pulse pair stimulated-echo pulse sequence (BPPSTE).48 The 90° pulse length was typically 7 to 9 μs. The gradient strength (g) was varied from 1 G/cm to 30 or 32 G/cm. The bipolar pulse gradient duration (δ) was 2 ms (1 ms duration of individual pulses), the gradient recovery delay (τ) 300 μs, the diffusion period (Δ) was 300 ms in order to attenuate the signal intensity to approximately 10% of the value obtained at 1 G/cm. At least 16 transients were collected for each increment step with a relaxation delay of 4s. The field gradient strength was calibrated by measuring the self-diffusivity value (Ds) of 10 mol % 1 H2O in D2O, Ds = (1.92 ( 0.06) 10-9 m2/s at 25 °C.49 For the BPPSTE pulse sequence, the resonance intensity (I) is related to the selfdiffusion coefficient, Ds by " # I δ τ ð1Þ ¼ exp -Ds γ2 g 2 δ2 Δ - Io 3 2 Where the quantity γδg is often referred to as K. From a suitable plot of the intensity versus K2, one can determine the self-diffusivity. The difussion-ordered spectroscopy (DOSY) analysis transfers the onedimensional (1D) NMR stack plots of attenuated spectra into twodimensional (2D) spectra with chemical shifts on one axis and the distribution of diffusion coefficients on the other (see Figure S1 and Figure S2 in the Supporting Information). Details of the DOSY processing algorithm are well documented.50-53 In this work, DOSY processing was used to resolve overlapping resonances and was performed using the Varian VNMR, VnmrJ operating system. The sweep width of the attenuated stack spectra was manually adjusted to approximately 3-4 kHz for data storage purposes. In the 1H dimension, the free induction decays (FIDs) were zero filled to 32 767 data points and processed with a decaying exponential apodization function equivalent to 0.2 Hz line broadening. One hundred twenty-eight complex increments were used in the diffusion dimension (plot of peaks corresponding to their diffusion coefficient values). Peak heights were utilized as signal intensity (I) in the analyses. When the experimental conditions were such that the signal-to-noise was poor (e.g., samples with small TAA cation concentration and long diffusion delay time (Δ)), the spectra were carefully examined to include only clearly identifiable peaks. Dynamic Light Scattering (DLS) Experiments. The DLS experiments were performed using a BIC ZetaPALS with a BI-9000AT correlator. The wavelength of the incident laser beam (λ) was 660 nm and the detector angle (θ) was 90°. In order to eliminate any dust, the tested samples were filtered using a 0.45 μm PES syringe filter (Corning Co.) prior to loading into the cuvette (VWR). For each sample, three measurements were performed, and the elapsed time was 5-10 min to ensure good signal-tonoise. The sampling and analysis were carried out in the self-beating mode. The delay-time increased from 2 μs to 100 ms and the measurement temperature was 25 °C. The intensity autocorrelation functions were analyzed with the non-negative constrained least-squares method (NNLS).54-56 NNLS fitting yields a particle size (d)/translational diffusion (Dt) distribution for polydisperse solutions from the following equation and the largest population is set to 100%. Dt ¼ 3850
1 τq2
ð2Þ
dx.doi.org/10.1021/la104648s |Langmuir 2011, 27, 3849–3858
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Figure 1. (Left) PXRD pattern and (right) TGA traces of solids obtained from 0.5TPA/C3 and 0.5TPA/xTMA/C3 mixtures (x = 0.5, 1.0) heated at 90 °C for 2 weeks.
Figure 2. FE-SEM images of solids synthesized from (left) 0.5TPA/C3, (center) 0.5TPA/0.5TMA/C3, and (right) 0.5TPA/1.0TMA/C3 mixtures. Scale bar is 1 μm in all images. where q = (4πn/λ) sin(θ/2) is the scattering vector and τ is relaxation time. The refractive index (n) of the solution was taken to be 1.33. Given the level of dilution in C3 background solution (approximately 1 vol % silica), particle-particle interactions can be reasonably neglected. The viscosity of the solutions is approximately that of deuterated water (1.097 cP at 298 K).
’ RESULTS AND DISCUSSION Materials Characterization. Mixed organocation-silica mixtures 0.5TPA/xTMA/C3 (x = 0.5, 1.0, 1.5) were heated at 90 °C to investigate the effect of TMA concentration on the growth of silicalite-1. Those mixtures were chosen as the dilute TAA content facilitates NMR measurements and the 0.5TPA/C3 mixture, i.e., in the absence of TMA, has been shown previously by our lab to form silicalite-1. After two weeks of heating, solid products were observed from the 0.5TPA/0.5TMA/C3 and 0.5TPA/1.0TMA/C3 mixtures, whereas the mixture with the highest TMA content (i.e., 0.5TPA/ 1.5TMA/C3) remained optically transparent, indicating the suppression of solid phase formation due to the TMA. These collected solids were further analyzed to verify phase purity. The PXRD data in Figure 1 (left) shows that the products obtained from TPATMA-silica mixtures are pure MFI-type materials. Figure 1(right) shows the weight loss curve of the products from 0.5TPA/xTMA/ C3 (x = 0.5, 1.0) and 0.5TPA/C3 mixtures versus temperature. For all the samples, approximately 12% weight loss in the temperature range 360-650 °C is observed due to the thermal decomposition of the organocation. The amount of the organic template corresponds
to four TPA molecules per unit cell, indicating the zeolite forms with the ideal unit-cell composition [(TPA)4(SiO2)96]. In addition, the two TPA-TMA-silica mixtures show a weight loss of approximately 2 wt % in the temperature ranges 25-350 °C. This weight loss is attributed to physically adsorbed water and organocation on the outer crystal surface. The results above indicate that silicalite-1 crystals can be synthesized from TPA-TMA-systems with limited TMA concentration.57 Figure 2 shows the SEM micrographs of silcalite-1 formed in pure TPA-silica mixtures and TPA-TMAsilica mixtures. All crystals are large (∼10 μm) with high length/ width ratio, consistent with previous reports on the growth of elongated flat crystals from synthesis mixtures with high H2O/SiO2 ratio and limited TPA concentration.58,59 Also the crystals formed in the mixed organocation systems are less uniform than those prepared from pure-TPA systems, indicating the possible influence of TMA on the crystal morphology beside the growth of silicalite-1. These TPA-TMA-silica mixtures were subjected to detailed measurements to study how the addition of TMA affects TPAsilicate interactions before and at the early stages of heating. DLS of TPA-TMA-Silica Mixtures. Figure 3 shows the particle size distribution of the mixed cation-silica mixtures and TPAsilica mixture heated at 90 °C for 24 h. The presence of primary particles with a diameter of approximately 6 nm was found in all the samples prior to heating and a slight increase in the particle size to approximately 10 nm is observed during heating period. Also the population of these heated primary particles remains high and stays stable for the first 24 h. These above observations 3851
dx.doi.org/10.1021/la104648s |Langmuir 2011, 27, 3849–3858
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Figure 3. Particle size distributions obtained from DLS data of organocation-silica mixtures prepared with deuterated water and heated at 90 °C for 24 h: (a) 0.5TPA/C3; (b) 0.5TPA/0.25TMA/C3; (c) 0.5TPA/0.5TMA/C3; (d) 0.5TPA/1.0TMA/C3.
are consistent with previous studies on TPA-TEOS-H2O systems upon hydrothermal treatment.1,2,23,60,61 1 H NMR of TPA-TMA-Silica Mixtures. Previous work from our lab has investigated the silica nanoparticle-TPA interactions in TPA-silica and TMA-silica mixtures.29 The aim of the present study is to quantify how the addition of TMA to the TPA systems affects the TPA-silica interactions and thus zeolite formation. TMA was chosen as the additional cation because it is more hydrophilic and is incapable of making pure silica-zeolites from clear solutions, whereas TPA can direct silicalite-1 formation after
