16298
J. Phys. Chem. C 2010, 114, 16298–16308
Combined Application of Tracer Zero Length Column Technique and Pulsed Field Gradient Nuclear Magnetic Resonance for Studies of Diffusion of Small Sorbate Molecules in Mesoporous Silica SBA-15 Amrish R. Menjoge,† Qinglin Huang,‡ Bendaoud Nohair,§ Mladen Eic,‡ Wei Shen,| Renchao Che,| Serge Kaliaguine,§ and Sergey Vasenkov*,† Department of Chemical Engineering, UniVersity of Florida, GainesVille, Florida 32611, Department of Chemical Engineering, UniVersity of New Brunswick, Fredericton, New Brunswick, E3B 5A3, Canada, De´partement de Ge´nie Chimique, UniVersite´ LaVal, Que´bec, G1 V 0A6, Canada, and Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Laboratory of AdVanced Materials, Fudan UniVersity, Shanghai, China ReceiVed: June 25, 2010; ReVised Manuscript ReceiVed: August 25, 2010
Tracer zero length column (TZLC) and pulsed field gradient (PFG) NMR techniques were used to study self-diffusion of toluene in two samples of SBA-15 silica. Analysis of the diffusion data allowed us to assign evaluated diffusivities to diffusion of toluene in pore systems of SBA-15 particles. It was observed that there is a large discrepancy between the values of the diffusivities obtained by the two techniques under very similar experimental conditions. The most likely reason of this discrepancy is related to the particular morphology of the SBA-15 particles, which form stringlike aggregates with lengths close to 20-30 µm. As a consequence of the formation of such aggregate structures some mesoporous channels can be as long as the length of these structures. These long channels are expected to have transport barriers at the points of intergrowth of primary particles. Other channels are much shorter and are not expected to exhibit any significant transport barriers. Under the experimental conditions used TZLC measurements are most sensitive to sorbate diffusion in the channels of the former type. At the same time, PFG NMR measurements are expected to be more sensitive to diffusion in the channels of the latter type where T2 NMR relaxation times are not shortened by the presence of multiple transport resistances. As a result, these two techniques can provide complementary diffusion data on sorbate transport in SBA-15 materials. 1. Introduction 1
Mesoporous silica SBA-15 attracts significant interest of the research community due to the possibility to use this type of mesoporous materials in catalysis and separations as well as for selective adsorption and immobilization of biomolecules.2-5 SBA-15 exhibits two-dimensional hexagonal p6mm symmetry with monodispersed cylindrical mesopores, which can be tailored to have diameters in the range between 4 and 22 nm. SBA-15 materials possess some intrawall porosity. Depending on the synthesis conditions, the type of the porosity can change from a corona of micropores to small mesopores that provide connections between the main mesoporous channels.6 Properties of sorbate transport in SBA-15 are quite important for the majority of the potential applications of these materials. Despite their importance, these properties have not been studied as much as those of zeolites. A previous study7 is one of a few recent investigations of molecular diffusion in SBA-15 materials. In this work the zero length column (ZLC) technique was applied to study diffusion of n-heptane in SBA-15 samples. Because of the limitation of this technique requiring that diffusion measurements are performed in the limiting case of low sorbate loadings, the data reported in this work corresponds, * To whom correspondence should be addressed: Phone: +1 352 392 0315. Fax: +1 352 392 0315. E-mail:
[email protected]. † University of Florida. ‡ University of New Brunswick. § Universite´ Laval. | Fudan University.
for the most part, to the surface diffusion of n-heptane along the walls of mesoporous channels. A high content of micropores in these walls is expected to lead to the resemblance of the diffusion process along/inside the mesopore walls to that in pure microporous materials. This expectation was found to be consistent with relatively low values of n-heptane diffusivities obtained by ZLC (10-14-10-13 m2/s at temperatures close to 300 K).7 It was observed that a decrease in the micropore content results in an increase in the measured diffusivities. Among microscopic techniques used to study transport properties of porous materials pulsed field gradient (PFG) NMR was shown to be particularly useful.8 This technique was recently applied to study self-diffusion of benzene and nitrobenzene in the pore systems of SBA-15.9,10 To exclude contribution of sorbate diffusion in the gaps between the SBA-15 particles to the recorded diffusion data, the PFG NMR measurements were performed at sufficiently low temperatures when the sorbate located between the SBA-15 particles was frozen. The diffusivities along the main mesoporous channels measured by PFG NMR for benzene and nitrobenzene were found to be several orders of magnitude higher than those obtained by ZLC for n-heptane.7 In particular, the PFG NMR measurements provided values around 10-10 m2/s for diffusion of nitrobenzene along the SBA-15 channels at 253 K. In contrast to the ZLC measurements, the PFG NMR studies were performed under conditions when the SBA-15 channels were completely saturated by the liquid sorbate. Under such conditions surface diffusion is not expected to play a dominant role in the overall diffusion
10.1021/jp105882s 2010 American Chemical Society Published on Web 09/10/2010
Combined Application of TZLC Technique and PFG NMR process because at any given time only a small fraction of sorbate molecules is located next to the mesopore walls. At the same time, as discussed above, surface diffusion is likely to be the main contributor to the diffusion process investigated by ZLC in ref 7. Such difference in the diffusion mechanisms certainly contributes to the large deviations between the diffusivities measured by the two techniques. Another contribution to this difference in diffusivities can arise from the fact that PFG NMR records self-diffusion coefficients (i.e., diffusivities in the absence of macroscopic concentration gradients), while ZLC measures transport diffusivities (i.e., diffusivities characterizing molecular fluxes under conditions of the existence of macroscopic concentration gradients). In this work tracer ZLC (TZLC),11,12 viz., a modified ZLC technique capable of self-diffusion measurements, together with PFG NMR were applied to study self-diffusion of the same sorbate in the same samples of SBA-15 under essentially identical measurement conditions. The data reported in this paper show the existence of large differences between the values of the sorbate diffusivities obtained by these two techniques for the same samples. This result, which at first glance appears to be quite surprising, is discussed in the paper. Although large discrepancies between sorbate diffusivities measured by microscopic techniques (such as PFG NMR) and macroscopic methods (such as TZLC) are well documented for zeolites;8 the existence and possible reasons of similar discrepancies for mesoporous materials are not well known. 2. Experimental Section Materials. SBA-15 mesoporous silica samples were synthesized using Pluronic P123 (EO20PO70EO20, BASF) as a structuredirecting agent and tetraethyl orthosilicate (Aldrich) as the silica source. Two samples of mesoporous silica SBA-15 were obtained following the procedure reported by Zhao et al.1,13 7.65 g of P123 was dissolved in 290 g of a 1.6 M aqueous HCl solution. Sixteen g of TEOS was then added to this solution dropwise under vigorous stirring and the synthesis was carried out for 20 h at a constant temperature of 35 °C. The obtained mixture was transferred to an autoclave for hydrothermal treatment (aging). The autoclave was kept at 80 or 130 °C for 24 h to form SBA-15 materials with different pore sizes. The resulting solids were filtered and washed 3 times with H2O. The product was dried under air at 80 °C. This was followed by calcination at 550 °C for 6 h with a heating ramp of 1 °C/ minute. The resulting calcined samples are referred to as SBA15-80 and SBA-15-130 where the last number after the dash indicates the temperature of aging in the autoclave expressed in °C, i.e., 80 or 130 °C. Characterization. Powder X-ray diffraction (XRD) patterns were recorded using a Bruker D4 X-ray diffractometer with nickel-filtered Cu KR radiation (λ ) 1.5418 Å). The tube voltage was 40 kV, while the current was 40 mA. Diffraction patterns were recorded with scan step of 0.02° for the values of 2θ between 0.5° and 5°. Transmission electron micrography (TEM) was obtained using a JEOL 2011 microscope operating at 200 kV. Scanning electron micrography (SEM) was recorded using a Philip-XL30 instrument operating at 20 kV. Nitrogen adsorption and desorption isotherms were measured at 77 K using a Quantachrome Autosorb1 volumetric adsorption analyzer. Prior to analysis, samples were activated by keeping them under vacuum at 523 K for 5 h. The BrunauerEmmett-Teller (BET) equation was used to calculate the surface area SBET from adsorption data obtained at P/P0 between 0.1 and 0.2. The total pore volume was estimated from the
J. Phys. Chem. C, Vol. 114, No. 39, 2010 16299 volume of N2 adsorbed at a relative pressure of P/P0 ) 0.99. Nonlocal density functional theory (NLDFT)14 analyses were also performed to evaluate surface area, pore size as well as micropore and total pore volume. The selected NLDFT kernel considers sorption of N2 on silica at 77 K assuming cylindrical pore geometry and the model of equilibrium isotherm based on the adsorption branch. Micropore volume was determined from NLDFT cumulative pore volumes estimated for pores 0.6. By use of the values of Deff/R2 and Deff/l2 (Table 2) obtained by matching the experimental ZLC curves with appropriate models (eqs 1-6), it is possible to estimate from above the diffusivities of toluene in the aggregates by substituting the values of the characteristic lengths of the longest aggregates representing around 30% of the sample volume for 2R and l. The resulting estimates are given in Table 2. It is important to note that these estimates from above for the toluene diffusivities reported in Table 2 are several orders of magnitude lower than the corresponding toluene self-diffusivity in liquid bulk at 298 K (2.8 × 10-9 m2/s).27 Figure 6 shows the PFG NMR attenuation curves measured at the same temperature and for the same toluene loadings in SBA-15-80 and SBA-15-130 as those used in the reported above TZLC studies. The measurements were performed for different diffusion times. In the presentation of Figure 6 the attenuation curves measured for different diffusion times have to coincide if the diffusion behavior and the corresponding sorbate diffusivities remain independent of diffusion time. Clearly, the data in Figure 6 show strong dependence on diffusion time. The pattern of changes of the attenuation curves with increasing diffusion time is similar to that observed for the transition from the intracrystalline to long-range diffusion regimes in beds of zeolite crystals.8 Thus, it can be assumed that the results in Figure 6 correspond to the transition between the following two types of diffusion: (i) intraparticle diffusion, which is expected for short diffusion times and represents diffusion inside uninterrupted pore systems of SBA-15 particles and aggregates, and (ii) long-range diffusion, which is expected for large diffusion times and represents diffusion for displacements larger than the
sizes of these particles and aggregates. It is important to note that the observed dependencies of the PFG NMR attenuation curves on diffusion time can be affected by the longitudinal NMR relaxation process if the time constant of this process (T1) for toluene is smaller than or comparable with the diffusion time. However, it was found that under our experimental conditions T1 g 4 s, i.e., much larger than the diffusion times used. Hence, the measured time dependencies of the attenuation curves were not perturbed by the T1 relaxation effects. The observed time dependencies of the PFG NMR attenuation curves are not affected by the transverse (T2) NMR relaxation process because the time intervals corresponding to this relaxation in the PFG NMR sequence were exactly the same for all attenuation curves measured for the same sample and temperature. Table 3 shows results of fitting of the attenuation curves in Figure 6 by eq 9. This equation corresponds to isotropic 3-dimensional diffusion of two ensembles of sorbate molecules. In addition to the diffusivities and fractions of sorbate ensembles, Table 3 also presents the corresponding values of the square roots of the mean square displacements (MSD). The MSD values were calculated using the Einstein relation
〈r2(teff)〉 ) 2nDteff
(11)
where n denotes dimensionality of the diffusion process (n ) 3 for three-dimensional diffusion). Ensemble 1 in Table 3 can be assigned to long-range diffusion based on the following considerations. It is seen that the fraction of this ensemble increases with increasing diffusion time and the root MSD values are in all cases comparable with or larger than the largest dimension (viz., characteristic length) of the SBA-15 aggregates in the studied samples. In contrast, the root MSD values of ensemble 2 are smaller than the characteristic length of the aggregates. However, these values are still larger than the characteristic diameter of the aggregates. Hence, this ensemble corresponds to intraparticle diffusion with some contribution from the diffusion in the gas phase between the particles. Such contribution is expected because some sorbate molecules can enter and exit the aggregates along the shorter dimension of the aggregates. Because of this contribution of the gas phase diffusion it is expected that true intraparticle diffusivities at 298 K are smaller than the corresponding values of D2 in Table 3. Obtaining more precise estimates of the intraparticle diffusivities at 298 K would require PFG NMR measurements for diffusion times much smaller than the smallest effective diffusion time used (1.9 ms). Under our experimental conditions measurements at teff < 1.9 ms were not technically possible. However, it was possible to perform PFG NMR studies at a lower temperature resulting in a reduction of the measured root MSD values for the same smallest diffusion time of 1.9 ms. The data obtained from such studies at 273 K are shown in Figure 7. It was verified that limitations imposed by short T2 NMR relaxation time
Combined Application of TZLC Technique and PFG NMR
J. Phys. Chem. C, Vol. 114, No. 39, 2010 16305
TABLE 2: TZLC Diffusion Data for Toluene at 298 K sample SBA-15-80 SBA-15-130
(Deff/l2)a for 1D diffusion (1/s) -4
9.4 × 10 6.4 × 10-4
Deff for 1D diffusion (m2/s) -12
