Hyperpolarized 129Xe NMR as an Alternative Approach for

Article pubs.acs.org/JPCC

Hyperpolarized 129Xe NMR as an Alternative Approach for Investigating Structure and Transport in Ordered Mesoporous Materials Prepared via Pseudomorphic Transformation Julia Hollenbach,† Christian Küster,‡ Hans Uhlig,§ Maximilian Wagner,∥ Bernd Abel,∥ Roger Glas̈ er,‡,§ Wolf-Dietrich Einicke,‡ Dirk Enke,‡ and Jörg Matysik*,† †

Institut für Analytische Chemie, and ‡Institut für Technische Chemie, University of Leipzig, 04103 Leipzig, Germany Institut für Nichtklassische Chemie e.V., 04103 Leipzig, Germany ∥ Leibniz Institut für Oberflächenmodifizierung, 04318 Leipzig, Germany §

S Supporting Information *

ABSTRACT: The stepwise pseudomorphic transformation of silica gel and porous glass into MCM-41 (Mobil Composition of Matter No. 41) has been investigated for the first time with hyperpolarized 129Xe NMR. The changes in structure were followed by changes in the line shape and chemical shift of the 129Xe NMR signals, whereby variable temperature (VT)-NMR and selective 1D exchange spectroscopy (EXSY) experiments revealed differences in the mechanism of the transformation, depending on the starting material.

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INTRODUCTION The pseudomorphic transformation was introduced by Galarneau et al.1 in 2002 as a new strategy for the synthesis of Mobil Composition of Matter (MCM-41,-48) type micelle templated silica (MTS), which, contrary to the classical template based silica gel condensation, allows for the shape control of the MCM-41/-48 material. The term is adapted from mineralogy, where the definition “pseudomorph” refers to a mineral having a particular outer shape (morphology) unrelated to the crystallographic space group as a result of the mineral undergoing various dissolution−precipitation reactions.2,3 The procedure has been applied to various starting materials, ranging from silica gels4,5 and zeolites6 to controlled pore glasses (CPG)7,8 and biogenic silica sources,9 whereby the starting porosity should be large enough to accommodate the volume expansion arising from the formation of the MCM-41/48 pore system. Additionally, the degree of transformation can be varied by the reaction conditions to achieve products with both the parent and the MCM-41/-48 pore system and thus opening the door toward hierarchically structured materials with well-defined morphology. During the synthesis, an amorphous silica species is transformed into an MCM-41/-48 structure by treatment with a surfactant solution under alkaline conditions, whereby the morphology of the starting material is maintained. The pore wall of the starting material is progressively dissolved in the basic solution and immediately precipitated due to the template effect of the structure-directing surfactant.5,7 Hence, the newly generated MCM-41/-48 porosity is integrated into the pristine pore walls of the starting system. For the synthesis of MCM-41 and MCM-48, the typical structure directing agent is a quaternary ammonium surfactant, e.g., cetyltrimethylammonium (CTA+). In the classic approach by Galarneau et al., © XXXX American Chemical Society

cetyltrimethylammonium bromide (CTAB) is dissolved in NaOH,1,3 whereby later strategies combine template and basicity within one molecule by using cetyltrimethylammoniumhydroxide (CTAOH), as described by Einicke et al.6,10 The detailed mechanism of the transformation is not entirely understood yet, although two general models have been proposed: (i) The core−shell model assumes the formation of the MCM-41/-48 system as a homogeneous layer at the surface or the core of the silica particle, growing toward the center of the sphere or vice versa with proceeding transformation.5,10 (ii) In the homogeneous dispersion model, the transformation is described via the formation of distinct MCM41/-48 domains, being homogeneously dispersed over the particle.10 The core−shell approach was developed based on the results from N2-sorption measurements and X-ray diffraction (XRD) of different transformation series, whereas the presence of distinct MCM-41/-48 domains was concluded from pulsed-field-gradient (PFG) NMR experiments.10 Furthermore, it is also assumed that the progression of a transformation depends on the size of the primary porosity and, thus, the ability of the starting material to accommodate the newly generated MCM-41/-48 phase and the volume expansion resulting from the transformation. In this context, also observations implying hybrid approaches based on the two main models have been reported in literature, e.g., the formation of domains at the surface of the particle, growing toward the core with proceeding transformation.7 A pseudomorphic transformation always involves changes in both the structure and transport properties of the materials. Received: May 8, 2017 Revised: June 18, 2017 Published: June 28, 2017 A

DOI: 10.1021/acs.jpcc.7b04365 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

NMR offers complementary information for the characterization of the pseudomorphic transformation and related materials. We also address both partial and completely transformed systems from different starting materials, aiming to investigate structure and transport properties of the samples. This data will be used to derive information about the mechanism of the transformation depending on the starting material. Finally, conclusions obtained from the HP−129Xe NMR experiments will be compared to the results obtained with standard methods, i.e., N2-sorption and SEM measurements.

So far, the products and transformation stages have been mainly characterized by “conventional” techniques such as N2sorption, Hg-porosimetry, X-ray diffraction, or electron microscopy. These techniques are used to follow and quantify structural and textural changes but they reflect a global or heavily averaged structural picture, making the recognition of local defects, features, or changes challenging.11 In addition, it is difficult to obtain information about changes in the transport properties and dynamic processes involved in the transformation procedure. To this end, PFG-NMR has already been employed to obtain a better insight into the transport properties.10,12 Galarneau et al. examined the diffusion off n-hexane in different samples prepared via pseudomorphic transformation to follow the changes in the mass transport properties of the materials, drawing conclusions about the internal structure of the intermediate products.12,13 A similar approach was used by Enke et al. to study the mechanism of the transformation of silica gel.10 PFG-NMR is a robust method to explore molecular diffusion in porous media. The information on the transport process is obtained via the attenuation of a measuring signal under the influence of an external magnetic field gradient. As the signal attenuation depends on the diffusivity of the fluid, it is only possible to distinguish between two adsorption sites within one sample if they have significantly different diffusion properties.14 In order to improve the characterization of both structural and transport properties and to complement the investigation of the mechanism, it is desirable to have a technique being able to study structural aspects as well as dynamics. In the last decades, 129Xe NMR has entered the world of materials science as an alternative approach for characterizing materials. The approach is based on the excellent environmental susceptibility of the chemical shift of the gas15−17 and has been used to examine a broad spectrum of porous materials, such as zeolites, silica gel, porous glasses, micelle templated silica, polymers, and modified systems.18−27 In general, the NMR parameters are correlated with the interactions of the gas, the symmetry of the voids, and also to the motion of the gas inside the material.28,29 129Xe NMR provides information about the pore structure, e.g., pore sizes and pore heterogeneity, and it can be used to probe the interconnectivity as well as dynamics of the voids. Additionally, PFG-methods can be applied to study the diffusion of the gas.30,31 Using hyperpolarized (HP-) 129Xe gas, the sensitivity can be enhanced by several orders of magnitude, allowing us to work with very low Xe concentrations in the sample.32 In this range, Xe−Xe interactions can be neglected providing direct access to the Xe-surface interactions. The chemical shift and the line shape of the Xe NMR signals are the main parameters to derive information about the pore structure of the sample. Due to the environmental sensitivity of the chemical shift, each Xe resonance corresponds to a distinct adsorption site in the material. In general, the chemical shift is increasing with decreasing pore size and there have been established several empirical models between the chemical shift and the pore diameter.18,33,34 Furthermore, the line shape is influenced by exchange processes and the heterogeneity of the adsorption site.35 To the best of our knowledge, 129Xe NMR studies applied to pseudomorphic transformations have not been reported in literature yet. Here, we aim to demonstrate that HP−129Xe

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EXPERIMENTAL METHODS Synthesis. Silica gel as starting material: LiChrospher 60 (Merck) was used as starting material for the synthesis of series A. CTAOH (cetyltrimethylamoniumhydroxide) as structure directing agent was generated via ion exchange (Amberjet 500 OH) of an 0.08 mol/L CTAB (cetyltrimethylamonium bromide) solution. Different transformation degrees were realized by using different amounts of CTAOH solution (Table 1). The reactions were performed in a Teflon bottle at

Table 1. Volume of CTAOH Solution, VCTAOH, and Molar Composition of Synthesis Mixtures for Pseudomorphic Transformation of LiChrospher Si 60 sample

VCTAOH/cm3 g‑1

SiO2

CTAOH

H2O

LiChr. 60 A-0.17 A-0.33 A-0.50 A-0.67 A-0.83 A-1.00

0 7 14 21 28 35 42

1 1 1 1 1 1 1

0 0.036 0.070 0.105 0.140 0.175 0.210

0 30.0 46.7 70.0 96.7 116.7 140.0

120 °C for 4 days followed by a thorough washing step. The materials were dried at 70 °C and calcined as follows: 200 °C for 1 h, 400 °C for 1 h, and 550 °C for 2 h (heating rate 5 K/ min). Controlled pore glass (CPG) as starting material: For the preparation of series B, a sodium borosilicate glass with a constitution of 70 wt % SiO2, 23 wt % B2O3, and 7 wt % Na2O was milled and fractionated into a grain class of 200−250 μm. Subsequently this material was annealed for phase separation at 590 °C for 24 h in a muffle furnace, followed by an acidic leaching step. The powder was stirred at 90 °C for 18 h in a 3 N HCl solution to remove the soluble phase, washed to neutral, and dried at 120 °C. To reveal the desired porosity and to purge the pore system, an alkaline treatment with 0.5 N NaOH solution at 30 °C was conducted for 6 h. Again the material was washed thoroughly and dried at 120 °C. For the generation of the MCM-41 system, a 0.08 molar CTAOH solution was prepared via the continuous ion exchange of CTABr inside an Ambersep OH 900 bed. For B-0.50, a slurry of 1 g of CPG and 21 mL of CTAOH was produced and transferred into a Teflon autoclave, while B-0.25 was transformed with 10.5 mL of reaction solution. The transformation took place in a 120 °C oven over 4 days, followed by a thorough washing step. Finally, both materials were dried at 70 °C and calcined as follows: 200 °C for 1 h, 400 °C for 1 h, and 550 °C for 2 h. Characterization. N2-sorption measurements of series A and B were carried out on an ASAP 2010 device from Micromeretics and an Autosorp iQ from Quantachrome at 77 B


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The Journal of Physical Chemistry C K. Prior to analysis, all materials were dried at 250 °C for 10 h under high vacuum to purge the pore system from physisorbed water and residues. Based on the isotherm, the specific surface area was determined by BET method (p/p0 = 0.05−0.20), the pore width distribution by DFT method, and the total pore volume was calculated at p/p0 = 0.995. The macropore structure of B-0.00 was analyzed according to BJH theory using the desorption branch of the isotherm, while the mesopores of the MCM-41 phase of series B were calculated with nonlocal density functional theory (NLDFT) applying equilibrium model (both branches). Hg-intrusion measurements were carried out using a Pascal 140/440 system from Porotec. The dried samples were filled into the measurement cells and low and high pressure measurements were conducted up to 4000 bar. Scanning electron microscopy (SEM) images were recorded using an ULTRA 55 microscope (Carl Zeiss SMT, Oberkochen) 1.5 kV with a SE2 detector. To look at the interior of the particles, the samples were slightly broken with a pestle prior to the fixation on a conducting carbon pad. HP−129Xe NMR Measurements. HP−129Xe was produced by spin exchange optical pumping (SEOP) in a home-designed polarizer, described elsewhere.36 All NMR measurements were performed on a 400 MHz spectrometer Bruker DRX400 equipped with a standard 5 mm BBO probe operated at 110.6 MHz for 129Xe. During the measurements, the HP−129Xe containing gas mixture coming from the polarizer was directly inserted into the sample tubes using a home-built gas-insertion cap. For each spectrum, 4−32 scans were accumulated with 9.5 μs (π/2)pulses and 20−50 s recycle delays. The variable and low temperature experiments were carried out using a PT temperature controller. The temperature inside the NMR coil was calibrated using a standard MeOH temperature calibration sample (Bruker). Selective 1D HP−129Xe exchange experiments were carried out at room temperature, using a Gaussian-shaped 90° pulse on resonance with the free gas. To prevent a flow effect of the streaming gas mixture, the gas flow was interrupted during each scan, using a home-designed solenoid valve system. All 129Xe NMR chemical shifts were referenced to the signal of the free gas (0 ppm). The absolute signal intensities between the different samples are not comparable as the filling height in the NMR tube and the polarization rate are not constant.

Figure 1. Room temperature HP−129Xe NMR spectra of the products of the stepwise transformation of the amorphous silica gel LiChrospher 60 into a MCM-41 material. The degree of transformation, going along with the content of MCM-41 inside the material, is increasing from bottom to the top (ns = 32; polarizing conditions: 79.8% He, 19.3% N2, 1.0% Xe, Tcell = 140 °C, Pcell = 1 bar).

LiChrospher 60, which is in good agreement with the data from nitrogen-sorption measurements (Supporting Information, S1). With increasing degree of transformation, the buildup of a sharp signal with pure Lorentzian line shape is observed (FWHM = 350−500 Hz), overlapping with a broad, asymmetric peak vanishing in sample A-0.67. The position of the sharp signal shifts from 90 to 85 ppm with increasing degree of transformation, and its’ narrow line shape is similar to other MCM-41 materials described in the literature.27,37 The shift of the signal is in agreement with the results of the nitrogen sorption measurements, indicating an increase in the pore width with progressing transformation (Table 2 and Figure 2a−d). Compared to the signal of LiChrospher 60, the maximum of the broad population is shifted toward lower ppm values and thus larger pore diameters, reflecting a material abrasion at the pristine pore walls during the transformation.5 This pore widening is a well-known effect for silica gels under hydrothermal conditions, being reinforced by the alkalinity of the surfactant solution.38 However, it is noteworthy that all spectra of the partially transformed samples are dominated by the sharp signal associated with the MCM-41 porosity, even at the early transformation stages, having a low content of MCM-41 phase. The low intensity of the signal from the starting pore system might occur due to an increased adsorption on the MCM-41 surface or by the transformation procedure itself. To investigate this effect in more detail, these results are compared to the spectra of a physical mixture of a MCM-41 material with similar pore size (dp = 4.5 nm) and LiChrospher 60 (Figure 3). The silica gel used for the mixtures originates from a different batch of LiChrospher 60, so that deviations in the mean pore size and the pore size distribution might occur, provoking differences in the 129Xe spectra of this compound. In the spectra of the physical mixture, the signals of both pore

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RESULTS AND DISCUSSION Transformation of LiChrospher into MCM-41 Material. Although the pseudomorphic transformation has been carried out using a multitude of different starting materials, the transformation of silica gel into MCM-41 is the most commonly established and studied system in material science.1,4,5 For that reason, the stepwise transformation of an amorphous silica gel was chosen as a starting system to explore the applicability of HP−129Xe NMR to this subject. This series will be denoted “series A” in the following, whereby the individual samples are labeled A-x, with x denoting the volume fraction of the transformation agent, going along with the content of MCM-41 phase in the material. The HP−129Xe spectra of series A are displayed in Figure 1 (spectra A−G). The starting material (Figure 1, spectrum G) shows an asymmetric signal at 90 ppm with a tailing toward lower chemical shift values and a line width of 2600 Hz. This peak shape reflects the broad pore size distribution of the C


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The Journal of Physical Chemistry C Table 2. HP−129Xe NMR and N2-sorption Texture Data of Series A and the Starting Material LiChrospher 60 HP−129Xe NMR data

N2-sorption data dp MCM/ nmb

BET/ m2 g−1c

δ129Xe/ ppm 91 90, 71d,e 92, 69d,e 92, 73d,e 90 88 85

sample

spectrum

MCM/ %a

LiChr. 60 A-0.17

G F

8

3.91

819 382

A-0.33

E

15

3.97

504

A-0.50

D

36

4.10

661

A-0.67 A-0.83 A-1.00

C B A

56 80 99

4.17 4.20 4.20

803 944 1170

fwhm/ Hzd,e 2600 970, 3298 1364, 2962 686, 2754 490 392 512

a

MCM-41 content of the material, determined via the pore volumes measures with N2 sorption. bPore diameter of the MCM-41 pore system, determined from N2-sorption by DFT-methods cBET surface area of the entire material, determined via N2-sorption d,e129Xe chemical shifts and line widths (full width at half maximum) as determined from the line shape analysis with MestreNova 10.0

Figure 3. Room temperature HP−129Xe NMR spectra of the pure samples and physical mixtures of LiChrospher 60 and MCM-41, containing 25%, 50%, and 75% of the latter sample (ns = 8; polarizing conditions: 80.9% He, 18.4% N2, 0.7% Xe, Tcell = 130 °C, Pcell = 1 bar).

to the silica gel although the adsorption sites are located in different particles might be due to the high surface area (1260 m2g−1) of the MCM-41 sample. Additionally, the broad, heterogeneous line shape of the LiChrospher 60 signal further decreases the overall intensity of this peak. In the spectra of the transformed samples, the broad signal already disappears at a MCM-41 content of 30% (sample A0.50). In the earlier transformation stages, the signal assigned to

systems can be detected clearly up to an MCM-41 content of 50%, whereas the mixture containing 75% MCM-41 solely shows the signal of the MCM-41 material. In the sample containing 25% MCM-41, the sharp signal is already around twice as intense as the broad signal associated with the silica gel, clearly demonstrating an adsorption preference for the MCM41 surface. The fact that Xe adsorbs to this surface rather than

Figure 2. (a) N2-sorption isotherms of series A, measured at 77 K. The isotherms reflect the transformation of a broad pore size distribution at larger pore diameter into an ordered mesopore structure, having a narrow pore size distribution. The desorption branch of the intermediate transformation stages shows a cavitation effect, indictated by the closing of the hystereris loop at P/P0 = 0.42. (b) Percentage of the parent pore system affected by cavitation (black) and the “free” pores of LiChrospher 60 (red) with increasing degree of transformation, indicated by the volume fraction of CTAOH. (c and d) Pore width distributions of series A, calculated from the N2-sorption data, using DFT methods. With progressing transformation, the mean pore width of the MCM-41 pores increases while the starting pore system is widened up. D


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Figure 4. (a) Schematic cross section of a silica gel particle during a progressing transformation. MCM-41 domains are distributed over the particle, where the new porosity blocks parts of the starting pore system. (b) Schematic 129Xe gas transport in a partially transformed system having a domain structure. The HP−129Xe gas is preferably adsorbed to the MCM-41 surface (solid arrows), either coming from the gas phase surrounding the particle or adjacent silica gel pores. In the MCM-41 domains, the initial porosity is only accessible via the MCM-41 pores, promoting the relaxation of the gas on the way through the ordered pore system.

Transformation of CPG into MCM-41 Material. To analyze a system with a larger difference in the pore size of the two pore systems and, thus, avoiding the overlap of characteristic 129Xe-signals, the partial transformation of a mesoporous CPG into MCM-41 has been studied. According to the chemical-shift−pore-size correlations, porous glasses with pore sizes in the large mesopore/small macropore range are characterized by a 129Xe chemical shift of 20−30 ppm, whereas the signal of a classical MCM-41 pore is expected around 70 ppm.33,39 Additionally, CPGs have a more uniform pore structure and different transport properties compared to silica gels.40 In the present study, a porous glass with a pore diameter of 53 nm (sample B-0.00) is partially transformed into the MCM41 phase with transformation degrees of 25% (B-0.25) and 50% (B-0.50). Preliminary HP−129Xe NMR results of this series have already been presented in a review article39 as an illustrative example to introduce the method. The room temperature HP−129Xe spectra of the individual samples are shown in Figure 5. Beside the adsorbent peak at 30 ppm, the starting CPG already shows a broad exchange signal

MCM-41 is significantly more pronounced compared to the peak in the physical mixture of equal or lower MCM-41 content. Hence, the predominance of the MCM-41 signal in this case might not only result from a general adsorption preference but also from the distribution of the MCM-41 phase in the material. The N2-sorption isotherms of the partially transformed samples (Figure 2b) reveal a cavitation effect, indicating that the pore entrance of some of the starting pores is blocked by smaller pores. This constriction induces a spontaneous evaporation of N2 at a relative pressure of 0.42 during the desorption procedure, resulting in the closing of the hysteresis loop between the adsorption and desorption branch of the isotherm.10,12 A detailed analysis of the isotherms shows an increase in the fraction of the pores affected by cavitation up to sample A-0.67, whereby the fraction of freely assessable pores is continuously decreasing. These findings are in good agreement with the results from mechanistic studies by Galarneau et al.12 and Einicke et al.,10 indicating that the transformation into MCM-41 occurs via the formation of distinct domains spread over the particle (Figure 4a). In these domains, the starting pore structure is only accessible for Xe via the surrounding MCM-41 pore system. The high adsorption affinity of the MCM-41 surface leads to an accumulation of Xe here rather than in the silica gel pore and, thus, also to the reduction of the gas diffusion into the starting pore system in the domain region (Figure 4b). Consequently, the number of 129Xe atoms in the constricted silica gel pores is reduced compared to the readily accessible pore, lowering the overall intensity of the signal compared to the physical mixture of similar MCM-41 content. Additionally, the effect of relaxation has to be considered, as the hyperpolarized 129Xe might already undergo relaxation and depolarization processes in the MCM-41 pores. The fact that the 129Xe NMR signal of the starting pore system is detected up to a MCM-41 content of 50% additionally supports the domain model rather than a core− shell approach, as the starting pore system in this case would be blocked for 129Xe similar to the pores inside a domain.

Figure 5. Room temperature spectra of the samples of the partial pseudomorphic transformation of a porous glass (B-0.00) into MCM41 with transformation degrees of 25% (B-0.25) and 50% (B-0.50). (ns = 16; polarizing conditions: 80.9% He, 18.4% N2, 0.7% Xe, Tcell = 135 °C, Pcell = 2 bar). E


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The Journal of Physical Chemistry C at 6 ppm, arising from a fast exchange between intra- and extraparticle Xe41 due to the large pore size of 53 nm at a particle size fraction of 250−500 μm. However, the chemical shift of the adsorbent signal is still in agreement with the value expected from the chemical-shift−pore-size correlation for CPG, so that the position of this signal is not yet affected by the exchange. The spectra of the partially transformed samples B-0.25 and B-0.50 show asymmetric adsorbent peaks at 69 ppm (B-0.25) and 75 ppm (B-0.50), respectively, whereby the asymmetry is more pronounced for B-0.25. The line shape of these signals implies the overlap of a sharp peak with a broad signal, forming the shoulder of the peak. Additionally, another broad signal with low intensity is observed close to 0 ppm. This signal is different from the exchange pattern observed in the starting CPG, though, no separate signal is detected whose chemical shift and line shape can be associated with the porosity of the starting material. In contrast to this, N2-sorption and Hgintrusion data clearly reveal the presence of large CPG pores in these samples (Figure 6a,b and Table 3). Consequently, it seems reasonable that the pseudomorphic transformation in case of a porous glass also induces significant changes in the transport properties of the material in terms of the accessibility of the starting pore system. These changes can be due to many reasons, e.g., an adsorption preference of the gas for MCM-41 phase, similar to the effect observed in the physical mixtures of LiChrospher 60 and MCM-41 materials. In this case, the gas directly diffuses to the MCM-41 phase integrated in the pristine pore walls, using the starting CPG pore system as “transport pathway”. In order to evaluate whether these modifications solely arise from presence of an ordered mesopore structure or the rearrangement during the transformation, a series of physical mixtures of a CPG with similar pore diameter (dp = 56 nm) and MCM-41 (dp = 3.9 nm) have been examined (Figure 7). These mixtures have the same MCM-41 volume fractions as the transformation products (25% MCM-41: B′-0.25, 50% MCM41: B′-0.50). In the mixtures, the signals of both CPG and the MCM-41 porosity are observed, whereby the chemical shifts correspond to the values measured in the pure materials. Similar to the system LiChrospher 60/MCM-41, the intensity of the MCM41 signal is several orders larger than for CPG despite a lower or equal MCM-41 content in the mixture. This can be traced back to the higher value of the specific surface area of 1260 m2 g−1 of this MCM-41 sample compared to 31 m2 g−1 for the CPG. According to Henry’s law of adsorption, the number of adsorbed Xe atoms and, thus, the number of spins contributing to the NMR signal increases with the specific surface area.42 Contrary to the transformed samples B-0.25 and B-0.50, exchange between free Xe and the CPG pores is not influenced by the presence of MCM-41 particles in the physical mixture. Thus, it can be excluded that the observations in the transformed samples solely arise from the adsorption properties of the two pore structures, and exchange effects due to structural rearrangements in the material have to be considered. In this case, the shoulders of the asymmetric adsorbent signals in B-0.25 and B-0.50 imply a fast exchange of the MCM-41 adsorption site, e.g., with adjacent CPG pores. A similar asymmetric pattern was observed by Zhang et al. for cocrystallized MCM-49/zeolite samples, who attributed the asymmetric signal to a fast exchange of 129Xe between the two

Figure 6. (a)N2 isotherms measured at 77 K and (b) pore size distributions determined by mercury intrusion for series B. The isotherms reflect the partial transformation of an in average terms macroporous material into an ordered mesoporous structure, having a narrow pore size distribution. The mercury intrusion measurements indicate partially homogeneous pore wall coverage by the MCM-41 layer due to a narrowing of the initial pore system as well as a reduced specific macropore volume.

porosities being more closely stacked compared to a physical mixture.43 While the arrangement of the different phases over particle in the cocrystallized sample is homogeneous, the distribution of the MCM-41 phase over the particle, going along with the transformation mechanism, has to be considered additionally in the case of a pseudomorphic transformation. If the exchange arises from fast exchange between the pores in statistically distributed domains, the CPG pores should be directly accessible for 129Xe, and a decrease in temperature might reveal the different adsorption sites. However, it has not been proven yet that the transformation of a porous glass solely occurs via the formation of these distributed domains, as observed for silica gels as starting material. Instead, several F


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Table 3. Texture Data Determined by N2-sorption and Hg-intrusion and HP−129Xe Chemical Shift Data of the Samples B-0.00, B-0.25, and B-0.50 129

sample

dp/nm

B-0.00 B-0.25 B-0.50

53,a 50b 4.1,a 45b 4.1,a 41b

2

‑1a

ABET/m g 67 327 596

Xe chemical shift/ppm

MCM/%

333 K

298 K

273 K

259 K

213 K

24.9 50

16, 6c 48 9 63, 6c

30, 6c 69,d 12 73,d 15

e 77d 80d

57, 12c 83d 84

95 97 97

a Determined from N2-sorption at 77 K (BJH dDesorption for B-0.00, NLDFT for B-0.25/50). bDetermined from Hg-intrusion at 303 K. cBroad signal with low intensity. dAsymmetric line shape with shoulder at lower ppm value. eThe spectrum of B-0.00 was not measured at 273 K.

HP−129Xe might already become depolarized before reaching the parent pore due to wall collisions on the way through the MCM-41 shell. To study the dynamical behavior of the gas in more detail and, thus, drawing conclusions about the distribution of the MCM-41 phase over the particle, variable temperature (VT)NMR experiments have been performed in the temperature range from 333 to 213 K (Figure 8). At 333 K, the shoulder of the adsorbent signal of B-0.25 and B-0.50 is significantly more distinct and the signal close to the resonance of the free gas appears more intense, having an increased line width. However, these effects are more pronounced in sample B-0.25 and indicate the acceleration of the exchange processes at higher temperature. Whereas this broad signal is not present anymore in the spectra of both samples at temperatures of 273 K, this exchange effect is still visible up to 259 K in the starting CPG sample (Supporting Information, S3). The adsorbent signal of samples B-0.25 and B-0.50 remains asymmetric at lower temperatures before turning into a sharp signal at 213 K (B-0.25) and 259 K (B-0.50), respectively, without the formation of a second population. The presence of a single, sharp signal indicates a change in the adsorption properties below 259 K, and it seems that Xe is not adsorbed to the starting CPG pores anymore. The same trend was observed in the low temperature spectra of the physical mixtures (Supporting Information, S4). As the asymmetric pattern of the

Figure 7. Room temperature HP−129Xe NMR spectra of the pure samples and physical mixtures of CPG and MCM-41 materials, containing 25% and 50% MCM-41 (ns = 8; polarizing conditions: 80.9% He, 18.4% N2, 0.7% Xe, Tcell = 130 °C, Pcell = 1 bar).

studies reported that the domain growth at the particle surface in these systems occurs faster than inside,4,7 indicating a combination of the core−shell approach and the domain model. In this case, the majority of CPG pores inside the particle is only accessible via the MCM-41 system, and

Figure 8. Variable temperature HP−129Xe NMR spectra of B-0.25 (left) and B-0.50 (right) in the temperature range between 333 and 213 K (ns = 32; polarizing conditions: 90.1% He, 9.3% N2, 0.6% Xe, Tcell = 140 °C, Pcell = 3 bar). The presence of the asymmetry of the adsorbent signal at low temperatures indicates an exchange process fast on the NMR time scale between the two areas of different pore structure inside the particle. G


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The Journal of Physical Chemistry C MCM-41 signal remains visible until a temperature where the CPG surface is not sampled by 129Xe NMR anymore, it is reasonable to assume that the transformed samples contain two different MCM-41 species. One species is affected by the exchange with surrounding CPG pores, representing the shoulder of the adsorbent signal, and the other species creates the sharp component of the peak not being influenced by exchange. However, a signal in the chemical shift range expected for the starting pore system is not observed at lower temperature. The absence of the signal arising from the starting pore system of the CPG is the case, e.g., when the particle is covered by an MCM-41 layer followed by distinct domains located toward the center of the sphere as it has been observed for some transformations of porous glasses described in the literature.5 Here, the remaining starting pore structure is located in the center of the particle. Hence, the hyperpolarized gas would already undergo relaxation on the way through the particle, preventing the detection of a signal attributed to the starting pore system. At the same time, the findings could also be explained by the presence of large MCM-41 domains, whereby the sharp component of the adsorbent signal corresponds to the MCM-41 pores in the inner part of the domains. Assuming this model, the CPG porosity should be directly accessible for 129 Xe, but since the gas has higher affinity for the MCM-41 surface, it is preferably migrating to the MCM-41 containing domains. In order to obtain a better understanding of the pore interconnectivity and the exchange processes and, thus, to derive information about the underlying mechanistic model, selective HP−129Xe exchange spectroscopy (EXSY) experiments have been performed with sample B-0.50. In general, the EXSY experiment is a robust tool to probe exchange and pore interconnectivity. Numerous 129Xe NMR studies of porous materials used the two-dimensional approach.44,45 However, the performance of a full set of 2D experiments with different mixing times is extremely time consuming.46 Thus, a conventional selective 1D approach was chosen in this work to limit the experimental time. In this experiment, the signal of the free gas was selectively excited, and the buildup of the adsorbent signal during the mixing time is observed. The spectra are displayed in Figure 9 and enable a qualitative description of the exchange processes. At the lowest mixing time of 0.1 ms, only a sharp signal could be observed, changing the line shape to the asymmetric pattern observed in the normal 129Xe NMR spectrum of the sample with increasing mixing time. This sharp signal corresponds to the sharp component of the asymmetric adsorbent signal, indicating that 129 Xe first diffuses into the MCM-41 species not affected by exchange. In the case of the homogeneously dispersed MCM41 domains, it seems reasonable that the signal corresponding to the CPG pore structure should be observed at least at the shortest mixing time, as these pores are freely accessible for 129 Xe and the exchange with the MCM-41 domains, indicated by the shoulder of the asymmetric signal, has not proceeded yet. However, this signal was not observed in the 1D EXSY spectra, confirming that absence of a distinct “CPG” signal is associated with relaxation effects most likely arising from the location of the porosity in the inner part of the particle rather than solely being caused by exchange effects.

Figure 9. Selective 1D EXSY HP−129Xe spectra of B-0.50, using mixing times in the range of 0−50 ms. The signal of the free gas was excited selectively, using a Gaussian-shaped 90° pulse on resonance. With increasing mixing time, the signal of the adsorbed 129Xe gas is building up due to diffusion of the excited 129Xe atoms to the adsorption sites in the sample. For the shortest mixing time of 0.1 ms, only the sharp signal at 73 ppm is observed, indicating the presence of the MCM-41 surface layer directly accessible for the gas (ns = 8; polarizing conditions: 80.9% He, 18.4% N2, 0.7% Xe, Tcell = 140 °C, Pcell = 1 bar).

Together with the data of the VT-NMR experiments these findings demonstrate, that, in the case of porous glass particles with a pore diameter of 50 nm as starting material, the partial pseudomorphic transformation occurs preferentially in the outer regions of the particle which can lead to the formation of a MCM-41 layer at the surface of the particle, followed by distinct MCM-41 domains inside the material (Figure 10a). Considering the nitrogen sorption isotherm, this layer is not dense because nitrogen can access it without hindering the CPG pores of that material. The model proposed here is in excellent agreement with scanning electron microscopy (SEM) images of the surface of crushed samples (Figure 11). In the area of the breaking edges it is possible to obtain an image of the inner parts of the sample, revealing the formation of the MCM-41 layer at the surface and the underlying domains. In the case of B-0.25 the layer is less dense and contains scattered larger glass pores, giving rise to the broad signal observed at room temperature and above close to the resonance of the free gas. The presence of the surface layer and distinct MCM-41 domains influences detectability of the CPG pores underneath this layer in two ways (Figure 10b). In these domains, the MCM-41 porosity is directly integrated into the pristine pore walls of the starting material, enabling a fast exchange. The time regime of this exchange is so fast on the NMR time scale that it is solely observed by line broadening and a slight shift of this MCM-41 component to lower chemical shift values, forming the shoulder of the asymmetric signal. Since the gas favorably adsorbs to the MCM-41 surface, the 129 Xe population in CPG pores around the domains is so low that the remaining signal is no longer detectable. The residual CPG pore system is located in the core of the particle; only accessible via these domains and the MCM-41 layer. It is very likely that the hyperpolarized gas is efficiently H


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Figure 10. (a) Schematic cross-section of the particles during a pseudomorphic transformation of a porous glass into MCM-41. The transformation proceeds via the formation of MCM-41 domains, whereby the domain growth is faster at the surface of the particle, leading to a layer of MCM-41 at the outer surface (A), followed by a zone consisting of distinct domains (B). The residual starting pore system is located toward the core of the particle (C). (b) Schematic 129Xe gas transport inside a partially transformed porous glass. The gas is favorably adsorbed to the MCM-41 surface layer (A), creating the sharp component of the adsorbent signal. In the area of the domains (B), the gas adsorbed to the MCM-41 pores is in fast exchange with the surrounding CPG porosity, generating the shoulder of the adsorbent signal. The initial porosity (C) is mainly accessible via zones A and B, whereby the hyperpolarized gas already undergoes relaxation during the migration through the particle.

phic transformation include the diffusion of the involved transformation solution through the particle. Diffusion processes in porous samples are not only determined by microscopic properties such as the pore structure and geometry but also by macroscopic properties, namely the shape and size of the particles.47 It has already been demonstrated that the transport of 129Xe in porous materials strongly depends on the bulk properties of the sample, affecting the chemical shift and line shape in 129Xe NMR spectra.41 Hence, we intend to study the influence of the particle size on the mechanism of the pseudomorphic transformation in more detail in future work.

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CONCLUSION HP−129Xe NMR has been applied for the first time to study the transformation of different materials into ordered MCM-41 structures. It was demonstrated that the change in the structure and transport properties of the materials can be followed by changes in the number, chemical shift, and line shape of the 129 Xe NMR signals. The effect of differences in Xe adsorption due to different surface areas and relaxation effects has to be considered for the evaluation of the results. In addition to conventional characterization techniques such as N2-sorption, HP−129Xe NMR provides the possibility to study transport processes and local heterogeneities, contributing to the experimental validation of the mechanistic models of the pseudomorphic transformation, which are still discussed controversial in materials research. For the small-pored silica LiChrospher 60, the transformation proceeds via the formation of distinct MCM-41 domains, blocking the initial porosity. In the case of a porous glass with small macropores, HP−129Xe NMR results revealed the presence of a higher amount of MCM-41 phase at the surface of the particle and distinct domains in the inner part, indicating a faster progress of the transformation at the surface of the particle. Thus, structural, dynamical, and mechanistic information can be obtained with one method, whereas different conventional techniques are necessary to explore the different aspects.

Figure 11. SEM images of the crushed particles of B-0.25 (A and B) and B-0.50 (C and D), revealing the formation of an MCM-41 layer at the surface of the particle. With progressing transformation, the layer becomes denser.

depolarized due to wall interactions on the way to the center of the particle, hence preventing the detection of the isolated CPG pores. Consequently, the mechanism of the transformation and, thus, the transport properties of the intermediate transformation stages are different from the transformation of a silica gel, occurring via distinct domains. These varieties might be due to the differences in the pore size of the starting material and, hence, the ability to accommodate the newly generated MCM-41 system.39 Another aspect which has not been investigated so far in the present literature about pseudomorphic transformation is the influence of the particle size. The CPG sample and the silica gel chosen in this study do not only differ in the pore size but also in the size of the particles: Whereas LiChrospher 60 particles have an average size of 15 μm, the CPG sample greatly exceeds this value, having a particle size of 200−250 μm. The dissolution and precipitation reactions during the pseudomorI


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Furthermore, the influence of exchange processes on the Xe signals offers a potential to explore the transport properties of partially transformed, hierarchically structured samples, being essential for the practical application of these materials, e.g., as column materials.5

129

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04365. N 2 -sorption isotherms of LiChrospher 60, XRD spectrum of sample A-1.00, VT HP−129Xe NMR spectra of both sample B-0.00 and the physical mixtures of MCM-41/CPG. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Prager for help with the SEM measurements. M.W. gratefully acknowledges support from an ESF fellowship. Furthermore, we would like to thank Prof. S. Berger for the critical discussion about the VT-NMR experiments.

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REFERENCES

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Hyperpolarized 129Xe NMR as an Alternative Approach for

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