Ozone Treatment: A Versatile Tool for the Postsynthesis Modification

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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Ozone Treatment: A Versatile Tool for the Postsynthesis Modification of Porous Silica-Based Materials Hrishikesh Joshi, Daniel Jalalpoor, Cristina Ochoa-Hernań dez, Wolfgang Schmidt, and Ferdi Schüth* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany

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S Supporting Information *

ABSTRACT: Facile synthesis of silica-based functional materials at low temperatures has remained a challenge in materials science. To this end, we demonstrate the use of a gaseous ozone stream, generated via an electric discharge method, as a versatile tool for the postsynthesis modification of silica-based functional nanomaterials. First, a parametric study is conducted with a mesoporous model material to obtain basic insights into the reaction of the organics with ozone. The study is then extended to a number of distinct silica-based inorganic materials. The scope of ozone treatment can be broadly classified into three categories: (a) elimination of templates or structure directing agents (SDAs) from materials with pore sizes ranging from 0.5 to 10 nm, (b) selective transformation of organic groups functionalized on the mesoporous silica, and (c) simultaneous elimination of intermediate polymeric shells and template from the outer shells to obtain yolk−shell type materials. Each material studied here requires different parameters (temperature, time, and concentration of ozone) depending on its physical and chemical properties which have been carefully examined. Overall, the study demonstrates the potential of ozone treatment in tailoring functional materials at low temperature and provides vital insights into the reaction of ozone with silica-based materials. The study shows that gaseous ozone treatment is not limited to only one type of materials but can be applied to many systems, and we are convinced that the methodology can be applied to a multitude of organic@inorganic systems way beyond the scope of materials presented here.

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INTRODUCTION Ozone is often used as an oxidizing agent in organic synthesis and water treatment, owing to its higher oxidation potential, longer lifetime, and unique addition reaction compared to other oxidizing agents.1,2 Molecularly, ozone is a bent molecule with a standard electrical potential of 2.07 V, and it can be generated by electric discharge (ED-O3) methods or UVassisted methods (UV-O3).1−3 Technological advancements have also enabled the generation of ozone plasma or air plasma (Plasma-Air/O3) by charging and accelerating the gas mixture in an electric field.4,5 Ozone generated from these methods primarily reacts in two different ways, first, via a direct oxidation process and, second, via an advanced oxidation process (AOP), involving radicals generated from the degradation of ozone.1,2,6 Mesoporous silicas, since their discovery,7−9 have garnered a lot of interest with regards to the challenges in synthesis and functional modifications for catalytic applications.10 The essential element of the synthesis is the addition of (soft)templates that leads to a co-operative assembly, typically resulting in ordered arrangements.11 Following synthesis, normally a thermal treatment under air is employed to obtain the corresponding mesoporous material by eliminating the organic groups. However, the silanol density of the resulting © XXXX American Chemical Society

mesoporous material is quite low, which imposes a limitation for postfunctionalization or impregnation.12 Synthesis of microporous materials such as zeolites also entails the use of organic groups called structure directing agents (SDAs), which are most commonly also eliminated by a thermal treatment. In addition to the decreased silanol density, thermal treatment inflicts certain changes to the porous materials such as pore shrinkage in mesoporous silica13 and formation of extra framework aluminum species (EFAl) in aluminosilicates14 which are undesirable in certain applications. Over the years, alternative routes have been developed to remove the template effectively. These methods provide high silanol densities and preserve the morphological or chemical aspects of the porous materials. These routes include, for instance, Soxhlet extraction, piranha treatment, acid catalyzed ethanol reflux, sulfuric acid treatment, and HNO3/H2O2 treatment.15−19 The limitations of these chemical approaches are that they may require long time, or multiple cycles, and they are ineffective for certain templates such as octadecyltrimethoxysilane (OTMS). Moreover, the use of concentrated acids raises Received: September 27, 2018 Revised: November 19, 2018

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DOI: 10.1021/acs.chemmater.8b04113 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

etry/differential scanning calorimetry coupled with mass spectrometry (TG/DSC-MS). On the basis of the aforementioned results, we then extend the use of ozone treatment to a range of materials from microporous materials (zeolites) to mesoporous materials with pore sizes of 4 and 10 nm. Further, ozone treatment is also employed to selectively oxidize functional mesoporous silica and to obtain yolk−shell type materials.

concerns for post-treatment washing of materials and disposal of used acidic mixtures. Techniques such as microwave irradiation, DBD plasma, and glow discharge plasma have also been used to remove template or SDAs from porous materials where the reaction occurs at lower temperatures, yielding high silanol densities.20,21 Conversely, these techniques involve the use of sophisticated equipment that makes the processes cost-intensive. In addition, these techniques involve the use of plasmas that contain a multitude of species, such as electrons, radicals, singlet oxygens, ozone molecules, and plasmons, which pose a challenge for deeper mechanistic insight. Oxidation of organic groups embedded within inorganic matrixes by UV-O3 or ED-O3 yields porous or functional materials.21−27 Specifically for UV-O3, absorption of 185 nm UV light generates the O3 molecules which dissociate into reactive species upon absorption of 254 nm UV light.25,26,28 However, this method is inefficient because, first, the lifetime of UV-O3 is short, second, the ozone generation cannot be controlled easily and, third, the scattering of UV light limits its applicability to surface reactions. On the other hand, ED-O3 generation can be controlled precisely from a flow of pure oxygen by varying the strength of the electrical discharge. Seminal work by the group of Lllewellyn et al. demonstrated the use of ED-O3 to remove the template (cetyltrimethylammonium bromide) from MCM-41 type materials and also addressed the challenges associated with UV-O3. They removed almost 85% of the template by redispersing the asmade material in water and thereafter treating it with ozone in the solution for 24 h, yielding a stable pore structure with high silanol density.25,26 However, a longer treatment with ozone compromised the pore structure to some extent. Heng et al. reported a study that showed the use of ozone to eliminate SDAs from tubular ZSM-5 membranes in a flow setup.27 Though these reports are limited in their scope, they have demonstrated the prospects of ozone treatment in material synthesis which we aim to explore and expand. Numerous accounts have been reported in the literature on the probable mechanisms of the oxidation of functional organic molecules such as alkenes, alcohols, and carbonyls by ozone.29−32 The widely accepted mechanism for ozone reaction with alkenes was proposed by Criegee et al. It involves the formation of a five-membered cyclic intermediate with ozone and unsaturated carbon atoms.29 However, very few reports provide insight into the oxidation of alkanes with ozone in the gas phase due to their kinetically limited interaction. Hamilton et al. proposed a probable reaction mechanism of ozone with alkanes in the gas phase which proceeds via stereoselective formation of alcohols and hence prompts the involvement of ozone molecules in the ratedetermining step rather than the free oxygen radicals.32 In addition, ozone reactions within the pores of porous materials suffer from mass transfer limitations, where several gas molecules may compete for the reaction. Furthermore, the interaction between the organic templates or structuredirecting agents with the inorganic network may play a crucial role in the reaction.17,27 Thus, we begin the exploration of scope with a model material to standardize the parameters required for the ozone treatment and also to gain insight into the reaction. This investigation is based on infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy which are complemented by other characterization techniques such as elemental analysis, N2 sorption analysis, and thermogravim-

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EXPERIMENTAL SECTION

Materials. Syntheses of silica-based materials with different morphological properties are described in detail in the Supporting Information.33−38 Each sample tested in this study has been abbreviated as described in Table 1. A schematic representation of the inorganic−organic hybrid materials is provided in Scheme 1.

Table 1. Description of the Sample Names 1 2 3 4 5 6

full description of sample

abbreviationa

Mesoporous silica - octadecyltrimethoxysilane Nano ZSM-5-tetrapropylammonium hydroxide Mesoporous silica - cetyltrimethylammonium chloride SBA - 15 - pluronic P123 SBA - 15 - thiol (SH) Solid silica @ resorcinol-formaldehyde @ mesoporous silica-CTAC

mS-OTMS n-ZSM-5-TPAOH mS-CTAC SBA-15-P123 SBA-15-SH S@RF@mS-CTAC

The sample name is appended with “oz” for indicating an ozone treatment, with “cal” for indicating that the sample is calcined, and with “reflux” for indicating a reflux treatment.

a

Ozone Treatment of Materials. Ozone treatment of these materials was performed in the setup as shown in Figure S1. Typically, 100 mg of as-made material was dried in a box oven at 80 °C overnight. The material was then placed in a 50 mL 3-neck roundbottom flask, and the inlets and outlet were connected as shown. Argon flow and oxygen flow were switched on with a flow rate of 250 mL/min at S.T.P. for both streams unless specified otherwise. Ozone was supplied by an ozone generator (Argentox, Ozone Generator G1) connected to an oxygen (O2) gas cylinder (≥99.5 vol %, Air Liquide). The applied voltage was gradually increased and set to 3.35 V. The temperature of the reaction was controlled by immersing the flask in an oil bath set at the required temperature and the time was varied accordingly. In some cases, the moisture in the system was increased by humidifying the argon flow by bubbling through water. After the ozone treatment, materials were either washed with ethanol (2 × 30 mL) or refluxed with ethanol (80 °C, 25 mL, 120 min). Subsequently, the solid material was dried in a box oven at 80 °C overnight. The argon flow was essential for generating a homogeneous mixture of gases, good flow profile, and also to vary the moisture content in the reaction vessel. It is important to note that rubber tubes were not used in the entire setup as they could get oxidized on contact with the ozone. Detailed lists of parameters tested for each sample are provided in the Supporting Information, and the final standardized parameter set for each material in Table 2. Characterization. Fourier-transform infrared (FTIR) spectra were measured in attenuated total reflectance mode (FTIR-ATR) with a Nicolet Magna 560 FTIR spectrometer using a MercuryCadmium-Telluride (MCT) detector. Samples were directly deposited on the ATR module. The spectra were collected with 32 scans at 4 cm−1 resolution. TEM micrographs were obtained with a Hitachi H-7100 microscope (100 kV). All samples were placed on a lacy carbon film supported by a copper grid. Samples were suspended in ethanol, followed by an ultrasonication treatment, and then deposited onto the grids. EDX mapping of samples was recorded on a Hitachi HD-2700 scanning transmission electron microscope operated at 200 kV equipped with an EDAX Octane T Ultra W EDX detector. Physisorption measurements were carried out on a B


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Scheme 1. Schematic Representation of the Various Types of Inorganic−Organic Materials Treated by Gas Phase Ozonea,b

a

In each case, the organic species involved in the oxidation reaction is highlighted along with its structural arrangement within the inorganic network. (a−c) mesoporous silica with varying pore diameter, (d) microporous materials (n-ZSM-5-TPAOH), (e) multilayered core−shell structure S@RF@mS, and (f) thiol functionalized mesoporous silica (SBA-15-SH). b#: Alternative methods (noncalcination/chemical routes) for the corresponding materials are known in the literature.17,33,37 °C unless the sample contained organic groups, in which case the activation was done for 10 h at 120 °C. Thermogravimetric analysis

Micromeritics 3Flex and ASAP 2010 instruments. Prior to the analysis, materials were activated under vacuum for at least 8 h at 300 C


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Chemistry of Materials Table 2. Standardized Reaction Parameters for Ozone Treatment of Materials 1 2 3 4 5 6

sample

f1/f 2a

Tpa

tma

O3a

mS-OTMS-oz6 mS-CTAC-oz2-reflux SBA-15-P123-oz2 nZSM-5-TPA-oz6 S@RF@mS-CTAC-oz3 SBA-15-SH-oz1

1 1 1 0.2 1 1

120 80 80 185 120 25

120 120 120 180 120 15

48 48 48 48 48 25

f1, argon flow (mL/min); f 2, (O3 + O2) flow (250 mL/min, constant); Tp, temperature (°C); tm, time of reaction (min); O3, concentration of ozone (g/Nm3). Nm3, normal cubic meter. a

Figure 1. (a) Thermogravimetric analysis (TG) of mS-OTMS, mSCTAC, and SBA-15-P123 under air with a heating rate of 10 °C/min within a temperature range of 50−900 °C. (b) Normalized ATR-IR spectra from 3350 to 1250 cm−1 of mS-OTMS after sequential ozone treatment at changing temperature, moisture content, and time. All spectra are normalized to the band of the Si-O-Si stretching at 1070 cm−1.

coupled with mass spectrometry (TG-MS) was carried out in a Netzsch STA 449 F3 Jupiter thermal analysis instrument connected to a Netzsch QMS 403 D Aëolos mass spectrometer. Approximately 5 mg of sample was heated in 40 mL/min gas flow (argon or synthetic air) with an additional protective flow of 20 mL/min of argon. The heating rate was 10 °C/min for a temperature range of 50−900 °C. Mass spectra were collected in scan mode or in multiple ion detection (MID) mode. Solid-state NMR spectra were recorded on a Bruker Avance III HD 500 XWB spectrometer using a double-bearing standard MAS probe (DVT BL4) operating at resonance frequencies of 125.8 MHz for 13C and 99.4 MHz for 29Si. Compressed air was used to spin the samples at a rate of 10 kHz. The experimental conditions for 13C CP MAS NMR were as follows: 2 s recycle delay, 36 000−72 000 scans, 1 ms contact time, and 3.3 μs 1H π/2 pulse. The 29Si MAS NMR spectra were measured using single π/6-pulses (2.0 μs) with a recycle delay of 30 s (7200−8000 scans). Doubling the recycle delay did not lead to any significant changes in the relative intensities of the lines assigned to the different Qn groups. High-power proton decoupling (spinal64 for 13C and cw for 29Si) was applied for all spectra. The chemical shift was referenced with respect to neat tetramethylsilane (TMS) in a separate rotor. CHN analysis (carbon, nitrogen, and hydrogen) was done using a microanalyzer (vario MICRO Cube) from Elementar. To ensure full combustion in the presence of phosphorus, V2O5 was added to the analysis.

Figure 1b shows the ATR-IR spectra of materials after the ozone treatment and subsequent washing with ethanol. The vibration bands from 3000 to 2800 cm−1 and 1800 cm−1 to 1680 cm−1 can be assigned to the −C-H and −CO stretching modes, respectively. At room temperature (r.t.), the reaction was quite slow (mS-OTMS-oz1), oxidizing only 9% of the template in 30 min, whereas, on increasing the temperature to 80 °C (mS-OTMS-oz2), over 70% of the template was removed as derived from the ratio of the intensities at 2880 cm−1. The time was then increased to 60 min (mS-OTMSoz3), which further decreased the intensity to a residual 10%. However, a closer look at the spectrum suggested that the carbon-containing molecules were not completely eliminated from the material but converted into carbonyl groups as evident by the bands at ca. 1725 cm−1. Therefore, the temperature and time were further increased to 120 °C and 120 min, respectively (mS-OTMS-oz6), to also eliminate these carbonyl species. As shown in the Figure 1b, the intensity of the carbonyl stretching band (1725 cm −1) decreased considerably, suggesting that the carbon species may have been removed effectively. The final material treated at 120 °C for 120 min (mS-OTMS-oz6) was characterized by 29Si MAS NMR (Figure S3), and the silanol groups were quantified. Elemental analysis of this sample revealed that only 5% of carbon remained in the material (Table 3, entry 2). Further increasing the reaction time did not improve the removal substantially, and an increase in temperature led to a rapid exothermic reaction, resulting in pressure build up. In an attempt to further improve the reaction, the concentration of ozone in the reaction vessel was increased by decreasing the flow rate of argon, which allowed for a longer reaction time as the overall flow rates were decreased. However, these parameters resulted in similar temperature rise as during the high temperature reactions. Therefore, the parameters best suited for OTMS were 120 °C and 120 min at 48 g/Nm3 of ozone concentration (Table 2, entry 1). The BET surface area of mS-OTMS-oz6 was 945 m2/g, quite comparable to that of a calcined sample (926 m2/g, as shown in Table 3, entry 3). Interestingly, resonance lines associated with T groups were not seen in the 29Si NMR for mS-OTMS-oz6 and the lines for Qn groups were well-defined showing the Q3, Q2, and Q4 distinctly (Figure S3). Explicitly, the concentration of silanol groups was much higher than that of organics, and hence, this material deems well suitable for postfunctionalization.

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RESULTS AND DISCUSSION In order to standardize the parameters required for ozone treatment, the mesoporous model silica material was synthesized using octadecyltrimethoxysilane (OTMS) as template. OTMS was chosen as the organic template for these investigations because, first, its thermal stability was found to be higher than that of other templates such as cetyltrimethylammonium chloride (CTAC) or block copolymers (P123) as shown in Figure 1a. Second, it consists of only carbon and hydrogen atoms in its hydrophobic segment and any subsequent changes to the hydrophobic chain upon reaction with ozone are easily monitored via spectroscopic tools. Third, the OTMS group is attached to the silica network by a Si−C bond as shown in Scheme 1a, unlike other templates that interact by supramolecular or electrostatic forces, making OTMS suitable for solid state analysis. Fourth, OTMS is not removed effectively by other methods reported in the literature17 to obtain silanol-rich-silica. Strong oxidizing mixtures (piranha solutions) were successful in removing the template; however, repeated cycles were required for effective removal (Figure S2). A sequential gas phase ozone treatment was performed on mS-OTMS (Figure 1b) for finding suitable parameters for eliminating OTMS while retaining or generating more silanol groups on the silica surface. D


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Table 3. Thermogravimetric Analysis, Textural Properties, Silanol Density, and Carbon Content of Porous Silica Materials Treated with Ozone (-oz) and Calcined under Air (-cal)c

1 2 3 4 5 6 7 8 9 10 11 12

sample

wt. ls. (%)b

mS-OTMS mS-OTMS-oz6 mS-OTMS-cal n-ZSM-5-TPAOH n-ZSM-5-TPAOH-oz6 n-ZSM-5-TPAOH-cal mS-CTAC mS-CTAC-oz2-reflux mS-CTAC-cal SBA-15-P123 SBA-15-P123-oz2 SBA-15-P123-cal

41.0 11.0 1.4 14.4 4.3 1.8 47.0 9.0 1.5 50.0 6.0 1.3

BET area (m2/g)b

pore size (nm)b

DFT pore size (nm)b

945 926

2.8 3.0

3.0 3.2 0.6 0.6

955 1136

2.9 2.9

3.9 3.7

711 749

10.0 8.6

12.0 11.0

micro. vol. (cm3/g)b

tot. vol. (cm3/g)b 0.732 0.769

0.125 0.131

0.217 0.251 0.909 1.076

0.051 0.035

1.163 1.146

C% 34.2 5.2 0.2 9.7 1.5 0.2 37.3 2.3 0.1 29.6 3.0 0.5

{Si}a 0.43

0.57

0.44

a

Silanol content ({Si}) = (2Q2 + Q3)/(Q2 + Q3 + Q4) from quantification of 29Si NMR. bTG weight loss (wt. ls.), BET surface area (BET area) determined from relative pressure between 0.1 and 0.25, BJH adsorption pore size (pore size), DFT pore size calculated with a cylinder geometry and a N2-cylindrical pores-oxide surface model (DFT pore size), t-plot micropore volume (micro. vol.), total pore volume (tot. vol.) calculated at P/Po = 0.95 and carbon weight % obtained from elemental analysis (C%). P/Po = 0.95 was chosen for pore volume calculations as mS-OTMS and mS-CTAC isotherms showed hysteresis above 0.95 from the interparticle pore adsorption. cAs-made materials are also included for comparison.

Figure 2. Spectroscopic investigation after sequential ozone treatment of mS-OTMS at 80 °C with equal flow rate of ozone and argon of 250 mL/ min and ozone concentration of 48 g/Nm3. (a) ATR-IR spectra from 3000 to 2800 cm−1 and 1800 cm−1 to 1600 cm−1 after 20, 40, 60, and 240 min. All the ATR-IR spectra are normalized to the band of the Si-O-Si stretching at 1070 cm−1. (b) Textural properties and carbon content after different times and temperatures. (c) Solid state 13C CP MAS NMR after 0, 20, 40 min. The symbols show the different regions in the 13C spectra. (d) Schematic diagram proposing probable products of the gas phase ozone reaction with disordered mesoporous silica prepared by using OTMS (mS-OTMS). The circular structure represents the pore structure within the inorganic material created by the organic template (OTMS). R group in the final state represents the residual carbon in the material.

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these factors, nano-ZSM-5 with a silicon to aluminum ratio (SAR) of 50 (n-ZSM-5-TPAOH) was synthesized as a model system.35 ZSM-5 zeolite shows a three-dimensional micropore channel system with pore diameters of about 0.55 nm. The TPAOH is located in the micropores in the as-made ZSM-5. Similar to the previous treatment of the mesoporous material, n-ZSM-5-TPAOH was treated with ozone at a temperature of 120 °C (n-ZSM-5-TPAOH-oz1 to n-ZSM-5-TPAOH-oz3) with increasing concentration of ozone passing through the reaction vessel. However, no change was observed in the characteristic vibrational bands of the TPAOH (Figure S8). This suggests that a temperature of 120 °C is not sufficient to induce a reaction between ozone and the organic species encapsulated in the zeolite channels. Therefore, the temperature was increased to 185 °C (n-ZSM-5-TPAOH-oz4) where a decrease in vibrational intensities of the organic groups was observed after 120 min. Increasing the concentration by changing flow rates (n-ZSM-5-TPAOH-oz5) and extending the total treatment time to 180 min (n-ZSM-5-TPAOH-oz6) removed basically all TPAOH from the micropores of the zeolite as shown in Table 3 (entry 5). The wide angle XRD patterns of a calcined sample and an ozone treated sample are identical (Figure 3), suggesting that the crystal structure of the

To extend the scope of ozone treatment over a wide range of materials, a deeper understanding of this reaction was essential. Thus, mS-OTMS was further subjected to spectroscopic investigation. For this investigation, a treatment temperature of 80 °C was chosen (Figure 2a) as the reaction at 120 °C did not allow for any mechanistic insights (Figure S4). The ATRIR spectra of mS-OTMS treated at 80 °C with increasing time (20 min to 120 min) showed a gradual decrease of −C-H vibration bands (at ca. 2880 cm−1) and an increase of carbonyl signals (at ca. 1725 cm−1) up to a treatment time of 60 min. This change can be attributed to the oxidation of hydrocarbon chains before they are removed. In order to gain further insight into these species, NMR spectroscopy was used (Figure 2c). A broad resonance line at 175 ppm was observed after 40 min which can be attributed to carboxylate groups (terminal acid groups or ester groups) on the carbon chain. Further, the resonance lines at 30 ppm which can be assigned to long aliphatic carbon chains in the template decreased in intensity and broadened significantly with time (0 min to 40 min). The line broadening is likely caused by the addition of hydroxyl groups on the aliphatic carbon chains and the intensity changes by the removal of the hydrocarbon chain.39 Concomitantly, the textural properties of these materials were assessed (Figure 2b). After a treatment for 20 min, no change in pore volume was observed and the change in carbon content was relatively small. This suggests that the majority of the carbon chains were still retained in the material. After treatment for 40 min, the pore volume increased to 0.3 cm3/g and the pore size to ca. 2.5 nm, suggesting that most of the hydrocarbon was eliminated, leaving behind short hydrocarbon chains. Overall, the sequential changes to mS-OTMS are shown in a schematic representation in Figure 2d. Further, to assess the effect of moisture/water on the reaction, an experiment was conducted where the argon flow was humidified by bubbling through water while keeping the remaining parameters constant (mSOTMS-oz11, mS-OTMS-oz16). The ATR-IR spectra showed a similar decrease in −C-H vibration bands; however, the carbonyl bands were slightly higher than in the corresponding case without water (Figures S5 and S6). This indicates that the presence of moisture hinders the ozone reaction, causing an ineffective oxidation that leads to the formation of aldehyde groups as confirmed by 1H NMR (Figure S7). When all results are collated, it can be inferred that, for effective ozone treatment, moisture should be avoided in the system as it makes the reaction inefficient. Furthermore, the formation of C-OH groups proceeds without the elimination/ segmentation of the hydrocarbon chains and the C−C bond cleavage begins only after the carbon chain is oxygenated (Figure 2d). Therefore, in principle, templates containing oxygen atoms may react at lower temperature or at faster rates and segment immediately. This hypothesis is elaborated later by studying different types of templates and SDAs. After the elimination of the organic groups, distinct silanol species are obtained in high concentration which can be used for postmodifications as confirmed by 29Si NMR spectra. Microporous Materials − Aluminosilicate Zeolite. Interaction of the structure directing agents (SDAs) or templates with the inorganic network is an important factor which is not investigated above. This interaction is further strengthened by thepresence of atoms like aluminum (Al) or iron (Fe).17,27 Typical examples for these materials are zeolites (aluminosilicates) which also have pore sizes below 1 nm. The narrow pores may cause mass transfer restrictions. Considering

Figure 3. (a) Wide angle XRD patterns of ozone treated (n-ZSMTPAOH-oz6) and calcined (n-ZSM-TPAOH-cal) zeolites.

material does not change during the treatment with ozone. The pore volumes of the samples are comparable, and they have identical pore sizes (Table 3, entries 5 and 6). Zeolites require higher temperatures for the reaction as the pore confinement and the arrangement of SDAs within the pore may limit the accessibility of the ozone molecules to the SDA. Further, a strong electrostatic interaction exists between the aluminosilicate network and the SDA which may require high temperature to remove the molecules. Ordered Mesoporous Silica (OMS). Use of ozone for removal of cationic tetraalkylammonium halide surfactants has been demonstrated previously.25,26 However, the approach required an ozone treatment in solution for over 15 h. As shown before (Figure 1b) H2O may hinder the associated oxidation process. Therefore, ordered mesoporous silica (mSCTAC) synthesized using CTAC was subjected to an ozone treatment similar to that used for mSiO2-OTMS. Though CTAC resembles OTMS in terms of its hydrophobic segment, F


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vibration bands decreased in intensity considerably, confirming the removal of the template. The silanol density was quantified by 29Si MAS NMR (Table 3, entry 11) which also showed presence of distinct silanol species (Q3 and Q2). The final product had a BET surface area of 711 m2/g, which is comparable to the surface area of the calcined sample (Table 3, entry 12). The DFT and BJH pore size distribution of the ozone treated sample were narrower and were centered at higher values (by 1 nm) than those of the calcined sample. The differences in pore size distributions were quite significant (Figure S12), showing that the ozone treatment preserves the morphological aspects of a material by avoiding pore shrinkage. A comparison of the different morphological aspects (particle size and pore arrangement) of mS-OTMS-oz6, mS-CTACoz2-reflux, and SBA-15-P123-oz2 are shown by the TEM images (Figure 4a−c). The pore structures of mS-CTAC-oz2-

a key difference between both materials is the interaction of the template with silica. OTMS is covalently bound to the silica network, whereas CTAC interacts via electrostatic interaction of its positively charged headgroups (Scheme 1a,b). Owing to the difference in interaction of the template with the inorganic network, it was expected that a lower temperature would be sufficient to either oxidize the template completely or add polar compounds on the hydrophobic segments, reducing its tendency to form micelles and subsequently allowing the removal of the template by washing. Therefore, mS-CTAC was subjected to an ozone treatment at 80 °C for 120 min and subsequent washing with ethanol (mSCTAC-oz1). The ATR-IR spectrum of the material after this treatment showed the presence of a template with prominent bands at ca. 2880 and 1225 cm−1 (Figure S9) which can be attributed to −C-H and −C-N stretching vibrations, respectively. Interestingly, a reflux treatment for 120 min with ethanol (mS-CTAC-oz2-reflux) after the same ozone treatment improved the removal significantly. Elemental analysis mS-CTAC-oz2-reflux revealed only 2% of carbon was present in this material (Table 3, entry 8). It is well-known in the literature that CTAC can be removed by a reflux treatment with acidic ethanol for 12 h17 but a treatment of nonacidic ethanol for 120 min does not affect the template as shown in Figure S9. The ozone reaction at 80 °C adds polar functional groups to the hydrophobic part which requires the reflux conditions (80 °C for 120 min) to break the micellar structure and extract the organics from the pores. A set of additional parameters was tested to improve the ozonolysis and also to circumvent the use of the reflux treatment (mS-CTAC-oz3, mS-CTAC-oz4); however, these changes did not improve the removal. This further suggests that the template−silica network interaction influences the ozonolysis, and species such as the tetraalkylammonium with its electrostatic interaction may have a detrimental effect on the reaction and prevent full template removal. The parameters best suited for mS-CTAC were a treatment at 80 °C for 120 min, followed by reflux for 120 min at the same temperature. The textural properties for mS-CTAC, mS-CTAC-oz2-reflux, and mS-CTAC-cal are given in Table 3, entries 7−9. Ordered Hexagonal Mesoporous Silica. Ozone may react at a faster rate with oxygenated organics as hypothesized earlier, and the reaction will be less exothermic.30−32 To investigate the parameters required for such type of materials, mesoporous silica with block copolymers (SBA-15-P123) was synthesized that consists of hydrophobic (polyethylene oxide) and hydrophilic (polypropylene oxide) segments (Scheme 1c). An ozone treatment for 120 min at 80 °C (SBA-15-P123-oz2) removed the template almost completely with only 3% carbon remaining in the silica material (Table 3, entry 11). In the case of SBA-15-P123, a reflux treatment was not required after the ozone treatment. This could be explained by the changes of the template during reaction. The polymer (P123) reacts with the ozone easily due to the highly oxygenated carbon backbone. During the reaction, the polymeric chains get broken down into smaller segments (oligomers) or get further oxidized via alcoholic and carbonyl groups. These oligomers/oxidized chains are easily removed by the washing with ethanol. Moreover, P123 is a neutral template which has weaker interactions with the silica network (unlike CTAC) and, hence, electrostatic interaction does not affect the reaction of ozone. The final material (SBA-15-P123-oz2) was further characterized by IR spectroscopy (Figure S10) where the C-H

Figure 4. TEM micrographs of ozone treated (a) disordered mesoporous silica (mS-OTMS-oz6), (b) ordered mesoporous silica (mS-CTAC-oz2-reflux), (c) SBA-type materials (SBA-15-P123-oz2), and (d) yolk−shell type material (S@RF@mS-CTAC-oz3). Inset images show the magnified micrographs.

reflux and SBA-15-P123-oz2 were highly ordered, unlike the pore structure of mS-OTMS-oz6 which can be characterized as a disordered pore structure. The N2 sorption isotherms for these materials and their corresponding BJH adsorption pore size distributions are shown in Figures S11 and S12. Functional Mesoporous Silica. All the aforementioned examples require strong oxidizing conditions which enable the elimination of a variety of organic molecules from weakly interacting templates to strongly interacting SDAs. However, ozone generated by an electric discharge method can also be tuned to selectively oxidize organic groups grafted on silica. Therefore, thiol functionalized SBA-15 was synthesized on high silanol density silica as described in the literature (SBA15-SH).37 A mild oxidation of this material at room temperature with an ozone concentration of 25 g/Nm3 for 15 min selectively oxidized the thiol groups (SBA-15-SH-oz1). The final material was characterized by TG-MS analysis (Figure 5a) where the total mass loss from the material changed from 10% to 11.3%. This change in weight loss may be attributed to the oxidation of R-SH. The coupled MS analysis shows the presence of thiol groups (SH+) in SBA-15SH and the presence of an SO2+ signal for SBA-15-SH-oz1. G


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rubber, as the ozone reacted with the polymeric network within the span of 120 min even at the outlet. This encouraged us to extend the application range of the ozone treatment and try to remove polymeric shells, which had been used as sacrificial templates to obtain yolk−shell type materials. Simultaneous oxidative elimination of an intermediate resin and the template from the outer shell would enable the synthesis of these nanostructures with high silanol density in the silica shells. In order to demonstrate this, a multilayered core shell structure was synthesized as described in the literature with some modifications.38 These nanostructures consisted of solid nonporous silica spheres, a resorcinol-formaldehyde resin shell, and a porous silica shell (S@RF@mS-CTAC) as the core, first shell, and second shell, respectively. An ozone treatment for 120 min at 120 °C (S@RF@mS-CTAC-oz3) removed the template and the polymeric shell simultaneously as shown in the TEM image (Figure 4d) with only 3 wt % of carbon remaining in the material as confirmed by elemental analysis. The ATR-IR spectra of the calcined and ozone treated samples are similar (Figure S15) and the TEM images (Figure S16) show that the cores have identical sizes, indicating that the resorcinol formaldehyde shell was oxidized effectively. EDX mapping of Si and O (Figure S17) shows that the intensity of O in the intermediate space is very low, confirming the removal of the resin, as the resin contains oxygen atoms in its polymeric network. The N2 sorption isotherms of S@RF@mSCTAC-cal and S@RF@mS-CTAC-oz3 are identical (Figure S18) which suggests that the textural properties of these materials are comparable. On lowering the temperature to 100 °C, the removal was ineffective, as the color of the sample was slightly brown, indicating the presence of polymer or coke within the structure.

Figure 5. (a) Thermogravimetric coupled mass spectrometric analysis of SBA-15-SH and ozone treated material SBA-15-thiol-oz1 under argon flow of 40 mL/min and a ramp rate of 10 °C/min for a temperature range of 50−900 °C. Ion current signals corresponding to m/z = 33 (SH+) and m/z = 64 (SO2+) are shown. Dashed curves represent signals from SBA-15-SH, and solid curves represent signals from SBA-15-oz1. (b) 13C CP MAS NMR spectra of SBA-15-SH-oz1 and SBA-15-SH. The solid line in the structure represents the silica network to which the functional groups are bound.

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CONCLUSIONS In summary, ozone treatment in gas phase can be used as a versatile tool in the postsynthesis transformation of various types of porous materials. The ozone treatment is easily controllable and allows for low temperature synthesis and modification of different types of porous materials. Mesoporous materials with different pore sizes and templates were successfully obtained without the use of high temperature thermal removal of templates. The treatment with ozone preserved/generated silanol groups on silica, which made the final materials quite suitable for postfunctionalization. For instance, disordered mesoporous silica with high silanol density (mS-OTMS-oz6) was obtained quite easily after a treatment with ozone for only 120 min at 120 °C. Most of the chemical leaching approaches17−20 reported in the literature were not as efficient and effective as ozone for the removal of this template. This material was taken as a model material to study the ozone reaction in some more detail with the aim to extend the treatment procedure to other materials. Two important observations obtained from the study were: (a) elimination of organic groups should be performed preferentially under conditions where moisture in the system is excluded, and (b) atoms which have an affinity to ozone, unlike saturated hydrocarbons, seem to react faster/preferentially with ozone. The latter was demonstrated by selective oxidation of functional groups grafted on silica. Specifically, an ozone treatment of thiol-functionalized silica (r.t., 15 min, 25 g/Nm3) led to the selective oxidation of thiol groups to SO3H. Furthermore, the treatment was also instrumental in the

Simultaneously, the figure shows the absence of thiol signals and SO 2+ signals in SBA-15-SH-oz1 and SBA-15-SH, respectively. This suggests that the thiol has oxidized either to SO2H or to SO3H groups. To identify the oxidized species, solid state 13C MAS NMR was employed. The α′, β′, and γ′ carbons of SBA-15-SH (Figure 5b) can be attributed to the carbons of the 3-mercaptopropyl group.40 The resonance line at 53 ppm for SBA-15-SH-oz1 shown in Figure 5b can be attributed to the carbon (γ) attached to the sulfonic acid group (SO3H).41 The carbon attached to the silicon atom (α) does not show any shift in comparison to SBA-15-SH (α′), whereas the β and γ show significant shifts due the oxidation of thiol group. Ozone treatment for 15 min with an ozone concentration of 48 g/Nm3 oxidized the carbon chain to carbonyl groups as identified from the ATR-IR spectrum (Figure S13). Therefore, a treatment for 15 min with 25 g/ Nm3 was sufficient to selectively oxidize the thiol group to sulfonic acid groups without affecting the remaining carbon chain. The N2 sorption isotherms (Figure S14) for SBA-15-SH and SBA-15-SH-oz1 are very similar, indicating no substantial change in the textural properties of both materials. Yolk−Shell Type Materials. One of the precautions stated in the description of the ozone setup was against the use of H


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synthesis of yolk−shell type materials (S@RF@mS-CTACoz3), as it simultaneously eliminated the intermediate polymeric shell and the template of the outermost layer. On the basis of parameters for each of the materials, it can be concluded that the interaction of the organic group with the network and the properties of the template are important parameters that dictate the temperature required for the oxidative transformations. Overall, these examples demonstrate the versatile nature of dry ozone treatment that can be used for the preparation of porous materials. This study lays a solid foundation for further investigations to gain deeper insights via in situ studies that may expand the use of ozone treatment beyond the scope presented here.

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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.chemmater.8b04113.

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Material synthesis, detailed description of ozone treatment, ATR-IR spectra of materials, N2 sorption isotherms, pore size distributions, and EDX mapping of yolk−shell materials (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 208 3062373. Fax: +49 208 3062995. ORCID

Hrishikesh Joshi: 0000-0001-8728-0495 Cristina Ochoa-Hernández: 0000-0002-3203-7137 Wolfgang Schmidt: 0000-0001-5166-1202 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Ozone treatment was performed by an ozone generator from the group of A. Fürstner (Max-Planck-Institut für Kohlenforschung) and is gratefully acknowledged. We would like to thank B. Zibrowius (Max-Planck-Institut für Kohlenforschung) for measuring 13C, 29Si solid state NMR spectra and also for the discussions on the mechanistic aspects. We would also like to thank N. Pfänder (Max-Planck-Institut für Chemische Energiekonversion) for EDX mapping of samples and P. Springer (Mikoranalytics Lab, Kolbe) for CHN analysis of the samples. H.J. would like to thank IMPRS-RECHARGE for financial support. D.J. thanks the IMPRS-SURMAT for financial support.

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Ozone Treatment: A Versatile Tool for the Postsynthesis Modification

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