Communication pubs.acs.org/crystal
Investigation of High-Pressure and Temperature Behavior of Surfactant-Containing Periodic Mesostructured Silicas Manik Mandal,† Vincenzo Stagno,‡ Yingwei Fei,‡ and Kai Landskron*,† †
Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, United States
‡
S Supporting Information *
ABSTRACT: Surfactant-containing periodic mesostructured silica materials, namely SBA-16 and FDU-12, were studied under pressures between 1 and 4 GPa and temperatures between 100 and 400 °C. At 4 GPa crystallization of coesite can be achieved already at 200 °C. The mild transition of amorphous to crystalline silica is believed to be accomplished by the inbuilt hydroxyl groups present in the starting material. At 2 GPa the crystallization of quartz is accomplished at a temperature of 400 °C. Both quartz and coesite are obtained in nanocrystalline form.
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However, no studies are available for surfactant-containing PMS materials under high pressure and higher temperature. Temperature plays a significant role when combined with pressure in the transformation of amorphous to crystalline phase. It was of interest to us to understand the P−T behavior of such PMS materials. In this regard, here, we want to communicate the high-pressure and temperature behavior of assynthesized (surfactant-containing) PMS materials. Our main hypothesis was that the silanol groups of as-synthesized periodic mesostructured silica materials can facilitate crystallization (and as such phase transition) by the formation of mineralizing water during condensation. Because the silanol groups are homogeneously dispersed in the material, one would expect the same for the water. This could allow for the crystallization of the silica at milder pressure and temperature conditions compared to our previously studied periodic mesostructured silica/carbon composites. A periodic mesostructured silica/carbon composite does not have silanol groups because of the high temperature needed for carbonization of the carbon precursor infiltrated into the silicas. In addition, it was of interest how the surfactant can substitute carbon as a support to stabilize the mesostructure at high pressure. To test our hypothesis, we prepared SBA-16 silica in its assynthesized form, i.e. with Pluronic F127 inside the pores. This material has a body-centered cubic structure (Im3m symmetry), in which the pores are connected with each other to form a 3-D network with a unit cell parameter of ∼16 nm.11,12 This material was subjected to high pressure and temperature in piston-cylinder apparatuses (details can be found in the
ince the discovery of surfactant-templated periodic mesostructured silica-based materials,1 many efforts have been made to explore various applications of these materials such as use as adsorbent, as catalyst support, and for the immobilization of guest molecules.2 The initial motivation for their synthesis came from the fact that such materials would be able to fill the gap between ordered microporous (50 nm) materials and that they could be superior compared to zeolites for the cracking of petroleum because of their better mass transport properties.1 However, soon it was realized that due to the amorphous nature of the pore walls of such silica materials, they do not suffice in terms of the hydrothermal stability that is required for such applications. Only very recently, the first siliceous periodic mesoporous materials having crystalline pore walls were synthesized.3 Tsapatis, Ryoo, and Corma reported the first zeolitic alumosilicates with periodic mesopores.4−6 Our own group reported recently the first periodic mesoporous SiO2 materials with crystalline coesite and quartz pore walls.3,7 The periodic mesoporous quartz and coesite materials were synthesized by a high-pressure nanocasting route. Hereby, the mesoporous silica is filled with a carbon support before the high-pressure synthesis to produce a periodic mesostructured silica/carbon composite. The carbon has the role to stabilize the mesostructure at high pressure. After the high-pressure synthesis, the carbon is removed by oxidation in air. Previously, the high pressure behavior of surfactant containing periodic mesostructured silica (PMS) materials was studied by Tolbert et al.8−10 In their studies, it was shown that under high pressure PMS materials do not lose mesostructural order and the silica walls remain amorphous. © 2012 American Chemical Society
Received: October 1, 2012 Published: December 17, 2012 15
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Supporting Information). At a pressure of 2 GPa and a temperature of 400 °C applied for 6 h, the amorphous silica was converted into crystalline silica as seen from XRD pattern (Figure 1). All the reflections can be assigned to α-quartz.
Figure 3. FTIR spectra for (A) as-synthesized SBA-16 silica after P−T experiment at 2 GPa and 400 °C and (B) after calcination of sample at 550 °C under air.
Figure 1. XRD pattern (Cu Kα) for α-quartz synthesized from assynthesized SBA-16 at 2 GPa and 400 °C before calcination.
indicates that the silanol groups have been eliminated during the phase transformation and the produced water has likely catalyzed the crystallization. The calcined material was white, indicating that the degraded block-copolymers are removed by oxidation under air. A nitrogen adsorption isotherm at −196 °C was collected for the material after calcination at 550 °C under air (to burn out residual block copolymers). Material was found to be nonporous with a small BET surface area (∼10 m2 g−1). The measured surface area is consistent with the adsorption of nitrogen gas molecules to the surface of the nanocrystalline quartz. Apparently, the material lost its mesoporous structure during the high-pressure experiment, which is also confirmed by the featureless small-angle X-ray scattering pattern (SAXS) (see Supporting Information Figure S3). To determine the lower limit of the crystallization temperature, an experiment at 300 °C was carried out. The product material shows a featureless pattern in the XRD, indicating that the silica remains amorphous with no specific morphology according to TEM and SEM (see Supporting Information Figure S4). An additional experiment at 1 GPa and 400 °C was performed in order to determine the lower pressure limit for the crystallization of the materials. No crystalline phase was formed, as there was only a broad reflection at 2θ ∼ 23° in the WXRD pattern (see Supporting Information Figure S5). The SAXS data shows the loss of mesostructural ordering (see Supporting Information Figure S5). According to N 2 adsorption data the mesostructure was not retained and the material is essentially nonporous. In order to understand the effect of pressure on crystallization temperature for as-synthesized ordered mesostructured silica materials, the pressure was increased. It was observed that, with increasing pressure, the crystallization temperature was lowered significantly. At a pressure of 4 GPa and a temperature of 300 °C, it was possible to obtain pure coesite, one of the high pressure polymorphs of SiO2, as indicated by XRD (Figure 4). All the peaks in the XRD pattern can be assigned to coesite. Similar to quartz, no specific morphology was observed for coesite, as seen from SEM and TEM (Figure 4). IR spectroscopy of the product material after calcination under air showed bands at 1226, 1153, 1059, 1034, 791, and 680 cm−1 (see Supporting Information Figure S6). Bands above 1000 cm−1 are due to asymmetric stretching vibration of Si−O bonds, and bands at 791 and 680 cm−1 are due to Si−O−Si
SEM shows that the α-quartz consists of intergrown crystals (Figure 2). These quartz crystals have sizes of ∼0.2−2 μm, as seen from the SEM image.
Figure 2. SEM (left, scale bar = 10 μm), TEM image (right), and SAED (inset) for α-quartz synthesized from as-synthesized SBA-16 at 2 GPa and 400 °C before calcination.
TEM shows that the quartz is nonporous, as seen by the homogeneous electronic contrast. The selected-area electron diffraction (SAED) shows spots indicating that the particles are single crystalline (Figure 2, inset). Elemental analysis (EDX) studies for this material confirm that the material is composed of silica (see Supporting Information Figure S1). The FTIR spectrum for the product material before calcination shows bands corresponding to vibrations of the Si−O bonds of silica, as well as C−C and C−H bonds of the F127 block copolymers (Figure 3A). Bands at 1161 and 1068 cm−1 are due to stretching vibrations of the Si−O bonds, while bands at 798, 774, and 692 cm−1 are due to the deformation of the Si−O−Si bond angles.13 Bands at 1372, 1451, 2886, 2921, and 2955 cm−1 are due to the vibrational modes of C−C and C−H bonds from block-copolymers that remained in the material.14 The color of the material after P−T treatment was black by visual inspection. Presumably, the block-copolymers were already partially decomposed. This is additionally corroborated by examining FTIR for the same sample after calcination at 550 °C for 5 h under air. This sample showed bands for quartz and no bands associated with C−C and C−H bonds (Figure 3B). No bands for silanol groups were observed for this material (Figure 3). However, the starting material contains a band at 947 cm−1 which is due to Si−OH stretch vibrations (see Supporting Information Figure S2).14 This 16
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which is similar to the SBA-16 material after such treatment. The loss of mesostructual ordering for this material was also observed from the SAXS analysis. Lowering the pressure to 1 GPa and a temperature of 400 °C did not produce a crystalline phase, as seen from XRD, which is similar to the case of as-synthesized SBA-16 material after such treatment. Moreover, a pressure of 2 GPa and a temperature of 300 °C also did not produce any crystalline material (see Supporting Information Figure S11). While this material was mostly nonporous, some particles had signatures of mesostructural ordering, as seen from TEM images (see Supporting Information Figure S11). This could be related to the onset of crystallization of the silica material under the P−T conditions used here. When pressure was increased to 4 GPa and a temperature of 200 °C was applied, coesite was obtained (see Supporting Information Figure S12). This result is very similar to the case of as-synthesized SBA-16 silicas under similar P−T experiments. There is also an amorphous background indicating that the crystallization is not fully complete. The calcined material showed the presence of mesoporosity in some regions of the TEM image (see Supporting Information Figure S12). However, the electron diffraction (ED) pattern for this region showed no diffraction spots, indicating that the porous particles belong to the fraction of the sample which is not yet crystallized. On the basis of these results, it appears that the phase transition from amorphous to crystalline silica occurs within a temperature range of 300−400 °C when a pressure of 2 GPa is applied, while the temperature is only 100−200 °C at 4 GPa. The crystallization temperatures are much lower than those required for the crystallization of the siliceous phase in periodic mesostructured silica/carbon composites at the same pressures.7 The crystallization temperatures are even lower than those for periodic mesoporous silicas without surfactant. This allows for the conclusion that the presence of silanol groups, which are absent in silica/carbon composites and periodic mesoporous silicas that have been calcined at >500 °C, significantly lowers the minimal crystallization temperature. It can also be clearly seen from our study that the increase of pressure can subsequently reduce the crystallization temperature. In conclusion, nanocrystalline silica (α-quartz) can be synthesized at 2 GPa and a temperature of 400 °C from assynthesized periodic mesoporous silicas SBA-16 and FDU-12. At 4 GPa coesite can be crystallized from a temperature of 200 °C. This phase transformation is believed to be precatalyzed by hydroxyl groups that are present in the starting material. The mesostructure is lost during the high-pressure synthesis, which lets us conclude that the surfactant cannot take over the role of carbon as support for the preparation of periodic mesoporous silicas with crystalline walls.
Figure 4. XRD pattern (top, Mo Kα), SEM (bottom left, scale bar = 2 μm), and TEM image (bottom right) for coesite synthesized from assynthesized SBA-16 at 4 GPa and 300 °C before calcination.
bending vibrations.13 The product material is nonporous according to N2 adsorption data. Coesite was obtained even at a temperature of 200 °C (see Supporting Information Figure S7). This is the lowest temperature of crystallization for coesite reported so far. Previously, coesite was obtained from calcined (i.e., surfactant free) SBA-16 silica at a temperature of 300 °C, and at a much higher pressure of 12 GPa.15 Even at such high pressure (i.e., 12 GPa), no crystalline phase was obtained at 200 °C. Further lowering of the temperature to 100 °C did not yield any crystalline phase (see Supporting Information Figure S8). This allows for the conclusion that the minimal crystallization temperature of coesite from as-synthesized periodic mesoporous SBA-16 at 4 GPa is in the range between 100 and 200 °C. It is interesting to note that the as-synthesized SBA-16 material, treated at a pressure of 4 GPa and temperature of 100 °C, showed the presence of periodic mesoporosity, as seen from TEM image (see Supporting Information Figure S8). Some particles with no porosity were also observed. To evaluate the generality of the behavior of surfactantcontaining periodic mesostructured silicas at high pressure, additional experiments were carried out with as-synthesized large-pore FDU-12 silica (unit cell parameter ∼ 35 nm) as a starting material. This material was synthesized using Pluronic F127 block copolymers as a template and has a face-centeredcubic (Fm3m symmetry) structure.16 It was observed that, at a pressure of 2 GPa and a temperature of 400 °C applied for 6 h, the material was converted completely to crystalline quartz, as seen from the XRD pattern (see Supporting Information Figure S9). SEM shows that the quartz crystal sizes are in the range 0.2−1.0 μm. These crystals are intergrown and multifaceted in nature, as seen by SEM (see Supporting Information Figure S9). The material was further characterized by TEM, which indicates that the mesostructure of the material is lost (see Supporting Information Figure S9). SAED confirms the crystalline nature of the material (see Supporting Information Figure S10). The N2 adsorption isotherm for the calcined material shows no porosity and a negligible BET surface area,
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ASSOCIATED CONTENT
* Supporting Information S
Synthesis of starting material, experimental details, and characterization of materials. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +1-610-758-5788. Fax: +1 610-758-6536. 17
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported as part of EFree, an Energy Frontier Research Centre funded by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award #DE-SG0001057. We thank Lehigh University and Carnegie Institution of Washington for additional financial support of the project. Also, we would like to thank Liwei Deng and Renbiao Tao for technical assistance.
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REFERENCES
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