High-Pressure ChemistryDOI: 10.1002/anie.201001114

Direct Formation of Mesoporous Coesite Single Crystals from PeriodicMesoporous Silica at Extreme Pressure**Paritosh Mohanty, Volkan Ortalan, Nigel D. Browning, Ilke Arslan, Yingwei Fei, andKai Landskron*

A very fundamental principle in nature is close-packed solid-state structures that minimize the “empty space” betweenatoms. Accordingly, porous structures form only under specialconditions that are able to overcome the tendency of closepacking. In most cases porosity is created either by templatingstrategies or by the use of directional bonding effects.[1, 2] Theformer effect is widely used for the preparation of zeolites andmesoporous oxides, while the latter effect is extensivelyapplied for the preparation of porous metal–organic frame-works.[1, 2] Most porous materials are only metastable underambient conditions.[3] Porous materials exposed to highpressure (> 1 GPa) typically undergo facile pore collapse.At extreme pressure (> 10 GPa) pore collapse can alreadyoccur at room temperature. For example, periodic mesopo-rous silica MCM-41 undergoes pore collapse at room temper-ature when the pressure exceeds 12 GPa. At high pressureand elevated temperature the tendency for pore collapsebecomes even more pronounced because of the enhancedkinetic activation of the chemical bonds.[4]

Recently, we have reported the formation of stishovitenanocrystals from SBA-16 at 12 GPa and 400 8C.[5] Herein, wereport the direct formation of a mesoporous coesite fromperiodic mesoporous silica SBA-16 at 12 GPa and 300 8C

without the aid of a template and within a reaction time ofonly five minutes. The experiment was carried out in a multi-anvil high-pressure assembly placed into a 1500-ton hydraulicpress. The experiment yielded well crystalline coesite accord-ing to wide-angle powder X-ray diffraction (WAXS) (Fig-ure 1a). All reflections could be indexed and refined to latticeconstants of a = 7.157(3) �, b = 12.371(4) �, c = 7.181(6) �,and b = 120.288, which are typical values for coesite (JCPDSfile number: 79-0445). The formation of coesite was furtherproven by Raman spectroscopy, which shows the typicalbands for coesite at wavenumbers of 112, 151, 173, 202, 271,328, 358, 429, 469, and 521 cm�1 (Figure 1b). These values are

Figure 1. a) Wide-angle X-ray diffraction pattern obtained with CoKa

radiation and b) Raman spectrum of mesoporous coesite obtainedfrom SBA-16.

[*] Dr. P. Mohanty, Prof. Dr. K. LandskronDepartment of Chemistry, Lehigh UniversityBethlehem, PA 18015 (USA)Fax: (+ 1)610-758-6536E-mail: kal205@lehigh.eduHomepage: http://www.lehigh.edu/~kal205

Dr. V. Ortalan, Prof. Dr. I. ArslanDepartment of Chemical Engineering and Materials ScienceUniversity of California at DavisDavis, CA 95616-5294 (USA)

Prof. N. D. BrowningDepartment of Chemical Engineering and Materials ScienceDepartment of Molecular and Cellular BiologyUniversity of California at DavisDavis, CA 95616 (USA)andCondensed Matter and Materials DivisionPhysical and Life Sciences DirectorateLaurence Livermore National LaboratoryLivermore, CA 94550 (USA)

Dr. Y. FeiGeophysical Laboratory, Carnegie Institution of WashingtonWashington, DC, 20015 (USA)

[**] We thank Lehigh University and the Carnegie Institution ofWashington for financial support.

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/anie.201001114.

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in good accordance with literatureRaman data for coesite and can beassigned to the Bg, Ag, Bg, A1, Bg,Ag + Bg, Ag, Bg, A1, and Bg vibra-tions.[6] According to the phasediagram of silica, stishovite is theexpected phase at the respectivepressure and temperature condi-tions. The formation of coesite inthe stability field of stishovite indi-cates that the crystallization iskinetically controlled. The crystal-lization pathway leads through acoesite intermediate. Owing to themild crystallization temperature of300 8C, the coesite phase is stableenough not to convert further intostishovite at a significant rate. Theobservation of coesite in the stabil-ity field of stishovite is in accord-ance with the Ostwald step rule,which states that a system tends tocrystallize first in its least stablepolymorph.

To elucidate the morphology ofthe coesite obtained, we investi-gated the product by transmissionelectron microscopy (TEM). Toour surprise, the TEM images indi-cated a highly porous morphologywith a mesoscale texture (Fig-ure 2a–c). The images indicatedspherically to elliptically shapedpores with diameters betweenapproximately 2 and 50 nm. Thepore system does not exhibit peri-odic order. The pore wall diame-ters are in the same size regime asthe pores. No specific particleshape was observed. To investigatewhether the porous particles werecrystalline we performed selectedarea electron diffraction (SAED).The electron diffraction patternrevealed bright regular diffractionspots which indicate that the crys-tals are magnitudes larger than themesopores (Figure 2d). The crys-tals can therefore be described asmesoporous single crystals. Theporosity was further studied byscanning electron microscopy(SEM). SEM confirmed the pres-ence of mesoporosity (Figure 3).The images show spherical andelliptical pores and pore wallswith diameters of approximately15–50 nm. The smaller pores(< 15 nm) could not be clearly

Figure 2. TEM images (a–c) and SAED pattern of mesoporous coesite obtained from SBA-16.

Figure 3. SEM images of mesoporous coesite obtained from SBA-16.

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observed in the SEM images due to the lower resolution ofthe SEM images compared to the images obtained with theTEM technique. Figure 3c shows that the regular mesoporos-ity extends over a wide area, demonstrating that the sample ishomogeneously mesoporous. No appreciable amounts ofnonporous particles or nonporous areas within a specificparticle were observed.

To confirm the porous nature of the coesite and to obtainmore quantitative information about porosity and the poresystem we attempted to perform electron tomography.Unfortunately, the specimen is susceptible to beam damageduring the measurements and the pores collapsed withinseconds. Therefore, a through-focal series of TEM imageswere collected by aberration corrected electron microscopythat allowed for optically sectioning the specimen at lowelectron doses.[7] We calculated the total volume of oneparticular particle to be 14.301 � 10�14 cm3 and the totalvolume of the pores within this particle to be 6.994 �10�14 cm3. Because of the elongated probe profile in theelectron beam direction a correction of these values isrequired. Including the effect of elongation in the probe thetotal volume of the probe was calculated to be 6.994� 1.390 �

10�14 cm3 (Figure 4a). Based on this analysis we found thevolume percentage of the pores to be (49� 9)%. Likely, theporosity is in the upper range of this value (ca. 55%) becausethe lower limit of the porosity assumes that all pores areperfect spheres. The surface area was calculated to be53 m2 g�1. The relatively low surface area is plausible consid-ering the single crystallinity of the material which is consistentwith low surface roughness. The pore size distribution wascalculated by using three different particles for more reliablestatistics and was found to be centered around 4 nm, clearlydemonstrating that the sample is mesoporous (Figure 4b). Asmall additional maximum in the histogram was also found forthe pore sizes at around 30 nm. This maximum correspondswell to the larger mesopores seen by SEM.

A question is: What guides the pore formation at anextreme pressure of 12 GPa? If the observed porosity is aresidual porosity of the starting material SBA-16, a meso-porous material should also be obtained at a temperaturelower than 300 8C. To clarify this mechanistic question, weconducted an experiment in which we exposed SBA-16 to200 8C at 12 GPa. The experiment yielded a glassy, almosttransparent, monolith which is noncrystalline according to X-ray diffraction (see Figure S1a in the Supporting Informa-tion). The entire absence of crystalline coesite was confirmedby Raman spectroscopy (see Figure S1b in the SupportingInformation). These findings show that the minimal temper-ature to crystallize the coesite from SBA-16 lies between 200and 300 8C, which is a very low crystallization temperature fora SiO2 material. TEM investigations of the sample recoveredfrom 200 8C did not reveal any mesostructural texture,suggesting that the mesopores are collapsed (see Figure S2in the Supporting Information). The absence of electroniccontrast also confirms that no significant amounts of meso-genic templating species, for example, residual F127 oradsorbed vapors (e.g. adsorbed organic vapors or water)were present during the high-pressure experiment. It istherefore suggested that the mesoporous coesite singlecrystals form via an intermediate glassy state which isnonporous. The mesoporosity is likely created during thecrystallization of the coesite from the nonporous glass. Thisraises the question: What drives the pore formation duringcrystallization? Tolbert et al. have shown that the mesoporesof MCM-41 can elastically and reversibly deform at highpressure and ambient temperature.[8] Because pore collapse isthe ultimate state of pore deformation, it can be assumed thatelastic strain exists in the collapsed pore structure of theintermediate glass formed from SBA-16 at 12 GPa. Uponcrystallization, the material becomes significantly stiffer andthe pores are able to reform. Simultaneously, the crystalliza-tion-induced volume shrinkage may further aid the formationof porosity. After the crystallization is complete, the porouscoesite remains metastable due to the higher stiffness andinertness of the crystalline coesite channel walls at the mildtemperature conditions, and the short reaction times (5 min).The mechanism is shown schematically in Figure 5. Thismechanism is supported by the observation of spherical poresthat are present in both the SBA-16 as well as the porouscoesite material. Moreover, the pore size distribution of theproduct is centered around 4 nm, which is similar to that of

Figure 4. Depth-sectioned electron microscopy (a) and pore sizedistribution (b) of mesoporous coesite obtained from SBA-16.

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the SBA-16 starting material (5.1 nm). The elastic strain in theglassy intermediate can be clearly seen by polarizationmicroscopy, which shows an anisotropic structure for theglass (see Figure S3 in the Supporting Information). Theexistence of strain in the glass is further supported by IRspectroscopy (see Figure S4 in the Supporting Information) ofthe glass and the same glass that has been post-treated at1000 8C at ambient pressure. The bands for the symmetric(around 800 cm�1)[8] and asymmetric Si�O stretching vibra-tions (around 1100 (transverse vibration)[8] and 1200 cm�1

(longitudinal vibration))[8] of the post-treated sample areshifted to higher wavenumbers in comparison to those of theas-synthesized sample, which indicates a decrease of the Si-O-Si intertetrahedral bond angle and a decrease of strain. Theas-synthesized material also exhibits an additional band at950 cm�1 that completely disappears upon heating. Densitymeasurements of the glass revealed a density of 2.18 g cm�3,which is only slightly lower to that of quartz glass (ca.2.2 gcm�3).[9] This corroborates that no templating species arepresent in the glass and that the mesoporous coesite formsfrom a strained nonporous intermediate glass during crystal-lization without the aid of a template. A conceivablealternative mechanism would assume the formation of themesopores upon decompression. However, this appears veryunlikely because in this case also the recovered glass formedat 200 8C would be porous.

We were further interested whether the formation ofporous coesite is unique to SBA-16 or if it can be observedalso from other periodic mesoporous silica materials. Toprobe this, we have performed high-pressure experimentswith KIT-6 at 12 GPa and temperatures of 200 and 300 8Crespectively. For the experiment at 300 8C excellent crystal-lization was achieved and phase-pure coesite was obtainedaccording to WAXS (see Figure S5 in the SupportingInformation). TEM investigations revealed that KIT-6 alsoyields mesoporous particles (see Figure S6a in the SupportingInformation). The pore sizes appear somewhat less regularcompared to those of the product obtained from SBA-16.Next to large pores with diameters of approximately 20–30 nm there are pores that are about an order of magnitudesmaller (around 2–3 nm). SAED showed very bright spotsdemonstrating that the porous particles are highly crystalline(Figure S6b). The diffraction spots are somewhat less regularcompared to the coesite obtained from SBA-16 indicatingsome degree of polycrystallinity. However, the limitednumber of diffraction spots indicates that only a fewcrystallites are present in the selected area. SEM of the

sample corroborates the mesoporosity. Thepores in the 20–30 nm size regime are visible(see Figure S6c in the Supporting Informa-tion). Pores as small as 3 nm are below theresolution of an scanning electron micro-scope and therefore could not be observed.The experiment performed at 200 8C yieldedagain an X-ray amorphous glassy product(see Figure S7a in the Supporting Informa-tion). Absence of crystalline coesite wasfurther confirmed by Raman spectroscopy(see Figure S7b). Transmission electron mi-

croscopy of the product revealed neither mesostructural norany other nanoscopic texture (see Figure S7c). Furthermore,the polarization microscopy image (see Figure S7d) shows ananisotropic structure for the glass similar to that of the glassysample obtained from SBA-16 (see Figure S3 in the Support-ing Information). This confirms that the observed porosity ofthe coesite is not a residual porosity of the initial mesostruc-ture. Overall, the analogous behavior of SBA-16 and KIT-6suggests the mesoporous coesite forms by the same formationmechanism.

In conclusion we have demonstrated that single-crystal-line mesoporous high-pressure silica phases (coesite) canform directly from periodic mesoporous silicas without theaid of templates. The direct formation of a highly porousmaterial at extreme pressure is very surprising because naturestrongly favors dense over porous structures at high pressure.The phenomenon can be explained by the crystallization of anonporous glassy silica intermediate that “remembers” theoriginal mesoporosity. The memory effect can be explained byelastic strain in the collapsed intermediate structure andcrystallization-induced volume shrinkage. The mesoporouscoesite is able to exist in a metastable state at the mildtemperature of 300 8C (12 GPa).

Experimental SectionChemicals: Triblock copolymer EO106PO70EO106 Pluronic F127(BASF), tetraethyl orthosilicate (TEOS, Sigma–Aldrich), hydrochlo-ric acid (EMD Chemicals), butanol (Alfa Aesar). All the chemicalswere used as-received without further purification. KIT-6 and SBA-16were prepared according to literature.[10, 11]

Multi-anvil experiment: The experiments were carried out in amulti-anvil assembly with a 1500 t hydraulic press. The samples wereencapsulated in Pt capsules of 2.5 mm diameter and 3 mm length. Acapsule was placed inside an alumina sleeve, a cylindrical Re heater,and a zirconia sleeve for thermal insulation. This assembly was placedinside a Cr2O3-doped MgO octahedron with an edge length of 8 mmand a diameter of 14 mm. The octahedron was placed between eightcorner-truncated tungsten carbide cubes with pyrophyllite gaskets.The resulting cubic assembly was placed into the press. In thefollowing, the sample was compressed to the final pressure at a rate of2 GPah�1. After the final pressure was reached, the sample washeated to 200 and 300 8C, respectively, at a heating rate of100 K min�1. A sample was kept at the final temperature for 5 minand then quenched. The pressure was released at a rate of 3 GPah�1.After normal pressure was reached, the samples were extracted fromthe Pt capsule.

Characterization of the materials: The formation of the productphase, and the study of its structure and microstructures were carriedout by X-ray diffraction (XRD), transmission electron microscopy

Figure 5. Proposed formation mechanism of mesoporous coesite from periodic mesopo-rous silicas.

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(TEM), scanning electron microscopy (SEM), Raman, and Fouriertransform infrared (FT-IR) spectroscopy. The TEM images weretaken on a JEOL JEM-2000 electron microscope operated at 200 kV.Samples for the TEM analysis were prepared by dispersing theparticles in acetone and dropping a small volume of it onto a holeycarbon film on a copper grid. SEM images of the specimen wererecorded on a Hitachi S-4300 SEM. The XRD patterns were recordedby using a Bruker High-Star diffractometer with a CoKa radiationsource and a Rigaku Rapid II diffractometer with MoKa radiation.The Raman spectrum of the specimen was collected by using aHoriba-Jobin Yvon LabRam-HR spectrometer equipped with aconfocal microscope (Olympus BX-30), a 532 nm notch filter, and asingle-stage monochromator. The Raman spectrum was collectedwith 532 nm excitation (20 mW, YAG laser) in the 100–1200 cm�1

region. The spectrum was collected at ambient conditions.

Received: February 23, 2010Published online: May 6, 2010

.Keywords: coesite · high-pressure chemistry ·mesoporous materials · mesoporous silica · silicon

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