Microporous and Mesoporous Materials 152 (2012) 214–218

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Microporous and Mesoporous Materials

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Large-pore periodic mesoporous silicas with crystalline channel wallsand exceptional hydrothermal stability synthesized by a generalhigh-pressure nanocasting route

Paritosh Mohanty a, Berenika Kokoszka a, Cong Liu a, Manuel Weinberger a, Manik Mandal a,Vincenzo Stagno b, Yingwei Fei b, Kai Landskron a,⇑a Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USAb Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 August 2011Received in revised form 15 November 2011Accepted 16 November 2011Available online 23 November 2011

Keywords:Crystalline wallsMesoporous silicaHydrothermal stabilityNanocastingPetroleum cracking

1387-1811/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.micromeso.2011.11.031

⇑ Corresponding author. Fax: +1 610 758 6536.E-mail address: kal205@lehigh.edu (K. Landskron)

Periodic mesoporous quartz was synthesized by a generally applicable nanocasting method at industri-ally achievable pressure (2–4 GPa) and temperature (750 �C) conditions. The materials have fcc lattice ofinterconnected spherical mesopores with diameters of ca. 15–18 nm, BET surface areas of 234–238 m2 g�1, and pore volumes of 0.70–0.84 cm3 g�1. They show exceptional hydrothermal stability ofat least 800 �C in pure steam which appears suitable for practical applications in petroleum cracking.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

Crystalline microporous silicas and aluminosilicates, zeolites,are one of the most diverse and useful inorganic materials withapplications ranging from catalytic hydrocarbon conversion, toion exchange, to gas separation [1–3]. Much of their utility derivesfrom the crystalline nature of their pore walls. For instance, highhydrothermal stability and high acidity is a direct consequence oftheir crystallinity which is crucial for applications such as petro-leum cracking [4]. General synthesis routes to zeolites are welldeveloped, most prominently hydrothermal synthesis routesemploying structure directing agents (SDAs). The existence of gen-eral, inexpensive, and industrially compatible synthesis methodsfor zeolites has been of paramount importance for both the aca-demic interest in zeolites as well as their industrial deployment[1].

There has been a long-standing quest for zeolites with ultra-large pores because the small micropores of zeolites severely re-strict diffusion, which limits the performance of zeolites in manyapplications, e.g. catalysis [2]. A major breakthrough in this areaoccurred in 1992 with the discovery of periodic mesoporous silicamaterials by Mobil Oil [5]. It was envisaged that these larger pore

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materials would soon replace zeolites in many applications, be-cause of their far superior mass transport properties compared tomicroporous zeolites [6,7]. However, periodic mesoporous silicashave the disadvantage of non-crystalline pore walls, which has se-verely restricted their utility. For instance, their hydrothermal sta-bility is inferior compared to zeolites which has prohibited many oftheir uses, e.g. in crude petroleum cracking [6,7]. Up to date thereare only three examples of zeolites with periodic mesopores [9–11]which have been synthesized by Corma, Ryoo, and Tsapatsis,respectively.

Recently, we have introduced the concept of ‘‘nanocasting athigh pressure’’ [8] in which mechanical pressure and temperatureis applied to a periodic mesostructured composite to produce aperiodic mesostructured high-pressure phase. After the selectiveremoval of one phase at ambient pressure, a periodic mesoporoushigh-pressure phase is obtained. Using nanocasting at high-pres-sure we have recently synthesized periodic mesoporous coesiteas the first example of a periodic mesoporous high-pressure phase[8]. Periodic mesoporous coesite is also the first periodic mesopor-ous SiO2 material with crystalline channel walls and the first peri-odic mesoporous silica material with crystalline dense channelwalls (the channel walls of the materials reported by Corma, Ryoo,and Tsapatsis are microporous, i.e. not dense). Thus, nanocasting athigh-pressure appeals as a general synthetic strategy to crystallizethe channel walls of mesostructures that are difficult or impossible

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to crystallize at ambient pressure. Pressure can facilitate crystalli-zation because mechanical stress can activate bonds and crystal-line phases are usually denser phases compared to amorphousphases. Periodic mesoporous coesite was synthesized from peri-odic mesostructured SBA-16/carbon composites at 12 GPa. Thispressure is beyond industrially applicable pressure and requires aspecialized laboratory to perform synthesis at the milligram scale.Bulk industrial syntheses and generally applicable routine labora-tory scale syntheses require a pressure of 4 GPa or lower [12]. Thus,a successful demonstration of a periodic mesoporous silica mate-rial with crystalline channel walls at 4 GPa or lower would suggestthe commercial viability of nanocasting at high pressure. In addi-tion, milligram to gram scale syntheses could be performed easilyin a standard laboratory because 4 GPa can be reached in bench-top high-pressure apparatuses. These syntheses should not be re-stricted to silicas and generally applicable to virtually any meso-structured material.

2. Experimental section

2.1. Materials and methods

2.1.1. ChemicalsTriblock copolymer Pluronic F127 (BASF), tetraethyl orthosili-

cate (Sigma–Aldrich), conc. hydrochloric acid (EMD Chemicals).All the chemicals were used as-received without furtherpurification.

2.1.2. Synthesis of LP-FDU-12The synthesis procedure has been done as described by Zhao

and co-workers [13].0.5 g of F127 and 2.5 g of KCl, 30 mL of 2 M HCl were mixed and

cooled to 15 �C. After 1 h 0.6 g of mesitylene was added. The solutionwas further stirred for 1 h. Then 2.08 g of TEOS was added dropwiseusing a dropping funnel. Time for dropping the TEOS was typically15–20 min. The reaction solution is then stirred for another 24 h at15 �C. Afterwards the mixture is transferred into an autoclave andaged at 120 �C for 2 days. During filtration of the aged sample, theproduct is washed with 2 times or 3 times the volume of the reactantsolution with water. The product is then dried for 24 h in the fumehood. Calcination was performed by heating the powder in air to550 �C within 5 h. The sample was kept at that temperature foranother 5 h and cooled to room temperature.

2.1.3. Synthesis of FDU-12/carbon composite1.6 g of HP grade AR-mesophase pitch (Mitsubishi Gas and

Chemical) with a softening point of 302 �C and 1 g of mesoporousfcc silica FDU-12 were dispersed in ethanol. The mixture was stir-red for few hours and the ethanol was evaporated at 80 �C. The ob-tained solid mixture was then heated in a tube furnace at 302 �Cwith a heating rate of 1 �C/min for 4 h under a continuous flowof nitrogen gas. After this thermal treatment the sample was car-bonized by raising the temperature of the furnace to 900 �C witha heating rate of 2 �C per min and the carbonization was donefor 2 h in flowing nitrogen.

2.1.4. High-pressure multi-anvil experimentsThe experiments were carried out in a multi-anvil assembly

with a 1500-ton hydraulic press. The samples were encapsulatedin Pt capsules of 2.5 mm diameter and 3 mm length. A capsulewas placed inside an alumina sleeve, a cylindrical Re heater, anda zirconia sleeve for thermal insulation. This assembly was placedinside a Cr2O3 doped MgO octahedron with an edge length of18 mm. The temperature was measured directly at the Pt capsulewith a W/Re thermocouple which was implemented into this

assembly. The octahedron was placed between eight corner-trun-cated tungsten carbide cubes with pyrophyllite gaskets. We useda truncated edge length of 11 mm for the experiments. Theresulting cubic assembly was placed into the press. In the follow-ing, the sample was compressed to the final pressure with a rateof 2 GPa/h. After the final pressure was reached, the sample washeated to the final temperature with a heating rate of 100 K/min.The sample was kept at the final temperature for 3 h and thenquenched. The pressure was released with a rate of 2 GPa/h. Afternormal pressure was reached, the samples were extracted from thePt capsule.

2.1.5. High-pressure piston-cylinder experimentsThe 1 GPa and 2 GPa experiments were conducted in the end-

loaded piston-cylinder apparatus using graphite as heater. Thestarting material was loaded and sealed in gold capsules. The sam-ples were pressurized to either 1 GPa or 2 GPa at room tempera-ture and then heated to 750 �C at 100 �C/min and kept at thetarget pressure–temperature conditions for 6 h. The samples werethen quenched to ambient conditions for characterization.

2.1.6. CharacterizationThe TEM images were taken on a JEOL JEM-2000 microscope

operated at 200 kV. A JEOL JEM-2200 electron microscope operatedat 200 kV was used for STEM and HR-STEM investigations. Samplesfor the TEM analysis were prepared by dispersing the particles inacetone and dropping a small volume of it onto a holey carbon filmon copper grid. SEM images were taken using a Hitachi S4200instrument at an operating voltage of 5 kV. The wide angle X-raydiffraction pattern was recorded on a Rigaku Rapid II X-ray diffrac-tometer using Mo Ka radiation and a Rigaku MiniFlex II (Cu Ka).Small angle diffraction was carried out with a Rigaku Rotaflexinstrument and Cu Ka radiation and a Bruker Nanostar instrument(Cu Ka). The N2 sorption experiments were performed using aQuantachrome Autosorb-1 MP instrument. Before the measure-ments, the samples were degassed in vacuum at 150 �C forovernight. NLDFT methods (adsorption branch model for sphericalmesopores) were used to calculate the pore size distribution. Thesurface area was determined using the multipoint BET method.5 points were selected between relative pressures of 0.01and 0.35.

3. Results and discussion

Herein, we report the synthesis of large-pore crystallineperiodic mesoporous quartz LP-FDU-12-Q at 2 and 4 GPa. LP-FDU-12-Q has been first synthesized at a pressure of 4 GPa and atemperature of 750 �C using cubic large-pore periodic mesoporoussilica LP-FDU-12 with an fcc lattice of spherical mesopores (19 nmsize) as starting material. The LP-FDU-12 was synthesized similarlyto a method reported by Zhao et al. [13]. Then, the mesopores ofthe FDU-12 were filled by infiltration of molten mesophase pitchas carbon source at its softening point (302 �C) and subsequent car-bonization at 900 �C in N2 atmosphere [14]. The role of the carbonis to prevent mesostructural collapse at high pressure and hightemperature. The resulting silica/carbon mesostructured compos-ite had negligible surface area and no measurable micro- or meso-porosity. In the following step the silica/carbon composite wasplaced into a platinum capsule, which was inserted into a 18/11multi-anvil assembly [15]. Details about the experimental set-upcan be found in the materials and methods section. The pressurewas then ramped up to 4 GPa with a rate of 1 GPa/h. The samplewas heated subsequently to 750 �C for a time period of 3 h. Afterthat, the sample was quenched and depressurized at a rate of1 GPa/h.

Fig. 1. (a) SAXS (Cu Ka) and (b) WXRD (Mo Ka) patterns of periodic mesoporousquartz.

Fig. 3. N2 isotherm and pore size distribution (inset) of large-pore periodicmesoporous quartz synthesized at 4 GPa.

216 P. Mohanty et al. / Microporous and Mesoporous Materials 152 (2012) 214–218

Heat-treatment at 550 �C in air for 5 h was used to oxidize thecarbon. The small angle X-ray diffraction (SAXS) pattern of theproduct material (Fig. 1a) shows a clear peak at 2h = 0.39�, whichdemonstrates the high periodic mesoscale order of the specimen.The lattice constant is 39.1 nm assuming a face-centered cubicstructure. In order to study the crystallinity of the pore walls ofthe specimen, wide-angle X-ray diffraction (WXRD) was per-formed. The WXRD pattern (Fig. 1b) shows sharp and well-resolved diffraction peaks, which can be attributed to a-quartz.At first glance, the formation of quartz is surprising because at4 GPa and 750 �C coesite is the thermodynamically stable phaseaccording to the phase diagram of silica. The formation of quartzcan be explained best by the Ostwald step rule, which states thatthe least stable polymorph of a compound tends to crystallize first.Because the synthesis temperature is relatively low, quartz can ex-ist as a metastable intermediate. The activation barrier at this tem-perature is too high to allow for a quartz–coesite phase transitionat a significant rate. Surface area effects are unlikely the reason for

Fig. 2. (a) TEM image and (b and c) SEM images of p

the formation of quartz because surface stresses add to the pres-sure and typically favor the high-pressure polymorph over thelow-pressure polymorph [16].

In order to further study the mesostructure, the specimen wasinvestigated by transmission electron microscopy (TEM). TEM(Fig. 2a) demonstrates excellent periodic mesostructural order.No noteworthy amounts of disordered particles were detected.The pore sizes and the pore wall thicknesses are ca. 10–13 nmaccording to TEM. The fourier transform image (FFT) (Fig. 2a, lowerinset) clearly shows that the cubic symmetry of the mesostructurewas retained. The pore and channel wall diameters measured byTEM are further confirmed by scanning electron microscopy(SEM) (Fig. 2b and c). In addition, the low magnification imageFig. 2c shows that the periodic mesoporosity extends over a widearea. The crystallinity of the pore walls was further investigatedby selected area electron diffraction (SAED) (upper inset ofFig. 2a). The SAED patterns revealed regular arrays of diffractionspots that can be assigned to quartz and confirm the single crystal-linity of the pore walls.

N2 sorption at �196 �C revealed a type-IV isotherm with a steepN2 uptake at p/p0 of 0.75–0.85 which is typical for periodic meso-porous materials. The pore size distribution (PSD) was calculatedby non-local Density Functional Theory (model for spherical mes-opores, Quantachrome AS1 software) and showed a bimodal poresize distribution centered at 8 and 15 nm, respectively (Fig. 3 in-set). The two maxima can be interpreted as the average pore en-trance sizes (8 nm) and the actual diameter of the sphericalmesopores (15 nm), respectively [17]. The average of these valuesis in reasonable agreement with the pore sizes estimated from TEMand SEM images (10–13 nm). As expected, the pores are somewhatsmaller than those of the FDU-12 parent silica (19 nm) due to thepressure and crystallization induced volume shrinkage. BJH

eriodic mesoporous quartz synthesized at 4 GPa.

Fig. 4. (a) SEM, (b) TEM images of hydrothermally treated periodic mesoporous quartz and (c) SAXS patterns of products obtained at 2 and 1 GPa, respectively.

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methods as well as NLDFT methods designed for cylindrical mes-opores also gave a narrow pore size distribution but underesti-mated the pore size by ca. 50%. This effect is typical for sphericalpore systems and has also been observed for SBA-16 and othermesostructures with spherical mesopores [17]. The BET surfacearea and pore volume was found to be 238 m2 g�1 and 0.70 cc g�1,respectively. These values are somewhat smaller than those of theparent silica (350 m2 g�1 and 0.79 cc g�1). The decreased surfacearea can be explained by the smoother pore surface of the crystal-line periodic mesoporous quartz compared to the LP-FDU-12 withamorphous walls. Furthermore, the higher density of the quartz(theoretical density = 2.6 cc g�1) compared to amorphous silica(2.2 cc g�1) contributes to the lower specific surface area. Thisinterpretation is further supported by the fact that the pore volumeof the mesoporous quartz has not decreased as much as the surfacearea.

To investigate the hydrothermal stability of the material, LP-FDU-12-Q was treated in a pure steam stream (vapor pressure ofH2O = 1 atm) at 800 �C for 2 h. For comparison, the LP-FDU-12starting material with amorphous channel walls was subjected tothe same conditions. The mesoporous quartz was recovered with-out obvious visual change. In contrast, the LP-FDU-12 startingmaterial was found to be depolymerized and volatilized com-pletely (our alumina boat was found empty after the treatment)indicating that the crystallization has a great effect on the hydro-thermal stability. SEM and TEM (Fig. 4a and b) of the hydrother-mally treated LP-FDU-12-Q material clearly showed that themesopores withstood this hydrothermal treatment. Among well-ordered particles also some particles with non-periodic mesoporeswere found (Fig. S1) which suggests beginning deterioration of thesample at these conditions. SAXS still showed a clear reflectionconfirming that the periodicity of the mesopores as well as crystal-line quartz phase (as seen from XRD pattern) was largely main-tained (Fig. S2). The reflection shifted from 0.39� to 0.45� 2hindicating that some lattice contraction occurs during the hydro-thermal treatment which may be interpreted as beginning porecollapse in the materials.

To investigate if LP-FDU-12-Q can be synthesized at even lowerpressure we have performed two additional experiments at 2 GPaand 1 GPa in a piston-cylinder apparatus. At 2 GPa and 750 �C, acrystalline quartz phase formed according to XRD while at 1 GPano significant crystallization was observed (Fig. S3). The datashows that the minimal crystallization pressure is within the rangeof 1–2 GPa. SAXS of the calcined product materials demonstratesthat the periodic mesostructure is intact as visible from a reflectioncentered at 0.39� 2h (Fig. 4c). However, the broadening of the peaksindicates that the order of the materials was slightly decreased incomparison to the material synthesized at 4 GPa. The fact that theperiodicity of the non-crystalline material is reduced indicates thatthe distortion is not induced by the crystallization but is due to theless uniform isostatic pressure in a piston-cylinder apparatus ascompared to a multi-anvil assembly. The presence of periodic

order was further confirmed by SEM, TEM, and STEM (Fig. S4). N2

sorption data shows a type IV isotherm (Fig. S5) with a steep cap-illary condensation step for LP-FDU-12-Q (2 GPa). The pore sizedistribution calculated by DFT methods for spherical mesoporesis centered at 18 nm with a pore entrance size of 8 nm. The surfacearea is 234 m2 g�1 and the pore volume was found to be0.87 cm3 g�1. These values are in excellent agreement with thedata obtained for the material produced at 4 GPa. The somewhatlarger pores can be explained by the lower pressure that has beenapplied. High resolution bright field STEM was performed to con-firm the crystallinity of the channel walls. The experiment wasvery challenging due to severe beam damage. Nonetheless, it waspossible to capture the lattice fringes of quartz in the channel wallsof the mesostructure when the beam exposure time was smallerthan 5 s (Fig. S6 and S7). The lattice fringes can be assigned tothe 101 lattice planes of quartz and have a spacing ofd = 0.34 nm (Fig. S7), which is in accordance with the PC-PDF file65-0466.

4. Conclusion

In conclusion we have shown that nanocasting at high pressurecan be performed at industrially compatible pressures of 2–4 GPa.These pressures are also low enough to allow for bench-top syn-theses of in principle any mesostructure of interest in standard lab-oratories. Moreover, we have demonstrated the first large-poreperiodic mesoporous silica material with crystalline channel walls.For the first time the hydrothermal stability of periodic mesopor-ous silicas with crystalline channel walls with those of amorphouschannel walls was compared. The hydrothermal stability wasgreatly enhanced for the crystalline materials and reached excep-tional values of at least 800 �C in pure steam for at least 2 h. Thesestabilities appear suitable for applications in petroleum cracking.

Acknowledgements

Since June 2011, this work was supported as part of EFree, anEnergy Frontier Research Center funded by the U.S. Departmentof Energy Office of Science, Office of Basic Energy Sciences underAward #DE-SG0001057. In addition, we thank Lehigh University,and the Carnegie Institution of Washington for additional financialsupport of the project. Dr. Chris Kiely and Qian He are gratefullyacknowledged for assistance with STEM and HR-STEM imaging.We gratefully acknowledge Dr. Shi Jin, and Dr. Michal Kruk forSAXS measurements of hydrothermally treated LP-FDU-12-Q usingthe Bruker Nanostar Instrument funded by NSF CHE-0723028.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2011.11.031.

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