HKUST-1 silica aerogel composites: novel materials for the separation of saturated and unsaturated hydrocarbons by conventional liquid chromatography

Abstract

HKUST-1 silica aerogel composite (HKUST-1@SiO₂) has been studied as a stationary phase for the efficient separation of unsaturated hydrocarbons from saturated aliphatics by conventional liquid chromotography (LC). HKUST-1@SiO₂ has been prepared via an advanced sol–gel method and subsequent drying in supercritical CO₂ to minimize the deterioration of the individual properties of the MOF and silica aerogel structures and tune its properties to be acceptable for flow mode. The synthesized composite was characterized by X-ray diffraction, FT-IR spectroscopy, XPS, scanning electron microscopy with EDAX mapping and low-temperature nitrogen adsorption. According to these data, the composite represents physically dispersed domains of HKUST-1 in the silica aerogel network. It was shown that the HKUST-1@SiO₂ composite can be used as a highly efficient stationary phase for conventional liquid chromatographic separation of cyclohexene or benzene from cyclohexane. This is the first time a MOF composite has been used for the separation of organic molecules by LC, demonstrating new vistas for the application of these materials in the flow mode.

---

Introduction

The separation of olefins and paraffins with the same number of carbon atoms in the molecules as well as mixtures of benzene and cyclohexane with similar boiling points of the components, presently relies on energy-intensive distillation-based technologies.¹ Adsorptive separation is considered to be a more efficient choice for these processes.² Metal–organic frameworks (MOFs) combining well-defined structural characteristics, large surface areas, tunable surface properties and pore size are promising candidates for separation and purification of liquid mixtures.³ It has been shown recently that a MOF HKUST-1 which consists of copper paddlewheel metal clusters linked by benzene-1,3,5-tricarboxylate ligands⁴ is an efficient sorbent for the liquid-phase separation of mono-, di- and polyaromatic hydrocarbons,^5a unsaturated alkylaromatics from their saturated analogues,^5a–c olefins from corresponding paraffins^5d–f and aromatics from aliphatics.^5f,g

HKUST-1 as well as other MOFs is mostly obtained in the form of fine powder of polycrystalline particles (the size D50 of HKUST-1, specifically Basolite C300 produced by BASF, is 15.96 μm (ref. 6)) which are extremely difficult to form into larger pellets.⁷ It blocks the development and implementation of any efficient processes based on these materials in flow regime and blanks the most advantages and unique properties of the MOFs. Therefore a key challenge for packed bed applications of MOFs in continuous processes is how to prepare them in a shape suitable to limit the backpressure and material loss.^5c,7,8 Various methods for shaping pure MOFs such as granulation using binder,⁷ preparation of thin films⁹ and hollow MOF capsules,¹⁰ synthesis of hierarchical porous MOF monoliths^5c and aerogel structures¹¹ are described. Alternative approach of MOFs shaping is the synthesis of MOF composite materials containing the MOF as the dispersed phase in an organic or inorganic matrix.^8,12

Much attention has been paid to synthesis of MOF composites with silica using different approaches to improve the mechanical properties of MOF's.^8,12e–k One of the approaches is based on the synthesis of HKUST-1 from the molecular precursors directly in the pores of silica-based support, such as silica beads used in chromatography,^8a–c mesoporous silica^12h–j and other pre-synthesized silica-based materials.^8d The presence of MOF's nanoparticle inside silica pores was shown to decrease the initial mesopore volume,^8a,d for instance 25 wt% of HKUST-1 can occupy about 30% of mesopore.^8d As alternative to this two-stage and time-consuming procedure, one-step syntheses of composites were developed through sol–gel method starting from molecular precursor^12k or commercially produced MOFs.^12f,g The series of HKUST-1 silica aerogel composites was synthesised using sol–gel process involving hydrolysis, condensation, gelation, curing, solvent exchange and supercritical drying steps.^12f The microporosity of composites increased with the increasing amount of HKUST-1 however blockage of some fraction of micropores of the MOF was observed.

In some cases MOF silica composites have been used as a stationary phase for the high-performance liquid chromatography (HPLC).^8a–c So, monodisperse composite spheres of silica-HKUST-1 with a particle size of 3 μm prepared by embedding HKUST-1 into the pores of silica beads can be used as a stationary phase for liquid chromatographic separation of ethylbenzene/styrene and p-ethyltoluene/p-methylstyrene.^8a In another study HKUST-1-silica microspheres were used as packing materials for fast and efficient liquid chromatographic separation of toluene/ethylbenzene/styrene, toluene/o-xylene/thiophene and xylene isomers.^8b However, these materials cannot be used for conventional liquid chromatography (LC) on a preparative scale due to small particle size (3–7 μm).^8a–c

In the present work, the potential of HKUST-1 silica aerogel composite as the stationary phase for the separation of saturated and unsaturated hydrocarbons has been investigated by conventional liquid chromatography (LC). To avoid the micropore blockage an advanced approach was developed for synthesis of HKUST-1 silica aerogel composites (HKUST-1@SiO₂) using preliminary prepared SiO₂ sol as a silica precursor. Due to the benefit of combining MOF and silica aerogel structures without deterioration of their individual properties this type of material can be used as a sorbent in various fields such as gas storage and separation.

Experimental

Materials

The metal–organic framework HKUST-1 (Basolite® C300) was purchased from Sigma-Aldrich. Other commercial reagents and solvents were used without additional purification or distillation: tetraethoxysilane (TEOS, 98+%, Acros Organics), isopropanol (i-PrOH, ≥99.7%, FCC, Sigma-Aldrich), hydrochloric acid solution (HCl 32% aq., Sigma-Aldrich), ammonium hydroxide solution (NH₄OH 25% aq., Sigma-Aldrich), n-heptane (99.0%, HPLC-grade, Panreac), and n-octane (99+%, Acros Organics).

Synthetic procedure of HKUST-1@SiO₂ aerogel composite preparation

HKUST-1@SiO₂ aerogel composite was prepared by sol–gel method using supercritical CO₂ drying. Initially, the acid prehydrolysis of TEOS was carried out by mixing TEOS (15.0 mL), i-PrOH (15.6 mL) and diluted HCl aq. of pH = 2 (3.0 mL, prepared by dissolving 0.1 mL of HCl 32 wt% aq. in 90 mL of distilled water). A molar ratio of H₂O/Si of the resulting mixture was 2.5 : 1 with total silicon concentration C_Si = 2 mol L⁻¹. For aging, the mixture was maintained for two days at room temperature in a tightly closed flask (to prevent a solvent evaporation). For further polycondensation reaction and gelling of the SiO₂ sol, NH₃ solution in aqueous isopropanol (4.9 mL) prepared by mixing 0.5 mL of NH₄OH 25% aq., 2.7 mL of distilled water and 45.4 mL of isopropanol was added dropwise for 6 min to a stirred portion of the acid-prehydrolyzed TEOS (5 mL) at room temperature. At the next step, a suspension of the HKUST-1 in isopropanol prepared by vigorous stirring of 150, 257, 400, or 600 mg of HKUST-1 and 2.1 mL of isopropanol were added in one portion at ambient temperature. The resulting mixture was agitated thoroughly before the subsequent gelling during 30–40 min and maintained for two days at ambient temperature without agitation. Final molar ratios of Si/NH₃ and H₂O/Si in the gel were 14.8 : 1 and 4.2 : 1, correspondingly, at a concentration of SiO₂ about 50 g L⁻¹. So, the amount of the HKUST-1 was 20, 30, 40 and 50 wt% of the total weight of composite assuming no MOF was lost during the synthesis.

After gelling, the HKUST-1 silica aerogel without its mother liquor was carefully placed into the autoclave (300 mL, Autoclave Engineers, USA) charged with 50 mL of isopropanol. During the further drying procedure at supercritical condition in CO₂, isopropanol was slowly displaced by liquid CO₂ compressed up to 70 atm and supplied to the autoclave by syringe pump Teledyne Isco 260D (USA). After closing the autoclave, the liquid CO₂ (∼100 mL) under pressure of 70 atm was pumped thereto for 5 min. After maintaining for 2 h, a mixed CO₂/isopropanol solvent was partially released for ∼30 min at a low release rate. The release was stopped no sooner than ca. 25 mL of condensed isopropanol was collected in a graduated cylinder as receiving flask. Then a fresh portion of the pressured liquid CO₂ (∼75 mL) was pumped for 5 min. The procedure was repeated four times in 6, 16, 32, and 48 hours after starting the drying. At the end, the total amount of isopropanol collected in receiving flask was 47–48 mL. Thus, the replacement of isopropanol took ∼48 hours. Then, the temperature inside the autoclave was raised up to 60 °C at a rate of 30 °C h⁻¹, and the pressure was maintained at 100 atm. After reaching the temperature (60 °C), the autoclave was slowly depressurized by releasing the pressure at a rate of 0.4 atm min⁻¹. The heating was stopped after reducing the pressure down to 5–10 atm. After being fully depressurized, the autoclave was opened, and the dried HKUST-1@SiO₂ aerogel composite was applied for our purpose as synthetized.

For the synthesis of pure silica aerogel, the prehydrolysis of TEOS, the aging, the gelling of the SiO₂ sol and supercritical CO₂ drying were carried out in the same way as described for HKUST-1@SiO₂ aerogel composite preparation.

Characterization of the HKUST-1@SiO₂ aerogel composite

Powder XRD patterns were obtained on a Bruker D8 Advance diffractometer (Cu K_α radiation, λ = 0.15418 nm) equipped with a LynxEye position sensitive detector. The data were collected in the 2θ range of 5–55° with a step size of 0.05° and a collection time of 3 s. Textural characteristics were determined from low-temperature nitrogen adsorption isotherms obtained on a Micromeritics ASAP® 2400 analyzer. Investigation of the surface morphology of the samples was performed using a scanning electron microscope (SEM) JSM-6460 LV JEOL with energy-dispersive spectrometer INCA Energy350 (Oxford Instruments). The FT-IR spectroscopic analysis was performed using Bruker Vertex 70v spectrometer equipped with a diamond ATR accessory (Specac, Ltd., U.K.) and DLaTGS detector. X-ray photoelectron spectra (XPS) were recorded on a SPECS (Germany) photoelectron spectrometer using a hemispherical PHOIBOS-150-MCD-9 analyzer (Mg K* radiation, h* = 1253.6 eV, 100 W).

General procedure for the breakthrough experiments

Before the experiments, the monolithic HKUST-1@SiO₂ aerogel composite was powdered in a mortar and sifted to collect the fractions with a certain particle size (45–80, 80–200 and 200–500 μm). The breakthrough experiments were performed at room temperature on a 70 mm stainless steel column with an internal diameter of 4 mm (cartridge CatCart®70) filled with HKUST-1@SiO₂ or pure silica aerogel and placed in H-Cube Pro instrument (Thalesnano, Hungary) with an HPLC pump.^5g Sorbents were vacuum dried at 200 °C during 3 hours before the experiment. A 0.05 M solution of individual compounds (benzene, cyclohexene, p-xylene or styrene) in n-heptane containing n-decane (0.5 vol%) as an internal standard was fed into the column at a flow rate of 0.15 mL min⁻¹. Samples of 0.30–0.45 mL were taken directly at the column outlet, and concentrations were determined by GC analysis (Agilent 6890N instrument equipped with a 19091S-416 HP 5-MS capillary column 60.0 m × 320 μm × 0.25 μm) using n-decane as an internal standard.

Column chromatographic separation

The monolithic HKUST-1@SiO₂ aerogel composite was powdered in a mortar and sifted to collect the fraction of particle size 45–200 μm. The latter was activated at 200 °C in a vacuum for 3 h and suspended in n-octane. The suspension containing 2.0 g of the grinded HKUST-1@SiO₂ was transferred in a glass tube (15 mm inner diameter) to obtain a 60 mm long column. A mixture containing cyclohexane (1 μL) and cyclohexene or benzene (1 μL) was placed on the top of the column and eluted by n-octane at 25 °C. The elution rate was 3 mL h⁻¹. Samples of 1.5 mL were collected every 30 minutes, and concentrations were determined by GC. Based on these data, curves of the concentrations of each compound vs. eluted volume were plotted. Chromatographic separation of a mixture of cyclohexane and cyclohexene on a column packed with pure silica aerogel was performed in the same way except the length of the resulting column (90 mm).

Results and discussion

Preparation of the HKUST-1@SiO₂ aerogel composites

The sol–gel methods are widely used for the preparation of silica aerogel composites with metal oxides, metals, MOF etc. The matter is that sol can be synthesized with different particle sizes allowing to control the textural properties of composites (surface area, pore volume, porosity) through the adjusting synthetic conditions.¹³ One-step sol–gel syntheses of MOF-containing composites were developed early starting from TEOS as silica precursor and commercially produced HKUST-1 (ref. 12f) and ZIF-8 (ref. 12g). It was found that the microporosity of composites were increased with the increasing amount of HKUST-1 from 5 to 30 wt%, but blockage of some fraction of MOF's micropore was evidenced by BET data.^12f On our mind, the contact of HKUST-1 particles with the reaction media containing TEOS and HCl could negatively influence on their structure leading to the blockage of micropores.

To avoid a negative influence of the reaction media on the HKUST-1 structure the synthetic procedure was modified to minimize the residence time of HKUST-1 in the reaction vessel. As a consequence the powder of commercial HKUST-1 was introduced into a colloid solution of pre-synthesized SiO₂ sol right before the gelation starts that favour the preservation of the HKUST-1 original structure. The variation of the water concentration during the sol synthesis allows governing the size of the sol particles, while gelation time was minimized using optimal concentration of ammonia.

In accordance with the above described procedure the samples of HKUST-1@SiO₂ aerogel composites with different content of HKUST-1 (20, 30, 40, 50 wt%) were synthesized via sol–gel method and subsequent drying in supercritical CO₂. In order to do it, a suspension of HKUST-1 in isopropanol was added to SiO₂ sol which was previously prepared from acid-prehydrolysed TEOS. Earlier the influence of both SiO₂ sol characteristics and features of the following aerogel synthesis on the properties of the resulting silica aerogel was studied.^13a In particular it was shown that the gelation time strongly depends on ammonia, water and TEOS concentrations.

Characterization of the HKUST-1@SiO₂ aerogel composites

The HKUST-1@SiO₂ samples containing 20 or 30 wt% of MOF (hereinafter referred to as HKUST-1(20%)@SiO₂ or HKUST-1(30%)@SiO₂) had a good strength and other mechanical properties but the composites with the higher content of HKUST-1 were found to be a fragile material that can be easily crumbled and broken into small pieces. HKUST-1(30%)@SiO₂ sample was chosen for the further investigation as a compromise between the intention to improve the sorption properties by increasing the MOF content and to design a MOF-containing composite material with the proper mechanical properties. This material was characterized by physical methods and used in the sorption experiments.

Photography of monolithic HKUST-1(30%)@SiO₂ aerogel composite is shown in Fig. 1. One can see the composite looks like an opaque aerogel of blue colour. The characteristic peaks of pure crystalline HKUST-1 can be seen in XRD pattern of composite (Fig. 2) that confirms the preservation of individual MOF structure in synthesised material. The intensity of the appropriate XRD peaks in the composite was found to be less than the intensity of the peaks in the spectra of pure HKUST-1 because of the lower concentration of HKUST-1 in the amorphous silica aerogel. A similar decrease in intensity was documented in recent studies dealing with the synthesis of HKUST-1 silica composites.^8d,12f Based on these results, the conclusion can be made that the crystalline structure of HKUST-1 is stable in the conditions used during the composite preparation.

Fig. 1 Photography of monolithic HKUST-1@SiO₂ aerogel composite.

Fig. 2 XRD patterns of samples HKUST-1(30%)@SiO₂ and HKUST-1.

FT-IR and XPS spectroscopies showed no noticeable interactions between HKUST-1 and SiO₂ (ESI†) and no covalent bonding between MOF fragments and silica matrix. Thus, HKUST-1(30%)@SiO₂ composite represents physically dispersed domains of HKUST-1 in the silica aerogel network.

The surface morphology of the samples was investigated by SEM. The results showed that starting HKUST-1 represented the crystals with sizes from 1 to 20 μm (Fig. 3a). HKUST-1@SiO₂ composite is a monolithic silica aerogel with dispersed domains of HKUST-1 (Fig. 3b and c). SEM images of HKUST-1(30%)@SiO₂ with EDS mapping are presented in ESI.†

Fig. 3 SEM images of HKUST-1 (a) and HKUST-1(30%)@SiO₂ (b, c).

The low-temperature nitrogen adsorption isotherms of HKUST-1, pure silica aerogel and HKUST-1(30%)@SiO₂ composite are shown in Fig. 4. Adsorption isotherm of HKUST-1 revealed a type I of IUPAC classification which is typical of microporous materials. Adsorption isotherms of HKUST-1(30%)@SiO₂ and pure silica aerogel are a type IV that characterizes mesoporous adsorbents.¹⁴ Textural characteristics of the samples are shown in Table 1. The HKUST-1(30%)@SiO₂ composite has both a higher specific BET surface area and a higher degree of microporosity than pure silica aerogel. The micropore volume as well as the fraction of micropore surface area to BET surface area are directly proportional to the HKUST-1 weight percent in the composite pointing out the availability of MOF's micropore in the silica network.

Fig. 4 Adsorption isotherms of N₂ on HKUST-1, pure silica aerogel and HKUST-1(30%)@SiO₂ composite at 77 K.

Table 1 Textural characteristics of the samples

Sample	BET surface area, m^2 g^−1	Micropore area, m^2 g^−1	Micropore volume, cm^3 g^−1
HKUST-1	1673	1650	0.67
SiO2 aerogel	978	80	0.05
HKUST-1(30%)@SiO2	1655	551	0.18

---

Liquid-phase sorption and chromatographic separation of hydrocarbons on HKUST-1@SiO₂ aerogel composite

The HKUST-1(30%)@SiO₂ composite with different particle size (45–80 μm, 80–200 μm and 200–500 μm) was used for the liquid-phase sorption of benzene. The breakthrough experiments were performed using a column packed with HKUST-1(30%)@SiO₂ and pure silica aerogel (particle size of 80–200 μm) and 0.05 M solution of benzene in n-heptane (Fig. 5). The HKUST-1(30%)@SiO₂ composite was found to be able to sorb a significant amount of benzene before its detecting in the effluent stream. Benzene breakthrough occurred at ∼8 mL per g-sorbent in experiments with 45–80 μm and 80–200 μm fractions of HKUST-1(30%)@SiO₂. In the case of the fraction with larger particle size (200–500 μm) the breakthrough point is slightly lower, obviously due to diffusion limitations. Meanwhile, breakthrough point of pure silica aerogel was significantly lower (∼2 mL per g-sorbent). It means that the HKUST-1 particles of the composite are the main contributors to the sorption observed.

Fig. 5 Breakthrough experiments with a 0.05 M solution of benzene in n-heptane on a 7 cm column filled with HKUST-1(30%)@SiO₂ or pure silica aerogel (granules of 80–200 μm) at 25 °C.

The ability of HKUST-1(30%)@SiO₂ to sorb other unsaturated hydrocarbons, like cyclohexene, p-xylene and styrene is illustrated by the breakthrough profiles presented in Fig. 6. The sorption capacity determined by integrating above the breakthrough profile increased in the following order: cyclohexene < benzene ∼ p-xylene < styrene. The sorption of the individual compounds from n-heptane solution is likely to be governed mainly by the total number of π-electrons in the sorbate molecule that can coordinate to the copper sites in the framework of HKUST-1 with the π-complex formation.^2b,5

Fig. 6 Breakthrough experiments with a 0.05 M solutions of individual unsaturated hydrocarbons in n-heptane on a 7 cm column filled with HKUST-1(30%)@SiO₂ (granules of 80–200 μm) at 25 °C.

After the breakthrough experiments, the spent HKUST-1(30%)@SiO₂ composite can be regenerated by vacuum drying at 200 °C without loss of its sorption capacity.

A sorption capacity of HKUST-1(30%)@SiO₂ composite to unsaturated hydrocarbons is lower than that of the initial HKUST-1. So, benzene breakthrough occurred at ∼15.1 mL per g-sorbent in the same experimental conditions with HKUST-1. However, due to good packing properties of HKUST-1(30%)@SiO₂ this material may be of interest for use in separation of unsaturated hydrocarbons from their saturated analogues in preparative scale. To evaluate the potential of the HKUST-1(30%)@SiO₂ composite for LC, a glass tube of 15 mm inner diameter was charged with a suspension of HKUST-1(30%)@SiO₂ (2.0 g, granules of 45–200 μm) in n-octane to obtain a 6 cm high column. A probe containing 1 μL of cyclohexane and 1 μL of cyclohexene or benzene was placed on the top of the column and eluted by n-octane at 25 °C. The elution rate was 3 mL h⁻¹. The experimental chromatograms demonstrate a clear peak resolution in both cases (Fig. 7a and b). Thus, the HKUST-1(30%)@SiO₂ can be used as a stationary phase for separation of olefins or aromatics from corresponding paraffins and cycloparaffins by LC under atmospheric pressure. In previously published works chromatographic separations on a column packed with MOF composite were realized only by using HPLC technique at high pressure.^8a–c

Fig. 7 Chromatographic separation of mixtures: (a) cyclohexane and cyclohexene on a HKUST-1(30%)@SiO₂ column; (b) cyclohexane and benzene on a HKUST-1(30%)@SiO₂ column; (c) cyclohexane and cyclohexene on a pure silica aerogel column. The inset photo shows the real chromatographic column packed with HKUST-1(30%)@SiO₂ composite.

The separation of cyclohexane and cyclohexene has not been reached on a column packed with pure silica aerogel under similar conditions (Fig. 7c). It should be noted that chromatographic separation on same column with pure HKUST-1 (Basolite C300) is impossible because the sorbent layer 6 cm in height consisting of particles with sizes 1–20 μm completely blocks the eluent flow.

Conclusions

An advanced approach for the synthesis of HKUST-1 silica aerogel composites (HKUST-1@SiO₂) via sol–gel method and subsequent drying in supercritical CO₂ has been developed to minimize the deterioration of the individual properties of MOF and silica aerogel structures and tune its properties to be acceptable for flow mode. The synthesized HKUST-1@SiO₂ aerogel composite was characterized by X-ray diffraction, FT-IR spectroscopy, XPS, scanning electron microscopy with EDAX mapping and low-temperature nitrogen adsorption. XRD analysis indicated that HKUST-1 retained its crystal structure in the composite. It was registered no noticeable interactions between HKUST-1 and SiO₂. Thus, HKUST-1(30%)@SiO₂ composite represents physically dispersed domains of HKUST-1 in the silica aerogel network.

It was found that the HKUST-1@SiO₂ composite with 30 wt% of HKUST-1 can be used as a highly efficient stationary phase for conventional liquid chromatographic separation of cyclohexene or benzene from cyclohexane. This is the first time MOF composite has been used for the separation of organic molecules by LC. The HKUST-1 particles of the composite are the main contributors to the sorption of unsaturated hydrocarbons. Meanwhile, the role of silica is to ensure good packing properties of the material. The data obtained illustrate the great potential of MOF silica aerogel composites as the material that transfers unique properties of the MOFs into continuous flow processes.

Acknowledgements

This work was performed within the SB RAS program “II.2. Integration & Development” (project No. 0303-2015-0009).

References

1. (a) R. B. Eldridge, Ind. Eng. Chem. Res., 1993, 32, 2208; (b) J. P. Garcia Villaluenga and A. Tabe-Mohammadi, J. Membr. Sci., 2000, 169, 159.
2. (a) J. Padin, R. Yang and C. Munson, Ind. Eng. Chem. Res., 1999, 38, 3614; (b) A. Takahashi and R. T. Yang, AIChE J., 2002, 48, 1457.
3. (a) J.-R. Li, J. Sculley and H.-C. Zhou, Chem. Rev., 2012, 112, 869; (b) B. van de Voorde, B. Bueken, J. Denayer and D. De Vos, Chem. Soc. Rev., 2014, 43, 5766; (c) K. A. Cychosz, R. Ahmad and A. J. Matzger, Chem. Sci., 2010, 1, 293.
4. S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen and I. D. Williams, Science, 1999, 283, 1148.
5. (a) R. Ahmad, A. G. Wong-Foy and A. J. Matzger, Langmuir, 2009, 25, 11977; (b) M. Maes, F. Vermoortele, L. Alaerts, J. F. M. Denayer and D. E. De Vos, J. Phys. Chem. C, 2011, 115, 1051; (c) A. Ahmed, M. Forster, R. Clowes, P. Myers and H. Zhang, Chem. Commun., 2014, 50, 14314; (d) M. Maes, L. Alaerts, F. Vermoortele, R. Ameloot, S. Couck, V. Finsy, J. F. M. Denayer and D. E. De Vos, J. Am. Chem. Soc., 2010, 132, 2284; (e) L. Alaerts, M. Maes, M. A. van der Veen, P. A. Jacobs and D. E. De Vos, Phys. Chem. Chem. Phys., 2009, 11, 2903; (f) Z. R. Herm, E. D. Bloch and J. R. Long, Chem. Mater., 2014, 26, 323; (g) A. L. Nuzhdin and G. A. Bukhtiyarova, Mendeleev Commun., 2015, 25, 153.
6. http://www.sigmaaldrich.com/catalog/product/aldrich/688614.
7. V. Finsy, L. Ma, L. Alaerts, D. E. De Vos, G. V. Baron and J. F. M. Denayer, Microporous Mesoporous Mater., 2009, 120, 221.
8. (a) R. Ameloot, A. Liekens, L. Alaerts, M. Maes, A. Galarneau, B. Coq, G. Desmet, B. F. Sels, J. F. M. Denayer and D. E. De Vos, Eur. J. Inorg. Chem., 2010, 3735; (b) A. Ahmed, M. Forster, R. Clowes, D. Bradshaw, P. Myers and H. Zhang, J. Mater. Chem. A, 2013, 1, 3276; (c) K. Tanaka, T. Muraoka, D. Hirayama and A. Ohnish, Chem. Commun., 2012, 48, 8577; (d) A. Sachse, R. Ameloot, B. Coq, F. Fajula, B. Coasne, D. De Vos and A. Galarneau, Chem. Commun., 2012, 48, 4749.
9. (a) F. Jeremias, S. K. Henninger and C. Janiak, Chem. Commun., 2012, 48, 9708; (b) Y. Yoo and H.-K. Jeong, Chem. Commun., 2008, 2441.
10. R. Ameloot, F. Vermoortele, W. Vanhove, M. B. J. Roeffaers, B. F. Sels and D. De Vos, Nat. Chem., 2011, 3, 382.
11. (a) M. R. Lohe, M. Rose and S. Kaskel, Chem. Commun., 2009, 6056; (b) L. Li, S. Xiang, S. Cao, J. Zhang, G. Ouyang, L. Chen and C.-Y. Su, Nat. Commun., 2013, 4, 1774.
12. (a) N. Stock and S. Biswas, Chem. Rev., 2012, 112, 933; (b) Q.-L. Zhu and Q. Xu, Chem. Soc. Rev., 2014, 43, 5468; (c) M. Wickenheisser and C. Janiak, Microporous Mesoporous Mater., 2015, 204, 242; (d) M. Wickenheisser, A. Herbst, R. Tannert, B. Milow and C. Janiak, Microporous Mesoporous Mater., 2015, 215, 143; (e) X. Hu, X. Yan, M. Zhou and S. Komarneni, Microporous Mesoporous Mater., 2016, 219, 311; (f) Z. Ulker, I. Erucar, S. Keskin and C. Erkey, Microporous Mesoporous Mater., 2013, 170, 352; (g) A. Prabhu, A. Al Shoaibi and C. Srinivasakannan, Mater. Lett., 2015, 146, 43–46; (h) A. Kondo, S. Takanashi and K. Maeda, J. Colloid Interface Sci., 2012, 384, 110; (i) A. M. B. Furtado, J. Liu, Y. Wang and M. D. LeVan, J. Mater. Chem., 2011, 21, 6698; (j) Z. Li and H. C. Zeng, J. Am. Chem. Soc., 2014, 136, 5631; (k) X. Yan, X. Hu and S. Komarneni, RSC Adv., 2014, 4, 57501.
13. (a) G. M. Pajonk, E. Elaloui, P. Achard, B. Chevalier, J.-L. Chevalier and M. Durant, J. Non-Cryst. Solids, 1995, 186, 1; (b) I. A. Rahman and V. Padavettan, J. Nanomater., 2012, 2012, 132424; (c) V. P. Pakharukova, A. S. Shalygin, E. Y. Gerasimov, S. V. Tsybulya and O. N. Martyanov, J. Solid State Chem., 2016, 233, 294.
14. ISO 9277:2010 (en), Determination of the specific surface area of solid by gas adsorption–BET method.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06522h

---