Synthesis of zeolite Beta, MFI, and MTW using imidazole, piperidine, and pyridine based quaternary ammonium salts as structure directing agents

Abstract

The synthesis of zeolites is investigated using a variety of quaternary ammonium salts of imidazole, piperidine, and pyridine based structure directing agents. The materials were characterized by a complementary combination of X-ray diffraction, N₂-adsorption/desorption, scanning electron microscopy, and transmission electron microscopy. Only suitable quaternary ammonium salts were able to form zeolites Beta, MFI, and MTW. Zeolites (especially Beta) obtained using these quaternary ammonium salts are nanocrystalline in nature and exhibited high surface area and pore volume compared to conventional zeolites. Experimental evidence and density functional theory studies suggest that a limited conformational freedom, provided by the cyclic structure of the quaternary ammonium salts, is an important prerequisite and acts as a structure directing agent for the synthesis of zeolites.

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Introduction

Zeolites are crystalline microporous materials, which are widely used as molecular sieves, ion exchangers, and solid acid catalysts.^1–5 Zeolites are excellent solid acid catalytic materials due to their crystalline framework structures that are ordered at the atomic level.^1,4 Structure directing agents (SDA) for the synthesis of zeolites are able to form zeolites with pore sizes below 15 Å. The microporous nature of zeolites restricts their use as molecular sieves or catalysts in processes that involve large reactant molecules.^4,6 Thus, zeolites with controlled shape and large porosity are preferred to minimize diffusion constraints. To overcome these limitations, large pore mesoporous materials via a variety of supramolecular templating methods have been developed.^7–10 Recently, several review articles have appeared, which highlight various strategies to create additional porosity in these microporous materials.^2,5,11

Meso- and/or macropores can be created in the zeolite via either non-templating (through a post-synthesis treatment by steaming or partial leaching of zeolite lattice elements^12,13) or templating synthesis procedures.^2,5,11 Templating (hard/soft) procedures offer the opportunity to generate a controlled and possibly variable mesopore system, determined by the size and/or amount of the structure directing agents. Among the templating methods, hard templating has been first employed to create mesopores inside zeolites.^14–17 Mesoporous carbon and carbon nanotubes have been used as hard templates to form mesoporous MFI.^15,16,18,19 A truly intra-crystalline mesoporous MFI zeolite was obtained by using amphiphilic organosilane as the template.²⁰ A mesoporous MFI zeolite with a mesoporous/microporous hierarchical structure shows a remarkably improved resistance to the deactivation of catalytic activities in various reactions.²¹ Another approach to improve the diffusion in the zeolite is to minimize the size of the zeolite crystals and thereby shorten the diffusion path.²² Mesoporous MFI nanoparticle aggregates (termed as nanocrystalline MFI) have been synthesized using various triethoxyalkylsilanes.²³ Mesoporous MFI has also been synthesized using linear Gemini-type templates consisting of several bridged ammonium cations terminated by long alkyl chains.^24,25 Most of these studies were focused on the synthesis of nanocrystalline MFI. Only a few efforts have been made to prepare nanocrystalline Beta and MTW.^25–34 It has been reported that hierarchically porous Beta can be prepared by using 3,10-diazoniabicyclo[10.2.2]hexadeca-12,14,15-triene-3,3,10,10-tetramethyl-dichloride^29,27 or by using a suitable silylating agent such as phenylaminopropyl-trimethoxysilane.²⁸ Nanosized Beta with various particle sizes and high surface areas were synthesized with tetraethylammonium hydroxide (TEAOH) and cetyltrimethylammonium surfactants using a dry-gel method.²⁹

The literature survey suggests that soft templates, which are cationic in nature, are able to form hierarchical zeolites of different framework structures. Our research is focused on the synthesis and applications of ionic liquids.^30–35 The above mentioned observations stimulated us to design several quaternary ammonium salts of imidazole, piperidine and pyridine based SDA for the synthesis of zeolites. In this study, a variety of piperidine, imidazole, and pyridine based mono-quaternary and di-quaternary ammonium salts were prepared and utilized as a SDA for the synthesis of zeolites. In our preliminary communication, the synthesis of zeolite Beta using a few di-quaternary ammonium salts was reported.³⁵ In this study, the work has been extended to other kinds of quaternary ammonium salts for the synthesis of zeolites of different framework structures. A detailed investigation of the influence of textural properties of the resultant materials was made by using different types of precursors for the zeolite synthesis, synthesis parameters, and the role of several new quaternary ammonium salt based SDAs.

Experimental

Synthesis of quaternary ammonium salts of imidazole, piperidine and pyridine based structure directing agents

Since the quaternary ammonium salts of imidazole, piperidine and pyridine have ionic liquid-like properties, they have been designated as IL. IL2, IL4–IL9, and IL11–IL13 were synthesized by following previously reported procedures (see Scheme 1 and Scheme 2).^30,35,36

Scheme 1 Schematic representation for the synthesis of IL1–IL6.

For the synthesis of IL3, first the sodium salt of imidazole (NaIm) was synthesized according to the reported procedure.³⁶ In a typical synthesis, imidazole (4 mmol) and NaOH (4 mmol) were mixed and stirred at 383 K for 4 h under solvent-free conditions (Scheme 1). After the reaction, the reaction mixture was cooled and the by-product H₂O was removed under reduced pressure to obtain solid NaIm (yield= 94%). 1,6-Dibromohexane (5 mmol) was added to a solution of NaIm (5 mmol) and dry THF (30 mL) under nitrogen atmosphere. The reaction mixture was stirred for 12 h at room temperature. The obtained solid was filtered and the filtrate was concentrated under reduced pressure. The residue was subjected to column chromatography (silica gel (100–200 mesh), hexane/ethyl acetate (80/20) to afford 1-(6-bromo-hexyl)-imidazole as a colorless liquid (yield= 81%). The liquid was dissolved in acetonitrile (40 mL) and refluxed for 6 d. Upon completion of the reaction, the solvent was evaporated under vacuum. The residue was washed with ethyl acetate 3–4 times and then dried under vacuum at 353 K (yield= 87%).

Scheme 2 Schematic representation for the synthesis of IL7-IL16. Structure of a, b, c is provided in Scheme 1.

IL10 was prepared using a similar procedure to that adopted for the synthesis of IL7. In a typical synthesis, N,N′-hexamethylenebis(piperidine) (10 mmol) was reacted with 1,10-dibromo-decane (10 mmol) to obtain IL10 (yield= 88%) (Scheme 2).

IL14–IL16 were prepared using a similar procedure to that adopted for the synthesis of IL11. For their synthesis, 4,4′-Trimethylene bis(pyridine) (10 mmol) was reacted with 1,1′-dibromo-p-xylene (10 mmol), benzyl chloride (20 mmol) or 1,6-dibromohexane (10 mmol) in toluene to obtain IL14 (yield= 89%), IL15 (yield= 92%) or IL16 (yield= 90%) (Scheme 2).

IL3. IR (KBr, υ, cm⁻¹) = 3400, 2936, 1626, 1560, 1452, 1327, 1160, 1043, 867, 643. ¹H NMR (D2O) δ = 9.02 (s, 1H), 7.3 (s, 2H), 4.2 (t, 4H), 1.82 (m, 4H) 1.30 (m, 4H). ¹³C NMR δ = 135.6, 122.3, 49.4, 29.0, 24.8. Elemental analysis found: C, 46.37; H, 6.67; N, 12.07. Calc. for C₉H₁₅N₂Br: C, 46.75; H, 6.49; N, 12.12%.

IL10. IR (KBr, υ, cm⁻¹) = 3397, 2938, 1619, 1425, 1052, 941, 892, 861, 830,734. ¹H NMR (D2O) δ = 4.09–3.91 (m, 16H), 1.73–1.19 (m, 36H). ¹³C NMR δ = 59.1, 54.5, 29.0, 28.2, 27.9, 26.7, 22.6, 20.1. Elemental analysis found: C, 56.45; H, 9.61; N, 5.01. Calc. for C₂₆H₅₂N₂Br₂: C, 56.52; H, 9.49; N, 5.07%.

IL14. IR (KBr, υ, cm⁻¹) = 3415, 3037, 2948, 2880, 1633, 1469, 1292, 1158, 767. ¹H NMR (D2O) δ = 8.7 (d, 4H), 7.25–7.78 (m, 8H), 5.12 (s, 4H), 2.51 (t, 4H), 1.88 (m, 2H). ¹³C NMR δ = 159.2, 145.0, 134.1, 131.2, 129.7, 129.1, 60.1, 35.6, 29.1. Elemental analysis found: C, 54.48; H, 4.97; N, 5.93. Calc. for C₂₁H₂₂N₂Br₂: C, 54.57; H, 4.8; N, 6.06%.

IL15. IR (KBr, υ, cm⁻¹) = 3398, 3040, 2949, 2882, 1633, 1470, 1294, 1159, 763. ¹H NMR (D2O) δ = 8.75 (d, 4H), 7.20–7.82 (m, 14H), 5.92 (s, 4H), 2.64 (t, 4H), 1.90 (m, 2H). ¹³C NMR δ = 156.8, 144.9, 136.8, 131.1, 130.3, 128.9, 125.6, 62.7, 36.1, 28.2. Elemental analysis found: C, 71.72; H, 6.42; N, 6.14. Calc. for C₂₇H₂₈N₂Cl₂: C, 71.84; H, 6.25; N, 6.21%.

IL16. IR (KBr, υ, cm⁻¹) = 3398, 3039, 2948, 2882, 1633, 1474, 1293, 1158, 764. ¹H NMR (D₂O) δ = 8.8 (d, 4H), 7.89 (d, 4H), 4.01 (t, 4H), 2.60 (t, 4H), 1.99 (m, 2H), 1.87 (m, 4H), 1.31 (m, 4H). ¹³C NMR δ = 157.2, 143.5, 128.3, 60.9, 34.3, 30.2, 28.5, 24.8. Elemental analysis found: C, 51.41; H, 6.09; N, 5.84. Calc. for C₁₉H₂₆N₂Br₂: C, 51.6; H, 5.93; N, 6.33%.

Syntheses of Zeolites

For the synthesis of zeolites using sodium-silicate, first a sodium silicate solution was made using Ludox (40% aqueous solution, Aldrich) as the silica source. The silica source was diluted with a dilute NaOH solution yielding a sodium silicate solution with a molar ratio of 100SiO₂ : 30Na₂O. Two kinds of aluminum source, Al₂(SO₄)₃ and Al(NO₃)₃, were investigated in this study. The aluminum source was dissolved in dilute sulfuric acid (x : z : 1500, Aluminum source : H₂SO₄ : H₂O). The two solutions were mixed via vigorous stirring at room temperature. The resultant mixture was a clear solution. When IL was added slowly into this solution, under vigorous stirring at room temperature, the mixture gelled immediately. The molar gel composition can be represented as 30Na₂O : xAl₂O₃ : 100SiO₂ : yIL : zH₂SO₄ : 6000H₂O (where 0 ≤ x ≤ 2.5, 0 ≤ y ≤ 10, z = 15 or 20). After aging for 6 h at room temperature, the reaction mixture converted to a free flow liquid gel (less viscous), which was transferred to a Teflon-coated stainless-steel autoclave (100 mL capacity) and heated at 443 K with stirring. Precipitates were filtered, washed with deionised water, dried at 383 K, and calcined at 823 K for 4 h under flowing air. For the synthesis of Beta using tetraethylorthosilicate (TEOS), the required amount of H₂SO₄, followed by IL11 was added to a conventional alkaline mixture containing NaOH, TEOS, and distilled water to make the molar composition 30Na₂O:2.5Al₂O₃:100SiO₂:10IL11:15H₂SO₄:6000H₂O. When TEOS was used as silica source, ethanol formed was removed (after 6 h of reaction) during the hydrolysis of TEOS, before being loaded into autoclave. Conventional MFI³⁴ and Beta³⁷ were synthesized by following the reported procedure.

Characterization

X-ray diffraction (XRD) patterns were recorded in the 2θ range of 5–50° with a scan speed of 2° min⁻¹ on a PANalytical X'PERT PRO diffractometer using Cu Kα radiation (λ = 0.1542 nm, 40 kV, 40 mA) and a proportional counter detector. Nitrogen adsorption measurements at 77 K were performed by Autosorb-IQ Quantachrome Instruments volumetric adsorption analyzer. The sample was out-gassed at 573 K for 2 h in the degas port of the adsorption apparatus. The specific surface area was determined by BET method using the data points of P/P₀ in the range of about 0.05–0.3. The pore diameter was estimated using the Barret–Joyner–Halenda (BJH) model. The Transmission electron microscopy (TEM) investigation was carried out using a FEI, TECNAI G² 20, S-TWIN microscope operating at 200 kV, equipped with a GATAN CCD camera. The calcined sample was dispersed in ethanol using an ultrasonic bath, mounted on a carbon coated Cu grid, dried, and then used for the TEM measurement. Scanning electron microscopy (SEM) measurements were carried out on a JEOL JSM-6610LV to investigate the morphology.

Density function theory (DFT) calculation

The minimum-energy geometries of the quaternary ammonium salts were obtained by performing DFT geometry optimizations at the B3LYP/6-31G level using the Gaussian09 program.³⁸ Vibrational analysis was performed to ensure the absence of negative frequencies, which verify the existence of a true minimum. Theoretical studies were also performed by considering temperature in the calculations. The Born–Oppenheimer molecular dynamics (BOMD) simulation model was used to ensure the stability of the geometrically optimized structures of quaternary ammonium salts at the zeolite synthesis temperature (443 K) by using the method of Nose and Hoover.

Result and discussion

In this study, several mono-quaternary and di-quaternary ammonium salts of imidazole, piperidine and pyridine were synthesized using a multi-step synthetic route (Scheme 1 and Scheme 2). The quaternary ammonium salts investigated in this study are classified in five categories, they are: (1) imidazole based mono-cationic quaternary ammonium salts (IL1, IL2, and IL3), (2) imidazole based di-quaternary ammonium salts (IL4, IL5, and IL6), (3) piperidine based di-quaternary ammonium salts (IL7, IL8, IL9, and IL10), (4) 4,4′-trimethylenebis(1-methylpiperidine) based di-quaternary ammonium salts (IL11, IL12, and IL13), and (5) 4,4′-trimethylenebis(pyridine) based di-quaternary ammonium salts (IL14, IL15 and IL16). The quaternary ammonium salts were utilized as SDAs for the preparation of zeolites under hydrothermal conditions using sodium silicate and aluminum sulfate as the zeolite precursors. The effects of synthetic variables such as Si : Al molar ratio, amount of sulfuric acid, role of silica and aluminum source, and hydrothermal temperatures were investigated. Zeolites were synthesized under strong basic conditions using the molar composition 30Na₂O : xAl₂O₃ : 100SiO₂ : yIL : zH₂SO₄ : 6000H₂O, where 0 ≤ x ≤ 2.5, y = 0–10, z = 15 or 20. We are interested in obtaining the synthesis conditions by which zeolite materials having Beta, MFI, and MTW framework structures with high surface area and mesoporosity are obtained. The XRD patterns of the as synthesized form of the zeolites (Beta, MFI and MTW) obtained using IL are similar to the XRD patterns of the calcined materials. The XRD patterns match well with the reported XRD pattern of zeolites (Beta, MFI and MTW), which clearly indicates the high purity of the obtained zeolite materials. CHN analysis confirms that only a negligible amount of organic content (<0.1%) is present in the calcined form of the zeolite.

Synthesis of zeolites using imidazole based mono-cationic quaternary ammonium salts (IL1–IL3)

First, the mono-cationic quaternary ammonium salts were explored for the synthesis of zeolites under our synthesis conditions. The investigation was started using commercially available 1-butyl,3-methyl-imidazolium bromide (IL1), which is known to form zeolite Beta and MFI.³⁹ However, under our synthesis conditions, an unknown crystalline silicate phase was observed (Fig. S1, ESI†). Using IL2, no crystalline phase was obtained under a variety of the synthesis conditions investigated in this study. IL3 was able to form zeolite MFI [designated as MFI-IL3-1 (when 15H₂SO₄ was used), and MFI-IL3-2 (when 20H₂SO₄ was used)] under the synthesis composition 30Na₂O : 0.625Al₂O₃ : 100SiO₂ : 10IL3 : 15 or 20H₂SO₄ : 6000H₂O (Fig. 1a). N₂-adsorption studies show that both materials exhibited type IV isotherms with H4 hysteresis (Fig. 1b), which shows that the material contains both micropores and mesopores. The isotherm of MFI-IL3-1 clearly evidences capillary condensation in the mesopores of the material, which is reflected in their textural properties (Table 1). The morphology of MFI-IL3-1 and MFI-IL3-2 was found to be spheroidal (Fig. 1c). The particle sizes of MFI-IL3-1 were found to be smaller than those of MFI-IL3-2. Agglomeration of the small particles is reflected in the properties of the obtained materials. Due to the relatively small particle size, the surface area of MFI-IL3-1 was found to be larger when compared to MFI-IL3-2. The agglomeration of the small particles in MFI-IL3-1 is reflected in its higher mesopore volume compared to MFI-IL3-2. Mesoporous void spaces are formed due to the crystal packing of these small particles. It is, further, interesting to note that when the synthesis was performed in the absence of Al (30Na₂O : 0Al₂O₃ : 100SiO₂ : 10IL3 : 15H₂SO₄ : 6000H₂O), MTW was obtained (designated as MTW-IL3, Fig. 1a). MFI-IL3-1 exhibited significantly larger surface area and mesopore volume compared to conventional MFI, which signifies that MFI-IL3-1 is nanocrytsalline in nature. One structure directing agent is able to form zeolites of two different framework structures (MFI and MTW). The structure of IL3 was optimized using density functional theory (DFT) method employing B3LYP with 6-31G basis set (Fig. 1d and Table 2).³⁸ The molecular dimension of IL3 is larger than the pore apertures of the MFI and MTW zeolites (maximum diameters of a sphere that can be included in MFI and MTW framework structure are 6.36 and 6.08 Å, respectively). Hence, theoretical study confirms that the size of the SDA is not the important factor for the synthesis of MFI and MTW structures. Very recently, it has been reported that for the formation of zeolite Beta, the molecular dimension of the SDA should be less than the pore channel of the 12 member ring of Beta (pore dimensions: 6.6 × 6.7 Å for [100] and 5.6 × 5.6 Å for [001]).²⁶ The molecular size of the IL3 is less than the pore aperture of zeolite Beta, but it was unable to form zeolite Beta. Therefore, it can be concluded that the critical size of the SDA is not the only necessary condition for the formation of zeolite Beta.

Fig. 1 (a) XRD patterns of MTW-IL3, MFI-IL3-1 and MFI-IL3-2, (b) N₂-adsorption isotherms of MFI-IL3-1 and MFI-IL3-2 (inset shows pore size distributions), (c) SEM images of MFI-IL3-1, MFI-IL3-2 and MTW-IL3 and (d) Optimized structure of IL3 using B3LYP/6-31G.

Table 1 Textural properties of various zeolites (except Beta) synthesized using a variety of quaternary ammonium salt based structure directing agents. Unless otherwise stated, sodium silicate and Al₂(SO₄)₃ were used as the silica and aluminium sources, respectively

Zeolite	Molar composition	S BET (m^2 g^−1)	V micro (mL g^−1)	V meso (mL g^−1)
MFI-IL3-1	30Na2O : 0.625Al2O3 : 100SiO2 : 10IL3 : 15H2SO4 : 6000H2O	497	0.14	0.24
MFI-IL3-2	30Na2O : 0.625Al2O3 : 100SiO2 : 10IL3 : 20H2SO4 : 6000H2O	326	0.12	0.10
MFI	30Na2O : 0.625Al2O3 : 100SiO2 : 10TPABr : 20H2SO4 : 6000H2O	328	0.13	0.08
MTW-IL3	30Na2O : 0Al2O3 : 100SiO2 : 10IL3 : 15H2SO4 : 6000H2O	204	0.10	0.04
MTW-IL6-1	30Na2O : 0.625Al2O3 : 100SiO2 : 10IL3 : 20H2SO4 : 6000H2O	280	0.11	0.16
MTW-IL6-2	30Na2O : 0.625Al2O3 : 100SiO2 : 10IL3 : 15H2SO4 : 6000H2O	256	0.10	0.13
MTW-IL11-1	30Na2O : 0Al2O3 : 100SiO2 : 10IL11 : 15H2SO4 : 6000H2O	301	0.11	0.12
MTW-IL11-2	30Na2O : 0.625Al2O3 : 100SiO2 : 10IL11 : 20H2SO4 : 6000H2O	230	0.10	0.06
MTW-IL11-3	30Na2O : 0Al2O3 : 100SiO2 : 10IL11 : 20H2SO4 : 6000H2O	287	0.11	0.11
MTW-IL13	30Na2O : 0.63Al2O3 : 100SiO2 : 10IL16 : 15H2SO4 : 6000H2O	298	0.12	0.11

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Table 2 Structural properties of quaternary ammonium salts investigated in this study

Quaternary ammonium salt	Dipole moment (Debye)	Energy (a.u)	Molecular size (Å)
IL1	5.82	−423.06	10.21 × 4.3 × 3.38
IL2	7.60	−536.15	10.1 × 4.42 × 4.33
IL3	2.32	−460.62	6.46 × 5.02 × 3.74
IL4	1.29	−995.36	10.46 × 7.6 × 4.62
IL5	5.52	−1227.60	21.0 × 7.76 × 5.23
IL6	0.82	−921.58	10.88 × 8.24 × 3.82
IL7	0.17	−1047.34	16.34 × 5.52 × 5.89
IL8	3.27	−1279.59	13.84 × 5.80 × 5.30
IL9	1.04	−973.57	13.99 × 6.84 × 4.63
IL10	4.79	−1130.80	15.3 × 8.33 × 5.41
IL11	4.16	−1008.06	12 × 5 × 6.8
IL12	2.33	−1240.30	16.49 × 8.3 × 6.82
IL13	4.01	−934.27	12.16 × 8.1 × 4.78
IL14	1.37	−922.21	7.11 × 6.91 × 4.54
IL15	2.07	−1154.48	21.0 × 4.75 × 4.81
IL16	0.62	−848.44	8.3 × 7.9 × 4.63

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Synthesis of zeolites using imidazole based di-quaternary ammonium salts (IL4–IL6)

Imidazole based di-quaternary ammonium salts (IL4–IL6) were investigated as SDAs for the synthesis of zeolites. MTW was obtained using all the imidazole based quaternary ammonium salts with molar composition (30Na₂O : xAl₂O₃ : 100SiO₂ : 10IL : 15 or 20H₂SO₄ : 6000H₂O; 75 < Si/Al < ∞). It was observed that MTW was formed under low Al content and this is consistent with the reported literature, which states that MTW is thermodynamically favored under high-silica synthesis conditions.⁴⁰ In this manuscript, physico-chemical data is provided only for MTW synthesized using IL6 (MTW-IL6-1, MTW-IL6-2) (Fig. 2a and Table 1).

Fig. 2 (a) XRD patterns of BETA-IL6, MTW-IL6-1, MTW-IL6-2 and MTW-IL6-3, (b) N₂-adsorption isotherm of BETA-IL6 (inset shows pore size distribution) and (c) SEM and TEM images of BETA-IL6.

It is very interesting to note that zeolite Beta was obtained using IL6 (designated as Beta-IL6), when synthesis was performed using the gel composition 30Na₂O : 2.5Al₂O₃ : 100SiO₂ : 10IL6 : 15H₂SO₄ : 6000H₂O (Fig. 2a), whereas IL4 and IL5 were unable to form any crystalline phase under this synthesis condition. The broad nature of the XRD peak of Beta-IL6 clearly shows that its crystal sizes are smaller than conventional Beta (Table 3). Zeolite Beta obtained in this condition is not the traditional zeolite Beta but a mixture of polymorphs of A and B phases. Traditional zeolite Beta is highly disordered. It is made of the random intergrowth of polymorph A and polymorph B in the ratio of ca. 45 : 55.^41,42 Both the polymorphs A and B are constructed from the same centrosymmetric, tertiary building unit (TBU) arranged in layers successively, which interconnect in either a left handed (L) or right handed (R) manner. Polymorph A represents an uninterrupted sequence of either left handed or right handed, whereas the polymorph B has an alternative stacking sequence. TBU has no preference for either mode of connection, which leads to an almost equal possibility of each in the structure of Beta, which enhances the growth of a faulty structure. It is reported in the literature that if a mixture of polymorph A and polymorph B of zeolite beta is formed then the broad peak at low angle splits into two sharp peaks of different intensity showing their percentage composition in the structure.^41,42 Based on the XRD obtained for Beta-IL6, one can say that Beta-IL6 consists of mixture of polymorph B and polymorph A. The N₂-adsorption study shows that Beta-IL6 exhibited a type IV isotherm with a hysteresis loop corresponding to capillary condensation in the mesopores (Fig. 2b). Furthermore, the zeolite exhibited a bi-modal distribution of mesopores in the range of 3.1 to 6.3 nm. N₂-adsorption studies show that the material has very high surface area and mesopore volume compared to conventional Beta (Table 3). SEM and TEM analysis showed that Beta-IL6 is composed of zeolite nanocrystals of 16-18 nm of diameter (Fig. 2c).

Table 3 Textural properties of zeolite Beta synthesized using a variety of quaternary ammonium salts based structure directing agents with molar composition 30Na₂O : xAl₂O₃ : 100SiO₂ : yIL : zH₂SO₄ : 6000H₂O

Zeolite	x : y : z	Time (d)	Temp. (K)	Crystallite size (Å)	SBET (m^2 g^−1)	Vmicro (mL g^−1)	Vmeso (mL g^−1)
a Synthesized by following the reported procedure, b TEOS was used as silica source, c Al2(NO3)3 was used aluminum source.
Beta^a	—	—	—	29	495	0.18	0.23
Beta-IL6	2.5 : 10 : 15	2	443	13.6	670	0.18	0.63
Beta-IL11-1	2.5 : 10 : 15	2	443	13.5	685	0.18	0.65
Beta-IL11-2	0.625 : 10 : 15	2	443	17.5	645	0.17	0.62
Beta-IL11-3	2.5 : 5 : 15	4	443	16.3	663	0.18	0.62
Beta-IL11-4	2.5 : 10:15	7	403	15.4	672	0.18	0.63
Beta-IL11-5^b	2.5 : 10:15	2	443	24.0	528	0.17	0.32
Beta-IL11-6^c	2.5 : 10:15	2	443	16.9	648	0.18	0.62
Beta IL12	2.5 : 10:15	4	443	23.1	480	0.17	0.21
Beta-IL13	0 : 10:15	2	443	16.3	590	0.18	0.5

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To understand why only IL6 is able to form zeolite Beta, structures of IL4–IL6 were optimized (Fig. 3) and structural parameters are provided in Table 2. The molecular dimensions of IL4–IL6 were found to be larger than the pore aperture of MTW and Beta (maximum diameter of a sphere that can be included in Beta framework structure is 6.68 Å). Formation of zeolite Beta using IL6 confirms that the molecular size is not a prerequisite for the formation of zeolite Beta. The optimized structures reveal that the imidazole rings in IL4–IL6 are situated at different planes (Fig. 3). Two imidazole rings in IL4, IL5, and IL6 make an angle of 58°, 73°, and 110°, respectively. Based on the experimental and theoretical studies, one can conclude that Beta can be obtained using imidazole containing cyclic SDA, when two imidazole rings make an angle of 110°. Such an observation can lead us to conclude that structural rigidity is an important factor, which leads to the formation of a zeolite of suitable framework structure. Since MTW is formed by IL4–IL6, hence it can be concluded that such structural rigidity is not a prerequisite for the formation of the MTW framework.

Fig. 3 Optimized structure of IL4–IL6 using B3LYP/6-31G.

Synthesis of zeolites using piperidine based di-quaternary ammonium salts (IL7–IL10)

Having found success in the synthesis of zeolites of various framework structures using imidazole based quaternary ammonium salts (especially di-quaternary ammonium salts), several piperidine based di-quaternary ammonium salts (IL7–IL10) were investigated in the synthesis of zeolites. Piperidine based SDA (IL7–IL10) fails to produce any known zeolite framework structure under the variety of synthesis compositions investigated in this study. Several unknown silicate phases were obtained for the materials synthesized using IL7–IL10 (Fig. S1, ESI†). The structures of IL7–IL10 were optimized to understand the reason for their inability to form zeolite Beta or MTW and structural parameters are provided in Table 2. The optimized structures reveal that the piperidine rings exist in the twist-boat conformation in IL7–IL10 (Fig. 4 and Fig. 5).

Fig. 4 Optimized structures of IL7 and IL8 using B3LYP/6-31G.

Fig. 5 Optimized structures of IL9 and IL10 using B3LYP/6-31G.

Synthesis of zeolites using 4,4′-trimethylenebis(1-methylpiperidine) based di-quaternary ammonium salts (IL11–IL13)

4,4′-trimethylenebis(1-methylpiperidine) based SDAs (IL11–IL13) were found to be excellent for the synthesis of zeolite Beta and MTW. Preliminary investigation shows that all these SDAs are able to produce zeolite Beta.³⁵ Detailed investigation on the influence of silica and aluminum sources, synthesis temperature, and the amount of H₂SO₄ are discussed in the following section.

Under the synthesis composition 30Na₂O : xAl₂O₃ : 100SiO₂ : 10IL11 : 15H₂SO₄ : 6000H₂O (with Si/Al < 100), zeolite Beta (Beta-IL11-1 and Beta-IL11-2) was obtained, when IL11 was used as the SDA (Fig. 6a, Table 3). The N₂-adsorption study shows that Beta-IL11-1 exhibited a type IV isotherm with H2 hysteresis loop corresponding to capillary condensation in the mesopores (Fig. 6b). Furthermore, the zeolite exhibited a broad distribution of mesopore diameters (2.7 to 8.3 nm). The crystallite size of Beta-IL11-1 calculated from XRD was found to be 13.5 Å, which matches well with the TEM investigation (Fig. 6c). Crystallite size, surface area, and pore volume of Beta-IL11-2 were found to be 17.5 Å, 645 m² g⁻¹, and 0.79 mL g⁻¹, respectively (Table 3). The influence of the amount of IL11 in the synthesis was investigated. It was found that, when the synthesis was performed using SDA : SiO₂ = 1 : 20 instead of 1 : 10, Beta (designated as Beta-IL11-3) synthesis took a longer time (4 d). Crystallite size of the Beta-IL11-3 calculated from XRD was found to be larger (16.3 Å) than Beta-IL11-1, whereas surface area and pore volume of Beta-IL11-3 was found to be less compared to Beta-IL11-1 (Table 3). When synthesis was performed using SDA : SiO₂ = 1 : 40, Beta did not form even after 4 d of hydrothermal treatment. Based on the above investigation, it can be concluded that a good quality Beta sample can be obtained when SDA : SiO₂ = 1 : 10. When the synthesis was carried out at 403 K instead of 443 K, 7 d are required to form zeolite Beta (Beta-IL11-4) (Fig. 6a, Table 3). Based on these investigations one can conclude that either by decreasing the amount of SDA or by decreasing the synthesis temperature, the synthesis of Beta required a longer time. An interesting result was observed when the synthesis was performed using TEOS, which required 4 d to form zeolite Beta (Beta-IL11-5) (Fig. 7a). The crystallite size of Beta-IL11-5 was found to be comparatively larger (24 Å) than Beta-IL11-1. The N₂-sorption study shows that Beta-IL11-5 exhibited a type IV isotherm with H3 hysteresis loop corresponding to capillary condensation in mesopores (Fig. 6b). Capillary condensation in Beta-IL11-5 was less prominent compared to Beta-IL11-1. Although zeolite Beta was obtained using Al(NO₃)₂ in 2 d (designated as Beta-IL11-6), it exhibits slightly less surface area and pore volume as compared to Beta-IL11-1 (Table 3). After evaluating all the synthesis parameters, it can be concluded that a high quality Beta sample can be obtained by using sodium silicate as the silica source and Al₂(SO₄)₃ as the alumina source with the synthesis composition (30Na₂O : 2.5Al₂O₃ : 100SiO₂ : 10IL14 : 15H₂SO₄ : 6000H₂O) in 2 d of hydrothermal treatment at 443 K. Though it was difficult to differentiate the morphology and particle sizes of various Beta samples prepared in this study using SEM, a clear differentiation can be obtained for the sample synthesized using TEOS and samples synthesized using sodium silicates (compare SEM images in Fig. 7b with Fig. 6c). A large particle size of about 1 μm was observed in the SEM micrograph of Beta-IL11-5, which is made up of several small nanocrystals. TEM investigation gives clear evidence that Beta-IL11-5 synthesized using TEOS is made up of small nanocrystals of about 25–28 nm (Fig. 7b), whereas the Beta-IL11-1 mesoporous framework is composed of zeolite nanocrystals of 14–15 nm in diameter (Fig. 6c). These diameters are in agreement with the crystallite sizes calculated from XRD by using the Scherrer equation.

Fig. 6 (a) XRD patterns of various Beta samples synthesized using IL11 under different conditions, (b) N₂-adsorption isotherms of Beta-IL11-1, Beta-IL11-5 and MTW-IL11-1 (inset shows pore size distribution), (c) SEM and TEM images of Beta-IL11-1.

Fig. 7 (a) XRD patterns of Beta-IL11-1, Beta-IL11-5 and Beta-IL11-6, and (b) SEM and TEM images of Beta-IL11-5.

Under a variety of synthesis conditions, IL11 leads to the formation of zeolite Beta. However, when the synthesis was performed by using a large amount of H₂SO₄, or under high Si/Al condition (Si/Al > 100), it resulted in the formation of zeolite MTW. For example: under the same synthesis composition but without Al [(30Na₂O : 0Al₂O₃ : 100SiO₂ : 10IL14 : 15H₂SO₄ : 6000H₂O), IL11 forms MTW (MTW-IL11-1)] (Fig. 8a). When the amount of H₂SO₄ was increased from 15H₂SO₄ to 20H₂SO₄, then zeolite MTW was obtained (0 < Si/Al < ∞) (designated as MTW-IL11-2 and MTW-IL11-3). SEM investigations confirm that the morphology of samples is varied by varying the synthesis composition. MTW-IL11-2 exhibits nanowire like morphology, whereas other two samples of this series exhibited spheroidal morphology (Fig. 8c). The N₂-adsorption study of MTW-IL11-1 shows that the material exhibited a type IV isotherm with a broad distribution of mesopore diameters (3 to 10 nm) (Fig. 6b). The hysteresis loop of MTW-IL11-1 confirms that the material has less mesopore volume compared to Beta-IL11-1. Textural properties obtained from the N₂-adsorption studies are summarized in Table 1, which confirms that the MTW materials have less surface area and lower pore volumes compared to Beta samples synthesized using IL11. The formation of MTW zeolite at low Al content in the present work is consistent with the reported literature.⁴⁰

Fig. 8 (a) XRD patterns of MTW-IL11-1, MTW-IL11-2 and MTW-IL11-3, (b) XRD patterns of Beta-IL12, Beta-IL13 and MTW-IL13, (c) SEM images of MTW-IL11-1, MTW-IL11-2, MTW-IL11-3 and MTW-IL13 and TEM images of MTW-IL11-1.

Under the synthesis composition 30Na₂O : xAl₂O₃ : 100SiO₂ : 10IL12 : 15H₂SO₄ : 6000H₂O (with Si/Al < 50) using IL12, zeolite Beta was obtained (Beta-IL12). Either increasing the Si/Al or increasing the amount of H₂SO₄ leads to the formation of a mixture of zeolites (BETA + MTW). Beta (designated as Beta-IL13) was obtained using IL13 using the synthesis composition 30Na₂O : 0Al₂O₃ : 100SiO₂ : 10IL13 : 15H₂SO₄ : 6000H₂O, in the absence of Al. Under high Al content (Si/Al < 50) or using a high concentration of H₂SO₄, no zeolite phase was obtained using IL13. Under low aluminium content (70 < Si/Al < ∞), MTW was obtained using IL13 (MTW-IL13). The textural properties obtained from the N₂-adsorption studies are provided in Table 1 and Table 3. The structures of IL11–IL13 were optimized (Fig. 9) and their structural properties are provided in Table 2.

Fig. 9 Optimized structures of IL11–IL13 using B3LYP/6-31G.

Synthesis of zeolites using 4,4′-trimethylenebis(pyridine) based di-quaternary ammonium salts (IL14–IL16)

The previous section demonstrates that 4,4′-trimethylenebis(1-methylpiperidine) based SDAs are best suited for the synthesis of zeolite Beta. It was also noticed that only a suitable type of imidazole based SDA (IL6) was able to form zeolite Beta. Hence, to understand the role of different kinds of heterocyclic compounds and ring rigidity, structurally homologous pyridine based SDAs were prepared. Under the variety of synthesis conditions investigated in this study, pyridine based di-quaternary ammonium salts failed to produce any crystalline phase. Structures of IL14–IL16 were optimized to understand their inability to form zeolites (Fig. 10), and their structural properties are provided in Table 2.

Fig. 10 Optimized structures of IL14–IL16 using B3LYP/6-31G.

Role of quaternary ammonium salts as SDAs for the synthesis of zeolites

It has been demonstrated that quaternary ammonium salts of imidazole and piperidine can be used not only as functional solvents but also as templates for the preparation of various inorganic nanomaterials with novel and improved properties.⁴³ Molecular interactions such as H bonding,⁴⁴ π–π stacking,⁴⁵ and other dispersive forces (such as van der Waals interactions)⁴⁶ are responsible for their pre-organized structures and templating action in the synthesis of various nanoporous materials such as mesoporous silica and metal oxides.^47–49 The physico-chemical properties of quaternary ammonium salts of imidazole and piperidine are much more diverse than those of conventional SDAs of zeolites and, hence, they can be used as the SDA for the synthesis of zeolite.³⁹

Control experiments reveal that the initial gel phase (0 h) contained only 2.5 wt% of TEA⁺ and the final zeolite phase contained 7.7 wt% TEA⁺, when zeolite Beta was synthesized using conventional TEA⁺ as the SDA. This demonstrates that when using TEA⁺ as the SDA, Na⁺ was the predominant cation in the initial gel, and this was replaced with TEA⁺ after prolonged hydrothermal crystallization. The similar composition of IL11 in the initial phase (16.5%) and final zeolite phase (19.2%) suggests that the initial phase consists of zeolite like organized silicate species around IL11. Such a pre-organized zeolite like assembly is responsible for the facile synthesis of zeolite Beta using IL11. It is highly possible that the di-quaternary ammonium salts are able to generate a large number of zeolite seeds when compared to their mono-ammonium analogues TEA⁺ or 1-butyl,3-methyl-imidazolium cation.³⁹ This was confirmed from the XRD analysis of the products obtained using IL11, which show that a pre-organized zeolite like assembly was formed during gelation (0 h, before crystallization) (Fig. 11). This process reduced the nucleation time and further crystal growth took place rapidly to form nanocrystalline zeolite Beta. The synthesis of nanocrystalline zeolites using di-quaternary ammonium salts indicates that the presence of two ammonium groups in the same molecule could be the most important factor for the generation of nanocrystalline morphologies and mesoporosity.

Fig. 11 XRD patterns of the product obtained during the crystallization of zeolite Beta using IL11.

DFT calculations helped us to understand the ability and inability of the quaternary ammonium salts investigated in this study to form zeolite Beta. Very recently, it has been reported that for the formation of zeolite Beta the molecular dimension of the SDA should be less than the pore channel of the 12 member ring of Beta.²⁶ However, our results, and previous reports²⁶ clearly demonstrate that this may not be the necessary condition for the formation of Beta. To understand the reason behind our observations, the geometrically optimized structures were carefully analyzed. The piperidine rings of IL7–IL10 exist in twist-boat conformation, whereas the piperidine rings of IL11–IL13 exist in chair conformation. Since only IL11–IL13 are able to form zeolite Beta and MTW, we can conclude that a suitable conformation (chair form) is required for the formation of Beta and MTW. Such a limited conformational freedom is known as an important prerequisite for the selective structure direction of zeolites.⁵⁰ The BOMD simulation method of Nose and Hoover indicates that conformational change by thermal rotation and vibration is greatly suppressed, even at zeolite synthesis temperature (443 K), due to rigid cyclic geometry. For instance, results of the simulation model for IL11 are provided in the ESI section (Fig. S2†). The molecular structure of IL11 (Fig. S3, ESI†) at different stages of the potential energy diagram confirms that the chair confirmation is stable even at the zeolite synthesis temperature (443 K). In the above section, it has been described that among the imidazole based SDA, only IL6 was able to form zeolite beta, in which two imidazole rings make an angle of 110°. However, all imidazole based SDA (IL4–IL6) were able to form zeolite MTW. Further, pyridine based SDA (IL14–IL16) were unable to form Beta or MTW. Based on these observations, one can conclude that structural rigidity varies from ring to ring and therefore their ability to form a suitable kind of zeolite framework also varies. After analyzing all the results, it can be concluded that the size of the SDA (investigated in this study) is not an important factor to modulate the crystallization process for a suitable kind of framework structure, but it requires a suitable special orientation of imidazole and piperidine rings, which enables the formation of a zeolite of a particular framework structure.

Conclusion

A variety of mono-quaternary and di-quaternary ammonium salts of imidazole, piperidine and pyridine were utilized as structure directing agents for the synthesis of zeolite Beta, MFI, and MTW. 4,4′-trimethylenebis(1-methylpiperidine) and imidazole based di-quaternary ammonium salts were found to be very effective SDAs for zeolites having a 12-membered pore channel, i.e., Beta and MTW, whereas the mono-quaternary ammonium salts of the imidazole based cyclic compounds generated MTW and nanocrystalline MFI zeolites. The zeolites thus obtained were shown to be nanocrystalline materials with high surface areas and large mesopore volumes. The presence of the mesopores between adjacent nanocrystals is known to be an important factor, leading to significantly improved molecular diffusion and hence catalytic performance (e.g., lifetime, activity and selectivity). The synthetic cost of Beta is very high due to the high cost for the preparation of organic SDAs in the hydroxide form. In this study, Beta was synthesized using low cost structure directing agents (in halide form), sodium silicate as the silica source, and aluminum sulphate as the aluminum source. Experimental evidence and DFT calculations suggest that structural rigidity along with a specific spatial orientation of imidazole and piperidine rings of quaternary ammonium salts are important factors for delivering zeolites of a particular framework structure.

Acknowledgements

The authors are thankful to the Department of Science and Technology, New Delhi, for financial assistance (DST grant SR/S1/PC/31/2009). We are grateful to Dr T. J. Dhilip Kumar, Chemistry Department, IIT Ropar for providing Gaussian 09 software and valuable suggestions. The authors are also thankful to Prof. M. K. Surappa, Director IIT Ropar for his constant encouragement.

References

1. A. Corma, Chem. Rev., 1997, 97, 2373.
2. Y. Tao, H. Kanoh, L. Abrams and K. Kaneko, Chem. Rev., 2006, 106, 896.
3. G. Basca, Chem. Rev., 2007, 107, 5366.
4. R. Rinaldi and F. Schüth, Energy Environ. Sci., 2009, 2, 610.
5. C. S. Cundy and P. A. Cox, Chem. Rev., 2003, 103, 663.
6. J. S. Beck, J. C. Vartuli, G. J. Kennedy, C. T. Kresge, W. J. Roth and S. E. Schramm, Chem. Mater., 1994, 6, 1816.
7. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 1992, 359, 710.
8. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Frederickson, B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548.
9. P. T. Tanev and T. J. Pinnavaia, Science, 1995, 267, 865.
10. T. W. Kim, F. Klitz, B. Paul and R. Ryoo, J. Am. Chem. Soc., 2005, 127, 7601.
11. J. Pérez-Ramírez, C. H. Christensen, K. Egeblad, C. H. Christensen and J. C. Groen, Chem. Soc. Rev., 2008, 37, 2530.
12. A. H. Janssen, A. J. Koster and K. P. de Jong, Angew. Chem., Int. Ed., 2001, 40, 1102.
13. R. Srivastava, N. Iwasa, S-I. Fujita and M. Ara, Catal. Lett., 2009, 130, 655.
14. B. T. Holland, L. Abrams and A. Stein, J. Am. Chem. Soc., 1999, 121, 4308.
15. C. J. H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt and A. Carlsson, J. Am. Chem. Soc., 2000, 122, 7116.
16. Y. S. Tao, H. Kanoh and K. Kaneko, J. Am. Chem. Soc., 2003, 125, 6044.
17. K. Egeblad, H. Christina Christensen, M. Kustova and C. H. Christensen, Chem. Mater., 2008, 20, 946.
18. A. Sakthivel, S. J. Huang, W. H. Chen, Z. H. Lan, K. H. Chen, T. W. Kim and R. Ryoo, Chem. Mater., 2004, 16, 3168.
19. I. Schmidt, A. Boisen, E. Gustavsson, K. Stáhl, S. Pehrson, S. Dahl, A. Carlsson and C. J. H. Jacobsen, Chem. Mater., 2001, 13, 4416.
20. M. Choi, H. S. Cho, R. Srivastava, C. Venkatesan, D. H. Choi and R. Ryoo, Nat. Mater., 2006, 5, 718.
21. R. Srivastava, M. Choi and R. Ryoo, Chem. Commun., 2006, 4489.
22. R. Srivastava, N. Iwasa, S-I. Fujita and M. Arai, in Progress in porous media Research, ed. K. S. Tian and H. J. Shu, Nova Science Publisher, 2009, Chapter 1, 1-53.
23. R. Srivastava, N. Iwasa, S. I. Fujita and M. Arai, Chem.–Eur. J., 2008, 14, 9507.
24. K. Na, C. Jo, J. Kim, K. Cho, J. Jung, Y. Seo, R. J. Messinger, B. F. Chmelka and R. Ryoo, Science, 2011, 333, 328.
25. W. Park, D. Yu, K. Na, K. E. Jelfs, B. Slater, Y. Sakamoto and R. Ryoo, Chem. Mater., 2011, 23, 5131.
26. M. Choi, K. Na and R. Ryoo, Chem. Commun., 2009, 2845.
27. K. Na, M. Choi and R. Ryoo, J. Mater. Chem., 2009, 19, 6713.
28. D. P. Serrano, J. Aguado, J. M. Escola, J. M. Rodrìguez and A. Peral, Chem. Mater., 2006, 18, 2462.
29. A. Sakthivel, A. Iida, K. Komura, Y. Sugi and K. V. R. Chary, Microporous Mesoporous Mater., 2009, 119, 322.
30. R. Kore and R. Srivastava, J. Mol. Catal. A: Chem., 2011, 345, 117.
31. R. Kore and R. Srivastava, Catal. Commun., 2011, 12, 1420.
32. R. Kore and R. Srivastava, Tetrahedron Lett., 2012, 53, 3245.
33. R. Kore, T. J. D. Kumar and R. Srivastava, J. Mol. Catal. A: Chem., 2012, 360, 61.
34. R. Kore and R. Srivastava, Catal. Commun., 2012, 18, 11.
35. R. Kore, B. Satpati and R. Srivastava, Chem.–Eur. J., 2011, 17, 14360.
36. J. E. Bara, Ind. Eng. Chem. Res., 2011, 50, 13614.
37. M. A. Camblor and J. P. Pariente, Zeolites, 1991, 11, 202.
38. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Gaussian, Inc., Wallingford CT, Revision A.1, 2009.
39. L. M. Marcelo, O. S. Michele, B. C. P. Sibele, F. S. Roberto and B. G. Katia, Appl. Catal., A, 2010, 374, 26.
40. P. M. Piccione, S. Yang, A. Navrotsky and M. E. Davis, J. Phys. Chem. B, 2002, 106, 3629.
41. A. Corma, M. Moliner, A. Cantin, M. J. Diaz-Cabanas, J. L. Jorda, D. Zhang, J. Sun, K. Jansson, S. Hovmoller and X. Zou, Chem. Mater., 2008, 20, 3218.
42. M. D. Kadgaonkar, M. W. Kasture, D. S. Bhange, P. N. Joshi, V. Ramaswamy and R. Kumar, Microporous Mesoporous Mater., 2007, 101, 108.
43. M. Antonietti, D. B. Kuang, B. Smarsly and Z. Yong, Angew. Chem., Int. Ed., 2004, 43, 4988.
44. A. K. Boal, F. Ilhan, J. E. DeRouchey, T. T. Albrecht, T. P. Russell and V. M. Rotello, Nature, 2000, 404, 746.
45. J. Jin, T. Iyoda, C. Cao, Y. Song, L. Jiang, T. Li and D. Zhu, Angew. Chem., Int. Ed., 2001, 40, 2135.
46. R. P. Andres, J. D. Biefield, J. I. Henderson, D. B. Janes, V. R. Kolagunta, C. P. Kubiak, W. J. Mahoney and R. G. Osifchin, Science, 1996, 273, 1690.
47. B. G. Trewyn, C. M. Whitman and V. S. Y. Lin, Nano Lett., 2004, 4, 2139.
48. Y. Zhou, J. H. Schattka and M. Antonietti, Nano Lett., 2004, 4, 477.
49. Y. Liu, J. Li, M. Wang, Z. Li, H. Liu, P. He, X. Yang and J. Li, Cryst. Growth Des., 2005, 5, 1643.
50. Y. Kubota, M. M. Helmkamp, S. I. Zones and M. E. Davis, Microporous Mater., 1996, 6, 213.

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra20437a

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