Factors Governing the Formation of Hierarchically and Sequentially

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Factors Governing the Formation of Hierarchically and Sequentially Intergrown MFI Zeolites by Using Simple Diquaternary Ammonium Structure-Directing Agents Sye Hoe Keoh, Watcharop Chaikittisilp, Koki Muraoka, Rino R. Mukti, Atsushi Shimojima, Prashant Kumar, Michael Tsapatsis, and Tatsuya Okubo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03887 • Publication Date (Web): 26 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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Chemistry of Materials

Factors Governing the Formation of Hierarchically and Sequentially Intergrown MFI Zeolites by Using Simple Diquaternary Ammonium Structure-Directing Agents Sye Hoe Keoh,† Watcharop Chaikittisilp,*,† Koki Muraoka,† Rino R. Mukti,‡ Atsushi Shimojima,§ Prashant Kumar,⊥ Michael Tsapatsis,⊥ and Tatsuya Okubo*,† †

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Division of Inorganic and Physical Chemistry and Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, JI. Ganesha 10, Bandung 40132, Indonesia § Department of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan ‡

⊥

Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455, USA ABSTRACT: Zeolites with hierarchical structures are of particular interest because such structures can improve molecular diffusion, particularly that of bulky molecules. N,N,N,N’,N’,N’-Hexapropylpentanediammonium cations (Pr6-diquat-5), a simple diquaternary ammonium organic structure-directing agent (OSDA), can direct the formation of hierarchically and sequentially intergrown MFI zeolites without employing any mesoporogens. In this paper, the effects of OSDAs having structures similar to Pr6-diquat-5 but different lengths of alkyl spacers and/or different substituting groups on the phase selectivity and morphology of the resulting zeolites are presented. It was revealed that the number of carbon atoms between two charged nitrogens in the OSDAs significantly affected the intergrowth and morphology of the crystals formed. In addition, the propyl-substituted OSDAs were found to be very selective to the formation of MFI zeolite, while the butyl-substituted OSDAs were not. For Pr6-diquat-5, the condition for the formation of hierarchically and sequentially intergrown MFI zeolites was somewhat narrow with the optimized molar composition of 1 SiO2: 0.2 Pr6-diquat-5: 0.375–0.500 KOH: 200 H2O: 4 EtOH. Defect lines observed on the obtained zeolite crystals by a transmission electron microscope were considered to be connectors for such intergrowths. The unique intergrowth formed by Pr6-diquat-5 was surmised to be due to the unusual fitting of Pr6-diquat-5 inside the channels of MFI zeolite, which was explained by comparing molecular dimensions and stabilization energies of each OSDA.

INTRODUCTION Zeolites, a class of crystalline microporous materials with well-defined micropores, high surface area, large pore volume, and high hydrothermal stability, are one of key industrial materials that have been utilized for various applications, for instance, as catalysts, ion-exchangers, adsorbents, and separation membranes.1–4 However, zeolites encounter limitation of molecular diffusion and transport through the zeolite bodies due to the sole presence of micropores in their framework structures.5,6 Such problems become more pronounced when zeolites are used in the applications involving bulky molecules. Syntheses of zeolites having larger micropores or with shortened the diffusion path lengths are two main strategies to overcome the molecular transport problems. In particular, there are various attempts to synthesize extra-large pore zeolites,7,8 to decrease crystal sizes of zeolites into the nanometer length scale,9,10 and to fabricate hierarchical zeolites with secondary meso- and/or macropores.11–13 Among these attempts, hierarchical zeolites with different levels of porosity are highly promising because the interconnection between their intrinsic micropores through mesopores can increase the reaction rates for diffusion-limited reactions and improve the regeneration of zeolite catalysts due to coke deposition.14,15

Two general methods, top-down and bottom-up, can be used to prepare hierarchical zeolites. A top-down or postsynthetic approach involves treatments of zeolites under acidic or basic conditions, generally referred to as dealumination or desilication, respectively, to dissolve parts of the zeolites and simultaneously generate mesopores.16 This method has successfully been applied to MFI, FAU, MOR, *BEA, AST, and FER zeolites with a wide range of Si/Al molar ratios.17–20 However, the formation of uniform mesopores by the postsynthetic approach remains challenging.21 On the other hand, a bottom-up or templating approach employs “templates” to fabricate hierarchical zeolites, which can be divided into hardand soft-templating. Hard-templating introduces solid templates inside the zeolite growth matrix. Generally, the solid templates are nanostructured carbons. After the completion of zeolite formation, these solid templates are removed by calcination, thereby producing mesopores. Carbon templates include carbon nanoparticles,22,23 carbon aerogels,24 and threedimensionally ordered mesoporous (3DOm) carbons.25,26 However, this hard-templating method requires multi-step synthesis, which may be difficult to apply in large-scale production. Alternatively, soft templates can yield hierarchical zeolites with well-controlled mesopores in a single-step manner. Sev-

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eral amphiphilic, bifunctional molecules have been designed and used as mesopore-generating agents (mesoporogens) for the fabrication of hierarchical zeolites and zeolite nanosheets.27–31 Interestingly, without using such complex organic molecules, the formation of self-pillared MFI zeolite nanosheets by repetitive branching has been recently achieved by using tetrabutylphosphonium (TBP) or tetrabutylammonium (TBA) cations as the non-mesoporogen, organic structuredirecting agents (OSDAs).32,33 Similarly, it was reported at almost the same time that simple diquaternary ammonium cations (N,N,N,N’,N’,N’-hexapropylpentanediammonium (Pr6diquat-5), see Figure 1a) can direct the formation of hierarchically and sequentially intergrown MFI zeolites.34 A main difference between both cases is that the intergrowths in the selfpillared MFI zeolite nanosheets happen at a single-unit-cell level,32,33 while the zeolite plates are thicker and the spacing between intergrowth events is lower for the hierarchically and sequentially intergrown MFI zeolites.34 This can imply that although hierarchical MFI zeolites are formed via the intergrowth process in both cases, their formation mechanisms are still unclear and may be different.

Figure 1. Chemical structures of the OSDAs used in this study, (a) Pr6-diquat-n (n = 4–10 and 12) and (b) Bu6-diquat-m (m = 4– 6).

Our motivation for using Pr6-diquat-5 originally came from its ability to form very thin plate-like crystals of MFI zeolite.35 As is well known, the 90° rotational intergrowth is often formed during the synthesis of MFI zeolite.36 By employing Pr6-diquat-5 as the OSDA, we have expected that if the 90° intergrowth can be enhanced, zeolites with house-ofcards-like, hierarchical morphology will be obtained. Although the use of Pr6-diquat-5 for the synthesis of hierarchically and sequentially intergrown MFI zeolites was demonstrated previously,34 the synthesis was conducted under very narrow conditions. It is thus still unclear whether Pr6-diquat-5 is mainly responsible for such unique intergrowth or there are other more critical factors such as synthesis conditions that primarily involve. In addition, it was suggested previously that Pr6diquat-5 is imperfectly fitted to the MFI framework, resulting in the intergrowth, but the supporting evidence was not provided.34 Herein, aiming to identify the critical factors governing the formation of such unique hierarchically and sequentially intergrown zeolites, we explore the synthesis of zeolites by employing Pr6-diquat-5 and other diquaternary ammoniums having structures similar to Pr6-diquat-5 but with different lengths of alkyl spacers between two nitrogens and/or different substituents as the OSDAs (see Figure 1). When using Pr6-diquat5, the effects of synthesis parameters are also investigated. Computational modeling and high-resolution transmission electron microscopy (TEM) observation are conducted to provide some better understanding of the formation process. Only Pr6-diquat-5 directs the formation of hierarchically and sequentially intergrown MFI zeolites and the chemical compositions of the reactants significantly influence the frequency of

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sequential intergrowth formed. Besides, the zeolite morphology and the phase selectivity highly depend on the structures of OSDAs.

EXPERIMENTAL SECTION Chemicals. The following chemicals were used as provided for synthesis of OSDAs: 1,4-diiodobutane (C4H8I2, Tokyo Chemical Industry Co., Ltd. (TCI)), 1,5-diiodopentane (C5H10I2, TCI), 1,6-diiodohexane (C6H12I2, TCI), 1,7diaminoheptane (C7H18N2, Alfa Aesar), 1,8-diiodooctane (C8H16I2, TCI), 1,9-diaminononane (C9H22N2, Sigma-Aldrich Co. LLC (Sigma-Aldrich)), 1,10-diiododecane(C10H20I2, TCI), 1,12-diaminododecane (C12H28N2, Wako Pure Chemical Industries, Ltd. (Wako)), tripropylamine (C9H21N, SigmaAldrich), tributylamine (C12H27N, Sigma-Aldrich), 1iodopropane (C3H7I, Sigma-Aldrich), dehydrated ethanol (C2H6O, Wako), acetone (C3H6O, Wako), hexane (C6H14, Wako), and potassium carbonate (K2CO3, Wako). For synthesis of zeolite, tetraethyl orthosilicate (TEOS, Wako) and potassium hydroxide (KOH, 1 mol/L, Wako) were used as received. Synthesis of OSDAs by nucleophilic substitution. N,N,N,N’,N’,N’-Hexapropylbutanediammonium (Pr6-diquat4), N,N,N,N’,N’,N’-hexapropylpentanediammonium (Pr6diquat-5), N,N,N,N’,N’,N’-hexapropylhexanediammonium (Pr6-diquat-6), N,N,N,N’,N’,N’-hexapropyloctanediammonium (Pr6-diquat-8), N,N,N,N’,N’,N’-hexapropyldecanediammonium (Pr6-diquat-10), N,N,N,N’,N’,N’-hexabutylbutanediammonium N,N,N,N’,N’,N’(Bu6-diquat-4), hexabutylpentanediammonium (Bu6-diquat-5), and N,N,N,N’,N’,N’-hexabutylhexanediammonium (Bu6-diquat-6) cations were synthesized by reacting the corresponding diiodoalkane with tripropylamine or tributylamine. Synthesis procedures of Pr6-diquat-4 is described here as an example. Typically, dehydrated ethanol and tripropylamine (3 equiv) were added to a nitrogen-filled, oven-dried, two-necked flask equipped with a condenser and a magnetic stirring bar. Subsequently, 1,4-diiodobutane (1 equiv) was added slowly to the solution. This mixture was heated slowly to 90 °C and kept in dark while stirring at this temperature for 48 h. Then, the solution was cooled to room temperature and ethanol was evaporated by a rotary evaporator, yielding semi-solid. This semisolid was purified by recrystallization in ethanol and ethyl acetate. The recrystallized solid was washed with acetone or hexane and dried under atmospheric condition. Detailed characterization results of OSDAs are shown in the Supporting Information. Synthesis of OSDAs by exhaustive alkylation. N,N,N,N’,N’,N’-Hexapropylheptanediammonium (Pr6-diquat7), N,N,N,N’,N’,N’-hexapropylnonanediammonium (Pr6diquat-9), and N,N,N,N’,N’,N’hexapropyldodecanediammonium (Pr6-diquat-12) cations were synthesized by exhaustive alkylation of the corresponding diamine with 1-iodopropane. Typically, K2CO3 (3 equiv) was added into a two-necked flask equipped with a condenser and a magnetic stirring bar. The reaction flask was evacuated and refilled with nitrogen gas. These procedures were repeated three times. Then, dehydrated ethanol and diamine (1 equiv) were added. While stirring, 1-iodopropane (7.5 equiv) was slowly added to the reaction mixture. Then, the reaction mixture was slowly heated to reflux (kept in dark). After 48 h, the mixture was cooled to room temperature and then filtrated to


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remove solid (mainly K2CO3). About a half of filtrate was evaporated by a rotary evaporator, yielding liquid with precipitates. The precipitates (mainly KI) were then removed by filtration. The remaining filtrate was further evaporated to yield semi-solid. This semi-solid was purified by the same procedures described above for Pr6-diquat-4. Characterization results of OSDAs can be found in the Supporting Information. Synthesis of zeolites. For studying the effects of OSDA structures, the synthesis mixture was prepared with a molar composition of 1 SiO2: 0.2 OSDA: 0.375 KOH: 200 H2O: 4 EtOH (OSDAs = Pr6-diquat-n, where n = 4–10 and 12, and Bu6-diquat-m, where m = 4–6). For studying the effects of synthesis gel compositions, the synthesis mixture was prepared with molar compositions of 1 SiO2: x Pr6-diquat-5: y KOH: z H2O: 4 EtOH, where x = 0.1–1.0, y = 0.250–0.750, and z = 100–250. In a typical synthesis, the OSDA (in a diiodide form) was dissolved in a KOH aqueous solution under stirring. Then, TEOS was added dropwise to the stirring solution. This reaction mixture was stirred for 24 h at room temperature. Then, the resulting mixture was transferred into a 23mL Teflon-lined stainless autoclave. The hydrothermal synthesis was carried out at 150 °C for 216 h under rotation of 20 rpm. After completing the synthesis, the autoclave was cooled to room temperature. The product was recovered by filtration, washed with deionized water until the filtrate reached pH 7, and then dried at 80 °C overnight. The calcination was conducted, if need, at 550 °C for 10 h under dried air. Characterization. Solution-state 13C NMR spectra were recorded by a JEOL JNM-EX-270 spectrometer at 67.80 MHz. The phase purity and crystallinity of the zeolite products were determined by powder XRD patterns collected on a Rigaku Ultima IV diffractometer using Cu Kα radiation (λ = 1.54056 Å, 40 kV, 40 mA) at a scanning rate of 10°/min over a range of 3° to 50°. The morphology of the crystals was observed by a field emission scanning electron microscope (JEOL JSM7000F) with an accelerating voltage of 15 kV. Prior to the observation, the powder was coated with osmium. Nitrogen physisorption was performed on an Autosorb-iQ (Quantachrome Instruments) at −196 °C. Thermogravimetry analysis (TGA) was performed on a PU 4K (Rigaku) with a heating rate of 10 K/min under a flow of 10% O2 and 90% He. Elemental analysis (carbon, hydrogen, and nitrogen) was performed on a CE-440 elemental analyzer (Exeter Analytical). Solid-state NMR spectra were recorded by a JEOL JNM ECA400 spectrometer. 13C cross-polarization magic-angle-spinning (CP/MAS) NMR spectra were recorded at 100.52 MHz with a recycle delay of 5 s, a contact time of 5 ms, and a spinning rate of 10 kHz. 29Si MAS NMR spectra were recorded at 79.42 MHz with a pulse delay of 80 s (45° pulse). The amounts of silicon and potassium in the products were quantified on an inductively coupled plasma–atomic emission spectrometer (ICP-AES, Thermo Scientific iCAP-6300) and an atomic absorption spectrometer (AAS, Hitachi Z-2000), respectively. For transmission electron microscopy (TEM) observations, 60–100 nm thin electron transparent sections were prepared by microtoming (Leica EM UC6 Ultramicrotome) the zeolite powder sample embedded in resin cured overnight at 60 °C. These sections were placed on carbon film supported copper mesh TEM grids (TedPella) for examination under the microscope. FEI Tecnai G2 F30 (S)TEM equipped with TWIN pole piece (Cs=2 mm) and a Schottky field emission electron gun operating at 300 kV with extraction voltage of 4,000 V was used for bright-field TEM imaging. Images were acquired

using a Gatan 4k x 4k Ultrascan CCD. Low-dose setup of the microscope was used to minimize beam exposure of the zeolite sample. Computational modeling. To evaluate the interaction energies between OSDAs and MFI zeolite, computational modeling was performed using a GULP37 package. A crystal structure of MFI zeolite downloaded from the website of International Zeolite Association38 was optimized under the constant volume condition using the Sanders–Leslie–Catlow (SLC) potential,39 which is a widely tested force field for various zeolites.40–42 A single-point calculation was performed for the SLC-optimized structure to estimate the energy of MFI zeolite. Each OSDA was submitted to the energy minimization algorithm using the charge-less Dreiding force field43 to obtain the energy of the OSDA. Two diquaternary ammonium OSDAs per unit cell were introduced into the considered channels (either straight or sinusoidal) of MFI zeolite. Molecular dynamics (MD) simulation using an NVT ensemble was performed on this zeolite–OSDA complex at 343 K using the Dreiding force field for 1 ps to approach the global minimum.44 The final structure of MD was further optimized with the Dreiding force field, while fixing the coordinates of the MFI structure and the cell parameters at the SLC-optimized values. The so-called stabilization energy was calculated by subtracting the final energy of the zeolite–OSDA complex with the energy of empty zeolite and the energy of free OSDA.44,45 For comparison, the similar procedure was applied to tetrapropylammonium (TPA) cations but with four cations located at the channel intersections of MFI zeolite for each unit cell.

RESULTS AND DISCUSSION Effects of the structures of OSDAs on the phase selectivity and morphology of zeolites. Several simple diquaternary ammonium cations have been employed as the OSDAs for the synthesis of zeolite. Some of the reported diquaternary OSDAs can affect shape and size of zeolite crystals; in particular, pyrrolidine- and hexamethyleneimine-derived diquaternary OSDAs can yield very small crystals (20–30 nm) of MFI and *BEA zeolites.46,47 However, zeolites with hierarchical structure have not been obtained. Since Pr6-diquat-5 can direct the formation of hierarchically and sequentially intergrown MFI zeolite,34 the effects of other OSDAs having structure similar to Pr6-diquat-5 were investigated. Powder XRD patterns of the products synthesized with propyl- (Pr6-diquat-n) and butyl-substituted OSDAs (Bu6-diquat-m) are presented in Figures S1 and S2 in the Supporting Information, respectively. In the case of propylsubstituted OSDAs, OSDAs with n = 5–10 and 12 can direct the formation of MFI zeolites, while amorphous solid was obtained when Pr6-diquat-4 was used as the OSDA (we did not examine Pr6-diquat-11 because we cannot find the commercial sources of its starting reagent). Note that the synthesis of zeolite with Pr6-diquat-12 required longer time (336 h) to obtain highly crystallized MFI zeolite. When the hydrothermal synthesis was done for 216 h, a mixture of amorphous solid and MFI zeolite (as a minor product) was obtained (see Figure S3 in the Supporting Information). It is suggested that the OSDAs with longer alkyl chain length (higher C/N+ ratios) tend to decrease the crystallization rate as they become more hydrophobic (less soluble in water).48,49 As shown in Figure S2 in



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Figure 2. SEM images of as-synthesized products: (a) hierarchically and sequentially intergrown MFI zeolite by Pr6-diquat-5, (b) aggregated, small leaf-like MFI zeolite by Pr6-diquat-6, (c) oval-shaped MFI zeolite with 90° intergrowth by Pr6-diquat-7, (d) nanometer-sized, round-shaped MFI zeolite by Pr6-diquat-8, (e) submicrometer-sized, round-shaped MFI zeolite by Pr6-diquat-9, (f) aggregated, coffinshaped MFI zeolite with 90° intergrowth by Pr6-diquat-10, (g) almond-shaped, intergrown MFI zeolite by Pr6-diquat-12 and (h) long leaflike MFI zeolite with 90° intergrowth by Bu6-diquat-6.

the Supporting Information, in the case of butyl-substituted OSDAs, only Bu6-diquat-6 directed the formation of MFI zeolite whereas Bu6-diquat-4 and Bu6-diquat-5 yielded only amorphous solids. Figure 2 shows SEM images of the products synthesized with Pr6-diquat-n (n = 5–10 and 12) and Bu6-diquat-6 as the OSDAs, revealing that the morphology of the obtained zeolites highly depended on the structures of OSDAs, similar to the previous observation.35 Interestingly, the hierarchical MFI zeolites with unique intergrowth can be obtained only with Pr6-diquat-5 (Figure 2a). When Pr6-diquat-6, Pr6-diquat-7, Pr6diquat-8, Pr6-diquat-9, Pr6-diquat-10, and Pr6-diquat-12 were employed as the OSDAs, aggregated, small leaf-like (Figure 2b), oval-shaped with 90° intergrowth (Figure 2c), nanometersized, round-shaped (Figure 2d), submicrometer-sized, roundshaped (Figure 2e), aggregated, coffin-shaped with 90° intergrowth (Figure 2f), and almond-shaped, intergrown (Figure 2g) MFI zeolites were observed, respectively. In the case of butyl-substituted OSDAs, Bu6-diquat-6 gave the crystals with long leaf-like morphology (Figure 2h). These observations strongly suggested that the OSDA having five methylene carbons connecting two nitrogen atoms somehow interacts with the inorganic zeolite frameworks and directs the growth in a way that can enhance the 90° intergrowths sequentially. The presence of OSDAs inside the zeolite products was confirmed by thermogravimetry and elemental analyses (carbon, hydrogen, and nitrogen). As summarized in Table S1 in the Supporting Information, 2.1–2.2 molecules of Pr6-diquat-n, where n = 6–10 and 12, per unit cell of MFI zeolite were occluded in the products, which is close to the value of tetrapropylammonium (TPA) cation, the most typical OSDA for MFI zeolite (4.0 molecules per unit cell). In the case of Pr6-diquat-5, on the contrary, a higher amount of 2.7 molecules (equivalent to 5.4 TPA) per unit cell was observed. We have surmised that with this unique, hierarchical and sequential intergrowth, non-zeolitic micro- and/or mesopores generated by the intergrowths should be present in the obtained

zeolite bodies and accordingly can provide additional spaces for accommodating the higher amount of OSDAs. At high temperature and alkaline conditions, it is well known that quaternary ammonium cations can be cleaved via Hoffman elimination, producing tertiary amine, olefin, and water. To confirm that the OSDAs are occluded intact in the resulting zeolites, 13C cross-polarization magic-angle-spinning (CP/MAS) nuclear magnetic resonance (NMR) analyses were performed. Figure 3 shows 13C CP/MAS NMR spectra of the as-synthesized products obtained from Pr6-diquat-5, Pr6diquat-8, and Pr6-diquat-12, in comparison with the solutionstate 13C NMR spectra of the employed OSDAs. If the OSDAs are decomposed, the signals due to the products of the Hoffman degradation should be observed at chemical shifts (δ) of ca. 120 and 140 for the β-olefinic carbon (–CH2CH=CH2) and the α-olefinic carbon (–CH2CH=CH2), respectively. As can be seen in Figure 3, the 13C CP/MAS NMR spectra of the assynthesized zeolites matched well with their corresponding OSDAs without the presence of the Hoffman degradation products, suggesting that the OSDAs are stable under this synthesis condition and occluded intact in the as-synthesized products. This can imply that the unique hierarchically and sequentially intergrown morphology produced by Pr6-diquat-5 is not a result of degraded OSDAs. 29 Si MAS NMR spectra of the as-synthesized products obtained from Pr6-diquat-5, Pr6-diquat-8, and Pr6-diquat-12 are shown in Figure S4 in the Supporting Information. Two resonances are observed at δ = −112 and −101 ppm, attributed to Q4 (Si(OSi)4) and Q3 (Si(OSi)3(OH) or Si(OSi)3O−) silicons, respectively. The contents of Q3 defects can be estimated from the signal ratios, Q3/(Q3+Q4), summarized in Figure S4 in the Supporting Information. Surprisingly, high amounts of Q3 defects were observed for all samples, suggesting that under the present synthesis condition the MFI products somehow tended to contain more Q3 defects than usual. Potassium cations found in the product synthesized with TPA (K/Si = 0.02) are considered to be responsible in part for such high amounts of Q3 sites. The product synthesized by using Pr6-diquat-12


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had the Q3 content comparable to that obtained from the conventional OSDA (TPA), while the products synthesized with Pr6-diquat-5 and Pr6-diquat-8 contained more Q3 sites. These higher degrees of Q3 silicon found in the products of Pr6diquat-5 and Pr6-diquat-8 are probably attributed to the higher content of 90° intergrowths34 and the nanometer-sized crystals,50 respectively.

N-to-N distances being 6.6, 7.9, 9.2, 10.5, 11.8, 13.1, 14.4, and 17.0 Å for n = 4–10 and 12, respectively. Considering the channel system of MFI zeolite, the distances between the centers of two intersections along the sinusoidal (a-axis) and the straight (b-axis) channels were estimated to be about 12 and 10 Å, respectively. However, OSDAs are not necessary to sit at these intersection centers because they should be located at the optimal positions to maximize their structure-direction as described by the stabilization energy.44,45 As a reference, we performed the computational modeling for the TPA–MFI zeolite and found that the distances between two nitrogen atoms of TPA along the a- and b-axes in the optimized structure were 10.5 and 10.7 Å, respectively. Based on these calculations, the length of Pr6-diquat-4 is too short to be accommodated inside the channels of MFI zeolite; as a result, under our synthesis conditions, Pr6-diquat-4 cannot direct the MFI formation. Surprisingly, OSDAs having very short (7.9 Å for Pr6-diquat-5) and very long (17.0 Å for Pr6-diquat-12) alkyl spacers directed the formation of MFI zeolite with unique morphologies. These suggested that OSDAs can undergo the conformational changes to fit in the zeolite channels, thereby stabilizing the zeolite framework. Such unusual fitting probably enhances the unique intergrowths observed in the MFI zeolite products. For a better understanding of the framework stabilization of each OSDA, the stabilization energies when OSDAs lay along the a- and b-axes of MFI zeolite were calculated and summarized in Table 1 with the optimized structures depicted in Figure 4. The stabilization energies of Pr6-diquat-5 for MFI zeolite was highest (least stable). Compared to the stabilization energy of TPA (−8.78 kJ/mol Si), most of diquaternary ammonium OSDAs can provide lower stabilization energies (more stable), except for Pr6-diquat-5 located along the a-axis (i.e., the sinusoidal channel) and Pr6-diquat-12 located along the b-axis (i.e., the straight channel). In addition, only Pr6diquat-5 showed the preferential location along the b-axis while for other OSDAs the stabilization energies when sitting along the a-axis were comparable to or lower than when sitting along the b-axis. Table 1. Stabilization energies for the OSDAs employed in this work OSDA

Ea-axis (kJ/mol Si)a

Eb-axis (kJ/mol Si)b

−8.78c

TPA

Ea-axis−Eb-axis (kJ/mol Si) n.a.d

Pr6-diquat-5

−8.65

−9.43

0.78

Figure 3. (a, c, e) Solution-state 13C NMR spectra of OSDAs and (b, d, f) solid-state 13C CP/MAS NMR spectra of their corresponding as-synthesized zeolites: (a, b) Pr6-diquat-5, (c, d) Pr6diquat-8, and (e, f) Pr6-diquat-12.

Pr6-diquat-6

−10.51

−10.48

−0.03

Pr6-diquat-7

−11.10

−11.02

−0.08

Pr6-diquat-8

−11.74

−11.46

−0.28

In a viewpoint of structure-direction, the shape and size of OSDAs are of primary importance as the OSDAs have to provide enough stabilization to direct the formation of particular zeolite frameworks.42,44,45,51 Similar to our observation, the lengths of alkyl spacers have shown to be an important factor in zeolite synthesis.35,51–53 Therefore, we have estimated the sizes (lengths) of each OSDA and compared them to the sizes of the microporous cavities of MFI zeolite. As depicted in Figure S5 in the Supporting Information, the optimized structures of the Pr6-diquat-n OSDAs revealed their intramolecular

Pr6-diquat-9

−12.68

−11.93

−0.75

Pr6-diquat-10

−12.45

−11.36

−1.09

Pr6-diquat-12

−10.86

−6.08

−4.78

a

Stabilization energy when OSDAs are located along the sinusoidal channels (a-axis). bStabilization energy when OSDAs are located along the straight channels (b-axis). cIn the case of TPA, stabilization energy is independent of located channels. dNot applicable.



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Figure 4. Optimized structures of OSDA–MFI zeolite complexes: (a) TPA, (b) Pr6-diquat-5, (c) Pr6-diquat-6, (d) Pr6-diquat-7, (e) Pr6diquat-8, (f) Pr6-diquat-9, (g) Pr6-diquat-10, and (h) Pr6-diquat-12.

As can be seen in Figure 4, the nitrogen atoms of Pr6diquat-8, Pr6-diquat-9, and Pr6-diquat-10 were located in the channel intersections at the positions close to the nitrogen atoms of TPA. In contrast, for shorter and longer Pr6-diquat-n OSDAs, their nitrogen atoms were located at the positions different from those of TPA as they were located closer to the zeolite framework (also see Figures S6 and S7 in the Support-

ing Information). The averages of the shortest intermolecular distances between the nitrogen atoms of OSDAs and the frameworked oxygens for Pr6-diquat-5 laying along the a- and b-axes were 3.89 and 3.94 Å, respectively. These averages of the shortest NOSDA···Oframework distances for other lengthened OSDAs became longer, except for Pr6-diquat-12. We have therefore speculated that the unusual fitting of Pr6-diquat-5 in


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the channels of MFI zeolite, perhaps occluded preferentially along the b-axis, may cause the internal framework stresses. As a consequence, the 90° intergrowth should be generated to relax the framework stresses or distortions caused by Pr6diquat-5 somehow in a sequential fashion, resulting in the unique hierarchical MFI zeolite. Note that some changes in lattice parameters and unit cell volumes of the MFI zeolites synthesized by using Pr6-diquat-5, Pr6-diquat-6, and Pr6diquat-7 observed by the synchrotron X-ray diffractions were reported previously.35 It should be noted that the stabilization energy alone cannot fully explain the crystal morphology of zeolite because the adsorption of OSDAs on the crystal surfaces during the crystal growth process can also influence the crystal growth rates of a particular crystal face and the final crystal morphology.35,54,55 More computational works for OSDAs are needed to clarify the relations of the stabilization energy, the framework distortion, and the crystal morphology, which merits future investigation. In addition, more realistic zeolite models such as the three-dimensional structures representing this unique intergrowth and the structures containing silanol Q3 sites should be considered. Effects of the synthesis conditions on the formation of hierarchically and sequentially intergrown MFI zeolite with Pr6-diquat-5 as the OSDA. Pr6-diquat-5 was used for the synthesis of MFI zeolite previously but only thin plate-like morphology without intergrowths was observed.35 Under our synthesis conditions, however, hierarchically and sequentially intergrown MFI zeolite can be obtained.34 These suggested that the synthesis condition is very crucial for the enhancement of the 90° intergrowth. To maximize the intergrowth, the effects of several synthesis parameters were studied. The effect of the Pr6-diquat-5/SiO2 ratios was investigated by using the initial reactant composition of 1 SiO2: x Pr6diquat-5: 0.375 KOH: 200 H2O: 4 EtOH, where x = 0.1–1.0. XRD patterns of the as-synthesized products are shown in Figure 5. When the Pr6-diquat-5 amount was very low (x = 0.1), a broad peak representing the amorphous solid with some diffraction peaks assigned to the MFI zeolite phase was observed, indicating that at the low amount of Pr6-diquat-5 longer synthesis time is needed to achieve highly crystallized MFI zeolite. When the Pr6-diquat-5 amount was slightly increased to x = 0.2, the resulting product was highly crystallized MFI zeolite without any impurities. No significant difference in the XRD patterns was observed when x was further increased to 1.0. Figure 6 shows SEM images of the products synthesized at different Pr6-diquat-5/SiO2 ratios. When x = 0.1, highly intergrown crystals with amorphous solids were observed (Figure 6a), which is in agreement with its XRD pattern. Highly developed intergrowth was observed when x = 0.2 (Figure 6b). At higher Pr6-diquat-5/SiO2 ratios, in contrast, SEM images showed less pronounced intergrown zeolites. As shown in Figure 6c–f, the degree of intergrowth was decreased as the Pr6-diquat-5/SiO2 ratio increased. These suggested that the excess amounts of Pr6-diquat-5 somehow diminish the zeolite intergrowth. Interestingly, the sizes of crystal plates when x = 0.2–1.0 were almost identical. Also, it should be noted that the zeolite products obtained from the reactants with different amounts of Pr6-diquat-5 had similar solid yields.

Figure 5. XRD patterns of products synthesized from reactant mixtures of 1 SiO2: 0.1–1.0 Pr6-diquat-5: 0.375 KOH: 200 H2O: 4 EtOH.

Figure 6. SEM images of products synthesized at Pr6-diquat5/SiO2 of (a) 0.1, (b) 0.2, (c) 0.4, (d) 0.6, (e) 0.8, and (f) 1.0 (scale bar: 1 µm).

The amount of KOH determines the alkalinity of the synthesis mixture and can regulate the solubility of silicates. The effect of KOH concentration in the synthesis mixture was investigated with the compositions of 1 SiO2: 0.2 Pr6-diquat-5: y KOH: 200 H2O: 4 EtOH (y = 0.250–0.750). As summarized in Table S2 in the Supporting Information, the solid yields (10– 78%) decreased as the KOH/SiO2 ratio was increased. When y = 0.250, no Bragg diffraction peaks were observed (see Figure S8 in the Supporting Information). Figure 7a shows XRD patterns of the as- synthesized products as a function of KOH amounts. Well-defined XRD peaks representing MFI zeolite



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were observed at y = 0.375–0.500, indicating that the crystallinity is independent of the KOH amounts within this range. However, the crystallinity decreased as y was increased to 0.625. When the alkalinity was further increased to y = 0.750, a broad peak with slight diffractions from MFI zeolite was observed, indicating that the product was a mixture of amorphous solid and MFI zeolite (as minor phase).

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ture (H2O/SiO2 = 100) were smaller and less intergrown than those synthesized from more diluted composition (H2O/SiO2 = 200).

Figure 8. SEM images of products synthesized at H2O/SiO2 = (a) 100 and (b) 200 (scale bar: 1 µm).

Figure 7. (a) XRD patterns of products synthesized from reactant mixtures of 1 SiO2: 0.2 Pr6-diquat-5: 0.375–0.750 KOH: 200 H2O: 4 EtOH. SEM images of products synthesized at KOH/SiO2 of (b) 0.375, (c) 0.500, (d) 0.625, and (e) 0.750 (scale bar: 2 µm).

The morphology of the products synthesized with different KOH/Si ratios was shown in Figure 7b–e. At y = 0.375–0.500, fully developed, hierarchically and sequentially intergrown MFI crystals were observed (see Figure 7b,c). At higher alkalinity, although highly intergrown crystals were observed, there existed many amorphous-like solids, which is in agreement with the XRD results. In addition, silica nanoparticles were observed at very high alkalinity (y = 0.750). In the previous reports,34,56,57 silica nanoparticles are suggested to be formed at the early stage of zeolite crystallization. These silica nanoparticles then supply, either directly or indirectly, growth nutrients for crystal growth. As is well documented, the alkalinity of the synthesis mixture highly influences the solubility of silica and accordingly affects the crystallization rate of zeolite.58 Under high alkalinity, the formation of silica nanoparticles and their subsequent aggregation to form zeolite34,56,57 may be slowed down due to the higher solubility of silica. As a result, the fully developed, intergrown crystals were not achieved under the present synthesis time. It is noteworthy that MFI zeolite having similar morphology can also be synthesized from a synthesis mixture containing NaOH instead of KOH under the identical condition (see Figure S9 in the Supporting Information). In hydrothermal synthesis of zeolite, besides involving in the Si–O–Si bond formation and cleavage via dehydration condensation and hydrolysis, respectively, water controls the overall concentration of reactants. To study the effect of water contents, the H2O/SiO2 ratios were varied from 100 to 250 while fixing other compositions at 1 SiO2: 0.2 Pr6-diquat-5: 0.375 KOH: 4 EtOH. Figure S10 in the Supporting Information shows XRD patterns of the as-synthesized samples obtained at different H2O/SiO2 ratios. All samples were highly crystallized MFI zeolites without any significant differences in their diffraction peaks. As shown in Figure 8, however, the MFI crystals obtained from more concentrated synthesis mix-

At lower amounts of water (i.e., more concentrated), more nuclei should be formed at the beginning of crystallization and the silicate precursors were then consumed during the crystal growth, resulting in smaller and less intergrown crystals. On the contrary, higher amounts of water probably led to less spontaneous nucleation. The more remaining silicate species then supplied for the crystal growths during which surface (heterogeneous) nucleation occurred, resulting in larger, sequentially intergrown crystals. Note that crystallization of zeolite under more concentrated condition (H2O/SiO2 = 100) occurred faster as compared to the diluted synthesis composition (H2O/SiO2 = 200) (see Figure S11 in the Supporting Information). Evolution of products during the crystallization process. As described above, the optimal reactant composition towards crystal intergrowth was found to be 1 SiO2: 0.2 Pr6diquat-5: 0.375–0.500 KOH: 200 H2O: 4 EtOH. To further examine the intergrowth process, we followed the textural evolution of the products synthesized at the optimal composition. As can be seen in XRD patterns (Figure S12 in the Supporting Information), the products were amorphous even after 84 h of hydrothermal synthesis as no diffraction peaks can be observed. The visible MFI diffraction peaks appeared after 120 h of hydrothermal synthesis. Fully crystallized MFI zeolite was then obtained after 216 h. Nitrogen adsorption–desorption measurements were conducted to evaluate the pore characteristics of the products synthesized for 120, 144, 168, and 216 h. Figure 9a shows their nitrogen sorption isotherms with those plotted in a semilog scale provided in Figure S13 in the Supporting Information. As the hydrothermal synthesis time was increased, the nitrogen uptakes at low relative pressure (P/Po < 0.10) increased, indicating the increases in micropore volumes (also see the cumulative pore volume plots shown in Figure S14 in the Supporting Information). The presence of hysteresis loops at relative pressure higher than 0.8 observed in the products synthesized for 120, 144, and 168 h was presumably due to the interparticular mesoporous voids. The isotherms of the highly crystallized product (216 h) showed a substantial uptake in a low relative pressure range, a hysteresis loop at P/Po > 0.40 and a slight uptake at higher relative pressure, indicating the presence of micro- and mesopores in this sample. The apparent specific Brunauer–Emmett–Teller (BET) area and the total pore volume (at P/Po = 0.99) of this sample were calculated to be 570 m2/g and 0.50 cm3/g, respectively. Its micropore volume was estimated to be 0.18 cm3/g. The BET areas and the


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pore volumes summarized in Table 2 showed that as prolonging the synthesis time, the BET areas and the micropore volumes increased, while the total pore volumes and the mesopore volumes decreased (also see Figure S14 in the Supporting Information), supporting the gradual evolution from amorphous solids with interparticular mesopores into hierarchically porous zeolites.

intergrown MFI zeolite starts with the formation of plate-like particles. After a certain period of hydrothermal synthesis, the 90° intergrowth occurred on the plate-like particles. Subsequently, this 90° intergrowth was formed in a sequential and repeating manner. Finally, the zeolite crystals were fully developed into hierarchically and sequentially intergrown structures. MFI–MEL intergrowth has been suggested to be an origin of orthogonal intergrowth leading to the formation of selfpillared, hierarchical zeolite.32,33 To directly observe the 90° intergrowth on the plate-like crystals, the MFI product synthesized at high Pr6-diquat-5/SiO2 of 1.0 was selected for transmission electron microscopy (TEM) observation. We selected this sample because it contained less intergrowth and therefore can be observed by TEM more easily (i.e., increasing chances for imaging a single perpendicular intergrowth). As shown in

Figure 9. (a) Nitrogen adsorption–desorption isotherms and (b) SEM images of products synthesized at 150 °C for different periods of time from reactant mixtures of 1 SiO2: 0.2 Pr6-diquat-5: 0.375 KOH: 200 H2O: 4 EtOH.

Table 2. Porous properties of products synthesized for different periods of time. Synthesis time (h)

SBET (m2/g)a

Vtotal (cm3/g)b

Vmicro (cm3/g)c

Vmeso (cm3/g)d

120

180

1.26

0.00

1.26

144

210

1.14

0.03

1.11

168

310

0.86

0.07

0.79

216

570

0.50

0.18

0.32

a

b

Specific BET area. Total pore volume at P/Po = 0.99. cMicropore volume calculated by a nonlocal density functional theory (NL-DFT) method. dMesopore volume = Vtotal−Vmicro.

SEM images of the products synthesized for 120, 144, 168, and 216 h are depicted in Figure 9b. Aggregates of nanoparticles and plate-like particles, likely representing amorphous solid and zeolite, respectively, were observed after 120 h of hydrothermal synthesis. When the synthesis time was increased to 144 h, plate-like zeolites with the 90° intergrowth appeared together with amorphous nanoparticles. With increased synthesis time, the plate-like zeolites with the 90° intergrowth were gradually developed and grown into hierarchically and sequentially intergrown MFI zeolites. These suggested that the formation of hierarchically and sequentially

Figure 10. (a) BF-TEM image of the plate-like particles synthesized at Pr6-diquat-5/SiO2 = 1.0, inset shows the FFT of the image section in the white box and (b) magnified image of the area highlighted in (a) showing a linear defect in the crystal structure.



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Figure 10, a defect was clearly observed but the orientation of the defect was not very clear. As shown in Figure S15 in the Supporting Information, the symmetrical appearance of the defect line (Figures 10b and S15a) with respect to the fringe pattern in the regions 1 and 2 was observed (see Figure S15b). Since the TEM fringe pattern is identical on both sides of the defect, we believe that this could be a connector for the perpendicular intergrowth (90° intergrowth). An origin of our hierarchically and sequentially intergrown MFI zeolite may be different from the proposed MFI–MEL intergrowth32,33 with one possibility being the epitaxial overgrowth of a-plane on bplane without MEL as a connector.36 Based on the overall observations, thin crystalline plates were initially formed. Subsequently, the unusual fitting of Pr6-diquat-5 in the bchannel of MFI zeolite may cause some structure distortion or framework stress, thereby enhancing the defects. These defects can act as connectors for sequential 90° intergrowths.

CONCLUSIONS Propyl-substituted diquaternary ammonium organic structuredirecting agents, Pr6-diquat-n, where n = 5–10 and 12, were found to be very selective for the formation of MFI zeolite, while butyl-substituted OSDAs were not, as only Bu6-diquat-6 yielded MFI zeolite under the present synthesis condition. The lengths of alkyl spacers between two charged nitrogen cations in OSDAs affected the morphology of the obtained crystals. Only Pr6-diquat-5 directed the formation of hierarchically and sequentially intergrown MFI zeolite. The synthesis condition was somewhat narrow with the optimum molar composition and synthesis condition being 1 SiO2: 0.2 Pr6-diquat-5: 0.375– 0.500 KOH: 200 H2O: 4 EtOH at 150°C for 216 h. The unique intergrowth formed by Pr6-diquat-5 was explained in terms of its molecular dimension and stabilization energy, resulting in the unusual fitting of Pr6-diquat-5 inside the channels of MFI zeolite. This can create the defects and accordingly enhance the 90° intergrowths repeatedly and sequentially. In a viewpoint of applications especially in catalysis, incorporation of aluminum into the frameworks of hierarchical zeolites is of importance. Although the present study has focused on a pure silica composition, aluminosilicate MFI zeolites with similar morphology can also be synthesized under the identical condition as reported previously.34 The effects of the synthesis parameters on the degree of intergrowth of aluminosilicate MFI zeolites and their comparison with the results of the pure silica counterpart may merit future investigation.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Experimental details for synthesis of OSDAs, elemental analysis results, thermogravimetry analysis results, additional XRD patterns, 29Si MAS NMR spectra, optimized configurations of OSDAs and OSDA–zeolite complexes, N2 adsorption–desorption isotherms in a semilog scale, and cumulative pore volume plots.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (W. C.) *E-mail: [email protected] (T. O.)

Notes

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The authors declare no competing financial interest.

ACKNOWLEDGMENT S. H. K. thanks Panasonic Scholarship and Honjo International Scholarship Foundation for financial support during his graduate study. This work was supported in part by Grant-in-Aids for Scientific Research (A) (JSPS KAKENHI Grant Number: 26249118) and for Young Scientists (B) (JSPS KAKENHI Grant Number: 16K18284) by the Japan Society for the Promotion of Science (JSPS). Parts of this work were conducted at the Center for Nano Lithography & Analysis at The University of Tokyo, which is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and at the University of Minnesota Characterization Facility, which receives partial support from the NSF through the NNIN program.

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Factors Governing the Formation of Hierarchically and Sequentially

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