Effective Fabrication of Catalysts from Large-Pore, Multidimensional

Jan 3, 2014 - As noted above, Itabashi et al.(15) proposed a working hypothesis for the seed-assisted, OSDA-free synthesis of zeolites on the basis of...
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Effective Fabrication of Catalysts from Large-Pore, Multidimensional Zeolites Synthesized without Using Organic Structure-Directing Agents Yoshihiro Kubota,*,† Keiji Itabashi,‡ Satoshi Inagaki,† Yuji Nishita,† Raita Komatsu,† Yasuyuki Tsuboi,† Shoma Shinoda,† and Tatsuya Okubo*,‡ †

Division of Materials Science and Chemical Engineering, Yokohama National University, Yokohama 240-8501, Japan Department of Chemical System Engineering, The University of Tokyo, Tokyo 113-8656, Japan

‡

S Supporting Information *

ABSTRACT: An Al-rich zeolite beta with *BEA topology and a Si/Al ratio as low as 6−7 was synthesized without the use of an organic structure-directing agent (OSDA) and subsequently treated by steam followed by heating with nitric acid for the purposes of dealumination, so as to prepare a catalyst. The steaming process played an important role in stabilizing the *BEA framework, presumably by repairing site defects with migrating silicon species. Steaming at around 700 °C was observed to produce optimal stabilization of the zeolite and allowed subsequent acidic dealumination while maintaining an intact framework. A second demonstration of successful OSDA-free synthesis and effective catalyst fabrication through postsynthetic modification involved the fabrication of a 12−10−10-ring zeolite having an MSE-type framework. This represented the first successful synthesis of an Al-rich MSE-type zeolite (with a Si/Al ratio as low as 6−7) using seed crystals in the absence of any OSDA. The gel composition as well as the crystallization temperature and time were optimized for the purpose of this synthesis such that a pure MSE phase could be obtained in a relatively short crystallization period of only 45 h. Longer crystallization periods and inadequate aging times gave mordenite as an impurity and as a major phase, respectively. These results offer further support for the so-called “composite building unit” hypothesis. As with the zeolite beta, direct dealumination of the MSE-type zeolite by acid treatment resulted in the collapse of the framework, which was avoided by steaming at 700 °C. After stabilization by steaming, acidic dealumination without framework collapse became possible. The dealuminated versions of the Al-rich beta and MSE-type zeolites were shown to be effective catalysts for the hexane cracking reaction, affording propylene in high selectivity. The MSEtype zeolite exhibited a particularly high level of coking resistance in addition to a significant yield of propylene, indicating that zeolites synthesized without using an OSDA show promise for industrial applications as highly selective and long-lived catalysts.

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INTRODUCTION Multidimensional, large-pore zeolites that contain at least one 12-ring (12-R) micropore are useful materials, both in scientific and in industrial contexts. Zeolite beta1,2 is one of the most important zeolites because of its exceptional properties and its unique pore structure. Typical zeolite beta represents an intergrowth between polymorphs A and B2,3 with an exact framework type code (FTC) of *BEA.1 For simplicity, the topological abbreviation “BEA” will be used for the “*BEA” in this Article. This structure may be described as a threedimensionally intersecting channel system with 12-R micropores, in which the dimension of a typical pore is 6.6−6.7 Å along the a and b axes and 5.6−5.6 Å along the c axis.1 Various methods for the synthesis of zeolite beta have been reported, including common hydrothermal synthesis in an alkaline medium,4 synthesis in a fluoride medium (the so-called fluoride method),5 and dry-gel conversion (the DGC method).6 The first synthesis of zeolite beta not requiring the use of an organic © 2014 American Chemical Society

structure-directing agent (OSDA) was reported by Xiao et al. in 2008.7 OSDA-free synthesis is generally desirable mainly because the OSDA process is not appreciated from the environmental and economical points of view.8 Since Xiao’s report, there have been other examples of the OSDA-free syntheses of zeolites that originally required OSDA for their production, such as ZSM-349 and RUB-13.10 Later, other successful syntheses of zeolite beta were independently disclosed;11,12 Okubo et al. extensively investigated the reproducible synthesis of zeolite beta and succeeded in developing an OSDA-free, seed-assisted technique.12,13 They were also successful in the OSDA-free synthesis of several other zeolites, such as ZSM-12,14 and described the synthetic strategy in a systematic manner (vide infra).15 Received: November 15, 2013 Revised: January 3, 2014 Published: January 3, 2014 1250

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[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidinium diiodide (TEBOP2+(I−)2) as the OSDA. The gel composition window for the successful crystallization of pure MCM-68 is very narrow, and the Si/Al ratio of the product is limited to the range of 9− 12.19,22,25 The synthesis of MCM-68 also requires a prolonged crystallization period of more than 10 days. The alteration of the chemical composition of the product through the direct crystallization of a pure MSE phase is difficult to accomplish and tends to instead generate other phases such as BEA, MTW, or MOR. We have previously succeeded in overcoming this limitation by utilizing the steam-assisted crystallization (SAC) method6 with some postsynthetic modification to obtain a pure-silica version of MSE (YNU-2).22,31 In this process, the crystallization period is ca. 5 days. We were subsequently successful in the synthesis of an Al-rich MSE-type zeolite (YNU-3) with a Si/Al molar ratio of approximately 7, requiring a relatively short crystallization period of only 3 days, via the hydrothermal conversion of an FAU-type zeolite.32 In all cases, however, the use of TEBOP2+ as an OSDA was still necessary, and the efficiency of crystallization as well as the phaseselection was sensitive to the synthesis conditions. These facts indicate the challengingness of the MSE synthesis even with the use of OSDA. As noted above, Itabashi et al.15 proposed a working hypothesis for the seed-assisted, OSDA-free synthesis of zeolites on the basis of the results of the OSDA-free syntheses of beta, mordenite, and ferrierite. This hypothesis also involved structural consideration of the common composite building units (CBUs) between the seed zeolite and the zeolites obtained from seed-free gels. By applying this theory, seedassisted, OSDA-free syntheses of ZSM-5, ZSM-11, and ZSM-12 were successfully performed.14,15 Furthermore, an alternative successful route to the OSDA-free synthesis of ZSM-12 (CBU: bik, jbw, mtw, cas) accomplished with the aid of beta seeds was reasonably explained by considering the combination of the CBUs in the beta seeds (mtw) and the gel-yielding ZSM-5 (cas) when no seeds were added. On the basis of these results, requirements for the successful seed-assisted, OSDA-free synthesis of zeolites have been proposed.15 It should be noted that this working hypothesis is not for the understanding of the crystallization mechanism of zeolites but for the consideration strategy for the zeolite syntheses under the seed-zssisted, OSDA-free conditions to avoid exhaustive trialand-error experiments. Building on the results of catalyst preparation from zeolite beta using the OSDA-free synthetic procedures discussed above, we report herein the surprisingly successful crystallization of a pure MSE phase without the use of any OSDA. The crystallization of an MSE-type zeolite in this manner not only significantly broadens the range of zeolite types that may be obtained by seed-assisted OSDA-free synthesis but also leads to a generalization of the “CBU hypothesis”. Subsequent stabilization/dealumination of the BEA and MSE materials to allow applications in the catalytic cracking of n-hexane is also reported.

From an industrial point of view, zeolite beta is a promising catalytic material representing one of the five most popular zeolite framework types (FAU, MFI, MOR, FER, and BEA) from which a majority of industrial zeolite catalysts are drawn.16 Because the reactions in which these catalysts are used and the catalyst regeneration processes are carried out at high temperatures in many processes, the design and preparation of heat- and steam-resistant catalysts are necessary. In general, syntheses employing OSDAs tend to produce materials with high silica contents, while the OSDA-free syntheses give materials with much lower silica contents, which are less hydrophobic. In this regard, the two syntheses complement one another. The low-silica zeolites are useful as ion-exchangers because they incorporate numerous ion-exchange sites; however, they are not suitable as catalysts for organic transformations due to the lack of framework stability and hydrophobicity, and appropriate postsynthesis treatments would be necessary to eliminate this drawback. As pointed out by Valtchev et al.,17 the widening window of postsynthetic options enables the preparation of zeolites with application-specific properties. To utilize the Al-rich materials as catalysts, dealumination by treatment with acid is often necessary. Very recently, De Vos et al. reported the preparation of a catalyst from zeolite beta under OSDA-free conditions, using a combination of steam dealumination and acid leaching.18 They investigated typical alkylation or acylation reactions of aromatics over this acid catalyst at 90−150 °C, as well as the hydroconversion of ndecane at 170−280 °C over a bifunctional catalyst derived from zeolite beta obtained under OSDA-free conditions. To the best of our knowledge, however, there are no other examples of the fabrication or applications of catalysts from zeolite beta synthesized without the use of OSDAs. An important aspect of the development of such new catalysts is the systematic examination of the effects of steam or nitric acid treatment on the extent of dealumination as well as the framework stability of zeolite beta samples that are crystallized in the absence of an OSDA. Because the catalysts applied to paraffin cracking are typically employed at temperatures as high as 650 °C, extremely stable catalysts are required. Another demonstration of the successful application of OSDA-free synthesis to the effective fabrication of catalysts by postsynthetic modification is the development of a 12−10−10ring zeolite having an MSE-type framework.1,19 MCM-68 is a representative MSE-type zeolite (the “type material” of MSE)1 first synthesized by Mobil researchers;20 UZM-3521 and YNU222 are related materials with the same topology. These three materials are examples of new multidimensional zeolites with a 12−10−10-R pore system. These structures contain a straight 12-R channel, which intersects two independent tortuous 10-R channels as well as an 18 × 12-R supercage accessible only through 10-R channels.19 This framework also possesses eight distinct T-sites.1,19 Zeolites of this type are known to exhibit unique acid-catalytic properties23,24 and are potentially useful as shape-selective catalysts for the alkylation of aromatics,25−27 as well as for the production of propylene by paraffin cracking.28 Their use as hydrocarbon traps has also been reported.29 In addition, titanium-substituted MCM-68 has demonstrated performance superior to that of TS-1 ([Ti]-MFI) for the oxidation of phenol and olefins when using H2O2 as the oxidant.30 The original synthesis of MCM-68 was possible only under hydrothermal conditions using N,N,N′,N′-tetraethylbicyclo-

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EXPERIMENTAL SECTION

Chemicals and Materials. The commercially available reagents were used as received without further purification. The suppliers and any appropriate cautions are noted in each following section when and where necessary. Measurements. The crystallinity and phase purity of each zeolite catalyst were examined by powder X-ray diffraction (XRD) on an

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distilled water followed by the addition of NaOH and KOH to form a clear solution. Cab-O-Sil M5 and the MSE seeds were slowly and simultaneously added to this solution, and the resulting mixture was homogenized using a mortar and pestle. The amount of MSE seeds used was 10 wt % relative to the silica source, and the total weight of all aluminosilicate gel was adjusted to obtain the desired gel composition. The seed-embedded (Na, K)-aluminosilicate gel was transferred to a 60-mL stainless steel autoclave and subjected to hydrothermal treatment at 140 °C for various periods of time under static conditions and autogenous pressure. The product was then filtered, washed with hot distilled water, and dried at 60 °C. The MSEtype zeolite thus obtained was denoted as MSEOSDAF. Typical Large-Scale Synthesis of MSE without Using an ODSA. Aqueous solutions of NaOH (6.32 mmol g−1; 12.68 g, 84.06 mmol) and KOH (5.96 mmol g−1; 996 mg, 5.94 mmol) were mixed with distilled water (43.84 g, 3.00 mol) in a Teflon cup, and sodium aluminate (Al/NaOH = 0.77; 443 mg) was dissolved in the mixture. The resulting clear solution was transferred to a mortar, and calcined MCM-68 seeds (901 mg) were added. After the entire mixture was homogenized using a mortar and pestle for 10 min, Cab-O-Sil M5 (9.01 g, 150 mmol) was added, and the combined ingredients were again homogenized in the same manner for 20−30 min. The mixture was then transferred to a 60-mL stainless steel autoclave and subjected to hydrothermal treatment at 140 °C for 48 h under static conditions and autogenous pressure. The product was subsequently filtered, washed thoroughly with hot distilled water, and dried at 60 °C to obtain pure MSEOSDAF. Postsynthetic Treatment of the Zeolites (BEA and MSE) Synthesized by the ODSA-Free, Seed-Assisted Method. Direct Acid Treatment. As-synthesized BEAOSDAF or MSEOSDAF were treated with HNO3 solutions of concentration x mol L−1 (where x = 0.1, 0.5, 1.0, 2.0, or 6.0) (60 mL (g-sample)−1) in a 200 mL round-bottom flask under reflux conditions in a 130 °C oil bath for 24 h. The acid-treated samples were denoted as BEAOSDAF_AT(x) or MSEOSDAF_AT(x), where x was the acid concentration in mol L−1. Ion Exchange. Ion exchange of the calcined samples to the NH4+form was performed using an NH4NO3 solution as follows. NH4NO3 (4.0 g) and the calcined sample (2.0 g) were suspended in H2O (100 mL) in a 250 mL polypropylene bottle. The bottle was capped tightly and allowed to stand at 80 °C for 24 h with occasional release of pressure and careful shaking. After being cooled, the sample was separated by filtration and washed with deionized water. This process was repeated twice, after which the sample was dried overnight at room temperature. The resulting zeolite was again calcined in a muffle furnace, during which the temperature was raised from ambient to 550 °C over a period of 4 h, and kept at the same temperature for 6 h to give the sample in H+ form. The NH4+-forms of the BEA- and MSEtype zeolites are denoted as BEAOSDAF_IE and MSEOSDAF_IE, respectively. Direct calcination of the BEAOSDAF_IE and MSEOSDAF_IE gave acidic H+-forms that are designated as BEAOSDAF_IE_cal and MSEOSDAF_IE_cal, respectively. Steaming of NH4+-Form Samples. Steaming of the NH4+-form samples was carried out utilizing the equipment shown in Supporting Information Figure S1. Steam (7.3−12.3 kPa) was supplied at various temperatures (y °C) for 24 h, and the resultant BEA- and MSE-type materials were denoted BEAOSDAF_IE_ST(y) and MSEOSDAF_IE_ST(y), respectively, where y is the steam temperature in °C. Screening identified an optimized steam temperature of 700 °C, and (y) may be omitted for y = 700. Dealumination by Acid Treatment Before and After Steaming. The dealumination of the BEAOSDAF_IE_ST or MSEOSDAF_IE_ST was typically carried out by treatment with a 6.0 mol L−1 HNO3 solution (60 mL (g-sample)−1) in a 200-mL round-bottom flask at 80 °C for 2 h in the case of BEA samples or under reflux conditions in a 130 °C oil bath for 24 h for MSE samples. The acid-treated samples were denoted BEAOSDAF_IE_ST_AT(x) or MSEOSDAF_IE_ST_AT(x), where x is the acid concentration in mol L−1, and employed in the catalytic reactions. Reaction Procedure. An appropriate amount of each zeolite catalyst was pelletized without any binder, roughly crushed, and then

Ultima-IV (Rigaku) using Cu Kα radiation at 40 kV and 20 mA. The Si/Al molar ratios in the bulk materials were measured by means of inductively coupled plasma-atomic emission spectrometry (ICP-AES, ICPE-9000, Shimadzu). Nitrogen adsorption and desorption isotherms at −196 °C were obtained for samples pretreated under vacuum at 400 °C for 2−6 h on a Belsorp Max gas adsorption instrument. Specific surface areas (SBET) and micropore volumes (Vmicro) were calculated using the BET method and the t-plot method, respectively. Solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were acquired using an AVANCEIII 600 (Bruker) operating at 600 MHz for 1H, 119.2 MHz for 29Si, and 156.4 MHz for 27Al. All of the MAS NMR spectra were recorded at room temperature in a 4 mm diameter ZrO2 tube. The 29Si chemical shifts were determined on the basis of that of hexamethylcyclotrisiloxane at −9.66 ppm. Dipolar-decoupling (DD) MAS NMR measurements were carried out using 1024 pulses with a recycle time of 30 s at a spinning rate of 10 kHz. The 27Al chemical shifts were determined using an aqueous Al(NO3)3 solution, the resonance peak of which was adjusted to 0 ppm. The direct-excitation (DE) MAS NMR measurements were carried out using 1024 pulses with a recycle time of 0.5 s at a spinning rate of 13 kHz. The morphologies of the zeolite catalysts were observed by scanning electron microscopy (SEM) using a JSM-7001F (JEOL) or a Hitach S-4800. The coke contents of the used catalysts were determined by thermogravimetry (TG, Thermo plus EVO II TG8120, Rigaku), during which the weight loss of the used catalyst over the range of 400−800 °C was defined as the amount of coke formed during the catalytic reaction. OSDA-Free Synthesis of Zeolite Beta. OSDA-assisted Al-beta (Si/Al = 12.0, Na/Al = 0.04) was hydrothermally crystallized from an aluminosilicate gel containing Et4N+OH− (TEA+OH−) with the following chemical composition: SiO2 −0.0357Na2O−0.175TEA2O− 0.0286Al2O3−14H2O. The obtained zeolite beta was then calcined and subsequently used as seed crystals. OSDA-free synthesis was carried out by the addition of the calcined beta seeds (Si/Al = 12.0) to an OSDA-free Na+-aluminosilicate gel prior to the hydrothermal treatment. The chemical composition of the Na+-aluminosilicate gel was adjusted to SiO2−0.275Na2O−0.025Al2O3−25H2O. The zeolite beta product typically crystallized after heating the gel at 140 °C for 60 h. The beta zeolite obtained in this manner was denoted BEAOSDAF. A more detailed description of this synthesis is contained in previous reports.12,13 Synthesis of MCM-68 Seed Crystals. Conventional MCM-68 zeolite was synthesized as follows. Colloidal silica (Ludox HS-40, DuPont, 40 wt % SiO2, 6.01 g, 100.0 mmol), deionized water (40 mL), and Al(OH)3 (Pfaltz and Bauer, 0.78 mg, 10.0 mmol) were mixed in a 180 mL Teflon beaker and stirred for 10 min. Aqueous KOH solution (5.93 mmol g−1, 6.32 g, 37.5 mmol) was added to the solution, and the mixture was stirred for a further 30 min, after which N,N,N′,N′tetraethyl-exo,exo-bicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidinium diiodide, TEBOP2+(I−)2 (10.0 mmol; see refs 20 and 32 for detailed synthetic procedures), was added as the OSDA, and the mixture was stirred for another 4 h. The resulting mixture, having the molar composition 1.0SiO2−0.1TEBOP2+(I−)2−0.375KOH−0.1Al(OH)3− 30H2O, was placed in a 125 mL Teflon-lined autoclave within a convection oven and maintained at 160 °C for 16 days. After the autoclave was cooled to room temperature, the obtained solid was separated by centrifugation, washed several times with deionized water, and dried overnight. The as-synthesized MCM-68 zeolite was obtained as a white powder (6.04 g). To remove any OSDA occluded in the pores, the as-synthesized MCM-68 was heated in a muffle furnace, raising the temperature from ambient to 650 °C at a rate of 1 °C min−1 and maintained at that same temperature for 10 h. Finally, the sample was cooled to room temperature to give the calcined product as a white powder (Si/Al = 11). OSDA-Free Synthesis of MSE-type Zeolite. OSDA-free syntheses of MSE zeolites were carried out by adding the calcined MCM-68 seeds (Si/Al = 11), as described in the previous section, to OSDA-free (Na, K)-aluminosilicate gels with the molar composition 1.0SiO2−0.3[Na2O + K2O]−0.01Al2O3−20H2O. The general preparation procedure was as follows. Sodium aluminate was dissolved in 1252

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sieved to obtain particles 500−600 μm in size. Hexane cracking was performed under atmospheric pressure in a down-flow quartz-tube microreactor with an 8 mm inner diameter. Prior to running the reaction, 100 mg of catalyst pellets was packed in the fixed bed of the reactor and preheated at 650 °C for 1 h in a stream of air. The reaction was performed at 650 °C for 305 min in a stream of helium containing an appropriate amount of hexane (W/F = 19.6 g-cat. h (molhexane)−1). Following the reaction at 650 °C, the system was cooled to room temperature in a helium stream, and the used catalyst was recovered. The reactants and products were analyzed on an HP-PLOT Q capillary column (i.d. 0.53 mm; length 30 m; Agilent Technology) using a GC-14B (Shimadzu) with a flame ionization detector. The conversion of hexane and the selectivity of each catalyst were calculated on a carbon basis relative to the initial amount of hexane.

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RESULTS AND DISCUSSION Stabilization and Dealumination of Zeolite Beta Synthesized under OSDA-Free Conditions. In the case of a conventional zeolite beta hydrothermally synthesized by an OSDA-assisted method (typically with a Si/Al ratio around 12−15 corresponding to 4−5 Al sites/unit cell), the framework aluminum atoms are readily removed by liquid phase nitric acid treatment at 80−100 °C. It is relatively difficult, however, to obtain the framework dealumination of materials synthesized by the OSDA-free method (in which the typical Si/Al ratio is 5−7, corresponding to 8−11 Al sites/unit cell). In this work, the Si/Al ratio of the as-synthesized BEAOSDAF was 6.4, and, as shown in Figure 1a and b, almost no dealumination took place

Figure 2. XRD patterns of (a) BEAOSDAF, (b) BEAOSDAF_IE, (c) BEA O S D A F _IE_ST(150), (d) BEA O S D A F _IE_ST(250), (e) BEA O S D A F _IE_ST(350), (f) BEA O S D A F _IE_ST(450), (g) BEA O S D A F _IE_ST(550), (h) BEA O S D A F _IE_ST(650), (i) BEAOSDAF_IE_ST(750), and (j) BEAOSDAF_IE_ST(850). Si/Al ratios of the samples are also shown. Abbreviations are explained in the text.

patterns obtained following this acid treatment are shown in Figure 3. Significant framework collapse or loss of crystallinity

Figure 1. XRD patterns of (a) as-synthesized BEAOSDAF, (b) BEAOSDAF_AT(0.1), (c) BEAOSDAF_AT(0.5), (d) BEAOSDAF_AT(1.0.), (e) BEAOSDAF_AT(2.0), and (f) BEAOSDAF_AT(6.0). Si/Al ratios of the samples are also shown. Abbreviations are explained in the text.

in 0.1 mol L−1 nitric acid. When the acid concentration was increased, the framework collapsed concurrent with dealumination (Figure 1c−f). To avoid this collapse, we attempted to steam the OSDA-free zeolite beta following ion-exchange of Na+ with NH4+. Removing as much Na+ as possible is desirable, and some examples of ion-exchange just prior to steaming have been reported during the preparation of USY33 and zeolite beta.18 Figure 2 shows the effects of steaming temperature; after steaming at 150−750 °C, no collapse was observed, and, in addition, the Si/Al ratio (as determined by ICP) of the sample did not change significantly, although a slight decrease in XRD crystallinity was observed at 850 °C. Each steamed sample was treated with dilute nitric acid (0.1 mol L−1), and the XRD

Figure 3. XRD patterns of (a) BEA OSDAF _AT(0.1), (b) BEAOSDAF_IE_AT(0.1), (c) BEAOSDAF_IE_ST(150)_AT(0.1), (d) BEAOSDAF_IE_ST(250)_AT(0.1), (e) BEAOSDAF_IE_ST(350)_AT(0.1), (f) BEAOSDAF_IE_ST(450)_AT(0.1), (g) BEAOSDAF_IE_ST(550)_AT(0.1), (h) BEA OSDAF _IE_ST(650)_AT(0.1), (i) BEAOSDAF_IE_ST(750)_AT(0.1), and (j) BEAOSDAF_IE_ST(850) _AT(0.1), showing the effects of steaming temperature followed by dilute acid treatment at 80 °C for 2 h. Si/Al ratios of the samples are also shown. Abbreviations are explained in the text. 1253

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was observed when the steaming temperature was below 500 °C (Figure 3c−f), while efficient stabilization was achieved when the steaming temperature was between 550 and 750 °C (Figure 3g−i). Dealumination by dilute acid treatment after steaming was most efficient when the steaming temperature was in a range of 650−750 °C. Additional investigation of the steaming temperature indicated that dealumination by dilute acid treatment was most effective when the steam was applied at 700 °C (Figure 4d).

Figure 5. 29Si DD-MAS NMR spectra of (a) BEAOSDAF, Si/Al = 6.4; (b) BEAOSDAF_IE, Si/Al = 6.3; (c) BEAOSDAF_IE_ST(700), Si/Al = 5.3; and (d) BEAOSDAF_IE_ST(700)_AT(1.0), Si/Al = 60. Spectra were obtained from 1024 pulses with a recycle time of 30 s at a spinning rate of 10 kHz. Abbreviations are explained in the text.

the intensity of the Q3 site signals as well as sharpening of the Q4 signals subsequent to steaming. The low level of mesopore formation detected by N2 adsorption−desorption measurements (Supporting Information Figure S2) is also consistent with Si-migration.31 The 27Al MAS NMR spectra of BEAOSDAF and BEAOSDAF_IE as well as samples steam-treated at various temperatures are shown in Figure 6. Figure 6a−c clearly shows major peaks only in the region of 50−65 ppm, which correspond to tetrahedral aluminum and indicate that all of the aluminum in BEAOSDAF and BEAOSDAF_IE is incorporated into the framework. Upon steaming, the framework Al signal gradually decreases, while the signal attributed to extra-framework Al (octahedral Al) at around 0 ppm becomes evident. Both tetrahedral and octahedral aluminum peaks disappear, however, after thorough steaming at higher temperatures such as 750 °C. The decrease in the tetrahedral Al signal intensity is due to the hydrolytic removal of framework Al, while the decrease in the signal intensity of octahedral Al may be due to a lack of unity in the extra-framework aluminum species. It is interesting to note that both BEAOSDAF and BEAOSDAF_IE exhibit a shoulder peak at around 54 ppm accompanied by a major peak at around 59 ppm. According to the literature,34 the lower-field resonance (left peak) in the 27Al MAS NMR spectrum corresponds to a narrower T−O−T angle (T = Al or Si) in the framework. Because the T1 and T2 sites in the BEA framework35 (the numbering is based on refs 35, 36) are associated with the widest T−O−T angles (ca. 153.5° and 152.1°, respectively, on average; see Table S1 in the Supporting Information) in comparison with the other seven possible T sites (ca. 147.1− 151.7° on average, Supporting Information Table S1), the tetrahedral aluminum assigned to the shoulder at 54 ppm would correspond to the T1 and T2 sites in the BEA framework.36 The aluminum atoms in these sites tend to remain in the framework, while the aluminum atoms at other T sites (assigned to the major peak at around 59 ppm) disappear more rapidly, and thus the initially minor peak (the shoulder) becomes a major peak upon steaming (Figure 6). For any given T site, there are four adjacent T−O−T angles. The differences (the mean deviations) between the four T−O−T angles are

Figure 4. XRD patterns and Si/Al ratios of (a) BEAOSDAF, (b) BEAOSDAF_IE, (c) BEAOSDAF_IE_ST(700), (d) BEAOSDAF_IE_ST(700)_AT(0.1), (e) BEA OSDAF _IE_ST(700)_AT(1.0), (f) BEAOSDAF_IE_ST(700)_AT(8.0), (g) BEAOSDAF_IE_ST(700)_AT(8.0), and (h) BEAOSDAF_IE_ST(700)_AT(13.4), showing the effects of acid treatment at 80 °C for 2 h after steaming at 700 °C for 24 h. In the case of samples (g) and (h), prolonged 24 h acid treatment under reflux was applied. Abbreviations are explained in the text.

Subsequent nitric acid treatment of the sample that was steamed at 700 °C resulted in an increase in the analytical value of Si/Al depending on the acid concentration (Figure 4), indicating that optimizing both the steaming temperature (at 700 °C) and the concentration of acid applied during dealumination can tune the Si/Al ratio of the final product. It should be noted that there was no evidence of framework collapse, indicating that the framework was made more robust by steaming. A possible explanation for the observed results is as follows. Hydrolysis of the Al−O bond results in removal of Al from the framework and the concurrent formation of site defects to a maximum 8.6 sites per unit cell. This large number of site defects is likely the cause of the framework collapse. During steam treatment, the migration of Si(OH)4 species takes place through the micropores from elsewhere in the crystal lattice to the defect positions, and the condensation of these species is able to repair the lattice defects.31 This hypothesis is supported by some characterization results, including the NMR and adsorption data. Figure 5 shows the 29 Si MAS NMR spectra of BEA OSDAF , BEA OSDAF _IE, BEAOSDAF_IE_ST(700), and BEAOSDAF_IE_ST(700)_AT(1.0) samples, which clearly demonstrate the significant decrease in 1254

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Figure 6. 27Al DD-MAS NMR spectra of (a) BEAOSDAF, Si/Al = 6.4; (b) BEAOSDAF_IE, Si/Al = 6.2; (c) BEAOSDAF_IE_ST(150), Si/Al = 6.4; (d) BEAOSDAF_IE_ST(250);, Si/Al = 6.5; (e) BEAOSDAF_IE_ST(350), Si/Al = 6.5; (f) BEAOSDAF_IE_ST(450), Si/Al = 6.3; (g) BEAOSDAF_IE_ST(550), Si/Al = 5.8; and (h) BEAOSDAF_IE_ST(750), Si/Al = 5.4. Spectra were obtained from 1024 pulses with a recycle time of 0.5 s at a spinning rate of 13 kHz. Spectra c′−h′ are Y-gain-expanded versions of spectra c−h, respectively. Abbreviations are explained in the text.

crystallization of MOR along with the MSE phase still took place to some extent (Supporting Information Figure S5). Partial replacement of Na by K suppressed the formation of MOR to a remarkable extent (Figure 7 and Supporting

particularly small for T1 and T2 sites, meaning that bond cleavage by hydrolysis is more difficult in the case of the T1 and T2 aluminums, which may explain the slow dealumination at the T1 and T2 sites. Observations via scanning electron microscopy (SEM) of the BEAOSDAF samples at various postsynthesis stages demonstrated that the material is composed of particles approximately 100− 200 nm in size, which means that these particles are at the same level in size as the OSDA-assisted BEA seed crystals, although the particles differ in terms of morphology (see Figure S3 in the Supporting Information). More precisely, however, the OSDAassisted seed crystals polycrystalline aggregates are built of much smaller crystallites than the BEAOSDAF particles, which is clearly recognized in the FE-SEM images shown in our previous papers.12,13 During the hydrothermal treatment, zeolite beta would not crystallize directly from the OSDAfree gel but mainly crystallizes on the surface of the residual beta seeds.13 The beta seeds would be partly dissolved and disaggregated into small pieces13 to form two types of seed:8 (i) a single isolated bred released from the aggregates and (ii) complex aggregates comprising a large number of single-bred nuclei. Although both types (i) and (ii) could exist, aggregated product may be a result of type (ii) seed. Regardless of the crystallization mechanism, the particle size of BEAOSDAF in this work is sufficiently small to allow catalytic applications of the material. Optimization of Conditions for the OSDA-Free Synthesis of MSE-type Zeolite. We initially attempted a typical synthesis of an MSE phase from Na-aluminosilicate. Although very slight increases in the MSE phase peaks were observed in the powder XRD patterns, as shown in Supporting Information Figure S4, these were not significant, and the mordenite (MOR) phase became prominent after 60 h, accompanied by a small amount of zeolite beta (BEA). In this case, therefore, the resulting seed crystals included very small amounts of BEA phase impurities. When pure MCM-68 was used as seed crystals, contamination by BEA was avoided, although

Figure 7. XRD patterns of (a) calcined MCM-68 seed crystals and products obtained by heating the gel at 140 °C for (b) 49 h, (c) 57 h, (d) 64 h, and (e) 72 h. The starting gel composition was 1.0SiO2− 0.28Na2O−0.02K2O−0.01Al2O3−20H2O. The peaks marked with asterisks indicate the presence of a mordenite (MOR) phase. Abbreviations are explained in the text.

Information Figure S6). As a typical example, Figure 7 shows the time-course of XRD patterns during crystallization using 10 wt % seed crystals in the presence of a very small amount of K (molar fraction of K within the total alkali content is 0.067). In this case, a pure MSE phase was obtained after a time span of 40−64 h. In contrast to the K-free synthesis, a small amount of BEA contamination in the seed crystals did not have any negative effects. An overly long crystallization period was also found to result in contamination by an MOR phase. The fact that the appearance of the MOR phase was delayed when K was added suggests that the potassium ion inhibits the crystallization of MOR. Although the original MCM-68 1255

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synthesis required the use of KOH rather than NaOH, the addition of too much potassium retarded the overall crystallization process and eventually resulted in crystallization of MOR as the predominant phase (Supporting Information Figures S7−S10). On the basis of these results, the optimal ratio of K/[Na+K] is considered to be 0.067. Using this ratio, quite reproducible synthesis was possible even when the synthetic scale was increased such that the quantities involved were 5 times larger than our initial investigations (Figure 8),

Figure 9. Effects of postsynthesis treatments. XRD patterns and Si/Al ratios of (a) as-synthesized [Al]-MSEOSDAF, Si/Al = 6.8; (b) sample (a) treated with 6 mol L−1 HNO3 under reflux for 2 h; (c) [Al]MSEOSDAF_IE, Si/Al = 6.5; (d) [Al]-MSEOSDAF_IE_ST, Si/Al = 6.8; and (e) De[Al]-MSEOSDAF_IE_ST_AT(6.0), Si/Al = 66.9. Abbreviations are explained in the text.

Figure 8. Powder XRD patterns of as-synthesized products obtained under OSDA-free conditions at 140 °C over 44−48 h on different scales. Starting SiO2 amounts were (a) 30 mmol, (b) 100 mmol, and (c) 150 mmol.

affording pure MSE phases in constant yields. A detailed experimental procedure is described in the Supporting Information. The product yield was typically 20%, although it varied depending on the relationship between the starting gel composition and chemical composition of the product. Dealumination Behavior of MSE Synthesized without Using an OSDA. In the case of conventional MSE hydrothermally synthesized by the OSDA-assisted method (typical Si/Al is around 11, corresponding to 9−10 Al sites/ unit cell), framework aluminum atoms are readily removed by liquid phase nitric acid treatment at 80−100 °C to give dealuminated sample (denoted MCM-68_AT). With the MSE synthesized using an OSDA-free method (Figure 9a; typical Si/ Al is 6−7 corresponding to 14−16 Al sites/unit cell), however, the framework exhibits significant collapse during dealumination (Figure 9b). The steaming of an NH4+-form was found to be an effective means of avoiding this collapse, according to the results obtained for BEAOSDAF, as discussed in the previous section Stabilization and Dealumination of Zeolite Beta Synthesized under OSDA-Free Conditions. The MSEOSDAF underwent ion exchange with an NH4NO3 solution at 80 °C to give the NH4+form (MSEOSDAF_IE; Figure 9c). During steaming of this NH4+-form, the steam (7.3−12.3 kPa) was typically supplied at 700 °C for 24 h, and the resulting material was denoted MSEOSDAF_IE_ST (Figure 9d), as noted in the section Postsynthetic Treatment of the Zeolites (BEA and MSE) Synthesized by the ODSA-Free, Seed-Assisted Method. No framework collapse was observed during this steaming, while the Si/Al ratio (as per ICP data) of the sample did not increase. Subsequent HNO3 treatment resulted in a remarkable increase in the Si/Al ratio (Figure 9e) even though the MSE framework remained intact. The acid treatment procedure consisted of heating the steamed sample at 80 °C for 2 h in a 6 mol L−1 HNO3 solution. In this manner, dealuminated samples were obtained and denoted MSEOSDAF_IE_ST(700)_AT(6.0).

There is a common reason for the framework stabilization of BEAOSDAF and MSEOSDAF, which was observed to result from steaming. As discussed in the previous section, we believe that the migration of Si(OH)4 takes place during steaming to repair site defects by condensation reactions between Si(OH)4 and silanols.31 The Q3 signals seen in the 29Si MAS NMR spectra were significantly smaller than expected on the basis of the initial aluminum amounts in the as-synthesized MSEOSDAF sample (Supporting Information Figure S11) as well as the results of our previous investigation.31 Figure 10 shows the Si/Al ratio of the sample as a function of the concentration of HNO3 used for acid treatment. The conventional MCM-68 (initial Si/Al = 11.0) was quite readily dealuminated with dilute HNO3, whereas the dealumination of YNU-3 (an MSE analogue with initial Si/Al = 7.2, see

Figure 10. Effects of the HNO3 concentration on the extent of dealumination of (a) MCM-68, (b) YNU-3, and (c) MSEOSDAF_IE_ST during treatment at 80 °C for 24 h. YNU-3 represents an MSE-type zeolite (see Introduction and ref 28). Abbreviations are explained in the text. 1256

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also exhibited smaller external surface areas (38−44 m2 g−1) than MCM-68 (59 m2 g−1),31 a result that is consistent with their relative particle sizes. Hexane Cracking over BEA and MSE Catalysts. Figure 12 shows the variation over time in both hexane conversion and

Introduction)32 and MSEOSDAF_IE_ST (initial Si/Al = 7.8) required much more concentrated acid, and moreover the maximum Si/Al ratio did not exceed 100 even when using concentrated HNO3. Although a minor signal assignable to octahedral Al was observed in the 27Al MAS NMR spectra after acid treatment of the steamed sample, the major signal remained that of tetrahedral Al (Supporting Information Figure S12). On the basis of our previous investigations and a general knowledge of catalysis, the appropriate Si/Al ratio for catalytic use is very often in the range of 50−100. The fact that the MSEOSDAF materials exhibit durability during additional dealumination is therefore highly desirable and promising with regard to future catalytic applications. There may be both microscopic (such as “siting” of Al in the framework) and macroscopic (related to particle size and morphology) reasons for the observed dealumination resistance. With regard to macroscopic reasons, scanning electron microscopy (SEM) observations of MSEOSDAF revealed that the material is composed of particles approximately 0.5 μm in size, and so these particles are larger than the MCM-68 seed crystals, which were ca. 50−100 nm (Figure 11). The MSEOSDAF therefore shows a defined morphology accompanied by particle sizes sufficiently small for catalytic applications.

Figure 12. Conversion and yield from the cracking of n-hexane catalyzed by (a) BEA O S D A F _IE_cal, Si/Al = 6.7; (b) BEAOSDAF_IE_ST(700), Si/Al = 5.8; (c) BEAOSDAF_IE_ST(700) _AT(1.0), Si/Al = 60.0; (d) OSDA-assisted beta, Si/Al = 56.7; and (e) commercially available beta (Tosoh HSZ-940 HOA), Si/Al = 55.5. In the case of sample (c), the Si/Al ratio was adjusted by steaming at 700 °C followed by treatment with 1.0 mol L−1 HNO3. In the case of sample (d), the Si/Al ratio was adjusted by treatment with 1.0 mol L−1 HNO3. Abbreviations are explained in the text.

product yield during the cracking of n-hexane at 650 °C over various BEA-type zeolite catalysts. Product distribution is also shown in Supporting Information Figure S14. The parent BEAOSDAF catalyst exhibited sudden deactivation, while the steamed sample (BEAOSDAF_IE_ST(700)) still showed deactivation (Figure 12a and b). In contrast, the optimally treated BEAOSDAF samples, such as BEAOSDAF_IE_ST(700)_AT(1.0), showed a remarkable steadiness in conversion as well as in propylene yield and selectivity (Figure 12c). For comparison, an OSDA (tetraethylammonium)-assisted zeolite beta and a high-quality commercial zeolite beta (Tosoh HSZ-940HOA), which had their Si/Al ratios adjusted to equivalent levels with 1.0 mol L−1 nitric acid, were used under the same reaction conditions. These materials exhibited either gradual or relatively fast deactivation along with lower propylene yields (Figure 12d and e), emphasizing the superiority of the BEAOSDAF-related catalysts over the conventional BEA catalysts under certain conditions. Figure 13 shows the variation over time in both hexane conversion and product yield during the cracking of n-hexane at 650 °C over various MSE-type zeolite catalysts. Product distribution is also shown in Supporting Information Figure S15. Although the system composed of the dealuminated version of MCM-68 (Si/Al = 64.5; denoted as MCM-68_AT) exhibited catalytic activity sufficient for the hexane cracking process, it was significantly deactivated such that the hexane conversion at 305 min was below 50%. This deactivation was likely caused by the large amount of coke (33.4 mg-coke (gcatalyst)−1 after 305 min of reaction) formed on the catalyst. In contrast, the dealuminated version of YNU-3 (Si/Al = 69.4; denoted YNU-3_AT) maintained its initial catalytic activity after 305 min and showed a minimal amount of coke formation

Figure 11. Typical FE-SEM images of (a) MCM-68 seed crystals (calcined), (b) YNU-3 (calcined), and (c) MSEOSDAF. Abbreviations are explained in the text.

The MSEOSDAF samples before and after acid treatment gave typical type-I nitrogen adsorption−desorption isotherms (Supporting Information Figure S13), and the corresponding micropore volumes estimated by the t-plot method were in the range of 0.17−0.18 cm3 g−1, which is similar to that of calcined MCM-68 seed crystals (0.19 cm3 g−1).28 MSEOSDAF samples 1257

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CONCLUSIONS BEA-type zeolites synthesized under OSDA-free conditions were stabilized and dealuminated by steaming followed by heating in a nitric acid solution. The optimal temperature for the steaming process was around 700 °C. The Si/Al ratios of the products were controlled by tuning the acid concentration. MSE-type zeolites were also successfully synthesized under OSDA-free conditions. The as-synthesized Al-rich products were stabilized and dealuminated in a manner similar to the process applied to the zeolite beta, using a combination of steaming and heating in a HNO3 solution. With both the BEA and the MSE materials, Si-migration during the steaming process is suggested. The preparation of acid catalysts from BEA and MSE materials crystallized under OSDA-free conditions was thus successful, and the resulting catalysts showed promising catalytic performance in hexane cracking reactions. The stabilization process reported here is also expected to be effective in the case of other large-pore zeolites synthesized without using an OSDA. The achievements reported in this Article allow for the possibility of the OSDA-free synthesis of new zeolites and demonstrate the catalytic applicability of aluminum-rich zeolites that have been synthesized without using an OSDA and that are postsynthetically stabilized.

Figure 13. Conversion and yield from cracking of n-hexane catalyzed by (a) dealuminated MCM-68 (MCM-68_AT, Si/Al = 64.5), (b) dealuminated-YNU-3 (YNU-3_AT, Si/Al = 69.4), and (c) MSEOSDAF_IE_ST(700)_AT(6.0), Si/Al = 66.9. Abbreviations are explained in the text.

(1.8 mg-coke (g-catalyst)−1 after 305 min of reaction). Surprisingly, the dealuminated version of MSEOSDAF (Si/Al = 66.9; MSEOSDAF_IE_ST(700)_AT(6.0)) showed much improved catalytic activity and comparatively reduced coke formation (3.3 mg-coke (g-catalyst)−1 after 305 min of reaction) as previously reported.28 Hexane conversion with this material was always much higher than that over dealuminated YNU-3 (YNU-3_AT). Looking at the product selectivity at the point of 70% hexane conversion at 650 °C, the dealuminated version of MSEOSDAF (=MSEOSDAF_IE_ST(700) _AT(6.0)) exhibited higher propylene selectivity (ca. 44%; the same level as the less active YNU-3_AT) and lower BTX selectivity (1%) as compared to dealuminated MCM-68 (ca. 41% and 3%, respectively). As far as propylene yield is concerned, the reaction over the stabilized, dealuminated version of MSEOSDAF (i.e., MSEOSDAF_IE_ST(700)_AT(6.0)) gave propylene in much higher yields than that over the other MSE-type catalysts at any point after 1 h in the reaction (Figure 13), indicating that properly modified MSEOSDAF may be a viable candidate for application as a long-lived paraffin-cracking catalyst for the selective production of propylene. The superiority of MSEOSDAF_IE_ST(700)_AT(6.0)) over dealuminated MCM-68 and dealuminated YNU-3 can be explained by considering the order of initial activity and that of deactivation rate as follows. Assuming a uniform distribution of active sites, the effects of external surface area as a function of particle size on the initial catalytic activity and on the catalyst lifetime are in a trade-off relationship. Initial activity depends on the number of pore-entrance as a function of external surface area, and catalyst lifetime depends on the heavy coke formation that is considered to occur mainly on the outer surface of the particle.37,38 Within the current series of MCM-68, YNU-3, and MSEOSDAF cases, MCM-68 has the smallest particle size and accordingly shows the highest initial activity with the fastest deactivation rate, whereas YNU-3 has the largest particle size and shows the lowest initial activity with the slowest deactivation rate. The MSEOSDAF has an intermediate particle size so that it shows relatively high initial activity while keeping slow deactivation rate, resulting in a nearly optimum catalytic performance.

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ASSOCIATED CONTENT

S Supporting Information *

Further details, including a summary of T−O−T angles, a description of the apparatus used for steam treatment of catalysts, nitrogen adsorption−desorption isotherms, FE-SEM images, additional XRD patterns obtained during the synthesis of MSE-type materials, 29Si and 27Al solid NMR spectra, and the results of hexane cracking (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to acknowledge the Nippon Chemical Industrial Co., Ltd. for partly supporting this work through financial assistance and also for assisting in performing FE-SEM observations. This work was supported in part by New Energy and Industrial Development Organization (NEDO) .

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REFERENCES

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