Crystallization of Ti-Rich *BEA Zeolites by the Combined Strategy of

Article pubs.acs.org/crystal

Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystallization of Ti-Rich *BEA Zeolites by the Combined Strategy of Using Ti−Si Mixed Oxide Composites and Intentional Aluminum Addition/Post-Synthesis Dealumination Hirofumi Horikawa,† Takayuki Iida,† Ryota Osuga,‡ Koji Ohara,§ Junko N. Kondo,‡ and Toru Wakihara*,† †

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259-R1-10 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan § Japan Synchrotron Radiation Research Institute/SPring-8, Kouto 1-1-1, Sayo-gun, Hyogo 679-5198, Japan ‡

S Supporting Information *

ABSTRACT: Titanosilicate zeolites are well-known catalysts for selective oxidation using hydrogen peroxide, an environmentally friendly oxidant. To effectively synthesize these materials with high Ti contents, we have focused on using a Ti−Si mixed oxide composite as the ingredient along with intentional addition of an aluminum source to promote crystallization. Ti-beta, a *BEA-type zeolite containing titanium at the framework sites, was chosen as a model zeolite. First, (Ti, Al)-beta, a *BEA-type zeolite containing both Ti and Al, was prepared; the occluded aluminum inside the product was subsequently removed by an acid treatment. This treatment not only lead to the reduction of the aluminum content to trace levels but also improved the states of the titanium species to the desired tetrahedral coordination state. Thus, Ti-beta zeolites with little extra-framework Ti were successfully obtained with molar compositions up to Ti/(Ti + Si) = 4.0 mol %. As a titanosilicate zeolite catalyst, high functionality was demonstrated based on the oxidation of cyclooctene, confirming the positive impact of having high titanium content with low aluminum content. Finally, investigation of the intermediates during the crystallization process was performed to understand the behavior of titanium species throughout the crystallization and to propose the critical factors for achieving efficient Ti introduction.

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INTRODUCTION Zeolites are crystalline porous framework materials comprised of tetrahedral metal oxide units, such as [SiO4] and [AlO4]. Due to their high structural stabilities and diverse functionalities, zeolites have played a crucial role in numerous industrial chemical processes.1,2 Controlled synthesis of different catalytic active sites inside the zeolite framework is the key for developing efficient catalysts; such controlled synthesis is being demanded from fields ranging from biomass conversion3 to fine chemicals production.4 While the acid sites are easily produced by the introduction of aluminum into the siliceous zeolite framework,2 being a very common composition found in nature, oxidation sites (using H2O2 as oxidant) are often developed by the introduction of tetravalent heteroatom cations, such as titanium5 and tin,6 into the framework sites. Ever since the first studies on titanium silicalite-1 (TS-1, titanosilicate MFI-type zeolite)7,8 reporting its remarkable oxidation ability by activating H2O2 using framework Ti sites, a vast number of academic contributions have focused on the incorporation of titanium into various zeolite frameworks. While TS-1 has been widely known as a catalyst for many of the oxidation reactions involving aliphatic hydrocarbons, such as for the production of propylene oxide,8 other titanosilicates © XXXX American Chemical Society

with different frameworks are capable of having higher selectivites when applied to other reactions.5 For example, Ticontaining MOR-type zeolite has been shown to possess high performance and stability for ammoximation reactions,9 and Ticontaining MSE-type zeolite is known to possess high selectivity in phenol oxidation to p-hydroquinone.10 Ti-beta is known to possess a higher catalytic activity for some of the reactions that use larger substrates due to its three-dimensional large pore structure (12-membered ring).11 Ti-beta also possesses higher structural stability compared to Ti-MCM-41, a mesoporous silica having isolated Ti sites in the amorphous silicate walls.12,13 Ti-beta has recently gained additional attention as a catalyst for oxidative desulfurization, a process that utilizes oxidation to remove sulfur-containing species, such as dibenzothiophene, from crude oil.14 In this process, sulfurcontaining compounds are oxidized and removed by the subsequent distillation, thus preventing reactor corrosion caused by H2 in the conventional hydro-desulfurization process. For such cases where the desired reaction selectivity can be Received: November 22, 2017 Revised: February 19, 2018

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DOI: 10.1021/acs.cgd.7b01621 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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combination with the subsequent dealumination treatment is an appealing method for obtaining Ti-rich zeolites having little TiOx impurities.

easily achieved, the production of zeolites having higher titanium content per mass is desired. The first Ti-beta zeolites were typically synthesized by adding a Ti source, such as titanium chloride or alkoxide, into the ingredient mixture used for the crystallization of zeolite crystals. Such synthetic methods are often denoted as the direct synthesis method.15−17 Currently, in addition to the direct synthesis method, alternative techniques for incorporating Ti, namely, postsynthetic treatment procedures, have also been reported. In these methods, aluminum is first removed from an aluminosilicate zeolite to prepare a siliceous framework with much defect sites, and a subsequent treatment is then performed to incorporate Ti atoms into the framework defects. Treatment procedures include vapor phase atom-planting (typically using TiCl4),18,19 liquid phase insertion using various titanium sources, such as (NH4)2TiF620 and titanium oxalate,21 and solid-state ion exchange/dry impregnation using Ti(OEt)422 or titanocene dichloride.23 The preparation of some of the titanosilicate frameworks, such as Ti-MOR, by the direct synthesis method is considered difficult and has mainly been achieved using post-synthetic treatment procedures.19 Some methods have realized the formation of Ti-beta zeolites with molar compositions up to 5 wt %,23 but the formation of extraframework TiOx can be an issue in these procedures.21−23 In this work, we have focused on the use of a Ti−Si mixed oxide composite as the ingredient for obtaining Ti-beta zeolites with high Ti content. A mechanochemical treatment method to obtain Ti−Si mixed oxide composites for synthesizing various titanosilicates,24−26 including Ti-beta,26 has previously been reported by Yamamoto et al. The product possessed similar Ti loadings (Ti/(Ti + Si) < 2 mol %) and catalytic/structural properties with those prepared by conventional methods. In contrast, in the current work, we aimed to first obtain Ticontaining aluminosilicate beta zeolites ((Ti, Al)-beta) with high Ti content (4.0 mol %) through the intentional addition of aluminum. In the case of *BEA zeolite syntheses, the presence of aluminum in the ingredient is highly beneficial, or indispensable, for promoting crystallization inside the fluoride-free alkaline solution.15 For example, a previous study27 has shown that the addition of aluminum (Si/Al = 50) into the ingredient was mandatory for converting zeolite precursors into the *BEA crystals during the hydrothermal treatment. Synthesis of Ti-beta in the absence of alkalinity, with little aluminum, was made possible by the addition of dealuminated pure silica *BEA zeolite seeds,16,17 yet the crystallization took ∼2 weeks. The use of highly toxic fluoride reagents (such as HF) makes the production of pure silica frameworks with minimal structural defect sites possible,28 but is avoided for large scale productions. Thereby, aluminum is a major contributor for the crystallization of *BEA zeolites. However, since aluminum is also an element that is detrimental for many of the reactions involving titanosilicate oxidation catalysts,17 we have performed a subsequent dealumination treatment for obtaining Ti-beta with little aluminum content for this work. We have not only confirmed the reduction of framework Al to trace levels using this method, but have also observed the fixation of Ti atoms to the desired tetrahedral states. The behavior of titanium throughout the crystallization process was also monitored by collecting the products after shortened crystallization periods to investigate influential factors for the efficient crystallization of Ti-beta zeolites. Through these experiments, it was proposed that the synthetic route based on the use of mixed oxides composite as an ingredient in

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EXPERIMENTAL SECTION

Material Synthesis. Titanium oxide (rutile type, TiO2, Wako Chemicals), fumed silica (Cab-O-Sil, M5), aluminum nitrate nonahydrate (Al(NO3)3·9H2O, Wako Chemicals), nitric acid (HNO3, 60 wt %, Wako Chemicals), acetonitrile (CH3CN, 99.5%, Wako Chemicals), hydrogen peroxide (H2O2, 30.0−35.5 wt %, Kishida Chemical), aqueous tetraethylammonium hydroxide solution (TEAOH, 35 wt %, Sigma-Aldrich), cyclooctene (95%, TCI), and tetrabutyl orthotitanate (TBOT, Wako Chemicals) were purchased commercially and used without further purification. The Ti−Si mixed oxide composite was obtained from the mechanochemical treatment of TiO2 and fumed silica. The mixture of TiO2 and fumed silica, with the composition of Ti/(Ti + Si) = 2.5, 3, 4, or 5 mol %, was ground using a planetary ball mill (Fritsch P6). Balls (diameter: 5 mm) and pots (internal volume: 500 mL) made of silicon nitride ceramics were used for the treatment. The reagents were added so that the total amount of TiO2 and fumed silica would be 25 g, and the solid mixture was treated at 600 rpm for 24 h. Planetary ball milling was performed by a repetition of 15 min milling and 15 min cooling to prevent excess heating of the instrument, and this cycle was repeated 96 times. Each mixture was designated as “mixture x mol % y h”, where x represents the titanium content Ti/(Ti + Si) inside the mixture in mol % and y represents the elapsed milling time, excluding rest, in hours. For comparison, fumed silica was planetary ball milled at 600 rpm for 24 h under identical conditions and was designated as “milled fumed silica”. (Ti, Al)-beta was synthesized from the thus obtained Ti−Si mixed oxide composite. Al(NO3)39H2O and the Ti−Si mixed oxide composite were dispersed into TEAOHaq, and the hydrothermal synthesis was performed at 160 °C for selected time periods inside a Teflon-lined autoclave. The molar ratio of the synthetic gel was as follows, SiO2/xTiO2/0.02Al(NO3)3/0.54TEAOH/8.4H2O. After the hydrothermal synthesis, the product was collected by centrifugation, washed with deionized water, and dried inside an 80 °C oven overnight. The samples were designated as (Ti, Al)-beta x mol % y days, where x and y represent the Ti content of the composite used, and the hydrothermal synthesis period, respectively. For the synthesis of (Ti, Al)-beta 2 mol % 3 days, additional fumed silica was mixed with the Ti−Si mixed oxide composite (mixture 2.5 mol % 24 h) to achieve Ti/(Ti + Si) = 2 mol %, and was used as the ingredient for zeolite crystallization. (Ti, Al)-beta zeolite was also crystallized using the direct synthesis method. The synthesis was performed using milled fumed silica, TEAOHaq, Al(NO3)3·9H2O, TBOT, and deionized water as the ingredients. Molar compositions of the ingredient materials were as follows, SiO 2 /TiO 2 /Al 2 O 3 /TEAOH/H 2 O = 1:0.017 or 0.053:0.02:0.54:10.2. These mixtures were hydrothermally treated at 160 °C under static conditions. The products were washed with deionized water and dried at 80 °C. This sample was designated as (Ti, Al)-beta DirectSyn_x mol %, where x represents the mol % of Ti added (x = 2 or 5). Postsynthetic dealumination was performed by acid treatment using concentrated nitric acid.22 The obtained (Ti, Al)-beta zeolites were dispersed in 60 wt % nitric acid (mass ratio of HNO3 aq to zeolite: ∼15), and the suspension was heated at 90 °C overnight inside a Teflon-lined autoclave. After the treatment, the zeolite was collected by centrifugation, washed with deionized water, and dried inside an 80 °C oven overnight. A suffix “_AT” was added to the samples that underwent the acid treatment. All products were calcined at 550 °C for 10 h, after 3 h of ramping to the designated temperature. Characterization. Powder X-ray diffraction (XRD) patterns were collected using a Rigaku Ultima IV diffractometer and Cu Kα radiation (40 kV, 40 mA) for a 2θ range of 3°− 40°. Elemental analysis was performed using an inductively coupled plasma atomic emission B



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spectrometry (ICP-AES; iCAP DUO-6300, Thermo Fisher Scientific) after dissolving the samples in hydrofluoric acid (HF). Diffuse reflectance (DR) UV−vis spectra were recorded using a JASCO V-670 spectrometer in the 200−500 nm wavelength range at a scan rate of 100 nm/min. BaSO4 was used as the background reference. N2 adsorption−desorption measurements were conducted using an Autosorb-iQ2-MP analyzer (Quantachrome) at 77 K. The samples were degassed at 400 °C for 6 h under a vacuum before the measurement. SEM observation was conducted using a JSM-7000F microscope (JEOL). Fourier transformation infrared (FT-IR) spectra were taken using JASCO FT/IR-6100. The spectra were measured after diluting inside KBr pellets. The high-energy X-ray total scattering (HEXTS) measurements were performed on powdered samples in a quartz capillary at room temperature using a horizontal two-axis diffractometer at the BL04B2 high-energy X-ray diffraction beamline (SPring-8, Japan). The energy of incident X-rays was 61.43 keV (λ = 0.2018 Å). The maximum Q (Q = 4π sin θ/λ), Qmax, collected in this study was 20 Å−1. The obtained data were subjected to well-established analysis procedures, such as absorption, background, and Compton scattering corrections, and subsequently normalized to give a Faber−Ziman total structure factor S(Q).29,30 These collected data were used to calculate the pair distribution function, G(r), using the following function: G(r ) = 4πr[ρ(r ) − ρ0 ] =

2 π

Figure 1. XRD patterns of mixtures comprised of TiO2 and SiO2 with different Ti/(Ti+Si) molar ratios after ball milling treatment for 24 h, and a physical mixture with Ti/(Ti + Si) = 5 mol % as the reference (mixture 5 mol % 0 h). Peaks from β-Si3N4 were confirmed at 2θ = 26.7, 33.5, and 36.0° in the ball milled mixtures due to the introduction of silicon nitride from the milling media during the treatment. The peaks from TiO2 (rutile) were only confirmed in the physical mixture (mixture 5 mol % 0 h).

Q max

∫Q min

Q [S(Q ) − 1] sin(Qr ) dQ

where ρ is the atomic number density. The epoxidation reaction was performed in a glass reactor immersed in a 60 °C oil bath by using H2O2 as the oxidant. In a typical run, the reaction was performed with the catalyst (25 mg), substrate (1.0 mmol), and H2O2 (1.0 mmol) in acetonitrile (2.0 mL) under vigorous stirring for 1 h. The mixture was analyzed by gas chromatography with a flame ionization detector (Shimadzu, GC2014). The following definitions were used to quantify experimental data:

conversion [%] = selectivity [%] = yield [%] =

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moles of reactant consumed × 100 moles of reactant fed moles of product produced × 100 moles of reactant consumed

moles of product × 100 moles of reactant fed Figure 2. DR UV−vis spectra of mixtures comprised of TiO2 and SiO2 with different Ti/(Ti + Si) molar ratios after ball milling treatment for 24 h, and a physical mixture with Ti/(Ti + Si) = 5 mol % as a reference (mixture 5 mol % 0 h).

RESULTS AND DISCUSSION 1. Mechanochemical Treatment for the Preparation of the Ti−Si Mixed Oxide Composite. The XRD patterns and DR UV−vis spectra of the TiO2 and fumed silica mixture before and after the mechanochemical treatments are shown in Figures 1 and 2, respectively. In the XRD patterns, after the planetary ball mill grinding, the diffraction peaks attributed to TiO2 (rutile) at 2θ = 27.8, 36.4, 39.5° disappeared in all of the products. The DR UV−vis results of the mechanochemically treated products exhibited an absorption peak in the region in between 200−240 nm, assigned to the isolated tetrahedral titanium species.15,17,31 In these mixtures, the absorption peaks assigned to the octahedral Ti (260 nm) or bulk polymeric TiO2 (320−330 nm) were not detected, signifying that most of the Ti were in a tetrahedral coordination environment, similar to the state of isolated tetrahedral Ti embedded inside silicate glass.32,33 Conversely, the absorption spectrum of mixture 5 mol % 0 h (also shown in Figure 2) reveals an evident absorption peak extending up to 400 nm that was ascribed to the titanium species inside the rutile crystal. Thereby, the absorption in between 200−240 nm in the DR UV−vis spectra was a clear indication of the changes in the degree of isolation of the titanium species. The presence of an absorption at 960

cm−1 was confirmed in the FT-IR measurements of mixture 5 mol % 24 h, corresponding to the Si−O−Ti vibration (Figure S1), in commensurate with the DR UV−vis results. Having confirmed that the amorphization of the rutile phase proceeded beyond the detection limits of XRD, the overall structural information on the composites was investigated on the basis of pair distribution function, G(r), obtained from the X-ray scattering measurements (Figure 3). HEXTS measurements provide an established methodology for analyzing structures of amorphous solids, and the pair distribution function, G(r), derived from this measurement visualizes the possibility of finding certain interatomic distances, r, inside a unit volume of the analyte sample. The Faber-Ziman total structure factor, S(Q), used for the calculation of pair distribution function, G(r), are summarized in Figure S2. In the TiO2/fumed silica hand mixture (mixture 5 mol % 0 h), the first nearest neighbor distances of Si−O and Ti−O were assigned to the correlations at ca. 1.6 and 2.1 Å, respectively. The peak observed at 2.6 Å was assigned to the O−[Si]−O C



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Figure 5. DR UV−vis spectra of (Ti, Al)-beta zeolites prepared from Ti−Si mixed oxide composites with different Ti/(Ti+Si) molar ratios, and (Ti,Al)-beta DirectSyn_2 mol % prepared using the conventional synthesis method.

Figure 3. (A) Pair distribution function, G(r), of mixtures comprised of TiO2 and SiO2 with Ti/(Ti + Si) = 4 and 5 mol % after ball milling treatment for 24 h, and a physical mixture with Ti/(Ti + Si) = 5 mol % as a reference. (B) Theoretical pair distribution function, G(r), of TiO2 (rutile) calculated using PDFgui software.35 Only the correlations between Ti−Ti are linked with dotted lines for brevity.

Figure 4. XRD patterns of (Ti, Al)-beta zeolites prepared from Ti−Si mixed oxide composites with different Ti/(Ti + Si) molar ratios, and (Ti, Al)-beta DirectSyn_2 mol % prepared using the conventional synthesis method. All of the diffraction peaks observed originated from the *BEA zeolite crystal structure.

Figure 6. DR UV−vis spectra of (Ti, Al)-beta 5 mol % 6 days before and after the acid treatment (Ti, Al)-beta 5 mol % 6 days_AT) and the Ti−Si mixed oxide composite (mixture 5 mol % 24 h) as a reference.

Table 1. ICP-AES Results for (Ti, Al)-Beta Zeolites with Different Ti/(Ti + Si) Molar Ratios and (Ti, Al)-Beta 5 mol % after Dealumination via the Acid Treatment

(Ti, (Ti, (Ti, (Ti, (Ti, (Ti,

Al)-beta Al)-beta Al)-beta Al)-beta Al)-beta Al)-beta

2.5 mol % 3 days 3 mol % 3 days 4 mol % 3 days 5 mol % 6 days 5 mol % 6 days_AT DirectSyn_2 mol %

Si/Ti [−]

Ti/(Ti + Si) [mol %]

Si/Al [−]

41 41 25 24 24 56

2.4 2.4 3.8 4.0 4.0 1.7

38 34 37 39 960 37

Figure 7. SEM images of (Ti, Al)-beta 5 mol % 6 days before and after the acid treatment.

after 24 h of mechanochemical treatment, indicating a decrease in the atomic ordering for these interatomic distances inside the mixture. These peaks were found to be derived from the Ti−Ti, Ti−O, and even O−O interatomic distances of rutile, found by comparison with the theoretical pair distribution function calculated from the PDFgui software35 (Figure 3B, only the correlations between Ti−Ti are linked with dotted lines for brevity). Even the correlation at r = 3.6 Å, which is the first

correlation, while the Si−Si (Ti) and O−[Ti]−O correlations overlapped at 3.1 Å. The theoretical pair distribution function of TiO2 (rutile) was calculated using PDFgui software40 based on the crystal model reported by Howard et al.34 (Figure 3B). The pair distribution function of both the 4 and 5 mol % mixture after 24 h milling is also described in Figure 3A. The correlations at r values of 3.1, 3.6, and 4.7 Å were undetectable D



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Table 2. Results for the Oxidation Reaction of Cyclooctene to Cyclooctane Oxide Using H2O2 as the Oxidant and Various Titanosilicate Zeolite Catalystsa sample

Si/Ti [−]

Ti/(Ti + Si) [mol %]

conversion [%]

selectivity [%]

(Ti, Al)-beta 3 mol % 3 days_AT (Ti, Al)-beta 4 mol % 3 days_AT (Ti, Al)-beta 5 mol % 6 days_AT (Ti, Al)-beta 5 mol % 6 days (Ti, Al)-beta 2 mol % 3 days_AT (Ti,Al)-beta DirectSyn_2 mol %_AT

37 31 24 24 47 57

2.6 3.1 4.0 4.0 2.1 1.7

67 57 30 15 44 36

86 85 65 35 74 78

a Reaction conditions: catalyst loading, 25 mg; substrate (cyclooctene), 1 mmol; H2O2, 1 mmol; solvent (CH3CN), 2 mL; reaction temperature, 60 °C; reaction time, 1 h.

Table 3. Composition of Ingredient Materials and the Product (Ti, Al)-Beta Zeolites Prepared by Hydrothermal Synthesis

sample (Ti, Al)-beta 4 mol % (Ti, Al)-beta DirectSyn_2 mol % (Ti, Al)-beta DirectSyn_5 mol %

Si/Ti of the ingredient [−]

Ti/(Ti + Si) of the ingredient [mol %]

Si/Ti of the product [−]

Ti/(Ti + Si) of the product [mol %]

25

3.9

24

3.8

60a

1.6a

56

1.7

19a

5.0a

44

2.2

a

Molar ratio calculated from the weight of the reagents added to the ingredient. Other results were calculated from ICP-AES analysis of the solid samples.

Figure 8. XRD patterns of (zeolite) products obtained after different synthesis periods using the Ti−Si mixed oxide composite with Ti/(Ti + Si) = 4 mol %.

Figure 10. DR UV−vis results of the solid products formed after different synthesis periods from the Ti−Si mixed oxide composite with Ti/(Ti + Si) = 4 mol %.

indicate that neither the rutile nanocrystals nor the amorphous aggregates of titanium oxide were present inside the mixture. The composites thus obtained were used as the ingredients for the synthesis of (Ti, Al)-*BEA zeolites. 2. Synthesis of Ti-rich *BEA Zeolites Using the Mechanochemical Composite. The synthesis of (Ti, Al)beta zeolites was tested using the Ti−Si mixed oxide composites having the molar compositions of Ti/(Ti + Si) between 2.5−5 mol %. The XRD patterns of the crystallized (Ti, Al)-beta zeolites from mixed oxide composites, as well as the (Ti, Al)-beta DirectSyn_2 mol %, prepared using the conventional direct synthesis method, are summarized in Figure 4. Under the identical synthetic conditions, i.e., 3 days of hydrothermal treatment, all of the composites provided *BEA crystals as the products. The crystallization of (Ti, Al)-beta

Figure 9. Crystallinity and surface and bulk Ti/(Ti + Si) molar ratios of the solid products formed after different synthesis periods. The crystallinity was calculated from the XRD peak height at 2θ = 22.4°, and the bulk and the surface composition were measured by ICP-AES and XPS, respectively.

nearest neighbor Ti−Ti correlation, was no longer observed. The data also implied the dispersion of Ti in the atomic levels inside the silicate network to form a Ti−Si mixed oxide composite. These characterization studies clarify the formation of a disordered admixture of Ti, Si, and O from the mechanochemical treatment in the composition region between Ti/(Ti + Si) = 2.5−5 mol %. XRD patterns, DR UV−vis spectra, and pair distribution functions derived from HEXTS measurements all E



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Scheme 1. Proposed Route for the Effective Formation of (Ti, Al)-Beta Zeolites from Ti−Si Mixed Oxide Composite Mediating Two Different Incorporation Routes: (1) Crystallization of Zeolites from Composites without Ti Dissolution and (2) Insertion of Ti from the Liquid Phase during Crystallization

increased with the Ti content. The assignment of the absorption in this wavelength region has been controversial, and previous works have assigned the absorption to both the extraframework titanium and the octahedral hydrated titanium at the zeolite surface.31,38,39 Acid treatment in nitric acid was performed on (Ti, Al)-beta 5 mol % 6 days to remove the aluminum species from the framework. Molar compositions of the treated sample were calculated using ICP-AES (also shown in Table 1). Successful removal of aluminum from the framework to trace levels (Si/Al = 960) was confirmed, while the preservation of Ti inside the sample was also verified. The N2 adsorption results (also listed in Table S1) showed that the micropore volume was not affected by the nitric acid treatment, indicating that the acid treatment has no negative influence on the crystallinity of the structure. To investigate on the coordination environment of the titanium species, DR UV−vis measurement was performed, as shown in Figure 6. The absorption peak around 260 nm disappeared after the acid treatment, while the absorption peak between 200−240 nm was still present. This is a strong evidence to justify that the final products contained little extraframework titanium species. These results also show that the acid treatment can selectively remove the Al inside the zeolites, while the state of Ti can be fixed to tetrahedral coordination state. Additionally, the titanium species exhibiting an absorption peak at 260 nm can thus be assigned to the hydrated or the octahedral titanium that could not undergo condensation with the adjacent silanol to be placed in the correct framework sites of the zeolite. The SEM image of (Ti, Al)-beta 5 mol % before and after the acid treatment are shown in Figure 7. The Ti-beta crystallites were found to be nanoparticles having particle sizes in the range of 70−230 nm. No apparent change in the morphology was observed after the acid treatment. The utility of the material as a titanosilicate catalyst was finally assessed using cyclooctene oxidation as the model reaction. The olefin oxidation reaction is a key chemical process for producing various fine chemicals, and the undesired hydrolysis of the epoxide ring can easily proceed in the presence of Brö nsted acid sites.22 Acid treatment was performed to all the (Ti,Al)-beta zeolites for obtaining catalysts that only differ in the Ti contents. The catalytic performances of Ti-beta zeolites with different Ti loadings are shown in Table 2. First, though the activity increased with the Ti loadings at relatively low Ti contents, the conversion took a maximum with (Ti, Al)-beta 3 mol % 3 days_AT, and the performance decreased at higher Ti loadings. Similar phenomenon has been

from the composite having Ti/(Ti + Si) = 5 mol % took synthesis periods up to 6 days to reach the maximum crystallinity. As titanium is not considered to possess structure-directing abilities, such as Al,36 the above result is a reflectance of the hindering effect of Ti on the crystallization of zeolite structures. The crystallinities of the zeolites were assessed based on the internal micropore volumes calculated based on the N2 adsorption−desorption measurement to complement the crystallinity calculated from the diffraction peak intensities. The measured BET specific surface areas and micropore volumes are summarized in Table S1 in the Supporting Information. Products prepared using the mechanochemical method showed equivalent micropore volumes with the product prepared by the direct synthesis method, which were typically between 0.24 and 0.25 cc g−1. The formation of apparent titanium oxide phases, such as rutile and anatase, was not confirmed in the XRD patterns in any of the samples. The β-Si3N4 that was introduced from the ball mill equipment, confirmed in the XRD patterns of the composites, was no longer observed. This was due to the susceptibility of βSi3N4 against basic conditions to form SiO2, an ingredient for zeolite crystallization. In our previous report,37 the presence of elements from the equipment was confirmed, and especially aluminum (typically Si/Al ≈ 100) may influence the crystallization. However, in the current work, the effect is limited since twice as much Al is intentionally added into the ingredient mixture. To confirm this, crystallization was tested without the additional Al, and in this case, solid products with poor crystallinities were obtained (Figure S3). The yields of the samples were typically above 85 wt %, while the yield for (Ti, Al)-beta DirectSyn_2 mol % was 89 wt % after 3 days of synthesis, showing the high yields that can be achieved using mechanochemically treated materials as zeolite ingredients. The atomic composition of the ingredient composite and the product (Ti, Al)-beta zeolites calculated from ICP-AES measurement results are summarized in Table 1. The titanium content of the (Ti, Al)-beta zeolites increased with the titanium content in the composites. However, similar Ti compositions were obtained for (Ti, Al)-beta 5 mol % 6 days and (Ti, Al)beta 4 mol % 3 days, signifying a limitation in the incorporation of Ti species during crystallization. The DR UV−vis spectra of the (Ti, Al)-beta zeolite samples are summarized in Figure 5. All of the samples exhibited a large absorption peak around 200−240 nm, which is the typical absorption arising from the charge transfer band of O2− to tetrahedral Ti4+. Furthermore, the absorption shouldering around 260 nm was also observed, and the absorption intensity F



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synthesized by the direct method. Despite increasing the Ti content in the initial ingredient from 1.7 mol % to 5.0 mol %, only a slight increase was confirmed in the product (from 1.7 mol % to 2.2 mol %, shown in Table 3). This suggests that there is a certain limitation in the Ti incorporation achievable by the conventional direct synthesis method under the given synthesis time frame, further emphasizing the advantage of using the Ti−Si mixed oxide composite. The incorporation mechanism of titanium during the crystallization of Ticontaining zeolites has been investigated by several researchers, and it is postulated that in conventional direct synthesis methods, Ti is inserted into the zeolite defect sites after an (alumino)silicate framework is formed.41,42 Other synthetic routes, such as the YNU method,42 have enabled the crystallization of TS-1 at higher titanium loadings, and the crystallization is speculated to proceed from a preformed titanosilicate solid. Thus, the enhanced incorporation of Ti confirmed in this work was considered to occur presumably due to the stronger binding between Ti and silica inside the composite, in contrast to the direct synthesis route where such interatomic connectivity does not exist in the ingredient state. The transition in the coordination environment of Ti was investigated through DR UV−vis spectroscopy (Figure 10). Formation of the absorption peak at 260 nm was confirmed in the product after 12 h, but this absorption band became minor after 24 h, indicating partial formation of hydrated or octahedral Ti species at the initial stage of the crystallization. On the basis of the above-mentioned findings, a proposed crystallization scheme was developed (Scheme 1). In the initial crystallization step of the *BEA zeolites (∼24 h), titanium exists in two states, either incorporated inside the zeolite framework, or in the liquid phase (that is, in a form that is difficult be to collected through centrifugation). However, the latter titanium species are brought back to the zeolite framework after longer synthesis periods, as in most titanosilicate zeolite crystallizations.41,42 It is worth highlighting here that the intermediate after 12 h (crystallinity ∼12%) still contains a high amount of Ti (Ti/(Ti + Si) = 1.6 mol %), comparable with the Ti content of fully crystallized (Ti, Al)beta DirectSyn_2 mol %. Homogeneous incorporation of Ti into the *BEA crystal framework from the liquid phase occurs thereafter, as confirmed from the characterization results of the intermediates after 48 and 72 h of hydrothermal treatment. The advantage of using a composite structure as the ingredient lies in enabling Ti to be accommodated into the zeolite structure from the initial stages of crystallization.

confirmed in other studies on titanosilicate zeolite catalysts for epoxidation using H2O2.40 Influence due to the reduced hydrophobicity inside the zeolite framework was suggested as the reason for such behavior of the catalyst. The reactivity of (Ti, Al)-beta 5 mol %_AT was compared with that of (Ti, Al)beta 5 mol % to see the effect of dealumination. By introducing the acid treatment, selectivity to cyclooctene oxide increased significantly from 35% to 65% presumably due to much reduction in Brönsted acid sites. The selectivity to epoxide product using (Ti, Al)-beta 3 mol % 3 days_AT catalyst reached up to 86%. The increment in the conversion from 15% to 30% was also observed, which can be accounted by the increased hydrophobicity by the reduction of framework Al.36 Finally, the effect of the synthetic method was investigated based on the comparison of (Ti, Al)-beta 2 mol % 3 days_AT and (Ti, Al)-beta DirectSyn_2 mol %_AT. (Ti, Al)-beta DirectSyn_2 mol % was prepared by using a mixture of “mixture 2.5 mol % 24 h” and “fumed silica” (the final composition was fixed to Ti/(Ti + Si) = 2 mol %), and an identical acid treatment procedure was performed to produce this control catalyst. These catalysts having similar Ti loadings exhibited comparable performances thus showing that the influence by the preparation route does not pose significant difference in the catalytic outcome. The high activity and selectivity in cyclooctene epoxidation using hydrogen peroxide confirms the catalysts prepared in this work function as a titanosilicate oxidation catalyst that show comparable or improved performances in comparison to those prepared by the conventional routes. 3. Crystallization Behavior of Ti-Beta. Having confirmed that it is indeed possible to expand the titanium loading inside the titanosilicate zeolites using these synthetic methods, a tentative crystallization mechanism was finally investigated with special focus on the behavior of Ti throughout the hydrothermal treatment. The crystallinity, Ti content (at the surface and the bulk), and the coordination environment of titanium in the intermediates during crystallization were analyzed. Comparison with the (Ti, Al)-beta produced using the direct synthesis method was also performed to clarify the impact of the composite formation. Fixing the Ti−Si mixed oxide composite to be used to Ti/(Ti + Si) = 4 mol %, the hydrothermal synthesis of *BEA-type zeolites were performed under various treatment periods at 160 °C. XRD results of the products after the different hydrothermal treatment periods (12, 24, 48, and 72 h) are shown in Figure 8. The peak intensity in the XRD pattern increased with hydrothermal treatment periods. The crystallinity calculated from the XRD results (the height of the peak at 2θ = 22.4°), the bulk, and the surface composition, measured by ICP-AES and XPS, respectively, are summarized in Figure 9. Under the applied synthesis conditions, the crystallization of *BEA was confirmed after 24 h of hydrothermal treatment, and the calculated crystallinity maximized after 48 h. Both the bulk and the surface composition of the products transcended in a similar manner, showing that the intermediates during crystallization have a highly homogeneous nature in composition throughout the synthesis. While there was an initial drop in the Ti content after 24 h of crystallization, it was brought back to the original ingredient composition (4 mol %) after a longer synthesis time (72 h). In contrast to the products prepared from composite oxides, the final Ti loading was limited in the case of (Ti, Al)-beta

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CONCLUSIONS The preparation of *BEA-type zeolites containing titanium in the framework crystal sites at high loadings was achieved by the combined usage of (1) Ti−Si mixed oxide composite as the starting material and (2) intentional aluminum addition to promote the crystallization of the zeolite structure. After the subsequent dealumination treatment, it was found that the framework aluminum sites are effectively removed without loss in titanium content. Furthermore, the extraframework titanium, presumably that is weakly bound to the crystal sites, is fixed into the desired tetrahedral coordination state, leading to high Ti content and quality. The obtained titanosilicate *BEA zeolites possessed superior catalytic activity compared to those prepared by the direct synthesis method, and the advantage of using acid treatment was also reflected in the catalytic outcome. It was finally confirmed that the use of composite played an G



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effective role for incorporation of Ti species from the initial stages of *BEA zeolite crystallization under the hydrothermal synthetic conditions. Such preparation procedure is expected to be applicable for obtaining other heteroatom-containing zeolites with higher flexibility in the framework compositions.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge on the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01621. BET specific surface areas, micropore volumes, total structure factor, S(Q), used for calculating the pair distribution functions, G(r), FT-IR spectra used in this study (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-3-5841-7368; fax: +81-3-5800-3806; e-mail: [email protected]. ORCID

Junko N. Kondo: 0000-0002-7940-1266 Toru Wakihara: 0000-0002-3916-3849 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant for Advanced Industrial Technology Development (2011) from the New Energy and Industrial Technology Development Organization of Japan. The High Energy Total X-ray Scattering experiments conducted at SPring-8 were approved by the Japan Synchrotron Radiation Research Institute (proposal nos. 2015B0115 and 2016A0115). T.I. thanks the Japan Society for the Promotion of Science for a Grant-in-aid for Scientific research (this work was supported by JSPS KAKENHI Grant Number 15J07161).

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Crystallization of Ti-Rich *BEA Zeolites by the Combined Strategy of

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