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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6390−6397

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High-Rota Synthesis of Single-/Double-/Multi-Unit-Cell Ti-HSZ Nanosheets To Catalyze Epoxidation of Large Cycloalkenes Efficiently Yarong Zhao, Dan Zhou,* Tianjun Zhang, Yun Yang, Ke Zhan, Xinchao Liu, Hui Min, Xinhuan Lu, Renfeng Nie, and Qinghua Xia* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials & Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, 368 Youyi Avenue, Wuhan 430062, China S Supporting Information *

ABSTRACT: This work first reports high-efficiency epoxidation of large cycloalkenes (carbon number ≥ 7) with tert-butyl hydroperoxide (TBHP) over single-/double-/multi-unit-cell nanosheet-constructed hierarchical zeolite, which is synthesized by one-step hydrothermal crystallization using piperidine as the structure-directing agent of the microporous structure. The excellent catalytic property of the material is ascribed to its unique structural characteristic. Plenty of surface titanols or silanols on the surface of MWW nanosheets are beneficial for the formation of transitionstate intermediates; a large number of intercrystalline mesopores in the shell of the material not only facilitate the formation of the intermediate for TBHP but also have nearly no hindrance for the diffusion and mass transfer of bulky cycloalkene to the above intermediates; the 12-MR side cups penetrating into the crystals from the external surface are exposed as much as possible to the reaction system because of the single-/double-/multiunit-cell MWW nanosheet, serving as the primary reaction space for the epoxidation of bulky cyclic alkene and oxidants and providing enough space for the transition state of Ti−OOtBu and bulky cycloalkane. Moreover, an efficient calcination-free catalytic reaction−regeneration method is developed to overcome the challenge for the recyclability of microporous Ti-zeolite in the catalytic epoxidation of bulky cycloalkenes. KEYWORDS: hierarchical, nanosheet, epoxidation, cycloalkene, Ti-containing zeolite, calcination-free, reaction−regeneration

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INTRODUCTION Zeolite, a type of microporous crystalline material, has found wide application in the chemical industry as the most important solid catalyst.1 Because of the steric limitation of zeolite micropores, catalytic applications of zeolites are mainly limited in selective conversion of small organic molecules. In recent decades, great efforts have been directed to the efficient catalytic conversion of bulky organic molecules by zeolite materials through increasing the accessibility to the active sites and reducing the diffusional problems of bulkier reactants.2−4 Epoxides are versatile and useful intermediates for organic synthesis because of the high reactivity of their three-membered ring structure, so the epoxidation of alkenes or organic molecules with double bonds has been an indispensable chemical process in synthetic organic chemistry and chemical industry.5−11 Particular efforts are being made to develop sustainable heterogeneous catalytic processes, replacing homogeneous and stoichiometrical reactions that lead to serious environmental problems. Ti-containing zeolite, one of the most important members of the zeolite family, has attracted scientific and technological interests because of its ability to catalyze environmentally © 2018 American Chemical Society

benign selective epoxidation of alkenes. Although the selective epoxidation of linear or small alkenes has been successfully conducted over Ti-containing zeolites,12−14 the catalytic epoxidation of bulky alkenes (especially that of large cycloalkenes with the number of C atoms ≥7) is still a great challenge. Although various methods have been attempted to increase the catalytic activity of Ti-containing zeolite for bulky cycloalkenes,15−20 the reported results are not satisfactory yet. Moreover, the recycling of the catalyst for the epoxidation of bulky cycloalkenes has not been realized, which is another challenge. To increase the efficient utilization of catalytic active sites in microporous zeolite materials, zeolite nanosheets have been synthesized;21−24 however, various organic surfactants or structure-directing agents have to be used. For example, for the synthesis of nanosheet TS-1,21 a diquaternary ammonium surfactant (C16H33−N+(CH3)2−C6H12−N+(CH3)2−C6H13) has to be initially prepared, then used in the preparation of Received: December 9, 2017 Accepted: January 29, 2018 Published: January 29, 2018 6390

DOI: 10.1021/acsami.7b18734 ACS Appl. Mater. Interfaces 2018, 10, 6390−6397

Research Article

ACS Applied Materials & Interfaces

radiation. Scanning electron microscopy (SEM) images were obtained on a JEOL JSM-6510A. Transmission electron microscopy (TEM) images were obtained using FEI Tecnai G20. High-resolution transmission electron microscopy (HRTEM) images were obtained using FEI Titan G2 60-300. The surface areas and pore volumes of the calcined samples were measured by means of N2 adsorption at 77 K on a Quantachrome iQ-MP gas adsorption analyzer. Before the nitrogen adsorption, samples were dehydrated at 523 K for 10 h. The total surface area was calculated via the Brunauer−Emmett−Teller (BET) equation, and the pore distribution and total pore volume (at a relative pressure of P/P0 = 0.98) were calculated by the Barrett−Joyner− Halenda (BJH) method. Fourier transform infrared (FT-IR) spectra of framework vibrations were recorded on a PerkinElmer Spectrum One FTIR spectrophotometer. The OH-region IR spectra collected under vacuum conditions were measured on a Bruker Tensor 27 spectrometer. Before measuring the spectra, a self-supported sample wafer (15 mg) was prepared and evacuated at ca. 2.0 × 10−2 to 1.0 × 10−3 Pa. X-ray photoelectron spectroscopy (XPS) spectra were determined on a PerkinElmer PHI ESCA system. The X-ray source was a standard Mg anode (1253.6 eV) at 12 kV and 300 W. Solid-state 29Si NMR spectra were recorded on a Bruker AVANCE III HD 400WB (9.4 T) NMR spectrometer with a commercial doubleresonance MAS probe at a Larmor frequency of 79.5 MHz for 29Si. Solid-state 29Si DP/MAS (direct-polarization magic-angle spinning) NMR spectra were recorded with a single pulse excitation using a short tip angle (π/4) to obtain quantitative results and a recycle delay of 50 s. Solid-state 1H−29Si CP/MAS (cross-polarization magic-angle spinning) NMR spectra were recorded with a 90° 1H-pulse width of 4.0 μs and a contact time of 2 ms. The MAS speeds were 5 and 8 kHz in 29Si DP/MAS and CP/MAS NMR measurements, respectively. 29Si chemical shifts were determined using a solid external reference kaolin resonance at −91.5 ppm relative to tetramethylsilane (TMS). 1H chemical shifts were determined using a solid external reference adamantane resonance at 1.78 ppm relative to TMS. Catalytic and Recycling Experiments. The catalytic epoxidation of cycloalkenes with aqueous TBHP was carried out in an autoclave at a rotating speed of 35 rpm (Figure S2). In a typical run, 10 mmol of cycloalkene, 10 mmol of TBHP (65% aqueous solution), 10.0 mL of acetonitrile, and 50 mg of Ti-HSZ-N-AT catalyst were mixed in the autoclave and heated to the desired temperature under rotation at 35 rpm. The reaction was continued for several hours. After the reaction, samples of the reaction mixture were qualitatively analyzed by a Shimadzu 2010 GC, equipped with a 30m capillary column (Rtx-1) and an FID detector. Different catalyst regeneration methods were conducted, including conventional open system (Figure S3) and closed system used in this work (Figure S4). In an open system, the used catalyst (1.0 g) was washed with H2O2/ethanol solution (30 g of 30% H2O2 in 30 g of ethanol) at 343 K for 23 h. The catalyst was recovered by filtration and dried at 353 K overnight for next use. In a closed system, the used catalyst and H2O2/ethanol solution were added in the autoclave and heated to the desired temperature under rotation at 35 rpm. The reaction was continued for several hours. To maintain the amount of catalyst used in every run uniform, two parallel experiments were conducted synchronously (shown in Supporting Information Figure S5).

reaction gel, and removed by calcination at a high temperature before catalytic applications. A dual-template method is also developed to synthesize zeolite nanosheets, such as DS-ITQ-2 with MWW monolayers22 and SAPO-34 nanosheet,23 for which organic structure-directing agents (N-hexadecyl-N′-methylDABCO) or organosilane surfactant [3-(trimethoxysilyl) propyl] octadecyl dimethyl ammonium chloride have to be used as the mesoporogen beside small-molecule organic amines as the microporous template, which have to be removed by calcination as well. The above methods not only waste sources but also consume large amount of energy. In 2016, a Ti-containing hollownest-structured zeolite (TiHSZ) material was synthesized through the one-step hydrothermal rota-crystallization mode at a relatively low rotation rate (100 rpm) (Scheme 1). No additional diquaternary ammonium surfactant Scheme 1. Schematic Diagram of the Ti-HSZ-N Material

or complex organic structure-directing agent is used, and only piperidine (PI) is needed for the structure-directing of microporous MWW structure of Ti-HSZ-N. The unique structural characteristics endow the material with a predominant catalytic epoxidation property for bulky cycloalkenes (carbon number ≥ 7). Moreover, a calcination-free catalytic reaction−regeneration method is developed to overcome the challenge for the recyclability of microporous Ti-zeolite in the catalytic epoxidation of bulky cycloalkenes.

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EXPERIMENTAL SECTION

Materials. Boric acid (H3BO3, 99.5%), PI (99%), tert-butyl hydroperoxide (TBHP, 65%), and hydrogen peroxide (H2O2, 30%) were purchased from Sinopharm; cycloheptene (96%), 1-methylcyclohexene (98%), 4-methylcyclohexene (98%), and cyclododecene (98%, mixture of trans- and cis-isomers) were purchased from TCI. Other used materials included fumed silica (aerosil-200 SiO2, 99.9%, Degussa), tetra-butyl orthotitanate (98%, Feida), deionized water, nitric acid (HNO3, 66%, Shuangrun), acetonitrile (99.5%, Bodi), cyclooctene (95%, Aladdin), acetone (99.5%, Aosheng), and ethanol (99.7%, Aosheng). Zeolite Synthesis. The synthesis route of the Ti-HSZ-N material is shown in Figure S1. Note that the molar composition of the gel is 1.0 SiO2/0.038 TiO2/0.67 B2O3/1.48 PI/19.0−40 H2O, and the rotating crystallization speed is higher than 100 rpm. Ti-HSZ-N-AT catalyst was obtained through acid treatment (AT) of the precursor TiHSZ-N, followed by washing with ethanol until pH ≈ 7.0 and drying. Ti-HSZ-N-Cal and Ti-HSZ-N-AT-Cal were prepared by calcinations of Ti-HSZ-N and Ti-HSZ-N-AT at 550 °C for 10 h, respectively. For comparison, conventional Ti-MWW, TS-1, and Ti-beta zeolite materials were also prepared according to the literature.12,26−28 Characterization. X-ray diffraction (XRD) patterns of samples were recorded on a Bruker D8A25 diffractometer with Cu Kα

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RESULTS AND DISCUSSION Single-/double-/multi-unit-cell Ti-HSZ nanosheets have the following three obvious structural and morphology characteristics, including MWW-type crystals, hollownest morphology, and single-/double-/multi-unit-cell nanosheets. For the formation of single-/double-/multi-unit-cell Ti-HSZ nanosheets, it is found that both the rotating rate of the autoclave and the H2O/Si ratio of the reaction gel are important, and detailed discussions are shown in the Supporting Information, inclusive of Figures S6−S8 and Table S1. Ti-HSZ-N-AT catalyst was obtained through simple AT of the precursor Ti-HSZ-N 6391

DOI: 10.1021/acsami.7b18734 ACS Appl. Mater. Interfaces 2018, 10, 6390−6397

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Figure 1. XRD patterns of Ti-HSZ-N, Ti-HSZ-N-AT, Ti-HSZ-N-Cal, and Ti-HSZ-N-AT-Cal. Figure 3. UV−vis spectra of samples Ti-HSZ-N, Ti-HSZ-N-Cal, TiHSZ-N-AT, and Ti-HSZ-N-AT-Cal.

Figure 2. HRTEM images of the Ti-HSZ-N-AT catalyst ((a) hollownest morphology; (b) MWW flaky crystal; and (c,d) single-/ double-/multi-unit-cell nanosheets).

Figure 4. Ti 2p XPS spectra of Ti-HSZ-N, Ti-HSZ-N-Cal, Ti-HSZ-NAT, and Ti-HSZ-N-AT-Cal.

without calcination at a high temperature.19 For comparison, Ti-HSZ-N-Cal and Ti-HSZ-N-AT-Cal were also prepared by calcinations of Ti-HSZ-N and Ti-HSZ-N-AT at 550 °C for 10 h, respectively. The powder XRD pattern of Ti-HSZ-N synthesized under a 120 rpm rotating speed (see Figure 1) shows an MWW topology. The intensities of the 001 and 002 diffraction lines are decreased as a result of partial disappearance of the lamellar structure for the sample Ti-HSZ-N-AT. The elemental analysis results listed in Table S2 reveal that the majority of PI molecules (ca. 91%) can be removed, implying the efficient removal of the molecules of the structure-directing agent PI by AT. Ti-HSZ-N-Cal and Ti-HSZ-N-AT-Cal show XRD patterns similar to that of Ti-HSZ-N-AT.

Ti-HSZ-N-AT displays a hollownest morphology (Figure S9), the shell of which is constructed by randomly intergrown flaky crystals, as shown in the HRTEM images (Figures 2a,b and S10). There exist a number of hierarchical pores constructed by the stacking of the intergrown MWW nanosheet crystals. As observed in Figure 2c,d, there exist a number of double-layered nanosheets with a crystal thickness of 5.0 nm and single-unit-cell nanosheets with a thickness of 2.5 nm, and the percentage of single-/double-layered nanosheets is calculated to be about 12−15%. The physical parameters of the samples Ti-HSZ-N-AT, TiHSZ-N-Cal, and Ti-HSZ-N-AT-Cal are listed in Table S3. The BJH pore distribution indicates an average mesopore size of 6392

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Figure 5. Single-pulse 29Si MAS (a) and 1H−29Si CP/MAS (b) NMR spectra of Ti-HSZ, Ti-HSZ-Cal, Ti-HSZ-AT, and Ti-HSZ-AT-Cal.

Table 1. Fraction (%) of QnSi Population Determined by Quantitative Data of Solid-State One-Pulse 29Si MAS NMR Spectra samples Ti-HSZ-N Ti-HSZ-N-AT Ti-HSZ-N-Cal Ti-HSZ-N-AT-Cal

chemical shift range −86.4 −89.9 −96.0 −92.4

to to to to

−118.2 −120.2 −120.2 −120.2

(Q2 + Q3)/Q4 ratio

(Q2 + Q3)/Qn (%)

2.22 0.44 0.38 0.48

69 31 27 32

sample. For the sample Ti-HSZ-N-Cal, the occurrence of a broad 318 nm band indicates the formation of an anatase-like phase after the calcination at a high temperature. After AT, only 212 and 230 nm bands are observed in the UV−vis spectrum of Ti-HSZ-N-AT, implying that the majority of nonframework Ti species has been efficiently removed. Ti-HSZ-N-AT-Cal shows a UV spectrum similar to that of Ti-HSZ-N-AT. In the IR spectrum (Figure S13), the band at 960 cm−1 is ascribed to Si− O−Ti stretching vibrations of the sample.25 XPS, a surface-sensitive technique for the analysis of elements and their oxidation states, was employed to study the state of the Ti species (Figure 4). For Ti-HSZ-N and TiHSZ-N-Cal, owing to the presence of the majority of nonframework Ti(VI) oxides in the samples, only the binding energies of Ti 2p3/2 at 458.4 or 458.7 eV are observed. For TiHSZ-N-AT, two different types of Ti species in the tetrahedrally coordinated state are visible. The peak at 459.2 eV is associated with open Ti species, and the peak at 460.2 eV is ascribed to closed Ti sites,20 which is consistent with the UV−vis analysis results. After calcination, some of the open Ti sites are transformed to closed ones (Ti-HSZ-N-AT-Cal). Solid-state 29Si MAS NMR experiments were conducted to characterize the chemical environments of Si atoms in various samples. In the single-pulse 29Si MAS NMR spectra shown in Figure 5a, the chemical shift ranges of 29Si MAS NMR for different samples are different, which are listed in Table 1. For Ti-HSZ-N, the resonances emerge in the range of −86.4 to −118.2 ppm. After AT (sample Ti-HSZ-N-AT), the resonances are located in the range of −89.9 to −120.2 ppm. For Ti-HSZN-Cal and Ti-HSZ-N-AT-Cal, the chemical shift range has shifted to the range of −96.0 to −120.2 and −92.4 to −120.2 ppm, respectively, implying that a portion of defect sites disappears after the calcination at a high temperature. To identify different Si species, 1H−29Si CP/MAS NMR experiments were conducted to provide information regarding

17−19 nm. The external BET surface area of Ti-HSZ-N-AT is comparable to that of Ti-HSZ-N-Cal, implying that the majority of PI molecules trapped in the mesopores can be removed by AT, which is consistent with the elemental analysis result. Compared with that for Ti-HSZ-N-AT, a slight increase in the microporous BET surface area for Ti-HSZ-N-Cal is ascribed to the removal of residual PI molecules in the micropores. For the sample Ti-HSZ-N-AT-Cal, the external and microporous BET surface areas are the highest, suggesting the entire removal of PI molecules from hierarchical pores. The hysteresis loop at a relative pressure of P/P0 = 0.5−0.98 observed in the N2 adsorption−desorption isotherm of TiHSZ-N-AT (Figure S11) is ascribed to the capillary condensation in textural stacking pores assigned to intercrystalline voids in the delaminated MWW layers and confirms the HRTEM analysis result. In the UV−vis spectrum of Ti-HSZ-N shown in Figure 3, the main broad band at 260 nm is attributed to octahedral Ti species in nonframework positions and the shoulder bands at 212 and 230 nm result from a high dispersion of the tetrahedrally coordinated Ti in the framework,15 which are characteristic of a perfect closed-site Ti(OSi)4 and a defective open-site Ti(OSi)3(OH) (Figure S12), respectively.19,20 It is found that the content of nonframework Ti species is much more than that of framework Ti species in the precursor 6393

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ACS Applied Materials & Interfaces Table 2. Catalytic Reaction Results for the Epoxidation of Bulky Cycloalkenes over Various Catalystsa

Scheme 2. Proposed Catalytic Mechanism for the Epoxidation of Cyclooctene over Ti-Containing Zeolite

the resonances located in the range of −107.6 to −120 ppm are ascribed to the Q4 [(SiO)4Si] Si species. The quantitative data of various samples are listed in Table 1. As shown in Table 1, about 69% Si species in the precursor are Q2 or Q3 types, including framework and nonframework Si species. This implies that rapid rotating crystallization is beneficial for the generation of titanols or silanols on the surface of single-/double-/multi-layered nanosheet crystals. After AT, the nonframework Si and Ti species have been removed from the sample Ti-HSZ-N-AT. The percentage of (Q2 + Q3) in Ti-HSZ-N-AT is about 31%. It indicates that a number of hydroxyl groups are reserved after the AT, which is beneficial for the formation of transition-state intermediates. After being calcined, still 32% (Q2 + Q3) sites are preserved for Ti-HSZ-N-AT-Cal. The FT-IR spectra of Ti-HSZ-N-AT and Ti-HSZ-N-AT-Cal catalyst collected under vacuum conditions are shown in Figure S14. For samples heated at 400 °C for 50− 60 min, the rich vibration for Si−OH groups is still visible. This implies that rapid rotating crystallization is beneficial for the generation of silanols or titanols on the surface of single-/ double-/multi-layered nanosheet crystals, which is consistent with the solid-state 29Si MAS NMR results. The catalytic epoxidation results of the Ti-HSZ-N-AT catalyst for bulky cycloalkenes are listed in Table 2. Although TBHP as the oxidant is normally inactive for the epoxidation of alkenes when using crystalline microporous zeolite materials as the catalyst because of the steric constraint, Ti-HSZ-N-AT can convert about 66.8% cycloheptene at 110 °C. The epoxidation of 1-methylcyclohexene and 4-methylcyclohexene over TiHSZ-N-AT is also effective. Noticeably, 98.0% conversion of cyclooctene can be attained at a reaction temperature of 120 °C. Furthermore, 66.9% cyclododecene can be converted into the corresponding epoxide by TBHP as the oxidant over TiHSZ-N-AT (120 °C/8 h). The reactivity of cycloalkenes decreases with increasing carbon number. The catalytic epoxidation for various cycloalkenes are also conducted over Ti-HSZ-N-AT-Cal. It is found that Ti-HSZ-NAT-Cal presents a catalytic reactive activity comparable with that of Ti-HSZ-N-AT under the same reaction conditions. This suggests that the majority of the intergrown single-/double-/ multi-unit-cell MWW nanosheet crystals have been maintained

a

Reaction conditions: cycloalkene, 10 mmol; TBHP, 10 mmol; acetonitrile, 10 mL; catalyst, 50 mg; reaction time, 8 h.

the connectivity of hydrogen nuclei with silicon nuclei. Owing to the existence of PI molecules in the precursor Ti-HSZ-N, the line shape of the 1H−29Si CP/MAS NMR spectrum has no obvious difference from that of the single-pulse 29Si MAS NMR spectrum. On the basis of the comparison of the single-pulse 29 Si and 1H−29Si CP/MAS NMR spectra of Ti-HSZ-N-AT, it is found that the intensity of the resonances located in the range of −96 to −105.6 ppm has increased much after the 1H−29Si cross-polarization. A similar phenomenon is also observed for samples Ti-HSZ-Cal and Ti-HSZ-AT-Cal. This implies that these resonances should be ascribed to Si species having intense connectivity with hydrogen nuclei. The resonances located at −107.6 ppm for Ti-HSZ-N and −107.5 ppm for Ti-HSZ-N-AT are attributed to Q4 Si species because no obvious intensity increase is observed for the two resonances in the 29Si CP MAS spectra. On the basis of the above analysis, the resonances located in the range of −86.4 to −105.6 ppm are ascribed to the Q2 [(SiO)2Si(OH)2] or Q3 [(SiO)3SiOH] Si species, whereas 6394

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ACS Applied Materials & Interfaces Scheme 3. Schematic Diagram of the Catalytic Epoxidation of Bulky Cycloalkene over Ti-HSZ-N-AT

Figure 6. Recycling tests of cyclooctene (a) and cyclododecene (b) over Ti-HSZ-N-AT (reaction conditions: catalyst, 50 mg; acetonitrile, 10 mL; alkene, 10 mmol; TBHP, 10 mmol; time, 8 h; temperature, 120 °C).

actual reaction occurs only when the substrate molecules can reach the intermediates. The existence of a number of intercrystalline mesopores (ca. 18 nm) in the shell of TiHSZ-N-AT not only facilitates the formation of the intermediate for TBHP but also has nearly no hindrance for the diffusion and mass transfer of bulky cycloalkene to the above intermediates. Second, the larger the substrate, the more open reaction space is needed. The 12-MR side cups penetrating into the crystals from the external surface can be exposed as much as possible to the reaction system because of the single-/double-/ multi-unit-cell MWW nanosheet structure of Ti-HSZ-N (Scheme 3), serving as the primary reaction space for the epoxidation of bulky cyclic alkene and oxidants and providing enough space for the transition state of Ti−OOtBu and bulky cycloalkene (I in Scheme 2). Third, the transition state involving bulky cycloalkene needs a higher activation temperature than small alkenes. A higher reaction temperature is not suitable for the H2O2 reaction system, which is generally below 70 °C, but suitable for the TBHP reaction system (100−120 °C). However, for conventional Ti-MWW or Ti-IEZ-MWW-RSC-Cal,18 the Ti active sites are still difficult to be accessed by bulky TBHP and cycloalkene molecules by merely increasing the reaction

intact even after the calcination, as shown in the HRTEM image (Figure S15), and the closed Ti active sites are still accessible in the single-/double-unit-cell MWW nanosheet crystals. The low catalytic activity for the epoxidation of cycloalkenes over Ti-HSZ-Cal is ascribed to the existence of plenty of anatase-like phase in the samples decomposing TBHP (TBHP conversion and efficiency are listed in Table S4), and that over Ti-HSZ-N is ascribed to the negligible influence of a large number of PI molecules trapped in the pores. When traditional TS-1, Ti-MWW-AT, or Ti-beta is used as the catalyst, the low catalytic activity for the epoxidation of cycloalkenes is ascribed to the steric limitation of zeolite micropores. In addition, by comparing the activity of the TiHSZ-AT material prepared under a rotating rate of 56 rpm (Table S5), its higher catalytic properties over Ti-HSZ-N-AT prepared under a high rotating rate present the importance of single-/double-unit-cell nanosheets in the improvement of the epoxidation efficiency for bulky cycloalkenes. The prominent catalytic activity of Ti-HSZ-N-AT in the epoxidation of bulky cyclic alkenes with TBHP involves the following three reasons: First, it is well-known that the titanosilicate-catalyzed reactions involve a cyclic intermediate of a five-membered ring, and the intermediate for TBHP (i in Scheme 2) is much bulkier than that for H2O2 because of the tert-butyl group.24 An 6395

DOI: 10.1021/acsami.7b18734 ACS Appl. Mater. Interfaces 2018, 10, 6390−6397

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this catalyst can be reused five times without an appreciable drop of the conversion and selectivity. This work may open up one new methodology for catalytic transformation of large cyclic molecules over hierarchical zeolite catalysts constructed smartly by microporous zeolite crystals.

temperature because of the limited space expansion between MWW layers. Recycling tests of cyclooctene and cyclododecene have been conducted. As shown in Table S6, by the conventional catalystregeneration method in an open system (Figure S3) at a low temperature,13,21 1.0 g of used catalyst was washed by H2O2/ ethanol solution. For Ti-HSZ-N-AT and Ti-HSZ-N-AT-Cal in the second reaction run, an obvious decrease in the conversion of cyclooctene to 74.6 and 44.1%, respectively, is observed from Table S6 (entries 1 and 2). Consequently, a closed system for the catalyst regeneration was attempted (shown in Figure S4). The reuses of Ti-HSZ-NAT (five times) do not appreciably decrease the conversion of cyclooctene (see Figure 6a and Table S7), in which the conversion of about 88−98 mol % can be maintained with an almost constant selectivity of 100% (entry 3 in Table S6). The UV−vis (Figure S16) and Ti 2p XPS (Figure S17) spectra of Ti-HSZ-N-AT (reused) indicate that the oxidation states of Ti active sites have been maintained. The high efficiency of catalyst regeneration under a closed system is ascribed to the relatively higher treatment temperature and autogenous pressure, which is beneficial for the efficient removal of substrate and other organic molecules. In addition, recycling tests of cyclododecene have also been conducted by the same method. As shown in Figure 6b, the cyclododecene conversion can be kept about 50−66 mol % with an almost constant selectivity of 99%. For comparison, recycling tests of cyclooctene over traditional Ti-MWW zeolite materials (Ti-MWW-AT and TiMWW-AT-Cal) were also conducted (entries 4 and 5 in Table S6). Note that traditional Ti-MWW catalyst was difficult to be recycled. In the first use, 82.6 or 85.9% conversions of cyclooctene were obtained over Ti-MWW-AT (acid-treated TiMWW precursor) and Ti-MWW-AT-Cal (AT and calcination), which were sharply decreased to 56.7 and 64.5% in the second use and to 16.3 and 19.7% in the third use, respectively. The low efficiency for the regeneration of traditional Ti-MWW zeolite is assumed to be due to the steric limitation of zeolite micropores for removing bulky organic molecules efficiently. This comparison proves the advantages of Ti-HSZ zeolite nanosheets as an efficient catalyst for the oxidation of bulky alkene molecules and as a recyclable heterogeneous catalyst. The above method is proven to be a feasible and efficient route for the regeneration of hierarchical catalyst Ti-HSZ-N-AT constructed by nanosheets in the epoxidation of bulky cycloalkenes. Moreover, the calcination at a high temperature is not necessary for the treatment of the precursor and for the reaction−regeneration recycles, which is attainable not only for small alkenes25 but also for bulky cycloalkene molecules in this work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18734. Elemental analysis, physical parameters, reaction−regeneration recycling results, synthesis route of zeolite, catalytic reaction setup, catalyst regeneration system, SEM, HRTEM, N2 adsorption−desorption isotherm, Ti active site, FT-IR, UV−vis, and Ti 2p XPS (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.Z.). *E-mail: [email protected]. Phone: 0086-27-8866-2747. Fax: 0086-27-8866-3043 (Q.X.). ORCID

Qinghua Xia: 0000-0002-4698-7889 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support by National Natural Science Foundation of China (21571055, 21673069, and 21503074) and Hubei Province Outstanding Youth Foundation (2016CFA040) and the technical support by State Center for Magnetic Resonance in Wuhan for the solid-state NMR experiments.

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REFERENCES

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CONCLUSIONS A single-/double-/multi-unit-cell MWW nanosheet-constructed hierarchically structured Ti-zeolite material is successfully synthesized by one-step hydrothermal rota-crystallization under a relatively high rotation speed (>100 rpm) and a H2O/Si ratio of ca. 40. The excellent catalytic activity of this material for the catalytic epoxidation of bulky cycloalkenes with TBHP is ascribed to the easily accessible Ti active sites on the single-/double-unit-cell MWW nanosheets and to the hierarchical porous structure promoting the diffusion and mass transfer of bulky reaction molecules. Furthermore, a calcination-free catalyst-regeneration method is developed, by which 6396

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Research Article

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DOI: 10.1021/acsami.7b18734 ACS Appl. Mater. Interfaces 2018, 10, 6390−6397