Multi-Unit-Cell Ti-HSZ

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High-rota-synthesis of Single-/double/multi-unit-cell Ti-HSZ Nanosheets to Catalyze Epoxidation of Large Cycloalkenes Efficiently Zhao YaRong, Dan Zhou, Tianjun Zhang, Yun Yang, Ke Zhan, Xinchao Liu, Hui Min, Xinhuan Lu, Renfeng Nie, and Qinghua Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18734 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Applied Materials & Interfaces

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 ABSTRACT: This work firstly reports high-efficiency epoxidation of large cycloalkenes (carbon number ≥ 7) with TBHP over single-/double/multi-unit-cell nanosheet constructed hierarchical zeolite, which have been one-step crystallized hydrothermally using piperidine as the structure-directing agent of microporous structure. The excellent catalytic property of the material is ascribed to its unique structure characteristic. Plenty of surface titanols or silanols on the surface of MWW nanosheets is beneficial for the formation of transition-state intermediates; a large number of intercrystalline mesopores in the shell of the material 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; the 12-MR side cups penetrating into the crystals from the external surface are exposed as much as possible to the reaction system due to the single/double/multi-unit-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, cycloalkane, Ti-containing zeolite, calcination-free, reactionregeneration INTRODUCTION Ti-containing zeolite for bulky cycloalkenes,15-20 the reported results are not satisfactory yet. Moreover, the recyZeolite, a type of microporous crystalline materials, is cling of the catalyst for the epoxidation of bulky cycloalfound wide application in chemical industry as the most kenes has not been realized, which is another challenge. important solid catalysts.1 Due to steric limitation of zeolite micropores, the catalytic applications of zeolites are mainly limited in selective conversion of small organic molecules. In the recent decades, great efforts have been directed to catalytic conversion of bulky organic molecules efficiently 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 due to the high reactivity of threemembered ring structure, so that the epoxidation of alkenes or organic molecules with double bond 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 zeolite family, has shown scientific and technological interests due to the ability catalyzing environmentally-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 of large cycloalkenes with the number of C atoms ≥ 7) is still a great challenge. In spite of various methods have been attempted to increase the catalytic activity of

In order 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 TS1,21 diquaternary ammonium surfactant (C16H33-N+(CH3)2C6H12-N+(CH3)2-C6H13) has to be initially prepared and then used in the preparation of reaction gel, and removed by calcinations at high temperature before catalytic applications. Dual-template method is also developed to synthesis zeolite nanosheets, such as DS-ITQ-2 with MWW monolayers22 and SAPO-34 nanosheet23, by 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 amine as the microporous template, which have to be removed by calcinations as well. The above methods not only waste sources but also consume large amount of energy. In 2016, a Ti-containing hollownest-structured zeolite (Ti-HSZ) material was synthesized through one-step hydrothermal rota-crystallization mode at relatively low rotation rate (100 rpm) (Scheme 1). No additional diquaternary ammonium surfactant or complex organic structuredirecting agent is used, and only piperidine (PI) is needed for the structure-directing of microporous MWW structure of Ti-HSZ-N. The unique structure characteristics endow the material with predominant catalytic epoxidation property for bulky cycloalkenes (carbon number ≥ 7). Moreover, a calcination-free catalytic reactionregeneration 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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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 relative pressure of P/P0= 0.98) was calculated by the Barrett-Joyner-Halenda (BJH) method. FTIR spectra of framework vibrations were recorded on a Perkin Elmer Spectrum One FTIR spectrophotometer. The OH-region IR spectra collected under vacuum condition were measured on a Bruker Tensor 27 spectrometer. Before measuring the spectra, a self-supported sample wafer (15 mg) -2 -3 was prepared and evacuated at ca. 2.0×10 ~1.0×10 Pa. X-ray photoelectron spectra (XPS) 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. 29

Scheme 1. Schematic diagram of the Ti-HSZ-N material.

EXPERIMENTAL SECTION Materials. Boric acid (H3BO3, 99.5%), piperidine (PI, 99%), tert-butyl hydroperoxide (TBHP, 65%), and hydrogen peroxide (H2O2, 30%) were purchased from Sinopharm, cycloheptene (96%), 1-methylcyclohexene (98%), 4methylcyclohexene (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 (TBOT, 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: 19.0~40H2O, and the rotating crystallization speed is higher than 100 rpm. Ti-HSZ-N-AT catalyst was obtained through acid treatment (AT) of the precursor Ti-HSZ-N, followed by washing with ethanol until the pH ≈ 7.0 and drying. Ti-HSZ-N-Cal and Ti-HSZ-N-AT-Cal were prepared o by calcinations of Ti-HSZ-N and Ti-HSZ-N-AT at 550 C for 10 hours, respectively. For comparison, conventional TiMWW, TS-1, and Ti-Beta zeolite materials are also prepared 12,26-28 according to the literature. Characterization. X-ray diffraction (XRD) patterns of samples were recorded on a Bruker D8A25 diffractometer with Cu Kα radiation. Scanning electron microscope (SEM) were imaged on a JEOL JSM-6510A. Transmission electron microscope (TEM) images were obtained using FEI Tecnai G20. High resolution transmission electron microscope (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

The solid-state Si NMR spectra were recorded on a Bruker Avance III HD 400WB (9.4 T) NMR spectrometer with a commercial double resonance MAS probe at Larmor 29 29 frequency of 79.5 MHz for Si. Solid-state Si DP/MAS (Direct-polarization magic angle spinning) NMR spectra were recorded with a single pulse excitation using a short tip angle (pi/4) to obtain quantitative results, and a recycle delay 1 29 of 50 s. Solid-state H- Si CP/MAS (Cross-polarization magic angle spinning) NMR spectra were recorded with a 90 1 degree H-pulse width of 4.0 µs and a contact time of 2 ms. 29 The MAS speed were 5 kHz and 8 kHz in Si DP/MAS and 29 CP/MAS NMR measurements, respectively. The Si chemical shifts were determined using a solid external reference Kaolin resonance at -91.5 ppm relative to tetramethylsilane 1 (TMS). H chemical shifts were determined using a solid external reference adamantane resonate at 1.78 ppm relative to tetramethylsilane (TMS). Catalytic and recycling Experiments. The catalytic epoxidation of cycloalkenes with aqueous TBHP were carried out in the 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 TiHSZ-N-AT catalyst were mixed in the autoclave and heated to the desired temperature under rotation of 35 rpm. The reaction was continued for several hours. After the reaction, samples of the reaction were qualitatively analyzed by a Shimadzu 2010 GC, equipped with a 30m capillary column (Rtx-1) and an FID detector. Different catalyst regeneration methods have been conducted, including conventional open system (Figure S3) and closed system used in this work (Figure S4). In open system, the used catalyst (1.0 g) was washed by H2O2/ethanol solution (30 g 30%H2O2 in 30 g ethanol) at 343K for 23 h. The catalyst was recovered by filtration and dried at 353 K overnight for next use. In closed system, the used catalyst and H2O2/ethanol solution were added in the autoclave and heated to the desired temperature under rotation of 35 rpm. The reaction was continued for several hours. In order to keep the amount of catalyst used in every run uniformly, two parallel experiments were conducted synchronously (shown in the supporting information Figure S5).

RESULTS AND DISCUSSION Single-/double/multi-unit-cell Ti-HSZ nanosheets have the following three obvious structure and morphol-

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ogy characteristics, including MWW type crystals, hollownest morphology, and single-/double/multi-unit-cell nanosheets. For the formation of single/double/multiunit-cell Ti-HSZ nanosheets, it is found that both the rotating rate of autoclave and the H2O/Si ratio of reaction gel are important, and the detailed discussions are shown in the supporting information, inclusive of Figure S6~S8 and Table S1. Ti-HSZ-N-AT catalyst was obtained through simple acid treatment (AT) of the precursor Ti-HSZ-N without calcination at 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 oC for 10 hours, respectively.

nm, and the percentage of single/double-layered nanosheets is calculated to be about 12~15%.

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

Figure 1. XRD patterns of Ti-HSZ-N, Ti-HSZ-N-AT, Ti-HSZ-N-Cal and Ti-HSZ-N-AT-Cal. The powder XRD pattern of Ti-HSZ-N synthesized under 120 rpm rotating speed (see Figure 1) is MWW topology. The intensity of the 001 and 002 diffraction lines are decreased as a result of partial disappearance of the lamellar structure for 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 structure-directing agent PI molecules by acid treatment. Ti-HSZ-N-Cal and Ti-HSZN-AT-Cal show similar XRD pattern to Ti-HSZ-N-AT. Ti-HSZ-N-AT displays hollownest morphology (Figure S9), and the shell of which is constructed by randomly intergrown flaky crystals, as shown in the HRTEM images (Figure 2 a,b and Figure S10). There exists the number of hierarchical pores constructed by the stacking of the intergrown MWW nanosheet crystals. As observed in Figure 2 (c) and (d), there exists the number of doublelayered nanosheets with the crystal thickness of 5.0 nm and single-unit-cell nanosheet with the thickness of 2.5

The physical parameters of the samples Ti-HSZ-N-AT, Ti-HSZ-N-Cal, and Ti-HSZ-N-AT-Cal are listed in Table S3. The BJH pore distribution indicates the average mesopore size of 17~19 nm. The external BET surface area of TiHSZ-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 acid treatment, consistent with the element analysis result. Compared with Ti-HSZ-N-AT, the 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 sample Ti-HSZ-N-ATCal, the external and microporous BET surface areas are the highest, suggesting the entirely removal of PI molecules from hierarchical pores. The hysteresis loop at relative pressure of P/P0=0.5−0.98 observed in the N2 adsorption-desorption isotherm of Ti-HSZ-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 bond at 260 nm is attributed to octahedral Ti species in non-framework positions, and the shoulder bands at 212 nm and 230 nm result from high dispersion of the tetrahedrally coordinated Ti in the framework,15 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 sample. For sample Ti-


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HSZ-N-Cal, the occurrence of a broad 318-nm bond indicates the formation of anatase-like phase after the calcination at high temperature. After acid treatment, only 212-nm and 230-nm bands are observed in the UV–vis spectrum of Ti-HSZ-N-AT, implying that the majority of non-framework Ti species has been efficiently removed. Ti-HSZ-N-AT-Cal shows similar UV spectrum to Ti-HSZN-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

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

Figure 3. UV-vis spectra of samples Ti-HSZ-N, TiHSZ-N-Cal, Ti-HSZ-N-AT, and Ti-HSZ-N-AT-Cal. X-ray photoelectron spectroscopy (XPS), a surfacesensitive 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 Ti-HSZ-N-Cal, owing to the presence of the majority of non-framework Ti(VI) oxides in the samples, only the binding energy (BE) of Ti 2p3/2 at 458.4 eV 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 consistent with the UV-vis analysis results. After calcination, some of open Ti sites are transformed to closed ones (Ti-HSZ-NAT-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 5 (a), the chemical shift range of 29 Si MAS NMR for different samples is different, which is listed in the Table 1. For Ti-HSZ-N, the resonances emerge in the range of -86.4 to -118.2 ppm. After acid treatment (sample Ti-HSZ-N-AT), the resonances locate in the range of -89.9 to -120.2 ppm. For Ti-HSZ-N-Cal and Ti-HSZ-N-AT-Cal, the chemical shift range has shifted to the range of -96.0 ~ -120.2 and -92.4 ~ -120.2 ppm, implying that a portion of defect sites disappears after the calcinations at high temperature. In order to identify different Si species, 1H-29Si CP/MAS NMR experiments were conducted to give out the information regarding 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 1 H-29Si CP/MAS NMR spectrum has no obvious difference from that of single pulse 29Si MAS NMR spectrum. Based on the comparison of the single pulse 29Si 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~-105.6ppm has increased much after the 1H-29Si cross polarization. Similar phenomenon is also observed for samples Ti-HSZ-Cal and Ti-HSZ-AT-Cal. It 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 is attributed to Q4 Si species because no obvious intensity increase is observed for the two reso-


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nances in the 29Si CP MAS spectra. Based on the above analysis, the resonances located in the range of -86.4 ~ 105.6 ppm are ascribed to the Q2 [(SiO)2Si(OH)2] or Q3 [(SiO)3SiOH] Si species, while the resonances located in the range of -107.6 ~ -120 ppm are ascribed to the Q4 [(SiO)4Si] Si species.

Table 1. Fraction(%) of QnSi population determined by quantitative data of solid-state one pulse 29Si MAS NMR spectra.

Ti-HSZ-N Ti-HSZ-N-AT

Chemical shift range -86.4~-118.2 -89.9~-120.2

(Q2+Q3)/Q4 ratio 2.22 0.44

(Q2+Q3)/Qn (%) 69 31

Ti-HSZ-N-Cal

-96.0~-120.2

0.38

27

Ti-HSZ-N-AT-Cal

-92.4~-120.2

0.48

32

Samples

Table 2. The catalytic reaction results for the epoxidation of bulky cycloalkenes over various catalysts.a Substrate and product

Figure 5. Single-pulse 29Si MAS (a) and 1H-29Si CP/MAS (b) NMR spectra of Ti-HSZ, Ti-HSZ-Cal, Ti-HSZAT and Ti-HSZ-AT-Cal. 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 non-framework Si species. It 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 acid treatment, the nonframework Si and Ti species have been removed from sample Ti-HSZ-N-AT. The percentage of (Q2+Q3) in the Ti-HSZ-N-AT is about 31%. It indicates that a number of hydroxyl groups are reserved after the acid treatment, 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 condition are shown in Figure S14. For samples heated at 400 oC for 50~60 min, the rich vibration for Si-OH groups is still visible. It implies that rapid rotating crystallization is beneficial for the generation of silanols or titanols on the surface of single/double/multi-layered nanosheet crystals, consistent with the solid-state 29Si MAS NMR results.

Catalyst Ti-HSZ-N-AT

Temp. (oC) 80

Conv. (%) 31.1

Sele. (%) 96.2

110

66.8

98.4

Ti-HSZ-N-AT-Cal

110

67.7

99.2

Ti-HSZ-N-Cal

110

18.4

96.3

TS-1

110

14.9

97.3

Ti-MWW-AT

110

51.9

99.3 99.8

Ti-Beta

110

16.3

Ti-HSZ-N-AT

100

54.0

95.9

Ti-HSZ-N-AT-Cal

100

52.7

98.9

Ti-HSZ-N-Cal

100

24.2

97.2

TS-1

100

6.7

99.5

Ti-MWW-AT

100

31.4

98.7 96.1

Ti-HSZ-N-AT

100

37.2

Ti-HSZ-N-AT-Cal

100

33.1

96.3

Ti-HSZ-N-Cal

100

9.2

99.2

TS-1

100

2.9

96.7

Ti-MWW-AT

100

22.6

98.3

Ti-HSZ-N-AT

100

54.0

100

120

98.0

100

Ti-HSZ-N-AT-Cal

120

96.4

100

Ti-HSZ-N-Cal

120

59.6

99.6

Ti-HSZ-N

120

3.0

69.9

TS-1

120

21.1

96.9

Ti-MWW-AT

120

82.6

100

Ti-Beta

120

22.6

99.8

Ti-HSZ-N-AT

100

39.1

98.9

Ti-HSZ-N-AT-Cal

100

37.7

96.1

Ti-HSZ-N-Cal

100

21.9

99.2

Ti-HSZ-N-AT

120

66.9

98.7

Ti-HSZ-N-AT-Cal

120

64.2

96.8

Ti-HSZ-N-Cal

120

42.2

98.9

TS-1

120

13.6

98.5

Ti-MWW-AT

120

36.7

98.3

a

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

The catalytic epoxidation results of Ti-HSZ-N-AT catalyst for bulky cycloalkenes are listed in Table 2. Although tert-butyl hydroperoxide (TBHP) as the oxidant is normally inactive for the epoxidation of alkenes when using crystalline microporous zeolite materials as the catalyst because of steric constraint; however, Ti-HSZ-N-AT can convert about 66.8% cycloheptene at 110 oC. The epoxidation of 1-methylcyclohexene and 4-methylcyclohexene over Ti-HSZ-N-AT is also effective. Noticeably, 98.0%



conversion of cyclooctene can be attained at the reaction temperature of 120oC. Furthermore, 66.0% cyclododecene can be converted into the corresponding epoxide by TBHP as the oxidant over Ti-HSZ-N-AT (120oC/8h). 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 TiHSZ-N-AT-Cal presents a comparable catalytic reactive activity with Ti-HSZ-N-AT under the same reaction conditions. It suggests that the majority of the intergrown single-/double-/multi-unit-cell MWW nanosheet crystals have been maintained intact even though by the calcinations, 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 TiHSZ-Cal is ascribed to the existence of plenty of anataselike phase in the samples decomposing TBHP (TBHP conversion and efficiency is listed in Table S4), and that over Ti-HSZ-N is ascribed to the negligible influence of 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 TiHSZ-AT material prepared under rotating rate of 56 rpm (Table S5), the higher catalytic properties over Ti-HSZ-NAT prepared under high rotating rate present the importance of single-/double-unit-cell nanosheets on the improvement of the epoxidation efficiency for bulky cycloalkenes.

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the H2O2 reaction system, which is generally below 70 oC, but suitable for the TBHP reaction system (100~120oC). However, for conventional Ti-MWW or Ti-IEZ-MWWRSC-cal,18 the Ti active sites are still difficult to be accessible by bulky TBHP and cycloalkene molecules by merely increasing reaction temperature, due to limited space expansion between MWW layers.

Scheme 2. Proposed catalytic mechanism for the epoxidation of cyclooctene over Ti-containing zeolite.

The prominent catalytic activity of Ti-HSZ-N-AT in the epoxidation of bulky cyclic alkenes with TBHP involves the following three reasons: Firstly, it is well-known that the titanosilicatecatalyzed reactions involve a cyclic intermediate of a 5membered ring, and the intermediate for TBHP (i in Scheme 2) is much bulkier than that for H2O2 due to tertbutyl group.24 An actual reaction occurs only when the substrate molecules can reach the intermediates. The existence of the number of intercrystalline mesopores (ca. 18 nm) in the shell of Ti-HSZ-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. Secondly, the larger the substrate is, 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 due to 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). Thirdly, the transition state involving bulky cycloalkene needs the higher activation temperature than small alkenes. Higher reaction temperature is not suitable for

Scheme 3. Schematic diagram of the catalytic epoxidation of bulky cycloalkene over Ti-HSZ-N-AT. Recycling tests of cyclooctene and cyclododecene have been conducted. As shown in Table S6, by conventional catalyst-regeneration method in an open system (Figure S3) at low temperature,13,21 1.0 g of used catalyst was washed by H2O2/ethanol solution. For Ti-HSZ-N-AT or Ti-HSZ-N-AT-Cal in the second reaction run, an obvious decrease in the conversion of cyclooctene to 74.6% or 44.1% is observed from Table S6 (entry 1 and 2). Consequently, a closed system for the catalyst regeneration was attempted (shown in Figure S4). The reuses of Ti-HSZ-N-AT (5 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 main-


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tained with 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 closed system is ascribed to relatively higher treatment temperature and autogenous pressure, which is beneficial for the removal of substrate and other organic molecules efficiently. 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 almost constant selectivity of 99%.

CONCLUSIONS Single/double/multi-unit-cell MWW nanosheet constructed hierarchically-structured Ti-zeolite material is successfully synthesized by one-step hydrothermal rotacrystallization under relatively high rotation speed (>100 rpm) and H2O/Si ratio of ca. 40. The excellent catalytic activity over this material for the catalytic epoxidation of bulky cycloalkenes with TBHP is ascribed to easily accessible Ti active sites on the single-/double-unit-cell MWW nanosheets, and to hierarchical porous structure promoting the diffusion and mass transfer of bulky reaction molecules. Furthermore, a calcination-free catalystregeneration method is developed, by which this catalyst can be reused 5 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.

ASSOCIATED CONTENT Supporting Information Elemental analysis, physical parameters, reactionregeneration recycling results, synthesis route of zeolite, catalytic reaction setup, catalyst regeneration systerm, SEM, HRTEM, N2 adsorption-desorption isotherm, Ti active site, FTIR, UV-vis, Ti 2p XPS (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. 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, 120oC). For comparison, recycling tests of cyclooctene over traditional Ti-MWW zeolite materials (Ti-MWW-AT and Ti-MWW-AT-Cal) were also conducted (entry 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% conversion of cyclooctene was obtained over Ti-MWWAT (acid treated Ti-MWW precursor) and Ti-MWW-ATCal (acid treatment plus calcination), which was sharply decreased to 56.7% or 64.5% in the second use, and to 16.3% or 19.7% in the third use. The low efficiency for the regeneration of traditional Ti-MWW zeolite is assumed due to the steric limitation of zeolite micropores for removing bulky organic molecules efficiently. This comparison proves the advantage of Ti-HSZ zeolite nanosheets as efficient catalyst for the oxidation of bulky alkene molecules and as recyclable heterogeneous catalyst. The above method is proven to be a feasible and efficient route for the regeneration of hierarchical catalyst TiHSZ-N-AT constructed by nanosheets in the epoxidation of bulky cycloalkenes. Moreover, the calcination at 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.

Corresponding Author *E-mail: [email protected], [email protected]. Tel: 0086-27-8866-2747. Fax: 0086-27-8866-3043. Present Addresses Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan, 430062, P. R. China 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 interests.

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

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Table of Contents


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Multi-Unit-Cell Ti-HSZ

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