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. 2020 Apr 23;5(17):9912–9919. doi: 10.1021/acsomega.0c00184

MWW-Type Titanosilicate Synthesized by Simply Treating ERB-P Zeolite with Acidic H2TiF6 and Its Catalytic Performance in a Liquid Epoxidation of 1-Hexene with H2O2

Shujuan Guo †,*, Yanjie Zhang †, Yuansong Ye †, Jinzhe Song †, Mengru Li ‡
PMCID: PMC7203989  PMID: 32391478

Abstract

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Synthesis of a Ti-incorporated zeolite using a simple and economical method has recently become a focus of attention. The direct hydrothermal synthesis of Ti-MWW is most commonly applied; however, it is challenging to perform and exhibits low titanium utilization. An innovative strategy of synthesizing Ti-MWW is proposed in the present study by simply treating the ERB-1 precursor of an MWW-type boron silicate with a H2TiF6/HNO3 solution. This significantly shortens the Ti grafting process from 5 days to only a few hours and reduces the use of the structure-directing agent hexamethyleneimine (HMI); furthermore, no extraframework Ti is observed in the precursor, indicating good atomic economy. Typically, a piperidine (PI)-treated sample Ti-MWW2–1-PI exhibits a higher conversion (76.6%) than the original Ti-MWW (44.8%) in the epoxidation of 1-hexene. X-ray diffraction (XRD), inductively coupled plasma (ICP), and transmission electron microscopy (TEM) techniques are used to explain in detail the probable mechanism underlying the incorporation of Ti species into the MWW framework. X-ray photoelectron spectroscopy (XPS) is employed to study the coordinate state of the Ti and F species in the samples after treatment with a piperidine solution. This method can be applied to synthesize other kinds of lamellar-structured zeolites with heteroatoms.

1. Introduction

Titanosilicates have been widely applied in the production of various oxygenates with the use of H2O2 as an environmentally friendly oxidant in olefin liquid epoxidation,1,2 ammoximation,3,4 and phenol hydroxylation5,6 processes. Highly efficient selective oxidation is associated with a uniform pore structure, hydrophobicity,7,8 and, most importantly, unique tetracoordinated titanium species.9−11 The hydrothermal method is commonly applied to synthesize zeolites with tetracoordinated titanium species, but this method requires a larger amount of organic structure-directing agent (OSDA)12 than pure silicate.13

To simplify the synthesis process, a new approach to introduce Ti active sites into the framework has been established: grafting titanium in highly dealuminized silicate.14−18 With either a solid–gas treatment, with, for example, TiCl4, or a solid–liquid modification, with, for example, a Ti(acac)2OiPr2 isopropanol solution, it is not difficult to synthesize a Ti-incorporated zeolite and conserve the use of OSDA. Hence, the presence of extraframework octahedral-coordinated titanium species severely hinders the catalytic performance of epoxidation with hydrogen peroxide.18,19 These zeolite types are preferred to catalyze bulk molecules with tert-butyl hydroperoxide (TBHP). Therefore, a catalyst that exhibits only tetracoordinated titanium species by hydrothermal synthesis is highly desirable.

Wu et al.20−22 promoted a postsynthesis method to obtain a simultaneous isomorphous substitution. This innovative method was applied in a zeolite with unique properties, in which the structure could reversibly change between a two-dimensional (2D) and three-dimensional (3D) topology. Highly deboronated ERB-1 was stirred with a piperidine (PI) water solution containing Ti species at the crystallization temperature. At this temperature, piperidine/tetrabutyl orthotitanate (TBOT) was present in the interlayer region and could facilitate the immigration of Ti species into the framework.23,24 Organic Ti species were later hydrolyzed to form silica hydroxyl nests during a deboronation process. The Ti-MWW precursor was further acid-washed to eliminate hexacoordinated Ti species and then calcined at 823 K for 6 h. Therefore, we expect that the synthesis of Ti-MWW will not be difficult if we can complete the simultaneous acidic deboronation and titanium hydrolysis of an MWW 2D-layered structure.

According to a previous study, organic Ti, such as TBOT, plays an important role as a Ti source in the synthesis of titanosilicate. Thus, it is difficult to control its hydrolysis rate, especially under acidic conditions. Therefore, the proposed method has extensive application prospects for synthesizing zeolite from inorganic reactants. The proposed method refers to the previous postsynthesis,25,26 wherein inorganic titanium materials (NH4)2TiF6 and H2TiF6 are employed as a Ti source and the structure of a boron-containing MWW framework is reversed to a Ti-MWW-type catalyst. The mechanism can also be described as follows: silica hydroxyl nests are formed by eliminating silica atoms in the framework, while simultaneously, inorganic Ti hydrolysis occurs in the structure. The above method selectively produces tetracoordinated titanium species. TS-1, which was prepared with TPABr, colloidal silica, and Ti(SO4)2 in a hydrothermal synthesis process,27 performed well in epichlorohydrin epoxidation; however, the as-synthesized samples needed to be acid-washed to eliminate extraframework Ti species before they could exhibit high catalytic performance.

In this work, a B-containing MWW-type precursor ERB-p was prepared by a dry-gel conversion (DGC) method. A series of Ti-MWW zeolites could be obtained by treating the ERB precursor with a certain amount of H2TiF6/HNO3 solution, made with different concentrations of HNO3 solution and with various reaction times. Ti-MWW could be obtained by direct calcination because the as-synthesized samples exhibited no extraframework titanium species.

2. Results and Discussion

2.1. Influence of Ti Content on Epoxidation of 1-Hexene

To optimize the Si/Ti ratio, ERB-p was washed with different concentrations of H2TiF6 in a 2 M HNO3 solution for 1 h at 378 K. Ti-MWW, with a Si/Ti ratio in the range of 10–160, was prepared.

Figure 1 shows the results of Ti-MWW synthesized with different Ti contents in the epoxidation of 1-hexene. The 1-hexene conversion first increased but then decreased with increasing Ti content. Interestingly, the selectivity of epoxide was found to increase dramatically from 54.7 to 84.0% when the Si/Ti ratio slightly changed from 40 to 20, which could be attributed to the interlayer expansion that was observed in the zeolite; the above result can be confirmed in Figure 2 with the appearance of the 002 diffraction peak. The structure–activity relationship will be discussed later. Nevertheless, the yield of epoxide exhibited volcano-type behavior with the change in the Ti content and reached a maximum at a ratio of 20. Thus, Ti-MWW with a Ti content ratio of 20 was further investigated to reveal the mechanism for the introduction of H2TiF6 into the framework.

Figure 1.

Figure 1

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Comparison of epoxidation activity of 1-hexene between the Ti-MWW samples synthesized with different Ti contents.

Figure 2.

Figure 2

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XRD pattern of Ti-MWW with different Si/Ti ratios: (a) 40, (b) 20, and (c) 10.

2.2. Mechanism of Introducing H2TiF6 into the Framework in Different Treatments

A postsynthesis method20−22 was investigated by replacing boron in the boron-containing zeolite with Ti. However, the procedure needs to be separated into two steps: an acid treatment step and an additional step for the conversion of the 3D crystalline structures, which are caused by the direct calcination,19 into a 2D lamellar structure. Thus, the above procedure requires an extensive investigation to convert these two complicated steps to a procedure with only one step. An unstable interlayer structure provided a new approach to synthesize Ti-MWW from ERB-p, which required replacing a piperidine molecule in the interlayer region of ERB-p with a TiF62– ion. A cation acid treatment of zeolite was essential in this work because the OSDA/TiF62–-ion replacement would not occur if the conditions were too mild; the framework would collapse due to the simultaneous HF corrosion. X-ray diffraction (XRD) patterns, scanning electron microscopy, and ICP static analyses were used to investigate the mechanism of introducing titanium into the framework, and the results are shown in Figures 3, 4, and Table 1, respectively.

Figure 3.

Figure 3

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Influence of acid concentration and treatment time on the ion-exchange synthesis of Ti-MWW: (a) Ti-MWW0–1, (b) Ti-MWW1–1, (c) Ti-MWW2–1, (d) Ti-MWW2–0.25, (e) Ti-MWW2–2, (f) Ti-MWW2–4, and (g) Ti-MWW2–7.

Figure 4.

Figure 4

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Scanning electron micrographs of samples synthesized in different conditions: (A) ERB-1, (B) Ti-MWW1–1, (C) Ti-MWW2–1, (D) Ti-MWW2–2, (E) Ti-MWW2–4, and (F) Ti-MWW2–7.

Table 1. Chemical Composition of Ti-MWW Samples and the Results of Epoxidation of 1-Hexene with H2O2.

    epoxidation of 1-hexene
sample name product (Si/Ti ratio)a 1-hexene conversion (%) TONb (mol (mol–1)) epoxide selectivity (%) H2O2c selectivity (%)
Ti-MWW0–1 26 8.2 13 43.6 46.7
Ti-MWW1–1 65 39.2 147 90.7 90.3
Ti-MWW2–1 62 56.6 201 84.0 88.2
Ti-MWW2–2 46 53.5 142 82.5 87.8
Ti-MWW2–4 34 49.2 99 76.2 87.5
Ti-MWW2–7 69 41.0 163 86.9 84.6
Ti-MWWd 54 44.8 214 99.0 93.0
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a

Given by ICP.

b

TON in mol (mol Ti)−1.

c

Efficiency of H2O2 = (amount of product/amount of H2O2 consumed) × 100%.

d

Data of Ti-MWW were from ref (17).

Interestingly, the entire series of catalysts showed 002 diffraction peaks in the XRD pattern except for the low-acid-treated sample named Ti-MWW0–1; this significant characteristic was distinguishable from the original Ti-MWW and B-MWW. It is common that direct calcination in air burns off the OSDA molecules between the layers and causes interlayer condensation, which converts the original structure to a 3D crystalline structure and results in the disappearance of the [001] and [002] diffraction peaks.17 Moreover, the intensity of the [002] peak increases with increasing HNO3 concentration and duration of acid treatment. The relationship between the number of active sites (Ti) and the structure of the zeolite was investigated by the XRD patterns and ICP data. Figure 5 shows that a high Ti content is related to the increasing expansion of the interlayer structure (increasing intensity of the crystal surface I002). The structure belonging to the 002 surfaces benefited the introduction of Ti species into the MWW structure. The XRD patterns shifted to a lower degree, which also indicated that Ti atoms were planted in the framework.

Figure 5.

Figure 5

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Ratio of the [002] peak and [100] peak intensity and the corresponding Si/Ti ratios in the series of catalysts: (A) Ti-MWW1–1, (B) Ti-MWW2–0.25, (C) Ti-MWW2–1, (D) Ti-MWW2–2, and (E) Ti-MWW2–4.

However, the acidic H2TiF6 treatment was accompanied by an unexpectedly strong desilication process, as shown in Figure 3. The morphology of ERB-1 was considered to be primarily assembled from thin platelet crystals of the MWW phase.28 ERB-1 had an average particle size of 3.5 μm, and the large secondary crystals, which were measured to be 1 μm, collided into the primary platelet crystals during the series of acid treatments. As shown in Figure 4D and 3g, the crystallinity of the MWW structure can be destroyed by acid treatment conditions that are too severe.

Ultraviolet–visible (UV–vis) spectra were obtained to analyze the coordinate state of Ti-MWW synthesized under different conditions. The band at approximately 220 nm was assigned to the ligand-to-metal charge transfer originating from the oxide ligand to Ti4+ in a tetrahedral (Td) coordination.29Figure 6 shows that no extraframework Ti species emerge during the acidic grafting, which is different from the as-synthesized Ti-MWW. This phenomenon indicated that the titanium atom was selectively inserted into the framework and produced Ti-MWW in a new atom-economical method. It also showed that the existence of an interlayer structure and proper acid treatment conditions dominated the grafting process as Ti-MWW0–1, without HNO3 assistance, and ERB-12–1 (using ERB-1 instead ERB-p), with no obvious [002] peak, appeared as extraframework Ti (band at approximately 260 nm) in the anatase phase (band at approximately 330 nm). As shown in Figure 7, Ti was not completely introduced into the framework because of the absence of piperidine in the interlayer and the assistance of HNO3.

Figure 6.

Figure 6

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UV–vis spectra of (a) Ti-MWW2–1, (b) as-synthesized Ti-MWW2–1, and (c) as-synthesized Ti-MWW.

Figure 7.

Figure 7

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UV–vis spectra of (a) Ti-MWW1–1, (b) Ti-MWW0–1, and (c) ERB2–1 (using ERB-1 instead of ERB-p).

In addition, Ti-MWW prepared by treating ERB-p with acidic H2TiF6 exhibited an IR band at 945 cm–1 (Figure 8), while the characteristic IR band at 960 cm–1 was assigned to the stretching of Si–O–Ti bonds in the [Ti(OSiO3)4] unit. This result indicated that the Si–O–Ti bonds might be influenced by fluorine in the framework.

Figure 8.

Figure 8

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Fourier transform infrared (FT–IR) spectra of Ti-MWW and Ti-MWW2–1.

The mechanism of Ti grafting could be concluded as follows: the desilication and deboronation (Si/B > 1000) process provided hydroxyl nests. During this procedure, acidic H2TiF6 partly replaced the piperidine and prevented the silanol in the interlayer from forming a condensation interlayer-expanded30 structure, and inorganic Ti hydrolysis on the hydroxyl nest selectively produced framework Ti, at the same time. The proposed mechanism for the incorporation of Ti species is shown in Figure 9.

Figure 9.

Figure 9

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Mechanism of Ti grafting.

The epoxidation of 1-hexene with H2O2 was selected as the probe reaction in this study. The samples could not show high performance without proper acid treatment conditions. Ti-MWW0–1 achieved only 8.2% conversion and 43.6% selectivity, which could be attributed to the extraframework Ti. Ti-MWW synthesized by 1 M HNO3 showed 39.2% conversion and 90.7% epoxide selectivity. The conversion increased sharply with an increasing concentration of HNO3 (2 M) but demonstrated a low selectivity (84.0%) due to the inevitable simultaneous insertion of Ti and F atoms. The decrease in epoxide yield with the extension of the acid treatment time also indicated that overused F atoms in the framework and zeolite structure caused a collapse in the structure due to its application in epoxidation reactions. For instance, no catalytic activity was observed with acid-treated ERB-1 because the structure transformed from a 2D lamellar structure to a 3D MWW structure after calcination, which inhibited Ti insertion.

2.3. Influence of PI Treatment on the Ti Coordinate State of Ti-MWW

To overcome the negative impact of F– in the framework and further improve the catalytic performance of Ti-MWW in the epoxidation of 1-hexene, a piperidine treatment was used. The amount of piperidine required was calculated by the amount of Ti species; for instance, one Ti ligand required one PI molecule. The state of the Ti and F species was analyzed by X-ray photoelectron spectroscopy (XPS), a surface-sensitive technique for the analysis of elements and their oxidation states. The results are shown in Figures 10 and 11. The binding energy of Ti 2p changed from 460.2 to 459.2 and 458.1 eV after the PI treatment, indicating that the Ti species in the tetrahedrally coordinated state may transform into two different forms.31 The low binding energy indicated the negative charge distribution of Ti atoms in Ti-MWW2–1-PI. An F–-modified hydrothermal synthesis of Ti-MWW29−32 exhibited SiF6–, SiO4/2F– units, and SiO3/2F units. The SiO3/2F units were considered beneficial to the epoxidation of 1-hexene in the literature; the above observation is due to the high electronegativity of the SiO3/2F unit withdrawing the electron around the Ti nucleus, thus reinforcing the activity of Ti4+. Hence, Ti-MWW synthesized by a direct acidic H2TiF6 treatment with ERB-p only had SiO4/2F– and SiF6– units in the framework, which was considered undesirable for the epoxidation reaction. After the piperidine treatment, the binding energy of F1S split into two peaks: 685 eV (F– in the SiF6– and SiO4/2F– units33) and 688 eV (F in the SiO3/2F units). Furthermore, the F– units significantly transitioned to SiO3/2F.

Figure 10.

Figure 10

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Ti 2p XPS spectra of (A) Ti-MWW2–1 and (B) Ti-MWW2–1-PI.

Figure 11.

Figure 11

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F 1s XPS spectra of (A) Ti-MWW2–1 and (B) Ti-MWW2–1-PI.

The catalytic performance was greatly enhanced by a simple piperidine treatment. The conversion of 1-hexene increased up to 76.6% when Ti-MWW2–1 was used as the catalyst. The selectivity of epoxide and H2O2 also increased to 94.3 and 94.4%, respectively. The activity of the Ti-MWW2–7 sample improved and was even higher than that of Ti-MWW2–1 after the PI treatment; however, the selectivity of epoxide became even worse due to its poor crystallinity. We speculated that piperidine arranged F– into the framework and formed a more stable SiO3/2F structure and that the ligands with open-site six-coordinate Ti species promoted the activation of H2O2 over Ti-MWW; thus, the above arrangement significantly enhanced the catalytic performance of the epoxidation of 1-hexene (Table 2).

Table 2. Results of Epoxidation of 1-Hexene with H2O2.

    epoxidation of 1-hexene
sample name product (Si/Ti ratio)a 1-hexene conversion (%) TONb (mol (mol–1)) epoxide selectivity (%) H2O2c selectivity (%)
Ti-MWW2–1 62 56.6 201 84.0 88.2
Ti-MWW2–1-PI   76.6 273 94.3 94.4
Ti-MWW2–7 69 41.0 163 86.9 84.6
Ti-MWW2–7-PI   78.7 313 78.5 87.9
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a

Given by ICP.

b

TON in mol (mol Ti)−1.

c

Efficiency of H2O2 = (amount of product/amount of H2O2 consumed) × 100%.

As shown in Table 3, the catalytic activity of Ti-MWW2–1-PI was higher than that of other typical titanium silicates, such as nanosized TS-1 and Ti-β, as well as other synthesized Ti-incorporated MWW-type zeolites. TS-1 in ref (34) is also an anatase-free type zeolite, which is fair for comparison with Ti-MWW synthesized in this work. TS-1 exhibited several advantages, including high epoxide and H2O2 selectivity and only a slight difference in the 1-hexene epoxidation conversion compared with Ti-MWW2–1. The above results indicate that the TS-1 catalyst could also be a suitable candidate for linear olefin epoxidation in the absence of anatase in the framework. Ti-D-ERB-1-L-5h had a higher Ti content but lower 1-hexene conversion than Ti-MWW2–1-PI. The higher catalytic performance in this work was attributed to the MWW topology and the absence of anatase and extraframework Ti.

Table 3. Epoxidation of 1-Hexene with H2O2 over Various Titanosilicatesa.

catalyst Si/Ti conversion (%) H2O2c selectivity (%)
Ti-MWW2–1-PI 62 76.6 94.4
TS-1-HRb 81 52.6 97.6
Ti-βc 59 40.3 100
Ti-D-ERB-1-L-5hd 43 35.16 100
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a

Reaction conditions: catalyst, 50 mg; 1-hexene, 10 mmol; H2O2 (30 wt % aqueous solution), 10 mmol; MeCN, 10 mL; temp., 333 K; time, 2 h.

b

Data from ref (34) CH3OH (10 mL) as solvent.

c

Reaction conditions:35 16.5 mmol of 1-hexene, 11.8 g of solvent, 4.1 mmol of H2O2, 100 mg of catalyst, 50 °C, 2 h.

d

Data from ref (18).18

3. Conclusions

In this study, an innovative strategy was discovered: Ti-MWW could be successfully synthesized by replacing the piperidine molecule in the interlayer region of ERB-p with a TiF62– ion. This method was much more efficient than a hydrothermal synthesis. The acidic H2TiF6 treatment conditions could be neither too mild nor too severe. Ti-MWW2–1 was optimized as a desired catalyst, showing higher catalytic performance than Ti-MWW in the epoxidation of 1-hexene with H2O2. Furthermore, the conversion of 1-hexene increased from 56.6 to 76.6% when the catalyst was treated with PI. Most importantly, if the 2D lamellar structure is treated with acidic H2TiF6, which is associated with the zeolite framework properties, then the proposed method might be applied to different zeolite types.

4. Experimental Section

4.1. Preparation of Ti-MWW

4.1.1. Synthesis of ERB-1 Precursor

The ERB-1 precursor was synthesized by the DGC method.28 Hexamethyleneimine (HMI), boric acid, and ERB-136 were employed as the structure-directing agent (SDA), crystallization-supporting agent, and seed, respectively. In the DGC method, 2.0 g of SiO2 (A200, Degussa), 0.257 g of boric acid (SCRC), and 0.2 g of ERB-1 as the seed were mixed together and dissolved in 10 mL of deionized water to form a homogeneous gel. The resultant homogeneous gel was placed in a 373 K oil bath under stirring until the water was completely evaporated to get the silica-boric compound. The silica-boric compound was then transferred into a small container, which was then placed into a Teflon-lined autoclave containing 2 g of HMI and 4 g of deionized water. Crystallization was conducted in this autoclave at 443 K for 2 days. The solid product was washed with deionized water and dried at 383 K overnight, which was denoted as ERB-p.

4.1.2. ERB-p Treated with H2TiF6/HNO3

The ERB precursor consists of SiO2, HMI, and water, to synthesize Ti-MWW with a certain Si/Ti ratio. The amount of pure SiO2 was determined by thermogravimetry. In a typical synthesis procedure, 1.37 g of ERB-p (1 g pure SiO2) was mixed with 0.273 g of H2TiF6 (50% in aqueous, Aladdin) in 20 mL of X mole HNO3 solution. The mixture where the Si/Ti mole ratio equaled 20 was then placed in a 25 mL flask at 378 K for Y hour with a condenser. The as-synthesized Ti-MWW was obtained by washing and drying at 383 K overnight, and further calcined at 823 K for 6 h. The product prepared by ERB-p was denoted as Ti-MWWx–y. The piperidine modification experiment was conducted by treating 0.1 g of Ti-MWWx-y with 10 mL of a 10 mmol PI–water solution at 378 K with a condenser for 1 h, and Ti-MWWx–y-PI was obtained by additional filtration and drying.

4.2. Catalyst Characterization

X-ray diffraction (XRD) patterns were obtained using a Rigaku MiniFlex 600 X-ray diffractometer with Cu Kα radiation at 45 kV and 20 mA over a 2θ range of 5–45°. The UV–vis diffuse reflectance spectra were recorded on a Lambda 950 spectrophotometer using BaSO4 as the reference. The micromorphology of the samples was characterized by a Hitachi S-8010 scanning electron microscope. X-ray photoelectron spectroscopy (XPS) patterns were carried out using an ESCALAB 250Xi (Thermo Scientific) instrument. The Ti content was measured by inductively coupled plasma optical emission (ICP) spectroscopy (PerkinElmer OPTIMA 8000). FT-IR spectra were recorded by a Nicolet 6700 FT–IR spectrometer using the KBr technique, and the wafer was prepared with the same amount of sample and KBr under the same pressure.

4.3. Epoxidation of 1-Hexene

The liquid-phase oxidation of 1-hexene with hydrogen peroxide was carried out in a 25 mL flask with a condenser. In a typical run, 50 mg of catalyst, 10 mmol of H2O2 (30% aqueous solution, SCRC), 10 mmol of 1-hexene (Aladdin), and 10 mL of acetonitrile (99%, SCRC) were mixed in a reactor. The reaction was carried out at 333 K for 2 h under magnetic stirring. The catalyst was filtered, and the final reaction mixture was analyzed by a gas chromatograph (Agilent 6890) and quantified using cyclohexanone as an internal standard. The residual H2O2 was determined by a standard titration method with 0.1 M sodium thiosulfate.

Acknowledgments

The authors sincerely acknowledge the support of Department of Education, Fujian Province (No. JT180397), the National Natural Science Foundation of China (21503105), and the Scientific Research Fund of Higher Education of Fujian Province of China (JK 2017037).

The authors declare no competing financial interest.

References

  1. Hulea V.; Dumitriu E. Styrene oxidation with H2O2 over Ti-containing molecular sieves with MFI, BEA and MCM-41 topologies. Appl. Catal., A 2004, 277, 99–106. 10.1016/j.apcata.2004.09.001. [DOI] [Google Scholar]
  2. Schmidt I.; Krogh A.; Wienberg K.; Carlsson A.; Brorson M.; Jacobsen C. J. H. Catalytic epoxidation of alkenes with hydrogen peroxide over first mesoporous titanium-containing zeolite. Chem. Commun. 2000, 21, 2157–2158. 10.1039/b006460m. [DOI] [Google Scholar]
  3. Dal Pozzo L.; Fornasari G.; Monti T. TS-1, catalytic mechanism in cyclohexanone oxime production. Catal. Commun. 2002, 3, 369–375. 10.1016/S1566-7367(02)00145-0. [DOI] [Google Scholar]
  4. Song F.; Liu Y. M.; Wu H. H.; He M. Y.; Wu P.; Tatsumi T. A novel titanosilicate with MWW structure: Highly effective liquid-phase ammoximation of cyclohexanone. J. Catal. 2006, 237, 359–367. 10.1016/j.jcat.2005.11.018. [DOI] [Google Scholar]
  5. Pang C. L.; Xiang J. H.; Li G. Y.; Hu C. W. Direct ring C-H bond activation to produce cresols from toluene and hydrogen peroxide catalyzed by framework titanium in TS-1. J. Catal. 2018, 366, 37–49. 10.1016/j.jcat.2018.07.038. [DOI] [Google Scholar]
  6. Wu P.; Komatsu T.; Yashima T. Hydroxylation of aromatics with hydrogen peroxide over titanosilicates with MOR and MFI structures: Effect of Ti peroxo species on the diffusion and hydroxylation activity. J. Phys. Chem. B 1998, 102, 9297–9303. 10.1021/jp982951t. [DOI] [Google Scholar]
  7. Bhaumik A.; Tatsumi T. Selective Dihydroxylation over Titanium Silicate Molecular Sieves. J. Catal. 1998, 176, 305–309. 10.1006/jcat.1998.2055. [DOI] [Google Scholar]
  8. Park S.; Cho K. M.; Youn M. H.; Seo J. G.; Jung J. C.; Baeck S. H.; Kim T. J.; Chung Y. M.; Oh S. H.; Song I. K. Direct epoxidation of propylene with hydrogen peroxide over TS-1 catalysts: Effect of hydrophobicity of the catalysts. Catal. Commun. 2008, 9, 2485–2488. 10.1016/j.catcom.2008.07.003. [DOI] [Google Scholar]
  9. Jin S.; Wang Z.; Tao G.; Zhang S.; Liu W.; Fu W.; Zhang B.; Sun H.; Wang Y.; Yang W. UV resonance Raman spectroscopic insight into titanium species and structure-performance relationship in boron-free Ti-MWW zeolite. J. Catal. 2017, 353, 305–314. 10.1016/j.jcat.2017.07.032. [DOI] [Google Scholar]
  10. Tozzola G.; Mantegazza M. A.; Ranghino G.; Petrini G.; Bordiga S.; Ricchiardi G.; Lamberti C.; Zulian R.; Zecchina A. On the structure of the active site of Ti-silicalite in reactions with hydrogen peroxide: A vibrational and computational study. J. Catal. 1998, 179, 64–71. 10.1006/jcat.1998.2205. [DOI] [Google Scholar]
  11. Yuan S.; Si H.; Fu A.; Chu T.; Tian F.; Duan Y.-B.; Wang J. Location of Si Vacancies and Ti(OSi)(4) and Ti(OSi)(3)OH Sites in the MFI Framework: A Large Cluster and Full Ab Initio Study. J. Phys. Chem. A 2011, 115, 940–947. 10.1021/jp110977m. [DOI] [PubMed] [Google Scholar]
  12. Thangaraj A.; Eapen M. J.; Sivasanker S.; Ratnasamy P. Studies on the synthesis of titanium silicalite, TS-1. Zeolites 1992, 12, 943–950. 10.1016/0144-2449(92)90159-M. [DOI] [Google Scholar]
  13. Persson A. E.; Schoeman B. J.; Sterte J.; Otterstedt J.-E. The synthesis of discrete colloidal particles of TPA-silicalite-1. Zeolites 1994, 14, 557–567. 10.1016/0144-2449(94)90191-0. [DOI] [Google Scholar]
  14. Chang C. C.; Jin F.; Jang L.-Y.; Lee J.-F.; Cheng S. Effect of the grafting agent on the structure and catalytic performance of Ti-MCM-22. Catal. Sci. Technol. 2016, 6, 7631–7642. 10.1039/C6CY01591C. [DOI] [Google Scholar]
  15. Charbonneau L.; Kaliaguine S. Epoxidation of limonene over low coordination Ti in Ti-SBA-16. Appl. Catal., A 2017, 533, 1–8. 10.1016/j.apcata.2017.01.001. [DOI] [Google Scholar]
  16. Fadhli M.; Khedher I.; Fraile J. M. Modified Ti/MCM-41 catalysts for enantioselective epoxidation of styrene. J. Mol. Catal. A: Chem. 2016, 420, 282–289. 10.1016/j.molcata.2016.05.001. [DOI] [Google Scholar]
  17. Korzeniowska A.; Grzybek J.; Roth W. J.; Kowalczyk A.; Michorczyk P.; Čejka J.; Přech J.; Gil B. Incorporation of Ti as a pyramidal framework site in the mono-layered MCM-56 zeolite and its oxidation activity. ChemCatChem 2019, 11, 520–527. 10.1002/cctc.201800981. [DOI] [Google Scholar]
  18. Yan M.; Jin F.; Ding Y.; Wu G.; Chen R.; Wang L.; Yan Y. Synthesis of Titanium-Incorporated MWW Zeolite by Sequential Deboronation and Atom-Planting Treatment of ERB-1 as an Epoxidation Catalyst. Ind. Eng. Chem. Res. 2019, 58, 4764–4773. 10.1021/acs.iecr.8b05836. [DOI] [Google Scholar]
  19. Wu P.; Tatsumi T.; Komatsu T.; Yashima T. A novel titanosilicate with MWW structure. II. Catalytic properties in the selective oxidation of alkenes. J. Catal. 2001, 202, 245–255. 10.1006/jcat.2001.3278. [DOI] [Google Scholar]
  20. Wang L.; Liu Y.; Wei X.; Wu H.; Li X.; Mingyuan He A.; Peng W. Improving the Hydrophobicity and Oxidation Activity of Ti–MWW by Reversible Structural Rearrangement. J. Phys. Chem. C 2008, 112, 6132–6138. 10.1021/jp712155k. [DOI] [Google Scholar]
  21. Fan W.; Peng W. U.; Namba S.; Tatsumi T. Synthesis and catalytic properties of a new titanosilicate molecular sieve with the structure analogous to MWW-type lamellar precursor. J. Catal. 2006, 243, 183–191. 10.1016/j.jcat.2006.07.003. [DOI] [Google Scholar]
  22. Wu P.; Fan W.; Nuntasri D.; Tatsumi T. MWW-type titanosilicate: Novel preparation and high efficiency in the epoxidation of various alkenes. Stud. Surf. Sci. Catal. 2004, 154, 2581–2588. 10.1016/s0167-2991(04)80527-5. [DOI] [Google Scholar]
  23. Leonowicz M. E.; Lawton J. A.; Lawton S. L.; Rubin M. K. MCM-22: a molecular sieve with two independent multidimensional channel systems. Science 1994, 264, 1910–1913. 10.1126/science.264.5167.1910. [DOI] [PubMed] [Google Scholar]
  24. Yoshioka M.; Yokoi T.; Tatsumi T. Effectiveness of the reversible structural conversion of MWW zeolite for preparation of interlayer-expanded Ti-MWW with high catalytic performance in olefin epoxidation. Microporous Mesoporous Mater. 2014, 200, 11–18. 10.1016/j.micromeso.2014.08.007. [DOI] [Google Scholar]
  25. Oumi Y.; Manabe T.; Sasaki H.; Inuzuka T.; Sano T. Preparation of Ti incorporated Y zeolites by a post-synthesis method under acidic conditions and their catalytic properties. Appl. Catal., A 2010, 388, 256–261. 10.1016/j.apcata.2010.08.063. [DOI] [Google Scholar]
  26. Yang B.; Jiang J.-G.; Zhang K.; Peng W. Synthesis of Novel Titanosilicate Catalysts by Simultaneous Isomorphous Substitution and Interlayer Expansion of Zeolitic Layered Silicates. Chem. Mater. 2016, 28, 5295–5303. 10.1021/acs.chemmater.6b00750. [DOI] [Google Scholar]
  27. Wang M.; Zhou J.; Mao G.; Zheng X. Synthesis of TS-1 from an Inorganic Reactant System and Its Catalytic Properties for Allyl Chloride Epoxidation. Ind. Eng. Chem. Res. 2012, 51, 12730–12738. 10.1021/ie202524t. [DOI] [Google Scholar]
  28. Wu P.; Miyaji T.; Liu Y. M.; He M. Y.; Tatsumi T. Synthesis of Ti-MWW by a dry-gel conversion method. Catal. Today 2005, 99, 233–240. 10.1016/j.cattod.2004.09.045. [DOI] [Google Scholar]
  29. Zhang F. Z.; Guo X. W.; Wang X. S.; Li G.; Zhou J. C.; Yu J. Q.; Li C. The active sites in different TS-1 zeolites for propylene epoxidation studied by ultraviolet resonance Raman and ultraviolet visible absorption spectroscopies. Catal. Lett. 2001, 72, 235–239. 10.1023/a:1009061910392. [DOI] [Google Scholar]
  30. Fang X.; Wang Q.; Zheng A.; Liu Y.; Lin L.; Wu H.; Deng F.; He M.; Wu P. Post-synthesis, characterization and catalytic properties of fluorine-planted MWW-type titanosilicate. Phys. Chem. Chem. Phys. 2013, 15, 4930 10.1039/c3cp44700f. [DOI] [PubMed] [Google Scholar]
  31. Xu L.; Huang D.-D.; Li C.-G.; Ji X.; Jin S.; Feng Z.; Xia F.; Li X.; Fan F.; Li C.; Wu P. Construction of unique six-coordinated titanium species with an organic amine ligand in titanosilicate and their unprecedented high efficiency for alkene epoxidation. Chem. Commun. 2015, 51, 9010–9013. 10.1039/C5CC02321A. [DOI] [PubMed] [Google Scholar]
  32. Fang X.; Sun L.; Lin L.; Wu L.; Liu Y. Enhanced catalytic oxidation performance of F-Ti-MWW through the synergistic effect of anion and cation. Catal. Commun. 2017, 96, 54–57. 10.1016/j.catcom.2017.04.007. [DOI] [Google Scholar]
  33. Yu J. C.; Yu; Ho; Jiang; Zhang Effects of F- Doping on the Photocatalytic Activity and Microstructures of Nanocrystalline TiO2 Powders. Chem. Mater. 2002, 14, 3808. 10.1021/cm020027c. [DOI] [Google Scholar]
  34. Zhang T.; Chen X.; Chen G.; Chen M.; Bai R.; Jia M.; Yu J. Synthesis of anatase-free nano-sized hierarchical TS-1 zeolites and their excellent catalytic performance in alkene epoxidation. J. Mater. Chem. A 2018, 6, 9473–9479. 10.1039/C8TA01439F. [DOI] [Google Scholar]
  35. Blasco T.; Camblor M. A.; Corma A.; Esteve P.; Guil J. M.; Martínez A.; Perdigón-Melón J. A.; Valencia S. Direct Synthesis and Characterization of Hydrophobic Aluminum-Free Ti?Beta Zeolite. J. Phys. Chem. B 1998, 102, 75–88. 10.1021/jp973288w. [DOI] [Google Scholar]
  36. Millini R.; Perego G.; Parker W. O. Jr.; Bellussi G.; Carluccio L. Layered structure of ERB-1 microporous borosilicate precursor and its intercalation properties towards polar molecules. Microporous Mater. 1995, 4, 221–230. 10.1016/0927-6513(95)00013-Y. [DOI] [Google Scholar]

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