High-Performance Microwave Synthesized Mesoporous TS-1 Zeolite

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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High-Performance Microwave Synthesized Mesoporous TS‑1 Zeolite for Catalytic Oxidation of Cyclic Olefins Seung-Kyun Kim,† Benjaram M. Reddy,†,‡ and Sang-Eon Park*,† †

Laboratory of Nano-Green Catalysis & Nano Center for Fine Chemicals Fusion Technology, Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea ‡ Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500 007, India ABSTRACT: A distinctive mesoporous titanium silicalite-1 (TS-1) was prepared by microwave-assisted postsynthetic treatment with H2O2 to generate hierarchical pore structure. For comparison, mesoporous TS-1 was also prepared with alkali or fluoride postsynthetic treatment. Synthesized catalysts were characterized by various techniques, namely, XRD, XPS, FTIR, UV−vis DRS, and others and evaluated for oxidation of cyclic olefins. The post-treated TS-1 with H2O2 and microwave irradiation exhibited a high catalytic activity in comparison to the parent TS-1. Both microwave irradiation time and temperature during postsynthetic treatment showed influence on the oxidation activity of the catalyst. The H2O2 coupled microwave irradiation generated mesoporosity in the microporous TS-1 crystals and improved its catalytic activity by the creation of external Ti species located on the TS-1 surface. Alkali or fluoride postsynthetic treated TS-1 catalysts also exhibited similar activity with that of H2O2 post-treated sample. In particular, the mesoporous TS-1 samples exhibited prominent shape selectivity in the oxidation of cyclododecene bulky molecule.

1. INTRODUCTION 1

There are two prominent approaches to generate the porosity in the zeolite structure. One is the direct synthesis, and the other approach is the postsynthetic treatment. In the direct synthetic approach, which is also classified as the bottomup method, the preparation of mesoporous zeolites was achieved by employing various soft or hard templates such as polymeric soft template,20 soft dual template,21 single-multifunctionalized template,22 and hard template.23 In addition, other approaches were also often used for the direct synthesis such as silanization,24,25 quasi-solid-state crystallization, and so on.26 On the contrary, the postsynthetic approach or nontemplating top-down method is another effective way to create mesoporous structure through additional reactions.27 Recently, the dissolution−recrystallization method as a mixed approach was also reported by various research groups.28,29 Among different approaches, it was realized from extensive investigations that microwave-assisted synthesis is one of the promising methods to make various materials including zeolites with defined structure and porosity. Microwave irradiation as energy source for the fabrication of zeolites has been utilized since its first report by Mobil in 1980s.30 Microwave heating in chemical reactions has its own advantages. In the zeolite synthetic system, microwave energy

2

Various kinds of titanium silicates, namely, Ti-MFI, Ti-BEA, Ti-MOR,3 Ti-MWW,4 Ti-MCM-41,5 and Ti-SBA-15 (ref 6), have been widely investigated and explored for different catalytic reactions in the last few decades. Catalytic oxidation over various titanosilicates has been carried out mainly using hydrogen peroxide (H2O2) as the oxidant under mild reaction conditions, since it is a benign and environmentally friendly oxidant.7,8 Therefore, these materials have drawn a lot of attention as green catalysts for sustainable chemistry and the environment. Especially, titanium silicalite-1 (TS-1) is one of the remarkable catalysts owing to unique characteristics of the Ti species in the MFI zeolite framework such as stability of Lewis acidic sites in an aqueous system, uniform pore structure, and hydrophobicity.9,10 Owing to these advantages, the TS-1 has been extensively applied for manufacturing of various kinds of oxygenates based on very prominent reactions in the chemical industry with H2O2, which include hydroxylation of benzene,11 epoxidation of olefins,12−17 and ammoximation of cyclohexanone.18,19 However, it is not possible to extend the same analogy for the oxidation of bulky molecules due to the diffusion limitations of the reactants caused by the small pore size of TS-1 (∼0.5−0.6 nm). Therefore, many research groups have shown a lot of interest in this subject and made some progress toward overcoming the aforesaid disadvantage.6,7,9 One of the best strategies identified was the introduction of mesoporous structure into the TS-1 crystallites. © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

November 2, 2017 February 13, 2018 February 19, 2018 February 19, 2018 DOI: 10.1021/acs.iecr.7b04556 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

SiO2:0.015TiO2:0.2TPA+:IPA:22.2H2O. The synthesis gel was transferred to 100 mL of a Teflon lined microwave vessel (XP1500). The vessel was moved into the microwave oven (CEM MARS-5) and heated to 438 K with 800 W microwave irradiation with increasing temperature and maintained at the same temperature for 90 min. After crystallization, the obtained solid was filtered, washed with deionized water, and dried at 353 K. Lastly, the as-synthesized sample was calcined to remove organic template at 823 K for 6 h in air atmosphere to obtain the parent TS-1. 2.2. Postsynthetic Treatment of TS-1. The calcined TS-1 sample synthesized in the previous step was used as the parent compound for further processing. Postsynthetic treatment with microwave radiation and H2O2 was carried out in a microwave reactor (CEM MARS-5) with 800 W power by varying the temperature (343 to 423 K) and irradiation time (15 to 60 min). The reaction vessel (XP-1500) was filled with 30 mL of H2O2 aqueous solution (34.5 wt %, Samchun) and 1 g of TS-1 sample and sealed. It was placed in the microwave reactor and irradiated with varying irradiation times and temperature. After the treatment, the resulting solid was filtered and washed with deionized water until free from H2O2. Finally, the obtained sample was dried at 373 K for 12 h and calcined at 823 K for 6 h in air atmosphere. For comparison, the parent sample was also treated with NaOH or NH4F according to the modified procedure described elsewhere.37 In brief, 1 g of parent TS-1 and 30 mL of 0.2 M NaOH (or 0.2 M NH4F) solution were mixed and heated at 338 K for 30 min followed by calcination at 823 K for 6 h in the air atmosphere. Notations used throughout this article for these prepared samples are NaOH_MTS-1 and NH4F_MTS-1, respectively. The list of catalysts synthesized with their synthetic conditions and the notations used to describe them are presented in Table 1.

could be introduced directly which facilitates rapid heating rate compared with the conventional hydrothermal methods.31 As a result, it is possible to achieve selective and fast crystallization of the zeolites in high yields.32 Further, the microwave energy effectively controls the crystal morphology and particle size33,34 and can lead to enhanced hydrophobicity in the zeolites.35 There are several reports on the application of microwave irradiation during the synthesis of various zeolites. For example, Pavel and Schmidt reported synthesis of hierarchical titanosilicate ETS-10 by microwave irradiation treatment for photodegradation of aromatic compounds,36 and Abelló and ́ prepared mesoporous MFI zeolite with microPérez-Ramirez wave-mediated desilication under alkaline conditions.37 Interestingly, the application of microwave irradiation in conjunction with NaOH solution accelerated the formation of intracrystalline mesoporosity in the commercial ZSM-5 zeolites compared to the standard conventional heating. This effect was attributed to an efficient transfer of thermal energy to the synthetic zeolite solution, thus enhancing the rate of silicon extraction. Hierarchical zeolites with mesoporous surface areas of around 230 m2g−1 with ∼10 nm pore diameter were also prepared within short exposure times (3−5 min) and preserving the crystallinity of the parent samples.37 However, the application of microwave irradiation in the postsynthetic treatment for the generation of mesoporous structure is relatively scarce in the literature. The present study was undertaken against the aforementioned background. In this investigation, microwave irradiation was proficiently explored for the synthesis and postsynthetic treatment of TS-1 in combination with H2O2 and applied for oxidation of cyclic olefins. In order to optimize the synthetic procedure of mesoporous TS-1, the influence of microwave irradiation temperature and exposure time was also thoroughly investigated. For the purpose of comparison, mesoporous TS-1 was also synthesized with alkali or fluoride treatment and evaluated for oxidation of cyclic olefins under identical conditions. All the synthesized catalysts were characterized by various techniques, namely, X-ray diffraction, transmission electron microscopy, UV−vis diffuse reflectance spectroscopy, FT-infrared spectroscopy, X-ray photoelectron spectroscopy, BET surface area, and BJH pore size distribution method. Finally, an attempt was made to correlate the catalytic activity of three different mesoporous TS-1 catalysts (postsynthetic treatment with H2O2, NaOH, and NH4F) with their characterization results.

Table 1. List of Prepared Samples and Treatment Conditions

2. EXPERIMENTAL SECTION 2.1. Synthesis of Titanium Silicalite-1 (TS-1). The TS-1 samples were synthesized by using tetraethyl orthosilicate (TEOS, Sigma-Aldrich), tetrabutyl orthotitanate (TBOT, TCI), tetrapropylammonium hydroxide (TPAOH, 20−25 wt % aqueous solution, TCI), isopropyl alcohol (IPA, J.T. Baker), and deionized water. All these commercial guaranteed reagents (GR grade) were used without further purification. The TS-1 was prepared following the same procedure described elsewhere.38 About 24.48 g of TEOS was mixed with an aqueous solution of TPAOH and stirred for 2 h. Around 0.65 g of TBOT was dissolved in 7.2 g of isopropyl alcohol and added to the above mixture solution. The resulting mixture was heated under stirring at 353 K for 1 h to remove the isopropyl alcohol from the gel. Finally, a small amount of water was added to compensate for the vaporized portion during the heating step. The molar composition of the synthesis gel was

a

catalyst

treatment

conc. (N)

TS-1 H2O2_MTS-1_A H2O2_MTS-1_B H2O2_MTS-1_C H2O2_MTS-1_D H2O2_MTS-1_E H2O2_MTS-1_E2 H2O2_MTS-1_E3 H2O2_MTS-1_E4 NaOH_MTS-1 NH4F_MTS-1

H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 NaOH NH4F

1.0a 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.2 0.2

time (min) temperature (K) 60 60 60 60 60 45 30 15 30 30

343 363 383 403 423 423 423 423 338 338

1.0 N H2O2 aqueous solution = 34.5 wt % H2O2 aqueous solution.

2.3. Characterization of Mesoporous TS-1. Powder Xray diffraction patterns of various synthesized samples were recorded using Rigaku Miniflex diffractometer with Cu Kα radiation (1.540 Å) at 30 kV and 15 mA in the 2θ range of 5− 40° with a 2θ step size of 0.02° and a step time of 2.4 s. Textural properties of the samples were estimated using N2 gas adsorption by Micromeritics ASAP 2020 surface area analyzer at liquid nitrogen temperature (77 K). Prior to the measurements, degassing of samples was carried out at 473 K for 4 h. The BET method was applied to calculate the total surface area, which was used for comparison. The t-plot method was used to B

DOI: 10.1021/acs.iecr.7b04556 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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other words, the H2O2_MTS-1_E sample exhibited the highest conversion of cyclohexene. Figure 2 shows the influence of

discriminate between micro- and mesoporosity. The mesopore size distribution was obtained by BJH model applied to the desorption branch of the isotherm. Transmission electron microscopy images were recorded with a JEOL JEM-3011 electron microscope operated at 200 kV and equipped with a CCD camera. The diffuse reflectance UV−vis spectrum with continuous N2 purging by using N2 gas generator was recorded on a Shimadzu SOLIDSPEC-3700 DUVUV−vis−NIR spectrophotometer employing BaSO4 as the standard. The Fourier transform infrared spectrum was recorded using KBr pellet technique on a Nicolet 6700 FT-IR spectrometer under N2 atmosphere by cooling down to 77 K using liquid nitrogen. Xray photoelectron spectroscopy measurements were performed on a Thermo Scientific K-Alpha equipped with a monochromatic aluminum X-ray source. 2.4. Catalytic Oxidation of Cyclic Olefins. Catalytic oxidation of various cyclic olefins [cyclohexene (Sigma-Aldrich, 99%), cyclooctene (TCI, 99%), and cyclododecene (TCI, 99%)] was carried out in EYELA ChemiStation Personal Organic Synthesizer at 343 K. Before the reaction, all catalysts were activated at 423 K for 2 h. First, the reaction mixture of 100 mg of catalyst, 20 mmol of cyclic olefins, and 10 mL of acetonitrile (Deajung, 99%) or ethanol (Deajung, 99%) as solvent was heated until the reactant temperature reached 343 K. After the temperature reached 343 K, 20 mmol of hydrogen peroxide (30 wt % aqueous solution, Sigma-Aldrich) was added dropwise to the reactor during 30 min. The molar composition of resulting mixture (H2O2:Cyclic olefin) was 1:1. The reaction was carried out during 5 h, and the reaction products were analyzed by Agilent 6890 gas chromatograph with flame ionization detector and HP-5 capillary column. Products were identified with the help of GC-MS.

Figure 2. Effect of microwave temperature on various samples for catalytic oxidation of cyclohexene. Sample notation details are in Table 1.

microwave treatment temperature on the cyclohexene oxidation. Since microwave irradiation time was optimized (60 min) in the previous experiment, the catalytic reaction was carried out over various samples at different temperatures. As can be noted from Figure 2, the cyclohexene conversion was also found to increase with increasing microwave heating temperature for H2O2 postsynthetic treatment. Similar to the reaction trend as observed from Figure 1, the cyclohexene conversion over the TS-1 sample that was treated at 434 K (H2O2_MTS-1_E) for 60 min also exhibited the highest oxidation activity. Interestingly, the catalytic performance of H2O2_MTS-1_E2 was dramatically increased compared to that of H2O2_MTS-1_E3 catalyst (Figure 1 and Table 1). This observation gives an impression that the H2O2 treatment time required should be at least over 30 min for effective postsynthesis modification. It is well-known in the literature that postsynthesis treatment of zeolites with alkaline solution generates hierarchical pore structure by selective removal of Si atoms, which facilitate suppression of diffusional limitations caused by zeolitic microporosity and result in an enhanced catalytic activity.39 Postsynthesis treatment with fluoride ions also improves the activity of catalysts by increasing the surface hydrophobicity.40 In a similar way, it was presumed that microwave-assisted H2O2 treatment should also facilitate an enhanced catalytic activity. To establish this concept, the cyclic olefin oxidation over microwave-assisted H2O2 post-treated TS-1 was carried out for comparison with NaOH and NH4F treated TS-1 samples which were already demonstrated in the earlier work.39 In addition, the catalytic activity of the TS-1 parent sample was also carried out to confirm the enhanced catalytic activity. In the previous paragraphs, it was demonstrated that the H2O2_MTS-1_E sample exhibits the highest cyclohexene conversion. Therefore, it was considered as an optimized catalyst for evaluation of other cyclic olefins oxidation, and for comparison with other equivalent catalysts. Therefore, the catalytic activity of all these samples was evaluated through the catalytic oxidation of bulky molecules such as cyclooctene and cyclododecene in addition to the cyclohexene. The catalytic reaction was carried out at 343 K for 5 h in a batch reactor as

3. RESULTS AND DISCUSSION 3.1. Catalytic Evaluation of Microwave Synthesized TS-1. In order to understand and optimize the H2O2 posttreatment influence on the parent TS-1, first, cyclohexene oxidation was carried out over various H2O2_MTS-1_423_y (y denotes microwave exposure time, Table 1) catalysts that were prepared with different microwave irradiation times at 423 K. As shown in Figure 1, the cyclohexene conversion increased linearly with the increase of microwave irradiation time. In

Figure 1. Effect of microwave irradiation time on various samples for catalytic oxidation of cyclohexene. Sample notation details are in Table 1. C


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sample for cyclododecene oxidation shows not only enhanced cyclododecene conversion but also increased selectivity toward cyclododecene oxide. The reason for different catalytic behavior of postsynthetic treated samples for bulky molecules oxidation is described in the later paragraphs. 3.2. Characterization of Microwave Synthesized TS-1. The XRD patterns of calcined TS-1 parent sample along with other post-treatment samples (H2O2, NaOH, and NH4F treated) are shown in Figure 3. The powder XRD patterns of

described in the Experimental Section by using H2O2 as the oxidant. Table 2 shows the conversion and product selectivity in the oxidation of various cyclic olefins over various prepared Table 2. Oxidation of Cyclohexene (A), Cyclooctene (B), and Cyclododecene (C) over Various Prepared Catalystsa (A) Cyclohexene Oxidationb product distribution (%) catalyst TS-1 H2O2_MTS-1_E NaOH_MTS-1 NH4F_MTS-1

conversion (%)

epoxidec

-diold

10.6 25.9 28.8 27.1 (B) Cyclooctene

59 36.2 26.8 67.4 22.4 68.1 28 67.7 Oxidationb

conversion (%)

epoxideg

-enone

-enolf

1.8 1.3 3.4 1.7

2.6 2.1 6.1 2.6

product distribution (%) catalyst TS-1 H2O2_MTS-1_E NaOH_MTS-1 NH4F_MTS-1

-enonh

-enoli

38.0 61.2 57.6 51.7

20.6 8.7 17.7 17.7

6.7 41.4 9.5 30.1 12.0 25.3 10.1 30.6 (C) Cyclododecene Oxidationb

product distribution (%) catalyst

conversion (%)

epoxidej

-enonk

-enoll

otherm

TS-1 H2O2_MTS-1_E NaOH_MTS-1 NH4F_MTS-1

1.4 2.6 3.7 6.1

37.1 47.8 49.7 44.0

57.8 44.0 29.8 31.1

5.1 8.2 2.6 7.1

15.3 15.9

Figure 3. X-ray diffraction patterns of TS-1 (a), H2O2_MTS-1_E (b), NaOH_MTS-1 (c), and NH4F_MTS-1 (d).

a

Reaction conditions: catalyst, 100 mg; acetonitrile (solvent), 10 mL; reactant, 20 mmol; cyclohexene:H2O2 = 1:1; temperature, 343 K; time, 5 h. bSample notations are in Table 1. cCyclohexene oxide. d1,2Cyclohexadiol. e2-Cyclohexene-1-one. f2-Cyclohexen-1-ol. gCyclooctene oxide. h2-Cyclooctene-1-one. i2-Cycloocten-1-ol. jCyclododecene oxide. k2-Cyclododecen-1-one. l2-Cyclododecen-1-ol. mOther = 1,2cyclododecadiol, 2-ethoxycylcododecanol.

various samples show the characteristic peaks of MFI structure without any impurities.38 In comparison to the TS-1 parent sample, the zeolitic structure of the postsynthetic treated samples also remained the same even after postsynthetic treatment with various soft and harsh reagents. The TEM images of these four different samples are shown in Figure 4. All samples show Hockey puck shaped morphology with a range of particulate sizes from 200 to 300 nm and the stacked fibrous morphology caused by microwave induced interaction between the TS-1 particles.38,43 In addition, the postsynthetic treated TS-1 catalysts exhibited randomly oriented mesopore structure in the range of 2−10 nm resulted during microwaveassisted post-treatment. The pore size distribution of various samples was calculated by using desorption branch of N2 sorption isotherms by the BJH method. As shown in Figure 5, the pore size distribution measurements of all samples revealed relatively narrow pore size distribution of around 2−4 nm pore diameter. Figure 6 shows the N2 adsorption− desorption isotherms of various synthesized catalysts including microwave-assisted postsynthetic treatment sample. The isotherms of the parent TS-1 sample reveal typical microporous nature, whereas those of post-treatment samples exhibit hysteresis loops at above P/P0 = 0.4. Occurrence of hysteresis loops at less than P/P0 = 0.2 is caused by the “fluid-crystalline phase transition” due to the adsorption phase change by microporosity.44 As shown in Table 3, the textural properties of the postsynthetic treatment samples reveal an enhanced external surface area and external pore volume. Such increased external porosity is expected to promote the cyclic olefin oxidation activity. The total surface area (SBET) of posttreatment samples was found to decrease slightly, except for that of the NH4F_MTS-1 sample. It may be due to a slight

catalysts. As can be noted from Table 2, the conversion of cyclic olefins over postsynthesis treatment TS-1 catalysts is higher than that of TS-1 parent sample. It indicates that postsynthetic treatment by microwave irradiation promotes the catalytic activity. As shown in Table 2A, the conversion of cyclohexene was found to increase up to 25.9% over the H2O2_MTS-1_E sample in comparison to the parent TS-1. Further, the H2O2 treated sample exhibited somewhat similar activity with that of alkaline and fluoride treated (NaOH_MTS1 and NH4F_MTS-1) TS-1 samples. Interestingly, the product selectivity was found to change as a result of postsynthetic treatment. Selectivity of 1,2-cyclohexadiol was increased in the case of post-treated TS-1 samples (H 2 O 2 _MTS-1_E, NaOH_MTS-1, and NH4F_MTS-1), whereas the yield (conversion × selectivity) of cyclohexene oxide remained same or was slightly enhanced. This observation could be explained based on earlier reported literature that defect Ti or Si sites generated by the postsynthetic treatment facilitate the formation of Brønsted acid sites, which lead to the ring opening reaction of the cyclic olefins.41,42 As shown in Table 2B, the cyclooctene oxidation also followed a similar trend with that of cyclohexene oxidation. On the other hand, the catalytic results pertaining to the oxidation of cyclododecene exhibited different behavior in comparison to other substrates as described above (Table 2C). The catalytic activity of the H2O2_MTS-1_E D


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Figure 6. N2 sorption isotherms of TS-1 (black), H2O2_MTS-1_E (red), NaOH_MTS-1 (green), and NH4F_MTS-1 (blue) samples. Figure 4. Transmission electron microscopy (TEM) images of TS-1 (a), H2O2_MTS-1_E (b), NaOH_MTS-1 (c), and NH4F_MTS-1 (d) samples.

Table 3. Textural Properties of Parent TS-1 and Postsynthetic Treatment TS-1 Samples catalyst

SBETa (m2/g)

Smicrob (m2/g)

Sextb (m2/g)

Vtotalc (cm3/g)

Vextd (cm3/g)


440 400 377 451

435 299 294 316

5 101 83 135

0.23 0.21 0.22 0.22

0.02 0.10 0.11 0.10

a

Calculated by BET method. bCalculated by the t-plot method based on the BET surface area. cSingle-pointed total pore volume at P/P0 = 0.99. dVtotal − (Vmicro calculated by t-plot).

Figure 5. Pore size distribution measurements of TS-1 (a), H2O2_MTS-1_E (b), NaOH_MTS-1 (c), and NH4F_MTS-1 (d) samples (obtained by BJH method).

structural damage during the microwave-assisted post-treatment and harsh conditions, such as NaOH solution. Various spectroscopy techniques were also utilized to understand the nature of active Ti species after post-treatment with various reagents and microwave irradiation. The UV−vis spectroscopy results of various samples are shown in Figure 7. Generally, the transition metal species incorporated into the zeolite frameworks show “ligand to metal charge-transfer (LMCT)” transitions in the UV region.37 The Ti species incorporated in the TS-1 framework also showed the “LMCT”

Figure 7. UV−vis spectra of TS-1 (a), H2O2_MTS-1_E (b), NaOH_MTS-1 (c), and NH4F_MTS-1 (d) samples.

transitions in the range of 210−230 nm,45 which indicate the presence of tetrahedrally coordinated active Ti species in the zeolite framework. As can be noted from Figure 7, the UV−vis spectra of the TS-1 parent sample shows a band centered at 220 nm, which originates from the electronic transfer of the pi−pi E


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XPS analysis was also carried out to investigate the chemical state of Ti species and the surface Ti content in various samples. The XP spectra are shown in Figure 9 and the peak

transitions between the framework titanium species and the oxygen atoms.42 It is the characteristic of isolated tetrahedralcoordinated Ti4+ cations, indicating that no extra-framework anatase is present in the TS-1 samples. It means that Ti atoms are well-incorporated in the framework with the tetrahedral coordination state. On the other hand, “red shift” which means a portion of tetrahedral-coordinated Ti species, moved to the octahedral-coordinated or nonframework phase is apparent in the case of postsynthetic treatment samples. In addition, no absorption band at 350 nm indicates that nonframework TiO2 anatase phase is present. FTIR spectra of various samples were obtained to understand the degree of Ti incorporation in the framework. The spectra are shown in Figure 8, and the corresponding band intensity

Figure 9. XP spectra of TS-1 (a), H2O2_MTS-1_E (b), NaOH_MTS1 (c), and NH4F_MTS-1 (d) samples.

intensity ratios along with elemental composition are presented in Table 5. As shown in Table 5, the ratio of peak area 1 to the Table 5. Relative Peak Intensity Ratio and Surface Elemental Contents As Determined from XPS Measurements surface atomic %

Figure 8. FT-IR spectra of TS-1 (a), H2O2_MTS-1_E (b), NaOH_MTS-1 (c), and NH4F_MTS-1 (d) samples. a

TS-1

H2O2_MTS-1_E

NaOH_MTS-1

NH4F_MTS-1

1.21

1.07

1.02

1.32

Ap1/Ap2a

Si2p

Ti2p

Si/Ti


10.83 0.46 2.46 1.29

34.91 36.13 35.27 35.74

0.39 0.84 0.57 0.41

89.51 43.01 61.88 87.17

Calculated by eq 2.

peak area 2 (Ap1/Ap2) in the XP spectra (Figure 9) are calculated by eq 2: A p1 peak area of peak 1 framework Ti species = ≈ A p2 peak area of peak 2 extraframework Ti species

Table 4. Relative Intensity Ratio of the Band at 960 cm−1 to 800 cm−1 (I960/I800) I960/I800

catalyst

(2)

The surface Ti content of H2O2_MTS-1_E sample was the highest as presented in Table 5. The Si/Ti ratio of the postsynthetic treatment samples was lower than the TS-1 parent sample. Further, the XP spectra of other mesoporous TS-1 samples (NaOH_MTS-1 and NH4F_MTS-1) also show higher Ti contents than that of TS-1 parent sample. It was reported in the literature that peak 1 indicates the framework Ti species with the binding energy of Ti2p at around 460 eV, and peak 2 represents the extra framework Ti species with the binding energy of Ti2p at approximately 458 eV.47 As can be noted from Table 5, the Ap1/Ap2 ratio of all TS-1 samples was dramatically decreased after the postsynthetic treatment. This change could be due to the transformation of framework Ti species on the surface of the catalysts to extra framework Ti species during postsynthetic treatment. In addition, the Ti species inside the TS-1 particles could be exposed by postsynthetic treatment, which could then be changed to the extra framework species. Consequently, the extra framework Ti species located on the surface was increased after the

ratios are shown in Table 4. As can be noted from Figure 8, the characteristic band at 960 cm−1 which is assigned to the asymmetric stretching mode of Si−O−Ti groups was clearly observed even after the post-treatment. The relative intensity ratio of the characteristic band at 960 cm−1 to another band at 800 cm−1 (I960/I800) could be used as a criterion to estimate the Ti incorporation into the zeolite framework.46 As presented in Table 4, the relative intensity ratio for TS-1 parent sample and the postsynthetic treatment samples was calculated by eq 1: I960/I800 =

height of the peak at 960 cm−1 height of the peak at 800 cm−1

(1)

Compared to the TS-1 parent sample, the ratio was slightly decreased for all samples, except NH4F_MTS-1, which could be considered as an evidence of Ti extraction from the framework by microwave-assisted or other post-treatment method employed. F


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postsynthetic treatment, and it was expected to enhance the catalytic activity for cyclic olefins oxidation. Binding energy of the Ti2p peak in the case of the NH4F_MTS-1 sample was found to increase, in contrast to the previous reports where binding energy was decreased after fluoride treatment.48 The characterization results suggest an impression that some of the observed anomalies could be the critical factors for the experimental catalytic activity in the cyclic olefins oxidation. It is also known from the literature that the presence of titanyl groups, on which the H2O2 is activated by the formation of titanium peroxo complexes, is a prerequisite for oxygenation activity.41 Further, in order to afford high selectivities, the TS-1 should be free of impurities which cause acid catalyzed side reactions or H2O2 decomposition.41 On the whole, the present catalytic activity results on various TS-1 samples are in line with the earlier reported literature.

4. CONCLUSIONS Mesoporous titanium silicalite-1 was successfully synthesized by H2O2 mediated microwave-assisted postsynthetic treatment. The obtained mesoporous TS-1 showed enhanced catalytic activity for oxidation of various cyclic olefins with H2O2 as the oxidant. Both microwave irradiation time and temperature of irradiation play an important role during postsynthetic treatment to obtain the final active catalyst. As revealed by various characterization results, the H2O2 coupled microwave irradiation treatment generates mesoporosity in the microporous TS-1 crystals and improves its catalytic activity. The H2O2 mediated microwave-assisted postsynthetic treatment catalyst exhibits similar catalytic behavior with that of alkaline (NaOH) or fluoride (NH4F) postsynthetic treatment samples. Microwave-assisted postsynthetic treatment with H2O2 could be considered as a simplest approach to generate mesoporosity and enhanced catalytic activity in the TS-1 catalysts. Further, mesoporous TS-1 exhibits good shape selectivity in the oxidation of bulky cyclic olefins such as cyclododecene.

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AUTHOR INFORMATION

Corresponding Author

*(S.-E.P.) E-mail: separk@inha,ac.kr. Tel.: +82 32 860 6775. ORCID

Benjaram M. Reddy: 0000-0002-5451-7289 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by C1 Refinery Program through the National Research Foundation of Korea (NRF) funded by the ministry of Education (2016M3DA1A01913275). B.M.R. thanks Korea Federation of Science and Technology (KOFST) for the offer of Invitation Scientist position under the Brain Pool program.

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High-Performance Microwave Synthesized Mesoporous TS-1 Zeolite

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