Epoxidation of methyl oleate and unsaturated FAMEs obtained from

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

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Epoxidation of Methyl Oleate and Unsaturated Fatty Acid Methyl Esters Obtained from Vegetable Source over Ti-Containing Silica Catalysts Yue Wei, Gang Li,* Qiang Lv, Chuanying Cheng, and Hongchen Guo State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

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S Supporting Information *

ABSTRACT: With different oxidants (aqueous H2O2 or cumene hydroperoxide), Ti-containing silica catalysts (hierarchical TS-1, TiMCM-41, and Ti-HMS) were applied as catalysts for the epoxidation of methyl oleate (MO) and vegetable-derived unsaturated fatty acid methyl esters (FAMEs). We found that the catalytic performances of titanium silicalites were highly dependent on oxidant types. For the epoxidation of MO over hierarchical TS-1 (HTS-1), aqueous H2O2 is a better oxidant compared with cumene hydroperoxide (CHP). Benefiting from good molecular accessibility and high activity of Ti sites, combined with high hydrophobicity, ultra-high-yield epoxides were obtained over HTS-1. In contrast, CHP was more favorable for Ti-HMS catalyst, which was beneficial to selective conversion of MO into epoxide under solvent-free conditions. The superior catalytic performance was ascribed to the high surface area and pore volume of Ti-HMS. Furthermore, the corresponding structure−catalytic performance relationships and reusability of various catalysts were discussed in consideration of the effect of oxidants.

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INTRODUCTION Benefiting from the inexhaustibility and low cost, renewable feedstocks have been regarded as a promising alternative to traditional fossil fuels affording the global economic and industrial demand.1 As the largest renewable platform, vegetable oils play significantly important role in chemical industry owing to their universal availability, low cost, natural biodegradability, and environmental friendliness.2 Currently, much emphasis has been placed on the production of biodiesel-fatty acid methyl esters (FAMEs). As a benign fuel, high-purity FAMEs have been produced and used extensively for various requirements.3 Meanwhile, epoxidized FAMEs also unlocked tremendous application possibilities, like employed as indispensable intermediates for production of plasticizers4 and stabilizers5 during the production of PVP, lubricants,6 coatings,7 polyurethane,8 cosmetics,9 and so on. On the basis of either pre- or in situ formed organic percarboxylic acids, commercialized epoxy fatty acid derivatives are mainly obtained via Prileshajev reaction. Carboxylic acids (typically formic and acetic acids) are employed as active oxygen carriers, meanwhile, H2O2 and strong mineral acids (H2SO4, HNO3, and H3PO3) are used as oxygen donor and catalysts, respectively.10,11 In this procedure, several remarkable drawbacks remain unsolved so far, such as low selectivity resulted from oxirane ring opening in acidic reaction medium, corrosion problems caused by acids, and intractable homogeneous catalysts separation after the reaction. Therefore, considerable efforts have been made to develop proper heterogeneous catalytic systems with more effective and © XXXX American Chemical Society

cleaner process for epoxidation of fatty acids and their esters. Recently, series of catalysts have been explored, including Ticontaining materials (Ti-silicas, Ti-MCM-41, nanosized HTS1),12−16 Nb-based materials,17,18 methyltrioxorhenium(VII) supported on niobia,19 acidic ion-exchange resin,20 alumina and alumina-supported group VI metal oxide,21 immobilized peroxycarboxylic acid,22 and immobilized Novozym 435.23 Among these, titanium-silica catalysts with regularly tetrahedrally coordinated Ti are currently the most promising options, due to their high efficiency, good versatility, and reusability. As the pioneer of titanium-silica catalysts, titanosilicate TS-1 is a highly selective and environmentally benign catalyst partnered with H2O2 as an oxidant for the selective oxidation of alkenes, alkanes, aromatics, alcohols, and other substrates.24 However, when applied for the oxidation of bulky molecules, the catalyst shows inferior performance due to the diffusion limitation imposed by its small pores. Exploring hierarchically porous zeolites with high crystallinity is an effective strategy to enhance catalytic performance via bottom-up (hard-/softtemplating method) and top-down (selective desilication in basic media) routes to create secondary pores.25−27 Although the top-down strategy can introduce secondary pores in the crystalline structure effectively,28 these aggressive methods often result in collapsed zeolitic skeleton and poorly controlled Received: Revised: Accepted: Published: A

August 28, 2018 November 3, 2018 November 8, 2018 November 8, 2018 DOI: 10.1021/acs.iecr.8b04155 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

nium hydroxide (TPAOH) and polyquaternium-6 were employed to create micropores and additional meso-/macropores, respectively.35 When using H2O2 as oxidant, the synthesized HTS-1 showed excellent catalytic activities for the oxidation of benzothiophene and also displayed a removal rate of 100% in thiophene oxidation within 2 h. Most recently, HTS-1 with proper SiO2/TiO2 mole ratio has been prepared by a similar method36 and demonstrated connective secondary pores, regularly isolated, tetrahedral Ti species, and high crystallinity. As a consequence, it showed excellent catalytic performance for the epoxidation of MO with aqueous H2O2 as a green oxidant. Specifically, as high as 100% conversion of MO coupled with 94.9% epoxide selectivity were achieved under the optimal conditions. Such results indicate the great potential of as-made HTS-1 for catalytic bulky alkenes epoxidation; thus, a clear understanding on the structure− performance relationship of HTS-1 remains an encouraging research focus. In this work, we report a systematic investigation into the effect of SiO2/TiO2 mole ratio and polyquaternium-6/SiO2 mass ratio on the structure−catalytic performance of HTS-1 for MO epoxidation. The catalytic properties of HTS-1 were also compared with that of other Ti-containing catalysts (conventional TS-1 (CTS-1), large-grained hierarchical TS-1 (L-HTS-1), mesoporous titanosilicates Ti-MCM-41, and TiHMS). Given the practicability, the effect of oxidants on the target catalyst performance was carefully evaluated with CHP and aqueous H2O2 under solvent or solvent-free conditions. In addition, the epoxidation of other unsaturated FAMEs were also studied with three different oils (cottonseed oil, castor oil, and soy bean oil), which were low-cost, easily available, and distinct from each other as the substrates. As a result, HTS-1 displayed high activity and selectivity for all substrates tested with H2O2 oxidant, while Ti-HMS was the best candidate for MO epoxidation with CHP as oxidant under solvent-free conditions. Otherwise, the reusability and stability of the catalysts using different oxidants were also presented. The exhibited research could pave the way for the development of other more efficient catalysts for vegetable oils FAMEs epoxidation.

mesopore volume/diameter since such pores are predominant cavities in the zeolite crystals instead of cylindrical pores connecting the external surface with the inner part of the crystals.29 Alternatively, HTS-1 obtained by hard templates (carbon nanotubes, carbon black, mesoporous carbon, carbon aerogels)30 and soft templates (surfactants, organosilane, cationic polymers)29 generally possesses highly active, isolated, four-coordinated Ti species, and well-defined morphology, exhibiting enhanced catalytic performance for heterogeneous oxidation reactions. Besides, amounts of mesoporous titanosilicates (Ti-MCM-41 and Ti-HMS) have been investigated as oxidation catalysts for bulky substrates. Such mesoporous structure with high external surface area is beneficial to the diffusion of bulky molecules and their related catalysis. These materials are vulnerable to the water environment, usually resulting in lower catalytic recyclability with aqueous H2O2 oxidant because of the serious catalyst degradation with the noncrystalline framework damage. However, in the case of tertbutyl- (TBHP) and cumene hydroperoxide (CHP) as oxidants under anhydrous conditions, the epoxide yields can come up to high level without leaching of titanium observed in successive reaction cycles.31,32 Přech et al.33 also found that more open structures of mesoporous titanosilicates brought higher yield using TBHP oxidant compared with TS-1. When TBHP instead of H2O2 is used as oxidant under water free conditions, the only driving parameter for epoxidation remains diffusion. Such results suggested that the attainment of optimal catalytic performance and reusability for different catalytic systems need to combine with proper oxidants. Generally, to comply with the green chemistry principles, the industrial processes are required to be not only efficient, selective, and high yielding but also environmentally benign. The utilization of nontoxic chemicals, renewable materials, less toxic or useful byproduct production and solvent-free conditions are the key issues in the green synthetic strategy. As for renewable FAMEs epoxidation, hydrogen peroxide and organic hydroperoxides (TBHP or CHP) are widely used oxidants. H2O2 is usually regarded as more economic and environmentally friendly. However, the use of H2O2 under aqueous conditions will inevitably bring side-reactions such as hydrolysis and nonideal catalyst stability. However, CHP oxidation under anhydrous conditions can effectively avoid the negative impact of water. Besides, from the perspective of green chemistry, CHP could also be considered since the good recyclability of their reduction product (alcohol). For example, an innovative process for PO production (CHPO) with CHP as the oxidant has been commercialized by Sumitomo Chemical Co. Ltd. successfully,34 which can avoid concomitant other organic substances through a two-step redox oxygen carrier (cumene) recycling, although in recent studies catalytic epoxidations of FAMEs have been reported using H2O2 or organic hydroperoxides over various Ti-containing silica catalysts. The synergic relationship of catalyst and oxidant types has not been studied systematically. In terms of solvent, up to now, majority of the unsaturated FAMEs epoxidation processes have been implemented with an organic solvent (e.g., acetonitrile, acetone, ethyl acetate, methanol, dichloromethane, chloroform, toluene, etc.).11−13,15−17 Therefore, diminishing or avoiding organic solvents utilization is one of the challenges of green chemistry to be taken into account. In our previous work, HTS-1 zeolites were synthesized by an effective template-assisted structure-directing strategy and used as catalysts for oxidative desulfurization. Tetrapropylammo-

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EXPERIMENTAL SECTION Catalyst Preparation. HTS-1 was synthesized according to the literature35 with polyquaternium-6 as mesopore template. The detailed procedure is shown in the Supporting Information. HTS-1 zeolites were prepared by varying SiO2/ TiO2 mole ratio and polyquaternium-6/SiO2 mass ratio, while all other conditions were same. The as-obtained materials were designed as HTS-1(a, b), where a and b indicate SiO2/TiO2 mole ratio and polyquaternium-6/SiO2 mass ratio, respectively. For contrast, the reference sample CTS-1 with the SiO2/TiO2 ratio of 50 was also synthesized following a similar procedure except without the addition of polyquaternium-6. Besides, to elucidate the effect of particle size of HTS-1 on the catalytic activity for the epoxidation of unsaturated FAMEs and the limitation of mass transfer on bulky FAMEs molecules, largegrained hierarchical TS-1 with sucrose-derived carbon materials as mesopore template was prepared according to the literature with slight modification,37 denoted as L-HTS-1. The other two reference samples, mesoporous titaniumcontaining silicates Ti-MCM-41 and Ti-HMS with the SiO2/ TiO2 mole ratio of 50, were prepared according to the literature reported previously.38,39 B

DOI: 10.1021/acs.iecr.8b04155 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Catalyst Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2400 diffractometer using Cu Kα radiation run at 40 kV and 100 mA. The electronic state of Ti in the titanosilicate samples was analyzed using a JASCO UV550 spectrometer. FT-IR spectra were recorded on a Bruker EQUINOX55 spectrometer. Solid state 29Si magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectra were measured on an Agilent DD2 500 MHz spectrometer. The titanium content of the samples was determined by elemental analysis via optical emission spectrometry with inductively coupled plasma (ICP-OES, PerkinElmer/Nex ION 300D). The crystal size and morphology were determined by a NOVA NanoSEM 450 scanning electron microscopy (SEM) from FEI Company. Transmission electron microscopy (TEM) images were taken on FEI Company Tecnai G2 20 Stwin instrument. N2 physical adsorption−desorption measurements were carried out at 77 K using a Quantachrome Autosorb-1MP after degassing of the sample under vacuum at 350 °C. The total specific surface area and mesopore pore-size distribution were calculated from adsorption data employing the Brunauer−Emmett−Teller (BET) method and Barrett−Joyner−Halenda (BJH) adsorption algorithm, respectively. Total pore volume was estimated from the amount of nitrogen adsorbed at the relative pressure of 0.99 and micropore volume was determined by the t-plot method. Catalytic Reactions. The catalytic reactions were carried out batchwise36 using acetonitrile as solvent and the detailed procedures are shown in Supporting Information. For comparison, epoxidation tests under solvent-free condition using low concentration MO (70 wt %, J&K) were also carried out following the same procedure. To study the effect of oxidant, an equivalent molar amount of CHP (85 wt %, Aladdin) instead of H2O2 was added to the reaction system, while the used oxidant in all other sections is H2O2. The product analyses method is the same as that in our previous work36 (Supporting Information). The unsaturated FAMEs mixtures obtained from vegetable oils (cottonseed oil, castor oil, and soy bean oil, respectively) were prepared by transesterification with methanol using NaOCH3 as catalyst. The iodine value (IV) and composition of each FAME mixture were determined by titration tests and GC, respectively. Epoxidation of FAMEs mixtures from three oils were carried out in a similar protocol as mentioned above.

Figure 1. XRD patterns of CTS-1 and HTS-1 with different SiO2/ TiO2 mole ratio and polyquaternium-6/SiO2 mass ratio.

Figure 2. UV−vis spectra of CTS-1 and HTS-1 with different polyquaternium-6/SiO2 mass ratio (a) and SiO2/TiO2 mole ratio (b).

formation of Ti−O−Ti bonds.40 The band intensity at 330 nm is low, which indicates the presence of trace amounts of anatase type Ti species. Apparently, the band at 210−220 nm indicates bathochromic shift and spreads out with the increase of polyquaternium-6/SiO2 mass ratio, indicating the increment of high coordinated nonframework Ti species. Additionally, the formation of distorted tetra-coordinated Ti species (OH)Ti(OSi)3 may account for this broadened band. However, with the increase of titanium content, both the framework Ti peaks and extra-framework Ti peaks become intense, suggesting the presence of higher amount of framework Ti and extraframework Ti species. The FT-IR spectra of HTS-1, Ti-MCM-41, and Ti-HMS are shown in Figure 3a. The broad peak around 3500 cm−1 should be attributed to the characteristic stretching band of hydrogen bonded hydroxyl group at defect sites, confirming the presence of numerous silanol groups on the external surface of TiMCM-41 and Ti-HMS. Thus, Ti-MCM-41 and Ti-HMS show higher hydrophilicity compared with that of HTS-1, which could significantly influence their catalytic epoxidation performances. Figure 3b exhibits the 29Si MAS NMR spectrum of HTS-1(50, 2/3); the strong signal at −115 ppm is associated with typical Si(OSi)4 (Q4) species. The absence of signal at −104 ppm in the spectrum, corresponding to the Si species functionalized with hydroxyl groups (Q3), proves that there are nearly no silanol groups in HTS-1(50, 2/3). The N2 adsorption−desorption isotherms of the obtained materials are shown in Figure. 4. It can be clearly seen that CTS-1 (Figure 4a) shows a type-I isotherms of microporous materials while that of HTS-1 (Figure 4a) is characteristic of

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RESULTS AND DISCUSSION Catalyst Characterization. The powder X-ray diffraction patterns of CTS-1 and a series of HTS-1 with different SiO2/ TiO2 mole ratio and polyquaternium-6/SiO2 mass ratio after calcination at 823 K are shown in Figure 1. It can be clearly seen that CTS-1 and all the HTS-1 exhibit characteristic reflections of MFI topology. The crystallinity levels of HTS1(50, 2/3) and HTS-1(33, 2/3) are analogous to that of CTS1, while the crystallinity of other HTS-1 samples is lower. The diffraction peak at 25.4° corresponding to anatase species shows a negligible intensity, suggesting a presence of trace amounts of anatase type Ti phases. The electronic state of Ti in the titanosilicate zeolites was characterized by a diffusereflectance (DR) UV−vis spectrometer (Figure 2). Compared with CTS-1, the obtained HTS-1 zeolites after calcination show absorption bands centered at 260−300 nm, which is related to hexacoordinated Ti species caused by partially polymerization of nonframework Ti species through the C


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Figure 3. FT-IR spectra of HTS-1(50, 2/3), Ti-HMS, and Ti-MCM-41 (a); 29Si MAS NMR spectrum of HTS-1(50, 2/3) (b).

Figure 4. N2 adsorption−desorption isotherms of CTS-1 and HTS-1 (50, 2/3) (a), Ti-HMS and Ti-MCM-41 (b); BJH pore size distributions of HTS-1 (50, 2/3) (a, inset), Ti-HMS and Ti-MCM-41 (b, inset).

Figure 5. SEM images of CTS-1 (a) and HTS-1 with different SiO2/TiO2 mole ratios and polyquaternium-6/SiO2 mass ratios: HTS-1(33, 2/3) (b); HTS-1(50, 1/3) (c); HTS-1(50, 2/3) (d); HTS-1(50, 4/3) (e); TEM images of HTS-1(50, 3/3) (f, g).

both type-I and type-IV, implying coexistence of intrinsic micropores and intracrystal mesopores created by removing polyquaternium-6 template. The N2 adsorption isotherms of reference samples Ti-HMS and Ti-MCM-41 are shown in Figure 4b; apparently Ti-MCM-41 and Ti-HMS give typical isotherm of type-IV. The pore size distribution curves show TiMCM-41 and Ti-HMS have uniform mesopores with the size centered at 3.2 and 2.3 nm, respectively (Figure 4b inset).

SEM images of synthetic CTS-1 and HTS-1 are displayed in Figure 5. Clearly, CTS-1 (Figure 5a) is uniformly spheroidic crystal with regular size around 200−300 nm. Samples HTS1(33, 2/3), HTS-1(50, 1/3), and HTS-1(50, 2/3) (Figure 5b−d) are composed of hexagonal or cake-like particles with a rough surface. It can be clearly seen that the dosage of polyquaternium-6 can influence the morphology of the materials significantly. With the increase of polyquaternium-6 D


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methyl 9,10-epoxy stearate (ME), several byproducts were formed, predominantly in consecutive reactions (Scheme 1). Interestingly, in all the reactions with solvent, hydrolysis products (diols) of fatty epoxides were barely observed in GC analyses, as proved by 1H NMR (absence of the peak at about 3.4 ppm for CH(OH), Figure S1). Such a phenomenon could be caused by the superhydrophobicity of the catalyst. Besides, peaks at 9−10 ppm in 1H NMR demonstrate the formation of aldehydes byproducts. In FT-IR spectra (Figure S2), the peak at 3020 cm−1 is ascribed to the CC double bond, which disappears after the epoxidation reaction, whereas the epoxy group peak at 820 cm−1 appears after epoxidation, confirming the formation of methyl epoxy stearate. In terms of SiO2/TiO2 mole ratio (Table 1, entry 1−3), HTS-1 with SiO2/TiO2 mole ratio of 50 displayed an excellent activity. Typically, the significantly important factor for the activity of Ti-containing materials is the Ti species coordination. On the basis of the XRD patterns and UV−vis spectra, the better catalytic behavior of HTS-1(50, 2/3) can be attributed to the more regularly isolated, tetrahedral Ti species and high crystallinity compared with those of HTS-1(33, 2/3) and HTS-1(100, 2/3). For HTS-1(33, 2/3), the increase of titanium content did not lead to a detectable improvement of catalytic activity. It is inferred that large amounts of extra framework Ti species in HTS-1(33, 2/3) can give rise to H2O2 unproductive decomposition and other side reactions. As for polyquaternium-6/SiO2 mass ratio (Table 1, entry 2, 4−6), HTS-1(50, 2/3) exhibited the optimal catalytic performance. Although more macro- and mesopores can be introduced in HTS-1 with the increase of polyquaternium-6 mass, this is at the price of reduced active Ti species, lowered crystallinity, and enlarged particle size (SEM, Figure 5), which is disadvantageous for the epoxidation of MO. For the catalysts obtained by using large amount of polyquaternium-6, more titanium species pertain to the distorted, tetrahedral Ti species and show poor activity. The slightly low crystallinity of HTS-1(50, 1/3) should account for its lower catalytic activity. Finally, HTS-1(50, 2/3) (HTS-1 for short) with the best catalytic activity was chosen as typical catalyst in the following study.

mass in the synthesis (Figure 5c−e), the morphology varies from hexagon or disc with blunt edge to angular rectangular rod. Besides, the surface of crystals becomes smooth and a larger particle size is obtained at higher polyquaternium-6/ SiO2 mass ratio. The intergrowth of crystal particles and the strong interaction between electropositive polyquaternium-6 and negatively charged aggregate silicate species can account for this cross-link effect, which results in the larger particles. Nevertheless, a large amount of polyquaternium-6 can suppress crystallization process and result in amorphous domain, which is consistent with the XRD results (Figure 1). From TEM images (Figure 5f,g), the presence of intracrystalline mesopore in the sample HTS-1(50, 3/3) can be demonstrated by the bright spots in rectangular morphology. Catalytic Tests. Effect of SiO2/TiO2 Mole Ratio and Polyquaternium-6/SiO2 Mass Ratio of HTS-1. HTS-1 prepared with different SiO2/TiO2 mole ratios and polyquaternium-6/SiO2 mass ratios were used as catalysts for epoxidation of MO (Table 1). Besides the major product Table 1. Effect of SiO2/TiO2 Mole Ratio and Polyquaternium-6/SiO2 Mass Ratio of HTS-1 on Catalytic Performance in the Epoxidation of MO entry

Si/Ti mole ratio

polyquaternium6/SiO2 mass ratio

XMO (%)

SME (%)

YME (%)

S1 (%)

S2 (%)

1 2 3 4 5 6

100 50 33 50 50 50

2/3 2/3 2/3 1/3 3/3 4/3

42.6 72.8 69.4 67.7 27.3 4.5

94.1 94.9 92.9 93.6 91.1 100

40.1 69.1 64.5 63.4 24.9 4.5

5.9 3.4 5.2 6.4 8.9 0

0 1.7 1.9 0 0 0

a Reaction conditions: 10 mg of HTS-1, 20 μL of MO (99 wt %), 10.5 μL of aqueous H2O2 (50 wt %), H2O2/double bond molar ratio = 2.9:1, CH3CN 5 mL, 9 h, 90 °C. S1: selectivity of methyl oxooctadecanoate byproduct, S2: selectivity of methyl 9-oxononanate byproduct.

Scheme 1. Proposed Reaction Scheme for the Epoxidation of MO with H2O2 over HTS-1

E


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Figure 6. Conversion of MO XMO, selectivity SME, and yield YME of epoxide in the epoxidation of MO over different catalysts in acetonitrile (a) and solvent-free conditions (b).(Reaction conditions: a: 10 mg of catalyst, 20 μL of MO (99 wt %), 5 mL of CH3CN; b: 50 mg of catalyst, 4.4 mL of MO (70 wt %), aqueous H2O2 (50 wt %), H2O2/double bond molar ratio = 2.9:1, 9 h, 90 °C).

Effect of the Different Ti-Containing Catalysts. Herein, the catalytic performance of the obtained HTS-1 for MO epoxidation was compared with mesoporous Ti-HMS and Ti-MCM-41. Moreover, L-HTS-1 was also chosen to determinate the effect of particle size. As shown in Figure 6a, HTS-1 exhibited higher catalytic activity compared with LHTS-1, which can be explained by the enhanced diffusion limitation endowed by long diffusion path in large-grained HTS-1. The conversion of MO was rather low when the reaction was performed over Ti-MCM-41 or Ti-HMS, although they possess large mesopore volume coupled with high specific surface area. Such results can be attributed to the following reasons. First, as evidenced by the peak at 3500 cm−1 in FT-IR spectra (Figure 3a), the presence of numerous silanol groups on the mesopore walls can cause H2O2 decomposition.41 Probably, hydrophilic character of these solids with Si−OH promotes surface hydrolysis of the Ti−O−Si bonds and then the aggregation of the Ti sites into TiO2 large domains, leading to the detrimental H2O2 decomposition.42 The hydrophility of Ti-MCM-41 and Ti-HMS could also decrease the adsorption of the organic substrate. We found that when Ti-HMS was modified by methyl trichlorosilane more epoxide could be obtained which mainly contributed to the improved hydrophobicity of the catalyst. Second, the less active sites in Ti-HMS and Ti-MCM-41 should also account for their poor catalytic performance partially. It has been reported that there are two kinds of Ti species: tetrapodal (SiO)4Ti and tripodal (SiO)3Ti(OH) structures.43 The former is predominant in TS-1 which can generate more active titanium A-type species for olefins epoxidation, while the latter is predominant in Ti-MCM-41. In order to deeply understand the distinction of the catalysts in MO epoxidation, further tests have been conducted under solvent-free conditions with 70 wt % MO as substrate (Figure 6b). However, under solvent-free conditions, Ti-MCM-41 and Ti-HMS exhibited comparable even higher conversion than HTS-1, but their selectivity was inferior to HTS-1. The possible mechanism for this is discussed in detail below. In the case of olefin epoxidation in liquid phase, both molecular accessibility and site activity are the key factors. Typically, catalytic site activity is closely related to the configuration of Ti site (tetrahedral) in zeolite framework, chemical nature of active species (Ti-OOH), the nature of the surface (hydrophobic/hydrophilic), and crystallinity of the zeolite, while molecular accessibility is usually

determined by channel architectures, viz., total pore volume, meso- or macropore diameter, and specific surface area. For the oxidation under solvent conditions, low concentration of substrate presents and only minor active sites are needed. Besides, the molar ratio of Ti active sites to substrate molecules in the test is high, which makes highly active sites more pivotal than the accessibility of active species. Thus, mesoporous molecular sieves Ti-MCM-41 and Ti-HMS with amorphous framework and the resulting low catalytic ability cannot catalyze this reaction effectively in spite of their high specific area and large pore volume. Guidotti et al.44 also reported that the morphology and texture of Ti-containing catalysts grafted on different silica supports only show negligible influence on the MO epoxidation. In solvent-free oxidation, a significant amount of bulky substrate presents in a biphase liquid reaction system, which makes the mass transfer limited, so it is the molecular accessibility rather than catalytic activity of Ti-sites that governs the reaction. Although Ti-MCM-41 and Ti-HMS with large mesopore volume could afford the diffusion of bulky MO to the active Ti species, the large dosage of water accompanied by H2O2 could enhance the ring-opening side reaction (Scheme 2) on acidic silanol groups of Ti-MCM-41 and Ti-HMS, and oxidative cleavage reaction could occur further, leading to a nonideal selectivity. Therefore, compared with Ti-MCM-41 and Ti-HMS, the outstanding catalytic performance of HTS-1 under both solvent and solvent-free conditions with H2O2 can be attributed to its high Ti site Scheme 2. Reaction Scheme of MO Epoxidation over Mesoporous Titanosilicates Ti-MCM-41 and Ti-HMS

F


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Industrial & Engineering Chemistry Research activity and outstanding hydrophobicity, but under solvent-free conditions, the hydrolysis side reaction catalyzed by the relatively strong acid sites in HTS-1 (evidenced by the desorption peak at about 385 °C in NH3-TPD spectrum, Figure S3) and the presence of high amount of water accompanied by H2O2 result in the dissatisfactory selectivity of epoxide. The epoxidation of MO over CTS-1 was also investigated and the results are shown in Figure 6. The conversion of MO over CTS-1 with acetonitrile as solvent (Figure 6a) was only slightly lower than that of HTS-1. In theory, MO can hardly diffuse into the channel of CTS-1 as the molecular dimension of MO with elongated quasi-linear shape is much larger than the micropore diameters of CTS-1. This anomalous phenomenon is ascribed to that the active sites on the external surface or near the pore mouth of CTS-1 can catalyze the epoxidation of low concentrated substrate under solvent conditions. Thus, it is the catalytically active Ti-sites rather than molecular accessibility that govern the reaction. Nevertheless, the catalytic performance of CTS-1 under solvent-free conditions (Figure 6b) decreased significantly, since the limited active sites on the external of catalyst are far from sufficient for the epoxidation of numerous MO molecules. Compared with HTS-1, the selectivity toward epoxide over CTS-1 under solvent-free conditions was superior. Such a result could be ascribed to the nearly absence of extra-framework Ti species (Figure 2a), which can catalyze H2O2 decomposition and ring opening secondary reaction.45 Above all, we draw a similar conclusion as Bregante et al.46 There are two orthogonal design criteria for the alkene epoxidation catalysts. The first one is great electron affinities (i.e., stronger Lewis acids) which could generate more electrophilic M−(O2) intermediates that accelerate alkene epoxidation. The second is the pore environment (i.e., pore diameter and hydrophobic/hydrophilic character) surrounding the active sites that could facilitate the interactions with the desired alkene and stabilize the transition state for epoxidation. Extrapolation to More Complex FAMEs Mixtures Derived from Vegetable Oils. From the practical viewpoint, the research into the epoxidation of unsaturated FAMEs mixtures obtained from vegetable sources is profoundly meaningful. First of all, to discern the effect of free fatty acid (FFA) in vegetable oils on the epoxidaion reaction, the epoxidation of MO with the addition of oleic acid in reaction system has been carried out (SI). We found that the feed of minor oleic acid can remarkably accelerate the reaction progress, which is of vital significance for the epoxidation of vegetable-oils-derived unsaturated FAMEs without purification of predeacidification. The epoxidation was carried out (Table. 2) using unsaturated FAMEs mixtures derived from vegetable oils (cottonseed oil, castor oil, and soy bean oil) as substrate. Under optimum conditions, the FAMEs mixtures of soy bean oil and cottonseed oil on HTS-1 exhibit similar reactivity, with conversion up to 95.1 and 96.2%, respectively. This can be ascribed to their analogical chemical constituents. In contrast, compared with pure MO, their conversion and selectivity are slightly low, which can be attributed to their complex composition.47 Moreover, methyl linoleate with two double bonds occupies a large proportion of cottonseed oil and soy bean oil FAMEs, which is harder to epoxidize than MO. Such a result is caused by the decreased electron density on the double bond owing to inductive effect of the extra double bond.48,49 When castor-oil-derived FAME mixtures mainly

Table 2. Epoxidation of the MO and FAMEs Mixtures Obtained from Various Vegetable Oils entry

substrate

conversion (%)

selectivity (%)

yield (%)

1 2 3 4 5

MOa soybean oil FAMEsb soybean oil FAMEsa cottonseed oil FAMEsa castor oil FAMEsa

100 65.4 95.1 96.2 87.1

94.9 76.0 89.4 91.8 78.7

94.9 49.7 85.0 88.3 68.5

a

Reaction conditions: 30 mg of HTS-1, aqueous H2O2 (50 wt %), H2O2/double bond molar ratio = 5.4:1, 110 °C. bReaction conditions: 10 mg of HTS-1, aqueous H2O2 (50 wt %), H2O2/ double bond molar ratio = 2.9:1, 90 °C; 20 μL of substrate, 5 mL of CH3CN, 9 h.

composed of methyl ricinoleate were used as substrate, the activity of HTS-1 was poorer than in the other two substrate systems (cottonseed oil and soy bean oil), which should result from the surface hydrophobicity of HTS-1 (methyl ricinoleate molecules are relatively hydrophilic). Effect of the Oxidant. With the aim to meet the requirements of green chemistry, high selectivity to the target product epoxides under solvent-free conditions should be achieved. As discussed earlier, the relatively low selectivity of epoxides could be caused by the disturbance of water added with aqueous H2O2. Herein, we conducted reactions using organic oxidant CHP to exclude water from the catalytic system. Under this reaction condition, Ti-HMS showed significantly high catalytic activity (Table 3, entry 1−3), Table 3. Epoxidation of MO with CHP over Various Titanosilicates without Solventa entry

cata

CHP/CC molar ratio

XMO (%)

SME (%)

YME (%)

TON

1 2 3 4 5

Ti-HMS Ti-HMS Ti-HMS HTS-1 HTS-1

2.9 1.5 1.0 2.9 1.5

77.2 78.9 71.0 46.2 17.9

82.8 85.5 93.0 21.7 46.2

63.9 67.4 66.0 10.0 8.3

646 660 595 481 186

a

Reaction conditions: 50 mg of catalyst, 4.4 mL of MO (70 wt.%), CHP (85 wt.%), 9 h, 90 °C. TON: moles of the substrate converted after 9 h of reaction/mol of Ti.

obtaining 71.0% MO conversion and 93.0% epoxides selectivity, respectively, even mole ratio of oxidant to substrate was as low as 1.0 mol mol−1. In contrast, the MO conversion and epoxy MO selectivity obtained over HTS-1 were much lower than those on mesoporous titanium-containing silicate Ti-HMS, which was distinct from the results achieved with H2O2. Unlike H2O2, the bulky molecular dimension of both substrate MO and oxidant CHP restricts the accessibility of active Ti sites. As a result, mesoporous titanium-silicate TiHMS with extremely high specific surface area (966 m2 g−1) and pore volume (0.57 cm3 g−1) can provide enough mesopore channels for the diffusion of reactants and oxidants, making the Ti active sites more available. As for HTS-1, the lack of ordered mesopore imposes steric hindrance for bulky molecules, particularly when large amount of bulky substrate presents under solvent-free conditions, resulting in the lower MO conversion. On the other hand, CHP can be decomposed into phenol and acetone under the catalytic system. We found that the introduction of acetone led to markedly decrease of G


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Industrial & Engineering Chemistry Research the MO conversion (Table S3, entry 1−3). Moreover, the decrease was more severe for HTS-1 than Ti-HMS system, confirming that the presence of acetone is more detrimental to MO epoxidation over HTS-1. This result is also consistent with the lower MO conversion using acetone solvent in Table S1. In the case of selectivity, HTS-1 showed very low selectivity for epoxide with CHP, while Ti-HMS demonstrated a very high selectivity, exhibiting completely opposite trend with that using H2O2 oxidant. As discussed above, for the hydrophilic amorphous framework, Ti-HMS showed an extremely low selectivity of epoxy MO with H2O2, which confirms that the hydrophilic environment of Ti species is unfavorable for epoxidation in the presence of water. However, when epoxidation is carried out with CHP under anhydrous media, the negative impact of water can be eliminated, and the sidereactions could be effectively restrained. However, the relatively low selectivity toward epoxide formation over HTS-1 may be attributed to the following reasons: (1) Compared with Ti-HMS that with disordered framework, HTS-1 with regularly isolated, tetrahedral Ti species possesses strong catalytic oxidizing activity, not only for epoxidation but also for other undesirable side-reactions such as oxidative cleavage and rearrangement. This speculation was further suggested by the results obtained under acetonitrile solvent conditions using 99 wt.% MO substrate (XMO = 3.3%, Sketone = 26.8%, Saldehyde = 16.5%). (2) The epoxidation reaction of MO oxidized by CHP could be related to the radicals chain steps. To provide experimental evidence for the generation of reactive radical species, selective radical quenching tests were carried out using p-benzoquinone for HO• radical traps (Table S3, entry 4). The presence of p-benzoquinone suppresses the reaction, which is a proof of the formation of HO• radicals in the system. The proposed possible reaction profiles are shown in Scheme 3. As reported previously by Clerici50 and Yoon,51 TS-1 can catalyze the hydroxylation of allylic alkanes characterized by the typical marks of radical mechanisms and Ti−O• radicals are speculated to be involved in the oxidation. Briefly, the CHP undergoes a thermally or catalytically driven deperoxidation or dehydroxyl reaction and creates reactive free radicals, which can initiate a radical chain reaction and yield ketone or diols byproducts, resulting in the remarkably low selectivity for epoxides over HTS-1. The effects of CHP/CC molar ratio were also investigated comprehensively for HTS-1 and Ti-HMS. It turned out that the MO conversion improved and selectivity for epoxy MO declined markedly varying CHP/ CC molar ratio from 1.5 to 2.9 for HTS-1 (Table 3, entries 4 and 5), similar to the result obtained using H2O2 as oxidant,36 whereas little variation of the values has been found over Ti-HMS catalyst in this process (Table 3, entries 1−3). Surprisingly, Ti-HMS afforded high yield (∼66%) for epoxide with stoichiometric amount of CHP and without solvent. Above all, the reaction without solvent over Ti-HMS gives a good result with TON value reaching up to 595 mol mol−1. It is in attractive from an industrial viewpoint and that of green chemistry, which seeks precisely to limit the use of environmentally unfriendly solvents such as acetonitrile. Moreover, if using CHP as the oxidant, then the byproduct cumyl alcohol could be easily converted to cumene, acting as the precursor of CHP, which can be fed back into the selective oxidation cycles. Through comparing the results for epoxidations by H2O2 and CHP over different catalysts, clearly, the oxidant choice strongly affects the performance of the catalyst. It can be

Scheme 3. Proposed Possible Radical Reaction Mechanism

concluded that HTS-1 is inclined to H2O2 while Ti-HMS prefers organic hydroperoxides CHP oxidizing agent in MO epoxidation. Titanium-Peroxo Intermediates. For alkene epoxidation, the main pathway proceeds over the isolated tetrahedral Ti sites to produce catalytically active hydroperoxo and peroxo intermediates, usually confirmed by DR UV−vis spectroscopy. HTS-1 and Ti-HMS samples were mixed with 50 wt % H2O2, heated at 90 °C for 1 h under stirring, and then dried under vacuum. The UV−vis spectra of the treated samples are shown in Figure S4. The materials exhibited an additional broad signal from 300 to 500 nm, assigned to Ti-OO(H) species. Meanwhile, the signals centered at 260 nm enhance significantly, especially for Ti-HMS system, indicating the interaction of H2O2 with the Ti centers to give penta- or pseudo-octahedral complexes. These strengthened bands are usually ascribed to (H2O)Ti(η1-OOH), (H2O)Ti(η2-OOH), and/or Ti(η2-O2) side-on species (Scheme 4).52,53 Externally, upon adding aqueous H2O2 to HTS-1 and Ti-HMS, an intense yellow color appeared immediately, and then the color disappeared after vacuum drying. Bonino et al.54 held the opinion that the interconversion between Ti(η1-OOH) and Ti(η2-O2) occurs in the TS-1/H2O2/H2O system, with the former hydroperoxo species being colorless and the latter peroxo species being yellow. It has been suggested that both the two Ti-(O2) species coexist in Ti-based materials and that the alkenes epoxidation generally proceeds with the Ti-OOH intermediate. Bregante et al.46 studied the isomeric product distributions of Z-stilbene epoxidation with H2O2 by in situ UV−vis experiments and came to the same conclusion. In all peroxide treated samples, the coordination of Ti-sites can be restored to tetrahedral by calcination accompanying the UV− vis spectra completely recovers. Recycling Tests. To better fulfill the green chemistry guideline, the reusability and regeneration of the catalysts were H


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activity and selectivity could be nearly completely recovered. All the results were consistent with the XRD and FT-IR analyses (Figure S5), suggesting its outstanding reusability. The recycling performances of the Ti-MCM-41 and Ti-HMS catalysts for MO epoxidation were also investigated (Figure 7b). Unlike HTS-1, the activity of Ti-MCM-41 and Ti-HMS decreased greatly in the second run. After calcination, the conversions of MO over Ti-MCM-41 and Ti-HMS did not increase to a large extent. The structure of Ti-MCM-41 or TiHMS was nearly destroyed completely after 1 run of reaction, indicating that the presence of water accompanying with H2O2 was detrimental to their structure. The durability of the Ti-HMS was also studied in MO epoxidation without solvent and oxidized by CHP. In the first 3 cycles of reuse, the used catalyst was collected and washed with ethanol and then subjected to the next run. As shown in Figure 7c, the MO conversion decreased gradually with increasing the number of recycles over Ti-HMS, which could be due to the substrates deposition on the active sites. When it was regenerated by calcination, the catalytic activity of the catalyst was restored, which implies that the leach of active titanium and framework collapse in anhydrous media are negligible. All the results correspond with the XRD and UV− vis analyses (Figure S6). Above results verify that catalysts exhibit different reusability when using different oxidants. In general, crystalline zeolitic framework of HTS-1 results in its superior stability over titanium-silicate mesoporous molecular sieves under H2O2 oxidized conditions. Nevertheless, meso-

Scheme 4. Active Ti Species

studied. The catalyst was separated from liquid mixtures by centrifugation and drying and then was used in next run. As shown in Figure 7a, MO conversion over HTS-1 with H2O2 dropped sharply after the second run, mainly caused by the organic residues (high-molecular-weight oligomer, condensed reactant, and fatty epoxides) blocking the pore.55 In contrast, the selectivity of epoxide changed slightly. By calcinating the catalyst in air at 823 K for 8 h after the fourth run, the initial

Figure 7. Recyclability tests of HTS-1 (a) (the catalyst was calcined before run 5) as well as Ti-MCM-41 and Ti-HMS (b) (the catalysts were calcined before the third run) with H2O2 combined with acetonitrile solvent and Ti-HMS using CHP as oxidant without solvent (c) in the epoxidation of MO. Reaction conditions: (a, b) 10 mg of catalyst, 20 μL of MO (99 wt %), 10.5 μL of aqueous H2O2 (50 wt %), 5 mL of CH3CN. (c) 50 mg of catalyst, 4.4 mL of MO (70 wt %), CHP (85 wt %), oxidant/double bond molar ratio = 2.9:1, 9 h, 90 °C. I


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porous Ti-containing silicate Ti-HMS also exhibits excellent reusability in the epoxidation of MO by CHP.

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CONCLUSIONS In summary, various Ti-containing silica zeolites (HTS-1, LHTS-1, Ti-MCM-41, and Ti-HMS) were synthesized and used as the catalysts in the epoxidation of MO and vegetablederived unsaturated FAMEs mixtures. The epoxidation reactions were inspected under both solvent and solvent-free conditions with either H2O2 or CHP as oxidant. The results suggested that the activity and selectivity of titanosilicates for the epoxidation were highly associated with the oxidants. Highefficiency epoxidation of FAMEs with aqueous H2O2 reckons on highly active and tetrahedrally coordinated Ti species in crystalline zeolite with superhydrophobic surface. In contrast, epoxidation with bulky CHP oxidant in anhydrous conditions is closely correlated to the interconnected pore structure, which affords the efficient diffusion of substrate and oxidant to the active sites. This work would provide important guidance for the construction of new catalysts as well as the selection of proper oxidant for the related reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b04155. Syntheses of the various titanosilicates; details of the catalytic reactions and products characterization; 1H NMR spectra, FT-IR spectra, NH3-TPD profile, XRD patterns, and UV−vis spectra of the samples; catalytic results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel. (Fax): +86-411-84986113. E-mail: [email protected]. cn. ORCID

Gang Li: 0000-0003-0741-8023 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from Evonik Industries AG and the Program for New Century Excellent Talents in University (NCET-04-0270).

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REFERENCES

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Epoxidation of methyl oleate and unsaturated FAMEs obtained from

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