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Selective oxidation of cyclopentene with H2O2 by using H3PW12O40 and TBAB as a phase transfer catalyst
Accepted Manuscript Selective oxidation of Cyclopentene with H2O2 by using H3PW12O40 and TBAB as a phase transfer catalyst
Yaling Luo, Changjun Liu, Hairong Yue, Siyang Tang, Yingming Zhu, Bin Liang PII: DOI: Reference:
S1004-9541(18)31072-3 https://doi.org/10.1016/j.cjche.2018.10.014 CJCHE 1301
To appear in:
Chinese Journal of Chemical Engineering
Received date: Revised date: Accepted date:
19 July 2018 11 October 2018 15 October 2018
Please cite this article as: Yaling Luo, Changjun Liu, Hairong Yue, Siyang Tang, Yingming Zhu, Bin Liang , Selective oxidation of Cyclopentene with H2O2 by using H3PW12O40 and TBAB as a phase transfer catalyst. Cjche (2018), https://doi.org/10.1016/ j.cjche.2018.10.014
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ACCEPTED MANUSCRIPT
Selective Oxidation of Cyclopentene with H2O2 by Using ☆
H3PW12O40 and TBAB as a Phase Transfer Catalyst
Yaling Luo1,2, Changjun Liu1, Hairong Yue1, Siyang Tang1, Yingming Zhu1, Bin Liang1* Low-carbon Technology and Chemical Reaction Engineering Laboratory,
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School of Chemical Engineering, Northwest Minzu University, Lanzhou 730000,
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School of Chemical Engineering, Sichuan University, Chengdu 610065, China
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China ☆
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Supported by the National Natural Science Foundation of China (21406146).
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*Corresponding author: [email protected]
ACCEPTED MANUSCRIPT Abstract The selective oxidation of cyclopentene by aqueous H2O2 using H3PW12O40 and tetrabutyl ammonium bromide (TBAB) as a phase transfer catalyst has been investigated. The results show that the presence of TBAB significantly improved the
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oxidation selectivity of cyclopentene. The effects of the reaction conditions on the conversion of cyclopentene were investigated in detail. The optimal reaction
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conditions are as follows: the H3PW12O40 to TBAB molar ratio, 1:1~1:3; H3PW12O40
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to cyclopentene molar ratio, 0.54:100~0.64:100; and molar ratio of H2O2 to
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cyclopentene, 1.6:1. The conversion reached 59.8% in 4 hours at 35.0 ℃, while the selectivity of glutaraldehyde was 38.0% and the selectivity of 1,2-cyclopentanediol
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was 55.6%. In addition, an route for oxidation of cyclopentene by aqueous H2O2 using a heteropoly acid and quaternary ammonium salt as a phase transfer catalyst was
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proposed.
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Keywords: Glutaraldehyde; Cyclopentene oxidation; Phase-transfer；Heteropoly acid
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ACCEPTED MANUSCRIPT 1. Introduction The chemicals 1,2-epoxycyclopentane (CPO), glutaraldehyde (GA) and 1,2-cyclopentanediol (1,2-diol) are high value-added chemicals in the polymer, fragrance, pharmaceutical, and agricultural industries [1]. Glutaraldehyde is
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particularly important in the medical and hygiene fields as an efficient and safe disinfectant [2-4]. Oxidation reactions are of fundamental importance in nature [5-7],
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such as cyclohexane [8] and toluene oxidations [9, 10]. The selective partial oxidation
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of cyclopentene (CPE) provides a potentially economic and environmentally benign
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production route for CPO, GA and 1,2-diol due to their wide availability from both the petrochemical and coke industries [2]. Glutaraldehyde can be synthesized from
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CPE by selective oxidation using tungstic acid and a heteropoly acid as homogeneous oxidation catalysts and hydrogen peroxide as an oxidant, giving a high oxidation
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activity under mild conditions [11-13]. However, solvents, such as tert-butyl alcohol
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[12], are normally needed for this reaction because of the immiscibility of substrate and oxidant, thus resulting in extra operational costs for solvent recovery and catalyst
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recycling.
Traditionally, the combination of one reagent with another which is initially
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located in queous/organic biphase is the phase transfer catalysts (PTCs) [14]. PTCs were investigated over nearly half a century and extensive work in which the phase of the catalysts change from the liquid phase to the solid phase in an organic reaction has been reported [15-17]. Polyoxometalates (POMs), macrocyclic polyethers (crown ethers), open chain polyethers (polyethylene glycols, glymes), poly (ionic liquid)s, ionic liquids (ILs) and so on are used as PTCs in the oxidation reaction [18-22]. In the
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ACCEPTED MANUSCRIPT selective oxidation reaction of CPE in the absence of solvent, the reaction occurs in the oil phase, and the oxidant H2O2 and catalytic intermediates transfer from the water phase to the oil phase. Using a phase-transfer agent can accelerate the mass transfer of both the oxidant and intermediates through the inter-phase. On the other hand, the products are water-soluble, and they are extracted from the organic phase
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continuously during the reaction, which may thermodynamically enhance the reaction
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[23, 24]. The aim of this work is to develop a liquid-liquid heterogeneous reaction
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process without the use of any solvent for the selective oxidation of CPE. Polyoxometalates (POMs) have been found to be efficient catalysts in the
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selective oxidation of alkanes, olefins, sulfides and alcohols by using hydrogen
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peroxide as an oxidant [7, 16, 17]. H3PMo12O40 and H3PW12O40 have been intensively investigated, and they are active in olefin oxidation to epoxides, alcohols and
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aldehydes [25-28]. In the presence of a phase-transfer catalyst, both H3PMo12O40 and
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H3PW12O40 showed high activities in the selective catalytic epoxidation of alkenes, in which the organic phase contained chloroform, 1,2-dichloroethane and olefin, while the aqueous phase contained H2O2 and H3PMo12O40 or H3PW12O40 [29]. H3PW12O40
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exhibited a higher activity than H3PMo12O40 in the epoxidation of CPE [27].
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Quaternary ammonium cations are often used as PTCs in many heterogeneous liquid-liquid reactions [30-32], and their phase transfer efficiencies depend on their carbon chains, both the length [31, 32] and structure [33, 34]. Even though Meshechkina reported that Katamin AB ([(CH3)2C6H5CH2NC14H29]Cl) and Adogen 464 ([CH3(C9H19)3N]Cl) showed higher activity than tetraethylammonium bromide ([(CH3CH2)4N]Br) and tetraethylammonium iodide [(CH3CH2)4N]I) [33], the anions of quaternary ammonium salts normally have less impact on the performance of PTCs 4
ACCEPTED MANUSCRIPT [32, 35]. In the epoxidation/oxidation of alkenes, such as cycloolefins (cyclohexene, cyclooctene [29] and CPE [36]), 1,7-octadiene [31], 1-octene [37] and 5-vinyl-2-norbornene [32], quaternary ammonium salts with C4-C18 alkyl groups are often used as phase-transfer catalysts. Shan et al. [38] reported the influence of the lipophilic capability on the PTC performance. Screening quaternary ammonium PTCs
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is the key for the heterogeneous oxidation.
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In this work, we focused on the selective oxidation of CPE with hydrogen
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peroxide by using tetrabutyl ammonium bromide (TBAB, (C4H9)4NBr) as a phase transfer catalyst and H3PMo12O40 and H3PW12O40 as oxidation catalysts. The
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heterogeneous liquid-liquid reaction was explored, and the reaction parameters, such
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as the molar ratio of H3PW12O40 to CPE, the molar ratio of H3PW12O40 to TBAB, and the molar ratio of H2O2 to CPE as well as the reaction temperature and reaction time,
2. Experimental
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2.1. Materials
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were systematically investigated and optimized.
Cyclopentene (CPE, 99%), hydrogen peroxide (30 wt%), phosphotungstic acid
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(H3PW12O40, 99.97%), 1,2-dichloroethane (DCE, CH2ClCH2Cl, 99%), tetrabutyl ammonium chloride [TBAC, (C4H9)4NСl, 98%], tetrabutyl ammonium bromide [TBAB, (C4H9)4NBr, 99%], and cetyltrimethyl ammonium bromide [CTMAB, (C16H33)N(CH3)3Br, 99%] were purchased from Chengdu Kelong Chemical Reagent (Chengdu, China). Phosphomolybdic acid (H3PMo12O40, 98%) was purchased from the Aladdin Industrial Corporation (Shanghai, China). Dimethyldioctadecyl
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ACCEPTED MANUSCRIPT ammonium bromide [DMDOAB, (C18H37)2(CH3)2NBr, 98%] was obtained from ScienMax Inc. (Shanghai, China). Cetylpyridinium chloride [CPC, C16H33NC5H5Cl, 98.5%] was purchased from the Tianjin Guangfu Institute of Superfine Chemical Industry (Tianjin, China). All of the chemicals were used as purchased without further
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purification. Solvents used in this study were strictly redistilled.
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2.2. Oxidation of cyclopentene
The selective oxidation of CPE was carried out in a 100 ml three-neck flask
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equipped with a magnetic stirrer and a reflux condenser. The reaction temperature was
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kept at 35.0±0.1 ℃ by a super thermostatic water bath. Typically, 1.0 g of H3PW12O40 (0.35 mmol) was dissolved in 12.2 g of aqueous H2O2 (30.0 wt%) (108
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mmol H2O2) at 35.0 ± 0.1 ℃ under stirring for 15 min. Then, 30 ml of 1,2-dichloroethane (DCE) along with a given amount of quaternary ammonium salt
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was fed into the reaction flask.
The reaction was then initiated by adding 3.7 g of CPE (54 mmol) into the
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system under vigorous stirring [26]. After reacting for a given time, the reaction mixture was cooled down to room temperature and settled to be separated as two
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phases. The products were identified by GC-MS (GCMS-QP2010 Plus, NIST08.LIB) and quantified by a gas chromatograph (FL GC 9790II equipped with a FID detector and a 30 m × 0.25 mm × 0.25 μm FFAP capillary column) using cyclohexanone as the internal standard. The objective products are CPO, 1,2-diol and GA, and their selectivities are calculated according to equations (1)-(3):
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ACCEPTED MANUSCRIPT Selectivity of CPO mol %
Selectivity of GA mol %
n(CPO)(mol) 100 (n(CPO)+n(GA)+n(1,2-diol))(mol)
(1)
n(GA)(mol) 100 (n(CPO)+n(GA)+n(1,2-diol))(mol)
(2)
n(1,2-diol)(mol) 100 (n(CPO)+n(GA)+n(1,2-diol))(mol)
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Selectivity of 1,2-diol mol %
(3)
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The conversion of CPE is calculated by equation (4), since CPO, 1,2-diol and
(n(CPO)+n(GA)+n(1,2-diol))(mol) 100 (4) n 0 (CPE)(mol)
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The conversion of CPE mol %
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GA were the only identified liquid products:
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where n0(CPE) is the initial amount of CPE (54 mmol) and n(CPO), n(1,2-diol)
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3. Results and Discussion
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and n(GA) are the amounts of CPO, 1,2-diol and GA, respectively.
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3.1 Catalytic activities of H3PW12O40 and H3PMo12O40
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The catalytic activities of H3PW12O40 and H3PMo12O40 were compared under identical reaction conditions using TBAC as the PTC. The results (Table 1) showed that both H3PW12O40 and H3PMo12O40 were active catalysts for the selective oxidation of CPE with aqueous H2O2. Only CPO, GA and 1,2-diol were identified in the oxidation products. The selectivities of CPO, GA and 1,2-diol on H3PW12O40 were 17.0%, 23.4% and 59.6%, respectively. The higher selectivity of 1,2-diol on H3PW12O40 may due to its higher acidity, which catalyzes the hydrolysis of CPO [11]. 7
ACCEPTED MANUSCRIPT The catalytic activity of H3PW12O40 was approximately 140% higher than that of H3PMo12O40 (Table 1). This observation is similar to that in the epoxidation of 1-octene [30]. Thus, H3PW12O40 is considered to be a better catalyst for the selective oxidation of CPE in the presence of a phase transfer catalyst. The catalytic
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performance of H3PW12O40 in the selective oxidation of CPE under phase transfer
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conditions is further studied thereafter.
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At the same time, the difference between TBAC and TBAB reactivity was also
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compared. The selectivity of CPO, GA, 1,2-diol were 14.4%, 37.1%, 48.5%, respectively, and the conversion of CPE was 37.7% in the H2O2/H3PW12O40/TBAB
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system. Obviously, the reactivity of TBAB is higher than that of TBAC (Table 1). Therefore, TBAB was choosed as phase transfer agent in the following study.
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Table 1 Catalytic activities of H3PW12O40 and H3PMo12O40a
Run
Catalyst
Conversion of CPE/%
CPO
GA
1,2-diol
H3PW12O40
17.0
23.4
59.6
22.4
2
H3PMo12O40
33.5
23.2
43.3
9.4
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1
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a
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Selectivity /%
Reaction conditions: Aqueous H2O2 (30wt%) was added dropwise over 35 min. CPE = 54
mmol, DCE = 30ml, n(CPE):n(H2O2) = 1:2, catalyst = 0.35 mmol, TBAC as the PTC, n(catalyst):n(PTC) = 1:3, reaction temperature = 35.0±0.1 ℃, reaction time = 4 h, and stirring speed = 960 rpm.
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ACCEPTED MANUSCRIPT 3.2 Effect of the H3PW12O40 loading
The effect of the H3PW12O40 loading was studied while keeping n(H3PW12O40): n(TBAB) = 1:3. The results (Fig. 1) showed that a higher H3PW12O40 loading leads to a higher CPE conversion rate and higher selectivity of GA. Increasing the ratio of
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H3PW12O40 to CPE from 0.0008:1 to 0.0054:1 led to an increase in the CPE
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conversion from 22.0% to 54.7% along with an increase in the GA selectivity from
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24.6% to 36.3%. However, further increasing the ratio of H3PW12O40 to CPE did not
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increase the conversion of CPE or selectivity of the product. The concentration of the catalyst complex in the aqueous phase was limited by the initial concentration of
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H3PW12O40. With a low H3PW12O40 ratio, less catalyst complex was transferred from the aqueous phase to the organic phase. Therefore, the conversion of CPE and
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selectivity of GA were low because the oxidation was mainly carried out in the
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organic phase [36]. With the increase of the amount of H3PW12O40, more peroxide complex transferred into the organic phase, and there it accelerated the reaction in the
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organic phase, resulting in increases in the conversion and GA selectivity. When the
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molar ratio of H3PW12O40 to CPE was 0.0054-0.0064, the complex concentration in the organic phase tended to be saturated and showed less effect in the reaction. Starks et al. [34] reviewed the epoxidation of olefins with H2O2, which was catalyzed by phase-transfer catalysts/sodium tungstate/phosphoric acid. They indicated that the effectiveness of epoxidation increased as the pH value in the aqueous phase decreased, although the high acidity caused hydrolysis of the epoxide product. However, the hydrolytic cleavage of the oxirane ring was significantly 9
ACCEPTED MANUSCRIPT prevented under acidic conditions because of phase separation. In our work, the generation of HBr during the reaction with increasing H3PW12O40 concentrations increased the acidity of the aqueous phase, and as a result, the selectivity of 1,2-diol significantly increased at a low H3PW12O40 ratio and gradually decreased at higher
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ratios. The hydrolysis of epoxide was retarded in the acidic aqueous phase. In our experiments, the CPO yield was obviously lower compared with those of
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GA and 1,2-diol. Meshechkina et al. [36] investigated the kinetics of the reaction
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between CPE and aqueous hydrogen peroxide in presence of a phase transfer catalyst, and Dai et al. [39] reported the oxidation process of CPE. It was found that CPO was
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possibly a main intermediate from which GA was formed via the further oxidation of
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CPO in the organic phase, and 1,2-diol was formed via the further reaction of CPO with water. Therefore, CPO extracted from the organic phase into aqueous phase
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formed 1,2-diol through a hydration reaction. Further oxidation of CPO in the organic
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phase formed GA. The low CPO selectivity indicates that the transfer of CPO between phases was fast, and it was rapidly hydrolyzed to form 1,2-diol [13]. On the other hand, the low CPO yield may be ascribed to that the CPE that was consumed in
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a stoichiometric amount with hydrogen peroxide.
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ACCEPTED MANUSCRIPT
60
40
Selectivity of CPO Selectivity of GA Selectivity of 1,2-diol Conversion of CPE
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0 0.000
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20
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Selectivity ( % )
40
0.004
0.008
Conversion of cyclopentene ( % )
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0 0.012
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molar ratio of H3PW12O 40/ CPE ( mol )
Fig. 1 Influence of the molar ratio of H3PW12O40 to CPE on oxidation of CPE
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Reaction conditions: CPE = 54 mmol, n(CPE):n(H2O2) = 1:2, TBAB as the PTC, n(H3PW12O40):n(PTC) = 1:3, DCE = 30 ml, reaction temperature = 35.0 ± 0.1 ℃, reaction time =
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4 h, and stirring speed = 960 rpm.
3.3 Effects of various quaternary ammonium salts
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In PTC promoted multiphase oxidation, quaternary ammonium salts are critical for the interphase transfer of “active oxygen”, which is generally in the form of
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peroxometalates [40], such as Q3[(PO4){W2O2(μ-O2)2(O2)2}2]. Therefore, the structure and chemical properties of the quaternary ammonium influence the distribution of such species as well as the distribution of certain products or intermediates. The promotion effects of CPC, CTMAB, DMDOAB and TBAB were investigated. The results are shown in Table 2. CPC led to an approximately 9% higher conversion of CPE than the other quaternary ammonium salts. The electron-donating ability of the
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ACCEPTED MANUSCRIPT groups are in the order of: CPC > CTMAB > DMDOAB ˃ TBAB, and the higher electron-donating effect may be the reason for the higher activity [41]. However, TBAB gave a slightly higher GA selectivity and an almost equal or lower CPO and 1,2-diol selectivity compared to the other salts. TBAB ([(C4H9)4N+]Br)
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has stronger interaction with tungsten species and transports sufficient active components into the organic phase, which enhanced the oxidation reaction of CPO in
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the organic phase. Its symmetrical and bulky structure also provide good
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anion-activation ability [34], which is important for the selectivity of the oxidation reaction in the organic phase. With the aim of maximizing the production of GA, the
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combination of TBAB and H3PW12O40 was further studied thereafter.
a
31.2 31.8 31.8 37.1
53.6 54.1 53.2 48.5
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15.2 14.1 15.0 14.4
CPO 1.3 2.2 2.4 3.1
GA
1,2diol
2.8 4.7 5.1 8.7 5.1 8.6 8.1 10.6
Wbo
Wbw
×102
×102
Wo/Ww ×10
(mol/L) (mol/L) (mol/L) 7.9 5.4 4.3 3.3
9.3 20.0 22.1 23.2
Conversion of CPE(%)
8.5 2.7 2.0 1.4
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CTMAB CPC DMDOAB TBAB
Products (mol)
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Selectivity (%) Quaternary Ammonium 1,2Salts CPO GA diol
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Table 2 Effect of quaternary ammonium salt on the oxidation of cyclopentenea
Reaction conditions: CPE = 54 mmol, n(CPE):n(H2O2) = 1:2, H3PW12O40 = 0.35 mmol,
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n(H3PW12O40): n(PTC) = 1:3, DCE = 30 ml, reaction temperature = 35.0 ± 0.1 ℃, reaction time = 4 h, and stirring speed = 960 rpm. Wo = the concentration of tungsten species in the organic phase; Ww = the concentration of tungsten species in the water phase. b
1.01 g of H3PW12O40 (0.35 mmol) was added to DCE (30 ml) within 15 min under stirring
at 35.0 ± 0.1 ℃. Then, 12.24 g of aqueous H2O2 (30 wt%) (108 mmol) along with a given amount of quaternary ammonium salt (1.05 mmol) were fed into the reaction vessel, and the mixture was stirred at 35.0 ± 0.1 ℃ for 2 h.
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36.8 45.9 37.2 37.7
ACCEPTED MANUSCRIPT 3.4 Effect of the H3PW12O40 to TBAB ratio Since H2O2 is immiscible in the organic phase, the reaction of CPE is slow and the oxidation conversion is low in the absence of TBAB (see Fig. 2). The CPE conversion was 14.8%, and the selectivities of CPO, GA and 1,2-diol were 20.6%,
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30.9%, 48.5% in 4 h, respectively. Fig. 2 also shows the oxidation conversions as well as the selectivities of CPO, GA and 1,2-diol in the presence of TBAB. When the
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molar ratio of H3PW12O40 to TBAB increased to 1:2, the conversion increased from
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14.8% to 54.2%. Further increasing TBAB did not increase the conversion. The selectivities of GA and 1,2-diol significantly increased in the presence of TBAB, but
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the selectivity of CPO decreased. The selectivities of CPO, GA and 1,2-diol did not
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change with further increasing the TBAB. TBAB, as a phase transfer catalyst, greatly increased the reaction activity due to its ability to transport oxidation species from the
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aqueous phase into the organic phase. On the other hand, the main reason is that only
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tungsten complexes, obtained from the reaction of phosphotungstic acid, TBAB, and H2O2, possesses high reactivity. It entered the organic phase and enhanced the oxidation of CPO in the organic phase. Therefore, the CPO was converted to GA. At
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the same time, CPO entered the aqueous phase by extraction, resulting in the
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hydrolysis of CPO, and it forms 1,2-diol in the aqueous phase. Using a fixed amount of H3PW12O40, the amount of synthesized active intermediate from the catalyst is a fixed value. An extra amount of TBAB did not increase the amount of the active intermediate. Thus, the molar ratio of H3PW12O40 to TBAB is suggested to be 1:1~1:3 for the oxidation of CPE.
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Selectivity of CPO Selectivity of GA Selectivity of 1,2-diol Conversion of CPE
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40
40
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Selectivity ( % )
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Conversion of cyclopentene ( % )
80
80
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0 0
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20
1:2
1:1
1:3
1:4
1:6
0
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molar ratio of H3PW12O40/TBAB ( mol )
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Fig. 2 Effect of H3PW12O40 to TBAB molar ratio Reaction conditions: CPE = 54 mmol, n(CPE):n(H2O2) = 1:2, H3PW12O40 = 0.35 mmol, TBAB as the PTC, DCE = 30 ml, reaction temperature = 35.0 ± 0.1 ℃, reaction time = 4 h, and stirring
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speed = 960 r·min-1.
3.5 Effect of the H2O2 to CPE ratio
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Fig. 3 shows the influence of the hydrogen peroxide concentration on the oxidative reaction results in the presence of H3PW12O40 and TBAB. When the molar
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ratio of H2O2:CPE was changed from 1 to 1.6, the conversion increased from 45.3% to 59.8%. The H2O2:CPE ratio did not show an obvious influence on the conversion when it was further increased. The selectivity of GA slightly decreased with the increasing H2O2 amount, while the selectivity of 1,2-diol increased. In the presence of the phase-transfer catalyst, the selectivity of CPO remained low and gradually increased with increases in the H2O2 amount. Thus, the optimum ratio of H2O2:CPE was 1.6:1. When the molar ratio of hydrogen peroxide to CPE increased, it indicates 14
ACCEPTED MANUSCRIPT that the aqueous phase increased [42] and more CPO was extracted, resulting in an increase of 1,2-diol by the hydrolysis of CPO, and a decrease of GA by reducing CPO oxidation in the organic phase.
80
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Selectivity of CPO Selectivity of GA Selectivity of 1,2-diol Conversion of CPE
60
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40
40
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Selectivity ( % )
60
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20
0 1.0:1
1.6:1
1.8:1
20
Conversion of cyclopentene ( % )
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0 2.0:1
2.2:1
3.0:1
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molar ratio of H2O2/CPE ( mol )
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Fig. 3 Influence of the molar ratio of H2O2 to CPE on oxidation of CPE Reaction conditions: CPE = 54 mmol, H3PW12O40 = 0.35 mmol, TBAB as the PTC, n(H3PW12O40):n(PTC) = 1:3, DCE = 30 ml, reaction temperature = 35.0 ± 0.1 ℃, reaction time =
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4 h, and stirring speed = 960 r·min-1.
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3.6 Effects of the reaction temperature and reaction time Fig. 4 illustrates the experimental results under different temperatures of 25.0, 30.0, 35.0 and 40.0 ℃. Temperature positively affected the reactivity. Since the oxidation of CPE is an exothermic reaction [39], the increase of the conversion with increasing temperature means that the reaction is far from chemical equilibrium. The selectivity of 1,2-diol slightly increased and the selectivity of GA slightly decreased 15
ACCEPTED MANUSCRIPT when the temperature increased from 25.0 to 40.0 ℃. The hydration of CPO is an endothermal reaction, and increasing the temperature shows positive effects. On the other hand, it shows a negative effect on the oxidation of CPO in the organic phase that forms GA as the product.
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Table 3 shows that the total yields of CPO, 1,2-diol and GA first increased from approximately 52% to approximately 60% as the reaction time increased from 2 hours
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to 4 hours, and then, the total yield decreased to approximately 53% (8 h). This
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suggests that certain degree of over oxidation took place as the reaction time increases. Therefore, the optimal reaction time is 4 h, which is shorter than that catalyzed by
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tungsten-substituted molybdophosphoric acids (the optimal reaction time is 8 h) [13].
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Hence, the optimal conditions are as follows: 5.4:1000~6.4:1000 for the molar ratio of H3PW12O40 to CPE; 1:1.6 for the molar ratio of CPE to H2O2; 1:1~1:3 for the
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molar ratio of H3PW12O40 to TBAB; 4 h for the reaction time and 35.0 ℃ for the
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temperature.
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ACCEPTED MANUSCRIPT
80
60
40
40
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Selectivity ( % )
60
20
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20
0 25.0
30.0
35.0
Conversion of cyclopentene ( % )
80
Selectivity of CPO Selectivity of GA Selectivity of 1,2-diol Conversion of CPE
0 40.0
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Temperatures ( ºC )
Fig. 4 Effect of temperatures on oxidation of CPE
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Reaction conditions: CPE = 54 mmol, H3PW12O40 = 0.35 mmol, TBAB as the PTC, n(H3PW12O40):n(PTC) = 1:3, n(CPE):n(H2O2) = 1:1.6, DCE = 30 ml, reaction time = 4 h, and
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stirring speed = 960 r·min-1.
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Table 3 Influence of reaction time on oxidation of CPEa Yield (%)
t (h)
GA
1,2-diol
2
4.2
19.3
28.4
4
4.0
22.5
33.3
8
4.3
19.4
29.6
a
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CE
CPO
Reaction conditions: CPE = 54 mmol , H3PW12O40 = 0.35 mmol, TBAB as the PTC,
n(H3PW12O40):n(PTC) = 1:3, n(CPE):n(H2O2) = 1:1.6, DCE = 30 ml, reaction temperature = 35.0 ℃, and stirring speed = 960 r·min-1.
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ACCEPTED MANUSCRIPT 3.7 Discussion on the schematic reaction pathway The selective partial oxidation of CPE was conducted in a liquid-liquid reaction system by using a PTC and heteropoly acids as catalysts. As illustrated in Scheme 1, the reactant CPE was fed as an organic phase and hydrogen peroxide was fed in
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aqueous solution. Excess H2O2 reacted with [PM12O40]3- to form peroxo complexes that served as active components in this process [43]. The phase-transfer agent serves
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as an intermediator that complexes with peroxides and brings the “active oxygen” in
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the form of a peroxo group from the aqueous phase to the organic phase. Therefore, the peroxide complex reacts with CPE in the organic phase to form the intermediate
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CPO and further to form GA. The product GA is soluble in aqueous solution and was
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continuously extracted from the organic phase to the aqueous phase, thus promoting the forward reaction. The intermediate CPO, an oxo compound, can also be extracted
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into the aqueous phase, where CPO is oxidized into another product 1,2-diol [36, 38].
Scheme 1. Mechanism of CPE oxidation with H2O2 catalyzed by heteropolyacid in a biphasic system
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ACCEPTED MANUSCRIPT 4. Conclusions The combination of TBAB and H3PW12O40 was found to be an efficient phase transfer catalyst for the selective oxidation of CPE. GA and 1,2-diol were formed with high selectivity under mild reaction conditions. The optimal phase transfer catalytic
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oxidation conditions are: H3PW12O40 to TBAB molar ratios of 1:1~1:3, H3PW12O40 to CPE molar ratios of 0.0054:1~0.0064:1, and a hydrogen peroxide to CPE molar ratio
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of 1.6:1. At 35.0 ℃, the conversion reached 59.8%, and the selectivities of GA and
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1,2-diol reached 38.0% and 55.6%, respectively. A schematic reaction pathway in the
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oxidation of cyclopentene was discussed.
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Yaling Luo, Changjun Liu, Hairong Yue, Siyang Tang, Yingming Zhu, Bin Liang PII: DOI: Reference:
S1004-9541(18)31072-3 https://doi.org/10.1016/j.cjche.2018.10.014 CJCHE 1301
To appear in:
Chinese Journal of Chemical Engineering
Received date: Revised date: Accepted date:
19 July 2018 11 October 2018 15 October 2018
Please cite this article as: Yaling Luo, Changjun Liu, Hairong Yue, Siyang Tang, Yingming Zhu, Bin Liang , Selective oxidation of Cyclopentene with H2O2 by using H3PW12O40 and TBAB as a phase transfer catalyst. Cjche (2018), https://doi.org/10.1016/ j.cjche.2018.10.014
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ACCEPTED MANUSCRIPT
Selective Oxidation of Cyclopentene with H2O2 by Using ☆
H3PW12O40 and TBAB as a Phase Transfer Catalyst
Yaling Luo1,2, Changjun Liu1, Hairong Yue1, Siyang Tang1, Yingming Zhu1, Bin Liang1* Low-carbon Technology and Chemical Reaction Engineering Laboratory,
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1
School of Chemical Engineering, Northwest Minzu University, Lanzhou 730000,
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2
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School of Chemical Engineering, Sichuan University, Chengdu 610065, China
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China ☆
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Supported by the National Natural Science Foundation of China (21406146).
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*Corresponding author: [email protected]
ACCEPTED MANUSCRIPT Abstract The selective oxidation of cyclopentene by aqueous H2O2 using H3PW12O40 and tetrabutyl ammonium bromide (TBAB) as a phase transfer catalyst has been investigated. The results show that the presence of TBAB significantly improved the
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oxidation selectivity of cyclopentene. The effects of the reaction conditions on the conversion of cyclopentene were investigated in detail. The optimal reaction
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conditions are as follows: the H3PW12O40 to TBAB molar ratio, 1:1~1:3; H3PW12O40
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to cyclopentene molar ratio, 0.54:100~0.64:100; and molar ratio of H2O2 to
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cyclopentene, 1.6:1. The conversion reached 59.8% in 4 hours at 35.0 ℃, while the selectivity of glutaraldehyde was 38.0% and the selectivity of 1,2-cyclopentanediol
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was 55.6%. In addition, an route for oxidation of cyclopentene by aqueous H2O2 using a heteropoly acid and quaternary ammonium salt as a phase transfer catalyst was
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proposed.
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Keywords: Glutaraldehyde; Cyclopentene oxidation; Phase-transfer；Heteropoly acid
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ACCEPTED MANUSCRIPT 1. Introduction The chemicals 1,2-epoxycyclopentane (CPO), glutaraldehyde (GA) and 1,2-cyclopentanediol (1,2-diol) are high value-added chemicals in the polymer, fragrance, pharmaceutical, and agricultural industries [1]. Glutaraldehyde is
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particularly important in the medical and hygiene fields as an efficient and safe disinfectant [2-4]. Oxidation reactions are of fundamental importance in nature [5-7],
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such as cyclohexane [8] and toluene oxidations [9, 10]. The selective partial oxidation
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of cyclopentene (CPE) provides a potentially economic and environmentally benign
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production route for CPO, GA and 1,2-diol due to their wide availability from both the petrochemical and coke industries [2]. Glutaraldehyde can be synthesized from
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CPE by selective oxidation using tungstic acid and a heteropoly acid as homogeneous oxidation catalysts and hydrogen peroxide as an oxidant, giving a high oxidation
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activity under mild conditions [11-13]. However, solvents, such as tert-butyl alcohol
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[12], are normally needed for this reaction because of the immiscibility of substrate and oxidant, thus resulting in extra operational costs for solvent recovery and catalyst
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recycling.
Traditionally, the combination of one reagent with another which is initially
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located in queous/organic biphase is the phase transfer catalysts (PTCs) [14]. PTCs were investigated over nearly half a century and extensive work in which the phase of the catalysts change from the liquid phase to the solid phase in an organic reaction has been reported [15-17]. Polyoxometalates (POMs), macrocyclic polyethers (crown ethers), open chain polyethers (polyethylene glycols, glymes), poly (ionic liquid)s, ionic liquids (ILs) and so on are used as PTCs in the oxidation reaction [18-22]. In the
3
ACCEPTED MANUSCRIPT selective oxidation reaction of CPE in the absence of solvent, the reaction occurs in the oil phase, and the oxidant H2O2 and catalytic intermediates transfer from the water phase to the oil phase. Using a phase-transfer agent can accelerate the mass transfer of both the oxidant and intermediates through the inter-phase. On the other hand, the products are water-soluble, and they are extracted from the organic phase
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continuously during the reaction, which may thermodynamically enhance the reaction
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[23, 24]. The aim of this work is to develop a liquid-liquid heterogeneous reaction
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process without the use of any solvent for the selective oxidation of CPE. Polyoxometalates (POMs) have been found to be efficient catalysts in the
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selective oxidation of alkanes, olefins, sulfides and alcohols by using hydrogen
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peroxide as an oxidant [7, 16, 17]. H3PMo12O40 and H3PW12O40 have been intensively investigated, and they are active in olefin oxidation to epoxides, alcohols and
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aldehydes [25-28]. In the presence of a phase-transfer catalyst, both H3PMo12O40 and
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H3PW12O40 showed high activities in the selective catalytic epoxidation of alkenes, in which the organic phase contained chloroform, 1,2-dichloroethane and olefin, while the aqueous phase contained H2O2 and H3PMo12O40 or H3PW12O40 [29]. H3PW12O40
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exhibited a higher activity than H3PMo12O40 in the epoxidation of CPE [27].
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Quaternary ammonium cations are often used as PTCs in many heterogeneous liquid-liquid reactions [30-32], and their phase transfer efficiencies depend on their carbon chains, both the length [31, 32] and structure [33, 34]. Even though Meshechkina reported that Katamin AB ([(CH3)2C6H5CH2NC14H29]Cl) and Adogen 464 ([CH3(C9H19)3N]Cl) showed higher activity than tetraethylammonium bromide ([(CH3CH2)4N]Br) and tetraethylammonium iodide [(CH3CH2)4N]I) [33], the anions of quaternary ammonium salts normally have less impact on the performance of PTCs 4
ACCEPTED MANUSCRIPT [32, 35]. In the epoxidation/oxidation of alkenes, such as cycloolefins (cyclohexene, cyclooctene [29] and CPE [36]), 1,7-octadiene [31], 1-octene [37] and 5-vinyl-2-norbornene [32], quaternary ammonium salts with C4-C18 alkyl groups are often used as phase-transfer catalysts. Shan et al. [38] reported the influence of the lipophilic capability on the PTC performance. Screening quaternary ammonium PTCs
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is the key for the heterogeneous oxidation.
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In this work, we focused on the selective oxidation of CPE with hydrogen
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peroxide by using tetrabutyl ammonium bromide (TBAB, (C4H9)4NBr) as a phase transfer catalyst and H3PMo12O40 and H3PW12O40 as oxidation catalysts. The
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heterogeneous liquid-liquid reaction was explored, and the reaction parameters, such
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as the molar ratio of H3PW12O40 to CPE, the molar ratio of H3PW12O40 to TBAB, and the molar ratio of H2O2 to CPE as well as the reaction temperature and reaction time,
2. Experimental
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2.1. Materials
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were systematically investigated and optimized.
Cyclopentene (CPE, 99%), hydrogen peroxide (30 wt%), phosphotungstic acid
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(H3PW12O40, 99.97%), 1,2-dichloroethane (DCE, CH2ClCH2Cl, 99%), tetrabutyl ammonium chloride [TBAC, (C4H9)4NСl, 98%], tetrabutyl ammonium bromide [TBAB, (C4H9)4NBr, 99%], and cetyltrimethyl ammonium bromide [CTMAB, (C16H33)N(CH3)3Br, 99%] were purchased from Chengdu Kelong Chemical Reagent (Chengdu, China). Phosphomolybdic acid (H3PMo12O40, 98%) was purchased from the Aladdin Industrial Corporation (Shanghai, China). Dimethyldioctadecyl
5
ACCEPTED MANUSCRIPT ammonium bromide [DMDOAB, (C18H37)2(CH3)2NBr, 98%] was obtained from ScienMax Inc. (Shanghai, China). Cetylpyridinium chloride [CPC, C16H33NC5H5Cl, 98.5%] was purchased from the Tianjin Guangfu Institute of Superfine Chemical Industry (Tianjin, China). All of the chemicals were used as purchased without further
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purification. Solvents used in this study were strictly redistilled.
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2.2. Oxidation of cyclopentene
The selective oxidation of CPE was carried out in a 100 ml three-neck flask
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equipped with a magnetic stirrer and a reflux condenser. The reaction temperature was
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kept at 35.0±0.1 ℃ by a super thermostatic water bath. Typically, 1.0 g of H3PW12O40 (0.35 mmol) was dissolved in 12.2 g of aqueous H2O2 (30.0 wt%) (108
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mmol H2O2) at 35.0 ± 0.1 ℃ under stirring for 15 min. Then, 30 ml of 1,2-dichloroethane (DCE) along with a given amount of quaternary ammonium salt
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was fed into the reaction flask.
The reaction was then initiated by adding 3.7 g of CPE (54 mmol) into the
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system under vigorous stirring [26]. After reacting for a given time, the reaction mixture was cooled down to room temperature and settled to be separated as two
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phases. The products were identified by GC-MS (GCMS-QP2010 Plus, NIST08.LIB) and quantified by a gas chromatograph (FL GC 9790II equipped with a FID detector and a 30 m × 0.25 mm × 0.25 μm FFAP capillary column) using cyclohexanone as the internal standard. The objective products are CPO, 1,2-diol and GA, and their selectivities are calculated according to equations (1)-(3):
6
ACCEPTED MANUSCRIPT Selectivity of CPO mol %
Selectivity of GA mol %
n(CPO)(mol) 100 (n(CPO)+n(GA)+n(1,2-diol))(mol)
(1)
n(GA)(mol) 100 (n(CPO)+n(GA)+n(1,2-diol))(mol)
(2)
n(1,2-diol)(mol) 100 (n(CPO)+n(GA)+n(1,2-diol))(mol)
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Selectivity of 1,2-diol mol %
(3)
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The conversion of CPE is calculated by equation (4), since CPO, 1,2-diol and
(n(CPO)+n(GA)+n(1,2-diol))(mol) 100 (4) n 0 (CPE)(mol)
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The conversion of CPE mol %
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GA were the only identified liquid products:
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where n0(CPE) is the initial amount of CPE (54 mmol) and n(CPO), n(1,2-diol)
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3. Results and Discussion
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and n(GA) are the amounts of CPO, 1,2-diol and GA, respectively.
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3.1 Catalytic activities of H3PW12O40 and H3PMo12O40
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The catalytic activities of H3PW12O40 and H3PMo12O40 were compared under identical reaction conditions using TBAC as the PTC. The results (Table 1) showed that both H3PW12O40 and H3PMo12O40 were active catalysts for the selective oxidation of CPE with aqueous H2O2. Only CPO, GA and 1,2-diol were identified in the oxidation products. The selectivities of CPO, GA and 1,2-diol on H3PW12O40 were 17.0%, 23.4% and 59.6%, respectively. The higher selectivity of 1,2-diol on H3PW12O40 may due to its higher acidity, which catalyzes the hydrolysis of CPO [11]. 7
ACCEPTED MANUSCRIPT The catalytic activity of H3PW12O40 was approximately 140% higher than that of H3PMo12O40 (Table 1). This observation is similar to that in the epoxidation of 1-octene [30]. Thus, H3PW12O40 is considered to be a better catalyst for the selective oxidation of CPE in the presence of a phase transfer catalyst. The catalytic
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performance of H3PW12O40 in the selective oxidation of CPE under phase transfer
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conditions is further studied thereafter.
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At the same time, the difference between TBAC and TBAB reactivity was also
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compared. The selectivity of CPO, GA, 1,2-diol were 14.4%, 37.1%, 48.5%, respectively, and the conversion of CPE was 37.7% in the H2O2/H3PW12O40/TBAB
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system. Obviously, the reactivity of TBAB is higher than that of TBAC (Table 1). Therefore, TBAB was choosed as phase transfer agent in the following study.
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Table 1 Catalytic activities of H3PW12O40 and H3PMo12O40a
Run
Catalyst
Conversion of CPE/%
CPO
GA
1,2-diol
H3PW12O40
17.0
23.4
59.6
22.4
2
H3PMo12O40
33.5
23.2
43.3
9.4
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1
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a
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Selectivity /%
Reaction conditions: Aqueous H2O2 (30wt%) was added dropwise over 35 min. CPE = 54
mmol, DCE = 30ml, n(CPE):n(H2O2) = 1:2, catalyst = 0.35 mmol, TBAC as the PTC, n(catalyst):n(PTC) = 1:3, reaction temperature = 35.0±0.1 ℃, reaction time = 4 h, and stirring speed = 960 rpm.
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ACCEPTED MANUSCRIPT 3.2 Effect of the H3PW12O40 loading
The effect of the H3PW12O40 loading was studied while keeping n(H3PW12O40): n(TBAB) = 1:3. The results (Fig. 1) showed that a higher H3PW12O40 loading leads to a higher CPE conversion rate and higher selectivity of GA. Increasing the ratio of
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H3PW12O40 to CPE from 0.0008:1 to 0.0054:1 led to an increase in the CPE
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conversion from 22.0% to 54.7% along with an increase in the GA selectivity from
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24.6% to 36.3%. However, further increasing the ratio of H3PW12O40 to CPE did not
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increase the conversion of CPE or selectivity of the product. The concentration of the catalyst complex in the aqueous phase was limited by the initial concentration of
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H3PW12O40. With a low H3PW12O40 ratio, less catalyst complex was transferred from the aqueous phase to the organic phase. Therefore, the conversion of CPE and
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selectivity of GA were low because the oxidation was mainly carried out in the
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organic phase [36]. With the increase of the amount of H3PW12O40, more peroxide complex transferred into the organic phase, and there it accelerated the reaction in the
CE
organic phase, resulting in increases in the conversion and GA selectivity. When the
AC
molar ratio of H3PW12O40 to CPE was 0.0054-0.0064, the complex concentration in the organic phase tended to be saturated and showed less effect in the reaction. Starks et al. [34] reviewed the epoxidation of olefins with H2O2, which was catalyzed by phase-transfer catalysts/sodium tungstate/phosphoric acid. They indicated that the effectiveness of epoxidation increased as the pH value in the aqueous phase decreased, although the high acidity caused hydrolysis of the epoxide product. However, the hydrolytic cleavage of the oxirane ring was significantly 9
ACCEPTED MANUSCRIPT prevented under acidic conditions because of phase separation. In our work, the generation of HBr during the reaction with increasing H3PW12O40 concentrations increased the acidity of the aqueous phase, and as a result, the selectivity of 1,2-diol significantly increased at a low H3PW12O40 ratio and gradually decreased at higher
PT
ratios. The hydrolysis of epoxide was retarded in the acidic aqueous phase. In our experiments, the CPO yield was obviously lower compared with those of
RI
GA and 1,2-diol. Meshechkina et al. [36] investigated the kinetics of the reaction
SC
between CPE and aqueous hydrogen peroxide in presence of a phase transfer catalyst, and Dai et al. [39] reported the oxidation process of CPE. It was found that CPO was
NU
possibly a main intermediate from which GA was formed via the further oxidation of
MA
CPO in the organic phase, and 1,2-diol was formed via the further reaction of CPO with water. Therefore, CPO extracted from the organic phase into aqueous phase
D
formed 1,2-diol through a hydration reaction. Further oxidation of CPO in the organic
PT E
phase formed GA. The low CPO selectivity indicates that the transfer of CPO between phases was fast, and it was rapidly hydrolyzed to form 1,2-diol [13]. On the other hand, the low CPO yield may be ascribed to that the CPE that was consumed in
AC
CE
a stoichiometric amount with hydrogen peroxide.
10
ACCEPTED MANUSCRIPT
60
40
Selectivity of CPO Selectivity of GA Selectivity of 1,2-diol Conversion of CPE
20
0 0.000
SC
RI
20
PT
Selectivity ( % )
40
0.004
0.008
Conversion of cyclopentene ( % )
60
0 0.012
NU
molar ratio of H3PW12O 40/ CPE ( mol )
Fig. 1 Influence of the molar ratio of H3PW12O40 to CPE on oxidation of CPE
MA
Reaction conditions: CPE = 54 mmol, n(CPE):n(H2O2) = 1:2, TBAB as the PTC, n(H3PW12O40):n(PTC) = 1:3, DCE = 30 ml, reaction temperature = 35.0 ± 0.1 ℃, reaction time =
PT E
D
4 h, and stirring speed = 960 rpm.
3.3 Effects of various quaternary ammonium salts
CE
In PTC promoted multiphase oxidation, quaternary ammonium salts are critical for the interphase transfer of “active oxygen”, which is generally in the form of
AC
peroxometalates [40], such as Q3[(PO4){W2O2(μ-O2)2(O2)2}2]. Therefore, the structure and chemical properties of the quaternary ammonium influence the distribution of such species as well as the distribution of certain products or intermediates. The promotion effects of CPC, CTMAB, DMDOAB and TBAB were investigated. The results are shown in Table 2. CPC led to an approximately 9% higher conversion of CPE than the other quaternary ammonium salts. The electron-donating ability of the
11
ACCEPTED MANUSCRIPT groups are in the order of: CPC > CTMAB > DMDOAB ˃ TBAB, and the higher electron-donating effect may be the reason for the higher activity [41]. However, TBAB gave a slightly higher GA selectivity and an almost equal or lower CPO and 1,2-diol selectivity compared to the other salts. TBAB ([(C4H9)4N+]Br)
PT
has stronger interaction with tungsten species and transports sufficient active components into the organic phase, which enhanced the oxidation reaction of CPO in
RI
the organic phase. Its symmetrical and bulky structure also provide good
SC
anion-activation ability [34], which is important for the selectivity of the oxidation reaction in the organic phase. With the aim of maximizing the production of GA, the
NU
combination of TBAB and H3PW12O40 was further studied thereafter.
a
31.2 31.8 31.8 37.1
53.6 54.1 53.2 48.5
PT E
15.2 14.1 15.0 14.4
CPO 1.3 2.2 2.4 3.1
GA
1,2diol
2.8 4.7 5.1 8.7 5.1 8.6 8.1 10.6
Wbo
Wbw
×102
×102
Wo/Ww ×10
(mol/L) (mol/L) (mol/L) 7.9 5.4 4.3 3.3
9.3 20.0 22.1 23.2
Conversion of CPE(%)
8.5 2.7 2.0 1.4
CE
CTMAB CPC DMDOAB TBAB
Products (mol)
D
Selectivity (%) Quaternary Ammonium 1,2Salts CPO GA diol
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Table 2 Effect of quaternary ammonium salt on the oxidation of cyclopentenea
Reaction conditions: CPE = 54 mmol, n(CPE):n(H2O2) = 1:2, H3PW12O40 = 0.35 mmol,
AC
n(H3PW12O40): n(PTC) = 1:3, DCE = 30 ml, reaction temperature = 35.0 ± 0.1 ℃, reaction time = 4 h, and stirring speed = 960 rpm. Wo = the concentration of tungsten species in the organic phase; Ww = the concentration of tungsten species in the water phase. b
1.01 g of H3PW12O40 (0.35 mmol) was added to DCE (30 ml) within 15 min under stirring
at 35.0 ± 0.1 ℃. Then, 12.24 g of aqueous H2O2 (30 wt%) (108 mmol) along with a given amount of quaternary ammonium salt (1.05 mmol) were fed into the reaction vessel, and the mixture was stirred at 35.0 ± 0.1 ℃ for 2 h.
12
36.8 45.9 37.2 37.7
ACCEPTED MANUSCRIPT 3.4 Effect of the H3PW12O40 to TBAB ratio Since H2O2 is immiscible in the organic phase, the reaction of CPE is slow and the oxidation conversion is low in the absence of TBAB (see Fig. 2). The CPE conversion was 14.8%, and the selectivities of CPO, GA and 1,2-diol were 20.6%,
PT
30.9%, 48.5% in 4 h, respectively. Fig. 2 also shows the oxidation conversions as well as the selectivities of CPO, GA and 1,2-diol in the presence of TBAB. When the
RI
molar ratio of H3PW12O40 to TBAB increased to 1:2, the conversion increased from
SC
14.8% to 54.2%. Further increasing TBAB did not increase the conversion. The selectivities of GA and 1,2-diol significantly increased in the presence of TBAB, but
NU
the selectivity of CPO decreased. The selectivities of CPO, GA and 1,2-diol did not
MA
change with further increasing the TBAB. TBAB, as a phase transfer catalyst, greatly increased the reaction activity due to its ability to transport oxidation species from the
D
aqueous phase into the organic phase. On the other hand, the main reason is that only
PT E
tungsten complexes, obtained from the reaction of phosphotungstic acid, TBAB, and H2O2, possesses high reactivity. It entered the organic phase and enhanced the oxidation of CPO in the organic phase. Therefore, the CPO was converted to GA. At
CE
the same time, CPO entered the aqueous phase by extraction, resulting in the
AC
hydrolysis of CPO, and it forms 1,2-diol in the aqueous phase. Using a fixed amount of H3PW12O40, the amount of synthesized active intermediate from the catalyst is a fixed value. An extra amount of TBAB did not increase the amount of the active intermediate. Thus, the molar ratio of H3PW12O40 to TBAB is suggested to be 1:1~1:3 for the oxidation of CPE.
13
ACCEPTED MANUSCRIPT
Selectivity of CPO Selectivity of GA Selectivity of 1,2-diol Conversion of CPE
60
40
40
PT
Selectivity ( % )
60
Conversion of cyclopentene ( % )
80
80
20
0 0
SC
RI
20
1:2
1:1
1:3
1:4
1:6
0
NU
molar ratio of H3PW12O40/TBAB ( mol )
MA
Fig. 2 Effect of H3PW12O40 to TBAB molar ratio Reaction conditions: CPE = 54 mmol, n(CPE):n(H2O2) = 1:2, H3PW12O40 = 0.35 mmol, TBAB as the PTC, DCE = 30 ml, reaction temperature = 35.0 ± 0.1 ℃, reaction time = 4 h, and stirring
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D
speed = 960 r·min-1.
3.5 Effect of the H2O2 to CPE ratio
CE
Fig. 3 shows the influence of the hydrogen peroxide concentration on the oxidative reaction results in the presence of H3PW12O40 and TBAB. When the molar
AC
ratio of H2O2:CPE was changed from 1 to 1.6, the conversion increased from 45.3% to 59.8%. The H2O2:CPE ratio did not show an obvious influence on the conversion when it was further increased. The selectivity of GA slightly decreased with the increasing H2O2 amount, while the selectivity of 1,2-diol increased. In the presence of the phase-transfer catalyst, the selectivity of CPO remained low and gradually increased with increases in the H2O2 amount. Thus, the optimum ratio of H2O2:CPE was 1.6:1. When the molar ratio of hydrogen peroxide to CPE increased, it indicates 14
ACCEPTED MANUSCRIPT that the aqueous phase increased [42] and more CPO was extracted, resulting in an increase of 1,2-diol by the hydrolysis of CPO, and a decrease of GA by reducing CPO oxidation in the organic phase.
80
PT
Selectivity of CPO Selectivity of GA Selectivity of 1,2-diol Conversion of CPE
60
RI SC
40
40
NU
Selectivity ( % )
60
MA
20
0 1.0:1
1.6:1
1.8:1
20
Conversion of cyclopentene ( % )
80
0 2.0:1
2.2:1
3.0:1
D
molar ratio of H2O2/CPE ( mol )
PT E
Fig. 3 Influence of the molar ratio of H2O2 to CPE on oxidation of CPE Reaction conditions: CPE = 54 mmol, H3PW12O40 = 0.35 mmol, TBAB as the PTC, n(H3PW12O40):n(PTC) = 1:3, DCE = 30 ml, reaction temperature = 35.0 ± 0.1 ℃, reaction time =
CE
4 h, and stirring speed = 960 r·min-1.
AC
3.6 Effects of the reaction temperature and reaction time Fig. 4 illustrates the experimental results under different temperatures of 25.0, 30.0, 35.0 and 40.0 ℃. Temperature positively affected the reactivity. Since the oxidation of CPE is an exothermic reaction [39], the increase of the conversion with increasing temperature means that the reaction is far from chemical equilibrium. The selectivity of 1,2-diol slightly increased and the selectivity of GA slightly decreased 15
ACCEPTED MANUSCRIPT when the temperature increased from 25.0 to 40.0 ℃. The hydration of CPO is an endothermal reaction, and increasing the temperature shows positive effects. On the other hand, it shows a negative effect on the oxidation of CPO in the organic phase that forms GA as the product.
PT
Table 3 shows that the total yields of CPO, 1,2-diol and GA first increased from approximately 52% to approximately 60% as the reaction time increased from 2 hours
RI
to 4 hours, and then, the total yield decreased to approximately 53% (8 h). This
SC
suggests that certain degree of over oxidation took place as the reaction time increases. Therefore, the optimal reaction time is 4 h, which is shorter than that catalyzed by
NU
tungsten-substituted molybdophosphoric acids (the optimal reaction time is 8 h) [13].
MA
Hence, the optimal conditions are as follows: 5.4:1000~6.4:1000 for the molar ratio of H3PW12O40 to CPE; 1:1.6 for the molar ratio of CPE to H2O2; 1:1~1:3 for the
D
molar ratio of H3PW12O40 to TBAB; 4 h for the reaction time and 35.0 ℃ for the
AC
CE
PT E
temperature.
16
ACCEPTED MANUSCRIPT
80
60
40
40
PT
Selectivity ( % )
60
20
SC
RI
20
0 25.0
30.0
35.0
Conversion of cyclopentene ( % )
80
Selectivity of CPO Selectivity of GA Selectivity of 1,2-diol Conversion of CPE
0 40.0
NU
Temperatures ( ºC )
Fig. 4 Effect of temperatures on oxidation of CPE
MA
Reaction conditions: CPE = 54 mmol, H3PW12O40 = 0.35 mmol, TBAB as the PTC, n(H3PW12O40):n(PTC) = 1:3, n(CPE):n(H2O2) = 1:1.6, DCE = 30 ml, reaction time = 4 h, and
D
stirring speed = 960 r·min-1.
PT E
Table 3 Influence of reaction time on oxidation of CPEa Yield (%)
t (h)
GA
1,2-diol
2
4.2
19.3
28.4
4
4.0
22.5
33.3
8
4.3
19.4
29.6
a
AC
CE
CPO
Reaction conditions: CPE = 54 mmol , H3PW12O40 = 0.35 mmol, TBAB as the PTC,
n(H3PW12O40):n(PTC) = 1:3, n(CPE):n(H2O2) = 1:1.6, DCE = 30 ml, reaction temperature = 35.0 ℃, and stirring speed = 960 r·min-1.
17
ACCEPTED MANUSCRIPT 3.7 Discussion on the schematic reaction pathway The selective partial oxidation of CPE was conducted in a liquid-liquid reaction system by using a PTC and heteropoly acids as catalysts. As illustrated in Scheme 1, the reactant CPE was fed as an organic phase and hydrogen peroxide was fed in
PT
aqueous solution. Excess H2O2 reacted with [PM12O40]3- to form peroxo complexes that served as active components in this process [43]. The phase-transfer agent serves
RI
as an intermediator that complexes with peroxides and brings the “active oxygen” in
SC
the form of a peroxo group from the aqueous phase to the organic phase. Therefore, the peroxide complex reacts with CPE in the organic phase to form the intermediate
NU
CPO and further to form GA. The product GA is soluble in aqueous solution and was
MA
continuously extracted from the organic phase to the aqueous phase, thus promoting the forward reaction. The intermediate CPO, an oxo compound, can also be extracted
AC
CE
PT E
D
into the aqueous phase, where CPO is oxidized into another product 1,2-diol [36, 38].
Scheme 1. Mechanism of CPE oxidation with H2O2 catalyzed by heteropolyacid in a biphasic system
18
ACCEPTED MANUSCRIPT 4. Conclusions The combination of TBAB and H3PW12O40 was found to be an efficient phase transfer catalyst for the selective oxidation of CPE. GA and 1,2-diol were formed with high selectivity under mild reaction conditions. The optimal phase transfer catalytic
PT
oxidation conditions are: H3PW12O40 to TBAB molar ratios of 1:1~1:3, H3PW12O40 to CPE molar ratios of 0.0054:1~0.0064:1, and a hydrogen peroxide to CPE molar ratio
RI
of 1.6:1. At 35.0 ℃, the conversion reached 59.8%, and the selectivities of GA and
SC
1,2-diol reached 38.0% and 55.6%, respectively. A schematic reaction pathway in the
NU
oxidation of cyclopentene was discussed.
MA
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24