Efficient hydrolysis of cyclohexyl acetate to cyclohexanol catalyzed by

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Kinetics, Catalysis, and Reaction Engineering

Efficient hydrolysis of cyclohexyl acetate to cyclohexanol catalyzed by dual-SO3H functionalized heteropolyacid-based solid acids Yong Liu, Weihua Liu, Lin Wang, Miaojun Su, and Fujian Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00240 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Industrial & Engineering Chemistry Research

Efficient hydrolysis of cyclohexyl acetate to cyclohexanol catalyzed by dual-SO3H functionalized heteropolyacid-based solid acids Yong Liu *†, Weihua Liu †, Lin Wang#, Miaojun Su†, Fujian Liu*‡ †

Henan Key Laboratory of Polyoxometalate, College of Chemistry and Chemical

Engineering, Henan University, Kaifeng, 475004, PR China ‡

National Engineering Research Center of Chemical Fertilizer Catalyst (NERC-CFC),

School of Chemical Engineering, Fuzhou University, Fuzhou, 350002, PR China. #

Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan

430071, China. Corresponding Author * E-mail: [email protected], [email protected]. ABSTRACT We report here novel dual-SO3H functionalized heteropolyacid-based solid acids (HPA-ILs) with different Keggin heteropolyacids anion and investigate their catalytic applications in hydrolyze cyclohexyl acetate into cyclohexanol. Compared with a variety of the traditional acidic catalysts, [Bis-Bs-BDMAEE]HPMo12O40 displayed the higher catalytic acidities and good repeated utilization. The effect of various parameters including reaction temperatures, catalysts dosage, initial reactants volume ratio, and reaction time on the hydrolysis of cyclohexyl acetate was studied in detail. The optimal condition was obtained and the cyclohexyl acetate conversion reached up to 90.56 % with 94.86 % selectivity for cyclohexanol. More interestingly, the

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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reusability of [Bis-Bs-BDMAEE]HPMo12O40 was good because it was reused five times without a significant loss in catalytic activity. KEYWORDS:

Dual-SO3H

functionalization;

Heteropolyacids;

Solid

acids;

Hydrolysis; Reusability. 1. INTRODUCTION As an important chemical intermediate, cyclohexanol can synthesize adipic acid, hexamethylenediamine, caprolactam, and cyclohexanone. It has been widely used in organic chemical, coating, and textile industry.1 Three main methods for producing cyclohexanol are cyclohexane oxidation,2,3 phenol hydrogenation,4 and cyclohexene direct hydration5,6. At present, the main industrial method for production of cyclohexanol is cyclohexane oxidation. However, this method shows serious disadvantages such as low selectivity, high energy consumption, and explosion hazards, which limits its development.7 In addition, the hydrogenation of phenol develop slowly due to the high cost of phenol and the large demand of hydrogen energy.8,9 Among of them, the production of cyclohexanol by direct hydration of cyclohexene is considered to be the most promising method because this process has some advantages of atom economy, and high selectivity. Unfortunately, cyclohexanol conversion of the direct hydration reaction is not satisfying because of the extremely poor miscibility of water and cyclohexene. The solubility of cyclohexene in water is only 0.02 % w/w at 298 K,10 which leads to very slow reaction rate. In order to overcome the defects of the method of cyclohexene direct hydration, the preparation of cyclohexnol from cyclohexene via cyclohexyl carboxylate has been proposed. This reaction proceeds via two steps. For the first step, the cyclohexyl carboxylate is formed by reaction of cyclohexene with the carboxylic acid, such as formic acid, acetic acid. In the second step, the cyclohexyl carboxylate is hydrolyzed 2


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to yield cyclohexanol. Steyer et al.11,12 reported the indirect hydration of cyclohexene to cyclohexanol from cyclohexyl formate using ion-exchange resins as catalysts in a column reactor. Li et al.13,14 studied the hydrolysis of cyclohexyl acetate to cyclohexanol catalyzed by the carbonaceous solid acids and the acid ionic liquids of 1-sulfobutyl-3-methyl-imidazolium

hydrogen

sulfate

([HSO3bmim]HSO4),

respectively. However, reaction temperature of 130 °C leads to the poor selectivity and wastes energy.

Scheme 1. The reaction mechanism of cyclohexyl acetate hydrolysis.

The reaction mechanism of cyclohexyl acetate hydrolysis was shown in Scheme 1. Cyclohexyl acetate hydrolysis is a nucleophile substitution reaction. Firstly, the acidic catalysts protonate the ester carbonyl group of cyclohexyl acetate and make it more electrophilic. Secondly, it forms the tetrahedral intermediate by nucleophilic attack of the oxygen atom of water. Thirdly, cyclohexanol is obtained after the tetrahedral intermediate goes through proton transfer. Finally, acetic acid is obtained due to elimination of the proton. If the reaction temperature is too high, however, the pyrolysis of cyclohexyl acetate is more likely to occur. Cyclohexyl acetate pyrolysis is a β-elimination reaction that finally obtains cyclohexene and acetic acid. So it is

3



very necessary to investigate a kind of effective and environmentally friendly catalysts for the hydrolysis of cyclohexyl acetate to cyclohexanol. In recent years, functional ionic liquids (ILs), as a kind of clean solvents and new catalysts, have many advantages of low vapor pressure, remarkable solubility, and high chemical stability, which have been widely used in hydrolysis, esteriﬁcation and transesterification reaction14-19. So, ILs have received much attention by many scholars. However, several disadvantages of the ILs still limit their applications such as plenty of ILs needed as reaction medium, and the difficult recovery of ILs from the reaction media. On the other hand, Keggin structures heteropolyacids (HPAs), as multifunctional catalysts, are a kind of multi-core inorganic polymers with oxygen bridge, which show many good properties due to its unique cage structure. However, the low surface areas and the solubility of HPAs hinder their practical applications.20,21 To solve those problems, it is an effective way to improve their performance by immobilization of HPAs.22 However, the immobilization of HPAs also suffer from weak binding force and easy losing of active sites, which lead to low reaction rate.23,24 Due to attracted advantages of both ILs and heteropolyacids, heteropolyacid-based ionic liquids (HPA-ILs) catalysts have received wide attention, which show good catalytic activities, and could be easily revovered as solid catalysts. HPA-ILs have been applied to use as catalysts in esterification, transesterification, and oxidation,25-30 which show excellent catalytic performance. In our previous works, we have been synthesized a series of HPA-ILs, which have been applied in synthesis of diethylene glycol monobutyl ether acetate through esterification reaction and transesterification reaction of methyl acetate with isooctyl alcohol.31,32 We successfully developed a variety of dual-SO3H functioned HPA-ILs with different Keggin heteropolyacids anion in this work, which were used in cyclohexanol 4


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by hydrolysis of cyclohexyl acetate. The impact for various parameters of catalyst type, reaction temperature, catalyst dosage, initial reactant volume ratio, reaction time and reusability of catalysts were investigated in detail. The synthesized [Bis-Bs-BDMAEE]HPMo12O40 exhibit excellent activities and enhanced reusability in cyclohexanol by hydrolysis of cyclohexyl acetate in comparision with various reported acid catalysts. 2. EXPERIMENTAL SECTION 2.1. Materials. Phosphotungstic acid (H3PW12O40), n-butyl bromide (C4H9Br), silicotungstic acid (H4SiW12O40), and phosphomolybdic acid (H3PMo12O40) were obtained from Sinopharm Chemical Reagent, Shanghai, China. Bis(2-dimethylaminoethyl) ether (purity ≥ 98 %) was obtained from J&K Scientific Ltd., Beijing, China. 1,4-Butane sultone (purity ≥ 99 %) and 1-Methylimidazole (purity ≥ 99 %) were purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Other reagents were of analytical grade and were directly used without any further treatment. 2.2. Preparation and characterization of dual-SO3H functionalized HPA-ILs. Three dual-SO3H functioned HPA-ILs with different Keggin heteropolyacids anions were synthesized in this work (Scheme 2). The synthesis of HPA-ILs ([Bis-Bs-BDMAEE]HPMo12O40)

was

performed

as

follows:

0.01

mol

Bis(2-dimethylaminoethyl) ether and 0.02 mol 1,4-butane sultone were poured in a 50 mL round-bottom flask. Then, the mixtures were gradually heated to 60 °C under stirring, and the product is white solid. The unreacted components attached to white solid were removed by washing with ethyl acetate and ether for several times, and the production was vacuum dried at 80 °C for 24 h. After that, an aqueous solution of H3PMo12O40 (0.01 mol) was added, and the mixtures were agitated at 80 °C for 18 h. 5



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Water was removed by vacuum drying at 80 °C to give the product as a green solid. The

other

HPA-ILs

[Bis-Bs-BDMAEE]HPW12O40

and

[Bis-Bs-BDMAEE]H2SiW12O40 were also prepared according to the process above described for [Bis-Bs-BDMAEE]HPMo12O40. 1H NMR (400 MHz, D2O) δ: 1.70-1.82 (m, 4H), 1.88-2.04 (m, 4H), 2.91 (t, J=7.6 Hz, 4H), 3.20 (s, 12H), 3.40 (t, J=5.4 Hz, 4H), 3.69 (s, 4H), 4.09 (s, 4H).

Scheme 2. Structures of dual-SO3H functioned HPA-ILs

The dual-SO3H functioned HPA-ILs solid acids were studied by using 1H nuclear magnetic resonance (NMR, Bruker DPX-400). Fourier transform infrared (FT-IR) spectroscopies were performed on Bruker Optics (Germany). Thermogravimetric analysis (TG)-differential scanning calorimetry was performed on Mettler Toledo (Switzerland). Elemental analysis (C, S, H, O, N) of the dual-SO3H functioned HPA-ILs were conducted with an elemental analyzer (Vario EL cube, Elemental Analysis System GmbH, Hanau, Germany). The acidity of the catalysts were characterized by using temperature programmed desorption of ammonia (NH3-TPD, Autochem II 2920, Micromeritics, USA). The solid-state

31

P NMR spectra of the

samples were performed with following steps: the samples were dehydrated into glass 6


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tubes under a vacuum condition before the sorption of trimethylphosphine oxide (TMPO) probe molecule. The treatment was kept at 453 K for 24 h, and the pressure was below 10-3 Pa. The TMPO adsorbed samples was prepared by using Zheng’s method

33

. Before the NMR experiments, the treated samples were put into NMR

rotors with a Kel-F end cap with the protection of dry nitrogen in a glove box.

31

P

NMR experiments were operated on Bruker Ascend-500 spectrometer with the resonance frequency of 202.34 MHz, where a 4 mm triple-resonance MAS probe at a spinning rate of 12 kHz was used. Pulse width (π/2) for 31P was 4.5 µs. With a recycle delay of 30 s, 31P MAS NMR spectra with high power proton decoupling were tested. The chemical shift of 31P resonance was recorded based on 1 M aqueous H3PO4. 2.3. Apparatus and Procedure. The hydrolysis of cyclohexyl acetate was conducted in a 50 mL round-bottom flask equipped with a magnetic stirrer and a reflux condenser. In a typical run, 2 mL cyclohexyl acetate and 6 mL H2O were placed into the reactor. After the reaction temperature increased to the set value, the catalysts were added to the reactor, and the mixtures were stirred 8 h. In each experiment, samples were taken out from the reactor at specific time intervals. Finally, in order to avoid any further reaction, the samples were cooled rapidly in the ice water bath, and then analyzed with gas chromatography (GC). 2.4. Analysis. The samples were analyzed using a gas chromatograph (Fuli, 9790) with capillary column DB-1 (30 m × 0.539 mm × 1.50 µm). The temperature of detector with a hydrogen flame ionization detector (FID) was set to be 250 °C using nitrogen as the carrier gas. The injector temperature was set to be 250 °C. The column temperature was set from 80 °C to 200 °C by means of program with heating rate 10 °C /min. 7



3. RESULTS AND DISCUSSION 3.1. Catalyst Characterizations Figure 1 shows FT-IR spectra (Vertex 70 FT-IR instrument in the 4000-450 cm-1 region) of the synthesized solid acid catalysts. Notably, these dual-SO3H functionalized HPA-ILs have the similar FT-IR spectra because of the same cationic unit. The bands at 3450 and 3014 cm-1 are assigned to the stretching vibration of O-H and C-H. The bands at 2927 and 2870 cm-1 are assigned to the stretching vibration of -CH2-. The absorption peaks of 1479 cm-1 is assigned to bending vibration of -CH2-. In addition, the absorption peaks at 1163 and 1043 cm-1 associated with the asymmetrical and symmetric stretching vibration of the –S=O to –SO3H, could also be observed. From Figure 2b, several characteristic peaks at the wave number of 1065 cm-1(P-Oa stretching vibration), 962 cm-1 (Mo-Ot stretching vibration), 869 cm-1 (Mo-Ob-Mo stretching vibration), and 786 cm-1 (Mo-Oc-Mo stretching vibration) were also found. By comparison of Figure 1a and b, [Bis-Bs-BDMAEE]HPMo12O40 has the same peaks, showing [Bis-Bs-BDMAEE]HPMo12O40 retains the Keggin structure of H3PMo12O40. Similarly, [Bis-Bs-BDMAEE]HPW12O40 has peaks at the wave numbers of 1079 (P-O stretching vibration), 978 (W=O stretching vibration), 893 (W-Ob-W stretching vibration) and 805 cm-1 (W-Oc-W stretching vibration), showing [Bis-Bs-BDMAEE]HPW12O40 retains the Keggin structure of H3PW12O40 and [Bis-Bs-BDMAEE]H2SiW12O40 has peaks at wave number of 1020 (Si-O stretching vibration), 981 (W=Ot stretching vibration), 926 (W-Oe-W stretching vibration) and 783 cm-1(W-Oc-W stretching vibration), showing [Bis-Bs-BDMAEE]H2SiW12O40 retains the Keggin structure of H4SiW12O40. The FT-IR spectra show that these dual-SO3H functioned HPA-ILs of Figure 1a, c and e retain both of the Keggin structure of heteropolyacid and the organic cation structure. 8


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As seen in Figure 2, TG curves of the synthesized acid catalysts were recorded. Notably, all of dual-SO3H functioned HPA-ILs are gradually decomposed. The weight loss of catalysts was caused by evaporation of water from them before 280 °C. The weight loss in the range from 280 to 800 °C was attributed to the decomposition of [Bis-Bs-BDMAEE]. The main weight loss associated with the decomposition of heteropolyacid anion was observed above 800 °C.

Figure 1. The FT-IR spectra of (a) [Bis-Bs-BDMAEE]HPMo12O40, (b) H3PMo12O40, (c) [Bis-Bs-BDMAEE]HPW12O40, (d) H3PW12O40, (e) [Bis-Bs-BDMAEE]H2SiW12O40 and (f) H4SiW12O40.

9



Figure 2. TG curves of (a) [Bis-Bs-BDMAEE]HPMo12O40, (b) [Bis-Bs-BDMAEE]HPW12O40 and (c) [Bis-Bs-BDMAEE]H2SiW12O40.

The other HPA-ILs were also characterized by 1H NMR, which was as follows: [Bsmim]3PMo12O40/[Bsmim]3PW12O40/[Bsmim]4SiW12O40: 1H NMR (400 MHz, DMSO) δ 9.13 (s, 2H), 7.74 (dt, J=27.0, 1.7 Hz, 2H), 4.19 (t, J=7.0 Hz, 2H), 3.86 (s, 3H), 2.54-2.43 (m, 2H), 1.95–1.79 (m, 2H), 1.54 (d, J=7.3 Hz, 2H). [Bmim]3PMo12O40/[Bmim]3PW12O40/[Bmim]4SiW12O40: 1H NMR (400 MHz, DMSO) δ 9.10 (s, 1H), 7.75 (dd, J=14.6, 13.0 Hz, 2H), 4.17 (t, J=7.2 Hz, 2H), 3.86 (s, 3H), 1.83–1.72 (m, 2H), 1.26 (dt, J=14.7, 7.4 Hz, 2H), 0.91 (t, J=7.4 Hz, 3H). [Mim]3PMo12O40/[Mim]3PW12O40/[Mim]3SiW12O40: 1H NMR (400 MHz, DMSO) δ 9.00 (s, 1H), 7.63 (s, 1H), 7.56 (s,1H), 3.92 (s, 3H). The results of elemental analysis are illustrated in Table 1. Notably, the measured contents of C, H, O, N, and S were in good agreement with the calculated values. Table 1 Elemental analysis of various catalysts 10


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Catalyst

[Bis-Bs-BDMAEE]HPMo12O40

[Bis-Bs-BDMAEE]HPW12O40

[Bis-Bs-BDMAEE]H2SiW12O40

Element

Measured value/ %

Calculated value/ %

C

9.87

8.51

H

2.68

1.74

O

38.28

33.30

N

1.35

1.24

S

3.18

2.84

C

6.53

5.80

H

1.80

1.19

O

21.68

22.70

N

0.94

0.85

S

2.08

1.94

C

5.91

5.80

H

1.63

1.19

O

22.92

22.71

N

0.85

0.85

S

1.95

1.94

The acidic performances of the synthesized catalysts are evaluated by NH3-TPD measurements. The desorption curves of ammonia are shown in Figure 3, which suggests relatively strong acid strength of [Bis-Bs-BDMAEE]X based solid acids. Various peaks were identified in the measurement range of 50-600 °C, corresponding to ammonia desorption from surface acid sites of different strength. There is a positive correlation between desorption temperature and acid strength of the synthesized catalysts. It can be seen that the order acid strength of catalysts is [Bis-Bs-BDMAEE]HPMo12O40

Efficient hydrolysis of cyclohexyl acetate to cyclohexanol catalyzed by

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