A cleaner ammoximation of cyclohexanone over Ti-MWW in dimethyl carbonate

Journal Pre-proof A cleaner ammoximation of cyclohexanone over Ti-MWW in dimethyl carbonate

Shuxian Zhang, Huijuan Wei, Xin Gao, Mengtian Huang, Xiangyu Wang, Yiqiang Wen PII:

S1566-7367(20)30044-3

DOI:

https://doi.org/10.1016/j.catcom.2020.105968

Reference:

CATCOM 105968

To appear in:

Catalysis Communications

Received date:

6 September 2019

Revised date:

22 February 2020

Accepted date:

23 February 2020

Please cite this article as: S. Zhang, H. Wei, X. Gao, et al., A cleaner ammoximation of cyclohexanone over Ti-MWW in dimethyl carbonate, Catalysis Communications (2019), https://doi.org/10.1016/j.catcom.2020.105968

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© 2019 Published by Elsevier.

Journal Pre-proof

A cleaner ammoximation of cyclohexanone over Ti-MWW in dimethyl carbonate Shuxian Zhang, Huijuan Wei, Xin Gao, Mengtian Huang, Xiangyu Wang * , Yiqiang Wen* Green Catalys is Center, and College of Chemistry, Zhengzhou University, Zhengzhou 450001, P.R. China *

Corresponding author.

Abstract: In this paper, dimethyl carbonate was selected as the solvent for the cyclohexanone ammoximation over Ti-MWW. Under the optimized reaction conditions, high

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conversion of cyclohexanone (99.8%) and high selectivity to oxime (99.6%) were achieved. The results of reuse of the catalyst showed that the Ti-MWW in this

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reaction system had better stability. Though the deactivation of the catalyst occurred after being used five times, the selectivity to cyclohexanone oxime was still kept

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above 99.9%, and calcining could easily recover catalytic activity. This work may provide a more environmentally benign solution for the industry production of

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cyclohexanone oxime.

Keywords: Ti-MWW, Cyclohexanone, Cyclohexanone oxime, Ammoximation,

1. Introduction

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Dimethyl carbonate.

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Cyclohexanone oxime is an important intermediate for the production of

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ε-caprolactam [1, 2]. The conventional technology of oxime production includes the preparation of hydroxylamine salt [3] and the noncatalytic oximation of cyclohexanone, which are energy intensive [4], multi-step, and environmentally unfavourable [5]. In 1987, a single-step ammoximation process was developed by Enichem using cyclohexanone, NH3, and H2 O2 to synthesize cyclohexanone oxime with titanium silicate-1 (TS-1) catalysts [6]. The effect of organic solvents on the ammoximation of cyclohexanone was studied by Roffia [7] and Wang [8], and the results revealed that t-butanol is a suitable solvent. However, t-butanol is toxic [9], and toluene is used as the extractant to separate cyclohexanone oxime and water in the industrial process, which has environmental drawbacks and causes an increase of industrial costs [10]. Therefore, an urgent requirement is the development of a much cleaner process for heterogeneous cyclohexanone ammoximation. Luo et al. [11] successfully conducted a solvent- free liquid-liquid-solid three-phase reaction in a

1

Journal Pre-proof microreaction system containing two T-junction microreactors. Wu et al. [12] applied Ti-MWW to the liquid-phase ammoximation of cyclohexanone and found that it is quite effective in a clean solvent of water. Xing et al. [13] developed a novel dual- membrane airlift reactor for the cyclohexanone ammoximation over TS-1 without the addition of extra solvent. Dimethyl carbonate (DMC) is a highly oxygenated compound and a good solvent, and can be used as a high-efficiency reaction medium for oxidations of a wide range of substrates by using H2 O2 as the oxidant. Bernini et al. investigated the applicability of using DMC as a solvent for the oxidations of various organic compounds in H2 O2

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/methyltrioxorhenium catalytic oxidation system, and these oxidations proceeded with good conversions and in good yields, even at a lower temperature (room temperature~60) [14]. Kamata et al. also investigated the epoxidation of alkenes with

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H2 O 2 , and discovered that DMC was the best solvent [15]. Moreover, DMC has a

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polarity similar to that of cyclohexanone [16], which promotes the dissolution of cyclohexanone. Compared to t-butanol, which is used as a solvent in conventional

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ammoximation process, DMC is a more environmentally benign solvent due to its safety [17] and high biodegradability.

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Ti-silicalite is considered to be a zeolite catalyst that is very active in oxidation reactions due to the presence of tetrahedrally coordinated Ti (Ti (IV)) [18]. Moreover,

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in the ammoximation of ketones, H2 O2 first interacts with the Ti (IV) sites to form

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Ti-OOH species, then reacts with NH3 molecules that attack this active site to form NH2 OH, and finally NH2 OH is released from the Ti site and reacts with ketone to yield the oxime [19-21]. Ti-MWW is a zeolite of titanosilicate with the MWW structure composed of independent 10-MR channels, 12-MR supercages and side cups, and shows thin platelet crystals [12, 22-23]. Therefore, it possesses the large specific surface area, micropore and mesopore volume. It is well known that the mesopores in heterogeneous catalysts are beneficial to the catalytic reaction. The selectivity and activity of porous materials applied in catalytic reaction depend greatly on the diffusion properties [24], and microporous zeolites present severe limitations when large reactant molecules are involved, especially in liquid-phase systems [25]. Therefore, we investigated the feasibility of DMC and several other solvents for Ti-MWW in the cyclohexanone ammoximation. The effects of critical process parameters, such as temperature, concentrations of catalyst, and ratio of NH3 /cyclohexanone on the ammoximation were also studied. Afterward, the 2

Journal Pre-proof durability and reusability of Ti-MWW in DMC were further investigated. 2. Experimental Colloidal silica was purchased from Zhejiang Yuda Chemical Co., Ltd. Other materials were purchased from Sinopharm Chemical Reagent Co., Ltd. 2.1. Catalyst preparation 2.1.1. Synthesis of Ti-MWW The Ti-MWW zeolite was prepared according to the method in Ref [26] by using boric acid and piperidine (PI) as crystallization-supporting agent and structure directing agent (SDA), respectively. The synthetic gels with molar composition of 1.0

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SiO2 : 0.03 TiO2 : 1.4 PI: 0.67 B2 O 3 : 19 H2 O were hydrothermally crystallized at 443 K for 7 days. The obtained powder was refluxed in a 2 M HNO 3 aqueous solution to remove extra-framework Ti species and part of framework boron as well. Finally, the

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acid-washed product was filtered, washed with deionized water, dried at 393 K for 12

2.1.2. Synthesis of micron-sized TS-1

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h, and calcined at 823 K for 6 h.

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Micro-sized TS-1 was synthesized using the following method [27]. The precursor with molar composition of 1 SiO 2 : 0.03 TiO 2 : 0.1 TPABr: 0.5 ethanolamine: 30 H2 O

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was hydrothermally crystallized at 448 K for 3 days. The obtained powder was refluxed in a 2 M HNO 3 aqueous solution to remove extraframework Ti species.

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Finally, the acid-washed product was filtered, washed with deionized water, dried at

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393 K for 12 h, and calcined at 823 K for 6 h. The sample was denoted as MTS-1. The micron-sized hollow TS-1 was synthesized according to the following method [28]. One gram of the synthesized TS-1 powder was mixed with 7.2 mL ethanolamine and TPABr solution. After stirring at room temperature for 2 h, the mixture was transferred into a PTFE- lined autoclave and maintained at 443 K for 2 days with stirring. Subsequently, the product was refluxed in a 2 M HNO 3 aqueous solution, filtered, washed with deionized water, dried at 373 K overnight, and calcined at 823 K for 6 h. The sample was denoted as MHTS-1. 2.1.3. Synthesis of nano-sized TS-1 Nano-sized TS-1 was synthesized according to the following method [29]. The precursor with molar composition of 1 SiO 2 : 0.03 TiO2 : 0.1 TPAOH: 0.5 ethanolamine: 30 H2 O was hydrothermally crystallized at 448 K for 3 days. The obtained powder was refluxed in a 2 M HNO 3 aqueous solution to remove extraframework Ti species. Finally, the acid-washed product was filtered, washed with deionized water, dried at 3

Journal Pre-proof 393 K overnight, and calcined at 823 K for 6 h. The sample was denoted as NTS-1. The nano-sized hollow TS-1 was prepared according to the following method [30]. The NTS-1 sample was further modified in the solution consisting of TPAOH and NaOH at 448 K for 2 days. The product was refluxed in a 2 M HNO 3 aqueous solution, filtered, washed with distilled water, dried at 393 K, and calcined at 823 K for 6 h. The sample was denoted as NHTS-1. 2.2. Characterizations Powder X-ray diffraction (XRD) was performed on a Panalytical X’ pert PRO Diffractometer with Cu Kα (λ = 1.5406 Å). Fourier transform infrared (FT-IR) spectra

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were recorded in the region of 4000-400 cm−1 on a Nicolet Nexus 470 FT-IR spectrometer, and the samples to be measured were ground with KBr and pressed into thin wafers. Ultraviolet- visible diffuse reflectance (UV-vis) spectra were obtained on

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an Agilent Cary 5000 spectrometer from 190 to 800 nm, and pure BaSO 4 was spent as

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reference. The element compositions were determined by inductively coupled plasma emission spectrometry (ICP) on a Thermo IRIS Intrepid II atomic emission

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spectrometer. The N 2 sorption isotherms were measured on an ASAP 2420 surface area analyzer (Micromeritics, USA) at 77 K, and pore size distribution was calculated

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by using a desorption curve according to the Barrett-Joyner-Halenda (BJH) method.

microanalyzer.

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Scanning electron microscopy (SEM) images were obtained with a n S-4800 scanning

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2.3. Cyclohexanone ammoximation Cyclohexanone ammoximation was carried out in a three-necked flask (100 mL) equipped with the magnetic stirrer and condenser. In a typical run, cyclohexanone, solvent and catalyst were added into the flask. The mixture was heated to the desired temperature, and then the reaction was initiated b y adding aqueous H2 O2 (27.5 wt%) and aqueous ammonia (25 wt%) at a constant rate with two micropumps, respectively. After 1.5 h, the reaction was quenched by cooling, and the products were analyzed by using a GC-9790 plus gas chromatography equipped with a flame ionization detector (FID) and an OV-1701 capillary column (30 m × 0.32 mm × 0.25 μm). The byproducts were determined on GC-MS.

3. Results and discussion 3.1. Characterization of zeolites Fig. 1 shows the powder XRD patterns of Ti-MWW and TS-1 samples. The characteristic peaks of the Ti-MWW at 2θ =7.2°, 8.0°, 10.0°, 14.4°, 22.8°, and 26.2° 4

Journal Pre-proof were well corresponded with the MWW standard pattern. The powder XRD patterns of TS-1 samples all show diffraction peaks at 7.9°, 8.8°, 23.1°, 23.9°, and 24.4°, well

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corresponding to the MFI zeolite structure.

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Fig.1. XRD patterns of Ti-MWW and TS-1 samples. Fig. S1 shows the SEM images of titanosilicate zeolites. The Ti-MWW sample

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shows typical thin plate- like shape morphology with a thickness of around 50 nm. MTS-1 sample shows rounded-boat morphology and the particle size is about

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1.01-1.24 × 0.50-0.61 × 0.20-0.22 µm. After hydrothermal treatment in TPAOH solution, the obtained MHTS-1 sample shows coffin-shaped morphology and the

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range of particle size is about 1.33-1.46 × 0.61-0.70 × 0.23-0.25 µm. The NTS-1 and

respectively.

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NHTS-1 samples are ball- like with diameter ranges of 184-244 nm, and 200-253 nm,

Fig. S2 and Fig. S3 present the UV-vis and FT-IR spectra of these samples, respectively. It can be clearly observed that all titanosilicate samples show the characteristic IR band around at 960 cm−1 in IR spectra. This peak has been widely acknowledged as the evidence for incorporating Ti species into the zeolite framework [31]. Moreover, the UV- vis spectra of Ti-MWW and TS-1 samples contain predominant adsorption band at about 210 nm, which can be assigned to the tetra-coordinated titanium Ti (TiO 4 ) species. The band at about 330 nm that is attributed to anatase-like phase is remarkable in the UV-vis spectra of NTS-1 and NHTS-1. However, this band is almost negligible for Ti-MWW and micro-sized TS-1, indicating that there is little anatase- like phase in these samples. Thus, the Ti content in MHTS-1 may be higher than other samples of TS-1. The replacement of Si by the 5

Journal Pre-proof larger Ti in the tetrahedral zeolite framework causes a slight expansion in the unit cell parameters（Table S5）, which leads to a slight displacement of the XRD peaks [29,32]. Nitrogen sorption isotherms (A) and pore size distributions (B) are illustrated in Fig. 2, respectively. All the samples show a type IV isotherm. The MHTS-1 and NHTS-1 samples have an obvious type H3 hysteresis loop, indicating the presence of mesopores. There is a sharp increase at high relative pressure (P/P 0 >0.9) for Ti-MWW, which might be related to the interparticle voids formed by crystallites. Besides, the textural properties and elemental compositions of Ti-MWW and TS-1 samples are

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illustrated in Table S1. Ti-MWW gave the largest specific surface area and micropore volume. For MTS-1, the specific surface area and micropore volume change little,

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while the total pore volume slightly increases after hydrothermal treatment in TPAOH

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solution.

Fig.2. (A) N 2 adsorption-desorption isotherms and (B) pore size distribution of TS-1 and Ti-MWW samples at 77 K. 3.2. Screening of catalysts and solvents Table 1 A comparison of ammoximation of cyclohexanone in various solvents* Entry

Catalyst

Solvent

Conversion (%)

Selectivity (%)

1

Ti-MWW

T-butanol

92.1

99.4

2

Ti-MWW

Cyclohexane

98.5

99.4

3

Ti-MWW

DMC

99.8

99.6

4

Ti-MWW

Toluene

95.8

99.0

5

Ti-MWW

Water

98.8

99.4

6

Ti-MWW

Ethanol

99.7

90.1

6

Journal Pre-proof Ti-MWW

Methanol

99.1

79.1

8

NTS-1

T-butanol

58.8

99.8

9

NTS-1

DMC

30.9

97.2

10

MTS-1

T-butanol

8.3

94.7

11

MTS-1

DMC

6.0

91.7

12

MHTS-1

T-butanol

34.0

99.5

13

MHTS-1

Cyclohexane

17.1

98.8

14

MHTS-1

Toluene

26.3

98.7

15

MHTS-1

Water

31.8

81.8

16

MHTS-1

DMC

19.6

97.2

17

NHTS-1

T-butanol

18

NHTS-1

DMC

19

NHTS-1

Cyclohexane

20

NHTS-1

Toluene

21

NHTS-1

Water

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7

99.1

56.7

97.6

57.9

99.0

82.4

98.7

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87.2

85.2

98.8

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*Reaction conditions: cyclohexanone (57 mmol), catalyst (300 mg), solvent (172 mmol), NH3 (86 mmol), H2 O2 (62.7 mmol), 343 K, 1.5 h.

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Table 1 shows the results of cyclohexanone ammoximation over various catalysts. The moles of solvent have been held constant for the experiments. Ti-MWW exhibits

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better catalytic performance in various solvents than the samples of TS-1. The main

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reason is that Ti-MWW possesses the larger specific surface area, micropore and mesopore volume (Table S1). The larger surface areas and mesoporous structure could enhance the diffusion of reactant and product [25, 33]. Furthermore, TS-1 is a typical of medium pore zeolite of 10-MR, while Ti- MWW possesses a pore system that consists of intralayer 10- membered ring (MR) channels and 12-MR side cups and two independent interlayer. Therefore, TS-1 is more likely to suffer mass transfer restrictions [34]. In view of solvent investigation results, Ti-MWW yielded the best catalytic performance when DMC was used as the solvent (Table 1, entry 3), but the best suitable solvent for TS-1 is t-butanol. The likely reason is that the key step of this reaction is the production of the intermediate NH2 OH from the catalytic oxidation of NH3 with H2 O2 over catalyst [18]. The external surface of Ti-MWW is more hydrophilic than that of TS-1 [22]. When DMC, cyclohexane, or toluene was used as the solvent, the adsorbed solvent on the external surface of TS-1 may prevent NH3 7

Journal Pre-proof and H2 O 2 from approaching catalytic active sites. However, the more hydrophilic external surface of Ti-MWW may adsorb less solvent. Thus, the hydrophilicity may be one of the reasons why Ti-MWW exhibits better catalytic performance. Although cyclohexane and water are also good solvents for cyclohexanone ammoximation over Ti-MWW, one study proposed that cyclohexane is toxic [35]. When using water as solvent, cyclohexanone oxime crystals precipitated after reaction and were mixed with the catalyst particles, resulting in the difficult separation of catalyst and target product. A precise investigation on the time course is carried out among DMC, t-butanol, and water to check the solvent effect on the ammoximation reaction rate. Cyclohexanone

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conversion and oxime yield in these three solvents present similar increasing tendency with the prolonging of reaction time (Fig. S4). Therefore, DMC was chosen as the solvent for cyclohexanone ammoximation over Ti-MWW.

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3.3. Effect of Operation Conditions

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3.3.1 Effect of temperature

The catalytic performances of Ti-MWW at different temperatures are presented in

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Fig. 3. With the temperature increasing from 333 to 343 K, cyclohexanone conversion increased from 41.8% to 99.8%. When the temperature further increased to 353 K,

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both cyclohexanone conversion and oxime selectivity dropped slightly. The first reason is that some organic byproduct species was found at the reaction temperature

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of 343 K and 353 K (seen in Table S3 and S4). The second reason is that H2 O2 easily

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underwent nonproductive decomposition at higher temperatures [12]. In addition, methanol was detected in the reaction solution due to the decomposition of DMC (seen in Table S3 and S4). The amount of methanol was 0.12% and 0.85% of DMC in reaction mixtures of 343K and 353K, respectively. Therefore, the subsequent experiments were carried out at the temperature of 343K.

8

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Fig. 3. Effect of temperature on the cyclohexanone ammoximation over Ti- MWW. Reaction conditions: cyclohexanone (57 mmol), Ti- MWW (300 mg), DMC (14.5 mL),

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NH3 (175 mmol), H2 O2 (62.7 mmol), 343 K.

3.3.2 Effect of NH3 /cyclohexanone molar ratio

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As shown in Fig. 3, the NH3 /cyclohexanone molar ratio has little effect on the selectivity of oxime within the range from 1: 1 to 3: 1, and the selectivity of oxime maintained

over 99.5%.

Although the

cyclohexanone ammoximation

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was

stoichiometrically requires equivalent moles of ammonia and cyclohexanone,

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cyclohexanone conversion was only 54.9% when the NH3 /cyclohexanone molar ratio

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was 1.0. When the NH3 /cyclohexanone ratio increases to 1.5, the cyclohexanone conversion reaches 99.8%, and the conversion no longer increases with the further increase of the NH3 / cyclohexanone ratio. It is well known that more ammonia can reduce the loss caused by vaporization of ammonia and provide an appropriate basic reaction environment. Our results suggest that the cyclohexanone ammoximation proceeded most effectively at the NH3 /cyclohexanone ratio of 1.5-2.5.

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Fig. 4. Effect of NH3 /Cyclohexanone ratio on the cyclohexanone ammoximation over (14.5 mL), H2 O2 (62.7 mmol), 343 K. 3.3.3 Effect of catalyst concentration

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Ti-MWW. Reaction conditions: cyclohexanone (57 mmol), Ti-MWW (300 mg), DMC

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As shown in Fig. 5, cyclohexanone conversion increased significantly with an increasing concentration of Ti- MWW, reached a maximum of 99.8% at 5.0 g/L, and

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changed little when further increasing the concentration from 5.0 to 5.5 g/L. Therefore,

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the most suitable catalyst amount was 5.0 g/L.

Fig. 5. Effect of catalyst concentration on the cyclohexanone ammoximation over Ti-MWW. Reaction conditions: cyclohexanone (57 mmol), DMC (14.5 mL), NH3 (86 mmol), H2 O2 (62.7 mmol), 343 K. 10

Journal Pre-proof 3.4. The durability and reusability of Ti-MWW in DMC The stability of Ti-MWW in cyclohexanone ammoximation with DMC as solvent was investigated. Fig. S8 showed that the catalytic performance of Ti-MWW did not significantly decrease during the previous five cycles, and catalyst deactivation occurred at the sixth cycle. The deactivated Ti-MWW was calcined and reused in the cyclohexanone ammoximation, and the calcined Ti-MWW recovered its activity. The deactivated and regenerated Ti- MWW catalysts were characterized by XRD, FT-IR, and N 2 sorption isotherms. These results indicate that the deactivated catalyst still possesses characteristic peaks of Ti-MWW, but the surface area and pore

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volume decreased remarkably because the zeolite channels were blocked by organic compounds (seen Fig. S9, S10 and Table S2). After the deactivated catalyst was regenerated by simple calcination, the surface area and pore volumes were almost

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completely recovered; thus, the catalytic performance of the Ti-MWW was restored.

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4. Conclusion

In summary, Ti-MWW and DMC was used in the cyclohexanone ammoximation

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as catalyst and solvent, respectively, and the satisfactory results were obtained. High conversion of cyclohexanone (99.8%) and selectivity to oxime (99.6%) were achieved.

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Meanwhile, the suitable reaction conditions for the cyclohexanone ammoximation over Ti- MWW were investigated. Furthermore, Ti-MWW was recycled in

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cyclohexanone ammoximation with DMC as solvent and exhibits better reusability.

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This work may provide a more environmentally-benign solution for the production of cyclohexanone oxime. Acknowledgements

Financial support from the Innovation Fund for Elitists of Henan Province, China (Grant No. 0221001200), the Natural Science Foundation of China ( Grant No. 21773215), and the Joint Project of Zhengzhou University and Hebei Meibang Engineering Technology Co., Ltd. for the clean production of cyclohexanone oxime is acknowledged. The authors are highly indebted to teams of collaborators from both Zhengzhou University and Hebei Meibang Engineering Technology Co., Ltd.

Author contributions: S.X. Zhang contributed the central idea, performed the research, analysed most of the data, and wrote the initial draft of the paper. Y. Q. Wen and X.Y. Wang contributed to refining the ideas, carrying out additional 11

Journal Pre-proof analyses and finalizing this paper. S.X. Zhang, Y. Q. Wen, X.Y. Wang, H. J. Wei, X. Gao and M. T. Huang contributed to the revisions. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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(2011) 1-8.

[35] L.A. Malley, J.R. Bamberger, J.C. Stadler, G.S. Elliott, J.F. Hansen, T. Chiu, J.S. Grabowski, K.L.

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Pavkov, Drug. Chem. Toxicol. 23 (2000) 513-537.

Fig. 1. XRD patterns of Ti-MWW and TS-1 samples.

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Fig. 2. (A) N 2 adsorption-desorption isotherms and (B) pore size distribution of TS-1

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and Ti-MWW samples at 77 K.

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Fig. 3. Effect of temperature on the cyclohexanone ammoximation over Ti-MWW.

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Fig. 4. Effect of NH3 /Cyclohexanone ratio on the cyclohexanone ammoximation over Ti-MWW

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Fig. 5. Effect of catalyst concentration on the cyclohexanone ammoximation over Ti-MWW

Entry

Catalyst

Solvent

Conversion (%)

Selectivity (%)

1

Ti-MWW

T-butanol

92.1

99.4

2

Ti-MWW

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Pr

Table 1 A comparison of ammoximation of cyclohexanone in various solvents*

98.5

99.4

3

Ti-MWW

DMC

99.8

99.6

Ti-MWW

Toluene

95.8

99.0

Ti-MWW

Water

98.8

99.4

6

Ti-MWW

Ethanol

99.7

90.1

7

Ti-MWW

Methanol

99.1

79.1

8

NTS-1

T-butanol

58.8

99.8

9

NTS-1

DMC

30.9

97.2

10

MTS-1

T-butanol

8.3

94.7

11

MTS-1

DMC

6.0

91.7

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Cyclohexane

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MHTS-1

T-butanol

34.0

99.5

13

MHTS-1

Cyclohexane

17.1

98.8

14

MHTS-1

Toluene

26.3

98.7

15

MHTS-1

Water

31.8

81.8

16

MHTS-1

DMC

19.6

97.2

17

NHTS-1

T-butanol

87.2

99.1

18

NHTS-1

DMC

56.7

19

NHTS-1

Cyclohexane

20

NHTS-1

Toluene

21

NHTS-1

Water

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57.9

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82.4

99.0 98.7 98.8

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85.2

97.6

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Graphical abstract:

Highlights

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A clean approach for catalytic ammoximation of cyclohexanone to oxime has been explored. Dimethyl carbonate is a suitable solvent for cyclohexanone ammoximation over Ti-MWW.

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Ti-MWW showed excellent catalytic activity and recyclability for cyclohexanone ammoximation with dimethyl carbonate as the solvent.

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 

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