A magnetically recyclable TS-1 for ammoximation of cyclohexanone

Catalysis Communications 49 (2014) 20–24

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Short Communication

A magnetically recyclable TS-1 for ammoximation of cyclohexanone Tong Liu, Lei Wang, Hui Wan ⁎, Guofeng Guan ⁎ College of Chemistry and Chemical Engineering and State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China

a r t i c l e

i n f o

Article history: Received 18 December 2013 Received in revised form 29 January 2014 Accepted 2 February 2014 Available online 6 February 2014 Keywords: Fe3O4@TS-1 catalyst Ammoximation Cyclohexanone oxime Mass recovery

a b s t r a c t In this paper, Fe3O4 nanoparticles coated with titanium silicalite-1 (designated Fe3O4@TS-1) were successfully prepared and used as catalysts for ammoximation of cyclohexanone. Characterizations demonstrated that the magnetic catalyst was coated with a thin TS-1 layer of ~30 nm in thickness. The catalyst still displayed excellent catalytic activity after introducing Fe3O4 core. Recovery experiments revealed that Fe3O4@TS-1 catalyst could be easily recovered by adding an external magnetic field. Moreover, no appreciable catalytic deactivation was observed after four times of recycling. This work provided a promising way to overcome the recycle problem during the application of TS-1. © 2014 Elsevier B.V. All rights reserved.

2. Experimental

were added into the flask loaded with ethylene glycol (80 mL) with stirring. Then sodium acetate was added into the mixture and the obtained solution was transferred into a PTFE-lined stainless-steel autoclave. The autoclave was heated at 200 °C for 10 h, and then cooled down to room temperature. The products were then washed with deionized water for 3 times and dried at 60 °C under vacuum overnight. Fe3O4 was firstly mixed with deionized water and treated under ultrasonic environment to disperse uniformly. Then the mixture of tetraethyl orthosilicate (denoted as TEOS) and tetrapropylammonium hydroxide (denoted as TPAOH) was slowly injected into the solution of Fe3O4 under vigorous stirring. After hydrolyzing for 1 h, the mixture of tetrabutyl titanate (denoted as TBOT) and isopropanol (denoted as IPA) was added slowly. After removing the alcohol at 80 °C, the mixture with the following composition SiO2: TiO2: TPAOH: IPA: H2O: H2O2: Fe3O4 = 1: 0.025: 0.3: 0.4: (20 ~ 30): (1 ~ 2): (10 ~ 30) was transferred into a PTFE-lined stainless-steel autoclave and heated at 170 °C under static conditions for 2 days. Afterwards, the obtained products were washed with deionized water and dried at 110 °C overnight. Then the sample was calcined at 520 °C in air for 16 h to get Fe3O4@TS-1 catalyst [18]. TS-1 was synthesized according to the procedure raised by A. Thangaraj et al. [19]. The synthetic method was as same as method in preparation part of Fe3O4@TS-1 mentioned above without adding Fe3O4.

2.1. Sample preparation

2.2. Catalyst characterization

The super paramagnetic Fe3O4 nanoparticles were synthesized according to the reference [17]. Typically, FeCl3 and sodium citrate

XRD was obtained on a SmartLab-9 using CuKa2 radiation (40 kV, 100 mA) with steps of 0.02° and 1 s per step. N2 adsorption-desorption measurements were carried out on a Micromeritics ASAP 2020 system model instrument at 77 K. Brunauer–Emmett–Teller (BET) method was used to measure the specific surface areas. The pore size distribution

1. Introduction Cyclohexanone oxime, an important intermediate, is used in the production of ε-caprolactam. Traditionally, cyclohexanone-hydroxylamine process is employed in the commercial process and large amount of low valued ammonium sulfate is produced [1]. Reactions catalyzed by TS-1 meet the requirement of green chemical engineering because only H2O is generated [2–7]. High activity is observed only for TS-1 nano crystals (b0.3 μm) [8], whereas the crystals larger than 1 μm are almost inactive [8,9]. Membrane separation has to be used to separate TS-1 nano crystals, which greatly increases the cost of separation. Recently, nano-sized particle with magnetic cores and functional shell has attracted great attention for its unique property. It has been applied in various fields such as catalysts, drug release and removal of metal ions [10–15]. The introduction of magnetic core makes it easy to separate composites from the reaction system by adding an external magnetic field. The functionalized material still remains in nano scale after coating on the surface of magnetic core. It can be a promising way to solve the separation problem through introducing magnetic core into TS-1 [15,16]. In this paper, Fe3O4 nanoparticle was introduced as the core into TS-1 catalysts. High catalytic activity and good mass recovery were achieved for ammoximation of cyclohexanone.

⁎ Corresponding authors. Tel.: +86 25 83587198. E-mail addresses: [email protected] (H. Wan), [email protected] (G. Guan). 1566-7367/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2014.02.004

T. Liu et al. / Catalysis Communications 49 (2014) 20–24

320

21

4

a

Intensity

a

10

20

30

40

50

(533)

(440)

(422) (551)

(400)

(331)

(220)

b

60

c

70

3

dVp/ddp

Volume Absorbed (cm /g)

280

b 2 1

240

0 0.0

200

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Pore diameter (nm)

160

a

120

b

80 40 0

80

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

2 Theta(dgree)

Fig. 3. N2 adsorption/desorption isotherms of different catalysts, a: TS-1, b: Fe3O4@TS-1. Insert was the pore size distribution of a: TS-1 and b: Fe3O4@TS-1 calculated using Saito–Foley model.

Fig. 1. XRD patterns of samples, a: Fe3O4@TS-1; b: TS-1; c: Fe3O4.

and micropore volumes were calculated using SF method and from t-plot, respectively. FT-IR spectra were collected on a Thermo Nicolet 870 spectrophotometer using anhydrous KBr as dispersing agent. The morphologies of prepared samples were studied with SEM and TEM (SEM, Hitachi S-4800; TEM, JEOL, model 794). The iron content in Fe3O4@TS-1 catalysts was detected by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 2000 DC). Magnetization measurements were conducted at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7407) with a maximum magnetic field of 18000 Oe.

2.4. Catalyst recycle The used catalysts were separated from the reaction system by centrifuge and adding an external magnetic field for TS-1 and Fe3O4@TS-1, respectively. All used catalysts were washed using tert-butanol three times and dried overnight for reuse. The quantity of catalyst was calculated according to the weight before and after reaction. The deactivated catalysts were recovered by calcining at 520 °C for regeneration. 3. Results and discussion

2.3. Ammoximation of cyclohexanone 3.1. Catalysts characterization Ammoximation was carried out in a 50 mL round-bottom flask equipped with the magnetic stirrer and condenser. Typically, cyclohexanone, tert-butanol and deionized water were firstly added into the flask containing TS-1 catalysts. The amount of catalyst was 10 g/mol cyclohexanone. The mixture was vigorously stirred and heated to 80 °C. Then hydrogen peroxide (30 wt.%) and ammonia hydroxide (25 wt.%) were then injected continuously through the syringe pumps, and this time was recorded as the start of reaction. The final molar ratio of the reaction mixture was in the range of 1.0 cyclohexanone: (1.0–1.2) H2O2: (2.5–2.7) NH3. Product was analyzed by an SP6890 gas chromatograph equipped with a FID detector.

a b 977

c 973

XRD patterns of Fe3O4@TS-1, TS-1 and Fe3O4 were shown in Fig. 1a, b and c, respectively. Fe3O4@TS-1 exhibited the typical MFI peaks of TS-1 around 23.1°, 23.3°, 23.7°, 24.1°, 24.4°, indicating the MFI structure of TS-1 was not affected by the introduction of Fe3O4. The sharp peaks at 2θ = 30.3°, 35.6°, 43.3°, 53.3°, 57.1° and 62.8° (Fig. 1a and c) were assigned to the Fe3O4 crystals. Fe3O4@TS-1 also possessed the characteristic peaks of Fe3O4, meaning that the Fe3O4 core in the Fe3O4@TS-1 was well preserved after coating procedure [20]. Fig. 2 clearly demonstrated the FT-IR spectra of functional groups of Fe3O4, TS-1 and Fe3O4@TS-1, respectively. The intense absorption peaks around 550 and 1228 cm−1 were characteristic peaks of MFI type zeolite structure. The vibrational mode around 970 cm−1 was considered as the fingerprint that titanium successfully entered into the framework of molecular sieve [21–23]. Similar spectrum was obtained for Fe3O4@TS-1 and no characteristic peak of Fe3O4 was observed in the sample (Fig. 2b). It suggested that the MFI type structure was not affected after introducing Fe3O4 and TS-1 layer was coated well on the surface of Fe3O4. Band shifting was observed after introducing Fe3O4 core. According to Fig. 2, the wavenumber at 973 cm−1 for TS-1 shifted to 977 cm−1 for Fe3O4@TS-1. The shift of wavenumber indicated that the strength of titanium oxygen bond decreased after introducing

800 550

1228

450

Table 1 The textual properties of TS-1 and Fe3O4@TS-1 catalysts. Sample

BET surface area/m2 g−1

1100

3500

3000

2500

2000

1500

1000

wavenumber/cm-1

500

TS-1 Fe3O4@TS-1 a

Fig. 2. FT-IR spectra of samples, a: Fe3O4; b: Fe3O4@TS-1; c: TS-1.

b

465 488.7a

Average pore diameter/nm 0.59 0.41

Calculated only for mass of TS-1 component. Calculated from t-plot.

Pore volume (cm3∙g−1) Vtotal

Vmicrob

Vmeso

0.4590 0.4143

0.2016 0.2335

0.2574 0.1808

22

T. Liu et al. / Catalysis Communications 49 (2014) 20–24

Fig. 4. SEM and TEM images of the Fe3O4@TS-1 (Si/Fe = 20). A: stir for 24 h; B: stir for 6 h; C: low magnification of TEM; D: HRTEM image of Fe3O4@TS-1.

Fe3O4 core. Interactions between metals and TS-1 increased the electrophilicity of Ti center and therefore H2O2 was converted effectively [24]. It might contribute to the catalytic performance. The N2 adsorption–desorption isotherms of TS-1 and Fe3O4@TS-1 were shown in Fig. 3. The isotherm of Fe3O4@TS-1 exhibited a sharp uptake at low relative pressure, indicating the microporous structure of the catalyst. The textual properties of the two samples were given in Table 1. The average pore diameter of Fe3O4@TS-1 decreased a little, which might be the squeezing of Fe3O4. It was conscious that Fe3O4 core did not contribute to the surface area significantly. The actual surface area of Fe3O4@TS-1 and pore volume only for mass of TS-1 component were calculated assisted with ICP (CFe = 31.2467 mg∙L− 1). No

much difference of surface areas and pore volumes of the two materials was observed, which suggested that introducing Fe3O4 core would not affect the original textual properties of TS-1. The results of XRD, FT-IR and BET illustrated that the structure still kept intact after introducing Fe3O4. SEM images (Fig. 4A, B) showed that the currently synthesized Fe3O4@TS-1 composites had a uniform morphology with a mean size of ca. 250 × 150 × 100 nm. The morphology of catalyst remained uniform after functionalization. The surface of the catalysts was smoother along with the longer stirring time in the process of gel preparation. This phenomenon further proved that the magnetic samples were formed by the coating of TS-1 nanocrystals outside Fe3O4 core. TEM images of Fe3O4@TS-1 showed that the as-synthesized magnetic composites were uniform with a mean diameter of about 250 nm which agreed with SEM images. The dark section was the Fe3O4 core and the bright section was TS-1. The thickness of TS-1 membrane was ca. 30 nm (Fig. 4C). The evidences shown above suggested the successfully formation of core-shell structure. The insertion of Fe3O4 enabled the catalyst to achieve the satisfied magnetic separation ability. It was the key to control the thickness of TS-1 membrane outside of Fe3O4 core, because too much functional shell would decrease the ability of magnetic separation. The hysteresis loops shown in Fig. 5 suggested that the samples were all supermagnetic

Table 2 Test results of ammoximation of cyclohexanone with different catalysts.

Fig. 5. Room temperature magnetization hysteresis loops of the Fe3O4@TS-1 with different Si/Fe ratios, a: Si/Fe = 10, b: Si/Fe = 20 and c: Si/Fe = 30. Insert was the separation process, Fe3O4@TS-1 of suspension solution (left); magnetic capture by adding an external magnetic field (right).

Experiment

Recycling times

Catalyst

Recovery/%

Conversion/%

Selectivity/%

1 2 3 4 5 6 7 8

– – 1 2 3 4 5 Regeneration

TS-1 Fe3O4 Fe3O4@TS-1 Fe3O4@TS-1 Fe3O4@TS-1 Fe3O4@TS-1 Fe3O4@TS-1 Fe3O4@TS-1

– – 99.2 98.9 96.5 99.1 97.4 98.9

100 2.59 100 99.83 94.48 93.6 52.69 100

97.1 0 99.4 91.3 97.2 95.4 60.7 93.2

T. Liu et al. / Catalysis Communications 49 (2014) 20–24

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Table 3 Ammoximation of cyclohexanone with different catalysts. Reaction conditions Catalyst amount to reactant

Temperature/°C

Time/h

solvent

5.1 wt.% Ti-MWW-PSc 10 wt% Ti-MWW-HTSc 5.1 wt.% Ti-Meso-MORd 20 wt.% hierarchical TS-1 2.8 wt.% K6PW9V3O40∙4H2O 10 wt.% TS-2 20 wt.% Ti-Beta 17.3 wt% TS-1e 33 wt.% Clay-Based TS-1 10.2 wt.% TS-1f 9.4 wt.% Fe3O4@TS-1 9.4 wt.% Fe3O4@TS-1g

65 65 60 75 25 80 65 75 80 80 80 80

1.5 1.5 2 5.5 10 5 5 1.5 2.5 6 3 1

H2O H2O H2O/tert-butanol tert-butanol isopropanol tert-butanol H2O tert-butanol H2O/tert-butanol H2O/tert-butanol H2O/tert-butanol H2O/tert-butanol

a b c d e f g

Canonea/%

Soximeb/%

Reference

99.4 97.0 97.7 91.0 86.0 74.6 15.0 96.0 100 98.5 100 95.3

99.4 99.9 99.3 92.0 N97 69.1 4.0 95.3 97 99.4 99.4 97.6

[25] [25] [26] [27] [28] [29] [30] [31] [32] [33] This work This work

Conversion of cyclohexanone. Selectivity of cyclohexanone oxime. H2O2 was added dropwise at a constant within 1 h. NH3 and H2O2 was added to the reaction within 2 h. Prepared using pretreated titanium dioxide as a titanium source. Molar composition: SiO2:0.033TiO2: 0.05TPAOH: 0.6 n-butylamine: 35H2O; The NH3 and H2O2 was injected in 0.5 h and reacted another 0.5 h.

ones. The saturated magnetization of Fe3O4@TS-1 with Si/Fe ratio of 10, 20 and 30 were 35.010, 20.079 and 13.622 emu/g, respectively. The decrease of saturated magnetization for different magnetic catalysts was mainly attributed to the decreased Fe3O4 content in the catalysts. Trace remanence and coercivity were observed for these samples, which suggested that they could be well dispersed without aggregation when no external magnetic field was added. The high saturated magnetization ensured the strong magnetic sensitivity of the magnetic nanohybrids. So Fe3O4@TS-1 could be easily removed from the reaction system by adding an external magnetic field (Fig. 5 insert).

easily recovered after high temperature calcination which meant that the deactivation of TS-1 was mainly caused by channel blocking. It was frequently caused by by-products generated in the reaction such as tert-butylcyclohexanone, 2-(tert-butylperoxy)-cyclohexamine and 2-cyclohexlidenecylohexanone [34]. The selectivity of regenerated Fe3O4@TS-1 dropped a little than the fresh one, which might be attributed to the irreversible dissolution loss of TS-1 framework [35].

3.2. Catalytic activity

Fe3O4@TS-1 was successfully prepared by introducing Fe3O4 particles in the gel preparation process and used as catalyst for ammoximation of cyclohexanone. The magnetic catalyst had the core-shell structure with uniform size. The synthesis method was easy to handle and reproducible. The Fe3O4@TS–1 catalyst synthesized had a high magnetization, which could help to separate them from the reaction system by simply adding an external magnetic field. The catalytic activity of Fe3O4@TS-1 was comparable or even better than that for TS-1.

Ammoximation of cyclohexanone was used to evaluate the catalytic activity of Fe3O4@TS-1 and TS-1. High conversion and selectivity were achieved for both catalysts (Exp. 1 and 3 in Table 2), which suggested that the catalytic activity of Fe3O4@TS-1 was not affected after introducing Fe3O4. As shown in Table 2, Fe3O4 had almost no catalytic activity. It in return demonstrated that catalytic activity of Fe3O4@TS-1 was attributed to the TS-1 component on the surface of Fe3O4 core. The space time yield (STY) of the cyclohexanone oxime was 3.81 g g−1 h−1 for Fe3O4@ TS-1 and 3.73 g g−1 h−1 for TS-1, respectively. The catalytic performance of Fe3O4@TS-1 was even better than that of TS-1. The improvement of Fe3O4@TS-1 catalyst might be attributed to the increase of electrophilicity of Ti center and enhanced active oxygen in H2O2. Results of ammoximation with various catalysts were investigated which were shown in Table 3 [25–33]. Almost existing operations offered high catalytic activity for most Ti containing catalysts except TS-2 and Ti-Beta. Though high catalytic activities were achieved in the reaction with hierarchical TS-1 and Clay-Based TS-1, high amount of catalysts had to be used. For K6PW9V3O40∙4H2O, long reaction time was needed. Additional experiment (9.4 wt.% Fe3O4@TS-1, 1 h) was conducted to compare the catalytic activities with other catalysts. Catalytic activity as high as other catalysts (e.g. Ti-MWW-PS, Ti-Meso-MOR, TS-1) was achieved for ammoximation of cyclohexanone. It further proved that introducing of Fe3O4 would not affect the catalytic activity. The mass recovery and reusability of both catalysts were further investigated. As shown in Table 2, the mass recovery of Fe3O4@TS-1 (ca. 98%) was much higher than that of TS-1(ca. 90%, not shown). No iron ion was detected in the reaction system which qualified the purity requirement for cyclohexanone oxime. That in turn demonstrated that Fe3O4 core was coated well after reaction. The Fe3O4@TS-1 catalyst still showed excellent catalyst activity for ammoximation after four times of recycling. Catalytic activity of deactivated Fe3O4@TS-1 was

4. Conclusions

Acknowledgements We gratefully acknowledge the support from the National Science Foudation of China (Grant No. 21306082), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (12KJB530004) and the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech Univerisity (ZK201305). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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