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一种高效的多孔氮掺杂碳基气凝胶催化剂及其催化乙苯氧化性能

更新时间:2009-03-28

The selective oxidation of ethylbenzene (EB) to acetophenone (AP) is one of greatly important commercial reactions because AP is an important raw material in the syntheses of pharmaceuticals, resins, flavoring agents and perfumes[1-2]. Typical industrial synthesis of AP is the liquid-phase oxidation of EB by oxygen in the air. However, by-productions are numerous and the catalyst is hard to separate in this method[1-3]. Hence, the development of new environmentally friendly and sufficient procedures for the selective oxidation of EB is very urgent. Oxidations and catalysts are crucial in the reaction, so it may be promising methods which using peroxide oxidants and heterogeneous catalysts.

Carbon materials which can be used as catalysts or supports mainly include activated carbon, carbon nanotubes, graphite and graphene. Graphene, as novel two-dimensional carbon nanomaterial, is a single layer of sp2 carbon atoms and it has many outstanding electrical and thermal properties[4]. Graphene aerogels (GA), as graphene-based three-dimensional (3D) materials, have the merit of high porosity and retain the own advantages of graphene[5-6]. Hence, GA have aroused more and more concern to explore. More significantly, the introduction of heteroatoms can improve the cataly-tic performance of carbon materials[7]. And nitrogen has attracted more and more interests due to its valence electron for bonding and suitable atomic size[8].

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Biomasses also have been utilized to prepare various functional carbon materials due to abundant, cheap and environmental factors[9]. Cattail wools (CW) are the flowers of cattail which is a kind of perennial, aquatic plant widespread available biomass. CW may serve as both the template and the precursor for the preparation of carbon material by the facile carbonization procedure and the further chemical or physical activation[10-11]. Therefor, it is a good and reasonable idea to develop porous N-doped carbon-based compo-site aerogels combining GA and CW.

In this paper, we synthesized GA-CW-X compo-site aerogels through different amounts of CW immobilized on GA via the processes of hydrothermal synthesis and calcination. And CW were pre-treated by carbonization and activation. XRD, Raman, FT-IR and TEM were used to characterize these catalysts. The selective oxidation of EB to AP with TBHP as oxidant was carried out to investigate the catalytic performance of these catalysts. The selectivity of AP was up to 92.7% and the conversion of EB was up to 87.6% with GA-CW-0.5 as catalyst at 80 ℃.

1 Experimental

1.1 Materials and reagents

CW were collected from cattail in Zhengzhou University, Henan Province, China. All the other chemicals were purchased from commercial sources and used without further treatment. Methanol was of chromatographic reagent grade and the other chemicals were of analytical reagent grade.

1.2 Preparation of GA, CW and GA-CW-X

The graphene oxide (GO) was prepared by a modified Hummer method[12]. The obtained GO aqueous dispersion was 5 g/L. 30 mL GO aqueous dispersion was sealed in a 100 mL Teflon-lined autoclave and put into the oven at 180 ℃ for 24 h. The obtained hydrogels were freeze-dried for 2 d to obtain graphene aerogels, denoted as GA.

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Fig.1 shows a schematic diagram for the preparation of GA-CW-X. The prepared 10% CW was disper-sed in 15 mL ethanol based on the mass ratio of 10% CW∶Fe(NO3)3·9H2O∶GO=X∶0.25∶1 (X=0.25, 0.5, 0.75, 1, respectively), which denoted as A-X solution. B solution was commixed with 2.0 g urea, 15 mL deionized water and 30 mL GO aqueous dispersion. A-X solution and B solution were mixed under ultrasonic treatment for 2 h, followed by adding 37.5 mg Fe(NO3)3·9H2O. After kept at ultrasonication for 5 h, the mixed aqueous dispersion was sealed in a 100 mL Teflon-lined autoclave and treated at 180 ℃ for 24 h. The autoclave was cooled down to the room temperature. Then the gel was taken out and freeze-dried. After heating the sample at 850 ℃ for 2 h under N2 atmosphere with the heating rate of 5 ℃/min, the dark sample was washed with HCl and water until pH=7.0, followed by dried at 110 ℃. Finally, the obtain product was porous N-doped carbon-based composite aerogels, denoted as GA-CW-X (X=0.25, 0.5, 0.75, 1, respectively).

2.0 g CW was immersed into the mixed solution of 10 mL Ni(NO3)2 solution (10%) and 10 mL ethanol for several hours and dried at 80 ℃. Carbonized process was performed at 600 ℃ for 4 h in a tube furnace with the heating speed of 5 ℃/min under N2 atmosphere. The carbonized CW was immersed into 2 mol/L KOH based on mass ratio of KOH: sample =4∶1 to activation. After dried at 80 ℃, the sample was heated at 800 ℃ for 1 h with a heating rate of 3 ℃/min under N2 atmosphere. Subsequently, the dark sample was washed with HCl and water until pH=7.0, followed by dried[13-14]. The obtained product was 10% CW.

  

Fig.1 Schematic representation for the preparation of GA-CW-X

1.3 Catalytic activity tests

Typically, 1.0 mmol EB, 10.0 mg catalyst, 3.0 mmol tert-butyl hydroperoxide (TBHP, 70% in H2O) and 3.0 mL ultrapure water were sealed in a 30 mL quartz glass tube and remained at 80 ℃ for 24 h with stirring in an oil bath. After that, the reaction solution was diluted to 25 mL with ethanol. Subsequently, the catalyst was filtered and the filtrate was reserved for the further analyzed by a high performance liquid chromatography (HPLC, Wufeng LC) with Agilent TC-C18 (2) column, 5 μm, 4.6 × 150 mm. The mobile phase was methanol and water.

1.4 Catalysts characterizations

X-ray powder diffraction (XRD) patters were performed on Bruker D8 Advance X-ray powder diffractometer (Germany) using Cu-Kα, λ=0.154 18 nm, 2θ=5°~90°. Raman spectra were collected on Renishaw in Via Laser microscopy Raman spectrometer (UK). Fourier transform infrared (FT-IR) spectra were carried out using Bruker VERTEX 70 Fourier transform infrared spectrometer (Germany) with the KBr disc technique. Transmission electron microscopy (TEM) was conducted on a FEI Talos F200S. For TEM analyses, samples were dispersed in ethanol under ultrasonic treatment, followed by putting a drop of the solution on a Cu grid.

2 Results and discussion

2.1 Catalyst characterizations

The morphology and structure of CW, GA and GA-CW-0.5 were examined by transmission electron microscopy (TEM), as shown in Fig.5. GA-CW-0.5 is talked as a typical sample because of the similar morphology of GA-CW-X. As shown in Fig.5, many pores distribute over the whole disordered nanosheets of GA-CW-0.5, which can create more surface area and active sites. The TEM image of GA (Fig.6a) exhibits few-layered sheet with wrinkles and numerous small pores. The TEM image of CW (Fig.6b) shows the disordered nanosheets of CW.

The ID/IG ratio of GA-CW-X is higher than that of CW and GA, indicating that N atoms derived from urea and the interaction between GA and CW exist in the composites aerogels[20,23]. What’s more, the ID/IG va-lue of GA-CW-0.5 is the maximum of these catalysts, and the catalytic performance of GA-CW-0.5 is the best one among them for EB oxidation. It’s worth noting that the weak 2D band at 2 890 cm-1 is typical peak of graphitic carbon materials[24].

  

Fig.2 XRD patterns of GA, CW, GA-CW-0.25, GA-CW-0.5, GA-CW-0.75, GA-CW-1

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由表4可以计算出CNN-1网络训练参数为 4 930,CNN-2网络训练参数为35 900,CNN-1相比较于CNN-2参数减少了86%。两个CNN框架均基于caffe平台构建,训练样本为MSTAR的10类数据,训练过程最大迭代次数为60 000次,训练和测试的训练误差和测试正确率曲线如图4和图5所示。

  

Fig.3 FT-IR spectra of GA, CW, GA-CW-0.25, GA-CW-0.5, GA-CW-0.75, GA-CW-1

Raman spectroscopy is a useful tool to understand the structure of carbon materials. As shown in Fig.4, all the Raman spectra show two typical peaks at 1 357 and 1 595 cm-1, which presents D band and G band, respectively[17,21]. The D band refers to the structural disorder and defects, and G band originates from the vibration of sp2-hybridized carbon atoms in the graphi-tic lattice[13,15]. The intensity ratio of the D band and G band (ID/IG) is the measure of the graphitic ordering of carbon materials[13,22].

  

Fig.4 Raman spectra of GA, CW, GA-CW-0.25, GA-CW-0.5, GA-CW-0.75, GA-CW-1

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The chemistry structure and crystal phase were evaluated by X-ray powder diffraction (XRD). From the XRD patterns of GA and GA-CW-X in Fig.2, a broad weak at 25.6° is observed, which presents the (002) planes of graphitic carbon[8,15]; a weak peak at 43.1° is attributed to the (100) facet[16]. The two peaks all manifest that the reduction of GO has happened during this synthesis procedure. The peak at 24.2° in the XRD pattern of CW indicates the amorphous nature of carbon in CW[17]. However, the strong diffraction peaks at 28.9°, 29.5° and 41.0° are contributed to NH4NO3, KNO3 and Ni(NO3)2 (JCPDS ICCD card PDF#08-0452, PDF#36-1188, PDF#14-0593), respectively, which are the residual impurities from the courses or the substances contained by CW itself.

  

Fig.5 TEM image of GA-CW-0.5

  

Fig.6 TEM images of GA (a) and CW (b)

2.2 Catalytic performance

Reaction conditions: EB (1.0 mmol), TBHP (3.0 mmol, 70% in H2O), catalyst (10.0 mg), ultrapure water (3.0 mL), 80 ℃, 24 h. The conversion and selectivity are detected by HPLC.

FT-IR spectra were used to investigate the functional groups in these catalysts, as shown in Fig.3. The typical absorption peak at 3 435 cm-1 is assigned to the stretching vibration of O-H[18] and the weak peak at 2 924 cm-1 is considered as the asymmetric vibration of C-H[19]. Many other characteristic functional groups of GA-CW-X can be found at 1 630 cm-1 (C=C)[19], 1 578 cm-1 (C=N)[20], 1 399 cm-1 (C-OH)[6] and 1 210 cm-1 (C-N)[5]. The results prove that nitrogen species exist in the GA-CW-X. The peak of CW at 1 616 cm-1 presents the groups of C=C[19] and the peak at 1 400 cm-1 is attributed to the vibration of C-H[18]. The peaks of GA at 1 634 and 1 398 cm-1 are corresponded to C=C and C-OH[20], respectively.

 

Table 1 Catalytic activities of catalysts for theselective oxidation of EB

  

EntryCatalystConversionofEB/%SelectivityofAP/%1blank16.99.92CW27.615.53GA49.956.64GA-CW-0.2591.086.55GA-CW-0.587.692.76GA-CW-0.7581.675.87GA-CW-182.381.6

The selective oxidation of EB using TBHP as an oxidant was performed to measure the catalytic activities of prepared catalysts, as displayed in Table 1. AP is the main product. The selectivity of AP is only 9.9% (Table 1, entry 1) in the oxidation of EB without any catalyst. When CW and GA were used as the catalyst (Table 1, entries 2, 3), the selectivity of AP increases to 15.5% and 56.6%, separately. It is found that GA-CW-X (Table 1, entries 4-7) have greatly catalytic activities than GA and CW. What’s more, the selectivity of AP is 92.7% and the conversion of EB is 87.6% when GA-CW-0.5 was used as catalyst (Table 1, entry 5). This could be attributed to more defects in GA-CW-0.5, which can be verified from that the ratio of ID/IG is 1.16 (Fig.4). It is note that the conversion of EB with GA-CW-0.25 as catalyst is a little higher than that of GA-CW-0.5 used as catalyst, but the yield and selectivity of AP are lower (Table 1, entries 4, 5). We proposed that GA-CW-0.25 is so active in the reaction that there are more other products generate.

The excellent catalytic performance of GA-CW-X can be ascribed to the following reasons. Firstly, the porous structure could provide more catalytic sites to accelerate electrons transport[9], which can be observed in the Fig.5. Secondly, the N-doping could generate more defects and more adsorption surface. And nitrogen atoms can activate the benzylic C-H bond[25]. The FT-IR and Raman spectra prove that nitrogen atoms have been successfully introduced. Thirdly, the interaction between GA and CW could remain their own outstanding features and promote the contact with reactants.

The results of the selective oxidation of EB at di-fferent temperature with GA-CW-0.5 as the catalyst are displayed in Table 2. When the temperature increased from 40 to 80 ℃, the conversion of EB increases obviously and the selectivity of AP approximately increases also. The increasing of temperature improved decomposition of TBHP which can create more active radicals with the increasing of temperature. Active radicals play an important role in the oxidation reaction.

 

Table 2 Catalytic activity of GA-CW-0.5 for oxidation of EB at different temperature

  

t/℃ConversionofEB/%SelectivityofAP/%8087.692.77080.581.06078.572.55071.676.74061.860.9

Reaction conditions: EB (1.0 mmol), TBHP (3.0 mmol, 70% in H2O), catalyst (10.0 mg), ultrapure water (3.0 mL), 24 h. The conversion and selectivity are detected by HPLC.

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Taking GA-CW-0.5 as example, the reusability experiments were tested, as shown in Fig.7. Typically, 1.0 mmol EB, 10.0 mg catalyst, 3.0 mmol TBHP and 3.0 mL ultrapure water are mixed, sealed and kept at 80 ℃ for 24 h. The catalytic performance of GA-CW-0.5 displays obviously fall. The selectivity of AP is nearly 63% after five runs and the catalytic perfor-mance trends to stable. The reason is that the structure of GA-CW-0.5 after five runs has been damaged that some local pores disappear, which can be observed from Fig.8. As a word, the results prove that GA-CW-X have high catalytic activity and general stability in the selective oxidation of EB with TBHP as the oxidant.

  

Fig.7 Reusability tests of GA-CW-0.5 for oxidation of EB

  

Fig.8 TEM images of GA-CW-0.5 before reaction (a) and after five runs (b)

3 Conclusions

In this work, we offer a novel method to convert biomass waste (CW) into carbon catalyst for the production of AP. GA-CW-X composite aerogels have been prepared through different amounts of CW immobilized on GA by the processes of hydrothermal synthesis and calcination. The selective oxidation of EB to AP with TBHP as oxidant was performed to test the cataly-tic activities of these carbon catalysts. The selectivity of AP was up to 92.7% and the conversion of EB was up to 87.6% with GA-CW-0.5 as catalyst in 80 ℃ for 24 h. The porous structure and nitrogen atoms of GA-CW-X can create more defects and more active sites, which are of benefit to the reaction. What’s more, the interaction between GA and CW promotes the formation of AP. This work offers a green and efficient carbon material as catalyst for oxidation of EB to AP.

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孙 倩,苏小利,高 丽,陈 威,郑修成,毛立群,杨敬贺
《化学研究》 2018年第01期
《化学研究》2018年第01期文献

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