- 1 - Liquid phase conversion of phenols into aromatics over magnetic

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Liquid phase conversion of phenols into aromatics over magnetic Pt/NiO-Al2O3@Fe3O4 catalysts via a coupling process of hydrodeoxygenation and dehydrogenation Guohua Zhu, Kui Wu, Liang Tan, Weiyan Wang, Yanping Huang, Diyu Liu, and Yunquan Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01419 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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ACS Sustainable Chemistry & Engineering

Liquid phase conversion of phenols into aromatics over magnetic Pt/NiOAl2O3@Fe3O4 catalysts via a coupling process of hydrodeoxygenation and dehydrogenation

Guohua Zhu†, Kui Wu†, Liang Tan, Weiyan Wang*, Yanping Huang, Diyu Liu, Yunquan Yang*

School of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, 411105, PR China

E-mail: [email protected] (Guohua Zhu), [email protected] (Kui Wu), [email protected] (Liang Tan), [email protected] (Weiyan Wang), Yanping Huang: [email protected] (Yanping Huang), [email protected] (Diyu Liu), [email protected] (Yunquan Yang)

†

These two authors contributed equally to this work and were considered co-first

authors.

* To whom correspondence should be addressed. E–mail: [email protected] (W. Wang), [email protected] (Y. Yang) Tel: (+86) 731–58298581 Fax: (+86) 731–58293284 -1ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

ABSTRACT Magnetic colloidal sol was firstly prepared and added in the mixed solution of Ni(NO3)2 and Al(NO3)3. After the constant-pH co-precipitation and calcination, magnetic NiOAl2O3@Fe3O4 mixed oxides were formed and used as supports for Pt based catalysts. The activities of these magnetic Pt/NiO-Al2O3@Fe3O4 catalysts were tested in the hydrodeoxygenation (HDO) of phenols such as 4-ethylphenol, 4-methylphenol and guaiacol. These exhibited high HDO activity and aromatic hydrocarbons were produced through a coupling process of hydrodeoxygenation and dehydrogenation. The addition of Fe enhanced both of the conversion and aromatic hydrocarbons selectivity, which was attributed to the electron transfer between metallic Pt and Fe. For example, after the liquid phase HDO of 4-ethylphenol at 300 °C and 2.0 MPa for 1.5 h, 97.9% conversion and 18.3% toluene selectivity were obtained. When the reaction time prolonged to 8 h, ethylbenzene selectivity increased to 88.0%. Moreover, these Pt/NiOAl2O3@Fe3O4 catalysts were easily recycled from the liquid reaction mixture by employing an external magnet, but the dehydrogenation stability needs to be further improved. KEYWORDS: Pt; Magnetism; Hydrodeoxygenation; Phenols; Dehydrogenation; Coupling process

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INTRODUCTION Bio-oil was considered as the sole renewable and sustainable resource for hydrocarbon fuel and chemicals and attracted much attention in recent years.1, 2 However, this bio-oil contains many oxygen-containing compounds such as phenols, furans, aldehydes, and so on, contributing to a high oxygen content, which causes to a low heat value and then hampers its extensive utilization as a fuel.3, 4 Hydrodeoxygenation (HDO) is an efficient technology for selectively removing oxygen from the bio-oil with the presence of hydrogen.5, 6 Because the phenolic Caromatic–O bond is stronger than other C–O bonds, the difficulty for the deoxygenation is increased.7 In addition, phenols are dominant and typical compounds in the lignin-derived bio-oils. Hence, phenols were usually selected as model compounds to study the HDO activity of the prepared catalysts and the corresponded reaction mechanism.8 Previous literatures had concluded that the HDO of phenols mainly proceeded with two routes: direct deoxygenation via the scission of Caromatic–OH bond (DDO) and indirect deoxygenation via saturation of aromatic ring followed by dehydration (HYD),9-11 as shown in Scheme 1, where the the latter consumed much more precious hydrogen than the former.

Scheme 1 Reaction routes in the HDO of phenols Until now, many catalysts, including noble metals,12-18 borides,19, 20 transition metal sulfides,21-24 carbides25, 26 and nitrides,27, 28 have been employed into the HDO reactions,



and exhibited different catalytic properties. Sulfide catalysts present high HDO activity, but the easy deactivation via the oxygen–sulfur exchange at the edge sites in no sulfurcontaining HDO process is the greatest drawback for their commercialization.29, 30 In contrast, noble metal catalysts possess high hydrogenation activity, leading to that HYD is the main reaction route for the HDO of phenols, which can lower the reaction temperature but increase the hydrogen consumption because of the saturation of phenyl ring.12, 31 Recently, some efforts have been made on decreasing hydrogen consumption in the HDO of phenols on noble catalysts. For example, Noronha et al.32, 33 had studied the effects of supports on the HDO activity of Pd catalysts and concluded that the oxophilic ZrO2 favored to inhibit the formation of benzene ring hydrogenated products, where the aromatics selectivity reached 44.2%. Fe-based catalysts, charactering of the advantages of being inexpensive and environmentally friendly, are also used as potential candidates for the HDO of phenols. Dufour et al.34, 35 had reported that 74% guaiacol conversion and 38% aromatics yield were obtained at 400 °C. Wang et al.36, 37 had found that Fe/C showed a promising HDO activity and the addition of Pd greatly enhanced its HDO activity, where the aromatics yield reached to 83.2% at 450 °C. These Fe species in the catalysts act as oxophilic sites, which can selectively adsorb oxygenated compounds and then form strong bond with the oxygen functionality and final facilitate the cleavage of C–O bonds. In addition, Fe oxides have magnetic property, which is known for the facile recycling by introducing a magnetic field and usually use as support or additive for the heterogeneous catalysts.3840

Recently, there had been reported that Pt supported catalysts had high activity for the


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dehydrogenation of cycloalkanes into aromatics.41, 42 Based on these, a coupling process of hydrodeoxygenation and dehydrogenation is proposed for the HDO of phenols. That is, the hydrogenation products (cycloalkanes) were further dehydrogenated into aromatics via releasing hydrogen. Rinaldi et al.43 had also reported a similar strategy for the conversion of phenols into aromatics through coupling the HDO and dehydrogenation processes. Hence, in this study, magnetic NiO-Al2O3@Fe3O4 mixed metal oxides were prepared and used as supports for Pt based catalysts, and then applied into the HDO of phenols. The effects of Fe oxides content in the catalyst on the HDO activity and products distribution were studied in detail. EXPERIMENTAL SECTION Materials and agents All the agents were of analytical grade having purity above 99%. Ni(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH, Na2CO3 and ethanol were purchased from Tianjin Hengxing Chemical Reagent Co. (Tianjin, China). FeCl2 and FeCl3 were bought from Sinopharm Chemical Reagent Co. (Shanghai, China). The other agents were ordered by Aladdin Chemical Reagent Co. (Shanghai, China). Preparation of Pt supported catalysts Magnetic colloidal sol FeCl2 (1.075g) and FeCl3 (2.95g) were dissolved in a 50 mL water and placed in a 100 mL three-necked flask. Ammonium hydroxide was added this flask at 65 °C to adjust the pH value to about 10, and then kept reacting for 1 h. The magnetic colloidal sol was obtained.



NiO-Al2O3@Fe3O4 supports NiO-Al2O3@Fe3O4 mixed metal oxides were prepared by a constant-pH co-precipitation method employing NaOH/Na2CO3 as the precipitating agent.44 In a typical synthesis, 4.7 mL magnetic colloidal sol, Ni(NO3)2·6H2O (4.36 g) and Al(NO3)3·9H2O (1.88 g) were dissolved in a 40 mL water and placed in a 250 mL three-necked flask. 50 mL NaOH solution (1.2 mol/L) was added dropwise with vigorous agitation. To ensure the pH value to about 10, Na2CO3 solution as a buffer reagent was also added simultaneously. After reaction, the precipitate was washed with water and aged at 95 °C for 10 h, and then washed with anhydrous ethanol again, and dried at 120 °C in an oven for 4 h, and finally calcined in a muffle oven at 500 °C for 5 h to ensure its conversion into mixed metal oxides. These supports were denoted as Fe-Ni-Al-X, where X presented the theoretical weight content of Fe oxides in the Fe-Ni-Al-X mixed metal oxides. Pt supported on NiO-Al2O3@Fe3O4 catalysts Pt supported on NiO-Al2O3@Fe3O4 catalysts were prepared by an incipient wetness impregnation method with chloroplatinic acid solution, where the Pt content in the catalyst was 2.0 wt %. After impregnation, the mixture was dried in an oven at 120 °C for 10 h and calcined at 400 °C with a ramping rate of 5 °C/min for 2 h in air, and then put in a stainless steel tubular flow reactor and reduced by H2 (30 mL/min (STP)) at 350 °C with a ramping rate of 3 °C/min for 4 h. These resultant catalysts were named as Pt-Fe-Ni-Al-X. Catalyst characterization The structure of the catalysts were determined by X–ray diffraction (XRD) technology


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on a D/max2550 18KW Rotating anode X–Ray Diffractometer with monochromatic Cu Kα radiation (λ= 1.5418 Å) radiation at voltage and current of 40 kV and 300 mA. Fourier transforminfrared spectroscopy (FT-IR) characterization of the samples mixed with spectroscopy grade KBr was recorded on a NICOLET 380 FT-IR at room temperature, in the range of 4000–400 cm−1. The specific surface area was measured by a Quantachrome's NOVA–2100e Surface Area instrument by physisorption of nitrogen at –196 °C. The samples were dehydrated at 300 °C using vaccum degassing for 12 h before experiments. Thermogravimetric analysis was carried out with a METTLER TGA/DSC1/1600HT apparatus in air atmosphere at a heating rate of 10 °C/min. Raman spectral experiments were carried out using an inVia Reflex Laser Micro-Raman spectroscope with 532 nm excitation source. The laser power was kept at 0.3 mW during the experiment. The surface electronic state was analyzed by X–ray photoelectron spectroscopy (XPS) using Kratos Axis Ultra DLD instrument at 160eV pass energy. Al Kα radiation was used to excited photoelectrons. The binding energy value of each element was corrected using C1s = 284.6 eV as a reference. The XP spectra of each element was deconvoluted using a Gaussian–Lorentz curve–fitting program. The magnetic property was measured using vibrating sample magnetometer (VSM) analysis at 25 °C with an applied field from -30 000 to +30 000 Oe. Catalyst activity measurement The HDO activity tests were carried out in a 100–mL batch reactor. The catalyst (0.06 g), 4-ethylphenol (3.61 g) and dodecane (25 g) were placed into the batch reactor. Air in the autoclave was evacuated by pressurization–depressurization cycles with (2) (1) (3) 7 ACS Paragon Plus Environment


nitrogen and subsequently with hydrogen. The system was heated to 300 °C, and then pressurized with hydrogen to 2.0 MPa and adjusted the stirring speed to 900 rpm. During the reaction, liquid samples were withdrawn from the reactor and analysed by Agilent 6890/5973N GC–MS and 7890 gas chromatography using a flame ionization detector (FID) with a 30 m AT–5 capillary column. The experiments have been repeated twice at least. The mean standard deviation for these experiments was within 3% and the carbon balance in each experiment was higher than 95%. The conversion, selectivity and deoxygenation degree for each experiment were calculated as follows: Conversion (mol %) = (1 − Selectivity (A, mol%) =

moles of residual reactant ) × 100% moles of initial reactant

moles of product (A) × 100% moles of reacted reactant

Deoxygenation degree (D. D. , wt%) = (1 −

oxygen content in the final organic compounds ) × 100% total oxygen content in the initial mixture

RESULTS AND DISCUSSION Characterization of Pt supported catalysts


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110 113

018

009 015

006

Intensity(a.u.)

(a)

Fe-Ni-Al-0 Fe-Ni-Al-5 Fe-Ni-Al-10 Fe-Ni-Al-15 10

20

30

40

50

60

70

80

90

2Theta(degree)

(b)

◆

Fe3O4 ＃ NiO

＃

◆

＃

＃

◆

Intensity(a.u.)

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


003

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Pt-Fe-Ni-Al-15

Pt-Fe-Ni-Al-10

Pt-Fe-Ni-Al-5 Pt-Fe-Ni-Al-0 10

20

30

40

50

60

70

80

90

2Theta(degree) Fig. 1 XRD patterns of (a) support precursor and (b) Pt-Fe-Ni-Al-X Fig. 1 presents the XRD patterns of support precursor and Pt supported on Fe-Ni-Al-O mixed metal oxides catalysts. Several diffraction peaks were observed at 2θ = 11°, 23°, 35°, 39°, 46°, 61°and 62°in the XRD patterns of support precursor, indexing to the (003), (006), (009), (015), (018), (110) and (113) planes of a typical layered double hydroxides, respectively.45 According to the synthesis procedure, there must produce Fe3O4 as a magnetic substance in the support precursors, but which was not detected in



Fig. 1(a). This might be resulted from the coverage of Ni-Al layered double hydroxides on Fe3O4 or its amorphous state. Because these support precursors had been calcined at 500 °C before the incipient wetness impregnation, the characteristic diffraction peaks to the layered double hydroxides structure were disappeared, as shown in Fig. 1(b), and there emerged three evident diffraction peaks at 2θ = 37°, 43°and 63°, corresponding to the diffractions of the NiO phase.46 These demonstrated that Ni-Al layered double hydroxides had been completely converted into mixed metal oxides. In addition, with the increase of Fe3O4 content, two diffraction peaks at 2θ = 30°and 36°, assigning to (220) and (311) planes of Fe3O4,47, 48 respectively, became obvious, which indicated that the destruction of layered double hydroxides structure exposed Fe3O4 on the catalyst surface or the growth of Fe3O4 particles after a calcination at high temperature. However, Fig. 1(b) presented no diffraction peak to Al2O3, which might be caused by its high dispersion or amorphous state because of the layered double hydroxides structure of the support precursors. No diffraction peak to metal Pt was attributed to its low content and high dispersion. The double hydroxides structure was further confirmed by FT IR, as shown in Fig. 2. Two broad adsorption peaks at 3554 cm-1 and 3446 cm-1 were corresponded to the O–H stretching vibrations in the hydrotalcite layers and interlayer water and physically adsorbed water, and the broad adsorption peaks at 1382 cm-1 and 752 cm-1 were assigned to stretching and bending modes of interlayer carbonate species, and the bands at 550610 cm-1 were attributed to the vibration of Al-O and Ni-O,49, 50 respectively. These indicated that the support precursor had a characteristic double hydroxides structure. For


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Pt supported catalysts, two peaks to carbonate species at 1382 cm-1 and 752 cm-1 and a peak at 3554 cm-1 to water in the hydrotalcite layers completely disappeared. These demonstrated that the Fe-Ni-Al layered double hydroxides were transformed into mixed metal oxides after the calcination at 500 °C before using as supports for Pt based catalysts.49, 50 The bands at 420-610 cm-1 were assigned to the Al-O and Ni-O bond vibrations of the mixed metal oxides. In addition, two peaks at 1630 cm-1 and 3446 cm1

were attributed to the physically adsorbed water on the catalyst surface.

Fe-Ni-Al-0 hydrotalcite

Transmittance (a. u.)

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


Pt-Fe-Ni-Al-15 Pt-Fe-Ni-Al-10 Pt-Fe-Ni-Al-5 Pt-Fe-Ni-Al-0

4000 3500 3000 2500 2000 1500 1000

500

-1

Wave numbers(cm )

Fig. 2 FT IR spectra of Fe-Ni-Al-0 hydrotalcite and Pt supported catalysts To further confirm the complete transformation of layered double hydroxides into the stable mixed metal oxides, thermogravimetric analyses were taken under an air atmosphere. As seen in Fig. 3, the DTA-TG curves of support precursor for Pt-Fe-NiAl-15 showed that its decomposition underwent three crucial stages.46, 51 The initial weight loss under about 100 °C was attributed to the removal of physically adsorbed water. The second one between 100 °C and 225 °C was mainly connected with the removal of interlayer water. The third stage around 311 °C was corresponded to the 11 ACS Paragon Plus Environment


dehydroxylation of layered double hydroxides and decomposition of interlayer carbonate anions. After increasing the temperature above 500 °C, the weight loss was very slight, which indicated that the decomposition process was finished at about 500 °C. Therefore, the final form of the supports in the resultant catalyst were mixed metal oxides. In the third stage, the decomposition of interlayer carbonate anions released gaseous CO2, which could maintain the original structure and hinder the sintering of the oxide components during the calcination process. Moreover, the release of CO2 left vacancy. These would result in the high surface area of Pt supported catalysts.50, 52 As measured by nitrogen physisorption, the surface area of Pt-Fe-Ni-Al-0, Pt-Fe-Ni-Al-5, Pt-Fe-Ni-Al-10 and Pt-Fe-Ni-Al-15 was 105.6 m2/g, 117.8 m2/g, 100.4 m2/g and 112.0 m2/g, respectively. From these data, it could be concluded that the Fe oxides content had little effect on the catalyst surface area. 100 500

225

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70 60

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DTG (% / oC)

90

Weight (%)

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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Temperature (oC) Fig. 3 DTA-TG curves of support precursor for Pt-Fe-Ni-Al-15


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Pt-Fe-Ni-Al-15

Ni 2p

Intensity (a. u.)

(a)

Pt-Fe-Ni-Al-0

868

864

860

856

852

Electron Binding Energy (eV)

Pt-Fe-Ni-Al-15

Fe 2p

710.5

Intensity (a. u.)

(b)

714.7

730

725

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715

708.6

710

705


(c)

Intensity (a.u.)

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


Pt 4f

Pt-Fe-Ni-Al-0

Pt-Fe-Ni-Al-15 86 84 82 80 78 76 74 72 70 68 66 64


Fig. 4

XPS spectra of (a) Ni 2p and (b) Fe 2p levels in Pt-Fe-Ni-Al-X catalysts

The valence state of the elements in Pt-Fe-Ni-Al-0 and Pt-Fe-Ni-Al-15 were analysed by XPS, as shown in Fig. 4. Because the binding energy of Pt 4f is very close to that of 13 ACS Paragon Plus Environment


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Al 2p, it was very difficult to accurately analyse the chemical states of Pt in the coexistence of Al. In comparison with the standard binding energy of metal Ni (853.0 eV),53 Fig. 4 (a) presented that no Ni0 existed in the catalysts, which might be resulted from that Ni0 was easily oxidized in the air because it was inevitable to contact with oxygen before the characterization. For the XP spectra of Fe 2p, as shown in Fig. 4 (b), three peaks at the binding energy of 708.4 eV, 711.2 eV and 714.7 eV were observed, corresponding to the Fe0, Fe2+, Fe3+,48, 53 respectively. These indicated that some of Fe oxides were reduced to Fe0, which might be caused by that the presence of Pt promoted the reduction of Fe oxides. However, the binding energy of Fe0 peak in Pt-Fe-Ni-Al-15 shifted to higher energy value comparing with the standard binding energy metal Fe (707.3 eV).53 Moreover, although the XP spectrums of Pt 4f level of Pt-Fe-Ni-Al-0 and Pt-Fe-Ni-Al-15 were difficult to be deconvoluted, a slightly negative shift of binding energy of Pt was observed in Pt-Fe-Ni-Al-15 in comparison with Pt-Fe-Ni-Al-0, as shown in Fig. 1 (c). These demonstrated that there existed an electron transfer between metallic Pt and Fe. This electron transfer had also been reported in other bimetal catalysts,54,

55

which was expected to enhance the hydrodeoxygenation and

dehydrogenation activity. To determinate the magnetic property, Pt-Fe-Ni-Al-15 catalyst was measured using vibrating sample magnetometer (VSM) analysis at 25 °C with an applied field from −30 000 to +30 000 Oe. Its magnetization curve, seen in Fig. 5a, did not show hysteresis, indicating that Pt-Fe-Ni-Al-15 was paramagnetism.39,

47

A rapid increase of

magnetization with increasing applied magnetic field was observed, and Pt-Fe-Ni-Al-15


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had a saturation magnetization of 1.9 emu/g, which was sufficient for the magnetic separation by a permanent magnet.38 As displayed in Fig. 5b, Pt-Fe-Ni-Al-15 was thoroughly collected from liquid phase using a simple laboratory magnet, but Pt-Fe-NiAl-0 could not. These indicated that adding Fe oxides into the supports could obtain magnetic catalysts, which favoured to separate the catalyst from the reaction solution.

Fig. 5 (a) Hysteresis loops for Pt-Fe-Ni-Al-15 at 27 °C and (b) magnetism image of PtFe-Ni-Al-15 and Pt-Fe-Ni-Al-0 HDO activity of Pt supported catalysts The HDO performances of Pt supported on Fe-Ni-Al-O mixed metal oxides were firstly measured using 4-ethylphenol as a model compound. Fig. 6a presents that the products were ethylcyclohexane and ethylbenzene and no oxygen-containing product was detected under the conditions of 300 °C and 2.0 MPa hydrogen pressure, indicating a high deoxygenation activity for Pt-Fe-Ni-Al-0. According to the changes of products with reaction time, although no oxygen-containing intermediate was observed, the high hydrogenation of Pt made the HDO of 4-ethylphenol mainly proceeded with HYD route. Moreover, ethylcyclohexane selectivity decreased while ethylbenzene selectivity



increased with reaction time, even after the conversion had reached 100%, suggesting that ethylcyclohexane was further converted into ethylbenzene via a dehydrogenation procedure. To confirm this, ethylcyclohexane was used as a reactant to replace 4ethylphenol under the same reaction conditions. After reaction for 8 h, the conversion of ethylcyclohexane reached to 91.9% and ethylbenzene selectivity was almost 100%. Therefore, the plausible reaction routes for the HDO of 4-ethylphenol on these catalysts were proposed and displayed in Scheme 2. 4-Ethylphenol was firstly converted into ethylcyclohexane via hydrogenation and dehydration processes, and then was further converted into ethylbenzene via the dehydrogenation. In this case, the hydrogen was initially stored in benzene ring via the hydrogenation, and then released via the dehydrogenation, which was attributed to the differences on the active components and supports in Pt/NiO-Al2O3@Fe3O4 in comparison with previous catalysts.

Scheme 2 Plausible reaction routes for the HDO of 4-ethylphenol The HDO of 4-ethylphenol on no catalyst and NiO-Al2O3@Fe3O4 support were also carried out under the same conditions. Both the conversions were very low (< 7%), suggesting that the active phase for the HDO reaction was Pt. The effects of Fe oxides content in the final catalysts on the HDO activity are shown in Fig. 6. With the increase 16 ACS Paragon Plus Environment

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of Fe oxides content, both the conversion rate of 4-ethylphenol and ethylbenzene selectivity were enhanced. For example, 97.0% conversion with a selectivity of 23.6% ethylbenzene and 99.4% conversion with a selectivity of 19.7% ethylbenzene were obtained on Pt-Fe-Ni-Al-0 and Pt-Fe-Ni-Al-15 after reaction at 300 °C for 2 h, respectively. When the reaction time prolonged to 8 h, ethylbenzene selectivity on PtFe-Ni-Al-0 and Pt-Fe-Ni-Al-15 were increased to 55.5% and 90.8% respectively. These demonstrated that the addition of Fe enhanced the HDO and dehydrogenation activities. The high ethylbenzene selectivity on these catalyst decreased the hydrogen consumption for the removal of oxygen from the same weight of 4-ethylphenol in comparison with the single HYD route, which might be attributed to the electron transfer between metallic Pt and Fe. This electron transfer leaded to the less negative charge of Pt, strengthening the interaction between the electron donor and Pt active sites and then promoting the adsorption of ethylcyclohexane for the dehydrogenation reaction.56 (a)100

(b)100

90

Conversion Ethylcyclohexane Ethylbenzene

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Conversion/Selectivity (mol %)


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Fig. 6 HDO of 4-ethylphenol on (a) Pt-Fe-Ni-Al-0, (b) Pt-Fe-Ni-Al-5, (c) Pt-Fe-Ni-Al10 and (d) Pt-Fe-Ni-Al-15 at 300 °C Table 1 HDO of 4-methylphenol on Pt-Fe-Ni-Al-0 and Pt-Fe-Ni-Al-15 [a] Pt-Fe-Ni-Al-0

Pt-Fe-Ni-Al-15

Catalyst 1.5 h

8h

1.5 h

8h

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4-Methylcyclohexanol

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88.0

Conversion Product selectivity (mol %)

[a]

Reaction conditions: 0.06 g catalyst, 3.2 g 4-methylphenol, 25.0 g dodecan, 300 °C

and 2 MPa hydrogen pressure Apart from 4-ethylphenol, some other phenols such as 4-methylphenol and guaiacol were also selected as model compounds to study the coupling process of hydrodeoxygenation and dehydrogenation over Pt-Fe-Ni-Al-15. The results are summarized in Table 1 and 2. After reaction at 300 °C and 2 MPa for 1.5 h, the 18 ACS Paragon Plus Environment

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conversion reached 97.9% with a selectivity of 18.3% toluene. 4-Methylcyclohexanol, as an oxygen-containing intermediate for the HDO of 4-methylphenol, was also observed in the products, which confirmed the HYD route. When the reaction time was prolonged to 8 h, toluene selectivity was increased to 88.0% while methylcyclohexane selectivity was decreased to 12.0%, demonstrating that the conversion of 4methylphenol into toluene

was

proceeded

with

the coupling process

of

hydrodeoxygenation and dehydrogenation. In the HDO of guaiacol, cyclohexanol and cyclohexanone were also observed. Benzene selectivity increased from 26.2% to 66.5% after reaction at 300 °C for 8 h when the Fe oxides content was increased from 0 to 15%. These results also confirmed that Pt-Fe-Ni-Al-15 had high HDO activity and dehydrogenation activity. Table 2 HDO of guaiacol on Pt-Fe-Ni-Al-0 and Pt-Fe-Ni-Al-15 [a] Pt-Fe-Ni-Al-0

Pt-Fe-Ni-Al-15

Catalyst 1.5 h

8h

1.5 h

8h

85.5

100

99.8

100

Phenol

4.1

2.0

0.4

0

Cyclohexanol

19.6

0

0

0

Cyclohexanone

19.6

4.7

0.5

0

Cyclohexane

34.3

69.7

58.3

33.4

Benzene

28.1

26.2

40.8

66.5

Conversion Product selectivity (mol %)

[a]

Reaction conditions: 0.06 g catalyst, 1.84 g guaiacol, 25.0 g dodecan, 300 °C and 2 19 ACS Paragon Plus Environment


MPa hydrogen pressure One of the most attracting advantages of magnetic catalysts was its facile recycling. Hence, Pt-Fe-Ni-Al-15 was employed to study the catalyst stability in the HDO of 4ethylphenol. The spent Pt-Fe-Ni-Al-15 catalyst was quickly separated from the reaction solution with the assistance of an external magnet, which was re-used in a subsequent run. After 3 runs, both the conversion and deoxygenation degree were still higher than 99% at 300 °C (Fig. 7), presenting the high stability on the HDO activity. However, ethylbenzene selectivity dropped dramatically after the first run, indicating a very low stability on dehydrogenation activity. To further reveal the deactivation reasons, the spent Pt-Fe-Ni-Al-15 was characterized by XPS and Raman. Fig. 8 (a) presented that no peak to metallic Fe was observed, which might be resulted from the poisoning effect of the produced water on the Fe0. During the liquid HDO reactions, the produced water was not removed immediately, which deactivated the metallic Fe component for dehydrogenation reaction. Moreover, Lien et al.57 had also reported that zero-valent iron formed an effective redox couple with water yielding ferrous iron and hydrogen gas under anaerobic environments. In addition, carbon deposition was usually considered as another significant reason for catalyst deactivation. In Fig. 8 (b), no band was observed in the range of 1200−1800 cm−1, indicating that no carbon deposited on the spent catalyst surface after the HDO reaction.58 These demonstrated that the dramatic reduction on the dehydrogenation activity was attributed to the change of Fe valence state with the presence of water. To further support this conclusion, Pt-Fe-Ni-Al-15 was also applied into the vapor phase dehydrogenation of ethylcyclohexane in a fixed-bed reactor. The


Page 20 of 34

Page 21 of 34

results showed that the product was ethylbenzene and its selectivity was higher than 99.9% after reaction at 300 °C for 10 h. In vapor phase HDO reaction, the produced water could be removed immediately and the oxidization of Fe0 could be inhibited under hydrogen atmosphere. Consequently, the stability of Pt-Fe-Ni-Al-O on dehydrogenation activity might be improved in the vapor phase HDO reactions.

(a) Conversion/Selectivity (%)

100

Conversion Ethylbenzene

60 40 20 0

(b)

100

2

Run (times)

Conversion Ethylbenzene

3

Ethylcyclohexane

80 60 40 20 0 1

Fig. 7

Ethylcyclohexane

80

1

Conversion/Selectivity (%)

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


2

Run (times)

3

HDO of 4-ethylphenol on Pt-Fe-Ni-Al-15 at 300 °C for (a) 3 h and (b) 8 h



Fe 2p

Intensity (a. u.)

(a)

740

735

730

725

720

715

710

705

700


(b) Intensity (a.u.)

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

Page 22 of 34

1000 1100 1200 1300 1400 1500 1600 1700 1800

-1 Raman Shift (cm )

Fig. 8 (a) Fe 2p level and (b) Raman spectra of spent Pt-Fe-Ni-Al-15 CONCLUSIONS Magnetic NiO-Al2O3@Fe3O4 supports with different Fe contents were prepared via the calcination of the mixture of Ni-Al layered double hydroxides and Fe hydroxide and used as supports for Pt based catalysts. Metallic Fe and a positive shift of Fe on the binding energy were observed in these Pt supported on NiO-Al2O3@Fe3O4 catalysts. During the liquid phase HDO of phenols, these catalysts presented high deoxygenation


Page 23 of 34 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


activity. The hydrogenated products such as cycloalkanes could be further converted into aromatics, presenting a good dehydrogenation activity. Aromatics selectivity was increased with the content of Fe in the catalysts, which reached up to 90.8%. Moreover, because of the magnetism of supports, these catalysts were easily separated from the reaction mixture by an external magnet and presented high stability on the HDO activity, but the dehydrogenation activity was dramatic decreased because of the change of metallic Fe into Fe oxides after the HDO reaction, which might be improved in the vapor phase reactions. AUTHOR INFORMATION Corresponding Authors *Phone: (+86) 731-58298581. Fax: (+86)731-58293284. Email: [email protected] (W. Wang); [email protected] (Y. Yang) ORCID Weiyan Wang: 0000-0003-4372-8248 Author Contributions Guohua Zhu and Kui Wu contributed equally to this work and should be considered cofirst authors. Notes The authors declare no competing financial interest ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21776236, 21676225), Natural Science Foundation of Hunan Province (2018JJ2384)



Page 24 of 34

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For Table of Contents Use Only Magnetic Pt/NiO-Al2O3@Fe3O4 catalysts were prepared and presented high activity in the hydrodeoxygenation of phenols as model compounds of renewable bio-oil.

δ+

δ–

Pt Fe

Pt

NiO-Al2O3@Fe3O4

Pt

ACS Paragon Plus Environment

Pt

- 1 - Liquid phase conversion of phenols into aromatics over magnetic

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