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Carbon nanotubes catalysts
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Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts Feng Li, Wenxi Zhu, Shanshan Jiang, Yue Wang, Hua Song, Cuiqin Li* Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing, 163318, Heilongjiang, China
highlights
graphical abstract
Fe3O4 modified Ru/CNTs was successfully prepared. Catalytic transfer hydrogenation of furfural was carried out over Ru eFe3O4/CNTs. Mechanism for catalytic transfer hydrogenation
of
furfural
was
studied.
article info
abstract
Article history:
In this study, magnetic Fe3O4 modified Ru/Carbon nanotubes (CNTs) catalysts were used to
Received 31 March 2019
achieve the catalytic transfer hydrogenation of furfural (FF) to furfuryl alcohol (FFA), with
Received in revised form
alcohols as the solvent and hydrogen donors. According to the result of the catalyst
13 November 2019
characterization, Fe3O4 promoted the formation of Ru0 species. The effects of Fe3O4 loading
Accepted 18 November 2019
and different hydrogen donors on the catalytic transfer hydrogenation of FF were tested,
Available online xxx
and the reaction parameters and catalyst stability were also analyzed. It is found that Fe3O4 effectively enhanced the activity of Ru/CNTs in catalytic transfer hydrogenation of FF, the
Keywords:
catalytic activity was optimized at the Fe3O4 loading of 5 wt%, and the optimal hydrogen
Furfural
donor was i-propanol. Moreover, the RueFe3O4/CNTs could be easily collected for further
Catalytic transfer hydrogenation
use and possessed excellent stability. The mechanism of the catalytic transfer hydroge-
Ru
nation of FF using RueFe3O4/CNTs was discussed, and the corresponding catalyst activity
Fe3O4
groups included metal Ru sites and RuOx-Fe3O4 Lewis acid sites, which account for the
Carbon nanotubes
excellent catalytic activity of transfer hydrogenation. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing, 163318, China. E-mail address: [email protected] (C. Li). https://doi.org/10.1016/j.ijhydene.2019.11.139 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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Introduction
Experimental section
Because of the worldwide fossil resource crisis and growing environmental issues, the study on the catalytic conversion of renewable biomass and biomass-derivatives to high valueadded platform chemicals has aroused great attention. Furfural (FF), which is obtained from the C5 sugars in lignocellulosic biomass, has been extensively investigated and considered one of the most interesting platform molecules. FF contains both C]O in the branched chain structure and C]C in the furan ring, so it can be further processed via hydrogenation [1] and hydrodeoxygenation [2] to produce furfuryl alcohol (FFA), 2-methylfuran (2-MF), furan, tetrahydrofurfuryl alcohol, 2-methyltetrahydrofuran, and tetrahydrofuran. The hydrogenation of FF to FFA, a crucial industrial chemical and intermediate, represents a key synthetic transformation for FF exploitation [3e6]. In the existing industrial process of FF hydrogenation to FFA, CueCr catalysts with H2 have been used. However, the main disadvantage of CueCr catalysts is the high toxicity, which causes serious environmental pollution [7e9]. Moreover, the use of H2 still presents several issues, such as H2 storage, safety and transportation. Accordingly, an effective catalytic system using less toxic catalyst without H2 is urgently demanded for the selective hydrogenation of FF to FFA. Recently, the catalytic transfer hydrogenation has emerged as an alternative approach for the catalytic hydrogenation, using hydrogen donors such as alcohols and acids as hydrogen sources to replace molecular hydrogen [10e14]. The catalytic transfer hydrogenation of FF using various catalysts has also been reported, and the main products are FFA [6,15,16] and methyl furan [17,18]. Though the catalytic transfer hydrogenation of FF using various catalysts has been studied, this process needs further improvement. Besides, the catalytic transfer hydrogenation of FF has only performed under liquid-phase conditions so far. The catalyst recyclability is vital for the practical industrial applications. Usually, it is difficult to collect the nano-sized catalysts while avoiding the weight loss using the conventional methods (such as filtration or centrifugation) during the recycling process. In contrast, magnetic catalysts, overcome the drawback effectively due to the unique separation and recycle recovery properties using an external magnet. Ferrite is an important type of magnetic materials, and particularly, magnetite (Fe3O4) is characterized by low cost, availability, nontoxicity, and easy functionalization with other metallic species or organocatalysts. Adding Fe3O4 into catalysts endows the catalysts with excellent magnetic properties and even further improves the catalytic performances [19e23]. In this study, a series of magnetic Fe3O4-modified Ru/Carbon nanotubes (CNTs) catalysts were prepared. Also, we analyzed the catalytic transfer hydrogenation of FF to FFA using the magnetic RueFe3O4/CNTs catalysts with alcohols as the hydrogen donor. The effects of reaction parameters and catalyst stability were studied. Furthermore, the mechanism underlying the catalytic transfer hydrogenation of FF using RueFe3O4/ CNTs was discussed.
Materials and methods CNTs were obtained from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China, with purity of >97.0%). The other chemicals were purchased from Aladdin Chemicals Co. Ltd (Shanghai, China).
Catalysts preparation The RueFe3O4/CNTs were prepared with 5 wt% Ru and different Fe3O4 loading on the CNTs support with hydrazine hydrate as a reducing agent. Typically, raw CNTs were treated with concentrated HNO3 reflux at 398 K for 6 h, washed with distilled water and dried at 373 K for 12 h. An appropriate amount of FeCl3$6H2O was dissolved in 10 ml of deionized water, and the pH was adjusted to 4.8 by adding dilute ammonia water. Subsequently, CNTs and 8.2 mg of RuCl3 were added into the solution and stirred for 0.5 h. Next, 1.5 ml of 50 wt% hydrazine hydrate was added dropwise at 323 K and stirred for 1 h. The solid was collected by magnetically separating the solution, and then it was washed with deionized water and ethanol in turn. Finally, the resulting solid was dried at 323 K under N2 flow (50 ml/min). The catalysts assynthesized are denoted as Ru-xFe3O4/CNTs, where x ¼ 2, 5, 10 and 15 represents the theoretical loading of Fe3O4. The Ru/ CNTs and Fe3O4/CNTs with 5 wt% Ru and 5 wt% Fe3O4 were prepared using the same method, respectively, for comparison.
Characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ Max-2200 X-ray diffractometer operated at 40 kV and 40 mA using Cu Ka radiation. The morphology was determined through transmission electron microscope (TEM, JEOL JEM-
Intensity (A.U.)
Ru Fe3O4 CNTs Fe3O4/CNTs Ru-15Fe3O4/CNTs Ru-10Fe3O4/CNTs Ru-5Fe3O4/CNTs Ru-2Fe3O4/CNTs Ru/CNTs 10
20
30
40
50
60
70
80
2θ (Degree) Fig. 1 e XRD patterns of Ru/CNTs, Ru-xFe3O4/CNTs and Fe3O4/CNTs.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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4000EX). N2 adsorption/desorption isotherms were obtained by using a Tristar II3020 surface area and porosity analyzer. The specific surface areas were determined by the BrunauerEmmett-Teller (BET) method, and the pore size distribution was calculated by the Barrett-Joyner-Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) spectra were analyzed using a Thermo Fisher Scientific K-Alpha instrument. The Ru and Fe content was analyzed by a Shimadzu ICPS-7510 spectrometer using inductively coupled plasma atomic emission spectroscopy (ICP).
Catalytic experiments The catalytic transfer hydrogenation of FF to FFA using different catalysts was explored. In a typical reaction, 20 mg of catalyst, 1 mmol of FF, 10 ml of hydrogen donor were added to a 50 ml stainless steel autoclave with a quartz lining. After purging the reactor with N2 six times, the reactor was pressured to 2 MPa N2 and then heated to 453 K. After the reaction for 4 h, the reactor was quickly cooled in an ice-water bath. The products were analyzed using a gas chromatograph (Shimadzu GC-14C, FID, DB-Wax capillary column), and then
identified using gas chromatography/mass spectrometry (Agilent 7890/5975C).
Results and discussion Catalyst characterization Fig. 1 shows the XRD patterns of Ru/CNTs, Ru-xFe3O4/CNTs and Fe3O4/CNTs. Obviously, the diffraction peaks at 2q of 26.1 , 43.1 and 53.5 are present in all samples, corresponding to (002), (100) and (004) crystal faces of the graphited carbon tube wall [24], respectively. For the Ru/CNTs, besides the peaks of CNTs, the weak peak at 2q of 44.0 can be attributed to the hexagonal phase of Ru (101) [25]. For the Fe3O4/CNTs, the diffraction peaks at 2q of 18.3 , 30.1 , 35.4 , 53.4 , 56.9 and 62.5 can be assigned to the diffraction of (111), (220), (311), (422), (511), and (440) crystal planes of Fe3O4 [26], respectively, which implies the successful synthesis of Fe3O4. The RuxFe3O4/CNTs shows the characteristic diffraction peaks of Ru, Fe3O4 and CNTs, and the peak intensity of Fe3O4 was gradually enhanced with the increase in Fe3O4 loading.
Fig. 2 e TEM images of (a) Ru/CNTs, (b) Fe3O4/CNTs and (c) Rue5Fe3O4/CNTs.
(b)
Ru/CNTs Ru-Fe O /CNTs
CNTs Fe O /CNTs
15
Pore volume (×10 cm /g)
CNTs Fe O /CNTs
150
100
50
0 0.0
9
6
0.4
0.6
Relative pressue (p/p0)
0.8
1.0
12 9 6 3 0
3
0.2
Ru/CNTs Ru-Fe O /CNTs
15
12
-3
3
200
Pore volume (cm /g)
250
3
Absorebed amount (cm /g STP)
(a)
0
0
3
6
9
12
15
Pore diameter (nm)
0
40
80
120
160
200
Pore diameter (nm)
Fig. 3 e (a) N2 adsorptionedesorption isotherms and (b) pore size distribution curves of CNTs, Ru/CNTs, Rue5Fe3O4/CNTs and Fe3O4/CNTs.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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the mesoporous structures. For all samples, the pores were distributed uniformly, and the dominant mesopore was about 3 nm in size. Note that the pore volume of Rue5Fe3O4/CNTs at pore size of 3 nm increased obviously, suggesting that Rue5Fe3O4/CNTs has more mesoporous structure compared with Ru/CNTs and Fe3O4/CNTs. The textural properties of CNTs and catalysts are listed in Table 1. Compared with Ru/ CNTs and Fe3O4/CNTs, the specific surface areas of Rue5Fe3O4/CNTs were enlarged obviously. The Rue5Fe3O4/CNTs has the largest pore volume and the smallest pore diameter among all samples, which further confirms that Rue5Fe3O4/ CNTs has more mesoporous structures than Ru/CNTs and Fe3O4/CNTs. This was because the Ru particles of Ru/CNTs were small-sized and a part of Ru particles entered the CNTs, which blocked the tubes and reduced the specific surface areas. In contrast, the Fe3O4 particles in Fe3O4/CNTs were large-sized and could not enter the CNTs, whereas they could only be loaded outside the CNTs, which slightly affects the specific surface areas. During the preparation of Rue5Fe3O4/ CNTs, Ru reacted with Fe3O4, which enlarged the pore structures of Fe3O4, thereby expanding the specific surface areas. The full-survey XPS spectra of Ru/CNTs, Rue5Fe3O4/CNTs and Fe3O4/CNTs reveal the presence of C, O, Ru and Fe elements (Fig. 4a). The peak of Ru 3d3/2 at about 284.5 eV in Ru/CNTs and
Table 1 e The textural properties of CNTs and catalysts. Sample
SBET (m2/g)
Vpore (cm3/g)
dpore (nm)
107.6 80.6 87.0 98.4
0.39 0.34 0.37 0.38
13.4 16.9 17.1 15.3
CNTs Ru/CNTs Fe3O4/CNTs Rue5Fe3O4/CNTs
Fig. 2 shows the TEM images of Ru/CNTs, Fe3O4/CNTs and Rue5Fe3O4/CNTs. Ru particles distribute evenly in Ru/CNTs and the particle sizes ranges within 1e1.5 nm Fe3O4 particles in Fe3O4/CNTs distribute mainly inside and outside the CNTs, and the particle size is large, ranging mainly within 7e11 nm. The particle size in Rue5Fe3O4/CNTs is mainly about 3.5 nm, showing particle aggregation to some degree. However, compared with Fe3O4/CNTs, no particles as large as Fe3O4 in Fe3O4/CNTs were found in Rue5Fe3O4/CNTs. Fig. 3 shows N2 adsorption-desorption isotherms and pore size distribution curves of CNTs, Ru/CNTs, Rue5Fe3O4/CNTs and Fe3O4/CNTs. Clearly, all samples exhibit a type II hysteresis loop according to the IUPAC classification (Fig. 3a), which suggests the presence of a mesoporous structure in the samples. The pore size distribution curve in Fig. 3b further proves
(a)
(b) C 1s Ru 3d
C 1s Ru 3d3/2
Intensity (a.u.)
Intensity (a.u.)
O 1s Ru/CNTs
Ru 3p
Fe 2p
Ru-5Fe3O4/CNTs
700
600
500
400
300
200
100
0
288
Binding Energy (eV) (c) 461.7
285
Ru-5Fe3O4/CNTs
710.8
464
462
460
Binding Energy (eV)
458
456
Intensity (a.u.)
Intensity (a.u.)
Ru/CNTs
466
279
(d) 719
461.3
468
282
Binding Energy (eV)
Run+
470
Ru 3d5/2
Ru-5Fe3O4/CNTs
Ru/CNTs
Fe3O4/CNTs 800
Fe3O4/CNTs
716
713.3
719 716
735
730
725
720
715
Ru-5Fe3O4/CNTs 710.2
710.5 Fe3O4/CNTs 712.5 709.1
710
705
700
Binding Energy (eV)
Fig. 4 e XPS spectra patterns of Ru/CNTs, Rue5Fe3O4/CNTs and Fe3O4/CNTs. (a) Full survey, (b) C 1s and Ru 3d spectra, (c) Ru 3p spectra, and (d) Fe 2p spectra.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
international journal of hydrogen energy xxx (xxxx) xxx
Conversion
FOL
2-MF
others
80 60 40 Fe3O4/CNTs
Conversion / Selectivity (%)
100
20 0
0
2
5
10
15
5
The Fe 2p peaks (Fig. 4d) of Fe3O4/CNTs show two broad peaks at 710.4 and 724.0 eV, corresponding to Fe 2p3/2 and Fe 2p1/2 states, respectively, and consistent with the reported XPS of Fe3O4 [30,31]. Furthermore, the spectrum can fit three main peaks and two satellite peaks in the Fe 2p3/2. The peak at 709.1 eV is assigned to Fe2þ octahedral species, with a satellite peak at 716.0 eV. The peaks at 710.5 and 712.5 eV are attributed to Fe3þ octahedral species and Fe3þ tetrahedral species, respectively. The satellite peak at 719.0 eV is assigned to Fe3þ. The Fe2þ/Fe3þ ratio was found 0.43 for the Fe 2p3/2 transition, close to the stoichiometry of Fe3O4.
Modification effects of Fe3O4 on Ru/CNTs 5
Loading of Fe3O4 (wt%) Fig. 5 e Effect of Fe3O4 loading on catalytic transfer hydrogenation of FF over Ru/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 2 MPa N2, 453 K, 4 h.
Rue5Fe3O4/CNTs is overlapped with that of C 1s at 284.6 eV (Fig. 4b), which complicates the accurate analysis [27]. The binding energy of Ru 3d5/2 in Ru/CNTs and Rue5Fe3O4/CNTs shifted negatively compared to the standard binding energy of Ru 3d5/2, which implies the occurrence of electron transfer. For the Ru 3p3/2 peaks (Fig. 4c) of Ru/CNTs, two peaks around 461.3 and 465.0 eV can be attributed to Ru0 and Runþ species [28,29], respectively. Compared to the standard binding energy of Ru 3p3/2 (461.3 eV), the corresponding binding energy of Ru 3p3/2 (461.7 eV) in Ru/CNTs shifted positively, indicating that some electrons were transferred from Ru to CNTs in Ru/CNTs to form electron-deficient Ru sites. The composition percentages of Ru0 and Runþ calculated based on the deconvoluted peak areas were 86.5% and 13.5%, respectively. In Rue5Fe3O4/CNTs, there composition percentages were 91.2% and 8.8%, respectively. Furthermore, the Ru0 peak intensity of RueFe3O4/CNTs is obviously higher than that of Ru/CNTs, suggesting that adding Fe3O4 can promote the formation of Ru0 species.
According to the result of blank experiment without catalyst, there was no reaction between FF and i-propanol. The catalytic transfer hydrogenation of FF using different catalysts is shown in Fig. 5. For Ru/CNTs, the FF conversion and FFA selectivity are 46.9% and 95.4%, respectively. Compared with Ru/CNTs, the addition of small amount of Fe3O4 can significantly enhance the activity of catalytic transfer hydrogenation. The FF conversion increases from 81.8% to 98.9% when the Fe3O4 loading increases from 2 wt% to 5 wt%. With further loading of Fe3O4, the FF conversion rate slightly rose and the corresponding selectivity of FFA slightly declined. Besides, the catalytic performance of Fe3O4/CNTs with Fe3O4 loading of 5 wt% was also examined. A 30.8% FF conversion and 100% FFA selectivity were obtained over Fe3O4/CNTs.
Catalytic transfer hydrogenation of FF FF contains both C]O and C]C bonds, and the reaction networks of the catalytic transfer hydrogenation of FF with ipropanol as the hydrogen donor are shown in Scheme 1. During this process, the chemical properties of hydrogen donors determine the evolution ability of active H from the alcohol dehydrogenation by a catalyst, which will further affect the catalytic performance. Fig. 6 shows the effect of various alcohols on the catalytic transfer hydrogenation of FF. As the alkyl chain length was extended, the FF conversion rate first rose and then declined. Under the same carbon number, when secondary alcohols served as the hydrogen donor, the FF conversion was higher than that of primary alcohols. This
Scheme 1 e Reaction pathway of the catalytic transfer hydrogenation of FF.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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international journal of hydrogen energy xxx (xxxx) xxx
2-MF
others
80 60
n-Butanol
n-Propanol
i-Butanol
Ethanol
20
i-Propanol
40 Methanol
Conversion / Selectivity (%)
FOL
0
FOL
2-MF
others
3
4
4(10 h)
80 60 40 20 0
Hydrogen Donor
Conversion
100
Conversion / Selectivity (%)
Conversion
100
1
2
Amount of Furfural (mmol)
Fig. 6 e Effect of hydrogen donors on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), alcohol (10 ml), catalyst (20 mg), 2 MPa N2, 453 K, 4 h.
Fig. 8 e Effect of FF amount on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), catalyst (20 mg), 2 MPa N2, 453 K, 4 h.
can be explained by the hydrophilicity and reduction potentials of alcohols. From the perspective of hydrophilic properties, Rue5Fe3O4/CNTs exhibits hydrophilic characteristics due to the presence of different oxygenated groups on CNTs and metal oxides. However, as the alkyl chain length was prolonged, the hydrophilic properties of alcohols were reduced, which was unfavorable for the adsorption of alcohols onto catalyst surfaces. From the perspective of reduction potential, the reduction potential of different alcohols decreased following methanol > n-propanol > ethanol > n-butanol > ipropanol > i-butanol [12,32]. The primary alcohols have higher reduction potentials than secondary alcohols, or namely, secondary alcohols exhibit stronger hydrogen donating capability. When i-propanol served as the hydrogen donor, the
hydrogenation activity of FF was the highest among all alcohols. Moreover, acetone as the dehydrogenation product of ipropanol and almost the same amount of FFA were detected, which clearly suggested i-propanol was the best hydrogen donor for the catalytic transfer hydrogenation of FF to FFA. Figs. 7 and 8 show the effects of catalyst and FF amount on the catalytic transfer hydrogenation of FF, respectively. The FF conversion dramatically increased from 26.4% to 98.9% when the catalyst amount rose from 5 to 20 mg. Subsequently, change in FF conversion is insignificant with a catalyst amount of 25 mg. When the catalyst dosage was constant at 20 mg, the FF conversion rate declined with the increase in the FF amount. When the FF amount was 4 mmol, the FF conversion rate after 4 h of reaction was 42.2%, and as the reaction time was further prolonged to 10 h, the FF conversion rate rose to 91.0%. Unlike the conversion of FF, the selectivity of FFA remained unchanged regardless of the catalyst or FF amount. The effect of reaction temperature on the catalytic transfer hydrogenation of FF was investigated (Fig. 9). Reaction temperature at 393 K allowed for a low catalytic activity and a 2.1% FF conversion. When the reaction temperature exceeded 413 K, the catalytic reactivity was rapidly enhanced with the rise in the temperature, and at the reaction temperature of 453 K, the FF conversion reached 98.9%. When the reaction temperature was further raised to 493 K, an increased FF conversion of 99.4% with a slight decreased FFA selectivity of 91.1% was obtained. Generally, the higher the reaction temperature, the higher the catalytic activity will be. Moreover, Wang et al. [33] studied the catalytic transfer hydrogenation of FF over hydroxyapatite-encapsulated magnetic g-Fe2O3 and found that higher reaction temperature could accelerate the dehydrogenation of i-propanol. During the FF catalytic conversion hydrogenation, the N2 pressure of the reaction system did not affect the FFA selectivity seriously, and the selectivity remained higher than 97% (Fig. 10). However, when the N2 pressure exceeded 2 MPa, the
Conversion
Conversion / Selectivity (%)
100
FOL
2-MF
others
15
20
25
80 60 40 20 0
5
10
Amount of Catalysts (mg) Fig. 7 e Effect of catalyst amount on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), 2 MPa N2, 453 K, 4 h.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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international journal of hydrogen energy xxx (xxxx) xxx
Conversion
FOL
2-MF
others
80 60 40 20 0
393
413
433
453
4 73
Conversion
100
Conversion / Selectivity (%)
Conversion / Selectivity (%)
100
2-MF
others
80 60 40 20 0
493
FOL
0.5
1
2
3
4
6
10
24
Reaction Time (h)
Reaction Temperature (K) Fig. 9 e Effect of reaction temperature on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 2 MPa N2, 4 h.
Fig. 11 e Effect of reaction time on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 0.1 MPa N2, 453 K.
FF conversion slightly declined. Lower N2 pressure is suitable for enhancing the catalytic activity. Fig. 11 shows the effect of reaction time on the catalytic transfer hydrogenation of FF. The FF conversion increased gradually during the reaction process, especially at the early reaction stage. When the reaction time exceeded 4 h, the FF conversion remained at nearly 99.4%, whereas, the selectivity of FFA decreased gradually. When the reaction time was extended to 24 h, the FF conversion and the selectivity of FFA were 99.6% and 91.8%, respectively. This was because with the extension of reaction time, the hydrogenation of FF would be excessive to form 2-MF and other by-products.
Reusability of catalyst
Conversion
FOL
2-MF
others
80 60 40 20 0
0.1
0.5
1
2
3
4
N2 Pressure (MPa) Fig. 10 e Effect of N2 pressure on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 453 K, 4 h.
Conversion
100
Conversion / Selectivity (%)
Conversion / Selectivity (%)
100
Catalyst stability is vital for industrial applications. The stability of Rue5Fe3O4/CNTs determined by five consecutive catalytic tests is shown in Fig. 12. After each recycling experiment, the used catalyst was collected from the reaction mixture using the external magnet, washed with i-propanol and then used directly in the next run. After the first cycle, the conversion of FF decreased slightly from 99.4% to 98.1%. From the second cycle, however, the Rue5Fe3O4/CNTs shows a significant stability without a significant loss in catalytic activity.
FOL
2-MF
others
80 60 40 20 0
0.5
1
2
3
4
6
10
24
Reaction Time (h) Fig. 12 e Recycling of the Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 0.1 MPa N2, 453 K, 4 h.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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international journal of hydrogen energy xxx (xxxx) xxx
Mechanism for the catalytic transfer hydrogenation of FF over RueFe3O4/CNTs
Fig. 13 e XRD patterns of fresh and spent Rue5Fe3O4/CNTs.
To explain the catalyst deactivation in the first cycle, fresh catalyst was used again for the catalytic test to obtain the used catalyst after the first recycle. According to the result of XRD, compared with the fresh Rue5Fe3O4/CNTs, the catalyst after cyclic use showed the peaks of Ru0, Fe3O4 and CNTs, but no other peaks (Fig. 13), whereas after one cycle of use, the peak of Ru0 at 2q of 44.0 in the catalyst decreased slightly. After five cycles of use, however, the peak intensity of Ru0 in the catalyst did not significantly decline any more. ICP showed the Ru content in the catalyst declined from 4.6 wt% to 4.4 wt% and then to 4.3 wt% after one and five cycles of use, and the loss of Ru primarily occurred in the first recycle. This was because in the RueFe3O4/CNTs, the Ru particles was physically adsorbed onto CNTs. Under vigorous stirring, the gently-adsorbed and unstable Ru particles onto CNTs fell off, which primarily occurred during the first recycle. However, no significant loss of Fe was observed, which can be attributed to the magnetism of Fe3O4 and the use of magnet during the catalyst recycle.
The catalytic transfer hydrogenation of unsaturated compounds with i-propanol as the hydrogen donor has been extensively studied over various catalysts, but the views concerning the active components of catalysts and the corresponding hydrogen transfer mechanisms are conflicting. In the existing research, there are two reaction routes about the catalytic transfer hydrogenation on metal-mediated hydrogenation and Lewis acid-mediated intermolecular hydride transfer, which correspond to the active components of metal sites and Lewis acid sites, respectively. Kobayashi et al. [34] suggested that RuO2 is the active species, rather than Ru metal nanoparticles, in the transfer hydrogenation of cellulose to sugar alcohol over Ru/C catalyst. Chen et al. [35] reported that both reaction pathways have occurred in the catalytic transfer hydrogenation of FF over Cu/MgOeAl2O3, i.e. metal Cu sites and Lewis acid sites, respectively. Vlachos' group [36] have studied the catalytic transfer hydrogenation of FF over Ru/C catalyst and suggested that Ru and RuOx are the active phases. However, further mechanism investigations have shown that the hydrogenation of carbonyl group proceeds via the Lewis acid-catalyzed intermolecular hydride transfer mechanism rather than dehydrogenation of the i-propanol, followed by hydrogenation of the carbonyl group on metal sites [37,38]. In this study, the catalytic transfer hydrogenation conversion of FF over Ru/CNTs was 46.9%. XRD and XPS reveal that the Ru in Ru/CNTs primarily existed in the form of metal Ru and very rarely as RuOx (Lewis acid). Thus, it could be stated that metal Ru played the role of catalysis. Fe3O4 exhibits both magnetic property and Lewis acid function [39]. Compared with Ru/CNTs, the Rue5Fe3O4/CNTs further enhanced the Lewis acid concentration when the Ru concentration was unchanged, and the catalytic transfer hydrogenation of FF rose from 46.9% to 98.9%. Given the catalytic transfer hydrogenation of FF using Fe3O4/CNTs reached 30.8%, it could be stated that the metal Ru sites and RuOx-Fe3O4 Lewis acid sites of Rue5Fe3O4/CNTs were also capable of catalysis, which primarily proceeds through metal-mediated hydrogenation
Fig. 14 e Mechanism for catalytic transfer hydrogenation of FF over RueFe3O4/CNTs. Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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and Lewis acid-mediated intermolecular hydride transfer, respectively. The underlying mechanism of catalysis is shown in Fig. 14. During metal-mediated hydrogenation, H atoms transferred are first adsorbed onto the Ru metallic sites, and then the adsorbed H atoms were added to the C]O bond starting with CeH and followed by OeH bond formation. In contrast, the Lewis acid-mediated intermolecular hydride was transferred following the Meerwein-Ponndorf-Verley (MPV) mechanism. The MPV mechanism began via the formation of a propanol-FF six-membered ring configuration by the adsorbed i-propanol and FF on the Lewis acid sites, through which the b-H of i-propanol was transferred to the carbonyl C atom of FF.
[5]
[6]
[7]
[8]
Conclusions The catalytic transfer hydrogenation of FF to FFA using the Fe3O4-modified Ru/CNTs was successfully achieved. The resulting RueFe3O4/CNTs shows 99.4% FF conversion and 100% FFA selectivity using i-propanol under 453 K and 0.1 MPa N2 for 4 h. Adding Fe3O4 could promote the formation of Ru0 species. Furthermore, adding Fe3O4 endowed the catalysts with magnetism, so that the catalysts can be easily recovered from the reaction mixture using a magnet without significant loss of catalytic activity. In the meantime, Fe3O4 could also catalyze the active components. The catalytic transfer hydrogenation of FF over RueFe3O4/CNTs proceeds through metal-mediated hydrogenation (Ru metal) and Lewis acidmediated intermolecular hydride transfer (RuOx-Fe3O4 Lewis acid), respectively.
[9]
[10]
[11]
[12]
Acknowledgments [13]
We gratefully acknowledge the financial supports from Northeast Petroleum University (ts26180228) and Heilongjiang Natural Science Foundation of China (E2018012).
[14]
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Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
Available online at www.sciencedirect.com
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Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts Feng Li, Wenxi Zhu, Shanshan Jiang, Yue Wang, Hua Song, Cuiqin Li* Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing, 163318, Heilongjiang, China
highlights
graphical abstract
Fe3O4 modified Ru/CNTs was successfully prepared. Catalytic transfer hydrogenation of furfural was carried out over Ru eFe3O4/CNTs. Mechanism for catalytic transfer hydrogenation
of
furfural
was
studied.
article info
abstract
Article history:
In this study, magnetic Fe3O4 modified Ru/Carbon nanotubes (CNTs) catalysts were used to
Received 31 March 2019
achieve the catalytic transfer hydrogenation of furfural (FF) to furfuryl alcohol (FFA), with
Received in revised form
alcohols as the solvent and hydrogen donors. According to the result of the catalyst
13 November 2019
characterization, Fe3O4 promoted the formation of Ru0 species. The effects of Fe3O4 loading
Accepted 18 November 2019
and different hydrogen donors on the catalytic transfer hydrogenation of FF were tested,
Available online xxx
and the reaction parameters and catalyst stability were also analyzed. It is found that Fe3O4 effectively enhanced the activity of Ru/CNTs in catalytic transfer hydrogenation of FF, the
Keywords:
catalytic activity was optimized at the Fe3O4 loading of 5 wt%, and the optimal hydrogen
Furfural
donor was i-propanol. Moreover, the RueFe3O4/CNTs could be easily collected for further
Catalytic transfer hydrogenation
use and possessed excellent stability. The mechanism of the catalytic transfer hydroge-
Ru
nation of FF using RueFe3O4/CNTs was discussed, and the corresponding catalyst activity
Fe3O4
groups included metal Ru sites and RuOx-Fe3O4 Lewis acid sites, which account for the
Carbon nanotubes
excellent catalytic activity of transfer hydrogenation. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. College of Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing, 163318, China. E-mail address: [email protected] (C. Li). https://doi.org/10.1016/j.ijhydene.2019.11.139 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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Introduction
Experimental section
Because of the worldwide fossil resource crisis and growing environmental issues, the study on the catalytic conversion of renewable biomass and biomass-derivatives to high valueadded platform chemicals has aroused great attention. Furfural (FF), which is obtained from the C5 sugars in lignocellulosic biomass, has been extensively investigated and considered one of the most interesting platform molecules. FF contains both C]O in the branched chain structure and C]C in the furan ring, so it can be further processed via hydrogenation [1] and hydrodeoxygenation [2] to produce furfuryl alcohol (FFA), 2-methylfuran (2-MF), furan, tetrahydrofurfuryl alcohol, 2-methyltetrahydrofuran, and tetrahydrofuran. The hydrogenation of FF to FFA, a crucial industrial chemical and intermediate, represents a key synthetic transformation for FF exploitation [3e6]. In the existing industrial process of FF hydrogenation to FFA, CueCr catalysts with H2 have been used. However, the main disadvantage of CueCr catalysts is the high toxicity, which causes serious environmental pollution [7e9]. Moreover, the use of H2 still presents several issues, such as H2 storage, safety and transportation. Accordingly, an effective catalytic system using less toxic catalyst without H2 is urgently demanded for the selective hydrogenation of FF to FFA. Recently, the catalytic transfer hydrogenation has emerged as an alternative approach for the catalytic hydrogenation, using hydrogen donors such as alcohols and acids as hydrogen sources to replace molecular hydrogen [10e14]. The catalytic transfer hydrogenation of FF using various catalysts has also been reported, and the main products are FFA [6,15,16] and methyl furan [17,18]. Though the catalytic transfer hydrogenation of FF using various catalysts has been studied, this process needs further improvement. Besides, the catalytic transfer hydrogenation of FF has only performed under liquid-phase conditions so far. The catalyst recyclability is vital for the practical industrial applications. Usually, it is difficult to collect the nano-sized catalysts while avoiding the weight loss using the conventional methods (such as filtration or centrifugation) during the recycling process. In contrast, magnetic catalysts, overcome the drawback effectively due to the unique separation and recycle recovery properties using an external magnet. Ferrite is an important type of magnetic materials, and particularly, magnetite (Fe3O4) is characterized by low cost, availability, nontoxicity, and easy functionalization with other metallic species or organocatalysts. Adding Fe3O4 into catalysts endows the catalysts with excellent magnetic properties and even further improves the catalytic performances [19e23]. In this study, a series of magnetic Fe3O4-modified Ru/Carbon nanotubes (CNTs) catalysts were prepared. Also, we analyzed the catalytic transfer hydrogenation of FF to FFA using the magnetic RueFe3O4/CNTs catalysts with alcohols as the hydrogen donor. The effects of reaction parameters and catalyst stability were studied. Furthermore, the mechanism underlying the catalytic transfer hydrogenation of FF using RueFe3O4/ CNTs was discussed.
Materials and methods CNTs were obtained from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China, with purity of >97.0%). The other chemicals were purchased from Aladdin Chemicals Co. Ltd (Shanghai, China).
Catalysts preparation The RueFe3O4/CNTs were prepared with 5 wt% Ru and different Fe3O4 loading on the CNTs support with hydrazine hydrate as a reducing agent. Typically, raw CNTs were treated with concentrated HNO3 reflux at 398 K for 6 h, washed with distilled water and dried at 373 K for 12 h. An appropriate amount of FeCl3$6H2O was dissolved in 10 ml of deionized water, and the pH was adjusted to 4.8 by adding dilute ammonia water. Subsequently, CNTs and 8.2 mg of RuCl3 were added into the solution and stirred for 0.5 h. Next, 1.5 ml of 50 wt% hydrazine hydrate was added dropwise at 323 K and stirred for 1 h. The solid was collected by magnetically separating the solution, and then it was washed with deionized water and ethanol in turn. Finally, the resulting solid was dried at 323 K under N2 flow (50 ml/min). The catalysts assynthesized are denoted as Ru-xFe3O4/CNTs, where x ¼ 2, 5, 10 and 15 represents the theoretical loading of Fe3O4. The Ru/ CNTs and Fe3O4/CNTs with 5 wt% Ru and 5 wt% Fe3O4 were prepared using the same method, respectively, for comparison.
Characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ Max-2200 X-ray diffractometer operated at 40 kV and 40 mA using Cu Ka radiation. The morphology was determined through transmission electron microscope (TEM, JEOL JEM-
Intensity (A.U.)
Ru Fe3O4 CNTs Fe3O4/CNTs Ru-15Fe3O4/CNTs Ru-10Fe3O4/CNTs Ru-5Fe3O4/CNTs Ru-2Fe3O4/CNTs Ru/CNTs 10
20
30
40
50
60
70
80
2θ (Degree) Fig. 1 e XRD patterns of Ru/CNTs, Ru-xFe3O4/CNTs and Fe3O4/CNTs.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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4000EX). N2 adsorption/desorption isotherms were obtained by using a Tristar II3020 surface area and porosity analyzer. The specific surface areas were determined by the BrunauerEmmett-Teller (BET) method, and the pore size distribution was calculated by the Barrett-Joyner-Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) spectra were analyzed using a Thermo Fisher Scientific K-Alpha instrument. The Ru and Fe content was analyzed by a Shimadzu ICPS-7510 spectrometer using inductively coupled plasma atomic emission spectroscopy (ICP).
Catalytic experiments The catalytic transfer hydrogenation of FF to FFA using different catalysts was explored. In a typical reaction, 20 mg of catalyst, 1 mmol of FF, 10 ml of hydrogen donor were added to a 50 ml stainless steel autoclave with a quartz lining. After purging the reactor with N2 six times, the reactor was pressured to 2 MPa N2 and then heated to 453 K. After the reaction for 4 h, the reactor was quickly cooled in an ice-water bath. The products were analyzed using a gas chromatograph (Shimadzu GC-14C, FID, DB-Wax capillary column), and then
identified using gas chromatography/mass spectrometry (Agilent 7890/5975C).
Results and discussion Catalyst characterization Fig. 1 shows the XRD patterns of Ru/CNTs, Ru-xFe3O4/CNTs and Fe3O4/CNTs. Obviously, the diffraction peaks at 2q of 26.1 , 43.1 and 53.5 are present in all samples, corresponding to (002), (100) and (004) crystal faces of the graphited carbon tube wall [24], respectively. For the Ru/CNTs, besides the peaks of CNTs, the weak peak at 2q of 44.0 can be attributed to the hexagonal phase of Ru (101) [25]. For the Fe3O4/CNTs, the diffraction peaks at 2q of 18.3 , 30.1 , 35.4 , 53.4 , 56.9 and 62.5 can be assigned to the diffraction of (111), (220), (311), (422), (511), and (440) crystal planes of Fe3O4 [26], respectively, which implies the successful synthesis of Fe3O4. The RuxFe3O4/CNTs shows the characteristic diffraction peaks of Ru, Fe3O4 and CNTs, and the peak intensity of Fe3O4 was gradually enhanced with the increase in Fe3O4 loading.
Fig. 2 e TEM images of (a) Ru/CNTs, (b) Fe3O4/CNTs and (c) Rue5Fe3O4/CNTs.
(b)
Ru/CNTs Ru-Fe O /CNTs
CNTs Fe O /CNTs
15
Pore volume (×10 cm /g)
CNTs Fe O /CNTs
150
100
50
0 0.0
9
6
0.4
0.6
Relative pressue (p/p0)
0.8
1.0
12 9 6 3 0
3
0.2
Ru/CNTs Ru-Fe O /CNTs
15
12
-3
3
200
Pore volume (cm /g)
250
3
Absorebed amount (cm /g STP)
(a)
0
0
3
6
9
12
15
Pore diameter (nm)
0
40
80
120
160
200
Pore diameter (nm)
Fig. 3 e (a) N2 adsorptionedesorption isotherms and (b) pore size distribution curves of CNTs, Ru/CNTs, Rue5Fe3O4/CNTs and Fe3O4/CNTs.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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the mesoporous structures. For all samples, the pores were distributed uniformly, and the dominant mesopore was about 3 nm in size. Note that the pore volume of Rue5Fe3O4/CNTs at pore size of 3 nm increased obviously, suggesting that Rue5Fe3O4/CNTs has more mesoporous structure compared with Ru/CNTs and Fe3O4/CNTs. The textural properties of CNTs and catalysts are listed in Table 1. Compared with Ru/ CNTs and Fe3O4/CNTs, the specific surface areas of Rue5Fe3O4/CNTs were enlarged obviously. The Rue5Fe3O4/CNTs has the largest pore volume and the smallest pore diameter among all samples, which further confirms that Rue5Fe3O4/ CNTs has more mesoporous structures than Ru/CNTs and Fe3O4/CNTs. This was because the Ru particles of Ru/CNTs were small-sized and a part of Ru particles entered the CNTs, which blocked the tubes and reduced the specific surface areas. In contrast, the Fe3O4 particles in Fe3O4/CNTs were large-sized and could not enter the CNTs, whereas they could only be loaded outside the CNTs, which slightly affects the specific surface areas. During the preparation of Rue5Fe3O4/ CNTs, Ru reacted with Fe3O4, which enlarged the pore structures of Fe3O4, thereby expanding the specific surface areas. The full-survey XPS spectra of Ru/CNTs, Rue5Fe3O4/CNTs and Fe3O4/CNTs reveal the presence of C, O, Ru and Fe elements (Fig. 4a). The peak of Ru 3d3/2 at about 284.5 eV in Ru/CNTs and
Table 1 e The textural properties of CNTs and catalysts. Sample
SBET (m2/g)
Vpore (cm3/g)
dpore (nm)
107.6 80.6 87.0 98.4
0.39 0.34 0.37 0.38
13.4 16.9 17.1 15.3
CNTs Ru/CNTs Fe3O4/CNTs Rue5Fe3O4/CNTs
Fig. 2 shows the TEM images of Ru/CNTs, Fe3O4/CNTs and Rue5Fe3O4/CNTs. Ru particles distribute evenly in Ru/CNTs and the particle sizes ranges within 1e1.5 nm Fe3O4 particles in Fe3O4/CNTs distribute mainly inside and outside the CNTs, and the particle size is large, ranging mainly within 7e11 nm. The particle size in Rue5Fe3O4/CNTs is mainly about 3.5 nm, showing particle aggregation to some degree. However, compared with Fe3O4/CNTs, no particles as large as Fe3O4 in Fe3O4/CNTs were found in Rue5Fe3O4/CNTs. Fig. 3 shows N2 adsorption-desorption isotherms and pore size distribution curves of CNTs, Ru/CNTs, Rue5Fe3O4/CNTs and Fe3O4/CNTs. Clearly, all samples exhibit a type II hysteresis loop according to the IUPAC classification (Fig. 3a), which suggests the presence of a mesoporous structure in the samples. The pore size distribution curve in Fig. 3b further proves
(a)
(b) C 1s Ru 3d
C 1s Ru 3d3/2
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O 1s Ru/CNTs
Ru 3p
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710.5 Fe3O4/CNTs 712.5 709.1
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Binding Energy (eV)
Fig. 4 e XPS spectra patterns of Ru/CNTs, Rue5Fe3O4/CNTs and Fe3O4/CNTs. (a) Full survey, (b) C 1s and Ru 3d spectra, (c) Ru 3p spectra, and (d) Fe 2p spectra.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
international journal of hydrogen energy xxx (xxxx) xxx
Conversion
FOL
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others
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Conversion / Selectivity (%)
100
20 0
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5
The Fe 2p peaks (Fig. 4d) of Fe3O4/CNTs show two broad peaks at 710.4 and 724.0 eV, corresponding to Fe 2p3/2 and Fe 2p1/2 states, respectively, and consistent with the reported XPS of Fe3O4 [30,31]. Furthermore, the spectrum can fit three main peaks and two satellite peaks in the Fe 2p3/2. The peak at 709.1 eV is assigned to Fe2þ octahedral species, with a satellite peak at 716.0 eV. The peaks at 710.5 and 712.5 eV are attributed to Fe3þ octahedral species and Fe3þ tetrahedral species, respectively. The satellite peak at 719.0 eV is assigned to Fe3þ. The Fe2þ/Fe3þ ratio was found 0.43 for the Fe 2p3/2 transition, close to the stoichiometry of Fe3O4.
Modification effects of Fe3O4 on Ru/CNTs 5
Loading of Fe3O4 (wt%) Fig. 5 e Effect of Fe3O4 loading on catalytic transfer hydrogenation of FF over Ru/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 2 MPa N2, 453 K, 4 h.
Rue5Fe3O4/CNTs is overlapped with that of C 1s at 284.6 eV (Fig. 4b), which complicates the accurate analysis [27]. The binding energy of Ru 3d5/2 in Ru/CNTs and Rue5Fe3O4/CNTs shifted negatively compared to the standard binding energy of Ru 3d5/2, which implies the occurrence of electron transfer. For the Ru 3p3/2 peaks (Fig. 4c) of Ru/CNTs, two peaks around 461.3 and 465.0 eV can be attributed to Ru0 and Runþ species [28,29], respectively. Compared to the standard binding energy of Ru 3p3/2 (461.3 eV), the corresponding binding energy of Ru 3p3/2 (461.7 eV) in Ru/CNTs shifted positively, indicating that some electrons were transferred from Ru to CNTs in Ru/CNTs to form electron-deficient Ru sites. The composition percentages of Ru0 and Runþ calculated based on the deconvoluted peak areas were 86.5% and 13.5%, respectively. In Rue5Fe3O4/CNTs, there composition percentages were 91.2% and 8.8%, respectively. Furthermore, the Ru0 peak intensity of RueFe3O4/CNTs is obviously higher than that of Ru/CNTs, suggesting that adding Fe3O4 can promote the formation of Ru0 species.
According to the result of blank experiment without catalyst, there was no reaction between FF and i-propanol. The catalytic transfer hydrogenation of FF using different catalysts is shown in Fig. 5. For Ru/CNTs, the FF conversion and FFA selectivity are 46.9% and 95.4%, respectively. Compared with Ru/CNTs, the addition of small amount of Fe3O4 can significantly enhance the activity of catalytic transfer hydrogenation. The FF conversion increases from 81.8% to 98.9% when the Fe3O4 loading increases from 2 wt% to 5 wt%. With further loading of Fe3O4, the FF conversion rate slightly rose and the corresponding selectivity of FFA slightly declined. Besides, the catalytic performance of Fe3O4/CNTs with Fe3O4 loading of 5 wt% was also examined. A 30.8% FF conversion and 100% FFA selectivity were obtained over Fe3O4/CNTs.
Catalytic transfer hydrogenation of FF FF contains both C]O and C]C bonds, and the reaction networks of the catalytic transfer hydrogenation of FF with ipropanol as the hydrogen donor are shown in Scheme 1. During this process, the chemical properties of hydrogen donors determine the evolution ability of active H from the alcohol dehydrogenation by a catalyst, which will further affect the catalytic performance. Fig. 6 shows the effect of various alcohols on the catalytic transfer hydrogenation of FF. As the alkyl chain length was extended, the FF conversion rate first rose and then declined. Under the same carbon number, when secondary alcohols served as the hydrogen donor, the FF conversion was higher than that of primary alcohols. This
Scheme 1 e Reaction pathway of the catalytic transfer hydrogenation of FF.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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2-MF
others
80 60
n-Butanol
n-Propanol
i-Butanol
Ethanol
20
i-Propanol
40 Methanol
Conversion / Selectivity (%)
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FOL
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3
4
4(10 h)
80 60 40 20 0
Hydrogen Donor
Conversion
100
Conversion / Selectivity (%)
Conversion
100
1
2
Amount of Furfural (mmol)
Fig. 6 e Effect of hydrogen donors on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), alcohol (10 ml), catalyst (20 mg), 2 MPa N2, 453 K, 4 h.
Fig. 8 e Effect of FF amount on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), catalyst (20 mg), 2 MPa N2, 453 K, 4 h.
can be explained by the hydrophilicity and reduction potentials of alcohols. From the perspective of hydrophilic properties, Rue5Fe3O4/CNTs exhibits hydrophilic characteristics due to the presence of different oxygenated groups on CNTs and metal oxides. However, as the alkyl chain length was prolonged, the hydrophilic properties of alcohols were reduced, which was unfavorable for the adsorption of alcohols onto catalyst surfaces. From the perspective of reduction potential, the reduction potential of different alcohols decreased following methanol > n-propanol > ethanol > n-butanol > ipropanol > i-butanol [12,32]. The primary alcohols have higher reduction potentials than secondary alcohols, or namely, secondary alcohols exhibit stronger hydrogen donating capability. When i-propanol served as the hydrogen donor, the
hydrogenation activity of FF was the highest among all alcohols. Moreover, acetone as the dehydrogenation product of ipropanol and almost the same amount of FFA were detected, which clearly suggested i-propanol was the best hydrogen donor for the catalytic transfer hydrogenation of FF to FFA. Figs. 7 and 8 show the effects of catalyst and FF amount on the catalytic transfer hydrogenation of FF, respectively. The FF conversion dramatically increased from 26.4% to 98.9% when the catalyst amount rose from 5 to 20 mg. Subsequently, change in FF conversion is insignificant with a catalyst amount of 25 mg. When the catalyst dosage was constant at 20 mg, the FF conversion rate declined with the increase in the FF amount. When the FF amount was 4 mmol, the FF conversion rate after 4 h of reaction was 42.2%, and as the reaction time was further prolonged to 10 h, the FF conversion rate rose to 91.0%. Unlike the conversion of FF, the selectivity of FFA remained unchanged regardless of the catalyst or FF amount. The effect of reaction temperature on the catalytic transfer hydrogenation of FF was investigated (Fig. 9). Reaction temperature at 393 K allowed for a low catalytic activity and a 2.1% FF conversion. When the reaction temperature exceeded 413 K, the catalytic reactivity was rapidly enhanced with the rise in the temperature, and at the reaction temperature of 453 K, the FF conversion reached 98.9%. When the reaction temperature was further raised to 493 K, an increased FF conversion of 99.4% with a slight decreased FFA selectivity of 91.1% was obtained. Generally, the higher the reaction temperature, the higher the catalytic activity will be. Moreover, Wang et al. [33] studied the catalytic transfer hydrogenation of FF over hydroxyapatite-encapsulated magnetic g-Fe2O3 and found that higher reaction temperature could accelerate the dehydrogenation of i-propanol. During the FF catalytic conversion hydrogenation, the N2 pressure of the reaction system did not affect the FFA selectivity seriously, and the selectivity remained higher than 97% (Fig. 10). However, when the N2 pressure exceeded 2 MPa, the
Conversion
Conversion / Selectivity (%)
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FOL
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others
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20
25
80 60 40 20 0
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Amount of Catalysts (mg) Fig. 7 e Effect of catalyst amount on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), 2 MPa N2, 453 K, 4 h.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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international journal of hydrogen energy xxx (xxxx) xxx
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Reaction Time (h)
Reaction Temperature (K) Fig. 9 e Effect of reaction temperature on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 2 MPa N2, 4 h.
Fig. 11 e Effect of reaction time on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 0.1 MPa N2, 453 K.
FF conversion slightly declined. Lower N2 pressure is suitable for enhancing the catalytic activity. Fig. 11 shows the effect of reaction time on the catalytic transfer hydrogenation of FF. The FF conversion increased gradually during the reaction process, especially at the early reaction stage. When the reaction time exceeded 4 h, the FF conversion remained at nearly 99.4%, whereas, the selectivity of FFA decreased gradually. When the reaction time was extended to 24 h, the FF conversion and the selectivity of FFA were 99.6% and 91.8%, respectively. This was because with the extension of reaction time, the hydrogenation of FF would be excessive to form 2-MF and other by-products.
Reusability of catalyst
Conversion
FOL
2-MF
others
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0.1
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2
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N2 Pressure (MPa) Fig. 10 e Effect of N2 pressure on catalytic transfer hydrogenation of FF over Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 453 K, 4 h.
Conversion
100
Conversion / Selectivity (%)
Conversion / Selectivity (%)
100
Catalyst stability is vital for industrial applications. The stability of Rue5Fe3O4/CNTs determined by five consecutive catalytic tests is shown in Fig. 12. After each recycling experiment, the used catalyst was collected from the reaction mixture using the external magnet, washed with i-propanol and then used directly in the next run. After the first cycle, the conversion of FF decreased slightly from 99.4% to 98.1%. From the second cycle, however, the Rue5Fe3O4/CNTs shows a significant stability without a significant loss in catalytic activity.
FOL
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others
80 60 40 20 0
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2
3
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Reaction Time (h) Fig. 12 e Recycling of the Rue5Fe3O4/CNTs. Reaction conditions: FF (1 mmol), i-propanol (10 ml), catalyst (20 mg), 0.1 MPa N2, 453 K, 4 h.
Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
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international journal of hydrogen energy xxx (xxxx) xxx
Mechanism for the catalytic transfer hydrogenation of FF over RueFe3O4/CNTs
Fig. 13 e XRD patterns of fresh and spent Rue5Fe3O4/CNTs.
To explain the catalyst deactivation in the first cycle, fresh catalyst was used again for the catalytic test to obtain the used catalyst after the first recycle. According to the result of XRD, compared with the fresh Rue5Fe3O4/CNTs, the catalyst after cyclic use showed the peaks of Ru0, Fe3O4 and CNTs, but no other peaks (Fig. 13), whereas after one cycle of use, the peak of Ru0 at 2q of 44.0 in the catalyst decreased slightly. After five cycles of use, however, the peak intensity of Ru0 in the catalyst did not significantly decline any more. ICP showed the Ru content in the catalyst declined from 4.6 wt% to 4.4 wt% and then to 4.3 wt% after one and five cycles of use, and the loss of Ru primarily occurred in the first recycle. This was because in the RueFe3O4/CNTs, the Ru particles was physically adsorbed onto CNTs. Under vigorous stirring, the gently-adsorbed and unstable Ru particles onto CNTs fell off, which primarily occurred during the first recycle. However, no significant loss of Fe was observed, which can be attributed to the magnetism of Fe3O4 and the use of magnet during the catalyst recycle.
The catalytic transfer hydrogenation of unsaturated compounds with i-propanol as the hydrogen donor has been extensively studied over various catalysts, but the views concerning the active components of catalysts and the corresponding hydrogen transfer mechanisms are conflicting. In the existing research, there are two reaction routes about the catalytic transfer hydrogenation on metal-mediated hydrogenation and Lewis acid-mediated intermolecular hydride transfer, which correspond to the active components of metal sites and Lewis acid sites, respectively. Kobayashi et al. [34] suggested that RuO2 is the active species, rather than Ru metal nanoparticles, in the transfer hydrogenation of cellulose to sugar alcohol over Ru/C catalyst. Chen et al. [35] reported that both reaction pathways have occurred in the catalytic transfer hydrogenation of FF over Cu/MgOeAl2O3, i.e. metal Cu sites and Lewis acid sites, respectively. Vlachos' group [36] have studied the catalytic transfer hydrogenation of FF over Ru/C catalyst and suggested that Ru and RuOx are the active phases. However, further mechanism investigations have shown that the hydrogenation of carbonyl group proceeds via the Lewis acid-catalyzed intermolecular hydride transfer mechanism rather than dehydrogenation of the i-propanol, followed by hydrogenation of the carbonyl group on metal sites [37,38]. In this study, the catalytic transfer hydrogenation conversion of FF over Ru/CNTs was 46.9%. XRD and XPS reveal that the Ru in Ru/CNTs primarily existed in the form of metal Ru and very rarely as RuOx (Lewis acid). Thus, it could be stated that metal Ru played the role of catalysis. Fe3O4 exhibits both magnetic property and Lewis acid function [39]. Compared with Ru/CNTs, the Rue5Fe3O4/CNTs further enhanced the Lewis acid concentration when the Ru concentration was unchanged, and the catalytic transfer hydrogenation of FF rose from 46.9% to 98.9%. Given the catalytic transfer hydrogenation of FF using Fe3O4/CNTs reached 30.8%, it could be stated that the metal Ru sites and RuOx-Fe3O4 Lewis acid sites of Rue5Fe3O4/CNTs were also capable of catalysis, which primarily proceeds through metal-mediated hydrogenation
Fig. 14 e Mechanism for catalytic transfer hydrogenation of FF over RueFe3O4/CNTs. Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139
international journal of hydrogen energy xxx (xxxx) xxx
and Lewis acid-mediated intermolecular hydride transfer, respectively. The underlying mechanism of catalysis is shown in Fig. 14. During metal-mediated hydrogenation, H atoms transferred are first adsorbed onto the Ru metallic sites, and then the adsorbed H atoms were added to the C]O bond starting with CeH and followed by OeH bond formation. In contrast, the Lewis acid-mediated intermolecular hydride was transferred following the Meerwein-Ponndorf-Verley (MPV) mechanism. The MPV mechanism began via the formation of a propanol-FF six-membered ring configuration by the adsorbed i-propanol and FF on the Lewis acid sites, through which the b-H of i-propanol was transferred to the carbonyl C atom of FF.
[5]
[6]
[7]
[8]
Conclusions The catalytic transfer hydrogenation of FF to FFA using the Fe3O4-modified Ru/CNTs was successfully achieved. The resulting RueFe3O4/CNTs shows 99.4% FF conversion and 100% FFA selectivity using i-propanol under 453 K and 0.1 MPa N2 for 4 h. Adding Fe3O4 could promote the formation of Ru0 species. Furthermore, adding Fe3O4 endowed the catalysts with magnetism, so that the catalysts can be easily recovered from the reaction mixture using a magnet without significant loss of catalytic activity. In the meantime, Fe3O4 could also catalyze the active components. The catalytic transfer hydrogenation of FF over RueFe3O4/CNTs proceeds through metal-mediated hydrogenation (Ru metal) and Lewis acidmediated intermolecular hydride transfer (RuOx-Fe3O4 Lewis acid), respectively.
[9]
[10]
[11]
[12]
Acknowledgments [13]
We gratefully acknowledge the financial supports from Northeast Petroleum University (ts26180228) and Heilongjiang Natural Science Foundation of China (E2018012).
[14]
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Please cite this article as: Li F et al., Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.139