Hydrogenation of Biomass-Derived

Subscriber access provided by UNIV OF CAMBRIDGE

Kinetics, Catalysis, and Reaction Engineering

Catalytic transfer hydrogenolysis/hydrogenation of biomassderived 5-formyloxymethylfurfural to 2, 5-dimethylfuran over Ni-Cu bimetallic catalyst with formic acid as a hydrogen donor Yong Sun, Caixia Xiong, Quanchang Liu, Jiaren Zhang, Xing Tang, Xianhai Zeng, Shijie Liu, and Lu Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05960 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 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

Industrial & Engineering Chemistry Research

Catalytic transfer hydrogenolysis/hydrogenation of biomass-derived 5-formyloxymethylfurfural to 2, 5-dimethylfuran over Ni-Cu bimetallic catalyst with formic acid as a hydrogen donor Yong Sun*,†,# Caixia Xiong†,#, Quanchang Liu§, Jiaren Zhang

‖,*

, Xing Tang†, Xianhai Zeng‡, Shijie

Liu┴, Lu Lin†

†

Xiamen Key Laboratory of Clean and High-valued Utilization for Biomass, College of Energy,

Xiamen University, Xiamen, 361102, P. R. China ‡Fujian

Engineering and Research Center of Clean and High-valued Technologies for Biomass,

Xiamen 361102, PR China, 361102, P. R. China §China ‖

Chemical Industry News, Beijing 100120, P. R. China

PetroChina Petrochemical Research Institute, Beijing 102206, China

┴Department

of Paper and Bioprocess Engineering, State University of New York College of

Environmental Science and Forestry, Syracuse, New York 13210, United States

Corresponding Author * E-mail:

[email protected]

* E-mail:

[email protected]

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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 Compared to biomass-derived 5-hydroxymethylfurfural (HMF), 5-formyloxymethylfurfural (FMF) is highly potential as feedstock to produce 2, 5-dimethylfuran (DMF) due to its properties of higher stability and enhanced hydrophobicity. Herein, catalytic conversion of FMF to DMF over Ni-Cu bimetallic catalyst with formic acid (FA) as hydrogen donor was investigated. A favorable DMF yield of 71.0% was obtained by catalyst 36Ni-12Cu/SBA-15. The conversion of FMF to DMF was controlled by surface reaction on the catalyst. In addition, catalytic mechanism investigation showed that the excellent catalytic performance and recyclability of catalyst were attributed to the formation of Cu-Ni alloy and its synergistic role. The incorporation of Cu and Ni substantially improved the selectivity of hydrogenolysis towards C-O ester bond. Meanwhile, FMF exhibited a better advantage to substitute HMF for producing DMF over Ni-Cu bimetallic catalyst with formic acid as a hydrogen donor.

KEYWORDS：5-formyloxymethylfurfural; 2, 5-dimethylfuran; formic acid; Ni-Cu bimetallic catalysts

TOC GRAPHIC

2


Page 2 of 22

Page 3 of 22 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


1. INTRODUCTION Transformation of renewable lignocellulosic biomass to biofuels has captured great interest in recent years. Noteworthily, 2, 5-Dimethylfuran (DMF), as a promising new generation of alternative biofuel derived from biomass, is particularly attractive due to the merits of high energy density (30 kJcm-3), high octane number (RON = 119), nearly ideal boiling point (94 °C), excellent miscibility with gasoline and low solubility in water (2.3 g L−1). 1, 2 At present, the most common way to produce DMF is the hydrodeoxygenation (HDO) of the biomass-based platform chemical 5-hydroxymethylfurfural (HMF) with H2 in the presence of metalbased catalyst. In 2007, Dumesic et al.3 provided a catalytic strategy for the production of 2, 5dimethylfuran from fructose via intermediate HMF, creating a route for transforming abundant renewable biomass resources into liquid fuel. In this multistep route, Cu-Ru/C catalyst was developed to catalyze the conversion of HMF to DMF, giving 71 % yield of DMF from 5 wt% HMF at 220 C under 6.8 atm of H2. And in 2013, Nishimura et al.4 reported that the selective hydrogenation of HMF over the carbon-supported palladium-gold (Pd-Au/C) in the presence of hydrochloric acid (HCl) under atmospheric hydrogen pressure gave excellent DMF yield as high as 96 %. More recently, Saha et al.5 developed a bimetallic catalyst containing Zn and Pd/C components for the conversion of HMF to DMF with a high conversion of 99 % and selectivity of 85 %, however, the deactivation of the catalyst was observed after the 4th cycle. It was also reported that Ru-MoOx/C catalyst brought about a DMF yield of 79.4% at 180 C under 1.5 MPa of H2.6 Zu et al.7 reported an excellent yield of DMF (93.4%) from HMF using Ru/Co3O4 as catalyst. From these studies, we find that noble metals, such as Pd, Au and Ru, are always good candidates for catalyzing HMF to DMF due to their high catalytic activity and selectivity. On the other hand, considering the high cost, scarcity and long-term availability of noble metals, great efforts have been made to develop non-noble metal catalysts recently. Huang et al.8 developed Ni-W2C/AC catalytic system for the conversion of HMF to DMF, which achieved a DMF yield as high as 96 % (130 C, 3 h, 4.0 MPa H2). Ni/Co3O4 catalyst developed by Yang et al. also achieved a 76% yield of DMF from HMF under relatively mild conditions (130 C, 24 h, 1.0 MPa H2). Lately, it was reported that Cu-Co bimetallic nanoparticles coated with carbon layers showed excellent catalytic capacity for the HDO of HMF to DMF, achieved a DMF yield of 99.4% at 180 C for 8.0 h under 5.0 MPa 3



H2.9 Molecular hydrogen is widely employed as the hydrogen donor in most HDO processes of HMF, due to its wide availability and easy activation on metal surface. However, the low solubility in most solvents makes high pressure H2 necessary to achieve a desired DMF yield, accompanied by

safety concerns and high cost.10 Compared to H2, formic acid (FA), as a renewable liquid-phase hydrogen donors that can be produced from lignocellulosic biomass, have attracted extensive attention for catalytic transfer hydrogenation (CTH) reactions.11 In 2010, Thananatthanachon and Rauchfuss first investigated the selective hydrogenation of HMF using formic acid as hydrogen donor together with catalysts Pd/C and sulfuric acid, achieving an excellent DMF yield.12 De et al.13 reported the one pot conversion of lignocellulosic and algal biomass to DMF with Ru/C together with H2SO4 as catalysts and FA as hydrogen donor, obtaining a yield of 32%. However, the addition of H2SO4 in two reports above-mentioned poses a challenge for the durability of material and catalysts. In 2017, Wang and co-workers 11 studied the CTH/hydrogenolysis of HMF to DMF over Ni-Co/C with formic acid as hydrogen donor, the excellent DMF yield of 90.0 % was obtained (210 C, 24 h), while the catalytic activity of the catalyst decreased sharply after the 1st run due to the loss of metal constituents. As is well known, isolation/storage of HMF is still a challenge. Among the reported purification methods available, distillation is not suitable to purify heat-sensitive HMF due to the decomposition and polymerization caused by the high active hydroxyl and aldehyde group. Compared to HMF, FMF with -OOCH in place of -OH possesses the properties of lower polarity, higher stability and enhanced hydrophobicity (approximate 80 g water is required to dissolve 1 g of FMF), favoring its separation form the reaction mixture by vacuum distillation and further purification. Our lab had developed the preparation process of FMF with the purity of 98.8% based on the one-pot pathway of cellulose and two-step pathway of fructose.14, 15 In addition, in some studies of converting HMF to DMF using formic acid as hydrogen donor, FMF was deemed as a key intermediate.12 The conversion is proposed to proceed through hydrogenolysis of the ester bond in intermediates 5-formyloxymethylfurfural and 2formyloxymethyl-5-methylfuran. In our work, based on the advantages of FMF described above, the endeavor to produce DMF by substituting HMF with FMF over non-noble Ni-Cu/SBA-15 4


Page 4 of 22

Page 5 of 22 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


bimetallic catalyst was made through CTH process with formic acid as hydrogen donor.

2. EXPERIMENTAL SECTION 2.1. Materials FMF was prepared from cellulose with formic acid by the method reported in our previous literature.14 HMF (99%) was supplied by Sigma-Aldrich China Co. Ltd. (Shanghai, China). Formic acid was purchased from Aladdin Chemical Technology Co. Ltd. (Shanghai, China). Zeolite SBA15 was purchased from Nanjing JCNANO Technology Co. Ltd (Nanjing, China). Cu(NO3)2·3H2O, Ni(NO3)2·6H2O and other chemicals were supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used without further treatment. 2.2. Catalyst Preparation The nickel, copper and nickel-copper catalysts were prepared by an incipient wetness impregnation method16. Typically, the bimetallic catalyst 36Ni-12Cu/SBA-15 (the mass ratio of Ni (0.36 g) and Cu (0.12 g) to SBA-15 (1.00 g) was 0.48:1) was prepared as following: 1.78 g Ni(NO3)2·6H2O and 0.45 g Cu(NO3)·3H2O were dissolved in 0.90 g deionized water. Then, the resulting solution was impregnated into 1.00 g SBA-15, followed by aging for 10 h at room temperature. Next, the supported SBA-15 catalyst was dried at 110 °C for 10 h and calcined at 400 °C for 4 h under air atmosphere. Finally, the catalyst was reduced at 400 °C for 4 h in a hydrogen atmosphere (hydrogen flow rate at 530 mL/min). 2.3. Catalyst Characterizations X-ray diffraction (XRD) patterns of the catalysts were carried out on an Ultima IV Advance X-ray diffractometer with a Cu Kα radiation source operated at 40 kV and 40 mA. Data were collected in the 2θ range between 10° and 90° with a step size of 0.02° at a scanning speed of 10° min-1. BET surface areas were measured by N2 adsorption–desorption isotherms at -196 °C on an ASAP 2020 HD 88 surface area and porosimetry analyzer (Micromeritics). The samples were degassed in vacuum at 373 K for 4 h before N2 adsorption. H2 temperature-programmed reduction (H2-TPR) were carried out on a Micromeritics AutoChem II Chemisorption Analyzer 2920 equipped with a thermal conductivity detector (TCD). The catalysts was reduced with 5% H2/Ar mixture at a flow rate of 100 mL/min by heating at 5



10 °C/min form 100°C to 800 °C. Scanning electron microscopy combinedwith energy dispersive X-ray analysis (SEM–EDX) analysiswas was performed on a SUPRA55 operated at an accelerating voltage of 20 kV. 2.4. Typical Procedure for Production of DMF A typical experiment was carried out in a 25 mL reactor heated by oil bath with a magnetic stirrer. 0.18 g FMF, 10 mL THF, 0.10 g solid catalyst and 400 μL formic acid (98%) were mixed in the reactor. Then the sealed reactor was heated to 220 °C with a stirring of 500 rpm for 5 h. After reaction, the reactor was cooled to room temperature. The liquid products were centrifuged at 7000 rpm for 2 min, the resulting supernatant liquid was analyzed by GC. The qualitative analysis of sample was determined by a GC-MS (Thermo Trace 1300 and ISQ LT) instrument equipped with a TR-5MS column of 15 m × 0.25 mm × 0.25 μm and electron impact ionization (EI). DMF, FMF and other products in the reaction mixture were analyzed by an Agilent 7890 series equipped with a HP-5 capillary column (30.0 m 6 320 mm 6 0.25 mm) and a flame ionization detector (FID) operated at 543 K. The carrier gas was N2 at a flow rate of 1.0 mL min-1. The following programmed temperature was used in the analysis: 313 K (4 min)-15 K min-1-623 K (5 min). The yield of DMF and the conversion of FMF were calculated using the following equations: FMF conversion (%) = (1 − DMF yield (%) =

𝑀𝑜𝑙𝑒 𝑜𝑓 𝐹𝑀𝐹 )×% 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑜𝑙𝑒 𝑜𝑓 𝐹𝑀𝐹

𝑀𝑜𝑙𝑒 𝑜𝑓 𝐷𝑀𝐹 ×% 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑜𝑙𝑒 𝑜𝑓 𝐹𝑀𝐹

3. RESULTS AND DISCUSSION 3.1 Catalyst Characterization 3.1.1 X-ray Diffraction The XRD patterns of catalysts 48Ni/SBA-15, 48Cu/SBA-15 and 36Ni-12Cu/SBA-15 before and after reduction were obtained for crystallinity analysis. As observed in Figure 1(a), a diffuse and broad peak at 22.9º observed in the XRD patterns for all the catalysts was attributed to SBA15. While the diffraction peaks at 35.5º, 38.8º and 48.8º was assigned to the oxide phases of copper (JCPDS05-0661), and the peaks at 37.2º, 43.3º and 62.9º related to the oxide phases of nickel were also observed in the XRD patterns.17,18 As for the bimetallic 36Ni-12Cu/SBA-15, no obvious 6


Page 6 of 22

Page 7 of 22

diffraction peaks attributed to copper species were detected, but these peaks were found in the catalyst 24Ni-24Cu/SBA-15, indicating that the diffraction peaks of copper oxide were overlapped by those of nickel oxide due to its low content. The change tendency of peak intensity with the CuO/NiO ratio also showed that species of copper and nickel were highly dispersed on SBA-15. a

CuO Cu2O 

36Ni12Cu

 NiO

 

24Ni24Cu

Intensity (a.u)

 

33

34

35 

36

37

38 





39



40

48Ni

36Ni12Cu 24Ni24Cu







 

 

 

12Ni36Cu 48Cu

20

30

40

50

60

70

80

90

2θ (degree)

 NiCu

b 

40Ni8Cu



Cu Ni

NiO





36Ni12Cu

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


24Ni24Cu 12Ni36Cu

 48Ni

 







 

48Cu

30

40

50

60

70

80

2θ (degree)

Figure 1. X-ray diffraction patterns of the calcined catalysts (a) and reduced catalysts (b) The XRD patterns of the reduced catalysts with different Ni/Cu ratio are showed in Figure 1(b). Obviously, for all the bimetallic samples, the Cu-Ni alloy formed during reduction under H2 flow at 400 ºC.18-20 Meanwhile, the shoulder peaks around 2θ = 44.4º and 51.6º attributed to Cu–Ni alloy became sharper with the increasing nickel content, but the intensity of peaks related to copper declined gradually (2θ = 43.4º, 50.5º and 74.1º). In addition, the peaks of CuO and NiO phases presented initially in the calcined samples were not detected for all the reduced bimetallic samples. However, a small proportion of the NiO phase was observed in 48Ni/SBA-15 catalyst (reduced at 400 ºC) due to its higher reduction temperature (405 ºC). When reduction temperature was increased 7



to 800 ºC, all NiO phases were reduced (Figure S1). As for the catalysts with same Ni/Cu ratio (3:1), the peaks of Cu–Ni alloy also became sharper with the increasing total metal loading (Figure S2). Besides the formation of Ni-Cu alloy, the doping of Cu to Ni catalyst also affected the average crystallite size of catalyst particles (Table 1). The average crystallite size of Ni-Cu alloy particles was obvious less than those of separate Cu or Ni over SBA-15, especially for Cu. This result also verified the real fusion of Ni and Cu over the support SBA-15. 3.1.2 Temperature-Programed Reduction 197

222

12Ni-36Cu

24Ni-24Cu

TCD Signal (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 8 of 22

193

236 36Ni-12Cu

247 405

48Cu 48Ni

100

200

300

400

500

600

Temperature (°C )

Figure 2. H2-TPR profiles of the calcined catalysts. H2-TPR was conducted to investigate the reduction properties of the calcined catalysts (Figure 2). The calcined catalyst 48Cu/SBA-15 showed the reduction peak for copper oxide centered at 247 ºC. The reduction peaks for nickel oxide was observed at 403 ºC, as reported, the reduction of the bulk nickel oxide occurred at 400-450 ºC.19 Nevertheless, the reduction peaks of the bimetallic catalysts with the different Ni/Cu ratio shifted toward a lower temperature (190 ºC-240 ºC), which might result from a strong interaction between copper atom and nickel atom.21 It was also in agreement with previous result that adding copper to nickel catalysts promoted the reduction of nickel oxide 19, and the reduction temperature (lower than 297 ºC) of nickel species was significantly decreased.21, 22 In addition, the emergence of shoulder peaks seemed to be the merging of two reduction peaks corresponding to copper oxide and nickel oxide, suggesting the synergistic reduction process of CuO and NiO took place on the surface of SBA-15. 3.1.3 N2-Physisorption Table 1. Physicochemical properties of reduced catalysts 8


Page 9 of 22

Entry

Sample

BET

Pore Volume

Pore Size

crystallite size

(m2/g)a

(cm3/g)b

(nm)c

(nm)d

1

SBA-15

804.52

1.03

5.10

-

2

48Ni/SBA-15

360.74

0.53

5.88

20.19

3

36Ni-12Cu/SBA-15

351.37

0.60

6.28

18.69

4

48Cu/SBA-15

369.66

0.56

6.07

38.81

5

used catalyst

288.75

0.49

6.77

14.11

a: BET surface area; b: Total pore volume measured at P/Po= 0.99; c: Pore Size calculated by BJH method. d: The average crystallite size of metallic copper particles was calculated based on XRD patterns using the Scherrer equation.

3

Pore volume (cm /g/nm)

0.35

3

Quantity Absorbed (cm /g STP)

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


SBA-15 48Ni 48Cu 36Ni-12Cu

0.30 0.25 0.20 0.15 0.10 0.05

SBA-15

0.00 0

2

4

6

8

10

12

14

Pore diameter (nm)

48Cu 36Ni-12Cu

48Ni

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Figure 3. N2 adsorption–desorption isotherms and pore size distribution curves (inset) of the samples The physicochemical properties of the reduced catalysts are listed in Table 1. The SBA-15 support exhibited a highest BET surface area, while substantial decline for the supported catalysts was observed due to the deposition of metal particles (Cu, Ni). The N2 adsorption-desorption isotherms and pore size distributions in the samples are shown in Figure 3. All the samples exhibited typical type IV isotherms with an H1-type hysteresis loop in line with the IUPAC classification, which was the typical characteristic of ordered mesoporous materials with narrow pore size distribution. The adsorption branches were located at relative pressures in the range from 0.55 to 0.85, except the 48Ni/SBA-15 catalyst (in the range from 0.40 to 0.80). And the sharp increase in 9



the N2 adsorbed volume was the distinctive feature of capillary condensation in mesopores.23, 24 The pore size distributions of the samples (inset, Figure 3) were estimated by the BJH method, revealing a narrow pore distribution in the range of 6-10 nm similar to that of SBA-15. 3.1.4 SEM Analysis

Figure 4. SEM image of fresh and used 36Ni-12Cu/SBA-15 catalysts (A) fresh, (B) used for 6 cycles, and the elemental mapping of fresh 36Ni-12Cu/SBA-15: (a) oxygen (b) nickel (c) copper. With the main objective of gaining more information about Ni and Cu dispersion on the support surface, SEM analysis was performed for the reduced 36Ni-12Cu catalyst. The metals were uniformly deposited on the surface of SBA-15 support with channel structure and high surface area (Figure 4A). Furthermore, the face scanning EDS further confirmed a homogeneity of nanocomposition. (Figure 4a, b, c). From the SEM photograph, the size of Ni-Cu alloy ranged from several nanometers to about several tens nanometers, which was consistent with the result of XRD analysis. Meanwhile, we found that the size of Ni-Cu alloy became larger with the increasing loading (Figure S3). As for the catalyst recycled for 6 times, the size appeared to become smaller relative to the fresh catalyst. The smooth circular particles of catalyst looked more irregular. This was also verified by the result of XRD analysis. Furthermore, some fragments were observed on the surface of the used catalyst, suggesting that the SBA-15 support might be slightly damaged due to collision during reaction (Figure 4B). 10


Page 10 of 22

Page 11 of 22

3.2. Hydrogenolysis of FMF 3.2.1 Effect of Ni/Cu Ratio 90 100 HMF 5-MFA DMF 5-MF

70 60

80

60

50 40

40 30 20

Conversion rate (%)

80

Yield (%)

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


20

10 0

0 0Ni48Cu 12Ni36Cu 24Ni24Cu 36Ni12Cu 40Ni8Cu 48Ni0Cu

Ni/Cu ratio

Figure 5. Effect of Ni/Cu ratio on the conversion of FMF into DMF. To determine the optimum Ni/Cu ratio of the catalyst, a series of Ni-Cu/SBA-15 catalysts were prepared at a constant mass ratio of metals to SBA-15 (0.48:1) with different Ni/Cu ratios. These catalysts were tested in the hydrogenolysis of FMF at 220 °C under self-pressure using formic acid as hydrogen donor. The tested Ni-Cu bimetallic catalysts showed excellent catalytic activity for FMF conversion (Figure 5), while the DMF yield significantly varied with the different Ni/Cu ratios. Although the conversion of FMF reached 82%, the poor yield of DMF was observed using the monometallic catalyst 48Cu/SBA-15. Meanwhile, the main intermediate 5-methyl-furfural (5-MF) generated from the hydrogenolysis of FMF was obtained with the yield of 17.4%, meaning that copper monometallic catalyst had a better activity towards the hydrogenolysis of the formyloxy group in FMF. In addition, the maximum DMF yield of 71% was obtained over the 36Ni12Cu/SBA-15 catalyst based on the synergistic effect between copper and nickel species. In the case of Ni-Cu bimetallic catalyst, the incorporation of Cu and Ni improved the hydrogenation selectivity and hydrogenolysis capacity of Ni, which was possible attributed to the formation of Cu-Ni alloy. 3.2.2 Effect of Total Metal Loading The catalytic performance of Ni-Cu/SBA-15 for hydrodeoxygenation of FMF with different total metal loading are presented in Figure 6. At least 96.0 % of FMF was converted using 3Ni1Cu/SBA-15 catalyst, while only 18.7 % of DMF was yielded. The main by-product was 5-MF with the yield of 19.1 %. Meanwhile, HMF and 5-methyl-furfuryl alcohol (5-MFA) were also detected 11



with the yields of 16.4 % and 7.6 %, respectively. When increasing the metal loading at the constant Ni/Cu ratio of 3:1, the enhanced catalytic performance with the increasing DMF yield and almost complete FMF conversion was achieved, which indicated the hydrogenolysis and hydrogenation efficiency increased simultaneously with the loading of catalyst. When the metal loading was increased to the metal/SBA-15 mass ratio of 0.48:1, the yield of DMF reached a favorable value of 71.0 %. In addition, SEM analysis showed that the catalyst size became larger with increasing loading. Although more catalyst might accelerate the HMF conversion, larger size might affect the catalytic selectivity. Therefore, further increasing the loadings of copper and nickel did not bring positive effect on the yield of FMF. 90 100 80 DMF 5-MF 5-MFA HMF

60

80

60

50 40

40 30 20

Conversion rate (%)

70

Yield (%)

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 12 of 22

20

10 0

0 3Ni1Cu

15Ni5Cu

27Ni9Cu 36Ni12Cu 48Ni16Cu 57Ni19Cu

Different metal loading

Figure 6. Effect of different metal loading on the conversion of FMF into DMF.

3.2.3 Effect of Reaction Time Since HMF was invariably used as substrate for producing DMF, the comparative experiments were conducted to figure out a distinction between FMF and HMF in producing DMF with time course (Figure 7). We found that FMF was almost totally converted in the initial 3 h with the DMF yield of 59.4 %. Meanwhile, a little of HMF, generated form the hydrolysis of FMF, was converted gradually to DMF and disappeared after 3 h. Prolonging the reaction time to 5 h, an excellent DMF yield of up to 71.0 % was achieved. Disappointedly, extending reaction time from 5 h to 7 h, the DMF yield declined gradually. In contrast, the hydrodehydration of HMF proceeded quickly within 3 h, giving the DMF yield of 59.7 % with the HMF conversion of 98.5 %. Then, the reaction rate gradually slowed down. After 5 h, the HMF conversion of 100 % and highest DMF yield of 60.5 % were reached respectively. Further, the DMF yield was not enhanced by prolonging the reaction 12


Page 13 of 22

time to 7 h. These results indicated that FMF possessed a greater advantage than HMF to yield DMF over Ni-Cu/SBA-15 catalyst using FA as hydrogen donor.

a

90

100

80

Yield (%)

70 60

60

DMF 5-MF 5-MFA HMF

50 40 30

40

Conversion rate (%)

80

20 20 10 0

0 0

1

2

3

4

5

6

7

8

Time (h) 90

100

b

80

70 60 50

60

DMF 5-MF 5-MFA FMF

40 30

40

20

Conversion rate (%)

80

Yield (%)

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


20 10 0

0 0

1

2

3

4

5

6

7

8

Time (h)

Figure 7. Time course of the CTH of FMF (a) and HMF (b). Especially, we noticed that the reaction proceeded to at least 65 % of complete conversion of FMF or HMF within half hour. While the yields of DMF was about a half of feedstock conversion. Even if considering a small amount of known intermediates, there still were large gaps between the DMF yield and feedstock conversion. With the elapsing of reaction time, DMF was gradually generated. From these phenomena, we believed that FMF or HMF was first adsorbed on the surface of catalyst, leading to the feedstock concentration in solution declined quickly, then with declining surface concentration of DMF, the desorption rate of DMF gradually slowed down. This result suggested the CTH of FMF was controlled by the surface reaction and very depended on the adsorption of feedstock and desorption of DMF, which provided the direction to improve the 13



catalytic performance of Ni-Cu catalyst. 3.3. Catalyst Recyclability The stability and recyclability of the 36Ni-12Cu/SBA-15 catalyst were also evaluated through 6 reaction cycles (Figure 8). After the first cycle, the catalyst was separated by centrifugation followed by washing with fresh THF and directly used for the next cycle. The used catalyst remained still the high catalytic performance after 6 cycles with a slight decline in the DMF yield. Further, the used catalyst was characterized by XRD, BET, TG and SEM analyses. Compared with the fresh catalyst, the Cu–Ni alloy was still detected in the used catalyst, while the average crystallite size of used catalyst became smaller (Figure S4, Table 2). Although the particles became irregular and the structure of SBA-15 was slightly damaged due to collision, the structure of SBA-15, as well as the metal particles supported on the surface of SBA-15, could be discerned clearly in the SEM photograph (Figure 4B). Moreover, only 2 % mass change was also observed form the TG curve (Figure S5), suggesting a few organics, which possibly was DMF, was adsorbed on the surface based on the phenomena that the CTH of FMF very depended on the adsorption of feedstock and desorption of DMF, which probable contributed to the slight decline in the surface area and pore volume of used catalyst (Table 1). Altogether, the structure of catalysts hadn't been changed much before and after the reaction, which possibly contributed its excellent recyclability.

100 DMF yield FMF conversion

80

Percent (%)

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 14 of 22

60

40

20

0 1

2

3

4

5

6

Run number

Figure 8. Recyclability of 36Ni-12Cu/SBA-15 catalyst in the conversion of FMF into DMF 3.4. Reaction Mechanism Study To shed light on the reaction mechanism, some experiments were elaborated to investigate the effect of various reaction conditions on products distribution (Table 2). When the reaction was 14


Page 15 of 22 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


catalyzed by 36Ni-12Cu/SBA-15 at a relatively low reaction temperature (180 °C), the low conversion of FMF and yield of DMF were obtained (Table 2, entry 1). The main by-product was HMF with the yield of 32.8 %. The reaction temperature raised to 220 °C, the DMF yield increased sharply to 71.0 % with the complete FMF conversion. Continuously raising reaction temperature to 240 °C led to a slight decrease in the DMF yield instead. Table 2. Effect of reaction temperature on the conversion of FMF into DMF.a Yield (%) Entry Substrate

Catalyst

T (°C)

X (%) HMF

FMF

5-MF

5-MFA

DMF

1

FMF

36Ni-12Cu

180

57.9

32.8

-

0.9

0.6

11.6

2

FMF

36Ni-12Cu

200

86.3

26.8

-

7.4

3.4

22.9

3

FMF

36Ni-12Cu

220

100

0

-

0

2.6

71.0

4

FMF

36Ni-12Cu

240

100

0

-

0

2.3

62.8

5b

FMF

36Ni-12Cu

180

79.1

14.8

-

4.7

1.5

41.5

6

FMF

220

100

12.8

-

6.0

17.4

25.8

Physical mixing

7

FMF

48Cu

220

83.7

46.1

-

17.4

2

1.5

8b

FMF

48Cu

220

83.7

45.3

-

21.3

2.2

3.8

9

FMF

48Ni

220

98.1

5.1

-

6.4

1.5

45.1

10d

FMF

48Ni

150

97.4

2.0

-

1.1

41.5

21.0

11

HMF

48Ni

220

100

-

0

0

1.3

58.8

12

HMF

48Cu

220

49.5

-

22.5

4.1

Hydrogenation of Biomass-Derived

Recommend Documents