Selective Transfer Hydrogenation of Biomass-Based Furfural and 5

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Selective transfer hydrogenation of biomass-based furfural and 5-hydroxymethylfurfural over hydrotalcite-derived copper catalysts using methanol as hydrogen donor Jun Zhang, and Jinzhu Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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ACS Sustainable Chemistry & Engineering

Selective transfer hydrogenation of biomass-based furfural and 5-hydroxymethylfurfural over hydrotalcite-derived copper catalysts using methanol as hydrogen donor

Jun Zhang† and Jinzhu Chen†,§,*

†

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences. CAS Key Laboratory of

Renewable Energy, Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development. No. 2 Nengyuan Rd, Wushan, Tianhe District, Guangzhou 510640, P.R. China. §

College of Chemistry and Materials Science, Jinan University. No. 601 Huangpu Avenue West,

Tianhe District, Guangzhou 510632, P.R. China * Corresponding author, Tel.: (+86)-20-8522-2191, Fax: (+86)-20-8522-0223, E-mail address: [email protected] (J. Chen)

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ABSTRACT: A series of inexpensive copper-based catalysts, derived from hydrotalcite precursors, were screened for selective transfer hydrogenation of biomass-based furfural (FFR) to furfuryl alcohol (FA) and 2-methyl furan (MF) with methanol as both the solvent and hydrogen donor. The specific textural characteristics of the prepared catalysts were characterized by various techniques such as XRD, SEM, TEM, TGA/DTA, nitrogen physisorption, NH3-TPD, H2-TPR, IR spectra of pyridine adsorption and XPS. The copper catalyst showed excellent transfer hydrogenation selectivity towards FFR-to-FA transformation by giving a FA yield of 94.0 mol% at 473 K in methanol. Treatment the copper catalyst with H2/N2 at 773 K led to formation metallic Cu species in the activated sample. Interestingly, the resulting activated copper catalyst favored FFR-to-MF transformation by producing MF yield of 94.1 mol% at 513 K. The FFR-to-MF transformation involved a tandem reaction of FFR hydrogenation and successive FA hydrogenolysis; while, a synergistic effect between metallic Cu species and acidic sites on the catalyst surface played a key role on MF formation. In addition, the activated copper catalyst exhibited outstanding performance towards another biomass-related important conversion of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF) with the desired DMF yield of 96.7 mol%. The recycling experiments revealed that both the copper and the activated copper catalysts maintained good activity and stability after five-time recycling. The research thus highlights a new perspective for a green, efficient and biomass-related hydrogenation reaction using readily available and inexpensive catalytic system of copper-methanol, while, without any external hydrogen supplies.

KEYWORDS:

copper

catalyst;

furfural;

5-hydroxymethylfurfural;

hydrogenation 2 ACS Paragon Plus Environment

methanol;

transfer

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INTRODUCTION The depletion of fossil carbon resources, huge energy demands and environmental deterioration have encouraged the development of alternative renewable sources as well as novel approaches to the sustainable supply of fuels and fine chemicals.1–3 In this regard, the catalytic conversion of lignocellulosic biomass into a wide variety of biofuels and chemicals has recently gained increasing interests. A recent review critically discussed nano-scale metal-catalysts for valorization of biomass-related cellulose, chitin, lignin and lipids.3 Among these biomass-derived platform molecules, furfural (FFR), primarily obtained by acid-catalyzed dehydration of biomass-derived pentoses,4–6 was identified as a promising building block for valuable products. Particularly, selective transformation of FFR provides an important route to prepare a wide range of target derivatives, such as furfuryl alcohol (FA), tetrahydrofurfuryl alcohol (THFA), 2-methyl furan (MF), and cyclopentanone.7–9 These FFR-derived compounds are broadly employed in polymer industry, synthetic fibers, rubber-resins, energy chemical industry, and so on.10–12 Conventional process for FFR hydrogenation requires the use of pressurized molecular hydrogen in combination with various metal catalysts, such as Pd/SiO2, Ru/C, Pt/C, Cu/SBA-15, Cu-MgO, Mo2C and Ni-Fe-B.13–20 Generally, the precious metal catalysts exhibit appreciable activity towards FFR conversion, and FFR can be easily and quantitatively consumed after the reaction. However, several noticeable drawbacks, such as poor selectivity of the transformations due to undesired side reactions and high cost of the catalysts caused by the limited abundance of precious metals, are prominent barriers for large-scale applications in FFR conversion. In comparison, non-precious copper-based catalysts have received attractive attention and are reported to be very active for FFR hydrogenation. For instance, Lesiak et al. found that addition of Cu into 3 ACS Paragon Plus Environment


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Pd/SiO2 significantly improved the hydrogenation rate of FFR and reduced the decarbonylation rate.21 Zhu et al. prepared three supported Cu-catalysts (Cu/SiO2, Cu/Al2O3 and Cu/ZnO) by a typical precipitation method for the selective hydrogenation of FFR, and Cu/SiO2 exhibited the best catalytic performance affording MF yield of 89.5% under the synergistic effect of metal and weak Lewis acid sites of the catalyst.22 Yan et al. reported that Cu-Cr bimetallic catalysts, derived from hydrotalcite precursors, revealed synergistic effect in selective hydrogenation of FFR yielding 95% yield of FA with 99% conversion of FFR at the temperature of 473 K.23 Notably, the hydrotalcite has a clear advantage to produce high loading amount of the supported Cu-Cr catalysts through proper calcination and reduction. Though high conversion and excellent product selectivity for FFR hydrogenation were achieved over copper-based catalysts, the use of high pressure molecular hydrogen still put forward a number of obvious issues, such as H2 storage, safety, transportation, solubility, and so on. On this basis, alternative hydrogen donors of formic acid and alcohols for catalytic transfer hydrogenation (CTH) purpose have received extensive concern, especially in recent years.24–29 Particularly, the utilization of alcohols as hydrogen donor offers significant benefits when compared with formic acid in some ways related to corrosiveness. In fact, alcohols, such as methanol, ethanol, 1-butanol, 2-propanol and 2-butanol, are extensively applied in the reductive upgrading of biomass as hydrogen donor. Generally, secondary alcohols including 2-propanol and 2-butanol require lower reaction temperature and sometimes help to reduce side reactions. For instance, Hermans reported Pd/Fe2O3-promoted transfer hydrogenation/hydrogenolysis for reductive upgrading of FFR and 5-hydroxymethylfurfural (HMF) to MF and 2,5-dimethylfuran (DMF), respectively, using 2-propanol or 2-butanol as hydrogen donors.30 Certainly, methanol is also a preferred hydrogen 4 ACS Paragon Plus Environment

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source from the perspectives of cost reduction and environmental protection. Moreover, methanol can be produced from renewable biomass resources and the co-products generated in copper-catalyzed methanol reforming are gaseous compounds, i.e. CO, CO2 and CH4 (Scheme S1).31 Thus, extensive efforts have been devoted to seeking efficient catalysts for upgrading of biomass using methanol as a clean H-transfer agent. For example, a Cu-doped porous metal oxide (Cu-PMO) from hydrotalcite-like precursor was developed for hydrogenation of HMF via a hydrogen transfer from clean methanol, the transformation afforded a combined yield of 61% to DMF, 2,5-dimethyltetrahydrofuran (DMTHF) and 2-hexanol.32 Furthermore, Cu-PMO also served as an effective catalyst for the depolymerization of organosolv lignin in methanol medium by giving good-to-excellent yields of bio-oils with similar molecular weight distributions.33 More recently, Lin et al. examined commercial CuO for methyl levulinate (ML) conversion into γ-valerolactone (GVL) using methanol as both a solvent and hydrogen donor, and a GVL selectivity of 87.6% was attained at nearly a quantitative ML conversion.34 Inspired by the above findings, herein, we reported the CTH of FFR to FA and MF with methanol as both a solvent and hydrogen donor using inexpensive copper-based catalysts derived from hydrotalcite precursors. A systematic characterization of the catalysts including structural properties, copper valence state, surface acid amount and surface acid type were conducted; moreover, various reaction parameters on the CTH reaction were investigated. The copper catalyst showed excellent selectivity towards FFR-to-FA transformation by giving a FA yield of 94.0 mol%. Treatment the copper catalyst with H2/N2 led to formation metallic Cu species in the activated sample. Interestingly, the resulting activated copper catalyst favored FFR-to-MF transformation by producing MF yield of 94.1 mol%. The FFR-to-MF transformation involved a tandem reaction of 5 ACS Paragon Plus Environment


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FFR hydrogenation and successive FA hydrogenolysis; while, a synergistic effect between metallic Cu species and acidic sites on the catalyst surface played a key role on MF formation. In addition, the activated copper catalyst, again, efficiently promoted another biomass-related important conversion of HMF to DMF with the target product yield of 96.7 mol%. The recycling experiments revealed that both the copper and the activated copper catalysts maintained good activity and stability after five-time recycling. EXPERIMENTAL Materials Furfuryl alcohol (FA, 98%), 2-methyl furan (MF, 98%), 5-hydroxymethylfurfural (HMF, 99%) and 2,5-dimethylfuran (DMF, 99%) were purchased from Aladdin Reagent Co. Ltd. (Shanghai, P. R. China) and used as received. CuO (AR), MeOH (AR) and all of other chemicals were provided by Guangzhou Chemical Factory (Guangzhou, P. R. China) and used without further purification. Furfural (FFR) was obtained from Aladdin Reagent Co. Ltd. (Shanghai, P. R. China) and distilled under vacuum before use. Catalyst synthesis Four samples of copper-based hydrotalcite-like compounds (HTlc) with Cu/Al molar ratios of 1:1, 2:1, 3:1 and 4:1 were synthesized by the co-precipitation method described earlier.35 Two aqueous solutions, one containing appropriate amounts of metal nitrates [Cu(NO3)2·6H2O and Al(NO3)3·9H2O] and the other containing precipitating agent (NaOH and Na2CO3), were dropped slowly at a pH value around 9. After precipitation, the resulting suspension was maintained at 348 K for 10 h with stirring. The precipitate was washed with distilled water until the pH value of the ﬁltrate was around 7 and then dried at 383 K for 12 h under air flow. The precipitate was ground to 6 ACS Paragon Plus Environment

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fine powders and then calcined at 773 K in muffle furnace for 3 h. The samples obtained in this way were designated as CuxAl, where x was the Cu/Al molar ratio. For example, a calcined copper catalyst, derived from hydrotalcite precursor, with a Cu/Al molar ratio of 2 was denoted as Cu2Al. The resulting Cu2Al sample further activated in a 40% H2/N2 flow at 773 K was labeled as Cu2Al-A. The activation process was performed in a heating rate of 2 K min-1 under a flowing of 40% H2/N2 with the final temperature maintained for 3 h. Typical procedure for the CTH of FFR in methanol The experiments were performed in a 50 mL cylindrical stainless steel high-pressure reactor. In a typical run, a methanol (15 mL) solution of furfural (115 mg, 1.2 mmol) and certain amount of copper-based catalysts were added into the reactor. The sealed reactor was purged three times with N2 and finally pressurized to 1 MPa. Thereafter, the reactor was brought to the desired temperature by means of a heating jacket. When the reaction was completed at desired times, the mixture was cooled, filtered and stored for analysis. In addition, the gaseous products of the reaction were collected for GC analysis. Catalyst characterization X-ray powder diffraction (XRD) was performed on a Bruker D8 Advance diffractometer using Cu Kα radiation generated at 40 kV and 40 mA. X-ray photoelectron spectra (XPS) and Auger electron spectroscopy were made on a Kratos Ultra system employing an Al Kα radiation source. The binding energies for each spectrum were calibrated with a C1s spectrum of 284.6 eV. Notably, the sample was exposed to an air atmosphere at 298 K before XPS measurement. The morphology of the fresh and recycled samples was observed by scanning electron microscopy (SEM, Hitachi S-4800). High resolution transmission electron microscopy (HR-TEM, JEM-2100HR) was 7 ACS Paragon Plus Environment


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conducted for morphological analysis of copper-based catalysts. As for sample preparation, a small amount of catalysts was firstly dispersed in ethanol solution, and then a drop of the suspension was placed onto carbon-coated molybdenum grids, followed by evaporating the solvent. Chemical analyses of metal elements were measured by a Thermo Elemental ICP-AES spectrometer. The Brunauer-Emmett-Teller (BET) surface area measurements were performed with N2 adsorption-desorption isotherms at 77 K (SI-MP-10, Quantachrome). Before test, the samples were degassed under vacuum at 473 K for 18 h. The measurements of catalyst acidity were carried out on a

Micromeritics

Autochem

II

2920

chemisorption

analyzer

following

an

ammonia

temperature-programmed desorption (NH3-TPD) method. For each run, the sample was heated up to 573 K at a rate of 10 K min-1 and kept for 0.5 h in a He flow to remove adsorbed impurities. Then the sample was cooled down to 373 K for the adsorption of NH3. After flushing with He for 1 h to remove physically adsorbed NH3, the TPD data were collected from 373 K to 1023 K with a ramp of 20 K min-1. The hydrogen temperature-programmed reduction (H2-TPR) analysis of the prepared copper catalysts was performed on Autochem II 2920. In a typical experiment, around 0.15 g of catalyst was placed in a quartz sample cell, the sample was then flushed with pure argon at a flow rate of 50 mL min-1 and 573 K for 0.5 h and then cooled down to room temperature. Subsequently, a 10% H2/Ar (30 mL min-1) was flown through the sample while the temperature was increased to 800 K at 10 K min-1 ramp rate and held at final temperature for 0.5 h. Pyridine adsorption Fourier-transform infrared (Py-IR) analysis was conducted on Bruker FT-IR Tensor 27 equipped with BX-5 in situ transmission FT-IR device to identify acid type and concentration of prepared catalyst. About 20 mg sample was pressed into a self-supporting disk of 13 mm diameter, and then the disk was placed in a quartz cell connected to a conventional closed gas-circulation system. The 8 ACS Paragon Plus Environment

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sample disk was pretreated at 473 K for 2 h in a pressure of 1.0 × 10-3 Pa and then the temperature was cooled to 313 K for collecting single channel background spectrum. For adsorption experiment, pyridine was introduced to the sample for 20 min. IR spectra were measured at 313 K after adsorption balance, followed by evacuation to remove adsorbed pyridine for collecting IR spectra at 423

K

(heating

rate,

10

K

min-1)

and

673

K

(heating

rate,

10

K

min-1).

Thermogravimetric/differential thermic analyses (TGA/DTA) analyses were performed by using a SDT Q 600 instrument over fresh and spent catalysts in order to identify the amounts of adsorbed organic compounds over the catalyst surface. Specifically, a sample of around 100 mg was typically employed for the measurement at temperatures from room temperature up to 1173 K with a heating rate of 20 K min-1 in air. Products analysis The quantity of collected samples was analyzed by Fuli GC 9790II equipped with a flame ionization detector (FID) and a KB-5 capillary column (30.0 m × 0.32 µm × 0.25 µm) using nitrogen as the carrier gas. The GC-MS analysis was recorded on Trace GC-MS 2000 equipped with a 30 m HP-5 (0.25 mm internal diameter). The operating conditions for GC-MS were as follows: injector port temperature 533 K, column temperature and initial temperature 323 K (3 min), gradient rate 10 K min-1, final temperature 493 K (2 min), flow rate 75 mL min-1. The assignment of all of the products and intermediates was confirmed by database for each MS chart. The analysis of gaseous products was performed on Agilent 7890A equipped with an Agilent CP-7429 column, a FID and a thermal conductivity detector (TCD). The carrier gas was helium and the column temperature was maintained at 333 K for 15.2 min for the quantitative analysis of in situ generated H2, CO and CO2 according to the calibrated curves of standard gases. 9 ACS Paragon Plus Environment


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The corresponding formulas for calculating conversion and yield were defined as below: Conversion = [mole of reactant converted / mole of reactant fed] × 100% Yield = [mole of product produced / mole of reactant fed] × 100% In addition, the product formation rate was defined as the formed target product with the unit of µmol per gram of copper in the catalyst per min, in which copper component was determined by ICP-AES analysis and presumably contributed to the reactions. RESULTS AND DISCUSSION Catalyst characterization Figure 1 shows the XRD patterns of the CuxAl-HTlc. The prepared copper catalyst precursors revealed a highly ordered crystalline structure with distinct hydrotalcite features consistent with the literature.36 Featured diffraction peaks at 11.6o, 23.3o, 35.6o, 40.1o, 48.0o, 58.3o and 61.7o could be clearly found, which were assigned to (003), (006), (012), (015), (018), (110), and (113) reflections of hydrotalcite (JCPDS files no. 22-0700), respectively.37,38 Furthermore, isolated phases of individual metal hydroxides were undetected in the CuxAl-HTlc samples with an x range from 1 to 3, suggesting the formation of a pure hydrotalcite phase. Evidently, over-excess addition of Cu component in the CuxAl-HTlc would destroy the layered hydrotalcite structure due to crystal form change, as shown in Figure 1 for Cu4Al-HTlc. In addition, Bragg reflection at 2θ of 38.8o in Cu4Al-HTlc was attributed to the phase of gibbsite (JCPDS files no. 07-0324), further indicating the formation of amorphous hydrate phase.39


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Hydrotalcite precursors

(003)

Cu4Al HTlc Cu3Al HTlc

(012)

Cu2Al HTlc

(015)

Intensity / a.u.

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(006) (009)

15

Cu1Al HTlc

(018) (110)

30

45

(113)

60

75

2θ / degrees

Figure 1. Powder XRD patterns of CuxAl-HTlc.

Copper catalyst CuxAl was obtained by calcination of the corresponding CuxAl-HTlc at 773 K for 3 h in the air. While, the activated copper catalyst (CuxAl-A) was prepared by treatment CuxAl with 40% H2/N2 flow at 773 K. Figure 2 gives the XRD patterns of the CuxAl and CuxAl-A samples. Initially, CuO was investigated for comparison purpose (Figure 2). Diffraction peaks at 32.4°, 35.7°, 38.9°, 48.6°, 53.7°, 58.1°, 61.5°, 66.1°, 67.9°, 72.3°, and 75.1° were observed in commercial CuO, which revealed the typical reflections ascribed to monoclinic CuO (JCPDS files no. 65-2309).40 As for CuxAl (x = 1–4) samples, characteristic peaks assigned to monoclinic CuO could also be obviously found. Notably, the peaks corresponding to copper hydroxide and other copper species were unidentified. In the case of Cu3Al-A sample, typical crystallization peaks centered at 43.4°, 50.6°, and 74.2° were ascribable to the (111), (200), and (220) planes of fcc crystal structures of metallic Cu (JCPDS04-0836).41,42 Interestingly, the XRD patterns of spent Cu2Al showing metallic Cu planes demonstrating that a portion of Cu element existed as Cu0 species after the hydrogen transfer reaction. In addition, color change for fresh and recycled Cu2Al evidently suggested the variation of copper valence state (Figure S1). Therefore, XRD analysis 11 ACS Paragon Plus Environment


revealed the presence of a pure hydrotalcite phase for CuxAl-HTlc samples (x = 1–3), monoclinic CuO for CuxAl samples (x = 1–4), and fcc crystal structures of metallic Cu for Cu3Al-A sample. Moreover, a portion of Cu component in Cu2Al was reduced from the oxidation state of Cu(II) to Cu(0) after the Cu2Al-promoted hydrogen transfer reaction.

Cu4Al

(200)

Cu3Al

Cu2Al

15

30

(020) (202) (113)

(202)

CuO

(110)

Cu1Al

45

Cu CuO

(311) (220) (311) (220) (222)

(002) (111)

Cu2Al, used

(111)

Cu3Al-A

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

60

75

2θ / degrees

Figure 2. Powder XRD patterns of CuxAl and CuxAl-A.

XPS characterizations were undertaken to investigate the surface chemical state of the copper and the activated copper catalysts. As depicted in Figure 3A, full spectra of Cu2Al including C 1s, O 1s, Al 2p, Cu 2p and Cu LMM were conducted. In the cases of Cu2Al and Cu3Al catalysts, the peaks located at the binding energies of 933.7 eV and 953.7 eV with strong satellite peaks were assigned as Cu 2p3/2 and Cu 2p1/2, respectively (Figures 3B1 and 3B2), which indicates the presence of oxidation state of Cu2+ species.43 The results were in good accordance with XRD analysis (Figure 2). While, for spent Cu3Al, both Cu0 (35.6%) and Cu+ (50.7%) species were additionally observed after the reaction (Figure 3B3), suggesting a partial reduction of surface Cu2+ species during the transfer hydrogenation. In the case of Cu3Al-A sample, the binding energies of Cu 2p3/2 and Cu 2p1/2 down shifted separately to 932.4 eV and 952.2 eV (Figure 3B4), if compared with Cu3Al. 12 ACS Paragon Plus Environment

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Besides, the intensity of the satellite peaks in Cu3Al-A became rather weakened. As is well known, Cu0 species cannot be distinctly distinguished from Cu+ through Cu 2p XPS analysis,44 thus Cu LMM Auger spectra were performed to accurately clarify the copper species. According to the Cu LMM spectra as shown in Figure 3C, the peak with a kinetic energy of around 918.6 eV was indexed to metallic copper; while, the peak at 916.9 eV was assigned to Cu+,45,46 demonstrating the presence of both Cu0 and Cu+ species in Cu3Al-A sample. An in-depth study of Cu 2p spectra further confirmed the existence of small amount of Cu+ (6.8%) and Cu2+ (9.9%) species on the surface of the fresh Cu3Al-A (Figure 3B4). However, the H2-TPR analysis indicated that prepared copper catalysts could be completely activated in a H2 atmosphere at around 600 K (Figure S2). Therefore, it was inferred that the existence of Cu+ and Cu2+ species in the Cu3Al-A sample was presumably attributed to a partial oxidation of surface Cu0 species upon the sample exposure to air before XPS measurement. This assumption was further supported by an evident color change from dark red for the fresh Cu3Al-A to almost black after the air exposure of Cu3Al-A (Figure S3). Generally, the XPS results were in line with XRD analysis, revealing the oxidation state of Cu2+ for both Cu2Al and Cu3Al, while, the presence of Cu0, Cu+ and Cu2+ species for both Cu3Al-A and spent Cu3Al. The temperature-programmed desorption profiles of desorbed ammonia on CuxAl and CuxAl-A samples were described in Figure 4 with the inset of the assigned acid amount. Generally, the higher desorption temperature of NH3-TPD reflects the stronger acid strength of the catalyst. With respect to CuxAl (x = 1–4) samples, characteristic peak at desorption temperature of nearly 670 K was clearly observed (Figure 4A). Calculated results further demonstrated that doping of Al component significantly enhanced the catalyst acidity. As a result, Cu1Al catalyst, containing the highest levels 13 ACS Paragon Plus Environment


of aluminum site in the investigated samples, presented the maximum acid amount. In the case of CuxAl-A samples (x = 1–4), the intensity of desorption peaks at high temperature zone became weaker, thus indicating the moderate acid strength (Figure 4B). This observation can be attributed to a reduced Cu valence state for CuxAl-A samples, which gave much metallic Cu species, however, at the expense of both acidic CuO species and acidic OH groups.

(A)

Cu 2p

O 1s

Cu3Al-A Cu3Al, used once

Intensity / a.u.

Al 2p C 1s Cu3Al-A

Intensity / a.u.

Cu LMM

(C)

Cu Auger

Cu3Al, used once

Cu3Al

Cu3Al Cu2Al

Cu2Al

0

300

600

900

905

1200

910

Binding energy / eV (B1)

915

920

925

930

Kinetic energy / eV

2p3/2 2p1/2

(B2)

Cu2Al

2p3/2 2p1/2

Cu3Al 2+

2+

Cu

Cu

2+

2+

Cu : 100%

Cu : 100%

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 39

930 2p 3/2

940

950

960

930

Cu3Al, used once

(B3)

(B4)

940

2p1/2

0

Cu : 35.6%

950

2p3/2

2p1/2 0

Cu : 83.3%

0

Cu + Cu 2+ Cu

0

Cu + Cu 2+ Cu

+

Cu : 50.7% 2+

Cu : 13.7%

930

940

950

960 Cu3Al-A

960

+

Cu : 6.8% 2+

Cu : 9.9%

930

940

950

960

Binding energy / eV

Figure 3. XPS spectra of the prepared samples: (A) full spectra; (B) Cu 2p; (C) Cu LMM. 14 ACS Paragon Plus Environment

Cu1Al

Sample

(A)

Cu2Al

Cu1Al Cu2Al Cu3Al Cu4Al CuO

Cu3Al

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


Cu4Al CuO

450

600

750

Acid amount -1 (mmol g ) 2.02 1.79 1.29 0.59 0.19

900

Temperature / K Cu1Al-A

Sample

(B)

Cu2Al-A

Cu1Al-A Cu2Al-A

Cu3Al-A Cu4Al-A

Cu3Al-A Cu4Al-A

Intensity / a.u.

Page 15 of 39

450

600

750

Acid amount -1 (mmol g ) 1.06 1.13 0.43 0.19

900

Temperature / K

Figure 4. NH3-TPD profiles of (A) CuxAl and (B) CuxAl-A.

With in-depth study, the IR spectra of adsorbed pyridine (Py-IR) on some typical samples were performed to characterize acid type and the corresponding concentration. As indicated in Figure 5, two bands at around 1442 and 1490 cm−1 attributed to adsorbed pyridine molecules were apparently displayed, suggesting the presence of large amounts of Lewis acid (L) sites in Cu2Al, Cu2Al-A and Cu3Al-A. In addition, a weak absorption peak at 1552 cm−1 depicted as Brønsted acid (B) sites was also observed, which indicated a lower concentration of acid sites. Notably, Lewis acid and Brønsted acid sites, obtained by Py-IR (Figure 5), on the sample surface revealed much lower 15 ACS Paragon Plus Environment


acidity in comparison with those described in NH3-TPD analysis (Figure 4), which could be attributed to the presence of a large quantity of acidic OH groups on the sample surface.47

Absorbance

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

0.6

Sample

0.5

Cu2Al 0.0203 0.217 Cu2Al-A 0.00232 0.253 Cu3Al-A 0.0440 0.205

-1

L (mmol g )

B

0.4

L

B

Cu2Al

0.3 0.2 Cu Al-A 2 0.1 Cu3Al-A

0.0 1700

1650

1600

1550

1500

1450

Wavenumbers / cm-1

Figure 5. IR spectra of pyridine adsorbed on Cu2Al, Cu2Al-A and Cu3Al-A.

The mesoporosity of copper-based catalysts was confirmed by N2 adsorption-desorption isotherm analysis, as illustrated in Figure S4. All samples displayed type IV isotherms with a type H3 hysteresis loop, which was characteristic of mesoporous structure in a size range of 2-50 nm. The physicochemical properties of the prepared samples were listed in Table 1. Generally, the Brunauer-Emmett-Teller (BET) surface area trended down with the increase in Cu loading for both CuxAl (Table 1, Entries 1-4) and CuxAl-A (Table 1, Entries 5-7) samples; while, the average pore diameter increased significantly with Cu loading (Table 1, Entries 1-4 and 5-7). Recycled Cu2Al catalyst gave higher BET surface area if compared with the fresh one (Table 1, Entries 9 and 2), which could be attributable to the in situ formed copper nanoparticles. High-temperature calcination was considered to be an effective way to remove adsorbed organics on the catalyst surface. On the other hand, the calcination treatment led to particle sintering, which is presumably responsible for 16 ACS Paragon Plus Environment

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Table 1. Structural properties of CuxAl and CuxAl-A samples

a

BET surface area

P. D. e

P. V. f

m2 g-1

nm

cm3 g-1

Entry

Sample

1 2 3 4

Cu1Al Cu2Al Cu3Al Cu4Al

113.3 66.5 69.6 50.4

6.5 7.9 38.8 44.0

0.31 0.30 0.35 0.32

5 6

Cu1Al-A Cu2Al-A

96.3 61.7

6.6 9.3

0.28 0.26

7 8 9 10 11 12

Cu3Al-A Cu4Al-A Cu2Al a Cu2Al b Cu3Al-Ac Cu3Al-Ad

56.4 66.7 153.7 85.6 46.9 44.0

37.5 20.9 3.8 3.7 13.1 18.8

0.18 0.19 0.31 0.27 0.23 0.19

: used once; b: used once and calcined again; c:used twice; d: used twice, calcined and reduced

again; e: average pore diameter; f: total pore volume.

the decline of BET surface area (Table 1, Entries 9 and 10). For regenerated Cu3Al-A under a H2 atmosphere, both BET surface area and average pore diameter of the sample were relatively lower, namely 44.0 m2 g-1 and 18.8 nm, respectively (Table 1, Entry 12). Catalyst regeneration presumably accelerated particle agglomeration during calcination and reduction procedures. The surface structure changes for recycled Cu2Al and Cu3Al-A catalysts were also verified by SEM (Figure S5) and high resolution transmission electron microscopy (HR-TEM, Figure S6) images. In addition, HR-TEM analysis further indicated that the copper particles existed in an irregular form in Cu3Al-A sample. In order to investigate the carbonaceous components on the catalyst surface, we analyzed the 17 ACS Paragon Plus Environment


fresh and spent Cu2Al using TGA/DTA technique which was performed by heating the samples from room temperature to 1173 K in air. As shown in Figure 6, the analysis results revealed an initial marginal weight loss (1–4 wt.%) before heating to 450 K, due to physisorbed water and surface hydroxyl groups. A further weight loss of 2–3 wt.% was registered for fresh and spent Cu2Al over the temperature range of 535–560 K, probably owing to the removal of intercalated water. As the temperature rose, two exothermic peaks at around 642 and 845 K were observed in DTA profile of the spent catalyst as compared with the fresh one, indicating the presence of organic compounds adsorbed on the Cu2Al surface after the reaction. Therefore, effectively removing carbon deposits was necessary for catalyst regeneration when concerning recycling experiments.

551

100

DTA

TGA

98

Fresh Cu2Al

Fresh Cu2Al

Spent Cu2Al

Spent Cu2Al

96 412

94

DTA

TGA, weight / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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448 434

92

537 642

845

90 88

450

600

750

938

900

1050

Temperature / K

Figure 6. TGA-DTA profiles for the fresh and spent Cu2Al.

Activity test The developed CuxAl and CuxAl-A catalysts were investigated for the transfer hydrogenation of FFR with methanol as both the solvent and the hydrogen source, and the results were summarized in Table 2. Initially, commercially available CuO was investigated as the catalyst for comparison purpose and a FFR conversion of 86.5% with 41.8 mol% FA yield was obtained (Table 18 ACS Paragon Plus Environment

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2, Entry 1). In contrast, full conversions of FFR were observed over CuxAl catalysts (Table 2, Entries 2-5). However, the selectivity to target product FA was still not improved significantly, which could be attributed to the presence of parallel reactions such as acetal, hemiacetal, etherification and aldol (Figure S7) induced by acid sites of the catalyst (Figures 4 and 5). Among the CuxAl series catalysts examined, Cu2Al revealed preferred activity towards FA production via FFR transfer hydrogenation (Table 2, Entry 3). Based on XRD and XPS analyses of CuxAl before and after the reaction (Figures 2 and 3), Cu component in the catalyst initially existed in the oxidation state of Cu2+ and promoted methanol decomposition to produce hydrogen molecule. Meanwhile, Cu2+ species would consume certain amounts of in situ generated hydrogen to form active Cu0 center for the subsequent FFR hydrogenation. Therefore, lower copper loading in CuxAl series catalysts (such as Cu1Al) led to insufficient hydrogenation ability toward FFR. On the other hand, much effort should be devoted to the reduction of Cu2+ into Cu0 species at the expense of generated hydrogen with respect to higher Cu loading samples (such as Cu3Al and Cu4Al), which was evidently not conducive to the subsequent hydrogenation of FFR. Thus, among the CuxAl series catalysts examined, Cu2Al catalyst was the most active one for FA production. Both FA and MF were observed in CuxAl-promoted transfer hydrogenation of FFR. The hydrogenation and saturation of C=O double bond in FFR molecule led to the formation of FA; while, the subsequent hydrogenolysis of saturated C–O bond in FA gave MF product. However, as shown in Table 2, only trace amount of MF was detected with the CuxAl series catalysts (Table 2, Entries 2-5), indicating that the catalyst with more active Cu0 species was required for FA hydrogenolysis to give MF. Therefore, CuxAl-A series samples, obtained by treatment CuxAl series samples with H2/N2 flow at 773 K, were further investigated for FFR hydrogenation. Surprisingly, 19 ACS Paragon Plus Environment


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Table 2. Catalytic activities of CuxAl and CuxAl-A on transfer hydrogenation of FFR

Entry

Catalyst

FFR Conversion %

FA Yield mol%

MF Yield mol%

1 2 3 4

CuO Cu1Al Cu2Al Cu3Al

86.5 > 99 > 99 > 99

41.8 47.4 54.7 44.9

3.5 1.9 4.7 3.7

5 6

Cu4Al Cu1Al-A

98.7 98.1

38.0 36.9

0.87 16.5

7 8 9

Cu2Al-A Cu3Al-A Cu4Al-A

> 99 97.7 98.0

26.6 4.1 21.0

47.9 88.2 40.7

Reaction conditions: catalyst (90 mg), FFR (115 mg, 1.2 mmol), methanol (15 mL), 513 K, N2 (1 MPa), 600 rpm, 1.5 h.

the MF yield was remarkably increased under the identical reaction conditions when using CuxAl-A catalyst for FFR transformation (Table 2, Entries 6-9). A reasonable explanation is that more active Cu0 species in CuxAl-A catalyst participated in the reaction. These Cu0 species efficiently promoted the FA hydrogenolysis to give MF as well as methanol decomposition to afford hydrogen (Figure S8 and Table S1). Also, properly increasing Cu loading level facilitated the transformation of FA into MF, presumably under the synergistic action of active Cu0 centers and acid sites on the catalyst surface. However, over high Cu loading in the catalyst (such as Cu4Al-A) did not favor MF formation (Table 2, Entry 9), due to the weaker acidity of Cu4Al-A as confirmed by NH3-TPD analysis. Previous work also verified acid sites play a crucial role in deoxygenation procedure.48,49 20 ACS Paragon Plus Environment

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By comparison, Cu2Al, possessing a proper copper loading level, was the most active catalyst for FFR-to-FA transformation by producing FA yield 54.7 mol% at 513 K (Table 2, Entry 3). While, under the identical reaction conditions, Cu3Al-A, rendering more active Cu0 and moderate amounts of acid sites, was the most active catalyst for FFR-to-MF transformation by resulting in a MF yield of 88.2 mol% (Table 2, Entry 8). The FFR-to-MF transformation involves a tandem reaction of FFR hydrogenation by the saturation of C=O double bond in FFR molecule and subsequent FA hydrogenolysis by the deoxygenation of saturated C–O bond in FA compound. On the basis of the above observations, Cu2Al and Cu3Al-A were chosen for the following work to find the optimum conditions. Herein, the influence of reaction temperature on FFR conversion was investigated in the range of 463−518 K and the results were shown in Figure 7. In the case of FA formation over Cu2Al catalyst (Figure 7A), increasing reaction temperature promoted transfer hydrogenation of FFR in the initial stage, which was mainly attributed to continuous hydrogen supply from rapid methanol decomposition under elevated temperatures. Satisfied result of 67.5 mol% FA yield was achieved at a temperature of 473 K. Higher temperatures promoted the formation of active Cu0 sites resulting in subsequent FA hydrogenolysis to MF. In addition, higher temperatures were associated with a chain of undesired reactions such as aldol condensation and etherification.27 In terms of MF production over Cu3Al-A (Figure 7B), low yield of MF was observed at relatively lower temperatures with appreciable amount of intermediate FA. Notably, a quantitative transformation of the intermediate FA was observed with the rise in the reaction temperature. Therefore, abundant active Cu0 sites in Cu3Al-A combined with a sufficient supply of H2, originated from methanol reforming under higher temperatures, accounted for this aspect. In this case, a promising MF yield of 88.2 mol% with a FFR conversion of 97.7% was reported at a 21 ACS Paragon Plus Environment


temperature of 513 K. As expected, a further increased temperature to 518 K allowed a full FFR conversion; however, both yield and selectivity towards the targeted MF declined due to deep hydrogenolysis. Taking into consideration of practical cost and efficiency, the optimal temperature for this reaction was suggested to be 513 K.

(A)

100

80

Yield / mol%

60 FFR conversion FA yield MF yield

40

60

40 20

Conversion / %

80

20 0 460

470

480

490

500

510

Reaction temperature / K 100

(B)

80

FFR conversion MF yield FA yield

60

60 40 40

Conversion / %

80

Yield / mol%

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 20 0

460

470

480

490

500

510

520

Reaction temperature / K

Figure 7. Effect of reaction temperature on FFR transfer hydrogenation over (A) Cu2Al and (B) Cu3Al-A catalysts. Reaction conditions: catalyst (90 mg), FFR (115 mg, 1.2 mmol), methanol (15 mL), N2 (1 MPa), 600 rpm, 1.5 h.

The influence of catalyst dosage on the CTH of FFR was subsequently surveyed to find the suitable conditions to improve the yields of FA and MF. As depicted in Figure 8, the target 22 ACS Paragon Plus Environment

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compound yields for both FA (Figure 8A) and MF (Figure 8B) were adequately enhanced for higher catalyst dosage during the initial stage of the reaction, owing to the fact that much more H2 molecules were available from methanol reforming with the sufficient loading of copper catalysts. As for FA production, a significant rise in FA yield from 39.0 to 63.9 mol% was attained with the increase in the catalyst dosage from 75 to 105 mg after 1 h (Figure 8A). Under the optimized reaction conditions, a FA yield of 94.0 mol% with a full conversion of FFR was obtained by using a dosage of 105 mg Cu2Al at reaction temperature of 473 K after 2.5 h in methanol. Increasing Cu2Al dosage remarkably promoted FFR-to-FA transformation. However, longer reaction time of 2.5 h was still required to achieve maximum FA yield regardless of Cu2Al dosage (Figure 8A), which is attributed to the rate-determining step of Cu2+ species reduction to active Cu0 in the Cu2Al catalyst rather than FFR hydrogenation. A further increase in reaction time from 2.5 to 5.0 h led to a steep drop in FA yield, which is elucidated as subsequent hydrogenolysis of FA to produce around 16.5 mol% MF and the formation of various etherification products (Figure S7). As regards MF formation, a higher Cu3Al-A dosage greatly promoted FFR-to-MF transformation (Figure 8B), since high dosage rendered abundant active Cu0 site for both hydrogen supply and subsequent hydrogenation/hydrogenolysis within a relatively short period. Therefore, an excellent MF yield of 94.1 mol% was easily attained at a Cu3Al-A dosage of 105 mg in 1.5 h at 513 K in methanol (Figure 8B). Extending reaction time was unfavorable to the raise of MF yield due to the formation of side products, such as 2-methyltetrahydrofuran, 2-pentanol, THFA, and so on (Figure S9). Notably, a prolonged time was necessary to achieve reaction equilibrium when using lower catalyst dosages of 75 and 90 mg. Therefore, the Cu2Al catalyst, derived from hydrotalcite precursor, showed outstanding transfer hydrogenation selectivity towards the FFR-to-FA transformation by 23 ACS Paragon Plus Environment


giving a FA yield of 94.0 mol% with methanol as hydrogen donor. While, the activated Cu3Al-A catalyst favored the FFR-to-MF transformation by producing MF yield of 94.1 mol%. Recently, Yan and co-workers reported selective hydrogenation of FFR using hydrotalcite-derived Cu catalyst and H2 by giving 90% yield of FA and 51% yield of MF.50 Evidently, the hydrotalcite-derived Cu2Al and Cu3Al-A catalysts in this research showed comparable or even higher catalytic performance in transfer hydrogenation than in the case of previously reported hydrogenation.

(A)

FA yield / mol%

90

60 Catalyst dosage 75 mg 90 mg 105 mg

30

1

2

3

4

5

4

5

Reaction time / h (B)

90

MF yield / mol%

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

75 Catalyst dosage 75 mg 90 mg 105 mg

60

45

1

2

3

Reaction time / h

Figure 8. Effect of catalyst dosage on FFR transfer hydrogenation over (A) Cu2Al and (B) Cu3Al-A catalysts. Reaction conditions: FFR (115 mg, 1.2 mmol), methanol (15 mL), N2 (1 MPa), 600 rpm, (Cu2Al and 473 K) for (A), (Cu3Al-A and 513 K) for (B). 24 ACS Paragon Plus Environment

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Table 3. Transfer hydrogenation of biomass-derived HMF into DMF

a

Entry

T [K]

Usage [mg]

Conversion a [%]

Yield b [mol%]

1 2 3 4

473 483 493 503

90 90 90 90

80.1 92.0 93.4 96.8

13.5 34.7 49.9 78.1

35.0 90.2 129.7 203.0

5 6

513 513

90 75

> 99 95.7

94.9 60.1

246.6 187.4

7

513

105

> 99

96.7

215.4

Product formation rate [µmolDMF gCu-1 min-1]

: HMF conversion; b: DMF yield. Reaction conditions: Cu3Al-A (90-105 mg), HMF (151 mg, 1.2

mmol), methanol (15 mL), N2 (1 MPa), 600 rpm, 1.5 h.

Given the outstanding performance of copper-methanol catalytic system in FFR transformation, another important biomass-based platform chemical HMF was also subjected to this system for the synthesis of highly valuable DMF without any external hydrogen supplies. As shown in Table 3, the temperature was found, again, to play a key role in HMF-to-DMF transformation over Cu3Al-A with methanol as both the solvent and hydrogen source. Low reaction temperature gave low selectivity

of

DMF

(Table

3,

Entries

1

and

2),

many

intermediates

including

5-hydroxymethyl-2-methyl furan and a series of condensation compounds were detected as confirmed by GC-MS analysis (Figure S10). An exciting result on the desired DMF production was observed at 513 K, which afforded an excellent DMF yield of 96.7 mol% with the highest formation rate of 246.6 µmolDMF gCu-1 min-1 (Table 3, Entry 5). Also, a low catalyst dosage of 75 mg gave a poor DMF yield of 60.1 mol% (Table 3, Entry 6). Therefore, the copper catalyst developed in this 25 ACS Paragon Plus Environment


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

research showed excellent performance towards biomass-related transfer hydrogenation reaction, by giving a FA yield of 94.0 mol% in FFR-to-FA transformation with Cu2Al as the catalyst. In the case of activated copper catalyst of Cu3Al-A, a MF yield of 94.1 mol% was obtained from FFR-to-MF transformation and a DMF yield of 96.7 mol% was observed from HMF-to-DMF conversion. To further examine the stability and reusability of catalyst, recycling experiments on the transfer hydrogenation of FFR were carried out with Cu2Al and Cu3Al-A catalysts under the optimized conditions. In a typical cycle, the spent catalyst was separated from the reaction mixture by filtration after the reaction, dried in air and then reactivated before reuse. As shown in Figure 9A, recovered Cu2Al, after regeneration via calcination at 773 K in an air atmosphere, was found to sustain good catalytic activity in the consecutive cycles with only slight decreases in both FFR conversion and FA yield. TGA/DTA analysis indicated most of the adsorbed carbon deposition was removed after calcination treatment (Figure 6). Furthermore, N2 adsorption-desorption isotherm analysis demonstrated that regenerated Cu2Al still maintained high BET surface area and pore volume, which contributes to mass transfer during FFR conversion (Table 1, Entries 9 and 10). In the case of Cu3Al-A recycling for FFR-to-MF transformation, calcination treatment at 773 K was first performed to remove the adsorbed organic compounds, then followed by reactivation at 773 K under hydrogen atmosphere before being subjected to the next cycle. As seen from Figure 9B, an acceptable decrease for the activity of Cu3Al-A after repeated use was observed, presumably owing to a minor decline in BET surface area caused by particle sintering during reactivation procedure. Thus, sufficient contact between FFR and active metal centers could be partially suppressed. Notably, both Cu2Al and Cu3Al-A catalysts exhibited exceptional stability with almost no leaching of Cu (< 0.1 mg L-1) and Al (< 0.1 mg L-1), as confirmed by ICP-AES analysis on the reaction 26 ACS Paragon Plus Environment

Page 26 of 39

mixtures. Therefore, our research revealed the investigated Cu2Al and Cu3Al-A were promising catalysts for selective transformation of FFR using methanol as the hydrogen source.

FA yield

(A)

FFR conversion

100

75

mol%

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


50

25

0

1

2

3

4

Cycles MF yield

(B)

FFR conversion

100

75

mol%

Page 27 of 39

50

25

0

1

2

3

4

Cycles

Figure 9. Effect of catalysts Cu2Al (A) and Cu3Al-A (B) recycling on transfer hydrogenation of FFR. Reaction conditions: FFR (115 mg, 1.2 mmol), methanol (15 mL), N2 (1 MPa), 600 rpm, [Cu2Al (105 mg), 473 K and 2.5 h] for (A), [Cu3Al-A (105 mg), 513 K and 1.5 h] for (B).

A plausible mechanism for the transfer hydrogenation of FFR was proposed (Figure 10), in which methanol steam reforming took place on copper oxide species to provide hydrogen molecule and active Cu0 species. Initially, dimethyl ether and water molecules were generated through acid sites catalyzed etherification of methanol. Then methanol steam reforming would occur in the 27 ACS Paragon Plus Environment


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Figure 10. Proposed mechanism for transfer hydrogenation of FFR in methanol media over copper catalysts.

presence of water molecule and Cu(II) species, which produced several gaseous components such as H2 and CO2.51,52 Subsequently, Cu(II) species were reduced into Cu(I) and Cu(0) with the aid of generated H2 molecule. Herein, the in situ generated Cu0 active center also promoted methanol steaming reforming to give much more hydrogen molecules. Under hydrogen atmosphere, the C=O double bond in furfural molecule was attacked by two adsorbed hydrogen atoms which located on the metallic Cu surface through two active intermediates.9,53,54 As described by Resasco et al., a first H attacking to the C atom of the carbonyl favored the formation of an alkoxide intermediate (route a); while, a first H attacking to the O atom of the carbonyl yielded a hydroxyalkyl species (route b).9,53 And then these two compounds adsorbed on Cu surface were hydrogenated into FA via H attack. During this process, hemiacetal, acetal, aldol condensation and etherification also occurred due to the presence of numerous B/L acid sites, resulting in a series of byproducts such as 2-methoxymethyl-furan,

2-(dimethoxymethyl)-furan

and

5-furfurylfurfuryl

alcohol,

as

demonstrated by GC-MS analysis (Figures S7-2, S7-4 and S7-5). In the step of FA hydrogenolysis, 28 ACS Paragon Plus Environment

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FA was first protonated by the hydrogen on the atomic Cu surface. Subsequently, the C–O bond at α-position was dissociated to produce a molecule of H2O in the presence of L acid site, and then another hydrogen atom on the Cu surface attacked C atom at the α-position yielding the target compound MF.48,55,56

CONCLUSIONS We have developed a series of cheap copper-based catalysts, derived from hydrotalcite precursors, for the selective transfer hydrogenation of biomass-based FFR and HMF using methanol as both the solvent and hydrogen donor. The valence of active Cu species, catalyst acidity and reaction temperature played an important role in product selectivity. Under optimized condition, a FA yield of 94.0 mol% was obtained from FFR-to-FA transformation with Cu2Al as the catalyst. While, a MF yield of 94.1 mol% was observed from FFR-to-MF transformation and a DMF yield of 96.7 mol% was achieved from HMF-to-DMF conversion by using Cu3Al-A catalyst. The recovery and reutilizations of catalysts in successive reactions suggested that catalysts maintained good activity and stability after five-time recycling. The research thus highlights a new perspective for a green, efficient and biomass-related hydrogenation reaction using readily available and inexpensive catalytic system of copper-methanol, while, without any external hydrogen supplies. Future work on CTH of mixtures from methanolysis without separation and purification are still ongoing, in view of the obvious advantages of copper-methanol catalytic system.

ASSOCIATED CONTENT: Supporting Information 29 ACS Paragon Plus Environment


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

This material is available free of charge via the Internet at http://pubs.acs.org. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xxxxxxx. Reaction routes for methanol reforming, photography for fresh and used Cu2Al, TPR analysis of copper based catalysts, photographs for color change of fresh Cu3Al-A sample after several hours in the air, nitrogen adsorption–desorption isotherms of fresh and recycled copper catalysts, SEM images of Cu2Al and Cu3Al-A, TEM images of Cu1Al-A, Cu2Al-A, Cu3Al-A and Cu4Al-A, GC-MS analysis of reaction products and intermediates, the concentration of gaseous components under varied reaction conditions, residual methanol volume, gaseous component and final pressure after reaction (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Tel. (+86)-20-8522-2191. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of Guangdong Province, China (2016A030313173, 2014A030310386, 2015A030312007), National Natural Science Foundation of China (21472189), Science and Technology Planning Project of Guangzhou City, China (201707010238).


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to 2-methylfuran. ACS Catal. 2015, 5, 3988−3994.

For Table of Contents Use Only

Selective transfer hydrogenation of biomass-based furfural and 5-hydroxymethylfurfural over hydrotalcite-derived copper catalysts using methanol as hydrogen donor

Jun Zhang and Jinzhu Chen

Hydrotalcite-derived copper catalysts were developed for selective transfer hydrogenation of furfural using methanol as both solvent and hydrogen donor.


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TOC 705x257mm (96 x 96 DPI)

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Selective Transfer Hydrogenation of Biomass-Based Furfural and 5

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