The hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF) with sol-gel Ru-Co/SiO2 catalyst

(1)

ORIGINAL PAPER: SOL-GEL AND HYBRID MATERIALS FOR CATALYTIC, PHOTOELECTROCHEMICAL AND SENSOR APPLICATIONS

The hydrogenation of 5-hydroxymethylfurfural (HMF) to

2,5-dimethylfuran (DMF) with sol

–gel Ru-Co/SiO

2

catalyst

Merve Esen1●Solmaz Akmaz1●Serkan Naci Koç1●Mehmet Ali Gürkaynak1 Received: 11 April 2019 / Accepted: 6 June 2019 / Published online: 15 June 2019

Abstract

Sol–gel Ru/SiO2, Co/SiO2, and Ru-Co/SiO2catalysts were prepared for 5-hydroxymethylfurfural (HMF) hydrogenation to

2,5-dimethylfuran (DMF). Catalysts were characterized by BET, XRD, TPR, TEM, and XPS. Reactions were run with catalysts that were reduced at 500 °C for 1 h. Reaction time, temperature and hydrogen pressure effects were studied for hydrogenation. In the presence of both Ru and Co, the easier reduction was observed at Ru-Co/SiO2catalyst. Although no

DMF yield was observed with Ru/SiO2and Co/SiO2, 96% DMF was obtained at 180 °C, under 15 bar H2pressure for 2 h.

Moreover >99.9% DMF yield was achieved with Ru-Co/SiO2 at 120 °C for 8 h and at 140 °C for 4 h, respectively. DMF

yield did not change signiﬁcantly after third use, however deactivation was observed after ﬁfth use of catalyst. It may be attributed to oxidation of cobalt active sites during recycle runs.

Graphical Abstract

SiO2

Ru/Co CoOx

Highlights

● _{A new Sol}–gel Ru-Co/SiO₂_{catalyst was tested for 5-HMF hydrogenation to DMF.} ● >99.9% DMF yield was achieved at 120 °C under 15 bar H2pressure for 8 h.

● _{Synergetic effect was observed between Ru and Co in reduction that affects the catalytic properties.} Keywords Biofuel● _HMF● _DMF●_{Catalytic hydrogenation} ●_Sol–gel catalyst

1 Introduction

Fossil fuels supply almost all transportation energy demand in the World. Moreover, crude oil and its intermediates are basic sources for thousands of chemicals. However, the

increasing world population and energy demand and the depletion of fossil resources stimulate scientists to search alternative energy sources [1]. Researching of new pro-duction routes of important fuel chemicals has gained great importance to produce energy from biomass [2].

Biomass is cheap and renewable source for alternative energy production. It has been estimated that bio-based industries will widen around the World with developing effective biomass conversion technologies [3]. For example, it has been suggested that the use of biofuel will increase to 36 billion gallon and its 16 billion parts will be produced from cellulose in United States [4]. Carbohydrates can be * Serkan Naci Koç

nacik@istanbul.edu.tr

1 _{Depatment of Chemical Engineering, Istanbul}

University-Cerrahpaşa, Avcilar, 34320 Istanbul, Turkey

123456789

0();,:

123456789

(2)

dehydrated to furan compounds and one of them namely 5-Hydroxymethylfurfural is on the “Top 10 Bio-based Che-mical List” of US DOE [5].

5-HMF is the platform chemical that can be used to produce many important chemicals. 2,5-Dimethyl furan (DMF) is one of the most important chemicals that can be derived from 5-HMF. DMF has more advantages than ethanol as a fuel. The volumetric energy density of DMF is 31.5 MJ/L and this value is 40% higher than that of ethanol. It has high boiling point that is advantage for safe trans-portation and storage. Moreover DMF has DSI character-istics similar to gasoline for internal combustion engines [6]. DMF can be produced from 5-HMF by catalytic hydro-genation reaction [7]. DMF production route from biomass has four steps [8,9];

Pretreatment of cellulosic biomass and hydrolysis to glucose.

Isomerization of glucose to fructose.

5-HMF from fructose with releasing three moles of water. DMF from 5-HMF with catalytic selective hydrogenation

Reaction from 5-HMF to 2,5-DMF is shown in Scheme1. Reactions occur in two steps: (1) hydrogenation of aldehyde group of 5-HMF and (2) hydrogenolysis of hydroxymethyl group. Due to the fact that HMF has dif-ferent functional groups, many types of competitive reac-tions can be observed such as partial hydrogenation, ring hydrogenation and ring opening side reactions [7]. In the

ﬁrst step, 5-HMF is hydrogenated to

bis-hydroxymethylfurfural (BHMF) and it is suggested that DMF is produced from this BHMF route [9]. BHMF intermediate is converted to 2-(hydroxymethyl)-5-methyl-furan (MFA) and then DMF was synthesized from MFA by releasing H2O from structure. Although BHMF route is

more favorable from HMF to DMF [10–12], the formation of MF by hydrogenolysis of HMF is also possible then MF can be hydrogenation to FA and DMF sequentially.

The studies on DMF synthesis were begun in 2007 by Roman-Leshkov et al. [13]. In their study, DMF has been synthesized from HMF in the presence of 1-buthanol sol-vent at 220 °C and 6.8 bar H2 atmosphere. They have

reported 71% DMF yield with CuRu/C catalyst in 10 h reaction time. After this work, the studies on DMF synthesis reaction have increased in literature. Zu et al. [14] has

reported 93.4% DMF yield at 220 °C and 0.7 MPa H2

pressure for 24 h. Chen et al. [15] have achieved 99.4% DMF yield at 180 °C and 5 MPa H2 pressure for 8 h with

Cu-Co@C (Cu:Co= 1:3) catalyst. Li et al. [16] has used partially reduced Co3O4catalysts and achieved 83.3% DMF

yield at 170 °C for 12 h. An et al. [17] has reported 74.2% DMF yield with 11.8% Co-(ZnO-ZnAl2O4) catalyst at mild

130 °C with 0.7 MPa pressure for 24 h. Gao et al. [18] has prepared RuCo/CoOx for HMF hydrogenation and reported 96.5% DMF yield at 200 °C and 0.5 MPa H2 pressure for

2 h. Gao et al. [19] has used cyclohexanol as hydrogen source for HMF hydrogenation to DMF and 96.1% DMF yield has been achieved at 220 °C for 30 min under N2

atmosphere with nitrogen doped carbon supported copper catalyst in their other study. Srivastava et al. [20] has achieved 78% DMF yield with Cu‐Co/Al2O3 catalyst at

220 °C, 3 MPa H2 pressure for 8 h. Hu et al. [21] has

achieved 94.7% DMF yield at 200 °C for 2 h with Ru/C catalyst. Nishimura et al. [22] has prepared PdAu/C catalyst and achieved over 99.96% DMF yield under mild 60 °C and atmospheric hydrogen but with the aid of homogeneous HCI acid. Shi et al. [23] has reported 73.2% DMF yield with 100% conversion of HMF at 120 °C under 3.0 MPa H2

and for 2 h with Pt/rGO catalyst. Talpade et al. [24] (2019) has reported 85% DMF yield with magnetically separable Fe-Pd/C bimetallic catalyst at 150 °C, under 20 bar H2

pressure.

In this study, sol–gel Ru-Co/SiO2 catalyst has been

prepared for HMF hydrogenation to DMF under mild temperature and hydrogen pressure conditions.

2 Experimental

2.1 Preparation of Ru-Co/SiO

2

catalyst by sol

–gel

method

Cobalt(II) acetate and ruthenium(III) chloride were dis-solved in ethanol. TEOS (Tetra ethyl ortho silicate) was added to this mixture by dropwise at 50 °C and con-tinuously mixed at this temperature for 30 min. Then H2O

was added to this mixture by dropwise. pH of mixture was adjusted to 5 with acetic acid then solution was mixed overnight. Gel was handed and dried at 110 °C and calcined at 400 °C for 4 h in air then reduced under10% H2/N2

mixture for 1 h at 500 °C. The prepared Ru-Co/SiO2catalyst

contains 4% Ru and 7.9% Co by weight. The 3.9 and 11.8% Co containing forms of Ru-Co/SiO2 were also prepared

with same method. All catalysts were run in reactions with their reduced forms. The 7.9% Co containing catalyst is named as Ru-Co/SiO2in the text.

O O O H 5-HMF 3 H (g) O CH3 C H3 2,5-DMF -2 H O Scheme 1 5-HMF to 2,5-DMF reaction

(3)

2.2 Catalyst characterization

Nitrogen physisorption measurements were carried out at −196 °C with a Quantachrome Nova 3200e BET surface area analyzer after vacuum degassing of the catalysts at 300 °C. XRD results were obtained on a Rigaku D/Max-2200 diffractometer using Cu–Kα irradiation (λ = 1.5404 Å). XPS analyzes were done using Thermo Scien-tiﬁc K-Alpha x-ray photoelectron spectrometer. All binding energies were referenced to the C1s line (286.4 eV). JEOL JEM 2100 HRTEM was used for TEM measurements at 200 kV. Temperature programmed reduction (TPR) of cat-alysts were measured with HIDEN CATLABTM system. Before reduction experiments, catalysts wereﬂushed with nitrogen gas at 300 °C for 30 min then cooled. TPR were run with 5% H2/N2mixture at a rate of 5 °C/min. Hydrogen

was monitored with QIC-20 mass spectrometer.

2.3 Reaction studies

Parr 4598 model, high pressure reactor system with a volume of 100 mL was used for reactions. 0.5 g HMF was

dissolved in 23 mL THF solvent and then 0.013 gr naph-thalene as internal standard and 0.2 g catalyst were added to the reaction mixture, respectively. Reactor was purged several times with hydrogen before running the experi-ments. Afterwards, reactor was pressurized with hydrogen and heated to the reaction temperature. After the reaction has been completed, reactor was cooled to room tempera-ture and reaction products were measured with Agilent Technologies 7890A Series Gas Chromatography and Agilent Technologies 5975C Series Mass Spectroscopy (GC-MS) system.

HMF conversion and DMF yield were calculated by using the Eqs. (1) and (2), respectively:

HMF conversionð Þ ¼% Consumed HMF moleð Þ Initial HMF moleð Þ 100 ð1Þ DMF yieldð Þ ¼% DMF moleð Þ Initial HMF moleð Þ 100 ð2Þ 0 10 20 30 40 50 60 70 80 In te n si ty ( a.u .) ♠ ♠ ♠ ♥ ♥

•

• ♦ ♦ ♥ ♥ •

Co

♠

Ru

♦

CoO

♣

Co

₂

SiO

₄ ♥

_Co3O4

0 10 20 30 40 50 60 70 80 In te ns ity ( a.u .) In te ns ity (a .u .) 2θ/ 2θ/ 2θ/

(3.9 % Co)

(11.8 % Co)

♠ ♠

(7.86 % Co)

♠ ♦ ♥ ♥ ♥ ♥ ♥ 0 10 20 30 40 50 60 70 80 ♦ ♦ ♠ ♠ ♠ ♠ ♠ ♠ • ♦ ♥ ♥ ♥ ♥ ♥

(4)

3 Results and discussion

3.1 Catalyst characterization

BET surface area of Ru-Co/SiO2catalyst was measured as

249 m2/g. XRD patterns of reduced Ru-Co/SiO2are seen in

Fig. 1. 2Ɵ 38.4°, 44.2°, 58.5°, and 70.0° peaks can be attributed to Ru metal (JCPDS Card No: 06-0663), 2Ɵ 36.5° and 42.5o and 78.0o peaks were attributed to CoO phases (JCPDS Card No: 780431) and 2Ɵ 44.2o peak represents Co metal overlapping with Ru metal (JCPDS Card No: 150806). 2Ɵ 38.4opeak could also be attributed to Co2SiO4 crystal phase (JCPDS Card No: 700323).

Similarly Co2SiO4 observation has been reported with

sol–gel Co/SiO2catalyst [25]. The peaks at 2Ɵ 18.7°, 31.1°,

36.5°, 58.5°, and 65.1obelong to Co3O4(JCPDS Card No:

00-042-1467).

XPS peaks of ruthenium of reduced fresh andﬁfth used Ru-Co/SiO2are shown in Fig.2a, b. Since C1s and Ru 3d3/2

orbitals can not be splitted, only Ru3d5/2was evaluated. The

peak at 279.3 eV corresponds to Ru(0) [26–28]. Co 2p3/2

XPS analyses of reduced fresh Ru-Co/SiO2 catalysts are

seen in Fig.3a. The binding energies of 779.6, 781.6, and 783.5 eV peaks can be attributed to Co(0), Co(3+), and Co(2+), respectively [29–31]. Spin-orbit splitting (ΔE) are 14.9 and 15.2 eV for Co(3+) and Co(2+), respectively [32,33]. First satellite peak also contributes Co(2+) forma-tion in the catalyst. As shown in Fig.3b, XPS analysis of ﬁfth used catalyst was also measured. Co(2+)_{formation may}

come from CoO and Co2SiO4 formations. Since catalysts

were prepared by sol–gel method, Si–O–Co bonds may also form. Similarly, it has been reported that Co2SiO4 peaks

were observed for Co/SiO2 synthesized by sol–gel method

[34–37].

TPR peaks of Co/SiO2, Ru/SiO2, and Ru-Co/SiO2

cata-lysts are seen in Fig.4. TPR peaks of Ru/SiO2between 150

and 220 °C can be attributed to ruthenium oxide reduction to metallic ruthenium. First reduction peak of Co/SiO2

288 286 284 282 280 278 276 274 Inte nsity ( a .u .)

Binding Energy (eV) a) C1s and Ru3d Ru3d Ru(0) 288 286 284 282 280 278 276 274 In te n s it y ( a .u .)

Binding Energy (eV) b)

Fig. 2 a Ruthenium (Ru 3d) XPS result of fresh Ru-Co/SiO2catalyst,b Ruthenium (Ru 3d) XPS result of ﬁfth used Ru-Co/SiO2catalyst

810 800 790 780 770 2p3/2 Co(+2) Co(+3) Co(+2) Co(+3) Intensity (a .u .)

Binding Energy (eV)

Co(0) satellite 2p_1/2 a) 810 800 790 780 770 Int e ns it y ( a .u.)

Binding Energy (eV) b)

Fig. 3 a Cobalt (Co 2p) XPS result of fresh Ru-Co/SiO2 catalyst,

(5)

catalyst that is completed at 400 °C can be attributed to Co(3+) reduction to Co(2+) and then Co metallic formation begins [16, 37, 38]. Since TPR study was cut at 600 °C, only the beginning Co metal formation was observed. Similar reduction properties have been reported for sol–gel Co/SiO2 catalyst [31]. Co(2+) to Co(0) reduction begins at

around 400 °C in this study. On the other hand, reduction is completed at around 380 °C with Ru-Co/SiO2 catalyst. It

means that Ruthenium oxide is easily reduced to Ru metal and these Ru metal phases provide hydrogen to cobalt oxide

sites by hydrogen spillover and make the reduction of cobalt oxide phases highly easier. The shifting of cobalt oxide reduction to the lower temperatures with noble metal addition has similarly been reported for Co-M/SiO2(M: Ru,

Re) and Ir-Co/SiO2 catalysts [39, 40]. The amounts of

hydrogen consumption of three catalysts (up to 400 °C for Co/SiO2) were calculated from MS signals after each TPR

experiments. The values are 1.47, 1.02, and 9.48 mmol H2/g

catalyst for Ru/SiO2, Co/SiO2, and Ru-Co/SiO2,

respec-tively. Since the reduction shifted to lower temperatures and the amount of hydrogen consumption increased approxi-mately nine times for Ru-Co/SiO2than that of Ru/SiO2, Co/

SiO2 catalysts.

TEM pictures of Ru-Co/SiO2are seen in Fig.5.

3.2 Comparative reaction studies of sol

–gel Ru/SiO

2

,

Co/SiO

2

, and Ru-Co/SiO

2

catalysts

Ru/SiO2, Co/SiO2, and Ru-Co/SiO2 prepared by sol–gel

method were compared for DMF synthesis from HMF. Hydrogenation reactions were carried out at 180 °C, under 15 bar H2pressure for 2 h. THF was used as solvent. DMF

yields and HMF conversions are seen in Table1. It is highly interesting that although HMF conversion was observed in all cases, no DMF was detected with GC-MS for Ru/SiO2

and Co/SiO2 catalysts in this reaction conditions. On the

other hand, 96% DMF yield at 100% HMF conversion was achieved with Ru-Co/SiO2. It can clearly be said that the

presence of both Ru and Co provides high activity and selectivity for DMF formation from HMF hydrogenation.

3.3 The effect of catalyst amount on reaction with

RuCo/SiO

2

catalyst

The effect of catalyst amount on DMF yield and HMF conversion with Ru-Co/SiO2 are seen in Table 2. HMF

conversion is the same as 100% for the amounts of catalysts between 0.1 and 0.25 g. However, the maximum DMF yield was observed with 0.2 g catalyst and then further reactions were run with this amount.

3.4 Temperature and reaction time effects on DMF

synthesis with Ru-Co/SiO

2

catalyst

Different reaction temperatures (120, 140, 160, 180, and 200 °C) and reaction times (0.5, 1, 2, 3, 4, 6, and 8 h) were studied with Ru-Co/SiO2catalyst. As seen in Fig.6, a clear

trend was observed between reaction time and temperature for DMF formation. At lower temperatures (e.g., 120 °C), the higher DMF yields were obtained for longer reaction times. For example, >99.9% DMF yield was observed at 120 °C for 8 h. However, >99.9% DMF yield was achieved at 140 °C for shorter reaction time namely 4 h. When

0 100 200 300 400 500 600 Intensity (a.u.) Co/SiO₂ 0 100 200 300 400 500 600 Intensity (a.u.) 0 100 200 300 400 500 600 Ru/SiO₂ Ru-Co/SiO₂ Intensity (a.u)

(6)

temperature increased to 160 °C, maximum DMF yield was reached to 97.6% for 1 h then yield decreased. As seen from Fig. 6, DMF yield shows volcano plot and yield decreases with further temperature increase. It can be said that Ru-Co/ SiO2 is highly active and selective catalyst for DMF

synthesis from HMF hydrogenation in this reaction condi-tions. In summary, it can be said that >99.9% DMF yield can be achieved at mild 120 °C but longer reaction times. This yield can be achieved at higher temperatures (e.g., 140 °C) at shorter reaction times but DMF yield decreases at longer reaction times. It may be due to further by-product formation from DMF at higher temperatures. When the literature is examined, >99.9% DMF yield is one of the highest DMF yields in the literature. Nishimura et al. [19] has reported >99.96% DMF yield with PdAu/C catalyst at atmospheric H2 pressure and milder reaction temperature

(e.g., 60 °C). However they have added homogeneous HCI acid in reaction medium. Gao et al. [18] has reported 96.5% DMF yield with RuCo/CoOx at 200 °C and 0.5 MPa H2

pressure for 2 h.

3.4.1 The effect of hydrogen pressure on reaction with Ru-Co/SiO2catalyst

It is known that hydrogen pressure is also effective on hydrogenation reactions as reaction time and temperature. Thus, different hydrogen pressures were also studied for HMF hydrogenation to DMF. Results are seen in Fig. 7. Although HMF conversion is 100% in each hydrogen pressure, the change of DMF yield shows again volcano plot. 38.8% DMF yield increased to >99.9% under 20 bar hydrogen pressure and then decreased to 89.6% at 25 bar.

An increase in hydrogen pressure increases the amount of dissolved hydrogen in reaction environment and facil-itates the DMF formation. However, DMF yield decreased with further increase in hydrogen pressure. The similar result has been reported that DMF may be decomposed to Fig. 5 TEM picture of reduced

fresh Ru-Co/SiO2catalyst

Table 1 The effect of catalyst type on DMF yield and HMF conversion

Ru/SiO2 Ru-Co/SiO2 Co/SiO2

DMF yield (%) Not detected 96.0 Not detected HMF conversion (%) 31.1 100 38.1 Reaction conditions: HMF: 0.5 g, THF: 23 g, naphthalene (internal standard): 0.013 g, 180 °C, 2 h, 15 bar H2

Table 2 Effect of catalyst amount on reaction with Ru-Co/SiO2

Catalyst weight (g)

0.1 0.15 0.2 0.25

DMF yield (%) 84.5 94.3 96.0 82.3

HMF conversion (%) 100 100 100 100

Reaction conditions: HMF: 0.5 g, THF: 23 g, naphthalene (internal standard): 0.013 g, 180 °C, 2 h, 15 bar H2. 0 1 2 3 4 5 6 7 8 9 0 20 40 60 80 100 % D M F Yi e ld Time (h) 120 °C 140 °C 160 °C 180 °C 200 °C

Fig. 6 Reaction temperature and time effect on reaction with Ru-Co/ SiO2 catalyst (Catalyst: 0.2 g, HMF:0.5 g, THF: 23 g, Naphthalene

(7)

by-products with the higher amount of hydrogen in reaction medium at higher hydrogen pressures [21].

3.5 The effect of cobalt content in Ru-Co/SiO

2

catalyst on reactions

Ru-Co/SiO2 containing 3.9 and 11.8% Co were also tested

for DMF synthesis. Reaction studies results are seen in Table3. HMF conversion is 100% for catalysts containing 3.9 and 7.9% Co. Moreover, DMF yields are nearly same in both cases e.g., 83.9 and 82.1%, respectively. However, both HMF conversion decreased to 88.7% and DMF yield decreased to 41.1% with further increase in the amount of cobalt content.

As aforementioned in catalyst characterization section, reduced Ru-Co/SiO2catalyst contains Ru metal, Co metal,

CoOx forms, and also Co2SiO4 phase. As it has been

pro-posed in literature, cobalt metal adsorbs hydrogen and HMF is adsorbed on the oxygen-deﬁcient cobalt oxide sites then hydrogenation/hydrogenolysis occurs with dissociated hydrogen on oxygen deﬁcient-cobalt oxide sites [16, 18]. However, further explanation on reaction mechanism with sol–gel Ru-Co/SiO2may be needed. Although interestingly,

any DMF was not detected with Ru/SiO2 and Co/SiO2 in

this study, it has been reported in literature that high DMF

yield has been observed with partially reduced Co3O4 and

also reduced Ru/Co3O4catalysts [14]. This result may come

from the nature of sol–gel environment. Strong Co–O–Si bonds may suppress the oxygen deﬁcient site formation that responsible for HMF adsorption. Moreover, as it is seen in TPR peaks, the easier reduction of sol–gel Ru-Co/SiO2 is

the possible reason of >99.9% DMF yield with sol–gel Ru-Co/SiO2catalyst. This can be explained by hydrogen

spil-lover effect from Ru to cobalt oxides providing easier reduction of cobalt oxide sites that creates oxygen de ﬁ-ciency in Co–CoOx interphase. In summary, it can be said that there is strong synergy between Ru metal and these Co–CoOx catalytic sites.

3.6 Reusability studies of Ru-Co/SiO

2

catalyst

According to the reaction results, Ru-Co/SiO2prepared by

sol–gel method is highly active and selective for DMF synthesis from HMF hydrogenation and this catalyst gives one of the highest DMF yields in literature as discussed above. For this reason, as seen in Table 4, reusability stu-dies were run with this catalyst. Catalyst was filtered from reaction medium and washed with THF and ethanol then dried at 110 °C under nitrogen atmosphere before each run. Under selected reaction conditions, HMF conversion decreased from 100 to 92.4% and DMF yield decreased from 96% to 90.1 after second use and then DMF yield decreased to 80.2% after third use. However, sharp decrease to 5.5% was observed on DMF yield afterfifth run. It means that catalyst was deactivated after fifth run. As aforemen-tioned in XPS analysis results, Co(0), Co(2+), and Co(3+) intensities of fresh Ru-Co/SiO2 catalyst change after

ﬁfth use. The Co(3+)_/Co(0)_{, Co}(2+)_/Co(0) _{and Co}(3+)_/Co(2+)

ratios of fresh catalyst were calculated from deconvoluted XPS spectra as 2.13, 1.85, and 1.15, respectively. The Co(3+)/Co(0), Co(2+)/Co(0), and Co(3+)/Co(2+) of fifth used catalyst were calculated as 4.63, 1.75, and 2.63, respec-tively. Thus, it is seen that although Co(2+) and Co(0)XPS intensities decreased, their ratio did not change significantly for fresh and fifth use. However, both Co(+3)intensity and also Co(3+)/Co(0)ratio clearly increased after thefifth use of the catalyst. The possible reason of deactivation may be due to oxidation of both Co(0) metal and Co(+2) sites during recycle experiments. Since it has been proposed that oxygen

5 10 15 20 25 30 0 20 40 60 80 100 DMF Y iel d (%) Pressure (bar)

Fig. 7 Hydrogen pressure effect on reaction with Ru-Co/SiO2catalyst

(Catalyst: 0.2 g, HMF:0.5 g, THF: 23 g, naphthalene (internal stan-dard): 0.013 g, 160 °C, 0.5 h)

Table 3 The effect of cobalt content on reaction with Ru-Co/SiO2

catalyst Co content (%)

3.9 7.9 11.8

DMF yield (%) 83.9 82.1 41.1

HMF conversion (%) 100 100 88.7

Reaction conditions: catalyst: 0.2 g, HMF: 0.5 g, THF: 23 g, naphthalene (internal standard): 0.013 g, 160 °C, 0.5 h, 15 bar H2

Table 4 Reusability studies of Ru-Co/SiO2catalyst

Recycle

1 2 3 4 5

DMF yield (%) 96.0 90.1 80.2 22.1 5.5 HMF conversion (%) 100 100 92.4 57.3 33.9 Reaction conditions: HMF: 0.5 g, THF: 23 g, naphthalene (internal standard): 0.013 g, 180 °C, 2 h, 15 bar H2

(8)

deficient sites are responsible for deoxygenation of HMF molecule [16,18], the oxidation of these oxygen deficient sites may be the reason of catalyst deactivation. Oxygen comes from HMF molecule deoxygenation and may also come from air during recycle procedure. Nevertheless, this is thefirst report on sol–gel Ru-Co/SiO2 catalyst that has

great potential for HMF to DMF hydrogenation. However, different catalyst preparation, reduction and reaction con-ditions are needed for long term stability of this highly active and selective catalyst in recycle studies. Anyway these ﬁndings may open the ways to new approaches and studies for HMF to DMF reaction (Table5).

4 Conclusion

Ru/SiO2, Co/SiO2, and Ru-Co/SiO2 catalysts prepared by

sol–gel method were comparatively studied for DMF synthesis from HMF hydrogenation. Although no DMF was observed with Ru/SiO2 and Co/SiO2 catalysts, 96% DMF

yield was achieved with Ru-Co/SiO2 catalyst at the same

reaction condition. It shows that when Ru and Co both are used together, they have cooperative synergetic activity and selectivity in hydrogenation/hydrogenolysis reactions for DMF synthesis from HMF. Particularly, reaction tempera-ture has strong effect on DMF formation. The >99.9% DMF yield was achieved at 120 °C for 8 h and at 140 °C for 4 h with 15 bar hydrogen pressure. The DMF yield did not change significantly after third use. However, faster deac-tivation was observed after fifth use of catalyst. Never-theless, it is thefirst report on sol–gel Ru-Co/SiO2catalyst

for HMF hydrogenation to DMF. Thus, it can be said that sol–gel Ru-Co/SiO2 is promising catalyst for DMF

synth-esis from HMF hydrogenation. However, further studies should be held for better catalyst recyclability properties.

Acknowledgements This work was supported by Scientiﬁc Research Projects Coordination Unit of Istanbul University. (Project number: FYL-2017-24265) and The Scientiﬁc and Technological Research Council of Turkey (TUBITAK) (Project No: 214M149)

Compliance with ethical standards

Conﬂict of interest The authors declare that they have no conﬂict of interest.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afﬁliations.

References

1. Raymond LR, Deming CP, Nichols MW (2007) Hard tTruths, facing the hard truths about energy. A report of the National Petroleum Council, Library of Congress Control Number: 2007937013

2. Perlack RD, Wright LL, Turhollow AF, Graham RL, Stokes JB, Erbach CD (2005) Biomass as feedstock for a bioenergy and bioproducts industry: the technical feasibility of a billion-ton annual supply, DOE/GO-102005-2135

3. Chheda JN, Huber GW, Dumesic JA (2007) Angew Chem Int Ed 46:7164–7183

4. Coyle W (2010) Next-generation biofuels, near-term challenges and implications for agriculture. Economic Research Service/ USDA, Amber Waves

5. Putten Van R-J, Waal Van der JC, Jong de E, Rasrendra CB, Heeres HJ, Vries de JG (2013) Chem Rev 113:1499–1597 6. Somers KP, Simmie JM, Gillespie F, Burke U, Connolly J,

Metcalfe WK, BattinLeclerc F, Dirrenberger P, Herbinet O, Glaude PA, Curran HJ (2013) Proc Combust Inst 34:225–232 7. Chen Z, Wan C (2017) Renew Sust Energ Rev 73:610–621 8. Binder JB, Raines RT (2009) J Am Chem Soc 131:1979–1985 9. Saha B, Abu-Omar MM (2015) ChemSusChem 8:1133–1142 10. Yang Y, Liu Q, Li D, Tan J, Zhang Q, Wang C, Ma L (2017) RSC

Adv 7:16311–16318

11. Nagpure AS, Venugopal AK, Nishita L, Marimuthu M, Raja T, Satyanarayana C (2015) Catal Sci Technol 5:1463–1472 Table 5 Different catalysts for

HMF to DMF reaction Solvent Temp.

(°C) H2/Bar Time (h) HMF conversion DMF yield Ref. no Cu-Ru/C 1-Butanol 220 6.8 10 100 71.0 13 Ru/Co3O4 THF 220 7 24 100 93.4 14 Cu-Co@C Ethanol 180 50 8 100 99.4 15

Partial reduced Co3O4 Dioxane 170 10 12 97.0 83.0 16

Co-(ZnO-ZnAl2O4) THF 130 70 24 99.9 74.2 17 Cu-Co/Al2O3 THF 220 30 8 100 78.0 20 Ru/C THF 200 20 2 100 94.7 21 PdAu/C (Homogeneous HCl) THF 60 Atm. 12 >99 >99.96 22 Pt/rGO 1-Butanol 120 30 2 100 73.2 23 Fe-Pd/C THF 150 20 3 100 85.0 24

(9)

12. Verevkin SP, Emel’yanenko VN, Stepurko EN, Ralys RV, Zaitsau DH (2009) Ind Eng Chem Res 48:10087–10093

13. Roman-Leshkov Y, Barrett CJ, Liu ZY, Dumesic JA (2007) Nature 447:982–986

14. Yanhong Z, Panpan Y, Jianjian W, Xiaohui L, Jiawen R, Guanzhong L, Yanqin W (2014) Appl Catal B 146:244–248 15. Bingfeng C, Fengbo L, Huang Z, Yuan G (2017) Appl Catal B

200:192–199

16. Li D, Liu Q, Zhu C, Wang H, Cui C, Wang C, Ma L (2019) J Energy Chem 30:34–41

17. An Z, Wang W, Dong S, He J (2019) Catal Today 319:128–138 18. Gao Z, Fan G, Liu M, Yang L, Li F (2018) Appl Catal B

237:649–659

19. Gao Z, Li C, Fan G, Yang L, Li F (2018) Appl Catal B 226:523–533

20. Srivastava S, Jadeja GC, Parikh J (2017) Chin J Catal 38:699–709 21. Hu L, Tang X, Xu J, Wu Z, Lin L, Liu S (2014) Ind Eng Chem

Res 53:3056–3064

22. Nishimura S, Ikeda N, Ebitani K (2014) Catal Today 232:89–98 23. Shi J, Wang Y, Yu X, Du W, Hou Z (2016) Fuel 163:74–79 24. Talpade AD, Tiwari MS, Yadav GD (2019) Mol Catal 465:1–15 25. Leo IM, Granados ML, Fierro JLG, Mariscal R (2014) Chin J

Catal 35:614–621

26. Eblagon KM, Tsang SCE (2014) Appl Catal B 160–161:22–34

27. Chakroune N, Viau G, Ammar S, Poul L, Veautier D, Chehimi MM, Mangeney C, Villain F, Fievet F (2005) Langmuir 21:6788–6796 28. Wang Y, Hou B, Li D, Chen J, Sun Y (2012) Reac Kinet Mech

Cat 106:217–224

29. Huang J, Qian W, Ma H, Zhang H, Ying W (2017) RSC Adv 7:33441–33449

30. Li S, Peng S, Huang L, Cui X, Al-Enizi AM, Zheng GACS (2016) Appl Mater Interfaces 8:20534–20539

31. Xie S, Liu Y, Deng J, Yang J, Zhao X, Han Z, Zhang K, Dai H (2017) J Catal 352:282–292

32. Bianchi CL, Martini F, Moggi P (2001) Catal Lett 76:65–69 33. Riva R, Miessner H, Vitali R, Del Piero G (2000) Appl Catal A

196:111–123

34. Gao P, Wang C (2015) RSC Adv 5:70661–70667

35. Sewell GS, Stean E, O’Connor CT (1996) Catal Lett 37:255–260 36. Lucredio AF, Bellido JDA, Zawadzki A, Assaf EM (2011) Fuel

90:1424–1430

37. James OO, Maity S (2016) J Pet Technol Altern Fuels 7:1–12 38. Batista MS, Santos RKS, Assaf EM, Assaf JM, Ticianelli EA

(2004) J Power Sources 134:27–32

39. Iida H, Sakamoto K, Takeuchi M, Igarashi A (2013) Appl Catal A 466:256–263

40. Okabe K, Li X, Wei M, Arakawa H (2004) Catal Today 89:431–438