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Chemoselective hydrogenation of biomass-derived 5hydroxymethylfurfural into the liquid biofuel 2,5-dimethylfuran Lei Hu, Lu Lin, and Shijie Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5013807 • Publication Date (Web): 23 May 2014 Downloaded from http://pubs.acs.org on June 2, 2014

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Industrial & Engineering Chemistry Research

Chemoselective hydrogenation of biomass-derived 5-hydroxymethylfurfural into the liquid biofuel 2,5-dimethylfuran

Lei Hu,*,† Lu Lin,‡ and Shijie Liu§



Jiangsu Key Laboratory for Biomass-based Energy and Enzyme Technology, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223300, China ‡

§

School of Energy Research, Xiamen University, Xiamen 361005, China

Department of Paper and Bioprocess Engineering, State University of New York, College of Environmental Science and Forestry, Syracuse 13210, NY, USA

*Corresponding Author: [email protected] Telephone/Fax: +86-0517-83526983

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Abstract

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In recent years, 2,5-dimethylfuran (DMF), which is produced by the selective

3

hydrogenation of biomass-derived 5-hydroxymethylfurfural (HMF) and is considered

4

as a new-fashioned liquid biofuel for transportation, has received much more attention

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from many researchers in the world. Compared to the current market-leading

6

bioethanol, DMF possesses a higher energy density, a higher boiling point, a higher

7

octane number and is immiscible with water. At present, the study on the selective

8

hydrogenation of HMF into DMF is still in its infancy, however, it has been becoming

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a very important research orientations in the field of bioenergy. In consideration of the

10

excellent physicochemical properties, the momentous application values and the

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broad market prospects of DMF, the reported catalytic systems and the latest research

12

achievements for the selective hydrogenation of HMF into DMF in the light of the

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diversity of hydrogen donors such as molecular hydrogen, formic acid, alcohols and

14

water are systematically summarized and discussed, and the reaction mechanism of

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DMF synthesis and the combustion performance and safety of DMF are also outlined

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in this critical review. Moreover, some potential research trends in the future studies

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are prospected to offer the valuable ideas and advices for the selective hydrogenation

18

of HMF and provide the theoretical references and technical supports for the practical

19

production and application of DMF.

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Keywords: Biomass; Liquid biofuel; Hydrogen donor; 5-Hydroxymethylfurfural;

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Slective hydrogenation; 2,5-Dimethylfuran

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1. INTRODUCTION

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As is known to all, the non-renewable fossil resources such as coal, oil and

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natural gas are the cornerstone of fuels, chemicals and materials industries in the

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world today, which make great and tremendous contributions for the development and

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prosperity of human society.1-3 However, with the diminishing fossil resource reserves

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and the increasing fossil resource prices along with the growing concerns about

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environmental pollution and global warming, it is very important to search the reliable

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and renewable resources that can be used to gradually replace fossil resources.4-9 In

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recent years, biomass, as a diverse, widespread, abundant and inexpensive renewable

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resource, has attracted much more attention in both scientific and industrial

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communities. Nature produces 200 billion metric tons of biomass with an energy

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content of 3×1018 KJ per year by photosynthesis, which is around 10 times the present

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and annual energy consumption of the world.10 Under the drive of the huge potential

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of biomass, many countries in the world have formulated the corresponding research

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and development plans such as America's “Energy Farm”, Brazil’s “Alcohol

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Program”, India's “Renewable Energy Scenario” and Japan's “Sunshine Project”.11

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More importantly, the transformation of biomass into fuels, chemicals and materials

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has a great significance to decrease the excessive dependence on fossil resources,

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alleviate the energy crisis, reduce the environmental pollution and promote the

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sustainable development of the whole human society.

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Lignocellulose, which is the most abundant biomass resource on the earth, is

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mainly composed of three components, cellulose (40~50%), hemicellulose (25~35%) 3

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and lignin (15~20%).12-14 Recently, a process involving the raw material pretreatment

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of lignocelluloses, the selective separation of three components and the oriented

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transformation of each component is perceived as one of the most effective pathways

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for the utilization of biomass resource. Among the various desired compounds via the

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oriented transformation,15-19 5-hydroxymethylfurfural (HMF) is considered as a

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versatile platform compound (Scheme 1) and a crucial intermediate for connecting

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biomass resource and fossil industry,20-24 and this is because that it can be further

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transformed into a series of high-quality fuels such as ethyl levulinate (EL),25-27

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5-ethoxymethylfurfural (EMF),28-30 2,5-dimethylfuran (DMF)31-33 and C9~C15

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alkanes34-36

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2,5-dihydroxymethylfuran

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2,5-furandicarboxylic acid (FDCA).46-48 Among the above-mentioned derivatives,

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DMF, which is produced by the selective hydrogenation of HMF, is particularly

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attractive. Compared to the current market-leading bioethanol, DMF possesses a

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higher energy density (31.5 MJ/L),49 a higher octane number (119),50 a lower

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volatility (bp 92~94 °C)32 and a lower separation energy consumption,51 and it is

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immiscible with water,52 which is more similar to gasoline (Table 1), these excellent

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performances make DMF a more appropriate, ideal and promising biomass-derived

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liquid biofuel for transportation53 as well as a renewable source for the production of

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p-xylene via the Diels-Alder reaction with ethylene.54-57

and

high-value

chemicals

(DHMF),40-42

such

as

levulinic

2,5-diformylfuran

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>

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acid

(LA),37-39

(DFF)43-45

and

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Currently, the research on the selective hydrogenation of HMF into DMF is still

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at a preliminary stage, however, it has been becoming a hot issue in the field of

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bioenergy. In the light of the excellent physicochemical properties, the momentous

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application values and the broad market prospects of DMF, we systematically

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summarize the various catalytic systems and the latest research progresses for the

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selective hydrogenation of HMF into DMF from the perspective of hydrogen donors

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such as molecular hydrogen, formic acid, alcohols and water in this critical review.

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And then, we discuss the reaction mechanism of DMF synthesis and the combustion

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performance and safety of DMF. Moreover, we also point out some potential research

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orientations on the basis of the main problems encountered in recent researches.

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2. SELECTIVE HYDROGENATION OF HMF INTO DMF

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HMF, containing an aldehyde group, a hydroxyl group and a furan ring, is very

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reactive, and its hydrogenation products are very complicated.58 Therefore, how to

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ensure the hydrogenation priorities of aldehyde group and hydroxyl group and avoid

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the further hydrogenation of furan ring are the principal issues in the selective

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hydrogenation of HMF into DMF. However, to solve these issues, it is crucial to

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choose a appropriate catalytic system. According to the recent research results, the

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various catalytic systems and the state-of-the-art research progresses for the selective

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hydrogenation of HMF into DMF in terms of diverse hydrogen donors are firstly

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summarized in the following section (Table 2).

86 87

2.1. Molecular hydrogen as hydrogen donor. In 2007, Román-Leshkov et al.51 5

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launched a pioneering study on the selective hydrogenation of HMF over the catalyst

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of copper chromite (CuCrO4) using molecular hydrogen (H2) as hydrogen donor in

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1-butanol, the conversion of HMF with 100% and the yield of DMF with 61% were

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obtained at 220 °C for 10 h. Disappointingly, CuCrO4 was easily deactivated by

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chloride ions at a low level or even a p.p.m. level in the reaction solvent. To alleviate

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the poisoning of the copper catalyst, the authors developed a chloride-resistant

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carbon-supported copper-ruthenium (CuRu/C) catalyst, which led to 100% HMF

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conversion and 71% DMF yield. More notably, when the reaction was performed in

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1-butanol containing 1.6 mmol/L chloride ions, CuRu/C also gave 100% HMF

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conversion and 61% DMF yield (Scheme 2). Although CuRu/C was affected to some

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extent by the presence of chloride ions, its performance was markedly superior to that

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of CuCrO4. In 2009, the above catalytic system consisting of CuRu/C, H2 and

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1-butanol was adopted by Binder and Raines to explore the selective hydrogenation of

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the crude HMF from corn stover, the yield of DMF with 49% was observed at 220 °C

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for 10 h,1 which further demonstrated a wide applicability of CuRu/C in the selective

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hydrogenation of HMF into DMF. Subsequently, in the presence of 1-butanol and H2,

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the catalyst of hollow carbon sphere-supported platinum-cobalt (PtCo@HCS)

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designed by Wang et al. was used for the selective hydrogenation of HMF, resulting

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in 98% DMF yield with 100% HMF conversion at 180 °C for 2 h.59 Furthermore,

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when PtCo@HCS was reused in the second cycle, the conversion of HMF and the

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yield of DMF were still 100% and 98%, respectively. For comparison, activated

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carbon-supported platinum (Pt/AC) and graphitized carbon-supported platinum 6

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(Pt/GC) catalysts were also employed under the same reaction conditions.

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Unfortunately, only 9% and 56% DMF yields were obtained, respectively. However,

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when the two catalysts were modified with cobalt to form PtCo/AC and PtCo/GC, the

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conversion of HMF and the yield of DMF were increased to 100% and 98%,

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respectively,59 proving that the alloy is crucial for the selective hydrogenation of

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HMF into DMF.

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>

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More recently, Hu et al.,32 Huang et al.,60 Nishimura et al.61 and Zu et al.62

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reported the selective hydrogenation of HMF over Ru/C, active carbon-supported

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nickel-tungsten carbide (Ni-W2C/AC), carbon-supported palladium-gold (PdAu/C)

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and cobalt oxide-supported ruthenium (Ru/Co3O4) in the presence of tetrahydrofuran

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(THF) and H2, excellent DMF yields as high as 94.7%, 96%, 96% and 93.4% with

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100% HMF conversion were reached at 200 °C for 2 h, 180 °C for 3 h, 60 °C for 6 h

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and 130 °C for 24 h, respectively. In addition, these catalysts exhibited good

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recyclabilities, when they were reused several times (at least 3 times), almost no

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decrease in the stabilities and activities were found. More satisfactorily, Chatterjee et

126

al.63 proposed a novel catalytic strategy for the selective hydrogenation of HMF by

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using supercritical carbon dioxide (CO2) and water (H2O) as reaction medium, which

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could easily produce various main compounds by tuning the CO2 pressure and H2

129

pressure (Scheme 3). When 10 MPa CO2 and 1 MPa H2 were used, a marvellous yield

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of DMF with 100% was achieved using Pd/C as catalyst at a lower reaction

131

temperature of 80 °C for 2 h.63 Furthermore, it is interesting to note that the selectivity 7

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of DMF was found to largely depend on the mole ratio of CO2 to H2O, and an

133

excessive CO2 or H2O would reduce the selectivity of DMF. Hence, an optimum mole

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ratio of CO2 to H2O = 1:0.32 was mandatory for the synthesis of targeted DMF with

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high selectivity.63 In addition, it should be pointed out that the combination of CO2

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and H2O is an example of green and sustainable reaction medium for the selective

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hydrogenation of HMF into DMF. However, further studies such as NMR analysis of

138

reaction process and pH analysis of reaction medium are necessary to understand the

139

detailed reaction pathway for the selective hydrogenation of HMF into DMF and the

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real reason for the outstanding performance in the presence of CO2 and H2O.

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Furthermore, the selective hydrogenation of HMF was also investigated in ionic

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liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl), only 15% DMF yield with

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47% HMF conversion was gained under the pressure of H2 using carbon-supported

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palladium (Pd/C) as catalyst at 120 °C for 1 h.64 Compared to the catalytic system in

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1-butanol, THF and supercritical CO2 and H2O, the lower DMF yield in [EMIM]Cl

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was thought to be attributed to the lower reaction temperature and reaction time as

147

well as the lower solubility of H2 in ionic liquid.64 In addition, the authors also found

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that the source of HMF had very little effect on the selective hydrogenation of HMF

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or the distribution of products.64

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>

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Although HMF can be readily hydrogenated into DMF, the above-mentioned

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catalytic systems possess the same problem, which is that H2 is used as hydrogen

153

donor. As is well-known, H2 is mainly derived from the non-renewable fossil 8

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resources, its production cost is very high. Moreover, H2 is readily dispersed and

155

ignited in air, its storage and transportation are very inconvenient. Additionally, H2 is

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difficultly dissolved in various solvents especially in ionic liquid, its atom utilization

157

is very low. Thus, from the point of view of the practical production, it is uneconomic

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and unsafe for the selective hydrogenation of HMF into DMF using H2 as hydrogen

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donor.

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2.2. Formic acid as hydrogen donor. In 2010, Thananatthanachon and

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Rauchfuss proposed a mild catalytic system, in which formic acid (FA) was firstly

162

used as hydrogen donor for the selective hydrogenation of HMF (Scheme 4).65 When

163

the reaction was carried out in THF over the catalysts of sulfuric acid (H2SO4) and

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Pd/C, more than 95% DMF yield with 100% HMF conversion was observed at 70 °C

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for 15 h. In view of the transformation of HMF into DMF, a one-pot process for the

166

synthesis of DMF from fructose was also investigated (Scheme 5). In the presence of

167

FA, H2SO4, Pd/C and THF, fructose was initially dehydrated at 150 °C for 2 h, and

168

the generated HMF was subsequently hydrogenated at 70 °C for 15 h, which gave 51%

169

DMF yield.65 Moreover, it is worth noting that FA has three distinct functions in this

170

catalytic system. Namely, FA is an acid catalyst for the dehydration of fructose into

171

HMF and a reagent for the deoxygenation of furanylmethanols as well as a hydrogen

172

donor for the hydrogenation of HMF into DHMF.65 In 2012, under the same catalytic

173

system, De et al. studied the conversion of fructose into DMF via HMF by using the

174

heating method of microwave radiation, the yield of DMF with 32% was gained at

175

150 °C and 75 °C for 10 min and 45 min, respectively.50 In addition, it is worth noting 9

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that although FA, which can also be formed from HMF in its hydration reaction to LA,

177

is a renewable and potential hydrogen donor,66-68 it has a strong acidity and

178

corrosivity. Furthermore, when FA is used for the selective hydrogenation of HMF, a

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stronger acidic and corrosive H2SO4 is necessary to get the high yield of DMF. Hence,

180

using FA as hydrogen donor for the practical production of DMF, a series of special

181

corrosion-resistant equipments are needed, and then the corresponding costs will be

182

increased, which restrain a wide applications of FA to a large extent.

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>

184

>

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2.3. Alcohols as hydrogen donor. In 2012, a new approach was reported by

186

Hansen et al. for the selective hydrogenation of HMF via the method of catalytic

187

transfer hydrogenation,69 in which methanol was used as hydrogen donor and reaction

188

medium. When the reaction was performed over Cu-doped porous metal oxide

189

(Cu-PMO) catalyst, 34% DMF yield with 100% HMF conversion was obtained at

190

300 °C for 0.75 h.69 It should be noted that compared to H2 and FA, the production

191

cost can be reduced and the operation security can be improved to a certain extent

192

when methanol is used as hydrogen donor. However, in the reaction process, the

193

critical temperature of methanol is very high (as high as 300 °C) and the selectivity of

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DMF is very low (only 34%). Therefore, to overcome the shortcomings of methanol,

195

isopropanol, another hydrogen donor as well as reaction medium,70-72 was

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alternatively introduced into this new approach by Jae et al.33 When the reaction was

197

conducted over the catalyst of Ru/C, the conversion of HMF with 100% and the yield 10

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of DMF with 81% were achieved at 190 °C for 6 h. Unfortunately, when the

199

recovered Ru/C was reused in the second cycle, HMF conversion and DMF yield

200

were significantly decreased to 47% and 13%, respectively, showing a considerable

201

deactivation of Ru/C even after its first use, which might be due to the formation of

202

high molecular weight byproducts on ruthenium surfaces.33 More recently, a more

203

stable catalyst of ferric oxide-supported palladium (Pd/Fe2O3) was prepared and used

204

by Scholz et al.,73 the conversion of HMF with 100% and the yield of DMF with 72%

205

were achieved in a continuous-flow reactor at 180 °C. More excitingly, under the

206

reaction conditions, when Pd/Fe2O3 was continuously used for 47 h, it also exhibited

207

an initial activity.73 In addition, it is observed that isopropanol is better than methanol

208

in the selective hydrogenation of HMF into DMF via the method of catalytic transfer

209

hydrogenation, which can not only decrease the reaction temperature by more than

210

100 °C, but also improve the the selectivity of DMF by more than 38%. However,

211

isopropanol also has several deficiencies, for instance, the hydrogen transfer reaction

212

of isopropanol is reversible and the high-pressure nitrogen is needed in the reaction

213

process.

214

2.4. Water as hydrogen donor. From the above descriptions, whether H2, FA,

215

methanol or isopropanol is used as hydrogen donor for the selective hydrogenation of

216

HMF into DMF, all the reaction temperatures are more than 60 °C even as high as

217

300 °C. More gratifyingly, Nilges and Schröder presented a room-temperature and

218

atmospheric-pressure electrocatalytic hydrogenation technique for the selective

219

hydrogenation of HMF into DMF.74 As illustrated in Scheme 6, the reaction process 11

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can be understood as a series of the consecutive 2-electron/2-proton reduction steps,

221

which require a total of six electrons and six protons. Moreover, the results

222

demonstrated that copper electrode materials were much more effective than other

223

electrode materials such as nickel, platinum, carbon, iron, lead and aluminium, and

224

the addition of acetonitrile or ethanol on the basis of sulfuric electrolyte solution not

225

only suppressed the formation of H2 but also improved the yield of DMF and the

226

coulombic efficiency of the electrocatalytic hydrogenation.74 Therefore, when a

227

combination of copper electrodes and 0.5 M H2SO4 in a 1:1 mixture of water and

228

ethanol was used, the selectivities of DMF, DHMF and MFA were 35.6%, 33.8% and

229

11.1%, respectively. In addition, the authors also stated that DHMF and MFA are the

230

intermediates in the selective hydrogenation of HMF into DMF, when the reaction

231

time is extended, DHMF and MFA will be further transformed into DMF.74 Finally, it

232

should be pointed out that this is the first time for the direct electrochemical

233

hydrogenation of HMF into DMF, and such a electrochemical hydrogenation process

234

provides a path to convert electric energy from other renewable resources such as

235

wind power or photovoltaics into liquid biofuels as well as provides a path to replace

236

the use of H2 in the production of biochemicals. >

237 238

3.

REACTION

MECHANISM

FOR

239

HYDROGENATION OF HMF INTO DMF

THE

SELECTIVE

240

Up to now, the researchers have successfully developed a variety of the catalytic

241

systems for the selective hydrogenation of HMF into DMF. However, whichever the 12

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catalytic system is used, three consecutive steps are compulsive (Scheme 7): (I)

243

C2-aldehyde group and C5-hydroxyl group in HMF are firstly hydrogenated into

244

DHMF and 5-methylfurfural (MF), respectively; (II) DHMF and MF are subsequently

245

hydrogenated into the same intermediate MFA; (III) MFA is further hydrogenated

246

into the targeted product DMF. Furthermore, it should be pointed out that from the

247

thermodynamic point of view, the bond energy of C=O and the bond energy of C=C

248

are 715 KJ·mol−1 and 615 KJ·mol−1, respectively, indicating that the hydrogenation of

249

C=C bond is more easier than C=O bond.75 However, the presence of conjugated

250

furan ring make the hydrogenation of C=O bond relatively easier than C=C bond in

251

the selective hydrogenation of HMF into DMF.58 As previously stated, HMF is very

252

reactive, and it will generate the varied hydrogenation products. That is, in addition to

253

the targeted product DMF, many other byproducts such as furfuryl alcohol (FA),

254

2,5-dihydroxymethyltetrahydrofuran (DHMTHF), 5-methyltetrahydrofurfuryl alcohol

255

(MTHFA), 2,5-dimethyltetrahydrofuran (DMTHF), 2,5-hexanedione (HDN) and

256

2-hexanol (HAO) may also be detected in the complicated hydrogenation products of

257

HMF (Scheme 7). It is important to note that in the selective hydrogenation of HMF

258

into DMF, although there are many possible byproducts, these byproducts are hard to

259

form under the appropriate reaction conditions. Moreover, in the light of numerous

260

reported research results,1, 32, 33, 50, 51, 59-65, 69, 73, 74, 76-84 a integrated reaction pathway for

261

the transformation of a variety of biomass-derived carbohydrates into DMF via HMF

262

is presented in Scheme 8 to provide the theoretical references and technical supports

263

for the practical production of DMF in the near future. 13

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>

265

>

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4. COMBUSTION PERFORMANCE AND SAFETY OF DMF

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DMF is being considered as a green and potential transportation biofuel, and its

268

combustion performance and safety should be tested prior to the practical application.

269

In the earliest studies, Zhong et al.85 and Tian et al.86 showed that various combustion

270

properties such as spray characteristic, fuel consumption rate, thermal efficiency,

271

laminar burning velocity and exhaust emission were proved to be similar to gasoline

272

in a direct-injection engine, and no apparent adverse effects on the engine were

273

detected in the duration of the tests. Thus, the authors concluded that no major

274

modifications or adjustments to the existing gasoline-type engines would be needed

275

when the pure DMF was used as a fuel.85 In the subsequent studies, Christensen et

276

al.87 confirmed that DMF had a good potential as a suitable candidate to blend with

277

gasoline through measuring its vapour pressure, vapour lock protection, distillation,

278

density, viscosity and octane value. In comparison to the pure gasoline, the emissions

279

of unburnt hydrocarbon and carcinogenic formaldehyde were reduced and the

280

properties of anti-wear, anti-friction, anti-knock and exhaust-gas temperature were

281

improved when the blend of gasoline and DMF was used in a spark ignition

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engine.88-92

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Furthermore, in the investigation of the toxicological and ecotoxicological

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potencies, Zeiger et al.93 found that DMF could pass the Ames test in salmonella

285

mutagenicity assays, which demonstrated that it was probably not carcinogenic. 14

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According to PubChem BioAssays, DMF was inactive in 135 out of 138 bioactivity

287

tests.53 In the US Environmental Protections Agency’s fathead minnow acute toxicity

288

test with regard to baseline narcosis (50% lethal concentration: 71 mg·L−1), DMF was

289

moderately active.53 By contrast, saturated hydrocarbons showed high to moderate

290

activity. Phuong et al.94 observed that DMF had only a moderate aquatic toxicity,

291

whereas its combustion intermediates posed a much broader range of hazards than

292

DMF itself. That is, nine of 49 intermediates were found to have a association with 26

293

tumors and systemic diseases. However, considering that the intermediates included

294

1,3-butadiene and benzene, these results are not surprising. Actually, all hydrocarbons

295

and oxygenates containing carbon-carbon bonds, whether biomass-derived or not, are

296

prone to produce these intermediates in the process of combustion under the

297

appropriate conditions.95 Therefore, the direct relevance to DMF is somewhat

298

questionable. More recently, Fromowitz et al.96 examined the toxicity of DMF to the

299

bone marrow, the results showed that DMF might induce chromosome breakage

300

(clastogenic) and be genotoxic to hematopoietic cells, and the authors urged that more

301

authoritative, thorough and detailed toxicological studies on DMF should be

302

conducted to ensure public and occupational safety before it is approved to produce in

303

mass quantities. Furthermore, it should be pointed out that although there are no direct

304

head-to-head studies, the limited available information indicated that DMF is not

305

more toxic than current fuel components.51

306

5. CONCLUSIONS AND PERSPECTIVES

307

As mentioned in Introduction, HMF is a versatile platform compound, DMF is a 15

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new-fashioned liquid biofuel, and the selective hydrogenation of biomass-derived

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HMF into DMF is a promising research orientations in the field of bioenergy.

310

Although the recent research advancements have shown some exciting results, the

311

large-scale production of DMF has not been accomplished until now. In order to

312

accelerate this process, some potential points should be addressed in the future studies:

313

(I) The exploration of clean and comprehensive coupling reaction systems. For

314

instance, the dehydrogenation reactions of a great many compounds especially

315

alcohols such as 1,4-butanediol, cyclohexanol and phenethyl alcohol have been

316

systematically and thoroughly studied by many research groups in the past few years.

317

However, the generated H2 in these dehydrogenation processes has not been used in a

318

appropriate and reasonable way. Expectingly, it is possible to complete the in-situ

319

utilization of generated H2 and the effective synthesis of DMF if the dehydrogenation

320

of compounds and the selective hydrogenation of HMF are coupled in the same

321

reactor. (II) The preparation of multifunctional catalysts. According to the

322

above-mentioned coupling reactions and transfer hydrogenation reactions, preparing a

323

recoverable multifunctional catalyst especially containing the magnetic material,

324

which can not only be used for the dehydrogenation and hydrogen transfer of

325

compounds, but also can be used for the selective hydrogenation of HMF, has a great

326

realistic significance for the practical production of DMF. (III) The establishment of

327

high-efficiency and energy-efficient separation and purification technologies. In the

328

process of HMF hydrogenation, the products are very complicated, how to separate

329

DMF from the complicated products is the prerequisite for the practical application of 16

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DMF. However, a very little attention is paid to this issue. Therefore, in addition to get

331

a higher DMF yield, developing a simple and efficient method for the separation and

332

purification of DMF by using a combination of filtration, distillation, condensation,

333

extraction and decoloration on the basis of different physicochemical properties of

334

various products is a necessary step for the practical application of DMF. (IV) The

335

design of consecutive reaction equipments. Whichever hydrogenation approach is

336

adopted, a consecutive reaction equipment involving the selective hydrogenation of

337

HMF and the separation and purification of DMF should be designed in line with

338

various hydrogen donors to ensure the practical production of DMF. All in all, the

339

synthesis of DMF, whether HMF or biomass-derived carbohydrate is used as raw

340

material, should be moved in the direction of green, efficient, simple and inexpensive

341

technology, and the research on hydrogen donors, catalysts and catalytic systems

342

should also be further enhanced. With the advent of various advanced technologies

343

and with the deepening of comprehensive studies, the synthesis of DMF will make a

344

greater progress and breakthrough, and play an important role in the transportation

345

sector. Despite facing many difficulties and challenges, we still believe that the future

346

is remarkably bright.

347

ACKNOWLEDGEMENTS

348

This work was financially supported by the Scientific Research Foundation for

349

the Doctoral Scholars of Huaiyin Normal University (31HL001), the National Natural

350

Science Foundation of China (21106121) and the National Basic Research Program of

351

China (2010CB732201). 17

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burning velocities of 2,5-dimethylfuran compared with ethanol and gasoline.

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636 637

Scheme 1. HMF as a platform compound for the synthesis of various derivatives.

31

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638 639

Scheme 2. Diagram for the conversion of fructose into DMF. Diagram includes the

640

selective dehydration of fructose into HMF in a biphasic reactor (R1); the evaporation

641

of water and HCl from the liquid solvent containing HMF, leading to precipitation of

642

NaCl (E1); the selective hydrogenation of HMF into DMF over CuRu/C (R2); and the

643

separation of DMF from the extracting solvent and unreacted intermediates (S1).

644

Reprinted with permission from ref. 51. Copyright 2007 Nature Publishing Group.

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645 646

Scheme 3. Reaction pathway of HMF hydrogenation in supercritical CO2 and H2O.63

33

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647 648

Scheme 4. Pathway for the synthesis of DMF from HMF.65

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649 650

Scheme 5. One-pot process to generate DMF from fructose.65

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-

+

H +2

+2

+2

e +2

H+

H+

+2

e-

-

+2 e-

+

+2

H

+2

e

651 652

Scheme 6. Illustration of the electrocatalytic hydrogenation of HMF into DMF.74

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Scheme 7. Plausible reaction mechanism for the hydrogenation of HMF.

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OH O OH HO O

O

O HO

OH O

OH

O H OH OHOH

OH OH O

HO HO

OH H Sucrose

n

OH

OH

O

O

O HO

OH

OH OHO

Hydrolysis

Cellulose

OH O n

Starch Hydrolysis

Hydrolysis

OH OH

OH O HO

O

OH

OH

Isomerization

OH

HO HO

OH Glucose OH

Cellobiose

O

HO

O

HO HO

Hydrolysis

H

HO H

OH OH

Hydrolysis

O

HO

O

O HO OH

OH

OH

HO H Fructose

Maltose

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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HO HO

OH

O

Dehydration

OH

OH

OH

O

O

O

O

O Hydrogenation

DHMF

Hydrogenation

MF

HMF Hydrogenation

Hydrogenation

Hydrogenation

OH O MFA Hydrogenation

O Biofuels

Separation

Separation

Biofuels

DMF

655 656

Scheme 8. Integrated reaction pathway for the transformation of biomass-derived

657

carbohydrates into DMF.

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Table 1. Comparision of DMF, gasoline and bioethanol

658

Property

DMF

Gasoline

Bioethanol

Molecular formula

C6H8O

C4~C12

C 2 H6 O

Molecular mass (g/mol)

96.13

100~105

46.07

Liquid density (kg/m3, 20 °C)

889.7

744.6

790.9

Relative vapor density

3.31

3~4

1.59

Latent heat of vaporization (KJ/mol, 20 °C)

31.91

38.51

43.25

Energy density (MJ/L)

31.5

35

23

Boiling point (101 KPa)

92~94

96.3

78.4

Water solubility (25 °C)

Immiscible

Immiscible

Miscible

Research octane number

119

95.8

110

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Table 2. The selective hydrogenation of HMF into DMF in various catalytic systems

659

Entry 1 2 3a 4 5 6 7 8 9 10 11 12 13b 14b 15 16 17 18 660

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a

Hydrogen donor H2

Solvent

Catalyst

Temperature (°C)

Time (h)

1-Butanol

CuCrO4

220

10

HMF conversion (%) 100

H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 Formic acid Formic acid

1-Butanol 1-Butanol 1-Butanol 1-Butanol THF THF THF THF CO2/H2O [EMIM]Cl THF THF

CuRu/C CuRu/C Ru/C PtCo@HCS Ru/C Ni-W2C/AC PdAu/C Ru/Co3O4 Pd/C Pd/C Pd/C/H2SO4 Pd/C/H2SO4

220 220 260 180 200 180 60 130 80 120 70 150/70

10 10 1.5 2 2 3 6 24 2 1 15 2/15

Formic acid Methanol Isopropanol Isopropanol

THF Methanol Isopropanol Isopropanol

H2O

H2SO4 solution

Ru/C//H2SO4 Cu-PMO Ru/C Pd/Fe2O3 Cu electrode

150/75 300 190 180 R. T.

2/15 0.75 6 — —

Crude HMF was used as an initial material; b Fructose was used as an initial material. 40

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DMF yield (%)

Ref.

61

51

100 — 99.8 100 100 100 100 100 100 47 100 —

71 49 60.3 98 94.7 96 96 93.4 100 15 95 51

51 1 9 59 32 60 61 62 63 64 65 65

— 100 100 100 —

30 34 81 72 35.6

50 69 33 73 74