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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
1
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Abstract
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In recent years, 2,5-dimethylfuran (DMF), which is produced by the selective
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hydrogenation of biomass-derived 5-hydroxymethylfurfural (HMF) and is considered
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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
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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
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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
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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
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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
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of HMF and provide the theoretical references and technical supports for the practical
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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
64
>
65
4 ACS Paragon Plus Environment acid (LA),37-39 (DFF)43-45 and Page 5 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 66 Currently, the research on the selective hydrogenation of HMF into DMF is still 67 at a preliminary stage, however, it has been becoming a hot issue in the field of 68 bioenergy. In the light of the excellent physicochemical properties, the momentous 69 application values and the broad market prospects of DMF, we systematically 70 summarize the various catalytic systems and the latest research progresses for the 71 selective hydrogenation of HMF into DMF from the perspective of hydrogen donors 72 such as molecular hydrogen, formic acid, alcohols and water in this critical review. 73 And then, we discuss the reaction mechanism of DMF synthesis and the combustion 74 performance and safety of DMF. Moreover, we also point out some potential research 75 orientations on the basis of the main problems encountered in recent researches. 76 2. SELECTIVE HYDROGENATION OF HMF INTO DMF 77 HMF, containing an aldehyde group, a hydroxyl group and a furan ring, is very 78 reactive, and its hydrogenation products are very complicated.58 Therefore, how to 79 ensure the hydrogenation priorities of aldehyde group and hydroxyl group and avoid 80 the further hydrogenation of furan ring are the principal issues in the selective 81 hydrogenation of HMF into DMF. However, to solve these issues, it is crucial to 82 choose a appropriate catalytic system. According to the recent research results, the 83 various catalytic systems and the state-of-the-art research progresses for the selective 84 hydrogenation of HMF into DMF in terms of diverse hydrogen donors are firstly 85 summarized in the following section (Table 2). 86 87 2.1. Molecular hydrogen as hydrogen donor. In 2007, Román-Leshkov et al.51 5 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 88 launched a pioneering study on the selective hydrogenation of HMF over the catalyst 89 of copper chromite (CuCrO4) using molecular hydrogen (H2) as hydrogen donor in 90 1-butanol, the conversion of HMF with 100% and the yield of DMF with 61% were 91 obtained at 220 °C for 10 h. Disappointingly, CuCrO4 was easily deactivated by 92 chloride ions at a low level or even a p.p.m. level in the reaction solvent. To alleviate 93 the poisoning of the copper catalyst, the authors developed a chloride-resistant 94 carbon-supported copper-ruthenium (CuRu/C) catalyst, which led to 100% HMF 95 conversion and 71% DMF yield. More notably, when the reaction was performed in 96 1-butanol containing 1.6 mmol/L chloride ions, CuRu/C also gave 100% HMF 97 conversion and 61% DMF yield (Scheme 2). Although CuRu/C was affected to some 98 extent by the presence of chloride ions, its performance was markedly superior to that 99 of CuCrO4. In 2009, the above catalytic system consisting of CuRu/C, H2 and 100 1-butanol was adopted by Binder and Raines to explore the selective hydrogenation of 101 the crude HMF from corn stover, the yield of DMF with 49% was observed at 220 °C 102 for 10 h,1 which further demonstrated a wide applicability of CuRu/C in the selective 103 hydrogenation of HMF into DMF. Subsequently, in the presence of 1-butanol and H2, 104 the catalyst of hollow carbon sphere-supported platinum-cobalt (PtCo@HCS) 105 designed by Wang et al. was used for the selective hydrogenation of HMF, resulting 106 in 98% DMF yield with 100% HMF conversion at 180 °C for 2 h.59 Furthermore, 107 when PtCo@HCS was reused in the second cycle, the conversion of HMF and the 108 yield of DMF were still 100% and 98%, respectively. For comparison, activated 109 carbon-supported platinum (Pt/AC) and graphitized carbon-supported platinum 6 ACS Paragon Plus Environment Page 6 of 40 Page 7 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 110 (Pt/GC) catalysts were also employed under the same reaction conditions. 111 Unfortunately, only 9% and 56% DMF yields were obtained, respectively. However, 112 when the two catalysts were modified with cobalt to form PtCo/AC and PtCo/GC, the 113 conversion of HMF and the yield of DMF were increased to 100% and 98%, 114 respectively,59 proving that the alloy is crucial for the selective hydrogenation of 115 HMF into DMF. 116 > 117 More recently, Hu et al.,32 Huang et al.,60 Nishimura et al.61 and Zu et al.62 118 reported the selective hydrogenation of HMF over Ru/C, active carbon-supported 119 nickel-tungsten carbide (Ni-W2C/AC), carbon-supported palladium-gold (PdAu/C) 120 and cobalt oxide-supported ruthenium (Ru/Co3O4) in the presence of tetrahydrofuran 121 (THF) and H2, excellent DMF yields as high as 94.7%, 96%, 96% and 93.4% with 122 100% HMF conversion were reached at 200 °C for 2 h, 180 °C for 3 h, 60 °C for 6 h 123 and 130 °C for 24 h, respectively. In addition, these catalysts exhibited good 124 recyclabilities, when they were reused several times (at least 3 times), almost no 125 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 127 using supercritical carbon dioxide (CO2) and water (H2O) as reaction medium, which 128 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 130 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 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 132 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 134 ratio of CO2 to H2O = 1:0.32 was mandatory for the synthesis of targeted DMF with 135 high selectivity.63 In addition, it should be pointed out that the combination of CO2 136 and H2O is an example of green and sustainable reaction medium for the selective 137 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 140 real reason for the outstanding performance in the presence of CO2 and H2O. 141 Furthermore, the selective hydrogenation of HMF was also investigated in ionic 142 liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl), only 15% DMF yield with 143 47% HMF conversion was gained under the pressure of H2 using carbon-supported 144 palladium (Pd/C) as catalyst at 120 °C for 1 h.64 Compared to the catalytic system in 145 1-butanol, THF and supercritical CO2 and H2O, the lower DMF yield in [EMIM]Cl 146 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 148 that the source of HMF had very little effect on the selective hydrogenation of HMF 149 or the distribution of products.64 150 > 151 Although HMF can be readily hydrogenated into DMF, the above-mentioned 152 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 ACS Paragon Plus Environment Page 8 of 40 Page 9 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 154 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 156 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 158 and unsafe for the selective hydrogenation of HMF into DMF using H2 as hydrogen 159 donor. 160 2.2. Formic acid as hydrogen donor. In 2010, Thananatthanachon and 161 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 164 Pd/C, more than 95% DMF yield with 100% HMF conversion was observed at 70 °C 165 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 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 176 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 179 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. 183 > 184 > 185 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 194 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 196 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 ACS Paragon Plus Environment Page 10 of 40 Page 11 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 198 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 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 12 of 40 220 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 ACS Paragon Plus Environment Page 13 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 242 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 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 264 > 265 > 266 4. COMBUSTION PERFORMANCE AND SAFETY OF DMF 267 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 282 engine.88-92 283 Furthermore, in the investigation of the toxicological and ecotoxicological 284 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 ACS Paragon Plus Environment Page 14 of 40 Page 15 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 286 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 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 308 new-fashioned liquid biofuel, and the selective hydrogenation of biomass-derived 309 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 ACS Paragon Plus Environment Page 16 of 40 Page 17 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 330 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 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 352 REFERENCES 353 (1) Binder, J. B.; Raines, R. T. Simple chemical transformation of lignocellulosic 354 biomass into furans for fuels and chemicals. J. Am. Chem. Soc. 2009, 131, 355 1979-1985. 356 (2) Serrano-Ruiz, J. C.; Dumesic, J. A. Catalytic routes for the conversion of biomass 357 into liquid hydrocarbon transportation fuels. 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Results from the testing of 311 chemicals. Environ. Mol. 626 Mutagen. 1992, 19, 2-141. 627 (94) Phuong, J.; Kim, S.; Thomas, R.; Zhang, L. P. Predicted toxicity of the biofuel 628 candidate 2,5-dimethylfuran in environmental and biological systems. Environ. 629 Mol. Mutagen. 2012, 53, 478-487. 630 631 (95) Simmie, J. M. Detailed chemical kinetic models for the combustion of hydrocarbon fuels. Progr. Energy Combust. Sci. 2003, 29, 599-634. 632 (96) Fromowitz, M.; Shuga, J.; Wlassowsky, A. Y.; Ji, Z.; North, M.; Vulpe, C. D.; 633 Smith, M. T.; Zhang, L. P. Bone marrow genotoxicity of 2,5-dimethylfuran, a 634 green biofuel candidate. Environ. Mol. Mutagen. 2012, 53, 488-491. 635 30 ACS Paragon Plus Environment Page 30 of 40 Page 31 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 636 637 Scheme 1. HMF as a platform compound for the synthesis of various derivatives. 31 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 638 639 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. 32 ACS Paragon Plus Environment Page 32 of 40 Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 645 646 Scheme 3. Reaction pathway of HMF hydrogenation in supercritical CO2 and H2O.63 33 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 647 648 Scheme 4. Pathway for the synthesis of DMF from HMF.65 34 ACS Paragon Plus Environment Page 34 of 40 Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 649 650 Scheme 5. One-pot process to generate DMF from fructose.65 35 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 - + 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 36 ACS Paragon Plus Environment Page 36 of 40 Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Industrial & Engineering Chemistry Research 653 654 Scheme 7. Plausible reaction mechanism for the hydrogenation of HMF. 37 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 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 Page 38 of 40 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. 38 ACS Paragon Plus Environment Page 39 of 40 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 Industrial & Engineering Chemistry Research 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 39 ACS Paragon Plus Environment Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 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 Page 40 of 40 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 ACS Paragon Plus Environment 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