Renewable and Sustainable Energy Reviews 103 (2019) 227-247
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Renewable and Sustainable Energy Reviews
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Recent advances in catalytic conversion of biomass to 5- hydroxymethylfurfural and 2, 5-dimethylfuran
Haiyong Wanga,b,c,d, Changhui Zhua,b,c,d, Dan Lia,b,ce, Qiying Liua,b,c,f*, Jin Tana,b,e, Chenguang Wanga,b,e, Chiliu Caia,b,c, Longlong Maa,b,c,
a Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China b Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, PR China
Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China d University of Chinese Academy of Sciences, Beijing 100049, PR China
*Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, Jiangsu, PR China Dalian National Laboratory for Clean Energy, Dalian 116023, PR China
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ABSTRACT
Keywords: HMF DMF Catalyst Sugar Biomass
Reaction system
With the growing shortage of fossil energy and the increasing of concerns over global climate changes and environmental problems have driven the development of alternative energy sources. Recently, great interest has been oriented towards the development of sustainable resources, especially the utilization of lignocellulosic biomass, a renewable and the most abundant source of biomass originating from plant photosynthesis in nature. Catalytic conversion of renewable cellulosic biomass can produce a series of compounds such as 5-hydro- xymethylfurfural (HMF) and 2,5-dimethylfuran (DMF) which are important platform compounds and ideal re- newable alternative to fossil fuels. To obtain the renowned bio-based platform molecules, various catalysts and reaction systems have been used in the past decade years. To fully understand current biomass to HMF and DMF development, it is necessary to have an overview and comparison of different homogeneous and heterogeneous catalysts. The reaction systems also exhibit a remarkable impact on the yield and distribution of products with different catalysts. General trends and future research directions of using biomass for HMF, DMF production are also discussed systematically.
1. Introduction
nature. The glucose can isomerize to fructose, one kind of hexose, due
to its furan ring, its a better material to convert to HMF. HMF is a Energy, especially fossil energy, playing an important role in de-
crucial platform compound which can be convert into many chemical velopment of human society, is the basic industry of the national
products through breaking C-C bonds and losing three molecules water economy. The fast growing energy consumption and declining energy
of hexose. It is very useful as the precursor for the biofuel 2, 5-di- reserves have caused a series of problems. Particularly, greenhouse
methylfuran (DMF). DMF, with high energy density (31.5 MJ/L), is si- gases which are, to a great extent, responsible for climate change.
milar to that of gasoline (35 MJ/L) and diesel (33 MJ/L). In addition, Finding a renewable resource to replace fossil energy has been become
the high boiling point (bp 365-367 K) of DMF is less volatile than an urgent need for researchers. Biomass, with an estimated global
ethanol (bp 351 K) and the DMF is immiscible with water [2,3]. These production of around 1.1 x 1011 t per year, is a kind of carbon-con-
attributes bode well for the use of DMF as an alternative liquid fuel for taining renewable resources. It includes lignocellulose, cellulose, and
transportation. Therefore, how to make monosaccharides (such as hemicellulose. Among them, cellulose, as the most abundant non-food
glucose, fructose) and polysaccharides into DMF with high efficiency biomass on earth, is a promising renewable feedstock for production of
for improving environment problems and optimize the energy-resource fuels and chemicals. In particular, conversion from renewable raw
structure is becoming an imminent task [4,5]. biomass resources into high-quality liquid biofuels and high value-
HMF has been described as one of the 12 key platform molecules added chemicals is undoubtedly the most attractive approach [1].
derived from biomass by the US Department of Energy. It is recognized Glucose (CgH12Og) is the most widely distributed monosaccharide in
as a versatile intermediate
to link up the biomass resources and
*Corresponding authors at: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail addresses: liuqy@ms. http://giec.ac.cn(Q. Liu), mall@ms.giec.ac.cn (L. Ma).
https://doi.org/10.1016/j.rser.2018.12.010Received 24 August 2018; Received in revised form 6 December 2018; Accepted 7 December 2018 1364-0321/O 2018 Published by Elsevier Ltd.
H. Wang et al.
Renewable and Sustainable Energy Reviews 103 (2019) 227-247
DMTHF [H]
[H]
HA
DMF
Glucose
Сон
[H]
LA
GVL
гон [0] HMF
[0]
DFF
Fructose
OH
[0]
HMFA
FDCA
EMF
Fig. 1. Synthesis of various derivatives from HME.
chemical industry. As it contains an aldehyde group and a hydro-
the engine. Furthermore, it is immiscible with water and is easier to xymethyl group, HMF can be further converted into a series of high-
blend with gasoline than ethanol [24,25]. This is advantageous to quality fuels such as DMF and 5-ethoxymethyfurfural (EMF) and high-
transportation and storage of DMF. These features make DMF as a value chemicals such as levulinic acid (LA), y-valerolactone (GVL), 2,5-
better choice for liquid fuel. dimethyltetrahydrofuran (DMTHF), and 2-hexanol (HA) [6-10]. A
Currently, much more interest and attention are paid to the pro- series of furan derivatives such as 2,5-diformylfuran (DFF), 5-formyl-2-
duction of HMF, DMF and conventional derivatives. The synthetic furancarboxylic acid (FFCA), 5-hydroxymethyl-2-furoic acid (HMFA)
methods, mechanistic aspects, biphasic solvent for HMF production and furan-2,5-dicarboxylic acid (FDCA) also can be obtained by further
[26,27], physicochemical properties and commercial prospects of HMF oxidation of HMF (Fig. 1) [11-20]. HMF can be serve as an antisickling
and DMF [28], have been extensively reviewed by many researchers agents that particularly bind to intracellular sickle haemoglobin (HbS)
[8,25,29-32]. However, to the best of our knowledge, there are no without inhibition by plasma and tissue proteins or
other undesirable
special reviews on the biomass pretreatment, mechanisms, reaction consequences, it will be a key player in the bio-based renaissance
systems, catalysts and process economy of HMF and DMF. In addition, it [21,22].
is particularly necessary to point out that lots of new technologies are DMF, which is a renewable oxygen liquid fuels, and an useful
increasingly applied and many important achievements are con- platform molecule. It has excellent physicochemical properties
tinuously obtained in this research area. Hence, a comprehensive and (Table 1), which considered as a promising new generation of alter-
real-time review is also needed. native fuel with high energy density, high octane number (RON = 119)
In this review, the transformation of biomass into HMF or DMF is and lower volatility [23]. DMF has higher energy density than ethanol
cost competitive with petro-chemical technologies and for that is re- and butanol and is more efficient as fuel; its boiling point falls in be-
quired the development of new catalysts and simplify technology by tween ethanol and butanol, making DMF have proper gasification
reducing the number of reactions, purification and isolation processes. performance, which is conducive to inhibiting the air resistance at the
Herein, a comprehensive review about the catalytic conversion of bio- engine inlet and satisfying the low temperature start-up performance of
mass such as glucose, fructose and cellulose into HMF and DMF have been presented in this paper. The homogeneous and heterogeneous catalysts, and the reaction systems for the production of HMF and DMF developed in the last few years are discussed. The scale-up conversion of biomass and the process economy analysis of HMF and DMF pro- duction are also discussed in this review.
Table 1
Comparison of physicochemical properties of DMF, ethanol, butanol and ga- soline [25].
Properties
DMF
Ethanol
1-Butanol
Gasoline Molecular formula
CHg
C2Ho
CHioo Molecular mass (g/mol)
96.13
46.07
74.12
100-105 Oxygen content (%)
16.67
34.78
21.6 Hydrogen content (%)
8.32
13.02
13.49 Carbon content (%)
75.01
52.2
64.91 Stoichiometric air-fuel
I ratio
10.72
8.95
11.2
14.7 Liquid density (kg/m, 293 K)
889.7
790.9
810
744.6 Latent heat vaporization (kJ/kg from
389.1
919.6
707.9
351 298 K)
Lower heat value (MJ/kg)
33.7
26.9
33.2
42.9 Boiling point (K)
366.2
350.4
390.4
300-498 Water solubility (wt%, 293 K)
0.26
miscible
7.7
negligible Research octane number (RON)
119
110
98
90-100 Cetane number
25
10-15 Surface tension (mN/m)
25.9
22.3
24.6
20.0 m Kinematic viscosity (cSt, 293 K)
0.57
1.5
3.6
0.37-0.44 Auto-ignition temperature (K)
559
707
658
693
2. Biomass pretreatment
The availability of inexpensive feedstock holds the key to viable large-scale production of HMF and DMF. Lignocellulosic biomass, in- cluding agricultural residues, hardwoods, softwoods, and grasses, is the most abundant raw material suitable for HMF and DMF production. However, due to the recalcitrance of lignocellulosic biomass, directly conversion these materials to HMF or DMF is rather difficult. The main goals of biomass pretreatment are to reducing cellulose crystallinity, increasing cellulose porosity, and improve sugar availability. Successful pretreatment methods are necessary for reducing the cost of biomass derived chemicals. The biomass pretreatment methods mainly include physical and chemical methods.
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Renewable and Sustainable Energy Reviews 103 (2019) 227-247 2.1. Physical methods
transformation of recovered biomass of wood chips after a diluted
NaOH pretreatment. The yield of HMF obtained from untreated bio- Physical methods are simple and widely applied in biomass pre-
mass was 41% [46]. Bases pretreatment has obvious advantages, treatment. As well known, microwave radiation, ultrasound and ball
however, it also needs further treatment before used for HMF and DMF milling are common physical pretreatments for HMF production.
production. Microwave pretreatment is an energy-efficient and environmentally
benign technology that can be used to reduce the recalcitrance of
3. Catalytic conversion of biomass to HMF complex biomass structure [33-35]. After microwave treatment, the
adaptability of lignocellulosic feedstock to subsequent reaction is en-
Currently, the process to produce HMF always used acid as catalysts hanced. However, the cost of equipment investment is high, and the
and limited fructose as raw feedstock. While, using glucose as raw structure of the equipment is complex. These limit the application in
feedstock to produce HMF, the results are always unsatisfactory. As in large-scale production of HMF and DMF.
the catalytic conversion of glucose to HMF, an effective catalyst should Ultrasound can open crystalline regions of cellulose, decompose
be capable of fulfilling different functions, including sugar isomeriza- lignin molecules, and significantly improve accessibility and chemical
tion, and intermediate dehydration. The isomerization of glucose could reactivity of cellulose. Ultrasonic treatment can decompose hemi-
happen in the presence of bases or Lewis acids including metal halides cellulose, causing a decrease in fiber-to-surface area ratio, which are
and solid acid [49], and the dehydration reaction can be fulfilled over beneficial on subsequent hydrolysis and HMF production [36,37]. A
conventional acids containing Brönsted and Lewis acids. The key step in high production yields of HMF and furfural were attained for native
the catalytic con version of glucose to fructose is the isomerization re- cellulose when an ultrasonic pretreatment was used prior to a micro-
action. Because these two reactions take place on different active sites, wave treatment with stirring. The yield was significantly increased from
effective coupling of such reactions can give high yields of HMF pro- 24.5% to 53.2% [38]. However, the ultrasonic vibration energy is too
ducts in glucose conversion. Therefore, the catalysts involving high low to change the surface conformation of the raw material particle.
activity and selectivity for in situ isomerization to fructose are required. The combination of ultrasound and other treatments (acid or base)
Recent years, many types of catalysts have been investigated for the would be a good choice.
conversion of biomass to HMF such as metal salts, inorganic acids, solid Cellulose crystallinity can be physically decreased by the ball mil-
acids/alkali and ionic liquids. ling pretreatment, leading to disruption of hydrogen bonding and a
concomitant increase in the number of B-1,4-glycosidic bonds acces-
3.1. Reaction mechanisms for production of HMF sible to the catalyst involved in cellulose hydrolysis [39].
The catalytic production of HMF from different raw feedstocks have 2.2. Chemical methods
been studied from view of chemistry in the last decades [50,51]. Recent
years studies have shown that sugars (especially fructose and glucose) Chemical methods applied on biomass pretreatment plays a key
are the main reactants to produce HMF [52]. The reaction route of role. Usually, hot-compressed water (HCW, including sub-critical water
carbohydrate dehydrating to HMF is shown in Fig. 2. (SCW), super-critical water (SPW)), organic solvents, and catalysts
Fructose is active and easier to be dehydrated and converted into which could be roughly divided into acids, bases, ionic liquids are used
HMF, Antal et al. [53] proposed two possible reaction mechanisms of for biomass pretreatment [40-48].
fructose to HMF with proton acid: annular dehydration and chain de- Water is the cheapest, non-flammable, non-toxic, and clean solvent,
hydration mechanisms, as shown in Fig. 3 and Fig. 4. For annular de- which increases the economic feasibility of the process in single-phase
hydration mechanism: the intermediate (a) of the enol type was gen- method [40]. At the critical points, water exhibits different properties
erated firstly in the fructose dehydration; the intermediate (a) was from that under normal conditions. Due to these properties, especially
dehydrated another H20 to generate (b). Finally, the third H20 was the higher concentration of hydrogen and hydroxide ions [41], HCW is
removed form (b) to generate HMF. In Amarasekaras study [54], the one of the most promising alternative medium for biomass pretreat-
key intermediate (b) was identified by H and 13C NMR spectra during ment [42].
the reaction. It provides evidence for the mechanism of annular dehy- Organic treated could improve the sugar availability and increasing
dration. the HMF yield from biomass compared to untreated biomass. In
Using fructose as feedstock, the yield of HMF is up to 99%, while the Martels work, biomass was pre-treated by methanol, the yield of HMF
fructose is limited in nature, and cost is high. Glucose can be obtained was improved from less than 1% for untreated biomass to 18.0% for
by hydrolysis of lignocellulose, which are not only cheaper but has a methanol treated biomass [43].
rich reserves. The lignocellulose is the most abundant form of biomass, In acid pretreatment, inorganic acids (sulfuric, nitric, hydrochloric,
with an annual production of around 170 billion metric tons [55]. and phosphoric acids) and organic acids (formic, acetic, and propionic
While, the yield of using glucose as raw material for HMF is low and the acids) are most used. With the H protons help, the cellulose and
side reactions are serious. It is of great significance to obtain high yield hemicellulose are rapidly hydrolyzed to glucose and xylose, respec-
HMF from glucose or even lignocellulose. tively. In addition, HMF also could be obtained at high temperature in
Its a pioneering work that Zhao et al. used metal halides in these pretreatments process (see Section 3.2.2). Currently, inorganic
[EMIM]CI as catalysts to catalytic conversion of sugars, the highest acids are the most widely used in pretreatment biomass for subsequent
yield of HMF with glucose as feedstock was near 70%. They found that reactions. However, pretreatments efficiency in these acid are relatively
when glucose dissolved in [EMIM]CI solvent, it predominantly exists as slow. Furthermore, majority of acids are highly toxic and corrosive, so
a-anomer form (Fig. 5) [56]. It is generally accepted that solvation of the acid must be recovered, and the equipment should have acid cor-
sugars occurs through hydrogen bonding of chloride ions of the solvent rosion resistance [44].
with the carbohydrate hydroxyl groups [57], in spite of that this in- Bases pretreatment mainly depends on the solubility performance of
teraction is insufficient to cause mutarotation. To explain the reaction lignin in the bases solution. NaOH, KOH, Ca(OH)2, and ammonium
mechanism, 13C and H NMR were used to study the reaction system. hydroxide are suitable for the pretreatment of lignocellulose. For bases
The 13C NMR indicated that metal halides could promote the inter- pretreatment, the ester bonds connects the hemicellulose and other
conversion of the a- and ß-anomers (Fig. 5). They also studied the -OH components (such as between lignin and other hemicelluloses) can be
resonances with CuCl2 as catalyst by H NMR, the results showed that break by saponification reaction [45]. In Chi Vans report, the highest
the resonances shifted up field and were very broad, indicating ex- HMF yield (79%) for lignocellulosic
biomass was obtained from the
change through interactions with the metal. In [EMIM]CI, the
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H. Wang et al.
Renewable and Sustainable Energy Reviews 103 (2019) 227-247
Lignocellulose
Hemicellulose
*Cellulose
Lignin
hydrodeoxygenation
O
OH
HO
Cellulose
Он
hydrolysis
dehydration
тон
HOT
LOн Он
isomerization
HO,
Glucose
Fig. 2. The reaction pathway of cellulose dehydrated to HMF/DMF.
glyceraldehyde exists as a hemiacetal, representing a good mimic for
[59-61]. In this section, the catalysts effective for the i跟着化石能源的日益欠缺以及对全球天气变革和情况问题的日益存眷，鞭策了替代能源的开展。比来，人们对可持续资本的开发产生了极大的兴趣，出格是对木量纤维素生物量的操纵，那是一种可再生的、源自天然界动物光合感化的最丰硕的生物量来源。可再生纤维素生物量的催化转化能够产生一系列化合物，如5-羟甲基糠醛（HMF）和2,5-二甲基呋喃（DMF），它们是重要的平台化合物和抱负的可从头替代化石燃料。为了获得出名的生物基平台分子，在过去的十年中已经利用了各类催化剂和反响系统。为了充实领会目前生物量转化为HMF和DMF的开展，有需要对差别的均相和异相催化剂停止概述和比力。在差别的催化剂下，反响系统对产物的产量和散布也表示出显著的影响。还系统地讨论了操纵生物量消费HMF、DMF的一般趋向和将来研究标的目的。
somerization and glucopyranose. They also studied the kinetic behavior of metal halides,
dehydration reaction are focused. results showed that the coordination of CuCl2 with sugar is different
from CrCl2. Above all they suggested that CrCls anion plays an im- portant part in proton transfer, which is benefit of mutarotation of
3.2.1. Metal salts catalyst glucose in [EMIM]CI. CrClg is vital to a formal hydride transfer, leading
As a kind of homogeneous catalyst, metal salts have been widely to isomerization of glucose to fructose (Fig. 6).
used in the formation of HMF from sugar because of their obviously An alternative mechanism using deuterium labels have been tested
catalytic activity. Different experiments have been conducted to test the by Binder and co-workers. First, they used D20 as a deuteron source.
catalytic property of metal salts. The investigations of previous catalysts Next, they used H20 as a proton source to convert glucose-2-d into
were listed in Table 1. The metal salts such as CrCls, CrCl2, FeCl2, FeCl3, HMF. The H NMR results suggested that about 33% deuterium in-
CuCl2, ZnCl2 and other complex metal salt catalysts have good catalytic corporation at C-1 of HMF, confirmed by the appearance of a signal in
performance. keeping with the aldehydic deuteron in the H NMR spectrum. As a
Metal salt such as CrCl2, CrCl3, FeCl2, FeCl3, CuCI, CuCl2, VCl3, result, these results supports 1, 2-hydride shift mechanism consisting
MoCl3, PdCl2, PtCI2, PtCl4, RuCls and RhCls showed better catalytic with ketose formation (Fig. 7) [58].
performance on conversion fructose to HMF, HMF yields ranging from The researchers proposed various reaction mechanisms for different
63% to 83%. While only CrCl2 was able to directly catalyze the dehy- catalytic and reaction systems. Their studies on the mechanism of HMF
dration of glucose with the highest yield close to 70% [56]. The work synthesis from glucose show that isomerization of glucose is an im-
was pioneering and established the first step in an ultimate goal of portant step in the catalytic reaction. While, most of the intermediates
developing a system to generate HMF from complex biomass, such as that sugars isomerization and dehydration are still undetected.
cellulose and lignocellulose. Developing new on-line detection methods and testing methods to
The ligands and additives exhibited a remarkable impact on the monitor and capture intermediates during the reaction process may
distribution of products in metal salts catalytic systems. Metal salts provide more support for the study of reaction mechanism.
combined with ionic liquids, mineral acids and organic solvents can exhibit superior performances.
NHC/metal (NHC: N-heterocyclic carbene) complexes were also 3.2. Catalysts for biomass conversion to HMF
selected as catalysts for the sugar dehydration reaction. The ligands
offer a great deal of flexibility as the catalytic activity can be modified Catalyst, especially acid catalyst, plays a key role in conversion of
by varying the stereo and electronic properties of the NHCs. The ac- biomass to HMF. Many kinds of acid catalysts are used for the con-
tivity of different NHC/metal catalysts in [BMIM]CI while using fruc- version of biomass, such as metal salts, inorganic acid and solid acid.
tose and glucose as feedstock were tested. The highest HMF yield of For conversion of glucose to HMF, the mechanisms (see Section 3.1)
both fructose (HMF yield 96%) and glucose (HMF yield 81%) was suggest that the reaction included two steps: glucose isomerization and
performed when using 6/CrCl2 (Fig. 8) as catalyst. It is interesting that fructose dehydration. Hence, the catalyst should be a single one con-
with the ligand of 8, the yield of HMF is much lower, indicating that a taining multifunctional components for biomass such as glucose, cel-
sterically complex would have a relatively lower activity in binding lulose conversion to HMF. Metal salt and inorganic acid are the most
with substrates and initiating the reaction [62,63]. In Chun and his co- used catalysts, and they show good catalytic performance in ionic li-
workers study, CrCl2 and ZnCl2 have been used as catalysts in 1-me- quids and polar non-proton organic solvents. As the most studied and
thyl-3-octylimidazolium chloride ([MOIM]CI, Fig. 9) system. They most diverse catalyst, solid acid have better performance in water
found that the rate of sucrose hydrolysis was relatively much faster in H
HQ
H
OH
Fig. 3. The annular reaction pathway of fruc- QH
tose dehydrated to HMF [53]. O
H*
H* HO.
HO, -H
-H2
-H2 о
Oн
оH
ОH
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H. Wang et al.
Renewable and Sustainable Energy Reviews 103 (2019) 227-247
H2C—OH
HC—OH
HÇ=0
HÇ=0
HÇ=0 =0
-он
-он
Ç=0
=0 HO-CH
H HO-CH
HO
HO- HỌ-OH
HС—OH
-Hao
HỌ-OH
H2o
-OH -H20
-он HỌ-OH
HỌ-OH
HỌ-OH
HỌ-OH
HỌ-OH HC-OH
HC-OH
HC-OH
HC-OH
H2C-OH Fig. 4. The chain reaction mechanism of fructose dehydrated to HMF [53].
QH
HC- -CH
OH
HO
HO1
OH
QH
OH
QH
ГOн Hó
H Glucose
a-glucopyranose anomer
-Oн Гон
HO
enolization HO
H Glucose
B-glucopyranose anomer
он
он но
OH
enediol(ate)
+[EMIM]MCI,
OH
+[EMIM]MCI,
QH
OH
OH
OH
OH
Hо1
HO-
hydride shift
HO
тон
Hó
OH H
H
он ketose
M= various metals
Fig. 5. Interconversion of a-and B-glucopyran anomers in [EMIM]CI with metal halide [56].
HO
TH/H
the reactions with HCI than without it. It is suggested that HCl play an important part in conversion of
sucrose. The highest yield
Fig. 7. The 1, 2-hydride shift mechanism of aldose to HMF [58]. (82.0 + 3.9 wt%) in HMF production was reached in the HCl and CrCI
mixture solvent [52]. This can be guessed that the interplay of [MOIM]CI and metal halides could cause glucose isomerization and
fructose isomerization and the hexoses dehydration [67]. While, the fructose dehydration more rapidly to produce HMF with high yield
loss of water-sensitive metal salts are unavoidable. Hence it can be seen [64]. Yang et al. investigated the conversion of cellulose and glucose
that the reactions using metal salts are mostly carried out in ionic li- into HMF over AlCls catalyst in water-THF system. 37% HMF yield was
quids or organic solvents. Few works about metal salts catalyzing the obtained at 453 K for 30 min under microwave heating, while 61%
conversion of biomass to HMF in aqueous solutions are reported. HMF yield was achieved at 433 K for 10 min. Moreover, the addition of
Heeres and co-workers used a series of metal salts as catalysts for de- NaCl has reduced the formation of lactic acid [65]. An excellent HMF
hydration reactions of D-glucose in aqueous solutions. Their works show yield near to 92% could be achieved from glucose over Germanium (IV)
that Al (III) and Cr (II) salts gave the highest conversion of D-glucose. Chloride under mild condition. The enhanced interplay of GeCl4 and
Using A13+ as catalyst the main product was lactic acid, while for Cr2+ glucose at elevated reaction temperatures urge ring-opening of inter-
the main products were LA and HMF, and while for Zn2+ the main mediates to straight-chain intermediate, forming fructose subsequently
products were HMF and fructose. The results suggested that in aqueous (Entry 5, Table 2) [66].
solution, Zn2+ has a catalytic effect on glucose isomerization to fructose According to the hydrolysis constant of metal salts in aqueous so-
[60]. lution, they are classified into "water-compatible" and “water-sensi-
Strong Lewis acid resulted from metal cations combining with tive". The water-sensitive metal salts could improve both the glucose/
strong Brönsted acid can efficiently influence isomerization of glucose and dehydration of fructose. The controlled surface area and pore size
оH
QH
HO-
m
HO
то
Гон
Hó
он
Oн
Im
он
HO
-[EMIM]CrCl,
HO
Гон
OH
OH
Fig. 6. CrClz leads to the isomerization of glucopyranose to fructofuranose, followed by dehydration to HMF [56].
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H. Wang et al.
Renewable and Sustainable Energy Reviews 103 (2019) 227-247
Fig. 8. Structures of N-heterocydic carbene (NHC) ligands used [63].
(CHzh-CHy
Generally, Brönsted acids include acidic ion-exchange resins such as CCI
Amberlyst-15, heteropoly acids and
their derivatives such as Fig. 9. 1-methyl-3-octylimidazolium chloride ([MOIM]CI).
Ag3PW12040. The mixed Brönsted and Lewis acidic catalysts include zeolites such as Sn-Beta. A series of experiments have been im-
plemented to explore the efficiency and stability of solid acid and base are beneficial for improving the selectivity of HMF [27,68,69]. Al-
catalysts. In Oharas study, HMF from glucose and fructose was carried though much higher HMF yield can be achieved via using metal salts as
out in the presence of Amberlyst-15 in N, N-dimethylformamide sol- catalysts, however, the separation of HMF from products is rather dif-
vent. Amberlyst-15 functioned as solid acid catalyst can influence an- ficult. Moreover, the subsequent processing is complex due to toxicity
hydroglucose with high selectivity (58% HMF selectivity). Meanwhile, of metal salts which can cause environmental problems.
glucose is isomerized to fructose in presence of hydrotalcite which
acted as solid base. With the coexistence of solid acid and base, 58% 3.2.2. Inorganic acid
HMF selectivity and 73% glucose conversion achieved at 373K (Entry Inorganic acids mainly include phosphoric acid, hydrochloric acid,
30, Table 2) [73]. Amberlyst-15 powder in a size of 0.15-0.053 mm was sulfuric acid and other common liquid acids. Due to their distinctive
used catalyst, and showed 100% HMF yield (Entry 11, Table 2) at high characteristics, its easy to be dissolved in water and organic solvents,
fructose concentration (50 wt% in DMSO) [74]. which can effect HMF yield companied with the changing of feedstocks
Qi et al. have explored the production of HMF from glucose and concentration, type and reaction temperature and time [70].
fructose under microwave irradiation by using TiO2 and ZrO2 as cata- As a strong Brönsted acid, H2SO4 played a significantly role in
lysts. 30.5% HMF yield and 65% fructose conversion were obtained catalyzing process. Using HzSO4 as catalyst to catalytic conversion of
when using Zro2 as catalyst. 38.1% HMF yield and 83.6% fructose fructose into HMF, 53% HMF yield was obtained in 32s at 523K in
conversion were obtained when using TiO2 as catalyst (Entry 12, water, while the result showed that both selectivity of HMF and de-
Table 2). Furthermore, ZrO2 was more effective to promote the iso- gradation have declined without H2SO4 (Entry 8, Table 2). Dumesic
merization of glucose to fructose in which 50% glucose conversion and et al. have implemented a series of experiments testing the catalytic
more than 60% fructose selectivity in reaction [75]. It has also been efficiency of HCI. They investigated the dehydration of fructose through
studied that the coexistence of Brönsted and Lewis acidic sites could HCI in water-DMSO system, with the addition of poly (1-vinyl-2-pyr-
obtain higher yield of HMF than the catalysts only contained Brönsted rolidinone) (PVP) aimed to enhance the selectivity of HMF. More than
acidic or Lewis acidic sites. While the highest HMF yield of 72.8% and 80% HMF selectivity was gained at 453 K for 2.5-3 min. Besides it, they
selectivity of 93.6% were obtained when choosing fructose as feedstock also found that the trends in modifier impact was similar to HCI when
over sulfated zirconia catalyst which contained Brönsted acidic or Lewis using resin catalyst (Entry 1, Table 2) [71]. Although HCl and other
acidic sites in DMSO (Entry 14, Table 2) [76]. The functionalization inorganic salts have been widely used, it could reduce the selectivity of
with acid groups in the zeolite, SiO2, Al2O3 which widely used as a HMF through rehydrating to format LA in presence of H* and it easily
support and exhibits good catalytic properties. The functionalization corrode equipment with strong acidity [72].
can increases the acidity in these solid surface, and most of these acid Though inorganic acids have many advantages such as cheaper cost,
sites of Brönsted are found in a more available way, improving the easy to get and suitable for large-scale industrial production, they still
selectivity to HMF in fructose dehydration (Entry 4, 14, 17, 19, 20, also have many drawbacks just like the equipment corrosion and en-
Table 2). vironmental pollution.
Zeolites have already shown potential in various Brönsted acidic reactions in condensed phase [77]. Ordomsky et al. studied the dehy-
dration of glucose in the presence of HZSM-5 zeolites at 383K in a 3.2.3. Solid acid
[Bmim]Cl solvent, 45% HMF
F yield was obtained [78]. Dealuminated Compared with homogeneous catalysts, heterogeneous catalysts
Beta zeolites, which had Lewis and Brönsted acid sites, are effective. have shown
superior behaviors in conversion of biomass to HMF, such
catalysts in transformation of glucose to HMF. Isomerization of glucose as easy to separate with products, and environment-friendly. Many
types of heterogeneous catalysts, consisted of both the Brönsted acid
to fructose was promoted by Lewis acid sites through intramolecular
hydride transfer and then fructose was dehydrated to HMF over and Lewis acids sites, have been developed for HMF production.
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H. Wang et al.
Renewable and Sustainable Energy Reviews 103 (2019) 227-247
Table 2
Experimental results under which previous catalysts investigations. Entry
Feedstock
Medium
Catalyst
Condition
Selectivity(mo1%)
Yield (mol%)
Ref fructose
(HO-DMSO): PVP, MIBK - 2-butanol
HCI
453 K, 3 min
85
75.7
[71] fructose
THF
HCI
433 K, 50 min
89
[88] fructose
THF-[BMIM]CI
WCI
323 K, 4h HO-2-butanol
(89] fructose
TA-P
433 K, 100 min
95.7
90
[90] fructose
[BMIM]CI
Gecl
373 K, 5 min
92.1
92.1 MeTHF/H
(661 fructose
Glu-TsOH-Ti
453 K, 10 min
59.6
59
[91] fructose
HO-MIBK
Ag
393 K, 1h
93.8
77.7
[85] fructose
SO
523 K, 32 s
53
[54] fructose
DMA-LİCI-[EMIM]CI
Cuc
393 K, 1.5h
93
83
[92] 10
fructose
DMSO
CsH PW 2.50512 40
373 K, 2h
91
91
[74] 11
fructose
DMSO
Amberlyst-15-P
393 K, 2h
100
100
[74] 12
fructose
H20
Tio
473 K, 5 min
45.7
38.1
[75] 13
fructose
H2O-MIBK
40
388 K, 1h
94.7
74
[84] 14
fructose
cetone-DMSO
Sulfated Zirconia
453 K, 20 min
77.8
72.8
[76] 15
fructose
HO-butanol
formic acid, NaCI
443 K, 70 min
70.4
69.2
[93] 16
fructose
[BMIM]CI
6/CrCI
373 K, 6h
96
[63] 17
fructose
water/1,4-dioxane
SPAN-11
413 K, 3h
71.7
7V
[94] 18
fructose
Choline chloride
Ha
373 K, 4h
91.2
90.3
[95] 19
fructose
DMSO
Aquivion@silica
363 K, 2h
85
85
[96] 20
fructose
H20-MIBK - 2-butanol
functionalized alumina
453 K, 30 min
55
39.6
[97] 21
fructose
Water-DMSO
Beta(OF)-Cal500
453 K, 30 min
83
76
[98] 22
fructose
[bmim]Br
KL-80C-1 ha
393 K, 30 min
99.1
99.1
[99] 23
glucose
[EMIM]C
CCI
373 K, 3h
78
83
[56] 24
glucose
DMA-LiBr
ÇrBr
373 K, 6h
80
[92] 25
glucose
DMSO
S02O
403 K, 6h
47.9
47.9
[100] 26
glucose
H20
2
473 K, 5 min
17.6
10.0
[75] 27
glucose
[BMIM]CI
/Crc
373 K, 6h
৪1
[63] 28
glucose
THF-water
Sn-Beta
453 K, 70 min
72
56.9
[101] 29
glucose
Water-MIBK
A
403 K, 4 h
85.2
76.3
[85] 30
glucose
N, N-dimethylformamide
Amberlyst- 15, hydrotalcite
373 K, 3h
58
42
[73] 31
glucose
HO-THF
AIC.H2O
433 K, 30 min
66.3
65
[65] 32
glucose
[BMIM]C
Gec
373 K, 5 min
92
92
[66] 33
glucose
(EMIM]BF
Snc
373 K, 3h
60
60
[102] 34
glucose
[BMIM]C
YboT)
413 K, 6h
37
24
[103] 35
glucose
H20
Nbw
393 K, 2h
52.1
18.8
[104] 36
glucose
DMSO-HO
UIO-66
433 K, 30 min
37
(105] 37
cellobiose
HO-THE
AICIH2O
453 K, 30 min
58
[65] 38
cellulose
DMA-LİCI-[EMIMICI
CrCl2, HCI
413 K, 2h
5A
[92) 39
cellulose
HO-THF
AIcI
453 K, 30 min
37
[65] 40
cellulose
HO-THF
NaHSO, ZnSO
433 K, 1h
55.2
53
[106] 41
cellulose
GVL
H2SO4
483 K, 30 min
11.9
[107] 42
cellulose
DMSO: water and MIBK: butanol
SGQD
443 K, 2h
28.1
22.2
[108] 43
corncob
GVL
SPTPA
448 K, 30 min
32.3
[109] PVP: ; TA-p: tantalum hydroxide treated with 1 M phosphoric acid and calcined at 573 K; Amberlyst-15-P: Amberlyst-15 powder in a size of 0.15-0.053 mm; DMSO: dimethylsulfoxide; MIBK: methyl isobutyl ketone; MeTHF: methyltetrahydrofura; SPAN-11: sulfonated polyaniline; GVL: gamma-valerolactone; SPTPA: porous polytriphenylamine-SOgH.
": KL zeolite treated with 1 M NHANO3 solution at 353 K for 1 h.
Brönsted acid sites. In Otomo work, they performed calcination at high
structure [82]. CS2.5Ho.sPW is considered to be an environmentally temperature or steam treatment on Beta zeolite to form Al species out of
friendly and water-tolerant catalyst [83]. 74% HMF yield and 94.7% the framework by cleavage of Si-O-Al bonds in the framework. The
selectivity were obtained when used Cs2.5Ho.5PW1204o to catalyze treated Beta zeolite showed 55% selectivity to HMF at 78% conversion
fructose (Entry 13, Table 2) [84]. Fan etal. have tested the performance of glucose [79]. It has been demonstrated that catalytic systems that
of Ag3PW12O4o in HMF production with glucose and fructose as feed- contain both Lewis and Brönsted acidity are more beneficial for HMF
stocks. The acid strength of H3PW12040 could be modified by ex- production than Lewis or Brönsted acidic catalysts alone. Swift et al.
changing H* with Ag*. 77.7% HMF yield was achieved with fructose conducted a combined experimental and computational study to reveal
as feedstock over AgsPW12040 in H2O-MIBK (Entry 7, Table 2). The the kinetics of tandem glucose isomerization and fructose dehydration
catalyst also showed excellent performance using glucose as substrate, to HMF over a bifunctional zeolite H-BEA-25 in water. According to
the HMF yield was 76.3% [85]. Different catalytic performance under their work, when the ratio of Lewis to Brönsted acid sites of 0.3, HMF
the same condition were also compared. Compared with Ag3PW12040, degradation reactions were suppressed, the maximize HMF rate pro-
both HCI and H3PW12O40 give high conversion and low selectivity of duced from glucose and the highest HMF yield (60%) would be
hexose. This is due to their strong acidity,
which could induce HMF achieved at 403 K. These predictions provide a framework for under-
rehydrating to by-products such as LA and formic acid (FA) [86]. standing and improving tandem reactions catalyzed by heterogeneous
In conclusion, metal salt and inorganic acids are cheap, easy to get catalysts [80]. While the hydrothermal instability of zeolite is a dis-
and suitable for large-scale industrial production, but the using of them advantage for their application in HMF production, for the deal-
usually have more problems such as corrosion equipment, and en- umination and collapse of framework under water vapor.
vironmental pollution caused by separation of metal salts. Solid acids Heteropoly acids are believed to have higher activity in both
with tolerating high temperature and adaptation of surface acidity are homogeneous and heterogeneous reactions [81]. They can change the
beneficial to improve the selectivity of products. In addition, they can ability to receive or release electrons by changing their chemical
be easy to separate from reactant and be reused, which is conducive to
დ 2
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H. Wang et al
Renewable and Sustainable Energy Reviews 103 (2019) 227-247
their application to industry [32,87]. However, it is worth noting that they are easily to produce some byproducts such as humins at higher concentrations. Developing low-cost solid catalystswith balanced Lewis and Brönsted acidity are more promising for HMF production.
3.3. Reaction system for biomass conversion to HMF
Since 19th century, HMF has gotten more and more attention. In the
3.3.2. Ionic liquids (ILs) early 1964, Moye has reviewed the synthetic methods and the physi-
ILs have been widely employed in the dehydration of sugar to HMF cochemical properties of HMF [110]. After almost 60 years research,
(Fig. 10 (b)), as it has negligible vapor pressure, high thermodynamic the reaction systems of producing HMF can divide into several parts
stability, and closes to infinite structural variation [114]. Moreover, the including biphasic system like water-organic system, such as water-
butanol, water-methylisobutyl ketone (MIBK), acetone-dimethyl sulf-
superior solubilizing ability of them made them dissolve almost all of
polymers [115,116]. Imidazole ILs, such as [EMIM]CI, [EMIM]BF4, oxide (DMSO), water-n-butyl alcohol -THF, and monophasic system like
[AMIM]CI (Fig. 11), is a kind of commonly used IL medium. For ex- ionic liquids (ILs), etc.
ample, the aqueous phase is unnecessary in the system when using
[AMIM]Cl as solvent as the presence of aqueous phase will reduce the 3.3.1. Biphasic system
dehydration during the catalytic reaction [56]. Zhao et al. investigated Water, widely distributed in nature, that always used as solvent
the HMF yields from fructose and glucose during the temperature without pollution to environment [111]. However, the production of
353-393 K in [EMIM]CI system over metal halides in 2007. Chromium HMF is always low for accompanying with by-products such as FA,
(II) chloride was found to be most effective, resulting in the conversion humins and LA. In order to prevent the decomposition of HMF, organic
of glucose to HMF with yield near to 70%. They also studied the effi- solvents are often used as extracting agents. In biphasic system (Fig. 10
ciency of different metal halides for dehydration fructose to HMF. The (a)), HMF could be extracted quickly from the organic system by water,
yield of HMF ranged from 63% to 83% at 353 K. The highest HMF yield improving yield by hydrophobicity system. Dumesic and co-workers
was achieved over CrCl2 (Entry 21, Table 2) [56]. Zhang et al. reported have made a great contribution to the development of biphasic system.
the influence of catalyst types, reaction temperature, and water content They systematically compared the effect of extracting agents including
on the HMF yield by using glucose as raw material in ILs system. Their alcohols and ketones with C3-Cc atoms as well as THF, and found that
work showed that the highest HMF yield of 92% was obtained with organic phase with the carbon number of 4 exhibited the highest effi-
10% GeCl4 when using [BMIM]Cl at 373 K for 5 min (Entry 5, Table 2) ciency. The highest selectivity was obtained by using THF as extracting
[66]. Hu et al. reported that a relatively high HMF yield can be gained agent [23,71,88]. Yang et al. used glucose and fructose as raw material
in [EMIM]BF4 system over SnCl4. Their new studied showed that for- to investigate the effect of catalyst content and reaction temperature on
mation of the Cs ring chelate complex of Sn atom and glucose con- HMF yield in water-butanol system. The results showed that the max-
tribute in the high HMF yield. Even the concentration of glucose more imum HMF yield (90%) was achieved at a temperature of 433 K, the
than 23 wt%, the system was still efficient with a HMF yield of 61%. mass ratio of catalyst to n-butanol was 0.1:1.2 (Entry 4, Table 2) [90].
Moreover, the combined of [EMIM]BF4 and SnCl4 was also tested in Jiang et al. tested the effect of FA on conversion of fructose into HMF in
other sugars conversion (Entry 33, Table 2) [102]. Stahlberg et al. H2O-butanol system. The results showed that the HMF yield was higher
investigated the conversion of glucose into HMF in ILs over lanthanide in a shorter time with the presence of FA, 69.2% HMF yield was gained
catalysts. The yield of HMF in [BMIM]CI with Yb(OTf)s as catalyst was at 443 K for 70 min (Entry 15, Table 2) [93]. Moreover, with the in-
24% (Entry 34, Table 2) [103]. The lanthanide (III) ions are excellent creasing of FA amount, the fallen down of HMF yield implying the
Lewis acid catalysts giving HMF without accompanying an undesirable further FA concentration could induce the degradation of HMF product
further decomposition to LA, a process which is often observed upon [112]. Pagan-Torres et al. combined Lewis acid (e.g., AIClg) with
dehydration with conventional Brönsted acids [117,118]. Brönsted acid (HCI) to catalyze glucose dehydration into HMF in H2O-
Although higher yields can be obtained in ILs under a mild condi- THF biphasic system. The pH led to the formation of different products
tion, the Synthesis of ILs is typically complex, high cost, high-energy and the tunable Lewis acidity of metal halides [113].
consumption and the dehydration products are difficult to separate. The biphasic system utilizes the characters of high hydrophilicity of
Besides, the reusability of solvent is rather difficult, which adversely sugars and the relative hydrophobicity of HMF to drive converted HMF
obstacle its large-scale utilization in industry. in the aqueous phase into the organic phase timely, avoiding further
decomposition to by-products. Such biphasic systems for HMF pre- paration are cost-saving and have been widely concerned in recent
3.3.3. Polar aprotic organic solvent years.
In addition to ILs, some polar aprotic organic solvents have been
BFA [EMIM]BFA
[AMIM]CI
Fig. 11. Ionic liquid EMIM]BF4 and [AMIM]CI.
DMF
(extracting phase)
fructoseHMF) (aqueous phase)
Ionic Liquids ([EMIM]CI/[BMIM]CI/ [EMIM]BFA)
- Polar Aprotic Organic Solvent (DMSO/DMF/DMA)
Fig. 10. (a) Biphasic system (b) ILs system and (c) Polar aprotic organic solvent system.
234
H. Wang et al.
Renewable and Sustainable Energy Reviews 103 (2019) 227-247
OH
LOH
dehydration
hydrogenolysis
OH
HOT
HO, isomerization
Hó
Hò
он
OH
HO
он
-HO
HOV
о
HO"
Humins
OH
oi
Fig. 12. Reaction pathways for producing HMF from sugar dehydration, along with routes to major side products. employed as the reaction medium for sugar dehydration to produce
ChCl has a good capture capacity of CO2, leading to the generation HMF. According to all above, HMF can be produced through sugar
carbonic acid which further catalyzed the dehydration of fructose to 5- dehydration in aqueous solution, nevertheless the yield is low due to
HMF with a yield of 72% [125]. In Lins work, they researched the side products formation, especially humins (Fig. 12). To improve HMF
conversion the fructose to HMF in DESs system using extremely low yield, many groups have investigated alternative polar aprotic organic
concentration of hydrochloric acid as the catalyst. The highest HMF and mixed solvents. Dimethyl sulfoxide (DMSO), Dimethylacetamide
yield was obtained at 373 K (Entry 18, Table 2). The system showed (DMA), Dimethylformamide (DMF) and Caprolactam (CPL) are the
excellent recyclability, which could be directly reused for multiple most frequently reported reaction solvent (Fig. 10 (c)). Coexistent H*
times without downgrading of the HMF yield. After the reaction, by can effectively inhibit the degradation of HMF and improve its yield
adding butanone to reduce the polarity of the organic solution, HMF [119]. The better efficacious systems reported are DMA-LiCI system
product with 98% purity could be obtained after further evaporation. [92] and CPL-LiCI system [120]. Binder et al. reported the DMA-LiCI
Although many groups have investigated the reaction in alternative system enable synthesis HMF with highest yield about 83% from fruc-
solvents [126-128], the fundamental role of the reaction medium has tose, 62% from glucose and 54% from cellulose in one-step (Entry 9, 22
not been understood yet. First, we still know little about the mechan- and 36, Table 2). The formation of DMA-Li* macrocations induced a
istic details of HMF degradation. Second, it is still unclear how local high concentration of weakly ion-paired chloride ions, which could
solvent-substrate interactions affect the electronic and vibrational form hydrogen bonds with the hydroxyl groups of cellulose and im-
structure of HMF especially the stability and structure of the rate-de- peded other extensive network of intra- and interchain hydrogen bonds
termining transition states [126,129]. Tyler et al. tried to solve these [92]. Chen et al. found that CPL-LiCl is an effective solvent system for
two questions from the solvent-induced frequency shifts (SIFS), and the dehydration of glucose to HMF as well. A higher HMF yield of
they investigated the SIFS of the carbonyl stretching vibration v (C=O) 66.7% is achieved at 373 K for 3h in CPL-LiCI mixtures (3:1 mol ratio)
of HMF in different solvents (protic, polar aprotic, and nonpolar sol- [120]. In other single polar aprotic organic solvent, Shimizu et al. re-
vents) system. Their study showed that strong SIFS could make the H- searched the conversation of fructose to HMF at 393 K for 2 h in the
bonding interactions stronger. In addition, H-bonding solvent *(X-H) presence of Cs2.5Ho.5PW12040 with DMSO as solvent. The yield of HMF
orbitals could delocalize the lone-pair electrons of carbonyl, increasing is 91% (Entry 10, Table 2) [74]. However, it was difficult to separate
the charged density and decreasing local potential energy [126]. HMF from DMSO due to the high boiling point of DMSO. Yan et al.
In conclusion, water is a widely used solvent, however, the yield of obtained an optimized HMF yield of 47.6% at 403K over
HMF is always low for accompanying with by-products such as FA, SO-/ZrO2-Al2O3 with Zr-Al mole ratio of 1:1 in DMSO system (Entry
humins and LA. For biphasic system, the organic phase acts as an ex- 25, Table 2) [100].
tracting phase for continuous accumulation of HMF into the organic phase immediately after its formation in aqueous phase. Then, lower concentration of HMF in the aqueous phase limits the rate of side re- actions and improves HMF yields. This method allows easy separation and reusability of the reactive aqueous phase containing spent homo- geneous or heterogeneous catalysts. Polar aprotic organic solvents can improve HMF yield by preventing the rehydration of HMF, however, high boiling point of polar aprotic solvents make the separation of HMF more difficult and the presence of N, S, etc. reduce the purity of HMF.
3.3.4. Deep eutectic solvents (DESs)
The solvent systems inevitably have negative effects on HMF synthesis from biomass. Water is a cheaply and widely used solvent, and it has been widely applied in the production of HMF. However, it undesirably promotes the further conversion of HMF to LA [121]. Ionic liquids provide excellent reaction performances, while its application is limited by the economic inefficiency and difficult reutilization [122]. Polar organic solvent such as DMSO and DMF effectively inhibit the further dehydration of HMF to LA, while it bring more difficulties for
3.4. Density functional theory (DFT) study of biomass dehydration to HMF the isolation and purification of HMF product [122]. More and more
researchers are beginning to look for other reaction systems for HMF
DFT study could provide more information to understand the pro- synthesis from biomass. Deep eutectic solvents (DESs) are systems
cesses of the con version of biomass to HMF, such as isomerization of formed from a eutectic mixture of Lewis or Brönsted acids and bases
glucose/fructose, dehydration of biomass and the activity sites of cat- which can contain a variety of anionic and/or cationic species [123].
alyst. Ionic liquid 1-butyl-3-methylimidazolium chloride ([BMIM]CI, Thus, DES has been applied in many cases for the conversion of car-
Fig. 13) combined with transition metal salts ([BMIM]/MoCl3, bohydrates. A 90% yield of HMF was obtained from fructose in a DES
[BMIM]/WCl3, [BMIM]/FeClg) on glucose isomerization and fructose mixture formed by citric acid and ChoCI [124]. Lius study showed that
dehydrations into HMF were studied by
density functional theory
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H. Wang et al.
Renewable and Sustainable Energy Reviews 103 (2019) 227-247
enediol form as followed. In the end, the removal of a third water molecule and [BMIM]/MCI3 catalytic center lead to the final product HMF [130].
Fig. 13. 1-butyl-3-methylimidazolium chloride ([BMIM]CI).
Li et al. [131] studied a new mechanism for glucose dehydration to
HMF without isomerization to fructose for surface models of anatase (DFT). The experiments have indicated that during the isomerization of
TiO2 using periodic DFT calculations. Activation of the glucose at glu- glucose (Fig. 14), the stabilities of intermediates and energy barrier are
coses C3-OH position by titania started the reaction resulting in ad- sensitive to the mental center during each reaction step [130]. They
sorbed 3-deoxyglucosone. Comparison of different surface models concluded that over different metal chloride catalysts, the isomerization
shows that the presence of tetrahedral Ti+ species on a defective TiO2 of glucose decreased in the order of WCIs > MoCls > CrCls > FeCl.
(101) anatase surface was essential for explaining the activity. Synergy According their study, the mechanism of glucose isomerization and
between a strong Lewis acidic Ti site and a vicinal basic oxygen site of a fructose dehydration as shown in Fig. 14. Firstly, the interactions be-
TiOH group was essential to establish the direct conversion of glucose tween the hydroxyl groups in glucopyranose and Cl atoms in [BMIM]/
to HMF. This work also suggested that a balanced Lewis and Brönsted MClx catalyst is generated to form a complex 1. The intermediate 2 is
acidity was more advantageous
for the dehydration of glucose to HMF. generated by opening the pyranose ring in complex 1, characterized by
Yue et al. [132] regulated the acidity of the tungsten oxide by con- binding of -CHO group in the open form of glucose to MClx. Then
trolled the amount of introduction of Nb. They found that, the strong complex 2 is transformed into intermediate 3, which involves the for-
Lewis acidity and weak Brönsted acidity caused the reaction to proceed mation of a five-membered-ring chelate structure
between the two
through isomerization to fructose and dehydration of fructose to a neighboring hydroxyl groups in glucose and metal atoms. The next step
partially dehydrated intermediate in THF/water. The intermediate had occurs with the conversion of the 1,2-enediol intermediate to the open
been identified by LC-ESI-MS. The DFT calculations was used to de- form of complex 4 that as a part of a metal containing complex, which
monstrate how Lewis acidic w and Nb sites catalyze glucose iso- can finally release the [BMIM]/MCIx moiety and is converted into
merization. The calculation results showed that the Lewis acid centers fructofuranose via a ring closure.
on the tungstite surface could isomerize glucose into fructose. Sub- Reaction starts from active site where two oxygen atoms bonded
stitution of W by Nb could lower the overall activation barrier for with Cr center strongly active C1-O2 bond slightly in formatting HMF
glucose isomerization by stabilizing the deprotonated glucose ad- from fructose in [BMIM]/CrCl3. Then a series of intermediates such as
sorbate.
HỌ
HO
HO
Гон
OH
+L/MC Oн
HO L=ionic liquid or NHC M= various metals
OH
он
HO,
HO,
-он
HO1
OH
OH
OH
HO
HO.
O
он
Он
HO,
-он
OH
он
Fig. 14. Plausible mechanism of glucopyranose isomerization to produce fructofuranose catalyzed by MClx in [BMIM]CI [130].
236
H. Wang et al
Renewable and Sustainable Energy Reviews 103 (2019) 227-247 4. Conversion of biomass to DMF
intermediates by using each of these intermediates as a substrate
(Fig. 16). Firstly, MF is obtained by hydrodeoxygenation of HMF, and High energy density, high octane number, high boiling point and
the rate is fast. MF converts principally to MFA and DMF during a short low freezing point make DMF become an ideal replacement and/or
time. While once DHMF is generated, the yield of DMF decrease dopant for the presently used gasoline. In addition, enhancing com-
sharply, as the DHMF inhibits the conversion of MFA to DMF, due to the bustion efficiency and reducing contaminant emission make it en-
competitive adsorption on the surface of Pd. This work also showed that vironment-friendly.
HMF is produced via glucose dehydration could be converted to DMF Nowadays, various types of the catalytic system have been devel-
directly over hydrogenation catalyst [136]. oped for selective dehydrogenation (HDO) of HMF into DMF [133]. The
common catalytic systems include single translation-metal such as Pt,
4.2. Catalyst for biomass conversion to DMF Ru, Mo, Cu, etc. and bimetallic catalyst such as Ni-Co, Cu-Ru, Pt-Au,
etc. All above catalysts are better candidate to get relatively high yields
Catalyst played a key role in producing DMF with high yield from of DMF. Compared to non-noble metals, noble metals and bimetallic
biomass, such as HMF, glucose, fructose, cellulose, lignocellulose, etc. catalysts exhibit good catalytic properties and stability. Moreover,
Bifunctional catalysts, containing a hydrogenation metal as well as a under hydrogen atmosphere, translation metal oxide is easier to form
deoxygenation component, are usually required for DMF production. Lewis acid sites, which could
1 adsorb alcohol hydroxyl and aldehyde
The recent developments in catalysts, and reaction system are listed in groups, giving higher DMF selectivity through breaking C-O bond.
Table 3. For metal/acid catalyst, the synergistic effect of metal-acid is
the key for HDO of HMF. The formation of DMF from HMF usually 4.1. Reaction pathway of DMF production
needs balanced metal-acid sites and requires highly dispersed metal
sites with few acidic sites [137]. The Lewis acid site in metal-acid DMF, producing from selective HDO reaction of biomass derived
catalysts are more active for the dehydration of C-OH groups in HMF, HMF platform [134], is not only an important intermediate of solvents
which would effectively accelerate the cleavage of the C-O bond [96]. and spices, but an ideal substitute for fossil fuels. The generally reaction
Single metal catalytic systems, such as Ru, Pd, Pt,
and Ni have been pathway of glucose and fructose conversion into DMF as follows.
widely employed in the catalytic HDO of HMF. Using glucose as sub- Firstly, glucose or fructose dehydrating to HMF by removing three
strate, 15% selectivity of DMF selectivity was obtained in [EMIM]Cl in oxygen atoms. Secondly, 5-methylfurfural (MF) and 2, 5-dihydrox-
the presence of Pd/C at 393 K for 60 min. The addition of acetonitrile ymethylfuran (DHMF) from by selective HDO, and the two products
could improve the selectivity of DMF to 32%, with 60.3% DMF yield undergo hydrogenolysis and with 2-hydroxymethyl-5-methylfuran
obtained at 553 K in the presence of Ru/C (Entry 3, Table 3) [138]. Pt (HHMF) generated. Finally, DMF is generated by HHMF hydro-
based catalyst are rarely used in the catalytic HDO of HMF. Pt/rGO is deoxygenation. The pathways of monosaccharide conversion to DMF is
used as catalyst for HDO of HMF, the highest yield of DMF reached shown in Fig. 15 [135].
73.2% with a 100% conversion of HMF at 393 K, 3.0 MPa H2 and 2.0h Intermediates such as MF, 5-methylfurfyl alcohol (MFA), and DHMF
reaction time [139]. The high yield of DMF over Pt/rGO is attributed to were observed in the HDO reaction of HMF to DMF. In order to identify
the highly dispersed Pt NPs, micropore-free structure, and the selective the reaction pathways from HMF to DMF, Chidambaram and Bell ex-
adsorption of HMF. Compared with Ru, Pd based catalysts, the DMF plored the relative rate
of formation and consumption of
these
yields are low over Pt based catalyst.
OH
HO,
—OH
HO1
isomerization
OH
он Гон
OH
H
о
OH
HMF
QH
QH
OH
MF
DHMF
HHMF
H
DMF
DHMF: 2,5-dihydroxymethylfuran; HHMF: 5-methyl-2-furanmethanol; MF: 5-methylfurfural
Fig. 15. The pathways of DMF from monosaccharide.
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Renewable and Sustainable Energy Reviews 103 (2019) 227-247
OMBM
+H
+2H2 Slow
-HO AL
DMF
HO,
Fast +H
HMF
Fast +H-HO
MF
MFA +2H -H
Fast
Slow +H2
MTHFA
+H2
Slow
Fast
Slow
HO
LOH
DMF
HD
DHMF
MF: 5-methy furfural; DHMF: 2,5-dihydroxymethylfuran; OMBM: 5,5-(oxybis(mythylene))bis(2-methylfuran); MTHFA: 5-methyltetrahydrofurfuryl alcohol; HD: 2,5-hexanedione; AL: Angelicalactone
Fig. 16. Reaction pathways for HDO of HMF [136].
Table 3
Results of direct conversion of biomass into DMF. Entry
Feedstock
Medium
Catalyst
Condition
Selectivity (mol%)
Yield (mo1%)
Ref HMF
MeOH
Cu-PMO
573 K, 45 min
34
34
[162] HMF
isopropanol
Ru/C
463 K, 6h
৪0
80
[163] HMF
1-butanol
Ru/C
533 K, 1.5h
60.4
60.3
[138] HMF
THF
7Ni-30WC/AC
453 K, 3h
96
96
[145] HMF
THF
Pd/C/Zn
423 K, 8h
85
85
[148] HMF
THF
Ni/Coo
403 K, 24 h
76
76
[151] HMF
THF
Ru/Coo
403 K, 24h
93.4
93.4
[152] HMF
THF
2 wt% Ru-NaY
493 K, 1h
78
78
[156] 10
HMF
DMSO
Ru/Cu
423 K, 20 min
91
[164] HMF
MeOH
Cu-PMO
533 K, 45 min
48
48
[162] 12
HMF
HO-dixoane
Pd/C
393 K, 15h
89.5
85
[165] 13
HMF
n-tetradecane
LF-N20
503 K, 6h
98.3
98.3
[166] 14
HMF
1-butanol
Pt/rGO
393 K, 2h
73.2
73.2
[167] 15
HMF
1,4-dioxane
Ni/Al
453 K, 3h
91.5
91.5
[142] 16
HMF
ethanol
Cu-Co@c
453 K, 8h
99.4
99.4
[149] 17
HMF
THF
PdAu4/GC800
423 K, 4h
94.4
81.9
[168] 18
HMF
THF
Ni-Co/C
483 K, 24h
90.9
90
[169] 19
HMF
1,4-dioxane
NiZnAI
453 K, 15h
93.6
93.6
[170] 20
HMF
n-butanol
Fe-L1/C-800
513 K, 4h
86.2
86.2
[144) 21
HMP
ethanol
Ni@SAZn_PC
423 K
86
68
[137] 22
HMP
1-butanol
25c-Co/3c-Pt/MWCNTs
453 K, 8h
92.3
92.3
(171] 23
HMF
dioxane
Ru/CNT
423 K, 1h
86.1
83.5
(172] 24
fructose
[BMIM]CI THF
Ru/C
403 K, 30 min/493 K, 5 h
50
50
[173] 25
fructose
HCI-KCI- 1-butanol
Cu-Ru/C
493 K, 10h
82
73
[23] 26
fructose
GBL-H20
HT-Cu/ZnO/AI-
513K
40.6
40.6
[170] 27
fructose
THF
Pd/C
343 K, 15h
51
51
[24] 28
fructose
THF
Cu/Alo 2
343 K, 15h
51
51
[27] 29
fructose
THF
2.4 Pd/UiO-66 @SGO
433 K, 3h
83.8
68.6
[174] 30
fructose
FA-ethanol
Amberlyst 15/ Ni@wc.
383 K, 423
38.5
38.5
[175] 31
glucose
THF
4.8 Pd/UIO-66 @SGO
433 K, 3h
63.8
45.3
[174] Note: Cu-PMO, Cu-doped porous metal oxide; GBL, y-butyrolactone; DMA, dimethylacetamide.
Noble metal catalysts have high reactivity and good catalytic effect,
have great potential in HMF conversion. Ni/C catalyst was used to while the high price limits their application in industrial production.
substitute noble metal catalysts for HMF HDO reaction in Gyngazovas Developing efficient non-noble metal catalysts to replace noble metal
work, about 80% DMF yield was obtained with 100% conversion of catalysts has become a research hotspot in catalytic field. Kong and co-
HMF at 453 K in 2-methyl-THF solvent [143]. In Li and coworkers workers [140-142] develop a series of Ni based catalysts, they use
work, they introduced inexpensive Fe-based catalyst into HDO reaction Raney Ni, Ni-Al2O3 and NiSi-PS as catalyst with dioxane as solvent. The
of HMF. Under the optimal reaction conditions, complete conversion of highest yields of DMF are 88.5%, 91.5% and 64.3%, respectively. The
HMF was achieved with 86.2% selectivity to DMF (Entry 20, Table 3) performance of Ni based catalysts show that non noble metal catalysts
[144].
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Renewable and Sustainable Energy Reviews 103 (2019) 227-247
In addition, bifunctional catalysts have been further examined in the HDO of HMF. A bifunctional catalyst can lead much higher yield when modifying a nickel-based catalyst with another metal, which equipped with Lewis acid sites and HDO ability. Nickel-tungsten car- bide catalyst on active carbon (Ni-W2C/AC) was used for selective HDO
DMF of HMF into DMF. Synergistic effect between Ni and W2C gave the
catalyst ability of hydrogenation and deoxygenation. Moreover, in this catalyst, W2C contains both acidic sites and metallic sites which can
Catalyst catalyze both the deoxygenation and hydrogenation reactions, giving a
higher DMF yield: 96% DMF yield
I was achieved over 7%Ni-30%W2C/
Coox AC in THF (Entry 4, Table 3) [145]. Mo metal has the same functions
with W metal, 79.4% DMF yield can be obtain over the Ru-MoOx/C catalyst [146]. Pd/C was an effective catalyst for HDO of monomeric lignin surrogate molecules under Lewis acidic ZnCl2 [147]. In addition, Pd/Zn/C was used to convert HMF into DMF, Pd/C/Zn catalyst re- presented the most effective result among the several catalysts, and 85% DMF yield was achieved at 423 K for 8 h over Pd-ZnO/C catalyst
HMF (Entry 5, Table 3) [148]. Cu-Co bimetallic nanoparticles coated with
carbon layers was used for HDO reaction of HMF, 99.4% DMF yield
Fig. 18. The synergistic effect of Co-CoOx is the key for HDO of HMF [153]. with 100% HMF conversion was achieved at 453 K in ethanol. The bi-
metallic nanoparticles were entrapped by carbon shells (Fig. 17) which
HMF over Ni/ZrP catalyst. As shown in Fig. 19. Ni/ZrP is facilely could protect them from oxidation and sintering. The catalytic of this
converted into Ni and ZrP with metal and acid functionalities for acti- catalyst was stable and no decline after six-run recycling test (Entry 16,
vating H2 and -CH2OH groups, respectively. The oxygen in the CH2OH Table 3） [149].
of HMF is activated by the Lewis acidic sites of Zr+ (originated from CoOx could break C-O bond effectively [150], 76% DMF yield was
the vacant orbit of Zr), which is easily hydrodeoxygenated to MF as the obtained over Ni/CosO4 catalyst in THF system at 403K for 24 h, and
primary intermediate because of attacking of H atoms activated by the 93.4% DMF yield over Ru/CosO4 (Entry 6-7, Table 3) [151,152]. It is
neighboring Ni metal. The oxygen in the carbonyl group of MF con- already demonstrated that Ni can selectively hydrogenated of C-O, C-C,
tinues to be activated by Zr4+ species, followed by hydrogenolysis via and C-O bonds, respectively. In our previous work [153], bi-functional
MFA, which is a precursor to be converted into DMF. Co-CoOx catalysts was used for HDO of HMF, the metal/acid catalyst
Zeolites are excellent catalysts and applied in many reactions be- showed high efficiency in selective hydrogenolysis of HMF to DMF (the
cause of their high hydrothermal stability and shape-selectivity [155]. DMF yield of 83%) via the synergistic effect of metallic Co and acidic
Zeolite combined with other metal catalyst is useful for biomass con- CoOx (Fig. 18). Kong et al. launched experiments to test the efficiency of
version into DMF. Nagpure et al. implemented the experiment of DMF non-noble such as Ni. They reported that hydrotalcite derived Ni-Al203
from HMF over Ru-NaY, giving 78% DMF yield (Entry 9, Table 3) catalysts could selectively catalytic conversion of HMF into DMF. The
[156]. HY zeolite plays a vital role in improving HDO reaction of HMF highly dispersed Ni and Al2O3 particles gave 91.5% DMF at 453 K for
and ring opening through breaking of the furanic C-O bond. One-Step 3h (Entry 15, Table 3) [142].
continuous conversion of fructose to DMF was achieved over combined Lewis acidity on metal/acid catalysts plays a key role in HDO of
HY zeolite and inexpensive hydrotalcite (HT)-Cu/ZnO/Al2O3 in a fixed- HMF. In our previous work [154], we studied the scheme of HDO of
bed reactor. The high yields of DMF (40.6%) were obtained by simply
100 nm
10 nm
10 nm
nm
Fig. 17. (a) TEM image of Cu-Co@Carbon nanoparticles at low magnification. (b-d) TEM images of the selected single bimetallic nanoparticle [149].
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Renewable and Sustainable Energy Reviews 103 (2019) 227-247
Hydrogenafion
• Hydrodeoxygenation
CH
MF
HOT
Over-hy
nation
MTHFA
MFA
=0
HMF
Hydrodeoxygenation
Furan ring op
ing and hydrationi DMF
MF: 5-methyfurfural; MFA: 5-methyl-2-furanmethanol; MTHFA: 5-methyltetrahydrofurfuryl alcohol
Fig. 19. Proposed Reaction Mechanism for the HDO of HMF to DMF via MF [154].
tuning the reaction temperature [157].
development perspective. Electrocatalysis, which is usually carried out in moderate reaction
conditions, has been used on the hydrogenation of numerous com-
4.3. Reaction solvent for biomass conversion to DMF pounds for many years. Kwon et al. studied the electrocatalytic hy-
drogenation of HMF to 2,5-dihydroxymethylfuran on solid metal elec-
For direct biomass and the derivatives conversion to DMF, many trodes in neutral media, both in the absence and in the presence of
processes such as hydrolysis, isomerization, and HDO are involved. As glucose. They also pointed out that the first electron-transfer step
the solvents show significance influence on
different process, the sui- during HMF reduction was a non-catalytic reaction with proton transfer
table solvent are needed for DMF production. directly from water in the electrolyte [158]. Nilges firstly reported the
conversion of HMF into DMF by an electrocatalytic method. The highest
4.3.1. Monophasic solvent for HMF conversion to DMF DMF selectivity (35.6%) was achieved in 0.5 M H2SO4 in a mixture of
HMF is a key intermediate for the biomass conversion to DMF, and water and ethanol. Though the selectivity was still low, it makes the
it also has been studied for many researchers. The recent researches are electrocatalytic hydrogenation become a potential approach for the
listed in Table 3. Using HMF as feedstock, monophasic solvents, in- conversion of HMF into DMF [159]. Yu et al. produced DMF from HMF
cluding organic solvent and ILs, are widely used in the HDO reaction. on a novel ZrOz-doped graphite electrode by a two-step method. The
Particularly, the ionic liquid solvent, which are widely applied in HMF was obtained from fructose using ZnCl2 as catalyst, the highest
conversion of biomass to value-added chemicals [176,177]. An research yield of HMF reached 96%. Then, the effective con version of DMF from
was carried out in [BMIM]CI system over heterogeneous Ni, Ru, Pt, and HMF was achieved on the ZrO2-doped graphite electrode, and the
Ir-based catalysts under mild condition. The yield of furan-based diols highest yield of DMF reached 30.7% in this report [160]. The electro-
from HMF was obtained from 34.0% to 89.3%, showing ILs playing an catalytic hydrogenation has great advances in the HDO reaction of
important part in cellulose hydrolysis [178]. For Chidambaram and HMF. Reaction temperature is low, always room temperature; water as
Bells study, Pd/C was applied as catalyst used for different HMF source, proton source, neednt additional hydrogen; and the process is green
neat HMF, extracted HMF and HMF in EMIMCI-CH3CN. The highest and environmentally friendly. However, it still has the problems of poor
yields of DMF was obtained nearly 15% with HMF in EMIMCI-CH3CN as selectivity and low yield. We need to pay more attention to the new,
substrate. Their research also showed that the sources of HMF has no catalytically efficient electrode materials to solve these problems.
effect on DMF yield [136]. Noble metal catalysts show good
1 catalytic performances,
while some drawbacks,
such as high cost and easy deactivation, limit their
4.3.2. Biphasic solvent for biomass conversion to DMF practical applications [161]. Therefore, developing noble metal-free,
When using sugars, or cellulose as feedstock in one pot conversion cheap, efficient and stable heterogeneous catalysts and reaction system
to DMF, because involving the dehydration of sugars to HMF, the bi- are highly desirable for both an economic and a sustainable
phasic solvent may
be a better choice. In the early time, the formation
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Renewable and Sustainable Energy Reviews 103 (2019) 227-247
of DMF from sugar always involves two steps. The first step was the sugar conversion into HMF, and the second step is situ hydrogenation of HMF to DMF. In 2007, Dumesic et al. have launched a pioneering work of DMF from fructose in biphasic system: the first step involved dehy- dration of fructose into HMF with acid catalyst, then organic phase extracted HMF immediately and HMF was converted into DMF by breaking C-O bonds over Cu-Ru/C catalysts. The highest DMF yield was obtained by adding NaCl into reactor over CuRu/C catalyst (Cu: Ru = 3:2, Entry 25, Table 3) [23]. Zhu et al. developed a one-step re- action system to converted fructose to DMF over combined HY zeolite and inexpensive hydrotalcite -Cu/ZnO/Al2O3 in a water-y-butyr- olactone system directly. 40.6% DMF yield was gained at 513K (Entry 26, Table 3). Li et al. have implemented one-step production of DMF and DMTHF from fructose. They concluded that the organic solvent in IL contained binary solvent mixture and the water amount strongly affected the reaction, in a certain range, the yield of furan-based fuels increased with increasing water amount. Much higher amount of DMF (66% yield) and DMTF (20% yield) were obtained in biphasic [BMIM]CI/THF solvent system with the modified Ru catalyst [173]. Mitra et al. described the efficient formation DMF from HMF in H2O- dioxane mixture over Pd/C catalyst and FA. FA improved the conver- sion of HMF into DMF. IH NMR spectroscopy indicated the hydro- genation first occurs on the formyl group during the HDO of HMF [165].
Fructose
OH
HCO,H -3HO
DMF
-cO,
HMF
FMMF
HCO,H H
|HCO,H -H QH
4.4. Hydrogen transfer for biomass conversion to HMF
Although molecular hydrogen has many advantages like wide availability and easy activation on many metal surfaces, the employ- ment of hydrogen gas presents a number of challenges including transport, process economy and sustainability. Moreover, the low so- lubility of molecular hydrogen in most solvents results in high H2 pressure that leads to a considerable safety hazard [179]. Catalytic transfer hydrogenation is a reaction system which using FA, alcohol and other substance such as hydrocarbon, ammonia, and hydrazine as hy- drogen donor rather than H2. Using these organic compounds as hy- drogen source, not only the production cost can be reduced but also the security can be improved [176].
Thananatthanachon et al. have used FA as a hydrogen donor for hydrogenation of HMF and fructose. In their experiments, FA served as catalyst and a source of hydrogen and a deoxygenation agent, as shown in Fig. 20. 95% of DMF yield was obtained from HMF, and they also directly converted fructose into DMF in the presence of FA over Pd/C in THF, with DMF yield of 51% (Fig. 21) [24]. Both the two pathways, the
2HCO,H PA/C THE
-2CO FMF
-HO
.HMME. HHMF: 5-methyl-2-furanmethanol; FMF, 5-formylfuranyl-2-formate
Fig. 21. One -pot process to generate DMF from fructose [24].
conversion is proposed to proceed through the intermediate of HMMF its monoformate ester (FMMF). In the reaction, FA has three roles, first one, as an acid catalyst for dehydration of fructose to HMF, second one, as a hydrogen source for hydrogenation, and third one, as a reagent for the deoxygenation of furanylmethanols. While, addition of H2SO4 in the above report poses the challenge for the material and catalysts dur- ability. Yang et al. applied the catalytic transfer hydrogenation process for the reductive upgrading of HMF using a non-noble Ni-Co/C catalyst with formic acid as hydrogen source without H2SO4, the highest DMF yield 90.0%, was achieved at 483 K in THF (Entry 18, Table 3) [169]. Although FA is a renewable and very promising source of hydrogen, the strong corrosiveness of FA and H2SO4 limits its large-scale appli- cation in the preparation of DMF. Alcohols can also act as a source of hydrogen and without corrosive.
Hansen et al. have used alcohols as hydrogen donor, and 34% DMF yield was achieved at 573 K for 45 min over Cu-doped porous metal oxide (Cu-PMO) (Entry 1, Table 3) [162]. Jae et al. have investigated the conversion of DMF from HMF over Ru/C with secondary alcohols. The yield of DMF can reach up to 80% under the reaction condition of 463 K for 6h (Entry 2, Table 3) [163]. The solvents were all derived from biomass, which provide a green access to convert HMF to DMF. However, the products are difficult to be separated due to producing by-products. Li et al. investigated the reaction pathway with n-butanol as additional hydrogen source, and they pointed out that the
QH
OH HCOH Pd/C
ỘH
QH HCOH
HSO， —HO
HMF
BHMF
Pd/c- CO2
O HCOH
HSO4 —H2
HCOH HSO4 -CO2
DMF
>95%
BHMF: 2,5-bis(hydroxymethyl)furan; HHMF: 5-methyl-2-furanmethanol
Fig. 20. Pathway for DMF from HMF [24].
HMMP
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Renewable and Sustainable Energy Reviews 103 (2019) 227-247
Pd or AuPd catalysts
QH
OH
OH
O2 free H2
This work: NiCu-alloy, N2 atmosphere, vapor-phase coupling reaction
H2 free
+ H2 / formic acid / alcohol
Ru or Cu catalysts
Fig. 22. The dehydrogenation-hydrogenation coupling process for the syn- chronized production of phenol and DMF [180].
processes were divided into four and six processing areas respectively that include synthesis and purification sections. The HMF process em- ploys a biphasic (aqueous-organic phases) continuously stirred tank reactor (CSTR), and the DMF process has an additional 3-train fixed bed catalytic PFTR. The organic phase of the resultant HMF solution that contains 58% HMF was then flashed from the aqueous-organic se- parator to remove excess butanol. The vapor phase through the va- porizer was fed into PFTR in which hydrogen was added for the con- version of HMF to DMF with Cu-Ru/C as catalyst. The catalyst needed regeneration after every 10 cycles, which was treated by H2. They also gave a more complete block diagram for the preparation of DMF from fructose (Fig. 25).
Zhu et al. [170] developed a continuous DMF production tech- nology. Direct conversion of fructose to DMF over combined HY zeolite. and hydrotalcite (HT)-Cu/ZnO/Al2O3 in a continuous fixed-bed reactor (Fig. 24). Fructose mixed with water (15 wt%) and y-butyrolactone (GBL, 82 wt%) solvent. The fructose-GBL-water solution was con- tinuously pumped to reactor using a HPLC pump. Simultaneously, pure H2 was introduced into reactor using a mass-flow controller. The final products were condensed and collected in a gas-liquid separator. They also pointed out that a decrease in fructose conversion occurred when the fructose WHSV is more than 0.02h-1, whereas the yield of HMF increased and then declined with increasing WHSV.
In scale-up conversion biomass to HMF and DMF, sugars are usually as feedstock. As sugars are easily dissolved in the solvent, the mass transfer between the catalysts activity sites and sugars are easy. Both homogeneous and heterogeneous catalysts can be used for the dehy- dration of sugars. While, for solid feedstock, such as cellulose and lig- nocellulose, the limited contact between activity sites of heterogeneous catalysts and solid feedstocks significant influence the efficiency of this kind catalyst. Hence, homogeneous may be a better choice for con- version cellulose and lignocellulose. In other hand, the key to improve. the efficiency of heterogeneous is to enhance the mass transfer between activity sites of heterogeneous catalysts between the solid feedstock.
hydrogenation of the C-O bond in HMF was demonstrated to be the rate-determining step during the hydrodeoxygenation, which could be accelerated greatly by using alcohol solvents as additional H-donors (Entry 20, Table 3) [144]. Li et al. reported a creative work, they es- tablished a vapor-phase dehydrogenation-hydrogenation coupling process over bimetallic Ni-Cu alloy nano-catalysts for the synchronized production of phenol and DMF (Fig. 22). At the optimal condition (513 K, N2, 0.1 MPa, LHSV = 1.0 h-1), the yields of phenol and DMF were simultaneously maximized,
98% and 99%, respectively. The as developed coupling process provides a new and promising avenue for the green production of industrial chemicals and biofuels from funda- mental raw materials [180].
Currently, the main source of hydrogen is nonrenewable petro-
6. Process economy analysis chemical resources. The cost of hydrogen is high, the high dispersion
and flammability makes it is difficult to store and transport.
The process economy of most of the reported HMF and DMF pro- Furthermore, the solubility of hydrogen in various solvents makes the
duction methods is unknown. Very few researchers reported the cost availability of hydrogen is low. Herein, it is imperative to develop safe,
analysis using an Aspen Plus model based on different reaction condi- efficient, green and environment-friendly hydrogen sources.
tions. Dumesic et al. reported a techno-economic analysis using an
Aspen Plus model based on a two-step integrated process. In this pro- 5. Scale-up conversion of biomass to HMF and DMF
cess, fructose solution was first dehydrated with HCl in a biphasic
solvent [185]. The processing capacities were 300 metric ton per day of Currently, much more researches are done for the production of
fructose and the plants would operate for 20 years. Installed equipment HMF and DMF from biomass, but the scale-up conversion of biomass to
costs were estimated as $102.4 million for the HMF (US $ 2007), and these high value-added products is still a big obstacle in the practical
$121.9 million for the DMF process. The minimum selling prices (MSP) utilization. Only few work has been done on the scale-up of HMF and
for HMF production with fructose as feedstock was estimated as $1.33/ DMF production. We summarized the methods in the reports (23,
L ($5.03 per gallon). [181-184]), and the continuous process rote for producing HMF from
Based on a 96.6 metric tons per day production target for DMF, the biomass as shown in Fig. 23.
MSP for DMF was calculated at $2.02 ($7.63 per gallon). In this work, Typically, biomass such as fructose, glucose, cellulose etc., which
they pointed out that the most significant parameters for MSP of DMF are obtained from lignocellulosic (see Section 2.), water, acid catalyst
were feedstock cost, product yields, catalysts cost and total purchased and organic solvent are mixed in a reactor and then converting in the
equipment costs. For HMF, increased the HMF yields, inexpensive reactor. After reaction, the mixture was pumped into solvent separation
fructose, lower capital costs and higher price for LA could lower its reactor. In this reactor, a portion of the product, unconverted fructose
price and helped establish it as a bio-based commodity chemical for a and the organic solvent are removed by extraction. Part of the solvent
range of other applications. For DMF, sensitivity analysis showed that was transferred into the reaction reactor, and the remaining mixture
the low DMF yield was the most aggravating factor for a high MSP. A was pumped into the third reactor. In the third reactor, unconverted
20% increase in the yield could lower the MSP by 16.7%. The process biomass was separated and subsequently recycled into the reaction
for conversion of HMF to DMF used an expensive catalyst in this work. reactor by extraction. Then through separation of acid catalyst and
The catalyst performance at the level didnt not seem viable for com- filtering, the product HMF was obtained. After further purification, the
mercial application of DMF as a fuel. Development of less expensive and HMF can be used for DMF production. Dumesic and coworkers [185]
more active catalysts with lower noble metal composition was essential have further processed HMF in a conventional fixed bed catalytic plug
for this fuel applications. flow tubular reactor (PFTR) to produce DMF. HMF and DMF production
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Renewable and Sustainable Energy Reviews 103 (2019) 227-247
Biomass
Unconverted biomass
catalyst
solvent
HMF organic phase
Biomass aqueous phase
oseud snoonbe/oseud iueo
HMF unconverted Biomass By products H,O/acid
by products (humins)
HMF Unconverted Biomass Extracting phase Aqueous phase (acid)
solvent separation
acid catalyst recovery
filter
Biomass: fructose, glucose, cellulose; acid: H,SO4, HCI, solid acid
Fig. 23. Conversion of biomass to HMF in a continuous production system.
HMF+HO
વo
н,он
+ H2
has been attracted more attention. Much higher yields of DMF could be obtained through reducing by-products. Besides, ILs gives a high HMF and DMF yield. Nevertheless, the high cost restricts its industry appli- cation. To achieve the goal of commercial process, there are still many challenges to face:
HY zeolite 413 K
HY zeolite 413 K
HT-Cu/ZnO/Al,03
413 K
HT-Cu/ZnO/Al,O
513 K
DHMF
DMF + HO/GBL
Fig. 24. Direct conversion of fructose to DHMF and DMF in a fixed-bed reactor (modified from Fig. 1 of Ref. [170]).
7. Conclusion and perspectives
Catalytic conversion HMF and DMF from biomass is playing a vital role in replacing fossil-based energy. Many researches have been taken to achieve a high yield of HMF and DMF. Among different catalyst systems, metal salts have been widely used in the formation of HMF from sugars because of their obviously catalytic activity. Solid acids have a relatively high efficiency due to the balanced Lewis/Brönsted acidity and stability. Among different reaction systems, biphasic system
(1) The reaction mechanism during the process is still need to study. To research the reaction mechanism, many new detection methods should be used, such as in situ nuclear magnetic resonance (NMR), mass spectrum (MS), electrospray ionization (ESI) and other ana- lytical means. Considering the complicated reaction of biomass transformation, it is also necessary to develop more advanced si- mulated methodologies, such as First-principles simulation, mo- lecular dynamics and other methods to explain the reaction more clearly.
(II) The highly efficient coupled catalytic systems are needed to study. Although HMF and DMF could be obtained with a high yield from sugars, the yield is rather low with cellulose or other cellulose- based biomass as raw materials. Practically, the industry scale production of HMF directly from raw biomass has not been rea- lized yet. One-pot process to generate DMF using cellulose as feedstock involves hydrolysis, isomerization, and HDO, each re- action requires different catalyst and reaction system, so how to combine the catalyst and the reaction system with high DMF yield from biomass need more efforts. Acid-functionalized metal catalyst could improve the selectivity of the HDO reaction, while how to match metal and acid component need more researches.
(III) Based on the economy process analysis, the catalysts performance is a key fact for the MSP of DMF. Although many reported catalysts have showed good performances, more efforts need to design ef- ficient, economical, recyclable, and environmentally friendly cat- alysts to achieve high selectivity of target products. For scale-up HMF and DMF production, using heterogeneous catalyst is a pro- mising way. Developing heterogeneous catalysts would focus on active sites, high surface area and accessible acidic moieties. (IV) Hydrogen donors are still need to research in generating DMF. FA and alcohols have been used as hydrogen source in many hydro- genation reactions, while researches in DMF production are still limited. Developing multifunctional catalysts, which can be used for reforming hydrogen production and hydrogen transfer, as well as selective HDO of HMF, is of great significance for the prepara- tion of DMF from biomass.
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Recyclo BOH Stream (1376 MTid)
Rocycle BOH stream (310 MTId) Rocycle BOH stroam (1394 MT)
Recycle water stream (224 MTid)
H Recycle Stream (63 MTM)
Area 200
Area 400
--- Area 100
Tow 908 MTid）
Fiser
4556 MTId
Dissiation Column
--- Area 300 H2
(5.7 MTId)
PFTR
Fructose (300MTId)- BOH (0.95 MTId-
Biphasic HCI (3.74 MTId)
Rsctor (CSTR w Ftration
Makeup Water (782 MT)
PAW K
Flash Sep.
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) M HO8
Flash
DMF separator
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(96.6 MTId Distitation
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porizer
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(คIพ
- Area 600
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Distilation Col
Area 500
oS MTId
Distillation Colum
Distillation Colum
Evaporator
Byproduct （S04 MTId）
676 MTId
Recycle Fructose Stream (71 MTId)
Levulinle Acld (8 MTd)
BOH: butanol; WWT: wastewater treatment.
Fig. 25. DMF production process block diagram [185].
Acknowledgement
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