Renewable and Sustainable Energy Reviews 130 (2020) 109944
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
journal homepage: http://www.elsevier.com/locate/rser
Recent advances in mechanochemical production of chemicals and carbon
materials from sustainable biomass resources
Feng Shen a, Xinni Xiong c, Junyan Fu b, Jirui Yang a, Mo Qiu a, Xinhua Qi b, d, **, Daniel C.
W. Tsang c, *
a
Agro-Environmental Protection Institute, Ministry of Agriculture and Rural Affairs, No. 31, Fukang Road, Nankai District, Tianjin, China
College of Environmental Science and Engineering, Nankai University, No. 38 Tongyan Road, Jinnan District, Tianjin, China
Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
d
National & Local Joint Engineering Research Center of Biomass Resource Utilization, Tianjin 300350, China
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Biomass valorization
Ball milling
Lignocellulose
Porous carbon
Sustainable biorefinery
Waste management
Biomass resources have been considered as one of the most promising renewable feedstocks to replace fossil
resources. However, valorization of biomass is still challenging due to concerns about environmental sustain­
ability and low efficiency of conversion processes. Mechanical ball milling technology, which has emerged as an
efficient and environmentally sound alternative to traditional method, can overcome this obstacle to facilitate
biomass valorization. Mechanical energy in the ball milling process can induce chemical reactions of biomass in
solvent-less/-free conditions. This work reviews the latest advances in the mechanochemical conversion of
biomass into chemicals and carbon materials. The initial pretreatment of biomass, catalytic transformation
process of biomass, and synthesis of biomass-derived carbon materials (ordered mesoporous carbons, hierar­
chically porous carbons, carbon/metal composites, etc.) are discussed in detail. Mechanisms, development his­
tory, key influencing factors, and technology readiness level of ball milling on biomass valorization are also
elucidated. Limitations and opportunities associated with this green technology are highlighted for future
research directions.
1. Introduction
Currently, most of the chemicals, fuels, and materials used by
humans are produced from non-renewable fossil resources such as oil,
natural gas, and coal. These non-renewable resources are declining in
amount on Earth, while the world’s demand for energy, chemicals, and
materials is surging. The consumption of fossil resources can lead to
serious environmental pollution and global climate change due to the
release of large amounts of greenhouse gases and potentially toxic ele­
ments [1]. The price fluctuation of petroleum-derived fuels has inter­
fered the global economy and sustainable development [2]. It is urgent
to look for renewable alternatives of fossil resources for producing
chemicals, fuels, and materials.
Biomass is the most abundant renewable resource on Earth. It is
estimated that the global biomass production is approximately 130
billion tons per year [3]. Through biological and/or thermochemical
technologies, the biomass resources can be transformed into sustainable
chemicals, fuels, and materials [4–6]. The International Energy Agency
suggests that 10% of the world’s energy can be provided by biomass by
2035, and 27% of transportation fuel can be replaced by
biomass-derived energy by the year of 2050 [2]. Energy production from
biomass is an environmentally benign process with low impacts on the
natural environment in view of its carbon-neutral characteristics.
Biomass can be transformed into chemicals, fuels, and carbon materials
with different methods including bio-chemical, thermo-chemical, and
physio-chemical processes [7–9]. Waste biomass can also be used to
produce biochar, which emerges as a green and sustainable material for
soil amendment, chemical synthesis, wastewater treatment, and energy
recovery [10–14]. In order to improve the properties and performance
of biochar, more and more studies explore cost-efficient routes of
Abbreviations: BM, ball milling; DP, degree of polymerization; 5-HMF, 5-hydroxymethylfurfural; HPCs, hierarchically porous carbons; h-BN, two-dimensional
hexagonal boron nitride; LMWC, chitosan with low molecular weight; OMCs, ordered mesoporous carbons.
* Corresponding author
** Corresponding author. College of Environmental Science and Engineering, Nankai University, No. 38 Tongyan Road, Jinnan District, Tianjin, China.
E-mail addresses: qixinhua@nankai.edu.cn (X. Qi), dan.tsang@polyu.edu.hk (D.C.W. Tsang).
https://doi.org/10.1016/j.rser.2020.109944
Received 6 February 2020; Received in revised form 14 May 2020; Accepted 27 May 2020
Available online 10 June 2020
1364-0321/© 2020 Elsevier Ltd. All rights reserved.
F. Shen et al.
Renewable and Sustainable Energy Reviews 130 (2020) 109944
biochar synthesis and modification, such as metal impregnation
[15–17]. With thermochemical technologies, a broad range of valuable
products can be generated from biomass as exemplified in Fig. 1. A brief
overview on valorization of biomass including the pretreatment of
biomass, catalytic process of biomass into chemicals, synthesis and
application of biomass-derived carbons are listed in Table 1.
At present, only small percentages of biomass feedstock are used for
commercial purposes. This is because the robust structure of biomass
limits its conversion. As such, much effort has been made to improve the
conversion efficiency of biomass such as biomass pretreatment tech­
nology [19,43,44], new solvent systems such as ionic liquid [45] and
deep eutectic solvent [46,47], microwave-assisted catalytic process
[48], and mechanical-assisted conversion process [49]. Among them,
ionic liquid, deep eutectic solvent, and microwave technology for
biomass valorization have been comprehensively reviewed in previous
literature [24,27,50,51]. Therefore, this report critically reviews the
latest advances of biomass valorization using mechanochemical re­
actions. Mechanochemical reactions refers to the chemical reactions
induced by the direct absorption of mechanical energy including impact,
compression, shearing, friction, and stretching [52]. Ball milling is the
most important instrument for practical mechanochemistry. As shown in
Fig. 2, changes in solid reactants under mechanical forces start with
elastic deformation. Prolonged milling leads to irreversible plastic
deformation, which is induced by shear stress. New surfaces of the re­
actants with active sites are formed and they are easier to contact,
coalesce, and react [53,54]. The hallmark of mechanochemistry is that
chemical reactions can be achieved either by dry milling in solid state
without bulk dissolution of reactants or by wet milling under
solvent-less conditions. Therefore, mechanochemical methods are more
efficient, greener, and more sustainable than the conventional methods
of chemical reactions (e.g., thermochemistry, electrochemistry, sono­
chemistry, or photochemistry) [55].
The development history of the mechanochemistry is illustrated in
Fig. 3. It is widely accepted that the book ‘‘On Stones’’ by Aristotle’s
student and successor in 315 B.C is the first document mentioning
mechanochemistry [57]. It is recorded that elemental mercury could be
obtained from cinnabar and acetic acid via solid-state reaction by
grinding in a mortar. After that, there was no explicit record about
mechanochemistry for the next 2000 years. In 1820, Faraday published
a literature work about the reduction of silver chloride by a “dry way”
method in the presence of zinc, tin, etc [58]. This work started the study
on mechanochemistry. M. Carey Lea (1823–1897) is considered as “the
father of mechanochemistry” since he performed systematic
Table 1
Recent reviews on biomass valorization.
Category
Content
References
Pretreatment of
biomass
Pretreatment of biomass via fungal and
physicochemical processes
Pretreatment technologies of biomass for
bioethanol production
Pretreatments to enhance the digestibility
of biomass
Pretreatment of biomass by alkaline
Pretreatment of biomass by organic solvent
Catalysts for conversion of cellulose into
platform chemicals
Biomass conversion with microwave
technology
Acid catalytic conversion of biomass to
levulinic acid
Pyrolysis of biomass into materials and
chemicals
Transformation of biomass into chemicals
in ionic liquids
Valorization of biomass to biofuels with
bifunctional solid catalysts
Transformation of biomass into
γ-valerolactone
Catalytic fast pyrolysis of biomass
Conversion of biomass into chemicals over
metal catalysts
Conversion of biomass to value-added
chemicals over heterogeneous catalysts
Conversion of biomass into 5hydroxymethylfurfural
Conversion of food waste into value-added
chemicals
Biomass-derived activated carbons via
hydrothermal treatment
Biomass-derived activated carbons via
microwave-assisted method
Biomass-derived heterogeneous catalyst
for biodiesel production
Biomass-derived catalysts for chemical
synthesis, biofuel production, and
pollution control
Biomass-derived activated carbon for
electrodes of supercapacitors
Biomass-derived activated carbon for air
pollution control
Biomass-derived carbon nanofiber
Lignin-derived carbon for adsorption
[18]
Catalytic conversion of
biomass into
chemicals
Biomass-derived
carbon materials
Fig. 1. Conversion of biomass into valuable products by thermochemical technologies.
2
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
F. Shen et al.
Renewable and Sustainable Energy Reviews 130 (2020) 109944
such as twin-screw extrusion [63]. In the setup of twin-screw extrusion,
solid materials are mechanical ground together by two counter-rotatory
screws and the materials are transported along the extrusion path
through a barrel. Kilograms of products can be obtained with this
equipment, achieving large-scale production via mechanochemistry.
There are some typical critical reviews concerning mechanisms and
applications of mechanochemistry in different industries [52–54,56,57,
61,64–68]. However, as an emerging application in the biorefinery field,
the state-of-the-art and challenges of mechanochemistry technology on
the transformation of biomass into valuable chemicals, fuels, and carbon
materials have not been comprehensively reviewed. Therefore, this re­
view places specific focus on the pretreatment of biomass via ball
milling,
catalytic
transformation
of
biomass
by
mechanochemical-assisted methods, and synthesis of biomass-derived
carbon materials with ball milling technology, as illustrated in Scheme
1.
2. Mechanochemical pretreatment of biomass
Fig. 2. Illustration of mechanochemical reactions [56].
Among different kinds of biomass, lignocellulosic biomass is the most
abundant biomass on Earth, but its conversion into chemicals and fuels
is still challenging due to the robust structure. Therefore, pretreatment is
an essential step for efficient conversions. The aim of the pretreatment is
to break down and remove the lignin component of biomass, thus dis­
rupting the crystalline structure of cellulose and enhancing catalytic
accessibility to the cellulose and hemicellulose during hydrolysis [19]. A
large number of pretreatment approaches have been investigated in a
wide variety of biomass feedstock types, which were reviewed in the
investigations on the chemical effects of mechanical action [59]. He
found that mechanochemical reactions could give different products
from normal thermal heating. In 1891, Wilhelm Ostwald defined
“mechanochemistry” as one of the four chemistry disciplines along with
photochemistry, thermochemistry, and electrochemistry [52]. In 1894,
Gerard Heinicke defined the mechanochemistry as “a branch of chem­
istry concerned with chemical and physical changes of solids induced by
the action of mechanical influence” [60]. Most intensive developments
on mechanochemistry occurred in the past two decades, especially for its
applications in synthetic organic chemistry [56].
The equipment for mechanochemistry also changed from the original
manual mortar and pestle to automated mill devices (Fig. 3). Manual
grinding reactions by mortar and pestle are easily influenced by
different individuals and the environment [53]. Meanwhile, mechano­
chemical reactions are better controlled when performed with auto­
mated ball mills including vibration mills and planetary mills [52]. For
vibration mills, the reaction jar is oscillated from one side to another
causing the enclosed ball bearing to shear and grind the substrates
together. For the planetary mills, the reaction jar is spun at a high speed,
counter-rotatory to the main spinning ‘‘sun wheel”, which results in the
grinding of balls with solids [61]. In the process of energy-intensive ball
milling, the efficiency of energy transfer between the balls and materials
is adjusted by milling media, milling speed, ball-to-power weight ratio,
style of milling, milling atmosphere, running time, and the surface
topography of filler [62].
Nowadays, mechanochemistry is widely used in various fields
including inorganic materials synthesis, organic compounds synthesis,
metals recycling, crystal engineering, supramolecular aspects, etc.
Mechanochemical reaction is also developed for industrial applications
Scheme 1. Scope of the review.
Fig. 3. History of mechanochemistry.
3
F. Shen et al.
Renewable and Sustainable Energy Reviews 130 (2020) 109944
recent literature [43,69–73]. Dilute acids or alkalis are the most widely
used solvents to pretreat lignocellulosic biomass, but they would pro­
duce a large amount of wastewater for treatment and discharge. In
recent years, ball milling has emerged as an efficient and green method
to pretreat lignocellulosic biomass. Despite the high demand of energy,
the pretreatment using ball milling inducing mechanochemical re­
actions can greatly reduce the particle sizes of biomass via changing the
crystalline structures, decreasing the degree of polymerization of cel­
lulose, and distorting the chemical bonding. Ball milling technology can
be used individually or coupled with other techniques (such as micro­
wave and ozonolysis) to pretreat lignocellulosic biomass as summarized
in Fig. 4.
Table 2
Mechanical milling pretreatment on the product yields from different types of
biomass.
2.1. Mechanical milling pretreatment
2.1.1. Efficiency of mechanical milling pretreatment
Mechanical milling has been commonly used for the pretreatment of
biomass to enhance its conversion efficiently into value-added products
[74–92]. Table 2 summarizes the effect of ball milling pretreatment on
the conversion of lignocellulosic biomass into different chemicals and
fuels. It can be seen that ball milling pretreatment has great impacts on
the transformation of lignocellulosic biomass. With ball milling pre­
treatment, the product yields could be improved by 1.1–14.8 folds
compared with untreated samples. For example, Zakaria et al. used oil
palm biomass from empty fruit bunch to produce glucose and xylose via
enzymatic hydrolysis [87]. After dry ball milling for 60 min at 250 rpm
with a planetary ball mill, the empty fruit bunch was directly hydrolyzed
with Acremonium cellulase. The maximum yields of glucose and xylose
were 67.5% and 80.1%, respectively, which were much higher than that
from untreated empty fruit bunch (15.9% and 5.4%, respectively). Pang
et al. reported that ball milling pretreatment was efficient to promote
the catalytic conversion of lignocellulosic biomass to ethylene glycol.
After dry ball milling for 6 h using a planetary ball mill, the Miscanthus
was converted by a binary catalyst of tungstic acid and Ru/AC in water.
The maximum ethylene glycol yield from Miscanthus reached 52.4%,
which was much higher than that from untreated Miscanthus (22.5%)
[93].
Compared with other lignocellulosic biomass pretreatment tech­
niques, ball milling has many advantages such as easy-operation, and
avoidance of solvent and post treatments [90]. Zhang et al. compared
different biomass pretreatment methods including microwave-assisted
alkali, ultrasound-assisted alkali, and ball milling to pretreat the diges­
ted residue of rice straw for ethanol production [95]. They found that
the highest ethanol yield was obtained from digested residue after ball
milling pretreatment, because it substantially reduced the crystallinity
of biomass. Jiang et al. compared the effects of ionic liquid and ball
milling pretreatment on the glucose yield from cellulose [84]. The
glucose yield of untreated cellulose was only 20.9%, whereas both ball
milling and ionic liquid pretreated cellulose exhibited notably higher
Biomass
substrate
Product
Yield without
Pretreatment
Yield with BM
pretreatment
Reference
Sugarcane
bagasse
Rice straw
Cotton wool
Tissue paper
Douglas-fir
forest
residuals
Jatropha hulls
Cellulose
Oil palm empty
fruit bunch
Corn stover
glucose
~80 mg/g
338.6 mg/g
[74]
glucose
sorbitol
sorbitol
glucose
23.4%
66 mg/g
83 mg/g
~14%
89.4%
183 mg/g
338 mg/g
~60%
[77]
[79]
[79]
[80]
TRS
glucose
xylose
29.5%
20.9%
5.4%
35.4%
84.5%
80.1%
[82]
[84]
[87]
ethyl
levulinate
levulinic
acid
ethylene
glycol
ethanol
15.59%
20.46%
[91]
46.0%
52.2%
[94]
22.5%
52.4%
[93]
30.8%
116.65%
[95]
Cellulose
Miscanthus
Digested
residue of
rice straw
cellulose digestibility for efficient enzymatic hydrolysis. The yield from
ball milling pretreated cellulose (84.5%) was higher than that from ionic
liquid pretreated cellulose (78.0%). In addition to high efficiency, ball
milling pretreatment is environmentally friendly with simple opera­
tions. Ball milling is a physical process that eliminates the use of
potentially hazardous chemical reagents. The pretreated sample could
be used directly without washing and filtration steps. For example, it
was reported that conventional sodium hydroxide and aqueous
ammonia pretreatment processes showed a rice straw loss of 34.2% and
14.8%, respectively. Comparatively, no biomass loss was observed with
ball milling [90]. Besides, no by-products are formed after ball milling
pretreatment, whereas chemical pretreatment generated phenolic and
heterocyclic compounds that inhibited downstream fermentation [96].
2.1.2. Mechanisms of mechanical milling pretreatment
The high conversion efficiency of ball milling pretreated lignocel­
lulosic biomass could be attributed to the destruction of the rigid
structure of biomass via mechanical forces such as impact, compression,
shearing, and friction. After pretreatment, the cellulose and hemicellu­
lose are much more readily accessible to enzymes or chemical catalysts.
As shown in Fig. 5, the structure changes of lignocellulosic biomass due
to ball milling include the crystallinity index of cellulose [44,89], degree
of polymerization [83,97], surface area [86,91], thermal stability [98],
and particle size [76,87,89].
Fig. 4. Biomass pretreatment with sole mechanical milling or in the presence of assistants.
4
F. Shen et al.
Renewable and Sustainable Energy Reviews 130 (2020) 109944
Planetary ball mill was proved to modify the cellulose morphology,
structure, and properties [76]. The crystallinity index and the mean size
of the cellulose crystallite domains were calculated from Wide Angle
X-Ray Diffraction (WAXD) analysis, and both showed a significant
decrease associated with ball milling. For instance, the crystallinity
index decreased from 0.53 to 0.15 after ball milling for 60 min.
Process conditions such as substrate-to-ball ratio and milling time
have some effects on the pretreatment efficiency of biomass, while the
material of grinding balls may exert a greater influence [89]. It was
found that ball milling pretreatment reduced the size and crystallinity of
the corn stover from different balls made of alumina, zirconia, and steel.
Among them, steel balls were more effective for crushing biomass.
Alumina balls had the lowest specific gravity and showed higher texture
aspect ratio than the others. It seems that the balls with higher specific
gravity and texture aspect ratio are more appropriate for improving the
enzymatic hydrolysis of biomass. Therefore, alumina balls are preferable
options if the ball mill pretreatment is solely used, while steel balls are
the most suitable if the ball mill is combined with other treatments such
as chemical pretreatment to reduce milling time [89].
2.2. Mechanochemical-assisted pretreatment of biomass
2.2.1. Mechanochemical-assisted acid/alkaline pretreatment
Mechanical milling pretreatment usually requires a long time and
great amount of energy to reduce the crystalline degree of cellulose, and
it marginally reduces the molecular mass of biomass components. In
order to improve the efficiency of mechanical milling and reduce energy
consumption, other biomass pretreatment methods are used in combi­
nation. Acids (H2SO4, HCl, H3PO4, oxalic acid, etc.) and alkalis (NaOH)
are commonly used auxiliary reagents during ball milling process
[99–106]. For instance, Eucalyptus wood was mixed with different
concentrations of diluted acid (HCl or CH3COOH) at room temperature
for 20 h. After moisture removal, the biomass was treated with a plan­
etary ball mill at 450 rpm for different durations. The pretreated
biomass was then enzymatically hydrolyzed with Novozymes [106].
Fig. 6 shows that ball milling pretreatment has a great influence on the
crystallinity index (CrI) of Eucalyptus wood. Sole ball milling for 2 h
reduced the CrI from 34.60% to 16.56%. The presence of HCl (0.5%)
during ball milling has a negligible effect. However, the addition of 2.5%
acetic acid during ball milling promoted the decrease of CrI (12.89%)
and facilitated the decrease of degree of polymerization compared to
sole ball milling pretreatment (37 vs 129.9) [106].
In addition to mineral acids, organic acids such as oxalic acid, maleic
acid, malonic acid, benzoic acid, and salicylic acid are also used to assist
ball milling pretreatment of biomass [99,107]. Among them, oxalic
acid-assisted ball milling pretreatment showed the best performance.
Impregnation procedure could be eliminated by employing oxalic acid
in a solid-state during ball milling pretreatment of biomass [99].
Compared with liquid acid, solid acid is milder, thereby reducing the
undesired side-reactions during ball milling process. The oxalic acid
could also be used as in situ catalyst to convert biomass into chemicals.
Dicarboxylic organic acids such as oxalic acid could hydrolyze β-(1,
4)-bonds of cellulose more selectively than liquid sulfuric acid [108],
Fig. 6. Effect of acid-assisted ball milling on the crystallinity indexes of Euca­
lyptus wood. (“BM” represents sole ball milling, “A1” represents concentration
of CH3COOH (1.0%, v/v), “H0.5” represents concentration of HCl (0.5%, v/v),
and “2 h” represents ball milling time) [106].
which can be attributed to their characteristics of two pKa values [109].
Liquid alkaline can also be used as an additive during ball milling
pretreatment of lignocellulosic biomass, which could be more effective
than liquid acid [103]. Lin et al. evaluated the efficiency of different
acid- and alkaline-assisted ball milling pretreatments of corn stover. As
shown in Fig. 7, the sugar yields from corn stover pretreated by ball
milling in the presence of different alkalis (NaOH, KOH, Ca(OH)2,
NH4OH) were all higher than those with different acids (HNO3, HCl,
H2SO4, H3PO4, CH3COOH). This can be explained by the fact that alkalis
are much more efficient than acid in removing lignin from lignocellu­
losic biomass, thus improving the reactivity of the exposed cellulose and
hemicellulose with catalysts [110].
2.2.2. Mechanochemical-assisted metal salt pretreatment
Metal salts have been used for the pretreatment of biomass by
Fig. 5. Structure changes of lignocellulosic biomass caused by mechanical milling pretreatment.
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Renewable and Sustainable Energy Reviews 130 (2020) 109944
conversion of microcrystalline cellulose compared to ball milling alone
for 6 h [116]. More importantly, the combination of microwave irradi­
ation and ball milling reduced 77.4% energy consumption.
Hot compressed water [117], ozonolysis [118], and fungal treatment
[119] are also employed to assist ball milling pretreatment of biomass.
Similarly, the purpose of these auxiliary methods is to reduce ball
milling time and save energy. Hot compressed water could act as acids at
approximately 200 � C, and dissolve most of hemicellulose and part of
lignin from lignocellulosic biomass [120], thus facilitating the ball
milling process and reducing the milling time. For example, eucalyptus
biomass pretreated with hot compressed water at 160 � C for 30 min
followed by ball milling for 20 min exhibited an approximately 70%
yield of total reducing sugars, which was comparable to the yields from
eucalyptus pretreated with hot compressed water at a higher tempt­
erature (200 � C, 30 min) or ball milling for a longer time (40 min) [117].
Ozonolysis is another useful method, in which the strong oxidizing
property of ozone could effectively remove lignin from biomass [121].
Pretreatment of biomass combining ozonolysis and ball milling could
significantly improve the efficiency of enzymatic hydrolysis than solely
using either one [118].
Fig. 7. Acid or alkaline-assisted ball milling pretreatment of corn stover for
sugar production [103].
reducing the crystallinity of cellulose for the enhancement of biomass
valorization [111,112]. Metal salts are less corrosive compared to
mineral acid or alkaline. Zhang et al. studied the effects of fifteen
common metal salts on the ball milling pretreatment of sugarcane
bagasse at 500 rpm for 60 min [113,114]. The resulting mixture was
dried without the removal of metal salts. Afterwards the pretreated
sugarcane bagasse was hydrolyzed by enzymes for 48 h at 50 � C. As
shown in Fig. 8, the saccharification yields increased from 10.3% to
34.8% after ball milling pretreatment, while the addition of metal salts
further gave significant enhancement of yields. Among them, the per­
formances of AlCl3, FeCl3, Al(NO3)3, and Fe(NO3)3 in pretreating
biomass were particularly outstanding. This is because that NO3 and Cl
anions had a higher possibility to combine with the hydrogen of hy­
droxyl groups in cellulose molecules [115]. Instantaneous high tem­
perature induced by ball milling process could cause the melting of
metal salt, thus increasing its fluidity and permeability in lignocellulosic
biomass. As a result, the metal salts with lower melting point tempera­
ture showed better synergistic combination with ball milling
pretreatment.
3. Mechanochemical conversion of biomass into chemicals
3.1. Mechanical milling with solid acid catalysts
Solid catalysts play an important role in green synthesis of chemicals
due to its advantages of recyclability and mildness [17,122,123].
However, solid acids showed unsatisfactory performance for robust
biomass conversion since they are both insoluble in water. Therefore, in
practice, most catalytic conversions of biomass make use of solvents
such as ionic liquid to overcome the solid-solid mass transport among
solid acid catalysts [124,125]. By using mechanical milling, it is possible
to overcome the diffusion barrier in the absence of solvent [126]. With
the aid of high-energy mechanical forces, solid catalysts present better
interactions with substrates, thus ensuring efficient cleavage of glyco­
sidic bonds. In the first report on mechanocatalytic decomposition of
biomass with solid catalyst under the solvent-free condition, up to 84%
of cellulose can be hydrolyzed into water-soluble sugars by ball milling
with kaolinite catalyst [126]. After that, mechanochemical conversion
of biomass in the presence of solid acids has been extensively employed
to produce various chemicals including sorbitol [97], glucose [127,
128], xylose [129], levogluosan [130], and N-acetyl-D-glucosamine
[131]. Ball milling of biomass together with catalysts showed much
better performance than catalytic conversion of sole ball milling pre­
treated biomass. For instance, the sorbitol selectivity from catalytic
conversion of ball milled cellulose via Ru/AC catalyst was 50%. How­
ever, the selectivity increased to 80% when cellulose was ball milled
together with the catalyst of Ru/AC [97]. The rate-determining step of
hydrolytic hydrogenation is the hydrolysis of cellulose into glucose. Ball
milling can improve the contact and interactions between catalyst and
cellulose, and meanwhile promote the hydrolysis of cellulose into
glucose [132]. The closed mixed state of solid catalyst and cellulose can
continue in the subsequent hydrolytic hydrogenation because of the
insoluble properties of the catalyst and the substrate. In this way, higher
substrate conversion and product yield could be obtained.
Interestingly, different milling modes showed a great influence on
the product formation [130]. As shown in Fig. 9, glucose was the main
product when cellulose was ball milled with solid acid in the rolling
mode. However, the shaking mode produced levogluosan as the main
product under the same condition. This phenomenon indicates that
there is an optimal energetic region for the production of glucose. The
energy input required for this technology closely depends on the type of
equipment used. Most of the reported studies of mechanocatalytic
depolymerization of biomass used rolling milling equipment such as
planet ball mill.
Qiu et al. prepared a –SO3H functionalized solid acid by the co-
2.2.3. Other combinations
The combination of microwave irradiation and ball milling is
considered to be an energy-saving and highly efficient pretreatment
technology. For example, it was found that ball milling for 1 h followed
by 20 min of microwave irradiation achieved higher efficiency of
Fig. 8. Saccharification yields of untreated and ball milling pretreated sugar­
cane bagasse in the presence of different metal salts [113].
6
F. Shen et al.
Renewable and Sustainable Energy Reviews 130 (2020) 109944
cellulose. The excellent physical contact between the biomass and solid
catalysts driven by ball milling is crucial. Further study revealed that
mixing ball milled cellulose with weakly acidic carbonaceous catalyst in
0.015 wt% HCl aqueous solution gave an increased glucose yield of 74%,
compared with 59.3% glucose yield without liquid acid [127]. Similarly,
a carbon-based catalyst bearing only –COOH and –OH groups was pre­
pared by pyrolysis of black liquor from rice straw. High yield of glucose
and xylose (52.1% and 66.5%, respectively) could be obtained from rice
straw by ball milling of substrate and weakly acidic solid catalyst fol­
lowed by 0.015 wt% HCl solution at 200 � C for 60 min [136].
Therefore, there is a synergism between dilute acid and weakly
acidic solid. Cellulose was firstly hydrolyzed into soluble oligosaccha­
rides by the weakly acidic carbon due to good contact between the
catalyst and the cellulose via mix-milling, and the subsequently formed
oligosaccharides were hydrolyzed to glucose by the mineral acid in
aqueous solution [127].
Compared to strongly acidic solid catalysts, weakly acidic solids are
easy to synthesize and are friendly to equipment and the environment.
Besides, weakly acidic sites such as –COOH and –OH are much more
stable under the hydrothermal reaction conditions compared to strongly
acidic groups such as –SO3H [137]. Large-scale milling process has
already been available in food industries, thus showing promising
applicability of weakly acidic solids assisted ball milling method in the
biorefinery industry.
While many solid acids such as carbonaceous materials [127] and
metal oxides [138] have been used in the mechanocatalytic conversion
of biomass, solid acids with layered structures are particularly suitable
for the mechanocatalytic processes [126]. For example, natural clay
kaolinite,
which
is
an
aluminosilicate
consisting
of
aluminum-containing (as AlO6 units) layers covalently bound to
silicon-containing (as SiO4 units) layers in a ratio of 1:1, showed the best
catalytic activity for cellulose hydrolysis by mechanocatalytic processes.
This is because the layered structures can be easily broken by ball
milling, enhancing the specific surface area and exposure of the active
sites [139]. The surface area of active sites is crucial during mechano­
catalytic processes [140]. Takagaki et al. reported that up to 72% yield
of water-soluble sugars could be obtained from cellulose by ball milling
in the presence of HNbMoO6, which was another layered metal oxide
with strong acidity [138].
Fig. 9. Effect of milling mode on the product formation by mechanocatalytic
depolymerization of cellulose [130].
carbonization of sucralose and p-toluenesulfonic acid and used it for the
conversion of cellulose into glucose in pure water (Fig. 10). A 35.6%
yield of glucose was obtained when the cellulose was pretreated by sole
ball milling and then hydrolyzed with the as-prepared solid acid catalyst
in water. In comparison, when the cellulose was ball milled together
with the –SO3H functionalized solid acid and then hydrolyzed in
aqueous solution at the same condition, a maximum glucose yield of
52.8% could be obtained [133]. The excellent catalytic activity for the
production of glucose from cellulose was ascribed to the decreased de­
gree of crystallinity of the cellulose and the improved contact between
the active sites and the β-1,4-glycosidic bonds in the cellulose during ball
milling of cellulose and solid acid.
Although the above strong acid (-SO3H) modified solid catalysts
were widely used in biomass valorization, the preparation process was
complex and environmentally unfriendly. Weakly acidic solid catalyst
bearing –COOH and –OH groups could be synthesized and showed po­
tential application in biomass conversion [134,135]. For example,
Fukuoka et al. developed a weakly acidic solid catalyst based
mechanochemical-assisted method for biomass valorization in aqueous
solution [127–129,136]. As shown in Fig. 11 [132], cellulose was first
ball milled together with weakly acidic solids (-OH or –COOH functional
carbons) in this method. After adding a trace amount of HCl (<0.012 wt
%), an extraordinary yield of glucose (>88%) was obtained from
3.2. Mechanical milling with solid alkali catalysts
Mechanically ball milling enables a new way to induce the solid-solid
reactions. This method also applies to solvent-free production of
chemicals from biomass by ball milling with alkali catalysts. As shown in
Fig. 10. Hydrolysis of cellulose into glucose by mechanochemical process [133].
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Fig. 11. Mechanochemical-assisted hydrolysis of cellulose into glucose [132].
Fig. 12, a yield of 35.6% lactic acid can be produced from biomassderived glucose by ball milling together with solid Ba(OH)2 without
further reactions [141]. In this method, the reaction time was much
shortened compared with that of lactic acid formation from glucose
using aqueous Ba(OH)2 solution.
Lignin is one of the most promising renewable sources for the pro­
duction of aromatic chemicals [142], which accounted for 15–30% of
dry biomass [143]. However, lignin has been typically underused as a
waste stream in the current biorefinery industry because its conversion
is difficult due to the presence of three-dimensional aromatic rings. The
poor solubility of lignin in common solvents limits its conversion in
solution. Solvent-free mechanochemistry provides a new means to
overcome this [144,145]. Bolm et al. proved that phenol could be pro­
duced from lignin by base-assisted ball milling (Fig. 13) [144]. It was
shown from NMR spectroscopy that lignin was degraded in the same
manner with those of solvent-based procedures, but the ball milling
process achieved metal- and solvent-free conditions with inexpensive
and readily available base catalysts such as solid NaOH.
Chitosan has a great potential for biomedical and pharmaceutical
applications [146–148]. It can be produced from crustacean shells,
skeletons of insects, fungi, etc. [149]. Traditionally, chitosan with a low
molecular weight (LMWC) is produced by solvent-based multi-step
methods along with a large amount of concentrated and corrosive basic
solutions [150]. Enzymes can also be used to produce LMWC [151,152],
but the cost is high and the reaction time is long. Ball milling was pro­
posed to produce LMWC from chitin and shrimp shells [153]. High yield
of LMWC with purity of ca. 90% could be obtained in a single step by ball
milling of chitin or crude shrimp shell in presence of solid NaOH
(Fig. 14). Compared with traditional base solution catalytic approach,
base-catalyzed mechanochemical transformation of biomass showed
higher efficiency and mitigated adverse environmental impacts. Base
usage was reduced to about 1/10, and the range of molecular weights
Fig. 13. Mechanochemical production of phenol from lignin with solid base
catalyst [144].
was narrowed among the obtained LMWC products.
3.3. Mechanical milling of liquid acid-impregnated biomass
Solid catalysts can hardly achieve complete conversion of biomass
due to their relatively low reactivity even with the assistance of me­
chanical ball milling. However, impregnation of biomass with liquid
acid followed by ball milling of the acidic biomass could achieve nearly
full conversion of biomass into ‘water-soluble lignocellulose’ [49,
154–156]. Fig. 15 shows a typical schematic representation of mecha­
nocatalytic conversion of acid-impregnated biomass proposed by
Rinaldi et al. [154]. This strategy contains three key steps: (1) impreg­
nation of the substrate with liquid acid; (2) solvent removal; and (3)
mechanocatalytic depolymerization of acid-impregnated biomass
through ball milling process. This method, for example, achieved more
than 90% conversion of pine sawdust into water-soluble sugars [155].
An alternative procedure to the liquid acid-impregnation was to
expose the biomass to gaseous HCl. The gas-phase procedure eliminates
the need for solvent removal and recycling, thus improving the energy
efficiency of the process [157].
In cellulose hydrolysis by ball milling, the effects of impregnation
using different acids can be diverse [156]. It was found that acids with
low strength (pKa >0) (e.g., acetic acid, oxalic acid, and phosphoric
acid) showed little effect on the solubility of cellulose, while acids with
high strength (pKa <0) (e.g., HCl, HNO3, and H2SO4) significantly
improved the solubility of cellulose after ball milling. H2SO4 was the
most efficient acid during ball milling process followed by HNO3. This
can be attributed to the high retention of H2SO4 on surface of cellulose
after drying. A higher concentration of acid and a longer impregnation
time also facilitate hydrolysis of cellulose by ball milling [49].
Ball milling of acid-impregnated cellulose can produce oligomers
with high solubility due to the acid-catalyzed formation of new glyco­
sidic linkages [158]. The newly formed glucan oligomers are α(1 → 6)
glycosidically linked branches, which are absent from the original cel­
lulose [156]. Acid plays an important role in the synthesis of these newly
formed α(1 → 6) glucan oligomers during ball milling process [159].
Addition of glucose during ball milling treatment of acid-impregnated
cellulose would greatly reduce the milling time compared with adding
acid alone [158], which can be attributed to the quick formation of α(1
→ 6) branches in the presence of glucose.
Fig. 12. Mechanochemical production of lactic acid from biomass-derived
glucose with solid Ba(OH)2 [141].
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Renewable and Sustainable Energy Reviews 130 (2020) 109944
Fig. 14. Mechanochemical production of phenol from chitin with solid base catalyst [153].
Fig. 15. Mechanochemical depolymerization of liquid acid impregnated biomass [46].
The water-soluble oligomers obtained by the ball milling of acidimpregnated cellulose could be easily converted into value-added
chemicals [158,160,161]. Fig. 16 shows a typical procedure for
glucose formation from cellulose by using the mechanochemical
method. Cellulose was firstly impregnated with H2SO4 solution, high
yield of water-soluble oligomers could be obtained from ball milling of
the acidic cellulose. After adding the solid catalyst (e.g. sulfonated
carbons) in the reaction medium, the oligomers could be selectively
hydrolyzed into glucose in aqueous solution with yield up to 91.2%
[158].
5-hydroxymethylfurfural (5-HMF), one of the most important plat­
form chemicals, can be produced from biomss with high yields using this
method. Water-soluble oligosaccharides obtained from mechanical
depolymerization of acidic biomass could produce HMF with a high
yield [161]. As shown in Fig. 17, the solid biomass was firstly
impregnated with diluted HCl solution, followed by the production of
water-soluble oligosaccharides during ball milling process. Subse­
quently, 4-propylguaiacol and NaCl were added to the reaction medium
to form biphasci solvent system, achieving the maximum yield of 5-HMF
at 79% when using AlCl3 as catalyst.
3.4. Mechanical milling with metal salts
It has been proven that metal salts have positive effect on ball milling
pretreatment of biomass [113,114]. Some metal salts (e.g., AlCl3,
Al2(SO4)3, LiBr) are widely used for catalytic transformtion of biomass
into chemicals [162–164]. This leads to a synergistic effect on the
biomass conversion via ball milling. For example, a metal salt-assisted
mechanochemical method for the production of 5-HMF from cellulose
was developed. The metal salt (Al2(SO4)3) not only acted as catalyst for
Fig. 16. Mechanochemical production of glucose by mechanical milling of acid-impregnated cellulose [158].
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Fig. 17. Mechanochemical production of 5-HMF by mechanical milling of acid-impregnated biomass [161].
its Lewis and Brønsted acidity, but also assisted the disruption of cellu­
lose during ball milling pretreatment (Fig. 18). In this way, high yield of
5-HMF could be obtained from cellulose [165]. Similarly, AlCl3 was used
for the mechanochemical production of fructooligosaccharide from
inulin [166], which produced fructan-type polysaccharides via ball
milling strategy. It was found that AlCl3-assissed ball milling increased
the overall contents of mono-, di-, and tri-saccharides by nearly 30-fold
compared to AlCl3-free milling process.
In summary, mechanochemical-assisted method is an efficient way
to improve the product yields and reduce the reaction time of biomass
conversion compared to solution-based catalytic method. Some typical
examples of the improvement of product yields from biomass via
mechanochemical-assisted method are summarized in Table 3.
Table 3
Comparison of product yields from biomass in the presence or absence of BMassistance.
4. Mechanochemical synthesis of biomass-derived carbons
With carbon content estimated at 45–50 wt% [168], biomass is
regarded as a promising and sustainable precursor to prepare advanced
carbonaceous materials. Carbon materials have been widely used in
many fields such as catalysts, adsorbents, and energy storage. Synthesis
of carbon materials involves various methods including pyrolysis [169],
hydrothermal carbonization [170], ionothermal carbonization [171],
and molten salt carbonization [172]. In order to enrich the functionality
of carbonaceous materials or to improve the preparation efficiency,
microwave and ball milling are also employed during the above syn­
thetic process. Ball milling methods for preparing biomass-derived
Biomass
substrate
Product
Catalyst
Performance
without BMassistance
Yield with
BMassistance
Reference
cellulose
cellulose
cellulose
rice
straw
Rice
straw
cellulose
glucose
sorbitol
hexitols
xylose
biochar
Ru/AC
H2SO4
biochar
28.9%
49.4%
44.7%
41.1
52.8%
68.0%
87.6%
61.5%
[127]
[97]
[160]
[136]
glucose
biochar
3.7%
19.4%
[136]
glucose
35.6%
52%
[133]
cellulose
cellulose
HMF
glucose
39.8%
36%
44.6%
57%
[165]
[132]
HMF
DHMF
sulfonated
carbon
Al2(SO4)
activated
carbon
NaOH
67% (24 h)
64 (5 min)
[167]
carbon materials are illustrated in Fig. 19 and discussed in detail in the
following sections.
4.1. Ordered mesoporous carbons
Since the first synthesis in 1999 by Ryoo and Hyeon et al. [173,174],
ordered mesoporous carbons (OMCs) have been widely used in various
Fig. 18. Al2(SO4)3-assisted mechanochemical production of 5-HMF from cellulose [165].
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self-assembly process of biomass-derived polyphenol (tannin) with
Pluronic®F127 was accomplished by mechanical energy including
impact, compression, shearing, and friction provided by ball milling,
thus avoiding the use of acid or base catalysts. The typical process of
mechanochemical synthesis of OMCs is shown in Fig. 20. Firstly, the
template of F127 and carbon precursor of tannin are ball milled to form
a gel of micelle. Then, metal salts such as Zn(OAc)2, Ni(OAc)2, Mg
(OAc)2, Ca(OAc)2 and Co(OAc)2, are added as cross-linker to promote
the crosslinking of the micelles. OMCs are obtained after removing the
soft template and carbonizing the ball-milled mixture by thermal
treatment in inert gas. In the process, metal nanoparticles could also be
in situ reduced and preserved inside the pores by controlling the
carbonization temperature [180]. The metal-doped OMCs can be
directly used as catalysts, adsorbents, or electrodes of supercapacitor. As
shown in Fig. 21, the pyrolysis of Tannin-F127-Zn(OAc)2 composite
displayed a type IV isotherm and H1-type hysteresis loop with narrowed
pores size distribution, which verified the presence of mesoporous
structure. However, the carbon derived from Tannin-F127 or tannin-Zn
(OAc)2 were in a disordered structure with a broad range of pore size. In
this respect, F127 interacted with Tannin through hydrogen bonds to
form an ordered mesophase, and the metal ions crosslinked with the
micelle units around them. Therefore, both F127 and metal salts are
essential for the designed assembly of mesoporous structure from
biomass via ball milling.
Metal salt is necessary for ball milling synthesis of OMCs [182,183].
As shown in Fig. 22, the Mimosa tannin and F127 was used as carbon
precursor and soft template, respectively. A small amount of water was
employed to promote the micelle formation between tannin and F127,
while no cross-linker such as metal salt was used. The results showed
that F127 determined the success of forming mesoporous structure,
whereas water was essential to form micelles by enhancing the inter­
action between tannin and F127. Typical mesoporous structure of the
synthesized OMCs is shown in Fig. 23. The resultant OMC had a perfect
ordered mesoporous structure and high stability up to 1500 � C, thus
showing a great potential in applications such as oil spills remediation,
CO2 adsorption, and supercapacitors.
There are further applications of OMCs after activation; OMCs can be
activated with an increased surface area and become an ideal material as
adsorbent or electrode of supercapacitors. For instance, OMCs synthe­
sized by water-assisted ball milling was found to have their surface area
Fig. 19. Representation of biomass-derived carbon materials prepared via
mechanical milling process for different applications.
fields such as adsorption, catalysis, separation, sensors, and energy
storage due to their intriguing structure of uniform and tunable pore
channels, high surface area, large pore volume, high stability, etc. [175,
176]. The fundamentals and synthesis methods of OMCs have been well
summarized [177]. In general, OMCs are synthesized by either hard or
soft template methods using poisonous regents such as HF, concentrated
NaOH, or formaldehyde. These wet processing methods require a large
amount of solvent and restrict the widespread application of OMCs. The
primary self-assembly process for soft-templating synthesis of OMCs is
also induced by the acid or base catalyst in solvent. To overcome these
drawbacks, a simple solid-state synthesis of biomass-derived OMCs
using ball milling strategy was developed [178–181]. In this method,
Fig. 20. Schematic scheme of mechanical milling synthesis of OMCs in the presence of metal ions [180].
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OMCs can be synthesized by mechanical milling with hard templates
[184–186]. The typical process is illustrated in Fig. 25. The seed fats
derived from soybean, colza, and peanut were used as renewable and
low-cost carbon precursors while ordered mesoporous silica SBA-15 was
used as a hard template. The seed fats and SBA-15 are ground together
with FeCl3⋅6H2O by ball milling to form homogeneous mixtures, which
was subsequently carbonized under inert gas atmosphere. The hard
template of SBA-15 was then removed with NaOH solution. The FeCl3
could be reduced in situ during the high temperature carbonization
process and was embedded in the channels of OMCs endowing magnetic
properties. This is useful for recycling the OMCs after usage.
4.2. Hierarchically porous carbons
In addition to OMCs, mechanical milling has been used to prepare
other types of biomass-derived carbons. For example, hierarchically
porous carbons (HPCs) are synthesized from biomass by ball milling
with activating agents such as KHCO3, K2CO3, and ZnCl2 [187–189].
Biomass (e.g., fallen flowers [187], myriophyllum aquaticum [189],
lignin [188], rice straw [190], dairy manure [169]) and activating
agents are ball milled to form uniform mixtures, which undergo
carbonization to form HPCs. In this process, ball milling plays a vital role
in the formation of hierarchical structure. Subjected to the mechanical
forces of ball milling, crystals of activating agents are embedded into the
framework of biomass, leading to the formation of macropores after
crystals removal. During the carbonization process, activating agents
such as KHCO3 decompose into CO2 and H2O vapor. These gases corrode
the biomass at a high temperature and thus micropores and mesopores
are formed.
Another strategy was developed to prepare biomass-derived HPCs
Fig. 21. Nitrogen adsorption-desorption isotherms (77 K) of control samples
and the pore size distributions [180].
significantly increased from ~500 to ~2000 m2/g after CO2 activation
without destructing the ordered mesoporous structure (Fig. 24). As an
electrode of supercapacitor, OMCs show high-rate capability and longterm stability in both aqueous and organic electrolytes [183], which
can be attributed to their large surface area and ordered porous
structure.
In addition to the above-mentioned soft template-based method,
Fig. 22. Schematic of water-assisted mechanochemical synthesis of OMCs [182].
Fig. 23. Typical TEM images of OMCs synthesized by water-assisted mechanical milling [182].
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Renewable and Sustainable Energy Reviews 130 (2020) 109944
Fig. 24. Schematic water-assisted mechanical milling synthesis of OMCs with large surface area [183].
Fig. 25. Illustration of the synthesis of magnetic OMCs by mechanical milling [186].
using mechanical milling with the assistance of CaCO3 or eggshell [169,
190]. As shown in Fig. 26, waste dairy manure and eggshell were
employed as carbon precursor and hard templates. They were pretreated
via ball milling at 350 rpm for 30 min. Afterwards, the mixtures were
pyrolyzed at a high temperature (>800 � C), and part of CaCO3 decom­
posed into CO2 which acted as an activation agent to produce micro­
pores in the carbon. The subsequent removal of residual CaCO3 and
newly formed CaO by HCl solution led to the formation of mesopores. In
this way, carbon materials with hierarchical pores structure could be
obtained, which showed great potentials to be used as electrodes of
supercapacitor [169] or adsorbent for mercury removal [190].
This method can be modified by employing nitrogen-rich precursors
or adding nitrogen-containing reagents for the production of nitrogendoped HPCs [187,188,191]. Typical synthesis of nitrogen-doped HPCs
by ball milling is shown in Fig. 27. Such one-pot synthesis process of
nitrogen-doped HPCs is efficient and solvent-free, making it as an
environmentally friendly and sustainable approach to produce elec­
trodes of supercapacitor for energy storage [192,193].
Ball milling treatment of biomass (e.g., agricultural by-products) also
has significant effects on the surface area of the produced carbon ma­
terials [194]. As shown in Fig. 28, direct carbonization of raw biomass
usually results in a low surface area (~5 m2/g), while ball milling
treatment of biomass followed by carbonization provides carbon mate­
rials with total surface area as large as 1771 m2/g. The increase in
surface area could be attributed to the mechanochemical process that
breaks the monolithic biomass into fine powders (~2.2 μm) with bulk
defects. The tiny microparticles facilitate the release of volatile com­
pounds formed during carbonization and hence increase the surface
Fig. 26. Synthesis of HPCs from dairy manure and eggshell by mechanical milling method [169].
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Renewable and Sustainable Energy Reviews 130 (2020) 109944
area. This can effectively improve the properties of carbon materials and
avoid using the usage of chemical activating agents. The mechano­
chemical strategy gives a new direction for synthesis of biomass-derived
porous carbons for future studies.
4.3. Biomass-derived carbon/metal composites
Biomass-derived carbons have been widely used as the supports for
metal catalysts due to their low cost and sustainability [195]. In general,
carbon-supported metal catalysts are synthesized by wet impregnation
method [17,196,197], which requires a large amount of time and sol­
vents. Mechanochemical ball milling provides a green and efficient
route for the preparation of carbon/metal composites in solvent-free
conditions. There are two common ways to prepare carbon/metal
composites via mechanical milling method, as shown in Fig. 29.
In the first route, the biomass was pyrolyzed at a high temperature,
Fig. 27. Schematic scheme of mechanical milling synthesis of HPCs [188].
Fig. 28. Synthesis of carbon materials by carbonization of agricultural wastes with/without mechanical milling treatment [194].
Fig. 29. Two representative methods for mechanochemical synthesis of carbon/metal composites via two routes.
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Renewable and Sustainable Energy Reviews 130 (2020) 109944
followed by ball milling with metal oxides (e.g., Fe3O4 [198]) to obtain
carbon/metal composites. In the other route, the biomass was ball
milled with metal precursors (e.g. FeCl2 [199]), afterwards the mixture
of biomass and metal precursors was pyrolyzed at a high temperature
producing the carbon/metal composites. Both methods are solvent-free
and time-efficient. Typical examples of carbon/metal composites pre­
pared by ball milling method and their applications are listed in Table 4.
In addition, the ball milling technology can also be used to prepare
other materials from biomass such as nanofibers and carbon quantum
dots [206–208], and can increase the surface area and
oxygen-containing functional groups of biomass-derived biochars
[209–212]. We believe that ball milling as a green and efficient tool will
open more doors to conversion of biomass into new valuable materials.
methods such as aerobic and anaerobic pretreatment are still at lab
scale, and their TRL are 4–6 [214]. Steam explosion has been widely
used in the biomass pretreatment due to their features of high efficiency
and low environmental risk [215]. The TRL of steam explosion also
reaches the value of 6–8. Although acid and alkaline are widely used in
biomass pretreatment in pilot and commercial scale, they suffer from the
problems of inhibitors/by-products formation, corrosion to equipment,
and high environmental risks. Their TRL are reported to be 5–7. Me­
chanical milling, as discussed in this review, has many superior ad­
vantages such as high efficiency, absence of inhibitors production, and
low environmental risks. It is evaluated to have a TRL of 5–6 in
consideration of the barrier of high energy consumption [216].
Although mechanical milling has been used for biomass pretreatment in
pilot scale, its applications for downstream conversion of biomass to
chemicals and carbon materials are still at lab scale, and no data of
industrialized applications are currently available. Thus, TRL levels are
estimated to be 2–3.
5. Technology assessment
There are many advantages of mechanochemical process over
solution-based wet process. Mechanochemical processing not only cir­
cumvents the requirement of reagents solubility and large consumption
of solvent, but also avoids complex post-treatment steps such as solvent
removal and product purification. Most of the mechanochemical re­
actions are rapid within minutes to hours (Fig. 30). For example,
polysaccharides-supported Nb2O5 can be synthesized via ball milling
within 30 min [201], which requires several hours by wet impregnation
method. Another advantage of mechanochemistry is its operation at
room temperature. Therefore, from the perspectives of environment and
economics, mechanochemistry is a green and labor- and time-saving
approach for the synthesis of chemicals and materials from biomass.
Technology readiness level (TRL) is a widely used tool to evaluate
the maturity of a technology or concept on a scale of 1–9 from basic
principles to commercialization. TRL 1–3 are identified as lab scale, TRL
4–6 as pilot scale, and 7–9 as commercial scale [213]. Biomass valori­
zation is a complicated process involving multiple steps with different
technologies. Pretreatment is a crucial step to all value chains from
biomass, which is recognized as one of the most expensive processing
steps for biomass valorization. Therefore, TRL of mechanical milling
reviewed in this article together with other technologies for biomass
pretreatment are discussed as a typical case, and the results are shown in
Table 5. Enzyme pretreatment of biomass is a mature technology used in
many industries. However, high cost of enzyme limits its widespread
full-scale application, and TRL are identified as 7–8. Other biological
6. Conclusions and outlook
Renewable biomass can substitute the exhausted fossil resources to
produce chemicals and materials with green technology, the efficiency
and environmental sustainability of biomass conversion, however,
remain as a big challenge. Mechanical milling, as a prominent approach
of mechanochemical processes, is one of the most environmentally
friendly instruments with advantages of high efficiency, simplicity, high
reproducibility, and mild reaction conditions. This work provides a
critical review of state-of-the-art knowledge on the milling technology
for biomass valorization, including initial pretreatment, catalytic con­
version process, and design of biomass-derived materials.
Mechanical milling can be used to pretreat lignocellulosic biomass,
and the performance is comparable with traditional acid/base pre­
treatment method, while it offers the advantages of easy operation and
solvent-free conditions. Ball milling pretreatment allows for a high ef­
ficiency of lignocellulosic biomass conversion, due to its effects on the
crystallinity index, degree of polymerization, surface area, thermal
stability, and particle size of the biomass. Dilute acid, microwave, liquid
nitrogen, etc., can further reduce the energy consumption of ball milling
process.
The mechanical energy in the ball milling process can induce
chemical reactions of biomass. The mechanochemical milling-assisted
depolymerization of biomass is demonstrated to be a highly attractive
entry-point method for biorefinery processes. This strategy is especially
suitable for solid catalysts and overcomes the solid-solid mass transfer
barrier. Solid acids with layered structures give excellent performance in
the mechanocatalytic depolymerization of cellulose due to its exposure
of larger surface areas. Weakly acidic solid catalysts (-COOH, –OH)
could effectively convert waste biomass into chemicals through ball
milling, thus avoiding the use of corrosive concentrated H2SO4. It is also
found that parameters such as ball milling time, rotation speed, ball mill
beads, milling mode, etc., have significant influences on the product
formation and selectivity.
Mechanochemistry also paves a new path to design and synthesize
biomass-derived materials (especially ordered mesoporous carbons, hi­
erarchically porous carbons, and metal/carbon composites) with pre­
cursors in solid state. Compared with traditional wet processing,
mechanical milling method requires much shorter time and avoids the
use of toxic reagents and complicated procedures. ball milling also
shows an excellent ability for simple, green, one-pot mechanosynthesis
of carbon materials with active sites such as heteroatoms and metal el­
ements. Therefore, ball milling-assisted synthesis can be widely used in
energy storage, adsorbents, and catalysts.
Despite the remarkable potential of mechanochemistry in biomass
valorization, the high consumption of energy in mechanochemical
processes poses an obstacle to its large-scale production. Combining
mechanical milling with other technologies to reduce energy
Table 4
Carbon/metal composites synthesized by mechanical milling from different
biomass.
Biomass
precursor
Metal
precursor
Ball
milling
conditions
Application
Reference
Humins
FeCl2 or Fe
(NO3)3
350 rpm,
45 min
[199]
Rice straw
Eggshell
(CaCO3)
Ammonium
niobate (V)
oxalate
HAuCl4,
PdCl2 RuCl3,
Re(CO)5Br
Fe, α-Fe2O3
or Fe3O4
30 min
Oxidation of
isoeugenol to
vanillin
Adsorption of
phosphate
Oxidation of
isoeugenol to
vanillin
_
Polysaccharide
Lignin
Coconut,
pinenut and
walnut shells
Lignin
Urea and h-BN
Hickory
AgNO3
Cu
(NO3)2⋅3H2O
MgO
350 rpm,
30 min
29.5 Hz,
90 min
[200]
[201]
[202]
550 rpm
for 6 h
Pharmaceutical
removal
[198]
30 Hz for
90 min
800 rpm
for 8 h
500 rpm
for 12 h
Antimicrobial
filters
Depolymerization
of lignins
Removal of
phosphate and
methylene blue
[203]
[204]
[205]
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Renewable and Sustainable Energy Reviews 130 (2020) 109944
Fig. 30. Comparison of wet- and mechanochemical processing for biomass valorization.
Young Scholars of Tianjin (No. 17JCJQJC45500), and PolyU Project of
Strategic Importance.
Table 5
TRL of the biomass pretreatment with different technologies ([214,216,217]).
Technology
TRL
Description
Mechanical milling
5–6
Steam explosion
6–8
Ammonia fiber
explosion
Dilute acid
3–5
Alkaline
5–7
Ionic liquids
2–3
Supercritical-CO2
2–4
Enzymes
7–8
Reduce the crystallinity, no inhibitors, without
solvent removal, low environmental risk
❑ High energy
Lignin and hemicelluloses removal, low
environmental risk
❑ Catalysts are required, formation of inhibitors
Low inhibitors formation, increasing surface area
❑ High cost (solvent)
Effective hemicellulose removal
❑ Formation of inhibitors and by-products, neutrali­
zation needed
Low inhibitor formation, low cost
❑ Residue generated, neutralization needed
Low temperature/energy, dissolution of all
components
❑ High cost, difficulty in purification
Green solvent, low inhibitors formation
❑ High temperature, high pressure, high cost
Low temperature, effective hemicellulose/lignin
removal
❑ High cost, long time
5–7
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Besides, the impact of mechanical milling on the transformation of
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CrediT author statement
Feng Shen: Conceptualization, Methodology, Data curation, WritingOriginal draft preparation, Visualization. Xinni Xiong: Data curation,
Investigation, Writing- Original draft preparation, Visualization. Junyan
Fu, Jirui Yang, Mo Qiu: Data curation, Investigation, Writing- Original
draft preparation. Xinhua Qi: Conceptualization, Methodology, Re­
sources, Supervision, Project administration, Validation, Funding
acquisition. Daniel C.W. Tsang: Conceptualization, Resources, Supervi­
sion, Writing- Reviewing and Editing, Funding acquisition.
Acknowledgements
The authors appreciate the financial support from the National
Natural Science Foundation of China (NSFC, No. 21706139, No.
21876091 and No. 21577073), Natural Science Fund for Distinguished
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