Mechanochemistry-assisted hydrolysis of softwood over stable sulfonated carbon catalysts in a semi-batch process

Article abstract of DOI:10.1039/c9ra07668a

The hydrolysis of lignocellulose is the first step in saccharide based bio-refining. The recovery of homogeneous acid catalysts imposes great challenges to the feasibility of conventional hydrolysis processes. Herein, we report a strategy to overcome these limitations by using stable sulfonated carbons as solid acid catalysts in a two-step process, composed of mechanocatalytic pretreatment and secondary hydrolysis in a semi-batch reactor. Without mechanocatalytic pre-treatment the hydrolysis of the insoluble substrate largely occurs through homogeneously catalyzed reactions. Ball-milling induced amorphization promotes a substantially higher substrate reactivity, because homogeneous hydrolysis occurs preferentially from less ordered structural domains in cellulose. In contrast, concerted ball-milling (CBM) of cellulose with the sulfonated carbon promotes a heterogeneously catalyzed hydrolysis to soluble oligosaccharides. By performing an in-depth physicochemical characterization of cellulose subjected to CBM treatment with different carbons, we reveal the crucial role of strong Br?nsted acid sites in facilitating mechanocatalytic depolymerization. Recyclability experiments confirmed that despite being subject to profound structural changes during repeated pre-treatment/semi-batch hydrolysis cycles, the sulfonated carbon retained its catalytic activity. The combination of mechanocatalytic pretreatment with strong solid acids and hydrolysis in the semi-batch reactor was successfully extrapolated for the first time to the hydrolysis of real lignocellulose to achieve quantitative yields in C₅ and high yields in C₆ derived products.

Full text of DOI:10.1039/c9ra07668a

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Mechanochemistry-assisted hydrolysis of softwood
over stable sulfonated carbon catalysts in a semi-
batch process†
Cite this: RSC Adv., 2019, 9, 33525
ab
ab
ad
Jingwei Xie,c Oliver Krocher
and Frederic Vogel
´ ´
*
David Scholz,
¨
The hydrolysis of lignocellulose is the first step in saccharide based bio-refining. The recovery of
homogeneous acid catalysts imposes great challenges to the feasibility of conventional hydrolysis
processes. Herein, we report a strategy to overcome these limitations by using stable sulfonated carbons
as solid acid catalysts in a two-step process, composed of mechanocatalytic pretreatment and
secondary hydrolysis in a semi-batch reactor. Without mechanocatalytic pre-treatment the hydrolysis of
the insoluble substrate largely occurs through homogeneously catalyzed reactions. Ball-milling induced
amorphization promotes a substantially higher substrate reactivity, because homogeneous hydrolysis
occurs preferentially from less ordered structural domains in cellulose. In contrast, concerted ball-milling
(CBM) of cellulose with the sulfonated carbon promotes a heterogeneously catalyzed hydrolysis to
soluble oligosaccharides. By performing an in-depth physicochemical characterization of cellulose
subjected to CBM treatment with different carbons, we reveal the crucial role of strong Brønsted acid
sites in facilitating mechanocatalytic depolymerization. Recyclability experiments confirmed that despite
being subject to profound structural changes during repeated pre-treatment/semi-batch hydrolysis
cycles, the sulfonated carbon retained its catalytic activity. The combination of mechanocatalytic
pretreatment with strong solid acids and hydrolysis in the semi-batch reactor was successfully
extrapolated for the first time to the hydrolysis of real lignocellulose to achieve quantitative yields in C5
and high yields in C6 derived products.
Received 30th September 2019
Accepted 3rd October 2019
DOI: 10.1039/c9ra07668a
rsc.li/rsc-advances
but suffer from slow reaction rates, limited catalyst stability and
high costs for the catalysts.7 Even in the absence of catalysts,
Introduction
cellulose undergoes auto-hydrolysis in hot compressed water at
temperatures around 230–240 ꢀC.8 In this case, the protons
required for hydrolysis are provided by the self-ionization
equilibrium of water which reaches a maximum at approxi-
mately 300 ꢀC.9 Hydrolysis can also be performed at milder
conditions through the use of homogeneous Brønsted acid
catalysts. During the 20th century different attempts were made
to implement hydrolysis processes for the production of
monosaccharides from lignocellulose, using concentrated10,11
or diluted12–14 mineral acid catalysts. Although some of these
processes have yielded saccharides with impressive yields, none
of them have so far been commercialized due to unfavourable
process economics resulting from the recovery or the neutrali-
zation of the homogeneous acid.14 To this end, the use of solid
acid catalysts would offer clear advantages. Heterogeneous
catalysts can be more easily recycled and are less prone to
induce corrosion in reactor materials.15 Nonetheless, the use of
solid acid catalysts also imposes new challenges.
Owing to its widespread abundance, lignocellulosic biomass is
regarded as a promising substitute to replace fossil-resources as
a feedstock for the production of fuels and chemicals.1 Cellu-
lose (40–55%),2 the most abundant component of lignocellu-
lose, is
a syndiotactic homopolymer consisting of b-D-
glucopyranose units connected via b-1,4-glycosidic bonds. The
polymer chains are held together by an extended intra- and
inter-chain hydrogen bonding network that facilitates the
occurrence of chemically recalcitrant crystalline domains.3
Chemical valorisation of cellulose can be accomplished by
hydrolytic cleavage of the glycosidic bond. Different approaches
based on enzyme catalysts,4 thermochemical processing5 and
Brønsted acid catalysts6 have been studied. Hydrolysis
processes relying on the action of enzymes yield mono-
saccharides with high selectivity at mild reaction conditions,
aPaul Scherrer Institute, 5232 Villigen PSI, Switzerland. E-mail: frederic.vogel@psi.ch
b
´
´ ´
Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland
cGeorgia Institute of Technology, Atlanta, GA, 30332, USA
dFachhochschule Nordwestschweiz, 5210 Windisch, Switzerland
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra07668a
Firstly, the metal-oxide,16,17 zeolite17,18 or polymer19,20 based-
materials that have been tested in this application, all lack
long-term stability under the hydrothermal conditions typical of
lignocellulose hydrolysis processes.21–24 Substantial progress in
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this regard has been made in recent years through the devel- homogeneous catalysts should also apply to solid acid cata-
opment of acid functionalized carbons. Carbon materials lysts, that is to say, materials featuring a high density of strong
display higher stability in the liquid phase because poly- Brønsted acid sites should display higher activity during
aromatic systems do not undergo hydrolytic attack,25 which can mechanocatalytic depolymerization. This hypothesis is sup-
lead to dissolution. Different oxidation26,27 and sulfonation28 ported by previous studies that have investigated the efficacy
strategies have been developed to introduce carboxylic or of various solid acids during CBM treatment. Kobayashi et al.
sulfonic acid surface functionalities into carbons. Both of these showed that CBM treatment with a carboxylic acid function-
display activity in cellulose conversion, however, hydrolysis with alized carbon does not induce mechanocatalytic depolymer-
carboxylic acid functionalized materials typically requires ization but enhances the propensity of cellulose to undergo
higher temperatures (180–230 ꢀC)26,29–32 to proceed at acceptable hydrolysis.29 The CBM treated substrate (substrate/catalyst (S/
ꢀ
rates as compared to materials containing sulfonic acid groups C) ¼ 6.5, 60 rpm, 48 h) could be hydrolysed at 180 C to yield
(100–150 ꢀC).17,33–36 This observation can be rationalized with 21.3% and 70.0% mono- and oligosaccharides, respectively, as
the fact that hydrolysis proceeds via protonation of the weakly opposed to 3.8% and 10.0% when individually ball-milled
basic glycosidic oxygen which has been shown to be favoured by cellulose was converted under the same conditions.29 In this
strong Brønsted acids (pKa # ꢁ3).37
study, a higher selectivity to monosaccharides could only be
Another marked difference between carboxylic and sulfonic achieved when a secondary aqueous phase hydrolysis was
acid group functionalized materials is, that the latter was performed in presence of strong homogeneous Brønsted
believed to be less stable due to the propensity of sulfonic acid acids. When CBM is performed with solid acids featuring
groups to leach in hydrothermal media and to undergo deac- strong Brønsted acid sites, cellulose undergoes extensive
tivation via ion exchange induced proton leaching.38 However, mechanocatalytic depolymerization. Mechanocatalysis with
we recently demonstrated that stable sulfonated carbons can be metal-oxides (niobium molybdate45 or kaolinite43) or resins
prepared by exploiting the dependence of sulfonic acid group (Amberlyst-70 (ref. 30) and Naon46) has been shown to afford
stability on the structure of the carbonaceous support.39 The soluble oligosaccharides at 72.0–99.0% yield (S/C ¼ 1–6.5, 60–
stability of sulfonic acid groups in carbons is directly linked to 800 rpm, 3–48 h). Although these results indicate the principal
their proximity to activating/deactivating substituents that feasibility of this approach for overcoming the restrained
increase/decrease the tendency of the C–S bond to undergo substrate–catalyst contact, it has also shown that metal-
solvolysis. Furthermore, their deactivation via proton leaching oxides47 and Amberlyst30 are not recyclable aer the CBM
can be fully overcome by the addition of complexation agents to treatment due to structural degradation and deactivation of
the reaction medium which suppress the ability of cations to the materials. Consequently, so far, no catalytic processes have
participate in ion exchange processes.39 Both modes of deacti- been developed that employ mechanocatalytic pre-treatment
vation are particularly relevant for biomass conversion with strong solid acid catalysts. Furthermore, previous
processes as these are typically performed in hydrothermal studies have focused on studying the mechanocatalytic
media and because most biomass feedstocks contain 0.3–1.0% formation of oligosaccharides from model substrates rather
of minerals.40
than real lignocellulosic biomass.
A second challenge is related to the constraints imposed on
The issues addressed before demonstrate the benecial
the reaction system by the insolubility of cellulose. The insol- effect of employing strong Brønsted acid catalysts for the
ubility should be anticipated to strongly impede a heteroge- hydrolysis of cellulose and lignocellulose. The lacking under-
neously catalysed hydrolysis. Although mechanisms involving standing of the mechanism of this reaction and the unavail-
a collision of particles in suspension29,30,41 or adsorption35,36 of ability of stable catalysts has so far hampered the ability to
cello-oligosaccharides have been proposed, neither of these design continuous-ow catalytic processes for the conversion of
have been underlined by experimental results. Nonetheless, real lignocellulosic biomass. Therefore, in this study, we
several previous studies have reported that good yields (50.4– develop a semi-batch hydrolysis process using a hydrothermally
74.5%) can be achieved during the hydrolysis of pristine35,42 or and mechanically stable sulfonated carbon as a strong solid
mechanically activated18 (ball-milled) cellulose with solid acids. acid catalyst. We shed light on the crucial role of homoge-
It however remains uncertain to what degree the reaction truly neously catalysed reactions in this application and reveal the
occurred over heterogenized acid sites and to what extent structural features of cellulose that dictate its reactivity. In
homogeneously catalysed reactions (i.e. auto-hydrolysis or a second step, we investigate the efficacy of the sulfonated
hydrolysis by leached species) contributed to product carbon catalyst in facilitating a mechanocatalytic depolymer-
formation.
ization and identify the active sites over which the reaction
A strategy proposed to overcome the mass-transfer limita- occurs. By demonstrating the high stability of the sulfonated
tions is mechanocatalytic depolymerization, which can be carbon catalyst we form the basis for designing a two-step
achieved by the concerted ball-milling (CBM) of a solid acid catalytic process, in which mechanocatalytically pretreated
with cellulose.43 When mechanocatalytic depolymerization is cellulose is hydrolysed in a semi-batch reactor. We extrapolate
performed with homogeneous catalysts,44 the rate of depoly- this methodology for the rst time to the conversion of real
merization has been demonstrated to exhibit a similar lignocellulose and show that the obtained yields are competitive
dependence on the pKa of the employed acid, as found for the with those achieved with state-of-the-art homogeneous acid
aqueous phase hydrolysis.37 The characteristics of effective based processes.
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installed in series with the heat-exchanger. The reactor effluent
was ltered through a 500 nm porous metal-frit (Mottcorp
4001130–005). During a typical experiment the sulfonated carbon
catalyst (2 g), the substrate (7 g) and the eluent were charged into
the reactor and the owrate of the HPLC pump was set to 7
mL minꢁ1. Aer the system was pressurized the reaction was
initialized, by heating to 165 ꢀC. The reactor effluent was
collected in a Schott bottle, which was placed on a balance
(Sartorius LC6201S) to monitor the effluent ow-rate. The reac-
tion progress was monitored by taking samples in regular inter-
vals at the outlet of the back-pressure valve or directly from the
effluent collection bottle. Aer a 6 h experiment, the ow-rate of
the HPLC pump was reduced to 2 mL minꢁ1 and the reaction was
quenched by cooling the reactor with pressurized air for 0.5 h.
Experimental
Materials
D-(+)-cellobiose (Sigma-Aldrich, $98%), D-(+)-glucose mono-
hydrate (Sigma-Aldrich, 99%), D-(+)-fructose (Fluka, >99%), D-
(+)-mannose (Sigma-Aldrich, 99%), D-(+)-galactose (Sigma-Aldrich,
99%), 5-HMF (Sigma-Aldrich, 99%), 1,6-anhydro-b-D-glucose
(Sigma-Aldrich, 99%), levulinic acid (Sigma-Aldrich, 99%), formic
acid (Sigma-Aldrich, 98%), D-(+)-xylose (Sigma-Aldrich, $99%), L-
(+)-arabinose (Sigma-Aldrich, $99%) and furfural (Sigma-Aldrich,
99%) were used as external calibration standards.
Analysis
Analyte quantication was achieved by means of an HPLC
system (Agilent 1260 Innity) equipped with an Aminex Biorad
monosaccharide analysis column (HPX-87C, 300 ꢂ 7.8 mm)
and a refractive index detector. The columꢁn1 was operated at
80 ꢀC and with a owrate of 0.45 mL min using Millipore
water as the eluent.
Ball-milling and concerted ball-milling
Mechanical and mechanocatalytic pre-treatment of cellulose
was performed in a sialon pot (80 mL) with sialon spheres (25
pcs. with 1 cm diameter) at 300 rpm by using a Fritsch Pul-
verisette 6. The total mass of cellulose and catalyst charged into
the ball-mill was kept constant for all pre-treatments (9 g). The
temperature in the ball-mill was monitored in regular intervals
to exclude that a temperature induced degradation of the
sample occurred.
The conversion (X) of cellulose (Sigma-Aldrich, Avicel PH-
101) was calculated by gravimetric analysis according to the
following equation, X ¼ 1 ꢁ (ms ꢁ mcat)/mCel. ms denotes the dry
mass of solids recovered aer an experiment and mcat and mCel
the catalyst and cellulose mass charged into the reactor,
respectively. The yield of reaction product i (Yi) was calculated
by using the following formula, Yi ¼ ciVp/nGluc. in which ci
denotes the concentration of reaction product i, Vp the total
volume of reaction products and nGluc. the moles of glucose
bound in cellulose (calculated by assuming a molecular mass of
Mw ¼ 162 g molꢁ1 for cellulose-bound glucose). The selectivity
(Si) in formation of reaction product i was calculated using the
following formula, Si ¼ Yi/X. Oligosaccharides were quantied
by post-hydrolysis according to the method described by
NREL.48 Identication of unknown volatile substances was
accomplished with an Agilent 5977A Series GC/MSD equipped
with a quadrupole MS detector and a HP-5MS UI column.
Oligosaccharides were identied by measuring the ESI total-ion
chromatograms on a Thermo Scientic Ultimate 3000 equipped
with a QExactive Focus mass spectrometer.
Characterization of pretreated cellulose
Particle size: The primary particle size distribution was
measured on a Horiba LA-950V2 laser diffraction particle size
analyser. Powder XRD: Powder XRD patterns of cellullose were
measured on a D8 ADVANCE (Bruker) diffractometer over the
range 2q ¼ 5–70ꢀ using Cu Ka radiation (l ¼ 0.154 nm).
Deconvolution was performed on baseline-corrected XRD
patterns by assuming Gaussian peak shape for all diffraction
peaks.49 A Levenberg–Marquardt minimization algorithm opti-
mized the peak position, width and area such as to achieve an
optimal t to the diffraction patterns (R2 $ 0.994 for all
deconvoluted diffraction patterns). The crystallite size was
calculated by using a shape factor of 0.94 in the Scherrer
equation.50 FTIR spectroscopy: FTIR spectra were recorded in the
range 400–4000 cmꢁ1 with a resolution of 2 cmꢁ1 by using
a Bruker Vertex 80 V FTIR spectrometer. FTIR spectra of the
cellulose samples were recorded by preparing 1 wt% KBr pellets.
Prior to preparing the pellets, cellulose samples and KBr were
dried overnight in a convection oven set to 110 ꢀC. Elemental
analysis: The carbon, sulphur, nitrogen and hydrogen content
of the different samples were measured by using an Elementar
Vario EL Device. SEM-EDX: Electron microscopy and energy
dispersive X-ray spectroscopy was performed on a Zeiss Ultra 55
eld-emission-gun scanning electron microscope. To reduce
charging effects, all samples were applied on conductive carbon
Catalyst synthesis and characterization
The detailed procedures for preparation and characterization of
the sulfonated carbon catalyst are described elsewhere.39
Deactivation of the as prepared catalyst was achieved through
three consecutive ion exchange treatments (1 g per 15 mL
ꢀ
solution, 30 C, 16.5 h) using an aqueous solution containing
0.25 wt% of each Na+, K+, Ca2+ and Mg2+
.
Semi-batch reactor
The hydrolysis of cellulose and sowood was performed in tape and subsequently sputter coated (48 mA, 90 s) by using
a stirred high pressure reactor vessel (Parr Instrument Company a Leica EM SCD 500 and an Au : Pd alloy (80 : 20) or chromium
HP/HT 4576A, 250 mL) connected at the inlet to an HPLC pump for EDX mapping. Images were acquired using an acceleration
(Knauer P4.1S) and at the outlet to an in-house made heat- voltage of 7.0 kV and a magnication of 100, 500 and 2500ꢂ.
exchanger. The pressure (20 bar) over the system was main- Solubility of cellulose samples: The solubility of pre-dried
tained by a back pressure valve (Swagelok KHB1W0A4C2P60000) samples (2 h, 110 ꢀC) was measured by suspending cellulose
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in Millipore water (1 g per 10 mL, 25 ꢀC, 1 h) under continuous in –SO3H groups, respectively.55 To verify that the carbon
stirring (500 rpm). Aer 1 h, the samples were centrifuged framework of C-350-SO3H was hydrothermally stable its crys-
(14 000 rpm, 15 min) and the ltered (0.22 mm, PTFE) super- talline and molecular carbon structure were characterized with
natant was analysed for its oligosaccharide, monosaccharide X-ray diffraction (XRD) and Raman spectroscopy, respectively.
and dehydration product concentration.
Both the XRD patterns (Fig. S1.†B) and the Raman spectra
(Fig. S1.†C) conrmed that the amorphous carbon structure did
not undergo structural transformation during hydrothermal
treatment. Measurement of the specic surface area (SSA) using
Krypton as an adsorbate (Fig. S1D†) further showed that the
sample preserved its textural properties (Table S1†) during the
treatment in hot-compressed water.
Synthesis of amorphous cellulose
Amorphous cellulose was synthesized through precipitation of
Avicel PH-101 dissolved in a 50/50 mixture NaSCN (Sigma-
Aldrich, $98%) and ethylenediamine (Sigma-Aldrich,
$99.5%) in accordance with a procedure described elsewhere.51
Semi-batch reactor
Adsorption of cellobiose
To enable the hydrolysis of cellulose under continuous-ow
conditions and using solid acid catalysts, we developed
a semi-batch process (Fig. 1).
The reactor is composed of a continuously stirred tank,
connected at the inlet to an HPLC pump and at the outlet to
a back-pressure valve. To retain the unreacted substrate and the
catalyst, the reactor effluent was ltered through a 500 nm
porous metal-frit installed at the outlet of the reactor. To
prevent the frit from clogging with particles, it was installed
close (2 mm) to the impeller. We found that this design feature
Adsorption of cellobiose (20 mM) onto C-350-SO3H-HT (100 mg
catalyst per 1 mL solution) was performed in 2.5 mL centrifuꢀge
vials placed in an Eppendorf Thermomix Comfort set to 25 C
and 4000 rpm. Aer a set amount of time, the vials were
centrifuged (14 000 rpm, 15 min) and the ltered (0.22 mm,
PTFE) supernatant was analysed for its cellobiose
concentration.
Recyclability experiments
During recyclability experiments the catalyst was recovered was crucial for maintaining a lter surface free of deposited
from the mixture containing unreacted cellulose, by dissolving particles and thus for maintaining a low pressure drop over the
the latter during 80 min in 72% H2SO4 (15 mL per 1 g of powder frit. We attribute this effect to the shear forces at the surface of
mixture) heated to 30 ꢀC. Subsequently, the catalyst was ltered the lter originating from the turbulent uid environment
from the solution and washed with 1 L of deionized water at induced by the rotating impeller. The formation of a turbulent
room temperature and 2 L of hot (90–95 ꢀC) deionized water. To uid environment is conrmed by considering the impeller
ensure complete removal of cellulose the dissolution procedure Reynolds number (NRe(600 rpm) ¼ 2.5 ꢂ 106) for the system
was performed twice.
which is two orders of magnitude greater than the boundary
value56 for fully turbulent uids in stirred tank reactors.
The continuous product removal is an essential feature of
the reactor system that enables decoupling the residence times
of the substrate and the hydrolysis products (mean residence
time, s ¼ 35 min). Thereby the undesired decomposition reac-
tions57 of monosaccharides that typically limit the attainable
reaction yields can be minimized.
Compositional analysis
Compositional analysis was performed in accordance with the
methodologies described by NREL.48,52–54 The mass balance for
the compositional analysis of the spruce r wood was closed to
106%. The spruce r wood was chopped on the 15th of
¨
September 2015 in the forest of Wurenlingen, Switzerland.
Results and discussion
Sulfonated carbon catalyst
The sulfonated carbon catalyst utilized in this study was
prepared by carbonization of cellulose at 350 ꢀC (denoted C-350)
and sulfonation of the resulting carbonaceous material (deno-
ted C-350-SO3H) using oleum (20% SO3/H2SO4). Sulfonation of
C-350 led to the introduction of both hydrothermally stable and
instable sulfonic acid groups (–SO3H: 2068 ꢃ 57 mmol gꢁ1).
ꢀ
Hydrothermal pre-treatment (180 C, 16.5 h) of the sulfonated
materials removed the instable groups and resulted in a mate-
rial (denoted C-350-SO3H-HT) with 869 ꢃ 29 mmol gꢁ1 stable
–SO3H groups. The chemical identity of sulphur species in the
catalyst was validated with Fourier-transform infrared (FTIR)
spectroscopy. The FTIR spectra (Fig. S1A†) show two clearly
identiable absorption bands at 1040 and 1168 cmꢁ1, assigned
Fig. 1 Semi-batch process for hydrolysis of cellulose and softwood
to the symmetric and asymmetric stretching modes of O]S]O using sulfonated carbon catalysts.
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Due to the tendency of the sulfonated carbon catalyst to temperatures than used in this study. Hydrolysis could also be
deactivate in presence of cationic impurities,39 the eluent was homogeneously catalysed by protons provided by the dissocia-
enriched with 1 mM 15-crown-5 polyether (15-C-5) and ethyl- tion of EDTA. Our initial experiments therefore aimed at
enediaminetetraacetic acid (EDTA) as complexation agents for elucidating the contribution of homogeneous reaction path-
the mono- and divalent cations, respectively. The cellulose used ways to the hydrolysis of cellulose.
in this study had an ash content of 0.2% and is therefore ex-
The contribution of auto-hydrolysis was determined through
pected to liberate cations into the reaction medium during the an experiment in which the reactor was only charged with
course of the reaction.
cellulose and Millipore water was used as the solvent. Even at
the mild temperature (165 ꢀC) employed in this work, auto-
hydrolysis led to 8.9% conversion (X) to yield (Y) 4.6% oligo-
saccharides, 1.5% monosaccharides and 0.1% dehydration
products (Fig. 2).
Hydrolysis pathways
The Brønsted acid catalysed hydrolysis of cellulose is initialized
with the formation of soluble and insoluble oligosaccharides
(Scheme 1). It is well established in literature that solubility is
dened by the degree of polymerization (DP).58 Whereas oligo-
saccharides with a DP of 2–6 are soluble in water at room
temperature, glucans with 7–13 monomeric units are only
partially soluble in hot compressed water. The formed oligo-
saccharides can subsequently undergo secondary hydrolysis to
form glucose. The formation of aldose (mannose and galactose)
and ketose (fructose) isomerization products of glucose is
attributed to a Lobry de Bruyn-van Ekenstein transformation.59
Finally, the formed monosaccharides can undergo intra- or
inter-molecular dehydration to form levoglucosan and 5-
(hydroxymethyl)furfural (5-HMF), respectively. This typically
unwanted side reaction of C6 monosaccharides should however
be limited by the continuous product removal from the semi-
batch reactor.
Cellulose and the sulfonated carbon catalysts are both
insoluble in water at room temperature. The question thus
arises how the reaction between the solid acid and cellulose
should proceed. It has been proposed that the reaction occurs
through a collision induced event29,30,41 or through adsorption of
the substrate onto the catalysts surface.35,36
It is however important to highlight that the formation of
soluble reaction products could also occur via homogeneously
catalysed reactions. For example, it is well known that cellulose
readily undergoes auto-hydrolysis,8,60 albeit, typically at higher
When the eluent was enriched with 1 mM EDTA and 15-C-5,
the conversion of cellulose increased marginally to 9.5% due to
the higher concentration of [H3O+] ions present in this case (pH
3). Furthermore, the formed oligosaccharides increasingly
underwent secondary hydrolysis. Oligosaccharides, mono-
saccharides, and dehydration products were formed with 3.6%,
4.7%, and 0.1% yield. The slightly increased conversion
conrms that the protons liberated by EDTA (pKa ¼ 0, 1.5, 2.0,
and 2.66)61 contributed to the formation of soluble reaction
products. The overall increase was however limited due to the
fact that the hydrolysis reaction is initiated by the protonation
of the weakly basic glycosidic oxygen15 and thus, typically,
requires the use of high concentrations of strong Brønsted acids
(pKa # ꢁ3),37 to achieve acceptable reaction rates.
In presence of both EDTA and the sulfonated carbon catalyst,
the conversion of cellulose increased further to 15.5%. Over the
course of the reaction, oligosaccharides, monosaccharides and
dehydration products were formed with 6.3%, 7.1%, and 0.2%
yield (Fig. 2). Notably, even in presence of the sulfonated carbon
catalyst the yield of dehydration product remained low. This
conrms that the continuous product removal from the semi-
batch reactor effectively suppressed secondary reactions of the
formed monosaccharides.
Fig. 2 Conversion ( ) and yields of oligosaccharides ( ), mono-
saccharides ( ) and dehydration products ( ) achieved with pristine
cellulose in presence and in absence of EDTA and C-350-SO3H-HT.
Scheme
1
Reaction pathway for cellulose, oligosaccharides and Reaction conditions: 165 ꢀC, 6.5 h, 3 wt% cellulose, s ¼ 35 min,
monosaccharides in the presence of Brønsted acids.
impeller speed ¼ 600 rpm.
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The fact that the conversion increased when the hydrolysis affect various structural properties of cellulose, including the
was performed in presence of the sulfonated carbon catalyst, particle size and crystallinity.64 To understand how structural
suggests that cellulose can additionally undergo a heteroge- properties evolved with treatment time and how these corre-
neously catalysed reaction pathway. However, despite the lated with reaction yields, we prepared samples subjected to
presence of strong Brønsted acid sites in the catalyst, the different durations of ball-milling (denoted BM-x h).
conversion and achieved yields were low as compared to those
Changes in the crystal structure were assessed by measuring
reported in previous studies (50.4–74.5%).18,35,42 We attribute the XRD patterns of the various samples. The diffraction pattern
this nding to the fact that the reaction with a solid acid catalyst of pristine cellulose (Fig. 3A) shows three peaks centred at 2q ¼
is strongly inhibited by the substrates insolubility and therefore 15, 22.5 and 34.5ꢀ that are characteristic of the naturally occurring
predominantly occurs under participation of leached active cellulose allomorph. The three peaks form due to overlapping
ꢀ
sites from the catalyst. The sulfonated carbon catalyst employed diffractions associated with the (1 0 1), (1 0 1), (0 2 1), (0 0 2) and (0
in this work leached only 16% of its sulfonic acid groups during 0 4) crystal planes.49 Additionally, it is generally recognized that
the reaction, which corresponds to a hypothetical H2SO4 a broad peak at 2q ¼ 20ꢀ is characteristic of para-crystalline
concentration of 1 mM in the reaction medium. The release of domains.49 This assignment was conrmed by measuring the
H2SO4 into the reaction medium would invoke a further diffraction pattern of an amorphous cellulose sample (Fig. 3A).
increase in the concentration of protons (pH 2.7), leading to an
The amorphization during ball-milling can be directly
acceleration of homogeneously catalysed reactions. Thus, the inferred from the decreasing intensity of the peaks at 2q ¼ 15,
true contribution of a heterogeneously catalysed reaction to the 22.5 and 34.5ꢀ and the increase in intensity at 2q z 20ꢀ. To
conversion of the substrate could not be determined accurately. quantify the change in crystallinity, the diffraction patterns
However these experiments clearly show that homogeneous were deconvoluted into the contributions from the different
reaction pathways (auto-hydrolysis, hydrolysis by EDTA and crystal planes (Fig. 3B). A relative measure of crystallinity can
hydrolysis by leached species) signicantly contribute to the then be calculated as the quotient of the total area of crystalline
conversion of cellulose.
reexes to the total area of the diffraction pattern.49 Following
Following from the hypothesis that the reaction between this methodology the crystallinity index (CI) of pristine cellulose
cellulose and the catalyst occurs via a collision-induced was determined to be 65.3%, which is consistent with values
event,29,30,41 we investigated the effect of the collision reported previously.49 The CI of pretreated cellulose decreased
frequency on the hydrolysis reaction rate. We anticipated that monotonously with the duration of ball-milling (Fig. 5) and
the collision frequency between the substrate and the catalyst reached 56.5% in the sample BM-54 h. The CI initially rapidly
should be correlated to the degree of turbulence in the reaction decreased and then began to plateau at longer treatment times.
medium. To test this hypothesis we performed several experi- This is tentatively attributed to the tendency of the substrate to
ments in which we varied the impeller rotational speed (75–600 adhere to the milling media at longer treatment times. The
rpm). Surprisingly, the conversion and the total yield remained substrate layer is expected to dampen the mechanical forces
constant irregardless of the impeller speed (Fig. S3†). This during impact, which would consequently reduce the efficiency
consequently suggests that the initial formation of soluble of the pre-treatment. Similar observations have also been made
oligosaccharides via a heterogeneously catalysed reaction by Rinaldi and co-workers.65
pathway does not proceed via a collision-induced event, or that
the collision of the substrate with the catalyst is not the rate
determining step in the reaction mechanism.
The effect of ball-milling on the crystal structure of cellulose
The comparatively low yields achieved with pristine cellulose
are a direct manifestation of its pronounced chemical recalci-
trance. The recalcitrance is linked to the presence of an
extended intra- and inter-chain hydrogen bonding network that
facilitates the formation of partially crystalline domains.3
Glycosidic bonds in crystalline domains are shielded from the
chemical environment and are thus signicantly less reactive62
than such in para-crystalline domains. The latter are commonly
referred to as amorphous domains and the two terms are used
interchangeably in this study.
The higher reactivity of glycosidic bonds in para-crystalline
domains suggests that amorphization of cellulose should be
an effective strategy to enhance its propensity to undergo
hydrolysis. Among the different pre-treatments used for
enhancing the reactivity of cellulose, ball-milling has been
identied as one of the most effective.63 Ball-milling is known to sum of the fitted peaks ( ).
Fig. 3 (A) Powder X-ray diffraction patterns of pristine ( ), BM-1 h
( ),BM-4 h ( ), BM-8 h ( ),BM-18 h ( ),BM-54 h ( ),BM-18 h + 5% H2O
( ) and amorphous cellulose ( ). (B) Powder X-ray diffraction pattern of
ꢀ
pristine cellulose ( ), the fitted ① – (1 0 1) ( ), ② – (1 0 1) ( ), ③ – (0 2 1)
( ), ④ – para-crystalline ( ), ⑤ – (0 0 2) ( ), ⑥ – (0 0 4) ( ) peaks and
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Nonetheless, the determined CI's conrm that ball-milling
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The effect of ball-milling on the hydrogen bond network
promoted the amorphization and that this structural trans-
formation was most pronounced in samples subjected to longer
treatment times. The increase in structural heterogeneity
during ball-milling is also exemplied by considering the full
width at half maximum (FWHM) of the deconvoluted peaks. It
is commonly accepted in literature that peak broadening in the
diffraction pattern of cellulose is related to an increased
amorphous content.49 With the exception of the (0 0 2) diffrac-
tion, the FWHM of the deconvoluted peaks indeed increased
with prolonged ball-milling duration (Table S2†). Even in
samples subjected to prolonged pre-treatment, but displaying
only minor changes in the CI (BM-18 h and BM-54 h), ball-
milling lead to continued peak broadening, that is to say
amorphization.
It should be noted that peak broadening may also be related
to changes in the sizes of crystallites. To assess this effect on the
FWHM, the crystallite size in the various samples was estimated
with the Scherrer equation. The average crystallite size deter-
mined from the crystal planes in pristine cellulose was 4.8 nm,
which is consistent with values reported in literature.49 The
crystallite size in ball-milled samples (Table S3†) initially
decreased with the duration of ball-milling and remained
constant at 3.5 nm at longer treatment times (18 h and 54 h).
Consequently, it can be concluded that at least a part of the
increased FWHM is associated with a reduction in the crystallite
size. However, it is important to point out that ball-milling
induced dislocations, faults or stress in the lattice would like-
wise contribute to peak-broadening.
The formation of crystalline domains in cellulose is facilitated by
an extended inter- and intra-chain hydrogen bonding network.
Therefore, changes in the crystallinity should be accompanied by
alterations in the hydrogen bond network (Scheme 2).
To study these changes, FTIR spectra of the different
samples in the 4000–3000 cmꢁ1 wavenumber region (Fig. S7†)
were recorded. According to literature, absorption in this region
can be assigned to the n(O–H) stretching vibration of inter- (O6–
H–O3 – 3310–3230 cmꢁ1) and intra-chain (O3–H–O5 – 3375–
3340 cmꢁ1 and O2–H–O6 – 3460–3405 cmꢁ1) hydrogen bonded
as well as free –OH groups (3580–3550 cmꢁ1).67 Visualization of
changes in the spectra was accomplished by determining the
normalized difference spectra of the pretreated samples, rela-
tive to pristine cellulose (Fig. 4). The difference spectra of the
samples BM-18 h and BM-54 h showed two strongly overlapping
negative peaks centred at 3345 and 3283 cmꢁ1, assigned to the
valence vibration of intra-chain O3–H–O5 and inter-chain O6–
H–O3 H-bonded –OH groups, respectively. The fact that the
difference spectra had negative intensity indicates that the O3–
H–O5 and O6–H–O3 H-bonds had been partially disrupted in
these samples. Additionally, a broad positive peak centred at
3576 cmꢁ1 with a shoulder of weak intensity at 3445 cmꢁ1 was
formed in these samples. The peaks are assigned to the
stretching vibration of free and of intra-chain (O2–H–O6)
bonded –OH groups, respectively. The positive nature of the
peak at 3576 cmꢁ1 shows that these samples contained more
free –OH groups, as would be expected from the disruption of
O3–H–O5 and inter-chain O6–H–O3 H-bonded –OH groups.
Similar structural changes in the H-bond network were also
found in the amorphous cellulose and the sample that was ball-
milled with 5% H2O. However, in both samples the negative
absorption peaks assosciated with O3–H–O5 and O6–H–O3 H-
bonded –OH groups had higher intensity, suggesting that
these H-bonds had been disrupted to a greater extent. These
results thus demonstrate that besides reducing the crystallinity
and the particle size, ball-milling has a measurable effect on the
nature of the H-bond network. Furthermore, this effect is most
pronounced in samples that experienced the highest reduction
in crystallinity, which conrms that amorphization occurs hand
in hand with a disruption of the H-bond network.
Besides changing the crystallite size, ball-milling also had
a pronounced effect on the particle size distribution. Aer 1 h of
ball-milling, the mean of the particle size distribution (d50,V
)
changed from 50.8 mm to 22.4 mm. Interestingly, longer treat-
ment times did not lead to a further reduction in the particle
size distribution (Fig. S4C†), but to the formation of increas-
ingly spherical particles (Fig. S4B†).
Although ball-milling promoted amorphization, the CI
showed that the samples retained a substantial fraction of their
crystallinity. Therefore in a next step, means to enhance the
amorphization during ball-milling were investigated. Based on
the ability of water molecules to form H-bonds with cellulose,66
it was postulated that the addition of water during ball-milling
may aid in disrupting crystalline structures. To test this
hypothesis, cellulose was ball-milled for 18 h with prior addi-
tion of 2–25 wt% H2O. As postulated, the CI index decreased
when up to 10 wt% H2O was added during ball-milling
(Fig. S5A†). The lowest CI index (52.0%) was achieved when
5% H2O was added. At higher water loadings the CI index
increased and at 25 wt% approached the value for pristine
cellulose. The amount of added water also had a pronounced
effect on the particle size (Fig. S5C†) and morphology
(Fig. S5B†). The addition of 2–10% H2O promoted the formation
of larger and highly spherical particles, whereas at 25 wt% H2O
added the characteristics of the particle size distribution
approached the values found for pristine cellulose. The latter
effect is related to the formation of a viscous paste that retarded
the motion of the grinding media.
The interrelation between structure and specic reactivity
With the aim to identify structure–reactivity relations, we pro-
gressed to studying the hydrolysis of the pretreated cellulose
Scheme 2 Hydrogen bonding in cellulose.
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(34.0%). To substantiate the hypothesis that reactivity is related
to the CI, we tested the propensity of the amorphous cellulose
(CI ¼ 20.1%) to undergo hydrolysis. To our surprise the ach-
ieved conversion and yields (Table S3†) were comparable to the
ones achieved with cellulose that was ball-milled for 18 h (CI ¼
57.4%). This clearly indicates that other factors must be inu-
encing the reactivity of cellulose. Besides the crystallinity, the
reactivity may also be strongly determined by the particle size.
Smaller particles would be expected to show higher reactivity
because the surface area where a reaction could occur would be
larger. This effect may be responsible for the lower reactivity of
the amorphous cellulose. Although the sample featured the
lowest CI of all tested samples, the mean of its particle size
distribution (d50,V) was nearly four times as large as that of the
BM-18 h (Table S3†).
To further elucidate the structural origin of reactivity, we
investigated the solubility of the pretreated samples. It is well
known that ball-milling can induce mechanochemistry via
bond breakage and formation.68 On that basis, we hypothesized
that the enhanced reactivity of ball-milled cellulose could be
related to the cleavage of glycosidic bonds, leading to the
formation of smaller and therefore partially soluble oligosac-
charides. The solubility of pristine cellulose in Millipore water
Fig. 4 FTIR difference spectra of BM-1 h ( ), BM-4 h ( ), BM-8 h ( ),
BM-18 h ( ), BM-54 h ( ), BM-18 h + 5% H2O ( ) and of amorphous
cellulose ( ).
samples. Fig. 5 shows the conversion and yields achieved with
ball-milled cellulose in presence of C-350-SO3H-HT. The
conversion initially increased with the duration of ball-milling
and then plateaued at longer treatment times. Interestingly,
the CI's of the samples displayed a similar trend. That is to say,
the CI initially decreased and then reached a constant value at
longer treatment times.
The trends in the reaction yields and in the CI's, suggests
that the degree of crystallinity is a descriptor for the specic
reactivity of cellulose. This hypothesis is also supported by the
results obtained with wet ball-milled samples. In these samples,
it was found that an increase/decrease in the CI (Fig. S5A†) was
directly correlated with a decrease/increase in the total reaction
yields (Fig. S6†). The sample ball-milled with 5% H2O had the
lowest CI (52.0%) and achieved the highest level of conversion
ꢀ
at 25 C was 0.05%, and thus, negligible. With increasing ball-
milling duration, the solubility monotonously increased and
reached 0.79% aer 54 h (Table S3†). The soluble fraction was
composed of oligosaccharides (94%) and monosaccharides
(6%). No traces of dehydration products (5-HMF and levoglu-
cosan) could be detected in any of the samples. Analysis of the
soluble fraction using liquid chromatography coupled to a mass
spectrometer (LC-MS) enabled identication of oligosaccha-
rides with a DP of up to 9 (Fig. S8B†). We do not exclude that
larger oligosaccharides formed during ball-milling. However,
due to the dependence of solubility on the DP it is improbable
that larger oligosaccharides dissolved in sufficiently high
concentration to enable reliable analysis. We stress that we
found oligosaccharides of comparable size (DP ¼ 8) in the
soluble fraction of pristine cellulose (Fig. S8A†), although at
lower concentration.
The formation of soluble oligosaccharides conrms that
ball-milling indeed induced glycosidic bond cleavage. The
increased solubility undoubtedly aids in the conversion of
cellulose during the subsequent hydrolysis in the semi-batch
reactor. Additionally, it is reasonable to assume that the solu-
bility of the pretreated samples was even higher under the
ꢀ
conditions in the semi-batch reactor (165 C). However, due to
the tendency of cellulose to undergo auto-hydrolysis at elevated
temperatures it is not possible to measure the solubility of
samples under the experimental conditions. Nevertheless, we
investigated if the enhanced reactivity can be ascribed to an
enhanced tendency of cellulose to undergo homogeneous
hydrolysis pathways. Fig. 6 shows the conversion and yields
achieved with the sample BM-18 h in presence and in absence of
EDTA and the sulfonated carbon catalyst. In comparison with
pristine cellulose (Fig. 2) the conversion through auto-
hydrolysis increased from 8.9% to 17.4%. The major reaction
products were the same as with pristine cellulose;
Fig. 5 Crystallinity index ( ), conversion ( ) and yields of oligosac-
charides ( ), monosaccharides ( ) and dehydration products ( ) ach-
ieved with ball-milled cellulose in presence of C-350-SO3H-HT.
Reaction conditions: 165 ꢀC, 6.5 h, 3 wt% BM-cellulose, S/C ¼ 7/2,
1 mM EDTA & 15-C-5, t ¼ 35 min, impeller speed ¼ 600 rpm.
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oligosaccharides (Y ¼ 9.9%) that only partially underwent cellulose, the contribution of homogeneously catalysed reaction
secondary hydrolysis to monosaccharides (Y ¼ 3.1%). When the is overlooked. The ndings presented here, however show that
hydrolysis was performed in presence of EDTA the conversion even under the comparatively mild reaction conditions chosen
(X ¼ 21.6%) further increased and the formed oligosaccharides in this study, amorphized cellulose readily undergoes homo-
(Y ¼ 9.4%) increasingly underwent secondary hydrolysis to geneously catalysed hydrolysis (auto-hydrolysis and hydrolysis
monosaccharides (Y ¼ 9.2%). In presence of both EDTA and the by EDTA).
sulfonated carbon catalyst, the conversion reached 27.2%
(15.7% with pristine cellulose). Thus, as during experiments
with pristine cellulose the conversion increased by approxi-
Mechanocatalytic depolymerization of cellulose
mately 6% when the reaction was performed in presence of the
sulfonated carbon catalyst.
Despite the higher yields achieved with ball-milled cellulose,
the majority of product formation occurs via homogeneously
catalysed reactions and only to a limited extent via reaction with
the sulfonated carbon catalyst.
The pathway experiments clearly exemplify that the higher
specic reactivity of ball-milled cellulose can be explained with
an enhanced tendency of the substrate to undergo homoge-
neous hydrolysis (auto-hydrolysis and hydrolysis by EDTA). The
enhanced reactivity can be rationalized with the increased
amorphous content (lower CI) and the higher propensity of the
amorphous fraction to undergo homogeneous hydrolysis. The
preferential hydrolysis of glycosidic bonds in amorphous
domains by homogeneous reactions is conrmed by the CI
determined for the unconverted residues. The CI increased
from 57.4% to 72.1% in the unconverted residue recovered aer
the auto-hydrolysis experiment with BM-18 h (Fig. S9B†). An
increase in the CI was also found in the unconverted residue
recovered aer experiments with pristine cellulose (Fig. S9A†).
These results thus conclusively explain why ball-milled cellulose
is oen found to exhibit a higher specic reactivity during
a solid acid catalysed hydrolysis. The higher reactivity does not
result from an enhanced reaction with the solid acid, but rather
from the higher tendency of the substrate to undergo homo-
geneously catalysed reactions. Thus, homogeneous reaction
pathways play a critical role in the hydrolysis of cellulose over
solid acid catalyst. It is important to point out that in many
studies dealing with the solid acid catalysed conversion of
A truly heterogeneously catalysed hydrolysis should however
occur during the concerted ball-milling (CBM) of cellulose with
a solid acid. To investigate the efficacy of the sulfonated carbon
catalyst in facilitating a mechanocatalytic depolymerization and
to understand the inuence on the structure and reactivity of
cellulose, we subjected the two powders to CBM treatment (S/C
¼ 7/2, 300 rpm, 18 h). Fig. 7 shows the yields in oligosaccha-
rides, monosaccharides and dehydration products achieved
with CBM-7/2 treated cellulose. Compared with cellulose sub-
jected to individual ball-milling, the CBM treated cellulose
much more readily underwent conversion to yield 17.4%, 35.3%
and 1.2% oligo-, monosaccharides and dehydration products
during hydrolysis in the semi-batch reactor.
To elucidate the phenomena leading to the enhanced reac-
tivity, structural changes in the CBM treated cellulose were
characterized. In a rst step the XRD patterns of the CBM
treated samples were recorded and the respective contribution
of cellulose to these was determined. The latter was accom-
plished by assuming that the diffraction pattern of the powder
mixture can be described as a weight-fraction weighted linear
combination of the diffraction patterns of the individual
components.69 This approach was veried by comparing the
measured and the calculated diffraction pattern of a 7/2
Fig. 6 Conversion ( ) and yields of oligosaccharides ( ), mono-
saccharides ( ) and dehydration products ( ) achieved with 18 h ball- Fig. 7 Yields of oligosaccharides ( ), monosaccharides ( ) and dehy-
milled cellulose in presence and in absence of EDTA and C-350- dration products ( ) achieved with CBM-7/2 treated cellulose. Reac-
SO3H-HT. Reaction conditions: 165 ꢀC, 6.5 h, 3 wt% BM-18 h, s ¼ tion conditions: 165 ꢀC, 6.5 h, 3 wt% CBM-7/2, 1 mM EDTA & 15-C-5, s
35 min, impeller speed ¼ 600 rpm.
¼ 35 min, impeller speed ¼ 600 rpm.
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physical mixture of individually ball-milled cellulose and indi- amorphization is not solely induced by the action of the
vidually ball-milled C-350-SO3H-HT (Fig. S10†). sulfonic acid groups, i.e. via glycosidic bond cleavage, but may
The diffraction pattern of CBM treated cellulose (Fig. 8A) was rather be related to an interaction between the carbon material
composed of a single broad peak centred at 2q z 20ꢀ, previously and cellulose. Furthermore, because deep amorphization could
assigned to para-crystalline structures. The broad peak featured not be achieved by physically mixing the two solids (Fig. S10†),
a shoulder at 2q z 15ꢀ associated with overlapping diffractions the interaction between the two powders appears to be facili-
ꢀ
from the (1 0 1) and (1 0 1) crystal planes. A very weak signal tated by the CBM treatment.
detected at 2q ¼ 34.5ꢀ is associated with the (0 0 4) diffraction.
Previous studies have provided compelling evidence for the
Comparing the diffraction patterns of CBM-treated and indi- ability of poly-aromatic systems in carbon materials to adsorb
vidually ball-milled cellulose, illustrates that the former expe- oligosaccharides from aqueous solutions.70,71 The adsorption
rienced
a
much higher degree of amorphization. process occurs on hydrophobic sites on the carbon material
Correspondingly, the CI determined from the deconvoluted (polyaromatic domains) and is driven by entropically favoured
peak areas of CBM-treated cellulose (31.6%) was signicantly hydrophobic interactions combined with CH–p hydrogen
lower than of individually ball-milled cellulose (57.4%).
bonding.72 The affinity of oligosaccharides for carbons has been
claimed to be higher than the affinity between oligosaccha-
rides.72 Consistent with these studies, adsorption tests per-
formed in the scope of this study showed that the sample C-350-
SO3ꢁH1-HT had an adsorption capacity of 0.79 mg cellobiose
gcat (Fig. S11†). It may therefore be possible that the CBM
treatment led to sufficient proximity between the carbon
material and the sulfonated carbon catalyst to facilitate
adsorption and thereby amorphization. In an effort to validate
this analysis, a FTIR spectroscopy based investigation of the
liquid-phase and mechanochemical adsorption process was
performed. Spectroscopic evidence of the adsorption process by
comparing the position of the n(C–H) absorption band in free
and adsorbed cellobiose/cellulose could however not be estab-
lished due to the to the low adsorbate loading and the strong IR
absorptivity of the carbon material.
The pronounced affiliation between the substrate and the
catalyst can however be seen in the SEM images of the CBM-7/2
mixture (Fig. 9). The images show strongly aggregated particles
in which the grain boundaries of the substrate and the catalyst
cannot be identied. To visualize the strongly aggregated nature
of the powder mixture, energy-dispersive X-ray spectroscopy
(EDX) was employed to map the distribution of sulphur (origi-
nating solely from the catalyst) within the particles. The EDX
mapping shows that sulphur is evenly distributed within the
The role of catalyst surface functionalities during
mechanocatalytic depolymerization
To understand the processes involved in amorphizing cellulose
and the role of the catalyst's surface functionalities in these, the
CBM treatment was repeated with carbon materials featuring
variations in their surface chemistries. The CBM treatment was
repeated with rstly a non-sulfonated carbon material (denoted
CBM-7/2-no sulf.) and secondly with a deactivated sulfonated
carbon catalyst (denoted CBM-7/2-deac.). The deactivated cata-
lyst was prepared by exchanging the protons of the sulfonic acid
groups with cations, via three consecutive ion-exchange treat-
ments with an aqueous solution containing commonly occur-
ring cationic impurities (Na+, K+, Ca2+ and Mg2+ 0.25 wt% each).
Interestingly, in both cases the CBM treatment led to compa-
rable degrees of amorphization as when the as synthesized C-
350-SO3H-HT was used in the treatment (Fig. 8A). The corre-
sponding CI's of CBM-treated cellulose with the non-sulfonated
or the deactivated materials were 35.9% and 34.7%, respec-
tively. The fact that similar reductions in crystallinity occurred
regardless of the material's surface chemistry, indicates that
Fig. 8 (A) Powder X-ray diffraction patterns of pristine ( ), BM-18 h ( ),
CBM-7/2 ( ), CBM-7/2-no sulfonation ( ) and CBM-7/2-deactivated
catalyst ( ). (B) Powder X-ray diffraction pattern of CBM-7/2 ( ), the
ꢀ
fitted ① – (1 0 1) ( ), ② – (1 0 1) ( ), ④ – para-crystalline ( ), ⑤ – (0 0 2)
( ), ⑥ – (0 0 4) ( ) peaks and sum of the fitted peaks ( ).
Fig. 9 SEM images and sulphur EDX mapping of CBM-7/2.
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particles, which indicates that the CBM treatment promoted the stability of the catalyst was therefore studied during repeated
formation of a solid–solid suspension of the substrate and the CBM treatment & hydrolysis cycles. The S/C ratio was chosen
catalyst. The strongly affiliated nature of the CBM components such as to achieve levels of conversion far from 100%.
is also supported by the fact that the particle size distribution of
Fig. 10A shows the conversion and yields achieved during
the mixture was unimodal (Fig. S12†) and had a mean particle four consecutive CBM-hydrolysis cycles. Aer each cycle, the
size (13.5 mm) that was nearly half as large as that of individually cellulose-catalyst mixture was recovered and the former was
ball-milled cellulose (24.6 mm). The higher degree of amorph- removed through dissolution in a 72% H2SO4 solution. The
ization as well as the further reduction in particle size achieved recycled catalyst and fresh cellulose were then subjected to
during CBM, as compared to standard ball-milling, is thus repeated CBM treatment and hydrolysis in the semi-batch
tentatively attributed to an interaction of the two powders reactor. During the rst cycle a minor increase/decrease in the
facilitated by the strong mechanical forces exerted upon them selectivity of forming oligosaccharides/monosaccharides
during CBM treatment.
occurred, which can be attributed to the leaching of –SO3H
Although similar degrees of amorphization were achieved groups (Fig. 10B). The density of active sites decreased from 869
during the CBM treatment with C-350, C-350-SO3H-HT and C- mmol gꢁ1 in the fresh catalyst to 556 mmol gꢁ1 during the rst
350-SO3H-HT-deac, the samples displayed fundamentally two cycles. Importantly, the remaining sulfonic acid groups of
different reactivity (Table S4†). When the non-sulfonated carbon the catalyst proved to be stable during the third and fourth
was used for the CBM treatment, the obtained yields in oligo-, cycle. The chemical identity of the remaining sulphur in the
monosaccharides and dehydration products were 12.8%, 12.3% catalyst was conrmed by recording FTIR spectra of the spent
and 0.3%, at 26.9% conversion, respectively. The conversion and materials (Fig. S13B†). The FTIR spectra of the spent materials
yields are comparable to those obtained when hydrolysing indi- show an absorption band at 1040 cmꢁ1, previously assigned to
vidually ball-milled cellulose in absence of the sulfonated carbon the symmetric stretching mode of O]S]O in –SO3H groups.
catalyst (Fig. 6). Thus, in this case, conversion occurred
Despite leaching of –SO3H groups, the catalytic activity
predominantly via homogeneously catalysed pathways (auto- remained stable over the four cycles. Consequently, leached
hydrolysis and hydrolysis by EDTA). Similarly, the conversion sulphur containing species did not contribute to product
and yields increased only marginally when the CBM treatment formation during the CBM treatment or during hydrolysis in the
was performed with the deactivated catalyst (Table S4†). Conse- semi-batch.
quently, –SO3H groups must play a prominent role in catalysing
the formation of soluble reaction products during the CBM reduced –SO3H density is not due to the leaching of sulphur
treatment or the subsequent hydrolysis process. from the material but is rather caused by changes in the
It is also important to point out that a part of the apparently
Solubility tests performed with the CBM treated samples elemental composition of the catalyst. Individual ball-milling of
revealed that mechanocatalytic depolymerization only occurred the catalyst led to an increase in the oxygen content (fresh:
when the active sulfonated carbon catalyst was used during the 31.8%; ball-milled: 37.5%) which consequently also reduced the
treatment. When the CBM treatment was conducted with the sulfonic acid group density (869 to 788 mmol gꢁ1). Besides
non-sulfonated or the deactivated materials the solubilities leaching, the catalyst also underwent a reduction in its particle
were 0.7% and 1.9%, respectively (Table S4†). In contrast, 32% size during the recyclability experiments. SEM images of the
of the sample CBM-7/2 was soluble in Millipore water at room recovered catalyst (Fig. S13A†) show that ball-milling led to
temperature. The soluble fraction was composed of oligosac-
charides (95%) with a DP up to 10 (Fig. S8C†) with additional
minor amounts of monosaccharides (4%) and dehydration
products (1%) formed. Based on the nding that individually
ball-milled cellulose or cellulose subjected to CBM treatment
with the non-sulfonated or deactivated carbon displayed only
marginal solubility it is concluded that the enhanced solubility
can be attributed to a mechanochemical depolymerization
catalysed by –SO3H groups. It is further noteworthy, that unlike
the soluble fraction from ball-milled cellulose, the sample CBM-
7/2 contained dehydration products. This can be interpreted as
a further indication that the CBM treatment induces sufficient
proximity between the catalyst and reaction products to facili-
tate a reaction.
Stability of the sulfonated carbon catalyst
Fig. 10 (A) Conversion ( ) and yields of oligosaccharides ( ), mono-
saccharides ( ) and dehydration products ( ) during recyclability
The CBM treatment and the subsequent hydrolysis process
expose the sulfonated carbon catalyst to harsh mechanical
forces and hydrothermal conditions. Thus, the catalyst may be
experiments. (B) Sulphur content of catalyst before ( ) and after (
a reaction cycle. Reaction conditions: 165 ꢀC, 6.5 h, 3 wt% CBM-7/2,
subject to deactivation by attrition or –SO3H leaching. The 1 mM EDTA & 15-C-5, s ¼ 35 min, impeller speed ¼ 600 rpm.
)
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a reduction in size and changed the morphology of the parti- analysis48,53,54 revealed that the spruce r wood was composed of
cles. The median of the particle size distribution aer the rst hemicellulose (20.5%), cellulose (43.3%) and lignin (32.3%) as
cycle was reduced to 16.7 mm. During the second and third cycle well as minor amounts of extractives (3.2%) and ash (0.5%).
the median decreased further to 13.8 and 13.3 mm, respectively. Both hemicellulose and cellulose undergo acid catalysed
hydrolysis following very similar chemical pathways. Like C6-
based compounds C5-containing oligosaccharides undergo
hydrolysis to form the corresponding monosaccharides that can
subsequently undergo acid catalysed dehydration to furfural.
However, unlike cellulose, the hemicellulose in the spruce wood
is a branched heteropolymer composed of mannose (10.7%),
xylose (7.7%) and arabinose (2.1%) units. The branching of the
backbone hinders the aggregation of the polymer chains and
thereby prevents the formation of inter-chain H-bonds. Conse-
quently, hemicellulose exhibits a signicantly higher tendency
to undergo auto-8 or acid catalysed14 hydrolysis. In contrast to
the cellulosic constituents, lignin is a crosslinked hetero-
polymer composed of phenyl-propanoid building blocks inter-
connected through different carbon–oxygen and carbon–carbon
bonds. The presence of carbon–carbon bonds implies that
lignin cannot be fully depolymerized using only Brønsted acids
under the conditions employed in this study. Fig. 12 shows the
yields for C5- and C6-derived reaction products achieved with
CBM-treated spruce wood (S/C ¼ 1/1) during hydrolysis in the
semi-batch reactor. As evidenced by the higher yields, the C5-
containing hemicellulose was signicantly more reactive. The
corresponding yields for C5 (xylose and arabinose) derived
oligosaccharides, monosaccharides and dehydration products
were 15.7%, 84.9% and 4.9%, respectively.
Inuence of the substrate-to-catalyst ratio during
mechanocatalytic depolymerization
Having established that the catalyst was stable, in a next step
efforts were made to optimize the conversion of the substrate. To
this end, we investigated the inuence of the S/C ratio on the
mechanocatalytic depolymerization and the hydrolysis in the
semi-batch reactor. Regardless of the S/C ratio, CBM treatment
led to extensive amorphization, whereas higher catalyst loadings
resulted in slightly lower CI's (Table S4†). However, the S/C ratio
directly dened the amount of soluble oligosaccharides formed
during CBM treatment. Whereas individually ball-milled cellu-
lose was only 0.4% soluble, the sample CBM-10/1 exhibited
a tenfold higher solubility (3.9%). The solubility of CBM treated
cellulose increased further with the inverse of the S/C ratio and
reached 64.1% for CBM-1/1 treated cellulose (Fig. 11A).
Following CBM treatment at varying S/C ratios, the samples
were hydrolysed in the semi-batch reactor (Fig. 11B). During
hydrolysis in the semi-batch reactor the oligosaccharides
formed during CBM treatment were increasingly converted to
monosaccharides at higher S/C ratios. At a S/C ratio of 1/1,
78.2% of cellulose was converted to yield 17.4%, 50.0% and
1.4% oligosaccharides, monosaccharides and dehydration
products, respectively.
The yields in C6 (glucose and mannose) derived oligosac-
charides, monosaccharides and dehydration products were
21.4%, 52.6% and 2.2%, respectively. As expected, lignin did not
undergo extensive depolymerization. Only traces of the lignin
depolymerization markers vanillin, 2-methoxy-4-propyl phenol
and 4-acetoxy-3-methoxy acetophenone could be identied in
the reactor effluent. Solubility tests further showed that the
CBM-treated sowood underwent mechanocatalytic hydrolysis
Hydrolysis of sowood
To assess the applicability of the developed conversion strategy
to real lignocellulose, in a nal step the hydrolysis of spruce r
wood (picae abies) was investigated. Compositional
Fig. 11 Conversion ( ) and yields of oligosaccharides ( ), mono-
saccharides ( ) and dehydration products ( ) achieved with CBM
treated cellulose as a function of the substrate-to-catalyst ratio, during Fig. 12 Yields of (A) C5- and (B) C6-derived oligosaccharides ( ),
(A) solubility tests (25 ꢀC, 1 g CBM-cellulose per 10 mL Millipore water, monosaccharides ( ) and dehydration products ( ) achieved with
1 h) and (B) hydrolysis in the semi-batch reactor (165 ꢀC, 6.5 h, 3 wt% CBM-1/1 treated softwood. Reaction conditions: 165 ꢀC, 6.5 h, 2 wt%
CBM-cellulose, 1 mM EDTA & 15-C-5, s ¼ 35 min, impeller speed ¼ CBM-1/1 softwood, 1 mM EDTA & 15-C-5, s ¼ 35 min, impeller speed
600 rpm).
¼ 600 rpm.
33536 | RSC Adv., 2019, 9, 33525–33538
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to yield 8.2% and 6.1%, C5- and C6-derived compounds,
respectively. The high degree of depolymerization of both
cellulose and hemicellulose conrm that the developed meth-
odology can be extrapolated to real lignocellulose. Furthermore,
the achieved yields in C5- and C6-derived products are the
highest ever reported in literature for a continuous hydrolysis
process that does not rely on the use of strong homogeneous
Brønsted acids. Remarkably, the yields even show that the
developed conversion strategy can compete with those achieved
with conventional diluted homogeneous acid processes (C5:
88%, C6: 50–60%).12–14
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A two-step process composed of mechanocatalytic pre-treatment
and secondary hydrolysis in a semi-batch reactor was developed
using sulfonated carbons as recyclable solid acid catalyst. The
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Innovation Agency lnnosuisse (Grant Number 2150001269) and
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