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Journal ArticleDOI

1D Lignin-Based Solid Acid Catalysts for Cellulose Hydrolysis to Glucose and Nanocellulose

28 Aug 2015-ACS Sustainable Chemistry & Engineering (American Chemical Society)-Vol. 3, Iss: 10, pp 2566-2574

AbstractOne-dimensional (1D) solid acid catalysts have been synthesized from lignin-based activated carbon fibers via sulfonation and hydrothermal treatment to be mesoporous and contain 0.56 mmol/g sulfonic and 0.88 mmol/g total acid for direct hydrolysis of highly crystalline rice straw cellulose (CrI = 72.2%). Under optimal hydrothermal conditions of 150 °C and 5 atm, 69.8% of cellulose was hydrolyzed in three consecutive runs, yielding 64% glucose at 91.7 selectivity as well as 8.1% cellulose nanofibrils (2.1 nm thick, 3.1 nm wide, and up to 1 μm long). These 1D acid catalysts could be used repetitively to hydrolyze the remaining cellulose as well as be easily separated from products for hydrolysis of additional cellulose. In essence, complete valorization of rice straw cellulose has been demonstrated by direct hydrolysis with these 1D acid catalysts to superior glucose selectivity while generating high value cellulose nanofibrils.

Topics: Cellulose (66%), Nanocellulose (62%), Hydrolysis (56%), Lignin (55%)

Summary (1 min read)

■ INTRODUCTION

  • Heterogeneous solid acid catalysts are particularly suited for large-scale industrial operations due to their many advantages over homogeneous liquid phase catalysts.
  • 28 An exceptional 74% glucose yield was achieved under hydrothermal conditions using sulfonated ordered mesoporous carbon derived from a silica template.
  • Therefore, solid acid catalysts with high acid densities and yet geometries that allow easy separation for reuse would be of significant interest.
  • 30 This study aimed to synthesize one-dimensional (1D) solid acid catalysts for direct hydrolysis of crystalline cellulose for simultaneous production of sugars and high valued nanocellulose.
  • The 1D solid acid catalysts were prepared from sulfonation of submicron-sized activated carbon fibers (ACFs) using concentrated sulfuric acid.

■ EXPERIMENTAL SECTION

  • For repetitive hydrolysis, the dried HTSACF catalysts along with the remaining cellulose were redispersed to 32 mL of water and heated at 150 °C for a second and then a third time with drying and weighing in between to determine the extent of cellulose hydrolysis for each repetition.
  • Overall, the SACFs carbonized at higher 900 °C still had a much higher acid density than SCFs carbonized at lower 600 °C and therefore were hydrothermally treated to prepare stable solid acid catalysts.
  • Hydrothermal treatment reduced both BJH mesopore and tplot-derived micropore surface areas to 299 and 197 m 2 /g and corresponding pore volumes to 0.69 and 0.08 cm 3 /g, respectively, suggesting ∼90% of the pores were mesopores in the 5−50 nm range.

ACS Sustainable Chemistry & Engineering

  • Showing consistent HTSACF catalytic activity and persistent cellulose hydrolysis over repeated hydrolysis cycles.
  • At 150 °C, it is postulated that hydronium ions could diffuse faster into the amorphous regions and interspacing between crystallites while simultaneous defibrillation of partially hydrolyzed cellulose into nanofibrils was facilitated by the increased pressure.
  • Both yields and morphologies of CNFs from hydrolysis catalyzed by HTSACFs at 150 °C were compared with CNCs from sulfuric acid hydrolysis 42 and CNFs from TEMPO oxidation, 33, 43 all from the same rice straw cellulose.
  • Instead, the hydronium ions diffuse from HTSACFs into the contacting surface layer of cellulose to cleave the hydrogen bond among amorphous chains and crystallites to peel off thin nanofibrils from the cellulose surface.

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UC Davis Previously Published Works
Title
1D lignin-based solid acid catalysts for cellulose hydrolysis to glucose and nanocellulose
Permalink
https://escholarship.org/uc/item/1t4608p8
Journal
ACS Sustainable Chemistry and Engineering, 3(10)
ISSN
2168-0485
Authors
Hu, S
Jiang, F
Hsieh, YL
Publication Date
2015-10-05
DOI
10.1021/acssuschemeng.5b00780
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

1D Lignin-Based Solid Acid Catalysts for Cellulose Hydrolysis to
Glucose and Nanocellulose
Sixiao Hu, Feng Jiang, and You-Lo Hsieh*
Fiber and Polymer Science, University of California-Davis, One Shields Avenue, Davis, California 95616, United States
*
S
Supporting Information
ABSTRACT: One-dimensional (1D) solid acid catalysts have
been synthesized from lignin-based activated carbon bers via
sulfonation and hydrothermal treatment to be mesoporous and
contain 0.56 mmol/g sulfonic and 0.88 mmol/g total acid for
direct hydrolysis of highly crystalline rice straw cellulose (CrI =
72.2%). Under optimal hydrothermal conditions of 150 °C
and 5 atm, 69.8% of cellulose was hydrolyzed in three
consecutive runs, yielding 64% glucose at 91.7 selectivity as
well as 8.1% cellulose nanobrils (2.1 nm thick, 3.1 nm wide,
and up to 1 μm long). These 1D acid catalysts could be used
repetitively to hydrolyze the remaining cellulose as well as be
easily separated from products for hydrolysis of additional cellulose. In essence, complete valorization of rice straw cellulose has
been demonstrated by direct hydrolysis with these 1D acid catalysts to superior glucose selectivity while generating high value
cellulose nanobrils.
KEYWORDS: Solid acid catalysts, Mesoporous, Activated carbon bers, Hydrolysis, Glucose, Cellulose nanobrils
INTRODUCTION
Heterogeneous solid acid catalysts are particularly suited for
large-scale industrial operations due to their many advantages
over homogeneous liquid phase catalysts. Heterogeneous solid
acid catalysts are noncorrosive and safe and can function more
eciently under continuous ow than the typical batch
operations of homogeneous liquid acid catalysts.
1
In solid
acid catalysis, the reactants can be activated directly by protons
generated from the acid sites or via hydrogen spillover from
other active phases in the system.
2
The products could be easily
separated to allow recovery and reuse of the solid catalysts.
Various solid acid catalysts, such as zeolite,
36
silica,
712
hydrous zirconia,
12
naon
1315
and carbons,
1619
have been
developed for common chemical reactions, including ester-
ication,
3,12,16
alcohol dehydration,
7,13
and hydrolysis.
18,19
Solid catalysts have also been reported in the hydrolysis of
cellulose for biofuel production, including naon,
15
zeolite,
20
Fe
3
O
4
magnetic nanoparticles bearing mesoporous silica,
21
nano Zn-Ca-Fe oxide,
22
as well as a wide range of cellulose
chars and activated carbon particulates.
18,19,2329
Among them,
carbon solid acid catalysts have exhibited particularly high
activity because of their high acid densities and strong
interactions with the β-1,4-glycosidic bonds in cellulose.
28
An
exceptional 74% glucose yield was achieved under hydro-
thermal conditions using sulfonated ordered mesoporous
carbon derived from a silica template.
18
However, either ball
milling
18,26,29
or ionic liquid dissolution of cellulose
27
was
necessary to enhance contact or diusion to destruct the
crystalline structure of cellulose. To overcome the accessibility
issue, smaller Zn-Ca-Fe oxide nanoparticle catalysts have been
shown to signicantly increase the hydrolysis of crystalline
cellulose into glucose and oligosaccharides.
22
Separation of
zero-dimensional nanocatalysts from the hydrolysate products
is, however, dicult. Therefore, solid acid catalysts with high
acid densities and yet geometries that allow easy separation for
reuse would be of signicant interest. Furthermore, solid acid
hydrolysis of cellulose should generate nanocellulose, but this
has only been reported on solid cationic exchange polystyrene
resin beads (NKC-9 with 4.7 mmol/g acid density).
30
This study aimed to synthesize one-dimensional (1D) solid
acid catalysts for direct hydrolysis of crystalline cellulose for
simultaneous production of sugars and high valued nano-
cellulose. The 1D solid acid catalysts were prepared from
sulfonation of submicron-sized activated carbon bers (ACFs)
using concentrated sulfuric acid. The ultrane ACFs were
electrospun from lignin and simultaneously carbonized and
activated at 900 °C to high specic surface (1400 m
2
/g) and
pore volume (0.7 cm
3
/g).
31
The solid acid catalysts from lignin-
based ACFs have several advantages and desired attributes over
other solid acid catalysts for hydrolysis of crystalline cellulose.
Lignin, an abundant and underutilized biomass fraction, is an
excellent carbon precursor, and lignin chars are less recalcitrant
than rigid graphitic carbons, such as carbon nanotubes,
18
to be
readily sulfonated by concentrated sulfuric acid, a much safer
alternative than the commonly used fuming sulfuric acid for
sulfonation of graphitic carbons. These 1D solid acid catalysts
Received: July 29, 2015
Revised: August 21, 2015
Published: August 21, 2015
Research Article
pubs.acs.org/journal/ascecg
© 2015 American Chemical Society 2566 DOI: 10.1021/acssuschemeng.5b00780
ACS Sustainable Chem. Eng. 2015, 3, 2566 2574

can be directly mixed with untreated semicrystalline cellulose.
ACFs, with the high aspect ratio, submicron diameters, and
porous structures are expected to be in more intimate contact
with cellulose, aording shorter access to the active sites and
better interconnected diusive pathways for rapid and ecient
hydrolysis while minimizing deactivation of the catalysts. The
continuous brous form of the solid acid catalysts can be easily
separated from the soluble products via simple ltration for
repetitive use.
EXPERIMENTAL SECTION
Materials. Pure cellulose was isolated from rice straw (Calrose
variety) by extraction with 2:1 v/v toluene/ethanol and subsequent
dissolution of lignin and hemicellulose/silica with acidied NaClO
2
(1.4 wt %, 70 °C, 5 h) and alkaline KOH (5 wt %) at 90 °C for 2 h
resulting in 36% yield and 72.2% crystallinity.
32,33
Poly(ethylene
oxide) (PEO) (M
w
= 600 kDa), alkali lignin (low sulfonate) (AL
ls
)
(M
w
= 60 kDa, spruce origin) were acquired from Sigma-Aldrich
(USA), sodium hydroxide (anhydrous NaOH pallets, A.C.S. grade, 85
wt % minimum purity), aqueous NaOH (1N), sulfuric acid (9598 wt
% purity), dinitrosalicylic acid (98 wt % purity), sodium sulte
anhydrous (98 wt % minimum purity), potassium sodium tartrate
tetrahydrate (A.C.S. grade), and water (HPLC grade) were from
Fisher Scientic (USA). All of the chemicals were used as received.
Synthesis of Solid Acid Catalysts. Both carbon bers (CFs) and
activated carbon bers (ACFs) were synthesized according to a
previously reported approach.
31
Briey, aqueous 9:1 w/w AL
ls
/PEO
(10 wt % total concentration) mixtures without and with NaOH at a
1:2 w/w NaOH/lignin ratio were electrospun into CF and ACF
precursor bers, respectively. The precursor bers were placed in a
quartz tube (2 cm inner diameter) of an electric furnace (Mini-Mite,
Lindberg/Blue), heated at 10 °C min
1
under owing N
2
at 100 mL
min
1
to 105 °C, held for 0.5 h to drive o moisture, and then to 600
(CFs) or 900 °C (CFs and ACFs), held for another 0.5 h, and nally
cooled to ambient temperature under owing N
2
at 100 mL min
1
.
CFs and ACFs were then washed with deionized water repeatedly to
remove residual alkali metals and other small hydrocarbon impurities
followed by oven drying at 60 °C for 12 h.
Dried CFs and ACFs were sulfonated with concentrated sulfuric
acid at 110 or 150 °C for 20 h (5 mg/mL solid/liquid ratio, 1 atm) to
obtain sulfonated CF (SCFs) and ACFs (SACFs), respectively. SACFs
were rinsed thoroughly with 70 °C water to remove residual sulfuric
acid and then heated in water at 150 °C and 5 atm for 24 h using a 100
mL poly(tetrauoroethylene) cylindrical reactor to obtain the
hydrothermal-treated SACFs. Hydrothermal-treated SACFs sulfonated
at 150 °C were further characterized and used in the hydrolysis of
cellulose and were denoted as HTSACFs unless specied otherwise.
Hydrolysis of Cellulose. Solid HTSACFs catalyst (160 mg) and
cellulose (40 mg) were dispersed in 32 mL of water in a 100 mL PTFE
cylindrical reactor, hermetically sealed, and heated in 110, 130, and
150 °C oil baths for 24 h with corresponding pressures of 1.4, 2.8, and
5.0 atm, respectively. The catalysts and remaining cellulose were
vacuum ltered (Whatman No. 1, dried, and weighed to 0.1 mg). The
average mass of the remaining cellulose and the catalysts (m
cell+HTSACF
)
from ve reactions were used to determine the extent of cellulose
hydrolysis via eq 1. For repetitive hydrolysis, the dried HTSACF
catalysts along with the remaining cellulose were redispersed to 32 mL
of water and heated at 150 °C for a second and then a third time with
drying and weighing in between to determine the extent of cellulose
hydrolysis for each repetition. The ltrates were thoroughly dialyzed
(1214 kDa MWCO, Fisher brand) to exchange the sugars in the
aqueous environm ent and retain nanocellulose in the dialysis
membrane. The glucose produced was quantied by the colorimetric
assay as described later. The nanocellulose yield was determined
gravimetrically by weighing dried nanocellulose and reported as a
percentage of the original cellulose.
=
×
+
m
cellulose hydrolysis (wt %)
160
40
100%
cell HTSACF
(1)
Analytical Methods. The chemical structures of ACFs, SACFs,
and HTSACFs were examined by Fourier transform infrared
spectroscopy (FTIR) (Nicolet 6700, Thermo Scientic), and their
morphologies were characterized by scanning electron microscope
(SEM) (FEI-XL 30, FEI) and atomic compositions by energy-
dispersive X-ray spectroscopy (EDX) adjunct to the SEM. All samples
were washed thoroughly with deionized water and dried at 60 °C for at
least 12 h before characterization. All FTIR spectra were collected
from samples pressed with anhydrous KBr powders into pellets. All
samples were sputter coated with gold for 1 min and imaged by SEM,
and noncoated samples were analyzed by EDX under a working
voltage of 5 kV. The acid density of the catalysts was analyzed via
conductometric titration using a pH/con ductivi ty meter (510,
OAKTON). For each titration, 10 mg of the acid catalysts were
dispersed in 15 mL of water, and 0.02 N NaOH was added to
neutralize the acids. The surface acid density (σ, in mmol/g of the
solid acid catalyst) was calculated from eq 2, where c is the NaOH
concentration (in N), m is the mass of the catalyst (in g), and V is the
volume of NaOH (in mL) used to neutralize the sulfonic acid and total
acids on the catalysts.
σ =
V
m
c
(2)
For surface area and pore characterization, the ACFs, SACFs, and
HTSACFs were dried at 50 °C for 48 h and then measured at 77 K by
a nitrogen adsorptiondesorption analyzer (ASAP 2020, Micro-
metics). The single point total pore volume was estimated from
nitrogen adsorption at a relative pressure P/P
o
close to 1.The
BrunauerEmmettTeller (BET) surface area was calculated from the
isotherm in the BET linear region where relative pressure P/P
o
ranged
from 0.05 to 0.3. The mesopore surface area was derived from the
adsorption branch whereas pore and neck size distributions were
derived from both adsorption and desorption branches of the isotherm
using the BarretJoynerHalenda (BJH) method. To derive micro-
pore characteristics from the adsorptiondesorption isotherm with
incomplete data below 0.05 P/P
o
relative pressure, micropore surface
area and pore hydraulic diameter distributions from 0.7 to 1.6 nm were
derived from a t-plot using the Mikhail, Brunauer, and Bodor MP
method
34
and the Harkins and Jura equation.
35
Micropore volume
(V
mp
) was derived from the tangent line of a contiguous range of the t-
plot using the surface area of the lled pores via eq 3
=
−+
×
+−
V
SS tt()()
2
15.47
nn nn
mp
11
(3)
where S
n
and t
n
are the surface area derived from the slope of the
tangent and the thickness of the adsorbed layer at the n point in the t-
plot, respectively, and 15.47 was the constant for converting gas
volume to liquid volume at STP.
The mixed HTSACF catalysts/cellulose (4:1) before and after three
consecutive reactions (32 mL of water and 150 °C) were sonicated in
a water bath (Branson 2510) for 10 min and then visualized under a
polarizing light microscope (PLM, LEICA DM 2500). The nano-
cellulose separated from the catalyst by dialysis was diluted 10×,of
which 10 μL was deposited onto a freshly cleaved mica surface, air-
dried, and scanned using an atomic force microscope (AFM, MFP 3D,
Asylum Research) under ambient air conditions in the tapping mode
with OMCL-AC160TS standard silicon probes at 1 Hz scan rate and
512 × 512 pixel resolution. The height image and pro le were
processed with Igor Pro 6.21 loaded with MFP3D 090909 + 1409, and
the average thickness was determined from 100 individual
nanocellulose. Using a transmission electron microscope (TEM), a
diluted nanocellulose suspension (5 μ L) was deposited onto glow-
discharged carbon-coated grids (300 mesh copper, Formvar-carbon,
Ted Pella Incorporated, Redding, CA) with excess liquid being
removed by blotting with lter paper after 10 min, negatively stained
with 2 wt % uranyl acetate solution for 5 min, and observed using a
Philip CM12 TEM operated at 100 kV accelerating voltage. The width
ACS Sustainable Chemistry & Engineering Research Article
DOI: 10.1021/acssuschemeng.5b00780
ACS Sustainable Chem. Eng. 2015, 3, 2566 2574
2567

of the nanocellulose was measured from 100 individual nanobrils
using an image analyzer (ImageJ, NIH, USA).
The water-soluble hydrolysates (10 μL) were injected into a 0.32
mm × 100 mm Hypercarb column, and the compositions were
analyzed by LC-MS (Thermo Scientic, USA). A standard reverse-
phase gradient utilizing 5 mM ammonium formate as solvent A and
acetonitrile as solvent B was run at a 25 μL/min ow rate for 12 min.
The eluent was monitored for negative ions by an orbitrap XL
(Thermo Scientic, USA) operated in the centroid mode at 4.5 kV
spray voltage, 275 °C capillary temperature, and 20 sheath gas setting.
Spectral data were acquired at a 30,000 fwhm resolution setting with
the lockmass feature, which has a typical mass accuracy of <2 ppm.
The glucose concentration was derived from the colorimetric assay.
36
Briey, 100 mL of 1 wt % dinitrosalicylic acid reagent (DNS) solution
was prepared by mixing 1 g of dinitrosalicylic acid, 1 g of sodium
hydroxide, and 50 mg of sodium sulte. Then, 1.5 mL of DNS solution
and 1.5 mL of hydrolysate were mixed and then heated at 90 °C for 10
min to develop the red-orange color, which was stabilized by
subsequently adding 1 mL of 40 wt % potassium sodium tartrate
solution. U pon cooling to ambient temperature, the UV vis
absorbance of the stabilized solution was measured (Evolution 600,
Thermo Scientic) at 575 nm. Their sugar concentrations were
derived from the standard calibration using glucose solutions in 0.125
to 1.25 mg/mL concentrations (Figure S1). The glucose yield and
selectivity were calculated based on the original cellulose (40 mg) via
eqs 4 and 5, respectively, where C
g
is the glucose concentration (mg/
mL) determined by colorimetric titration and V
g
is the volume of the
hydrolysate (mL).
=
×
×
CV
glucose yield (%)
40
100%
gg
(4)
glucose selectivity (%)
glucose yield
cellulose hydrolysis
100%
(5)
RESULTS AND DISCUSSION
Acid Density and Chemical Structures of Solid Acid
Catalysts. Sulfonated carbon bers (SCFs) and sulfonated
activated carbon bers (SACFs) were prepared under two
carbonization (T
c
= 600 or 900 °C) and sulfonation (T
s
= 110
or 150 °C) temperatures (Table 1). The SACFs carbonized at
900 °C were also hydrothermal-treated (150 °C, 5 atm, 24 h)
to obtain HTSACFs. The acid densities were determined by
conductometric titration where conductivity typically decreased
sharply from neutralization of sulfonic acid groups, then stayed
constant as the weaker carboxylic acid was consumed, and
nally increased after all acid sites were neutralized with NaOH
addition (Figure S2). In all titration curves, sulfonic acid was
shown to be dominant with much lower extents of carboxylic
acid groups. Although sulfonic acid is expected from the
sulfonation reaction, carboxylic acid groups are likely from
oxidation of the reductive hydroxyl groups during prolonged
sulfonation.
Both sulfonic and total acid densities were higher for SCFs
carbonized at the lower 600 °C T
c
and for all bers sulfonated
at the higher 150 °C T
s
(Table 1). SCFs carbonized at the
higher 900 °C had approximately 1/4 to 1/3 of the respective
sulfonic and total acid densities, i.e., 0.3 and 0.41 mmol/g at T
s
= 110 °C and 0.52 and 0.68 mmol/g at T
s
= 150 °C,
respectively, of those carbonized at 600 °C. Sulfonation of
ACFs (T
c
= 900 °C) at the respective T
s
of 110 and 150 °Con
SACFs, on the other hand, doubled and tripled the sulfonic and
total acid quantities relative to the corresponding SCFs.
Although the more rigid carbon structures produced at the
higher carbonization temperature of 900 °C are more dicult
to be sulfonated, hence lower acid densities, higher specic
surface ACF could only be produced at 900 °C. The greater
sulfonation eects on ACFs are due to their higher internal
surfaces as well as the carbon microstructure defects introduced
by alkali hydroxide activation.
37
Most signicantly, the higher
sulfonation temperature produced increases in sulfonic and
total acids on the activated SACFs that were more pronounced
than on the SCFs. Overall, the SACFs carbonized at higher 900
°C still had a much higher acid density than SCFs carbonized at
lower 600 °C and therefore were hydrothermally treated to
prepare stable solid acid catalysts. Following hydrothermal
washing (150 °C, 5 atm, 24 h), the sulfonic and total acid
densities of HTSACFs sulfonated at 110 and 150 °C declined
signicantly by 55% (0.31 and 0.48 mmol/g) and 65% (0.56
mmol/g and 0.88 mmol/g), whereas their mass decreased to
lesser degrees of 5 and 9%, respectively. The substantial losses
of sulfonic and total acids coupled with some mass losses from
hydrothermal washing suggest that sulfonation of the pore
surfaces may have led to fragmentation and solubilization of
small hydrocarbons. Both surface acid densities and catalyst
mass remained unchanged following a second hydrothermal
washing, showing no further leaching. That these acid sites
remain stable under the hydrothermal catalysis condition is a
critical attribute for HTSACF solid acid catalysts to be eective
in repetitive use. HTSACFs sulfonated at 150 °C with higher
sulfonic (0.56 mmol/g) and acid (0.88 mmol/g) densities was
used as the solid acid catalyst for the hydrolysis of cellulose to
be described later.
The presence of carboxylic and sulfonic acid groups on
ACFs, SACFs, and HTSACFs was further conrmed by FTIR
and EDX. Both FTIR spectra of SACFs and HTSACFs showed
aCO peak at 1720 cm
1
but no evidence of the COH peak
at 1630 cm
1
(Figure 1a), indicating complete oxidation of C
OH to carbonyl and carboxylic acid by the concentrated
sulfuric acid. Aromatic skeletal st retching at 1580 cm
1
,
expected from the polycyclic aromatic structures in all three
activated carbons, was also clearly present. In addition, EDX
showed the O content to be tripled from 4.4 wt % in ACFs to
12.7 wt % in SACFs in which 2.8 wt % of S was also detected
(Figure 1b). The hydrothermal treatment signicantly lowered
the respective O and S contents in SACFs to 7.3 and 0.9 wt %
in HTSACFs. Such elemental com position changes were
consistent with the decreases in sulfonic and total acid densities
presented previously and attributed to the dissolution of the
highly oxidized and sulfonated hydrocarbons from the
hydrothermal treatment.
Morphologies and Porous Structures of the Solid
Acid Catalysts. ACFs were 500 nm to 2 μm wide and most
were over 100 μm long (Figure 2a and b). Sulfonation caused
SACFs to fragment into a few tens of micrometers in length.
This obvious length reduction is attributed to the extreme
Table 1. Sulfonic and Total Acid Density (mmol/g) of
Sulfonated Carbon Fibers (SCFs) and Sulfonated Activated
Carbon Fibers (SACFs) Prepared at the Prescribed
Carbonization (T
c
) and Sulfonation (T
s
) Temperatures
T
s
= 110 °C T
s
= 150 °C
sample T
c
(°C) sulfonic acid total acid sulfonic acid total acid
SCF 600 1.2 1.4 1.4 1.9
SCF 900 0.3 0.41 0.52 0.68
SACF 900 0.7 1.11 1.62 2.43
HTSACF 900 0.31 0.48 0.56 0.88
ACS Sustainable Chemistry & Engineering Research Article
DOI: 10.1021/acssuschemeng.5b00780
ACS Sustainable Chem. Eng. 2015, 3, 2566 2574
2568

oxidation in concentrated sulfuric acid and the mechanical
forces from ma gnetic agitation (Figure 2c and d). The
hydrothermally washed HTSACFs remained similarly frag-
mented but showed no further morphological changes (Figure
2e and f). All three bers, i.e., ACFs, SACFs, and HTSACFs,
appeared similarly porous with surface macropores in the tens
of nanometers range (Figure 2b, d, and f).
The porous structures of ACFs, SACFs, and HTSACFs were
further analyzed by BET nitrogen adsorption to all exhibit type
IV adsorption desorption isotherms (Figure 3a) typical of
microporous and mesoporous materials. Each showed a step-
down at 0.45 relative pressure in the desorption isotherm
(Figure 3a) and an articial peak at 4 nm (Figure 3b),
suggesting an ink-bottle porous structure with a neck size
smaller than 5 nm.
38,39
The sulfonation reaction slightly and
hydrothermal treatment more prominently shifted the meso-
pores to larger pore sizes and wider ranges ( Figure 3b and c).
Sulfonation slightly increased the mesopore (530 nm) surface
area and volume from 407 m
2
/g to 416 m
2
/g and from 0.71 to
0.75 cm
3
/g, respectively (Figure 3c), but signicantly decreased
micropore (0.50.8 nm) surface area and volume from 444 to
305 m
2
/g and 0.18 to 0.13 cm
3
/g, respectively (Figure 3d),
resulting in a slight decrease in the total BET surface area (885
to 752 m
2
/g) and pore volume (0.9 to 0.88 cm
3
/g).
Figure 1. (a) FTIR and (b) EDX of ACF, SACF, and HTSACF. Inset in (b): elemental composition (wt %).
Figure 2. SEM of (a,b) ACFs, (c,d) SACFs, and (e,f) HTSACFs.
ACS Sustainable Chemistry & Engineering Research Article
DOI: 10.1021/acssuschemeng.5b00780
ACS Sustainable Chem. Eng. 2015, 3, 2566 2574
2569

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Journal ArticleDOI
Feng Shen1, Richard L. Smith2, Luyang Li1, Lulu Yan1, Xinhua Qi1 
Abstract: Microcrystalline cellulose could be effectively converted into levulinic acid in pure water at 180 °C in 12 h without additives in a maximum yield of 51.5% with a cellulase-mimetic solid acid catalyst prepared without the use of sulfuric acid. Ball-milling pretreatment of cellulose improved levulinic acid yields by only a few percent, showing that the cellulose binding sites (−Cl) and catalytic sites (−SO3H) of the catalyst are key to the activity of the catalyst. The spent catalyst could be regenerated with H2O2 solution after recycling for 5 times to maintain more than 95% of its catalytic activity. Glucose used as starting material under the same reaction conditions and with the same cellulase-mimetic solid acid gave a yield of 61.5% levulinic acid. The conversion route for carbohydrates to levulinic acid in pure water with the biomimetic catalyst prepared with a H2SO4-free method provides an environmentally friendly method for producing biobased-platform chemicals from renewable resources.

75 citations


Journal ArticleDOI
Abstract: Nanocellulose is gaining evident interest from researchers and engineers because of its renewability, biocompatibility, biodegradability, high mechanical strength, abundant hydroxyl groups for potential functionality, and extensive raw materials. Versatile sources are accordingly explored like harvested wood, annual plants, and agricultural residues. However, an abundant shrub plant, Amorpha fruticosa Linn., has not yet been reported for isolating nanocellulose. We accordingly propose a green method with low energy consumption to extract nanocellulose from the vast shrub source via combined grinding and successive homogenization treatments. The derived nanocellulose possesses a fine structure with a diameter of ∼10 nm and an aspect ratio over 1000, high thermal stability with a maximum decomposition temperature of 337 °C, and similar composition with a hydroxyl group and a crystal I structure to that of natural cellulose. The demonstrated nanopaper presents visible light transmittance over 90% and haze be...

44 citations


Journal ArticleDOI
Abstract: This review intends to introduce the application of lignin-derived catalyst for green organic synthesis over latest two decades and aims to present a renewable alternative for conventional catalyst for future industry application. The structure of lignin is initially introduced in this review. Then, various pretreatment and activation technologies of lignin are systematically presented, which includes physical activation for the formation of well-developed porosity and chemical activation to introduce catalytic active sites. Finally, the catalytic performances of various lignin-derived catalysts are rationally assessed and compared with conventional catalysts, which involves lignin-derived solid acids for hydrolysis, hydration, dehydration (trans)esterification, multi-component reaction and condensation, lignin-derived solid base for Knoevenagel reaction, lignin-derived electro-catalysts for electro-oxidation, oxygen reduction reaction, and lignin-derived supported transition metal catalysts for hydrogenation, oxidation, coupling reaction, tandem reaction, condensation reaction, ring-opening reaction, Friedel-Crafts-type reaction, Fischer–Tropsch synthesis, click reaction, Glaser reaction, cycloaddition and (trans)esterification. The above lignin-derived catalysts thus successfully promote the transformations of organic compounds, carbon dioxide, biomass-based cellulose, saccharide and vegetable oil into valuable chemicals and fuels. At the end of this review, some perspectives are given on the current issues and tendency on the lignin-derived catalysts for green chemistry.

39 citations


Journal ArticleDOI
24 Jul 2018
Abstract: A complete bibliometric analysis of the Scopus database was performed to identify the research trends related to lignin valorization from 2000 to 2016. The results from this analysis revealed an exponentially increasing number of publications and a high relevance of interdisciplinary collaboration. The simultaneous valorization of the three main components of lignocellulosic biomass (cellulose, hemicellulose, and lignin) has been revealed as a key aspect and optimal pretreatment is required for the subsequent lignin valorization. Research covers the determination of the lignin structure, isolation, and characterization; depolymerization by thermal and thermochemical methods; chemical, biochemical and biological conversion of depolymerized lignin; and lignin applications. Most methods for lignin depolymerization are focused on the selective cleavage of the β-O-4 linkage. Although many depolymerization methods have been developed, depolymerization with sodium hydroxide is the dominant process at industrial scale. Oxidative conversion of lignin is the most used method for the chemical lignin upgrading. Lignin uses can be classified according to its structure into lignin-derived aromatic compounds, lignin-derived carbon materials and lignin-derived polymeric materials. There are many advances in all approaches, but lignin-derived polymeric materials appear as a promising option.

35 citations


Cites background from "1D Lignin-Based Solid Acid Catalyst..."

  • ...The chemical functionalization of the carbon material itself [470] or the incorporation of metal precursors and nanoparticles [471] and enzymes [472] result in promising catalysts with several potential applications, including the treatment of lignocellulosic biomass [473]....

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References
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Journal ArticleDOI
Abstract: It is possible to say that solid acid catalysis involves the largest amounts of catalysts used and the largest economical effort in the oil refining and chemical industry. In this review the author has tried to describe perhaps the most important solid acids based on inorganic oxides, going from their preparation procedures and characterization, to their catalytic activity for a series of hydrocarbon reactions. The review starts with an introductory part in where the nature of the acid sites and their physicochemical characterization is described. Then the classification to the different catalysts is initiated with the older amorphous silica-alumina and aluminum phosphates and followed by catalysts with more interest at present which are discussed in order of increasing acid strength: zeolites, heteropoly acids, and sulfated metal oxides. The aim of this review is to present an extended summary of the state of the art and the current and the future tendencies in the field. 720 refs.

2,647 citations


Journal ArticleDOI
Abstract: Physical gas adsorption is extensively used in the characterization of micro- and mesoporous materials and is often considered as a straightforward-to-interpret technique. However, physical phenomena like the tensile strength effect, adsorbate phase transitions, and monolayer formation in combined micro- and mesoporous materials frequently lead to extra contributions in the adsorption isotherm. Models for pore size determination mostly do not account for this, and assignment to real pores leads to improper analysis of adsorption data. In this review, common pitfalls and limitations in the analysis of pore size distributions derived from adsorption isotherms of micro- and mesoporous materials are identified and discussed based on new results and examples reported in the recent literature.

1,566 citations


Journal ArticleDOI

1,036 citations


Journal ArticleDOI
TL;DR: The carbon catalyst can be readily separated from the saccharide solution after reaction for reuse in the reaction without loss of activity, and the catalytic performance of the carbon catalyst is attributed to the ability of the material to adsorb beta-1,4 glucan, which does not adsorb to other solid acids.
Abstract: The hydrolysis of cellulose into saccharides using a range of solid catalysts is investigated for potential application in the environmentally benign saccharification of cellulose. Crystalline pure cellulose is not hydrolyzed by conventional strong solid Bronsted acid catalysts such as niobic acid, H-mordenite, Nafion and Amberlyst-15, whereas amorphous carbon bearing SO 3H, COOH, and OH function as an efficient catalyst for the reaction. The apparent activation energy for the hydrolysis of cellulose into glucose using the carbon catalyst is estimated to be 110 kJ mol (-1), smaller than that for sulfuric acid under optimal conditions (170 kJ mol (-1)). The carbon catalyst can be readily separated from the saccharide solution after reaction for reuse in the reaction without loss of activity. The catalytic performance of the carbon catalyst is attributed to the ability of the material to adsorb beta-1,4 glucan, which does not adsorb to other solid acids.

833 citations


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Q1. What are the contributions in "1d lignin-based solid acid catalysts for cellulose hydrolysis to glucose and nanocellulose" ?

One-dimensional solid acid catalysts have been synthesized from lignin-based activated carbon fibers via sulfonation and hydrothermal treatment to be mesoporous and contain 0.56 mmol/g sulfonic and 0.88 mmol /g total acid for direct hydrolysis of highly crystalline rice straw cellulose this paper.