SBA-15 supported ionic liquid phase (SILP) with H2PW12O40- for the hydrolytic catalysis of red macroalgal biomass to sugars

Article abstract of DOI:10.1039/c6ra03740b

A supported ionic liquid phase (SILP) catalyst for biomass hydrolysis was prepared via immobilization of an acidic ionic liquid (IL) with a phosphotungstic counter-anion H₂PW₁₂O₄₀^- (HPW) on ordered mesoporous silica (SBA-15). Characterization results from XRD, N₂ physisorption, FT-IR, TGA and SEM/TEM image analyses confirmed the successful preparation of the SILP catalyst (SBA-IL-HPW). Meanwhile, its catalytic performance was evaluated in terms of sugar production from the hydrolysis of different biomasses in water. Under optimal hydrolysis conditions, SBA-IL-HPW yielded 73% d-galactose from agarose and 58% d-glucose from cellobiose. Moreover, SBA-IL-HPW effectively hydrolyzed the red macroalgae G. amansii as it afforded 55% total reducing sugar and 38% d-galactose yields. SBA-IL-HPW was easily separated from the hydrolysates after reaction and was re-used five times without significant loss of activity. Overall findings reveal the potential of SBA-IL-HPW as a durable, environmentally benign catalyst for sugar production from renewable resources.

Full text of DOI:10.1039/c6ra03740b

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SBA-15 supported ionic liquid phase (SILP) with
ꢀ
H2PW12O40 for the hydrolytic catalysis of red
Cite this: RSC Adv., 2016, 6, 33901
macroalgal biomass to sugars†
Lenny B. Malihan, Grace M. Nisola, Neha Mittal, Seong-Poong Lee, Jeong Gil Seo,
*
Hern Kim and Wook-Jin Chung
A supported ionic liquid phase (SILP) catalyst for biomass hydrolysis was prepared via immobilization of an
acidic ionic liquid (IL) with a phosphotungstic counter-anion H2PW12O40ꢀ (HPW) on ordered mesoporous
silica (SBA-15). Characterization results from XRD, N2 physisorption, FT-IR, TGA and SEM/TEM image
analyses confirmed the successful preparation of the SILP catalyst (SBA-IL–HPW). Meanwhile, its catalytic
performance was evaluated in terms of sugar production from the hydrolysis of different biomasses in
water. Under optimal hydrolysis conditions, SBA-IL–HPW yielded 73% D-galactose from agarose and 58%
D-glucose from cellobiose. Moreover, SBA-IL–HPW effectively hydrolyzed the red macroalgae G. amansii
as it afforded 55% total reducing sugar and 38% D-galactose yields. SBA-IL–HPW was easily separated
from the hydrolysates after reaction and was re-used five times without significant loss of activity.
Overall findings reveal the potential of SBA-IL–HPW as a durable, environmentally benign catalyst for
sugar production from renewable resources.
Received 10th February 2016
Accepted 24th March 2016
DOI: 10.1039/c6ra03740b
www.rsc.org/advances
the reaction occurs only at the diffusion layer of the IL-catalyst
phase rather than in the bulk phase.7
1. Introduction
Supported ionic liquid phase (SILP) catalysis has been
developed to address these issues. SILP materials are prepared
by overlaying an IL lm over a solid support with a high surface
area. The resulting material appears as dry solid but up-close
contains the active species as a thin liquid lm in its pores.8
SILP catalysts combine the benets of high activity and selec-
tivity from homogeneous catalysts with that of the recyclability,
large surface area and hydrothermal stability of heterogeneous
systems.6 Various organic reactions such as hydroformylation,9
esterication,10 polymerization,11 amination,12 dehydration6
and metal scavenging systems13 have already been performed
using SILP catalysts. In biomass pretreatment, SILP catalysts
bearing Brønsted acidic groups could be more advantageous
since they can effectively hydrolyze the biomass and be re-used
Biomass have been widely utilized as renewable energy
resources for the production of bio-fuel and bio-based chem-
icals. For example, lignocellulosic agricultural residues and
forest woody feedstocks have already been employed as
substrates for bio-ethanol production.1 Aquatic biomasses such
as Laminaria, Saccorhiza, Alaria and Gelidium have also shown
potential as sugar resources due to their high carbohydrate
contents. These macroalgal species are more viable than
lignocellulosic materials because of their low lignin content,
abundance and simple cultivation environment.2
Pretreatment strategies involving dilute or concentrated
acids, enzymes and supercritical water have been utilized to
enhance the digestibility of biomass substrates.3 Recently,
environmentally benign ionic liquids (IL) have been used to
efficiently dissolve macroalgal biomass for their subsequent
hydrolysis to sugars.4,5 However, IL-based homogeneous catal-
ysis has several drawbacks such as utilization of large amounts
of expensive ILs, their complex recovery from the reaction
mixture and possible mass transfer limitations due to their high
viscosity.6 In effect, only a small fraction of the IL is utilized and
ꢀ
multiple times. Acidic counter-anions such as HSO4
anion6,14–16 and polyoxometalate species17,18 have already been
employed as SILP catalysts and used in various reactions.
However, the use of SILP catalysts in biomass hydrolysis has
been scarce. A sulfonic acid functionalized IL supported on
silica gel was reported previously for the hydrolysis of cellulose
using 1-n-butyl-3-methylimidazolium chloride ([BMIM]Cl) as
solvent.19 But the use of a large amount of IL as solvent during
hydrolysis would not make this system economically feasible.
Heteropolyacids (HPAs) have been particularly used in
cellulose hydrolysis20,21 due to their strong acidity making them
promising IL counter-anions. Among the Keggin HPAs, phos-
photungstic acid (H3PW12O40, HPW) has the highest acidity
Energy and Environment Fusion Technology Center (E2FTC), Department of Energy
Science and Technology (DEST), Myongji University, 116 Myongji-ro, Cheoin-gu,
Yongin City, Gyeonggido 17058, South Korea. E-mail: wjc0828@gmail.com; Fax:
+82-31-337-2902; Tel: +82-31-330-6687
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra03740b
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approaching the superacid region. In fact, HPW exhibits 500 ꢁC in air (2.5 ꢁC minꢀ1) for 5 h. Prior functionalization, SBA-
ꢁ
stronger acidity as compared to mineral acids (e.g. H2SO4, HNO3 15 was activated at 80 C in vacuo for 4 hours.
and HCl) as evidenced by its low deprotonation enthalpy from
2.2.2 Synthesis of SBA-IL. The IL, 1-(tri-ethoxy-silyl-propyl)-
density functional theory calculations.21 Meanwhile, the meso- 3-methylimidazolium chloride, was prepared following a re-
porosity of SBA-15 (2–50 nm) and its large surface area (500– ported procedure.24 In a typical reaction, 0.10 mol (8.21 g) of N-
1000 m2 gꢀ1) could facilitate convenient diffusion of large methylimidazole and 0.10 mol (24.08 g) of 3-chloropropyl-
ꢁ
substrate molecules while its thick walls (3.1–6.4 nm) make it triethoxysilane were stirred at 95 C for 24 h. The product was
a thermally stable support. Moreover, it has abundant silanol extracted twice with diethyl ether and dried at room tempera-
groups available for functionalization.22
ture. Meanwhile, SBA-15 was functionalized with the IL
Herein, an HPW-based acidic ionic liquid anchored on according to a previous method.24 In a typical reaction, 1.5 g of
ordered mesoporous SBA-15 (SBA-IL–HPW) was synthesized the synthesized IL was added to 3 g of vacuum-dried SBA-15
and used as an SILP catalyst for the hydrolysis of several types of dispersed in 90 mL of anhydrous toluene. The mixture was
biomasses for sugar production. The SILP catalyst was exten- stirred for 16 h at 90 ꢁC under a nitrogen atmosphere. The
sively characterized by X-ray diffraction (XRD), N2 adsorption– functionalized SBA-15 was then ltered, washed with toluene
desorption, thermogravimetric analysis (TGA), Fourier and vacuum-dried at 50 ꢁC for 12 h. Excess IL was removed by 24
transform-infrared (FT-IR) spectroscopy, scanning electron h Soxhlet extraction using boiling dichloromethane followed by
microscopy (SEM) and transmission electron microscopy (TEM) vacuum drying at 50 ꢁC. The resulting material was designated
to conrm its successful preparation. The applicability of SBA- as SBA-IL.
IL–HPW for sugar production was investigated via hydrolysis of
2.2.3 Immobilization of HPW onto SBA-IL (SBA-IL–HPW).
agarose, cellobiose and a red macroalgae, Gelidium amansii. The SILP catalyst was obtained via anion exchange of 1 mmol
ꢁ
Moreover, the long term hydrothermal stability of the catalyst HPW with 1 g of vacuum-dried (80 C, 1 h) SBA-IL. HPW was
was studied through a series of hydrolytic cycles.
initially dissolved in 50 mL DI water before addition of SBA-IL.
The dispersion was stirred for 12 h at room temperature. Aer
the reaction, the solid product was ltered off, washed with DI
water (4 ꢂ 25 mL) to remove the residual HPW and further dried
at 50 ꢁC in vacuo for 24 h. The resulting SILP material was
designated as SBA-IL–HPW.
2. Experimental
2.1 Materials
All chemicals were of analytical grade and were used as
received. Poly(ethylene oxide)–poly(propylene oxide) or
Pluronic® 123 (average molecular weight ¼ 5800 g molꢀ1), 3-
chloropropyl-triethoxysilane (CPTES), toluene (anhydrous,
99.8%) were purchased from Sigma Aldrich (USA). Tetraethyl
orthosilicate (TEOS, 98%), N-methylimidazole, n-butylamine
and the heteropolyacid hydrate of H3PW12O40 were procured
from Acros Organics (Belgium). Hydrochloric acid (37% HCl)
was from Showa (Japan). Agarose, glucose and galactose were
supplied by Acros Organics (Belgium), whereas cellobiose was
from Sigma Aldrich (USA).
Dried Gelidium amansii was supplied by Natural Food (http://
0808.or.kr) from Sanjeong-dong, Mokpo, South Korea. Samples
were washed with deionized (DI) water to remove residual salts
and dried at 40 ꢁC until the moisture content was #10 wt%. The
dried samples were pulverized and stored in a desiccator when
not in use.
2.3 Catalyst characterization
Low angle XRD patterns were measured in a PANalytical
EMPYREAN SWAX diffractometer operated at 40 kV and 25 mA,
calibrated with a standard silicon sample using Ni-ltered Cu-
Ka (l ¼ 0.15406 nm) within 0.3–10ꢁ 2q range. Wide angle
powder XRD patterns were acquired using PANalytical, X'Pert-
Pro diffractometer with Cu-Ka radiation operated at 40 mA
and 40 kV with 0.03ꢁ step size in the 2q range of 5–50ꢁ. Nitrogen
adsorption–desorption isotherms were obtained using a Bel-
sorp-mini II (Bel Japan, Inc.) system at 77 K. The samples
ꢁ
were degassed under vacuum at 60 C for 2 h prior measure-
ments. The specic surface area of the samples was calculated
using the Brunauer–Emmett–Teller (BET) method using the
adsorption isotherms acquired at P/P0 ¼ 0.05–0.15. The pore
volume and pore diameter were determined via Barrett–Joyner–
Halenda (BJH) method from the adsorption branch of the
isotherms. Mean pore size was obtained from the maximum
BJH pore size distributions. The morphology of the samples was
2.2 Catalyst synthesis
2.2.1 Preparation of mesoporous SBA-15. Ordered meso- examined under Hitachi S-3500 N SEM and eld emission TEM
porous silica SBA-15 was synthesized according to methods re- (JEOL, JEM-2000EX) operated at 200 kV. FT-IR spectra of the
ported elsewhere23 using tri-block copolymer Pluronic® 123 as samples were recorded on a VARIAN FTS 2000 (Scimitar Series)
a structure directing agent. Approximately 10 g of Pluronic® 123 FT-IR spectrophotometer using anhydrous KBr as standard (1
was dissolved in 325 mL of DI water and 60 mL of HCl at 40 ꢁC. wt% of the sample). The tungsten (W) content in SBA-IL–HPW
Aer dissolution, 20.6 g of TEOS was added and stirred for 2 h at was determined using inductively coupled plasma-atomic
800 rpm. The suspension was statically aged in an oven at 40 ꢁC emission spectroscopy (ICP-AES). Thermogravimetric analysis
ꢁ
for 24 h and further at 80 C for another 24 h. The produced (TGA) was carried out on a Mettler Toledo (DSC823e) analyzer
SBA-15 was repeatedly washed with hot (80 ꢁC) DI water and operated from 35 ꢁC to 1000 ꢁC with a heating rate of 10 ꢁC
dried at room temperature for 4 days followed by calcination at minꢀ1 in N2, supplied at 50 mL minꢀ1. The acidic strength and
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acid sites of the catalyst were quantied by potentiometric
titration with 0.05 M n-butylamine in acetonitrile using
a TitroLine® 7000 system from SI analytics. Initial electrode
potential (Ei) indicates the maximum acid strength of the
sample which is classied as: Ei > 100 mV (very strong acid
sites), 0 < Ei < 100 mV (strong sites), ꢀ100 < Ei < 0 mV (weak
sites) and Ei < ꢀ100 (very weak sites).25 The total number of acid
sites present in the solid sample was determined at the equiv-
alence point (mmol amine per g catalyst).
Msugar (mg) ¼ CHPLC (mg mLꢀ1) ꢂ V1 (mL)
Msugar or MTRS
(2)
Yield ðwt%Þ ¼
ꢂ 100 (3)
ðMA ꢂ 0:5Þ or MC or ðMGA ꢂ AÞ
2.6 Recycling of the catalyst
Aer each hydrolysis run, SBA-IL–HPW was separated from the
reaction mixture by centrifugation at 4000 rpm for 10 min. The
retrieved SILP was washed with DI water and dried at 60 C in
2.4 Biomass hydrolysis
ꢁ
vacuo for 24 h prior re-use.
The hydrolytic activity of the SILP catalyst was evaluated using
substrate models such as agarose and cellobiose and a red
macroalgae, G. amansii. All experiments were carried out in
a Teon-lined steel autoclave reactor. In a typical reaction,
a mixture of 1 wt% biomass suspension in water and known
amount of SBA-IL–HPW were placed in the reactor. The sample
was stirred briey at room temperature. The reactor was then
placed in an oven at a specied temperature and time. The
reaction was quenched by rapidly cooling the reactor in an ice
bath and the catalyst was retrieved by centrifugation. The sugar
content in the supernatant was analyzed as detailed in Section
2.5. For the hydrolysis of G. amansii, its initial sugar composi-
tion was determined using a modied two-step acid hydrolysis
described previously.5 Compositional analysis showed a glucose
and galactose content of 18.14 and 28.48 wt%, respectively.
Sugar yields were calculated based on this initial carbohydrate
content. The total reducing sugar in G. amansii hydrolysates
were calculated based on the total glucose and galactose
content of 46.62 wt%.
2.7 Stability of the catalyst
A modied hot ltration experiment was performed as a leach-
ing test to investigate the stability of the SILP catalyst during
hydrolysis.27 A pre-weighed sample of SBA-IL–HPW SILP was
autoclaved several cycles in DI water. For each leaching cycle,
fresh water was used; the SILP was separated from the water via
gravity ltration using a lter paper. The autoclaved water,
collected from each cycle, was loaded with agarose and then re-
autoclaved for the hydrolysis reaction. Samples were collected
and galactose yields were quantied as detailed in Section 2.5.
High galactose yield would suggest elution of HPW into the
water from the SILP.
3. Results and discussion
3.1 Synthesis of the catalyst
The SBA-IL–HPW SILP catalyst was prepared by covalent
attachment of the pre-synthesized chloride-containing IL on
mesoporous SBA-15 (Scheme 1). This method avoids the
destruction of the support material in contrast to immobiliza-
tion via the anion and decreases the possible leaching of the
active sites.24 Under benign reaction conditions, the acid sites
were created on SBA-IL by anion exchange between chloride and
H3PW12O40 to afford the SILP catalyst. The successful prepara-
tion of SBA-IL–HPW was determined through a series of char-
acterization techniques discussed in Section 3.2.
2.5 Analysis of products and quantication of sugar yields
Total reducing sugars (TRS) were quantied using the DNS
method described elsewhere.26 The TRS concentration (CTRS) of
G. amansii hydrolysates was calculated based on a standard
curve obtained with glucose. The concentration of mono-
saccharides and other reaction products (CHPLC) were quanti-
ed using High Performance Liquid Chromatography (HPLC)
equipped with a Refractive Index Detector (HPLC-RID) and
a Biorad Aminex HPX-87H Ion Exclusion Column (300 mm ꢂ
7.8 mm). Column and detector temperatures were kept at 60
and 40 ꢁC, respectively. The 5 mM H2SO4 mobile phase was
ltered, degassed and delivered at 0.6 mL minꢀ1. The mass of
TRS (MTRS) was calculated using eqn (1) whereas that of the
monosaccharides (Msugar) was calculated using eqn (2) wherein
V1 is the volume of the solution. Sugar yields were calculated
using eqn (3) given the mass of agarose (MA), cellobiose (MC)
and G. amansii (MGA) where A is the individual sugar content in
the red macroalgae. Galactose yields from agarose were calcu-
lated by multiplying MA with 0.5 assuming it contains equi-
molar amounts of galactose and 3,6-anhydro-L-galactose (3,6-
AHG). Reported sugar yields are average values of at least
duplicate runs.
3.2 Characterization of the prepared catalysts
3.2.1 Structural and morphological properties. Low angle
XRD patterns of pristine SBA-15 (Fig. 1) revealed three well-
resolved peaks at 2q ¼ 0.88ꢁ, 1.54ꢁ and 1.78ꢁ, which can be
indexed as (1 0 0) (strong), (1 1 0) (weak) and (2 0 0) (weak),
respectively. These peaks are characteristic of hexagonal (p6mm)
SBA-15 which has a signicant degree of long-range order and
a well-formed two dimensional meso-structure.28 IL modica-
tion (SBA-IL) and further incorporation of the heteropolyacid
(SBA-IL–HPW) exhibited lower peak intensities especially with
the (1 0 0) signal. This is indicative of pore-lling effect in SBA-
15 which is consistent with the increased wall thickness
(Table 1) of both modied SBA-15 samples. Diffraction peaks at
higher 2q degrees were still evident in the functionalized
materials (inset Fig. 1) albeit at lower intensities. This suggests
ꢂ
ꢃ
ꢀ
ꢁ
5
MTRSðmgÞ ¼ CTRS mg mLꢀ1
ꢂ
ꢂ V1ðmLÞ
(1)
0:5
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Scheme 1 Reaction scheme for the synthesis of SBA-IL–HPW.
Fig. 2 Wide angle XRD patterns of bulk HPW, SBA-15, SBA-IL and SBA-
IL–HPW.
Fig. 1 Low angle XRD patterns of SBA-15, SBA-IL and SBA-IL–HPW.
IL). The SILP catalyst, SBA-IL–HPW exhibited a similar pattern
with a broad band centered at 25ꢁ. Compared with bulk HPW
which showed various crystalline peaks, no prominent peaks
related to the Keggin anion was observed in SBA-IL–HPW. This
suggests that HPW were present as charge compensating
anions of the IL and were not in their crystalline form.27
The mesoporosity of all SBA-based samples (Fig. 3A) was
evidenced by the irreversible type IV N2 adsorption–desorption
an intact SBA-15 structure aer immobilization, as further evi-
denced by the unaltered d100-spacing ¼ 9.9 nm in all samples.
To further observe the presence of HPW on SBA-15, Fig. 2
shows the wide angle XRD patterns for bulk HPW, SBA-15, SBA-
IL and SBA-IL–HPW. SBA-15 revealed a broad band at 22.5ꢁ
corresponding to amorphous silica. No considerable changes
were observed when SBA-15 was functionalized with the IL (SBA-
Table 1 Physico-chemical properties of SBA-15, SBA-IL and SBA-IL–HPW
Ave. pore
Samples
Surface areaa (m2 gꢀ1
)
Pore volumeb (cm3 gꢀ1
)
diameterc (nm)
Wall thicknessd (nm)
SBA-15
SBA-IL
SBA-IL–HPW
812
366
159
1.06
0.63
0.26
4.03
3.53
2.71
7.49
7.99
8.81
a
Calculated by the BET equation at P/P0 ¼ 0.05–0.15. b BJH pore adsorption volume. c Adsorption average pore diameter. d Wall thickness ¼ a0 ꢀ
pffiffiffi
pore size; a0 ¼ 2dð100Þ
=
3.27
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a lower relative pressure which could be due to the coating and
lling of the pores of SBA-15.29
From the N2 adsorption–desorption curves, the structural
properties of the samples including BET surface area and pore
dimensions were acquired (Table 1). The surface area of SBA-15
(SA ¼ 812 m2 gꢀ1) decreased aer IL functionalization (SA ¼ 366
m2 gꢀ1) and further upon anion exchange with HPW (SA ¼ 159
m2 gꢀ1). The presence of the acidic IL in SBA-IL–HPW was also
evidenced by the decrease in pore volume (Table 1) and pore
diameter (Fig. 3B). All the samples exhibited narrow pore size
distributions indicating structural consistency before and aer
immobilization. The decrease in surface area and pore volume,
together with the increase in wall thickness of SBA-15 conrmed
the successful incorporation of the acidic IL in its pores.
The morphologies of bare SBA-15 and SBA-IL–HPW were
examined by SEM (Fig. 4). SBA-15 exhibited wavy or rope-like
mesopore domains (Fig. 4a), which aggregated into a wheat-
like structure.30 SBA-IL–HPW (Fig. 4b) showed similar
morphology with its parent support but TEM images reveal the
difference between the samples. Pristine SBA-15 (Fig. 4e)
exhibited highly ordered hexagonal meso-structure whereas an
amorphous IL layer (inset: Fig. 4f) was evident on the surface of
SBA-IL–HPW. This further conrms the successful attachment
of the acidic IL on SBA-15 and the formation of an SILP
material.
Fig. 3 N2 adsorption–desorption isotherms (A) and pore size distri-
bution curves (B) of SBA-15, SBA-IL and SBA-IL–HPW.
3.2.2 Surface chemical properties. FT-IR spectra (Fig. 5) of
all samples conrmed the successful functionalization of SBA-
15 with the IL and subsequent anion exchange with HPW.
Pristine SBA-15 featured the symmetric and asymmetric
isotherms with H1-type hysteresis loops (as dened by IUPAC)
associated with capillary condensation taking place in the
channels. A wide hysteresis loop at P/P0 ¼ 0.6–0.8 was observed
in SBA-15 while those in SBA-IL and SBA-IL–HPW shied to
stretching of Si–O–Si bands at 806 and 1000–1200 cmꢀ1
,
Fig. 4 SEM images of (a) SBA-15 and (b) SBA-IL–HPW; TEM images of SBA-15 (c and e) and SBA-IL–HPW (d and f).
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Fig. 6 Thermogravimetric curves of SBA-15, SBA-IL and SBA-IL–
HPW.
Fig. 5 FT-IR spectra of SBA-15, SBA-IL, bulk HPW and SBA-IL–HPW.
SBA-IL–HPW showed an overall weight loss of 12.72%. The
weight loss at T > 300 ꢁC can be ascribed to the decomposition
of the cation as well as partial loss of the heteropolyacid.34 The
lower weight loss in SBA-IL–HPW than in SBA-IL is mainly due
to the addition of inorganic and relatively more thermally stable
HPW. DTG curve of SBA-IL–HPW (Fig. S1†) showed the rapid
loss of the acidic IL from 280–460 ꢁC. Hence, the catalyst is
respectively. The stretching vibrations of terminal ^Si–OH in
SBA-15 is evident at 968 cmꢀ1 while the bending vibrations of
the Si–O unit could be observed at 470 cmꢀ1 31 The peak at 3357
.
cmꢀ1 could be due to OH– stretching of silanols and adsorbed
water. Characteristic peaks in SBA-IL conrmed the successful
binding of the IL on the silica support. The lower absorbance at
966 cmꢀ1 in SBA-IL could be due to the interaction of the ethoxy
groups from the IL with the surface silanols in SBA-15. More-
over, the presence of peaks at 1459 and 1575 cmꢀ1 in SBA-IL
could be attributed to the presence of the C]N and aromatic
C]C stretching of the imidazolium cation, respectively. The
weak bands at 2931 and 2808 cmꢀ1 are due to the methylene
vibrations of the propyl chain in the organosilane moiety.10,31,32
ꢁ
thermally stable at temperatures below 280 C.
3.2.4 Acidity of SBA-IL–HPW. SBA-15 exhibited very weak
acid sites having an initial electrode potential (Ei) of ꢀ130.0 mV.
On the other hand, SBA-IL–HPW exhibited very strong acid sites
having an Ei of +120.8 mV corresponding to 0.1649 mmol gꢀ1
(Fig. S2†).
3ꢀ
The structure of PW12O40 consists of a PO4 tetrahedron sur-
rounded by four W3O13 groups which are connected by corner-
sharing oxygens. Bulk HPW features four characteristic bands
at ca. 1080, 985, 892 and 827 cmꢀ1, attributable to the asym-
metric stretching of P–O, terminal W]O, corner-sharing W–Oc–
W and edge-sharing W–Ob–W bonds, respectively.33 Though
most of the characteristic peaks of HPW were masked in the
spectra due to the strong absorption bands of silica, the pres-
ence of W–O–W band at 897 cmꢀ1 and the higher intensity of
the band at 954 cmꢀ1 suggest the presence of HPW in SBA-IL–
HPW. Moreover, ICP-AES analysis of SBA-IL–HPW revealed an
estimated W content of 2158 ppm (i.e. 0.98 mmol gꢀ1 HPW).
These ndings further assert the successful immobilization of
the acidic IL onto SBA-15, an SILP acidic catalyst.
3.2.3 Thermal stability. Thermal gravimetric curves of bare
and functionalized SBA-15 (Fig. 6) reveal weight losses at
temperature # 100 ꢁC, which can be ascribed to the removal of
moisture and residual solvents. Among the samples, pristine
SBA-15 exhibited the least overall weight loss of 1.35 wt% at
>600 ꢁC due to the condensation of surface silanols to siloxanes.
The presence of the IL moiety explains the higher weight
reduction in SBA-IL. The gradual weight decline (19.77%) at
150–700 ꢁC was mainly due to the decomposition of the IL
which is equivalent to 0.61 mmol gꢀ1 IL on SBA-15. Meanwhile,
3.3 Catalytic activity of SBA-IL–HPW
The catalytic activity of SBA-IL–HPW was demonstrated in the
hydrolysis of red macroalgae and its representative model
compounds: agarose and cellobiose using water as the solvent.
Control hydrolysis runs with 50 mg of agarose in an autoclave
for 12 h at T ¼ 120 ꢁC in the presence of SBA-15 and SBA-IL
yielded solidied samples when cooled to room temperature.
This suggests that both materials were unable to depolymerize
and hydrolyze the substrate. These ndings verify that neither
material had active acidic sites which could catalyze agarose
hydrolysis. Meanwhile, galactose yields of 80% from agarose
and 56% from G. amansii were obtained from the hydrolysates
in aqueous systems loaded with pure HPA (acid concentration ¼
1.98 mmol Lꢀ1). This result conrms that, as the anionic form
of HPA, HPW would be the sole active component in the SBA-IL–
HPW SILP that would be responsible for the acidic hydrolysis of
the substrates.
3.3.1 Effect of catalyst amount on agarose hydrolysis. The
effect of catalyst amount on sugar yields was evaluated using
agarose as a substrate which contains equimolar repeating
units of a-(1,3)-D-galactose and b-(1,4)-3,6-anhydro-L-galactose.35
A representative hydrolysis temperature of 120 C was adapted
for agarose based on a previous study.5
ꢁ
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Table 2 Galactose yields (wt%) from the hydrolysis of agarose using
different amounts of SBA-IL–HPWa
from the tested substrates, followed by sugar degradation
(Scheme 2).5 Extreme reaction conditions such as high
acidity, prolonged reaction time and high hydrolysis
temperature may drive the reaction towards the formation of
degradation products such as 5-HMF, formic acid and levu-
linic acid.39 Fig. 7A shows the galactose yields from the
hydrolysis of agarose. Maximum galactose yield (73 wt%) was
observed aer 12 h of reaction, which plateaued at 71% aer
24 h. The main decomposition product of agarose hydrolysis
was 5-HMF (Table S1†) which started to form aer 6 h of the
reaction. This could be due to the hydrolysis of the acid labile
3,6-AHG, the other major component of agarose.40 The yield
of 5-HMF decreased aer 24 h of reaction due to the forma-
tion of its re-hydration products such as formic and levulinic
acids.41
Catalyst amount
(mg)
Galactose yield
(wt%)
Entry
1
2
3
4
5
6
0
20
40
60
80
0
39.7
65.4
69.6
56.0
49.7
120
a
Reaction conditions: 50 mg agarose, 5 mL DI water, autoclaved for 6 h
at 120 ꢁC.
Table 2 shows the D-galactose yields from the hydrolysis of
agarose by varying the amount of SBA-IL–HPW. An increase in
the sugar production concurrent with the increase in SBA-IL–
HPW was observed. A maximum galactose yield of 70% was
achieved with 60 mg of SBA-IL–HPW aer 6 h. Further increase
in SBA-IL–HPW reduced the galactose yield which can be
ascribed to the formation of degradation compounds due to
increased acidity in the system. Thus, 60 mg of SBA-IL–HPW
was utilized for subsequent reactions unless otherwise stated.
3.3.2 Effect of reaction time on biomass hydrolysis. The
hydrolysis performance of SBA-IL–HPW was further evaluated
using cellobiose and a red macroalgae, G. amansii. Cellobiose,
a sub-unit of cellulose, is a disaccharide with b-1,4-linked
glucose units.36 Meanwhile, the primary polysaccharides in G.
amansii are brin (cellulose) and agarose.37 Hydrolysis of
cellobiose was carried out at a slightly higher temperature ꢁ(i.e.
Glucose yields from the hydrolysis of cellobiose were found
to be maximum aer 12 h of reaction (Fig. 7B). The highest
glucose yield of 58% was released from cellobiose which
declined to 33% aer 24 h. The presence of decomposition
products such as formic acid and 5-HMF were observed aer 18
h of reaction (Table S2†). While glucose production declined,
cellobiose attained ꢃ99% conversion aer 24 h, which suggests
formation of sugar degradation products specically levulinic
acid (275.84 mg gꢀ1 cellobiose) from the released glucose. The
higher galactose yield from agarose has been consistent with
previous reports as it is more acid-labile than substrates con-
taining b-linkages such as cellulose or cellobiose.5 A similar
trend was observed in the hydrolysis of G. amansii (Fig. 7C)
reaching a maximum galactose yield of 38% and a TRS yield of
58% aer 12 h. There was no considerable increase in the sugar
yields when the reaction was extended until 24 h. The main
decomposition products formed during G. amansii hydrolysis
are 5-HMF and formic acid (Table S3†). Meanwhile, no levulinic
acid was detected in the hydrolysates suggesting that the
galactose released from the hydrolysis of agarose is more
susceptible to degradation as compared to that of G. amansii.
Moreover, the absence of glucose from the hydrolysis of G.
amansii has also been observed in earlier reports which involved
ꢁ
140 C) as compared with agarose and G. amansii (i.e. 120 C).
The higher temperature requirement was due to the presence of
b-1,4 linkages in cellobiose which are not readily hydrolyzed as
compared with the a-linkages found in agarose.38
The acid-catalyzed hydrolysis of red macroalgal poly-
saccharides has been modeled as a pseudo-rst order
sequential reaction releasing sugars as galactose or glucose
Scheme 2 Putative mechanism of acid-catalyzed hydrolysis of red macroalgal polysaccharides for sugar production and subsequent formation
of degradation products using Brønsted acid catalysts.
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Fig. 7 Sugar yields (wt%) from the hydrolysis of agarose (A), cellobiose (B) and G. amansii (C) using SBA-IL–HPW at different time intervals.
Reaction conditions: 50 mg substrate, 60 mg SBA-IL–HPW, 5 mL DI water, 120 ꢁC for agarose and G. amansii, 140 ꢁC for cellobiose, autoclave.
thermal acid hydrolysis systems since the polysaccharide galactose yield at cycle 2 (Fig. 8) was due to the release of loosely
present in this macroalgae is cellulose, which is more resistant bound HPW residuals from the SILP. The steady performance of
than cellobiose.42
the SILP in the succeeding hydrolytic cycles (Fig. 8) also agrees
with the leaching test; the meager galactose yields at the latter
cycles (Table S4†) suggest that HPW was not further leached out
from the SILP aer the rst leaching cycle. In this regard, the
temporary elution of HPW (aer rst hydrolytic run) had no
signicant deteriorating effect on the long term performance of
the SILP.
Moreover, inspection of used SBA-IL–HPW via FT-IR analysis
(Fig. S3†) reveal no chemical alteration even aer ve hydrolytic
cycles. This further conrms that majority of the HPW
remained intact in the SILP. Overall results demonstrate the
reusability and stability of SBA-IL–HPW, and its potential as
a resilient SILP catalyst for biomass-based sugars production.
3.4 Re-usability and stability of SBA-IL–HPW
The hydrothermal stability of SBA-IL–HPW was demonstrated
using the optimal conditions in the hydrolysis of agarose, i.e. 12
h at 120 ꢁC. The catalyst was easily recovered aer each
hydrolysis reaction by centrifugation and re-used without
further activation. Cycled experiment (Fig. 8) reveal a consistent
performance (>70%) except aer the second cycle, which
showed a slightly higher galactose yield. Result from the
modied hot ltration test (Table S5†) was consistent with this
nding as the galactose yield in autoclaved water system
(leaching solvent) was also highest (13%) at the rst leaching
cycle (Table S4†). These ndings suggest that the increase of
4. Conclusions
Supported ionic liquid phase catalyst based on acidic HPW
anion (SBA-IL–HPW) was successfully synthesized and applied
for the hydrolysis of macroalgal biomass. Various character-
ization techniques conrmed that the acidic ionic liquid was
covalently attached in the pores of ordered mesoporous SBA-15
rendering the SILP catalyst hydrothermally stable. The SILP
catalyst effectively released sugars (e.g. galactose and glucose)
from agarose, cellobiose and G. amansii with optimal sugar
yields aer 12 h of reaction. Moreover, SBA-IL–HPW could be
easily recycled without signicant loss of activity which makes it
more industrially feasible as compared to homogeneous IL
systems. Overall ndings demonstrate the potential of SBA-IL–
HPW as an SILP catalyst for biomass processing for sugar
production.
Acknowledgements
Fig. 8 Recyclability of SBA-IL–HPW in the hydrolysis of agarose.
Reaction conditions: 50 mg agarose, 60 mg SBA-IL–HPW, 5 mL DI
water, 120 ꢁC, 12 h, autoclave.
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
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funded by the Ministry of Education (2009-0093816) and by the 20 J. Tian, J. Wang, S. Zhao, C. Jiang, X. Zhang and X. Wang,
Ministry of Science, ICT
2015R1C1A2A01054605).
&
Future Planning (No.
Cellulose, 2010, 17, 587.
21 K. Shimizu, H. Furukawa, N. Kobayashi, Y. Itaya and
A. Satsuma, Green Chem., 2009, 11, 1627.
22 B. Zou, Y. Hu, D. Yu, J. Xia, S. Tang, W. Liu and H. Huang,
Biochem. Eng. J., 2010, 53, 150.
23 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson,
B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548.
References
1 A. Limayem and S. C. Ricke, Prog. Energy Combust. Sci., 2012,
38, 449.
¨
24 M. H. Valkenberg, C. de Castro and W. F. Holderich, Green
2 R. P. John, G. S. Anisha, L. Madhavan Nampoothiri and
A. Pandey, Bioresour. Technol., 2011, 102, 186.
3 R. Kumar, S. Singh and O. V. Singh, J. Ind. Microbiol.
Biotechnol., 2008, 35, 377.
4 L. B. Malihan, G. M. Nisola and W. J. Chung, Bioresour.
Technol., 2012, 118, 545.
Chem., 2002, 4, 88.
25 A. S. Khder and A. I. Ahmed, Appl. Catal., A, 2009, 354, 153.
26 C. Li and Z. K. Zhao, Adv. Synth. Catal., 2007, 349, 1847.
27 R. Gao, Q. Zhu, W. L. Dai and K. Fan, RSC Adv., 2012, 2, 6087.
28 N. Y. He, C. S. Woo, H. G. Kim and H. I. Lee, Appl. Catal., A,
2005, 281, 167.
5 L. B. Malihan, G. M. Nisola, N. Mittal, J. G. Seo and
W. J. Chung, Renewable Energy, 2014, 66, 596.
6 K. B. Sidhpuria, A. L. Daniel-da-Silva, T. Trindade and
J. A. P. Coutinho, Green Chem., 2011, 13, 340.
7 A. Riisager, R. Fehrmann, S. Flicker, R. van Hal,
M. Haumann and P. Wasserscheid, Angew. Chem., Int. Ed.,
2005, 44, 815.
29 P. Krawiec, C. Weidenthaler and S. Kaskel, Chem. Mater.,
2004, 16, 2869.
30 A. Lapkin, B. Bozkaya, T. Mays, L. Borello, K. Edler and
B. Crittenden, Catal. Today, 2003, 81, 611.
31 S. K. Kundu, J. Mondal and A. Bhaumik, Dalton Trans., 2013,
42, 10515.
32 L. T. Aany Soa, A. Krishnan, M. Sankar, N. K. Kala Raj,
P. Manikandan, P. R. Rajamohanan and T. G. Ajithkumar,
J. Phys. Chem. C, 2009, 113, 21114.
33 X. Han, W. Yan, K. Chen, C. T. Hung, L. L. Liu, P. H. Wu,
S. J. Huang and S. B. Liu, Appl. Catal., A, 2014, 485, 149.
34 S. Choi, Y. Wang, Z. Nie, J. Liu and C. H. F. Peden, Catal.
Today, 2000, 55, 117.
¨
8 R. Ochsner, M. J. Schneider, C. Meyer, M. Haumann and
P. Wassercheid, Appl. Catal., A, 2011, 399, 35.
9 C. P. Mehnert, R. A. Cook, N. C. Dispenziere and M. Afeworki,
J. Am. Chem. Soc., 2002, 124, 12932.
10 K. Qiao, H. Hagiwara and C. Yokoyama, J. Mol. Catal. A:
Chem., 2006, 246, 65.
11 W. Och˛edzan-Siodłak and K. Dziubek, Appl. Catal., A, 2014,
484, 134.
35 C. N. Jol, T. G. Neiss, B. Penninkhof, B. Rudolph and G. A. De
Ruiter, Anal. Biochem., 1999, 268, 213.
12 A. H. Tamboli, A. A. Chaugule, F. A. Sheikh, W. J. Chung and
H. Kim, Chin. J. Catal., 2015, 36, 1365.
13 W. Chen, Y. Zhang, L. Zhu, J. Lan, R. Xie and J. You, J. Am.
Chem. Soc., 2007, 129, 13879.
36 J. A. Bootsma and B. H. Shanks, Appl. Catal., A, 2007, 327, 44.
37 Y. Kim, D. Kim, T. Kim, M. K. Shin, Y. J. Kim, J. J. Yoon and
I. S. Chang, Biomass Bioenergy, 2013, 56, 38.
38 M. Kitano, D. Yamaguchi, S. Suganuma, K. Nakajima,
H. Kato, S. Hayashi and M. Hara, Langmuir, 2009, 25, 5068.
39 N. Mittal, G. M. Nisola and W. J. Chung, Tetrahedron Lett.,
2012, 53, 3149.
40 B. Kim, J. Jeong, S. Shin, D. Lee, S. Kim, H. J. Yoon and
J. K. Cho, ChemSusChem, 2010, 3, 1273.
41 T. S. Jeong, Y. S. Kim and K. K. Oh, Bioresour. Technol., 2011,
102, 10529.
14 J. Miao, H. Wan and G. Guan, Catal. Commun., 2011, 12, 353.
15 S. Rostamnia, A. Hassankhani, H. G. Hossieni, B. Gholipour
and H. Xin, J. Mol. Catal. A: Chem., 2014, 395, 463.
16 Q. Zhang, J. Luo and Y. Wei, Green Chem., 2010, 12, 2246.
17 J. Kasai, Y. Nakagawa, S. Uchida, K. Yamaguchi and
N. Mizuno, Chem.–Eur. J., 2006, 12, 4176.
18 J. Xiong, W. Zhu, W. Ding, L. Yang, M. Zhang, W. Jiang,
Z. Zhao and H. Li, RSC Adv., 2015, 5, 16847.
19 A. S. Amarasekara and O. S. Owereh, Catal. Commun., 2010,
11, 1072.
42 H. Cho, C. H. Ra and S. K. Kim, J. Microbiol. Biotechnol., 2014,
24, 264.
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