Sustainable conversion of cellulosic biomass to chemicals under visible-light irradiation

Article abstract of DOI:10.1039/c5ra16616k

For the first time, the conversion of crystalline cellulose to valuable chemicals was enhanced by visible-light irradiation using zeolite-based gold nanoparticles (Au-NPs). This plasmon-enhanced photocatalytic conversion significantly improved processing efficiency and achieved a high yield of 60% at relatively low temperature. Moreover, the photocatalytic properties of the photocatalysts varied with the light intensity and the irradiation wavelength.

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Sustainable conversion of cellulosic biomass to chemicals under
visible-light irradiation
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
c
d
b
b
Lina Wang, Zhanying Zhang, Lixiong Zhang, Song Xue, William O. S. Doherty, Ian M. O’Hara
a
and Xuebin Ke*
DOI: 10.1039/x0xx00000x
www.rsc.org/
For the first time, the conversion of crystalline cellulose to
The attention is turning to the solid acid catalysts in
valuable chemicals was enhanced by visible-light irradiation using companying with ever-increasing demands for energy and
9,10
7
zeolite-based gold nanoparticles (Au-NPs). This plasmon-enhanced environment. Solid acid catalysts, such as metal oxides,
,8
1
1,12
13,14
photocatalytic conversion significantly improved processing metal phosphate,
efficiency and achieved a high yield of 60% at relative low sulphonated carbonaceous acids,
polymer based acid resins,
1
5,16
17-19
heteropoly acids,
H-
2
0
temperature. Moreover, the photocatalytic properties of the form zeolites, normally carry a high concentration of strong
photocatalysts varied directly with the light intensity and the Brønsted acid sites embedded into an acidic environment.
irradiation wavelength.
Moreover, these solid acid catalysts possess a high affinity to
2
poly-saccharides structure, suitable for cellulose hydrolysis.
1
With diminishing fossil resources and increasing concerns
about environmental issues, biomass conversion becomes
Great progresses on solid acid catalysis have been made in
biomass conversion to produce chemicals or fuels in terms of
catalyst structures, active sites, reaction media, and pre-
treatment technologies. However, the mass diffusion between
crystalline cellulose and solid catalysts seriously impedes
practical productions of chemicals from cellulose. Without the
assistance of milled or microwaved treatment, the yields are
often below 50% and get primary products of glucose and
reducing sugars of wide distributions. With more easily
degraded feedstock, such as amorphous cellulose, cellobiose,
glucose and fructose, the conversion can be conducted at a
1
,2
attractive for the production of fuels and chemicals.
Cellulose, the most abundant non-food biomass on earth, is a
promising renewable feedstock. It is estimated that the world-
wide production of cellulose amounts to 110 billion
3
,4
tons/year.
Cellulose is
composed of glucose monomers linked by
a
water insoluble saccharide polymer
-1,4 glycosidic
β
bonds. Apart from enzymatic hydrolysis, acid catalysis is
another promising option for cellulose conversions, which
initially produce chemical intermediates or valuable chemicals
and subsequently proceed to biofuels. Mineral acids including
o
3,4
mild condition (<120 C). Current processes for cellulose
conversion (including pyrolysis) always require high
temperatures (>200 °C) and/or high pressures (>20 bar), which
are suffering from low product selectivity, high energy
consumption and catalyst instability. Thus, it is imperative to
develop energy-efficient processes for cellulose conversion.
H SO , HCl and HBr, have demonstrated a high efficiency
2
4
5
,6
toward the hydrolysis of cellulose, up to 80% of yield.
Generally, homogeneous catalysts can exhibit higher yields at
relative mild conditions. However, the catalysts themselves are
subject to environmental concerns (corrosion and pollution) in
product separation and device maintenance.
Scheme 1. Advanced nanostructure for cellulose conversion: a)
Zeolites on nanofibres (zeolite#NFs); b) Metals coated
zeolite#NFs with oriented open pores and enhanced
3
6
0
electromagnetic fields, E/E =10 -10 times.
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In this communication, for the first time photocatalytic to some extent. Generally, the conversion of cellulose went
+
conversions were proposed to enhance cellulose hydrolysis through several key stages (Scheme 2). When H ions of solid
utilising a new plasmonic nanostructure, which was composed acids attached to the
β-1,4-glycosidic bonds, it caused a split
of TiO nanofibres (NFs) supported H-form Y-zeolites (HY) in the cellulose chain to from glucose. Subsequently the
2
decorated with Au-NPs (Au-HYT). Firstly, highly efficient dehydration of glucose produced 5-hydrooxymethylfurfural
nanocatalysts were designed to enhance the mass transfer (HMF). Furthermore, the hydrolysis of HMF may produce
5
-8
(Scheme 1). Nanozeolites (< 100 nm) were grown on ultrathin Levulinic acid (LeA) or Humins.
nanofibres, where the open pores of the nanozeolites were
The acid strength of the catalyst was an important factor in
controlled, to expose active sites and porous channels. This activating crystalline cellulose towards hydrolysis. However,
design facilitated the dissolved/degraded reactants access to the network structure of cellulose endowed it with high
active sites inside channels, therefore benefiting catalyst chemical stability, thus processing must be performed under
2
2,23
selectivity.
More importantly, the new nanocatalysts could harsh conditions of high temperature and/or digestion by
be decorated with plasmonic metals, such as Au-NPs, which strong liquid acids. Under these reaction conditions, control of
functioned as “antennas” to efficiently absorb visible light. The product selectivity was impossible due to the thermal
localised surface plasmon resonance (LSPR) effect is a physical instabilities of cellulose derived sugars. Many side-products
absorption of light, which produces an enhancement of were commonly fabricated, including the important “platform
3
6
3-8
electromagnetic field up to 10 -10 folds locally. Plasmonic molecule” 5-hydroxymethylfurfural (HMF), LeA and humins.
metals have recently been explored for efficient In this work, the reaction yield indicated the production of
2
4,25
photochemical synthesis.
We have found that Au-NPs can valuable chemicals (glucose and HMF).
significantly enhance zeolite photocatalysis with high product
6,27
selectivity.
2
Table 1. The yield (%) of cellulose conversion with various
This research aims to develop new catalysts materials and catalysts
innovative process. The plasmonic structure of Au-HYT was Catalysts
originally explored and firstly used for cellulose conversion
[a]
Light on
Light-off
glucose HMF Total glucose HMF Total
under light irradiation. Various solid acid catalysts, such as Au-
Au-HYT
48.1
30.2
14
10.6 58.7 15.5
2.6 32.8 10.2
10.1 24.1 11.1
5.7 30.6 19.6
19.1 39.3 18.9
5.7 20.6 10.6
0
15.5
10.2
18.6
24.2
NPs decorated zeolite Y on TiO NFs (Au-YT), TiO supported
2
2
Au-YT
HYT
YT
0
HY zeolites (HYT), TiO supported Y zeolites (YT), HY zeolites
2
7.5
4.6
and acid treatment TiO NFs (HT), were also compared.
2
24.9
20.2
14.9
HY
17.1 36
4.9 15.5
HT
[
a] Reaction conditions: 50 mg cellulose, 50 mg catalyst, 500
mg water, 4.5 g EMIMCl, 0.5 W cm-2 visible light, at 140 °C for
6 hours.
1
Plasmonic metal nanoparticles were loaded onto the solid
acid catalysts to absorb visible light to enhance catalyst activity
and product selectivity. This is feasible because light irradiation
can intensify the strongly polarised electrostatic fields of
2
6,27
zeolite extra-framework ions.
Table 1 showed the results of
cellulose conversion with various catalysts, including Au-HYT,
Au-YT, HYT, YT, HY and HT. The conversion of cellulose
proceeded effectively on these catalysts even at mild reactive
conditions. The catalytic conversions improved significantly on
Au-HYT under visible light irradiation; particularly, the yields of
glucose and HMF increased from 15.5% to 58.7% on Au-HYT.
However, with light off Au-NPs have not such an effect and
even impeded the conversion to HMF. This could be concluded
by the comparison of Au-HYT and HYT. The similar situation
happened on Au-YT and YT. These results indicated that Au-
NPs themselves were not active sites under dark conditions
and confirmed that visible light and Au-NPs were necessities to
sharply boost cellulose conversion.
Scheme 2. Catalytic procedures of cellulose conversion.
Cellulose; . Glucose; . 5-hydrooxymethylfurfural (HMF);
Levulinic acid (LeA).
1.
2
3
4.
The pre-treatment of feedstock, reactive media, and
reaction conditions were decided according to relative
3
,4
documents. The experimental procedure was detailed in
supplementary information (SI). Generally, crystalline cellulose
was firstly dissolved in 1-ethyl-3-methylimidazolium chloride
(EMIMCl) solution. Subsequently, cellulose hydrolysis was
initiated with addition of water and catalysts as well as light
irradiation. If only with water as solvent, no products were
detected. The water amount had a profound effect on
2
8
Moreover, the products mainly consisted of glucose and
HMF, and no LeA produced under light irradiation on Au-HYT.
It can be concluded that both LSPR effect and acid strength of
new catalysts played important roles in the serial process for
cellulose solubility and swelling. The 10% concentration of
water in ionic liquids was selected, which assisted hydrolysis of
cellulose as well as maintained cellulose swelling and solubility
2
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2
cellulose conversion. The TiO nanofibres had large external while catalysts of Au-NPs loads had strong absorption around
2
-1
surface areas (~50 m g ) and large aspect ratios (200 nm in 530 nm in the visible-light region. This LSPR effect can be
diameter and 10 μm in length), which were easily to form harnessed in new catalyst systems therefore enhanced the
networking structures and facilitated the dispersion and catalytic conversion of cellulosic biomass. The loads of Au-NPs
recycling of catalysts from liquid solution. Many efforts were were about 3% in weight and the atom ratio of Si/Al was
made to reveal possible function mechanisms on cellulose around 4.5 detected by the SEM-EDS, confirming the
conversion using the optimal catalysts of Au-HYT.
formation of zeolite Y in consistent with the XRD analysis. The
TEM image indicated that the size of Au-NPs was around 5 nm
(not shown).
e
Au(111)
Figure 3 showed the yield of valuable chemicals for
Au(200)
cellulose conversion. The products mainly consisted of glucose
and HMF under light irradiation and the produced LeA could
be neglected. It indicated that the yields of valuable chemicals
increase with higher temperature. The yield of glucose and
HMF achieved 58.7% at 16 hours at 140 °C (Fig. 3a). After
reaction time of 16 hours, the yields at 120 °C and at 130 °C
continued to increase. In contrast, the reaction at 140 °C
displayed a decline tendency after 16 hours (Fig. 3b-3d). The
maximum yield of glucose and HMF at 16 hours at 140 °C can
be attributed to the further conversions of HMF to generate
solid humins, which are main side-products under light
d
c
b
a
1
0
20
30
40
50
2
θ /
2
9,30
irradiation.
The yield at the same reaction condition but
Figure 1. XRD patterns of various catalysts: a) HT; b) HY; c) with light off was 3.5% at 140 °C for 24 hours.
YT/HYT; d) Au-YT; e) Au-HYT.
a60
b
70
60
LeA
HMF
Glucose
Glucose
HMF
LeA
The crystal structures of various catalysts were detected by
X-ray diffraction (XRD) patterns (Fig. 1), confirming that the
zeolite structure preserved well after loads of Au-NPs. The
characteristic peaks of zeolite Y at 2θ of 6.3°, 15.9° and 24.0°
were reflections from the (111), (133) and (266) planes of
zeolite Y, respectively. It indicated that the zeolite nano-
50
5
4
3
0
0
0
40
Total
30
2
0
0
0
20
1
1
0
0
o
120
C
o
C
o
0
4
8
12
16
20
24
130
140 C
Time/ hours
2
2,23
crystals on the nanofibres were well crystallized.
A couple
of new peaks (Fig. 1) around 38.2° and 44.5° were observed
and assigned to the characteristic diffraction peaks of Au-NPs,
corresponding to the Au (111) and (200) planes,
c
70
60
d
70
60
50
40
30
20
10
0
Glucose
HMF
LeA
Glucose
HMF
LeA
5
4
0
0
Total
Total
2
6,27
respectively.
30
2
1
0
0
0
0
4
8
12
16
20
24
0
4
8
12
16
20
24
Time/ hours
Time/ hours
Figure 3. The yield for cellulose conversion on Au-HYT. a) The
yields of valuable chemicals at different temperatures at 16
hours; b) The product distributions at 120 °C; c) The product
distributions at 130 °C; d) The product distributions at 140 °C.
Reaction conditions: 50 mg cellulose, 50 mg catalyst, 500 mg
e
d
c
-2
a
water, 4.5 g EMIMCl, 0.5 W cm visible light, 24 hours.
b
One may argue that the acidity of zeolites plays a more
important role than LSPR effect in the cellulose conversion
+
because Brønsted acid sites (H ) are the catalytic sites.
3
00 400 500 600 700 800
Wavelength/ nm
However, this study conveyed a clear message that the
polarised electromagnetic field due to LSPR effect of Au-NPs
did contribute to improving the catalytic performances of
zeolites under visible light irradiation. To illuminate the
impacts of light irradiation on the cellulose conversions, the
light intensity and the range of wavelength were investigated
on Au-HYT at 130 °C with light on.
Figure 2. UV/Vis spectra of of various catalysts: a) HT; b) HY; c)
YT/HYT; d) Au-YT; e) Au-HYT.
2
As shown in Figure 2, both TiO NFs and Y-zeolites with or
+
without acid treatment (H exchange) did not have obvious
absorption in the wavelength range of visible light region,
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The light wavelength was also investigated using glass
filters to block photons with wavelengths below the filter
threshold (see SI, Section 3). For example, the cut-off
wavelength of 400 nm means that a majority of the light with
wavelength smaller than 400 nm was blocked. Figure 5
showed that the light wavelength significantly influenced the
catalytic activity of Au-HYT. The yield decreased remarkably,
dropping from 34.4% to 6.4% while the wavelength ranges
were narrowed from full wavelength to the wavelength large
than 600 nm. The low yield of the latter was corresponding to
the low light absorption of Au-NPs in this light ranges (the
curve e in Figure 2). This result demonstrated a wavelength-
selective enhancement on Au-HYT, and proved that visible
light with wavelength around 530 nm could induce the LSPR
effect of Au-NPs and made a primary contribution to boost the
catalytic activity.
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
-2
0
0
0
.7Wcm
.5Wcm
.3Wcm
-
2
2
-
4
8
12
16
20
24
Reaction time/ hours
Figure 4. The yields of valuable chemicals for cellulose
conversion varying with light intensity. Reaction conditions: 50
mg cellulose, 50 mg catalyst, 500 mg water, 4.5 g EMIMCl, at
100
130 °C for 24 hours.
8
6
4
0
0
0
Figure 4 illustrated the yields of valuable chemicals for
cellulose conversion varying with light intensity. Hydrolysis of
cellulose to glucose and subsequent formation of HMF were
observed under light irradiation with different light intensities.
The yield of LeA could be neglected, which was detailed in
supplementary information (SI, Section 1). The yield of glucose
-2
20
0
and HMF at light intensity of 0.7 W cm demonstrated the
highest efficiency; achieving a total yield of 69.8% at 130 °C for
2
0
3
4 hours. The reaction rate at lower light intensity, such as at
-2
-2
1
2
3
4
5
.5 W cm and 0.3 W cm , showed relative low conversions of
4.4% and 29.8%, respectively. It indicated that the conversion
Runs
of cellulose can be activated or driven by a plasmonic Figure 6. The yields of valuable chemicals for cellulose
nanostructure. Higher light intensity meant that more photons conversion with recycled catalysts. Reaction conditions: 50 mg
or more energy input originated from incident light at a certain cellulose, 50 mg catalyst, 500 mg water, 4.5 g EMIMCl, 0.5 W
-2
excitation wavelength, which may efficiently interact with Au- cm visible light, at 140 °C for 16 hours.
NPs in the catalysts. This result indicated that increasing light
intensity could enhance the LSPR effect on Au-HYT thus
contributing to boosting the catalytic activity.
Figure 6 showed the conversions of cellulose on Au-HYT
recycled for four times. As far as we can see, the efficiency was
reducing gradually from a yield around 60% to about 50% after
5
-run use. The catalysts can maintain about 80% of the initial
35
30
25
20
15
10
5
0
Full wavelength
400 nm
> 450 nm
reactive activity after 5 runs. It indicated that the catalyst was
>
very stable and the supported Au-NPs could be reused without
26,27
>
>
500 nm
600 nm
leaching.
From the TEM image (see SI, Fig. S4), the 5 nm of
Au-NPs, 100 nm of zeolites, and fibrous TiO can be cleanly
2
seen without obvious aggregations or amorphology. In thought
that the recycling of catalysts was only conducted with drying
procedure, the treatment of recycled catalysts, such as the
removal of deposited carbon by calcination, should further
improve the activity of recycled catalysts.
A mechanism for the hydrolysis of cellulose to glucose was
+
outlined in Scheme 3. Basically solid acids (proton H )
4
8
12
16
20
24
Reaction time/ hours
originated from zeolites or TiO2 NFs, played a key role in
hydrolysis of cellulose. Particularly, the LSPR effect of Au-NPs
under visible light irradiation enhanced the polarity electrical
Figure 5. The yields of valuable chemicals for cellulose
conversion varying with wavelength ranges. Reaction
conditions: 50 mg cellulose, 50 mg catalyst, 500 mg water, 4.5
2
6,27
field of the extra-framework cations in zeolites.
The
polarity electrical field benefited to acid strength as well as
+
stretched polarized bonds while attacked by proton H (step I).
-2
g EMIMCl, 0.5 W cm visible light, at 130 °C for 24 hours.
4
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The stretching further enhanced the negative character of the at 130 °C for 24 h. Both the acid strength and LSPR effect of
oxygen atom in the -1, 4 glycosidic bond. Consistent with Au-NPs have key impacts on processing efficiency. The
thermal reactions, the C1’ O bond in the -1, 4 glycosidic processing mechanism was proposed with the acid strength
β
–
β
linkage was stretched and broke down to glucose and a enhanced by the polarized electrical fields of zeolites due to
positive carbon cation (step II). With polarized water the LSPR effect. The outcomes would significantly decrease
molecular, the hydrolysis reaction at the positive carbon cation processing costs, reduce energy consumption, and contribute
+
easily proceeded to produce glucose and release a proton H
3
to the utilisation of broad biomass resources. This research has
great potential to make biofuel and chemical productions
renewable. More significantly, due to the solid nature of the
catalysts, the separation and recycling of catalysts from the
liquid product solution becomes more readily after complete
conversion of cellulose.
1
(step III).
OH
OH
OH
6
5
H
O
II
4
HO
O
O
O
O
OH
O
HO
HO
HO
Glucose
OH
3
2
1
OH
OH
hv
OH
+
6'
Zeolite/TiO2
4'
I
H+
5'
O
Au
O
HO
2'
1'
3'
H
H
OH
References
O
OH
OH
OH
6'
5'
III
1
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'
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O
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'
n
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'
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5
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Similarly, the subsequent conversion from glucose to HMF
went through a dehydration process assisted by solid acid
catalysts (see SI, Scheme S1). Apart from the solid acid (proton
+
H ) attacking the O in C-O-C (step I in SI, Scheme S1), the
glucose underwent an opening-cycle reaction. Meanwhile, a
carbonyl bond is formed (step II in SI, Scheme S1), which could
be enhanced by plasmonic structure (Au-NPs) and had been
well documented by our previous works as well as other
groups. To make it simple, this process to form a carbonyl
bond is similar to the oxidation of alcohols, where the
electromagnetic fields enhanced by LSPR effect of Au-NPs
produced energetic electrons and thus promote the hydrogen-
1
1
1
1
1
1
1
1
2
6,27,32-34
abstracting process.
inter-molecular dehydration processes proceeded. After losing
water molecules in total (steps II-IV in SI, Scheme S1), the
Subsequently, the continuous
3
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and LSPR effect played crucial roles. The effect of light
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4
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2
2
1
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2
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Table of Contents Graphic and Synopsis
Sustainable conversion of cellulosic biomass to chemicals under visible-light
irradiation
a
b
c
d
b
b
Lina Wang, Zhanying Zhang, Lixiong Zhang, Song Xue, William O. S. Doherty, Ian M. O’Hara and
a
Xuebin Ke*
Plasmonic nanostructure: a high conversion (> 60%) of crystalline cellulose to chemicals was achieved with
3
6
0
enhanced electromagnetic fields, E/E =10 -10 times.
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