Selective Microwave-Assisted Pyrolysis of Cellulose towards Levoglucosenone with Clay Catalysts

Article abstract of DOI:10.1002/cssc.201903026

Levoglucosenone (anhydrosugar) is one of the most promising chemical platforms derived from the pyrolysis of biomass. It is a chiral building block for pharmaceuticals as well as an intermediate in the production of solvents and polymers. Therefore, the development of cost-efficient, low-energy production methods is vital for a future sustainable biorefinery. Here, a novel, green approach to the production of levoglucosenone was developed by using a microwave (MW)-assisted pyrolysis of cellulose in the presence of readily available clays. It was shown that natural and pillared clays in the presence of MW irradiation significantly increase the yield of levoglucosenone from cellulose. Both the water content and the presence of acid centres are critical characteristics that influence the yield and distribution of catalysed products. A unique experiment was designed by using a synergetic effect between different types of catalysts, which enhanced the levoglucosenone yield to 12.3 wt % with 63 % purity.

Full text of DOI:10.1002/cssc.201903026

Accepted Article
Title: Selective Microwave-assisted Pyrolysis of Cellulose towards
Levoglucosenone using Clay Catalysts
Authors: Alisa Doroshenko, Ihor Pylypenko, Karl Heaton, Stephen
Cowling, James Clark, and Vitaliy L. Budarin
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To be cited as: ChemSusChem 10.1002/cssc.201903026
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10.1002/cssc.201903026
ChemSusChem
COMMUNICATION
Selective Microwave-assisted Pyrolysis of Cellulose towards
Levoglucosenone using Clay Catalysts
Alisa Doroshenko,[a] Ihor Pylypenko,[b] Karl Heaton,[c] Stephen Cowling,[d] James Clark,*[a]
and Vitaliy Budarin*[a]
Abstract: Levoglucosenone (anhydrosugar) is one of the most
promising chemical platforms derived from the pyrolysis of
biomass. It is a chiral building block for pharmaceuticals as well
as an intermediate in the production of solvents and polymers.
Therefore, the development of cost-efficient, low-energy
production methods are vital for a future sustainable biorefinery.
Here we report a novel, green approach to the production of
levoglucosenone using a microwave (MW) assisted pyrolysis of
cellulose in the presence of readily available clays. We showed
that natural and pillared clays in the presence of MW irradiation
compounds is impossible, while the application of
chromatography columns is an expensive decision for the large-
scale chemical industry.
A possible solution is selective in-situ targeting of the
desired compounds during pyrolysis of biomass (which typically
contains hemicellulose, cellulose and lignin). At present, most of
the problems associated with refining hemicellulose have been
addressed,[2] while refining cross-linked lignin requires significant
further developments. Hence our focus on cellulose. Pyrolysis of
cellulose to
a complex mixture of chemicals is already
significantly increase the yield of levoglucosenone from cellulose. developed but a controllable and sustainable production of the
Both the water content and the presence of acid centers are
critical characteristics which influence the yield and distribution
of catalyzed products. A unique experiment was designed using
a synergetic effect between different types of catalysts which
platform
molecules
levoglucosenone
(LGO)
and
5-
hydroxymethylfurfural (5-HMF) is not. Conventional pyrolysis of
cellulose proceeds at high temperatures (T>360°C), inducing
secondary reactions and therefore producing a complex mixture
enhanced the levoglucosenone yield to 12.3wt% with 63% purity. of products.
One of the ways to increase selectivity during pyrolysis is
catalysis.[3] Natural aluminosilicates such as zeolites are widely
reported to preferentially catalyze the conversion of biomass to
aromatics.[4,5] However, although cost-efficient and readily-
available, clays have not attracted significant attention. Pillared
clays (a class of swollen clays modified with a variety of large
polynuclear hydroxo-complexes) are of particular interest.[6]
Another way to improve the pyrolysis selectivity is the
application of MW-assisted heating, which proceeds at lower
temperatures (~165-220°C) than conventional pyrolysis.
Bio-oil is a complex organic mixture resulting from thermal
processing (pyrolysis) of biomass and bio-waste and represents
an alternative renewable source for chemicals and fuels. The
majority of individual compounds in bio-oil are multi-functional
oxygen-containing chemicals. Some of them are attractive
platform molecules ready for industrial use without any pre-
functionalization.[1] These platform molecules could form the
core of a sustainable and efficient biorefinery.
Currently, one of the biggest challenges for such
a
In this paper we investigated the synergy between MW-
assisted cellulose pyrolysis in the presence of natural bentonite
and pillared bentonite. We chose Al-pillared bentonite (Al-PILC)
as pillared clay because of its commercial availability, simple
synthesis and high acidity, caused by the intercalated Al13
biorefinery is separation of the complex bio-oil to individual
compounds. The direct distillation of the oxygen-containing
hydroxo-complexes (AlO4Al12(OH)24(H2O)12)7+, acting as
heteropoly acid.
a
[a]
Alisa Doroshenko, Prof. James Clark, Dr. Vitaliy Budarin
Green Chemistry Centre of Excellence
Department of Chemistry
Successful intercalation of the Al13 hydroxo-complexes into
native bentonite (Na-form) increases the basal spacing (d001
University of York
)
Heslington, York, UK, YO10 5DD
E-mail: vitaliy.budarin@york.ac.uk, james.clark@york.ac.uk
Homepage: www.vitaliybudarin.com
Dr. Ihor Pylypenko
Department of Chemical Technology of Ceramic and Glass
The National Technical University of Ukraine “Igor Sikorsky Kyiv
Polytechnic Institute”
Peremohy Ace, 37, Kyiv, Ukraine, 03056
Karl Heaton
MS Service
from 15.20Å to 18.11Å (Figure 1a-b). Furthermore, thermally-
resolved Small-Angle X-ray Scattering (SAXS) (Figure 1a-b,
insertion graphs) shows the difference in water behavior
between the pillared and native bentonite. The step-wise
changes (w2, w1, w0) of the basal spacing for the bentonite are
related to a serial removal of discrete water sheets in the
interlamellar space of this catalyst.[7] The absence of such
phenomenon for an Al-PILC (Figure 1b, insertion graph)
indicates that Al13-complexes are acting as supportive pillars
between the clay sheets, preventing their collapse. Additionally,
the N2 adsorption results indicate the formation of a larger
number of micro- and meso-pores for Al-PILC in comparison to
the non-pillared clay leading to ca. a six-fold increase in BET
surface area and smaller water diffusion rate (Figure 1c and S6).
The different environment for water within the samples is
shown by thermal gravimetric analysis. The bentonite desorbs
[b]
[c]
[d]
Department of Chemistry
University of York
Heslington, York, UK, YO10 5DD
Dr. Stephen Cowling
Liquid Crystals and Materials Chemistry Group
Department of Chemistry
University of York
Heslington, York, UK, YO10 5DD
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201903026
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COMMUNICATION
twice as much water as Al-PILC (4.2wt%) below 180°C, (the
temperature of cellulose pyrolysis in a MW-reactor, Figure 1d).
Mass-loss of the Al-PILC is stretched out in time and
temperature because of a slower water diffusion compared to
the original macro-porous bentonite (Figure 1d).[8,9]
levoglucosan is an intermediate for the production of sugars via
interaction with internally produced water.[14]
Figure 1. Characterisation of the catalysts: a), b) SAXS analysis and
thermally-resolved SAXS data; c) N2 adsorption data; d) thermal analysis
Figure 2. Experimental data: a), b) – composition of the bio-oils resulted from
the MW-assisted pyrolysis of cellulose mixed with bentonite and Al-PILC
respectively (all yields are calculated with respect to the cellulose weight); c)
GC-FID data of the extracted bio-oil for cellulose (top), 50wt% of bentonite
(middle) and 50wt% of Al-PILC (bottom)
The acid properties of materials were confirmed to be
different through pyridine titration and temperature-programed
desorption of ammonia (Figure S8-9). The original bentonite
contains mainly Lewis species while Al-PILC has both Lewis and
Bronsted acid centers.
The influence of the clay catalysts on the MW-assisted
cellulose pyrolysis has been systematically investigated. The
catalysts were intensively mixed together with cellulose at
different loads and subjected to microwave-assisted pyrolysis
(Figure S10).
The production of sugars in the presence of bentonite
slows down at >20wt% of the catalyst, while the yield of 5-HMF
starts to increase up to 4wt% (Figure 2a). It is reasonable to
assume that 5-HMF is a product of sugars conversion.[15,16] In
contrast, the use of Al-PILC gives maximum yield of 5-HMF at
20wt% of the catalyst (Figure 2b). This pronounced difference in
HMF yields could be due to differences in both the water content
and nature of acid centers of these catalysts. Recently Lewis
acid species were reported to play a significant role in glucose
transformation to 5-HMF,[14,17] and bentonite, incorporating
Lewis sites, could be a promising candidate for this reaction. In
contrast, Al-PILC contains many more Bronsted centers.
Moreover, additional acid centers are produced during thermal
decomposition of the Al13-complex towards Al2O3 (Figure 3). We
did not find any evidence of Al2O3 having a catalytic effect, while
the Al13-complex produced acid sites could promote reactions
typical for Bronsted species such as LGO production – a well-
known product of cellulose pyrolysis in the presence of Bronsted
acids.[8,9] This explains a nearly negligible LGO yield with
bentonite (Figure 2a) and its substantial growth to 6.3wt% (40%
purity) at 50wt% of Al-PILC (Figure 2b). Conventional pyrolysis
of cellulose mixed with the same Al-PILC loading resulted in
higher LGO yield (10.1%), but much lower selectivity of ~26%
(Figure S15). Thus, our experiments show that MW irradiation
The results obtained clearly show that the Al13 pillaring
procedure significantly changes the catalytic performance of the
bentonite (Figure 2a-b). Low bentonite loading (<20wt%)
increases bio-oil production (Figure 2a), which is in good
agreement with previous studies.[10,11] However, we found that at
higher loading, the bio-oil yield is decreased. This effect can be
attributed to competition between the cellulose hydrolysis
process and the bio-oil’s interaction with clay (charring and
adsorption). The moisture produced below 180°C from
macropore bentonite promotes cellulose hydrolysis and
increases yields of sugars and bio-oil (Figure 2a). In contrast to
the bentonite, the Al-PILC (which releases low amounts of water
4.2wt%, Figure 1d), leads to a gradual decrease in the bio-oil
yield (Figure 2a).
Interestingly, we found that levoglucosan yields are
proportional to bio-oil yields independent of the nature and
quantity of the catalyst (Figure 2a-b). This observation confirms
levoglucosan as being a primary intermediate for low molecular
weight organic volatiles (M˂162 g/mol) through its
decomposition
in
cellulose
pyrolysis.[12,13]
Moreover,
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10.1002/cssc.201903026
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improves pyrolysis selectivity and that the production of 5-HMF
and LGO are competitive (Figure 2c) and depend on the amount
of locally available water and type of acid centers.[18]
production of glucose, 5-HMF, levulinic and lactic acids (Figure
S18). The Al-PILC was successfully regenerated three times
losing a total of ~34% of its activity.
Stoichiometrically the formation of levoglucosan (C6H10O5)
from cellulose (C6H10O5)n does not require H2O, but surprisingly
its production needs a catalytic amount of water, while an
excess of H2O promotes cellulose hydrolysis to sugars
(Figure 3). Final yield of the pyrolysis of dry cellulose favors
charring reactions.[19] We believe that the ratio between
levoglucosan and sugars determines the possible yields of LGO
and 5-HMF and is controlled by the amount of available water.
The final yield of LGO is controlled by the presence of Bronsted
acid sites, such as those present in Al-PILC (Figure 3). The
catalytic formation of LGO can occur directly from levoglucosan,
or indirectly via 1,4:3,6-dianhydro-α-D-glycopyranose (DGP)
intermediate.[20]
Figure 4. Synergetic effect of bentonite and Al-PILC to enhance the yield of
LGO (notably, the retention time of the LGO is longer compared to that one on
Figure 2c, because the spectra were collected at different times)
In conclusion, through MW-assisted pyrolysis of cellulose
in the presence of bentonite and Al-PILC, we have identified a
competitive nature between the production of 5-HMF and LGO.
We believe the majority of LGO and 5-HMF comes from
levoglucosan and sugars respectively. The ratio between sugars
and levoglucosan is controlled mainly by the amount of water
produced during the pyrolysis stage. This water favors
hydrolysis reactions, increasing the yields of sugars, which are
partially converted to 5-HMF on the Lewis acid sites. The
Bronsted acid sites of Al-PILC promote the production of 40%
pure LGO at 6.3wt% yield at 50wt% of the catalyst loading. We
also found a synergetic effect between both catalysts to
enhance the yield of LGO to 12.3wt%, which is comparable with
our conventional pyrolysis experiment but the selectivity towards
LGO is three times higher. The Al-PILC catalysts is considered a
good candidate for LGO production and in the MW-assisted
pyrolysis of paper waste was regenerated three times.
Figure 3. The suggested scheme of MW-assisted pyrolysis of cellulose
Therefore, based on these observations, we designed a
“layer” experiment to enhance the yield of LGO (Figure 4a)
emphasizing a synergetic effect between both catalysts. The
vessel was packed with of bentonite at the bottom to generate
water, enhancing the bio-oil yield during MW-assisted pyrolysis
(Figure 4a). The bentonite loading (20wt%) was chosen based
on the highest yield of bio-oil obtained in the previous
experiments (Figure 2a). Cellulose was mixed with Al-PILC and
a layer of pure Al-PILC catalyst was placed on the top (Figure
4a). The yield of the bio-oil increased and the composition
analysis showed the presence of 12.3wt% of LGO of ~60%
purity in the bio-oil mixture (Figure 4b).
Experimental Section
All experimental details are available in the Supplementary Materials.
Keywords: Microwave chemistry • Biomass • Pyrolysis • Clays •
Platform molecules
To demonstrate a significant biorefinery potential, we
successfully performed MW-assisted pyrolysis of paper waste
simply mixed with 50wt% of the Al-PILC, giving 4.8wt% of LGO
(Figure S16). To reactivate the catalyst, we followed a two-step
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Entry for the Table of Contents
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Alisa Doroshenko, Ihor Pylypenko,
Karl Heaton, Stephen Cowling, James
Clark,* and Vitaliy Budarin*
Page No. – Page No.
Text for Table of Contents
Selective
pyrolysis
microwave-assisted
cellulose towards
of
levoglucosenone using clay catalysts
Clays can be superior catalysts to obtain platform molecules from biomass via microwave pyrolysis.
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