The potential of microwave technology for the recovery, synthesis and manufacturing of chemicals from bio-wastes

Article abstract of DOI:10.1016/j.cattod.2013.11.058

Through a series of case studies it is demonstrated that microwave dielectric heating can be a powerfultool to recover and synthesize valuable molecules from a wide range of biomass types. In addition, undermicrowave irradiation the production of chemicals from biomass proceeds at markedly lower temper-atures (up to 150?C) compared to conventional heating. This has a secondary benefit in that moleculeswith a high degree of functionality are produced while conventional heating tends to produce a greatproportion of lower value gases. Furthermore, the technical set-up of a microwave reactor can easilyaccommodate for an in-situ separation of acids and valuable products therewith improving the shelf lifeof the latter. The benefits of combining hydrothermal conditions with microwave irradiation are alsoillustrated. In addition, a specialized case of selective heating in a biphasic reaction system is discussed,allowing for improved yields and selectivity.

Full text of DOI:10.1016/j.cattod.2013.11.058

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
The potential of microwave technology for the recovery, synthesis
and manufacturing of chemicals from bio-wastes
a,∗
c
b
a
a
Vitaliy L. Budarin , Peter S. Shuttleworth , Mario De bruyn , Thomas J. Farmer ,
a
a
a
Mark J. Gronnow , Lucie Pfaltzgraff , Duncan J. Macquarrie , James H. Clark
a
Green Chemistry Centre of Excellence, Department of Chemistry, University of York, York, UK
Departamento de Física de Polímeros, Elastómeros y Aplicaciones Energéticas, Instituto de Ciencia y Tecnología de Polímeros, CSIC, Madrid, Spain
Biorenewables Development Centre, The Biocentre, York Science Park, York, UK
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2013
Received in revised form 15 October 2013
Accepted 30 November 2013
Available online xxx
Through a series of case studies it is demonstrated that microwave dielectric heating can be a powerful
tool to recover and synthesize valuable molecules from a wide range of biomass types. In addition, under
microwave irradiation the production of chemicals from biomass proceeds at markedly lower temper-
◦
atures (up to 150 C) compared to conventional heating. This has a secondary benefit in that molecules
with a high degree of functionality are produced while conventional heating tends to produce a great
proportion of lower value gases. Furthermore, the technical set-up of a microwave reactor can easily
accommodate for an in-situ separation of acids and valuable products therewith improving the shelf life
of the latter. The benefits of combining hydrothermal conditions with microwave irradiation are also
illustrated. In addition, a specialized case of selective heating in a biphasic reaction system is discussed,
allowing for improved yields and selectivity.
Keywords:
Microwaves
Biomass
Pyrolysis
Hydrolysis
Chemicals
Energy
© 2013 Elsevier B.V. All rights reserved.
1
. Introduction
research [4]. In very general terms the biorefinery approach encom-
passes two major techniques: thermochemical (see Fig. 1) and
biochemical [5]. The former is able to accept a wide range of
biomass types and convert them into a broad spectrum of chemi-
1.1. Biorefineries concept
The global concern over the future oil supplies is fuelling the
cals and materials, and as such, forms the basis of this paper. The
thermochemical treatment of biomass also holds vast potential to
apply cleaner technologies, promoting the use of greener chemi-
cal practices [6]. It is typically conducted at elevated temperatures
under (a) pyrolytic conditions i.e. inert atmosphere or vacuum and
with only a limited amount of water present or (b) hydrothermally
i.e. under significant pressure and in the presence of an excess of
water (see Fig. 1).
The products of the thermal treatment of biomass span solids
(char), liquids and gases. The yield of these fractions can be con-
trolled by parameters such as rate of heating, temperature, pressure
and the presence of additives. Typically, increasing the heating rate
facilitates the production of volatiles.
Chemicals can be obtained from both gas and liquid fractions.
However, the complete deconstruction of the original biopolymer
matrix to gaseous molecules followed by their recombination is
energy intensive and less sustainable compared to a tuned direct
manufacturing approach. At present the thermochemical route
requires high initial capital costs despite the fact that it is fast and
able to accept a wide variety of biomass types. The costs relate to
the micronization of the biomass, a very intensive pre-treatment,
return to a bio-based economy with an emphasis on the use of non-
food related lignocellulosic waste material [1]. This approach has
two principle strategic ambitions: (a) the replacement of imported
petroleum by renewable domestic raw materials (the energy goal)
and (b) the establishment of a robust bio-based industry (the eco-
nomic goal). The former is already addressed through the current
push for bio-fuels (e.g. ethanol, butanol, (algal) biodiesel) displac-
ing a part of the transportation related gasoline and diesel. Despite
the high volumes of fuel produced, its relatively low value strongly
limits the possible return-on-investment for new bio-fuel related
technologies [2]. Therefore, there has also been a push towards the
production of value-added chemicals from biomass. These drivers
for bio-fuels and bio-derived chemicals are powering the develop-
ment of new zero-waste biorefinery technologies [3]. Conceptually,
a biorefinery applies a hybrid of technologies from different fields
including (polymer) chemistry, bio-engineering and agricultural
∗ Corresponding author. Tel.: +44 1904 324479.
E-mail address: Vitaliy.budarin@york.ac.uk (V.L. Budarin).
0
920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.11.058
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Fig. 2. Comparison of convection and microwave heat distributions within a sample
matrix.
accurately [15]. This allows for a safe and precise control, even
when applying very high heating rates. A large number of examples
highlighting the efficiency of microwave-mediated reactions have
been described in the literature and then particularly in the areas
of organic synthesis [16], polymers [17], and green chemistry [18].
The advantages of microwave technology in terms of mobility,
using small scale processors, and towards waste remediation have
also been highlighted [19].
Fig. 1. The thermochemical approach in a hypothetical biorefinery. The striped
block refers to a more limited application scope.
Typically, microwave-assisted processing of biomass is con-
◦
and the high temperatures involved (>500 C). In addition, conven-
◦
ducted at high temperatures (between 450 and 600 C), which is
tional pyrolysis is unable to separate the acidic aqueous fraction
from the organic one in-situ, thus requiring additional upgrading
steps. This, combined with a low degree of functionality of the
products, restricts the industrial application of conventional pyrol-
ysis of biomass. Currently microwave-assisted pyrolysis of biomass
is gaining interest as in-situ separation of the products is possi-
ble. Hydrothermal processing on the other hand requires often the
presence of strong acids and/or high pressures.
◦
similar to conventional pyrolysis (>350 C), with the prime focus
being on pyrolysis, gasification and liquefaction to fuels. However,
microwaves are not only useful as an alternative method of heating,
there is also good evidence that they allow for different pyrolytic
mechanisms [20]. In this respect, the fact that microwave irradi-
ation already interacts with different types of biomass (including
wood, grasses, agricultural residues and food waste) at relatively
◦
low temperatures (<200 C), producing bio-char and bio-oil, is of
significant importance [21–23]. These mild pyrolysis conditions can
substantially change the composition of the produced bio-oils and
benefit future bio-refineries.
This article explores these emerging microwave-assisted biore-
finery technologies/opportunities for processing a wide range of
biomasses.
1
.2. Microwave heating
Many routes have been explored to improve the pyrolysis pro-
cess including the use of catalysts, more uniform temperature
distributions and increased rates of heating. These though are not
yet allowing the processing of larger particles. Microwave (MW)
activation is a more recent approach holding great potential to
overcome these issues. The use of microwave irradiation is well
established in many industrial and commercial applications for
over sixty years: industrial scale microwave systems [7] are avail-
able for wood drying [8] and food processing [9,10]. However, there
are still a number of drawbacks e.g. the limited penetration depth of
microwaves and the overall energy consumption. As such further
refinement of the equipment and operating procedures is neces-
sarily and this particularly towards pyrolysis [11,12].
The microwave region of the electromagnetic spectrum lies
between the infrared and radio frequencies, and corresponds to
wavelengths between 1 cm and 1 m (i.e. frequencies of 30 GHz
to 300 MHz, respectively). Microwave dielectric heating uses the
intrinsic potential of (polar) compounds to transform electromag-
netic energy into heat, therewith provoking and driving chemical
reactions. In contrast to conventional heating this method does not
involve radiant heat and indeed the materials themselves absorb
and dissipate the “energy” making the heating process more volu-
metric and hence appreciably faster and selective (see Fig. 2). This
feature of microwaves is very important for the processing of poorly
thermal conducting materials such as wood.
2. Material and methods
2.1. Microwave processing of materials
2.1.1. Microwave rape meal and wheat straw pyrolysis
2.1.1.1. CEM discovery laboratory microwave. Samples of rape meal
or wheat straw (typically 300 mg) were weighed out into a
microwave tube, and then sealed using the microwave tube lid.
The sample was placed in the microwave and irradiated under
varying conditions: typical power outputs and temperatures were,
◦
respectively between 100–300 W and 100–300 C. After microwave
treatment, the sample was removed from the microwave and
washed with acetone to remove condensed (non)-volatile compo-
nents.
2.1.1.2. Microwave biomass pyrolysis. Larger scale microwave pyrol-
yser. Milestone ROTO SYNTH rotative solid phase microwave reactor
(Milestone Srl., Italy) (2 L vessel). Average sample mass was around
200 g. Samples were exposed to a maximum microwave power of
1200 W with an operating microwave frequency of 2.45 GHz (wave-
length 12.2 cm). The process temperature was maintained below
It has been shown recently that microwave technology can be
an energy- and cost-efficient method of heating [13]. As such it has
gained wide acceptance as a mild and controllable processing tool
both at the laboratory and industrial scale [14]. Microwave heating
can be controlled instantly and the power applied can be regulated
◦
200 C, as measured by infrared temperature probes. The process
vacuum was less than 100 mbar and monitored continuously. Liq-
uid fractions were collected in a series of round bottom flasks fitted
within the vacuum line.
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2
.1.2. Microwave biomass hydrolysis
The latter temperature was maintained for a further 10 min. The
transfer line temperature was set at 270 C, the ion source at 240 C
and EI energy at 70 eV, in positive ion mode.
◦
◦
2.1.2.1. Microwave–assisted extraction/hydrolysis of orange peel.
TM
Multimode CEM “MARS” microwave reactor with One Touch
technology using EasyPrepTM Plus Teflon 100 mL closed vessels.
Typically six vessels were run simultaneously at a power input of
2
5
.1.7. Comparison of conventional and microwave heating for
-chloromethyl furfural (CMF) production from d-fructose
For the microwave protocol, 500 mg of fructose was added to
◦
8
00 W to a temperature between 120 and 200 C with a holding
time of 20 min. The applied microwave frequency was 2.45 GHz.
This microwave is fitted with a dual infrared/fibre optic system for
accurate temperature measurements.
a CEM 35 mL microwave tube containing 5 mL concentrated HCl
(Fisher Chemicals), 10 mL 1,2-dichloroethane (DCE, Sigma-Aldrich)
Sairem Miniflow 200SS, 200 W, 2450 MHz. This device was used to
gain accurate information on the energy efficiency of microwave-
assisted hydrothermal experiments. Two experiments were carried
out in pyrex glass closed vessels (20 mL): a blank one consisting of
distilled water and one containing orange peel (OP) in water (1:10
weight ratio). Each vessel was microwaved at 100 W (fixed forward
power) until a specific temperature was reached, at which point
the reflected power was recorded. The temperature was measured
◦
and a magnetic stirrer. The reaction mixture was heated to 70 C
(constant temperature mode) for 10 min at medium stirring speed.
Following the allotted reaction time the mixture was cooled rapidly
with compressed air and transferred to a separating funnel where
the organic layer was removed. The aqueous layer was washed
twice with 2 × 10 mL DCE and the three organic layers combined
and dried over magnesium sulphate. The solvent was removed to
leave a yellow oil (341 mg, 85% yield, 98% purity by GC). 1H NMR
®
externally using a Calex PyroUSB 151 infrared sensor.
(CDCl , 400 MHz): 4.59 (s, 2H, CH ), 6.55 (d, J = 3.6 Hz, 1H, Ar–H),
3 2
.16 (d, J = 3.6 Hz, 1H, Ar–H), 9.58 (s, 1H, CHO) ppm. C NMR (CDCl3,
13
7
1
5
2.1.2.2. Pectin isolation. After microwave-assisted extraction of the
00 MHz): 36.6 (CH ), 112.1, 121.9 (Ar), 152.9 (Ar), 156.1 (Ar), 177.8
2
pectin, the latter is precipitated using an excess of ethanol. The
mixture was left standing overnight and the precipitated pectin
was subsequently removed by centrifugation. The recovered pectin
was washed twice with acetone followed by a hot ethanol filtration
in order to remove the neutral sugars. Finally the pectin was dried
5 (CHO) ppm. The purity of the isolated product was determined
by GC-FID and the peaks assigned using GC–MS. The rates of heating
of the two layers were determined by individually heating 10 mL of
liquid (conc. HCl or DCE) for 10 min at 50 W (fixed power) and the
temperature recorded by the internal infrared temperature probe
of the CEM microwave reactor.
◦
using a stainless steel Virtex freeze dryer equipped with a −105 C
condenser.
2.1.3. Elemental analysis
3. Results and discussions
CHN analysis was carried out using an Exeter Analytical (War-
wick, UK) CE440 Elemental Analyser, calibrated against acetanilide
3.1. Microwave-assisted low temperature pyrolysis of biomass
with an S-benzyl-thiouronium chloride internal standard.
3.1.1. Case study: Rape meal
2.1.4. Preparative thermogravimetric analysis
Pyrolysis of rape meal, as shown in Fig. 3, illustrates nicely the
Conventional bio-char samples were prepared using a thermo-
gravimetric analyser STA 409 (Netzsch, Germany). Typically 80 mg
product distribution obtained from the interaction of microwave
irradiation with biomass at low temperature. Rape meal obtained
by cold pressing of rape seed contains up to 20% residual, and
thus unrecovered, primary oil along with a variety of structural
components (cellulose, hemicellulose and lignin). Interestingly the
interaction of microwaves with the rape meal leads to distinct
sequential conversions representing the complex structure of this
biomass. In the first step, primary oil (virgin rape seed oil) and
residual water are removed via microwave steam distillation at
−
1
of sample was heated under a constant N2 flow (100 mL min ) to
◦
−1
the target temperature at a heating rate of 10 C min and was held
at this temperature for 2 h. For this specific preparative method it
was impossible to use heating rates higher than 10 C min while
this would have led to significant sample overheating.
◦
−1
2
.1.5. Characterisation of gas fraction
The characterization of the off-gases from the microwave pyrol-
◦
∼100 C. This offers a potential alternative for the generally applied
ysis of wheat straw pellets was performed by connecting the
microwave device to an IR gas cell. The IR cell and transfer pipe were
hexane extraction step, therewith also adding to the sustainability
◦
profile of the products and the process. Between 100 and 250 C
◦
heated to 200 C. Typically 5 g of wheat straw was placed inside the
the dry rape meal is pyrolysed producing a secondary oil (bio-oil),
non-condensable gases, (chemisorbed) water and char. The resid-
ual solid (bio-char) has a significantly increased calorific value and
makes an excellent fuel.
microwave reactor and a continuous power of 300 W was applied
during the process for 6 min.
2
.1.6. Characterisation of liquid fractions
Pyrolysis-GC was performed using a CDS Analytical Pyroprobe
000 interfaced to an Agilent Technologies 6890 N Gas Chromato-
The generation of volatiles correlates well with the sample tem-
perature profile (Fig. 3B, sections I, IIa and IIb). From graph 3 C
it can be inferred that the initial microwave-assisted steam dis-
tillation step corresponds to a maximal sample heating rate of
2
graph, using a CDS 1500 interface. The oven was fitted with a 61.3 m
−
1
◦
(
(
length), 0.25 mm (internal diameter), 0.25 ␮m (phase thickness)
14% cyanopropylphenyl, 86% dimethyl polysiloxane) Rtx 1701
150 K min at a temperature of 70 C. Pyrolysis of the biomass and
the generation of the secondary oil (bio-oil) is more complex, being
dependent on the biomass structure. As such two different maxi-
capillary column. An Agilent Technologies 5975B Inert XL Mass
Selective Detector was coupled to the GC.
◦
mal heating rate peaks at ca. 145 and 190 C could be identified.
GC–MS of the microwave oils was performed using a Perkin
Elmer Clarus 500 gas chromatogram with a Clarus 560S mass
spectrometer. All samples were prepared in DCM. A Phenomenex
Zebron DB5-HT column (dimensions 30 m × 0.25 mm × 0.25 ␮m)
was used for all the separations. The method used an injection vol-
ume of 0.5 ␮L, with a split ratio of 50:1 at a constant flow rate of
It is known that under conventional heating, the decomposition of
biomass can be divided into three stages, each corresponding to
a particular structural component. The thermal decomposition of
hemicellulose, cellulose and lignin occurs, respectively, between
◦
◦
◦
220–350 C, 150–450 C and a wider domain of 150–700 C [24].
◦
Therefore, it was proposed that the peaks between 100 and 200 C
−1
1
0 mL min carrier gas (helium). The GC heating program was: a
shown in Fig. 3B correspond to the decomposition of the polysac-
charide components within the rape meal. This was further verified
◦
◦
−1
◦
1
min hold at 60 C followed by a ramp rate of 8 C min to 360 C.
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Fig. 3. Overview of the microwave pyrolysis of rape meal using the CEM Discovery microwave. (A) Schematic representation of the product yield at different stages of
the microwave pyrolysis process. (B) Graph of temperature versus pressure emphasising the three major regions where products are obtained. (C) Graph showing the
temperatures at which maximum heating rate occurs, and their relation to the three stages of product formation as shown in part B.
Fig. 4. Influence of the carbonisation temperature on the heat of combustion of char obtained both by microwave assisted and conventional methods from (A) cellulose and
(
B) hemicellulose. Van Krevelen diagram of char obtained from (C) cellulose and (D) hemicellulose. CEM Discovery laboratory microwave.
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Table 1
from the heat of combustion profiles for cellulose and hemi-
cellulose (see Fig. 4A and B). It can also be seen that in the
presence of microwave irradiation both hemicellulose and cellulose
decompose to char at significantly lower temperatures than under
conventional heating. The maximum calorific values are obtained
Chemical composition of wheat straw microwave-pyrolysis oil (Agilent Technolo-
gies 6890N).
Compound
Elution time
Percentage
area (%)
Cumulative
yield (%)
(
min)
◦
at temperatures which are significantly lower (150 and 100 C,
Acetic acid
n-Hexadecanoic acid
Vinyl guaiacol
7.194
0.84
0.75
10.7
0.57
0.50
4.54
3.71
2.84
2.73
2.45
Acids
1.6%
Furans
51.871
33.715
16.057
14.194
35.586
46.636
38.196
24.703
42.779
respectively) than when convection heating is used. Interestingly
the Van Krevelen diagrams, represented in Fig. 4C and D, show
clearly that the bio-chars obtained from cellulose through both
conventional and microwave-assisted pyrolysis have a very sim-
ilar composition–irrespective the temperature difference. It is also
noteworthy that the hydrogen content of the char obtained from
microwave-assisted pyrolysis of hemicellulose, is higher than the
one obtained by conventional pyrolysis. These results could explain
the higher maximum calorific value for the MW generated bio-char
derived from hemicellulose (Fig. 4B).
2-Furanmethanol
11.8%
Furfural
Syringol
Methoxyeugenol
trans-Isoeugenol
Guaiacol
Phenols
23.8%
4-Methyl-2,5-
dimethoxybenzaldehyde
Phenol
p-Ethylguaiacol
p-Cresol
Creosol
m-Cresol
o-Cresol
Vanillin
24.154
31.701
27.455
28.657
27.370
26.002
38.980
46.309
32.943
2.00
1.57
1.50
1.38
1.23
1.16
1.13
27.87
1.27
3.1.2. Case study: Wheat straw biorefinery
Agricultural residues comprise a major component of the total
biomass available in the world. Wheat straw is one of the most
promising candidates for utilisation in a biorefinery because it is
abundantly available as a co-product of the food industry. Annually
this mounts to a global availability of 350 million tonnes of which
Levoglucosan
Sugars
31.6%
1,4:3,6-Dianhydro-.alpha.-
d-glucopyranose
1
30 million tonnes in Europe and approximately 6 million tonnes
in the UK [25]. Importantly, this by-product only has a low-value
usage, making this crop one of the most promising candidates for
biorefining.
Furthermore, under microwave conditions significant amounts
of long chain acids including hexadecanoic acid were obtained
(Fig. 6B and Table 1).
Wheat straw does not contain significant amounts of primary
oil and only low quantities of high boiling waxes. The major com-
ponent of wheat straw is cellulose, making that a wheat straw
biorefinery has to focus predominantly on the production of high
value products from cellulose. As such, the microwave-assisted
breakdown of wheat straw focused mainly on the activation of
its major structural components, cellulose and hemicellulose, in
3.1.3. Influence of biomass nature on the results of microwave
induced pyrolysis
Based on different microwave pyrolysis screening conditions
four physical parameters were identified as being critical to opti-
mal liquid yield: sample mass [21], microwave power [28], water
content [29] and the biomass density [30]. Ultimately all these
parameters affect the heating rate, with higher rates generally
being better for bio-oil production. Following optimisation of these
parameters, bio-oil was obtained from a range of biomass feed-
stocks with the results of these experiments shown in Table 2. One
of the most significant advantages of microwave pyrolysis is the
in-situ separation of bio-oil into a minimum of two fractions based
on differences in their boiling points. The first fraction contains
water, aldehydes and acids, whereas the second fraction consists
solely of valuable organics. As can be seen, the yields of both frac-
tions are highly dependent on the feedstock adopted. However, the
microwave bio-oil yields after optimisation were independent from
the type of microwave used and comparable to those obtained by
flash pyrolysis in all four cases. For example, the 60% liquid frac-
tion yield obtained during microwave-assisted pyrolysis of Spruce
is equivalent to the bio-oil yields obtained during fast-pyrolysis,
data of which can be found in the recent literature [31,32]. GC–MS
spectra for five analysed biomasses are shown in Fig. 7. While paper
pyrolysis results mainly in the production of levoglucosan, the use
of reed canary grass and barley dust gives predominantly pheno-
lics. Pyrolysis of spruce and bracken tends to give a mixture of
phenolics and sugars. The many phenolics detected point at a sig-
nificant breakdown of the lignin. Overall the presence of catechol
(1,2-dihydroxyphenol) is particularly prominent.
◦
the 100–200 C temperature range. The microwave pyrolysis of the
original untreated wheat straw gave 14% gas, 21% bio-oil, 30% char
and a further 35% of an aqueous fraction (see Fig. 5). To quantify the
effectiveness of the microwave pyrolysis process, the mass balance
and the retained carbon have been calculated. The gas fraction con-
tains H , CO , CO, CH and a mixture of lower alkanes/alkenes. Vast
2
2
4
potential exists to use this mixture as synthesis gas or as a lower
grade energy source. The aqueous fraction contains predominantly
water, derived from the breakdown of the lignocellulosic compo-
nents, and low molecular weight acids and aldehydes (around 10%).
The formed bio-char is ideal for combustion as it has both a good
−1
calorific value (CV) of 27.2 kJ g and does not contain any added
sand. From Fig. 5 it can be seen that this fraction contains 43% of the
−
1
original feedstock energy (wheat straw CV = 18 kJ g ) and 42% of its
carbon. The bio-oil fraction accounts for 21% of the mass yield and
2
9% of the feedstock carbon. Therefore, the total recovered usable
carbon from low temperature microwave pyrolysis is ∼70%. This
constitutes a significant improvement compared to conventional
pyrolysis where most of the carbon is lost as CO2 and CO.
The composition of the pyrolysis oils produced from conven-
◦
tional high temperature (600 C) pyrolysis and low temperature
microwave processing of wheat straw are significantly different
(
see Fig. 6). The conventional pyrolysis oil includes large amounts
of furfural and acetic acid (from the cellulosic components), and
phenols and methoxyphenols (from the lignin) but only small quan-
tities of sugars (Fig. 6A). In contrast, the microwave bio-oil contains
appreciably less furfural and acetic acid, compared to the amount of
phenolic compounds. Also, the microwave-generated bio-oil gen-
erally shows a smaller number of compounds. Moreover, around
a third of the total peak area can be assigned to levoglucosan and
vinyl guaiacol, both of which are valuable chemical intermediates
3.2. Hydrothermal microwave treatment of biomass
3.2.1. Orange peel biorefinery: Pectin, HMF, sugars, flavonoids,
limonene and terpenes/terpinols
Globally around 31.2 million tonnes of citrus fruit are processed
annually by the food manufacturing industry, yielding 15.6 mil-
lion tonnes of citrus peel waste (CPW) [33]. This constitutes an
ideal renewable feedstock for an integrated biorefinery [34–36].
[
26,27].
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Fig. 5. Mass and carbon balance of low temperature microwave pyrolysis of wheat straw. Milestone Microwave Reactor.
Table 2
Product distribution obtained in the microwave pyrolysis of a variety of biomasses. Milestone Microwave Reactor.
Sample
Microwave pyrolysis product distribution (%)
Mass recovery (%)
Bio-char
Aqueous
Bio-oil
Liquida
Bracken
Paper
Spruce
47.9
41.5
26
15.3
17.6
41
22.8
23.4
19
38.1
42.2
60
70.7
64.9
45
Barley dust
Reed canary grass
45.7
45.6
15.9
15.9
18.9
23.0
34.8
38.9
80.5
68.6
a
Liquid yield—combination of bio-oil and aqueous fractions.
Microwave technology offers great potential for the recovery of
the individual components of CPW. In view of the overall consti-
tution of CPW this concerns terpenes, waxes, monosaccharides,
pectin, antioxidants (and cellulose). Through consecutive use of
a microwave-driven (steam) distillation and a microwave facil-
itated hydrolysis step, it was found possible to recover these
compounds. More specifically, the microwave-driven (steam) dis-
tillation allowed for the recovery of a highly pure limonene fraction
[37], this water based process constitutes a source of HCl-free
pectin. In Table 4 commercially derived orange pectin (CDOP) is
compared with our microwave hydrothermally extracted pectin
(MWHEP). It is apparent that the microwave hydrothermal route
gives rise to both a higher molecular weight (Mw) and a markedly
lower polydispersity index (Mw/Mn) compared to commercial
orange pectin.
(
fraction A) from orange peel waste (OPW). Interestingly, the
composition of fraction A differs from conventionally obtained
limonene, therefore imparting a different scent. The four most
prominent compounds of fraction A are listed in Table 3. Impor-
tantly, fraction A was recovered from the orange peel using only
the water already present in the peel.
Table 3
GC–MS analysis of microwave driven steam distillation of OPW. The four most
prominent compounds are shown including their relative proportions.
Compound
Structure
Quantity (%)
The non-volatile fraction of the microwave steam distilled
orange peel (MSD-OP) could be obtained from an acetone
soxhlet. Interestingly, MSD-OP was found to be rich in 5-
hydroxymethylfurfural (HMF). This is a secondary decomposition
product of glucose. As such it can be concluded that the microwave
treatment of orange peel does not only engage in the (steam)
distillation of limonene but catalyses “simultaneously” also the
transformation of glucose into HMF (Fig. 8). This process is favoured
by the natural acidity of OPW and the continuous removal of water.
The acetone extracted MSD-OP was consequently subjected to a
d-Limonene
94.83
␣-Myrcene
2.18
0.84
H
H
␣-Pinene
microwave-assisted hydrothermal process at temperatures below
◦
1
50 C. This allowed for the extraction of the pectin embedded
O H
Linalool
0.59
in the cellulose matrix. While commercial pectin is traditionally
obtained through strong acid-mediated extraction (HCl, pH 2−4)
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◦
◦
Fig. 6. Comparative GC–MS trace. (A) the fast pyrolysis of wheat straw at 600 C (B) wheat straw microwave-pyrolysis oil at 200 C.
As to prove the energy efficiency of microwave technology for
hydrothermal applications the reflected energies were measured
using a specially designed microwave (SAIREM). Microwaving
multifunctional bio-derived furan [38,39]. The conditions used for
this process can be viewed as low-temperature hydrothermal and
are thus firmly placed in the field of thermochemical biomass con-
version. Mascal’s research group has also demonstrated the value
of CMF as a platform chemical, illustrating its use as a building block
in the formation of fuels, solvents, monomers, pharmaceuticals (e.g.
1
0 mL of distilled water in a closed vessel at 100 W (fixed power)
◦
to both 115 and 130 C gave a consistently reflected power of
1
5 W. Upon addition of 1 g OPW to the water, the reflected power
◦
◦
®
decreased to 5 W (at 115 C) and 7 W (at 130 C). This suggests that
in a typical hydrothermal extraction of orange peel over 93% of
the microwave energy is effectively absorbed, demonstrating the
energy efficiency of the process.
Randitidine ), herbicides and insecticides (␦-aminolevulinic acid).
Also, it can be readily converted to HMF [40,41]. The latter has long
been touted as a bio-derived platform molecule of great potential
and this not in the least as it can be converted via oxidation to 2,5-
furandicarboxylic acid, a monomer used for the production of the
highly promising PET replacement, polyethylene furanoate (PEF)
3.2.2. CMF formation and the role of microwaves
[
42]. However, due to the high reactivity of HMF the yields and
Pioneering research by Mascal et al. developed a biphasic reac-
selectivity have remained low. The replacement of the hydroxyl
group on HMF for a chlorine results in a more stable compound.
In addition, the use of the biphasic reaction system allows for
the rapid removal of the CMF product from the reactive environ-
ment of the concentrated HCl, and into the selected non-aqueous
phase (typically dichloroethane DCE). Using conventional heating,
CMF formation from a range of mono/polysaccharides has been
demonstrated with remarkable selectivity and high yields utilising
such biphasic reaction system. In this process the carbohydrates
are charged into the concentrated HCl, dehydrated to furans (i.e.
tion system that can convert a range of carbohydrates (sugars and
polysaccharides) to 5-chloromethyl furfural (CMF), a versatile and
Table 4
Comparison of the weight average (Mw) and number average (Mn) molecular weights
and the polydispersity index (Mw/Mn) of CODP and MwHEP.
−
−1
)
Pectin type
Mn (g mol 1
)
Mw (g mol
Mw/Mn
4
4
CDOP
MwHEP
3.15 × 10
8.93 × 10
2.83
1.71
5
5
1.31 × 10
2.23 × 10
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Fig. 7. Influence of the nature of biomass on the composition of microwave derived bio-oil (Milestone Microwave Reactor). GC spectra of pyrolysis oils of reed canary grass,
spruce, bracken pellets, paper waste and barley dust (Perkin Elmer Clarus 500 GC).
HMF) and subsequently halogenated to CMF. As the latter has a
reduced miscibility in the aqueous layer, it quickly transfers to the
organic phase where it is protected from further reaction. Under
these standard conditions both layers of the biphasic reaction mix-
ture are heated at the same rate and to the same temperature. As
the non-aqueous layer contains, apart from the solvent, only the
desired CMF product, heating of this layer leads to an unnecessary
expenditure of energy as well as a risk for unwanted side-reactions.
Details of a microwave-enhanced variation of this reaction have
been recently published which show that the reaction times are
considerably reduced while retaining high yields and selectivity.
That way CMF with purity greater than 95% is produced in only a
matter of minutes, the key results of which are shown in Table 5
[43]. As evident from these results higher yields were obtained
in the microwave compared to conventional heating for both sol-
vents investigated. This was attributed to the improved heating rate
observed for microwave reactions as discussed above (i.e. the direct
heating of the solvent/substrate layer as opposed to heating of the
reaction vessel first). In the case of DCE as the solvent, the improved
reaction yield was accompanied by a marginally improved selectiv-
ity and in earlier work it was postulated that this was the result of
microwaves heating polar solvents (such as conc. HCl) at a greater
Table 5
Comparison of conventional versus microwave heating for the conversions of D-fructose to CMF in cyclohexane or 1,2-dichloromethane (DCE). Data previously published in
reference [38].
O
HO
O
O
OH
conc. HCl, DCE or cyclohexane,
MW or conventional heating, 70 C,
10 mins
Cl
HO
HO
OH
5
-chloromethyl
D-fructose
furfural (CMF)
Solvent
Heating type
% Yield
% Selectivity
Cyclohexane
Cyclohexane
DCE
Microwave
Conventional
Microwave
Conventional
75
42
85
77
91
98
98
96
DCE
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therewith improving the product selectivity (reduced side reac-
tions of the CMF in the cooler organic layer) and generating CMF at
a faster rate (improved heating rate of the polar organic layer) com-
pared to equivalent conventional heating protocols. Interestingly,
when cyclohexane is used as the solvent, the difference in yield
between conventional and microwave heating (42% versus 75%,
respectively) is far greater than in the case of DCE (77% versus 85%).
The low yield of CMF under conventional heating when cyclohex-
ane was used, was attributed to an excessive formation of a black,
insoluble, carbonaceous humic material which resided in the aque-
ous layer. Initially it was proposed that this carbonaceous humic
material was formed from the degradation of CMF, caused by exces-
sive heating of the organic layer combined with the presence of
concentrated acid. As the dielectric constant of cyclohexane (2.03)
is lower than the one of DCE (10.4), suggesting that DCE would
have the greater interaction with microwaves, the lower yield of
CMF is unlikely linked to unwanted side reactions of the CMF in
the organic layer, as the opposite would have been expected [44].
Alternatively, the difference in yields may have been the result of
a differing solubility of water (or more appropriately HCl) in the
organic layers. However, both solvents have very low solubilities
in water; the molar ratio for water dissolved in DCE and cyclo-
◦
hexane (at 25 C) being, respectively 0.0019 and 0.0024 [45,46].
Fig. 8. Schematic overview of the combined simultaneous microwave-driven steam
distillation and conversion of glucose into HMF from OPW.
Another possible cause relates to the kinetics of CMF transfer from
the acidic aqueous layer to the organic layer, though this would fail
to account for the differences observed between microwave and
conventional heating. Nevertheless, it has been demonstrated that
microwave heating compares favourably to conventional heating
when producing CMF in a biphasic reaction system. Further work on
larger scale production of CMF will undoubtedly further exemplify
this selective heating. This example goes some way to demonstrate
how microwaves can be used in an alternative fashion to those dis-
cussed previously in this article, i.e. the selective heating of layers
in a biphasic system for the production of chemicals from biomass.
Through the application of similar biphasic methodologies further
significant advancements in the valorisation of waste biomass and
conversions of chemicals can be made.
rate than non-polar equivalents (such as DCE). To investigate this
theory further, the heating rates of the two layers (conc. HCl and
DCE) were compared under the same microwave power (50 W), the
profiles for which are shown in Fig. 9. Evidently, the aqueous layer
containing the substrate was found to heat at a faster rate than the
◦
organic layer; the concentrated HCl phase is found to reach 97 C
(
◦
50 W, 10 min), while the DCE phase only reaches 84 C. This agrees
with the known microwave heating theory in that more polar sol-
vents with higher dissipation factors interact to a greater extent
with the microwaves and are therefore heated at a greater rate. This
selective heating allows for a reduced heating of the organic layer,
Fig. 9. Temperature profiles occurring in the biphasic mono/polysaccharide to CMF reaction system.
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4
. Conclusions
Drying, Shijiazhuang Development Zone Daxin Electronic Science And
Technology Co., Ltd, 2013, Alibaba.com: http://www.alibaba.com/product-
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The above case studies show that microwave pyrolysis can be
[
[
[
successfully applied to the pyrolysis of a wide range of lignocellu-
losic biomasses. As this involves moderate temperature conditions
◦
(
100–200 C) real potential exists towards their implementation
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need for a further removal of water and acidic compounds at a
later stage, constitutes a real cost benefit. From a hydrothermal
perspective the case study of waste orange peel illustrates nicely
the possible recovery of a range of products in a minimal amount
of steps without adding further water and acids. Finally the CMF
case illustrates that ‘selective’ microwave heating of a biphasic sys-
tem allows for markedly improved selectivities and higher yields.
In summary, we have highlighted a few examples illustrating and
emphasizing the real and sustainable potential of microwaves for
the valorisation of waste biomass and chemical conversions to
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PS gratefully acknowledges the Ministerio de Ciencia e Innovación
for the concession of a Juan de la Cierva (JCI-2011-10836) contract.
The authors would also like to thank Sundance, a fresh fruit juicer
in New Covent Garden Market, London, for supplying orange peel
and microwave producer SAIREM (www.sairem.com, Lyon, France)
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Please cite this article in press as: V.L. Budarin, et al., The potential of microwave technology for the recovery, synthesis and manufacturing of
chemicals from bio-wastes, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2013.11.058