Application of mild autohydrolysis to facilitate the dissolution of wood chips in direct-dissolution solvents

Article abstract of DOI:10.1039/c6gc00183a

Wood is not fully soluble in current non-derivatising direct-dissolution solvents, contrary to the many reports in the literature quoting wood 'dissolution' in ionic liquids. Herein, we demonstrate that the application of autohydrolysis, as a green and economical wood pre-treatment method, allows for a massive increase in solubility compared to untreated wood. This is demonstrated by the application of two derivitising methods (phosphitylation and acetylation), followed by NMR analysis, in the cellulose-dissolving ionic liquids 1-allyl-3-methylimidazolium chloride ([amim]Cl) and 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]). In addition, the non-derivitising tetrabutylphosphonium acetate ([P₄₄₄₄][OAc]) : DMSO-d6 electrolyte also allowed for dissolution of the autohydrolysed wood samples. By combination of different particle sizes and P-factors (autohydrolysis intensity), it has been clearly demonstrated that the solubility of even wood chips can be drastically increased by application of autohydrolysis. The physiochemical factors affecting wood solubility after autohydrolysis are also discussed.

Full text of DOI:10.1039/c6gc00183a

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PAPER
Application of mild autohydrolysis to facilitate the
dissolution of wood chips in direct-dissolution
solvents†
Cite this: DOI: 10.1039/c6gc00183a
a
a
a
a
Somdatta Deb, Sara R. Labafzadeh, Unna Liimatainen, Arno Parviainen,
b
c
c
a
a
Lauri K. J. Hauru, Shoaib Azhar, Martin Lawoko, Tuomas Kulomaa, Tia Kakko,
Juha Fiskari, Marc Borrega, Herbert Sixta, Ilkka Kilpeläinen* and
Alistair W. T. King*
a
b
b
a
a
Wood is not fully soluble in current non-derivatising direct-dissolution solvents, contrary to the many
reports in the literature quoting wood ‘dissolution’ in ionic liquids. Herein, we demonstrate that the appli-
cation of autohydrolysis, as a green and economical wood pre-treatment method, allows for a massive
increase in solubility compared to untreated wood. This is demonstrated by the application of two derivi-
tising methods (phosphitylation and acetylation), followed by NMR analysis, in the cellulose-dissolving
ionic liquids 1-allyl-3-methylimidazolium chloride ([amim]Cl) and 1,5-diazabicyclo[4.3.0]non-5-enium
acetate ([DBNH][OAc]). In addition, the non-derivitising tetrabutylphosphonium acetate ([P4444][OAc]) :
DMSO-d6 electrolyte also allowed for dissolution of the autohydrolysed wood samples. By combination
of different particle sizes and P-factors (autohydrolysis intensity), it has been clearly demonstrated that
the solubility of even wood chips can be drastically increased by application of autohydrolysis. The
physiochemical factors affecting wood solubility after autohydrolysis are also discussed.
Received 19th January 2016,
Accepted 11th February 2016
DOI: 10.1039/c6gc00183a
www.rsc.org/greenchem
without some kind of mechanical or chemical pre-treatment.
Introduction
It is suggested that this is in part due to the lignin–carbo-
1
0–13
Wood is often reported to be the most abundant renewable hydrate complexes (LCCs).
These are often regarded to be
feedstock for future production of materials, chemicals and covalent linkages between mainly hemicelluloses and lignin
energy. Wood is known to be ‘recalcitrant’ towards chemical and render the LC matrix resistant to dissolution. However,
conversion but the well-documented cellulose dissolution the presence of LCCs, as covalent linkages, in native
1
–4
5–7
capability of basic ionic liquids
and electrolytes
is com- (untreated) wood is disputed.
In any case, in order to maintain the molecular weight
monly used to justify the ability to dissolve wood in similar sol-
vents. Kilpeläinen et al. showed that wood can be dissolved (MW) of the constituent polymers during homogenisation,
8
and chemically modified from chloride ionic liquids, such as methods for selective deconstruction of the wood matrix are
[amim]Cl, but the degree of degradation caused during these required. This would allow for the application of these high
dissolution and modification steps was not apparent. At least MW polymers in a wide range of potential material appli-
a highly acetylated and chloroform-soluble polymeric material cations, i.e. preparation of synthetic regenerated fibres, films,
9
was identified by NMR analysis. However, Kyllönen et al. have thermoplastics, compositing and formation of polymeric
subsequently shown that wood is simply not soluble in non- blends, before or after further chemical modification.
1
4
derivatising direct-dissolution solvents, such as ionic liquids,
Previously Hauru et al. showed that a better solubilisation
of wood and more efficient fractionation was possible after
initial autohydrolysis pre-treatment, followed by ionic liquid
fractionation, compared to the untreated birch wood. This was
demonstrated for particle sizes similar to sawdust-size. Auto-
hydrolysis is the treatment of wood under hydrothermal con-
ditions, by which some components of wood are extracted.
Acetic acid released during the initial hydrolysis of acetate
groups mainly on hemicelluloses, further catalyse hydrolysis
a
Laboratory of Organic Chemistry, Department of Chemistry, University of Helsinki
A.I. Virtasen aukio 1 (Chemicum), P.O. Box 55, 00014 Helsinki, Finland.
E-mail: alistair.king@helsinki.fi, ilkka.kilpeläinen@helsinki.fi; Tel: +358505279446
b
Department of Forest Products Technology, School of Chemical Technology, Aalto
University, Finland
c
Wallenberg Wood Science Center (WWSC), Department of Fiber and Polymer
Technology, Royal Institute of Technology (KTH), Stockholm, Sweden
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
1
5,16
c6gc00183a
and release of the wood components.
There are several
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synonyms or related processes, such as hot water extraction, much more thorough dissolution and chemical modification
pressurised hot water extraction, steam pre-treatment, steam of wood chips, as the most suitable low-cost feedstock for bulk
explosion, etc. All result in extraction of hemicelluloses and industrial application.
some lignin, to different degrees. Autohydrolysis is also
implemented in industry, as a pre-treatment prior to kraft
pulping, to produce dissolving-grade pulp (high purity and
molecular weight cellulose designed for polymeric chemical or
Experimental
textile uses). The resulting pulp is typically called pre-hydro-
lysis kraft (PHK) pulp. As a stand-alone process there are
several advantages to the use of autohydrolysis: (i) the process
milling steps (Fig. 1).
is relatively ‘green’ as water is used, (ii) hemicelluloses may be
Various wood samples of different particle sizes were prepared
by combinations of Wiley milling, sieving and planetary
recovered and converted to a wide range of useful chemicals,
Wood samples: ‘Sawdusts’
such as xylitol, prebiotics, antioxidants, pharmaceutical and
1
7
Silver birch (Betula pendula) chips and autohydrolysed birch
chips were Wiley milled to pass a 22-mesh (1 mm) sieve.
cosmetic ingredients, etc. Most PHK mills however simply
combust the extract for its calorific value. (iii) Solid wood
residue enriched in cellulose and lignin can be further utilised
17
in alternative applications, such as in biocomposites.
Wood samples: planetary milled wood
As the aim of this study is to assess the effect of autohydro-
lysis on the solubility of wood in non-derivatising direct-
dissolution solvents, 3 experimental elements were combined:
Soxhlet-extracted (acetone, 24 h) Norway spruce (Picea abies)
and birch wood chips were Wiley milled to pass a 40-mesh
(
0.5 mm) screen. These samples were then planetary milled
(1) preparation of wood samples of different particle sizes to
using a PM400 Retsch Planetary Ball Mill, to the required
degree (Fig. 1). The following conditions were used: 36 g of
extractive-free wood samples were planetary ball milled at 3000
rpm from 0–24 h.
assess how wider particle size ranges affect solubility, (2) auto-
hydrolysis of wood samples to varying degrees to assess how
the intensity of autohydrolysis affects solubility and (3) correct
choice of analytical procedures to quantify wood solubility.
This last point was satisfied by using ionic liquids to dissolve/
swell the wood samples, prior to NMR analysis. Two of these
Wood samples: larger particle size-ranges
methods also involved chemical derivatisation and one did First, the unextracted wood chips and autohydrolysed wood
not, to demonstrate that dissolution was not an artefact of the chips were Wiley milled without sieving to collect the bulk
chemical modification steps. Therefore, the goals of the effluent (Fig. 1). This was then sized using a sieve stack
present manuscript concern the investigation of autohydroly- (<0.16 mm, 0.16–0.4 mm, 0.4–1.0 mm, 1–3 mm, 3–6 mm and
sis, as a sustainable wood pre-treatment method, to allow for >6 mm, Fig. 2).
Fig. 1 Methods for the production of the particle-sizes used in this study.
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Fig. 2 Large particle sizes produced during Wiley milling and sieving.
Wood samples: autohydrolysed wood chips
then used in the present publication after the various mecha-
nical treatments and sieving steps. The conditions used to
prepare the samples analysed in this study are presented in
Autohydrolysed birch wood was prepared by extracting the
wood chips with hot water at various intensities (by varying
the extraction temperature and isothermal extraction time),
18
Table 1 and are based on Borrega et al.
1
8
according to Borrega et al. The intensity of hot water extrac- 31
P NMR procedure
tions is typically described using the P-factor, in units of
3
1
1
9
Preparation of [amim]Cl and the P NMR analysis were per-
hours. This is a measure of cumulative conversion of hemi-
cellulose (based on hydrolysis kinetics) under the given temp-
erature conditions and over time. It is derived by the time-
dependent integration of the rate constant from a modified
Arrhenius equation, using the activation energy for xylan
9
formed according to Kyllönen et al. (ESI). Briefly, wood
samples were heated at various times and temperatures in dry
[amim]Cl. This allows for significant swelling of the biomass
and complete dissolution in some cases. Hydroxyl groups were
then functionalized as phosphite esters (Fig. 3) and the whole
spectral region (151.5–133 ppm) was then integrated against
−1
18
hydrolysis (125.6 kJ mol (ref. 19)). In Borrega et al., pres-
surised hot water was re-circulated through the chip bed (1 kg
of chips, 3 kg of deionised water) in a batch reactor (10 L).
After the treatment, the water extract was drained and the
wood residue was recovered and stored in a freezer. These were
the internal standard to give a value of ‘available hydroxyls’
1
(
mmol g− ). This is essentially a measure of the solubility of
the wood samples as those hydroxyl groups that are still in the
solid and gel states are not visible using this solution-state
NMR technique. For the >3 mm particle size range the par-
ticles may have required splitting to fit into an 8 ml sample
vial for the derivatisation reaction.
18
Table 1 Parameters for autohydrolysis of the birch chips
Isothermal
reaction time
(min)
Ionic liquid acetylation and NMR analysis
a
Reactor set
temp. (°C)
Reactor heat-
up time (min)
P-factor
(h)
Sample
Acetylation was performed by dissolving the wood samples in
[
(
DBNH][OAc] followed by acetylation with isopropenyl acetate
Fig. 4). [DBNH][OAc] was prepared according to Parviainen
1
2
1
1
2
2
2
2
80 : 0
00 : 0
180
200
180
180
200
220
200
220
0
0
10
30
10
2
18
21
27
47
29
25
49
47
51
167
262
80 : 10
80 : 30
00 : 10
20 : 2
00 : 30
20 : 25
et al. (1 : 1 addition of DBN and acetic acid in the absence of
solvent). Freshly prepared [DBNH][OAc] (1.90 g) was then
added to the wood samples (100 mg) in a screw-top glass vial
and heated, with stirring at 130 °C, for 1 h under argon. The
mixture was allowed to cool to 80 °C and isopropenyl acetate
766
3
1057
1322
3600
8166
30
25
a
(
670 μl) was added. The reaction mixture was then stirred vig-
This is the reaction time at the set temperature, after the reactor heat
up time.
orously with a spatula and after a few minutes the vial was left
3
1
Fig. 3
P NMR analysis procedure for quantifying wood hydroxyls.
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Fig. 4 Mild acetylation procedure applied to the acetylation of autohydrolysed birch samples.
in an oil bath at 80 °C for 30 min. Water (50 ml) was added and fully soluble (Fig. 6), with ∼12 mmol g− representing the total
the mixture was refluxed for 1 h to remove traces of the ionic number of hydroxyl groups in wood. This is consistent with
liquid. The product was filtered and dried under vacuum. All what was published previously for Norway spruce by Kyllönen
1
9
samples had changed from the original dark brown to a light et al. The results show that for both hard and softwood
brown colour. The product was extracted with chloroform species mechanical degradation is required to reach full solu-
(20 ml) with gentle heating and filtered through a G3 glass bility under mild dissolution conditions (90 °C, 3 h) in [amim]
sinter. Chloroform was removed by rotary evaporation. The Cl. The solubility was also investigated as a function of temp-
dried filtrate and the undissolved material were weighed and erature and time (Fig. 7). If the samples are heated at 100 °C or
yields were determined.
below over an extended period there is no increase in the avail-
1
The ATR-IR and H NMR spectra (chloroform-d1) were able hydroxyls as a function of time.
recorded for all samples. DEPT-edited HSQC were also
recorded for some samples.
However, if the pre-dissolution temperature is increased
from 90 °C to 140 °C, for a dissolution time of 3 h the solubi-
lity increases. If the dissolution time at 120 °C is increased
from 1–24 h the solubility also increases. As this increase in
Ionic liquid electrolyte dissolution and NMR analysis
1
H NMR analysis of dissolved wood was possible, under non- solubility is not observed at <100 °C this is likely due to degra-
derivatising conditions, through the use of [P4444][OAc] (Fig. 5) dation occurring at the elevated temperature.
as the DMSO-d6 electrolyte. [P4444][OAc] : DMSO-d6 stock solu-
This is quite consistent with early reports of the depolymeri-
2
0,21
tion (200 μl, 50 : 50 wt%) was added to the wood samples sation of cellulose or lignin in chloride ionic liquids
and
(
33.8 mg) in an 8 ml sealable vial. Further DMSO-d6 (400 μl) confirms the need for lower temperatures (100 °C or lower) to
was added and the samples were heated, with stirring, for 3 h preserve polymer molecular weights. This is particularly rele-
at 100 °C. The solutions (600 μl) were transferred to a 5 mm vant for analytical procedures using [amim]Cl and other chlor-
1
NMR tube while hot, via a needle and syringe. H NMR spectra ide anion analogues. Other analogues, in particular containing
(
at 65 °C) were recorded for all solutions. [P4444][OAc] was pre- the acetate anion, are also shown to be reactive towards cellu-
7
pared according to Holding et al. (ESI).
lose. This topic has been thoroughly discussed in previous
2
2,23
reports.
Results and discussion
Solubility of unextracted wood (low particle size-range)
Both spruce and birch were planetary milled from the sawdust
31
and subjected to the P NMR analysis conditions. The results
show that only after extended milling times are the samples
Fig. 6 Increase in solubility of spruce and birch after extended plane-
31
tary milling, as determined by [amim]Cl dissolution and P NMR analy-
9
Fig. 5 Structure of [P4444][OAc] used for NMR analysis of wood
sis. Solubility is represented by the ‘available hydroxyls’ corresponding
samples as the DMSO-d6 electrolyte under non-derivatising conditions.
to the number of measurable phosphite esters.
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Fig. 7 Solubility of birch wood sawdust after heating in [amim]Cl at different temperatures over 3 h heating time (left) and at 120 °C for extended
heating times (right).
Solubility of autohydrolysed birch (sawdust)
To measure the effect of autohydrolysis intensity on wood solu-
bility, autohydrolysis was then applied to birch chips according
1
8
to Borrega et al. (Table 1). The chips were then Wiley milled
to pass a 1 mm sieve (sawdust size) and were then subjected to
3
1
P NMR analysis (Fig. 8). For comparison, the theoretical OH
content was also calculated based on the chemical compo-
1
8
sition of the different biomass samples, before and after
autohydrolysis (cellulose content, xylan content, lignin content
and acetyl content). The values were determined using eqn (1)
and (2):
OH
¼ OHðCellÞ þ OHðXylÞ þ OHðLigÞ ꢀ OHðAcÞ
ð1Þ
ðThÞ
Fig. 8 Solubility (available OH) of autohydrolysed sawdust samples,
with increasing P-factor, compared to the theoretical available OH
values from sugar, lignin and acetyl analyses.
XOH ꢁ %
18
ðXÞ
OH
¼
ð2Þ
ðXÞ
1
00
where OH(X) (mmol g− ) are the different theoretical
1
hydroxyl contributions to eqn (1), %(X) are the different mass
the percentage by mass of lignin in the samples (based on
Klason and acid-soluble lignin), %(Ac) is the percentage by
mass of acetyl species in the biomass samples. All values are
given in the ESI.†
The results in Fig. 8 clearly show that application of a
P-factor close to 200 h allows for practically complete solubili-
sation of the sawdust samples, as opposed to the almost com-
pletely insoluble samples for the 0 h P-factor (untreated
samples). P-factors of around 800–1000 h are typically those
which are applied during pre-hydrolysis for PHK pulp pro-
duction. Clearly mild autohydrolysis is a good economic sub-
stitute for costly planetary milling when one wishes to achieve
a more complete chemical modification of wood.
percentages of those species in the wood samples and XOH
1
(
mmol g− ) are the available hydroxyls for the different
species: OH(Th) is the theoretical hydroxyl content of the whole
biomass, OH(Cell) is the theoretical cellulose hydroxyl content,
OH(Xyl) is the theoretical xylan hydroxyl content without acetyl
functionalities and OH(Ac) is the theoretical hydroxyl content
consumed by residual acetate groups. In the second equation,
−
1
CellOH (18.5 mmol g ) is the available hydroxyls per unit
mass of cellulose (3/162). XylOH is the available hydroxyls per
−
1
unit mass of xylan (14.2 mmol g ) and was determined by
2
4
King et al. based on data published by Timell concerning the
2
5
sugar composition of xylan. LigOH is the available hydroxyls
per unit mass of lignin (ESI) and are mostly predicted values
3
1
based on P NMR analysis of milled wood lignin, after the
2
6
Dissolution vs. ‘reactive dissolution’
autohydrolysis of birch wood at increasing P-factors. Only
3
1
lower P-factors were analysed but LigOH values for higher The reagent used in the P NMR phosphitylation procedure is
P-factors are estimated based on the fact that higher intensity rather reactive and some degree of ‘reactive dissolution’ may
treatments cause significant condensation of lignin. AcOH is be expected. Reactive dissolution has been observed previously
the unavailable hydroxyls per unit mass of acetyl (23.3 mmol for cellulose modification where cellulose can be dissolved, in
−
1
g
) incorporated into the biomass, as acetate esters. %(Cell) the absence of direct-dissolution solvents, particularly using
2
7,28
is the percentage by mass of cellulose in the samples, %(Xyl) acid chlorides as acylation reagents.
However, in this
is the percentage by mass of xylan in the samples, %(Lig) is context we wish also to avoid other possible degradation
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3
1
for the untreated wood sample. As with the P NMR analysis
this indicates a very significant increase in the solubility of
wood after an autohydrolysis P-factor of around 200 h. All
samples after extraction with chloroform and drying in a rotary
evaporator are film-forming (ESI†). For direct comparison of
3
1
the [amim]Cl- P NMR results with the [DBNH][OAc]-acetyl-
3
1
ation results, the P NMR results can be plotted against the
acetylation results (ESI†). While some error exists, clearly there
is a strong correlation between the two analytical procedures,
indicating that reactive dissolution during the phosphitylation
procedure does not have a significant effect when dissolving
the minimally treated (autohydrolysed) samples. However, the
yields from acetylation are not close to 100%, as in the case of
Fig. 9 Yields after [DBNH][OAc] dissolution, acetylation (Fig. 5) and
solubilisation of autohydrolysed sawdust samples into chloroform.
3
1
the P NMR procedure indicating that there is some effect.
Importantly the increase in solubility on going from untreated
effects produced by the high reactivity or the phosphitylation up to a P-factor of around 200 h is significant using both pro-
reagent. Therefore, two additional mild procedures were cedures. NMR analysis was performed on the acetylated
1
tested: (1) acetylation in [DBNH][OAc] with isopropenyl acetate, materials (Fig. 10). This included H NMR on all acetylated
and (2) direct-dissolution into [P4444][OAc] : DMSO-d6. NMR samples and DEPT-edited HSQC on the untreated sample (0 h
analysis was performed on the acetylated materials after extrac- P-factor) and the sample autohydrolysed to a P-factor of 766 h.
tion with chloroform and also on the biomass dissolved in the From Fig. 10 we can see that as the P-factor increases the cellu-
1
[
P4444][OAc] : DMSO-d6 electrolyte solutions. [DBNH][OAc] has lose in the samples ( H: C3 – 5.07 ppm, C2 – 4.80 ppm, C1 –
been used previously for the acetylation of hemicelluloses, 4.42 ppm, gem-C6 – 4.38 & 4.06 ppm, C4 – 3.71 ppm, C5 –
2
9
with acetic anhydride as the reagent. An even milder pro- 3.54 ppm) clearly becomes soluble. A relatively significant
1
13
cedure has however been developed using the lower reactivity peak which may correspond to the lignin methoxy ( H/ C:
3
0
acetate ester, isopropenyl acetate (Fig. 9). Acetylation using 3.76/56.25 ppm) is present. Some unidentified peaks upfield
1
these reagents is expected to avoid any issues with reactive dis- of the cellulose backbone ( H: 3.5 ppm) are also present. At
solution. The sawdust samples (5 wt%) were first dissolved this point in the 766 h P-factor sample these are thought to be
over 1 h at 130 °C to pre-swell the biomass. Some degradation lignin resonances which are possibly methylene species (CH2),
is expected under these conditions but is not expected to be alpha to both ketone and oxygen. This is however speculative
significant due to the shorter heating times and degradation without much more detailed analyses. For the untreated
seems to occur to a much lesser extent in [DBNH][OAc], com- sample (0 h P-factor) there are more unknown resonances that
pared to chloride ionic liquids. However, no accurate study has may be related to extractives, saccharides and lignin. While
been done on this to date. Isopropenyl acetate was then intro- there are peaks that roughly correspond to acetylated cellulose,
duced and the reaction quenched. The filtered and dried pro- these are rather weak, relative to the unknown resonances,
ducts were then extracted with chloroform to give an indicating minimal solubilisation of cellulose during the
additional measure of the solubility of the samples. The yields acetylation and chloroform extraction. There are also several
after acetylation and chloroform extraction are given in Fig. 9. unidentified saccharide resonances, consistent with the
Here we can clearly see that there is a dramatic increase in the solubilisation of mainly the lower molecular weight fractions
chloroform solubility of the acetylated autohydrolysed and not the cellulose.
samples, compared to the untreated samples. The weight
percent gains (WPGs) compared to the theoretical WPGs are
Dissolution under ‘non-derivatising’ conditions
all above 75%, even for the untreated sample. This nicely cor- As a further demonstration of how reactive dissolution does
responds with the ATR-IR analysis (ESI†) showing high degrees not effect solubility at lower P-factors, the samples were dis-
of acetylation for all samples. At intermediate P-factors the solved in the ionic liquid electrolyte [P4444][OAc] : DMSO-d6.
WPGs are almost 100% of the theoretical WPGs indicating Similar electrolytes have been used for the analysis of high
more thorough acetylation. At higher P-factors (>3000 h) the molecular weight pulp samples, containing mainly cellu-
7
,31
yields slightly decrease again indicating that further degra- lose.
All components of wood are soluble in these electro-
7
dation of the samples may reduce yields, after aqueous precipi- lytes, as demonstrated previously by Holding et al. Therefore,
tation. It is commonly known that low molecular weight if it is truly possible to solubilize autohydrolysed wood, this
samples may not fully precipitate upon the addition of a non- medium should allow for this. 5 wt% samples were dissolved
1
solvent. However, as the samples were only acetylated on a into the electrolyte solutions and H NMR analysis was per-
1
00 mg scale, there may also be error due to unavoidable formed (Fig. 11). Clearly for the extended P-factors the cellu-
losses during filtration and transfer of samples. Above a lose in the samples is dissolving, indicating a much more
P-factor of around 260 h the chloroform-soluble portion of the complete solubilisation of the samples. For the 0 and 51 h
recovered acetylated material reaches 93%, compared to 14% P-factors little cellulose is visible indicating poor solubility of
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Fig. 11 H NMR saccharide backbone region for the different P-factors
of autohydrolysed birch wood dissolved in [P4444][OAc] : DMSO-d6. The
concentrations are the same for all spectra and normalised against the
[P
4444][OAc] signals, i.e. solubility is represented by cellulose backbone
signal intensity. The assignments are for the different cellulose back-
1
7,31
bone H signals, according to Holding et al.
Fig. 12 Solubility (available OH) of increasing particle size ranges, up to
chip-size, after autohydrolysis of the birch wood chips at a P-factor of
1057 h. 24 h PM refers to the experimental value for the untreated birch
sample which was fully soluble, after 24 h of planetary milling.
1
Fig. 10 H NMR saccharide backbone region for the [DBNH][OAc]-
acetylated CDCl
hydrolysed birch wood (middle). The 2D spectra are DEPT-edited (CH &
CH in red, CH in blue) HSQC spectra (above and below). Assignments
3
-soluble fractions for the different P-factors of auto-
3
2
in the 766 h P-factor HSQC are for the cellulose backbone peaks.
were then subjected to [amim]Cl dissolution, phosphite ester
31
derivatisation and the P NMR analysis procedure (Fig. 12).
From the results presented in Fig. 12 it is clear that decreasing
particle size slightly improves the solubility of the untreated
chips. However, after autohydrolysis to a P-factor of 1057 h, all
particle size ranges are equally soluble. Some error exists but
the results clearly show that autohydrolysis can be used to dras-
tically enhance the solubility of wood chips over the untreated
chips, with the untreated chips being practically insoluble.
the whole biomass sample. This is entirely consistent with the
P and acetylation results.
3
1
Solubility of autohydrolysed birch (chips)
From a techno-economic standpoint, sawdust is not a suitable
feedstock to use for bulk application. While it is readily avail-
able as a low-cost by-product from saw mills, it is only avail-
able on a limited scale industrially. Industrial chips are much
more common and at a much lower cost compared to the
Mechanism causing increased solubility
9
additional milling steps required to reach lower particle sizes, In Kyllönen et al. it was argued that the insolubility of wood
such as sawdust. As such, we wished to investigate larger was due to the recalcitrant ‘lignin–carbohydrate matrix (LCM)’.
particle-size ranges to see if they could also be solubilised after LCCs are of course implicated in this but it was not specifically
autohydrolysis. To achieve this, the starting untreated and argued that these were actual branching covalent linkages,
autohydrolysed wood chips (1057 h P-factor) were Wiley milled between polysaccharide and lignin. Extensive covalent LCCs
and sieved to give different particle size ranges (Fig. 2). These would bind the wood matrix components to rapidly approach
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Green Chemistry
a molecular weight equivalent of Avogadro’s number, for that 3-methylimidazolium chloride ([bmim]Cl) was shown to be an
molar quantity of material (much too high MW to be soluble). alternative structure that did not form this adduct, presumably
2
2
As mentioned in the previous article the scepticism about the due to the reduced basicity of the anion. Similar reports can
occurrence of covalent LCCs in native (unprocessed) woods is also be found for reaction with lignin.
mainly due to the inability to isolate LCC fractions for high
Potential applications and sustainability
resolution analyses, without the need for chemically or
mechanically harsh treatments. This is the Schrödinger’s cat
of wood chemistry. As a consequence, we would like to present
several options for discussion concerning the solubility of
autohydrolysed wood:
Autohydrolysis in itself is a pre-treatment method performed
nd
in water on a 2 generation bio-based feedstock. It is both
green and sustainable as it is already implemented for the pro-
duction of PHK. It yields an extractive fraction which can be
combusted for its calorific value (steam or electricity gene-
ration) or used for chemicals production. Under mild autohy-
drolysis conditions this fraction typically contains acetic acid
and hemicelluloses, both valuable chemicals. In terms of
chemistry, the hemicellulose (xylan) fraction may be chemi-
cally modified to polymeric chemicals or it is also already
industrially converted to xylose (for xylitol production) and fur-
fural. Polymeric hemicelluloses also already have wide appli-
cation in the food industry. Regarding the sustainability of the
subsequent direct-dissolution step on the residual chips, the
most obvious applications are in further chemical modifi-
cation of the wood matrix or regeneration to films and fibres,
e.g. a lyocell using the ionic liquid as the direct-dissolution
(1) Increased porosity – it has been known for some time
that during autohydrolysis of wood, hemicelluloses and lignin
3
2
are extracted. Smiljanski and Stanković have shown that there
is an increase in the sub-microscopic porosity of beech wood
(Fagus moesica) as glucuronoxylans are extracted during pre-
hydrolysis. This increase in porosity would clearly allow for
much more rapid penetration of the wood matrix with reagents.
(2) Increased fibrillation – after autohydrolysis the wood
chips become much softer and easier to break apart, parallel
with the fibre orientation. In addition, the chips fibrillate
quite rapidly in ionic liquid ([amim]Cl and [DBNH][OAc]) as
opposed to unextracted chips, which essentially do not fibril-
late at all. This effect clearly also rapidly increases surface
area, similar to increased porosity. This is mainly a result of
the removal of acid-soluble lignin and pectins from the middle
lamella, which acts as a cement keeping the adjacent fibres
together. It is not clear if this, combined with removal of hemi-
cellulose (binder), is enough to allow for complete dissolution
of the remaining wood matrix.
3
5,36
solvent.
Provided low toxicity ionic liquids and reagents
are used, the main stumbling block to sustainability is the
removal of water from ionic liquids during the recycling stage,
as typically water is used to regenerate the biomass. Therefore,
the value of the product must be high enough to offset this
cost or advances made that either minimise the use or energy
consumption of water use.
(3) LCC breakage – autohydrolysis, by definition, is an acid-
catalysed process for the degradation and extraction of hemi-
celluloses. Therefore, acid-labile linkages are expected to be
degraded during this process. The proposed LCC structures
Conclusions
1
0–13
(benzyl ethers, benzyl esters and phenyl glycosides)
will
have variable stabilities under pH conditions. If they are actu- In this paper, we demonstrate that mild autohydrolysis can be
ally present in native wood then particular types are expected applied to allow for almost complete solubilisation of wood
to be cleaved, potentially diminishing the overall molecular chips in ionic liquid. Without the autohydrolysis pre-treatment
weight of the LCM low enough to be rendered into solution.
both wood chips and particle sizes much lower than sawdust
(
4) Lignin condensation – at higher P-factors lignin starts to (<0.16 mm) are mostly insoluble, until you get down to the
26
condense. This may have the opposite effect of LCC breakage, very small particle sizes that extended planetary milling
i.e. leading to reduced solubility of the matrix, if the condensed affords. As autohydrolysis is an existing green and economical
fragments become large enough in size. Therefore, keeping to pre-treatment method, typically applied prior to kraft pulping,
low P-factors will likely be beneficial for many reasons.
this could now open the door to much more thorough chemi-
Aside from the structural changes in wood, ionic liquids cal modification of wood chips, as a low cost feedstock. This
themselves are often the culprits when it comes to enhanced could most obviously lead to new applications in thermoplas-
‘solubility’ of wood in seemingly non-derivatising ionic tics, films, polymer blends, composite materials etc. It could
liquids. There is now quite much evidence demonstrating that also lead to novel dissolution/regeneration applications, such
certain ionic liquids react with wood functionalities or catalyse as a wood-based lyocell process where high tensile strength
reactions. This reactivity often goes hand-in-hand with the wood fibres are regenerated. The factors affecting the solubility
cellulose dissolution capability as both depend on anion basi- of autohydrolysed chips over unextracted chips are also dis-
2
2
city. A good example of this is the conjugation of 1-ethyl- cussed, with no significant conclusions other than to further
3
-methylimidazolium acetate ([emim][OAc]) with reducing clarify the different potential physiochemical factors involved.
3
3,34
ends of cellulose.
This has recently been developed by Understanding all of these processes is required to make pre-
2
3
Clough et al. where it has been shown that [emim][OAc] dictions about wood recalcitrance. However, clearly there will
allows for conversion of the carbon in the glucopyranose ring be interesting opportunities in the future to start dissecting
into an imidazolium C1–formaldehyde adduct. 1-Butyl- the different contributions to the recalcitrance of wood. New
Green Chem.
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economical pre-treatments, e.g. mild acid/base, ammonolysis, 16 G. Garrote, H. Domiguez and J. C. Parajo, J. Chem. Technol.
electron beam, redox processes, etc., may also prove to be more Biotechnol., 1999, 74, 1101–1109.
effective or economical than autohydrolysis and offer different 17 P. Gullon, A. Romani, C. Vila, G. Garrote and J. C. Parajo,
application areas.
Biofuels, Bioprod. Biorefin., 2012, 6, 219–232.
1
1
8 M. Borrega, L. K. Tolonen, F. Bardot, L. Testova and
H. Sixta, Bioresour. Technol., 2013, 135, 665–671.
9 H. Sixta, A. Potthast and A. W. Krotschek, in Chemical
Pulping Processes: Handbook of Pulp, ed. H. Sixta, Wiley-
VCH, Weinheim, 2006, vol. 1, pp. 343–345.
Acknowledgements
We would like to acknowledge FIBIC (The Finnish Bioeconomy
Cluster) Oy, now known as CLIC Innovation, for funding under
the FuBio (Future Biorefinery) program. We would also like to
thank the WWSC (Wallenberg Wood Science Center) for
additional financial support.
2
2
2
0 G. Bentivoglio, T. Röder, M. Fasching, M. Buchberger,
H. Schottenberger and H. Sixta, Lenzinger Ber., 2006, 86,
154–161.
1 G. Laus, G. Bentivoglio, H. Schottenberger, V. Kahlenberg,
H. Kopacka, T. Röder and H. Sixta, Lenzinger Ber., 2005, 84,
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Green Chem.