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dark slurry obtained from this continuous process was filtered, and
the collected liquid stream was referred to as the pilot liquor (PL)
herein. A fraction of PL was treated by following the procedure
mentioned above to obtain the pilot-liquor diluted (PLD) and pilot-
liquor concentrated (PLC) samples. Scheme 3 presents a schematic
diagram for the preparation of the liquor samples used herein.
sion of xylose into furfural. However, the presence of soluble
polymeric lignin compounds (from Miscanthus lignin and alkali
lignin) in the reaction mixture significantly enhanced the para-
sitic side reactions of pentoses that led to the formation of un-
desired condensation products. This valuable information is es-
sential for the further design and optimization of downstream
processes for the valorization of hemicellulose, which can be
integrated into formic acid based pretreatment plants during
fractionation of lignocellulosic biomass.
Dehydration experiments
The dehydration reactions were carried out in ACE glass pressure
tubes (25.4 mm O.D.10.2 cm), comprising polytetrafluoroethylene
(PTFE) plugs and FETFE O-rings, with aliquots (5 mL) of the liquor
solution. After sealing, each tube was placed for a defined period
of time in an oil bath set at a particular reaction temperature (be-
tween 130 and 1708C). When the reaction time was completed,
each tube was removed from the oil bath and immersed in a cold
water bath to quench the reaction. The effective concentration of
acid as hydronium ions ([H+]) was calculated from the dissociation
constant and activity coefficients evaluated at the reaction temper-
ature by using the Debye–Hückel equation and a temperature-de-
pendent function of formic acid dissociation constant proposed by
Kim et al.[21]
Experimental Section
Materials and chemicals
XYL (>99%), d-glucose (>99%), ARA (>99%), formic acid
(>98.0%), LA (>99.0%), HMF (>98.0%), furfural (>98.0%), 1 (>
99.0%), 2 (>98.0%), 3 (>97.0%), 4 (99.0%), and alkali lignin were
of analytical grade and purchased from Sigma–Aldrich. All reacting
solutions and dilutions for HPLC analysis were prepared with de-
ionized water (18.2 MWcm).
Biomass feedstocks (Miscanthus x giganteus, spruce bark, hemp,
and sawdust) were sourced from local biomass suppliers. Prior to
the utilization of biomass, the materials were milled, sieved to
a particle size of <425 mm, and allowed to dry under ambient
room conditions. A lignin solid sample (FP-2) isolated from the hy-
drogen peroxide based fractionation process of Miscanthus de-
scribed elsewhere[14] was also used for the experiments. This lignin
sample consisted of 70.8 wt% Klason lignin, 23.8 wt% acid-soluble
lignin, 0.2 wt% pentoses, 1.1 wt% d-glucose, and 4.1 wt% ash.
A standard procedure was followed to quantify the initial oligo-
meric sugar contents in the liquor solutions.[14] After precipitating
and removing lignin, a certain amount of 96% sulfuric acid was
added to a liquor sample (5 g) to reach an optimized H2SO4 con-
centration between 1.5 and 3.0 wt%. The sample was heated for
1 h at 1208C under autogenous pressure. The content of mono-
meric sugars before and after hydrolysis with H2SO4 was measured
by HPLC, as described below. Standard solutions containing XYL,
ARA, and d-glucose were hydrolyzed under the same conditions to
measure the extent of degradation of each monosaccharide during
the hydrolysis of the oligomers. The amount of oligosaccharides
was quantified by using Equation (1):
Liquor preparation by autothermal H2O2/HCOOH fractiona-
tion of biomass
ꢀ
ꢂ
ꢃꢁ
Five different liquor samples from Miscanthus were prepared
under two different operating modes: batch and continuous. In
batch-operating mode, biomass (300 g) was mixed (9–10 wt% dry
solid loading) with a solution of hydrogen peroxide (4.0–5.0 wt%)
in concentrated formic acid. The reactor system and experimental
procedure for this operating mode have been described in detail
elsewhere.[14] A dark slurry with a cellulosic pulp in suspension was
obtained after the fractionation of biomass materials. The pulp was
filtered to retain particles of >20–25 mm. The filtrate, which con-
tained dissolved lignin and hemicellulose sugars, was collected and
referred to as batch liquor (BL) herein. A fraction of BL was treated
by adding deionized water to precipitate the lignin. The suspen-
sion formed was filtered under vacuum to separate particles of
>1.6 mm. Subsequently, filtered solutions were centrifuged to sep-
arate any coagulated lignin that had not been retained in the filtra-
tion. The BL fraction obtained after lignin removal was referred to
batch-liquor diluted (BLD) herein. A fraction of BLD was evaporated
at 408C under vacuum to increase the sugar concentration in solu-
tion, and this sample was referred to as batch-liquor concentrated
(BLC) herein.
100 À Deg%i
COL;i ¼ Cf;i À Co;i
Â
ð11Þ
100
in which COL,i is the concentration of oligosaccharides as the equiv-
alent monomeric sugars; Co,i and Co,f are the concentrations of the
monomeric sugar measured before and after hydrolysis with
H2SO4, respectively; and Deg%i is the percentage loss of monomer-
ic sugars measured in the standard solutions.
Analytical measurements
The concentrations of XYL, ARA, d-glucose, HMF, LA, and furfural
in the samples were quantified by using HPLC. The measurement
of sugars was conducted in an ICS-3000 system (Dionex Corp.,
Sunnyvale, CA) equipped with an AS50 autosampler (10 mL sample-
loop injection), dual pump, column oven, and integrated electro-
chemical detector for the quantification of carbohydrates. In addi-
tion, a diode array detector was used in series (DAD-3000RS) to
measure furanic compounds and organic acids. An ion-exchange
column (Hi-Plex H 7.7300 mm, 8 mm) was used for the combined
elution of the monosaccharides, furfural, and HMF. The column op-
erated at 658C by isocratic elution with deionized water
(0.65 mLminÀ1) and a postcolumn flow of sodium hydroxide
(300 mm, 0.1 mLminÀ1) for the electrochemical detector. LA was
detected by using Dionex Acclaim Organic Acid guard (5 mm, 4
10 mm) and analytical (5 mm, 4250 mm) columns connected in
Liquor samples prepared in continuous operating mode were pro-
vided by the Technology Centre for Biorefining and Bioenergy
(TCBB; Ireland), which operated a pilot plant (60–180 kghÀ1) for
the fractionation of lignocellulosic biomass in Nenagh, Co. Tipper-
ary, Ireland. Miscanthus, hemp, and sawdust (4 wt% dry solid load-
ing) were fractionated by using a mixture of formic acid (67–
74 wt%) and hydrogen peroxide (3–5 wt%) in a continuous tubular
reactor operating at temperatures between 100 and 1208C. The
ChemSusChem 2016, 9, 492 – 504
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