One-pot integrated processing of biopolymers to furfurals in molten salt hydrate: Understanding synergy in acidity

Article abstract of DOI:10.1039/c7gc01709j

The manufacture of furfural and 5-hydroxymethylfurfural (HMF), promising renewable platforms for bio-products, with high carbon efficiency is a challenge because of their undesired polymerization to humins. Here, we present an integrated process in inorganic salt solution to overcome this challenge. First, sugar biopolymers yield high monosaccharides (>90%) via efficient saccharification at low temperature. Second, monosaccharides in hydrolysates undergo efficient dehydration in biphasic solvents, assisted by the Br?nsted acidity of the salt solution and the added inorganic Lewis acid. The salt solution exhibits high carbon efficiency in furfural and 5-hydroxymethylfurfural (HMF) products (>80 mol%) when compared with the analogous non-salt systems due to the cooperativity of the dual acidity and the higher viscosity of the salt solution than water and the concurrent reactive extraction of products. The process integration makes the separation of the aqueous phase easy, demonstrating nearly comparative activity upon recycling. Intact lignocellulose also achieves high yields of the dehydration products via one-pot depolymerization and saccharification, followed by dehydration. In addition, we elucidate the reasons for the high saccharification efficiency of the salt solution by correlating the effective acidity from the electrode potential and Hammett acidity measurements.

Full text of DOI:10.1039/c7gc01709j

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One-pot Integrated Processing of Biopolymers to Furfurals in Molten Salt Hydrate:
Understanding Synergy in Acidity
Sunitha Sadula, Owen Oesterling, Andrew Nardone, Brian Dinkelacker, Basudeb Saha*
Catalysis Center for Energy Innovation and Department of Chemical and Biomolecular
Engineering, University of Delaware, Newark, DE 19716, USA
Corresponding author’s email: bsaha@udel.edu
Abstract
Furfural and 5ꢀhydroxymethylfurfual (HMF), promising renewable platforms for bioꢀproducts,
manufacturing with high carbon efficiency is a challenge because of their undesired
polymerization to humins. Here, we present an integrated process in inorganic salt solution to
overcome this challenge. First, sugar biopolymers yield high monosaccharides (>90%) via
efficient saccharification at low temperature. Second, monosaccharides in hydrolysates undergo
efficient dehydration in the presence of an organic solvent, assisted by the Brønsted acidity of the
salt solution and added inorganic Lewis acid. The salt solution exhibits high carbon efficiency in
furfural and 5ꢀhydroxymethylfurfural (HMF) products (>80 mol%) when compared with
analogous nonꢀsalt systems due to cooperativity of the dual acidity and higher viscosity of the
salt solution than water and concurrent reactive extraction of products. The process integration
makes easy separation of the aqueous phase, demonstrating nearly comparative activity upon
recycling. Intact lignocellulose also achieves high yields of the dehydration products via oneꢀpot
depolymerization and saccharification, followed by dehydration. In addition, we elucidate the
reasons for high saccharification efficiency of the salt solution by correlating the effective acidity
from electrode potential and Hammett acidity measurements.
Keywords: Saccharification, molten salt hydrates, dehydration, Hammett acidity, electrode
potential, lignocellulose.
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Introduction
Nonꢀfood lignocellulosic biomass is a promising renewable carbon source for bioenergy,
biomaterials and sustainable products. This bioꢀrenewable carbon feed can overcome challenges
associated with possible petroleum shortage, price volatility caused by demandꢀsupply
imbalance, global warming by minimizing carbon emissions, and also offer carbon credits to
produced products.1 Xylan, a main component of hemicellulose with roughly one fifth of carbon
by dry weight of biomass, is composed of C5 sugar, xylose. Xylose upgrading to advanced
biofuels and chemicals such as furfural, xylitol, xylaric acid, monohydroxymethylfuran and other
products is of great interest in recent years. One example is furfural, which has a large market.
Current annual market demand of furfural is about 550 million lbs, which is expected to reach
1100 million lbs by 2020 because of its high demand for furan resin, furoic acid, methylfuran,
tetrahydrofuran, methyltetrahydrofuran, jet and diesel fuels, lubricate oils, adhesives and
medicines.2,3, 4 Insufficient supply of furfural currently limits commercialization of downstream
technologies using furfural as a feed stream.
Furfural syntheses from xylose, hemicellulose and woody biomass using high pressure of
CO2,5, 6 acidic ionic liquids7ꢀ9 and microporous silicoaluminophosphates10 have been reported in
biphasic and monophasic solvents. Furfural is commercially synthesized from agricultural
residues, e.g., corncob, sugarcane bagasse, rice husk and wheat bran,11 in a twoꢀstep process
which involves mineral acid depolymerization of biomass xylans to xylose followed by its
dehydration.12ꢀ14 China is the largest producer of furfural using Westproꢀmodified Huaxia
Technology which operates at considerably high temperature (170 °Cꢀ 185 °C) in the presence of
sulphuric acid. The yield is moderate (50%) after 3 h of reaction time.2 The acidic processes have
several disadvantages including low carbon efficiency due to formation of humins byꢀproduct,
high energy consumption, equipment corrosion and handling of acidic waste. Kinetic studies of
acid assisted saccharification and dehydration suggested faster degradation of xylose occurs in
sulfuric acid.15ꢀ17 Improvement of carbon efficiency is necessary for better process economics
and environmental sustainability.
An alternative approach is biomass xylan saccharification in an inorganic molten salt
hydrate (MSH) followed by Lewis and Brønsted acids assisted conversion of xylose to improve
carbon efficiency in furfural. Recently, MSHs have emerged as promising media to breakdown
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biopolymers to sugars.18 MSH medium, a highly concentrated solution of inorganic salt (Csalt
≥
50%), has water to salt molar ratio closer to the coordination number of the cations of the salt. At
this condition, the cations of the salts are shielded from anions by one hydration sphere and ionꢀ
water interactions predominantly occurs in the hydration sphere.19 As a result, the hydrated
cations can polarize water molecules making their protons acidic. The anions of the salt enhances
the acidity by deshielding the protons and increasing their tendency to leave water.20, 21 Figure 1
illustrates the interaction patterns in the hydration sphere of the salt solutions with changes of the
salt concentrations.
Figure 1. Illustration of the interaction patterns in the hydration sphere with changes of the salt
concentrations.
MSHs are (1) easy to prepare, (2) environmentally friendly due to their high boiling point
and low vapor pressure and (3) less expensive than common ionic liquids. Because of these
advantages, MSHs media have been used for biopolymer, e.g. crystalline cellulose
saccharification.22, 23 The efficiency of the media depends on the nature of the salts used. The
o
reported methods at high temperatures (>120 C) or high acid concentrations (>3 wt%) caused
degradation of sugars and carbon loss in the form of humins.24,25
Our center has recently reported18 that LiBrꢀbased MSH system is effective for
saccharification of crystalline cellulose to soluble sugars (>90% yield) with high yield of
monosaccharide (82%) at low temperature (85 °C) and shorter time (30 min). This reaction
conditions minimized undesired reaction pathway for humins formation, representing high
carbon efficiency in produced products. The results demonstrate that there could be a synergy
between the MSH media and added acid additive. However, the extent of the synergy remains
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elusive because of lack of accurate measurement of the acidity of the solutions. Efforts to
compare the pH of the solution by a pH meter, measuring negative pH values which is outside
the range of a pH electrode, with electrode potential (EP) values of the solution by a
potentiometer was not conclusive.18 In addition, the separation of highly soluble sugars from the
homogenous MSH solution remains a critical challenge despite some development in prior
studies reporting salt extraction26 and sugars separation via complexationꢀdecomplexation,27
antisolvents,28 membranes29 and ion exclusion chromatography.30 The reported separation
methods are either slow or expensive and are impractical for large scale separation.
Building on our prior study of selective conversion of crystalline cellulose, herein LiBrꢀ
MSH is used for saccharification of xylan. Kinetic studies are performed as a function of acid
additive (H2SO4), LiBr and substrate concentrations and temperature. Synergy in acidity
between salt and acid additive is examined by measuring effective hydrogenꢀion activity of the
solution from the Hammett acidity function. In addition, the challenge associated with xylose
separation is addressed by integrating the saccharification step with a dehydration step for
concurrent separation of product via reactive extraction in a biphasic solvent. Further the benefit
of the integrated process has been extended for the production of 5ꢀhydroxymethylfurfural
(HMF) and furfural from crystalline cellulose and intact lignocellulose.
Experimental Section
Material
Lithium bromide, 5 M sulfuric acid, aluminum chloride hexahydrate, ethyl acetate, mꢀcresol,
oxalic acid, ethanol, 2ꢀmethyl tetrahydrofuran (MeTHF), nitroanilines and HPLC standards
(xylose, glucose, acetic acid, formic acid, levulinic acid, 5ꢀhydroxymethylfurfural, furfural) were
purchased from SigmaꢀAldrich. Glucoꢀ and xyloꢀoligosaccharides with degree of polymerization
(DP) of 2ꢀ6 with >95% purity were purchased from Megazymes. All chemicals were used as
received. Unless otherwise mentioned, deionized water (Millipore model DirectꢀQ3 UV R) was
used for preparation of all solutions. Syringe filter discs (Nylon, 0.2 ꢁm) for filtration of
solutions for HPLC analysis were purchased from Fisher Scientific. Beech wood xylan
containing 6.5% moisture was purchased from TCI America.
Methods
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Saccharification. Saccharification of xylan was conducted in high pressure glass vials. The vial
was loaded with calculated amount of LiBr and 0.05 M aqueous sulfuric acid. Upon dissolving
LiBr, calculated amount of substrate (crystalline cellulose, xylan or biomass) was added into the
vial and the mixture was vortexed for about 30 sec. The vial was sealed with an Alꢀcrimp cap
and placed in a preheated heating block to start the reaction at 85 °C (or other desired
temperature) with continuous stirring using a magnetic bar. Upon completion of reaction for the
set time, the vial was removed from the heating block and quenched in an ice bath. The
hydrolysate was diluted in DI water by 10 times and filtered for HPLC analysis.
Xylose hydrolysate dehydration. C5 sugar in the hydrolysate, obtained from xylan, was
dehydrated to furfural in a biphasic solvent. Typically, 1 mL hydrolysate was mixed with
calculated amount of a Lewis acid (AlCl3) and an organic extracting solvent (ethyl acetate, EA)
in a high pressure glass vial. The ratios of the hydrolysate (reactive phase) to organic solvent
were varied in the range of 1 – 3 (v/v). Upon addition of a small magnetic bar into the vial for
stirring, the vial was sealed with an Alꢀcrimp cap and placed in a preheated stainless steel
heating block preset at desired temperature. After completion of the reaction for the set time, the
vial was removed from the heating block and quenched in an ice bath. The solution was allowed
to settle to ensure separation of the two phases. The organic phase containing furfural was
decanted in a collection flask and the aqueous phase containing MSH was washed two times
with EA (2 × 5 mL) to extract remaining HMF or furfural or both. Both EA and MSH phases
were filtered for analysis by HPLC. The MSH phase was diluted 10 times with DI water before
analysis.
Recycling MSH. Recyclability of the MSH was tested for the saccharification of pure cellulose
as a model system. First, crystalline cellulose was saccharified in MSH at LiBr to water molar
ratio of 3.25 (1.46 g LiBr, 1 mL 0.05 M acid, 39.5 mg cellulose, total volume 1.5 mL). After
saccharification of cellulose at 85 °C for 30 min, the hydrolysate containing soluble glucose was
subjected to dehydrate in MSH/EA (1:3 (v/v)) biphasic system at 120 °C for 1 h using AlCl3 (50
mol% with respect to the molar amount of glucose, 32.7 mg glucose in 1.5 mL, 22 mg
AlCl3.6H2O). After decantation of the organic phase containing HMF and washing the aqueous
phase twice with EA, the recovered solution containing MSH and Alꢀspecies was filtered
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through a syringe filter and reused in the 2nd cycle. The yields of glucose and HMF after the
hydrolysis and the dehydration steps were quantified by HPLC. The unconverted amount of
glucose that remained in the hydrolysate after the 1st cycle of dehydration was accounted during
addition of cellulose in the 2nd cycle. EA was added to adjust the ratio of EA to MSH phase to 3
(v/v) and the mixture was heated at 120 °C for 1 h. Cellulose saccharification and glucose
dehydration takes place in tandem in the 2nd cycle as the recovered aqueous phase from the 1st
cycle contains both MSH and AlCl3. Upon completion of the 2nd cycle, the organic and aqueous
phases were worked out by following the same procedure discussed above and the reactive phase
was reused.
Selection of organic solvents for furfural extraction. Partition coefficient (R) values of
different organic solvents was determined for selection of the best organic solvent for extraction
of furfurals from the MSH phase. These experiments using HMF and furfural were performed in
a jacketed glass reactor equipped with a thermostated bath to set desired temperature of the
reaction mass. MSH solution was prepared by mixing 8.22 g LiBr in 10 mL H2O (water to LiBr
molar ratio is 3.25). HMF or furfural (5 wt% ) was added to the MSH solution and dissolved by
stirring the solution. To this, calculated amount of an organic solvent was added to adjust the
ratios of organic and aqueous phase of 1 and 3 (v/v) in order to evaluate the effect of volume of
the extracting solvent on HMF and furfural partitioning. The mixture was stirred at 60 °C or 120
°C for 1 h and settled overnight to ensure complete phase separation. HMF or furfural
concentrations in both phases were quantified by HPLC upon dilution. All experiments were
conducted in triplicate.
Hammett acidity function measurement. First, base solutions of concentration 15 mg/L were
prepared by dissolving calculated amounts of different base indicators (nitro and chloroꢀnitro
derivatives of aniline) in DI water and the absorbance of these solutions were measured on an
Agilent UVꢀVis spectrophotometer (Cary 60) using a 1 cm path length quartz cuvette. Then
different amounts of preꢀcalculated LiBr were added to the base solutions to adjust desired salt
concentrations and the mixtures were stirred to dissolve salt completely. Each solution was
filtered using a syringe filter, stirred for 10 min upon adding a calculated amount of acid additive
and transferred into the cell cuvette for UVꢀVis spectrophotometric analysis. The measured
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absorbance was used to calculate the Hammett acidity function (H0) using the following
equation, where BH+ is the conjugate acid of a base B, pKBH+ is –log (KBH+) for dissociation of
H0 =pKBH+ + log([B]/[BH+])
BH+. For 2,6ꢀdinitroaniline base, a molar extinction coefficient (ɛ0) value of 9670 l molꢀ1 cmꢀ131
and a pKBH+ value of ꢀ5.5431 were used for calculation of unprotonated base concentration and
H0.
pH and electrode potential measurements. pH and electrode potential (mV) of dilute acid
additive, LiBr solutions without acid additive and MSH solutions containing acid additive and
different amount of LiBr were measured using a pH meter (Fisher Scientific accumet Basic
AB15) having capability of both pH and electrode potential measurement features. The electrode
was calibrated with standard solutions of appropriate pH values before each measurement. Each
solution was measured in triplicate for reproducibility in data.
Analysis and quantification of various products. xylose hydrolysates and the aqueous and
organic phases from xylose hydrolysate dehydration reactions were diluted 10ꢀfold, unless
otherwise mentioned, and analyzed on a Waters HPLC instrument (model e2695) equipped with
a photodiode array (PDA) detector (Waters 2998) as well as a refractive index (RI) detector
(Waters 2414). Three HPLC columns of following specifications were used for analysis of
different sugar and furfurals products at different operating conditions. (1) A Bioꢀrad Aminex
HPXꢀ87H (7.8 × 300 mm, 9 µm) column operating at column oven temperature of 55 °C, an
aqueous solution of H2SO4 (0.005 M) as a mobile phase at flow rate of 0.6 mL/min was used for
detection and quantification of xylose (10.14 min), xylulose (10.41) and furfural (45.58 min)
using the RI and PDA (254 nm) detectors. The number in parenthesis refers to the retention time
of the species. (2) An Agilent Hiplex Na (7.7 × 300 mm, 10 µm) column operating at column
oven temperature of 85 °C and DI water a mobile phase at flow rate of 0.2 mL/min was used for
analysis of oligosaccharides using the RI detector. This column and HPLC operating conditions
detected glucoꢀ and xyloꢀoligosaccharides of DP (degree of polymerization) up to 6. Retention
times for detected oligosaccharides are given in Table S1. (3) An Agilent Zorbax SB C18 (4.6 x
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250 mm, 5 µm) column operating at column oven temperature of 25 °C and acetonitrileꢀwater
mixture (1:1 v/v) as a mobile phase at a flow rate of 0.3 mL/min was used for analysis of HMF
in the organic phase from sugar hydrolysate dehydration experiments using a PDA detector (254
nm).
The characteristic peaks for organic products and sugar monosaccharides and
oligosaccharides were identified from the retention times of the authentic samples. Each peak
was integrated, and the actual concentrations of each product were calculated from their
respective preꢀcalibrated plots of peak areas vs. concentrations. The yields of various products
(C5 monosaccharides, oligosaccharides, HMF, furfural and other soluble products) from xylan
saccharification were calculated by following equations.
C5 monosaccharides and oligosaccharides yields from xylan saccharification
ꢖꢍꢕꢌꢔꢏ ꢍꢋ ꢉꢎ ꢉꢕꢐꢏꢗꢓꢕꢔ ꢘꢓꢊꢉ ꢙꢚꢛꢇ
ꢀꢁꢂꢃꢄ (ꢅꢆ%)ꢇꢈ ꢉꢊꢋꢊ/ꢊꢌꢍꢎꢊꢏꢐꢑꢑꢒꢐꢓꢍꢔꢕꢏ
=
× 100
(1)
(2)
ꢜꢝꢌꢐꢋ ꢍꢋ ꢉꢎ ×ꢞꢟ
where F2 is Mw of monoꢀ or oligosaccharides/132
Furfural and xylulose yields from dehydration of xylose hydrolysate
ꢖꢍꢕꢌꢔ ꢍꢋ ꢉꢊꢌ ꢉꢕꢐꢏꢗꢓꢕꢔ ꢘꢓꢊꢉ ꢙꢚꢛꢇ
ꢀꢁꢂꢃꢄ (ꢠꢡꢃ%)ꢞꢗꢓꢘꢗꢓꢐꢌ,ꢢꢝꢌꢗꢌꢊꢏꢕ
=
ꢣꢋꢍꢤꢍꢐꢌ ꢢꢝꢌꢊꢏꢕ ꢍꢋ ꢒꢝꢔꢓꢊꢌꢝꢏꢐꢤꢕꢏ (ꢍꢋ ꢉꢊꢌ)
× 100
HMF Partition coefficient was calculated using the following equation
ꢙꢫꢞ ꢊꢓ ꢘꢗꢓꢘꢗꢓꢐꢌ
ꢬꢭꢮꢯꢰꢱꢲ ꢳꢴꢯꢵꢶ
ꢥꢦꢧꢆꢁꢆꢁꢡꢨ ꢩꢡꢂꢪꢪꢁꢩꢁꢂꢨꢆꢙꢫꢞ ꢊꢓ ꢘꢗꢓꢘꢗꢓꢐꢌ
=
(3)
ꢙꢫꢞ ꢊꢓ ꢘꢗꢓꢘꢗꢓꢐꢌ
ꢷꢸꢹ ꢳꢴꢯꢵꢶ
Results and Discussion
Xylan saccharification. Xylan is present in both hardwood (15ꢀ30 wt%) and softwood (5ꢀ
10%).32 Saccharification of xylan to xylose occurs rapidly as xyloꢀoligosaccharides peaks are
absent in the HPLC chromatogram of the hydrolysate collected in 5 min (Figure 2a), unlike
hydrolysate from cellulose saccharification showing significant glucoꢀoligosaccharides (Figure
S1). The major product in 2.5 min is xylose with 43 wt% yield. The conversion of the remaining
xylan progresses as it gets dissolved in solution. Finally, nearly quantitative yield of xylose is
obtained in 20 min. Figure 2b shows xylan saccharification occurs faster in MSH than that of
crystalline cellulose due to its amorphous nature, lower DP (degree of polymerization), linear
structure and weaker ether linkages33 compared with crystalline cellulose. Importantly, xylose
yield is selective in 20 min with insignificant furanic products (<0.5% furfural) due to mild
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reaction temperature (85 °C) and very low acid additive (0.05 M). A small fraction of xylose
degraded to furfural (4%) over 2 h (Figure 2c).
Figure 2. Xylan saccharification results in MSH: (a) Stack plot of HPLC chromatograms
showing xyloꢀoligosaccharide peaks (XH = Xylohexaose, XP = Xylopentaose, XTt =
Xylotetraose, XTt = Xylotriose, XB = Xylobiose) disappear quickly. (b) Comparison of glucose
and xylose yields from crystalline cellulose and xylan. (c) Yields of xylose and other products.
(d) Xylose yields at different concentrations of acid additive: (I) no acid, (II) 0.005 M, (III)
0.0075 M, (IV) 0.01 M, (V) 0.025 M, (VI) 0.05 M. (e) Color appearance of hydrolysates
showing less humins formation at lower concentration of acid additive; (1) 0.05 M (2) 0.025 M
(3) 0.01 M (f) Dependence of initial rates as a function of the concentrations of acid additive.
Total HPLC yield in Figure (c) corresponds to the total soluble sugars and furfural. Reaction
conditions: 1.3 wt% xylan or 1.58 wt% cellulose, 59 wt% LiBr, water/salt molar ratio 3.25, 0.05
M H2SO4, 85 °C. Acid variation experiments were performed at fixed LiBr (59 wt%)
concentration.
The saccharification rates depend on the concentration of acid additive (Figure 2d).
Although the rates are faster at higher concentrations of the additive, the faster rates are not
necessarily beneficial for xylose selectivity as they lead to its degradation. Noteworthy, low
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concentrations of the additive (0.01ꢀ0.025 M) achieved the best yield of xylose (90 wt%) with
high selectivity in 2 h as the degradation of xylose is reduced and the color of the sugar solution
is much lighter, indicating less degradation products, as opposed to darker coloration of the
hydrolysates at high concentration of the acid additive (Figure 2e). Small amount of dimer
(xylobiose; <3 wt%) and furfural (<3 wt%) are also formed with xylose to account the total
carbon yield of 96%. Initial rates of xylan saccharification at low conversion (<20 wt%) increase
linearly with the concentration of the acid additive, suggesting the saccharification follows firstꢀ
order dependence on the additive (Figure 2f)
The reaction temperature has a profound effect on the saccharification (Figures 3a). The
initial rates are slow at temperatures below 60 °C (60 wt% xylose yield for 2 h). High
temperatures (≥100 °C) accelerated the reaction rates with high xylose yield (~90% at 100 °C) in
shorter times but concurrent degradation of xylose to furfural (Figure S2) and humins results in
the yield loss of xylose. A reaction at 140 °C shows that the salt solution itself is effective to
saccharify xylan, producing 50% xylose within 10 min without the acid additive. This suggests
that the acidity of the salt solution increases with increase in temperature due to higher
polarization of water molecules in the hydration sphere of the MSH by the hydrated cations.
Thus, acid additive was not required at 140 °C. Interestingly, the acid additive was found to have
a detrimental effect. Xylose yield dropped immediately to 10 wt% with appearance of black
colored humins in the solution when 0.05 mM acid additive was added. Brønsted acid assisted
dehydration of xylose in the latter reaction results in furfural formation (27 wt%) which is
degraded to humins within 20 min.
Figure 3b plots the initial rates for xylan saccharification as a function of inverse
temperature. The apparent activation energy calculated from the Arrhenius plot is 100kJ/mol,
which is considerably lower than that reported for xylan saccharification in acidic water (H2SO4;
127 kJ/mole)32. The activation energy for xylan conversion is also lower than crystalline
cellulose conversion under similar reaction conditions in MSH.18
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Figure 3. Xylan saccharification at varying temperatures (a,b), LiBr concentrations (c) and xylan
amounts (d). Temperatures in Figure (a) are denoted by numbers as follows: (I) = 50 °C, (II) =
60 °C, (III) = 70 °C, (IV) = 85 °C, (V) = 100 °C, (VI) = 110 °C, (VII) = 140°C. Figure (b) shows
dependence of initial rates as a function of 1/T. Reaction conditions: 1.3 wt% xylan, 0.05 M
acid additive, water/salt molar ratio = 3.25, 59 wt% LiBr. LiBr and xylan variation experiments
were conducted at 85 °C.
The saccharification also depends on the salt concentration. Higher LiBr loading (59
wt%; water to LiBr molar ratio = 3.25) in MSH results in faster xylan conversion and higher
xylose yield (Figure 3c). This emphasizes that MSH is effective at water to salt molar ratio of
equal or higher than the coordination number of the cations of the salts, which is 3 for LiBr. This
condition makes water acidic and enables faster xylan saccharification. The reaction becomes
slow at LiBr loading ≤ 50 wt%, as the water to LiBr molar ratio is low (2.35 and 1.88 for 50 wt%
and 40 wt% LiBr loadings, respectively) and the solutions no longer possess MSH properties. A
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control experiment without LiBr shows very little reaction (2 wt% xylose yield for 2 h),
suggesting the acid additive (0.05 M) alone is not sufficient for saccharification. Our results
reveal a synergy between the salt and acid in enhancing xylan saccharification.
Xylan concentration variation experiments at fixed acid additive, LiBr and water to salt
molar ratio show faster saccharification rates and higher xylose yield (90% within 20 min) at
lower xylan loading (Figure 3d), which is consistent with prior results for crystalline cellulose
saccharification.18 The reactions for prolonged time cause some degradation of xylan, owing to
xylose isomerization (to xylulose) and dehydration (to furfural). The solution is almost colorless,
indicating humins formation via oligomerization of sugar intermediate/furfural did not occur.
High saccharification rates with high xylose yield at low xylan loadings (0.65 – 1.3 wt%) can be
attributed to xylan’s faster dissolution in MSH, resulting in faster saccharification. At higher
loadings, the degree of saccharification could be limited by xylan’s initial dissolution, which is
consistent with observed agglomeration of xylan when large amount was added at once in MSH.
Synergy in acidity and its measurement. As aforementioned, xylan saccharification is very slow
without LiBr in 0.05 M aqueous acid additive (2% xylose yield for 2 h). Conversely, 59 wt%
LiBr alone results in very slow saccharification without acid additive (1.7% xylose yield). The
results indicate that there is a synergy between LiBr and acid additive. To evaluate this, pH of
MSH containing 0.05 M acid additive and 59 wt% LiBr was measured. The measured pH value
(ꢀ2) is outside the normal pH range of a pH electrode. Notably, pH values of 0.05 M acid
additive and 59 wt% LiBr solutions separately measured as 1.3 and 5.9, respectively. Because of
negative pH of the MSH, the acidity of the solution was measured using EP and Hammett acidity
methods in order to establish a correlation between the two methods.
First EP of MSH, acid additive and salt solutions were measured separately. EP values of
0.05 M acid additive without LiBr and 59 wt% LiBr solution without acid additive are 340 mV
and 55 mV, respectively. In contrast, EP value of the MSH solution containing 0.05 M acid
additive and 59 wt% LiBr measured as 618 mV, as opposed to 423 mV for combined value of
the two separate solutions (acid additive and LiBr). Increased EP of MSH strongly correlates
with enhanced xylan saccharification in MSH than that in LiBr solution and acid additive alone.
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To validate the EP data, Hammett acidity function (H0) was determined. This method has
been used to measure acidity for very concentrated solutions of strong acids, e.g., super acids,
and nonꢀaqueous and saltꢀwater solutions.21, 34 Using this method, Duffy et al. demonstrated the
acidity of ZnCl2ꢀwater solution at water to salt ratio of 9 is higher than that of orthophosphoric
acid.21 In another report, the authors found an increase in acidity of a salt solution with
increasing the salt concentrations using 2ꢀnitroaniline and 4ꢀchloroꢀ2ꢀnitro aniline as base
indicators for Hammett titration,35 although maximum salt concentration used was lower than
that required the solution to possess MSH properties.
Initial experiments using 2ꢀnitroaniline and 4ꢀchloroꢀ2ꢀnitroaniline as base indicators,
which has been used in the past,35 suggest that these bases are not effective for LiBrꢀMSH at
water to LiBr molar ratio of 3.25, likely because of its high acidity. 2,6ꢀdinitroaniline was used
as a base, which shows an increase in acidity of MSH, based on calculated Hammett function
(H0) values from measured absorbance (Figure S3), with increasing acid additive concentrations
(0ꢀ0.05 M) at fixed LiBr (59 wt%) (Table S2, Figure 4). Conversely, the effective acidity of the
solutions increased with an increase of LiBr concentration at fixed acid additive (0.05 M; Table
S3). Figure 4a shows a good correlation between H0 and EP values of the MSH solution. The
results validate that EP value can be considered to measure the relative acidity of strong acids as
this method is simpler than the Hammett acidity measurement.
700
0.05 M
600
500
400
0.005 M
300
0.002 M
200
0.001 M
100
ꢀ5.6
ꢀ5.4
ꢀ5.2
ꢀ5.0
ꢀ4.8
Hammett Acidity (H0)
Figure 4. Correlation of H0 for LiBrꢀMSH solutions with their measured EP values showing an
increased acidity of the solutions with an increase in acid additive (0ꢀ0.05 M; Table S2 tabulates
data used for the plot).
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Process integration for xylose separation via dehydration and reactive extraction. Separation
of xylose or MSH is a critical step to ensure recycling the reactive phase and selective processing
of xylose to high value products in the downstream steps. Prior arts employing crown ethers
extraction for salts26 and boronic acid complexationꢀdecomplexation,27 antisolvents,28 liquid
membranes29 and ion exclusion chromatography30 for sugar separation are not scalable. Another
approach for separation is the conversion of xylose to furfural in biphasic solvents and reactive
extraction of furfural, which will enable easy separation of product and recycling of the MSH
phase (Figure S4).
Xylose conversion to furfural progresses via isomerization and dehydration reactions, and
requires Lewis and Brønsted acid sites.36 The Lewis acid isomerizes xylose to xylulose, while the
Brønsted acid dehydrates the isomerized intermediate to furfural. Xylose dehydration to furfural
37
have been reported using metal oxides , metal chlorides (e.g. AlCl3, CrCl3, Zr(O)Cl2)38, 39,40ꢀ42
and solid acids41, 43, 44 catalysts in monoꢀ and biphasic solvents45. Among these, metal chlorides
demonstrated high activity in sugar isomerization,46ꢀ48 owing to facilitated hydride transfer from
O1 oxygen to O2 of xylose through formation of a cyclic intermediate.48
Following our reported method, AlCl3 and a biphasic solvent was used to convert xylose
hydrolysate to furfural.49 First, several extracting solvents of high partition coefficients predicted
by COSMOꢀSAC50 were screened and then the reaction conditions were optimized for xylose
dehydration in MSH hydrolysate. Usually inorganic salts in the aqueous phase improve
partitioning of furfural into the organic phase due to the salting out effect.51, 52 Interestingly MSH
behaves negatively with most organic solvents in partitioning furfural. For example, HMF
partitioning into methylisobutylketone (MIBK) enhanced in a MIBK/water biphasic solvent
(partition coefficient = 1.2) in which water is saturated with 30 wt% NaCl.52 Noteworthy, our
experiment using pure HMF as a model furan for solvent selection shows very low partition
coefficient for HMF (0.5) in MIBK/MSH (1:3 (v/v)). Similarly, the COSMOꢀSAC model
predicted a high partition coefficient for HMF (33.8) in mꢀcresol/water, a finding which was
experimentally validated.52 However, in the mꢀcresol/MSH system, furfural forms a
condensation product with mꢀcresol, which agrees with a previous report.53 A controlled
experiment validates condensation between pure HMF and mꢀcresol in MSH under comparable
reaction conditions, resulting in fourꢀfold decrease in HMF peak in the HPLC chromatogram.
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This results in erroneous partition coefficient values. Furanic solvents (tetrahydrofuran (THF)
and 2ꢀmethyl tetrahydrofuran (2ꢀMeTHF)), which form separate phases with pure water or NaCl
saturated water, are miscible with MSH. The screening experiments identified ethyl acetate (EA)
as the best extracting solvent for HMF extraction from MSH hydrolysate. Solvents screening for
furfural extraction at two different temperatures (60 °C and 120 °C) with and without AlCl3
reveal that EA has slightly higher partition coefficient values than MIBK at both temperatures
(Table S4).
After selection of EA as the organic solvent for furfural extraction, the reaction
conditions were optimized for xylose dehydration in EA/MSH by varying the reaction
temperature (100ꢀ140 °C) and AlCl3 concentrations (10ꢀ50 mol% on the basis of xylose amount).
A hydrolysate (1.5 mL) containing 30 mg xylose, obtained from xylan saccharification at
optimal reaction conditions (water/salt molar ratio of 3.25, 0.05 M acid additive, 69 at% LiBr, 85
°C, 20 min), was used for the dehydration. Assuming lithium ions of the MSH could act a Lewis
acid for the dehydration, an initial experiment was conducted without AlCl3. This reaction at 140
°C and hydrolysate to EA molar ratio of 1:3 formed maximum 16% furfural for 2 h. The
hydrolysates prepared using less amount of acid additive (0.01 and 0.025 M) yield less furfural
(11 and 14%, respectively) under comparable conditions. Xylulose is not observed as an
intermediate from xylose isomerization. Assuming lithium ions could be effective at lower
concentration, 10 times diluted hydrolysate was used for dehydration without AlCl3, which gives
worse furfural yield. These results indicate that lithium ions are ineffective for xylose
isomerization, leading to poor furfural yield.
The dehydration with 50 ml% AlCl3 improved furfural yield significantly (84 mol%) at
quantitative conversion of xylose. The reaction progresses via xylulose intermediate (Figure S5)
as established by us.54 Some carbon loss can be attributed to formation of humins as the solution
turned brownish during reaction, indicating oligomerization of furfural with xylose occurs,
although this byꢀproduct was not quantified. The reactions at varying temperatures (100 – 140
°C) and AlCl3 concentrations show (Figure 5) that both xylose conversion and furfural yield
improved with increasing temperature and the catalyst amount. The dehydrations at 120 °C and
140 °C occur faster than at 100 °C (Figures 5a and 5b). Xylose conversion is higher at 140 °C
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than at 120 °C while furfural yields at these two temperatures are comparable. This suggests
higher carbon loss at 140 °C, possibly in the form of humins.
100
80
60
40
20
0
100
80
60
40
20
0
(a)
(b)
140 oC
120 oC
100 oC
140 oC
120 oC
100 oC
0
30
60
90
120
0
30
60
90
120
Time (min)
Time (min)
100
80
60
40
20
0
100
80
60
40
20
0
(d)
(c)
50%
25%
10%
50%
25%
10%
0
20 40 60 80 100 120
0
30
60
90
120
Time (min)
Time (min)
Figure 5. The effects of reaction temperatures and AlCl3 concentrations on xylose conversion
(a,c) and furfural yields (b,d). Reaction conditions: 1.5 mL xylose hydrolysate containing 30 mg
xylose, 4.5 mL EA. The temperature and AlCl3 dependence experiments were performed at
fixed AlCl3 (50 mol%) and temperature (120 °C), respectively. The yields are on molar basis.
In case of AlCl3 variation experiments, the reaction at 50 mol% AlCl3 shows similar
profile in xylose conversion and furfural yield (Figures 5c and 5d), although total carbon yield of
furfural is slightly lower for the reasons discussed above. At lower AlCl3 (10 mol%), especially
at 120 °C, there is a significant variation in carbon yield in furfural and xylose conversion,
indicating humins formation could be predominate at this condition. While the exact reason is
not clear, xylose isomerization to xylulose, an intermediate that is eventually dehydrates to
furfural, is expected to occur slowly in the presence of lower concentration of Lewis acid. Thus,
oligomerization between furfural and xylose could dominate at this condition.46, 55
Next we evaluate the integrated process for glucose dehydration and separation of
product from glucose hydrolysate obtained from crystalline cellulose saccharification in MSH.
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Although HMF production from C6 monosaccharide, cellulose and biomass has been extensively
studied using monoꢀ and bifunctional acidic catalysts in biphasic solvent systems,51 such studies
are rare in the MSH system, except a recent report.56 A glucose hydrolysate of concentration 33
mg glucose/mL hydrolysate was prepared by saccharification of crystalline cellulose at optimal
conditions developed for xylan saccharification, i.e., 59 wt% LiBr, water to salt molar ratio of
3.25, 0.05 M acid additive, 85 °C, 30 min reaction time and 1.58 wt% crystalline cellulose. The
dehydration at the best conditions of xylose hydrolysate, but at higher temperature (140 °C),
yields less HMF in 30 min when compared with xylose dehydration results (Figure S6). The
reaction for prolonged time caused a loss of HMF yield, suggesting oligomerization between
glucose and HMF takes place at faster rates at 140 °C. Lowering the reaction temperature to 120
°C improves HMF yield significantly (85% for 2 h; Figure 6). However, the total HPLC yield,
representing the combined yield of HMF, fructose and unconverted glucose decreased over time,
100
80
60
Glucose
40
20
0
HMF
Fructose
Total HPLC Yield
0
20 40 60 80 100 120
Time (min)
Figure 6. Dehydration results of glucose hydrolysate (obtained from saccharification of
crystalline cellulose in MSH) with AlCl3 in EA/MSH solvent. Reaction conditions: glucose
hydrolysate to EA ratio 1:3 (v/v), 120 °C, 50 mol% AlCl3. Total HPLC yield in Figure (a) is the
combined yield of HMF, combined yield of HMF, fructose and unconverted glucose.
indicating some carbon loss in the form of humins occurs. Fructose is detected as an intermediate
after reaction for 30 min, which suggests the reaction progresses through isomerization of
57
glucose to fructose,
and corroborates with our observation for xylose conversion. Fructose
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concentration in 30 min is low because of its fast dehydration to HMF as it is formed. HMF
rehydration forms levulinic acid (LA) in MSH monophasic system; however, LA formation is
not evident in EA/MSH biphasic reaction (Figure S7). Because of low partition coefficients of
HMF in MIBK/MSH (MIBK = methyl isobutyl ketone) and DMF/MSF (DMF = 2,5ꢀ
dimethylfuran) biphasic solvents, glucose hydrolysate dehydration in these solvents give poor
yields of HMF (Table S5 and Figure S8). A controlled experiment in nonꢀMSH system
(MIBK/water) achieves higher HMF yield (23%), confirming complexities of the MSH system
with MIBK as an extracting solvent, as observed in the solvent selection experiments (vide
supra).
Comparison of our results in MSH with the literature results in nonꢀMSH systems show
that maximum reported HMF and furfural yields in biphasic solvents containing nonꢀMSH as an
aqueous phase are 68 mol%58 and 84 mol%59 at 140 °C and 170 °C, respectively. Similarly,
prior study reported maximum 62% HMF from fructose in water/MIBK at 130 °C.49 In contrast,
we observed considerably higher HMF and similar amount of furfural in MSH at lower
temperature (120 °C). This comparison suggests that the cooperative effect of Lewis and
Brønsted acids and high viscosity (3 mPa.S)60 of the MSH medium is beneficial for enhanced
HMF and furfural yields.
Next the scope of the integrated process is extended to more realistic feedstock. In this
case, intact poplar wood (particle size 1 mm × 1 mm × 1 mm) containing 45.3% glucan, 18%
xylan including small amount of acetate xylan, 26% lignin, 0.5 % ash and 5% extractives (based
on dry weight of wood following NREL compositional analysis)61 is depolymerized and
saccharified into soluble monosaccharides (C5/C6 sugars) in oneꢀstep using MSH. At optimal
xylan saccharification condition, intact poplar wood gives maximum 43 wt% glucose and 15
wt% xylose yields on the basis of dry weight of wood, which corresponds to 95 wt% and 81%,
respectively on the basis of theoretical glucan and xylan amount in wood (Figure 7b). The
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Figure 7. Oneꢀpot integrated process for poplar wood conversion to HMF and furfural in MSH.
(a) HPLC chromatogram showing glucoꢀoligosaccharides/monosaccharide peaks during biomass
depolymerization and saccharification. CB, CTr, CTt and CP represent cellobiose, cellotriose,
cellotetrose and cellopentose. (b) monosaccharides, glucoꢀoligosaccharide and solid lignin yields
in wt% based on dry weight of poplar wood. Small amount acetic acid, soluble lignin and
furfural are also formed. (c) Reaction profile for glucose and oligosaccharides formation during
poplar wood conversion. (d) Dehydration of sugar hydrolysates to HMF and furfural in
EA/MSH.
reaction is slow in the beginning with majority of depolymerized cellulose remained in the form
of glucoꢀoligosaccharides (Figure 7a). The concentration of total glucoꢀoligosaccharides
increased to 36 wt% in 0.5 h of which cellopentose is 11 wt%. Higher Mw soluble
oligosaccharides (DP>5) could be formed during depolymerization, which could not be
determine using the HPLC method. Majority of xylose is formed within 0.5 h. Prolonged
reaction for 2 h achieves maximum glucose yield via depoymerization of remaining biomass and
saccharification of glucoꢀoligosaccharides. Subsequent dehydration of the resulting hydrolysate
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in EA/MSH solvent gives maximum 78 mol% furfural and 62 mol% HMF. The rate of formation
of furfural takes place faster than that of HMF. The reaction is almost complete within 0.5 h.
continuing the reaction for a longer time (2 h) shows very slow degradation of furfurals. Side
products (LA, humins, FA) are formed in the MSH phase (Figure S7) that accounts for carbon
loss.
Recycling MSH. The recycling efficiency of MSH was tested for the conversion of glucose
hydrolysate. Following the procedure discussed in the experimental section, the MSH phase was
recovered after the 1st cycle. The next cycle starts with the recovered MSH, calculated amount
of crystalline cellulose and EA. The saccharification and dehydration occur in tandem fashion in
the subsequent cycles as the Alꢀcatalyst was not separated. Initial recycling experiments shows a
loss of HMF yield from cycle 1 to 3 (Figure 8a). We hypothesize the loss of effective acid or
LiBr or Alꢀcatalyst concentrations could be reasons for the activity loss. ICPꢀMS analysis of
crude HMF product shows a little loss of Li (0.04% wt%) and negligible loss of Al from the
MSH phase to the organic phase. EP values dropped from cycle 1 (618 mV) to cycle 2 (483 mV),
indicating a decrease in acidity of the MSH phase. Repeat recycling experiments with a small
amount of added acid additive from 2nd cycle to keep EP value constant achieve nearly
comparable HMF yield in the 2nd and 3rd cycles (Figure 8b). A little activity loss in the 3rd cycle
(~9 % HMF yield loss) could be attributed to the mass loss of the MSH phase during recovery.
Glucose conversion
HMF yield
Glucose conversion
HMF yield
100
80
60
40
20
0
100
80
60
40
20
0
(a)
(b)
1
2
3
1
2
3
Cycle
Cycle
Figure 8. MSH recycling results for the conversion of crystalline cellulose to HMF in ethyl
acetate (EA)/MSH solvent (a). A small amount of acid additive was added to the recovered
MSH phase for experiments in Figure (b) to keep EP value constant. Reaction conditions: 1.5
mL hydrolysate, 50 mol% AlCl3, 4.5 mL EA, 120 °C, 1 h.
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Conclusions
An integrated process for efficient conversion of sugar biopolymers and intact biomass to
furfural and HMF with high carbon efficiency in LiBrꢀMSH has been demonstrated. First, sugar
biopolymers, e.g., xylan undergoes efficient saccharification to xylose with >90% yield within
20 min at low temperature, which is consistent with lower activation energy (100 kJ/mol) for
saccharification in MSH than prior report in mineral acid. The saccharification rates increase
with increasing the concentrations of LiBr and acid additive, and achieve maximum saccharified
xylose yield at water to LiBr molar ratio of 3.25 where the solution possesses MSH properties.
Controlled experiments suggest the saccharification is much more effective when LiBr and acid
additive are present together, which correlates with high acidity of the solution as determined
from electrode potential and Hammett acidity function. Subsequent dehydration of sugars in
MSH hydrolysate by the Brønsted acidity of the medium and added inorganic Lewis acid in the
presence of an extracting solvent achieves higher furfural (>80%) and HMF yields than the
analogues nonꢀMSH systems. MSH behaves differently in partitioning the dehydration products
in most extracting solvents predicted by the COSMOꢀSAC model for nonꢀMSH medium. Ethyl
acetate exhibits the highest partition coefficient in MSH. The process integration and reactive
extraction of products enable multiple recycling of the reactive phase without involving complex
separation. The integrated process also achieved high yields of HMF and furfural from intact
poplar wood in oneꢀpot.
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Green Chemistry
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