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Kinetics, Catalysis, and Reaction Engineering
Polymeric Solid Acid Catalysts for Lignocellulosic Biomass Fractionation Anh Vu, S. Ranil Wickramasinghe, and Xianghong Qian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05286 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018
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Polymeric Solid Acid Catalysts for Lignocellulosic Biomass Fractionation
Anh Vu1, S. Ranil Wickramasinghe1, Xianghong Qian2*
1
2
Department of Chemical Engineering, University of Arkansas, Fayetteville, AR 72701
Department of Biomedical Engineering, University of Arkansas, Fayetteville, AR 72701 *Corresponding author, Email:
[email protected]; Tel: 4795758401
Abstract Lignocellulosic biomass fractionation has been conducted using a synthetic polymeric solid acid catalyst consisting of dual polymer chains. The acidic polymeric chain, poly (styrene sulfonic acid) (PSSA) catalyzes biomass hydrolysis. A neighboring poly (vinyl imidazolium chloride) (PIL) chain helps solubilize lignocellulosic biomass and enhance the catalytic activity. Hydrolysis was conducted for crystalline cellulose and acid, base or steam pretreated cornstover samples in ionic liquids (IL) and mixtures of IL with H2O or γ-valerolactone (GVL) or other organic solvents. Near quantitative total reducing sugar (TRS) yields for cellulose hydrolysis as well as pretreated cornstover biomass were achieved at mild conditions and in less than 12 hours. Our designed polymeric solid acid catalysts are superior to cellulases as they can be operated at a higher temperature and at a much higher hydrolysis rate. These catalysts are stable and maintain high catalytic activity after repeated runs. Moreover, they can be easily regenerated and are environmental friendly.
Keywords: Polymeric Acid Catalysts, Lignocellulosic Biomass, Solvent, Hydrolysis, Sugar
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1. Introduction Cellulosic biomass represents an abundant renewable resource for the production of bio-based products and biofuels. Cellulosic biomass is composed mainly of hemicelluloses (~15-32%), cellulose (~30-50%) and lignin (~15-25%). Hemicelluloses, mostly xylan, are natural polymers of β-D-xylose and other minor sugars; whereas, cellulose is made of β-D-glucose. Lignin is a polymer composed of non-fermentable phenyl-propene monomer units. Cellulose is a very complex substrate with amorphous, semi-crystalline and crystalline structures
1, 2
. Crystalline
cellulose possesses extensive and strong hydrogen bonding networks 2, 3. The cooperativity of the hydrogen bonding interactions in crystalline cellulose enhances the hydrogen bonding interaction energy by as much as 50% compared to non-cooperative hydrogen bonding energies in polysaccharides, making it the most recalcitrant substrate to hydrolysis 4. Thermo-chemical pretreatment of biomass opens up the biomass structure and has long been recognized as a critical step to produce cellulose with acceptable enzymatic digestibility 5. Various technologies including dilute acid ammonia fiber explosion
12, 13
, and lime
6, 7
, alkaline or base
14, 15
8, 9
, hot water or steam
10, 11
,
pretreatment methods have been developed to
accomplish pretreatment 16. Dilute sulfuric acid pretreatment (~0.5-3.0 wt%) is the leading technology, and sulfuric acid is currently the most cost-effective agent used in pretreatment to hydrolyze hemicelluloses and relocate lignin
1, 8, 17-26
. Typically, dilute acid pretreatment of biomass is carried out at an
elevated temperature of 160-200°C. Due to the corrosive nature of sulfuric acid, the costs to construct pilot and commercial scale dilute acid pretreatment plants are high. Further, the recovery and separation of homogenous acid catalysts are expensive with major environmental concerns. A solid acid catalyst replacing the sulfuric acid catalyst will have tremendous
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advantage. Solid acid catalysts will be easy to separate and recycle. They are much less corrosive and environmental benign with enormous economic and environmental benefits. For the pretreated biomass samples, a cocktail of enzymes is needed to hydrolyze cellulose completely during the subsequent enzymatic conversion due to the complexity of cellulose substrate 1. These enzymes are relatively expensive to produce and generally cannot be reused. In addition, enzymatic digestion of cellulose, particularly crystalline cellulose is slow and may take from a few days to a couple of weeks to accomplish. A solid acid catalyst that is capable of mimicking the action of cellulase enzymes will be highly desirable. The hydrolysis reaction catalyzed by an enzyme mimic catalyst can be operated at a temperature much higher than the enzymatic conversion process thereby increasing the conversion rate and reducing the hydrolysis time significantly. Solid acid catalysts such as zeolites and polymeric solid acid catalysts have been investigated as potential catalysts for biomass processing
27-30
. However, most commercially
available solid acid catalysts demonstrated limited TRS yields of less than 80% from crystalline cellulose hydrolysis 29, 30. Previously 27 we have designed an enzyme mimic polymeric solid acid catalyst which demonstrated high catalytic activity in deconstructing cellulose with over 97% TRS yield achieved in ionic liquid (IL) solvent at mild condition of 130oC. The polymeric solid acid catalyst consists of two randomly grafted polymer chains on a membrane or glass substrate. The acidic polymeric chain, poly (styrene sulfonic acid) (PSSA) synthesized via atom-transfer radical polymerization (ATRP) catalyzes biomass hydrolysis. A neighboring poly (vinyl imidazolium chloride) (PIL) chain grafted via UV-initiated radical polymerization helps solubilize lignocellulosic biomass and enhance the catalytic activity. However, only crystalline cellulose substrate was investigated for the hydrolysis reaction using the rather expensive 1-
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ethyl-3-methylimidazolium chloride ([EMIM]Cl) solvent. There have been many studies using IL solvents for biomass processing. Among their many superior properties especially as green and environmentally friendly solvents, ILs such as [EMIM]Cl and 1-n-butyl-3-methylimidazolium chloride ([BMIM]Cl) are found to be able to dissolve cellulose and even lignocellulosic biomass. Our previous study
31
shows that purified
[EMIM]Cl with small amount of water (equivalent to 4 glucose units) can hydrolyze cellulose with TRS and glucose yields reaching 97% and 19% respectively in the absence of any added acid catalyst. Further our theoretical calculations show that increased [H+] concentration from enhanced auto-dissociation of water molecules in IL facilitates the biomass hydrolysis reaction. Even though no additional catalyst used, the purity of [EMIM]Cl has to be so high as to make the process impractical. The other main disadvantages of using ILs are their high cost and high viscosity which make them economically unviable. Our recent studies demonstrate
27, 32-36
that solvent and solvent mixtures play a critical
role during biomass fractionation and conversion. The barriers for hydrolysis and subsequent sugar dehydration and degradation are largely solvent induced. Besides ILs, other organic solvents such as γ-valerolactone (GVL) 37, dimethyl sulfoxide (DMSO) 35 have been investigated mainly as solvent, or as ionic liquid cosolvent for biomass processing. Earlier study
38, 39
demonstrates that H2O/IL mixtures containing up to 50 wt% or more water are effective as pretreatment media leading to fermentable sugar yields of 81% during the subsequent enzymatic hydrolysis, a significant enhancement over 67% sugar yields obtained with pretreatment using pure ILs. This enhanced hydrolysis is due to the increased solubilization of lignin in IL/H2O mixtures. Not only IL/H2O mixtures are able to dissolve cellulose and lignocellulosic biomass, but also they are excellent solvents for lignin 38, 39, one of the main polymers in biomass to hinder
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hemicellulose and cellulose hydrolysis. Here previously optimized PSSA/PIL polymeric solid acid catalysts were used to investigate the hydrolysis reactions of crystalline cellulose in IL and IL mixtures with GVL and several other organic solvents. Moreover, hydrolysis reactions of acid, base and steam pretreated cornstover samples were investigated using these solid acid catalysts in IL and IL/H2O mixed solvents as water is the most inexpensive, green and environmental friendly solvent. Near theoretical TRS yields were obtained for crystalline cellulose as well as acid, base and steam pretreated lignocellulosic biomass at mild temperatures with high efficiency.
2. Materials and Methods 2.1 Materials and reagents DI water (0.06 µS/cm) was obtained from Water Pro/RO reverse osmosis or Pro Plus deionization purification system from Labconco Corp. (Kansas City, MO). Benzoin ethyl ether (BEE, 97%), 1-butyl-3-methyl-imidazolium chloride ([BMIM]Cl, 99%), 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl, 95%), 3-aminopropyltriethoxysilane (APTES, 99%), ethyl acetate (EtOAc, anhydrous, 99.8%), 2,2’-bipyridine (bpy, 99%), copper(I) chloride (CuCl, anhydrous, ≥ 99.99% trace metal basis), copper(II) chloride (CuCl2, anhydrous, ≥ 99.99% trace metal basis), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloric acid (EDC.HCl, ≥ 98%), 2-bromo-2-methylpropionyl bromide (or α-bromoisobutyrylbromide) (Bib, 98%), and sodium 4-styrenesulfonate (NaStS, ≥90%), boric anhydride (99.99% trace metal basis), dimethyl sulfoxide (DMSO, ≥99% ), trimethylamine (TEA, ≥ 99%), were purchased from Millipore Sigma (Saint Louis, MO). Acetonitrile (ACN, reagent, HPLC grade) and hydrochloric acid (12N), ethanol (EtOH, pure, 200 proof) were purchased from Decon Labs, Inc. (King of Prussia, PA), and sodium hydroxide (NaOH, ACS grade) were purchased from VWR (West Chester, PA). 5
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Potassium hydroxide (KOH, 98% extra pure), tetrahydrofuran (THF, 99.9%, stabilized), and γvalerolactone (GVL, 98%) were purchased from Acros Organics (Fisher Science Education, Hanover Park, IL). Acetic acid (glacial, ACS grade) was purchased from EMD Millipore (Darmstadt, Germany). N-Vinyl imidazole (VI, 99%) and N, N-dimethylacetamide (DMAc, 99%) were purchased from Alfa Aesar (Tewsbury, MA). All chemicals were used without further purification. α-cellulose (white powder, 99%) and D-glucose (99%) were purchased from Millipore Sigma (Saint Louis, MO). Acid, base and steam pretreated cornstover samples were obtained from the National Renewable Energy Laboratory (NREL). These samples were dried and grinded before hydrolysis. Anopore aluminum oxide disc membranes with 0.2 µm pore and 47 mm diameter were purchased from GE Healthcare Life Science (Pittsburgh, PA). Silica oxide cylindrical tube membranes with 0.15 cm inner diameter (ID) and 10 nm pore size were obtained from T3 Scientific (Minneapolis, Minnesota). Silica oxide membranes including cylindrical tube membranes with 10 nm pore size and 0.15 cm ID, 0.1 µm pore size and 0.6 cm as well as 1 cm ID, disc membranes with 7 nm pore size and 152 cm diameter were obtained from Atech Innovations GmbH (Gladbeck, Germany).
2.2 Catalysts Synthesis 2.2.1
UV initiator Synthesis
UV initiator BEE-COOH was synthesized following the same protocol as described in our previous work 27. The basic reaction mechanism is shown in Scheme 1 below.
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UV initiator (BEE-COOH) synthesis
Scheme 1
O
NH2
O
O O
OH
O
Br
Si SAM
OH OH OH O
O
O
O
O O
HN
HN
O
2. ATRP Initiator Immobilization
NH2
Si
1. UV Initiator Immobilization
Br
O
Br
SO3H
Br
N Cl
n
N Cl
m
O
SO3H
O
O
N
SO-3Na+
O
O
N
SO-3Na+
Br
H
O
n
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Br O HN
HN
O
N
O ATRP
HN
HN
O
O
N HN
HN
O
1. UV 15 min 2. HCl
SO-3Na+
Scheme 2 Polymeric PSSA/PIL solid acid catalyst synthesis from the glass or membrane substrate. The PSSA chain is grafted by atom transfer radical polymerization (ATRP). The PIL chain is grafted by UV initiated polymerization.
2.2.2
PSSA/PIL Polymeric Solid Acid Catalysts Synthesis
Catalyst synthesis initiated by forming a self-assembled monolayer (SAM) of 3aminopropyltriethoxysilane on glass or ceramic membrane substrates as shown in Scheme 2.
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Subsequently UV initiator (BEE-COOH) and ATRP initiator (Bib) were immobilized on the substrate by forming amide bonds with the amino groups from the SAM layer. The densities of UV and ATRP initiators and the corresponding PIL and PSSA chains can be tuned independently by varying the initiator concentrations and/or immobilization time. The PSSA chain was grafted by atom-transfer radical polymerization (ATRP) followed by UV initiated free radical polymerization to form PIL. The details for the PSSA and PIL syntheses have been described in our previous publication
27
. Our earlier studies indicate that UV initiator immobilization for 60
min and ATRP initiator immobilization for 8 h followed by 24 h ATRP and 15 min UV-initiated polymerization produce the best performing catalyst at the conditions described previously
27
.
All the hydrolysis reactions performed here used PSSA/PIL catalysts synthesized at this optimized condition.
2.3
Catalyst Characterization
The synthesized catalysts before the final step used in this study were characterized using x-ray photoelectron spectroscopy (XPS). Figure 1 shows the XPS of the unmodified glass substrate, substrate modified with a SAM layer and subsequently grafted with PSSA and PIL dual polymer chains at the optimized condition as described previously. The XPS spectra were recorded before the final step of immersing the modified substrate in 12 N HCl to generate active catalyst. For the unmodified glass substrate, the Si 2s and 2p peaks are clear and distinctive. The reduction of the Si 2s and 2p peaks after SAM layer modification and their complete disappearance after grafting the PSSA/PIL polymer precursors indicate the successful modification of the polymer chains. In addition, the appearance of the N 1s peak after SAM layer immobilization and its increased magnitude after PSSA/PIL grafting suggest a successful PIL surface modification. The
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appearance of S 2s and 2p peaks after PSSA/PIL modification again suggests the successful ATRP modification. However, the appearance or the disappearance of these peaks are necessary but not sufficient to confirm the successful modification of the substrate with the PIL/PSSA chains. Additional surface characterization will be needed for the further confirmation.
XPS results for glass substrate C1s O1s PSSA +PIL S2s
S2p
c/s
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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N1s SAM layer Si2s
Si2p Unmodified
600
500
400
300
200
100
0
Binding Energy (eV) Figure 1
XPS of unmodified glass substrate, after SAM layer modification, substrate
modification with PIL and PSSA chains. PIL and PSSA polymer chains are synthesized with 60 mins of UV initiator immobilization, 8 h of ATRP initiator immobilization, 24 h ATRP, and 15 mins UV polymerization. 2.4
Lignocellulosic Biomass Preprocessing
The as-received acid, base and steam pretreated NREL cornstover samples contain moisture. They were dried under vacuum oven at 40oC and subsequently grinded. The mass of both the wet
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and dried/grinded samples were measured to estimate the contents of the biomass in these samples. Table 1 shows the percentage biomass for acid, base and steam pretreated NREL biomass samples.
Table 1 Percentage of dried biomass from NREL pretreated cornstover samples Cornstover sample
Acid-pretreated
Base-pretreated
Steam-pretreated
Biomass content (%)
39.7
34.8
38.7
The composition of the pretreated corn stover samples were determined following a twostep acid hydrolysis method recommended by NREL
40, 41
. Briefly, 300 mg biomass samples
were hydrolyzed by 72% (w/w) sulfuric acid at 30 °C for 60 min followed by adding 84 mL DI water to dilute sulfuric acid concentration to 4%. The samples were autoclaved at 121°C for 60 min. After the samples were cooled down to room temperature, filtration was conducted to separate the slurry into solid and liquid fractions. The solid fraction was used to determine the acid-insoluble lignin and the liquid fraction was used to analyzed the sugar contents and acidsoluble lignin. The lignin content is the sum of acid-soluble lignin and acid-insoluble lignin. Table 2 shows the compositions of acid-, base- and steam-pretreated corn stover samples. Table 2 The percentage compositions of the acid, base and steam pretreated cornstover biomass samples.
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Corn-stover
Glucan
Xylan
Lignin
Base pretreated
61.9
17.9
8.8
Acid pretreated
59.3
9.3
22.5
Steam pretreated
46.1
16.6
23.9
Dried and grinded samples were used for the subsequent biomass hydrolysis reactions. When a mixed [EMIM]Cl/H2O solvent was used, the biomass samples were first mixed with the solvent and were stirred at 60°C for 3 h before hydrolysis was performed. 2.5
Crystalline Cellulose Hydrolysis
A total of 0.1 g of cellulose was first dissolved in 10 mL [EMIM]Cl or a mixture of [EMIM]Cl and a cosolvent in a batch reactor at 60°C for 3 h. Polymeric solid acid catalyst immobilized on a glass or membrane substrate was then also submerged into the solution. With both substrates, catalyst loading was kept at 1%. The batch reactor was then tightly sealed and placed into a sand bath. Reaction was conducted at 130oC for the specified amount of time. When the reactor had cooled down to room temperature, the products were diluted with 100 mL of DI water to precipitate out the unreacted cellulose. The precipitated cellulose was then filtered and subsequently dried under vacuum oven. The total reducing sugar (TRS) was then determined with the DNS reagent for all conditions except when the cosolvent was GVL. When IL/GVL mixed solvents were used, mass balance approach as described in section 2.7.2 was used to determine TRS yield.
2.6
NREL Pretreated Biomass Hydrolysis in IL/H2O Mixtures
A total of 0.1 g dried and grinded NREL cornstover pretreated biomass sample was first dissolved in certain amount of [EMIM]Cl in a batch reactor at 60°C overnight. A specified
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amount of water was then added into IL mixture in order to obtain total 10 mL of liquid volume. The amount of [EMIM]Cl and H2O used was based on the predetermined ratio of [EMIM]Cl/H2O as mixed solvent. Polymeric solid acid catalyst immobilized on ceramic membrane substrate was then also submerged into the solution. A 3% catalyst loading was used for the catalytic conversion. The batch reactor was then tightly sealed and placed into a sand bath. Reaction was conducted at 95oC, 100oC or 105oC for a specified amount of time respectively. When the reactor had cooled down to room temperature, the products were diluted with 100 mL of DI water. The unreacted cellulose was then precipitated, filtered and dried under vacuum oven. The total reducing sugar (TRS) was determined with the DNS reagent. 2.7
TRS Yield Determination
2.7.1
TRS yield determination from hydrolysates in IL and IL mixed solvents
For all hydrolysis reactions except the ones with GVL as a cosolvent, the TRS yields were determined using DNS assay. The detailed procedure is described below. After the hydrolysis reaction, 10 mL of hydrolysate was diluted with 100 mL of DI water. The solution was then filtered with a 0.22 µm pore size PES membrane to remove unreacted cellulose and other solids from the hydrolysate. The solids and the membrane were dried in vacuum oven at 40°C overnight. The weight of solids was then measured. The TRS yield in hydrolysate was measured with a 3,5-dinitrosalicylic acid (DNS) assay. A DNS reagent was prepared following the procedure described by Miller
42
. A mixture of 0.5 mL DNS reagent and 1 mL of diluted
hydrolysate was heated in a boiling water bath for 5 min and was then cooled down to room temperature. The absorbance of the mixture was measured using a UV-visible spectrometer at 540 nm. The concentration of TRS in the sample was determined using a standard curve based
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on glucose as shown in Figure 2. TRS yield was calculated based on the total amount of dried biomass.
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
DNS Assay Standard Curve y = 5.541x - 0.1816 R² = 0.9996
0
0.05
0.1 0.15 0.2 Glucose Concentration (mg/mL)
0.25
Figure 2 Standard curve for DNS assay to determine the TRS concentration in hydrolysate 2.7.2
TRS yield determination from hydrolysates in IL/GVL mixed solvents
Since the presence of GVL will affect the activity of DNS reagent, it is not possible to determine the TRS yield directly using DNS assay for hydrolysis reaction conducted in IL/GVL mixed solvents. However, based on the measured products from crystalline cellulose hydrolysis in [EMIM]Cl, over 90% mass balance was achieved accounting for unreacted cellulose, TRS, and trace amount of HMF/furfural using our immobilized catalysts. Since one of the advantages of using GVL as a co-solvent is to suppress the production of the difficult-to-quantify humins, and the fact that near quantitative mass balance was achieved based on measurable known products in IL solvents, mass balance was used to estimate the TRS yield in IL/GVL mixed solvents. The amount of unreacted cellulose and potentially small amount of humins produced were determined by precipitation as described previously. The amount of HMF/furfural produced was determined by UV absorbance at 280 nm. Therefore, the amount of TRS produced was estimated 13
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by the mass of total cellulose minus the mass of solids after the reaction and the amount of HMF/furfural produced during the reaction. This is a rather crude estimate as the addition or removal of water molecules during hydrolysis or dehydration was not taken into account. The other degradation products from HMF and furfural such as formic acid, levulinic acid were also neglected since the hydrolysis reaction was conducted at mild temperature of 130oC with at most only a few percent of HMF produced during the reaction period investigated. 2.7.3
Glucose yield determination
Glucose yield based on its concentration was determined using glucose assay kit. Glucose oxidase/peroxidase reagent was prepared by dissolving 1 glucose oxidase/peroxidase capsule with 39.2 mL of DI water. o-dianisidine reagent was prepared by dissolving 5 mg of odianisidine dihydrochloride with 1 mL of DI water. A total of 0.8 mL of the o-dianisidine reagent was added into to the brown flask which contained 39.2 mL of glucose oxidase/peroxidase reagent and mixed well as assay solution. A total of 2 mL of assay solution was then added into a test tube which contained 1 mL of the hydrolysate sample. The solution was gently mixed. Then, the test tube was placed in a water bath at 37°C for 30 min. The reaction was then terminated by adding 2 mL of 12 N sulfuric acid into the solution and cooled down to room temperature. Finally, the solution was measured with UV absorbance at 540 nm. A standard curve for the glucose assay is shown in Figure 3.
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Glucose Assay Standard Curve 0.5 UV absorbance
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y = 0.1512x + 0.0043 R² = 0.9979
0.4 0.3 0.2 0.1 0 0
Figure 3
0.5
1 1.5 2 Concentration (mg/mL)
2.5
3
Standard curve for glucose assay to determine the glucose concentration in
hydrolysate 2.8
Catalyst Regeneration
It is not possible to determine the strength of the solid acid catalyst due to the difficulty in measuring the acidity at the liquid-solid interface. Nevertheless, a pH value of about 3-4 was measured for the catalysts equilibrated in water at the 1-3% loading. Even though our solid acid catalysts are recyclable and demonstrate high catalytic activity even after multiple repeated reactions in ionic liquid and its mixture with GVL, the catalysts will start to lose activity after cellulose/biomass hydrolysis reactions have been conducted in water or IL/H2O mixtures. Even in IL or IL/H2O mixed solvents, the catalysts will also slowly degrade due to the deposition of hydroxymethylfurfural (HMF) or the coverage of the polymeric dark brown humins. These membranes were then regenerated by soaking in several different solvents sequentially to remove these dark brown colored humins. These solvents include 2 M NaOH solution, GVL and DMAc solvents. Thereafter, the membranes were again soaked in 12 N HCl to regenerate the catalysts.
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3.
Results and Discussions
3.1 The effects of solvent on cellulose hydrolysis Cellulose hydrolysis with 1% cellulose loading was conducted initially in [EMIM]Cl and [BMIM]Cl solvents separately using PIL/PSSA catalysts immobilized on the glass substrates following the protocol described earlier. The PIL/PSSA catalysts consist of two polymeric chains PSSA and PIL randomly immobilized on membrane/glass substrates. The polymer chain length and chain density as well as the relative ratio of the chain densities can be independently controlled by varying the initiator immobilization time/concentration and polymerization time. The PSSA chain possessing the sulfonic acid groups catalyzes the biomass hydrolysis whereas the PIL chain helps solubilize the biomass substrate and enhance the catalytic activity. The catalytic activity and selectivity can be tuned by varying the polymer chain length, chain density and their relative density ratios. The TRS yields as a function of time are shown in Figure 4 for both solvents. It can be seen that TRS yield increases initially reaching a maximum value then it starts to decrease. Moreover, a TRS yield of over 95% can be achieved after 8 h of reaction in [EMIM]Cl in agreement with our earlier results
27
. The yield is slightly less at about 90% after
7.5 h or reaction in [BMIM]Cl. Our earlier results
27
show that over 95% of TRS yield can be
achieved in about 6 hours for cellulose hydrolysis with 1% loading in [EMIM]Cl with catalysts immobilized on ceramic membrane substrates. This increase in reaction time when PIL/PSSA catalysts immobilized on glass substrates is likely due to the reduced surface area of the flat glass substrate compared to the porous silica membrane substrate.
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Cellulose Hydrolysis in Ionic Liquids 1% cellulose, glass substrate, 130oC 100
Fig 80
TRS Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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ure 60
[EMIM]Cl [BMIM]Cl
Total
4
40 20
Our 0 6
7
7.5
8
8.5
9
earli
Reaction Time (h) er theoretical and experimental results indicate that solvent or solvent mixture play a critical role in biomass processing and sugar conversions particularly in acid catalyzed reactions 32-36, 43-45. The first step in hydrolysis or sugar reaction is the protonation of the functional groups including the β-1,4-glycosidic bond, or the ring oxygen or the hydroxyl groups on the sugar ring depending on the reaction pathways. If the solvent in the reaction medium has a relative strong affinity for proton, they could compete with the glycosidic bond or other functional groups on the sugar ring for proton thereby reducing proton availability for the hydrolysis or other sugar reactions. Subsequently it will increase the activation barrier for the protonation of glycosidic bond inhibiting the hydrolysis reaction. This have also been shown for sugar molecule’s dehydration, condensation, isomerization and ring opening reactions.
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Cellulose Hydrolysis in [EMIM]Cl
%TRS
1% cellulose, glass substrate, 130oC, 8 h reaction time
%Glucose
100 80
Sugar Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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60 40 20 0 1st
2nd
3rd
4th
5th
6th
7th
8th
9th
10th
11th
12th
No. of Runs Figure 5
TRS and glucose yields for cellulose hydrolysis with 1% loading in [EMIM]Cl
using PIL/PSSA catalyst immobilized on glass substrate after 8 h of reaction at 130oC for a total of 12 repeated runs without regeneration.
The advantage of using a solid acid catalyst is the possibility to recycle and reuse the catalyst. The catalytic activity of the PIL/PSSA catalysts was evaluated by conducting cellulose hydrolysis repeatedly with the same catalysts. Figure 5 shows the TRS and glucose yields for 12 repeated runs using the same PIL/PSSA catalyst immobilized on glass substrate with 1% cellulose loading and 1% catalyst loading in [EMIM]Cl at 130oC. The yields were determined after 8 h of reaction which was shown to give the best results. It can be seen that the catalytic activity remains more or less the same even after 12 runs with the TRS yield reaching over 90%. Glucose yield also maintains at about 20% at the 12th run, similar to the previous runs. These results are consistent with our previous data
27
that the catalysts are stable and can be used
repeatedly in aprotic ionic liquid solvents.
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Cellulose Hydrolysis in 70:30 IL/ACN 1% cellulose, silica membrane, 130oC
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Cellulose Hydrolysis in 70:30 IL/DMAc 1% cellulose, silica membrane, 130oC
%TRS %TRS
%Glucose
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80
Sugar Yield (%)
%Glucose
Sugar Yield (%)
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Figure 6
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Reaction time (h)
The TRS and glucose yields for cellulose hydrolysis in 70:30 IL/ACN and
IL/DMAc mixed solvents with 1% cellulose loading at 130oC. The catalysts were immobilized on silica membrane substrates.
Since solvent and solvent mixtures are critical to biomass processing and that ILs are generally too expensive for commercial biomass conversion applications, different organic solvents were used as cosolvents for partial replacement of IL during cellulose hydrolysis reactions. Moreover, as was discussed previously, solvent and solvent mixtures play a critical role in the carbohydrate conversions. Here acetonitrile (ACN), dimethylacetamide (DMAc) and GVL were added to [EMIM]Cl as a cosolvent to investigate cellulose hydrolysis in the mixed solvents. Figure 6 shows the TRS and glucose yields for cellulose hydrolysis with 1% biomass and 1% catalyst loading in 70:30 IL/ACN (left panel) and 70:30 IL/DMAc (right panel) using PIL/PSSA catalyst immobilized on silica membrane substrate at 130oC. It can be seen that TRS and glucose yields of over 95% and 30% have reached respectively in IL/ACN mixed solvent 19
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after 6 h of reaction. Replacing up to 30% of the expensive [EMIM]Cl with inexpensive ACN is possible. On the other hand, only about 78% TRS and 21% glucose yields were obtained in 70:30 IL/DMAc. Our results clearly demonstrate that solvent composition plays a critical role for cellulose deconstruction in support of our previously mentioned theoretical findings.
Cellulose Hydrolysis in 70:30 IL/GVL 1% cellulose, silica membrane, 130oC %TRS %Glucose
100
100
Cellulose Hydrolysis in 50:50 IL/GVL 1% cellulose, silica membrane, 130oC %TRS %Glucose
80 Sugar Yield (%)
80
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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60 40 20
60 40 20
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Figure 7
2
2.5
3
Reaction Time (h)
Reaction Time (h)
The TRS and glucose yields for cellulose hydrolysis in 70:30 (left) and 50:50
IL/GV mixed solvents at 130oC. The PIL/PSSA catalysts were immobilized on silica membrane substrates.
Since GVL is a green and environmental friendly solvent and has been shown to be effective for biomass processing
46
, cellulose hydrolysis in various ratios of IL/GVL mixtures
with 1% biomass and 1% catalyst loading were conducted at 130oC using PIL/PSSA catalysts immobilized on silica membranes. Figure 7 shows two representative TRS and glucose yields in 70:30 (left panel) and 50:50 (right panel) IL/GVL mixed solvents respectively. About 97% TRS and 30% glucose yields were achieved after 4 h of reaction in 70:30 IL/GVL solvent mixture. On
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the other hand, almost the same TRS (96%) and glucose (32%) yields were obtained only after 2.5 h of reaction in 50:50 IL/GVL mixed solvent. It can be seen that replacing IL with the inexpensive and green GVL solvent does not affect sugar yields for cellulose hydrolysis using our PIL/PSSA catalysts. Our results (not shown) demonstrate that over 90% TRS yield can be achieved by replacing almost 80% of IL with GVL. Moreover, the addition of GVL appears to speed up the hydrolysis reaction due likely to the reduced activation barrier for hydrolysis since the activation barrier is largely solvent induced as discussed previously. The reaction time to achieve maximum sugar yield decreased from 6 h in 100% IL to 4 h in 70:30 IL/GVL and down to 2.5 h in 50:50 IL/GVL solvent mixture. Our results agree with earlier studies 46, 47 using GVL as a solvent for biomass hydrolysis with mineral acids as catalysts. GVL appears to promote hydrolysis reaction leading to superior performance observed for this solvent. However, small amount of IL is needed in order to reach quantitative yield for cellulose hydrolysis. Further increase in GVL percentage to over 90% in the mixed solvent reduces the sugar yield. This is probably due to the relative poor solubility of cellulose in GVL compared to that of IL at the mild temperature of 130oC used here.
3.2 The effects of biomass loading on cellulose hydrolysis Since biomass processing at higher loading is more desirable, cellulose hydrolysis with 5% biomass loading and 1% catalyst loading was conducted using PIL/PSSA catalysts immobilized on silica membrane substrates. Figure 8 exhibits TRS and glucose yields for hydrolysis performed in 100% [EMIM]Cl at 130oC. The highest TRS yield achieved is only about 84.7% and 29.9% glucose yield after 5 h of reaction. Compared to the quantitative yield obtained with 1% biomass loading, higher biomass content appears to decrease the performance of our
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polymeric solid acid catalysts. The reduced TRS yield can be accounted by 10.6% of solids and 2.6% HMF measured. The solids consist of unreacted cellulose and small amount of humins formed. Further increase in the reaction time to 6 h leads to almost 14.5% of solids and 5.9% of HMF with TRS and glucose yields reduced to 78.3% and 25.2% respectively. The increase in solids is likely due to the further formation of the humins from glucose degradation product HMF. It is known that GVL can suppress the production of humins. Cellulose hydrolysis at 5% loading in IL/GVL mixed solvents were subsequently performed.
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Cellulose Hydrolysis in [EMIM]Cl 5% cellulose, silica membrane, 130oC
80
%TRS %Glucose
Sugar Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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60 40 20 0 1
Figure 8
2
3 4 5 Reaction Time (h)
6
The TRS and glucose yields for cellulose hydrolysis in [EMIM]Cl with 5%
cellulose loading at 130oC
Figure 9 shows TRS and glucose yields for 5% loading cellulose hydrolysis in 50:50 (left) and 20:80 (right) IL/GVL mixed solvents respectively using our PIL/PSSA catalysts immobilized on silica membrane substrates. With 50% GVL cosolvent, TRS and glucose yields of 92.7% and 32.2% were achieved after 5 h of reaction at 130oC. Clearly replacing 50% IL with GVL enhances the hydrolysis reaction and suppresses the formation of humins. Further increase 22
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in GVL to 80% leads to an even higher TRS of 94.5% with 30.2% glucose produced after about 6 h of reaction. Clearly longer hydrolysis reaction time is needed in order to completely hydrolyze cellulose at higher biomass loading at the same temperature and catalytic condition. However, the longer residence time leads to the degradation of glucose to HMF thereby reducing the TRS yield. That is probably why only 85% maximum TRS yield is achieved in 100% [EMIM]Cl. Partial replacing IL with GVL leads to the suppression of humin production which increases TRS yields at longer reaction time with continued hydrolysis reaction. That is also probably why the reaction time to reach maximum TRS yield increases from 3-4 h to about 5-6 h with 5% biomass loading in IL/GVL mixed solvents at the same catalyst loading and reaction temperature.
100
Cellulose Hydrolysis in 50:50 IL/GVL 100
Cellulose Hydrolysis in 20:80 IL/GVL 5% cellulose, silica membrane, 130°C
5% cellulose, silica membrane, 130°C
%TRS
%TRS 80
80
%Glucose
%Glucose
Sugar Yield (%)
Sugar Yield (%)
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Reaction Time (h)
Figure 9
2
The TRS and glucose yields for cellulose hydrolysis in 50:50 and 20:80 IL/GVL
mixed solvents with 5% cellulose loading at 130oC.
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3.3 Lignocellulosic biomass hydrolysis using PIL/PSSA catalysts Our PIL/PSSA catalysts have high catalytic activity and selectivity for cellulose hydrolysis achieving near quantitative TRS yields in IL, IL/ACN and IL/GVL mixed solvents at mild reaction temperature and in less than 10 hours. In addition, it has been shown that our catalysts can be reused many times without losing its catalytic activity. Further, up to 80% of the IL can be replaced by the green and environmental friendly solvent GVL without compromising the performance of the catalyst. In order to further evaluate our synthetic polymeric acid catalysts as a replacement for cellulase enzymes, acid, base and steam pretreated cornstover biomass were obtained from the NREL. Samples of the pretreated biomass cornstover were dried and percentage of biomass contents determined. As shown previously in Table 1, the percentages of dried biomass in three differently pretreated biomass materials are between 35-40%. As shown in Table 2, different pretreatment methods lead to different amount of glucan, xylan and lignin in the pretreated cornstover. It can be seen that alkali pretreated cornstover has the highest glucan content with the lowest amount of lignin whereas acid pretreated sample contains the least amount of xylan among the three pretreated cornstover biomass. The composition of steam pretreated cornstover resembles the closest to the original raw samples.
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100% [EMIM]Cl, silica membrane, 95°C 90
Acid treated steam treated Alkali treated
80 70 TRS Yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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60 50 40 30 20 10 0.5
Figure 10
1
1.5
2 2.5 Time (h)
3
3.5
4
TRS yields for acid, base and steam pretreated NREL cornstovers using our
PIL/PSSA catalysts immobilized on silica membrane substrate in 100% [EMIM]Cl at 95oC. PIL/PSSA catalysts were used to hydrolyze acid, base and steam pretreated NREL cornstover samples in 100% [EMIM]Cl at 130oC with 1% biomass loading and 3% catalyst loading. However, significant amount of humins were found in all the reactions. As a result, temperatures as low as 95oC was used to carry out the hydrolysis reaction. Figure 10 shows the TRS yields for the three pretreated biomass samples at 95oC. Here the TRS yield is reported as the percentage of total reducing sugar measured after hydrolysis with respect to the total dry biomass in the sample including lignin. It can be seen that the highest TRS yields were achieved after only about 3 h of reaction. The TRS yields reached over 77.8%, 76% and 55.9% for acid, base and steam pretreated samples respectively. These are close to the theoretical yields of 68.6%, 79.8%, and 62.7% for the three corresponding pretreatment conditions. For acidpretreated sample, a slight higher TRS yield than the theoretical value was obtained. This is due
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to the degradation of some of the glucose molecules during composition analysis using dilute sulfuric acid hydrolysis. Since the base-pretreated sample has the highest glucan content and lowest lignin content, highest TRS yield was obtained compared to other two pretreated cornstover samples. For steam-pretreated samples, glucan content is the lowest and lignin content the highest among the three pretreatment methods, a lower TRS yield was obtained during the catalytic conversion. The amount lignin in the biomass sample affects the efficiency and sugar yield for the hydrolysis of lignocellulosic biomass. All in all, it can be seen that our PIL/PSSA catalysts are very efficient in hydrolyzing these pretreated cornstover biomass samples. Hydrolysis experiments were also performed at even lower temperatures, but 95oC was found to be the optimal condition for the hydrolysis reaction.
80:20 IL/H2O, silica membrane, 95oC
70:30 IL/H2O, silica membrane, 95oC
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70 60 50 40
60 50 40
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Time (h)
Time (h)
Figure 11 TRS yields for acid, base and steam pretreated NREL cornstover biomass using our PIL/PSSA catalysts immobilized on silica substrate in 20:80 and 70:30 IL/H2O mixed solvents at 95oC. Since water is the cheapest and best solvent for biomass processing, it is desirable to have
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as much water as possible in the IL solvent mixtures. However, as discussed in our previous studies
27
, water can slow down the hydrolysis reaction dramatically and reduce the TRS yield
significantly. The presence of water also tends to deactivate the PIL/PSSA catalysts due to water molecule’s high affinity for proton. The amount of water in the solvent mixture needs to be carefully optimized. Figure 10 shows the TRS yields for acid, base and steam pretreated cornstover samples using our PIL/PSSA catalysts immobilized on silica membrane substrate in 80:20 (left panel) and 70:30 (right panel) IL/H2O mixed solvents for reactions conducted at 95oC. The catalyst loading was kept at 3%. It can be seen that the highest TRS yield was obtained with base pretreatment followed by acid and steam pretreatment in both 80:20 and 70:30 IL/H2O mixed solvents. The highest TRS yield was obtained after 7 h of hydrolysis reaction for all three pretreatment conditions in 80:20 IL/H2O mixed solvent. Indeed, the addition of water as a cosolvent slows down the hydrolysis reaction substantially compared to the reactions conducted in 100% [EMIM]Cl. The highest TRS yields were measured to be 72.6%, 65.2% and 56.1% for base, acid and steam pretreated samples respectively. These are slightly lower than the yields obtained in pure [EMIM[Cl but remain similar to the theoretical yields of 79.8%, 68.6% and 62.7% for the three corresponding pretreatment conditions. Here again shows that lignin content affects TRS yields of these differently pretreated lignocellulosic biomass samples. The effects are more pronounced IL/H2O mixed solvents. The highest TRS yield was obtained for the lowest lignin sample with base pretreatment. Increasing the amount of water in the solvent mixture to 70:30 IL/H2O, the highest TRS yields obtained are 69.1%, 60.1% and 54.4% after 17 h of reaction for base, acid and steam pretreated samples respectively. Again, it can be seen that water slows down the hydrolysis reaction significantly. As discussed previously from our earlier theoretical results
27
32-36, 43
, the activation barriers for cellulose
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hydrolysis and subsequent glucose conversion to HMF or isomerization to fructose are induced largely by the solvent due to solvent’s competition for proton. As water has high affinity for proton, the presence of water molecules will increase the barrier and slow down the reaction. However, water is also the reactant for the hydrolysis reaction, small amount of water is needed in order for the hydrolysis to occur. 80:20 IL/H2O, silica membrane, 100oC
70:30 IL/H2O, silica membrane, 100oC
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Time (h)
Figure 12
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Time (h)
TRS yields for acid, base and steam pretreated NREL cornstover biomass using
PIL/PSSA catalysts immobilized on silica substrate in 70:30 (left) and 80:20 (right) IL/H2O mixed solvents at 100oC.
As the reaction becomes significantly slow when the volume percentage of the water increases from 20 to 30, the reaction temperature was correspondingly increased by 5oC to 100oC and another 5oC to 105oC. Figures 12 and 13 show the TRS yields for the three pretreated biomass samples in 80:20 and 70:30 IL/H2O mixed solvents at 100oC and 105oC respectively. For reactions conducted in 70:30 IL/H2O mixed solvents, it can be seen that the highest TRS
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yields are obtained after 14 h of reaction for all three samples at 100oC and after only 9 h of reaction at 105oC. The reaction rate increases as temperature increases. However, the highest yields are only 60.2%, 63.8% and 45.5% for base, acid and steam pretreated biomass samples respectively in 100oC, lower compared to the reactions conducted at 95oC. TRS yields of 66.9%, 65.2% and 46.2% are obtained for the three corresponding pretreated samples at 105oC slightly higher than the reactions at 100oC, but noticeably lower than the corresponding yields at 95oC. This is due to the increase in the side reactions such as sugar degradation reactions. Our earlier results 34, 43 show that glucose dehydration to HMF has a higher activation barrier than that of the cellulose hydrolysis reaction. Higher temperature increases the rate of hydrolysis reaction, but also tends to favor the dehydration reaction. For example, at 105oC in 70:30 IL/H2O mixed solvent, significant amount of HMF (~20%) was obtained after 15 h of reaction for acid pretreated sample. The percentages of HMF produced in 70:30 IL/H2O mixed solvent at 100oC and 95oC were significantly less at only a few percent at the same reaction time. For hydrolysis reaction conducted in 80:20 IL/H2O mixed solvents, the highest TRS yields are obtained after only 6 to 8 h of reaction for all three temperatures tested. The highest TRS yields obtained are 64.9%, 63.2% and 53.1% for base, acid and steam pretreated samples at 100oC. The corresponding yields are 56.7%, 66.2% and 48.7% at 105oC. As discussed previously, increase in temperature will speed up the hydrolysis reaction, but also tends to reduce the TRS yield. More interestingly, higher temperature tends to favor the hydrolysis reaction for acid pretreated cornstover sample compared to base or steam pretreated samples as shown in Figures 11-13. Our results indicate that the PIL/PSSA catalysts are effective in hydrolyzing pretreated cornstover samples in IL/H2O mixed solvents. Near quantitative TRS yields were obtained with
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little product degradation at mild temperatures. As demonstrated in our earlier work
27
, PIL is
necessary to achieve biomass hydrolysis at high selectivity and efficiency. The solubilizing effects of PIL enhances the catalytic activity and overall sugar yields. 80:20 IL/H2O, silica membrane, 105oC
70:30 IL/H2O, silica membrane, 105oC
Acid treated steam treated Alkali treated
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0
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Figure 13
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TRS yields for acid, base and steam pretreated NREL cornstover biomass using
PIL/PSSA catalysts immobilized on silica membrane substrate in 20:80 (left) panel and 30:70 (right panel) IL/H2O mixed solvents at 105oC.
Since the PSSA/PIL catalyst is a general type of the BrØsted acid catalyst, it can be used to catalyze many other acid-catalyzed reactions. Our future work will focus on optimizing this catalyst for other sugar reactions including dehydration reaction and isomerization reaction. Since it is a solid acid catalyst that can be reused and regenerated, it is a green and environmental friendly catalyst that has potential applications beyond carbohydrate or lignocellulosic materials.
4.
Conclusions
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Polymeric solid acid PIL/PSSA catalysts immobilized on substrates have been successfully synthesized and evaluated for cellulose hydrolysis in IL/cosolvent mixtures at mild temperature. Optimized catalysts demonstrate over 95% TRS and 20% glucose yields in ILs. TRS yield of ~ 95% were also achieved by replacing up to 80% with the inexpensive green solvent GVL at 5% biomass loading. Hydrolysis with pretreated cornstover samples from NREL were conducted in IL/H2O mixed solvents at temperatures close to 100oC. Near quantitative TRS yields were obtained. Our results demonstrate that our PIL/PSSA catalysts are effective and efficient in processing lignocellulosic biomass. 5.
Acknowledgement
Funding from NSF (CBET 1264896) are gratefully acknowledged. The authors would also like to thank Dan Schell from the National Renewable Energy Laboratory for providing pretreated cornstover samples. Corn-stover composition analysis by Dr. Benqun Qi is also appreciated.
6.
(1)
References
Himmel, M. E.; Ding, S. Y.; Johnson, D. K.; Adney, W. S.; Nimlos, M. R.; Brady, J. W.;
Foust, T. D., Biomass recalcitrance: Engineering plants and enzymes for biofuels production. Science 2007, 315, 804-807. (2)
Qian, X. H.; Ding, S. Y.; Nimlos, M. R.; Johnson, D. K.; Himmel, M. E., Atomic and
electronic structures of molecular crystalline cellulose I beta: A first-principles investigation. Macromolecules 2005, 38, 10580-10589. (3)
Nishiyama, Y.; Langan, P.; Chanzy, H., Crystal structure and hydrogen-bonding system in
cellulose 1 beta from synchrotron X-ray and neutron fiber diffraction. JACS 2002, 124, 90749082. 31
ACS Paragon Plus Environment
Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(4)
Qian, X. H., The effect of cooperativity on hydrogen bonding interactions in native
cellulose I beta from ab initio molecular dynamics simulations. Mol. Simul. 2008, 34, 183-191. (5)
Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y. Y.; Holtzapple, M.; Ladisch, M.,
Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 2005, 96, 673-686. (6)
Knappert, D.; Grethlein, H.; Converse, A., Partial acid hydrolysis of cellulosic materials
as a pretreatment for enzymic hydrolysis. Biotechnol. Bioeng. 1980, 22, 1449-1463. (7)
Knappert, D. R. Partial acid hydrolysis pretreatment for enzymic hydrolysis of cellulose:
a process development study for ethanol production. 1981. (8)
Hsu, T. A., Pretreatment of biomass. In Handbook on Bioenthanol Production and
Ultlization, Wyman, C., Ed. Taylor & Francis: Washington, D.C., 1996. (9)
Clarke, M. A.; Edye, L. A.; Eggleston, G., Sucrose decomposition in aqueous solution,
and losses in sugar manufacture and refining. In Advances in Carbohydrate Chemistry and Biochemistry 1997, 52, 441-470. (10)
Heitz, M.; Capekmenard, E.; Koeberle, P. G.; Gagne, J.; Chornet, E.; Overend, R. P.;
Taylor, J. D.; Yu, E., FRACTIONATION OF POPULUS-TREMULOIDES AT THE PILOTPLANT SCALE - OPTIMIZATION OF STEAM PRETREATMENT CONDITIONS USING THE STAKE-II TECHNOLOGY. Bioresour. Technol. 1991, 35, 23-32. (11)
Saddler, J. N.; Ramos, L. P.; Breuil, C., Steam pretreatment of lignocellulosic residues. In
Bioconversion of Forest and Agricultural Plant Residues, Saddler, J. N., Ed. CAB International: Oxford, 1993; pp 73-92. (12)
Dale, B. E.; Moreira, M. J., A FREEZE-EXPLOSION TECHNIQUE FOR
INCREASING CELLULOSE HYDROLYSIS. Biotechnol. Bioeng. 1982, 31-43.
32
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Page 32 of 37
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(13)
Dale, B. E.; Leong, C. K.; Pham, T. K.; Esquivel, V. M.; Rios, I.; Latimer, V. M.,
Hydrolysis of lignocellulosics at low enzyme levels: Application of the AFEX process. Bioresour. Technol. 1996, 56, 111-116. (14)
Chang, V. S.; Burr, B.; Holtzapple, M. T., Lime pretreatment of switchgrass. Appl.
Biochem. Biotechnol 1997, 63-5, 3-19. (15)
Chang, V. S.; Nagwani, M.; Holtzapple, M. T., Lime pretreatment of crop residues
bagasse and wheat straw. Appl. Biochem. Biotechnol. 1998, 74, 135-159. (16)
Wyman, C. E.; Dale, B. E.; Elander, R. T.; Holtzapple, M.; Ladisch, M. R.; Lee, Y. Y.,
Coordinated development of leading biomass pretreatment technologies. Bioresour. Technol. 2005, 96, 1959-1966. (17)
Esteghlalian, A. R.; Hashimoto, A. G.; Fenske, J. J.; Penner, M. H., Modeling and
optimization of the dilute sulfuric acid pretreatment of corn stover, poplar and switchgrass. Bioresour. Technol. 1997, 59, 129-136. (18)
Shiang, M.; Linden, J. C.; Mohagheghi, A.; Grohmann, K.; Himmel, M. E.,
REGULATION OF CELLULASE SYNTHESIS IN ACIDOTHERMUS-CELLULOLYTICUS. Biotechnol. Progr. 1991, 7, 315-322. (19)
Kim, S. B.; Lee, Y. Y., Kinetics in acid-catalyzed hydrolysis of hardwood hemicellulose.
Biotechnol. Bioeng. Symp. 1987, 17, 71-. (20)
Maloney, M. T.; Chapman, T. W.; Baker, A. J., DILUTE ACID-HYDROLYSIS OF
PAPER BIRCH - KINETICS STUDIES OF XYLAN AND ACETYL-GROUP HYDROLYSIS. Biotechnol. Bioeng. 1985, 27, 355-361. (21)
Mayans, O.; Scott, M.; Connerton, I.; Gravesen, T.; Benen, J.; Visser, J.; Pickersgill, R.;
Jenkins, J., Two crystal structures of pectin lyase A from Aspergillus reveal a pH driven
33
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conformational change and striking divergence in the substrate-binding clefts of pectin and pectate lyases. Structure 1997, 5, 677-689. (22)
Chen, R.; Lee, Y. Y.; Torget, R., Kinetic and modeling investigation on two-stage reverse-
flow reactor as applied to dilute-acid pretreatment of agricultural residues. Appl. Biochem. Biotechnol. 1996, 57/58, (Seventeenth Symposium on Biotechnology for Fuels and Chemicals, 1995), 133-146. (23)
Liu, C. G.; Wyman, C. E., The effect of flow rate of very dilute sulfuric acid on xylan,
lignin, and total mass removal from corn stover. Ind. Eng. Chem. Res. 2004, 43, 2781-2788. (24)
Keller, F. A.; Hamilton, J. E.; Nguyen, Q. A., Microbial pretreatment of biomass.
Potential for reducing severity of thermochemical biomass pretreatment. Appl. Biochem. Biotechnol. 2003, 105-108, 27-41. (25)
Torget, R.; Walter, P.; Himmel, M.; Grohmann, K., Dilute-acid pretreatment of corn
residues and short-rotation woody crops. Appl. Biochem. Biotechnol. 1991, 28-29, 75-86. (26)
Torget, R.; Walter, P.; Himmel, M.; Grohmann, K., DILUTE-ACID PRETREATMENT
OF CORN RESIDUES AND SHORT-ROTATION WOODY CROPS. Appl. Biochem. Biotechnol. 1991, 28-9, 75-86. (27)
Qian, X.; Lei, J.; Wickramasinghe, S. R., Novel polymeric solid acid catalysts for
cellulose hydrolysis. RSC Advances 2013, 3, 24280-24287. (28)
Alonso, D. M.; Gallo, J. M. R.; Mellmer, M. A.; Wettstein, S. G.; Dumesic, J. A., Direct
conversion of cellulose to levulinic acid and gamma-valerolactone using solid acid catalysts. Catal. Sci. Technol. 2013, 3, 927-931. (29)
Amarasekara, A. S.; Owereh, O. S., Synthesis of a sulfonic acid functionalized acidic
ionic liquid modified silica catalyst and applications in the hydrolysis of cellulose. Catal.
34
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Page 34 of 37
Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Industrial & Engineering Chemistry Research
Commun. 2010, 11, 1072-1075. (30)
Rinaldi, R.; Palkovits, R.; Schüth, F., Depolymerization of Cellulose Using Solid
Catalysts in Ionic Liquids. Angew. Chem. Int. Ed. 2008, 47, 8047-8050. (31)
Zhang, Y. T.; Du, H. B.; Qian, X. H.; Chen, E. Y. X., Ionic Liquid-Water Mixtures:
Enhanced K-w for Efficient Cellulosic Biomass Conversion. Energy & Fuels 2010, 24, 24102417. (32)
Dong, H. T.; Nimlos, M. R.; Himmel, M. E.; Johnson, D. K.; Qian, X. H., The Effects of
Water on beta-D-Xylose Condensation Reactions. J. Phys. Chem. A 2009, 113, 8577-8585. (33)
Liu, D. J.; Nimlos, M. R.; Johnson, D. K.; Himmel, M. E.; Qian, X. H., Free Energy
Landscape for Glucose Condensation Reactions. J. Phys. Chem. A 2010, 114, 12936-12944. (34)
Qian, X., Mechanisms and energetics for acid catalyzed β-D-glucose conversion to 5-
hydroxymethylfurfurl. J. Phys. Chem. A 2011, 115, 11740-11748. (35)
Qian, X.; Liu, D., Free energy landscape for glucose condensation and dehydration
reactions in dimethyl sulfoxide and the effects of solvent. Carbohydr. Res. 2014, 388, (Supplement C), 50-60. (36)
Qian, X. H.; Wei, X. F., Glucose Isomerization to Fructose from ab Initio Molecular
Dynamics Simulations. J. Phys. Chem. B 2012, 116, 10898-10904. (37)
Mellmer, M. A.; Sener, C.; Gallo, J. M. R.; Luterbacher, J. S.; Alonso, D. M.; Dumesic, J.
A., Solvent effects in acid‐catalyzed biomass conversion reactions. Angew. Chem. Int. Ed. 2014, 53, 11872-11875. (38)
Fu, D.; Mazza, G., Aqueous ionic liquid pretreatment of straw. Bioresour. Technol. 2011,
102, 7008-7011. (39)
Fu, D.; Mazza, G., Optimization of processing conditions for the pretreatment of wheat
35
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straw using aqueous ionic liquid. Bioresour. Technol. 2011, 102, 8003-8010. (40)
Bhagia, S.; Nunez, A.; Wyman, C. E.; Kumar, R., Robustness of two-step acid hydrolysis
procedure for composition analysis of poplar. Bioresour. Technol. 2016, 216, 1077-1082. (41)
Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D.; Crocker, D.
Determination of structural carbohydates and lignin in biomass; National Renewable Energy Laboratory, CO, USA, 2008. (42)
Miller, G. L., Use of dinitrosalicylic acid reagent for determination of reducing sugar.
Anal. Chem. 1959, 31, 426-428. (43)
Qian, X. H., Mechanisms and Energetics for Bronsted Acid-Catalyzed Glucose
Condensation, Dehydration and Isomerization Reactions. Top. in Catal. 2012, 55, 218-226. (44)
Qian, X. H., Free Energy Surface for Bronsted Acid-Catalyzed Glucose Ring-Opening in
Aqueous Solution. J. Phys. Chem. B 2013, 117, 11460-11465. (45)
Qian, X. H.; Johnson, D. K.; Himmel, M. E.; Nimlos, M. R., The role of hydrogen-
bonding interactions in acidic sugar reaction pathways. Carbohydr. Res. 2010, 345, 1945-1951. (46)
Mellmer, M. A.; Alonso, D. M.; Luterbacher, J. S.; Gallo, J. M. R.; Dumesic, J. A.,
Effects of γ-valerolactone in hydrolysis of lignocellulosic biomass to monosaccharides. Green Chem. 2014, 16, 4659-4662. (47)
Motagamwala, A. H.; Won, W.; Maravelias, C. T.; Dumesic, J. A., An engineered solvent
system for sugar production from lignocellulosic biomass using biomass derived γ-valerolactone. Green Chem. 2016, 18, 5756-5763.
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TOC 80 Acid treated Steam treated Alkali treated
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TRS Yield (%)
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60 50 40 30 20 2
3
4
5
6
7
8
Time (h)
TRS yields for acid, base and steam pretreated NREL cornstover biomass using our PIL/PSSA catalysts immobilized on silica substrate in 20:80 H2O/[EMIM]Cl mixed solvents at 95oC. TRS yields were calculated based on the total dry biomass used for the conversion.
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