Electronic Supplementary Material (ESI) for Energy & Environmental Science This journal is © The Royal Society of Chemistry 2012
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Supporting Information A Mechanistic Model of Fast Pyrolysis of Glucose-Based Carbohydrates to Predict Bio-oil Composition R. Vinu and Linda J. Broadbelt* Department of Chemical and Biological Engineering, Northwestern University 2145 Sheridan Road, Evanston, IL 60208, United States
*
Corresponding Author. Tel.: +1-847-491-5351; Fax: +1-847-491-3728.
E-mail:
[email protected] (L. J. Broadbelt)
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OH
O
OH
O
HO HO
O
HO
OH O
OH
HO
O
O
OH
OH
OH
O
O
OH
OH
OH
OH O
O
OH
HO
O OH
OH
O OH
HO HO
O
HO
OH
OH
OH
O
HO
OH
O OH
O
OH
OH
OH
O
HO HO
O
OH HO
OH
OH
O
O OH
OH
OH HO
O HO HO
OH
OH
OH
OH HO
OH
OH
O
HO HO
OH O
OH
O
OH O
OH OH
O O
HO
OH HO HO
OH
OH
OH HO
O
HO HO
OH
OH
OH
OH HO
O
OH
OH
Scheme SI 1. Mechanism of formation of various dimeric products, anhydrosugars and glucose from cellobiose through dehydration, retro Diels-Alder and glycosidic bond cleavage reactions. 1,2-Dehydration and retro Diels-Alder reactions involve the formation of one molecule of water and glycolaldehyde, respectively. All cellobiose-derived dimers can also react with water to cleave the glycosidic linkage and form the respective monomeric compounds.
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Electronic Supplementary Material (ESI) for Energy & Environmental Science This journal is © The Royal Society of Chemistry 2012
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SI Appendix A. Continuous Distribution Kinetic Rate Equations for Chain Reactions. Reactions of the cellulosic chain in Schemes 1 and 2 can be modeled using continuous distribution kinetics which takes into account the chain length dependency of the rates of polymeric species that are undergoing pyrolytic cleavage. Continuous distribution kinetics has been widely used to model polymerization and degradation of synthetic polymers.63-65 The chain reactions involved in cellulose fast pyrolysis model can be broadly classified as (i) initiation (random chain cleavage), (ii) depropagation and end-chain initiation (specific chain cleavage), and (iii) hydrolysis. Rate equations for the above reaction classes are given below. Detailed derivation of these equations can be found elsewhere.63,65 1. Initiation kinit ( x ) C ( x) CLVG ( x x ') C '( x ')
(A.1)
Reaction (I) in Scheme 1 falls under this category. This reaction involves the cleavage of glycosidic bond at any random position in the cellulose chain, C, of chain length x, resulting in the formation of a cellulose chain terminated by an LG end, C LVG, of chain length x-x’, and another cellulose chain C’ of chain length x’. The rate of this reaction is linearly dependent on chain length, kinit(x) = kinit*x.65 The even distribution of products along all x’≤x is given by the random stoichiometric kernel, ΩRand(x’,x) = 1/x.65 The rate equations, after applying moments on the initial population balance equation, are given by
dC ( m ) kinit C ( m1) dt
(A.2)
m dCLVG k dC '( m) init C ( m1) dt dt m 1
(A.3)
The moments, C(0), C(1) and C(2) denote molar concentration, mass concentration, and second moment, respectively. Number average (Mn) and weight average molecular weight (Mw) of
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cellulose are given by C(1)/C(0) and C(2)/C(1), respectively. The Saidel-Katz approximation was utilized to close the moments for the third moment, C(3), as shown in Eq. (A.4).64
C
(3)
C (2)C (2) C (2)C (1) 2 C (1) C (0)
(A.4)
2. Depropagation and end-chain initiation deprop C ( x) CLVG ( x 1) LVG
k
(A.5)
This reaction involves unzipping of monomers (levoglucosan or glucose) from either ends of the cellulose chain according to reactions (II), (III) and (IV) in Scheme 1. As this reaction results in continuous evolution of monomers of unit chain length, the product distribution follows a Dirac delta function given by, ΩDeprop(x-1, x) = δ(x-1).63 The rate equations are as follows.
dC ( m) kdeprop C ( m ) dt
(A.6)
( m) m m dCLVG kdeprop (1) j C ( m j ) dt j 0 j
(A.7)
d [ LVG] kdeprop C (0) dt
(A.8)
3. Dehydration or retro-Diels-Alder reaction anywhere in the cellulosic chain Dehyd rDA C ( x) Cmid ( x xH2O / xGA ) H 2O/Glycolaldehyde (GA)
k
/k
(A.9)
Dehydration and subsequent retro Diels-Alder reaction in the dehydrated unit of the cellulose chain result in the formation of glycolaldehyde (reactions (i) and (ii) in Scheme 2). The rate equations for this class of reactions are the same as that of depropagation, except that these reactions mostly take place anywhere in the middle of the cellulose chain and involve a product whose molecular weight is less than that of the monomer. Hence, the ratio of molecular weight
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of the evolved product with respect to the monomer (xH2O or xGA) is included in the rate expressions.
dC ( m ) kDehyd C ( m ) dt
(A.10)
(m) m m dCmid kDehyd ( xH2O ) j C ( m j ) dt j 0 j
(A.11)
d [ H 2O ] kDehyd C (0) dt
(A.12)
4. Hydrolysis of cellulosic chain to form glucose (G) from chain end k'
Hyd Hyd C ( x) H 2O [C ( x) H 2O]* C '( x xG ) G
k
(A.13)
Hydrolysis at either ends of the cellulose chain results in the formation of glucose according to reaction (V) in Scheme 1. Moment solutions do not exist for bimolecular reactions involving the formation of two products. Hence, in deriving the rate equations for reaction (A.13), we need to assume an intermediate short-lived species [C(x)–H2O]* for which the pseudo-steady-state approximation (PSSA) can be applied. The rate coefficient k’Hyd was only utilized for derivation purposes, and the final rate equations in terms of final species concentrations and kHyd are given by the following.
dC ( m ) kHyd C ( m ) [ H 2O] dt
d [ H 2O] d [G] kHyd C (0) [ H 2O] dt dt
q m m q dC '( m) kHyd ( xG ) j ( xH 2O ) p C ( q p ) [ H 2O] dt j 0 j p 0 p
(A.14)
(A.15)
(A.16)
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Table SI 1. Size of fast pyrolysis model in terms of the number of species and reactions for cellulose, maltohexaose, cellobiose and glucose. Carbohydrate
No. of species
No. of reactions
Cellulose/Maltohexaose
52
99
Cellobiose
49
65
Glucose
40
41
Sum of yields of major products, wt.%
100
Expt. Model 80
60
40
20
0
400
450
500 550 o Temperature, C
600
650
Figure SI 1. Comparison of total yield of the major products (levoglucosan + glycolaldehyde + formic acid + HMF + furfural) obtained from the model with experiments at different temperatures.
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7 32 30
Mw
4
Molecular weight, x10 g mol
-1
28 26 24 22
OH
20
OH HO HO
18 16
Mn
14
O
OH
O
O
O
HO OH
OH
HO O
21
O
12
O OH
10
HO HO
8
O
O O
HO OH
OH
6 4 2 0
0.01
0.1
1
10
100
1000
10000
Pyrolysis time, millisecond
Figure SI 2. Model time evolution of number average (Mn) and weight average (Mw) molecular weights of cellulose at 500 oC. The predominant species during 100 ms and 600 – 700 ms are shown in the figure. 10000 1000 100
k, s
-1
10 1 0.1 0.01 1E-3
Levoglucosan from glucose (17) Dehydration (10),(21) Grob fragmentation (5),(9) Retro Aldol reaction of glucose (2) Glucose ring opening (1) Glucose ring flipping (18) Retro Diels-Alder (16) 1.10
1.15
1.20
1.25
1.30
1.35
1.40
1.45
1.50
1000/T, 1/K
Figure SI 3. Variation of rate coefficients with temperature for selected reactions that compete with dehydration of glucose to form levoglucosan. The reaction indices refer to Scheme 3.
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Polymer + melt phase LMWP yield, wt.%
100 90 80
No change in k k(ring-flip) doubled k(ring-flip) halved k(ring-opening) doubled k(ring-opening) halved
70 60 50 40 30 20
Char + anhydro sugars
10 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
Pyrolysis time, s
Figure SI 4. Sensitivity of model time evolution of the remaining solid fraction (polymer + melt phase) after reaction for various values of rate constants of ring opening and ring flipping at 500 o
C.
60 Levoglucosan
No change in k k(ring-flip) doubled k(ring-flip) halved k(ring-opening) doubled k(ring-opening) halved
55 50
Yield, wt.%
12 10 8
Formic acid
Glycolaldehyde
Char
6 4 2
HMF Furfural
0
Figure SI 5. Sensitivity of model yields of the major products for various values of rate constants of ring opening and ring flipping at 500 oC.
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12
Glycolaldehyde
0.10 s
Formic acid CHAR Levoglucosan 2-Hydroxy-3oxobutanal
Product yield, wt.%
10
8
HMF 6
MW = 86 4
Furfural Formaldehyde Glyoxal Acetaldehyde
2
0 0
1
2
3
4
5
6
7
8
9
10
Pyrolysis time, s
Figure SI 6. Model time evolution of various fast pyrolysis products from glucose at 500 oC.
