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Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis
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Proceedings of the Combustion Institute 000 (2016) 1–8 www.elsevier.com/locate/proci
Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis Erwei Leng, Yang Wang, Xun Gong∗, Biao Zhang, Yang Zhang, Minghou Xu∗∗ State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037, Luoyu Road, Wuhan 430074, China Received 4 December 2015; accepted 27 June 2016 Available online xxx
Abstract The influence of KCl and CaCl2 on the primary reactions of cellulose pyrolysis is studied using a wiremesh reactor from 250 °C to 600 °C, focusing on the reaction intermediates. A pre- column derivatization with benzoyl chloride prior to HPLC analysis is applied for the quantification of anhydro-sugars (levoglucosan, cellobiosan, maltosan) from pyrolysis. At low temperatures, the additions of inorganics salts, especially CaCl2 , weakens hydrogen bonds, resulting in high yields of levoglucosan and cellobiosan from the cleavage of glycosidic bonds rather than from dehydration reactions. At elevated temperatures, dehydration reactions in the sugar units are mainly responsible for the destruction of sugar rings followed by the scission of pyran rings, leading to the weight-loss of CaCl2 -loaded cellulose in the form of low molecular weight organic species. Meanwhile, accumulated unsaturated structures suppress the cleavage of glycosidic bonds, leading to the formation of char. However, KCl appears to catalyze the cleavage of glycosidic bonds or the scission of pyran rings directly, which perhaps occurs through a homolytic mechanism, leading to low molecular weight species. Furthermore, maltosan is shown to be a secondary product and is catalyzed by KCl and CaCl2 indirectly through the repolymerization of levoglucosan in the solid phase. A modified mechanism is also proposed regarding cellulose pyrolysis and the primary catalysis of KCl and CaCl2 . © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Cellulose; Primary reactions; Reaction intermediates; KCl; CaCl2
1. Introduction Biomass, as a superior alternative energy to fossil fuels, is gaining increasing attention. Much re∗
Corresponding author. Fax: +86 27 87545526. Corresponding author. Fax: +86 27 87544779. E-mail addresses: [email protected] (X. Gong), [email protected] (M. Xu). ∗∗
search has been performed on its thermochemical conversions (combustion, gasification, pyrolysis etc.), during which pyrolysis is always the initial stage [1]. Therefore, the mechanism of pyrolysis is crucial for biomass industrialization. Alkali and alkali earth metals (AAEM), such as K, Ca, Na, and Mg, broadly exist in biomass and strongly impact the pyrolysis of cellulose [2]. Previous works [2–4] have shown improved yields of char and low
http://dx.doi.org/10.1016/j.proci.2016.06.167 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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molecular species at the expense of anhydro-sugars. However, alkalis present stronger cracking catalysis, leading to more low molecular species [5] compared to alkaline earth metals [4]. As a commonly accepted mechanism, the pyrolysis of cellulose goes through an intermediate state and further turns into gases, volatiles and char [6]. The intermediates are critical during fast pyrolysis, which mostly consist of anhydro-sugars as well as a few sugars [7]. A recent work using a drop-tube/fixed-bed quartz reactor [8] proposed that MgCl2 is more effective in accelerating the formation of intermediates at low temperatures than NaCl, which may be responsible for its ability to sharply reduce the temperature of cellulose decomposition, as observed in other works [4]. However, catalysis effects at higher temperatures (>300 °C) have not been discussed in detail, and secondary reactions may lead to incorrect conclusions [2]. Py-GC–MS have provided some meaningful results [2], but have failed to analyze the evolution of intermediates in char during pyrolysis. Therefore, in this study, KCl and CaCl2 were wet-impregnated into cellulose and fast pyrolysis of three samples was carried out in a wire-mesh reactor (WMR) to minimize secondary reactions. Moreover, a pre-column derivatization with benzoyl chloride before HPLC-MS was applied in anhydro-sugars analysis for the first time for quantification. The objective of this study is to explore the primary catalysis of KCl and CaCl2 on the formation and characteristics of intermediates during fast pyrolysis of cellulose. 2. Experiments 2.1. Materials and fast pyrolysis experiments Microcrystalline cellulose (Avicel PH-102, FMC Biopolymer) with a size between 106 and 150 μm was used in this study. Prior to use, cellulose was washed by deionized water to remove any water-soluble portions and dried at 105 °C. Then, both KCl-loaded and CaCl2 -loaded samples were prepared through wet impregnation with a loading level of 0.025 mol for each mol of glucose. The details of the impregnation are described elsewhere [8]. Pyrolysis experiments were carried out on a WMR used by Gong et al. [7] to extrude the primary catalysis of KCl and CaCl2 . Each time, ∼8 mg of cellulose sample was sandwiched between two wire-mesh layers for fast pyrolysis (100 °C/s) with various holding times at the final temperature (250– 600 °C). During pyrolysis, a stream of nitrogen was passing through the sample holder at a linear gas velocity of 0.1 m/s (4 L/min) to sweep out the generated volatiles, which was quenched by liquid nitrogen and collected in a Teflon filled trap. Then, a mixture of chloroform and methanol (4:1 by volume) was used to wash the tar off the trap, which
was evaporated at 35 °C to determine the yield of non-evaporable liquid products on a weight basis. Each experiment was conducted in triplicate. The triplicate experiments were treated and mixed well for the analyses below. More details regarding the WMR are described elsewhere [7]. 2.2. Characterization of solid and liquid products 2.2.1. Yield of water-soluble intermediates in the solid residues The solid residues on the sample holder were washed by deionized water to obtain “watersoluble intermediates” after filtration at ambient temperature. The elemental compositions of the solid residues (before washing) were analyzed by an elemental analyzer (Elementar Vario Micro cube), while the total organic carbon (TOC) of the watersoluble intermediates was analyzed by a TOC analyzer (Analytikjena multi N/C 2100). Then, the yield of it was obtained by normalizing the TOC in the water-soluble intermediates to the total carbon in the loaded cellulose. 2.2.2. Yield of water-soluble liquid The trap loaded with primary volatiles was thoroughly washed out to obtain a solution of all anhydro-sugars in the condensed liquid products, which is referred to hereafter as “water-soluble liquid.” The water-soluble liquids were analyzed by the same TOC analyzer. Then, the TOC in the water-soluble liquid was normalized to the total carbon in the loaded cellulose, resulting in the yield of water-soluble liquid. 2.3. Derivatization of anhydro-sugars with benzoyl chloride for quantification Usually, HPLC is used to characterize anhydrosugars from cellulose pyrolysis. However, it is difficult and inaccurate for conventional HPLC analysis to quantify anhydro-sugars from lowly loaded cellulose. In this work, a pre-column derivatization with benzoyl chloride for ultraviolet-absorption was first applied in the analysis of anhydro-sugars, according to its previous applications on sugars and sugar alcohols [9]. A 50-μl liquid sample was combined with 40 μl of an undiluted benzoyl chloride solution (Aladdin), mixed slightly, combined with 160-μl NaOH (7 mol/L), and mixed well. After 15 min, 80-μl H3 PO4 (6 mol/L) and 400-μl ethyl acetate were added with mixing, followed by centrifugation for 10 min. Then, 200 μl of the supernatant was extracted and analyzed by HPLC (Agilent 1100 LC) with a UV detector. A Waters Xterra RP C18 separation column was used for linear gradient elution at 30 °C with 75/25 (v/v) methanol/water before 30 min and 90/10 (v/v) methanol/water from 30 to 45 min as the mobile phase. The UV detection wavelength was 232 nm.
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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Table 1 Analytical data for the test compounds. Analyte
tR a
y=ax+b
R2
DLb
Levoglucosan Cellobiosan Maltosan
9.9 20.5 25.2
y=32.5x−0.1 y=26.6x−0.5 y=35.1x−0.4
0.998 0.998 0.991
80 100 100
a
retention time (min). detection Limit (ng/ml): three times the signal-tonoise ratio. b
Fig. 2. Yields of water-soluble intermediates in solid residues at various temperatures.
the initiation temperature of weight-loss, as well as the final temperature of pyrolysis, and CaCl2 performs more efficient catalysis. In addition, higher yields of char and low molecular weight (MW) species (water, gases, formic acid, etc.) are also expected after the addition of KCl and CaCl2 , as proposed in a previous work [2]. Nevertheless, KCl is more efficient for the formation of low MW species, while the catalysis effects on char formation are similar, with the same yield of ∼12.8% at above 500 °C. Relative to complete conversion, it is expected that the yields of solid products will all increase slightly at lower temperatures because low temperatures promote exothermic reactions of char formation and restrain endothermic reactions producing pyrolysis vapors [10]. However, temperature nearly does not influence the yields of non-evaporable liquid products. Fig. 1. Yields of solid products (a) and non-evaporable liquids (b) as a function of pyrolysis temperature at zero holding (solid symbols) and complete conversion (hollow symbols).
Nine mixed standard solutions of three anhydrosugars (levoglucosan, cellobiosan, and maltosan) in the same concentration range (4–1000 ppm) were analyzed. Table 1 shows the analytical data. 3. Results and discussion 3.1. Yields of liquid and solid products from pyrolysis Figure 1 shows the yields of solid and nonevaporable liquid products of the three samples during pyrolysis on the WMR; complete conversion means that the holding time is long enough for a fixed yield of solid products at a constant temperature. As shown, both KCl and CaCl2 reduce
3.2. Characteristics of solid products To further understand the catalysis effects of KCl and CaCl2 , various analyses were performed on the solid and liquid products. The yields of water-soluble intermediates are presented in Fig. 2 based on the total carbon in loaded cellulose. A comparison of Fig. 1a and Fig. 2 shows that the fastest weight-loss point is consistent with the maximum yield of water-soluble intermediates for all samples. Therefore, the volatilization of intermediates is always primarily responsible for the weightloss of cellulose. Moreover, the formation of intermediates at low temperatures is improved by inorganic species, especially by CaCl2 (a maximum of 26% at 350 °C), as shown in Fig. 2. This results in the reduced initiation temperature of weightloss, which is consistent with previous works on TG [4,11]. Furthermore, the elemental compositions of solid products collected after pyrolysis are presented in a Van Krevelen diagram [12] (Fig. 3). It
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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Fig. 3. Van Krevelen diagram for the solid residues at various temperatures.
is shown that the elemental compositions of raw cellulose do not change significantly with increasing temperature when secondary reactions are almost averted on the WMR. This is notably different from a previous fast pyrolysis work using a droptube/fixed-bed reactor with considerable secondary reactions present [13]. Usually, two main competitive pathways (dehydration and depolymerization) exist during cellulose pyrolysis [14]. However, the result reveals that considerable dehydration does not seem to occur inside the char during fast pyrolysis. Mamleev et al. [15] concluded that dehydration is a fast secondary reaction with respect to cellulose depolymerization. Therefore, in conventional reactors, dehydration reactions mainly come from the secondary decomposition and polymerization of primary products. Pyrolysis of the two impregnated samples shows that the elemental compositions obviously shift along the dehydration line. Then, it is plausible that KCl and CaCl2 promote dehydration reactions in primary catalysis, and CaCl2 seems to be much more efficient. The quantitative results of the yields (as percentages of the total carbon in raw cellulose) and selectivities (as percentages of the total carbon in water-soluble intermediates) of levoglucosan and cellobiosan in the solid residues are presented in Fig. 4. At low temperatures, both inorganic salts accelerate the cleavage of glycosidic bonds, resulting in higher yields of levoglucosan and cellobiosan. For example, CaCl2 -loaded and KCl-loaded cellulose respectively lead to levoglucosan yields of 1.59% and 0.21%, and cellobiosan yields of 0.13% and 0.05%, compared to 0.15% and 0.03% from raw cellulose at 300 °C. This is consistent with the enhanced formation of intermediates in the solid residues. The explanation is that coordination between oxygen atoms and AAEM [4,8,16] can weaken the hydrogen bonds during drying after wet-impregnation and pyrolysis [13], resulting in the instability of glycosidic bonds and hydroxyl groups, which probably promote the cleavage of
Fig. 4. Yields and selectivities of anhydro-sugars in water-soluble intermediates at various temperatures. Solid symbols: levoglucosan; hollow symbols: cellobiosan.
glycosidic bonds and dehydration reactions. Furthermore, Ca2+ , which is more active than K+ as a Lewis acid, is more effective in catalysis. Nevertheless, there is minimal dehydration among the impregnated samples below 300 °C, as shown in Fig. 3, which reveals that the cleavage of glycosidic bonds is much faster than dehydration reactions [2]. The destruction of sugar structures is also improved by K+ and Ca2+ , leading to more nonsaccharides, mostly above 300 °C. As shown in Fig. 4b, the selectivities of levoglucosan from both impregnated samples follow a similar tendency as those from raw cellulose at the initial temperature, despite the advanced depolymerization of cellulose by CaCl2 . Subsequently, they decline sharply above 300 °C, which reveals the active destruction of sugar structures. According to Fig. 3, the prominent dehydration of CaCl2 -loaded cellulose above 300 °C may be responsible for this process. However, as for KCl-loaded cellulose, there is a contradiction between the low selectivity of anhydro-sugars and low degree of dehydration, suggesting a different mechanism of catalysis by KCl compared to CaCl2 (see more discussion below).
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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Fig. 5. Yields of water-soluble liquids (a) and detected carbon percentage (b) at various temperatures. Hollow symbols: complete conversion; solid symbols: zero holding.
3.3. Characteristics of liquid products Figure 5 shows the yield of water-soluble liquids and detected carbon percentage, and there are two important findings. First, when secondary reactions are inhibited during fast pyrolysis, the conversion of cellulose carbon into gases and waterinsoluble portions is probably promoted by the addition of KCl, but not by CaCl2 . Figure 5b shows the total detected carbon (carbon in liquid detected by TOC plus carbon in solid detected by elemental analyzer) versus pyrolysis temperature. As shown, ∼80% of the carbon is in the water-soluble portions and char for both raw and CaCl2 -loaded cellulose, while ∼20% of the carbon disappears as a result of KCl-loading. The promoted dehydration reactions by CaCl2 can be mostly attributed to intramolecular dehydration of sugar units forming C=C and not the cross-linking reactions forming C–O–C, which aids in the formation of CO and CO2 . This is supported by other works [4,17] regarding K as a cracking catalyst. Second, temperature plays an important role on the carbon conversion of raw and KCl-loaded cellulose. As shown in Fig. 5, the yields of water-soluble liquid and the detected car-
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bon percentage at complete conversion attain their maxima at 400 °C or 450 °C. It is accepted that a higher temperature may accelerate the cleavage of pyran rings to form gases or water-insoluble species [2], which is further enhanced by KCl. In the case of lower temperatures, as mentioned above, the exothermic reaction of char formation, as well as gas formation, through decarbonylation and decarboxylation is preferred, as indicated by the BroidoShafizadeh Model [6]. There are two mechanisms for interpreting this process. One is the repolymerization of reaction intermediates, which is avoided on the WMR. The other is that low temperatures are adverse for the decomposition of cellulose to produce volatiles, which results in promoted crosslinking reactions and char formation. Regarding CaCl2 -loaded cellulose, a deeply accelerated decomposition of cellulose at low temperatures probably weakens this process. However, fast pyrolysis (heating rates > 2000 °C/s) on Py-GC-MS suggests that the influence of temperature on pyrolysis speciation from cellulose only becomes prominent at 500 °C and higher [2]. It seems that the heating rate really makes a difference. The mechanism is that the rearrangement of the cellulose structure during the preheating process at low temperatures promotes char formation [18] through dehydration and cross-linking reactions [19]. It was reported [20] that longer preheating at low temperature from 250 to 300 °C can raise the char yield from cellulose pyrolysis. During flash pyrolysis (>2000 °C/s), this rearrangement process and its effect on the consequent reactions are weakened sharply compared to the pyrolysis in this work (100 °C/s), resulting in the difference. The quantification of levoglucosan and cellobiosan in primary volatiles is shown in Fig. 6. Higher yields at low temperatures and lower yields at high temperatures of zero holding after addition are expected. The lower yields of levoglucosan at 500 °C (∼27.4%, ∼4.5%, and ∼16.3% from raw, KCl-loaded, and CaCl2 -loaded cellulose, respectively) in this study compared to a previous work [2] (60%, ∼15%, ∼30% with a heating rate of ∼2000 °C/s) are perhaps due to the lower heating rate (100 °C/s). For levoglucosan formation during pyrolysis, the mechanisms have been mainly summarized as [21] homolysis, heterolysis, hydrolysis, and transglycosylation. As shown in Fig. 6, it is worth noting that most of levoglucosan and cellobiosan from raw cellulose is produced from 400 to 450 °C, together with an abrupt increase of cellobiosan, which behaves as a chain propagator after chain initiation. The results in [21] suggest a large energy barrier gap between the homolytic cleavage of glycosidic bonds and levoglucosan. By considering the large gap between the yields of levoglucosan and cellobiosan shown in Figs. 4 and 6, the drastic formations of levoglucosan and cellobiosan are probably due to a homolytic [22] or heterolytic [23]
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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Fig. 7. Pi-j of levoglucosan and cellobiosan in various temperature intervals. Solid symbols: levoglucosan; hollow symbols: cellobiosan.
Fig. 6. Yields of levoglucosan (a) and cellobiosan (b) in liquid products. Hollow symbols: complete conversion; solid symbols: zero holding.
cleavage of glycosidic bonds. After the addition of KCl or CaCl2 , this process is slowed. To explore the influences of various temperature intervals, an index Pi-j is introduced as: Pi− j = Y j − Yi / C l j − C li × 100Yc −1 (1) where Yi(j) is the yield of anhydro-sugar at a certain temperature i(j), Yc is the yield at 500 °C at complete conversion, and Cli(j) is the conversion level on the weight of cellulose at a certain temperature i(j). Pi− j is shown in Fig. 7 versus the temperature interval, where two important observations can be made. First, the destruction of pyran rings of KClloaded cellulose is expedited significantly above 400 °C. Obviously, there is a sharp decline from P350-400 to P400-450 for KCl-loaded cellulose, which means that more weight-loss occurs due to the cleavage of pyran rings, forming low MW species and gases above 400 °C. However, for CaCl2 -loaded cellulose, Pi-j does not change that much at various temperatures. Considering the abrupt decline of the detected carbon percentage of KCl-loaded cellulose at 400 °C, as shown in Fig. 5b, KCl is deemed to catalyze the cleavage of glycosidic bonds directly, together with gas formation and subordinate dehydration reactions. Furthermore, such catalysis likely occurs through a homolytic route [2] rather than as the result of ion-coordination, as men-
tioned above. In contrast, dehydration reactions followed by scission reactions (such as retro Diels Alder reactions) probably take place for CaCl2 loaded cellulose. Second, the P450-500 values of levoglucosan and cellobiosan from CaCl2 -loaded cellulose are much higher than those from raw cellulose. It seems that the glycosidic bond becomes more “stable” at high temperatures after the addition of CaCl2 despite its acceleration for the destruction of sugar structures. This also supports the inefficiency of CaCl2 on the cleavage of glycosidic bonds and is perhaps due to the condensed structures that result from accumulated dehydration reactions at elevated temperatures by CaCl2 , which suppress the sequential cleavage of glycosidic bonds forming levoglucosan and lead to char formation. This can be interpreted as a more effective inhibition of CaCl2 on cellobiosan than levoglucosan. For example, the yield of levoglucosan in liquid at 500 °C from CaCl2 -loaded cellulose divided by that of cellobiosan is ∼9.4, that is, nearly double the ∼5.8 and ∼4.9 values from raw and KCl-loaded cellulose, respectively. In addition, the low yield and selectivity of cellobiosan from CaCl2 -loaded cellulose, as shown in Fig. 4, suggest that more condensed structures suppress the formation of cellobiosan on the terminals of cellulose chains. Furthermore, the above discussion is also supported by the yield of levoglucosan at complete conversion, as shown in Fig. 6a. Here, the yield at 400 °C from raw and KCl-loaded cellulose show an increase of 46.8% and 18.0%, respectively, compared to the yield at 500 °C, while that from CaCl2 loaded cellulose decreases from 16.3% at 500 °C to 13.8% at 350 °C. This result is consistent with the yield of water-soluble liquid shown in Fig. 4. Cellobiosan transfers via an aerosol route [24], which is quite different from the evaporation of levoglucosan. Lower yields of cellobiosan from impregnated samples can perhaps make this process more dependent on the temperature physically.
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[27] during cellulose pyrolysis and is transferred to the liquid perhaps through aerosol, similar to cellobiosan. 3.5. Discussions on the reaction mechanism of cellulose pyrolysis
Fig. 8. Yield (solid symbols) and selectivity (hollow symbols) of maltosan in water-soluble intermediates at various temperatures.
3.4. The formation of maltosan In this study, when the secondary reactions are minimized, very low contents of maltosan can be quantified by the derivatization method. Considering that the polymerization of levoglucosan prefers the formation of α-D-glycosidic linkages to βD-glycosidic linkages [25], maltosan formation is more likely through secondary reactions. Figure 8 shows the yield and selectivity of maltosan in the solid residues from the three samples versus the pyrolysis temperature, which follow a similar tendency as that of levoglucosan, as shown in Fig. 4. It seems that inorganic species affect the formation of maltosan indirectly by affecting the formation of levoglucosan. However, direct effects of inorganic species on the formation of maltosan are not obvious in this study because secondary reactions are minimized, although a previous work proposed an auxo-action on the polymerization of levoglucosan [26]. Moreover, maltosan also appears in liquid with yields of 0.25%, 0.04% and 0.11% for raw, KClloaded and CaCl2 -loaded cellulose, respectively, above 500 °C. Because the mass transfer from volatiles to solid residues is forbidden, it is plausible that maltosan is produced from the repolymerization of levoglucosan in the solid phase
During fast pyrolysis of cellulose, levoglucosan is generally accepted as the dominant product from “active cellulose,” but not the primary product [7,10]. This is also demonstrated in this study by considering the obvious yield of cellobiosan with the secondary reactions minimized. From the drastic formation of levoglucosan at 400 °C to 500 °C, as shown in Fig. 6a, it seems that a heterolytic [22] or homolytic [23] cleavage of glycosidic bonds is responsible for the sequential formation of levoglucosan. Moreover, this process may be accompanied by the formation of cellobiosan on the chain terminal, resulting in an increase at 400 °C to 500 °C. Afterwards, levoglucosan would repolymerize or convert to oligosaccharides (such as maltosan) or other isomerides through secondary reactions in the solid phase. However, a hydrogen bonding network can reinforce the glycosidic bonds and hydroxyl groups in cellulose, resulting in a higher decomposition temperature and hardly any dehydration in the primary reactions. For impregnated samples, K+ and Ca2+ can interact with oxygen atoms to weaken hydrogen bonds [13] during drying and pyrolysis, which makes hydroxyl groups and glycosidic bonds unstable, leading to the cleavage of glycosidic bonds and dehydration reactions. This results in the advanced decomposition of impregnated cellulose at low temperatures. However, the random cleavage of glycosidic bonds followed by chain propagation reactions seems to be much faster than dehydration reactions during fast pyrolysis [10]. This benefits for the formation of active terminals and is responsible for the high yield and selectivity of levoglucosan in solid residues at 300 °C from CaCl2 -loaded cellulose, as shown in Fig. 4, because Ca2+ , which is a divalent cation, is more efficient in weakening hydrogen bonds compared to K+ .
Fig. 9. A modified mechanism of cellulose pyrolysis and primary catalysis of KCl and CaCl2 .
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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With increasing temperature, CaCl2 catalyzes the intramolecular dehydration of sugar units, forming C=C, followed by the scission of pyran rings to form low MW species. However, accumulated unsaturated structures may suppress the cleavage of glycosidic bonds forming levoglucosan and further enhance the condensation of solid residues. As for KCl-loaded cellulose, KCl appears to catalyze the cleavage of glycosidic bonds [11] or the scission of pyran rings [28] directly through a homolytic mechanism to produce low MW species against the formation of levoglucosan, as well as other anhydro-sugars, especially above 400 °C. A modified mechanism based on a previous work [29] is proposed in Fig. 9 regarding cellulose pyrolysis and the primary catalysis of KCl and CaCl2 .
4. Conclusions Fast pyrolysis was performed on a WMR for raw and two wet-impregnated cellulose samples with KCl and CaCl2 . The anhydro-sugars (levoglucosan, cellobiosan, and maltosan) during pyrolysis were quantified with a derivatization method. In this study, the different catalysis effects of CaCl2 and KCl during cellulose fast pyrolysis were clarified according to different temperature ranges. Some important conclusions can be drawn, as follows: (1) At low temperatures, both inorganic salts, especially CaCl2 , can weaken the hydrogen bonds and stabilities of hydroxyls and glycosidic bonds. However, the cleavage of glycosidic bonds to form levoglucosan seemed much faster, resulting in the sharply accelerated decomposition of CaCl2 -loaded cellulose without obvious dehydration. (2) At elevated temperatures, dehydration reactions in the sugar unit were significantly promoted by CaCl2 , resulting in the formation of low MW organic species. This was mainly responsible for the destruction of pyran rings. Moreover, accumulated unsaturated structures may suppress the cleavage of glycosidic bonds and further enhance the condensation of solid residues. In contrast, KCl presented a different catalytic behavior with increasing temperature, that is, direct homolytic cleavage of glycosidic bonds or pyran rings promoted by KCl may be responsible for the major weight-loss, rather than dehydration reactions. (3) Maltosan was more likely produced through secondary reactions, such as the repolymerization of levoglucosan, and indirect auxoaction on the formation of maltosan by inorganic species through levoglucosan was observed.
Acknowledgments This research is supported by the National Natural Science Foundation of China (51306062 and U1261204). The authors are grateful to the Analytical and Testing Center at Huazhong University of Science and Technology. References [1] L. Zhang, C.C. Xu, P. Champagne, Energy Convers. Manag. 51 (5) (2010) 969–982. [2] P.R. Patwardhan, J.A. Satrio, R.C. Brown, Bioresour. Technol 101 (12) (2010) 4646–4655. [3] D.J. Nowakowski, J.M. Jones, J. Anal. Appl. Pyrol. 83 (1) (2008) 12–25. [4] N. Shimada, H. Kawamoto, S. Saka, J. Anal. Appl. Pyrol. 81 (1) (2008) 80–87. [5] D.J. Nowakowski, J.M. Jones, R. Brydson, Fuel 86 (15) (2007) 2389–2402. [6] A.G.W. Bradbury, Y. Sakai, F. Shafizadeh, J. Appl. Polym. Sci. 23 (11) (1979) 3271–3280. [7] X. Gong, Y. Yu, X. Gao, Energy Fuel 28 (8) (2014) 5204–5211. [8] Y. Yu, D. Liu, H. Wu, Energy Fuel 28 (1) (2014) 245–253. [9] J. Oehlke, M. Brudel, I.E. Blasig, J. Chromatogr. B 655 (1) (1994) 105–111. [10] P.R. Patwardhan, J.A. Satrio, R.C. Brown, J. Anal. Appl. Pyrol. 86 (2) (2009) 323–330. [11] S. Wang, Q. Liu, Y. Liao, Korean J. Chem. Eng. 24 (2) (2007) 336–340. [12] D.W.V. Krevelen, Fuel 29 (29) (1950) 269–284. [13] D. Liu, Y. Yu, Y. Long, Proc. Combust. Inst 35 (2) (2015) 2381–2388. [14] W. Chaiwat, I. Hasegawa, T. Tani, Energy Fuel 23 (3) (2009) 5765–5772. [15] V. Mamleev, S. Bourbigot, M.L. Bras, J. Anal. Appl. Pyrol. 84 (1) (2009) 1–17. [16] A. Saddawi, J.M. Jones, A. Williams, Fuel Process. Technol. 104 (2012) 189–197. [17] W. Pan, G.N. Richards, J. Anal. Appl. Pyrol. 16 (2) (1989) 117–126. [18] G. Maschio, C. Koufopanos, A. Lucchesi, Bioresour. Technol 42 (3) (1992) 219–231. [19] D.F. Arseneau, Can. J. Chem. 49 (4) (1971) 632–638. [20] A. Broido, M.A. Nelson, Combust. Flame 24 (1975) 263–268. [21] X. Zhang, W. Yang, C. Dong, J. Anal. Appl. Pyrol. 104 (2013) 19–27. [22] G.R. Ponder, G.N. Richards, T.T. Stevenson, J. Anal. Appl. Pyrol. 22 (92) (1992) 217–229. [23] M.S. Mettler, S.H. Mushrif, A.D. Paulsen, Energy Environ. Sci. 5 (1) (2012) 5414–5424. [24] A.R. Teixeira, K.G. Mooney, J.S. Kruger, Energy Environ. Sci. 4 (10) (2011) 4306–4321. [25] Y. Houminer, S. Patai, Isr. J. Chem. 7 (4) (1969) 535–546. [26] H. Kawamoto, D. Yamamoto, S. Saka, J. Wood Sci. 54 (3) (2008) 242–246. [27] T. Hosoya, H. Kawamoto, S. Saka, J. Anal. Appl. Pyrol. 83 (1) (2008) 64–70. [28] G.N. Richards, J. Anal. Appl. Pyrol. 10 (3) (1987) 251–255. [29] E. Ranzi, A. Cuoci, T. Faravelli, Energy Fuel 22 (6) (2008) 4292–4300.
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Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis Erwei Leng, Yang Wang, Xun Gong∗, Biao Zhang, Yang Zhang, Minghou Xu∗∗ State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037, Luoyu Road, Wuhan 430074, China Received 4 December 2015; accepted 27 June 2016 Available online xxx
Abstract The influence of KCl and CaCl2 on the primary reactions of cellulose pyrolysis is studied using a wiremesh reactor from 250 °C to 600 °C, focusing on the reaction intermediates. A pre- column derivatization with benzoyl chloride prior to HPLC analysis is applied for the quantification of anhydro-sugars (levoglucosan, cellobiosan, maltosan) from pyrolysis. At low temperatures, the additions of inorganics salts, especially CaCl2 , weakens hydrogen bonds, resulting in high yields of levoglucosan and cellobiosan from the cleavage of glycosidic bonds rather than from dehydration reactions. At elevated temperatures, dehydration reactions in the sugar units are mainly responsible for the destruction of sugar rings followed by the scission of pyran rings, leading to the weight-loss of CaCl2 -loaded cellulose in the form of low molecular weight organic species. Meanwhile, accumulated unsaturated structures suppress the cleavage of glycosidic bonds, leading to the formation of char. However, KCl appears to catalyze the cleavage of glycosidic bonds or the scission of pyran rings directly, which perhaps occurs through a homolytic mechanism, leading to low molecular weight species. Furthermore, maltosan is shown to be a secondary product and is catalyzed by KCl and CaCl2 indirectly through the repolymerization of levoglucosan in the solid phase. A modified mechanism is also proposed regarding cellulose pyrolysis and the primary catalysis of KCl and CaCl2 . © 2016 by The Combustion Institute. Published by Elsevier Inc. Keywords: Cellulose; Primary reactions; Reaction intermediates; KCl; CaCl2
1. Introduction Biomass, as a superior alternative energy to fossil fuels, is gaining increasing attention. Much re∗
Corresponding author. Fax: +86 27 87545526. Corresponding author. Fax: +86 27 87544779. E-mail addresses: [email protected] (X. Gong), [email protected] (M. Xu). ∗∗
search has been performed on its thermochemical conversions (combustion, gasification, pyrolysis etc.), during which pyrolysis is always the initial stage [1]. Therefore, the mechanism of pyrolysis is crucial for biomass industrialization. Alkali and alkali earth metals (AAEM), such as K, Ca, Na, and Mg, broadly exist in biomass and strongly impact the pyrolysis of cellulose [2]. Previous works [2–4] have shown improved yields of char and low
http://dx.doi.org/10.1016/j.proci.2016.06.167 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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molecular species at the expense of anhydro-sugars. However, alkalis present stronger cracking catalysis, leading to more low molecular species [5] compared to alkaline earth metals [4]. As a commonly accepted mechanism, the pyrolysis of cellulose goes through an intermediate state and further turns into gases, volatiles and char [6]. The intermediates are critical during fast pyrolysis, which mostly consist of anhydro-sugars as well as a few sugars [7]. A recent work using a drop-tube/fixed-bed quartz reactor [8] proposed that MgCl2 is more effective in accelerating the formation of intermediates at low temperatures than NaCl, which may be responsible for its ability to sharply reduce the temperature of cellulose decomposition, as observed in other works [4]. However, catalysis effects at higher temperatures (>300 °C) have not been discussed in detail, and secondary reactions may lead to incorrect conclusions [2]. Py-GC–MS have provided some meaningful results [2], but have failed to analyze the evolution of intermediates in char during pyrolysis. Therefore, in this study, KCl and CaCl2 were wet-impregnated into cellulose and fast pyrolysis of three samples was carried out in a wire-mesh reactor (WMR) to minimize secondary reactions. Moreover, a pre-column derivatization with benzoyl chloride before HPLC-MS was applied in anhydro-sugars analysis for the first time for quantification. The objective of this study is to explore the primary catalysis of KCl and CaCl2 on the formation and characteristics of intermediates during fast pyrolysis of cellulose. 2. Experiments 2.1. Materials and fast pyrolysis experiments Microcrystalline cellulose (Avicel PH-102, FMC Biopolymer) with a size between 106 and 150 μm was used in this study. Prior to use, cellulose was washed by deionized water to remove any water-soluble portions and dried at 105 °C. Then, both KCl-loaded and CaCl2 -loaded samples were prepared through wet impregnation with a loading level of 0.025 mol for each mol of glucose. The details of the impregnation are described elsewhere [8]. Pyrolysis experiments were carried out on a WMR used by Gong et al. [7] to extrude the primary catalysis of KCl and CaCl2 . Each time, ∼8 mg of cellulose sample was sandwiched between two wire-mesh layers for fast pyrolysis (100 °C/s) with various holding times at the final temperature (250– 600 °C). During pyrolysis, a stream of nitrogen was passing through the sample holder at a linear gas velocity of 0.1 m/s (4 L/min) to sweep out the generated volatiles, which was quenched by liquid nitrogen and collected in a Teflon filled trap. Then, a mixture of chloroform and methanol (4:1 by volume) was used to wash the tar off the trap, which
was evaporated at 35 °C to determine the yield of non-evaporable liquid products on a weight basis. Each experiment was conducted in triplicate. The triplicate experiments were treated and mixed well for the analyses below. More details regarding the WMR are described elsewhere [7]. 2.2. Characterization of solid and liquid products 2.2.1. Yield of water-soluble intermediates in the solid residues The solid residues on the sample holder were washed by deionized water to obtain “watersoluble intermediates” after filtration at ambient temperature. The elemental compositions of the solid residues (before washing) were analyzed by an elemental analyzer (Elementar Vario Micro cube), while the total organic carbon (TOC) of the watersoluble intermediates was analyzed by a TOC analyzer (Analytikjena multi N/C 2100). Then, the yield of it was obtained by normalizing the TOC in the water-soluble intermediates to the total carbon in the loaded cellulose. 2.2.2. Yield of water-soluble liquid The trap loaded with primary volatiles was thoroughly washed out to obtain a solution of all anhydro-sugars in the condensed liquid products, which is referred to hereafter as “water-soluble liquid.” The water-soluble liquids were analyzed by the same TOC analyzer. Then, the TOC in the water-soluble liquid was normalized to the total carbon in the loaded cellulose, resulting in the yield of water-soluble liquid. 2.3. Derivatization of anhydro-sugars with benzoyl chloride for quantification Usually, HPLC is used to characterize anhydrosugars from cellulose pyrolysis. However, it is difficult and inaccurate for conventional HPLC analysis to quantify anhydro-sugars from lowly loaded cellulose. In this work, a pre-column derivatization with benzoyl chloride for ultraviolet-absorption was first applied in the analysis of anhydro-sugars, according to its previous applications on sugars and sugar alcohols [9]. A 50-μl liquid sample was combined with 40 μl of an undiluted benzoyl chloride solution (Aladdin), mixed slightly, combined with 160-μl NaOH (7 mol/L), and mixed well. After 15 min, 80-μl H3 PO4 (6 mol/L) and 400-μl ethyl acetate were added with mixing, followed by centrifugation for 10 min. Then, 200 μl of the supernatant was extracted and analyzed by HPLC (Agilent 1100 LC) with a UV detector. A Waters Xterra RP C18 separation column was used for linear gradient elution at 30 °C with 75/25 (v/v) methanol/water before 30 min and 90/10 (v/v) methanol/water from 30 to 45 min as the mobile phase. The UV detection wavelength was 232 nm.
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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Table 1 Analytical data for the test compounds. Analyte
tR a
y=ax+b
R2
DLb
Levoglucosan Cellobiosan Maltosan
9.9 20.5 25.2
y=32.5x−0.1 y=26.6x−0.5 y=35.1x−0.4
0.998 0.998 0.991
80 100 100
a
retention time (min). detection Limit (ng/ml): three times the signal-tonoise ratio. b
Fig. 2. Yields of water-soluble intermediates in solid residues at various temperatures.
the initiation temperature of weight-loss, as well as the final temperature of pyrolysis, and CaCl2 performs more efficient catalysis. In addition, higher yields of char and low molecular weight (MW) species (water, gases, formic acid, etc.) are also expected after the addition of KCl and CaCl2 , as proposed in a previous work [2]. Nevertheless, KCl is more efficient for the formation of low MW species, while the catalysis effects on char formation are similar, with the same yield of ∼12.8% at above 500 °C. Relative to complete conversion, it is expected that the yields of solid products will all increase slightly at lower temperatures because low temperatures promote exothermic reactions of char formation and restrain endothermic reactions producing pyrolysis vapors [10]. However, temperature nearly does not influence the yields of non-evaporable liquid products. Fig. 1. Yields of solid products (a) and non-evaporable liquids (b) as a function of pyrolysis temperature at zero holding (solid symbols) and complete conversion (hollow symbols).
Nine mixed standard solutions of three anhydrosugars (levoglucosan, cellobiosan, and maltosan) in the same concentration range (4–1000 ppm) were analyzed. Table 1 shows the analytical data. 3. Results and discussion 3.1. Yields of liquid and solid products from pyrolysis Figure 1 shows the yields of solid and nonevaporable liquid products of the three samples during pyrolysis on the WMR; complete conversion means that the holding time is long enough for a fixed yield of solid products at a constant temperature. As shown, both KCl and CaCl2 reduce
3.2. Characteristics of solid products To further understand the catalysis effects of KCl and CaCl2 , various analyses were performed on the solid and liquid products. The yields of water-soluble intermediates are presented in Fig. 2 based on the total carbon in loaded cellulose. A comparison of Fig. 1a and Fig. 2 shows that the fastest weight-loss point is consistent with the maximum yield of water-soluble intermediates for all samples. Therefore, the volatilization of intermediates is always primarily responsible for the weightloss of cellulose. Moreover, the formation of intermediates at low temperatures is improved by inorganic species, especially by CaCl2 (a maximum of 26% at 350 °C), as shown in Fig. 2. This results in the reduced initiation temperature of weightloss, which is consistent with previous works on TG [4,11]. Furthermore, the elemental compositions of solid products collected after pyrolysis are presented in a Van Krevelen diagram [12] (Fig. 3). It
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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Fig. 3. Van Krevelen diagram for the solid residues at various temperatures.
is shown that the elemental compositions of raw cellulose do not change significantly with increasing temperature when secondary reactions are almost averted on the WMR. This is notably different from a previous fast pyrolysis work using a droptube/fixed-bed reactor with considerable secondary reactions present [13]. Usually, two main competitive pathways (dehydration and depolymerization) exist during cellulose pyrolysis [14]. However, the result reveals that considerable dehydration does not seem to occur inside the char during fast pyrolysis. Mamleev et al. [15] concluded that dehydration is a fast secondary reaction with respect to cellulose depolymerization. Therefore, in conventional reactors, dehydration reactions mainly come from the secondary decomposition and polymerization of primary products. Pyrolysis of the two impregnated samples shows that the elemental compositions obviously shift along the dehydration line. Then, it is plausible that KCl and CaCl2 promote dehydration reactions in primary catalysis, and CaCl2 seems to be much more efficient. The quantitative results of the yields (as percentages of the total carbon in raw cellulose) and selectivities (as percentages of the total carbon in water-soluble intermediates) of levoglucosan and cellobiosan in the solid residues are presented in Fig. 4. At low temperatures, both inorganic salts accelerate the cleavage of glycosidic bonds, resulting in higher yields of levoglucosan and cellobiosan. For example, CaCl2 -loaded and KCl-loaded cellulose respectively lead to levoglucosan yields of 1.59% and 0.21%, and cellobiosan yields of 0.13% and 0.05%, compared to 0.15% and 0.03% from raw cellulose at 300 °C. This is consistent with the enhanced formation of intermediates in the solid residues. The explanation is that coordination between oxygen atoms and AAEM [4,8,16] can weaken the hydrogen bonds during drying after wet-impregnation and pyrolysis [13], resulting in the instability of glycosidic bonds and hydroxyl groups, which probably promote the cleavage of
Fig. 4. Yields and selectivities of anhydro-sugars in water-soluble intermediates at various temperatures. Solid symbols: levoglucosan; hollow symbols: cellobiosan.
glycosidic bonds and dehydration reactions. Furthermore, Ca2+ , which is more active than K+ as a Lewis acid, is more effective in catalysis. Nevertheless, there is minimal dehydration among the impregnated samples below 300 °C, as shown in Fig. 3, which reveals that the cleavage of glycosidic bonds is much faster than dehydration reactions [2]. The destruction of sugar structures is also improved by K+ and Ca2+ , leading to more nonsaccharides, mostly above 300 °C. As shown in Fig. 4b, the selectivities of levoglucosan from both impregnated samples follow a similar tendency as those from raw cellulose at the initial temperature, despite the advanced depolymerization of cellulose by CaCl2 . Subsequently, they decline sharply above 300 °C, which reveals the active destruction of sugar structures. According to Fig. 3, the prominent dehydration of CaCl2 -loaded cellulose above 300 °C may be responsible for this process. However, as for KCl-loaded cellulose, there is a contradiction between the low selectivity of anhydro-sugars and low degree of dehydration, suggesting a different mechanism of catalysis by KCl compared to CaCl2 (see more discussion below).
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Fig. 5. Yields of water-soluble liquids (a) and detected carbon percentage (b) at various temperatures. Hollow symbols: complete conversion; solid symbols: zero holding.
3.3. Characteristics of liquid products Figure 5 shows the yield of water-soluble liquids and detected carbon percentage, and there are two important findings. First, when secondary reactions are inhibited during fast pyrolysis, the conversion of cellulose carbon into gases and waterinsoluble portions is probably promoted by the addition of KCl, but not by CaCl2 . Figure 5b shows the total detected carbon (carbon in liquid detected by TOC plus carbon in solid detected by elemental analyzer) versus pyrolysis temperature. As shown, ∼80% of the carbon is in the water-soluble portions and char for both raw and CaCl2 -loaded cellulose, while ∼20% of the carbon disappears as a result of KCl-loading. The promoted dehydration reactions by CaCl2 can be mostly attributed to intramolecular dehydration of sugar units forming C=C and not the cross-linking reactions forming C–O–C, which aids in the formation of CO and CO2 . This is supported by other works [4,17] regarding K as a cracking catalyst. Second, temperature plays an important role on the carbon conversion of raw and KCl-loaded cellulose. As shown in Fig. 5, the yields of water-soluble liquid and the detected car-
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bon percentage at complete conversion attain their maxima at 400 °C or 450 °C. It is accepted that a higher temperature may accelerate the cleavage of pyran rings to form gases or water-insoluble species [2], which is further enhanced by KCl. In the case of lower temperatures, as mentioned above, the exothermic reaction of char formation, as well as gas formation, through decarbonylation and decarboxylation is preferred, as indicated by the BroidoShafizadeh Model [6]. There are two mechanisms for interpreting this process. One is the repolymerization of reaction intermediates, which is avoided on the WMR. The other is that low temperatures are adverse for the decomposition of cellulose to produce volatiles, which results in promoted crosslinking reactions and char formation. Regarding CaCl2 -loaded cellulose, a deeply accelerated decomposition of cellulose at low temperatures probably weakens this process. However, fast pyrolysis (heating rates > 2000 °C/s) on Py-GC-MS suggests that the influence of temperature on pyrolysis speciation from cellulose only becomes prominent at 500 °C and higher [2]. It seems that the heating rate really makes a difference. The mechanism is that the rearrangement of the cellulose structure during the preheating process at low temperatures promotes char formation [18] through dehydration and cross-linking reactions [19]. It was reported [20] that longer preheating at low temperature from 250 to 300 °C can raise the char yield from cellulose pyrolysis. During flash pyrolysis (>2000 °C/s), this rearrangement process and its effect on the consequent reactions are weakened sharply compared to the pyrolysis in this work (100 °C/s), resulting in the difference. The quantification of levoglucosan and cellobiosan in primary volatiles is shown in Fig. 6. Higher yields at low temperatures and lower yields at high temperatures of zero holding after addition are expected. The lower yields of levoglucosan at 500 °C (∼27.4%, ∼4.5%, and ∼16.3% from raw, KCl-loaded, and CaCl2 -loaded cellulose, respectively) in this study compared to a previous work [2] (60%, ∼15%, ∼30% with a heating rate of ∼2000 °C/s) are perhaps due to the lower heating rate (100 °C/s). For levoglucosan formation during pyrolysis, the mechanisms have been mainly summarized as [21] homolysis, heterolysis, hydrolysis, and transglycosylation. As shown in Fig. 6, it is worth noting that most of levoglucosan and cellobiosan from raw cellulose is produced from 400 to 450 °C, together with an abrupt increase of cellobiosan, which behaves as a chain propagator after chain initiation. The results in [21] suggest a large energy barrier gap between the homolytic cleavage of glycosidic bonds and levoglucosan. By considering the large gap between the yields of levoglucosan and cellobiosan shown in Figs. 4 and 6, the drastic formations of levoglucosan and cellobiosan are probably due to a homolytic [22] or heterolytic [23]
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Fig. 7. Pi-j of levoglucosan and cellobiosan in various temperature intervals. Solid symbols: levoglucosan; hollow symbols: cellobiosan.
Fig. 6. Yields of levoglucosan (a) and cellobiosan (b) in liquid products. Hollow symbols: complete conversion; solid symbols: zero holding.
cleavage of glycosidic bonds. After the addition of KCl or CaCl2 , this process is slowed. To explore the influences of various temperature intervals, an index Pi-j is introduced as: Pi− j = Y j − Yi / C l j − C li × 100Yc −1 (1) where Yi(j) is the yield of anhydro-sugar at a certain temperature i(j), Yc is the yield at 500 °C at complete conversion, and Cli(j) is the conversion level on the weight of cellulose at a certain temperature i(j). Pi− j is shown in Fig. 7 versus the temperature interval, where two important observations can be made. First, the destruction of pyran rings of KClloaded cellulose is expedited significantly above 400 °C. Obviously, there is a sharp decline from P350-400 to P400-450 for KCl-loaded cellulose, which means that more weight-loss occurs due to the cleavage of pyran rings, forming low MW species and gases above 400 °C. However, for CaCl2 -loaded cellulose, Pi-j does not change that much at various temperatures. Considering the abrupt decline of the detected carbon percentage of KCl-loaded cellulose at 400 °C, as shown in Fig. 5b, KCl is deemed to catalyze the cleavage of glycosidic bonds directly, together with gas formation and subordinate dehydration reactions. Furthermore, such catalysis likely occurs through a homolytic route [2] rather than as the result of ion-coordination, as men-
tioned above. In contrast, dehydration reactions followed by scission reactions (such as retro Diels Alder reactions) probably take place for CaCl2 loaded cellulose. Second, the P450-500 values of levoglucosan and cellobiosan from CaCl2 -loaded cellulose are much higher than those from raw cellulose. It seems that the glycosidic bond becomes more “stable” at high temperatures after the addition of CaCl2 despite its acceleration for the destruction of sugar structures. This also supports the inefficiency of CaCl2 on the cleavage of glycosidic bonds and is perhaps due to the condensed structures that result from accumulated dehydration reactions at elevated temperatures by CaCl2 , which suppress the sequential cleavage of glycosidic bonds forming levoglucosan and lead to char formation. This can be interpreted as a more effective inhibition of CaCl2 on cellobiosan than levoglucosan. For example, the yield of levoglucosan in liquid at 500 °C from CaCl2 -loaded cellulose divided by that of cellobiosan is ∼9.4, that is, nearly double the ∼5.8 and ∼4.9 values from raw and KCl-loaded cellulose, respectively. In addition, the low yield and selectivity of cellobiosan from CaCl2 -loaded cellulose, as shown in Fig. 4, suggest that more condensed structures suppress the formation of cellobiosan on the terminals of cellulose chains. Furthermore, the above discussion is also supported by the yield of levoglucosan at complete conversion, as shown in Fig. 6a. Here, the yield at 400 °C from raw and KCl-loaded cellulose show an increase of 46.8% and 18.0%, respectively, compared to the yield at 500 °C, while that from CaCl2 loaded cellulose decreases from 16.3% at 500 °C to 13.8% at 350 °C. This result is consistent with the yield of water-soluble liquid shown in Fig. 4. Cellobiosan transfers via an aerosol route [24], which is quite different from the evaporation of levoglucosan. Lower yields of cellobiosan from impregnated samples can perhaps make this process more dependent on the temperature physically.
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[27] during cellulose pyrolysis and is transferred to the liquid perhaps through aerosol, similar to cellobiosan. 3.5. Discussions on the reaction mechanism of cellulose pyrolysis
Fig. 8. Yield (solid symbols) and selectivity (hollow symbols) of maltosan in water-soluble intermediates at various temperatures.
3.4. The formation of maltosan In this study, when the secondary reactions are minimized, very low contents of maltosan can be quantified by the derivatization method. Considering that the polymerization of levoglucosan prefers the formation of α-D-glycosidic linkages to βD-glycosidic linkages [25], maltosan formation is more likely through secondary reactions. Figure 8 shows the yield and selectivity of maltosan in the solid residues from the three samples versus the pyrolysis temperature, which follow a similar tendency as that of levoglucosan, as shown in Fig. 4. It seems that inorganic species affect the formation of maltosan indirectly by affecting the formation of levoglucosan. However, direct effects of inorganic species on the formation of maltosan are not obvious in this study because secondary reactions are minimized, although a previous work proposed an auxo-action on the polymerization of levoglucosan [26]. Moreover, maltosan also appears in liquid with yields of 0.25%, 0.04% and 0.11% for raw, KClloaded and CaCl2 -loaded cellulose, respectively, above 500 °C. Because the mass transfer from volatiles to solid residues is forbidden, it is plausible that maltosan is produced from the repolymerization of levoglucosan in the solid phase
During fast pyrolysis of cellulose, levoglucosan is generally accepted as the dominant product from “active cellulose,” but not the primary product [7,10]. This is also demonstrated in this study by considering the obvious yield of cellobiosan with the secondary reactions minimized. From the drastic formation of levoglucosan at 400 °C to 500 °C, as shown in Fig. 6a, it seems that a heterolytic [22] or homolytic [23] cleavage of glycosidic bonds is responsible for the sequential formation of levoglucosan. Moreover, this process may be accompanied by the formation of cellobiosan on the chain terminal, resulting in an increase at 400 °C to 500 °C. Afterwards, levoglucosan would repolymerize or convert to oligosaccharides (such as maltosan) or other isomerides through secondary reactions in the solid phase. However, a hydrogen bonding network can reinforce the glycosidic bonds and hydroxyl groups in cellulose, resulting in a higher decomposition temperature and hardly any dehydration in the primary reactions. For impregnated samples, K+ and Ca2+ can interact with oxygen atoms to weaken hydrogen bonds [13] during drying and pyrolysis, which makes hydroxyl groups and glycosidic bonds unstable, leading to the cleavage of glycosidic bonds and dehydration reactions. This results in the advanced decomposition of impregnated cellulose at low temperatures. However, the random cleavage of glycosidic bonds followed by chain propagation reactions seems to be much faster than dehydration reactions during fast pyrolysis [10]. This benefits for the formation of active terminals and is responsible for the high yield and selectivity of levoglucosan in solid residues at 300 °C from CaCl2 -loaded cellulose, as shown in Fig. 4, because Ca2+ , which is a divalent cation, is more efficient in weakening hydrogen bonds compared to K+ .
Fig. 9. A modified mechanism of cellulose pyrolysis and primary catalysis of KCl and CaCl2 .
Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167
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With increasing temperature, CaCl2 catalyzes the intramolecular dehydration of sugar units, forming C=C, followed by the scission of pyran rings to form low MW species. However, accumulated unsaturated structures may suppress the cleavage of glycosidic bonds forming levoglucosan and further enhance the condensation of solid residues. As for KCl-loaded cellulose, KCl appears to catalyze the cleavage of glycosidic bonds [11] or the scission of pyran rings [28] directly through a homolytic mechanism to produce low MW species against the formation of levoglucosan, as well as other anhydro-sugars, especially above 400 °C. A modified mechanism based on a previous work [29] is proposed in Fig. 9 regarding cellulose pyrolysis and the primary catalysis of KCl and CaCl2 .
4. Conclusions Fast pyrolysis was performed on a WMR for raw and two wet-impregnated cellulose samples with KCl and CaCl2 . The anhydro-sugars (levoglucosan, cellobiosan, and maltosan) during pyrolysis were quantified with a derivatization method. In this study, the different catalysis effects of CaCl2 and KCl during cellulose fast pyrolysis were clarified according to different temperature ranges. Some important conclusions can be drawn, as follows: (1) At low temperatures, both inorganic salts, especially CaCl2 , can weaken the hydrogen bonds and stabilities of hydroxyls and glycosidic bonds. However, the cleavage of glycosidic bonds to form levoglucosan seemed much faster, resulting in the sharply accelerated decomposition of CaCl2 -loaded cellulose without obvious dehydration. (2) At elevated temperatures, dehydration reactions in the sugar unit were significantly promoted by CaCl2 , resulting in the formation of low MW organic species. This was mainly responsible for the destruction of pyran rings. Moreover, accumulated unsaturated structures may suppress the cleavage of glycosidic bonds and further enhance the condensation of solid residues. In contrast, KCl presented a different catalytic behavior with increasing temperature, that is, direct homolytic cleavage of glycosidic bonds or pyran rings promoted by KCl may be responsible for the major weight-loss, rather than dehydration reactions. (3) Maltosan was more likely produced through secondary reactions, such as the repolymerization of levoglucosan, and indirect auxoaction on the formation of maltosan by inorganic species through levoglucosan was observed.
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Please cite this article as: E. Leng et al., Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.06.167