Tar and char can be regarded as unwanted byproducts during the gasification process. In this study, three types of catalyst, i.e., biomass char (bio-char), nickel supported on biomass (Ni+bio-char), and nickel supported on bio-char (bio-char+Ni), were studied to compare the catalytic effects of different preparation methods on tar model compound removal. The structural characteristics of the three catalysts were also investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) methods. The results revealed that Ni+bio-char catalyst showed much higher activity for the reformation of toluene (C7H8) as a tar model compound than the other two catalysts. Toluene could be completely converted to small gas molecules at a conversion rate of 99.92% at 800 °C, and the maximum yield of gas was 432 mL/(mL C7H8). In particular, the H2 and CH4 yields were 339 and 85 mL/(mL C7H8) at 850 °C, respectively. An N2 absorption-desorption experiment demonstrated that the specific surface area of Ni+bio-char was 32.87 times that of bio-char and 8.39 times that of bio-char+Ni. Moreover, metallic nickel (Ni0) particles could be generated in the carbon matrix of Ni+bio-char catalyst. SEM analysis confirmed that the Ni+bio-char catalyst had a more porous structure. Nickel supported on biomass might be a promising catalyst for tar reformation because of its excellent catalytic activities.
Experimental and Theoretical Analysis of the Pyrolysis Mechanism of a Dimeric Lignin Model Compound with α-O-4 Linkage
Chao Liu, Yubin Deng, Shubin Wu,* Ming Lei, and Jiajin Liang
In this study, 4-benzyloxyphenol was introduced as an α-O-4 dimeric lignin model compound to explore the pyrolysis mechanism of α-O-4 linkage in lignin. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), density functional theory (DFT) calculations, and collision theory were employed to illustrate the pyrolysis process experimentally and theoretically. The results suggest that the pyrolysis of 4-benzyloxyphenol starts from the Cα-O bond homolysis because it has the lowest bond dissociation energy (BDE) (164.9 kJ/mol). Among the four main products, as well as the primary products, the formation of bibenzyl and toluene depend on the probability of molecular collisions, while the formation of p-benzoquinone and hydroquinone is influenced by thermodynamic factors. The minor products, which were basically generated from the secondary pyrolysis of the main products and consisted of oxygenated compounds and polycyclic compounds, were only observed at 600 °C. The energy barrier, the enthalpy change, and their combined effects determined the formation of minor products.
Keywords: Dimeric lignin model compound; α-O-4 linkage; Pyrolysis mechanism; DFT method; Collision theory
Contact information: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong 510640, PR China; *Corresponding author: email@example.com
With increasing depletion of fossil resources (coal, oil, and gas) and the deterioration of the environment (acid rain, greenhouse effect, ozone hole, etc.), new sources of energy are being sought. As a renewable and sustainable resource, biomass is considered to be a substitute for fossil resources. For this purpose, more attention has been paid to the development and utilization of biomass in recent decades. Among the forms of utilization, pyrolysis is regarded as a promising method of converting biomass into bio-oil (Bridgwater et al. 1999). The pyrolysis of different raw materials (Jiang et al. 2010; Lv et al. 2013; Liang et al. 2015) with various equipment (Lédé et al. 2007; Guo et al. 2012; Zhang et al. 2013) with factors such as temperature (Jiang et al. 2010; Lou et al. 2010), residence time ( Marsh et al. 2004; Guo and Wang 2015), additive and catalyst (Guo et al. 2012; Zhang et al. 2013; Shao et al. 2014), and atmosphere (Chen et al. 2008), has been widely investigated. However, the intermediate pyrolysis processes are still obscure. Hence, it is necessary to investigate the pyrolysis mechanism of biomass.
Because of its diverse composition, biomass has complicated pyrolysis reactions. Therefore, a mechanistic study on a single component of biomass is realistic. Of the three major components of biomass (cellulose, lignin, and hemicelluose), lignin has a more complex chemical structure and pyrolysis mechanism than cellulose (Shen and Gu 2009) or hemicellulose (Wang et al. 2013), whose pyrolysis mechanisms have been thoroughly investigated already. Therefore, for the in-depth understanding of biomass pyrolysis, it is necessary to explore the mechanism of lignin pyrolysis. Furthermore, in the process of exploring the pyrolysis mechanism, pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) is widely used because of its speediness and repeatability (Guo and Wang 2015). In addition, for the further understanding of the lignin pyrolysis mechanism in theory, density functional theory (DFT) is often introduced to determine the pyrolysis active site and the potential energy (Beste and Buchanan 2009; Kim et al. 2011; Huang et al. 2014a).
Lignin is composed of methoxylated phenylpropane units, including p-coumaryl alcohols, coniferyl alcohols, and sinapyl alcohols, which are crosslinked by ether bonds (β-O-4, α-O-4, 4-O-5’) and carbon-carbon bonds (β-5, 5-5’, β-1, β–β’) (Kang et al. 2013). Among these bonds, the content of the β-O-4 linkage is the highest (45% to 60%) (Adler 1977; Chakar and Ragauskas 2004; Rodrigues Pinto et al. 2010; Shen et al. 2010; Pandey and Kim 2011). Therefore, in previous mechanism studies, dimeric lignin model compounds with β-O-4 linkage were preferred as research objects (Drage et al. 2002; Liu et al. 2011; Chu et al. 2013). The α-O-4 linkage, namely the second most abundant ether bond, also has a relatively high chemical activity, similar to the β-O-4 linkage, and can affect the pyrolysis process and products distribution as well. However, studies on this topic are rare, and the corresponding results only based on the experiments (Kawamoto et al. 2007; Kim et al. 2014). Hence, exploring the pyrolysis of the α-O-4 linkage experimentally and theoretically is meaningful for the further investigation of the pyrolysis mechanism of lignin.
Therefore, in this study, Py-GC/MS was introduced to identify the pyrolytic products from the α-O-4 dimeric lignin model compound. Moreover, DFT calculations and collision theory were applied to speculate on the pyrolysis process of feedstock and simulate the pyrolysis mechanism of the α-O-4 linkage in lignin.
The model compound used in this study was 4-benzyloxyphenol, which can be regarded as an analogue for dimeric structures within lignin. It was purchased from Aladdin Industrial Corporation (Shanghai, China) and used directly without further purification. Figure 1 shows the chemical structural formula of the α-O-4 lignin dimer model.
Fig. 1. Chemical structural formula of 4-benzyloxyphenol
Py-GC/MS was evaluated on a CDS 5200 pyrolyzer (CDS Analytical, USA) connected to a 7890A gas chromatograph coupled with a 5975C mass spectrometer (Agilent Technologies, USA). The sample was pyrolyzed at the set temperatures of (400, 500, and 600 °C) for 15 s with a heating rate of 10000 °C/s using helium as the carrier gas. The split ratio was 50:1, and the ﬂow rate was 1.00 mL/min. The Py oven was set at 100 °C as to not volatilize the feedstock, and all others (the injector, detector, and interface temperatures) were set at 250 °C. HP-5MS capillary column (30 m × 0.25 mm × 0.25 μm, Agilent Technologies, USA) was selected as the separation column. The column temperature was programmed from 50 to 250 °C (5 min) with a heating rate of 8 °C/min. The mass spectrometer was set at an ionizing voltage of 70 eV, and the mass range from m/z 5 to 400 was scanned with a speed of 1.0 s/decade.
DFT calculations were carried out on the Gaussian 03 program (Frisch et al. 2004). A B3LYP/6-31G(d,p) basis set was used to optimize the equilibrium geometries of reactants, intermediates, transition states, and products (Huang et al. 2014b). The transition states were located by transition state theory method and were confirmed by visual inspection of the imaginary frequency using Gauss view and IRC calculations. Activation energies (reaction energy barriers) for reactions were estimated with the relative energies, including the zero-point energy correction, between the transition state and the reactant. For homolytic reactions that had no transition states, bond dissociation energy was used as a substitute for the activation energy.
RESULTS AND DISCUSSION
Pyrolytic Products Distribution
The GC/MS chromatograms of 4-benzyloxyphenol pyrolysis at 400, 500, and 600 °C are shown in Fig. 2. In these chromatograms, the volatilization of the feedstock can be clearly recognized during pyrolysis, and changes in the species and yields of pyrolytic products from 4-benzyloxyphenol are also intuitively displayed with increasing temperature. Yields (percentage of integral area) of the detected compounds after volatilized feedstock deduction are summarized in Table 1.
From Fig. 2 and Table 1, it can be seen that at 400 °C, only bibenzyl was detected. Hence, the relative yield of bibenzyl was 100%. When the temperature rose to 500 °C, the yield of bibenzyl decreased quickly to 51.42%. At the same time, toluene, p-benzoquinone, and hydroquinone were generated in large quantities. However, the species of pyrolytic products at 600 °C was up to 11 and the major products were still bibenzyl, toluene, p-benzoquinone, and hydroquinone, whose total yields were up to 98.51%. As for these minor products formed at 600 °C, the related yields were quite low; and with the exception of (E)-stilbene (0.78%), the yields of other minor products were approximately 0.1%.
Judging from the chemical structure of these pyrolytic products, a relationship can be proposed in which the four main compounds (bibenzyl, toluene, p-benzoquinone, and hydroquinone) are separately derived from the benzyl unit and the hydroquinone unit in 4-benzyloxyphenol. This assumes that these main products were produced from the cleavage of the α-O-4 linkage in 4-benzyloxyphenol. Also, the minor products may have benefitted from the secondary pyrolysis of these main compounds.
Fig. 2. GC/MS chromatograms of 4-benzyloxyphenol pyrolysis at 400, 500, and 600 °C
Table 1. Pyrolytic Products from 4-Benzyloxyphenol Detected by Py-GC/MS at Various Temperatures
a volatilized feedstock was deleted.
-: Not detected.
Primary Products Formation
Previous theoretical research has shown that the bond dissociation energies (BDEs) of ether linkages in lignin and its model compounds are lower than those of other linkages. Thus, these ether bonds are considered to be the active sites of lignin pyrolysis (Beste and Buchanan 2009; Kim et al. 2011; Huang et al. 2014a). For our research object, the calculated BDEs of main bonds (except for the Ar-H bonds) are listed in Table 2. As can be seen from the data, the Cα-O bond has the lowest BDE (164.9 kJ/mol) and was considered to be the active site for the primary reaction. Therefore, the thermal degradation of 4-benzyloxyphenol was considered to start from the Cα-O bond dissociation.
Table 2. Dissociation Energy of the Main Bonds in 4-Benzyloxyphenol (kJ/mol)
The pyrolysis process of 4-benzyloxyphenol is indicated in Fig. 3, which combines the experimental results and free-radical theory. As shown in Fig. 3, homolytic cleavage of the Cα-O bond was found in the fast pyrolysis condition. The benzyl radical (intermediate A, abbreviated IMA) and the hydroquinone radical (IMB) were generated from homolysis. Because of the homolytic reaction, the energy barrier of this step (S1) was the BDE of the Cα-O bond, 164.9 kJ/mol.
Fig. 3. Formation processes of main pyrolytic products from 4-benzyloxyphenol
Molecular collision is the basis for the further reactions of the generated IMA. When two IMA approached each other, the coupling reaction was realized with the formation of the Cα-Cα’ bond and bibenzyl (A1). During the coupling step, the released energy was 238.9 kJ/mol. Thus, the total enthalpy change of A1 formation was less than 0 (-74.0 kJ/mol). As shown in Table 1, A1 was a major product. Therefore, the possibility of this thermodynamic process was highly in accordance with the experimental results. Furthermore, the IMA collided with the H radical and was quenched into toluene (A2). The total enthalpy change of A2 formation was -197.0 kJ/mol. The quenching pathway was also an exothermic process.
As for these two products, A1 and A2 both originated from the aryl ring A (Fig. 3). The energy barriers of the two pathways were the same. The released energy in the quenching step was more than that in the coupling step, but the experimental yield of A1 was much higher than that of A2. The inconsistency was caused by the probability of collision. After the Cα-O bond break, large amounts of IMA were formed while there were not enough H radicals in the system. Hence, the probability of coupling was much higher than that of quenching and the actual yield of A1 was higher than that of A2. Overall, the experimental results provided a strong support for the collision theory.
When the collision theory was introduced to explain the further reactions of IMB, it was found that the active peroxide bond (-O-O-) would form by the coupling reaction of two IMB. However, the generated peroxide bond was broken up into IMB under the pyrolysis condition. Hence, it is not necessary to discuss the coupling reaction in IMB. Furthermore, the bimolecular disproportionation reaction should be found in IMB with hydroquinone (B1) and p-benzoquinone (B2) both formed equally. This process was not in accordance with the experimental results.
To explain the formation of B1 and B2, thermodynamic factors should be introduced. The generated IMB could combine with the H radical and be reduced into B1. Meanwhile, H radical could be removed with IMB, being oxidized into B2. Because of the different energy requirements for the reduction and oxidation reactions (-304.9 and 227.2 kJ/mol, respectively), the gap between the total enthalpy change of B1 formation and B2 formation was large. The formation route of B1 was an exothermic process with an energy of 140.0 kJ/mol released, while the formation of B2 was an endothermic reaction, which needed energy of 392.1 kJ/mol. The thermodynamic explanation agreed with the experimental results in Table 1, in which the yield of B1 was several times greater than that of B2.
Secondary Products Formation
Minor products were thought to be generated from the secondary pyrolysis of the primary products. The formation of secondary products is inevitable but ideally should be avoided during the thermal conversion of biomass. For example, with 4-benzyloxyphenol, some oxygenated compounds (e.g. benzaldehyde and phenol), and polycyclic compounds (e.g., diphenylmethane and 9,10-dihydrophenanthrene) were observed at 600 °C. However, decreasing the oxygen content and the yields of polycyclic compounds is the focus of bio-oil upgrading. Hence, the formation mechanisms of these oxygenated and polycyclic compounds deserve to be discussed.
There is no doubt that phenol (B3) was formed through the dehydroxylation reaction in B1 (Fig. 4). However, the total enthalpy change of B3 formation was low (-154.4 kJ/mol) and the energy barrier of the dehydroxylation reaction approached 447.3 kJ/mol, which severely restricted the formation of B3.
As shown in Fig. 5, the active IMA also combined with the OH radical to form benzyl alcohol (IM2). The generated IM2 went on being oxidized into benzaldehyde (A3) under the thermochemical condition through the transition state 1 (TS1). The formation of the CHO group is a common reaction type in hydroxyl compounds pyrolysis (Harman-Ware et al. 2013; Kotake et al. 2013). The total enthalpy change of A3 formation was 300.3 kJ/mol. This reaction process was considered the oxidation of the benzyl radical, which was a transferring form of the oxygen during pyrolysis. The formation of A3 originated from the combination of the OH radical; therefore, the yield of A3 was determined by the possibility of the combination, which was related to the concentration of the OH radical. Thereby, the formation of A3 was controlled by the OH radical. When the pyrolysis temperature was set at 600 °C, phenol (B3) was produced, which released an equal amount of OH radical (Fig. 4). These OH radicals contributed to the formation of A3. Furthermore, almost the same yield of B3 and A3, as listed in Table 1, further confirmed the formation pathways shown in Figs. 4 and 5.
Fig. 4. Secondary pyrolysis of hydroquinone
Fig. 5. Proposed formation pathway of benzaldehyde
As the most abundant pyrolytic product, bibenzyl (A1) had the highest probability to undergo secondary reactions. Other minor products were realized from the further degradation of A1. As shown in Fig. 6, secondary reactions on A1 were divided into three processes, including the homolysis of the C1-Cα bond (Fig. 6(a)), the formation of the C2-C2’ bond (Fig. 6(b)), and the formation of the Cα=Cα’ bond (Fig. 6(c)). Inevitably, the Cα-Cα bond was broken down during pyrolysis and IMA was formed. Then, the generated IMA reacted along the routes mentioned above, therefore, these details were not repeated.
The energy barrier of the homolytic pyrolysis of the C1-Cα bond in A1 was 402.5 kJ/mol. After the homolysis, a phenethyl radical (IM3) and a phenyl radical (IM4) were formed. Because of the lack of H radical in the system, the subsequent reaction of IM3 had to undergo the route with high-energy barrier to produce styrene (A4) (Fig. 6(a)). Hence, the total enthalpy change of A4 formation reached 458.8 kJ/mol.
Because of the active chemical structure, the generated IM4 would combine with other radicals. As determined from the experimental products in Table 1, IM6 might be quenched into benzene (A5) by the H radical, which would release energy of 456.7 kJ/mol. The active IM6 might react with OH radical as well, with phenol (A6/B3) formed and energy of 449.6 kJ/mol released. In addition, IM6 could even combine with the benzyl radical (IMA), transferred into diphenylmethane (A7), which would release energy of 344.2 kJ/mol.
Although the yield of 9,10-dihydrophenanthrene (A8) was low, only 0.09%, it might be transferred into phenanthrene through dehydrogenation. Phenanthrene could be further polymerized into polycyclic compounds and induce the coke formation. Hence, it was necessary to investigate the formation of A8. Figure 6(b) gave out the formation pathway of A8. Firstly, the Cα-Cα’ bond was twisted to let C2 and C2’ get close. Then, the C2-C2’ bond was formed through a cyclic transition state. The energy barrier of A8 formation was 500.5 kJ/mol, which severely restricted the formation of A8.
As the most abundant secondary product, (E)-stilbene (A9) was produced along Fig. 6(c). The formation pathway of A9 was also an endothermic process, whose total enthalpy change was 441.2 kJ/mol and energy barrier only 351.4 kJ/mol, which was lower than any other barriers during the secondary pyrolysis.
Fig. 6. Secondary pyrolysis of bibenzyl: (a) homolysis of the C1-Cα bond, (b) formation of the C2-C2’ bond, and (c) formation of the Cα=Cα’ bond.
A dimeric lignin model compound with α-O-4 linkage was investigated by experiments (Py-GC/MS) and theories (DFT calculations and collision theory) in order to explore the pyrolysis mechanism of lignin. The following results could make the α-O-4 linkage dissociation and the subsequent pyrolysis reactions much clearer.
- The DFT results suggested that the BDE of the Cα-O bond in 4-benzyloxyphenol was the lowest (164.9 kJ/mol). Thus, pyrolysis started from the Cα-O bond homolysis, which was consistent with the Py-GC/MS results.
- Among pyrolytic products, bibenzyl, toluene, p-benzoquinone, and hydroquinone were the main products. The formation of bibenzyl and toluene depended on the probability of molecular collision; while the formation of p-benzoquinone and hydroquinone were influenced by the thermodynamic factors.
- Small amounts of oxygenated compounds and polycyclic compounds were produced from the secondary pyrolysis of main products at 600 °C. The energy barriers and total enthalpy changes of the minor products formation were high, which led to their low yields.
This work was supported by the National Major Fundamental Research Program of China (973 program, No. 2013CB228101) and the State Natural Sciences Foundation of China (No. 31270635).
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Article submitted: October 15, 2015; Peer review completed: January 30, 2016; Revised version received and accepted: February 19, 2016; Published: February 29, 2016.