Journal of the Korean Wood Science and Technology

The Korean Society of Wood Science & Technology (한국목재공학회)
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Pyrolysis of Lignin Obtained from Cinnamyl Alcohol Dehydrogenase (CAD) Downregulated Arabidopsis Thaliana

  • Kim, Kwang Ho (Clean Energy Research Center, Korea Institute of Science and Technology) ;
  • Kim, Jae-Young (Graduate School of International Agricultural Technology and Institute of Green-Bio Science and Technology, Seoul National University) ;
  • Kim, Chang Soo (Clean Energy Research Center, Korea Institute of Science and Technology) ;
  • Choi, Joon Weon (Graduate School of International Agricultural Technology and Institute of Green-Bio Science and Technology, Seoul National University)
  • Received : 2019.03.25
  • Accepted : 2019.07.08
  • Published : 2019.07.25
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Abstract

Despite its potential as a renewable source for fuels and chemicals, lignin valorization still faces technical challenges in many aspects. Overcoming such challenges associated with the chemical recalcitrance of lignin can provide many opportunities to innovate existing and emerging biorefineries. In this work, we leveraged a biomass genetic engineering technology to produce phenolic aldehyde-rich lignin structure via downregulation of cinnamyl alcohol dehydrogenase (CAD). The structurally altered lignin obtained from the Arabidopsis thaliana CAD mutant was pyrolyzed to understand the effect of structural alteration on thermal behavior of lignin. The pyrolysis was conducted at 400 and $500^{\circ}C$ using an analytical pyrolyzer connected with GC/MS and the products were systematically analyzed. The results indicate that aldehyde-rich lignin undergoes fragmentation reaction during pyrolysis forming a considerable amount of C6 units. Also, it was speculated that highly reactive phenolic aldehydes facilitate secondary repolymerization reaction as described by the lower yield of overall phenolic compounds compared to wild type (WT) lignin. Quantum mechanical calculation clearly shows the higher electrophilicity of transgenic lignin than that of WT, which could promote both fragmentation and recondensation reactions. This work provides mechanistic insights toward biomass genetic engineering and its application to the pyrolysis allowing to establish sustainable biorefinery in the future.

Keywords

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Fig. 1. The main pathway involved in monolignol biosynthesis in biomass cell wall. The three monolignol precursors shown with a gray background resulted from the downregulation of CAD gene. PAL, phenylalanine ammonia lyase; C4H, cinnamate 4-hydroxylase; 4CL, 4-coumaric acid-CoA ligase; HCT, hydroxycinnamoyl-CoA shikimate hydroxycinnamoyl transferase; pC3H, p-coumarate 3-hydroxylase; CCR, cinnamoyl-CoA reductase; CCOMT, caffeoyl-CoA O-methyltransferase; F5H, ferulate 5-hydroxylase; COMT, caffeic acid O-methyltransferase; CAD, cinnamyl alcohol dehydrogenase.

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Fig. 3. Gas chromatograms obtained from the pyrolysis of WT and CAD mutant at 500 ℃.

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Fig. 4. Yields of lignin derivatives based on number of carbons in side chain obtained at 400 ℃ (left) and 500 ℃ (right).

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Fig. 5. Yields of lignin derivatives based on lignin subunits (H, G and S units) obtained at 400 ℃ (left) and 500 ℃ (right).

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Fig. 6. Optimized geometry of (A) β-O-4’ (guaiacylglycerol-β-guaiacyl ether) structure found in WT and (B) 8-O-4’ dimer (guaiacylacrylaldehyde-β-guaiacyl ether) found in CAD downregulated biomass.

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Fig. 7. Electrophilicity index of β-O-4’ structure found in WT and 8-O-4’ dimer found in CAD downregulated biomass.

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Fig. 2. (A) Main structures of typical lignin subunits and (B) new structures present in the lignin of CAD downregulated biomass (Zhao et al., 2013).

Table 1. The yield of pyrolysis products from WT and CAD mutant lignin at 400 and 500 ℃ (μg/mg lignin).

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