Wet explosion pretreatment of loblolly pine leads to an increase in methoxylation of the lignin

Abstract

Background: In biorefineries, various pretreatments traditionally employ hazardous chemicals (ammonia, sulfuric acid, sulfite, etc.) for opening the softwood structure and to facilitate easy accessibility of the cellulose for further downstream processing. The resultant lignin (known as technical lignin) after extraction of the carbohydrate fraction as sugars has been either burned as fuel or used in biochemical or biofuel production. It has been observed that the technical lignin after such biomass pretreatments is often more condensed and, hence, cannot be easily used to produce fine chemicals of high value. In this study, we examine lignin after wet explosion pretreatment where the biomass in subjected to oxygen to understand how these interactions will affect lignin utilization for biochemical production.

Oxidative degradation of biorefinery lignin obtained after pretreatment of forest residues of Douglas Fir

“…The dry biomass sample was milled using a Retsch cutting mill SM 200 (Retsch Inc. Newtown, PA, USA), screened and sieved through a 1.6 mm mesh. The FS-10 sample was then wet exploded according to the method previously described Rana et al, 2015), using a temperature of 185°C, 25 min residence time and 5% O2 loading, followed by enzymatic hydrolysis using 40 mg enzyme /g cellulose with a 90/10…”

“…During this process, all the lignin-derived organic compounds will be selectively extracted into the ethyl acetate phase and can be recovered by properly pipetting out the supernatant after cooling to room temperature conditions. The GC-MS conditions for analysis were similar to described in previous work (Rana et al, 2015). Briefly, 1 μL sample was injected into helium (used as carrier gas) flowing at 60 mL/min which then passes through a DB-5 (30m length X 250μm I.D.…”

“…Lignin is a highly branched amorphous heteropolymer connected through a system of C-C and phenylpropane linkages and is made up from oxidative coupling of hydroxyphenyl (H), guaiacyl (G) and syringyl (S) components (Himmel et al, 2007). While the SPORL pretreatment of the forest residues results in formation of lignosulfonates, studies have indicated that the lignin obtained from wet explosion pretreatment retained its native structure with limited modifications primarily since there were no harmful chemicals added during the pretreatment process (Rana et al, 2015).…”

“…Briefly, wet explosion pretreatment is a combination of wet oxidation and steam explosion using oxygen as main oxidant to degrade lignocellulosic biomass structure Rana et al, 2012). Previous studies on wet exploded loblolly pine lignin at optimal conditions for sugar production indicated that structural modifications within the lignin molecule occurred in the form of methoxylation of the hydroxyphenyl components present in lignin (Rana et al, 2015). These studies indicated that an increase in the methoxylation of the lignin components resulted in a reduced recondensation and reploymerization of the lignin making it an ideal feedstock for further conversions, which potentially could lead to bio-products of high value, which could bring down the cost of biofuels production.…”

“…These studies indicated that an increase in the methoxylation of the lignin components resulted in a reduced recondensation and reploymerization of the lignin making it an ideal feedstock for further conversions, which potentially could lead to bio-products of high value, which could bring down the cost of biofuels production. Also in most studies on lignin, the sugars are separated from the lignin fraction using either alkali treatment (Rocha et al, 2012) or using organic solvents (Rana et al, 2015;Ke et al, 2011) resulting in a cleaner lignin fraction for further deconstruction to biochemicals. However, these purification/separation processes can significantly affect process economics and the absence of data on the structure of lignin from forestry residues further complicates process design.…”