Hydrothermal Conversion Of Lignin And Application Studies Of Its Products

Recently, studies on high-value chemicals and fuels from biomass have become aresearch hot spot, as biomass is widely existed, abundant and renewable. Hydrothermalconversion is a promising and environmentally friendly technology, with which the biomasscan be depolymerized and platform chemicals can be obtained. However, most of theresearchers have focused on the process studies of hydrothermal gasification and liquefactionof biomass. The biomass hydrothermal carbonization technologies as well as the applicationstudies of hydrothermal conversion products (liquefied products, hydrochar, etc.) are lacking.In this work, hydrothermal conversion of lignin was conducted, and the antioxidant abilitiesof the liquefied products were explored, and the liquefied products were successfullyclassified separated. The lignin derived hydrochars were tested as solid fuel and carbon-basedadsorption materials, respectively. Moreover, various biomass constituents (lignin, cellulose,etc.) were hydrothermally carbonized, and all of these biomass derived hydrochars werecharacterized and compared. These hydrochars were further newly applied for carbon basedsulfonated catalyst preparation.Black liquor alkaline lignin and lignosulfonate were liquefied from280-350℃. Theantioxidant abilities of liquefaction products were tested and compared with the raw materials.Results showed that after hydrothermal liquefaction, both the total phenol content and unitantioxidant power of the two lignin liquefaction products were improved, and alkaline ligninliquefaction products had a larger increase than lignosulfonate liquefaction products. Theinfluences of reaction time and temperature on oil yield, total phenol content and antioxidantpower of alkaline lignin liquefaction products were discussed. The total phenol content wasfound to have certain relationships with the antioxidant abilities. The suitable conditions foralkaline lignin hydrothermal liquefaction were at300℃,30min or320℃,15min. Moreover,thermal oxidation stability tests of tung oil showed that the addition of lignin liquefactionproducts obviously increased the oxidation stability time of tung oil at a relative lowtemperature (130℃), however, the antioxidant activity of lignin liquefaction products at150℃was somewhat low. So, it can be inferred that lignin hydrothermal liquefaction productshave the potential to become the valueable antioxidant. Alkaline lignin was liquefed under hydrothermal conditions at270-330℃, and the totaloil yield reached28.3wt%at a reaction temperature of300℃. The liquefed products wereeffectively separated into four main types of substances: benzenediols, monophenolichydroxyl products, weak-polar products, and water-soluble products (low-molecular-weightorganic acids, alcohols, etc.). The production process and yield of each classifed productswere discussed. More than half of the oil products were phenolics. A mechanism for phenolicproducts production from lignin liquefaction was proposed. The results indicated that thedecomposition of lignin under hydrothermal conditions occured mainly by three steps:hydrolysis and cleavage of the ether bond and the C-C bond, demethoxylation, and alkylation.Moreover, we found that higher temperature would favour the demethoxylation and alkylationreactions.Hydrothermal carbonization of cellulose, lignin, D-xylose (substitute for hemicelluloses),and wood meal (WM) was experimentally conducted between225and265℃, and thechemical and structure properties of the hydrochars were investigated. The hydrochar yieldwas between45and60%, and the yield trend of the feedstock was lignin> WM> cellulose>D-xylose. The hydrochars seemed stable before300℃, and aromatic structure was formed inall of these hydrochars. The C content, C recovery, energy recovery, ratios of C/O and C/H inall of these hydrochars were among63-75%,80-87%,78-89%,2.3-4.1, and12-15,respectively. The higher heating value (HHV) of the hydrochars was among24-30MJ/kg,with an increase of45-91%compared with the corresponding feedstock. Temperature wasvery important in the hydrothermal carbonization process. Higher temperatures generallyaccelerated the hydrothermal carbonization of biomass, resulting in hydrochars with loweryield, lower volatile matter content, lower ion exchange capacity, lower O-containingfunctional groups, but higher C content. The lignin hydrothermal carbonization mechanismwas that lignin hydrothermal carbonization products were made of polyaromatic hydrocharsand phenolic hydrochars.Formaldehyde was originally used as a polymerization agent to perform hydrothermalcarbonization of black liquor for solid fuel production from220to285℃. Compared tohydrochar prepared without formaldehyde, hydrochar produced in the presence of a2.8wt%formaldehyde solution (hydrochar-F) had0.27~1.13times higher yield,0.021~0.36times higher heating value (HHV),0.20~1.31times higher C recovery efficiency,0.20~1.44timeshigher total energy recovery efficiency,0.36~0.49times lower sulfur content, and0.11~0.52times lower ash content. The HHV of hydrochar-Fs were ranged from2.2×104to3.0×104kJ/kg, while the HHV of hydrochar-F produced at285℃was1.90times greater than that ofthe raw material (black liquor solid). Hydrothermal conversion of lignosulfonate with orwithout formaldehyde as polymerization agent was conducted at320℃. Compared with thehydrochar produced without formaldehyde addition (WF-hydrochar), the yield and surfacearea of the hydrochar produced with formaldehyde addition (F-hydrochar) increased33.8%and46.4%respectively. The Langmuir saturated adsorption capacity of Cu2+, methyl orange(MO) for F-hydrochar was19.6,16.3mg·g-1respectively. The Langmuir saturated adsorptioncapacity of MO for WF-hydrochar was18.9mg·g-1.Amorphous hydrochar sulfonated catalysts were generated from four kinds of biomass(lignin, cellulose, wood meal and D-xylose) by hydrothermal carbonization at varioustemperatures (225,245and265℃) followed by sulfonation, with a yield of36-56%. All ofthese catalysts owned aromatic structure, hydroxyl and carboxyl groups, and with a density ofSO3H groups between0.56and0.87mmol/g.5-Hydroxymethylfurfural (HMF) was directlyproduced from inulin in ionic liquids (ILs) through one step with the addition of hydrocharsulfonated catalysts, with a factual yield of47-65%at100℃,60min. Moderate extension ofreaction time (from30to90min) and increase of temperature (from80to120℃) promotedHMF production. The hydrochar sulfonated catalysts showed high reusability (the factualyield retained61%even after being used five times), as well as good catalytic activitycompared with traditional solid acid catalysts.Hydrolysis of two kinds of lignin model compounds was originally conducted with sixdifferent solid acid catalysts (732cation resin, HZSM-5, sulfated zirconia, Amberlyst-15cation resin, C225-SO3H and L225-SO3H). Monobenzone was used as the-O-4bond type oflignin model compound, and hydroquinone was the main product with a yield andproductivity of among5.6-47.7%,57.1-92.0%, respectively. Guaiacylglycerol--guaiacylether (GG) was employed as the-O-4bond type of lignin model compound, while guaiacolwas found the main product with a yield and productivity of among22.2-38.0%and63.0-100%, respectively. Compared with the homogeneous catalyst H2SO4, Amberlyst-15 cation resin, C-SO3H and L-SO3H showed comparable efficiency of conversion ratio butmuch higher selectivity of the phenolic products. These hydrochar sulfonated catalystsshowed unique advantages in the catalytic degradation and hydrolysis reactions, consideringthe renewable, aboundant, and low-cost properties of biomass, application of hydrocharsulfonated catalysts should be one of the potential industrialization way for biomasshydrothermal conversion.