Effect of water–carbon dioxide ratio on the selectivity of phenolic compounds produced from alkali lignin in sub- and supercritical fluid mixtures

Abstract

Lignin is considered as the most abundant renewable carbon source after cellulose and noncommercialized waste product with constantly growing annual production exceeding 50 million tons per year. This article is focused on a newly developed selective synthesis of high-value phenolic products from lignin in presence of supercritical carbon dioxide (scCO 2) known as sustainable, non-flammable, naturally abundant, and catalytically active solvent. Depending on the synthesis conditions, such as temperature (250, 300, and 350°C) and water-to-scCO 2 ratio (1:5, 1:2, 1:1, and 2:1), high yield of the specific phenolic compounds such as phenol, guaiacols, and vanillin has been achieved. The GCMS analysis reveales the trend of the increased total phenolic yield with temperature and strong dependence of the selectivity on the water-to-scCO 2 ratio. The maximum selectivity toward formation of the specific phenolic products, such as guaiacol and vanillin was observed at the highest H 2 O:scCO 2 =1:5 ratio. At 350 °C the relative yield of guaiacol was ~39% whereas at 250 °C vanillin was a dominant phenolic monomer with a relative yield of ~33%. Moreover, at the intermediate temperature of 300 °C both guiacol and

Hydrothermal Depolymerization of Biorefinery Lignin-Rich Streams: Influence of Reaction Conditions and Catalytic Additives on the Organic Monomers Yields in Biocrude and Aqueous Phase

“…Even though the mass of depolymerized material was almost constant, the synergetic effect of sCO 2 together with subcritical water was highlighted by an enhanced generation of phenol, methoxyphenols and dimethoxyphenols compared to the results in absence of catalysts, as reported in Figure 10. Similar catalytic effect has been explained by Numan-Al-Mobin et al [24], observing that sCO 2 in hydrothermal medium acts like acid homogeneous catalyst promoting the selectivity towards phenolic compounds. Compared to other homogeneous catalysts, the use of fluids at supercritical state with catalytic effects, like carbon dioxide, reduces the mass transportation limitations due to diffusivity and lower density that permits a higher penetration in the feedstock's pores.…”

“…Furthermore, the use of CO 2 at its supercritical state has been extensively investigated as selective solvent for phenolic compounds separation and extraction [20][21][22][23]. Moreover, Numan-Al-Mobin [24] converted alkali lignin using a mixture of subcritical water and supercritical carbon dioxide, varying temperature (250, 300, and 350 • C) and water-to-sCO 2 ratio (1:5, 1:2, 1:1, and 2:1) showing an enhanced yield of specific phenolic compounds and suggesting a conversion mechanism driven by the heterogeneous acid catalytic activity of the dissolved CO 2 . In another study, Chan et al [25] studied the effect of supercritical carbon dioxide on the liquefaction of palm kernel shell, suggesting an improved biocrude yield at lower temperature due to the higher CO 2 dissolution in water enhancing the protons availability in the water medium.…”

Kraft Lignin–Tannic Acid as a Green Stabilizer for Oil/Water Emulsion

“…It has abundant functional groups, mostly aliphatic hydroxyl and phenolic hydroxyl groups. Annually, more than 50 million tons of lignin are produced in the pulping industry . The majority of lignin is combusted to produce energy, but its utilization as a filler or binder accounts for a share of 2–5% only.…”

Catalytic in Situ Hydrogenolysis of Lignin in Supercritical Ethanol: Effect of Phenol, Catalysts, and Reaction Temperature

“…Various thermochemical methods for the effective depolymerization of lignin into liquid fuel (named bio-oil) or chemicals have been investigated, including pyrolysis, − hydrolysis, − and hydrogenolysis. ,− Among those methods, the pyrolysis process has been successfully confirmed to be uncontrollable, leading to inevitable recondensation as it always involved different kinds of free-radical reactions. − The hydrolysis process, mainly catalyzed over either acidic or basic catalysts (e.g., H 2 SO 4 , NaOH), was always accomplished through the hydrolytic cleavage of the aryl–alkyl ether bonds in lignin, which would also lead to repolymerization into either oligomers or char due to the presence of reactive phenolic monomers. , Hydrogenolysis of lignin has been shown to be effective and feasible for the conversion of lignin to liquid fuel or other high-valued chemicals. However, the hydrogenolysis process is always in need of supplying external hydrogen and recycling and/or purification of the unreacted hydrogen gas, which would add to the cost for the conversion. − The in situ hydrogenolysis of lignin has been extensively studied recently using hydrogen-donor solvents (such as ethanol, 2-propanol, formic acid, etc.)…”