Metal Ion‐Catalyzed Hydrothermal Liquefaction of Calcium Lignosulfonate in Subcritical Water

Three metal ion catalysts, namely, Na⁺, K⁺, and Ca²⁺, were tested to improve liquefaction of calcium lignosulfonate, and their effects on product distribution were specifically investigated. Results showed that metal ion catalysts favored the production of hydrogen as well as of phenolic compounds. The total contents of phenolic compounds catalyzed by metal ions were generally higher than 75 %. The catalysis abilities of Na⁺ and K⁺ were better than that of Ca²⁺. The neutral‐alkaline condition was much more beneficial to calcium lignosulfonate liquefaction. Compared to the hydrochars with Na⁺ and K⁺ catalysts, the hydrochars with Ca²⁺ catalyst had higher carbon content and a higher heating value.