Hydrothermal liquefaction of cellulose in subcritical water—the role of crystallinity on the cellulose reactivity

The hydrothermal liquefaction of cellulose in subcritical water using reaction times from 25 to 110 minutes is studied. Major emphasis is put on the impact of the cellulose crystallinity on the degree of liquefaction, on the yields of the major reaction products, glucose and 5-(hydroxymethyl)furfural (5-HMF), and on the composition of the solid reaction residues. It is demonstrated that the crystallinity index (CI) of the used cellulose starting material plays a crucial role in the reaction mechanisms, the reaction rates and obtained product yields. Amorphous starting materials show a larger extent of liquefaction. Thus, 64% total liquid carbon is obtained for cellulose with a CI of 42 (MCC42), whereas that of MCC74 (CI 74) is 45%. The major liquefaction products are glucose and 5-HMF. It is shown that the cellulose conversion commences with a fast decomposition of the amorphous material domains, followed by a slower conversion of the crystalline cellulose domains.