关键词: 纤维素, 慢速热解, 炭化, 分子内/间氢键, 二维相关红外光谱

Abstract:

The slow pyrolysis of cellulose was conducted in a fixed bed reactor at a temperature ranging of 250—550℃. The formation and evolution characteristics of residual char was investigated by elemental composition analysis and two-dimensional infrared correlation spectroscopy. The inter- and intra-molecular hydrogen bonding networks in cellulose were identified and the change of hydrogen bonding patterns during the initial stage of pyrolysis was investigated. The decomposition of cellulose during slow pyrolysis was mainly focused on 250—360℃ with a maximum decomposition rate at about 300℃. The predominant reaction of cellulose carbonization at 250—300℃ was dehydration. Furthermore, the O(3)H…O(5) intra-molecular H-bonds were found to be the most sensitive to temperature, and it changes before that of the methylene groups. However, it changes after that of O(2)H…O(6) intra-molecular and O(6)H…O(3) inter-molecular H-bonds. In the initial stage of carbonization, the cleavage of O(6)H…O(3) intra-molecular H-bonds resulted in the formation of free hydroxyl groups and the tight cellulose structure was loosen up. With temperature rising, the cleavage of intra-molecular H-bonds and the subsequent dehydration generated large amount of carbonyl, double bonds and cyclic ethers in the char. At 300℃, the ring-opening of pyran structure together with the cleavage of glycosidic bond gave rise to the drastic degradation of cellulose. As a result, substantial amount of aliphatic hydrocarbons with double bonds and carbonyl functional groups were generated in the char. These groups were further underwent molecular rearrangement, condensation and aromatization and resulted in the formation of aromatic ring and aryl alkyl ethers. In 300—460℃, the predominant reaction of cellulose carbonization was deoxygenation, such as decarbonylation and decarboxylation. Meanwhile, the content of aliphatic hydrocarbons in char decreased gradually and that of the aromatic structure increased. The predominant carbonization reaction shifted from deoxygenation toward dehydrogenation as temperature exceeded 460℃, such as demethylation and demethylenation. The aromaticity of char increased with temperature and a highly condensed aromatic structures emerged in the char.

Key words: cellulose, slow pyrolysis, carbonization, intra-/inter-molecular hydrogen bond, two-dimensional infrared correlation spectroscopy