Effect of KCl and CaCl2 loading on the formation of reaction intermediates during cellulose fast pyrolysis

Abstract

The influence of KCl and CaCl2 on the primary reactions of cellulose pyrolysis is studied using a wire-mesh reactor from 250°C to 600°C, focusing on the reaction intermediates. A pre- column derivatization with benzoyl chloride prior to HPLC analysis is applied for the quantification of anhydro-sugars (levoglucosan, cellobiosan, maltosan) from pyrolysis. At low temperatures, the additions of inorganics salts, especially CaCl2, weakens hydrogen bonds, resulting in high yields of levoglucosan and cellobiosan from the cleavage of glycosidic bonds rather than from dehydration reactions. At elevated temperatures, dehydration reactions in the sugar units are mainly responsible for the destruction of sugar rings followed by the scission of pyran rings, leading to the weight-loss of CaCl2-loaded cellulose in the form of low molecular weight organic species. Meanwhile, accumulated unsaturated structures suppress the cleavage of glycosidic bonds, leading to the formation of char. However, KCl appears to catalyze the cleavage of glycosidic bonds or the scission of pyran rings directly, which perhaps occurs through a homolytic mechanism, leading to low molecular weight species. Furthermore, maltosan is shown to be a secondary product and is catalyzed by KCl and CaCl2 indirectly through the repolymerization of levoglucosan in the solid phase. A modified mechanism is also proposed regarding cellulose pyrolysis and the primary catalysis of KCl and CaCl2.