Thermal degradation of carbohydrates, cellulose and their esters with boric acid

Abstract

Thermal degradation of hydrocarbons, cellulose and wood pulp has been studied for a long time and has attracted much attention [1–4]. Pyrolysis of cellulose above the temperature of chemical decomposition is accompanied by carbonization. The fibrous structure is retained in the carbon-graphite compounds [5]. Cellulose pyrolysis in the temperature range of carbonization is affected by exothermal reactions according to DTA data and by the formation of new C-C bonds with heat energy liberation (186 kcal/mole). The formation of chemical bonds between the carbon atoms in the sp 3 hybrid state in the form of tetrahedral σ-bonds produces a diamond structure. Carbon atoms which are in the sp 2-hybrid state form laminar structures of aromatic rings. If the layers are oriented in relation to each other, we are dealing with a graphite structure, the type of which depends on the law of alternation of layers [6]. The carbonization mechanism of cellulose owing to its complexity, has not been examined. Assumptions can only be made in this respect concerning the participation of individual fragments from chemical decomposition of cellulose in the formation of carbocyclic condensed structures. Unsaturated compounds probably play the main role, being formed as intermediate products of decomposition, and take part in the Diels-Alders thermal reactions of condensation. These processes are often favoured by a disturbance in electron equilibrium, e.g. the electron-acceptor effect of carbonyl groups of dienophyls, as shown by Kato [7]. Various, structures of products of decomposition of cellulose, consisting of four carbon atoms [8] and compounds with aromatic properties—furane, styrene and anthracene—may take part in the reaction as dienes. The presence of diene and dienophyl thus promotes carbocyclization by the mechanism of circular electron transition. The formation of carbon compounds is considerably influenced by factors related to the formation of nuclei of carbon particles. This is promoted by various catalysts of heterogeneous topochemical reactions, e.g. transition metal carbides. ▪