A study of mesophase formation from a low temperature coal tar pitch using formaldehyde as a promoter for polymerisation

摘要

Low temperature coal tar pitches (LTCTP) develop mesophase rapidly, due to the presence of high contents of heteroatoms and aliphatic groups [1,2]. The mesophase originating from these pitches displays a high tendency to coalescence, because of both the low viscosity and the low primary quinoline insoluble content. Furthermore, low carbonisation yields are obtained due to the low aromaticity of the pitch [1]. In this study, areduction in the excessively high coalescence tendency of this kind of pitch is attempted by increasing the viscosity of the reaction media [3], while improving the carbonisation yields. Taking into account the high content of phenolic compounds of LTCTP the authors considered their polymerisation with formaldehyde as a possible approach to increase viscosity. This is the typical reaction in the synthesis of phenol-formaldehyde resins [4,5], which can be performed at moderate temperature with acid or basic catalysts and involves the creation of methylene bridges through the formation of methylol groups, finally leading to larger aromatic molecules.The characteristics of the LTCTP used in this study (CTP1) are listed in Table 1. It has a low C/H atomic ratio and low contents of insoluble fractions, indicatinga low degree of aromatic condensation. Accordingly, a high concentration of aliphatic side chains is indicated by the high proportion of aliphatic hydrogen. Finally, the pitch has a high content of oxygen, mainly in -phenolic compounds [1,2].Polymerisation reactions were carried out with formaldehyde (Paraformaldehyde 95 percent, Aldrich) in an experimental set-up described elsewhere [2]. Typically, 100 g of pitch were heated at 2 deg C mkr~(-1) up to 80-170 deg C. The formaldehyde (5 percent or 13.3 percent) and the catalyst (20 ml of 4 percent wt/wt NaOH/ethanol solution for basic catalysis and 2 g of oxalic acid for acidic catalysis) was then added and the reaction mass stirred at 160 rpm.