Chemical Structure of Coal and Petroleum Pitches

Abstract

The chemical structure of four pitches is discussed. Each pitch was separated into nine fractions by solvent fractionation at room temperature with pyridine, quinoline and a series of binary solvents consisting of n-hexane and benzene.The average structure of the fractions obtained was estimated by elementary analysis, MW, GPC, IR, UV and NMR.Results of the characterisations of four pitches are as follows:(1) Coal tar pitch, (i) Coal tar pitch contains about 25% of the quinoline soluble and insoluble fractions. It is suggested that this pitch contains a much condensed aromatic ring system from these fractions.(ii) Fractions from n-hexane soluble to pyridine soluble have the largest aromaticity and lowest H/C ratio compared with the corresponding fractions of the other pitches.(iii) On the basis of the parameter Cp/Ca (or Hau/Ca), the average structural unit of each fraction tends to increase in the number of the aromatic rings with increasing fraction number, from 3-4 to 8-9 rings. Moreover, it was suggested from the parameters σ and Ln, that about 10% of the peripheral carbon atoms in the aromatic sheet are replaced by methyl groups.(iv) The average molecule of the fractions appears to consist of less than two average structural units, particularly, the aromatic rings in the average molecule for Fr. 1 to Fr. 4 seem to have nearly equal numbers of average structural units.(2) Naphtha cracking tar pitch; (i) Fractions of this pitch have the largest average molecular weight and lowest H/C ratio of all pitches studied. It is suggested that naphtha cracking tar pitch contains fairly aliphatic structures.(ii) The average structural units of each fraction contain from 3 to 8 rings varying with descending order of the fraction and consisting of condensed aromatic systems. It was suggested from a and Ln that about 20-30% of peripheral carbon atoms in the average structural unit are replaced by ethyl groups.(iii) The average molecule of the fractions appears to consist of condensed aromatic ring systems with 2-3 average structural units for Fr. 1 Fr. 5 and more than 4 average structural units for Fr. 6 and Fr. 7.(3) Catalytic cracking tar pitch of crude oil; The characteristics of this pitch appear to resemble coal tar pitch. However, the H/C ratio and molecular weight were slightly larger than coal tar pitch.(4) Prepared pitch from polymerized naphthalene; This pitch is different from the other pitches described above. The structure of fractions of this pitch appears to consist of a mixture, with more than three units of naphthalene bonded at the 2-and 6-positions of the naphthalene nucleus and perylene type products. Moreover, these products appear to contain some aliphatic structures.