16
Thermal
Conversion
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 14, 2016 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch016
Aromatic Compounds
of to
Polynuclear Carbon
1
I. C. Lewis and L. S. Singer
Carbon Products Division, Parma Technical Center, Union Carbide Corporation, Cleveland, O H 44101
The pyrolysis of polynuclear aromatic compounds and their mixtures produces carbonaceous pitchlike residues and ultimately carbon and graphite. Reaction studies on model aromatic hydrocarbons can be used to illustrate the chemical changes involved in this complex process. The reactions largely involve aromatic polymerizations accompanied by bond cleavage and rearrangement. The tendency of large polynuclear aromatic compounds to associate into a liquid crystalline state plays a key role in their conversion to carbon. Studies using electron paramagnetic resonance—electron nuclear double resonance can clarify the nature of the stable free radicals that develop during carbonization. These aspects are reviewed to provide a clearer understanding of the chemistry involved in the transformation of polynuclear aromatic compounds to carbon.
P O L Y N U C L E A R A R O M A T I C C O M P O U N D S can be transformed thermally into
carbonaceous residues and ultimately to carbon and graphite by a process called carbonization. The chemistry of carbonization is exceedingly complex and encompasses a wide variety of reaction types including bond cleavage, polymerization, molecular rearrangement, and hydrogen transfer. Even with a single aromatic hydrocarbon as a starting material, pyrolysis leads initially to a diversity of products. Detailed thermal reaction studies have been carried out on a number of polynuclear aromatic compounds (1-5). Various experimental as well as theoretical techniques have been used in attempting to clarify the nature of these reactions (6-8). The literature on the thermal Current address: 525 Race Street, Berea, OH 44017 0065-2393/88/0217-0269$06.00/0 © 1988 American Chemical Society
Ebert; Polynuclear Aromatic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 14, 2016 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch016
270
POLYNUCLEAR AROMATIC COMPOUNDS
reactions of polynuclear aromatic mixtures as they occur naturally i n petro leum and coal-derived products is extensive. Scheme I is a generalized scheme for the transformation of a polynuclear aromatic hydrocarbon to carbon and graphite. Heat treatment at about 350500 °C leads to a complex reaction product mixture designated as pitch. Further reaction at temperatures near 500 °C results in an infusible poly meric hydrocarbon mixture designated as coke. As the heat-treatment proc ess continues, the remaining hydrogen is removed, and a two-dimensional carbon polymer is formed. Finally, at temperatures near 3000 °C, threedimensionally ordered graphite is produced.
I AROMATIC HYDROCARBON | J(
300-500° C )
PITCH (COMPLEX,FUSIBLE HYDROCARBON MIXTURE) (~500 C) e
COKE (INFUSIBLE POLYMERIC HYDROCARBON MIXTURE) (~1000 C) e
CARBON (TWO DIMENSIONAL CARBON POLYMER) (~3000°C)
Scheme I. Transformation of a polynuclear aromatic hydrocarbon to carbon and graphite.
Providing complete reaction mechanisms for any of these stages has not been possible. However, from detailed studies of the initial thermal reac tions, general concepts pertaining to the thermal conversion of polynuclear
Ebert; Polynuclear Aromatic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
16.
LEWIS & SINGER
271
Thermal Conversion to Carbon
aromatic compounds to carbon have been developed. These studies are reviewed in this chapter with the objective of clarifying the chemistry of the carbonization process.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 14, 2016 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch016
Interactions in Polynuclear Aromatic Mixtures (Nature of Pitch) Pitches are complex fusible aromatic mixtures generally obtained from the pyrolysis of coal- and petroleum-derived fractions. However, the pyrolysis of even a single aromatic hydrocarbon results in a product almost as complex as pitches derived from coal or petroleum. Pitches have some unique properties that are important for the sub sequent transformation to carbon. Solid pitches behave as eutectic glasses. They exhibit glass transitions and melt to a low viscosity liquid over a broad temperature range. The results of a difierential scanning calorimetric (DSC) experiment for measuring the glass transition temperature of a naphthalenederived pitch are shown i n Figure 1. Pitches exhibit another significant feature, namely, a tendency to form a liquid crystalline state. The devel opment of a mesophase stage during conversion of pitches or aromatic hy drocarbons to coke was first discovered by Brooks and Taylor (9). Depending on structure, mixtures of polynuclear aromatic compounds of a sufficiently large size form a nematic liquid crystalline phase. This phenomenon is i l lustrated i n Figure 2, which contains polarized light photomicrographs of a petroleum pitch derived from decant oil as it is heat treated at 400 °C. This heat treatment results in volatilization of lower molecular weight components and polymerization of the more reactive species. The higher molecular
0
50
100
150
200
TEMPERATURE
