High-pressure pyrolysis of isoprenoid hydrocarbon p-menthane in a tandem micro-reactor with online GC–MS/FID

Abstract

Isoprenoids have been synthesized in large quantities through metabolic engineering method using some microorganisms as hosts and biomass as carbon resources. An isoprenoid cyclic hydrocarbon, p-menthane, is considered as a promising “drop-in” fuel to substitute traditional cycloalkane components, however, its pyrolysis data is lacking so far which is a significant part of the fuel combustion chemistry, especially under high pressures. In this work, pyrolysis experiments of p-menthane were carried out in a tandem micro-reactor under temperature of 450–800 °C and pressure of 2 MPa. 39 species were identified and quantified by online GC–MS/FID, based on which some initial pyrolysis pathways were determined. The early production of 4-methyl-1-cyclohexene and 1-isopropyl-1-cyclohexene indicates that methyl and isopropyl scission happen at the initial stage followed by H-transfer reaction and dehydrogenation. The activation energy Ea and pre-exponential factor A of the overall reaction were calculated as 225 kJ mol−1 and 1.17 × 1012 s−1, respectively. For secondary reactions, many conjugated dienes such as 1,3-butadiene and 2-methyl-1,3-butadiene and small olefins such as ethylene and propene are generated through the ring-opening pathways. Benzene and toluene are the most abundant products among aromatics detected which are formed through dehydrogenation and demethylation of some early species like 4-methyl-1-cyclohexene and side-chain scission of aromatics. A particular product 1,3-hexadien-5-yne was detected which is generated through ring-opening pathways followed by H-transfer and demethylation reactions and could isomerized to benzene. A lumped kinetic model was proposed to describe the pyrolysis of p-menthane which exhibited good agreement with the experimental data.