Unveiling the low-temperature oxidation chemistry of dipropyl carbonate

Abstract

Dialkyl carbonates (DACs) own an environmentally friendly synthesis route, making them potential candidates as alternative fuels. However, for DACs to be widely accepted as an alternative fuel, a comprehensive understanding of their combustion behavior is essential. Dipropyl carbonate (DPrC) represents a transition from short-chain to mid-chain carbonates, understanding its combustion behaviors holds significance in unraveling the combustion chemistry of carbonates. In this study, the oxidation of DPrC was investigated with the initial fuel mole fraction of 0.5% at three equivalence ratios of 0.5, 1.0, and 2.0 within a temperature range of 550–1100 K in a jet-stirred reactor for the first time. Gas chromatography was utilized for the quantitative detection of reactants, intermediates, and products. A detailed DPrC mechanism was first developed, and good agreements between measurements and simulations were obtained. A notable negative temperature coefficient (NTC) behavior was first observed in the oxidation of DACs. Such NTC phenomenon occurred at fuel-lean conditions in the temperature range of 620–660 K, while only a weak low-temperature consumption was observed at the stoichiometric condition. Kinetic modeling studies showed that this unique low-temperature chemistry of DPrC can be attributed to the differences in the RO₂ isomerization reactions between DPrC and short-chain DACs. The RO₂ isomerization via a six-member ring transition state could happen in DPrC oxidation but not in dimethyl carbonate and diethyl carbonate oxidation, due to the different fuel molecular structure. Therefore, the subsequent reaction pathways via QOOH → O₂QOOH → HO₂Q = O + OH → OQ = O + OH were promoted and two OH radicals were released in this process. Moreover, it is conceivable that mid or long-chain DACs could also exhibit an NTC phenomenon due to the increased potential for RO₂ isomerization via a six- or seven-member ring transition state, thereby increasing the likelihood of RO₂ isomerization occurrence.