{"title":"The influence of molecular structure on the oxidation reactivity of long-chain ethers: Experimental observation and theoretical analysis","authors":"Shiliang Wu, Jiajing Bao, Hongfei Bie, Yuan Liu","doi":"10.1016/j.fuel.2024.132345","DOIUrl":null,"url":null,"abstract":"<div><p>Long-chain ethers have been proposed as promising biofuels for advanced combustion methods, yet their low-temperature oxidation (LTO) characteristics remain poorly understood. In this study, the LTO reactivities of <em>n</em>-heptane and five long-chain ethers with different structures, including dibutyl ether (DBE), diethylene glycol dimethyl ether (DGM), polyoxymethylene dimethyl ethers (PODE), 1,2-dimethoxyethane (1,2-DME), and dipropyleneglycol dimethyl ether (DPGDE) are investigated using a cooperative fuel research (CFR) engine, which is closer to the actual internal combustion engine operating environment. Products formed from LTO of ethers and <em>n</em>-heptane are investigated over a wide range of compress ratios (CRs), and their relationship to the global oxidation reactivity is suggested based on the quantum chemistry calculation. The presence of oxygen atoms in long-chain ethers significantly enhances oxidation reactivity, with the global oxidation reactivity ranking as follows: DGM > DPGDE > 1,2-DME > DBE > PODE > n-heptane. The key intermediate species generated during the LTO of <em>n</em>-heptane include aldehydes, ketones, and cyclic ethers, while oxygen atoms in ethers facilitate the formation of acids. The species pathway analysis and quantum chemistry calculations reveal that (1,5) and (1,6) H-transfers of alkylperoxy radical (<span><math><mrow><mtext>R</mtext><mtext>O</mtext><mover><mtext>O</mtext><mo>̇</mo></mover></mrow></math></span>) are critical chain-propagation channels, playing pivotal roles in the LTO process. The impact of oxygen on oxidation reactivity can be attributed to two primary factors: accelerating the H-abstraction of <span><math><mrow><mover><mtext>O</mtext><mo>̇</mo></mover><mtext>H</mtext></mrow></math></span> and H-transfer of <span><math><mrow><mtext>R</mtext><mtext>O</mtext><mover><mtext>O</mtext><mo>̇</mo></mover></mrow></math></span> by weakening the neighboring C-H bonds, and enhancing the branching ratio of H-transfer of <span><math><mrow><mtext>R</mtext><mtext>O</mtext><mover><mtext>O</mtext><mo>̇</mo></mover></mrow></math></span>. The arrangement of two oxygen atoms between every two carbon atoms (–OCH<sub>2</sub>CH<sub>2</sub>O–) is optimal for oxidation reactivity, while the arrangement of oxygen and carbon (–OCH<sub>2</sub>OCH<sub>2</sub>–) weakens the oxidation activity due to fewer available hydrogens for H-transfer. The methyl group in branching-chain, exhibit reduced oxidation reactivity due to the strength of C-H bonds. Additionally, for ethers with the same structure, a longer carbon chain allows for more available hydrogens, resulting in stronger oxidation reactivity.</p></div>","PeriodicalId":325,"journal":{"name":"Fuel","volume":null,"pages":null},"PeriodicalIF":6.7000,"publicationDate":"2024-06-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Fuel","FirstCategoryId":"5","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0016236124014935","RegionNum":1,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"ENERGY & FUELS","Score":null,"Total":0}
引用次数: 0
Abstract
Long-chain ethers have been proposed as promising biofuels for advanced combustion methods, yet their low-temperature oxidation (LTO) characteristics remain poorly understood. In this study, the LTO reactivities of n-heptane and five long-chain ethers with different structures, including dibutyl ether (DBE), diethylene glycol dimethyl ether (DGM), polyoxymethylene dimethyl ethers (PODE), 1,2-dimethoxyethane (1,2-DME), and dipropyleneglycol dimethyl ether (DPGDE) are investigated using a cooperative fuel research (CFR) engine, which is closer to the actual internal combustion engine operating environment. Products formed from LTO of ethers and n-heptane are investigated over a wide range of compress ratios (CRs), and their relationship to the global oxidation reactivity is suggested based on the quantum chemistry calculation. The presence of oxygen atoms in long-chain ethers significantly enhances oxidation reactivity, with the global oxidation reactivity ranking as follows: DGM > DPGDE > 1,2-DME > DBE > PODE > n-heptane. The key intermediate species generated during the LTO of n-heptane include aldehydes, ketones, and cyclic ethers, while oxygen atoms in ethers facilitate the formation of acids. The species pathway analysis and quantum chemistry calculations reveal that (1,5) and (1,6) H-transfers of alkylperoxy radical () are critical chain-propagation channels, playing pivotal roles in the LTO process. The impact of oxygen on oxidation reactivity can be attributed to two primary factors: accelerating the H-abstraction of and H-transfer of by weakening the neighboring C-H bonds, and enhancing the branching ratio of H-transfer of . The arrangement of two oxygen atoms between every two carbon atoms (–OCH2CH2O–) is optimal for oxidation reactivity, while the arrangement of oxygen and carbon (–OCH2OCH2–) weakens the oxidation activity due to fewer available hydrogens for H-transfer. The methyl group in branching-chain, exhibit reduced oxidation reactivity due to the strength of C-H bonds. Additionally, for ethers with the same structure, a longer carbon chain allows for more available hydrogens, resulting in stronger oxidation reactivity.
期刊介绍:
The exploration of energy sources remains a critical matter of study. For the past nine decades, fuel has consistently held the forefront in primary research efforts within the field of energy science. This area of investigation encompasses a wide range of subjects, with a particular emphasis on emerging concerns like environmental factors and pollution.