Theoretical Study on the Hydrogenation Mechanisms of Model Compounds of Heavy Oil in a Plasma-Driven Catalytic System

Abstract

Heavy oil will likely dominate the future energy market. Nevertheless, processing heavy oils using conventional technologies has to face the problems of high hydrogen partial pressure and catalyst deactivation. Our previous work reported a novel method to upgrade heavy oil using hydrogen non-thermal plasma under atmospheric pressure without a catalyst. However, the plasma-driven catalytic hydrogenation mechanism is still ambiguous. In this work, we investigated the intrinsic mechanism of hydrogenating heavy oil in a plasma-driven catalytic system based on density functional theory (DFT) calculations. Two model compounds, toluene and 4-ethyltoluene have been chosen to represent heavy oil, respectively; a hydrogen atom and ethyl radical have been chosen to represent the high reactivity species generated by plasma, respectively. DFT study results indicate that toluene is easily hydrogenated by hydrogen atoms, but hard to hydrocrack into benzene and methane; small radicals, like ethyl radicals, are prone to attach to the carbon atoms in aromatic rings, which is interpreted as the reason for the increased substitution index of trap oil. The present work investigated the hydrogenation mechanism of heavy oil in a plasma-driven catalytic system, both thermodynamically and kinetically.Our previous work [7] reported a novel method to upgrade heavy oil using hydrogen non-thermal plasma under atmospheric pressure without a catalyst. Plasma is an electroneutral mixture and contains electroneutral gas molecules, as well as electrons, ions, atoms, radicals, photons and excited molecules, which are chemically and physically active species [8]. Due to the high reactivity, plasma has been used in many fields, such as gas cleaning, surface treatment and ozone production [9][10][11][12][13][14][15][16][17][18].In our previous work [7], although it has been demonstrated that non-thermal plasma could increase light oil yield significantly and add hydrogen to heavy oil under atmospheric pressure without a catalyst, the plasma-driven catalytic hydrogenation mechanism is still ambiguous, especially the reason for the increased substitution index of trap oil.The density functional theory (DFT) has been used to study the thermodynamics associated with steam reforming of small organic molecules (e.g., DiMethyl Ether and Ethanol) under cold plasma conditions [19,20]. These studies focused on the dissociation of original reactants forming radicals through the highly energetic electrons within cold plasmas and the following radical reactions. The calculation results showed that the thermodynamic obstacle was easy to be overcome under cold plasma conditions. A DFT study has also been used to study the interaction of plasma species with the catalyst surface to reveal the effect of plasma and explain the reaction mechanism under plasma conditions [21][22][23].The hydrogenation mechanisms of heavy oil on traditional catalysts were also widely investigated using DFT [24][25][26]. Due to the complex of reaction and reactants, model compounds, such a...

Low-temperature and atmospheric pressure plasma for palm biodiesel hydrogenation

“…3.6 . This produced ethyl and methyl radicals 22 from eighteen carbon chains (C18) to form sixteen carbon chain (C16) radicals. After that, the C16 radicals accepted the hydrogen radicals to form stable fatty acid methyl esters.…”

“…According to the previous studies, the catalytic hydrogenation reaction must be performed at high pressure (0.3–0.6 MPa) and elevated temperature (80–120 °C). It perhaps faces problems with high operating and maintenance costs from the high hydrogen pressure environment, as well as a slow deactivation of catalyst performance from the deposition of carbon or metals on the catalyst surface 22 . To avoid these issues, one of the alternative methods is to utilize plasma technology.…”

Upgrading palm biodiesel properties via catalyst‐free partial hydrogenation using needle‐plate dielectric barrier discharge plasma torch

“…The catalysts may also be deactivated by various mechanisms such as fouling or coking, poisoning, sintering, etc. 18 During the hydrogenation process, the conversion of cis-isomers to trans-isomers is a significant limitation of the hydrogenation process by catalysts. 13 When compared to the cis-isomer, the trans-isomer exhibits worse cloud point properties due to its higher crystallization point.…”

“…It can cause problems with high operating and maintenance costs as a result of using high hydrogen pressure. The catalysts may also be deactivated by various mechanisms such as fouling or coking, poisoning, sintering, etc 18 . During the hydrogenation process, the conversion of cis ‐isomers to trans ‐isomers is a significant limitation of the hydrogenation process by catalysts 13 .…”

Effect of dielectric barrier discharge plasma hydrogenation on oxidation stability of biodiesel derived from vegetable oils

“…According to the previous studies, the catalytic hydrogenation reaction must be performed at high pressure (0.3 -0.6 MPa) and elevated temperature (80 -120°C). It perhaps faces problems with high operating and maintenance costs from the high hydrogen pressure environment, as well as a slow deactivation of catalyst performance from the deposition of carbon or metals on the catalyst surface [85]. To avoid these issues, one of the alternative methods is to utilize plasma technology.…”