A ReaxFF Molecular Dynamics Study of the Pyrolysis Mechanism of

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A ReaxFF Molecular Dynamics Study of the

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Pyrolysis Mechanism of Oleic-type Triglycerides

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Ying Zhang1, 2, Xuelei Wang3, Qingmin Li*1, 2, Rui Yang1, 2, and Chengrong Li1, 2

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1 State Key Lab of Alternate Electrical Power System with Renewable Energy Sources, North

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China Electric Power University, Beijing 102206, China

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2 Beijing Key Lab of HV and EMC, North China Electric Power University, Beijing 102206,

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China

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3 State Grid Shandong Electric Power Research Institute

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ABSTRACT: Reactive Force Field (ReaxFF) method is employed in the molecular dynamics

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(MD) simulation of oleic-type triglyceride (OTG) pyrolysis for the first time. The complex

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pyrolysis mechanism of OTG at high temperature, especially focusing on the multi-channel

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pyrolysis pathways of OTG and radical-related evolution mechanisms of products, is intensively

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investigated at atomistic level by performing a series of ReaxFF MD simulations. Based on

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simulation trajectory analysis, we find that the initiation decomposition of OTG pyrolysis is

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through C-O bond fission to release the straight oleic acid radical (C18H33O2•). The

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decomposition of C18H33O2• radical is mainly started through multi-channel pathways: the

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decarboxylation reaction to form long-chain hydrocarbon radical (C17H33•) and CO2, and C-C 1 ACS Paragon Plus Environment

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bond cleavages at α, β-C position to form hydrocarbon radicals and ester radicals. C-C bond β-

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scissions and conjugation reactions play important roles in the subsequent decomposition of

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C18H33O2• radical. ReaxFF MD simulations lead to reasonable decomposition pathways for OTG

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pyrolysis compared with experimental results and further confirmed by calculating the standard

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reaction enthalpies based on density functional theory. The temperature effect on distributions of

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various products is also analyzed. C2H4 is the most abundant stable product. Certain amounts of

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CO and H2O are first discovered at high temperature. The product evolution tendencies with

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temperature are reasonable compared with the experimental observations. Based on similar

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evolution characters, the dominant products are categorized into three groups: the stable

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products, the reactive radical products, and the temperature-dependent products. In particular,

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detailed radical-related evolution behaviors of three representative products (C2H4, CH3• radical

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and CO) are discussed systematically at atomistic level. Besides, the activation energy and pre-

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exponential factor for the pyrolysis of oleic-type triglycerides extracted from the ReaxFF MD

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simulations are in good agreement with the experimental results. This work demonstrates that

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ReaxFF method is a computationally feasible and reliable approach to elucidate the intricate

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pyrolysis mechanism of oleic-type triglycerides.

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Key Words: Oleic-type triglycerides; ReaxFF MD simulation; Pyrolysis mechanism; Standard

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reaction enthalpy; Kinetic modeling

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1. Introduction

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As a resourceful energy that can be largely extracted from plant seeds, triglyceride in vegetable

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oil has been considered an excellent raw material for conversion to renewable fuels and

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chemicals used as petroleum and fossil alternatives.1 Besides its eminent biodegradability and 2 ACS Paragon Plus Environment

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similar heating value to that of fossil-derived hydrocarbons, plant-based triglyceride is higher in

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energy content compared to other biomass sources. 2 At present, under the proper processing

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conditions, triglyceride converted into diesel-type fuels has been widely used in industrial

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applications (such as diesel engines 3 , turbines 4 and power transformers 5 ). Therefore, the

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mechanisms of decomposing triglyceride into hosts of bio-products have drawn worldwide

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attention.6-7

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In general, pyrolysis, a severe method of thermal cracking in the absence of air, has been

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extensively applied to investigate the decomposition pathways of transforming triglycerides into

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hydrocarbons. Through analyzing the pyrolysis products of vegetable oil using Gas

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Chromatography-Mass Spectrometer (GC-MS) analysis, large amounts of studies focusing on

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the decomposition schemes of triglyceride molecules have been reported.8-10 Chang and Wan8

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proposed a reaction mechanism for the thermal cracking of saturated triglycerides through tung

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oil pyrolysis, which included 16 types of reactions. Schwab et al. 9 postulated a pyrolysis scheme

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of unsaturated triglycerides to account for the formation of hydrocarbon gases, alkanes, alkenes,

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dienes, aromatics and carboxylic acids through the soybean oil pyrolysis. O. Idem et al. 10 studied

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the thermal cracking of canola oil in a flow type reactor and reported a reaction model to

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demonstrate a decomposition mechanism for both saturated and unsaturated triglycerides. All of

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these works have afforded thoughtful insights into the decomposition behaviors of triglyceride-

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based vegetable oil. However, for the pyrolysis experiments of vegetable oil, the pyrolysis

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product distributions can be significantly affected by pyrolysis material and operating variables

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(such as temperature, pressure and space velocity), and the identified and quantified product

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species are usually restricted to the function of GC-MS analysis. Therefore, conflicting views

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involving the types of intermediate products and the order of intermediate reactions still exist in 3 ACS Paragon Plus Environment

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the proposed pyrolysis schemes,8-10 in spite that these pyrolysis schemes are generally similar to

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each other. For example, different heavy oxygenated hydrocarbons, including long-chain fatty

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acids (RCOOH), esters (RCOOR’), ketones (RCOR’), and aldehydes (RCHO), were proposed as

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the initial decomposition products of triglyceride.8-10 Chang and Wan8 proposed that the odd

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hydrocarbon products were produced via decarboxylation upon RCOOH and subsequent C2H4

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elimination, and the even hydrocarbon products were formed by releasing a ketene from RCOR’

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and following C2H4 elimination; Schwab et al.9 postulated that the formation of hydrocarbon

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products were through decarboxylation and then followed by C2H4 elimination upon RCOOH.

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Due to the different formation concentrations of the paraffin hydrocarbons with carbon number

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larger than 7 (i.e., C7+) detected during pyrolysis experiments, Chang and Wan8 proposed that

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decarboxylation would take place before C-C bond cleavage for oxygenated hydrocarbons, while

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Schwab et al.9 proposed that the opposite reaction process is also possible. Moreover, although

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the pyrolysis process of triglycerides has been commonly accepted to be radical-dominated,11

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direct descriptions of the radical-related reactions concerning the formation mechanisms of

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important products have rarely been reported in the previous works,8-10 for free radicals are

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difficult to be detected through current experimental methods. Therefore, due to the

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heterogeneous nature of triglycerides and the complexity of the pyrolysis process, it is of great

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challenge to investigate the short-lived intermediate products, multi-channel decomposition

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reactions, and radical-related evolution mechanisms of important products that are related to the

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pyrolysis of OTG by practical pyrolysis experiments alone. Consequently, atomistic level

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descriptions of triglyceride pyrolysis, including the critical decomposition reactions, the high

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temperature behaviors, and the kinetic modeling, have not been well established so far.

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By virtue of its veracity, efficiency and practicability, computer molecular simulation 4 ACS Paragon Plus Environment

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technology cannot only calculate the atomistic parameters of concerned materials accurately, but

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also help to analyze the atomistic descriptions of reaction mechanisms profoundly.12 In theory,

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quantum mechanics (QM) can provide dependable configurations, thermochemical properties,

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and kinetic analysis for all chemical systems. 13 However, QM method is generally used to

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simulate small molecular systems that contain no more than 100 atoms. Its high-precision

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calculation makes it too expensive to carry out simulations for large-scale triglyceride model

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necessary to capture the complicated triglyceride pyrolysis process. On the other hand, by

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describing the relationship of energy and geometry with a set of simplified potential functions,

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conventional molecular dynamics (CMD) is applicable to much larger molecular system with

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relatively lower calculation quantities. 14 - 15 However, CMD method, employing fixed partial

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charges and static bonds between atoms, is mainly applied to systems at or around their

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equilibrium configuration, which is therefore difficult to describe dynamic chemical reactions for

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triglyceride pyrolysis. Furthermore, CMD method demands that the parameters used in its

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potential functions should be fitted against a series of data gathered from experimental results or

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QM sources, which makes the empirical force field resulting from this fitting procedure much

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more unreliable.

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Currently, the bond-order (BO) based ReaxFF method developed by van Duin et al.16 is built

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to bridge the gap between QM method and CMD method. Without fixing the rigid connectivity

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between atoms inside molecules, ReaxFF method allows chemical bonds to form and break

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freely, which overcomes the defects of CMD method. The parameters of ReaxFF are

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continuously scrutinized by checking against the QM data to diminish major discrepancies,

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which preserves the computational accuracy of QM method. ReaxFF method also has the

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advantage of fast calculation and can be used in large simulation systems for dynamical atomistic 5 ACS Paragon Plus Environment

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analysis for a sufficiently long time, which settles the expensive computational cost problem for

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QM method. At present, ReaxFF method has already been successfully applied to large-scale

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reactive chemical MD simulations, such as combustion and pyrolysis of hydrocarbons, 17 - 19

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pyrolysis of polymers, 20 - 21 and explosion of high energy materials (TATP and RDX). 22 - 23

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Therefore, ReaxFF is a good transferable method that can be applied in varieties of dynamic

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chemical systems. With regard to the area of pyrolysis of triglycerides, Zhang et al. used ReaxFF

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method to study the pyrolysis of tripalmitin (C51H101O6, C16:0)24 and trilinolenin (C57H92O6,

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C18:3)25 at high temperature, and mainly investigated the initial decomposition reaction and the

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formation mechanism of cyclic hydrocarbons. Their successful applications have laid the

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foundations of using ReaxFF method to investigate the pyrolysis behaviors of triglyceride

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materials. Therefore, in order to get a further in-depth understanding of the intricate pyrolysis

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mechanism of triglyceride at atomistic level, this paper presents great efforts of using ReaxFF

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MD simulations to investigate the overall pyrolysis profile, the detailed multi-channel pyrolysis

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pathways, the distributions and radical-related evolution mechanisms of products, and the

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pyrolysis kinetic modelling during the triglyceride pyrolysis process under different temperature

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conditions, especially focusing on the multi-channel pyrolysis pathways of OTG and evolution

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mechanisms of products which could not be systematically studied by current experimental

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approaches.

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In the present work, a series of MD simulations employing ReaxFF method are performed on

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an oleic-type triglyceride (OTG) model that is composed of three oleic acid chains attaching to a

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glycerol group through ester bonds. This paper is organized as follows. First, the basic concept

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and theory of ReaxFF method are briefly introduced in Section 2. Afterwards, simulation details

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and computational methods of OTG pyrolysis are presented in Section 3. Subsequently, Section 6 ACS Paragon Plus Environment

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4 is devoted to discuss the simulation results of OTG pyrolysis. Finally, Section 5 presents the

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main conclusions of the OTG pyrolysis with suggestions of some possible applications. It is

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expected that the present work could provide a detailed description of the pyrolysis mechanism

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of oleic-type triglycerides at atomistic level and afford some helpful references for practical

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triglyceride conversion use.

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2. Brief Overview of ReaxFF MD Simulations

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ReaxFF is a BO-based reactive force field developed by van Duin et al.16 for research use in

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MD simulations. Having all bonds defined explicitly, empirical force fields are unable to

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simulate reactive chemical reactions. On account of this, ReaxFF eschews explicit bond

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interactions but rather in favour of BOs, which allows for bond breaking and bond forming

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continuously. In ReaxFF method, the general system energy Esystem is divided into various

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potential energy contributions, as demonstrated in Equation (1).

Esystem = Ebond + Eover + Eunder + Eval + Epen

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+ Econj + Etors + EvdWaals + ECoulomb

(1)

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As in Equation (1), Ebond denotes the bond energy, Eover and Eunder denote the atom over- and

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under- coordination terms, Eval denotes valence angle terms, Epen denotes penalty energy, Etors

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denotes torsion angle energy, Econj denotes the contribution of conjugation effects to molecular

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energy, EvdWaals denotes non-bonded van der Waals interactions, and ECoulomb denotes Coulomb

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interactions.16

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The potential energy contributions listed in Equation (1) can all be calculated by potential

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energy functions associated with BO. By updating the coordinate of each atom at every

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iteration, ReaxFF method uses a general relationship amongst bond distance, bond order and 7 ACS Paragon Plus Environment

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bond energy that leads to proper dissociation of bonds to separate atoms. Moreover, the

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Electronegativity Equalization Method (EEM),26 a semi-empirical approach rooted in density

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functional theory, is introduced to determine the distribution of atomic charges and the energy

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required to polarize the atoms. Therefore, combined with the parameters derived from QM

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calculations on bond dissociation and reactions, ReaxFF method can provide accurate atomic-

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level descriptions of various reactive chemical reactions and is adopted here to use in MD

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simulations to study the intricate OTG pyrolysis behavior.

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3. Simulation Details and Computational Methods

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3.1 The Molecular Model of Oleic-type Triglycerides

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As there are hundreds of saturated and unsaturated triglycerides inside vegetable oil, a direct

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ReaxFF MD simulation of their pyrolysis behaviors is not feasible. In natural vegetable oil, the

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fatty acid chains of triglycerides are prone to be straight chains and have an even number of

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carbons generally between 16 and 20.27 Moreover, the unsaturation degree of the fatty acid

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chains would largely influence the oxidation stability and condensation point of vegetable

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oil.28 Therefore, in order to reduce the scale of pyrolysis simulation, oleic-type triglyceride

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with mono C=C bond, which possesses both low solidifying point and excellent oxidation

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resistance,27 is chosen to serve as an applicable surrogate to investigate the pyrolysis behaviors

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of triglycerides.

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The oleic-type triglyceride with a molecular formula of C57H104O6 is composed of a glycerol

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backbone linking with three C18 mono-unsaturated fatty acids through ester bonds. In order to

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construct the molecular model of OTG, Amsterdam Density Functional (ADF), 29 a MD

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simulation software, was employed in this work. Figure 1 displayed the model of this typical

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triglyceride that was built and conformation/energy-optimized using ADF software. The gray,

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white, and red spheres represented carbon, hydrogen, and oxygen atoms respectively. Due to

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the symmetrical structure of OTG molecule, we labeled the three carbon atoms of the glycerol

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backbone sequentially as C-1, C-2 and C-3 to have a better understanding of the OTG structure.

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Figure 1. Snapshot of a model of the oleic-type triglyceride with three C18 mono-unsaturated

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fatty acid chains attached to a glycerol backbone through ester bonds; the bottom left corner is

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the chemical formula of the oleic-type triglyceride; three carbon atoms of the glycerol

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backbone are sequentially labeled as C-1, C-2 and C-3; color code: Carbon-grey, Hydrogen-

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white, Oxygen-red.

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3.2 Cook-off Simulation of Pyrolysis of Oleic-type Triglycerides

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In order to obtain an overall pyrolysis profile of the temperature-induced decomposition

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behavior of oleic-type triglycerides, cook-off simulation employing ReaxFF method was first

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performed on pure OTG molecules. In the cook-off simulation, 10 optimized OTG molecules 9 ACS Paragon Plus Environment

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were placed in a periodic cubic unit cell measuring 40Å×40Å×40Å with a density of 0.23 g/cm3.

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As have been reported in various studies of ReaxFF MD simulations such as the pyrolysis and

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combustion of n-dodecane17 and the oxidation of toluene18, the density would not affect the

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reaction mechanisms, but only has influence on the reaction rate which is much weaker than the

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influence of temperature.

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The cook-off simulation was performed in the temperature range 300K-2800K for a total of

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100 picosecond (ps). In reactive MD simulations by applying QM and CMD methods, 30 the

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typical time step for the motion of atoms is 0.1 femtosecond (fs), i.e. 10-16s, while the typical

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time for an elementary reaction to occur is 0.1ps, i.e. 10-13s, which gives rise to the phenomenon

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that the occurrence probability of a chemical reaction is very low and the direct simulation of the

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chemical reaction is too time-consuming. Therefore, the settings of high temperature are favored

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in reactive MD simulations in order to promote sufficient atomic motion and molecular

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collisions, which could prominently increase the reaction rate and reduce the simulation time.30

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Similarly in ReaxFF MD simulations, the enhanced simulation temperature could significantly

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accelerate the reaction rate and maintain the simulation time within reasonable range. Although it

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might bring some uncertainty in chemical mechanism analysis, sufficient research achievements

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of ReaxFF MD simulations have proved that the simulation results acquired by elevating the

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simulation temperature are in good agreement with experimental observations.17-23 The 100ps

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cook-off simulation of pyrolysis of OTG was conducted through three steps. In the first step, in

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order to guarantee the structural stability of this cellular system, the simulation system was

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dynamically equilibrated at 300K for 10ps using the canonical ensemble (also known as the NVT

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ensemble, which stands for constant particle number, constant volume and constant temperature)

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with a time step of 0.1fs, and assurance was made that no chemical reactions would take place at 10 ACS Paragon Plus Environment

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this temperature. The 0.1fs time step would be sufficient in this work, as Wang et al.17 and

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Chenoweth et al.31 had employed this time step in ReaxFF MD simulations for pyrolysis and

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oxidation of hydrocarbons and proved that this time step was appropriate. In the second step, the

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equilibrated simulation system was used in the cook-off procedure where the temperature was

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heated to 2800K at a fast rate of 50 K/ps within 50ps. In the final step, the simulation system was

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equilibrated at 2800K for 40ps. During the whole simulation, the temperature was maintained by

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the Berendsen thermostat with a temperature damping constant of 100fs. The heating process

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would not influence the pyrolysis mechanism, but just had effects on the initial decomposition

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time of reactants.32 As the simulation settings in Ref. [19] had provided a good description of

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complex reactions in hydrocarbon systems, the criterion value of BO for the breaking and

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formation of chemical bonds was set as 0.3 in this work. The simulation settings had proved to

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have no influence on simulation itself.31,33-34 Besides, the ReaxFF parameters trained from QM

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calculations in Ref. [34] were adopted here without modifications. To reduce the simulation

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randomness and improve the statistical accuracy, the above cook-off simulation was repeated for

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10 times within independent and stochastic pyrolysis systems.

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3.3 Unimolecular Pyrolysis Simulations

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In order to study the short-lived intermediate products and multi-channel decomposition

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reactions of triglyceride pyrolysis, unimolecular pyrolysis simulation employing ReaxFF method

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was performed on an OTG system, with only one OTG molecule placed in the same unit cell

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used in cook-off simulation. Firstly, the equilibrated operation for cook-off simulation was

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applied in this unimolecular system. Then a series of ReaxFF MD simulations using NVT

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ensemble with the temperature of 1800K~2800K at 200K intervals (namely at 1800K, 2000K,

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2200K, 2400K, 2600K, and 2800K respectively) were performed with the same time step of

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0.1fs. A total of 100ps data were recorded in a single trajectory of pyrolysis simulation. The

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temperature was maintained using the Berendsen thermostat method with a temperature damping

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constant of 100fs and the BO cutoff was set as 0.3. Additionally, to reduce the simulation

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randomness caused by temperature differences, the unimolecular pyrolysis simulation of OTG

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was repeated for 10 times with independent pyrolysis systems at each temperature respectively.

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Therefore, the detailed pyrolysis mechanisms of OTG could be exhaustively studied by tracing

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the decomposition reactions of every OTG molecule.

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3.4 Temperature-dependent Pyrolysis Simulations

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In order to investigate the detailed distributions and radical-related evolution mechanisms of

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important products at high temperature, temperature-dependent pyrolysis simulations employing

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ReaxFF method were carried out on pure oleic-type triglycerides. The same equilibrated system

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with 10 OTG molecules for cook-off simulation was applied in temperature-dependent pyrolysis

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simulations. The temperature setting was the same as used in the previous unimolecular pyrolysis

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simulations. The temperature-dependent pyrolysis simulation at each temperature was also

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repeated for 10 times using independent simulation system. As a result, the distributions and

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evolution mechanisms of important products, which change over time and vary with temperature,

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could be investigated in details.

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3.5 Quantum Mechanics Calculations

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In order to ensure the validity and transferability of the ReaxFF method, quantum mechanics

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calculations were performed on principal reactions observed during the OTG pyrolysis process.

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Standard enthalpy change of reaction (∆E) is defined as the enthalpy difference between the 12 ACS Paragon Plus Environment

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produced fragments and reactant under standard condition (101.3 kPa, 298 K), which can be

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used to determine the reactive complexity of a reaction. Therefore, standard reaction enthalpies

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of important reactions in OTG pyrolysis were calculated based on density functional theory

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(DFT). The ORCA program 35 was employed in the present work to perform all the DFT

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calculations. The hybrid functional Becke’s three-parameter gradient-corrected exchange

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potential combined with Lee-Yang-Parr gradient-corrected correlation potential (B3LYP)36-37 has

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provided reliable QM values at a computational-efficient cost.38-39 Thus, it was adopted in the

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DFT calculations in the following work. The split-valence triple-zeta basis set (6-311G)40 added

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with diffuse and polarization functions was used as the basis set throughout the DFT calculations,

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for it has shown reasonable frequency and energy analysis for organic compounds.40-41

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The B3LYP functional DFT calculation was conducted through four steps. In the first step,

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geometry optimization of each molecule was calculated at B3LYP level with 6-311G basis set

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supplemented by diffuse functions, one set of d functions on heavy atoms and one set of p

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functions on hydrogens. In the second step, harmonic frequency analysis was performed at the

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same level to guarantee that each geometry-optimized configuration could correspond to the true

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local minimal value of energy, and to calculate the zero-point energy and thermo-correction

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values. In the third step, the electronic energy of each structure was obtained by using single-

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point energy calculations at B3LYP level with 6-311G basis set supplemented by diffuse

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functions, 3 sets of d functions and one set of f functions on heavy atoms, and 2 sets of p

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functions on hydrogens. In the final step, the reaction enthalpies at standard condition were

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calculated with the computed DFT energy data.

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4. Results and Discussion

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4.1 Cook-off Simulation of Oleic-type Triglyceride Pyrolysis

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To get an overview of the pyrolysis of oleic-type triglycerides, cook-off simulation was

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performed on OTG molecular system for the first time. From our ReaxFF MD simulation, a total

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of 78 different species were captured, including the short-lived intermediates and the stable

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products. In this work, the observed product species were divided into 10 categories as C0, C1,

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C2, C3, C4, C5, C6, C7+, CO2 and Oxides. C0 included the H• radical and H2; Ci (i = 1~6)

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represented for the hydrocarbon species whose carbon atom number was i; C7+ contained the

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hydrocarbon species whose carbon atom number was no less than 7; CO2 was carbon dioxide;

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and Oxides denoted the oxygenated hydrocarbon species. The distributions of the reactant and

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each product category, and the temperature profile of cook-off simulation as a function of time

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were given in Figure 2. Due to the abundant generation of species in C2 category, the molecular

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number curve of C2 category displayed in Figure 2 was only one third of the production amount

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of C2 category, in order to reduce the size of the figure.

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Figure 2. Evolution of various product species and temperature as a function of time in the cook-

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off simulation; the curve of 1/3C2 represents one third of the production amount of C2 category.

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It could be seen that, as the simulation proceeded, pre-equilibrated OTG molecules began to

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decompose at 15.4ps at a temperature around 570K. The whole consumption of the OTG

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reactants took about 10ps to accomplish. In the light of time-dependent distributions of product

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species, the pyrolysis process could be divided into four stages. The first stage was from the

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beginning of the cook-off process (10ps) to 30ps. In this stage, following the consumption of

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OTG reactants, the production of Oxides species was very quickly and reached its maximum at

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this stage. The second stage was from 30ps to around 50ps. During this stage, the largely

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generated Oxides species gradually consumed. Meanwhile, CO2 and C7+ species began to

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accumulate and reached their maximum contents at the end of this stage. We could see in Figure 15 ACS Paragon Plus Environment

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Page 16 of 46

1

2 that the curves of CO2 and C7+ species in the ascending stage shared almost the same trend.

2

The third stage was from 50ps to 90ps. In this stage, the C7+ species was gradually consumed

3

and reached its stabilization, and the yield of C2 species was witnessed a rapid increase with

4

time. The fourth stage was the rest of the simulation process (90ps~100ps). After 90ps of

5

pyrolysis, the production and consumption of all product categories approached to their

6

equilibrium and the pyrolysis process of cook-off simulation came to an end.

7

By analyzing the cook-off simulation trajectory thoroughly, the first observed initiation

8

reaction of OTG pyrolysis was the breakage of the C-O bond between the glycerol backbone and

9

the oleic acid chain to form two radicals. Ethylene (C2H4) was the most abundant and stable

10

product whose amount was observed a continual increase in quantities and reached its stable

11

state at the end of the simulation. Carbon dioxide was another important stable product, for

12

almost all the oxygen atoms had transformed into CO2. Large amounts of unstable intermediates

13

were produced during the pyrolysis process. The large oxygen-containing radicals, especially the

14

oleic acid radical (C18H33O2•), were critical intermediate products in the thermal decomposition

15

process of OTG pyrolysis. Long-chain hydrocarbon radicals represented by C17H33•, C11H21•, etc.

16

played important roles in intermediate reactions to dissociate into smaller alkanes, alkenes and

17

alkynes. These intermediates were found to reach their maximum amounts at the middle of the

18

simulation and then progressively consume with time.

19

To further investigate the proportional distributions of the total product species generated

20

during the cook-off simulation, the corresponding percentage of each category produced at the

21

end of the cook-off simulation was plotted in Figure 3. The C2 category had the largest

22

proportion, which could account for about 57.75%. The C3 category whose percentage was

23

8.14% contained more species. CO2 as a stable product during the pyrolysis process had a 16 ACS Paragon Plus Environment

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1

percentage over 11.24%. The C0 and C1 category had the percentage all larger than 4%. The C4

2

category had 8.53% proportion due to the generation of alkadienes, which were structural stable

3

compounds. Moreover, the product species of C5 category whose percentage was larger than 2%

4

was found to have the largest species number, mainly due to the reciprocal recombination

5

between the small products of C0~C3. Therefore, it could be deduced that at the end of the

6

simulation, the distribution of the total product species was small molecular-dominated, which

7

the total percentage of C0~C3 categories together with CO2 could account for about 86% of the

8

total products.

9 10

Figure 3. Percentage distribution of each category after 100ps cook-off simulation of OTG

11

pyrolysis.

12

13

4.2 Detailed Analysis of the Pyrolysis Pathway in Unimolecular Pyrolysis

14

Simulations

15

As the detailed atomistic description of OTG pyrolysis pathway could not be explicitly 17 ACS Paragon Plus Environment

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Page 18 of 46

1

described under current experimental techniques, the complete pyrolysis pathway of OTG,

2

including the short-lived intermediates and multi-channel reactions, was intensively investigated

3

by analyzing the decomposition trajectory of every OTG molecule during unimolecular pyrolysis

4

simulations. Moreover, the standard reaction enthalpies of the captured reactions were calculated

5

using DFT calculations at B3LYP level. Detailed trajectory analysis showed that the primary

6

pyrolysis pathway observed for individual OTG molecule was roughly alike during different

7

temperature simulations. As a result, the principal decomposition reactions of the complete OTG

8

pyrolysis pathway collected during the unimolecular pyrolysis simulations and their

9

corresponding standard reaction enthalpies were elaborately displayed in Figure 4.

18 ACS Paragon Plus Environment

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Energy & Fuels

1 2

Figure 4. The principal decomposition reactions of the complete OTG pyrolysis pathway

3

collected during unimolecular pyrolysis simulations and their corresponding standard reaction

4

enthalpies calculated using DFT calculations at B3LYP level.

5 19 ACS Paragon Plus Environment

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1

The initial decomposition of OTG molecules was found caused by the release of C18H33O2•

2

radicals. Namely, the initial cleavage of OTG was related to the fission of the C-O bond between

3

the glycerol backbone and the oleic acid chain (C18H33O2-). It indicated that the C-O bond of

4

OTG is weaker on bond strength and could be the crucial spot on OTG thermal stability. For the

5

purpose of further elucidation of the initial decomposition mechanism, we calculated the

6

standard reaction enthalpy (∆E) of the fission of C-O bond and that of O-C(=O) bond, in order to

7

compare the activation barrier height of these two possible reactions. The calculated ∆E value of

8

the fission of C-O bond was 307 kJ/mol while that of O-C(=O) bond was 430 kJ/mol, which

9

implied that the O-C(=O) bond breakage pathway had a lower possibility. This DFT calculation

10

was well consistent with our ReaxFF simulation result that OTG was more likely to initially

11

decompose into fatty acids rather than alcohols and aldehydes. Schwal et al.9 also proposed the

12

same initial decomposition reaction via analyzing the thermal decomposition of soybean oil

13

using GC-MS analysis.

14

As the simulation proceeded, the decomposition process was mainly concentrated on the

15

thermal cracking of C18H33O2• radicals, which could be regarded as the secondary thermal

16

cracking phase of OTG. Three principal decomposition pathways of C18H33O2• radicals were

17

observed in ReaxFF MD simulations, which were demonstrated in Figure 4. Pathway p1 was

18

started with decarboxylation reaction by removing the carboxyl group from C18H33O2• radical,

19

leading to the formation of CO2 and C17H33• radical. The C17H33• radical, upon C-C bond β-

20

scission reactions and conjugation reaction, gave the subsequent odd-numbered hydrocarbon

21

radicals and large amounts of C2H4. Pathway p2 and p3 were respectively started through the C-

22

C bond cleavages at position of α, β-C to C=C bond, contributing to the production of ester

23

radicals (C6H12COO• and C5H10COO•) and olefin radicals (C11H21• and C12H23•). The produced 20 ACS Paragon Plus Environment

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1

ester and olefin radicals would also undergo decarboxylation reactions, β-scission reactions and

2

conjugation reactions to develop into smaller molecules and radicals. According to the collected

3

simulation trajectories, the most frequent reaction pathway observed in the pyrolysis was p1,

4

which had a reaction probability of nearly 70%. The occurrence probability of pathway p2 was a

5

bit larger than that of p3, and the occurrence probabilities of both p2 and p3 were found highly

6

temperature-dependent, which would rise with the increase of the temperature. This could be

7

explained by the standard reaction enthalpies shown in Table 1 and marked in Figure 4. The

8

calculated ∆E of the start reaction of p1 was 342 kJ/mol, which was lower than that of p2 (393

9

kJ/mol) and p3 (415 kJ/mol). During the thermal cracking of stearic acids, Maher et al.42 detected

10

that appreciable amounts of n-heptadecane with concurrent production of CO2 were produced

11

before other products, and therefore proposed that decarboxylation reaction could be the first

12

step in stearic acid pyrolysis. O. Idem et al.10 reported that C-C bond cleavages at α, β-C position

13

took place before decarboxylation reactions for some unsaturated oxygenated hydrocarbons

14

during the thermal cracking of canola oil. Schwab et al.9 studied the thermal cracking of high

15

oleic safflower oil and proposed that the presence of unsaturated C=C bond could facilitate the

16

starting cleavages of fatty acids at α, β-C position. Therefore, combined with our simulation

17

observations, it was fairly to conclude that the decarboxylation reaction would be the primary

18

start decomposition reaction for all types of fatty acid radicals, while the unsaturation degree of

19

triglyceride would enhance the C-C bond cleavages at α, β-C position, resulting in diversified

20

intermediate products and multi-channel pathways. Moreover, as the temperature would affect

21

the start decomposition channel of the released fatty acids, the pyrolysis operating conditions

22

should be carefully controlled in order to obtain the desired fuel components and chemicals,

23

especially when high-unsaturated vegetable oil is chosen as the pyrolysis material. 21 ACS Paragon Plus Environment

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Page 22 of 46

1

Furthermore, throughout the decomposition process of C18H33O2• radicals, we found that β-

2

scission reaction, proceeding via eliminating a molecule of C2H4 from hydrocarbon radicals, was

3

the most dominant decomposition reaction. Due to its close proximity of enthalpy values at

4

different C sites, β-scission reaction was only presented with the average ∆E in Table 1. The ∆E

5

value of β-scission reaction was particularly lower than the other reactions, which was well

6

consistent with the dominance of β-scission reactions. Besides the dominated β-scission

7

reactions, conjugations were also important reactions that would result in the formation of

8

conjugated double bond structures, which were 1,3-butadiene (H2C=CH-HC=CH2) and 1,3-

9

pentadiene (H3C-HC=CH-HC=CH2) observed during the simulations. It was vital to observe the

10

presence of alkadienes in the pyrolysis process, for they were crucial compounds to react with

11

alkenes in the formation of cyclic products and subsequent aromatic products through Diels-

12

Alder addition mechanisms.43 The ∆E values of conjugation reactions were also calculated and

13

listed in Table 1. This decomposition mechanism of decarboxylated fatty acids developing into

14

small odd carbon-numbered alkanes and alkenes through conjugations and β-scission reactions

15

was well consistent with the pyrolysis experimental observations of Chang and Wang8 and

16

Greensfelder et al.44.

17

The above-mentioned reactions were explicitly intramolecular decomposition reactions and

18

endothermic. O. Idem et al.10 observed that the amount of total gases generated during the

19

thermal cracking of canola oil would increase with the temperature, and proposed that

20

decomposition reactions (e.g., decarboxylation, β-scission reaction, and C-C bond cleavage) that

21

would result in the formation of gas phase products are endothermic, which indicated that the

22

OTG pyrolysis pathway in the present work was reasonable. The observed pyrolysis pathway

23

could also account for the phenomenon that the majority of the ultimate products collected in the 22 ACS Paragon Plus Environment

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1

Energy & Fuels

cook-off simulation of OTG pyrolysis were small molecules or radicals.

2

In general, the entire pyrolysis process was radical-dominated. The majority of the

3

intermediate reactions were through intramolecular cracking reactions. The intermolecular

4

reactions, who played important parts in the region of forming multi-channel reactions and

5

diversified intermediate products, were noted to occur rarely, resulting in the monotone change

6

of intermediates. This was mainly because that only one OTG molecule was placed in the unit

7

cell, which could not generate sufficient free radicals and enough molecular collisions. Based on

8

our observation in the subsequent simulations, various intermolecular reactions were observed

9

and analyzed in the evolution mechanisms of important products.

10 11

Table 1. Standard reaction enthalpies ∆E (kJ/mol) of important decomposition reactions

12

observed during the unimolecular pyrolysis simulations. Reaction Type

13

Reaction C18H33O2• → C17H33• + CO2 Start decomposition C18H33O2• → C11H21• + C6H12COO• C18H33O2• → C12H23• + C5H10COO• β-scission RCH2CH2• → R• + C2H4 C11H21• → C4H6 + C7H15• Conjugation C12H23• → C5H8 + C7H15• C6H12COO• →C6H12 + CO2 Decarboxylation C5H10COO• → C5H10 + CO2 Note: R• represented various hydrocarbon radicals.

∆E 342 393 415 93 140 109 350 360

14

15

4.3 Temperature-dependent Pyrolysis of Oleic-type Triglycerides

16

4.3.1 Chemical composition and different types of dominant products

17

As discussed in the previous subsection, the majority of the product species remained at the

23 ACS Paragon Plus Environment

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Page 24 of 46

1

end of the pyrolysis were small molecules and radicals. In order to get a better understanding of

2

the temperature influence on product distributions, the chemical composition and different types

3

of the small-molecular products were analyzed at the end of the 100ps temperature-dependent

4

pyrolysis simulations in temperatures from 1800K to 2800K. As given in Table 2, we collected

5

the statistics of the detailed composition and quantity of the fragment whose carbon atom

6

number was below four. The number in front of each fragment was its corresponding molecular

7

quantity detected at the end of the pyrolysis simulation.

8 9 10

Table 2. Detailed chemical compositions of small molecules/radicals observed at the end of the 100ps temperature-dependent pyrolysis simulations in temperatures from 1800K to 2800K. C0

C1

1800K 2000K

C2

C3

63C2H4

9C3H5• 6C3H6 9C3H5• 10C3H6 3C3H7• 9C3H5• 15C3H6 3C3H7• 5C3H4 6C3H5• 13C3H6 1C3H8 9C3H4 9C3H5• 17C3H6

3CH3•

70C2H4 2C2H3• 92C2H4 1C2H6 4C2H2 2C2H3• 133C2H4 1C2H5• 3C2H2 13C2H3• 116C2H4 3C2H6 2C2H• 5C2H2 16C2H3• 123C2H4 1C2H5• 1C2H6

2200K

5H2

5CH3• 1CH4

2400K

6H2

9CH3• 2CH4

2600K

18H2

4CH3• 5CH4

2800K

24H2

4CH3• 14CH4

3C3H3• 15C3H4 5C3H5• 11C3H6

Small Oxides 30CO2

Total Fragments 108

30CO2

125

29CO2 1•COOH

163

28CO2 1CO 1H2O 1•COOH 27CO2 3CO 3H2O

213

24CO2 6CO 6H2O

260

230

11

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1

As explicitly shown in Table 2, with the rising of the temperature, the total number of the

2

small molecular fragments was constantly increasing, though it showed a tendency of saturation,

3

which was also well consistent with our DFT calculations that most of the reactions resulting in

4

the formation of gas products were endothermic. According to our simulation trajectory analysis,

5

besides the different chemical properties and reaction behaviors during the pyrolysis process,

6

similar evolution mechanism characters could be found for diversified species of the small

7

molecular fragments. The observed small species could be roughly divided into three groups: (1)

8

the stable products, (2) the reactive radical products, and (3) the temperature-dependent products,

9

which were closely correlated to one another.

10

For the stable products, their production rate was always bigger than their consumption rate,

11

which could remain fairly large amounts in the final products. The stable products mainly

12

contained the small olefin molecules (e.g., C2H4 and C3H6) and small molecules (e.g., H2, CH4

13

and CO2), which could also be easily collected during thermal decomposition experiments.8-10,45

14

As could be seen from Table 2, the generation amounts of H2, CH4, C2H4 and C3H6 were

15

constantly increasing with the rise of the temperature. Obviously, C2H4 was the most dominant

16

stable product under every simulation temperature. Zámostný et al.45 analyzed the products of

17

steam cracking (short residence time, temperature over 800°C) of C18 mono-unsaturated

18

triglycerides by using GC-MS analysis and detected that C2H4 was the most abundant product,

19

which had a yield percentage of over 30%. Therefore, our simulation results were in good

20

agreement with the experimental data. Although CO2 was witnessed a declination with the

21

increase of temperature, CO2 was a meaningful stable product, for it was produced directly

22

through decarboxylation reactions upon fatty acid radicals (RCOO•) and accompanied with the

23

formation of hydrocarbon radicals (R•) which were important derivatives of triglycerides to 25 ACS Paragon Plus Environment

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1

Page 26 of 46

produce desirable biodiesel.

2

For the group of reactive radical products, it contained most of the small radicals, such as H•,

3

CH3•, C2H3•, C3H5• etc. Reactive radical products were observed to generate throughout the

4

whole pyrolysis simulation yet would not remain in large quantities by the end of pyrolysis, for

5

their producing process was always accompanied with their consuming process. Most of them

6

were produced through intramolecular reactions and direct decompositions from reactant, and

7

were consumed through intermolecular reactions to form more stable species. According to our

8

reaction analysis of pyrolysis simulations, H• radical and CH3• radical were very abundant

9

products which could be observed in all the stages of OTG pyrolysis, yet there were no H•

10

radicals and only a small amount of CH3• radicals could be collected at the end of all

11

simulations. These short-lived radicals, which could hardly be detected through practical

12

experimental methods, were important intermediates that related to most of the intermediate

13

reactions.

14

The last group was the temperature-dependent products, which mainly contained alkyne

15

molecules (e.g., C2H2, C3H4) and small oxides (e.g., CO, H2O). The generation of these products

16

was highly related to the temperature, for they could only be spotted to accumulate considerable

17

amounts of quantities at higher temperatures. Alkyne molecules showed the similar formation

18

pattern, for they were prone to generate at much higher temperatures mainly due to the sequential

19

dehydrogenation process of the olefin molecules (e.g., C2H4 →C2H3• + H•, C2H3• → C2H2 +

20

H•). This feature could commonly be observed in most of the thermal decomposition process of

21

hydrocarbons. 46 - 47 For small oxides, the generation of CO and H2O was largely due to the

22

sufficient free radicals and much more intense collisions between molecules under higher

23

temperatures. Though only small amounts of CO and H2O were captured, according to the 26 ACS Paragon Plus Environment

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1

formation mechanism analysis through pyrolysis simulation trajectories, the production

2

quantities of these small oxides would be much larger if given longer simulation time.

3

4.3.2 Evolution mechanisms of important products

4

Investigation of the evolution reactions associated with radicals is essential to get a thorough

5

perspective of the pyrolysis mechanism of OTG. As the representative products in stable

6

products, reactive radical products, and temperature-dependent products respectively, the radical-

7

related evolution mechanisms of C2H4, CH3• radical and CO were typically investigated through

8

elaborate trajectory analysis.

9

Ethylene, with the highest yield, was the most important stable product of OTG pyrolysis.

10

Figure 5 displayed the time-dependent distributions of C2H4 under each simulation temperature.

11

As shown in Figure 5, the maximum value of the C2H4 molecule number kept increasing and the

12

reaction time used to reach the formation saturation point continuously brought forward with the

13

rise of the temperature. We collected and categorized the main reaction channels of C2H4

14

molecules, including the production channels and the consumption channels, during the 2400K

15

pyrolysis simulation as displayed in Figure 6. Among Figure 6, R1• represented the long alkene

16

radicals, which contained C15H29•, C13H25• and C11H21•; R2• represented the alkane radicals,

17

which contained C5H11•, C3H7• and CH3•; R3• represented the small radicals, which contained H•

18

and CH3•. According to our simulation trajectory analysis, the primary reactions to produce C2H4

19

molecules were through β-scission reactions, proceeding through eliminating a C2H4 fragment

20

from the hydrocarbon radical (RCH2CH2•), i.e.,

21

RCH2CH2• → R• + CH2=CH2

22

It was easily expected, as the β-scission reactions were found to be the most crucial and

23

dominant intramolecular cracking reactions in the OTG pyrolysis pathway as shown in Figure 4. 27 ACS Paragon Plus Environment

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Page 28 of 46

1

This formation mechanism of C2H4 had been confirmed by Rice and Kossiakoff using the

2

available experimental data of hydrocarbon pyrolysis. 48 On the other hand, the consumption

3

channels of C2H4 molecules were mainly through intermolecular reactions, including the C2H4

4

addition reactions (R3• + CH2=CH2 → R3CH2CH2•) and the cyclization reaction (CH2=CH2 +

5

H2C=CH-HC=CH2 →

6

reaction (CH2=CH2 → CH2=CH• + H•). These consumption channels of C2H4 molecules were

7

found to be much more reactive at higher temperatures, as could be seen in Figure 5 that the

8

development trend of the time-dependent distributions of C2H4 molecules at 2600K and 2800K

9

was witnessed an obvious declination at the end stage of the pyrolysis process.

), and intramolecular reactions, including the dehydrogenation

10 11

Figure 5. The time-dependent distributions of C2H4 molecules obtained from temperature-

12

dependent pyrolysis simulations at 1800K~2800K.

13

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1 2

Figure 6. The main production channels and consumption channels of C2H4 molecules observed

3

during the 2400K pyrolysis simulation; red and blue columns denoted the production and

4

consumption reactions respectively; R1• represented the long alkene radicals, which contained

5

C15H29•, C13H25• and C11H21•; R2• represented the alkane radicals, which contained C5H11•,

6

C3H7• and CH3•; R3• represented the small radicals, which contained H• and CH3•.

7 8

Methyl radical was found to be a representative reactive radical product in the present work.

9

Plotted in Figure 7, we presented the temperature-dependent distributions of the total produced

10

and consumed CH3• radical numbers collected under each simulation temperature. It could be

11

seen that the total production of the CH3• radicals increased significantly with the rising of the

12

temperature while the consumption of the CH3• radicals shared the same trend, which resulted in

13

only a small amount of CH3• radicals observed at the end of the 100ps pyrolysis simulations as

14

shown in Table 2. According to our simulation trajectory analysis, the production and 29 ACS Paragon Plus Environment

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1

consumption of CH3• radicals happened almost simultaneously. The primary reaction channels to

2

produce CH3• radicals were through intramolecular β-scission reactions (e.g., C3H7• → C2H4 +

3

CH3•) and C-C bond cleavage upon hydrocarbon radicals (e.g., C4H9• → C3H6 + CH3•). The

4

primary reaction channels to consume CH3• radicals were via intermolecular addition reactions,

5

including hydrogen addition reactions (e.g., CH3• + H• → CH4), radical addition reactions (e.g.,

6

CH3• + C2H3• → C3H6), and molecule addition reactions (e.g., CH3• + C2H4 → C3H7•). Some of

7

the channels (e.g., C2H4 + CH3• → C3H7•) are observed to be reversible. The reaction CH3• + H•

8

→ CH4 and CH3• + C2H3•→ C3H6 were important pathways to generate methane and propane,

9

respectively. The above evolution mechanism of the short-lifetime and active-chemical

10

properties of CH3• radicals was well consistent with the free-radical mechanism of paraffinic

11

hydrocarbon pyrolysis proposed by Rice and Kossiakoff48 and the reactive molecular dynamics

12

study of hydrocarbon pyrolysis of Ding et al.19.

13 14

Figure 7. Temperature-dependent distributions of the produced and consumed CH3• radical

15

numbers at 1800K~2800K; red and blue columns denoted the produced and consumed CH3•

16

radicals respectively. 30 ACS Paragon Plus Environment

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Energy & Fuels

1 2

Carbon monoxide (CO) was an important small oxide species besides CO2 and had been

3

detected in various pyrolysis experiments of vegetable oil.7-10 The formation mechanism of CO

4

observed in this work was found mainly through two reaction schemes, which was explicitly

5

displayed in Figure 8. For Scheme 1, with the rising of the temperature, the ester radical (e.g.

6

C6H12COO•), which was dissociated from C18H33O2• fatty acid radicals through pathway p2 and

7

p3 as displayed in Figure 4, tended to absorb an active H• radical, leading to the formation of a

8

carboxylic acid (e.g. C6H12COOH). After successive eliminations of C2H4 molecules from the

9

carboxylic acid, the remained carboxyl radical (•COOH) would eventually break into a molecule

10

of CO and a hydroxyl radical (OH•) via C-O fission. For Scheme 2, CO2 was largely generated

11

through decarboxylation reactions upon C18H33O2• fatty acid radicals. Accompanied by the much

12

more drastic production and depletion of H• radicals at higher temperatures, the produced CO2

13

was inclined to react with a brisk H• radical to form a •COOH radical. Like Scheme 1, the

14

•COOH radical would finally break into CO and OH• radical through C-O fission. It could be

15

concluded that enough H• radicals and sufficient collisions between molecules were essential

16

conditions to the generation of CO molecules. Supported by the concentration of CO detected in

17

the thermal cracking experiment of deoxygenating stearic fatty acids, Mäki-Arvela et al. 49

18

proposed that the fatty acids would decompose into hydrocarbons and release CO through

19

decarbonylation reaction by removing the carboxyl group, which was well consistent with

20

Scheme 1. By analyzing the pyrolysis product distributions of canola and soybean oil, Kubatova

21

et al.50 proposed that hydrogen would facilitate the cracking by reducing CO2 to form CO, which

22

was well consistent with Scheme 2. Therefore, combined with our simulation observations, both

23

schemes of CO formation were possible and should be added to the pyrolysis mechanism of 31 ACS Paragon Plus Environment

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OTG to expand the radical-related descriptions of the evolution mechanisms of temperature-

2

dependent pyrolysis products. Additionally, the produced OH• radical was an important reactive

3

intermediate, for it was observed to continuously react with adjacent fragments and would

4

eventually form into a stable molecule of H2O by combining with a brisk H• radical. According

5

to the thermal cracking experiments of canola and soybean oil in Ref. [50] and the thermal

6

hydrolysis study of canola oil in Ref. [51], the production of CO would accompany with the

7

formation of H2O and the reduction of hydrogen, which was well consistent with our simulation

8

observations. On the other hand, as the formation of CO would give rise to the generation of

9

water, a by-product which is incompatible with the production of biofuels, the pyrolysis time and

10

temperature should be carefully controlled to avoid its formation during the practical triglyceride

11

pyrolysis process.

12

In conclusion, although the free radicals are difficult to be detected by current experimental

13

means, the above direct descriptions of the radical-related pyrolysis reactions are reasonable

14

compared to the practical pyrolysis experimental observations, which suggests that ReaxFF MD

15

simulation is a promising method to study the radical-dominated pyrolysis process of oleic-type

16

triglycerides.

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1 2

Figure 8. Flow diagram of formation schemes of CO molecules; the color code for every

3

snapshot is: Carbon-grey, Hydrogen-white, and Oxygen-red; the regions of interest are presented

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in sphere-cylinder mode and highlighted by dashed boxes, which are also corresponded to the

2

dashed boxes on the molecular formula presented on the upper side of the snapshots.

3

4.4 Kinetic Analysis of the Pyrolysis of Oleic-type Triglycerides

4

To investigate the kinetic properties of OTG pyrolysis at high temperature, the time evolution

5

profiles of OTG, which carried out in the previous temperature-dependent pyrolysis simulations

6

at the temperature range 1800K~2800K, were used to study the first-order kinetics of OTG

7

pyrolysis. The use of the consumption rate of reactants to study the first-order kinetics of

8

pyrolysis had been thoroughly reported in large amounts of researches.17-19 The considered

9

kinetic model to describe the pyrolysis of OTG was presented in the following scheme of

10

reaction: k Oleic-type triglyceride  →products

11

(2)

12

In the present work, the concentration of OTG reactant was simply substituted by the

13

molecular number of OTG. The rate constant k at each constant temperature T was calculated

14

from the linear fitting of the molecular number Nt against the simulation time t, as demonstrated

15

by Equation (3). The symbol N0 denoted the original molecular number of OTG, which was 10 in

16

this work.

ln Nt − ln N0 = kt

17

(3)

18

The Napierian logarithm of rate constant k (i.e., ln k) was then applied into a linear fitting

19

against the reciprocal of constant temperature T (i.e., 1/T) based on the Arrhenius expression, as

20

shown in Formula (4), to calculate the activation energy (Ea) and the pre-exponential factor (A).

21

The symbol R in Formula (4) is the molar gas constant, which has an approximate value of

22

8.314.

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ln k = ln A − E a / RT

(4)

2

The rate constant k at each temperature was calculated by carrying out the simulation for

3

several times. The average rate constant k at each temperature was selected to fit the Arrhenius

4

expression. The fitted Arrhenius plot of the OTG pyrolysis was shown in Figure 9. The fitted

5

slope and Y-intercept of Formula (4) were -24.56 × 103 K*s-1 and 31.97 s-1 respectively.

6

Therefore, the activation energy Ea and the pre-exponential factor A of this work were 204

7

kJ/mol and 7.66×1013 s−1 respectively.

8 9

Figure 9. Fitted Napierian logarithm of rate constant k versus inversed temperature T obtained

10

from 100ps temperature-dependent simulations of OTG pyrolysis at 1800K~2800K.

11 12

To estimate the reliability of ReaxFF method using in the present work, the calculated Ea and A

13

extracted from ReaxFF MD simulations were compared with the experimental results as shown

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in Table 3. As the kinetic models used to extrapolate the experimental data in Ref. [52]~[55]

2

were mainly multi-step models which generally involved in the process of volatilization,

3

decomposition and intercross, the cited Arrhenius parameters of triglyceride pyrolysis in Table 3

4

were measured in the decomposition process. By analyzing the thermal cracking of sunflower oil

5

using thermogravimetric method, Santos et al.52 obtained the activation energy between 155 and

6

220 kJ/mol at the reaction order between 0.95 and 1.82; Melo et al.53 obtained that the range of

7

the activation energy vary from 170 to 210 kJ/mol; and Gouveia de Souza et al.54 obtained that

8

the activation energy and the pre-exponential factor were 205 to 216 kJ/mol and 1.5×1013 to 1.2

9

×1018 s-1 respectively. Font and Rey55 obtained that the activation energy and pre-exponential

10

factor for the decomposition process of olive oil were 194.6 kJ/mol and 3.54 × 1013 s-1

11

respectively. Therefore, under different experimental treatments, the reported practical activation

12

energy of triglyceride varied from 155 to 220 kJ/mol. The simulation temperature of OTG

13

pyrolysis in the present work ranged from 1800K to 2800K, while there were no existing

14

experimental results that had been reported under such high temperature. Considering the

15

temperature difference between the practical experiments and our ReaxFF MD simulations, the

16

calculated Arrhenius parameters of the present work were well consistent with the experimental

17

values overall.

18 19

Table 3. Fitted Arrhenius parameters of OTG pyrolysis. Ref. [52] Ref. [53] Ref. [54] Ref. [55] this work

Temperature range (K) Oil type A (s-1) Ea (kJ/mol) 593-763 sunflower oil 155-220 603-771 sunflower oil 170-210 13 18 653-753 sunflower oil 1.5×10 -1.2×10 205-216 13 550-725 olive oil 194.6 3.54×10 13 1800-2800 OTG 7.66×10 204

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Energy & Fuels

5. Conclusion

2

In the present work, a series of ReaxFF MD simulations are carried out to investigate the

3

pyrolysis mechanism of oleic-type triglyceride for the first time. The OTG model, which is

4

composed of three C18 mono-unsaturated fatty acids attaching to a glycerol group by ester

5

linkages, is constructed and used in the pyrolysis simulations. The overall pyrolysis profile,

6

detailed pyrolysis pathway, evolutions of important products, and pyrolysis kinetic modelling

7

which are related to the OTG pyrolysis are intensively investigated.

8

Through the 100ps cook-off simulation, the decomposition reaction of OTG pyrolysis is

9

observed to initiate through the breakage of C–O bond between the glycerol backbone and the

10

oleic acid chain at a temperature around 570K. A total of 78 different species are observed,

11

including the short-lived intermediates and the stable products. C2H4 is the most abundant

12

product and almost all the oxygen atoms are transferred into CO2. The large oxygen-containing

13

radicals and long-chain hydrocarbon radicals are important intermediate products to decompose

14

into smaller radicals and molecules. At the end of the cook-off simulation, the ultimate products

15

are small molecular-dominated, which the small molecular categories (C0~C3 and CO2) could

16

account for about 86% of the total products.

17

By systematically analyzing the unimolecular pyrolysis simulations, an overall pyrolysis

18

pathway of OTG, especially focusing on the short-lived intermediate products and multi-channel

19

decomposition reactions, is analyzed exhaustively at atomistic level. It is found that the initial

20

decomposition reaction of OTG is caused by C-O fission, resulting in the release of C18H33O2•

21

radical. As the secondary thermal cracking of OTG, the decomposition of C18H33O2• radical is

22

found to start through multi-channel decomposition reactions: decarboxylation reactions and C-C

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bond cleavages at α, β-C position, which is largely affected by the unsaturation degree of

2

triglyceride and pyrolysis temperature. C-C bond β-scission reactions are the most dominant

3

intramolecular cracking reactions in the decomposition process of OTG pyrolysis. The observed

4

reactions during our ReaxFF MD simulations are well consistent with the experimental

5

observations and further confirmed by DFT calculations, which suggests that ReaxFF method

6

can correctly provide multi-channel decomposition pathways and their reactivity trend for OTG

7

pyrolysis.

8

Based on similar evolution characters shown in the temperature-dependent pyrolysis

9

simulations, principal products collected in the temperature range of 1800K~2800K are divided

10

into three groups: the stable products, the reactive radical products and the temperature-

11

dependent products. C2H4 is the most abundant stable product. C-C bond β-scission reactions are

12

found to be the primary reactions to produce C2H4. CH3• radical is a representative reactive

13

radical product which is found to generate throughout the pyrolysis yet would not accumulate

14

large amounts in the final products for its active chemical property. Moreover, CO is an

15

important temperature-dependent product, which is first discovered during the ReaxFF MD

16

simulations of OTG pyrolysis. Detailed simulation trajectory analysis shows that the formation

17

of CO is mainly through two radical-related schemes: (1) decarbonylation reaction upon ester

18

radicals by removing the carboxyl group to release CO, and (2) hydrogen addition reaction by

19

reducing CO2 to form CO. The detected fragments (e.g., C2H4, CH3• radical and CO) and their

20

evolution mechanisms are in good agreement with the GC-MS experimental results, which

21

suggests that ReaxFF method can predict most of the product species and their evolution

22

channels, and can be used to assist the investigation of radical-related evolution mechanisms of

23

important products. 38 ACS Paragon Plus Environment

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1

Finally, the first-order kinetic modelling is applied to describe the pyrolysis of oleic-type

2

triglyceride. The calculated activation energy and pre-exponential factor of OTG pyrolysis

3

obtained from ReaxFF MD simulations are 204 kJ/mol and 7.66×1013 s−1 respectively at

4

temperature range of 1800K~2800K, which are reasonably consistent with the experimental

5

results. This validated result further suggests that ReaxFF is a promising and reliable method to

6

unravel the intricate pyrolysis mechanism of oleic-type triglycerides at atomistic level.

7 8 9

AUTHOR INFORMATION

10

Corresponding Author

11

Beijing Key Lab of HV and EMC, North China Electric Power University, Beijing 102206,

12

China.

13

* Tel./fax: +86 10 61772040. E-mail address: [email protected] (Q. Li).

14

Author Contributions

15

The manuscript was written through contributions of all authors. All authors have given

16

approval to the final version of the manuscript.

17

Notes

18

The authors declare no competing financial interest.

19

ACKNOWLEDGMENT

20

This research work was supported by National Natural Science Foundation of China (Grant

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1

51477051) and Beijing Natural Science Foundation (Grant 3142018).

2

ABBREVIATIONS

3

ReaxFF, reactive force field; MD, molecular dynamics; OTG, oleic-type triglyceride; DFT,

4

density functional theory; GC-MS, gas chromatography-mass spectrometer; QM, quantum

5

mechanics; CMD, conventional molecular dynamics; BO, bond order; ADF, Amsterdam Density

6

Functional; NVT, constant particle number, constant volume and constant temperature; B3LYP,

7

Becke’s three-parameter gradient-corrected exchange potential combined with Lee-Yang-Parr

8

gradient-corrected correlation potential; 6-311G, split-valence triple-zeta basis set.

9

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