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Oil shale is an important potential energy source, consisting of an inorganic mineral matrix containing organic matter. The organic matter is generally divided into two fractions: bitumen and kerogen. Kerogen is insoluble in normal organic solvents and believed to be the source material for oil and gas that formed during the oil shale thermal process.1−5 The composition of kerogen depends upon the organic matter origin, the conditions of preservation of organic matter during sedimentation, and the thermal maturation. According to the van Krevelen diagram, kerogen can be classified into four types on the basis of their ratios of H/C and O/C. In the past 2 decades, much efforts have been devoted to study kerogen, focusing on the research of the molecular structure,6−27 kerogen pyrolysis,28−50 and natural oil generation.51−80 The chemical structure features of kerogen are of great practical significance to understand the pyrolysis mechanism and guide the actual industrial processes. Development of a two-dimensional (2D) model of kerogen provides a reasonable starting point for understanding the chemical structure of oil shale.65−72 According to the structural information obtained from elemental analysis, electron microscopy, 13C nuclear magnetic resonance (NMR), thermogravimetry, functional analysis, and pyrolysis, Behar and Vandenbroucke66 proposed the models for kerogens of type I, type II, and type III at different evolution stages (beginning of diagenesis, beginning of catagenesis, and end of catagenesis) with the molecular weight of about 25 000, respectively. Siskin et al.71 proposed a 2D model of kerogen for the Green River oil shale with a chemical formula of C645H1017N19O17S4. The data of the model by Siskin et al. were mainly obtained by NMR and mass spectroscopy of materials isolated under mild conditions. 13C NMR quantified the specific carbon-containing functional groups, and mass spectrometry analyzed the gas evolution and species during kerogen pyrolysis. This model was also compared to the results of NMR, X-ray photoemission spectroscopy (XPS), and sulfur X-ray absorption near edge structure (XANES). Later, Lille et al.11 evaluated the chemical structure of Estonian kukersite kerogen using a simulation of 13C magic angle spinning (MAS) NMR spectra. In comparison to the 2D model, a three-dimensional (3D) structural model not only defines the structural information but also provides a new way to determine the pyrolysis reaction mechanism and active sites as well as predict the reaction trend.73−75 Orendt et al.76 recently developed a 3D structural model of Green River kerogen based on the 2D structure of

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Three-Dimensional Structure of a Huadian Oil Shale Kerogen Model:

An Experimental and Theoretical Study

Xiao-Hui Guan, Yao Liu, Di Wang, Qing Wang, Ming-Shu Chi, Shuang Liu, and Chun-Guang Liu*

College of Chemical Engineering, Northeast Dianli University, Jilin, Jilin 132012, People’s Republic of China

Supporting Information

ABSTRACT: The molecular structural information on a kerogen isolated from Huadian oil shale was obtained using solid-state

C nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), and X-ray

diﬀraction (XRD) techniques. Then, a series of Huadian kerogen isomers were constructed on the basis of these structural data.

The possible carbon skeleton isomer and the substituted position eﬀects of the aromatic ring, aliphatic ether bond, carboxylic

acid, and carboxylic acid derivative as well as the quantity of tertiary and quaternary carbons on Huadian kerogen model stability

have been systematically studied on the basis of density functional theory (DFT) calculations. For the carbon skeleton isomer,

the calculated total energy decreases with the increasing number of the closed annular space (grid), which is constituted by

connecting the aliphatic chain to the aromatic cluster or other aliphatic chain. DFT calculations show an about 16.8 kcal mol

−1

decrease in total energy for every grid increase when the number of grids increases from 2 to 11. A signiﬁcant break in the

decrease of the total energy has been obtained for an isomer with 11 grids, which means that a proper number of grids (11 grids

is appropriate in this paper) in carbon skeleton should be considered for building the chemical structure of Huadian kerogen. For

the substituted position eﬀects, aliphatic ether bonding to quaternary carbon, carboxylic acid attaching to secondary carbon, and

carboxylic acid derivative bonding to quaternary carbon seem to give a lower energy structure than other connections. Besides, a

high quantity of tertiary and quaternary carbons is conducive to a stable model for Huadian kerogen. The aromatic cluster

dispersed distribution also makes a contribution to improve the stability of the model. According to these results, we proposed a

relatively stable Huadian kerogen three-dimensional (3D) model. Moreover, this 3D model was testiﬁed reasonablely through

the match between calculated and experimental

C NMR spectra.

1. INTRODUCTION

Oil shale is an important potential energy source, consisting of

an inorganic mineral matrix containing organic matter. The

organic matter is generally divided into two fractions: bitumen

and kerogen. Kerogen is insoluble in normal organic solvents

and believed to be the source material for oil and gas that

formed during the oil shale thermal process.

1−5

The

composition of kerogen depends upon the organic matter

origin, the conditions of preservation of organic matter during

sedimentation, and the thermal maturation. According to the

van Krevelen diagram, kerogen can be classiﬁed into four types

on the basis of their ratios of H/C and O/C. In the past 2

decades, much eﬀorts have been devoted to study kerogen,

focusing on the research of the molecular structure,

6−27

kerogen pyrolysis,

28−50

and natural oil generation.

51−80

The

chemical structure features of kerogen are of great practical

signiﬁcance to understand the pyrolysis mechanism and guide

the actual industrial processes.

Development of a two-dimensional (2D) model of kerogen

provides a reasonable starting point for understanding the

chemical structure of oil shale.

65−72

According to the structural

information obtained f rom elemental analysis, electron

microscopy,

C nuclear magnetic resonance (NMR), thermog-

ravimetry, functional analysis, and pyrolysis, Behar and

Vandenbroucke

proposed the models for kerogens of type

I, type II, and type III at diﬀerent evolution stages (beginning of

diagenesis, beginning of catagenesis, and end of catagenesis)

with the molecular weight of about 25 000, respectively. Siskin

et al.

proposed a 2D model of kerogen for the Green River oil

shale with a chemical formula of C

645

1017

. The data

of the model by Siskin et al. were mainly obtained by NMR and

mass spectroscopy of materials isolated under mild conditions.

C NMR quantiﬁed the speciﬁc carbon-containing functional

groups, and mass spectrometry analyzed the gas evolution and

species during kerogen pyrolysis. This model was also

compared to the results of NMR, X-ray photoemission

spectroscopy (XPS), and sulfur X-ray absorption near edge

structure (XANES). Later, Lille et al.

evaluated the chemical

structure of Estonian kukersite kerogen using a simulation of

C magic angle spinning (MAS) NMR spectra.

In comparison to the 2D model, a three-dimensional (3D)

structural model not only deﬁnes the structural information but

also provides a new way to determine the pyrolysis reaction

mechanism and active sites as well as predict the reaction

trend.

73−75

Orendt et al.

recently developed a 3D structural

model of Green River kerogen based on the 2D structure of

Siskin et al. using a combination of quantum chemistry and

molecular dynamic calculations. In their work, the molecular

dynamics were employed to achieve the initial monomer

conformations. Then, a simulated annealing procedure was

repeatedly employed to obtain the lowest energy monomer

conformations. The minimum energy structure was further

Received: December 23, 2014

Revised: June 18, 2015

Published: June 18, 2015

Article

pubs.acs.org/EF

Energy Fuels 2015, 29, 4122−4136

optimized at the restricted Hartree−Fock (HF) level of the

quantum chemical theory using the minimal STO-3G basis sets.

On the basis of optimized geometries of the 3D structure,

NMR was calculated using density functional theory (DFT)

PBE1PBE exchange correlation functional and 4-31G basis sets.

This work provided a general methodology to develop a 3D

model based on a known 2D model of kerogen. Li and co-

workers

has deﬁned the lowest energy conformation of the

Huadian kerogen molecular structure using a simulated

annealing procedure. Structu ral parameters of the lowest

energy conformation were further analyzed. On the basis of

the above molecular dynamics simulation met hods, they

proposed the possible reaction sites and pyrolysis process of

kerogen.

For the 3D molecular simulation, the interaction between

kerogen and other molecular units coming from the organic

matter of oil shale should also be considered. In general, the

organic matter mainly composes of kerogen (the most

abundant), asphaltenes, resins, hydrocarbons, and other ﬂuids

(such as carbon dioxide, water, and nitrogen). Galliero et al.

constructed a 3D molecular model of organic matter presenting

a type II oil shale in the middle of the oil generation window

using molecular dynamics simulations (NPT molecular

dynamics and force ﬁelds). Their results provide a lot of

valuable information at the molecular levels, such as the ﬂuid

distribution within the organic matter, pore size distributions,

isothermal compressibility, and dynamic of the ﬂuids within the

kerogen matrix. Very recently, a molecular dynamic simu-

lation

has been applied to a representative set of kerogen for

the purpose of obtaining quantitative predictions of volumetric

properties, which also take into account the interaction among

them. The density results are well in agreement with the well-

documented trends of kerogen density with thermal maturity

and organic type.

As mentioned above, both Orendt and Li adopted a

simulated annealing algorithm to seek the lowest energy

conformation of kerogen. However, the simulated annealing

algorithm

is based on a given 2D model with ﬁ xed

connections and structural characteristics to obtain a more

compact and superior conformation than the initial model.

That is, molecular dynamic simulation considers the conforma-

tional isomer rather than the geometric isomer. The kerogen

model has numerous geometric isomers. Only considering one

connection case to seek the lowest conformation is not

convincing and suﬃcient. Besides, it is impracticable to

endlessly optimize all possible geometric isomers for a large-

sized kerogen molecule because of limitations in computational

resources.

80−82

Thus, it is valuable to explore the relationship

between isomer stability and molecular geometries of kerogen.

The HF method provided a reasonable starting point to

probe the chemical structure for macromolecular systems but

failed to consider the electron correlation eﬀects, which is

important for calculating structure and energies of a molecule.

To overcome the weakness in electron correlation eﬀects,

coupled cluster, conﬁguration interaction, and perturbation

theory have been developed. However, they are not suitable for

macromolecular systems because of the high computational

cost. In contrast, the DFT method has been proven to be

eﬃcient in evaluation of the physicochemical property for a

wide range of compounds with a large size because of proper

consideration of electron correlation and moderate computa-

tional cost. Currently, DFT is a widespread acceptance method

for understanding physicochemical properties of macromolec-

ular systems. It should be stressed that the DFT method turned

out to be insuﬃcient for describing the weak intra- or

intermolecular interactions, but it is the most accurate method

thatcanbeusedonlargemoleculesatanaﬀordable

computational cost.

During the past few years, our group did research in the

combustion characteristics and physicochemical properties of

oil shales, combustion and co-combustion characteristics of

semi-coke, etc.

83−99

Semi-coke mentioned here refers to the

solid waste left after oil shale retorting, which contains phenols,

polycyclic aromatic hydrocarbons (PAHs), and oil prod-

ucts.

100,101

However, few studies were focused on the chemical

structure of kerogen in oil shale. Therefore, the aim of this

paper is (i) to obtain the detailed structural information on

Huadian kerogen through a variety of experimental methods,

(ii) and on the basis of these experimental data, a series of

Huadian kerogen 3D isomer models have been constructed to

consider the carbon skeleton isomerization, the substituted

position eﬀects of the aromatic ring, aliphatic ether bond,

carboxylic acid, and carboxylic acid derivative, and the quantity

of tertiary and quaternary carbons on model stability using

DFT calculations.

Research in this paper probably provides a scientiﬁc guide to

build and ﬁnd the kerogen 3D model. Although the constructed

kerogen model in this paper is not the lowest energy

conformation among inﬁnite kerogen geometric isomers, it is

still valuable for building the model of kerogen. What we

emphasize is that, during the process of building a model of

kerogen, the carbon skeleton isomerization, substituted

position eﬀects, and quantity of tertiary and quaternary carbons

should be taken into reasonable considerations. For most of the

cases in the current, these factors were not considered or

inadequately described in building a 3D model of kerogen.

2. EXPERIMENTAL SECTION

2.1. Sample Preparation. The oil shale sample used in this paper

was obtained from Huadian mine located in Jilin, China. Large oil

shale blocks were ﬁrst crushed and sieved to 0.2 mm to obtain the

experimental oil shale sample. Bitumen was extracted from the shale

using chloroform. Then, the oil shale sample was demineralized by a

four-step extraction procedure using HCl−HF−HNO

−HCl. The

demineralization eﬀect has been checked using the X-ray diﬀraction

(XRD) technique (see below).

The results of elemental analysis of Huadian kerogen are shown in

Table 1. According to the atomic H/C and O/C ratios,

51,102

Huadian

kerogen belongs to type I, although the atomic H/C and O/C ratios of

our kerogen diﬀer from other Huadian kerogen reported in previous

studies.

23,77

This may be due to the fact that the reported sample

preparation process did not remove asphaltenes. The various structure

of kerogen obtained in the same mine but diﬀerent sedimentary layers

Table 1. Elemental Analysis of Huadian Kerogen (wt %, Dry

and Ash-Free Basis)

C 71.73

H 8.885

O 11.033

N 1.29

2.257

H/C

1.486

O/C

0.115

N/C

0.189

S/C

0.012

Total sulfur.

Atomic ratio.

Energy & Fuels Article

DOI: 10.1021/ef502759q

Energy Fuels 2015, 29, 4122−4136

4123

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