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\DOI{10.5802/crchim.237}
\datereceived{2023-01-27}
\daterevised{2023-05-18}
\dateaccepted{2023-06-20}
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\dateposted{2023-09-13}
\begin{document}

%\copyeditor{RK}

\begin{noXML}

\RubricEF{Research article}{Article de recherche}

\title{Aquathermolysis of heavy oil catalyzed by transition metal salts
and clay}

\alttitle{Aquathermolyse de p\'etrole lourd catalys\'ee par des sels de
m\'etaux de transition et de l'argile}

\author{\firstname{Yingna} \lastname{Du}}
\address{Shaanxi Province Key Laboratory of Environmental Pollution
Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou
University, Xi'an, 710065, China} 
\address{Shaanxi University Engineering Research Center of Oil and Gas
Field Chemistry, Xi'an Shiyou University, Xi'an, 710065, China}
\email[Y. Du]{1294138205@qq.com}

\author{\firstname{Liyuan} \lastname{Zhang}}
\addressSameAs{1}{Shaanxi Province Key Laboratory of Environmental Pollution
Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou
University, Xi'an, 710065, China} 
\address{No.11 Oil Production Plant, PetroChina Changqing Oilfield
Company, Xi'an, 710060, China}
\email[L. Zhang]{zlyuan\_cq@petrochina.com.cn}

\author{\firstname{Rui} \lastname{Jing}}
\address{Xi'an Changqing Chemical Group Co., Ltd, PetroChina Changqing
Oilfield Company, Xi'an, 7160065, China}
\email[R. Jing]{1156947308@qq.com}

\author{\firstname{Yongfei} \lastname{Li}}
\addressSameAs{1}{Shaanxi Province Key Laboratory of Environmental Pollution
Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou
University, Xi'an, 710065, China} 
\email[Y. Li]{yfli@xsyu.edu.cn}

\author{\firstname{Bo} \lastname{Yang}}
\addressSameAs{1}{Shaanxi Province Key Laboratory of Environmental Pollution
Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou
University, Xi'an, 710065, China} 
\addressSameAs{2}{Shaanxi University Engineering Research Center of Oil and Gas
Field Chemistry, Xi'an Shiyou University, Xi'an, 710065, China}
\email[B. Yang]{youngbo1120@hotmail.com}

\author{\firstname{Gang} \lastname{Chen}\CDRorcid{0000-0002-3711-4250}\IsCorresp}
\addressSameAs{1}{Shaanxi Province Key Laboratory of Environmental Pollution
Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou
University, Xi'an, 710065, China} 
\addressSameAs{2}{Shaanxi University Engineering Research Center of Oil and Gas
Field Chemistry, Xi'an Shiyou University, Xi'an, 710065, China}
\email[G. Chen]{gangchen@xsyu.edu.cn}

\shortrunauthors

\thanks{This research was funded by National Science Foundation of
China (50874092) and the Youth Innovation Team of Shaanxi University
and Postgraduate Innovation Fund Project of Xi'an Shiyou University
(YCS21211045).}

\begin{abstract}
Currently,
researchers have indicated that inorganic minerals in 
reservoirs, such as clay minerals, carbonates and quartz, can catalyze
the evolution of organic matter into oil and gas. Therefore it is
reasonable to believe that the minerals in reservoirs may act as a
 catalyst support with the metal-containing catalyst
added from outside during the thermal
recovery of heavy oil. This paper studied the aquathermolysis of heavy
oil catalyzed by minerals and transition metal. The reaction conditions
of two heavy oil samples were investigated. The results show that the
optimal reaction conditions of heavy oil from Xinjiang Baikouquan
Oilfield (XBO) are the reaction temperature of 250~\textdegree C and the
reaction time of 6~h; for the crude oil from Xinjiang Tahe Oilfield
(XTO), the optimal reaction conditions are determined to be the
reaction temperature of 250~\textdegree C and the reaction time of 12~h,
the water--oil ratio of the two oils is 0.3. Under optimal conditions,
viscosity and pour point of heavy oil are significantly reduced.
Differential scanning calorimetry (DSC), GC-MS analysis,
thermogravimetric analysis (TGA), and elemental analysis were used to
study the properties of the two heavy oil samples before and after
reaction to explore the mechanism of the catalyzed aquathermolysis of
heavy oil. This work will benefit the related heavy oil recovery work
in this field.
\end{abstract}

\begin{altabstract}
Des travaux r\'ecents indiquent que les min\'eraux inorganiques
pr\'esents dans le gisement, tels que les min\'eraux argileux, les
carbonates et le quartz, peuvent catalyser l'\'evolution de la
mati\`ere organique en p\'etrole et en gaz. Il est donc raisonnable de
penser que ces min\'eraux peuvent agir comme cocatalyseurs avec le
catalyseur ajout\'e pendant la r\'ecup\'eration thermique du p\'etrole
lourd. Cet article a \'etudi\'e l'aquathermolyse de p\'etrole lourd
catalys\'ee par des min\'eraux et des m\'etaux de transition. Les
conditions de r\'eaction de deux \'echantillons de p\'etrole lourd ont
\'et\'e \'etudi\'ees. Les r\'esultats montrent que les conditions
optimales de r\'eaction du p\'etrole lourd du champ p\'etrolif\`ere de
Xinjiang Baikouquan (XBO) sont une temp\'erature de r\'eaction de
250~{\textdegree}C et un dur\'ee de r\'eaction de 6~heures ; pour le
p\'etrole brut du champ p\'etrolif\`ere de Xinjiang Tahe (XTO), les
conditions optimales de r\'eaction sont une temp\'erature de r\'eaction
de 250~{\textdegree}C et une dur\'ee de r\'eaction de 12~heures, le
rapport eau-huile des deux p\'etroles est de 0,3. Dans les conditions
optimales, la viscosit\'e et le point d'\'ecoulement du p\'etrole lourd
sont consid\'erablement r\'eduits. La calorim\'etrie diff\'erentielle
\`a balayage (DSC), l'analyse GC-MS, l'analyse thermogravim\'etrique
(TGA) et l'analyse \'el\'ementaire ont \'et\'e utilis\'ees pour
\'etudier les propri\'et\'es des deux \'echantillons de p\'etrole lourd
avant et apr\`es la r\'eaction afin d'explorer le m\'ecanisme de
l'aquathermolyse catalys\'ee du p\'etrole lourd. Ce travail est une
contribution au domaine de la r\'ecup\'eration du p\'etrole lourd.
\end{altabstract}

\keywords{\kwd{Aquathermolysis}
\kwd{Viscosity reduction}
\kwd{Heavy oil}
\kwd{Transition metal}
\kwd{Clay}
\kwd{Catalysis}}

\altkeywords{\kwd{Aquathermolyse}
\kwd{R\'eduction de la viscosit\'e}
\kwd{P\'etrole lourd}
\kwd{M\'etal de transition}
\kwd{Argile}
\kwd{Catalyse}}

\maketitle

\clearpage

\twocolumngrid

\end{noXML}

\section{Introduction} 

The current heavy oil reserves are approximately 5.6 trillion
barrels~\cite{1,2,3,4}. However, heavy oil has the\break characteristics of
high density, high viscosity, poor fluidity and complex composition,
which lead to difficulties in its extraction, transportation and
subsequent processing~\cite{5,6}. Therefore, it is necessary to carry
out research on the viscosity reduction technology of heavy oil, and it
is necessary to solve the problems existing in the process of
exploitation and transportation~\cite{7,8}. The initial method was to
use superheated water or steam to heat heavy oil to reduce its
viscosity and make it easier to flow, thereby enhancing
heavy oil recovery~\cite{9,10}. There are the two main types of heavy
oil viscosity reduction technologies: physical and chemical. The former
 is primarily based on the addition of crude oil flow
improvers during the hot injection process, which is a simple and
low-cost method. Gu~\cite{11} and Chen \etal~\cite{12,13} investigated
and synthesized a variety of alkylbenzene sulfonates, some of which were
able to achieve a viscosity reduction rate higher than 90\%. When the
alkylbenzene sulfonate co-crystallizes with the saturated hydrocarbon in
crude oil, it causes disordering of the 
{wax particles}
and results in a lower pour point.
Nonionic surfactants were also studied as a means of
improving the viscosity and pour point of crude oil~\cite{14}. In their
research, Zhou \etal~\cite{15} utilized barium alkylbenzene sulfonate as
the main synthetic material. It was shown that crude oil flow improvers
can prevent the crystallization of saturated hydrocarbons in heavy oil
and effectively reduce the viscosity of heavy oil. In addition, many
researchers have explored polymers as potential viscosity reducing
agents. Lv \etal~\cite{16}, synthesized a functional copolymer
viscosity reducer with mixed esters, 4-vinylpyridine and styrene
monomers, indicating that the steric hindrance of copolymer viscosity
reducer molecules destroys the interlocking structure of asphaltenes,
which helps to reduce the viscosity of heavy oil. Zhang \etal~\cite{17} 
modified waste polystyrene with acetic anhydride, which
can effectively reduce environmental pollution while also reducing
heavy oil  viscosity and improving crude oil fluidity. The purpose of
chemical viscosity reduction technology is to reduce the viscosity of
heavy oil by incorporating a catalyst into the aquathermolysis process.
At present, there are four types of catalysts; transition metal
water-soluble catalysts, transition metal oil-soluble catalysts,
acid--base catalysts, and ionic liquid catalysts~\cite{18}. In the process
of steam injection, the addition of catalyst facilitates the pyrolysis of
asphaltene and resin molecules into small molecular
aquathermolysis products~\cite{19}, thereby improving the viscosity and
fluidity of heavy oil~\cite{20,21,22} and solving the problems of
difficulty and high cost of heavy oil exploitation. Therefore, this
method is widely used~\cite{24}. Many researchers have also done a lot
of research work on heavy oil aquathermolysis and prepared a series of
catalysts to conduct aquathermolysis viscosity reduction experiments on
heavy oil from different oilfields in China, and the viscosity
reduction rate can reach about 80\%~\cite{27,28}. 
Chen \etal~\cite{25,26}, investigated aquathermolysis of heavy oil at low
temperature catalyzed by Zn (II) coordination complexes and Co (II)
complexes, respectively. The reaction of catalyst and alcohol occurred
at low temperature, which proved that the combination of catalyst and
alcohol had a synergistic effect, which was helpful to greatly reduce
the viscosity of heavy oil and improve its quality.
Chen~\cite{29,30} conducted in-depth and meticulous research on the
catalyzed reaction of aquathermolysis at the molecular level, deduced a
large number of catalyzed mechanisms of the cracking reaction using
kinetics, and proposed several new catalyzed reaction models. Ming 
\etal~\cite{38}, developed an in situ synthesis strategy using
well-dispersed CuO nanoparticles as an aquathermolysis catalyst to
study the mechanism of viscosity reduction of heavy oil in the
catalyzed \mbox{system.}\looseness=1

Although researchers have conducted extensive studies on catalyzed
aquathermolysis of heavy oil, the effect of the indoor simulation
experiment cannot be replicated in the oil field. This is 
because the indoor simulation experiment does not consider the effect of
minerals in the oil reservoir on the addition of exogenous catalysts.
On the other hand, in the traditional heavy oil viscosity
reduction technology, inorganic salts and water are added to the heavy
oil, but some metal cations will combine with OH$^{-}$ in
water to form hydroxide precipitates, resulting in poor viscosity
reduction effect. The transition metal cation may adsorb on/in
bentonite, which forms a new catalyst in the aquathermolysis. There are
many kinds of minerals in the oil reservoir of oilfield. At present,
studies have shown that inorganic minerals such as clay minerals,
carbonates, and quartz have catalytic effects on the transformation of
organic matter into oil and gas. When the exogenous catalyst enters the
oil reservoir, the oil reservoir minerals may adsorb or exchange ions to
form a complex, which catalyzes the aquathermolysis reaction. A typical
representative of clay reservoir minerals can adsorb metal cations by 
lattice substitution at the negatively charged surface.
Therefore, we studied the aquathermolysis catalyzed by
bentonite-supported transition metals to evaluate the influence of
reservoir minerals on the external catalyst.

\section{Materials and methods} 

\subsection{Materials}

The chlorides used in the experiment, such as ferrous chloride, ferric
chloride, cobalt chloride, and copper chloride were purchased from
National Group Chemical Reagent Co., Ltd. The liquid reagents are
analytically pure reagents without further purification. The heavy oil
used for evaluation is from Xinjiang Baikouquan Oilfield (XBO) and Xinjiang
Tahe Oilfield (XTO), properties of which are shown in\break Table~\ref{tab1}.

\begin{table*}
\caption{\label{tab1}Crude oil physical parameters}
\begin{tabular}{ccccccc}
\thead
{Crude oil} & 
\parbox[t]{2cm}{\centering Pour point (\textdegree C)} & 
\parbox[t]{2.2cm}{\centering Water content (\%)} & 
\parbox[t]{2cm}{\centering Saturated HC (\%)} & 
\parbox[t]{2cm}{\centering Aromatic HC (\%)} & 
\parbox[t]{1cm}{\centering Resin (\%)} & 
\parbox[t]{1.8cm}{\centering Asphaltene (\%)}\vspace*{2pt} \\ 
\endthead
XBO & \05.3 & 11.41 & 48.50 & 21.00 & 19.07 & \00.02 \\ 
XTO & 14.4 & \00.17 & 42.13 & 19.70 & 21.83 & 13.44
\botline
\end{tabular}
\end{table*}

Each fraction (saturates, aromatics, resins and asphaltenes) content in
the heavy oil sample was quantitatively analyzed based on the standard
of SY/T 5119-2008~\cite{31}. The pour point was measured according to
SY/T 2541-2009~\cite{32}. The viscosity of the crude oil sample was
measured according to the standard of SY/T 0520-2008~\cite{33}.

\subsection{Initial crude oil reaction, product separation and analysis}

The water/oil mass ratio of the aquathermolysis of the two oil samples
was selected according to the method described in~\cite{34}; the
temperature and duration of the aquathermolysis were selected according to
the method described in~\cite{35}.

\subsection{Preparation catalyst}

Calcium bentonite and sodium bentonite were hydrated for 
6~h. The transition metal chloride was added to the hydrated bentonite
in a 1.1 mass ratio
(the bentonite:chloride molar ratio  is 1:1.5). After stirring for
3~h, vacuum distillation, washing, drying, filtering and grinding 
the residue, the
bentonite-supported transition metal ion catalyst was obtained. The
names of the catalysts are shown in Table~\ref{tab2}.

\begin{table}
\caption{\label{tab2}List of catalysts}
\begin{tabular}{ccc}  
\thead
\parbox[t]{1.5cm}{\centering Metal chloride} & 
\parbox[t]{1.5cm}{\centering Calcium bentonite} & 
\parbox[t]{1.5cm}{\centering Sodium bentonite}\vspace*{2pt} \\ 
\endthead
Ferrous chloride & CB-1 & NB-1 \\ 
Ferric chloride & CB-2 & NB-2 \\ 
Cobalt chloride & CB-3 & NB-3 \\ 
Copper chloride & CB-4 & NB-4 
\botline
\end{tabular}
{\vspace*{4pt}}
\end{table}

\subsection{Catalyzed aquathermolysis reaction of heavy oil}

Heavy oil and water were added successively with a mass ratio of 0.3
into the reactor. After catalyst addition, 
the reactants were heated to 180~\textdegree C
and stirred. The mixture was cooled,
and then the pour point and viscosity of the crude oil were
evaluated~\cite{36}.

\subsection{Elemental analysis}

The elemental compositions (C, H, N and S) of initial oil and treated
oil were measured by an element analyzer in accordance with China Petroleum
Industry Standard SY/T 5119-2016.

\subsection{Thermogravimetric analysis}

The TGA was conducted on a TGA/DSC 8220 (Mettler Toledo,
Switzerland) instrument over a temperature range of 30--600~\textdegree
C, under nitrogen atmosphere at a heating rate of 10~\textdegree
C/min~\cite{37}.

\subsection{Differential scanning calorimetry (DSC)\newline analysis}

Different scanning calorimetry (DSC) analysis of waxy crude oil samples
was performed on a DSC822e DSC (Mettler Toledo, Switzerland) in a
nitrogen atmosphere at a flow rate of 20 ml/min at a scan rate of ${-}$30
to 80~\textdegree C at 11~\textdegree C/min, and a cooling rate of
8~\textdegree C/min. The wax precipitation point of heavy oil was
measured according to SY/T 0545-2012.

\subsection{GC-MS analysis of saturated hydrocarbon components of heavy oil.}

A HP-5 type capillary chromatographic column (0.32~mm ${\times}$ 30~m,
0.25~m) was used. The temperature of the injection port was 300~\textdegree C,
the temperature of the detector was 280~\textdegree C. The temperature
was programmed as follows: the starting temperature was 40~\textdegree C, held for
0.5~min; the temperature was increased to
120~\textdegree C at a rate of 5~\textdegree C/min, and then 
to 280~\textdegree C at 10~\textdegree C/min, and 
then maintained for 2.00~min.

\section{Results and discussion}

\subsection{Characterizations of catalyst}

It can be seen from Figure~\ref{fig1} that the metal-containing crystallites
are tightly adsorbed on the scaly surface of bentonite and the inner
wall of the pores, which increases the contact site of metal
crystals with crude oil. The catalytic activity of a solid catalyst is
directly proportional to its specific surface area. Therefore, by
loading metals onto bentonite, its catalytic efficiency can be
significantly improved.

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}SEM images of calcium bentonite (left) and sodium
bentonite (right) before and after loading (a) before loading; (b) after
loading.}
\end{figure*}

\subsection{The effect of catalyzing heavy oil aquathermolysis upgrading}

Heavy oil was added to the reactor with a water/oil ratio of 0.3 and
the catalysts were then screened with a 0.05\% mass ratio to heavy oil
under the optimized conditions mentioned above. During aquathermolysis
of XBO, it can be seen in Figure~\ref{fig2}(a,b) that the prepared
catalysts did show some desired impact. The viscosity reduction effect
of catalyst CB-4 was the best. Compared with the blank oil sample, 
the \mbox{viscosity} of XBO was reduced by 37.1\% (50~\textdegree C), and the
viscosity of XBO was reduced by 21.5\% (50~\textdegree C). However, the
effect on XTO was obviously reflected in Figure~\ref{fig2}(c,d) CB-1
can catalyze aquathermolysis of XTO effectively with a
viscosity-reduction rate of 68.2\% (50~\textdegree C) and NB-1 can
catalyze the aquathermolysis of XTO effectively with a viscosity
reduction rate of 66.2\% (50~\textdegree C).

\begin{figure*}
{\vspace*{3pt}}
\includegraphics{fig02}
{\vspace*{3pt}}
\caption{\label{fig2}Viscosity effects of calcite and sodium clay
supported catalysts for XBO treatment at different temperatures (a,b).
Viscosity effects of calcite and sodium clay supported catalysts for
XTO treatment at different temperatures (c,d).}
{\vspace*{6pt}}
\end{figure*}

Comparison of the experiments above show that the aquathermolysis
process of XBO and XTO catalyzed by this series of catalysts contains
the cracking reaction and the polymerization reaction at the same time.
The cause of the low reduction in viscosity is hypothesized to be
either the complex reaction of metal ions with the heavy oil, which
results in the formation of coke particles and lumps, or the free
radical polymerization of the C--S intermediates produced by
aquathermolysis, resulting in a significant amount of macromolecular
substances. Nickel could affect the quality of crude oil~\cite{22} and
pollute refining catalysts, so the bentonite-supported Ni (II) catalyst
was not investigated further. Zn (II) has no activity and is
not listed here.

\subsection{Differential scanning calorimetry (DSC)\newline analysis}

Differential scanning calorimeter (DSC) analysis was conducted to study
the wax formation process in untreated and treated XTO crude oil. CB-1
and NB-1 with relatively high catalytic activity for XTO were selected
for DSC analysis of XTO before and after the catalyzed reaction. It can
be seen from Figure~\ref{fig3} that the DSC curve of XTO shifts
significantly to the left after the catalyzed aquathermolysis. The wax
precipitation point decreased from 30.8~\textdegree C to
27.1~\textdegree C and then to 26.9~\textdegree C. This demonstrates that after
the reaction, a portion of the heavy components in the oil sample are
decomposed into lighter components, while the lighter components
additionally dissolve a \mbox{portion} of the heavy components, slowing the
precipitation of 
{wax particles}
and lowering the oil sample's wax precipitation point.

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}DSC curves of XTO before and after
aquathermolysis.}
\end{figure}

\subsection{Thermogravimetric analysis}

CB-1 and NB-1 have a high catalytic effect on XTO as was seen during
thermogravimetric analysis of XTO before and after the catalyzed
reaction. Figure~\ref{fig4} (catalyst vs. oil sample) shows that the weight loss
curve of XTO shifted to the left before and after the catalyzed
aquathermolysis. The weight loss rates of XTO before and after adding
the catalysts CB-1 and NB-1 to the aquathermolysis at
50--200~\textdegree C were 19.9\%, 23.0\%, and 22.5\%, respectively, and
the weight loss rates at 200--350~\textdegree C were 33.7\%, 32.9\%, and
32.9\%, respectively, 32.5\%, and the weight loss rates at
350--500~\textdegree C were 32.4\%, 29.3\% and 29.5\%,
respectively. As can be seen, the oil sample's low-temperature portion
weight loss rate increased while its high-temperature part weight loss
rate reduced before and after the aquathermolysis. This shows that the
catalyzed aquathermolysis cracks a part of the high-boiling,
difficult-to-decompose macromolecular substances in the heavy oil into
small molecular substances with low boiling points.

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}TGA curves of XTO oil samples under different
reaction conditions.}
\end{figure}

\subsection{Elemental analysis}

The change in element contents (C, H, N, and S) of XTO before and after
aquathermolysis reaction was determined by elemental analysis. It can be
seen from Table~\ref{tab3} that the total proportion of C and H
elements in XTO increases before and after CB-1 catalyzed reaction,
indicating that CB-1 has higher catalyzed activity. After the
aquathermolysis of heavy oil in the presence of water, 
C, H and, N content\break increases while S content decreases
and the C/H\break ratio decreases, indicating that water provides hydrogen
for the heavy oil aquathermolysis reaction. The~proportion of S element
fell by 0.31\% with aquathermolysis, by 0.38\% with CB-1-catalyzed
reaction. Since heteroatom-containing compounds are only found in
colloid and asphaltene, the proportion of C and H elements in heavy oil
increases after the reaction, while the proportion of S element
decreases significantly, and the trend is essentially\break consistent
with the viscosity reduction trend of heavy oil. This shows that the
aquathermolysis process is a series of reactions such as cracking,
rearrangement, and hydrogenation of the C--S bonds of the
sulfur-containing macromolecules in the resin asphaltenes. Finally, the
macromolecules are cracked into small molecules and the S element is
converted into H$_{2}$S gas, reducing the S content in the
oil\break sample.

\begin{table}
\caption{\label{tab3}Element contents (C, H, N, and S) of XTO before
and after the aquathermolysis reaction}
\tabcolsep=3.5pt
\begin{tabular}{ccccccc}
\thead
 & {Name} & {C (\%)} & {H (\%)} & {N (\%)} & {S (\%)} & {C:H (\%)} \\ 
\endthead
\morerows{4}{XTO} & Blank & 79.12 & 11.21 & 1.70 & 2.45 & 7.06 \\
 & CB-1 & 85.93 & 11.99 & 1.47 & 2.07 & 7.17 \\
 & CB-2 & 78.94 & 11.02 & 1.55 & 2.47 & 7.16 \\
 & CB-3 & 80.71 & 11.21 & 1.69 & 2.14 & 7.20 \\
 & CB-3 & 80.71 & 11.21 & 1.69 & 2.14 & 7.20
 \botline
\end{tabular}
\end{table}

\subsection{GC-MS analysis of saturated hydrocarbon components of heavy oil}

The saturated hydrocarbon components were separated from XBO and XTO 
before and after the reaction by the national standard method, and the
components were analyzed by GC-MS. The sample was made by dissolving 5\%
oil in chromatographically pure n-heptane, the peak detection limit was
2\% of the maximum peak area, the solvent peak was excluded, and the
maximum peak area matching compound is toluene. Figure~\ref{fig5} and
Table~\ref{tab4} illustrate that the proportion of low-carbon components 
grew, the proportion of medium-carbon components increased and then reduced,
and the proportion of high-carbon components increased and declined
following the catalyzed reaction of XTO aquathermolysis. After adding
CB-1, the components below C$_{10}$ increased from 33.15\% to 34.85\%,
the components between C$_{10}$--C$_{15}$ increased from 44.29\%
to 47.00\%, and the components between C$_{15}$--C$_{20}$
decreased from 19.36\% to 16.85\%. The components above C$_{20}$ are
reduced from 3.20\% to 1.20\%. After adding NB-1, the components below
C$_{10}$ increased from 33.15\% to 37.58\%, the components between
C$_{10}$--C$_{15}$ decreased from 44.29\% to 40.35\%, the
components between C$_{15}$--C$_{20}$ increased from 19.36\% to
20.50\%, and the components above C$_{20}$ are reduced from 3.20\% to
1.57\%.

\begin{table}
\caption{\label{tab4}Carbon number distribution results of XTO before
and after the reaction}
\begin{tabular}{cccc}
\thead
\parbox[t]{1.5cm}{\centering Carbon number} & 
\parbox[t]{1.7cm}{\centering XTO blank (\%)} & 
\parbox[t]{1cm}{\centering CB-1 (\%)} & 
\parbox[t]{1cm}{\centering NB-1 (\%)}\vspace*{2pt} \\ 
\endthead
${<}$C$_{10}$ & 33.15 & 34.85 & 37.58 \\ 
C$_{10}$--C$_{15}$ & 44.29 & 47.00 & 40.35 \\ 
C$_{15}$--C$_{20}$ & 19.36 & 16.85 & 20.50 \\ 
${>}$C$_{20}$ & \03.20 & \01.20 & \01.57 
\botline
\end{tabular}
\end{table}

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}GC curves of XTO before and after the reaction (a)
XTO, (b) CB-1-catalyzed aquathermolysis XTO, (c) NB-1-catalyzed
aquathermolysis XTO.}
\end{figure*}

The results show that the catalyst CB-1 has a higher cracking
effect on the molecules of ${>}$C$_{15}$ in XTO, while the
effect on the molecules of ${<}$C$_{15}$ is not
obvious. The molecular catalysis effect of NB-1 on
C$_{10}$--C$_{15}$ in XTO is obvious but, in addition to catalyzing the
cracking reaction, it also catalyzes the polymerization reaction, so
that the proportion of C$_{15}$--C$_{20}$ components increases after the
reaction. The~GC peaks of the oil samples were assigned by mass
spectrometry in order to analyze the alterations of XTO before and
after the reaction from the molecular structure. Table~\ref{tab5}
lists some new molecules generated after the aquathermolysis.

The GC-MS results show that after reaction, the monocyclic and bicyclic
compounds in n-alkanes and aromatic hydrocarbons increase, and a large
amount of unsaturated hydrocarbons and heterocyclic aromatic
hydrocarbons containing N, O elements 
{is generated}
at the same time, which is due to the resin asphaltenes. The
straight-chain hydrocarbon fragments resulting from ring-opening and
side chain scission during aquathermolysis
enter the saturated hydrocarbons while the cyclic
cracking fragments enter the aromatic hydrocarbons.

\begin{table*}[t!]
\caption{\label{tab5}List of new molecules produced after the
aquathermolysis reaction}
\tabcolsep=2.5pt
\begin{tabular}{cccc}  
\thead
Retention time (min) & Compound formula CB-1 & Retention time (min) & Compound formula NB-1 \\ 
\endthead
\05.277 & \raisebox{2.7em}{} \parbox{4.5cm}{\centering \inlinefig{fx01}} & \04.649 & \parbox{4.5cm}{\centering \inlinefig{fx02}}\vspace*{7pt}\\
\07.388 & \parbox{4.5cm}{\centering \inlinefig{fx03}} & \06.760 & \parbox{4.5cm}{\centering \inlinefig{fx04}}\vspace*{7pt}\\
\07.454 & \parbox{4.5cm}{\centering \inlinefig{fx05}} & \04.013 & \parbox{4.5cm}{\centering \inlinefig{fx06}}\vspace*{7pt}\\
\07.663 & \parbox{4.5cm}{\centering \inlinefig{fx07}} & \07.393 & \parbox{4.5cm}{\centering \inlinefig{fx08}}\vspace*{7pt}\\
10.281 & \parbox{4.5cm}{\centering \inlinefig{fx09}}  & 10.281  & \parbox{4.5cm}{\centering \inlinefig{fx10}}\vspace*{7pt}\\
11.060 & \parbox{4.5cm}{\centering \inlinefig{fx11}}  & 11.057  & \parbox{4.5cm}{\centering \inlinefig{fx12}}\vspace*{7pt}\\
11.907 & \parbox{4.5cm}{\centering \inlinefig{fx13}}  & 14.790  & \parbox{4.5cm}{\centering \inlinefig{fx14}}\vspace*{7pt}\\
15.718 & \parbox{4.5cm}{\centering \inlinefig{fx15}}  & 15.720  & \parbox{4.5cm}{\centering \inlinefig{fx16}}\vspace*{7pt}\\
17.607 & \parbox{4.5cm}{\centering \inlinefig{fx17}}  & 17.609  & \parbox{4.5cm}{\centering \inlinefig{fx18}}\vspace*{7pt}\\
22.309 & \parbox{4.5cm}{\centering \inlinefig{fx19}}  & 20.769  & \parbox{4.5cm}{\centering \inlinefig{fx20}}\vspace*{7pt}\\
24.524 & \parbox{4.5cm}{\centering \inlinefig{fx21}}\vspace*{3pt}  &         & 
\botline
\end{tabular}
\end{table*}

\subsection{Mechanism analysis}

This catalyst includes metal ions and bentonite carriers. The loose
pores and scaly structure of bentonite not only help to fix the
catalyst center, but also help to disperse the crude oil, increase
the surface area of the catalyst in contact with the resin and
asphaltene molecules, boost the catalyst's selectivity and the
efficiency of the aquathermolysis reaction. Metal ions promote the
formation of free radicals by breaking the C--S bond, as shown in
Figure~\ref{fig6}. However, the
viscosity of the oil sample decreases significantly after
aquathermolysis catalyzed by calcium bentonite-supported catalyst,
while it remains relatively unchanged after aquathermolysis catalyzed
by sodium bentonite-supported catalyst. This may be due to the
different pore sizes of the two kinds of bentonite, resulting in a
greater difference in the degree of contact between water molecules and
heavy oil. Water vapor in sodium bentonite cannot effectively spread to
heavy oil, and there is insufficient active hydrogen\break to inhibit free
radical polymerization. Therefore, the heavy oil coked severely after
the reaction, which also increases the viscosity. According to the TGA
curve analysis, some of the high boiling point and challenging to
decompose macromolecular substances in heavy oil will be split into low
boiling point small molecules during catalyzed aquathermolysis before
and after the addition of exogenous catalyst.
It is speculated from the structure of
the compounds assigned by mass spectrometry that the following
reactions may have occurred during the aquathermolysis catalysis process:

\begin{figure*}
{\vspace*{3pt}}
\includegraphics{fig06}
{\vspace*{3pt}}
\caption{\label{fig6}Catalytic
mechanism of in situ and exogenous catalyst.}
{\vspace*{3pt}}
\end{figure*}

\noindent (1) Isomerization reaction\vspace*{3pt}

\noindent \inlinefig{fx22}

\noindent (2) Hydrogenation reaction\vspace*{3pt}

\noindent \inlinefig{fx23}

\noindent (3) Ring opening reaction\vspace*{3pt}

\noindent \inlinefig{fx24}

\noindent (4) Fragmentation reaction\vspace*{3pt}

\noindent \inlinefig{fx25}

\noindent (5) Oxidation reaction\vspace*{3pt}

\noindent \,\inlinefig{fx26}

\noindent (6) Alcoholization reaction\vspace*{3pt}

\noindent \,\inlinefig{fx27}

\noindent (7) Hydrolysis reaction of ester\vspace*{3pt}

\noindent \inlinefig{fx28}


\section{Conclusions}

In this work, a series of catalysts used in the aquathermolysis of heavy
oil was prepared. The viscosity of the oil sample decreases after
aquathermolysis catalyzed by calcium bentonite-supported catalyst, but
not significantly after aquathermolysis catalyzed by sodium 
bentonite-supported catalyst. It has almost no catalytic activity on XBO but a
high activity on XTO, reducing viscosity by 68.2\%. This series of
catalysts\break catalyzes the aquathermolysis to effectively \pagebreak break the C--S
bond of resin and asphaltene molecules in the XTO and crack the
macromolecules into small molecules of alkanes, olefins, and aromatic
hydrocarbons, lowering the viscosity and pour point of heavy oil. This
work will benefit related work in this\break field.

\section*{Conflicts of interest}

The authors declare no conflicts of interest.

\section*{Acknowledgments}

We thank the work of Modern Analysis and Testing Center of
Xi'an Shiyou University.

\CDRGrant[NSFC]{50874092}
\CDRGrant[XSYU]{YCS21211045}

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