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\DOI{10.5802/crchim.269}
\datereceived{2023-05-05}
\daterevised{2023-09-02}
\datererevised{2023-10-24}
\dateaccepted{2023-10-24}
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\dateposted{2024-05-14}
\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Materials and Energy Valorization of Biomass and Waste : The Path for Sustainability and Circular Economy Promotion}
%\def\cdr@specialtitle@french{Valorisation \'energ\'etique et mati\`ere de la biomasse et des d\'echets : la voie de la durabilit\'e et de la promotion de l'\'economie circulaire}
%\makeatother

\CDRsetmeta{articletype}{research-article}

\title{Gel--sol synthesis of hierarchical CaO using pollen as
biotemplate for biodiesel production}

\alttitle{Synth\`{e}se gel--sol de CaO hi\'{e}rarchique utilisant le
pollen comme biomatrice pour la production de biodiesel}

\author{\firstname{Jingdi} \lastname{Zheng}}
\address{Shaanxi Province Key Laboratory of Environmental Pollution
Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou
University, Xi'an, China}

\author{\firstname{Shuang} \lastname{Tao}}
\addressSameAs{1}{Shaanxi Province Key Laboratory of Environmental Pollution
Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou
University, Xi'an, China}

\author{\firstname{Ying} \lastname{Yang}}
\addressSameAs{1}{Shaanxi Province Key Laboratory of Environmental Pollution
Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou
University, Xi'an, China}

\author{\firstname{Ying} \lastname{Tang}\IsCorresp}
\addressSameAs{1}{Shaanxi Province Key Laboratory of Environmental Pollution
Control and Reservoir Protection Technology of Oilfields, Xi'an Shiyou
University, Xi'an, China}
\address{Engineering Research Center of Oil and Gas Field Chemistry,
Universities of Shaanxi Provence, Xi'an Shiyou University, Xi'an, China}
\email[Y. Tang]{tangying78@xsyu.edu.cn}

\keywords{\kwd{Biodiesel}
\kwd{Rape pollen}
\kwd{Sol--gel method}
\kwd{Tri-component coupling transesterification}
\kwd{CaO}}

\altkeywords{\kwd{Biodiesel}
\kwd{Pollen de colza}
\kwd{M\'{e}thode sol--gel}
\kwd{Transest\'{e}rification par couplage \`{a} trois composants}
\kwd{CaO}}

\shortrunauthors

\thanks{National Natural Science Foundation of China (No. 51974252) and
Youth Innovation Team of Shaanxi University, Xi'an Shiyou University
Graduate Innovation Project (YCS22213106).} 

\begin{abstract}
In this paper, a novel hierarchical calcium-based catalyst marked as
CaO(S) has been established to catalyze oil-methyl acetate-methanol for
no-glycerol biodiesel production. The CaO(S) was synthesized through
sol--gel-biotemplate method using calcium nitrate-ethanol-citric acid
as precursor and rape pollen as biotemplate, followed by the
characterizations such as N\textsubscript{2}-Brunauer Emmet Teller (N\textsubscript{2}-BET),
X-ray diffraction (XRD), Fourier transform-infrared (FT-IR),
CO\textsubscript{2}-temperature programmed desorption (CO\textsubscript{2}-TPD),
Thermogravimetry differential thermal analysis (TG-DTA) and scanning
electron microscopy (SEM). The results indicate that pollen as a
biotemplate could be used to improve the basicity and textural
parameters of calcium oxide particles, especially for 1/1-CaO(S)-700,
presented by its total basicity of 5.90 mmol/g, the specific surface
area of 16.47~m\textsuperscript{2}/g, the pore volume of 0.04~cm\textsuperscript{3}/g, the
average pore size of 10.28~nm, and the widespread distribution of pore
size (2--12.5~nm). Moreover, the yield of no-glycerol biodiesel,
96.62\%, has been investigated after 3 h under optimized conditions
(temperature = 65~\textdegree C, dosage of CaO(S) = 10 wt\%, 
oil/methyl acetate/methanol = 1/1/8), by the yield presence of
91.37\% after 3 cycles, which indicated that the 1/1-CaO(S)-700
prepared by sol--gel-biotemplate method had good stability.
\end{abstract}

\begin{altabstract}
Dans cet article, un nouveau catalyseur \`{a} structure
hi\'{e}rarchique \`{a} base de calcium, appel\'{e} CaO(S), a
\'{e}t\'{e} cr\'{e}\'{e} pour catalyser l'huile, l'ac\'{e}tate de
m\'{e}thyle et le m\'{e}thanol en vue de la production de biodiesel
sans glyc\'{e}rol. Le CaO(S) a \'{e}t\'{e} synth\'{e}tis\'{e} par la
m\'{e}thode sol--gel-biomatrice en utilisant le nitrate de
calcium-\'{e}thanol-acide citrique comme pr\'{e}curseur et le pollen de
colza comme biomatrice, suivi par les caract\'{e}risations telles que
N\textsubscript{2}-Brunauer Emmet Teller (N\textsubscript{2}-BET), diffraction des rayons
X (XRD), spectroscopie infrarouge \`{a} transform\'{e}e de Fourier
(FT-IR), d\'{e}sorption programm\'{e}e en temp\'{e}rature du CO\textsubscript{2}
(CO\textsubscript{2}{-TPD), analyse thermique diff\'{e}rentielle par
thermogravim\'{e}trie (TG-DTA) et microscopie \'{e}lectronique \`{a}
balayage (SEM). Les r\'{e}sultats indiquent que le pollen en tant que
biomatrice pourrait \^{e}tre utilis\'{e} pour am\'{e}liorer la
basicit\'{e} et les param\`{e}tres texturaux des particules d'oxyde de
calcium, en particulier pour le 1/1-CaO(S)-700, pr\'{e}sent\'{e} par sa
basicit\'{e} totale de 5,90 mmol/g, la surface sp\'{e}cifique de 16,47
m}\textsuperscript{2}/g, le volume des pores de 0,04 cm\textsuperscript{3}/g, la taille
moyenne des pores de 10,28 nm, et la distribution \'{e}tendue de la
taille des pores (2--12,5 nm). En outre, le rendement du biodiesel sans
glyc\'{e}rol, 96,62\%, a \'{e}t\'{e} \'{e}tudi\'{e} apr\`{e}s 3 heures
dans des conditions optimis\'{e}es (temp\'{e}rature = 65~\textdegree C,
dosage de CaO(S) = 10 wt\%, huile/ac\'{e}tate de
m\'{e}thyle/m\'{e}thanol = 1/1/8), par la pr\'{e}sence de rendement de
91,37\% apr\`{e}s 3 cycles, ce qui indiquait que le 1/1-CaO(S)-700
pr\'{e}par\'{e} par la m\'{e}thode sol--gel-biomatrice avait une bonne
stabilit\'{e}.
\end{altabstract}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}

Energy as the backbone of the country's future development, its
industrialization and per capita consumption has become an important
measure of human technology and living standards, especially in the
rapid development of industrialization today~\cite{1}. The
dieselization of vehicles is a major driving force for the reform of
the fuel market, which makes the promotion and development of the
diesel industry become the focus of public attention~\cite{2}. It can
be observed that petrochemical diesel is mainly a derivative from crude
oil after a series of processes such as distillation, catalytic
cracking, thermal cracking, hydrocracking, and petroleum coking, and
most of them are hydrocarbons~\cite{3}. However, petrochemical diesel
has two major constraints of high sulfur and high nitrogen contents,
which have seriously shaken the ecological civilization, typically of
the atmospheric environment~\cite{4}. As a model of biomass energy,
biodiesel has dual benefits, which can not only avoid price
fluctuations caused by traditional non-renewable fuels, but also
achieve structural energy conservation and emission reduction~\cite{5}.
At present, the most mature theory of biodiesel is based on the
development of oil (Triglycerides, TG) and low-chain alcohol (Methanol,
MeOH) into generate biodiesel (Fatty Acid Methyl Ester, FAME) with
carbon chain lengths ranging from 16 to 20~\cite{6}. The oil--alcohol
transesterification with the participation of base catalyst will
generate a large amount of glycerol, among which, there is 1/10 of
glycerol in every 10 t of methyl ester fuel, affecting its combustion
performance and low temperature flow, and even the operation of diesel
engines~\cite{7}. Methyl acetate (MeOAc), an organic acid that is
miscible with alcohol, is often used as an acetylating solvent to react
with glycerol to prepare glycerol acetate~\cite{8}. Therefore, the
introduction of methyl ester reagent in oil--alcohol not only achieves
the in situ conversion of glycerol, but also improves its performance. 

Calcium oxide material, as one of the conventional heterogeneous
catalysts, is an oxide composed of divalent oxygen anion and the most
active divalent calcium ion in alkaline earth metals, most of which are
prepared by calcinating calcium-containing minerals, mollusk shells,
eggshells, and bones, etc~\cite{9}. The melting temperature of CaO is
2572~\textdegree C, and its ionic spacing and crystal structure is similar to
NaCl, which belongs to a typical ionic crystal with cubic structure.
Among them, calcium ions and two adjacent oxygen ions are cross-linked
through electrostatic attraction and three-dimensionally extended into
a cubic structure~\cite{10}. Wang et al. used modified CaO by
bromobenzene to catalyze the Aldol reaction 
(acetophenone--benzaldehyde) 
to generate chalcone with the yield of 98.9\%, and also
proved that the Aldol reaction was performed by Ca$^{2+}$
activated the carbonyl group of acetophenone and O$^{2-}$
activated the aldehyde group of benzaldehyde~\cite{11}. Subsequently,
the CaO as a catalyst was added in a mixed solution of
glycerol-dimethyl carbonate (1/2) so as to investigate the mechanistic
pathways to the {formation} of glycerol carbonate, as studied by
Simanjuntak et~al.~\cite{12}. Besides, CaO is also widely involved in
transesterification reactions. Leandro et~al.\ \mbox{investigated} the
\mbox{catalytic} ability of natural waste biomass calcium oxide (crab shell
and egg shell) after calcinating. It is found that the resulting
nano-CaO materials play an important role in the transformation of FAME
from TG and MeOH, which presents the yields of FAME are
$83.10\pm0.27$ wt\% and $97.75\pm0.02$ wt\%,
\mbox{respectively}~\cite{13}. Thus, CaO is a valuable and promising solid
base catalyst, which is widely used in various organic reactions.
However, commercial calcium oxide is almost obtained by calcining
limestone, resulting in poor dispersibility and poor internal pore
structure~\cite{14}. Therefore, an increasing number of researchers
have gradually shifted their study focus to the synthesis and
application of highly dispersed metal oxide with hierarchical
structure, in order to solve the defects of traditional calcium\break 
\mbox{oxide}.

The addition of template can regulate the morphology, structure and
even dispersion, so as to further achieve the purpose of improving its
catalytic properties. It is necessary initially to classify the
templates into certain types, such as physical, chemical, and
biological template~\cite{15}. Li et~al.~synthesized three kinds of
CaO-based sorbents using Ca(CH$_{3}$COO)$_{2}$, Ca(OH)$_{2}$ and
CaCO$_{3}$ as precursors by a wet mixing method with the help of
aluminate cement and organic fiber, and the cyclic performance of the
sorbent decomposed from Ca(CH$_{3}$COO)$_{2}$ is better. It also
verifies that it plays an important role in CO$_{2}$ capture~\cite{16}.
It is secondary that there are abundant biological templates, which can
quickly and controllably prepare many metal oxides with different
compositions and biological forms. The most typical ones are yeast,
cotton, wood, filter paper, jute fiber, pomelo, birch, rice, pollen,
etc.~\cite{17,18,19,20,21,22,23,24,25}. Rape pollen, with a special
network structure and three-dimensional pores, is of great inspiration
for the preparation of highly dispersed solid bases, and there are a
certain number of hydroxyl and carboxyl groups on its surface, which
helps the metal ions diffuse to the surface uniformly, thereby forming
nucleation sites for further meristems and\break assembly~\cite{26}.

This study aims to investigate the gel--sol synthesis of hierarchical
CaO using pollen as a template and calcium nitrate-ethanol-citric acid
as precursor. The application of the derived CaO(S) in catalytic
oil-methyl acetate-methanol production of no-glycerol biodiesel was
studied. The CaO(S)-based catalysts were characterized and analyzed to
elucidate the effects of their preparation conditions on pore
structure, basicity, and dispersion and their relationship to catalytic
performance.

\section{Experimental details}

\subsection{Reagents}

The required analytical reagents, namely methyl acetate
(C$_{3}$H$_{6}$O$_{2}$, Kelon Co.), anhydrous methanol (CH$_{3}$OH,
Fuyu Co.), sodium hydroxide (NaOH, Tianli Co.), cyclohexane
(C$_{6}$H$_{12}$, Tianli Co.), methyl heptadecanoate
(C$_{18}$H$_{38}$O$_{2}$, TCI Co.), anhydrous ethanol
(C$_{2}$H$_{5}$OH, Fuyu Co.), calcium nitrate tetrahydrate
(Ca(NO$_{3}$)$_{2}\cdot$4H$_{2}$O, Damao Co.), citric acid
(C$_{6}$H$_{8}$O$_{7}\cdot$H$_{2}$O, Kelon Co.), were purchased from
China. The rapeseed oil was obtained from Shanxi Jianxing Co., China.
The rape pollen broken at ultra-low temperature was obtained from Henan
Changge Yanyuan Bee Products Co., China.

\subsection{Characterization techniques}

The textural properties of CaO(S) were determined by Micromeritics ASAP
2020 HD88 (Norcross, Georgia, USA)~\cite{27}. The crystalline
structures of CaO(S) were identified by D8 ADVAHCL (Germany)
(Cu-K${\upalpha}$, $\uplambda=1.5406$~\AA{}) with a scan voltage of 40
kV, a current of 30 mA and a 2${\theta}$ range of
10\textdegree--80\textdegree~\cite{28}. The FT-IR spectras of CaO(S)
were recorded by Nicolet 5700 (Madison, Wisconsin, USA) in the
wavenumber ranging from 4000 to 400~cm$^{-1}$~\cite{29}. The
thermogravimetric experiments of CaO(S) were performed by TGA-SDTA851
analyzer (Shanghai, China) in a temperature range of 25--800~\textdegree
C at a rate of 10~\textdegree C/min with a nitrogen flow rate of 50
mL/min~\cite{30}. The CO$_{2}$-TPD curves and its total basicity of
CaO(S) were obtained by ChemiSorb 2750 (Norcross, Georgia, USA) and the
morphology of CaO(S) was determined by JSM-6390A (Japan)~\cite{31,32}.

\subsection{Gel--sol synthesis of hierarchical CaO}

An ethanol solution of a predetermined concentration of calcium nitrate
tetrahydrate (Ca(NO$_{3}$)$_{2}{\cdot}$4H$_{2}$O) with 1~mol/L, denoted by
CN, as a Ca$^{2+}$ precursor and porous template (rape pollen after
ultrasonic cleaning with ethanol) were magnetically stirred in a water
bath at 30~\textdegree C, followed by the addition of citric acid (CA)
and stirring at 70~\textdegree C for 3 h to form gels based on the
pollen skeleton. The solutions with three different molar ratios of CN
to CA from 1/0.5 to 1/1.5, denoted at X-CaO(S) where X is the CN/CA
ratio, S is the sol--gel method containing different citric acid
concentrations of 0.5~mol/L, 1~mol/L and 1.5~mol/L were prepared and then
centrifuged and dried, finally calcined to obtain powder. The
calcination conditions of the muffle furnace (KFS-7-12, Shanghai) were
a calcination temperature in range of 600--800~\textdegree C, a
calcination time of 3 h under an air atmosphere. Target sample CaO was
obtained, denoted X-CaO(S)-Y, Y represents the calcination temperature.
Schematic diagram of the specific preparation process of CaO(S) by
sol--gel method is shown in Figure~\ref{fig1}. The synthetic reactions
are shown in Equations~(\ref{eq1})--(\ref{eq5}). Commercial CaO (Kelon
Co.), used as a benchmark, was calcined at 700~\textdegree C under an
air atmosphere for 3 h (denoted as Commercial CaO-700).
{\begin{eqnarray}
&amp;&amp;\label{eq1}\hspace*{-6pt}
\mathrm{Ca}^{2+}+2\mathrm{C}_{6}\mathrm{H}_{7}{\mathrm{O}_{7}}^{-}\rightarrow 
\mathrm{Ca}(\mathrm{C}_{6}\mathrm{H}_{7}\mathrm{O}_{7})_{2}\Seqnsplit
&amp;&amp;\hspace*{-6pt}
\mathrm{Ca}(\mathrm{C}_{6}\mathrm{H}_{7}\mathrm{O}_{7})_{2}+\mathrm{OH}^{-}\rightarrow \mathrm{Ca}(\mathrm{OH})(\mathrm{C}_{6}\mathrm{H}_{7}\mathrm{O}_{7})
+\mathrm{C}_{6}\mathrm{H}_{7}{\mathrm{O}_{7}}^{-}\nonumber\\
\label{eq2}\Seqnsplit
&amp;&amp;\label{eq3}\hspace*{-6pt}
\mathrm{Ca}(\mathrm{OH})(\mathrm{C}_{6}\mathrm{H}_{7}\mathrm{O}_{7})+\mathrm{OH}^{-}\rightarrow 
\mathrm{Ca}(\mathrm{OH})_{2}+\mathrm{C}_{6}\mathrm{H}_{7}{\mathrm{O}_{7}}^{-}\Seqnsplit
&amp;&amp;\label{eq4}\hspace*{-6pt}
\mathrm{Ca}(\mathrm{OH})_{2}+\mathrm{Ca}(\mathrm{OH})_{2}\rightarrow \mathrm{Ca}-\mathrm{O}-\mathrm{Ca}-\mathrm{O}+2\mathrm{H}_{2}\mathrm{O}\Seqnsplit
&amp;&amp;\label{eq5}\hspace*{-6pt}
\mathrm{Ca}(\mathrm{OH})_{2}\rightarrow \mathrm{CaO}+\mathrm{H}_{2}\mathrm{O}\uparrow  
\end{eqnarray}}\unskip


\begin{figure*}
{\vspace*{5pt}}
\includegraphics{fig01}
{\vspace*{5pt}}
\caption{\label{fig1}Schematic diagram of CaO(S) by sol--gel method.}
{\vspace*{5pt}}
\end{figure*}


\subsection{Preparation of no-glycerol biodiesel}

The refining process of rapeseed oil was carried out in a beaker with a
mass ratio of oil and lye (1~mol/L NaOH) of 4:1 or 5:1, in order to
avoid the catalytic performance of free fatty acids in oil to solid
base. Therefore, the raw oil should be reacted with the lye under
magnetic stirring for 5 h, and then washed with boiled water and
separated in a separatory funnel until the pH of the lower water was
about 7.0, followed by the addition of quantitative cyclohexane. It was
dehydrated in a device equipped with a water separator and a reflux
condenser, and finally the excess cyclohexane was removed by using a
rotary evaporator so as to purify oil.

To investigate the catalytic performance of CaO (S), the no-glycerol
biodiesel generated from the tri-component coupling transesterification
reaction was used as the target product, typically of synthesis in a
250 mL three-neck round bottom flask equipped with a reflux condenser.
A certain amount of CaO (S) was added to a mixed solution consisting of
oil-methanol-methyl acetate, and the water bath temperature was
controlled at 65~\textdegree C for 3.5~h. Subsequently, the biodiesel
was collected every 20 or 30 min by centrifugation (4000 rpm, 30 min)
and vacuum rotation evaporation (85~\textdegree C, 5 min) to remove
solid CaO(S), unreacted methanol (boiling point: 64.5~\textdegree C)
and methyl acetate (boiling point:\break 57.8~\textdegree C).


A quantitative analysis of biodiesel (FAME) was performed by gas
chromatography (Yiyou-GC7860, N$_{2}$ as carrier gas, FID detector,
KB-Wax capillary column) based on internal standard method (methyl
heptadecanoate as internal standard, cyclohexane as solvent). The oven,
injector, and detector temperatures were set to 100~\textdegree C,
250~\textdegree C, and 280~\textdegree C, respectively. The temperature
program was divided into three stages, heating from 100~\textdegree C
to 200~\textdegree C at 20~\textdegree C/min and holding for 3 min,
then heating to 220~\textdegree C at 10~\textdegree C/min and keeping
for 3 min, finally heating to 240~\textdegree C at 10~\textdegree C/min
and staying for 3 min, among of which the entire analysis process
lasted 20~min. The calculation formula of FAME yield is shown in
Equation~(\ref{eq6}). $\sum A_{i}$, $A_{\mathrm{MH}}$ and
$C_{\mathrm{MH}}$ are the peak areas of all FAMEs, the peak areas of
methyl heptadecanoate and the concentration of methyl heptadecanoate
(1~mg/mL). $V_{\mathrm{MH}}$ is the volume of internal standard
solution (1 mL) and $W$ is the net mass (mg) of the product.
{\begin{equation}
\label{eq6}
\mathrm{yield}~(\% )=\left[\frac{\left(\sum
A_{i}-A_{\mathrm{MH}}\right)}{A_{\mathrm{MH}}}\right]
\frac{C_{\mathrm{MH}}V_{\mathrm{MH}}\times 100}{W}.
\end{equation}}\unskip


\section{Results and discussion}

\subsection{Characterization of the CaO(S) catalyst}

\subsubsection{N$_{2}$-BET analysis}

Table~\ref{tab1} shows that when the calcination temperature is changed
from 600~\textdegree C to 700~\textdegree C, the specific surface area
of CaO(S) increases from 9.77 m$^{2}$/g to 16.47 m$^{2}$/g, and when
the calcination temperature is further increased to 800~\textdegree C,
the CaO(S) will be greatly reduced to 8.84 m$^{2}$/g, indicating that
within the calcination temperature of CaO(S), an appropriate increase
is beneficial to improve its pore structure. As shown in
Figure~\ref{fig2}(a), all CaO(S) have mesoporous structures (IV-type
isotherm, H$_{3}$-type hysteresis loop)~\cite{33}, and when the
calcination temperature is 700~\textdegree C, the amount of nitrogen
adsorption generated by 1/1-CaO(S)-700 is the largest at
$P/P_{0}=0\ndash 1$, which includes the monolayer adsorption of nitrogen
on the surface of CaO(S) ($P/P_{0}&lt;0.4$) and the multimolecular
adsorption of nitrogen ($P/P_{0}&gt;0.4$)~\cite{34}. Besides, it is
available from Figure~\ref{fig2}(b), 1/1-CaO (S)-700 produces more
mesopores with an average pore diameter concentrated at 3.5 nm and a
relatively wide change (2--12.5~nm), indicating a hierarchical pore
structure.


\begin{figure*}
{\vspace*{3pt}}
\includegraphics{fig02}
{\vspace*{3pt}}
\caption{\label{fig2}(a)~N$_{2}$-adsorption--desorption isotherms and
(b)~pore size distribution curve of CaO(S) under different calcination
condition.}
{\vspace*{8pt}}
\end{figure*}


\begin{table*}
\caption{\label{tab1} Pore structure properties of CaO(S) under
different calcination conditions and commercial CaO}
\begin{tabular}{cccc}
\thead
Type of catalyst &amp; BET surface area  (m$^{2}$/g) &amp; Pore volume 
(cm$^{3}$/g) &amp; Average pore diameter (nm) \\
\endthead
1/1-CaO(S)-600 &amp; \09.77 &amp; 0.05 &amp; 20.89 \\ 
1/1-CaO(S)-700 &amp; 16.47 &amp; 0.04 &amp; 10.28 \\ 
1/1-CaO(S)-800 &amp; \08.84 &amp; 0.03 &amp; 11.88 \\ 
Commercial CaO-700 &amp; 11.45  &amp; 0.03 &amp; 11.31
\botline
\end{tabular}
\end{table*}


Table~\ref{tab2} shows the pore parameters of CaO(S) under different
molar ratios of CN-CA, and Figure~\ref{fig3}(a,b) is its
N$_{2}$-adsorption--desorption isotherm and pore size distribution. It
can be seen from Table~\ref{tab2} that when the molar ratio of 
CN-CA is changed
from 1/0.5 to 1/1, the specific surface area of CaO(S) increases from
9.90 m$^{2}$/g to 16.47 m$^{2}$/g, and the pore volume is given by 0.02
cm$^{3}$/g increased to 0.04 cm$^{3}$/g, and the average pore diameter
increased from 9.16 nm to 10.28 nm, which is an attribute of citric
acid by providing a large amount of H$^{+}$ in the ethanol
solution, promoting the hydrolysis of calcium salts and the
polycondensation so as to a preferred performance of the CaO(S)
gel~\cite{35}. When the CN-CA ratio is further increased to 1/1.5, the
specific surface area of CaO(S) decreases from 16.47 m$^{2}$/g to 11.29
m$^{2}$/g, and the average pore diameter increases from 10.28 nm to
14.70 nm, indicating that the excessive citric acid can reduce the
specific surface area and the number of pores, which is not conducive
to efficient diffusion of reactants of different sizes in heterogeneous
reaction systems. 


\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig03}
{\vspace*{2pt}}
\caption{\label{fig3}(a) N$_{2}$-adsorption--desorption isotherms and
(b) pore size distribution curve of CaO(S) under different mole ratio
of nitrate and citric acid.}
{\vspace*{2pt}}
\end{figure*}

\subsubsection{XRD analysis}

In order to study the effects of calcination conditions and the molar
ratio of CN-CA on the crystalline structures of CaO(S), X-ray scanning
was performed to obtain its XRD patterns, as shown in
Figure~\ref{fig4}(a), which can enable a longitudinal comparison of the
changes of the crystalline structures of the CaO(S) calcined at
600~\textdegree C, 700~\textdegree C and 800~\textdegree C,
respectively. It can be seen that the calcination conditions are
related to the phase composition, particle size and even the dispersion
of CaO(S) particles. Among them, the diffraction peaks of
1/1-CaO(S)-600 at $2{\theta}=22.96\text{\textdegree}$, 29.29\textdegree,
35.86\textdegree, 39.43\textdegree, 43.18\textdegree, 47.38\textdegree, and
48.48\textdegree\ match well with the standard card JCPDS 33-0268
classified as the typical calcite CaCO$_{3}$, which can be inferred
that when the calcination temperature is 600~\textdegree C, the CaO
crystals are not completely formed and mainly exist in the form of
carbonate~\cite{36}. However, when the calcination temperature is
further increased to 700~\textdegree C or 800~\textdegree C, the
carbonate gradually decomposes to form calcium oxide, resulting in a
more perfect crystal structure and an increase in particle size
(Scherrer formula: $D = K{\uplambda}/B\cos{\theta}$), which can be
found in the crystal plane comparison of CaO (JCPDS 48-1467, $2{\theta}
= 32.21\text{\textdegree}$, 37.36\textdegree, 53.86\textdegree,
64.19\textdegree and 67.38\textdegree)~\cite{37}. Notably, the
1/1-CaO(S)-800 has sharp low-angle peak at $2{\theta}=32.21\text{\textdegree}$,
37.36\textdegree, which is consistent with the relatively poor particle
dispersion caused by sintering of the surface of the CaO(S)
\mbox{particles}.


\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig04}
{\vspace*{2pt}}
\caption{\label{fig4}XRD patterns of the synthesized CaO(S) and
Commercial CaO-700 (a) calcination conditions; (b) the mole ratio of
nitrate and citric acid.}
{\vspace*{2pt}}
\end{figure*}

\begin{table*}
\caption{\label{tab2} Pore structure properties of CaO(S) under
different mole ratio of CN-CA and commercial CaO}
\begin{tabular}{cccc}
\thead
Type of catalyst &amp; BET surface area  (m$^{2}$/g) &amp; Pore volume 
(cm$^{3}$/g) &amp; Average pore diameter (nm) \\
\endthead
1/0.5-CaO(S)-700 &amp; \09.90 &amp; 0.02 &amp; \09.16 \\ 
1/1-CaO(S)-700 &amp; 16.47 &amp; 0.04 &amp; 10.28 \\ 
1/1.5-CaO(S)-700 &amp; 11.29 &amp; 0.04 &amp; 14.70 \\ 
Commercial CaO-700 &amp; 11.45 &amp; 0.03 &amp; 11.31
\botline
\end{tabular}
\end{table*}

Figure~\ref{fig4}(b) shows the XRD pattern of 1/0.5-CaO(S)-700,
1/1-CaO(S)-700, 1/1.5-CaO(S)-700 obtained from CN-CA solution at
different molar ratios. It can be seen from Figure~\ref{fig4}(b) that
1/0.5-CaO(S)-700 has a stronger CaO diffraction peak than Commercial
CaO-700, and there are also miscellaneous peaks similar to Ca(OH)$_{2}$
and CaCO$_{3}$. When the amount of CA increases, the crystallinity and
\mbox{structural} \mbox{regularity} of CaO in CaO(S) improve, which is attributed to
the fact that CA not only adjusts the pH of the ethanol solution, but
also reacts with calcium salts to form calcium citrate, resulting in
avoiding the precipitation of calcium ions and promoting the hydrolysis
of calcium salts and the polycondensation reaction. When the molar
ratio of CN-CA is further increased to 1/1.5, the CaO peaks of
1/1.5-CaO(S)-700 at $2{\theta}=32.21\text{\textdegree}$, 37.36\textdegree,
53.86\textdegree, 64.19\textdegree, and 67.38\textdegree\ are stronger,
suggesting a large particle size and poor dispersion~\cite{37}.

\subsubsection{TG-DTA analysis}

TG-DTA analysis was performed on CaO(S) prepared at different
calcination temperatures and impregnated with a 1/1 molar ratio of
CN-CA, and the results are shown in Figure~\ref{fig5}(a,b). According to
Figure~\ref{fig5}(a), the weight loss rates of Ca(OH)$_{2}$
(370--450~\textdegree C) and CaCO$_{3}$
(550--785~\textdegree C) in the presence of 1/1-CaO(S)-600 are 1.71\%
and 23.31\%~\cite{38,39}. When the calcination temperature is
700~\textdegree C, 1/1-CaO(S)-700 has the weight loss rates of
Ca(OH)$_{2}$ and CaCO$_{3}$ with 7.09\% and 3.48\%. Further increasing
the calcination temperature to 800~\textdegree C, the weight loss of
Ca(OH)$_{2}$ and CaCO$_{3}$ of CaO(S) are presented by 5.69\% and
3.42\%, implying that the erosion of CaO(S) is gradually weakened by
CO$_{2}$ in the air, and the erosion of H$_{2}$O is gradually enhanced.


\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig05}
{\vspace*{2pt}}
\caption{\label{fig5}TG-DTA curves of the synthesized CaO(S) (a,b)
calcination conditions; (c,d) the mole ratio of nitrate and citric
acid.}
{\vspace*{2pt}}
\end{figure*}




Figure~\ref{fig5}(c--d) is the TG-DTA curves of CaO(S) prepared from
CN-CA solutions with different molar ratios and calcinated at
700~\textdegree C. It is available from Figure~\ref{fig5}(c), when the
molar ratio of CN-CA is 1/0.5, the weight loss rates of Ca(OH)$_{2}$
(360--450~\textdegree C) and CaCO$_{3}$
(490--640~\textdegree C) corresponding to
1/0.5-CaO(S)-600 are 7.19\% and 3.59\%~\cite{38,39}. Further increasing
the molar ratio of CN-CA to 1/1, the weight loss rates of Ca(OH)$_{2}$
and CaCO$_{3}$ derived from 1/1-CaO(S)-700 are 7.09\% and 3.48\%. When
the molar ratio of CN-CA is 1/1.5, the weight loss rates of
Ca(OH)$_{2}$ and CaCO$_{3}$ produced by 1/1.5-CaO(S)-700 are 4.10\% and
3.07\%, which shows that the erosion of CO$_{2}$ and H$_{2}$O on the
CaO(S) surface is weakened as the CN-CA ratio changes from 1/0.5 to
1/1.5. It can be seen from Figure~\ref{fig5}(d) that with the increase
of the amount of CA, the endothermic peak temperature of CaCO$_{3}$ of
CaO(S) is inclined to the direction of 800~\textdegree C, indicating
that the thermal stability of the CaO(S) gel is improved. The
decomposition temperatures of Ca(OH)$_{2}$ and CaCO$_{3}$ over the
prepared CaO(S) are 400~\textdegree C and 613~\textdegree C,
respectively, as shown in Figure~\ref{fig5}(b), which are relatively
lower than the results studies reported by Dj. Vujicic et~al.~\cite{40}
reflecting Ca(OH)$_{2}$ and CaCO$_{3}$ decomposition temperatures of
480~\textdegree C and 740~\textdegree C, respectively. Similar results
has been found in previous research which indicated that low
temperature decomposition usually related to higher
dispersion~\cite{41}.



\subsubsection{FT-IR analysis}

FT-IR analysis of CaO(S) from different calcination conditions was
performed in the range of 4000--500~cm$^{-1}$. As shown in
Figure~\ref{fig6}(a), there are two types of functional groups are
present, corresponding to the stretching vibration peaks of \mbox{--OH}
(3648~cm$^{-1}$, 3448~cm$^{-1}$ and 1647~cm$^{-1}$) and CO$_{3}^{2-}$
(1459~cm$^{-1}$, 871~cm$^{-1}$ and 713~cm$^{-1}$),
respectively~\cite{42,43}. Compared with 1/1-CaO(S)-700 and
1/1-CaO(S)-800, the CO$_{3}^{2-}$-type peak of 1/1-CaO(S)-600 is
relatively sharp, indicating that the main crystal composition is
carbonate in the presence of 1/1-CaO(S)-600. When the calcination
temperature increases from 600~\textdegree C to 800~\textdegree C, the
stretching vibration peak of CO$_{3}^{2-}$ gradually becomes weaker,
and the peak of --OH gradually becomes sharper, indicating that the
strong hydroxylation and weak carbonation of CaO(S) as the calcination
temperature ranging from 600~\textdegree C to 800~\textdegree C, as
described by TG-DTA analysis.


\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig06}
{\vspace*{2pt}}
\caption{\label{fig6}FT-IR spectras of the synthesized CaO(S) and
Commercial CaO-700 (a) calcination conditions; (b) the mole ratio of
nitrate and citric acid.}
{\vspace*{2pt}}
\end{figure*}


Figure~\ref{fig6}(b) shows the FT-IR spectras of CaO(S) prepared from
different CN-CA molar ratios under the same calcination conditions
(700~\textdegree C, 3 h). It can be observed from Figure~\ref{fig6} that
as the CN-CA molar ratio changes from 1/0.5 to 1/1.5, the intensities
of the stretching vibration peaks of --OH (3648~cm$^{-1}$,
3448~cm$^{-1}$, and 1647~cm$^{-1}$) and CO$_{3}^{2-}$
(1459~cm$^{-1}$, 871~cm$^{-1}$ and 713~cm$^{-1}$) from CaO(S) are
reduced respectively~\cite{42,43}, which was attributed to the
formation of calcium citrate complexes between CA and Ca$^{2+}$,
avoiding the precipitation of Ca$^{2+}$ and promoting the hydrolysis
and polycondensation of calcium salts, indicating that with the
increase of the amount of CA, the surface of CaO(S) is less eroded by
CO$_{2}$ and H$_{2}$O in the air.

\subsubsection{CO$_{2}$-TPD analysis}

The CO$_{2}$-TPD curves of CaO(S) were measured by
temperature-programmed desorption by using CO$_{2}$ as the acid probe
molecule. Its results are shown in Figure~\ref{fig7}(a,b) and the total
basicity is presented in \mbox{Table}~\ref{tab3}. It can be seen from
Figure~\ref{fig7}(a) that all the peaks of the CO$_{2}$-TPD curves of
CaO(S) appear in the high temperature region (600--800~\textdegree C),
which is a strong chemical adsorption site, indicating that CaO(S)
belongs to a strong solid base. When the calcination temperature is
600~\textdegree C, the crystal phase of 1/1-CaO(S)-600 is carbonate
with \mbox{rhombohedral} structure. When the calcination temperature is
changed from 600~\textdegree C to 700~\textdegree C, the pyrolysis of
CaO(S) gel is more sufficient, and the active CaO grains gradually
nucleate and grow so as to the total basicity is 5.90~mmol/g. However,
the reduced total basicity (3.89~mmol/g) exhibited by 1/1-CaO(S)-800 is
due to the sintering of its surface, which reduces the specific surface
area. 


\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}CO$_{2}$-TPD curves of the synthesized CaO(S) and
Commercial CaO-700 (a) calcination conditions; (b) the mole ratio of
nitrate and citric acid.}
\end{figure*}


As presented in Figure~\ref{fig7}(b), when the CN-CA molar ratio
increases from 1/0.5 to 1/1, the CO$_{2}$ desorption peak area of
CaO(S) becomes larger, and the total basicity increases in range of
2.10--5.90~mmol/g. The 1/1.5-CaO(S)-700 produced by the CN-CA molar
ratio of 1/1.5 has a decreased total basicity (3.21~mmol/g), which is
due to the increase of CA improves the stability of CaO(S) gel and
deteriorates the dispersion of CaO particles.


\begin{table*}
\caption{\label{tab3} Total basicity of CaO(S) and Commercial
CaO-700{\vspace*{2pt}}}
\begin{tabular}{ccc}
\thead
Type of catalyst &amp; Total basicity (mmol/g) &amp; Desorption peaks (area\%) \\
\endthead 
1/1-CaO(S)-600 &amp; - &amp; - \\ 
1/1-CaO(S)-700 &amp; 5.90 &amp; 1.37 \\ 
1/1-CaO(S)-800 &amp; 3.89 &amp; 0.90 \\ 
1/0.5-CaO(S)-700 &amp; 2.10 &amp; 0.49 \\ 
1/1.5-CaO(S)-700 &amp; 3.21 &amp; 0.75 \\ 
Commercial CaO-700 &amp; 0.79 &amp; 0.18 
\botline
\end{tabular}
{\vspace*{2pt}}
\end{table*}



\subsubsection{SEM analysis}

Figure~\ref{fig8} shows the SEM images of the pollen before and after
ethanol ultrasonic treatment (a,b), CaO(S) (c--e) prepared at different
calcination temperatures and Commercial CaO-700 (f). It can be seen from
Figure~\ref{fig8} that when the calcination temperature is
600~\textdegree C, the CaO(S) prepared by the sol--gel method can still
see the embryonic form of the pollen, and its particle size
distribution is in range of 0.47--0.85
${\upmu}$m~\cite{44}. When the calcination temperature is
700~\textdegree C, the particles of CaO(S) are more uniformly
dispersed, and the particle size distribution is presented by 0.11--0.26
${\upmu}$m. However, a calcination temperature of 800~\textdegree C can
trigger the sintering and agglomeration of the CaO(S) surface, and the
particle size distribution is 0.37--0.68 ${\upmu}$m.


\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig08}
\caption{\label{fig8}SEM of CaO(S) under different calcination
conditions (a) original pollen (b) pretreated pollen (c) 1/1-CaO(S)-600
(d) 1/1-CaO(S)-700 (e) 1/1-CaO(S)-800 (f) Commercial CaO-7.}
\end{figure*}


\subsection{Catalytic performance of the CaO(S) catalysts}

The catalytic activity of CaO(S) derived from different calcination
temperatures was investigated at 65~\textdegree C, n(oil/methyl
acetate/methanol) ${=}$ 1/1/8, and CaO(S) dosage ${=}$ 10 wt\%. It can be seen
from Figure~\ref{fig9}(a) that when the calcination temperature is
600~\textdegree C, 700~\textdegree C and 800~\textdegree C,
respectively, the yields of CaO(S) catalyzed transesterification to
FAME can reach 3.45\%, 96.62\% and 64.99\%, the reaction times are
3.5~h, 3~h, and 3~h, indicating that an increase in the calcination
temperature from 600~\textdegree C to 700~\textdegree C is advantageous
to increase the reaction rate of transesterification and shorten the
reaction time, which is related to its increased specific surface
area~\cite{45}.


\begin{figure*}
{\vspace*{3pt}}
\includegraphics{fig09}
{\vspace*{3pt}}
\caption{\label{fig9}Catalytic activity of CaO(S) under different (a)
calcination condition; (b) mole ratio of calcium nitrate and citric
acid.}
{\vspace*{3pt}}
\end{figure*}


Based on the results of Figure~\ref{fig9}(a), the effects of different
CN-CA molar ratios on the catalytic activity of CaO(S) at
700~\textdegree C calcinating is described at 65~\textdegree C,
n(oil/methyl acetate/methanol)  ${=}$ 1/1/8, CaO(S) dosage ${=}$ 10 wt\%,
as shown in Figure~\ref{fig9}(b). It can be seen from
Figure~\ref{fig9}(b) that when the CN-CA molar ratio is 1/0.5, 1/1 and
1/1.5, the FAMEs produced by the corresponding 1/0.5-CaO(S)-700,
1/1-CaO(S)-700 and 1/1.5-CaO(S)-700 can reach the highest in 3 h, which
are 89.61\%, 96.62\% and 92.38\%. There is an explanation confirmed
that appropriately changing the ratio of CN-CA can increase the
reaction rate of transesterification, which is attributed to the fact
that citric acid easily forms a complex with Ca$^{2+}$ in ethanol
solution, and the CaO(S) gel becomes stable~\cite{46}.



The relationship between different doses of 1/1-CaO(S)-700 and FAME
yield at 65~\textdegree C, n(oil/ester/alcohol) ${=}$ 1/1/8 is presented
in Figure~\ref{fig10}(a). From Figure~\ref{fig10}(b), a trend of
increasing FAME yield of 1/1-CaO(S)-700 with dosage \mbox{ranging} from 5~wt\%
to 10~wt\% was observed over time within 3~h of transesterification.
Specifically, when the dosage of 1/1-CaO(S)-700 is 5 wt\% and 10 wt\%,
the yield of FAME within 3 h is 73.07\% and 96.62\%. Extending the
reaction time to 3.5 h, the FAME yield of 1/1-CaO(S)-700 at 5 wt\%
still increase slowly, to 74.90\%, while the FAME yield of
1/1-CaO(S)-700 at 10 wt\% decrease slightly, to 92.63\%, which suggests
that increasing the amount of 1/1-CaO(S)-700 is conducive to providing
more basic sites, thus increasing the reaction rate and enhancing the
catalytic effect. However, the generation of unfavorable reactions
(saponification reaction) would reduce the transesterification effect
with the participation of 1/1-CaO(S)-700 (15 wt\%), which reduced the
FAME yield to 78.81\% at 3 h~\cite{47}.


\begin{figure*}
\includegraphics{fig10}
\caption{\label{fig10}Effect of reaction parameters of CaO(S) on FAME
yield (a) the catalyst amount (b) the reaction temperature (c) the
oil/ester/alcohol ratio.}
\end{figure*}


The effect of the reaction temperature of 1/1-CaO(S)-700 on the FAME
yield at CaO(S) dosage ${=}$ 10 wt\%, n(oil/ester/alcohol) ${=}$ 1/1/8 was
investigated, as shown in Figure~\ref{fig10}(b). It is available from
Figure~\ref{fig10}(b) that in the presence of 1/1-CaO(S)-700, the FAME
yields within 3 h at 60~\textdegree C, 65~\textdegree C and
70~\textdegree C are 49.52\%, 96.62\% and 81.19\%. Subsequently, when
the reaction time is extended to 3.5 h, the FAME yield increased to
61.32\% at 60~\textdegree C, while the FAME yields decrease slightly at
65~\textdegree C and 70~\textdegree C, as presented by 92.62\% and
79.11\%, which indicates that an increase of the reaction temperature
by 1/1-CaO(S)-700 is favourable to encourage the transesterification
reaction (endothermic reaction). When the reaction temperature of
1/1-CaO(S)-700 continues to 70~\textdegree C, the volatilization of
low-boiling methanol is induced~\cite{48}.

The carbon chain length, steric effect and dosage of lower alcohols
have important influence on the transesterification reaction. Methanol,
as one of the alcohols with the shortest carbon chain and the smallest
steric resistance, has the strongest nucleophilicity. Therefore, the
effect of methanol dosage on FAME yield was examined at 65~\textdegree
C, $1/1-C$aO(S)-700 dosage ${=}$ 10 wt\%, and the results are shown in
Figure~\ref{fig10}(c). It can be seen from Figure~\ref{fig10}(c) that
when n(oil/ester/alcohol) is $1/1/6$, $1/1/8$, and $1/1/10$, the FAME
yields within 3 h reach 71.70\%, 96.62\%, 6.64\%, respectively, which
is due to the dilution of the system caused by excess methanol
molecules~\cite{49}. Methyl acetate added in the reaction can directly
react with the resulting glycerol, achieving online conversion of
glycerol at the stoichiometric ratio of 1/1. The amount of methanol was
investigated in detail due to it is a great important factor to improve
the reaction to shift to the right, thereby improving the conversion
rate\break of oil~\cite{50}.

Moreover, the 1/1-CaO(S)-700 from the reacted mixture can be
regenerated by centrifugation and washing with ethanol and calcination
of 700~\textdegree C in order to remove the oil and glial molecules
inside the catalyst. After regeneration, the cyclic catalytic ability
of 1/1-CaO(S)-700 and commercial CaO-700 was explored under optimal
conditions, as shown in Figure~\ref{fig11}(a). It can been seen that the
yields of FAME catalyzed by 1/1-CaO(S)-700 are 96.62\%, 95.40\% and
91.37\%, all of which remains above 90\% in 3 cycles, and the excellent
recyclability of 1/1-CaO(S)-700 indicates its good catalytic stability
under transesterification reaction. In commercial CaO, the FAME yield
maintains on a relatively low level after reused in three cycles, no
more than 56\%. Furthermore, it can been found that 1/1-CaO(S)-700
prepared by sol--gel method showed stable catalytic activity than
commercial CaO, as implied by Figure~\ref{fig11}(b). Some properties of
prepared biodiesel produced by CaO(S) including relative density,
viscosity, flash point, ester content and free glycerol are listed in
Table~\ref{tab4}. It can be seen that the various performance
\mbox{indicators} of the biodiesel meet the European biodiesel
standards~\cite{51}. Furthermore, it is evident that the main resource
of obtained CaO(S) is derived from commercial CaO which has brought
important benefits for industrial applications due to its excellent
basicity~\cite{52} and low cost at approximately \$2.9/kg compared with
the value of KOH, at approximately \$3.3/kg.


\begin{figure*}
\includegraphics{fig11}
\caption{\label{fig11}(a) Regeneration analysise of 1/1-CaO(S)-700 and
Commercial CaO-700 in transesterification; (b) Catalytic activity of
commercial CaO-700 and CaO(S).}
\end{figure*}




\begin{table}
\caption{\label{tab4} Fuel properties of biodiesel}
\begin{tabular}{ccc}
\thead
Properties  &amp; Test method &amp; EN14214 \\
\endthead
Relative density, 298 K  &amp; 0.89 &amp; 0.86--0.90 \\ 
Viscosity, 313 K (mm$^{2}$/s)  &amp; 4.6 &amp; 3.5--5.0 \\ 
Flashpoint (K)  &amp; 437 &amp; ${&gt;}$373 \\ 
Ester content (\%) &amp; 91.8--99.8 &amp; 96.5 \\ 
Free glycerol (\%, m/m)  &amp; 0.018 &amp; ${&lt;}$0.02
\botline
\end{tabular}
\end{table}

\section{Conclusions}

In summary, series of CaO(S) materials were prepared based on a
combination of pollen and \mbox{sol--gel} method via solvation, alcohololysis,
condensation, followed by the calcination decomposition. Compared with
commercial CaO calcinated at 700~\textdegree C, as-prepared
1/1-CaO(S)-700 possesses larger BET surface areas of 16.4746 m$^{2}$/g,
hierarchical structure with pore size distribution of 2--12.5 nm, \mbox{total}
\mbox{basicity} of 5.9018 mmol/g and strong basic sites (CO$_{2}$-desorption
temperature around 700~\textdegree C), thereby promoting the diffusion
of reactants in heterogeneous reaction systems and improving their
reaction efficiency. Consequentially, the resulting 1/1-CaO(S)-700 is
predicted considerable activity in the transesterification reaction of
oil-methyl acetate-methanol. The yield of biodiesel catalyzed by
1/1-CaO(S)-700 via diffusing or depositing of Ca$^{2+}$ in confined
spaceas (pollen) by the sol--gel approach is obviously higher than that
by commercial CaO, as presented by 96.62\% after 3~h under
65~\textdegree C, the dosage of 1/1-CaO(S)-700 of 10~wt\%, oil/methyl
acetate/methanol of 1/1/8. Furthermore, its regeneration performance
shows that 1/1-CaO(S)-700 has significant catalytic stability with the
yield of 91.37\% after 3 cycles. This study provides a facile and
efficient synthetic path for calcium oxide with hierarchical structure
and improves total basicity, opening up more opportunities for its
application in biodiesel production or other organic reactions
involving calcium oxide.

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}

The work was supported financially by the National Natural Science
Foundation of China (51974252), the Youth Innovation Team of Shaanxi
University and Xi'an Shiyou University Graduate Innovation Project
(YCS22213106). 

\section*{Acknowledgments}

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

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