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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Chem.</journal-id>
<journal-title>Frontiers in Chemistry</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Chem.</abbrev-journal-title>
<issn pub-type="epub">2296-2646</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">882235</article-id>
<article-id pub-id-type="doi">10.3389/fchem.2022.882235</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Chemistry</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>A Highly Effective Biomass-Derived Solid Acid Catalyst for Biodiesel Synthesis Through Esterification</article-title>
<alt-title alt-title-type="left-running-head">Zhang et&#x20;al.</alt-title>
<alt-title alt-title-type="right-running-head">Biomass-Derived Solid Acid Catalyst</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author">
<name>
<surname>Zhang</surname>
<given-names>Songdang</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Pan</surname>
<given-names>Hu</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1312687/overview"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Huang</surname>
<given-names>Jinshu</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Li</surname>
<given-names>Yuncong</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Zhang</surname>
<given-names>Heng</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1551723/overview"/>
</contrib>
</contrib-group>
<aff id="aff1">
<sup>1</sup>
<institution>State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering</institution>, <institution>Key Laboratory of Green Pesticide and Agricultural Bioengineering</institution>, <institution>State-Local Joint Laboratory for Comprehensive Utilization of Biomass</institution>, <institution>Center for Research and Development of Fine Chemicals</institution>, <institution>Ministry of Education</institution>, <institution>Guizhou University</institution>, <addr-line>Guiyang</addr-line>, <country>China</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>College of Biological</institution>, <institution>Chemical Science and Engineering</institution>, <institution>Jiaxing University</institution>, <addr-line>Jiaxing</addr-line>, <country>China</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/765992/overview">Yaqiong Su</ext-link>, Xi&#x2019;an Jiaotong University, China</p>
</fn>
<fn fn-type="edited-by">
<p>
<bold>Reviewed by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1591902/overview">Yulin Hu</ext-link>, University of Prince Edward Island, Canada</p>
<p>
<ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1621457/overview">Kai Cai</ext-link>, Guizhou Academy of Tobacco Science, China</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Heng Zhang, <email>hzhang23@gzu.edu.cn</email>
</corresp>
<fn fn-type="other">
<p>This article was submitted to Green and Sustainable Chemistry, a section of the journal Frontiers in Chemistry</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>16</day>
<month>03</month>
<year>2022</year>
</pub-date>
<pub-date pub-type="collection">
<year>2022</year>
</pub-date>
<volume>10</volume>
<elocation-id>882235</elocation-id>
<history>
<date date-type="received">
<day>23</day>
<month>02</month>
<year>2022</year>
</date>
<date date-type="accepted">
<day>28</day>
<month>02</month>
<year>2022</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2022 Zhang, Pan, Huang, Li and Zhang.</copyright-statement>
<copyright-year>2022</copyright-year>
<copyright-holder>Zhang, Pan, Huang, Li and Zhang</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these&#x20;terms.</p>
</license>
</permissions>
<abstract>
<p>Efficient valorization of renewable liquid biomass for biodiesel production using the desirable biomass-based catalysts is being deemed to be an environmentally friendly process. Herein, a highly active biomass-based solid acid catalyst (SiO<sub>2</sub>@Cs-SO<sub>3</sub>H) with renewable chitosan as raw material through sulfonation procedure under the relatively mild condition was successfully manufactured. The SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyst was systematically characterized, especially with a large specific surface area (21.82&#xa0;m<sup>2</sup>/g) and acidity (3.47&#xa0;mmol/g). The catalytic activity of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H was evaluated by esterification of oleic acid (OA) and methanol for biodiesel production. The best biodiesel yield was acquired by Response Surface Methodology (RSM). The optimized reaction conditions were temperature of 92&#xb0;C, time of 4.1&#xa0;h, catalyst dosage of 6.8 wt%, and methanol to OA molar ratio of 31.4, respectively. In this case, the optimal experimental biodiesel yield was found to be 98.2%, which was close to that of the predicted value of 98.4%, indicating the good reliability of RSM employed in this study. Furthermore, SiO<sub>2</sub>@Cs-SO<sub>3</sub>H also exhibited good reusability in terms of five consecutive recycles with 87.0% biodiesel yield. As such, SiO<sub>2</sub>@Cs-SO<sub>3</sub>H can be considered and used as a bio-based sustainable catalyst of high-efficiency for biodiesel production.</p>
</abstract>
<kwd-group>
<kwd>biodiesel</kwd>
<kwd>renewable bio-based catalyst</kwd>
<kwd>response surface methodology</kwd>
<kwd>sulfonation</kwd>
<kwd>esterification</kwd>
</kwd-group>
</article-meta>
</front>
<body>
<sec id="s1">
<title>1 Introduction</title>
<p>In today&#x2019;s society, due to the rapid development of industry, the accompanying sharp increase in the consumption of fossil fuels has led to a series of social problems including energy shortages and environmental pollution (<xref ref-type="bibr" rid="B23">Li et&#x20;al., 2017</xref>; <xref ref-type="bibr" rid="B22">Li et&#x20;al., 2019</xref>; <xref ref-type="bibr" rid="B58">Zhao et&#x20;al., 2019</xref>; <xref ref-type="bibr" rid="B16">Hu et&#x20;al., 2021</xref>). Therefore, it is of great significance to develop the renewable bioenergy of huge potential in terms of green and efficiency, aiming to replace the increasingly scarce traditional fossil energy (<xref ref-type="bibr" rid="B29">Liu et&#x20;al., 2021</xref>; <xref ref-type="bibr" rid="B24">Li et&#x20;al., 2022</xref>). Biodiesel, a kind of green and clean energy with low pollutant emission, non-toxic, good renewability and biodegradability, is regarded as a promising alternative to conventional fossil fuels (<xref ref-type="bibr" rid="B4">Atadashi et&#x20;al., 2013</xref>; <xref ref-type="bibr" rid="B35">Pan et&#x20;al., 2019</xref>; <xref ref-type="bibr" rid="B43">Tan et&#x20;al., 2021</xref>). Biodiesel is mainly composed of various fatty acid alkyl esters, which are related to the composition of raw materials (<xref ref-type="bibr" rid="B34">Pan et&#x20;al., 2018</xref>; <xref ref-type="bibr" rid="B46">Wang et&#x20;al., 2018</xref>; <xref ref-type="bibr" rid="B3">Alagumalai et&#x20;al., 2021</xref>). In general, biodiesel can be produced mainly through the transesterification of triglycerides of edible oil, vegetable oil, and waste edible oil, such as soybean (<xref ref-type="bibr" rid="B17">Hu et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B52">Xie et&#x20;al., 2021b</xref>; <xref ref-type="bibr" rid="B59">Zhu et&#x20;al., 2021</xref>), rapeseed (<xref ref-type="bibr" rid="B14">H&#xe1;jek et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B11">Gaidukevi&#x10d; et&#x20;al., 2021</xref>), and <italic>Jatropha curcas</italic> oil (<xref ref-type="bibr" rid="B13">Guti&#xe9;rrez-L&#xf3;pez et&#x20;al., 2021</xref>; <xref ref-type="bibr" rid="B19">Laskar et&#x20;al., 2021</xref>), as well as the esterification of free fatty acids (FFAs) such as oleic acid (OA) with methanol. Recently, on account of the merits of microalgae, biodiesel production directly from microalgae oil has also attracted attention (<xref ref-type="bibr" rid="B56">Zhang et&#x20;al., 2015</xref>; <xref ref-type="bibr" rid="B5">Bauer et&#x20;al., 2017</xref>; <xref ref-type="bibr" rid="B1">Abdala et&#x20;al., 2020</xref>). However, considering the issues of cost, environmental pollution, and food safety, producing biodiesel from waste edible oil has gradually become a research hotspot. It is worth noting that although the use of waste edible oil can reduce the relevant process cost and environmental pollution, the high levels of FFA in waste edible oil make its biodiesel technology difficult (<xref ref-type="bibr" rid="B53">Zhang et&#x20;al., 2019</xref>; <xref ref-type="bibr" rid="B6">Bhatia et&#x20;al., 2020</xref>; <xref ref-type="bibr" rid="B41">Shatesh et&#x20;al., 2020</xref>). Therefore, to solve this problem, it is necessary to design a catalyst that can efficiently transform the high levels of FFA feedstock into biodiesel. Usually, the conventional catalysts used widely for biodiesel production are divided into two types, namely homogeneous and heterogeneous groups. Homogeneous catalysts restrict their further development due to the obvious disadvantages including difficult separation, difficult regeneration, and high requirements on experimental equipment (<xref ref-type="bibr" rid="B12">Guldhe et&#x20;al., 2017</xref>; <xref ref-type="bibr" rid="B31">Negm et&#x20;al., 2019</xref>; <xref ref-type="bibr" rid="B21">Li et&#x20;al., 2020</xref>). Hence, heterogeneous catalysts are being regarded as the more extensive way for biodiesel production using low-quality oil feedstocks.</p>
<p>A negative saponification reaction occurs when biodiesel is produced directly from oil with high FFA content under the catalysis of an alkali catalyst. As such, acid pretreatment <italic>via</italic> esterification should be required to reduce acid content, and esterification of FFAs is also deemed as an effective manner to produce biodiesel (<xref ref-type="bibr" rid="B55">Zhang et&#x20;al., 2017</xref>; <xref ref-type="bibr" rid="B50">Xie et&#x20;al., 2021</xref>). To this end, it is a good choice to employ heterogeneous acid catalysts that can effectively catalyze the esterification reactions for the preparation of biodiesel. Solid acid catalysts containing sulfonic acid groups generally showed better catalytic activity for biodiesel production, especially in the esterification of FFAs with methanol. This should be attributed to the strong acidity of -SO<sub>3</sub>H (<xref ref-type="bibr" rid="B27">Liu et&#x20;al., 2020</xref>). Nonetheless, compared with homogeneous catalysts, the heterogeneous acid catalysts usually depict lower catalytic efficiency, of which more stringent reaction conditions are required and the relatively higher catalyst production cost should also be carried out to achieve the desired effect. As a consequence, it is of great importance to developing low-cost and efficient solid acid catalysts that catalyze the esterification of FFAs to produce biodiesel.</p>
<p>With respect to this, biomass-derived catalysts have been widely used in the biodiesel production on account of their advantages of renewability, non-toxic, and environmental protection property. Nevertheless, the common biomass-based catalysts [bagasse (<xref ref-type="bibr" rid="B2">Akinfalabi et&#x20;al., 2020</xref>), rice husk carbon (<xref ref-type="bibr" rid="B8">Chen et&#x20;al., 2013</xref>) and bamboo (<xref ref-type="bibr" rid="B44">Tang and Niu, 2019</xref>), etc.] are mainly prepared by carbonization at high temperatures (&#x3e;300&#xb0;C) and sulfonation with corrosive sulfuric acid. The as-mentioned protocol usually requires harsh reaction conditions and also brings about pollution to the environment. Consequently, the relatively simple synthesis methods and the more environmentally friendly organic sulfonation reagents should be adopted to manufacture biomass-based catalysts. <italic>p</italic>-Toluenesulfonic acid, a non-oxidizing strong organic acid (one million times more acidic than benzoic acid), is considered to be a better organic sulfonation reagent (<xref ref-type="bibr" rid="B49">Wang et&#x20;al., 2015</xref>).</p>
<p>Chitosan (Cs), the product of natural polysaccharide chitin after the removal of acetyl group, has demonstrated various attractive advantages such as biodegradability, biocompatibility, and non-toxicity. It is widely applied in many fields such as food additives, environmental protection, cosmetics, antimicrobial agent, drug development, and other daily chemical industry (<xref ref-type="bibr" rid="B28">Liu et&#x20;al., 2015</xref>; <xref ref-type="bibr" rid="B10">Dhakshinamoorthy et&#x20;al., 2021</xref>). Cs, as a weakly alkaline polysaccharide, is rich in -NH<sub>2</sub> and -OH functional groups and also shows strong modification potential. Therefore, of particular interest is the rational synthesis of biomass-based catalysts from Cs for biodiesel production. Recently, supported solid acid catalysts have shown great application potential in the production of biodiesel owing to their excellent catalytic performance and strong design ability. With regard to this, nano-SiO<sub>2</sub> is considered an excellent carrier because of its large pore size and specific surface area, along with strong stability even in the acidic&#x20;media.</p>
<p>In this study, a novel biomass-derived solid acid catalytic material SiO<sub>2</sub>@Cs-SO<sub>3</sub>H was successfully prepared using <italic>p</italic>-toluenesulfonic acid as the sulfonation reagent, and nano-SiO<sub>2</sub> as the solid support. The synthetic procedure used in this study was simple, mild, and environmentally friendly. Furthermore, the catalytic performance of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H was assessed through the esterification of OA and methanol for biodiesel production.</p>
</sec>
<sec id="s2">
<title>2 Materials and Methods</title>
<sec id="s2-1">
<title>2.1 Materials</title>
<p>Chitosan (&#x3e;99%, the degree of deacetylation &#x2265;95%), <italic>p</italic>-toluenesulfonic acid monohydrate (AR, 99%), and methanol (AR, &#x2265;99.7%) were obtained from Chengdu Jinshan Chemical Reagent Co., Ltd. Oleic acid (AR, &#x2265;99%) and nano-SiO<sub>2</sub> were bought from Shanghai Macklin Biochemical Co., Ltd. Acetic acid (AR, &#x2265;99.5%) was supplied by Chongqing Chuandong Chemical Co., Ltd. 50% of glutaraldehyde (AR, &#x2265;99%) was purchased from Tianjin Damao Chemical Reagent Factory. Ethanol (AR, &#x2265;99.5%) and petroleum ether (AR, 60&#x2013;90&#xb0;C) were obtained from Tianjin Fuyu Fine Chemical Co., Ltd. KOH (AR, &#x2265;85.0%) was purchased from Shanghai Titan Scientific Co.,&#x20;Ltd.</p>
</sec>
<sec id="s2-2">
<title>2.2 Catalyst Preparation</title>
<p>The synthesis diagram of biomass-based solid acid catalyst SiO<sub>2</sub>@Cs-SO<sub>3</sub>H is shown in <xref ref-type="fig" rid="F7">Scheme&#x20;1</xref>.</p>
<fig id="F7" position="float">
<label>SCHEME 1</label>
<caption>
<p>Schematic illustration for the synthesis of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H.</p>
</caption>
<graphic xlink:href="fchem-10-882235-g007.tif"/>
</fig>
<sec id="s2-2-1">
<title>2.2.1 Sulfonation of Chitosan</title>
<p>0.5&#xa0;g chitosan was added to 50&#xa0;ml 1 wt% acetic acid solution and stirred magnetically for 10&#xa0;min at room temperature. Then, 5.7&#xa0;g <italic>p</italic>-toluenesulfonic acid monohydrate was added to the chitosan acetic acid solution and strongly stirred until the solid was dissolved.</p>
</sec>
<sec id="s2-2-2">
<title>2.2.2 Preparation of SiO<sub>2</sub> @Cs-SO<sub>3</sub>H Catalyst</title>
<p>0.1&#xa0;g SiO<sub>2</sub> and 1&#xa0;ml glutaraldehyde were added to the sulfonated chitosan solution followed by stirring at room temperature for 2&#xa0;h. The resulting solution was then dried overnight at 80&#xb0;C, and the as-synthesized solid materials were ground and repeatedly washed with anhydrous ethanol until the filtrate became pH &#x3d; 7. Finally, the so-called catalyst SiO<sub>2</sub>@Cs-SO<sub>3</sub>H was obtained after being dried at 80&#xb0;C for 4&#xa0;h.</p>
</sec>
</sec>
<sec id="s2-3">
<title>2.3 Catalyst Characterization</title>
<p>X-ray diffraction (XRD) was measured by Tongda TD-3500 X-ray diffractometer (Cu K<italic>&#x3b1;</italic> radiation) to characterize the crystallinity and structure of the catalyst. The functional groups of the catalyst were determined by KBr compression using Nicolet 360&#x20;FT-IR (Fourier-transform infrared apparatus). The thermal stability of the catalyst in the nitrogen atmosphere was detected by PerkinElmer TGA 47 thermogravimetric analyzer. The ammonia temperature-programmed desorption (NH<sub>3</sub>-TPD) measurements were performed on the AutoChem 2920 chemisorption apparatus with a thermal conductivity detector (TCD) to detect the acidity of the catalyst. The Bruner-Emmett-Teller (BET) method and N<sub>2</sub> adsorption-desorption apparatus (ASAP 2460, McMerric Instruments Co., Ltd.) were used to check the physical properties of the catalytic material (specific surface area, pore-volume, and pore diameter, respectively).</p>
</sec>
<sec id="s2-4">
<title>2.4 Esterification of Oleic Acid With Methanol</title>
<p>The catalytic performance of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H was determined by the esterification of oleic acid and methanol for biodiesel synthesis. To be specific, a mixture of 1&#xa0;g oleic acid (acid value: 200&#xa0;mg KOH/g), 4.3&#xa0;ml methanol, and 0.06&#xa0;g catalyst were added into a 15&#xa0;ml thick wall pressure tube (Beijing Synthware Glass Instrument Co. Ltd.), and the encapsulated tube was allowed to proceed in an oil bath at 650&#xa0;rpm using different temperatures for a specific time. After the reaction, petroleum ether was added to extract the reaction products. Finally, the petroleum ether was removed by vacuum distillation. The acid value (AV) of the product was detected by 0.1&#xa0;M KOH solution titration using phenolphthalein as an indicator (<xref ref-type="bibr" rid="B26">Lin et&#x20;al., 2021</xref>). The yield of biodiesel was determined according to the change of AV before and after oleic acid reaction, and the calculation formula was as follows:<disp-formula id="e1">
<mml:math id="m1">
<mml:mtable columnalign="left">
<mml:mtr>
<mml:mtd>
<mml:mi mathvariant="normal">B</mml:mi>
<mml:mi mathvariant="normal">i</mml:mi>
<mml:mi mathvariant="normal">o</mml:mi>
<mml:mi mathvariant="normal">d</mml:mi>
<mml:mi mathvariant="normal">i</mml:mi>
<mml:mi mathvariant="normal">e</mml:mi>
<mml:mi mathvariant="normal">s</mml:mi>
<mml:mi mathvariant="normal">e</mml:mi>
<mml:mi mathvariant="normal">l</mml:mi>
<mml:mo>&#xa0;</mml:mo>
<mml:mi mathvariant="normal">Y</mml:mi>
<mml:mi mathvariant="normal">i</mml:mi>
<mml:mi mathvariant="normal">e</mml:mi>
<mml:mi mathvariant="normal">l</mml:mi>
<mml:mi mathvariant="normal">d</mml:mi>
<mml:mrow>
<mml:mo>(</mml:mo>
<mml:mo>%</mml:mo>
<mml:mo>)</mml:mo>
</mml:mrow>
<mml:mo>&#x3d;</mml:mo>
<mml:mfrac>
<mml:mrow>
<mml:mi mathvariant="normal">A</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">V</mml:mi>
<mml:mi mathvariant="normal">0</mml:mi>
</mml:msub>
<mml:mo>&#x2212;</mml:mo>
<mml:mi mathvariant="normal">A</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">V</mml:mi>
<mml:mi mathvariant="normal">1</mml:mi>
</mml:msub>
</mml:mrow>
<mml:mrow>
<mml:mi mathvariant="normal">A</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">V</mml:mi>
<mml:mi mathvariant="normal">0</mml:mi>
</mml:msub>
</mml:mrow>
</mml:mfrac>
<mml:mo>&#xd7;</mml:mo>
<mml:mi mathvariant="normal">100</mml:mi>
<mml:mo>%</mml:mo>
</mml:mtd>
</mml:mtr>
<mml:mtr>
<mml:mtd>
<mml:mi mathvariant="normal">A</mml:mi>
<mml:msub>
<mml:mi mathvariant="normal">V</mml:mi>
<mml:mi mathvariant="normal">1</mml:mi>
</mml:msub>
<mml:mo>&#x3d;</mml:mo>
<mml:mfrac>
<mml:mrow>
<mml:mn>56.1</mml:mn>
<mml:mo>&#xd7;</mml:mo>
<mml:mn>0.1</mml:mn>
<mml:mo>&#xd7;</mml:mo>
<mml:msub>
<mml:mi mathvariant="normal">V</mml:mi>
<mml:mrow>
<mml:mi mathvariant="normal">K</mml:mi>
<mml:mi mathvariant="normal">O</mml:mi>
<mml:mi mathvariant="normal">H</mml:mi>
</mml:mrow>
</mml:msub>
</mml:mrow>
<mml:mi mathvariant="normal">m</mml:mi>
</mml:mfrac>
</mml:mtd>
</mml:mtr>
</mml:mtable>
</mml:math>
<label>(1)</label>
</disp-formula>Where AV<sub>0</sub> and AV<sub>1</sub> are the acid value of oleic acid and products, respectively; The numbers 56.1 and 0.1 in <xref ref-type="disp-formula" rid="e1">Eq. 1</xref> are the relative molecular mass and solution concentration (0.1&#xa0;mol/L) of KOH, respectively; V<sub>
<italic>KOH</italic>
</sub> is the volume of KOH titrated; <inline-formula id="inf1">
<mml:math id="m2">
<mml:mi>m</mml:mi>
</mml:math>
</inline-formula> is the mass of the product.</p>
</sec>
<sec id="s2-5">
<title>2.5 Response Surface Methodology (RSM)</title>
<p>The factors affecting the esterification of oleic acid with methanol were optimized by the Box-Behnken design (BBD) experiment of RSM. In this process, four factors and three levels were adopted to design the experiment, in which the biodiesel yield was regarded as the response value, and the temperature, time, methanol to oleic acid molar ratio, and the catalyst dosage were served as the main four experimental parameters. Accordingly, a total of 29 experiments were designed in a random order manner. The detailed symbols, names of each parameter, and their ranges and levels are shown in <xref ref-type="table" rid="T1">Table&#x20;1</xref>. Moreover, the relationship between response values and parameters was studied by the least square multiple regression method. The second-order polynomial was fitted to establish the model (<xref ref-type="bibr" rid="B18">Kumar and Singh, 2019</xref>; <xref ref-type="bibr" rid="B15">Hossain et&#x20;al., 2021</xref>).<disp-formula id="e2">
<mml:math id="m3">
<mml:mtable columnalign="left">
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<mml:mtd>
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<mml:mo>&#x3d;</mml:mo>
<mml:msub>
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</mml:msub>
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<label>(2)</label>
</disp-formula>Where Y represents the response prediction value, <italic>&#x3b2;</italic>
<sub>
<italic>0</italic>
</sub> refers the intercept term, <italic>&#x3b2;</italic>
<sub>
<italic>1</italic>
</sub> to <italic>&#x3b2;</italic>
<sub>
<italic>14</italic>
</sub> are the linear effect coefficient, the quadratic coefficient of the crossover and square effect, and <italic>&#x3b5;</italic> implies the&#x20;error.</p>
<table-wrap id="T1" position="float">
<label>TABLE 1</label>
<caption>
<p>Experimental parameters and range of response surface methodology optimization.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th rowspan="2" align="left">Independent variables</th>
<th rowspan="2" align="center">Symbols</th>
<th colspan="3" align="center">Range and levels</th>
</tr>
<tr>
<th align="center">&#x2212;1</th>
<th align="char" char=".">0</th>
<th align="char" char=".">1</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">Temperature</td>
<td align="center">A</td>
<td align="center">80</td>
<td align="char" char=".">90</td>
<td align="char" char=".">100</td>
</tr>
<tr>
<td align="left">Time</td>
<td align="center">B</td>
<td align="center">3</td>
<td align="char" char=".">4</td>
<td align="char" char=".">5</td>
</tr>
<tr>
<td align="left">Catalyst dosage</td>
<td align="center">C</td>
<td align="center">4</td>
<td align="char" char=".">6</td>
<td align="char" char=".">8</td>
</tr>
<tr>
<td align="left">Methanol to oleic acid molar ratio</td>
<td align="center">D</td>
<td align="center">25</td>
<td align="char" char=".">30</td>
<td align="char" char=".">35</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>Analysis of variance (ANOVA) was used to perform statistical analysis on the model, and the influence of independent experimental variables along with their interactions on the response value (biodiesel yield) was also investigated. The <italic>p</italic>-value plays a decisive role in the probability error of the model and the significance of each variable. When the <italic>p</italic>-value is below 0.05%, it indicates that the variable has statistical significance. In addition, a response surface diagram was drawn to demonstrate the influence of the interaction between univariate and bivariate on the yield of biodiesel, and the optimal value was also determined accordingly.</p>
</sec>
<sec id="s2-6">
<title>2.6 Reusability</title>
<p>The reusability of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H was evaluated under the optimal experimental conditions. After the reaction was finished, the catalyst was separated by centrifugation followed by washing with anhydrous ethanol 3&#x20;times to remove the impurities attached to the catalyst. After washing, the catalyst was dried in a vacuum drying oven at 80&#xb0;C for 8&#xa0;h to remove the solvent. Then, the as-dried recycled catalyst was employed for the next optimization experiment. The recycling process of the catalyst was repeated for five consecutive&#x20;runs.</p>
</sec>
</sec>
<sec sec-type="results|discussion" id="s3">
<title>3 Results and Discussion</title>
<sec id="s3-1">
<title>3.1 Catalyst Characterization</title>
<p>The structure and crystallinity SiO<sub>2</sub>@Cs-SO<sub>3</sub>H samples were characterized by XRD. As shown in <xref ref-type="fig" rid="F1">Figure&#x20;1A</xref>, SiO<sub>2</sub>@Cs-SO<sub>3</sub>H samples showed a broad diffraction peak within the range of 2<italic>&#x3b8;</italic> &#x3d; 10&#xb0;&#x2013;30&#xb0;, corresponding to the characteristic peaks of Cs (<xref ref-type="bibr" rid="B38">Ranjani et&#x20;al., 2019</xref>; <xref ref-type="bibr" rid="B39">Sabar et&#x20;al., 2020</xref>). However, the absorption peak of SiO<sub>2</sub> was not highlighted in the figure, which may be due to the amorphous state of nano-SiO<sub>2</sub> (<xref ref-type="bibr" rid="B45">Thangaraj et&#x20;al., 2019</xref>). With respect to this, the existence of SiO<sub>2</sub> was further proved by FT-IR spectroscopy. <xref ref-type="fig" rid="F1">Figure&#x20;1B</xref> shows the FT-IR spectra of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H and Cs. The absorption peaks observed in both the two samples at 1,380&#xa0;cm<sup>&#x2212;1</sup> and 1,598&#xa0;cm<sup>&#x2212;1</sup> are attributed to the C-O and N-H functional groups, respectively, and these should be typical characteristic peaks of Cs (<xref ref-type="bibr" rid="B40">Saengprachum et&#x20;al., 2019</xref>). The obvious peaks of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H at 1,182&#xa0;cm<sup>&#x2212;1</sup> and 1,035&#xa0;cm<sup>&#x2212;1</sup> are derived from the stretching vibration of O&#x3d;S&#x3d;O, indicating the successful functionalization of the active sulfonic acid group on the Cs (<xref ref-type="bibr" rid="B30">Nata et&#x20;al., 2017</xref>). Meanwhile, the absorption peaks shown at 1,076&#xa0;cm<sup>&#x2212;1</sup>, 795&#xa0;cm<sup>&#x2212;1</sup>, and 680&#xa0;cm<sup>&#x2212;1</sup> correspond to the anti-symmetrical stretching vibration, symmetrical stretching, and bending of Si-O-Si, respectively (<xref ref-type="bibr" rid="B33">Owoeye et&#x20;al., 2020</xref>). These indicate that the sulfonated chitosan has been successfully coated onto&#x20;SiO<sub>2</sub>.</p>
<fig id="F1" position="float">
<label>FIGURE 1</label>
<caption>
<p>
<bold>(A)</bold> XRD spectrum of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H, <bold>(B)</bold> FI-IR spectra of Cs and SiO<sub>2</sub>@Cs-SO<sub>3</sub>H</p>
</caption>
<graphic xlink:href="fchem-10-882235-g001.tif"/>
</fig>
<p>The thermal stability of the prepared catalyst was studied by TGA analysis. From <xref ref-type="fig" rid="F2">Figure&#x20;2A</xref>, it can be observed that the weight loss of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H mainly occurs in two stages. When the temperature is below 205&#xb0;C, the weight loss of the catalyst determined as 6.95 wt% should be ascribed to the loss of water adsorbed on the catalyst. The second peak existing in the range of 205&#x2013;540&#xb0;C (weight loss of 42.97 wt%) was related to the polymeric network decomposition and the formation of carbon materials. However, the mass of the catalyst did not decrease significantly after 540&#xb0;C, indicating that the carbon material produced gradually became stable. In short, the catalyst showed good thermal stability and it can completely meet the demand of catalytic esterification reaction to produce biodiesel.</p>
<fig id="F2" position="float">
<label>FIGURE 2</label>
<caption>
<p>SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyst diagram of <bold>(A)</bold> TGA, <bold>(B)</bold> NH<sub>3</sub>-TPD, <bold>(C)</bold> BET, and <bold>(D)</bold> pore size distribution.</p>
</caption>
<graphic xlink:href="fchem-10-882235-g002.tif"/>
</fig>
<p>NH<sub>3</sub>-TPD was used to investigate the relative acidity density and acidic strength of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H. As depicted in <xref ref-type="fig" rid="F2">Figure&#x20;2B</xref>, the peaks shown in the profile were derived from the desorption of NH<sub>3</sub> at acidic sites on the sample, and the desorption temperature was related to the acid strength of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H. As illustrated in <xref ref-type="fig" rid="F2">Figure&#x20;2B</xref>, the NH<sub>3</sub>-TPD profile shows three peaks within the range of 50&#x2013;600&#xb0;C. The peaks in the temperature ranges of 110&#x2013;200&#xb0;C, 310&#x2013;400&#xb0;C, and 450&#x2013;520&#xb0;C correspond to the sites of a weak acid (-OH), medium strong acid (-SO<sub>3</sub>H) and strong acid (-SO<sub>3</sub>H), respectively. Meanwhile, the measurement result showed that SiO<sub>2</sub>@Cs-SO<sub>3</sub>H possessed a higher acid density of 3.47&#xa0;mmol/g, which were superior to the reported sulfonated solid acid catalysts such as SAC (1.50&#xa0;mmol/g) (<xref ref-type="bibr" rid="B20">Lathiya et&#x20;al., 2018</xref>), ASHC-SO<sub>3</sub>H (1.40&#xa0;mmol/g) (<xref ref-type="bibr" rid="B7">Cao et&#x20;al., 2021</xref>), and PMB-SO<sub>3</sub>H (1.92&#xa0;mmol/g) (<xref ref-type="bibr" rid="B37">Quah et&#x20;al., 2020</xref>). The specific surface area, pore size, and pore volume of the SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyst were measured by BET-BJH, as shown in <xref ref-type="fig" rid="F2">Figures 2C,D</xref>. According to the results, the catalyst exhibited a specific surface area of 21.82&#xa0;m<sup>2</sup>/g, a pore volume of 0.022&#xa0;cm<sup>2</sup>/g, and a pore size distribution between 2 and 20&#xa0;nm. Compared with the sulfonated chitosan catalysts that were reported in the previous literature (<xref ref-type="bibr" rid="B47">Wang et&#x20;al., 2019</xref>; <xref ref-type="bibr" rid="B57">Zhang and Sun, 2020</xref>), SiO<sub>2</sub>@Cs-SO<sub>3</sub>H demonstrated a benign porous structure, implying that SiO<sub>2</sub>@Cs-SO<sub>3</sub>H will be conducive to esterification for biodiesel synthesis. It is worth mentioning that when nano-SiO<sub>2</sub> with good surface efficiency and large surface area was used as the suitable carrier, of crucial significance was to improve the specific surface area of the catalyst, thus leading to better catalytic performance accordingly.</p>
</sec>
<sec id="s3-2">
<title>3.2 Esterification of Oleic Acid to Biodiesel</title>
<p>The prepared SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyst from sulfonated chitosan was further applied to catalyze the esterification reaction of oleic acid with methanol for the production of biodiesel. The effects of reaction temperature (70&#x2013;110&#xb0;C), reaction time (1&#x2013;5&#xa0;h), catalyst dosage (0&#x2013;8 wt%), and the methanol to OA molar ratio (15/1&#x2013;35/1) on biodiesel yield were studied by single-factor experiments.</p>
<sec id="s3-2-1">
<title>3.2.1 Reaction Temperature</title>
<p>Generally, the reaction temperature is a significant variable affecting the reaction rate in the esterification of OA with methanol (<xref ref-type="bibr" rid="B54">Zhang et&#x20;al., 2018</xref>). As shown in <xref ref-type="fig" rid="F3">Figure&#x20;3A</xref>, the influence of temperature on the yield of biodiesel was investigated in the temperature range of 70&#x2013;110&#xb0;C. It was found that with the temperature rising from 70 to 90&#xb0;C, the biodiesel yield gradually increased to more than 90%. Subsequently, as the temperature continued to rise, the yield of biodiesel decreased slightly, which may be due to the decrease of methanol feedstock caused by the higher temperature. Therefore, 90&#xb0;C was selected as the appropriate reaction temperature.</p>
<fig id="F3" position="float">
<label>FIGURE 3</label>
<caption>
<p>Single-factor optimization of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyst for oleic acid esterification reaction: <bold>(A)</bold> reaction temperature, <bold>(B)</bold> reaction time, <bold>(C)</bold> catalyst dosage, and <bold>(D)</bold> methanol to oleic acid molar&#x20;ratio.</p>
</caption>
<graphic xlink:href="fchem-10-882235-g003.tif"/>
</fig>
</sec>
<sec id="s3-2-2">
<title>3.2.2 Reaction Time</title>
<p>The reaction time also has a certain influence on biodiesel yield. From the point of kinetic, the esterification of OA with methanol requires a certain amount of time for mass transfer to reach an equilibrium state (<xref ref-type="bibr" rid="B36">Pan et&#x20;al., 2017</xref>). <xref ref-type="fig" rid="F3">Figure&#x20;3B</xref> shows the effect of reaction time on biodiesel yield at 90&#xb0;C, methanol to OA molar ratio of 30/1, and catalyst dosage of 6 wt%, and the highest yield of 96% was obtained after 4&#xa0;h of reaction. Thus, 4&#xa0;h was determined as the optimal reaction&#x20;time.</p>
</sec>
<sec id="s3-2-3">
<title>3.2.3 Catalyst Dosage</title>
<p>The amount of catalyst is also an important parameter to evaluate its catalytic capacity for OA esterification reaction, wherein the number of active sites is closely related to the yield of biodiesel (<xref ref-type="bibr" rid="B48">Wang et&#x20;al., 2021</xref>). As shown in <xref ref-type="fig" rid="F3">Figure&#x20;3C</xref>, only 8.57% of the biodiesel yield was obtained without any catalyst. With the increase of catalyst amount from 0 wt% to 6 wt%, the corresponding biodiesel yield reached over 90%. However, when the catalyst amount exceeded 6 wt%, the biodiesel yield showed a downward trend. This could be explained that since the increase of the catalyst dosage proceeded, the number of catalytic active sites also raised, thus promoting the esterification reaction towards the biodiesel synthesis. It should be noted that too much catalyst would hinder mass transfer between the catalyst and the reactants, slowing down the reaction rate ultimately. Therefore, 6 wt% was selected as the suitable dosage of catalyst.</p>
</sec>
<sec id="s3-2-4">
<title>3.2.4 Methanol to OA Molar Ratio</title>
<p>Apart from the above three factors, the amount of methanol also played a crucial role in the esterification of OA. It is certain that appropriately increasing the amount of alcohol can promote the forward reaction, thus improving the yield of biodiesel accordingly (<xref ref-type="bibr" rid="B51">Xie and Wang, 2021</xref>). As shown in <xref ref-type="fig" rid="F3">Figure&#x20;3D</xref>, the influence of alcohol dosage on biodiesel yield was studied with different molar ratios (15/1&#x2013;35/1), while remaining the other variables as the same. It was determined that the biodiesel yield showed an upward trend with the increase of molar ratio, of which the highest yield of 95% was observed at 30/1. However, excessive methanol ultra-dilution the concentration of the reaction system and reduces the chance of OA arriving at the catalytic active site, thus reducing the yield of biodiesel. Thus, 30/1 was determined as the appropriate molar&#x20;ratio.</p>
</sec>
</sec>
<sec id="s3-3">
<title>3.3 Optimization of Biodiesel Production by RSM</title>
<p>To further optimize the reaction conditions of the SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyzing oleic acid esterification reaction to obtain the best biodiesel yield, we further employed the response surface method to randomly design 29 sets of experiments on the basis of the single-factor experimental results in the previous section. The obtained results and the corresponding predicted values of the above <xref ref-type="disp-formula" rid="e2">Eq. 2</xref> are shown in <xref ref-type="table" rid="T2">Table&#x20;2</xref>. The specific second-order polynomial equation of the design model based on the predicted response value of the four parameters is depicted in <xref ref-type="disp-formula" rid="e3">Eq. 3</xref>.<disp-formula id="e3">
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<mml:msup>
<mml:mrow>
<mml:mtext>.</mml:mtext>
<mml:mn>15</mml:mn>
<mml:mtext>D</mml:mtext>
</mml:mrow>
<mml:mn>2</mml:mn>
</mml:msup>
</mml:mtd>
</mml:mtr>
</mml:mtable>
</mml:math>
<label>(3)</label>
</disp-formula>
</p>
<table-wrap id="T2" position="float">
<label>TABLE 2</label>
<caption>
<p>Experimental design and results of oleic acid esterification reaction catalyzed by SiO<sub>2</sub>@Cs-SO<sub>3</sub>H.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Run</th>
<th align="center">A: Temperature (&#xb0;C)</th>
<th align="center">B: Time (h)</th>
<th align="center">C: Catalyst dosage (wt%)</th>
<th align="center">D: Methanol to oleic acid molar ratio</th>
<th align="center">Experimental biodiesel yield (%)</th>
<th align="center">Predicted biodiesel yield (%)</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">1</td>
<td align="center">90</td>
<td align="center">3</td>
<td align="center">4</td>
<td align="center">30</td>
<td align="center">92.64</td>
<td align="center">92.53</td>
</tr>
<tr>
<td align="left">2</td>
<td align="center">90</td>
<td align="center">4</td>
<td align="center">6</td>
<td align="center">30</td>
<td align="center">97.33</td>
<td align="center">97.33</td>
</tr>
<tr>
<td align="left">3</td>
<td align="center">80</td>
<td align="center">4</td>
<td align="center">6</td>
<td align="center">35</td>
<td align="center">94.77</td>
<td align="center">94.43</td>
</tr>
<tr>
<td align="left">4</td>
<td align="center">100</td>
<td align="center">3</td>
<td align="center">6</td>
<td align="center">30</td>
<td align="center">97.34</td>
<td align="center">97.67</td>
</tr>
<tr>
<td align="left">5</td>
<td align="center">90</td>
<td align="center">4</td>
<td align="center">4</td>
<td align="center">25</td>
<td align="center">92.96</td>
<td align="center">92.68</td>
</tr>
<tr>
<td align="left">6</td>
<td align="center">80</td>
<td align="center">4</td>
<td align="center">4</td>
<td align="center">30</td>
<td align="center">90.24</td>
<td align="center">90.17</td>
</tr>
<tr>
<td align="left">7</td>
<td align="center">80</td>
<td align="center">5</td>
<td align="center">6</td>
<td align="center">30</td>
<td align="center">96.44</td>
<td align="center">96.13</td>
</tr>
<tr>
<td align="left">8</td>
<td align="center">100</td>
<td align="center">5</td>
<td align="center">6</td>
<td align="center">30</td>
<td align="center">97.17</td>
<td align="center">96.87</td>
</tr>
<tr>
<td align="left">9</td>
<td align="center">80</td>
<td align="center">4</td>
<td align="center">6</td>
<td align="center">25</td>
<td align="center">92.66</td>
<td align="center">92.84</td>
</tr>
<tr>
<td align="left">10</td>
<td align="center">90</td>
<td align="center">4</td>
<td align="center">8</td>
<td align="center">25</td>
<td align="center">95.23</td>
<td align="center">94.8</td>
</tr>
<tr>
<td align="left">11</td>
<td align="center">90</td>
<td align="center">5</td>
<td align="center">6</td>
<td align="center">35</td>
<td align="center">97.98</td>
<td align="center">98.07</td>
</tr>
<tr>
<td align="left">12</td>
<td align="center">100</td>
<td align="center">4</td>
<td align="center">6</td>
<td align="center">35</td>
<td align="center">97.54</td>
<td align="center">97.3</td>
</tr>
<tr>
<td align="left">13</td>
<td align="center">90</td>
<td align="center">4</td>
<td align="center">6</td>
<td align="center">30</td>
<td align="center">96.34</td>
<td align="center">97.33</td>
</tr>
<tr>
<td align="left">14</td>
<td align="center">80</td>
<td align="center">3</td>
<td align="center">6</td>
<td align="center">30</td>
<td align="center">92.27</td>
<td align="center">92.6</td>
</tr>
<tr>
<td align="left">15</td>
<td align="center">90</td>
<td align="center">5</td>
<td align="center">4</td>
<td align="center">30</td>
<td align="center">93.49</td>
<td align="center">93.66</td>
</tr>
<tr>
<td align="left">16</td>
<td align="center">90</td>
<td align="center">4</td>
<td align="center">4</td>
<td align="center">35</td>
<td align="center">91.59</td>
<td align="center">92.05</td>
</tr>
<tr>
<td align="left">17</td>
<td align="center">90</td>
<td align="center">5</td>
<td align="center">6</td>
<td align="center">25</td>
<td align="center">94.50</td>
<td align="center">94.81</td>
</tr>
<tr>
<td align="left">18</td>
<td align="center">100</td>
<td align="center">4</td>
<td align="center">8</td>
<td align="center">30</td>
<td align="center">97.27</td>
<td align="center">97.38</td>
</tr>
<tr>
<td align="left">19</td>
<td align="center">90</td>
<td align="center">5</td>
<td align="center">8</td>
<td align="center">30</td>
<td align="center">98.15</td>
<td align="center">98.2</td>
</tr>
<tr>
<td align="left">20</td>
<td align="center">90</td>
<td align="center">4</td>
<td align="center">6</td>
<td align="center">30</td>
<td align="center">97.82</td>
<td align="center">97.33</td>
</tr>
<tr>
<td align="left">21</td>
<td align="center">90</td>
<td align="center">3</td>
<td align="center">6</td>
<td align="center">35</td>
<td align="center">95.26</td>
<td align="center">94.99</td>
</tr>
<tr>
<td align="left">22</td>
<td align="center">100</td>
<td align="center">4</td>
<td align="center">4</td>
<td align="center">30</td>
<td align="center">94.85</td>
<td align="center">94.68</td>
</tr>
<tr>
<td align="left">23</td>
<td align="center">100</td>
<td align="center">4</td>
<td align="center">6</td>
<td align="center">25</td>
<td align="center">95.51</td>
<td align="center">95.78</td>
</tr>
<tr>
<td align="left">24</td>
<td align="center">90</td>
<td align="center">3</td>
<td align="center">6</td>
<td align="center">25</td>
<td align="center">95.20</td>
<td align="center">95.15</td>
</tr>
<tr>
<td align="left">25</td>
<td align="center">90</td>
<td align="center">4</td>
<td align="center">6</td>
<td align="center">30</td>
<td align="center">97.54</td>
<td align="center">97.33</td>
</tr>
<tr>
<td align="left">26</td>
<td align="center">90</td>
<td align="center">4</td>
<td align="center">8</td>
<td align="center">35</td>
<td align="center">98.14</td>
<td align="center">97.54</td>
</tr>
<tr>
<td align="left">27</td>
<td align="center">90</td>
<td align="center">4</td>
<td align="center">6</td>
<td align="center">30</td>
<td align="center">97.62</td>
<td align="center">97.33</td>
</tr>
<tr>
<td align="left">28</td>
<td align="center">90</td>
<td align="center">3</td>
<td align="center">8</td>
<td align="center">30</td>
<td align="center">96.82</td>
<td align="center">96.59</td>
</tr>
<tr>
<td align="left">29</td>
<td align="center">80</td>
<td align="center">4</td>
<td align="center">8</td>
<td align="center">30</td>
<td align="center">95.88</td>
<td align="center">96.08</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>Analysis of variance was performed on the experimental design model as exhibited in <xref ref-type="table" rid="T2">Table&#x20;2</xref>, and the corresponding results are presented in <xref ref-type="table" rid="T3">Table&#x20;3</xref>. The F and <italic>p</italic>-value represent the significance of the entire model and the model term respectively. The larger the F value, the more significant the model and the better the fit. The smaller the <italic>p</italic>-value, the more important the model term is for the response value. In general, the <italic>p</italic>-value &#x3c; 0.05 is highly significant of the model variables (<xref ref-type="bibr" rid="B42">Sulaiman et&#x20;al., 2017</xref>). According to the results in <xref ref-type="table" rid="T3">Table&#x20;3</xref>, it can be observed that the reaction parameters including A, B, C, and D can be determined to be highly significant. Meanwhile, for the high F value (45.57) and low <italic>p</italic>-value (&#x3c;0.0001) of the model, it can be seen that the selected quadratic polynomial regression model to optimize the yield of biodiesel is significant. The &#x201c;Lack of Fit F-value&#x201d; of 0.47 indicates that lack of fit is not significant compared with the pure error, showing that the model fits the experimental data sufficiently. Furthermore, the coefficient of determination <italic>R</italic>
<sup>2</sup> of 97.85% indicates that the obtained model can explain 97.85% of the response variability, indicating that the model has high reliability. The adjusted <italic>R</italic>
<sup>2</sup> of 95.71% and the predicted <italic>R</italic>
<sup>2</sup> of 91.76% are considered to be reasonably consistent because their difference is less than 0.2. Thus, it can be concluded that the fit of the model is more significant. To explain the reliability and applicability of the model, the experimental real value is compared with the predicted value (<xref ref-type="fig" rid="F4">Figure&#x20;4</xref>). There is a low deviation between the predicted value and the real value. Therefore, these have proven that the quadratic polynomial regression model can match the data well, and the system response is generated within the scope of independent variable research.</p>
<table-wrap id="T3" position="float">
<label>TABLE 3</label>
<caption>
<p>ANOVA on the RSM experimental design&#x20;data.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="left">Source</th>
<th align="center">Sum&#x20;of squares</th>
<th align="center">Df</th>
<th align="center">Mean square</th>
<th align="center">F value</th>
<th align="center">
<italic>p</italic>-value prob&#x20;&#x3e;&#x20;F</th>
<th align="left"/>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="left">Model</td>
<td align="center">134.32</td>
<td align="center">14</td>
<td align="center">9.59</td>
<td align="center">45.57</td>
<td align="center">&#x3c;0.0001</td>
<td align="left">significant</td>
</tr>
<tr>
<td align="left">A: Temperature</td>
<td align="center">25.29</td>
<td align="center">1</td>
<td align="center">25.29</td>
<td align="center">120.11</td>
<td align="center">&#x3c;0.0001</td>
<td align="left"/>
</tr>
<tr>
<td align="left">B: Time</td>
<td align="center">5.6</td>
<td align="center">1</td>
<td align="center">5.6</td>
<td align="center">26.61</td>
<td align="center">0.0001</td>
<td align="left"/>
</tr>
<tr>
<td align="left">C: Catalyst dosage</td>
<td align="center">55.56</td>
<td align="center">1</td>
<td align="center">55.56</td>
<td align="center">263.88</td>
<td align="center">&#x3c;0.0001</td>
<td align="left"/>
</tr>
<tr>
<td align="left">D: Methanol/OA molar ratio</td>
<td align="center">7.24</td>
<td align="center">1</td>
<td align="center">7.24</td>
<td align="center">34.38</td>
<td align="center">&#x3c;0.0001</td>
<td align="left"/>
</tr>
<tr>
<td align="left">AB</td>
<td align="center">4.71</td>
<td align="center">1</td>
<td align="center">4.71</td>
<td align="center">22.37</td>
<td align="center">0.0003</td>
<td align="left"/>
</tr>
<tr>
<td align="left">AC</td>
<td align="center">2.59</td>
<td align="center">1</td>
<td align="center">2.59</td>
<td align="center">12.31</td>
<td align="center">0.0035</td>
<td align="left"/>
</tr>
<tr>
<td align="left">AD</td>
<td align="center">0.0016</td>
<td align="center">1</td>
<td align="center">0.0016</td>
<td align="center">0.0076</td>
<td align="center">0.9318</td>
<td align="left"/>
</tr>
<tr>
<td align="left">BC</td>
<td align="center">0.058</td>
<td align="center">1</td>
<td align="center">0.058</td>
<td align="center">0.27</td>
<td align="center">0.6091</td>
<td align="left"/>
</tr>
<tr>
<td align="left">BD</td>
<td align="center">2.92</td>
<td align="center">1</td>
<td align="center">2.92</td>
<td align="center">13.89</td>
<td align="center">0.0023</td>
<td align="left"/>
</tr>
<tr>
<td align="left">CD</td>
<td align="center">4.8</td>
<td align="center">1</td>
<td align="center">4.8</td>
<td align="center">22.78</td>
<td align="center">0.0003</td>
<td align="left"/>
</tr>
<tr>
<td align="left">A<sup>2</sup>
</td>
<td align="center">7.69</td>
<td align="center">1</td>
<td align="center">7.69</td>
<td align="center">36.55</td>
<td align="center">&#x3c;0.0001</td>
<td align="left"/>
</tr>
<tr>
<td align="left">B<sup>2</sup>
</td>
<td align="center">1.17</td>
<td align="center">1</td>
<td align="center">1.17</td>
<td align="center">5.54</td>
<td align="center">0.0337</td>
<td align="left"/>
</tr>
<tr>
<td align="left">C<sup>2</sup>
</td>
<td align="center">17.91</td>
<td align="center">1</td>
<td align="center">17.91</td>
<td align="center">85.07</td>
<td align="center">&#x3c;0.0001</td>
<td align="left"/>
</tr>
<tr>
<td align="left">D<sup>2</sup>
</td>
<td align="center">8.6</td>
<td align="center">1</td>
<td align="center">8.6</td>
<td align="center">40.86</td>
<td align="center">&#x3c;0.0001</td>
<td align="left"/>
</tr>
<tr>
<td align="left">Residual</td>
<td align="center">2.95</td>
<td align="center">14</td>
<td align="center">0.21</td>
<td align="left"/>
<td align="left"/>
<td align="left"/>
</tr>
<tr>
<td align="left">Lack of Fit</td>
<td align="center">1.6</td>
<td align="center">10</td>
<td align="center">0.16</td>
<td align="center">0.47</td>
<td align="center">0.8455</td>
<td align="left">not significant</td>
</tr>
<tr>
<td align="left">Pure Error</td>
<td align="center">1.35</td>
<td align="center">4</td>
<td align="center">0.34</td>
<td align="left"/>
<td align="left"/>
<td align="left"/>
</tr>
<tr>
<td align="left">Cor Total</td>
<td align="center">137.27</td>
<td align="center">28</td>
<td align="left"/>
<td align="left"/>
<td align="left"/>
<td align="left"/>
</tr>
</tbody>
</table>
</table-wrap>
<fig id="F4" position="float">
<label>FIGURE 4</label>
<caption>
<p>Comparison of experimental and predicted values.</p>
</caption>
<graphic xlink:href="fchem-10-882235-g004.tif"/>
</fig>
<p>Through the three-dimensional surface graph, the influence of the interaction of any two independent variables on biodiesel yield was studied, while the coding values of the other two parameters were kept at zero, as shown in <xref ref-type="fig" rid="F5">Figure&#x20;5</xref>. <xref ref-type="fig" rid="F5">Figure&#x20;5A</xref> displays the interaction effect of time and temperature, and catalyst dosage was 6 wt% and methanol to oleic acid molar ratio was 30:1. The extension of reaction time shows no significant impact on the yield of biodiesel in the lower reaction temperature range, which may be that the reaction requires a higher temperature to improve the reaction rate (<xref ref-type="bibr" rid="B46">Wang et&#x20;al., 2018</xref>). However, with the increase of the temperature, it can be found that higher biodiesel yield can be reached in a relatively short time, indicating that the influence of reaction temperature on biodiesel yield was more significant than the reaction time. This can also be observed from the higher F value (120.11) of the reaction temperature. <xref ref-type="fig" rid="F5">Figure&#x20;5B</xref> and <xref ref-type="fig" rid="F5">Figure&#x20;5C</xref> show the interaction of temperature with catalyst dosage and molar ratio, respectively. For low temperature and catalyst dosage, the biodiesel yield was low. However, when the amount of catalyst and temperature increased, the biodiesel yield gradually moved to the high-value region, and the effect of the catalyst dosage on the yield of biodiesel was greater than that of the reaction temperature. This is attributed to the addition of catalytic active sites that can promote the reaction more effectively and quickly, but a superfluous amount will inversely hinder the mass transfer between substances, slowing down the reaction rate ultimately (<xref ref-type="bibr" rid="B7">Cao et&#x20;al., 2021</xref>). Similarly, <xref ref-type="fig" rid="F5">Figure&#x20;5C</xref> shows that the biodiesel yield increases with the increase of reaction temperature and the molar ratio of methanol to oleic acid, but when the molar ratio increases to a certain extent, the biodiesel yield takes on a downtrend. This was because the increase of methanol dosage is conducive to the positive occurrence of the reaction, thus resulting in the improvement of product yield. However, excessive methanol will dilute the concentration of the reaction system, and the inverse reaction rate will be increased with the increase of the number of product esters, both of which are unfavorable for the formation of products (<xref ref-type="bibr" rid="B25">Lim et&#x20;al., 2020</xref>). In addition, compared with the molar ratio, the influence of temperature on the yield of biodiesel was more significant.</p>
<fig id="F5" position="float">
<label>FIGURE 5</label>
<caption>
<p>Surface response plots of different experimental variables against biodiesel yield. <bold>(A)</bold> Temperature and time, <bold>(B)</bold> time and catalyst dosage, <bold>(C)</bold> methanol to oleic acid molar ratio and temperature, <bold>(D)</bold> catalyst dosage and methanol to oleic acid molar ratio, <bold>(E)</bold> catalyst dosage and time, and <bold>(F)</bold> methanol to oleic acid molar ratio and time.</p>
</caption>
<graphic xlink:href="fchem-10-882235-g005.tif"/>
</fig>
<p>The conjugate effect of the catalyst amount and methanol/OA molar ratio on biodiesel yield was depicted in <xref ref-type="fig" rid="F5">Figure&#x20;5D</xref>. The reaction temperature and time were kept at 90&#xb0;C and 4&#xa0;h, respectively. It can be seen from <xref ref-type="fig" rid="F5">Figure&#x20;5D</xref> that the yield of biodiesel was constantly increased with the molar ratio of methanol to oleic acid and catalyst dosage increased. In the esterification reaction, the low molar ratio of methanol to oleic acid is adopted, the increase of catalyst dosage demonstrated a weak effect on the improvement of biodiesel yield. This should be due to the fact that the reversibility of the esterification reaction usually requires a higher methanol/OA molar ratio. However, an excessive amount of methanol will dilute the concentration of catalyst in the whole system, resulting in a decrease in the concentration of active center per unit volume (<xref ref-type="bibr" rid="B32">Nongbe et&#x20;al., 2017</xref>). <xref ref-type="fig" rid="F5">Figure&#x20;5E</xref> and <xref ref-type="fig" rid="F5">Figure&#x20;5F</xref> show the interaction of time with the catalyst amount and the molar ratio of methanol to OA, respectively. As mentioned earlier, the biodiesel yield continues to increase with the increment of time, the amount of catalyst, and the molar&#x20;ratio.</p>
<p>The experimental parameters were optimized according to the RSM to obtain the best biodiesel yield. The optimum conditions were obtained as follows: reaction temperature of 92&#xb0;C, reaction time of 4.1&#xa0;h, catalyst dosage of 6.8 wt%, and methanol to oleic acid molar ratio of 31.4/1. Correspondingly, the predicted biodiesel yield was as high as 98.4%. For the effectiveness of the model, the corresponding experiments were conducted under the predicted conditions, and the experimental biodiesel yield of 98.2% was very close to the predicted value, verifying that the model had certain reliability and effectiveness in predicting the biodiesel yield in this&#x20;study.</p>
</sec>
<sec id="s3-4">
<title>3.4 Reusability of Catalyst</title>
<p>Reusability is considered to be an important guideline to assess the performance of the catalyst, and efficient catalyst recycle can also reduce the biodiesel cost, to some extent (<xref ref-type="bibr" rid="B9">Dai et&#x20;al., 2021</xref>). Therefore, we conducted five consecutive reactions under the optimized experimental conditions to determine the reusability of the SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyst in oleic acid esterification reaction. The specific steps are as follows: after each reaction, the catalyst obtained through centrifugation was washed 3&#x20;times with anhydrous ethanol to remove the impurities attached to the catalyst, and then dried at 80&#xb0;C to remove the solvent. The dried catalyst was used for the next experiment. The tested results of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyst after five recycles are shown in <xref ref-type="fig" rid="F6">Figure&#x20;6A</xref>, wherein 87.0% biodiesel yield for the fifth recycle can still be reached, showing the relatively good reusability of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H. More importantly, <xref ref-type="fig" rid="F6">Figure&#x20;6B</xref> shows the infrared spectra of the catalysts before and after use, whereas the important absorption peak of the sulfonic acid -SO<sub>3</sub>H group still exists in the reused catalyst. Nonetheless, the intensity of the absorption peak is relatively weak compared with the fresh catalyst, indicating that part of -SO<sub>3</sub>H was lost in the recycling process, which may be the reason for the decrease of biodiesel&#x20;yield.</p>
<fig id="F6" position="float">
<label>FIGURE 6</label>
<caption>
<p>
<bold>(A)</bold> Reusability of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyst, <bold>(B)</bold> FI-IR spectra before and after repeated use of SiO<sub>2</sub>@Cs-SO<sub>3</sub>H (Methanol to oleic acid molar ratio of 31.4/1, 6.8 wt% and 92&#xb0;C for 4.1&#xa0;h).</p>
</caption>
<graphic xlink:href="fchem-10-882235-g006.tif"/>
</fig>
</sec>
</sec>
<sec id="s4">
<title>4 Conclusion</title>
<p>Sulfonated chitosan-derived SiO<sub>2</sub>@Cs-SO<sub>3</sub>H solid acid catalyst was synthesized by an easy method at room temperature. The SiO<sub>2</sub>@Cs-SO<sub>3</sub>H catalyst exhibited a high acidity (3.47&#xa0;mmol/g) and large specific surface area (21.82&#xa0;m<sup>2</sup>/g), showing high catalytic activity for the esterification of oleic acid and methanol. The reaction temperature, reaction time, catalyst amount, and molar ratio of methanol to oleic acid were optimized and analyzed by RSM to obtain the best biodiesel yield. Accordingly, the ideal reaction conditions were determined: 92&#xb0;C, 4.1&#xa0;h, catalyst dosage of 6.8 wt%, and methanol to oleic acid molar ratio of 31.4/1. In this case, the maximum biodiesel yield (98.2%) was obtained. In addition, the catalyst also had good reusability, and the biodiesel yield of 87.0% can still be reached after repeated use of 5 times. In conclusion, the SiO<sub>2</sub>@Cs-SO<sub>3</sub>H solid acid catalyst prepared from Cs biomass had excellent catalytic performance for the esterification of FFAs, which shows a good application prospect in the biodiesel production&#x20;field.</p>
</sec>
</body>
<back>
<sec id="s5">
<title>Data Availability Statement</title>
<p>The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.</p>
</sec>
<sec id="s6">
<title>Author Contributions</title>
<p>Writing&#x2014;original draft preparation, SZ. Revising&#x2014;manuscript, HP. Figure drawing, JH. Data curation, YL. Writing&#x2014;reviewing, supervision, HZ. All authors have read and agreed to the published version of the manuscript.</p>
</sec>
<sec id="s7">
<title>Funding</title>
<p>This work was supported by the National Natural Science Foundation of China (21908033), Fok Ying-Tong Education Foundation (161030), Natural Science Special Foundation of Guizhou University ((2021)16 Special Post B), and GZU (Guizhou University) Found for Cultivation ((2020)73).</p>
</sec>
<sec sec-type="COI-statement" id="s8">
<title>Conflict of Interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec sec-type="disclaimer" id="s9">
<title>Publisher&#x2019;s Note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
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