Preparation of ILe@Cu@MOF Catalyst and Its Application in Biodiesel Catalysis

Round 1

Reviewer 1 Report

why the authors used this catalyst. please compare it with similar catalyst. 

Is the catalyst, recylable. If ok, explore it. 

explaine the best optimizaed condition for production of biodesel

Author Response

Dear editor,

Thank you for your message. The manuscript was revised according to your advice. I made some additional changes and marked those changes with red color in revised manuscript. I am looking forward to your reply.

 

 

With best wishes,

 

Yinan Hao Ph.D.

College of Materials Science and Art Design

Inner Mongolia Agricultural University

Hohhot, 010018

P.R. China

 

Major revision taking into account the response point by point for the following:

Reviewer #1:

1. Why the authors used this catalyst. please compare it with similar catalyst. Is the catalyst, recylable. If ok, explore it, explaine the best optimizaed condition for production of biodesel

 

Thanks a lot for your advice. Based on your advice, the specific explanation is as follows：

MOFs are ideal catalysts for biodiesel preparation due to their high specific surface area, large porosity, adjustable pore size and pore volume [1,2]. However, most MOFs exhibit poor hydrothermal stability, and their catalytic activity and reusability are often reduced due to hydrolysis in the process of preparing biodiesel, resulting in higher costs, which limits their application [3].

In the process of producing biodiesel, water will appear in both the raw materials and by-products, which is detrimental to the catalyst. The traditional treatment method is mostly to introduce hydrophobic molecules into the MOFs framework, but the synthesis process which interferes with the crystallization of MOFs is complex, and some hydrophobic molecules are toxic and expensive. Amino acids hinder the attack of water molecules on metal active site of and prevent the weakening of metal and linker coordination bond which finally helps in enhancement of hydrothermal stability of Cu @MOF. Graft hydrophobic polymer Isoleucine on Cu@MOF to prepare ILe@Cu@MOF to improve the hydrothermal stability of MOFs. In this paper, various characterization techniques such as X-Ray Diffraction (XRD), Fourier-Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), contact angle test, Brunauer Emmett Teller (BET) indicated successful modification of Cu@MOF with Isoleucine. Thermogravimetric analysis (TGA), contact angle and SEM results after hydrothermal conditioning showed enhanced water stability for glycine, lysine and tyrosine functionalized Cu@MOF compared to pristine MOF. The graft of isoleucine on the surface of Cu@MOF made the material change from a hydrophilic material to a hydrophobic material, and the hydrothermal stability of Cu@MOF was greatly improved.

As shown in Figure 1，the production of biodiesel from Xanthoceras sorbifolia bunge oil by transesterification was studied using ILe@Cu@MOF as acid catalyst, the optimal esterification conditions are as follows: the amount of catalyst was 3 wt%, the molar ratio of methanol to oil was 35 : 1, the reaction temperature was 50 °C, and the reaction time was 4h.At this time, the biodiesel yield was up to 82.85 %. Moreover, after five cycles of ILe@Cu@MOF, the yield still reached 73.4 %.

 

Figure 1. (a) Effect of ILe addition on the yield of biodiesel, (b) Effect of the amount of catalyst on yield of biodiesel, (c) Effect of molar ratio of methanol to oil on yield of biodiesel, (d) Effect of reaction temperature on yield of biodiesel, (e) Effect of reaction time on yield of biodiesel, (f) Biodiesel yield using the regenerated catalyst for various cycles.

 

 

Reference

[1] Rui, S.; Li, L.; Yue, S.; Hao Y.; Jia, Shie. Catalysts from renewable resources for biodiesel production. Energy Conversion and Management, 2018, 178, 277–289.

[2] Niu, S.; Ning, Y.; Lu, C.; Han, K.; Yu, H.; Zhou,Y. Esterification of oleic acid to produce biodiesel catalyzed by sulfonated activated carbon from bamboo. Energy Convers Manage 2018, 163, 59–65.

[3] Unza, J.; Asif, Husain, K.; Rabia, L. et.al. Copper and calcium-based metal organic framework (MOF) catalyst for biodiesel production from waste cooking oil: A process optimization study. Energy Conversion and Management. 2020, 215, 112934.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Dear Authors:

This paper, presented a very interesting report for Preparation of ILe@Cu@MOF catalyst and its application in biodiesel catalysis. The overall structure and writing indeed require a significant improvement. I really would like to see this article in Coatings after minor revisions.

 

1.      It is better to provide some information about biodiesel catalysis in the introduction.

2.      How do you ensure that the catalyst synthesis reaction is finished and the catalyst is synthesized? Do you use TLC?

3.      The content you presented in the conclusion section is related to the results and discussion section. Correct this section.

Best Regards

Author Response

Dear editor,

Thank you for your message. The manuscript was revised according to your advice. I made some additional changes and marked those changes with red color in revised manuscript. I am looking forward to your reply.

 

 

With best wishes,

 

Yinan Hao Ph.D.

College of Materials Science and Art Design

Inner Mongolia Agricultural University

Hohhot, 010018

P.R. China

 

Major revision taking into account the response point by point for the following:

Reviewer #2:

1. It is better to provide some information about biodiesel catalysis in the introduction.

 

Thanks a lot for your advice. Based on your advice, the introduction had been revised in revised manuscript.

As the global economy and industry continue to grow, coupled with rapid population urbanization, people rely heavily on energy from non-renewable resources such as fossil fuels. [1-3] Environmental pollution and climate change caused by the production and use of fossil fuels have prompted people to look for alternative fuels for oil [4-8]. Based on this, biodiesel is recognized as a promising liquid diesel biofuel due to its renewability and environmental friendliness. As a substitute for petroleum fuel oil, biodiesel has the advantages of non-toxicity and biodegradability. Therefore, a biodiesel made from renewable resources is needed [9-14].

At present, the production of biodiesel mainly adopts transesterification method, which mainly includes acid catalysis and alkali catalysis. The acid catalytic reaction can ignore the influence of free fatty acids, which is more feasible and simple in the process flow. However, there are some problems such as long reaction time, difficult product separation and serious environmental pollution [15-18]. In contrast, due to the fast rate of alkali-catalyzed reaction, it is widely used in actual industrial production. The alkali catalytic method can be divided into homogeneous alkali catalytic method and heterogeneous solid alkali catalytic method. Catalysts commonly used in homogeneous base catalysis include: NaOH, sodium methoxide, etc., which have the advantages of fast reaction speed and high conversion rate. However, homogeneous base catalysts are difficult to recycle, complex post-treatment, and will cause secondary pollution to the environment [19]. In order to solve these problems, solid base catalysts have emerged as the times require. Its reaction conditions are simple, the catalyst life is long, continuous and automated production can be achieved, and environmental pollution can be effectively avoided. The commonly used types of alkali catalysts are: alkali metals, hydrotalcites, alkaline earth metal oxides, hydrotalcite-like solid bases, supported solid bases, etc. At present, many kinds of solid bases have been applied to the preparation of biodiesel under laboratory conditions, which has become a new hot spot in the preparation of biodiesel [20-22].

To improve the biodiesel yield and ensuring sustainable process, various biodiesel catalyst systems such as homogenous and heterogeneous catalysts have been practiced [23,24]. The transesterification for biodiesel reaction requires a catalyst, such as an alkaline catalyst an acid catalyst. Reports indicate that the reaction with the alkaline catalyst is faster than the reaction with the acid catalyst, and that acid catalytic reactions require a high molar ratio of alcohol to oil to perform the reaction with good efficiency at a reasonable time. Alkaline metals carbonates, alkaline earth metal carbonates, alkaline earth metal oxides, Transition metal oxides and a mixture of metal oxides, have been used as alkaline heterogeneous catalysts in the transesterification method [25]. To date, Homogenous catalysts such as KOH, NaOH or CH₃ONa are relatively moderate in the activity due to soap formation during the purification process which contributes to the inability to recover the glycerol [26]. For raw materials with high FFA content, strong acidic catalysts are preferable because of their ability to inhibit soap formation which results in high biodiesel yield. A heterogeneous catalyst is a more suitable option for easy separation and reusability with better catalytic activity and stability being reported [27]. Various heterogeneous catalyst systems have been employed i.e alkali doped materials, transition metal oxides, hydrotalcite and silica based materials. Heterogeneous catalysts offer several advantages, including their reusability, higher reaction rate and selectivity, simplification of the crude biodiesel purification, eliminating the formation of soap, short process time, relatively lower operating temperature recyclability, eco-friendliness and low cost. Metal organic frameworks (MOFs) are the emerging heterogeneous materials with a moderate surface area and more sites for the catalytic application, making them a promising alternative for stable biodiesel production.

Metal-organic frameworks (MOFs) are emerging heterogeneous materials, which have been widely used in energy storage, catalytic activity, electrochemical magnetism and other scientific research fields [28]. MOFs are porous solids assembled by coordination bonds between metal ions and organic ligands [29]. MOFs have a moderate specific surface area and more catalytic application sites, making them a promising alternative for the stable production of biodiesel [30]. As a heterogeneous catalyst, copper-based MOF can efficiently oxidize alcohols and convert them into corresponding products, which can be used as a promising catalyst for strengthening and stabilizing biodiesel production [31]. The synthesis of ILe@Cu@MOF catalyst with general precursor as isoleucine is beneficial to the processability and optimization of the transesterification process [32].

In this study, the synthesis of structured catalyst (ILe@Cu@MOF) catalyzed the preparation of biodiesel from Xanthoceras sorbifolia bunge oil was introduced. The structure and related properties of the synthetic catalyst were studied. The specific process of catalytic biodiesel was further studied. The prepared biodiesel has good quality and provides a new way for biomass energy in the future.

 

Reference

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[2] Gielen, D.; Boshell, F.; Saygin, D.; Bazilian, MD.; Wagner, N.; Gorini, R. The role of renewable energy in the global energy transformation. Energy Strategy Reviews 2019, 24, 38-50.

[3] Sinha, SK.; Subramanian, K.; Singh, HM.; Tyagi, V.; Mishra, A. Progressive trends in biofuel policies in India: targets and implementation strategy. Biofuels 2019, 10, 155-166.

[4] Pereira, LG.; Cavalett, O.; Bonomi, A.; Zhang, Y.; Warner, E.; Chum, HL. Comparison of biofuel life-cycle GHG emissions assessment tools: the case studies of ethanol produced from sugarcane, corn, and wheat. Renew Sustain Energy Rev 2019, 110, 1–12.

[5] Aghbashlo, M.; Tabatabaei, M.; Hosseinpour, S. On the exergoeconomic and exergoenvironmental evaluation and optimization of biodiesel synthesis from waste cooking oil (WCO) using a low power, high frequency ultrasonic reactor. Energy Convers Manage 2018, 164, 385-398.

[6] Sivakumar, P.; Anbarasu, K.; Renganathan, S. Bio-diesel production by alkali catalyzed transesterification of dairy waste scum. Fuel 2011, 90, 147-151.

[7] Sohail, M.; Yun, YN.; Lee, E.; Kim, SK.; Cho, K.; Kim, JN. Synthesis of Highly Crystalline NH₂-MIL-125 (Ti) with S-Shaped Water Isotherms for Adsorption Heat Transformation. Cryst Growth Des 2017, 17, 1208–1213.

[8] Chen, L.; Yin, P.; Liu, X.; Yang, L.; Yu, Z.; Guo, X. Biodiesel production over copper vanadium phosphate. Energy 2011, 36, 175–180.

[9] Danmaliki, GI.; Saleh, TA.; Shamsuddeen, AA. Response surface methodology optimization of adsorptive desulfurization on nickel/activated carbon. Chem Eng J 2017, 313, 993-1003.

[10] Wang, S.; Ye, B.; An, C.; Wang, J.; Li, Q. Synergistic effects between Cu metal–organic framework (Cu-MOF) and carbon nanomaterials for the catalyzation of the thermal decomposition of ammonium perchlorate (AP). J Mater Sci 2018, 54, 4928-4941.

[11] Khoja, AH.; Tahir, M.; Saidina, Amin, NA. Evaluating the Performance of a Ni Catalyst Supported on La₂O₃-MgAl₂O₄ for Dry Reforming of Methane in a Packed Bed Dielectric Barrier Discharge Plasma Reactor. Energy Fuels 2019, 33, 11630–11647.

[12] Bayat, A.; Baghdadi, M.; Bidhendi, GN. Tailored magnetic nano-alumina as an efficient catalyst for transesterification of waste cooking oil: Optimization of biodiesel production using response surface methodology. Energy Convers Manage 2018, 177, 395–405.

[13] Khan, IA.; Badshah, A.; Nadeem, MA.; Haider, N.; Nadeem, MA. A copper based metalorganic framework as single source for the synthesis of electrode materials for highperformance supercapacitors and glucose sensing applications. Int J Hydrogen Energy 2014, 39, 19609–19620.

[14] Drenth, AC.; Olsen, DB.; Denef, K. Fuel property quantification of triglyceride blends with an emphasis on industrial oilseeds camelina, carinata, and pennycress. Fuel 2015, 153, 19–30.

[15] Menon, SS.; Chandran, SV.; Koyappayil, A.; Berchmans, S. Copper- Based Metal-Organic Frameworks as Peroxidase Mimics Leading to Sensitive H₂O₂ and Glucose Detection. Chemistryselect 2018, 3, 8319–8324.

[16] Yang, J.; Zhao, F.; Zeng, B. One-step synthesis of a copper-based metal–organic framework–graphene nanocomposite with enhanced electrocatalytic activity. RSC Adv 2015, 5, 22060–22065.

[17] Zhu, H.; Wu, Z.; Chen, Y.; Zhang, P.; Duan, S.; Liu, X. Preparation of Biodiesel Catalyzed by Solid Super Base of Calcium Oxide and Its Refining Process. Chin J Catal 2006, 27, 391–396.

[18] Devaraj, K.; Veerasamy, M.; Aathika, S.; Mani, Y.; Thanarasu, A.; Dhanasekaran, A. Study on effectiveness of activated calcium oxide in pilot plant biodiesel production. J Cleaner Prod 2019, 225, 18–26.

[19] Karmakar, R.; Kundu, K.; Rajor, A. Fuel properties and emission characteristics of biodiesel produced from unused algae grown in India. Pet Sci 2018,15, 385–395.

[20] Al-Jammal, N.; Al-Hamamre, Z.; Alnaief, M. Manufacturing of zeolite based catalyst from zeolite tuft for biodiesel production from waste sunflower oil. Renewable Energy 2016, 93, 449–459.

[21] Rattanaphra, D.; Harvey, A.; Srinophakun P. Simultaneous conversion of triglyceride/free fatty acid mixtures into biodiesel using sulfated zirconia. Top. Catal. 2010, 53, 773–782.

[22] Jacobson, K.; Gopinath, R.; Meher, LC.; Dalai, AK. Solid acid catalyzed biodiesel production from waste cooking oil. Appl Catal B 2008, 85, 86–91.

[23] Tang, ZE.; Lim, S.; Pang, YL.; Ong, HC.; Lee, KT. Synthesis of biomass as heterogeneous catalyst for application in biodiesel production: state of the art and fundamental review. Renew Sust Energ Rev 2018, 92, 235–253.

[24] Rui, S.; Li, L.; Yue, S.; Hao Y.; Jia, Shie. Catalysts from renewable resources for biodiesel production. Energy Conversion and Management, 2018, 178, 277–289.

[25] Sivakumar, P.; Anbarasu, K.; Renganathan, S. Bio-diesel production by alkali catalyzed transesterification of dairy waste scum. Fuel 2011, 90, 147–151.

[26] Chouhan, APS,; Sarma, AK. Modern heterogeneous catalysts for biodiesel production: A comprehensive review. Renew Sustain Energy Rev 2011, 15, 4378–4399.

[27] Perego, C.; Bosetti, A. Biomass to fuels: The role of zeolite and mesoporous materials. Microporous Mesoporous Mater 2011, 144, 28–39.

[28] Chouhan, AS.; Sarma, AK. Modern heterogeneous catalysts for biodiesel production: A comprehensive review. Renewable and Sustainable Energy Reviews 2011, 15, 4378–4399.

[29] Le, Zhong.; Yuxiao, Feng.; Hongtong, Hu. Enhanced enzymatic performance of immobilized lipase on metal organic frameworks with superhydrophobic coating for biodiesel production[J]. Journal of colloid and interface science 2021, 602, 426-436.

[30] Valentino, Bervia.; Lunardi, Fransiska.; Gunawan, Felycia.; Edi, Soetaredjo. Efficient One-Step Conversion of a Low-Grade Vegetable Oil to Biodiesel over a Zinc Carboxylate Metal–Organic Framework ACS Omega 2021, 6, 1834-1845.

[31] Dhawane, SH.; Kumar, T.; Halder, G. Process optimisation and parametric effects on synthesis of lipase immobilised carbonaceous catalyst for conversion of rubber seed oil to biodiesel. Energy Convers Manage 2018, 176, 55–68.

[32] Balajii, M.; Niju, S. A novel biobased heterogeneous catalyst derived from Musa acuminata peduncle for biodiesel production–Process optimization using central composite design. Energy Convers Manage 2019, 189, 118–131.

 

 

2. How do you ensure that the catalyst synthesis reaction is finished and the catalyst is synthesized? Do you use TLC?

 

Thanks a lot for your advice. Based on your advice, the part had been revised in revised manuscript.

Biodiesel could be potentially manufactured by a transesterifification reaction using vegetable oils, animal fat cooking oils or algae oils[1,2]. The transesterification requires Cu@MOF catalyst process a triglyceride (fat/oil) reacts with methanol.

The transesterification reaction is represented as:

 

 

 

Catalyst

                                                                                                        R₁COOCH₃

                    + 3 (CH₃OH)                              +  R₂COOCH₃          

                                                            R₃COOCH₃

 

           Oil                  Methanol                Glycerol        Biodiesel

 

The products of the transesterification reaction are brown glycerol in the lower layer and light yellow biodiesel in the upper layer, transesterification is observed if no stratification occurs. The Cu@MOF catalyst was performed by co-precipitation method. The process continues by stirring with 100 mL of 10 % of N, N-dimethylformamide (DMF) solution and 100 mL of 10 % CuSO₄ solution in a round-bottom flask at 85 °C for 24 h. Afterwards, the mixture was filtrated and washed by deionized water repeatedly. Finally, the sample Cu@MOF catalyst was dried at 110 °C for 7 h. The preparation flow chart is shown in Figure 1a.

A certain mass fraction（15 , 25, 30 , 45 , 55 ) wt% of  L-isoleucine and 2g of Cu@MOF were added into the three-port flask with 100 mL ethanol stirred at 50 °C for 2 h until dissolved.Afterwards, the mixture was filtrated and washed by 100mL of anhydrous ethanol. Finally, the sample ILe@Cu@MOF catalyst was dried at 110°C for 7 h. The preparation flow chart is shown in Figure 1b.

In this paper, Fatty acid methyl esters of biodiesel were analysed by Gas Chromatography (GC) coupled with Mass Spectroscopy (MS) instead of Thin layer chromatography (TLC).

Figure 1. (a) The synthesis process of Cu@MOF catalyst, (b) The synthesis process of ILe@Cu@MOF catalyst, (c) The process and determination of biodiesel preparation from Xanthoceras sorbifolia bunge oil by catalyst.

 

 

3. The content you presented in the conclusion section is related to the results and discussion section. Correct this section.

 

Thanks a lot for your advice. Based on your advice, the part had been revised in revised manuscript.

 

Although various plant resources have been used to produce biofuels, due to the huge demand for oil resources around the world, it is more necessary to use renewable plant resources to develop diesel fuel. In this work, an eco-friendly transesterification biodiesel production process using Xanthoceras sorbifolia bunge oil was developed. The new synthesized ILe@Cu@MOF in this work showed superior performance as compared to the other heterogeneous catalysts. SEM images exhibited many flaky forms of ILe are attached to the surface of Cu@MOF which is successfully functionalized with ILe. EDS depicts the content of N element from ILe increased from 0.4 wt % to 3.6 wt %. FTIR and Raman both confirmed that Cu@MOF was successfully functionalized by amino acids espicially additional peak at 3247 cm^-1 due to the stretching vibration mode in the N-H group. TG depicted the temperature of ILe@Cu@MOF should not exceed 360℃, otherwise it will lead to the collapse of Cu@ MOF structure.XRD results indicated the formation ILe@Cu@MOF with the main diffraction peak intensity of Cu@MOF decreased, and some diffraction angles also shifted respectively. The specific surface area and pore size of ILe@Cu@MOF with 19.687 m²/g and 31.74 nm respectively. The water contact angle of Cu@MOF after grafting the ILe increased from 86° to 121°, the hydrophobicity was improved obviously.According to the results obtained from experimental design, the maximum yield of biodiesel of 82.85% was achieved at the catalyst weight percentage of 3%, methanol/oil molar ratio of 35:1, reaction temperature at 50 ℃, and reaction time of 4h. The yield of biodiesel still reached 73.40 % after the catalyst be reused five cycles. ¹³C NMR and ¹H NMR spectroscopy affirmed the transformation of the Xanthoceras sorbifolia bunge oil to excellent quality and purity biodiesel. GC-MS confirmed the biodiesel was composed of five fatty acid methyl esters.

 

Author Response File: Author Response.docx

Reviewer 3 Report

Dear Authors,

I have read your article and I would like to recommend your work for publication. However, a minor revision is necessary, e.g. the quality of selected figures is not good enough. In details, the font size shown in Figures 2, 5 and 8 should be increased significantly. Also, a list of abbreviations (nomenclature) used in the work should be included at the end of your work. This will make it easier to understand abbreviations presented e.g. in the abstract (line 15). I have no other comments about your work. Its layout is typical for research work. It presents a literature analysis, methods and materials used, research results and conclusions related to the purpose of the work. The overall quality is good enough and for this reason I would like to recommend this paper for publication.

Kind regards, Reviewer

Author Response

Dear editor,

Thank you for your message. The manuscript was revised according to your advice. I made some additional changes and marked those changes with red color in revised manuscript. I am looking forward to your reply.

 

 

With best wishes,

 

Yinan Hao Ph.D.

College of Materials Science and Art Design

Inner Mongolia Agricultural University

Hohhot, 010018

P.R. China

 

Major revision taking into account the response point by point for the following:

Reviewer #3:

1. I have read your article and I would like to recommend your work for publication. However, a minor revision is necessary, e.g. the quality of selected figures is not good enough. In details, the font size shown in Figures 2, 5 and 8 should be increased significantly. Also, a list of abbreviations (nomenclature) used in the work should be included at the end of your work. This will make it easier to understand abbreviations presented e.g. in the abstract (line 15). I have no other comments about your work. Its layout is typical for research work. It presents a literature analysis, methods and materials used, research results and conclusions related to the purpose of the work. The overall quality is good enough and for this reason I would like to recommend this paper for publication.

 

Thanks a lot for your advice. Based on your advice, the problems had been revised in revised manuscript.

Heterogeneous catalysts play a dual role in transesterification due to their advantages of separable and reusable. In recent years, heterogeneous catalysts derived from renewable resources have received more attention. In this paper, the production of biodiesel from Xanthoceras sorbifolia bunge oil by transesterification was studied under the action of ILe@Cu@MOF catalyst. Fourier-Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy, scanning electron microscope (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), X-ray diffraction (XRD), Brunauer Emmett Teller (BET), Thermogravimetric (TG) and other characterization methods were used to characterize the microstructure and thermal stability of the catalyst, and further study the ILe@Cu@MOF catalyst for the preparation of biodiesel from Xanthoceras sorbifolia bunge oil. The results show that the surface of ILe@Cu@MOF catalyst is attached with a sheet-like structure of isoleucine (ILe), which mainly contains Cu, O, C and N elements. The specific surface area is 19.687 m²/g, and the average pore size is 31.74 nm, which belongs to mesoporous material. The pyrolysis temperature of ILe@Cu@MOF reached 360 °C, indicating that the grafting of ILe had a protective effect on Cu@MOF and increased the pyrolysis temperature of Cu@MOF. At the same time, the water contact angle increased from 86° to 121°, and the material was hydrophobic. The optimum conditions for the preparation of biodiesel were as follows : the amount of catalyst was 3 wt%, the molar ratio of methanol to oil was 35 : 1, the reaction temperature was 50 °C, and the reaction time was 4h.At this time, the biodiesel yield was up to 82.85 %. Moreover, after five cycles of ILe@Cu@MOF, the yield still reached 73.4 %. GC-MS and MNR studies showed that the quality of biodiesel after catalysis was higher. The prepared catalyst can make biodiesel products more sustainable, environmentally friendly and economical, and provide future prospects for the energy utilization of renewable resources.

Author Response File: Author Response.docx

Reviewer 4 Report

1. Synthesis of Cu@MOF described unclear. "Firstly, 100 mL of N, N-dimethylformamide (DMF) solution with mass concentration of 10 % and 100 mL of copper sulfate (CuSO4) solution with mass concentration of 10 % were prepared". Solution of what in DMF?

2. 2.3.Preparation of biodiesel: what alcohol was used in experiments? 

3. Please provide specific surface area and isoterms for Cu@MOF.

4. Please compare experimental results with Cu@MOF.

5. NMR spectra are discussed very poorly. Please assign peaks with actual groups in atoms in biodiesel and triglycerides molecules.

6. Please discuss the role of isoleycine in reaction mechanism.

Extensive editing is needed. Examples: "Esterification transesterification is an endothermic process...", "This is because increasing methanol promotes...", "the temperature makes methanol in a gasification state..." and so on.

Author Response

Dear editor,

Thank you for your message. The manuscript was revised according to your advice. I made some additional changes and marked those changes with red color in revised manuscript. I am looking forward to your reply.

 

 

With best wishes,

 

Yinan Hao Ph.D.

College of Materials Science and Art Design

Inner Mongolia Agricultural University

Hohhot, 010018

P.R. China

 

Major revision taking into account the response point by point for the following:

Reviewer #4:

1. Synthesis of Cu@MOF described unclear. "Firstly, 100 mL of N, N-dimethylformamide (DMF) solution with mass concentration of 10 % and 100 mL of copper sulfate (CuSO4) solution with mass concentration of 10 % were prepared". Solution of what in DMF?

 

Thanks a lot for your advice. Based on your advice, the part had been revised in revised manuscript.

 

The synthesis of the Cu@MOF was performed by using a modified co-precipitation method as shown in Figure 1a. 10 % concentration of copper sulfate (CuSO₄) and 10 % concentration of phthalic acid (H₂BDC) were dispersed in 100 mLwater and 100 mL DMF respectively.

 

 

2. 2.3. Preparation of biodiesel: what alcohol was used in experiments? 

 

Thanks a lot for your advice. A detailed explanation is as follows:

 

During the transesterifification reaction, Xanthoceras sorbifolia bunge oil can be reacted with methanol under specifific reaction conditions, to form fatty acid esters (biodiesel) in the presence of ILe@Cu@MOF catalyst.

The transesterification reaction is represented as:

 

 

 

Catalyst

                                                                                                        R₁COOCH₃

                    + 3 (CH₃OH)                              +  R₂COOCH₃          

                                                            R₃COOCH₃

 

           Oil                  Methanol                Glycerol        Biodiesel

 

 

3. Please provide specific surface area and isoterms for Cu@MOF.

 

Thanks a lot for your advice. Based on your advice, the part had been has been provided.

 

The specific surface area and pore size of Cu@MOF were analyzed by analyzing the N₂ adsorption-desorption isotherms. As shown in Figure.1, there is no obvious inflection point in the downward concave of the isotherm. This is because the interaction between Cu@MOF is stronger than the interaction between ILe@Cu@MOF and N₂, which leads to the difficulty of Cu@MOF adsorbing N₂ in the initial stage of adsorption. After that, the relative pressure increases and the adsorption capacity rises rapidly. Therefore, it can be judged that the curve is a type III isotherm. After amino acid modification, the specific surface area of Cu@MOF increased to 5.07 m²/g, and the average pore size was 23.64 nm. Therefore, Cu@MOF is a mesoporous material.

 

Figure 1. N2-physisorption isotherms and pore size distribution of Cu@MOF

 

 

4. Please compare experimental results with Cu@MOF.

 

Thanks a lot for your advice.

As shown in Fig.2, the maximum yield of biodiesel of 82.85% was achieved at ILe@Cu@MOF catalyst weight percentage of 3%, methanol/oil molar ratio of 35:1, reaction temperature at 50℃, and reaction time of 4h. The yield of biodiesel still reached 73.40% after the catalyst be reused five cycles.

As shown in Fig.3 and Fig.4 , the maximum yield of biodiesel of 72.36% was achieved at Cu@MOF catalyst weight percentage of 1%, methanol/oil molar ratio of 20:1, reaction temperature at 40 ℃, and reaction time of 2 h. The yield of biodiesel reached 65.68% after the catalyst be reused five cycles.

 

Figure 2. (a) Effect of ILe addition on the yield of biodiesel, (b) Effect of the amount of catalyst on yield of biodiesel, (c) Effect of molar ratio of methanol to oil on yield of biodiesel, (d) Effect of reaction temperature on yield of biodiesel, (e) Effect of reaction time on yield of biodiesel, (f) Biodiesel yield using the regenerated catalyst for various cycles.

 

Figure 3. (a) Effect of the amount of catalyst on yield of biodiesel, (b) Effect of molar ratio of methanol to oil on yield of biodiesel, (c) Effect of reaction time on yield of biodiesel, (d) Effect of reaction temperature on yield of biodiesel.

 

Figure 4. Biodiesel yield using the regenerated catalyst for various cycles.

 

 

 

 

 

5. NMR spectra are discussed very poorly. Please assign peaks with actual groups in atoms in biodiesel and triglycerides molecules.

 

Thanks a lot for your advice. Based on your advice, the part had been revised in revised manuscript.

 

By comparing and analyzing Figure 8b and d, it can be found that in Figure 8d, two double  peaks of (-CH₂) at 4.32 ppm and（-CH₂CH₃）at 4.15 ppm related to Xanthoceras sorbifolia bunge oil molecules appear, which are almost disappeared in the map of biodiesel in Figure 8b, and a single peak of the methoxy protons as a singlet （-OCH₃）is added at 3.61 ppm assure the conversion of Xanthoceras sorbifolia bunge oil into biodiesel. The peak of Xanthoceras sorbifolia bunge oil at 0.90-2.80 ppm is shifted to 0.8-2.72 ppm in diesel, which proves that Xanthoceras sorbifolia bunge oil can be converted into biodiesel. Comparing figure 8a and c, it can be found that in the Xanthoceras sorbifolia bunge oil map, the single peak of (-CH₃) at -82.15 ppm becomes a multiple peak of（-OCH₃）at -85.96 ppm in the biodiesel map; the multiple peaks at -66.46 to -73.65 ppm were shifted to -67.73 to -77.16 ppm in biodiesel [1,2]. A single peak of −46.00 ppm was added to biodiesel; the peaks of -18.84 to -19.48 ppm in Xanthoceras sorbifolia bunge oil moved to -19.91 to -20.55 ppm, and 31.65-33.95 ppm moved to 31.67 ppm. In addition, there are almost no miscellaneous peaks in the carbon spectrum of biodiesel, indicating that the purity is high and the reaction is thorough [3].

 

 

Reference

[1] Chowdhury, Z.Z., Hamid, S.B.A., Das, R., Hasan, M.R., Zain, S.M., Khalid, K., Uddin, M. N. Preparation of carbonaceous adsorbents from lignocellulosic biomass and their use in removal of contaminants from aqueous solution. BioResources 2013, 8, 6523–6555.

[2] Dehghani, S., Haghighi, M. Sono-enhanced dispersion of CaO over Zr-Doped MCM-41 bifunctional nanocatalyst with various Si/Zr ratios for conversion of waste cooking oil to biodiesel. Renew. Energy 2020,153, 801–812.

[3 ] di Bitonto, L., Reynel-Avila, ´ H.E., Mendoza-Castillo, D.I., Bonilla-Petriciolet, A., Dur´ anValle, C.J., Pastore, C. Synthesis and characterization of nanostructured calcium oxides supported onto biochar and their application as catalysts for biodiesel production. Renew. Energy 2020, 160, 52–66.

 

 

6. Please discuss the role of isoleycine in reaction mechanism.

 

Thanks a lot for your advice. Based on your advice, the part had been revised in revised manuscript.

 

The Xanthoceras sorbifolia bunge oil contains about 6% water, water will cause the deactivation of the Cu@MOF catalyst and reduce the catalytic efficiency. The new synthesized ILe @Cu@MOF in this work showed superior performance especially hydrophobicity and pyrolysis temperature as compared to the other heterogeneous catalysts.The contact Angle of water increased from 86° to 121°, indicating that the ILe@Cu@MOF catalyst was hydrophobic. The pyrolysis temperature of ILe@Cu@MOF reached 360℃, indicating that grafting ILe had protective effect on Cu@MOF and increased the pyrolysis temperature of Cu@MOF. The maximum yield of biodiesel of 82.85% and 72.36% using ILe@Cu@MOF and Cu@MOF  respectively. Cu@MOF catalysts are porous solids with a well-structured assembly of coordinated bonds between metal ions and organic ligands. Cu@MOF catalysts have a moderate surface area and more sites for the catalytic application, making them a promising alternative for stable biodiesel production. .therefore,in this work, Cu@MOF component in ILe@Cu@MOF catalyst played a catalytic role for transforming Xanthoceras sorbifolia bunge oil into biodiesel.Cu@MOF efficiently oxidizes alcohols as heterogeneous catalysts, converting them into corresponding products which can be a promising catalyst for enhanced and stable biodiesel production. As a Lewis acid catalyst, the reaction mechanism of ILe@Cu@MOF is as follows：ILe@Cu@MOF provides unsaturated open sites, and methanol molecules coordinate with the central Cu atoms of ILe@Cu@MOF particles. Methanol is adsorbed on the Lewis base (B) of the catalyst to form oxygen anions. The positions of methanol and triglyceride are close to each other, so their interaction is easy to occur. The nucleophilic oxygen atom from the hydroxyl group of methanol then attacks the electrophilic carbon at the triacylglycerol ester group, resulting in transesterification to produce a tetrahedral intermediate. The hydroxyl group is then broken to form two esters, producing glycerol by-products. The generated alkoxy anion obtains the H atom, and the generated Lewis base continues to catalyze the next reaction [1,2].

 

Reference

[1] dos Santos, L.K., Hatanaka, R.R., de Oliveira, J.E., Flumignan, D.L. Production of biodiesel from crude palm oil by a sequential hydrolysis/esterification process using subcritical water. Renew. Energy 2019, 130, 633–640.

[2] Elkelawy, M., Alm-Eldin Bastawissi, H., Esmaeil, K.K., Radwan, A.M., Panchal, H., Sadasivuni, K.K., Ponnamma, D., Walvekar, R. Experimental studies on thebiodiesel production parameters optimization of sunflower and soybean oil mixture and DI engine combustion, performance, and emission analysis fueled with diesel/biodiesel blends. Fuel 2019, 255, 115791.

 

Author Response File: Author Response.docx

Round 2

Reviewer 4 Report

1. Please integrate answers on comments #2, 3 and 4 to the manuscript.

2. There's no way, that 1H NMR signals of methyl- and methylene groups are in the 4-4,5 ppm region. Please properly assign all major signals to groups in molecules.

Moderate editing is needed. Examples: "Esterification transesterification is an endothermic process...", "This is because increasing methanol promotes...", "the temperature makes methanol in a gasification state..." and so on.

Author Response

Dear editor,

Thank you for your message. The manuscript was revised according to your advice. I made some additional changes and marked those changes with red color in revised manuscript. I am looking forward to your reply.

 

 

With best wishes,

 

Yinan Hao Ph.D.

College of Materials Science and Art Design

Inner Mongolia Agricultural University

Hohhot, 010018

P.R. China

 

Minor revision taking into account the response point by point for the following:

Reviewer #4:

1. Please integrate answers on comments #2, 3 and 4 to the manuscript.

 

Thanks a lot for your advice. Based on your advice, comments # 2, 3 and 4 have been added in revised manuscript.

 

 

Preparation and determination of biodiesel

Ten grams of Xanthoceras sorbifolia bunge oil and ILe@Cu@MOF catalysts with mass ratios of 1wt %, 2wt %, 3wt %, 4wt % and 5wt % were added to three flasks, inserted into a condenser tube, and stirred in a water bath at a certain temperature. When the oil phase temperature reached a certain value (50 °C, 60 °C, 70 °C, 80 °C, 90 °C), a certain amount of methanol (methanol-oil molar ratio: 15 :1, 20 : 1,25 : 1,30 : 1,35 : 1) was added. After cooling and centrifugation, the catalyst was separated, and the liquid was poured into the separation funnel to stand for stratification. The upper liquid was heated to 80 °C, and the residual methanol was evaporated. Then anhydrous sodium sulfate was added to remove water and weighed. The yield of biodiesel was measured by weighing method. The test process is shown in Figure 1c.

 

Figure 1. (a) The synthesis process of Cu@MOF catalyst, (b) The synthesis process of ILe@Cu@MOF catalyst, (c) The process and determination of biodiesel preparation from Xanthoceras sorbifolia bunge oil by catalyst.

 

 

Effect of the production process parameters on the yield of biodiesel

Different amounts of ILe were grafted onto Cu@MOF to prepare ILe@Cu@MOF catalyst and used to catalyze the preparation of biodiesel. As shown in Figure 2a, with the increase of ILe addition, the biodiesel yield showed a trend of increasing first and then leveling off. When the amount of ILe added to 45 wt%, the biodiesel yield reached the maximum. At this time, the solubility of ILe in the solution reached saturation, and then increased the amount of ILe added, the biodiesel yield did not increase. Therefore, the optimum addition amount of ILe was 45 %. At this time, the biodiesel yield was 71.99 %. The effect of the amount of catalyst on the yield was investigated under the experimental conditions of molar ratio of alcohol to oil of 20 : 1, reaction temperature of 60 °C and reaction time of 2 h. As shown in Figure 2b, with the increase of the amount of ILe@Cu@MOF catalyst, the biodiesel yield showed a trend of increasing first and then decreasing. This is due to the increase of the catalyst, the reactive sites also increase. After the addition of 3 wt% catalyst, the amount of catalyst continues to increase, the reaction system becomes thicker, the mass transfer capacity is weakened, and the biodiesel yield is reduced. Therefore, the amount of ILe@Cu@MOF catalyst should be 3 wt%. Under the experimental conditions of ILe@Cu@MOF catalyst addition of 3 wt%, reaction temperature of 60 °C and reaction time of 2 h, the effect of molar ratio of alcohol to oil on biodiesel yield was analyzed. As shown in Figure.2c, with the increase of the molar ratio of alcohol to oil, the trend of biodiesel yield increased gradually and then tended to be gentle. This is because increasing methanol promotes the reaction in the direction of producing fatty acid methyl ester. When the molar ratio of methanol to oil is 30 : 1 to 35 : 1, the curve gradually flattens out. This is because the gradual increase of methanol will dilute the reaction solution and reduce the contact between the reactants. Therefore, the molar ratio of alcohol to oil was 35 : 1. The experimental conditions of ILe@Cu@MOF addition amount of 3 wt%, molar ratio of methanol to oil of 35 : 1 and reaction time of 2 h were selected to explore the effect of reaction temperature on biodiesel yield. As shown in Figure 2d, with the increase of reaction temperature, the yield of biodiesel showed a trend of gradually decreasing. This is because increasing the temperature makes methanol in a gasification state, reducing the contact of methanol with the catalyst and Xanthoceras sorbifolia bunge oil, so the biodiesel yield is reduced. Esterification transesterification is an endothermic process, and too low reaction temperature is not conducive to improving the yield of biodiesel. Considering comprehensively, the most suitable reaction temperature for this reaction is 50 °C. The effect of reaction time on the yield of biodiesel was discussed under the experimental conditions of 3 wt% ILe@Cu@MOF, 35 : 1 molar ratio of alcohol to oil and 50 °C reaction temperature. As shown in Figure.2e, with the increase of reaction time, the yield of biodiesel increased gradually. When the reaction time was 4 h to 5 h, the yield curve of biodiesel tended to be flat. This is because the longer the reaction time, the grafted ILe on the ILe@Cu@MOF catalyst gradually leached, exposing the active sites of ILe@Cu@MOF, increasing the catalytic activity and increasing the yield. Therefore, the best reaction time is 4 h. Combined with the above test results, the optimum reaction conditions can be obtained : the amount of catalyst added is 3 wt%, the molar ratio of alcohol to oil is 35 : 1, the reaction temperature is 50 °C, and the reaction time is 4h. Under the reaction conditions, the biodiesel yield reached 82.85 %.

ILe@Cu@MOF was used as a catalyst to react under the best experimental conditions. After the reaction, the catalyst was extracted by centrifugation and put into Xanthoceras sorbifolia bunge oil again for reaction, repeated five times, and the biodiesel yield after each catalysis was calculated. As shown in Fig.2f, the yield of ILe@Cu@MOF catalyst still reached 73.4 % after five cycles. This proves that ILe@Cu@MOF is a catalyst with high catalytic efficiency and good reusability. The yield of ILe@Cu@MOF biodiesel can reach 73.4 % after five times of use.

The specific surface area and pore size of Cu@MOF were analyzed by analyzing the N₂ adsorption-desorption isotherms. As shown in Figure.3a, there is no obvious inflection point in the downward concave of the isotherm. This is because the interaction between Cu@MOF is stronger than the interaction between ILe@Cu@MOF and N₂, which leads to the difficulty of Cu@MOF adsorbing N₂ in the initial stage of adsorption. After that, the relative pressure increases and the adsorption capacity rises rapidly. Therefore, it can be judged that the curve is a type III isotherm. After amino acid modification, the specific surface area of Cu@MOF increased to 5.07 m²/g, and the average pore size was 23.64 nm. Therefore, Cu@MOF is a mesoporous material.

As shown in Figure.3b-f, the maximum yield of biodiesel of 82.85% was achieved at ILe@Cu@MOF catalyst weight percentage of 3%, methanol/oil molar ratio of 35:1, reaction temperature at 50℃, and reaction time of 4h. The yield of biodiesel still reached 73.40% after the catalyst be reused five cycles. In contrast, the yield of ILe@Cu@MOF biodiesel can reach a higher level.

 

 

 

 

 

 

Figure 2. (a) Effect of ILe addition on the yield of biodiesel, (b) Effect of the amount of catalyst on yield of biodiesel, (c) Effect of molar ratio of methanol to oil on yield of biodiesel, (d) Effect of reaction temperature on yield of biodiesel, (e) Effect of reaction time on yield of biodiesel, (f) Biodiesel yield using the regenerated catalyst for various cycles.

 

Figure 3. (a) N₂-physisorption isotherms of Cu@MOF and pore size distribution of ILe@Cu@MOF, (b) Effect of the amount of Cu@MOF catalyst on yield of biodiesel, (c) Effect of molar ratio of methanol to oil on yield of biodiesel, (d) Effect of reaction time on yield of biodiesel, (e) Effect of reaction temperature on yield of biodiesel, (f) Biodiesel yield using the regenerated catalyst for various cycles.

 

 

2. There's no way, that 1H NMR signals of methyl- and methylene groups are in the 4-4,5 ppm region. Please properly assign all major signals to groups in molecules.

 

Thanks a lot for your advice. Based on your advice, the part had been revised in revised manuscript.

Thanks a lot for your advice. Based on your advice, the part had been revised in revised manuscript.

 

 

We supplemented the NMR spectra of Xanthoceras sorbifolia bunge oil and biodiesel. The peak at 1.28 ppm was caused by the hydrogen in -CH₂-, the peak at 1.64 ppm was caused by β-methylene, the peak at 2.00 ppm was caused by the methylene on the double peak, the peak at 0.88 ppm was caused by-CH₃, and the peak at 5.32 ppm was caused by the hydrogen atom on the double bond.By comparing and analyzing Figure 9b and 9d, it can be found that in Figure 9d, two double peaks of (-CH₂) at 4.32 ppm and（-CH₂CH₃）at 4.15 ppm related to Xanthoceras sorbifolia bunge oil molecules appear, which are almost disappeared in the map of biodiesel in Figure 9b, and a single peak of the methoxy protons as a singlet (-OCH₃)is added at 3.61 ppm assure the conversion of Xanthoceras sorbifolia bunge oil into biodiesel. The peak of Xanthoceras sorbifolia bunge oil at 0.90-2.80 ppm is shifted to 0.8-2.72 ppm in diesel, which proves that Xanthoceras sorbifolia bunge oil can be converted into biodiesel.

Comparing figure 9a and c, it can be found that in the Xanthoceras sorbifolia bunge oil map, the single peak of (-CH₃) at -82.15 ppm becomes a multiple peak of（-OCH₃）at -85.96 ppm in the biodiesel map; the multiple peaks at -66.46 to -73.65 ppm were shifted to -67.73 to -77.16 ppm in biodiesel. A single peak of −46.00 ppm was added to biodiesel; the peaks of -18.84 to -19.48 ppm in Xanthoceras sorbifolia bunge oil moved to -19.91 to -20.55 ppm, and 31.65-33.95 ppm moved to 31.67 ppm [1-2]. In addition, there are almost no miscellaneous peaks in the carbon spectrum of biodiesel, indicating that the purity is high and the reaction is thorough [3].

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4. (a) ¹³C NMR spectra of biodiesel, (b) ¹H NMR spectra of biodiesel, (c) ¹³C NMR spectra of sorbifolia oil, (d) ¹H NMR spectra of sorbifolia oil.

Reference

[1] Chowdhury, Z.Z., Hamid, S.B.A., Das, R., Hasan, M.R., Zain, S.M., Khalid, K., Uddin, M. N. Preparation of carbonaceous adsorbents from lignocellulosic biomass and their use in removal of contaminants from aqueous solution. BioResources 2013, 8, 6523–6555.

[2] Dehghani, S., Haghighi, M. Sono-enhanced dispersion of CaO over Zr-Doped MCM-41 bifunctional nanocatalyst with various Si/Zr ratios for conversion of waste cooking oil to biodiesel. Renew. Energy 2020,153, 801–812.

[3] di Bitonto, L., Reynel-Avila, ´ H.E., Mendoza-Castillo, D.I., Bonilla-Petriciolet, A., Dur´ anValle, C.J., Pastore, C. Synthesis and characterization of nanostructured calcium oxides supported onto biochar and their application as catalysts for biodiesel production. Renew. Energy 2020, 160, 52–66.

 

Author Response File: Author Response.docx