Progress of Methylation of C_6-8~Arene with Methanol: Mechanism, Catalysts, Kinetic/Thermodynamics and Perspectives

Round 1

Reviewer 1 Report

The manuscript of Dong and co-workers reviews the methylation reactions of benzene/toluene and xylene with methanol to produce p-xylene and trimethylbenzene, respectively, in presence of zeolites as catalysts.

In general, the information included in the manuscript is correct. However, the review is poorly organized and subtopics are sometimes repeated in different sections. In some sections, the work requires a more detailed analysis of the information. The manuscript needs to be restructured so that is concise and more orderly and understandable. Additionally, it seems that the manuscript focuses only on the chemical part of the methylation of arenes with methanol, and omits or barely treats aspects of the process, which is the main topic of the journal.

The current form of the review is not suitable and it is suggested to reject the manuscript for publication in Processes or reconsider after major revision. In any case, the authors are invited to take into account the points indicated in the attached report.

Comments for author File: Comments.pdf

Author Response

Dear reviewer, First of all, thank you for your valuable advice. Those comments are all valuable and very helpful for revising and improving our paper. We studied the comments carefully and revised the manuscript in accordance with the reviewers’ comments and suggestions. I have revised it and upload it. The changes are shown in yellow. Yours sincerely, Wang Comments to the Author: • Affiliations are not indicated for each author. Reply: Has been marked. • Notation for xylene is not homogeneous. Sometimes it is denoted simply as xylene (X) without specifying to which isomer it refers. Sometimes it is denoted as para-xylene (PX). Reply: In the article X stands for o-xylene, m-xylene and p-xylene, while PX stands for p-xylene. • It is not clear if the notation B/T indicates a mixture of benzene with toluene, or benzene and toluene as independent reactants. Clarify this throughout the text. Reply: PX was prepared by the alkylation of benzene or toluene as independent reactants with methanol. It has been modified in the “ABSTRACT” section. • Along the text, the term MT-PX appears several times, and is sometimes replaced by the term TM-PX. Neither term is described or identified. Reply: The methylation reaction of benzene with methanol to obtain high-purity p-xylene (BM-PX); the methylation reaction of toluene with methanol to obtain high-purity p-xylene (TM-PX). The full text has been revised (yellow). • Chemical and mathematical equations are not numbered before section 8. Reply: In the article, most expressions of seemingly reactive equations were all mechanisms, so they were classified by category. However, this part mainly determined the basic reactive equations through the previous mechanism and experimental research, which could better explain the thermodynamic problems, so it was numbered. This section had been adjusted to section 3.2 of the article。 • The reaction network presented in Figure 1 is somewhat chaotic. It is known that this kind of process implies several parallel and consecutive reactions, so the reaction network must be easy to follow and understandable. In this case, it would be useful to explain in detail the reaction network in section 2. Nevertheless, the need for section 2 should also be reconsidered, since the reaction network is not mentioned again in the rest of the manuscript and more details about the reaction are presented in the rest of the sections. In any case, this part could be used to justify the reactions considered in section 8. Thermodynamic analysis of methylation reaction 2. Reply: Fig 1 has been replanned. Thanks for the guidance of the experts, we reintegrated the diagram into a plan to reflect its role. For example, Fig. 1~4, 9 were used in the discussion on the reaction mechanism of methylation system (4.4). • The review would be more understandable if the sections followed a logical sequence according to the process development. As stated in the previous point, it is desirable that the section on the reaction network precede the section on the thermodynamics of the reaction. The reason for this is that the thermodynamic study provides the theoretical foundation for the experimental development of a reactive system, specifically the reaction conditions applicable to catalytic activity tests. Reply: We especially recognize the opinions of expert teachers, and the whole paper has been redesigned. • The order of sections 4, 5, 6 and 7 is suitable. However, section 9, which deals with the reaction mechanism regarding theoretical calculations by DFT, should be included in section 3. Reply: The content has been adjusted. • Titles and subsections of sections 4, 5 and 6 are not homogeneous. The idea of classifying and analyzing the methylation reactions of benzene, toluene and xylene is suitable and useful for their understanding. However, these sections cover different non-analog points between sections, resulting in an incomplete view of the process. For example, section 6 does not address any of the points included in sections 4 and 5. Reply: Titles and subsections of sections 4, 5 (After the revision, sections 5,6 in the article) have been modified. However, there were few literatures about the preparation of trimethylbenzene (After the revision, section 7 in the article), and it is difficult to support as much as the first two sections. • Regarding the catalysts, the review requires an in-depth analysis of the effect of the physicochemical properties (mainly zeolite structure, acidity and textural properties) of the catalysts on the methylation reactions of benzene, toluene and xylene (conversion, selectivity, yield, deactivation, amount of coke). A table summarizing the information, as well as the respective analysis and discussion, should be included. Reply: It has been filled in Table 2 and Table 3. • The manuscript lacks a section referring to the effect of process variables, which is the main topic of the journal. Reply: It has been filled in section 5.5 and section 6.4. • Reaction kinetics is of critical importance in chemical process engineering. However, the manuscript does not reflect such importance for the methylation reactions of benzene, toluene, and xylene. It is necessary to include the kinetic models reported in the literature, and classify them according to their type, whether power law, Langmuir-Hinshelwood or Eley-Rideal. Similarly, the values of the kinetic parameters of these models (reaction rate constants, adsorption-desorption equilibrium constants and activation energies) must be reported depending on the catalysts. Tables are a good option to summarize this type of information. Reply: It has been filled in section 8 (Table 5). • Parameters and variables in subsection 8.1 are not indicated. Reply: Due to the severity of COVID-19, relevant parameters are obtained from books (no electronic version), which are in the school library and cannot be entered into the school, so relevant data cannot be obtained. But the data are the same as those in the book [1]. [1] P. S. Ma, The experimental data manual of organic compound[M]. Chemical Industry Press, Beijing, 2006. • The title in subsections 8.4.2 and 8.4.3 is repeated. Reply: Has been modified. • The author contributions statement only includes two authors. The contribution of the rest of authors is not mentioned. Reply: Contributions from other authors have been added. • The format of the references requires revision since it is not consistent with the template of the journal. Reply: The format of the references has been completely revised.

Author Response File: Author Response.docx

Reviewer 2 Report

In this work, a review about the methylation of benzene from methanol to give toluene, xylene and trimethylbenzene (among other products) is reported. The manuscript is very complete, covering main and side reactions involved in the methylation processes, as well as providing an overview about the reaction mechanisms proposed to date, catalysts employed (with an special emphasis on heterogeneous catalysis), and the kinetic and thermodynamic aspects related to the process.

Please, find some possible minor changes and suggestions in the document attached.

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

       Thanks for your letter and for the referees’ comments concerning our manuscript. Those comments are all valuable and very helpful for revising and improving our paper. We studied the comments carefully and revised the manuscript in accordance with the reviewers’ comments and suggestions. I have revised it and upload it. The changes are shown in yellow.

 

Yours sincerely,

Wang

 

 

Comments from the editors and reviewers:

First of all, thank you for your valuable advice. Those comments are all valuable and very helpful for our paper and future work.

1) At the ending of the line 63, please add a reference about alkylation reactions.

Reply: References have been added.

2) In the Figure 1, I don’t understand well the “orbital-like” icon and the small red arrow in the R21 cycle. Please, check this figure if it is needed.

Reply: The small red arrow was removed.

3) In the lines 145-146, according to the Figure 4, the “methyl radical” would be a methylene radical or carbene (:CH₂), which is different of methyl radical (·CH₃).

Reply: Indeed, there's a methylene radical or Carbene(:CH₂), which has been modified.

4) I do not understand well the lines 298-299: “Among them, the acidity required for the disproportionation of toluene was the strongest and the acidity was too weak for the methylation”. Please, change this text in order to make easier its understanding.

Reply: The sentence has been modified.

5) In the line 658, are the “level 1” and “level 0” the partial orders of the reaction for benzene and methanol respectively?

Reply: Yes.

[1]V. D. M. Jeroen, V. Melina, O. Unni, et al., Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts[J]. Journal of Catalysis, 2012, 292: 201-212．

 

6) In the lines 90-91, and 781-783, the sentences “Toluene was formed when protonated toluene was deprotonated” and “According to the principle of entropy increase, it could be seen that the entropy principle was an important principle for restricting chemical reactions” are some redundant. Please change these sentences in order to a avoid repetitions.

Reply: It has been modified according to the opinions of expert teachers.

 

 

If there is something wrong, we hope the teacher kindly give us your advice and instruct us.

Best wish!

Wang

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have made significant changes to the manuscript, mainly editing, making it easier to read. However, the review requires special attention to the thermodynamic analysis and full consideration of the points mentioned in the document attached. For which, it should be reconsidered after Major revision before its publication in Processes.

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

First of all, thank you for your valuable advice.

Those comments are all valuable and very helpful for revising and improving our paper. We studied the comments carefully and revised the manuscript in accordance with the reviewers’ comments and suggestions. I have revised it and upload it. The changes are shown in yellow.

 

Yours sincerely,

Wang

 

Comments to the Author:

1. Words or phrases such as "believed" or "speaking relatively" are used throughout the text. Please avoid the use of these types of phrases as they are ambiguous and detract from the seriousness of the manuscript.

Reply: Has been modified.

 

2. Figure 1 is briefly mentioned in section 2 and needs to be explained in detail, as it is the starting point for thermodynamic, kinetic and reaction mechanism analyses.

Reply: Thank you very much for your comments and suggestions. Fig. 1 involves all reaction types of the reaction system. The types of reactions were illustrated at the bottom of Fig. 1.

 

3. Figures 1 and 2 represent essentially the same information, with different notation for the reactions and different representation of the structure of the compounds. Unify both reaction systems into one, and present it in section 2 indicating each reaction as in Figure 2 of the second version of the manuscript.

Reply: Thank you very much for your comments and suggestions. We would like to say that Figure 1 and Figure 2 are fundamentally different. Figure 1 involves all reaction types of the reaction system. However, according to literature research, our experimental results and calculations, specific reactions were given under the precondition of relevant catalysts (Fig. 2) and relevant calculations were carried out. If you put two pictures together it's very messy. For example, MTO reaction includes methanol to dimethyl ether step in Fig 2, but that didn't appear in Fig. 1. Benefits of separate presentation: diagram content was not complex, simple and clear, easy to obtain.

But we changed it accordingly.

 

4. As requested in the first review report of the manuscript, parameters and variables in equations 1-6 (subsection 3.1) are not indicated. It is not necessary to include the numerical values of the parameters. Indicate only each term of the equations. These are fairly well known and can be found in a wide variety of texts on thermodynamics. The thermodynamic analysis section requires special attention as mentioned below:

Reply: Thanks for your guidance. We follow the teacher's advice and it has been modified.

 

5. The objective of a review is to analyze the information on the topic of interest published in articles, books and patents. However, the section on the thermodynamic analysis of the process does not represent such an analysis. Rather, it presents unvalidated results from the estimation of the thermodynamic properties of the reactions involved. It is understandable (and to some extent valid) that given the lack of published2 information on the thermodynamics of the reacting system, that authors estimate and report this information. However, this study needs to be carried out in greater detail. In principle, the thermodynamic analysis of a reaction system begins with the collection of information on the thermodynamic properties of the reactants and products involved in the system. When this information is not available, it is appropriate to estimate the thermodynamic properties of pure compounds by group contribution methods. In the case of this manuscript, the authors mention the use of the Benson method. However, the use of such a method is not justified. In addition, it is known that the different group contribution methods provide results with variable reliability depending on the type of molecules under study [1], for which a statistical comparison between the results provided by different group contribution methods is necessary, for example: Benson, Joback, and Constantinou-Gani, as reported in the appendix A of reference [2]. Once the method with the least deviation has been selected, proceed to perform the calculations according to equations 1-6. The choice of the method with the best fit is important, since small deviations of Cp would affect exponentially the estimation of the change of Gibbs free energy of reaction and of the equilibrium constant [1].

Reply: The teacher's judgment was sound. Therefore, we consulted the literature for modification. Through literature research, this method was generally adopted for methylation reaction. Meanwhile, we had made a comparison of data change trend, which was basically consistent.

The literature found was as follows:

1. Feng, Z.W.; Hu, B.; Guo, X.Q. Thermodynamic analysis of methanol and benzene alkylation by ASPEN PLUS. SHANXI Chem. Ind. (China) 2019, 1, 16-21.
2. Xu Y.R.; Xu, X.L.; Zhu, X.D. Thermodynamic analysis on alkylation reaction of benzene and methanol. React. Eng. Technol. 2015, 31(5), 475-480.
3. Ren, J.; Zhou, Z.J. Thermodynamic analysis of toluene methylation reaction system. Acta Petrolei Sinica (Petroleum Processing Section), 2020, 36(2): 340-348.

 

6. In Table 1, the Gibbs free energy change (ΔG_T) data indicate that increasing the temperature from 623 K to 723 K favors reaction 1. In contrast, the data of equilibrium constants (ln K_p) suggest the opposite. Mathematically, the numerical values of ln K_p should increase with the decrease of ΔG_T. It is possible, but not certain, that there is an influence of the group contribution method used. Check and/or correct these data for the entire set of reactions taking into account the group contribution method selected in the point 5 mentioned above. From the rest of the manuscript:

Reply: The change trend of the above data was consistent with the literature.

 

7. In line 191 of page 6, the text says "the acid active site of molecules sieve could". The correct term is molecular sieve.

Reply: It has been corrected.

 

8. The format of the title of section 4.4 differs from the indicated format. Additionally, the need for section 4.4 should also be reconsidered. It is suggested not to include it since its contribution to the manuscript is minor and sections 4.1, 4.2 and 4.3 discuss well the reaction mechanism of the methylation system. In any case, sections 4.1, 4.2 and 4.3 should be enriched with greater details of the experimental studies on the reaction mechanism using the appropriate analytical techniques (gas chromatography coupled to mass spectrometry, infrared spectroscopy, nuclear magnetic resonance, etc). In the same way, the theoretical aspects of the reaction mechanisms that are already mentioned require enrichment taking into account the transition states, bond lengths and energies, and, where appropriate, the interaction of reactants and products with the active sites of the catalysts, for example bond lengths and adsorption/desorption energies of reactants/products. If necessary.

Reply: It has been modified. Thank you for your advice. Although this part is the result of our final combination of experiments, it is of little value in the review paper, so we finally decide to delete it.

The appropriate analytical techniques (gas chromatography coupled to mass spectrometry, infrared spectroscopy, nuclear magnetic resonance, etc). was used. In addition, isotope experiments were commonly used in section 4.1. Such as: Van et al. [15] studied the methylation reaction of ¹²C-labeled benzene and ¹³C-labeled methanol, 95% of toluene in the product contained only one ¹³C, and 5% of toluene was generated by indirect methylation of methanol through hydrocarbon pools. It indicated that there was MTO reaction in the methylation reaction system of benzene with methanol, that is, C2~C5 olefins were obtained through the MTO reaction process (Fig. 2, Fig. 5).

In situ experimental studies had shown that the adsorption of methanol at the acid active site of molecular sieve could indeed generate methoxyl intermediate species at appropriate temperature [5], which could easily be methylated with aromatic hydro-carbon molecules, in line with the step-by-step mechanism.

 

9. Information in tables 2, 3, 4 and 5 is correct. However, its presentation is confusing. It is not possible to appreciate where the rows begin and end, and to what level the rest of the information belongs. Please edit these tables to make them understandable and neat.

Reply: It has been adjusted and modified.

 

10. Figures 12 and 14 are barely mentioned in sections 5.5 and 6.4, respectively. They need to be explained in a logical sequence besides the advantages of these processes.

Reply: Fig. 12 and 14 are barely mentioned in sections 5.5 and 6.4. They were the result of our research on the basis of previous predecessors. The logical sequence of this article has been modified.

 

 

 

If there is something wrong, we hope the teacher kindly give us your advice and instruct us.

Best wish!

Wang

 

Author Response File: Author Response.docx