A Review on the Effects of Organic Structure-Directing Agents on the Hydrothermal Synthesis and Physicochemical Properties of Zeolites
Round 1
Reviewer 1 Report
The article a review on the effects of organic structure-directing agents in hydrothermal synthesis and physicochemical properties of zeolites is an interesting and fits the subject of Chemistry. This mini-review has represented the effect of SDAs on zeolite physicochemical properties. Although in the last decades, several new SDAs have been invented, defining optimum properties for a SDA molecule is not straightforward. To look more closely at SDAs development, one can see that several parameters interplay in this process namely the interaction between SDA and zeolite precursors or with solvents, structural stability (rigidity), degrees of hydrophobicity, etc. The impact of quantity and nature of SDA is also important in the zeolite synthesis procedure and should be taken into account. Developing an in-depth knowledge about these features is still in progress and requires in depth insight into their multi-task role in zeolite crystallization. Altogether, having all these complexities, the combination of computational and experimental approaches is essential for the assessment of available SDAs as well as modelling of new species to outline protocols for the synthesis of new alternative SDAs. The result analysis is accurate and adequate. The authors, using the appropriate equipment, have thoroughly investigated the issue. Therefore, the manuscript can be recommended for publication in Chemistry, but after a minor corrections. The following points should be taken into account:
- If possible, it would be good to complete the information on the size of the crystallites calculated from the results of the XRD analysis.
- There are no references to the figures in the publication text: 2,3,7,8.
- Novelty elements should be better highlighted in the introduction. Papers should be cited in Introduction section; for example:
Simultaneous Removal of Pb2+ and Zn2+ Heavy Metals Using Fly Ash Na-X Zeolite and its Carbon Na-X(C) Composite, Materials, 14, 2832, 2021
Adsorption mechanism of poly (vinyl alcohol) on the surfaces of synthetic zeolites: sodalite, Na-P1 and Na-A, Adsorption, 25, 567-574, 2019
Comparison of Physicochemical Properties of Fly Ash Precursor, Na-P1(C) Zeolite–Carbon Composite and Na-P1 Zeolite—Adsorption Affinity to Divalent Pb and Zn Cations, Materials, 14, 3018, 2021
Comments for author File: 
Comments.pdf
Author Response
We thank the reviewer for his/her compliments on the quality, novelty and originality of our article, and for recommending publication in Chemistry after minor revision.
Comment 1: If possible, it would be good to complete the information on the size of the crystallites calculated from the results of XRD analysis.
Response 1: We agree with the reviewer and thank him/her for enhancing the manuscript. Indeed, we have calculated the mean average crystalline domain size using the Scherrer equation: L = , in which L is the average crystallite size, K is Scherrer constant and equal to 0.9 nm, λ is the wavelength of the Cu-K α X-ray source radiation (nm) and equal to 0.15406 nm, β is FWHM (radians) and θ is the peak positions (radians). We calculated the β (FWHM or Full Width at Half Maximum) of two most intense peaks of each sample using origin software in degrees and converted it to radians. The values were plugged in the Scherrer equation and the crystallite size obtained as the average of two results.
Based on this we measured the crystal size and the data is added in Page 5, Paragraph 1, Lines 166-176:
“The size of crystallites of both zeolite Y samples was calculated based on Scherrer equation and the results showed an average crystallite size of 43.28 nm in the case of zeolites synthesized in the presence of TMA+ and 84.05 nm in the sample synthesized in the absence of TMA+.
In addition, the particle size visualized by SEM technique (is not shown here) and reflected that the presence of TMA+ as SDA notably increases the size of zeolite Y particles (from the average size of 3.5 µm in the absence of SDA to an average size of 74 µm in the presence of TMA+). However, it should be noticed that due to the semi-amorphous nature of the zeolite Y spheres prepared in the absence of TMA+, the size of the particles visualized by SEM is approximately measured (in some parts amorphous phases covered the surface of particles)”.
In addition, we discussed the effect of organic SDA (TMA+) on the particle size of zeolite Y beads obtained by SEM imaging in the following part of this section, line 169.
Comment 2: There are no references to figures 2,3,7,8 in the publication text.
Response 2: We thank the reviewer for her/his comment. Figure 2, 3 and 7 were generated by authors themselves and were not reproduced from literature. Figure 3 is the XRD pattern of two zeolites synthesized in our lab. There is no Figure 8 in this review.
Comment 3: Novelty elements should be better highlighted in the introduction. Papers should be cited in introduction section:
Simultaneous Removal of Pb2+ and Zn2+ Heavy Metals Using Fly Ash Na-X Zeolite and its Carbon Na-X(C) Composite, Materials, 14, 2832, 2021
Adsorption mechanism of poly (vinyl alcohol) on the surfaces of synthetic zeolites: sodalite, Na-P1 and Na-A, Adsorption, 25, 567-574, 2019
Comparison of Physicochemical Properties of Fly Ash Precursor, Na-P1(C) Zeolite– Carbon Composite and Na-P1 Zeolite—Adsorption Affinity to Divalent Pb and Zn
Cations, Materials, 14, 3018, 2021.
Response 3: References mentioned by reviewer were cited in the last section of the introduction part, Page 3, Paragraph 1, Line 84 (references [35-37]), as requested.
Author Response File: Author Response.pdf
Reviewer 2 Report
In this manuscript, the authors have reviewed several aspects of organic molecules, known as organic structure-directing agents, used to direct the formation of zeolites. Such organic structure-directing agents are of significance for the formation of zeolites; therefore, there are many excellent review papers, in addition to original papers, available in many journals. The present manuscript is not so well organized. I have found that several points did not describe accurately (which may be because the authors are not specialized in zeolite research). Basically, organics used in zeolite synthesis can be categorized, based on their functions in the zeolite formation (crystallization), into i) real template, ii) structure-directing agent, and iii) pore-filling agents. In this manuscript, it is very confused about the functions of organics. In addition, figures were not so related to the contents. Also, figures were not described properly in the text. Overall, I think this manuscript, as a review article, needs substantial improvement. Therefore, at the current form, I cannot recommend for the publication of “Chemistry” journal.
Author Response
Response: We thank the reviewer for his/her comments. Although the reviewer feedback reflected some negative points of our review in a general way, we believe that we looked at the topic from a point of view that is new. Based on our knowledge up to now, there is no review gathered on the effects of structure-directing agents in several physicochemical properties of zeolites, although, there are some excellent reviews in this field to discuss professionally the synthesis or advances in SDAs molecules or written with a special point of view which was not the target of our study. We focus in this review on the organic SDAs in each case of zeolite that is reported in our work and when we talk about pore size and topology of the formed zeolite framework, implicitly, we are pointing to the structure-directing effect and pore-filling role of the organic species at the same time. We scientifically, do not think that real template is a practical terminology in this case. Because all organic cations mentioned in the review can play the role of template for shaping or structuring the zeolitic frameworks (e.g., channels, pores and cavities). It is not clear to us the point raised by the reviewer about ‘real templates’. All figures are referred to in the text and they are schematic figures which can be fully understood by readers. Furthermore, each image has an explanation, highlighting the concept behind the figure. Therefore, we would kindly appreciate it if the reviewer can go through more details in the feedback or at least clarify which parts need to be discussed in more details.
Reviewer 3 Report
The manuscript entitled “A review on the effects of organic structure-directing agents in hydrothermal synthesis and physicochemical of zeolites” reports an interesting review on organic structure-directing agents (SDAs) in the synthesis of zeolites.
The work is well organized and enjoyable to read. The topic is very specific because it deals exclusively with all the effects of the SDAs. The argument is consistent with the purpose of the journal and I believe it could be a good reference to researchers working in this area of research.
Just a small note: for a more complete introduction, I recommend considering an important class of zeotypes, such as the Engelhard titanium silicate, on which the effect of SDAs has been extensively studied. For this reason I recommend to consider:
*)Turta , N.A.; De Luca , P.; Bilba , N.; B.Nagy J.; Nastro , A. Synthesis of titanosilicate ETS-10 in presence of cetyltrimethylammonium bromide. Microporous and Mesoporous Materials, 2008, 112 (1-3), pp 425-431.
*)Pavel, C.C.; B.Nagy, J.; Bilba N..; Nastro, A.; Perri C.; D. Vuono; De Luca, P.; Asaftei , I,V. Influence of TAABr salts on the crytstallization of ETS-10. Microporous and Mesoporous Materials, 2004, 71 (1-3),pp 77-85
In light of the above, I recommend that the manuscript be published only after a minor revision.
Author Response
Response: We thank the reviewer for his/her compliments on the well-organized and enjoyable text of our review article, recommending publication after minor revision and also for his/her suggestion to add Engelhard titanium silicate to introduction section. The discussion about Engelhard Titanosilicates type of zeolites is added to the introduction part, bottom of page 2 paragraph 1 and top of page 3, paragraph 1, Line 75 (reference 27 & 28), as requested by the reviewer.
In addition, we refined the text from the fluency point of view once more and all amendments are incorporated using tack changes which are easily visible in the text.
Round 2
Reviewer 2 Report
I still remain with my previous decision.
I have worked on the zeolite synthesis, especially in the presence of organic molecules, for more than 15+ years. I still think that this review paper is not well organized. I will never recommend my students and junior colleagues to read it because this paper can lead them to some misunderstanding.
At first, the authors should explain how they catagorize the functions of organics in zeolite synthesis.
For example:
1. Organics for crystallization of zeolites with specific framework structure
In this case, researchers in zeolite communities classify the organics based on their structure-directing ability into i) true template, ii) structure-derecting agent, and iii) pore-filling agent. This is well accepted and well known for community. If the authors have never heard about this classification, I recommend them to study several papers by Prof. M. E. Davis and Prof. A. Corma published in the 1990's.
2. Organics for modification of crystal morphology
In this case, organics are not always the "structure"-directing agents. Based on the formation mechanisms, the organics should be called modififers.
3. Organics for .....
The other major problem of this review is that many examples described therein are not exclusively the effect of organics because other synthesis conditions were also different. This can make the readers confusing and can lead to misunderstanding.
Author Response
Response:
We thank the reviewer for his/her comments. We believe we have complied with his request and the review article is now clear and there is no confusion.
We address here true templates as SDAs that we used in our review which can crystallize zeolites. See also ref. 27, 28, 29 and 30 of this review. So, the term true template is not introducing a new principal or definition beyond SDAs and it is not providing information totally different from SDAs. For the sake of clarity, it has been used for particular zeolitic frameworks which can be synthesized only with a limited number of SDAs or a single type of SDA. In this respect, a few organic species are available as true templates (See ref. 31) but they are still SDAs and their functionality do not differ from SDAs. It should be emphasized that terms true template and SDAs do not have a rigid boundary of definition and different scientists look at them somehow differently. For example, while some species have been recognized as true SDAs, some as pore filler and some others even as intermediate position indicating that they are not 100 % pore-fillers, SDA or true template and more likely a combination of these duties are fulfilled by them (See ref 32). In addition, we would like to mention that even in ref 32, term true structure directing template is used not true template but in no way we were confused during the reading of this paper as these terms are conveying the purpose of author of applying these terms. We also refer an article from Prof. Corma published in year 2001 clarifies the word true template has some limitations for application compared with SDAs. Furthermore, we believe that true template has been used as representative of SDAs more in older articles from 90’s decade or earlier and nowadays SDAs is more in common use for the same concept. To converge above-mentioned discussion, we do not believe the readers of this review are confused when we speak directly and particularly about organic SDAs (i.e. those species that are structuring and stabilizing zeolitic framework and thus have the ability to crystallize zeolites). No need to mention that the title of our review is the effects of organic structure-directing agents on zeolite synthesis not organics that would need to be more classified as fillers, SDAs and modifiers. So, certainly, we are discussing about a special type of organics (i.e. organic SDAs) in zeolite synthesis.
The term modifier in zeolite synthesis is applied when an additive is used to alter the interactions, for example, with zeolite Al species (see ref 34), or orienting zeolite films on a surface (in this case surface modifier like organic surfactants are used, see ref. 35)
Nevertheless, we respect the point of view of the reviewer and the concern about clarification bout different organics which is a more detailed look to a wide range of species in zeolite synthesis. Although discussion about all of these species was not the scope of our review, we added a paragraph on page 2 of this review, line 64 onwards, to explain the difference between these species with emphasizing that our target in this review is discussing the role of SDAs in zeolite formation. We referred to some articles to clarify as high as possible this issue that can be found in blue color in reference section (line 452 onwards).
So, once more we emphasize that all organic species that we discussed here are SDA species not pore-filler or modifiers.
In addition, we have done some minor changes in different pages to use the phrase SDA as high as possible, correct which are mentioned here:
Page 9, line 298, page 10, line 332, page 10, line 342, page 10, line 346, page 10, line 351, page 10, line 356, page 10, line 357, page 10, line 357.
And three references also corrected:
Page 17, line 684, ref. 128 is deleted. Page 17, line 686, number of reference is changed from 129 to 128. Page 17, line 688, new ref 129 is added.
Author Response File: 
Author Response.docx
