Synthesis and investigation of physico-chemical properties of dicationic ionic liquids

A series of dicationic ionic liquids (ILs) including [C4(MIM)2][PF6]2, [C5(MIM)2][PF6]2, [C6(MIM)2][PF6]2 and [C4(PYR)2][PF6]2 were synthesized. Their thermal stability and melting points were analysed. It was found that dicationic ILs presented important implications in the design of homogeneous and heterogeneous system with water. A homogeneous system of dicationic ILs with water could be formed at a relatively high temperature and then a heterogeneous system was formed when the solution was cooled to a low temperature. The ILs recovered by altering the temperature were obtained in high percentage yields of [C4(MIM)2][PF6]2 (97.6%), [C5(MIM)2][PF6]2 (97.3%), [C6(MIM)2][PF6]2 (98.0%) and [C4(PYR)2][PF6]2 (94.2%). On the other hand, [C4(MIM)2][PF6]2 and [C5(MIM)2][PF6]2 exhibited good solubility in acetonitrile and acetone. A homogeneous system could be achieved with imidazolium-based ILs with a relatively low amount of water and acetonitrile at room temperature. All of the properties of dicationic ILs have a strong correlation with the nature of dications, the linkage chain and the symmetry of dications. Dicationic ILs may provide a new opportunity for some specific applications in order to enable the effective separation and isolation of products.


Introduction
The main goal of investigation should be described in detail. At present the main problems in chemistry of studied IL are unclear  correlations between amount content (ml) and temperature. It will be better to rebuild Y-axis as ratio of H2O/ILs (g/ml or g/g) -Figure3. Caption is unclear. As I understand you allocate IL from ILs/H2O solution at 100oC by cooling solution. Note that amount of H2O in mixture should be added I suggest to revise, for example, The recovery of dicationic ILs - Figure 4 (Miscibility of dicationic ILs with water and acetonitrile at room temperature).  correlations between amount content (ml) and temperature. It will be better to rebuild Y-axis as ratio of H2O/ILs (g/ml or g/g) -Figure3. Caption is unclear. As I understand you allocate IL from ILs/H2O solution at 100oC by cooling solution. Note that amount of H2O in mixture should be added I suggest to revise, for example, The recovery of dicationic ILs - Figure 4 (Miscibility of dicationic ILs with water and acetonitrile at room temperature). Comments to the Author(s) In this manuscript, Sun, Y. et al. prepared a series of dicationic ionic liquids and investigated the miscibilities of these ionic liquids with water. They tested the thermal stability and melting points of these ionic liquids and studied their solubilities and recovery rates. In this re-submitted version, authors didn't address my previous comments seriously and properly. The writing is worse, and authors even didn't delete the journal template instruction in the second paragraph. Many spaces between words are missed. More importantly, in my previous comments, I asked the authors to explain why monocationic pyridinium-based ILs had higher decomposition temperature than that of dicationic pyridinium-based ILs with the same anion, while dicationic imidazolium-based ILs had higher decomposition temperatures than that of monocationic imidazolium-based ILs. It is not a suitable way to address this comment by simply deleting the discussion of the decomposition temperatures of monocationic and dicationic pyridinium-based ILs. Based on the above comments, I don't consider this manuscript as a serious scientific paper.

RSOS-181230.R0
Review form: Reviewer 1 (Maria Timofeeva) Is the manuscript scientifically sound in its present form? Yes Introduction I suggest to specify the main goal of your investigation. For example, "The main goal of this investigation was to increase knowledge in physicochemical properties of dicationic Ionic Liquids. The main attentions were focused on the investigations (a) its thermal stability, (b) its melting points and (c) its solubility in water and water-organic mixtures. All these parameters can be important in a future their applying as solvents or catalysts".
Figure3 was not revised. What do you mean when write "IL recovery rate" (Y-axis)? Percent (%) can be dimension of rate (!!!).

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Is the language acceptable? Yes
Is it clear how to access all supporting data? Not Applicable

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation? Accept as is
Comments to the Author(s) In this revised manuscript, authors have carefully revised their manuscript, corrected the format of the paper, and provided the detailed explanation to address my previous comments. So I recommend it can be accepted for publication. Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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The reviewers and handling editors have recommended publication, but also suggest some minor revisions to your manuscript. Therefore, I invite you to respond to the comments and revise your manuscript.
Because the schedule for publication is very tight, it is a condition of publication that you submit the revised version of your manuscript before 18-Oct-2018. Please note that the revision deadline will expire at 00.00am on this date. If you do not think you will be able to meet this date please let me know immediately.
To revise your manuscript, log into https://mc.manuscriptcentral.com/rsos and enter your Author Centre, where you will find your manuscript title listed under "Manuscripts with Decisions". Under "Actions," click on "Create a Revision." You will be unable to make your revisions on the originally submitted version of the manuscript. Instead, revise your manuscript and upload a new version through your Author Centre.
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Introduction
The main goal of investigation should be described in detail. At present the main problems in chemistry of studied IL are unclear.
Results and discussion -Page 3, Lines 20-21. You wrote that "As mentioned in the experimental section, yields of ionic liquids followed the order of [C 6 (MIM) 2 ][PF 6  In this manuscript, Sun, Y. et al. prepared a series of dicationic ionic liquids and investigated the miscibilities of these ionic liquids with water. They tested the thermal stability and melting points of these ionic liquids and studied their solubilities and recovery rates. In this re-submitted version, authors didn't address my previous comments seriously and properly. The writing is worse, and authors even didn't delete the journal template instruction in the second paragraph. Many spaces between words are missed. More importantly, in my previous comments, I asked the authors to explain why monocationic pyridinium-based ILs had higher decomposition temperature than that of dicationic pyridinium-based ILs with the same anion, while dicationic imidazolium-based ILs had higher decomposition temperatures than that of monocationic imidazolium-based ILs. It is not a suitable way to address this comment by simply deleting the discussion of the decomposition temperatures of monocationic and dicationic pyridinium-based ILs. Based on the above comments, I don't consider this manuscript as a serious scientific paper.

Answer:
We are really sorry about the version of the manuscript. When we uploaded our manuscript, all the formations of the file were changed, because we used a "WPS" word system, leading to the missing of many spaces between words. It is our fault and we are sorry for the inconvenience to read. We really appreciate you still could give us an opportunity.
Thank you for your previous comments. You asked us to explain why monocationic pyridinium-based ILs had higher decomposition temperature than that of dicationic pyridinium-based ILs with the same anion, while dicationic imidazolium-based ILs had higher decomposition temperatures than that of monocationic imidazolium-based ILs. It is really not a suitable way to simply deleting the discussion. However, could you give us an opportunity to explain it please?
In our original manuscript, we described the above discussion, because wecited a literature ( shows the T d,onset at 483 K." However, the author emphasized "Unexpected results were observed" with these two ILs, because very few pyridinium-based ionic liquids were found in this case compared with most other literatures. These dicationic pyridinium-based ILs have low thermal stability, because there is an ether bond in the linkage chain of their dications that is unstable, which are different with the chemical structure of ILs in our work. Thus, we cited wrong information in our original manuscript and that is why we deleted the discussion in the re-submitted manuscript. Fortunately, you help us to find our mistake and thank you very much for your reminding.
In order to identify the thermal stability of monocationic ILs and dicationic ILs for both imidazolium based ILs and pyridinium based ILs, we did further experiments to confirm this. In our work, [C 4 (MIM)]PF 6 presented T onset at 426-427 o C. Dicationic imidazolium-based ILs generated the thermal stability in a range of 443-445 o C, which were higher than monocationic ILs. In a similar case, dicationic [C 4 (PYR) 2 ][PF 6 ] 2 exhibited T onset at 414 o C, which was higher than monocationic [C 4 (PYR)]PF 6 at 395-396 o C. It was reported that the thermal stability of 39 types of dicationic ionic liquids were found to be significantly higher than those that have been observed for many traditional imdazolium-based ionic liquids. (Anderson JL, Ding R, Ellern A, Armstrong DW. 2005 Structure and properties of high stability geminal dicationic ionic liquids. J. Am. Chem. Soc.127,593-604. doi:10.1021/ja046521u). In their paper, the highest thermal stability (> 400 o C was obtained with [C 9 (mpy) 2 ]NTf 2 (1-methylpyrrolidinium dication) and the lowest decomposition temperature (330 o C) was observed for [C 9 (bpy) 2 ]NTf 2 (1-butylpyrrolidium dication. Our work was consistent with Dr. Anderson et al. We have described all data in the manuscript. Thank you.
Introduction I suggest to specify the main goal of your investigation. For example, "The main goal of this investigation was to increase knowledge in physicochemical properties of dicationic Ionic Liquids. The main attentions were focused on the investigations (a) its thermal stability, (b) its melting points and (c) its solubility in water and water-organic mixtures. All these parameters can be important in a future their applying as solvents or catalysts".
We have corrected the Introduction according to reviewer's advices.
Thank you for your help.
Figure3 was not revised. What do you mean when write "IL recovery rate" (Y-axis)? Percent (%) can be dimension of rate (!!!).
We have modified Figure 3 and re-write the contents in 4.6.
Referee: 2 In this revised manuscript, authors have carefully revised their manuscript, corrected the format of the paper, and provided the detailed explanation to address my previous comments. So I recommend it can be accepted for publication. Thanks very much for your kind work and consideration on publication of our paper. On behalf of my co-authors, we would like to express our great appreciation to reviewers.
Thank you and best regards.