Synthesis of Quaternary Ammonium Room-Temperature Ionic Liquids and their Application in the Dissolution of Cellulose

Round 1

Reviewer 1 Report

This is an interesting work where novel aqueous based systems for cellulose dissolution and processing have been developed and characterized. The paper is reasonably well written but it should be submitted to a professional proof reading. In what follows, a few questions /comments are enumerated which need to be addressed before the paper is suitable for publication.

The authors state that “NMMO is the common cellulose dissolving solvent currently.”It should be noted that despite being highly efficient in dissolving cellulose and the main ingredient of the Lyocel process, viscose still dominates, by far, the market of cellulose man-made fibers. So NMMO is an interesting solvent but the drawbacks presented (as the ones the authors enunciate) plus the major investments in safety are still favoring the viscose process. This should be highlighted.

The “greenness” of ionic liquids is questionable. For instance, some show toxic and poisonous properties to aquatic organisms. See for instance the following references:

Wells A S , Coombe V T . On the Freshwater Ecotoxicity and Biodegradation Properties of Some Common Ionic Liquids. Organic Process Research & Development, 2006, 10(4):794-798;

QUIJANO, Guillermo, Annabelle, et al. Toxicity and biodegradability of ionic liquids: New perspectives towards whole-cell biotechnological applications. Chemical Engineering Journal, 2013, 174(1):27-32.;

Zhang C, Zhu L, Wang J, et al. The acute toxic effects of imidazolium-based ionic liquids with different alkyl-chain lengths and anions on zebrafish (Danio rerio). Ecotoxicol Environ Saf, 2017, 140:235-240.;

Jing C Q, Liu Y T, Shen X Y, et al. Toxic evaluation of the alkylmethylimidazolium-based ionic liquids in yeast Saccharomyces cerevisiae by flow cytometry. Toxin Reviews, 2012, 31(3-4):27-31.

Poor reference selection. Much more key references are needed.

Please note that work has been done in the past regarding the synthesis of novel quaternarium ammonium ILs and their implementation in cellulose and lignocellulose dissolution. This should be cited.

Moreover, some relevant works of alkylammonium hydroxide solutions (also known as onium hydroxides) and their interaction with lignocelluloses should be considered:

Abe, M.; Fukaya, Y.; Ohno, H. Fast and facile dissolution of cellulose with tetrabutylphosphonium hydroxide containing 40 wt% water. Chem. Commun. 2012, 48 (12), 1808−1810.

Abe, M.; Kuroda, K.; Ohno, H. Maintenance-Free Cellulose Solvents Based on Onium Hydroxides. ACS Sustainable Chem. Eng. 2015, 3 (8), 1771−1776.

Ema, T.; Komiyama, T.; Sunami, S.; Sakai, T. Synergistic effect of quaternary ammonium hydroxide and crown ether on the rapid and clear dissolution of cellulose at room temperature. RSC Adv. 2014, 4 (5), 2523−2525.

Zhong, C.; Wang, C. M.; Huang, F.; Jia, H. H.; Wei, P. Wheat straw cellulose dissolution and isolation by tetra-n-butylammonium hydroxide. Carbohydr. Polym. 2013, 94 (1), 38−45.

Abe, M.; Yamada, T.; Ohno, H. Dissolution of wet wood biomass without heating. RSC Adv. 2014, 4 (33), 17136−17140.

Hyvakko, U.; King, A. W. T.; Kilpelainen, I. Extraction of Wheat Straw with Aqueous Tetra-n-Butylphosphonium Hydroxide. BioResources 2013, 9 (1), 1565−1577.

Abe, M.; Yamanaka, S.; Yamada, H.; Yamada, T.; Ohno, H. Almost complete dissolution of woody biomass with tetra-nbutylphosphonium hydroxide aqueous solution at 60 degrees C. Green Chem. 2015, 17 (8), 4432−4438.

Gubitosi, M.; Duarte, H.; Gentile, L.; Olsson, U.; Medronho, B. On cellulose dissolution and aggregation in aqueous tetrabutylammonium hydroxide. Biomacromolecules 2016, 17, 2873-2881

Lines 138-139: How was it found that the synthesized QA RTILs could break H bonds? What is the role of the cation? If the cation has no role then one would simply need a low viscosity acetate-base compound, such as acetic acid, which unfortunately does not work as direct cellulose solvent.

Figure 1f. Dissolution is not perfect, even after 20min, since a lot of particles and birefringent cellulose fragments are still observable. Therefore, dissolution seems to be incomplete. Please comment.

Line 177. The authors state that the transition from cellulose I to cellulose II was also reported in another solvent system. I should add that the majority of cellulose solvents convert native cellulose I in the polymorph cellulose II. So nothing remarkable is reported here.

What is the scale in figure 5, not visible.

During the recovery and recycle of the IL the authors claim IL adheres to the distillation bottle thus reducing the amount of recovered IL. Could this be minimized? How much IL is estimated to be impregnated in the film after regeneration?

Use the same scale in figure 6. Let’s say from 0.5 to 4 ppm.

It is claimed in the conclusions that the regenerated films exhibited good mechanical properties but NO proper tests have been performed to support this statement. These needs to be done to support any mechanical analysis.   

The authors state that these novel solvents are “ideal green solvents” but again no tests were performed regarding their hazardous effects to living organisms or degradation rate in the soil or in aqueous environment. This is far too speculative.

One of my major concerns is related to the poor discussion done regarding the effect of the cation. The authors mention on the viscosity effect (which is only related to kinetics) and the capacity on these systems to break H bonds. However, nowhere in the paper they show clear evidence supporting this mechanism. On the other hand, the authors completely neglect the amphiphilic properties of cellulose and of these cations and how that may contribute to the overall picture. This is even more striking because the anion is kept constant and only the cation is changed and therefore it is strange that the discussion does not focus on the differences on the cations (apart from the viscosity effect).

Author Response

Apr. 15, 2019

Replay to reviewers’ comment

Manuscript ID: applsci-462475

Revised manuscript title: Synthesis of Quaternary Ammonium Room-Temperature Ionic Liquids and its Application in Dissolution of Cellulose

Dear Sir/Madam,

We thank you for giving us valuable suggestions to improve the quality of our manuscript. We have revised our manuscript by following your suggestion. We have prepared revised manuscript and responded to your questions. The following are list of amendments to your queries.

***************************************************************************

Revision

Reviewer #1: This is an interesting work where novel aqueous based systems for cellulose dissolution and processing have been developed and characterized. The paper is reasonably well written but it should be submitted to a professional proof reading. In what follows, a few questions /comments are enumerated which need to be addressed before the paper is suitable for publication.

The authors state that “NMMO is the common cellulose dissolving solvent currently.” It should be noted that despite being highly efficient in dissolving cellulose and the main ingredient of the Lyocel process, viscose still dominates, by far, the market of cellulose man-made fibers. So NMMO is an interesting solvent but the drawbacks presented (as the ones the authors enunciate) plus the major investments in safety are still favoring the viscose process. This should be highlighted.

Response: Many thanks to Reviewer’s kindly corrections and remind. The statement regarding NMMO was revised in the introduction of the revised manuscript.

The “greenness” of ionic liquids is questionable. For instance, some show toxic and poisonous properties to aquatic organisms. See for instance the following references:

Wells A S , Coombe V T . On the Freshwater Ecotoxicity and Biodegradation Properties of Some Common Ionic Liquids. Organic Process Research & Development, 2006, 10(4):794-798;

QUIJANO, Guillermo, Annabelle, et al. Toxicity and biodegradability of ionic liquids: New perspectives towards whole-cell biotechnological applications. Chemical Engineering Journal, 2013, 174(1):27-32.;

Zhang C, Zhu L, Wang J, et al. The acute toxic effects of imidazolium-based ionic liquids with different alkyl-chain lengths and anions on zebrafish (Danio rerio). Ecotoxicol Environ Saf, 2017, 140:235-240.;

Jing C Q, Liu Y T, Shen X Y, et al. Toxic evaluation of the alkylmethylimidazolium-based ionic liquids in yeast Saccharomyces cerevisiae by flow cytometry. Toxin Reviews, 2012, 31(3-4):27-31.

Response: Many thanks to Reviewer’s kindly corrections and remind. The “greenness” of ionic liquids herein indicate their recyclable property. The toxicity of certain quaternary ammonia cations and related anions should be concerned. The statement regarding greenness of ionic liquids was revised in the introduction of the revised manuscript.

Poor reference selection. Much more key references are needed.

Please note that work has been done in the past regarding the synthesis of novel quaternarium ammonium ILs and their implementation in cellulose and lignocellulose dissolution. This should be cited. Moreover, some relevant works of alkylammonium hydroxide solutions (also known as onium hydroxides) and their interaction with lignocelluloses should be considered:

Response: Many thanks to Reviewer’s kindly remind. The onium hydroxide solutions also exhibit good dissolution ability to cellulose and provide information for dissolution mechanism. The key references, as shown in following, will be added into the revised manuscript as Ref. 8-15.

Abe, M.; Fukaya, Y.; Ohno, H. Fast and facile dissolution of cellulose with tetrabutylphosphonium hydroxide containing 40 wt% water. Chem. Commun. 2012, 48 (12), 1808−1810.

Abe, M.; Kuroda, K.; Ohno, H. Maintenance-Free Cellulose Solvents Based on Onium Hydroxides. ACS Sustainable Chem. Eng. 2015, 3 (8), 1771−1776.

Ema, T.; Komiyama, T.; Sunami, S.; Sakai, T. Synergistic effect of quaternary ammonium hydroxide and crown ether on the rapid and clear dissolution of cellulose at room temperature. RSC Adv. 2014, 4 (5), 2523−2525.

Zhong, C.; Wang, C. M.; Huang, F.; Jia, H. H.; Wei, P. Wheat straw cellulose dissolution and isolation by tetra-n-butylammonium hydroxide. Carbohydr. Polym. 2013, 94 (1), 38−45.

Abe, M.; Yamada, T.; Ohno, H. Dissolution of wet wood biomass without heating. RSC Adv. 2014, 4 (33), 17136−17140.

Hyvakko, U.; King, A. W. T.; Kilpelainen, I. Extraction of Wheat Straw with Aqueous Tetra-n-Butylphosphonium Hydroxide. BioResources 2013, 9 (1), 1565−1577.

Abe, M.; Yamanaka, S.; Yamada, H.; Yamada, T.; Ohno, H. Almost complete dissolution of woody biomass with tetra-nbutylphosphonium hydroxide aqueous solution at 60 degrees C. Green Chem. 2015, 17 (8), 4432−4438.

Gubitosi, M.; Duarte, H.; Gentile, L.; Olsson, U.; Medronho, B. On cellulose dissolution and aggregation in aqueous tetrabutylammonium hydroxide. Biomacromolecules 2016, 17, 2873-2881

Lines 138-139: How was it found that the synthesized QA RTILs could break H bonds? What is the role of the cation? If the cation has no role then one would simply need a low viscosity acetate-base compound, such as acetic acid, which unfortunately does not work as direct cellulose solvent.

Response: Many thanks to Reviewer’s kindly remind and suggestion. The intermolecular and intramolecular hydrogen bonds of cellulose are broken by QA RTILs, resulting in the change of crystalline structure of regenerated cellulose. The conjunction with the spatial heterogeneity in QA RTILs with an amphiphilic cation due to the aggregation of alkyl tails. The hydrophobic tail aggregation implies a trend to make other parts more hydrophilic and polar. Hence, an appropriate increase in alkoxy chains leads QA RTILs to be more powerful cellulose solvents, through increasing the spatial heterogeneity. [added as Ref. 16 and 17] The low viscosity, acetic acid, dissolve the cellulose very slightly under the room-temperature condition. The statement for effect of cation will be addressed in the revised manuscript.

Figure 1f. Dissolution is not perfect, even after 20min, since a lot of particles and birefringent cellulose fragments are still observable. Therefore, dissolution seems to be incomplete. Please comment.

Response: Many thanks to Reviewer’s kindly remind. The dissolution of cellulose is not incomplete under relatively mild condition (50oC) as shown in Figure 1. The dissolution will be complete at higher temperature for 1 h of reaction. The one of goals in this work is the preparation of smooth regenerated cellulose film, as shown in Figure 5. The solution of Figure 1(f) is a proper coating material, so the complete dissolution is not emphasized in this manuscript. The complete dissolution is very important factor for biological fermentation issue and we will study it in the future.

Line 177. The authors state that the transition from cellulose I to cellulose II was also reported in another solvent system. I should add that the majority of cellulose solvents convert native cellulose I in the polymorph cellulose II. So nothing remarkable is reported here.

Response: Many thanks to Reviewer’s kindly suggestion. The crystal structure of regenerated cellulose is a proper parameter to indicate the dissolution ability of cellulose solvent and the proper cellulose concentration in RTILs for the coating process.

What is the scale in figure 5, not visible.

Response: Many thanks to Reviewer’s kindly remind. The scale bars are added into the revised figure.

During the recovery and recycle of the IL the authors claim IL adheres to the distillation bottle thus reducing the amount of recovered IL. Could this be minimized? How much IL is estimated to be impregnated in the film after regeneration?

Response: Many thanks to Reviewer’s kindly suggestion. In the lab test, the recovery of the IL is difficult to reach 99%. The recovery rate should be reasonably increased in a large-scale production process, and the recovery ratio cannot be estimated form present data precisely. The regenerated cellulose film was analyzed by an elemental analyzer for CHON shown in the following table. The result indicates the residual IL in the film is trace and not estimated correctly.

Table. EA analysis of CTA-5 and CNA-5 films

Use the same scale in figure 6. Let’s say from 0.5 to 4 ppm.

Response: Many thanks to Reviewer’s kindly correction. The statement is rewritten in the revised manuscript.

It is claimed in the conclusions that the regenerated films exhibited good mechanical properties but NO proper tests have been performed to support this statement. These needs to be done to support any mechanical analysis.  

Response: Many thanks to Reviewer’s kindly remind. The mechanical properties of the films are examined roughly by hand-tearing test. The films with rough morphology and fiber texture, such as CA-5 and CQA-5, are broken easily. A blade coater should be applied to control the film thickness and is not available in our lab now. The regular stretching test will be carried out in the future work.

The authors state that these novel solvents are “ideal green solvents” but again no tests were performed regarding their hazardous effects to living organisms or degradation rate in the soil or in aqueous environment. This is far too speculative.

Response: Many thanks to Reviewer’s kindly remind. The “greenness” of ionic liquids herein indicate their recyclable property. The statement regarding greenness of ionic liquids will be revised in the revised manuscript.

One of my major concerns is related to the poor discussion done regarding the effect of the cation. The authors mention on the viscosity effect (which is only related to kinetics) and the capacity on these systems to break H bonds. However, nowhere in the paper they show clear evidence supporting this mechanism. On the other hand, the authors completely neglect the amphiphilic properties of cellulose and of these cations and how that may contribute to the overall picture. This is even more striking because the anion is kept constant and only the cation is changed and therefore it is strange that the discussion does not focus on the differences on the cations (apart from the viscosity effect).

Response: Many thanks to Reviewer’s kindly remind and suggestion. The conjunction with the spatial heterogeneity in QA RTILs with an amphiphilic cation due to the aggregation of alkyl tails. The hydrophobic tail aggregation implies a trend to make other parts more hydrophilic and polar. Hence, an appropriate increase in alkoxy chains leads QA RTILs to be more powerful cellulose solvents, through increasing the spatial heterogeneity. The discussion regarding cations were added into the revised manuscript.

Reviewer 2 Report

This article on the dissolution of cellulose using quaternary ammonium room-temperature ionic liquids is interesting. The authors found ILs capable of dissolving cellulose whic are easy to synthesize and that are recyclable.

Some comments :

- in the introduction part, the authors wrote that room-temperature ionic liquids (RTILs) are excellent green solvents. This sentence has to be modulate since it is not always the case.

- It could be interesting to study the amount of water that these RTILS can stand without loosing their efficiency in the dissolution of cellulose, based on the fact that biomass contains always water.

Author Response

Apr. 15, 2019

Replay to reviewers’ comment

Manuscript ID: applsci-462475

Revised manuscript title: Synthesis of Quaternary Ammonium Room-Temperature Ionic Liquids and its Application in Dissolution of Cellulose

Dear Sir/Madam,

We thank you for giving us valuable suggestions to improve the quality of our manuscript. We have revised our manuscript by following your suggestion. We have prepared revised manuscript and responded to your questions. The following are list of amendments to your queries.

***************************************************************************

Revision

This article on the dissolution of cellulose using quaternary ammonium room-temperature ionic liquids is interesting. The authors found ILs capable of dissolving cellulose whic are easy to synthesize and that are recyclable. Some comments:

- in the introduction part, the authors wrote that room-temperature ionic liquids (RTILs) are excellent green solvents. This sentence has to be modulate since it is not always the case.

- It could be interesting to study the amount of water that these RTILS can stand without loosing their efficiency in the dissolution of cellulose, based on the fact that biomass contains always water.

Response: Many thanks to Reviewer’s kindly remind and suggestion. We will keep on doing this work.

Round 2

Reviewer 1 Report

The authors have provided acceptable replies to the majority of my comments and the manuscript has been improved. Therefore I believe the paper is now suitable for publication.