Cleanup and Conversion of Biomass Liquefaction Aqueous Phase to C₃–C₅ Olefins over Zn_xZr_yO_z Catalyst

Round 1

Reviewer 1 Report

The manuscript deals with the zinc-zirconium oxide catalysts for biomass conversion in liquid reaction medium. There is a minor remark:

1). In name of manuscript «to C3 to C5 olefins over Zn_xZr_yO_z catalyst» should be replaced by «to C3-C5 olefins over zinc-zirconium oxide catalyst».

2). In «Keywords» the chemical symbols (Zn_xZr_yO_z) should not be used.

3). What is the correct name for the catalyst? Zn_xZr_yO_z (lines 15, 22, 24, etc.) or Zn₁Zr_2.5O (line 297)? I recommend choosing the Zn₁Zr_2.5O.

4). The characteristics of the zinc-zirconium oxide catalysts should be added to the manuscript. For example, the phase composition, zinc and zirconium content, functional groups on the catalyst surface, etc. This data can be given in table form.

5). The dot is placed at the end of the sentence after the reference is given. Check all the text!!!

6). The numbers should be separated from the unit by a space. Check all the text!!!

7). Tables should be prepared according to the journal rules.

8). In Figs. 2 and 4, some words and lines are not visible.

9). What is the experimental error of kinetic measurements?

10). In the hydrogen enrichment of the gas phase the acetone yield is increased. This fact should be explained taking into account the mechanism of catalytic interaction.

11). What is the role of the catalyst in the change of reaction route observed in the different medium? In this manuscript (for journal «Catalysts»), this question remained unanswered.

In my opinion, the interesting problem is discussed but the manuscript must be reviewed. I recommend this paper to be accepted for the publication in Journal «Catalysts» with changes.

Best regards, Reviewer.

Author Response

1). In name of manuscript «to C3 to C5 olefins over Zn_xZr_yO_z catalyst» should be replaced by «to C3-C5 olefins over zinc-zirconium oxide catalyst».

We have updated the tittle to be “Cleanup and Conversion of Biomass Liquefaction Aqueous Phase to C₃ – C₅ Olefins over Zn_xZr_yO_z Catalyst” We have chosen to leave Zn_xZr­_yO_z to be consistent with other literature in the field.

2). In «Keywords» the chemical symbols (Zn_xZr_yO_z) should not be used.

We have replaced Zn_xZr_yO_z with Zinc-Zirconia in the keywords

3). What is the correct name for the catalyst? Zn_xZr_yO_z (lines 15, 22, 24, etc.) or Zn₁Zr_2.5O (line 297)? I recommend choosing the Zn₁Zr_2.5O.

Have changed Zn_xZr_yO_z to Zn₁Zr_2.5O when discussing the catalyst used in this paper but have left general Zn_xZr_yO_z when discussing the catalyst system in general

4). The characteristics of the zinc-zirconium oxide catalysts should be added to the manuscript. For example, the phase composition, zinc and zirconium content, functional groups on the catalyst surface, etc. This data can be given in table form.

As we are not presenting the Zn_xZr_yO_z catalyst as our discovery we instead have added citations to other works where detailed characterizations of the catalyst system are available.

5). The dot is placed at the end of the sentence after the reference is given. Check all the text!!!

This has been corrected, thank you,

6). The numbers should be separated from the unit by a space. Check all the text!!!

This has been corrected, thank you.

7). Tables should be prepared according to the journal rules.

This has been corrected, thank you.

8). In Figs. 2 and 4, some words and lines are not visible.

We have updated the figures with larger text.

9). What is the experimental error of kinetic measurements?

Experimental error is 1% or less on all measurements. Text addressing this has been added to methods section and is copied below:

“For all reactions total carbon balance was >95% and measurement errors in composition are <1%.”

10). In the hydrogen enrichment of the gas phase the acetone yield is increased. This fact should be explained taking into account the mechanism of catalytic interaction.

This is a good question.  They key mechanistic insight is that upon hydrogen co-feed intermediate acetone is increasingly hydrogenated to form propene, while some acetone still undergoes condensation and decomposition reaction to form isobutene.  This was addressed in Section 2.3 where we highlight this mechanistic insight, and reference our prior work that details this catalysis, per below:

”…when the N₂ carrier gas was replaced by H₂ in in 25 vol% increments, the propene (C₃H₆) selectivity increased roughly linearly with H₂ composition from 5.40 to 18.6%_, while the isobutene selectivity decreased from 43.1 to 32.6%. The change in product distribution is because the hydrogenation of acetone (and subsequent dehydration to produce propene) is enhanced in the presence of H₂ while acetone self-condensation (to produce isobutene) is inhibited as depicted in Schematic 1 and described in our previous work [25]. Further, the CO₂ selectivity also decreased as a result of inhibiting the isobutene formation as expected from the reaction network described in Schematic 1.”

11). What is the role of the catalyst in the change of reaction route observed in the different medium? In this manuscript (for journal «Catalysts»), this question remained unanswered.

As described above there are mechanistic consequences when hydrogen is used instead of nitrogen co-feed  Regarding the role of the catalyst on the mechansism, more details were added to the introduction where Schematic 1 (the mechanisms) was discussed:

“…Ethanol first undergoes ethanol dehydrogenation and ketonization reactions thus producing acetone. ZnO addition offers the necessary basic sites while also suppressing most of the strong acid sites responsible for undesirable ethanol dehydration. Acetone then undergoes aldol condensation and C–C cleavage over acid sites, while the formation of acetone decomposition products (CH4 and CO2) is largely suppressed [25].”…

In my opinion, the interesting problem is discussed but the manuscript must be reviewed. I recommend this paper to be accepted for the publication in Journal «Catalysts» with changes.

Reviewer 2 Report

I have found the current study very interesting, especially considering the previous batch processing was integrated with a continuous flow liquid-liquid extraction. However, in detailed there are some points needed to be revised. Especially, the introduction part should be revised. The authors should highlight the importance of the study, differences with the other studies, the reasons why they focused on this research by taking the research questions given down below into account.

Here are comments on the current version of the paper:

Abstract

In general, the abstract seem OK but there is definitely room for some improvements by including some of the highlighted results, which are important to the field. Considering most of the researchers firstly read the abstracts, it is important to give the significant points instead of mostly describing the procedure (3 sentences max. are enough for the general description).

Introduction

The introduction is well written in terms of narrowing down to the specific research topic, however still too generic. In the final paragraph, authors are describing the main aim of the study as “feasibility of directly using biomass-liquefaction-derived aqueous phase for the direct production of olefin”. However, some of the below questions should be asked and answered:

Why olefins are important? What are their potential application areas? What are the some other ways of producing them? Why the method used in this study is important for producing them? Comparing the pros and cons with the other methods in the literature (by giving some brief literature research) would strengthen the paper. Novelty should be clarified. The important finding should be highlighted/summarized at the end of the introduction.

Results and Discussion

The long version of the abbreviations should be given at first when it appears in the text, so that the abbreviations can be used at the further pages. Please check all the abbreviations throughout the text. A section as “abbreviations” can be also practical for the readers.

Line 94: MTBE

Table 1: BDL

Line 76: PNNL-HTL

Line 76: KIT-SP

Line 76: USDA-FP and so on...

What is the basis of the total organic concentration (wt.%)? Biomass basis, bio-oil, dry basis etc.?

Line 77: “the feedstocks had a different total organic concentration, 26.2 wt % for KIT-SP, 14.6 wt% for USDA-FP, and 32.6 wt% for PNNL-HTL.”

Materials and Methods

Authors described the carbon treatment step as the first step of their procedure in order to remove colorful bodies and any other impurities from the samples. As given in the following sentences, they gave a correlation (simply) between the number of the carbon treatment and achieving the desired color. It would be nice to integrate the before/after photos (if it is possible) so that it would give a clear understanding. In case it is not possible, maybe some previous work can be cited.

LINE 80: “…….a carbon treatment was first performed to remove color bodies and other compounds from the as received samples as it has been previously reported to cause deactivation during catalytic upgrading….”

LINE 264: “For USDA-FP, two carbon treatments were sufficient to achieve the desired color change of the feedstock. For KIT-SP, five carbon treatments were required to achieve the desired color change of the feedstock.”

 References

Please give the required previous work reference or the relevant reference at the end of the sentence, in case it is indicated as “can be found elsewhere” as given below.

Line 288: “Mixed oxide catalysts were synthesized via wet impregnation of a Zn(NO3)2 · 6H2O solution on Zr(OH)4 as described elsewhere.”

Period (or comma depending on the sentence) should come after the references. The citation of the references within the text needs to be revised as follows:

Current version: Sentence . [x-x]

Correct version: Sentence [x-x] .

           

Author Response

I have found the current study very interesting, especially considering the previous batch processing was integrated with a continuous flow liquid-liquid extraction. However, in detailed there are some points needed to be revised. Especially, the introduction part should be revised. The authors should highlight the importance of the study, differences with the other studies, the reasons why they focused on this research by taking the research questions given down below into account.

Here are comments on the current version of the paper:

Abstract

In general, the abstract seem OK but there is definitely room for some improvements by including some of the highlighted results, which are important to the field. Considering most of the researchers firstly read the abstracts, it is important to give the significant points instead of mostly describing the procedure (3 sentences max. are enough for the general description).

More precise details from the text have been added to the abstract (please see track changes).

Introduction

The introduction is well written in terms of narrowing down to the specific research topic, however still too generic. In the final paragraph, authors are describing the main aim of the study as “feasibility of directly using biomass-liquefaction-derived aqueous phase for the direct production of olefin”. However, some of the below questions should be asked and answered:

Why olefins are important? What are their potential application areas? What are the some other ways of producing them? Why the method used in this study is important for producing them? Comparing the pros and cons with the other methods in the literature (by giving some brief literature research) would strengthen the paper. Novelty should be clarified. The important finding should be highlighted/summarized at the end of the introduction.

 

We have added some uses and citations for isobutene to the introduction. As the focus of the work here is demonstration of aqueous biomass liquefaction streams, we feel that further delving into fossil olefin production is beyond the scope of this work.

Results and Discussion

The long version of the abbreviations should be given at first when it appears in the text, so that the abbreviations can be used at the further pages. Please check all the abbreviations throughout the text. A section as “abbreviations” can be also practical for the readers.

Line 94: MTBE

Table 1: BDL

Line 76: PNNL-HTL

Line 76: KIT-SP

Line 76: USDA-FP and so on...

A list of abbreviations used has been added after the conclusion section.

What is the basis of the total organic concentration (wt.%)? Biomass basis, bio-oil, dry basis etc.?

Line 77: “the feedstocks had a different total organic concentration, 26.2 wt % for KIT-SP, 14.6 wt% for USDA-FP, and 32.6 wt% for PNNL-HTL.”

The basis of total organic concentration is simply the summation of all organic compounds identified via LC. Text has been added clarifying this and is copied below:

“As received, the feedstocks had a different total organic concentration (based on LC composition), 26.2 wt% for KIT-SP, 14.6 wt% for USDA-FP, and 32.6 wt% for PNNL-HTL.”

Materials and Methods

Authors described the carbon treatment step as the first step of their procedure in order to remove colorful bodies and any other impurities from the samples. As given in the following sentences, they gave a correlation (simply) between the number of the carbon treatment and achieving the desired color. It would be nice to integrate the before/after photos (if it is possible) so that it would give a clear understanding. In case it is not possible, maybe some previous work can be cited.

LINE 80: “…….a carbon treatment was first performed to remove color bodies and other compounds from the as received samples as it has been previously reported to cause deactivation during catalytic upgrading….”

LINE 264: “For USDA-FP, two carbon treatments were sufficient to achieve the desired color change of the feedstock. For KIT-SP, five carbon treatments were required to achieve the desired color change of the feedstock.”

Images of the KIT-SP feedstock before and after the carbon treatment have been added to the supporting information as well as reference to the prior work which also has images of the PNNL-HTL feedstock, before and after carbon treatment. Unfortunately, images of the USDA-FP feedstock before and after carbon treatment are not available.

 References

Please give the required previous work reference or the relevant reference at the end of the sentence, in case it is indicated as “can be found elsewhere” as given below.

Line 288: “Mixed oxide catalysts were synthesized via wet impregnation of a Zn(NO3)2 · 6H2O solution on Zr(OH)4 as described elsewhere.”

Reference has been added, we apologize for this oversight.

Period (or comma depending on the sentence) should come after the references. The citation of the references within the text needs to be revised as follows:

Current version: Sentence . [x-x]

Correct version: Sentence [x-x] .

This has been corrected, thank you.

Round 2

Reviewer 2 Report

The points which are addressed to improved the paper has been fully achieved by the authors.