Acidic and Catalytic Properties of Zeolites Modified by Zinc in the Conversion Process of Lower C₃–C₄ Alkanes

Round 1

Reviewer 1 Report

Metal-modified zeolites catalyze the conversion of light alkanes to aromatics. The role of the metal species and acid sites of a zeolite in the enhancement of light alkane conversion to aromatics is now accepted and could summarized as follow:

-          the metal species are responsible for the dehydrogenation of alkanes

-          the Brønsted acid sites of the zeolite are responsible for the the oligomerization and cyclization of olefins.

It would be nice if the authors discuss the formation of metal-alkyl species and compared the ZnO and ZnS based zeolites. This point is mentioned in the conclusion, but there is no comment of this particular point in the discussion.

Numerous metal-modified zeolites have been described in the literature, and among Ga-modified zeolites showed interesting properties (see ref 4 and 27 cited by the authors among others). Furthermore, it has been noticed that zinc catalyst suffers from vaporization from zeolites due to the reduction of the zinc oxide into metals that have a low melting point and boiling point (See ref 8 cited by the authors). In this publication dealing with rhenium-modified HZSM-5 catalyst, it is conclude “zinc-promoted zeolites, gallium-promoted catalysts are more effective for propane transformation” (introduction first paragraph). Authors should discuss that particular point and justify the use of Zn-based zeolites.

Did the authors checked the potential reduction of ZnO into Zn during the process and performed time on stream study?

Did the authors noticed a difference between ZnO and ZnS ?

A comparison with all prepared materials to an academia standard such as Ga-modified zeolites could be interesting.

Most of the discussion of the paper is related to the description of tables. As such no clear trend could be found: It was claimed in the conclusion that higher yield of aromatic are produced using ZnS-modified zeolites. However, when looking at the composition of the liquid phase as the function of the amount of ZnS, no clear trend could be found between ZnS concentration and BTX yields. The same is true in table 1. The authors should improved the discusion of their results.

Could the authors can detailed what are “pseudocumulus” in table 1 and 2.

 

Author Response

1. In our works we deal with different modifying additives to increase the activity of the zeolite catalysts.

In this paper, to increase the activity of ZSM-5 zeolites, we used ZnO (T about 2000 ^oC) and ZnS as modifiers. Under the conditions of synthesis, formation and catalysis, the formation of metallic zinc from possible different oxygen-containing zinc compounds is not observed. In the section "Introduction" we considered the various possible oxygen-containing zinc compounds formed during the synthesis and formation of Zn-containing catalysts in details.

 

2. As for comparison of ZnO and ZnS modifiers. The oxides and sulphides of zinc compared to the metallic zinc are very heat resistant substances. In addition, we were the first to propose zinc sulfide ZnS (T – 1850 ^oC at 15 Mpa) as a modifier, it is a very thermostable substance and a more active modifier for the formation of arenes from the propane-butane fraction compared with ZnO. The introduction of 3-5% ZnS to the H-ZKE-AF zeolite allows increasing the yield of aromatic hydrocarbons to 60.6% in the liquid products of PBF conversion, which is 9% higher than the initial H-ZKE-AF and 4-6% more than with 1-3% ZnO/99-97% of H-ZKE-AF.

 

3. We agree with the comment of the reviewer regarding pseudocumene. In the tables 1 and 2 pseudocumene is added (1,2,4-trimethylbenzene).

 

 

 

On behalf of all the authors, I express our sincere gratitude for the review of the article.

Respectfully,

Professor V.I. Erofeev

Reviewer 2 Report

I have carefully read paper catalysts-464304, with title "Acidic and Catalytic Properties of Zeolites Modified by Zinc in the Conversion Process of Lower C3-C4 Alkanes" to find it worthy of publication in Catalysts. It is a sound addition to the synthesis and characterisation of catalysts focused on the production of valuable platform chemicals out of methane, ethane and other low molecular weight hydrocarbons found in natural gas and similar fossil sources. There is, however, some space for improvement:

1) The authors used ZnO and ZnS as modifiers for their catalysts. However, these catalysts are synthetised in the presence of an organic fraction rich in alcohols that is a subproduct of e-caprolactam process. With this in mind, is the composition of this fraction always the same? Being a subproduct, its composition could change and this would affect the structure and catalytic behaviour of the catalyst obtained. Maybe some more comments on this modifier should be included in an introduction section that is short as it is now.

2) There are a phlethora of numbers in tables 1 and 2, but no error value is introduced. Could the authors give an idea of the error they have in the determination of yields, conversions and percentages? Maybe not in the tables, which have numbers enough by now, but in the text, when commenting such values or just before. This is just to state that the differences found are statistically consistent.

3) Minor comment: please, pass all table 3 to page 6, so it is not divided between two pages.

4) In the "Author Contributions" section, do not use, please, cyrilic characters. Write everything in English so to state clearly the contributions of all authors to the paper.

Author Response

 

1. The article in the sections concerning the synthesis of the zeolites and the materials provides the clarifications on the use of the "alcoholic fraction" for the synthesis of the zeolites.

 

2. The section 3.4. specifies the error of gaseous and liquid hydrocarbon determination by the gas chromatography method (2.5 %).

 

3. The tables 1-3 are placed on different pages without any text breaks.

 

4. The section Author Contributions is translated into English.

 

 

On behalf of all the authors, I express our sincere gratitude for the review of the article.

 

 

Respectfully,

Professor V.I. Erofeev

Reviewer 3 Report

Thank you for inviting me to review this manuscript. I think that is an interesting investigation. However, there are in the text a several corrections or comments. These are my corrections and suggestions for improving the manuscript:

Comments for author File: Comments.pdf

Author Response

Response to the reviewer 3

19.04.2019

 

 

1. The paragraphs highlighted in yellow mean the author’s corrections and additions.

 

2. Regarding the absence of the modified zeolite X-ray patterns in the article. Due to the fact that the content of the modifiers in the zeolite is slight - 1-5%, they are not seen on the X-ray patterns of the modified zeolites. This is stated in the article.

 

3. The IR spectrum of the pure zeolite is given in the article; the IR spectra of the modified zeolites are similar to the pure zeolite and therefore are not shown. The main bands of the IR spectrum are interpreted in the article.

 

4. The article presents a correlation between the surface properties of the catalysts and their acidity.

 

4. In Tables 1 and 2, the separation lines are made for the catalysts.

 

5. In figures 3 and 4, the numbers are set for each curve.

 

6. The article provides interpretations of various acid sites of the zeolite catalysts. Pyridine adsorption was not studied on the zeolite catalysts under question.

 

7. Synthesis of the zeolite powders with the same particle size was not studied.

 

8. The scheme of the catalytic unit is given in the article and briefly described.

 

 

 

On behalf of all the authors, I express our sincere gratitude for the review of the article.

 

 

 

Respectfully,

Professor V.I. Erofeev

Round 2

Reviewer 1 Report

I really appreciate that the authors change “pseudocumulus” in table 1 and 2 by “pseudocumene”.

I do not understand what mean “their composition meet the technological regulations” line 206. It would be better to give information on the “multicomponent organic structure-forming additive of the alcohol fraction (a byproduct of caprolactam production)”, i.e. MW, … since all the information are in a Russian patent (Erofeev, V.I.; Koval, L.M. Pat. 2313486 RU. 2007; ref 22).

In my review, I was asking if the authors:

- “checked the potential reduction of ZnO into Zn during the process and performed time on stream study?”

- A comparison of prepared materials to an academia standard such as Ga-modified zeolites

- Did the authors noticed a difference between ZnO and ZnS ?

 

I have no comments on these questions. For the last question, the comparison of Table 1 and 2, for example 5%ZnO and 5% ZnS at 575°C gave similar results for major products :

 

Liq phase wt%	ZnO   5wt%	ZnS   5wt%
Benzene	20.7	21.8
Toluene	40.0	40.9
Ethylbenzene	1.1	1.3
m-Xylene	8.1	8.6
p-Xylene	5.7	3.8
o-Xylene	3.2	4.1
Pseudocumene	0.3	0.5
Naphthalene	9.4	7.9
α-methylnaphthalene	0.3	3.9
β- methylnaphthalene	5.7	1.7

 

The differences between benzene and toluene are very low. What is the standard error?

The authors should comment the inversion of selectivity between α-methylnaphthalene and β- methylnaphthalene using ZnO and ZnS. The same question for the selectivity of xylene. Same amount of m-Xylene but higher amount of p-Xylene with ZnO and higher amount of o-Xylene with ZnS. Comments?

 

I am still waiting response of the authors on a point by point basis before the publication could be published in Catalysts.

Author Response

Response to the review

28.03.2019

 

 

1. The chemical composition of such an organic additive as the alcoholic fraction for the zeolite synthesis is given in the article in the section concerning the synthesis of zeolites and materials.

 

2. Concerning a decrease in content of ZnO in Zn during conversion process of propane-butane fraction into arenes in the article, we specify in the introduction that during formation of ZnO-zeolite catalysts at small concentrations of 0.5-5%ZnO in the zeolite under the conditions of propane conversion process, formation of surface ZnOH⁺ and ZnO is most probable. Formation of the bulk phases of Zn on the zeolite surface at a small concentration of ZnO is very improbable. We did not study this question in the work, as well as we did not compare ZnO with the Ga-modified zeolites. However, we studied these catalysts before, and the Ga-modified zeolites are more active than the Zn-zeolite catalysts.

 

3. As for the difference of ZnO and ZnS during conversion process of propane-butane fraction into arenes in the article, in the section concerning conversion process of propane-butane fraction and in the annotation it is specified that there is 9% more of liquid products (arenes) with 3 and 5% ZnS-zeolite catalysts than with the ZnO-zeolite catalysts.

 

4. As for a small difference in an output of some products: benzene and toluene with the ZnO and ZnS-zeolite catalysts. As a part of liquid products we obtain not two substances – benzene and toluene, but more than 10-15 components. It leads to an increase in the product output by 9% that is presented by these 10-15 components, and as a result, on average, it turns out to be 0.5 - 2% each.

 

5. The question of "selectivity inversion" of some isomers: o-, m-. p-xylols and methylnaphthalenes was not specifically studied in the article as it is not essentially important. The purpose of the work was not to obtain p-xylol or α-methylnaphthalene, but to obtain liquid hydrocarbons (arenes) from gaseous raw materials – the propane-butane fraction. The difference in formation of these isomers can be apparently explained by probability and spatial factors which really play an important role in the zeolite catalysts and reaction.

 

 

 

On behalf of all the authors, I express our sincere gratitude for the review of the article.

 

 

Respectfully,

Professor V.I. Erofeev

Reviewer 3 Report

  The autors have made all the proposed changes                                                                                                                                   

Round 3

Reviewer 1 Report

- I still do not understand what mean “their composition meet the technological regulations” line 207. I appreciate that the authors gave the composition of the alcohol fraction. Concentration of cyclohexanone vary form 2 to 22 wt%? This is no influence of such parameter on the properties of the catalyst? 2-heptanone instead of "heptanone - 2".

- I understand that "The purpose of the work was not to obtain p-xylol or α-methylnaphthalene, but to obtain liquid hydrocarbons (arenes) from gaseous raw materials". But the composition of the liquid phase and the ratio of the "10-15 components" could give valuable information of the effect of ZnO vs ZnS. In conclusion authors said line 193  "The presence of the free 3 d-orbitals in zinc sulfides makes the electrons more mobile and reactive ..." and this could be illustrated with the isomers ...  May be but should try ...

Furthermore, when only looking to the "Yield of liquid products" in table 1 (ZnO) and table 2 (ZnS) no comparison could be found in the text. Why the yield is decreasing when increasing the amount of ZnO althought the yield is constant when increasing the amount of ZnS. The authors should at least try to explain this trend using the section  2.3. "Acid Propertis of Catalysts".

In table 1 (ZnO) and table 2 (ZnS) at 550°C, we have a yield of  50.7% and 50.5% respectively.

Again what is the difference between ZnO and ZnO?

Author Response

Response to the reviewer 1

19.04.2019

 

 

1. As for the first three comments, we have corrected the article.

 

2. Regarding the last fourth comment on the same arene output of 50.7% and 50.5% at 550 °C for ZnS and ZnO, we can say that the yields of the liquid products from the propane-butane fraction on the studied catalysts were similar at 550 °C, but the contents of the gaseous and liquid products were different.

 

 

 

On behalf of all the authors, I express our sincere gratitude for the review of the article.

 

 

 

Respectfully,

Professor V.I. Erofeev