Kinetic and Spectroscopic Studies of Methyl Ester Promoted Methanol Dehydration to Dimethyl Ether on ZSM-5 Zeolite
Round 1
Reviewer 1 Report
The article investigates the effect of esters on the methanol dehydration reaction on zeolite catalysts. The topic is interesting and relevant. The work is well structured. However, while reading, I have a few questions:
- The need for 16 reactors and 24 hours of operation is not clear. What is the reproducibility of the experiment? Do any process parameters change over time?
- Why a number of studies (kinetic studies, FT-IR) were carried out on SAR80 zeolite, but NMR on SAR23?
- Methylhexanoate has a boiling point of 149.5C, while kinetic experiments are carried out at temperatures up to 130C. Thus, it is impossible to talk about its partial pressure based on added ammount.
- According to the reaction scheme (Fig. 11), acid is the intermediate product. However, the authors do not write whether they observed an acid in the reaction products?
- lines 210-213, Figure 3. It is not clear at what partial pressure of methanol the dependencies are given.
- lines 337-234. It is not clear how the TOF values were calculated. What reaction order was assumed?
- It seemed to me that it would be interesting to compare the maximum DME yields achieved on a catalyst without promotion, with promotion, and DME yield according to thermodynamic equilibrium. But this is on the discretion of the authors.
Author Response
We thank for the referee for their expert comments, which we address below.
- The need for 16 reactors and 24 hours of operation is not clear. What is the reproducibility of the experiment? Do any process parameters change over time?
This has been addressed in the revised experimental section for the kinetic experiments. The use of high throughput experimentation for the parallel testing catalysts at small scale has been routine in our laboratories for well over a decade. Modern HTE equipment (such as the Flowrence unit from Avantium BV) gives excellent reproducibility (see the revised manuscript), provided the equipment is thoroughly cleaned between each experiment following the protocol described for these experiments. This is particularly important given the potency of some of the promoters found in our work with zeolite catalysts for methanol dehydration to DME. HTE also allows control measurements (with a blank reactor and a benchmark catalyst consistently tested at the same condition) to be made in parallel in the same experiment. The HTE unit we used in our work is equipped with 4 temperature blocks, each with 4 reactors, allowing the H-ZSM-5 catalyst to be simultaneously tested (i.e. in parallel) at different temperatures.
- Why a number of studies (kinetic studies, FT-IR) were carried out on SAR80 zeolite, but NMR on SAR23?
The NMR studies used the SAR23 catalyst to improve the sensitivity of the measurements, by allowing more methyl ester to be adsorbed on the increased number of BA sites in this material. Note the methyl ester used in our experiments (unlike the methanol) was not 13C enriched. In our previous work we have shown the methyl n-hexanoate strongly promotes DME formation on ZSM-5 with a SAR 23, as well as with a SAR of 80; this has been noted in the revised manuscript.
- Methylhexanoate has a boiling point of 149.5C, while kinetic experiments are carried out at temperatures up to 130C. Thus, it is impossible to talk about its partial pressure based on added amount.
The methyl n-hexanoate was tested by diluting it in methanol, with the partial pressure of the ester below the Dew point for all the conditions tested. This has been addressed in the revised experimental section. The maximum partial pressure of methyl n-hexanoate tested was 0.165 bar, which is below its saturated vapour pressure at the lowest temperature (110 oC) tested in this work.
- According to the reaction scheme (Fig. 11), acid is the intermediate product. However, the authors do not write whether they observed an acid in the reaction products?
The methanol is in large excess to the methyl n-hexanoate in all the kinetic experiments. In addition, the methanol conversion is low (< 35 % and much lower than that in most of the measurements reported) in all the experiments conducted. We have previously shown that under such low methanol conversion conditions methyl esters largely pass over the ZSM-5 catalyst unconverted and that carboxylic acids are rapidly esterified [see reference 18]. Comments have been accordingly added to experimental section in the revised manuscript.
- lines 210-213, Figure 3. It is not clear at what partial pressure of methanol the dependencies are given.
This has been addressed in the revised manuscript. The methanol partial pressure in these experiments was 1.1 bar.
- lines 337-234. It is not clear how the TOF values were calculated. What reaction order was assumed?
This has been addressed in the revised manuscript, with the equation used to calculate the TOF values added.
Note : The apparent activation energy measurements were taken at conditions where the reaction was zero order in methanol and methyl n-hexanoate. As noted in the manuscript the methanol conversion for these measurements was below 10%.
- It seemed to me that it would be interesting to compare the maximum DME yields achieved on a catalyst without promotion, with promotion, and DME yield according to thermodynamic equilibrium. But this is on the discretion of the authors.
All the experiments are performed with methanol conversions significantly away from equilibrium conversion of methanol to DME (95% at 110 oC); this now noted in the revised manuscript. We have previously shown in the ESI for the original communication [reference 18] related to this work that the methyl ester promoters are effective over a wide range of methanol conversions.
Reviewer 2 Report
In overall, this is a high level research paper. I think this manuscript needs no further revisions. I agree to be considered for publication in the current form.
Author Response
In overall, this is a high level research paper. I think this manuscript needs no further revisions. I agree to be considered for publication in the current form.
- Nothing to address, we thank the referee for their expert, succinct and fair review.
Reviewer 3 Report
Comments on Chemistry-2204744
In this paper, reaction kinetics and in-situ spectroscopic studies have been carried out to study to further elucidate the promotional mechanism of the carboxylate esters in methanol dehydration to DME on zeolites. Overall, the article is well organized and its presentation is good. However, some minor issues still need to be improve.
1. There are a few grammar errors in this paper. It will be good to do a thorough proof-reading of the manuscript again.
2. XRD are marked with detailed diagrams to facilitate readers to better understand your expression, and it will be more professional.
3. The references should be expanded. Some new literatures might be help the authors to further deepen the understanding of reaction mechanism as well as newest developing in this field (Journal of Environmental Management, 2023, 326: 116790 Regeneration mechanism of a novel high-performance biochar mercury adsorbent directionally modified by multimetal multilayer loading).
Author Response
We thank for the referee for their expert comments, which we address below.
- There are a few grammar errors in this paper. It will be good to do a thorough proof-reading of the manuscript again.
The grammar and phraseology in the paper has been reviewed and changed where appropriate, see the revised manuscript.
- XRD are marked with detailed diagrams to facilitate readers to better understand your expression, and it will be more professional.
The XRD diagram has been updated accordingly, see the revised manuscript.
- The references should be expanded. Some new literatures might be help the authors to further deepen the understanding of reaction mechanism as well as newest developing in this field (Journal of Environmental Management, 2023, 326: 116790 Regeneration mechanism of a novel high-performance biochar mercury adsorbent directionally modified by multimetal multilayer loading).
We fully agree with the referee that a bit more context could be provided on the established mechanisms for zeolite catalysed methanol dehydration to DME. We have added a short paragraph to the with 5 additional references to the introductory section of the manuscript. One of these references is from 2021, so presents some of the latest thinking in this extensively studied area. We respectfully do not see the direct relevance of the reference provided by the referee to the work we report here though the reference provided is very clearly an important topic to study.
