Bifunctional Hybrid FTS Catalyst Mixed with SAPO-34 Zeolite for Application in the GTL-FPSO Process

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript on Fischer-Tropsch synthesis (FTS) using hybrid catalysts, which explores the integration of Co/γ-alumina catalyst with various zeolites to enhance hydrocarbon production, presents interesting insights into the gas-to-liquid process. The investigation into the physicochemical properties of these catalysts and their performance in a fixed-bed reactor is commendable. Particularly, the findings on the hybrid catalysts producing lighter hydrocarbons and the role of SAPO-34 in hydrocracking heavy hydrocarbons are intriguing.

 

However, after careful consideration, I recommend a major revision of the manuscript. Below are some key areas that need to be addressed:

 

1.     It is crucial to include electron microscopy images to ascertain whether Co has been successfully loaded onto the supports through physical mixing. This will provide direct visual evidence of the catalyst's morphology and the distribution of Co on the supports.

2.     Stability is a critical aspect of FTS reactions, given the potential for carbon deposition on the catalyst surface. I suggest conducting X-ray diffraction (XRD) analyses of the catalysts before and after the reaction. This would help in understanding any structural changes that may contribute to catalyst deactivation.

3.     The manuscript currently relies heavily on literature references to explain the reaction mechanism. For a more rigorous and comprehensive understanding, it is essential to incorporate analysis using X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). These techniques will provide a deeper insight into the surface chemistry and functional groups involved in the reaction.

Comments on the Quality of English Language

I have reviewed the English language used in your manuscript and am pleased to report that the quality of writing is commendable. Your manuscript adheres well to the conventions of scientific research paper writing, which greatly facilitates understanding and engagement for readers.

Author Response

Thank you for your insightful review. The current study aims to identify the optimal ratio of zeolites in a hybrid catalyst that produces light hydrocarbons by applying different commercial and prepared zeolites. Also, when designing the hybrid catalysts, we used Co/γ-alumina catalysts that we had already made in large quantities, which is why we see the same numbers in the characterization. I am preparing a follow-up study that subdivides the types of zeolites. When conducting these experiments, I will apply precise analysis techniques (such as XPS and FTIR) to produce more detailed, higher-quality research results based on your suggestions.

Reviewer 2 Report

Comments and Suggestions for Authors

FTS is a well-studied process for obtaining liquid fuels. In order to improve selectivity to desired products, the use of an acid catalyst has been studied since the 1970s. This paper is interesting. However, major revision is required due to the following reasons.

1. The Si/Al ratios in zeolite beta, ZSM-5, and SAPO-34 are 38, 280, and 0.7, respectively. Why have these acid catalysts and such different Si/Al ratios been selected for the process?

2. As it is observed in Table 2, ZSM5 zeolite has a very low strong acidity. The main objective is to obtain C5+ hydrocarbons. The reason for using this catalyst is therefore not well understood. Wouldn't a ZSM5 zeolite with a lower Si/Al ratio be better?

3. Following with the previous aspect, ZSM5 zeolites with different Si/Al ratios should be studied.

4. The effect temperature has on CO conversion, and products yield and selectivity should be studied in more detail.

5. Even if in a qualitative way, some comments about the following conclusions of the paper should be included in the text:

- The origin of the deactivation. How does deactivation affect to the higher methane yield observed after 30 h of reaction? Is there a phase change in the Co? Coke deposition?

- Different ways to mitigate the deactivation of the solid should be indicated in the text.

6. The results should be compared with those obtained by other authors in recent papers using bifunctional catalysts of similar nature.

Comments on the Quality of English Language

Minor editing of English language is required

Author Response

Thanks for your insightful review. As you requested, I added a brief explanation of the origin of the deactivation in the discussion section(in 3.2 at third and fifth paragraphs). I also added a simple comparison to other studies on bifunctional catalysts of a similar nature based on the closest reaction conditions. In addition, in a follow-up study, the quality of the results will be improved through a more detailed analysis of the hybrid catalysts employing zeolites with various Si/Al ratios, and the reaction conditions will be further optimized to prevent coke deposition.

Reviewer 3 Report

Comments and Suggestions for Authors

The presented paper gives results of using physical mixture of Co/γ-alumina with SAPO-34, ZSM-5 and zeolite beta (hybrid catalysts) for FTS and shows that hybrid systems have some advantages such as the formation of lighter products than those obtained over the Co/γ-alumina.

The manuscript includes 45 references; most of them (38  -  84 % ) are older than five years, 5 of the rest are self-citations by authors. The most of the references are relevant to the topic (including self-citations - 10 in total).

 The part of the manuscript (Characterization of Hybrid FTS Catalysts,  Figs. 1-3, Table 1,2)  has been already published [29]; the authors do not use TPR reduction data of Co/alumina catalyst (Fig.2, lines 140-150) for discussion and therefore the description of TPR is needless and irrelevant to the scope under discussion.

The second part (Performance of Hybrid FTS Catalysts Mixed with Zeolites) presents new experimental results. This part is very difficult to read, and it seems to be a good idea to divide the text (lines 196-250) into fragments with a single statement in a separate paragraph.

The following principle points should be mentioned concerning the manuscript content:

 1          Conversion of synthesis gas (CO + H2) to hydrocarbon products in the fuel range has attracted enormous attention in recent years and a lot of review work has been published on the process. Therefore, one should compare the catalytic performance with catalyst which is commonly used in commercial FTS reactions (for ex. See [45]) to “…advance the development of an improved GTL-FPSO process…” (line 266).

 2          When describing catalytic activities and hydrocarbon selectivities of the prepared catalysts (Table 3) the authors said that “…the CO conversion and hydrocarbon selectivity were determined under the testing conditions” (line 184). This statement is very uncertain taking into account the fast deactivation of catalysts.  It is necessary to define exactly “the testing conditions” to be sure that the comparison is correct.   

3            “…As shown in Table 3, the hybrid FTS catalyst prepared using Co/γ-alumina and zeolite beta alone is high-performing, but the mixed catalysts incorporating both ZSM-5 and SAPO-34 perform even better…” (lines 234-236).  That is not true.  Co/Al2O3+Beta hybrid catalyst has highest C5+ selectivity (85%); only Co/Al2O3+SAPO-34 (2) -ZSM-5 (8) composition is more active than Co/Al2O3+Beta ( CO conversions 73% and 69%, correspondingly). But the C5+ yield (conversion *selectivity, a fraction of CO converted to C5+ products) has the highest value for Co/Al2O3+Beta (59%) compared to 58, 50 and 34% for Co/Al2O3+ SAPO-34 (2) -ZSM-5 (8), Co/Al2O3+ SAPO-34 (5) -ZSM-5 (5) and Co/Al2O3+ SAPO-34 (8) -ZSM-5 (2), accordingly.

Specific comments:

It is necessary to give radius of catalyst pellet and supports

Table 1 and 3: Please, adjust significant digits (see, for ex. [29])

Table 2: Area of NH3 TPD peak is not a widespread way to measure acidity, arbitrary units are better.

Author Response

Thank you for your insightful review.

1. We added brief a comparison with other hybrid catalysts under the closest reaction conditions, as also suggested by Reviewer 2.
2. As requested, I replaced the phrase “under the testing conditions” in the footnotes of Table 3 to convey that these values are averages from stabilization to deactivation (20–50 h).
3. In terms of the C₅₊ yield, the reviewer is correct, but in terms of producing lighter hydrocarbons, SAPO-34:ZSM-5 with a lower alpha number (ratio of 2:8) was more effective.

The manuscript has also been revised based on specific comments Lines 1 and 2. Regarding the comment on line 3, we have shown the results from a previous study to compare the acidity of the supports. In a subsequent study, we will determine this property using a more sophisticated method.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I commend your thorough approach and innovative research on optimizing zeolite ratios in hybrid catalysts. And, I recommend accepting this manuscript for its contribution to the field of FTS catalyst.

Reviewer 2 Report

Comments and Suggestions for Authors

The suggestions of the first revision and new relevant comments have been included to the text.

Reviewer 3 Report

Comments and Suggestions for Authors

I do recommend to change significant figures in Table l (BET,Avg. pore dia., d(Co3O4), similar to [44]) to  agree with measurement uncertainties.