Exploring Perhydro-Benzyltoluene Dehydrogenation Using Sulfur-Doped PtMo/Al₂O₃ Catalysts

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study developed a sulfur-doped PtMo/Al₂O₃ catalysts for organic dehydrogenation. I suggest major revision and some points are listed below.

1. More key data should be included in the abstract.
2. The introduction should show the merit of their work compared to the previous work, and summarize their novelty in one or two sentences.
3. Why choose metal-based catalyst? Literature review should be done to emphasize the importance, and some works about metal based catalyst could be referred to: J Environ Chem Eng, 2024, 12: 114446. J Environ Manage. 2024, 359, 120938. ACS ES&T Eng, 2024, 4: 2642-2656. NPJ Clean Water. 2024, 7: 38. Sep Purif Technol. 2024, 341: 126938. Chem Eng J. 2024, 479: 147320.
4. The author said based on their previous work, what’s the difference between these two works?
5. Please listed the aims of the work point by point.
6. In the section of 2.1, the original text is ‘Platinum was then added using (NH₃)₄ (NO₃) ₂(Aldrich, 99.995%)’, there is an error in the reagent writing. According to the supporting information, the correct substance should be (NH₃)₄Pt(NO₃)₂.
7. In the section of ‘Dehydrogenation tests’, the substance H0-BT was mentioned, it is recommended to give the chemical name of this substance when it appears for the first time. The same applies to the following substances.
8. In the section of 3.1, ‘The effect of sulfur incorporation was studied by preparing a series of catalysts containing 0.2 wt.% Pt and 0.5 wt.% Mo’, please explain why catalysts containing 0.2 wt.% Mo were prepared.
9. The same section, distinguishing between Mo and S contributions, what does it mean?
10. In the section of 3.3, catalysts performance was evaluated at same dehydrogenation conditions 533 K,1 bar,1200 rpm and 9.7 g_LOHC/g_cat’, does this condition refer to the optimal dehydrogenation condition? Is it evidenced by the experimental data?
11. Activity results indicated that catalytic activity increased with the addition of sulfur, especially during the first two hours of reaction, except for the sample containing 0.2 wt.% S, the reasons for the abnormal activity of the sample with 0.2 wt.% S were not deeply explored in this section. Only a simple statement was made, and the analysis was not in - depth enough.
12. ‘while H12-BT concentrations vary across the different samples, the consumption of H6-BT does not follow a direct two-step reaction sequence (H12-BT → H6-BT → H0-BT)’, there is no research on the change of H12-BT concentration in the preceding and following texts. So, how was the correlation between the two obtained here?
13. In section 3.4 ‘For this model, all reactions were assumed to follow first-order kinetics’, When establishing the dehydrogenation reaction kinetic model, it is assumed that the reaction follows first - order kinetics and the formation of some intermediate products is neglected. How can the rationality of this assumption be verified in the context of this research system? If the actual reaction does not conform to this assumption, what deviations will occur in the model prediction results and the reaction mechanism analysis?

Author Response

Comment 1: More key data should be included in the abstract.

 

Response 1: Thank you for your suggestion. The abstract has been revised to include additional key results:

 

This study investigates the dehydrogenation of perhydrobenzyltoluene, a Liquid Organic Hydrogen Carrier (LOHC), using sulfur-doped bimetallic PtMo/Al₂O₃ catalysts. Based on previous research that highlighted the superior performance of PtMo catalysts over monometallic Pt catalysts, this work focuses on minimizing byproduct formation, specifically methylfluorene, through sulfur doping. Catalysts with low platinum content (< 0.3 wt.%) were synthesized using the wet impregnation method varying sulfur concentrations to study their impact on catalytic activity. Characterization techniques, including CO–DRIFT and CO–TPD, revealed the role of sulfur in selectively blocking low-coordinated Pt sites, thus, improving selectivity and maintaining high dispersion. Catalytic tests revealed that samples with ≥ 0.1 wt.% sulfur achieved up to a threefold reduction in methylfluorene formation compared to the unpromoted PtMo/Al₂O₃ sample, with a molar fraction below 2% at 240 minutes. In parallel, these samples reached a degree of dehydrogenation (DoD) above 85% within 240 minutes, demonstrating that improved selectivity can be achieved without compromising catalytic performance.

 

 

Comment 2: The introduction should show the merit of their work compared to the previous work and summarize their novelty in one or two sentences.

 

Response 2: We thank the reviewer for the helpful comment. The novelty of this work compared to our previous study is now clearly stated in the last paragraph of Section 1 – Introduction:

 

Therefore, in the present work, we explore the influence of sulfur incorporation into PtMo/Al₂O₃ catalysts as a strategy to suppress methylfluorene formation while maintaining high dehydrogenation rates. In addition, we investigate catalysts with reduced Pt content (below 0.25 wt.%) which is significantly lower than values commonly reported in the literature [44], aiming to decrease both catalyst cost and environmental impact, with total metal loading below 0.5 wt.%.

 

 

Comment 3: Why choose metal-based catalyst? Literature review should be done to emphasize the importance, and some works about metal based catalyst could be referred to: J Environ Chem Eng, 2024, 12: 114446. J Environ Manage. 2024, 359, 120938. ACS ES&T Eng, 2024, 4: 2642-2656. NPJ Clean Water. 2024, 7: 38. Sep Purif Technol. 2024, 341: 126938. Chem Eng J. 2024, 479: 147320.

 

Response 3: We thank the reviewer for this comment and for the suggested references. In response, we have expanded the literature background in the Introduction to better justify the choice of metal-based heterogeneous catalysts. A new sentence has been added to the paragraph discussing catalytic dehydrogenation, emphasising the benefits of metal-based heterogeneous systems over homogeneous catalysts. Additionally, two new references have been included to support this discussion.

 

[30] J.-Y. Cho, H. Kim, J.-E. Oh, B. Y. Park, Catalysts. 2021, 11 (12). DOI: https://doi.org/10.3390/catal11121497.

[31] Y. Yang, M. Liu, X. You, Y. Li, H. Lin, J. P. Chen, Chemical Engineering Journal. 2024, 479, 147320. DOI: https://doi.org/10.1016/j.cej.2023.147320.

 

The revised paragraph now reads as follows:

 

As previously mentioned, the dehydrogenation of LOHCs is a catalytic process. Among the different catalytic approaches, metal-based heterogeneous catalysts stand out over homogeneous systems due to their broader operational applicability, improved catalytic stability, and easier separation and recyclability [30, 31]. The dehydrogenation of homocyclic compounds involves selective activation of C-H bonds while preventing C-C bond cleavage. Among the various metals noble-metals [32] platinum remains the most widely used, owing to its high activity and selectivity in C–H bond cleavage [33]. Furthermore, metal oxides are recognized for their thermal stability and their ability to promote dehydrogenation through hydrogen spillover [34–36]. Consequently, Pt/Al₂O₃ is commonly used as a catalyst in LOHC dehydrogenation processes [13, 18, 21, 37, 38].

 

 

Comment 4: The author said based on their previous work, what’s the difference between these two works?

 

Response 4: We appreciate the reviewer’s note. This clarification indeed helps to better highlight the novelty and contribution of the present work in relation to our previous study. The last paragraph of Section 1 -Introduction has been revised as follows:

 

In this study, we examine the perhydrobenzyltoluene dehydrogenation (H12-BT) focusing on the byproduct formation during the reaction. Based on our previous work [43] molybdenum promoted PtMo/Al2O3 catalysts showed higher dehydrogenation activity of benzyltoluene, achieving higher conversion and selec-tivity than monometallic 1 wt.% Pt/Al2O3 sample. Therefore, Mo-promoted Pt catalyst was selected as the base system for further optimization. The same study also revealed that catalysts with higher activity led to increased formation of methylfluorene, an undesired byproduct generated from highly dehydrogenated benzyltoluene species (H0-BT). Therefore, in the present work, we explore the influence of sulfur incorporation into PtMo/Al₂O₃ catalysts as a strategy to suppress methylfluorene formation while maintaining high dehydrogenation rates. In addition, we investigate catalysts with reduced Pt content (below 0.25 wt.%) which is significantly lower than values commonly reported in the literature [44], aiming to decrease both catalyst cost and environmental impact, with total metal loading below 0.8 wt.%.

 

 

Comment 5: Please listed the aims of the work point by point.

 

Response 5: The aims of this work are:

• Synthesise bimetallic PtMo catalysts with low platinum content, using molybdenum as a catalytic promoter.
• Reduce the platinum loading in the developed catalysts below 0.3 wt.%, in order to improve materials with a noble metal content that is competitive with those reported in the literature.

• Investigate the effect of sulphur incorporation on these catalysts, particularly regarding the formation of methylfluorene as a by-product during the dehydrogenation reaction, with the aim of minimising its formation and supporting the potential for benzyltoluene recyclability in future hydrogen storage and release cycles.

 

 

Comment 6: In the section of 2.1, the original text is ‘Platinum was then added using (NH₃)₄ (NO₃)₂ (Aldrich, 99.995%)’, there is an error in the reagent writing. According to the supporting information, the correct substance should be (NH₃)₄Pt(NO₃)₂.

 

Response 6: We thank the reviewer for pointing out this error. The Pt precursor has now been corrected to (NH₃)₄Pt(NO₃)₂ in Section 2.1 – Catalyst preparation.

 

 

Comment 7: In the section of ‘Dehydrogenation tests’, the substance H0-BT was mentioned, it is recommended to give the chemical name of this substance when it appears for the first time. The same applies to the following substances.

 

Response 7: We appreciate the reviewer’s comment. The substances names have been added when first mentioned in Section 2.3 – Dehydrogenation tests. Additionally, the captions of Figures 5 and 6 have been corrected accordingly.

 

 

Comment 8: In the section of 3.1, ‘The effect of sulfur incorporation was studied by preparing a series of catalysts containing 0.2 wt.% Pt and 0.5 wt.% Mo’, please explain why catalysts containing 0.2 wt.% Mo were prepared.

 

Response 8: We thank the reviewer for this observation. We believe the reviewer is referring to the Pt content, not  Mo. To clarify this point, we have revised the paragraph in Section 3.1 as follows:

 

The effect of sulfur incorporation was studied by preparing a series of catalysts containing 0.2 wt.% Pt and 0.5 wt.% Mo, while varying the sulfur content from 0 wt.% to 0.2 wt.%. Based on our previous results, a Mo loading close to 0.5 wt.% was shown to be effective as a promoter for perhydrobenzyltoluene dehydrogenation, and was therefore selected for this study [43]. As a novel approach, we reduced the noble metal content to 0.2 wt.% Pt, lower than the values typically reported in the literature for this reaction [24, 38, 46], while maintaining a consistent Pt:Mo atomic ratio to enable a clear evaluation of sulfur’s influence. The XRF results confirm the successful incorporation of metals during catalyst preparation, with measured metal contents closely matching the nominal values. Importantly, the Pt:Mo atomic ratio remained consistent. This consistency in Pt ratios ensures that the influence of sulfur incorporation can be analyzed without interference from variations in the catalyst's overall composition (Table 1).

 

 

Comment 9: The same section, distinguishing between Mo and S contributions, what does it mean?

 

Response 9: We thank the reviewer for this question. In this context, “contributions” refers to the Kα signals associated with Mo and S. Using HAADF-STEM imaging, our aim was to observe the dispersion and distribution of sulfur species on the catalyst surface. However, due to the overlapping energy ranges of the Mo and S Kα signals, it was not possible to clearly distinguish their individual contributions, which limited the interpretation of sulfur dispersion in the presence of molybdenum. To avoid confusion, we have revised the paragraph in Section 3.2. The modified paragraph now reads as follows:

 

Figure 2 presents the HAADF maps for the unpromoted PtMo sample and the catalyst containing 0.2 wt.% sulfur. In the unpromoted sample, Pt is homogeneously distributed over the support, whereas the sulfur-containing catalyst (PtMo-0.2S) shows some Pt agglomeration (red dots). Although the PtMo-0.2S sample exhibited more intense contrast in the HAADF images, differentiating between molybdenum and sulfur in the mapping is challenging due to the overlap of the molybdenum Lα and sulfur Kα signals [50]. This spectra overlap, combined with the low overall metal content in the samples, limits the signal-to-background ratio and prevents clear identification of sulfur distribution in the presence of molybdenum. For a complete comparison, HAADF maps of all synthesized samples are included in the Supplementary Information Document.

 

 

Comment 10: In the section of 3.3, catalysts performance was evaluated at same dehydrogenation conditions 533 K,1 bar,1200 rpm and 9.7 g_LOHC/g_cat’, does this condition refer to the optimal dehydrogenation condition? Is it evidenced by the experimental data?

 

Response 10: We thank the reviewer for this observation. The operating conditions used in this study (533 K, 1 bar, 1200 rpm, and 9.7 g_LOHC/g_cat) do not correspond to the optimal dehydrogenation conditions. These conditions were selected based on the technical limitations of our experimental setup. It has been reported in the literature that pressurized systems can enhance dehydrogenation performance by enabling operation at higher temperatures and improving dehydrogenation rate (DOI: 10.1039/d1se01767e). However, since our setup is limited to atmospheric pressure, the tests were performed under these constraints to ensure consistent comparison between the catalysts studied.

 

 

Comment 11: Activity results indicated that catalytic activity increased with the addition of sulfur, especially during the first two hours of reaction, except for the sample containing 0.2 wt.% S, the reasons for the abnormal activity of the sample with 0.2 wt.% S were not deeply explored in this section. Only a simple statement was made, and the analysis was not in - depth enough.

 

Response 11: We thank the reviewer for this comment. As suggested, we have expanded the discussion in Section 3.4 “Effect of sulfur doping on bimetallic PtMo/Al₂O₃ catalysts activity” to provide a more in-depth explanation of the dehydrogenation activity observed for the PtMo-0.2S sample.

 

Since, the primary effect of sulfur is to block low-coordinated Pt sites, therefore, as Pt particle size increases, the number of low-coordinated centers decreases, meaning less sulfur is required to achieve the same effect [41, 67]. In the case of the PtMo-0.15S sample, the sulfur content was lower than the optimal level for the catalyst’s particle size (<1.5 nm). While this sample exhibited higher dehydrogenation productivity, the sulfur incorporation was insufficient to effectively block these Pt active sites, resulting in a higher formation of methylfluorene. In contrast, the PtMo-0.2S sample showed lower dehydrogenation activity during the first two hours of reaction. This performance was attributed to a more extensive coverage of low-coordinated Pt sites, which are suggested to play a key role in the direct dehydrogenation pathway from fully hydrogenated H12-BT to fully dehydrogenated H0-BT, as discussed in Section 3.4 ‘Dehydrogenation pathway’. To further investigate Pt–S interactions, preliminary XPS analyses were conducted. However, due to the low metal content and the resulting high noise-to-signal ratio, the data were inconclusive, and no additional analyses were performed.

 

 

Comment 12: While H12-BT concentrations vary across the different samples, the consumption of H6-BT does not follow a direct two-step reaction sequence (H12-BT → H6-BT → H0-BT)’, there is no research on the change of H12-BT concentration in the preceding and following texts. So, how was the correlation between the two obtained here?

 

Response 12: We thank the reviewer’s comment. The statement regarding the deviation from a direct two-step reaction pathway (H12-BT → H6-BT → H0-BT) was based on the kinetic modelling performed using a sequential two-step reaction scheme. A discrepancy was observed between the model and the experimental data, particularly during the first two hours of reaction, where the model underestimated the formation of H0-BT and overestimated the accumulation of H6-BT. This result suggested that a portion of H12-BT may be directly converted to H0-BT.

 

The kinetic model, including the system of equations (S1–S10) and the simulation results (Figure S1), has been included in the Supporting Information Document. The modelling was performed using the PtMo-0.2S sample as a reference, given its lower by-product formation. Furthermore, the paragraph in Section 3.3 Dehydrogenation activity has been revised to reflect this explanation more clearly.

 

Although the incorporation of sulfur has been shown to effectively reduce methylfluorene formation, achieving complete conversion of the intermediate product (H6-BT) remains a challenge for maximizing dehydrogenation yield. Notably, while H12-BT concentrations vary across different samples, the consumption of H6-BT does not follow a direct two-step reaction sequence (H12-BT → H6-BT → H0-BT). The formation of H0-BT increases over time, whereas the H6-BT concentration decreases only gradually (Figure 6). The kinetic model based on the sequential two-step pathway showed that, particularly during the first two hours of reaction, this model underestimated the concentration of H0-BT and overestimated the accumulation of H6-BT compared to the experimental data. This discrepancy suggests that H12-BT may not exclusively convert through the H6-BT intermediate but may also undergo a direct dehydrogenation to H0-BT (H12-BT → H0-BT), as previously reported by Wang et al. [55]. The system of equations used in the kinetic model (Equations S-1 to S-10) and the corresponding simulation results (Figure S1) are provided in the Supporting Information document.

 

 

Comment 13: In section 3.4 ‘For this model, all reactions were assumed to follow first-order kinetics’, When establishing the dehydrogenation reaction kinetic model, it is assumed that the reaction follows first - order kinetics and the formation of some intermediate products is neglected. How can the rationality of this assumption be verified in the context of this research system? If the actual reaction does not conform to this assumption, what deviations will occur in the model prediction results and the reaction mechanism analysis?

 

Response 13: We thank the reviewer for this reference regarding the kinetic model. On one hand, the simplified reaction mechanism, based on two irreversible first-order steps, was developed following the pseudo-homogeneous approach proposed by Wang et al. (2024) [https://doi.org/10.1016/j.cej.2024.148591]. In their work, these authors justified the use of first-order kinetics for the dehydrogenation of H12-BT by assuming that the concentration of dissolved hydrogen in the liquid phase is negligible, due to its rapid transfer to the gas phase. This simplifies the kinetic treatment by eliminating the need to account for hydrogen in the rate expressions.

On the other hand, the decision to exclude intermediates such as H4-BT and H10-BT was based on both literature and experimental observations. Specifically, Leinweber and Müller (2018) [https://doi.org/10.1002/ente.201700376] reported that these partially hydrogenated species form and convert rapidly during benzyltoluene hydrogenation, indicating that they do not accumulate significantly. Although our study focuses on the reverse (dehydrogenation) reaction, GC-MS analysis of our samples did not reveal measurable signals corresponding to H4-BT or H10-BT. This suggests that their formation and consumption are fast relative to H6-BT, and thus they do not significantly influence the overall reaction rate under the conditions studied. Additionally, no by-products aside from methylfluorene were detected in our system. Methylfluorene was included in the model to account for side reactions that affect selectivity.

We acknowledge that assuming first-order kinetics and excluding minor intermediates represents a simplification of the actual reaction mechanism. However, the experimental data aligns reasonably well with this assumption, without significant deviations. Despite its simplified nature, the model provides valuable insight into the dominant reaction routes and serves as a basis for comparison.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript deals with the investigation of the dehydrogenation of perhydrobenzyltoluene over sulfur-doped bimetallic PtMo/Al₂O₃ catalysts. Catalysts were characterized by employing various techniques including XRF, BET, CO pulse chemisorption, STEM-HAADF, H₂-TPR, CO-DRIFTS and CO-TPD. The incorporation of sulfur led to enhanced catalytic performance for perhydrobenzyltoluene dehydrogenation by selectively blocking low-coordinated Pt sites, thus generating fewer but more selective active centers. Among the promoted catalysts, the one containing 0.2 wt.% sulfur exhibited the highest conversion of the partially dehydrogenated intermediate (H6–BT) and significantly suppressed methylfluorene formation, achieving a threefold reduction compared to the unpromoted PtMo/Al₂O₃ catalyst. The manuscript is well-written and well-structured, presenting interesting results. However, some issues should be addressed prior to publication, considering the following comments.

 

(1) The manuscript contains several formatting issues, such as incorrect formatting of subscripts. Please make the necessary corrections.

( 2 ) Why did the authors use the CO chemisorption technique to determine metal dispersion instead of H₂ chemisorption?

( 3 ) Although the work is very interesting, sulfur-doped Pt/Al₂O₃ catalysts have been previously studied in LOHC dehydrogenation reactions. Therefore, I suggest the authors clearly highlight the novelty of their work at the end of the Introduction section.

Author Response

Comment 1: The manuscript contains several formatting issues, such as incorrect formatting of subscripts. Please make the necessary corrections.

Response 1: Thank you for pointing this out. We have carefully revised the manuscript and corrected all formatting and typo errors.

Comment 2: Why did the authors use the CO chemisorption technique to determine metal dispersion instead of H₂ chemisorption?

Response 2: Thank you for this insightful question. Based on our previous studies, the incorporation of molybdenum into Pt-based catalysts affects the dissociation and chemisorption capacity of hydrogen on Pt active centers, which can lead to an overestimation of the number of active Pt sites when using H₂ chemisorption (https://doi.org/10.1016/j.apcatb.2024.124349). For this reason, CO chemisorption was preferred, as it provides a more reliable estimation of accessible Pt sites under these conditions. While we acknowledge that the CO:Pt stoichiometry may deviate from the 1:1 mol:mol used in our calculations, the resulting dispersion values are in good agreement with those obtained from electron microscopy images.

Comment 3: Although the work is very interesting, sulfur-doped Pt/Al₂O₃ catalysts have been previously studied in LOHC dehydrogenation reactions. Therefore, I suggest the authors clearly highlight the novelty of their work at the end of the Introduction section.

Response 3: Thank you for this valuable suggestion. We have revised the last paragraph of Section 1 Introduction to clearly highlight the novelty of our work. In particular, we emphasize the differences compared to our previous studies using PtMo/Al₂O₃ catalysts for the dehydrogenation of perhydrobenzyltoluene, as well as the significant reduction in platinum content compared to catalysts typically reported in the literature for the same reaction. The paragraph now reads as follows:

In this study, we examine the perhydrobenzyltoluene dehydrogenation (H12-BT) focusing on the byproduct formation during the reaction. Based on our previous work [43] in which the incorporation of molybdenum as a promoter in PtMo/Al₂O₃ catalysts significantly enhanced the dehydrogenation activity of benzyltoluene, achieving higher conversion and selectivity compared to a monometallic 1 wt.% Pt/Al₂O₃ sample prepared by the same method, we selected a Mo-promoted Pt catalyst as the base system for further optimization. However, that same study also revealed that catalysts with higher activity led to increased formation of methylfluorene, an undesired byproduct generated from highly dehydrogenated benzyltoluene species (H0-BT). Therefore, in the present work, we explore the influence of sulfur incorporation into PtMo/Al₂O₃ catalysts as a strategy to suppress methylfluorene formation while maintaining high dehydrogenation rates. In addition, we investigate catalysts with reduced Pt content (below 0.25 wt.%) which is significantly lower than values commonly reported in the literature [44], aiming to decrease both catalyst cost and environmental impact, with total metal loading below 0.8 wt.%.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The merits and the significance of the work compared to the previous work was still not clarify.  The introduction was not well improved with sufficient refs. Comment 3, 4, 5, 10, 13 were not well addressed.

Author Response

REPLY TO THE REVIEWER 1:

The merits and the significance of the work compared to the previous work was still not clarify.

Answer to comment:

We appreciate the reviewer for the additional comment. In response, we have further clarified the merit and significance of our work in the last paragraph of the Introduction (Section 1). The revised paragraph now explicitly states how this study differs from previous work and what its contribution is to the field:

"While previous studies on PtMo/Al₂O₃ catalysts have demonstrated high activity for benzyltoluene dehydrogenation, they often suffer from the formation of undesirable by-products such as methylfluorene. In this work, we propose sulphur incorporation as a strategy to suppress these side reactions while preserving catalytic performance. Moreover, catalyst formulations were developed with significantly reduced platinum content (<0.25 wt.%), below literature values [50] aiming to: i) synthesise bimetallic PtMo catalysts with low platinum content, using molybdenum as a catalytic promoter; ii) reduce the platinum loading in the developed catalysts below 0.25 wt.%, in order to improve materials with a competitive noble metal content if compared with those reported in the literature; iii) investigate the effect of sulphur incorporation on catalytic performance, particularly regarding the formation of methylfluorene as a by-product during the dehydrogenation reaction, with the aim of minimising its formation and supporting the potential for benzyltoluene recyclability in future hydrogen storage and release cycles. These advances represent a step forward toward more selective and sustainable dehydrogenation catalysts with lower cost and environmental impact."

 

The introduction was not well improved with sufficient refs.

Answer to comment:

We are grateful for the reviewer for the comment regarding the improvement of the Introduction. The new references cover key aspects of LOHC development, including practical implementations [Energies 2024, 17(8), 1940], conceptual and catalytic insights [Molecules 2024, 29(20), 4938], recent catalytic advancements [Catalysts 2025, 15(5), 427], and comparative studies of homogeneous vs. heterogeneous systems [Catalysts 2021, 11(12), 1497]. In addition, we included works addressing the techno-economic and environmental assessments of LOHCs [Catalysts 2022, 12(10), 1113], intensification strategies [Catalysts 2025, 15(1), 44], and broader hydrogen carrier challenges [Energies 2023, 16(16), 6035].

These references have been integrated to support the motivation of our study and to clearly justify the relevance of our approach in the current research context.

 

Comment 3, 4, 5, 10, 13 were not well addressed

Answer to comment 3:

We thank the reviewer for the constructive feedback and for providing valuable references. In the revised manuscript, we have further elaborated metal-based heterogeneous catalysis emphasizing their advantages in dehydrogenation processes and referring to the literature suggested. New references have been included:

“As previously mentioned, the dehydrogenation of LOHCs is a catalytic process. Among the different catalytic approaches, metal-based heterogeneous catalysts stand out over homogeneous systems due to their broader operational applicability, improved catalytic stability, and easier separation and recyclability [30, 34]. Compared to homogeneous systems, they also enable more scalable and sustainable operation. Among the various metals noble-metals [35, 36] platinum remains the most widely used, owing to its high activity and selectivity in C–H bond cleavage and is widely reported in the literature as benchmark systems [37].”

 

Regarding the references recommended by the reviewer (one of which was already cited):

• Environ. Chem. Eng., 2024, 12(6), 114446; DOI: 10.1016/j.jece.2024.114446
• Environ. Manage., 2024, 359, 120938; DOI: 10.1016/j.jenvman.2024.120938
• ACS ES&T Eng., 2024, 4: 2642–2656; https://doi.org/10.1021/acsestengg.4c00321
• NPJ Clean Water, 2024, 7:38; https://doi.org/10.1038/s41545-024-00333-6
• Purif. Technol., 2024, 341:126938; https://doi.org/10.1016/j.seppur.2024.126938

These are high quality and very interesting works developed by Yi Yang (Faculty of Arts and Sciences, Beijing Normal University, Zhuhai, 519087, China) and coauthors in the field of water treatment through innovative materials and processes withouth a direct link to the manuscript.

We sincerely appreciate the reviewer suggestion and acknowledge the high quality and relevance of these contributions by Yi Yang and co-authors in the field of advanced water treatment technologies. These works present valuable progress in materials science and environmental engineering. However, their specific scope and application areas differ from the core focus of our manuscript, which centers on LOHC dehydrogenation for energy storage applications.

Answer to comment 4:

As mentioned before, we appreciate the reviewer for the additional comment and we have further clarified the difference between these two works in the last paragraph of the Introduction (Section 1). The revised paragraph now explicitly states how this study differs from previous work and what its contribution is to the field:

"While previous studies on PtMo/Al₂O₃ catalysts have demonstrated high activity for benzyltoluene dehydrogenation, they often suffer from the formation of undesirable by-products such as methylfluorene. In this work, we propose sulphur incorporation as a strategy to suppress these side reactions while preserving catalytic performance. Moreover, catalyst formulations were developed with significantly reduced platinum content (<0.25 wt.%), below literature values [44] aiming to: i) synthesise bimetallic PtMo catalysts with low platinum content, using molybdenum as a catalytic promoter; ii) reduce the platinum loading in the developed catalysts below 0.25 wt.%, in order to improve materials with a competitive noble metal content if compared with those reported in the literature; iii) investigate the effect of sulphur incorporation on catalytic performance, particularly regarding the formation of methylfluorene as a by-product during the dehydrogenation reaction, with the aim of minimising its formation and supporting the potential for benzyltoluene recyclability in future hydrogen storage and release cycles. These advances represent a step forward toward more selective and sustainable dehydrogenation catalysts with lower cost and environmental impact."

 

Answer to comment 5:

The aims of the work listed have been incorporated in the revised manuscript (last paragraph of the Introduction - Section 1).

 

Answer to comment 10:

We appreciate the reviewer’s valuable comment. The conditions used in our catalytic dehydrogenation tests (533 K, 1 bar, 1200 rpm, and 9.7 g_LOHC/g_cat) were not selected as the optimal dehydrogenation conditions, but rather as standardized operating parameters defined by the technical capabilities of our current experimental setup. These conditions were applied consistently for all catalysts to ensure reliable comparative analysis and this is how it is mentioned in the manuscript and taking into account the information of the literature (Lee: https://doi.org/10.3390/app14135803). To clarify this point, we have revised the manuscript (Section 3.3) to explicitly state that these are benchmark conditions constrained by the setup, and not the result of a prior optimization process.

“Then, catalysts performance was evaluated at same dehydrogenation conditions 533 K, 1 bar, 1200 rpm and 9.7 g_LOHC/g_cat selected as benchmark conditions.”

 

Answer to comment 13:

We appreciate the reviewer’s comment regarding the assumptions used in the kinetic modelling.

To clarify, the assumption of first-order kinetics was initially based on prior studies, Wang et al. 2024, DOI: 10.1016/j.cej.2024.148591, who applied a pseudo-homogeneous model to the dehydrogenation of H12-BT. In their work, the simplification to first-order kinetics was justified by the rapid desorption of hydrogen and its negligible solubility in the liquid phase, which allowed for excluding hydrogen concentration from the rate law without compromising model accuracy. Additionally, no accumulation of partially hydrogenated intermediates such as H4-BT or H10-BT was observed in our GC-MS analyses. This is consistent with prior findings by Leinweber and Müller 2018, DOI: 10.1002/ente.201700376, who reported that these intermediates form and convert rapidly, without significantly impacting the overall concentration profile.

To verify the rationality of our model assumptions, we initially tested a sequential first-order mechanism (H12-BT → H6-BT → H₀0BT), as shown in the revised Supporting Information (Figure S1). However, this model failed to reproduce the experimental results. The predicted initial H0-BT concentrations were lower than experimental values, and the final H6-BT concentrations were significantly underestimated. These discrepancies resulted in low correlation coefficients, now reported in Table S2. Therefore, we proposed an alternative model involving parallel first-order reactions, where H12-BT dehydrogenates directly to both H6-BT and H0-BT. This model significantly improved the fit to the experimental data and better reproduced the observed concentration trends, as shown in Figure 7 of the main manuscript and in Figure S1 of the Supporting Information.

 

If any comment is not well addressed, please, do not hesitate to answer again detailing the info required to properly address the comments.

Author Response File: Author Response.pdf