Hydrodesulfurization of Thiophene in n-Heptane Stream Using CoMo/SBA-15 and CoMo/AlSBA-15 Mesoporous Catalysts
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis manuscript described hydrodesulfurization of thiophene using CoMo/SBA-15 and CoMo/AlSBA-15 mesoporous catalysts. The research approach/results are interesting and represented nicely in the MS. It can be acceptable for publication after careful revisions following the comments below.
Abstract: The abstract is too long and lack of insight! It should be concise and easily readable to find the novelty of the work! Please revise the abstract thoroughly within 200~250 words!
Introduction: I don’t understand, why there is chapter 1.1 when there is no 1.2 or so? What is the advantage of HDS over other methods including oxidative/adsorptive desulfurization? Moreover, the introduction is lack of focusing on any specific problem that was solved in this study. Please re-write some parts by addressing a significant challenge that was solved through this research! Moreover, to choose thiophene as a substrate might have more specific/scientific reason, that should be highlighted in the introduction.
Experimental: What is the meaning of “necessary amounts of cobalt nitrate” and how the author controls the “loading of 15% by weight” of active metal phase?
Results and Discussion:
I am curious either the catalysts had any micropore or not! If yes, at least the information should be provided! Surely, catalysis might be facilitated by the mesopore!
So far, the CoMo/AlSBA-15 was regarded as the best catalysts for HDS of thiophene. The reason for higher activity, main active site should be explained with proper physiochemical perspective of catalysts and substrate. This is somehow lacking here! Please, explain the main active site, make and illustration to show the active site and possible reaction pathway.
The CoMo/AlSBA-15 catalyst comprised three different crystal phases including MoO3 (JCPDS Registry: 35-06609) with an orthorhombic structure, Co3O4 (JCPDS Registry: 35-06609) with a cubic structure and mixed oxides of cobalt and molybdenum in the form of CoMoO4 (JCPDS Registration: 21-0868) with monoclinic structure. Which metal species is responsible for higher HDS catalysis? If two or more are responsible, could you explain how they contributed in HDS reaction!
The relative performance of the developed catalysts with the reported catalysts in the HDS of thiophene should be shown to understand the competitiveness of the material.
Typos free MS is highly appreciated. There are some….! Please check carefully and revise them!
Comments on the Quality of English LanguageModerate editing of English language required
Author Response
ANSWER TO REVIEWERS
REVIEWER 1
This manuscript described hydrodesulfurization of thiophene using CoMo/SBA-15 and CoMo/AlSBA-15 mesoporous catalysts. The research approach/results are interesting and represented nicely in the MS. It can be acceptable for publication after careful revisions following the comments below.
Q1) Abstract: The abstract is too long and lack of insight! It should be concise and easily readable to find the novelty of the work! Please revise the abstract thoroughly within 200~250 words!
A1) Thank you for comments. The Abstract was re-written and now has 248 words.
The new Abstract is:
“Heterogeneous catalysts containing cobalt and molybdenum supported on mesoporous materials type SBA-15 and AlSBA-15 were synthesized for application in the HDS reactions of thiophene in n-heptane stream. The materials were synthesized by hydrothermal method using Pluronic P123 as template. The calcined SBA-15 and AlSBA-15 supports were submitted to co-impregnation with solutions of cobalt nitrate and ammonium heptamolybdate, aiming the production of 15% in mass of metal loading with atomic ratio of [Co/(Co+Mo)]=0.45. The obtained materials were dried and calcined for obtaining the mesoporous catalysts in the form of CoMo/SBA-15 and Co-Mo/AlSBA-15. The catalysts were characterized by XRD, TG/DTG, SEM and nitrogen adsorption. From XRD analysis, it was verified that after decomposition of the cobalt and molybdenun salts, were formed MoO3, Co3O4 and CoMoO4 oxides on the supports, being attributed to these chemical species the activity for the HDS reactions. The catalytic activity of the obtained catalysts was evaluated in a continuously flowing tubular fixed-bed microreactor coupled on-line to a gas chromatograph, using n-heptane stream containing 12070 ppm of thiophene (ca. 5100 ppm of sul-fur) as a model compound. The synthesized catalysts presented good activity for HDS reaction, and the main obtained products were cis- and trans-2-butene, 1-butene, n-butane and low amounts of isobutane. The presence of 1,3-butadiene and tetrahydrothiophene (THT) were not de-tected. A mechanism of the primary and secondary reactions and subsequent formation of the ole-fins and paraffins, in the CoMo/SBA-15 and CoMo/AlSBA-15 mesoporous catalysts were pro-posed, considering steps of desulfurization, hydrogenation, dehydrogenation, THT decyclization and isomerization.”
Q2) Introduction: I don’t understand, why there is chapter 1.1 when there is no 1.2 or so?
A2) It was corrected. There is no more 1.1.
Q3) What is the advantage of HDS over other methods including oxidative/adsorptive desulfurization? Moreover, the introduction is lack of focusing on any specific problem that was solved in this study. Please re-write some parts by addressing a significant challenge that was solved through this research!
A3) The following text was included:
“The advantage of HDS over oxidative or adsorptive desulfurization methods lies in its effectiveness in removing a wide range of sulfur compounds from hydrocarbon streams at relatively mild conditions. HDS operates at high temperatures and pressures, typically using metal catalysts such as molybdenum or cobalt, which enables efficient removal of sulfur compounds including thiols, sulfides, and thiophenes. This method is effective in reducing sulfur content to meet stringent environmental regulations, and it does not produce harmful waste products as in some oxidative methods. Additionally, HDS is well-suited for treating a wide variety of feedstocks, making it a versatile and widely used desulfurization method in the petroleum industry. Specifically, some of the key advantages of HDS include selectivity, efficiency and compatibility with existing infrastructure. HDS is highly selective for sulfur removal, allowing for the removal of sulfur compounds without significantly affecting the hydrocarbon components. This selectivity is crucial for refining high-quality fuels. HDS processes can achieve high desulfurization efficiency, even at relatively mild operating conditions, which can result in lower energy consumption compared to other methods. These advantages make HDS a popular choice for desulfurization in the petroleum refining industry.”
Q4) Moreover, to choose thiophene as a substrate might have more specific/scientific reason, that should be highlighted in the introduction.
A4) The following text was included in the introduction:
“Thiophene is commonly used in hydrodesulfurization (HDS) research for several specific/ scientific reasons: (i) thiophene is a heterocyclic compound containing sulfur, which is structurally similar to many sulfur-containing compounds found in crude oil. Therefore, studying thiophene allows researchers to understand the behavior of more complex sulfur-containing compounds found in petroleum; (ii) thiophene serves as a model compound for studying the mechanisms and kinetics of hydrodesulfurization reactions. Its relatively simple structure makes it suitable for detailed experimental and theoretical investigations, providing insights into the fundamental processes involved in HDS reactions; (iii) researchers use thiophene as a probe molecule to evaluate the performance of various catalysts in hydrodesulfurization processes. Thun, by studying the effect of different catalysts on the conversion of thiophene to its desulfurized products, it is possible identify catalysts with improved activity, selectivity, and stability for industrial HDS applications. Thiophene is also employed in mechanistic studies aimed at understanding the detailed reaction pathways involved in hydrodesulfurization. The use of thiophene in hydrodesulfurization research allows to elucidate fundamental principles concerning sulfur removal from fossil fuels, leading to the development of more efficient and environmentally friendly HDS processes.”
Q5) Experimental: What is the meaning of “necessary amounts of cobalt nitrate” and how the author controls the “loading of 15% by weight” of active metal phase?
A5) This was re-written and explained. In the Manuscript: the following comment was included:
“The metal impregnation procedure consisted of weighing ca. 0.5 g of the support (SBA-15 or AlSBA-15) considering the relative humidity. The amounts of cobalt nitrate, Co(NO3)2.6H2O, and ammonium hepatmolybidate, (NH4)6Mo7O24.4H2O, were weighed in a porcelain crucible and solubilized in 20 mL of absolute ethanol using a glass rod. After solubilization of the salts, the support was slowly added, stirring with the glass rod. The crucible with the suspension was transferred to the heating mantle at 70 oC, homogenizing periodically to evaporate the excess solvent. After evapora-tion of excess ethanol, the crucible was transferred to the oven and dried at 100 oC for 6 hours. The depositions of the metallic phases were carried out to obtain a loading of 15% by weight of active phase, with a [Co/(Co+Mo)] atomic ratio of 0.45.
The synthesis process allows the cobalt and molybdenum ions from the solution to be absorbed into the pores of the support material. After impregnation, the solvent was evaporated, leaving behind the metals distributed on the surface and within the pores of the support materials. The impregnated materials were dried to remove any remaining moisture. The dried catalysts were subjected to calcination at 500 oC, under atmosphere of air flowing at 100 mL/min. This step serves to decompose the cobalt and molybdenum salts and convert them into cobalt and molybdenum oxides on the support material. Finally, before the catalytic evaluation, the calcined materials were subjected to a reduction step, in a hydrogen atmosphere. This reduces the cobalt and molybdenum species to their metallic form, the active metal phase required for catalysis.”
Q6) Results and Discussion:
I am curious either the catalysts had any micropore or not! If yes, at least the information should be provided! Surely, catalysis might be facilitated by the mesopore!
A6) Thank you. The required information was included in the text:
“SBA-15 and AlSBA-15 are types of mesoporous materials known for their well-defined pore structures. These materials contain both micropores and mesopores, which contribute to their unique properties and applications. The micropores are primarily formed within the walls of the silica framework, arising from the arrangement of silica units during the synthesis process. They provide additional surface area and can influence the adsorption and diffusion of small molecules and ions. The presence of micropores enhances the overall surface area and adsorption capacity of the material. However, the mesopores are the dominant type of pores and are responsible for the ordered and uniform pore structures characteristic of these materials. Mesopores are formed between the silica walls, creating channels or void spaces that allow for the efficient diffusion of molecules within the material. These larger pores enable the transport of larger molecules and facilitate mass transfer processes. The presence of both micropores and mesopores in SBA-15 and AlSBA-15 contributes to their high surface area, large pore volume, and uniform pore size distribution. This combination of pore characteristics makes them attractive materials for a HDS applications, where precise control over pore structure and surface properties is essential.”
Q7) So far, the CoMo/AlSBA-15 was regarded as the best catalysts for HDS of thiophene. The reason for higher activity, main active site should be explained with proper physiochemical perspective of catalysts and substrate. This is somehow lacking here! Please, explain the main active site, make and illustration to show the active site and possible reaction pathway.
A7) Thank you. See comments bellow:
“In CoMo/AlSBA-15 catalysts, the main active sites for HDS reactions are typically associated with the Co-Mo sulfide clusters supported on the mesoporous AlSBA-15 material. These active sites provide the necessary coordination environment and surface properties conducive to the efficient adsorption, activation, and subsequent desulfurization of thiophene molecules. The Co and Mo species form metal sulfide clusters on the surface of the AlSBA-15 support. These clusters act as the active sites for HDS reactions. The interaction between Co and Mo species can lead to the formation of Co-Mo sulfide phases, which are particularly effective in promoting thiophene desulfurization. Both Co and Mo species provide coordination sites for the adsorption and activation of thiophene molecules. The metal sites facilitate the breaking of carbon-sulfur bonds in thiophene, initiating the desulfurization process. The mesoporous structure of AlSBA-15 support enhances the dispersion of Co and Mo species, maximizing the exposure of active sites to reactant molecules. Additionally, the interaction between metal sulfide clusters and the support surface can stabilize the active sites and improve catalytic performance.
For the reaction pathway, the following steps were considered: (i) adsorption and activation: thiophene molecules adsorb onto the catalysts via coordination with Co and Mo species. The metal sites activate thiophene by breaking the carbon-sulfur bonds; (ii) desulfurization: activated thiophene undergoes desulfurization, where the sulfur atom is removed as hydrogen sulfide (H2S), leading to the formation of butane. This step involves hydrogenation and subsequent sulfur elimination reactions facilitated by the active metal sites; (iii) desorption: the desulfurized products desorb from the active sites and diffuse away from the catalyst surface, making room for new reactant molecules.”
Q8) The CoMo/AlSBA-15 catalyst comprised three different crystal phases including MoO3 (JCPDS Registry: 35-06609) with an orthorhombic structure, Co3O4 (JCPDS Registry: 35-06609) with a cubic structure and mixed oxides of cobalt and molybdenum in the form of CoMoO4 (JCPDS Registration: 21-0868) with monoclinic structure. Which metal species is responsible for higher HDS catalysis? If two or more are responsible, could you explain how they contributed in HDS reaction!
A8) Ok. Explained and the following paragraph was included:
“Several metal species are known to be effective catalysts for hydrodesulfurization (HDS) reactions. Among them, molybdenum (Mo) and cobalt (Co) are commonly used. These metals play different roles in the HDS process, and sometimes catalysts are composed of combinations of these metals for enhanced performance. Mo facilitates the breaking of carbon-sulfur bonds in sulfur-containing compounds, promoting desulfurization. It has a high affinity for sulfur, enabling it to form stable sulfide compounds on the catalyst surface. Mo also helps in hydrogenation reactions, which are often coupled with desulfurization in HDS processes. Co-based catalysts are less common than Mo-based catalysts but is still used in certain HDS applications. Co facilitates desulfurization reactions by breaking carbon-sulfur bonds. Co-based catalysts also exhibit good selectivity towards thiophene conversion. In many cases, catalysts used in industrial HDS processes contain combinations of these metal species, such as Co-Mo to leverage their synergistic effects. Thus, Mo facilitates the initial breaking of carbon-sulfur bonds, while Co promotes further desulfurization and hydrogenation reactions. Additionally, the presence of mesoporous silica support (SBA-15 or AlSBA-15) can enhance the catalyst ability, improving the sulfur removal efficiency.”
Reviewer 2 Report
Comments and Suggestions for AuthorsThe article describes synthesis, characterisation and catalytic activity of the CoMo/SBA-15 and CoMo/AlSBA-15 samples in hydrodesulfurisation of thiophene. The article is clearly written, although it contains come minor spelling / grammar mistakes, which should be corrected. The reported results are supported by the results.
I recommend the article for publication after a few issues are explained / discussed in the manuscript:
How was the wall thickness determined?
How do you define conversion and selectivity?
Are you able to measure the acidity of the catalysts and connect it to their catalytic activity?
Authors observed that after one hour of the reaction with CoMo/SBA-15 catalyst the selectivity to isobutane increased with selectivity to n-butane decreasing and attributted the reaction to isomerisation. The effect was not observed in case of CoMo/AlSBA-15, which contains acid sites, typically connected with this type of reactivity. Could you please think of the explanation of the observed effect?
What is the fate of sulfur removed in the reaction? Is it possible to detect MoS2 in the spent catalysts?
The introduction part devoted to the catalysts already used for the reaction should be slightly enlarged by discussing CoMo-based zeolites / mesoporous materials.
Comments on the Quality of English Language
There are some minor language errors or misspelling which should be corrected, see e.g.:
- line 160 "heating hate kinetic models".
- Ea ranges should be listed in an increasing order (line 162).
Author Response
ANSWER TO REVIEWERS
REVIEWER 2
The article describes synthesis, characterization and catalytic activity of the CoMo/SBA-15 and CoMo/AlSBA-15 samples in hydrodesulfurisation of thiophene. The article is clearly written, although it contains come minor spelling / grammar mistakes, which should be corrected. The reported results are supported by the results.
I recommend the article for publication after a few issues are explained / discussed in the manuscript:
Q1) How was the wall thickness determined?
A1) The following comment and Figure were inserted in the manuscript:
“The determining of the wall thickness (Wt) of SBA-15 and AlSBA-15 mesoporous materials can be achieved through various experimental techniques and characterization methods. Transmission Electron Microscopy (TEM) offers a direct approach, enabling precise measurements of thin sections of the material. Small-Angle X-ray diffraction provides structural insights by analyzing X-ray patterns, while nitrogen adsorption desorption isotherms can indirectly infer the wall thickness through BET analysis. Gas sorption techniques, like nitrogen physisorption, and Solid-State NMR spectroscopy offer additional means to determine pore size distribution and local atomic connectivity, contributing to wall thickness estimation. Scanning Electron Microscopy (SEM) provide surface topography images that aid in measurements, though the latter may be less effective compared to TEM. The combination of these methods is often utilized to comprehensively characterize the structure and properties of SBA-15 materials, considering factors such as sample preparation requirements and thermal treatments.”
Figure 9. Hexagonal arrangement of the mesoporous structure of the SBA-15, showing the mesoporous parameter (a0), wall thickness (Wt) and pore size diameter (Dp).
Q2) How do you define conversion and selectivity?
A2) Thank you. The definition of conversion and selectivity were inserted in the manuscript.
Where “mol(i)” and “mol(f)” represent the initial and final molar quantities, obtained by analyzing the chromatograms at the beginning and end of the reaction, respectively. The letter “P” represents the reaction product, measured at the reactor outlet, which may be butane, isobutane, 1-butene, cis-2-butene or trans-2-butene. The paraffin/olefin ratio was determined considering the selectivity of (n-butane + isobutane) divided by the selectivity of (1-butene + cis-2-butene + trans-2-butene).
Q3) Are you able to measure the acidity of the catalysts and connect it to their catalytic activity?
Authors observed that after one hour of the reaction with CoMo/SBA-15 catalyst the selectivity to isobutane increased with selectivity to n-butane decreasing and attributted the reaction to isomerization. The effect was not observed in case of CoMo/AlSBA-15, which contains acid sites, typically connected with this type of reactivity. Could you please think of the explanation of the observed effect?
A3) For this answer, a plausive comment and a Figure were included in the manuscript, as supported by two references, as stated below:
“One probable explanation for the presence of acid sites on the CoMo/SBA-15 catalyst can be attributed to silanol groups (≡Si-O-H) inside the micropores (Si-OH bulk), with acid character due to the formation of hydrogen bonds, in addition to moderate acidity on the surface of the mesopores (associated Si-OH), forming hydrogen bonds on the external surface of silicon oxide [72]. Another possibility that must be considered is the reactions of H2S, generated in the HDS reaction, with oxygen from the silicate groups, forming sulfated silica with surface proton sites [73]. In this case, we would have the formation of Bronsted super-acidic sites on the internal surface of the mesopores, as proposed in Figure XX, with the sulfated CoMo/SBA-15 catalyst presenting silanol groups capable of donating or accepting hydrogen bonds with different strengths, consequently promoting an increasing in the concentration of isobutane, from the isomerization process of n-butane.“
Figure 17. Proposed scheme for the silanol groups and acidity on the CoMo/SBA-15 catalyst, evidencing the formation of different hydrogen bonds and acid sites: (a) Bronsted superacid site due to presence of sulfate on the SBA-15 surface; (ii) isolated silanol; (iii) associated silanol; (iv) bulk silanol, showing an acid site on the microporous surface.
[72] Tsyganenko, A.A.; Storozheva, E.N.; Manoilova, O.V.; Lesage, T.; Daturi, M.; Lavalley, J.-C. Brønsted acidity of silica silanol groups induced by adsorption of acids. Catalysis Letters 2000, 70, 159–163.
[73] Wijaya, K.; Lammaduma Malau, M.L.; Utami, M.; Mulijani, S.; Patah, A.; Wibowo, A.C.; Chandrasekaran, M.; Rajabathar, J.R.; Al-Lohedan, H.A. Synthesis, Characterizations and Catalysis of Sulfated Silica and Nickel Modified Silica Catalysts for Diethyl Ether (DEE) Production from Ethanol towards Renewable Energy Applications. Catalysts 2021, 11, 1511.
Q4) What is the fate of sulfur removed in the reaction? Is it possible to detect MoS2 in the spent catalysts?
A4) Unfortunately, these analyzes were not carried out in this work.
Q5) There are some minor language errors or misspelling which should be corrected, see e.g.:
- line 160 "heating hate kinetic models".
- Ea ranges should be listed in an increasing order (line 162).
A5) Thank you. They were corrected.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThank you for the insightful revisions. To enhance readability and clarity, the authors is recommended to consider providing a more concise manuscript that will greatly benefit readers in grasping the key findings efficiently.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe article is very much ameliorated and I recommend it for publication.
