F,O,S-Codoped Graphitic Carbon Nitride as an Efficient Photocatalyst for the Synthesis of Benzoxazoles and Benzimidazoles
, Eva Sánchez-Hernández 1, Celia Andrés-Juan 2, Pedro Chamorro-Posada 3, Guillermo Antorrena 4, Jesús Martín-Gil 1 and Pablo Martín-Ramos 1,*Round 1
Reviewer 1 Report
Alberto Santiago‐Aliste and coworkers reported the Acidified and F,S,O‐codoped graphitic carbon nitride as an efficient photocatalyst for the synthesis of benzoxazoles and benzimidazolesthe. From the methodology and the results, I think it can be accepted after addressing the following comments:-Figure 1 should be indicated as a) and b), as these are two individual figures.
-In figure 2, label a) and c) is missing.
-Figure 4 should be indicated as a), b), c), ……for better understating.
-Figure 5 should also be indicated as a) and b).
-Authors have stated the catalyst as acidified catalyst. However, no characterization data have been presented to support this statement such as TPD-NH3. If catalyst acidity can affect the reaction rate and yield of the product, it should be discussed in the discussion section. Else, I would suggest the authors to omit the acidified word.
-A reaction needs to be performed in the absence of light source and discuss the findings in the main text of the manuscript.
-Some suitable references need to be cited in the characterization section.
-In supporting information, affiliations of authors are missing.
-Author should include XRD and FTIR spectra of reused catalyst to observe any changes in the chemical nature after the reaction and discuss the results in terms of catalyst stability.
Author Response
Alberto Santiago‐Aliste and coworkers reported the Acidified and F,S,O‐codoped graphitic carbon nitride as an efficient photocatalyst for the synthesis of benzoxazoles and benzimidazoles. From the methodology and the results, I think it can be accepted after addressing the following comments:
Q1. Figure 1 should be indicated as a) and b), as these are two individual figures.
Response: Corrected.
Q2. In figure 2, label a) and c) is missing.
Response: Corrected.
Q3. Figure 4 should be indicated as a), b), c), ……for better understating.
Response: Corrected.
Q4. Figure 5 should also be indicated as a) and b).
Response: Corrected.
Q5. Authors have stated the catalyst as acidified catalyst. However, no characterization data have been presented to support this statement such as TPD-NH3. If catalyst acidity can affect the reaction rate and yield of the product, it should be discussed in the discussion section. Else, I would suggest the authors to omit the acidified word.
Response: The word ‘acidified’ has been omitted throughout the ms., as suggested by the Reviewer.
Q6. A reaction needs to be performed in the absence of light source and discuss the findings in the main text of the manuscript.
Response: In the absence of light the reactions did not progress. The yield of the reactions was negligible. This has been clarified in subsection 2.2.1 of the revised version of the ms., just below Table 3.
Q7. Some suitable references need to be cited in the characterization section.
Response: Six additional references have been included.
Q8. In supporting information, affiliations of authors are missing.
Response: Affiliations have been indicated in the supporting information file.
Q9. Author should include XRD and FTIR spectra of reused catalyst to observe any changes in the chemical nature after the reaction and discuss the results in terms of catalyst stability.
Response: After five recycle runs, the intensity of two bands in the FTIR spectra of the catalyst decreased, both presumably associated with C-F bonds: the one at 639 cm-1 (a wagging mode) and the one at 1027 cm-1 (attributable to semi-ionic C-FII bonds) [Xu et al. (2016), DOI:10.1021/acs.analchem.6b00115], thus suggesting some role of F-doping in the catalytic behavior. No changes were observed in the XRPD pattern. To clarify this point, the following paragraph has been added: “Throughout these runs, the structural characteristics remained unaltered, but a change was observed in the FTIR spectra (Figure S7): a decrease in the intensity of the bands at 639 and 1027 cm-1 (attributed to C-F and C-O vibrations), suggesting some role of fluorine and oxygen active sites/species in the catalytic behavior.” A new supplementary figure has been added to show the change in the FTIR spectra (Figure S7).
Reviewer 2 Report
The authors synthesized the acidified and F, S, O-co-doped carbon nitride photocatalysts, characterized their partial physicochemical properties, and evaluated their photocatalytic activities for the formation of benzoxazoles and benzimidazoles via condensation–aromatization of 2-aminophenol or 1,2-phenylenediamine with suitable aldehydes. This work contains some new results and could be considered for publication. However, the authors should revise their manuscript before acceptance for publication according to the following comments:
1. What are the contents of F, S, and O in the F, S, O co-doped g-C3N4 samples, rather than their surface compositions determined by XPS but by ICP-AES?
2. What are the F, S, and O atoms located in the g-C3N4 samples?
3. What are the main products and their selectivities of condensation–aromatization of the reactants in the present work?
4. What are the active sites for active species or the addressed reaction?
5. What are the roles of F, S, O dopants and g-C3N4 in catalyzing the addressed reaction?
6. How about the photocatalytic stability of the typical sample?
7. What are the photocatalytic mechanisms?
8. A comparison on photocatalytic activity of the as-obtained typical sample should be made with those of the related samples reported in the literature.
9. There are some inappropriate English words or expressions in the manuscript. The authors should carefully polish the English of the whole manuscript.
Author Response
The authors synthesized the acidified and F, S, O-co-doped carbon nitride photocatalysts, characterized their partial physicochemical properties, and evaluated their photocatalytic activities for the formation of benzoxazoles and benzimidazoles via condensation–aromatization of 2-aminophenol or 1,2-phenylenediamine with suitable aldehydes. This work contains some new results and could be considered for publication. However, the authors should revise their manuscript before acceptance for publication according to the following comments:
Q1. What are the contents of F, S, and O in the F, S, O co-doped g-C3N4 samples, rather than their surface compositions determined by XPS but by ICP-AES?
Response: Even though the techniques used in this study (XPS and SEM-EDS) may be deemed appropriate (they appear in most of the works that investigate g-C3N4 as a catalyst), in view of the discrepancies between the results of the two techniques, we agree with the reviewer on the need to use an additional technique. Given that we do not have access to ICP-AES, we have conducted an elemental microanalysis for C, H, N, and S, and used total combustion-ion chromatography (C-IC) for F content determination. The materials and methods section and the characterization results section have been updated accordingly.
Please kindly note that, on the basis of these contents, ‘F,S,O’ has been replaced with ‘F,O,S’ throughout the ms.
Q2. What are the F, S, and O atoms located in the g-C3N4 samples?
Response: The presence of the F, O, and S atoms in the g-C3N4 samples is a result of the decomposition of triflic acid. These heteroatoms would replace N atoms, given that, according to Chu et al. (DOI: 10.1016/j.carbon.2020.07.053), S- or O-doping creates C‒S or C‒O bonds, respectively, by replacing N atoms. Also, according to van Phuc (DOI: 10.4028/www.scientific.net/AMM.889.24), F atoms should replace N atoms. The FTIR spectrum supports the presence of such C-F, C-S, C-O bonds (consistent with the F-, S- and O-doping) due to the presence of a shoulder at 1082 cm−1, attributable either to C‒F bonds or C‒S/C-O bonds; a band at 1277-1263 cm-1, which can be assigned to C-F and C-O modes (although it may be overlapped with a C-N mode); and the bands at 639 and 1027 cm-1, which can be ascribed either to C-F (wagging) and C-O (stretching) modes, respectively. Concerning the distribution of the doping elements in the photocatalyst, it would be homogeneous, according to the EDS elemental mapping. The infrared spectroscopy and EDS subsections have been updated to include this information.
Q3. What are the main products and their selectivities of condensation–aromatization of the reactants in the present work?
Response: The main reaction products were presented in Figure 8 and Figure 9. Given that the location of the materials and methods section after the results and discussion sections is defined by the journal’s guidelines (which do not follow the usual IMRaD sequence), we have moved the chemical structures of the main products to a new figure (Figure 6), at the very beginning of subsection 2.2.1. This should facilitate the reading of the manuscript and address the first query raised by the Reviewer. Concerning the selectivities, in response to the Reviewer’s request, they have been determined by HRMS (ESI) for the benzimidazole and the benzoxazole for which the highest yields were obtained, viz. 2-phenylbenzoxazole and 2-2(hydroxyphenyl)benzimidazole, following a procedure analogous to that presented in the recent paper by Pham et al. [Catalysts 2022, 12, 1394. doi:10.3390/catal12111394]. A new paragraph has been included in subsection 2.2.1 commenting on the intermediate product/desired final product ratios (slightly worse than those obtained using, for instance, [CholineCl][Oxalic Acid] as a catalyst), together with a new figure in the supporting information file (Figure S6). The materials and methods section has been updated accordingly.
Q4. What are the active sites for active species or the addressed reaction?
Response: As for the identification of the active sites of the photocatalyst, according to Li et al. [Design and application of active sites in g-C3N4-based photocatalysts. Journal of Materials Science & Technology, DOI:10.1016/j.jmst.2020.03.033], ‒NH‒ and ‒C=O/C‒O active functional groups, N defects, and heteroatom-dopants can act as active sites for photocatalysts.
N atoms supply the oxidation and reduction active sites, and C atoms tend to provide the reduction active sites in photocatalytic process. However, separating these two types of atoms for oxidation and reduction is difficult due to their chemical bonding that generates the delocalized π-conjugated structure and consequently decreases the active sites. In fact, conventional g-C3N4 has few reactive sites and adsorption sites due to its stacked bulk structure and stable π-conjugated system (“[…] Conventional g-C3N4 prepared through calcination of nitrogen-containing photocatalysts exhibits the stacked bulk structure with small specific surface area, which further reduces the active sites on its surface [32-36]. In addition to the smaller specific surface area, the less surface-active substances and the weaker substrate absorption capacity all reduced the mass and quantity of active sites on the surface of the conventional g-C3N4. Owing to these factors, the conventional g-C3N4 exhibits only moderate photocatalytic activity […]”).
According to the same author, conventional g-C3N4 does not thoroughly adsorb most of the reaction substrates due to its few adsorption sites. Doping of atoms with specific adsorption to the reaction substrate can increase the adsorption site of g-C3N4 and consequently its active surface sites (“[…] Introducing an active substance that can specifically adsorb the reaction substrates will increase the adsorption sites of g-C3N4. The active substance can be heteroatoms, co-catalysts, photocatalysts, and functional groups […]” and “[…] Pristine g-C3N4 exhibits low adsorption capacity for reaction substrate due to its limited specific surface area and restricted active sites. Some types of heteroatom doping can increase the adsorption activity sites of g-C3N4 to specifically adsorb some reaction substrates [171-175].”).
The most relevant points of the two previous paragraphs have been included in a new subsection of the discussion (3.1) on the role of doping elements.
Concerning the active species involved in the reaction, the hypothesized pathway that involves superoxide radicals (•O2−) as the main active species in the photocatalytic synthesis of benzoxazoles and benzimidazoles is a reasonable assumption, supported by the findings of other authors (e.g., Li et al.). We are aware that other alternative hypotheses may be proposed: for instance, by referring to the S+ centers, which could work as Lewis acid sites, as P+ centers do in P-doping (Katsumata et al. DOI: 10.1016/j.matlet.2021.130068); or that a mechanism based on the concept of cooperative catalysis may be proposed [Kalel et al., DOI: 10.1016/j.jics.2022.100550], in which the interaction of aldehydes with the acidic site of the catalyst leads to protonation of the carbonyl oxygen of aldehyde, which then would react with the -NH2 group of the 2-amino phenol or the 2-phenylenediamine, etc. However, in our view, they are more speculative. Hence, after evaluating the mechanisms reported in the bibliography for related samples, we have chosen to adopt Li's work as a reference and to include only the soundest statements.
Q5. What are the roles of F, S, O dopants and g-C3N4 in catalyzing the addressed reaction?
Response: The dopants would be involved in the modification of the inter-layer spacing (an effective way to promote the migration of photogenerated electrons between the g-C3N4 structural units) and would provide more active sites. The modifications would be a result of the radius of doping atoms being larger than those of C and N ones. According to the literature, among the doping atoms, S would be the one that, after P, is the most capable of boosting the catalytic activity of g-C3N4: it has a higher covalent radius (105 pm) than those of C (76 pm) and N (71 pm) and has an electronegativity close to that of C (2.55). According to Li et al. (Design and application of active sites in g-C3N4-based photocatalysts. Journal of Materials Science & Technology, doi:10.1016/j.jmst.2020.03.033), heteroatom doping promotes the migration of photogenerated electrons between structural units, in such a way that the active surface sites (associated with the active functional groups introduced) can have sufficient photogenerated carriers to participate in photocatalysis. Furthermore, introducing heteroatoms can enhance the adsorption of reaction substrates (e.g., in the case of F-doping, F can increase the adsorption activity sites of g-C3N4 to specifically adsorb some reaction substrates (Li et al., DOI: 10.1016/j.jmst.2020.03.033)). Moreover, according to Katsumata et al. (DOI: 10.1016/j.matlet.2021.130068), some of the incorporated dopants (such as P and S) can promote visible light absorption and the separation of charge carriers. This has been discussed in a new subsection of the discussion (3.1).
Q6. How about the photocatalytic stability of the typical sample?
Response: As an indicator of the photocatalytic stability of F,O,S-codoped g-C3N4, its recyclability for the synthesis of 2-phenylbenzoxazole and 2-(2-hydroxyphenyl)benzimidazole was studied, confirming four recycle runs without significant loss of its photocatalytic activity. Please kindly note that, as per the request of another Reviewer, we have characterized the photocatalyst after five recycle runs, finding that two weak bands in the FTIR spectra of the catalyst decreased (see spectra below), both presumably associated with C-F bonds: the one at 639 cm-1 (a wagging mode) and the one at 1027 cm-1 (attributable to semi-ionic C-FII bonds) [Xu et al. (2016), DOI:10.1021/acs.analchem.6b00115], thus suggesting some role of F-doping in the catalytic behavior. Nevertheless, no changes were observed in the XRPD pattern. We have clarified this in the FTIR results section. We believe that the observed photochemical stability should be ascribed to the preferred mediation of •O2– radicals, in line with the findings of other authors, such as Xiao et al., 2020 (DOI: 10.1021/acs.accounts.9b00624), who reported that the mediation of these radicals did not introduce any instability for a g-C3N4 type catalyst. This point has also been clarified in the revised text, in which we have added the following sentence after Figure 8: “The predominance of •O2– radicals (with a negligible intervention of •OH radicals) would contribute to the preservation of the stability of the F,O,S-doped g-C3N4 catalyst: as noted by Xiao et al., 2020 (DOI: 10.1021/acs.accounts.9b00624), the mediation of these radicals does not introduce any instability for a g-C3N4 type catalyst.”
Q7. What are the photocatalytic mechanisms?
Response: The hypothesized photocatalytic mechanism was described in a paragraph (lines 279-287) and in Figure 8, but we agree with the reviewer on the advisability of expanding it with additional details, as follows: “Under visible light irradiation, F,O,S-doped g-C3N4 absorbs photons and electron-hole pairs are generated. The photogenerated holes (h+) in the valence band of the catalyst do not have the ability to oxidize water to hydroxyl radicals (•OH) (because the oxidation potential is not sufficient for the oxidation of H2O), but the photogenerated electrons (e-) in the conduction band of F,O,S-codoped g-C3N4 would be able to reduce the molecular oxygen to form reactive superoxide radical ions (•O2−) [Kobkeatthawin et al. DOI: 10.3390/catal12020120]. Based on this feature […]”
Q8. A comparison on photocatalytic activity of the as-obtained typical sample should be made with those of the related samples reported in the literature.
Response: In the reviewed version, references to DOI:10.1016/j.jmst.2020.03.033, 10.1016/j.matlet.2021.130068, 10.1021/acs.accounts.9b00624, 10.1016/j.carbon.2020.07.053, 10.3390/catal12020120, and 10.4028/www.scientific.net/AMM.889.24 have been included, for comparison purposes.
Q9. There are some inappropriate English words or expressions in the manuscript. The authors should carefully polish the English of the whole manuscript.
Response: Since no specific errors were indicated by the Reviewer, we have revised the use of the English language throughout the ms. and double-checked it with GrammarlyPro and Writefull.
Round 2
Reviewer 2 Report
After carefully checked the responses and modifications of the revised manuscript, I think that the authors have properly modified their manuscript according to the Reviewers' comments, and it is now acceptable for publication.
