Sulfonic Resins as Catalysts for the Oxidation of Alcohols with H₂O₂/KBr

Round 1

Reviewer 1 Report

The manuscript presented by Dorado et al. may be of interest for the readers of Catalysts. The work outlines the use of heterogeneous catalysts for the oxidation of alcohols showing good results to replace the conventional homogeneous catalysts. In general, the manuscript is well written and the results are clearly explained and discussed. However, some issues must be addressed to improve this future article:

1. Since the performance of the resins is mainly attributed to their properties, more information is needed about their textural and acidic characteristics. If possible, you may conduct characterization tests and report the results. You may also report (in a table or graph) the properties you have from the providers, highlighting those that explain the resin’s performance.

2. To make the results easier to be seen, the authors must arrange in a table some results from some studies only reported in the text. For example, in line 58, you mentioned and reported the comparative results with homogeneous catalysts, and in line 98 you mentioned and reported the results for the type of bromide source. It would be interesting to put all these results in tables, so that they can be seen easily.

3. As the authors mentioned in line 92, the amount of catalyst is indeed very important and, in my viewpoint, that deserves a specific study. Actually, the amount of actual sites is important and all the reaction tests must be compared on the same basis of actual active sites available for the reaction (analogous to the concept of turnover frequency (TOF)).

4. In Figure 1, please use a legend to easily read the meaning of the blue bar. Although you have indicated the meaning in the figure’s title, it would be easier to read in a legend even if the figure is printed or seen in greyscale.

Author Response

The manuscript presented by Dorado et al. may be of interest for the readers of Catalysts. The work outlines the use of heterogeneous catalysts for the oxidation of alcohols showing good results to replace the conventional homogeneous catalysts. In general, the manuscript is well written and the results are clearly explained and discussed. However, some issues must be addressed to improve this future article:

1. Since the performance of the resins is mainly attributed to their properties, more information is needed about their textural and acidic characteristics. If possible, you may conduct characterization tests and report the results. You may also report (in a table or graph) the properties you have from the providers, highlighting those that explain the resin’s performance.

A new table in Materials and Methods section (Table 4) has been added, with some important properties of the resins. The data sources (webpages) have been included as additional references (35-40). The acidities, determined by back titration (lines 229-235), have been included. The particle size from the supplier has been compared with that measured in dry state and swollen with the solvent mixture (Figure S1 in supporting information) used in the reaction. Some possible effects have been explained (as a hypothesis) in lines 123-138.

2. To make the results easier to be seen, the authors must arrange in a table some results from some studies only reported in the text. For example, in line 58, you mentioned and reported the comparative results with homogeneous catalysts, and in line 98 you mentioned and reported the results for the type of bromide source. It would be interesting to put all these results in tables, so that they can be seen easily.

The results previously shown only in the text have been included in Table 1 (entries 8 to 11).

3. As the authors mentioned in line 92, the amount of catalyst is indeed very important and, in my viewpoint, that deserves a specific study. Actually, the amount of actual sites is important and all the reaction tests must be compared on the same basis of actual active sites available for the reaction (analogous to the concept of turnover frequency (TOF)).

The reactions were designed by maintaining constant the amount of sulfonic sites, considering the functionalization of each catalyst, determined by back titration (Table 4). This point has been stressed in the text (lines 80-85) and also by including the amount of catalyst (in mg) in Table 1. In that way, the conversion at the same reaction time can be directly compared in a similar way as TOF.

4. In Figure 1, please use a legend to easily read the meaning of the blue bar. Although you have indicated the meaning in the figure’s title, it would be easier to read in a legend even if the figure is printed or seen in greyscale.

The legend has been included.

Reviewer 2 Report

19^th December, 2023.

Referee report:

Manuscript ID: Catalysts-2782132

Title: Sulfonic resins as catalysts for the oxidation of alcohols with H₂O₂/KBr

Authors: Vincente Dorado, Clara I Herrerias, and Jose M Fraile*

Summary: Fraile and coworkers  developed an oxidation process of alcohols using sulfonic acid resins as heterogeneous catalyst in the presence of H₂O₂ and KBr.  Various alcohols used as substrates are 9 (10) - hydroxyl stearate, 1-dodecanol, 1-octadecanol, 2-methyl cyclohexanol, (-)-menthol.  Structural features of the catalysts (namely the degree of crosslinking, the length of the side chains) were found be a crucial factor more important than the acid amount or acid strength of the catalyst. Treatment of the sulfonated resin with HCl at 80 °C for 2 h is proposed to be a viable strategy for the recovering of the activity of the solid acid catalyst. Best results were obtained with the gel-type poly (styrene-divinyl benzene) resin. Cyclic secondary alcohols were oxidized to ketones.  Primary fatty alcohols were oxidized to carboxylic acids which were subsequently esterified with the substrate alcohol.  The solid acid resin catalyst showed activity for 8 reaction runs. Owing to the originality and usefulness of the results, the work of Fraile and coworkers can be accepted for publication in “Catalysts” after minor revision.

Minor revision required:  The authors may consider the following issues while revising the manuscript.

1. The terms “structural” and “textural” were used synonymously through they are different.  In the current context, the features like degree of crosslinking and others are structural rather than textural. No evaluation of the textural properties like the surface area of the catalyst and porosity were made and so the authors should refrain from using the term “textural”.

2. The treatment of sulfonic acid resin with HCl for 3 h at 80 °C is effective for regeneration.  The authors can comment on the chemistry of regeneration upon HCl treatment.

3. Can the acidity amount and acid strength of the resins be examined by either FTIR or TPD?

In addition to the above, the following aspects can also be considered.

Author Response

Minor revision required:  The authors may consider the following issues while revising the manuscript.

1. The terms “structural” and “textural” were used synonymously through they are different.  In the current context, the features like degree of crosslinking and others are structural rather than textural. No evaluation of the textural properties like the surface area of the catalyst and porosity were made and so the authors should refrain from using the term “textural”.

The reviewer is right about the concepts structural and textural, but in the case of resins both concepts are intimately connected. In fact, high cross-linked resins are macroreticular, with a more rigid structure, permanent porosity and poor swelling behavior, whereas low cross-linking resins are gel-type, with a much more flexible structure, without permanent porosity and high solvent dependent swelling capacity. After revision of the manuscript, the term textural was only used twice. In this version, the last one (in Conclusions section) has been substituted by “structural”.

The evaluation of surface area and porosity is not currently used with resins due to the pointed swelling ability. The BET measurement of a gel-type resin would give very low values of surface area and porosity, whereas in the reaction medium, in the presence of a suitable solvent system, the structure would be swollen, increasing both parameters in a significant, but unmeasurable, manner. We have included the new Table 4, with some properties of the resins, and in the Supporting Material we have included some pictures of the swelling ability in the reaction solvent system.

2. The treatment of sulfonic acid resin with HCl for 3 h at 80 °C is effective for regeneration.  The authors can comment on the chemistry of regeneration upon HCl treatment.

It is due to a simple cationic exchange. The deactivation-reactivation mechanism has been explain in a more detailed way in lines 147-152 and 154-155, as well as in the new Scheme 3.

3. Can the acidity amount and acid strength of the resins be examined by either FTIR or TPD?

Again, these methods used for inorganic catalytic materials are not suitable for resins. In the case of FTIR, the bands of the resin will probably mask the bands corresponding to the adsorbed probe molecule (e.g. pyridine). Regarding TPD, the instability of the resins at moderately high temperatures precludes its use.

As the nature of the sites is clear (sulfonic groups), the determination of the type and strength of the sites is not so important, and only functionalization is usually determined by back titration, as described in lines 229-235. The results are collected in Table 4.

In addition to the above, the following aspects can also be considered.

The mistakes have been corrected. Other cases of temperature without spacing have been also corrected.

The indicated superscript was in a reference, and the reference manager used (Mendeley) does not allow superscripts. As it was a footnote, we have eliminated the number.

Regeneration of active sites: as already discussed, the process is described in lines 147-152 and 154-155, as well as in the new Scheme 3.

The titration method is of common use for characterization of the proton (cation) exchange capacity of resins. One reference (34) has been added where a similar method is used for proton exchange determination in carbon materials.

The term “0 min” indicated that ramp2 followed ramp 1 without any plateau. In order to prevent any misunderstanding, the oven temperature program for GC has been expressed more clearly.

Additional information about commercially available compounds, used as reagents, has been added. The oxidized products were identified by NMR and/or GC-MS, by comparison with the spectra described in the literature or collected in the libraries. The methyl hydroxystearate was prepared in our laboratory from methyl oleate, as described in the experimental section.

Both ¹H and ¹³C NMR spectra were registered in the same spectrometer. This point has been clearly stated in the text.

The role of KBr is crucial, as already described in the experiments in homogeneous phase (ref. 24), where the control experiments without KBr were performed, and also observed in some of our experiments with TBAB. The proposed mechanism, adapted from the one described in the literature, has been included in a new Scheme 2, also commented in the text (lines 114-138). Some additional references (28, 32 and 33) that support some aspects of the mechanism have been also included.