Investigating the Role of Cs Species in the Toluene–Methanol Side Chain Alkylation Catalyzed by CsX Catalysts

Round 1

Reviewer 1 Report

The paper present interesting data on the activity of Cs-modified NaX zeolites in side-chain toluene alkylation with methanol. In general, the conclusions drawn by the authors are supported by the physicochemical data. However, some questions should be addressed in the revision of the manuscript.

-          Reflexes of Cs2O should be indicated clearly on the XRD patterns of the zeolites. Or the signals at 12.3° and 25.6° should be clearly assigned to Cs2O in the text.

-          NH3-TPD and CO2 TPD data for the initial NaX zeolite should be presented in Figs. 4,5. Surprisingly, the content of medium-strength acid sites in the zeolites is the same with no dependence on the Cs loading. The authors should explain this fact.

-          The IR spectrum of NaX with adsorbed methanol and TPD data for methanol decomposition for NaX would be also informative for comparison with Cs samples.

-          In discussing the catalytic data, obviously, the effect of pore blocking by excessive Cs should be discussed at high Cs loadings. It is also important to compare the found Cs loading with the stoichiometric Cs content corresponding to complete ions exchange of Na ions. Some data on the distribution of Cs between ion-exchanged forms and Cs2O should be also given.

Minor editing is required

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The side-chain alkylation of toluene with methanol is a key catalytic process to produce ethylbenzene, or even styrene. In this sense, the present work addresses the preparation of Cs-exchanged X zeolites, varying the Cs loading, and their use as catalysts for this catalytic process. Although the topic is interesting, the following issues should be addressed to improve the information provided in the manuscript: 

1.    The labels of catalysts should facilitate the identification of the exchange process. For instance, CsX1 could be Cs_1_60, where 1 indicates the number of exchange cycles and 60 is the reaction temperature in ºC, or Cs_10, where 10 is the wt% of the Cs content.

2.    (p. 3, l. 1) Cs(NO3)3 must be CsNO3.

3.    Experimental evidence must be provided for the existence of the three types of Cs sites present in the different materials: (1) Cs ions within the zeolite cage; (2) Cs+ interacting with defective sites in the zeolite framework, leading to the formation of cesium  silicate-like species or causing slight changes in the zeolite structure; and (3) highly dispersed Cs2O on the surface of the zeolite, forming a thin layer of Cs2O]. Moreover, the quantification of this type of Cs species must be carried out to explain the different catalytic behaviour of the prepared materials.

4.    The formation of a thin layer of Cs2O has not been demonstrated. In fact, the average size of Cs2O crystallites must be determined from their diffraction peaks associated.

5.    The concentration of acid and basis sites should be determined from the NH3- and CO2-TPD data, respectively, and used to explain the catalytic performance, as well as the absorption bands observed in the FTIR spectra after adsorption of methanol.

6.    Revise the footnote of Table 2, since the element Fe is not included. Furthermore, information regarding the elements Al and O should be included in this Table and consequently discussed. Nevertheless, the chemical composition obtained from SEM-EDX not only includes information about the surface, but also form the inner region of solids. Perhaps X-ray photoelectron spectroscopy (XPS) is a more suitable tool to obtain information on the surface of catalysts and, in fact, this is its main use in the physical-chemical characterization of materials.

7.    The magnification of the SEM micrographs (Figure 3) should be improved, since hardly any differences are observed between sodium zeolite and cesium-exchanged samples, such as the small amount of fine particles of Cs2O (smaller than 0.5 μm) attached to the surface of larger zeolite grains.

8.    (Table 3) The meaning of the different columns, mainly those related to the catalytic data, must be indicated. The discussion of these data should be improved.

9.    The present catalytic results must be compared with those already reported in the literature about Cs-exchanged zeolites for the side-chain alkylation of toluene with methanol.

In summary, it is necessary to clarify all these points before recommending the manuscript for publication.

English must be revised in depth, since there are several types, and very similar sentences, such as, for example, the first paragraph of section 2.2.1, and the description of the results regarding the TPD decomposition of methanol, displayed in Figure 7.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Most of questions have been answered, so the manuscript can be accepted in the present format.