Novel Complex Titanium NASICON-Type Phosphates as Acidic Catalysts for Ethanol Dehydration

Round 1

Reviewer 1 Report

The manuscript entitled “Novel Complex Titanium NASICON-Type Phosphates as Acidic Catalysts for Ethanol Dehydration” reports the catalysis of ethanol dehydration by complex titanium phosphate MAlTiP (M0.5(1+x)AlxTi2-x(PO4)3 (M = Ni, Mn; x = 0; 0,2)) catalysts with NASICON-type structure synthesized by the sol-gel method. The experimental results will be of interest to readers in the field of catalysis. Therefore, I think it is appropriate to be published in Catalysts with minor revisions.

1. Symbols for physical quantities should be uniformly italicized. There should be a space between the numerical value and the unit of physical quantities.

2. The diffraction peaks of the standard card should be given in Figure 1 for comparison.

3. The TEM images are suggested to characterize the samples.

4. Authors should compare the catalysis performances of the samples with those in other references.

5. There are some errors or mistakes in the text. Please check carefully and correct them.

Author Response

1. We are grateful for the Reviewer of the constructive comments. All correction has been made.

2. Thank you for this suggestion. We have added literature XRD data for the sample Mn_0.5Ti₂(PO₄)₃ (MnTiP) simulated basing on the structural data [Figure 1b].

3. The focus of our research was on the determination of relationship between acidity of solid catalysts and their catalytic activity. We consider that the characterization of texture properties given in the work is sufficient and TEM will not add additional information to the conclusions.

4. We have added Table 4. The results obtained for ethanol dehydration are summarized and compared other NASICON-type catalysts from the literature in Table 4.

5. The correction has been made.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this manuscript, the complex titanium phosphate MAlTiP (M_0.5(1+x)Al_xTi_2-x(PO₄)₃ (M=Ni, Mn; x=0, 0.2) for dehydration of ethanol into ethylene and diethyl ether (DEE). The study is significant and interesting. The authors in-detail investigated the reason for the difference in the activity of the prepared catalysts based on the multiple characterizations. The conclusion is supported by the data. In my opinion, this manuscript is suitable for the publication in the journal of Catalysts. However, minor revisions are proposed as follows.

(1)It should be explained why the spectral band intensity at 2196cm^-1 in Figure 5b increases significantly, while the spectral band intensity at 2022cm^-1 in Figure 5a does not.

(2) Since the acidic properties are quite important for catalyzing the present reaction, the quantitative analysis on them should be concerned. The present FTIR for CO and C6H6 adsorption are not enough. Therefore, I advise that NH₃-TPD characterization is adopted to understand the distribution of acidic sites.

(3) Following on from the previous question, the infrared characterization of pyridine should also be used to quantitatively calculate BAS and LAS, as well as interpret the catalytic mechanism.

Author Response

1. Response 1: We provided additional explication in the text «Note, that with the increase in CO pressure the band intensity of absorbed CO increases due to increasing the degree of filling of the LAS by CO molecules.  Simultaneously, the CO molecules may displace other adsorbed molecules or hydroxyl groups. The significant increase in band intensity at 2196 cm^-1 in Figure 5b is due to the displacement of part of the OH-groups from LAS.  For Ni-containing system such a process does not play a significant role (Figure 5a, band intensity at 2022 cm^-1 ).

2. Response for 2 and 3:We agree with the reviewer that FTIR for CO and C₆H₆ can't determine the number of acid sites.  We really appreciate the reviewer's idea of application NH3-TPD or IR-Py. We observe the lack of publications devoted to this technique for NASICON catalysts especially comparing with catalytic activity. Our idea was to demonstrate the possibility to use the C₆H₆ as probe molecule for BAS determination for framework phosphates, to calculate the proton affinity for BAS characterization and demonstrate the correlation “acidity - activity” for NASICON catalysts.

Author Response File: Author Response.pdf

Reviewer 3 Report

In the manuscript, the ethanol dehydration was studied on complex titanium phosphate MAlTiP (M0.5(1+x)AlxTi2-x(PO4)3 with M=Ni, Mn (x = 0; 0,2)) catalysts with NASICON-type structure. The MnAlTiP exhibited the highest ethylene selectivity among other catalysts with 87% at 420 °C. It was shown that the activity of complex phosphates in ethanol dehydration increases with the strength of the Brønsted acid sites (BAS). This work is relatively well-organized, a revision is needed before its publication. Here are some of my concerns.

1. The standard JCPDS card should be include in the Fig. 1 for the XRD pattern of MnTP. Is there any XRD peak shift for these samples in Fig. 1?

2. It is difficult to say the valence information based on FTIR, the authors should measure the XPS spetra to make it sure. 

3. For the phosphates, the analyses of three types of OH-groups in the FTIR spectrum should refer to some reports: such as Adv. Mater., 2022, 34, 2203615.

4. A comparison table or bar plots listing your performance of different samples should be provided to make it more clearly. 

Author Response

1. Thank you for this suggestion. We have added literature XRD data for the sample Mn_0.5Ti₂(PO₄)₃ (MnTiP) simulated basing on the structural data [Figure 1b]. XRD peak shifts for MnTiP were within the uncertainty in 2θ angle registration (0.02°). There were some peak shifts for MnAlTiP compared with MnTiP due to differences in their chemical compositions.

2. We agree with the reviewer to say the valence information based on FTIR is difficult. We state that the nature of LA sites is different on the surface of Ni- and Mn-containing triple phosphates and these sites change under the CO pressure.  Since this finding is not key to explaining catalysis data but explains the appearance of new bands at exposure in CO, we suggest the XPS study is unnecessary. Unfortunately, the reference [36] with classification was incorrect and has been changed by A. Davydov. Molecular spectroscopy of oxide catalyst surfaces (edited by N.T.Sheppard), Willey, 2003, ISBN 047198731X.

3. We have added to the existing reference some recent publications such as Liu, Y.; Chen, Y.; Tian, Y.; Sakthivel, T.; Liu, H.; Guo, S.; Zeng, H.; Dai, Z., Synergizing Hydrogen Spillover and Deprotonation by Internal Polarization Field in a MoS2/NiPS3 Vertical Heterostructure for Boosted Water Electrolysis. Advanced Materials, 2022, 34, 2203615.   doi:10.1002/adma.202203615. The number of added reference is [37].

4. We have added Table 4. The results obtained for ethanol dehydration are summarized and compared other NASICON-type catalysts from the literature in Table 4.

Author Response File: Author Response.pdf