In Situ X-ray Absorption Spectroscopy Cells for High Pressure Homogeneous Catalysis

Round 1

Reviewer 1 Report

This paper is an interesting technical note concerning the building of two new environmental cells for liquids for operando XAS/XANES and is suitable for publication in catalysts after the authors have considered the comments below.

 1. It would be very beneficial for the reader (and probably also for the authors themselves) if a carton/illustration of the cell under operating conditions, including the environment and the beam path, could be added to Figs. 1 and 2.

 2. A discussion on the potential for the application of EXAFS would also be appreciated.

Author Response

Point 1. This paper is an interesting technical note concerning the building of two new environmental cells for liquids for operando XAS/XANES and is suitable for publication in catalysts after the authors have considered the comments below.

Responce 1. We thank the reviewer for his/her positive report.

Point 2. It would be very beneficial for the reader (and probably also for the authors themselves) if a carton/illustration of the cell under operating conditions, including the environment and the beam path, could be added to Figs. 1 and 2.

Responce 2. We fully agree with this suggestion and have updated figures 1 and 2 in the revised version showing additional panels with cells under operating conditions, including the environment and the beam path.

Point 3. A discussion on the potential for the application of EXAFS would also be appreciated.

Responce 3. We have added the EXAFS results for Rh catalyst (see Figure 6b, chapter 2.4) and the following description:

“…Extended X-ray absorption fine structure (EXAFS) of the same sample is shown in Figure 6b. It confirms that initial single peak corresponded to the first shell of CO molecules split into two overlapped peaks: less coordinated CO and shell with larger inter-atomic distances, that might be attributed to the presence of Et3N group…”

Reviewer 2 Report

The manuscript deals with an interesting subject, operando X-ray absorption spectroscopy in higher pressure cells focusing on homogeneous catalysis. The authors nicely describe the set-up of the high pressure cells for transmission and fluorescence experiments. This part seems to be well suited for publication.

However, I have a couple of partly serious comments regarding the experiments and the reaction conditions.

1.  chap. 2.3  Ru-based homogeeous catalysis

The reaction is studied, as stated in the manuscript, under 5 bar CO and 20 bar H2. Why do the authors use this gas mixture ? I assume, CO was used at 5 bar to get a higher amount of the gas dissolved in the liquid for the reaction (although this is never mentioned). The use of 20 bar H2 is unclear since hydrogen does not seem to be used for the reaction. The authors could have used an inert gas to enhance the gas pressure and to dissolve more CO (if that is what they intended to do). The data in Fig. 5 are okay and I would agree with the conclusions.

2. chap. 2.4 Rh-based homogeneous catalysis

minor remark: please correct the figure caption, replace "Ru" by "Rh"

more serious remark: again I do not understand the reaction conditions (total pressure: 100 bar , reaction mixture CO : H2 1 : 1).  From my point of view, it is for sure not beneficial for the assumed reaction to offer more CO in the liquid phase and to assume that CO is being replaced in the Rh complex by Et3N. Again the presence of hydrogen in unclear. Such a process seems extremely unlikely to me, and moreover, a replacement might also affect the oxidation state of Rh.

There is absolutely no evidence for assuming that Et3N is replacing one CO in the Rh complex. The authors might, of course, show a reference spectrum of this substance for comparison to support their "suggestion". If this cannot be done, they simply have two different spectra showing something has happened. However, such a result cannot be published.

3.   2.4 Ru K-edge XANES.....

I am sorry, but it is really unacceptable to try to publish such noisy spectra (lower part of Fig. 7). There is no excuse for doing so, and the sentence  reading "the lower quality of spectra at room temperature is explained by inhomogeneity" does not rectify such a procedure. One cannot obtain ANY information from those 5-6 spectra in the lower part of Fig. 7, and the statement of a change of the oxidation state is not visible. I am also missing reference spectra to support the statements.

Such a "result" or, better said, a sequence of spectra, cannot be published in any journal. This need to be improved before considering publication.

4. finally, my last remark: to my understanding operando X-ray absorption spectroscopy means more than recording XAS spectra. The authors must relate the data / spectra to a chemical analysis of the reaction. In heterogeneous catalysis (solid-gas), for instance, one has to measure the concentration of reactants and products via mass spectroscopy, GC, or FTIR, to correlate the state of the reaction with the spectra. I cannot find any comment to this point.

To summarize: The article is technically interesting and might be of importance for the readership of this journal. I agree more or less with the first (technical) part. The second part, where the authors intend to demonstrate the capability of their set-up at different reactions has major drawbacks that need to be improved before considering publication. That also includes my final remark regarding "Operando XAS".

Author Response

The manuscript deals with an interesting subject, operando X-ray absorption spectroscopy in higher pressure cells focusing on homogeneous catalysis. The authors nicely describe the set-up of the high pressure cells for transmission and fluorescence experiments. This part seems to be well suited for publication.

However, I have a couple of partly serious comments regarding the experiments and the reaction conditions.

We would like to thank the reviewer for his/her, in general, positive feedback, and appreciate the criticism which helped to improve our work. We also would like to stress that the focus of this work is on technical possibilities of the cells for potential applications in catalysis that is why we do not discuss in detail the chemical procedure and structural changes in the systems which were taken as examples – this is a topic for independent reports.

Point 1. chap. 2.3  Ru-based homogeeous catalysis

The reaction is studied, as stated in the manuscript, under 5 bar CO and 20 bar H2. Why do the authors use this gas mixture ? I assume, CO was used at 5 bar to get a higher amount of the gas dissolved in the liquid for the reaction (although this is never mentioned). The use of 20 bar H2 is unclear since hydrogen does not seem to be used for the reaction. The authors could have used an inert gas to enhance the gas pressure and to dissolve more CO (if that is what they intended to do). The data in Fig. 5 are okay and I would agree with the conclusions.

Responce 1. The answer for this question is in the type of the reaction for which the catalyst is developed – this is hydrodeoxygenation reaction, which requires hydrogen as the main reagent. Therefore, the presence of H₂ as the main reagent should not be surprising. The question, instead, should be in the use of CO, which is not involved in the reaction. As we have shown earlier based on ex situ data (Stalpaert et al. ACS Catal. 2020, ref 34), the catalyst becomes inactive in the absence of CO, which was hypothetically assigned to the formation of ruthenium carbonyl complexes, which are necessary for the active catalyst. Now, this was directly confirmed by in situ measurements. We have added the following comment to the chapter 2.3.:

The presence of CO is necessary to form the active species for hydrodeoxygenation reaction.

Point 2. chap. 2.4 Rh-based homogeneous catalysis

minor remark: please correct the figure caption, replace "Ru" by "Rh"

The caption has been corrected.

more serious remark: again I do not understand the reaction conditions (total pressure: 100 bar , reaction mixture CO : H2 1 : 1).  From my point of view, it is for sure not beneficial for the assumed reaction to offer more CO in the liquid phase and to assume that CO is being replaced in the Rh complex by Et3N. Again the presence of hydrogen in unclear. Such a process seems extremely unlikely to me, and moreover, a replacement might also affect the oxidation state of Rh.

There is absolutely no evidence for assuming that Et3N is replacing one CO in the Rh complex. The authors might, of course, show a reference spectrum of this substance for comparison to support their "suggestion". If this cannot be done, they simply have two different spectra showing something has happened. However, such a result cannot be published.

Responce 2. As in the previous comment, the answer to the question is in the name of the reaction. In this case we deal with hydroformylation reaction, which by definition requires hydrogen and CO as reagents. We fully agree with the Reviewer’s remark, that from structural point of view there is not enough analysis performed to claim the substitution of the ligand, but as we noted above, this is beyond the scope of this manuscript, which is focused on the cell and its potential applications.

We have included the EXAFS results for Rh catalyst (see Figure 6b), showing the difference between the first shell of CO molecules at 25 °C and mixed phase at 80 °C.

As was shown before, that hydroformylation, and particularly reductive hydroformylation reactions, are best performed with homogenous Rh/N systems [10.1016/j.mcat.2021.112010, 10.1016/j.jcat.2021.06.001], that allow single-step conversion of olefins to alcohols. The developed cell allows investigating such process in situ and harsh conditions which are close to those used in industrial processes. In particular, for hydroformylation over non-modified Rh-catalysts and Rh/N systems the typical pressures of syngas (1:1 – 1:2 CO:H2) are 50 – 150 bars. We agree that the structural conclusions made in the section are preliminary, because more extensive screening of the reaction conditions, measurement of vast reference database and extensive theoretical modelling are required which is a topic for independent investigation. Therefore, according to your comment, we modified the statement about the structural modifications around Rh, by putting softer conclusion about the structure and highlighting the importance of the conditions achieved in the developed cell: 
 
Therefore, the performed study demonstrates that the local structure of rhodium sites is changing upon reaction and the developed cell can be successfully used to track this evolution under the conditions relevant for industrial hydroformylation of Rh-based homogenous catalysts, which typically requires 50-150 bar of CO:H2 (1:1 – 1:2)

Point 3. 2.4 Ru K-edge XANES.....

I am sorry, but it is really unacceptable to try to publish such noisy spectra (lower part of Fig. 7). There is no excuse for doing so, and the sentence  reading "the lower quality of spectra at room temperature is explained by inhomogeneity" does not rectify such a procedure. One cannot obtain ANY information from those 5-6 spectra in the lower part of Fig. 7, and the statement of a change of the oxidation state is not visible. I am also missing reference spectra to support the statements.

Such a "result" or, better said, a sequence of spectra, cannot be published in any journal. This need to be improved before considering publication.

Responce 3. Here, we have to strongly disagree with Reviewer. One can obtain a lot of information from such data, otherwise the analysis of in situ, time-resolved XAS for many low concentrated systems would be impossible. As a group with strongest expertise in XAS we have added to the section 2.4. just one of several possible routes for the analysis and extraction of useful information of such kind of data:

To overcome the issues of poor signal-to-noise ratio, which complicates the interpretation of individual spectra, we have applied PCA analysis using PyFitIt code, to decompose the whole dataset into statistically relevant components. Subsequent target matrix transformation allows one to convert the above components into physically meaningful spectra. It can be seen from the Figure 7b, that the whole dataset shown in Figure 7a, is represented by the two independent states, corresponding to Ru(IV) and Ru(II) oxidation states according to the position of the edge. The concentration profiles of these two states are shown in Figure 7c.

Point 4. finally, my last remark: to my understanding operando X-ray absorption spectroscopy means more than recording XAS spectra. The authors must relate the data / spectra to a chemical analysis of the reaction. In heterogeneous catalysis (solid-gas), for instance, one has to measure the concentration of reactants and products via mass spectroscopy, GC, or FTIR, to correlate the state of the reaction with the spectra. I cannot find any comment to this point.

Responce 4. We agree with the reviewer. Operando methodology implies simultaneous measurement of the reaction efficiency together with the collection of structural data. We agree that it was not done in the present work, although both cells do allow such procedure (e.g. by using a pump to circulate the catalyst through another cell for FTIR or UV-Vis measurement). According to this remark, we always use term in situ, when describing the performed experiments.

Reviewer 3 Report

The paper reports on the design, building, and test of two autoclave-type cells for operando X-ray absorption
spectroscopy experiments in transmission and fluorescence modes.
Such cells capable of operating at high-pressure and high-temperature conditions are required to study
the catalytic processes under relevant industrial conditions.

The paper is well written and is suitable for publication in the Catalysts journal after minor corrections.

1) The photo of the fluorescence cell should be added in Fig. 3 for completeness.

2) The directions of the incoming and outgoing X-rays should be indicated by arrows in Figs. 1 and 2
to make the schemes clear for the reader.

3) The information on the linear sizes of the cells should be added.

4) Why was the aluminium tube used only in the transmission cell?

5) Please indicate the possible energy range accessible with both cells.

Author Response

The paper reports on the design, building, and test of two autoclave-type cells for operando X-ray absorption spectroscopy experiments in transmission and fluorescence modes. Such cells capable of operating at high-pressure and high-temperature conditions are required to study the catalytic processes under relevant industrial conditions.
The paper is well written and is suitable for publication in the Catalysts journal after minor corrections.

We thank reviewer for his/her positive report.

Point 1. The photo of the fluorescence cell should be added in Fig. 3 for completeness.

Responce 1. The photo of the fluorescence cell has been added (see Fig. 3a).

Point 2. The directions of the incoming and outgoing X-rays should be indicated by arrows in Figs. 1 and 2 to make the schemes clear for the reader.

Responce 2. We have updated figures 1 and 2 in the revised version showing additional panels with cells under operating conditions, including the environment and the beam path.

Point 3. The information on the linear sizes of the cells should be added.

Responce 3. We have added scale bars in figures 1 and 2.

Point 4. Why was the aluminium tube used only in the transmission cell?

Responce 4. The transmission geometry allows having very small window which is required for the direct beam to enter the cell. The dimensions of the beam are typically within 1 mm (and even smaller in horizontal axis). With such small hole, a thin layer of aluminum was enough to prevent deflection of the inner Teflon liner. For bigger window size, which is required for fluorescence (the optimal conditions are reached if the beam is striking the sample at 45 ° and fluorescence is collected at 90° with respect to the incident beam), a thinker aluminum layer is required whose total absorption at energies 20-24 keV is higher than that of the used carbon windows.

We have added this remark to the description of the transmission cell (chapter 2.1, p.3).

Point 5. Please indicate the possible energy range accessible with both cells.

Responce 5. The total absorption of the transmission cell (from aluminum and Teflon) is 1.4 at Zr K-edge, while the absorption of water and organic liquids at this energy is already negligible. Thus, starting from Zr K-edge the sample can be optimized for high-quality transmission measurements. For the fluorescence cell at the same energy with collection of Zr K_α emission, the total absorption is 1.2, which is also low enough for the measurement.

We have added this remark at the end of chapter 2.1 (p.4).

Round 2

Reviewer 2 Report

In the revized version, the authors have made  a couple of important modifications and improvements of the data evaluation.  One part was related to a better understanding of the reaction conditions for persons not being very familiar with organic chemistry and homogeneous catalysis. On the other hand, the authors significantly improved the of figs. 6 and 7. I am well aware that the group is able to perform a proper analysis of XAS data.

Finally, I suggest to slightly change the title of the manuscript from "Operando" (the authors do not show any data refering to Operando for the manuscript) to "In-situ".

I otherwise agree with the modifications and recommend to publish the manuscript in Catalysts