Dehydrogenation of Propane to Propylene Using Promoter-Free Hierarchical Pt/Silicalite-1 Nanosheets

Round 1

Reviewer 1 Report

The article discusses the preparation and testing of eolite-nanosheet supproted catalysts as unpromoted alternatives to current industrial catalysts. The paper is clearly written and all results are discussed in detail.

There are, however, a few points the authors should elaborate on:

In Figure 1, the SEM/TEM pictures should be at the same scale, bot better comparison.

Related to that, have the catalysts been pressed/crushed/sieved to ensure similar particle sizes when comparing then ? Otherwise it would be obvious that only from using smaller particles one would achieve high selectivities/conversion. Have different particle size fractions been tested to investigate the influence of internal pore diffusion ?

How is the mechanical stability of those nano-sheets ? Can they be pressed to pellets, are binders required ?

Further, related to this, the high surface and support acidic properties provide a high dispersion of the platinum( and precursor), forming small clusters of Pt. In commercial systems the promotion bin tin has a similar effect. The question is whether the remaining acid sites can be blocked to minimize any cracking reactions ? Has this been looked into ?

Could the authors mention or add a line for the theoretical equilibrium coversion at the used temperature/pressure ? It should roughly be around 45 %. So the experimental values are well below that, which allows the comparison of different catalysts.

The authors mention high conversion and selectivity at the chosen conditions. Those conditions are often chosen in published works to (intentionally?) favor high conversion and selectivity. At more industrial conditions (1-3 bars, 600 degC, 20 % steam) how would the catalysts perform ? Can you give an indication whether the selectivity would still reach 90 % ?

Related to that, the authors compare their catalyst support to other silicates, but those are a poor reference. Could you add some discussion comparing the developed system to state-of-the-art or industrial catalysts, such as Pt/Zn on alumina, PtSn on hydrotalcites etc. (Akporiaye et al.) ? Do the nano-sheet supported systems perform similarly well ?

The catalysts still need to be frequently regenerated by air treatment. Has the stability been tested, especially with respect to platinum sintering ?

Author Response

Journal: Catalysts
Manuscript ID: catalysts-436072
Title: " Dehydrogenation of propane to propylene using promoter-free

hierarchical Pt/silicalite-1 nanosheets"
Author(s): Wannaruedee Wannapakdee, Thittaya Yutthalekha, Pannida Dugkhuntod, Kamonlatth Rodponthukwaji, Anawat Thivasasith, Somkiat Nokbin, Thongthai Witoon, Sitthiphong Pengpanich, Chularat Wattanakit

           We would like to thank the reviewer for a very nice consideration of our manuscript and giving us very useful comments. Herein, we would like to express and revise our manuscript according the reviewer's comments as shown below:

Reviewer 1: Questions / comments (italics), together with our answers:

Reviewer 1:

 

The article discusses the preparation and testing of zeolite-nanosheet supported catalysts as unpromoted alternatives to current industrial catalysts. The paper is clearly written and all results are discussed in detail.

 

There are, however, a few points the authors should elaborate on:

 

Q1:  In Figure 1, the SEM/TEM pictures should be at the same scale, both better comparison.

 

A1: Revised manuscript

            Thank you very much the referee for very useful comments. We have already improved the manuscript by adding the SEM/TEM images in the same scale bar as shown in Figure 1.

Fig. 1 Characterizations of various Pt supported catalysts: A) XRD patterns, B-D) SEM images of Pt supported on hierarchical silicalite-1 nanosheets (1%Pt-Si-MFI-NS), conventional silicalite-1 (1%Pt-Si-MFI-CON) and alumina (1%Pt-Al₂O₃), respectively, E-G) TEM images of 1%Pt-Si-MFI-NS, 1%Pt-Si-MFI-CON and 1%Pt-Al₂O₃, respectively and H) N₂ sorption isotherms and BJH pore size distributions (see inset).

 

Q2: Related to that, have the catalysts been pressed/crushed/sieved to ensure similar particle sizes when comparing then ? Otherwise it would be obvious that only from using smaller particles one would achieve high selectivities/conversion. Have different particle size fractions been tested to investigate the influence of internal pore diffusion?

 

A2: Revised manuscript and authors’ explanation:

            Thank you very much the referee for very useful comments. Prior to the reaction testing, we prepared the catalysts with the similar particles size by pressed/crushed/sieved process to control the particle size of all samples in the range of 425 – 850 µm to make sure that we can use the hierarchical zeolite to improve the internal pore diffusion. In addition, we have already improved the manuscript according to referee suggestion as the highlighted sentence:

 

“The dehydrogenation of propane to propylene was carried out in a continuous-flow fixed-bed reactor. Prior to the reaction testing, the similar particles size of a catalyst was controlled in the range of 425 – 850 µm. A 0.2 g of catalyst was placed into the reactor and activated at 550 °C under the flow of H₂ (2.4 ml.min^-1) for 2 h…”

 

Q3: How is the mechanical stability of those nano-sheets? Can they be pressed to pellets, are binders required?

 

A3: Authors’ explanation:

            Thank you very much the referee for very useful comments. We carried out to test the mechanic stability of the hierarchical zeolite nanosheet including the thermal stability after the reaction. It was found that the mechanical stability of a catalyst after pressed/crushed/sieved process was similar to what has been observed as the prepared one. It means that it is possible to be pressed to pellets. However, after the reaction, we observed a significant decrease in intensity as shown in Fig. R1. The intensity of catalyst after testing reaction remains representing the characteristic peaks of ZSM-5 structure, attributing to MFI framework. The results indicate that the thermal stability of nanosheet catalysts has remained as shown the zeolite structures with the crystallinity of 41.7% compared to nanosheet catalyst before doing the reaction.

Figure R1. XRD patterns of 1 wt.% Pt Si-MFI-NS catalysts before (black) and after (red) the PDH reaction.

 

Q4: Further, related to this, the high surface and support acidic properties provide a high dispersion of the platinum( and precursor), forming small clusters of  Pt. In commercial systems the promotion bin tin has a similar effect. The question is whether the remaining acid sites can be blocked to minimize any cracking reactions? Has this been looked into?

 

A4: Revised manuscript and Authors’ explanation:

            Thank you very much the referee for very useful comments. We carried out additional experiments regarding NH₃-TPD to observe the acidity of a hierarchical zeolite support. Indeed, we can also take the benefit of silicalite-1 nanosheet as a solid support because in this case it contains only small amount of strong acid sites due to the absence of Al active species. From NH₃-TPD curves and the amount of acid sites obtained by the deconvoluted curves (Fig. R2 and Table R1), it clearly shows that the number of strong acid sites of hierarchical MFI nanosheets is obviously lower than that of Al₂O₃. This behavior also causes the suppression of side catalytic cracking. In addition, we have already included the NH₃-TPD profiles in Figure S3.

 

Figure R2. NH₃-TPD profiles of various supports, including 1wt%Pt hierarchical silicalite-1 nanosheets (1%Pt-Si-MFI-NS), 1wt%Pt conventional silicalite-1 (1%Pt-Si-MFI-CON) and 1%Pt alumina (Al₂O₃) (1%Pt-Al₂O₃).

 

Table R1. The acidity characterization of the Pt-based catalysts.

Samples	Number of acid sites (mmol.g-1)a
	Weak (Peak I)	Medium (Peak II)	Strong (Peak III)
1wt%Pt Al2O3	0.110	0.057	0.132
1wt%Pt Si-MFI-CON	0.058	0.071	0.037
1wt%Pt Si-MFI-NS	0.027	0.042	0.006

 ^aObtained from the deconvoluted NH₃-TPD analysis.

 

Q5: Could the authors mention or add a line for the theoretical equilibrium conversion at the used temperature/pressure? It should roughly be around 45 %. So the experimental values are well below that, which allows the comparison of different catalysts.

 

A5: Revised manuscript

Thank you very much the referee for very useful comments. We have included the information according to the theoretical equilibrium conversion at the used temperature/pressure as following highlighted sentences into page no. 7, line 215 – 216: 

 “The conversion of propane was in the range of 30-35% and it agreed well with the theoretical equilibrium value at 550°C is ~37%...”

In our experiments, propane conversion ranged from 2 to 36% at 550 °C under atmospheric pressure, which was nearly the equilibrium conversion of propane dehydrogenation of ~37% under the similar experimental condition [1, 2] as shown in Fig. R3.

Figure R3. Temperatures required to achieve 10 and 40% equilibrium conversion of C₂ – C₁₅ n-paraffins at 1 atm [1,2].

References:

[1] M.M. Bhasin, J.H McCain, B.V Vora, T. Imai, P.R Pujadó, Dehydrogenation and oxydehydrogenation of paraffins to olefins, Appl. Catal., A., 2001, 221, 397–419.

[2] Neil M. Schweitzer, Bo Hu, Ujjal Das, Hacksung Kim, Jeffrey Greeley, Larry A. Curtiss, Peter C. Stair, Jeffrey T. Miller, Adam S. Hock, Propylene Hydrogenation and Propane Dehydrogenation by a Single-Site Zn²⁺ on Silica Catalyst, ACS Catal., 2014, 4, 1091−1098.

 

Q6: The authors mention high conversion and selectivity at the chosen conditions. Those conditions are often chosen in published works to (intentionally?) favor high conversion and selectivity. At more industrial conditions (1-3 bars, 600 degC, 20 % steam) how would the catalysts perform? Can you give an indication whether the selectivity would still reach 90 %?

 

A6: Authors’ explanation:

Thank you very much the referee for very useful comments. Based on available literatures, they also indicated the reaction study in the temperature condition in the range of 550 to 625 °C. The increase of temperature also affects high propane conversion and high propylene selectivity. However, at the temperature above 625 °C the cracking product is pronounced.

            In case of industrial conditions (1-3 bars, 600 degC, 20 % steam), the Pt-zeolite with promoters such as Sn, Na, Fe, Zn, K, Zn, Mn, etc. can also increase the reaction activity with the steam. The researchers [1] claimed that “..the most serious problem was dealumination of the support during regeneration with steam as ZSM-5 has an Al based structure. Herein, we report using silicalite-1 based catalysts which should perform under industrial conditions due to high Si based structure, which can resist the hydrothermal treatment.

From literature [2] and our obtained results, we can reach 90% which is already close to the theoretical equilibrium conversion. As the reaction is highly endothermic, an increase in temperature enhances the conversion of propane due to both kinetic and thermodynamic factors. However, higher temperatures could promote noncatalytic gas-phase reactions, which enhance side reactions such as propane cracking and coke formation but normally, they are predominant at the reaction temperature above 625 °C.

Figure R4. Influence of temperature on equilibrium propane conversion (dash line) and catalyst performance.[2]

References :

[1] Z. Nawaz, X. Tang, Q. Zhang, D.Wang, W. Fei, SAPO-34 supported Pt–Sn-based novel catalyst for propane dehydrogenation to propylene, Catal. Comm., 2009, 10, 1925–1930.

[2] F.T. Zangeneh, A. Taeb, K. Gholivand, S. Sahebdelfar, Kinetic study of propane dehydrogenation and catalyst deactivation over Pt-Sn/Al₂O₃ catalyst, J. Ener. Chem., 2013, 22, 726 – 732.

 

Q7: Related to that, the authors compare their catalyst support to other silicates, but those are a poor reference. Could you add some discussion comparing the developed system to state-of-the-art or industrial catalysts, such as Pt/Zn on alumina, PtSn on hydrotalcites etc. (Akporiaye et al.)? Do the nano-sheet supported systems perform similarly well?

 

A7: Revised manuscript

Thank you very much the referee for very useful comments. Based on the literatures, the dispersion of (mono- or bimettallic) metals is one of the most important factors on PDH reaction, such as the well-dispersed of Pt/Sn on hydrotalcite layers (or Mg(Al)O). The hydrotalcite has a high surface area (typical 160-220 m²/g) and shows a much higher resistance to sintering under steam-rich conditions than pure MgO, which is suitable for being basic catalysts and hydrogenation catalysts. [1] As can be seen in the experimental data, we believe that hierarchical zeolite structures could be the good candidate to enhance the catalytic activity by increasing the metal dispersion and/or metal-metal interaction. In addition, we have already added some discussion following the referee’s suggestion as following highlighted sentences:

“In addition, the hierarchical structure provided a good benefit for promoting the propylene production, resulting in the suppression of the further oligomerization as a side reaction due to the shortening of the diffusion path length of catalysts. These observations may have the benefit of an increase in metal dispersion and it is complementary to the development of the well-dispersed metallic nanopartiles on solid supports in PDH reaction [56]”.

 

Reference :

[1] D. Akporiaye, S. F. Jensen, U. Olsbye, F. Rohr, E. Rytter, M. Rønnekleiv, A. I. Spjelkavik, A novel, Highly efficient catalyst for propane dehydrogenation, Ind. Eng. Chem. Res., 2001, 40, 4741-4748.

 

Q8: The catalysts still need to be frequently regenerated by air treatment. Has the stability been tested, especially with respect to platinum sintering?

 

A8: Revised manuscript

Thank you very much the referee for very useful comments. We checked the Pt sintering effect of Pt-loaded on Silicalite-1 nanosheets after the catalytic reaction. Compared with a fresh catalyst (Fig. R6(A)), it clearly shows that the Pt metal nanoparticles supported on zeolite nanosheets have remained the well-dispersion (Fig. R6(B)) with the similar size of 2.1 ± 0.7 nm. It is therefore reasonable to assume that the stacking-layered zeolite nanosheets inhibit the metal aggregation. In addition, we have already improved the manuscript following the referee’s suggestion in Figure S7 and following highlighted sentences:

“In addition, the Pt metal nanoparticles supported on zeolite nanosheets have remained the well-dispersion with the similar size of 2.1 ± 0.7 nm (Fig. S7).”

 

Figure R6. HRTEM images of 1%wtPt Si-MFI-NS (A) before and (B) after the PDH reaction.

 

 

We thank the editor and referees for their very constructive comments and kind consideration,

Best regards,

Chularat Wattanakit

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript described a new catalyst platform of hierarchical Pt-doped silicalite-1 nanosheets for propane dehydrogenation. The Pt/silicalite-1 nanosheet, as well as the conventional silicalite-1 and alumina, were thoroughly characterized, and the catalysis performances were compared. The Pt/silicalite-1 nanosheets shower superior catalytic performance and low side effects. The authors also discussed the rationale of the results. Overall, this is a well-written paper with solid experiment data and good discussion. I would agree to publish this paper with a minor correction:

Fig. 1H, the pore diameter unit is missing. 

Author Response

Journal: Catalysts
Manuscript ID: catalysts-436072
Title: " Dehydrogenation of propane to propylene using promoter-free

hierarchical Pt/silicalite-1 nanosheets"
Author(s): Wannaruedee Wannapakdee, Thittaya Yutthalekha, Pannida Dugkhuntod, Kamonlatth Rodponthukwaji, Anawat Thivasasith, Somkiat Nokbin, Thongthai Witoon, Sitthiphong Pengpanich, Chularat Wattanakit

 

We would like to thank the reviewer for a very nice consideration of our manuscript and giving us very useful comments. Herein, we would like to express and revise our manuscript according the reviewer's comments as shown below:

 

 

Reviewer 2: Questions / comments (italics), together with our answers:

Reviewer 2:

The manuscript described a new catalyst platform of hierarchical Pt-doped silicalite-1 nanosheets for propane dehydrogenation. The Pt/silicalite-1 nanosheet, as well as the conventional silicalite-1 and alumina, were thoroughly characterized, and the catalysis performances were compared. The Pt/silicalite-1 nanosheets shower superior catalytic performance and low side effects. The authors also discussed the rationale of the results. Overall, this is a well-written paper with solid experiment data and good discussion. I would agree to publish this paper with a minor correction:

 

Q1: Fig. 1H, the pore diameter unit is missing.

 

 

A1: Revised manuscript

Thank you very much the referee for very nice comments and useful suggestion. We have already improved the manuscript by adding the unit of pore diameter in Figure 1H.

 

Fig. 1 Characterizations of various Pt supported catalysts: A) XRD patterns, B-D) SEM images of Pt supported on hierarchical silicalite-1 nanosheets (1%Pt-Si-MFI-NS), conventional silicalite-1 (1%Pt-Si-MFI-CON) and alumina (1%Pt-Al₂O₃), respectively, E-G) TEM images of 1%Pt-Si-MFI-NS, 1%Pt-Si-MFI-CON and 1%Pt-Al₂O₃, respectively and H) N₂ sorption isotherms and BJH pore size distributions (see inset).

 

 

 

We thank the editor and referees for their very constructive comments and kind consideration,

Best regards,

Chularat Wattanakit

Author Response File: Author Response.pdf