H-ZSM-5 Materials Embedded in an Amorphous Silica Matrix: Highly Selective Catalysts for Propylene in Methanol-to-Olefin Process

Round 1

Reviewer 1 Report

This manuscript by Prof. Narasimharao and Mokhtar et al describes the synthesis of new doped H-ZSM-5 materials with different Si/Al ratios. The materials were fully characterized by spectroscopy. The synthesized zeolite materials exhibited lower density of Bronsted acid sites, which are key to the MTO process, in which hydride transfer reactions are suppressed and less oligomerization reactions occur. Three newly synthesized materials displayed better performance of the conversion of methanol to propylene compared with Zeolyst. However, the formation of HPC species was enhanced for the synthetic materials. The authors argued that more Lewis acid sites and less structural hindrance were responsible for the result. There were few evidences to support the statement.

 

 

Author Response

Reviewer (1):

 

Comment: The formation of HPC species was enhanced for the synthetic materials. The authors argued that more Lewis acid sites and less structural hindrance were responsible for the result. There were few evidences to support the statement.

 

Response:

Thanks a lot for the reviewer’s comment. Our argument was built on the reported literature [1] who proved that a synergy of extra-framework aluminum (EFAl) Lewis and Brønsted sites in zeolite catalysts facilitates initial C–C bond formation in the initiation step of the MTO reaction via the Al–COH₂ ⁺ intermediate. In addition, the less the structural hindrance provided by the synthesized ZSM-5 embedded amorphous silica with its meso-macro porous nature and voids (See Fig. 4b) resulted in enhancing HPC formation.

We present here CO-FTIR adsorption for all the investigated samples to clarify the presence of strong Lewis acid sites on the synthesized materials only. Kondo, J. et. al. [2]reported that the Lewis acid (Al³⁺) sites coordinated to CO shows a weak stretching vibration at 2230 cm^-1, this peak represents the strong Lewis acid sites. As shown in the (figure.1) all the synthesized ZSM-5 samples exhibited weak band at ~ 2232 cm^-1, however this band was not observed on Zeolyst sample.

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Reviewer 2 Report

In this work, Kamaluddin et al. report the synthesis and the phys-chemical characterization of amourphous ZSM-5 materials having different Si/Al ratio and the catalytic assessment in methanol to olefin process. The paper is well organized and it may be interesting for the readers of Catalysts but major reviosion is necessary before pubblication. Herewith, please the authors may find my comments: 1) The introduction part should be revised in the light of the following studies: - Haw et al., ACS Catalysis, 2018, 8, 8199; - Palcic et al., Chemistry, A European J., 2018, 24, 13136; - Wakihara et al., Phys.l Chem. Chem. Phys., 2006, 8, 224; - Catizzone et al., Catal Today, 2018, 304, 39; - Svelle et al., J. Am. Chem. Soc., 2006, 128, 14770; - Losch et al., Appl. Catal. A, 2016, 509, 30; 2) Experimental - the authors shall add all the information needed to replicate the synthesis of the materials (such as gel composition, amount of reactants, etc); - BJH: why the authors use the desorption branches? Usually, the adsoprtion braches is more accurrate. For instance, by using desorption branch a peak at below 5 nm is present, but it is not real. - More informations about NH3-TPD measurements are required (e.g. adsoprtion T, etc); - FT-IR: are self-supported pellets or KBr is used? All of these details have to be added; - Which is the concentration of methanol in the feed of the reactor? - The unit of WHSV shoudl be [h-1]; 3) Results 2.1 - FT-IR: The intense band at 3749 cm-1 may be related to the presence of amorphous; -The micropore volume of investigated materials shoudl be calculated and discussed; - BJH PSD shuuld be discussed in the light of the above comments; - line 207: -112 ppm is associated to Si(4Si); 4) Results 2.2 -The NH3 desorption temeprature is NOT directly indicative of a strength of acid sites. The following papers may be useful: Lònyi et al., Microp. Mesop. Mater., 2001, 47, 293; Giordano et al., J. Energ. Chem. 2019, 30, 162; Katada et al., J. Phys. Chem. B 1997, 101, 5969; Niwa et al., The Chemical Record, 2013, 13, 432; Simon et al., Appl. Catal. A., 2007, 328, 174-182; - FT-IR of adsorbed CO is not able to determine Lewis acid sites? The authors shoudl revise and discuss this part; 5) Results 2.3 - is the conversion referred to both Methanol and dimethy ether? the selectivitity towards DME shoudl be added; - May the auhors add to the SI the GC graph in order to verify the formation of propane or ethane? these compounds are usually present in MTO and is the a specific column is not used Propane and Propylene mat share the same GC peak. - The claim reported in line 336-338 should be strongly supported from experimental evidences. - line 341: EFAl+ species may have no Lewis acidity; the authors should revise this aspect and FT-IR may be useful. 6) Conclusion: "Mesopores H-ZSM-5 zeolites" are an other type of material: crystalline materials with a micro/meso structure. 7) May the authors test an amourphous alumosilicate sample? Even a discussion in the light of other studies may be useful.

Author Response

Reviewer (2):

 

1) Comment: The introduction part should be revised in the light of the following studies:

Haw et al., ACS Catalysis, 2018, 8, 8199; - Palcic et al., Chemistry, A European J., 2018, 24, 13136; - Wakihara et al., Phys.l Chem. Chem. Phys., 2006, 8, 224; - Catizzone et al., Catal Today, 2018, 304, 39; - Svelle et al., J. Am. Chem. Soc., 2006, 128, 14770; - Losch et al., Appl. Catal. A, 2016, 509, 30;

Response: we appreciate the reviewer comment on developing the introduction part. We revised the introduction according to the reviewer suggestions and some references were added (see the revised version).

2) Comment: Experimental

2.1. The authors shall add all the information needed to replicate the synthesis of the materials (such as gel composition, amount of reactants, etc.);

Response:

All the needed information regarding the gel composition and amounts of reactants were added to the experimental section and highlighted in yellow colour.

2.2. Comment: BJH: why the authors use the desorption branches? Usually, the adsorption braches is more accurate. For instance, by using desorption branch a peak at below 5 nm is present, but it is not real.

Response:

We appreciate the reviewer’s comment and according to the suggestion we used NLDFT for the adsorption branch of the synthesized materials. The obtained results were given in Figure 4b and Table 1. Only Zeolyst, which is microporous zeolite showed peak in the microporous range.

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Author Response File: Author Response.pdf

Reviewer 3 Report

This paper deals with MTO catalytic properties of H-ZSM-5 synthesized with a short hydrothermal period. It is explained that the synthesized H-ZSM-5 provided high propylene selectivity due to a small number of Brfnsted acid sites and their weak acidity. I wonder if a small number of Brfnsted acid sites and their weak acidity really influenced the MTO catalytic activities, and why the synthesis method employed in this study provided a small number of Brfnsted acid sites and their weak acidity. If a short hydrothermal period was useful, the influence of hydrothermal period on catalytic properties should be examined. Moreover, the presence of amorphous phase possibly influences the catalytic properties. Since long hydrothermal treatment decreases the amount of amorphous phase, the hydrothermal synthesis for different periods would provide useful insight into the influence of amorphous phase. Furthermore, it is explained in lines 341 to 345 that more Lewis acids sites were present in the synthesized samples, and the synergetic effect of Brfnsted acid and Lewis acid influenced the catalytic properties. However, I do not understand which results had relation to the amount of Lewis acid. So, I do not feel that this paper reach the level suitable for publication. Other comments are shown below:

 

1.       It is known that for MTO process, Lewis acids retards the formation of coke, while strong Brfnsted acids provide a large amount of coke. In this study, the synthesized samples provided larger amount of coke than commercial Zeolyst did, despite the results that the synthesis method employed in this study provided a small number of Brfnsted acid sites and their weak acidity. Why?

2.       It is explained in the Introduction section that since the catalytic properties of nano-sized ZSM-5 embedded in amorphous silica have not been explored, their application to a catalyst in MTO process was examined. However, the ZSM-5 crystals synthesized in this study were large but did not have nanometer scale, because their XRD peaks were very sharp and the SEM images showed micrometer-sized crystals.

3.       I wonder why the particles shown in Figure 3d did not have a characteristic shape of ZSM-5. Moreover, it is explained in some parts that the synthesized ZSM-5 crystals had mesopores. However, as shown in Figure 3a-c, the synthesized ZSM-5 crystals did not have mesopores. BET measurements regard the voids between particles as mesopores.

4.       It is explained in lines 198 to 199 that all the samples showed a major resonance at -112 ppm; however, the major resonance in Figure 5B (b) is at higher field than -112 ppm. Moreover, all shoulder peaks in Figure 5B (a) to (c) were assigned to Si(2Si,2Al). I wonder why two shoulder peaks with different chemical shifts were assigned to the same species. Is this assignment right?

 

Author Response

Reviewer (3):

1. Comment: This paper deals with MTO catalytic properties of H-ZSM-5 synthesized with a short hydrothermal period. It is explained that the synthesized H-ZSM-5 provided high propylene selectivity due to a small number of Brønsted acid sites and their weak acidity. I wonder if a small number of Brønsted acid sites and their weak acidity really influenced the MTO catalytic activities, and why the synthesis method employed in this study provided a small number of Brønsted acid sites and their weak acidity.

Response: We appreciate the reviewer’s comment

(1)   The Brønsted acid sites are crucial on forming active hydrocarbon pool, consequently derive the formation of olefin products. Moreover, one sample of ZSM-5 (Si/Al = 40) without ion exchange using ammonia was tested for the MTO reaction. Figure 4, showed that the catalytic activity of this particular catalyst is very low as there is no Brønsted functional group [Al-OH-Si] that is usually formed after ion-exchange of ZSM-5.

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Round 2

Reviewer 2 Report

2.5. Comment: Which is the concentration of methanol in the feed of the reactor? 

Response: The concentration of methanol used for catalytic testing was, HPLC grade (99.99 % pure), obtained from Acros Organics. This part was added to the materials section.

Comment by the reviewer: the authors report that helium was used as carrier. Therefore, which is the methanol molar fraction in the stream at the reactor inlet?

Author Response

I would like to thank the prominent effort by the distinguished reviewers who added a lot to the quality of the current revised version.

2.5.1 Comment by the reviewer: the authors report that helium was used as carrier. Therefore, which is the methanol molar fraction in the stream at the reactor inlet?  

Response:

The He carrier gas (87%) flow with MeOH saturation (ca.13%). The molar fraction of MeOH was 0.0184.

Calculations:

 Methanol saturation = 13%

He carrier gas = 87%

 

 

The total number of moles in mixture gas =

0.40625 mol MeOH + 21.734 mol He = 22.1403 mol total

 

The mole fraction of MeOH ( in a gas mixture

 

Author Response File: Author Response.pdf

Reviewer 3 Report

The revisions improved this paper a little; however, I do not think that the authors addressed all of my comments. I do not understand why a slight change in Si/Al ratio of precursors (Si/Al = 40 to 50) had significant influence on the properties of resulting samples. Since some parameters, such as the degree of crystallization, the Si/Al ratio, reproducibility etc., influence their properties, it is hard to understand how the parameters influenced. Thus, I previously suggested the preparation of samples by hydrothermal synthesis for different hydrothermal periods at a fixed Si/Al ratio; however, the authors said that they are planning to study it in the upcoming research. Moreover, I do not think that the authors figure out 29Si NMR spectra. As I mentioned previously, I do not understand why two shoulder peaks with different chemical shifts in 29Si NMR spectra were assigned to the same species. In this paper, all the peaks in 29Si NMR spectra are assigned to Q4 species (in this paper, they are represented as Si(nSi, (4-n)Al)); however, as FT-IR spectra exhibited absorptions assigned to OH, the amorphous phase contained a large amount of hydroxy groups. 29Si NMR spectra should exhibit Q3, Q2, or Q1 species. Possibly, 29Si CP-MAS NMR measurement distinguish Q4 peaks from Si species with SiOH bonds, such as Q3, Q2, and Q1.

 

Author Response

We appreciate the precise comments given by the reviewer that will not only add to the quality of the work, but will make it highly presentable into the bargain. 

Author Response File: Author Response.pdf