Syngas to light olefins conversion with high olefin/paraffin ratio using ZnCrOx/AlPO-18 bifunctional catalysts

Direct synthesis of light olefins from syngas (STO) using a bifunctional catalyst composed of oxide and zeolite has attracted extensive attention in both academia and industry. It is highly desirable to develop robust catalysts that could enhance the CO conversion while simultaneously maintain high selectivity to C2-C4 olefins. Herein, we report a bifunctional catalyst consisting of ZnCr binary oxide (ZnCrOx) and low-Si AlPO-18 zeolite, showing both satisfying selectivity to C2-C4 olefins of 45.0% (86.7%, CO2 free) and high olefin/paraffin ratio of 29.9 at the CO conversion of 25.2% under mild reaction conditions (4.0 MPa, 390 °C). By optimizing the reaction conditions, the CO conversion could be markedly increased to 49.3% with a slight drop in selectivity. CD3CN/CO-FTIR characterizations and theoretical calculations demonstrate that low-Si AlPO-18 zeolite has lower acid strength, and is therefore less reactive toward the hydride transfer in the STO reaction, leading to a higher olefin/paraffin ratio.

In summary, this is an interesting paper that would be much more interesting if the authors focussed on highlighting challenges (CO2 selectivity is a very important one) rather than in trying to sell their work as commercially viable when it probably is not yet there. In that line, performing experiments at different CO:H2 ratios would be quite wise. It goes without saying that it is highly recommended to report not only CO conversion but also H2 conversion.
Reviewer #2 (Remarks to the Author): He and co-workers report on direct conversion of synthesis gas to lower olefins using a combination of ZnCr-oxide and low-Si AlPO-18 zeolite. By increasing reaction pressure they are able to achieve higher CO conversion levels than reported before. High selectivities to lower olefins and low selectivities to methane are obtained. This research can be considered as an important step to arrive at direct conversion of synthesis gas to lower olefins. Publication in NC is supported.
A number of comments are to be taken into account, however.
1. Under all circumstances high selectivities to CO2 are apparent (Table S1) which limits the application of this process to CO-rich synthesis gas. If H2-rich synthesis gas is used recycle of CO2 will be necessary. These considerations should be mentioned in the main text. 2. General literature references 2-4 involve literature from the 1990-ies which is okay but should be extended to more recent reviews and results, e.g., Angew. Chem. Int. Ed. 2016, 55, 2-4;ACS Catal. 2013, 3, 2130-2149Science 335, 835 (2012). 3. Mechanistic considerations (using DFT calculations) are restricted to hydride transfer. However, a lively debate is ongoing on the nature of the intermediate (methanol versus ketene) is ongoing in the literature. The authors should comment and contribute in this respect. 4. The stability reported in Figure 6 is promising. Next to conversion stability also catalyst coking should be considered. What are typical coke levels after catalysis?
Reviewer #3 (Remarks to the Author): This manuscript reports a bifunctional catalyst for syngas to olefins and the catalyst consists of ZnCr binary oxide and low-Si AlPO-18 zeolite. Similar bifunctional catalysts have been reported in many previous papers including the previous work by the authors. The key claim of this work is the use of low Si AlPO-18 zeolite with AEI topology instead of SAPO-34 with CHA topology. The lower density and weaker strength of acid sites may be responsible for the high olefin selectivity of the current catalyst with low-Si AlPO-18 zeolites. The performance is quite good but not extraordinary. The high CO conversion of 40-50% can only be obtained at a very high syngas pressure (10 MPa). From the scientific sense, the insights offered by this work do not represent a significant advance in its field. Hence, this work lacks sufficient novelty/insights to meet the scientific level required by Nat. Commun. It is recommended that the manuscript is more appropriate for a specialized journal.

Reviewer #1 (Remarks to the Author):
This is an interesting manuscript that may have potential for Nature Communications but not in its current form.
First of all, the manuscript is misleading: all selectivities reported are in a CO2 free basis, considering that CO2 selectivities (only reported in the caption of figure 1) are in the order of a 50%, this means that selectivities to all the other products are actually half of those reported. When put in perspective, catalytic results are not far from those obtained when doing classical FTS chemistry on K promoted Fe catalysts (see for instance Nature Commun. 6 (2015) 6451). This is in principle fine, with CO:H2 ratios of 1, one would want to make CO2 to account for the H2 poor feed that may come from i.e. coal gasification. However, I do not see the need to hide this high CO2 selectivity just to further contribute to the hype of multifunctional systems for syngas to olefins. I do understand that previous works have (successfully) follow the same strategy to get published in high profile journals, but there has to be an end to it.
A: We agree with the reviewer's comments. The selectivity to CO 2 , hydrocarbons and other products were recalculated, and updated in Figure 2 of the revised manuscript.
Accordingly, the data in Figure 3 and Figure 6 were updated. In the revised manuscript, both the selectivity in the presence and absence of CO 2 were addressed in order to compare to the previous results.

Apart from this point, there are other issues that the authors have to address:
The authors claim: "Given that the number of exposed 8-membered ring channels in AIPO-18 is higher than the SAPO-34 zeolite, a faster diffusion of reactants and products in AIPO-18 could be expected, which will likely affect the catalytic performance of bifunctional catalysts" This may be true, however, particles of both zeolites are very different in size, while primary AlPO-18 particles seem to be sub-nanometric, SAPO-34 crystals are all larger than 2-3 micron. Experiments with similar primary particles should be perform to further strengthen or withdraw this hypothesis A: Yes, the particle size of zeolites is another important factor influencing the catalytic performance of bifunctional catalysts. Only using this kind of nanosheet low Si AlPO-18 zeolite can we obtain both high CO conversion and high selectivity to olefins. We synthesized a low Si AlPO-18 zeolite sample with lager particle size to investigate the size effect of zeolite on the reaction performance of the bifunctional catalysts, and the results were added in Figure S1 and Table S1 of the revised manuscript. We found that the zeolite particle size significantly affects CO conversion and oxygenate selectivity, but slightly influences C2-C4 olefin/paraffin ratio. So in the revised manuscript, the following sentences "a faster diffusion of reactants and products in AlPO-18 could be expected, which will likely affect the catalytic performance of bifunctional catalysts"were deleted, and the following sentences on the particle size effect were added in page 6.
"The lower CO conversion and oxygenate selectivity imply that the driving force of AlPO-18 zeolite is insufficient for this reaction, possibly relevant with two aspects of the particle size and acidity. Regarding the former aspect, we further synthesized an AlPO-18 zeolite with 1-2 μm particle size (AlPO-18-L, Figure S1) to investigate the size effect of zeolite on the catalytic performance (

804-812
A: Thanks for this suggestion. We read carefully this recommended paper and found that the findings in this paper highly support the issue of the effect of acidity of our manuscript. We added a discussion in page 10 of the revised manuscript.
"Interestingly, previous work 31 have shown that the isolation of Brønsted acid sites is beneficial to maximize olefin selectivity by preventing secondary reactions in MTO process. In this aspect, the decrease of acid density also helps to promote the acid site isolation. Considering the catalytic difference in  zeolites, we demonstrated that reducing the acid density of zeolites and/or promoting the isolation of acid sites is beneficial to slightly increase the olefin/paraffin ratio, consistent with previous work 19,20,31 ." In summary, this is an interesting paper that would be much more interesting if the authors focussed on highlighting challenges (CO2 selectivity is a very important one) rather than in trying to sell their work as commercially viable when it probably is not yet there. In that line, performing experiments at different CO:H2 ratios would be quite wise. It goes without saying that it is highly recommended to report not only CO conversion but also H2 conversion.
A: We highly appreciate the reviewer's comments considering this is an interesting paper. We deleted related sentences on the commercially viable and then highlighted the scientific challenges in syngas to olefins conversion including CO 2 selectivity. The detail as follows: To delete: "The coupling of several cascade reactions into a direct process using one reactor could break the inherent thermodynamic equilibrium limit of each individual reaction and improve the overall conversion efficiency. From the perspective of industrial applications, this could simplify the reaction-separation process and reduce the investment cost." in page 2. Besides, the catalytic activity over ZnCr/low-Si AlPO-18 with different H 2 /CO mole ratio was tested and listed in Table S3 of the revised SI. The H 2 conversion was included and the corresponding discussion was added as can be seen in page 7.

Reviewer #2 (Remarks to the Author):
He and co-workers report on direct conversion of synthesis gas to lower olefins using a combination of ZnCr-oxide and low-Si AlPO-18 zeolite. By increasing reaction pressure they are able to achieve higher CO conversion levels than reported before. High selectivities to lower olefins and low selectivities to methane are obtained. This research can be considered as an important step to arrive at direct conversion of synthesis gas to lower olefins. Publication in NC is supported.
A number of comments are to be taken into account, however.
1. Under all circumstances high selectivities to CO2 are apparent (Table S1) which limits the application of this process to CO-rich synthesis gas. If H2-rich synthesis gas is used recycle of CO2 will be necessary. These considerations should be mentioned in the main text.
A: Thanks for the suggestion. The following sentences were added in the revised manuscript to discuss the effect of CO 2 selectivity on application of this process to H 2 -rich syngas and CO-rich (H 2 lean) syngas in page 3.
"Second, the effective utilization of carbon resources is low as the result of undesired

General literature references 2-4 involve literature from the 1990-ies which is
okay but should be extended to more recent reviews and results, e.g., Angew. A: These recent reviews and articles were cited in the revised manuscript.  ketene, since relatively larger amounts of oxygenates (methanol or DME) are produced on pure ZnCr oxides and ketene is less reactive than methanol for the chain propagation in the hydrocarbon pool mechanism 32,34 ." in page 13. Figure 6 is promising. Next to conversion stability also catalyst coking should be considered. What are typical coke levels after catalysis?

The stability reported in
A: We agree with reviewer's suggestion. The TGA curves of used ZnCrOx oxide and low-silica AlPO-18 zeolite were included in Figure S3. We also added the discussion part in page14.
"The TGA curves ( Figure S2) (ChemCatChem, 2018(ChemCatChem, , 10, 1536(ChemCatChem, -1541. Similar to those reported data in references, we found that it is extremely difficult to simultaneously improve CO conversion and maintain high selectivity to C2-C4 olefin (seesaw-type effect), which is particularly important in industrial catalysis. Aiming to a special type of products, like paraffins, aromatics, the zeolite components were selected using ZSM-5 zeolites. But in the case of the CO conversion into light olefins (C2-C4 olefins), we found that the attention were mainly focused on tailoring or optimizing the oxide components. It should be mentioned that SAPO-34 zeolite is the best choice for the MTO conversion, but it may not be the best choice in the bifunctional catalysts for the STO conversion to light olefins. According to the understanding that oxide component mainly contributes to the conversion of CO while zeolite component mainly dedicates to the product distribution, we believe that the above-mentioned seesaw-type effect between CO conversion and selectivity to light olefins could be alleviated by modulating zeolite component, which is the key content of this manuscript for developing novel bifunctional catalytic systems to light olefins.
In this work, our research direction is quite different from previous studies. We designed a novel bifunctional catalyst containing zeolite component with AEI structure, and simultaneously achieved high CO conversion (~50%) and C2-C4 olefin selectivity (> 80%) with an unprecedented high olefin ratio (> 20), which makes the bifunctional catalyst more competitive in industrialization. By in situ characterizations of acidity and DFT calculations, we systematically studied the correlation between the formation of paraffin by-products and the properties (acidity and cage structure) of small porous zeolite in the direct synthesis of olefins from syngas. Based on these results, we proposed that hydrogen transfer in zeolites is one of the important ways dedicating to the paraffins formation and the zeolite with weak performance (less acid sites, weak acidity strength) would be much better than those with strong performance.
We believe that this scientific insight (less or weak is better) would offer anther different idea for the development of novel bifunctional catalysts. The novelty and importance of this manuscript, we believe, are tenable.