A unique Co@CoO catalyst for hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran

The development of precious-metal-free catalysts to promote the sustainable production of fuels and chemicals from biomass remains an important and challenging target. Here, we report the efficient hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran over a unique core-shell structured catalyst, Co@CoO that affords the highest productivity among all catalysts, including noble-metal-based catalysts, reported to date. Surprisingly, we find that the catalytically active sites reside on the shell of CoO with oxygen vacancies rather than the metallic Co. The combination of various spectroscopic experiments and computational modelling reveals that the CoO shell incorporating oxygen vacancies not only drives the heterolytic cleavage, but also the homolytic cleavage of H2 to yield more active Hδ− species, resulting in the exceptional catalytic activity. Co@CoO also exhibits excellent activity toward the direct hydrodeoxygenation of lignin model compounds. This study unlocks, for the first time, the potential of simple metal-oxide-based catalysts for the hydrodeoxygenation of renewable biomass to chemical feedstocks.

catalyst. In addition, the labels in Fig. 2a and 2b are too small. 3. The authors state that "The hydrogenolysis of HMMF has a reaction order of ca. 0.9 for H2, but close to 0 for HMMF, indicating that the activation of H2 instead of HMMF is the rate-determining step (Supplementary Figure 9), in agreement with the DFT analysis.". However, the DFT results in the manuscript cannot conclude that the H2 activation is the rate determining step. The complete reaction pathways of the HMF to DFF reaction should be investigated to confirm the rate-determining step of the reaction. 4. In Fig. 3a, the y-axis of the energy profile should change from energy to relative energy. Is the -Eads of HMF on CoO(100)-Ov, -1.36eV, wrong? There are only two data points for the Eads of HMF and they are not relevant to the energy profile of H2. They should be removed from Fig. 3 and describing those values in the text would be better. 5. More configurations of HMF adsorption should be tested. For example, the bridge adsorption configuration might be formed via hydroxyl and formyl groups binding with a surface proposed by Kim et al. [Applied Surface Science, 456, 2018, 174-183] The Bader charges of atoms in HMF on catalyst should be analyzed. 6. In the DFT method, the information of a CoO crystal (i.e. space group) and size of a slab model, in Å, should be given.

Reviewer #3 (Remarks to the Author):
This contribution of the Wang team is an interesting work toward the better understanding of catalyst design for desired biomass deoxygenation reactions, in particular C-O hydrogenolysis, C=O hydrogenation as well as hydrodeoxygenation. Despite the relative simplicity of the substrates, this work addresses a central question, important in this field now for decades: how to increase efficiency for desired selective deoxygenation processes by more rational catalyst design. The team has found a unique core shell type Cobalt containing catalyst with activity outperforming even noble metal catalysts for the same reaction. Here the Co-oxide has been involved in the hydrogen activation, which as an important finding. I recommend the paper to be published after the questions below are addressed: 1.) It is known in the literature that conversion of HMF to DMF goes through many intermediates, and is known for side reactions. The side reactions are mainly char formation through self-condensation, undesired ring opening and subsequent condensation -these lead to loss of mass balance. Another set of side reactions are the ring hydrogenation products as well as ring opening and consecutive hydrogenation products. The reaction intermediates involve the corresponding furfural derivatives, and derived hydrogenation and hydrogenolysis products that are all part of the reaction network. There are typically more intermediates compared to what is shown on the scheme in Table 1. It would be great to comment on these aspects, contrast to or acknowledge already existing literature findings, also with references either in main text or in supporting. Have the authors not observed these phenomena? The recondensation is trackable by strictly following the mass balance. Were there no ring hydrogenation intermediates? This is surprising given the fact that the lignin b-O-4 model compound products include cyclohexanol. Data on internal standard/ mass balance and GC-MS/FID (representative) figures should be provided in the supporting information. 2.) Figure 1a: The comparison with existing systems is appreciated and appears to be complete. Can the area in the graph be somewhat enlarged, since it is a bit difficult to read. Also it appears that system 14 and system 19 are missing on the graph. 3.) It has been earlier observed in the literature that the 5-HMF hydrogenation is solvent dependent. Some prior art has been demonstrated in alcohols as solvents and have shown superior activities. In this work THF was used for the 5-HMF conversion and dioxane for the lignin b-O-4 conversion. Regarding this: a.) for future application, and given that the topic of the paper focuses on sustainability and moving from noble metal catalysts, do the authors think that such solvent choice is suitable? b.) a solvent evaluation table would be useful to add in supporting information c.) with such high hydrogenolysis activity of the catalyst why the choice of the ethers as solvents? Did they not suffer hydrogenolysis? A blank reaction with these solvents along should be added to the table. d.) Are these solvents innocent in catalysis or did they play any role in interacting with the catalyst surface, potentially coordinating to active metal sites etc..? And these aspects should be commented if possible supported by data or calculation. 4.) In the conclusions, the authors state: 'The superior activity of the Co@CoO catalyst originates from the unique CoO species with suitable oxygen vacancies, which can strongly adsorb HMF and catalyse the homolytic/heterolytic splitting of H2 molecules'. I assume they mean strong C=O interaction. In my view there was an extensive study on the H2 activation with this catalyst, but relatively less information was given related to the substrate -to -catalyst interactions. Can the authors measure some of these aspects? And especially aspects related to hydrogenolysis. It is also stated earlier in the manuscript that HMF interacts strongly with C=O and is rapidly hydrogenated (which may explain the lack of self-condensation) but perhaps the even more interesting question is the hydrogenolysis step. How does a C-O bond interact with the catalyst? Is there an interaction with the aromatic rings with this catalyst? (furan as well as phenol derivatives). The authors should provide more details on these aspects, experimental or calculation, to strengthen the narrative of the paper.

Reviewer #1:
The Manuscript " Co@CoO: a Unique Catalyst for Hydrogenolysis of Biomass-derived 5-Hydroxymethylfurfural to 2,5-Dimethylfuran" by Yanqin Wang et al. is quite an interesting one, however, few of the points may be clarified for the benefit of the readers. My views are appended below.
1. In abstract authors claimed that this is the first report on metal oxide catalyst for bio derived fuels, I would suggest to recheck the statement.
Reply: The statement has been revised: "This study unlocks, for the first time, the potential of simple metal-oxide-based catalysts for the hydrodeoxygenation of renewable biomass to chemical feedstocks." 2. Authors claimed that the biotransformation occurs over partially reduced Co3O4, and in the form of metallic core and oxide surface, I would suggest authors may perform XAS and temperature program studies.
Reply: The Co K-edge XAS spectra of Co3O4-200, Co3O4-250 and Co3O4-300 have been measured and the spectra in R space are shown in Fig. 1    3. The results with CoO and reduced CoO (Co?) are compared with partially reduced Co3O4 very different why! Reply: We thank the reviewer for raising this interesting question. The catalytic performance can be significantly influenced by the particle size, morphology, and surface properties of the catalysts. For the catalysts studied in this work, the commercial CoO is a bulky material with low surface area (12 m 2 g −1 ), while Co3O4 has a high surface area (30 m 2 g −1 ). More importantly, reduced commercial CoO appear like large particles (Fig. 3), while Co3O4-250 is a core-shell structured catalyst and the CoO shell is very thin, ca. 2.5-3.5 nm, which may contribute positively to its catalytic activity (e.g., through the quantum size effect [3][4][5] ). The detailed influence of CoO particle size and morphology may require future extensive investigations, which we aim to publish separately. This work mainly focused on the study of the CoO shell with oxygen vacancies that can dissociate H2 and activate HMF, giving rise to high activity and stability. Reply: We thank the reviewer for raising this concern. The HRTEM result suggests that the thickness of the CoO shell is 2.5 to 3.5 nm. In addition, as shown in Table 1, compared with the Co@CoO (Co3O4-250) catalyst (89.2% yield of DMF), Co3O4-400 that is mainly a metallic Co catalyst has very poor activity (1.3% yield of DMF), indicating that it is the CoO that results in the high activity. Furthermore, a combination of XPS, EPR and INS analyses also suggest that the shell of CoO with Ov is the main active phase. Thus, we sought to reveal the unique activity of the CoO-Ov site in this work, and the core of Co was not included in the theoretical modelling.

Reviewer #2:
This manuscript written by Yanqin Wang, Xue-Qing Gong, et al. synthesized Co@CoO coreshell catalysts for the 5-hydroxymethylfurfural to 2,5-dimethylfuran reaction. The synthesized core-shell catalyst, namely Co3O4-250, revealed the superior activity among metaloxide catalysts. Many techniques were used in experimental part to prove that the oxygen vacancies on the CoO shell promote the catalytic activity. The DFT calculations were applied to clarify the homolytic and heterolytic cleavage of H2 over the vacancy site in CoO. After reviewing this paper, I recommend that this paper is possible to be published in Nature Communications subjected to major revision. Some parts need to be clarified. My comments are listed below.
1. In table 1, the reduced commercial CoO provided the 45.0% yield of BHMF and 1.1% yield of DMF. According to those results, the heterolytic and homolytic cleavage of H2 around the vacancy sites of CoO may not be the only reason for the superior activity of Co@CoO. The confinement effect, morphologies, active facets as well as charge transfer between an inner core and an outer shell should influence the activity of the unique Co@CoO core-shell too. In addition, the slab models in the DFT part can represent the commercial CoO catalyst but they might not represent the Co@CoO catalyst.
Reply: We thank the reviewer for these valuable comments. We fully agree with the reviewer that the particle size, morphology, surface properties and confinement effect can affect the activity of the catalyst. In this work, Co3O4-250 shows better catalytic performance than the reduced commercial CoO catalyst, and this could be (partially) originated from the CoO shell with oxygen vacancies of the former. The thickness of the CoO shell is very thin, ca. 2.5-3.5 nm, which may contribute to the exceptional activity (e.g., through quantum size effect 1-3 ).
Also, the influence of morphology and charge transfer may be non-trivial. This will deserve a full future investigation. Honestly, we focused on exploring the unique role of the CoO species with suitable oxygen vacancies and revealed that such active sites can strongly adsorb HMF, and also catalyze the homolytic/heterolytic splitting of H2 molecules to produce highly active CoO(100)-Ov surfaces. Notably, the CoO(111) surface is a polar surface with O-terminated structure, which tends to form stable OH species. Even though, H2 on the CoO(111)-OV surface can be still split into one H δ+ and one H δ-with the help of the OV, as confirmed by the Bader charge analysis (Fig. 4), and such heterolytic dissociation of H2 was calculated to be exothermic (1.59 eV) and with an energy barrier of 0.41 eV. In comparison, the CoO(100)-OV surface is more favourable to produce a large number of active H δ-species due to the strong adsorption for H2 (1.63 eV), and the H δ-species can be also produced in a homolytic way (Fig. 7). All these results show that the hydride can be produced by H2 dissociation on both the reduced CoO(111) and Co(100)surfaces. The related discussions are now added in the revised manuscript (main text and Supplementary Figures 14 and 15).  3. The authors state that "The hydrogenolysis of BHMF has a reaction order of ca. 0.9 for H2, but close to 0 for BHMF, indicating that the activation of H2 instead of BHMF is the ratedetermining step (Supplementary Figure 9), in agreement with the DFT analysis.". However, the DFT results in the manuscript cannot conclude that the H2 activation is the rate determining step. The complete reaction pathways of the HMF to DMF reaction should be investigated to confirm the rate-determining step of the reaction.
Reply: We thank the reviewer for this suggestion. We have conducted new calculations and revised the statement: "The hydrogenolysis of BHMF has a reaction order of ca. 0.9 for H2, but close to 0 for BHMF, indicating that the activation of H2 is a critical step (see Supplementary   Figure 17)" The complete reaction pathways of HMF to DMF on the CoO(100)-OV surface were investigated ( Fig. 5 and Fig. 6). The DFT calculations showed that HMF is adsorbed on the CoO(100)-OV surface first as its adsorption is exothermic by about 2.22 eV, which is higher than the adsorption of H2 (exothermic of 1.63 eV). In addition, it is adsorbed vertically on the   4. In Fig. 3a, the y-axis of the energy profile should change from energy to relative energy. Is the -Eads of HMF on CoO(100)-Ov, -1.36eV, wrong? There are only two data points for the Eads of HMF and they are not relevant to the energy profile of H2. They should be removed from Fig. 3 and describing those values in the text would be better.
Reply: We thank the reviewer for raising this concern. The adsorption energy of HMF (Eads) at CoO(100)-Ov is exothermic by about 2.22 eV (Fig.8 (g)). As shown in Fig. 7, the two data points for the adsorption energy of HMF (Eads) have been removed. Please see our reply to comment 5 below for a detailed explanation.

Reviewer #3:
This contribution of the Wang team is an interesting work toward the better understanding of catalyst design for desired biomass deoxygenation reactions, in particular C-O hydrogenolysis, C=O hydrogenation as well as hydrodeoxygenation. Despite the relative simplicity of the substrates, this work addresses a central question, important in this field now for decades: how to increase efficiency for desired selective deoxygenation processes by more rational catalyst design. The team has found a unique core shell type Cobalt containing catalyst with activity outperforming even noble metal catalysts for the same reaction. Here the Co-oxide has been involved in the hydrogen activation, which as an important finding.
I recommend the paper to be published after the questions below are addressed: 1.) It is known in the literature that conversion of HMF to DMF goes through many intermediates, and is known for side reactions. The side reactions are mainly char formation through self-condensation, undesired ring opening and subsequent condensation -these lead to loss of mass balance. Another set of side reactions are the ring hydrogenation products as well as ring opening and consecutive hydrogenation products. The reaction intermediates involve the corresponding furfural derivatives, and derived hydrogenation and hydrogenolysis products that are all part of the reaction network. There are typically more intermediates compared to what is shown on the scheme in Table 1. It would be great to comment on these aspects, contrast to or acknowledge already existing literature findings, also with references either in main text or in supporting.
Reply: We thank the reviewer for the helpful advice. The extended reaction network and corresponding discussions have been added in the revised manuscript (main text and Figure 1).
Have the authors not observed these phenomena? The recondensation is trackable by strictly Reply: We thank the reviewer for raising this concern. No ring hydrogenation or opening products were observed in this system. The data on internal standard/mass balance and representative GC-MS/FID figures are shown below and has been added in the revised supporting information (Supplementary Figure 2). The results confirm that only DMF, HMMF and BHMF were observed. 2.) Figure 1a: The comparison with existing systems is appreciated and appears to be complete.
Can the area in the graph be somewhat enlarged, since it is a bit difficult to read. Also it appears that system 14 and system 19 are missing on the graph.
Reply: We thank the reviewer for the kind reminder, and we have corrected these issues in the revised manuscript.  Table 1).   Fig. 11/Fig. 12 and added in the revised manuscript ( Supplementary Figures 3 and 4).  Reply: We thank the reviewer for raising this comment. From the data and discussion given above, we find that THF is better than conventional solvents. To illustrate the reasons, we calculated the adsorptions of these solvent molecules (THF and ethanol) on the CoO(100)-OV surface (Fig. 13) and found that, (a) the calculated highest adsorption energies of THF and ethanol on the CoO(100)-OV surface are 1.27 eV and 2.16 eV, respectively, both of which are below that of HMF (2.22 eV); (b) as expected, the adsorption energy of ethanol is very close to that of HMF, and the competitive adsorption likely leads to the low yield of DMF in alcohols.
These results are now added in the revised manuscript (Supplementary Figure 5). In my view there was an extensive study on the H2 activation with this catalyst, but relatively less information was given related to the substrate -to -catalyst interactions. Can the authors measure some of these aspects? And especially aspects related to hydrogenolysis. It is also stated earlier in the manuscript that HMF interacts strongly with C=O and is rapidly hydrogenated (which may explain the lack of self-condensation) but perhaps the even more interesting question is the hydrogenolysis step. How does a C-O bond interact with the catalyst?
Is there an interaction with the aromatic rings with this catalyst? The authors should provide more details on these aspects, experimental or calculation, to strengthen the narrative of the paper.
Reply: We appreciate the reviewer for the suggestion. Additional DFT calculations show that C=O and C-O bonds in HMF prefer to be vertically adsorbed on the catalyst surface.
Furthermore, HMMF-adsorption-IR analysis was carried out and the results are shown in Fig. 14. There is a red shift of the signal at 1023 cm -1 for C-O stretching in the alkoxy functional group 1,2 . This phenomenon indicated that Co3O4-250 has an activating effect on the C-O bonds.
In comparison, the characteristic peaks at 1500-1650 cm -1 for C=C stretching and 1198 cm -1