Liquid marble-derived solid-liquid hybrid superparticles for CO2 capture

The design of effective CO2 capture materials is an ongoing challenge. Here we report a concept to overcome current limitations associated with both liquid and solid CO2 capture materials by exploiting a solid-liquid hybrid superparticle (SLHSP). The fabrication of SLHSP involves assembly of hydrophobic silica nanoparticles on the liquid marble surface, and co-assembly of hydrophilic silica nanoparticles and tetraethylenepentamine within the interior of the liquid marble. The strong interfacial adsorption force and the strong interactions between amine and silica are identified to be key elements for high robustness. The developed SLHSPs exhibit excellent CO2 sorption capacity, high sorption rate, long-term stability and reduced amine loss in industrially preferred fixed bed setups. The outstanding performances are attributed to the unique structure which hierarchically organizes the liquid and solid at microscales.

4. The authors spent a large section on the mechanical stability of the material. Yes, it is important. But I think the authors should also strengthen the CO2 adsorption section, which was highlighted in the title of the manuscript (not mechanical stability per se). 5. One key feature of CO2 adsorption is selectivity, in particular over N2 or CH4. Can the authors demonstrate that? 6. Another issue is the "CO2 adsorption efficiency", ie CO2:N ratio. That will show how much amine is directly engaging with CO2 molecules. There is no such information in the manuscript. 7. It is excellent to see the material lasted 60 cycles. The authors had examined the "spent" particles but only with SEM. I will be surprised if SEM can tell us a lot except morphology. Since the authors had gone that far for testing, the material should be thoroughly tested in terms of TGA (ie composition, amine content etc), FTIR etc. One interesting question is that are the TEPA component still TEPA after many cycles of heating and cooling? Has the TEPA polymerised?
Reviewer #3 (Remarks to the Author): This is a well prepared and comprehensive report which, in my view, also represents a significant fundamental advance in the development of composite materials for adsorption purposes (and, potentially, other purposes). I believe it is of interest to Nature Communications readers and that It should be published subject to addressing the issues detailed below. Mostly these relate to improving the clarity of the discussion.
Lines 64-65: I question the authors assertion that the highest CO2 capacity ever recorded by amine supported silica materials is 4.3 mmol.g-1. See, for example, Qi et al, Nat. Commun. 2014, 5, 5796. The introduction should be revised accordingly.
Lines 87-88: I am concerned that the authors should provide a little more detail about the nature of the MSP and MSP-O materials that they have utilised as components for the creation of the marbles. There is a description of the synthetic methods in the supplementary material and some basic N2 adsorption isotherms, but they have not reported the full BJH pore size distributions from this data (only ave pore size data). Is this pore size data consistent with the TEM/SEM data? I feel this warrants a little more discussion.
Line 106: 'discussed' should be 'discuss'. Remove 'the' from 'the TEPA'. Further, there seems to be no detailed description in the Supplementary material about the specifics of the preparation of TEPA marbles. What is the concentration of TEPA in water? What was the size of the droplets? Perhaps it was the same as for the SLHSP, but this is not explicitly stated. Further clarity is required.
Line 115, also line 119: Does 'commonly used mesoporous silica' refer to the prepared MSP or some other form of mesoporous material. This needs to be made clear. If it is MSP, why isn't it referred to by this acronym? Also, it is not clear whether this 'commonly used mesoporous silica' refers to a suspension of silica in the syringe or silica on the bed (glass slide). These details must also be made very clear to the reader.
Line 144: 'somewhat' should be 'some' Lines 148-149: I don't understand the logic of how the images in Supp Fig 5a confirm  Response: Thank you very much for your valuable comments and suggestions. We completely agree with your view of the choice of organic amines. From the viewpoint of amine loss, PEI (polyethyleneimine) is absolutely a better choice for CO 2 capture because of its lower volatility in comparison to tetraethylenepentamine (TEPA). It is worth mentioning that in this work, with one of aims at investigating the impact of the hydrophobic shell of the developed SLHSP on the amine loss during multiple adsorption-desorption cycles, we chose TEPA. Nevertheless, according to your constructive suggestion, PEI with an average molecular weight of (M W = 800) was used to examine our proposed method.
As the appearance and SEM images in Supplementary Figs. 12a and 12b show (also shown below), the SLHSPs prepared from PEI possess uniform spherical morphology and relatively smooth surface, even in the presence of the high dosage of PEI (ca. 82 wt%, Supplementary Fig.   12d). From the cross-sectional SEM image, one can see plenty of void spaces stemming from the interstices between MSPs (Supplementary Fig. 12c). As such, we can reach a conclusion that our method is really applicable to the highly viscous PEI as well.  With the uniform PEI-based SLHSP in hand, we measured its sorption capacity and recyclability in a fixed-bed reactor using a simulated flue gas (15 vol % CO 2 in N 2 with relative humidity 40%). The obtained results are presented in Supplementary Fig. 13a (also displayed below). The sorption capacity of PEI-based SLHSP was determined to be as high as 5.7 mmol g −1 .

Supplementary
Over the subsequent 19 adsorption-desorption cycles, CO 2 sorption capacity was always maintained at ca. 5.2-5.7 mmol g −1 , without apparent loss. After 20 cycles the amine loss was examined by TGA to be 2.3 wt % as compared to the fresh PEI-based SLHSP ( Supplementary Fig.  13a). Such an amine loss is lower than the case of TEPA, making it better to regenerate the sorbent at higher temperatures. The negligible amine loss is attributed to the low volatility, which verify your inference.  Supplementary Fig. 12c). The sorption capacity of PEI-based SLHSP was determined to be as high as 5.7 mmol g −1 . Over the subsequent 19 adsorption-desorption cycles, the CO 2 sorption capacity was always maintained at ca. 5.2-5.7 mmol g −1 , without apparent loss ( Supplementary Fig. 13a). After 20 cycles the amine loss was examined by TGA to be 2.3 wt% relative to the fresh PEI-based SLHSP ( Supplementary Fig. 13b), highlighting the potential for practical applications. The negligible amine loss is attributed to the low volatility of PEI."

Reviewer 2
We are grateful for your positive comments and valuable suggestions. We answer your questions as follows.
1. In the abstract, the authors highlighted the CO 2 adsorption capacity (6.1 mmol/g) as "outstanding". MOF has shown >7 mmol/g while PEI/MCM41 (a compatible material to this work) has also shown ca. 5 mmol/g. Both were reported several years ago. I am not sure "outstanding" is accurately used here. Also, the authors mentioned "excellent sorption rate".
Does it refer to the CO 2 adsorption kinetics? Based on the kinetic data shown in 5E, the samples took around 20 mins to achieve near maximum. This is indeed rather slow. Nonetheless, the 4 maximum capacity achieved here is good. I am impressed by the number of cycles that had been repeated. That takes some work.

Response:
We fully agree with you that many reported solid sorbents exhibit high adsorption capacity. However, most of these mentioned solid adsorbents are particles with nanometer sizes.
The tiny particles are difficult to install directly into industrial settings (for example, fixed bed rectors) because there is a high risk to plug fixed beds with great pressure drop. To meet the practical requirements, these nanoparticles must be shaped to large sized objects with uniform morphology through compression or spray-drying with assistance of binders. However, addition of binders often leads the sorbents both to being unpredictable in structure and to decrease in sorption capacity. To solve these problems, we attempt to prepare micron-to-millimeter sized sorbents, which are amenable to being packed directly in fixed-bed reactors. Compared to the reported micron-to-millimeter sized sorbents, our adsorption capacity is impressive. It is the same case with the adsorption rate. When considering the large size of our SLHSP sorbent, the rate is still good. For example, the rate is still higher than that of the nanosized TEPA-impregnated mesoporous materials (Fig. 5E).
All the same, we have changed the word "outstanding" to "excellent" in our revised version according to your suggestion.

Figure 1 was supposed to show how the material is formed but it is far from clear. There are arrows that had no labels and it is difficult to follow.
Response: Thank you very much for your advice. In  Response: The hydrophobic MSP-Os were used to fabricate liquid marbles because the hydrophobic nanoparticles can be attached at the gas/liquid interface through interfacial adsorption, forming a hydrophobic shield layer around liquid droplets. This shield layer can prevent the droplet coalesce, being favorable to maintaining of spherical morphology. As evidenced by our experiment, evaporation of the droplet (without MSP-Os on the surface) led to a doughnut-like shape due to the so-called coffee loop effect, whereas in the presence of MSP-Os uniform microspheres were obtained. Moreover, the hydrophobic layer made of MSP-O can prevent the evaporation of the enclosed hydrophilic TEPA during the course of CO 2 desorption, as shown in Fig. 6 in the revised manuscript.
The hydrophilic MSPs can be well dispersed in hydrophilic TEPA, and are subsequently subjected to assembly within the liquid marble due to the hydrogen-bonding interactions. In doing so, it is possible to obtain hierarchical interior structures and "porous liquid". As the SEM image of the cross-section of SLHSP shows ( Supplementary Fig. 5), plenty of macropores stemming from the interstices between MSPs were observed. That is to say, without MSP it is impossible to yield void spaces within liquid droplets. At the same time, MSPs act as pillars to support the superparticle architectures, thus reinforcing the solid-liquid hybrid superparticle.
The respective roles of MSP-Os and MSP are discussed on pages 6, 7, and 16.

The authors spent a large section on the mechanical stability of the material. Yes, it is
important. But I think the authors should also strengthen the CO 2 adsorption section, which was highlighted in the title of the manuscript (not mechanical stability per se).
Response: According to your advice, we strengthen discussion of the CO 2 adsorption in four aspects. Firstly, to further test our new concept, we chose PEI (M W = 800) to prepare solid-liquid hybrid superparticles again, and then discussed the adsorption performances (page 16 to 17, highlighted in yellow). "As another proof of the concept, our strategy is adaptable to the case of polyethyleneimine (PEI, M W = 800, see Supplementary Experimental Section). The SLHSPs prepared from PEI possess uniform spherical morphology and relatively smooth surface (Supplementary Figs. 12a and 12b,even in the presence of the high dosage of PEI (ca. 82 wt%, Supplementary Fig. 12d). From the cross-sectional SEM image, one can see plenty of void spaces 6 stemming from the interstices between MSPs (Supplementary Fig. 12c). The sorption capacity of PEI-based SLHSP was determined to be as high as 5.7 mmol g −1 . Over the subsequent 19 adsorption-desorption cycles, the CO 2 sorption capacity was always maintained at ca. 5. 2-5.7 mmol g −1 , without apparent loss (Supplementary Fig. 13a). After 20 cycles the amine loss was examined by TGA to be 2.3 wt% relative to the fresh PEI-based SLHSP (Supplementary Fig. 13b), highlighting the potential for practical applications. The negligible amine loss is attributed to the low volatility of PEI." Secondly, the SLHSP after 60 adsorption-desorption cycles has been characterized with SEM, TEM, 13 C NMR, 1 H NMR and FT-IR ( Supplementary Figs. 10 and 11). With these supplementary results, we discussed the morphology and structure of the "spent" SLHSP and the reasons for the decrease in adsorption capacity over 60 cycles. These contents are supplied on page 15 in our revised manuscript (highlighted in yellow). "To clarify the structural and compositional changes of SLHSP over multiple cycles, we characterized SLHSPs after 60 cycles with SEM, TEM and N 2 physical sorption analysis ( Supplementary Fig. 10), and analyzed the structural changes of TEPA that was collected after 60 th cycles with 13 C NMR, 1 H MNR and FT-IR ( Supplementary Fig. 11). Response: According to your suggestions, the CO 2 /N 2 selectivity was measured with TG-MS analyzer during the course of CO 2 desorption. The discussion of the CO 2 /N 2 selectivity has been added in our revised manuscript (page 13, highlighted in yellow). "The CO 2 /N 2 selectivity is estimated to be up to 187 from the TG-MS measurement. The high selectivity is due to the strong CO 2 -amine chemical interactions." 6. Another issue is the "CO 2 adsorption efficiency", ie CO 2 :N ratio. That will show how much amine is directly engaging with CO 2 molecules. There is no such information in the manuscript.

Response:
We agree with you that "amine efficiency" is important. The discussion of amine efficiency is supplied in our revised version. Amine efficiency is defined as the ratio of the moles of captured CO 2 to the total moles of N on the sorbent. Since the TEPA content in SLHSP is ~80.3 wt%, the N content was estimated to be 21.2 mmol N g -1 . Under dry conditions, the amine efficiency (the CO 2 /N ratio) of the SLHSP is ca. 0.25, which is lower than the theoretical amine efficiency (0.5 mol CO 2 per mol N under dry condition). The discrepancy could be explained by the fact that the high amine loading leads to the lowered accessibility of a portion of amines.
We have increased the discussion of amine efficiency in our revised manuscript (page 13, highlighted in yellow Response: Thank you very much for your comments. In addition to the SEM images of the recovered SLHSP, TGA results have been provided in our manuscripts, which revealed a 15 wt% amine loss after 60 cycles, compared to the fresh SLHSP (Fig. 6D). The loss of sorption capacity after 60 cycles was estimated to be ca. 14% of the initial capacity (Fig. 6B). Based on the results of the amine loss and the adsorption capacity decrease, we can infer that the decrease in adsorption capacity is mainly caused by the TEPA evaporation during multiple cycles.
According to your suggestions, we further characterized the recovered SLHSPs after washing with methanol with SEM, TEM and N 2 physical sorption analysis ( Supplementary Fig. 10, as presented below), and characterized TEPA that was collected from the SLHSPs after 60 th cycles 8 with 13 C NMR, 1 H MNR and FT-IR ( Supplementary Fig. 11, as presented below). The results of the SEM, TEM and N 2 physical sorption analysis revealed that the spherical morphology, interior architecture, and pore structure were well maintained although the surface area had a light decrease (from 539 to 459 m 2 g -1 ) after 60 cycles. which can be explained by the fact that we conducted CO 2 desorption at a moderate temperature (100 o C). The structural maintenance is in accord with our above inference that the loss of adsorption capacity is mainly caused by the TEPA evaporation.

Supplementary
According to your advice, we have added these discussions in the revised manuscript (page 15 to 16, highlighted in yellow). "To clarify the structural and compositional changes of SLHSPs over multiple cycles, we characterized SLHSPs after 60 cycles with SEM, TEM and N 2 physical sorption analysis (Supplementary Fig. 10), and analyzed the structural change of TEPA that was collected from the 60 th cycles with 13 C NMR, 1 H MNR and FT-IR ( Supplementary Fig. 11)  Thank you so much for your positive comments and suggestions. We have revised our manuscript according to your suggestions.
1. Lines 64-65: I question the authors assertion that the highest CO 2 capacity ever recorded by amine supported silica materials is 4.3 mmol.g -1 . See, for example, Qi et al, Nat. Commun. 2014, 5, 5796. The introduction should be revised accordingly.

Response:
We agree with you that many developed solid sorbents have shown high adsorption capacity. Most of these mentioned solid adsorbents are particles with nanometer sizes. The tiny particles are difficult to install directly into industrial settings (for example fixed bed) because there is a high risk to plug fixed beds (great pressure drop). To meet these technical requirements, the nanoparticles must be shaped into large sized objects with uniform morphology through compression or spray-drying with assistance of binders. However, addition of binders often leads the sorbents both to being unpredictable in structure and to decrease in sorption capacity. To solve these problems, we attempt to prepare micron-to-millimeter sized sorbents, which are amenable to being packed directly in fixed-bed reactors. Compared to the reported micron-to-millimeter sized sorbents, our adsorption capacity is impressive (the value of 4.3 mmol.g -1 is only for micron-to-millimeter sized sorbents). All the same, we have changed the word "outstanding" to "excellent" in the abstract in our revised version according to your suggestion. After modification with octyltrimethoxysilane, while the isotherm of MSP-O is still similar to the initial MSP indicating that the original mesoporous structure was maintained, the BJH pore size is observed to decrease from 15.5 nm to 12.7 nm. This is a result of the introduction of octyl groups into the mesoporous channels. The determined pore size of MSP is broadly consistent with the TEM observation (15−19 nm, Supplementary Fig. 1e). We have updated the corresponding discussion in our revised version (page 5, highlighted in yellow). "MSPs have particle sizes from hundreds of nanometer to one micron, and their pore size is centered around 15−19 nm (Fig. 1A), which is broadly consistent with the BJH pore size (Supplementary Fig. 1)." Perhaps it was the same as for the SLHSP, but this is not explicitly stated. Further clarity is required.

Supplementary
Response: Thank you for your suggestions. We have corrected these misspellings in our revised version. The procedure for preparation of the TEPA marbles has been added in the revised Supplementary Information (page S19, highlighted in yellow): "Typically, an aqueous solution of 30 wt% TEPA was transferred into a syringe with an orifice diameter of 50 μm and then continuously dropped onto a bed made of MSP-Os with the assistance of the syringe pump followed by rolling of the droplets on the bed, yielding TEPA marbles." The size of TEPA marbles is estimated to be ca. 100-200 μm from the optical micrographs (Supplementary Figs. 2b and 2c), which is smaller than that of SLHSPs (containing 9 wt% MSP).
This size change could be explained in terms of the increase in viscosity when solid particles are added. The increase in viscosity provides a larger resistance for breakage, thus leading to larger sized droplets. Response: Thank you for your advice. Here, mesoporous silica nanospheres (MSNs) with pore size of 8 nm were used in a control experiment for explaining the reason why we chose MSP with larger pores to prepare liquid marbles. We compared the stability of liquid marble stabilized by 13 hydrophobicated MSPs and MSNs (modified with the same amount hydrophobic octyltrimethoxysilane). Such a comparison highlights the unique role of the chosen MSP in the formation of high-quality liquid marble, as discussed on page 6 (the first paragraph). The procedure of the MSNs preparation is included in the Supplementary Sectional Section (page S18). The detailed characterization of MSNs is presented in Supplementary Fig. 3. To make it clear, the corresponding sentence on page 5 was revised as "In contrast, for the aqueous TEPA marbles prepared with another mesoporous silica MSNs (with pore size of 8 nm, modified with the same amount of octyl organosilane, Supplementary Fig. 3a), their surfaces were observed to be partially bare even as early as 7 min following their preparation ( Supplementary Fig. 3b)."

Line 144: 'somewhat' should be 'some'
Response: Thank you for careful checking. The word "somewhat" has been corrected as "some" in the revised manuscript.
6. Lines 148-149: I don't understand the logic of how the images in Supp Fig 5a confirm  Response: To clearly inspect a hierarchical mesoporous-macroporous network formed inside the interior of SLHSP, we removed the liquid TEPA through calcination (the calcination procedure can not affect the porous structures). As you know, the liquid TEPA is subjected to evaporation under high vacuum, which is detrimental to TEM equipment. Another consideration is that the location of the liquid in the pore will reduce the contrast between pore wall and pore during the TEM observation.

Lines 168: 'widened' should probably be 'broadened'
Response: Thank you. The word "widened" was replaced with "broadened" in the revised manuscript.
8. Lines 169-173: The reference to 'grafted TEPA' is unclear. How was this 'grafted TEPA' prepared. There is no reference quoted and I can't find anything in the supplementary material either. This requires clarification.
Response: According to your suggestion, the procedure for grafting TEPA onto MSP has been described in the revised Supplementary Information on page S19. "3 mmol Response: Thank you. The word "somewhat" was corrected to "some" in the revised manuscript.  Am. Chem. Soc. 1998, 120, 6024−6036], which is cited in Supplementary Information. The characterization results of SBA-15 with SEM, TEM and N 2 sorption are provided in Supplementary Fig. 7. There are differences in terms of pore size, pore structure and surface area between SBA-15 and MSP, which can be found from the characterization results. The porosity of the two materials is dominantly contributed by the mesoporosity based on the t-Plot analysis, which is stated in the footnotes of Supplementary Fig. 7 and Supplementary Fig. 1, respectively. The aim of using SBA-15 here is to further confirm that our liquid marble method as a general approach can improve the CO 2 adsorption performance, rather than investigation of the influence of the porous structure on adsorption performance. 12. Line 320: 'larger' should be 'large'