Demonstrating Effectual Catalysis of Corncob with Solid Acid Sn-NUS-BH in Cyclopentyl Methyl Ether–Water for Co-Producing Reducing Sugar, Furfural, and Xylooligosaccharides

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Paper Catalysts-3189259 "Demonstrating effectual catalysis of corncob with solid acid Sn-NUS-BH in cyclopentyl methyl ether-water for co-producing reducing sugar, furfural, and xylo-oligosaccharides" describes the preparation of a catalyst based on "biochar" and its application in degradation of corncob biomass. A very similar catalyst, based on rice husk instead of the here-used barley hull, was prepared and used for the same purpose before (ref. 53 in the literature list) and I am really disappointed that the authors do not describe their actual results in comparison with the results of their former work. But this is only one and not the most important problem.

My major concern is that there is no characterisation of the now produced catalyst material at all, not even an elemental analysis. This is unacceptable because it prevents from reproducing the work. No characaterisation--no  chance for any comparison and for checking the outcome of the procedure carried out in another lab. Moreover, the description of the procedure  (section 2.2) suffers from questionable (line 105) or missing (line 111) volumetric information. The paper cannot be published without fixing these major problems.

I also miss a blank experiment (behaviour of the CC substrate under "optimised" reaction conditions but without addition of Sn-NUS-BH), according to line 175 you immediately started with a catalyst loading of 0.6% instead of zero.

Equations 1 and 6 must be explained (in particular the numbers appearing therein), at least by an appropriate reference! They are not self-evident!

Figure 2a is unclear, as its explanatory text states "effect of temperature on enzymolysis effiacy..." but section 2.5 describes the reaction temperature as 50 deg (in my eyes more reasonable for an enzymatic reaction than the values in the figure). In general, it is very difficult to find out, from all of the presented figures, which of the parameters were kept constant and at which value. A comprehensive description alongside with the diagrams is highly desirable.

I further experienced difficulties to follow your reasoning in lines 219-236. You ascribe an improvement in enzymatic LCB digestion to the enchanced lignin degradation at elevated temperature. However, from figures 3a-d this is not evident to me since the lignin proportion in the CC residue after the first processing step is not reduced at all, at least not to a significant degree! So I am asking myself, are these data (and the statement in line 294) compatible with those from section 3.1?

Some small things in the end: Figure 6, the axes should be labelled with units. Line 38, ethanol butanol furfural = "high-value polymers"? Line 74, water-dimethyl sulfoxide = biphasic? Figure 1, in each of the panels another colour/type of representation is chosen for the same parameter (XOS, pH, FAL), this is not very helpful; the same for Figure 2. Figure 3 is O.K. in this respect...

 

 

 

 

Author Response

Reviewer 1#

Paper Catalysts-3189259 "Demonstrating effectual catalysis of corncob with solid acid Sn-NUS-BH in cyclopentyl methyl ether-water for co-producing reducing sugar, furfural, and xylo-oligosaccharides" describes the preparation of a catalyst based on "biochar" and its application in degradation of corncob biomass. A very similar catalyst, based on rice husk instead of the here-used barley hull, was prepared and used for the same purpose before (ref. 53 in the literature list) and I am really disappointed that the authors do not describe their actual results in comparison with the results of their former work. But this is only one and not the most important problem.

Response: Thanks for the good suggestion.

It is crucial to compare the actual results of this study with the results of previous works. In the revised manuscript, the advantages of Sn-NUS-BH catalyst in synergy with CPME-H₂O over the results of previous works were highlighted.

“In previous studies, UST-SN-RH prepared on rice husk (RH) as support was reported to catalyze the conversion of CC to FAL and the regenerated FAL was converted into furfuryl alcohol (FOL) via bioreduction. The highest yield of FAL of 40.9% was observed after 30 min of UST-SN-RH catalysis [56]. Yang et al. prepared AT-Sn-MMT to catalyze bamboo using montmorillonite K-10 and obtained 1.5 g/L of XOSs and 7.5 g/L of FAL [57]. Jia et al. used tin-based sulfonated diatomite (SO₄²⁻/Sn-DM) to effectively transform xylose to produce FAL in a NaCl-DMSO-water/toluene system, obtaining 81% of FAL yield at 180 ^oC [58]. Compared with previous studies, the reaction temperature and time of this study were lower, and FAL (11.7 g/L), XOSs (4.5 g/L) and reducing sugars (65.2%) were synergistically produced with considerable yields, fully realizing the efficient value-added transformation of CC.” was added before “Meanwhile, this work provided a new route for co-production of XOSs, FAL and reducing sugars through the pretreatment with tin-based biochar heterogeneous chemocatalysts in CPME-water.” in this section “3.5. Comprehensive evaluation of Sn-NUS-BH-catalyzing CC in CPME-H₂O”

[56] Q. Yang, Z. Tang, J. Xiong, Y. He, Sustainable Chemoenzymatic Cascade Transformation of Corncob to Furfuryl Alcohol with Rice Husk-Based Heterogeneous Catalyst UST-Sn-RH, Catalysts, 13(1) (2023) 37.

[57] Q. Yang, B. Fan, Y.-C. He, Combination of solid acid and solvent pretreatment for co-production of furfural, xylooligosaccharide and reducing sugars from Phyllostachys edulis, Bioresource Technology 395 (2024) 130398.

[58] Q. Jia, X. Teng, S. Yu, Z. Si, G. Li, M. Zhou, D. Cai, P. Qin, B. Chen, Production of furfural from xylose and hemicelluloses using tin-loaded sulfonated diatomite as solid acid catalyst in biphasic system, Bioresource Technology Reports 6 (2019) 145-151.

My major concern is that there is no characterization of the now produced catalyst material at all, not even an elemental analysis. This is unacceptable because it prevents from reproducing the work. No characterization—no chance for any comparison and for checking the outcome of the procedure carried out in another lab. Moreover, the description of the procedure (section 2.2) suffers from questionable (line 105) or missing (line 111) volumetric information. The paper cannot be published without fixing these major problems.

Response: Thanks for the good suggestion.

We have realized that catalyst characterization is essential. The characterization of the Sn-NUS-BH catalyst has been published in another paper on September 1 2024 (B. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583).

 

This manuscript (catalysts-3189259) was submitted this Special Issue “SI: Industrial Applications of High-Value Added Biomass Conversion”. This work aimed to that biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH was used for efficient catalytic conversion of corncob (CC) for and co-production of xylo-oligosaccharides, furfural and reducing sugars. By optimizing the system conditions (CPME-to-H₂O ratio, Sn-NUS-BH dosage, reaction time, and reaction temperature), the stubborn structure of corncobs was maximally disrupted. The chemical composition and structural characteristics (accessibility, lignin surface area, and hydrophobicity) of CC before and after treatment were assessed, demonstrating that the natural physical barriers of CC were disrupted and lignin was effectually eliminated. The natural physical barriers of CC were disrupted and lignin was effectually eliminated. The accessibility enhanced from 137.5 mg/g to 518.5 mg/g, the lignin surface area declined from 582.0 m²/g to 325.0 m²/g, and the hydrophobicity was changed from 4.7 L/g to 1.3 L/g. Through the treatment at 170 ^oC for 20 min, furfural (11.7 g/L) and xylo-oligosaccharides (4.5 g/L) were acquired in pretreatment liquor. The residual CC could be enzymatically saccharified into reducing sugars in the yield of 65.2%.

These characterizations of biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH, including SEM, FT-IR, XRD, XPS and NH₃-TPD, were reported in another work (B. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583) (SI: Novel Chemocatalysts and/or Biocatalysts for Sustainable Production of Value-Added Chemicals and Biofuels). This work aimed to that biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH was used for efficient catalytic conversion of corncob (CC) in a green biphasic system cyclopentyl methyl ether-water (CPME-H₂O), and co-production of xylo-oligosaccharides, furfural and reducing sugars was developed in this research. By using barley hulls (BHs) as biobased support, a heterogeneous biochar Sn-NUS-BH catalyst was created to transform corncob into furfural in cyclopentyl methyl ether–H₂O. Sn-NUS-BH had a fibrous structure with voids, a large comparative area, and a large pore volume, which resulted in more catalytic active sites. Through the characterization of the physical and chemical properties of Sn-NUS-BH, it was observed that the Sn-NUS-BH had tin dioxide (Lewis acid sites) and a sulfonic acid group (Brønsted acid sites). This chemocatalyst had good thermostability and catalytic activity to transform 75 g/L of corncob with ZnCl₂ (50 mM) to generate furfural (80.5% yield) in organic solvent-water biphasic system.

 

(see Catalysts 14 (2024) 583)

 

(see Catalysts 14 (2024) 583)

 

 

(see Catalysts 14 (2024) 583)

 

(see Catalysts 14 (2024) 583)

 

This submitted manuscript (catalysts-3189259: Demonstrating effectual catalysis of corncob with solid acid Sn-NUS-BH in cyclopentyl methyl ether-water for co-producing reducing sugar, furfural, and xylo-oligosaccharides) was submitted this Special Issue “SI: Industrial Applications of High-Value Added Biomass Conversion”, which is different work from this published work (B. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583) (SI: Novel Chemocatalysts and/or Biocatalysts for Sustainable Production of Value-Added Chemicals and Biofuels).

This submitted work is a continuation work reported in Catalysts (Catalysts 14 (2024) 583), and these two papers have different research topic and experiments These characterizations of biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH, including SEM, FT-IR, XRD, XPS and NH₃-TPD, have been published recently (Catalysts 14 (2024) 583). So, these information and images are not necessary to provide in this submitted manuscript (catalysts-3189259). In this revised version, some information about the characteristics about the specific surface and acid sites of Sn-NUS-BH (Sn-NUS-BH had tin dioxides (Lewis acid sites) and sulfonic acid groups (Brønsted acid sites) [4], which had two strong acid sites at 750 and 800 ^oC. Sn-NUS-BH had 63.2 m²/g of specific surface area, 0.15 cm³/g of pore volume, and 2.8 nm of pore size [4]) was given as below:

“2.2. Synthesis of Sn-NUS-BH

The barley hull (BH, 40-60 mesh) powder and used as the precursor for the carbonaceous solid acid catalyst. 200 g powder immersed in 800 mL ethanol containing NaOH (0.50 M), and this mixture was subjected to ultrasonication (100 W, 50 ^oC, 4 h) to obtain the NaOH-ultrasonicated barley hull (NUS-BH). The NUS-BH solid was collected, cleaned and then oven-dried (60 ^oC). 100 g of glucose and 0.50 L of DI water were mixed with the dried NUS-BH solid, and the treatment was implemented under the ultrasonication (100 W, 50 ^oC) for 1 h. The treated solid was collected, cleaned and then oven-dried (60 ^oC). The dried powder was calcined in a muffle furnace at a high temperature of 550 ^oC for 4 h. 600 mL of ethanol and 40.0 g of SnCl₄·5H₂O were added to the calcined powder and the pH was adjusted to 6.0 using 25.0 wt% ammonia water. The formed slurry was then oven-dried (80 ^oC) for 48 h, and the dried powder was treated through the sulfonation with 4.0 M H₂SO₄ (60 ^oC, 4 h) at a solid-liquid ratio of 1:15 (g:mL). The sulfonated medium was filtered, and the solid residue was further dried and then calcined (550 ^oC, 4 h) to acquire biochar-based solid acid catalyst Sn-NUS-BH. Sn-NUS-BH had tin dioxides (Lewis acid sites) and sulfonic acid groups (Brønsted acid sites) [4], which had two strong acid sites at 750 and 800 ^oC. Sn-NUS-BH had 63.2 m²/g of specific surface area, 0.15 cm³/g of pore volume, and 2.8 nm of pore size [4].” was added in this section “2. Materials and methods”.

 

Reference:

1. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583

In the preparation method (line 105), NUS-BH and glucose solution were mixed ultrasonically at 50 ^oC and dried, followed by high temperature calcination. The purpose was to allow glucose to attach to NUS-BH and form an irregular carbon shelf structure after dehydration and carbonization, thus sufficiently increasing the surface area of the catalyst.

The preparation process of Sn-NUS-BH catalyst (Section 2.2) should be described carefully and accurately. Thank you for your careful reading, which helped us to identify the errors and lack of volumetric information in the preparation method. In the revised manuscript, the corresponding descriptions have been modified and added.

“100 g of glucose and 0.50 mL of DI water were added to the dried NUS-BH solid, followed by 1 h of ultrasonication (100 W, 50 ^oC).” was revised to “100 g of glucose and 0.50 L of DI water were mixed with the dried NUS-BH solid, and the treatment was implemented under the ultrasonication (100 W, 50 ^oC) for 1 h.”.

“The formed slurry was then oven-dried (80 ^oC) for 48 h, and the dried powder was treated through the sulfonation with 4.0 M H₂SO₄ (60 ^oC, 4 h).” was revised to “The formed slurry was then oven-dried (80 ^oC) for 48 h, and the dried powder was treated through the sulfonation with 4.0 M H₂SO₄ (60 ^oC, 4 h) at a solid-liquid ratio of 1:15 (g:mL).” in this section “2.2. Synthesis of Sn-NUS-BH”.

 

I also miss a blank experiment (behaviour of the CC substrate under "optimised" reaction conditions but without addition of Sn-NUS-BH), according to line 175 you immediately started with a catalyst loading of 0.6% instead of zero.

Response: Thanks for the good suggestion.

The blank experiment (CC was pretreated with CPME-H₂O (2:1, v/v) for 20 min at 170 ^oC without the addition of Sn-NUS-BH catalyst.) in optimizing catalyst loading were added to the revised manuscript.

“However, without the addition of Sn-NUS-BH catalyst, the FAL and XOSs were 5.4 g/L and 2.6 g/L, respectively. It was clear that the combination of Sn-NUS-BH catalyst and CPME-H₂O could effectively improve the bioconversion efficiency.” was added after “To effectually improve the yields of FAL and XOSs, the combination of Sn-NUS-BH catalyst (3.6 wt%) and CPME-H₂O (2:1, v/v) afforded the good catalytic ability for conversion of CC at 170 ^oC for 20 min, acquiring FAL (11.7 g/L) and XOSs (4.5 g/L).”.

 

Equations 1 and 6 must be explained (in particular the numbers appearing therein), at least by an appropriate reference! They are not self-evident!

Response: Thanks for the good suggestion.

Thank you for your careful reading and for pointing out the deficiencies for us. In the revised manuscript, references were added to the corresponding equations and the parameters appearing in the equations were explained.

“The effects of reaction temperature and duration on the catalytic performance were evaluated using the severity factor (LogR0). LogR0 was calculated according to formula (1):

 ” was revised to “The effects of reaction temperature and duration on the catalytic performance were evaluated using the severity factor (LogR0) [32]. LogR0 was calculated according to formula (1):

where T represents the reaction temperature (^oC), t indicates the reaction time (min), 100 is the reference temperature (^oC), and 14.75 is the normal activation energy constant (^oC).”

“The released glucose concentration in the supernatant was determined using high-performance liquid chromatography (HPLC) to evaluate the enzymatic digestion efficacy.

” was revised to “The released glucose concentration in the supernatant was determined using high-performance liquid chromatography (HPLC) to evaluate the enzymatic digestion efficacy [34].

where 0.9 corresponds to the factor that converts glucose to equivalent glucan.”

 

[32] R. Mangione, R. Simões, H. Pereira, S. Catarino, J. Ricardo-da-Silva, I. Miranda, S. Ferreira-Dias, Potential Use of Grape Stems and Pomaces from Two Red Grapevine Cultivars as Source of Oligosaccharides, Processes, 10(9) (2022) 1896.

[34] Y. Chen, C. Ma, W. Tang, Y.-C. He, Comprehensive understanding of enzymatic saccharification of Betaine:Lactic acid-pretreated sugarcane bagasse, Bioresource Technology 386 (2023) 129485

 

Figure 2a is unclear, as its explanatory text states "effect of temperature on enzymolysis effiacy..." but section 2.5 describes the reaction temperature as 50 deg (in my eyes more reasonable for an enzymatic reaction than the values in the figure). In general, it is very difficult to find out, from all of the presented figures, which of the parameters were kept constant and at which value. A comprehensive description alongside with the diagrams is highly desirable.

Response: Thanks for the good suggestion. The test “"effect of temperature on enzymolysis effiacy..." is not clear. The “temperature” should be corrected to “pretreatment temperature”.

This Figure 2a was further corrected and organized, and this captions of Figure 2a was revised. In this revised manuscript, these revisions were given as below:

“Figure 2. Effect of temperature on enzymolysis efficacy and lignin elimination (a); Effect of time on enzymolysis efficacy and lignin elimination (b); The linear fitting about enzymolysis efficacy, lignin elimination and Log R₀ (c).” was revised to “Figure 2. Effects of different pretreatment temperatures from 150 to 190 ^oC on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 20 min; Enzymolysis condition: 50 ^oC, pH 4.8] (a); Effects of different pretreatment time from 10 to 50 min on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 170 ^oC; Enzymolysis condition: 50 ^oC, pH 4.8] (b); The linear fitting about enzymolysis efficacy, lignin elimination and Log R₀  [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v); Enzymolysis condition: 50 ^oC, pH 4.8] (c).”.

 

Figure 2. Effects of different pretreatment temperatures from 150 to 190 ^oC on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 20 min; Enzymolysis condition: 50 ^oC, pH 4.8] (a); Effects of different pretreatment time from 10 to 50 min on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 170 ^oC; Enzymolysis condition: 50 ^oC, pH 4.8] (b); The linear fitting about enzymolysis efficacy, lignin elimination and Log R₀  [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v); Enzymolysis condition: 50 ^oC, pH 4.8] (c).

 

I further experienced difficulties to follow your reasoning in lines 219-236. You ascribe an improvement in enzymatic LCB digestion to the enchanced lignin degradation at elevated temperature. However, from figures 3a-d this is not evident to me since the lignin proportion in the CC residue after the first processing step is not reduced at all, at least not to a significant degree! So I am asking myself, are these data (and the statement in line 294) compatible with those from section 3.1?

Response: Thanks for the good suggestion.

In Figure 3a, 3b, 3c & 3d, the lignin proportion in the pretreated CC residue are similar. However, these don’t represent that the lignin proportion in the CC residue after the first processing step is not reduced at all. The lignin removal was calculated based on the lignin content change and solid recovery [See Equation (2) and Equation (5)]. Different pretreatment conditions could cause different lignin content change and solid recovery.

 

According to Figure 2a and Figure 2b, it could be observed that the lignin removal was different under the different pretreatment temperature (Figure 2a) and pretreatment time (Figure 2b).

Figure 2. Effects of different pretreatment temperatures from 150 to 190 ^oC on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 20 min; Enzymolysis condition: 50 ^oC, pH 4.8] (a); Effects of different pretreatment time from 10 to 50 min on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 170 ^oC; Enzymolysis condition: 50 ^oC, pH 4.8] (b); The linear fitting about enzymolysis efficacy, lignin elimination and Log R₀  [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v); Enzymolysis condition: 50 ^oC, pH 4.8] (c).

 

Figure 3. Chemical composition of CC residue under different catalyst loading conditions (a); Chemical composition of CC residue under different CPME-H₂O volume ratio (b); Chemical composition of CC residue at different temperatures (c); Chemical composition of CC residue at different times (d).

 

The original manuscript suffered from poor language descriptions (lines 219-236), which made the analyses of enzymatic efficiency, lignin removal, and LogR0 incomprehensible. This was added and corrected in the revised manuscript.

“Lignin polymer is recognized to be an obstacle for enzymatic saccharification of LCB [42]. The fitting about lignin and enzymatic digestion was explored, and different operation temperature and reaction duration on influencing lignin elimination and enzymatic digestion efficacy were tested. As showcased in Figure 2a and 2b, lignin elimination and enzymatic digestion efficacy elevated with increasing operation temperature and reaction duration. Therefore, stronger severity factor (LogR₀) promoted the destruction of the physical barrier in LCB, thereby promoting the enzymatic digestion efficacy of cellulose. The reduction in the enzymatic digestion efficacy of cellulose was closely associated with the physical barrier fabricated by lignin. The existence of lignin not only blocks the contact of cellulase with cellulase, but also causes non-productively adsorption of cellulase on lignin [43]. The residue after reaction at 170 ^oC for 20 min was hydrolyzed with cellulase (10 FPU/g), and the enzymatic digestion efficacy and lignin elimination were 65.2% and 31.7%, respectively. As showcased in Figure 2c, the severity factor (LogR₀) manifested a good linear fitting with the enzymolysis efficacy (Y=26.55X-25.57, R²=0.96). LogR₀ had a good linear fitting with lignin removal (Y=22.07X-40.26, R²=0.84). Accordingly, Sn-NUS-BH catalyst (3.6 wt%) combined with CPME-H₂O (2:1, v/v) could help promote the elimination of CC lignin, thereby cleaving the natural physical barrier of CC and improving the enzymolysis efficacy.” was revised to “Lignin polymer is recognized to be an obstacle for enzymatic saccharification of LCB [42]. The effects of different reaction temperatures and time on delignification and enzymolysis efficiency were evaluated, and the relationships between enzymolysis efficiency, delignification and LogR0 were examined. As showcased in Figure 2a and 2b, delignification and enzymatic hydrolysis efficiency increased with increasing pretreatment temperature and time, with an overall increasing trend. The residue after reaction at 170 ^oC for 20 min was hydrolyzed with cellulase (10 FPU/g), and the enzymatic digestion efficacy and lignin elimination were 65.2% and 31.7%, respectively. The results showed that the Sn-NUS-BH catalyst combined with CPME-H₂O pretreatment disrupted the cross-linking structure among cellulose, hemicellulose and lignin in CC, resulting in more cellulose being exposed on the surface of CC, which improved the enzymatic hydrolysis efficiency. However, the increase in lignin removal and enzymatic hydrolysis efficiency was reduced at 180 ^oC, which may be due to the fact that high temperatures can lead to the condensation and deposition of lignin, thus affecting the enzymatic hydrolysis of cellulose [43]. The reduction in the enzymatic digestion efficacy of cellulose was closely associated with the physical barrier fabricated by lignin. The existence of lignin not only blocks the contact of cellulase with cellulase, but also causes non-productively adsorption of cellulase on lignin [44]. As showcased in Figure 2c, the severity factor (LogR₀) manifested a good linear fitting with the enzymolysis efficacy (Y=26.55X-25.57, R²=0.96). LogR₀ had a good linear fitting with lignin removal (Y=22.07X-40.26, R²=0.84). Accordingly, Sn-NUS-BH catalyst (3.6 wt%) combined with CPME-H₂O (2:1, v/v) could help promote the elimination of CC lignin, thereby cleaving the natural physical barrier of CC and improving the enzymolysis efficacy.” in this section “3.2. Relationship of enzymolysis efficacy and delignification after pretreatment”.

 

[43] Q. Yang, W. Tang, L. Li, M. Huang, C. Ma, Y.-C. He, Enhancing enzymatic hydrolysis of waste sunflower straw by clean hydrothermal pretreatment, Bioresource Technology 383 (2023) 129236.

 

In the original manuscript, there was a lack of rigor in the language description (Line 294), which was changed. In the revised manuscript, “Through high-pressure treatment, a large amount of lignin in CC was eliminated, resulting in the reduced specific surface area of lignin, weakened hydrophobicity, elevated accessibility, and improved enzymolysis efficacy [48].” was revised to “By pretreating CC with Sn-NUS-BH catalyst in CPME-H₂O at 170 °C for 20 min, a large amount of lignin in CC was eliminated, resulting in the reduced specific surface area of lignin, weakened hydrophobicity, elevated accessibility, and improved enzymolysis efficacy [48]. ”

 

Some small things in the end: Figure 6, the axes should be labelled with units. Line 38, ethanol butanol furfural = "high-value polymers"? Line 74, water-dimethyl sulfoxide = biphasic? Figure 1, in each of the panels another colour/type of representation is chosen for the same parameter (XOS, pH, FAL), this is not very helpful; the same for Figure 2. Figure 3 is O.K. in this respect...

Response: Thanks for the good suggestion.

Thank you for pointing out the error, in the original manuscript, Figure 6 was missing units. I have corrected it immediately. In the revised manuscript, the corresponding units have been added for Figure 6. The revised Figure 6 was shown below:

Figure 6. The radar image of lignin surface area (m²/g), enzymatic digestion efficiency (%), hydrophobicity (L/g), delignification (%) and accessibility (mg/g).

 

The original manuscript contains inappropriate language descriptions (line 38 and line 74). Ethanol, butanol, and furfural are not high-value polymers. Water-dimethyl sulfoxide is not a biphasic system. Corrected in the revised manuscript.

“Hemicellulose, consisting of various polysaccharides, can be hydrolyzed into hexoses, pentoses, and uronic acids, enabling the production of high-value polymers such as ethanol, butanol, furfural (FAL), 5-hydroxymethylfurfural (HMF), formic acid (FA), levulinic acid (LA), xylitol, and xylo-oligosaccharides (XOSs) [6-9] ” was revised to “Hemicellulose, consisting of various polysaccharides, can be hydrolyzed into hexoses, pentoses, and uronic acids, enabling the production of high-value biofuel and biobased chemicals such as ethanol, butanol, furfural (FAL), 5-hydroxymethylfurfural (HMF), formic acid (FA), levulinic acid (LA), xylitol, and xylo-oligosaccharides (XOSs) [6-9] ” in “Introduction”.

“While biphasic systems containing water and organic solvents, such as water-dimethyl sulfoxide (DMSO) [25], water-toluene [26], water-dichloromethane (DCM) [27], and water-methyl isobutyl ketone (MIBK) [28], not only elevate reaction efficacy but also minimize side-reactions for greatly raising the FAL productivity [24] ” was revised to “While the established systems containing water and organic solvents, such as water-dimethyl sulfoxide (DMSO) [25], water-toluene [26], water-dichloromethane (DCM) [27], and water-methyl isobutyl ketone (MIBK) [28], not only elevate reaction efficacy but also minimize side-reactions for greatly raising the FAL productivity [24]. ” in “Introduction”.

 

In the original manuscript, another color/type was chosen to represent the same parameters (XOS, pH, FAL) in each panel of Figures 1 and 2, which is not very helpful for the presentation of the results. Taking your advice, Figures 1 and 2 have been changed to show the same parameters in uniform color so that the figures can show the experimental results more directly. The modified Figures 1 and 2 are as follows:

Figure 1. Effects of CPME-H₂O volumetric ratio on forming FAL and XOSs  [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (1:3-3:1, v/v), 170 ^oC, 20 min] (a); Effect of catalyst load on forming FAL and XOSs [Treatment condition: Sn-NUS-BH catalyst (0.6-6.0 wt%), CPME-H₂O (2:1, v/v), 170 ^oC, 20 min] (b); Effects of temperature on forming FAL and XOSs [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 150-190 ^oC, 20 min] (c); Effect of time on forming FAL and XOSs [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 170 ^oC, 10-50 min] (d).

Figure 2. Effects of different pretreatment temperatures from 150 to 190 ^oC on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 20 min; Enzymolysis condition: 50 ^oC, pH 4.8] (a); Effects of different pretreatment time from 10 to 50 min on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 170 ^oC; Enzymolysis condition: 50 ^oC, pH 4.8] (b); The linear fitting about enzymolysis efficacy, lignin elimination and Log R₀  [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v); Enzymolysis condition: 50 ^oC, pH 4.8] (c).

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents the development of a barley hull biochar-based solid acid catalyst impregnated with SnCl4 for the catalytic conversion of corn cob into value-added products. While the concept is scientifically significant, the overall presentation is lacking, with insufficient information provided. Below are my observations that need to be addressed before this manuscript can be accepted for publication:

1. The characterization of the developed solid acid catalyst (Sn-NUS-BH) is entirely missing from the manuscript. Apart from the synthesis details, no additional information is provided.

2. Proper characterization of Sn-NUS-BH using techniques such as SEM, TEM, EDX, and FTIR should be conducted and included.

3. Quantification of the solid acid catalyst using NH3-TPD should be performed to estimate the Lewis and Brønsted acid sites of the catalyst.

4. A detailed mechanism for the conversion of corn cob into value-added products in the presence of the solid acid catalyst should be suggested in the manuscript.

5. The authors are also advised to provide a comparative analysis of existing solid acid catalysts, such as clays, with Sn-NUS-BH, including appropriate references and a discussion of its advantages over existing catalysts.

6. The figures should be improved, ensuring uniform text size and better resolution.

Author Response

Reviewer 2#

The manuscript presents the development of a barley hull biochar-based solid acid catalyst impregnated with SnCl4 for the catalytic conversion of corn cob into value-added products. While the concept is scientifically significant, the overall presentation is lacking, with insufficient information provided. Below are my observations that need to be addressed before this manuscript can be accepted for publication:

1. The characterization of the developed solid acid catalyst (Sn-NUS-BH) is entirely missing from the manuscript. Apart from the synthesis details, no additional information is provided.

Response: Thanks for the good suggestion.

We have realized that catalyst characterization is essential. The characterization of the Sn-NUS-BH catalyst has been published in another paper on September 1 2024 (B. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583).

This manuscript (catalysts-3189259) was submitted this Special Issue “SI: Industrial Applications of High-Value Added Biomass Conversion”. This work aimed to that biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH was used for efficient catalytic conversion of corncob (CC) for and co-production of xylo-oligosaccharides, furfural and reducing sugars. By optimizing the system conditions (CPME-to-H₂O ratio, Sn-NUS-BH dosage, reaction time, and reaction temperature), the stubborn structure of corncobs was maximally disrupted. The chemical composition and structural characteristics (accessibility, lignin surface area, and hydrophobicity) of CC before and after treatment were assessed, demonstrating that the natural physical barriers of CC were disrupted and lignin was effectually eliminated. The natural physical barriers of CC were disrupted and lignin was effectually eliminated. The accessibility enhanced from 137.5 mg/g to 518.5 mg/g, the lignin surface area declined from 582.0 m²/g to 325.0 m²/g, and the hydrophobicity was changed from 4.7 L/g to 1.3 L/g. Through the treatment at 170 ^oC for 20 min, furfural (11.7 g/L) and xylo-oligosaccharides (4.5 g/L) were acquired in pretreatment liquor. The residual CC could be enzymatically saccharified into reducing sugars in the yield of 65.2%.

These characterizations of biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH, including SEM, FT-IR, XRD, XPS and NH₃-TPD, were reported in another work (B. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583) (SI: Novel Chemocatalysts and/or Biocatalysts for Sustainable Production of Value-Added Chemicals and Biofuels). This work aimed to that biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH was used for efficient catalytic conversion of corncob (CC) in a green biphasic system cyclopentyl methyl ether-water (CPME-H₂O), and co-production of xylo-oligosaccharides, furfural and reducing sugars was developed in this research. By using barley hulls (BHs) as biobased support, a heterogeneous biochar Sn-NUS-BH catalyst was created to transform corncob into furfural in cyclopentyl methyl ether–H₂O. Sn-NUS-BH had a fibrous structure with voids, a large comparative area, and a large pore volume, which resulted in more catalytic active sites. Through the characterization of the physical and chemical properties of Sn-NUS-BH, it was observed that the Sn-NUS-BH had tin dioxide (Lewis acid sites) and a sulfonic acid group (Brønsted acid sites). This chemocatalyst had good thermostability and catalytic activity to transform 75 g/L of corncob with ZnCl₂ (50 mM) to generate furfural (80.5% yield) in organic solvent-water biphasic system.

 

(see Catalysts 14 (2024) 583)

 

 

(see Catalysts 14 (2024) 583)

 

 

(see Catalysts 14 (2024) 583)

 

 

(see Catalysts 14 (2024) 583)

 

This submitted manuscript (catalysts-3189259: Demonstrating effectual catalysis of corncob with solid acid Sn-NUS-BH in cyclopentyl methyl ether-water for co-producing reducing sugar, furfural, and xylo-oligosaccharides) was submitted this Special Issue “SI: Industrial Applications of High-Value Added Biomass Conversion”, which is different work from this published work (B. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583) (SI: Novel Chemocatalysts and/or Biocatalysts for Sustainable Production of Value-Added Chemicals and Biofuels).

This submitted work is a continuation work reported in Catalysts (Catalysts 14 (2024) 583), and these two papers have different research topic and experiments These characterizations of biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH, including SEM, FT-IR, XRD, XPS and NH₃-TPD, have been published recently (Catalysts 14 (2024) 583). So, these information and images are not necessary to provide in this submitted manuscript (catalysts-3189259). In this revised version, some information about the characteristics about the specific surface and acid sites of Sn-NUS-BH (Sn-NUS-BH had tin dioxides (Lewis acid sites) and sulfonic acid groups (Brønsted acid sites) [4], which had two strong acid sites at 750 and 800 ^oC. Sn-NUS-BH had 63.2 m²/g of specific surface area, 0.15 cm³/g of pore volume, and 2.8 nm of pore size [4]) was given as below:

“2.2. Synthesis of Sn-NUS-BH

The barley hull (BH, 40-60 mesh) powder and used as the precursor for the carbonaceous solid acid catalyst. 200 g powder immersed in 800 mL ethanol containing NaOH (0.50 M), and this mixture was subjected to ultrasonication (100 W, 50 ^oC, 4 h) to obtain the NaOH-ultrasonicated barley hull (NUS-BH). The NUS-BH solid was collected, cleaned and then oven-dried (60 ^oC). 100 g of glucose and 0.50 L of DI water were mixed with the dried NUS-BH solid, and the treatment was implemented under the ultrasonication (100 W, 50 ^oC) for 1 h. The treated solid was collected, cleaned and then oven-dried (60 ^oC). The dried powder was calcined in a muffle furnace at a high temperature of 550 ^oC for 4 h. 600 mL of ethanol and 40.0 g of SnCl₄·5H₂O were added to the calcined powder and the pH was adjusted to 6.0 using 25.0 wt% ammonia water. The formed slurry was then oven-dried (80 ^oC) for 48 h, and the dried powder was treated through the sulfonation with 4.0 M H₂SO₄ (60 ^oC, 4 h) at a solid-liquid ratio of 1:15 (g:mL). The sulfonated medium was filtered, and the solid residue was further dried and then calcined (550 ^oC, 4 h) to acquire biochar-based solid acid catalyst Sn-NUS-BH. Sn-NUS-BH had tin dioxides (Lewis acid sites) and sulfonic acid groups (Brønsted acid sites) [4], which had two strong acid sites at 750 and 800 °C. Sn-NUS-BH had 63.2 m²/g of specific surface area, 0.15 cm³/g of pore volume, and 2.8 nm of pore size.” was added in this section “2. Materials and methods”.

 

Reference:

1. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583

 

2. Proper characterization of Sn-NUS-BH using techniques such as SEM, TEM, EDX, and FTIR should be conducted and included.

Response: Thanks for the good suggestion.

These characterizations of biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH, including SEM, FT-IR, XRD, XPS and NH₃-TPD, were reported in another work (B. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583) (SI: Novel Chemocatalysts and/or Biocatalysts for Sustainable Production of Value-Added Chemicals and Biofuels). This work aimed to that biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH was used for efficient catalytic conversion of corncob (CC) in a green biphasic system cyclopentyl methyl ether-water (CPME-H₂O), and co-production of xylo-oligosaccharides, furfural and reducing sugars was developed in this research. By using barley hulls (BHs) as biobased support, a heterogeneous biochar Sn-NUS-BH catalyst was created to transform corncob into furfural in cyclopentyl methyl ether–H₂O. Sn-NUS-BH had a fibrous structure with voids, a large comparative area, and a large pore volume, which resulted in more catalytic active sites. Through the characterization of the physical and chemical properties of Sn-NUS-BH, it was observed that the Sn-NUS-BH had tin dioxide (Lewis acid sites) and a sulfonic acid group (Brønsted acid sites). This chemocatalyst had good thermostability and catalytic activity to transform 75 g/L of corncob with ZnCl₂ (50 mM) to generate furfural (80.5% yield) in organic solvent-water biphasic system.

 

This manuscript (catalysts-3189259) was submitted this Special Issue “SI: Industrial Applications of High-Value Added Biomass Conversion”. This work aimed to that biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH was used for efficient catalytic conversion of corncob (CC) for and co-production of xylo-oligosaccharides, furfural and reducing sugars. By optimizing the system conditions (CPME-to-H₂O ratio, Sn-NUS-BH dosage, reaction time, and reaction temperature), the stubborn structure of corncobs was maximally disrupted. The chemical composition and structural characteristics (accessibility, lignin surface area, and hydrophobicity) of CC before and after treatment were assessed, demonstrating that the natural physical barriers of CC were disrupted and lignin was effectually eliminated. The natural physical barriers of CC were disrupted and lignin was effectually eliminated. The accessibility enhanced from 137.5 mg/g to 518.5 mg/g, the lignin surface area declined from 582.0 m²/g to 325.0 m²/g, and the hydrophobicity was changed from 4.7 L/g to 1.3 L/g. Through the treatment at 170 ^oC for 20 min, furfural (11.7 g/L) and xylo-oligosaccharides (4.5 g/L) were acquired in pretreatment liquor. The residual CC could be enzymatically saccharified into reducing sugars in the yield of 65.2%.

This submitted manuscript (catalysts-3189259: Demonstrating effectual catalysis of corncob with solid acid Sn-NUS-BH in cyclopentyl methyl ether-water for co-producing reducing sugar, furfural, and xylo-oligosaccharides) was submitted this Special Issue “SI: Industrial Applications of High-Value Added Biomass Conversion”, which is different work from this published work (B. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583) (SI: Novel Chemocatalysts and/or Biocatalysts for Sustainable Production of Value-Added Chemicals and Biofuels).

This submitted work is a continuation work reported in Catalysts (Catalysts 14 (2024) 583), and these two papers have different research topic and experiments These characterizations of biochar-based tin-loaded heterogeneous catalyst Sn-NUS-BH, including SEM, FT-IR, XRD, XPS and NH₃-TPD, have been published recently (Catalysts 14 (2024) 583). So, these information and images are not necessary to provide in this submitted manuscript (catalysts-3189259). In this revised version, some information about the characteristics about the specific surface and acid sites of Sn-NUS-BH (Sn-NUS-BH had tin dioxides (Lewis acid sites) and sulfonic acid groups (Brønsted acid sites) [4], which had two strong acid sites at 750 and 800 ^oC. Sn-NUS-BH had 63.2 m²/g of specific surface area, 0.15 cm³/g of pore volume, and 2.8 nm of pore size [4]) was given as below:

“2.2. Synthesis of Sn-NUS-BH

The barley hull (BH, 40-60 mesh) powder and used as the precursor for the carbonaceous solid acid catalyst. 200 g powder immersed in 800 mL ethanol containing NaOH (0.50 M), and this mixture was subjected to ultrasonication (100 W, 50 ^oC, 4 h) to obtain the NaOH-ultrasonicated barley hull (NUS-BH). The NUS-BH solid was collected, cleaned and then oven-dried (60 ^oC). 100 g of glucose and 0.50 L of DI water were mixed with the dried NUS-BH solid, and the treatment was implemented under the ultrasonication (100 W, 50 ^oC) for 1 h. The treated solid was collected, cleaned and then oven-dried (60 ^oC). The dried powder was calcined in a muffle furnace at a high temperature of 550 ^oC for 4 h. 600 mL of ethanol and 40.0 g of SnCl₄·5H₂O were added to the calcined powder and the pH was adjusted to 6.0 using 25.0 wt% ammonia water. The formed slurry was then oven-dried (80 ^oC) for 48 h, and the dried powder was treated through the sulfonation with 4.0 M H₂SO₄ (60 ^oC, 4 h) at a solid-liquid ratio of 1:15 (g:mL). The sulfonated medium was filtered, and the solid residue was further dried and then calcined (550 ^oC, 4 h) to acquire biochar-based solid acid catalyst Sn-NUS-BH. Sn-NUS-BH had tin dioxides (Lewis acid sites) and sulfonic acid groups (Brønsted acid sites) [4], which had two strong acid sites at 750 and 800 °C. Sn-NUS-BH had 63.2 m²/g of specific surface area, 0.15 cm³/g of pore volume, and 2.8 nm of pore size.” was added in this section “2. Materials and methods”.

 

Reference:

1. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583

 

3. Quantification of the solid acid catalyst using NH3-TPD should be performed to estimate the Lewis and Brønsted acid sites of the catalyst.

Response: Thanks for the good suggestion.

NH₃-TPD can analyze the acid sites/acid amount of solid acid catalysts. This NH₃-TPD image has been published in another work (B. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583) (SI: Novel Chemocatalysts and/or Biocatalysts for Sustainable Production of Value-Added Chemicals and Biofuels).

In this revised version, the main result about NH₃-TPD of Sn-NUS-BH was described in short in this revised manuscript, and it was given as below:

“2.2. Synthesis of Sn-NUS-BH

The barley hull (BH, 40-60 mesh) powder and used as the precursor for the carbonaceous solid acid catalyst. 200 g powder immersed in 800 mL ethanol containing NaOH (0.50 M), and this mixture was subjected to ultrasonication (100 W, 50 ^oC, 4 h) to obtain the NaOH-ultrasonicated barley hull (NUS-BH). The NUS-BH solid was collected, cleaned and then oven-dried (60 ^oC). 100 g of glucose and 0.50 L of DI water were mixed with the dried NUS-BH solid, and the treatment was implemented under the ultrasonication (100 W, 50 ^oC) for 1 h. The treated solid was collected, cleaned and then oven-dried (60 ^oC). The dried powder was calcined in a muffle furnace at a high temperature of 550 ^oC for 4 h. 600 mL of ethanol and 40.0 g of SnCl₄·5H₂O were added to the calcined powder and the pH was adjusted to 6.0 using 25.0 wt% ammonia water. The formed slurry was then oven-dried (80 ^oC) for 48 h, and the dried powder was treated through the sulfonation with 4.0 M H₂SO₄ (60 ^oC, 4 h) at a solid-liquid ratio of 1:15 (g:mL). The sulfonated medium was filtered, and the solid residue was further dried and then calcined (550 ^oC, 4 h) to acquire biochar-based solid acid catalyst Sn-NUS-BH. Sn-NUS-BH had tin dioxides (Lewis acid sites) and sulfonic acid groups (Brønsted acid sites) [4], which had two strong acid sites at 750 and 800 °C. Sn-NUS-BH had 63.2 m²/g of specific surface area, 0.15 cm³/g of pore volume, and 2.8 nm of pore size.” was added in this section “2. Materials and methods”.

 

Reference:

1. Fan, L. Kong, Y. He, Highly efficient production of furfural from corncob by Barley hull biochar-based solid acid in cyclopentyl methyl ether–water system, Catalysts 14 (2024) 583

 

4. A detailed mechanism for the conversion of corn cob into value-added products in the presence of the solid acid catalyst should be suggested in the manuscript.

Response: Thanks for the good suggestion.

In the revised manuscript, a potential catalytic mechanism for the conversion of CC to FAL by Sn-NUS-BH in CPME-H₂O was proposed.

“3.6. Proposed catalytic mechanism for catalyzing CC into FAL with Sn-NUS-BH in CPME-H₂O

Sn-NUS-BH was used as a catalyst in combination with CPME-H₂O to catalyze CC transformation, and the potential mechanism for the production of FAL, XOSs and reducing sugars was proposed in this strategy (Figure 7). After Sn-NUS-BH combined with CPME-H₂O pretreatment, the natural dense anti-degradation structure of CC was destroyed, lignin was effectively dissociated. The accessibility of cellulose was significantly improved, and the surface area and hydrophobicity of lignin were reduced. Cellulose might be depolymerized into dextran under the catalysis of solid acid, and then hydrolyzed to produce glucose [59]. Water could promote the cleavage of intermolecular bonds (e.g., hydrogen bonds, ether bonds) present in CC through hydrolysis. The CPME and Sn-NUS-BH in the system might provide an acidic environment to promote the production of FAL [60]. Typically, hemicellulose could be converted into FAL and XOSs. The pentosan in hemicellulose may be hydrolyzed to obtain xylose, and then dehydrated to generate FAL. The formed FAL might be quickly extracted into CPME to minimize side reactions between FAL and intermediates or other compounds, thereby increasing the FAL yield [52]. In addition, enzymatic saccharification of the pretreated CC residue could obtain monomeric sugars, and the high reducing sugar yield (65.2%) was obtained due to the excellent effect of pretreatment. This study successfully developed the combined use of a new biochar-based solid acid catalyst Sn-NUS-BH and a green two-phase system CPME-H₂O to realize the conversion of CC into value-added products.

 

Figure 7. Possible mechanism involving Sn-NUS-BH -catalyzed biomass to furfural in CPME-H₂O.” was added in this section “3.5. Comprehensive evaluation of Sn-NUS-BH-catalyzing CC in CPME-H₂O”

[52] E. Cousin, K. Namhaed, Y. Pérès, P. Cognet, M. Delmas, H. Hermansyah, M. Gozan, P.A. Alaba, M.K. Aroua, Towards efficient and greener processes for furfural production from biomass: A review of the recent trends, Science of The Total Environment 847 (2022) 157599.

[59] J. Shen, R. Gao, Y.-C. He, C. Ma, Efficient synthesis of furfural from waste biomasses by sulfonated crab shell-based solid acid in a sustainable approach, Industrial Crops and Products 202 (2023) 116989.

[60] C.B.T.L. Lee, T.Y. Wu, A review on solvent systems for furfural production from lignocellulosic biomass, Renewable and Sustainable Energy Reviews 137 (2021) 110172.

5. The authors are also advised to provide a comparative analysis of existing solid acid catalysts, such as clays, with Sn-NUS-BH, including appropriate references and a discussion of its advantages over existing catalysts.

Response: Thanks for the good suggestion.

In recent years, heterogeneous processes catalyzed by solid acids have attracted extensive research attention due to their good thermal and chemical stability, easy separation and regeneration. Among them, clay and modified clay-based catalysts (such as bentonite, montmorillonite, hydrotalcite and halloysite clay) were widely used for organic transformations. A comparative analysis of Sn-NUS-BH and existing solid acid catalysts was added in the revised manuscript, highlighting the advantages of Sn-NUS-BH synergistic with CPME-H₂O in this study over the results of previous work.

The comparative analysis was given as below:

“In previous studies, UST-SN-RH prepared on rice husk (RH) as support was reported to catalyze the conversion of CC to FAL and the regenerated FAL was converted into furfuryl alcohol (FOL) via bioreduction. The highest yield of FAL of 40.9% was observed after 30 min of UST-SN-RH catalysis [56]. Yang et al. prepared AT-Sn-MMT to catalyze bamboo using montmorillonite K-10 and obtained 1.5 g/L of XOSs and 7.5 g/L of FAL [57]. Jia et al. used tin-based sulfonated diatomite (SO₄²⁻/Sn-DM) to effectively transform xylose to produce FAL in a NaCl-DMSO-water/toluene system, obtaining 81% of FAL yield at 180 ^oC [58]. Compared with previous studies, the reaction temperature and time of this study were lower, and FAL (11.7 g/L), XOSs (4.5 g/L) and reducing sugars (65.2%) were synergistically produced with considerable yields, fully realizing the efficient value-added transformation of CC.” was added before “Meanwhile, this work provided a new route for co-production of XOSs, FAL and reducing sugars through the pretreatment with tin-based biochar heterogeneous chemocatalysts in CPME-water.” in this section “3.5. Comprehensive evaluation of Sn-NUS-BH-catalyzing CC in CPME-H₂O”

 

[56] Q. Yang, Z. Tang, J. Xiong, Y. He, Sustainable Chemoenzymatic Cascade Transformation of Corncob to Furfuryl Alcohol with Rice Husk-Based Heterogeneous Catalyst UST-Sn-RH, Catalysts, 13(1) (2023) 37.

[57] Q. Yang, B. Fan, Y.-C. He, Combination of solid acid and solvent pretreatment for co-production of furfural, xylooligosaccharide and reducing sugars from Phyllostachys edulis, Bioresource Technology 395 (2024) 130398.

[58] Q. Jia, X. Teng, S. Yu, Z. Si, G. Li, M. Zhou, D. Cai, P. Qin, B. Chen, Production of furfural from xylose and hemicelluloses using tin-loaded sulfonated diatomite as solid acid catalyst in biphasic system, Bioresource Technology Reports 6 (2019) 145-151.

6. The figures should be improved, ensuring uniform text size and better resolution.

Response: Thanks for the good suggestion.

High resolution and uniform text size charts can provide readers with a better reading experience. All data charts in the manuscript were checked, adjusted and unified in details. The updated charts were as follows:

 

Figure 1. Effects of CPME-H₂O volumetric ratio on forming FAL and XOSs  [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (1:3-3:1, v/v), 170 ^oC, 20 min] (a); Effect of catalyst load on forming FAL and XOSs [Treatment condition: Sn-NUS-BH catalyst (0.6-6.0 wt%), CPME-H₂O (2:1, v/v), 170 ^oC, 20 min] (b); Effects of temperature on forming FAL and XOSs [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 150-190 ^oC, 20 min] (c); Effect of time on forming FAL and XOSs [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 170 ^oC, 10-50 min] (d).

 

Figure 2. Effects of different pretreatment temperatures from 150 to 190 ^oC on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 20 min; Enzymolysis condition: 50 ^oC, pH 4.8] (a); Effects of different pretreatment time from 10 to 50 min on enhancing enzymatic hydrolysis and lignin elimination [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v), 170 ^oC; Enzymolysis condition: 50 ^oC, pH 4.8] (b); The linear fitting about enzymolysis efficacy, lignin elimination and Log R₀  [Treatment condition: Sn-NUS-BH catalyst (3.6 wt%), CPME-H₂O (2:1, v/v); Enzymolysis condition: 50 ^oC, pH 4.8] (c).

 

Figure 4. The linear fitting about accessibility, lignin removal and hydrolysis efficacy (a); The linear fitting about surface lignin area, lignin removal and hydrolysis efficacy (b); The linear fitting about hydrophobicity, lignin removal and hydrolysis efficacy (c).

 

Figure 5. Effects of pretreatment temperature on the accessibility and hydrolysis efficiency (a), surface lignin area (b), and hydrophobicity (c) of lignin in raw and treated CC.

 

 

 

Figure 6. The radar image of lignin surface area (m²/g), enzymatic digestion efficiency (%), hydrophobicity (L/g), delignification (%) and accessibility (mg/g).

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have provided a revised version of "catalysts-3189259" and addressed and resolved all issues raised in my first review. So I do not have any objection against publication of the work. 

However, I am not really happy with the fact that preparation, application and characterisation of the catalyst are presented in separate papers. In former times, such a strategy had been considered "unnecessary fragmentation of work" and discouraged strongly by all journals and publishers... but times have changed.

Author Response

The authors have provided a revised version of "catalysts-3189259" and addressed and resolved all issues raised in my first review. So I do not have any objection against publication of the work. 

However, I am not really happy with the fact that preparation, application and characterisation of the catalyst are presented in separate papers. In former times, such a strategy had been considered "unnecessary fragmentation of work" and discouraged strongly by all journals and publishers... but times have changed.

Response: Thanks for the good suggestion. In the future work, we will present a complete and high-quality work about the preparation, application and characterisation of the catalyst.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for providing comprehensive reply to the raised question and necessary changes made in the manuscript. But it is unclear from the manuscript that this is a continuation work of the authors. Therefor I suggest to change the last paragraph of the revised manuscript so that it become clear the work is a continuation work with proper citation.

Author Response

Reviewer

Thank you for providing comprehensive reply to the raised question and necessary changes made in the manuscript. But it is unclear from the manuscript that this is a continuation work of the authors. Therefor I suggest to change the last paragraph of the revised manuscript so that it become clear the work is a continuation work with proper citation.

Response: Thanks for the good suggestion. The proper citation is given in the last paragraph in Section “3. Results and discussion” of the revised manuscript, and the clear description that this work is a continuation work of the authors is made as follows:

“3.6. Proposed catalytic mechanism for catalyzing CC into FAL with Sn-NUS-BH in CPME-H₂O

In our previous work, this heterogeneous biochar Sn-NUS-BH catalysts were prepared [4]. Various characterizations of Sn-NUS-BH revealed that Sn-NUS-BH had a large specific surface area and pore volume with tin dioxide (Lewis acidic site) and sulfonic acid groups (Brønsted acidic site), and it had good catalytic activity for transforming of CC in FAL with ZnCl₂ in organic solvent-water biphasic system. To efficiently valorize biomass into value-added chemicals, a continuation of Sn-NUS-BH catalyst application was implemented in this work. This Sn-NUS-BH was used to efficiently transform CC in a green biphasic system CPME-H₂O, and co-production of XOSs, FAL and reducing sugars was realized in this research. The CC chemical compositions, cellulose accessibility, surface lignin area, and lignin hydrophobicity before and after treatment were determined. The potential mechanism for the production of FAL, XOSs and reducing sugars was proposed (Figure 7). After Sn-NUS-BH combined with CPME-H₂O pretreatment, the natural dense anti-degradation structure of CC was destroyed, lignin was effectively dissociated. The accessibility of cellulose was significantly improved, and the surface area and hydrophobicity of lignin were reduced. Cellulose might be depolymerized into dextran under the catalysis of solid acid, and then hydrolyzed to produce glucose [59]. Water could promote the cleavage of intermolecular bonds (e.g., hydrogen bonds, ether bonds) present in CC through hydrolysis. The CPME and Sn-NUS-BH in the system might provide an acidic environment to promote the production of FAL [60]. Typically, hemicellulose could be converted into FAL and XOSs. The pentosan in hemicellulose may be hydrolyzed to obtain xylose, and then dehydrated to generate FAL. The formed FAL might be quickly extracted into CPME to minimize side reactions between FAL and intermediates or other compounds, thereby increasing the FAL yield [52]. After the treatment, the cellulose accessibility elevated from 137.5 to 518.5 mg/g, the surface lignin area reduced from 582.0 to 325.0 m²/g, and the lignin hydrophobicity declined from 4.7 to 1.3 L/g. The natural physical barrier of CC was clearly destroyed, and enzymatic saccharification of the pretreated CC residue could obtain monomeric sugars, acquiring the high reducing sugar yield (65.2%) due to the excellent effect of pretreatment. This study successfully developed the combined use of a new biochar-based solid acid catalyst Sn-NUS-BH and a green two-phase system CPME-H₂O to realize the conversion of CC into value-added products.”

Author Response File: Author Response.pdf