Sustainable Exploitation of Residual Cynara cardunculus L. to Levulinic Acid and n-Butyl Levulinate

Round 1

Reviewer 1 Report

I have been reading the article and it is quite good, I recommend its publication

Author Response

We thank the reviewer for his/her comment.

Best regards,

Anna Raspolli

Reviewer 2 Report

Defatted cardoon was employed as a raw material for the preparation of levulinic acid (LA) and its n-butyl ester (BL). The novelty probably resides on the use of this biomass residue to produce LA/BL, not precisely on the methodology(ies) used on the conversion. Nevertheless, interesting conversions into LA/BL were obtained. The sift of best catalytic conditions was carried out in the usual way by varying temperature/pressure parameters with several combinations and ratios of raw materials/catalysts, and the type of reactor (mono mode MW and a Parr reactor). The work may be reconsidered for publication after the following comments and concerns have been settled:

1. How the applied steam-explosion pre-treatment methodology can be justified on energetic grounds given the similar order of magnitude of LA/BL yields obtained directly from crude samples and those subjected to steam explosion?
2. How the authors intend to isolate and purify the levulinic acid or its ester from the crude reaction mixtures? And, from an industrial point of view, how can this be implemented in a cost-effective way as one looks to the composition of the reaction mixtures? This issue should have been tackled in the manuscript for a full understanding of the real possibilities of proposed route. Thus, (a) method(s) for the isolation of LA/BL should be carried out, and the characterization of the resulting compounds presented in the manuscript, as well as the isolated yields.
3. The authors should explain why only two strong mineral acids were used in their experiments. For a greener prospective, other acid catalysts should have been investigated.
4. While the title of the manuscript refers the preparation of LA and its n-butyl ester from cardoon, an extensive characterization was also devoted to the solid residue (a 30% by-product) obtained from the conversions. Albeit some prospective indications for their use were outlined, no real applications of it were demonstrated in the manuscript. As thus, it seems of little use to spend so many efforts with this kind of characterization and analysis.
5. The authors should explain why they state (lines 409-410) that “demonstrating the complete carbonization occurred with this technology” while at the beginning of the paragraph (lines 399-400) they mention that “confirm a limited beginning of carbonization”. How should one understand this?

Author Response

Dear Editor,

enclosed you find our review “Sustainable exploitation of residual Cynara cardunculus L. to levulinic acid and n-butyl levulinate”(Manuscript ID: catalysts-1345140) for publication in “Catalysts”. We have revised the paper according to all the suggestions of the referees and the new changes are red-highlighted in the manuscript. In particular:

 

Reviewer 2

• Defatted cardoon was employed as a raw material for the preparation of levulinic acid (LA) and its n-butyl ester (BL). The novelty probably resides on the use of this biomass residue to produce LA/BL, not precisely on the methodology(ies) used on the conversion. Nevertheless, interesting conversions into LA/BL were obtained. The sift of best catalytic conditions was carried out in the usual way by varying temperature/pressure parameters with several combinations and ratios of raw materials/catalysts, and the type of reactor (mono mode MW and a Parr reactor). The work may be reconsidered for publication after the following comments and concerns have been settled:

How the applied steam-explosion pre-treatment methodology can be justified on energetic grounds given the similar order of magnitude of LA/BL yields obtained directly from crude samples and those subjected to steam explosion?

 

We thank the reviewer for his/her comments and questions. Regarding the energetic aspect of the steam-explosion pre-treatment, first of all, it is important to underline that a complete energy balance is outside the scope of this manuscript. However, on the other side, the topic highlighted by the reviewer is very important and regarding this aspect, it is possible to specify that our work focuses mainly on an industrial perspective. It is true that LA yields obtained from crude cardoon (C sample) and from steam-exploded one (E sample) are similar, but the steam-explosion pre-treatment allows a significant enrichment of cellulose fraction in the treated sample, which results, at equal biomass loading, in marked increases of LA concentrations in the reaction mixture. In fact, if we compare for example run C3 with E3, the LA molar yields resulted 50.7 and 49.7 mol% respectively, which are comparable, but the LA concentrations are 15.9 and 25.5 g/L respectively, which are different. The possibility to have a more concentrated LA flow is a crucial aspect for the subsequent work-up, separation and purification processes respect to recover LA from diluted solutions in the perspective of industrial applications. In addition, the steam-explosion pre-treatment enables also the separation not only of the hemicelluloses fraction which can be separately exploited, but also the removal of extractives and ash which could make more difficult the exploitation of the cellulose fraction. In brief, the steam-explosion pre-treatment can be considered an initial fractionation of the starting biomass which allows a better valorization, including the ease of the product recovery, of the biomass fractions for industrial applications. In other words, the steam-explosion pre-treatment can help to adopt a High Gravity approach regarding the cellulose content (on dry biomass), approach already employed in our manuscript for biomass loading and as reported in the manuscript. This approach enables to achieve high products concentration, resulting particular promising in the perspective of industrial application. In fact, it increases the concentration of crude products, the costs for their purification and waste-water treatment.

On the above considerations, the manuscript has been modified as follows:

• “Both C and E samples show very different content of cellulose, about 38 and 65 wt%, respectively. In fact, after the steam-explosion pretreatment, the hemicellulose amount decreased from about 17 to 4 wt%, whereas extractives and ash were removed, as expected. On the other hand, lignin increased from about 17 up to 30 wt%, due to the reduction of the content of other components. The above-reported composition was determined on the dry biomasses and the enrichment of cellulose content in the steam-exploded sample resulted very important from an industrial point of view because at equal biomass loading, it enables to process a higher cellulose amount which results in marked increases of target products concentrations in the reaction mixture with subsequent ease for the successive work-up, separation and purification processes.”;
• “In the perspective of industrial application, the adoption of a high biomass loading is to be preferred, thus applying the High Gravity approach to achieve the highest products concentration. In other words, this is the same concept already applied regarding the cellulose content (on dry biomass) with the steam-explosion pretreatment. Such High Gravity method presents several advantages for an industrial perspective: it increases the concentration of crude products, reduces the costs for their purification and waste-water treatment.”;
• “As expected, higher LA concentrations were achieved from the cellulose-rich steam-exploded cardoon (E sample) rather than from that un-treated (C sample), justifying the adopted pretreatment in the industrial perspective: the maximum reached value was 59.0 g/L for E and 34.6 g/L for C.”.

 

• How the authors intend to isolate and purify the levulinic acid or its ester from the crude reaction mixtures? And, from an industrial point of view, how can this be implemented in a cost-effective way as one looks to the composition of the reaction mixtures? This issue should have been tackled in the manuscript for a full understanding of the real possibilities of proposed route. Thus, (a) method(s) for the isolation of LA/BL should be carried out, and the characterization of the resulting compounds presented in the manuscript, as well as the isolated yields.

 

We thank the referee for his/her valid suggestion on the discussion of the LA/BL purification, which is certainly strategic for better defining the economy of the process and favoring its next intensification. In the past, we had got experience in this topic from one of our collaborations with the company GFBiochemicals which produced LA on industrial scale. LA purification can be realized by solvent extraction and subsequent fractional distillation, according to, for example, the procedure reported in the Patent of Woestenborghs and Altink [1]. On this basis, LA purification from the crude hydrolysate deriving from the most promising experiment (run E7 of Table 2) was carried out, achieving the isolated LA yield of 20.3 wt% which was comparable to that determined by HPLC analysis, whereas the LA purity grade resulted 93%.

[1] Woestenborghs, P.L., Altink, R.L. Process for the isolation of levulinic acid, WO 2015/007602 A1.

 

Definitely, taking into account the right suggestion of the reviewer, the manuscript was modified as follows:

• “Lastly, in order to demonstrate the cost-effectiveness of the hydrolysis process, the purification of the crude hydrolysate deriving from run E7 of Table 2 was carried out on laboratory scale adopting 2-methyltetrahydrofuran as extraction solvent, followed by subsequent fractional distillation of the corresponding extract, according to the experimental procedure reported in Materials and Methods section. By this way, the final isolated LA yield of 20.3 wt% was ascertained, similar to that obtained from the HPLC analysis of the corresponding crude hydrolysate (run E7 of Table 2). The ascertained LA purity grade was 93%, estimated by both GC and HPLC technique, including FA, AA and angelica lactone as residual impurities. The latter compound originates from the acid-catalysed LA dehydration, followed by ring closure, which typically occurs upon LA heating, during the purification procedure”;
• “7 Purification of the crude mother liquor

         Crude mother liquor was extracted by continuous liquid/liquid extraction of the aqueous mixture with 2-methyltetrahydrofuran as the extraction solvent. In a typical procedure, 20 mL of the crude hydrolyzate was treated with 60 mL of 2-methyltetrahydrofuran in a continuous liquid/liquid extractor apparatus for 4 h and then, once the organic fraction was separated, the extract was subjected to a fractional distillation, in order to remove the solvent and recover the LA as pure product, according to the general procedure previously patented [62]. Briefly, the extract was subjected to a first distillation step, in order to remove the solvent, working under atmosferic pressure, until the bottom temperature was 100 °C. The bottom product was further distilled, in order to remove any lights, using a 50 cm Vigreux column, working at 100 mbar, progressively increasing the temperature of the oil bath from 75 to 130 °C (corresponding temperatures of the bottom zone of the distiller in the range 40-115 °C), and the distillation was stopped when no vapors reached the top of the distillation apparatus. Lastly, the bottom product was further distilled, working at 5 mbar, increasing the oil bath temperature from 185 to 195 °C (corresponding temperatures of the bottom zone of the distiller in the range 135-145 °C). The isolated top fraction was dried by a mechanical pump and the dried fraction was characterized by HPLC and GC chromatography. The LA purity grade of 93% was ascertained and on the basis of weighted amount and taking into account the LA purity grade, the isolated yield was determined. [62] Woestenborghs, P.L., Altink, R.L. Process for the isolation of levulinic acid, WO 2015/007602 A1, 2016.

 

 

• The authors should explain why only two strong mineral acids were used in their experiments. For a greener prospective, other acid catalysts should have been investigated.

 

We thank the reviewer for this observation because it enables us to explain better this aspect. For the present investigation, HCl and H₂SO₄ were selected as acid catalysts due to their generally recognized advantages in the perspective of industrial applications. In fact, they are characterized by low cost, abundant availability and high efficiency. Moreover, they are usually employed in high TRL chemical processes, especially for the production of levulinic acid, such as the Biofine process and GF Biochemicals plants [1,2]. According to the reviewer’s suggestion, we have also tested greener inorganic acids, FeCl₃ and AlCl₃, but we obtained lower LA yields with respect to the values achieved by using the two strong mineral acids described in the present work. Moreover, very low concentrations of mineral acids (HCl and H₂SO₄) were employed in our tests after the reaction optimization, supporting in this way the environmental sustainability of the proposed biorefinery process.

[1] Hayes, D.J., Fitzpatrick, S., Hayes, M.H.B., Ross, J.R.H. The biofine process –production of levulinic acid, furfural, and formic acid from lignocellulosic feedstocks. In Biorefineries–Industrial Processes and Product: Status Quo and Future Directions; Kamm, B., Gruber, P.R., Kamm, M., Eds.; WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2006; Volume 1, pp. 139-164.

[2] Morone, A., Apte, M., Pandey, R.A. Levulinic acid production from renewable waste resources: Bottlenecks, potential remedies, advancements and applications. Renew. Sustain. Energy Rev. 2015, 51, 548-565. doi.org/10.1016/j.rser.2015.06.032.

 

Based on these considerations, the manuscript has been revised as follows:

“The acid-catalyzed hydrolysis, assisted by microwave (MW) irradiation, was successfully performed for the two different samples of residual Cynara cardunculus L. and the effect of the main reaction parameters was investigated. Taking into account the high reached LA concentration, the hydrolysis of steam-exploded cardoon was also performed in a batch autoclave, in the perspective of a larger scaling-up. Moreover, in the same perspective of industrial applications, H₂SO₄ and HCl were selected as acid catalysts for the LA synthesis due to their generally recognized advantages. They are characterized by low cost, abundant availability and high efficiency. Furthermore, they are usually employed in high TRL chemical processes, especially for the production of levulinic acid, such as the Biofine process and GF Biochemicals plants [22,23]

[22] Hayes, D.J., Fitzpatrick, S., Hayes, M.H.B., Ross, J.R.H. The biofine process–production of levulinic acid, furfural, and formic acid from lignocellulosic feedstocks. In Biorefineries–Industrial Processes and Product: Status Quo and Future Directions; Kamm, B., Gruber, P.R., Kamm, M., Eds.; WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2006; Volume 1, pp. 139-164.

[23] Morone, A., Apte, M., Pandey, R.A. Levulinic acid production from renewable waste resources: Bottlenecks, potential remedies, advancements and applications. Renew. Sustain. Energy Rev. 2015, 51, 548-565. doi.org/10.1016/j.rser.2015.06.032.”.

 

• While the title of the manuscript refers the preparation of LA and its n-butyl ester from cardoon, an extensive characterization was also devoted to the solid residue (a 30% by-product) obtained from the conversions. Albeit some prospective indications for their use were outlined, no real applications of it were demonstrated in the manuscript. As thus, it seems of little use to spend so many efforts with this kind of characterization and analysis.

 

We thank the referee for his/her observation, which allows us to better specify some aspects related to the characterization of the char. In particular, the proposed characterization is rather preliminary, enabling us to consider also waste stream, up to now not adequately valued, in the perspective of improving the circular economy of the entire process. In this context, some of the reported data, in particular those related to TGA, but also those of FT-IR and elemental analysis, have been functional also to a better interpretation of the catalysis data (complete disappearance of the cellulosic component, thus demonstrating its complete conversion after a bulk and advanced carbonization of the solid phase). Moreover, elemental analysis has provided us to obtain useful information about the energetic properties of the synthesized char, thus considering its immediate use of this waste stream for energy recovery, for example, within the same process. On the other hand, in order to discuss the effective goodness of specific applications, which have been only mentioned as “proposals” in the final part of this manuscript, certainly extensive studies would be needed, which do not fall within the main scope of this work. On the basis of these considerations, in order to simplify the discussion about the char characterization, TGA curves have been moved to the revised Supplementary Section, keeping only the necessary discussion of the experimental data within the main manuscript, modifying the manuscript as follows:

• “This preliminary characterization of the obtained solid residues opens the way to their next Regarding this aspect, given the good HHV values of the synthesized lignite-like chars, certainly the most immediate use is the combustion for the energy recovery, but this choice is currently considered as the last option, preferring, when possible, its reuse within the scopes of the circular economy [54]. For agricultural uses, the application of biochar to the soil can mitigate climate change by promoting carbon sequestration and decrease greenhouse gas emissions [55]. Moreover, char has been advantageously proposed as a growing medium, to be used in combination with other components (vermicultite, clays, etc.) to improve physicochemical soil properties, such as increase in cation exchange capacity, water holding capacity, available water, improvement of soil structure, reduction in soil acidity, microbiological activity, quality, and yield of the crops [55]. The good functionalization of the produced chars, preliminarily demonstrated by FT-IR analysis, makes them good candidates as soil amendments for the recovery of contaminated soils, including stabilization of organic and inorganic contaminants [55]. Lately, more added-value char-based products are under development in many research fields, such as adsorption, catalysis and electrochemical energy storage (lithium-ion batteries, lithium-sulfur batteries, sodium-ion batteries and supercapacitors), after having properly tuned its physico-chemical properties, by choosing the appropriate starting feedstocks and optimizing the reaction conditions [56].”.
• “Moreover, the recovered residues and the starting biomasses were also characterized by thermogravimetric analysis and weight loss and weight loss thermograms are depicted in the Supplementary Section (Figure S8)”.

 

• The authors should explain why they state (lines 409-410) that “demonstrating the complete carbonization occurred with this technology” while at the beginning of the paragraph (lines 399-400) they mention that “confirm a limited beginning of carbonization”. How should one understand this?

 

Regarding this right observation of the referee, the first sentence (lines 399-400) is referred to the comparison between carbon content of cardoon C and E, considered both as starting feedstocks in Table 6 of the manuscript: therefore, carbon content of the steam-exploded cardoon (E) confirmed its limited occurred carbonization, when compared with the corresponding data of the crude sample (C). Instead, the second sentence (lines 409-410) is referred to the comparison between the carbon content of the solid residues (run C7, E7 and AE1, Table 6 of the manuscript) and those of the corresponding starting feedstocks (C and E - starting feedstocks, Table 6 of the manuscript), showing a greater variation of the carbon content, as a consequence of the performed hydrolysis/alcoholysis treatments. On this basis, taking into account the valuable referee’s observation, the text has been modified, as follows:

“The above data related to the starting feedstocks C and E confirm only a limited beginning of carbonization for the latter sample, as shown by the slight increase in its carbon content, occurred as a consequence of the mild steam-explosion treatment, aimed at the breakdown of the biomass matrix (cross-linking lignin), the bulk solubilization of the hemicellulose fraction and the removal of smaller hydrocarbon molecules (volatiles and gases) [36]. Instead, more advanced carbonization has occurred as a consequence of the acid-catalyzed hydrothermal treatment. H/C and O/C molar ratios of both solid residues at the end of runs C7 and E7 fall within the range reported in the literature for the hydrochars (H/C: ∼0.8–1.4 and O/C: ∼0.3–0.5) [37] and also in agreement with our previous work [38]. Therefore, the differences in the carbon content of the two different starting feedstocks (cardoon C and E, both as starting feedstocks in Table 6) have been attenuated by the acid-catalyzed hydrothermal treatment (solid residue of run C7 and E7, respectively, in Table 6), demonstrating the progressed carbonization occurred with this technology. Lastly, the post-alcoholysis residue (solid residue of run AE1 in Table 6) does not show significant compositional differences respect to the post-hydrolysis ones (solid residue of run C7 and E7 in Table 6), thus highlighting the similarity between the performed hydrolysis/alcoholysis treatments.”.

 

Hoping that this revision is enough for its publication, I send you my best personal regards.

Sincerely yours,

 

Prof. Anna Maria Raspolli Galletti

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and concerns have been properly addressed by the authors, and the manuscript main text and SM modified accordingly.

I recommend the acceptance of the manuscript in its present form, pending minor editing issues.