A process of preparation of sorbitol from cellulose or cellulose-based waste

Abstract

The present invention describes a single-step protocol for the hydrolytic hydrogenation of microcrystalline cellulose into sorbitol over commercial carbon- supported Ru, in the presence of gaseous CO2 as an acid source and molecular hydrogen as a reductant.

Description

[0001] A PROCESS OF PREPARATION OF SORBITOL FROM CELLULOSE OR CELLULOSE-BASED WASTE

[0002] FIELD OF THE INVENTION

[0003] The present invention refers to the field of hydrolytic catalytic hydrogenation of cellulose into sorbitol.

[0004] STATE OF THE ART

[0005] The depletion of fossil feedstocks and the enormous environmental issues posed by their combustion and chemical manipulation are among the most pressing concerns of our Society, to which the scientific / industrial community is responding through massive investments in the research of new strategies for a sustainable economy aimed to the conversion / valorisation of biomass into biofuels, renewable molecules and bio-based materials. In this context, given its abundance and its reduced impact on the food chain, lignocellulose (LC) is perhaps the best alternative to fossil resources, and even more specifically cellulose, the major component of LC, is the most promising biopolymer for the synthesis of a variety of chemicals. Among them, sorbitol and its derivatives have widespread applications as dispersing agents, humectants in pharmaceuticals, low-calorie sweeteners, cosmetics and textiles. Sorbitol is also one of the ten building block chemicals obtainable from cellulosic resources listed as strategic by the US Department of Energy.

[0006] The synthesis of sorbitol from cellulose is generally performed via a two-step reaction that includes an acid-catalysed hydrolysis of cellulose to glucose followed by the hydrogenation of glucose to sorbitol over metal catalysts (Scheme 1 ).

[0007]

[0008] Glucose

[0009] Hydrogenation i Cat, H2i

[0010] Sorbitol

[0011] Scheme 1 . Two-step catalytic cellulose conversion into sorbitol, via hydrolysis and hydrogenation.

[0012] Even though the robust crystalline structure of cellulose makes its hydrolytic breakdown to glucose still a challenge, many reaction protocols have been developed over the years. Homogeneous catalysts such as H2SO4 and HCI have been extensively described to this purpose. The use of strong liquid acids, however, is not sustainable from the environmental standpoint and suffers from serious drawbacks such as low selectivity, difficult product separation, corrosion and acid recovery.

[0013] As an alternative, several heterogeneous (acid) catalysts have been proposed for the direct conversion of cellulose to polyols [Negoi, A.; et al. Catal. Today 2014, 223, 122-128, doi.org / 10.1016 / j.cattod.2013.07.007; Kobayashi, H.;et al. Green Chem. 2011 , 13 (2), 326-333, doi.org / 10.1039 / c0gc00666a; Dhepe, P. L.; et al. ChemSusChem 2008, 1 (12), 969-975, doi.org / 10.1002 / cssc.200800129; Kaldstrom, M.; et al. Catal. Today 2011 , 167 (1 ), 91-95, doi.org / 10.1016 / j.cattod.2010.12.048; Ribeiro, L. S.; et al. Appl. Catal. B Environ. 2017, 217, 265-274, doi.org / 10.1016 / j.apcatb.2017.04.078]. In a first seminal study, Fukuoka and Dhepe described the hydrolytic hydrogenation of cellulose using different metal-based catalysts of which Pt / y-A^Os showed the best performance with a production of sorbitol and mannitol in 25% and 6% yield, respectively [Angew. Chemie - Int. Ed. 2006, 45 (31 ), 5161-5163. doi.org / 10.1002 / anie.200601921 ]. Subsequent studies highlighted that Ru-based catalysts, even commercial ones, were probably the best option for the conversion of cellulose into sorbitol, not only, for they displayed good activity and selectivity, but also for their competitive costs since Ru was available at a far lower price (ca~4%) with respect to other metals such as Au and Pt of comparable activity [Zhao, D.; et al. Mol. Catal. 2020, 495 (July), 111133, doi.org / 10.1016 / j.mcat.2020.111133], Luo et al. reported that at 245 °C and H2 at 60 bar, in the presence of carbon supported ruthenium (Ru / C), a high conversion of cellulose was achieved (86%) with a 30% sorbitol yield [Angew. Chemie - Int. Ed. 2007, 46 (40), 7636-7639, doi.org / 10.1002 / anie.200702661 ], though the high temperature caused both a partial degradation of glucose and the hydrogenolysis of sorbitol.

[0014] Several Ru catalysts supported on acidic carriers such as sulfonated carbon [Han, J. W.; et al. Catal. Commun. 2012, 19, 115-118. doi.org / 10.1016 / j.catcom.2011.12.032], phosphate [Xi, J.; et al. Appl. Catal. A Gen. 2013, 459, 52-58. doi.org / 10.1016 / j.apcata.2O13.03.047] and molecular sieves allowed significant improvements by making the reaction possible at lower temperatures and pressures (< 200 °C, 30-50 bar H2) and with a higher sorbitol yield up to 71 %.

[0015] Notwithstanding this, the process was slow due to the moderate acidity of the support that brought about a low hydrolysis rate. More performant catalysts were obtained using acid-Ru binary systems where the ratio of acidity to reduction activity could be adjusted: for example, heteropolyacids coupled with Ru / C could effectively improve cellulose conversion, giving a mixture of sorbitol and mannitol in 68% yield in only 1 h, at 180 °C under 50 bar H2 [Geboers, J.; et al. Green Chem. 2014, 16 (6), 3305-3312. doi.org / 10.1039 / c3gc42444h].

[0016] The poor water solubility of heteropolyacids, however, made difficult the catalyst handling / recovery, thereby hampering any large scale application of the procedure. To improve the reusability of the solid acid, zirconium phosphate (ZrP) instead of heteropolyacids was considered, in combination with Ru / C. This system was apparently highly active affording 85% yield of Ce alcohol in 2.5 h, at 190 °C and 50 bar H2[Liao, Y.; et al. Green Chem. 2014, 16 (6), 3305-3312. doi.org / 10.1039 / c3gc42444h] but it required an acidic pre-treatment of cellulose to reduce its crystallinity; otherwise, the rate-determining hydrolysis step was problematic. Deng et al. reported that the crystallinity of cellulose could be decreased by treating it with phosphoric acid. After this preliminary step, a 69% sorbitol yield was reached via a hydrolytic hydrogenation catalysed by Ru / CNT (carbon nanotubes) at 185 °C and pH2=50 bar [Deng, W.; et al. Catal. Letters 2009, 133 (1-2), 167-174. doi.org / 10.1007 / s10562-009-0136-3], Mechanochemical treatments were also evaluated to reduce the crystallinity index of cellulose, especially by ball-milling. A comparative analysis demonstrated that a catalytic mixture of Ru / C and L SiW-^C o allowed an 85% and a 36% yield of sugar alcohols starting from ball-milled cellulose and pristine microcrystalline cellulose, respectively [Geboers, J.; et al. Green Chem. 2014, 16 (6), 3305-3312. doi.org / 10.1039 / c3gc42444h]. In another work by Pereira et al., conversion close to 90% with 80% selectivity to sorbitol were reported by ball-milling of Ru / C and cellulose together, at 205 °C under 50 bar H2in 1 h [Ribeiro, L. S.; et al. Green Chem. 2015, 17 (5), 2973-2980. doi.org / 10.1039 / c5gc00039d].

[0017] The Chinese patent CN 10614 625 B describes a process for the preparation of sorbitol through the hydrogenation of cellulose, in the presence of compressed CO2using Pt-Ni supported on mesoporous zeolite (MESHZSM-5) as a catalyst. The disclosed process is carried out with a partial pressure of 5-60 bar of H2and with a partial pressure of 10-30 bar of CO2, for a reaction time of 10-300 minutes.

[0018] Whichever the approach, the use of acids, both as solid and even more as liquids, always implies concerns due to safety, corrosion (especially at high temperatures and pressure), and disposal. Therefore, the design of a more sustainable and low-environmental impact process for the direct conversion of cellulose into sorbitol remains a highly desirable target of a modem biorefinery.

[0019] SUMMARY OF THE INVENTION

[0020] Subject matter of the present invention is an efficient and robust one-step CO2- assisted hydrolytic hydrogenation process for the conversion of cellulose into sorbitol, said process comprising contacting cellulose or a cellulose-based substrate in water with 25-35 bar of H2 and 35-45 bar of CO2 in presence of a catalytic amount of a metal catalyst suitable for hydrogenation reactions at 200-250°C for at least 18 h to obtain sorbitol. Under these conditions, cellulose was first hydrolysed to glucose by reversibly formed carbonic acid in water and then instantaneously hydrogenated on the metal catalyst, preferably Ru / C.

[0021] Advantageously according to the present invention it was possible to obtain sorbitol in 81 % yield when starting from microcrystalline cellulose and in yield ranging from 56% to 72% when starting from a cellulose-based substrate, e.g. filter paper, cotton wool, cotton fiber and a pizza cardboard box.

[0022] Moreover, the use of pressurised CO2, as exclusive agent able to generate weakly acidic aqueous solutions by the formation of carbonic acid, avoids any issue related to more conventional acids, either liquids or solids, and more importantly, the acidity is reversible since carbonic acid is completely removed by venting CO2 from the reactor. This simple operation does not imply solvent discharge or additional treatments, and (weak) H2CO3 strongly limits any corrosion issue. Last but not least, CO2 is nontoxic and as a by-product of biorefinery processes for fuels and chemicals, it is available at almost no cost.

[0023] It therefore appears evident that CO2 is the only acidifying agent used in the process according to the present invention capable of promoting the hydrolytic step; while the metal catalyst is chosen from those known capable of promoting hydrogenation. DETAILED DESCRIPTION OF THE INVENTION

[0024] According to the invention a cellulose-based substrate is any substrate made of or comprising cellulose, not limiting examples are filter paper, paper, cotton wool, cotton fiber and a cardboard box. Preferably, in the process according to the invention, the metal catalyst is selected in group VI IB, VI I IB, IB or I IB of the periodic table, preferably the metal catalyst is selected in the group consisting of Ru, Pt, I r, Ni, Rh and Au, more preferably is Ru / C. According to the invention the metal catalyst can be homogeneous or heterogenous optionally supported on carbon or other suitable solid support. Preferably the metal catalyst is selected among those already known or even commercially available.

[0025] Preferably according to the process of the invention the temperature is 210-230 °C, more preferably 215-225 °C, even more preferably 220 °C.

[0026] Preferably according to the process of the invention the H2 partial pressure is 27-33 bar, more preferably 28-32 bar, even more preferably 30 bar.

[0027] Preferably according to the process of the invention the CO2 partial pressure is 37- 43 bar, more preferably 38-42 bar, even more preferably 40 bar.

[0028] Preferably according to the process of the invention the time is 18-30 h. more preferably 22-26 h, even more preferably 24h.

[0029] Preferably according to the process of the invention the catalyst is employed in an amount of 1 mg every 2 mg of the initial cellulose or cellulose-based substrate. The catalyst is preferably recycled, more preferably at least 6 times.

[0030] Preferably according to the process of the invention the water is used in an amount of 1 mL every 20 mg of cellulose or cellulose-based substrate.

[0031] The invention will be better understood in view of the following experimental section.

[0032] BRIEF DESCRIPTION OF THE FIGURES

[0033] Figure 1 - Effect of a) temperature; b) time and c) CO2 pressure in the hydrolysis of D-maltose. Reaction conditions: maltose (100 mg), H2O (5 mL). Conversion and selectivity were determined by HPAEC-MS.

[0034] Figure 2 - Effect of a) time and b) temperature in the CO2-assisted hydrogenation of maltose into sorbitol. Reaction conditions: maltose (100 mg), H2O (5 mL), 40 bar CO2, 30 bar H2, Ru / C (50 mg). Conversion and selectivity were determined by HPAEC-MS.

[0035] EXPERIMENTAL SECTION

[0036] Materials and equipment. D-(+)-maltose (>99%), microcrystalline cellulose, 5% Ru / C (lot #MKBW5890V), were commercially available compounds sourced from Sigma-Aldrich. If not otherwise specified, reagents were employed without further purification. Water was Milli-Q grade. H2 gas were purchased from SIAD, Italy. Quantitative analyses were performed by high pressure anion exchange chromatograph an ion chromatograph (Thermo Scientific™ Dionex™ ICS-5000) coupled to a single quadrupole mass spectrometer (Thermo Scientific™ MSQ Plus™) (HPAEC-MS). The crystallinity index of cellulose samples was investigated by X-Ray Diffraction (XRD) in the D8 Advance diffractometer of Broker® AXS, using the X-Ray source of the Cu Ka radiation, coupled to a Lynxeye detector, and monitoring the 29 within 10-80° at a rate of 0.08° min-1. All reactions were performed in duplicate to verify reproducibility.

[0037] Typical CO2-assisted hydrogenation experiments. Experiments were performed in a 25-mL tubular reactor of borosilicate glass (Pyrex), which was charged with microcrystalline cellulose (100 mg), 5% Ru / C (50 mg) and water (5 mL). The vessel was placed in a jacketed stainless-steel autoclave equipped with a manometer and two needle valves, and pressurised with hydrogen to 30 bar and CO2 to 40 bar (the final pressure was given by pH2 + PCO2 at room temperature). The autoclave was then heated by oil circulation at T= 150-250 °C, and the mixture was kept under magnetic stirring at a rate of 1500 rpm. After the desired reaction time (24h), the autoclave was cooled to room temperature and gently purged. The catalyst (Ru / C) the catalyst was filtered on PTFE (0.2 pm) and the products solutions was analysed by HPAEC-MS.

[0038] Products analysis. Cellulose conversion (CCellulose, %) was determined based on the weight of cellulose utilized in the reaction (mCellulose,0) and the solid recovered after reaction, taking into consideration the fraction of the solid catalyst in the remainings with m ceiiuiose=rriRecovered solid - mcataiyst (eq.1 ). 100

[0039] (eq 1 )

[0040] Products analysis and quantification was performed according to a validate method by Barbara et al. [Atmos. Environ. 2015, 118, 135-144. doi.org / 10.1016 / j.atmosenv.2015.07.047]

[0041] EXAMPLE 1 - C02-assisted Hydrolysis of D-Maltose into Glucose. Exploratory tests to investigate the use of wet CO2 were performed by employing D- (+)-maltose as a model compound. The CO2-assisted hydrolysis of maltose to glucose was studied (Scheme 2).

[0042] Maltose Glucose

[0043] Scheme 2. CO2-assisted hydrolysis of D-Maltose.

[0044] Experiments were carried out in a stainless-steel autoclave in which an aqueous solution of maltose (maltose: 100 mg; H2O: 5 mL) was set to react in the presence of CO2. The effects of temperature (T), time (h) and CO2 pressure (p) were investigated through three series of tests by varying: i) T in the range 25-150 °C at constant CO2 pressure (40 bar) and time t= 2h; ii) time in the range 2-15h at constant pressure (40 bar) and temperature (150 °C); iii) p in the range 5-40 bar, at constant temperature (150 °C) and time (12h). Maltose conversion and glucose selectivity were determined by HPAEC-MS. All the reported reactions were run in duplicate to ensure reproducibility: unless otherwise specified, conversions and selectivity differed by less than 5% from one test to another. Results are reported in Figure 1 . No maltose conversion was observed at pCC>2 = 40 bar in 2 h in the temperature range 25 - 100 °C; however, increasing T from 120 to 150 °C prompted the gradual increase in the conversion of maltose from 7% to 35%, respectively (Figure 1 , a). No products other than glucose, which was obtained in >99% selectivity, were observed under these conditions. Having set 150 °C as the operative temperature to continue this investigation, the effect of time was explored (Figurel , b). Increase in the reaction time caused an increase in maltose conversion, which became quantitative after 12 h. A small, but not negligible, amount of fructose (1 -2%) due to the isomerization of glucose was also detected. No further increase in conversion and selectivity was observed by extending the reaction time to 15 h. This allowed us to set T=150 °C and t=12 h as the conditions to study the effect of CO2 pressure (Figure 1 , c). Notably, high maltose conversion (ca. 68-70%) was achieved even at the lowest investigated CO2 pressure (5 bar), but only at to 40 bar, the reaction became quantitative with an excellent glucose selectivity (98-99%). A blank experiment was performed at 150 °C for 12 h in the absence of CO2. The reaction reached only 27% maltose conversion to glucose, thereby confirming the crucial role of the acidity provided by carbonic acid. The limited extent of the hydrolysis process observed without CO2, was due to thermal instability of the 3-O-4 glycosidic bond. In summary, parametric analysis showed that the CO2-assisted hydrolysis of maltose was strictly dependent on the reaction conditions: however, 98-99% glucose selectivity was obtained with quantitative maltose conversion, at 150 °C, under 40 bar CO2 for 12h.

[0045] EXAMPLE 2 - CO2-assisted Hydrolysis / Hydrogenation of D-Maltose into Sorbitol.

[0046] An initial experiment aimed at exploring the direct conversion of maltose into sorbitol was designed under the best conditions observed in example 1 for maltose hydrolysis (maltose: 100 mg, H2O: 5 mL, 150 °C, 40 bar CO2, 12 h) with the addition of Ru / C (50 mg) as a hydrogenation catalyst and hydrogen (30 bar). The final pressure was given by pH2 + PCO2 at room temperature, i.e. 70 bar. The reaction allowed a quantitative conversion of maltose into a mixture of maltitol (24%) and sorbitol (76%), respectively. Given this promising result, further tests were carried out by varying both the reaction time and the temperature. Results are reported in Figure 2.

[0047] At 150 °C, the prolongation of the reaction from 12, to 15, 18 and 24 h induced a gradual increase in sorbitol selectivity, up to 87%, at the expenses of maltitol (Figure 2a). The desired process was further favoured by slightly increasing the reaction temperature, from 150 to 170 °C: at quantitative conversion, sorbitol was obtained with an excellent 96% selectivity (Figure 2b).

[0048] EXAMPLE 3 - CO -assisted Hydrolysis / Hydrogenation of Cellulose into Sorbitol.

[0049] Cellulose Sorbitol Based on the results obtained in example 2 for the CO2-assisted hydrolytic hydrogenation of maltose, the next step was to apply the reaction to more complex cellulose-based substrates.

[0050] A suspension of microcrystalline cellulose (100 mg) in 5mL H2O was set to react under 40 bar CO2 and 30 bar H2 for 24 h, in the presence of Ru / C (50 mg). Different temperatures in the range 150-250 °C were explored. Cellulose conversion was determined gravimetrically by the difference in weight of cellulose employed in the reaction and the solid recovered after the reaction, taking into consideration also the weight of the solid catalyst. The yield of water-soluble polyols as sorbitol, mannitol and C4-C5 sugar alcohols was determined by HPAEC-MS (a detailed HPAEC-MS quantification protocol is reported in the SI Section). Results are reported in Table 1.

[0051] Table 1. CO2-assisted hydrolysis / hydrogenation of cellulose into sorbitol.

[0052] At the lower temperature of 150 °C, the conversion of cellulose reached 32%, but the observed products which included sorbitol, mannitol (as an isomerization product) and a mixture of C4-C5 polyols (hydrogenolysis products as erythritol, xylitol and arabitol), were obtained in poor, if not negligible yields of 7%, 2% and <1 %, respectively (entry 1 ). This result was ascribed to the formation of water-soluble oligomers which could not be detected by HPAEC-MS. An increase in the reaction temperature from 150 to 180 and 200 °C improved the conversion to >99%, and considerably favoured the formation of sorbitol which was achieved in up to 67% yield (entries 2-3). The concurrent formation of small amounts of mannitol (4%) and C4-C5 polyols (5%) was also observed. Finally, the yield of sorbitol was further increased to 81 % at 220 °C (entry 4), one of the best results reported so far for this reaction. Raising T to 250 °C, however, brought about an increase of hydrolysis products (C4-C5 sugar alcohols) which were obtained in a 26% yield at the expenses of sorbitol (54%, entry 5).

[0053] In summary, the hydrolytic hydrogenation protocol proved to be highly efficient in the conversion of cellulose into sorbitol, by using CO2 as acid precursor and molecular hydrogen as a reductant. Sorbitol was obtained with the highest yield of 81 % at 220 °C, under 40 bar CO2 and 30 bar H2 for 24 h. The formation of hydrogenolysis products such as erytritol, xylitol and arabitol (not exceeding 7% yield) was also observed.

[0054] Other commercially available carbon-supported metal catalysts typically employed for the conversion of cellulose into sorbitol including 5% Pt / C, 5% Pd / C and Raney Ni were also tested under the optimized reaction conditions found for Ru / C, i.e. 220 °C, under 40 bar CO2 and 30 bar H2 for 24 h. These catalytic systems proved to be highly efficient to this purpose, allowing the complete cellulose conversion in all cases. However, the sorbitol yield dramatically changed. More in details, over Pt / C a sorbitol yield of 68% was achieved. On the other hand, when Pd / C and Raney Ni were tested, sorbitol was obtained in 61 % and 57% yield, respectively.

[0055] EXAMPLE 4 - Catalyst Recycling

[0056] The cost of the catalyst in a liquid-phase reaction may represent up to a third of the total cost of the process, implying that its loss by leaching or other reasons is critical, and its recovery and reuse are crucial.

[0057] The stability and reusability of Ru / C were therefore investigated by designing recycling experiments under the conditions of Table 1 , entry 4 (cellulose: 100 mg, Ru / C: 50 mg, H2O: 5 mL, 220 °C, 40 bar CO2, 30 bar H2, 24 h). Once completed the first reaction, the catalyst was filtered off, washed with distilled water (30 mL) and dried overnight. The recovered catalyst was added to fresh microcrystalline cellulose (100 mg) and water (5 mL) and a new reaction was started. The recycling procedure was repeated six times, and the whole set of reactions was run twice to ensure reproducibility. The results are illustrated in Figure 3. Both the cellulose conversion and the sorbitol yield were steady at 97-99% and 77-81 %, respectively, during the six runs, thereby demonstrating that the overall performance of Ru / C was not altered over time by the reaction environment during the recycling tests and by the washing / restoring procedures.

[0058] EXAMPLE 5 - CO -assisted Hydrolysis / Hydrogenation of Cellulose Feedstock / Waste into Sorbitol

[0059] Other real-world cellulose sources were explored to investigate the applicability and scalability of the CO2-assisted hydrolysis / hydrogenation process of the invention. To the scope, cheap and largely available cellulosic feedstocks, including filter paper, cotton wool, cotton fiber and a common cellulose-based waste as pizza carton made of cardboard were selected. A mixture of finely ground cellulose feedstock (100 mg, size < 1 mm), Ru / C (50 mg) and H2O (5 mL) was set to react at 220 °C for 24 h, under 40 bar CO2 and 30 bar H2. Conversion and yield were determined by the same method described previously. Results are summarized in Table 2.

[0060] Table 2. CO2-assisted hydrolysis / hydrogenation of different cellulose feedstock.

[0061] Firstly, the commonly employed laboratory filter paper was tested. Such starting material was fully converted, allowing sorbitol formation in 62% yield with the concurrent formation of mannitol (3%) and, if compared with microcrystalline cellulose, a higher amount of C4-C5 polyols (21 %). A remarkable improvement in the sorbitol yield was observed when cotton wool and cotton fibers were tested. In these cases, sorbitol was obtained in 71 % and 72% yield, respectively with an almost equal amount of mannitol (5% and 4% respectively) and C4-C5 polyols (7% and 6%, respectively), while the conversion remained stable and quantitative in both cases. The pizza carton made of cardboard was also completely converted, with a sorbitol yield of 56%, while mannitol and C4-C5 polyols were observed in 6% and 13% yield, respectively. Overall, these results not only confirmed that the process of the invention was effective for the direct conversion of cellulose into sorbitol, but they also proved that the process was successfully applied to a wide range of cellulose feedstocks.

Claims

CLAIMS1 . A one-step hydrolytic hydrogenation process for the conversion of cellulose into sorbitol, said process comprising contacting cellulose or a cellulose-based substrate in water with 25-35 bar partial pressure of H2 and 35-45 bar partial pressure of CO2 in presence of a catalytic amount of a metal catalyst suitable for hydrogenation reactions at 200-250°C for at least 18 h to obtain sorbitol.

2. The process according to claim 1 , wherein CO2 is the only acidifying agent used in the aqueous solution.

3. The process according to claim 1 wherein the metal catalyst is selected in group VIIB, VII IB, IB or IIB of the periodic table, preferably the metal catalyst is selected in the group consisting of Ru, Pt, Ir, Ni, Rh and Au.

4. The process according to any one of claims 1 -3 wherein the metal catalyst is supported on carbon.

5. The process according to claim 4 wherein the metal catalyst is Ru / C.

6. The process according to anyone of claims 1-5 wherein the temperature is 210-230 °C, preferably 215-225 °C.

7. The process according to any one of claims 1 -6 wherein the H2 partial pressure is 27-33 bar, preferably 28-32 bar.

8. The process according to any one of claims 1 -7 wherein the CO2 partial pressure is 37-43 bar, preferably 38-42 bar.

9. The process according to any one of claims 1 -8 wherein the time is 18-30 h. preferably 22-26 h.

10. The process according to any one of claims 1 -9 wherein the catalyst is employed in an amount of 1 mg every 2 mg of the initial cellulose or cellulose-based substrate.

11. The process according to any one of claims 1 -10 wherein the catalyst is recycled, preferably at least 6 times.

12. The process according to any one of claims 1 -11 wherein the water is used in an amount of 1 mL every 20mg of cellulose-based substrate.

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