(98c) Effective Acidic Hydrothermal Degradation of Cellulose to Levulinic Acid through Pretreatment

The annual energy consumption in the world is increasing each year. We mostly rely on the fossil resources for the energy product and the mass consumption of these resources, leading to the exhaustion of them, is considered as the issue. To get out of the fossil resource society, biomass would be one of the solutions. Biomass is possibly utilized as an alternative energy resource or an alternative raw material to fossil resources. However, as the source of energy, biomass has huge barriers such as the relatively low energy content, seasonality, discrete geographic availability, high moisture content or high collecting cost. As an alternative raw material, on the other hand, biomass would be beneficial resource because chemical production requires far lower volumes of biomass to satisfy demand. The US Department of Energy identified 12 chemicals which should be made from biomass. These chemicals, which could be used as building block chemicals, potentially enable biomass to be main resource of making chemicals which derive from petroleum for now. Levulinic acid is one of these 12 chemicals and is able to be obtained through the hydrolysis of some kinds of saccharides such as glucose or fructose. The use of levulinic acid is quite various, for example, it can be the materials of nylon-like polymer, artificial rubber or plastics. To obtain levulinic acid from biomass, we need the degradation of cellulose, one of the main components of biomass, via glucose. Concerning the degradation of cellulose to glucose, enzymatic saccharification is one of the well-known methods. The huge advantage of enzymatic saccharification is high selectivity. However, the bioprocess requires high cost because we need high technology of sewage treatment and complicated control of reactor. In addition, sludge produced through the process decreases the efficiency and causes long reaction time. Considering these disadvantages of biochemical technology, we need to develop thermochemical technology. Under the present circumstances, thermochemical technology has the disadvantage of low selectivity. To overcome this situation, we pretreated cellulose to convert it into desirable, degradable precursor once for the following acidic thermochemical treatment, and investigated the effect of pretreatment on the yield and selectivity of levulinic acid.

Effect of pretreatment and difference between reagents.  Filter paper cellulose was pretreated in a sealed batch reactor at 50 ℃ for 24 h using H₃PO₄ or HCl as a reagent. After the pretreatment, pure water was added to the sample. As the second treatment, the reactor vessel was heated to 160-220 ℃ for 120 min. Products were analyzed using TOC analyzer and ion chromatography to identify the carbon conversion into solution and levulinic acid. To examine the effect of pretreatment, we conducted the experiment without the pretreatment for comparison. As the results, the main products through the treatment were levulinic acid, formic acid, glucose and cellobiose, and a little amount of HMF was also preduced. However, the yields of formic acid, glucose and cellobiose were very slight compare to that of levulinic acid, so that we discuss only about the yield of levulinic acid. Pretreated cellulose increased the carbon conversion into soluble component at all the hydrothermal temperature. The yield of levulinic acid was also increased through pretreatment at most by 10 C-% at 200 ℃ acidic hydrothermal degradation, reaching to 40 C-%. The selectivity also reached the highest score 55 % at 200 ℃ degradation with pretreatment, which is defined as (the yield of levulinic acid [C-%]) / (soluble fraction [C-%]). All the results indicate that the pretreatment worked effectively concerning the treatment with H₃PO₄. Then we conducted the same experiments using HCl as the reagents instead of H₃PO₄. Pretreated cellulose turned into soluble components more than untreated cellulose but the increase was quite slight compared to the case with H₃PO₄. However, the increase of the yield of levulinic acid was remarkable, especially at 200 ℃ and 210 ℃ by 17 C-% and 10 C-% respectively. The yield reached to 49.2 C-% at 210 ℃ of acidic hydrothermal degradation with the selectivity of 62.1 %. Comparing the results of H₃PO₄ and HCl, all the scores are higher when HCl was used as a reagent. As the summary of the section, the pretreatment with acid is very effective on producing levulinic acid from cellulose. As the result, two step treatment with HCl enhanced the yield of levulinic acid reaching to 49.2 C-% with the selectivity of 62.1 % at 210 ℃ acidic hydrothermal degradation.

Treatment with ionic liquid. Solubilization of cellulose with ionic liquid enables some reagent such as enzymes or acids to attack the active cites of cellulose more effectively. In addition, solubililzed cellulose is easily recovered as deposit by the addition of non-solubilizing reagent and this deposit is supposed to be changed to amorphous structure which has higher reactivity than crystalline cellulose. In this study, we chose 1-ethyl-3-methylimidazolium bromide ([EMIM]Br) and 1-ethyl-3-methylimidazolium methylphosphonate ([EMIM]P) for ionic liquid.

Treatment with [EMIM]Br. Cellulose solubilized with this ionic liquid is known to be recovered by the addition of water. We focused on this unique character which enables uncrystallization and easy separation of cellulose. As for the pretreatment, cellulose and [EMIM]Br were mixed and heated to 80 ℃ for 6 h. Immediately after heating, the products were cooled, then pure water was added for the extraction of the solute. The deposit appears as Br-cellulose hereafter. As for acidic hydrothermal treatment, Br-cellulose and 5 wt% H₂SO₄ were mixed and heated to 220 ℃ for 2 h. Products were analyzed using HPLC. As the results, after the pretreatment using [EMIM]Br at 80 ℃ for 6 h, no solid cellulose was recognized in the sample before the addition of pure water. Then, by the addition of the pure water, 99.0 wt% of cellulose was recovered as the deposit, Br-cellulose. Crystallinity of cellulose decreased by about 30 % through the pretreatment judging from the XRD curve. However, this decrease is not big considering that cellulose was completely solubilized once, so cellulose seemed to be recrystallized in the process of recovery. The elemental composition and TG curve showed little change through the pretreatment, which indicate that cellulose only changed its crystallinity through the pretreatment. Then, Br-cellulose was degraded under the acidic hydrothermal condition. For comparison, the acidic hydrothermal treatment was conducted using filter paper cellulose and amorphous cellulose as sample through the same method. More levulinic acid was produced from Br-cellulose than from filter paper cellulose. However, amorphous cellulose was turned into levulinic acid more than Br-cellulose, which indicates the crystallinity of cellulose is one of the important factors for producing levulinic acid.

Treatment with [EMIM]P.  [EMIM]P has very high ability of breaking the rigid structure of cellulose, which not only solubilizes cellulose but turns it into water-soluble cellulose. As for the pretreatment, cellulose and [EMIM]P were mixed and heated to 160 ℃ for 1 h. Immediately after heating, the products were cooled, then pure water was added. This cellulose solution appears as P-cellulose hereafter. As for acidic hydrothermal treatment, P-cellulose and 5 wt% H₂SO₄ were mixed and heated to 220 ℃ for 2 h. Products were analyzed using HPLC. As the results, different from the pretreatment with [EMIM]Br, cellulose pretreated at 150 ℃ for 1 h using [EMIM]P was solubilized completely and then never recovered by the addition of pure water. To see the change in crystallinity and thermo degradability, much ethanol was added to the sample and cellulose was recovered as P-cellulose-deposit. The yield of the deposit was 130.0 wt% containing 70 % of cellulose and 60 % of ionic liquid. Cellulose was uncrystallized very well and the thermal degradability was increased by the pretreatment judging from TG analyses and XRD analyses. Then, P-cellulose-deposit was degraded under the acidic hydrothermal condition using 5 wt% H₂SO₄. Since the recovery of cellulose requires plenty amounts of ethanol, we conducted the acidic hydrothermal degradation on water-added cellulose solution, P-cellulose as well. Both pretreated cellulose remarkably increased the yield of levulinic acid, especially P-cellulose reaching to 66 C-%. The yield 66 C-% is 79.2 % of maximum yield, according to the stoichiometry.

In conclusion, the pretreatment on cellulose, which is usually hard to degrade, changed its structure and enabled it to be converted into valuable chemical, levulinic acid. Especially, through the treatment with ionic liquid [EMIM]P, levulinic acid was obtained at very high yield, 66 C-%.