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The goal of the project is to develop an environmentally sound and industrially feasible cellulose hydrolysis method for the cellulosic-ethanol process. This goal would be achieved by the development of a recyclable acidic ionic liquid solvent system that can easily dissolve cellulose and act as the hydrolysis catalyst. The milestones targeted for the period were (a) Synthesis and characterization of six acidic ionic liquids and (b) Studying the dissolution and hydrolysis of cellulose in these acidic ionic liquids.  

The initial step of the project involved the synthesis of acidic ionic liquids (1-3) (Figure 1) with a sulfonic acid arm by condensation of the corresponding nitrogen bases with 1,3-propane or 1,4-butane sultones and then acidification of the salts with HCl. These ionic liquids were characterized by NMR spectroscopy.

Figure1. Acidic ionic liquids (1-3)

Dissolution of cellulose in these ionic liquids were tested at room temperature by mixing different cellulose samples and ionic liquids in glass vials and slow dissolving samples were allowed to stand at room temperature after initial mixing. Hydrolysis was carried out with or without preheating, by adding a controlled amount of water such that no precipitation of cellulose occurs, and warming the vials in a thermostated water bath. Next, the samples were diluted with water, neutralized, and then centrifuged to give clear supernatant solutions. These solutions were analyzed for total reducing sugars (TRS) using 3,5-dinitrosalicylic acid (DNS) method, and glucose by the Glucose Oxidase/Peroxidase assay using a Sigma GAGO-20 kit.

Dissolution of five different types cellulose samples in acidic ionic liquids was studied. The types of cellulose studied are; a-cellulose (DP ~ 100), microcrystalline (MC)-cellulose (DP ~ 240), Sigmacell cellulose (DP ~ 450), Whatman filter paper, and cotton wool. All five types of cellulose samples are found to dissolve in imidazolium type Brönsted acid ionic liquids (1a, b) within 2-5 min. at room temperature and atmospheric pressure.  These cellulose samples were found to dissolve in triethanolammonium (2a,b) and pyridinium ionic liquids (3a, b) when allowed to stand at room temperature for 24 hrs.           

Total amounts of reducing sugars and glucose formed in selected experiments are shown in the Table 1.

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                                                      Temperature ( °C) /time (min)                Yield (%)

                                                      before               after

Entry     IL/Cellulose                    adding H₂O      adding H₂O                    TRS      glucose

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1       1a/a-cellulose                        70/60              70/30                                  59          15

2       1a/MC-cellulose                    70/60              70/30                                  12            4

3       1a/Sigmacell                          70/60              70/30                                  62          14

4       1a/Sigmacell                              -                  70/30                                  39          12

5       1a/Sigmacell                          70/40              70/30                                  56          12

6       1a/Sigmacell                          70/60              70/60                                  42            7

7       1a/Sigmacell                          70/60              70/240                                29            4

8       1a/Sigmacell                              -                  50/960                                32            3

8       1a/Sigmacell                              -                  90/30                                  34            3

10     1a/Sigmacell                          90/30              90/30                                  26            2

11     1a/Sigmacell                              -                  90/240                                15            1

12     1b/a-cellulose                        70/60              70/30                                  32            -

13     1b/MC-cellulose                    70/60              70/30                                    7            -

14     1b/Sigmacell                          70/60              70/30                                  12            -

12     3b/a-cellulose                        70/60              70/30                                  32            -

13     3b/MC-cellulose                    70/60              70/30                                    7            -

14     3b/Sigmacell                          70/60              70/30                                  12            -

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Table 1. Average % yields of TRS and glucose produced in duplicate experiments. 10% w/w Cellulose in the acidic ionic liquid solutions, 2.0 equivalents of H₂O per glucose unit of cellulose were added in all hydrolysis experiments

Hydrolysis experiments using triethanolamine based acidic ionic liquids (2a, b) failed to give any reducing sugars. Immidazolium acidic ionic liquids gave better yields than pyridinium ionic liquids and 1-(1-propylsulfonic)-3-methylimidazolium chloride (1a) medium produced the highest yields of TRS and glucose with most of the cellulose samples studied. Both a-cellulose and Sigmacell cellulose produced moderate TRS yields, whereas microcrystalline (MC) cellulose, Whatman filter paper, and cotton wool produced poor yields in all of the hydrolysis experiments. The experiment with 1a/Sigmacell cellulose solution showed that highest TRS (62%) and glucose (14%) yields, and  was attained with 1hr. preheating at 70 °C and 30 min. heating at 70 °C, after adding water, as shown in the entry 3. Heating the sample at lower temperatures for a longer time (entry 8) or heating for a longer time at 70 °C (entry 6, 7) failed to give better yields.  Furthermore, longer heating times (entry 7) and higher temperatures (entry 10, 11) produced excessive charring of the sample, giving black residues.

In conclusion, key milestones were achieved for the first year of the project. The proposed forward path for the second year involves the following steps: (a) Optimization of the TRS and glucose yields (b) Testing the acidic ionic liquid dissolution and hydrolysis method on untreated raw biomass forms such as switchgrass and poplar (c) Recovery and reuse of the acidic ionic liquids.