Ananda S. Amarasekara, Ph. D
, Prairie View A&M University
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 system that can act as the hydrolysis catalyst. During the first year of the project we synthesized and characterized six acidic ionic liquids and studied the dissolution and hydrolysis of cellulose in these acidic ionic liquids. In the second phase of the project we have studied the possibility of using these ionic liquids in catalytic amounts in the aqueous medium for the hydrolysis of cellulose at moderate temperatures and pressures. The most effective ionic liquid from the first year study, 1-(1-propylsulfonic)-3-methylimidazolium chloride (1) was chosen for the initial study and we have compared the catalytic activity of this ionic liquid with aqueous sulfuric acid and p-toluenesulfonic acid solutions of the same molar H+ ion concentration.
Stock solutions of the acids, 1-(1-propylsulfonic)-3-methylimidazolium chloride, p-toluenesulfonic and sulfuric acid were prepared by dissolving appropriate amounts of these acids in deionized water to give acid concentration of 0.0321 mol H+/L in each solution. The accuracy of the concentrations was checked by titration with standardized aq. NaOH solution using phenolphthalein as the indicator. Sigmacell cellulose-type 101 (DP ~ 450) (0.030 g, 0.185 mmol of glucose unit of cellulose) were used in all experiments and was suspended in 2.00 mL of aqueous acid solution in a 25 mL high pressure stainless steel reaction kettle with Teflon inner sleeve. The reaction kettle was firmly closed and heated in a thermostated oven maintained at the desired temperature for a specified length of time for the degradation experiments. Then reaction kettle was removed from the oven and immediately cooled under running cold water to quench the reaction. The contents were transferred into a centrifuge tube and diluted to 10.0 mL with deionized water, neutralized by drop wise addition of 0.5 M aq. NaOH, and centrifuged at 3500 rpm for 6 min. to precipitate the solids before total reducing sugar (TRS) determination using 3,4-dinitrosalicylic acid (DNS) method. The glucose formed was measured using glucose oxidase/peroxidase enzymatic assay.
These experiments showed that cellulose samples heated in aqueous 1, and aqueous p-toluenesulfonic acid mediums at 150-170 °C temperature range produces higher total reducing sugar yields compared to the samples heated in aqueous sulfuric acid solutions of the same molar H+ ion concentration . At 170 °C, cellulose heated in aqueous p-TsOH, and ionic liquid 1 showed TRS yields of 32.6, and 28.5% respectively, compared to 22.0 % yield of TRS produced in aq. H2SO4, under identical conditions. This is a 48, and 30% enhancements of the catalytic activities in the degradation of cellulose due to the respective tolyl, and alkylimidazolium groups attached to the -SO3H function. This catalytic activity enhancement may be due to an adsorption of p-toluenesulfonic acid and 1-(1-propylsulfonic)-3-methylimidazolium chloride on the cellulose surface, which allows the facile disruption of the cellulose structure and hydrolysis of the glycosidic bonds. In a recent study on comparison of Pinaus radiate bark liquefacation in phenol using p-TsOH, and H2SO4 as catalysts, Mun has found  that in p-TsOH catalyzed liquefacation, cellulose residue had a lower crystallinity than in the H2SO4 catalyst samples. These results also support our observation of enhanced catalytic activity in the p-TsOH catalyzed degradation of cellulose.
It is interesting to note that total reducing sugar yields rapidly decreases above 170 °C in p-TsOH, and in ionic liquid 1 mediums, whereas the samples heated in aq. sulfuric acid continued to give constant TRS yields of about 22%. This may be due to decomposition of glucose and higher oligomers into non reducing sugar products in p-TsOH, and in ionic liquid 1 mediums at higher temperatures.
In conclusion, all targets were achieved for the second year of the project. We have found that aqueous solutions of acidic ionic liquids as well as p-TsOH are more effective catalysts than aqueous sulfuric acid of the same acid concentration. This important discovery can be considered as a lead for the development of a new catalytic system that can behave as a synthetic enzyme for the degradation of cellulose in aqueous medium. The proposed forward path for the third year involves the following steps: (a) Optimization of the TRS and glucose yields (b) Testing the aqueous solutions of acidic ionic liquid for the hydrolysis of untreated raw biomass forms such as switchgrass and poplar (c) Studying structural variations of acidic ionic liquids to improve the catalytic activity in the aqueous medium.
. Mun, S. P.; Jang, J. P. Liquefaction of cellulose in the presence of phenol using p- toluenesulfonic acid as a catalyst. J. Ind. Eng. Chem. Res. 2009, 15, 743-747.