Reports: UR149436-UR1: Cellulose Dissolution and Hydrolysis in Acidic Ionic Liquids

Ananda Amarasekara, PhD, 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 and second years of the project we synthesized, six acidic ionic liquids and studied the dissolution-hydrolysis of cellulose in these acidic ionic liquids.  In addition, 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 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. In the third year of the project we have studied the 1-(1-propylsulfonic)-3-methylimidazolium chloride for the hydrolysis of cellulose model compound in an attempt to study the cellulose hydrolysis mechanism.

(1). Hydrolysis of D-cellobiose using aqueous 1-(1-propylsulfonic)-3-methylimidazolium chloride and sulfuric acid solutions at 90, 105, and 120 °C

Catalytic activities of 1-(1-propylsulfonic)-3-methylimidazolium chloride was compared with the activities of sulfuric acid of the same molar H+ ion concentration, and according to Oscarson and Izatt’s expression on temperature dependence of the first and second dissociation constants of sulfuric acid in water, it is assumed that H2SO4 completely dissociates to give two H+ ions in the temperature range of the study [1, 2]. These experiments clearly showed that Brönsted acidic ionic liquid PSMIMCl has a higher catalytic activity than sulfuric acid of the same acid strength. Furthermore, catalytic activity enhancement is more significant at higher temperatures, for instance at 120 °C PSMIMCl produced 64.5% glucose yield after 40 min. reaction, whereas H2SO4 produced only 42.2% during the same period, and this is a 52.8% enhancement of catalytic activity due to the alkylimidazolium group attached to the sulfonic acid group. This catalytic activity enhancement may be due to an interaction of alkyl imidazolium group with hydroxyl groups of D-cellobiose. In fact Xiang has observed [3] similar interactions of 1-ethyl-3-methylimidazolium acetate with D-cellobiose in DMSO using 13C NMR spectroscopy.  Additionally, the glucose yield showed slight decrease with longer reaction times, especially at higher temperatures, and this could be due to well known decomposition of glucose to various products like 5-hydroxymethulfurfural, 1,6-anhydroglucose, levulinic acid, and formic acid in the acid medium [4,5].


(2). 1H NMR Studies of D-cellobiose hydrolysis in D2O using1-(1-propylsulfonic)-3-methylimidazolium chloride and sulfuric acid-d2 as catalysts

A series of 1H NMR spectra were recorded during the hydrolysis of D-cellobiose in 0.0321 mol H+/L 1-(1-propylsulfonic)-3-methylimidazolium chloride in D2O medium. D-Cellobiose sample heated in sulfuric acid-d2 medium also used to produce a similar series of spectra. The spectrum recorded at t = 0 h. showed a mixture of a and b anomers of D-cellobiose and only a trace amount of D-glucose. The anomeric composition of D-cellobiose in this mixture was calculated as a : b = 1.00 : 1.60 using the peak area ratio of two doublets at 4.97 and 4.40 ppm spectrum, and this value is compatible with the reported anomeric ratio [6] of D-cellobiose in water. A gradual increase in D-glucose and the disappearance of D-cellobiose is seen in the series of spectra recorded during the course of the reaction. 

Table 1


Time  Catalyst

h.  PSMIMCl  H2SO4

a : b  a : b


2  1:1.56  1:1.62 

6  1:1.65  1:1.58

12  1:1.60  1:1.59 

24  1:1.63   1:1.65


Calculated anomeric ratios of D-glucose produced remains approximately constant during the course of the reactions as seen in table1, and this may be due to a rapid equilibration of D-glucose formed. 1H NMR study of ionic liquid failed to reveal any reaction intermediates, and similarly NMR spectra of H2SO4 catalyzed reaction also showed no reaction intermediates. This can be explained by a mechanism involving a slow protonation of glycosidic oxygen in D-cellobiose and fast attack of water on the anomeric carbon, resulting fast hydrolysis D-cellobiose to D-glucose. In conclusion, we have shown that Brönsted acidic ionic liquid1-(1-propylsulfonic)-3-methylimidazolium chloride (PSMIMCl) has a higher catalytic activity than sulfuric acid in the hydrolysis of D-cellobiose to D-glucose in water at 90-120 °C. This catalytic activity enhancement is more significant at higher temperatures, and may be due to an interaction of alkylimidazolium group with D-cellobiose. 1H NMR monitoring of the PSMIMCl as well as sulfuric acid catalyzed D-cellobiose hydrolysis reactions at 90 °C failed to show any intermediates in the reaction, and this is possibly due to fast conversion of any intermediate(s) to product.



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