58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

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 catalyst system that can act as the hydrolysis catalyst. During the early stage 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 next phase of the project we have studied the use of neat acidic ionic liquids in the degradation of untreated biomass forms, as well as use of aqueous solutions of acidic ionic liquids with sulfonic acid groups, and alkyl/aryl sulfonic acids for hydrolysis of pure cellulose and lignocellulosic biomass. Two Graduate students worked on the project graduated during the period, and two undergraduates worked on the project during the academic year. The two main topics studied during the reporting period involves the degradation of switchgrass in acidic ionic liquids and use of ionic liquids as catalysts in aqueous ethanol medium.

(1). Degradation of untreated switchgrass biomass into reducing sugars in 1-(alkylsulfonic)-3-methylimidazolium Brönsted acidic ionic liquid medium under mild conditions

Switchgrass biomass samples collected at three different stages of maturity were seen degrading into reducing sugars and glucose when exposed to 1-(alkylsulfonic)-3-methylimidazolium Brönsted acidic ionic liquids under thermal and microwave conditions. The highest reducing sugar (58%) and glucose (15%) yields were obtained for switchgrass samples dissolved in  1-(butylsulfonic)-3-methylimidazolium chloride ionic liquid by heating at 70 °C for 1 hr. followed by treatment with 0.22 g water/g switchgrass, and then heating at 70 °C for  1 hr. for the hydrolysis of polysaccharides. The samples treated under microwave conditions produced relatively lower yields of reducing sugar (22-37%) and glucose (8-13%) yields, compared to heat treated samples.

RESULTS

Degradation of untreated switchgrass biomass into reducing sugars in 1-(alkylsulfonic)-3-methylimidazolium Brönsted acidic ionic liquid medium under mild conditions

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                                                                     Temp. ( °C) /time (min)              Yield (%)

                                    Ionic                           before             after                                                      

Entry  Switchgrass      liquid                       adding H₂O     adding H₂O         TRS   glucose       

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1              PV-1              1a                              70/60              70/60                  56       10         

2              PV-2              1a                              70/60              70/60                  48       13            

3              PV-3              1a                              70/60              70/60                  47       11         

4              PV-1              1b                              70/60              70/60                  50       10         

5              PV-2              1b                              70/60              70/60                  45       14           

6              PV-3              1b                              70/60              70/60                  58       15         

7              PV-3              1b                              70/60              70/30                  46        6  

8              PV-3              1b                              70/60              70/120                36        5         

9             PV-3               1b                               70/60             70/240                 32       5       

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Table 1. Average percent yields of TRS and glucose produced in duplicate experiments of degradation of untreated switchgrass under heat treatment conditions. 0.030 g of dried switchgrass powder in 0.300 g of Brönsted acidic ionic liquid and 6.7 mL of H₂O added in all experiments

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                                                                   Total microwave time (s)                 Yield (%)

                                      Ionic                      before             after                                                      

Entry    Switchgrass      liquid                   adding H₂O    adding H₂O              TRS   glucose       

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10              PV-1              1a                           60               60                               37       11         

11              PV-2              1a                           60               60                               25       13           

12              PV-3              1a                           60               60                               28       10         

13              PV-1              1b                           60               60                               26        8        

14              PV-2              1b                           60               60                               33       13           

15              PV-3              1b                           60               60                               34        9         

16              PV-1              1a                           60               30                               34       10         

17              PV-2              1a                           60               30                               22       11           

18              PV-3              1a                           60               30                               23         9         

19              PV-1              1a                           60               90                               35       10         

20              PV-2              1a                           60               90                               22         8            

21              PV-3              1a                           60               90                               24         9         

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Table 2. Average percent yields of TRS and glucose produced in duplicate experiments of degradation of untreated switchgrass under microwave conditions. 0.030 g of dried switchgrass powder in 0.300 g of Brönsted acidic ionic liquid and 6.7 mL of H₂O added in all experiments

(2). 1-(alkylsulfonic)-3-methylimidazolium Brönsted acidic ionic liquid catalyzed degradation cellulose in aqueous ethanol medium.

In these studies we have found that cellulose can be converted to a mixture of ethyl levulinate and levulinic acid by heating with a Brönsted acidic ionic liquid catalyst in aqueous ethanol medium in a one-pot operation under mild conditions. The highest ethyl levulinate yield of 15.1% was obtained for a reaction carried out at 150 °C, for 48 hr, in water-ethanol medium containing 23% water and using 1-(1-propylsulfonic)-3-methylimidazolium chloride as the catalyst. The levulinic acid yields continues to increase with increasing water content up to about 54% water in aqueous ethanol for reactions carried out at 150 °C for 48 hr, and the highest levulinic acid yield was 23.7%. The acidic ionic liquid catalyst used can be efficiently recovered (96%) from the water phase with negligible contamination and the stability of the catalyst was confirmed by comparison of the ¹H NMR spectrum of the recovered catalyst with fresh catalyst.