(687f) A New Route to Improved Glucose Yields in Cellulose Hydrolysis

Cellulose is the most abundant renewable carbon source available and can play a role in meeting our future energy needs if its reactivity can be improved. The crystal structure and hydrogen bonding in cellulose greatly limit access to b-1,4-glycosidic bonds by reactants and catalysts. Water is excluded almost completely from the crystalline regions in cellulose. This limitation makes cellulose hydrolysis much slower than starch. Cellulose decrystallization remains the bottleneck in cellulose conversion by chemical and biological process.

In our study of cellulose decrystallization using trifluoroacetic acid (TFA), an unusual inverse temperature-dependent swelling pathway was observed. Decreasing the TFA treatment temperature greatly accelerated the cellulose decrystallization process. It took only 100 minutes to completely decrystallize cellulose at 0 ^oC in TFA, a result not achieved for 48 hours at 25 ^oC in the same medium. Even though treatment at low temperatures eliminated the crystallinity, there were no observable morphology changes to cellulose's macrofibrils in SEM images.

The majority of TFA used in cellulose decrystallization can be recycled via a vacuum process. The small remaining TFA, deep in the cellulose matrix, can act as a catalyst in dilute acid hydrolysis. This was realized by adding water to the cellulose sample to make a 0.5% TFA solution. The hydrolysis of treated cellulose was compared to that for untreated cellulose and starch. Treated cellulose showed a dramatic increase in glucose yield than the untreated material. The lower temperatures and shorter reaction times led to reduced production of degradation products such as HMF and levulinic acid. In addition, the total TFA amount required in the hydrolysis process was lower than that of H₂SO₄ in traditional dilute acid process.

Alternatively, the remaining TFA could be completely removed by water. The cellulose, after water wash, remained highly amorphous and active toward hydrolysis. Reactivity of regenerated cellulose was compared with untreated cellulose and corn starch under typical dilute H₂SO₄ (1% and 0.5%) hydrolysis conditions at different temperatures and reaction times in batch reactors. The reactivity of the regenerated cellulose was six times higher than that of untreated cellulose and only slightly lower than that of corn starch. The regenerated cellulose is also expected to be very reactive in enzyme hydrolysis process.