Building on the work of Dr. Rajai Atalla and his co-workers, we have successfully used Raman microscopy to monitor changes in cellulose crystallinity that occur during acid pre-treatment. Abundant literature suggests that cellulose crystallinity correlates with observations of recalcitrance toward enzymatic hydrolysis. Using Raman microscopy, we found that acidic pre-treatment increased the apparent crystallinity of ball-milled microcrystalline cellulose as a clear trend in the crystallinity index over time was observed. Crystallinity index increases for approximately 60 min, at which time it ceases to change. Crystalline cellulose (filter paper) was subjected to temperature programmed hydrolysis, with crystallinity increasing at temperatures as low as 120 °C. We attribute these observations to preferential hydrolysis of amorphous cellulose, resulting in a residual solid enriched in crystalline cellulose. We tested this explanation by performing control experiments in the absence of acid and under dry thermal conditions. In the absence of acid, cellulose crystallinity index is stable over time; instead, distinct bands attributable to the O-H bending modes of water appear and grow over time. We attribute the appearance and growth of the O-H bending modes to swelling of amorphous cellulose by water; in the presence of acid, the O-H bending modes appear in the cellulose spectrum, but are significantly reduced in intensity compared to their appearance in the absence of acid. Under dry thermal conditions, we confirm that the bands used to track crystallinity index did not shift or broaden substantially, supporting their use under pre-treatment conditions. Furthermore, thermal conditions alone did not alter cellulose crystallinity (up to approximately 200 °C), again supporting the interpretation of the Raman spectra obtained under pre-treatment conditions. As supporting evidence, we performed cellulose hydrolysis experiments to track carbohydrate yield and infra-red spectroscopy under thermal conditions to complement Raman spectra. These results provide new understanding of cellulose chemistry during pre-treatment and once again show the versatility and utility Raman spectroscopy to probe cellulose structures.

In situ Raman Microscopy to monitor changes in cellulose crystallinity during acid pre-treatment

253rd National Meeting (2017)

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