Lignocellulosic biomass has potential as a renewable feedstock to displace, at least partly, fossil resources for the production of fuels and chemicals. Cellulose contains the majority of carbohydrate content in lignocellulosic biomass and maximizing its conversion to glucose, its monomeric constituent, is crucial for the economic success of the biorefinery concept. However, cellulose has a low reactivity towards hydrolytic depolymerization, resulting in low glucose release rates and harsh, energy-intensive conditions. Thus, understanding the structural parameters that determine cellulose recalcitrance is an important aspect for the emergence of economically sound reaction engineering schemes.  The major hypothesis in the literature is that amorphous cellulose has an order of magnitude higher hydrolysis rate constant than crystalline cellulose. However, an overlooked phenomenon in this kinetic mechanism is cellulose recrystallization in aqueous environment, especially since pretreatment and hydrolysis are typically carried out in the presence of water. This motivates revision of this model – if crystallinity is reaction parameter, then recrystallization should be considered in the mechanism.<br/>In this study we ball milled cellulose to obtain a series of samples with different crystallinities. Progressively ball-milled samples resulted in an increase of cellulose conversion and glucose yield in HCl catalyzed hydrolysis, consistent with the hypothesized effect of crystallinity. Recrystallization of ball-milled samples was carried out by treatment in water at temperatures that do not promote hydrothermal hydrolysis. Recrystallized cellulose resulted in only slightly lower glucose yield than ball-milled only cellulose. This observation contradicts the established hypothesis. Instead of crystallinity, another structural characteristic must explain the higher reactivity of ball-milled cellulose. Electron spin resonance measurements revealed that progressive ball-milling increases cellulose chain ends which results in a commensurate increase of glucose yield; the increased glucose yield is not recovered after cellulose recrystallization. Therefore, we concluded that the primary effect of ball milling to promote glucose yield is not amorphization, but instead partial chain breaking. The fundamental understanding gained from this study could help engineer pretreatment methods that increase the rate of lignocellulosic biomass conversion or reduce the energy requirements.

Recrystallizating ball-milled cellulose does not recover recalcitrance towards acid catalyzed hydrolysis

256th National Meeting (2018)

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