Effects of alkaline or liquid-ammonia treatment on crystalline cellulose: changes in crystalline structure and effects on enzymatic digestibility

Ashutosh Mittal,David K Johnson,Michael E Himmel,Rui Katahira

doi:10.1186/1754-6834-4-41

Ashutosh Mittal, David K Johnson + Show 2 more

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https://doi.org/10.1186/1754-6834-4-41

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Abstract

BackgroundIn converting biomass to bioethanol, pretreatment is a key step intended to render cellulose more amenable and accessible to cellulase enzymes and thus increase glucose yields. In this study, four cellulose samples with different degrees of polymerization and crystallinity indexes were subjected to aqueous sodium hydroxide and anhydrous liquid ammonia treatments. The effects of the treatments on cellulose crystalline structure were studied, in addition to the effects on the digestibility of the celluloses by a cellulase complex.ResultsFrom X-ray diffractograms and nuclear magnetic resonance spectra, it was revealed that treatment with liquid ammonia produced the cellulose IIII allomorph; however, crystallinity depended on treatment conditions. Treatment at a low temperature (25°C) resulted in a less crystalline product, whereas treatment at elevated temperatures (130°C or 140°C) gave a more crystalline product. Treatment of cellulose I with aqueous sodium hydroxide (16.5 percent by weight) resulted in formation of cellulose II, but also produced a much less crystalline cellulose. The relative digestibilities of the different cellulose allomorphs were tested by exposing the treated and untreated cellulose samples to a commercial enzyme mixture (Genencor-Danisco; GC 220). The digestibility results showed that the starting cellulose I samples were the least digestible (except for corn stover cellulose, which had a high amorphous content). Treatment with sodium hydroxide produced the most digestible cellulose, followed by treatment with liquid ammonia at a low temperature. Factor analysis indicated that initial rates of digestion (up to 24 hours) were most strongly correlated with amorphous content. Correlation of allomorph type with digestibility was weak, but was strongest with cellulose conversion at later times. The cellulose IIII samples produced at higher temperatures had comparable crystallinities to the initial cellulose I samples, but achieved higher levels of cellulose conversion, at longer digestion times.ConclusionsEarlier studies have focused on determining which cellulose allomorph is the most digestible. In this study we have found that the chemical treatments to produce different allomorphs also changed the crystallinity of the cellulose, and this had a significant effect on the digestibility of the substrate. When determining the relative digestibilities of different cellulose allomorphs it is essential to also consider the relative crystallinities of the celluloses being tested.

Highlights

In converting biomass to bioethanol, pretreatment is a key step intended to render cellulose more amenable and accessible to cellulase enzymes and increase glucose yields
The sample heated to 140°C (Figure 1a) contained cellulose IIII with high crystallinity, characterized by a narrow 002 peak with high intensity, whereas the sample treated at -33°C (Figure 1f) contained cellulose IIII that was more
Treatment of pure celluloses with alkali and ammonia produced changes in their crystalline structures, which impacted their digestibilities when tested with a commercial enzyme complex (GC 220)

Summary

Introduction

In converting biomass to bioethanol, pretreatment is a key step intended to render cellulose more amenable and accessible to cellulase enzymes and increase glucose yields. Four cellulose samples with different degrees of polymerization and crystallinity indexes were subjected to aqueous sodium hydroxide and anhydrous liquid ammonia treatments. The effects of the treatments on cellulose crystalline structure were studied, in addition to the effects on the digestibility of the celluloses by a cellulase complex. Many studies have shown that completely disordered or amorphous cellulose is hydrolyzed at a much faster rate than partially crystalline cellulose [7,8,9], which supports the idea that the initial degree of crystallinity is important in determining the enzymatic digestibility of a cellulose sample. Cellulose IIII and IIIII can be formed from cellulose I and II, respectively, by treatment with liquid ammonia; the reaction is, reversible [11]. A thorough review of cellulose crystalline allomorphs can be found elsewhere [13,14,15]

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