Fundamental Aspects of Dilute Acid Hydrolysis/Fractionation Kinetics of Hardwood Carbohydrates. 1. Cellulose Hydrolysis

Abstract

Previous kinetic modeling and bench-scale demonstration efforts using batch, percolation, or plug-flow reactors for the dilute sulfuric acid hydrolysis of cellulose have concluded that glucose yields above 70% of theoretical were not possible. This has been explained to be a result of reactions involving glucose or the cellulose itself in a destructive manner, as well as hydrolyzed soluble oligomers which have been modified chemically so as not to release glucose. However, recently, we have demonstrated that near-quantitative yields of glucose from cellulose can indeed be obtained using a bench-scale shrinking-bed percolation reactor in which an internal spring compresses the biomass as the reaction progresses. The present study was initiated to gain a fundamental understanding of the kinetic sequences involved in these high yields. Three reactor configurations (batch, percolation, and shrinking-bed percolation) were studied using similar hydrolysis severities to begin addressing chemical, physical, and hypothesized boundary layer phenomenon governing rate-limiting steps of glucose release from two prehydrolyzed yellow poplar cellulosic substrates. The characteristics of the logarithmic release of glucose as well as the logarithmic disappearance of cellulose as a linear function of time were found to be reactor dependent. Use of a percolation reactor was described where the initial hydrolysis rate constant for cellulose using 0.07% w/w sulfuric acid at 225 °C is enhanced 5-fold compared to a batch reactor. Additionally, when lower hydrolysis severities are used for hydrolyzing yellow poplar cellulose in batch mode, biphasic kinetics were observed. Several hypothesized boundary layer resistances, such as structured water, viscosity, and re-hydrogen bonding of released glucose, will be suggested as diffusion resistances for released glucose to the bulk medium, which would be a function of the reactor configuration and define potential glucose yields.

Microbial Cellulose Utilization: Fundamentals and Biotechnology

“…Recent studies have emphasized the importance of equilibrium and mass transfer effects in determining the rate of nonenzymatic hydrolysis of cellulose and hemicellulose. Torget et al (685) have proposed that a highly structured diffusion-resistant water layer on the surface of cellulose fibers and the limited solubility of highmolecular-weight oligomers are important factors limiting hydrolysis rates. Both Torget et al (685) and Jacobsen and Wyman (309) have hypothesized that removal of hydrolysis products and/or disruption of the structured water layer is important in accelerating hydrolysis rates.…”

“…Torget et al (685) have proposed that a highly structured diffusion-resistant water layer on the surface of cellulose fibers and the limited solubility of highmolecular-weight oligomers are important factors limiting hydrolysis rates. Both Torget et al (685) and Jacobsen and Wyman (309) have hypothesized that removal of hydrolysis products and/or disruption of the structured water layer is important in accelerating hydrolysis rates. Such physical chemical factors are presumably operative during enzymatic hydrolysis as well and may play a role in determining the relative effectiveness of CEM and CE complexes.…”

Direct Catalytic Conversion of Cellulose to 5-Hydroxymethylfurfural Using Ionic Liquids

“…Sulfuric acid has been used most frequently (at elevated temperatures), which is unsurprising due to its low cost. While higher temperatures and longer reaction times are required with dilute acid [45], the treatment of cellulose with concentrated acid accelerates the hydrolysis markedly, such that the polymer molecules depolymerise to monomers and this substantially increases dehydration of the monomeric sugars to HMF [46]. However, due to low selectivity and corrosion concerns, dilute acid is preferable for the controlled hydrolysis of cellulose.…”

Preliminary investigation into single stage enzyme free hydrolysis of hardwood sawdust.

“…This parasitic pathway has been circumvented through changing the reactor configuration. Torget et al (2000) achieved a 92% conversion using dilute acid hydrolysis and a novel shrinking bed reactor that keeps a constant pressure on the bed as it shrinks in volume due to the dissolution of the cellulose structure. Keeping the bed of biomass constantly packed leads to a shearing effect on the solid phase which is proposed to assist the transport of the product away from the cellulose structure.…”

“…Glucose yields in dilute acid hydrolysis using plug flow reactors have experimentally peaked at 70% of the theoretical maximum (Torget et al, 2000).…”