title: Insight into the evolution of the proton concentration during autohydrolysis and dilute-acid hydrolysis of hemicellulose.
abstract: BACKGROUND
  During pretreatment, hemicellulose is removed from biomass via proton-catalyzed hydrolysis to produce soluble poly- and mono-saccharides. Many kinetic models have been proposed but the dependence of rate on proton concentration is not well-defined; autohydrolysis and dilute-acid hydrolysis models apply very different treatments despite having similar chemistries. In this work, evolution of proton concentration is examined during both autohydrolysis and dilute-acid hydrolysis of hemicellulose from green bamboo. An approximate mathematical model, or "toy model", to describe proton concentration based upon conservation of mass and charge during deacetylation and ash neutralization coupled with a number of competing equilibria, was derived. The model was qualitatively compared to experiments where pH was measured as a function of time, temperature, and initial acid level. Proton evolution was also examined at room temperature to decouple the effect of ash neutralization from deacetylation.
  
  
  RESULTS
  The toy model predicts the existence of a steady-state proton concentration dictated by equilibrium constants, initial acetyl groups, and initial added acid. At room temperature, it was found that pH remains essentially constant both at low initial pH and autohydrolysis conditions. Acid is likely in excess of the neutralization potential of the ash, in the former case, and the kinetics of neutralization become exceedingly small in the latter case due to the low proton concentration. Finally, when the hydrolysis reaction proceeded at elevated temperatures, one case of non-monotonic behavior in which the pH initially increased, and then decreased at longer times, was found. This is likely due to the difference in rates between neutralization and deacetylation.
  
  
  CONCLUSIONS
  The model and experimental work demonstrate that the evolution of proton concentration during hydrolysis follows complex behavior that depends upon the acetyl group and ash content of biomass, initial acid levels and temperature. In the limit of excess added acid, pH varies very weakly with time. Below this limit, complex schemes are found primarily related to the selectivity of deacetylation in comparison to neutralization. These findings indicate that a more rigorous approach to models of hemicellulose hydrolysis is needed. Improved models will lead to more efficient acid utilization and facilitate process scale-up.
authors: Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, V6T 1Z3 Canada.
authors: Nuwan Sella
authors: NS
authors: Kapu
authors: Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, V6T 1Z3 Canada.
authors: Zhaoyang
authors: Z
authors: Yuan
authors: Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, V6T 1Z3 Canada ; Chemical and Environmental Technology, British Columbia Institute of Technology, 3700 Willingdon Ave, Burnaby, V5G 3H2 Canada.
authors: Xue Feng
authors: XF
authors: Chang
authors: Chemical and Environmental Technology, British Columbia Institute of Technology, 3700 Willingdon Ave, Burnaby, V5G 3H2 Canada ; Department of Wood Science, University of British Columbia, 2424 Main Mall, Vancouver, V6T 1Z4 Canada.
authors: Rodger
authors: R
authors: Beatson
authors: Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, V6T 1Z3 Canada.
authors: D Mark
authors: DM
authors: Martinez
authors: Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, V6T 1Z3 Canada.
authors: Heather L
authors: HL
authors: Trajano
keywords: Autohydrolysis; Biomass; Dilute-acid hydrolysis; Hemicellulose; Kinetic modeling; Pretreatment; Proton concentration