Xianghong Qian and Mark R. Nimlos

9.1 Background

With the depletion of fossil fuels and the increase in oil prices, biofuels have become ever more attractive as an alternative source of energy. The US Department of Energy (DOE) recently released its roadmap aiming to reach the goal of supplying 30% of the US motor vehicle gasoline from cellulosic ethanol (1). In addition, the European Union has a plan to produce 25% of its transportation fuels from biofuels (2). Toward these goals, significant progress has been made in biomass conversion of cellulose to fermentable sugars. Many schemes for the utilization of biomass as a source of renewable fuels and chemicals rely upon the ability to deconstruct the polysaccharides in plant cell walls into constituent sugars.

Acid hydrolysis is commonly used during pretreatment to break down the structures of plant cell walls and prepare plant polysaccharides for hydrolysis using biological catalysts. In this step, hemicellulose is hydrolyzed to monomeric sugars, the majority of which are pentosans, such as p-D-xylose. Depending on the severity (temperature and acidity) of this pretreatment process, some xylose molecules undergo an undesirable dehydration process, thus lowering the biomass conversion efficiency. In addition, the main degradation product, furfural, is a toxin for fermentative organisms (3, 4) and can polymerize to reduce access to other polysaccharides, such as cellulose. Thus, dehydration reactions in acid pretreatment of biomass present a barrier to economical conversion of biomass. In the past, sugar yield and acidic sugar degradation products were found to be strongly dependent upon the reactor configuration, the reaction media, and the reaction temperature (5-9) during dilute acid hydrolysis.

Xylan is a prevalent hemicellulose in many sources of biomass, and for example, makes up roughly 20% of corn stover (10). The polymer backbone of xylan is composed of xy­lose monomers joined by p-1,4 ether linkages, which hydrolyze to form xylo-oligomers and eventually xylose. Earlier studies (11) show a biphasic behavior of xylan hydrolysis, a fast breakdown of xylan to xylo-oligomer followed by a slow depolymerization of the residual xylan. The reasons for this biphasic behavior remain elusive. The hydrolysis of low degree of polymerization (DP) xylo-oligomer to xylose is typically fast (11). Reaction (9.1) shows the hydrolysis of a backbone section of xylan by an SN1 water substitution

Biomass Recalcitrance: Deconstructing the Plant Cell Wall for Bioenergy. Edited by Michael. E. Himmel © 2008 Blackwell Publishing Ltd. ISBN: 978-1-405-16360-6

mechanism. The mechanism involves addition of a proton to the oxygen atom of the ether linkage, which leads to the dissociation of the polymer to form a positively charged oxo — nium ion and a shorter xylan chain. A nearby water molecule quickly interacts with the oxonium ion forming a neutral xylan chain. The extra proton from the water molecule is recycled back into the solution by forming a hydronium ion with the surrounding water molecules. The stability of the oxonium ion suggests fast kinetics for the first half of the hydration reaction. As a result, xylo-oligomers and xylose should be released quickly during the acid pretreatment of xylan and biomass as long as protons are readily available and eas­ily transferred to the ether bond. The rate of protonation of the ether linkage is unknown. Most likely, it will depend on the macroscopic acid concentration and microscopic atomic and molecular environment, which may hinder/promote proton transport to the ether linkage.

The mechanisms of the dehydration or degradation reactions that lead to the destruction of sugar molecules have been less clearly understood. The decomposition of xylose in acidic aqueous solutions has been the focus of a number of studies dating back (12) to the 1930s. Furfural is found to be the main product from the decomposition reactions and this process is used in the industrial production of furfural from oat hulls. The mechanism for furfural formation was initially proposed (13-15) to occur via the open chain form of the sugar structure as is shown in (9.2). Even though xylose molecules have a predominant ring structure in water, the dehydration of the open chain form to furfural drives the equilibrium to the right. Moreover, the protonation of the ring oxygen opens up the ring structure. Degradation proceeds via the elimination of the two water molecules and eventual closure of the open chain to form a furan ring. No intermediate has been detected experimentally to validate this reaction mechanism.

Antal and coworkers (16) proposed another mechanism for xylose degradation to furfural via direct conversion from the six-carbon pyranose ring structure to the five-carbon furan ring structure. The existence of this mechanism was confirmed by recent ab initio molecular dynamics simulations and quantum mechanical calculations (17, 18). Two mechanisms, (9.3) and (9.4), were proposed (16) that involve direct rearrangement of the cyclic ring structure after the protonation ofthe hydroxyl groups and loss ofwater. Further elimination of the water molecules leads to the formation of furfural. The kinetics of furfural formation from xylose in acid solutions has been measured (19) and the reported activation energy is about 32 kcal mol-1. Direct dehydration or degradation of xylan has not been reported, though degradation ofxylo-oligomer, particularly xylotriose has been reported when certain inorganic salts are added to the solution (9). It is likely that these processes also have relatively high activation energies. Since hydrolysis is facile if protons are readily available for the (3-1,4 ether linkages, and dehydration reactions have high activation energies, it is theoretically possible to obtain high yields of xylose during dilute acid pretreatment of xylan and biomass.

However, the low yields of xylose (60-65%) (20-22), and high level of furfural formation (15%), suggest that more needs to be learned about the mechanisms and kinetics of hydrolysis