Lignocellulose pretreatment severity – relating pH to biomatrix opening

Abstract

In cellulose-to-ethanol processes a physico-chemical pretreatment of the lignocellulosic feedstock is a crucial prerequisite for increasing the amenability of the cellulose to enzymatic attack. Currently published pretreatment strategies span over a wide range of reaction conditions involving different pH values, temperatures, types of catalysts and holding times. The consequences of the pretreatment on lignocellulosic biomass are described with special emphasis on the chemical alterations of the biomass during pretreatment, especially highlighting the significance of the pretreatment pH. We present a new illustration of the pretreatment effects encompassing the differential responses to the pH and temperature. A detailed evaluation of the use of severity factor calculations for pretreatment comparisons signifies that the multiple effects of different pretreatment factors on the subsequent monosaccharide yields after enzymatic hydrolysis cannot be reliably compared by a one-dimensional severity factor, even within the same type of pretreatment strategy. However, a quantitative comparison of published data for wheat straw pretreatment illustrates that there is some correlation between the hydrolysis yields (glucose and xylose) and the pretreatment pH, but no correlation with the pretreatment temperature (90-200°C). A better recognition and understanding of the factors affecting biomatrix opening, and use of more standardized evaluation protocols, will allow for the identification of new pretreatment strategies that improve biomass utilization and permit rational enzymatic hydrolysis of the cellulose.

Impact of Different Lignin Fractions on Saccharification Efficiency in Diverse Species of the Bioenergy Crop Miscanthus

“…The severity (log M 0 ) of the pretreatment was 1.78 for the sodium hydroxide pretreatment and 1.99 for the dilute sulphuric acid pretreatment, as calculated by the following Eq. (3) [41]:…”

“…where t is the reaction time; C is the concentration of chemical catalyst (%w/v); n is the empirically determined constant fitted to be 0.849 and 3.90 for sodium hydroxide and sulfuric acid, respectively, [41,42]; and T is the reaction temperature in°C. Pretreated samples were then washed to neutral pH with deionized water (2×, 5 min, 50°C) and with 0.1 M sodium citrate buffer (pH 4.6, 5 min, 50°C).…”

Pressurized Hot Water Flow-through Extraction of Birch Sawdust with Acetate pH Buffer

“…The values of log R 0 at different extraction temperatures in a flow-through system were in a similar range as in PHW batch extractions used to make composites (Hosseinai et al 2011a,b;Hosseinai 2012;Paredes et al 2009). Alkaline and neutral lignocellulosic pretreatments were in a similar log R 0 range as a flow-through PHW extraction (Pedersen and Meyer 2010). The PHW extraction at 180 °C had a similar combined severity factor of 8.1 as a 10 min alkali treatment at 140 °C.…”

“…However, severity factor R 0 and P-factor have limitations on their usefulness, since pH values during extraction are not accounted for. Pedersen and Meyer (2010) used pH to calculate the severity of pretreatments. The combined severity factor (log(R´´)) was used to calculate the severity of pretreatment for different substrates, ranging from wheat straw to softwood and hardwood.…”

“…Because the R 0 and P-factor calculations used temperature and pressure, the buffered extractions had the same R 0 and P-values in both plain water and buffered extractions. With the combined severity factor, log(R" 0 ) and modified P-factor pH values had to be taken into account in the calculations according to Pedersen and Meyer (2010).…”