Optimization of Hydrothermal Pretreatment of Hardwood and Softwood Lignocellulosic Residues for Selective Hemicellulose Recovery and Improved Cellulose Enzymatic Hydrolysis

The sustainable utilization of lignocellulosic biomass as a renewable and abundant source lies at the core of the emerging biobased economy for the production of fuels, materials, and platform chemicals. The first step in the implementation of many biomass valorization technologies is the “pretreatment” that aims at biomass fractionation and recovery of its main structural components, i.e., cellulose, hemicellulose, and lignin, which can be then converted by downstream (bio)­catalytic processes to targeted high added value intermediate chemicals or final products. In this respect, hydrothermal pretreatment in pure water (also called liquid hot water or autohydrolysis) offers a method with low operational costs, free of organic solvents and corrosive acids or bases, and with no use of “external” liquid or solid catalysts. In the present work, the hydrothermal pretreatment of three types of lignocellulosic forestry and agricultural residues/byproducts was studied. They are representative of hardwood (residual poplar branches from logging operations and grapevine pruning) and softwood (pine sawdust) biomass. The pretreatment experiments were conducted in a batch-mode, high-pressure reactor under autogenous pressure at varying temperature (170–220 °C) and time (15–180 min) regimes and at liquid-to-solid ratio (LSR) of 15. The intensification of the process was expressed by the severity factor, log Ro. The process was optimized for increasing the recovery of hemicellulose in the form of monomeric sugars (xylose, mannose, galactose) or the respective oligo-saccharides, as well as for improving the production of glucose in the subsequent enzymatic hydrolysis of the pretreated biomass. Maximum hemicellulose recovery for poplar, grapevine, and pine in the liquid products was around 60% at ∼70%–85% hemicellulose removal, based on initial hemicellulose content of each biomass type, and was achieved at relatively moderate treatment severities (log Ro = 3.8–4.1). Formation of major degradation products, such as acids (i.e., formic and levulinic acid) and furans (i.e., furfural and HMF) was relatively low and below ca. 1 mg/mL for the whole range of pretreatment severities. Enzymatic hydrolysis of the parent lignocellulosic materials toward glucose was very low (i.e., 10%) and remained low for the pretreated pine biomass (16%) but was substantially improved for poplar (49%) and especially for grapevine (77%) as a result of hydrothermal pretreatment at the highest severity (log Ro = 4.7). The significant improvement of enzymatic hydrolysis of grapevine was attributed to the nearly complete removal of hemicellulose and to the changes in the morphological and textural characteristics of biomass particles, with the most pronounced one being the 9-fold increase in surface and pore volume