Theoretical And Experimental Studies On The Effect Of Branch Structure On The Fast Pyrolysis Mechanism Of Hemicellulose

The pyrolysis characteristics and mechanism of lignocellulosic biomass have attracted more and more attention.Hemicellulose is one of the main components of biomass.The diverse branched structures in hemicellulose lead to complicated pyrolysis products and the pyrolysis mechanism is difficult to be explored.An in-depth understanding of the pyrolysis reaction mechanism of hemicellulose branch structure is crucial to the study of the hemicellulose pyrolysis mechanism.In this paper,the reaction characteristics and mechanism of branched monosaccharides and the influence of branched units on the pyrolysis mechanism of hemicellulose were deeply explored by means of characterization analysis,pyrolysis experiment,and density functional theory(DFT)calculation.In this study,four main branched monosaccharides,i.e.,arabinose,galactose,galacturonic acid,and glucuronic acid were identified by high-performance liquid chromatography and X-ray diffraction in four typical xylan samples.The fast pyrolysis of these monosaccharides at 500℃ mainly produced 2,3-butanedione(DA),hydroxyacetaldehyde(HAA),hydroxyacetone(HA),furan-2-carbaldehyde(FF),and hydroxymethyl-furanone product.Based on experiments and calculations,it was determined for the first time that the hydroxymethyl-furanone product in the fast pyrolysis of monosaccharide was 5-(hydroxymethyl)furan-3(2H)-one(5-HMFO).TG-FTIR experiments showed that during the pyrolysis of monosaccharides containing uronic acid groups,the uronic acid groups were separated at the initial stage of pyrolysis and inhibited monosaccharides to form light aldehydes/ketones.The DFT calculations show that,in the pyrolysis process of each monosaccharide,there were little HAA and DA during the pyrolysis of all monosaccharides,due to the thermodynamic disadvantage of HAA generation and the high energy barrier of DA generation.Uronic acid groups in galacturonic acid and glucuronic acid increased the difficulty of methyl production in HA and restricted the HAA formation of C5-C6 fragments.Thus,the yields of light-oxygenated products from the pyrolysis of galacturonic acid and glucuronic acid were low.The interaction between the C4 hydroxyl space structure and the uronic acid group in galacturonic acid can reduce the difficulty of FF formation..In addition,the structural difference between pentose and hexose leads to the energy barrier of the dominant pathway of arabinose to 5-HMFO is 260.9 kJ/mol,while the energy barrier of the dominant pathway of hexose to 5-HMFO is only 242.2 kJ/mol.Based on the pyrolysis mechanism of branched monosaccharides,the pyrolysis mechanism of branched structure and its effect on the xylan pyrolysis were further studied.The main products of fast pyrolysis of four typical xylan hemicelluloses at 500℃ were HA,HAA,5-HMFO and FF.According to DFT theoretical calculations.The pyrolysis of branched-xylose models produces different monosaccharides and unsaturated monosaccharide fragments through C-C bond cleavage,and then continues to decompose to form various products.The arabinose-xylose model tends to generate light-oxygenated products and 5-HMFO.The galactose-xylose model tends to generate light-oxygenated products;The pyrolysis characteristics and mechanism of uronic acid-xylose unit are similar to those of corresponding branched monosaccharides.