Search

As an important component of organic solid wastes, disposal of plastic wastes (PWs) has always been a thorny issue. Thermochemical conversion is an efficient method. In-depth research on the influence of structural differences on their pyrolysis behavior is beneficial to promote the development of thermal treatment and conversion technology of PWs. The differences in the chemical structure of different polyolefin plastics at the molecule level are mainly reflected in their side chains. Herein, a series of typical polyolefin plastics with similar backbone structure and different side chain structures, including high-density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyvinyl alcohol (PVA) and ethylene-vinyl acetate copolymer (EVA), were selected for comprehensive pyrolysis characterization in this work. Advanced experimental analysis methods such as in-situ DRIFT and 2D-PCIS were adopted to investigate the evolution of two weak oxygen-containing side chains during pyrolysis of PVA and EVA, respectively. Besides, typical product distribution of six samples were systematically studied by combining SVUV-PIMS and GC/MS. Spectral analysis showed that OH side chain in macromolecular structure of PVA had the weakest thermal stability, and its oxidation reaction occurred earlier than dissociation and dehydration, while the acetoxy side chain in EVA polymers could not only be directly dissociated via deacetoxylation, but also be transformed into OH side chain via deacetylation that occurred simultaneously with deacetoxylation. Further combined with SVUV-PIMS and GC/MS, the distribution characteristics of pyrolysis products of polyolefin plastics and the rules of generation and release of typical products with temperature were obtained in detail. It was found that the release of typical oxygen-containing products was earlier or slightly earlier than the release of hydrocarbons in pyrolysis of PVA or EVA. However, the release of all hydrocarbons was almost simultaneous during the respective pyrolysis process of HDPE, PP and PS, suggesting that the evolution of oxygen-containing side chains with weak thermal stability occurred earlier under the drive of temperature. Based on the characterization results, the main evolution pathways of different side chains were proposed to explain the influence mechanism of different side chain structures on the pyrolysis behavior of polyolefin plastics. The cleavage of C C bonds mainly occurred in HDPE, PP and PS, while the evolution of side chains occurred first in PVC, PVA and EVA, and exerted different influence on the degradation of carbon skeleton. [ABSTRACT FROM AUTHOR]

• The structural differences of biomass had significant influence on pyrolysis mechanism. • Cleavage of glycosidic bond and dissociation of O-acetyl group in softwood hemicellulose were prone to occur. • The breakage of α-ether bond of hemicellulose-lignin bonds mainly occurred via Cα-O concerted cleavage. Biomass pyrolysis involves complex structural changes and numerous chemical reactions. At present, most studies on actual biomass pyrolysis focus on the macroscopic thermal degradation process, while the studies on microscopic pyrolysis rules are still very limited. This work provides a new insight into the pyrolysis mechanism of three types of actual biomass, namely hardwood (eucalyptus saligna), softwood (pinus sylvestris) and straw (corn straw), by combining in-situ characterization of the evolution of biomass structures with density functional theory (DFT) calculations of pyrolysis pathways. Firstly, the evolution of various characteristic structures during biomass pyrolysis was monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFT). It was found that the thermal stability of the same functional group in different biomass samples are not exactly the same, and the structural differences in biomass would lead to different pyrolysis behaviors. Then, considering the great structural difference of hemicellulose in different biomass, the key initial pyrolysis reaction paths including the cleavage of glycosidic bonds, the dissociation of side chains and the opening of sugar ring of hemicellulose were studied by DFT calculations. It was found that the cleavage of glycosidic bonds and the dissociation of O-acetyl side chains of glucomannan in softwood hemicellulose are more advantageous than those of xylan in hardwood and straw hemicellulose. In addition, the cleavage mechanism of hemicellulose-lignin bond (LC), a characteristic connection structure between different components of actual biomass, was investigated. The result showed that the cleavage of Cα-O mainly occurred by concerted mechanism for α-ether bond of LC. [ABSTRACT FROM AUTHOR]

Cellulose is one of the major components of biomass. The study on its pyrolysis process will be beneficial to the in-depth understanding of biomass pyrolysis mechanism. In this work, in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFT) combined with two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS) was first used to characterize the evolution process of the functional groups in cellulose during pyrolysis. The results showed that the degradation of carbon skeleton was prior to the dehydration of free hydroxyls after the destruction of hydrogen bond networks during pyrolysis. The thermal stability of C O in cellulose followed by the order of glycosidic bond

The in-depth study of the cleavage behaviors of linkages in biomass is important for the better understanding of biomass pyrolysis mechanism. This study aimed to clarify the pyrolysis mechanism of the typical ether linkages in biomass including β-1,4-glycosidic bond, α-O-4 bond and methoxyl using cellobiose, benzylphenyl ether and guaiacol as the model compounds. Combining the detection of the key intermediates, especially radicals by SVUV-PIMS and the evaluation of the reaction pathways by density functional theory (DFT) quantum chemical calculations, it was found that the concerted cleavage of β-1,4-glycosidic bond was more kinetically favorable than homolytic cleavage and heterolytic cleavage, and the ring opening of cellobiose via the breakage of C1′-O was likely to occur before the cleavage of glycosidic bond. The α-O-4 bond in benzylphenyl ether was mainly cleaved by Cα-O homolysis, and it was easy for the formed radicals to recombine with each other to yield phenolic dimers. In the initial evolution process of methoxyl in guaiacol, homolytic demethylation was the most important unimolecular reaction, while demethoxylation and radical-induced rearrangement reactions were difficult to occur due to their high energy barriers. In the presence of the formed methyl radicals from homolytic demethylation reaction, these two reactions would occur since their energy barriers were significantly reduced at this condition. [ABSTRACT FROM AUTHOR]