Sequential Oxidation Of Rice Husk, Wheat Straw And Their Extraction Residues

Biomass is acknowledged as the largest feedstock for the renewable energy, fuels andchemicals, and possesses advantages such as clean and renewable. The methodologies used forthe degradation of biomass are based on biochemical and thermochemical techniques; however,inferiorities such as high energy consumption, high cost, harsh production conditions, highresidue, complicated subsequent treatments, waste of precious components and environmentalpollution are rising to restrict the further development of those techniques. New methods for thedevelopment of utilization of biomass are necessary.Rice husk (RHP) and wheat straw (WSP) were degraded with sequential oxidation of NaOClaqueous solution under mild reaction conditions. The reaction products were analyzed by thecombination of sequential solvents extraction and gas chromatography/mass spectrometer(GC/MS). The residues from sequential oxidation were also analyzed with ultimate analyzer,Fourier transformed inferred spectrometer (FTIR), scanning electronic microscopy (SEM) andX-ray energy dispersive spectroscopy (EDS).The species identified in the extracts from sequential oxidation of RHP and WSP wereclassified as alkanes, arenes, aldehydes, ketones, phenols, carbonic acids (CAs), esters andnitrogen-containing organic compounds (NCOCs), as well as some alkenes and alcohols, etc.Among these groups of species, CAs, esters and ketones were the most abundant. Alkanes werederived from waxes in the biomass. Arenas, benzoic aldehydes and phenols were ascribed to thedegradation of lignin. Various ketones include furanones, cycloalkanones, pyrones, alkenonesand benzene-ring containing ones. The detected long chain fatty acids might be intrinsiccomponents as lipid in biomass, or be produced from the oxidation of waxes with NaOCl. Mostof the esters identified were phthalic esters and fatty acid phenyl esters, which could be interlinkspecies between lignin and waxes. The NCOCs can be classified into amides, N-heterocycliccompounds (NHCs), amines, nitriles, cloro(nitro)methanes, and sulfonamides..Sequential extraction with several solvents were carried out, and the species in the extractscould be classified into alkanes, alkenes, arenes, aldehydes, ketones, alcohols, esters, furans,NCOCs, CAs and steroids. About60%of the relative contents of the extracted species weresteroids, including cholesteric-and stigmastic-ones/anols/enols. Alkanes, alkenes and aldehydeswith long carbonic chain were attributed to the degradation of waxes. Alkyl substituted furansand benzofurans were derived from the hemicellulose and lignin, respectively.Oxidative degradation of residues from sequential extraction of RHP and WSP were alsocarried out. The species identified in the extracts were classified as alkyls, arenes, phenols,aldehydes, ketones, CAs, esters and other species. Short chain fatty acids and ketones were the most abundant species. However, the kinds and contents for most of the identified species weremuch different with that of unextractd RHP and WSP. Steroids and alkanes are water insolvablespecies, which may restraint the degradation of RHP and WSP with NaOCl aqueous solution.After extraction, the steroids and alkanes were extrated and separated, and the micro-structure ofbiomass might be changed; hence, the processes of oxidation of RHP and WSP were accelerated,and the extents of the degradation were also enhanced.The residues from sequential oxidation were characterized in detail. Also, the differencesbetween sequential oxidation with or without extraction pretreatment were confirmed. Theultimate analysis showed that the contents of C, H, N and S were decreased as well as H/C ratio.The SEM spectra presented vivid transformations of surfaces of the residues in the processes ofsequential degradation. Cellulose and hemicellulose were degraded and separated first, and thenthe lignin. The results form EDS analyses showed that Si, C and O were the main elements in theresidues, and SiO2should be the final component. These results for the residues were fullyaccordance with oxidation products in the liquid phase.Mechanisms basing on radical reactions between main components of biomass and radicaloxidative species were proposed. The degradation mechanism of hemicullulose was believed tobe similar with that of cellulose. The products of lignin with the oxidation of NaOCl includinghydroxyl-, methoxyl-and alkyl-substituted arenes and phenols; further oxidation of these speciesespecially on benzene-rings were also taken place to produce benzoquinones, phthalic acids andbenzoic acids. Reaction mechanisms of typical compounds in biomass were proposed, such asNCOCs. A model of lignin in RHP was proposed basing on the lignin-related species identifiedand their distribution as well as linkages.Most of the NCOCs, and various aldehydes, ketones, CAs and esters, were enriched in theCS2-extractable fraction because of the strong π-π interaction between C=S bond in CS2andC=O bonds in the extracted species.Main components in RHP and WSP were degraded effectively with sequential oxidation ofNaOCl under mild reaction conditions without additional energy consumption; furthermore,basing on the species identified in the extracts, the structure of biomass and the mechanisms ofthe oxidative degradation could be proposed. This investigation provides feedstock for theproduction of chemicals, and a possible way for value-added utilization of agricultural wastes.