ArticleMild oxidative degradation of spent auricularia auricular substrate and molecular composition of carboxylic acids in the resulting soluble portion
Introduction
Spent auricularia auricular substrate (SAAS) is a media residue after artificially growing auricularia auricular with wheat bran, oak wood sawdust, and other raw materials [1]. In most cases, SAAS has not been effectively developed and utilized, either directly placed in the open air or simply combusted, leading to serious environmental problems and huge waste [2], [3].
Like other lignocellulosic biomass, SAAS contains abundant oxygen-functional moieties that can be used to produce value-added oxygenated chemicals such as carboxylic acids (CAs) through mild oxidative degradation. The prevailing oxidants mainly consist of O2 [4], [5], [6], RuO4 (RuCl3 as precursor) [7], [8], NaOCl [9], [10], [11], [12], H2O2 [13], [14], and aqueous hydrogen peroxide/acetic anhydride (AHPO/AAH) [15], [16], [17], [18]. Among them, AHPO/AAH is relatively cheap, ecofriendly, and without other elements involved, and has relatively strong oxidizability. However, few reports are issued on the oxidative degradation of SAAS with AHPO/AAH. According to previous reports [19], [20], [21], [22], the oxidative degradation of AHPO/AAH mainly acts on aromatic carbon. Due to the action of peroxidase and laccase during the growth of auricularia auricular, the cellulose and hemicellulose in SAAS have been partially degraded. Therefore, SAAS contains 78% oak wood sawdust, which is rich in lignin. The p-hydroxyphenylpropane, guaiacol, and eugenol (H, G & S) are generally considered to be the major unit components of macromolecules from lignin [23], which can be converted into valuable CAs. Therefore, it is feasible for SAAS to be oxidatively degraded by AHPO/AAH.
In this study, we investigated oxidative degradation of SAAS using AHPO/AAH as the oxidant under mild conditions and analyzed the molecular compositions of the CAs obtained. We tried to explore the mechanisms for the oxidative degradation of SAAS and to optimize the reaction conditions for obtaining CAs in high yield and simple composition.
Section snippets
SAAS and reagents
SAAS consisting of 78% oak wood sawdust, 20% wheat bran, 1% sucrose, and 1% lime was collected from an edible mushroom planting base in Dunhua, Jilin Province, China. It was pulverized to pass through an 80-mesh sieve (particle size ≤ 180 μm) followed by desiccation in a vacuum at 80 °C for 24 h before use. Table 1 shows the proximate, ultimate, and group composition analyses of SAAS. AHPO (30%), AAH, acetone, CH2N2/diethyl ether solution, sodium thiosulfate, and potassium permanganate used in
Effect of the AHPO/AAH ratio
As Fig. 2 exhibits, ASP yield increased to peak, whereas the residue yield decreased to minimum at the AHPO/AAH ratio of 1:2. Nevertheless, when the AHPO/AAH ratio decreased to 2:1, ASP yield also decreased from 47.5% to 26.7%, while the GP yield increased rapidly to 24.2%, indicating that HO is derived from HPO decomposition of which acts on the oxidatively degraded products to continue to degrade into smaller molecules until it is a gas, so ASP yield is lowered and the GP yield is increased.
Conclusions
Up to 53.6% of the organic matter in SAAS was converted to the SPs, most of which was CAs. NADAs have much higher yield than the other group components, among NADAs, malonic acid, succinic acid, and nonanedioic acid are the most predominant. The composition and content of CAs obtained were significantly affected by the operating AHPO/AAH ratio and temperature. A series of acetoxy compounds indicate that CH3COO is also involved in the oxidative degradation of SAAS using AHPO/AAH. The oxidative
Nomenclature
- AAs
alkenoic acids
- AAAs
acetoxy alkanoic acids
- AADAs
alkylalkanedioic acids
- AAH
acetic anhydride
- ABAs
acetoxy benzoic acids
- ADAs
alkenedioic acids
- AHPO
aqueous hydrogen peroxide
- AISPs
acetone-insoluble portions
- AMBA
5-acetyl-2-methoxy benzoic acid
- ASPs
acetone-soluble portions
- ATAs
alkanetricarboxylic acids
- ATAs′
alkenetricarboxylic acids
- BAs
benzoic acids
- BDCAs
benzene dicarboxylic acids
- BMCAs
benzene monocarboxylic acids
- BR
benzene ring
- BTAs
benzenetrioic acids
- BTAs′
benzenetetroic acids
- BPCAs
benzenepolycarboxylic acids
- CAs
Acknowledgments
This work was supported by the Seed Fund from Jilin Agricultural Science and Technology University ([2016] No.Z02) and Undergraduate Scientific and Technical innovation of Jilin Province (Grant [2016] No. 030).
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