Enhanced Breaking of Lignin and Mesopore Formation in Zinc Chloride Assisted Hydrothermal Carbonization of Waste Biomasses

Abstract

Hydrochars from hydrothermal carbonization of different biowaste materials (dried dandelion, sawdust, coconut shell powder) formed in the presence of aqueous salt solutions were compared to those obtained by the common method in pure water. Hydrochars with increased carbon contents, pore volume and surface areas were specifically obtained from coconut shell powder in the presence of zinc chloride. Compositional and structural changes within the hydrochar products caused by the process conditions and/or the additive were characterized by solid state 13C NMR spectroscopy, proving that cellulose and, in particular, lignin units in the biomass are more easily attacked in the presence of the salt. Under saline conditions, a distinct particle break-up led to the creation of mesoporosity, as observable from hysteresis loops in nitrogen adsorption isotherms, which were indicative of the presence of pores with diameters of about 3 to 10 nm. The obtained hydrochars were still rich in functional groups which, together with the mesoporosity, indicates the compounds have a high potential for pollutant removal. This was documented by adsorption capacities for the methylene blue and methyl orange dyes, which exceeded the values obtained for other hydrochar-based adsorbers. A subsequent physical activation of the mesoporous hydrochars in steam at different temperatures and times resulted in a further drastic increase in the surface areas, of up to about 750 m2/g; however, this increase is mainly due to micropore formation coupled with a loss of surface functionality. Consequently, the adsorption capacity for the quite large dyes does not provide any further benefit, but the uptake of smaller gas molecules is favored.

Graphitized Biocarbon Derived from Hydrothermally Liquefied Low-Ash Corn Stover

“…The hydrochar obtained after HTL of the corn stover was chemically activated with the ZnCl 2 catalyst to increase the formation of mesopores and improve the SSA. ZnCl 2 catalyzes the degradation of the cellulose components and increases higher volumed pores on the hydrochar surfaces by promoting the decomposition of lignin polymeric structures . These stable oxygen species can be cleaved during the chemical activation process, when ZnCl 2 is solubilized and mixed with hydrochar via sonication in solvent water by restricting the formation of oxygen linkages as mentioned above.…”

“…ZnCl 2 catalyzes the degradation of the cellulose components and increases higher volumed pores on the hydrochar surfaces by promoting the decomposition of lignin polymeric structures. 47 These stable oxygen species can be cleaved during the chemical activation process, when ZnCl 2 is solubilized and mixed with hydrochar via sonication in solvent water by restricting the formation of oxygen linkages as mentioned above. In addition, this improves the binding of the OH-sites present on the dispersed hydrochar with the Zn 2+ ions to form Zn−O complexes.…”