Lignin is naturally abundant and a renewable precursor with the potential to be used in the production of both chemicals and materials. As many lignin conversion processes suffer from a significant production of solid wastes in the form of hydrochars, this study focused on transforming hydrochars into magnetic activated carbons (MAC). The hydrochars were produced via hydrothermal treatment of lignins together with formic acid. The activation of the hydrochars was performed chemically with KOH with a focus on the optimization of the MACs as adsorbents for CO2. MACs are potentially relevant to carbon capture and storage (CCS) and gas purification processes. In general, the MACs had high specific surface areas (up to 2875 m2/g), high specific pore volumes, and CO2 adsorption capacities of up to 6.0 mmol/g (1 atm, 0 °C). The textual properties of the MACs depended on the temperature of the activation. MACs activated at a temperature of 700 °C had very high ultramicropore volumes, which are relevant for potential adsorption-driven separation of CO2 from N2. Activation at 800 °C led to MACs with larger pores and very high specific surface areas. This temperature-dependent optimization option, combined with the magnetic properties, provided numerous potential applications of the MACs besides those of CCS. The hydrochar was derived from eucalyptus lignin, and the corresponding MACs displayed soft magnetic behavior with coercivities of <100 Oe and saturation magnetization values of 1–10 emu/g.
Highly microporous humins were synthesized from readily available sugars and bio-based polymers (monosaccharides, disaccharides, and polysaccharides) in sulfuric acid followed by a diethyl ether wash and heat treatment at 400 °C. The relative sustainability, costs of production, and availability of the starting materials were improved significantly as compared with the 5-hydroxymethylfurfural-based microporous humins recently studied by us. A multipronged approach was used to study the detailed characteristics of the adsorbents. Results from 1 H NMR, 13 C NMR, FTIR, WAXS, and elemental analysis were combined and showed that the adsorbents predominantly consisted of amorphous and aromatic carbon structures being rich in oxygen. They were highly porous, and the micropore volumes varied among the compositions as could be observed by analyzing CO 2 and N 2 gas adsorption data. Comparably high CO 2 uptakes of 4.25 and 1.94 mmol/g at 0 °C and 1 and 0.15 bar were observed. With the synthesis of microporous humins with varying porosities, the domain of potential applications of this class of materials could be expanded.
A new class of highly porous organic sorbents called microporous humins is presented. These microporous humins are derived from sustainable and industrially abundant resources, have high heat of CO 2 sorption, and could potentially be useful for the separation of carbon dioxide from gas mixtures. Their synthesis involves the polymerization of 5-hydroxymethyl furfural (HMF) in concentrated sulfuric acid and treatment with diethyl ether and heat. In particular, the porosities were tuned by the heat treatment. HMF is a potential platform chemical from biorefineries and a common intermediate in carbohydrate chemistry. A high uptake of CO 2 (up to 5.27 mmol/g at 0 °C and 1 bar) and high CO 2 -over-N 2 and CO 2 -over-CH 4 selectivities were observed. The microporous humins were aromatic and structurally amorphous, which was shown in a multipronged approach using 13 C nuclear magnetic resonance and Fourier transform infrared spectroscopies, elemental analysis, and wide-angle X-ray scattering.
Lignin conversion processes produce carbon-rich residues [J. Anal. Appl. Pyrolysis2015113713722; Chem. Rev.201011035523599] that can be converted into valuable materials such as magnetic activated carbons (MACs). Such lignin-derived MACs can be further used as functional substrates for hydrotreating NiMo catalysts. In this work, we studied the activity of different NiMo-MACs for the catalytic conversion of lignin in a formic acid/ethanol media (lignin-to-liquid, LtL, process). Two KOH-activated LtL hydrochars from eucalyptus (MACE) and Norwegian spruce (MACS) lignins were used as catalyst supports. In addition, the activity of the resulting NiMo-MACs, namely, C-MACE and C-MACS, was compared with a NiMo catalyst supported on a commercial activated carbon (AC). At reaction conditions of 340 °C and 6 h, the best result was obtained for the NiMo-MACS with a yield of 72.2 wt % of oil and 21.1 wt % of organic solids. At 300 °C and 10 h, both NiMo-MAC catalysts displayed higher hydrodeoxygenation (HDO) activities than their commercial counterpart, yielding considerably higher oil yields. The higher HDO activities are tentatively assigned to the formation of NiFe species on the catalytic surfaces of the NiMo-MAC catalysts. In addition, the magnetism exhibited by the C-MACS made it easy to recover the catalyst. However, a considerable loss of activity was observed upon recycling due to a chemical modification of the catalyst surface.
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