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A New Step Forward Nonseasonal 5G Biorefineries: Microwave-Assisted, Synergistic, Co-Depolymerization of Wheat Straw (2G Biomass) and Laminaria saccharina (3G Biomass)

Javier Remón, Samantha H. Danby, James H. Clark, Avtar S. Matharu
ACS sustainable chemistry & engineering 2020 v.8 no.33 pp. 12493-12510
Saccharina latissima, aqueous solutions, biomass, biorefining, carboxylic acids, catalysts, catalytic activity, depolymerization, feedstocks, gases, leaching, liquids, macroalgae, mass transfer, microwave treatment, sugars, synergism, temperature, wheat straw
This investigation explores the microwave-assisted, synergistic co-depolymerization of wheat straw (2G biomass) and Laminaria saccharina (3G biomass) as a novel strategy for the production of sugar-rich aqueous carriers. The effects of the feedstock (each material alone and all the possible binary mixtures) were carefully analyzed over a wide range of reaction temperatures and times. The optimization of the process revealed that 19 wt % of wheat straw and 46 wt % of L. saccharina could individually be converted into an aqueous, high-purity (85–95 C-wt %), sugar-rich solution at 190 °C using reaction times of 18 and 35 min, respectively. The reactivity of wheat straw can be synergistically increased by co-feeding this material a relative amount of seaweed varying between 36 and 57 wt % with respect to the total biomass content at 215 °C for 40 min. This allowed the transformation of 30 wt % of the feedstock mixture into a sugar-rich (90 C-wt %) aqueous solution. The higher reactivity of seaweed than that of wheat straw, as well as the synergistic effects that the former exerted on the depolymerization of the latter, was accounted for by the formation of “a pool of active catalytic species”. The mechanism involved an “in situ” metal-biomass, microwave-promoted catalysis with marginal mass transfer limitations followed by metal leaching, leading to a greater spread of the reactions occurring in the liquid phase. This resulted in the formation of new “in situ catalytic species”, i.e., carboxylic acids, which acted as homogeneous catalysts and ended up being transformed to gases so that they did not affect sugar purity. Therefore, this novel co-valorization strategy might represent a step-change for the development of novel “nonseasonal, nonfeedstock-dependent” 5G biorefineries and can help to render the entire biorefinery for 2G and 3G biomasses more logistically efficient and economically competitive.