Modular Flow Reactors for Valorization of Kraft Lignin and Low‐Voltage Hydrogen Production

Abstract

Recent studies have found that green hydrogen production and biomass utilization technologies can be combined to efficiently produce both hydrogen and value-added chemicals using biomass as an electron and proton source. However, the majority of them have been limited to proof-of-concept demonstrations based on batch systems. Here the authors report the design of modular flow systems for the continuous depolymerization and valorization of lignin and low-voltage hydrogen production. A redox-active phosphomolybdic acid is used as a catalyst to depolymerize lignin with the production of aromatic compounds and extraction of electrons for hydrogen production. Individual processes for lignin depolymerization, byproduct separation, and hydrogen production with catalyst reactivation are modularized and integrated to perform the entire process in the serial flow. Consequently, this work enabled a one-flow process from biomass conversion to hydrogen gas generation under a cyclic loop. In addition, the unique advantages of the fluidic system (i.e., effective mass and heat transfer) substantially improved the yield and efficiency, leading to hydrogen production at a higher current density (20.5 mA cm −2 ) at a lower voltage (1.5 V) without oxygen evolution. This sustainable eco-chemical platform envisages scalable co-production of valuable chemicals and green hydrogen for industrial purposes in an energy-saving and safe manner.

Critical Techniques for Overcoming the Diffusion Limitations in Heterogeneously Catalytic Depolymerization of Lignin

“…However, with regard to the industrial application, the employment of other mixing strategies such as modular flow and stirred slurry systems as alternatives assure the minimized mass diffusion of lignin. [52] Recent study designed the fluidic system, which can facilitate the mass and heat transfer of the overall process by (i) injection, mixing, and heating of reaction mixture for oxidative degradation, (ii) extraction of byproduct, (iii) segregation the water and oil phase and hydrogen production at low applied voltage. [52] Besides, increasing the mixing speed and maximizing the space velocity also contribute to the elimination of the external diffusion.…”

“…[52] Recent study designed the fluidic system, which can facilitate the mass and heat transfer of the overall process by (i) injection, mixing, and heating of reaction mixture for oxidative degradation, (ii) extraction of byproduct, (iii) segregation the water and oil phase and hydrogen production at low applied voltage. [52] Besides, increasing the mixing speed and maximizing the space velocity also contribute to the elimination of the external diffusion. [44,53] Solvent is essential to accelerate the heat and mass transfer and facilitates the reactants and catalysts to disperse homogeneously in reactors.…”

Polyoxometalate as an Effective Catalyst for Catalytic Lignin into Value‐Added Molecules

“…Kim al. [81] reported on flow-through systems that are modular and designed for continuous depolymerization, lignin upgrading and hydrogen production using H 3 PMo 12 O 40 as a catalyst (Figure 4). These advantages, particularly efficient mass and heat transfer, result in significantly higher yields and efficiencies, enabling hydrogen to be produced at higher current densities (20.5 mA cm À 2 ) and lower voltages (1.5 V) with no oxygen evolution.…”

Solar Biomass Reforming and Hydrogen Production with Earth‐Abundant Si‐Based Photocatalysts

“…While these studies relied on edible or highly processed biomass-derived chemicals, Reisner et al recently reported the utilization of non-edible biomass (e.g., cellulose and lignin) and visible-light-active CdS/CdO x photocatalysts for hydrogen production under simulated solar irradiation. [26] Despite promising results, these photocatalysts have critical limitations for practical applications, such as limited utilization of sunlight due to a large bandgap (2.4 eV for CdS and >3.2 eV for TiO 2 ), toxicity and photocorrosion (e.g., CdS), [27,28] and harsh conditions (e.g., 10.0 m NaOH). [26] Moreover, many studies have focused mostly on efficient solar hydrogen production and less on the selective production of biomass-derived chemicals, especially when using lignin.…”