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.


Preparation of PMA and lignin solution
PMA was dissolved to 0.5 M in 1 M H2SO4. After sonication for 2 h, a homogeneous yellow colored solution was obtained. The lignin solution was prepared by dissolving 9.2 g of lignin in 100 mL of 1 M H2SO4 (100 mL) and then by doubly filtering it via centrifugation at 4,000 RPM for 10 min and vacuum filtered to avoid microchannel blocking.

Fabrication of functional modules for continuous-flow reactors
Details on the design and optimization of each module are described in the manuscript and ESI.

Characterization
The concentrations of the reduced PMAs were confirmed by measuring the absorbance spectra with a V-730 UV−visible spectrophotometer (JASCO, Japan). The lignin structure was analyzed with a VNMRS 600 nuclear magnetic resonance (NMR) spectrometer (Agilent, USA).

Electrochemical analysis
Chronoamperometry (CA) was measured using a SP-150 Biologic potentiostat (BioLogic Science Instruments, France). Samples for GC analyses were collected from the outline of the tube from the cathode component and analyzed with a GC-2010 Plus gas chromatograph (Shimadzu Co., Japan).

Quantification of vanillin and acetovanillone
After the continuous separation process to extract vanillin and acetovanillone, 1 µL of n-decane was added to 2 mL of a chloroform solution as an internal standard for the GC-MS analysis. To set the calibration curve for vanillin and acetovanillone to verify the product yield, the intensity, according to the concentrations of the vanillin ( Figure S10b) and acetovanillone ( Figure S10c) solutions, was measured.
Split injections (1 µL) were performed with a GC Pal autosampler (CTC Analytics AG, Switzerland) at a split ratio of 25:1 using helium as a carrier gas.

Computational fluid dynamics simulation setup
The fluid flow inside a reactor and separator can be described by the incompressible Navier-stokes equation, together with a mass-conservation equation. Assuming steady state, the governing equation for fluid flow can be simplified to the following: ρv • ∇v = −∇p + µ∇ 2 + (Navier − Stokes equation) and (1)

Density-based phase-separation tank (DPT) for separating aqueous and organic phases
Chloroform, with a relatively higher density than water, was discharged to the outlet at the bottom. The pressure difference caused by the different heights of the outlets was controlled via the pressure control valve (Fig. S10a). For example, the organic and aqueous phases entering the DPT with the same flow rate ratio were discharged at the same quantity. Thus, by simply verifying the flow rate of the two outlets, we could modulate the liquid level in the DPT inner tank. However, only controlling the liquid level could not guarantee a 100 % separation efficiency because two immiscible phases can form an emulsion zone [3] that impedes effective separation. Therefore, to check the emulsion distribution inside the DPT and the actual separation performance, computational fluid dynamic (CFD) simulations and real experiments were performed to verify the separation efficiency of the DPT (Fig. S10b). Therefore, the emulsion zone formed with a thickness of 4.6 mm at the center height of the tank, indicating that the two liquid phases could be completely separated.   current density profiles during reoxidation of PMA 5at various applied voltages. Current densities abruptly decreased due to the rapid consumption of the reduced PMA 5-, whose initial concentration was 0.5 M. yield and on (b) vanillin selectivity. These graphs were drawn using the data [4] reported previously.