Ecient Electrocatalytic Upgrading of Furan-Based Biomass: Key Roles of Two-Dimensional Mesoporous Heterostructure and Ternary Electrolyte

: Development of high-performance electrocatalytic systems for efficient conversion of biomass to value-added chemicals under mild conditions and understanding of their mechanisms are of profound significance, but have remained a great challenge. Here, we report the first development of two-dimensional mesoporous electrocatalyst for biomass conversion. The electrocatalyst (meso-PA/PmPD/GO) consists of phytic acid (PA)-doped mesoporous poly(m-phenylenediamine) layers coated on graphene oxide nanosheets. Meanwhile, a high-performance ternary electrolyte containing 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF 4 ), acetonitrile and H 2 O is developed. The combination of meso-PA/PmPD/GO and the ternary electrolyte realizes highly efficient conversion of two important biomass derivatives at room temperature. One involves a hardly achieved oxidation of furfuryl alcohol to 6-hydroxy-2,3-dihydro-6H-pyrano-3-one with high faradic efficiency (FE: 83.7%) and selectivity (87.9%). The other involves the oxidation of furfural to 5-hydroxy-2(5H)-furanone with record-high FE (98.9%) and selectivity (93.6%). Mechanism study including DFT calculations unveils that N-heterocyclic carbenes (Bmim*) generated from BmimBF 4 act as the reaction-determining active species. Additionally, the synergistic effect of the PA doping, mesoporous structure and p-n heterojunction interface in meso-PA/PmPD/GO favors the mass transport and the transfer of generated holes to the outer layers, thus boosting the catalytic performance. selectivity for the of to with (98.9%) and selectivity (93.6%). Mechanism DFT collected using the potentiostatic mode at an open circuit potential of 100 kHz to 100 mHz with an amplitude of 5 mV. Electrochemical oxidations of furfuryl alcohol and furfural . Electrochemical oxidations of furfuryl alcohol and furfural were performed in a typical H-type cell at room temperature. The anodic and cathodic electrolytes were BmimBF 4 -MeCN-H 2 O and aqueous H 2 SO 4 solution (0.5 M), respectively. The amount of electrolyte in each chamber was 5.9 g in all experiments. Prior to electrolysis, the anolyte was bubbled with N 2 gas for 30 min under stirring. Then, furfuryl alcohol (0.45 mmol) or furfural (0.5 mmol) was added into the anolyte. The electrochemical reaction was started at a desired applied potential for 2 h. The liquid product was analyzed by 1 H NMR (Bruker Avance III 400 HD spectrometer) in DMSO- d 6 or D 2 O. The conversion and selectivity of the reaction were calculated in terms of 1 H NMR spectra.

With the ever-increasing depletion of global fossil resources, the utilization of green and renewable biomass to produce high value-added chemicals has attracted tremendous interest. [1][2][3][4][5] Among different types of biomass (e.g., lignocellulose, triglycerides, chitin, starch), lignocellulose is the most abundant resource. Derived from lignocellulose, furfuryl alcohol and furfural compounds have been recognized as indispensable and renewable feedstocks for sustainable chemical productions. 6 Compared with traditional thermocatalytic reactions, electrocatalytic oxidation represents a promising strategy for biomass conversion due to its mild reaction conditions including under ambient conditions and using H 2 O as the green oxygen source, etc. [7][8][9][10][11][12] However, at present most of electrocatalytic biomass conversions rely on metal-based catalysts; [13][14][15] numerous biomass conversions to high value-added chemicals have been preliminarily realized with low faradaic efficiency (FE)/selectivity, or even have been hardly achieved. For example, 6-hydroxy-2,3-dihydro-6H-pyrano-3-one (HDPO) is a key component in many biologically active compounds or the intermediates of numerous pharmaceuticals. [16][17][18] It has been primarily prepared from furan derivatives via thermal catalysis, which involves harsh conditions (-70 °C) and/or toxic oxidants. 19,20 Only one successful case was reported very recently on the electrocatalytic conversion of biomass (furfuryl alcohol) to HDPO by employing Ni-based catalyst. 21 It remains challenging to develop new electrocatalytic systems for achieving high FE/selectivity and understand the complicated reaction mechanism. On the other hand, for another important electrocatalytic oxidation of furfural to 5-hydroxy-2(5H)-furanone (HFO), which is a critical intermediate for the synthesis of pharmaceuticals, rubber additives, and agrochemical compounds, 22 it has still remained unsolved to simultaneously achieve both high FE and selectivity for HFO; the highest FEs of all the reported routes have reached only ~60% with selectivity of <83%. [23][24][25][26] The aforementioned problems mainly originate from the low utilization ratio of catalysts, because for most of reported electrocatalysts only the catalytic sites on the surface can be used.
Recently, two-dimensional (2D) mesoporous materials have aroused great attention in the field of electrocatalysis, in light of their high specific surface areas (SSAs), abundant catalytic sites, and short charge transport distance. [27][28][29] The mesopores enable the mass transportation into the internal of electrocatalysts and thus increase the utilization ratio of active sites, leading to high catalytic performance. However, to our knowledge, 2D mesoporous materials have not yet been explored as electrocatalysts for biomass conversion. This vacancy inspires the interest of study.
On the other hand, the efficiency of an electrocatalytic process also depends strongly on the electrolyte system. 30 Traditional electrolytes for biomass conversion are usually based on aqueous inorganic salt (such as K + , Na + , and NH 4 + , etc.) systems. However, these systems usually undergo severely competitive hydrogen or oxygen evolution effects. Furthermore, biomass derivatives usually possess multi-functional groups, which are difficult to be selectively activated in aqueous inorganic salt systems. Therefore, the development of high-performance electrolyte systems is also an essential branch in the area of biomass conversion.
The meso-PA/PmPD/GO nanosheets possess in-plane cylindrical mesopores with an average diameter of 10 ± 2 nm, a thickness of ca. 30 nm, and a high specific surface area of 258 m 2 g -1 .
Meanwhile, we also develop an efficient ternary electrolyte system containing 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF 4 ), acetonitrile and H 2 O. Strikingly, the combination of the meso-PA/PmPD/GO nanosheets and the ternary electrolyte enables excellent  and selectivity of 93.6% is achieved, which are much higher than all of the reported values.
Mechanism study including density functional theory (DFT) calculations unveil that N-heterocyclic carbenes (Bmim*) originated from BmimBF 4 act as the reaction-determining active species.
Meanwhile, the synergetic effect of the doped PA, mesoporous structure, and p-n heterojunction interface in the meso-PA/PmPD/GO nanosheets favors the mass transport, the electron-hole separation, and the transfer of holes to the outer layers, thus optimizing the catalytic performance.
This study opens a new avenue for the development of high-performance electrocatalytic systems for biomass conversion, and provides insightful clues for understanding complicated mechanisms of biomass conversion.
Typically, the P123 copolymer was firstly dissolved in water following reported procedures, forming cylindrical micelles. 38   were also evaluated for the electrochemical oxidation of furfuryl alcohol (Supplementary Table S1).
However, very low FE and selectivity of HDPO were obtained in these electrolyte systems, reflecting a crucial role of the ternary electrolyte in the highly efficient electrooxidation of furfuryl alcohol.  (Table 1). In addition, the performance differences suggest that both the PA incorporation and the mesoporous structure contribute remarkably to the increase of FE and selectivity. d denotes the average pore diameter obtained from TEM images; SSA represents the specific surface area obtained from the nitrogen adsorption-desorption isotherms by the BET method; Con. is the abbreviation of conversion.
Next, the cycling stability of meso-PA/PmPD/GO was examined. Fig. 3d shows no obvious decrease in the furfuryl alcohol conversion, FE and selectivity of HDPO after five catalytic cycles.
SEM and FTIR analyses reveal no obvious differences in the morphology and main characteristic vibration peaks of meso-PA/PmPD/GO after the catalytic cycles (Supplementary Figs. S10 and S11), suggesting the excellent cycling stability of meso-PA/PmPD/GO.
During the electrochemical oxidation of furfuryl alcohol, we also found that the furfural product could be further oxidized at higher potentials (>2.3 V vs NHE) in the ternary electrolyte.
Therefore, the electrochemical oxidation of furfural was also studied in the present work (Fig. 4a). 1 H NMR analysis revealed that HFO and HCOOH were the two main products of this conversion (Scheme 1b), along with a very small amount of maleic acid as byproduct ( Supplementary Fig. S12).
Impressively, the selectivity of HCOOH reached over 97%, while record-high FE (98.9%) and selectivity (93.6%) of HFO were also achieved at the optimal potential of 2.45 V vs NHE in our system (Fig. 4a). The cycling performance testing proved the excellent catalytic stability of meso-PA/PmPD/GO in the oxidation of furfural to HFO (Fig. 4b).
Similar to the electrocatalytic oxidation of furfuryl alcohol, the ternary electrolyte system also enables evidently higher FE and selectivity in the oxidation of furfural to HFO in comparison to those obtained in the aqueous inorganic salt electrolyte systems (Supplementary Table S2). In addition, among all the evaluated samples, meso-PA/PmPD/GO exhibits the best catalytic performance ( Table 2). As well, the performance differences indicate the notable contributions of the PA doping and the mesoporous structure to the high electrocatalytic performance of meso-PA/PmPD/GO.

Mechanism investigation.
To gain an insight into the reaction mechanisms of the electrocatalytic oxidations of furfuryl alcohol and furfural, the reaction pathways were investigated. For the electrochemical oxidation of furfuryl alcohol, a reasonable mechanism involving two pathways (Pathway I and II) is proposed in Fig. 5a, based on the fact that the major product HDPO along with byproducts including furfural and HCOOH was detected. In pathway I (as the main reaction), the furfuryl alcohol undergoes the opening and isomerization of furan ring to generate an intermediate 1.
Due  Table S3). In Fig. 5b   The reaction mechanism of the electrochemical oxidation of furfural to HFO was firstly explored by employing 5-methylfurfural (highly similar to furfural) as the reactant. The results and the corresponding reaction pathway are presented in the Supplementary Information (see Supplementary Figs. S18, S19 and the discussion in Page S14). Accordingly, the reaction pathway for the oxidation of furfural to HFO is proposed in Fig. 6a Fig. S20).
Finally, HFO is produced through the isomerization of 6. 49 DFT calculations (Fig. 6b) 50 Furthermore, Tafel plots were employed to probe the reaction kinetics. 51,52 The Tafel slopes obtained in the ternary electrolyte are smaller than those measured in the aqueous inorganic salts systems (Fig. 7a,b). This result proves the faster reaction kinetics in the ternary electrolyte system, which is favorable for the enhancement of the electrocatalytic performance.
Generally, the structure of electrocatalyst affects its catalytic performance. In our case, meso-PA/mPDA/GO shows better electrocatalytic performance than that of non-PA/mPDA/GO, which profits from its mesoporous structure with in-plane cylindrical pores. The in-plane cylindrical mesopores can afford meso-PA/mPDA/GO a higher specific surface area, and also facilitate the mass transport along the cylindrical pores into the stacked 2D nanosheets (see Fig. 8 for illustration).
As a line of evidence, the lower slope of the Z'-ω -1/2 line of meso-PA/PmPD/GO at the low-frequency region demonstrates that meso-PA/PmPD/GO is more favorable for mass transport, in this case the ternary electrolyte and the biomass molecules, in comparison to non-PA/PmPD/GO under similar conditions (Fig. 7c,d and Supplementary Fig. S23).  Fig. S25). Moreover, PA, as an active dopant, possesses the ability to improve the electrochemical activity of materials. [54][55][56][57] The electrochemical impedance spectra (EIS) give a smaller semicircle radius for meso-PA/PmPD/GO nanosheets than that for meso-PmPD/GO ( Fig. 7c and Supplementary Fig. S23a) UV-vis absorption spectra give a bandgap of 4.22 eV for GO ( Supplementary Fig. S28).
Accordingly, the CB value of GO is calculated to be -3.05 V vs NHE. Thereby, the CB and VB edges of PA/PmPD are enveloped within the energy gap of GO. Based on these results, a reverse type-I heterojunction mechanism is proposed for the charge separation and transfer at the interface of the meso-PA/PmPD/GO nanosheets during the electrocatalytic oxidations of the furan-based biomass (Fig. 8). Upon electrical excitation, the electrons on the CB of GO transfer to that of the PA/PmPD layer, while the holes on the VB of GO simultaneously hop to that of PA/PmPD. Due to the difference between the migration rates of electrons and holes, this leads to an efficient separation of the carriers at the interface. 60,61 In the aforementioned Bmim + -to-Bmim*-to-BmimH-to-Bmim + cycle, it is known that Bmim* generated from Bmim + triggers the oxidation of the furan-based molecules and thus transforms to BmimH. At this moment, the holes accumulated on the VB of the PA/PmPD layers capture the electrons from BmimH and convert it back to Bmim + , thereby promoting the Bmim + -to-Bmim*-to-BmimH-to-Bmim + cycle (Fig. 8). In stark contrast, for the meso-PmPD/GO nanosheets without the PA doping, the VB edge of the outer PmPD layers is higher than that of the inner GO ( Supplementary Fig. S27c). Therefore, a type-II heterojunction is formed in the meso-PmPD/GO nanosheets, where the generated holes on the outer PmPD layers move to the inner GO layers and thus are shielded by the PmPD layers ( Supplementary Fig. S29). This situation is not beneficial to the oxidation of BmimH back to Bmim + . This comparison verifies the remarkable contribution of the PA doping to the formation of the reverse type-I heterojunction in the sandwich-structured meso-PA/PmPD/GO nanosheets. The heterojunction structure enables the effective electron-hole separation and the transfer of the holes to the outer PA/PmPD layers, which favors the consecutive Bmim + -to-Bmim*-to-BmimH-to-Bmim + cycles and thus boosts the electrocatalytic performance. In summary, we constructed a phytic-acid-doped 2D metal-free mesoporous electrocatalyst and developed a high-performance ternary electrolyte containing BmimBF 4 , acetonitrile and H 2 O. Using water as the green oxygen source at room temperature, this new electrocatalytic system exhibited excellent catalytic performance for the electrochemical oxidation of furfuryl alcohol to HDPO with high FE (83.7%) and selectivity (87.9%), and also ultrahigh activity for the oxidation of furfural to HFO with record-high FE (98.9%) and selectivity (93.6%). Mechanism study including DFT calculations unveiled that the Bmim* originated from BmimBF 4 was the reaction-determining active species. Additionally, the synergetic effect of the PA doping, the mesoporous structure, and the p-n heterojunction interface in the 2D catalyst favored the mass transfer, the electron-hole separation, and the transport of holes to the outer layers, thus optimizing the catalytic performance.
This study provides insightful clues to develop more advanced electrocatalytic systems for efficient conversion of biomass to high value-added chemicals. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/max-2500 X-ray diffractometer using Cu Kα radiation (λ = 0.154 nm). Ultraviolet photoelectron spectroscopy (UPS) was conducted on the Scienta R3000 spectrometer system using using HeI 21.22 eV as exciting source with energy resolution of 50 meV. 1 H NMR spectra were recorded on a Bruker Avance III HD 400 MHz NMR spectrometer. Fourier transform infrared spectra (FTIR) were conducted using a Nicolet 6700. UV-Vis spectra were carried out on a UV/EV300 spectrophotometer (Thermo Fisher). Atomic Force Microscopy (AFM) images were collected on Nanonavi E-Sweep (Japan).

Materials
Raman spectroscopy was conducted on a Senterra R200-L (Bruker Optics). Gas  Figure 1 Schematic illustration of the synthesis of meso-PA/PmPD/GO nanosheets with in-plane cylindrical mesopores through an interfacial self-assembly strategy.   The electrochemical oxidation of furfural over the meso-PA/PmPD/GO catalyst in the ternary electrolyte.

Figures
(a) Applied potential and (b) cycling performance.

Supplementary Files
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