Photocatalytic Biorefinery to Lactic Acid: A Carbon Nitride Framework with O Atoms Replacing the Graphitic N Linkers Shows Fast Migration/Separation of Charge

Abstract

The rational regulation of the structure of photocatalysts with an enhanced visible-light absorption property and rapid charge dynamics is promising and challenging. Here, a facile method was developed for preparing oxygen-doped carbon nitride bundles with porous hollow trunk-shaped structures (Ox@CNB) using melamine with assistance of hydrochloric acid. Experimental results and density functional theory (DFT) calculations indicated that Ox@CNB exhibited controllable bandgaps (2.20-2.60 eV) and a low photoluminescence intensity. Interestingly, Ox@CNB displayed excellent photocatalytic activity for the synthesis of lactic acid (81.4 %) and universality for the reforming of biomass-derived monosaccharides to lactic acid. The oxidation active species of h + , * OH, * O 2 À and 1 O 2 all promoted the lactic acid production, and h + played a major role. A possible reaction mechanism of lactic acid synthesis was proposed by combining the experimental with the DFT results. A one-thousand-fold up-scaled experiment demonstrated the significant application potential of the system for the industrial production of lactic acid.

Potassium Molten Salt-Mediated In Situ Structural Reconstruction of a Carbon Nitride Photocatalyst

“…All samples showed a lower signal intensity than the control TEMPO under light and dark conditions, suggesting the lesser e – generation ability of samples. Interestingly, the TEMPO signal intensity of Br-PCN was high in comparison to PCN, Cl-CN, and I-CN, representing that Br-PCN has a low Fermi level, which increases the use of e – for subsequent photoreduction . These results infer that • OH, • O 2 – , • CH 3 – , and e – are taking part in photoreductions.…”

“…Interestingly, the TEMPO signal intensity of Br-PCN was high in comparison to PCN, Cl-CN, and I-CN, representing that Br-PCN has a low Fermi level, which increases the use of e − for subsequent photoreduction. 35 These results infer that…”

Heterogeneous photocatalysis for biomass valorization to organic acids

“…61,62 Through the use of inexpensive semiconductors (e.g., g-C 3 N 4 , TiO 2 , and Zn 1−x Cd x S) as photocatalysts, high yields (71.5-99%) of LA could be obtained from different biomass derivatives under mild conditions (e.g., room temperature to 70 °C, 0.4-8 h, and 0.15-2 mol L −1 alkali). [63][64][65][66][67][68][69][70][71][72][73][74][75]…”

“…65 In addition, O-doped carbon nitride (Ox@CNB) was also developed to be an efficient photocatalyst for producing LA (81.4% yield) from xylose (100% conversion) at 70 °C in 70 min. 66 The -NH 2 formed by incomplete polymerization of O-doped carbon nitride in Ox@CNB can be used as a good adsorption site for biomass substrate, markedly promoting the photocatalytic conversion of biomass-derived sugars into LA. Ox@CNB induced and accelerated the retroaldol condensation of xylose via electrostatic attraction with the -HN 2 electron donating group during photocatalysis.…”

“…Ox@CNB induced and accelerated the retroaldol condensation of xylose via electrostatic attraction with the -HN 2 electron donating group during photocatalysis. 66 The adsorption behaviors of the reaction molecules on Ox@CNB and the mechanisms involved in photocatalytic LA production were studied by DFT calculations. First, xylose is adsorbed on Ox@CNB and deprotonated to enolate over a strong base (e.g., hydroxyl ion).…”