Progress 04/01/24 to 03/31/25

Outputs
Target Audience:The research conducted for this project contributes to the training of one Ph.D. student or one post-doctor who has an exciting opportunity to be involved in interdisciplinary research including catalyst design, lignin catalytic conversion, and process control. The target audiences include graduate students or post-doctors, undergraduate students, and the broader academic communities through conference presentations, and published journals. Changes/Problems:No changes What opportunities for training and professional development has the project provided?This project aims to train two graduate students, one post-doctor and two undergraduate students by hands on research. How have the results been disseminated to communities of interest?The results have been disseminated through peer-reviewed publications, conference presentations, and hands on experiments in the lab. What do you plan to do during the next reporting period to accomplish the goals? Explore three metal catalyst to increase lignin depolymerization efficiency Explore new protocol to convert lignin to jet fuel and vanillin. Find the optimal metal ratios for CuFeZn hydrogenation catalysts using the neural network models. Use our AI-driven catalyst design method to design oxidation catalysts for the lignin degradation.

Impacts
What was accomplished under these goals? 1. We have successfully designed a highly active nano-CuO-modified graphene oxide (CuO/GO) catalyst, including Cu single atoms and CuO nanoparticles. This catalyst could catalyst over 95% lignin into monomeric aromatics (MA) and low molecular weight aromatics (LMWA) under a middle condition (140 oC) without any pre- and post-reaction procedures. 2. With this CuO/GO catalyst, we have successfully developed an effective one-pot oxidation method to depolymerize lignin towards isolated Phenolic Acids (PAs) under optimized conditions (10% catalyst, CaO2, H3PO4, 140 °C, ambient pressure) through only one step: Cu-modified graphene oxide (GO)-catalyzed lignin depolymerization. The solvent methanol plays an important role in protecting the produced phenolic acids (PAs) by forming methyl esters. The method exhibits high selectivity to PA and the total yields reached up to 10.8%. Notably, the phenolic hydroxyl groups keep inert during the whole process. 3. Apart from PA, we have also successfully found an effective method to upgrade the 75.8% LMWA from lignin oxidation into jet fuel through a Pt/C assisted hetedehydrogenation (HDO) process. The total yields of jet fuel from LMWA could be up to 91.27%, (corresponding to 65.90% lignin) 4. With comparative study in lignin depolymerization and lignin dimer dissociation, we have proved that the oxidation of enol ether (an important intermediate) on the surface of CuO nanoparticles is the key step to generate PA, and the nanosized CuO nanoparticles have decreased the conversion energy battery significantly. The introduction of CuO on GO enhanced the selectivity to PAs and avoid the over-oxidation of lignin into biochar. 5. We compared the catalytic efficiency of Cu-single-atom (Cu SA)-loaded GO, CuO nanoparticle (CuO NP) loaded GO, and Cu SA/CUO NP-loaded GO catalysts. Under optimized conditions, Cu SA/CUO NP-loaded GO catalysts exhibited the best efficiency for the oxidation depolymerization of lignin among the three catalysts. 5. We have characterized synthetic catalyst and produced lignin products through various advanced technologies, including X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Transition Electron Microscopy equipped with Energy Dispersive Spectroscopy (TEM-EDS), Scanning Electron Microscopy (SEM), Inductively coupled plasma mass analysis (ICP-MS), Atomic resolution high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray Absorption Fine Structure (XAFS), Gas Chromatography-Mass Spectrometry/filed ionization detector (GC-MS/FID), gel permeation chromatography (GPC), High-Performance Liquid Chromatography (HPLC), and near distance 1H-13C coupling 2D nuclear magnetic resonance (2D-HSQC) for both the products and the catalyst. These characterizations identified the structures and chemical status of Cu species on GO. The nanosized Cu species and unique chemical adsorption of epoxy Cu atoms provide active sites for the adsorption, activation, and depolymerization of lignin. GC-MS also confirmed the presence of enol ether as a significant intermediate in lignin depolymerization. 6. We have elucidated the mechanism of CuO/GO-catalyzed lignin depolymerization by dissociating a lignin dimer (veratrylglycero-β-guaiacyl ether) with CuO/GO catalyst, and explored possible reaction pathways. 7. We have performed density functional theory (DFT) study to calculate the energy levels of all the intermediates, transition states and products in lignin dimer dissociating, and thus obtaining the energy barriers in each step. Moreover, DFT calculations confirmed that the CuO species on GO could decrease the energy barriers for the depolymerization of enol ether into final products, and the oxygen species from CaO2 and enol ether would form a three-/four-member motifs to break the Cα-Cβ bonds selectively to give phenolic esters on CuO surfaces. 8. To design optimal hydrodeoxygenation (HDO) catalysts based on metallic atomic clusters for lignin, we computed the binding energies of phenol (model compound) on Pt1-Pt20 and Ni1-Ni20 clusters, supported on graphene. In general, the Pt clusters (n=1-20) have stronger binding energy to phenol than Ni clusters (n=1-20), leading stronger HDO ability in experiments for Pt clusters. This work will inspire us to design Ni-based alloy catalysts with strong binding to phenol, for the HDO of lignin. 9. In terms of performing AI-driven catalysts for hydrogenolysis or oxidation of lignin, we completed the Python codes of searching optimal atomic configuration in a periodic lattice with given metal ratio (e.g., Cu:Fe:Zn), based on genetic algorithm search and periodic DFT (using VASP) calculations. For the 1st test, we found the lowest-energy atomic configuration of Cu20Fe6Zn6 in a periodic lattice. This method will be used to search for the lowest-energies configuration for CuFeZn catalysts with different given metal ratios, for the purpose of buiding neural network models between metal ratios and reactant binding energies. 10. For designing heterogenous catalysts using inverse molecular design theory, we found a new catalyst CuFeZn alloy for the hydrogenation of lignin. The experimental results verified that the CuFeZn catalyst is more active than the Cu catalyst. By further experimental tests, we found that CuFeZn is indeed more active than Cu for the hydrogenation of guaiacol as well, indicating that our AI-driven design for hydrogenation was successful.

Publications

Progress 04/01/23 to 03/31/24

Outputs
Target Audience:The research conducted for this project contributes to the training of one Ph.D. student or one post-doctor who has an exciting opportunity to be involved in interdisciplinary research including catalyst design, lignin catalytic conversion, and process control. The target audience we will target includes graduate students or post-doctors, undergraduate students, and the broader academic communities through conference presentations, and published journals. Changes/Problems:There are no major changes/problems in the current stage. What opportunities for training and professional development has the project provided?This project has provided the opportunity to train two graduate students and several undergraduate students in the interdisciplinary areas including catalytic depolymerization of biopolymers, catalyst characterization, computational-aid catalyst design, DFT training, and process control protocol for sustainable processes. How have the results been disseminated to communities of interest?Yes. The results have been disseminated to communities of interest by publishing the journal papers. What do you plan to do during the next reporting period to accomplish the goals? We propose the idea of doping Cu species onto the surface of GO to further enhance its activity and selectivity towards lignin depolymerization, and we will synthesize a series of Cu- Cu-modified GO catalysts by attaching different Cu species to the GO surface. We will screen different Cu-modified GO catalysts using lignin model compounds for the best aldehyde selectivity and elucidate its mechanism. We will apply the selected catalysts to depolymerize lignin in waste lignocellulose (lignin-first approach) under optimized conditions. We will conduct data-driven catalyst design after we obtain some promising data for Cu-based catalysts. We will conduct the process control using reinforcement learning for this lignin depolymerized process after we have selected the correct catalyst for lignin depolymerization.

Impacts
What was accomplished under these goals? We have successfully developed an effective method to depolymerize lignin towards isolated Phenolic Acids (PAs) under mild conditions (<120 °C, ambient pressure). The method includes three steps: lignin acetylation, graphene oxide (GO)-catalyzed lignin depolymerization, and urea hydrogen peroxide (UHP) oxidation. The method exhibits a high selectivity and the total PA yields reached up to 20%. With comparative study in lignin depolymerization and lignin dimer dissociation We have proved that the acetylation step can significantly enhance the selectivity towards PAs by blocking the phenolic hydroxyl groups in lignin. The yield of PAs from pre-acetylated lignin was 5-7 times higher than that from non-acetylated lignin. We compared the catalytic efficiency of GO and two other metal-free catalysts: sodium persulfate (SP) and sodium formate (SF). In this study, GO exhibited the best efficiency and selectivity towards phenolic aldehydes (the precursors of PAs in the third step) among the three catalysts. We have proved that protective oxidation using the UHP oxidizer could convert aromatic aldehydes to a high yield of isolated PAs for both lignin model compounds (85-90%) and depolymerized lignin mixtures (75%), and the pre-acetylation of lignin avoided the undesirable side reaction to quinone due to the electron-withdrawing of the added acetyl groups. We have performed characterizations such as X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Gas Chromatography-Mass Spectrometry (GC-MS), and High-Performance Liquid Chromatography (HPLC) for both the products and the catalyst. XPS study revealed that GO catalyst was partially reduced during lignin depolymerization, and conspicuous reduction in the epoxy moieties was discerned, which can be attributed to their active involvement in the catalytic reaction. GC-MS analysis indicated the presence of enol ether as a significant intermediate in lignin depolymerization. We have elucidated the mechanism of GO-catalyzed lignin depolymerization by dissociating two lignin dimers (guaiacylglyerol-β-guaiacyl ether and veratrylglycero-β-guaiacyl ether) with GO, and proposed two possible reaction pathways. We have performed density functional theory (DFT) study to calculate the energy levels of all the intermediates, transition states and products in lignin dimer dissociating, and thus obtaining the energy barriers in each step. Moreover, DFT calculations confirmed that the oxygen species of GO surfaces can shift the lignin depolymerization thermodynamics towards aromatic aldehydes when the phenolic -OH groups in lignin are blocked by pre-acetylation.

Publications

• Type: Journal Articles Status: Accepted Year Published: 2023 Citation: Peng, W., Bao, H., Wang, Y., Cote, E., Sagues, W. J, Hagelin-Weaver, H., Gao, J., Xiao, D.*, Tong, Z.*, (2023) Selective Depolymerization of Lignin Towards Isolated Phenolic Acids Under Mild Conditions. ChemSusChem. e202300750.