Enhancing Aromatic Production from Reductive Lignin Disassembly: in Situ O‑Methylation of Phenolic Intermediates Jacob A. Barrett, † Yu Gao, ‡ Christopher M. Bernt, † Megan Chui, † Anthony T. Tran, † Marcus B. Foston,* ,‡ and Peter. C. Ford* ,† † Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106-9510, United States ‡ Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri 63130, United States * S Supporting Information ABSTRACT: The selective conversion of lignin into aromatic compounds has the potential to serve as a “green” alternative to the production of petrochemical aromatics. Herein, we evaluate the addition of dimethyl carbonate (DMC) to a biomass conversion system that uses a Cu-doped porous metal oxide (Cu 20 PMO) catalyst in supercritical methanol (sc- MeOH) to disassemble lignin with little to no char formation. While Cu 20 PMO catalyzes C-O hydrogenolysis of aryl-ether bonds linking lignin monomers, it also catalyzes arene methylation and hydrogenation, leading to product prolifer- ation. The MeOH/DMC co-solvent system significantly suppresses arene hydrogenation of the phenolic intermediates responsible for much of the undesirable product diversity via O-methylation of phenolic -OH groups to form more stable aryl-OCH 3 species. Consequently, product proliferation was greatly reduced and aromatic yields greatly enhanced with lignin models, 2-methoxy-4-propylphenol, benzyl phenyl ether, and 2- phenoxy-1-phenylethan-1-ol. In addition, organosolv poplar lignin (OPL) was examined as a substrate in the MeOH/DMC co- solvent system. The products were characterized by nuclear magnetic resonance spectroscopy ( 31 P, 13 C, and 2D 1 H- 13 C NMR) and gas chromatography-mass spectrometry techniques. The co-solvent system demonstrated enhanced yields of aromatic products. KEYWORDS: Lignin, Heterogeneous catalysis, Hydrotalcite, Porous metal oxides, Supercritical methanol, Dimethyl carbonate ■ INTRODUCTION Society’s dependence on fossil carbon resources is linked not only to energy needs but also to demand for chemical feedstocks. Petroleum accounts for ∼36% of annual energy consumption and 95% of organic chemicals produced in the United States. 1,2 The environmental impact of the resulting anthropogenic CO 2 emissions has motivated interest in sustainable technologies to meet our growing energy and manufacturing demands. In this context, significant activity has focused on developing large-scale biorefineries that would efficiently utilize lignocellulosic biomass for fuel and chemical production. 3,4 The lignin component of biomass has potential as a renewable source for industrially useful aromatic chemicals. 5 While technologies for selective conversion of the carbohydrate components of lignocellulosic biomass have been successful, 6 lignin is generally treated as waste and burned for low grade heat. The development of sustainable technologies for converting lignin to aromatic products in high yield, selectivity, and value would improve the economic feasibility and life cycle assessment for second generation (i.e., using lignocellulosic biomass feedstock) fermentative biorefineries. 7 Lignin is an aromatic macromolecule that comprises one of the three main components of lignocellulosic biomass, cellulose and hemicellulose being the others. 8 The polymeric structure of lignin results from radical polymerization of three hydrox- ycinnamyl alcohols: coniferyl alcohol (G), sinapyl alcohol (S), and coumaryl alcohol (H). The resulting macromolecule is naturally recalcitrant to biotic and abiotic degradation. The monomers of lignin are linked through several substructures that contain C-O bonds, in particular, aryl ether (β-O-4 and α- O-4), phenylcoumaran (β-5), and diaryl ether (4-O-5) linkages. 8 Other linkages are present in smaller amounts with abundances being highly dependent on the plant species, growth conditions, and lignin isolation technique (Figure 1). 9-11 Received: August 1, 2016 Revised: August 29, 2016 Published: September 15, 2016 Research Article pubs.acs.org/journal/ascecg © 2016 American Chemical Society 6877 DOI: 10.1021/acssuschemeng.6b01827 ACS Sustainable Chem. Eng. 2016, 4, 6877-6886 Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on December 28, 2018 at 03:52:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.