Hydrogenation of Levoglucosenone to Renewable Chemicals Results and Discussion Siddarth H. Krishna, Daniel J. McClelland, Quinn A. Rashke, James A. Dumesic, George W. At low temperature (40°C) over a Pd/Al2O3 catalyst, the C=C bond of LGO was Huber* hydrogenated to Cyrene in 100% selectivity, avoiding over-hydrogenation of the C=O bond. At Department of Chemical and Biological Engineering, University of Wisconsin-Madison, higher temperatures (100°C) over a Pd/Al2O3 catalyst, Cyrene was hydrogenated to Lgol in Madison, WI 53706, USA. *[email protected] 100% selectivity with an excess of the exo-Lgol isomer produced over the endo-Lgol isomer Introduction (exo/endo = 4.6). The stereoisomer ratio is not a function of conversion, temperature, or Lignocellulosic biomass has gained interest as a renewable feedstock for the particle size (measured by CO chemisorption) over Pd/Al2O3 catalysts, but is a function of the production of high-value oxygenated chemicals. Levoglucosenone (LGO) can be produced support material used (e.g. carbon, Al2O3), indicating that the stereoisomer ratio is driven by from cellulose in 50% yield using a polar aprotic solvent and sulfuric acid catalyst [1]. As kinetics rather than thermodynamics. We hypothesize that the excess of exo-Lgol results from shown in Scheme 1, LGO can be hydrogenated into a wide variety of chemicals. At low preferential hydrogen atom addition to the ketone group of Cyrene from the opposite side as temperatures (25-60oC) over a Pd/C catalyst, LGO is hydrogenated into Cyrene the anhydro-bridge. (dihydrolevoglucosenone), a non-toxic solvent with similar properties to environmentally harmful solvents [2]. DuPont has reported that LGO can be hydrogenated to levoglucosanol Lgol hydrogenolysis was carried out using a Pd/SiO2-Al2O3 catalyst at 150°C, with 58% (Lgol), followed by hydrogenolysis to tetrahydrofurandimethanol (THFDM) in 84% yield over selectivity towards THFDM. Lgol was unreactive in the presence of Pd/Al2O3 catalyst at a Pt/C catalyst [3]. THFDM can be further upgraded to 1,6-hexanediol in >80% yield using 150°C, indicating that acid sites are necessary for Lgol hydrogenolysis. A physical mixture of metal and acid catalysts. Lgol is a potential chiral precursor relevant to the fine chemicals and Pd/Al2O3 and SiO2-Al2O3 resulted in lower conversion (41%) and lower THFDM selectivity pharmaceuticals industries [4]. THFDM is an α,ω-diol and potential polymer precursor [5]. (29%) compared to the Pd/SiO2-Al2O3 catalyst (90% conversion, 58% THFDM selectivity), 1,6-hexanediol is a commodity chemical used in polyurethanes, coatings, and adhesives. indicating that close proximity of metal and acid sites promotes this reaction. Furthermore, the fundamental chemistry of LGO hydrogenation has not been explored in detail in the literature. Herein, we study the reaction network for LGO hydrogenation over supported The selectivities to THFDM (58%) as well as the side-products tetrahydropyran-2-methanol-5- palladium catalysts. We demonstrate that LGO can be used for the production of high-value hydroxyl (THP2M5H; 20% selectivity) and 2-methyl-tetrahydrofurfurylalcohol (2MTHFA; oxygenated chemicals, such as Cyrene, Lgol, or THFDM from lignocellulosic biomass. 17% selectivity) over the Pd/SiO2-Al2O3 catalyst were not a strong function of conversion, indicating that THFDM is unreactive under these conditions and that the side-products are produced in parallel with THFDM. Because we expect that both THFDM and THP2M5H can be ring-opened to 1,2,6-hexanetriol, the overall selectivity from LGO to 1,6-hexanediol precursors is 78%. A separate experiment using a mixture of Lgol and tetrahydropyran-2- methanol-5-ketone (THP2M5one, generated via the acid-catalyzed isomerization of Lgol) showed that THP2M5one is not a THFDM precursor, and is instead hydrogenated to THP2M5H. A 2.5:1 excess of cis-THFDM was produced over trans-THFDM. The THFDM stereoisomer ratio was found to be independent of the Lgol feed stereoisomer ratio, indicating that the mechanism passes through an intermediate which erases the stereochemistry of the feed. A mechanism is proposed for Lgol hydrogenolysis to THFDM which is consistent with the above findings. Significance Our results reveal the chemistry underlying the hydrogenation of LGO over Scheme 1: Reaction network for LGO hydrogenation. Green text indicates potential industrial supported palladium catalysts, and provide directions for designing catalytic systems to applications. Dotted lines indicate reactions not studied in this report. optimize the production of several valuable chemicals, including green solvents and polymer precursors, from lignocellulosic biomass. Materials and Methods References Supported palladium catalysts were synthesized by incipient wetness impregnation 1. Cao, F. et al. Energy Environ. Sci. 8, 1808 (2015). of a Pd(NO3)2 precursor on the appropriate support, followed by calcination under flowing air 2. Sherwood, J. et al. Chem. Commun. 50, 9650 (2014). o o at 400 C, reduction under flowing H2 at 260 C, and passivation using 1% O2/He. Reactions 3. Allgeier, A.M. et al. US Patent 8865,940 B2 (2014). were carried out in 45-75 mL Parr Hastelloy batch reactors equipped with dip-tubes to take 4. Zanardi, M. M., et al. Tetrahedron Lett. 50, 999 (2009). product samples over time. 13C NMR was used to identify the major reaction products. A 5. Moreau, C. et al. Top. Catal. 27, 1 (2004). combination of GC, HPLC, and quantitative 13C NMR were used to quantify the reaction products.