|Strategy for the conversion of solid cellulose to liquid hydrocarbon fuels. H+: acid sites; Me: metal sites; MeOx: metal oxide sites. Source: Serrano-Ruiz et al Click to enlarge.|
Researchers at the University of Wisconsin–Madison led by James Dumesic have developed a catalytic process to convert cellulose into liquid hydrocarbon fuels (diesel and gasoline), using a cascade strategy to achieve the progressive removal of oxygen from biomass, allowing the control of reactivity and facilitating the separation of products. A report on the process was published online 4 August in the journal Applied Catalysis B: Environmental.
Dumesic and his team earlier had reported a process for the catalytic upgrading of levulinic acid to 5-nonanone—a hydrophobic intermediate that can be catalytically upgraded to diesel and jet fuels—with the intermediate formation of γ-valerolactone (GVL). (Earlier post). Since levulinic acid is obtained by acid hydrolysis of lignocellulosic wastes, this route can potentially serve to produce liquid transportation fuels from biomass.
The new paper describes the extension of this route from solid biomass (cellulose) to liquid hydrocarbon fuels (diesel and gasoline) by developing a cascade approach.
The process starts with the deconstruction of solid cellulose in an aqueous solution of sulfuric acid yielding an equi-molar mixture of levulinic acid and formic acid. The team added solid cellulose to the reactor in progressive stages, thereby avoiding high levels of cellulose in the slurry at any given time and achieving effective mixing in the reactor over the entire period of deconstruction.
Sequential feeding also avoids high concentrations of glucose in the reactor, thereby minimizing undesirable polymerization reactions (favored at high concentrations of glucose) decrease the yield to levulinic acid. The levulinic acid yield is above 60% for the first cycle and gradually decreases with each cycle of cellulose addition, with a final yield of 52% after five cycles.
A Ru/C catalyst converts the formic acid in the mixture to CO2 and H2, combined with the reduction of levulinic acid to GVL; the H2 released by the decomposition of formic acid is utilized in the catalytic reduction of levulinic acid.
The formation of GVL allows strategies for the separation and recycling of the sulfuric acid used in the cellulose deconstruction step. This GVL product, with residual amounts of sulfur, can be upgraded to 5-nonanone with high yields (90%) in a single reactor by using a dual catalyst bed of Pd/Nb2O5 plus ceria-zirconia.
The 5-nonanone product is hydrophobic and separates spontaneously from water, yet possesses a functional group that can be used to control the structure and molecular weight of hydrocarbon fuel components formed in established downstream catalytic upgrading treatments: e.g., hydrogenation, dehydration, isomerization.
Following the conversion of cellulose into the monofunctional 5-nonanone, there are several catalytic upgrading approaches that can be applied to produce diesel and jet fuel components. This C9 ketone can be completely reduced to n-nonane by means of hydrogenation/dehydration cycles over a bifunctional metal-acid catalyst such as Pt/Nb2O5. This nonane product possesses an excellent cetane number (72), heating value (45 kJ/g) and viscosity (0.96 mm2/s) for use in high-speed diesel engines. Additionally, the low melting point of nonane (220 K) makes this compound a good seasonal blending agent to improve flow properties and decrease the cloud point of current diesel fuels during cold weather.
Moreover, the functional group of 5-nonanone can be used to control the structure and molecular weight of hydrocarbon fuel components formed in downstream catalytic upgrading treatment. For example, we have shown that 5-nonanol (the hydrogenation product of 5-nonanone) can be dehydrated and isomerized in a single step over an USY zeolite catalyst to produce a mixture of branched C9 alkenes which can be optionally hydrogenated to produce alkanes with appropriate structures and molecular weight for use in gasoline applications.
We note that the biomass derived 5-nonanone can also be converted into a C9 alkene stream (by means of hydrogenation and dehydration reactions) which can be oligomerized over an acid catalyst to achieve good yields of C18–C27 olefins, in the molecular weight range suitable for jet and diesel fuel applications.
Finally we note that the cascade approach described herein to convert cellulose can be integrated with existing processes to convert the hemi-cellulose and lignin fractions of lignocellulosic biomass. For example, hemi-cellulose can undergo hydrolysis to produce xylose, which can be used to produce H2 and CO2 by aqueous-phase reforming, or to produce furfural. The lignin fraction of biomass and solid humins (formed during the cellulose processing to GVL) can undergo gasification to produce H2, CO2, CO and CH4.
Importantly, the catalytic upgrading of GVL to pentanoic acid herein described can be carried out at atmospheric pressure, allowing the potential use of H2-enriched streams from biomass gasifiers for the GVL reduction to pentanoic acid, without the need of costly compression steps. In addition, pyrolysis could be employed to convert lignin and solid humins to bio-oils or to aromatic fuels.
—Serrano-Ruiz, et al
This work was supported in part by the US Department of Energy (DOE) Office of Basic Energy Sciences, and by the DOE Great Lakes Bioenergy Research Center.
J.C. Serrano-Ruiz, et al. (2010) Conversion of cellulose to hydrocarbon fuels by progressive removal of oxygen. Appl. Catal. B: Environ. doi:10.1016/j.apcatb.2010.07.029 doi: 10.1016/j.apcatb.2010.07.029