Aqueous phase processing (APP) is an exciting new technology for coupling solubilization of biomass with catalytic conversion of the resulting dissolved carbohydrate compounds into hydrocarbons and a variety of fuels and chemicals. Dumesic and co-workers have shown that APP can convert dissolved biomass derived compounds (including sugars, sugar alcohols, bio-oils, cellulose or even lignin) into hydrogen, light alkanes, liquid alkanes, and oxygenates.

We have recently identified the reaction chemistry for aqueous phase hydrodeoxygenation (APHDO) of sorbitol into C₁ to C₆ alkanes with bifunctional acid base catalysts. We have also shown how through modifying the reaction chemistry and the acid concentration of sites on the catalyst we can selectively produce a high octane gasoline blend from aqueous sorbitol solutions with a specially designed Pt/Zirconium phosphate (Pt/ZrP) catalyst.

While aqueous phase processing has been shown to be promising with pure carbohydrate solutions, it is vital to use less expensive aqueous solutions derived from lignocellulosic biomass, but we know of no study that has applied aqueous phase processing to streams resulting from biomass hydrolysis.

...it would be highly desirable to couple hydrolysis processes with aqueous phase hydrodeoxygenation to be able to use low cost lignocellulosic feedstocks for making hydrocarbon fuels.

The objective of this paper is to show how hydrolysis can be combined with APHDO to produce fuels from lignocellulosic biomass using maple wood as a representative feedstock. In this study, we seek to understand how to optimize the hydrolysis step to produce solutions that can be effectively upgraded to biofuels. Importantly, we will identify the key intermediates in the hydrolysis residues that are desirable and undesirable for APHDO and also determine whether hydrolysis residues can be processed using APHDO without significant catalyst deactivation.

—Li et al.

The team prepared aqueous carbohydrate solutions from maple wood using both hydrolysis in hot water and hydrolysis with dilute acids (H₂SO₄, oxalic acid). The aqueous carbohydrate solutions produced via hydrolysis were a mixture of xylose, water soluble hemicellulose oligomers, acetic acid, glucose, glucose oligomers, and probably some lignin polymers. Hydrolysis with hot water produced primarily hemicellulose oligomers; hydrolysis with acids (H₂SO₄, oxalic acid) produced mainly xylose and acetic acid.

These aqueous streams were then hydrodeoxygenated by a two-step catalytic process in which the first catalyst bed contained a Ru/C catalyst at 393 K and the second catalyst bed contained a Pt/zirconium phosphate (Pt/ZrP) catalyst at 518 K. The Ru/C catalyst was able to selectively hydrogenate xylose into xylitol but could not selectively hydrogenate the xylose oligomers.

Similar to the team’s work with APHDO of sorbitol and xylitol, three categories of products were generated:

1. light gases such as CO₂, methane, ethane, propane, and butane;

2. gasoline-range products including pentane, hexane, and C₂–C₆ monofunctional compounds such as alcohols, ketones, cyclic ethers, and small amount of carboxylic acids; and

3. aqueous phase products including methanol, 1,4-sorbitan, isosorbide, propanediol, butanediol, pentanediol, and hexanediol.

The estimated research octane number (RON) of the gasoline range products from the aqueous phase hydrodeoxygenation of glucose, xylose and maple wood hydrolysis is 82 to 96—similar to the RON of gasoline in the US market today. Thus, Li et al. noted, the majority of the monofunctional compounds could be used as high-octane gasoline additives or as precursors to produce longer chain diesel and jet fuel molecules.

They concluded that “aqueous phase hydrodeoxygenation of biomass derived hydrolysis residues is a promising option for the production of fuels and chemicals from lignocellulosic biomass that merits further research.”

Resources

• Ning Li, Geoffrey A. Tompsett, Taiying Zhang, Jian Shi, Charles E. Wyman and George W. Huber (2011) Renewable gasoline from aqueous phase hydrodeoxygenation of aqueous sugar solutions prepared by hydrolysis of maple wood. Green Chemistry 13, 91 doi: 10.1039/c0gc00501k