Reaction Mechanisms

Problem

The chemical reactions and mechanisms involved in the transformation of biomass to fuels are poorly understood, the thermodynamically and kinetically favorable reactions and pathways are not clear, and the catalyst deactivation mechanisms are controversial.

Approach

The conversion of ligno-cellulosic biomass to fuels and chemicals requires the effective utilization of both hemi-cellulose and cellulose components, consisting primarily of C5 and C6 sugars, respectively. Researchers have explored two classes of processing strategies — one in which the hemi-cellulose and cellulose fractions are processed together, and the other in which they are processed separately. While the simultaneous processing of all carbohydrates, such as in gasification or pyrolysis, offers the potential for simplicity of operation, the fractionation of hemi-cellulose and cellulose allows tailoring of the processes for each fraction, to take advantage of the different chemical and physical properties of these fractions. This fractionation also offers increased flexibility of operation.

With an understanding of these reaction mechanisms, scientists can develop the roadmaps for catalytic conversion of biomass. IACT is looking at four promising reaction mechanisms:

• Hydrogenation of furfural to furfuryl alcohol (FAL);
• Hydrogenolysis of etheric carbon—oxygen bonds;
• Conversion of furfuryl alcohol to levulinic acid and levulinate esters; and
• Conversion of fructose and hydroxymethylfurfural (HMF).

Furfural is an important intermediate in the conversion of hemi-cellulose to biofuels, but we must gain an improved understanding of the factors that control the reactivity and selectivity of sugars and furan compounds in liquid solvents, such as water and ethanol (biomass reaction conditions), and to make key connections to experimental results obtained by IACT's team of collaborators. A second goal is to understand the factors controlling the activity, selectivity, and stability of metal catalysts for the hydrogenation of furfural to furfuryl alcohol. This project employs state-of-the-art operando characterization and density functional theory to gain improved understanding of the reaction pathways.

Goals

IACT's overarching goals for the Reaction Mechanisms effort are to:

• Use and apply a variety of investigative tools and techniques to elucidate reaction intermediates;
• Perform density functional theory (DFT) calculations on mechanisms of furfuryl alcohol hydrogenation and decarbonylation, and lactic acid hydrogenation; and
• Investigate comprehensive reaction networks for glucose and fructose conversion.

Results

To date, the IACT Reaction Mechanisms team has:

1. Identified intermediates for FAL conversion, polymerization, and fructose dehydration;
2. Clarified the catalyst deactivation mechanism during the hydrogenation of furfural to furfuryl alcohol;
3. Elucidated the mechanism of acid-catalyzed fructose conversion, and
4. Established thermochemical trends for the reaction of lactic acid on the close-packed facets of late transition metals.

Future Directions

Going forward, IACT Reaction Mechanisms researchers will extend their earlier studies to the structural, energetic, electronic, and chemical reactivity properties of platinum (Pt), molybdenum (Mo), and Pt/Mo nanocatalysts on supports such as amorphous carbon and various oxides. They will elucidate and characterize the effects of the supports on the catalytic functionality of the particles. They will also perform studies aimed at finding sizes, compositions, and structures of Pt/M nanocatalysts and their supports with high selectivity for carbon-carbon (C-C), carbon-hydrogen (C-H), or carbon-oxygen (C-O) bond cleavage in various CxOyHz compounds, including those detected experimentally in the product stream of glycerol reforming; they are particularly interested in selectively breaking the C-O bond. They also will characterize the energetics and mechanisms of the different reaction pathways involved.

November 2012