In the new UMass approach, the hydroprocessing increases the intrinsic hydrogen content of the pyrolysis oil, producing polyols and alcohols. The zeolite catalyst then converts these hydrogenated products into light olefins and aromatic hydrocarbons in a yield as much as three times higher than that produced with the pure pyrolysis oil.

The yield of aromatic hydrocarbons and light olefins from the biomass conversion over zeolite is proportional to the intrinsic amount of hydrogen added to the biomass feedstock during hydroprocessing. The total product yield can be adjusted depending on market values of the chemical feedstocks and the relative prices of the hydrogen and biomass, the researchers said.

The integrated catalytic approach presented in this report can be tuned to produce different targeted distributions of organic small molecules that fit seamlessly into the existing petrochemical infrastructure. The products can be tuned to change with different market conditions. The C₆ to C₈ aromatic hydrocarbons can be high-octane gasoline additives or feedstocks for the chemical and polymer industries. The C₂ to C₄ olefins can also be used directly for polymer synthesis or can be modified to form other products, including alkylated aromatics and longer linear alpha olefins. The gasoline-range alcohols can be high-octane gasoline additives. The C₂ to C₆ diols can serve as feedstocks for the chemical and polymer industries. The chemical industry relies on seven primary building blocks that are all derived from petroleum-based processes: benzene, toluene, xylene, ethylene, propylene, 1,3-butadiene, and methanol. Our catalytic process produces five of these seven petrochemical feedstocks, which opens the door to a chemical industry based on renewable biomass feedstock.

—Vispute et al.

Reaction schematic for the integrated hydroprocessing and zeolite upgrading of pyrolysis oil. Solid arrows: Pyrolysis oil is directly passed over the zeolite catalyst. Long-dashed arrows: Pyrolysis oil is hydrogenated over Ru/C at 398 K and then passed over the zeolite catalyst. Short-dashed arrows: Pyrolysis oil is first hydrogenated over Ru/C at 398 K, then over Pt/C at 523 K, and then passed over the zeolite catalyst. The width of the vertical arrows represents the product carbon yield from a particular field. In addition to the product streams shown in the figure, oxygen is removed at the zeolite stage as a mixture of CO, CO2, and H2O. Boosting the hydrogen content of the zeolite feed (left to right) increases the thermal stability of the feed, resulting in a reduction in amount of coke and an increase in the yields of aromatic hydrocarbons and olefins. The addition of hydrogen also raises the proportion of oxygen lost as water relative to CO and CO2 and thereby further raises the proportion of carbon incorporated into marketable compounds. Credit: Science, Vispute et al. Click to enlarge.

The cost of hydrogen—which varies widely depending on location, mode of production and supply, and natural gas prices—is important in determining how much hydroprocessing should be done before deoxygenation with the zeolite catalyst, the authors note. The optimum H/C_eff ratio—the hydrogen-to-carbon atomic effective ratio—where the economic potential of the process is highest, decreases with increasing hydrogen cost.

Combining the hydrogenation steps with a zeolite conversion step reduces the overall hydrogen requirements as compared to using hydrogen for a complete deoxygenation of pyrolysis oil. A complete deoxygenation of pyrolysis oil by hydrodeoxygenation, with a 100% carbon yield to the corresponding hydrocarbons (that is, alkanes from C₁ to C₆ nonphenolic oxygenates and aromatics from phenolic compounds), requires 14 to 15 g of H₂/100 g of carbon in the feed, if the catalyst coking problems are overcome. In comparison, increasing the H/C_eff ratio of pyrolysis oil from 0 to 1.4 requires 11.7 g of H₂/100 g of carbon in the feed, reducing the hydrogen requirements as compared to complete hydrodeoxygenation by 20%.

Furthermore, during the hydrotreating process, large amounts of undesired methane are produced, which also can substantially increase the hydrogen requirements. Hydrogen required in these processes should preferably be obtained from renewable sources, such as by the reforming of biomass-derived feedstock. Alternatively, hydrogen can be obtained from coal gasification or from water splitting driven by carbon-free energy sources, such as solar, nuclear, and wind energy, as suggested by Agrawal et al.. Zeolite catalysts convert the biomass feedstocks into aromatics and olefins, which can fit easily into the existing infrastructure. Increasing the yield of petrochemical products from biomass therefore requires hydrogen. Thus, there exists an optimum solution for the economical maximum yield of petrochemical feedstocks products that is dictated by the cost of hydrogen. It is expected that future advances in the field of metal and zeolite catalysts, combined with reaction engineering, will allow us to design even more efficient and economical processes to convert biomass resources to renewable chemical industry feedstocks.

—Vispute et al.

Resources

• Tushar P. Vispute, Huiyan Zhang, Aimaro Sanna, Rui Xiao and George W. Huber (2010) Renewable Chemical Commodity Feedstocks from Integrated Catalytic Processing of Pyrolysis Oils. Science Vol. 330 no. 6008 pp. 1222-1227 doi: 10.1126/science.1194218