Tunable high-yield catalytic approach converts pyrolysis oil to bio-hydrocarbon chemical feedstocks including fuel additives
Researchers at University of Massachusetts-Amherst, with colleagues from Southeast University (China) and University of Nottingham (UK), have developed an integrated catalytic approach that combines hydroprocessing with zeolite catalysts to convert pyrolysis bio-oils into industrial commodity chemical feedstocks, including C2 to C6 monohydric alcohols and diols, C6 to C8 aromatic hydrocarbons, and C2 to C4 olefins with more than 60% overall carbon yields. A paper on the work led by Dr. George Huber was published in the 26 November issue of the journal Science.
Pyrolysis bio-oils are produced by the thermal decomposition of biomass by heating in the absence of oxygen at more than 500 °C; fast pyrolysis of biomass is much less expensive than biomass conversion technologies based on gasification or fermentation processes. However, the resulting bio-oils are typically high in oxygen, water, solids and acids. As a result, there is high interest in cost-effective pathways for the upgrading of bio-oils to more useful transportation fuel or commodity chemical components. (E.g., earlier post.)
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.
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/Ceff 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/Ceff 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.
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