New One-Pot Catalytic Process For Hydrogenation of Bio-Oil to Produce Alkanes
12 May 2009
| Plot of phenol conversion, cyclohexanol selectivity, and cyclohexanone selectivity for the aqueous-phase hydrogenation of phenol as a function of reaction time. Zhao et al. (2009) Click to enlarge. |
A team of German and Chinese scientists led by Johannes A. Lercher at the Technical University of Munich has developed a new catalytic process for the aqueous-phase hydrogenation of components of bio-oil directly into alkanes and methanol. As reported in the journal Angewandte Chemie, the process is based on a “one-pot reaction” catalyzed by a precious metal on a carbon support combined with an inorganic acid.
Bio-oil (or pyrolysis oil) is produced by fast pyrolysis or liquefaction of biomass. Although a promising second-generation renewable energy carrier, its high oxygen content, instability and lower energy content make direct use as an advanced liquid fuel not feasible. Consequently, there are a number of research initiatives underway exploring pathways for the efficient upgrading of bio-oil to a fungible hydrocarbon fuel. The US Department of Energy is also funding research in stabilizing bio-oils to support such upgrading. (Earlier post.)
Bio-oil is an aqueous, acidic, highly oxidized mixture (containing 15–30 wt% H2O, pH 2.5) and a highly oxygenated (nearly sulfur-free, ca. 30 wt% phenolic fraction). While not useful as a direct advanced fuel, it is an attractive feedstock for upgrading into alkanes.
Alkanes, commonly called paraffins, are saturated hydrocarbons; they are among the most important raw materials for chemical industry, and in particular as starting materials for the production of plastics. Furthermore, they are among the primary fuels in the world’s economy.
Hydrodeoxygenation is considered to be the most effective method for bio-oil upgrading. Conventional (sulfidebased) hydrotreating catalysts, however, contaminate products by incorporation of sulfur, deactivate rapidly by coke deposition, and are potentially poisoned by trace amounts of water. Therefore, conceptually the reductive upgrading of bio-oil in an acidic aqueous medium using metal catalysts offers a new attractive alternative route.
—Zhao et al. (2009)
Bio-oil contains a phenolic fraction consisting of compounds with the main framework being an aromatic ring made of six carbon atoms with some hydroxy (-OH) groups attached. The research team developed a new catalytic route with a bifunctional combination of a stable carbon-supported palladium catalyst with phosphoric acid as the proton source for the reaction.
The reaction is a one-pot reaction, meaning a one-step reaction whose partial reactions (hydrogenation, hydrolysis, and dehydration) occur in the same reactor, with no intermediate work-up.
This reaction pathway for the stepwise aqueous-phase hydrodeoxygenation of phenolic monomers is based on bifunctional catalysis, that is, coupling metal-catalyzed hydrogenation and acid-catalyzed hydrolysis and dehydration; this pathway differs drastically from that of C–O bond hydrogenolysis with sulfide catalysts.
—Zhao et al. (2009)
The end result is a mixture of various alkanes that separates into a second phase, making it easy to separate from the aqueous bio-oil phase.
The new route provides a feasible approach for the direct use of crude aqueous bio-oil mixture, facilitating an energy-efficient and atom-economic process.
—Zhao et al. (2009)
Resources
Chen Zhao, Yuan Kou, Angeliki A. Lemonidou, Xuebing Li, and Johannes A. Lercher (2009) Highly Selective Catalytic Conversion of Phenolic Bio-Oil to Alkanes. Angewandte Chemie International Edition 48, No. 22, 4047-4050, doi: 10.1002/anie.200900404
Very interesting.
In the paper, they show a lot of H2 is added for upgrading of the fuel. This adds a lot of energy to every carbon atom. Hydrogen can be made from biomass, but much more effectively, it will probably be made from wind, solar or nuclear. Biomass (even algae)is relatively to extremely inefficient in producing energy/square meter, so it is important to add as much energy as possible to every carbon atom.
Posted by: Alain | 12 May 2009 at 09:06 AM
This yields what advantages over current technology?
Posted by: kelly | 13 May 2009 at 06:31 AM
@ Kelly,
There is - by far - not enough biomass to replace all the liquid fuels we need. We may get there eventually if we transform huge parts of our planet to one big switchgrass monoculture. On the other hand, very soon, H2 can be produced relatively easy, abundantly, cheaply and environmently friendly using wind, solar, wave, nuclear,...
So as long as we need carbon-based fuels, it's important to have as much energy per carbon atom as possible. Upgrading biomass-carbon from low-energy content to high-energy content is essential to maximize its advantages. Even then there will not be enough biomass for a long time to come to replace all the fuels we need. And once we don't need as much carbon-based fuels anymore, we will surely start to transform part of the biomass to biochar and part of it to fuel and polymers. The more external H2 is added, the more biochar you can produce.
Since an area of switchgrass + windmills produces hundred time more wind-energy than bio-energy, it is very feasible to produce the H2 localy where the biomass is produced. As the price of windmills will come down, the H2 will be much cheaper than the biomass per Joule. This way we can have a great buffer capacity for the wind-energy. When demand (and price) for electricity is high, you sell the electricity, when demand is low, you produce H2 in cheap electrolisers and use it localy to upgrade fuel.
Posted by: Alain | 13 May 2009 at 08:56 AM