Research in the Biofuels and Environmental Catalysis group is focused on two principal objectives: to reduce the environmental impacts of fuel use and to develop renewable fuel sources. Central to these goals is the application of catalysis. In addition to the synthesis of novel catalysts, studies are conducted to probe catalyst structure, mode of action and deactivation mechanisms. The optimization of reaction conditions and the application of novel reactor technologies are also important activities.
Research is conducted in collaboration with a number of other university departments and external institutions, and is performed by both CAER staff and students. Collaboration between CAER and the various science and engineering departments at UK allows students to receive traditional graduate and undergraduate education, while being exposed to the unique multidisciplinary environment at CAER. The following provides an overview of the main topics currently being researched.
Low-cost, high capacity processes for the conversion of biomass into fuels and chemicals are essential for expanding the utilization of carbon neutral processes, reducing dependency on fossil fuel resources, and increasing rural income. Currently the production of ethanol by fermentation of corn-derived carbohydrates is the main technology used for the production of liquid fuels from biomass resources. Alternatively, biomass can be converted into fuels and chemicals indirectly (by gasification to syngas followed by catalytic conversion to liquid fuels) or directly to a liquid product by thermochemical means.
Direct thermochemical conversion processes include pyrolysis, liquefaction, and solvolysis. In collaboration with the Power Generation and Utility Fuels group and the Carbon Materials group, we are investigating a novel liquefaction-extraction method for biomass conversion to crude bio-oil. The proposed approach aims to utilize biomass on the farm for delayed transport to centralized bio-refineries. Given that biomass is expensive to transport, being able to pre-process and densify the biomass before transport to a centralized bio-refinery will save significant costs and potentially increase rural income.
The crude bio-oils afforded by thermochemical conversion processes such as pyrolysis are chemically complex and are typified by a high oxygen content. The oxygenated compounds present in raw bio-oils impart a number of unwanted characteristics such as thermal instability (reflected in increasing viscosity upon storage), corrosivity and low heating value. This instability is associated with the presence of reactive chemical species, notably alkenes, aldehydes, ketones, carboxylic acids and guaiacol-type molecules. Upon prolonged storage, condensation reactions involving these functional groups result in the formation of heavier compounds. The quality of bio-oils can be improved by the partial or total elimination of the oxygenated functionalities present. In this context, we are studying new approaches for catalyst-assisted stabilization of crude biomass-derived pyrolysis oils, for the ultimate production of fuels and high value chemicals. This work is performed in collaboration with the Department of Biosystems and Agricultural Engineering.
The production of biodiesel from vegetable oil represents another means of producing liquid fuels from biomass, and one which is growing rapidly in commercial importance. Commercially, biodiesel is produced from vegetable oils, including rapeseed, sunflower and soybean oil, as well as from animal fats. These oils and fats are typically composed of C14-C20 fatty acid triglycerides. In order to produce a fuel that is suitable for use in diesel engines, these triglycerides are converted to the respective alkyl esters (with glycerol as a co-product) by base-catalyzed transesterification with short chain alcohols.
Commercially homogeneous base catalysts are used, such as NaOH. However, solid base catalysts are attractive on the basis that their use should (i) result in a reduction in the amount of soaps and salts that need to be removed (thereby improving the quality of the glycerol co-product), and (ii) enable biodiesel production to be more readily performed as a continuous process. We are therefore studying the use of a variety of solid base catalysts for this purpose, such as layered double hydroxides.
Ongoing work at CAER is focused on reducing emissions of nitrogen oxides (NOx) present in lean exhaust gases (i.e., gases in which excess oxygen is present) such as those emitted by diesel engines. Lean NOx traps (LNTs) represent a promising technology for the abatement of NOx under lean conditions. Although LNTs have found application on lean-burn gasoline vehicles in Europe, the issue of catalyst durability remains problematic. LNT susceptibility to sulfur poisoning is the single most important factor determining effective catalyst lifetime. The NOx storage element of the catalyst has a greater affinity for SO2 than it does for NO2, and the resulting sulfate is more stable than the stored nitrate.
Although this sulfate can be removed from the catalyst by means of high temperature treatment under rich conditions, the required conditions give rise to deactivation mechanisms such as precious metal sintering, total surface area loss, and solid state reactions between the various oxides present. A principle objective of our work is to improve understanding of the mechanisms of lean NOx trap aging, and to understand the effect of washcoat composition on catalyst aging characteristics. The resulting insights may assist the design of more durable catalysts.
Another area of interest concerns the development of catalysts for NOx reduction under lean conditions using hydrocarbon reductants ("HC-SCR"). In this context, the properties of Pt/carbon nanotube catalysts are being explored. A further objective is to study structure-activity relationships in Pt-based metal alloy catalysts, so as to derive fundamental insights which may aid the design of HC-SCR catalysts.
Carbon materials represent an important class of catalyst supports. Whilst activated forms of carbon have traditionally been used in catalysis, in recent years new forms of carbon such as carbon nanofibers and nanotubes have become available. Studies on the use of these materials in catalysis are still relatively sparse, although some reports suggest that in catalytic applications they can prove superior to traditional carbon supports. In collaboration with the Carbon Materials group at CAER, we are exploring the use of carbon multi-walled nanotubes (MWNTs) in catalysis.
Other potential applications of carbon nanotubes include microelectronics, battery electrodes, drug delivery, RF shielding, conductive polymers and high strength composites (e.g., nanotube reinforced polymers). In collaboration with the Carbon Materials group we are studying the preparation of heteroatom-doped carbon nanotubes. Doping MWNTs with heteroatoms may result in useful chemical and electronic properties. For example, such doping may lead to the formation of electron-excess n-type semi-conductors (e.g., N-doped CNTs) or electron-deficient p-type materials (e.g., B-doped CNTs). In this context, we have recently prepared B-C-N MWNTs incorporating significantly higher concentrations of boron and nitrogen (up to 18 at.% and 17 at.%, respectively) than was hitherto possible. The electronic properties and growth mechanism of these B-C-N nanotubes are currently under investigation.
Contact: Mark Crocker