(6ap) Acid Catalysis of Lipids to Produce Green Fuels: Advancing Biofuels in a Fossil Fuel World

Authors: 
Benson, T. J., Mississippi State University


Development of renewable fuels is essential to global sustainability as Earth's fossil fuels are depleted, resulting in decreased supplies for transportation fuels and energy generation. Scientists have identified new renewable resources capable of displacing significant quantities of petroleum, the chief feedstock for transportation fuels and commodity chemicals. For example, lipids can be extracted from plants, animals, and microorganisms and used for fuel and specialty chemical production. Our research group has estimated that approximately 10 billion gallons of lipids could be generated and converted into fuels using current agricultural practices, wastewater treatment, and refining infrastructure.

During my master's work, I explored various bio-adsorbents for the removal of water from ethanol/water mixtures (1). Using a temperature swing adsorber, mixtures of 90, 95, and 97 wt% ethanol were volatilized and passed through a column bed packed with either hardwood sawdust, kenaf core, or bleached wood pulp. A glass adsorption column with an inside diameter of 2.54 cm was used to generate breakthrough curves to determine the effectiveness of the adsorbents and to allow comparability with starch based adsorbents that are currently used within the ethanol industry. Experimental results indicated that water was preferentially adsorbed allowing for complete dehydration of ethanol. Bleached wood pulp was a superior adsorbent over the kenaf core and hardwood sawdust in adsorbing water due to its large percentage of hydroxyl groups stemming from a high concentration of xylans. The bleached wood pulp was the only material that demonstrated similar water adsorbing capacities as compared to starch-based adsorbents such as corn grits.

My doctoral thesis has been devoted to the advancement of science in the cracking of lipid molecules into diesel-like and gasoline-like fuels. First, oleic acid, an unsaturated free fatty acid in many lipid sources, was reacted with triflic acid, a liquid superacid catalyst at 0°C (2). Instrumentation employed for sample analyzes included 1H-NMR, 13C-NMR, FTIR, and GC/MS (both EI and CI) to identify reaction products and to hypothesize a reaction pathway. The resultant products were saturated fatty acids with carbon lengths of C9-C14, C16, and C18. This work concluded that a Bronsted acid protonates the double bond causing methyl and hydride shifts that succumb to β-scissions resulting in linear and branched chain isomers of smaller carbon lengths than the original starting material. The cracking of oleic acid was in stark contrast to palmitic acid, a saturated fatty acid, which yielded no discernable products when reacted with triflic acid at temperatures up to 100°C.

The second step in this research was to evaluate the heterogeneous catalytic cracking chemistry of model lipid compounds, chiefly oleic acid, mono-, di-, and tri-olein. For this phase, a reactor/analyzer was constructed using a Varian 3600 GC which was reengineered to allow for reactions with online mass spectrometry analysis (3). This unit, which is capable of accepting a gas, liquid, or solid reactant, was equipped with a Saturn quadrupole ion trap mass spectrometer for mass spectral analysis in real-time as lipids were passed over a bed of highly acidic ZSM-5 catalyst. As this was an integrated reactor/analyzer, the Rxi-1ms GC column was exposed in the inlet to temperatures in excess of 400°C. This temperature is beyond the degradation point of the column, and so selective ion storage RF waveform was used to remove the siloxane masses m/z (207, 281, and 355) from the spectra. This produced lower detection limits and full scan data for identification. Electron impact and selected ejection chemical ionization were used for product identification and molecular weight confirmation. Hexane was reacted over H-ZSM-5 catalyst for instrument validation. This produced a series of alkanes and alkenes with distributions consistent with that reported for the cracking of hexane. The heterogeneous cracking of model compounds resulted in mixtures that were within the ranges of diesel and gasoline organics. Oleic acid was found to form chiefly BTEX and trimethylbenzene as initial cracking products. Decarboxylation is expected to follow a radical reaction pathway, yielding CO2. We plan to also publish these results in the Journal of Molecular Catalysis A: Chemical (4).

Along with research, I had the opportunity to teach a chemical engineering core course (CHE 3213 - Fluid Flow Operations) for two semesters. My average student evaluations (4.23/5.00) were above the average for the College of Engineering. I also had numerous opportunities to teach laboratory practices and experimental procedures to undergraduate and high school students who have worked with me to develop this research. My goal is to contribute to the education of future chemical engineers through research, teaching, and service to the community.

Refereed Publications

(1) T Benson and C George (2004), ?Cellulose Based Adsorbent Materials for the Dehydration of Ethanol Using Thermal Swing Adsorption,? Adsorption Journal Vol.11.

(2) T Benson, R Hernandez, T French, E Alley, and W Holmes, ?Reactions of Fatty Acids in Superacid Media: Identification of Equilibrium Products? Journal of Molecular Catalysis A: Chemical (Accepted for publication, May 2007).

(3) T Benson, W Holmes, R Hernandez, T French, and E Alley, ?Heterogeneous Catalytic Cracking Reactor Utilizing Online Mass Spectrometry Analysis? Journal of the American Society for Mass Spectrometry (Expected submission: June 2007)

(4) T Benson, R Hernandez, T French, E Alley, and W Holmes, ?Heterogeneous Reaction of Model Glycerides over H-ZSM5 Catalyst: Elucidation of Cracking Mechanisms? Journal of Molecular Catalysis A: Chemical (Expected submission: Summer 2007)