Catalyst Requires Pre-Sorting of Oxygenated Molecules
Multiple "pots" needed to turn bio-based materials into hydrocarbon fuels
Reducing carbon dioxide emissions and decreasing our nation's reliance on fossil fuels means designing processes that efficiently create energy-dense fuels from renewable sources. Scientists at the Institute for Integrated Catalysis uncovered challenges in creating such a method using the zeolite catalyst HZSM-5 and biomass. Enlarge Image.
Results: The culprits in halting a promising catalyst from turning biomass-derived ethanol into fuel are small oxygen-containing molecules, according to scientists at Pacific Northwest National Laboratory and Washington State University. The catalyst HZSM-5 removes oxygen from ethanol and oligomerizes, or stitches together, ethanol's remaining hydrogen-laden carbon backbones into desirable fuel, but slowly the catalyst fails and cannot be regenerated. The scientists found that for the catalyst to work effectively, the biomass-derived, carbon-rich feedstock must be pretreated to remove the following oxygenated molecules: ethyl acetate, acetic acid, and acetaldehyde.
"Each one requires different catalysts and different operating conditions," said Dr. Yong Wang, lead researcher on the study and the Voiland Distinguished Professor at WSU who holds a joint appointment with PNNL. "Competitive adsorptions on catalyst's active sites halt the conversion to the desired products."
Why It Matters: Replacing fossil fuels in industry, which is projected to consume more than half of the planet's delivered energy in 2040, means developing fuels that can perform within today's infrastructure. Industrial plants are typically built for fuels with low water content. Creating such a fuel from biomass efficiently demands a one-pot synthesis method that converts the water-packed feed streams into the desired products. This study, part of the work done at PNNL's Institute for Integrated Catalysis, highlights the challenges of using HZSM-5 in a one-pot method.
Methods: To ultimately develop an approach using zeolite catalysts to create light hydrocarbon fuels from aqueous bio-based feedstocks requires knowing what conditions permanently deactivate the catalyst. In this study, the researchers combined HZSM-5 with ethanol alone and saw the desired reactions and formation of hydrocarbons. Then, they mixed the catalyst with ethanol and one of three low-molecular-weight oxygenates typically found in biomass conversion processes: acetic acid, ethyl acetate, or acetaldehyde. The reactions were run at moderate conditions in a fixed-bed reactor, 360 °C and 300 pounds per square inch gauge (psig). Compared to an ethanol-alone experiment, adding the other oxygenates reduced catalyst life in the order of least to greatest impact: ethyl acetate, acetic acid, and then acetaldehyde.
In the reaction, the ethyl acetate first broke apart to form ethanol and acetic acid. The ethanol performed as expected with the catalyst, but the acetic acid generated an intermediate acetate ion that poisoned the catalyst by strongly adsorbing on the active sites. The acetaldehyde, which deactivated the catalyst more quickly than the other two, generated high molecular weight aromatic compounds quickly and these compounds blocked the catalyst's active sites. The scientists concluded that to remove the oxygen from the three oxygenates and oligomerize the remaining hydrocarbon require different catalytic active sites and different operating conditions.
"When using the HZSM-5 catalyst to turn biomass-derived feed stock into high-value chemicals, you must pre-treat the mixture to produce a stream that contains the same or similar compounds," said Wang.
What's Next: The researchers continue to delve into the fundamentals of zeolite catalysts to find an efficient, affordable one-pot synthetic route for converting biomass into fuels.
Sponsors: DOE Office of the Biomass Program (now called the Bioenergy Technologies Office) and the PNNL Laboratory Directed Research and Development Program.
Research Area: Chemical Sciences
Research Team: Mark A. Gerber and Matt Flake, Pacific Northwest National Laboratory; He Zhang, Washington State University; Karthi Ramasamy and Yong Wang, Pacific Northwest National Laboratory and Washington State University
Reference: Ramasamy KK, MA Gerber, M Flake, H Zhang, and Y Wang. 2013. "Conversion of Biomass-Derived Small Oxygenates over HZSM-5 and Its Deactivation Mechanism." Green Chemistry. Advance of print. DOI: 10.1039/C3GC41369A