PNNL

America runs on catalysts. And, the most common type of catalysts are zeolites—microporous, crystalline structures made primarily of silicon, aluminum, and oxygen. Zeolite catalysts play an integral part in our daily lives, from refining fuel, to producing plastics, to cleaning up radioactive waste. They are particularly important in the biofuel industry because of their ability to convert small oxygenates, such as ethanol, into hydrocarbons.

Zeolites make great catalysts because of their porosity, ability to withstand high temperatures and pressures, and resistance to oxidation and dissolving. However, as reliable as zeolite catalysts are, they remain susceptible to a phenomenon known as coking—the buildup of unwanted chemical deposits in the pores. Over time, as cokes accumulate in pores the catalyst becomes less effective, unable to convert chemicals into the desired products. The catalysts then need to undergo a thermal process to be reactivated, which can slow refining and increases costs.

By better understanding the formation of coke, scientists hope to improve catalyst design and prevent deactivation processes. But first, scientists need more effective means of “seeing” what leads to coking and where coking is located on the zeolite. According to a paper published in Scientific Reports, researchers at PNNL have found a new way using instruments at PNNL and Lawrence Berkeley National Laboratory (LBNL).

Traditionally, to analyze the composition of coke deposits, zeolite catalysts are treated with strong acids to break the pore structure and liberate the coke molecules. Researchers then examine the chemicals removed from the catalyst. Using such methods only helps to understand the chemical nature and composition of the coke molecules, but not the exact location where the coke molecules formed in the catalyst pores.

To achieve a more complete understanding of the location and the nature of the coke molecules, the PNNL team evaluated different state-of-the-art microscopy and spectroscopy methods—X-ray absorption spectroscopy at LBNL’s Advanced Light Source and nuclear magnetic resonance spectroscopy and atom probe tomography at EMSL, the Environmental Molecular Sciences Laboratory, at PNNL—to map out the catalyst and identify the location and chemical nature of the coke. Zeolite catalysts were first analyzed prior to being used. After being used to convert ethanol to hydrocarbon, the same catatlysts were examined again for signatures of coking. The team observed hydrocarbon molecules present in the pores of the catalyst at the same locations where aluminum concentrations were also the highest. Such coke molecules are known to lead to catalyst deactivation.

Based on their analysis, the team determined atom probe tomography to be a unique method for understanding coke formation mechanism at the nanoscale. The technique can be applied to any porous catalyst where coking or other poisoning mechanisms may be an issue.

A clearer view of coking could yield an unprecedented understanding of catalytic deactivation and in turn advance efforts to produce renewable fuels and chemicals from biomass. The insights gained from the project could lead to changes in catalyst synthesis methods to achieve a better distribution of aluminum or other elements, thus minimizing coking during the biofuel conversion processes.

The research was funded by DOE’s Bioenergy Technologies Office; Approved Program of Advanced Light source, a DOE’s Office of Basic Energy Sciences user facility; the Laboratory Directed Research & Development (LDRD) Program at PNNL; and EMSL, an Office of Science user facility by DOE’s Office of Biological and Environmental Research.

PNNL Research Team: Arun Devaraj, Vijayakumar Murugesan, Jie Bao, Mond F. Guo, Mirosław A. Derewinski, Zhijie Xu, Michel J. Gray, Sebastian Prodinger, and Karthikeyan K. Ramasamy