Creating a GPS for aluminum ions
Employing a combination of methods devised at Pacific Northwest National Laboratory and the Swiss Light Source, scientists at the two institutions were able to determine the distribution of aluminum ions in zeolites, tectosilicates consisting of an array of silicon-oxygen tetrahedra, which form a complex network of pores. The material is widely used by industry as an ion exchange material and solid acid, catalyzing a broad range of chemical reactions. In the first example of this approach, two chemical compositions of a structural variant, known as zeolite beta, have been analyzed. Aluminum atoms, which are critical to the catalytic activity, preferentially replace silicon atoms in the tetrahedra forming the zeolite lattice at only a few sites in the crystalline solid.
Zeolites used extensively in industry today are promising catalysts that turn biomass into transportation fuels, but the activity and stability of this class of materials (see sidebar) is challenging to understand and predict. Of particular interest is the location of the aluminum ions substituted for silicon in the zeolite lattice, eventually forming Brønsted acid sites. This study provides information to help design new catalysts and improve existing ones, increasing the efficiency of producing fuels and chemicals alike. In addition, the approach provides quantitative chemical insight about aluminum positions in a much broader field of important oxidic materials containing aluminum.
"This approach had never been applied to zeolites at this level of detail before," said Dr. Donald Camaioni, a PNNL chemist on the study. "It gave us an unprecedented level of structural detail regarding the placement of aluminum ions in the zeolite framework."
Methods: Two types of zeolites were selected for the study; these materials are part of a group the team is investigating for use in the conversion of bio-oils to transportation fuels. The team determined the distribution of aluminum by analyzing the extended x-ray absorption fine structure and the 27Al MAS-NMR spectra, and combined and interpreted both using advanced theory. While the 27Al MAS-NMR spectroscopy was performed at the NMR facility in EMSL, the x-ray absorption spectroscopy was performed at the Swiss Light Source, one of the few places in the world where these experiments are possible.
"When the theory and experiment converged on the same answer, we knew we'd captured the structure," said John Fulton, a PNNL scientist who worked on the study. "Without theoretical chemistry, we could not have unraveled the structure."
In the zeolites investigated, the aluminum predominantly occupies certain preferred tetrahedral sites. The aluminum atoms replace silicon atoms at tetrahedral sites on the framework's pentagonal or hexagonal rings. While both materials studied were, in principal, similar, the aluminum distribution was dramatically different because of the synthesis history.
"Now that we can determine with high accuracy the location of the catalyst's active sites, we can correlate catalytic performance with zeolite structure," said Aleksei Vjunov, a postmaster's research associate at PNNL who worked on the study. "This work has allowed insight into the complex zeolite chemistry and opens the door to zeolite synthesis by design, leading to more efficient and sustainable catalysts."
Led by Dr. Johannes Lercher, who directs the Institute for Integrated Catalysis at PNNL, the researchers will be using the new method to examine a range of zeolite samples. Their experiments could show how the activity and structure of these zeolites changes over the lifetime of the reaction. They are also working closely with PNNL's Dr. Miroslaw Derewinski, an internationally recognized expert in the field of zeolite synthesis and characterization.