"This uncertainty has led to a controversy in the scientific literature about the active phase and reaction mechanism of methane activation on these promising catalyst materials," said DICP's Bao.
Enter the world's largest NMR, uniquely capable of addressing this issue. The technological problem lay in the molybdenum oxide itself. To study this particular oxide with NMR, the chemists needed to pick up the signal from one variant of molybdenum, 95Mo; the ultra-high field of the NMR, housed at the DOE's Environmental Molecular Sciences Laboratory on the PNNL campus, allowed them to do so.
"The higher magnetic field improves the signal to noise," said Peden. "And its large sample volume allowed us to put enough catalyst into the spectrometer to overcome the poor sensitivity of 95Mo NMR."
The researchers painstakingly prepared catalysts with increasing concentrations of molybdenum in the zeolite scaffold and focused the 900 MHz NMR on the samples. The data revealed two different forms of the catalyst, as expected. One form contained the smaller nugget and the other form comprised the much larger clusters. When the concentration of molybdenum rose, more of these large clusters formed.
Then the team added methane and measured how much got converted into benzene by the catalysts. They found that when more smaller nuggets were present, more benzene was made, indicating the variety of one or two molybdenum oxide molecules was the reactive one.
Now, said Peden, the challenge is to design and produce the active form of the catalyst that could be used for large-scale benzene production, research that Bao and his group are alrea
|Contact: Mary Beckman|
DOE/Pacific Northwest National Laboratory