The direct conversion of methane to methanol using molecular oxygen as the oxidant is a grand challenge in the chemical sciences. Many catalytic materials have been investigated for this reaction. Copper-containing zeolites have emerged as one promising class of materials. Here we describe our efforts in using copper exchanged SSZ-39 zeolite for this chemistry and show that it is very promising material for this reaction and more active in our hands than the small-pore zeolite copper exchanged SSZ-13 under similar conditions. Initial testing at 225°C showed Cu-SSZ-39 with superior activity to the current zeolite literature benchmark catalyst for methane partial oxidation, Cu-SSZ-13. We have focused on 1) mapping out the catalytic behavior of this material over a range of operating conditions and metal loadings, and 2) understanding whether there are multiple types of active sites in this material. Maximum methanol productivity, both in terms of a mass catalyst basis and copper basis was observed at a Cu/Al ~ 0.2 with specific activity of 9.82 ± 0.23 mmolCH3OH.gcat-1.h and site time yield of 26.9 ± 0.8 molCH3OH.molCu-1.h *10-3. We should note that this site time yield significantly exceeds our prior work with Cu-SSZ-13. The lab also followed up on running this reaction in the absence of oxygen, per our prior work with SSZ-13. Results for the different catalysts with similar Cu/Al ~0.2 showed different behavior in the presence and absence of O2. The reason why, is not clear at the moment but hints towards the possibility of different copper sites in SSZ-39 compared to SSZ-13 or that their different shaped cages lead to different copper speciation. We are initiating a collaborative effort to perform X-ray absorption spectroscopy (XAS) to better understand this, and the data will be reported at a future time. Ongoing work also focuses on using NO binding using DRIFTS to study probe-molecule metal interactions.

Cu-SSZ-39: Copper exchanged zeolite for partial methane oxidation

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