Structure of Selective and Nonselective Dicopper (II) Sites in CuMFI for Methane Oxidation to Methanol

Abstract

The one-step valorization of natural gas remains a challenge. Methane conversion to methanol via chemical looping over copper-containing zeolites is a promising route, and CuMFI is among the earliest successfully applied. However, the structure of the active sites in CuMFI, as well as the effect of copper loading and Si/Al ratio on the copper speciation, are yet to be understood. We found that for CuMFI, the Cu/Al ratio determines the selectivity of methane conversion by governing the structure of the active dicopper sites. At a Cu/Al ratio below 0.3, copper-containing MFI materials host dimeric centers with a Cu–Cu separation of 2.9 Å and a UV/vis absorption band at 27 200 cm–1 capable of selective oxidation of methane to methanol in a wide temperature range (450–550 K). A higher Cu/Al ratio leads to the formation of mono-μ-oxo dicopper sites with Cu–Cu = 3.2 Å, which exhibit a characteristic band at 21 900 cm–1 and react with methane at lower temperatures (<450 K), yielding overoxidation products. Identifying distinctions in the structure of selective and nonselective copper sites will aid in the design of better-performing materials.

Quantifying the Limits of Methane Activation in Cu-exchanged Zeolites using Reactive and Interpretable Machine Learning based Potentials

“…Since 𝐸 , is directly and positively related to 𝑑 ()() , lower Cu-Cu distances result in lower 𝐸 , . Our conclusion agrees with experiment, where active sites with 2.9Å Cu-Cu separations are more selective than sites with 3.2Å separations 69.…”

“…These efforts, in part, have been motivated by the possibility of finding an elusive "Goldilocks" active site or topology that can outperform known catalysts while also maintaining selectivity towards methanol [61][62][63][64][65][66][67][68] . For example, recent results from van Bokhoven and Sushkevich suggest that certain types of [CuOCu] 2+ sites (with 2.9 Å Cu--Cu separations) are more selective than others in MFI 69 . These insights are based on detailed kinetic studies and characterization experiments including UV/Vis, XAS, FTIR and etc.…”

Structural Evolution of Copper-Oxo Sites in Zeolites upon the Reaction with Methane Investigated by Means of Cu K-edge X-ray Absorption Spectroscopy

“…Moreover, some recent studies suggest that not one particular type, but a mixture of different copper(II) sites may coexist in the copper-containing zeolite. 23,28,34 All this points to the existence of a variety of copper(II) sites, which likely behave differently under reactive conditions.…”

Mechanism of Hydrocarbon Formation in Methane and Methanol Conversion over Copper-Containing Mordenite

“…24,30 However, these products may undergo further oxidation to carbon mono-and dioxide. 23,29,31 The mentioned reaction pathways are summarized in Figure 1.…”

“…The promising approach for the latter process is methane conversion over copper-containing zeolites. The desired reaction involves methane partial oxidation to methanol, methoxy species, and dimethyl ether (DME), − which are stabilized being bonded to the Brønsted acid sites (BAS) in materials. , However, these products may undergo further oxidation to carbon mono- and dioxide. ,, The mentioned reaction pathways are summarized in Figure .…”