Department of Chemical Engineering , Massachusetts Institute of Technology , 77 Massachusetts Avenue , Cambridge , Massachusetts 02139 , United States.
ExxonMobil Research and Engineering , Annandale , New Jersey 08801 , United States.
J Am Chem Soc. 2019 Jul 24;141(29):11641-11650. doi: 10.1021/jacs.9b04906. Epub 2019 Jul 15.
Copper-exchanged zeolites can continuously and selectively catalyze the partial oxidation of methane to methanol using only oxygen and water at low temperatures, but the genesis and nature of the active sites are currently unknown. Herein, we demonstrate that this reaction is catalyzed by a [Cu-O-Cu] motif that forms via a hypothesized proton-aided diffusion of hydrated Cu ions within the cages of SSZ-13 zeolites. While various Cu configurations may be present and active for methane oxidation, a dimeric Cu motif is the primary active site for selective partial methane oxidation. Mechanistically, CH activation proceeds via rate-determining C-H scission to form a surface-bound C intermediate that can either be desorbed as methanol in the presence of HO/H or completely oxidized to CO by gas-phase O. High partial oxidation selectivity can be obtained with (i) high methane and water partial pressures and (ii) maximizing Cu dimer formation by using zeolites with high Al content and low Cu loadings.
铜交换沸石可以仅在低温下使用氧气和水连续且选择性地催化甲烷部分氧化为甲醇，但目前尚不清楚活性位的成因和性质。在此，我们证明该反应是由通过在 SSZ-13 沸石笼内水合 Cu 离子的假设质子辅助扩散形成的[Cu-O-Cu]基序催化的。虽然可能存在各种 Cu 构型并对甲烷氧化具有活性，但二聚 Cu 基序是选择性部分甲烷氧化的主要活性位。从机理上讲，CH 活化通过速率决定的 C-H 断裂进行，形成表面结合的 C 中间体，该中间体在存在 HO/H 的情况下可以解吸为甲醇，或者在气相 O 的存在下完全氧化为 CO。通过使用高 Al 含量和低 Cu 负载的沸石（i）提高甲烷和水的分压以及（ii）最大化 Cu 二聚体的形成，可以获得高的部分氧化选择性。
